the international scientific series the new physics and its evolution by lucien poincarÉ inspéctéur-general de l'instruction publique being the authorized translation of _la physique moderne, son Évolution_ new york d. appleton and company prefatory note m. lucien poincaré is one of the distinguished family of mathematicians which has during the last few years given a minister of finance to the republic and a president to the académie des sciences. he is also one of the nineteen inspectors-general of public instruction who are charged with the duty of visiting the different universities and _lycées_ in france and of reporting upon the state of the studies there pursued. hence he is in an excellent position to appreciate at its proper value the extraordinary change which has lately revolutionized physical science, while his official position has kept him aloof from the controversies aroused by the discovery of radium and by recent speculations on the constitution of matter. m. poincaré's object and method in writing the book are sufficiently explained in the preface which follows; but it may be remarked that the best of methods has its defects, and the excessive condensation which has alone made it possible to include the last decade's discoveries in physical science within a compass of some pages has, perhaps, made the facts here noted assimilable with difficulty by the untrained reader. to remedy this as far as possible, i have prefixed to the present translation a table of contents so extended as to form a fairly complete digest of the book, while full indexes of authors and subjects have also been added. the few notes necessary either for better elucidation of the terms employed, or for giving account of discoveries made while these pages were passing through the press, may be distinguished from the author's own by the signature "ed." the editor. royal institution of great britain, april . author's preface during the last ten years so many works have accumulated in the domain of physics, and so many new theories have been propounded, that those who follow with interest the progress of science, and even some professed scholars, absorbed as they are in their own special studies, find themselves at sea in a confusion more apparent than real. it has therefore occurred to me that it might be useful to write a book which, while avoiding too great insistence on purely technical details, should try to make known the general results at which physicists have lately arrived, and to indicate the direction and import which should be ascribed to those speculations on the constitution of matter, and the discussions on the nature of first principles, to which it has become, so to speak, the fashion of the present day to devote oneself. i have endeavoured throughout to rely only on the experiments in which we can place the most confidence, and, above all, to show how the ideas prevailing at the present day have been formed, by tracing their evolution, and rapidly examining the successive transformations which have brought them to their present condition. in order to understand the text, the reader will have no need to consult any treatise on physics, for i have throughout given the necessary definitions and set forth the fundamental facts. moreover, while strictly employing exact expressions, i have avoided the use of mathematical language. algebra is an admirable tongue, but there are many occasions where it can only be used with much discretion. nothing would be easier than to point out many great omissions from this little volume; but some, at all events, are not involuntary. certain questions which are still too confused have been put on one side, as have a few others which form an important collection for a special study to be possibly made later. thus, as regards electrical phenomena, the relations between electricity and optics, as also the theories of ionization, the electronic hypothesis, etc., have been treated at some length; but it has not been thought necessary to dilate upon the modes of production and utilization of the current, upon the phenomena of magnetism, or upon all the applications which belong to the domain of electrotechnics. l. poincarÉ. contents editor's prefatory note author's preface table of contents chapter i the evolution of physics revolutionary change in modern physics only apparent: evolution not revolution the rule in physical theory-- revival of metaphysical speculation and influence of descartes: all phenomena reduced to matter and movement-- modern physicists challenge this: physical, unlike mechanical, phenomena seldom reversible--two schools, one considering experimental laws imperative, the other merely studying relations of magnitudes: both teach something of truth--third or eclectic school-- is mechanics a branch of electrical science? chapter ii measurements § . metrology: lord kelvin's view of its necessity-- its definition § . the measure of length: necessity for unit-- absolute length--history of standard--description of standard metre--unit of wave-lengths preferable--the international metre § . the measure of mass: distinction between mass and weight--objections to legal kilogramme and its precision--possible improvement § . the measure of time: unit of time the second--alternative units proposed--improvements in chronometry and invar § . the measure of temperature: fundamental and derived units--ordinary unit of temperature purely arbitrary--absolute unit mass of h at pressure of m. of hg at ° c.--divergence of thermometric and thermodynamic scales--helium thermometer for low, thermo-electric couple for high, temperatures--lummer and pringsheim's improvements in thermometry. § . derived units and measure of energy: importance of erg as unit--calorimeter usual means of determination--photometric units. § . measure of physical constants: constant of gravitation--discoveries of cavendish, vernon boys, eötvös, richarz and krigar-menzel--michelson's improvements on fizeau and foucault's experiments-- measure of speed of light. chapter iii principles § . the principles of physics: the principles of mechanics affected by recent discoveries--is mass indestructible?--landolt and heydweiller's experiments --lavoisier's law only approximately true--curie's principle of symmetry. § . the principle of the conservation of energy: its evolution: bernoulli, lavoisier and laplace, young, rumford, davy, sadi carnot, and robert mayer--mayer's drawbacks--error of those who would make mechanics part of energetics--verdet's predictions--rankine inventor of energetics--usefulness of work as standard form of energy--physicists who think matter form of energy-- objections to this--philosophical value of conservation doctrine. § . the principle of carnot and clausius: originality of carnot's principle that fall of temperature necessary for production of work by heat-- clausius' postulate that heat cannot pass from cold to hot body without accessory phenomena--entropy result of this--definition of entropy--entropy tends to increase incessantly--a magnitude which measures evolution of system--clausius' and kelvin's deduction that heat end of all energy in universe--objection to this-- carnot's principle not necessarily referable to mechanics --brownian movements--lippmann's objection to kinetic hypothesis. § . thermodynamics: historical work of massieu, willard gibbs, helmholtz, and duhem--willard gibbs founder of thermodynamic statics, van t'hoff its reviver--the phase law--raveau explains it without thermodynamics. § . atomism: connection of subject with preceding hannequin's essay on the atomic hypothesis--molecular physics in disfavour--surface-tension, etc., vanishes when molecule reached--size of molecule--kinetic theory of gases--willard gibbs and boltzmann introduce into it law of probabilities--mean free path of gaseous molecules--application to optics--final division of matter. chapter iv the various states of matter § . the statics of fluids: researches of andrews, cailletet, and others on liquid and gaseous states-- amagat's experiments--van der waals' equation--discovery of corresponding states--amagat's superposed diagrams--exceptions to law--statics of mixed fluids-- kamerlingh onnes' researches--critical constants-- characteristic equation of fluid not yet ascertainable. § . the liquefaction of gases and low temperatures: linde's, siemens', and claude's methods of liquefying gases--apparatus of claude described--dewar's experiments--modification of electrical properties of matter by extreme cold: of magnetic and chemical-- vitality of bacteria unaltered--ramsay's discovery of rare gases of atmosphere--their distribution in nature--liquid hydrogen--helium. § . solids and liquids: continuity of solid and liquid states--viscosity common to both--also rigidity-- spring's analogies of solids and liquids--crystallization --lehmann's liquid crystals--their existence doubted --tamman's view of discontinuity between crystalline and liquid states. § . the deformation of solids: elasticity-- hoocke's, bach's, and bouasse's researches--voigt on the elasticity of crystals--elastic and permanent deformations--brillouin's states of unstable equilibria--duhem and the thermodynamic postulates-- experimental confirmation--guillaume's researches on nickel steel--alloys. chapter v solutions and electrolytic dissociation § . solution: kirchhoff's, gibb's, duhem's and van t'hoff's researches. § . osmosis: history of phenomenon--traube and biologists establish existence of semi-permeable walls--villard's experiments with gases--pfeffer shows osmotic pressure proportional to concentration-- disagreement as to cause of phenomenon. § . osmosis applied to solution: van t'hoff's discoveries--analogy between dissolved body and perfect gas--faults in analogy. § . electrolytic dissociation: van t'hoff's and arrhenius' researches--ionic hypothesis of--fierce opposition to at first--arrhenius' ideas now triumphant --advantages of arrhenius' hypothesis--"the ions which react"--ostwald's conclusions from this--nernst's theory of electrolysis--electrolysis of gases makes electronic theory probable--faraday's two laws--valency-- helmholtz's consequences from faraday's laws. chapter vi the ether § . the luminiferous ether: first idea of ether due to descartes--ether must be imponderable--fresnel shows light vibrations to be transverse--transverse vibrations cannot exist in fluid--ether must be discontinuous. § . radiations: wave-lengths and their measurements--rubens' and lenard's researches-- stationary waves and colour-photography--fresnel's hypothesis opposed by neumann--wiener's and cotton's experiments. § . the electromagnetic ether: ampère's advocacy of mathematical expression--faraday first shows influence of medium in electricity--maxwell's proof that light-waves electromagnetic--his unintelligibility--required confirmation of theory by hertz. § . electrical oscillations: hertz's experiments-- blondlot proves electromagnetic disturbance propagated with speed of light--discovery of ether waves intermediate between hertzian and visible ones--rubens' and nichols' experiments--hertzian and light rays contrasted--pressure of light. § . the x-rays: röntgen's discovery--properties of x-rays--not homogeneous--rutherford and m'clung's experiments on energy corresponding to--barkla's experiments on polarisation of--their speed that of light--are they merely ultra-violet?--stokes and wiechert's theory of independent pulsations generally preferred--j.j. thomson's idea of their formation-- sutherland's and le bon's theories--the n-rays-- blondlot's discovery--experiments cannot be repeated outside france--gutton and mascart's confirmation-- negative experiments prove nothing--supposed wave-length of n-rays. § . the ether and gravitation: descartes' and newton's ideas on gravitation--its speed and other extraordinary characteristics--lesage's hypothesis--crémieux' experiments with drops of liquids--hypothesis of ether insufficient. chapter vii wireless telegraphy § . histories of wireless telegraphy already written, and difficulties of the subject. § . two systems: that which uses the material media (earth, air, or water), and that which employs ether only. § . use of earth as return wire by steinheil --morse's experiments with water of canal--seine used as return wire during siege of paris--johnson and melhuish's indian experiments--preece's telegraph over bristol channel--he welcomes marconi. § . early attempts at transmission of messages through ether--experiments of rathenau and others. § . forerunners of ether telegraphy: clerk maxwell and hertz--dolbear, hughes, and graham bell. § . telegraphy by hertzian waves first suggested by threlfall--crookes', tesla's, lodge's, rutherford's, and popoff's contributions--marconi first makes it practicable. § . the receiver in wireless telegraphy--varley's, calzecchi--onesti's, and branly's researches-- explanation of coherer still obscure. § . wireless telegraphy enters the commercial stage-- defect of marconi's system--braun's, armstrong's, lee de forest's, and fessenden's systems make use of earth-- hertz and marconi entitled to foremost place among discoverers. chapter viii the conductivity of gases and the ions § . the conductivity of gases: relations of matter to ether cardinal problem--conductivity of gases at first misapprehended--erman's forgotten researches--giese first notices phenomenon--experiment with x-rays-- j.j. thomson's interpretation--ionized gas not obedient to ohm's law--discharge of charged conductors by ionized gas. § . the condensation of water-vapour by ions: vapour will not condense without nucleus--wilson's experiments on electrical condensation--wilson and thomson's counting experiment--twenty million ions per c.cm. of gas--estimate of charge borne by ion-- speed of charges--zeleny's and langevin's experiments--negative ions / of size of atoms--natural unit of electricity or electrons. § . how ions are produced: various causes of ionization--moreau's experiments with alkaline salts--barus and bloch on ionization by phosphorus vapours--ionization always result of shock. § . electrons in metals: movement of electrons in metals foreshadowed by weber--giese's, riecke's, drude's, and j.j. thomson's researches--path of ions in metals and conduction of heat--theory of lorentz--hesehus' explanation of electrification by contact--emission of electrons by charged body-- thomson's measurement of positive ions. chapter ix cathode rays and radioactive bodies § . the cathode rays: history of discovery--crookes' theory--lenard rays--perrin's proof of negative charge--cathode rays give rise to x-rays--the canal rays--villard's researches and magneto-cathode rays-- ionoplasty--thomson's measurements of speed of rays-- all atoms can be dissociated. § . radioactive substances: uranic rays of niepce de st victor and becquerel--general radioactivity of matter--le bon's and rutherford's comparison of uranic with x rays--pierre and mme. curie's discovery of polonium and radium--their characteristics--debierne discovers actinium. § . radiations and emanations of radioactive bodies: giesel's, becquerel's, and rutherford's researches--alpha, beta, and gamma rays--sagnac's secondary rays--crookes' spinthariscope--the emanation --ramsay and soddy's researches upon it--transformations of radioactive bodies--their order. § . disaggregation of matter and atomic energy: actual transformations of matter in radioactive bodies --helium or lead final product--ultimate disappearance of radium from earth--energy liberated by radium: its amount and source--suggested models of radioactive atoms--generalization from radioactive phenomena -le bon's theories--ballistic hypothesis generally admitted--does energy come from without--sagnac's experiments--elster and geitel's _contra_. chapter x the ether and matter § . the relations between the ether and matter: attempts to reduce all matter to forms of ether--emission and absorption phenomena show reciprocal action-- laws of radiation--radiation of gases--production of spectrum--differences between light and sound variations show difference of media--cauchy's, briot's, carvallo's and boussinesq's researches--helmholtz's and poincaré's electromagnetic theories of dispersion. § . the theory of lorentz:--mechanics fails to explain relations between ether and matter--lorentz predicts action of magnet on spectrum--zeeman's experiment --later researches upon zeeman effect-- multiplicity of electrons--lorentz's explanation of thermoelectric phenomena by electrons--maxwell's and lorentz's theories do not agree--lorentz's probably more correct--earth's movement in relation to ether. § . the mass of electrons: thomson's and max abraham's view that inertia of charged body due to charge--longitudinal and transversal mass--speed of electrons cannot exceed that of light--ratio of charge to mass and its variation--electron simple electric charge--phenomena produced by its acceleration. § . new views on ether and matter: insufficiency of larmor's view--ether definable by electric and magnetic fields--is matter all electrons? atom probably positive centre surrounded by negative electrons--ignorance concerning positive particles--successive transformations of matter probable --gravitation still unaccounted for. chapter xi the future of physics persistence of ambition to discover supreme principle in physics--supremacy of electron theory at present time--doubtless destined to disappear like others-- constant progress of science predicted--immense field open before it. index of names index of subjects chapter i the evolution of physics the now numerous public which tries with some success to keep abreast of the movement in science, from seeing its mental habits every day upset, and from occasionally witnessing unexpected discoveries that produce a more lively sensation from their reaction on social life, is led to suppose that we live in a really exceptional epoch, scored by profound crises and illustrated by extraordinary discoveries, whose singularity surpasses everything known in the past. thus we often hear it said that physics, in particular, has of late years undergone a veritable revolution; that all its principles have been made new, that all the edifices constructed by our fathers have been overthrown, and that on the field thus cleared has sprung up the most abundant harvest that has ever enriched the domain of science. it is in fact true that the crop becomes richer and more fruitful, thanks to the development of our laboratories, and that the quantity of seekers has considerably increased in all countries, while their quality has not diminished. we should be sustaining an absolute paradox, and at the same time committing a crying injustice, were we to contest the high importance of recent progress, and to seek to diminish the glory of contemporary physicists. yet it may be as well not to give way to exaggerations, however pardonable, and to guard against facile illusions. on closer examination it will be seen that our predecessors might at several periods in history have conceived, as legitimately as ourselves, similar sentiments of scientific pride, and have felt that the world was about to appear to them transformed and under an aspect until then absolutely unknown. let us take an example which is salient enough; for, however arbitrary the conventional division of time may appear to a physicist's eyes, it is natural, when instituting a comparison between two epochs, to choose those which extend over a space of half a score of years, and are separated from each other by the gap of a century. let us, then, go back a hundred years and examine what would have been the state of mind of an erudite amateur who had read and understood the chief publications on physical research between and . let us suppose that this intelligent and attentive spectator witnessed in the discovery of the galvanic battery by volta. he might from that moment have felt a presentiment that a prodigious transformation was about to occur in our mode of regarding electrical phenomena. brought up in the ideas of coulomb and franklin, he might till then have imagined that electricity had unveiled nearly all its mysteries, when an entirely original apparatus suddenly gave birth to applications of the highest interest, and excited the blossoming of theories of immense philosophical extent. in the treatises on physics published a little later, we find traces of the astonishment produced by this sudden revelation of a new world. "electricity," wrote the abbé haüy, "enriched by the labour of so many distinguished physicists, seemed to have reached the term when a science has no further important steps before it, and only leaves to those who cultivate it the hope of confirming the discoveries of their predecessors, and of casting a brighter light on the truths revealed. one would have thought that all researches for diversifying the results of experiment were exhausted, and that theory itself could only be augmented by the addition of a greater degree of precision to the applications of principles already known. while science thus appeared to be making for repose, the phenomena of the convulsive movements observed by galvani in the muscles of a frog when connected by metal were brought to the attention and astonishment of physicists.... volta, in that italy which had been the cradle of the new knowledge, discovered the principle of its true theory in a fact which reduces the explanation of all the phenomena in question to the simple contact of two substances of different nature. this fact became in his hands the germ of the admirable apparatus to which its manner of being and its fecundity assign one of the chief places among those with which the genius of mankind has enriched physics." shortly afterwards, our amateur would learn that carlisle and nicholson had decomposed water by the aid of a battery; then, that davy, in , had produced, by the help of the same battery, a quite unexpected phenomenon, and had succeeded in preparing metals endowed with marvellous properties, beginning with substances of an earthy appearance which had been known for a long time, but whose real nature had not been discovered. in another order of ideas, surprises as prodigious would wait for our amateur. commencing with , he might have read the admirable series of memoirs which young then published, and might thereby have learned how the study of the phenomena of diffraction led to the belief that the undulation theory, which, since the works of newton seemed irretrievably condemned, was, on the contrary, beginning quite a new life. a little later--in --he might have witnessed the discovery made by malus of polarization by reflexion, and would have been able to note, no doubt with stupefaction, that under certain conditions a ray of light loses the property of being reflected. he might also have heard of one rumford, who was then promulgating very singular ideas on the nature of heat, who thought that the then classical notions might be false, that caloric does not exist as a fluid, and who, in , even demonstrated that heat is created by friction. a few years later he would learn that charles had enunciated a capital law on the dilatation of gases; that pierre prevost, in , was making a study, full of original ideas, on radiant heat. in the meantime he would not have failed to read volumes iii. and iv. of the _mecanique celeste_ of laplace, published in and , and he might, no doubt, have thought that before long mathematics would enable physical science to develop with unforeseen safety. all these results may doubtless be compared in importance with the present discoveries. when strange metals like potassium and sodium were isolated by an entirely new method, the astonishment must have been on a par with that caused in our time by the magnificent discovery of radium. the polarization of light is a phenomenon as undoubtedly singular as the existence of the x rays; and the upheaval produced in natural philosophy by the theories of the disintegration of matter and the ideas concerning electrons is probably not more considerable than that produced in the theories of light and heat by the works of young and rumford. if we now disentangle ourselves from contingencies, it will be understood that in reality physical science progresses by evolution rather than by revolution. its march is continuous. the facts which our theories enable us to discover, subsist and are linked together long after these theories have disappeared. out of the materials of former edifices overthrown, new dwellings are constantly being reconstructed. the labour of our forerunners never wholly perishes. the ideas of yesterday prepare for those of to-morrow; they contain them, so to speak, _in potentia_. science is in some sort a living organism, which gives birth to an indefinite series of new beings taking the places of the old, and which evolves according to the nature of its environment, adapting itself to external conditions, and healing at every step the wounds which contact with reality may have occasioned. sometimes this evolution is rapid, sometimes it is slow enough; but it obeys the ordinary laws. the wants imposed by its surroundings create certain organs in science. the problems set to physicists by the engineer who wishes to facilitate transport or to produce better illumination, or by the doctor who seeks to know how such and such a remedy acts, or, again, by the physiologist desirous of understanding the mechanism of the gaseous and liquid exchanges between the cell and the outer medium, cause new chapters in physics to appear, and suggest researches adapted to the necessities of actual life. the evolution of the different parts of physics does not, however, take place with equal speed, because the circumstances in which they are placed are not equally favourable. sometimes a whole series of questions will appear forgotten, and will live only with a languishing existence; and then some accidental circumstance suddenly brings them new life, and they become the object of manifold labours, engross public attention, and invade nearly the whole domain of science. we have in our own day witnessed such a spectacle. the discovery of the x rays--a discovery which physicists no doubt consider as the logical outcome of researches long pursued by a few scholars working in silence and obscurity on an otherwise much neglected subject-- seemed to the public eye to have inaugurated a new era in the history of physics. if, as is the case, however, the extraordinary scientific movement provoked by röntgen's sensational experiments has a very remote origin, it has, at least, been singularly quickened by the favourable conditions created by the interest aroused in its astonishing applications to radiography. a lucky chance has thus hastened an evolution already taking place, and theories previously outlined have received a singular development. without wishing to yield too much to what may be considered a whim of fashion, we cannot, if we are to note in this book the stage actually reached in the continuous march of physics, refrain from giving a clearly preponderant place to the questions suggested by the study of the new radiations. at the present time it is these questions which move us the most; they have shown us unknown horizons, and towards the fields recently opened to scientific activity the daily increasing crowd of searchers rushes in rather disorderly fashion. one of the most interesting consequences of the recent discoveries has been to rehabilitate in the eyes of scholars, speculations relating to the constitution of matter, and, in a more general way, metaphysical problems. philosophy has, of course, never been completely separated from science; but in times past many physicists dissociated themselves from studies which they looked upon as unreal word-squabbles, and sometimes not unreasonably abstained from joining in discussions which seemed to them idle and of rather puerile subtlety. they had seen the ruin of most of the systems built up _a priori_ by daring philosophers, and deemed it more prudent to listen to the advice given by kirchhoff and "to substitute the description of facts for a sham explanation of nature." it should however be remarked that these physicists somewhat deceived themselves as to the value of their caution, and that the mistrust they manifested towards philosophical speculations did not preclude their admitting, unknown to themselves, certain axioms which they did not discuss, but which are, properly speaking, metaphysical conceptions. they were unconsciously speaking a language taught them by their predecessors, of which they made no attempt to discover the origin. it is thus that it was readily considered evident that physics must necessarily some day re-enter the domain of mechanics, and thence it was postulated that everything in nature is due to movement. we, further, accepted the principles of the classical mechanics without discussing their legitimacy. this state of mind was, even of late years, that of the most illustrious physicists. it is manifested, quite sincerely and without the slightest reserve, in all the classical works devoted to physics. thus verdet, an illustrious professor who has had the greatest and most happy influence on the intellectual formation of a whole generation of scholars, and whose works are even at the present day very often consulted, wrote: "the true problem of the physicist is always to reduce all phenomena to that which seems to us the simplest and clearest, that is to say, to movement." in his celebrated course of lectures at l'École polytechnique, jamin likewise said: "physics will one day form a chapter of general mechanics;" and in the preface to his excellent course of lectures on physics, m. violle, in , thus expresses himself: "the science of nature tends towards mechanics by a necessary evolution, the physicist being able to establish solid theories only on the laws of movement." the same idea is again met with in the words of cornu in : "the general tendency should be to show how the facts observed and the phenomena measured, though first brought together by empirical laws, end, by the impulse of successive progressions, in coming under the general laws of rational mechanics;" and the same physicist showed clearly that in his mind this connexion of phenomena with mechanics had a deep and philosophical reason, when, in the fine discourse pronounced by him at the opening ceremony of the congrès de physique in , he exclaimed: "the mind of descartes soars over modern physics, or rather, i should say, he is their luminary. the further we penetrate into the knowledge of natural phenomena, the clearer and the more developed becomes the bold cartesian conception regarding the mechanism of the universe. there is nothing in the physical world but matter and movement." if we adopt this conception, we are led to construct mechanical representations of the material world, and to imagine movements in the different parts of bodies capable of reproducing all the manifestations of nature. the kinematic knowledge of these movements, that is to say, the determination of the position, speed, and acceleration at a given moment of all the parts of the system, or, on the other hand, their dynamical study, enabling us to know what is the action of these parts on each other, would then be sufficient to enable us to foretell all that can occur in the domain of nature. this was the great thought clearly expressed by the encyclopædists of the eighteenth century; and if the necessity of interpreting the phenomena of electricity or light led the physicists of last century to imagine particular fluids which seemed to obey with some difficulty the ordinary rules of mechanics, these physicists still continued to retain their hope in the future, and to treat the idea of descartes as an ideal to be reached sooner or later. certain scholars--particularly those of the english school--outrunning experiment, and pushing things to extremes, took pleasure in proposing very curious mechanical models which were often strange images of reality. the most illustrious of them, lord kelvin, may be considered as their representative type, and he has himself said: "it seems to me that the true sense of the question, do we or do we not understand a particular subject in physics? is--can we make a mechanical model which corresponds to it? i am never satisfied so long as i have been unable to make a mechanical model of the object. if i am able to do so, i understand it. if i cannot make such a model, i do not understand it." but it must be acknowledged that some of the models thus devised have become excessively complicated, and this complication has for a long time discouraged all but very bold minds. in addition, when it became a question of penetrating into the mechanism of molecules, and we were no longer satisfied to look at matter as a mass, the mechanical solutions seemed undetermined and the stability of the edifices thus constructed was insufficiently demonstrated. returning then to our starting-point, many contemporary physicists wish to subject descartes' idea to strict criticism. from the philosophical point of view, they first enquire whether it is really demonstrated that there exists nothing else in the knowable than matter and movement. they ask themselves whether it is not habit and tradition in particular which lead us to ascribe to mechanics the origin of phenomena. perhaps also a question of sense here comes in. our senses, which are, after all, the only windows open towards external reality, give us a view of one side of the world only; evidently we only know the universe by the relations which exist between it and our organisms, and these organisms are peculiarly sensitive to movement. nothing, however, proves that those acquisitions which are the most ancient in historical order ought, in the development of science, to remain the basis of our knowledge. nor does any theory prove that our perceptions are an exact indication of reality. many reasons, on the contrary, might be invoked which tend to compel us to see in nature phenomena which cannot be reduced to movement. mechanics as ordinarily understood is the study of reversible phenomena. if there be given to the parameter which represents time,[ ] and which has assumed increasing values during the duration of the phenomena, decreasing values which make it go the opposite way, the whole system will again pass through exactly the same stages as before, and all the phenomena will unfold themselves in reversed order. in physics, the contrary rule appears very general, and reversibility generally does not exist. it is an ideal and limited case, which may be sometimes approached, but can never, strictly speaking, be met with in its entirety. no physical phenomenon ever recommences in an identical manner if its direction be altered. it is true that certain mathematicians warn us that a mechanics can be devised in which reversibility would no longer be the rule, but the bold attempts made in this direction are not wholly satisfactory. [footnote : i.e., the time-curve.--ed.] on the other hand, it is established that if a mechanical explanation of a phenomenon can be given, we can find an infinity of others which likewise account for all the peculiarities revealed by experiment. but, as a matter of fact, no one has ever succeeded in giving an indisputable mechanical representation of the whole physical world. even were we disposed to admit the strangest solutions of the problem; to consent, for example, to be satisfied with the hidden systems devised by helmholtz, whereby we ought to divide variable things into two classes, some accessible, and the others now and for ever unknown, we should never manage to construct an edifice to contain all the known facts. even the very comprehensive mechanics of a hertz fails where the classical mechanics has not succeeded. deeming this check irremediable, many contemporary physicists give up attempts which they look upon as condemned beforehand, and adopt, to guide them in their researches, a method which at first sight appears much more modest, and also much more sure. they make up their minds not to see at once to the bottom of things; they no longer seek to suddenly strip the last veils from nature, and to divine her supreme secrets; but they work prudently and advance but slowly, while on the ground thus conquered foot by foot they endeavour to establish themselves firmly. they study the various magnitudes directly accessible to their observation without busying themselves as to their essence. they measure quantities of heat and of temperature, differences of potential, currents, and magnetic fields; and then, varying the conditions, apply the rules of experimental method, and discover between these magnitudes mutual relations, while they thus succeed in enunciating laws which translate and sum up their labours. these empirical laws, however, themselves bring about by induction the promulgation of more general laws, which are termed principles. these principles are originally only the results of experiments, and experiment allows them besides to be checked, and their more or less high degree of generality to be verified. when they have been thus definitely established, they may serve as fresh starting-points, and, by deduction, lead to very varied discoveries. the principles which govern physical science are few in number, and their very general form gives them a philosophical appearance, while we cannot long resist the temptation of regarding them as metaphysical dogmas. it thus happens that the least bold physicists, those who have wanted to show themselves the most reserved, are themselves led to forget the experimental character of the laws they have propounded, and to see in them imperious beings whose authority, placed above all verification, can no longer be discussed. others, on the contrary, carry prudence to the extent of timidity. they desire to grievously limit the field of scientific investigation, and they assign to science a too restricted domain. they content themselves with representing phenomena by equations, and think that they ought to submit to calculation magnitudes experimentally determined, without asking themselves whether these calculations retain a physical meaning. they are thus led to reconstruct a physics in which there again appears the idea of quality, understood, of course, not in the scholastic sense, since from this quality we can argue with some precision by representing it under numerical symbols, but still constituting an element of differentiation and of heterogeneity. notwithstanding the errors they may lead to if carried to excess, both these doctrines render, as a whole, most important service. it is no bad thing that these contradictory tendencies should subsist, for this variety in the conception of phenomena gives to actual science a character of intense life and of veritable youth, capable of impassioned efforts towards the truth. spectators who see such moving and varied pictures passing before them, experience the feeling that there no longer exist systems fixed in an immobility which seems that of death. they feel that nothing is unchangeable; that ceaseless transformations are taking place before their eyes; and that this continuous evolution and perpetual change are the necessary conditions of progress. a great number of seekers, moreover, show themselves on their own account perfectly eclectic. they adopt, according to their needs, such or such a manner of looking at nature, and do not hesitate to utilize very different images when they appear to them useful and convenient. and, without doubt, they are not wrong, since these images are only symbols convenient for language. they allow facts to be grouped and associated, but only present a fairly distant resemblance with the objective reality. hence it is not forbidden to multiply and to modify them according to circumstances. the really essential thing is to have, as a guide through the unknown, a map which certainly does not claim to represent all the aspects of nature, but which, having been drawn up according to predetermined rules, allows us to follow an ascertained road in the eternal journey towards the truth. among the provisional theories which are thus willingly constructed by scholars on their journey, like edifices hastily run up to receive an unforeseen harvest, some still appear very bold and very singular. abandoning the search after mechanical models for all electrical phenomena, certain physicists reverse, so to speak, the conditions of the problem, and ask themselves whether, instead of giving a mechanical interpretation to electricity, they may not, on the contrary, give an electrical interpretation to the phenomena of matter and motion, and thus merge mechanics itself in electricity. one thus sees dawning afresh the eternal hope of co-ordinating all natural phenomena in one grandiose and imposing synthesis. whatever may be the fate reserved for such attempts, they deserve attention in the highest degree; and it is desirable to examine them carefully if we wish to have an exact idea of the tendencies of modern physics. chapter ii measurements § . metrology not so very long ago, the scholar was often content with qualitative observations. many phenomena were studied without much trouble being taken to obtain actual measurements. but it is now becoming more and more understood that to establish the relations which exist between physical magnitudes, and to represent the variations of these magnitudes by functions which allow us to use the power of mathematical analysis, it is most necessary to express each magnitude by a definite number. under these conditions alone can a magnitude be considered as effectively known. "i often say," lord kelvin has said, "that if you can measure that of which you are speaking and express it by a number you know something of your subject; but if you cannot measure it nor express it by a number, your knowledge is of a sorry kind and hardly satisfactory. it may be the beginning of the acquaintance, but you are hardly, in your thoughts, advanced towards science, whatever the subject may be." it has now become possible to measure exactly the elements which enter into nearly all physical phenomena, and these measurements are taken with ever increasing precision. every time a chapter in science progresses, science shows itself more exacting; it perfects its means of investigation, it demands more and more exactitude, and one of the most striking features of modern physics is this constant care for strictness and clearness in experimentation. a veritable science of measurement has thus been constituted which extends over all parts of the domain of physics. this science has its rules and its methods; it points out the best processes of calculation, and teaches the method of correctly estimating errors and taking account of them. it has perfected the processes of experiment, co-ordinated a large number of results, and made possible the unification of standards. it is thanks to it that the system of measurements unanimously adopted by physicists has been formed. at the present day we designate more peculiarly by the name of metrology that part of the science of measurements which devotes itself specially to the determining of the prototypes representing the fundamental units of dimension and mass, and of the standards of the first order which are derived from them. if all measurable quantities, as was long thought possible, could be reduced to the magnitudes of mechanics, metrology would thus be occupied with the essential elements entering into all phenomena, and might legitimately claim the highest rank in science. but even when we suppose that some magnitudes can never be connected with mass, length, and time, it still holds a preponderating place, and its progress finds an echo throughout the whole domain of the natural sciences. it is therefore well, in order to give an account of the general progress of physics, to examine at the outset the improvements which have been effected in these fundamental measurements, and to see what precision these improvements have allowed us to attain. § . the measure of length to measure a length is to compare it with another length taken as unity. measurement is therefore a relative operation, and can only enable us to know ratios. did both the length to be measured and the unit chosen happen to vary simultaneously and in the same degree, we should perceive no change. moreover, the unit being, by definition, the term of comparison, and not being itself comparable with anything, we have theoretically no means of ascertaining whether its length varies. if, however, we were to note that, suddenly and in the same proportions, the distance between two points on this earth had increased, that all the planets had moved further from each other, that all objects around us had become larger, that we ourselves had become taller, and that the distance travelled by light in the duration of a vibration had become greater, we should not hesitate to think ourselves the victims of an illusion, that in reality all these distances had remained fixed, and that all these appearances were due to a shortening of the rule which we had used as the standard for measuring the lengths. from the mathematical point of view, it may be considered that the two hypotheses are equivalent; all has lengthened around us, or else our standard has become less. but it is no simple question of convenience and simplicity which leads us to reject the one supposition and to accept the other; it is right in this case to listen to the voice of common sense, and those physicists who have an instinctive trust in the notion of an absolute length are perhaps not wrong. it is only by choosing our unit from those which at all times have seemed to all men the most invariable, that we are able in our experiments to note that the same causes acting under identical conditions always produce the same effects. the idea of absolute length is derived from the principle of causality; and our choice is forced upon us by the necessity of obeying this principle, which we cannot reject without declaring by that very act all science to be impossible. similar remarks might be made with regard to the notions of absolute time and absolute movement. they have been put in evidence and set forth very forcibly by a learned and profound mathematician, m. painlevé. on the particularly clear example of the measure of length, it is interesting to follow the evolution of the methods employed, and to run through the history of the progress in precision from the time that we have possessed authentic documents relating to this question. this history has been written in a masterly way by one of the physicists who have in our days done the most by their personal labours to add to it glorious pages. m. benoit, the learned director of the international bureau of weights and measures, has furnished in various reports very complete details on the subject, from which i here borrow the most interesting. we know that in france the fundamental standard for measures of length was for a long time the _toise du châtelet_, a kind of callipers formed of a bar of iron which in was embedded in the outside wall of the châtelet, at the foot of the staircase. this bar had at its extremities two projections with square faces, and all the _toises_ of commerce had to fit exactly between them. such a standard, roughly constructed, and exposed to all the injuries of weather and time, offered very slight guarantees either as to the permanence or the correctness of its copies. nothing, perhaps, can better convey an idea of the importance of the modifications made in the methods of experimental physics than the easy comparison between so rudimentary a process and the actual measurements effected at the present time. the _toise du châtelet_, notwithstanding its evident faults, was employed for nearly a hundred years; in it was replaced by the _toise du pérou_, so called because it had served for the measurements of the terrestrial arc effected in peru from to by bouguer, la condamine, and godin. at that time, according to the comparisons made between this new _toise_ and the _toise du nord_, which had also been used for the measurement of an arc of the meridian, an error of the tenth part of a millimetre in measuring lengths of the order of a metre was considered quite unimportant. at the end of the eighteenth century, delambre, in his work _sur la base du système métrique décimal_, clearly gives us to understand that magnitudes of the order of the hundredth of a millimetre appear to him incapable of observation, even in scientific researches of the highest precision. at the present date the international bureau of weights and measures guarantees, in the determination of a standard of length compared with the metre, an approximation of two or three ten-thousandths of a millimetre, and even a little more under certain circumstances. this very remarkable progress is due to the improvements in the method of comparison on the one hand, and in the manufacture of the standard on the other. m. benoit rightly points out that a kind of competition has been set up between the standard destined to represent the unit with its subdivisions and multiples and the instrument charged with observing it, comparable, up to a certain point, with that which in another order of ideas goes on between the gun and the armour-plate. the measuring instrument of to-day is an instrument of comparison constructed with meticulous care, which enables us to do away with causes of error formerly ignored, to eliminate the action of external phenomena, and to withdraw the experiment from the influence of even the personality of the observer. this standard is no longer, as formerly, a flat rule, weak and fragile, but a rigid bar, incapable of deformation, in which the material is utilised in the best conditions of resistance. for a standard with ends has been substituted a standard with marks, which permits much more precise definition and can be employed in optical processes of observation alone; that is, in processes which can produce in it no deformation and no alteration. moreover, the marks are traced on the plane of the neutral fibres[ ] exposed, and the invariability of their distance apart is thus assured, even when a change is made in the way the rule is supported. [footnote : the author seems to refer to the fact that in the standard metre, the measurement is taken from the central one of three marks at each end of the bar. the transverse section of the bar is an x, and the reading is made by a microscope.--ed.] thanks to studies thus systematically pursued, we have succeeded in the course of a hundred years in increasing the precision of measures in the proportion of a thousand to one, and we may ask ourselves whether such an increase will continue in the future. no doubt progress will not be stayed; but if we keep to the definition of length by a material standard, it would seem that its precision cannot be considerably increased. we have nearly reached the limit imposed by the necessity of making strokes of such a thickness as to be observable under the microscope. it may happen, however, that we shall be brought one of these days to a new conception of the measure of length, and that very different processes of determination will be thought of. if we took as unit, for instance, the distance covered by a given radiation during a vibration, the optical processes would at once admit of much greater precision. thus fizeau, the first to have this idea, says: "a ray of light, with its series of undulations of extreme tenuity but perfect regularity, may be considered as a micrometer of the greatest perfection, and particularly suitable for determining length." but in the present state of things, since the legal and customary definition of the unit remains a material standard, it is not enough to measure length in terms of wave-lengths, and we must also know the value of these wave-lengths in terms of the standard prototype of the metre. this was determined in by m. michelson and m. benoit in an experiment which will remain classic. the two physicists measured a standard length of about ten centimetres, first in terms of the wave-lengths of the red, green, and blue radiations of cadmium, and then in terms of the standard metre. the great difficulty of the experiment proceeds from the vast difference which exists between the lengths to be compared, the wave-lengths barely amounting to half a micron;[ ] the process employed consisted in noting, instead of this length, a length easily made about a thousand times greater, namely, the distance between the fringes of interference. [footnote : i.e. / of a millimetre.--ed.] in all measurement, that is to say in every determination of the relation of a magnitude to the unit, there has to be determined on the one hand the whole, and on the other the fractional part of this ratio, and naturally the most delicate determination is generally that of this fractional part. in optical processes the difficulty is reversed. the fractional part is easily known, while it is the high figure of the number representing the whole which becomes a very serious obstacle. it is this obstacle which mm. michelson and benoit overcame with admirable ingenuity. by making use of a somewhat similar idea, m. macé de lépinay and mm. perot and fabry, have lately effected by optical methods, measurements of the greatest precision, and no doubt further progress may still be made. a day may perhaps come when a material standard will be given up, and it may perhaps even be recognised that such a standard in time changes its length by molecular strain, and by wear and tear: and it will be further noted that, in accordance with certain theories which will be noticed later on, it is not invariable when its orientation is changed. for the moment, however, the need of any change in the definition of the unit is in no way felt; we must, on the contrary, hope that the use of the unit adopted by the physicists of the whole world will spread more and more. it is right to remark that a few errors still occur with regard to this unit, and that these errors have been facilitated by incoherent legislation. france herself, though she was the admirable initiator of the metrical system, has for too long allowed a very regrettable confusion to exist; and it cannot be noted without a certain sadness that it was not until the _ th july _ that a law was promulgated re-establishing the agreement between the legal and the scientific definition of the metre. perhaps it may not be useless to briefly indicate here the reasons of the disagreement which had taken place. two definitions of the metre can be, and in fact were given. one had for its basis the dimensions of the earth, the other the length of the material standard. in the minds of the founders of the metrical system, the first of these was the true definition of the unit of length, the second merely a simple representation. it was admitted, however, that this representation had been constructed in a manner perfect enough for it to be nearly impossible to perceive any difference between the unit and its representation, and for the practical identity of the two definitions to be thus assured. the creators of the metrical system were persuaded that the measurements of the meridian effected in their day could never be surpassed in precision; and on the other hand, by borrowing from nature a definite basis, they thought to take from the definition of the unit some of its arbitrary character, and to ensure the means of again finding the same unit if by any accident the standard became altered. their confidence in the value of the processes they had seen employed was exaggerated, and their mistrust of the future unjustified. this example shows how imprudent it is to endeavour to fix limits to progress. it is an error to think the march of science can be stayed; and in reality it is now known that the ten-millionth part of the quarter of the terrestrial meridian is longer than the metre by . millimetres. but contemporary physicists do not fall into the same error as their forerunners, and they regard the present result as merely provisional. they guess, in fact, that new improvements will be effected in the art of measurement; they know that geodesical processes, though much improved in our days, have still much to do to attain the precision displayed in the construction and determination of standards of the first order; and consequently they do not propose to keep the ancient definition, which would lead to having for unit a magnitude possessing the grave defect from a practical point of view of being constantly variable. we may even consider that, looked at theoretically, its permanence would not be assured. nothing, in fact, proves that sensible variations may not in time be produced in the value of an arc of the meridian, and serious difficulties may arise regarding the probable inequality of the various meridians. for all these reasons, the idea of finding a natural unit has been gradually abandoned, and we have become resigned to accepting as a fundamental unit an arbitrary and conventional length having a material representation recognised by universal consent; and it was this unit which was consecrated by the following law of the th july :-- "the standard prototype of the metrical system is the international metre, which has been sanctioned by the general conference on weights and measures." § . the measure of mass on the subject of measures of mass, similar remarks to those on measures of length might be made. the confusion here was perhaps still greater, because, to the uncertainty relating to the fixing of the unit, was added some indecision on the very nature of the magnitude defined. in law, as in ordinary practice, the notions of weight and of mass were not, in fact, separated with sufficient clearness. they represent, however, two essentially different things. mass is the characteristic of a quantity of matter; it depends neither on the geographical position one occupies nor on the altitude to which one may rise; it remains invariable so long as nothing material is added or taken away. weight is the action which gravity has upon the body under consideration; this action does not depend solely on the body, but on the earth as well; and when it is changed from one spot to another, the weight changes, because gravity varies with latitude and altitude. these elementary notions, to-day understood even by young beginners, appear to have been for a long time indistinctly grasped. the distinction remained confused in many minds, because, for the most part, masses were comparatively estimated by the intermediary of weights. the estimations of weight made with the balance utilize the action of the weight on the beam, but in such conditions that the influence of the variations of gravity becomes eliminated. the two weights which are being compared may both of them change if the weighing is effected in different places, but they are attracted in the same proportion. if once equal, they remain equal even when in reality they may both have varied. the current law defines the kilogramme as the standard of mass, and the law is certainly in conformity with the rather obscurely expressed intentions of the founders of the metrical system. their terminology was vague, but they certainly had in view the supply of a standard for commercial transactions, and it is quite evident that in barter what is important to the buyer as well as to the seller is not the attraction the earth may exercise on the goods, but the quantity that may be supplied for a given price. besides, the fact that the founders abstained from indicating any specified spot in the definition of the kilogramme, when they were perfectly acquainted with the considerable variations in the intensity of gravity, leaves no doubt as to their real desire. the same objections have been made to the definition of the kilogramme, at first considered as the mass of a cubic decimetre of water at ° c., as to the first definition of the metre. we must admire the incredible precision attained at the outset by the physicists who made the initial determinations, but we know at the present day that the kilogramme they constructed is slightly too heavy (by about / , ). very remarkable researches have been carried out with regard to this determination by the international bureau, and by mm. macé de lépinay and buisson. the law of the th july has definitely regularized the custom which physicists had adopted some years before; and the standard of mass, the legal prototype of the metrical system, is now the international kilogramme sanctioned by the conference of weights and measures. the comparison of a mass with the standard is effected with a precision to which no other measurement can attain. metrology vouches for the hundredth of a milligramme in a kilogramme; that is to say, that it estimates the hundred-millionth part of the magnitude studied. we may--as in the case of the lengths--ask ourselves whether this already admirable precision can be surpassed; and progress would seem likely to be slow, for difficulties singularly increase when we get to such small quantities. but it is permitted to hope that the physicists of the future will do still better than those of to-day; and perhaps we may catch a glimpse of the time when we shall begin to observe that the standard, which is constructed from a heavy metal, namely, iridium-platinum, itself obeys an apparently general law, and little by little loses some particles of its mass by emanation. § . the measure of time the third fundamental magnitude of mechanics is time. there is, so to speak, no physical phenomenon in which the notion of time linked to the sequence of our states of consciousness does not play a considerable part. ancestral habits and a very early tradition have led us to preserve, as the unit of time, a unit connected with the earth's movement; and the unit to-day adopted is, as we know, the sexagesimal second of mean time. this magnitude, thus defined by the conditions of a natural motion which may itself be modified, does not seem to offer all the guarantees desirable from the point of view of invariability. it is certain that all the friction exercised on the earth--by the tides, for instance--must slowly lengthen the duration of the day, and must influence the movement of the earth round the sun. such influence is certainly very slight, but it nevertheless gives an unfortunately arbitrary character to the unit adopted. we might have taken as the standard of time the duration of another natural phenomenon, which appears to be always reproduced under identical conditions; the duration, for instance, of a given luminous vibration. but the experimental difficulties of evaluation with such a unit of the times which ordinarily have to be considered, would be so great that such a reform in practice cannot be hoped for. it should, moreover, be remarked that the duration of a vibration may itself be influenced by external circumstances, among which are the variations of the magnetic field in which its source is placed. it could not, therefore, be strictly considered as independent of the earth; and the theoretical advantage which might be expected from this alteration would be somewhat illusory. perhaps in the future recourse may be had to very different phenomena. thus curie pointed out that if the air inside a glass tube has been rendered radioactive by a solution of radium, the tube may be sealed up, and it will then be noted that the radiation of its walls diminishes with time, in accordance with an exponential law. the constant of time derived by this phenomenon remains the same whatever the nature and dimensions of the walls of the tube or the temperature may be, and time might thus be denned independently of all the other units. we might also, as m. lippmann has suggested in an extremely ingenious way, decide to obtain measures of time which can be considered as absolute because they are determined by parameters of another nature than that of the magnitude to be measured. such experiments are made possible by the phenomena of gravitation. we could employ, for instance, the pendulum by adopting, as the unit of force, the force which renders the constant of gravitation equal to unity. the unit of time thus defined would be independent of the unit of length, and would depend only on the substance which would give us the unit of mass under the unit of volume. it would be equally possible to utilize electrical phenomena, and one might devise experiments perfectly easy of execution. thus, by charging a condenser by means of a battery, and discharging it a given number of times in a given interval of time, so that the effect of the current of discharge should be the same as the effect of the output of the battery through a given resistance, we could estimate, by the measurement of the electrical magnitudes, the duration of the interval noted. a system of this kind must not be looked upon as a simple _jeu d'esprit_, since this very practicable experiment would easily permit us to check, with a precision which could be carried very far, the constancy of an interval of time. from the practical point of view, chronometry has made in these last few years very sensible progress. the errors in the movements of chronometers are corrected in a much more systematic way than formerly, and certain inventions have enabled important improvements to be effected in the construction of these instruments. thus the curious properties which steel combined with nickel--so admirably studied by m.ch.ed. guillaume--exhibits in the matter of dilatation are now utilized so as to almost completely annihilate the influence of variations of temperature. § . the measure of temperature from the three mechanical units we derive secondary units; as, for instance, the unit of work or mechanical energy. the kinetic theory takes temperature, as well as heat itself, to be a quantity of energy, and thus seems to connect this notion with the magnitudes of mechanics. but the legitimacy of this theory cannot be admitted, and the calorific movement should also be a phenomenon so strictly confined in space that our most delicate means of investigation would not enable us to perceive it. it is better, then, to continue to regard the unit of difference of temperature as a distinct unit, to be added to the fundamental units. to define the measure of a certain temperature, we take, in practice, some arbitrary property of a body. the only necessary condition of this property is, that it should constantly vary in the same direction when the temperature rises, and that it should possess, at any temperature, a well-marked value. we measure this value by melting ice and by the vapour of boiling water under normal pressure, and the successive hundredths of its variation, beginning with the melting ice, defines the percentage. thermodynamics, however, has made it plain that we can set up a thermometric scale without relying upon any determined property of a real body. such a scale has an absolute value independently of the properties of matter. now it happens that if we make use for the estimation of temperatures, of the phenomena of dilatation under a constant pressure, or of the increase of pressure in a constant volume of a gaseous body, we obtain a scale very near the absolute, which almost coincides with it when the gas possesses certain qualities which make it nearly what is called a perfect gas. this most lucky coincidence has decided the choice of the convention adopted by physicists. they define normal temperature by means of the variations of pressure in a mass of hydrogen beginning with the initial pressure of a metre of mercury at ° c. m.p. chappuis, in some very precise experiments conducted with much method, has proved that at ordinary temperatures the indications of such a thermometer are so close to the degrees of the theoretical scale that it is almost impossible to ascertain the value of the divergences, or even the direction that they take. the divergence becomes, however, manifest when we work with extreme temperatures. it results from the useful researches of m. daniel berthelot that we must subtract + . ° from the indications of the hydrogen thermometer towards the temperature - ° c, and add + . ° to ° to equate them with the thermodynamic scale. of course, the difference would also become still more noticeable on getting nearer to the absolute zero; for as hydrogen gets more and more cooled, it gradually exhibits in a lesser degree the characteristics of a perfect gas. to study the lower regions which border on that kind of pole of cold towards which are straining the efforts of the many physicists who have of late years succeeded in getting a few degrees further forward, we may turn to a gas still more difficult to liquefy than hydrogen. thus, thermometers have been made of helium; and from the temperature of - ° c. downward the divergence of such a thermometer from one of hydrogen is very marked. the measurement of very high temperatures is not open to the same theoretical objections as that of very low temperatures; but, from a practical point of view, it is as difficult to effect with an ordinary gas thermometer. it becomes impossible to guarantee the reservoir remaining sufficiently impermeable, and all security disappears, notwithstanding the use of recipients very superior to those of former times, such as those lately devised by the physicists of the _reichansalt_. this difficulty is obviated by using other methods, such as the employment of thermo-electric couples, such as the very convenient couple of m. le chatelier; but the graduation of these instruments can only be effected at the cost of a rather bold extrapolation. m.d. berthelot has pointed out and experimented with a very interesting process, founded on the measurement by the phenomena of interference of the refractive index of a column of air subjected to the temperature it is desired to measure. it appears admissible that even at the highest temperatures the variation of the power of refraction is strictly proportional to that of the density, for this proportion is exactly verified so long as it is possible to check it precisely. we can thus, by a method which offers the great advantage of being independent of the power and dimension of the envelopes employed--since the length of the column of air considered alone enters into the calculation--obtain results equivalent to those given by the ordinary air thermometer. another method, very old in principle, has also lately acquired great importance. for a long time we sought to estimate the temperature of a body by studying its radiation, but we did not know any positive relation between this radiation and the temperature, and we had no good experimental method of estimation, but had recourse to purely empirical formulas and the use of apparatus of little precision. now, however, many physicists, continuing the classic researches of kirchhoff, boltzmann, professors wien and planck, and taking their starting-point from the laws of thermodynamics, have given formulas which establish the radiating power of a dark body as a function of the temperature and the wave-length, or, better still, of the total power as a function of the temperature and wave-length corresponding to the maximum value of the power of radiation. we see, therefore, the possibility of appealing for the measurement of temperature to a phenomenon which is no longer the variation of the elastic force of a gas, and yet is also connected with the principles of thermodynamics. this is what professors lummer and pringsheim have shown in a series of studies which may certainly be reckoned among the greatest experimental researches of the last few years. they have constructed a radiator closely resembling the theoretically integral radiator which a closed isothermal vessel would be, and with only a very small opening, which allows us to collect from outside the radiations which are in equilibrium with the interior. this vessel is formed of a hollow carbon cylinder, heated by a current of high intensity; the radiations are studied by means of a bolometer, the disposition of which varies with the nature of the experiments. it is hardly possible to enter into the details of the method, but the result sufficiently indicates its importance. it is now possible, thanks to their researches, to estimate a temperature of ° c. to within about °. ten years ago a similar approximation could hardly have been arrived at for a temperature of ° c. § . derived units and the measure of a quantity of energy it must be understood that it is only by arbitrary convention that a dependency is established between a derived unit and the fundamental units. the laws of numbers in physics are often only laws of proportion. we transform them into laws of equation, because we introduce numerical coefficients and choose the units on which they depend so as to simplify as much as possible the formulas most in use. a particular speed, for instance, is in reality nothing else but a speed, and it is only by the peculiar choice of unit that we can say that it is the space covered during the unit of time. in the same way, a quantity of electricity is a quantity of electricity; and there is nothing to prove that, in its essence, it is really reducible to a function of mass, of length, and of time. persons are still to be met with who seem to have some illusions on this point, and who see in the doctrine of the dimensions of the units a doctrine of general physics, while it is, to say truth, only a doctrine of metrology. the knowledge of dimensions is valuable, since it allows us, for instance, to easily verify the homogeneity of a formula, but it can in no way give us any information on the actual nature of the quantity measured. magnitudes to which we attribute like dimensions may be qualitatively irreducible one to the other. thus the different forms of energy are measured by the same unit, and yet it seems that some of them, such as kinetic energy, really depend on time; while for others, such as potential energy, the dependency established by the system of measurement seems somewhat fictitious. the numerical value of a quantity of energy of any nature should, in the system c.g.s., be expressed in terms of the unit called the erg; but, as a matter of fact, when we wish to compare and measure different quantities of energy of varying forms, such as electrical, chemical, and other quantities, etc., we nearly always employ a method by which all these energies are finally transformed and used to heat the water of a calorimeter. it is therefore very important to study well the calorific phenomenon chosen as the unit of heat, and to determine with precision its mechanical equivalent, that is to say, the number of ergs necessary to produce this unit. this is a number which, on the principle of equivalence, depends neither on the method employed, nor the time, nor any other external circumstance. as the result of the brilliant researches of rowland and of mr griffiths on the variations of the specific heat of water, physicists have decided to take as calorific standard the quantity of heat necessary to raise a gramme of water from ° to ° c., the temperature being measured by the scale of the hydrogen thermometer of the international bureau. on the other hand, new determinations of the mechanical equivalent, among which it is right to mention that of mr. ames, and a full discussion as to the best results, have led to the adoption of the number . to represent the number of ergs capable of producing the unit of heat. in practice, the measurement of a quantity of heat is very often effected by means of the ice calorimeter, the use of which is particularly simple and convenient. there is, therefore, a very special interest in knowing exactly the melting-point of ice. m. leduc, who for several years has measured a great number of physical constants with minute precautions and a remarkable sense of precision, concludes, after a close discussion of the various results obtained, that this heat is equal to . calories. an error of almost a calorie had been committed by several renowned experimenters, and it will be seen that in certain points the art of measurement may still be largely perfected. to the unit of energy might be immediately attached other units. for instance, radiation being nothing but a flux of energy, we could, in order to establish photometric units, divide the normal spectrum into bands of a given width, and measure the power of each for the unit of radiating surface. but, notwithstanding some recent researches on this question, we cannot yet consider the distribution of energy in the spectrum as perfectly known. if we adopt the excellent habit which exists in some researches of expressing radiating energy in ergs, it is still customary to bring the radiations to a standard giving, by its constitution alone, the unit of one particular radiation. in particular, the definitions are still adhered to which were adopted as the result of the researches of m. violle on the radiation of fused platinum at the temperature of solidification; and most physicists utilize in the ordinary methods of photometry the clearly defined notions of m. blondel as to the luminous intensity of flux, illumination (_éclairement_), light (_éclat_), and lighting (_éclairage_), with the corresponding units, decimal candle, _lumen_, _lux_, carcel lamp, candle per square centimetre, and _lumen_-hour.[ ] [footnote : these are the magnitudes and units adopted at the international congress of electricians in . for their definition and explanation, see demanet, _notes de physique expérimentale_ (louvain, ), t. iv. p. .--ed.] § . measure of certain physical constants the progress of metrology has led, as a consequence, to corresponding progress in nearly all physical measurements, and particularly in the measure of natural constants. among these, the constant of gravitation occupies a position quite apart from the importance and simplicity of the physical law which defines it, as well as by its generality. two material particles are mutually attracted to each other by a force directly proportional to the product of their mass, and inversely proportional to the square of the distance between them. the coefficient of proportion is determined when once the units are chosen, and as soon as we know the numerical values of this force, of the two masses, and of their distance. but when we wish to make laboratory experiments serious difficulties appear, owing to the weakness of the attraction between masses of ordinary dimensions. microscopic forces, so to speak, have to be observed, and therefore all the causes of errors have to be avoided which would be unimportant in most other physical researches. it is known that cavendish was the first who succeeded by means of the torsion balance in effecting fairly precise measurements. this method has been again taken in hand by different experimenters, and the most recent results are due to mr vernon boys. this learned physicist is also the author of a most useful practical invention, and has succeeded in making quartz threads as fine as can be desired and extremely uniform. he finds that these threads possess valuable properties, such as perfect elasticity and great tenacity. he has been able, with threads not more than / of a millimetre in diameter, to measure with precision couples of an order formerly considered outside the range of experiment, and to reduce the dimensions of the apparatus of cavendish in the proportion of to . the great advantage found in the use of these small instruments is the better avoidance of the perturbations arising from draughts of air, and of the very serious influence of the slightest inequality in temperature. other methods have been employed in late years by other experimenters, such as the method of baron eötvös, founded on the use of a torsion lever, the method of the ordinary balance, used especially by professors richarz and krigar-menzel and also by professor poynting, and the method of m. wilsing, who uses a balance with a vertical beam. the results fairly agree, and lead to attributing to the earth a density equal to . . the most familiar manifestation of gravitation is gravity. the action of the earth on the unit of mass placed in one point, and the intensity of gravity, is measured, as we know, by the aid of a pendulum. the methods of measurement, whether by absolute or by relative determinations, so greatly improved by borda and bessel, have been still further improved by various geodesians, among whom should be mentioned m. von sterneek and general defforges. numerous observations have been made in all parts of the world by various explorers, and have led to a fairly complete knowledge of the distribution of gravity over the surface of the globe. thus we have succeeded in making evident anomalies which would not easily find their place in the formula of clairaut. another constant, the determination of which is of the greatest utility in astronomy of position, and the value of which enters into electromagnetic theory, has to-day assumed, with the new ideas on the constitution of matter, a still more considerable importance. i refer to the speed of light, which appears to us, as we shall see further on, the maximum value of speed which can be given to a material body. after the historical experiments of fizeau and foucault, taken up afresh, as we know, partly by cornu, and partly by michelson and newcomb, it remained still possible to increase the precision of the measurements. professor michelson has undertaken some new researches by a method which is a combination of the principle of the toothed wheel of fizeau with the revolving mirror of foucault. the toothed wheel is here replaced, however, by a grating, in which the lines and the spaces between them take the place of the teeth and the gaps, the reflected light only being returned when it strikes on the space between two lines. the illustrious american physicist estimates that he can thus evaluate to nearly five kilometres the path traversed by light in one second. this approximation corresponds to a relative value of a few hundred-thousandths, and it far exceeds those hitherto attained by the best experimenters. when all the experiments are completed, they will perhaps solve certain questions still in suspense; for instance, the question whether the speed of propagation depends on intensity. if this turns out to be the case, we should be brought to the important conclusion that the amplitude of the oscillations, which is certainly very small in relation to the already tiny wave-lengths, cannot be considered as unimportant in regard to these lengths. such would seem to have been the result of the curious experiments of m. muller and of m. ebert, but these results have been recently disputed by m. doubt. in the case of sound vibrations, on the other hand, it should be noted that experiment, consistently with the theory, proves that the speed increases with the amplitude, or, if you will, with the intensity. m. violle has published an important series of experiments on the speed of propagation of very condensed waves, on the deformations of these waves, and on the relations of the speed and the pressure, which verify in a remarkable manner the results foreshadowed by the already old calculations of riemann, repeated later by hugoniot. if, on the contrary, the amplitude is sufficiently small, there exists a speed limit which is the same in a large pipe and in free air. by some beautiful experiments, mm. violle and vautier have clearly shown that any disturbance in the air melts somewhat quickly into a single wave of given form, which is propagated to a distance, while gradually becoming weaker and showing a constant speed which differs little in dry air at ° c. from . metres per second. in a narrow pipe the influence of the walls makes itself felt and produces various effects, in particular a kind of dispersion in space of the harmonics of the sound. this phenomenon, according to m. brillouin, is perfectly explicable by a theory similar to the theory of gratings. chapter iii principles § . the principles of physics facts conscientiously observed lead by induction to the enunciation of a certain number of laws or general hypotheses which are the principles already referred to. these principal hypotheses are, in the eyes of a physicist, legitimate generalizations, the consequences of which we shall be able at once to check by the experiments from which they issue. among the principles almost universally adopted until lately figure prominently those of mechanics--such as the principle of relativity, and the principle of the equality of action and reaction. we will not detail nor discuss them here, but later on we shall have an opportunity of pointing out how recent theories on the phenomena of electricity have shaken the confidence of physicists in them and have led certain scholars to doubt their absolute value. the principle of lavoisier, or principle of the conservation of mass, presents itself under two different aspects according to whether mass is looked upon as the coefficient of the inertia of matter or as the factor which intervenes in the phenomena of universal attraction, and particularly in gravitation. we shall see when we treat of these theories, how we have been led to suppose that inertia depended on velocity and even on direction. if this conception were exact, the principle of the invariability of mass would naturally be destroyed. considered as a factor of attraction, is mass really indestructible? a few years ago such a question would have seemed singularly audacious. and yet the law of lavoisier is so far from self-evident that for centuries it escaped the notice of physicists and chemists. but its great apparent simplicity and its high character of generality, when enunciated at the end of the eighteenth century, rapidly gave it such an authority that no one was able to any longer dispute it unless he desired the reputation of an oddity inclined to paradoxical ideas. it is important, however, to remark that, under fallacious metaphysical appearances, we are in reality using empty words when we repeat the aphorism, "nothing can be lost, nothing can be created," and deduce from it the indestructibility of matter. this indestructibility, in truth, is an experimental fact, and the principle depends on experiment. it may even seem, at first sight, more singular than not that the weight of a bodily system in a given place, or the quotient of this weight by that of the standard mass--that is to say, the mass of these bodies--remains invariable, both when the temperature changes and when chemical reagents cause the original materials to disappear and to be replaced by new ones. we may certainly consider that in a chemical phenomenon annihilations and creations of matter are really produced; but the experimental law teaches us that there is compensation in certain respects. the discovery of the radioactive bodies has, in some sort, rendered popular the speculations of physicists on the phenomena of the disaggregation of matter. we shall have to seek the exact meaning which ought to be given to the experiments on the emanation of these bodies, and to discover whether these experiments really imperil the law of lavoisier. for some years different experimenters have also effected many very precise measurements of the weight of divers bodies both before and after chemical reactions between these bodies. two highly experienced and cautious physicists, professors landolt and heydweiller, have not hesitated to announce the sensational result that in certain circumstances the weight is no longer the same after as before the reaction. in particular, the weight of a solution of salts of copper in water is not the exact sum of the joint weights of the salt and the water. such experiments are evidently very delicate; they have been disputed, and they cannot be considered as sufficient for conviction. it follows nevertheless that it is no longer forbidden to regard the law of lavoisier as only an approximate law; according to sandford and ray, this approximation would be about / , , . this is also the result reached by professor poynting in experiments regarding the possible action of temperature on the weight of a body; and if this be really so, we may reassure ourselves, and from the point of view of practical application may continue to look upon matter as indestructible. the principles of physics, by imposing certain conditions on phenomena, limit after a fashion the field of the possible. among these principles is one which, notwithstanding its importance when compared with that of universally known principles, is less familiar to some people. this is the principle of symmetry, more or less conscious applications of which can, no doubt, be found in various works and even in the conceptions of copernican astronomers, but which was generalized and clearly enunciated for the first time by the late m. curie. this illustrious physicist pointed out the advantage of introducing into the study of physical phenomena the considerations on symmetry familiar to crystallographers; for a phenomenon to take place, it is necessary that a certain dissymmetry should previously exist in the medium in which this phenomenon occurs. a body, for instance, may be animated with a certain linear velocity or a speed of rotation; it may be compressed, or twisted; it may be placed in an electric or in a magnetic field; it may be affected by an electric current or by one of heat; it may be traversed by a ray of light either ordinary or polarized rectilineally or circularly, etc.:--in each case a certain minimum and characteristic dissymmetry is necessary at every point of the body in question. this consideration enables us to foresee that certain phenomena which might be imagined _a priori_ cannot exist. thus, for instance, it is impossible that an electric field, a magnitude directed and not superposable on its image in a mirror perpendicular to its direction, could be created at right angles to the plane of symmetry of the medium; while it would be possible to create a magnetic field under the same conditions. this consideration thus leads us to the discovery of new phenomena; but it must be understood that it cannot of itself give us absolutely precise notions as to the nature of these phenomena, nor disclose their order of magnitude. § . the principle of the conservation of energy dominating not physics alone, but nearly every other science, the principle of the conservation of energy is justly considered as the grandest conquest of contemporary thought. it shows us in a powerful light the most diverse questions; it introduces order into the most varied studies; it leads to a clear and coherent interpretation of phenomena which, without it, appear to have no connexion with each other; and it supplies precise and exact numerical relations between the magnitudes which enter into these phenomena. the boldest minds have an instinctive confidence in it, and it is the principle which has most stoutly resisted that assault which the daring of a few theorists has lately directed to the overthrow of the general principles of physics. at every new discovery, the first thought of physicists is to find out how it accords with the principle of the conservation of energy. the application of the principle, moreover, never fails to give valuable hints on the new phenomenon, and often even suggests a complementary discovery. up till now it seems never to have received a check, even the extraordinary properties of radium not seriously contradicting it; also the general form in which it is enunciated gives it such a suppleness that it is no doubt very difficult to overthrow. i do not claim to set forth here the complete history of this principle, but i will endeavour to show with what pains it was born, how it was kept back in its early days and then obstructed in its development by the unfavourable conditions of the surroundings in which it appeared. it first of all came, in fact, to oppose itself to the reigning theories; but, little by little, it acted on these theories, and they were modified under its pressure; then, in their turn, these theories reacted on it and changed its primitive form. it had to be made less wide in order to fit into the classic frame, and was absorbed by mechanics; and if it thus became less general, it gained in precision what it lost in extent. when once definitely admitted and classed, as it were, in the official domain of science, it endeavoured to burst its bonds and return to a more independent and larger life. the history of this principle is similar to that of all evolutions. it is well known that the conservation of energy was, at first, regarded from the point of view of the reciprocal transformations between heat and work, and that the principle received its first clear enunciation in the particular case of the principle of equivalence. it is, therefore, rightly considered that the scholars who were the first to doubt the material nature of caloric were the precursors of r. mayer; their ideas, however, were the same as those of the celebrated german doctor, for they sought especially to demonstrate that heat was a mode of motion. without going back to early and isolated attempts like those of daniel bernoulli, who, in his hydrodynamics, propounded the basis of the kinetic theory of gases, or the researches of boyle on friction, we may recall, to show how it was propounded in former times, a rather forgotten page of the _mémoire sur la chaleur_, published in by lavoisier and laplace: "other physicists," they wrote, after setting out the theory of caloric, "think that heat is nothing but the result of the insensible vibrations of matter.... in the system we are now examining, heat is the _vis viva_ resulting from the insensible movements of the molecules of a body; it is the sum of the products of the mass of each molecule by the square of its velocity.... we shall not decide between the two preceding hypotheses; several phenomena seem to support the last mentioned--for instance, that of the heat produced by the friction of two solid bodies. but there are others which are more simply explained by the first, and perhaps they both operate at once." most of the physicists of that period, however, did not share the prudent doubts of lavoisier and laplace. they admitted, without hesitation, the first hypothesis; and, four years after the appearance of the _mémoire sur la chaleur_, sigaud de lafond, a professor of physics of great reputation, wrote: "pure fire, free from all state of combination, seems to be an assembly of particles of a simple, homogeneous, and absolutely unalterable matter, and all the properties of this element indicate that these particles are infinitely small and free, that they have no sensible cohesion, and that they are moved in every possible direction by a continual and rapid motion which is essential to them.... the extreme tenacity and the surprising mobility of its molecules are manifestly shown by the ease with which it penetrates into the most compact bodies and by its tendency to put itself in equilibrium throughout all bodies near to it." it must be acknowledged, however, that the idea of lavoisier and laplace was rather vague and even inexact on one important point. they admitted it to be evident that "all variations of heat, whether real or apparent, undergone by a bodily system when changing its state, are produced in inverse order when the system passes back to its original state." this phrase is the very denial of equivalence where these changes of state are accompanied by external work. laplace, moreover, himself became later a very convinced partisan of the hypothesis of the material nature of caloric, and his immense authority, so fortunate in other respects for the development of science, was certainly in this case the cause of the retardation of progress. the names of young, rumford, davy, are often quoted among those physicists who, at the commencement of the nineteenth century, caught sight of the new truths as to the nature of heat. to these names is very properly added that of sadi carnot. a note found among his papers unquestionably proves that, before , ideas had occurred to him from which it resulted that in producing work an equivalent amount of heat was destroyed. but the year is particularly memorable in the history of science as the year in which jules robert mayer succeeded, by an entirely personal effort, in really enunciating the principle of the conservation of energy. chemists recall with just pride that the _remarques sur les forces de la nature animée_, contemptuously rejected by all the journals of physics, were received and published in the _annalen_ of liebig. we ought never to forget this example, which shows with what difficulty a new idea contrary to the classic theories of the period succeeds in coming to the front; but extenuating circumstances may be urged on behalf of the physicists. robert mayer had a rather insufficient mathematical education, and his memoirs, the _remarques_, as well as the ulterior publications, _mémoire sur le mouvement organique et la nutrition_ and the _matériaux pour la dynamique du ciel_, contain, side by side with very profound ideas, evident errors in mechanics. thus it often happens that discoveries put forward in a somewhat vague manner by adventurous minds not overburdened by the heavy baggage of scientific erudition, who audaciously press forward in advance of their time, fall into quite intelligible oblivion until rediscovered, clarified, and put into shape by slower but surer seekers. this was the case with the ideas of mayer. they were not understood at first sight, not only on account of their originality, but also because they were couched in incorrect language. mayer was, however, endowed with a singular strength of thought; he expressed in a rather confused manner a principle which, for him, had a generality greater than mechanics itself, and so his discovery was in advance not only of his own time but of half the century. he may justly be considered the founder of modern energetics. freed from the obscurities which prevented its being clearly perceived, his idea stands out to-day in all its imposing simplicity. yet it must be acknowledged that if it was somewhat denaturalised by those who endeavoured to adapt it to the theories of mechanics, and if it at first lost its sublime stamp of generality, it thus became firmly fixed and consolidated on a more stable basis. the efforts of helmholtz, clausius, and lord kelvin to introduce the principle of the conservation of energy into mechanics, were far from useless. these illustrious physicists succeeded in giving a more precise form to its numerous applications; and their attempts thus contributed, by reaction, to give a fresh impulse to mechanics, and allowed it to be linked to a more general order of facts. if energetics has not been able to be included in mechanics, it seems indeed that the attempt to include mechanics in energetics was not in vain. in the middle of the last century, the explanation of all natural phenomena seemed more and more referable to the case of central forces. everywhere it was thought that reciprocal actions between material points could be perceived, these points being attracted or repelled by each other with an intensity depending only on their distance or their mass. if, to a system thus composed, the laws of the classical mechanics are applied, it is shown that half the sum of the product of the masses by the square of the velocities, to which is added the work which might be accomplished by the forces to which the system would be subject if it returned from its actual to its initial position, is a sum constant in quantity. this sum, which is the mechanical energy of the system, is therefore an invariable quantity in all the states to which it may be brought by the interaction of its various parts, and the word energy well expresses a capital property of this quantity. for if two systems are connected in such a way that any change produced in the one necessarily brings about a change in the other, there can be no variation in the characteristic quantity of the second except so far as the characteristic quantity of the first itself varies--on condition, of course, that the connexions are made in such a manner as to introduce no new force. it will thus be seen that this quantity well expresses the capacity possessed by a system for modifying the state of a neighbouring system to which we may suppose it connected. now this theorem of pure mechanics was found wanting every time friction took place--that is to say, in all really observable cases. the more perceptible the friction, the more considerable the difference; but, in addition, a new phenomenon always appeared and heat was produced. by experiments which are now classic, it became established that the quantity of heat thus created independently of the nature of the bodies is always (provided no other phenomena intervene) proportional to the energy which has disappeared. reciprocally, also, heat may disappear, and we always find a constant relation between the quantities of heat and work which mutually replace each other. it is quite clear that such experiments do not prove that heat is work. we might just as well say that work is heat. it is making a gratuitous hypothesis to admit this reduction of heat to mechanism; but this hypothesis was so seductive, and so much in conformity with the desire of nearly all physicists to arrive at some sort of unity in nature, that they made it with eagerness and became unreservedly convinced that heat was an active internal force. their error was not in admitting this hypothesis; it was a legitimate one since it has proved very fruitful. but some of them committed the fault of forgetting that it was an hypothesis, and considered it a demonstrated truth. moreover, they were thus brought to see in phenomena nothing but these two particular forms of energy which in their minds were easily identified with each other. from the outset, however, it became manifest that the principle is applicable to cases where heat plays only a parasitical part. there were thus discovered, by translating the principle of equivalence, numerical relations between the magnitudes of electricity, for instance, and the magnitudes of mechanics. heat was a sort of variable intermediary convenient for calculation, but introduced in a roundabout way and destined to disappear in the final result. verdet, who, in lectures which have rightly remained celebrated, defined with remarkable clearness the new theories, said, in : "electrical phenomena are always accompanied by calorific manifestations, of which the study belongs to the mechanical theory of heat. this study, moreover, will not only have the effect of making known to us interesting facts in electricity, but will throw some light on the phenomena of electricity themselves." the eminent professor was thus expressing the general opinion of his contemporaries, but he certainly seemed to have felt in advance that the new theory was about to penetrate more deeply into the inmost nature of things. three years previously, rankine also had put forth some very remarkable ideas the full meaning of which was not at first well understood. he it was who comprehended the utility of employing a more inclusive term, and invented the phrase energetics. he also endeavoured to create a new doctrine of which rational mechanics should be only a particular case; and he showed that it was possible to abandon the ideas of atoms and central forces, and to construct a more general system by substituting for the ordinary consideration of forces that of the energy which exists in all bodies, partly in an actual, partly in a potential state. by giving more precision to the conceptions of rankine, the physicists of the end of the nineteenth century were brought to consider that in all physical phenomena there occur apparitions and disappearances which are balanced by various energies. it is natural, however, to suppose that these equivalent apparitions and disappearances correspond to transformations and not to simultaneous creations and destructions. we thus represent energy to ourselves as taking different forms--mechanical, electrical, calorific, and chemical-- capable of changing one into the other, but in such a way that the quantitative value always remains the same. in like manner a bank draft may be represented by notes, gold, silver, or bullion. the earliest known form of energy, _i.e._ work, will serve as the standard as gold serves as the monetary standard, and energy in all its forms will be estimated by the corresponding work. in each particular case we can strictly define and measure, by the correct application of the principle of the conservation of energy, the quantity of energy evolved under a given form. we can thus arrange a machine comprising a body capable of evolving this energy; then we can force all the organs of this machine to complete an entirely closed cycle, with the exception of the body itself, which, however, has to return to such a state that all the variables from which this state depends resume their initial values except the particular variable to which the evolution of the energy under consideration is linked. the difference between the work thus accomplished and that which would have been obtained if this variable also had returned to its original value, is the measure of the energy evolved. in the same way that, in the minds of mechanicians, all forces of whatever origin, which are capable of compounding with each other and of balancing each other, belong to the same category of beings, so for many physicists energy is a sort of entity which we find under various aspects. there thus exists for them a world, which comes in some way to superpose itself upon the world of matter--that is to say, the world of energy, dominated in its turn by a fundamental law similar to that of lavoisier.[ ] this conception, as we have already seen, passes the limit of experience; but others go further still. absorbed in the contemplation of this new world, they succeed in persuading themselves that the old world of matter has no real existence and that energy is sufficient by itself to give us a complete comprehension of the universe and of all the phenomena produced in it. they point out that all our sensations correspond to changes of energy, and that everything apparent to our senses is, in truth, energy. the famous experiment of the blows with a stick by which it was demonstrated to a sceptical philosopher that an outer world existed, only proves, in reality, the existence of energy, and not that of matter. the stick in itself is inoffensive, as professor ostwald remarks, and it is its _vis viva_, its kinetic energy, which is painful to us; while if we possessed a speed equal to its own, moving in the same direction, it would no longer exist so far as our sense of touch is concerned. [footnote : "nothing is created; nothing is lost"--ed.] on this hypothesis, matter would only be the capacity for kinetic energy, its pretended impenetrability energy of volume, and its weight energy of position in the particular form which presents itself in universal gravitation; nay, space itself would only be known to us by the expenditure of energy necessary to penetrate it. thus in all physical phenomena we should only have to regard the quantities of energy brought into play, and all the equations which link the phenomena to one another would have no meaning but when they apply to exchanges of energy. for energy alone can be common to all phenomena. this extreme manner of regarding things is seductive by its originality, but appears somewhat insufficient if, after enunciating generalities, we look more closely into the question. from the philosophical point of view it may, moreover, seem difficult not to conclude, from the qualities which reveal, if you will, the varied forms of energy, that there exists a substance possessing these qualities. this energy, which resides in one region, and which transports itself from one spot to another, forcibly brings to mind, whatever view we may take of it, the idea of matter. helmholtz endeavoured to construct a mechanics based on the idea of energy and its conservation, but he had to invoke a second law, the principle of least action. if he thus succeeded in dispensing with the hypothesis of atoms, and in showing that the new mechanics gave us to understand the impossibility of certain movements which, according to the old, ought to have been but never were experimentally produced, he was only able to do so because the principle of least action necessary for his theory became evident in the case of those irreversible phenomena which alone really exist in nature. the energetists have thus not succeeded in forming a thoroughly sound system, but their efforts have at all events been partly successful. most physicists are of their opinion, that kinetic energy is only a particular variety of energy to which we have no right to wish to connect all its other forms. if these forms showed themselves to be innumerable throughout the universe, the principle of the conservation of energy would, in fact, lose a great part of its importance. every time that a certain quantity of energy seemed to appear or disappear, it would always be permissible to suppose that an equivalent quantity had appeared or disappeared somewhere else under a new form; and thus the principle would in a way vanish. but the known forms of energy are fairly restricted in number, and the necessity of recognising new ones seldom makes itself felt. we shall see, however, that to explain, for instance, the paradoxical properties of radium and to re-establish concord between these properties and the principle of the conservation of energy, certain physicists have recourse to the hypothesis that radium borrows an unknown energy from the medium in which it is plunged. this hypothesis, however, is in no way necessary; and in a few other rare cases in which similar hypotheses have had to be set up, experiment has always in the long run enabled us to discover some phenomenon which had escaped the first observers and which corresponds exactly to the variation of energy first made evident. one difficulty, however, arises from the fact that the principle ought only to be applied to an isolated system. whether we imagine actions at a distance or believe in intermediate media, we must always recognise that there exist no bodies in the world incapable of acting on each other, and we can never affirm that some modification in the energy of a given place may not have its echo in some unknown spot afar off. this difficulty may sometimes render the value of the principle rather illusory. similarly, it behoves us not to receive without a certain distrust the extension by certain philosophers to the whole universe, of a property demonstrated for those restricted systems which observation can alone reach. we know nothing of the universe as a whole, and every generalization of this kind outruns in a singular fashion the limit of experiment. even reduced to the most modest proportions, the principle of the conservation of energy retains, nevertheless, a paramount importance; and it still preserves, if you will, a high philosophical value. m.j. perrin justly points out that it gives us a form under which we are experimentally able to grasp causality, and that it teaches us that a result has to be purchased at the cost of a determined effort. we can, in fact, with m. perrin and m. langevin, represent this in a way which puts this characteristic in evidence by enunciating it as follows: "if at the cost of a change c we can obtain a change k, there will never be acquired at the same cost, whatever the mechanism employed, first the change k and in addition some other change, unless this latter be one that is otherwise known to cost nothing to produce or to destroy." if, for instance, the fall of a weight can be accompanied, without anything else being produced, by another transformation--the melting of a certain mass of ice, for example--it will be impossible, no matter how you set about it or whatever the mechanism used, to associate this same transformation with the melting of another weight of ice. we can thus, in the transformation in question, obtain an appropriate number which will sum up that which may be expected from the external effect, and can give, so to speak, the price at which this transformation is bought, measure its invariable value by a common measure (for instance, the melting of the ice), and, without any ambiguity, define the energy lost during the transformation as proportional to the mass of ice which can be associated with it. this measure is, moreover, independent of the particular phenomenon taken as the common measure. § . the principle of carnot and clausius the principle of carnot, of a nature analogous to the principle of the conservation of energy, has also a similar origin. it was first enunciated, like the last named, although prior to it in time, in consequence of considerations which deal only with heat and mechanical work. like it, too, it has evolved, grown, and invaded the entire domain of physics. it may be interesting to examine rapidly the various phases of this evolution. the origin of the principle of carnot is clearly determined, and it is very rare to be able to go back thus certainly to the source of a discovery. sadi carnot had, truth to say, no precursor. in his time heat engines were not yet very common, and no one had reflected much on their theory. he was doubtless the first to propound to himself certain questions, and certainly the first to solve them. it is known how, in , in his _réflexions sur la puissance motrice du feu_, he endeavoured to prove that "the motive power of heat is independent of the agents brought into play for its realization," and that "its quantity is fixed solely by the temperature of the bodies between which, in the last resort, the transport of caloric is effected"--at least in all engines in which "the method of developing the motive power attains the perfection of which it is capable"; and this is, almost textually, one of the enunciations of the principle at the present day. carnot perceived very clearly the great fact that, to produce work by heat, it is necessary to have at one's disposal a fall of temperature. on this point he expresses himself with perfect clearness: "the motive power of a fall of water depends on its height and on the quantity of liquid; the motive power of heat depends also on the quantity of caloric employed, and on what might be called--in fact, what we shall call--the height of fall, that is to say, the difference in temperature of the bodies between which the exchange of caloric takes place." starting with this idea, he endeavours to demonstrate, by associating two engines capable of working in a reversible cycle, that the principle is founded on the impossibility of perpetual motion. his memoir, now celebrated, did not produce any great sensation, and it had almost fallen into deep oblivion, which, in consequence of the discovery of the principle of equivalence, might have seemed perfectly justified. written, in fact, on the hypothesis of the indestructibility of caloric, it was to be expected that this memoir should be condemned in the name of the new doctrine, that is, of the principle recently brought to light. it was really making a new discovery to establish that carnot's fundamental idea survived the destruction of the hypothesis on the nature of heat, on which he seemed to rely. as he no doubt himself perceived, his idea was quite independent of this hypothesis, since, as we have seen, he was led to surmise that heat could disappear; but his demonstrations needed to be recast and, in some points, modified. it is to clausius that was reserved the credit of rediscovering the principle, and of enunciating it in language conformable to the new doctrines, while giving it a much greater generality. the postulate arrived at by experimental induction, and which must be admitted without demonstration, is, according to clausius, that in a series of transformations in which the final is identical with the initial stage, it is impossible for heat to pass from a colder to a warmer body unless some other accessory phenomenon occurs at the same time. still more correctly, perhaps, an enunciation can be given of the postulate which, in the main, is analogous, by saying: a heat motor, which after a series of transformations returns to its initial state, can only furnish work if there exist at least two sources of heat, and if a certain quantity of heat is given to one of the sources, which can never be the hotter of the two. by the expression "source of heat," we mean a body exterior to the system and capable of furnishing or withdrawing heat from it. starting with this principle, we arrive, as does clausius, at the demonstration that the output of a reversible machine working between two given temperatures is greater than that of any non-reversible engine, and that it is the same for all reversible machines working between these two temperatures. this is the very proposition of carnot; but the proposition thus stated, while very useful for the theory of engines, does not yet present any very general interest. clausius, however, drew from it much more important consequences. first, he showed that the principle conduces to the definition of an absolute scale of temperature; and then he was brought face to face with a new notion which allows a strong light to be thrown on the questions of physical equilibrium. i refer to entropy. it is still rather difficult to strip entirely this very important notion of all analytical adornment. many physicists hesitate to utilize it, and even look upon it with some distrust, because they see in it a purely mathematical function without any definite physical meaning. perhaps they are here unduly severe, since they often admit too easily the objective existence of quantities which they cannot define. thus, for instance, it is usual almost every day to speak of the heat possessed by a body. yet no body in reality possesses a definite quantity of heat even relatively to any initial state; since starting from this point of departure, the quantities of heat it may have gained or lost vary with the road taken and even with the means employed to follow it. these expressions of heat gained or lost are, moreover, themselves evidently incorrect, for heat can no longer be considered as a sort of fluid passing from one body to another. the real reason which makes entropy somewhat mysterious is that this magnitude does not fall directly under the ken of any of our senses; but it possesses the true characteristic of a concrete physical magnitude, since it is, in principle at least, measurable. various authors of thermodynamical researches, amongst whom m. mouret should be particularly mentioned, have endeavoured to place this characteristic in evidence. consider an isothermal transformation. instead of leaving the heat abandoned by the body subjected to the transformation--water condensing in a state of saturated vapour, for instance--to pass directly into an ice calorimeter, we can transmit this heat to the calorimeter by the intermediary of a reversible carnot engine. the engine having absorbed this quantity of heat, will only give back to the ice a lesser quantity of heat; and the weight of the melted ice, inferior to that which might have been directly given back, will serve as a measure of the isothermal transformation thus effected. it can be easily shown that this measure is independent of the apparatus used. it consequently becomes a numerical element characteristic of the body considered, and is called its entropy. entropy, thus defined, is a variable which, like pressure or volume, might serve concurrently with another variable, such as pressure or volume, to define the state of a body. it must be perfectly understood that this variable can change in an independent manner, and that it is, for instance, distinct from the change of temperature. it is also distinct from the change which consists in losses or gains of heat. in chemical reactions, for example, the entropy increases without the substances borrowing any heat. when a perfect gas dilates in a vacuum its entropy increases, and yet the temperature does not change, and the gas has neither been able to give nor receive heat. we thus come to conceive that a physical phenomenon cannot be considered known to us if the variation of entropy is not given, as are the variations of temperature and of pressure or the exchanges of heat. the change of entropy is, properly speaking, the most characteristic fact of a thermal change. it is important, however, to remark that if we can thus easily define and measure the difference of entropy between two states of the same body, the value found depends on the state arbitrarily chosen as the zero point of entropy; but this is not a very serious difficulty, and is analogous to that which occurs in the evaluation of other physical magnitudes--temperature, potential, etc. a graver difficulty proceeds from its not being possible to define a difference, or an equality, of entropy between two bodies chemically different. we are unable, in fact, to pass by any means, reversible or not, from one to the other, so long as the transmutation of matter is regarded as impossible; but it is well understood that it is nevertheless possible to compare the variations of entropy to which these two bodies are both of them individually subject. neither must we conceal from ourselves that the definition supposes, for a given body, the possibility of passing from one state to another by a reversible transformation. reversibility is an ideal and extreme case which cannot be realized, but which can be approximately attained in many circumstances. so with gases and with perfectly elastic bodies, we effect sensibly reversible transformations, and changes of physical state are practically reversible. the discoveries of sainte-claire deville have brought many chemical phenomena into a similar category, and reactions such as solution, which used to be formerly the type of an irreversible phenomenon, may now often be effected by sensibly reversible means. be that as it may, when once the definition is admitted, we arrive, by taking as a basis the principles set forth at the inception, at the demonstration of the celebrated theorem of clausius: _the entropy of a thermally isolated system continues to increase incessantly._ it is very evident that the theorem can only be worth applying in cases where the entropy can be exactly defined; but, even when thus limited, the field still remains vast, and the harvest which we can there reap is very abundant. entropy appears, then, as a magnitude measuring in a certain way the evolution of a system, or, at least, as giving the direction of this evolution. this very important consequence certainly did not escape clausius, since the very name of entropy, which he chose to designate this magnitude, itself signifies evolution. we have succeeded in defining this entropy by demonstrating, as has been said, a certain number of propositions which spring from the postulate of clausius; it is, therefore, natural to suppose that this postulate itself contains _in potentia_ the very idea of a necessary evolution of physical systems. but as it was first enunciated, it contains it in a deeply hidden way. no doubt we should make the principle of carnot appear in an interesting light by endeavouring to disengage this fundamental idea, and by placing it, as it were, in large letters. just as, in elementary geometry, we can replace the postulate of euclid by other equivalent propositions, so the postulate of thermodynamics is not necessarily fixed, and it is instructive to try to give it the most general and suggestive character. mm. perrin and langevin have made a successful attempt in this direction. m. perrin enunciates the following principle: _an isolated system never passes twice through the same state_. in this form, the principle affirms that there exists a necessary order in the succession of two phenomena; that evolution takes place in a determined direction. if you prefer it, it may be thus stated: _of two converse transformations unaccompanied by any external effect, one only is possible_. for instance, two gases may diffuse themselves one in the other in constant volume, but they could not conversely separate themselves spontaneously. starting from the principle thus put forward, we make the logical deduction that one cannot hope to construct an engine which should work for an indefinite time by heating a hot source and by cooling a cold one. we thus come again into the route traced by clausius, and from this point we may follow it strictly. whatever the point of view adopted, whether we regard the proposition of m. perrin as the corollary of another experimental postulate, or whether we consider it as a truth which we admit _a priori_ and verify through its consequences, we are led to consider that in its entirety the principle of carnot resolves itself into the idea that we cannot go back along the course of life, and that the evolution of a system must follow its necessary progress. clausius and lord kelvin have drawn from these considerations certain well-known consequences on the evolution of the universe. noticing that entropy is a property added to matter, they admit that there is in the world a total amount of entropy; and as all real changes which are produced in any system correspond to an increase of entropy, it may be said that the entropy of the world is continually increasing. thus the quantity of energy existing in the universe remains constant, but transforms itself little by little into heat uniformly distributed at a temperature everywhere identical. in the end, therefore, there will be neither chemical phenomena nor manifestation of life; the world will still exist, but without motion, and, so to speak, dead. these consequences must be admitted to be very doubtful; we cannot in any certain way apply to the universe, which is not a finite system, a proposition demonstrated, and that not unreservedly, in the sharply limited case of a finite system. herbert spencer, moreover, in his book on _first principles_, brings out with much force the idea that, even if the universe came to an end, nothing would allow us to conclude that, once at rest, it would remain so indefinitely. we may recognise that the state in which we are began at the end of a former evolutionary period, and that the end of the existing era will mark the beginning of a new one. like an elastic and mobile object which, thrown into the air, attains by degrees the summit of its course, then possesses a zero velocity and is for a moment in equilibrium, and then falls on touching the ground to rebound, so the world should be subjected to huge oscillations which first bring it to a maximum of entropy till the moment when there should be produced a slow evolution in the contrary direction bringing it back to the state from which it started. thus, in the infinity of time, the life of the universe proceeds without real stop. this conception is, moreover, in accordance with the view certain physicists take of the principle of carnot. we shall see, for example, that in the kinetic theory we are led to admit that, after waiting sufficiently long, we can witness the return of the various states through which a mass of gas, for example, has passed in its series of transformations. if we keep to the present era, evolution has a fixed direction--that which leads to an increase of entropy; and it is possible to enquire, in any given system to what physical manifestations this increase corresponds. we note that kinetic, potential, electrical, and chemical forms of energy have a great tendency to transform themselves into calorific energy. a chemical reaction, for example, gives out energy; but if the reaction is not produced under very special conditions, this energy immediately passes into the calorific form. this is so true, that chemists currently speak of the heat given out by reactions instead of regarding the energy disengaged in general. in all these transformations the calorific energy obtained has not, from a practical point of view, the same value at which it started. one cannot, in fact, according to the principle of carnot, transform it integrally into mechanical energy, since the heat possessed by a body can only yield work on condition that a part of it falls on a body with a lower temperature. thus appears the idea that energies which exchange with each other and correspond to equal quantities have not the same qualitative value. form has its importance, and there are persons who prefer a golden louis to four pieces of five francs. the principle of carnot would thus lead us to consider a certain classification of energies, and would show us that, in the transformations possible, these energies always tend to a sort of diminution of quality--that is, to a _degradation_. it would thus reintroduce an element of differentiation of which it seems very difficult to give a mechanical explanation. certain philosophers and physicists see in this fact a reason which condemns _a priori_ all attempts made to give a mechanical explanation of the principle of carnot. it is right, however, not to exaggerate the importance that should be attributed to the phrase degraded energy. if the heat is not equivalent to the work, if heat at ° is not equivalent to heat at °, that means that we cannot in practice construct an engine which shall transform all this heat into work, or that, for the same cold source, the output is greater when the temperature of the hot source is higher; but if it were possible that this cold source had itself the temperature of absolute zero, the whole heat would reappear in the form of work. the case here considered is an ideal and extreme case, and we naturally cannot realize it; but this consideration suffices to make it plain that the classification of energies is a little arbitrary and depends more, perhaps, on the conditions in which mankind lives than on the inmost nature of things. in fact, the attempts which have often been made to refer the principle of carnot to mechanics have not given convincing results. it has nearly always been necessary to introduce into the attempt some new hypothesis independent of the fundamental hypotheses of ordinary mechanics, and equivalent, in reality, to one of the postulates on which the ordinary exposition of the second law of thermodynamics is founded. helmholtz, in a justly celebrated theory, endeavoured to fit the principle of carnot into the principle of least action; but the difficulties regarding the mechanical interpretation of the irreversibility of physical phenomena remain entire. looking at the question, however, from the point of view at which the partisans of the kinetic theories of matter place themselves, the principle is viewed in a new aspect. gibbs and afterwards boltzmann and professor planck have put forward some very interesting ideas on this subject. by following the route they have traced, we come to consider the principle as pointing out to us that a given system tends towards the configuration presented by the maximum probability, and, numerically, the entropy would even be the logarithm of this probability. thus two different gaseous masses, enclosed in two separate receptacles which have just been placed in communication, diffuse themselves one through the other, and it is highly improbable that, in their mutual shocks, both kinds of molecules should take a distribution of velocities which reduce them by a spontaneous phenomenon to the initial state. we should have to wait a very long time for so extraordinary a concourse of circumstances, but, in strictness, it would not be impossible. the principle would only be a law of probability. yet this probability is all the greater the more considerable is the number of molecules itself. in the phenomena habitually dealt with, this number is such that, practically, the variation of entropy in a constant sense takes, so to speak, the character of absolute certainty. but there may be exceptional cases where the complexity of the system becomes insufficient for the application of the principle of carnot;-- as in the case of the curious movements of small particles suspended in a liquid which are known by the name of brownian movements and can be observed under the microscope. the agitation here really seems, as m. gouy has remarked, to be produced and continued indefinitely, regardless of any difference in temperature; and we seem to witness the incessant motion, in an isothermal medium, of the particles which constitute matter. perhaps, however, we find ourselves already in conditions where the too great simplicity of the distribution of the molecules deprives the principle of its value. m. lippmann has in the same way shown that, on the kinetic hypothesis, it is possible to construct such mechanisms that we can so take cognizance of molecular movements that _vis viva_ can be taken from them. the mechanisms of m. lippmann are not, like the celebrated apparatus at one time devised by maxwell, purely hypothetical. they do not suppose a partition with a hole impossible to be bored through matter where the molecular spaces would be larger than the hole itself. they have finite dimensions. thus m. lippmann considers a vase full of oxygen at a constant temperature. in the interior of this vase is placed a small copper ring, and the whole is set in a magnetic field. the oxygen molecules are, as we know, magnetic, and when passing through the interior of the ring they produce in this ring an induced current. during this time, it is true, other molecules emerge from the space enclosed by the circuit; but the two effects do not counterbalance each other, and the resulting current is maintained. there is elevation of temperature in the circuit in accordance with joule's law; and this phenomenon, under such conditions, is incompatible with the principle of carnot. it is possible--and that, i think, is m. lippmann's idea--to draw from his very ingenious criticism an objection to the kinetic theory, if we admit the absolute value of the principle; but we may also suppose that here again we are in presence of a system where the prescribed conditions diminish the complexity and render it, consequently, less probable that the evolution is always effected in the same direction. in whatever way you look at it, the principle of carnot furnishes, in the immense majority of cases, a very sure guide in which physicists continue to have the most entire confidence. § . thermodynamics to apply the two fundamental principles of thermodynamics, various methods may be employed, equivalent in the main, but presenting as the cases vary a greater or less convenience. in recording, with the aid of the two quantities, energy and entropy, the relations which translate analytically the two principles, we obtain two relations between the coefficients which occur in a given phenomenon; but it may be easier and also more suggestive to employ various functions of these quantities. in a memoir, of which some extracts appeared as early as , a modest scholar, m. massieu, indicated in particular a remarkable function which he termed a characteristic function, and by the employment of which calculations are simplified in certain cases. in the same way j.w. gibbs, in and , then helmholtz in , and, in france, m. duhem, from the year onward, have published works, at first ill understood, of which the renown was, however, considerable in the sequel, and in which they made use of analogous functions under the names of available energy, free energy, or internal thermodynamic potential. the magnitude thus designated, attaching, as a consequence of the two principles, to all states of the system, is perfectly determined when the temperature and other normal variables are known. it allows us, by calculations often very easy, to fix the conditions necessary and sufficient for the maintenance of the system in equilibrium by foreign bodies taken at the same temperature as itself. one may hope to constitute in this way, as m. duhem in a long and remarkable series of operations has specially endeavoured to do, a sort of general mechanics which will enable questions of statics to be treated with accuracy, and all the conditions of equilibrium of the system, including the calorific properties, to be determined. thus, ordinary statics teaches us that a liquid with its vapour on the top forms a system in equilibrium, if we apply to the two fluids a pressure depending on temperature alone. thermodynamics will furnish us, in addition, with the expression of the heat of vaporization and of, the specific heats of the two saturated fluids. this new study has given us also most valuable information on compressible fluids and on the theory of elastic equilibrium. added to certain hypotheses on electric or magnetic phenomena, it gives a coherent whole from which can be deduced the conditions of electric or magnetic equilibrium; and it illuminates with a brilliant light the calorific laws of electrolytic phenomena. but the most indisputable triumph of this thermodynamic statics is the discovery of the laws which regulate the changes of physical state or of chemical constitution. j.w. gibbs was the author of this immense progress. his memoir, now celebrated, on "the equilibrium of heterogeneous substances," concealed in in a review at that time of limited circulation, and rather heavy to read, seemed only to contain algebraic theorems applicable with difficulty to reality. it is known that helmholtz independently succeeded, a few years later, in introducing thermodynamics into the domain of chemistry by his conception of the division of energy into free and into bound energy: the first, capable of undergoing all transformations, and particularly of transforming itself into external action; the second, on the other hand, bound, and only manifesting itself by giving out heat. when we measure chemical energy, we ordinarily let it fall wholly into the calorific form; but, in reality, it itself includes both parts, and it is the variation of the free energy and not that of the total energy measured by the integral disengagement of heat, the sign of which determines the direction in which the reactions are effected. but if the principle thus enunciated by helmholtz as a consequence of the laws of thermodynamics is at bottom identical with that discovered by gibbs, it is more difficult of application and is presented under a more mysterious aspect. it was not until m. van der waals exhumed the memoir of gibbs, when numerous physicists or chemists, most of them dutch--professor van t'hoff, bakhius roozeboom, and others--utilized the rules set forth in this memoir for the discussion of the most complicated chemical reactions, that the extent of the new laws was fully understood. the chief rule of gibbs is the one so celebrated at the present day under the name of the phase law. we know that by phases are designated the homogeneous substances into which a system is divided; thus carbonate of lime, lime, and carbonic acid gas are the three phases of a system which comprises iceland spar partially dissociated into lime and carbonic acid gas. the number of phases added to the number of independent components--that is to say, bodies whose mass is left arbitrary by the chemical formulas of the substances entering into the reaction--fixes the general form of the law of equilibrium of the system; that is to say, the number of quantities which, by their variations (temperature and pressure), would be of a nature to modify its equilibrium by modifying the constitution of the phases. several authors, m. raveau in particular, have indeed given very simple demonstrations of this law which are not based on thermodynamics; but thermodynamics, which led to its discovery, continues to give it its true scope. moreover, it would not suffice merely to determine quantitatively those laws of which it makes known the general form. we must, if we wish to penetrate deeper into details, particularize the hypothesis, and admit, for instance, with gibbs that we are dealing with perfect gases; while, thanks to thermodynamics, we can constitute a complete theory of dissociation which leads to formulas in complete accord with the numerical results of the experiment. we can thus follow closely all questions concerning the displacements of the equilibrium, and find a relation of the first importance between the masses of the bodies which react in order to constitute a system in equilibrium. the statics thus constructed constitutes at the present day an important edifice to be henceforth classed amongst historical monuments. some theorists even wish to go a step beyond. they have attempted to begin by the same means a more complete study of those systems whose state changes from one moment to another. this is, moreover, a study which is necessary to complete satisfactorily the study of equilibrium itself; for without it grave doubts would exist as to the conditions of stability, and it alone can give their true meaning to questions relating to displacements of equilibrium. the problems with which we are thus confronted are singularly difficult. m. duhem has given us many excellent examples of the fecundity of the method; but if thermodynamic statics may be considered definitely founded, it cannot be said that the general dynamics of systems, considered as the study of thermal movements and variations, are yet as solidly established. § . atomism it may appear singularly paradoxical that, in a chapter devoted to general views on the principles of physics, a few words should be introduced on the atomic theories of matter. very often, in fact, what is called the physics of principles is set in opposition to the hypotheses on the constitution of matter, particularly to atomic theories. i have already said that, abandoning the investigation of the unfathomable mystery of the constitution of the universe, some physicists think they may find, in certain general principles, sufficient guides to conduct them across the physical world. but i have also said, in examining the history of those principles, that if they are to-day considered experimental truths, independent of all theories relating to matter, they have, in fact, nearly all been discovered by scholars who relied on molecular hypotheses: and the question suggests itself whether this is mere chance, or whether this chance may not be ordained by higher reasons. in a very profound work which appeared a few years ago, entitled _essai critique sur l'hypothese des atomes_, m. hannequin, a philosopher who is also an erudite scholar, examined the part taken by atomism in the history of science. he notes that atomism and science were born, in greece, of the same problem, and that in modern times the revival of the one was closely connected with that of the other. he shows, too, by very close analysis, that the atomic hypothesis is essential to the optics of fresnel and of cauchy; that it penetrates into the study of heat; and that, in its general features, it presided at the birth of modern chemistry and is linked with all its progress. he concludes that it is, in a manner, the soul of our knowledge of nature, and that contemporary theories are on this point in accord with history: for these theories consecrate the preponderance of this hypothesis in the domain of science. if m. hannequin had not been prematurely cut off in the full expansion of his vigorous talent, he might have added another chapter to his excellent book. he would have witnessed a prodigious budding of atomistic ideas, accompanied, it is true, by wide modifications in the manner in which the atom is to be regarded, since the most recent theories make material atoms into centres constituted of atoms of electricity. on the other hand, he would have found in the bursting forth of these new doctrines one more proof in support of his idea that science is indissolubly bound to atomism. from the philosophical point of view, m. hannequin, examining the reasons which may have called these links into being, arrives at the idea that they necessarily proceed from the constitution of our knowledge, or, perhaps, from that of nature itself. moreover, this origin, double in appearance, is single at bottom. our minds could not, in fact, detach and come out of themselves to grasp reality and the absolute in nature. according to the idea of descartes, it is the destiny of our minds only to take hold of and to understand that which proceeds from them. thus atomism, which is, perhaps, only an appearance containing even some contradictions, is yet a well-founded appearance, since it conforms to the laws of our minds; and this hypothesis is, in a way, necessary. we may dispute the conclusions of m. hannequin, but no one will refuse to recognise, as he does, that atomic theories occupy a preponderating part in the doctrines of physics; and the position which they have thus conquered gives them, in a way, the right of saying that they rest on a real principle. it is in order to recognise this right that several physicists--m. langevin, for example--ask that atoms be promoted from the rank of hypotheses to that of principles. by this they mean that the atomistic ideas forced upon us by an almost obligatory induction based on very exact experiments, enable us to co-ordinate a considerable amount of facts, to construct a very general synthesis, and to foresee a great number of phenomena. it is of moment, moreover, to thoroughly understand that atomism does not necessarily set up the hypothesis of centres of attraction acting at a distance, and it must not be confused with molecular physics, which has, on the other hand, undergone very serious checks. the molecular physics greatly in favour some fifty years ago leads to such complex representations and to solutions often so undetermined, that the most courageous are wearied with upholding it and it has fallen into some discredit. it rested on the fundamental principles of mechanics applied to molecular actions; and that was, no doubt, an extension legitimate enough, since mechanics is itself only an experimental science, and its principles, established for the movements of matter taken as a whole, should not be applied outside the domain which belongs to them. atomism, in fact, tends more and more, in modern theories, to imitate the principle of the conservation of energy or that of entropy, to disengage itself from the artificial bonds which attached it to mechanics, and to put itself forward as an independent principle. atomistic ideas also have undergone evolution, and this slow evolution has been considerably quickened under the influence of modern discoveries. these reach back to the most remote antiquity, and to follow their development we should have to write the history of human thought which they have always accompanied since the time of leucippus, democritus, epicurus, and lucretius. the first observers who noticed that the volume of a body could be diminished by compression or cold, or augmented by heat, and who saw a soluble solid body mix completely with the water which dissolved it, must have been compelled to suppose that matter was not dispersed continuously throughout the space it seemed to occupy. they were thus brought to consider it discontinuous, and to admit that a substance having the same composition and the same properties in all its parts--in a word, perfectly homogeneous--ceases to present this homogeneity when considered within a sufficiently small volume. modern experimenters have succeeded by direct experiments in placing in evidence this heterogeneous character of matter when taken in small mass. thus, for example, the superficial tension, which is constant for the same liquid at a given temperature, no longer has the same value when the thickness of the layer of liquid becomes extremely small. newton noticed even in his time that a dark zone is seen to form on a soap bubble at the moment when it becomes so thin that it must burst. professor reinold and sir arthur rücker have shown that this zone is no longer exactly spherical; and from this we must conclude that the superficial tension, constant for all thicknesses above a certain limit, commences to vary when the thickness falls below a critical value, which these authors estimate, on optical grounds, at about fifty millionths of a millimetre. from experiments on capillarity, prof. quincke has obtained similar results with regard to layers of solids. but it is not only capillary properties which allow this characteristic to be revealed. all the properties of a body are modified when taken in small mass; m. meslin proves this in a very ingenious way as regards optical properties, and mr vincent in respect of electric conductivity. m. houllevigue, who, in a chapter of his excellent work, _du laboratoire à l'usine_, has very clearly set forth the most interesting considerations on atomic hypotheses, has recently demonstrated that copper and silver cease to combine with iodine as soon as they are present in a thickness of less than thirty millionths of a millimetre. it is this same dimension likewise that is possessed, according to m. wiener, by the smallest thicknesses it is possible to deposit on glass. these layers are so thin that they cannot be perceived, but their presence is revealed by a change in the properties of the light reflected by them. thus, below fifty to thirty millionths of a millimetre the properties of matter depend on its thickness. there are then, no doubt, only a few molecules to be met with, and it may be concluded, in consequence, that the discontinuous elements of bodies--that is, the molecules-- have linear dimensions of the order of magnitude of the millionth of a millimetre. considerations regarding more complex phenomena, for instance the phenomena of electricity by contact, and also the kinetic theory of gases, bring us to the same conclusion. the idea of the discontinuity of matter forces itself upon us for many other reasons. all modern chemistry is founded on this principle; and laws like the law of multiple proportions, introduce an evident discontinuity to which we find analogies in the law of electrolysis. the elements of bodies we are thus brought to regard might, as regards solids at all events, be considered as immobile; but this immobility could not explain the phenomena of heat, and, as it is entirely inadmissible for gases, it seems very improbable it can absolutely occur in any state. we are thus led to suppose that these elements are animated by very complicated movements, each one proceeding in closed trajectories in which the least variations of temperature or pressure cause modifications. the atomistic hypothesis shows itself remarkably fecund in the study of phenomena produced in gases, and here the mutual independence of the particles renders the question relatively more simple and, perhaps, allows the principles of mechanics to be more certainly extended to the movements of molecules. the kinetic theory of gases can point to unquestioned successes; and the idea of daniel bernouilli, who, as early as , considered a gaseous mass to be formed of a considerable number of molecules animated by rapid movements of translation, has been put into a form precise enough for mathematical analysis, and we have thus found ourselves in a position to construct a really solid foundation. it will be at once conceived, on this hypothesis, that pressure is the resultant of the shocks of the molecules against the walls of the containing vessel, and we at once come to the demonstration that the law of mariotte is a natural consequence of this origin of pressure; since, if the volume occupied by a certain number of molecules is doubled, the number of shocks per second on each square centimetre of the walls becomes half as much. but if we attempt to carry this further, we find ourselves in presence of a serious difficulty. it is impossible to mentally follow every one of the many individual molecules which compose even a very limited mass of gas. the path followed by this molecule may be every instant modified by the chance of running against another, or by a shock which may make it rebound in another direction. the difficulty would be insoluble if chance had not laws of its own. it was maxwell who first thought of introducing into the kinetic theory the calculation of probabilities. willard gibbs and boltzmann later on developed this idea, and have founded a statistical method which does not, perhaps, give absolute certainty, but which is certainly most interesting and curious. molecules are grouped in such a way that those belonging to the same group may be considered as having the same state of movement; then an examination is made of the number of molecules in each group, and what are the changes in this number from one moment to another. it is thus often possible to determine the part which the different groups have in the total properties of the system and in the phenomena which may occur. such a method, analogous to the one employed by statisticians for following the social phenomena in a population, is all the more legitimate the greater the number of individuals counted in the averages; now, the number of molecules contained in a limited space-- for example, in a centimetre cube taken in normal conditions--is such that no population could ever attain so high a figure. all considerations, those we have indicated as well as others which might be invoked (for example, the recent researches of m. spring on the limit of visibility of fluorescence), give this result:--that there are, in this space, some twenty thousand millions of molecules. each of these must receive in the space of a millimetre about ten thousand shocks, and be ten thousand times thrust out of its course. the free path of a molecule is then very small, but it can be singularly augmented by diminishing the number of them. tait and dewar have calculated that, in a good modern vacuum, the length of the free path of the remaining molecules not taken away by the air-pump easily reaches a few centimetres. by developing this theory, we come to consider that, for a given temperature, every molecule (and even every individual particle, atom, or ion) which takes part in the movement has, on the average, the same kinetic energy in every body, and that this energy is proportional to the absolute temperature; so that it is represented by this temperature multiplied by a constant quantity which is a universal constant. this result is not an hypothesis but a very great probability. this probability increases when it is noted that the same value for the constant is met with in the study of very varied phenomena; for example, in certain theories on radiation. knowing the mass and energy of a molecule, it is easy to calculate its speed; and we find that the average speed is about metres per second for carbonic anhydride, for nitrogen, and for hydrogen at ° c. and at ordinary pressure. i shall have occasion, later on, to speak of much more considerable speeds than these as animating other particles. the kinetic theory has permitted the diffusion of gases to be explained, and the divers circumstances of the phenomenon to be calculated. it has allowed us to show, as m. brillouin has done, that the coefficient of diffusion of two gases does not depend on the proportion of the gases in the mixture; it gives a very striking image of the phenomena of viscosity and conductivity; and it leads us to think that the coefficients of friction and of conductivity are independent of the density; while all these previsions have been verified by experiment. it has also invaded optics; and by relying on the principle of doppler, professor michelson has succeeded in obtaining from it an explanation of the length presented by the spectral rays of even the most rarefied gases. but however interesting are these results, they would not have sufficed to overcome the repugnance of certain physicists for speculations which, an imposing mathematical baggage notwithstanding, seemed to them too hypothetical. the theory, moreover, stopped at the molecule, and appeared to suggest no idea which could lead to the discovery of the key to the phenomena where molecules exercise a mutual influence on each other. the kinetic hypothesis, therefore, remained in some disfavour with a great number of persons, particularly in france, until the last few years, when all the recent discoveries of the conductivity of gases and of the new radiations came to procure for it a new and luxuriant efflorescence. it may be said that the atomistic synthesis, but yesterday so decried, is to-day triumphant. the elements which enter into the earlier kinetic theory, and which, to avoid confusion, should be always designated by the name of molecules, were not, truth to say, in the eyes of the chemists, the final term of the divisibility of matter. it is well known that, to them, except in certain particular bodies like the vapour of mercury and argon, the molecule comprises several atoms, and that, in compound bodies, the number of these atoms may even be fairly considerable. but physicists rarely needed to have recourse to the consideration of these atoms. they spoke of them to explain certain particularities of the propagation of sound, and to enunciate laws relating to specific heats; but, in general, they stopped at the consideration of the molecule. the present theories carry the division much further. i shall not dwell now on these theories, since, in order to thoroughly understand them, many other facts must be examined. but to avoid all confusion, it remains understood that, contrary, no doubt, to etymology, but in conformity with present custom, i shall continue in what follows to call atoms those particles of matter which have till now been spoken of; these atoms being themselves, according to modern views, singularly complex edifices formed of elements, of which we shall have occasion to indicate the nature later. chapter iv the various states of matter § . the statics of fluids the division of bodies into gaseous, liquid, and solid, and the distinction established for the same substance between the three states, retain a great importance for the applications and usages of daily life, but have long since lost their absolute value from the scientific point of view. so far as concerns the liquid and gaseous states particularly, the already antiquated researches of andrews confirmed the ideas of cagniard de la tour and established the continuity of the two states. a group of physical studies has thus been constituted on what may be called the statics of fluids, in which we examine the relations existing between the pressure, the volume, and the temperature of bodies, and in which are comprised, under the term fluid, gases as well as liquids. these researches deserve attention by their interest and the generality of the results to which they have led. they also give a remarkable example of the happy effects which may be obtained by the combined employment of the various methods of investigation used in exploring the domain of nature. thermodynamics has, in fact, allowed us to obtain numerical relations between the various coefficients, and atomic hypotheses have led to the establishment of one capital relation, the characteristic equation of fluids; while, on the other hand, experiment in which the progress made in the art of measurement has been utilized, has furnished the most valuable information on all the laws of compressibility and dilatation. the classical work of andrews was not very wide. andrews did not go much beyond pressures close to the normal and ordinary temperatures. of late years several very interesting and peculiar cases have been examined by mm. cailletet, mathias, batelli, leduc, p. chappuis, and other physicists. sir w. ramsay and mr s. young have made known the isothermal diagrams[ ] of a certain number of liquid bodies at the ordinary temperature. they have thus been able, while keeping to somewhat restricted limits of temperature and pressure, to touch upon the most important questions, since they found themselves in the region of the saturation curve and of the critical point. [footnote : by isothermal diagram is meant the pattern or complex formed when the isothermal lines are arranged in curves of which the pressure is the ordinate and the volume the abscissa.--ed.] but the most complete and systematic body of researches is due to m. amagat, who undertook the study of a certain number of bodies, some liquid and some gaseous, extending the scope of his experiments so as to embrace the different phases of the phenomena and to compare together, not only the results relating to the same bodies, but also those concerning different bodies which happen to be in the same conditions of temperature and pressure, but in very different conditions as regards their critical points. from the experimental point of view, m. amagat has been able, with extreme skill, to conquer the most serious difficulties. he has managed to measure with precision pressures amounting to atmospheres, and also the very small volumes then occupied by the fluid mass under consideration. this last measurement, which necessitates numerous corrections, is the most delicate part of the operation. these researches have dealt with a certain number of different bodies. those relating to carbonic acid and ethylene take in the critical point. others, on hydrogen and nitrogen, for instance, are very extended. others, again, such as the study of the compressibility of water, have a special interest, on account of the peculiar properties of this substance. m. amagat, by a very concise discussion of the experiments, has also been able to definitely establish the laws of compressibility and dilatation of fluids under constant pressure, and to determine the value of the various coefficients as well as their variations. it ought to be possible to condense all these results into a single formula representing the volume, the temperature, and the pressure. rankin and, subsequently, recknagel, and then hirn, formerly proposed formulas of that kind; but the most famous, the one which first appeared to contain in a satisfactory manner all the facts which experiments brought to light and led to the production of many others, was the celebrated equation of van der waals. professor van der waals arrived at this relation by relying upon considerations derived from the kinetic theory of gases. if we keep to the simple idea at the bottom of this theory, we at once demonstrate that the gas ought to obey the laws of mariotte and of gay-lussac, so that the characteristic equation would be obtained by the statement that the product of the number which is the measure of the volume by that which is the measure of the pressure is equal to a constant coefficient multiplied by the degree of the absolute temperature. but to get at this result we neglect two important factors. we do not take into account, in fact, the attraction which the molecules must exercise on each other. now, this attraction, which is never absolutely non-existent, may become considerable when the molecules are drawn closer together; that is to say, when the compressed gaseous mass occupies a more and more restricted volume. on the other hand, we assimilate the molecules, as a first approximation, to material points without dimensions; in the evaluation of the path traversed by each molecule no notice is taken of the fact that, at the moment of the shock, their centres of gravity are still separated by a distance equal to twice the radius of the molecule. m. van der waals has sought out the modifications which must be introduced into the simple characteristic equation to bring it nearer to reality. he extends to the case of gases the considerations by which laplace, in his famous theory of capillarity, reduced the effect of the molecular attraction to a perpendicular pressure exercised on the surface of a liquid. this leads him to add to the external pressure, that due to the reciprocal attractions of the gaseous particles. on the other hand, when we attribute finite dimensions to these particles, we must give a higher value to the number of shocks produced in a given time, since the effect of these dimensions is to diminish the mean path they traverse in the time which elapses between two consecutive shocks. the calculation thus pursued leads to our adding to the pressure in the simple equation a term which is designated the internal pressure, and which is the quotient of a constant by the square of the volume; also to our deducting from the volume a constant which is the quadruple of the total and invariable volume which the gaseous molecules would occupy did they touch one another. the experiments fit in fairly well with the formula of van der waals, but considerable discrepancies occur when we extend its limits, particularly when the pressures throughout a rather wider interval are considered; so that other and rather more complex formulas, on which there is no advantage in dwelling, have been proposed, and, in certain cases, better represent the facts. but the most remarkable result of m. van der waals' calculations is the discovery of corresponding states. for a long time physicists spoke of bodies taken in a comparable state. dalton, for example, pointed out that liquids have vapour-pressures equal to the temperatures equally distant from their boiling-point; but that if, in this particular property, liquids were comparable under these conditions of temperature, as regards other properties the parallelism was no longer to be verified. no general rule was found until m. van der waals first enunciated a primary law, viz., that if the pressure, the volume, and the temperature are estimated by taking as units the critical quantities, the constants special to each body disappear in the characteristic equation, which thus becomes the same for all fluids. the words corresponding states thus take a perfectly precise signification. corresponding states are those for which the numerical values of the pressure, volume, and temperature, expressed by taking as units the values corresponding to the critical point, are equal; and, in corresponding states any two fluids have exactly the same properties. m. natanson, and subsequently p. curie and m. meslin, have shown by various considerations that the same result may be arrived at by choosing units which correspond to any corresponding states; it has also been shown that the theorem of corresponding states in no way implies the exactitude of van der waals' formula. in reality, this is simply due to the fact that the characteristic equation only contains three constants. the philosophical importance and the practical interest of the discovery nevertheless remain considerable. as was to be expected, numbers of experimenters have sought whether these consequences are duly verified in reality. m. amagat, particularly, has made use for this purpose of a most original and simple method. he remarks that, in all its generality, the law may be translated thus: if the isothermal diagrams of two substances be drawn to the same scale, taking as unit of volume and of pressure the values of the critical constants, the two diagrams should coincide; that is to say, their superposition should present the aspect of one diagram appertaining to a single substance. further, if we possess the diagrams of two bodies drawn to any scales and referable to any units whatever, as the changes of units mean changes in the scale of the axes, we ought to make one of the diagrams similar to the other by lengthening or shortening it in the direction of one of the axes. m. amagat then photographs two isothermal diagrams, leaving one fixed, but arranging the other so that it may be free to turn round each axis of the co-ordinates; and by projecting, by means of a magic lantern, the second on the first, he arrives in certain cases at an almost complete coincidence. this mechanical means of proof thus dispenses with laborious calculations, but its sensibility is unequally distributed over the different regions of the diagram. m. raveau has pointed out an equally simple way of verifying the law, by remarking that if the logarithms of the pressure and volume are taken as co-ordinates, the co-ordinates of two corresponding points differ by two constant quantities, and the corresponding curves are identical. from these comparisons, and from other important researches, among which should be particularly mentioned those of mr s. young and m. mathias, it results that the laws of corresponding states have not, unfortunately, the degree of generality which we at first attributed to them, but that they are satisfactory when applied to certain groups of bodies.[ ] [footnote : mr preston thus puts it: "the law [of corresponding states] seems to be not quite, but very nearly true for these substances [_i.e._ the halogen derivatives of benzene]; but in the case of the other substances examined, the majority of these generalizations were either only roughly true or altogether departed from" (_theory of heat_, london, , p. .)--ed.] if in the study of the statics of a simple fluid the experimental results are already complex, we ought to expect much greater difficulties when we come to deal with mixtures; still the problem has been approached, and many points are already cleared up. mixed fluids may first of all be regarded as composed of a large number of invariable particles. in this particularly simple case m. van der waals has established a characteristic equation of the mixtures which is founded on mechanical considerations. various verifications of this formula have been effected, and it has, in particular, been the object of very important remarks by m. daniel berthelot. it is interesting to note that thermodynamics seems powerless to determine this equation, for it does not trouble itself about the nature of the bodies obedient to its laws; but, on the other hand, it intervenes to determine the properties of coexisting phases. if we examine the conditions of equilibrium of a mixture which is not subjected to external forces, it will be demonstrated that the distribution must come back to a juxtaposition of homogeneous phases; in a given volume, matter ought so to arrange itself that the total sum of free energy has a minimum value. thus, in order to elucidate all questions relating to the number and qualities of the phases into which the substance divides itself, we are led to regard the geometrical surface which for a given temperature represents the free energy. i am unable to enter here into the detail of the questions connected with the theories of gibbs, which have been the object of numerous theoretical studies, and also of a series, ever more and more abundant, of experimental researches. m. duhem, in particular, has published, on the subject, memoirs of the highest importance, and a great number of experimenters, mostly scholars working in the physical laboratory of leyden under the guidance of the director, mr kamerlingh onnes, have endeavoured to verify the anticipations of the theory. we are a little less advanced as regards abnormal substances; that is to say, those composed of molecules, partly simple and partly complex, and either dissociated or associated. these cases must naturally be governed by very complex laws. recent researches by mm. van der waals, alexeif, rothmund, künen, lehfeld, etc., throw, however, some light on the question. the daily more numerous applications of the laws of corresponding states have rendered highly important the determination of the critical constants which permit these states to be defined. in the case of homogeneous bodies the critical elements have a simple, clear, and precise sense; the critical temperature is that of the single isothermal line which presents a point of inflexion at a horizontal tangent; the critical pressure and the critical volume are the two co-ordinates of this point of inflexion. the three critical constants may be determined, as mr s. young and m. amagat have shown, by a direct method based on the consideration of the saturated states. results, perhaps more precise, may also be obtained if one keeps to two constants or even to a single one-- temperature, for example--by employing various special methods. many others, mm. cailletet and colardeau, m. young, m.j. chappuis, etc., have proceeded thus. the case of mixtures is much more complicated. a binary mixture has a critical space instead of a critical point. this space is comprised between two extreme temperatures, the lower corresponding to what is called the folding point, the higher to that which we call the point of contact of the mixture. between these two temperatures an isothermal compression yields a quantity of liquid which increases, then reaches a maximum, diminishes, and disappears. this is the phenomenon of retrograde condensation. we may say that the properties of the critical point of a homogeneous substance are, in a way, divided, when it is a question of a binary mixture, between the two points mentioned. calculation has enabled m. van der waals, by the application of his kinetic theories, and m. duhem, by means of thermodynamics, to foresee most of the results which have since been verified by experiment. all these facts have been admirably set forth and systematically co-ordinated by m. mathias, who, by his own researches, moreover, has made contributions of the highest value to the study of questions regarding the continuity of the liquid and gaseous states. the further knowledge of critical elements has allowed the laws of corresponding states to be more closely examined in the case of homogeneous substances. it has shown that, as i have already said, bodies must be arranged in groups, and this fact clearly proves that the properties of a given fluid are not determined by its critical constants alone, and that it is necessary to add to them some other specific parameters; m. mathias and m. d. berthelot have indicated some which seem to play a considerable part. it results also from this that the characteristic equation of a fluid cannot yet be considered perfectly known. neither the equation of van der waals nor the more complicated formulas which have been proposed by various authors are in perfect conformity with reality. we may think that researches of this kind will only be successful if attention is concentrated, not only on the phenomena of compressibility and dilatation, but also on the calorimetric properties of bodies. thermodynamics indeed establishes relations between those properties and other constants, but does not allow everything to be foreseen. several physicists have effected very interesting calorimetric measurements, either, like m. perot, in order to verify clapeyron's formula regarding the heat of vaporization, or to ascertain the values of specific heats and their variations when the temperature or the pressure happens to change. m. mathias has even succeeded in completely determining the specific heats of liquefied gases and of their saturated vapours, as well as the heat of internal and external vaporization. § . the liquefaction of gases, and the properties of bodies at a low temperature the scientific advantages of all these researches have been great, and, as nearly always happens, the practical consequences derived from them have also been most important. it is owing to the more complete knowledge of the general properties of fluids that immense progress has been made these last few years in the methods of liquefying gases. from a theoretical point of view the new processes of liquefaction can be classed in two categories. linde's machine and those resembling it utilize, as is known, expansion without any notable production of external work. this expansion, nevertheless, causes a fall in the temperature, because the gas in the experiment is not a perfect gas, and, by an ingenious process, the refrigerations produced are made cumulative. several physicists have proposed to employ a method whereby liquefaction should be obtained by expansion with recuperable external work. this method, proposed as long ago as by siemens, would offer considerable advantages. theoretically, the liquefaction would be more rapid, and obtained much more economically; but unfortunately in the experiment serious obstacles are met with, especially from the difficulty of obtaining a suitable lubricant under intense cold for those parts of the machine which have to be in movement if the apparatus is to work. m. claude has recently made great progress on this point by the use, during the running of the machine, of the ether of petrol, which is uncongealable, and a good lubricant for the moving parts. when once the desired region of cold is reached, air itself is used, which moistens the metals but does not completely avoid friction; so that the results would have remained only middling, had not this ingenious physicist devised a new improvement which has some analogy with superheating of steam in steam engines. he slightly varies the initial temperature of the compressed air on the verge of liquefaction so as to avoid a zone of deep perturbations in the properties of fluids, which would make the work of expansion very feeble and the cold produced consequently slight. this improvement, simple as it is in appearance, presents several other advantages which immediately treble the output. the special object of m. claude was to obtain oxygen in a practical manner by the actual distillation of liquid air. since nitrogen boils at - ° and oxygen at - . ° c., if liquid air be evaporated, the nitrogen escapes, especially at the commencement of the evaporation, while the oxygen concentrates in the residual liquid, which finally consists of pure oxygen, while at the same time the temperature rises to the boiling-point (- . ° c.) of oxygen. but liquid air is costly, and if one were content to evaporate it for the purpose of collecting a part of the oxygen in the residuum, the process would have a very poor result from the commercial point of view. as early as , mr parkinson thought of improving the output by recovering the cold produced by liquid air during its evaporation; but an incorrect idea, which seems to have resulted from certain experiments of dewar--the idea that the phenomenon of the liquefaction of air would not be, owing to certain peculiarities, the exact converse of that of vaporization--led to the employment of very imperfect apparatus. m. claude, however, by making use of a method which he calls the reversal[ ] method, obtains a complete rectification in a remarkably simple manner and under extremely advantageous economic conditions. apparatus, of surprisingly reduced dimensions but of great efficiency, is now in daily work, which easily enables more than a thousand cubic metres of oxygen to be obtained at the rate, per horse-power, of more than a cubic metre per hour. [footnote : methode avec retour en arriere.--ed] it is in england, thanks to the skill of sir james dewar and his pupils--thanks also, it must be said, to the generosity of the royal institution, which has devoted considerable sums to these costly experiments--that the most numerous and systematic researches have been effected on the production of intense cold. i shall here note only the more important results, especially those relating to the properties of bodies at low temperatures. their electrical properties, in particular, undergo some interesting modifications. the order which metals assume in point of conductivity is no longer the same as at ordinary temperatures. thus at - ° c. copper is a better conductor than silver. the resistance diminishes with the temperature, and, down to about - °, this diminution is almost linear, and it would seem that the resistance tends towards zero when the temperature approaches the absolute zero. but, after - °, the pattern of the curves changes, and it is easy to foresee that at absolute zero the resistivities of all metals would still have, contrary to what was formerly supposed, a notable value. solidified electrolytes which, at temperatures far below their fusion point, still retain a very appreciable conductivity, become, on the contrary, perfect insulators at low temperatures. their dielectric constants assume relatively high values. mm. curie and compan, who have studied this question from their own point of view, have noted, moreover, that the specific inductive capacity changes considerably with the temperature. in the same way, magnetic properties have been studied. a very interesting result is that found in oxygen: the magnetic susceptibility of this body increases at the moment of liquefaction. nevertheless, this increase, which is enormous (since the susceptibility becomes sixteen hundred times greater than it was at first), if we take it in connection with equal volumes, is much less considerable if taken in equal masses. it must be concluded from this fact that the magnetic properties apparently do not belong to the molecules themselves, but depend on their state of aggregation. the mechanical properties of bodies also undergo important modifications. in general, their cohesion is greatly increased, and the dilatation produced by slight changes of temperature is considerable. sir james dewar has effected careful measurements of the dilatation of certain bodies at low temperatures: for example, of ice. changes in colour occur, and vermilion and iodide of mercury pass into pale orange. phosphorescence becomes more intense, and most bodies of complex structure--milk, eggs, feathers, cotton, and flowers--become phosphorescent. the same is the case with certain simple bodies, such as oxygen, which is transformed into ozone and emits a white light in the process. chemical affinity is almost put an end to; phosphorus and potassium remain inert in liquid oxygen. it should, however, be noted, and this remark has doubtless some interest for the theories of photographic action, that photographic substances retain, even at the temperature of liquid hydrogen, a very considerable part of their sensitiveness to light. sir james dewar has made some important applications of low temperatures in chemical analysis; he also utilizes them to create a vacuum. his researches have, in fact, proved that the pressure of air congealed by liquid hydrogen cannot exceed the millionth of an atmosphere. we have, then, in this process, an original and rapid means of creating an excellent vacuum in apparatus of very different kinds--a means which, in certain cases, may be particularly convenient.[ ] [footnote : professor soddy, in a paper read before the royal society on the th november , warns experimenters against vacua created by charcoal cooled in liquid air (the method referred-to in the text), unless as much of the air as possible is first removed with a pump and replaced by some argon-free gas. according to him, neither helium nor argon is absorbed by charcoal. by the use of electrically-heated calcium, he claims to have produced an almost perfect vacuum.--ed.] thanks to these studies, a considerable field has been opened up for biological research, but in this, which is not our subject, i shall notice one point only. it has been proved that vital germs--bacteria, for example--may be kept for seven days at - °c. without their vitality being modified. phosphorescent organisms cease, it is true, to shine at the temperature of liquid air, but this fact is simply due to the oxidations and other chemical reactions which keep up the phosphorescence being then suspended, for phosphorescent activity reappears so soon as the temperature is again sufficiently raised. an important conclusion has been drawn from these experiments which affects cosmogonical theories: since the cold of space could not kill the germs of life, it is in no way absurd to suppose that, under proper conditions, a germ may be transmitted from one planet to another. among the discoveries made with the new processes, the one which most strikingly interested public attention is that of new gases in the atmosphere. we know how sir william ramsay and dr. travers first observed by means of the spectroscope the characteristics of the _companions_ of argon in the least volatile part of the atmosphere. sir james dewar on the one hand, and sir william ramsay on the other, subsequently separated in addition to argon and helium, crypton, xenon, and neon. the process employed consists essentially in first solidifying the least volatile part of the air and then causing it to evaporate with extreme slowness. a tube with electrodes enables the spectrum of the gas in process of distillation to be observed. in this manner, the spectra of the various gases may be seen following one another in the inverse order of their volatility. all these gases are monoatomic, like mercury; that is to say, they are in the most simple state, they possess no internal molecular energy (unless it is that which heat is capable of supplying), and they even seem to have no chemical energy. everything leads to the belief that they show the existence on the earth of an earlier state of things now vanished. it may be supposed, for instance, that helium and neon, of which the molecular mass is very slight, were formerly more abundant on our planet; but at an epoch when the temperature of the globe was higher, the very speed of their molecules may have reached a considerable value, exceeding, for instance, eleven kilometres per second, which suffices to explain why they should have left our atmosphere. crypton and neon, which have a density four times greater than oxygen, may, on the contrary, have partly disappeared by solution at the bottom of the sea, where it is not absurd to suppose that considerable quantities would be found liquefied at great depths.[ ] [footnote : another view, viz. that these inert gases are a kind of waste product of radioactive changes, is also gaining ground. the discovery of the radioactive mineral malacone, which gives off both helium and argon, goes to support this. see messrs ketchin and winterson's paper on the subject at the chemical society, th october .--ed.] it is probable, moreover, that the higher regions of the atmosphere are not composed of the same air as that around us. sir james dewar points out that dalton's law demands that every gas composing the atmosphere should have, at all heights and temperatures, the same pressure as if it were alone, the pressure decreasing the less quickly, all things being equal, as its density becomes less. it results from this that the temperature becoming gradually lower as we rise in the atmosphere, at a certain altitude there can no longer remain any traces of oxygen or nitrogen, which no doubt liquefy, and the atmosphere must be almost exclusively composed of the most volatile gases, including hydrogen, which m.a. gautier has, like lord rayleigh and sir william ramsay, proved to exist in the air. the spectrum of the _aurora borealis_, in which are found the lines of those parts of the atmosphere which cannot be liquefied in liquid hydrogen, together with the lines of argon, crypton, and xenon, is quite in conformity with this point of view. it is, however, singular that it should be the spectrum of crypton, that is to say, of the heaviest gas of the group, which appears most clearly in the upper regions of the atmosphere. among the gases most difficult to liquefy, hydrogen has been the object of particular research and of really quantitative experiments. its properties in a liquid state are now very clearly known. its boiling-point, measured with a helium thermometer which has been compared with thermometers of oxygen and hydrogen, is - °; its critical temperature is - ° c.; its critical pressure, atmospheres. it is four times lighter than water, it does not present any absorption spectrum, and its specific heat is the greatest known. it is not a conductor of electricity. solidified at ° absolute, it is far from reminding one by its aspect of a metal; it rather resembles a piece of perfectly pure ice, and dr travers attributes to it a crystalline structure. the last gas which has resisted liquefaction, helium, has recently been obtained in a liquid state; it appears to have its boiling-point in the neighbourhood of ° absolute.[ ] [footnote : m. poincaré is here in error. helium has never been liquefied.--ed.] § . solids and liquids the interest of the results to which the researches on the continuity between the liquid and the gaseous states have led is so great, that numbers of scholars have naturally been induced to inquire whether something analogous might not be found in the case of liquids and solids. we might think that a similar continuity ought to be there met with, that the universal character of the properties of matter forbade all real discontinuity between two different states, and that, in truth, the solid was a prolongation of the liquid state. to discover whether this supposition is correct, it concerns us to compare the properties of liquids and solids. if we find that all properties are common to the two states we have the right to believe, even if they presented themselves in different degrees, that, by a continuous series of intermediary bodies, the two classes might yet be connected. if, on the other hand, we discover that there exists in these two classes some quality of a different nature, we must necessarily conclude that there is a discontinuity which nothing can remove. the distinction established, from the point of view of daily custom, between solids and liquids, proceeds especially from the difficulty that we meet with in the one case, and the facility in the other, when we wish to change their form temporarily or permanently by the action of mechanical force. this distinction only corresponds, however, in reality, to a difference in the value of certain coefficients. it is impossible to discover by this means any absolute characteristic which establishes a separation between the two classes. modern researches prove this clearly. it is not without use, in order to well understand them, to state precisely the meaning of a few terms generally rather loosely employed. if a conjunction of forces acting on a homogeneous material mass happens to deform it without compressing or dilating it, two very distinct kinds of reactions may appear which oppose themselves to the effort exercised. during the time of deformation, and during that time only, the first make their influence felt. they depend essentially on the greater or less rapidity of the deformation, they cease with the movement, and could not, in any case, bring the body back to its pristine state of equilibrium. the existence of these reactions leads us to the idea of viscosity or internal friction. the second kind of reactions are of a different nature. they continue to act when the deformation remains stationary, and, if the external forces happen to disappear, they are capable of causing the body to return to its initial form, provided a certain limit has not been exceeded. these last constitute rigidity. at first sight a solid body appears to have a finite rigidity and an infinite viscosity; a liquid, on the contrary, presents a certain viscosity, but no rigidity. but if we examine the matter more closely, beginning either with the solids or with the liquids, we see this distinction vanish. tresca showed long ago that internal friction is not infinite in a solid; certain bodies can, so to speak, at once flow and be moulded. m.w. spring has given many examples of such phenomena. on the other hand, viscosity in liquids is never non-existent; for were it so for water, for example, in the celebrated experiment effected by joule for the determination of the mechanical equivalent of the caloric, the liquid borne along by the floats would slide without friction on the surrounding liquid, and the work done by movement would be the same whether the floats did or did not plunge into the liquid mass. in certain cases observed long ago with what are called pasty bodies, this viscosity attains a value almost comparable to that observed by m. spring in some solids. nor does rigidity allow us to establish a barrier between the two states. notwithstanding the extreme mobility of their particles, liquids contain, in fact, vestiges of the property which we formerly wished to consider the special characteristic of solids. maxwell before succeeded in rendering the existence of this rigidity very probable by examining the optical properties of a deformed layer of liquid. but a russian physicist, m. schwedoff, has gone further, and has been able by direct experiments to show that a sheath of liquid set between two solid cylinders tends, when one of the cylinders is subjected to a slight rotation, to return to its original position, and gives a measurable torsion to a thread upholding the cylinder. from the knowledge of this torsion the rigidity can be deduced. in the case of a solution containing / per cent. of gelatine, it is found that this rigidity, enormous compared with that of water, is still, however, one trillion eight hundred and forty billion times less than that of steel. this figure, exact within a few billions, proves that the rigidity is very slight, but exists; and that suffices for a characteristic distinction to be founded on this property. in a general way, m. spring has also established that we meet in solids, in a degree more or less marked, with the properties of liquids. when they are placed in suitable conditions of pressure and time, they flow through orifices, transmit pressure in all directions, diffuse and dissolve one into the other, and react chemically on each other. they may be soldered together by compression; by the same means alloys may be produced; and further, which seems to clearly prove that matter in a solid state is not deprived of all molecular mobility, it is possible to realise suitable limited reactions and equilibria between solid salts, and these equilibria obey the fundamental laws of thermodynamics. thus the definition of a solid cannot be drawn from its mechanical properties. it cannot be said, after what we have just seen, that solid bodies retain their form, nor that they have a limited elasticity, for m. spring has made known a case where the elasticity of solids is without any limit. it was thought that in the case of a different phenomenon--that of crystallization--we might arrive at a clear distinction, because here we should he dealing with a specific quality; and that crystallized bodies would be the true solids, amorphous bodies being at that time regarded as liquids viscous in the extreme. but the studies of a german physicist, professor o. lehmann, seem to prove that even this means is not infallible. professor lehmann has succeeded, in fact, in obtaining with certain organic compounds-- oleate of potassium, for instance--under certain conditions some peculiar states to which he has given the name of semi-fluid and liquid crystals. these singular phenomena can only be observed and studied by means of a microscope, and the carlsruhe professor had to devise an ingenious apparatus which enabled him to bring the preparation at the required temperature on to the very plate of the microscope. it is thus made evident that these bodies act on polarized light in the manner of a crystal. those that m. lehmann terms semi-liquid still present traces of polyhedric delimitation, but with the peaks and angles rounded by surface-tension, while the others tend to a strictly spherical form. the optical examination of the first-named bodies is very difficult, because appearances may be produced which are due to the phenomena of refraction and imitate those of polarization. for the other kind, which are often as mobile as water, the fact that they polarize light is absolutely unquestionable. unfortunately, all these liquids are turbid, and it may be objected that they are not homogeneous. this want of homogeneity may, according to m. quincke, be due to the existence of particles suspended in a liquid in contact with another liquid miscible with it and enveloping it as might a membrane, and the phenomena of polarization would thus be quite naturally explained.[ ] [footnote : professor quincke's last hypothesis is that all liquids on solidifying pass through a stage intermediate between solid and liquid, in which they form what he calls "foam-cells," and assume a viscous structure resembling that of jelly. see _proc. roy. soc. a._, rd july .--ed.] m. tamman is of opinion that it is more a question of an emulsion, and, on this hypothesis, the action on light would actually be that which has been observed. various experimenters have endeavoured of recent years to elucidate this question. it cannot be considered absolutely settled, but these very curious experiments, pursued with great patience and remarkable ingenuity, allow us to think that there really exist certain intermediary forms between crystals and liquids in which bodies still retain a peculiar structure, and consequently act on light, but nevertheless possess considerable plasticity. let us note that the question of the continuity of the liquid and solid states is not quite the same as the question of knowing whether there exist bodies intermediate in all respects between the solids and liquids. these two problems are often wrongly confused. the gap between the two classes of bodies may be filled by certain substances with intermediate properties, such as pasty bodies and bodies liquid but still crystallized, because they have not yet completely lost their peculiar structure. yet the transition is not necessarily established in a continuous fashion when we are dealing with the passage of one and the same determinate substance from the liquid to the solid form. we conceive that this change may take place by insensible degrees in the case of an amorphous body. but it seems hardly possible to consider the case of a crystal, in which molecular movements must be essentially regular, as a natural sequence to the case of the liquid where we are, on the contrary, in presence of an extremely disordered state of movement. m. tamman has demonstrated that amorphous solids may very well, in fact, be regarded as superposed liquids endowed with very great viscosity. but it is no longer the same thing when the solid is once in the crystallized state. there is then a solution of continuity of the various properties of the substance, and the two phases may co-exist. we might presume also, by analogy with what happens with liquids and gases, that if we followed the curve of transformation of the crystalline into the liquid phase, we might arrive at a kind of critical point at which the discontinuity of their properties would vanish. professor poynting, and after him professor planck and professor ostwald, supposed this to be the case, but more recently m. tamman has shown that such a point does not exist, and that the region of stability of the crystallized state is limited on all sides. all along the curve of transformation the two states may exist in equilibrium, but we may assert that it is impossible to realize a continuous series of intermediaries between these two states. there will always be a more or less marked discontinuity in some of the properties. in the course of his researches m. tamman has been led to certain very important observations, and has met with fresh allotropic modifications in nearly all substances, which singularly complicate the question. in the case of water, for instance, he finds that ordinary ice transforms itself, under a given pressure, at the temperature of - ° c. into another crystalline variety which is denser than water. the statics of solids under high pressure is as yet, therefore, hardly drafted, but it seems to promise results which will not be identical with those obtained for the statics of fluids, though it will present at least an equal interest. § . the deformations of solids if the mechanical properties of the bodies intermediate between solids and liquids have only lately been the object of systematic studies, admittedly solid substances have been studied for a long time. yet, notwithstanding the abundance of researches published on elasticity by theorists and experimenters, numerous questions with regard to them still remain in suspense. we only propose to briefly indicate here a few problems recently examined, without going into the details of questions which belong more to the domain of mechanics than to that of pure physics. the deformations produced in solid bodies by increasing efforts arrange themselves in two distinct periods. if the efforts are weak, the deformations produced are also very weak and disappear when the effort ceases. they are then termed elastic. if the efforts exceed a certain value, a part only of these deformations disappear, and a part are permanent. the purity of the note emitted by a sound has been often invoked as a proof of the perfect isochronism of the oscillation, and, consequently, as a demonstration _a posteriori_ of the correctness of the early law of hoocke governing elastic deformations. this law has, however, during some years been frequently disputed. certain mechanicians or physicists freely admit it to be incorrect, especially as regards extremely weak deformations. according to a theory in some favour, especially in germany, i.e. the theory of bach, the law which connects the elastic deformations with the efforts would be an exponential one. recent experiments by professors kohlrausch and gruncisen, executed under varied and precise conditions on brass, cast iron, slate, and wrought iron, do not appear to confirm bach's law. nothing, in point of fact, authorises the rejection of the law of hoocke, which presents itself as the most natural and most simple approximation to reality. the phenomena of permanent deformation are very complex, and it certainly seems that they cannot be explained by the older theories which insisted that the molecules only acted along the straight line which joined their centres. it becomes necessary, then, to construct more complete hypotheses, as the mm. cosserat have done in some excellent memoirs, and we may then succeed in grouping together the facts resulting from new experiments. among the experiments of which every theory must take account may be mentioned those by which colonel hartmann has placed in evidence the importance of the lines which are produced on the surface of metals when the limit of elasticity is exceeded. it is to questions of the same order that the minute and patient researches of m. bouasse have been directed. this physicist, as ingenious as he is profound, has pursued for several years experiments on the most delicate points relating to the theory of elasticity, and he has succeeded in defining with a precision not always attained even in the best esteemed works, the deformations to which a body must be subjected in order to obtain comparable experiments. with regard to the slight oscillations of torsion which he has specially studied, m. bouasse arrives at the conclusion, in an acute discussion, that we hardly know anything more than was proclaimed a hundred years ago by coulomb. we see, by this example, that admirable as is the progress accomplished in certain regions of physics, there still exist many over-neglected regions which remain in painful darkness. the skill shown by m. bouasse authorises us to hope that, thanks to his researches, a strong light will some day illumine these unknown corners. a particularly interesting chapter on elasticity is that relating to the study of crystals; and in the last few years it has been the object of remarkable researches on the part of m. voigt. these researches have permitted a few controversial questions between theorists and experimenters to be solved: in particular, m. voigt has verified the consequences of the calculations, taking care not to make, like cauchy and poisson, the hypothesis of central forces a mere function of distance, and has recognized a potential which depends on the relative orientation of the molecules. these considerations also apply to quasi-isotropic bodies which are, in fact, networks of crystals. certain occasional deformations which are produced and disappear slowly may be considered as intermediate between elastic and permanent deformations. of these, the thermal deformation of glass which manifests itself by the displacement of the zero of a thermometer is an example. so also the modifications which the phenomena of magnetic hysteresis or the variations of resistivity have just demonstrated. many theorists have taken in hand these difficult questions. m. brillouin endeavours to interpret these various phenomena by the molecular hypothesis. the attempt may seem bold, since these phenomena are, for the most part, essentially irreversible, and seem, consequently, not adaptable to mechanics. but m. brillouin makes a point of showing that, under certain conditions, irreversible phenomena may be created between two material points, the actions of which depend solely on their distance; and he furnishes striking instances which appear to prove that a great number of irreversible physical and chemical phenomena may be ascribed to the existence of states of unstable equilibria. m. duhem has approached the problem from another side, and endeavours to bring it within the range of thermodynamics. yet ordinary thermodynamics could not account for experimentally realizable states of equilibrium in the phenomena of viscosity and friction, since this science declares them to be impossible. m. duhem, however, arrives at the idea that the establishment of the equations of thermodynamics presupposes, among other hypotheses, one which is entirely arbitrary, namely: that when the state of the system is given, external actions capable of maintaining it in that state are determined without ambiguity, by equations termed conditions of equilibrium of the system. if we reject this hypothesis, it will then be allowable to introduce into thermodynamics laws previously excluded, and it will be possible to construct, as m. duhem has done, a much more comprehensive theory. the ideas of m. duhem have been illustrated by remarkable experimental work. m. marchis, for example, guided by these ideas, has studied the permanent modifications produced in glass by an oscillation of temperature. these modifications, which may be called phenomena of the hysteresis of dilatation, may be followed in very appreciable fashion by means of a glass thermometer. the general results are quite in accord with the previsions of m. duhem. m. lenoble in researches on the traction of metallic wires, and m. chevalier in experiments on the permanent variations of the electrical resistance of wires of an alloy of platinum and silver when submitted to periodical variations of temperature, have likewise afforded verifications of the theory propounded by m. duhem. in this theory, the representative system is considered dependent on the temperature of one or several other variables, such as, for example, a chemical variable. a similar idea has been developed in a very fine set of memoirs on nickel steel, by m. ch. ed. guillaume. the eminent physicist, who, by his earlier researches, has greatly contributed to the light thrown on the analogous question of the displacement of the zero in thermometers, concludes, from fresh researches, that the residual phenomena are due to chemical variations, and that the return to the primary chemical state causes the variation to disappear. he applies his ideas not only to the phenomena presented by irreversible steels, but also to very different facts; for example, to phosphorescence, certain particularities of which may be interpreted in an analogous manner. nickel steels present the most curious properties, and i have already pointed out the paramount importance of one of them, hardly capable of perceptible dilatation, for its application to metrology and chronometry.[ ] others, also discovered by m. guillaume in the course of studies conducted with rare success and remarkable ingenuity, may render great services, because it is possible to regulate, so to speak, at will their mechanical or magnetic properties. [footnote : the metal known as "invar."--ed.] the study of alloys in general is, moreover, one of those in which the introduction of the methods of physics has produced the greatest effects. by the microscopic examination of a polished surface or of one indented by a reagent, by the determination of the electromotive force of elements of which an alloy forms one of the poles, and by the measurement of the resistivities, the densities, and the differences of potential or contact, the most valuable indications as to their constitution are obtained. m. le chatelier, m. charpy, m. dumas, m. osmond, in france; sir w. roberts austen and mr. stansfield, in england, have given manifold examples of the fertility of these methods. the question, moreover, has had a new light thrown upon it by the application of the principles of thermodynamics and of the phase rule. alloys are generally known in the two states of solid and liquid. fused alloys consist of one or several solutions of the component metals and of a certain number of definite combinations. their composition may thus be very complex: but gibbs' rule gives us at once important information on the point, since it indicates that there cannot exist, in general, more than two distinct solutions in an alloy of two metals. solid alloys may be classed like liquid ones. two metals or more dissolve one into the other, and form a solid solution quite analogous to the liquid solution. but the study of these solid solutions is rendered singularly difficult by the fact that the equilibrium so rapidly reached in the case of liquids in this case takes days and, in certain cases, perhaps even centuries to become established. chapter v solutions and electrolytic dissociation § . solution vaporization and fusion are not the only means by which the physical state of a body may be changed without modifying its chemical constitution. from the most remote periods solution has also been known and studied, but only in the last twenty years have we obtained other than empirical information regarding this phenomenon. it is natural to employ here also the methods which have allowed us to penetrate into the knowledge of other transformations. the problem of solution may be approached by way of thermodynamics and of the hypotheses of kinetics. as long ago as , kirchhoff, by attributing to saline solutions-- that is to say, to mixtures of water and a non-volatile liquid like sulphuric acid--the properties of internal energy, discovered a relation between the quantity of heat given out on the addition of a certain quantity of water to a solution and the variations to which condensation and temperature subject the vapour-tension of the solution. he calculated for this purpose the variations of energy which are produced when passing from one state to another by two different series of transformations; and, by comparing the two expressions thus obtained, he established a relation between the various elements of the phenomenon. but, for a long time afterwards, the question made little progress, because there seemed to be hardly any means of introducing into this study the second principle of thermodynamics.[ ] it was the memoir of gibbs which at last opened out this rich domain and enabled it to be rationally exploited. as early as , m. duhem showed that the theory of the thermodynamic potential furnished precise information on solutions or liquid mixtures. he thus discovered over again the famous law on the lowering of the congelation temperature of solvents which had just been established by m. raoult after a long series of now classic researches. [footnote : the "second principle" referred to has been thus enunciated: "in every engine that produces work there is a fall of temperature, and the maximum output of a perfect engine--_i.e._ the ratio between the heat consumed in work and the heat supplied--depends only on the extreme temperatures between which the fluid is evolved."--demanet, _notes de physique expérimentale_, louvain, , fasc. , p. . clausius put it in a negative form, as thus: no engine can of itself, without the aid of external agency, transfer heat from a body at low temperature to a body at a high temperature. cf. ganot's _physics_, th english edition, § .--ed.] in the minds of many persons, however, grave doubts persisted. solution appeared to be an essentially irreversible phenomenon. it was therefore, in all strictness, impossible to calculate the entropy of a solution, and consequently to be certain of the value of the thermodynamic potential. the objection would be serious even to-day, and, in calculations, what is called the paradox of gibbs would be an obstacle. we should not hesitate, however, to apply the phase law to solutions, and this law already gives us the key to a certain number of facts. it puts in evidence, for example, the part played by the eutectic point-- that is to say, the point at which (to keep to the simple case in which we have to do with two bodies only, the solvent and the solute) the solution is in equilibrium at once with the two possible solids, the dissolved body and the solvent solidified. the knowledge of this point explains the properties of refrigerating mixtures, and it is also one of the most useful for the theory of alloys. the scruples of physicists ought to have been removed on the memorable occasion when professor van t'hoff demonstrated that solution can operate reversibly by reason of the phenomena of osmosis. but the experiment can only succeed in very rare cases; and, on the other hand, professor van t'hoff was naturally led to another very bold conception. he regarded the molecule of the dissolved body as a gaseous one, and assimilated solution, not as had hitherto been the rule, to fusion, but to a kind of vaporization. naturally his ideas were not immediately accepted by the scholars most closely identified with the classic tradition. it may perhaps not be without use to examine here the principles of professor van t'hoff's theory. § . osmosis osmosis, or diffusion through a septum, is a phenomenon which has been known for some time. the discovery of it is attributed to the abbé nollet, who is supposed to have observed it in , during some "researches on liquids in ebullition." a classic experiment by dutrochet, effected about , makes this phenomenon clear. into pure water is plunged the lower part of a vertical tube containing pure alcohol, open at the top and closed at the bottom by a membrane, such as a pig's bladder, without any visible perforation. in a very short time it will be found, by means of an areometer for instance, that the water outside contains alcohol, while the alcohol of the tube, pure at first, is now diluted. two currents have therefore passed through the membrane, one of water from the outside to the inside, and one of alcohol in the converse direction. it is also noted that a difference in the levels has occurred, and that the liquid in the tube now rises to a considerable height. it must therefore be admitted that the flow of the water has been more rapid than that of the alcohol. at the commencement, the water must have penetrated into the tube much more rapidly than the alcohol left it. hence the difference in the levels, and, consequently, a difference of pressure on the two faces of the membrane. this difference goes on increasing, reaches a maximum, then diminishes, and vanishes when the diffusion is complete, final equilibrium being then attained. the phenomenon is evidently connected with diffusion. if water is very carefully poured on to alcohol, the two layers, separate at first, mingle by degrees till a homogeneous substance is obtained. the bladder seems not to have prevented this diffusion from taking place, but it seems to have shown itself more permeable to water than to alcohol. may it not therefore be supposed that there must exist dividing walls in which this difference of permeability becomes greater and greater, which would be permeable to the solvent and absolutely impermeable to the solute? if this be so, the phenomena of these _semi-permeable_ walls, as they are termed, can be observed in particularly simple conditions. the answer to this question has been furnished by biologists, at which we cannot be surprised. the phenomena of osmosis are naturally of the first importance in the action of organisms, and for a long time have attracted the attention of naturalists. de vries imagined that the contractions noticed in the protoplasm of cells placed in saline solutions were due to a phenomenon of osmosis, and, upon examining more closely certain peculiarities of cell life, various scholars have demonstrated that living cells are enclosed in membranes permeable to certain substances and entirely impermeable to others. it was interesting to try to reproduce artificially semi-permeable walls analogous to those thus met with in nature;[ ] and traube and pfeffer seem to have succeeded in one particular case. traube has pointed out that the very delicate membrane of ferrocyanide of potassium which is obtained with some difficulty by exposing it to the reaction of sulphate of copper, is permeable to water, but will not permit the passage of the majority of salts. pfeffer, by producing these walls in the interstices of a porous porcelain, has succeeded in giving them sufficient rigidity to allow measurements to be made. it must be allowed that, unfortunately, no physicist or chemist has been as lucky as these two botanists; and the attempts to reproduce semi-permeable walls completely answering to the definition, have never given but mediocre results. if, however, the experimental difficulty has not been overcome in an entirely satisfactory manner, it at least appears very probable that such walls may nevertheless exist.[ ] [footnote : see next note.--ed.] [footnote : m. stephane leduc, professor of biology of nantes, has made many experiments in this connection, and the artificial cells exhibited by him to the association française pour l'avancement des sciences, at their meeting at grenoble in and reproduced in their "actes," are particularly noteworthy.--ed.] nevertheless, in the case of gases, there exists an excellent example of a semi-permeable wall, and a partition of platinum brought to a higher than red heat is, as shown by m. villard in some ingenious experiments, completely impermeable to air, and very permeable, on the contrary, to hydrogen. it can also be experimentally demonstrated that on taking two recipients separated by such a partition, and both containing nitrogen mixed with varying proportions of hydrogen, the last-named gas will pass through the partition in such a way that the concentration--that is to say, the mass of gas per unit of volume-- will become the same on both sides. only then will equilibrium be established; and, at that moment, an excess of pressure will naturally be produced in that recipient which, at the commencement, contained the gas with the smallest quantity of hydrogen. this experiment enables us to anticipate what will happen in a liquid medium with semi-permeable partitions. between two recipients, one containing pure water, the other, say, water with sugar in solution, separated by one of these partitions, there will be produced merely a movement of the pure towards the sugared water, and following this, an increase of pressure on the side of the last. but this increase will not be without limits. at a certain moment the pressure will cease to increase and will remain at a fixed value which now has a given direction. this is the osmotic pressure. pfeffer demonstrated that, for the same substance, the osmotic pressure is proportional to the concentration, and consequently in inverse ratio to the volume occupied by a similar mass of the solute. he gave figures from which it was easy, as professor van t'hoff found, to draw the conclusion that, in a constant volume, the osmotic pressure is proportional to the absolute temperature. de vries, moreover, by his remarks on living cells, extended the results which pfeffer had applied to one case only--that is, to the one that he had been able to examine experimentally. such are the essential facts of osmosis. we may seek to interpret them and to thoroughly examine the mechanism of the phenomenon; but it must be acknowledged that as regards this point, physicists are not entirely in accord. in the opinion of professor nernst, the permeability of semi-permeable membranes is simply due to differences of solubility in one of the substances of the membrane itself. other physicists think it attributable, either to the difference in the dimensions of the molecules, of which some might pass through the pores of the membrane and others be stopped by their relative size, or to these molecules' greater or less mobility. for others, again, it is the capillary phenomena which here act a preponderating part. this last idea is already an old one: jager, more, and professor traube have all endeavoured to show that the direction and speed of osmosis are determined by differences in the surface-tensions; and recent experiments, especially those of batelli, seem to prove that osmosis establishes itself in the way which best equalizes the surface-tensions of the liquids on both sides of the partition. solutions possessing the same surface-tension, though not in molecular equilibrium, would thus be always in osmotic equilibrium. we must not conceal from ourselves that this result would be in contradiction with the kinetic theory. § . application to the theory of solution if there really exist partitions permeable to one body and impermeable to another, it may be imagined that the homogeneous mixture of these two bodies might be effected in the converse way. it can be easily conceived, in fact, that by the aid of osmotic pressure it would be possible, for example, to dilute or concentrate a solution by driving through the partition in one direction or another a certain quantity of the solvent by means of a pressure kept equal to the osmotic pressure. this is the important fact which professor van t' hoff perceived. the existence of such a wall in all possible cases evidently remains only a very legitimate hypothesis,--a fact which ought not to be concealed. relying solely on this postulate, professor van t' hoff easily established, by the most correct method, certain properties of the solutions of gases in a volatile liquid, or of non-volatile bodies in a volatile liquid. to state precisely the other relations, we must admit, in addition, the experimental laws discovered by pfeffer. but without any hypothesis it becomes possible to demonstrate the laws of raoult on the lowering of the vapour-tension and of the freezing point of solutions, and also the ratio which connects the heat of fusion with this decrease. these considerable results can evidently be invoked as _a posteriori_ proofs of the exactitude of the experimental laws of osmosis. they are not, however, the only ones that professor van t' hoff has obtained by the same method. this illustrious scholar was thus able to find anew guldberg and waage's law on chemical equilibrium at a constant temperature, and to show how the position of the equilibrium changes when the temperature happens to change. if now we state, in conformity with the laws of pfeffer, that the product of the osmotic pressure by the volume of the solution is equal to the absolute temperature multiplied by a coefficient, and then look for the numerical figure of this latter in a solution of sugar, for instance, we find that this value is the same as that of the analogous coefficient of the characteristic equation of a perfect gas. there is in this a coincidence which has also been utilized in the preceding thermodynamic calculations. it may be purely fortuitous, but we can hardly refrain from finding in it a physical meaning. professor van t'hoff has considered this coincidence a demonstration that there exists a strong analogy between a body in solution and a gas; as a matter of fact, it may seem that, in a solution, the distance between the molecules becomes comparable to the molecular distances met with in gases, and that the molecule acquires the same degree of liberty and the same simplicity in both phenomena. in that case it seems probable that solutions will be subject to laws independent of the chemical nature of the dissolved molecule and comparable to the laws governing gases, while if we adopt the kinetic image for the gas, we shall be led to represent to ourselves in a similar way the phenomena which manifest themselves in a solution. osmotic pressure will then appear to be due to the shock of the dissolved molecules against the membrane. it will come from one side of this partition to superpose itself on the hydrostatic pressure, which latter must have the same value on both sides. the analogy with a perfect gas naturally becomes much greater as the solution becomes more diluted. it then imitates gas in some other properties; the internal work of the variation of volume is nil, and the specific heat is only a function of the temperature. a solution which is diluted by a reversible method is cooled like a gas which expands adiabatically.[ ] [footnote : that is, without receiving or emitting any heat.--ed.] it must, however, be acknowledged that, in other points, the analogy is much less perfect. the opinion which sees in solution a phenomenon resembling fusion, and which has left an indelible trace in everyday language (we shall always say: to melt sugar in water) is certainly not without foundation. certain of the reasons which might be invoked to uphold this opinion are too evident to be repeated here, though others more recondite might be quoted. the fact that the internal energy generally becomes independent of the concentration when the dilution reaches even a moderately high value is rather in favour of the hypothesis of fusion. we must not forget, however, the continuity of the liquid and gaseous states; and we may consider it in an absolute way a question devoid of sense to ask whether in a solution the solute is in the liquid or the gaseous state. it is in the fluid state, and perhaps in conditions opposed to those of a body in the state of a perfect gas. it is known, of course, that in this case the manometrical pressure must be regarded as very great in relation to the internal pressure which, in the characteristic equation, is added to the other. may it not seem possible that in the solution it is, on the contrary, the internal pressure which is dominant, the manometric pressure becoming of no account? the coincidence of the formulas would thus be verified, for all the characteristic equations are symmetrical with regard to these two pressures. from this point of view the osmotic pressure would be considered as the result of an attraction between the solvent and the solute; and it would represent the difference between the internal pressures of the solution and of the pure solvent. these hypotheses are highly interesting, and very suggestive; but from the way in which the facts have been set forth, it will appear, no doubt, that there is no obligation to admit them in order to believe in the legitimacy of the application of thermodynamics to the phenomena of solution. § . electrolytic dissociation from the outset professor van t' hoff was brought to acknowledge that a great number of solutions formed very notable exceptions which were very irregular in appearance. the analogy with gases did not seem to be maintained, for the osmotic pressure had a very different value from that indicated by the theory. everything, however, came right if one multiplied by a factor, determined according to each case, but greater than unity, the constant of the characteristic formula. similar divergences were manifested in the delays observed in congelation, and disappeared when subjected to an analogous correction. thus the freezing-point of a normal solution, containing a molecule gramme (that is, the number of grammes equal to the figure representing the molecular mass) of alcohol or sugar in water, falls . ° c. if the laws of solution were identically the same for a solution of sea-salt, the same depression should be noticed in a saline solution also containing molecule per litre. in fact, the fall reaches . °, and the solution behaves as if it contained, not , but . normal molecules per litre. the consideration of the osmotic pressures would lead to similar observations, but we know that the experiment would be more difficult and less precise. we may wonder whether anything really analogous to this can be met with in the case of a gas, and we are thus led to consider the phenomena of dissociation.[ ] if we heat a body which, in a gaseous state, is capable of dissociation--hydriodic acid, for example--at a given temperature, an equilibrium is established between three gaseous bodies, the acid, the iodine, and the hydrogen. the total mass will follow with fair closeness mariotte's law, but the characteristic constant will no longer be the same as in the case of a non-dissociated gas. we here no longer have to do with a single molecule, since each molecule is in part dissociated. [footnote : dissociation must be distinguished from decomposition, which is what occurs when the whole of a particle (compound, molecule, atom, etc.) breaks up into its component parts. in dissociation the breaking up is only partial, and the resultant consists of a mixture of decomposed and undecomposed parts. see ganot's physics, th english edition, § , for examples.--ed.] the comparison of the two cases leads to the employment of a new image for representing the phenomenon which has been produced throughout the saline solution. we have introduced a single molecule of salt, and everything occurs as if there were . molecules. may it not really be said that the number is . , because the sea-salt is partly dissociated, and a molecule has become transformed into . molecule of sodium, . of chlorium, and . of salt? this is a way of speaking which seems, at first sight, strangely contradicted by experiment. professor van t' hoff, like other chemists, would certainly have rejected--in fact, he did so at first-- such a conception, if, about the same time, an illustrious swedish scholar, m. arrhenius, had not been brought to the same idea by another road, and, had not by stating it precisely and modifying it, presented it in an acceptable form. a brief examination will easily show that all the substances which are exceptions to the laws of van t'hoff are precisely those which are capable of conducting electricity when undergoing decomposition--that is to say, are electrolytes. the coincidence is absolute, and cannot be simply due to chance. now, the phenomena of electrolysis have, for a long time, forced upon us an almost necessary image. the saline molecule is always decomposed, as we know, in the primary phenomenon of electrolysis into two elements which faraday termed ions. secondary reactions, no doubt, often come to complicate the question, but these are chemical reactions belonging to the general order of things, and have nothing to do with the electric action working on the solution. the simple phenomenon is always the same--decomposition into two ions, followed by the appearance of one of these ions at the positive and of the other at the negative electrode. but as the very slightest expenditure of energy is sufficient to produce the commencement of electrolysis, it is necessary to suppose that these two ions are not united by any force. thus the two ions are, in a way, dissociated. clausius, who was the first to represent the phenomena by this symbol, supposed, in order not to shock the feelings of chemists too much, that this dissociation only affected an infinitesimal fraction of the total number of the molecules of the salt, and thereby escaped all check. this concession was unfortunate, and the hypothesis thus lost the greater part of its usefulness. m. arrhenius was bolder, and frankly recognized that dissociation occurs at once in the case of a great number of molecules, and tends to increase more and more as the solution becomes more dilute. it follows the comparison with a gas which, while partially dissociated in an enclosed space, becomes wholly so in an infinite one. m. arrhenius was led to adopt this hypothesis by the examination of experimental results relating to the conductivity of electrolytes. in order to interpret certain facts, it has to be recognized that a part only of the molecules in a saline solution can be considered as conductors of electricity, and that by adding water the number of molecular conductors is increased. this increase, too, though rapid at first, soon becomes slower, and approaches a certain limit which an infinite dilution would enable it to attain. if the conducting molecules are the dissociated molecules, then the dissociation (so long as it is a question of strong acids and salts) tends to become complete in the case of an unlimited dilution. the opposition of a large number of chemists and physicists to the ideas of m. arrhenius was at first very fierce. it must be noted with regret that, in france particularly, recourse was had to an arm which scholars often wield rather clumsily. they joked about these free ions in solution, and they asked to see this chlorine and this sodium which swam about the water in a state of liberty. but in science, as elsewhere, irony is not argument, and it soon had to be acknowledged that the hypothesis of m. arrhenius showed itself singularly fertile and had to be regarded, at all events, as a very expressive image, if not, indeed, entirely in conformity with reality. it would certainly be contrary to all experience, and even to common sense itself, to suppose that in dissolved chloride of sodium there is really free sodium, if we suppose these atoms of sodium to be absolutely identical with ordinary atoms. but there is a great difference. in the one case the atoms are electrified, and carry a relatively considerable positive charge, inseparable from their state as ions, while in the other they are in the neutral state. we may suppose that the presence of this charge brings about modifications as extensive as one pleases in the chemical properties of the atom. thus the hypothesis will be removed from all discussion of a chemical order, since it will have been made plastic enough beforehand to adapt itself to all the known facts; and if we object that sodium cannot subsist in water because it instantaneously decomposes the latter, the answer is simply that the sodium ion does not decompose water as does ordinary sodium. still, other objections might be raised which could not be so easily refuted. one, to which chemists not unreasonably attached great importance, was this:--if a certain quantity of chloride of sodium is dissociated into chlorine and sodium, it should be possible, by diffusion, for example, which brings out plainly the phenomena of dissociation in gases, to extract from the solution a part either of the chlorine or of the sodium, while the corresponding part of the other compound would remain. this result would be in flagrant contradiction with the fact that, everywhere and always, a solution of salt contains strictly the same proportions of its component elements. m. arrhenius answers to this that the electrical forces in ordinary conditions prevent separation by diffusion or by any other process. professor nernst goes further, and has shown that the concentration currents which are produced when two electrodes of the same substance are plunged into two unequally concentrated solutions may be interpreted by the hypothesis that, in these particular conditions, the diffusion does bring about a separation of the ions. thus the argument is turned round, and the proof supposed to be given of the incorrectness of the theory becomes a further reason in its favour. it is possible, no doubt, to adduce a few other experiments which are not very favourable to m. arrhenius's point of view, but they are isolated cases; and, on the whole, his theory has enabled many isolated facts, till then scattered, to be co-ordinated, and has allowed very varied phenomena to be linked together. it has also suggested--and, moreover, still daily suggests--researches of the highest order. in the first place, the theory of arrhenius explains electrolysis very simply. the ions which, so to speak, wander about haphazard, and are uniformly distributed throughout the liquid, steer a regular course as soon as we dip in the trough containing the electrolyte the two electrodes connected with the poles of the dynamo or generator of electricity. then the charged positive ions travel in the direction of the electromotive force and the negative ions in the opposite direction. on reaching the electrodes they yield up to them the charges they carry, and thus pass from the state of ion into that of ordinary atom. moreover, for the solution to remain in equilibrium, the vanished ions must be immediately replaced by others, and thus the state of ionisation of the electrolyte remains constant and its conductivity persists. all the peculiarities of electrolysis are capable of interpretation: the phenomena of the transport of ions, the fine experiments of m. bouty, those of professor kohlrausch and of professor ostwald on various points in electrolytic conduction, all support the theory. the verifications of it can even be quantitative, and we can foresee numerical relations between conductivity and other phenomena. the measurement of the conductivity permits the number of molecules dissociated in a given solution to be calculated, and the number is thus found to be precisely the same as that arrived at if it is wished to remove the disagreement between reality and the anticipations which result from the theory of professor van t' hoff. the laws of cryoscopy, of tonometry, and of osmosis thus again become strict, and no exception to them remains. if the dissociation of salts is a reality and is complete in a dilute solution, any of the properties of a saline solution whatever should be represented numerically as the sum of three values, of which one concerns the positive ion, a second the negative ion, and the third the solvent. the properties of the solutions would then be what are called additive properties. numerous verifications may be attempted by very different roads. they generally succeed very well; and whether we measure the electric conductivity, the density, the specific heats, the index of refraction, the power of rotatory polarization, the colour, or the absorption spectrum, the additive property will everywhere be found in the solution. the hypothesis, so contested at the outset by the chemists, is, moreover, assuring its triumph by important conquests in the domain of chemistry itself. it permits us to give a vivid explanation of chemical reaction, and for the old motto of the chemists, "corpora non agunt, nisi soluta," it substitutes a modern one, "it is especially the ions which react." thus, for example, all salts of iron, which contain iron in the state of ions, give similar reactions; but salts such as ferrocyanide of potassium, in which iron does not play the part of an ion, never give the characteristic reactions of iron. professor ostwald and his pupils have drawn from the hypothesis of arrhenius manifold consequences which have been the cause of considerable progress in physical chemistry. professor ostwald has shown, in particular, how this hypothesis permits the quantitative calculation of the conditions of equilibrium of electrolytes and solutions, and especially of the phenomena of neutralization. if a dissolved salt is partly dissociated into ions, this solution must be limited by an equilibrium between the non-dissociated molecule and the two ions resulting from the dissociation; and, assimilating the phenomenon to the case of gases, we may take for its study the laws of gibbs and of guldberg and waage. the results are generally very satisfactory, and new researches daily furnish new checks. professor nernst, who before gave, as has been said, a remarkable interpretation of the diffusion of electrolytes, has, in the direction pointed out by m. arrhenius, developed a theory of the entire phenomena of electrolysis, which, in particular, furnishes a striking explanation of the mechanism of the production of electromotive force in galvanic batteries. extending the analogy, already so happily invoked, between the phenomena met with in solutions and those produced in gases, professor nernst supposes that metals tend, as it were, to vaporize when in presence of a liquid. a piece of zinc introduced, for example, into pure water gives birth to a few metallic ions. these ions become positively charged, while the metal naturally takes an equal charge, but of contrary sign. thus the solution and the metal are both electrified; but this sort of vaporization is hindered by electrostatic attraction, and as the charges borne by the ions are considerable, an equilibrium will be established, although the number of ions which enter the solution will be very small. if the liquid, instead of being a solvent like pure water, contains an electrolyte, it already contains metallic ions, the osmotic pressure of which will be opposite to that of the solution. three cases may then present themselves--either there will be equilibrium, or the electrostatic attraction will oppose itself to the pressure of solution and the metal will be negatively charged, or, finally, the attraction will act in the same direction as the pressure, and the metal will become positively and the solution negatively charged. developing this idea, professor nernst calculates, by means of the action of the osmotic pressures, the variations of energy brought into play and the value of the differences of potential by the contact of the electrodes and electrolytes. he deduces this from the electromotive force of a single battery cell which becomes thus connected with the values of the osmotic pressures, or, if you will, thanks to the relation discovered by van t' hoff, with the concentrations. some particularly interesting electrical phenomena thus become connected with an already very important group, and a new bridge is built which unites two regions long considered foreign to each other. the recent discoveries on the phenomena produced in gases when rendered conductors of electricity almost force upon us, as we shall see, the idea that there exist in these gases electrified centres moving through the field, and this idea gives still greater probability to the analogous theory explaining the mechanism of the conductivity of liquids. it will also be useful, in order to avoid confusion, to restate with precision this notion of electrolytic ions, and to ascertain their magnitude, charge, and velocity. the two classic laws of faraday will supply us with important information. the first indicates that the quantity of electricity passing through the liquid is proportional to the quantity of matter deposited on the electrodes. this leads us at once to the consideration that, in any given solution, all the ions possess individual charges equal in absolute value. the second law may be stated in these terms: an atom-gramme of metal carries with it into electrolysis a quantity of electricity proportionate to its valency.[ ] [footnote : the valency or atomicity of an element may be defined as the power it possesses of entering into compounds in a certain fixed proportion. as hydrogen is generally taken as the standard, in practice the valency of an atom is the number of hydrogen atoms it will combine with or replace. thus chlorine and the rest of the halogens, the atoms of which combine with one atom of hydrogen, are called univalent, oxygen a bivalent element, and so on.--ed.] numerous experiments have made known the total mass of hydrogen capable of carrying one coulomb, and it will therefore be possible to estimate the charge of an ion of hydrogen if the number of atoms of hydrogen in a given mass be known. this last figure is already furnished by considerations derived from the kinetic theory, and agrees with the one which can be deduced from the study of various phenomena. the result is that an ion of hydrogen having a mass of . x ^{- } grammes bears a charge of . x ^{- } electromagnetic units; and the second law will immediately enable the charge of any other ion to be similarly estimated. the measurements of conductivity, joined to certain considerations relating to the differences of concentration which appear round the electrode in electrolysis, allow the speed of the ions to be calculated. thus, in a liquid containing / th of a hydrogen-ion per litre, the absolute speed of an ion would be / ths of a millimetre per second in a field where the fall of potential would be volt per centimetre. sir oliver lodge, who has made direct experiments to measure this speed, has obtained a figure very approximate to this. this value is very small compared to that which we shall meet with in gases. another consequence of the laws of faraday, to which, as early as , helmholtz drew attention, may be considered as the starting-point of certain new doctrines we shall come across later. helmholtz says: "if we accept the hypothesis that simple bodies are composed of atoms, we are obliged to admit that, in the same way, electricity, whether positive or negative, is composed of elementary parts which behave like atoms of electricity." the second law seems, in fact, analogous to the law of multiple proportions in chemistry, and it shows us that the quantities of electricity carried vary from the simple to the double or treble, according as it is a question of a uni-, bi-, or trivalent metal; and as the chemical law leads up to the conception of the material atom, so does the electrolytic law suggest the idea of an electric atom. chapter vi the ether § . the luminiferous ether it is in the works of descartes that we find the first idea of attributing those physical phenomena which the properties of matter fail to explain to some subtle matter which is the receptacle of the energy of the universe. in our times this idea has had extraordinary luck. after having been eclipsed for two hundred years by the success of the immortal synthesis of newton, it gained an entirely new splendour with fresnel and his followers. thanks to their admirable discoveries, the first stage seemed accomplished, the laws of optics were represented by a single hypothesis, marvellously fitted to allow us to anticipate unknown phenomena, and all these anticipations were subsequently fully verified by experiment. but the researches of faraday, maxwell, and hertz authorized still greater ambitions; and it really seemed that this medium, to which it was agreed to give the ancient name of ether, and which had already explained light and radiant heat, would also be sufficient to explain electricity. thus the hope began to take form that we might succeed in demonstrating the unity of all physical forces. it was thought that the knowledge of the laws relating to the inmost movements of this ether might give us the key to all phenomena, and might make us acquainted with the method in which energy is stored up, transmitted, and parcelled out in its external manifestations. we cannot study here all the problems which are connected with the physics of the ether. to do this a complete treatise on optics would have to be written and a very lengthy one on electricity. i shall simply endeavour to show rapidly how in the last few years the ideas relative to the constitution of this ether have evolved, and we shall see if it be possible without self-delusion to imagine that a single medium can really allow us to group all the known facts in one comprehensive arrangement. as constructed by fresnel, the hypothesis of the luminous ether, which had so great a struggle at the outset to overcome the stubborn resistance of the partisans of the then classic theory of emission, seemed, on the contrary, to possess in the sequel an unshakable strength. lamé, though a prudent mathematician, wrote: "_the existence_ of the ethereal fluid is _incontestably demonstrated_ by the propagation of light through the planetary spaces, and by the explanation, so simple and so complete, of the phenomena of diffraction in the wave theory of light"; and he adds: "the laws of double refraction prove with no less certainty that the _ether exists_ in all diaphanous media." thus the ether was no longer an hypothesis, but in some sort a tangible reality. but the ethereal fluid of which the existence was thus proclaimed has some singular properties. were it only a question of explaining rectilinear propagation, reflexion, refraction, diffraction, and interferences notwithstanding grave difficulties at the outset and the objections formulated by laplace and poisson (some of which, though treated somewhat lightly at the present day, have not lost all value), we should be under no obligation to make any hypothesis other than that of the undulations of an elastic medium, without deciding in advance anything as to the nature and direction of the vibrations. this medium would, naturally--since it exists in what we call the void--be considered as imponderable. it may be compared to a fluid of negligible mass--since it offers no appreciable resistance to the motion of the planets--but is endowed with an enormous elasticity, because the velocity of the propagation of light is considerable. it must be capable of penetrating into all transparent bodies, and of retaining there, so to speak, a constant elasticity, but must there become condensed, since the speed of propagation in these bodies is less than in a vacuum. such properties belong to no material gas, even the most rarefied, but they admit of no essential contradiction, and that is the important point.[ ] [footnote : since this was written, however, men of science have become less unanimous than they formerly were on this point. the veteran chemist professor mendeléeff has given reasons for thinking that the ether is an inert gas with an atomic weight a million times less than that of hydrogen, and a velocity of kilometres per second (_principles of chemistry_, eng. ed., , vol. ii. p. ). on the other hand, the well-known physicist dr a.h. bucherer, speaking at the naturforscherversammlung, held at stuttgart in , declared his disbelief in the existence of the ether, which he thought could not be reconciled at once with the maxwellian theory and the known facts.--ed.] it was the study of the phenomena of polarization which led fresnel to his bold conception of transverse vibrations, and subsequently induced him to penetrate further into the constitution of the ether. we know the experiment of arago on the noninterference of polarized rays in rectangular planes. while two systems of waves, proceeding from the same source of natural light and propagating themselves in nearly parallel directions, increase or become destroyed according to whether the nature of the superposed waves are of the same or of contrary signs, the waves of the rays polarized in perpendicular planes, on the other hand, can never interfere with each other. whatever the difference of their course, the intensity of the light is always the sum of the intensity of the two rays. fresnel perceived that this experiment absolutely compels us to reject the hypothesis of longitudinal vibrations acting along the line of propagation in the direction of the rays. to explain it, it must of necessity be admitted, on the contrary, that the vibrations are transverse and perpendicular to the ray. verdet could say, in all truth, "it is not possible to deny the transverse direction of luminous vibrations, without at the same time denying that light consists of an undulatory movement." such vibrations do not and cannot exist in any medium resembling a fluid. the characteristic of a fluid is that its different parts can displace themselves with regard to one another without any reaction appearing so long as a variation of volume is not produced. there certainly may exist, as we have seen, certain traces of rigidity in a liquid, but we cannot conceive such a thing in a body infinitely more subtle than rarefied gas. among material bodies, a solid alone really possesses the rigidity sufficient for the production within it of transverse vibrations and for their maintenance during their propagation. since we have to attribute such a property to the ether, we may add that on this point it resembles a solid, and lord kelvin has shown that this solid, would be much more rigid than steel. this conclusion produces great surprise in all who hear it for the first time, and it is not rare to hear it appealed to as an argument against the actual existence of the ether. it does not seem, however, that such an argument can be decisive. there is no reason for supposing that the ether ought to be a sort of extension of the bodies we are accustomed to handle. its properties may astonish our ordinary way of thinking, but this rather unscientific astonishment is not a reason for doubting its existence. real difficulties would appear only if we were led to attribute to the ether, not singular properties which are seldom found united in the same substance, but properties logically contradictory. in short, however odd such a medium may appear to us, it cannot be said that there is any absolute incompatibility between its attributes. it would even be possible, if we wished, to suggest images capable of representing these contrary appearances. various authors have done so. thus, m. boussinesq assumes that the ether behaves like a very rarefied gas in respect of the celestial bodies, because these last move, while bathed in it, in all directions and relatively slowly, while they permit it to retain, so to speak, its perfect homogeneity. on the other hand, its own undulations are so rapid that so far as they are concerned the conditions become very different, and its fluidity has, one might say, no longer the time to come in. hence its rigidity alone appears. another consequence, very important in principle, of the fact that vibrations of light are transverse, has been well put in evidence by fresnel. he showed how we have, in order to understand the action which excites without condensation the sliding of successive layers of the ether during the propagation of a vibration, to consider the vibrating medium as being composed of molecules separated by finite distances. certain authors, it is true, have proposed theories in which the action at a distance of these molecules are replaced by actions of contact between parallelepipeds sliding over one another; but, at bottom, these two points of view both lead us to conceive the ether as a discontinuous medium, like matter itself. the ideas gathered from the most recent experiments also bring us to the same conclusion. § . radiations in the ether thus constituted there are therefore propagated transverse vibrations, regarding which all experiments in optics furnish very precise information. the amplitude of these vibrations is exceedingly small, even in relation to the wave-length, small as these last are. if, in fact, the amplitude of the vibrations acquired a noticeable value in comparison with the wave-length, the speed of propagation should increase with the amplitude. yet, in spite of some curious experiments which seem to establish that the speed of light does alter a little with its intensity, we have reason to believe that, as regards light, the amplitude of the oscillations in relation to the wave-length is incomparably less than in the case of sound. it has become the custom to characterise each vibration by the path which the vibratory movement traverses during the space of a vibration--by the length of wave, in a word--rather than by the duration of the vibration itself. to measure wave-lengths, the methods must be employed to which i have already alluded on the subject of measurements of length. professor michelson, on the one hand, and mm. perot and fabry, on the other, have devised exceedingly ingenious processes, which have led to results of really unhoped-for precision. the very exact knowledge also of the speed of the propagation of light allows the duration of a vibration to be calculated when once the wave-length is known. it is thus found that, in the case of visible light, the number of the vibrations from the end of the violet to the infra-red varies from four hundred to two hundred billions per second. this gamut is not, however, the only one the ether can give. for a long time we have known ultra-violet radiations still more rapid, and, on the other hand, infra-red ones more slow, while in the last few years the field of known radiations has been singularly extended in both directions. it is to m. rubens and his fellow-workers that are due the most brilliant conquests in the matter of great wave-lengths. he had remarked that, in their study, the difficulty of research proceeds from the fact that the extreme waves of the infra-red spectrum only contain a small part of the total energy emitted by an incandescent body; so that if, for the purpose of study, they are further dispersed by a prism or a grating, the intensity at any one point becomes so slight as to be no longer observable. his original idea was to obtain, without prism or grating, a homogeneous pencil of great wave-length sufficiently intense to be examined. for this purpose the radiant source used was a strip of platinum covered with fluorine or powdered quartz, which emits numerous radiations close to two bands of linear absorption in the absorption spectra of fluorine and quartz, one of which is situated in the infra-red. the radiations thus emitted are several times reflected on fluorine or on quartz, as the case may be; and as, in proximity to the bands, the absorption is of the order of that of metallic bodies for luminous rays, we no longer meet in the pencil several times reflected or in the rays _remaining_ after this kind of filtration, with any but radiations of great wave-length. thus, for instance, in the case of the quartz, in the neighbourhood of a radiation corresponding to a wave-length of . microns, the absorption is thirty times greater in the region of the band than in the neighbouring region, and consequently, after three reflexions, while the corresponding radiations will not have been weakened, the neighbouring waves will be so, on the contrary, in the proportion of to , . with mirrors of rock salt and of sylvine[ ] there have been obtained, by taking an incandescent gas light (auer) as source, radiations extending as far as microns; and these last are the greatest wave-lengths observed in optical phenomena. these radiations are largely absorbed by the vapour of water, and it is no doubt owing to this absorption that they are not found in the solar spectrum. on the other hand, they easily pass through gutta-percha, india-rubber, and insulating substances in general. [footnote : a natural chlorate of potassium, generally of volcanic origin.--ed.] at the opposite end of the spectrum the knowledge of the ultra-violet regions has been greatly extended by the researches of lenard. these extremely rapid radiations have been shown by that eminent physicist to occur in the light of the electric sparks which flash between two metal points, and which are produced by a large induction coil with condenser and a wehnelt break. professor schumann has succeeded in photographing them by depositing bromide of silver directly on glass plates without fixing it with gelatine; and he has, by the same process, photographed in the spectrum of hydrogen a ray with a wave-length of only . micron. the spectroscope was formed entirely of fluor-spar, and a vacuum had been created in it, for these radiations are extremely absorbable by the air. notwithstanding the extreme smallness of the luminous wave-lengths, it has been possible, after numerous fruitless trials, to obtain stationary waves analogous to those which, in the case of sound, are produced in organ pipes. the marvellous application m. lippmann has made of these waves to completely solve the problem of photography in colours is well known. this discovery, so important in itself and so instructive, since it shows us how the most delicate anticipations of theory may be verified in all their consequences, and lead the physicist to the solution of the problems occurring in practice, has justly become popular, and there is, therefore, no need to describe it here in detail. professor wiener obtained stationary waves some little while before m. lippmann's discovery, in a layer of a sensitive substance having a grain sufficiently small in relation to the length of wave. his aim was to solve a question of great importance to a complete knowledge of the ether. fresnel founded his theory of double refraction and reflexion by transparent surfaces, on the hypothesis that the vibration of a ray of polarized light is perpendicular to the plane of polarization. but neumann has proposed, on the contrary, a theory in which he recognizes that the luminous vibration is in this very plane. he rather supposes, in opposition to fresnel's idea, that the density of the ether remains the same in all media, while its coefficient of elasticity is variable. very remarkable experiments on dispersion by m. carvallo prove indeed that the idea of fresnel was, if not necessary for us to adopt, at least the more probable of the two; but apart from this indication, and contrary to the hypothesis of neumann, the two theories, from the point of view of the explanation of all known facts, really appear to be equivalent. are we then in presence of two mechanical explanations, different indeed, but nevertheless both adaptable to all the facts, and between which it will always be impossible to make a choice? or, on the contrary, shall we succeed in realising an _experimentum crucis_, an experiment at the point where the two theories cross, which will definitely settle the question? professor wiener thought he could draw from his experiment a firm conclusion on the point in dispute. he produced stationary waves with light polarized at an angle of °,[ ] and established that, when light is polarized in the plane of incidence, the fringes persist; but that, on the other hand, they disappear when the light is polarized perpendicularly to this plane. if it be admitted that a photographic impression results from the active force of the vibratory movement of the ether, the question is, in fact, completely elucidated, and the discrepancy is abolished in fresnel's favour. [footnote : that is to say, he reflected the beam of polarized light by a mirror placed at that angle. see turpain, _leçons élementaires de physique_, t. ii. p. , for details of the experiment.--ed.] m.h. poincaré has pointed out, however, that we know nothing as to the mechanism of the photographic impression. we cannot consider it evident that it is the kinetic energy of the ether which produces the decomposition of the sensitive salt; and if, on the contrary, we suppose it to be due to the potential energy, all the conclusions are reversed, and neumann's idea triumphs. recently a very clever physicist, m. cotton, especially known for his skilful researches in the domain of optics, has taken up anew the study of stationary waves. he has made very precise quantitative experiments, and has demonstrated, in his turn, that it is impossible, even with spherical waves, to succeed in determining on which of the two vectors which have to be regarded in all theories of light on the subject of polarization phenomena the luminous intensity and the chemical action really depend. this question, therefore, no longer exists for those physicists who admit that luminous vibrations are electrical oscillations. whatever, then, the hypothesis formed, whether it be electric force or, on the contrary, magnetic force which we place in the plane of polarization, the mode of propagation foreseen will always be in accord with the facts observed. § . the electromagnetic ether the idea of attributing the phenomena of electricity to perturbations produced in the medium which transmits the light is already of old standing; and the physicists who witnessed the triumph of fresnel's theories could not fail to conceive that this fluid, which fills the whole of space and penetrates into all bodies, might also play a preponderant part in electrical actions. some even formed too hasty hypotheses on this point; for the hour had not arrived when it was possible to place them on a sufficiently sound basis, and the known facts were not numerous enough to give the necessary precision. the founders of modern electricity also thought it wiser to adopt, with regard to this science, the attitude taken by newton in connection with gravitation: "in the first place to observe facts, to vary the circumstances of these as much as possible, to accompany this first work by precise measurements in order to deduce from them general laws founded solely on experiment, and to deduce from these laws, independently of all hypotheses on the nature of the forces producing the phenomena, the mathematical value of these forces--that is to say, the formula representing them. such was the system pursued by newton. it has, in general, been adopted in france by the scholars to whom physics owe the great progress made of late years, and it has served as my guide in all my researches on electrodynamic phenomena.... it is for this reason that i have avoided speaking of the ideas i may have on the nature of the cause of the force emanating from voltaic conductors." thus did ampère express himself. the illustrious physicist rightly considered the results obtained by him through following this wise method as worthy of comparison with the laws of attraction; but he knew that when this first halting-place was reached there was still further to go, and that the evolution of ideas must necessarily continue. "with whatever physical cause," he adds, "we may wish to connect the phenomena produced by electro-dynamic action, the formula i have obtained will always remain the expression of the facts," and he explicitly indicated that if one could succeed in deducing his formula from the consideration of the vibrations of a fluid distributed through space, an enormous step would have been taken in this department of physics. he added, however, that this research appeared to him premature, and would change nothing in the results of his work, since, to accord with facts, the hypothesis adopted would always have to agree with the formula which exactly represents them. it is not devoid of interest to observe that ampère himself, notwithstanding his caution, really formed some hypotheses, and recognized that electrical phenomena were governed by the laws of mechanics. yet the principles of newton then appeared to be unshakable. faraday was the first to demonstrate, by clear experiment, the influence of the media in electricity and magnetic phenomena, and he attributed this influence to certain modifications in the ether which these media enclose. his fundamental conception was to reject action at a distance, and to localize in the ether the energy whose evolution is the cause of the actions manifested, as, for example, in the discharge of a condenser. consider the barrel of a pump placed in a vacuum and closed by a piston at each end, and let us introduce between these a certain mass of air. the two pistons, through the elastic force of the gas, repel each other with a force which, according to the law of mariotte, varies in inverse ratio to the distance. the method favoured by ampère would first of all allow this law of repulsion between the two pistons to be discovered, even if the existence of a gas enclosed in the barrel of the pump were unsuspected; and it would then be natural to localize the potential energy of the system on the surface of the two pistons. but if the phenomenon is more carefully examined, we shall discover the presence of the air, and we shall understand that every part of the volume of this air could, if it were drawn off into a recipient of equal volume, carry away with it a fraction of the energy of the system, and that consequently this energy belongs really to the air and not to the pistons, which are there solely for the purpose of enabling this energy to manifest its existence. faraday made, in some sort, an equivalent discovery when he perceived that the electrical energy belongs, not to the coatings of the condenser, but to the dielectric which separates them. his audacious views revealed to him a new world, but to explore this world a surer and more patient method was needed. maxwell succeeded in stating with precision certain points of faraday's ideas, and he gave them the mathematical form which, often wrongly, impresses physicists, but which when it exactly encloses a theory, is a certain proof that this theory is at least coherent and logical.[ ] [footnote : it will no doubt be a shock to those whom professor henry armstrong has lately called the "mathematically-minded" to find a member of the poincaré family speaking disrespectfully of the science they have done so much to illustrate. one may perhaps compare the expression in the text with m. henri poincaré's remark in his last allocution to the académie des sciences, that "mathematics are sometimes a nuisance, and even a danger, when they induce us to affirm more than we know" (_comptes-rendus_, th december ).] the work of maxwell is over-elaborated, complex, difficult to read, and often ill-understood, even at the present day. maxwell is more concerned in discovering whether it is possible to give an explanation of electrical and magnetic phenomena which shall be founded on the mechanical properties of a single medium, than in stating this explanation in precise terms. he is aware that if we could succeed in constructing such an interpretation, it would be easy to propose an infinity of others, entirely equivalent from the point of view of the experimentally verifiable consequences; and his especial ambition is therefore to extract from the premises a general view, and to place in evidence something which would remain the common property of all the theories. he succeeded in showing that if the electrostatic energy of an electromagnetic field be considered to represent potential energy, and its electrodynamic the kinetic energy, it becomes possible to satisfy both the principle of least action and that of the conservation of energy; from that moment--if we eliminate a few difficulties which exist regarding the stability of the solutions--the possibility of finding mechanical explanations of electromagnetic phenomena must be considered as demonstrated. he thus succeeded, moreover, in stating precisely the notion of two electric and magnetic fields which are produced in all points of space, and which are strictly inter-connected, since the variation of the one immediately and compulsorily gives birth to the other. from this hypothesis he deduced that, in the medium where this energy is localized, an electromagnetic wave is propagated with a velocity equal to the relation of the units of electric mass in the electromagnetic and electrostatic systems. now, experiments made known since his time have proved that this relation is numerically equal to the speed of light, and the more precise experiments made in consequence--among which should be cited the particularly careful ones of m. max abraham--have only rendered the coincidence still more complete. it is natural henceforth to suppose that this medium is identical with the luminous ether, and that a luminous wave is an electromagnetic wave--that is to say, a succession of alternating currents, which exist in the dielectric and even in the void, and possess an enormous frequency, inasmuch as they change their direction thousands of billions of times per second, and by reason of this frequency produce considerable induction effects. maxwell did not admit the existence of open currents. to his mind, therefore, an electrical vibration could not produce condensations of electricity. it was, in consequence, necessarily transverse, and thus coincided with the vibration of fresnel; while the corresponding magnetic vibration was perpendicular to it, and would coincide with the luminous vibration of neumann. maxwell's theory thus establishes a close correlation between the phenomena of the luminous and those of the electromagnetic waves, or, we might even say, the complete identity of the two. but it does not follow from this that we ought to regard the variation of an electric field produced at some one point as necessarily consisting of a real displacement of the ether round that point. the idea of thus bringing electrical phenomena back to the mechanics of the ether is not, then, forced upon us, and the contrary idea even seems more probable. it is not the optics of fresnel which absorbs the science of electricity, it is rather the optics which is swallowed up by a more general theory. the attempts of popularizers who endeavour to represent, in all their details, the mechanism of the electric phenomena, thus appear vain enough, and even puerile. it is useless to find out to what material body the ether may be compared, if we content ourselves with seeing in it a medium of which, at every point, two vectors define the properties. for a long time, therefore, we could remark that the theory of fresnel simply supposed a medium in which something periodical was propagated, without its being necessary to admit this something to be a movement; but we had to wait not only for maxwell, but also for hertz, before this idea assumed a really scientific shape. hertz insisted on the fact that the six equations of the electric field permit all the phenomena to be anticipated without its being necessary to construct one hypothesis or another, and he put these equations into a very symmetrical form, which brings completely in evidence the perfect reciprocity between electrical and magnetic actions. he did yet more, for he brought to the ideas of maxwell the most striking confirmation by his memorable researches on electric oscillations. § . electrical oscillations the experiments of hertz are well known. we know how the bonn physicist developed, by means of oscillating electric discharges, displacement currents and induction effects in the whole of the space round the spark-gap; and how he excited by induction at some point in a wire a perturbation which afterwards is propagated along the wire, and how a resonator enabled him to detect the effect produced. the most important point made evident by the observation of interference phenomena and subsequently verified directly by m. blondlot, is that the electromagnetic perturbation is propagated with the speed of light, and this result condemns for ever all the hypotheses which fail to attribute any part to the intervening media in the propagation of an induction phenomenon. if the inducing action were, in fact, to operate directly between the inducing and the induced circuits, the propagation should be instantaneous; for if an interval were to occur between the moment when the cause acted and the one when the effect was produced, during this interval there would no longer be anything anywhere, since the intervening medium does not come into play, and the phenomenon would then disappear. leaving on one side the manifold but purely electrical consequences of this and the numerous researches relating to the production or to the properties of the waves--some of which, those of mm. sarrazin and de la rive, righi, turpain, lebedeff, decombe, barbillon, drude, gutton, lamotte, lecher, etc., are, however, of the highest order--i shall only mention here the studies more particularly directed to the establishment of the identity of the electromagnetic and the luminous waves. the only differences which subsist are necessarily those due to the considerable discrepancy which exists between the durations of the periods of these two categories of waves. the length of wave corresponding to the first spark-gap of hertz was about metres, and the longest waves perceptible by the retina are / of a micron.[ ] [footnote : see footnote .] these radiations are so far apart that it is not astonishing that their properties have not a perfect similitude. thus phenomena like those of diffraction, which are negligible in the ordinary conditions under which light is observed, may here assume a preponderating importance. to play the part, for example, with the hertzian waves, which a mirror millimetre square plays with regard to light, would require a colossal mirror which would attain the size of a myriametre[ ] square. [footnote : i.e., , metres.--ed.] the efforts of physicists have to-day, however, filled up, in great part, this interval, and from both banks at once they have laboured to build a bridge between the two domains. we have seen how rubens showed us calorific rays metres long; on the other hand, mm. lecher, bose, and lampa have succeeded, one after the other, in gradually obtaining oscillations with shorter and shorter periods. there have been produced, and are now being studied, electromagnetic waves of four millimetres; and the gap subsisting in the spectrum between the rays left undetected by sylvine and the radiations of m. lampa now hardly comprise more than five octaves--that is to say, an interval perceptibly equal to that which separates the rays observed by m. rubens from the last which are evident to the eye. the analogy then becomes quite close, and in the remaining rays the properties, so to speak, characteristic of the hertzian waves, begin to appear. for these waves, as we have seen, the most transparent bodies are the most perfect electrical insulators; while bodies still slightly conducting are entirely opaque. the index of refraction of these substances tends in the case of great wave-lengths to become, as the theory anticipates, nearly the square root of the dielectric constant. mm. rubens and nichols have even produced with the waves which remain phenomena of electric resonance quite similar to those which an italian scholar, m. garbasso, obtained with electric waves. this physicist showed that, if the electric waves are made to impinge on a flat wooden stand, on which are a series of resonators parallel to each other and uniformly arranged, these waves are hardly reflected save in the case where the resonators have the same period as the spark-gap. if the remaining rays are allowed to fall on a glass plate silvered and divided by a diamond fixed on a dividing machine into small rectangles of equal dimensions, there will be observed variations in the reflecting power according to the orientation of the rectangles, under conditions entirely comparable with the experiment of garbasso. in order that the phenomenon be produced it is necessary that the remaining waves should be previously polarized. this is because, in fact, the mechanism employed to produce the electric oscillations evidently gives out vibrations which occur on a single plane and are subsequently polarized. we cannot therefore entirely assimilate a radiation proceeding from a spark-gap to a ray of natural light. for the synthesis of light to be realized, still other conditions must be complied with. during a luminous impression, the direction and the phase change millions of times in the vibration sensible to the retina, yet the damping of this vibration is very slow. with the hertzian oscillations all these conditions are changed--the damping is very rapid but the direction remains invariable. every time, however, that we deal with general phenomena which are independent of these special conditions, the parallelism is perfect; and with the waves, we have put in evidence the reflexion, refraction, total reflexion, double reflexion, rotatory polarization, dispersion, and the ordinary interferences produced by rays travelling in the same direction and crossing each other at a very acute angle, or the interferences analogous to those which wiener observed with rays of the contrary direction. a very important consequence of the electromagnetic theory foreseen by maxwell is that the luminous waves which fall on a surface must exercise on this surface a pressure equal to the radiant energy which exists in the unit of volume of the surrounding space. m. lebedeff a few years ago allowed a sheaf of rays from an arc lamp to fall on a deflection radiometer,[ ] and thus succeeded in revealing the existence of this pressure. its value is sufficient, in the case of matter of little density and finely divided, to reduce and even change into repulsion the attractive action exercised on bodies by the sun. this is a fact formerly conjectured by faye, and must certainly play a great part in the deformation of the heads of comets. [footnote : by this m. poincaré appears to mean a radiometer in which the vanes are not entirely free to move as in the radiometer of crookes but are suspended by one or two threads as in the instrument devised by professor poynting.--ed.] more recently, mm. nichols and hull have undertaken experiments on this point. they have measured not only the pressure, but also the energy of the radiation by means of a special bolometer. they have thus arrived at numerical verifications which are entirely in conformity with the calculations of maxwell. the existence of these pressures may be otherwise foreseen even apart from the electromagnetic theory, by adding to the theory of undulations the principles of thermodynamics. bartoli, and more recently dr larmor, have shown, in fact, that if these pressures did not exist, it would be possible, without any other phenomenon, to pass heat from a cold into a warm body, and thus transgress the principle of carnot. § . the x rays it appears to-day quite probable that the x rays should be classed among the phenomena which have their seat in the luminous ether. doubtless it is not necessary to recall here how, in december , röntgen, having wrapped in black paper a crookes tube in action, observed that a fluorescent platinocyanide of barium screen placed in the neighbourhood, had become visible in the dark, and that a photographic plate had received an impress. the rays which come from the tube, in conditions now well known, are not deviated by a magnet, and, as m. curie and m. sagnac have conclusively shown, they carry no electric charge. they are subject to neither reflection nor refraction, and very precise and very ingenious measurements by m. gouy have shown that, in their case, the refraction index of the various bodies cannot be more than a millionth removed from unity. we knew from the outset that there existed various x rays differing from each other as, for instance, the colours of the spectrum, and these are distinguished from each other by their unequal power of passing through substances. m. sagnac, particularly, has shown that there can be obtained a gradually decreasing scale of more or less absorbable rays, so that the greater part of their photographic action is stopped by a simple sheet of black paper. these rays figure among the secondary rays discovered, as is known, by this ingenious physicist. the x rays falling on matter are thus subjected to transformations which may be compared to those which the phenomena of luminescence produce on the ultra-violet rays. m. benoist has founded on the transparency of matter to the rays a sure and practical method of allowing them to be distinguished, and has thus been enabled to define a specific character analogous to the colour of the rays of light. it is probable also that the different rays do not transport individually the same quantity of energy. we have not yet obtained on this point precise results, but it is roughly known, since the experiments of mm. rutherford and m'clung, what quantity of energy corresponds to a pencil of x rays. these physicists have found that this quantity would be, on an average, five hundred times larger than that brought by an analogous pencil of solar light to the surface of the earth. what is the nature of this energy? the question does not appear to have been yet solved. it certainly appears, according to professors haga and wind and to professor sommerfeld, that with the x rays curious experiments of diffraction may be produced. dr barkla has shown also that they can manifest true polarization. the secondary rays emitted by a metallic surface when struck by x rays vary, in fact, in intensity when the position of the plane of incidence round the primary pencil is changed. various physicists have endeavoured to measure the speed of propagation, but it seems more and more probable that it is very nearly that of light.[ ] [footnote : see especially the experiments of professor e. marx (vienna), _annalen der physik_, vol. xx. (no. of ), pp. _et seq._, which seem conclusive on this point.--ed.] i must here leave out the description of a crowd of other experiments. some very interesting researches by m. brunhes, m. broca, m. colardeau, m. villard, in france, and by many others abroad, have permitted the elucidation of several interesting problems relative to the duration of the emission or to the best disposition to be adopted for the production of the rays. the only point which will detain us is the important question as to the nature of the x rays themselves; the properties which have just been brought to mind are those which appear essential and which every theory must reckon with. the most natural hypothesis would be to consider the rays as ultra-violet radiations of very short wave-length, or radiations which are in a manner ultra-ultra-violet. this interpretation can still, at this present moment, be maintained, and the researches of mm. buisson, righi, lenard, and merrit stewart have even established that rays of very short wave-lengths produce on metallic conductors, from the point of view of electrical phenomena, effects quite analogous to those of the x rays. another resemblance results also from the experiments by which m. perreau established that these rays act on the electric resistance of selenium. new and valuable arguments have thus added force to those who incline towards a theory which has the merit of bringing a new phenomenon within the pale of phenomena previously known. nevertheless the shortest ultra-violet radiations, such as those of m. schumann, are still capable of refraction by quartz, and this difference constitutes, in the minds of many physicists, a serious enough reason to decide them to reject the more simple hypothesis. moreover, the rays of schumann are, as we have seen, extraordinarily absorbable,--so much so that they have to be observed in a vacuum. the most striking property of the x rays is, on the contrary, the facility with which they pass through obstacles, and it is impossible not to attach considerable importance to such a difference. some attribute this marvellous radiation to longitudinal vibrations, which, as m. duhem has shown, would be propagated in dielectric media with a speed equal to that of light. but the most generally accepted idea is the one formulated from the first by sir george stokes and followed up by professor wiechert. according to this theory the x rays should be due to a succession of independent pulsations of the ether, starting from the points where the molecules projected by the cathode of the crookes tube meet the anticathode. these pulsations are not continuous vibrations like the radiations of the spectrum; they are isolated and extremely short; they are, besides, transverse, like the undulations of light, and the theory shows that they must be propagated with the speed of light. they should present neither refraction nor reflection, but, under certain conditions, they may be subject to the phenomena of diffraction. all these characteristics are found in the röntgen rays. professor j.j. thomson adopts an analogous idea, and states the precise way in which the pulsations may be produced at the moment when the electrified particles forming the cathode rays suddenly strike the anticathode wall. the electromagnetic induction behaves in such a way that the magnetic field is not annihilated when the particle stops, and the new field produced, which is no longer in equilibrium, is propagated in the dielectric like an electric pulsation. the electric and magnetic pulsations excited by this mechanism may give birth to effects similar to those of light. their slight amplitude, however, is the cause of there here being neither refraction nor diffraction phenomena, save in very special conditions. if the cathode particle is not stopped in zero time, the pulsation will take a greater amplitude, and be, in consequence, more easily absorbable; to this is probably to be attributed the differences which may exist between different tubes and different rays. it is right to add that some authors, notwithstanding the proved impossibility of deviating them in a magnetic field, have not renounced the idea of comparing them with the cathode rays. they suppose, for instance, that the rays are formed by electrons animated with so great a velocity that their inertia, conformably with theories which i shall examine later, no longer permit them to be stopped in their course; this is, for instance, the theory upheld by mr sutherland. we know, too, that to m. gustave le bon they represent the extreme limit of material things, one of the last stages before the vanishing of matter on its return to the ether. everyone has heard of the n rays, whose name recalls the town of nancy, where they were discovered. in some of their singular properties they are akin to the x rays, while in others they are widely divergent from them. m. blondlot, one of the masters of contemporary physics, deeply respected by all who know him, admired by everyone for the penetration of his mind, and the author of works remarkable for the originality and sureness of his method, discovered them in radiations emitted from various sources, such as the sun, an incandescent light, a nernst lamp, and even bodies previously exposed to the sun's rays. the essential property which allows them to be revealed is their action on a small induction spark, of which they increase the brilliancy; this phenomenon is visible to the eye and is rendered objective by photography. various other physicists and numbers of physiologists, following the path opened by m. blondlot, published during and manifold but often rather hasty memoirs, in which they related the results of their researches, which do not appear to have been always conducted with the accuracy desirable. these results were most strange; they seemed destined to revolutionise whole regions not only of the domain of physics, but likewise of the biological sciences. unfortunately the method of observation was always founded on the variations in visibility of the spark or of a phosphorescent substance, and it soon became manifest that these variations were not perceptible to all eyes. no foreign experimenter has succeeded in repeating the experiments, while in france many physicists have failed; and hence the question has much agitated public opinion. are we face to face with a very singular case of suggestion, or is special training and particular dispositions required to make the phenomenon apparent? it is not possible, at the present moment, to declare the problem solved; but very recent experiments by m. gutton and a note by m. mascart have reanimated the confidence of those who hoped that such a scholar as m. blondlot could not have been deluded by appearances. however, these last proofs in favour of the existence of the rays have themselves been contested, and have not succeeded in bringing conviction to everyone. it seems very probable indeed that certain of the most singular conclusions arrived at by certain authors on the subject will lapse into deserved oblivion. but negative experiments prove nothing in a case like this, and the fact that most experimenters have failed where m. blondlot and his pupils have succeeded may constitute a presumption, but cannot be regarded as a demonstrative argument. hence we must still wait; it is exceedingly possible that the illustrious physicist of nancy may succeed in discovering objective actions of the n rays which shall be indisputable, and may thus establish on a firm basis a discovery worthy of those others which have made his name so justly celebrated. according to m. blondlot the n rays can be polarised, refracted, and dispersed, while they have wavelengths comprised within . micron, and . micron--that is to say, between an eighth and a fifth of that found for the extreme ultra-violet rays. they might be, perhaps, simply rays of a very short period. their existence, stripped of the parasitical and somewhat singular properties sought to be attributed to them, would thus appear natural enough. it would, moreover, be extremely important, and lead, no doubt, to most curious applications; it can be conceived, in fact, that such rays might serve to reveal what occurs in those portions of matter whose too minute dimensions escape microscopic examination on account of the phenomena of diffraction. from whatever point of view we look at it, and whatever may be the fate of the discovery, the history of the n rays is particularly instructive, and must give food for reflection to those interested in questions of scientific methods. § . the ether and gravitation the striking success of the hypothesis of the ether in optics has, in our own days, strengthened the hope of being able to explain, by an analogous representation, the action of gravitation. for a long time, philosophers who rejected the idea that ponderability is a primary and essential quality of all bodies have sought to reduce their weight to pressures exercised in a very subtle fluid. this was the conception of descartes, and was perhaps the true idea of newton himself. newton points out, in many passages, that the laws he had discovered were independent of the hypotheses that could be formed on the way in which universal attraction was produced, but that with sufficient experiments the true cause of this attraction might one day be reached. in the preface to the second edition of the optics he writes: "to prove that i have not considered weight as a universal property of bodies, i have added a question as to its cause, preferring this form of question because my interpretation does not entirely satisfy me in the absence of experiment"; and he puts the question in this shape: "is not this medium (the ether) more rarefied in the interior of dense bodies like the sun, the planets, the comets, than in the empty spaces which separate them? passing from these bodies to great distances, does it not become continually denser, and in that way does it not produce the weight of these great bodies with regard to each other and of their parts with regard to these bodies, each body tending to leave the most dense for the most rarefied parts?" evidently this view is incomplete, but we may endeavour to state it precisely. if we admit that this medium, the properties of which would explain the attraction, is the same as the luminous ether, we may first ask ourselves whether the action of gravitation is itself also due to oscillations. some authors have endeavoured to found a theory on this hypothesis, but we are immediately brought face to face with very serious difficulties. gravity appears, in fact, to present quite exceptional characteristics. no agent, not even those which depend upon the ether, such as light and electricity, has any influence on its action or its direction. all bodies are, so to speak, absolutely transparent to universal attraction, and no experiment has succeeded in demonstrating that its propagation is not instantaneous. from various astronomical observations, laplace concluded that its velocity, in any case, must exceed fifty million times that of light. it is subject neither to reflection nor to refraction; it is independent of the structure of bodies; and not only is it inexhaustible, but also (as is pointed out, according to m. hannequin, by an english scholar, james croll) the distribution of the effects of the attracting force of a mass over the manifold particles which may successively enter the field of its action in no way diminishes the attraction it exercises on each of them respectively, a thing which is seen nowhere else in nature. nevertheless it is possible, by means of certain hypotheses, to construct interpretations whereby the appropriate movements of an elastic medium should explain the facts clearly enough. but these movements are very complex, and it seems almost inconceivable that the same medium could possess simultaneously the state of movement corresponding to the transmission of a luminous phenomenon and that constantly imposed on it by the transmission of gravitation. another celebrated hypothesis was devised by lesage, of geneva. lesage supposed space to be overrun in all directions by currents of _ultramundane_ corpuscles. this hypothesis, contested by maxwell, is interesting. it might perhaps be taken up again in our days, and it is not impossible that the assimilation of these corpuscles to electrons might give a satisfactory image.[ ] [footnote : m. sagnac (_le radium_, jan. , p. ), following perhaps professors elster and geitel, has lately taken up this idea anew.--ed.] m. crémieux has recently undertaken experiments directed, as he thinks, to showing that the divergences between the phenomena of gravitation and all the other phenomena in nature are more apparent than real. thus the evolution in the heart of the ether of a quantity of gravific energy would not be entirely isolated, and as in the case of all evolutions of all energy of whatever kind, it should provoke a partial transformation into energy of a different form. thus again the liberated energy of gravitation would vary when passing from one material to another, as from gases into liquids, or from one liquid to a different one. on this last point the researches of m. crémieux have given affirmative results: if we immerse in a large mass of some liquid several drops of another not miscible with the first, but of identical density, we form a mass representing no doubt a discontinuity in the ether, and we may ask ourselves whether, in conformity with what happens in all other phenomena of nature, this discontinuity has not a tendency to disappear. if we abide by the ordinary consequences of the newtonian theory of potential, the drops should remain motionless, the hydrostatic impulsion forming an exact equilibrium to their mutual attraction. now m. crémieux remarks that, as a matter of fact, they slowly approach each other. such experiments are very delicate; and with all the precautions taken by the author, it cannot yet be asserted that he has removed all possibility of the action of the phenomena of capillarity nor all possible errors proceeding from extremely slight differences of temperature. but the attempt is interesting and deserves to be followed up. thus, the hypothesis of the ether does not yet explain all the phenomena which the considerations relating to matter are of themselves powerless to interpret. if we wished to represent to ourselves, by the mechanical properties of a medium filling the whole of the universe, all luminous, electric, and gravitation phenomena, we should be led to attribute to this medium very strange and almost contradictory characteristics; and yet it would be still more inconceivable that this medium should be double or treble, that there should be two or three ethers each occupying space as if it were alone, and interpenetrating it without exercising any action on one another. we are thus brought, by a close examination of facts, rather to the idea that the properties of the ether are not wholly reducible to the rules of ordinary mechanics. the physicist has therefore not yet succeeded in answering the question often put to him by the philosopher: "has the ether really an objective existence?" however, it is not necessary to know the answer in order to utilize the ether. in its ideal properties we find the means of determining the form of equations which are valid, and to the learned detached from all metaphysical prepossession this is the essential point. chapter vii a chapter in the history of science: wireless telegraphy § i have endeavoured in this book to set forth impartially the ideas dominant at this moment in the domain of physics, and to make known the facts essential to them. i have had to quote the authors of the principal discoveries in order to be able to class and, in some sort, to name these discoveries; but i in no way claim to write even a summary history of the physics of the day. i am not unaware that, as has often been said, contemporary history is the most difficult of all histories to write. a certain step backwards seems necessary in order to enable us to appreciate correctly the relative importance of events, and details conceal the full view from eyes which are too close to them, as the trees prevent us from seeing the forest. the event which produces a great sensation has often only insignificant consequences; while another, which seemed at the outset of the least importance and little worthy of note, has in the long run a widespread and deep influence. if, however, we deal with the history of a positive discovery, contemporaries who possess immediate information, and are in a position to collect authentic evidence at first hand, will make, by bringing to it their sincere testimony, a work of erudition which may be very useful, but which we may be tempted to look upon as very easy of execution. yet such a labour, even when limited to the study of a very minute question or of a recent invention, is far from being accomplished without the historian stumbling over serious obstacles. an invention is never, in reality, to be attributed to a single author. it is the result of the work of many collaborators who sometimes have no acquaintance with one another, and is often the fruit of obscure labours. public opinion, however, wilfully simple in face of a sensational discovery, insists that the historian should also act as judge; and it is the historian's task to disentangle the truth in the midst of the contest, and to declare infallibly to whom the acknowledgments of mankind should be paid. he must, in his capacity as skilled expert, expose piracies, detect the most carefully hidden plagiarisms, and discuss the delicate question of priority; while he must not be deluded by those who do not fear to announce, in bold accents, that they have solved problems of which they find the solution imminent, and who, the day after its final elucidation by third parties, proclaim themselves its true discoverers. he must rise above a partiality which deems itself excusable because it proceeds from national pride; and, finally, he must seek with patience for what has gone before. while thus retreating step by step he runs the risk of losing himself in the night of time. an example of yesterday seems to show the difficulties of such a task. among recent discoveries the invention of wireless telegraphy is one of those which have rapidly become popular, and looks, as it were, an exact subject clearly marked out. many attempts have already been made to write its history. mr j.j. fahie published in england as early as an interesting work entitled the _history of wireless telegraphy_; and about the same time m. broca published in france a very exhaustive work named _la telegraphie sans fil_. among the reports presented to the congrès international de physique (paris, ), signor righi, an illustrious italian scholar, whose personal efforts have largely contributed to the invention of the present system of telegraphy, devoted a chapter, short, but sufficiently complete, of his masterly report on hertzian waves, to the history of wireless telegraphy. the same author, in association with herr bernhard dessau, has likewise written a more important work, _die telegraphie ohne draht_; and _la telegraphie sans fil et les ondes Électriques_ of mm. j. boulanger and g. ferrié may also be consulted with advantage, as may _la telegraphie sans fil_ of signor dominico mazotto. quite recently mr a. story has given us in a little volume called _the story of wireless telegraphy_, a condensed but very precise recapitulation of all the attempts which have been made to establish telegraphic communication without the intermediary of a conducting wire. mr story has examined many documents, has sometimes brought curious facts to light, and has studied even the most recently adopted apparatus. it may be interesting, by utilising the information supplied by these authors and supplementing them when necessary by others, to trace the sources of this modern discovery, to follow its developments, and thus to prove once more how much a matter, most simple in appearance, demands extensive and complex researches on the part of an author desirous of writing a definitive work. § the first, and not the least difficulty, is to clearly define the subject. the words "wireless telegraphy," which at first seem to correspond to a simple and perfectly clear idea, may in reality apply to two series of questions, very different in the mind of a physicist, between which it is important to distinguish. the transmission of signals demands three organs which all appear indispensable: the transmitter, the receiver, and, between the two, an intermediary establishing the communication. this intermediary is generally the most costly part of the installation and the most difficult to set up, while it is here that the sensible losses of energy at the expense of good output occur. and yet our present ideas cause us to consider this intermediary as more than ever impossible to suppress; since, if we are definitely quit of the conception of action at a distance, it becomes inconceivable to us that energy can be communicated from one point to another without being carried by some intervening medium. but, practically, the line will be suppressed if, instead of constructing it artificially, we use to replace it one of the natural media which separate two points on the earth. these natural media are divided into two very distinct categories, and from this classification arise two series of questions to be examined. between the two points in question there are, first, the material media such as the air, the earth, and the water. for a long time we have used for transmissions to a distance the elastic properties of the air, and more recently the electric conductivity of the soil and of water, particularly that of the sea. modern physics leads us on the other hand, as we have seen, to consider that there exists throughout the whole of the universe another and more subtle medium which penetrates everywhere, is endowed with elasticity _in vacuo_, and retains its elasticity when it penetrates into a great number of bodies, such as the air. this medium is the luminous ether which possesses, as we cannot doubt, the property of being able to transmit energy, since it itself brings to us by far the larger part of the energy which we possess on earth and which we find in the movements of the atmosphere, or of waterfalls, and in the coal mines proceeding from the decomposition of carbon compounds under the influence of the solar energy. for a long time also before the existence of the ether was known, the duty of transmitting signals was entrusted to it. thus through the ages a double evolution is unfolded which has to be followed by the historian who is ambitious of completeness. § if such an historian were to examine from the beginning the first order of questions, he might, no doubt, speak only briefly of the attempts earlier than electric telegraphy. without seeking to be paradoxical, he certainly ought to mention the invention of the speaking-trumpet and other similar inventions which for a long time have enabled mankind, by the ingenious use of the elastic properties of the natural media, to communicate at greater distances than they could have attained without the aid of art. after this in some sort prehistoric period had been rapidly run through, he would have to follow very closely the development of electric telegraphy. almost from the outset, and shortly after ampère had made public the idea of constructing a telegraph, and the day after gauss and weber set up between their houses in göttingen the first line really used, it was thought that the conducting properties of the earth and water might be made of service. the history of these trials is very long, and is closely mixed up with the history of ordinary telegraphy; long chapters for some time past have been devoted to it in telegraphic treatises. it was in , however, that professor c.a. steinheil of munich expressed, for the first time, the clear idea of suppressing the return wire and replacing it by a connection of the line wire to the earth. he thus at one step covered half the way, the easiest, it is true, which was to lead to the final goal, since he saved the use of one-half of the line of wire. steinheil, advised, perhaps, by gauss, had, moreover, a very exact conception of the part taken by the earth considered as a conducting body. he seems to have well understood that, in certain conditions, the resistance of such a conductor, though supposed to be unlimited, might be independent of the distance apart of the electrodes which carry the current and allow it to go forth. he likewise thought of using the railway lines to transmit telegraphic signals. several scholars who from the first had turned their minds to telegraphy, had analogous ideas. it was thus that s.f.b. morse, superintendent of the government telegraphs in the united states, whose name is universally known in connection with the very simple apparatus invented by him, made experiments in the autumn of before a special commission in new york and a numerous public audience, to show how surely and how easily his apparatus worked. in the very midst of his experiments a very happy idea occurred to him of replacing by the water of a canal, the length of about a mile of wire which had been suddenly and accidentally destroyed. this accident, which for a moment compromised the legitimate success the celebrated engineer expected, thus suggested to him a fruitful idea which he did not forget. he subsequently repeated attempts to thus utilise the earth and water, and obtained some very remarkable results. it is not possible to quote here all the researches undertaken with the same purpose, to which are more particularly attached the names of s.w. wilkins, wheatstone, and h. highton, in england; of bonetti in italy, gintl in austria, bouchot and donat in france; but there are some which cannot be recalled without emotion. on the th december , a physicist who has left in the university of paris a lasting name, m. d'almeida, at that time professor at the lycée henri iv. and later inspector-general of public instruction, quitted paris, then besieged, in a balloon, and descended in the midst of the german lines. he succeeded, after a perilous journey, in gaining havre by way of bordeaux and lyons; and after procuring the necessary apparatus in england, he descended the seine as far as poissy, which he reached on the th january . after his departure, two other scholars, mm. desains and bourbouze, relieving each other day and night, waited at paris, in a wherry on the seine, ready to receive the signal which they awaited with patriotic anxiety. it was a question of working a process devised by the last-named pair, in which the water of the river acted the part of the line wire. on the rd january the communication at last seemed to be established, but unfortunately, first the armistice and then the surrender of paris rendered useless the valuable result of this noble effort. special mention is also due to the experiments made by the indian telegraph office, under the direction of mr johnson and afterwards of mr w.f. melhuish. they led, indeed, in to such satisfactory results that a telegraph service, in which the line wire was replaced by the earth, worked practically and regularly. other attempts were also made during the latter half of the nineteenth century to transmit signals through the sea. they preceded the epoch when, thanks to numerous physicists, among whom lord kelvin undoubtedly occupies a preponderating position, we succeeded in sinking the first cable; but they were not abandoned, even after that date, for they gave hopes of a much more economical solution of the problem. among the most interesting are remembered those that s.w. wilkins carried on for a long time between france and england. like cooke and wheatstone, he thought of using as a receiver an apparatus which in some features resembles the present receiver of the submarine telegraph. later, george e. dering, then james bowman and lindsay, made on the same lines trials which are worthy of being remembered. but it is only in our own days that sir william h. preece at last obtained for the first time really practical results. sir william himself effected and caused to be executed by his associates--he is chief consulting engineer to the general post office in england-- researches conducted with much method and based on precise theoretical considerations. he thus succeeded in establishing very easy, clear, and regular communications between various places; for example, across the bristol channel. the long series of operations accomplished by so many seekers, with the object of substituting a material and natural medium for the artificial lines of metal, thus met with an undoubted success which was soon to be eclipsed by the widely-known experiments directed into a different line by marconi. it is right to add that sir william preece had himself utilised induction phenomena in his experiments, and had begun researches with the aid of electric waves. much is due to him for the welcome he gave to marconi; it is certainly thanks to the advice and the material support he found in sir william that the young scholar succeeded in effecting his sensational experiments. § the starting-point of the experiments based on the properties of the luminous ether, and having for their object the transmission of signals, is very remote; and it would be a very laborious task to hunt up all the work accomplished in that direction, even if we were to confine ourselves to those in which electrical reactions play a part. an electric reaction, an electrostatic influence, or an electromagnetic phenomenon, is transmitted at a distance through the air by the intermediary of the luminous ether. but electric influence can hardly be used, as the distances it would allow us to traverse would be much too restricted, and electrostatic actions are often very erratic. the phenomena of induction, which are very regular and insensible to the variations of the atmosphere, have, on the other hand, for a long time appeared serviceable for telegraphic purposes. we might find, in a certain number of the attempts just mentioned, a partial employment of these phenomena. lindsay, for instance, in his project of communication across the sea, attributed to them a considerable rôle. these phenomena even permitted a true telegraphy without intermediary wire between the transmitter and the receiver, at very restricted distances, it is true, but in peculiarly interesting conditions. it is, in fact, owing to them that c. brown, and later edison and gilliland, succeeded in establishing communications with trains in motion. mr willoughby s. smith and mr charles a. stevenson also undertook experiments during the last twenty years, in which they used induction, but the most remarkable attempts are perhaps those of professor emile rathenau. with the assistance of professor rubens and of herr w. rathenau, this physicist effected, at the request of the german ministry of marine, a series of researches which enabled him, by means of a compound system of conduction and induction by alternating currents, to obtain clear and regular communications at a distance of four kilometres. among the precursors also should be mentioned graham bell; the inventor of the telephone thought of employing his admirable apparatus as a receiver of induction phenomena transmitted from a distance; edison, herr sacher of vienna, m. henry dufour of lausanne, and professor trowbridge of boston, also made interesting attempts in the same direction. in all these experiments occurs the idea of employing an oscillating current. moreover, it was known for a long time--since, in , the great american physicist henry proved that the discharges from a leyden jar in the attic of his house caused sparks in a metallic circuit on the ground floor--that a flux which varies rapidly and periodically is much more efficacious than a simple flux, which latter can only produce at a distance a phenomenon of slight intensity. this idea of the oscillating current was closely akin to that which was at last to lead to an entirely satisfactory solution: that is, to a solution which is founded on the properties of electric waves. § having thus got to the threshold of the definitive edifice, the historian, who has conducted his readers over the two parallel routes which have just been marked out, will be brought to ask himself whether he has been a sufficiently faithful guide and has not omitted to draw attention to all essential points in the regions passed through. ought we not to place by the side, or perhaps in front, of the authors who have devised the practical appliances, those scholars who have constructed the theories and realised the laboratory experiments of which, after all, the apparatus are only the immediate applications? if we speak of the propagation of a current in a material medium, can one forget the names of fourier and of ohm, who established by theoretical considerations the laws which preside over this propagation? when one looks at the phenomena of induction, would it not be just to remember that arago foresaw them, and that michael faraday discovered them? it would be a delicate, and also a rather puerile task, to class men of genius in order of merit. the merit of an inventor like edison and that of a theorist like clerk maxwell have no common measure, and mankind is indebted for its great progress to the one as much as to the other. before relating how success attended the efforts to utilise electric waves for the transmission of signals, we cannot without ingratitude pass over in silence the theoretical speculations and the work of pure science which led to the knowledge of these waves. it would therefore be just, without going further back than faraday, to say how that illustrious physicist drew attention to the part taken by insulating media in electrical phenomena, and to insist also on the admirable memoirs in which for the first time clerk maxwell made a solid bridge between those two great chapters of physics, optics and electricity, which till then had been independent of each other. and no doubt it would be impossible not to evoke the memory of those who, by establishing, on the other hand, the solid and magnificent structure of physical optics, and proving by their immortal works the undulatory nature of light, prepared from the opposite direction the future unity. in the history of the applications of electrical undulations, the names of young, fresnel, fizeau, and foucault must be inscribed; without these scholars, the assimilation between electrical and luminous phenomena which they discovered and studied would evidently have been impossible. since there is an absolute identity of nature between the electric and the luminous waves, we should, in all justice, also consider as precursors those who devised the first luminous telegraphs. claude chappe incontestably effected wireless telegraphy, thanks to the luminous ether, and the learned men, such as colonel mangin, who perfected optical telegraphy, indirectly suggested certain improvements lately introduced into the present method. but the physicist whose work should most of all be put in evidence is, without fear of contradiction, heinrich hertz. it was he who demonstrated irrefutably, by experiments now classic, that an electric discharge produces an undulatory disturbance in the ether contained in the insulating media in its neighbourhood; it was he who, as a profound theorist, a clever mathematician, and an experimenter of prodigious dexterity, made known the mechanism of the production, and fully elucidated that of the propagation of these electromagnetic waves. he must naturally himself have thought that his discoveries might be applied to the transmission of signals. it would appear, however, that when interrogated by a munich engineer named huber as to the possibility of utilising the waves for transmissions by telephone, he answered in the negative, and dwelt on certain considerations relative to the difference between the periods of sounds and those of electrical vibrations. this answer does not allow us to judge what might have happened, had not a cruel death carried off in , at the age of thirty-five, the great and unfortunate physicist. we might also find in certain works earlier than the experiments of hertz attempts at transmission in which, unconsciously no doubt, phenomena were already set in operation which would, at this day, be classed as electric oscillations. it is allowable no doubt, not to speak of an american quack, mahlon loomis, who, according to mr story, patented in a project of communication in which he utilised the rocky mountains on one side and mont blanc on the other, as gigantic antennae to establish communication across the atlantic; but we cannot pass over in silence the very remarkable researches of the american professor dolbear, who showed, at the electrical exhibition of philadelphia in , a set of apparatus enabling signals to be transmitted at a distance, which he described as "an exceptional application of the principles of electrostatic induction." this apparatus comprised groups of coils and condensers by means of which he obtained, as we cannot now doubt, effects due to true electric waves. place should also be made for a well-known inventor, d.e. hughes, who from to followed up some very curious experiments in which also these oscillations certainly played a considerable part. it was this physicist who invented the microphone, and thus, in another way, drew attention to the variations of contact resistance, a phenomenon not far from that produced in the radio-conductors of branly, which are important organs in the marconi system. unfortunately, fatigued and in ill-health, hughes ceased his researches at the moment perhaps when they would have given him final results. in an order of ideas different in appearance, but closely linked at bottom with the one just mentioned, must be recalled the discovery of radiophony in by graham bell, which was foreshadowed in by c.a. brown. a luminous ray falling on a selenium cell produces a variation of electric resistance, thanks to which a sound signal can be transmitted by light. that delicate instrument the radiophone, constructed on this principle, has wide analogies with the apparatus of to-day. § starting from the experiments of hertz, the history of wireless telegraphy almost merges into that of the researches on electrical waves. all the progress realised in the manner of producing and receiving these waves necessarily helped to give rise to the application already indicated. the experiments of hertz, after being checked in every laboratory, and having entered into the strong domain of our most certain knowledge, were about to yield the expected fruit. experimenters like sir oliver lodge in england, righi in italy, sarrazin and de la rive in switzerland, blondlot in france, lecher in germany, bose in india, lebedeff in russia, and theorists like m.h. poincaré and professor bjerknes, who devised ingenious arrangements or elucidated certain points left dark, are among the artisans of the work which followed its natural evolution. it was professor r. threlfall who seems to have been the first to clearly propose, in , the application of the hertzian waves to telegraphy, but it was certainly sir w. crookes who, in a very remarkable article in the _fortnightly review_ of february , pointed out very clearly the road to be followed. he even showed in what conditions the morse receiver might be applied to the new system of telegraphy. about the same period an american physicist, well known by his celebrated experiments on high frequency currents--experiments, too, which are not unconnected with those on electric oscillations,--m. tesla, demonstrated that these oscillations could be transmitted to more considerable distances by making use of two vertical antennae, terminated by large conductors. a little later, sir oliver lodge succeeded, by the aid of the coherer, in detecting waves at relatively long distances, and mr rutherford obtained similar results with a magnetic indicator of his own invention. an important question of meteorology, the study of atmospheric discharges, at this date led a few scholars, and more particularly the russian, m. popoff, to set up apparatus very analogous to the receiving apparatus of the present wireless telegraphy. this comprised a long antenna and filings-tube, and m. popoff even pointed out that his apparatus might well serve for the transmission of signals as soon as a generator of waves powerful enough had been discovered. finally, on the nd june , a young italian, born in bologna on the th april , guglielmo marconi, patented a system of wireless telegraphy destined to become rapidly popular. brought up in the laboratory of professor righi, one of the physicists who had done most to confirm and extend the experiments of hertz, marconi had long been familiar with the properties of electric waves, and was well used to their manipulation. he afterwards had the good fortune to meet sir william (then mr) preece, who was to him an adviser of the highest authority. it has sometimes been said that the marconi system contains nothing original; that the apparatus for producing the waves was the oscillator of righi, that the receiver was that employed for some two or three years by professor lodge and mr bose, and was founded on an earlier discovery by a french scholar, m. branly; and, finally, that the general arrangement was that established by m. popoff. the persons who thus rather summarily judge the work of m. marconi show a severity approaching injustice. it cannot, in truth, be denied that the young scholar has brought a strictly personal contribution to the solution of the problem he proposed to himself. apart from his forerunners, and when their attempts were almost unknown, he had the very great merit of adroitly arranging the most favourable combination, and he was the first to succeed in obtaining practical results, while he showed that the electric waves could be transmitted and received at distances enormous compared to those attained before his day. alluding to a well-known anecdote relating to christopher columbus, sir w. preece very justly said: "the forerunners and rivals of marconi no doubt knew of the eggs, but he it was who taught them to make them stand on end." this judgment will, without any doubt, be the one that history will definitely pronounce on the italian scholar. § the apparatus which enables the electric waves to be revealed, the detector or indicator, is the most delicate organ in wireless telegraphy. it is not necessary to employ as an indicator a filings-tube or radio-conductor. one can, in principle, for the purpose of constructing a receiver, think of any one of the multiple effects produced by the hertzian waves. in many systems in use, and in the new one of marconi himself, the use of these tubes has been abandoned and replaced by magnetic detectors. nevertheless, the first and the still most frequent successes are due to radio-conductors, and public opinion has not erred in attributing to the inventor of this ingenious apparatus a considerable and almost preponderant part in the invention of wave telegraphy. the history of the discovery of radio-conductors is short, but it deserves, from its importance, a chapter to itself in the history of wireless telegraphy. from a theoretical point of view, the phenomena produced in those tubes should be set by the side of those studied by graham bell, c.a. brown, and summer tainter, from the year onward. the variations to which luminous waves give rise in the resistance of selenium and other substances are, doubtless, not unconnected with those which the electric waves produce in filings. a connection can also be established between this effect of the waves and the variations of contact resistance which enabled hughes to construct the microphone, that admirable instrument which is one of the essential organs of telephony. more directly, as an antecedent to the discovery, should be quoted the remark made by varley in , that coal-dust changes in conductivity when the electromotive force of the current which passes through it is made to vary. but it was in that an italian professor, signor calzecchi-onesti, demonstrated in a series of remarkable experiments that the metallic filings contained in a tube of insulating material, into which two metallic electrodes are inserted, acquire a notable conductivity under different influences such as extra currents, induced currents, sonorous vibrations, etc., and that this conductivity is easily destroyed; as, for instance, by turning the tube over and over. in several memoirs published in and , m. ed. branly independently pointed out similar phenomena, and made a much more complete and systematic study of the question. he was the first to note very clearly that the action described could be obtained by simply making sparks pass in the neighbourhood of the radio-conductor, and that their great resistance could be restored to the filings by giving a slight shake to the tube or to its supports. the idea of utilising such a very interesting phenomenon as an indicator in the study of the hertzian waves seems to have occurred simultaneously to several physicists, among whom should be especially mentioned m. ed. branly himself, sir oliver lodge, and mm. le royer and van beschem, and its use in laboratories rapidly became quite common. the action of the waves on metallic powders has, however, remained some what mysterious; for ten years it has been the subject of important researches by professor lodge, m. branly, and a very great number of the most distinguished physicists. it is impossible to notice here all these researches, but from a recent and very interesting work of m. blanc, it would seem that the phenomenon is allied to that of ionisation. § the history of wireless telegraphy does not end with the first experiments of marconi; but from the moment their success was announced in the public press, the question left the domain of pure science to enter into that of commerce. the historian's task here becomes different, but even more delicate; and he will encounter difficulties which can be only known to one about to write the history of a commercial invention. the actual improvements effected in the system are kept secret by the rival companies, and the most important results are patriotically left in darkness by the learned officers who operate discreetly in view of the national defence. meanwhile, men of business desirous of bringing out a company proclaim, with great nourish of advertisements, that they are about to exploit a process superior to all others. on this slippery ground the impartial historian must nevertheless venture; and he may not refuse to relate the progress accomplished, which is considerable. therefore, after having described the experiments carried out for nearly ten years by marconi himself, first across the bristol channel, then at spezzia, between the coast and the ironclad _san bartolommeo_, and finally by means of gigantic apparatus between america and england, he must give the names of those who, in the different civilised countries, have contributed to the improvement of the system of communication by waves; while he must describe what precious services this system has already rendered to the art of war, and happily also to peaceful navigation. from the point of view of the theory of the phenomena, very remarkable results have been obtained by various physicists, among whom should be particularly mentioned m. tissot, whose brilliant studies have thrown a bright light on different interesting points, such as the rôle of the antennae. it would be equally impossible to pass over in silence other recent attempts in a slightly different groove. marconi's system, however improved it may be to-day, has one grave defect. the synchronism of the two pieces of apparatus, the transmitter and the receiver, is not perfect, so that a message sent off by one station may be captured by some other station. the fact that the phenomena of resonance are not utilised, further prevents the quantity of energy received by the receiver from being considerable, and hence the effects reaped are very weak, so that the system remains somewhat fitful and the communications are often disturbed by atmospheric phenomena. causes which render the air a momentary conductor, such as electrical discharges, ionisation, etc., moreover naturally prevent the waves from passing, the ether thus losing its elasticity. professor ferdinand braun of strasburg has conceived the idea of employing a mixed system, in which the earth and the water, which, as we have seen, have often been utilised to conduct a current for transmitting a signal, will serve as a sort of guide to the waves themselves. the now well-known theory of the propagation of waves guided by a conductor enables it to be foreseen that, according to their periods, these waves will penetrate more or less deeply into the natural medium, from which fact has been devised a method of separating them according to their frequency. by applying this theory, m. braun has carried out, first in the fortifications of strasburg, and then between the island of heligoland and the mainland, experiments which have given remarkable results. we might mention also the researches, in a somewhat analogous order of ideas, by an english engineer, mr armstrong, by dr lee de forest, and also by professor fessenden. having thus arrived at the end of this long journey, which has taken him from the first attempts down to the most recent experiments, the historian can yet set up no other claim but that of having written the commencement of a history which others must continue in the future. progress does not stop, and it is never permissible to say that an invention has reached its final form. should the historian desire to give a conclusion to his labour and answer the question the reader would doubtless not fail to put to him, "to whom, in short, should the invention of wireless telegraphy more particularly be attributed?" he should certainly first give the name of hertz, the genius who discovered the waves, then that of marconi, who was the first to transmit signals by the use of hertzian undulations, and should add those of the scholars who, like morse, popoff, sir w. preece, lodge, and, above all, branly, have devised the arrangements necessary for their transmission. but he might then recall what voltaire wrote in the _philosophical dictionary_: "what! we wish to know what was the exact theology of thot, of zerdust, of sanchuniathon, of the first brahmins, and we are ignorant of the inventor of the shuttle! the first weaver, the first mason, the first smith, were no doubt great geniuses, but they were disregarded. why? because none of them invented a perfected art. the one who hollowed out an oak to cross a river never made a galley; those who piled up rough stones with girders of wood did not plan the pyramids. everything is made by degrees and the glory belongs to no one." to-day, more than ever, the words of voltaire are true: science becomes more and more impersonal, and she teaches us that progress is nearly always due to the united efforts of a crowd of workers, and is thus the best school of social solidarity. chapter viii the conductivity of gases and the ions § . the conductivity of gases if we were confined to the facts i have set forth above, we might conclude that two classes of phenomena are to-day being interpreted with increasing correctness in spite of the few difficulties which have been pointed out. the hypothesis of the molecular constitution of matter enables us to group together one of these classes, and the hypothesis of the ether leads us to co-ordinate the other. but these two classes of phenomena cannot be considered independent of each other. relations evidently exist between matter and the ether, which manifest themselves in many cases accessible to experiment, and the search for these relations appears to be the paramount problem the physicist should set himself. the question has, for a long time, been attacked on various sides, but the recent discoveries in the conductivity of gases, of the radioactive substances, and of the cathode and similar rays, have allowed us of late years to regard it in a new light. without wishing to set out here in detail facts which for the most part are well known, we will endeavour to group the chief of them round a few essential ideas, and will seek to state precisely the data they afford us for the solution of this grave problem. it was the study of the conductivity of gases which at the very first furnished the most important information, and allowed us to penetrate more deeply than had till then been possible into the inmost constitution of matter, and thus to, as it were, catch in the act the actions that matter can exercise on the ether, or, reciprocally, those it may receive from it. it might, perhaps, have been foreseen that such a study would prove remarkably fruitful. the examination of the phenomena of electrolysis had, in fact, led to results of the highest importance on the constitution of liquids, and the gaseous media which presented themselves as particularly simple in all their properties ought, it would seem, to have supplied from the very first a field of investigation easy to work and highly productive. this, however, was not at all the case. experimental complications springing up at every step obscured the problem. one generally found one's self in the presence of violent disruptive discharges with a train of accessory phenomena, due, for instance, to the use of metallic electrodes, and made evident by the complex appearance of aigrettes and effluves; or else one had to deal with heated gases difficult to handle, which were confined in receptacles whose walls played a troublesome part and succeeded in veiling the simplicity of the fundamental facts. notwithstanding, therefore, the efforts of a great number of seekers, no general idea disengaged itself out of a mass of often contradictory information. many physicists, in france particularly, discarded the study of questions which seemed so confused, and it must even be frankly acknowledged that some among them had a really unfounded distrust of certain results which should have been considered proved, but which had the misfortune to be in contradiction with the theories in current use. all the classic ideas relating to electrical phenomena led to the consideration that there existed a perfect symmetry between the two electricities, positive and negative. in the passing of electricity through gases there is manifested, on the contrary, an evident dissymmetry. the anode and the cathode are immediately distinguished in a tube of rarefied gas by their peculiar appearance; and the conductivity does not appear, under certain conditions, to be the same for the two modes of electrification. it is not devoid of interest to note that erman, a german scholar, once very celebrated and now generally forgotten, drew attention as early as to the unipolar conductivity of a flame. his contemporaries, as may be gathered from the perusal of the treatises on physics of that period, attached great importance to this discovery; but, as it was somewhat inconvenient and did not readily fit in with ordinary studies, it was in due course neglected, then considered as insufficiently established, and finally wholly forgotten. all these somewhat obscure facts, and some others--such as the different action of ultra-violet radiations on positively and negatively charged bodies--are now, on the contrary, about to be co-ordinated, thanks to the modern ideas on the mechanism of conduction; while these ideas will also allow us to interpret the most striking dissymmetry of all, i.e. that revealed by electrolysis itself, a dissymmetry which certainly can not be denied, but to which sufficient attention has not been given. it is to a german physicist, giese, that we owe the first notions on the mechanism of the conductivity of gases, as we now conceive it. in two memoirs published in and , he plainly arrives at the conception that conduction in gases is not due to their molecules, but to certain fragments of them or to ions. giese was a forerunner, but his ideas could not triumph so long as there were no means of observing conduction in simple circumstances. but this means has now been supplied in the discovery of the x rays. suppose we pass through some gas at ordinary pressure, such as hydrogen, a pencil of x rays. the gas, which till then has behaved as a perfect insulator,[ ] suddenly acquires a remarkable conductivity. if into this hydrogen two metallic electrodes in communication with the two poles of a battery are introduced, a current is set up in very special conditions which remind us, when they are checked by experiments, of the mechanism which allows the passage of electricity in electrolysis, and which is so well represented to us when we picture to ourselves this passage as due to the migration towards the electrodes, under the action of the field, of the two sets of ions produced by the spontaneous division of the molecule within the solution. [footnote : at least, so long as it is not introduced between the two coatings of a condenser having a difference of potential sufficient to overcome what m. bouty calls its dielectric cohesion. we leave on one side this phenomenon, regarding which m. bouty has arrived at extremely important results by a very remarkable series of experiments; but this question rightly belongs to a special study of electrical phenomena which is not yet written.] let us therefore recognise with j.j. thomson and the many physicists who, in his wake, have taken up and developed the idea of giese, that, under the influence of the x rays, for reasons which will have to be determined later, certain gaseous molecules have become divided into two portions, the one positively and the other negatively electrified, which we will call, by analogy with the kindred phenomenon in electrolysis, by the name of ions. if the gas be then placed in an electric field, produced, for instance, by two metallic plates connected with the two poles of a battery respectively, the positive ions will travel towards the plate connected with the negative pole, and the negative ions in the contrary direction. there is thus produced a current due to the transport to the electrodes of the charges which existed on the ions. if the gas thus ionised be left to itself, in the absence of any electric field, the ions, yielding to their mutual attraction, must finally meet, combine, and reconstitute a neutral molecule, thus returning to their initial condition. the gas in a short while loses the conductivity which it had acquired; or this is, at least, the phenomenon at ordinary temperatures. but if the temperature is raised, the relative speeds of the ions at the moment of impact may be great enough to render it impossible for the recombination to be produced in its entirety, and part of the conductivity will remain. every element of volume rendered a conductor therefore furnishes, in an electric field, equal quantities of positive and negative electricity. if we admit, as mentioned above, that these liberated quantities are borne by ions each bearing an equal charge, the number of these ions will be proportional to the quantity of electricity, and instead of speaking of a quantity of electricity, we could use the equivalent term of number of ions. for the excitement produced by a given pencil of x rays, the number of ions liberated will be fixed. thus, from a given volume of gas there can only be extracted an equally determinate quantity of electricity. the conductivity produced is not governed by ohm's law. the intensity is not proportional to the electromotive force, and it increases at first as the electromotive force augments; but it approaches asymptotically to a maximum value which corresponds to the number of ions liberated, and can therefore serve as a measure of the power of the excitement. it is this current which is termed the _current of saturation_. m. righi has ably demonstrated that ionised gas does not obey the law of ohm by an experiment very paradoxical in appearance. he found that, the greater the distance of the two electrode plates from each, the greater may be, within certain limits, the intensity of the current. the fact is very clearly interpreted by the theory of ionisation, since the greater the length of the gaseous column the greater must be the number of ions liberated. one of the most striking characteristics of ionised gases is that of discharging electrified conductors. this phenomenon is not produced by the departure of the charge that these conductors may possess, but by the advent of opposite charges brought to them by ions which obey the electrostatic attraction and abandon their own electrification when they come in contact with these conductors. this mode of regarding the phenomena is extremely convenient and eminently suggestive. it may, no doubt, be thought that the image of the ions is not identical with objective reality, but we are compelled to acknowledge that it represents with absolute faithfulness all the details of the phenomena. other facts, moreover, will give to this hypothesis a still greater value; we shall even be able, so to speak, to grasp these ions individually, to count them, and to measure their charge. § . the condensation of water-vapour by ions if the pressure of a vapour--that of water, for instance--in the atmosphere reaches the value of the maximum pressure corresponding to the temperature of the experiment, the elementary theory teaches us that the slightest decrease in temperature will induce a condensation; that small drops will form, and the mist will turn into rain. in reality, matters do not occur in so simple a manner. a more or less considerable delay may take place, and the vapour will remain supersaturated. we easily discover that this phenomenon is due to the intervention of capillary action. on a drop of liquid a surface-tension takes effect which gives rise to a pressure which becomes greater the smaller the diameter of the drop. pressure facilitates evaporation, and on more closely examining this reaction we arrive at the conclusion that vapour can never spontaneously condense itself when liquid drops already formed are not present, unless forces of another nature intervene to diminish the effect of the capillary forces. in the most frequent cases, these forces come from the dust which is always in suspension in the air, or which exists in any recipient. grains of dust act by reason of their hygrometrical power, and form germs round which drops presently form. it is possible to make use, as did m. coulier as early as , of this phenomenon to carry off the germs of condensation, by producing by expansion in a bottle containing a little water a preliminary mist which purifies the air. in subsequent experiments it will be found almost impossible to produce further condensation of vapour. but these forces may also be of electrical origin. von helmholtz long since showed that electricity exercises an influence on the condensation of the vapour of water, and mr c.t.r. wilson, with this view, has made truly quantitative experiments. it was rapidly discovered after the apparition of the x rays that gases that have become conductors, that is, ionised gases, also facilitate the condensation of supersaturated water vapour. we are thus led by a new road to the belief that electrified centres exist in gases, and that each centre draws to itself the neighbouring molecules of water, as an electrified rod of resin does the light bodies around it. there is produced in this manner round each ion an assemblage of molecules of water which constitute a germ capable of causing the formation of a drop of water out of the condensation of excess vapour in the ambient air. as might be expected, the drops are electrified, and take to themselves the charge of the centres round which they are formed; moreover, as many drops are created as there are ions. thereafter we have only to count these drops to ascertain the number of ions which existed in the gaseous mass. to effect this counting, several methods have been used, differing in principle but leading to similar results. it is possible, as mr c.t.r. wilson and professor j.j. thomson have done, to estimate, on the one hand, the weight of the mist which is produced in determined conditions, and on the other, the average weight of the drops, according to the formula formerly given by sir g. stokes, by deducting their diameter from the speed with which this mist falls; or we can, with professor lemme, determine the average radius of the drops by an optical process, viz. by measuring the diameter of the first diffraction ring produced when looking through the mist at a point of light. we thus get to a very high number. there are, for instance, some twenty million ions per centimetre cube when the rays have produced their maximum effect, but high as this figure is, it is still very small compared with the total number of molecules. all conclusions drawn from kinetic theory lead us to think that in the same space there must exist, by the side of a molecule divided into two ions, a thousand millions remaining in a neutral state and intact. mr c.t.r. wilson has remarked that the positive and negative ions do not produce condensation with the same facility. the ions of a contrary sign may be almost completely separated by placing the ionised gas in a suitably disposed field. in the neighbourhood of a negative disk there remain hardly any but positive ions, and against a positive disk none but negative; and in effecting a separation of this kind, it will be noticed that condensation by negative ions is easier than by the positive. it is, consequently, possible to cause condensation on negative centres only, and to study separately the phenomena produced by the two kinds of ions. it can thus be verified that they really bear charges equal in absolute value, and these charges can even be estimated, since we already know the number of drops. this estimate can be made, for example, by comparing the speed of the fall of a mist in fields of different values, or, as did j.j. thomson, by measuring the total quantity of electricity liberated throughout the gas. at the degree of approximation which such experiments imply, we find that the charge of a drop, and consequently the charge borne by an ion, is sensibly . x ^{- } electrostatic or . x ^{- } electromagnetic units. this charge is very near that which the study of the phenomena of ordinary electrolysis leads us to attribute to a univalent atom produced by electrolytic dissociation. such a coincidence is evidently very striking; but it will not be the only one, for whatever phenomenon be studied it will always appear that the smallest charge we can conceive as isolated is that mentioned. we are, in fact, in presence of a natural unit, or, if you will, of an atom of electricity. we must, however, guard against the belief that the gaseous ion is identical with the electrolytic ion. sensible differences between those are immediately apparent, and still greater ones will be discovered on closer examination. as m. perrin has shown, the ionisation produced by the x-rays in no way depends on the chemical composition of the gas; and whether we take a volume of gaseous hydrochloric acid or a mixture of hydrogen and chlorine in the same condition, all the results will be identical: and chemical affinities play no part here. we can also obtain other information regarding ions: we can ascertain, for instance, their velocities, and also get an idea of their order of magnitude. by treating the speeds possessed by the liberated charges as components of the known speed of a gaseous current, mr zeleny measures the mobilities, that is to say, the speeds acquired by the positive and negative charges in a field equal to the electrostatic unit. he has thus found that these mobilities are different, and that they vary, for example, between and centimetres per second for the two charges in dry gases, the positive being less mobile than the negative ions, which suggests the idea that they are of greater mass.[ ] [footnote : a full account of these experiments, which were executed at the cavendish laboratory, is to be found in _philosophical transactions_, a., vol. cxcv. ( ), pp. et seq.--ed.] m. langevin, who has made himself the eloquent apostle of the new doctrines in france, and has done much to make them understood and admitted, has personally undertaken experiments analogous to those of m. zeleny, but much more complete. he has studied in a very ingenious manner, not only the mobilities, but also the law of recombination which regulates the spontaneous return of the gas to its normal state. he has determined experimentally the relation of the number of recombinations to the number of collisions between two ions of contrary sign, by studying the variation produced by a change in the value of the field, in the quantity of electricity which can be collected in the gas separating two parallel metallic plates, after the passage through it for a very short time of the röntgen rays emitted during one discharge of a crookes tube. if the image of the ions is indeed conformable to reality, this relation must evidently always be smaller than unity, and must tend towards this value when the mobility of the ions diminishes, that is to say, when the pressure of the gas increases. the results obtained are in perfect accord with this anticipation. on the other hand, m. langevin has succeeded, by following the displacement of the ions between the parallel plates after the ionisation produced by the radiation, in determining the absolute values of the mobilities with great precision, and has thus clearly placed in evidence the irregularity of the mobilities of the positive and negative ions respectively. their mass can be calculated when we know, through experiments of this kind, the speed of the ions in a given field, and on the other hand--as we can now estimate their electric charge--the force which moves them. they evidently progress more slowly the larger they are; and in the viscous medium constituted by the gas, the displacement is effected at a speed sensibly proportional to the motive power. at the ordinary temperature these masses are relatively considerable, and are greater for the positive than for the negative ions, that is to say, they are about the order of some ten molecules. the ions, therefore, seem to be formed by an agglomeration of neutral molecules maintained round an electrified centre by electrostatic attraction. if the temperature rises, the thermal agitation will become great enough to prevent the molecules from remaining linked to the centre. by measurements effected on the gases of flames, we arrive at very different values of the masses from those found for ordinary ions, and above all, very different ones for ions of contrary sign. the negative ions have much more considerable velocities than the positive ones. the latter also seem to be of the same size as atoms; and the first-named must, consequently, be considered as very much smaller, and probably about a thousand times less. thus, for the first time in science, the idea appears that the atom is not the smallest fraction of matter to be considered. fragments a thousand times smaller may exist which possess, however, a negative charge. these are the electrons, which other considerations will again bring to our notice. § . how ions are produced it is very seldom that a gaseous mass does not contain a few ions. they may have been formed from many causes, for although to give precision to our studies, and to deal with a well ascertained case, i mentioned only ionisation by the x rays in the first instance, i ought not to give the impression that the phenomenon is confined to these rays. it is, on the contrary, very general, and ionisation is just as well produced by the cathode rays, by the radiations emitted by radio-active bodies, by the ultra-violet rays, by heating to a high temperature, by certain chemical actions, and finally by the impact of the ions already existing in neutral molecules. of late years these new questions have been the object of a multitude of researches, and if it has not always been possible to avoid some confusion, yet certain general conclusions may be drawn. the ionisation by flames, in particular, is fairly well known. for it to be produced spontaneously, it would appear that there must exist simultaneously a rather high temperature and a chemical action in the gas. according to m. moreau, the ionisation is very marked when the flame contains the vapour of the salt of an alkali or of an alkaline earth, but much less so when it contains that of other salts. arrhenius, mr c.t.r. wilson, and m. moreau, have studied all the circumstances of the phenomenon; and it seems indeed that there is a somewhat close analogy between what first occurs in the saline vapours and that which is noted in liquid electrolytes. there should be produced, as soon as a certain temperature is reached, a dissociation of the saline molecule; and, as m. moreau has shown in a series of very well conducted researches, the ions formed at about °c. seem constituted by an electrified centre of the size of a gas molecule, surrounded by some ten layers of other molecules. we are thus dealing with rather large ions, but according to mr wilson, this condensation phenomenon does not affect the number of ions produced by dissociation. in proportion as the temperature rises, the molecules condensed round the nucleus disappear, and, as in all other circumstances, the negative ion tends to become an electron, while the positive ion continues the size of an atom. in other cases, ions are found still larger than those of saline vapours, as, for example, those produced by phosphorus. it has long been known that air in the neighbourhood of phosphorus becomes a conductor, and the fact, pointed out as far back as by matteucci, has been well studied by various experimenters, by mm. elster and geitel in , for instance. on the other hand, in mr barus established that the approach of a stick of phosphorus brings about the condensation of water vapour, and we really have before us, therefore, in this instance, an ionisation. m. bloch has succeeded in disentangling the phenomena, which are here very complex, and in showing that the ions produced are of considerable dimensions; for their speed in the same conditions is on the average a thousand times less than that of ions due to the x rays. m. bloch has established also that the conductivity of recently-prepared gases, already studied by several authors, was analogous to that which is produced by phosphorus, and that it is intimately connected with the presence of the very tenuous solid or liquid dust which these gases carry with them, while the ions are of the same order of magnitude. these large ions exist, moreover, in small quantities in the atmosphere; and m. langevin lately succeeded in revealing their presence. it may happen, and this not without singularly complicating matters, that the ions which were in the midst of material molecules produce, as the result of collisions, new divisions in these last. other ions are thus born, and this production is in part compensated for by recombinations between ions of opposite signs. the impacts will be more active in the event of the gas being placed in a field of force and of the pressure being slight, the speed attained being then greater and allowing the active force to reach a high value. the energy necessary for the production of an ion is, in fact, according to professor rutherford and professor stark, something considerable, and it much exceeds the analogous force in electrolytic decomposition. it is therefore in tubes of rarefied gas that this ionisation by impact will be particularly felt. this gives us the reason for the aspect presented by geissler tubes. generally, in the case of discharges, new ions produced by the molecules struck come to add themselves to the electrons produced, as will be seen, by the cathode. a full discussion has led to the interpretation of all the known facts, and to our understanding, for instance, why there exist bright or dark spaces in certain regions of the tube. m. pellat, in particular, has given some very fine examples of this concordance between the theory and the facts he has skilfully observed. in all the circumstances, then, in which ions appear, their formation has doubtless been provoked by a mechanism analogous to that of the shock. the x rays, if they are attributable to sudden variations in the ether--that is to say, a variation of the two vectors of hertz-- themselves produce within the atom a kind of electric impulse which breaks it into two electrified fragments; _i.e._ the positive centre, the size of the molecule itself, and the negative centre, constituted by an electron a thousand times smaller. round these two centres, at the ordinary temperature, are agglomerated by attraction other molecules, and in this manner the ions whose properties have just been studied are formed. § . electrons in metals the success of the ionic hypothesis as an interpretation of the conductivity of electrolytes and gases has suggested the desire to try if a similar hypothesis can represent the ordinary conductivity of metals. we are thus led to conceptions which at first sight seem audacious because they are contrary to our habits of mind. they must not, however, be rejected on that account. electrolytic dissociation at first certainly appeared at least as strange; yet it has ended by forcing itself upon us, and we could, at the present day, hardly dispense with the image it presents to us. the idea that the conductivity of metals is not essentially different from that of electrolytic liquids or gases, in the sense that the passage of the current is connected with the transport of small electrified particles, is already of old date. it was enunciated by w. weber, and afterwards developed by giese, but has only obtained its true scope through the effect of recent discoveries. it was the researches of riecke, later, of drude, and, above all, those of j.j. thomson, which have allowed it to assume an acceptable form. all these attempts are connected however with the general theory of lorentz, which we will examine later. it will be admitted that metallic atoms can, like the saline molecule in a solution, partially dissociate themselves. electrons, very much smaller than atoms, can move through the structure, considerable to them, which is constituted by the atom from which they have just been detached. they may be compared to the molecules of a gas which is enclosed in a porous body. in ordinary conditions, notwithstanding the great speed with which they are animated, they are unable to travel long distances, because they quickly find their road barred by a material atom. they have to undergo innumerable impacts, which throw them first in one direction and then in another. the passage of a current is a sort of flow of these electrons in a determined direction. this electric flow brings, however, no modification to the material medium traversed, since every electron which disappears at any point is replaced by another which appears at once, and in all metals the electrons are identical. this hypothesis leads us to anticipate certain facts which experience confirms. thus j.j. thomson shows that if, in certain conditions, a conductor is placed in a magnetic field, the ions have to describe an epicycloid, and their journey is thus lengthened, while the electric resistance must increase. if the field is in the direction of the displacement, they describe helices round the lines of force and the resistance is again augmented, but in different proportions. various experimenters have noted phenomena of this kind in different substances. for a long time it has been noticed that a relation exists between the calorific and the electric conductivity; the relation of these two conductivities is sensibly the same for all metals. the modern theory tends to show simply that it must indeed be so. calorific conductivity is due, in fact, to an exchange of electrons between the hot and the cold regions, the heated electrons having the greater velocity, and consequently the more considerable energy. the calorific exchanges then obey laws similar to those which govern electric exchanges; and calculation even leads to the exact values which the measurements have given.[ ] [footnote : the whole of this argument is brilliantly set forth by professor lorentz in a lecture delivered to the electrotechnikerverein at berlin in december , and reprinted, with additions, in the _archives néerlandaises_ of .--ed.] in the same way professor hesehus has explained how contact electrification is produced, by the tendency of bodies to equalise their superficial properties by means of a transport of electrons, and mr jeans has shown that we should discover the existence of the well-known laws of distribution over conducting bodies in electrostatic equilibrium. a metal can, in fact, be electrified, that is to say, may possess an excess of positive or negative electrons which cannot easily leave it in ordinary conditions. to cause them to do so would need an appreciable amount of work, on account of the enormous difference of the specific inductive capacities of the metal and of the insulating medium in which it is plunged. electrons, however, which, on arriving at the surface of the metal, possessed a kinetic energy superior to this work, might be shot forth and would be disengaged as a vapour escapes from a liquid. now, the number of these rapid electrons, at first very slight, increases, according to the kinetic theory, when the temperature rises, and therefore we must reckon that a wire, on being heated, gives out electrons, that is to say, loses negative electricity and sends into the surrounding media electrified centres capable of producing the phenomena of ionisation. edison, in , showed that from the filament of an incandescent lamp there escaped negative electric charges. since then, richardson and j.j. thomson have examined analogous phenomena. this emission is a very general phenomenon which, no doubt, plays a considerable part in cosmic physics. professor arrhenius explains, for instance, the polar auroras by the action of similar corpuscules emitted by the sun. in other phenomena we seem indeed to be confronted by an emission, not of negative electrons, but of positive ions. thus, when a wire is heated, not _in vacuo_, but in a gas, this wire begins to electrify neighbouring bodies positively. j.j. thomson has measured the mass of these positive ions and finds it considerable, i.e. about times that of an atom of hydrogen. some are even larger, and constitute almost a real grain of dust. we here doubtless meet with the phenomena of disaggregation undergone by metals at a red heat. chapter ix cathode rays and radioactive bodies § . the cathode rays a wire traversed by an electric current is, as has just been explained, the seat of a movement of electrons. if we cut this wire, a flood of electrons, like a current of water which, at the point where a pipe bursts, flows out in abundance, will appear to spring out between the two ends of the break. if the energy of the electrons is sufficient, these electrons will in fact rush forth and be propagated in the air or in the insulating medium interposed; but the phenomena of the discharge will in general be very complex. we shall here only examine a particularly simple case, viz., that of the cathode rays; and without entering into details, we shall only note the results relating to these rays which furnish valuable arguments in favour of the electronic hypothesis and supply solid materials for the construction of new theories of electricity and matter. for a long time it was noticed that the phenomena in a geissler tube changed their aspect considerably, when the gas pressure became very weak, without, however, a complete vacuum being formed. from the cathode there is shot forth normally and in a straight line a flood within the tube, dark but capable of impressing a photographic plate, of developing the fluorescence of various substances (particularly the glass walls of the tube), and of producing calorific and mechanical effects. these are the cathode rays, so named in by e. wiedemann, and their name, which was unknown to a great number of physicists till barely twelve years ago, has become popular at the present day. about , hittorf made an already very complete study of them and put in evidence their principal properties; but it was the researches of sir w. crookes in especial which drew attention to them. the celebrated physicist foresaw that the phenomena which were thus produced in rarefied gases were, in spite of their very great complication, more simple than those presented by matter under the conditions in which it is generally met with. he devised a celebrated theory no longer admissible in its entirety, because it is not in complete accord with the facts, which was, however, very interesting, and contained, in germ, certain of our present ideas. in the opinion of crookes, in a tube in which the gas has been rarefied we are in presence of a special state of matter. the number of the gas molecules has become small enough for their independence to be almost absolute, and they are able in this so-called radiant state to traverse long spaces without departing from a straight line. the cathode rays are due to a kind of molecular bombardment of the walls of the tubes, and of the screens which can be introduced into them; and it is the molecules, electrified by their contact with the cathode and then forcibly repelled by electrostatic action, which produce, by their movement and their _vis viva_, all the phenomena observed. moreover, these electrified molecules animated with extremely rapid velocities correspond, according to the theory verified in the celebrated experiment of rowland on convection currents, to a true electric current, and can be deviated by a magnet. notwithstanding the success of crookes' experiments, many physicists-- the germans especially--did not abandon an hypothesis entirely different from that of radiant matter. they continued to regard the cathode radiation as due to particular radiations of a nature still little known but produced in the luminous ether. this interpretation seemed, indeed, in , destined to triumph definitely through the remarkable discovery of lenard, a discovery which, in its turn, was to provoke so many others and to bring about consequences of which the importance seems every day more considerable. professor lenard's fundamental idea was to study the cathode rays under conditions different from those in which they are produced. these rays are born in a very rarefied space, under conditions perfectly determined by sir w. crookes; but it was a question whether, when once produced, they would be capable of propagating themselves in other media, such as a gas at ordinary pressure, or even in an absolute vacuum. experiment alone could answer this question, but there were difficulties in the way of this which seemed almost insurmountable. the rays are stopped by glass even of slight thickness, and how then could the almost vacuous space in which they have to come into existence be separated from the space, absolutely vacuous or filled with gas, into which it was desired to bring them? the artifice used was suggested to professor lenard by an experiment of hertz. the great physicist had, in fact, shortly before his premature death, taken up this important question of the cathode rays, and his genius left there, as elsewhere, its powerful impress. he had shown that metallic plates of very slight thickness were transparent to the cathode rays; and professor lenard succeeded in obtaining plates impermeable to air, but which yet allowed the pencil of cathode rays to pass through them. now if we take a crookes tube with the extremity hermetically closed by a metallic plate with a slit across the diameter of mm. in width, and stop this slit with a sheet of very thin aluminium, it will be immediately noticed that the rays pass through the aluminium and pass outside the tube. they are propagated in air at atmospheric pressure, and they can also penetrate into an absolute vacuum. they therefore can no longer be attributed to radiant matter, and we are led to think that the energy brought into play in this phenomenon must have its seat in the light-bearing ether itself. but it is a very strange light which is thus subject to magnetic action, which does not obey the principle of equal angles, and for which the most various gases are already disturbed media. according to crookes it possesses also the singular property of carrying with it electric charges. this convection of negative electricity by the cathode rays seems quite inexplicable on the hypothesis that the rays are ethereal radiations. nothing then remained in order to maintain this hypothesis, except to deny the convection, which, besides, was only established by indirect experiments. that the reality of this transport has been placed beyond dispute by means of an extremely elegant experiment which is all the more convincing that it is so very simple, is due to m. perrin. in the interior of a crookes tube he collected a pencil of cathode rays in a metal cylinder. according to the elementary principles of electricity the cylinder must become charged with the whole charge, if there be one, brought to it by the rays, and naturally various precautions had to be taken. but the result was very precise, and doubt could no longer exist--the rays were electrified. it might have been, and indeed was, maintained, some time after this experiment was published, that while the phenomena were complex inside the tube, outside, things might perhaps occur differently. lenard himself, however, with that absence of even involuntary prejudice common to all great minds, undertook to demonstrate that the opinion he at first held could no longer be accepted, and succeeded in repeating the experiment of m. perrin on cathode rays in the air and even _in vacuo_. on the wrecks of the two contradictory hypotheses thus destroyed, and out of the materials from which they had been built, a theory has been constructed which co-ordinates all the known facts. this theory is furthermore closely allied to the theory of ionisation, and, like this latter, is based on the concept of the electron. cathode rays are electrons in rapid motion. the phenomena produced both inside and outside a crookes tube are, however, generally complex. in lenard's first experiments, and in many others effected later when this region of physics was still very little known, a few confusions may be noticed even at the present day. at the spot where the cathode rays strike the walls of the tube the essentially different x rays appear. these differ from the cathode radiations by being neither electrified nor deviated by a magnet. in their turn these x rays may give birth to the secondary rays of m. sagnac; and often we find ourselves in presence of effects from these last-named radiations and not from the true cathode rays. the electrons, when they are propagated in a gas, can ionise the molecules of this gas and unite with the neutral atoms to form negative ions, while positive ions also appear. there are likewise produced, at the expense of the gas still subsisting after rarefication within the tube, positive ions which, attracted by the cathode and reaching it, are not all neutralised by the negative electrons, and can, if the cathode be perforated, pass through it, and if not, pass round it. we have then what are called the canal rays of goldstein, which are deviated by an electric or magnetic field in a contrary direction to the cathode rays; but, being larger, give weak deviations or may even remain undeviated through losing their charge when passing through the cathode. it may also be the parts of the walls at a distance from the cathode which send a positive rush to the latter, by a similar mechanism. it may be, again, that in certain regions of the tube cathode rays are met with diffused by some solid object, without having thereby changed their nature. all these complexities have been cleared up by m. villard, who has published, on these questions, some remarkably ingenious and particularly careful experiments. m. villard has also studied the phenomena of the coiling of the rays in a field, as already pointed out by hittorf and plücker. when a magnetic field acts on the cathode particle, the latter follows a trajectory, generally helicoidal, which is anticipated by the theory. we here have to do with a question of ballistics, and experiments duly confirm the anticipations of the calculation. nevertheless, rather singular phenomena appear in the case of certain values of the field, and these phenomena, dimly seen by plücker and birkeland, have been the object of experiments by m. villard. the two faces of the cathode seem to emit rays which are deviated in a direction perpendicular to the lines of force by an electric field, and do not seem to be electrified. m. villard calls them magneto-cathode rays, and according to m. fortin these rays may be ordinary cathode rays, but of very slight velocity. in certain cases the cathode itself may be superficially disaggregated, and extremely tenuous particles detach themselves, which, being carried off at right angles to its surface, may deposit themselves like a very thin film on objects placed in their path. various physicists, among them m. houllevigue, have studied this phenomenon, and in the case of pressures between / and / of a millimetre, the last-named scholar has obtained mirrors of most metals, a phenomenon he designates by the name of ionoplasty. but in spite of all these accessory phenomena, which even sometimes conceal those first observed, the existence of the electron in the cathodic flux remains the essential characteristic. the electron can be apprehended in the cathodic ray by the study of its essential properties; and j.j. thomson gave great value to the hypothesis by his measurements. at first he meant to determine the speed of the cathode rays by direct experiment, and by observing, in a revolving mirror, the relative displacement of two bands due to the excitement of two fluorescent screens placed at different distances from the cathode. but he soon perceived that the effect of the fluorescence was not instantaneous, and that the lapse of time might form a great source of error, and he then had recourse to indirect methods. it is possible, by a simple calculation, to estimate the deviations produced on the rays by a magnetic and an electric field respectively as a function of the speed of propagation and of the relation of the charge to the material mass of the electron. the measurement of these deviations will then permit this speed and this relation to be ascertained. other processes may be used which all give the same two quantities by two suitably chosen measurements. such are the radius of the curve taken by the trajectory of the pencil in a perpendicular magnetic field and the measure of the fall of potential under which the discharge takes place, or the measure of the total quantity of electricity carried in one second and the measure of the calorific energy which may be given, during the same period, to a thermo-electric junction. the results agree as well as can be expected, having regard to the difficulty of the experiments; the values of the speed agree also with those which professor wiechert has obtained by direct measurement. the speed never depends on the nature of the gas contained in the crookes tube, but varies with the value of the fall of potential at the cathode. it is of the order of one tenth of the speed of light, and it may rise as high as one third. the cathode particle therefore goes about three thousand times faster than the earth in its orbit. the relation is also invariable, even when the substance of which the cathode is formed is changed or one gas is substituted for another. it is, on the average, a thousand times greater than the corresponding relation in electrolysis. as experiment has shown, in all the circumstances where it has been possible to effect measurements, the equality of the charges carried by all corpuscules, ions, atoms, etc., we ought to consider that the charge of the electron is here, again, that of a univalent ion in electrolysis, and therefore that its mass is only a small fraction of that of the atom of hydrogen, viz., of the order of about a thousandth part. this is the same result as that to which we were led by the study of flames. the thorough examination of the cathode radiation, then, confirms us in the idea that every material atom can be dissociated and will yield an electron much smaller than itself--and always identical whatever the matter whence it comes,--the rest of the atom remaining charged with a positive quantity equal and contrary to that borne by the electron. in the present case these positive ions are no doubt those that we again meet with in the canal rays. professor wien has shown that their mass is really, in fact, of the order of the mass of atoms. although they are all formed of identical electrons, there may be various cathode rays, because the velocity is not exactly the same for all electrons. thus is explained the fact that we can separate them and that we can produce a sort of spectrum by the action of the magnet, or, again, as m. deslandres has shown in a very interesting experiment, by that of an electrostatic field. this also probably explains the phenomena studied by m. villard, and previously pointed out. § . radioactive substances even in ordinary conditions, certain substances called radioactive emit, quite outside any particular reaction, radiations complex indeed, but which pass through fairly thin layers of minerals, impress photographic plates, excite fluorescence, and ionize gases. in these radiations we again find electrons which thus escape spontaneously from radioactive bodies. it is not necessary to give here a history of the discovery of radium, for every one knows the admirable researches of m. and madame curie. but subsequent to these first studies, a great number of facts have accumulated for the last six years, among which some people find themselves a little lost. it may, perhaps, not be useless to indicate the essential results actually obtained. the researches on radioactive substances have their starting-point in the discovery of the rays of uranium made by m. becquerel in . as early as niepce de st victor proved that salts of uranium impressed photographic plates in the dark; but at that time the phenomenon could only pass for a singularity attributable to phosphorescence, and the valuable remarks of niepce fell into oblivion. m. becquerel established, after some hesitations natural in the face of phenomena which seemed so contrary to accepted ideas, that the radiating property was absolutely independent of phosphorescence, that all the salts of uranium, even the uranous salts which are not phosphorescent, give similar radiant effects, and that these phenomena correspond to a continuous emission of energy, but do not seem to be the result of a storage of energy under the influence of some external radiation. spontaneous and constant, the radiation is insensible to variations of temperature and light. the nature of these radiations was not immediately understood,[ ] and their properties seemed contradictory. this was because we were not dealing with a single category of rays. but amongst all the effects there is one which constitutes for the radiations taken as a whole, a veritable process for the measurement of radioactivity. this is their ionizing action on gases. a very complete study of the conductivity of air under the influence of rays of uranium has been made by various physicists, particularly by professor rutherford, and has shown that the laws of the phenomenon are the same as those of the ionization due to the action of the röntgen rays. [footnote : in his work on _l'Évolution de la matière_, m. gustave le bon recalls that in he published several notes in the académie des sciences, in which he asserted that the properties of uranium were only a particular case of a very general law, and that the radiations emitted did not polarize, and were akin by their properties to the x rays.] it was natural to ask one's self if the property discovered in salts of uranium was peculiar to this body, or if it were not, to a more or less degree, a general property of matter. madame curie and m. schmidt, independently of each other, made systematic researches in order to solve the question; various compounds of nearly all the simple bodies at present known were thus passed in review, and it was established that radioactivity was particularly perceptible in the compounds of uranium and thorium, and that it was an atomic property linked to the matter endowed with it, and following it in all its combinations. in the course of her researches madame curie observed that certain pitchblendes (oxide of uranium ore, containing also barium, bismuth, etc.) were four times more active (activity being measured by the phenomenon of the ionization of the air) than metallic uranium. now, no compound containing any other active metal than uranium or thorium ought to show itself more active than those metals themselves, since the property belongs to their atoms. it seemed, therefore, probable that there existed in pitchblendes some substance yet unknown, in small quantities and more radioactive than uranium. m. and madame curie then commenced those celebrated experiments which brought them to the discovery of radium. their method of research has been justly compared in originality and importance to the process of spectrum analysis. to isolate a radioactive substance, the first thing is to measure the activity of a certain compound suspected of containing this substance, and this compound is chemically separated. we then again take in hand all the products obtained, and by measuring their activity anew, it is ascertained whether the substance sought for has remained in one of these products, or is divided among them, and if so, in what proportion. the spectroscopic reaction which we may use in the course of this separation is a thousand times less sensitive than observation of the activity by means of the electrometer. though the principle on which the operation of the concentration of the radium rests is admirable in its simplicity, its application is nevertheless very laborious. tons of uranium residues have to be treated in order to obtain a few decigrammes of pure salts of radium. radium is characterised by a special spectrum, and its atomic weight, as determined by madame curie, is ; it is consequently the higher homologue of barium in one of the groups of mendeléef. salts of radium have in general the same chemical properties as the corresponding salts of barium, but are distinguished from them by the differences of solubility which allow of their separation, and by their enormous activity, which is about a hundred thousand times greater than that of uranium. radium produces various chemical and some very intense physiological reactions. its salts are luminous in the dark, but this luminosity, at first very bright, gradually diminishes as the salts get older. we have here to do with a secondary reaction correlative to the production of the emanation, after which radium undergoes the transformations which will be studied later on. the method of analysis founded by m. and madame curie has enabled other bodies presenting sensible radioactivity to be discovered. the alkaline metals appear to possess this property in a slight degree. recently fallen snow and mineral waters manifest marked action. the phenomenon may often be due, however, to a radioactivity induced by radiations already existing in the atmosphere. but this radioactivity hardly attains the ten-thousandth part of that presented by uranium, or the ten-millionth of that appertaining to radium. two other bodies, polonium and actinium, the one characterised by the special nature of the radiations it emits and the other by a particular spectrum, seem likewise to exist in pitchblende. these chemical properties have not yet been perfectly defined; thus m. debierne, who discovered actinium, has been able to note the active property which seems to belong to it, sometimes in lanthanum, sometimes in neodynium.[ ] it is proved that all extremely radioactive bodies are the seat of incessant transformations, and even now we cannot state the conditions under which they present themselves in a strictly determined form. [footnote : polonium has now been shown to be no new element, but one of the transformation products of radium. radium itself is also thought to be derived in some manner, not yet ascertained, from uranium. the same is the case with actinium, which is said to come in the long run from uranium, but not so directly as does radium. all this is described in professor rutherford's _radioactive transformations_ (london, ).--ed.] § . the radiation of the radioactive bodies and the emanation to acquire exact notions as to the nature of the rays emitted by the radioactive bodies, it was necessary to try to cause magnetic or electric forces to act on them so as to see whether they behaved in the same way as light and the x rays, or whether like the cathode rays they were deviated by a magnetic field. this work was effected by professor giesel, then by m. becquerel, professor rutherford, and by many other experimenters after them. all the methods which have already been mentioned in principle have been employed in order to discover whether they were electrified, and, if so, by electricity of what sign, to measure their speed, and to ascertain their degree of penetration. the general result has been to distinguish three sorts of radiations, designated by the letters alpha, beta, gamma. the alpha rays are positively charged, and are projected at a speed which may attain the tenth of that of light; m.h. becquerel has shown by the aid of photography that they are deviated by a magnet, and professor rutherford has, on his side, studied this deviation by the electrical method. the relation of the charge to the mass is, in the case of these rays, of the same order as in that of the ions of electrolysis. they may therefore be considered as exactly analogous to the canal rays of goldstein, and we may attribute them to a material transport of corpuscles of the magnitude of atoms. the relatively considerable size of these corpuscles renders them very absorbable. a flight of a few millimetres in a gas suffices to reduce their number by one-half. they have great ionizing power. the beta rays are on all points similar to the cathode rays; they are, as m. and madame curie have shown, negatively charged, and the charge they carry is always the same. their size is that of the electrons, and their velocity is generally greater than that of the cathode rays, while it may become almost that of light. they have about a hundred times less ionizing power than the alpha rays. the gamma rays were discovered by m. villard.[ ] they may be compared to the x rays; like the latter, they are not deviated by the magnetic field, and are also extremely penetrating. a strip of aluminium five millimetres thick will stop the other kinds, but will allow them to pass. on the other hand, their ionizing power is , times less than that of the alpha rays. [footnote : this is admitted by professor rutherford (_radio-activity_, camb., , p. ) and professor soddy (_radio-activity_, london, , p. ). neither mr whetham, in his recent _development of physical science_ (london, ) nor the hon. r.j. strutt in _the becquerel rays_ (london, same date), both of whom deal with the historical side of the subject, seem to have noticed the fact.--ed.] to these radiations there sometimes are added in the course of experiments secondary radiations analogous to those of m. sagnac, and produced when the alpha, beta, or gamma rays meet various substances. this complication has often led to some errors of observation. phosphorescence and fluorescence seem especially to result from the alpha and beta rays, particularly from the alpha rays, to which belongs the most important part of the total energy of the radiation. sir w. crookes has invented a curious little apparatus, the spinthariscope, which enables us to examine the phosphorescence of the blende excited by these rays. by means of a magnifying glass, a screen covered with sulphide of zinc is kept under observation, and in front of it is disposed, at a distance of about half a millimetre, a fragment of some salt of radium. we then perceive multitudes of brilliant points on the screen, which appear and at once disappear, producing a scintillating effect. it seems probable that every particle falling on the screen produces by its impact a disturbance in the neighbouring region, and it is this disturbance which the eye perceives as a luminous point. thus, says sir w. crookes, each drop of rain falling on the surface of still water is not perceived as a drop of rain, but by reason of the slight splash which it causes at the moment of impact, and which is manifested by ridges and waves spreading themselves in circles. the various radioactive substances do not all give radiations of identical constitution. radium and thorium possess in somewhat large proportions the three kinds of rays, and it is the same with actinium. polonium contains especially alpha rays and a few gamma rays.[ ] in the case of uranium, the alpha rays have extremely slight penetrating power, and cannot even impress photographic plates. but the widest difference between the substances proceeds from the emanation. radium, in addition to the three groups of rays alpha, beta, and gamma, disengages continuously an extremely subtle emanation, seemingly almost imponderable, but which may be, for many reasons, looked upon as a vapour of which the elastic force is extremely feeble. [footnote : it has now been shown that polonium when freshly separated emits beta rays also; see dr logeman's paper in _proceedings of the royal society_, a., th september .--ed.] m. and madame curie discovered as early as that every substance placed in the neighbourhood of radium, itself acquired a radioactivity which persisted for several hours after the removal of the radium. this induced radioactivity seems to be carried to other bodies by the intermediary of a gas. it goes round obstacles, but there must exist between the radium and the substance a free and continuous space for the activation to take place; it cannot, for instance, do so through a wall of glass. in the case of compounds of thorium professor rutherford discovered a similar phenomenon; since then, various physicists, professor soddy, miss brooks, miss gates, m. danne, and others, have studied the properties of these emanations. the substance emanated can neither be weighed nor can its elastic force be ascertained; but its transformations may be followed, as it is luminous, and it is even more certainly characterised by its essential property, i.e. its radioactivity. we also see that it can be decanted like a gas, that it will divide itself between two tubes of different capacity in obedience to the law of mariotte, and will condense in a refrigerated tube in accordance with the principle of watt, while it even complies with the law of gay-lussac. the activity of the emanation vanishes quickly, and at the end of four days it has diminished by one-half. if a salt of radium is heated, the emanation becomes more abundant, and the residue, which, however, does not sensibly diminish in weight, will have lost all its radioactivity, and will only recover it by degrees. professor rutherford, notwithstanding many different attempts, has been unable to make this emanation enter into any chemical reaction. if it be a gaseous body, it must form part of the argon group, and, like its other members, be perfectly inert. by studying the spectrum of the gas disengaged by a solution of salt of radium, sir william ramsay and professor soddy remarked that when the gas is radioactive there are first obtained rays of gases belonging to the argon family, then by degrees, as the activity disappears, the spectrum slowly changes, and finally presents the characteristic aspect of helium. we know that the existence of this gas was first discovered by spectrum analysis in the sun. later its presence was noted in our atmosphere, and in a few minerals which happen to be the very ones from which radium has been obtained. it might therefore have been the case that it pre-existed in the gases extracted from radium; but a remarkable experiment by m. curie and sir james dewar seems to show convincingly that this cannot be so. the spectrum of helium never appears at first in the gas proceeding from pure bromide of radium; but it shows itself, on the other hand, very distinctly, after the radioactive transformations undergone by the salt. all these strange phenomena suggest bold hypotheses, but to construct them with any solidity they must be supported by the greatest possible number of facts. before admitting a definite explanation of the phenomena which have their seat in the curious substances discovered by them, m. and madame curie considered, with a great deal of reason, that they ought first to enrich our knowledge with the exact and precise facts relating to these bodies and to the effects produced by the radiations they emit. thus m. curie particularly set himself to study the manner in which the radioactivity of the emanation is dissipated, and the radioactivity that this emanation can induce on all bodies. the radioactivity of the emanation diminishes in accordance with an exponential law. the constant of time which characterises this decrease is easily and exactly determined, and has a fixed value, independent of the conditions of the experiment as well as of the nature of the gas which is in contact with the radium and becomes charged with the emanation. the regularity of the phenomenon is so great that it can be used to measure time: in seconds[ ] the activity is always reduced one-half. [footnote : according to professor rutherford, in . days.--ed] radioactivity induced on any body which has been for a long time in presence of a salt of radium disappears more rapidly. the phenomenon appears, moreover, more complex, and the formula which expresses the manner in which the activity diminishes must contain two exponentials. to find it theoretically we have to imagine that the emanation first deposits on the body in question a substance which is destroyed in giving birth to a second, this latter disappearing in its turn by generating a third. the initial and final substances would be radioactive, but the intermediary one, not. if, moreover, the bodies acted on are brought to a temperature of over °, they appear to lose by volatilisation certain substances condensed in them, and at the same time their activity disappears. the other radioactive bodies behave in a similar way. bodies which contain actinium are particularly rich in emanations. uranium, on the contrary, has none.[ ] this body, nevertheless, is the seat of transformations comparable to those which the study of emanations reveals in radium; sir w. crookes has separated from uranium a matter which is now called uranium x. this matter is at first much more active than its parent, but its activity diminishes rapidly, while the ordinary uranium, which at the time of the separation loses its activity, regains it by degrees. in the same way, professors rutherford and soddy have discovered a so-called thorium x to be the stage through which ordinary thorium has to pass in order to produce its emanation.[ ] [footnote : professor rutherford has lately stated that uranium may possibly produce an emanation, but that its rate of decay must be too swift for its presence to be verified (see _radioactive transformations_, p. ).--ed.] [footnote : an actinium x was also discovered by professor giesel (_jahrbuch d. radioaktivitat_, i. p. , ). since the above was written, another product has been found to intervene between the x substance and the emanation in the case of actinium and thorium. they have been named radio-actinium and radio-thorium respectively.--ed.] it is not possible to give a complete table which should, as it were, represent the genealogical tree of the various radioactive substances. several authors have endeavoured to do so, but in a premature manner; all the affiliations are not at the present time yet perfectly known, and it will no doubt be acknowledged some day that identical states have been described under different names.[ ] [footnote : such a table is given on p. of rutherford's _radioactive transformations_.--ed.] § . the disaggregation of matter and atomic energy in spite of uncertainties which are not yet entirely removed, it cannot be denied that many experiments render it probable that in radioactive bodies we find ourselves witnessing veritable transformations of matter. professor rutherford, professor soddy, and several other physicists, have come to regard these phenomena in the following way. a radioactive body is composed of atoms which have little stability, and are able to detach themselves spontaneously from the parent substance, and at the same time to divide themselves into two essential component parts, the negative electron and its residue the positive ion. the first-named constitutes the beta, and the second the alpha rays. the emanation is certainly composed of alpha ions with a few molecules agglomerated round them. professor rutherford has, in fact, demonstrated that the emanation is charged with positive electricity; and this emanation may, in turn, be destroyed by giving birth to new bodies. after the loss of the atoms which are carried off by the radiation, the remainder of the body acquires new properties, but it may still be radioactive, and again lose atoms. the various stages that we meet with in the evolution of the radioactive substance or of its emanation, correspond to the various degrees of atomic disaggregation. professors rutherford and soddy have described them clearly in the case of uranium and radium. as regards thorium the results are less satisfactory. the evolution should continue until a stable atomic condition is finally reached, which, because of this stability, is no longer radioactive. thus, for instance, radium would finally be transformed into helium.[ ] [footnote : this opinion, no doubt formed when sir william ramsay's discovery of the formation of helium from the radium emanation was first made known, is now less tenable. the latest theory is that the alpha particle is in fact an atom of helium, and that the final transformation product of radium and the other radioactive substances is lead. cf. rutherford, op. cit. passim.--ed.] it is possible, by considerations analogous to those set forth above in other cases, to arrive at an idea of the total number of particles per second expelled by one gramme of radium; professor rutherford in his most recent evaluation finds that this number approaches . x ^{ }.[ ] by calculating from the atomic weight the number of atoms probably contained in this gramme of radium, and supposing each particle liberated to correspond to the destruction of one atom, it is found that one half of the radium should disappear in years;[ ] and from this we may conceive that it has not yet been possible to discover any sensible loss of weight. sir w. ramsay and professor soddy attained a like result by endeavouring to estimate the mass of the emanation by the quantity of helium produced. [footnote : see _radioactive transformations_ (p. ). professor rutherford says that "each of the alpha ray products present in one gram of radium product (_sic_) expels . x ^{ } alpha particles per second." he also remarks on "the experimental difficulty of accurately determining the number of alpha particles expelled from radium per second."--ed.] [footnote : see rutherford, op. cit. p. .--ed.] if radium transforms itself in such a way that its activity does not persist throughout the ages, it loses little by little the provision of energy it had in the beginning, and its properties furnish no valid argument to oppose to the principle of the conservation of energy. to put everything right, we have only to recognise that radium possessed in the potential state at its formation a finite quantity of energy which is consumed little by little. in the same manner, a chemical system composed, for instance, of zinc and sulphuric acid, also contains in the potential state energy which, if we retard the reaction by any suitable arrangement--such as by amalgamating the zinc and by constituting with its elements a battery which we cause to act on a resistance--may be made to exhaust itself as slowly as one may desire. there can, therefore, be nothing in any way surprising in the fact that a combination which, like the atomic combination of radium, is not stable--since it disaggregates itself,--is capable of spontaneously liberating energy, but what may be a little astonishing, at first sight, is the considerable amount of this energy. m. curie has calculated directly, by the aid of the calorimeter, the quantity of energy liberated, measuring it entirely in the form of heat. the disengagement of heat accounted for in a grain of radium is uniform, and amounts to calories per hour. it must therefore be admitted that an atom of radium, in disaggregating itself, liberates , times more energy than a molecule of hydrogen when the latter combines with an atom of oxygen to form a molecule of water. we may ask ourselves how the atomic edifice of the active body can be constructed, to contain so great a provision of energy. we will remark that such a question might be asked concerning cases known from the most remote antiquity, like that of the chemical systems, without any satisfactory answer ever being given. this failure surprises no one, for we get used to everything--even to defeat. when we come to deal with a new problem we have really no right to show ourselves more exacting; yet there are found persons who refuse to admit the hypothesis of the atomic disaggregation of radium because they cannot have set before them a detailed plan of that complex whole known to us as an atom. the most natural idea is perhaps the one suggested by comparison with those astronomical phenomena where our observation most readily allows us to comprehend the laws of motion. it corresponds likewise to the tendency ever present in the mind of man, to compare the infinitely small with the infinitely great. the atom may be regarded as a sort of solar system in which electrons in considerable numbers gravitate round the sun formed by the positive ion. it may happen that certain of these electrons are no longer retained in their orbit by the electric attraction of the rest of the atom, and may be projected from it like a small planet or comet which escapes towards the stellar spaces. the phenomena of the emission of light compels us to think that the corpuscles revolve round the nucleus with extreme velocities, or at the rate of thousands of billions of evolutions per second. it is easy to conceive from this that, notwithstanding its lightness, an atom thus constituted may possess an enormous energy.[ ] [footnote : this view of the case has been made very clear by m. gustave le bon in _l'Évolution de la matière_ (paris, ). see especially pp. - , where the amount of the supposed intra-atomic energy is calculated.--ed.] other authors imagine that the energy of the corpuscles is principally due to the extremely rapid rotations of those elements on their own axes. lord kelvin lately drew up on another model the plan of a radioactive atom capable of ejecting an electron with a considerable _vis viva_. he supposes a spherical atom formed of concentric layers of positive and negative electricity disposed in such a way that its external action is null, and that, nevertheless, the force emanated from the centre may be repellent for certain values when the electron is within it. the most prudent physicists and those most respectful to established principles may, without any scruples, admit the explanation of the radioactivity of radium by a dislocation of its molecular edifice. the matter of which it is constituted evolves from an admittedly unstable initial state to another stable one. it is, in a way, a slow allotropic transformation which takes place by means of a mechanism regarding which, in short, we have no more information than we have regarding other analogous transformations. the only astonishment we can legitimately feel is derived from the thought that we are suddenly and deeply penetrating to the very heart of things. but those persons who have a little more hardihood do not easily resist the temptation of forming daring generalisations. thus it will occur to some that this property, already discovered in many substances where it exists in more or less striking degree, is, with differences of intensity, common to all bodies, and that we are thus confronted by a phenomenon derived from an essential quality of matter. quite recently, professor rutherford has demonstrated in a fine series of experiments that the alpha particles of radium cease to ionize gases when they are made to lose their velocity, but that they do not on that account cease to exist. it may follow that many bodies emit similar particles without being easily perceived to do so; since the electric action, by which this phenomenon of radioactivity is generally manifested, would, in this case, be but very weak. if we thus believe radioactivity to be an absolutely general phenomenon, we find ourselves face to face with a new problem. the transformation of radioactive bodies can no longer be assimilated to allotropic transformations, since thus no final form could ever be attained, and the disaggregation would continue indefinitely up to the complete dislocation of the atom.[ ] the phenomenon might, it is true, have a duration of perhaps thousands of millions of centuries, but this duration is but a minute in the infinity of time, and matters little. our habits of mind, if we adopt such a conception, will be none the less very deeply disturbed. we shall have to abandon the idea so instinctively dear to us that matter is the most stable thing in the universe, and to admit, on the contrary, that all bodies whatever are a kind of explosive decomposing with extreme slowness. there is in this, whatever may have been said, nothing contrary to any of the principles on which the science of energetics rests; but an hypothesis of this nature carries with it consequences which ought in the highest degree to interest the philosopher, and we all know with what alluring boldness m. gustave le bon has developed all these consequences in his work on the evolution of matter.[ ] [footnote : this is the main contention of m. gustave le bon in his work last quoted.--ed.] [footnote : see last note.--ed.] there is hardly a physicist who does not at the present day adopt in one shape or another the ballistic hypothesis. all new facts are co-ordinated so happily by it, that it more and more satisfies our minds; but it cannot be asserted that it forces itself on our convictions with irresistible weight. another point of view appeared more plausible and simple at the outset, when there seemed reason to consider the energy radiated by radioactive bodies as inexhaustible. it was thought that the source of this energy was to be looked for without the atom, and this idea may perfectly well he maintained at the present day. radium on this hypothesis must be considered as a transformer borrowing energy from the external medium and returning it in the form of radiation. it is not impossible, even, to admit that the energy which the atom of radium withdraws from the surrounding medium may serve to keep up, not only the heat emitted and its complex radiation, but also the dissociation, supposed to be endothermic, of this atom. such seems to be the idea of m. debierne and also of m. sagnac. it does not seem to accord with the experiments that this borrowed energy can be a part of the heat of the ambient medium; and, indeed, such a phenomenon would be contrary to the principle of carnot if we wished (though we have seen how disputable is this extension) to extend this principle to the phenomena which are produced in the very bosom of the atom. we may also address ourselves to a more noble form of energy, and ask ourselves whether we are not, for the first time, in presence of a transformation of gravitational energy. it may be singular, but it is not absurd, to suppose that the unit of mass of radium is not attached to the earth with the same intensity as an inert body. m. sagnac has commenced some experiments, as yet unpublished, in order to study the laws of the fall of a fragment of radium. they are necessarily very delicate, and the energetic and ingenious physicist has not yet succeeded in finishing them.[ ] let us suppose that he succeeds in demonstrating that the intensity of gravity is less for radium than for the platinum or the copper of which the pendulums used to illustrate the law of newton are generally made; it would then be possible still to think that the laws of universal attraction are perfectly exact as regards the stars, and that ponderability is really a particular case of universal attraction, while in the case of radioactive bodies part of the gravitational energy is transformed in the course of its evolution and appears in the form of active radiation. [footnote : in reality m. sagnac operated in the converse manner. he took two equal _weights_ of a salt of radium and a salt of barium, which he made oscillate one after the other in a torsion balance. had the durations of oscillation been different, it might be concluded that the mechanical mass is not the same for radium as for barium.] but for this explanation to be admitted, it would evidently need to be supported by very numerous facts. it might, no doubt, appear still more probable that the energy borrowed from the external medium by radium is one of those still unknown to us, but of which a vague instinct causes us to suspect the existence around us. it is indisputable, moreover, that the atmosphere in all directions is furrowed with active radiations; those of radium may be secondary radiations reflected by a kind of resonance phenomenon. certain experiments by professors elster and geitel, however, are not favourable to this point of view. if an active body be surrounded by a radioactive envelope, a screen should prevent this body from receiving any impression from outside, and yet there is no diminution apparent in the activity presented by a certain quantity of radium when it is lowered to a depth of metres under ground, in a region containing a notable quantity of pitchblende. these negative results are, on the other hand, so many successes for the partisans of the explanation of radioactivity by atomic energy. chapter x the ether and matter § . the relations between the ether and matter for some time past it has been the more or less avowed ambition of physicists to construct with the particles of ether all possible forms of corporeal existence; but our knowledge of the inmost nature of things has hitherto seemed too limited for us to attempt such an enterprise with any chance of success. the electronic hypothesis, however, which has furnished a satisfactory image of the most curious phenomena produced in the bosom of matter, has also led to a more complete electromagnetic theory of the ether than that of maxwell, and this twofold result has given birth to the hope of arriving by means of this hypothesis at a complete co-ordination of the physical world. the phenomena whose study may bring us to the very threshold of the problem, are those in which the connections between matter and the ether appear clearly and in a relatively simple manner. thus in the phenomena of emission, ponderable matter is seen to give birth to waves which are transmitted by the ether, and by the phenomena of absorption it is proved that these waves disappear and excite modifications in the interior of the material bodies which receive them. we here catch in operation actual reciprocal actions and reactions between the ether and matter. if we could thoroughly comprehend these actions, we should no doubt be in a position to fill up the gap which separates the two regions separately conquered by physical science. in recent years numerous researches have supplied valuable materials which ought to be utilized by those endeavouring to construct a theory of radiation. we are, perhaps, still ill informed as to the phenomena of luminescence in which undulations are produced in a complex manner, as in the case of a stick of moist phosphorus which is luminescent in the dark, or in that of a fluorescent screen. but we are very well acquainted with emission or absorption by incandescence, where the only transformation is that of calorific into radiating energy, or _vice versa_. it is in this case alone that can be correctly applied the celebrated demonstration by which kirchhoff established, by considerations borrowed from thermodynamics, the proportional relations between the power of emission and that of absorption. in treating of the measurement of temperature, i have already pointed out the experiments of professors lummer and pringsheim and the theoretical researches of stephan and professor wien. we may consider that at the present day the laws of the radiation of dark bodies are tolerably well known, and, in particular, the manner in which each elementary radiation increases with the temperature. a few doubts, however, subsist with respect to the law of the distribution of energy in the spectrum. in the case of real and solid bodies the results are naturally less simple than in that of dark bodies. one side of the question has been specially studied on account of its great practical interest, that is to say, the fact that the relation of the luminous energy to the total amount radiated by a body varies with the nature of this last; and the knowledge of the conditions under which this relation becomes most considerable led to the discovery of incandescent lighting by gas in the auer-welsbach mantle, and to the substitution for the carbon thread in the electric light bulb of a filament of osmium or a small rod of magnesium, as in the nernst lamp. careful measurements effected by m. fery have furnished, in particular, important information on the radiation of the white oxides; but the phenomena noticed have not yet found a satisfactory interpretation. moreover, the radiation of calorific origin is here accompanied by a more or less important luminescence, and the problem becomes very complex. in the same way that, for the purpose of knowing the constitution of matter, it first occurred to us to investigate gases, which appear to be molecular edifices built on a more simple and uniform plan than solids, we ought naturally to think that an examination of the conditions in which emission and absorption are produced by gaseous bodies might be eminently profitable, and might perhaps reveal the mechanism by which the relations between the molecule of the ether and the molecule of matter might be established. unfortunately, if a gas is not absolutely incapable of emitting some sort of rays by simple heat, the radiation thus produced, no doubt by reason of the slightness of the mass in play, always remains of moderate intensity. in nearly all the experiments, new energies of chemical or electrical origin come into force. on incandescence, luminescence is superposed; and the advantage which might have been expected from the simplicity of the medium vanishes through the complication of the circumstances in which the phenomenon is produced. professor pringsheim has succeeded, in certain cases, in finding the dividing line between the phenomena of luminescence and that of incandescence. thus the former takes a predominating importance when the gas is rendered luminous by electrical discharges, and chemical transformations, especially, play a preponderant rôle in the emission of the spectrum of flames which contain a saline vapour. in all the ordinary experiments of spectrum analysis the laws of kirchhoff cannot therefore be considered as established, and yet the relation between emission and absorption is generally tolerably well verified. no doubt we are here in presence of a kind of resonance phenomenon, the gaseous atoms entering into vibration when solicited by the ether by a motion identical with the one they are capable of communicating to it. if we are not yet very far advanced in the study of the mechanism of the production of the spectrum,[ ] we are, on the other hand, well acquainted with its constitution. the extreme confusion which the spectra of the lines of the gases seemed to present is now, in great part at least, cleared up. balmer gave some time since, in the case of the hydrogen spectrum, an empirical formula which enabled the rays discovered later by an eminent astronomer, m. deslandres, to be represented; but since then, both in the cases of line and band spectra, the labours of professor rydberg, of m. deslandres, of professors kayzer and runge, and of m. thiele, have enabled us to comprehend, in their smallest details, the laws of the distribution of lines and bands. [footnote : many theories as to the cause of the lines and bands of the spectrum have been put forward since this was written, among which that of professor stark (for which see _physikalische zeitschrift_ for , passim) is perhaps the most advanced. that of m. jean becquerel, which would attribute it to the vibration within the atom of both negative and positive electrons, also deserves notice. a popular account of this is given in the _athenæum_ of th april .--ed.] these laws are simple, but somewhat singular. the radiations emitted by a gas cannot be compared to the notes to which a sonorous body gives birth, nor even to the most complicated vibrations of any elastic body. the number of vibrations of the different rays are not the successive multiples of one and the same number, and it is not a question of a fundamental radiation and its harmonics, while--and this is an essential difference--the number of vibrations of the radiation tend towards a limit when the period diminishes infinitely instead of constantly increasing, as would be the case with the vibrations of sound. thus the assimilation of the luminous to the elastic vibration is not correct. once again we find that the ether does not behave like matter which obeys the ordinary laws of mechanics, and every theory must take full account of these curious peculiarities which experiment reveals. another difference, likewise very important, between the luminous and the sonorous vibrations, which also points out how little analogous can be the constitutions of the media which transmit the vibrations, appears in the phenomena of dispersion. the speed of propagation, which, as we have seen when discussing the measurement of the velocity of sound, depends very little on the musical note, is not at all the same in the case of the various radiations which can be propagated in the same substance. the index of refraction varies with the duration of the period, or, if you will, with the length of wave _in vacuo_ which is proportioned to this duration, since _in vacuo_ the speed of propagation is entirely the same for all vibrations. cauchy was the first to propose a theory on which other attempts have been modelled; for example, the very interesting and simple one of briot. this last-named supposed that the luminous vibration could not perceptibly drag with it the molecular material of the medium across which it is propagated, but that matter, nevertheless, reacts on the ether with an intensity proportional to the elongation, in such a manner as tends to bring it back to its position of equilibrium. with this simple hypothesis we can fairly well interpret the phenomena of the dispersion of light in the case of transparent substances; but far from well, as m. carvallo has noted in some extremely careful experiments, the dispersion of the infra-red spectrum, and not at all the peculiarities presented by absorbent substances. m. boussinesq arrives at almost similar results, by attributing dispersion, on the other hand, to the partial dragging along of ponderable matter and to its action on the ether. by combining, in a measure, as was subsequently done by m. boussinesq, the two hypotheses, formulas can be established far better in accord with all the known facts. these facts are somewhat complex. it was at first thought that the index always varied in inverse ratio to the wave-length, but numerous substances have been discovered which present the phenomenon of abnormal dispersion--that is to say, substances in which certain radiations are propagated, on the contrary, the more quickly the shorter their period. this is the case with gases themselves, as demonstrated, for example, by a very elegant experiment of m. becquerel on the dispersion of the vapour of sodium. moreover, it may happen that yet more complications may be met with, as no substance is transparent for the whole extent of the spectrum. in the case of certain radiations the speed of propagation becomes nil, and the index shows sometimes a maximum and sometimes a minimum. all those phenomena are in close relation with those of absorption. it is, perhaps, the formula proposed by helmholtz which best accounts for all these peculiarities. helmholtz came to establish this formula by supposing that there is a kind of friction between the ether and matter, which, like that exercised on a pendulum, here produces a double effect, changing, on the one hand, the duration of this oscillation, and, on the other, gradually damping it. he further supposed that ponderable matter is acted on by elastic forces. the theory of helmholtz has the great advantage of representing, not only the phenomena of dispersion, but also, as m. carvallo has pointed out, the laws of rotatory polarization, its dispersion and other phenomena, among them the dichroism of the rotatory media discovered by m. cotton. in the establishment of these theories, the language of ordinary optics has always been employed. the phenomena are looked upon as due to mechanical deformations or to movements governed by certain forces. the electromagnetic theory leads, as we have seen, to the employment of other images. m.h. poincaré, and, after him, helmholtz, have both proposed electromagnetic theories of dispersion. on examining things closely, it will be found that there are not, in truth, in the two ways of regarding the problem, two equivalent translations of exterior reality. the electrical theory gives us to understand, much better than the mechanical one, that _in vacuo_ the dispersion ought to be strictly null, and this absence of dispersion appears to be confirmed with extraordinary precision by astronomical observations. thus the observation, often repeated, and at different times of year, proves that in the case of the star algol, the light of which takes at least four years to reach us, no sensible difference in coloration accompanies the changes in brilliancy. § . the theory of lorentz purely mechanical considerations have therefore failed to give an entirely satisfactory interpretation of the phenomena in which even the simplest relations between matter and the ether appear. they would, evidently, be still more insufficient if used to explain certain effects produced on matter by light, which could not, without grave difficulties, be attributed to movement; for instance, the phenomena of electrification under the influence of certain radiations, or, again, chemical reactions such as photographic impressions. the problem had to be approached by another road. the electromagnetic theory was a step in advance, but it comes to a standstill, so to speak, at the moment when the ether penetrates into matter. if we wish to go deeper into the inwardness of the phenomena, we must follow, for example, professor lorentz or dr larmor, and look with them for a mode of representation which appears, besides, to be a natural consequence of the fundamental ideas forming the basis of hertz's experiments. the moment we look upon a wave in the ether as an electromagnetic wave, a molecule which emits light ought to be considered as a kind of excitant. we are thus led to suppose that in each radiating molecule there are one or several electrified particles, animated with a to-and-fro movement round their positions of equilibrium, and these particles are certainly identical with those electrons the existence of which we have already admitted for so many other reasons. in the simplest theory, we will imagine an electron which may be displaced from its position of equilibrium in all directions, and is, in this displacement, submitted to attractions which communicate to it a vibration like a pendulum. these movements are equivalent to tiny currents, and the mobile electron, when animated with a considerable velocity, must be sensitive to the action of the magnet which modifies the form of the trajectory and the value of the period. this almost direct consequence was perceived by lorentz, and it led him to the new idea that radiations emitted by a body ought to be modified by the action of a strong electromagnet. an experiment enabled this prevision to be verified. it was made, as is well known, as early as by zeeman; and the discovery produced a legitimate sensation. when a flame is subjected to the action of a magnetic field, a brilliant line is decomposed in conditions more or less complex which an attentive study, however, allows us to define. according to whether the observation is made in a plane normal to the magnetic field or in the same direction, the line transforms itself into a triplet or doublet, and the new lines are polarized rectilinearly or circularly. these are the precise phenomena which the calculation foretells: the analysis of the modifications undergone by the light supplies, moreover, valuable information on the electron itself. from the direction of the circular vibrations of the greatest frequency we can determine the sign of the electric charge in motion and we find it to be negative. but, further than this, from the variation of the period we can calculate the relation of the force acting on the electron to its material mass, and, in addition, the relation of the charge to the mass. we then find for this relation precisely that value which we have already met with so many times. such a coincidence cannot be fortuitous, and we have the right to believe that the electron revealed by the luminous wave which emanates from it, is really the same as the one made known to us by the study of the cathode rays and of the radioactive substances. however, the elementary theory does not suffice to interpret the complications which later experiments have revealed. the physicists most qualified to effect measurements in these delicate optical questions--m. cornu, mr preston, m. cotton, mm. becquerel and deslandres, m. broca, professor michelson, and others--have pointed out some remarkable peculiarities. thus in some cases the number of the component rays dissociated by the magnetic field may be very considerable. the great modification brought to a radiation by the zeeman effect may, besides, combine itself with other phenomena, and alter the light in a still more complicated manner. a pencil of polarized light, as demonstrated by signori macaluzo and corbino, undergoes, in a magnetic field, modifications with regard to absorption and speed of propagation. some ingenious researches by m. becquerel and m. cotton have perfectly elucidated all these complications from an experimental point of view. it would not be impossible to link together all these phenomena without adopting the electronic hypothesis, by preserving the old optical equations as modified by the terms relating to the action of the magnetic field. this has actually been done in some very remarkable work by m. voigt, but we may also, like professor lorentz, look for more general theories, in which the essential image of the electrons shall be preserved, and which will allow all the facts revealed by experiment to be included. we are thus led to the supposition that there is not in the atom one vibrating electron only, but that there is to be found in it a dynamical system comprising several material points which may be subjected to varied movements. the neutral atom may therefore be considered as composed of an immovable principal portion positively charged, round which move, like satellites round a planet, several negative electrons of very inferior mass. this conclusion leads us to an interpretation in agreement with that which other phenomena have already suggested. these electrons, which thus have a variable velocity, generate around themselves a transverse electromagnetic wave which is propagated with the velocity of light; for the charged particle becomes, as soon as it experiences a change of speed, the centre of a radiation. thus is explained the phenomenon of the emission of radiations. in the same way, the movement of electrons may be excited or modified by the electrical forces which exist in any pencil of light they receive, and this pencil may yield up to them a part of the energy it is carrying. this is the phenomenon of absorption. professor lorentz has not contented himself with thus explaining all the mechanism of the phenomena of emission and absorption. he has endeavoured to rediscover, by starting with the fundamental hypothesis, the quantitative laws discovered by thermodynamics. he succeeds in showing that, agreeably to the law of kirchhoff, the relation between the emitting and the absorbing power must be independent of the special properties of the body under observation, and he thus again meets with the laws of planck and of wien: unfortunately the calculation can only be made in the case of great wave-lengths, and grave difficulties exist. thus it cannot be very clearly explained why, by heating a body, the radiation is displaced towards the side of the short wave-lengths, or, if you will, why a body becomes luminous from the moment its temperature has reached a sufficiently high degree. on the other hand, by calculating the energy of the vibrating particles we are again led to attribute to these particles the same constitution as that of the electrons. it is in the same way possible, as professor lorentz has shown, to give a very satisfactory explanation of the thermo-electric phenomena by supposing that the number of liberated electrons which exist in a given metal at a given temperature has a determined value varying with each metal, and is, in the case of each body, a function of the temperature. the formula obtained, which is based on these hypotheses, agrees completely with the classic results of clausius and of lord kelvin. finally, if we recollect that the phenomena of electric and calorific conductivity are perfectly interpreted by the hypothesis of electrons, it will no longer be possible to contest the importance of a theory which allows us to group together in one synthesis so many facts of such diverse origins. if we study the conditions under which a wave excited by an electron's variations in speed can be transmitted, they again bring us face to face, and generally, with the results pointed out by the ordinary electromagnetic theory. certain peculiarities, however, are not absolutely the same. thus the theory of lorentz, as well as that of maxwell, leads us to foresee that if an insulating mass be caused to move in a magnetic field normally to its lines of force, a displacement will be produced in this mass analogous to that of which faraday and maxwell admitted the existence in the dielectric of a charged condenser. but m.h. poincaré has pointed out that, according as we adopt one or other of these authors' points of view, so the value of the displacement differs. this remark is very important, for it may lead to an experiment which would enable us to make a definite choice between the two theories. to obtain the displacement estimated according to lorentz, we must multiply the displacement calculated according to hertz by a factor representing the relation between the difference of the specific inductive capacities of the dielectric and of a vacuum, and the first of these powers. if therefore we take as dielectric the air of which the specific inductive capacity is perceptibly the same as that of a vacuum, the displacement, according to the idea of lorentz, will be null; while, on the contrary, according to hertz, it will have a finite value. m. blondlot has made the experiment. he sent a current of air into a condenser placed in a magnetic field, and was never able to notice the slightest trace of electrification. no displacement, therefore, is effected in the dielectric. the experiment being a negative one, is evidently less convincing than one giving a positive result, but it furnishes a very powerful argument in favour of the theory of lorentz. this theory, therefore, appears very seductive, yet it still raises objections on the part of those who oppose to it the principles of ordinary mechanics. if we consider, for instance, a radiation emitted by an electron belonging to one material body, but absorbed by another electron in another body, we perceive immediately that, the propagation not being instantaneous, there can be no compensation between the action and the reaction, which are not simultaneous; and the principle of newton thus seems to be attacked. in order to preserve its integrity, it has to be admitted that the movements in the two material substances are compensated by that of the ether which separates these substances; but this conception, although in tolerable agreement with the hypothesis that the ether and matter are not of different essence, involves, on a closer examination, suppositions hardly satisfactory as to the nature of movements in the ether. for a long time physicists have admitted that the ether as a whole must be considered as being immovable and capable of serving, so to speak, as a support for the axes of galileo, in relation to which axes the principle of inertia is applicable,--or better still, as m. painlevé has shown, they alone allow us to render obedience to the principle of causality. but if it were so, we might apparently hope, by experiments in electromagnetism, to obtain absolute motion, and to place in evidence the translation of the earth relatively to the ether. but all the researches attempted by the most ingenious physicists towards this end have always failed, and this tends towards the idea held by many geometricians that these negative results are not due to imperfections in the experiments, but have a deep and general cause. now lorentz has endeavoured to find the conditions in which the electromagnetic theory proposed by him might agree with the postulate of the complete impossibility of determining absolute motion. it is necessary, in order to realise this concord, to imagine that a mobile system contracts very slightly in the direction of its translation to a degree proportioned to the square of the ratio of the velocity of transport to that of light. the electrons themselves do not escape this contraction, although the observer, since he participates in the same motion, naturally cannot notice it. lorentz supposes, besides, that all forces, whatever their origin, are affected by a translation in the same way as electromagnetic forces. m. langevin and m. h. poincaré have studied this same question and have noted with precision various delicate consequences of it. the singularity of the hypotheses which we are thus led to construct in no way constitutes an argument against the theory of lorentz; but it has, we must acknowledge, discouraged some of the more timid partisans of this theory.[ ] [footnote : an objection not here noticed has lately been formulated with much frankness by professor lorentz himself. it is one of the pillars of his theory that only the negative electrons move when an electric current passes through a metal, and that the positive electrons (if any such there be) remain motionless. yet in the experiment known as hall's, the current is deflected by the magnetic field to one side of the strip in certain metals, and to the opposite side in others. this seems to show that in certain cases the positive electrons move instead of the negative, and professor lorentz confesses that up to the present he can find no valid argument against this. see _archives néerlandaises_ , parts and .--ed.] § . the mass of electrons other conceptions, bolder still, are suggested by the results of certain interesting experiments. the electron affords us the possibility of considering inertia and mass to be no longer a fundamental notion, but a consequence of the electromagnetic phenomena. professor j.j. thomson was the first to have the clear idea that a part, at least, of the inertia of an electrified body is due to its electric charge. this idea was taken up and precisely stated by professor max abraham, who, for the first time, was led to regard seriously the seemingly paradoxical notion of mass as a function of velocity. consider a small particle bearing a given electric charge, and let us suppose that this particle moves through the ether. it is, as we know, equivalent to a current proportional to its velocity, and it therefore creates a magnetic field the intensity of which is likewise proportional to its velocity: to set it in motion, therefore, there must be communicated to it over and above the expenditure corresponding to the acquisition of its ordinary kinetic energy, a quantity of energy proportional to the square of its velocity. everything, therefore, takes place as if, by the fact of electrification, its capacity for kinetic energy and its material mass had been increased by a certain constant quantity. to the ordinary mass may be added, if you will, an electromagnetic mass. this is the state of things so long as the speed of the translation of the particle is not very great, but they are no longer quite the same when this particle is animated with a movement whose rapidity becomes comparable to that with which light is propagated. the magnetic field created is then no longer a field in repose, but its energy depends, in a complicated manner, on the velocity, and the apparent increase in the mass of the particle itself becomes a function of the velocity. more than this, this increase may not be the same for the same velocity, but varies according to whether the acceleration is parallel with or perpendicular to the direction of this velocity. in other words, there seems to be a longitudinal; and a transversal mass which need not be the same. all these results would persist even if the material mass were very small relatively to the electromagnetic mass; and the electron possesses some inertia even if its ordinary mass becomes slighter and slighter. the apparent mass, it can be easily shown, increases indefinitely when the velocity with which the electrified particle is animated tends towards the velocity of light, and thus the work necessary to communicate such a velocity to an electron would be infinite. it is in consequence impossible that the speed of an electron, in relation to the ether, can ever exceed, or even permanently attain to, , kilometres per second. all the facts thus predicted by the theory are confirmed by experiment. there is no known process which permits the direct measurement of the mass of an electron, but it is possible, as we have seen, to measure simultaneously its velocity and the relation of the electric charge to its mass. in the case of the cathode rays emitted by radium, these measurements are particularly interesting, for the reason that the rays which compose a pencil of cathode rays are animated by very different speeds, as is shown by the size of the stain produced on a photographic plate by a pencil of them at first very constricted and subsequently dispersed by the action of an electric or magnetic field. professor kaufmann has effected some very careful experiments by a method he terms the method of crossed spectra, which consists in superposing the deviations produced by a magnetic and an electric field respectively acting in directions at right angles one to another. he has thus been enabled by working _in vacuo_ to register the very different velocities which, starting in the case of certain rays from about seven-tenths of the velocity of light, attain in other cases to ninety-five hundredths of it. it is thus noted that the ratio of charge to mass--which for ordinary speeds is constant and equal to that already found by so many experiments--diminishes slowly at first, and then very rapidly when the velocity of the ray increases and approaches that of light. if we represent this variation by a curve, the shape of this curve inclines us to think that the ratio tends toward zero when the velocity tends towards that of light. all the earlier experiments have led us to consider that the electric charge was the same for all electrons, and it can hardly be conceived that this charge can vary with the velocity. for in order that the relation, of which one of the terms remains fixed, should vary, the other term necessarily cannot remain constant. the experiments of professor kaufmann, therefore, confirm the previsions of max abraham's theory: the mass depends on the velocity, and increases indefinitely in proportion as this velocity approaches that of light. these experiments, moreover, allow the numerical results of the calculation to be compared with the values measured. this very satisfactory comparison shows that the apparent total mass is sensibly equal to the electromagnetic mass; the material mass of the electron is therefore nil, and the whole of its mass is electromagnetic. thus the electron must be looked upon as a simple electric charge devoid of matter. previous examination has led us to attribute to it a mass a thousand times less that that of the atom of hydrogen, and a more attentive study shows that this mass was fictitious. the electromagnetic phenomena which are produced when the electron is set in motion or a change effected in its velocity, simply have the effect, as it were, of simulating inertia, and it is the inertia due to the charge which has caused us to be thus deluded. the electron is therefore simply a small volume determined at a point in the ether, and possessing special properties;[ ] this point is propagated with a velocity which cannot exceed that of light. when this velocity is constant, the electron creates around it in its passage an electric and a magnetic field; round this electrified centre there exists a kind of wake, which follows it through the ether and does not become modified so long as the velocity remains invariable. if other electrons follow the first within a wire, their passage along the wire will be what is called an electric current. [footnote : this cannot be said to be yet completely proved. _cf_. sir oliver lodge, _electrons_, london, , p. .--ed.] when the electron is subjected to an acceleration, a transverse wave is produced, and an electromagnetic radiation is generated, of which the character may naturally change with the manner in which the speed varies. if the electron has a sufficiently rapid periodical movement, this wave is a light wave; while if the electron stops suddenly, a kind of pulsation is transmitted through the ether, and thus we obtain röntgen rays. § . new views on the constitution of the ether and of matter new and valuable information is thus afforded us regarding the properties of the ether, but will this enable us to construct a material representation of this medium which fills the universe, and so to solve a problem which has baffled, as we have seen, the prolonged efforts of our predecessors? certain scholars seem to have cherished this hope. dr. larmor in particular, as we have seen, has proposed a most ingenious image, but one which is manifestly insufficient. the present tendency of physicists rather tends to the opposite view; since they consider matter as a very complex object, regarding which we wrongly imagine ourselves to be well informed because we are so much accustomed to it, and its singular properties end by seeming natural to us. but in all probability the ether is, in its objective reality, much more simple, and has a better right to be considered as fundamental. we cannot therefore, without being very illogical, define the ether by material properties, and it is useless labour, condemned beforehand to sterility, to endeavour to determine it by other qualities than those of which experiment gives us direct and exact knowledge. the ether is defined when we know, in all its points, and in magnitude and in direction, the two fields, electric and magnetic, which may exist in it. these two fields may vary; we speak from habit of a movement propagated in the ether, but the phenomenon within the reach of experiment is the propagation of these variations. since the electrons, considered as a modification of the ether symmetrically distributed round a point, perfectly counterfeit that inertia which is the fundamental property of matter, it becomes very tempting to suppose that matter itself is composed of a more or less complex assemblage of electrified centres in motion. this complexity is, in general, very great, as is demonstrated by the examination of the luminous spectra produced by the atoms, and it is precisely because of the compensations produced between the different movements that the essential properties of matter--the law of the conservation of inertia, for example--are not contrary to the hypothesis. the forces of cohesion thus would be due to the mutual attractions which occur in the electric and magnetic fields produced in the interior of bodies; and it is even conceivable that there may be produced, under the influence of these actions, a tendency to determine orientation, that is to say, that a reason can be seen why matter may be crystallised.[ ] [footnote : the reader should, however, be warned that a theory has lately been put forth which attempts to account for crystallisation on purely mechanical grounds. see messrs barlow and pope's "development of the atomic theory" in the _transactions of the chemical society_, .--ed.] all the experiments effected on the conductivity of gases or metals, and on the radiations of active bodies, have induced us to regard the atom as being constituted by a positively charged centre having practically the same magnitude as the atom itself, round which the electrons gravitate; and it might evidently be supposed that this positive centre itself preserves the fundamental characteristics of matter, and that it is the electrons alone which no longer possess any but electromagnetic mass. we have but little information concerning these positive particles, though they are met with in an isolated condition, as we have seen, in the canal rays or in the x rays.[ ] it has not hitherto been possible to study them so successfully as the electrons themselves; but that their magnitude causes them to produce considerable perturbations in the bodies on which they fall is manifest by the secondary emissions which complicate and mask the primitive phenomenon. there are, however, strong reasons for thinking that these positive centres are not simple. thus professor stark attributes to them, with experiments in proof of his opinion, the emission of the spectra of the rays in geissler tubes, and the complexity of the spectrum discloses the complexity of the centre. besides, certain peculiarities in the conductivity of metals cannot be explained without a supposition of this kind. so that the atom, deprived of the cathode corpuscle, would be still liable to decomposition into elements analogous to electrons and positively charged. consequently nothing prevents us supposing that this centre likewise simulates inertia by its electromagnetic properties, and is but a condition localised in the ether. [footnote : there is much reason for thinking that the canal rays do not contain positive particles alone, but are accompanied by negative electrons of slow velocity. the x rays are thought, as has been said above, to contain neither negative nor positive particles, but to be merely pulses in the ether.--ed.] however this may be, the edifice thus constructed, being composed of electrons in periodical motion, necessarily grows old. the electrons become subject to accelerations which produce a radiation towards the exterior of the atom; and certain of them may leave the body, while the primitive stability is, in the end, no longer assured, and a new arrangement tends to be formed. matter thus seems to us to undergo those transformations of which the radio-active bodies have given us such remarkable examples. we have already had, in fragments, these views on the constitution of matter; a deeper study of the electron thus enables us to take up a position from which we obtain a sharp, clear, and comprehensive grasp of the whole and a glimpse of indefinite horizons. it would be advantageous, however, in order to strengthen this position, that a few objections which still menace it should be removed. the instability of the electron is not yet sufficiently demonstrated. how is it that its charge does not waste itself away, and what bonds assure the permanence of its constitution? on the other hand, the phenomena of gravitation remain a mystery. lorentz has endeavoured to build up a theory in which he explains attraction by supposing that two charges of similar sign repel each other in a slightly less degree than that in which two charges, equal but of contrary sign, attract each other, the difference being, however, according to the calculation, much too small to be directly observed. he has also sought to explain gravitation by connecting it with the pressures which may be produced on bodies by the vibratory movements which form very penetrating rays. recently m. sutherland has imagined that attraction is due to the difference of action in the convection currents produced by the positive and negative corpuscles which constitute the atoms of the stars, and are carried along by the astronomical motions. but these hypotheses remain rather vague, and many authors think, like m. langevin, that gravitation must result from some mode of activity of the ether totally different from the electromagnetic mode. chapter xi the future of physics it would doubtless be exceedingly rash, and certainly very presumptuous, to seek to predict the future which may be reserved for physics. the rôle of prophet is not a scientific one, and the most firmly established previsions of to-day may be overthrown by the reality of to-morrow. nevertheless, the physicist does not shun an extrapolation of some little scope when it is not too far from the realms of experiment; the knowledge of the evolution accomplished of late years authorises a few suppositions as to the direction in which progress may continue. the reader who has deigned to follow me in the rapid excursion we have just made through the domain of the science of nature, will doubtless bring back with him from his short journey the general impression that the ancient limits to which the classic treatises still delight in restricting the divers chapters of physics, are trampled down in all directions. the fine straight roads traced out by the masters of the last century, and enlarged and levelled by the labour of such numbers of workmen, are now joined together by a crowd of small paths which furrow the field of physics. it is not only because they cover regions as yet little explored where discoveries are more abundant and more easy, that these cross-cuts are so frequent, but also because a higher hope guides the seekers who engage in these new routes. in spite of the repeated failures which have followed the numerous attempts of past times, the idea has not been abandoned of one day conquering the supreme principle which must command the whole of physics. some physicists, no doubt, think such a synthesis to be impossible of realisation, and that nature is infinitely complex; but, notwithstanding all the reserves they may make, from the philosophical point of view, as to the legitimacy of the process, they do not hesitate to construct general hypotheses which, in default of complete mental satisfaction, at least furnish them with a highly convenient means of grouping an immense number of facts till then scattered abroad. their error, if error there be, is beneficial, for it is one of those that kant would have classed among the fruitful illusions which engender the indefinite progress of science and lead to great and important co-ordinations. it is, naturally, by the study of the relations existing between phenomena apparently of very different orders that there can be any hope of reaching the goal; and it is this which justifies the peculiar interest accorded to researches effected in the debatable land between domains hitherto considered as separate. among all the theories lately proposed, that of the ions has taken a preponderant place; ill understood at first by some, appearing somewhat singular, and in any case useless, to others, it met at its inception, in france at least, with only very moderate favour. to-day things have greatly changed, and those even who ignored it have been seduced by the curious way in which it adapts itself to the interpretation of the most recent experiments on very different subjects. a very natural reaction has set in; and i might almost say that a question of fashion has led to some exaggerations. the electron has conquered physics, and many adore the new idol rather blindly. certainly we can only bow before an hypothesis which enables us to group in the same synthesis all the discoveries on electric discharges and on radioactive substances, and which leads to a satisfactory theory of optics and of electricity; while by the intermediary of radiating heat it seems likely to embrace shortly the principles of thermodynamics also. certainly one must admire the power of a creed which penetrates also into the domain of mechanics and furnishes a simple representation of the essential properties of matter; but it is right not to lose sight of the fact that an image may be a well-founded appearance, but may not be capable of being exactly superposed on the objective reality. the conception of the atom of electricity, the foundation of the material atoms, evidently enables us to penetrate further into nature's secrets than our predecessors; but we must not be satisfied with words, and the mystery is not solved when, by a legitimate artifice, the difficulty has simply been thrust further back. we have transferred to an element ever smaller and smaller those physical qualities which in antiquity were attributed to the whole of a substance; and then we shifted them later to those chemical atoms which, united together, constitute this whole. to-day we pass them on to the electrons which compose these atoms. the indivisible is thus rendered, in a way, smaller and smaller, but we are still unacquainted with what its substance may be. the notion of an electric charge which we substitute for that of a material mass will permit phenomena to be united which we thought separate, but it cannot be considered a definite explanation, or as the term at which science must stop. it is probable, however, that for a few years still physics will not travel beyond it. the present hypothesis suffices for grouping known facts, and it will doubtless enable many more to be foreseen, while new successes will further increase its possessions. then the day will arrive when, like all those which have shone before it, this seductive hypothesis will lead to more errors than discoveries. it will, however, have been improved, and it will have become a very vast and very complete edifice which some will not willingly abandon; for those who have made to themselves a comfortable dwelling-place on the ruins of ancient monuments are often too loth to leave it. in that day the searchers who were in the van of the march after truth will be caught up and even passed by others who will have followed a longer, but perhaps surer road. we also have seen at work those prudent physicists who dreaded too daring creeds, and who sought only to collect all the documentary evidence possible, or only took for their guide a few principles which were to them a simple generalisation of facts established by experiments; and we have been able to prove that they also were effecting good and highly useful work. neither the former nor the latter, however, carry out their work in an isolated way, and it should be noted that most of the remarkable results of these last years are due to physicists who have known how to combine their efforts and to direct their activity towards a common object, while perhaps it may not be useless to observe also that progress has been in proportion to the material resources of our laboratories. it is probable that in the future, as in the past, the greatest discoveries, those which will suddenly reveal totally unknown regions, and open up entirely new horizons, will be made by a few scholars of genius who will carry on their patient labour in solitary meditation, and who, in order to verify their boldest conceptions, will no doubt content themselves with the most simple and least costly experimental apparatus. yet for their discoveries to yield their full harvest, for the domain to be systematically worked and desirable results obtained, there will be more and more required the association of willing minds, the solidarity of intelligent scholars, and it will be also necessary for these last to have at their disposal the most delicate as well as the most powerful instruments. these are conditions paramount at the present day for continuous progress in experimental science. if, as has already happened, unfortunately, in the history of science, these conditions are not complied with; if the freedoms of the workers are trammelled, their unity disturbed, and if material facilities are too parsimoniously afforded them,--evolution, at present so rapid, may be retarded, and those retrogressions which, by-the-by, have been known in all evolutions, may occur, although even then hope in the future would not be abolished for ever. there are no limits to progress, and the field of our investigations has no boundaries. evolution will continue with invincible force. what we to-day call the unknowable, will retreat further and further before science, which will never stay her onward march. thus physics will give greater and increasing satisfaction to the mind by furnishing new interpretations of phenomena; but it will accomplish, for the whole of society, more valuable work still, by rendering, by the improvements it suggests, life every day more easy and more agreeable, and by providing mankind with weapons against the hostile forces of nature. by the bibliothèque nationale de france (bnf/gallica) at http://gallica.bnf.fr. experimental researches in electricity by michael faraday, d.c.l. f.r.s. fullerian profesor of chemistry in the royal institution. corresponding member, etc. of the royal and imperial academies of science of paris, petersburgh, florence, copenhagen, berlin, gottingen, modena, stockholm, palermo, etc. etc. in two volumes vol. i. second edition reprinted from the philosophical transactions of - . london: richard and john edward taylor, printers and publishers to the university of london, red lion court, fleet street preface i have been induced by various circumstances to collect in one volume the fourteen series of experimental researches in electricity, which have appeared in the philosophical transactions during the last seven years: the chief reason has been the desire to supply at a moderate price the whole of these papers, with an index, to those who may desire to have them. the readers of the volume will, i hope, do me the justice to remember that it was not written as a _whole_, but in parts; the earlier portions rarely having any known relation at the time to those which might follow. if i had rewritten the work, i perhaps might have considerably varied the form, but should not have altered much of the real matter: it would not, however, then have been considered a faithful reprint or statement of the course and results of the whole investigation, which only i desired to supply. i may be allowed to express my great satisfaction at finding, that the different parts, written at intervals during seven years, harmonize so well as they do. there would have been nothing particular in this, if the parts had related only to matters well-ascertained before any of them were written:--but as each professes to contain something of original discovery, or of correction of received views, it does surprise even my partiality, that they should have the degree of consistency and apparent general accuracy which they seem to me to present. i have made some alterations in the text, but they have been altogether of a typographical or grammatical character; and even where greatest, have been intended to explain the sense, not to alter it. i have often added notes at the bottom of the page, as to paragraphs , , , , , , , , , for the correction of errors, and also the purpose of illustration: but these are all distinguished from the original notes of the researches by the date of _dec. _. the date of a scientific paper containing any pretensions to discovery is frequently a matter of serious importance, and it is a great misfortune that there are many most valuable communications, essential to the history and progress of science, with respect to which this point cannot now be ascertained. this arises from the circumstance of the papers having no dates attached to them individually, and of the journals in which they appear having such as are inaccurate, i.e. dates of a period earlier than that of publication. i may refer to the note at the end of the first series, as an illustration of the kind of confusion thus produced. these circumstances have induced me to affix a date at the top of every other page, and i have thought myself justified in using that placed by the secretary of the royal society on each paper as it was received. an author has no right, perhaps, to claim an earlier one, unless it has received confirmation by some public act or officer. before concluding these lines i would beg leave to make a reference or two; first, to my own papers on electro-magnetic rotations in the quarterly journal of science, . xii. . . . , and also to my letter on magneto-electric induction in the annales de chimie, li. p. . these might, as to the matter, very properly have appeared in this volume, but they would have interfered with it as a simple reprint of the "experimental researches" of the philosophical transactions. then i wish to refer, in relation to the fourth series on a new law of electric conduction, to franklin's experiments on the non-conduction of ice, which have been very properly separated and set forth by professor bache (journal of the franklin institute, . xvii. .). these, which i did not at all remember as to the extent of the effect, though they in no way anticipate the expression of the law i state as to the general effect of liquefaction on electrolytes, still should never be forgotten when speaking of that law as applicable to the case of water. there are two papers which i am anxious to refer to, as corrections or criticisms of parts of the experimental researches. the first of these is one by jacobi (philosophical magazine, . xiii. .), relative to the possible production of a spark on completing the junction of the two metals of a single pair of plates ( .). it is an excellent paper, and though i have not repeated the experiments, the description of them convinces me that i must have been in error. the second is by that excellent philosopher, marianini (memoria della societa italiana di modena, xxi. ), and is a critical and experimental examination of series viii, and of the question whether metallic contact is or is not _productive_ of a part of the electricity of the voltaic pile. i see no reason as yet to alter the opinion i have given; but the paper is so very valuable, comes to the question so directly, and the point itself is of such great importance, that i intend at the first opportunity renewing the inquiry, and, if i can, rendering the proofs either on the one side or the other undeniable to all. other parts of these researches have received the honour of critical attention from various philosophers, to all of whom i am obliged, and some of whose corrections i have acknowledged in the foot notes. there are, no doubt, occasions on which i have not felt the force of the remarks, but time and the progress of science will best settle such cases; and, although i cannot honestly say that i _wish_ to be found in error, yet i do fervently hope that the progress of science in the hands of its many zealous present cultivators will be such, as by giving us new and other developments, and laws more and more general in their applications, will even make me think that what is written and illustrated in these experimental researches, belongs to the by-gone parts of science. michael faraday. royal institution, march, . contents. par. series i. §. . induction of electric currents §. . evolution of electricity from magnetism §. . new electrical state or condition of matter §. . explication of arago's magnetic phenomena series ii. §. . terrestrial magneto-electric induction §. . force and direction of magneto-electric induction generally series iii. §. . identity of electricities from different sources ---- ---- i voltaic electricity ---- ---- ii ordinary electricity ---- ---- iii magneto-electricity ---- ---- iv thermo-electricity ---- ---- v animal electricity §. . relation by measure of common and voltaic electricity ---- note respecting ampère's inductive results after series iv. §. . new law of electric conduction §. . on conducting power generally series v. §. . electro-chemical decomposition ---- ¶ . new conditions of electro-chemical decomposition ---- ¶ . influence of water in such decomposition ---- ¶ . theory of electro-chemical decomposition series vi. §. . power of platina, &c. to induce combination series vii. §. .* electro-chemical decomposition continued (nomenclature) ---- ¶ . some general conditions of electro-chemical decomposition ---- ¶ . volta-electrometer ---- ¶ . primary and secondary results ---- ¶ . definite nature and extent of electro-chemical forces ---- ---- electro-chemical equivalents §. . absolute quantity of electricity in the molecules of matter series viii. §. . electricity of the voltaic pile ---- ¶ . simple voltaic circles ---- ¶ . electrolytic intensity ---- ¶ . associated voltaic circles; or battery ---- ¶ . resistance of an electrolyte to decomposition ---- ¶ . general remarks on the active battery series ix. §. . induction of a current on itself ---- inductive action of currents generally series x. §. . improved voltaic battery §. . practical results with the voltaic battery series xi. §. . on static induction ---- ¶ . induction an action of contiguous particles ---- ¶ . absolute charge of matter ---- ¶ . electrometer and inductive apparatus ---- ¶ . induction in curved lines ---- ---- conduction by glass, lac, sulphur, &c. ---- ¶ . specific inductive capacity ---- ¶ . general results as to the nature of induction ---- ---- differential inductometer series xii. ---- ¶ . conduction or conductive discharge ---- ¶ . electrolytic discharge ---- ¶ . disruptive discharge ---- ---- ---- insulation ---- ---- ---- as spark ---- ---- ---- as brush ---- ---- ---- positive and negative series xiii. ---- ---- ---- as glow ---- ---- ---- dark ---- ¶ . convection; or carrying discharge ---- ¶ . relation of a vacuum to electrical phenomena §. . nature of the electric current ---- ---- its transverse forces series xiv. §. . nature of the electric force or forces §. . relation of the electric and magnetic forces §. . note on electrical excitation index notes experimental researches in electricity. first series. § . _on the induction of electric currents._ § . _on the evolution of electricity from magnetism._ § . _on a new electrical condition of matter._ § . _on_ arago's _magnetic phenomena._ [read november , .] . the power which electricity of tension possesses of causing an opposite electrical state in its vicinity has been expressed by the general term induction; which, as it has been received into scientific language, may also, with propriety, be used in the same general sense to express the power which electrical currents may possess of inducing any particular state upon matter in their immediate neighbourhood, otherwise indifferent. it is with this meaning that i purpose using it in the present paper. . certain effects of the induction of electrical currents have already been recognised and described: as those of magnetization; ampère's experiments of bringing a copper disc near to a flat spiral; his repetition with electro-magnets of arago's extraordinary experiments, and perhaps a few others. still it appeared unlikely that these could be all the effects which induction by currents could produce; especially as, upon dispensing with iron, almost the whole of them disappear, whilst yet an infinity of bodies, exhibiting definite phenomena of induction with electricity of tension, still remain to be acted upon by the induction of electricity in motion. . further: whether ampère's beautiful theory were adopted, or any other, or whatever reservation were mentally made, still it appeared very extraordinary, that as every electric current was accompanied by a corresponding intensity of magnetic action at right angles to the current, good conductors of electricity, when placed within the sphere of this action, should not have any current induced through them, or some sensible effect produced equivalent in force to such a current. . these considerations, with their consequence, the hope of obtaining electricity from ordinary magnetism, have stimulated me at various times to investigate experimentally the inductive effect of electric currents. i lately arrived at positive results; and not only had my hopes fulfilled, but obtained a key which appeared to me to open out a full explanation of arago's magnetic phenomena, and also to discover a new state, which may probably have great influence in some of the most important effects of electric currents. . these results i purpose describing, not as they were obtained, but in such a manner as to give the most concise view of the whole. § . _induction of electric currents._ . about twenty-six feet of copper wire one twentieth of an inch in diameter were wound round a cylinder of wood as a helix, the different spires of which were prevented from touching by a thin interposed twine. this helix was covered with calico, and then a second wire applied in the same manner. in this way twelve helices were superposed, each containing an average length of wire of twenty-seven feet, and all in the same direction. the first, third, fifth, seventh, ninth, and eleventh of these helices were connected at their extremities end to end, so as to form one helix; the others were connected in a similar manner; and thus two principal helices were produced, closely interposed, having the same direction, not touching anywhere, and each containing one hundred and fifty-five feet in length of wire. . one of these helices was connected with a galvanometer, the other with a voltaic battery of ten pairs of plates four inches square, with double coppers and well charged; yet not the slightest sensible reflection of the galvanometer-needle could be observed. . a similar compound helix, consisting of six lengths of copper and six of soft iron wire, was constructed. the resulting iron helix contained two hundred and fourteen feet of wire, the resulting copper helix two hundred and eight feet; but whether the current from the trough was passed through the copper or the iron helix, no effect upon the other could be perceived at the galvanometer. . in these and many similar experiments no difference in action of any kind appeared between iron and other metals. . two hundred and three feet of copper wire in one length were coiled round a large block of wood; other two hundred and three feet of similar wire were interposed as a spiral between the turns of the first coil, and metallic contact everywhere prevented by twine. one of these helices was connected with a galvanometer, and the other with a battery of one hundred pairs of plates four inches square, with double coppers, and well charged. when the contact was made, there was a sudden and very slight effect at the galvanometer, and there was also a similar slight effect when the contact with the battery was broken. but whilst the voltaic current was continuing to pass through the one helix, no galvanometrical appearances nor any effect like induction upon the other helix could be perceived, although the active power of the battery was proved to be great, by its heating the whole of its own helix, and by the brilliancy of the discharge when made through charcoal. . repetition of the experiments with a battery of one hundred and twenty pairs of plates produced no other effects; but it was ascertained, both at this and the former time, that the slight deflection of the needle occurring at the moment of completing the connexion, was always in one direction, and that the equally slight deflection produced when the contact was broken, was in the other direction; and also, that these effects occurred when the first helices were used ( . .). . the results which i had by this time obtained with magnets led me to believe that the battery current through one wire, did, in reality, induce a similar current through the other wire, but that it continued for an instant only, and partook more of the nature of the electrical wave passed through from the shock of a common leyden jar than of the current from a voltaic battery, and therefore might magnetise a steel needle, although it scarcely affected the galvanometer. . this expectation was confirmed; for on substituting a small hollow helix, formed round a glass tube, for the galvanometer, introducing a steel needle, making contact as before between the battery and the inducing wire ( . .), and then removing the needle before the battery contact was broken, it was found magnetised. . when the battery contact was first made, then an unmagnetised needle introduced into the small indicating helix ( .), and lastly the battery contact broken, the needle was found magnetised to an equal degree apparently as before; but the poles were of the contrary kind. . the same effects took place on using the large compound helices first described ( . .). . when the unmagnetised needle was put into the indicating helix, before contact of the inducing wire with the battery, and remained there until the contact was broken, it exhibited little or no magnetism; the first effect having been nearly neutralised by the second ( . .). the force of the induced current upon making contact was found always to exceed that of the induced current at breaking of contact; and if therefore the contact was made and broken many times in succession, whilst the needle remained in the indicating helix, it at last came out not unmagnetised, but a needle magnetised as if the induced current upon making contact had acted alone on it. this effect may be due to the accumulation (as it is called) at the poles of the unconnected pile, rendering the current upon first making contact more powerful than what it is afterwards, at the moment of breaking contact. . if the circuit between the helix or wire under induction and the galvanometer or indicating spiral was not rendered complete _before_ the connexion between the battery and the inducing wire was completed or broken, then no effects were perceived at the galvanometer. thus, if the battery communications were first made, and then the wire under induction connected with the indicating helix, no magnetising power was there exhibited. but still retaining the latter communications, when those with the battery were broken, a magnet was formed in the helix, but of the second kind ( .), i.e. with poles indicating a current in the same direction to that belonging to the battery current, or to that always induced by that current at its cessation. . in the preceding experiments the wires were placed near to each other, and the contact of the inducing one with the buttery made when the inductive effect was required; but as the particular action might be supposed to be exerted only at the moments of making and breaking contact, the induction was produced in another way. several feet of copper wire were stretched in wide zigzag forms, representing the letter w, on one surface of a broad board; a second wire was stretched in precisely similar forms on a second board, so that when brought near the first, the wires should everywhere touch, except that a sheet of thick paper was interposed. one of these wires was connected with the galvanometer, and the other with a voltaic battery. the first wire was then moved towards the second, and as it approached, the needle was deflected. being then removed, the needle was deflected in the opposite direction. by first making the wires approach and then recede, simultaneously with the vibrations of the needle, the latter soon became very extensive; but when the wires ceased to move from or towards each other, the galvanometer-needle soon came to its usual position. . as the wires approximated, the induced current was in the _contrary_ direction to the inducing current. as the wires receded, the induced current was in the _same_ direction as the inducing current. when the wires remained stationary, there was no induced current ( .). . when a small voltaic arrangement was introduced into the circuit between the galvanometer ( .) and its helix or wire, so as to cause a permanent deflection of ° or °, and then the battery of one hundred pairs of plates connected with the inducing wire, there was an instantaneous action as before ( .); but the galvanometer-needle immediately resumed and retained its place unaltered, notwithstanding the continued contact of the inducing wire with the trough: such was the case in whichever way the contacts were made ( .). . hence it would appear that collateral currents, either in the same or in opposite directions, exert no permanent inducing power on each other, affecting their quantity or tension. . i could obtain no evidence by the tongue, by spark, or by heating fine wire or charcoal, of the electricity passing through the wire under induction; neither could i obtain any chemical effects, though the contacts with metallic and other solutions were made and broken alternately with those of the battery, so that the second effect of induction should not oppose or neutralise the first ( . .). . this deficiency of effect is not because the induced current of electricity cannot pass fluids, but probably because of its brief duration and feeble intensity; for on introducing two large copper plates into the circuit on the induced side ( .), the plates being immersed in brine, but prevented from touching each other by an interposed cloth, the effect at the indicating galvanometer, or helix, occurred as before. the induced electricity could also pass through a voltaic trough ( .). when, however, the quantity of interposed fluid was reduced to a drop, the galvanometer gave no indication. . attempts to obtain similar effects by the use of wires conveying ordinary electricity were doubtful in the results. a compound helix similar to that already described, containing eight elementary helices ( .), was used. four of the helices had their similar ends bound together by wire, and the two general terminations thus produced connected with the small magnetising helix containing an unmagnetised needle ( .). the other four helices were similarly arranged, but their ends connected with a leyden jar. on passing the discharge, the needle was found to be a magnet; but it appeared probable that a part of the electricity of the jar had passed off to the small helix, and so magnetised the needle. there was indeed no reason to expect that the electricity of a jar possessing as it does great tension, would not diffuse itself through all the metallic matter interposed between the coatings. . still it does not follow that the discharge of ordinary electricity through a wire does not produce analogous phenomena to those arising from voltaic electricity; but as it appears impossible to separate the effects produced at the moment when the discharge begins to pass, from the equal and contrary effects produced when it ceases to pass ( .), inasmuch as with ordinary electricity these periods are simultaneous, so there can be scarcely any hope that in this form of the experiment they can be perceived. . hence it is evident that currents of voltaic electricity present phenomena of induction somewhat analogous to those produced by electricity of tension, although, as will be seen hereafter, many differences exist between them. the result is the production of other currents, (but which are only momentary,) parallel, or tending to parallelism, with the inducing current. by reference to the poles of the needle formed in the indicating helix ( . .) and to the deflections of the galvanometer-needle ( .), it was found in all cases that the induced current, produced by the first action of the inducing current, was in the contrary direction to the latter, but that the current produced by the cessation of the inducing current was in the same direction ( .). for the purpose of avoiding periphrasis, i propose to call this action of the current from the voltaic battery, _volta-electric induction_. the properties of the second wire, after induction has developed the first current, and whilst the electricity from the battery continues to flow through its inducing neighbour ( . .), constitute a peculiar electric condition, the consideration of which will be resumed hereafter ( .). all these results have been obtained with a voltaic apparatus consisting of a single pair of plates. § . _evolution of electricity from magnetism._ . a welded ring was made of soft round bar-iron, the metal being seven-eighths of an inch in thickness, and the ring six inches in external diameter. three helices were put round one part of this ring, each containing about twenty-four feet of copper wire one twentieth of an inch thick; they were insulated from the iron and each other, and superposed in the manner before described ( .), occupying about nine inches in length upon the ring. they could be used separately or conjointly; the group may be distinguished by the letter a (pl. i. fig. .). on the other part of the ring about sixty feet of similar copper wire in two pieces were applied in the same manner, forming a helix b, which had the same common direction with the helices of a, but being separated from it at each extremity by about half an inch of the uncovered iron. . the helix b was connected by copper wires with a galvanometer three feet from the ring. the helices of a were connected end to end so as to form one common helix, the extremities of which were connected with a battery of ten pairs of plates four inches square. the galvanometer was immediately affected, and to a degree far beyond what has been described when with a battery of tenfold power helices _without iron_ were used ( .); but though the contact was continued, the effect was not permanent, for the needle soon came to rest in its natural position, as if quite indifferent to the attached electro-magnetic arrangement. upon breaking the contact with the batterry, the needle was again powerfully deflected, but in the contrary direction to that induced in the first instance. . upon arranging the apparatus so that b should be out of use, the galvanometer be connected with one of the three wires of a ( .), and the other two made into a helix through which the current from the trough ( .) was passed, similar but rather more powerful effects were produced. . when the battery contact was made in one direction, the galvanometer-needle was deflected on the one side; if made in the other direction, the deflection was on the other side. the deflection on breaking the battery contact was always the reverse of that produced by completing it. the deflection on making a battery contact always indicated an induced current in the opposite direction to that from the battery; but on breaking the contact the deflection indicated an induced current in the same direction as that of the battery. no making or breaking of the contact at b side, or in any part of the galvanometer circuit, produced any effect at the galvanometer. no continuance of the battery current caused any deflection of the galvanometer-needle. as the above results are common to all these experiments, and to similar ones with ordinary magnets to be hereafter detailed, they need not be again particularly described. . upon using the power of one hundred pairs of plates ( .) with this ring, the impulse at the galvanometer, when contact was completed or broken, was so great as to make the needle spin round rapidly four or five times, before the air and terrestrial magnetism could reduce its motion to mere oscillation. . by using charcoal at the ends of the b helix, a minute _spark_ could be perceived when the contact of the battery with a was completed. this spark could not be due to any diversion of a part of the current of the battery through the iron to the helix b; for when the battery contact was continued, the galvanometer still resumed its perfectly indifferent state ( .). the spark was rarely seen on breaking contact. a small platina wire could not be ignited by this induced current; but there seems every reason to believe that the effect would be obtained by using a stronger original current or a more powerful arrangement of helices. . a feeble voltaic current was sent through the helix b and the galvanometer, so as to deflect the needle of the latter ° or °, and then the battery of one hundred pairs of plates connected with a; but after the first effect was over, the galvanometer-needle resumed exactly the position due to the feeble current transmitted by its own wire. this took place in whichever way the battery contacts were made, and shows that here again ( .) no permanent influence of the currents upon each other, as to their quantity and tension, exists. . another arrangement was then employed connecting the former experiments on volta-electric induction ( - .) with the present. a combination of helices like that already described ( .) was constructed upon a hollow cylinder of pasteboard: there were eight lengths of copper wire, containing altogether feet; four of these helices were connected end to end, and then with the galvanometer ( .); the other intervening four were also connected end to end, and the battery of one hundred pairs discharged through them. in this form the effect on the galvanometer was hardly sensible ( .), though magnets could be made by the induced current ( .). but when a soft iron cylinder seven eighths of an inch thick, and twelve inches long, was introduced into the pasteboard tube, surrounded by the helices, then the induced current affected the galvanometer powerfully and with all the phenomena just described ( .). it possessed also the power of making magnets with more energy, apparently, than when no iron cylinder was present. . when the iron cylinder was replaced by an equal cylinder of copper, no effect beyond that of the helices alone was produced. the iron cylinder arrangement was not so powerful as the ring arrangement already described ( .). . similar effects were then produced by _ordinary magnets_: thus the hollow helix just described ( .) had all its elementary helices connected with the galvanometer by two copper wires, each five feet in length; the soft iron cylinder was introduced into its axis; a couple of bar magnets, each twenty-four inches long, were arranged with their opposite poles at one end in contact, so as to resemble a horse-shoe magnet, and then contact made between the other poles and the ends of the iron cylinder, so as to convert it for the time into a magnet (fig. .): by breaking the magnetic contacts, or reversing them, the magnetism of the iron cylinder could be destroyed or reversed at pleasure. . upon making magnetic contact, the needle was deflected; continuing the contact, the needle became indifferent, and resumed its first position; on breaking the contact, it was again deflected, but in the opposite direction to the first effect, and then it again became indifferent. when the magnetic contacts were reversed the deflections were reversed. . when the magnetic contact was made, the deflection was such as to indicate an induced current of electricity in the opposite direction to that fitted to form a magnet, having the same polarity as that really produced by contact with the bar magnets. thus when the marked and unmarked poles were placed as in fig. , the current in the helix was in the direction represented, p being supposed to be the end of the wire going to the positive pole of the battery, or that end towards which the zinc plates face, and n the negative wire. such a current would have converted the cylinder into a magnet of the opposite kind to that formed by contact with the poles a and b; and such a current moves in the opposite direction to the currents which in m. ampère's beautiful theory are considered as constituting a magnet in the position figured[a]. [a] the relative position of an electric current and a magnet is by most persons found very difficult to remember, and three or four helps to the memory have been devised by m. ampère and others. i venture to suggest the following as a very simple and effectual assistance in these and similar latitudes. let the experimenter think he is looking down upon a dipping needle, or upon the pole of the north, and then let him think upon the direction of the motion of the hands of a watch, or of a screw moving direct; currents in that direction round a needle would make it into such a magnet as the dipping needle, or would themselves constitute an electro-magnet of similar qualities; or if brought near a magnet would tend to make it take that direction; or would themselves be moved into that position by a magnet so placed; or in m. ampère's theory are considered as moving in that direction in the magnet. these two points of the position of the dipping-needle and the motion of the watch hands being remembered, any other relation of the current and magnet can be at once deduced from it. . but as it might be supposed that in all the preceding experiments of this section, it was by some peculiar effect taking place during the formation of the magnet, and not by its mere virtual approximation, that the momentary induced current was excited, the following experiment was made. all the similar ends of the compound hollow helix ( .) were bound together by copper wire, forming two general terminations, and these were connected with the galvanometer. the soft iron cylinder ( .) was removed, and a cylindrical magnet, three quarters of an inch in diameter and eight inches and a half in length, used instead. one end of this magnet was introduced into the axis of the helix (fig. .), and then, the galvanometer-needle being stationary, the magnet was suddenly thrust in; immediately the needle was deflected in the same direction as if the magnet had been formed by either of the two preceding processes ( . .). being left in, the needle resumed its first position, and then the magnet being withdrawn the needle was deflected in the opposite direction. these effects were not great; but by introducing and withdrawing the magnet, so that the impulse each time should be added to those previously communicated to the needle, the latter could be made to vibrate through an arc of ° or more. . in this experiment the magnet must not be passed entirely through the helix, for then a second action occurs. when the magnet is introduced, the needle at the galvanometer is deflected in a certain direction; but being in, whether it be pushed quite through or withdrawn, the needle is deflected in a direction the reverse of that previously produced. when the magnet is passed in and through at one continuous motion, the needle moves one way, is then suddenly stopped, and finally moves the other way. . if such a hollow helix as that described ( .) be laid east and west (or in any other constant position), and a magnet be retained east and west, its marked pole always being one way; then whichever end of the helix the magnet goes in at, and consequently whichever pole of the magnet enters first, still the needle is deflected the same way: on the other hand, whichever direction is followed in withdrawing the magnet, the deflection is constant, but contrary to that due to its entrance. . these effects are simple consequences of the _law_ hereafter to be described ( ). . when the eight elementary helices were made one long helix, the effect was not so great as in the arrangement described. when only one of the eight helices was used, the effect was also much diminished. all care was taken to guard against tiny direct action of the inducing magnet upon the galvanometer, and it was found that by moving the magnet in the same direction, and to the same degree on the outside of the helix, no effect on the needle was produced. . the royal society are in possession of a large compound magnet formerly belonging to dr. gowin knight, which, by permission of the president and council, i was allowed to use in the prosecution of these experiments: it is at present in the charge of mr. christie, at his house at woolwich, where, by mr. christie's kindness, i was at liberty to work; and i have to acknowledge my obligations to him for his assistance in all the experiments and observations made with it. this magnet is composed of about bar magnets, each fifteen inches long, one inch wide, and half an inch thick, arranged in a box so as to present at one of its extremities two external poles (fig. .). these poles projected horizontally six inches from the box, were each twelve inches high and three inches wide. they were nine inches apart; and when a soft iron cylinder, three quarters of an inch in diameter and twelve inches long, was put across from one to the other, it required a force of nearly one hundred pounds to break the contact. the pole to the left in the figure is the marked pole[a]. [a] to avoid any confusion as to the poles of the magnet, i shall designate the pole pointing to the north as the marked pole; i may occasionally speak of the north and south ends of the needle, but do not mean thereby north and south poles. that is by many considered the true north pole of a needle which points to the south; but in this country it in often called the south pole. . the indicating galvanometer, in all experiments made with this magnet, was about eight feet from it, not directly in front of the poles, but about ° or ° on one side. it was found that on making or breaking the connexion of the poles by soft iron, the instrument was slightly affected; but all error of observation arising from this cause was easily and carefully avoided. . the electrical effects exhibited by this magnet were very striking. when a soft iron cylinder thirteen inches long was put through the compound hollow helix, with its ends arranged as two general terminations ( .), these connected with the galvanometer, and the iron cylinder brought in contact with the two poles of the magnet (fig. .), so powerful a rush of electricity took place that the needle whirled round many times in succession[a]. [a] a soft iron bar in the form of a lifter to a horse-shoe magnet, when supplied with a coil of this kind round the middle of it, becomes, by juxta-position with a magnet, a ready source of a brief but determinate current of electricity. . notwithstanding this great power, if the contact was continued, the needle resumed its natural position, being entirely uninfluenced by the position of the helix ( .). but on breaking the magnetic contact, the needle was whirled round in the opposite direction with a force equal to the former. . a piece of copper plate wrapped _once_ round the iron cylinder like a socket, but with interposed paper to prevent contact, had its edges connected with the wires of the galvanometer. when the iron was brought in contact with the poles the galvanometer was strongly affected. . dismissing the helices and sockets, the galvanometer wire was passed over, and consequently only half round the iron cylinder (fig. .); but even then a strong effect upon the needle was exhibited, when the magnetic contact was made or broken. . as the helix with its iron cylinder was brought towards the magnetic poles, but _without making contact_, still powerful effects were produced. when the helix, without the iron cylinder, and consequently containing no metal but copper, was approached to, or placed between the poles ( .), the needle was thrown °, °, or more, from its natural position. the inductive force was of course greater, the nearer the helix, either with or without its iron cylinder, was brought to the poles; but otherwise the same effects were produced, whether the helix, &c. was or was not brought into contact with the magnet; i.e. no permanent effect on the galvanometer was produced; and the effects of approximation and removal were the reverse of each other ( .). . when a bolt of copper corresponding to the iron cylinder was introduced, no greater effect was produced by the helix than without it. but when a thick iron wire was substituted, the magneto-electric induction was rendered sensibly greater. . the direction of the electric current produced in all these experiments with the helix, was the same as that already described ( .) as obtained with the weaker bar magnets. . a spiral containing fourteen feet of copper wire, being connected with the galvanometer, and approximated directly towards the marked pole in the line of its axis, affected the instrument strongly; the current induced in it was in the reverse direction to the current theoretically considered by m. ampère as existing in the magnet ( .), or as the current in an electro-magnet of similar polarity. as the spiral was withdrawn, the induced current was reversed. . a similar spiral had the current of eighty pairs of -inch plates sent through it so as to form an electro-magnet, and then the other spiral connected with the galvanometer ( .) approximated to it; the needle vibrated, indicating a current in the galvanometer spiral the reverse of that in the battery spiral ( . .). on withdrawing the latter spiral, the needle passed in the opposite direction. . single wires, approximated in certain directions towards the magnetic pole, had currents induced in them. on their removal, the currents were inverted. in such experiments the wires should not be removed in directions different to those in which they were approximated; for then occasionally complicated and irregular effects are produced, the causes of which will be very evident in the fourth part of this paper. . all attempts to obtain chemical effects by the induced current of electricity failed, though the precautions before described ( .), and all others that could be thought of, were employed. neither was any sensation on the tongue, or any convulsive effect upon the limbs of a frog, produced. nor could charcoal or fine wire be ignited ( .). but upon repeating the experiments more at leisure at the royal institution, with an armed loadstone belonging to professor daniell and capable of lifting about thirty pounds, a frog was very _powerfully convulsed_ each time magnetic contact was made. at first the convulsions could not be obtained on breaking magnetic contact; but conceiving the deficiency of effect was because of the comparative slowness of separation, the latter act was effected by a blow, and then the frog was convulsed strongly. the more instantaneous the union or disunion is effected, the more powerful the convulsion. i thought also i could perceive the _sensation_ upon the tongue and the _flash_ before the eyes; but i could obtain no evidence of chemical decomposition. . the various experiments of this section prove, i think, most completely the production of electricity from ordinary magnetism. that its intensity should be very feeble and quantity small, cannot be considered wonderful, when it is remembered that like thermo-electricity it is evolved entirely within the substance of metals retaining all their conducting power. but an agent which is conducted along metallic wires in the manner described; which whilst so passing possesses the peculiar magnetic actions and force of a current of electricity; which can agitate and convulse the limbs of a frog; and which, finally, can produce a spark[a] by its discharge through charcoal ( .), can only be electricity. as all the effects can be produced by ferruginous electro-magnets ( .), there is no doubt that arrangements like the magnets of professors moll, henry, ten eyke, and others, in which as many as two thousand pounds have been lifted, may be used for these experiments; in which case not only a brighter spark may be obtained, but wires also ignited, and, as the current can pass liquids ( .), chemical action be produced. these effects are still more likely to be obtained when the magneto-electric arrangements to be explained in the fourth section are excited by the powers of such apparatus. [a] for a mode of obtaining the spark from the common magnet which i have found effectual, see the philosophical magazine for june , p. . in the same journal for november , vol. v. p. , will be found a method of obtaining the magneto-electric spark, still simpler in its principle, the use of soft iron being dispensed with altogether.--_dec. ._ . the similarity of action, almost amounting to identity, between common magnets and either electro-magnets or volta-electric currents, is strikingly in accordance with and confirmatory of m. ampère's theory, and furnishes powerful reasons for believing that the action is the same in both cases; but, as a distinction in language is still necessary, i propose to call the agency thus exerted by ordinary magnets, _magneto-electric_ or _magnelectric_ induction ( ). . the only difference which powerfully strikes the attention as existing between volta-electric and magneto-electric induction, is the suddenness of the former, and the sensible time required by the latter; but even in this early state of investigation there are circumstances which seem to indicate, that upon further inquiry this difference will, as a philosophical distinction, disappear ( ).[a] [a] for important additional phenomena and developments of the induction of electrical currents, see now the ninth series, - .--_dec. ._ § . _new electrical state or condition of matter._[a] [a] this section having been read at the royal society and reported upon, and having also, in consequence of a letter from myself to m. hachette, been noticed at the french institute, i feel bound to let it stand as part of the paper; but later investigations (intimated . . .) of the laws governing those phenomena, induce me to think that the latter can be fully explained without admitting the electro-tonic state. my views on this point will appear in the second series of these researches.--m.f. . whilst the wire is subject to either volta-electric or magneto-electric induction, it appears to be in a peculiar state; for it resists the formation of an electrical current in it, whereas, if in its common condition, such a current would be produced; and when left uninfluenced it has the power of originating a current, a power which the wire does not possess under common circumstances. this electrical condition of matter has not hitherto been recognised, but it probably exerts a very important influence in many if not most of the phenomena produced by currents of electricity. for reasons which will immediately appear ( .), i have, after advising with several learned friends, ventured to designate it as the _electro-ionic_ state. . this peculiar condition shows no known electrical effects whilst it continues; nor have i yet been able to discover any peculiar powers exerted, or properties possessed, by matter whilst retained in this state. . it shows no reaction by attractive or repulsive powers. the various experiments which have been made with powerful magnets upon such metals, as copper, silver, and generally those substances not magnetic, prove this point; for the substances experimented upon, if electrical conductors, must have acquired this state; and yet no evidence of attractive or repulsive powers has been observed. i have placed copper and silver discs, very delicately suspended on torsion balances in vacuo near to the poles of very powerful magnets, yet have not been able to observe the least attractive or repulsive force. . i have also arranged a fine slip of gold-leaf very near to a bar of copper, the two being in metallic contact by mercury at their extremities. these have been placed in vacuo, so that metal rods connected with the extremities of the arrangement should pass through the sides of the vessel into the air. i have then moved powerful magnetic poles, about this arrangement, in various directions, the metallic circuit on the outside being sometimes completed by wires, and sometimes broken. but i never could obtain any sensible motion of the gold-leaf, either directed to the magnet or towards the collateral bar of copper, which must have been, as far as induction was concerned, in a similar state to itself. . in some cases it has been supposed that, under such circumstances, attractive and repulsive forces have been exhibited, i.e. that such bodies have become slightly magnetic. but the phenomena now described, in conjunction with the confidence we may reasonably repose in m. ampère's theory of magnetism, tend to throw doubt on such cases; for if magnetism depend upon the attraction of electrical currents, and if the powerful currents at first excited, both by volta-electric and magneto-electric induction, instantly and naturally cease ( . . .), causing at the same time an entire cessation of magnetic effects at the galvanometer needle, then there can be little or no expectation that any substances not partaking of the peculiar relation in which iron, nickel, and one or two other bodies, stand, should exhibit magneto-attractive powers. it seems far more probable, that the extremely feeble permanent effects observed have been due to traces of iron, or perhaps some other unrecognised cause not magnetic. . this peculiar condition exerts no retarding or accelerating power upon electrical currents passing through metal thus circumstanced ( . .). neither could any such power upon the inducing current itself be detected; for when masses of metal, wires, helices, &c. were arranged in all possible ways by the side of a wire or helix, carrying a current measured by the galvanometer ( .), not the slightest permanent change in the indication of the instrument could be perceived. metal in the supposed peculiar state, therefore, conducts electricity in all directions with its ordinary facility, or, in other words, its conducting power is not sensibly altered by it. . all metals take on the peculiar state. this is proved in the preceding experiments with copper and iron ( .), and with gold, silver, tin, lead, zinc, antimony, bismuth, mercury, &c. by experiments to be described in the fourth part ( .), admitting of easy application. with regard to iron, the experiments prove the thorough and remarkable independence of these phenomena of induction, and the ordinary magnetical appearances of that metal. . this state is altogether the effect of the induction exerted, and ceases as soon as the inductive force is removed. it is the same state, whether produced by the collateral passage of voltaic currents ( .), or the formation of a magnet ( . .), or the mere approximation of a magnet ( . .); and is a strong proof in addition to those advanced by m. ampère, of the identity of the agents concerned in these several operations. it probably occurs, momentarily, during the passage of the common electric spark ( .), and may perhaps be obtained hereafter in bad conductors by weak electrical currents or other means ( . ). . the state appears to be instantly assumed ( .), requiring hardly a sensible portion of time for that purpose. the _difference_ of time between volta-electric and magneto-electric induction, rendered evident by the galvanometer ( .), may probably be thus explained. when a voltaic current is sent through one of two parallel wires, as those of the hollow helix ( .), a current is produced in the other wire, as brief in its continuance as the time required for a single action of this kind, and which, by experiment, is found to be inappreciably small. the action will seem still more instantaneous, because, as there is an accumulation of power in the poles of the battery before contact, the first rush of electricity in the wire of communication is greater than that sustained after the contact is completed; the wire of induction becomes at the moment electro-tonic to an equivalent degree, which the moment after sinks to the state in which the continuous current can sustain it, but in sinking, causes an opposite induced current to that at first produced. the consequence is, that the first induced wave of electricity more resembles that from the discharge of an electric jar, than it otherwise would do. . but when the iron cylinder is put into the same helix ( .), previous to the connexion being made with the battery, then the current from the latter may be considered as active in inducing innumerable currents of a similar kind to itself in the iron, rendering it a magnet. this is known by experiment to occupy time; for a magnet so formed, even of soft iron, does not rise to its fullest intensity in an instant, and it may be because the currents within the iron are successive in their formation or arrangement. but as the magnet can induce, as well as the battery current, the combined action of the two continues to evolve induced electricity, until their joint effect is at a maximum, and thus the existence of the deflecting force is prolonged sufficiently to overcome the inertia of the galvanometer needle. . in all those cases where the helices or wires are advanced towards or taken from the magnet ( . .), the direct or inverted current of induced electricity continues for the time occupied in the advance or recession; for the electro-tonic state is rising to a higher or falling to a lower degree during that time, and the change is accompanied by its corresponding evolution of electricity; but these form no objections to the opinion that the electro-tonic state is instantly assumed. . this peculiar state appears to be a state of tension, and may be considered as _equivalent_ to a current of electricity, at least equal to that produced either when the condition is induced or destroyed. the current evolved, however, first or last, is not to be considered a measure of the degree of tension to which the electro-tonic state has risen; for as the metal retains its conducting powers unimpaired ( .), and as the electricity evolved is but for a moment, (the peculiar state being instantly assumed and lost ( .),) the electricity which may be led away by long wire conductors, offering obstruction in their substance proportionate to their small lateral and extensive linear dimensions, can be but a very small portion of that really evolved within the mass at the moment it assumes this condition. insulated helices and portions of metal instantly assumed the state; and no traces of electricity could be discovered in them, however quickly the contact with the electrometer was made, after they were put under induction, either by the current from the battery or the magnet. a single drop of water or a small piece of moistened paper ( . .) was obstacle sufficient to stop the current through the conductors, the electricity evolved returning to a state of equilibrium through the metal itself, and consequently in an unobserved manner. . the tension of this state may therefore be comparatively very great. but whether great or small, it is hardly conceivable that it should exist without exerting a reaction upon the original inducing current, and producing equilibrium of some kind. it might be anticipated that this would give rise to a retardation of the original current; but i have not been able to ascertain that this is the case. neither have i in any other way as yet been able to distinguish effects attributable to such a reaction. . all the results favour the notion that the electro-tonic state relates to the particles, and not to the mass, of the wire or substance under induction, being in that respect different to the induction exerted by electricity of tension. if so, the state may be assumed in liquids when no electrical current is sensible, and even in non-conductors; the current itself, when it occurs, being as it were a contingency due to the existence of conducting power, and the momentary propulsive force exerted by the particles during their arrangement. even when conducting power is equal, the currents of electricity, which as yet are the only indicators of this state, may be unequal, because of differences as to numbers, size, electrical condition, &c. &c. in the particles themselves. it will only be after the laws which govern this new state are ascertained, that we shall be able to predict what is the true condition of, and what are the electrical results obtainable from, any particular substance. . the current of electricity which induces the electro-tonic state in a neighbouring wire, probably induces that state also in its own wire; for when by a current in one wire a collateral wire is made electro-tonic, the latter state is not rendered any way incompatible or interfering with a current of electricity passing through it ( .). if, therefore, the current were sent through the second wire instead of the first, it does not seem probable that its inducing action upon the second would be less, but on the contrary more, because the distance between the agent and the matter acted upon would be very greatly diminished. a copper bolt had its extremities connected with a galvanometer, and then the poles of a battery of one hundred pairs of plates connected with the bolt, so as to send the current through it; the voltaic circuit was then suddenly broken, and the galvanometer observed for any indications of a return current through the copper bolt due to the discharge of its supposed electro-tonic state. no effect of the kind was obtained, nor indeed, for two reasons, ought it to be expected; for first, as the cessation of induction and the discharge of the electro-tonic condition are simultaneous, and not successive, the return current would only be equivalent to the neutralization of the last portion of the inducing current, and would not therefore show any alteration of direction; or assuming that time did intervene, and that the latter current was really distinct from the former, its short, sudden character ( . .) would prevent it from being thus recognised. . no difficulty arises, i think, in considering the wire thus rendered electro-tonic by its own current more than by any external current, especially when the apparent non-interference of that state with currents is considered ( . .). the simultaneous existence of the conducting and electro-tonic states finds an analogy in the manner in which electrical currents can be passed through magnets, where it is found that both the currents passed, and those of the magnets, preserve all their properties distinct from each other, and exert their mutual actions. . the reason given with regard to metals extends also to fluids and all other conductors, and leads to the conclusion that when electric currents are passed through them they also assume the electro-tonic state. should that prove to be the case, its influence in voltaic decomposition, and the transference of the elements to the poles, can hardly be doubted. in the electro-tonic state the homogeneous particles of matter appear to have assumed a regular but forced electrical arrangement in the direction of the current, which if the matter be undecomposable, produces, when relieved, a return current; but in decomposable matter this forced state may be sufficient to make an elementary particle leave its companion, with which it is in a constrained condition, and associate with the neighbouring similar particle, in relation to which it is in a more natural condition, the forced electrical arrangement being itself discharged or relieved, at the same time, as effectually as if it had been freed from induction. but as the original voltaic current is continued, the electro-tonic state may be instantly renewed, producing the forced arrangement of the compound particles, to be as instantly discharged by a transference of the elementary particles of the opposite kind in opposite directions, but parallel to the current. even the differences between common and voltaic electricity, when applied to effect chemical decomposition, which dr. wollaston has pointed out[a], seem explicable by the circumstances connected with the induction of electricity from these two sources ( .). but as i have reserved this branch of the inquiry, that i might follow out the investigations contained in the present paper, i refrain (though much tempted) from offering further speculations. [a] philosophical transactions, , p. . . marianini has discovered and described a peculiar affection of the surfaces of metallic discs, when, being in contact with humid conductors, a current of electricity is passed through them; they are then capable of producing a reverse current of electricity, and marianini has well applied the effect in explanation of the phenomena of ritter's piles[a]. m.a. de la rive has described a peculiar property acquired by metallic conductors, when being immersed in a liquid as poles, they have completed, for some time, the voltaic circuit, in consequence of which, when separated from the battery and plunged into the same fluid, they by themselves produce an electric current[b]. m.a. van beek has detailed cases in which the electrical relation of one metal in contact with another has been preserved after separation, and accompanied by its corresponding chemical effects[c]. these states and results appear to differ from the electro-tonic state and its phenomena; but the true relation of the former to the latter can only be decided when our knowledge of all these phenomena has been enlarged. [a] annales de chimie, xxxviii. . [b] ibid. xxviii. . [c] ibid. xxxviii. . . i had occasion in the commencement of this paper ( .) to refer to an experiment by ampère, as one of those dependent upon the electrical induction of currents made prior to the present investigation, and have arrived at conclusions which seem to imply doubts of the accuracy of the experiment ( . &c.); it is therefore due to m. ampère that i should attend to it more distinctly. when a disc of copper (says m. ampère) was suspended by a silk thread and surrounded by a helix or spiral, and when the charge of a powerful voltaic battery was sent through the spiral, a strong magnet at the same time being presented to the copper disc, the latter turned at the moment to take a position of equilibrium, exactly as the spiral itself would have turned had it been free to move. i have not been able to obtain this effect, nor indeed any motion; but the cause of my failure in the _latter_ point may be due to the momentary existence of the current not allowing time for the inertia of the plate to be overcome ( . .). m. ampère has perhaps succeeded in obtaining motion from the superior delicacy and power of his electro-magnetical apparatus, or he may have obtained only the motion due to cessation of action. but all my results tend to invert the sense of the proposition stated by m. ampère, "that a current of electricity tends to put the electricity of conductors near which it passes in motion in the same direction," for they indicate an opposite direction for the produced current ( . .); and they show that the effect is momentary, and that it is also produced by magnetic induction, and that certain other extraordinary effects follow thereupon. . the momentary existence of the phenomena of induction now described is sufficient to furnish abundant reasons for the uncertainty or failure of the experiments, hitherto made to obtain electricity from magnets, or to effect chemical decomposition or arrangement by their means[a]. [a] the lycée, no. , for january st, has a long and rather premature article, in which it endeavours to show anticipations by french philosophers of my researches. it however mistakes the erroneous results of mm. fresnel and ampère for true ones, and then imagines my true results are like those erroneous ones. i notice it here, however, for the purpose of doing honour to fresnel in a much higher degree than would have been merited by a feeble anticipation of the present investigations. that great philosopher, at the same time with myself and fifty other persons, made experiments which the present paper proves could give no expected result. he was deceived for the moment, and published his imaginary success; but on more carefully repeating his trials, he could find no proof of their accuracy; and, in the high and pure philosophic desire to remove error as well as discover truth, he recanted his first statement. the example of berzelius regarding the first thorina is another instance of this fine feeling; and as occasions are not rare, it would be to the dignity of science if such examples were more frequently followed.--february th, . . it also appears capable of explaining fully the remarkable phenomena observed by m. arago between metals and magnets when neither are moving ( .), as well as most of the results obtained by sir john herschel, messrs. babbage, harris, and others, in repeating his experiments; accounting at the same time perfectly for what at first appeared inexplicable; namely, the non-action of the same metals and magnets when at rest. these results, which also afford the readiest means of obtaining electricity from magnetism, i shall now proceed to describe. § . _explication of arago's magnetic phenomena._ . if a plate of copper be revolved close to a magnetic needle, or magnet, suspended in such a way that the latter may rotate in a plane parallel to that of the former, the magnet tends to follow the motion of the plate; or if the magnet be revolved, the plate tends to follow its motion; and the effect is so powerful, that magnets or plates of many pounds weight may be thus carried round. if the magnet and plate be at rest relative to each other, not the slightest effect, attractive or repulsive, or of any kind, can be observed between them ( .). this is the phenomenon discovered by m. arago; and he states that the effect takes place not only with all metals, but with solids, liquids, and even gases, i.e. with all substances ( .). . mr. babbage and sir john herschel, on conjointly repeating the experiments in this country[a], could obtain the effects only with the metals, and with carbon in a peculiar state (from gas retorts), i.e. only with excellent conductors of electricity. they refer the effect to magnetism induced in the plate by the magnet; the pole of the latter causing an opposite pole in the nearest part of the plate, and round this a more diffuse polarity of its own kind ( .). the essential circumstance in producing the rotation of the suspended magnet is, that the substance revolving below it shall acquire and lose its magnetism in sensible time, and not instantly ( .). this theory refers the effect to an attractive force, and is not agreed to by the discoverer, m. arago, nor by m. ampère, who quote against it the absence of all attraction when the magnet and metal are at rest ( . .), although the induced magnetism should still remain; and who, from experiments made with a long dipping needle, conceive the action to be always repulsive ( .). [a] philosophical transactions, , p. . . upon obtaining electricity from magnets by the means already described ( .), i hoped to make the experiment of m. arago a new source of electricity; and did not despair, by reference to terrestrial magneto-electric induction, of being able to construct a new electrical machine. thus stimulated, numerous experiments were made with the magnet of the royal society at mr. christie's house, in all of which i had the advantage of his assistance. as many of these were in the course of the superseded by more perfect arrangements, i shall consider myself at liberty investigation to rearrange them in a manner calculated to convey most readily what appears to me to be a correct view of the nature of the phenomena. . the magnet has been already described ( .). to concentrate the poles, and bring them nearer to each other, two iron or steel bars, each about six or seven inches long, one inch wide, and half an inch thick, were put across the poles as in fig. , and being supported by twine from slipping, could be placed as near to or far from each other as was required. occasionally two bars of soft iron were employed, so bent that when applied, one to each pole, the two smaller resulting poles were vertically over each other, either being uppermost at pleasure. . a disc of copper, twelve inches in diameter, and about one fifth of an inch in thickness, fixed upon a brass axis, was mounted in frames so as to allow of revolution either vertically or horizontally, its edge being at the same time introduced more or less between the magnetic poles (fig. .). the edge of the plate was well amalgamated for the purpose of obtaining a good but moveable contact, and a part round the axis was also prepared in a similar manner. . conductors or electric collectors of copper and lead were constructed so as to come in contact with the edge of the copper disc ( .), or with other forms of plates hereafter to be described ( .). these conductors were about four inches long, one third of an inch wide, and one fifth of an inch thick; one end of each was slightly grooved, to allow of more exact adaptation to the somewhat convex edge of the plates, and then amalgamated. copper wires, one sixteenth of an inch in thickness, attached, in the ordinary manner, by convolutions to the other ends of these conductors, passed away to the galvanometer. . the galvanometer was roughly made, yet sufficiently delicate in its indications. the wire was of copper covered with silk, and made sixteen or eighteen convolutions. two sewing-needles were magnetized and fixed on to a stem of dried grass parallel to each other, but in opposite directions, and about half an inch apart; this system was suspended by a fibre of unspun silk, so that the lower needle should be between the convolutions of the multiplier, and the upper above them. the latter was by much the most powerful magnet, and gave terrestrial direction to the whole; fig. . represents the direction of the wire and of the needles when the instrument was placed in the magnetic meridian: the ends of the wires are marked a and b for convenient reference hereafter. the letters s and n designate the south and north ends of the needle when affected merely by terrestrial magnetism; the end n is therefore the marked pole ( .). the whole instrument was protected by a glass jar, and stood, as to position and distance relative to the large magnet, under the same circumstances as before ( .). . all these arrangements being made, the copper disc was adjusted as in fig. , the small magnetic poles being about half an inch apart, and the edge of the plate inserted about half their width between them. one of the galvanometer wires was passed twice or thrice loosely round the brass axis of the plate, and the other attached to a conductor ( .), which itself was retained by the hand in contact with the amalgamated edge of the disc at the part immediately between the magnetic poles. under these circumstances all was quiescent, and the galvanometer exhibited no effect. but the instant the plate moved, the galvanometer was influenced, and by revolving the plate quickly the needle could be deflected ° or more. . it was difficult under the circumstances to make the contact between the conductor and the edge of the revolving disc uniformly good and extensive; it was also difficult in the first experiments to obtain a regular velocity of rotation: both these causes tended to retain the needle in a continual state of vibration; but no difficulty existed in ascertaining to which side it was deflected, or generally, about what line it vibrated. afterwards, when the experiments were made more carefully, a permanent deflection of the needle of nearly ° could be sustained. . here therefore was demonstrated the production of a permanent current of electricity by ordinary magnets ( .). . when the motion of the disc was reversed, every other circumstance remaining the same, the galvanometer needle was deflected with equal power as before; but the deflection was on the opposite side, and the current of electricity evolved, therefore, the reverse of the former. . when the conductor was placed on the edge of the disc a little to the right or left, as in the dotted positions fig. , the current of electricity was still evolved, and in the same direction as at first ( . .). this occurred to a considerable distance, i.e. ° or ° on each side of the place of the magnetic poles. the current gathered by the conductor and conveyed to the galvanometer was of the same kind on both sides of the place of greatest intensity, but gradually diminished in force from that place. it appeared to be equally powerful at equal distances from the place of the magnetic poles, not being affected in that respect by the direction of the rotation. when the rotation of the disc was reversed, the direction of the current of electricity was reversed also; but the other circumstances were not affected. . on raising the plate, so that the magnetic poles were entirely hidden from each other by its intervention, (a. fig. ,) the same effects were produced in the same order, and with equal intensity as before. on raising it still higher, so as to bring the place of the poles to c, still the effects were produced, and apparently with as much power as at first. . when the conductor was held against the edge as if fixed to it, and with it moved between the poles, even though but for a few degrees, the galvanometer needle moved and indicated a current of electricity, the same as that which would have been produced if the wheel had revolved in the same direction, the conductor remaining stationary. . when the galvanometer connexion with the axis was broken, and its wires made fast to two conductors, both applied to the edge of the copper disc, then currents of electricity were produced, presenting more complicated appearances, but in perfect harmony with the above results. thus, if applied as in fig. , a current of electricity through the galvanometer was produced; but if their place was a little shifted, as in fig. , a current in the contrary direction resulted; the fact being, that in the first instance the galvanometer indicated the difference between a strong current through a and a weak one through b, and in the second, of a weak current through a and a strong one through b ( .), and therefore produced opposite deflections. . so also when the two conductors were equidistant from the magnetic poles, as in fig. , no current at the galvanometer was perceived, whichever way the disc was rotated, beyond what was momentarily produced by irregularity of contact; because equal currents in the same direction tended to pass into both. but when the two conductors were connected with one wire, and the axis with the other wire, (fig. ,) then the galvanometer showed a current according with the direction of rotation ( .); both conductors now acting consentaneously, and as a single conductor did before ( .). . all these effects could be obtained when only one of the poles of the magnet was brought near to the plate; they were of the same kind as to direction, &c., but by no means so powerful. . all care was taken to render these results independent of the earth's magnetism, or of the mutual magnetism of the magnet and galvanometer needles. the contacts were made in the magnetic equator of the plate, and at other parts; the plate was placed horizontally, and the poles vertically; and other precautions were taken. but the absence of any interference of the kind referred to, was readily shown by the want of all effect when the disc was removed from the poles, or the poles from the disc; every other circumstance remaining the same. . the _relation of the current_ of electricity produced, to the magnetic pole, to the direction of rotation of the plate, &c. &c., may be expressed by saying, that when the unmarked pole ( . .) is beneath the edge of the plate, and the latter revolves horizontally, screw-fashion, the electricity which can be collected at the edge of the plate nearest to the pole is positive. as the pole of the earth may mentally be considered the unmarked pole, this relation of the rotation, the pole, and the electricity evolved, is not difficult to remember. or if, in fig. , the circle represent the copper disc revolving in the direction of the arrows, and _a_ the outline of the unmarked pole placed beneath the plate, then the electricity collected at _b_ and the neighbouring parts is positive, whilst that collected at the centre _c_ and other parts is negative ( .). the currents in the plate are therefore from the centre by the magnetic poles towards the circumference. . if the marked pole be placed above, all other things remaining the same, the electricity at _b_, fig. , is still positive. if the marked pole be placed below, or the unmarked pole above, the electricity is reversed. if the direction of revolution in any case is reversed, the electricity is also reversed. . it is now evident that the rotating plate is merely another form of the simpler experiment of passing a piece of metal between the magnetic poles in a rectilinear direction, and that in such cases currents of electricity are produced at right angles to the direction of the motion, and crossing it at the place of the magnetic pole or poles. this was sufficiently shown by the following simple experiment: a piece of copper plate one fifth of an inch thick, one inch and a half wide, and twelve inches long, being amalgamated at the edges, was placed between the magnetic poles, whilst the two conductors from the galvanometer were held in contact with its edges; it was then drawn through between the poles of the conductors in the direction of the arrow, fig. ; immediately the galvanometer needle was deflected, its north or marked end passed eastward, indicating that the wire a received negative and the wire b positive electricity; and as the marked pole was above, the result is in perfect accordance with the effect obtained by the rotatory plate ( .). . on reversing the motion of the plate, the needle at the galvanometer was deflected in the opposite direction, showing an opposite current. . to render evident the character of the electrical current existing in various parts of the moving copper plate, differing in their relation to the inducing poles, one collector ( .) only was applied at the part to be examined near to the pole, the other being connected with the end of the plate as the most neutral place: the results are given at fig. - , the marked pole being above the plate. in fig. , b received positive electricity; but the plate moving in the same direction, it received on the opposite side, fig. , negative electricity: reversing the motion of the latter, as in fig. , b received positive electricity; or reversing the motion of the first arrangement, that of fig. to fig. , b received negative electricity. . when the plates were previously removed sideways from between the magnets, as in fig. , so as to be quite out of the polar axis, still the same effects were produced, though not so strongly. . when the magnetic poles were in contact, and the copper plate was drawn between the conductors near to the place, there was but very little effect produced. when the poles were opened by the width of a card, the effect was somewhat more, but still very small. . when an amalgamated copper wire, one eighth of an inch thick, was drawn through between the conductors and poles ( .), it produced a very considerable effect, though not so much as the plates. . if the conductors were held permanently against any particular parts of the copper plates, and carried between the magnetic poles with them, effects the same as those described were produced, in accordance with the results obtained with the revolving disc ( .). . on the conductors being held against the ends of the plates, and the latter then passed between the magnetic poles, in a direction transverse to their length, the same effects were produced (fig. .). the parts of the plates towards the end may be considered either as mere conductors, or as portions of metal in which the electrical current is excited, according to their distance and the strength of the magnet; but the results were in perfect harmony with those before obtained. the effect was as strong as when the conductors were held against the sides of the plate ( .). . when a mere wire, connected with the galvanometer so as to form a complete circuit, was passed through between the poles, the galvanometer was affected; and upon moving the wire to and fro, so as to make the alternate impulses produced correspond with the vibrations of the needle, the latter could be increased to ° or ° on each side the magnetic meridian. . upon connecting the ends of a plate of metal with the galvanometer wires, and then carrying it between the poles from end to end (as in fig. .), in either direction, no effect whatever was produced upon the galvanometer. but the moment the motion became transverse, the needle was deflected. . these effects were also obtained from _electro-magnetic poles_, resulting from the use of copper helices or spirals, either alone or with iron cores ( . .). the directions of the motions were precisely the same; but the action was much greater when the iron cores were used, than without. . when a flat spiral was passed through edgewise between the poles, a curious action at the galvanometer resulted; the needle first went strongly one way, but then suddenly stopped, as if it struck against some solid obstacle, and immediately returned. if the spiral were passed through from above downwards, or from below upwards, still the motion of the needle was in the same direction, then suddenly stopped, and then was reversed. but on turning the spiral half-way round, i.e. edge for edge, then the directions of the motions were reversed, but still were suddenly interrupted and inverted as before. this double action depends upon the halves of the spiral (divided by a line passing through its centre perpendicular to the direction of its motion) acting in opposite directions; and the reason why the needle went to the same side, whether the spiral passed by the poles in the one or the other direction, was the circumstance, that upon changing the motion, the direction of the wires in the approaching half of the spiral was changed also. the effects, curious as they appear when witnessed, are immediately referable to the action of single wires ( . .). . although the experiments with the revolving plate, wires, and plates of metal, were first successfully made with the large magnet belonging to the royal society, yet they were all ultimately repeated with a couple of bar magnets two feet long, one inch and a half wide, and half an inch thick; and, by rendering the galvanometer ( .) a little more delicate, with the most striking results. ferro-electro-magnets, as those of moll, henry, &c. ( .), are very powerful. it is very essential, when making experiments on different substances, that thermo-electric effects (produced by contact of the fingers, &c.) be avoided, or at least appreciated and accounted for; they are easily distinguished by their permanency, and their independence of the magnets, or of the direction of the motion. . the relation which holds between the magnetic pole, the moving wire or metal, and the direction of the current evolved, i.e. _the law_ which governs the evolution of electricity by magneto-electric induction, is very simple, although rather difficult to express. if in fig. , pn represent a horizontal wire passing by a marked magnetic pole, so that the direction of its motion shall coincide with the curved line proceeding from below upwards; or if its motion parallel to itself be in a line tangential to the curved line, but in the general direction of the arrows; or if it pass the pole in other directions, but so as to cut the magnetic curves[a] in the same general direction, or on the same side as they would be cut by the wire if moving along the dotted curved line;--then the current of electricity in the wire is from p to n. if it be carried in the reverse directions, the electric current will be from n to p. or if the wire be in the vertical position, figured p' n', and it be carried in similar directions, coinciding with the dotted horizontal curve so far, as to cut the magnetic curves on the same side with it, the current will be from p' to n'. if the wire be considered a tangent to the curved surface of the cylindrical magnet, and it be carried round that surface into any other position, or if the magnet itself be revolved on its axis, so as to bring any part opposite to the tangential wire,--still, if afterwards the wire be moved in the directions indicated, the current of electricity will be from p to n; or if it be moved in the opposite direction, from n to p; so that as regards the motions of the wire past the pole, they may be reduced to two, directly opposite to each other, one of which produces a current from p to n, and the other from n to p. [a] by magnetic curves, i mean the lines of magnetic forces, however modified by the juxtaposition of poles, which would be depicted by iron filings; or those to which a very small magnetic needle would form a tangent. . the same holds true of the unmarked pole of the magnet, except that if it be substituted for the one in the figure, then, as the wires are moved in the direction of the arrows, the current of electricity would be from n to p, and when they move in the reverse direction, from p to n. . hence the current of electricity which is excited in metal when moving in the neighbourhood of a magnet, depends for its direction altogether upon the relation of the metal to the resultant of magnetic action, or to the magnetic curves, and may be expressed in a popular way thus; let ab (fig. .) represent a cylinder magnet, a being the marked pole, and b the unmarked pole; let pn be a silver knife-blade, resting across the magnet with its edge upward, and with its marked or notched side towards the pole a; then in whatever direction or position this knife be moved edge foremost, either about the marked or the unmarked pole, the current of electricity produced will be from p to n, provided the intersected curves proceeding from a abut upon the notched surface of the knife, and those from b upon the unnotched side. or if the knife be moved with its back foremost, the current will be from n to p in every possible position and direction, provided the intersected curves abut on the same surfaces as before. a little model is easily constructed, by using a cylinder of wood for a magnet, a flat piece for the blade, and a piece of thread connecting one end of the cylinder with the other, and passing through a hole in the blade, for the magnetic curves: this readily gives the result of any possible direction. . when the wire under induction is passing by an electromagnetic pole, as for instance one end of a copper helix traversed by the electric current ( .), the direction of the current in the approaching wire is the same with that of the current in the parts or sides of the spirals nearest to it, and in the receding wire the reverse of that in the parts nearest to it. . all these results show that the power of inducing electric currents is circumferentially exerted by a magnetic resultant or axis of power, just as circumferential magnetism is dependent upon and is exhibited by an electric current. . the experiments described combine to prove that when a piece of metal (and the same may be true of all conducting matter ( .) ) is passed either before a single pole, or between the opposite poles of a magnet, or near electro-magnetic poles, whether ferruginous or not, electrical currents are produced across the metal transverse to the direction of motion; and which therefore, in arago's experiments, will approximate towards the direction of radii. if a single wire be moved like the spoke of a wheel near a magnetic pole, a current of electricity is determined through it from one end towards the other. if a wheel be imagined, constructed of a great number of these radii, and this revolved near the pole, in the manner of the copper disc ( .), each radius will have a current produced in it as it passes by the pole. if the radii be supposed to be in contact laterally, a copper disc results, in which the directions of the currents will be generally the same, being modified only by the coaction which can take place between the particles, now that they are in metallic contact. . now that the existence of these currents is known, arago's phenomena may be accounted for without considering them as due to the formation in the copper, of a pole of the opposite kind to that approximated, surrounded by a diffuse polarity of the same kind ( .); neither is it essential that the plate should acquire and lose its state in a finite time; nor on the other hand does it seem necessary that any repulsive force should be admitted as the cause of the rotation ( .). . the effect is precisely of the same kind as the electromagnetic rotations which i had the good fortune to discover some years ago[a]. according to the experiments then made which have since been abundantly confirmed, if a wire (pn fig. .) be connected with the positive and negative ends of a voltaic buttery, so that the positive electricity shall pass from p to n, and a marked magnetic pole n be placed near the wire between it and the spectator, the pole will move in a direction tangential to the wire, i.e. towards the right, and the wire will move tangentially towards the left, according to the directions of the arrows. this is exactly what takes place in the rotation of a plate beneath a magnetic pole; for let n (fig. .) be a marked pole above the circular plate, the latter being rotated in the direction of the arrow: immediately currents of positive electricity set from the central parts in the general direction of the radii by the pole to the parts of the circumference _a_ on the other side of that pole ( . .), and are therefore exactly in the same relation to it as the current in the wire (pn, fig. .), and therefore the pole in the same manner moves to the right hand. [a] quarterly journal of science, vol. xii. pp. . . . . . if the rotation of the disc be reversed, the electric currents are reversed ( .), and the pole therefore moves to the left hand. if the contrary pole be employed, the effects are the same, i.e. in the same direction, because currents of electricity, the reverse of those described, are produced, and by reversing both poles and currents, the visible effects remain unchanged. in whatever position the axis of the magnet be placed, provided the same pole be applied to the same side of the plate, the electric current produced is in the same direction, in consistency with the law already stated ( , &c.); and thus every circumstance regarding the direction of the motion may be explained. . these currents are _discharged or return_ in the parts of the plate on each side of and more distant from the place of the pole, where, of course, the magnetic induction is weaker; and when the collectors are applied, and a current of electricity is carried away to the galvanometer ( .), the deflection there is merely a repetition, by the same current or part of it, of the effect of rotation in the magnet over the plate itself. . it is under the point of view just put forth that i have ventured to say it is not necessary that the plate should acquire and lose its state in a finite time ( .); for if it were possible for the current to be fully developed the instant _before_ it arrived at its state of nearest approximation to the vertical pole of the magnet, instead of opposite to or a little beyond it, still the relative motion of the pole and plate would be the same, the resulting force being in fact tangential instead of direct. . but it is possible (though not necessary for the rotation) that _time_ may be required for the development of the maximum current in the plate, in which case the resultant of all the forces would be in advance of the magnet when the plate is rotated, or in the rear of the magnet when the latter is rotated, and many of the effects with pure electro-magnetic poles tend to prove this is the case. then, the tangential force may be resolved into two others, one parallel to the plane of rotation, and the other perpendicular to it; the former would be the force exerted in making the plate revolve with the magnet, or the magnet with the plate; the latter would be a repulsive force, and is probably that, the effects of which m. arago has also discovered ( .). . the extraordinary circumstance accompanying this action, which has seemed so inexplicable, namely, the cessation of all phenomena when the magnet and metal are brought to rest, now receives a full explanation ( .); for then the electrical currents which cause the motion cease altogether. . all the effects of solution of metallic continuity, and the consequent diminution of power described by messrs. babbage and herschel[a], now receive their natural explanation, as well also as the resumption of power when the cuts were filled up by metallic substances, which, though conductors of electricity, were themselves very deficient in the power of influencing magnets. and new modes of cutting the plate may be devised, which shall almost entirely destroy its power. thus, if a copper plate ( .) be cut through at about a fifth or sixth of its diameter from the edge, so as to separate a ring from it, and this ring be again fastened on, but with a thickness of paper intervening (fig. .), and if arago's experiment be made with this compound plate so adjusted that the section shall continually travel opposite the pole, it is evident that the magnetic currents will be greatly interfered with, and the plate probably lose much of its effect[b]. [a] philosophical transactions, , p. . [b] this experiment has actually been made by mr. christie, with the results here described, and is recorded in the philosophical transactions for , p. . an elementary result of this kind was obtained by using two pieces of thick copper, shaped as in fig. . when the two neighbouring edges were amalgamated and put together, and the arrangement passed between the poles of the magnet, in the direction parallel to these edges, a current was urged through the wires attached to the outer angles, and the galvanometer became strongly affected; but when a single film of paper was interposed, and the experiment repeated, no sensible effect could be produced. . a section of this kind could not interfere much with the induction of magnetism, supposed to be of the nature ordinarily received by iron. . the effect of rotation or deflection of the needle, which m. arago obtained by ordinary magnets, m. ampère succeeded in procuring by electro-magnets. this is perfectly in harmony with the results relative to volta-electric and magneto-electric induction described in this paper. and by using flat spirals of copper wire, through which electric currents were sent, in place of ordinary magnetic poles (ill.), sometimes applying a single one to one side of the rotating plate, and sometimes two to opposite sides, i obtained the induced currents of electricity from the plate itself, and could lead them away to, and ascertain their existence by, the galvanometer. . the cause which has now been assigned for the rotation in arago's experiment, namely, the production of electrical currents, seems abundantly sufficient in all cases where the metals, or perhaps even other conductors, are concerned; but with regard to such bodies as glass, resins, and, above all, gases, it seems impossible that currents of electricity, capable of producing these effects, should be generated in them. yet arago found that the effects in question were produced by these and by all bodies tried ( .). messrs. babbage and herschel, it is true, did not observe them with any substance not metallic, except carbon, in a highly conducting state ( .). mr. harris has ascertained their occurrence with wood, marble, freestone and annealed glass, but obtained no effect with sulphuric acid and saturated solution of sulphate of iron, although these are better conductors of electricity than the former substances. . future investigations will no doubt explain these difficulties, and decide the point whether the retarding or dragging action spoken of is always simultaneous with electric currents.[a] the existence of the action in metals, only whilst the currents exist, i.e. whilst motion is given ( . .), and the explication of the repulsive action observed by m. arago ( . .), are powerful reasons for referring it to this cause; but it may be combined with others which occasionally act alone. [a] experiments which i have since made convince me that this particular action is always due to the electrical currents formed; and they supply a test by which it may be distinguished from the action of ordinary magnetism, or any other cause, including those which are mechanical or irregular, producing similar effects ( .) . copper, iron, tin, zinc, lead, mercury, and all the metals tried, produced electrical currents when passed between the magnetic poles: the mercury was put into a glass tube for the purpose. the dense carbon deposited in coal gas retorts, also produced the current, but ordinary charcoal did not. neither could i obtain any sensible effects with brine, sulphuric acid, saline solutions, &c., whether rotated in basins, or inclosed in tubes and passed between the poles. . i have never been able to produce any sensation upon the tongue by the wires connected with the conductors applied to the edges of the revolving plate ( .) or slips of metal ( .). nor have i been able to heat a fine platina wire, or produce a spark, or convulse the limbs of a frog. i have failed also to produce any chemical effects by electricity thus evolved ( . ). . as the electric current in the revolving copper plate occupies but a small space, proceeding by the poles and being discharged right and left at very small distances comparatively ( .); and as it exists in a thick mass of metal possessing almost the highest conducting power of any, and consequently offering extraordinary facility for its production and discharge; and as, notwithstanding this, considerable currents may be drawn off which can pass through narrow wires, forty, fifty, sixty, or even one hundred feet long; it is evident that the current existing in the plate itself must be a very powerful one, when the rotation is rapid and the magnet strong. this is also abundantly proved by the obedience and readiness with which a magnet ten or twelve pounds in weight follows the motion of the plate and will strongly twist up the cord by which it is suspended. . two rough trials were made with the intention of constructing _magneto-electric machines_. in one, a ring one inch and a half broad and twelve inches external diameter, cut from a thick copper plate, was mounted so as to revolve between the poles of the magnet and represent a plate similar to those formerly used ( .), but of interminable length; the inner and outer edges were amalgamated, and the conductors applied one to each edge, at the place of the magnetic poles. the current of electricity evolved did not appear by the galvanometer to be stronger, if so strong, as that from the circular plate ( .). . in the other, small thick discs of copper or other metal, half an inch in diameter, were revolved rapidly near to the poles, but with the axis of rotation out of the polar axis; the electricity evolved was collected by conductors applied as before to the edges ( .). currents were procured, but of strength much inferior to that produced by the circular plate. . the latter experiment is analogous to those made by mr. barlow with a rotating iron shell, subject to the influence of the earth[a]. the effects obtained by him have been referred by messrs. babbage and herschel to the same cause as that considered as influential in arago's experiment[b]; but it would be interesting to know how far the electric current which might be produced in the experiment would account for the deflexion of the needle. the mere inversion of a copper wire six or seven times near the poles of the magnet, and isochronously with the vibrations of the galvanometer needle connected with it, was sufficient to make the needle vibrate through an arc of ° or °. the rotation of a copper shell would perhaps decide the point, and might even throw light upon the more permanent, though somewhat analogous effects obtained by mr. christie. [a] philosophical transactions, . p. . [b] ibid. . p. . . the remark which has already been made respecting iron ( .), and the independence of the ordinary magnetical phenomena of that substance and the phenomena now described of magneto-electric induction in that and other metals, was fully confirmed by many results of the kind detailed in this section. when an iron plate similar to the copper one formerly described ( .) was passed between the magnetic poles, it gave a current of electricity like the copper plate, but decidedly of less power; and in the experiments upon the induction of electric currents ( .), no difference in the kind of action between iron and other metals could be perceived. the power therefore of an iron plate to drag a magnet after it, or to intercept magnetic action, should be carefully distinguished from the similar power of such metals as silver, copper, &c. &c., inasmuch as in the iron by far the greater part of the effect is due to what may be called ordinary magnetic action. there can be no doubt that the cause assigned by messrs. babbage and herschel in explication of arago's phenomena is the true one, when iron is the metal used. . the very feeble powers which were found by those philosophers to belong to bismuth and antimony, when moving, of affecting the suspended magnet, and which has been confirmed by mr. harris, seem at first disproportionate to their conducting powers; whether it be so or not must be decided by future experiment ( .)[a]. these metals are highly crystalline, and probably conduct electricity with different degrees of facility in different directions; and it is not unlikely that where a mass is made up of a number of crystals heterogeneously associated, an effect approaching to that of actual division may occur ( .); or the currents of electricity may become more suddenly deflected at the confines of similar crystalline arrangements, and so be more readily and completely discharged within the mass. [a] i have since been able to explain these differences, and prove, with several metals, that the effect is in the order of the conducting power; for i have been able to obtain, by magneto-electric induction, currents of electricity which are proportionate in strength to the conducting power of the bodies experimented with ( .). §. _royal institution, november ._ _note._--in consequence of the long period which has intervened between the reading and printing of the foregoing paper, accounts of the experiments have been dispersed, and, through a letter of my own to m. hachette, have reached france and italy. that letter was translated (with some errors), and read to the academy of sciences at paris, th december, . a copy of it in _le temps_ of the th december quickly reached signor nobili, who, with signor antinori, immediately experimented upon the subject, and obtained many of the results mentioned in my letter; others they could not obtain or understand, because of the brevity of my account. these results by signori nobili and antinori have been embodied in a paper dated st january , and printed and published in the number of the _antologia_ dated november (according at least to the copy of the paper kindly sent me by signor nobili). it is evident the work could not have been then printed; and though signor nobili, in his paper, has inserted my letter as the text of his experiments, yet the circumstance of back date has caused many here, who have heard of nobili's experiments by report only, to imagine his results were anterior to, instead of being dependent upon, mine. i may be allowed under these circumstances to remark, that i experimented on this subject several years ago, and have published results. (see quarterly journal of science for july , p. .) the following also is an extract from my note-book, dated november , : "experiments on induction by connecting wire of voltaic battery:--a battery of four troughs, ten pairs of plates, each arranged side by side--the poles connected by a wire about four feet long, parallel to which was another similar wire separated from it only by two thicknesses of paper, the ends of the latter were attached to a galvanometer:--exhibited no action, &c. &c. &c.--could not in any way render any induction evident from the connecting wire." the cause of failure at that time is now evident ( .).--m.f. april, . second series. the bakerian lecture. § . _terrestrial magneto-electric induction._ § . _force and direction of magneto-electric induction generally._ read january , . § . _terrestrial magneto-electric induction._ . when the general facts described in the former paper were discovered, and the _law_ of magneto-electric induction relative to direction was ascertained ( .), it was not difficult to perceive that the earth would produce the same effect as a magnet, and to an extent that would, perhaps, render it available in the construction of new electrical machines. the following are some of the results obtained in pursuance of this view. . the hollow helix already described ( .) was connected with a galvanometer by wires eight feet long; and the soft iron cylinder ( .) after being heated red-hot and slowly cooled, to remove all traces of magnetism, was put into the helix so as to project equally at both ends, and fixed there. the combined helix and bar were held in the magnetic direction or line of dip, and (the galvanometer needle being motionless) were then inverted, so that the lower end should become the upper, but the whole still correspond to the magnetic direction; the needle was immediately deflected. as the latter returned to its first position, the helix and bar were again inverted; and by doing this two or three times, making the inversions and vibrations to coincide, the needle swung through an arc of ° or °. . when one end of the helix, which may be called a, was uppermost at first (b end consequently being below), then it mattered not in which direction it proceeded during the inversion, whether to the right hand or left hand, or through any other course; still the galvanometer needle passed in the same direction. again, when b end was uppermost, the inversion of the helix and bar in any direction always caused the needle to be deflected one way; that way being the opposite to the course of the deflection in the former case. . when the helix with its iron core in any given position was inverted, the effect was as if a magnet with its marked pole downwards had been introduced from above into the inverted helix. thus, if the end b were upwards, such a magnet introduced from above would make the marked end of the galvanometer needle pass west. or the end b being downwards, and the soft iron in its place, inversion of the whole produced the same effect. . when the soft iron bar was taken out of the helix and inverted in various directions within four feet of the galvanometer, not the slightest effect upon it was produced. . these phenomena are the necessary consequence of the inductive magnetic power of the earth, rendering the soft iron cylinder a magnet with its marked pole downwards. the experiment is analogous to that in which two bar magnets were used to magnetize the same cylinder in the same helix ( .), and the inversion of position in the present experiment is equivalent to a change of the poles in that arrangement. but the result is not less an instance of the evolution of electricity by means of the magnetism of the globe. . the helix alone was then held permanently in the magnetic direction, and the soft iron cylinder afterwards introduced; the galvanometer needle was instantly deflected; by withdrawing the cylinder as the needle returned, and continuing the two actions simultaneously, the vibrations soon extended through an arc of °. the effect was precisely the same as that obtained by using a cylinder magnet with its marked pole downwards; and the direction of motion, &c. was perfectly in accordance with the results of former experiments obtained with such a magnet ( .). a magnet in that position being used, gave the same deflections, but stronger. when the helix was put at right angles to the magnetic direction or dip, then the introduction or removal of the soft iron cylinder produced no effect at the needle. any inclination to the dip gave results of the same kind as those already described, but increasing in strength as the helix approximated to the direction of the dip. . a cylinder magnet, although it has great power of affecting the galvanometer when moving into or out of the helix, has no power of continuing the deflection ( .); and therefore, though left in, still the magnetic needle comes to its usual place of rest. but upon repeating (with the magnet) the experiment of inversion in the direction of the dip ( ), the needle was affected as powerfully as before; the disturbance of the magnetism in the steel magnet, by the earth's inductive force upon it, being thus shown to be nearly, if not quite, equal in amount and rapidity to that occurring in soft iron. it is probable that in this way magneto-electrical arrangements may become very useful in indicating the disturbance of magnetic forces, where other means will not apply; for it is not the whole magnetic power which produces the visible effect, but only the difference due to the disturbing causes. . these favourable results led me to hope that the direct magneto-electric induction of the earth might be rendered sensible; and i ultimately succeeded in obtaining the effect in several ways. when the helix just referred to ( . .) was placed in the magnetic dip, but without any cylinder of iron or steel, and was then inverted, a feeble action at the needle was observed. inverting the helix ten or twelve times, and at such periods that the deflecting forces exerted by the currents of electricity produced in it should be added to the momentum of the needle ( .), the latter was soon made to vibrate through an arc of ° or °. here, therefore, currents of electricity were produced by the direct inductive power of the earth's magnetism, without the use of any ferruginous matter, and upon a metal not capable of exhibiting any of the ordinary magnetic phenomena. the experiment in everything represents the effects produced by bringing the same helix to one or both poles of any powerful magnet ( .). . guided by the law already expressed ( .), i expected that all the electric phenomena of the revolving metal plate could now be produced without any other magnet than the earth. the plate so often referred to ( .) was therefore fixed so as to rotate in a horizontal plane. the magnetic curves of the earth ( . _note_), i.e. the dip, passes through this plane at angles of about °, which it was expected would be an approximation to perpendicularity, quite enough to allow of magneto-electric induction sufficiently powerful to produce a current of electricity. . upon rotation of the plate, the currents ought, according to the law ( . .), to tend to pass in the direction of the radii, through _all_ parts of the plate, either from the centre to the circumference, or from the circumference to the centre, as the direction of the rotation of the plate was one way or the other. one of the wires of the galvanometer was therefore brought in contact with the axis of the plate, and the other attached to a leaden collector or conductor ( .), which itself was placed against the amalgamated edge of the disc. on rotating the plate there was a distinct effect at the galvanometer needle; on reversing the rotation, the needle went in the opposite direction; and by making the action of the plate coincide with the vibrations of the needle, the arc through which the latter passed soon extended to half a circle. . whatever part of the edge of the plate was touched by the conductor, the electricity was the same, provided the direction of rotation continued unaltered. . when the plate revolved _screw-fashion_, or as the hands of a watch, the current of electricity ( .) was from the centre to the circumference; when the direction of rotation was _unscrew_, the current was from the circumference to the centre. these directions are the same with those obtained when the unmarked pole of a magnet was placed beneath the revolving plate ( .). . when the plate was in the magnetic meridian, or in any other plane _coinciding_ with the magnetic dip, then its rotation produced no effect upon the galvanometer. when inclined to the dip but a few degrees, electricity began to appear upon rotation. thus when standing upright in a plane perpendicular to the magnetic meridian, and when consequently its own plane was inclined only about ° to the dip, revolution of the plate evolved electricity. as the inclination was increased, the electricity became more powerful until the angle formed by the plane of the plate with the dip was °, when the electricity for a given velocity of the plate was a maximum. . it is a striking thing to observe the revolving copper plate become thus a _new electrical machine_; and curious results arise on comparing it with the common machine. in the one, the plate is of the best non-conducting substance that can be applied; in the other, it is the most perfect conductor: in the one, insulation is essential; in the other, it is fatal. in comparison of the quantities of electricity produced, the metal machine does not at all fall below the glass one; for it can produce a constant current capable of deflecting the galvanometer needle, whereas the latter cannot. it is quite true that the force of the current thus evolved has not as yet been increased so as to render it available in any of our ordinary applications of this power; but there appears every reasonable expectation that this may hereafter be effected; and probably by several arrangements. weak as the current may seem to be, it is as strong as, if not stronger than, any thermo-electric current; for it can pass fluids ( .), agitate the animal system, and in the case of an electro-magnet has produced sparks ( .). . a disc of copper, one fifth of an inch thick and only one inch and a half in diameter, was amalgamated at the edge; a square piece of sheet lead (copper would have been better) of equal thickness had a circular hole cut in it, into which the disc loosely fitted; a little mercury completed the metallic communication of the disc and its surrounding ring; the latter was attached to one of the galvanometer wires, and the other wire dipped into a little metallic cup containing mercury, fixed upon the top of the copper axis of the small disc. upon rotating the disc in a horizontal plane, the galvanometer needle could be affected, although the earth was the only magnet employed, and the radius of the disc but three quarters of an inch; in which space only the current was excited. . on putting the pole of a magnet under the revolving disc, the galvanometer needle could be permanently deflected. . on using copper wires one sixth of an inch in thickness instead of the smaller wires ( .) hitherto constantly employed, far more powerful effects were obtained. perhaps if the galvanometer had consisted of fewer turns of thick wire instead of many convolutions of thinner, more striking effects would have been produced. . one form of apparatus which i purpose having arranged, is to have several discs superposed; the discs are to be metallically connected, alternately at the edges and at the centres, by means of mercury; and are then to be revolved alternately in opposite directions, i.e. the first, third, fifth, &c. to the right hand, and the second, fourth, sixth, &c. to the left hand; the whole being placed so that the discs are perpendicular to the dip, or intersect most directly the magnetic curves of powerful magnets. the electricity will be from the centre to the circumference in one set of discs, and from the circumference to the centre in those on each side of them; thus the action of the whole will conjoin to produce one combined and more powerful current. . i have rather, however, been desirous of discovering new facts and new relations dependent on magneto-electric induction, than of exalting the force of those already obtained; being assured that the latter would find their full development hereafter. * * * * * . i referred in my former paper to the probable influence of terrestrial magneto-electric induction ( .) in producing, either altogether or in part, the phenomena observed by messrs. christie and barlow[a], whilst revolving ferruginous bodies; and especially those observed by the latter when rapidly rotating an iron shell, which were by that philosopher referred to a change in the ordinary disposition of the magnetism of the ball. i suggested also that the rotation of a copper globe would probably insulate the effects due to electric currents from those due to mere derangement of magnetism, and throw light upon the true nature of the phenomena. [a] christie, phil. trans. , pp. , , &c. barlow, phil. trans. , p. . . upon considering the law already referred to ( .), it appeared impossible that a metallic globe could revolve under natural circumstances, without having electric currents produced within it, circulating round the revolving globe in a plane at right angles to the plane of revolution, provided its axis of rotation did not coincide with the dip; and it appeared that the current would be most powerful when the axis of revolution was perpendicular to the dip of the needle: for then all those parts of the ball below a plane passing through its centre and perpendicular to the dip, would in moving cut the magnetic curves in one direction, whilst all those parts above that plane would intersect them in the other direction: currents therefore would exist in these moving parts, proceeding from one pole of rotation to the other; but the currents above would be in the reverse direction to those below, and in conjunction with them would produce a continued circulation of electricity. . as the electric currents are nowhere interrupted in the ball, powerful effects were expected, and i endeavoured to obtain them with simple apparatus. the ball i used was of brass; it had belonged to an old electrical machine, was hollow, thin (too thin), and four inches in diameter; a brass wire was screwed into it, and the ball either turned in the hand by the wire, or sometimes, to render it more steady, supported by its wire in a notched piece of wood, and motion again given by the hand. the ball gave no signs of magnetism when at rest. . a compound magnetic needle was used to detect the currents. it was arranged thus: a sewing-needle had the head and point broken off, and was then magnetised; being broken in halves, the two magnets thus produced were fixed on a stem of dried grass, so as to be perpendicular to it, and about four inches asunder; they were both in one plane, but their similar poles in contrary directions. the grass was attached to a piece of unspun silk about six inches long, the latter to a stick passing through a cork in the mouth of a cylindrical jar; and thus a compound arrangement was obtained, perfectly sheltered from the motion of the air, but little influenced by the magnetism of the earth, and yet highly sensible to magnetic and electric forces, when the latter were brought into the vicinity of the one or the other needle. . upon adjusting the needles to the plane of the magnetic meridian; arranging the ball on the outside of the glass jar to the west of the needles, and at such a height that its centre should correspond horizontally with the upper needle, whilst its axis was in the plane of the magnetic meridian, but perpendicular to the dip; and then rotating the ball, the needle was immediately affected. upon inverting the direction of rotation, the needle was again affected, but in the opposite direction. when the ball revolved from east over to west, the marked pole went eastward; when the ball revolved in the opposite direction, the marked pole went westward or towards the ball. upon placing the ball to the east of the needles, still the needle was deflected in the same way; i.e. when the ball revolved from east over to west, the marked pole wont eastward (or towards the ball); when the rotation was in the opposite direction, the marked pole went westward. . by twisting the silk of the needles, the latter were brought into a position perpendicular to the plane of the magnetic meridian; the ball was again revolved, with its axis parallel to the needles; the upper was affected as before, and the deflection was such as to show that both here and in the former case the needle was influenced solely by currents of electricity existing in the brass globe. . if the upper part of the revolving ball be considered as a wire moving from east to west, over the unmarked pole of the earth, the current of electricity in it should be from north to south ( . . .); if the under part be considered as a similar wire, moving from west to east over the same pole, the electric current should be from south to north; and the circulation of electricity should therefore be from north above to south, and below back to north, in a metal ball revolving from east above to west in these latitudes. now these currents are exactly those required to give the directions of the needle in the experiments just described; so that the coincidence of the theory from which the experiments were deduced with the experiments themselves, is perfect. . upon inclining the axis of rotation considerably, the revolving ball was still found to affect the magnetic needle; and it was not until the angle which it formed with the magnetic dip was rendered small, that its effects, even upon this apparatus, were lost ( .). when revolving with its axis parallel to the dip, it is evident that the globe becomes analogous to the copper plate; electricity of one kind might be collected at its equator, and of the other kind at its poles. . a current in the ball, such as that described above ( .), although it ought to deflect a needle the same way whether it be to the right or the left of the ball and of the axis of rotation, ought to deflect it the contrary way when above or below the ball; for then the needle is, or ought to be, acted upon in a contrary direction by the current. this expectation was fulfilled by revolving the ball beneath the magnetic needle, the latter being still inclosed in its jar. when the ball was revolved from east over to west, the marked pole of the needle, instead of passing eastward, went westward; and when revolved from west over to east, the marked pole went eastward. . the deflections of the magnetic needle thus obtained with a brass ball are exactly in the same direction as those observed by mr. barlow in the revolution of the iron shell; and from the manner in which iron exhibits the phenomena of magneto-electric induction like any other metal, and distinct from its peculiar magnetic phenomena ( .), it is impossible but that electric currents must have been excited, and become active in those experiments. what proportion of the whole effect obtained is due to this cause, must be decided by a more elaborate investigation of all the phenomena. . these results, in conjunction with the general law before stated ( .), suggested an experiment of extreme simplicity, which yet, on trial, was found to answer perfectly. the exclusion of all extraneous circumstances and complexity of arrangement, and the distinct character of the indications afforded, render this single experiment an epitome of nearly all the facts of magneto-electric induction. . a piece of common copper wire, about eight feet long and one twentieth of an inch in thickness, had one of its ends fastened to one of the terminations of the galvanometer wire, and the other end to the other termination; thus it formed an endless continuation of the galvanometer wire: it was then roughly adjusted into the shape of a rectangle, or rather of a loop, the upper part of which could be carried to and fro over the galvanometer, whilst the lower part, and the galvanometer attached to it, remained steady (plate ii. fig. .). upon moving this loop over the galvanometer from right to left, the magnetic needle was immediately deflected; upon passing the loop back again, the needle passed in the contrary direction to what it did before; upon repeating these motions of the loop in accordance with the vibrations of the needle ( .), the latter soon swung through ° or more. . the relation of the current of electricity produced in the wire, to its motion, may be understood by supposing the convolutions at the galvanometer away, and the wire arranged as a rectangle, with its lower edge horizontal and in the plane of the magnetic meridian, and a magnetic needle suspended above and over the middle part of this edge, and directed by the earth (fig. .). on passing the upper part of the rectangle from west to east into the position represented by the dotted line, the marked pole of the magnetic needle went west; the electric current was therefore from north to south in the part of the wire passing under the needle, and from south to north in the moving or upper part of the parallelogram. on passing the upper part of the rectangle from east to west over the galvanometer, the marked pole of the needle went east, and the current of electricity was therefore the reverse of the former. . when the rectangle was arranged in a plane east and west, and the magnetic needle made parallel to it, either by the torsion of its suspension thread or the action of a magnet, still the general effects were the same. on moving the upper part of the rectangle from north to south, the marked pole of the needle went north; when the wire was moved in the opposite direction, the marked pole went south. the same effect took place when the motion of the wire was in any other azimuth of the line of dip; the direction of the current always being conformable to the law formerly expressed ( .), and also to the directions obtained with the rotating ball ( .). . in these experiments it is not necessary to move the galvanometer or needle from its first position. it is quite sufficient if the wire of the rectangle is distorted where it leaves the instrument, and bent so as to allow the moving upper part to travel in the desired direction. . the moveable part of the wire was then arranged _below_ the galvanometer, but so as to be carried across the dip. it affected the instrument as before, and in the same direction; i.e. when carried from west to east under the instrument, the marked end of the needle went west, as before. this should, of course, be the case; for when the wire is cutting the magnetic dip in a certain direction, an electric current also in a certain direction should be induced in it. . if in fig. _dp_ be parallel to the dip, and ba be considered as the upper part of the rectangle ( .), with an arrow _c_ attached to it, both these being retained in a plane perpendicular to the dip,--then, however ba with its attached arrow is moved upon _dp_ as an axis, if it afterwards proceed in the direction of the arrow, a current of electricity will move along it from b towards a. . when the moving part of the wire was carried up or down parallel to the dip, no effect was produced on the galvanometer. when the direction of motion was a little inclined to the dip, electricity manifested itself; and was at a maximum when the motion was perpendicular to the magnetic direction. . when the wire was bent into other forms and moved, equally strong effects were obtained, especially when instead of a rectangle a double catenarian curve was formed of it on one side of the galvanometer, and the two single curves or halves were swung in opposite directions at the same time; their action then combined to affect the galvanometer: but all the results were reducible to those above described. . the longer the extent of the moving wire, and the greater the space through which it moves, the greater is the effect upon the galvanometer. . the facility with which electric currents are produced in metals when moving under the influence of magnets, suggests that henceforth precautions should always be taken, in experiments upon metals and magnets, to guard against such effects. considering the universality of the magnetic influence of the earth, it is a consequence which appears very extraordinary to the mind, that scarcely any piece of metal can be moved in contact with others, either at rest, or in motion with different velocities or in varying directions, without an electric current existing within them. it is probable that amongst arrangements of steam-engines and metal machinery, some curious accidental magneto-electric combinations may be found, producing effects which have never been observed, or, if noticed, have never as yet been understood. * * * * * . upon considering the effects of terrestrial magneto-electric induction which have now been described, it is almost impossible to resist the impression that similar effects, but infinitely greater in force, may be produced by the action of the globe, as a magnet, upon its own mass, in consequence of its diurnal rotation. it would seem that if a bar of metal be laid in these latitudes on the surface of the earth parallel to the magnetic meridian, a current of electricity tends to pass through it from south to north, in consequence of the travelling of the bar from west to east ( .), by the rotation of the earth; that if another bar in the same direction be connected with the first by wires, it cannot discharge the current of the first, because it has an equal tendency to have a current in the same direction induced within itself: but that if the latter be carried from east to west, which is equivalent to a diminution of the motion communicated to it from the earth ( .), then the electric current from south to north is rendered evident in the first bar, in consequence of its discharge, at the same time, by means of the second. . upon the supposition that the rotation of the earth tended, by magneto-electric induction, to cause currents in its own mass, these would, according to the law ( .) and the experiments, be, upon the surface at least, from the parts in the neighbourhood of or towards the plane of the equator, in opposite directions to the poles; and if collectors could be applied at the equator and at the poles of the globe, as has been done with the revolving copper plate ( .), and also with magnets ( .), then negative electricity would be collected at the equator, and positive electricity at both poles ( .). but without the conductors, or something equivalent to them, it is evident these currents could not exist, as they could not be discharged. . i did not think it impossible that some natural difference might occur between bodies, relative to the intensity of the current produced or tending to be produced in them by magneto-electric induction, which might be shown by opposing them to each other; especially as messrs. arago, babbage, herschel, and harris, have all found great differences, not only between the metals and other substances, but between the metals themselves, in their power of receiving motion from or giving it to a magnet in trials by revolution ( .). i therefore took two wires, each one hundred and twenty feet long, one of iron and the other of copper. these were connected with each other at their ends, and then extended in the direction of the magnetic meridian, so as to form two nearly parallel lines, nowhere in contact except at the extremities. the copper wire was then divided in the middle, and examined by a delicate galvanometer, but no evidence of an electrical current was obtained. . by favour of his royal highness the president of the society, i obtained the permission of his majesty to make experiments at the lake in the gardens of kensington-palace, for the purpose of comparing, in a similar manner, water and metal. the basin of this lake is artificial; the water is supplied by the chelsea company; no springs run into it, and it presented what i required, namely, a uniform mass of still pure water, with banks ranging nearly from east to west, and from north to south. . two perfectly clean bright copper plates, each exposing four square feet of surface, were soldered to the extremities of a copper wire; the plates were immersed in the water, north and south of each other, the wire which connected them being arranged upon the grass of the bank. the plates were about four hundred and eighty feet from each other, in a right line; the wire was probably six hundred feet long. this wire was then divided in the middle, and connected by two cups of mercury with a delicate galvanometer. . at first, indications of electric currents were obtained; but when these were tested by inverting the direction of contact, and in other ways, they were found to be due to other causes than the one sought for. a little difference in temperature; a minute portion of the nitrate of mercury used to amalgamate the wires, entering into the water employed to reduce the two cups of mercury to the same temperature; was sufficient to produce currents of electricity, which affected the galvanometer, notwithstanding they had to pass through nearly five hundred feet of water. when these and other interfering causes were guarded against, no effect was obtained; and it appeared that even such dissimilar substances as water and copper, when cutting the magnetic curves of the earth with equal velocity, perfectly neutralized each other's action. . mr. fox of falmouth has obtained some highly important results respecting the electricity of metalliferous veins in the mines of cornwall, which have been published in the philosophical transactions[a]. i have examined the paper with a view to ascertain whether any of the effects were probably referable to magneto-electric induction; but, though unable to form a very strong opinion, believe they are not. when parallel veins running east and west were compared, the general tendency of the electricity _in the wires_ was from north to south; when the comparison was made between parts towards the surface and at some depth, the current of electricity in the wires was from above downwards. if there should be any natural difference in the force of the electric currents produced by magneto-electric induction in different substances, or substances in different positions moving with the earth, and which might be rendered evident by increasing the masses acted upon, then the wires and veins experimented with by mr. fox might perhaps have acted as dischargers to the electricity of the mass of strata included between them, and the directions of the currents would agree with those observed as above. [a] . p. . . although the electricity obtained by magneto-electric induction in a few feet of wire is of but small intensity, and has not yet been observed except in metals, and carbon in a particular state, still it has power to pass through brine ( .); and, as increased length in the substance acted upon produces increase of intensity, i hoped to obtain effects from extensive moving masses of water, though quiescent water gave none. i made experiments therefore (by favour) at waterloo bridge, extending a copper wire nine hundred and sixty feet in length upon the parapet of the bridge, and dropping from its extremities other wires with extensive plates of metal attached to them to complete contact with the water. thus the wire and the water made one conducting circuit; and as the water ebbed or flowed with the tide, i hoped to obtain currents analogous to those of the brass ball ( .). . i constantly obtained deflections at the galvanometer, but they were very irregular, and were, in succession, referred to other causes than that sought for. the different condition of the water as to purity on the two sides of the river; the difference in temperature; slight differences in the plates, in the solder used, in the more or less perfect contact made by twisting or otherwise; all produced effects in turn: and though i experimented on the water passing through the middle arches only; used platina plates instead of copper; and took every other precaution, i could not after three days obtain any satisfactory results. . theoretically, it seems a necessary consequence, that where water is flowing, there electric currents should be formed; thus, if a line be imagined passing from dover to calais through the sea, and returning through the land beneath the water to dover, it traces out a circuit of conducting matter, one part of which, when the water moves up or down the channel, is cutting the magnetic curves of the earth, whilst the other is relatively at rest. this is a repetition of the wire experiment ( .), but with worse conductors. still there is every reason to believe that electric currents do run in the general direction of the circuit described, either one way or the other, according as the passage of the waters is up or down the channel. where the lateral extent of the moving water is enormously increased, it does not seem improbable that the effect should become sensible; and the gulf stream may thus, perhaps, from electric currents moving across it, by magneto-electric induction from the earth, exert a sensible influence upon the forms of the lines of magnetic variation[a]. [a] theoretically, even a ship or a boat when passing on the surface of the water, in northern or southern latitudes, should have currents of electricity running through it directly across the line of her motion; or if the water is flowing past the ship at anchor, similar currents should occur. . though positive results have not yet been obtained by the action of the earth upon water and aqueous fluids, yet, as the experiments are very limited in their extent, and as such fluids do yield the current by artificial magnets ( .), (for transference of the current is proof that it may be produced ( .),) the supposition made, that the earth produces these induced currents within itself ( .) in consequence of its diurnal rotation, is still highly probable ( , .); and when it is considered that the moving masses extend for thousands of miles across the magnetic curves, cutting them in various directions within its mass, as well as at the surface, it is possible the electricity may rise to considerable intensity. . i hardly dare venture, even in the most hypothetical form, to ask whether the aurora borealis and australia may not be the discharge of electricity, thus urged towards the poles of the earth, from whence it is endeavouring to return by natural and appointed means above the earth to the equatorial regions. the non-occurrence of it in very high latitudes is not at all against the supposition; and it is remarkable that mr. fox, who observed the deflections of the magnetic needle at falmouth, by the aurora borealis, gives that direction of it which perfectly agrees with the present view. he states that all the variations at night were towards the east[a], and this is what would happen if electric currents were setting from south to north in the earth under the needle, or from north to south in space above it. [a] philosophical transactions, , p. . § . _general remarks and illustrations of the force and direction of magneto-electric induction._ . in the repetition and variation of arago's experiment by messrs. babbage, herschel, and harris, these philosophers directed their attention to the differences of force observed amongst the metals and other substances in their action on the magnet. these differences were very great[a], and led me to hope that by mechanical combinations of various metals important results might be obtained ( .). the following experiments were therefore made, with a view to obtain, if possible, any such difference of the action of two metals, [b] philosophical transactions, , p. ; , p. . . a piece of soft iron bonnet-wire covered with cotton was laid bare and cleaned at one extremity, and there fastened by metallic contact with the clean end of a copper wire. both wires were then twisted together like the strands of a rope, for eighteen or twenty inches; and the remaining parts being made to diverge, their extremities were connected with the wires of the galvanometer. the iron wire was about two feet long, the continuation to the galvanometer being copper. . the twisted copper and iron (touching each other nowhere but at the extremity) were then passed between the poles of a powerful magnet arranged horse-shoe fashion (fig. .); but not the slightest effect was observed at the galvanometer, although the arrangement seemed fitted to show any electrical difference between the two metals relative to the action of the magnet, . a soft iron cylinder was then covered with paper at the middle part, and the twisted portion of the above compound wire coiled as a spiral around it, the connexion with the galvanometer still being made at the ends a and b. the iron cylinder was then brought in contact with the poles of a powerful magnet capable of raising thirty pounds; yet no signs of electricity appeared at the galvanometer. every precaution was applied in making and breaking contact to accumulate effect, but no indications of a current could be obtained. . copper and tin, copper and zinc, tin and zinc, tin and iron, and zinc and iron, were tried against each other in a similar manner ( ), but not the slightest sign of electric currents could be procured. . two flat spirals, one of copper and the other of iron, containing each eighteen inches of wire, were connected with each other and with the galvanometer, and then put face to face so as to be in contrary directions. when brought up to the magnetic pole ( .). no electrical indications at the galvanometer were observed. when one was turned round so that both were in the same direction, the effect at the galvanometer was very powerful. . the compound helix of copper and iron wire formerly described ( .) was arranged as a double helix, one of the helices being all iron and containing two hundred and fourteen feet, the other all copper and continuing two hundred and eight feet. the two similar ends aa of the copper and iron helix were connected together, and the other ends bb of each helix connected with the galvanometer; so that when a magnet was introduced into the centre of the arrangement, the induced currents in the iron and copper would tend to proceed in contrary directions. yet when a magnet was inserted, or a soft iron bar within made a magnet by contact with poles, no effect at the needle was produced. . a glass tube about fourteen inches long was filled with strong sulphuric acid. twelve inches of the end of a clean copper wire were bent up into a bundle and inserted into the tube, so as to make good superficial contact with the acid, and the rest of the wire passed along the outside of the tube and away to the galvanometer. a wire similarly bent up at the extremity was immersed in the other end of the sulphuric acid, and also connected with the galvanometer, so that the acid and copper wire were in the same parallel relation to each other in this experiment as iron and copper were in the first ( ). when this arrangement was passed in a similar manner between the poles of the magnet, not the slightest effect at the galvanometer could be perceived. . from these experiments it would appear, that when metals of different kinds connected in one circuit are equally subject in every circumstance to magneto-electric induction, they exhibit exactly equal powers with respect to the currents which either are formed, or tend to form, in them. the same even appears to be the case with regard to fluids, and probably all other substances. . still it seemed impossible that these results could indicate the relative inductive power of the magnet upon the different metals; for that the effect should be in some relation to the conducting power seemed a necessary consequence ( .), and the influence of rotating plates upon magnets had been found to bear a general relation to the conducting power of the substance used. . in the experiments of rotation ( .), the electric current is excited and discharged in the same substance, be it a good or bad conductor; but in the experiments just described the current excited in iron could not be transmitted but through the copper, and that excited in copper had to pass through iron: i.e. supposing currents of dissimilar strength to be formed in the metals proportionate to their conducting power, the stronger current had to pass through the worst conductor, and the weaker current through the best. . experiments were therefore made in which different metals insulated from each other were passed between the poles of the magnet, their opposite ends being connected with the same end of the galvanometer wire, so that the currents formed and led away to the galvanometer should oppose each other; and when considerable lengths of different wires were used, feeble deflections were obtained. . to obtain perfectly satisfactory results a new galvanometer was constructed, consisting of two independent coils, each containing eighteen feet of silked copper wire. these coils were exactly alike in shape and number of turns, and were fixed side by side with a small interval between them, in which a double needle could be hung by a fibre of silk exactly as in the former instrument ( .). the coils may be distinguished by the letters kl, and when electrical currents were sent through them in the same direction, acted upon the needle with the sum of their powers; when in opposite directions, with the difference of their powers. . the compound helix ( . .) was now connected, the ends a and b of the iron with a and b ends of galvanometer coil k, and the ends a and b of the copper with b and a ends of galvanometer coil l, so that the currents excited in the two helices should pass in opposite directions through the coils k and l. on introducing a small cylinder magnet within the helices, the galvanometer needle was powerfully deflected. on disuniting the iron helix, the magnet caused with the copper helix alone still stronger deflection in the same direction. on reuniting the iron helix, and unconnecting the copper helix, the magnet caused a moderate deflection in the contrary direction. thus it was evident that the electric current induced by a magnet in a copper wire was far more powerful than the current induced by the same magnet in an equal iron wire. . to prevent any error that might arise from the greater influence, from vicinity or other circumstances, of one coil on the needle beyond that of the other, the iron and copper terminations were changed relative to the galvanometer coils kl, so that the one which before carried the current from the copper now conveyed that from the iron, and vice versa. but the same striking superiority of the copper was manifested as before. this precaution was taken in the rest of the experiments with other metals to be described. . i then had wires of iron, zinc, copper, tin, and lead, drawn to the same diameter (very nearly one twentieth of an inch), and i compared exactly equal lengths, namely sixteen feet, of each in pairs in the following manner: the ends of the copper wire were connected with the ends a and b of galvanometer coil k, and the ends of the zinc wire with the terminations a and b of the galvanometer coil l. the middle part of each wire was then coiled six times round a cylinder of soft iron covered with paper, long enough to connect the poles of daniell's horse-shoe magnet ( .) (fig. .), so that similar helices of copper and zinc, each of six turns, surrounded the bar at two places equidistant from each other and from the poles of the magnet; but these helices were purposely arranged so as to be in contrary directions, and therefore send contrary currents through the galvanometer coils k and l, . on making and breaking contact between the soft iron bar and the poles of the magnet, the galvanometer was strongly affected; on detaching the zinc it was still more strongly affected in the same direction. on taking all the precautions before alluded to ( .), with others, it was abundantly proved that the current induced by the magnet in copper was far more powerful than in zinc. . the copper was then compared in a similar manner with tin, lead, and iron, and surpassed them all, even more than it did zinc. the zinc was then compared experimentally with the tin, lead, and iron, and found to produce a more powerful current than any of them. iron in the same manner proved superior to tin and lead. tin came next, and lead the last. . thus the order of these metals is copper, zinc, iron, tin, and lead. it is exactly their order with respect to conducting power for electricity, and, with the exception of iron, is the order presented by the magneto-rotation experiments of messrs. babbage, herschel, harris, &c. the iron has additional power in the latter kind of experiments, because of its ordinary magnetic relations, and its place relative to magneto-electric action of the kind now under investigation cannot be ascertained by such trials. in the manner above described it may be correctly ascertained[a]. [a] mr. christie, who being appointed reporter upon this paper, had it in his hands before it was complete, felt the difficulty ( .); and to satisfy his mind, made experiments upon iron and copper with the large magnet( .), and came to the same conclusions as i have arrived at. the two sets of experiments were perfectly independent of each other, neither of us being aware of the other's proceedings. . it must still be observed that in these experiments the whole effect between different metals is not obtained; for of the thirty-four feet of wire included in each circuit, eighteen feet are copper in both, being the wire of the galvanometer coils; and as the whole circuit is concerned in the resulting force of the current, tin's circumstance must tend to diminish the difference which would appear between the metals if the circuits were of the same substances throughout. in the present case the difference obtained is probably not more than a half of that which would be given if the whole of each circuit were of one metal. . these results tend to prove that the currents produced by magneto-electric induction in bodies is proportional to their conducting power. that they are _exactly_ proportional to and altogether dependent upon the conducting power, is, i think, proved by the perfect neutrality displayed when two metals or other substances, as acid, water, &c. &c. ( . .), are opposed to each other in their action. the feeble current which tends to be produced in the worse conductor, has its transmission favoured in the better conductor, and the stronger current which tends to form in the latter has its intensity diminished by the obstruction of the former; and the forces of generation and obstruction are so perfectly neutralize each other exactly. now as the obstruction is inversely as the balanced as to conducting power, the tendency to generate a current must be directly as that power to produce this perfect equilibrium. . the cause of the equality of action under the various circumstances described, where great extent of wire ( .) or wire and water ( .) were connected together, which yet produced such different effects upon the magnet, is now evident and simple. . the effects of a rotating substance upon a needle or magnet ought, where ordinary magnetism has no influence, to be directly as the conducting power of the substance; and i venture now to predict that such will be found to be the case; and that in all those instances where non-conductors have been supposed to exhibit this peculiar influence, the motion has been due to some interfering cause of an ordinary kind; as mechanical communication of motion through the parts of the apparatus, or otherwise (as in the case mr. harris has pointed out[a]); or else to ordinary magnetic attractions. to distinguish the effects of the latter from those of the induced electric currents, i have been able to devise a most perfect test, which shall be almost immediately described ( .). [a] philosophical transactions, . p. . . there is every reason to believe that the magnet or magnetic needle will become an excellent measurer of the conducting power of substances rotated near it; for i have found by careful experiment, that when a constant current of electricity was sent successively through a series of wires of copper, platina, zinc, silver, lead, and tin, drawn to the same diameter; the deflection of the needle was exactly equal by them all. it must be remembered that when bodies are rotated in a horizontal plane, the magnetism of the earth is active upon them. as the effect is general to the whole of the plate, it may not interfere in these cases; but in some experiments and calculations may be of important consequence. . another point which i endeavoured to ascertain, was, whether it was essential or not that the moving part of the wire should, in cutting the magnetic curves, pass into positions of greater or lesser magnetic force; or whether, always intersecting curves of equal magnetic intensity, the mere motion was sufficient for the production of the current. that the latter is true, has been proved already in several of the experiments on terrestrial magneto-electric induction. thus the electricity evolved from the copper plate ( .), the currents produced in the rotating globe ( , &c.), and those passing through the moving wire ( .), are all produced under circumstances in which the magnetic force could not but be the same during the whole experiments. . to prove the point with an ordinary magnet, a copper disc was cemented upon the end of a cylinder magnet, with paper intervening; the magnet and disc were rotated together, and collectors (attached to the galvanometer) brought in contact with the circumference and the central part of the copper plate. the galvanometer needle moved as in former cases, and the _direction_ of motion was the _same_ as that which would have resulted, if the copper only had revolved, and the magnet been fixed. neither was there any apparent difference in the quantity of deflection. hence, rotating the magnet causes no difference in the results; for a rotatory and a stationary magnet produce the same effect upon the moving copper. . a copper cylinder, closed at one extremity, was then put over the magnet, one half of which it inclosed like a cap; it was firmly fixed, and prevented from touching the magnet anywhere by interposed paper. the arrangement was then floated in a narrow jar of mercury, so that the lower edge of the copper cylinder touched the fluid metal; one wire of the galvanometer dipped into this mercury, and the other into a little cavity in the centre of the end of the copper cap. upon rotating the magnet and its attached cylinder, abundance of electricity passed through the galvanometer, and in the same direction as if the cylinder had rotated only, the magnet being still. the results therefore were the same as those with the disc ( .). . that the metal of the magnet itself might be substituted for the moving cylinder, disc, or wire, seemed an inevitable consequence, and yet one which would exhibit the effects of magneto-electric induction in a striking form. a cylinder magnet had therefore a little hole made in the centre of each end to receive a drop of mercury, and was then floated pole upwards in the same metal contained in a narrow jar. one wire from the galvanometer dipped into the mercury of the jar, and the other into the drop contained in the hole at the upper extremity of the axis. the magnet was then revolved by a piece of string passed round it, and the galvanometer-needle immediately indicated a powerful current of electricity. on reversing the order of rotation, the electrical current was reversed. the direction of the electricity was the same as if the copper cylinder ( .) or a copper wire had revolved round the fixed magnet in the same direction as that which the magnet itself had followed. thus a _singular independence_ of the magnetism and the bar in which it resides is rendered evident. . in the above experiment the mercury reached about halfway up the magnet; but when its quantity was increased until within one eighth of an inch of the top, or diminished until equally near the bottom, still the same effects and the _same direction_ of electrical current was obtained. but in those extreme proportions the effects did not appear so strong as when the surface of the mercury was about the middle, or between that and an inch from each end. the magnet was eight inches and a half long, and three quarters of an inch in diameter. . upon inversion of the magnet, and causing rotation in the same direction, i.e. always screw or always unscrew, then a contrary current of electricity was produced. but when the motion of the magnet was continued in a direction constant in relation to its _own axis_, then electricity of the same kind was collected at both poles, and the opposite electricity at the equator, or in its neighbourhood, or in the parts corresponding to it. if the magnet be held parallel to the axis of the earth, with its unmarked pole directed to the pole star, and then rotated so that the parts at its southern side pass from west to east in conformity to the motion of the earth; then positive electricity may be collected at the extremities of the magnet, and negative electricity at or about the middle of its mass. . when the galvanometer was very sensible, the mere spinning of the magnet in the air, whilst one of the galvanometer wires touched the extremity, and the other the equatorial parts, was sufficient to evolve a current of electricity and deflect the needle. . experiments were then made with a similar magnet, for the purpose of ascertaining whether any return of the electric current could occur at the central or axial parts, they having the same angular velocity of rotation as the other parts ( .) the belief being that it could not. . a cylinder magnet, seven inches in length, and three quarters of an inch in diameter, had a hole pierced in the direction of its axis from one extremity, a quarter of an inch in diameter, and three inches deep. a copper cylinder, surrounded by paper and amalgamated at both extremities, was introduced so as to be in metallic contact at the bottom of the hole, by a little mercury, with the middle of the magnet; insulated at the sides by the paper; and projecting about a quarter of an inch above the end of the steel. a quill was put over the copper rod, which reached to the paper, and formed a cup to receive mercury for the completion of the circuit. a high paper edge was also raised round that end of the magnet and mercury put within it, which however had no metallic connexion with that in the quill, except through the magnet itself and the copper rod (fig. .). the wires a and b from the galvanometer were dipped into these two portions of mercury; any current through them could, therefore, only pass down the magnet towards its equatorial parts, and then up the copper rod; or vice versa. . when thus arranged and rotated screw fashion, the marked end of the galvanometer needle went west, indicating that there was a current through the instrument from a to b and consequently from b through the magnet and copper rod to a (fig. .). . the magnet was then put into a jar of mercury (fig. .) as before ( .); the wire a left in contact with the copper axis, but the wire b dipped in the mercury of the jar, and therefore in metallic communication with the equatorial parts of the magnet instead of its polar extremity. on revolving the magnet screw fashion, the galvanometer needle was deflected in the same direction as before, but far more powerfully. yet it is evident that the parts of the magnet from the equator to the pole were out of the electric circuit. . then the wire a was connected with the mercury on the extremity of the magnet, the wire b still remaining in contact with that in the jar (fig. .), so that the copper axis was altogether out of the circuit. the magnet was again revolved screw fashion, and again caused the same deflection of the needle, the current being as strong as it was in the last trial ( .), and much stronger than at first ( .). . hence it is evident that there is no discharge of the current at the centre of the magnet, for the current, now freely evolved, is up through the magnet; but in the first experiment ( .) it was down. in fact, at that time, it was only the part of the moving metal equal to a little disc extending from the end of the wire b in the mercury to the wire a that was efficient, i.e. moving with a different angular velocity to the rest of the circuit ( .); and for that portion the direction of the current is consistent with the other results. . in the two after experiments, the _lateral_ parts of the magnet or of the copper rod are those which move relative to the other parts of the circuit, i.e. the galvanometer wires; and being more extensive, intersecting more curves, or moving with more velocity, produce the greater effect. for the discal part, the direction of the induced electric current is the same in all, namely, from the circumference towards the centre. * * * * * . the law under which the induced electric current excited in bodies moving relatively to magnets, is made dependent on the intersection of the magnetic curves by the metal ( .) being thus rendered more precise and definite ( . . .), seem now even to apply to the cause in the first section of the former paper ( .); and by rendering a perfect reason for the effects produced, take away any for supposing that peculiar condition, which i ventured to call the electro-tonic state ( .). . when an electrical current is passed through a wire, that wire is surrounded at every part by magnetic curves, diminishing in intensity according to their distance from the wire, and which in idea may be likened to rings situated in planes perpendicular to the wire or rather to the electric current within it. these curves, although different in form, are perfectly analogous to those existing between two contrary magnetic poles opposed to each other; and when a second wire, parallel to that which carries the current, is made to approach the latter ( .), it passes through magnetic curves exactly of the same kind as those it would intersect when carried between opposite magnetic poles ( .) in one direction; and as it recedes from the inducing wire, it cuts the curves around it in the same manner that it would do those between the same poles if moved in the other direction. . if the wire np (fig. .) have an electric current passed through it in the direction from p to n, then the dotted ring may represent a magnetic curve round it, and it is in such a direction that if small magnetic needles lie placed as tangents to it, they will become arranged as in the figure, _n_ and _s_ indicating north and south ends ( . _note_.). . but if the current of electricity were made to cease for a while, and magnetic poles were used instead to give direction to the needles, and make them take the same position as when under the influence of the current, then they must be arranged as at fig. ; the marked and unmarked poles _ab_ above the wire, being in opposite directions to those _a'b'_ below. in such a position therefore the magnetic curves between the poles _ab_ and _a'b'_ have the same general direction with the corresponding parts of the ring magnetic curve surrounding the wire np carrying an electric current. . if the second wire _pn_ (fig. .) be now brought towards the principal wire, carrying a current, it will cut an infinity of magnetic curves, similar in direction to that figured, and consequently similar in direction to those between the poles _ab_ of the magnets (fig. .), and it will intersect these current curves in the same manner as it would the magnet curves, if it passed from above between the poles downwards. now, such an intersection would, with the magnets, induce an electric current in the wire from _p_ to _n_ ( .); and therefore as the curves are alike in arrangement, the same effect ought to result from the intersection of the magnetic curves dependent on the current in the wire np; and such is the case, for on approximation the induced current is in the opposite direction to the principal current ( .). . if the wire _p'n'_ be carried up from below, it will pass in the opposite direction between the magnetic poles; but then also the magnetic poles themselves are reversed (fig. .), and the induced current is therefore ( .) still in the same direction as before. it is also, for equally sufficient and evident reasons, in the same direction, if produced by the influence of the curves dependent upon the wire. . when the second wire is retained at rest in the vicinity the principal wire, no current is induced through it, for it is intersecting no magnetic curves. when it is removed from the principal wire, it intersects the curves in the opposite direction to what it did before ( .); and a current in the opposite direction is induced, which therefore corresponds with the direction of the principal current ( .). the same effect would take place if by inverting the direction of motion of the wire in passing between either set of poles (fig. .), it were made to intersect the curves there existing in the opposite direction to what it did before. . in the first experiments ( . .), the inducing wire and that under induction were arranged at a fixed distance from each other, and then an electric current sent through the former. in such cases the magnetic curves themselves must be considered as moving (if i may use the expression) across the wire under induction, from the moment at which they begin to be developed until the magnetic force of the current is at its utmost; expanding as it were from the wire outwards, and consequently being in the same relation to the fixed wire under induction as if _it_ had moved in the opposite direction across them, or towards the wire carrying the current. hence the first current induced in such cases was in the contrary direction to the principal current ( . .). on breaking the battery contact, the magnetic curves (which are mere expressions for arranged magnetic forces) may be conceived as contracting upon and returning towards the failing electrical current, and therefore move in the opposite direction across the wire, and cause an opposite induced current to the first. . when, in experiments with ordinary magnets, the latter, in place of being moved past the wires, were actually made near them ( . .), then a similar progressive development of the magnetic curves may be considered as having taken place, producing the effects which would have occurred by motion of the wires in one direction; the destruction of the magnetic power corresponds to the motion of the wire in the opposite direction. . if, instead of intersecting the magnetic curves of a straight wire carrying a current, by approximating or removing a second wire ( .), a revolving plate be used, being placed for that purpose near the wire, and, as it were, amongst the magnetic curves, then it ought to have continuous electric currents induced within it; and if a line joining the wire with the centre of the plate were perpendicular to both, then the induced current ought to be, according to the law ( .), directly across the plate, from one side to the other, and at right angles to the direction of the inducing current. . a single metallic wire one twentieth of an inch in diameter had an electric current passed through it, and a small copper disc one inch and a half in diameter revolved near to and under, but not in actual contact with it (fig. ). collectors were then applied at the opposite edges of the disc, and wires from them connected with the galvanometer. as the disc revolved in one direction, the needle was deflected on one side: and when the direction of revolution was reversed, the needle was inclined on the other side, in accordance with the results anticipated. . thus the reasons which induce me to suppose a particular state in the wire ( .) have disappeared; and though it still seems to me unlikely that a wire at rest in the neighbourhood of another carrying a powerful electric current is entirely indifferent to it, yet i am not aware of any distinct _facts_ which authorize the conclusion that it is in a particular state. * * * * * . in considering the nature of the cause assigned in these papers to account for the mutual influence of magnets and moving metals ( .), and comparing it with that heretofore admitted, namely, the induction of a feeble magnetism like that produced in iron, it occurred to me that a most decisive experimental test of the two views could be applied ( .). . no other known power has like direction with that exerted between an electric current and a magnetic pole; it is tangential, while all other forces, acting at a distance, are direct. hence, if a magnetic pole on one side of a revolving plate follow its course by reason of its obedience to the tangential force exerted upon it by the very current of electricity which it has itself caused, a similar pole on the opposite side of the plate should immediately set it free from this force; for the currents which tend to be formed by the action of the two poles are in opposite directions; or rather no current tends to be formed, or no magnetic curves are intersected ( .); and therefore the magnet should remain at rest. on the contrary, if the action of a north magnetic pole were to produce a southness in the nearest part of the copper plate, and a diffuse northness elsewhere ( .), as is really the case with iron; then the use of another north pole on the opposite side of the same part of the plate should double the effect instead of destroying it, and double the tendency of the first magnet to move with the plate. . a thick copper plate ( .) was therefore fixed on a vertical axis, a bar magnet was suspended by a plaited silk cord, so that its marked pole hung over the edge of the plate, and a sheet of paper being interposed, the plate was revolved; immediately the magnetic pole obeyed its motion and passed off in the same direction. a second magnet of equal size and strength was then attached to the first, so that its marked pole should hang _beneath_ the edge of the copper plate in a corresponding position to that above, and at an equal distance (fig. .). then a paper sheath or screen being interposed as before, and the plate revolved, the poles were found entirely indifferent to its motion, although either of them alone would have followed the course of rotation. . on turning one magnet round, so that _opposite_ poles were on each side of the plate, then the mutual action of the poles and the moving metal was a maximum. . on suspending one magnet so that its axis was level with the plate, and either pole opposite its edge, the revolution of the plate caused no motion of the magnet. the electrical currents dependent upon induction would now tend to be produced in a vertical direction across the thickness of the plate, but could not be so discharged, or at least only to so slight a degree as to leave all effects insensible; but ordinary magnetic induction, or that on an iron plate, would be equally if not more powerfully developed in such a position ( .). . then, with regard to the production of electricity in these cases:--whenever motion was communicated by the plate to the magnets, currents existed; when it was not communicated, they ceased. a marked pole of a large bar magnet was put under the edge of the plate; collectors ( .) applied at the axis and edge of the plate as on former occasions (fig. .), and these connected with the galvanometer; when the plate was revolved, abundance of electricity passed to the instrument. the unmarked pole of a similar magnet was then put over the place of the former pole, so that contrary poles were above and below; on revolving the plate, the electricity was more powerful than before. the latter magnet was then turned end for end, so that marked poles were both above and below the plate, and then, upon revolving it, scarcely any electricity was procured. by adjusting the distance of the poles so as to correspond with their relative force, they at last were brought so perfectly to neutralize each other's inductive action upon the plate, that no electricity could be obtained with the most rapid motion. . i now proceeded to compare the effect of similar and dissimilar poles upon iron and copper, adopting for the purpose mr. sturgeon's very useful form of arago's experiment. this consists in a circular plate of metal supported in a vertical plane by a horizontal axis, and weighted a little at one edge or rendered excentric so as to vibrate like a pendulum. the poles of the magnets are applied near the side and edges of these plates, and then the number of vibrations, required to reduce the vibrating arc a certain constant quantity, noted. in the first description of this instrument[a] it is said that opposite poles produced the greatest retarding effect, and similar poles none; and yet within a page of the place the effect is considered as of the same kind with that produced in iron. [a] edin. phil. journal, , p. . . i had two such plates mounted, one of copper, one of iron. the copper plate alone gave sixty vibrations, in the average of several experiments, before the arc of vibration was reduced from one constant mark to another. on placing opposite magnetic poles near to, and on each side of, the same place, the vibrations were reduced to fifteen. on putting similar poles on each side of it, they rose to fifty; and on placing two pieces of wood of equal size with the poles equally near, they became fifty-two. so that, when similar poles were used, the magnetic effect was little or none, (the obstruction being due to the confinement of the air, rather,) whilst with opposite poles it was the greatest possible. when a pole was presented to the edge of the plate, no retardation occurred. . the iron plate alone made thirty-two vibrations, whilst the arc of vibration diminished a certain quantity. on presenting a magnetic pole to the edge of the plate ( .), the vibrations were diminished to eleven; and when the pole was about half an inch from the edge, to five. . when the marked pole was put at the side of the iron plate at a certain distance, the number of vibrations was only five. when the marked pole of the second bar was put on the opposite side of the plate at the same distance ( .), the vibrations were reduced to two. but when the second pole was an unmarked one, yet occupying exactly the same position, the vibrations rose to twenty-two. by removing the stronger of these two opposite poles a little way from the plate, the vibrations increased to thirty-one, or nearly the original number. but on removing it _altogether_, they fell to between five and six. . nothing can be more clear, therefore, than that with iron, and bodies admitting of ordinary magnetic induction, _opposite_ poles on opposite sides of the edge of the plate neutralize each other's effect, whilst _similar_ poles exalt the action; a single pole end on is also sufficient. but with copper, and substances not sensible to ordinary magnetic impressions, _similar_ poles on opposite sides of the plate neutralize each other; _opposite_ poles exalt the action; and a single pole at the edge or end on does nothing. . nothing can more completely show the thorough independence of the effects obtained with the metals by arago, and those due to ordinary magnetic forces; and henceforth, therefore, the application of two poles to various moving substances will, if they appear at all magnetically affected, afford a proof of the nature of that affection. if opposite poles produce a greater effect than one pole, the result will be due to electric currents. if similar poles produce more effect than one, then the power is _not_ electrical; it is not like that active in the metals and carbon when they are moving, and in most cases will probably be found to be not even magnetical, but the result of irregular causes not anticipated and consequently not guarded against. . the result of these investigations tends to show that there are really but very few bodies that are magnetic in the manner of iron. i have often sought for indications of this power in the common metals and other substances; and once in illustration of arago's objection ( .), and in hopes of ascertaining the existence of currents in metals by the momentary approach of a magnet, suspended a disc of copper by a single fibre of silk in an excellent vacuum, and approximated powerful magnets on the outside of the jar, making them approach and recede in unison with a pendulum that vibrated as the disc would do: but no motion could be obtained; not merely, no indication of ordinary magnetic powers, but none or _any electric current_ occasioned in the metal by the approximation and recession of the magnet. i therefore venture to arrange substances in three classes as regards their relation to magnets; first, those which are affected when at rest, like iron, nickel, &c., being such as possess ordinary magnetic properties; then, those which are affected when in motion, being conductors of electricity in which are produced electric currents by the inductive force of the magnet; and, lastly, those which are perfectly indifferent to the magnet, whether at rest or in motion. . although it will require further research, and probably close investigation, both experimental and mathematical, before the exact mode of action between a magnet and metal moving relatively to each other is ascertained; yet many of the results appear sufficiently clear and simple to allow of expression in a somewhat general manner.--if a terminated wire move so as to cut a magnetic curve, a power is called into action which tends to urge an electric current through it; but this current cannot be brought into existence unless provision be made at the ends of the wire for its discharge and renewal. . if a second wire move in the same direction as the first, the same power is exerted upon it, and it is therefore unable to alter the condition of the first: for there appear to be no natural differences among substances when connected in a series, by which, when moving under the same circumstances relative to the magnet, one tends to produce a more powerful electric current in the whole circuit than another ( . .). . but if the second wire move with a different velocity, or in some other direction, then variations in the force exerted take place; and if connected at their extremities, an electric current passes through them. . taking, then, a mass of metal or an endless wire, and referring to the pole of the magnet as a centre of action, (which though perhaps not strictly correct may be allowed for facility of expression, at present,) if all parts move in the same direction, and with the same angular velocity, and through magnetic curves of constant intensity, then no electric currents are produced. this point is easily observed with masses subject to the earth's magnetism, and may be proved with regard to small magnets; by rotating them, and leaving the metallic arrangements stationary, no current is produced. . if one part of the wire or metal cut the magnetic curves, whilst the other is stationary, then currents are produced. all the results obtained with the galvanometer are more or less of this nature, the galvanometer extremity being the fixed part. even those with the wire, galvanometer, and earth ( .), may be considered so without any error in the result. . if the motion of the metal be in the same direction, but the angular velocity of its parts relative to the pole of the magnet different, then currents are produced. this is the case in arago's experiment, and also in the wire subject to the earth's induction ( .), when it was moved from west to east. . if the magnet moves not directly to or from the arrangement, but laterally, then the case is similar to the last. . if different parts move in opposite directions across the magnetic curves, then the effect is a maximum for equal velocities. . all these in fact are variations of one simple condition, namely, that all parts of the mass shall not move in the same direction across the curves, and with the same angular velocity. but they are forms of expression which, being retained in the mind, i have found useful when comparing the consistency of particular phenomena with general results. _royal institution, december , ._ third series. § . _identity of electricities derived from different sources._ § . _relation by measure of common and voltaic electricity._ [read january th and th, .] § . _identity of electricities derived from different sources._ . the progress of the electrical researches which i have had the honour to present to the royal society, brought me to a point at which it was essential for the further prosecution of my inquiries that no doubt should remain of the identity or distinction of electricities excited by different means. it is perfectly true that cavendish[a], wollaston[b], colladon[c], and others, have in succession removed some of the greatest objections to the acknowledgement of the identity of common, animal and voltaic electricity, and i believe that most philosophers consider these electricities as really the same. but on the other hand it is also true, that the accuracy of wollaston's experiments has been denied[d]; and also that one of them, which really is no proper proof of chemical decomposition by common electricity ( . .), has been that selected by several experimenters as the test of chemical action ( . .). it is a fact, too, that many philosophers are still drawing distinctions between the electricities from different sources; or at least doubting whether their identity is proved. sir humphry davy, for instance, in his paper on the torpedo[e], thought it probable that animal electricity would be found of a peculiar kind; and referring to it, to common electricity, voltaic electricity and magnetism, has said, "distinctions might be established in pursuing the various modifications or properties of electricity in those different forms, &c." indeed i need only refer to the last volume of the philosophical transactions to show that the question is by no means considered as settled[f]. [a] phil. trans. , p. . [b] ibid. , p. . [c] annnles de chimie, , p. , &c. [d] phil. trans. , p. , note. [e] phil. trans. , p. . "common electricity is excited upon non-conductors, and is readily carried off by conductors and imperfect conductors. voltaic electricity is excited upon combinations of perfect and imperfect conductors, and is only transmitted by perfect conductors or imperfect conductors of the best kind. magnetism, if it be a form of electricity, belongs only to perfect conductors; and, in its modifications, to a peculiar class of them[ ]. animal electricity resides only in the imperfect conductors forming the organs of living animals, &c." [ ] dr. ritchie has shown this is not the case. phil. trans. , p. . [f] phil. trans. , p. . dr. davy, in making experiments on the torpedo, obtains effects the same as those produced by common and voltaic electricity, and says that in its magnetic and chemical power it does not seem to be essentially peculiar,--p. ; but he then says, p. , there are other points of difference; and after referring to them, adds, "how are these differences to be explained? do they admit of explanation similar to that advanced by mr. cavendish in his theory of the torpedo; or may we suppose, according to the analogy of the solar ray, that the electrical power, whether excited by the common machine, or by the voltaic battery, or by the torpedo, is not a simple power, but a combination of powers, which may occur variously associated, and produce all the varieties of electricity with which we are acquainted?" at p. of the same volume of transactions is dr. ritchie's paper, from which the following are extracts: "common electricity is diffused over the surface of the metal;--voltaic electricity exists within the metal. free electricity is conducted over the surface of the thinnest gold leaf as effectually as over a mass of metal having the same surface;--voltaic electricity requires thickness of metal for its conduction," p. : and again, "the supposed analogy between common and voltaic electricity, which was so eagerly traced after the invention of the pile, completely fails in this case, which was thought to afford the most striking resemblance." p. . . notwithstanding, therefore, the general impression of the identity of electricities, it is evident that the proofs have not been sufficiently clear and distinct to obtain the assent of all those who were competent to consider the subject; and the question seemed to me very much in the condition of that which sir h. davy solved so beautifully,--namely, whether voltaic electricity in all cases merely eliminated, or did not in some actually produce, the acid and alkali found after its action upon water. the same necessity that urged him to decide the doubtful point, which interfered with the extension of his views, and destroyed the strictness of his reasoning, has obliged me to ascertain the identity or difference of common and voltaic electricity. i have satisfied myself that they are identical, and i hope the experiments which i have to offer and the proofs flowing from them, will be found worthy the attention of the royal society. . the various phenomena exhibited by electricity may, for the purposes of comparison, be arranged under two heads; namely, those connected with electricity of tension, and those belonging to electricity in motion. this distinction is taken at present not as philosophical, but merely as convenient. the effect of electricity of tension, at rest, is either attraction or repulsion at sensible distances. the effects of electricity in motion or electrical currents may be considered as st, evolution of heat; nd, magnetism; rd, chemical decomposition; th, physiological phenomena; th, spark. it will be my object to compare electricities from different sources, and especially common and voltaic electricities, by their power of producing these effects. i. _voltaic electricity._ . _tension._--when a voltaic battery of pairs of plates has its extremities examined by the ordinary electrometer, it is well known that they are found positive and negative, the gold leaves at the same extremity repelling each other, the gold leaves at different extremities attracting each other, even when half an inch or more of air intervenes. . that ordinary electricity is discharged by points with facility through air; that it is readily transmitted through highly rarefied air; and also through heated air, as for instance a flame; is due to its high tension. i sought, therefore, for similar effects in the discharge of voltaic electricity, using as a test of the passage of the electricity either the galvanometer or chemical action produced by the arrangement hereafter to be described ( . .). . the voltaic battery i had at my disposal consisted of pairs of plates four inches square, with double coppers. it was insulated throughout, and diverged a gold leaf electrometer about one third of an inch. on endeavouring to discharge this battery by delicate points very nicely arranged and approximated, either in the air or in an exhausted receiver, i could obtain no indications of a current, either by magnetic or chemical action. in this, however, was found no point of discordance between voltaic and common electricity; for when a leyden battery ( .) was charged so as to deflect the gold leaf electrometer to the same degree, the points were found equally unable to discharge it with such effect as to produce either magnetic or chemical action. this was not because common electricity could not produce both these effects ( . .); but because when of such low intensity the quantity required to make the effects visible (being enormously great ( . .),) could not be transmitted in any reasonable time. in conjunction with the other proofs of identity hereafter to be given, these effects of points also prove identity instead of difference between voltaic and common electricity. . as heated air discharges common electricity with far greater facility than points, i hoped that voltaic electricity might in this way also be discharged. an apparatus was therefore constructed (plate iii. fig. .), in which ab is an insulated glass rod upon which two copper wires, c, d, are fixed firmly; to these wires are soldered two pieces of fine platina wire, the ends of which are brought very close to each other at _e_, but without touching; the copper wire c was connected with the positive pole of a voltaic battery, and the wire d with a decomposing apparatus ( . .), from which the communication was completed to the negative pole of the battery. in these experiments only two troughs, or twenty pairs of plates, were used. . whilst in the state described, no decomposition took place at the point _a_, but when the side of a spirit-lamp flame was applied to the two platina extremities at _e_, so as to make them bright red-hot, decomposition occurred; iodine soon appeared at the point _a_, and the transference of electricity through the heated air was established. on raising the temperature of the points _e_ by a blowpipe, the discharge was rendered still more free, and decomposition took place instantly. on removing the source of heat, the current immediately ceased. on putting the ends of the wires very close by the side of and parallel to each other, but not touching, the effects were perhaps more readily obtained than before. on using a larger voltaic battery ( .), they were also more freely obtained. . on removing the decomposing apparatus and interposing a galvanometer instead, heating the points _e_ as the needle would swing one way, and removing the heat during the time of its return ( .), feeble deflections were soon obtained: thus also proving the current through heated air; but the instrument used was not so sensible under the circumstances as chemical action. . these effects, not hitherto known or expected under this form, are only cases of the discharge which takes place through air between the charcoal terminations of the poles of a powerful battery, when they are gradually separated after contact. then the passage is through heated air exactly as with common electricity, and sir h. davy has recorded that with the original battery of the royal institution this discharge passed through a space of at least four inches[a]. in the exhausted receiver the electricity would _strike_ through nearly half an inch of space, and the combined effects of rarefaction and heat were such upon the inclosed air us to enable it to conduct the electricity through a space of six or seven inches. [a] elements of chemical philosophy, p. . the instantaneous charge of a leyden battery by the poles of a voltaic apparatus is another proof of the tension, and also the quantity, of electricity evolved by the latter. sir h. davy says[a], "when the two conductors from the ends of the combination were connected with a leyden battery, one with the internal, the other with the external coating, the battery instantly became charged; and on removing the wires and making the proper connexions, either a shock or a _spark_ could be perceived: and the least possible time of contact was sufficient to renew the charge to its full intensity." [a] elements of chemical philosophy, p. . . _in motion:_ i. _evolution of heat._--the evolution of heat in wires and fluids by the voltaic current is matter of general notoriety. . ii. _magnetism._--no fact is better known to philosophers than the power of the voltaic current to deflect the magnetic needle, and to make magnets according to _certain laws_; and no effect can be more distinctive of an electrical current. . iii. _chemical decomposition._--the chemical powers of the voltaic current, and their subjection to _certain laws_, are also perfectly well known. . iv. _physiological effects._--the power of the voltaic current, when strong, to shock and convulse the whole animal system, and when weak to affect the tongue and the eyes, is very characteristic. . v. _spark_.--the brilliant star of light produced by the discharge of a voltaic battery is known to all as the most beautiful light that man can produce by art. * * * * * . that these effects may be almost infinitely varied, some being exalted whilst others are diminished, is universally acknowledged; and yet without any doubt of the identity of character of the voltaic currents thus made to differ in their effect. the beautiful explication of these variations afforded by cavendish's theory of quantity and intensity requires no support at present, as it is not supposed to be doubted. . in consequence of the comparisons that will hereafter arise between wires carrying voltaic and ordinary electricities, and also because of certain views of the condition of a wire or any other conducting substance connecting the poles of a voltaic apparatus, it will be necessary to give some definite expression of what is called the voltaic current, in contradistinction to any supposed peculiar state of arrangement, not progressive, which the wire or the electricity within it may be supposed to assume. if two voltaic troughs pn, p'n', fig. , be symmetrically arranged and insulated, and the ends np' connected by a wire, over which a magnetic needle is suspended, the wire will exert no effect over the needle; but immediately that the ends pn' are connected by another wire, the needle will be deflected, and will remain so as long as the circuit is complete. now if the troughs merely act by causing a peculiar arrangement in the wire either of its particles or its electricity, that arrangement constituting its electrical and magnetic state, then the wire np' should be in a similar state of arrangement _before_ p and n' were connected, to what it is afterwards, and should have deflected the needle, although less powerfully, perhaps to one half the extent which would result when the communication is complete throughout. but if the magnetic effects depend upon a current, then it is evident why they could not be produced in _any_ degree before the circuit was complete; because prior to that no current could exist. . by _current_, i mean anything progressive, whether it be a fluid of electricity, or two fluids moving in opposite directions, or merely vibrations, or, speaking still more generally, progressive forces. by _arrangement_, i understand a local adjustment of particles, or fluids, or forces, not progressive. many other reasons might be urged in support of the view of a _current_ rather than an _arrangement_, but i am anxious to avoid stating unnecessarily what will occur to others at the moment. ii. _ordinary electricity._ . by ordinary electricity i understand that which can be obtained from the common machine, or from the atmosphere, or by pressure, or cleavage of crystals, or by a multitude of other operations; its distinctive character being that of great intensity, and the exertion of attractive and repulsive powers, not merely at sensible but at considerable distances. . _tension._ the attractions and repulsions at sensible distances, caused by ordinary electricity, are well known to be so powerful in certain cases, as to surpass, almost infinitely, the similar phenomena produced by electricity, otherwise excited. but still those attractions and repulsions are exactly of the same nature as those already referred to under the head _tension, voltaic electricity_ ( .); and the difference in degree between them is not greater than often occurs between cases of ordinary electricity only. i think it will be unnecessary to enter minutely into the proofs of the identity of this character in the two instances. they are abundant; are generally admitted as good; and lie upon the surface of the subject: and whenever in other parts of the comparison i am about to draw, a similar case occurs, i shall content myself with a mere announcement of the similarity, enlarging only upon those parts where the great question of distinction or identity still exists. . the discharge of common electricity through heated air is a well-known fact. the parallel case of voltaic electricity has already been described ( , &c.). . _in motion._ i. _evolution of heat._--the heating power of common electricity, when passed through wires or other substances, is perfectly well known. the accordance between it and voltaic electricity is in this respect complete. mr. harris has constructed and described[a] a very beautiful and sensible instrument on this principle, in which the heat produced in a wire by the discharge of a small portion of common electricity is readily shown, and to which i shall have occasion to refer for experimental proof in a future part of this paper ( .). [a] philosophical transactions, , p. . edinburgh transactions, . harris on a new electrometer, &c. &c. . ii. _magnetism._--voltaic electricity has most extraordinary and exalted magnetic powers. if common electricity be identical with it, it ought to have the same powers. in rendering needles or bars magnetic, it is found to agree with voltaic electricity, and the _direction_ of the magnetism, in both cases, is the same; but in deflecting the magnetic needle, common electricity has been found deficient, so that sometimes its power has been denied altogether, and at other times distinctions have been hypothetically assumed for the purpose of avoiding the difficulty[a]. [a] demonferrand's manuel d'electricité dynamique, p. . . m. colladon, of geneva, considered that the difference might be due to the use of insufficient quantities of common electricity in all the experiments before made on this head; and in a memoir read to the academie des sciences in [a], describes experiments, in which, by the use of a battery, points, and a delicate galvanometer, he succeeded in obtaining deflections, and thus establishing identity in that respect. mm. arago, ampère, and savary, are mentioned in the paper as having witnessed a successful repetition of the experiments. but as no other one has come forward in confirmation, mm. arago, ampère, and savary, not having themselves published (that i am aware of) their admission of the results, and as some have not been able to obtain them, m. colladon's conclusions have been occasionally doubted or denied; and an important point with me was to establish their accuracy, or remove them entirely from the body of received experimental research. i am happy to say that my results fully confirm those by m. colladon, and i should have had no occasion to describe them, but that they are essential as proofs of the accuracy of the final and general conclusions i am enabled to draw respecting the magnetic and chemical action of electricity ( . . . . &c.). [a] annales de chimie, xxxiii. p. . . the plate electrical machine i have used is fifty inches in diameter; it has two sets of rubbers; its prime conductor consists of two brass cylinders connected by a third, the whole length being twelve feet, and the surface in contact with air about square inches. when in good excitation, one revolution of the plate will give ten or twelve sparks from the conductors, each an inch in length. sparks or flashes from ten to fourteen inches in length may easily be drawn from the conductors. each turn of the machine, when worked moderately, occupies about / ths of a second. . the electric battery consisted of fifteen equal jars. they are coated eight inches upwards from the bottom, and are twenty-three inches in circumference, so that each contains one hundred and eighty-four square inches of glass, coated on both sides; this is independent of the bottoms, which are of thicker glass, and contain each about fifty square inches. . a good _discharging train_ was arranged by connecting metallically a sufficiently thick wire with the metallic gas pipes of the house, with the metallic gas pipes belonging to the public gas works of london; and also with the metallic water pipes of london. it was so effectual in its office as to carry off instantaneously electricity of the feeblest tension, even that of a single voltaic trough, and was essential to many of the experiments. . the galvanometer was one or the other of those formerly described ( . .), but the glass jar covering it and supporting the needle was coated inside and outside with tinfoil, and the upper part (left uncoated, that the motions of the needle might be examined,) was covered with a frame of wire-work, having numerous sharp points projecting from it. when this frame and the two coatings were connected with the discharging train ( .), an insulated point or ball, connected with the machine when most active, might be brought within an inch of any part of the galvanometer, yet without affecting the needle within by ordinary electrical attraction or repulsion. . in connexion with these precautions, it may be necessary to state that the needle of the galvanometer is very liable to have its magnetic power deranged, diminished, or even inverted by the passage of a shock through the instrument. if the needle be at all oblique, in the wrong direction, to the coils of the galvanometer when the shock passes, effects of this kind are sure to happen. . it was to the retarding power of bad conductors, with the intention of diminishing its _intensity_ without altering its _quantity_, that i first looked with the hope of being able to make common electricity assume more of the characters and power of voltaic electricity, than it is usually supposed to have. , the coating and armour of the galvanometer were first connected with the discharging train ( .); the end b ( .) of the galvanometer wire was connected with the outside coating of the battery, and then both these with the discharging train; the end a of the galvanometer wire was connected with a discharging rod by a wet thread four feet long; and finally, when the battery ( .) had been positively charged by about forty turns of the machine, it was discharged by the rod and the thread through the galvanometer. the needle immediately moved. . during the time that the needle completed its vibration in the first direction and returned, the machine was worked, and the battery recharged; and when the needle in vibrating resumed its first direction, the discharge was again made through the galvanometer. by repeating this action a few times, the vibrations soon extended to above ° on each side of the line of rest. . this effect could be obtained at pleasure. nor was it varied, apparently, either in direction or degree, by using a short thick string, or even four short thick strings in place of the long fine thread. with a more delicate galvanometer, an excellent swing of the needle could be obtained by one discharge of the battery. . on reversing the galvanometer communications so as to pass the discharge through from b to a, the needle was equally well deflected, but in the opposite direction. . the deflections were in the same direction as if a voltaic current had been passed through the galvanometer, i.e. the positively charged surface of the electric battery coincided with the positive end of the voltaic apparatus ( .) and the negative surface of the former with the negative end of the latter. . the battery was then thrown out of use, and the communications so arranged that the current could be passed from the prime conductor, by the discharging rod held against it, through the wet string, through the galvanometer coil, and into the discharging train ( ), by which it was finally dispersed. this current could be stopped at any moment, by removing the discharging rod, and either stopping the machine or connecting the prime conductor by another rod with the discharging train; and could be as instantly renewed. the needle was so adjusted, that whilst vibrating in moderate and small arcs, it required time equal to twenty-five beats of a watch to pass in one direction through the arc, and of course an equal time to pass in the other direction. . thus arranged, and the needle being stationary, the current, direct from the machine, was sent through the galvanometer for twenty-five beats, then interrupted for other twenty-five beats, renewed for twenty-five beats more, again interrupted for an equal time, and so on continually. the needle soon began to vibrate visibly, and after several alternations of this kind, the vibration increased to ° or more. . on changing the direction of the current through the galvanometer, the direction of the deflection of the needle was also changed. in all cases the motion of the needle was in direction the same as that caused either by the use of the electric battery or a voltaic trough ( ). . i now rejected the wet string, and substituted a copper wire, so that the electricity of the machine passed at once into wires communicating directly with the discharging train, the galvanometer coil being one of the wires used for the discharge. the effects were exactly those obtained above ( ). . instead of passing the electricity through the system, by bringing the discharging rod at the end of it into contact with the conductor, four points were fixed on to the rod; when the current was to pass, they were held about twelve inches from the conductor, and when it was not to pass, they were turned away. then operating as before ( .), except with this variation, the needle was soon powerfully deflected, and in perfect consistency with the former results. points afforded the means by which colladon, in all cases, made his discharges. . finally, i passed the electricity first through an exhausted receiver, so as to make it there resemble the aurora borealis, and then through the galvanometer to the earth; and it was found still effective in deflecting the needle, and apparently with the same force as before. . from all these experiments, it appears that a current of common electricity, whether transmitted through water or metal, or rarefied air, or by means of points in common air, is still able to deflect the needle; the only requisite being, apparently, to allow time for its action: that it is, in fact, just as magnetic in every respect as a voltaic current, and that in this character therefore no distinction exists. . imperfect conductors, as water, brine, acids, &c. &c. will be found far more convenient for exhibiting these effects than other modes of discharge, as by points or balls; for the former convert at once the charge of a powerful battery into a feeble spark discharge, or rather continuous current, and involve little or no risk of deranging the magnetism of the needles ( .). . iii. _chemical decomposition._--the chemical action of voltaic electricity is characteristic of that agent, but not more characteristic than are the _laws_ under which the bodies evolved by decomposition arrange themselves at the poles. dr. wollaston showed[a] that common electricity resembled it in these effects, and "that they are both essentially the same"; but he mingled with his proofs an experiment having a resemblance, and nothing more, to a case of voltaic decomposition, which however he himself partly distinguished; and this has been more frequently referred to by some, on the one hand, to prove the occurrence of electro-chemical decomposition, like that of the pile, and by others to throw doubt upon the whole paper, than the more numerous and decisive experiments which he has detailed. [a] philosophical transactions, , pp. , . . i take the liberty of describing briefly my results, and of thus adding my testimony to that of dr. wollaston on the identity of voltaic and common electricity as to chemical action, not only that i may facilitate the repetition of the experiments, but also lead to some new consequences respecting electrochemical decomposition ( . .). . i first repeated wollaston's fourth experiment[a], in which the ends of coated silver wires are immersed in a drop of sulphate of copper. by passing the electricity of the machine through such an arrangement, that end in the drop which received the electricity became coated with metallic copper. one hundred turns of the machine produced an evident effect; two hundred turns a very sensible one. the decomposing action was however very feeble. very little copper was precipitated, and no sensible trace of silver from the other pole appeared in the solution. [a] philosophical transactions, , p. . . a much more convenient and effectual arrangement for chemical decompositions by common electricity, is the following. upon a glass plate, fig. , placed over, but raised above a piece of white paper, so that shadows may not interfere, put two pieces of tinfoil _a, b_; connect one of these by an insulated wire _c_, or wire and string ( .) with the machine, and the other _g_, with the discharging train ( .) or the negative conductor; provide two pieces of fine platina wire, bent as in fig. , so that the part _d, f_ shall be nearly upright, whilst the whole is resting on the three bearing points _p, e, f_ place these as in fig. ; the points _p, n_ then become the decomposing poles. in this way surfaces of contact, as minute as possible, can be obtained at pleasure, and the connexion can be broken or renewed in a moment, and the substances acted upon examined with the utmost facility. . a coarse line was made on the glass with solution of sulphate of copper, and the terminations _p_ and _n_ put into it; the foil _a_ was connected with the positive conductor of the machine by wire and wet string, so that no sparks passed: twenty turns of the machine caused the precipitation of so much copper on the end _n_, that it looked like copper wire; no apparent change took place at _p_. . a mixture of equal parts of muriatic acid and water was rendered deep blue by sulphate of indigo, and a large drop put on the glass, fig. , so that _p_ and _n_ were immersed at opposite sides: a single turn of the machine showed bleaching effects round _p_, from evolved chlorine. after twenty revolutions no effect of the kind was visible at _n_, but so much chlorine had been set free at _p_, that when the drop was stirred the whole became colourless. . a drop of solution of iodide of potassium mingled with starch was put into the same position at _p_ and _n_; on turning the machine, iodine was evolved at _p_, but not at _n_. . a still further improvement in this form of apparatus consists in wetting a piece of filtering paper in the solution to be experimented on, and placing that under the points _p_ and _n_, on the glass: the paper retains the substance evolved at the point of evolution, by its whiteness renders any change of colour visible, and allows of the point of contact between it and the decomposing wires being contracted to the utmost degree. a piece of paper moistened in the solution of iodide of potassium and starch, or of the iodide alone, with certain precautions ( .), is a most admirable test of electro-chemical action; and when thus placed and acted upon by the electric current, will show iodine evolved at _p_ by only half a turn of the machine. with these adjustments and the use of iodide of potassium on paper, chemical action is sometimes a more delicate test of electrical currents than the galvanometer ( .). such cases occur when the bodies traversed by the current are bad conductors, or when the quantity of electricity evolved or transmitted in a given time is very small. . a piece of litmus paper moistened in solution of common salt or sulphate of soda, was quickly reddened at _p_. a similar piece moistened in muriatic acid was very soon bleached at _p_. no effects of a similar kind took place at _n_. . a piece of turmeric paper moistened in solution of sulphate of soda was reddened at _n_ by two or three turns of the machine, and in twenty or thirty turns plenty of alkali was there evolved. on turning the paper round, so that the spot came under _p_, and then working the machine, the alkali soon disappeared, the place became yellow, and a brown alkaline spot appeared in the new part under _n_. . on combining a piece of litmus with a piece of turmeric paper, wetting both with solution of sulphate of soda, and putting the paper on the glass, so that _p_ was on the litmus and _n_ on the turmeric, a very few turns of the machine sufficed to show the evolution of acid at the former and alkali at the latter, exactly in the manner effected by a volta-electric current. . all these decompositions took place equally well, whether the electricity passed from the machine to the foil _a_, through water, or through wire only; by _contact_ with the conductor, or by _sparks_ there; provided the sparks were not so large as to cause the electricity to pass in sparks from _p_ to _n_, or towards _n_; and i have seen no reason to believe that in cases of true electro-chemical decomposition by the machine, the electricity passed in sparks from the conductor, or at any part of the current, is able to do more, because of its tension, than that which is made to pass merely as a regular current. . finally, the experiment was extended into the following form, supplying in this case the tidiest analogy between common and voltaic electricity. three compound pieces of litmus and turmeric paper ( .) were moistened in solution of sulphate of soda, and arranged on a plate of glass with platina wires, as in fig. . the wire _m_ was connected with the prime conductor of the machine, the wire _t_ with the discharging train, and the wires _r_ and _s_ entered into the course of the electrical current by means of the pieces of moistened paper; they were so bent as to rest each on three points, _n, r, p; n, s, p_, the points _r_ and _s_ being supported by the glass, and the others by the papers; the three terminations _p, p, p_ rested on the litmus, and the other three _n, n, n_ on the turmeric paper. on working the machine for a short time only, acid was evolved at _all_ the poles or terminations _p, p, p_, by which the electricity entered the solution, and alkali at the other poles _n, n, n_, by which the electricity left the solution. . in all experiments of electro-chemical decomposition by the common machine and moistened papers ( .), it is necessary to be aware of and to avoid the following important source of error. if a spark passes over moistened litmus and turmeric paper, the litmus paper (provided it be delicate and not too alkaline,) is reddened by it; and if several sparks are passed, it becomes powerfully reddened. if the electricity pass a little way from the wire over the surface of the moistened paper, before it finds mass and moisture enough to conduct it, then the reddening extends as far as the ramifications. if similar ramifications occur at the termination _n_, on the turmeric paper, they _prevent_ the occurrence of the red spot due to the alkali, which would otherwise collect there: sparks or ramifications from the points _n_ will also redden litmus paper. if paper moistened by a solution of iodide of potassium (which is an admirably delicate test of electro-chemical action,) be exposed to the sparks or ramifications, or even a feeble stream of electricity through the air from either the point _p_ or _n_, iodine will be immediately evolved. . these effects must not be confounded with those due to the true electro-chemical powers of common electricity, and must be carefully avoided when the latter are to be observed. no sparks should be passed, therefore, in any part of the current, nor any increase of intensity allowed, by which the electricity may be induced to pass between the platina wires and the moistened papers, otherwise than by conduction; for if it burst through the air, the effect referred to above ( .) ensues. . the effect itself is due to the formation of nitric acid by the combination of the oxygen and nitrogen of the air, and is, in fact, only a delicate repetition of cavendish's beautiful experiment. the acid so formed, though small in quantity, is in a high state of concentration as to water, and produces the consequent effects of reddening the litmus paper; or preventing the exhibition of alkali on the turmeric paper; or, by acting on the iodide of potassium, evolving iodine. . by moistening a very small slip of litmus paper in solution of caustic potassa, and then passing the electric spark over its length in the air, i gradually neutralized the alkali, and ultimately rendered the paper red; on drying it, i found that nitrate of potassa had resulted from the operation, and that the paper had become touch-paper. . either litmus paper or white paper, moistened in a strong solution of iodide of potassium, offers therefore a very simple, beautiful, and ready means of illustrating cavendish's experiment of the formation of nitric acid from the atmosphere. . i have already had occasion to refer to an experiment ( . .) made by dr. wollaston, which is insisted upon too much, both by those who oppose and those who agree with the accuracy of his views respecting the identity of voltaic and ordinary electricity. by covering fine wires with glass or other insulating substances, and then removing only so much matter as to expose the point, or a section of the wires, and by passing electricity through two such wires, the guarded points of which were immersed in water, wollaston found that the water could be decomposed even by the current from the machine, without sparks, and that two streams of gas arose from the points, exactly resembling, in appearance, those produced by voltaic electricity, and, like the latter, giving a mixture of oxygen and hydrogen gases. but dr. wollaston himself points out that the effect is different from that of the voltaic pile, inasmuch as both oxygen and hydrogen are evolved from _each_ pole; he calls it "a very close _imitation_ of the galvanic phenomena," but adds that "in fact the resemblance is not complete," and does not trust to it to establish the principles correctly laid down in his paper. . this experiment is neither more nor less than a repetition, in a refined manner, of that made by dr. pearson in [a], and previously by mm. paets van troostwyk and deiman in or earlier. that the experiment should never be quoted as proving true electro-chemical decomposition, is sufficiently evident from the circumstance, that the _law_ which regulates the transference and final place of the evolved bodies ( . .) has no influence here. the water is decomposed at both poles independently of each other, and the oxygen and hydrogen evolved at the wires are the elements of the water existing the instant before in those places. that the poles, or rather points, have no mutual decomposing dependence, may be shown by substituting a wire, or the finger, for one of them, a change which does not at all interfere with the other, though it stops all action at the changed pole. this fact may be observed by turning the machine for some time; for though bubbles will rise from the point left unaltered, in quantity sufficient to cover entirely the wire used for the other communication, if they could be applied to it, yet not a single bubble will appear on that wire. [a] nicholson's journal, to. vol. i. pp. , . . . when electro-chemical decomposition takes place, there is great reason to believe that the _quantity_ of matter decomposed is not proportionate to the intensity, but to the quantity of electricity passed ( .). of this i shall be able to offer some proofs in a future part of this paper ( . .). but in the experiment under consideration, this is not the case. if, with a constant pair of points, the electricity be passed from the machine in sparks, a certain proportion of gas is evolved; but if the sparks be rendered shorter, less gas is evolved; and if no sparks be passed, there is scarcely a sensible portion of gases set free. on substituting solution of sulphate of soda for water, scarcely a sensible quantity of gas could be procured even with powerful sparks, and nearly none with the mere current; yet the quantity of electricity in a given time was the same in all these cases. . i do not intend to deny that with such an apparatus common electricity can decompose water in a manner analogous to that of the voltaic pile; i believe at present that it can. but when what i consider the true effect only was obtained, the quantity of gas given off was so small that i could not ascertain whether it was, as it ought to be, oxygen at one wire and hydrogen at the other. of the two streams one seemed more copious than the other, and on turning the apparatus round, still the same side in relation to the machine; gave the largest stream. on substituting solution of sulphate of soda for pure water ( .), these minute streams were still observed. but the quantities were so small, that on working the machine for half an hour i could not obtain at either pole a bubble of gas larger than a small grain of sand. if the conclusion which i have drawn ( .) relating to the amount of chemical action be correct, this ought to be the case. . i have been the more anxious to assign the true value of this experiment as a test of electro-chemical action, because i shall have occasion to refer to it in cases of supposed chemical action by magneto-electric and other electric currents ( . .) and elsewhere. but, independent of it, there cannot be now a doubt that dr. wollaston was right in his general conclusion; and that voltaic and common electricity have powers of chemical decomposition, alike in their nature, and governed by the same law of arrangement. . iv. _physiological effects._--the power of the common electric current to shock and convulse the animal system, and when weak to affect the tongue and the eyes, may be considered as the same with the similar power of voltaic electricity, account being taken of the intensity of the one electricity and duration of the other. when a wet thread was interposed in the course of the current of common electricity from the battery ( .) charged by eight or ten[a] revolutions of the machine in good action ( .), and the discharge made by platina spatulas through the tongue or the gums, the effect upon the tongue and eyes was exactly that of a momentary feeble voltaic circuit. [a] or even from thirty to forty. . v. _spark._--the beautiful flash of light attending the discharge of common electricity is well known. it rivals in brilliancy, if it does not even very much surpass, the light from the discharge of voltaic electricity; but it endures for an instant only, and is attended by a sharp noise like that of a small explosion. still no difficulty can arise in recognising it to be the same spark as that from the voltaic battery, especially under certain circumstances. the eye cannot distinguish the difference between a voltaic and a common electricity spark, if they be taken between amalgamated surfaces of metal, at intervals only, and through the same distance of air. . when the leyden battery ( .) was discharged through a wet string placed in some part of the circuit away from the place where the spark was to pass, the spark was yellowish, flamy, having a duration sensibly longer than if the water had not been interposed, was about three-fourths of an inch in length, was accompanied by little or no noise, and whilst losing part of its usual character had approximated in some degree to the voltaic spark. when the electricity retarded by water was discharged between pieces of charcoal, it was exceedingly luminous and bright upon both surfaces of the charcoal, resembling the brightness of the voltaic discharge on such surfaces. when the discharge of the unretarded electricity was taken upon charcoal, it was bright upon both the surfaces, (in that respect resembling the voltaic spark,) but the noise was loud, sharp, and ringing. . i have assumed, in accordance, i believe, with the opinion of every other philosopher, that atmospheric electricity is of the same nature with ordinary electricity ( .), and i might therefore refer to certain published statements of chemical effects produced by the former as proofs that the latter enjoys the power of decomposition in common with voltaic electricity. but the comparison i am drawing is far too rigorous to allow me to use these statements without being fully assured of their accuracy; yet i have no right to suppress them, because, if accurate, they establish what i am labouring to put on an undoubted foundation, and have priority to my results. . m. bonijol of geneva[a] is said to have constructed very delicate apparatus for the decomposition of water by common electricity. by connecting an insulated lightning rod with his apparatus, the decomposition of the water proceeded in a continuous and rapid manner even when the electricity of the atmosphere was not very powerful. the apparatus is not described; but as the diameter of the wire is mentioned as very small, it appears to have been similar in construction to that of wollaston ( .); and as that does not furnish a case of true polar electro-chemical decomposition ( .), this result of m. bonijol does not prove the identity in chemical action of common and voltaic electricity. [a] bibliothèque universelle, , tome xlv. p. . . at the same page of the bibliothèque universelle, m. bonijol is said to have decomposed, _potash_, and also chloride of silver, by putting them into very narrow tubes and passing electric sparks from an ordinary machine over them. it is evident that these offer no analogy to cases of true voltaic decomposition, where the electricity only decomposes when it is _conducted_ by the body acted upon, and ceases to decompose, according to its ordinary laws, when it passes in sparks. these effects are probably partly analogous to that which takes place with water in pearson's or wollaston's apparatus, and may be due to very high temperature acting on minute portions of matter; or they may be connected with the results in air ( .). as nitrogen can combine directly with oxygen under the influence of the electric spark ( .), it is not impossible that it should even take it from the potassium of the potash, especially as there would be plenty of potassa in contact with the acting particles to combine with the nitric acid formed. however distinct all these actions may be from true polar electro-chemical decompositions, they are still highly important, and well-worthy of investigation. . the late mr. barry communicated a paper to the royal society[a] last year, so distinct in the details, that it would seem at once to prove the identity in chemical action of common and voltaic electricity; but, when examined, considerable difficulty arises in reconciling certain of the effects with the remainder. he used two tubes, each having a wire within it passing through the closed end, as is usual for voltaic decompositions. the tubes were filled with solution of sulphate of soda, coloured with syrup of violets, and connected by a portion of the same solution, in the ordinary manner; the wire in one tube was connected by a _gilt thread_ with the string of an insulated electrical kite, and the wire in the other tube by a similar _gilt thread_ with the ground. hydrogen soon appeared in the tube connected with the kite, and oxygen in the other, and in ten minutes the liquid in the first tube was green from the alkali evolved, and that in the other red from free acid produced. the only indication of the strength or intensity of the atmospheric electricity is in the expression, "the usual shocks were felt on touching the string." [a] philosophical transactions, , p. . . that the electricity in this case does not resemble that from any ordinary source of common electricity, is shown by several circumstances. wollaston could not effect the decomposition of water by such an arrangement, and obtain the gases in _separate_ vessels, using common electricity; nor have any of the numerous philosophers, who have employed such an apparatus, obtained any such decomposition, either of water or of a neutral salt, by the use of the machine. i have lately tried the large machine ( .) in full action for a quarter of an hour, during which time seven hundred revolutions were made, without producing any sensible effects, although the shocks that it would then give must have been far more powerful and numerous than could have been taken, with any chance of safety, from an electrical kite-string; and by reference to the comparison hereafter to be made ( .), it will be seen that for common electricity to have produced the effect, the quantity must have been awfully great, and apparently far more than could have been conducted to the earth by a gilt thread, and at the same time only have produced the "usual shocks." . that the electricity was apparently not analogous to voltaic electricity is evident, for the "usual shocks" only were produced, and nothing like the terrible sensation due to a voltaic battery, even when it has a tension so feeble as not to strike through the eighth of an inch of air. . it seems just possible that the air which was passing by the kite and string, being in an electrical state sufficient to produce the "usual shocks" only, could still, when the electricity was drawn off below, renew the charge, and so continue the current. the string was feet long, and contained two double threads. but when the enormous quantity which must have been thus collected is considered ( . .), the explanation seems very doubtful. i charged a voltaic battery of twenty pairs of plates four inches square with double coppers very strongly, insulated it, connected its positive extremity with the discharging train ( .), and its negative pole with an apparatus like that of mr. barry, communicating by a wire inserted three inches into the wet soil of the ground. this battery thus arranged produced feeble decomposing effects, as nearly as i could judge answering the description mr. barry has given. its intensity was, of course, far lower than the electricity of the kite-string, but the supply of quantity from the discharging train was unlimited. it gave no shocks to compare with the "usual shocks" of a kite-string. . mr. barry's experiment is a very important one to repeat and verify. if confirmed, it will be, as far as i am aware, the first recorded case of true electro-chemical decomposition of water by common electricity, and it will supply a form of electrical current, which, both in quantity and intensity, is exactly intermediate with those of the common electrical machine and the voltaic pile. * * * * * iii. _magneto-electricity._ . _tension_.--the attractions and repulsions due to the tension of ordinary electricity have been well observed with that evolved by magneto-electric induction. m. pixii, by using an apparatus, clever in its construction and powerful in its action[a], was able to obtain great divergence of the gold leaves of an electrometer[b]. [a] annales de chimie, l. p. . [b] ibid. li. p . . _in motion_: i. _evolution of heat._--the current produced by magneto-electric induction can heat a wire in the manner of ordinary electricity. at the british association of science at oxford, in june of the present year, i had the pleasure, in conjunction with mr. harris, professor daniell, mr. duncan, and others, of making an experiment, for which the great magnet in the museum, mr. harris's new electrometer ( .), and the magneto-electric coil described in my first paper ( .), were put in requisition. the latter had been modified in the manner i have elsewhere described[a] so as to produce an electric spark when its contact with the magnet was made or broken. the terminations of the spiral, adjusted so as to have their contact with each other broken when the spark was to pass, were connected with the wire in the electrometer, and it was found that each time the magnetic contact was made and broken, expansion of the air within the instrument occurred, indicating an increase, at the moment, of the temperature of the wire. [a] phil, mag. and annals, , vol. xi. p. . . ii. _magnetism._--these currents were discovered by their magnetic power. . iii. _chemical decomposition._--i have made many endeavours to effect chemical decomposition by magneto-electricity, but unavailingly. in july last i received an anonymous letter (which has since been published[a],) describing a magneto-electric apparatus, by which the decomposition of water was effected. as the term "guarded points" is used, i suppose the apparatus to have been wollaston's ( . &c.), in which case the results did not indicate polar electro-chemical decomposition. signor botto has recently published certain results which he has obtained[b]; but they are, as at present described, inconclusive. the apparatus he used was apparently that of dr. wollaston, which gives only fallacious indications ( . &c.). as magneto-electricity can produce sparks, it would be able to show the effects proper to this apparatus. the apparatus of m. pixii already referred to ( .) has however, in the hands of himself[c] and m. hachctte[d], given decisive chemical results, so as to complete this link in the chain of evidence. water was decomposed by it, and the oxygen and hydrogen obtained in separate tubes according to the law governing volta-electric and machine-electric decomposition. [a] lond. and edinb. phil. mag. and journ., , vol. i. p. . [b] ibid. . vol. i. p. . [c] annales de chimie, li, p. . [d] ibid. li. p. . iv. _physiological effects._--a frog was convulsed in the earliest experiments on these currents ( .). the sensation upon the tongue, and the flash before the eyes, which i at first obtained only in a feeble degree ( .), have been since exalted by more powerful apparatus, so as to become even disagreeable. . v. _spark._--the feeble spark which i first obtained with these currents ( .), has been varied and strengthened by signori nobili and antinori, and others, so as to leave no doubt as to its identity with the common electric spark. * * * * * iv. _thermo-electricity._ . with regard to thermo-electricity, (that beautiful form of electricity discovered by seebeck,) the very conditions under which it is excited are such as to give no ground for expecting that it can be raised like common electricity to any high degree of tension; the effects, therefore, due to that state are not to be expected. the sum of evidence respecting its analogy to the electricities already described, is, i believe, as follows:--_tension._ the attractions and repulsions due to a certain degree of tension have not been observed. _in currents_: i. _evolution of heat._ i am not aware that its power of raising temperature has been observed. ii. _magnetism._ it was discovered, and is best recognised, by its magnetic powers. iii. _chemical decomposition_ has not been effected by it. iv. _physiological effects._ nobili has shown[a] that these currents are able to cause contractions in the limbs of a frog. v. _spark._ the spark has not yet been seen. [a] bibliothèque universelle, xxxvii. . . only those effects are weak or deficient which depend upon a certain high degree of intensity; and if common electricity be reduced in that quality to a similar degree with the thermo-electricity, it can produce no effects beyond the latter. * * * * * v. _animal electricity._ . after an examination of the experiments of walsh[a] ingenhousz[b], cavendish[c], sir h. davy[d], and dr. davy[e], no doubt remains on my mind as to the identity of the electricity of the torpedo with common and voltaic electricity; and i presume that so little will remain on the minds of others as to justify my refraining from entering at length into the philosophical proofs of that identity. the doubts raised by sir h. davy have been removed by his brother dr. davy; the results of the latter being the reverse of those of the former. at present the sum of evidence is as follows:-- [a] philosophical transactions, , p. . [b] ibid. , p. . [c] ibid. , p. . [d] ibid. , p. . [e] ibid. , p. . . _tension._--no sensible attractions or repulsions due to tension have been observed. . _in motion_: i. evolution of heat; not yet observed; i have little or no doubt that harris's electrometer would show it ( . .). . ii. _magnetism._--perfectly distinct. according to dr. davy[a], the current deflected the needle and made magnets under the same law, as to direction, which governs currents of ordinary and voltaic electricity. [a] philosophical transactions, , p. . . iii. _chemical decomposition._--also distinct; and though dr. davy used an apparatus of similar construction with that of dr. wollaston ( .), still no error in the present case is involved, for the decompositions were polar, and in their nature truly electro-chemical. by the direction of the magnet it was found that the under surface of the fish was negative, and the upper positive; and in the chemical decompositions, silver and lead were precipitated on the wire connected with the under surface, and not on the other; and when these wires were either steel or silver, in solution of common salt, gas (hydrogen?) rose from the negative wire, but none from the positive. . another reason for the decomposition being electrochemical is, that a wollaston's apparatus constructed with _wires_, coated by sealing-wax, would most probably not have decomposed water, even in its own peculiar way, unless the electricity had risen high enough in intensity to produce sparks in some part of the circuit; whereas the torpedo was not able to produce sensible sparks. a third reason is, that the purer the water in wollaston's apparatus, the more abundant is the decomposition; and i have found that a machine and wire points which succeeded perfectly well with distilled water, failed altogether when the water was rendered a good conductor by sulphate of soda, common salt, or other saline bodies. but in dr. davy's experiments with the torpedo, _strong_ solutions of salt, nitrate of silver, and superacetate of lead were used successfully, and there is no doubt with more success than weaker ones. . iv. _physiological effects._--these are so characteristic, that by them the peculiar powers of the torpedo and gymnotus are principally recognised. . v. _spark._--the electric spark has not yet been obtained, or at least i think not; but perhaps i had better refer to the evidence on this point. humboldt, speaking of results obtained by m. fahlberg, of sweden, says, "this philosopher has seen an electric spark, as walsh and ingenhousz had done before him in london, by placing the gymnotus in the air, and interrupting the conducting chain by two gold leaves pasted upon glass, and a line distant from each other[a]." i cannot, however, find any record of such an observation by either walsh or ingenhousz, and do not know where to refer to that by m. fahlberg. m. humboldt could not himself perceive any luminous effect. [a] edinburgh phil. journal, ii. p. . again, sir john leslie, in his dissertation on the progress of mathematical and physical science, prefixed to the seventh edition of the encyclopædia britannica, edinb. , p. , says, "from a healthy specimen" of the _silurus electricus,_ meaning rather the _gymnotus_, "exhibited in london, vivid sparks were drawn in a darkened room"; but he does not say he saw them himself, nor state who did see them; nor can i find any account of such a phenomenon; so that the statement is doubtful[a]. [a] mr. brayley, who referred me to those statements, and has extensive knowledge of recorded facts, is unacquainted with any further account relating to them. . in concluding this summary of the powers of torpedinal electricity, i cannot refrain from pointing out the enormous absolute quantity of electricity which the animal must put in circulation at each effort. it is doubtful whether any common electrical machine has as yet been able to supply electricity sufficient in a reasonable time to cause true electro-chemical decomposition of water ( . .), yet the current from the torpedo has done it. the same high proportion is shown by the magnetic effects ( . .). these circumstances indicate that the torpedo has power (in the way probably that cavendish describes,) to continue the evolution for a sensible time, so that its successive discharges rather resemble those of a voltaic arrangement, intermitting in its action, than those of a leyden apparatus, charged and discharged many times in succession. in reality, however, there is _no philosophical difference_ between these two cases. . the _general conclusion_ which must, i think, be drawn from this collection of facts is, that _electricity, whatever may be its source, is identical in its nature_. the phenomena in the five kinds or species quoted, differ, not in their character but only in degree; and in that respect vary in proportion to the variable circumstances of _quantity_ and _intensity_[a] which can at pleasure be made to change in almost any one of the kinds of electricity, as much as it does between one kind and another. [a] the term _quantity_ in electricity is perhaps sufficiently definite as to sense; the term _intensity_ is more difficult to define strictly. i am using the terms in their ordinary and accepted meaning. table of the experimental effects common to the electricities derived from different sources[a]. table headings a: physiological effects b: magnetic deflection. c: magnets made. d: spark. e: heating power. f: true chemical action. g: attraction and repulsion. h: discharge by hot air. _________________________________________________________ | | | | | | | | | | | | a | b | c | d | e | f | g | h | |_________________________|___|___|___|___|___|___|___|___| | | | | | | | | | | | . voltaic electricity | x | x | x | x | x | x | x | x | |_________________________|___|___|___|___|___|___|___|___| | | | | | | | | | | | . common electricity | x | x | x | x | x | x | x | x | |_________________________|___|___|___|___|___|___|___|___| | | | | | | | | | | | . magneto-electricity | x | x | x | x | x | x | x | | |_________________________|___|___|___|___|___|___|___|___| | | | | | | | | | | | . thermo-electricity | x | x | + | + | + | + | | | |_________________________|___|___|___|___|___|___|___|___| | | | | | | | | | | | . animal electricity | x | x | x | + | + | x | | | |_________________________|___|___|___|___|___|___|___|___| [a] many of the spaces in this table originally left blank may now be filled. thus with _thermo-electricity_, botto made magnets and obtained polar chemical decomposition: antinori produced the spark; and if it has not been done before, mr. watkins has recently heated a wire in harris's thermo-electrometer. in respect to _animal electricity_, matteucci and linari have obtained the spark from the torpedo, and i have recently procured it from the gymnotus: dr. davy has observed the heating power of the current from the torpedo. i have therefore filled up these spaces with crosses, in a different position to the others originally in the table. there remain but five spaces unmarked, two under _attraction_ and _repulsion_, and three under _discharge by hot air_; and though these effects have not yet been obtained, it is a necessary conclusion that they must be possible, since the _spark_ corresponding to them has been procured. for when a discharge across cold air can occur, that intensity which is the only essential additional requisite for the other effects must be present.--_dec. ._ § . _relation by measure of common and voltaic electricity._[a] [a] in further illustration of this subject see - in series vii.--_dec. ._ . believing the point of identity to be satisfactorily established, i next endeavoured to obtain a common measure, or a known relation as to quantity, of the electricity excited by a machine, and that from a voltaic pile; for the purpose not only of confirming their identity ( .), but also of demonstrating certain general principles ( , , &c.), and creating an extension of the means of investigating and applying the chemical powers of this wonderful and subtile agent. . the first point to be determined was, whether the same absolute quantity of ordinary electricity, sent through a galvanometer, under different circumstances, would cause the same deflection of the needle. an arbitrary scale was therefore attached to the galvanometer, each division of which was equal to about °, and the instrument arranged as in former experiments ( .). the machine ( .), battery ( .), and other parts of the apparatus were brought into good order, and retained for the time as nearly as possible in the same condition. the experiments were alternated so as to indicate any change in the condition of the apparatus and supply the necessary corrections. . seven of the battery jars were removed, and eight retained for present use. it was found that about forty turns would fully charge the eight jars. they were then charged by thirty turns of the machine, and discharged through the galvanometer, a thick wet string, about ten inches long, being included in the circuit. the needle was immediately deflected five divisions and a half, on the one side of the zero, and in vibrating passed as nearly as possible through five divisions and a half on the other side. . the other seven jars were then added to the eight, and the whole fifteen charged by thirty turns of the machine. the henley's electrometer stood not quite half as high as before; but when the discharge was made through the galvanometer, previously at rest, the needle immediately vibrated, passing _exactly_ to the same division as in the former instance. these experiments with eight and with fifteen jars were repeated several times alternately with the same results. . other experiments were then made, in which all the battery was used, and its charge (being fifty turns of the machine,) sent through the galvanometer: but it was modified by being passed sometimes through a mere wet thread, sometimes through thirty-eight inches of thin string wetted by distilled water, and sometimes through a string of twelve times the thickness, only twelve inches in length, and soaked in dilute acid ( .). with the thick string the charge passed at once; with the thin string it occupied a sensible time, and with the thread it required two or three seconds before the electrometer fell entirely down. the current therefore must have varied extremely in intensity in these different cases, and yet the deflection of the needle was sensibly the same in all of them. if any difference occurred, it was that the thin string and thread caused greatest deflection; and if there is any lateral transmission, as m. colladon says, through the silk in the galvanometer coil, it ought to have been so, because then the intensity is lower and the lateral transmission less. . hence it would appear that _if the same absolute quantity of electricity pass through the galvanometer, whatever may be its intensity, the dejecting force upon the magnetic needle is the same._ . the battery of fifteen jars was then charged by sixty revolutions of the machine, and discharged, as before, through the galvanometer. the deflection of the needle was now as nearly as possible to the eleventh division, but the graduation was not accurate enough for me to assert that the arc was exactly double the former arc; to the eye it appeared to be so. the probability is, that _the deflecting force of an electric current is directly proportional to the absolute quantity of electricity passed_, at whatever intensity that electricity may be[a]. [a] the great and general value of the galvanometer, as an actual measure of the electricity passing through it, either continuously or interruptedly, must be evident from a consideration of these two conclusions. as constructed by professor ritchie with glass threads (see philosophical transactions, , p. , and quarterly journal of science, new series, vol. i. p. .), it apparently seems to leave nothing unsupplied in its own department. . dr. ritchie has shown that in a case where the intensity of the electricity remained the same, the deflection of the magnetic needle was directly as the quantity of electricity passed through the galvanometer[a]. mr. harris has shown that the _heating_ power of common electricity on metallic wires is the same for the same quantity of electricity whatever its intensity might have previously been[b]. [a] quarterly journal of science, new series, vol. i. p. . [b] plymouth transactions, page . . the next point was to obtain a _voltaic_ arrangement producing an effect equal to that just described ( .). a platina and a zinc wire were passed through the same hole of a draw-plate, being then one eighteenth of an inch in diameter; these were fastened to a support, so that their lower ends projected, were parallel, and five sixteenths of an inch apart. the upper ends were well-connected with the galvanometer wires. some acid was diluted, and, after various preliminary experiments, that adopted as a standard which consisted of one drop strong sulphuric acid in four ounces distilled water. finally, the time was noted which the needle required in swinging either from right to left or left to right: it was equal to seventeen beats of my watch, the latter giving one hundred and fifty in a minute. the object of these preparations was to arrange a voltaic apparatus, which, by immersion in a given acid for a given time, much less than that required by the needle to swing in one direction, should give equal deflection to the instrument with the discharge of ordinary electricity from the battery ( . .); and a new part of the zinc wire having been brought into position with the platina, the comparative experiments were made. . on plunging the zinc and platina wires five eighths of an inch deep into the acid, and retaining them there for eight beats of the watch, (after which they were quickly withdrawn,) the needle was deflected, and continued to advance in the same direction some time after the voltaic apparatus had been removed from the acid. it attained the five-and-a-half division, and then returned swinging an equal distance on the other side. this experiment was repeated many times, and always with the same result. . hence, as an approximation, and judging from _magnetic force_ only at present ( .), it would appear that two wires, one of platina and one of zinc, each one eighteenth of an inch in diameter, placed five sixteenths of an inch apart and immersed to the depth of five eighths of an inch in acid, consisting of one drop oil of vitriol and four ounces distilled water, at a temperature about °, and connected at the other extremities by a copper wire eighteen feet long and one eighteenth of an inch thick (being the wire of the galvanometer coils), yield as much electricity in eight beats of my watch, or in / ths of a minute, as the electrical battery charged by thirty turns of the large machine, in excellent order ( . .). notwithstanding this apparently enormous disproportion, the results are perfectly in harmony with those effects which are known to be produced by variations in the intensity and quantity of the electric fluid. . in order to procure a reference to _chemical action_, the wires were now retained immersed in the acid to the depth of five eighths of an inch, and the needle, when stationary, observed; it stood, as nearly as the unassisted eye could decide, at - / division. hence a permanent deflection to that extent might be considered as indicating a constant voltaic current, which in eight beats of my watch ( .) could supply as much electricity as the electrical battery charged by thirty turns of the machine. . the following arrangements and results are selected from many that were made and obtained relative to chemical action. a platina wire one twelfth of an inch in diameter, weighing two hundred and sixty grains, had the extremity rendered plain, so as to offer a definite surface equal to a circle of the same diameter as the wire; it was then connected in turn with the conductor of the machine, or with the voltaic apparatus ( .), so as always to form the positive pole, and at the same time retain a perpendicular position, that it might rest, with its whole weight, upon the test paper to be employed. the test paper itself was supported upon a platina spatula, connected either with the discharging train ( .), or with the negative wire of the voltaic apparatus, and it consisted of four thicknesses, moistened at all times to an equal degree in a standard solution of hydriodate of potassa ( .). . when the platina wire was connected with the prime conductor of the machine, and the spatula with the discharging train, ten turns of the machine had such decomposing power as to produce a pale round spot of iodine of the diameter of the wire; twenty turns made a much darker mark, and thirty turns made a dark brown spot penetrating to the second thickness of the paper. the difference in effect produced by two or three turns, more or less, could be distinguished with facility. . the wire and spatula were then connected with the voltaic apparatus ( .), the galvanometer being also included in the arrangement; and, a stronger acid having been prepared, consisting of nitric acid and water, the voltaic apparatus was immersed so far as to give a permanent deflection of the needle to the - / division ( .), the fourfold moistened paper intervening as before[a]. then by shifting the end of the wire from place to place upon the test paper, the effect of the current for five, six, seven, or any number of the beats of the watch ( .) was observed, and compared with that of the machine. after alternating and repeating the experiments of comparison many times, it was constantly found that this standard current of voltaic electricity, continued for eight beats of the watch, was equal, in chemical effect, to thirty turns of the machine; twenty-eight revolutions of the machine were sensibly too few. [a] of course the heightened power of the voltaic battery was necessary to compensate for the bad conductor now interposed. . hence it results that both in _magnetic deflection_ ( .) and in _chemical force_, the current of electricity of the standard voltaic battery for eight beats of the watch was equal to that of the machine evolved by thirty revolutions. . it also follows that for this case of electro-chemical decomposition, and it is probable for all cases, that the _chemical power, like the magnetic force_ ( .), _is in direct proportion to the absolute quantity of electricity_ which passes. . hence arises still further confirmation, if any were required, of the identity of common and voltaic electricity, and that the differences of intensity and quantity are quite sufficient to account for what were supposed to be their distinctive qualities. . the extension which the present investigations have enabled me to make of the facts and views constituting the theory of electro-chemical decomposition, will, with some other points of electrical doctrine, be almost immediately submitted to the royal society in another series of these researches. _royal institution, th dec. ._ note.--i am anxious, and am permitted, to add to this paper a correction of an error which i have attributed to m. ampère the first series of these experimental researches. in referring to his experiment on the induction of electrical currents ( .), i have called that a disc which i should have called a circle or a ring. m. ampère used a ring, or a very short cylinder made of a narrow plate of copper bent into a circle, and he tells me that by such an arrangement the motion is very readily obtained. i have not doubted that m. ampère obtained the motion he described; but merely mistook the kind of mobile conductor used, and so far i described his _experiment_ erroneously. in the same paragraph i have stated that m. ampère says the disc turned "to take a position of equilibrium exactly as the spiral itself would have turned had it been free to move"; and further on i have said that my results tended to invert the sense of the proposition "stated by m. ampère, _that a current of electricity tends to put the electricity of conductors near which it passes in motion in the same direction._" m. ampère tells me in a letter which i have just received from him, that he carefully avoided, when describing the experiment, any reference to the direction of the induced current; and on looking at the passages he quotes to me, i find that to be the case. i have therefore done him injustice in the above statements, and am anxious to correct my error. but that it may not be supposed i lightly wrote those passages, i will briefly refer to my reasons for understanding them in the sense i did. at first the experiment failed. when re-made successfully about a year afterwards, it was at geneva in company with m.a. de la rive: the latter philosopher described the results[a], and says that the plate of copper bent into a circle which was used as the mobile conductor "sometimes advanced between the two branches of the (horse-shoe) magnet, and sometimes was repelled, _according_ to the direction of the current in the surrounding conductors." [a] bibliothèque universelle, xxi. p. . i have been in the habit of referring to demonferrand's _manuel d'electricité dynamique_, as a book of authority in france; containing the general results and laws of this branch of science, up to the time of its publication, in a well arranged form. at p. , the author, when describing this experiment, says, "the mobile circle turns to take a position of equilibrium as a conductor would do in which the current moved in the _same direction_ as in the spiral;" and in the same paragraph he adds, "it is therefore proved _that a current of electricity tends to put the electricity of conductors, near which it passes, in motion in the same direction._" these are the words i quoted in my paper ( .). le lycée of st of january, , no. , in an article written after the receipt of my first unfortunate letter to m. hachette, and before my papers were printed, reasons upon the direction of the induced currents, and says, that there ought to be "an elementary current produced in the same direction as the corresponding portion of the producing current." a little further on it says, "therefore we ought to obtain currents, moving in the _same direction_, produced upon a metallic wire, either by a magnet or a current. m. ampère _was so thouroughly persuaded that such ought to be the direction of the currents by influence_, that he neglected to assure himself of it in his experiment at geneva." it was the precise statements in demonferrand's manuel, agreeing as they did with the expression in m. de la rive's paper, (which, however, i now understand as only meaning that when the inducing current was changed, the motion of the mobile circle changed also,) and not in discordance with anything expressed by m. ampère himself where he speaks of the experiment, which made me conclude, when i wrote the paper, that what i wrote was really his avowed opinion; and when the number of the lycée referred to appeared, which was before my paper was printed, it could excite no suspicion that i was in error. hence the mistake into which i unwittingly fell. i am proud to correct it and do full justice to the acuteness and accuracy which, as far as i can understand the subjects, m. ampère carries into all the branches of philosophy which he investigates. finally, my note to ( .) says that the lycée, no. . "mistakes the erroneous results of mm. fresnel and ampère for true ones," &c. &c. in calling m. ampère's results erroneous, i spoke of the results described in, and referred to by the lycée itself; but _now_ that the expression of the direction of the induced current is to be separated, the term _erroneous_ ought no longer to be attached to them. april , . m.f.] fourth series. § . _on a new law of electric conduction._ § . _on conducting power generally._ received april ,--read may , . § . _on a new law of electric conduction._[a] [a] in reference to this law see further considerations at . . .--_dec. ._ . it was during the progress of investigations relating to electro-chemical decomposition, which i still have to submit to the royal society, that i encountered effects due to a very _general law_ of electric conduction not hitherto recognised; and though they prevented me from obtaining the condition i sought for, they afforded abundant compensation for the momentary disappointment, by the new and important interest which they gave to an extensive part of electrical science. . i was working with ice, and the solids resulting from the freezing of solutions, arranged either as barriers across a substance to be decomposed, or as the actual poles of a voltaic battery, that i might trace and catch certain elements in their transit, when i was suddenly stopped in my progress by finding that ice was in such circumstances a non-conductor of electricity; and that as soon as a thin film of it was interposed, in the circuit of a very powerful voltaic battery, the transmission of electricity was prevented, and all decomposition ceased. . at first the experiments were made with common ice, during the cold freezing weather of the latter end of january ; but the results were fallacious, from the imperfection of the arrangements, and the following more unexceptionable form of experiment was adopted. . tin vessels were formed, five inches deep, one inch and a quarter wide in one direction, of different widths from three eighths to five eighths of an inch in the other, and open at one extremity. into these were fixed by corks, plates of platina, so that the latter should not touch the tin cases; and copper wires having previously been soldered to the plate, these were easily connected, when required, with a voltaic pile. then distilled water, previously boiled for three hours, was poured into the vessels, and frozen by a mixture of salt and snow, so that pure transparent solid ice intervened between the platina and tin; and finally these metals were connected with the opposite extremities of the voltaic apparatus, a galvanometer being at the same time included in the circuit. . in the first experiment, the platina pole was three inches and a half long, and seven eighths of an inch wide; it was wholly immersed in the water or ice, and as the vessel was four eighths of an inch in width, the average thickness of the intervening ice was only a quarter of an inch, whilst the surface of contact with it at both poles was nearly fourteen square inches. after the water was frozen, the vessel was still retained in the frigorific mixture, whilst contact between the tin and platina respectively was made with the extremities of a well-charged voltaic battery, consisting of twenty pairs of four-inch plates, each with double coppers. not the slightest deflection of the galvanometer needle occurred. . on taking the frozen arrangement out of the cold mixture, and applying warmth to the bottom of the tin case, so as to melt part of the ice, the connexion with the battery being in the mean time retained, the needle did not at first move; and it was only when the thawing process had extended so far as to liquefy part of the ice touching the platina pole, that conduction took place; but then it occurred effectually, and the galvanometer needle was permanently deflected nearly °. . in another experiment, a platina spatula, five inches in length and seven eighths of an inch in width, had four inches fixed in the ice, and the latter was only three sixteenths of an inch thick between one metallic surface and the other; yet this arrangement insulated as perfectly as the former. . upon pouring a little water in at the top of this vessel on the ice, still the arrangement did not conduct; yet fluid water was evidently there. this result was the consequence of the cold metals having frozen the water where they touched it, and thus insulating the fluid part; and it well illustrates the non-conducting power of ice, by showing how thin a film could prevent the transmission of the battery current. upon thawing parts of this thin film, at _both_ metals, conduction occurred. . upon warming the tin case and removing the piece of ice, it was found that a cork having slipped, one of the edges of the platina had been all but in contact with the inner surface of the tin vessel; yet, notwithstanding the extreme thinness of the interfering ice in this place, no sensible portion of electricity had passed. . these experiments were repeated many times with the same results. at last a battery of fifteen troughs, or one hundred and fifty pairs of four-inch plates, powerfully charged, was used; yet even here no sensible quantity of electricity passed the thin barrier of ice. . it seemed at first as if occasional departures from these effects occurred; but they could always be traced to some interfering circumstances. the water should in every instance be well-frozen; for though it is not necessary that the ice should reach from pole to pole, since a barrier of it about one pole would be quite sufficient to prevent conduction, yet, if part remain fluid, the mere necessary exposure of the apparatus to the air or the approximation of the hands, is sufficient to produce, at the _upper surface_ of the water and ice, a film of fluid, extending from the platina to the tin; and then conduction occurs. again, if the corks used to block the platina in its place are damp or wet within, it is necessary that the cold be sufficiently well applied to freeze the water in them, or else when the surfaces of their contact with the tin become slightly warm by handling, that part will conduct, and the interior being ready to conduct also, the current will pass. the water should be pure, not only that unembarrassed results may be obtained, but also that, as the freezing proceeds, a minute portion of concentrated saline solution may not be formed, which remaining fluid, and being interposed in the ice, or passing into cracks resulting from contraction, may exhibit conducting powers independent of the ice itself. . on one occasion i was surprised to find that after thawing much of the ice the conducting power had not been restored; but i found that a cork which held the wire just where it joined the platina, dipped so far into the ice, that with the ice itself it protected the platina from contact with the melted part long after that contact was expected. . this insulating power of ice is not effective with electricity of exalted intensity. on touching a diverged gold-leaf electrometer with a wire connected with the platina, whilst the tin case was touched by the hand or another wire, the electrometer was instantly discharged ( .). . but though electricity of an intensity so low that it cannot diverge the electrometer, can still pass (though in very limited quantities ( .),) through ice; the comparative relation of water and ice to the electricity of the voltaic apparatus is not less extraordinary on that account, or less important in its consequences. . as it did not seem likely that this _law of the assumption of conducting power during liquefaction, and loss of it during congelation_, would be peculiar to water, i immediately proceeded to ascertain its influence in other cases, and found it to be very general. for this purpose bodies were chosen which were solid at common temperatures, but readily fusible; and of such composition as, for other reasons connected with electrochemical action, led to the conclusion that they would be able when fused to replace water as conductors. a voltaic battery of two troughs, or twenty pairs of four-inch plates ( .), was used as the source of electricity, and a galvanometer introduced into the circuit to indicate the presence or absence of a current. . on fusing a little chloride of lead by a spirit lamp on a fragment of a florence flask, and introducing two platina wires connected with the poles of the battery, there was instantly powerful action, the galvanometer was most violently affected, and the chloride rapidly decomposed. on removing the lamp, the instant the chloride solidified all current and consequent effects ceased, though the platina wires remained inclosed in the chloride not more than the one-sixteenth of an inch from each other. on renewing the heat, as soon as the fusion had proceeded far enough to allow liquid matter to connect the poles, the electrical current instantly passed. . on fusing the chloride, with one wire introduced, and then touching the liquid with the other, the latter being cold, caused a little knob to concrete on its extremity, and no current passed; it was only when the wire became so hot as to be able to admit or allow of contact with the liquid matter, that conduction took place, and then it was very powerful. . when chloride of silver and chlorate of potassa were experimented with, in a similar manner, exactly the same results occurred. . whenever the current passed in these cases, there was decomposition of the substances; but the electro-chemical part of this subject i purpose connecting with more general views in a future paper[a]. [a] in , sir h. davy knew that "dry nitre, caustic potash, and soda are conductors of galvanism when rendered fluid by a high degree of heat," (journals of the royal institution, , p. ,) but was not aware of the general law which i have been engaged in developing. it is remarkable, that eleven years after that, he should say, "there are no fluids known except such as contain water, which are capable of being made the medium of connexion between the metal or metals of the voltaic apparatus."--elements of chemical philosophy, p. . . other substances, which could not be melted on glass, were fused by the lamp and blowpipe on platina connected with one pole of the battery, and then a wire, connected with the other, dipped into them. in this way chloride of sodium, sulphate of soda, protoxide of lead, mixed carbonates of potash and soda, &c. &c., exhibited exactly the same phenomena as those already described: whilst liquid, they conducted and were decomposed; whilst solid, though very hot, they insulated the battery current even when four troughs were used. . occasionally the substances were contained in small bent tubes of green glass, and when fused, the platina poles introduced, one on each side. in such cases the same general results as those already described were procured; but a further advantage was obtained, namely, that whilst the substance was conducting and suffering decomposition, the final arrangement of the elements could be observed. thus, iodides of potassium and lead gave iodine at the positive pole, and potassium or lead at the negative pole. chlorides of lead and silver gave chlorine at the positive, and metals at the negative pole. nitre and chlorate; of potassa gave oxygen, &c., at the positive, and alkali, or even potassium, at the negative pole. [illustration] . a fourth arrangement was used for substances requiring very high temperatures for their fusion. a platina wire was connected with one pole of the battery; its extremity bent into a small ring, in the manner described by berzelius, for blowpipe experiments; a little of the salt, glass, or other substance, was melted on this ring by the ordinary blowpipe, or even in some cases by the oxy-hydrogen blowpipe, and when the drop, retained in its place by the ring, was thoroughly hot and fluid, a platina wire from the opposite pole of the battery was made to touch it, and the effects observed. . the following are various substances, taken from very different classes chemically considered, which are subject to this law. the list might, no doubt, be enormously extended; but i have not had time to do more than confirm the law by a sufficient number of instances. first, _water_. amongst _oxides_;--potassa, protoxide of lead, glass of antimony, protoxide of antimony, oxide of bismuth. _chlorides_ of potassium, sodium, barium, strontium, calcium, magnesium, manganese, zinc, copper (proto-), lead, tin (proto-), antimony, silver. _iodides_ of potassium, zinc and lead, protiodide of tin, periodide of mercury; _fluoride_ of potassium; _cyanide_ of potassium; _sulpho-cyanide_ of potassium. _salts._ chlorate of potassa; nitrates of potassa, soda, baryta, strontia, lead, copper, and silver; sulphates of soda and lead, proto-sulphate of mercury; phosphates of potassa, soda, lead, copper, phosphoric glass or acid phosphate of lime; carbonates of potassa and soda, mingled and separate; borax, borate of lead, per-borate of tin; chromate of potassa, bi-chromate of potassa, chromate of lead; acetate of potassa. _sulphurets._ sulphuret of antimony, sulphuret of potassium made by reducing sulphate of potassa by hydrogen; ordinary sulphuret of potassa. silicated potassa; chameleon mineral. . it is highly interesting in the instances of those substances which soften before they liquefy, to observe at what period the conducting power is acquired, and to what degree it is exalted by perfect fluidity. thus, with the borate of lead, when heated by the lamp upon glass, it becomes as soft as treacle, but it did not conduct, and it was only when urged by the blowpipe and brought to a fair red heat, that it conducted. when rendered quite liquid, it conducted with extreme facility. . i do not mean to deny that part of the increased conducting power in these cases of softening was probably due to the elevation of temperature ( . .); but i have no doubt that by far the greater part was due to the influence of the general law already demonstrated, and which in these instances came gradually, instead of suddenly, into operation. . the following are bodies which acquired no conducting power upon assuming the liquid state:-- sulphur, phosphorus; iodide of sulphur, per-iodide of tin; orpiment, realgar; glacial acetic acid, mixed margaric and oleic acids, artificial camphor; caffeine, sugar, adipocire, stearine of cocoa-nut oil, spermaceti, camphor, naphthaline, resin, gum sandarach, shell lac. . perchloride of tin, chloride of arsenic, and the hydrated chloride of arsenic, being liquids, had no sensible conducting power indicated by the galvanometer, nor were they decomposed. . some of the above substances are sufficiently remarkable as exceptions to the general law governing the former cases. these are orpiment, realgar, acetic acid, artificial camphor, per-iodide of tin, and the chlorides of tin and arsenic. i shall have occasion to refer to these cases in the paper on electro-chemical decomposition. . boracic acid was raised to the highest possible temperature by an oxy-hydrogen flame ( .), yet it gained no conducting powers sufficient to affect the galvanometer, and underwent no apparent voltaic decomposition. it seemed to be quite as bad a conductor as air. green bottle-glass, heated in the same manner, did not gain conducting power sensible to the galvanometer. flint glass, when highly heated, did conduct a little and decompose; and as the proportion of potash or oxide of lead was increased in the glass, the effects were more powerful. those glasses, consisting of boracic acid on the one hand, and oxide of lead or potassa on the other, show the assumption of conducting power upon fusion and the accompanying decomposition very well. . i was very anxious to try the general experiment with sulphuric acid, of about specific gravity . , containing that proportion of water which gives it the power of crystallizing at ° fahr.; but i found it impossible to obtain it so that i could be sure the whole would congeal even at ° fahr. a ten-thousandth part of water, more or less than necessary, would, upon cooling the whole, cause a portion of uncongealable liquid to separate, and that remaining in the interstices of the solid mass, and moistening the planes of division, would prevent the correct observation of the phenomena due to entire solidification and subsequent liquefaction. . with regard to the substances on which conducting power is thus conferred by liquidity, the degree of power so given is generally very great. water is that body in which this acquired power is feeblest. in the various oxides, chlorides, salts, &c. &c., it is given in a much higher degree. i have not had time to measure the conducting power in these cases, but it is apparently some hundred times that of pure water. the increased conducting power known to be given to water by the addition of salts, would seem to be in a great degree dependent upon the high conducting power of these bodies when in the liquid state, that state being given them for the time, not by heat but solution in the water[a]. [a] see a doubt on this point at .--_dec. ._ . whether the conducting power of these liquefied bodies is a consequence of their decomposition or not ( .), or whether the two actions of conduction and decomposition are essentially connected or not, would introduce no difference affecting the probable accuracy of the preceding statement. . this _general assumption of conducting power_ by bodies as soon as they pass from the solid to the liquid state, offers a new and extraordinary character, the existence of which, as far as i know, has not before been suspected; and it seems importantly connected with some properties and relations of the particles of matter which i may now briefly point out. . in almost all the instances, as yet observed, which are governed by this law, the substances experimented with have been those which were not only compound bodies, but such as contain elements known to arrange themselves at the opposite poles; and were also such as could be _decomposed_ by the electrical current. when conduction took place, decomposition occurred; when decomposition ceased, conduction ceased also; and it becomes a fair and an important question, whether the conduction itself may not, wherever the law holds good, be a consequence not merely of the capability, but of the act of decomposition? and that question may be accompanied by another, namely, whether solidification does not prevent conduction, merely by chaining the particles to their places, under the influence of aggregation, and preventing their final separation in the manner necessary for decomposition? . but, on the other hand, there is one substance (and others may occur), the _per-iodide of mercury_, which, being experimented with like the others ( .), was found to insulate when solid, and to acquire conducting power when fluid; yet it did not seem to undergo decomposition in the latter case. . again, there are many substances which contain elements such as would be expected to arrange themselves at the opposite poles of the pile, and therefore in that respect fitted for decomposition, which yet do not conduct. amongst these are the iodide of sulphur, per-iodide of zinc, per-chloride of tin, chloride of arsenic, hydrated chloride of arsenic, acetic acid, orpiment, realgar, artificial camphor, &c.; and from these it might perhaps be assumed that decomposition is dependent upon conducting power, and not the latter upon the former. the true relation, however, of conduction and decomposition in those bodies governed by the general law which it is the object of this paper to establish, can only be satisfactorily made out from a far more extensive series of observations than those i have yet been able to supply[a]. [a] see , &c. &c.--_dec. ._ . the relation, under this law, of the conducting power for electricity to that for heat, is very remarkable, and seems to imply a natural dependence of the two. as the solid becomes a fluid, it loses almost entirely the power of conduction for heat, but gains in a high degree that for electricity; but as it reverts hack to the solid state, it gains the power of conducting heat, and loses that of conducting electricity. if, therefore, the properties are not incompatible, still they are most strongly contrasted, one being lost as the other is gained. we may hope, perhaps, hereafter to understand the physical reason of this very extraordinary relation of the two conducting powers, both of which appear to be directly connected with the corpuscular condition of the substances concerned. . the assumption of conducting power and a decomposable condition by liquefaction, promises new opportunities of, and great facilities in, voltaic decomposition. thus, such bodies as the oxides, chlorides, cyanides, sulpho-cyanides, fluorides, certain vitreous mixtures, &c. &c., may be submitted to the action of the voltaic battery under new circumstances; and indeed i have already been able, with ten pairs of plates, to decompose common salt, chloride of magnesium, borax, &c. &c., and to obtain sodium, magnesium, boron, &c., in their separate states. § . _on conducting power generally._[a] [a] in reference to this § refer to in series viii., and the results connected with it.--_dec. ._ . it is not my intention here to enter into an examination of all the circumstances connected with conducting power, but to record certain facts and observations which have arisen during recent inquiries, as additions to the general stock of knowledge relating to this point of electrical science. . i was anxious, in the first place, to obtain some idea of the conducting power of ice and solid salts for electricity of high tension ( .), that a comparison might be made between it and the large accession of the same power gained upon liquefaction. for this purpose the large electrical machine ( .) was brought into excellent action, its conductor connected with a delicate gold-leaf electrometer, and also with the platina inclosed in the ice ( .), whilst the tin case was connected with the discharging train ( .). on working the machine moderately, the gold leaves barely separated; on working it rapidly, they could be opened nearly two inches. in this instance the tin case was five-eighths of an inch in width; and as, after the experiment, the platina plate was found very nearly in the middle of the ice, the average thickness of the latter had been five-sixteenths of an inch, and the extent of surface of contact with tin and platina fourteen square inches ( .). yet, under these circumstances, it was but just able to conduct the small quantity of electricity which this machine could evolve ( .), even when of a tension competent to open the leaves two inches; no wonder, therefore, that it could not conduct any sensible portion of the electricity of the troughs ( .), which, though almost infinitely surpassing that of the machine in quantity, had a tension so low as not to be sensible to an electrometer. . in another experiment, the tin case was only four-eighths of an inch in width, and it was found afterwards that the platina had been not quite one-eighth of an inch distant in the ice from one side of the tin vessel. when this was introduced into the course of the electricity from the machine ( .), the gold leaves could be opened, but not more than half an inch; the thinness of the ice favouring the conduction of the electricity, and permitting the same quantity to pass in the same time, though of a much lower tension. . iodide of potassium which had been fused and cooled was introduced into the course of the electricity from the machine. there were two pieces, each about a quarter of an inch in thickness, and exposing a surface on each side equal to about half a square inch; these were placed upon platina plates, one connected with the machine and electrometer ( .), and the other with the discharging train, whilst a fine platina wire connected the two pieces, resting upon them by its two points. on working the electrical machine, it was possible to open the electrometer leaves about two-thirds of an inch. . as the platina wire touched only by points, the facts show that this salt is a far better conductor than ice; but as the leaves of the electrometer opened, it is also evident with what difficulty conduction, even of the small portion of electricity produced by the machine, is effected by this body in the solid state, when compared to the facility with which enormous quantities at very low tensions are transmitted by it when in the fluid state. . in order to confirm these results by others, obtained from the voltaic apparatus, a battery of one hundred and fifty plates, four inches square, was well-charged: its action was good; the shock from it strong; the discharge would _continue_ from copper to copper through four-tenths of an inch of air, and the gold-leaf electrometer before used could be opened nearly a quarter of an inch. . the ice vessel employed ( .) was half an inch in width; as the extent of contact of the ice with the tin and platina was nearly fourteen square inches, the whole was equivalent to a plate of ice having a surface of seven square inches, of perfect contact at each side, and only one fourth of an inch thick. it was retained in a freezing mixture during the experiment. . the order of arrangement in the course of the electric current was as follows. the positive pole of the battery was connected by a wire with the platina plate in the ice; the plate was in contact with the ice, the ice with the tin jacket, the jacket with a wire, which communicated with a piece of tin foil, on which rested one end of a bent platina wire ( .), the other or decomposing end being supported on paper moistened with solution of iodide of potassium ( .): the paper was laid flat on a platina spatula connected with the negative end of the battery. all that part of the arrangement between the ice vessel and the decomposing wire point, including both these, was insulated, so that no electricity might pass through the latter which had not traversed the former also. . under these circumstances, it was found that, a pale brown spot of iodine was slowly formed under the decomposing platina point, thus indicating that ice could conduct a little of the electricity evolved by a voltaic battery charged up to the degree of intensity indicated by the electrometer. but it is quite evident that notwithstanding the enormous quantity of electricity which the battery could furnish, it was, under present circumstances, a very inferior instrument to the ordinary machine; for the latter could send as much through the ice as it could carry, being of a far higher intensity, i.e. able to open the electrometer leaves half an inch or more ( . .). . the decomposing wire and solution of iodide of potassium were then removed, and replaced by a very delicate galvanometer ( .); it was so nearly astatic, that it vibrated to and fro in about sixty-three beats of a watch giving one hundred and fifty beats in a minute. the same feebleness of current as before was still indicated; the galvanometer needle was deflected, but it required to break and make contact three or four times ( .), before the effect was decided. . the galvanometer being removed, two platina plates were connected with the extremities of the wires, and the tongue placed between them, so that the whole charge of the battery, so far as the ice would let it pass, was free to go through the tongue. whilst standing on the stone floor, there was shock, &c., but when insulated, i could feel no sensation. i think a frog would have been scarcely, if at all, affected. . the ice was now removed, and experiments made with other solid bodies, for which purpose they were placed under the end of the decomposing wire instead of the solution of iodide of potassium ( .). for instance, a piece of dry iodide of potassium was placed on the spatula connected with the negative pole of the battery, and the point of the decomposing wire placed upon it, whilst the positive end of the battery communicated with the latter. a brown spot of iodine very slowly appeared, indicating the passage of a little electricity, and agreeing in that respect with the results obtained by the use of the electrical machine ( .). when the galvanometer was introduced into the circuit at the same time with the iodide, it was with difficulty that the action of the current on it could be rendered sensible. . a piece of common salt previously fused and solidified being introduced into the circuit was sufficient almost entirely to destroy the action on the galvanometer. fused and cooled chloride of lead produced the same effect. the conducting power of these bodies, _when fluid_, is very great ( . .). . these effects, produced by using the common machine and the voltaic battery, agree therefore with each other, and with the law laid down in this paper ( .); and also with the opinion i have supported, in the third series of these researches, of the identity of electricity derived from different sources ( .). . the effect of heat in increasing the conducting power of many substances, especially for electricity of high tension, is well known. i have lately met with an extraordinary case of this kind, for electricity of low tension, or that of the voltaic pile, and which is in direct contrast with the influence of heat upon metallic bodies, as observed and described by sir humphry davy[a]. [a] philosophical transactions, , p. . . the substance presenting this effect is sulphuret of silver. it was made by fusing a mixture of precipitated silver and sublimed sulphur, removing the film of silver by a file from the exterior of the fused mass, pulverizing the sulphuret, mingling it with more sulphur, and fusing it again in a green glass tube, so that no air should obtain access during the process. the surface of the sulphuret being again removed by a file or knife, it was considered quite free from uncombined silver. . when a piece of this sulphuret, half an inch in thickness, was put between surfaces of platina, terminating the poles of a voltaic battery of twenty pairs of four-inch plates, a galvanometer being also included in the circuit, the needle was slightly deflected, indicating a feeble conducting power. on pressing the platina poles and sulphuret together with the fingers, the conducting power increased as the whole became warm. on applying a lamp under the sulphuret between the poles, the conducting power rose rapidly with the heat, and at last-the galvanometer needle jumped into a fixed position, and the sulphuret was found conducting in the manner of a metal. on removing the lamp and allowing the heat to fall, the effects were reversed, the needle at first began to vibrate a little, then gradually left its transverse direction, and at last returned to a position very nearly that which it would take when no current was passing through the galvanometer. . occasionally, when the contact of the sulphuret with the platina poles was good, the battery freshly charged, and the commencing temperature not too low, the mere current of electricity from the battery was sufficient to raise the temperature of the sulphuret; and then, without any application of extraneous heat, it went on increasing conjointly in temperature and conducting power, until the cooling influence of the air limited the effects. in such cases it was generally necessary to cool the whole purposely, to show the returning series of phenomena. . occasionally, also, the effects would sink of themselves, and could not be renewed until a fresh surface of the sulphuret had been applied to the positive pole. this was in consequence of peculiar results of decomposition, to which i shall have occasion to revert in the section on electro-chemical decomposition, and was conveniently avoided by inserting the ends of two pieces of platina wire into the opposite extremities of a portion of sulphuret fused in a glass tube, and placing this arrangement between the poles of the battery. . the hot sulphuret of silver conducts sufficiently well to give a bright spark with charcoal, &c. &c., in the manner of a metal. . the native grey sulphuret of silver, and the ruby silver ore, both presented the same phenomena. the native malleable sulphuret of silver presented precisely the same appearances as the artificial sulphuret. . there is no other body with which i am acquainted, that, like sulphuret of silver, can compare with metals in conducting power for electricity of low tension when hot, but which, unlike them, during cooling, loses in power, whilst they, on the contrary, gain. probably, however, many others may, when sought for, be found[a]. [a] see now on this subject, , .--_dec. ._ . the proto-sulphuret of iron, the native per-sulphuret of iron, arsenical sulphuret of iron, native yellow sulphuret of copper and iron, grey artificial sulphuret of copper, artificial sulphuret of bismuth, and artificial grey sulphuret of tin, all conduct the voltaic battery current when cold, more or less, some giving sparks like the metals, others not being sufficient for that high effect. they did not seem to conduct better when heated, than before; but i had not time to enter accurately into the investigation of this point. almost all of them became much heated by the transmission of the current, and present some very interesting phenomena in that respect. the sulphuret of antimony does not conduct the same current sensibly either hot or cold, but is amongst those bodies acquiring conducting power when fused ( .). the sulphuret of silver and perhaps some others decompose whilst in the solid state; but the phenomena of this decomposition will be reserved for its proper place in the next series of these researches. . notwithstanding the extreme dissimilarity between sulphuret of silver and gases or vapours, i cannot help suspecting the action of heat upon them to be the same, bringing them all into the same class as conductors of electricity, although with those great differences in degree, which are found to exist under common circumstances. when gases are heated, they increase in conducting power, both for common and voltaic electricity ( .); and it is probable that if we could compress and condense them at the same time, we should still further increase their conducting power. cagniard de la tour has shown that a substance, for instance water, may be so expanded by heat whilst in the liquid state, or condensed whilst in the vaporous state, that the two states shall coincide at one point, and the transition from one to the other be so gradual that no line of demarcation can be pointed out[a]; that, in fact, the two states shall become one;--which one state presents us at different times with differences in degree as to certain properties and relations; and which differences are, under ordinary circumstances, so great as to be equivalent to two different states. [a] annales de chimie, xxi. pp. , . . i cannot but suppose at present that at that point where the liquid and the gaseous state coincide, the conducting properties are the same for both; but that they diminish as the expansion of the matter into a rarer form takes place by the removal of the necessary pressure; still, however, retaining, as might be expected, the capability of having what feeble conducting power remains, increased by the action of heat. . i venture to give the following summary of the conditions of electric conduction in bodies, not however without fearing that i may have omitted some important points[a]. [a] see now in relation to this subject, -- .--_dec. ._ . all bodies conduct electricity in the same manner from metals to lac and gases, but in very different degrees. . conducting power is in some bodies powerfully increased by heat, and in others diminished, yet without our perceiving any accompanying essential electrical difference, either in the bodies or in the changes occasioned by the electricity conducted. . a numerous class of bodies, insulating electricity of low intensity, when solid, conduct it very freely when fluid, and are then decomposed by it. . but there are many fluid bodies which do not sensibly conduct electricity of this low intensity; there are some which conduct it and are not decomposed; nor is fluidity essential to decomposition[a]. [a] see the next series of these experimental researches. . there is but one body yet discovered[a] which, insulating a voltaic current when solid, and conducting it when fluid, is not decomposed in the latter case ( .). [a] it is just possible that this case may, by more delicate experiment, hereafter disappear. (see now, , , in relation to this note.--_dec. ._) . there is no strict electrical distinction of conduction which can, as yet, be drawn between bodies supposed to be elementary, and those known to be compounds. _royal institution, april , _. fifth series. § . _on electro-chemical decomposition._ ¶ i. _new conditions of electro-chemical decomposition._ ¶ ii. _influence of water in electro-chemical decomposition._ ¶ iii. _theory of electro-chemical decomposition._ received june ,--read june , . § . _on electro-chemical decomposition._[a] [a] refer to the note after , series viii.--_dec. ._ . i have in a recent series of these researches ( .) proved (to my own satisfaction, at least,) the identity of electricities derived from different sources, and have especially dwelt upon the proofs of the sameness of those obtained by the use of the common electrical machine and the voltaic battery. . the great distinction of the electricities obtained from these two sources is the very high tension to which the small quantity obtained by aid of the machine may be raised, and the enormous quantity ( . .) in which that of comparatively low tension, supplied by the voltaic battery, may be procured; but as their actions, whether magnetical, chemical, or of any other nature, are essentially the same ( .), it appeared evident that we might reason from the former as to the manner of action of the latter; and it was, to me, a probable consequence, that the use of electricity of such intensity as that afforded by the machine, would, when applied to effect and elucidate electro-chemical decomposition, show some new conditions of that action, evolve new views of the internal arrangements and changes of the substances under decomposition, and perhaps give efficient powers over matter as yet undecomposed. . for the purpose of rendering the bearings of the different parts of this series of researches more distinct, i shall divide it into several heads. ¶ i. _new conditions of electro-chemical decomposition._ . the tension of machine electricity causes it, however small in quantity, to pass through any length of water, solutions, or other substances classing with these as conductors, as fast as it can be produced, and therefore, in relation to quantity, as fast as it could have passed through much shorter portions of the same conducting substance. with the voltaic battery the case is very different, and the passing current of electricity supplied by it suffers serious diminution in any substance, by considerable extension of its length, but especially in such bodies as those mentioned above. . i endeavoured to apply this facility of transmitting the current of electricity through any length of a conductor, to an investigation of the transfer of the elements in a decomposing body, in contrary directions, towards the poles. the general form of apparatus used in these experiments has been already described ( . ); and also a particular experiment ( .), in which, when a piece of litmus paper and a piece of turmeric paper were combined and moistened in solution of sulphate of soda, the point of the wire from the machine (representing the positive pole) put upon the litmus paper, and the receiving point from the discharging train ( . .), representing the negative pole, upon the turmeric paper, a very few turns of the machine sufficed to show the evolution of acid at the former, and alkali at the latter, exactly in the manner effected by a volta-electric current. . the pieces of litmus and turmeric paper were _now_ placed each upon a separate plate of glass, and connected by an insulated string four feet long, moistened in the same solution of sulphate of soda: the terminal decomposing wire points were placed upon the papers as before. on working the machine, the same evolution of acid and alkali appeared as in the former instance, and with equal readiness, notwithstanding that the places of their appearance were four feet apart from each other. finally, a piece of string, seventy feet long, was used. it was insulated in the air by suspenders of silk, so that the electricity passed through its entire length: decomposition took place exactly as in former cases, alkali and acid appearing at the two extremities in their proper places. . experiments were then made both with sulphate of soda and iodide of potassium, to ascertain if any diminution of decomposing effect was produced by such great extension as those just described of the moist conductor or body under decomposition; but whether the contact of the decomposing point connected with the discharging train was made with turmeric paper touching the prime conductor, or with other turmeric paper connected with it through the seventy feet of string, the spot of alkali for an equal number of turns of the machine had equal intensity of colour. the same results occurred at the other decomposing wire, whether the salt or the iodide were used; and it was fully proved that this great extension of the distance between the poles produced no effect whatever on the amount of decomposition, provided the same _quantity_ of electricity were passed in both cases ( .). . the negative point of the discharging train, the turmeric paper, and the string were then removed; the positive point was left resting upon the litmus paper, and the latter touched by a piece of moistened string held in the hand. a few turns of the machine evolved acid at the positive point as freely as before. . the end of the moistened string, instead of being held in the hand, was suspended by glass in the air. on working the machine the electricity proceeded from the conductor through the wire point to the litmus paper, and thence away by the intervention of the string to the air, so that there was (as in the last experiment) but one metallic pole; still acid was evolved there as freely as in any former case. . when any of these experiments were repeated with electricity from the negative conductor, corresponding effects were produced whether one or two decomposing wires were used. the results were always constant, considered in relation to the _direction_ of the electric current. . these experiments were varied so as to include the action of only one metallic pole, but that not the pole connected with the machine. turmeric paper was moistened in solution of sulphate of soda, placed upon glass, and connected with the discharging train ( .) by a decomposing wire ( .); a piece of wet string was hung from it, the lower extremity of which was brought opposite a point connected with the positive prime conductor of the machine. the machine was then worked for a few turns, and alkali immediately appeared at the point of the discharging train which rested on the turmeric paper. corresponding effects took place at the negative conductor of a machine. . these cases are abundantly sufficient to show that electrochemical decomposition does not depend upon the simultaneous action of two metallic poles, since a single pole might be used, decomposition ensue, and one or other of the elements liberated, pass to the pole, according as it was positive or negative. in considering the course taken by, and the final arrangement of, the other element, i had little doubt that i should find it had receded towards the other extremity, and that the air itself had acted as a pole, an expectation which was fully confirmed in the following manner. . a piece of turmeric paper, not more than . of an inch in length and . of an inch in width, was moistened with sulphate of soda and placed upon the edge of a glass plate opposite to, and about two inches from, a point connected with the discharging train (plate iv. fig. .); a piece of tinfoil, resting upon the same glass plate, was connected with the machine, and also with the turmeric paper, by a decomposing wire _a_ ( .). the machine was then worked, the positive electricity passing into the turmeric paper at the point _p_, and out at the extremity _n_. after forty or fifty turns of the machine, the extremity _n_ was examined, and the two points or angles found deeply coloured by the presence of free alkali (fig. .). . a similar piece of litmus paper, dipped in solution of sulphate of soda _n_, fig. , was now supported upon the end of the discharging train _a_, and its extremity brought opposite to a point _p_, connected with the conductor of the machine. after working the machine for a short time, acid was developed at both the corners towards the point, i.e. at both the corners receiving the electricities from the air. every precaution was taken to prevent this acid from being formed by sparks or brushes passing through the air ( .); and these, with the accompanying general facts, are sufficient to show that the acid was really the result of electro-chemical decomposition ( .). . then a long piece of turmeric paper, large at one end and pointed at the other, was moistened in the saline solution, and immediately connected with the conductor of the machine, so that its pointed extremity was opposite a point upon the discharging train. when the machine was worked, alkali was evolved at that point; and even when the discharging train was removed, and the electricity left to be diffused and carried off altogether by the air, still alkali was evolved where the electricity left the turmeric paper. . arrangements were then made in which no metallic communication with the decomposing matter was allowed, but both poles (if they might now be called by that name) formed of air only. a piece of turmeric paper _a_ fig. , and a piece of litmus paper _b_, were dipped in solution of sulphate of soda, put together so as to form one moist pointed conductor, and supported on wax between two needle points, one, _p_, connected by a wire with the conductor of the machine, and the other, _n_, with the discharging train. the interval in each case between the points was about half an inch; the positive point _p_ was opposite the litmus paper; the negative point _n_ opposite the turmeric. the machine was then worked for a time, upon which evidence of decomposition quickly appeared, for the point of the litmus _b_ became reddened from acid evolved there, and the point of the turmeric _a_ red from a similar and simultaneous evolution of alkali. . upon turning the paper conductor round, so that the litmus point should now give off the positive electricity, and the turmeric point receive it, and working the machine for a short time, both the red spots disappeared, and as on continuing the action of the machine no red spot was re-formed at the litmus extremity, it proved that in the first instance ( .) the effect was not due to the action of brushes or mere electric discharges causing the formation of nitric acid from the air ( .). . if the combined litmus and turmeric paper in this experiment be considered as constituting a conductor independent of the machine or the discharging train, and the final places of the elements evolved be considered in relation to this conductor, then it will be found that the acid collects at the _negative_ or receiving end or pole of the arrangement, and the alkali at the _positive_ or delivering extremity. . similar litmus and turmeric paper points were now placed upon glass plates, and connected by a string six feet long, both string and paper being moistened in solution of sulphate of soda; a needle point connected with the machine was brought opposite the litmus paper point, and another needle point connected with the discharging train brought opposite the turmeric paper. on working the machine, acid appeared on the litmus, and alkali on the turmeric paper; but the latter was not so abundant as in former cases, for much of the electricity passed off from the string into the air, and diminished the quantity discharged at the turmeric point. . finally, a series of four small compound conductors, consisting of litmus and turmeric paper (fig. .) moistened in solution of sulphate of soda, were supported on glass rods, in a line at a little distance from each other, between the points _p_ and _n_ of the machine and discharging train, so that the electricity might pass in succession through them, entering in at the litmus points _b, b_, and passing out at the turmeric points _a, a_. on working the machine carefully, so as to avoid sparks and brushes ( .), i soon obtained evidence of decomposition in each of the moist conductors, for all the litmus points exhibited free acid, and the turmeric points equally showed free alkali. . on using solutions of iodide of potassium, acetate of lead, &c., similar effects were obtained; but as they were all consistent with the results above described, i refrain from describing the appearances minutely. . these cases of electro-chemical decomposition are in their nature exactly of the same kind as those affected under ordinary circumstances by the voltaic battery, notwithstanding the great differences as to the presence or absence, or at least as to the nature of the parts usually called poles; and also of the final situation of the elements eliminated at the electrified boundary surfaces ( .). they indicate at once an internal action of the parts suffering decomposition, and appear to show that the power which is effectual in separating the elements is exerted there, and not at the poles. but i shall defer the consideration of this point for a short time ( . .), that i may previously consider another supposed condition of electro-chemical decomposition[a]. [a] i find (since making and describing these results,) from a note to sir humphry davy's paper in the philosophical transactions, , p. , that that philosopher, in repeating wollaston's experiment of the decomposition of water by common electricity ( . .) used an arrangement somewhat like some of those i have described. he immersed a guarded platina point connected with the machine in distilled water, and dissipated the electricity from the water into the air by moistened filaments of cotton. in this way he states that he obtained oxygen and hydrogen _separately_ from each other. this experiment, had i known of it, ought to have been quoted in an earlier series of these researches ( .); but it does not remove any of the objections i have made to the use of wollaston's apparatus as a test of true chemical action ( .). ¶ ii. _influence of water in electro-chemical decomposition._ . it is the opinion of several philosophers, that the presence of water is essential in electro-chemical decomposition, and also for the evolution of electricity in the voltaic battery itself. as the decomposing cell is merely one of the cells of the battery, into which particular substances are introduced for the purpose of experiment, it is probable that what is an essential condition in the one case is more or less so in the other. the opinion, therefore, that water is necessary to decomposition, may have been founded on the statement made by sir humphry davy, that "there are no fluids known, except such as contain water, which are capable of being made the medium of connexion between the metals or metal of the voltaic apparatus[a]:" and again, "when any substance rendered fluid by heat, consisting of _water_, oxygen, and inflammable or metallic matter, is exposed to those wires, similar phenomena (of decomposition) occur[b]." [a] elements of chemical philosophy, p. , &c. [b] ibid. pp. , . . this opinion has, i think, been shown by other philosophers not to be accurate, though i do not know where to refer for a contradiction of it. sir humphry davy himself said in [a], that dry nitre, caustic potash and soda are conductors of galvanism when rendered fluid by a high degree of heat, but he must have considered them, or the nitre at least, as not suffering decomposition, for the statements above were made by him eleven years subsequently. in he also pointed out, that bodies not containing water, as _fused litharge_ and _chlorate of potassa_, were sufficient to form, with platina and zinc, powerful electromotive circles[b]; but he is here speaking of the _production_ of electricity in the pile, and not of its effects when evolved; nor do his words at all imply that any correction of his former distinct statements relative to _decomposition_ was required. [a] journal of the royal institution, , p. . [b] philosophical transactions, , p. . . i may refer to the last series of these experimental researches ( . .) as setting the matter at rest, by proving that there are hundreds of bodies equally influential with water in this respect; that amongst binary compounds, oxides, chlorides, iodides, and even sulphurets ( .) were effective; and that amongst more complicated compounds, cyanides and salts, of equal efficacy, occurred in great numbers ( .). . water, therefore, is in this respect merely one of a very numerous class of substances, instead of being the _only one_ and _essential_; and it is of that class one of the _worst_ as to its capability of facilitating conduction and suffering decomposition. the reasons why it obtained for a time an exclusive character which it so little deserved are evident, and consist, in the general necessity of a fluid condition ( .); in its being the _only one_ of this class of bodies existing in the fluid state at common temperatures; its abundant supply as the great natural solvent; and its constant use in that character in philosophical investigations, because of its having a smaller interfering, injurious, or complicating action upon the bodies, either dissolved or evolved, than any other substance. . the analogy of the decomposing or experimental cell to the other cells of the voltaic battery renders it nearly certain that any of those substances which are decomposable when fluid, as described in my last paper ( .), would, if they could be introduced between the metallic plates of the pile, be equally effectual with water, if not more so. sir humphry davy found that litharge and chlorate of potassa were thus effectual[a]. i have constructed various voltaic arrangements, and found the above conclusion to hold good. when any of the following substances in a fused state were interposed between copper and platina, voltaic action more or less powerful was produced. nitre; chlorate of potassa; carbonate of potassa; sulphate of soda; chloride of lead, of sodium, of bismuth, of calcium; iodide of lead; oxide of bismuth; oxide of lead: the electric current was in the same direction as if acids had acted upon the metals. when any of the same substances, or phosphate of soda, were made to act on platina and iron, still more powerful voltaic combinations of the same kind were produced. when either nitrate of silver or chloride of silver was the fluid substance interposed, there was voltaic action, but the electric current was in the reverse direction. [a] philosophical transactions, , p. . iii. _theory of electro-chemical decomposition._ . the extreme beauty and value of electro-chemical decompositions have given to that power which the voltaic pile possesses of causing their occurrence an interest surpassing that of any other of its properties; for the power is not only intimately connected with the continuance, if not with the production, of the electrical phenomena, but it has furnished us with the most beautiful demonstrations of the nature of many compound bodies; has in the hands of becquerel been employed in compounding substances; has given us several new combinations, and sustains us with the hope that when thoroughly understood it will produce many more. . what may be considered as the general facts of electrochemical decomposition are agreed to by nearly all who have written on the subject. they consist in the separation of the decomposable substance acted upon into its proximate or sometimes ultimate principles, whenever both poles of the pile are in contact with that substance in a proper condition; in the evolution of these principles at distant points, i.e. at the poles of the pile, where they are either finally set free or enter into union with the substance of the poles; and in the constant determination of the evolved elements or principles to particular poles according to certain well-ascertained laws. . but the views of men of science vary much as to the nature of the action by which these effects are produced; and as it is certain that we shall be better able to apply the power when we really understand the manner in which it operates, this difference of opinion is a strong inducement to further inquiry. i have been led to hope that the following investigations might be considered, not as an increase of that which is doubtful, but a real addition to this branch of knowledge. . it will be needful that i briefly state the views of electro-chemical decomposition already put forth, that their present contradictory and unsatisfactory state may be seen before i give that which seems to me more accurately to agree with facts; and i have ventured to discuss them freely, trusting that i should give no offence to their high-minded authors; for i felt convinced that if i were right, they would be pleased that their views should serve as stepping-stones for the advance of science; and that if i were wrong, they would excuse the zeal which misled me, since it was exerted for the service of that great cause whose prosperity and progress they have desired. . grotthuss, in the year , wrote expressly on the decomposition of liquids by voltaic electricity[a]. he considers the pile as an electric magnet, i.e. as an attractive and repulsive agent; the poles having _attractive_ and _repelling_ powers. the pole from whence resinous electricity issues attracts hydrogen and repels oxygen, whilst that from which vitreous electricity proceeds attracts oxygen and repels hydrogen; so that each of the elements of a particle of water, for instance, is subject to an attractive and a repulsive force, acting in contrary directions, the centres of action of which are reciprocally opposed. the action of each force in relation to a molecule of water situated in the course of the electric current is in the inverse ratio of the square of the distance at which it is exerted, thus giving (it is stated) for such a molecule a _constant force_[b]. he explains the appearance of the elements at a distance from each other by referring to a succession of decompositions and recompositions occurring amongst the intervening particles[c], and he thinks it probable that those which are about to separate at the poles unite to the two electricities there, and in consequence become gases[d]. [a] annales de chimie, , tom, lviii. p. . [b] ibid. pp. , , also tom. lxiii. p. . [c] ibid. tom. lviii. p. , tom, lxiii. p. . [d] ibid. tom. lxiii. p. . . sir humphry davy's celebrated bakerian lecture on some chemical agencies of electricity was read in november , and is almost entirely occupied in the consideration of _electro-chemical decompositions_. the facts are of the utmost value, and, with the general points established, are universally known. the _mode of action_ by which the effects take place is stated very generally, so generally, indeed, that probably a dozen precise schemes of electro-chemical action might be drawn up, differing essentially from each other, yet all agreeing with the statement there given. . when sir humphry davy uses more particular expressions, he seems to refer the decomposing effects to the attractions of the poles. this is the case in the "general expression of facts" given at pp. and of the philosophical transactions for , also at p. . again at p. of the elements of chemical philosophy, he speaks of the great attracting powers of the surfaces of the poles. he mentions the probability of a succession of decompositions and recompositions throughout the fluid,--agreeing in that respect with grotthuss[a]; and supposes that the attractive and repellent agencies may be communicated from the metallic surfaces throughout the whole of the menstruum[b], being communicated from _one particle to another particle of the same kind_[c], and diminishing in strength from the place of the poles to the middle point, which is necessarily neutral[d]. in reference to this diminution of power at increased distances from the poles, he states that in a circuit of ten inches of water, solution of sulphate of potassa placed four inches from the positive pole, did not decompose; whereas when only two inches from that pole, it did render up its elements[e]. [a] philosophical transactions, , pp. , . [b] ibid. p. . [c] ibid. p. . [d] ibid. p. . [e] ibid. p. . . when in sir humphry davy wrote again on this subject, he stated that he found nothing to alter in the fundamental theory laid down in the original communication[a], and uses the terms attraction and repulsion apparently in the same sense as before[b]. [a] philosophical transactions, , p. . [b] ibid. pp. , , . . messrs. riffault and chompré experimented on this subject in . they came to the conclusion that the voltaic current caused decompositions throughout its whole course in the humid conductor, not merely as preliminary to the recompositions spoken of by grotthuss and davy, but producing final separation of the elements in the _course_ of the current, and elsewhere than at the poles. they considered the _negative_ current as collecting and carrying the acids, &c. to the _positive_ pole, and the _positive_ current as doing the same duty with the bases, and collecting them at the _negative_ pole. they likewise consider the currents as _more powerful_ the nearer they are to their respective poles, and state that the positive current is _superior_ in power to the negative current[a]. [a] annales de chimie, , tom. lxiii. p. , &c. . m. biot is very cautious in expressing an opinion as to the cause of the separation of the elements of a compound body[a]. but as far as the effects can be understood, he refers them to the opposite electrical states of the portions of the decomposing substance in the neighbourhood of the two poles. the fluid is most positive at the positive pole; that state gradually diminishes to the middle distance, where the fluid is neutral or not electrical; but from thence to the negative pole it becomes more and more negative[b]. when a particle of salt is decomposed at the negative pole, the acid particle is considered as acquiring a negative electrical state from the pole, stronger than that of the surrounding _undecomposed_ particles, and is therefore repelled from amongst them, and from out of that portion of the liquid towards the positive pole, towards which also it is drawn by the attraction of the pole itself and the particles of positive _undecomposed_ fluid around it[c]. [a] précis elémentaire de physique, me édition, , tom. i. p. . [b] ibid. p. . [c] ibid. pp. , . . m. biot does not appear to admit the successive decompositions and recompositions spoken of by grotthuss, davy, &c. &c.; but seems to consider the substance whilst in transit as combined with, or rather attached to, the electricity for the time[a], and though it communicates this electricity to the surrounding undecomposed matter with which it is in contact, yet it retains during the transit a little superiority with respect to that kind which it first received from the pole, and is, by virtue of that difference, carried forward through the fluid to the opposite pole[b]. [a] précis elémentaire de physique, me édition, , tom. i. p. . [b] ibid. p, . . this theory implies that decomposition takes place at both poles upon distinct portions of fluid, and not at all in the intervening parts. the latter serve merely as imperfect conductors, which, assuming an electric state, urge particles electrified more highly at the poles through them in opposite directions, by virtue of a series of ordinary electrical attractions and repulsions[a]. [a] précis elémentaire de physique, me édition, , tom. i. pp. , . . m.a. de la rive investigated this subject particularly, and published a paper on it in [a]. he thinks those who have referred the phenomena to the attractive powers of the poles, rather express the general fact than give any explication of it. he considers the results as due to an actual combination of the elements, or rather of half of them, with the electricities passing from the poles in consequence of a kind of play of affinities between the matter and electricity[b]. the current from the positive pole combining with the hydrogen, or the bases it finds there, leaves the oxygen and acids at liberty, but carries the substances it is united with across to the negative pole, where, because of the peculiar character of the metal as a conductor[c], it is separated from them, entering the metal and leaving the hydrogen or bases upon its surface. in the same manner the electricity from the negative pole sets the hydrogen and bases which it finds there, free, but combines with the oxygen and acids, carries them across to the positive pole, and there deposits them[d]. in this respect m. de la rive's hypothesis accords in part with that of mm. riffault and chompré ( .). [a] annales de chimie, tom, xxviii. p. . [b] ibid. pp. , . [c] ibid. p. . [d] ibid. p. . . m. de la rive considers the portions of matter which are decomposed to be those contiguous to _both_ poles[a]. he does not admit with others the successive decompositions and recompositions in the whole course of the electricity through the humid conductor[b], but thinks the middle parts are in themselves unaltered, or at least serve only to conduct the two contrary currents of electricity and matter which set off from the opposite poles[c]. the decomposition, therefore, of a particle of water, or a particle of salt, may take place at either pole, and when once effected, it is final for the time, no recombination taking place, except the momentary union of the transferred particle with the electricity be so considered. [a] annales de chimie, tom, xxviii. pp. , . [b] ibid. pp. , . [c] ibid. p. . . the latest communication that i am aware of on the subject is by m. hachette: its date is october [a]. it is incidental to the description of the decomposition of water by the magneto-electric currents ( .). one of the results of the experiment is, that "it is not necessary, as has been supposed, that for the chemical decomposition of water, the action of the two electricities, positive and negative, should be simultaneous." [a] annales de chimie, tom, xxviii. tom. li. p. . . it is more than probable that many other views of electro-chemical decomposition may have been published, and perhaps amongst them some which, differing from those above, might, even in my own opinion, were i acquainted with them, obviate the necessity for the publication of my views. if such be the case, i have to regret my ignorance of them, and apologize to the authors. * * * * * . that electro-chemical decomposition does not depend upon any direct attraction and repulsion of the poles (meaning thereby the metallic terminations either of the voltaic battery, or ordinary electrical machine arrangements ( .),) upon the elements in contact with or near to them, appeared very evident from the experiments made in air ( , , &c.), when the substances evolved did not collect about any poles, but, in obedience to the direction of the current, were evolved, and i would say ejected, at the extremities of the decomposing substance. but notwithstanding the extreme dissimilarity in the character of air and metals, and the almost total difference existing between them as to their mode of conducting electricity, and becoming charged with it, it might perhaps still be contended, although quite hypothetically, that the bounding portions of air were now the surfaces or places of attraction, as the metals had been supposed to be before. in illustration of this and other points, i endeavoured to devise an arrangement by which i could decompose a body against a surface of water, as well as against air or metal, and succeeded in doing so unexceptionably in the following manner. as the experiment for very natural reasons requires many precautions, to be successful, and will be referred to hereafter in illustration of the views i shall venture to give, i must describe it minutely. . a glass basin (fig. .), four inches in diameter and four inches deep, had a division of mica _a_, fixed across the upper part so as to descend one inch and a half below the edge, and be perfectly water-tight at the sides: a plate of platina _b_, three inches wide, was put into the basin on one side of the division _a_, and retained there by a glass block below, so that any gas produced by it in a future stage of the experiment should not ascend beyond the mica, and cause currents in the liquid on that side. a strong solution of sulphate of magnesia was carefully poured without splashing into the basin, until it rose a little above the lower edge of the mica division _a_, great care being taken that the glass or mica on the unoccupied or _c_ side of the division in the figure, should not be moistened by agitation of the solution above the level to which it rose. a thin piece of clean cork, well-wetted in distilled water, was then carefully and lightly placed on the solution at the _c_ side, and distilled water poured gently on to it until a stratum the eighth of an inch in thickness appeared over the sulphate of magnesia; all was then left for a few minutes, that any solution adhering to the cork might sink away from it, or be removed by the water on which it now floated; and then more distilled water was added in a similar manner, until it reached nearly to the top of the glass. in this way solution of the sulphate occupied the lower part of the glass, and also the upper on the right-hand side of the mica; but on the left-hand side of the division a stratum of water from _c_ to _d_, one inch and a half in depth, reposed upon it, the two presenting, when looked through horizontally, a comparatively definite plane of contact. a second platina pole _e_, was arranged so as to be just under the surface of the water, in a position nearly horizontal, a little inclination being given to it, that gas evolved during decomposition might escape: the part immersed was three inches and a half long by one inch wide, and about seven-eighths of an inch of water intervened between it and the solution of sulphate of magnesia. . the latter pole _e_ was now connected with the negative end of a voltaic battery, of forty pairs of plates four inches square, whilst the former pole _b_ was connected with the positive end. there was action and gas evolved at both poles; but from the intervention of the pure water, the decomposition was very feeble compared to what the battery would have effected in a uniform solution. after a little while (less than a minute,) magnesia also appeared at the negative side: _it did not make its appearance at the negative metallic pole, but in the water_, at the plane where the solution and the water met; and on looking at it horizontally, it could be there perceived lying in the water upon the solution, not rising more than the fourth of an inch above the latter, whilst the water between it and the negative pole was perfectly clear. on continuing the action, the bubbles of hydrogen rising upwards from the negative pole impressed a circulatory movement on the stratum of water, upwards in the middle, and downwards at the side, which gradually gave an ascending form to the cloud of magnesia in the part just under the pole, having an appearance as if it were there attracted to it; but this was altogether an effect of the currents, and did not occur until long after the phenomena looked for were satisfactorily ascertained. . after a little while the voltaic communication was broken, and the platina poles removed with as little agitation as possible from the water and solution, for the purpose of examining the liquid adhering to them. the pole _c_, when touched by turmeric paper, gave no traces of alkali, nor could anything but pure water be found upon it. the pole _b_, though drawn through a much greater depth and quantity of fluid, was found so acid as to give abundant evidence to litmus paper, the tongue, and other tests. hence there had been no interference of alkaline salts in any way, undergoing first decomposition, and then causing the separation of the magnesia at a distance from the pole by mere chemical agencies. this experiment was repeated again and again, and always successfully. . as, therefore, the substances evolved in cases of electrochemical decomposition may be made to appear against air ( . .),--which, according to common language, is not a conductor, nor is decomposed, or against water ( .), which is a conductor, and can be decomposed,--as well as against the metal poles, which are excellent conductors, but undecomposable, there appears but little reason to consider the phenomena generally, as due to the _attraction_ or attractive powers of the latter, when used in the ordinary way, since similar attractions can hardly be imagined in the former instances. . it may be said that the surfaces of air or of water in these cases become the poles, and exert attractive powers; but what proof is there of that, except the fact that the matters evolved collect there, which is the point to be explained, and cannot be justly quoted as its own explanation? or it may be said, that any section of the humid conductor, as that in the present case, where the solution and the water meet, may be considered as representing the pole. but such does not appear to me to be the view of those who have written on the subject, certainly not of some of them, and is inconsistent with the supposed laws which they have assumed, as governing the diminution of power at increased distances from the poles. . grotthuss, for instance, describes the poles as centres of attractive and repulsive forces ( .), these forces varying inversely as the squares of the distances, and says, therefore, that a particle placed anywhere between the poles will be acted upon by a constant force. but the compound force, resulting from such a combination as he supposes, would be anything but a constant force; it would evidently be a force greatest at the poles, and diminishing to the middle distance. grotthuss is right, however, _in the fact_, according to my experiments ( . .), that the particles are acted upon by equal force everywhere in the circuit, when the conditions of the experiment are the simplest possible; but the fact is against his theory, and is also, i think, against all theories that place the decomposing effect in the attractive power of the poles. . sir humphry davy, who also speaks of the _diminution_ of power with increase of distance from the poles[a] ( .), supposes, that when both poles are acting on substances to decompose them, still the power of decomposition _diminishes_ to the middle distance. in this statement of fact he is opposed to grotthuss, and quotes an experiment in which sulphate of potassa, placed at different distances from the poles in a humid conductor of constant length, decomposed when near the pole, but not when at a distance. such a consequence would necessarily result theoretically from considering the poles as centres of attraction and repulsion; but i have not found the statement borne out by other experiments ( .); and in the one quoted by him the effect was doubtless due to some of the many interfering causes of variation which attend such investigations. [a] philosophical transactions, , p. . . a glass vessel had a platina plate fixed perpendicularly across it, so as to divide it into two cells: a head of mica was fixed over it, so as to collect the gas it might evolve during experiments; then each cell, and the space beneath the mica, was filled with dilute sulphuric acid. two poles were provided, consisting each of a platina wire terminated by a plate of the same metal; each was fixed into a tube passing through its upper end by an air-tight joint, that it might be moveable, and yet that the gas evolved at it might be collected. the tubes were filled with the acid, and one immersed in each cell. each platina pole was equal in surface to one side of the dividing plate in the middle glass vessel, and the whole might be considered as an arrangement between the poles of the battery of a humid decomposable conductor divided in the middle by the interposed platina diaphragm. it was easy, when required, to draw one of the poles further up the tube, and then the platina diaphragm was no longer in the middle of the humid conductor. but whether it were thus arranged at the middle, or towards one side, it always evolved a quantity of oxygen and hydrogen equal to that evolved by both the extreme plates[a]. [a] there are certain precautions, in this and such experiments, which can only be understood and guarded against by a knowledge of the phenomena to be described in the first part of the sixth series of these researches. . if the wires of a galvanometer be terminated by plates, and these be immersed in dilute acid, contained in a regularly formed rectangular glass trough, connected at each end with a voltaic battery by poles equal to the section of the fluid, a part of the electricity will pass through the instrument and cause a certain deflection. and if the plates are always retained at the _same distance from each other_ and from the sides of the trough, are always parallel to each other, and uniformly placed relative to the fluid, then, whether they are immersed near the middle of the decomposing solution, or at one end, still the instrument will indicate the same deflection, and consequently the same electric influence. . it is very evident, that when the width of the decomposing conductor varies, as is always the case when mere wires or plates, as poles, are dipped into or are surrounded by solution, no constant expression can be given as to the action upon a single particle placed in the course of the current, nor any conclusion of use, relative to the supposed attractive or repulsive force of the poles, be drawn. the force will vary as the distance from the pole varies; as the particle is directly between the poles, or more or less on one side; and even as it is nearer to or further from the sides of the containing vessels, or as the shape of the vessel itself varies; and, in fact, by making variations in the form of the arrangement, the force upon any single particle may be made to increase, or diminish, or remain constant, whilst the distance between the particle and the pole shall remain the same; or the force may be made to increase, or diminish, or remain constant, either as the distance increases or as it diminishes. . from numerous experiments, i am led to believe the following general expression to be correct; but i purpose examining it much further, and would therefore wish not to be considered at present as pledged to its accuracy. the _sum of chemical decomposition is constant_ for any section taken across a decomposing conductor, uniform in its nature, at whatever distance the poles may be from each other or from the section; or however that section may intersect the currents, whether directly across them, or so oblique as to reach almost from pole to pole, or whether it be plane, or curved, or irregular in the utmost degree; provided the current of electricity be retained constant in quantity ( .), and that the section passes through every part of the current through the decomposing conductor. . i have reason to believe that the statement might be made still more general, and expressed thus: that _for a constant quantity of electricity, whatever the decomposing conductor may be, whether water, saline solutions, acids, fused bodies, &c., the amount of electro-chemical action is also a constant quantity, i.e. would always be equivalent to a standard chemical effect founded upon ordinary chemical affinity_. i have this investigation in hand, with several others, and shall be prepared to give it in the next series but one of these researches. . many other arguments might be adduced against the hypotheses of the attraction of the poles being the cause of electro-chemical decomposition; but i would rather pass on to the view i have thought more consistent with facts, with this single remark; that if decomposition by the voltaic battery depended upon the attraction of the poles, or the parts about them, being stronger than the mutual attraction of the particles separated, it would follow that the weakest _electrical_ attraction was stronger than, if not the strongest, yet very strong _chemical_ attraction, namely, such as exists between oxygen and hydrogen, potassium and oxygen, chlorine and sodium, acid and alkali, &c., a consequence which, although perhaps not impossible, seems in the present state of the subject very unlikely. . the view which m. de la rive has taken ( .), and also mm. riffault and chompré ( .), of the manner in which electro-chemical decomposition is effected, is very different to that already considered, and is not affected by either the arguments or facts urged against the latter. considering it as stated by the former philosopher, it appears to me to be incompetent to account for the experiments of decomposition against surfaces of air ( . .) and water ( .), which i have described; for if the physical differences between metals and humid conductors, which m. de la rive supposes to account for the transmission of the compound of matter and electricity in the latter, and the transmission of the electricity only with the rejection of the matter in the former, be allowed for a moment, still the analogy of air to metal is, electrically considered, so small, that instead of the former replacing the latter ( .), an effect the very reverse might have been expected. or if even that were allowed, the experiment with water ( .), at once sets the matter at rest, the decomposing pole being now of a substance which is admitted as competent to transmit the assumed compound of electricity and matter. . with regard to the views of mm. riffault and chompré ( .), the occurrence of decomposition alone in the _course_ of the current is so contrary to the well-known effects obtained in the forms of experiment adopted up to this time, that it must be proved before the hypothesis depending on it need be considered. . the consideration of the various theories of electro-chemical decomposition, whilst it has made me diffident, has also given me confidence to add another to the number; for it is because the one i have to propose appears, after the most attentive consideration, to explain and agree with the immense collection of facts belonging to this branch of science, and to remain uncontradicted by, or unopposed to, any of them, that i have been encouraged to give it. . electro-chemical decomposition is well known to depend essentially upon the _current_ of electricity. i have shown that in certain cases ( .) the decomposition is proportionate to the quantity of electricity passing, whatever may be its intensity or its source, and that the same is probably true for all cases ( .), even when the utmost generality is taken on the one hand, and great precision of expression on the other ( .). . in speaking of the current, i find myself obliged to be still more particular than on a former occasion ( .), in consequence of the variety of views taken by philosophers, all agreeing in the effect of the current itself. some philosophers, with franklin, assume but one electric fluid; and such must agree together in the general uniformity and character of the electric current. others assume two electric fluids; and here singular differences have arisen. . mm. riffault and chompré, for instance, consider the positive and negative currents each as causing decomposition, and state that the positive current is _more powerful_ than the negative current[a], the nitrate of soda being, under similar circumstances, decomposed by the former, but not by the latter. [a] annales de chimie, , tom, lxiii. p. . . m. hachette states[a] that "it is not necessary, as has been believed, that the action of the two electricities, positive and negative, should be simultaneous for the decomposition of water." the passage implying, if i have caught the meaning aright, that one electricity can be obtained, and can be applied in effecting decompositions, independent of the other. [a] annales de chimie, , tom. li. p. . . the view of m. de la rive to a certain extent agrees with that of m. hachette, for he considers that the two electricities decompose separate portions of water ( .)[a]. in one passage he speaks of the two electricities as two influences, wishing perhaps to avoid offering a decided opinion upon the independent existence of electric fluids; but as these influences are considered as combining with the elements set free as by a species of chemical affinity, and for the time entirely masking their character, great vagueness of idea is thus introduced, inasmuch as such a species of combination can only be conceived to take place between things having independent existences. the two elementary electric currents, moving in opposite directions, from pole to pole, constitute the ordinary _voltaic current._ [a] annales de chimie, , tom, xxviii. pp. , . . m. grotthuss is inclined to believe that the elements of water, when about to separate at the poles, combine with the electricities, and so become gases. m. de la rive's view is the exact reverse of this: whilst passing through the fluid, they are, according to him, compounds with the electricities; when evolved at the poles, they are de-electrified. . i have sought amongst the various experiments quoted in support of these views, or connected with electro-chemical decompositions or electric currents, for any which might be considered as sustaining the theory of two electricities rather than that of one, but have not been able to perceive a single fact which could be brought forward for such a purpose: or, admitting the hypothesis of two electricities, much less have i been able to perceive the slightest grounds for believing that one electricity in a current can be more powerful than the other, or that it can be present without the other, or that one can be varied or in the slightest degree affected, without a corresponding variation in the other[a]. if, upon the supposition of two electricities, a current of one can be obtained without the other, or the current of one be exalted or diminished more than the other, we might surely expect some variation either of the chemical or magnetical effects, or of both; but no such variations have been observed. if a current be so directed that it may act chemically in one part of its course, and magnetically in another, the two actions are always found to take place together. a current has not, to my knowledge, been produced which could act chemically and not magnetically, nor any which can act on the magnet, and not _at the same time_ chemically[b]. [a] see now in relation to this subject, - .--_dec. ._ [b] thermo-electric currents are of course no exception, because when they fail to act chemically they also fail to be currents. . _judging from facts only_, there is not as yet the slightest reason for considering the influence which is present in what we call the electric current,--whether in metals or fused bodies or humid conductors, or even in air, flame, and rarefied elastic media,--as a compound or complicated influence. it has never been resolved into simpler or elementary influences, and may perhaps best be conceived of as _an axis of power having contrary forces, exactly equal in amount, in contrary directions_. * * * * * . passing to the consideration of electro-chemical decomposition, it appears to me that the effect is produced by an _internal corpuscular action_, exerted according to the direction of the electric current, and that it is due to a force either _super to_, or _giving direction to the ordinary chemical affinity_ of the bodies present. the body under decomposition may be considered as a mass of acting particles, all those which are included in the course of the electric current contributing to the final effect; and it is because the ordinary chemical affinity is relieved, weakened, or partly neutralized by the influence of the electric current in one direction parallel to the course of the latter, and strengthened or added to in the opposite direction, that the combining particles have a tendency to pass in opposite courses. . in this view the effect is considered as _essentially dependent_ upon the _mutual chemical affinity_ of the particles of opposite kinds. particles _aa_, fig. , could not be transferred or travel from one pole n towards the other p, unless they found particles of the opposite kind _bb_, ready to pass in the contrary direction: for it is by virtue of their increased affinity for those particles, combined with their diminished affinity for such as are behind them in their course, that they are urged forward: and when any one particle _a_, fig. , arrives at the pole, it is excluded or set free, because the particle _b_ of the opposite kind, with which it was the moment before in combination, has, under the superinducing influence of the current, a greater attraction for the particle _a'_, which is before it in its course, than for the particle _a_, towards which its affinity has been weakened. . as far as regards any single compound particle, the case may be considered as analogous to one of ordinary decomposition, for in fig. , _a_ may be conceived to be expelled from the compound _ab_ by the superior attraction of _a'_ for _b_, that superior attraction belonging to it in consequence of the relative position of _a'b_ and _a_ to the direction of the axis of electric power ( .) superinduced by the current. but as all the compound particles in the course of the current, except those actually in contact with the poles, act conjointly, and consist of elementary particles, which, whilst they are in one direction expelling, are in the other being expelled, the case becomes more complicated, but not more difficult of comprehension. . it is not here assumed that the acting particles must be in a right line between the poles. the lines of action which may be supposed to represent the electric currents passing through a decomposing liquid, have in many experiments very irregular forms; and even in the simplest case of two wires or points immersed as poles in a drop or larger single portion of fluid, these lines must diverge rapidly from the poles; and the direction in which the chemical affinity between particles is most powerfully modified ( . .) will vary with the direction of these lines, according constantly with them. but even in reference to these lines or currents, it is not supposed that the particles which mutually affect each other must of necessity be parallel to them, but only that they shall accord generally with their direction. two particles, placed in a line perpendicular to the electric current passing in any particular place, are not supposed to have their ordinary chemical relations towards each other affected; but as the line joining them is inclined one way to the current their mutual affinity is increased; as it is inclined in the other direction it is diminished; and the effect is a maximum, when that line is parallel to the current[a]. [a] in reference to this subject see now electrolytic induction and discharge, series xii. ¶ viii. - , &c.--_dec. ._ . that the actions, of whatever kind they may be, take place frequently in oblique directions is evident from the circumstance of those particles being included which in numerous cases are not in a line between the poles. thus, when wires are used as poles in a glass of solution, the decompositions and recompositions occur to the right or left of the direct line between the poles, and indeed in every part to which the currents extend, as is proved by many experiments, and must therefore often occur between particles obliquely placed as respects the current itself; and when a metallic vessel containing the solution is made one pole, whilst a mere point or wire is used for the other, the decompositions and recompositions must frequently be still more oblique to the course of the currents. . the theory which i have ventured to put forth (almost) requires an admission, that in a compound body capable of electro-chemical decomposition the elementary particles have a mutual relation to, and influence upon each other, extending beyond those with which they are immediately combined. thus in water, a particle of hydrogen in combination with oxygen is considered as not altogether indifferent to other particles of oxygen, although they are combined with other particles of hydrogen; but to have an affinity or attraction towards them, which, though it does not at all approach in force, under ordinary circumstances, to that by which it is combined with its own particle, can, under the electric influence, exerted in a definite direction, be made even to surpass it. this general relation of particles already in combination to other particles with which they are not combined, is sufficiently distinct in numerous results of a purely chemical character; especially in those where partial decompositions only take place, and in berthollet's experiments on the effects of quantity upon affinity: and it probably has a direct relation to, and connexion with, attraction of aggregation, both in solids and fluids. it is a remarkable circumstance, that in gases and vapours, where the attraction of aggregation ceases, there likewise the decomposing powers of electricity apparently cease, and there also the chemical action of quantity is no longer evident. it seems not unlikely, that the inability to suffer decomposition in these cases may be dependent upon the absence of that mutual attractive relation of the particles which is the cause of aggregation. . i hope i have now distinctly stated, although in general terms, the view i entertain of the cause of electro-chemical decomposition, _as far as that cause can at present be traced and understood_. i conceive the effects to arise from forces which are _internal_, relative to the matter under decomposition--and _not external_, as they might be considered, if directly dependent upon the poles. i suppose that the effects are due to a modification, by the electric current, of the chemical affinity of the particles through or by which that current is passing, giving them the power of acting more forcibly in one direction than in another, and consequently making them travel by a series of successive decompositions and recompositions in opposite directions, and finally causing their expulsion or exclusion at the boundaries of the body under decomposition, in the direction of the current, _and that_ in larger or smaller quantities, according as the current is more or less powerful ( .). i think, therefore, it would be more philosophical, and more directly expressive of the facts, to speak of such a body, in relation to the current passing through it, rather than to the poles, as they are usually called, in contact with it; and say that whilst under decomposition, oxygen, chlorine, iodine, acids, &c., are rendered at its negative extremity, and combustibles, metals, alkalies, bases, &c., at its positive extremity ( .), i do not believe that a substance can be transferred in the electric current beyond the point where it ceases to find particles with which it can combine; and i may refer to the experiments made in air ( .) and in water ( .), already quoted, for facts illustrating these views in the first instance; to which i will now add others. . in order to show the dependence of the decomposition and transfer of elements upon the chemical affinity of the substances present, experiments were made upon sulphuric acid in the following manner. dilute sulphuric acid was prepared: its specific gravity was . . a solution of sulphate of soda was also prepared, of such strength that a measure of it contained exactly as much sulphuric acid as an equal measure of the diluted acid just referred to. a solution of pure soda, and another of pure ammonia, were likewise prepared, of such strengths that a measure of either should be exactly neutralized by a measure of the prepared sulphuric acid. . four glass cups were then arranged, as in fig. ; seventeen measures of the free sulphuric acid ( .) were put into each of the vessels _a_ and _b_, and seventeen measures of the solution of sulphate of soda into each of the vessels a and b. asbestus, which had been well-washed in acid, acted upon by the voltaic pile, well-washed in water, and dried by pressure, was used to connect _a_ with _b_ and a with b, the portions being as equal as they could be made in quantity, and cut as short as was consistent with their performing the part of effectual communications, _b_ and a were connected by two platina plates or poles soldered to the extremities of one wire, and the cups _a_ and b were by similar platina plates connected with a voltaic battery of forty pairs of plates four inches square, that in _a_ being connected with the negative, and that in b with the positive pole. the battery, which was not powerfully charged, was retained in communication above half an hour. in this manner it was certain that the same electric current had passed through _a b_ and a b, and that in each instance the same quantity and strength of acid had been submitted to its action, but in one case merely dissolved in water, and in the other dissolved and also combined with an alkali. . on breaking the connexion with the battery, the portions of asbestus were lifted out, and the drops hanging at the ends allowed to fall each into its respective vessel. the acids in _a_ and _b_ were then first compared, for which purpose two evaporating dishes were balanced, and the acid from _a_ put into one, and that from _b_ into the other; but as one was a little heavier than the other, a small drop was transferred from the heavier to the lighter, and the two rendered equal in weight. being neutralized by the addition of the soda solution ( .), that from _a_, or the negative vessel, required parts of the soda solution, and that from _b_, or the positive vessel, required . parts. that the sum of these is not parts is principally due to the acid removed with the asbestus; but taking the mean of . parts, it would appear that a twenty-fourth part of the acid originally in the vessel _a_ had passed, through the influence of the electric current, from _a_ into _b_. . in comparing the difference of acid in a and b, the necessary equality of weight was considered as of no consequence, because the solution was at first neutral, and would not, therefore, affect the test liquids, and all the evolved acid would be in b, and the free alkali in a. the solution in a required . measures of the prepared acid ( .) to neutralize it, and the solution in b required also . measures of the soda solution ( .) to neutralize it. as the asbestus must have removed a little acid and alkali from the glasses, these quantities are by so much too small; and therefore it would appear that about a tenth of the acid originally in the vessel a had been transferred into b during the continuance of the electric action. . in another similar experiment, whilst a thirty-fifth part of the acid passed from _a_ to _b_; in the free acid vessels, between a tenth and an eleventh passed from a to b in the combined acid vessels. other experiments of the same kind gave similar results. . the variation of electro-chemical decomposition, the transfer of elements and their accumulation at the poles, according as the substance submitted to action consists of particles opposed more or less in their chemical affinity, together with the consequent influence of the latter circumstances, are sufficiently obvious in these cases, where sulphuric acid is acted upon in the _same quantity_ by the _same_ electric current, but in one case opposed to the comparatively weak affinity of water for it, and in the other to the stronger one of soda. in the latter case the quantity transferred is from two and a half to three times what it is in the former; and it appears therefore very evident that the transfer is greatly dependent upon the mutual action of the particles of the decomposing bodies[a]. [a] see the note to ( .),--_dec. ._ . in some of the experiments the acid from the vessels _a_ and _b_ was neutralized by ammonia, then evaporated to dryness, heated to redness, and the residue examined for sulphates. in these cases more sulphate was always obtained from _a_ than from _b_; showing that it had been impossible to exclude saline bases (derived from the asbestus, the glass, or perhaps impurities originally in the acid,) and that they had helped in transferring the acid into _b_. but the quantity was small, and the acid was principally transferred by relation to the water present. . i endeavoured to arrange certain experiments by which saline solutions should be decomposed against surfaces of water; and at first worked with the electric machine upon a piece of bibulous paper, or asbestus moistened in the solution, and in contact at its two extremities with pointed pieces of paper moistened in pure water, which served to carry the electric current to and from the solution in the middle piece. but i found numerous interfering difficulties. thus, the water and solutions in the pieces of paper could not be prevented from mingling at the point where they touched. again, sufficient acid could be derived from the paper connected with the discharging train, or it may be even from the air itself, under the influence of electric action, to neutralize the alkali developed at the positive extremity of the decomposing solution, and so not merely prevent its appearance, but actually transfer it on to the metal termination: and, in fact, when the paper points were not allowed to touch there, and the machine was worked until alkali was evolved at the delivering or positive end of the turmeric paper, containing the sulphate of soda solution, it was merely necessary to place the opposite receiving point of the paper connected with the discharging train, which had been moistened by distilled water, upon the brown turmeric point and press them together, when the alkaline effect immediately disappeared. . the experiment with sulphate of magnesia already described ( .) is a case in point, however, and shows most clearly that the sulphuric acid and magnesia contributed to each other's transfer and final evolution, exactly as the same acid and soda affected each other in the results just given ( , &c.); and that so soon as the magnesia advanced beyond the reach of the acid, and found no other substance with which it could combine, it appeared in its proper character, and was no longer able to continue its progress towards the negative pole. * * * * * . the theory i have ventured to put forth appears to me to explain all the prominent features of electro-chemical decomposition in a satisfactory manner. . in the first place, it explains why, in all ordinary cases, the evolved substances _appear only at the poles_; for the poles are the limiting surfaces of the decomposing substance, and except at them, every particle finds other particles having a contrary tendency with which it can combine. . then it explains why, in numerous cases, the elements or evolved substances are not _retained_ by the poles; and this is no small difficulty in those theories which refer the decomposing effect directly to the attractive power of the poles. if, in accordance with the usual theory, a piece of platina be supposed to have sufficient power to attract a particle of hydrogen from the particle of oxygen with which it was the instant before combined, there seems no sufficient reason, nor any fact, except those to be explained, which show why it should not, according to analogy with all ordinary attractive forces, as those of gravitation, magnetism, cohesion, chemical affinity, &c. _retain_ that particle which it had just before taken from a distance and from previous combination. yet it does not do so, but allows it to escape freely. nor does this depend upon its assuming the gaseous state, for acids and alkalies, &c. are left equally at liberty to diffuse themselves through the fluid surrounding the pole, and show no particular tendency to combine with or adhere to the latter. and though there are plenty of cases where combination with the pole does take place, they do not at all explain the instances of non-combination, and do not therefore in their particular action reveal the general principle of decomposition. . but in the theory that i have just given, the effect appears to be a natural consequence of the action: the evolved substances are _expelled_ from the decomposing mass ( . .), not _drawn out by an attraction_ which ceases to act on one particle without any assignable reason, while it continues to act on another of the same kind: and whether the poles be metal, water, or air, still the substances are evolved, and are sometimes set free, whilst at others they unite to the matter of the poles, according to the chemical nature of the latter, i.e. their chemical relation to those particles which are leaving the substance under operation. . the theory accounts for the _transfer of elements_ in a manner which seems to me at present to leave nothing unexplained; and it was, indeed, the phenomena of transfer in the numerous cases of decomposition of bodies rendered fluid by heat ( . .), which, in conjunction with the experiments in air, led to its construction. such cases as the former where binary compounds of easy decomposability are acted upon, are perhaps the best to illustrate the theory. . chloride of lead, for instance, fused in a bent tube ( .), and decomposed by platina wires, evolves lead, passing to what is usually called the negative pole, and chlorine, which being evolved at the positive pole, is in part set free, and in part combines with the platina. the chloride of platina formed, being soluble in the chloride of lead, is subject to decomposition, and the platina itself is gradually transferred across the decomposing matter, and found with the lead at the negative pole. . iodide of lead evolves abundance of lead at the negative pole, and abundance of iodine at the positive pole. . chloride of silver furnishes a beautiful instance, especially when decomposed by silver wire poles. upon fusing a portion of it on a piece of glass, and bringing the poles into contact with it, there is abundance of silver evolved at the negative pole, and an equal abundance absorbed at the positive pole, for no chlorine is set free: and by careful management, the negative wire may be withdrawn from the fused globule as the silver is reduced there, the latter serving as the continuation of the pole, until a wire or thread of revived silver, five or six inches in length, is produced; at the same time the silver at the positive pole is as rapidly dissolved by the chlorine, which seizes upon it, so that the wire has to be continually advanced as it is melted away. the whole experiment includes the action of only two elements, silver and chlorine, and illustrates in a beautiful manner their progress in opposite directions, parallel to the electric current, which is for the time giving a uniform general direction to their mutual affinities ( .). . according to my theory, an element or a substance not decomposable under the circumstances of the experiment, (as for instance, a dilute acid or alkali,) should not be transferred, or pass from pole to pole, unless it be in chemical relation to some other element or substance tending to pass in the opposite direction, for the effect is considered as essentially due to the mutual relation of such particles. but the theories attributing the determination of the elements to the attractions and repulsions of the poles require no such condition, i.e. there is no reason apparent why the attraction of the positive pole, and the repulsion of the negative pole, upon a particle of free acid, placed in water between them, should not (with equal currents of electricity) be as strong as if that particle were previously combined with alkali; but, on the contrary, as they have not a powerful chemical affinity to overcome, there is every reason to suppose they would be stronger, and would sooner bring the acid to rest at the positive pole[a]. yet such is not the case, as has been shown by the experiments on free and combined acid ( . .). [a] even sir humphry davy considered the attraction of the pole as being communicated from one particle to another of the _same_ kind ( .). . neither does m. de la rive's theory, as i understand it, _require_ that the particles should be in combination: it does not even admit, where there are two sets of particles capable of combining with and passing by each other, that they do combine, but supposes that they travel as separate compounds of matter and electricity. yet in fact the free substance _cannot_ travel, the combined one _can_. . it is very difficult to find cases amongst solutions or fluids which shall illustrate this point, because of the difficulty of finding two fluids which shall conduct, shall not mingle, and in which an element evolved from one shall not find a combinable element in the other. _solutions_ of acids or alkalies will not answer, because they exist by virtue of an attraction; and increasing the solubility of a body in one direction, and diminishing it in the opposite, is just as good a reason for transfer, as modifying the affinity between the acids and alkalies themselves[a]. nevertheless the case of sulphate of magnesia is in point ( . .), and shows that _one element or principle only_ has no power of transference or of passing towards either pole. [a] see the note to ( .).--_dec. ._ . many of the metals, however, in their solid state, offer very fair instances of the kind required. thus, if a plate of platina be used as the positive pole in a solution of sulphuric acid, oxygen will pass towards it, and so will acid; but these are not substances having such chemical relation to the platina as, even under the favourable condition superinduced by the current ( . .), to combine with it; the platina therefore remains where it was first placed, and has no tendency to pass towards the negative pole. but if a plate of iron, zinc or copper, be substituted for the platina, then the oxygen and acid can combine with these, and the metal immediately begins to travel (as an oxide) to the opposite pole, and is finally deposited there. or if, retaining the platina pole, a fused chloride, as of lead, zinc, silver, &c., be substituted for the sulphuric acid, then, as the platina finds an element it can combine with, it enters into union, acts as other elements do in cases of voltaic decomposition, is rapidly transferred across the melted matter, and expelled at the negative pole. . i can see but little reason in the theories referring the electro-chemical decomposition to the attractions and repulsions of the poles, and i can perceive none in m. de la rive's theory, why the metal of the positive pole should not be transferred across the intervening conductor, and deposited at the negative pole, even when it cannot act chemically upon the element of the fluid surrounding it. it cannot be referred to the attraction of cohesion preventing such an effect; for if the pole be made of the lightest spongy platina, the effect is the same. or if gold precipitated by sulphate of iron be diffused through the solution, still accumulation of it at the negative pole will not take place; and yet the attraction of cohesion is almost perfectly overcome, the particles are in it so small as to remain for hours in suspension, and are perfectly free to move by the slightest impulse towards either pole; and _if in relation_ by chemical affinity to any substance present, are powerfully determined to the negative pole[a]. [a] in making this experiment, care must be taken that no substance be present that can act chemically on the gold. although i used the metal very carefully washed, and diffused through dilute sulphuric acid, yet in the first instance i obtained gold at the negative pole, and the effect was repeated when the platina poles were changed. but on examining the clear liquor in the cell, after subsidence of the metallic gold, i found a little of that metal in solution, and a little chlorine was also present. i therefore well washed the gold which had thus been subjected to voltaic action, diffused it through other pure dilute sulphuric acid, and then found, that on subjecting it to the action of the pile, not the slightest tendency to the negative pole could be perceived. . in support of these arguments, it may be observed, that as yet no determination of a substance to a pole, or tendency to obey the electric current, has been observed (that i am aware of,) in cases of mere mixture; i.e. a substance diffused through a fluid, but having no sensible chemical affinity with it, or with substances that may be evolved from it during the action, does not in any case seem to be affected by the electric current. pulverised charcoal was diffused through dilute sulphuric acid, and subjected with the solution to the action of a voltaic battery, terminated by platina poles; but not the slightest tendency of the charcoal to the negative pole could be observed, sublimed sulphur was diffused through similar acid, and submitted to the same action, a silver plate being used as the negative pole; but the sulphur had no tendency to pass to that pole, the silver was not tarnished, nor did any sulphuretted hydrogen appear. the case of magnesia and water ( . .), with those of comminuted metals in certain solutions ( .), are also of this kind; and, in fact, substances which have the instant before been powerfully determined towards the pole, as magnesia from sulphate of magnesia, become entirely _indifferent to it_ the moment they assume their independent state, and pass away, diffusing themselves through the surrounding fluid. . there are, it is true, many instances of insoluble bodies being acted upon, as glass, sulphate of baryta, marble, slate, basalt, &c., but they form no exception; for the substances they give up are in direct and strong relation as to chemical affinity with those which they find in the surrounding solution, so that these decompositions enter into the class of ordinary effects. . it may be expressed as a general consequence, that the more directly bodies are opposed to each other in chemical affinity, the more _ready_ is their separation from each other in cases of electro-chemical decomposition, i.e. provided other circumstances, as insolubility, deficient conducting power, proportions, &c., do not interfere. this is well known to be the case with water and saline solutions; and i have found it to be equally true with _dry_ chlorides, iodides, salts, &c., rendered subject to electro-chemical decomposition by fusion ( .). so that in applying the voltaic battery for the purpose of decomposing bodies not yet resolved into forms of matter simpler than their own, it must be remembered, that success may depend not upon the weakness, or failure upon the strength, of the affinity by which the elements sought for are held together, but contrariwise; and then modes of application may be devised, by which, in _association_ with ordinary chemical powers, and the assistance of fusion ( . .), we may be able to penetrate much further than at present into the constitution of our chemical elements. . some of the most beautiful and surprising cases of electro-chemical decomposition and _transfer_ which sir humphry davy described in his celebrated paper[a], were those in which acids were passed through alkalies, and alkalies or earths through acids[b]; and the way in which substances having the most powerful attractions for each other were thus prevented from combining, or, as it is said, had their natural affinity destroyed or suspended throughout the whole of the circuit, excited the utmost astonishment. but if i be right in the view i have taken of the effects, it will appear, that that which made the _wonder_, is in fact the _essential condition_ of transfer and decomposition, and that the more alkali there is in the course of an acid, the more will the transfer of that acid be facilitated from pole to pole; and perhaps a better illustration of the difference between the theory i have ventured, and those previously existing, cannot be offered than the views they respectively give of such facts as these. [a] philosophical transactions, , p. . [b] ibid. p, , &c. . the instances in which sulphuric acid could not be passed though baryta, or baryta through sulphuric acid[a], because of the precipitation of sulphate of baryta, enter within the pale of the law already described ( . .), by which liquidity is so generally required for conduction and decomposition. in assuming the solid state of sulphate of baryta, these bodies became virtually non-conductors to electricity of so low a tension as that of the voltaic battery, and the power of the latter over them was almost infinitely diminished. [a] philosophical transactions, , p. , &c. . the theory i have advanced accords in a most satisfactory manner with the fact of an element or substance finding its place of rest, or rather of evolution, sometimes at one pole and sometimes at the other. sulphur illustrates this effect very well[a]. when sulphuric acid is decomposed by the pile, sulphur is evolved at the negative pole; but when sulphuret of silver is decomposed in a similar way ( .), then the sulphur appears at the positive pole; and if a hot platina pole be used so as to vaporize the sulphur evolved in the latter case, then the relation of that pole to the sulphur is exactly the same as the relation of the same pole to oxygen upon its immersion in water. in both cases the element evolved is liberated at the pole, but not retained by it; but by virtue of its elastic, uncombinable, and immiscible condition passes away into the surrounding medium. the sulphur is evidently determined in these opposite directions by its opposite chemical relations to oxygen and silver; and it is to such relations generally that i have referred all electro-chemical phenomena. where they do not exist, no electro-chemical action can take place. where they are strongest, it is most powerful; where they are reversed, the direction of transfer of the substance is reversed with them. [a] at and of series vii, will be found corrections of the statement here made respecting sulphur and sulphuric acid. at present there is no well-ascertained fact which proves that the same body can go directly to _either_ of the two poles at pleasure.--_dec. ._ . _water_ may be considered as one of those substances which can be made to pass to _either_ pole. when the poles are immersed in dilute sulphuric acid ( .), acid passes towards the positive pole, and water towards the negative pole; but when they are immersed in dilute alkali, the alkali passes towards the negative pole, and water towards the positive pole. . nitrogen is another substance which is considered as determinable to either pole; but in consequence of the numerous compounds which it forms, some of which pass to one pole, and some to the other, i have not always found it easy to determine the true circumstances of its appearance. a pure strong solution of ammonia is so bad a conductor of electricity that it is scarcely more decomposable than pure water; but if sulphate of ammonia be dissolved in it, then decomposition takes place very well; nitrogen almost pure, and in some cases quite, is evolved at the positive pole, and hydrogen at the negative pole. . on the other hand, if a strong solution of nitrate of ammonia be decomposed, oxygen appears at the positive pole, and hydrogen, with sometimes nitrogen, at the negative pole. if fused nitrate of ammonia be employed, hydrogen appears at the negative pole, mingled with a little nitrogen. strong nitric acid yields plenty of oxygen at the positive pole, but no gas (only nitrous acid) at the negative pole. weak nitric acid yields the oxygen and hydrogen of the water present, the acid apparently remaining unchanged. strong nitric acid with nitrate of ammonia dissolved in it, yields a gas at the negative pole, of which the greater part is hydrogen, but apparently a little nitrogen is present. i believe, that in some of these cases a little nitrogen appeared at the negative pole. i suspect, however, that in all these, and in all former cases, the appearance of the nitrogen at the positive or negative pole is entirely a secondary effect, and not an immediate consequence of the decomposing power of the electric current[a]. [a] refer for proof of the truth of this supposition to , , &c.--_dec. ._ . a few observations on what are called the _poles_ of the voltaic battery now seem necessary. the poles are merely the surfaces or doors by which the electricity enters into or passes out of the substance suffering decomposition. they limit the extent of that substance in the course of the electric current, being its _terminations_ in that direction: hence the elements evolved pass so far and no further. . metals make admirable poles, in consequence of their high conducting power, their immiscibility with the substances generally acted upon, their solid form, and the opportunity afforded of selecting such as are not chemically acted upon by ordinary substances. . water makes a pole of difficult application, except in a few cases ( .), because of its small conducting power, its miscibility with most of the substances acted upon, and its general relation to them in respect to chemical affinity. it consists of elements, which in their electrical and chemical relations are directly and powerfully opposed, yet combining to produce a body more neutral in its character than any other. so that there are but few substances which do not come into relation, by chemical affinity, with water or one of its elements; and therefore either the water or its elements are transferred and assist in transferring the infinite variety of bodies which, in association with it, can be placed in the course of the electric current. hence the reason why it so rarely happens that the evolved substances rest at the first surface of the water, and why it therefore does not exhibit the ordinary action of a pole. . air, however, and some gases are free from the latter objection, and may be used as poles in many cases ( , &c.); but, in consequence of the extremely low degree of conducting power belonging to them, they cannot be employed with the voltaic apparatus. this limits their use; for the voltaic apparatus is the only one as yet discovered which supplies sufficient quantity of electricity ( . .) to effect electro-chemical decomposition with facility. . when the poles are liable to the chemical action of the substances evolved, either simply in consequence of their natural relation to them, or of that relation aided by the influence of the current ( .), then they suffer corrosion, and the parts dissolved are subject to transference, in the same manner as the particles of the body originally under decomposition. an immense series of phenomena of this kind might be quoted in support of the view i have taken of the cause of electro-chemical decomposition, and the transfer and evolution of the elements. thus platina being made the positive and negative poles in a solution of sulphate of soda, has no affinity or attraction for the oxygen, hydrogen, acid, or alkali evolved, and refuses to combine with or retain them. zinc can combine with the oxygen and acid; at the positive pole it does combine, and immediately begins to travel as oxide towards the negative pole. charcoal, which cannot combine with the metals, if made the negative pole in a metallic solution, refuses to unite to the bodies which are ejected from the solution upon its surface; but if made the positive pole in a dilute solution of sulphuric acid, it is capable of combining with the oxygen evolved there, and consequently unites with it, producing both carbonic acid and carbonic oxide in abundance. . a great advantage is frequently supplied, by the opportunity afforded amongst the metals of selecting a substance for the pole, which shall or shall not be acted upon by the elements to be evolved. the consequent use of platina is notorious. in the decomposition of sulphuret of silver and other sulphurets, a positive silver pole is superior to a platina one, because in the former case the sulphur evolved there combines with the silver, and the decomposition of the original sulphuret is rendered evident; whereas in the latter case it is dissipated, and the assurance of its separation at the pole not easily obtained. . the effects which take place when a succession of conducting decomposable and undecomposable substances are placed in the electric circuit, as, for instance, of wires and solutions, or of air and solutions ( , .), are explained in the simplest possible manner by the theoretical view i have given. in consequence of the reaction of the constituents of each portion of decomposable matter, affected as they are by the supervention of the electric current ( .), portions of the proximate or ultimate elements proceed in the direction of the current as far as they find matter of a contrary kind capable of effecting their transfer, and being equally affected by them; and where they cease to find such matter, they are evolved in their free state, i.e. upon the surfaces of metal or air bounding the extent of decomposable matter in the direction of the current. . having thus given my theory of the mode in which electro-chemical decomposition is effected, i will refrain for the present from entering upon the numerous general considerations which it suggests, wishing first to submit it to the test of publication and discussion. _royal institution, june ._ sixth series. § . _on the power of metals and other solids to induce the combination of gaseous bodies._ received november , ,--read january , . . the conclusion at which i have arrived in the present communication may seem to render the whole of it unfit to form part of a series of researches in electricity; since, remarkable as the phenomena are, the power which produces them is not to be considered as of an electric origin, otherwise than as all attraction of particles may have this subtile agent for their common cause. but as the effects investigated arose out of electrical researches, as they are directly connected with other effects which are of an electric nature, and must of necessity be understood and guarded against in a very extensive series of electro-chemical decompositions ( .), i have felt myself fully justified in describing them in this place. . believing that i had proved (by experiments hereafter to be described ( .),) the constant and definite chemical action of a certain quantity of electricity, whatever its intensity might be, or however the circumstances of its transmission through either the body under decomposition or the more perfect conductors were varied, i endeavoured upon that result to construct a new measuring instrument, which from its use might be called, at least provisionally, a _volta-electrometer_ ( .)[a]. [a] or voltameter.--_dec. ._ . during the course of the experiments made to render the instrument efficient, i was occasionally surprised at observing a deficiency of the gases resulting from the decompositions of water, and at last an actual disappearance of portions which had been evolved, collected, and measured. the circumstances of the disappearance were these. a glass tube, about twelve inches in length and / ths of an inch in diameter, had two platina poles fixed into its upper, hermetically sealed, extremity: the poles, where they passed through the glass, were of wire; but terminated below in plates, which were soldered to the wires with gold (plate v. fig. .). the tube was filled with dilute sulphuric acid, and inverted in a cup of the same fluid; a voltaic battery was connected with the two wires, and sufficient oxygen and hydrogen evolved to occupy / ths of the tube, or by the graduation, parts. on separating the tube from the voltaic battery the volume of gas immediately began to diminish, and in about five hours only - / parts remained, and these ultimately disappeared. . it was found by various experiments, that this effect was not due to the escape or solution of the gas, nor to recombination of the oxygen or hydrogen in consequence of any peculiar condition _they_ might be supposed to possess under the circumstances; but to be occasioned by the action of one or both of the poles within the tube upon the gas around them. on disuniting the poles from the pile after they had acted upon dilute sulphuric acid, and introducing them into separate tubes containing mixed oxygen and hydrogen, it was found that the _positive_ pole effected the union of the gases, but the negative pole apparently not ( .). it was ascertained also that no action of a sensible kind took place between the positive pole with oxygen or hydrogen alone. . these experiments reduced the phenomena to the consequence of a power possessed by the platina, after it had been the positive pole of a voltaic pile, of causing the combination of oxygen and hydrogen at common, or even at low, temperatures. this effect is, as far as i am aware, altogether new, and was immediately followed out to ascertain whether it was really of an electric nature, and how far it would interfere with the determination of the quantities evolved in the cases of electro-chemical decomposition required in the fourteenth section of these researches. . several platina plates were prepared (fig. .). they were nearly half an inch wide, and two inches and a half long: some were / dth of an inch, others not more than / dth, whilst some were as much as / th of an inch in thickness. each had a piece of platina wire, about seven inches long, soldered to it by pure gold. then a number of glass tubes were prepared: they were about nine or ten inches in length, / ths of an inch in internal diameter, were sealed hermetically at one extremity, and were graduated. into these tubes was put a mixture of two volumes of hydrogen and one of oxygen, at the water pneumatic trough, and when one of the plates described had been connected with the positive or negative pole of the voltaic battery for a given time, or had been otherwise prepared, it was introduced through the water into the gas within the tube; the whole set aside in a test-glass (fig. .), and left for a longer or shorter period, that the action might be observed. . the following result may be given as an illustration of the phenomenon to be investigated. diluted sulphuric acid, of the specific gravity . , was put into a glass jar, in which was placed also a large platina plate, connected with the negative end of a voltaic battery of forty pairs of four-inch plates, with double coppers, and moderately charged. one of the plates above described ( .) was then connected with the positive extremity, and immersed in the same jar of acid for five minutes, after which it was separated from the battery, washed in distilled water, and introduced through the water of the pneumatic trough into a tube containing the mixture of oxygen and hydrogen ( .). the volume of gases immediately began to lessen, the diminution proceeding more and more rapidly until about / ths of the mixture had disappeared. the upper end of the tube became quite warm, the plate itself so hot that the water boiled as it rose over it; and in less than a minute a cubical inch and a half of the gases were gone, having been combined by the power of the platina, and converted into water. . this extraordinary influence acquired by the platina at the positive pole of the pile, is exerted far more readily and effectively on oxygen and hydrogen than on any other mixture of gases that i have tried. one volume of nitrous gas was mixed with a volume of hydrogen, and introduced into a tube with a plate which had been made positive in the dilute sulphuric acid for four minutes ( .). there was no sensible action in an hour: being left for thirty-six hours, there was a diminution of about one-eighth of the whole volume. action had taken place, but it had been very feeble. . a mixture of two volumes of nitrous oxide with one volume of hydrogen was put with a plate similarly prepared into a tube ( . .). this also showed no action immediately; but in thirty-six hours nearly a fourth of the whole had disappeared, i.e. about half of a cubic inch. by comparison with another tube containing the same mixture without a plate, it appeared that a part of the diminution was due to solution, and the other part to the power of the platina; but the action had been very slow and feeble. . a mixture of one volume olefiant gas and three volumes oxygen was not affected by such a platina plate, even though left together for several days ( . .). . a mixture of two volumes carbonic oxide and one volume oxygen was also unaffected by the prepared platina plate in several days ( , &c.). . a mixture of equal volumes of chlorine and hydrogen was used in several experiments, with plates prepared in a similar manner ( .). diminution of bulk soon took place; but when after thirty-six hours the experiments were examined, it was found that nearly all the chlorine had disappeared, having been absorbed, principally by the water, and that the original volume of hydrogen remained unchanged. no combination of the gases, therefore, had here taken place. . reverting to the action of the prepared plates on mixtures of oxygen and hydrogen ( .), i found that the power, though gradually diminishing in all cases, could still be retained for a period, varying in its length with circumstances. when tubes containing plates ( .) were supplied with fresh portions of mixed oxygen and hydrogen as the previous portions were condensed, the action was found to continue for above thirty hours, and in some cases slow combination could be observed even after eighty hours; but the continuance of the action greatly depended upon the purity of the gases used ( .). . some plates ( .) were made positive for four minutes in dilute sulphuric acid of specific gravity . : they were rinsed in distilled water, after which two were put into a small bottle and closed up, whilst others were left exposed to the air. the plates preserved in the limited portion of air were found to retain their power after eight days, but those exposed to the atmosphere had lost their force almost entirely in twelve hours, and in some situations, where currents existed, in a much shorter time. . plates were made positive for five minutes in sulphuric acid, specific gravity . . one of these was retained in similar acid for eight minutes after separation from the battery: it then acted on mixed oxygen and hydrogen with apparently undiminished vigour. others were left in similar acid for forty hours, and some even for eight days, after the electrization, and then acted as well in combining oxygen and hydrogen gas as those which were used immediately after electrization. . the effect of a solution of caustic potassa in preserving the platina plates was tried in a similar manner. after being retained in such a solution for forty hours, they acted exceedingly well on oxygen and hydrogen, and one caused such rapid condensation of the gases, that the plate became much heated, and i expected the temperature would have risen to ignition. . when similarly prepared plates ( .) had been put into distilled water for forty hours, and then introduced into mixed oxygen and hydrogen, they were found to act but very slowly and feebly as compared with those which had been preserved in acid or alkali. when, however, the quantity of water was but small, the power was very little impaired after three or four days. as the water had been retained in a wooden vessel, portions of it were redistilled in glass, and this was found to preserve prepared plates for a great length of time. prepared plates were put into tubes with this water and closed up; some of them, taken out at the end of twenty-four days, were found very active on mixed oxygen and hydrogen; others, which were left in the water for fifty-three days, were still found to cause the combination of the gases. the tubes had been closed only by corks. . the act of combination always seemed to diminish, or apparently exhaust, the power of the platina plate. it is true, that in most, if not all instances, the combination of the gases, at first insensible, gradually increased in rapidity, and sometimes reached to explosion; but when the latter did not happen, the rapidity of combination diminished; and although fresh portions of gas were introduced into the tubes, the combination went on more and more slowly, and at last ceased altogether. the first effect of an increase in the rapidity of combination depended in part upon the water flowing off from the platina plate, and allowing a better contact with the gas, and in part upon the heat evolved during the progress of the combination ( .). but notwithstanding the effect of these causes, diminution, and at last cessation of the power, always occurred. it must not, however, be unnoticed, that the purer the gases subjected to the action of the plate, the longer was its combining power retained. with the mixture evolved at the poles of the voltaic pile, in pure dilute sulphuric acid, it continued longest; and with oxygen and hydrogen, of perfect purity, it probably would not be diminished at all. . different modes of treatment applied to the platina plate, after it had ceased to be the positive pole of the pile, affected its power very curiously. a plate which had been a positive pole in diluted sulphuric acid of specific gravity . for four or five minutes, if rinsed in water and put into mixed oxygen and hydrogen, would act very well, and condense perhaps one cubic inch and a half of gas in six or seven minutes; but if that same plate, instead of being merely rinsed, had been left in distilled water for twelve or fifteen minutes, or more, it would rarely fail, when put into the oxygen and hydrogen, of becoming, in the course of a minute or two, ignited, and would generally explode the gases. occasionally the time occupied in bringing on the action extended to eight or nine minutes, and sometimes even to forty minutes, and yet ignition and explosion would result. this effect is due to the removal of a portion of acid which otherwise adheres firmly to the plate [a]. [a] in proof that this is the case, refer to .--_dec. ._ . occasionally the platina plates ( .), after being made the positive pole of the battery, were washed, wiped with filtering-paper or a cloth, and washed and wiped again. being then introduced into mixed oxygen and hydrogen, they acted apparently as if they had been unaffected by the treatment. sometimes the tubes containing the gas were opened in the air for an instant, and the plates put in dry; but no sensible difference in action was perceived, except that it commenced sooner. . the power of heat in altering the action of the prepared platina plates was also tried ( .). plates which had been rendered positive in dilute sulphuric acid for four minutes were well-washed in water, and heated to redness in the flame of a spirit-lamp: after this they acted very well on mixed oxygen and hydrogen. others, which had been heated more powerfully by the blowpipe, acted afterwards on the gases, though not so powerfully as the former. hence it appears that heat does not take away the power acquired by the platina at the positive pole of the pile: the occasional diminution of force seemed always referable to other causes than the mere heat. if, for instance, the plate had not been well-washed from the acid, or if the flame used was carbonaceous, or was that of an alcohol lamp trimmed with spirit containing a little acid, or having a wick on which salt, or other extraneous matter, had been placed, then the power of the plate was quickly and greatly diminished ( . .). . this remarkable property was conferred upon platina when it was made the positive pole in sulphuric acid of specific gravity . , or when it was considerably weaker, or when stronger, even up to the strength of oil of vitriol. strong and dilute nitric acid, dilute acetic acid, solutions of tartaric, citric, and oxalic acids, were used with equal success. when muriatic acid was used, the plates acquired the power of condensing the oxygen and hydrogen, but in a much inferior degree. . plates which were made positive in solution of caustic potassa did not show any sensible action upon the mixed oxygen and hydrogen. other plates made positive in solutions of carbonates of potassa and soda exhibited the action, but only in a feeble degree. . when a neutral solution of sulphate of soda, or of nitre, or of chlorate of potassa, or of phosphate of potassa, or acetate of potassa, or sulphate of copper, was used, the plates, rendered positive in them for four minutes, and then washed in water, acted very readily and powerfully on the mixed oxygen and hydrogen. . it became a very important point, in reference to the _cause_ of this action of the platina, to determine whether the _positive_ pole _only_ could confer it ( .), or whether, notwithstanding the numerous contrary cases, the _negative_ pole might not have the power when such circumstances as could interfere with or prevent the action were avoided. three plates were therefore rendered negative, for four minutes in diluted sulphuric acid of specific gravity . , washed in distilled water, and put into mixed oxygen and hydrogen. _all_ of them _acted_, though not so strongly as they would have done if they had been rendered positive. each combined about a cubical inch and a quarter of the gases in twenty-five minutes. on every repetition of the experiment the same result was obtained; and when the plates were retained in distilled water for ten or twelve minutes, before being introduced into the gas ( .), the action was very much quickened. . but when there was any metallic or other substance present in the acid, which could be precipitated on the negative plate, then that plate ceased to act upon the mixed oxygen and hydrogen. . these experiments led to the expectation that the power of causing oxygen and hydrogen to combine, which could be conferred upon any piece of platina by making it the positive pole of a voltaic pile, was not essentially dependent upon the action of the pile, or upon any structure or arrangement of parts it might receive whilst in association with it, but belonged to the platina _at all times_, and was _always effective_ when the surface was _perfectly clean_. and though, when made the _positive_ pole of the pile in acids, the circumstances might well be considered as those which would cleanse the surface of the platina in the most effectual manner, it did not seem impossible that ordinary operations should produce the same result, although in a less eminent degree. . accordingly, a platina plate ( .) was cleaned by being rubbed with a cork, a little water, and some coal-fire ashes upon a glass plate: being washed, it was put into mixed oxygen and hydrogen, and was found to act at first slowly, and then more rapidly. in an hour, a cubical inch and a half had disappeared. . other plates were cleaned with ordinary sand-paper and water; others with chalk and water; others with emery and water; others, again, with black oxide of manganese and water; and others with a piece of charcoal and water. all of these acted in tubes of oxygen and hydrogen, causing combination of the gases. the action was by no means so powerful as that produced by plates having been in communication with the battery; but from one to two cubical inches of the gases disappeared, in periods extending from twenty-five to eighty or ninety minutes. . upon cleaning the plates with a cork, ground emery, and dilute sulphuric acid, they were found to act still better. in order to simplify the conditions, the cork was dismissed, and a piece of platina foil used instead; still the effect took place. then the acid was dismissed, and a solution of _potassa_ used, but the effect occurred as before. . these results are abundantly sufficient to show that the mere mechanical cleansing of the surface of the platina is sufficient to enable it to exert its combining power over oxygen and hydrogen at common temperatures. . i now tried the effect of heat in conferring this property upon platina ( .). plates which had no action on the mixture of oxygen and hydrogen were heated by the flame of a freshly trimmed spirit-lamp, urged by a mouth blowpipe, and when cold were put into tubes of the mixed gases: they acted slowly at first, but after two or three hours condensed nearly all the gases. . a plate of platina, which was about one inch wide and two and three-quarters in length, and which had not been used in any of the preceding experiments, was curved a little so as to enter a tube, and left in a mixture of oxygen and hydrogen for thirteen hours: not the slightest action or combination of the gases occurred. it was withdrawn at the pneumatic trough from the gas through the water, heated red-hot by the spirit-lamp and blowpipe, and then returned when cold into the _same_ portion of gas. in the course of a few minutes diminution of the gases could be observed, and in forty-five minutes about one cubical inch and a quarter had disappeared. in many other experiments platina plates when heated were found to acquire the power of combining oxygen and hydrogen. . but it happened not infrequently that plates, after being heated, showed no power of combining oxygen and hydrogen gases, though left undisturbed in them for two hours. sometimes also it would happen that a plate which, having been heated to dull redness, acted feebly, upon being heated to whiteness ceased to act; and at other times a plate which, having been slightly heated, did not act, was rendered active by a more powerful ignition. . though thus uncertain in its action, and though often diminishing the power given to the plates at the positive pole of the pile ( .), still it is evident that heat can render platina active, which before was inert ( .). the cause of its occasional failure appears to be due to the surface of the metal becoming soiled, either from something previously adhering to it, which is made to adhere more closely by the action of the heat, or from matter communicated from the flame of the lamp, or from the air itself. it often happens that a polished plate of platina, when heated by the spirit-lamp and a blowpipe, becomes dulled and clouded on its surface by something either formed or deposited there; and this, and much less than this, is sufficient to prevent it from exhibiting the curious power now under consideration ( . .). platina also has been said to combine with carbon; and it is not at all unlikely that in processes of heating, where carbon or its compounds are present, a film of such a compound may be thus formed, and thus prevent the exhibition of the properties belonging to _pure_ platina[a]. [a] when heat does confer the property it is only by the destruction or dissipation of organic or other matter which had previously soiled the plate ( . . .).--_dec. ._ . the action of alkalies and acids in giving platina this property was now experimentally examined. platina plates ( .) having no action on mixed oxygen and hydrogen, being boiled in a solution of caustic potassa, washed, and then put into the gases, were found occasionally to act pretty well, but at other times to fail. in the latter case i concluded that the impurity upon the surface of the platina was of a nature not to be removed by the mere solvent action of the alkali, for when the plates were rubbed with a little emery, and the same solution of alkali ( .), they became active. . the action of acids was far more constant and satisfactory. a platina plate was boiled in dilute nitric acid: being washed and put into mixed oxygen and hydrogen gases, it acted well. other plates were boiled in strong nitric acid for periods extending from half a minute to four minutes, and then being washed in distilled water, were found to act very well, condensing one cubic inch and a half of gas in the space of eight or nine minutes, and rendering the tube warm ( .). . strong sulphuric acid was very effectual in rendering the platina active. a plate ( .) was heated in it for a minute, then washed and put into the mixed oxygen and hydrogen, upon which it acted as well as if it had been made the positive pole of a voltaic pile ( .). . plates which, after being heated or electrized in alkali, or after other treatment, were found inert, immediately received power by being dipped for a minute or two, or even only for an instant, into hot oil of vitriol, and then into water. . when the plate was dipped into the oil of vitriol, taken out, and then heated so as to drive off the acid, it did not act, in consequence of the impurity left by the acid upon its surface. . vegetable acids, as acetic and tartaric, sometimes rendered inert platina active, at other times not. this, i believe, depended upon the character of the matter previously soiling the plates, and which may easily be supposed to be sometimes of such a nature as to be removed by these acids, and at other times not. weak sulphuric acid showed the same difference, but strong sulphuric acid ( .) never failed in its action. . the most favourable treatment, except that of making the plate a positive pole in strong acid, was as follows. the plate was held over a spirit-lamp flame, and when hot, rubbed with a piece of potassa fusa (caustic potash), which melting, covered the metal with a coat of very strong alkali, and this was retained fused upon the surface for a second or two[a]: it was then put into water for four or five minutes to wash off the alkali, shaken, and immersed for about a minute in hot strong oil of vitriol; from this it was removed into distilled water, where it was allowed to remain ten or fifteen minutes to remove the last traces of acid ( .). being then put into a mixture of oxygen and hydrogen, combination immediately began, and proceeded rapidly; the tube became warm, the platina became red-hot, and the residue of the gases was inflamed. this effect could be repeated at pleasure, and thus the maximum phenomenon could be produced without the aid of the voltaic battery. [a] the heat need not be raised so much as to make the alkali tarnish the platina, although if that effect does take place it does not prevent the ultimate action. . when a solution of tartaric or acetic acid was substituted, in this mode of preparation, for the sulphuric acid, still the plate was found to acquire the same power, and would often produce explosion in the mixed gases; but the strong sulphuric acid was most certain and powerful. . if borax, or a mixture of the carbonates of potash and soda, be fused on the surface of a platina plate, and that plate be well-washed in water, it will be found to have acquired the power of combining oxygen and hydrogen, but only in a moderate degree; but if, after the fusion and washing, it be dipped in the hot sulphuric acid ( .), it will become very active. . other metals than platina were then experimented with. gold and palladium exhibited the power either when made the positive pole of the voltaic battery ( .), or when acted on by hot oil of vitriol ( .). when palladium is used, the action of the battery or acid should be moderated, as that metal is soon acted upon under such circumstances. silver and copper could not be made to show any effect at common temperatures. * * * * * . there can remain no doubt that the property of inducing combination, which can thus be conferred upon masses of platina and other metals by connecting them with the poles of the battery, or by cleansing processes either of a mechanical or chemical nature, is the same as that which was discovered by döbereiner[a], in , to belong in so eminent a degree to spongy platina, and which was afterwards so well experimented upon and illustrated by mm. dulong and thenard[b], in . the latter philosophers even quote experiments in which a very fine platina wire, which had been coiled up and digested in nitric, sulphuric, or muriatic acid, became ignited when put into a jet of hydrogen gas[c]. this effect i can now produce at pleasure with either wires or plates by the processes described ( . . .); and by using a smaller plate cut so that it shall rest against the glass by a few points, and yet allow the water to flow off (fig. .), the loss of heat is less, the metal is assimilated somewhat to the spongy state, and the probability of failure almost entirely removed. [a] annales de chimie, tom. xxiv. p. . [b] ibid. tom. xxiii. p. ; tom. xxiv. p. . [c] ibid. tom. xxiv. p. . . m. döbereiner refers the effect entirely to an electric action. he considers the platina and hydrogen as forming a voltaic element of the ordinary kind, in which the hydrogen, being very highly positive, represents the zinc of the usual arrangement, and like it, therefore, attracts oxygen and combines with it[a]. [a] tom. xxiv. pp. , . also bibliothèque universelle, tom. xxiv. p. . . in the two excellent experimental papers by mm. dulong and thenard[a], those philosophers show that elevation of temperature favours the action, but does not alter its character; sir humphry davy's incandescent platina wire being the same phenomenon with döbereiner's spongy platina. they show that _all_ metals have this power in a greater or smaller degree, and that it is even possessed by such bodies as charcoal, pumice, porcelain, glass, rock crystal, &c., when their temperatures are raised; and that another of davy's effects, in which oxygen and hydrogen had combined slowly together at a heat below ignition, was really dependent upon the property of the heated glass, which it has in common with the bodies named above. they state that liquids do not show this effect, at least that mercury, at or below the boiling point, has not the power; that it is not due to porosity; that the same body varies very much in its action, according to its state; and that many other gaseous mixtures besides oxygen and hydrogen are affected, and made to act chemically, when the temperature is raised. they think it probable that spongy platina acquires its power from contact with the acid evolved during its reduction, or from the heat itself to which it is then submitted. [a] annales de chimie, tom. xxiii. p. ; tom. xxiv. p, . . mm. dulong and thenard express themselves with great caution on the theory of this action; but, referring to the decomposing power of metals on ammonia when heated to temperatures not sufficient alone to affect the alkali, they remark that those metals which in this case are most efficacious, are the least so in causing the combination of oxygen and hydrogen; whilst platina, gold, &c., which have least power of decomposing ammonia, have most power of combining the elements of water:--from which they are led to believe, that amongst gases, some tend to _unite_ under the influence of metals, whilst others tend to _separate_, and that this property varies in opposite directions with the different metals. at the close of their second paper they observe, that the action is of a kind that cannot be connected with any known theory; and though it is very remarkable that the effects are transient, like those of most electrical actions, yet they state that the greater number of the results observed by them are inexplicable, by supposing them to be of a purely electric origin. . dr. fusinieri has also written on this subject, and given a theory which he considers as sufficient to account for the phenomena[a]. he expresses the immediate cause thus: "the platina determines upon its surface a continual renovation of _concrete laminæ_ of the combustible substance of the gases or vapours, which flowing over it are burnt, pass away, and are renewed: this combustion at the surface raises and sustains the temperature of the metal." the combustible substance, thus reduced into imperceptible laminæ, of which the concrete parts are in contact with the oxygen, is presumed to be in a state combinable with the oxygen at a much lower temperature than when it is in the gaseous state, and more in analogy with what is called the nascent condition. that combustible gases should lose their elastic state, and become concrete, assuming the form of exceedingly attenuated but solid strata, is considered as proved by facts, some of which are quoted in the giornale di fisica for [b]; and though the theory requires that they should assume this state at high temperatures, and though the _similar_ films of aqueous and other matter are dissipated by the action of heat, still the facts are considered as justifying the conclusion against all opposition of reasoning. [a] giornale di fisica, &c., , tom. viii. p. . [b] pp. , . . the power or force which makes combustible gas or vapour abandon its elastic state in contact with a solid, that it may cover the latter with a thin stratum of its own proper substance, is considered as being neither attraction nor affinity. it is able also to extend liquids and solids in concrete laminæ over the surface of the acting solid body, and consists in a _repulsion_, which is developed from the parts of the solid body by the simple fact of attenuation, and is highest when the attenuation is most complete. the force has a progressive development, and acts most powerfully, or at first, in the direction in which the dimensions of the attenuated mass decrease, and then in the direction of the angles or corners which from any cause may exist on the surface. this force not only causes spontaneous diffusion of gases and other substances over the surface, but is considered as very elementary in its nature, and competent to account for all the phenomena of capillarity, chemical affinity, attraction of aggregation, rarefaction, ebullition, volatilization, explosion, and other thermometric effects, as well as inflammation, detonation, &c. &c. it is considered as a form of heat to which the term _native calorie_ is given, and is still further viewed as the principle of the two electricities and the two magnetisms. . i have been the more anxious to give a correct abstract of dr. fusinieri's view, both because i cannot form a distinct idea of the power to which he refers the phenomena, and because of my imperfect knowledge of the language in which the memoir is written. i would therefore beg to refer those who pursue the subject to the memoir itself. . not feeling, however, that the problem has yet been solved, i venture to give the view which seems to me sufficient, upon _known principles_, to account for the effect. . it may be observed of this action, that, with regard to platina, it cannot be due to any peculiar, temporary condition, either of an electric or of any other nature: the activity of plates rendered either positive or negative by the pole, or cleaned with such different substances as acids, alkalies, or water; charcoal, emery, ashes, or glass; or merely heated, is sufficient to negative such an opinion. neither does it depend upon the spongy and porous, or upon the compact and burnished, or upon the massive or the attenuated state of the metal, for in any of these states it may be rendered effective, or its action may be taken away. the only essential condition appears to be a _perfectly clean_ and _metallic surface_, for whenever that is present the platina acts, whatever its form and condition in other respects may be; and though variations in the latter points will very much affect the rapidity, and therefore the visible appearances and secondary effects, of the action, i.e. the ignition of the metal and the inflammation of the gases, they, even in their most favourable state, cannot produce any effect unless the condition of a clean, pure, metallic surface be also fulfilled. . the effect is evidently produced by most, if not all, solid bodies, weakly perhaps by many of them, but rising to a high degree in platina. dulong and thenard have very philosophically extended our knowledge of the property to its possession by all the metals, and by earths, glass, stones, &c. ( .); and every idea of its being a known and recognised electric action is in this way removed. . all the phenomena connected with this subject press upon my mind the conviction that the effects in question are entirely incidental and of a secondary nature; that they are dependent upon the _natural conditions_ of gaseous elasticity, combined with the exertion of that attractive force possessed by many bodies, especially those which are solid, in an eminent degree, and probably belonging to all; by which they are drawn into association more or less close, without at the same time undergoing chemical combination, though often assuming the condition of adhesion; and which occasionally leads, under very favourable circumstances, as in the present instance, to the combination of bodies simultaneously subjected to this attraction. i am prepared myself to admit (and probably many others are of the same opinion), both with respect to the attraction of aggregation and of chemical affinity, that the sphere of action of particles extends beyond those other particles with which they are immediately and evidently in union ( .), and in many cases produces effects rising into considerable importance: and i think that this kind of attraction is a determining cause of döbereiner's effect, and of the many others of a similar nature. . bodies which become wetted by fluids with which they do not combine chemically, or in which they do not dissolve, are simple and well-known instances of this kind of attraction. . all those cases of bodies which being insoluble in water and not combining with it are hygrometric, and condense its vapour around or upon their surface, are stronger instances of the same power, and approach a little nearer to the cases under investigation. if pulverized clay, protoxide or peroxide of iron, oxide of manganese, charcoal, or even metals, as spongy platina or precipitated silver, be put into an atmosphere containing vapour of water, they soon become moist by virtue of an attraction which is able to condense the vapour upon, although not to combine it with, the substances; and if, as is well known, these bodies so damped be put into a dry atmosphere, as, for instance, one confined over sulphuric acid, or if they be heated, then they yield up this water again almost entirely, it not being in direct or permanent combination[a]. [a] i met at edinburgh with a case, remarkable as to its extent, of hygrometric action, assisted a little perhaps by very slight solvent power. some turf had been well-dried by long exposure in a covered place to the atmosphere, but being then submitted to the action of a hydrostatic press, it yielded, _by the mere influence of the pressure_, per cent. of water. . still better instances of the power i refer to, because they are more analogous to the cases to be explained, are furnished by the attraction existing between glass and air, so well known to barometer and thermometer makers, for here the adhesion or attraction is exerted between a solid and gases, bodies having very different physical conditions, having no power of combination with each other, and each retaining, during the time of action, its physical state unchanged[a]. when mercury is poured into a barometer tube, a film of air will remain between the metal and glass for months, or, as far as is known, for years, for it has never been displaced except by the action of means especially fitted for the purpose. these consist in boiling the mercury, or in other words, of forming an abundance of vapour, which coming in contact with every part of the glass and every portion of surface of the mercury, gradually mingles with, dilutes, and carries off the air attracted by, and adhering to, those surfaces, replacing it by other vapour, subject to an equal or perhaps greater attraction, but which when cooled condenses into the same liquid as that with which the tube is filled. [a] fusinieri and bellani consider the air as forming solid concrete films in these cases.--giornale di fisica, tom. viii, p. . . . extraneous bodies, which, acting as nuclei in crystallizing or depositing solutions, cause deposition of substances on them, when it does not occur elsewhere in the liquid, seem to produce their effects by a power of the same kind, i.e. a power of attraction extending to neighbouring particles, and causing them to become attached to the nuclei, although it is not strong enough to make them combine chemically with their substance. . it would appear from many cases of nuclei in solutions, and from the effects of bodies put into atmospheres containing the vapours of water, or camphor, or iodine, &c., as if this attraction were in part elective, partaking in its characters both of the attraction of aggregation and chemical affinity: nor is this inconsistent with, but agreeable to, the idea entertained, that it is the power of particles acting, not upon others with which they can immediately and intimately combine, but upon such as are either more distantly situated with respect to them, or which, from previous condition, physical constitution, or feeble relation, are unable to enter into decided union with them. . then, of all bodies, the gases are those which might be expected to show some _mutual_ action whilst _jointly_ under the attractive influence of the platina or other solid acting substance. liquids, such as water, alcohol, &c., are in so dense and comparatively incompressible a state, as to favour no expectation that their particles should approach much closer to each other by the attraction of the body to which they adhere, and yet that attraction must (according to its effects) place their particles as near to those of the solid wetted body as they are to each other, and in many cases it is evident that the former attraction is the stronger. but gases and vapours are bodies competent to suffer very great changes in the relative distances of their particles by external agencies; and where they are in immediate contact with the platina, the approximation of the particles to those of the metal may be very great. in the case of the hygrometric bodies referred to ( .), it is sufficient to reduce the vapour to the fluid state, frequently from atmospheres so rare that without this influence it would be needful to compress them by mechanical force into a bulk not more than / th or even / th of their original volume before the vapours would become liquids. . another most important consideration in relation to this action of bodies, and which, as far as i am aware, has not hitherto been noticed, is the condition of elasticity under which the gases are placed against the acting surface. we have but very imperfect notions of the real and intimate conditions of the particles of a body existing in the solid, the liquid, and the gaseous state; but when we speak of the gaseous state as being due to the mutual repulsions of the particles or of their atmospheres, although we may err in imagining each particle to be a little nucleus to an atmosphere of heat, or electricity, or any other agent, we are still not likely to be in error in considering the elasticity as dependent on _mutuality_ of action. now this mutual relation fails altogether on the side of the gaseous particles next to the platina, and we might be led to expect _à priori_ a deficiency of elastic force there to at least one half; for if, as dalton has shown, the elastic force of the particles of one gas cannot act against the elastic force of the particles of another, the two being as vacua to each other, so is it far less likely that the particles of the platina can exert any influence on those of the gas against it, such as would be exerted by gaseous particles of its own kind. . but the diminution of power to one-half on the side of the gaseous body towards the metal is only a slight result of what seems to me to flow as a necessary consequence of the known constitution of gases. an atmosphere of one gas or vapour, however dense or compressed, is in effect as a vacuum to another: thus, if a little water were put into a vessel containing a dry gas, as air, of the pressure of one hundred atmospheres, as much vapour of the water would _rise_ as if it were in a perfect vacuum. here the particles of watery vapour appear to have no difficulty in approaching within any distance of the particles of air, being influenced solely by relation to particles of their own kind; and if it be so with respect to a body having the same elastic powers as itself, how much more surely must it be so with particles, like those of the platina, or other limiting body, which at the same time that they have not these elastic powers, are also unlike it in nature! hence it would seem to result that the particles of hydrogen or any other gas or vapour which are next to the platina, &c., must be in such contact with it as if they were in the liquid state, and therefore almost infinitely closer to it than they are to each other, even though the metal be supposed to exert no attractive influence over them. . a third and very important consideration in favour of the mutual action of gases under these circumstances is their perfect miscibility. if fluid bodies capable of combining together are also capable of mixture, _they do combine_ when they are mingled, not waiting for any other determining circumstance; but if two such gases as oxygen and hydrogen are put together, though they are elements having such powerful affinity as to unite naturally under a thousand different circumstances, they do not combine by mere mixture. still it is evident that, from their perfect association, the particles are in the most favourable state possible for combination upon the supervention of any determining cause, such either as the negative action of the platina in suppressing or annihilating, as it were, their elasticity on its side; or the positive action of the metal in condensing them against its surface by an attractive force; or the influence of both together. . although there are not many distinct cases of combination under the influence of forces external to the combining particles, yet there are sufficient to remove any difficulty which might arise on that ground. sir james hull found carbonic acid and lime to remain combined under pressure at temperatures at which they would not have remained combined if the pressure had been removed; and i have had occasion to observe a case of direct combination in chlorine[a], which being compressed at common temperatures will combine with water, and form a definite crystalline hydrate, incapable either of being formed or of existing if that pressure be removed. [a] philosophical transactions, , p. . . the course of events when platina acts upon, and combines oxygen and hydrogen, may be stated, according to these principles, as follows. from the influence of the circumstances mentioned ( . &c.), i.e. the deficiency of elastic power and the attraction of the metal for the gases, the latter, when they are in association with the former, are so far condensed as to be brought within the action of their mutual affinities at the existing temperature; the deficiency of elastic power, not merely subjecting them more closely to the attractive influence of the metal, but also bringing them into a more favourable state for union, by abstracting a part of that power (upon which depends their elasticity,) which elsewhere in the mass of gases is opposing their combination. the consequence of their combination is the production of the vapour of water and an elevation of temperature. but as the attraction of the platina for the water formed is not greater than for the gases, if so great, (for the metal is scarcely hygrometric,) the vapour is quickly diffused through the remaining gases; fresh portions of this latter, therefore, come into juxtaposition with the metal, combine, and the fresh vapour formed is also diffused, allowing new portions of gas to be acted upon. in this way the process advances, but is accelerated by the evolution of heat, which is known by experiment to facilitate the combination in proportion to its intensity, and the temperature is thus gradually exalted until ignition results. . the dissipation of the vapour produced at the surface of the platina, and the contact of fresh oxygen and hydrogen with the metal, form no difficulty in this explication. the platina is not considered as causing the combination of any particles with itself, but only associating them closely around it; and the compressed particles are as free to move from the platina, being replaced by other particles, as a portion of dense air upon the surface of the globe, or at the bottom of a deep mine, is free to move by the slightest impulse, into the upper and rarer parts of the atmosphere. . it can hardly be necessary to give any reasons why platina does not show this effect under ordinary circumstances. it is then not sufficiently clean ( .), and the gases are prevented from touching it, and suffering that degree of effect which is needful to commence their combination at common temperatures, and which they can only experience at its surface. in fact, the very power which causes the combination of oxygen and hydrogen, is competent, under the usual casual exposure of platina, to condense extraneous matters upon its surface, which soiling it, take away for the time its power of combining oxygen and hydrogen, by preventing their contact with it ( .). . clean platina, by which i mean such as has been made the positive pole of a pile ( .), or has been treated with acid ( .), and has then been put into distilled water for twelve or fifteen minutes, has a _peculiar friction_ when one piece is rubbed against another. it wets freely with pure water, even after it has been shaken and dried by the heat of a spirit-lamp; and if made the pole of a voltaic pile in a dilute acid, it evolves minute bubbles from every part of its surface. but platina in its common state wants that peculiar friction: it will not wet freely with water as the clean platina does; and when made the positive pole of a pile, it for a time gives off large bubbles, which seem to cling or adhere to the metal, and are evolved at distinct and separate points of the surface. these appearances and effects, as well as its want of power on oxygen and hydrogen, are the consequences, and the indications, of a soiled surface. . i found also that platina plates which had been cleaned perfectly soon became soiled by mere exposure to the air; for after twenty-four hours they no longer moistened freely with water, but the fluid ran up into portions, leaving part of the surface bare, whilst other plates which had been retained in water for the same time, when they were dried ( .) did moisten, and gave the other indications of a clean surface. . nor was this the case with platina or metals only, but also with earthy bodies, rock crystal and obsidian would not wet freely upon the surface, but being moistened with strong oil of vitriol, then washed, and left in distilled water to remove all the acid, they did freely become moistened, whether they were previously dry or whether they were left wet; but being dried and left exposed to the air for twenty-four hours, their surface became so soiled that water would not then adhere freely to it, but ran up into partial portions. wiping with a cloth (even the cleanest) was still worse than exposure to air; the surface either of the minerals or metals immediately became as if it were slightly greasy. the floating upon water of small particles of metals under ordinary circumstances is a consequence of this kind of soiled surface. the extreme difficulty of cleaning the surface of mercury when it has once been soiled or greased, is due to the same cause. . the same reasons explain why the power of the platina plates in some circumstances soon disappear, and especially upon use: mm. dulong and thenard have observed the same effect with the spongy metal[a], as indeed have all those who have used döbereiner's instantaneous light machines. if left in the air, if put into ordinary distilled water, if made to act upon ordinary oxygen and hydrogen, they can still find in all these cases _that_ minute portion of impurity which, when once in contact with the surface of the platina, is retained there, and is sufficient to prevent its full action upon oxygen and hydrogen at common temperatures: a slight elevation of temperature is again sufficient to compensate this effect, and cause combination. [a] annales de chimie, tom. xxiv. p. . . no state of a solid body can be conceived more favourable for the production of the effect than that which is possessed by platina obtained from the ammonio-muriate by heat. its surface is most extensive and pure, yet very accessible to the gases brought in contact with it: if placed in impurity, the interior, as thenard and dulong have observed, is preserved clean by the exterior; and as regards temperature, it is so bad a conductor of heat, because of its divided condition, that almost all which is evolved by the combination of the first portions of gas is retained within the mass, exalting the tendency of the succeeding portions to combine. * * * * * . i have now to notice some very extraordinary interferences with this phenomenon, dependent, not upon the nature or condition of the metal or other acting solid, but upon the presence of certain substances mingled with the gases acted upon; and as i shall have occasion to speak frequently of a mixture of oxygen and hydrogen, i wish it always to be understood that i mean a mixture composed of one volume of oxygen to two volumes of hydrogen, being the proportions that form water. unless otherwise expressed, the hydrogen was always that obtained by the action of dilute sulphuric acid on pure zinc, and the oxygen that obtained by the action of heat from the chlorate of potassa. . mixtures of oxygen and hydrogen with _air_, containing one-fourth, one-half, and even two-thirds of the latter, being introduced with prepared platina plates ( . .) into tubes, were acted upon almost as well as if no air were present: the retardation was far less than might have been expected from the mere dilution and consequent obstruction to the contact of the gases with the plates. in two hours and a half nearly all the oxygen and hydrogen introduced as mixture was gone. . but when similar experiments were made with _olefiant gas_ (the platina plates having been made the positive poles of a voltaic pile ( .) in acid), very different results occurred. a mixture was made of . volumes hydrogen and . volumes oxygen, being the proportions for water; and to this was added another mixture of volumes oxygen and one volume olefiant gas, so that the olefiant gas formed but / th part of the whole; yet in this mixture the platina plate would not act in forty-five hours. the failure was not for want of any power in the plate, for when after that time it was taken out of this mixture and put into one of oxygen and hydrogen, it immediately acted, and in seven minutes caused explosion of the gas. this result was obtained several times, and when larger proportions of olefiant gas were used, the action seemed still more hopeless. . a mixture of forty-nine volumes oxygen and hydrogen ( .) with one volume of olefiant gas had a well-prepared platina plate introduced. the diminution of gas was scarcely sensible at the end of two hours, during which it was watched; but on examination twenty-four hours afterwards, the tube was found blown to pieces. the action, therefore, though it had been very much retarded, had occurred at last, and risen to a maximum. . with a mixture of ninety-nine volumes of oxygen and hydrogen ( .) with one of olefiant gas, a feeble action was evident at the end of fifty minutes; it went on accelerating ( .) until the eighty-fifth minute, and then became so intense that the gas exploded. here also the retarding effect of the olefiant gas was very beautifully illustrated. . plates prepared by alkali and acid ( .) produced effects corresponding to those just described. . it is perfectly clear from these experiments, that _olefiant gas_, even in small quantities, has a very remarkable influence in preventing the combination of oxygen and hydrogen under these circumstances, and yet without at all injuring or affecting the power of the platina. . another striking illustration of similar interference may be shown in _carbonic oxide_; especially if contrasted with _carbonic acid_. a mixture of one volume oxygen and hydrogen ( .) with four volumes of carbonic acid was affected at once by a platina plate prepared with acid, &c. ( .); and in one hour and a quarter nearly all the oxygen and hydrogen was gone. mixtures containing less carbonic acid were still more readily affected. . but when carbonic oxide was substituted for the carbonic acid, not the slightest effect of combination was produced; and when the carbonic oxide was only one-eighth of the whole volume, no action occurred in forty and fifty hours. yet the plates had not lost their power; for being taken out and put into pure oxygen and hydrogen, they acted well and at once. . two volumes of carbonic oxide and one of oxygen were mingled with nine volumes of oxygen and hydrogen ( .). this mixture was not affected by a plate which had been made positive in acid, though it remained in it fifteen hours. but when to the same volumes of carbonic oxide and oxygen were added thirty-three volumes of oxygen and hydrogen, the carbonic oxide being then only / th part of the whole, the plate acted, slowly at first, and at the end of forty-two minutes the gases exploded. . these experiments were extended to various gases and vapours, the general results of which may be given as follow. oxygen, hydrogen, nitrogen, and nitrous oxide, when used to dilute the mixture of oxygen and hydrogen, did not prevent the action of the plates even when they made four-fifths of the whole volume of gas acted upon. nor was the retardation so great in any case as might have been expected from the mere dilution of the oxygen and hydrogen, and the consequent mechanical obstruction to its contact with the platina. the order in which carbonic acid and these substances seemed to stand was as follows, the first interfering least with the action; _nitrous oxide, hydrogen, carbonic acid, nitrogen, oxygen_: but it is possible the plates were not equally well prepared in all the cases, and that other circumstances also were unequal; consequently more numerous experiments would be required to establish the order accurately. . as to cases of _retardation_, the powers of olefiant gas and carbonic oxide have been already described. mixtures of oxygen and hydrogen, containing from / th to / th of sulphuretted hydrogen or phosphuretted hydrogen, seemed to show a little action at first, but were not further affected by the prepared plates, though in contact with them for seventy hours. when the plates were removed they had lost all power over pure oxygen and hydrogen, and the interference of these gases was therefore of a different nature from that of the two former, having permanently affected the plate. . a small piece of cork was dipped in sulphuret of carbon and passed up through water into a tube containing oxygen and hydrogen ( .), so as to diffuse a portion of its vapour through the gases. a plate being introduced appeared at first to act a little, but after sixty-one hours the diminution was very small. upon putting the same plate into a pure mixture of oxygen and hydrogen, it acted at once and powerfully, having apparently suffered no diminution of its force. . a little vapour of ether being mixed with the oxygen and hydrogen retarded the action of the plate, but did not prevent it altogether. a little of the vapour of the condensed oil-gas liquor[a] retarded the action still more, but not nearly so much as an equal volume of olefiant gas would have done. in both these cases it was the original oxygen and hydrogen which combined together, the ether and the oil-gas vapour remaining unaffected, and in both cases the plates retained the power of acting on fresh oxygen and hydrogen. [a] philosophical transactions, , p. . . spongy platina was then used in place of the plates, and jets of hydrogen mingled with the different gases thrown against it in air. the results were exactly of the same kind, although presented occasionally in a more imposing form. thus, mixtures of one volume of olefiant gas or carbonic oxide with three of hydrogen could not heat the spongy platina when the experiments were commenced at common temperatures; but a mixture of equal volumes of nitrogen and hydrogen acted very well, causing ignition. with carbonic acid the results were still more striking. a mixture of three volumes of that gas with one of hydrogen caused _ignition_ of the platina, yet that mixture would not continue to burn from the jet when attempts were made to light it by a taper. a mixture even of _seven_ volumes of carbonic acid and _one_ of hydrogen will thus cause the ignition of cold spongy platina, and yet, as if to supply a contrast, than which none can be greater, _it cannot burn at a taper_, but causes the extinction of the latter. on the other hand, the mixtures of carbonic oxide or olefiant gas, which can do nothing with the platina, are _inflamed_ by the taper, burning well. . hydrogen mingled with the vapour of ether or oil-gas liquor causes the ignition of the spongy platina. the mixture with oil-gas burns with a flame far brighter than that of the mixture of hydrogen and olefiant gas already referred to, so that it would appear that the retarding action of the hydrocarbons is not at all in proportion merely to the quantity of carbon present. . in connexion with these interferences, i must state, that hydrogen itself, prepared from steam passed over ignited iron, was found when mingled with oxygen to resist the action of platina. it had stood over water seven days, and had lost all fetid smell; but a jet of it would not cause the ignition of spongy platina, commencing at common temperatures; nor would it combine with oxygen in a tube either under the influence of a prepared plate or of spongy platina. a mixture of one volume of this gas with three of pure hydrogen, and the due proportion of oxygen, was not affected by plates after fifty hours. i am inclined to refer the effect to carbonic oxide present in the gas, but have not had time to verify the suspicion. the power of the plates was not destroyed ( . .). . such are the general facts of these remarkable interferences. whether the effect produced by such small quantities of certain gases depends upon any direct action which they may exert upon the particles of oxygen and hydrogen, by which the latter are rendered less inclined to combine, or whether it depends upon their modifying the action of the plate temporarily (for they produce no real change on it), by investing it through the agency of a stronger attraction than that of the hydrogen, or otherwise, remains to be decided by more extended experiments. * * * * * . the theory of action which i have given for the original phenomena appears to me quite sufficient to account for all the effects by reference to known properties, and dispenses with the assumption of any new power of matter. i have pursued this subject at some length, as one of great consequence, because i am convinced that the superficial actions of matter, whether between two bodies, or of one piece of the same body, and the actions of particles not directly or strongly in combination, are becoming daily more and more important to our theories of chemical as well as mechanical philosophy[a]. in all ordinary cases of combustion it is evident that an action of the kind considered, occurring upon the surface of the carbon in the fire, and also in the bright part of a flame, must have great influence over the combinations there taking place. [a] as a curious illustration of the influence of mechanical forces over chemical affinity, i will quote the refusal of certain substances to effloresce when their surfaces are perfect, which yield immediately upon the surface being broken, if crystals of carbonate of soda, or phosphate of soda, or sulphate of soda, having no part of their surfaces broken, be preserved from external violence, they will not effloresce. i have thus retained crystals of carbonate of soda perfectly transparent and unchanged from september to january ; and crystals of sulphate of soda from may to the present time, november . if any part of the surface were scratched or broken, then efflorescence began at that part, and covered the whole. the crystals were merely placed in evaporating basins and covered with paper. . the condition of elasticity upon the exterior of the gaseous or vaporous mass already referred to ( . .), must be connected directly with the action of solid bodies, as nuclei, on vapours, causing condensation upon them in preference to any condensation in the vapours themselves; and in the well-known effect of nuclei on solutions a similar condition may have existence ( .), for an analogy in condition exists between the parts of a body in solution, and those of a body in the vaporous or gaseous state. this thought leads us to the consideration of what are the respective conditions at the surfaces of contact of two portions of the same substance at the same temperature, one in the solid or liquid, and the other in the vaporous state; as, for instance, steam and water. it would seem that the particles of vapour next to the particles of liquid are in a different relation to the latter to what they would be with respect to any other liquid or solid substance; as, for instance, mercury or platina, if they were made to replace the water, i.e. if the view of independent action which i have taken ( . .) as a consequence of dalton's principles, be correct. it would also seem that the mutual relation of similar particles, and the indifference of dissimilar particles which dalton has established as a matter of fact amongst gases and vapours, extends to a certain degree amongst solids and fluids, that is, when they are in relation by contact with vapours, either of their own substance or of other bodies. but though i view these points as of great importance with respect to the relations existing between different substances and their physical constitution in the solid, liquid, or gaseous state, i have not sufficiently considered them to venture any strong opinions or statements here[a]. [a] in reference to this paragraph and also , see a correction by dr. c. henry, in his valuable paper on this curious subject. philosophical magazine, . vol. vi. p. .--_dec. ._ . there are numerous well-known cases, in which substances, such as oxygen and hydrogen, act readily in their _nascent_ state, and produce chemical changes which they are not able to effect if once they have assumed the gaseous condition. such instances are very common at the poles of the voltaic pile, and are, i think, easily accounted for, if it be considered that at the moment of separation of any such particle it is entirely surrounded by other particles of a _different_ kind with which it is in close contact, and has not yet assumed those relations and conditions which it has in its fully developed state, and which it can only assume by association with other particles of its own kind. for, at the moment, its elasticity is absent, and it is in the same relation to particles with which it is in contact, and for which it has an affinity, as the particles of oxygen and hydrogen are to each other on the surface of clean platina ( . .). . the singular effects of retardation produced by very small quantities of some gases, and not by large quantities of others ( . . .), if dependent upon any relation of the added gas to the surface of the solid, will then probably be found immediately connected with the curious phenomena which are presented by different gases when passing through narrow tubes at low pressures, which i observed many years ago[a]; and this action of surfaces must, i think, influence the highly interesting phenomena of the diffusion of gases, at least in the form in which it has been experimented upon by mr. graham in and [b], and also by dr. mitchell of philadelphia[c] in . it seems very probable that if such a substance as spongy platina were used, another law for the diffusion of gases under the circumstances would come out than that obtained by the use of plaster of paris. [a] quarterly journal of science, , vol. vii. p. . [b] quarterly journal of science, vol. xxviii. p. , and edinburgh transactions, . [c] journal of the royal institution for , p. . . i intended to have followed this section by one on the secondary piles of ritter, and the peculiar properties of the poles of the pile, or of metals through which electricity has passed, which have been observed by ritter, van marum, yelin, de la rive, marianini, berzelius, and others. it appears to me that all these phenomena bear a satisfactory explanation on known principles, connected with the investigation just terminated, and do not require the assumption of any new state or new property. but as the experiments advanced, especially those of marianini, require very careful repetition and examination, the necessity of pursuing the subject of electro-chemical decomposition obliges me for a time to defer the researches to which i have just referred. _royal institution, november , ._ seventh series. § . _on electro-chemical decomposition, continued._[a] ¶ iv. _on some general conditions of electro-decomposition._ ¶ v. _on a new measurer of volta-electricity._ ¶ vi. _on the primary or secondary character of bodies evolved in electro-decomposition._ ¶ vii. _on the definite nature and extent of electro-chemical decompositions._ § . _on the absolute quantity of electricity associated with the particles or atoms of matter._ [a] refer to the note after , series viii.--_dec. ._ received january ,--read january , february and , . _preliminary._ . the theory which i believe to be a true expression of the facts of electro-chemical decomposition, and which i have therefore detailed in a former series of these researches, is so much at variance with those previously advanced, that i find the greatest difficulty in stating results, as i think, correctly, whilst limited to the use of terms which are current with a certain accepted meaning. of this kind is the term _pole_, with its prefixes of positive and negative, and the attached ideas of attraction and repulsion. the general phraseology is that the positive pole _attracts_ oxygen, acids, &c., or more cautiously, that it _determines_ their evolution upon its surface; and that the negative pole acts in an equal manner upon hydrogen, combustibles, metals, and bases. according to my view, the determining force is _not_ at the poles, but _within_ the body under decomposition; and the oxygen and acids are rendered at the _negative_ extremity of that body, whilst hydrogen, metals, &c., are evolved at the _positive_ extremity ( . .). . to avoid, therefore, confusion and circumlocution, and for the sake of greater precision of expression than i can otherwise obtain, i have deliberately considered the subject with two friends, and with their assistance and concurrence in framing them, i purpose henceforward using certain other terms, which i will now define. the _poles_, as they are usually called, are only the doors or ways by which the electric current passes into and out of the decomposing body ( .); and they of course, when in contact with that body, are the limits of its extent in the direction of the current. the term has been generally applied to the metal surfaces in contact with the decomposing substance; but whether philosophers generally would also apply it to the surfaces of air ( . .) and water ( .), against which i have effected electro-chemical decomposition, is subject to doubt. in place of the term pole, i propose using that of _electrode_[a], and i mean thereby that substance, or rather surface, whether of air, water, metal, or any other body, which bounds the extent of the decomposing matter in the direction of the electric current. [a] [greek: elektron], and [greek: -odos] _a way_. . the surfaces at which, according to common phraseology, the electric current enters and leaves a decomposing body, are most important places of action, and require to be distinguished apart from the poles, with which they are mostly, and the electrodes, with which they are always, in contact. wishing for a natural standard of electric direction to which i might refer these, expressive of their difference and at the same time free from all theory, i have thought it might be found in the earth. if the magnetism of the earth be due to electric currents passing round it, the latter must be in a constant direction, which, according to present usage of speech, would be from east to west, or, which will strengthen this help to the memory, that in which the sun appears to move. if in any case of electro-decomposition we consider the decomposing body as placed so that the current passing through it shall be in the same direction, and parallel to that supposed to exist in the earth, then the surfaces at which the electricity is passing into and out of the substance would have an invariable reference, and exhibit constantly the same relations of powers. upon this notion we purpose calling that towards the east the _anode_[a], and that towards the west the _cathode_[b]; and whatever changes may take place in our views of the nature of electricity and electrical action, as they must affect the _natural standard_ referred to, in the same direction, and to an equal amount with any decomposing substances to which these terms may at any time be applied, there seems no reason to expect that they will lead to confusion, or tend in any way to support false views. the _anode_ is therefore that surface at which the electric current, according to our present expression, enters: it is the _negative_ extremity of the decomposing body; is where oxygen, chlorine, acids, &c., are evolved; and is against or opposite the positive electrode. the _cathode_ is that surface at which the current leaves the decomposing body, and is its _positive_ extremity; the combustible bodies, metals, alkalies, and bases, are evolved there, and it is in contact with the negative electrode. [a] [greek: ano] _upwards_, and [greek: -odos] _a way_; the way which the sun rises. [b] [greek: kata] _downwards_, and [greek: -odos] _a way_; the way which the sun sets. . i shall have occasion in these researches, also, to class bodies together according to certain relations derived from their electrical actions ( .); and wishing to express those relations without at the same time involving the expression of any hypothetical views, i intend using the following names and terms. many bodies are decomposed directly by the electric current, their elements being set free; these i propose to call _electrolytes_.[a] water, therefore, is an electrolyte. the bodies which, like nitric or sulphuric acids, are decomposed in a secondary manner ( . .), are not included under this term. then for _electro-chemically decomposed_, i shall often use the term _electrolyzed_, derived in the same way, and implying that the body spoken of is separated into its components under the influence of electricity: it is analogous in its sense and sound to _analyse_, which is derived in a similar manner. the term _electrolytical_ will be understood at once: muriatic acid is electrolytical, boracic acid is not. [a] [greek: elektron], and [greek: lyo], _soluo_. n. electrolyte, v. electrolyze. . finally, i require a term to express those bodies which can pass to the _electrodes_, or, as they are usually called, the poles. substances are frequently spoken of as being _electro-negative_, or _electro-positive_, according as they go under the supposed influence of a direct attraction to the positive or negative pole. but these terms are much too significant for the use to which i should have to put them; for though the meanings are perhaps right, they are only hypothetical, and may be wrong; and then, through a very imperceptible, but still very dangerous, because continual, influence, they do great injury to science, by contracting and limiting the habitual views of those engaged in pursuing it. i propose to distinguish such bodies by calling those _anions_[a] which go to the _anode_ of the decomposing body; and those passing to the _cathode, cations_[b]; and when i have occasion to speak of these together, i shall call them _ions_. thus the chloride of lead is an _electrolyte_, and when _electrolyzed_ evolves the two _ions_, chlorine and lead, the former being an _anion_, and the latter a _cation_. [a] [greek: aniôn] _that which goes up._ (neuter participle.) [b] [greek: katiôn] _that which goes down._ . these terms being once well-defined, will, i hope, in their use enable me to avoid much periphrasis and ambiguity of expression. i do not mean to press them into service more frequently than will be required, for i am fully aware that names are one thing and science another. . it will be well understood that i am giving no opinion respecting the nature of the electric current now, beyond what i have done on former occasions ( . .); and that though i speak of the current as proceeding from the parts which are positive to those which are negative ( .), it is merely in accordance with the conventional, though in some degree tacit, agreement entered into by scientific men, that they may have a constant, certain, and definite means of referring to the direction of the forces of that current. [since this paper was read, i have changed some of the terms which were first proposed, that i might employ only such as were at the same time simple in their nature, clear in their reference, and free from hypothesis. ¶ iv. _on some general conditions of electro-chemical decomposition._ . from the period when electro-chemical decomposition was first effected to the present time, it has been a remark, that those elements which, in the ordinary phenomena of chemical affinity, were the most directly opposed to each other, and combined with the greatest attractive force, were those which were the most readily evolved at the opposite extremities of the decomposing bodies ( .). . if this result was evident when water was supposed to be essential to, and was present in, almost every case of such decomposition ( .), it is far more evident now that it has been shown and proved that water is not necessarily concerned in the phenomena ( .), and that other bodies much surpass it in some of the effects supposed to be peculiar to that substance. . water, from its constitution and the nature of its elements, and from its frequent presence in cases of electrolytic action, has hitherto stood foremost in this respect. though a compound formed by very powerful affinity, it yields up its elements under the influence of a very feeble electric current; and it is doubtful whether a case of electrolyzation can occur, where, being present, it is not resolved into its first principles. . the various oxides, chlorides, iodides, and salts, which i have shown are decomposable by the electric current when in the liquid state, under the same general law with water ( .), illustrate in an equally striking manner the activity, in such decompositions, of elements directly and powerfully opposed to each other by their chemical relations. . on the other hand, bodies dependent on weak affinities very rarely give way. take, for instance, glasses: many of those formed of silica, lime, alkali, and oxide of lead, may be considered as little more than solutions of substances one in another[a]. if bottle-glass be fused, and subjected to the voltaic pile, it does not appear to be at all decomposed ( .). if flint glass, which contains substances more directly opposed, be operated upon, it suffers some decomposition; and if borate of lead glass, which is a definite chemical compound, be experimented with, it readily yields up its elements ( .). [a] philosophical transactions, , p. . . but the result which is found to be so striking in the instances quoted is not at all borne out by reference to other cases where a similar consequence might have been expected. it may be said, that my own theory of electro-chemical decomposition would lead to the expectation that all compound bodies should give way under the influence of the electric current with a facility proportionate to the strength of the affinity by which their elements, either proximate or ultimate, are combined. i am not sure that that follows as a consequence of the theory; but if the objection is supposed to be one presented by the facts, i have no doubt it will be removed when we obtain a more intimate acquaintance with, and precise idea of, the nature of chemical affinity and the mode of action of an electric current over it ( . .): besides which, it is just as directly opposed to any other theory of electro-chemical decomposition as the one i have propounded; for if it be admitted, as is generally the case, that the more directly bodies are opposed to each other in their attractive forces, the more powerfully do they combine, then the objection applies with equal force to any of the theories of electrolyzation which have been considered, and is an addition to those which i have taken against them. . amongst powerful compounds which are not decomposed, boracic acids stand prominent ( .). then again, the iodide of sulphur, and the chlorides of sulphur, phosphorus, and carbon, are not decomposable under common circumstances, though their elements are of a nature which would lead to a contrary expectation. chloride of antimony ( . .), the hydro-carbons, acetic acid, ammonia, and many other bodies undecomposable by the voltaic pile, would seem to be formed by an affinity sufficiently strong to indicate that the elements were so far contrasted in their nature as to sanction the expectation that, the pile would separate them, especially as in some cases of mere solution ( . .), where the affinity must by comparison be very weak, separation takes place[a]. [a] with regard to solution, i have met with some reasons for supposing that it will probably disappear as a cause of transference, and intend resuming the consideration at a convenient opportunity. . it must not be forgotten, however, that much of this difficulty, and perhaps the whole, may depend upon the absence of conducting power, which, preventing the transmission of the current, prevents of course the effects due to it. all known compounds being non-conductors when solid, but conductors when liquid, are decomposed, with _perhaps_ the single exception at present known of periodide of mercury ( . .)[a]; and even water itself, which so easily yields up its elements when the current passes, if rendered quite pure, scarcely suffers change, because it then becomes a very bad conductor. [a] see now, , .--_dec. ._ . if it should hereafter be proved that the want of decomposition in those cases where, from chemical considerations, it might be so strongly expected ( , . .), is due to the absence or deficiency of conducting power, it would also at the same time be proved that decomposition _depends_ upon conduction, and not the latter upon the former ( .); and in water this seems to be very nearly decided. on the other hand, the conclusion is almost irresistible, that in electrolytes the power of transmitting the electricity across the substance is _dependent_ upon their capability of suffering decomposition; taking place only whilst they are decomposing, and being proportionate to the quantity of elements separated ( .). i may not, however, stop to discuss this point experimentally at present. . when a compound contains such elements as are known to pass towards the opposite extremities of the voltaic pile, still the proportions in which they are present appear to be intimately connected with capability in the compound of suffering or resisting decomposition. thus, the protochloride of tin readily conducts, and is decomposed ( .), but the perchloride neither conducts nor is decomposed ( .). the protiodide of tin is decomposed when fluid ( .); the periodide is not ( .). the periodide of mercury when fused is not decomposed ( .), even though it does conduct. i was unable to contrast it with the protiodide, the latter being converted into mercury and periodide by heat. . these important differences induced me to look more closely to certain binary compounds, with a view of ascertaining whether a _law_ regulating the _decomposability_ according to some _relation of the proportionals or equivalents_ of the elements, could be discovered. the proto compounds only, amongst those just referred to, were decomposable; and on referring to the substances quoted to illustrate the force and generality of the law of conduction and decomposition which i discovered ( .), it will be found that all the oxides, chlorides, and iodides subject to it, except the chloride of antimony and the periodide of mercury, (to which may now perhaps be added corrosive sublimate,) are also decomposable, whilst many per compounds of the same elements, not subject to the law, were not so ( . .). . the substances which appeared to form the strongest exceptions to this general result were such bodies as the sulphuric, phosphoric, nitric, arsenic, and other acids. . on experimenting with sulphuric acid, i found no reason to believe that it was by itself a conductor of, or decomposable by, electricity, although i had previously been of that opinion ( .). when very strong it is a much worse conductor than if diluted[a]. if then subjected to the action of a powerful battery, oxygen appears at the _anode_, or positive electrode, although much is absorbed ( .), and hydrogen and sulphur appear at the _cathode_, or negative electrode. now the hydrogen has with me always been pure, not sulphuretted, and has been deficient in proportion to the sulphur present, so that it is evident that when decomposition occurred water must have been decomposed. i endeavoured to make the experiment with anhydrous sulphuric acid; and it appeared to me that, when fused, such acid was not a conductor, nor decomposed; but i had not enough of the dry acid in my possession to allow me to decide the point satisfactorily. my belief is, that when sulphur appears during the action of the pile on sulphuric acid, it is the result of a secondary action, and that the acid itself is not electrolyzable ( .). [a] de la rive. . phosphoric acid is, i believe, also in the same condition; but i have found it impossible to decide the point, because of the difficulty of operating on fused anhydrous phosphoric acid. phosphoric acid which has once obtained water cannot be deprived of it by heat alone. when heated, the hydrated acid volatilizes. upon subjecting phosphoric acid, fused upon the ring end of a wire ( .), to the action of the voltaic apparatus, it conducted, and was decomposed; but gas, which i believe to be hydrogen, was always evolved at the negative electrode, and the wire was not affected as would have happened had phosphorus been separated. gas was also evolved at the positive electrode. from all the facts, i conclude it was the water and not the acid which was decomposed. . _arsenic acid_. this substance conducted, and was decomposed; but it contained water, and i was unable at the time to press the investigation so as to ascertain whether a fusible anhydrous arsenic acid could be obtained. it forms, therefore, at present no exception to the general result. . nitrous acid, obtained by distilling nitrate of lead, and keeping it in contact with strong sulphuric acid, was found to conduct and decompose slowly. but on examination there were strong reasons for believing that water was present, and that the decomposition and conduction depended upon it. i endeavoured to prepare a perfectly anhydrous portion, but could not spare the time required to procure an unexceptionable result. . nitric acid is a substance which i believe is not decomposed directly by the electric current. as i want the facts in illustration of the distinction existing between primary and secondary decomposition, i will merely refer to them in this place ( .). . that these mineral acids should confer facility of conduction and decomposition on water, is no proof that they are competent to favour and suffer these actions in themselves. boracic acid does the same thing, though not decomposable. m. de la rive has pointed out that chlorine has this power also; but being to us an elementary substance, it cannot be due to its capability of suffering decomposition. . _chloride of sulphur_ does not conduct, nor is it decomposed. it consists of single proportionals of its elements, but is not on that account an exception to the rule ( .), which does not affirm that _all_ compounds of single proportionals of elements are decomposable, but that such as are decomposable are so constituted. . _protochloride of phosphorus_ does not conduct nor become decomposed. . _protochloride of carbon_ does not conduct nor suffer decomposition. in association with this substance, i submitted the _hydro-chloride of carbon_ from olefiant gas and chlorine to the action of the electric current; but it also refused to conduct or yield up its elements. . with regard to the exceptions ( .), upon closer examination some of them disappear. chloride of antimony (a compound of one proportional of antimony and one and a half of chlorine) of recent preparation was put into a tube (fig. .) ( .), and submitted when fused to the action of the current, the positive electrode being of plumbago. no electricity passed, and no appearance of decomposition was visible at first; but when the positive and negative electrodes were brought very near each other in the chloride, then a feeble action occurred and a feeble current passed. the effect altogether was so small (although quite amenable to the law before given ( .)), and so unlike the decomposition and conduction occurring in all the other cases, that i attribute it to the presence of a minute quantity of water, (for which this and many other chlorides have strong attractions, producing hydrated chlorides,) or perhaps of a true protochloride consisting of single proportionals ( , .). . _periodide of mercury_ being examined in the same manner, was found most distinctly to insulate whilst solid, but conduct when fluid, according to the law of _liquido-conduction_ ( .); but there was no appearance of decomposition. no iodine appeared at the _anode_, nor mercury or other substance at the _cathode_. the case is, therefore, no exception to the rule, that only compounds of single proportionals are decomposable; but it is an exception, and i think the only one, to the statement, that all bodies subject to the law of liquido-conduction are decomposable. i incline, however, to believe, that a portion of protiodide of mercury is retained dissolved in the periodide, and that to its slow decomposition the feeble conducting power is due. periodide would be formed, as a secondary result, at the _anode_; and the mercury at the _cathode_ would also form, as a secondary result, protiodide. both these bodies would mingle with the fluid mass, and thus no final separation appear, notwithstanding the continued decomposition. . when _perchloride of mercury_ was subjected to the voltaic current, it did not conduct in the solid state, but it did conduct when fluid. i think, also, that in the latter case it was decomposed; but there are many interfering circumstances which require examination before a positive conclusion can be drawn[a]. [a] with regard to perchloride and periodide of mercury, see now , .--_dec. ._ . when the ordinary protoxide of antimony is subjected to the voltaic current in a fused state, it also is decomposed, although the effect from other causes soon ceases ( , .). this oxide consists of one proportional of antimony and one and a half of oxygen, and is therefore an exception to the general law assumed. but in working with this oxide and the chloride, i observed facts which lead me to doubt whether the compounds usually called the protoxide and the protochloride do not often contain other compounds, consisting of single proportions, which are the true proto compounds, and which, in the case of the oxide, might give rise to the decomposition above described. . the ordinary sulphuret of antimony its considered as being the compound with the smallest quantity of sulphur, and analogous in its proportions to the ordinary protoxide. but i find that if it be fused with metallic antimony, a new sulphuret is formed, containing much more of the metal than the former, and separating distinctly, when fused, both from the pure metal on the one hand, and the ordinary gray sulphuret on the other. in some rough experiments, the metal thus taken up by the ordinary sulphuret of antimony was equal to half the proportion of that previously in the sulphuret, in which case the new sulphuret would consist of _single_ proportionals. . when this new sulphuret was dissolved in muriatic acid, although a little antimony separated, yet it appeared to me that a true protochloride, consisting of _single_ proportionals, was formed, and from that by alkalies, &c., a true protoxide, consisting also of _single_ proportionals, was obtainable. but i could not stop to ascertain this matter strictly by analysis. . i believe, however, that there is such an oxide; that it is often present in variable proportions in what is commonly called protoxide, throwing uncertainty upon the results of its analysis, and causing the electrolytic decomposition above described[a]. [a] in relation to this and the three preceding paragraphs, and also , see berzelius's correction of the nature of the supposed now sulphuret and oxide, phil. mag. , vol. viii. : and for the probable explanation of the effects obtained with the protoxide, refer to , .--_dec. ._ . upon the whole, it appears probable that all those binary compounds of elementary bodies which are capable of being electrolyzed when fluid, but not whilst solid, according to the law of liquido-conduction ( .), consist of single proportionals of their elementary principles; and it may be because of their departure from this simplicity of composition, that boracic acid, ammonia, perchlorides, periodides, and many other direct compounds of elements, are indecomposable. . with regard to salts and combinations of compound bodies, the same simple relation does not appear to hold good. i could not decide this by bisulphates of the alkalies, for as long as the second proportion of acid remained, water was retained with it. the fused salts conducted, and were decomposed; but hydrogen always appeared at the negative electrode. . a biphosphate of soda was prepared by heating, and ultimately fusing, the ammonia-phosphate of soda. in this case the fused bisalt conducted, and was decomposed; but a little gas appeared at the negative electrode; and though i believe the salt itself was electrolyzed, i am not quite satisfied that water was entirely absent. . then a biborate of soda was prepared; and this, i think, is an unobjectionable case. the salt, when fused, conducted, and was decomposed, and gas appeared at both electrodes: even when the boracic acid was increased to three proportionals, the same effect took place. . hence this class of compound combinations does not seem to be subject to the same simple law as the former class of binary combinations. whether we may find reason to consider them as mere solutions of the compound of single proportionals in the excess of acid, is a matter which, with some apparent exceptions occurring amongst the sulphurets, must be left for decision by future examination. . in any investigation of these points, great care must be taken to exclude water; for if present, secondary effects are so frequently produced as often seemingly to indicate an electro-decomposition of substances, when no true result of the kind has occurred ( , &c.). . it is evident that all the cases in which decomposition _does not occur, may_ depend upon the want of conduction ( . .); but that does not at all lessen the interest excited by seeing the great difference of effect due to a change, not in the nature of the elements, but merely in their proportions; especially in any attempt which may be made to elucidate and expound the beautiful theory put forth by sir humphry davy[a], and illustrated by berzelius and other eminent philosophers, that ordinary chemical affinity is a mere result of the electrical attractions of the particles of matter. [a] philosophical transactions, , pp. , ; also , pp. , . ¶ v. _on a new measure of volta-electricity._ . i have already said, when engaged in reducing common and voltaic electricity to one standard of measurement ( .), and again when introducing my theory of electro-chemical decomposition ( . . .), that the chemical decomposing action of a current _is constant for a constant quantity of electricity_, notwithstanding the greatest variations in its sources, in its intensity, in the size of the _electrodes_ used, in the nature of the conductors (or non-conductors ( .)) through which it is passed, or in other circumstances. the conclusive proofs of the truth of these statements shall be given almost immediately ( , &c.). . i endeavoured upon this law to construct an instrument which should measure out the electricity passing through it, and which, being interposed in the course of the current used in any particular experiment, should serve at pleasure, either as a _comparative standard_ of effect, or as a _positive measurer_ of this subtile agent. . there is no substance better fitted, under ordinary circumstances, to be the indicating body in such an instrument than water; for it is decomposed with facility when rendered a better conductor by the addition of acids or salts; its elements may in numerous cases be obtained and collected without any embarrassment from secondary action, and, being gaseous, they are in the best physical condition for separation and measurement. water, therefore, acidulated by sulphuric acid, is the substance i shall generally refer to, although it may become expedient in peculiar cases or forms of experiment to use other bodies ( .). . the first precaution needful in the construction of the instrument was to avoid the recombination of the evolved gases, an effect which the positive electrode has been found so capable of producing ( .). for this purpose various forms of decomposing apparatus were used. the first consisted of straight tubes, each containing a plate and wire of platina soldered together by gold, and fixed hermetically in the glass at the closed extremity of the tube (plate v. fig. .). the tubes were about eight inches long, . of an inch in diameter, and graduated. the platina plates were about an inch long, as wide as the tubes would permit, and adjusted as near to the mouths of the tubes as was consistent with the safe collection of the gases evolved. in certain cases, where it was required to evolve the elements upon as small a surface as possible, the metallic extremity, instead of being a plate, consisted of the wire bent into the form of a ring (fig. .). when these tubes were used as measurers, they were filled with the dilute sulphuric acid, inverted in a basin of the same liquid (fig. .), and placed in an inclined position, with their mouths near to each other, that as little decomposing matter should intervene as possible; and also, in such a direction that the platina plates should be in vertical planes ( ). . another form of apparatus is that delineated (fig. .). the tube is bent in the middle; one end is closed; in that end is fixed a wire and plate, _a_, proceeding so far downwards, that, when in the position figured, it shall be as near to the angle as possible, consistently with the collection, at the closed extremity of the tube, of all the gas evolved against it. the plane of this plate is also perpendicular ( .). the other metallic termination, _b_, is introduced at the time decomposition is to be effected, being brought as near the angle as possible, without causing any gas to pass from it towards the closed end of the instrument. the gas evolved against it is allowed to escape. . the third form of apparatus contains both electrodes in the same tube; the transmission, therefore, of the electricity, and the consequent decomposition, is far more rapid than in the separate tubes. the resulting gas is the sum of the portions evolved at the two electrodes, and the instrument is better adapted than either of the former as a measurer of the quantity of voltaic electricity transmitted in ordinary cases. it consists of a straight tube (fig. .) closed at the upper extremity, and graduated, through the sides of which pass platina wires (being fused into the glass), which are connected with two plates within. the tube is fitted by grinding into one mouth of a double-necked bottle. if the latter be one-half or two-thirds full of the dilute sulphuric acid ( .), it will, upon inclination of the whole, flow into the tube and fill it. when an electric current is passed through the instrument, the gases evolved against the plates collect in the upper portion of the tube, and are not subject to the recombining power of the platina. . another form of the instrument is given at fig. . . a fifth form is delineated (fig. .). this i have found exceedingly useful in experiments continued in succession for days together, and where large quantities of indicating gas were to be collected. it is fixed on a weighted foot, and has the form of a small retort containing the two electrodes: the neck is narrow, and sufficiently long to deliver gas issuing from it into a jar placed in a small pneumatic trough. the electrode chamber, sealed hermetically at the part held in the stand, is five inches in length, and . of an inch in diameter; the neck about nine inches in length, and . of an inch in diameter internally. the figure will fully indicate the construction. . it can hardly be requisite to remark, that in the arrangement of any of these forms of apparatus, they, and the wires connecting them with the substance, which is collaterally subjected to the action of the same electric current, should be so far insulated as to ensure a certainty that all the electricity which passes through the one shall also be transmitted through the other. * * * * * . next to the precaution of collecting the gases, if mingled, out of contact with the platinum, was the necessity of testing the law of a _definite electrolytic_ action, upon water at least, under all varieties of condition; that, with a conviction of its certainty, might also be obtained a knowledge of those interfering circumstances which would require to be practically guarded against. . the first point investigated was the influence or indifference of extensive variations in the size of the electrodes, for which purpose instruments like those last described ( . . .) were used. one of these had plates . of an inch wide, and nearly four inches long; another had plates only . of an inch wide, and . of an inch long; a third had wires . of an inch in diameter, and three inches long; and a fourth, similar wires only half an inch in length. yet when these were filled with dilute sulphuric acid, and, being placed in succession, had one common current of electricity passed through them, very nearly the same quantity of gas was evolved in all. the difference was sometimes in favour of one and sometimes on the side of another; but the general result was that the largest quantity of gases was evolved at the smallest electrodes, namely, those consisting merely of platina wires. . experiments of a similar kind were made with the single-plate, straight tubes ( .), and also with the curved tubes ( .), with similar consequences; and when these, with the former tubes, were arranged together in various ways, the result, as to the equality of action of large and small metallic surfaces when delivering and receiving the same current of electricity, was constantly the same. as an illustration, the following numbers are given. an instrument with two wires evolved . volumes of mixed gases; another with plates . volumes; whilst the sum of the oxygen and hydrogen in two separate tubes amounted to . volumes. in another experiment the volumes were . , . , and . . . but it was observed in these experiments, that in single-plate tubes ( .) more hydrogen was evolved at the negative electrode than was proportionate to the oxygen at the positive electrode; and generally, also, more than was proportionate to the oxygen and hydrogen in a double-plate tube. upon more minutely examining these effects, i was led to refer them, and also the differences between wires and plates ( .), to the solubility of the gases evolved, especially at the positive electrode. . when the positive and negative electrodes are equal in surface, the bubbles which rise from them in dilute sulphuric acid are always different in character. those from the positive plate are exceedingly small, and separate instantly from every part of the surface of the metal, in consequence of its perfect cleanliness ( .); whilst in the liquid they give it a hazy appearance, from their number and minuteness; are easily carried down by currents, and therefore not only present far greater surface of contact with the liquid than larger bubbles would do, but are retained a much longer time in mixture with it. but the bubbles at the negative surface, though they constitute twice the volume of the gas at the positive electrode, are nevertheless very inferior in number. they do not rise so universally from every part of the surface, but seem to be evolved at different parts; and though so much larger, they appear to cling to the metal, separating with difficulty from it, and when separated, instantly rising to the top of the liquid. if, therefore, oxygen and hydrogen had equal solubility in, or powers of combining with, water under similar circumstances, still under the present conditions the oxygen would be far the most liable to solution; but when to these is added its well-known power of forming a compound with water, it is no longer surprising that such a compound should be produced in small quantities at the positive electrode; and indeed the blenching power which some philosophers have observed in a solution at this electrode, when chlorine and similar bodies have been carefully excluded, is probably due to the formation there, in this manner, of oxywater. . that more gas was collected from the wires than from the plates, i attribute to the circumstance, that as equal quantities were evolved in equal times, the bubbles at the wires having been more rapidly produced, in relation to any part of the surface, must have been much larger; have been therefore in contact with the fluid by a much smaller surface, and for a much shorter time than those at the plates; hence less solution and a greater amount collected. . there was also another effect produced, especially by the use of large electrodes, which was both a consequence and a proof of the solution of part of the gas evolved there. the collected gas, when examined, was found to contain small portions of nitrogen. this i attribute to the presence of air dissolved in the acid used for decomposition. it is a well-known fact, that when bubbles of a gas but slightly soluble in water or solutions pass through them, the portion of this gas which is dissolved displaces a portion of that previously in union with the liquid: and so, in the decompositions under consideration, as the oxygen dissolves, it displaces a part of the air, or at least of the nitrogen, previously united to the acid; and this effect takes place _most extensively_ with large plates, because the gas evolved at them is in the most favourable condition for solution, . with the intention of avoiding this solubility of the gases as much as possible, i arranged the decomposing plates in a vertical position ( . .), that the bubbles might quickly escape upwards, and that the downward currents in the fluid should not meet ascending currents of gas. this precaution i found to assist greatly in producing constant results, and especially in experiments to be hereafter referred to, in which other liquids than dilute sulphuric acid, as for instance solution of potash, were used. . the irregularities in the indications of the measurer proposed, arising from the solubility just referred to, are but small, and may be very nearly corrected by comparing the results of two or three experiments. they may also be almost entirely avoided by selecting that solution which is found to favour them in the least degree ( .); and still further by collecting the hydrogen only, and using that as the indicating gas; for being much less soluble than oxygen, being evolved with twice the rapidity and in larger bubbles ( .), it can be collected more perfectly and in greater purity. . from the foregoing and many other experiments, it results that _variation in the size of the electrodes causes no variation in the chemical action of a given quantity of electricity upon water_. . the next point in regard to which the principle of constant electro-chemical action was tested, was _variation of intensity_. in the first place, the preceding experiments were repeated, using batteries of an _equal_ number of plates, _strongly_ and _weakly_ charged; but the results were alike. they were then repeated, using batteries sometimes containing forty, and at other times only five pairs of plates; but the results were still the same. _variations therefore in the intensity_, caused by difference in the strength of charge, or in the number of alternations used, _produced no difference as to the equal action of large and small electrodes_. . still these results did not prove that variation in the intensity of the current was not accompanied by a corresponding variation in the electro-chemical effects, since the actions at _all_ the surfaces might have increased or diminished together. the deficiency in the evidence is, however, completely supplied by the former experiments on different-sized electrodes; for with variation in the size of these, a variation in the intensity must have occurred. the intensity of an electric current traversing conductors alike in their nature, quality, and length, is probably as the quantity of electricity passing through a given sectional area perpendicular to the current, divided by the time ( . _note_); and therefore when large plates were contrasted with wires separated by an equal length of the same decomposing conductor ( .), whilst one current of electricity passed through both arrangements, that electricity must have been in a very different state, as to _tension_, between the plates and between the wires; yet the chemical results were the same. . the difference in intensity, under the circumstances described, may be easily shown practically, by arranging two decomposing apparatus as in fig. , where the same fluid is subjected to the decomposing power of the same current of electricity, passing in the vessel a. between large platina plates, and in the vessel b. between small wires. if a third decomposing apparatus, such as that delineated fig. . ( .), be connected with the wires at _ab_, fig. , it will serve sufficiently well, by the degree of decomposition occurring in it, to indicate the relative state of the two plates as to intensity; and if it then be applied in the same way, as a test of the state of the wires at _a'b'_, it will, by the increase of decomposition within, show how much greater the intensity is there than at the former points. the connexions of p and n with the voltaic battery are of course to be continued during the whole time. . a third form of experiment, in which difference of intensity was obtained, for the purpose of testing the principle of equal chemical action, was to arrange three volta-electrometers, so that after the electric current had passed through one, it should divide into two parts, each of which should traverse one of the remaining instruments, and should then reunite. the sum of the decomposition in the two latter vessels was always equal to the decomposition in the former vessel. but the _intensity_ of the divided current could not be the same as that it had in its original state; and therefore _variation of intensity has no influence on the results if the quantity of electricity remain the same_. the experiment, in fact, resolves itself simply into an increase in the size of the electrodes ( .). . the _third point_, in respect to which the principle of equal electro-chemical action on water was tested, was _variation of the strength of the solution used_. in order to render the water a conductor, sulphuric acid had been added to it ( .); and it did not seem unlikely that this substance, with many others, might render the water more subject to decomposition, the electricity remaining the same in quantity. but such did not prove to be the case. diluted sulphuric acid, of different strengths, was introduced into different decomposing apparatus, and submitted simultaneously to the action of the same electric current ( .). slight differences occurred, as before, sometimes in one direction, sometimes in another; but the final result was, that _exactly the same quantity of water was decomposed in all the solutions by the same quantity of electricity_, though the sulphuric acid in some was seventy-fold what it was in others. the strengths used were of specific gravity . , and downwards. . when an acid having a specific gravity of about . was employed, the results were most uniform, and the oxygen and hydrogen ( .) most constantly in the right proportion to each other. such an acid gave more gas than one much weaker acted upon by the same current, apparently because it had less solvent power. if the acid were very strong, then a remarkable disappearance of oxygen took place; thus, one made by mixing two measures of strong oil of vitriol with one of water, gave forty-two volumes of hydrogen, but only twelve of oxygen. the hydrogen was very nearly the same with that evolved from acid of the specific gravity . . i have not yet had time to examine minutely the circumstances attending the disappearance of the oxygen in this case, but imagine it is due to the formation of oxywater, which thenard has shown is favoured by the presence of acid. . although not necessary for the practical use of the instrument i am describing, yet as connected with the important point of constant chemical action upon water, i now investigated the effects produced by an electro-electric current passing through aqueous solutions of acids, salts, and compounds, exceedingly different from each other in their nature, and found them to yield astonishingly uniform results. but many of them which are connected with a secondary action will be more usefully described hereafter ( .). . when solutions of caustic potassa or soda, or sulphate of magnesia, or sulphate of soda, were acted upon by the electric current, just as much oxygen and hydrogen was evolved from them as from the diluted sulphuric acid, with which they were compared. when a solution of ammonia, rendered a better conductor by sulphate of ammonia ( .), or a solution of subcarbonate of potassa was experimented with, the _hydrogen_ evolved was in the same quantity as that set free from the diluted sulphuric acid with which they were compared. hence _changes in the nature of the solution do not alter the constancy of electrolytic action upon water_. . i have already said, respecting large and small electrodes, that change of order caused no change in the general effect ( .). the same was the case with different solutions, or with different intensities; and however the circumstances of an experiment might be varied, the results came forth exceedingly consistent, and proved that the electro-chemical action was still the same. . i consider the foregoing investigation as sufficient to prove the very extraordinary and important principle with respect to water, _that when subjected to the influence of the electric current, a quantity of it is decomposed exactly proportionate to the quantity of electricity which has passed_, notwithstanding the thousand variations in the conditions and circumstances under which it may at the time be placed; and further, that when the interference of certain secondary effects ( . &c.), together with the solution or recombination of the gas and the evolution of air, are guarded against, _the products of the decomposition may be collected with such accuracy, as to afford a very excellent and valuable measurer of the electricity concerned in their evolution_. . the forms of instrument which i have given, figg. , , . ( . . .), are probably those which will be found most useful, as they indicate the quantity of electricity by the largest volume of gases, and cause the least obstruction to the passage of the current. the fluid which my present experience leads me to prefer, is a solution of sulphuric acid of specific gravity about . , or from that to . ; but it is very essential that there should be no organic substance, nor any vegetable acid, nor other body, which, by being liable to the action of the oxygen or hydrogen evolved at the electrodes ( . &c.), shall diminish their quantity, or add other gases to them. . in many cases when the instrument is used as a _comparative standard_, or even as _a measurer_, it may be desirable to collect the hydrogen only, as being less liable to absorption or disappearance in other ways than the oxygen; whilst at the same time its volume is so large, as to render it a good and sensible indicator. in such cases the first and second form of apparatus have been used, figg. , . ( . .). the indications obtained were very constant, the variations being much smaller than in those forms of apparatus collecting both gases; and they can also be procured when solutions are used in comparative experiments, which, yielding no oxygen or only secondary results of its action, can give no indications if the educts at both electrodes be collected. such is the case when solutions of ammonia, muriatic acid, chlorides, iodides, acetates or other vegetable salts, &c., are employed. . in a few cases, as where solutions of metallic salts liable to reduction at the negative electrode are acted upon, the oxygen may be advantageously used as the measuring substance. this is the case, for instance, with sulphate of copper. . there are therefore two general forms of the instrument which i submit as a measurer of electricity; one, in which both the gases of the water decomposed are collected ( . . .); and the other, in which a single gas, as the hydrogen only, is used ( . .). when referred to as a _comparative instrument_, (a use i shall now make of it very extensively,) it will not often require particular precaution in the observation; but when used as an _absolute measurer_, it will be needful that the barometric pressure and the temperature be taken into account, and that the graduation of the instruments should be to one scale; the hundredths and smaller divisions of a cubical inch are quite fit for this purpose, and the hundredth may be very conveniently taken as indicating a degree of electricity. . it can scarcely be needful to point out further than has been done how this instrument is to be used. it is to be introduced into the course of the electric current, the action of which is to be exerted anywhere else, and if ° or ° of electricity are to be measured out, either in one or several portions, the current, whether strong or weak, is to be continued until the gas in the tube occupies that number of divisions or hundredths of a cubical inch. or if a quantity competent to produce a certain effect is to be measured, the effect is to be obtained, and then the indication read off. in exact experiments it is necessary to correct the volume of gas for changes in temperature and pressure, and especially for moisture[a]. for the latter object the volta-electrometer (fig. .) is most accurate, as its gas can be measured over water, whilst the others retain it over acid or saline solutions. [a] for a simple table of correction for moisture, i may take the liberty of referring to my chemical manipulation, edition of , p. . . i have not hesitated to apply the term _degree_ ( .), in analogy with the use made of it with respect to another most important imponderable agent, namely, heat; and as the definite expansion of air, water, mercury, &c., is there made use of to measure heat, so the equally definite evolution of gases is here turned to a similar use for electricity. . the instrument offers the only _actual measurer_ of voltaic electricity which we at present possess. for without being at all affected by variations in time or intensity, or alterations in the current itself, of any kind, or from any cause, or even of intermissions of action, it takes note with accuracy of the quantity of electricity which has passed through it, and reveals that quantity by inspection; i have therefore named it a volta-electrometer. . another mode of measuring volta-electricity may be adopted with advantage in many cases, dependent on the quantities of metals or other substances evolved either as primary or as secondary results; but i refrain from enlarging on this use of the products, until the principles on which their constancy depends have been fully established ( . .); . by the aid of this instrument i have been able to establish the definite character of electro-chemical action in its most general sense; and i am persuaded it will become of the utmost use in the extensions of the science which these views afford. i do not pretend to have made its detail perfect, but to have demonstrated the truth of the principle, and the utility of the application[a]. [a] as early as the year , messrs. gay-lussac and thénard employed chemical decomposition as a measure of the electricity of the voltaic pile. see _recherches physico-chymiques_, p. . the principles and precautions by which it becomes an exact measure were of course not then known.--_dec. ._ ¶ vi. _on the primary or secondary character of the bodies evolved at the electrodes._ . before the _volta-electrometer_ could be employed in determining, as a _general law_, the constancy of electro-decomposition, it became necessary to examine a distinction, already recognised among scientific men, relative to the products of that action, namely, their primary or secondary character; and, if possible, by some general rule or principle, to decide when they were of the one or the other kind. it will appear hereafter that great mistakes inspecting electro-chemical action and its consequences have arisen from confounding these two classes of results together. . when a substance under decomposition yields at the electrodes those bodies uncombined and unaltered which the electric current has separated, then they may be considered as primary results, even though themselves compounds. thus the oxygen and hydrogen from water are primary results; and so also are the acid and alkali (themselves compound bodies) evolved from sulphate of soda. but when the substances separated by the current are changed at the electrodes before their appearance, then they give rise to secondary results, although in many cases the bodies evolved are elementary. . these secondary results occur in two ways, being sometimes due to the mutual action of the evolved substance and the matter of the electrode, and sometimes to its action upon the substances contained in the body itself under decomposition. thus, when carbon is made the positive electrode in dilute sulphuric acid, carbonic oxide and carbonic acid occasionally appear there instead of oxygen; for the latter, acting upon the matter of the electrode, produces these secondary results. or if the positive electrode, in a solution of nitrate or acetate of lead, be platina, then peroxide of lead appears there, equally a secondary result with the former, but now depending upon an action of the oxygen on a substance in the solution. again, when ammonia is decomposed by platina electrodes, nitrogen appears at the _anode_[a]; but though an _elementary_ body, it is a _secondary_ result in this case, being derived from the chemical action of the oxygen electrically evolved there, upon the ammonia in the surrounding solution ( .). in the same manner when aqueous solutions of metallic salts are decomposed by the current, the metals evolved at the _cathode_, though elements, are _always_ secondary results, and not immediate consequences of the decomposing power of the electric current. [a] annales de chimie, , tom. li. p. . . many of these secondary results are extremely valuable; for instance, all the interesting compounds which m. becquerel has obtained by feeble electric currents are of this nature; but they are essentially chemical, and must, in the theory of electrolytic action, be carefully distinguished from those which are directly due to the action of the electric current. . the nature of the substances evolved will often lead to a correct judgement of their primary or secondary character, but is not sufficient alone to establish that point. thus, nitrogen is said to be attracted sometimes by the positive and sometimes by the negative electrode, according to the bodies with which it may be combined ( . .), and it is on such occasions evidently viewed as a primary result[a]; but i think i shall show, that, when it appears at the positive electrode, or rather at the _anode_, it is a secondary result ( .). thus, also, sir humphry davy[b], and with him the great body of chemical philosophers, (including myself,) have given the appearance of copper, lead, tin, silver, gold, &c., at the negative electrode, when their aqueous solutions were acted upon by the voltaic current, as proofs that the metals, as a class, were attracted to that surface; thus assuming the metal, in each case, to be a primary result. these, however, i expect to prove, are all secondary results; the mere consequence of chemical action, and no proofs either of the attraction or of the law announced respecting their places[c]. [a] annales de chimie, , tom. li. p. . [b] elements of chemical philosophy, pp. . . [c] it is remarkable that up to it was the received opinion that the metals were reduced by the nascent hydrogen. at that date the general opinion was reversed by hisinger and berzelius (annales de chimie, , tom. li. p. ,), who stated that the metals were evolved directly by the electricity: in which opinion it appears, from that time, davy coincided (philosophical transactions, , p. ). . but when we take to our assistance the law of _constant electro-chemical action_ already proved with regard to water ( .), and which i hope to extend satisfactorily to all bodies ( .), and consider the _quantities_ as well as the _nature_ of the substances set free, a generally accurate judgement of the primary or secondary character of the results may be formed: and this important point, so essential to the theory of electrolyzation, since it decides what are the particles directly under the influence of the current, (distinguishing them from such as are not affected,) and what are the results to be expected, may be established with such degree of certainty as to remove innumerable ambiguities and doubtful considerations from this branch of the science. . let us apply these principles to the case of ammonia, and the supposed determination of nitrogen to one or the other _electrode_ ( . ,). a pure strong solution of ammonia is as bad a conductor, and therefore as little liable to electrolyzation, as pure water; but when sulphate of ammonia is dissolved in it, the whole becomes a conductor; nitrogen _almost_ and occasionally _quite_ pure is evolved at the _anode_, and hydrogen at the _cathode_; the ratio of the volume of the former to that of the latter varying, but being as to about or . this result would seem at first to imply that the electric current had decomposed ammonia, and that the nitrogen had been determined towards the positive electrode. but when the electricity used was measured out by the volta-electrometer ( . .), it was found that the hydrogen obtained was exactly in the proportion which would have been supplied by decomposed water, whilst the nitrogen had no certain or constant relation whatever. when, upon multiplying experiments, it was found that, by using a stronger or weaker solution, or a more or less powerful battery, the gas evolved at the _anode_ was a mixture of oxygen and nitrogen, varying both in proportion and absolute quantity, whilst the hydrogen at the _cathode_ remained constant, no doubt could be entertained that the nitrogen at the _anode_ was a secondary result, depending upon the chemical action of the nascent oxygen, determined to that surface by the electric current, upon the ammonia in solution. it was the water, therefore, which was electrolyzed, not the ammonia. further, the experiment gives no real indication of the tendency of the element nitrogen to either one electrode or the other; nor do i know of any experiment with nitric acid, or other compounds of nitrogen, which shows the tendency of this element, under the influence of the electric current, to pass in either direction along its course. . as another illustration of secondary results, the effects on a solution of acetate of potassa, may be quoted. when a very strong solution was used, more gas was evolved at the _anode_ than at the _cathode_, in the proportion of to nearly: that from the _anode_ was a mixture of carbonic oxide and carbonic acid; that from the _cathode_ pure hydrogen. when a much weaker solution was used, less gas was evolved at the _anode_ than at the _cathode_; and it now contained carburetted hydrogen, as well as carbonic oxide and carbonic acid. this result of carburetted hydrogen at the positive electrode has a very anomalous appearance, if considered as an immediate consequence of the decomposing power of the current. it, however, as well as the carbonic oxide and acid, is only a _secondary result_; for it is the water alone which suffers electro-decomposition, and it is the oxygen eliminated at the _anode_ which, reacting on the acetic acid, in the midst of which it is evolved, produces those substances that finally appear there. this is fully proved by experiments with the volta-electrometer ( .); for then the hydrogen evolved from the acetate at the _cathode_ is always found to be definite, being exactly proportionate to the electricity which has passed through the solution, and, in quantity, the same as the hydrogen evolved in the volta-electrometer itself. the appearance of the carbon in combination with the hydrogen at the positive electrode, and its non-appearance at the negative electrode, are in curious contrast with the results which might have been expected from the law usually accepted respecting the final places of the elements. . if the salt in solution be an acetate of lead, then the results at both electrodes are secondary, and cannot be used to estimate or express the amount of electro-chemical action, except by a circuitous process ( .). in place of oxygen or even the gases already described ( .), peroxide of lead now appears at the positive, and lead itself at the negative electrode. when other metallic solutions are used, containing, for instance, peroxides, as that of copper, combined with this or any other decomposable acid, still more complicated results will be obtained; which, viewed as direct results of the electro-chemical action, will, in their proportions, present nothing but confusion, but will appear perfectly harmonious and simple if they be considered as secondary results, and will accord in their proportions with the oxygen and hydrogen evolved from water by the action of a definite quantity of electricity. . i have experimented upon many bodies, with a view to determine whether the results were primary or secondary. i have been surprised to find how many of them, in ordinary cases, are of the latter class, and how frequently water is the only body electrolyzed in instances where other substances have been supposed to give way. some of these results i will give in as few words as possible. . _nitric acid._--when very strong, it conducted well, and yielded oxygen at the positive electrode. no gas appeared at the negative electrode; but nitrous acid, and apparently nitric oxide, were formed there, which, dissolving, rendered the acid yellow or red, and at last even effervescent, from the spontaneous separation of nitric oxide. upon diluting the acid with its bulk or more of water, gas appeared at the negative electrode. its quantity could be varied by variations, either in the strength of the acid or of the voltaic current: for that acid from which no gas separated at the _cathode_, with a weak voltaic battery, did evolve gas there with a stronger; and that battery which evolved no gas there with a strong acid, did cause its evolution with an acid more dilute. the gas at the _anode_ was always oxygen; that at the _cathode_ hydrogen. when the quantity of products was examined by the volta-electrometer ( .), the oxygen, whether from strong or weak acid, proved to be in the same proportion as from water. when the acid was diluted to specific gravity . , or less, the hydrogen also proved to be the same in quantity as from water. hence i conclude that the nitric acid does not undergo electrolyzation, but the water only; that the oxygen at the _anode_ is always a primary result, but that the products at the _cathode_ are often secondary, and due to the reaction of the hydrogen upon the nitric acid. . _nitre._--a solution of this salt yields very variable results, according as one or other form of tube is used, or as the electrodes are large or small. sometimes the whole of the hydrogen of the water decomposed may be obtained at the negative electrode; at other times, only a part of it, because of the ready formation of secondary results. the solution is a very excellent conductor of electricity. . _nitrate of ammonia_, in aqueous solution, gives rise to secondary results very varied and uncertain in their proportions. . _sulphurous acid._--pure liquid sulphurous acid does not conduct nor suffer decomposition by the voltaic current[a], but, when dissolved in water, the solution acquires conducting power, and is decomposed, yielding oxygen at the _anode_, and hydrogen and sulphur at the _cathode_. [a] see also de la rive, bibliothèque universelle, tom. xl. p. ; or quarterly journal of science, vol. xxvii. p, . . a solution containing sulphuric acid in addition to the sulphurous acid, was a better conductor. it gave very little gas at either electrode: that at the _anode_ was oxygen, that at the _cathode_ pure hydrogen. from the _cathode_ also rose a white turbid stream, consisting of diffused sulphur, which soon rendered the whole solution milky. the volumes of gases were in no regular proportion to the quantities evolved from water in the voltameter. i conclude that the sulphurous acid was not at all affected by the electric current in any of these cases, and that the water present was the only body electro-chemically decomposed; that, at the _anode_, the oxygen from the water converted the sulphurous acid into sulphuric acid, and, at the _cathode_, the hydrogen electrically evolved decomposed the sulphurous acid, combining with its oxygen, and setting its sulphur free. i conclude that the sulphur at the negative electrode was only a secondary result; and, in fact, no part of it was found combined with the small portion of hydrogen which escaped when weak solutions of sulphurous acid were used. . _sulphuric acid._--i have already given my reasons for concluding that sulphuric acid is not electrolyzable, i.e. not decomposable directly by the electric current, but occasionally suffering by a secondary action at the _cathode_ from the hydrogen evolved there ( .). in the year , davy considered the sulphur from sulphuric acid as the result of the action of the nascent hydrogen[a]. in , hisinger and berzelius stated that it was the direct result of the action of the voltaic pile[b], an opinion which from that time davy seems to have adopted, and which has since been commonly received by all. the change of my own opinion requires that i should correct what i have already said of the decomposition of sulphuric acid in a former series of these researches ( .): i do not now think that the appearance of the sulphur at the negative electrode is an immediate consequence of electrolytic action. [a] nicholson's quarterly journal, vol. iv. pp. , . [b] annales de chimie, , tom. li. p. . . _muriatic acid._--a strong solution gave hydrogen at the negative electrode, and chlorine only at the positive electrode; of the latter, a part acted on the platina and a part was dissolved. a minute bubble of gas remained; it was not oxygen, but probably air previously held in solution. . it was an important matter to determine whether the chlorine was a primary result, or only a secondary product, due to the action of the oxygen evolved from water at the _anode_ upon the muriatic acid; i.e. whether the muriatic acid was electrolyzable, and if so, whether the decomposition was _definite_. . the muriatic acid was gradually diluted. one part with six of water gave only chlorine at the _anode_. one part with eight of water gave only chlorine; with nine of water, a little oxygen appeared with the chlorine; but the occurrence or non-occurrence of oxygen at these strengths depended, in part, on the strength of the voltaic battery used. with fifteen parts of water, a little oxygen, with much chlorine, was evolved at the _anode_. as the solution was now becoming a bad conductor of electricity, sulphuric acid was added to it: this caused more ready decomposition, but did not sensibly alter the proportion of chlorine and oxygen. . the muriatic acid was now diluted with times its volume of dilute sulphuric acid. it still gave a large proportion of chlorine at the _anode_, mingled with oxygen; and the result was the same, whether a voltaic battery of pairs of plates or one containing only pairs were used. with acid of this strength, the oxygen evolved at the _anode_ was to the hydrogen at the _cathode_, in volume, as is to ; and therefore the chlorine would have been volumes, had it not been dissolved by the fluid. . next with respect to the quantity of elements evolved. on using the volta-electrometer, it was found that, whether the strongest or the weakest muriatic acid were used, whether chlorine alone or chlorine mingled with oxygen appeared at the _anode_, still the hydrogen evolved at the _cathode_ was a constant quantity, i.e. exactly the same as the hydrogen which the _same quantity of electricity_ could evolve from water. . this constancy does not decide whether the muriatic acid is electrolyzed or not, although it proves that if so, it must be in definite proportions to the quantity of electricity used. other considerations may, however, be allowed to decide the point. the analogy between chlorine and oxygen, in their relations to hydrogen, is so strong, as to lead almost to the certainty, that, when combined with that element, they would perform similar parts in the process of electro-decomposition. they both unite with it in single proportional or equivalent quantities; and the number of proportionals appearing to have an intimate and important relation to the decomposability of a body ( .), those in muriatic acid, as well as in water, are the most favourable, or those perhaps even necessary, to decomposition. in other binary compounds of chlorine also, where nothing equivocal depending on the simultaneous presence of it and oxygen is involved, the chlorine is directly eliminated at the _anode_ by the electric current. such is the case with the chloride of lead ( .), which may be justly compared with protoxide of lead ( .), and stands in the same relation to it as muriatic acid to water. the chlorides of potassium, sodium, barium, &c., are in the same relation to the protoxides of the same metals and present the same results under the influence of the electric current ( .). . from all the experiments, combined with these considerations, i conclude that muriatic acid is decomposed by the direct influence of the electric current, and that the quantities evolved are, and therefore the chemical action is, _definite for a definite quantity of electricity_. for though i have not collected and measured the chlorine, in its separate state, at the _anode_, there can exist no doubt as to its being proportional to the hydrogen at the _cathode_; and the results are therefore sufficient to establish the general law of _constant electro-chemical action_ in the case of muriatic acid. . in the dilute acid ( .), i conclude that a part of the water is electro-chemically decomposed, giving origin to the oxygen, which appears mingled with the chlorine at the _anode_. the oxygen _may_ be viewed as a secondary result; but i incline to believe that it is not so; for, if it were, it might be expected in largest proportion from the stronger acid, whereas the reverse is the fact. this consideration, with others, also leads me to conclude that muriatic acid is more easily decomposed by the electric current than water; since, even when diluted with eight or nine times its quantity of the latter fluid, it alone gives way, the water remaining unaffected. . _chlorides._--on using solutions of chlorides in water,--for instance, the chlorides of sodium or calcium,--there was evolution of chlorine only at the positive electrode, and of hydrogen, with the oxide of the base, as soda or lime, at the negative electrode. the process of decomposition may be viewed as proceeding in two or three ways, all terminating in the same results. perhaps the simplest is to consider the chloride as the substance electrolyzed, its chlorine being determined to and evolved at the _anode_, and its metal passing to the _cathode_, where, finding no more chlorine, it acts upon the water, producing hydrogen and an oxide as secondary results. as the discussion would detain me from more important matter, and is not of immediate consequence, i shall defer it for the present. it is, however, of _great consequence_ to state, that, on using the volta-electrometer, the hydrogen in both cases was definite; and if the results do not prove the definite decomposition of chlorides, (which shall be proved elsewhere,-- . . .,) they are not in the slightest degree opposed to such a conclusion, and do support the _general law_. . _hydriodic acid._--a solution of hydriodic acid was affected exactly in the same manner as muriatic acid. when strong, hydrogen was evolved at the negative electrode, in definite proportion to the quantity of electricity which had passed, i.e. in the same proportion as was evolved by the same current from water; and iodine without any oxygen was evolved at the positive electrode. but when diluted, small quantities of oxygen appeared with the iodine at the _anode_, the proportion of hydrogen at the _cathode_ remaining undisturbed. . i believe the decomposition of the hydriodic acid in this case to be direct, for the reasons already given respecting muriatic acid ( . .). . _iodides._--a solution of iodide of potassium being subjected to the voltaic current, iodine appeared at the positive electrode (without any oxygen), and hydrogen with free alkali at the negative electrode. the same observations as to the mode of decomposition are applicable here as were made in relation to the chlorides when in solution ( .). . _hydro-fluoric acid and fluorides._--solution of hydrofluoric acid did not appear to be decomposed under the influence of the electric current: it was the water which gave way apparently. the fused fluorides were electrolysed ( .); but having during these actions obtained _fluorine_ in the separate state, i think it better to refer to a future series of these researches, in which i purpose giving a fuller account of the results than would be consistent with propriety here[a]. [a] i have not obtained fluorine: my expectations, amounting to conviction, passed away one by one when subjected to rigorous examination; some very singular results were obtained; and to one of these i refer at .--_dec. ._ . _hydro-cyanic acid_ in solution conducts very badly. the definite proportion of hydrogen (equal to that from water) was set free at the _cathode_, whilst at the _anode_ a small quantity of oxygen was evolved and apparently a solution of cyanogen formed. the action altogether corresponded with that on a dilute muriatic or hydriodic acid. when the hydrocyanic acid was made a better conductor by sulphuric acid, the same results occurred. _cyanides._--with a solution of the cyanide of potassium, the result was precisely the same as with a chloride or iodide. no oxygen was evolved at the positive electrode, but a brown solution formed there. for the reasons given when speaking of the chlorides ( .), and because a fused cyanide of potassium evolves cyanogen at the positive electrode[a], i incline to believe that the cyanide in solution is _directly_ decomposed. [a] it is a very remarkable thing to see carbon and nitrogen in this case determined powerfully towards the positive surface of the voltaic battery; but it is perfectly in harmony with the theory of electro-chemical decomposition which i have advanced. . _ferro-cyanic acid_ and the _ferro-cyanides_, as also _sulpho-cyanic acid_ and the _sulpho-cyanides_, presented results corresponding with those just described ( .). . _acetic acid._--glacial acetic acid, when fused ( .), is not decomposed by, nor does it conduct, electricity. on adding a little water to it, still there were no signs of action; on adding more water, it acted slowly and about as pure water would do. dilute sulphuric acid was added to it in order to make it a better conductor; then the definite proportion of hydrogen was evolved at the _cathode_, and a mixture of oxygen in very deficient quantity, with carbonic acid, and a little carbonic oxide, at the _anode_. hence it appears that acetic acid is not electrolyzable, but that a portion of it is decomposed by the oxygen evolved at the _anode_, producing secondary results, varying with the strength of the acid, the intensity of the current, and other circumstances. . _acetates._--one of these has been referred to already, as affording only secondary results relative to the acetic acid ( .). with many of the metallic acetates the results at both electrodes are secondary ( . .). acetate of soda fused and anhydrous is directly decomposed, being, as i believe, a true electrolyte, and evolving soda and acetic acid at the _cathode_ and _anode_. these however have no sensible duration, but are immediately resolved into other substances; charcoal, sodiuretted hydrogen, &c., being set free at the former, and, as far as i could judge under the circumstances, acetic acid mingled with carbonic oxide, carbonic acid, &c. at the latter. . _tartaric acid._--pure solution of tartaric acid is almost as bad a conductor as pure water. on adding sulphuric acid, it conducted well, the results at the positive electrode being primary or secondary in different proportions, according to variations in the strength of the acid and the power of the electric current ( .). alkaline tartrates gave a large proportion of secondary results at the positive electrode. the hydrogen at the negative electrode remained constant unless certain triple metallic salts were used. . solutions, of salts containing other vegetable acids, as the benzoates; of sugar, gum, &c., dissolved in dilute sulphuric acid; of resin, albumen, &c., dissolved in alkalies, were in turn submitted to the electrolytic power of the voltaic current. in all these cases, secondary results to a greater or smaller extent were produced at the positive electrode. . in concluding this division of these researches, it cannot but occur to the mind that the final result of the action of the electric current upon substances, placed between the electrodes, instead of being simple may be very complicated. there are two modes by which these substances may be decomposed, either by the direct force of the electric current, or by the action of bodies which that current may evolve. there are also two modes by which new compounds may be formed, i.e. by combination of the evolving substances whilst in their nascent state ( .), directly with the matter of the electrode; or else their combination with those bodies, which being contained in, or associated with, the body suffering decomposition, are necessarily present at the _anode_ and _cathode_. the complexity is rendered still greater by the circumstance that two or more of these actions may occur simultaneously, and also in variable proportions to each other. but it may in a great measure be resolved by attention to the principles already laid down ( .). . when _aqueous_ solutions of bodies are used, secondary results are exceedingly frequent. even when the water is not present in large quantity, but is merely that of combination, still secondary results often ensue: for instance, it is very possible that in sir humphry davy's decomposition of the hydrates of potassa and soda, a part of the potassium produced was the result of a secondary action. hence, also, a frequent cause for the disappearance of the oxygen and hydrogen which would otherwise be evolved: and when hydrogen does _not_ appear at the _cathode_ in an _aqueous solution_, it perhaps always indicates that a secondary action has taken place there. no exception to this rule has as yet occurred to my observation. . secondary actions are _not confined to aqueous solutions_, or cases where water is present. for instance, various chlorides acted upon, when fused ( .), by platina electrodes, have the chlorine determined electrically to the _anode_. in many cases, as with the chlorides of lead, potassium, barium, &c., the chlorine acts on the platina and forms a compound with it, which dissolves; but when protochloride of tin is used, the chlorine at the _anode_ does not act upon the platina, but upon the chloride already there, forming a perchloride which rises in vapour ( . .). these are, therefore, instances of secondary actions of both kinds, produced in bodies containing no water. . the production of boron from fused borax ( . .) is also a case of secondary action; for boracic acid is not decomposable by electricity ( .), and it was the sodium evolved at the _cathode_ which, re-acting on the boracic acid around it, took oxygen from it and set boron free in the experiments formerly described. . secondary actions have already, in the hands of m. becquerel, produced many interesting results in the formation of compounds; some of them new, others imitations of those occurring naturally[a]. it is probable they may prove equally interesting in an opposite direction, i.e. as affording cases of analytic decomposition. much information regarding the composition, and perhaps even the arrangement, of the particles of such bodies as the vegetable acids and alkalies, and organic compounds generally, will probably be obtained by submitting them to the action of nascent oxygen, hydrogen, chlorine, &c. at the electrodes; and the action seems the more promising, because of the thorough command which we possess over attendant circumstances, such as the strength of the current, the size of the electrodes, the nature of the decomposing conductor, its strength, &c., all of which may be expected to have their corresponding influence upon the final result. . it is to me a great satisfaction that the extreme variety of secondary results has presented nothing opposed to the doctrine of a constant and definite electro-chemical action, to the particular consideration of which i shall now proceed. ¶ vii. _on the definite nature and extent of electro-chemical decomposition._ . in the third series of these researches, after proving the identity of electricities derived from different sources, and showing, by actual measurement, the extraordinary quantity of electricity evolved by a very feeble voltaic arrangement ( . .), i announced a law, derived from experiment, which seemed to me of the utmost importance to the science of electricity in general, and that branch of it denominated electro-chemistry in particular. the law was expressed thus: _the chemical power of a current of electricity is in direct proportion to the absolute quantity of electricity which passes_ ( .). [a] annales de chimie, tom, xxxv. p. . . in the further progress of the successive investigations, i have had frequent occasion to refer to the same law, sometimes in circumstances offering powerful corroboration of its truth ( . . .); and the present series already supplies numerous new cases in which it holds good ( . . . .). it is now my object to consider this great principle more closely, and to develope some of the consequences to which it leads. that the evidence for it may be the more distinct and applicable, i shall quote cases of decomposition subject to as few interferences from secondary results as possible, effected upon bodies very simple, yet very definite in their nature. . in the first place, i consider the law as so fully established with respect to the decomposition of _water_, and under so many circumstances which might be supposed, if anything could, to exert an influence over it, that i may be excused entering into further detail respecting that substance, or even summing up the results here ( .). i refer, therefore, to the whole of the subdivision of this series of researches which contains the account of the _volta-electrometer_ ( . &c.). . in the next place, i also consider the law as established with respect to _muriatic acid_ by the experiments and reasoning already advanced, when speaking of that substance, in the subdivision respecting primary and secondary results ( . &c.). . i consider the law as established also with regard to _hydriodic acid_ by the experiments and considerations already advanced in the preceding division of this series of researches ( . .). . without speaking with the same confidence, yet from the experiments described, and many others not described, relating to hydro-fluoric, hydro-cyanic, ferro-cyanic, and sulpho-cyanic acids ( . . .), and from the close analogy which holds between these bodies and the hydracids of chlorine, iodine, bromine, &c., i consider these also as coming under subjection to the law, and assisting to prove its truth. . in the preceding cases, except the first, the water is believed to be inactive; but to avoid any ambiguity arising from its presence, i sought for substances from which it should be absent altogether; and, taking advantage of the law of conduction already developed ( . &c.), i soon found abundance, amongst which _protochloride of tin_ was first subjected to decomposition in the following manner. a piece of platina wire had one extremity coiled up into a small knob, and, having been carefully weighed, was sealed hermetically into a piece of bottle-glass tube, so that the knob should be at the bottom of the tube within (fig. .). the tube was suspended by a piece of platina wire, so that the heat of a spirit-lamp could be applied to it. recently fused protochloride of tin was introduced in sufficient quantity to occupy, when melted, about one-half of the tube; the wire of the tube was connected with a volta-electrometer ( .), which was itself connected with the negative end of a voltaic battery; and a platina wire connected with the positive end of the same battery was dipped into the fused chloride in the tube; being however so bent, that it could not by any shake of the hand or apparatus touch the negative electrode at the bottom of the vessel. the whole arrangement is delineated in fig. . . under these circumstances the chloride of tin was decomposed: the chlorine evolved at the positive electrode formed bichloride of tin ( .), which passed away in fumes, and the tin evolved at the negative electrode combined with the platina, forming an alloy, fusible at the temperature to which the tube was subjected, and therefore never occasioning metallic communication through the decomposing chloride. when the experiment had been continued so long as to yield a reasonable quantity of gas in the volta-electrometer, the battery connexion was broken, the positive electrode removed, and the tube and remaining chloride allowed to cool. when cold, the tube was broken open, the rest of the chloride and the glass being easily separable from the platina wire and its button of alloy. the latter when washed was then reweighed, and the increase gave the weight of the tin reduced. . i will give the particular results of one experiment, in illustration of the mode adopted in this and others, the results of which i shall have occasion to quote. the negative electrode weighed at first grains; after the experiment, it, with its button of alloy, weighed . grains. the tin evolved by the electric current at the _cathode_: weighed therefore . grains. the quantity of oxygen and hydrogen collected in the volta-electrometer = . cubic inches. as cubic inches of oxygen and hydrogen, in the proportions to form water, may be considered as weighing . grains, the . cubic inches would weigh . of a grain; that being, therefore, the weight of water decomposed by the same electric current as was able to decompose such weight of protochloride of tin as could yield . grains of metal. now . : . :: the equivalent of water is to . , which should therefore be the equivalent of tin, if the experiment had been made without error, and if the electro-chemical decomposition _is in this case also definite_. in some chemical works is given as the chemical equivalent of tin, in others . . both are so near to the result of the experiment, and the experiment itself is so subject to slight causes of variation (as from the absorption of gas in the volta-electrometer ( .), &c.), that the numbers leave little doubt of the applicability of the _law of definite action_ in this and all similar cases of electro-decomposition. . it is not often i have obtained an accordance in numbers so near as that i have just quoted. four experiments were made on the protochloride of tin, the quantities of gas evolved in the volta-electrometer being from . to . cubic inches. the average of the four experiments gave . as the electro-chemical equivalent for tin. . the chloride remaining after the experiment was pure protochloride of tin; and no one can doubt for a moment that the equivalent of chlorine had been evolved at the _anode_, and, having formed bichloride of tin as a secondary result, had passed away. . _chloride of lead_ was experimented upon in a manner exactly similar, except that a change was made in the nature of the positive electrode; for as the chlorine evolved at the _anode_ forms no perchloride of lead, but acts directly upon the platina, it produces, if that metal be used, a solution of chloride of platina in the chloride of lead; in consequence of which a portion of platina can pass to the _cathode_, and would then produce a vitiated result. i therefore sought for, and found in plumbago, another substance, which could be used safely as the positive electrode in such bodies as chlorides, iodides, &c. the chlorine or iodine does not act upon it, but is evolved in the free state; and the plumbago has no re-action, under the circumstances, upon the fused chloride or iodide in which it is plunged. even if a few particles of plumbago should separate by the heat or the mechanical action of the evolved gas, they can do no harm in the chloride. . the mean of three experiments gave the number of . as the equivalent for lead. the chemical equivalent is . . the deficiency in my experiments i attribute to the solution of part of the gas ( .) in the volta-electrometer; but the results leave no doubt on my mind that both the lead and the chlorine are, in this case, evolved in _definite quantities_ by the action of a given quantity of electricity ( . &c.). . _chloride of antimony._--it was in endeavouring to obtain the electro-chemical equivalent of antimony from the chloride, that i found reasons for the statement i have made respecting the presence of water in it in an earlier part of these researches ( . . &c.). . i endeavoured to experiment upon the _oxide of lead_ obtained by fusion and ignition of the nitrate in a platina crucible, but found great difficulty, from the high temperature required for perfect fusion, and the powerful fluxing qualities of the substance. green-glass tubes repeatedly failed. i at last fused the oxide in a small porcelain crucible, heated fully in a charcoal fire; and, as it is was essential that the evolution of the lead at the _cathode_ should take place beneath the surface, the negative electrode was guarded by a green-glass tube, fused around it in such a _manner as to expose only the knob of platina_ at the lower end (fig. .), so that it could be plunged beneath the surface, and thus exclude contact of air or oxygen with the lead reduced there. a platina wire was employed for the positive electrode, that metal not being subject to any action from the oxygen evolved against it. the arrangement is given in fig. . . in an experiment of this kind the equivalent for the lead came out . , which is very much too small. this, i believe, was because of the small interval between the positive and negative electrodes in the oxide of lead; so that it was not unlikely that some of the froth and bubbles formed by the oxygen at the _anode_ should occasionally even touch the lead reduced at the _cathode_, and re-oxidize it. when i endeavoured to correct this by having more litharge, the greater heat required to keep it all fluid caused a quicker action on the crucible, which was soon eaten through, and the experiment stopped. . in one experiment of this kind i used borate of lead ( . .). it evolves lead, under the influence of the electric current, at the _anode_, and oxygen at the _cathode_; and as the boracic acid is not either directly ( .) or incidentally decomposed during the operation, i expected a result dependent on the oxide of lead. the borate is not so violent a flux as the oxide, but it requires a higher temperature to make it quite liquid; and if not very hot, the bubbles of oxygen cling to the positive electrode, and retard the transfer of electricity. the number for lead came out . , which is so near to . as to show that the action of the current had been definite. . _oxide of bismuth._--i found this substance required too high a temperature, and acted too powerfully as a flux, to allow of any experiment being made on it, without the application of more time and care than i could give at present. . the ordinary _protoxide of antimony_, which consists of one proportional of metal and one and a half of oxygen, was subjected to the action of the electric current in a green-glass tube ( .), surrounded by a jacket of platina foil, and heated in a charcoal fire. the decomposition began and proceeded very well at first, apparently indicating, according to the general law ( . .), that this substance was one containing such elements and in such proportions as made it amenable to the power of the electric current. this effect i have already given reasons for supposing may be due to the presence of a true protoxide, consisting of single proportionals ( . .). the action soon diminished, and finally ceased, because of the formation of a higher oxide of the metal at the positive electrode. this compound, which was probably the peroxide, being infusible and insoluble in the protoxide, formed a crystalline crust around the positive electrode; and thus insulating it, prevented the transmission of the electricity. whether, if it had been fusible and still immiscible, it would have decomposed, is doubtful, because of its departure from the required composition ( .). it was a very natural secondary product at the positive electrode ( .). on opening the tube it was found that a little antimony had been separated at the negative electrode; but the quantity was too small to allow of any quantitative result being obtained[a]. [a] this paragraph is subject to the corrective note now appended to paragraph .--_dec. ._ . _iodide of lead._--this substance can be experimented with in tubes heated by a spirit-lamp ( .); but i obtained no good results from it, whether i used positive electrodes of platina or plumbago. in two experiments the numbers for the lead came out only . and . , instead of . . this i attribute to the formation of a periodide at the positive electrode, which, dissolving in the mass of liquid iodide, came in contact with the lead evolved at the negative electrode, and dissolved part of it, becoming itself again protiodide. such a periodide does exist; and it is very rarely that the iodide of lead formed by precipitation, and well-washed, can be fused without evolving much iodine, from the presence of this percompound; nor does crystallization from its hot aqueous solution free it from this substance. even when a little of the protiodide and iodine are merely rubbed together in a mortar, a portion of the periodide is formed. and though it is decomposed by being fused and heated to dull redness for a few minutes, and the whole reduced to protiodide, yet that is not at all opposed to the possibility, that a little of that which is formed in great excess of iodine at the _anode_, should be carried by the rapid currents in the liquid into contact with the _cathode_. . this view of the result was strengthened by a third experiment, where the space between the electrodes was increased to one third of an inch; for now the interfering effects were much diminished, and the number of the lead came out . ; and it was fully confirmed by the results obtained in the cases of _transfer_ to be immediately described ( .). the experiments on iodide of lead therefore offer no exception to the _general law_ under consideration, but on the contrary may, from general considerations, be admitted as included in it. . _protiodide of tin._--this substance, when fused ( .), conducts and is decomposed by the electric current, tin is evolved at the _anode_, and periodide of tin as a secondary result ( . .) at the _cathode_. the temperature required for its fusion is too high to allow of the production of any results fit for weighing. . _iodide of potassium_ was subjected to electrolytic action in a tube, like that in fig. . ( .). the negative electrode was a globule of lead, and i hoped in this way to retain the potassium, and obtain results that could be weighed and compared with the volta-electrometer indication; but the difficulties dependent upon the high temperature required, the action upon the glass, the fusibility of the platina induced by the presence of the lead, and other circumstances, prevented me from procuring such results. the iodide was decomposed with the evolution of iodine at the _anode_, and of potassium at the _cathode_, as in former cases. . in some of these experiments several substances were placed in succession, and decomposed simultaneously by the same electric current: thus, protochloride of tin, chloride of lead, and water, were thus acted on at once. it is needless to say that the results were comparable, the tin, lead, chlorine, oxygen, and hydrogen evolved being _definite in quantity_ and electro-chemical equivalents to each other. * * * * * . let us turn to another kind of proof of the _definite chemical action of electricity_. if any circumstances could be supposed to exert an influence over the quantity of the matters evolved during electrolytic action, one would expect them to be present when electrodes of different substances, and possessing very different chemical affinities for such matters, were used. platina has no power in dilute sulphuric acid of combining with the oxygen at the _anode_, though the latter be evolved in the nascent state against it. copper, on the other hand, immediately unites with the oxygen, as the electric current sets it free from the hydrogen; and zinc is not only able to combine with it, but can, without any help from the electricity, abstract it directly from the water, at the same time setting torrents of hydrogen free. yet in cases where these three substances were used as the positive electrodes in three similar portions of the same dilute sulphuric acid, specific gravity . , precisely the same quantity of water was decomposed by the electric current, and precisely the same quantity of hydrogen set free at the _cathodes_ of the three solutions. . the experiment was made thus. portions of the dilute sulphuric acid were put into three basins. three volta-electrometer tubes, of the form figg. . . were filled with the same acid, and one inverted in each basin ( .). a zinc plate, connected with the positive end of a voltaic battery, was dipped into the first basin, forming the positive electrode there, the hydrogen, which was abundantly evolved from it by the direct action of the acid, being allowed to escape. a copper plate, which dipped into the acid of the second basin, was connected with the negative electrode of the _first_ basin; and a platina plate, which dipped into the acid of the third basin, was connected with the negative electrode of the _second_ basin. the negative electrode of the third basin was connected with a volta-electrometer ( .), and that with the negative end of the voltaic battery. . immediately that the circuit was complete, the _electro-chemical action_ commenced in all the vessels. the hydrogen still rose in, apparently, undiminished quantities from the positive zinc electrode in the first basin. no oxygen was evolved at the positive copper electrode in the second basin, but a sulphate of copper was formed there; whilst in the third basin the positive platina electrode evolved pure oxygen gas, and was itself unaffected. but in _all_ the basins the hydrogen liberated at the _negative_ platina electrodes was the _same in quantity_, and the same with the volume of hydrogen evolved in the volta-electrometer, showing that in all the vessels the current had decomposed an equal quantity of water. in this trying case, therefore, the _chemical action of electricity_ proved to be _perfectly definite_. . a similar experiment was made with muriatic acid diluted with its bulk of water. the three positive electrodes were zinc, silver, and platina; the first being able to separate and combine with the chlorine _without_ the aid of the current; the second combining with the chlorine only after the current had set it free; and the third rejecting almost the whole of it. the three negative electrodes were, as before, platina plates fixed within glass tubes. in this experiment, as in the former, the quantity of hydrogen evolved at the _cathodes_ was the same for all, and the same as the hydrogen evolved in the volta-electrometer. i have already given my reasons for believing that in these experiments it is the muriatic acid which is directly decomposed by the electricity ( .); and the results prove that the quantities so decomposed are _perfectly definite_ and proportionate to the quantity of electricity which has passed. . in this experiment the chloride of silver formed in the second basin retarded the passage of the current of electricity, by virtue of the law of conduction before described ( .), so that it had to be cleaned off four or five times during the course of the experiment; but this caused no difference between the results of that vessel and the others. . charcoal was used as the positive electrode in both sulphuric and muriatic acids ( . .); but this change produced no variation of the results. a zinc positive electrode, in sulphate of soda or solution of common salt, gave the same constancy of operation. . experiments of a similar kind were then made with bodies altogether in a different state, i.e. with _fused_ chlorides, iodides, &c. i have already described an experiment with fused chloride of silver, in which the electrodes were of metallic silver, the one rendered negative becoming increased and lengthened by the addition of metal, whilst the other was dissolved and eaten away by its abstraction. this experiment was repeated, two weighed pieces of silver wire being used as the electrodes, and a volta-electrometer included in the circuit. great care was taken to withdraw the negative electrodes so regularly and steadily that the crystals of reduced silver should not form a _metallic_ communication beneath the surface of the fused chloride. on concluding the experiment the positive electrode was re-weighed, and its loss ascertained. the mixture of chloride of silver, and metal, withdrawn in successive portions at the negative electrode, was digested in solution of ammonia, to remove the chloride, and the metallic silver remaining also weighed: it was the reduction at the _cathode_, and exactly equalled the solution at the _anode_; and each portion was as nearly as possible the equivalent to the water decomposed in the volta-electrometer. . the infusible condition of the silver at the temperature used, and the length and ramifying character of its crystals, render the above experiment difficult to perform, and uncertain in its results. i therefore wrought with chloride of lead, using a green-glass tube, formed as in fig. . a weighed platina wire was fused into the bottom of a small tube, as before described ( .). the tube was then bent to an angle, at about half an inch distance from the closed end; and the part between the angle and the extremity being softened, was forced upward, as in the figure, so as to form a bridge, or rather separation, producing two little depressions or basins _a, b_, within the tube. this arrangement was suspended by a platina wire, as before, so that the heat of a spirit-lamp could be applied to it, such inclination being given to it as would allow all air to escape during the fusion of the chloride of lead. a positive electrode was then provided, by bending up the end of a platina wire into a knot, and fusing about twenty grains of metallic lead on to it, in a small closed tube of glass, which was afterwards broken away. being so furnished, the wire with its lead was weighed, and the weight recorded. . chloride of lead was now introduced into the tube, and carefully fused. the leaded electrode was also introduced; after which the metal, at its extremity, soon melted. in this state of things the tube was filled up to _c_ with melted chloride of lead; the end of the electrode to be rendered negative was in the basin _b_, and the electrode of melted lead was retained in the basin _a_, and, by connexion with the proper conducting wire of a voltaic battery, was rendered positive. a volta-electrometer was included in the circuit. . immediately upon the completion of the communication with the voltaic battery, the current passed, and decomposition proceeded. no chlorine was evolved at the positive electrode; but as the fused chloride was transparent, a button of alloy could be observed gradually forming and increasing in size at _b_, whilst the lead at _a_ could also be seen gradually to diminish. after a time, the experiment was stopped; the tube allowed to cool, and broken open; the wires, with their buttons, cleaned and weighed; and their change in weight compared with the indication of the volta-electrometer. . in this experiment the positive electrode had lost just as much lead as the negative one had gained ( .), and the loss and gain were very nearly the equivalents of the water decomposed in the volta-electrometer, giving for lead the number . . it is therefore evident, in this instance, that causing a _strong affinity_, or _no affinity_, for the substance evolved at the _anode_, to be active during the experiment ( .), produces no variation in the definite action of the electric current. . a similar experiment was then made with iodide of lead, and in this manner all confusion from the formation of a periodide avoided ( .). no iodine was evolved during the whole action, and finally the loss of lead at the _anode_ was the same as the gain at the _cathode_, the equivalent number, by comparison with the result in the volta-electrometer, being . . . then protochloride of tin was subjected to the electric current in the same manner, using of course, a tin positive electrode. no bichloride of tin was now formed ( . .). on examining the two electrodes, the positive had lost precisely as much as the negative had gained; and by comparison with the volta-electrometer, the number for tin came out . . it is quite necessary in these and similar experiments to examine the interior of the bulbs of alloy at the ends of the conducting wires; for occasionally, and especially with those which have been positive, they are cavernous, and contain portions of the chloride or iodide used, which must be removed before the final weight is ascertained. this is more usually the case with lead than tin. . all these facts combine into, i think, an irresistible mass of evidence, proving the truth of the important proposition which i at first laid down, namely, _that the chemical power of a current of electricity is in direct proportion to the absolute quantity of electricity which passes_ ( . .). they prove, too, that this is not merely true with one substance, as water, but generally with all electrolytic bodies; and, further, that the results obtained with any _one substance_ do not merely agree amongst themselves, but also with those obtained from _other substances_, the whole combining together into _one series of definite electro-chemical actions_ ( .). i do not mean to say that no exceptions will appear: perhaps some may arise, especially amongst substances existing only by weak affinity; but i do not expect that any will seriously disturb the result announced. if, in the well-considered, well-examined, and, i may surely say, well-ascertained doctrines of the definite nature of ordinary chemical affinity, such exceptions occur, as they do in abundance, yet, without being allowed to disturb our minds as to the general conclusion, they ought also to be allowed if they should present themselves at this, the opening of a new view of electro-chemical action; not being held up as obstructions to those who may be engaged in rendering that view more and more perfect, but laid aside for a while, in hopes that their perfect and consistent explanation will ultimately appear. * * * * * . the doctrine of _definite electro-chemical action_ just laid down, and, i believe, established, leads to some new views of the relations and classifications of bodies associated with or subject to this action. some of these i shall proceed to consider. . in the first place, compound bodies may be separated into two great classes, namely, those which are decomposable by the electric current, and those which are not: of the latter, some are conductors, others non-conductors, of voltaic electricity[a]. the former do not depend for their decomposability upon the nature of their elements only; for, of the same two elements, bodies may be formed, of which one shall belong to one class and another to the other class; but probably on the proportions also ( .). it is further remarkable, that with very few, if any, exceptions ( . .), these decomposable bodies are exactly those governed by the remarkable law of conduction i have before described ( .); for that law does not extend to the many compound fusible substances that are excluded from this class. i propose to call bodies of this, the decomposable class, _electrolytes_ ( .). [a] i mean here by voltaic electricity, merely electricity from a most abundant source, but having very small intensity. . then, again, the substances into which these divide, under the influence of the electric current, form an exceedingly important general class. they are combining bodies; are directly associated with the fundamental parts of the doctrine of chemical affinity; and have each a definite proportion, in which they are always evolved during electrolytic action. i have proposed to call these bodies generally _ions_, or particularly _anions_ and _cations_, according as they appear at the _anode_ or _cathode_ ( .); and the numbers representing the proportions in which they are evolved _electro-chemical equivalents_. thus hydrogen, oxygen, chlorine, iodine, lead, tin are _ions_; the three former are _anions_, the two metals are _cations_, and , , , , , , are their _electro-chemical equivalents_ nearly. . a summary of certain points already ascertained respecting _electrolytes, ions_, and _electro-chemical equivalents_, may be given in the following general form of propositions, without, i hope, including any serious error. . i. a single _ion_, i.e. one not in combination with another, will have no tendency to pass to either of the electrodes, and will be perfectly indifferent to the passing current, unless it be itself a compound of more elementary _ions_, and so subject to actual decomposition. upon this fact is founded much of the proof adduced in favour of the new theory of electro-chemical decomposition, which i put forth in a former series of these researches ( . &c.). . ii. if one _ion_ be combined in right proportions ( .) with another strongly opposed to it in its ordinary chemical relations, i.e. if an _anion_ be combined with a _cation_, then both will travel, the one to the _anode_, the other to the _cathode_, of the decomposing body ( , . .). . iii. if, therefore, an _ion_ pass towards one of the electrodes, another _ion_ must also be passing simultaneously to the other electrode, although, from secondary action, it may not make its appearance ( .). . iv. a body decomposable directly by the electric current, i.e. an _electrolyte_, must consist of two _ions_, and must also render them up during the act of decomposition. . v. there is but one _electrolyte_ composed of the same two elementary _ions_; at least such appears to be the fact ( .), dependent upon a law, that _only single electro-chemical equivalents of elementary ions can go to the electrodes, and not multiples_. . vi. a body not decomposable when alone, as boracic acid, is not directly decomposable by the electric current when in combination ( .). it may act as an _ion_ going wholly to the _anode_ or _cathode_, but does not yield up its elements, except occasionally by a secondary action. perhaps it is superfluous for me to point out that this proposition has _no relation_ to such cases as that of water, which, by the presence of other bodies, is rendered a better conductor of electricity, and _therefore_ is more freely decomposed. . vii. the nature of the substance of which the electrode is formed, provided it be a conductor, causes no difference in the electro-decomposition, either in kind or degree ( . .): but it seriously influences, by secondary action ( .), the state in which the finally appear. advantage may be taken of this principle in combining and _ions_ collecting such _ions_ as, if evolved in their _free_ state, would be unmanageable[a]. [a] it will often happen that the electrodes used may be of such a nature as, with the fluid in which they are immersed, to produce an electric current, either according with or opposing that of the voltaic arrangement used, and in this way, or by direct chemical action, may sadly disturb the results. still, in the midst of all these confusing effects, the electric current, which actually passes in any direction through the body suffering decomposition, will produce its own definite electrolytic action. . viii. a substance which, being used as the electrode, can combine with the _ion_ evolved against it, is also, i believe, an _ion_, and combines, in such cases, in the quantity represented by its _electro-chemical equivalent_. all the experiments i have made agree with this view; and it seems to me, at present, to result as a necessary consequence. whether, in the secondary actions that take place, where the _ion_ acts, not upon the matter of the electrode, but on that which is around it in the liquid ( .), the same consequence follows, will require more extended investigation to determine. . ix. compound _ions_ are not necessarily composed of electro-chemical equivalents of simple _ions_. for instance, sulphuric acid, boracic acid, phosphoric acid, are _ions_, but not _electrolytes_, i.e. not composed of electro-chemical equivalents of simple _ions_. . x. electro-chemical equivalents are always consistent; i.e. the same number which represents the equivalent of a substance a when it is separating from a substance b, will also represent a when separating from a third substance c. thus, is the electro-chemical equivalent of oxygen, whether separating from hydrogen, or tin, or lead; and . is the electrochemical equivalent of lead, whether separating from oxygen, or chlorine, or iodine. . xi. electro-chemical equivalents coincide, and are the same, with ordinary chemical equivalents. . by means of experiment and the preceding propositions, a knowledge of _ions_ and their electro-chemical equivalents may be obtained in various ways. . in the first place, they may be determined directly, as has been done with hydrogen, oxygen, lead, and tin, in the numerous experiments already quoted. . in the next place, from propositions ii. and iii., may be deduced the knowledge of many other _ions_, and also their equivalents. when chloride of lead was decomposed, platina being used for both electrodes ( .), there could remain no more doubt that chlorine was passing to the _anode_, although it combined with the platina there, than when the positive electrode, being of plumbago ( .), allowed its evolution in the free state; neither could there, in either case, remain any doubt that for every . parts of lead evolved at the _cathode_, parts of chlorine were evolved at the _anode_, for the remaining chloride of lead was unchanged. so also, when in a metallic solution one volume of oxygen, or a secondary compound containing that proportion, appeared at the _anode_, no doubt could arise that hydrogen, equivalent to two volumes, had been determined to the _cathode_, although, by a secondary action, it had been employed in reducing oxides of lead, copper, or other metals, to the metallic state. in this manner, then, we learn from the experiments already described in these researches, that chlorine, iodine, bromine, fluorine, calcium, potassium, strontium, magnesium, manganese, &c., are _ions_ and that their _electro-chemical equivalents_ are the same as their _ordinary chemical equivalents_. . propositions iv. and v. extend our means of gaining information. for if a body of known chemical composition is found to be decomposable, and the nature of the substance evolved as a primary or even a secondary result ( . .) at one of the electrodes, be ascertained, the electro-chemical equivalent of that body may be deduced from the known constant composition of the substance evolved. thus, when fused protiodide of tin is decomposed by the voltaic current ( .), the conclusion may be drawn, that both the iodine and tin are _ions_, and that the proportions in which they combine in the fused compound express their electro-chemical equivalents. again, with respect to the fused iodide of potassium ( .), it is an electrolyte; and the chemical equivalents will also be the electro-chemical equivalents. . if proposition viii. sustain extensive experimental investigation, then it will not only help to confirm the results obtained by the use of the other propositions, but will give abundant original information of its own. . in many instances, the _secondary results_ obtained by the action of the evolved _ion_ on the substances present in the surrounding liquid or solution, will give the electro-chemical equivalent. thus, in the solution of acetate of lead, and, as far as i have gone, in other proto-salts subjected to the reducing action of the nascent hydrogen at the _cathode_, the metal precipitated has been in the same quantity as if it had been a primary product, (provided no free hydrogen escaped there,) and therefore gave accurately the number representing its electro-chemical equivalent. . upon this principle it is that secondary results may occasionally be used as measurers of the volta-electric current ( . .); but there are not many metallic solutions that answer this purpose well: for unless the metal is easily precipitated, hydrogen will be evolved at the _cathode_ and vitiate the result. if a soluble peroxide is formed at the _anode_, or if the precipitated metal crystallize across the solution and touch the positive electrode, similar vitiated results are obtained. i expect to find in some salts, as the acetates of mercury and zinc, solutions favourable for this use. . after the first experimental investigations to establish the definite chemical action of electricity, i have not hesitated to apply the more strict results of chemical analysis to correct the numbers obtained as electrolytic results. this, it is evident, may be done in a great number of cases, without using too much liberty towards the due severity of scientific research. the series of numbers representing electro-chemical equivalents must, like those expressing the ordinary equivalents of chemically acting bodies, remain subject to the continual correction of experiment and sound reasoning. . i give the following brief table of _ions_ and their electro-chemical equivalents, rather as a specimen of a first attempt than as anything that can supply the want which must very quickly be felt, of a full and complete tabular account of this class of bodies. looking forward to such a table as of extreme utility (if well-constructed) in developing the intimate relation of ordinary chemical affinity to electrical actions, and identifying the two, not to the imagination merely, but to the conviction of the senses and a sound judgement, i may be allowed to express a hope, that the endeavour will always be to make it a table of _real_, and not _hypothetical_, electro-chemical equivalents; for we shall else overrun the facts, and lose all sight and consciousness of the knowledge lying directly in our path. . the equivalent numbers do not profess to be exact, and are taken almost entirely from the chemical results of other philosophers in whom i could repose more confidence, as to these points, than in myself. . table of ions. _anions_. oxygen chlorine . iodine bromine . fluorine . cyanogen sulphuric acid selenic acid nitric acid chloric acid . phosphoric acid . carbonic acid boracic acid acetic acid tartaric acid citric acid oxalic acid sulphur (?) selenium (?) salpho-cyanogen _cations_. hydrogen potassium . sodium . lithium barium . strontium . calcium . magnesium . manganese . zinc . tin . lead . iron copper . cadmium . cerium cobalt . nickel . antimony . bismuth mercury silver platina . ? gold (?) ammonia potassa . soda . lithia baryta . strontia . lime . magnesia . alumina. (?) protoxides generally. quinia . cinchona morphia vegeto-alkalies generally. . this table might be further arrange into groups of such substances as either act with, or replace, each other. thus, for instance, acids and bases act in relation to each other; but they do not act in association with oxygen, hydrogen, or elementary substances. there is indeed little or no doubt that, when the electrical relations of the particles of matter come to be closely examined, this division must be made. the simple substances, with cyanogen, sulpho-cyanogen, and one or two other compound bodies, will probably form the first group; and the acids and bases, with such analogous compounds as may prove to be _ions_, the second group. whether these will include all _ions_, or whether a third class of more complicated results will be required, must be decided by future experiments. . it is _probable_ that all our present elementary bodies are _ions_, but that is not as yet certain. there are some, such as carbon, phosphorus, nitrogen, silicon, boron, alumium, the right of which to the title of _ion_ it is desirable to decide as soon as possible. there are also many compound bodies, and amongst them alumina and silica, which it is desirable to class immediately by unexceptionable experiments. it is also _possible_, that all combinable bodies, compound as well as simple, may enter into the class of _ions_; but at present it does not seem to me probable. still the experimental evidence i have is so small in proportion to what must gradually accumulate around, and bear upon, this point, that i am afraid to give a strong opinion upon it. . i think i cannot deceive myself in considering the doctrine of definite electro-chemical action as of the utmost importance. it touches by its facts more directly and closely than any former fact, or set of facts, have done, upon the beautiful idea, that ordinary chemical affinity is a mere consequence of the electrical attractions of the particles of different kinds of matter; and it will probably lead us to the means by which we may enlighten that which is at present so obscure, and either fully demonstrate the truth of the idea, or develope that which ought to replace it. . a very valuable use of electro-chemical equivalents will be to decide, in cases of doubt, what is the true chemical equivalent, or definite proportional, or atomic number of a body; for i have such conviction that the power which governs electro-decomposition and ordinary chemical attractions is the same; and such confidence in the overruling influence of those natural laws which render the former definite, as to feel no hesitation in believing that the latter must submit to them also. such being the case, i can have, no doubt that, assuming hydrogen as , and dismissing small fractions for the simplicity of expression, the equivalent number or atomic weight of oxygen is , of chlorine , of bromine . , of lead . , of tin , &c., notwithstanding that a very high authority doubles several of these numbers. § . _on the absolute quantity of electricity associated with the particles or atoms of matter._ . the theory of definite electrolytical or electro-chemical action appears to me to touch immediately upon the _absolute quantity_ of electricity or electric power belonging to different bodies. it is impossible, perhaps, to speak on this point without committing oneself beyond what present facts will sustain; and yet it is equally impossible, and perhaps would be impolitic, not to reason upon the subject. although we know nothing of what an atom is, yet we cannot resist forming some idea of a small particle, which represents it to the mind; and though we are in equal, if not greater, ignorance of electricity, so as to be unable to say whether it is a particular matter or matters, or mere motion of ordinary matter, or some third kind of power or agent, yet there is an immensity of facts which justify us in believing that the atoms of matter are in some way endowed or associated with electrical powers, to which they owe their most striking qualities, and amongst them their mutual chemical affinity. as soon as we perceive, through the teaching of dalton, that chemical powers are, however varied the circumstances in which they are exerted, definite for each body, we learn to estimate the relative degree of force which resides in such bodies: and when upon that knowledge comes the fact, that the electricity, which we appear to be capable of loosening from its habitation for a while, and conveying from place to place, _whilst it retains its chemical force_, can be measured out, and being so measured is found to be _as definite in its action_ as any of _those portions_ which, remaining associated with the particles of matter, give them their _chemical relation_; we seem to have found the link which connects the proportion of that we have evolved to the proportion of that belonging to the particles in their natural state. . now it is wonderful to observe how small a quantity of a compound body is decomposed by a certain portion of electricity. let us, for instance, consider this and a few other points in relation to water. _one grain_ of water, acidulated to facilitate conduction, will require an electric current to be continued for three minutes and three quarters of time to effect its decomposition, which current must be powerful enough to retain a platina wire / of an inch in thickness[a], red-hot, in the air during the whole time; and if interrupted anywhere by charcoal points, will produce a very brilliant and constant star of light. if attention be paid to the instantaneous discharge of electricity of tension, as illustrated in the beautiful experiments of mr. wheatstone[b], and to what i have said elsewhere on the relation of common and voltaic electricity ( . .), it will not be too much to say that this necessary quantity of electricity is equal to a very powerful flash of lightning. yet we have it under perfect command; can evolve, direct, and employ it at pleasure; and when it has performed its full work of electrolyzation, it has only separated the elements of _a single grain of water_. [a] i have not stated the length of wire used, because i find by experiment, as would be expected in theory, that it is indifferent. the same quantity of electricity which, passed in a given time, can heat an inch of platina wire of a certain diameter red-hot, can also heat a hundred, a thousand, or any length of the same wire to the same degree, provided the cooling circumstances are the same for every part in all cases. this i have proved by the volta-electrometer. i found that whether half an inch or eight inches were retained at one constant temperature of dull redness, equal quantities of water were decomposed in equal times. when the half-inch was used, only the centre portion of wire was ignited. a fine wire may even be used as a rough but ready regulator of a voltaic current; for if it be made part of the circuit, and the larger wires communicating with it be shifted nearer to or further apart, so as to keep the portion of wire in the circuit sensibly at the same temperature, the current passing through it will be nearly uniform. [b] literary gazette, , march and . philosophical magazine, , p. . l'institut, , p. . . on the other hand, the relation between the conduction of the electricity and the decomposition of the water is so close, that one cannot take place without the other. if the water is altered only in that small degree which consists in its having the solid instead of the fluid state, the conduction is stopped, and the decomposition is stopped with it. whether the conduction be considered as depending upon the decomposition, or not ( . .), still the relation of the two functions is equally intimate and inseparable. . considering this close and twofold relation, namely, that without decomposition transmission of electricity does not occur; and, that for a given definite quantity of electricity passed, an equally definite and constant quantity of water or other matter is decomposed; considering also that the agent, which is electricity, is simply employed in overcoming electrical powers in the body subjected to its action; it seems a probable, and almost a natural consequence, that the quantity which passes is the _equivalent_ of, and therefore equal to, that of the particles separated; i.e. that if the electrical power which holds the elements of a grain of water in combination, or which makes a grain of oxygen and hydrogen in the right proportions unite into water when they are made to combine, could be thrown into the condition of _a current_, it would exactly equal the current required for the separation of that grain of water into its elements again. . this view of the subject gives an almost overwhelming idea of the extraordinary quantity or degree of electric power which naturally belongs to the particles of matter; but it is not inconsistent in the slightest degree with the facts which can be brought to bear on this point. to illustrate this i must say a few words on the voltaic pile[a]. [a] by the term voltaic pile, i mean such apparatus or arrangement of metals as up to this time have been called so, and which contain water, brine, acids, or other aqueous solutions or decomposable substances ( .), between their plates. other kinds of electric apparatus may be hereafter invented, and i hope to construct some not belonging to the class of instruments discovered by volta. . intending hereafter to apply the results given in this and the preceding series of researches to a close investigation of the source of electricity in the voltaic instrument, i have refrained from forming any decided opinion on the subject; and without at all meaning to dismiss metallic contact, or the contact of dissimilar substances, being conductors, but not metallic, as if they had nothing to do with the origin of the current, i still am fully of opinion with davy, that it is at least continued by chemical action, and that the supply constituting the current is almost entirely from that source. . those bodies which, being interposed between the metals of the voltaic pile, render it active, _are all of them electrolytes_ ( .); and it cannot but press upon the attention of every one engaged in considering this subject, that in those bodies (so essential to the pile) decomposition and the transmission of a current are so intimately connected, that one cannot happen without the other. this i have shown abundantly in water, and numerous other cases ( . .). if, then, a voltaic trough have its extremities connected by a body capable of being decomposed, as water, we shall have a continuous current through the apparatus; and whilst it remains in this state we may look at the part where the acid is acting upon the plates, and that where the current is acting upon the water, as the reciprocals of each other. in both parts we have the two conditions _inseparable in such bodies as these_, namely, the passing of a current, and decomposition; and this is as true of the cells in the battery as of the water cell; for no voltaic battery has as yet been constructed in which the chemical action is only that of combination: _decomposition is always included_, and is, i believe, an essential chemical part. . but the difference in the two parts of the connected battery, that is, the decomposition or experimental cell, and the acting cells, is simply this. in the former we urge the current through, but it, apparently of necessity, is accompanied by decomposition: in the latter we cause decompositions by ordinary chemical actions, (which are, however, themselves electrical,) and, as a consequence, have the electrical current; and as the decomposition dependent upon the current is definite in the former case, so is the current associated with the decomposition also definite in the latter ( . &c.). . let us apply this in support of what i have surmised respecting the enormous electric power of each particle or atom of matter ( .). i showed in a former series of these researches on the relation by measure of common and voltaic electricity, that two wires, one of platina and one of zinc, each one-eighteenth of an inch in diameter, placed five-sixteenths of an inch apart, and immersed to the depth of five-eighths of an inch in acid, consisting of one drop of oil of vitriol and four ounces of distilled water at a temperature of about ° fahr., and connected at the other extremities by a copper wire eighteen feet long, and one-eighteenth of an inch in thickness, yielded as much electricity in little more than three seconds of time as a leyden battery charged by thirty turns of a very large and powerful plate electric machine in full action ( .). this quantity, though sufficient if passed at once through the head of a rat or cat to have killed it, as by a flash of lightning, was evolved by the mutual action of so small a portion of the zinc wire and water in contact with it, that the loss of weight sustained by either would be inappreciable by our most delicate instruments; and as to the water which could be decomposed by that current, it must have been insensible in quantity, for no trace of hydrogen appeared upon the surface of the platina during those three seconds. . what an enormous quantity of electricity, therefore, is required for the decomposition of a single grain of water! we have already seen that it must be in quantity sufficient to sustain a platina wire / of an inch in thickness, red-hot, in contact with the air, for three minutes and three quarters ( .), a quantity which is almost infinitely greater than that which could be evolved by the little standard voltaic arrangement to which i have just referred ( . .). i have endeavoured to make a comparison by the loss of weight of such a wire in a given time in such an acid, according to a principle and experiment to be almost immediately described ( .); but the proportion is so high that i am almost afraid to mention it. it would appear that , such charges of the leyden battery as i have referred to above, would be necessary to supply electricity sufficient to decompose a single grain of water; or, if i am right, to equal the quantity of electricity which is naturally associated with the elements of that grain of water, endowing them with their mutual chemical affinity. . in further proof of this high electric condition of the particles of matter, and the _identity as to quantity of that belonging to them with that necessary for their separation_, i will describe an experiment of great simplicity but extreme beauty, when viewed in relation to the evolution of an electric current and its decomposing powers. . a dilute sulphuric acid, made by adding about one part by measure of oil of vitriol to thirty parts of water, will act energetically upon a piece of zinc plate in its ordinary and simple state: but, as mr. sturgeon has shown[a], not at all, or scarcely so, if the surface of the metal has in the first instance been amalgamated; yet the amalgamated zinc will act powerfully with platina as an electromotor, hydrogen being evolved on the surface of the latter metal, as the zinc is oxidized and dissolved. the amalgamation is best effected by sprinkling a few drops of mercury upon the surface of the zinc, the latter being moistened with the dilute acid, and rubbing with the fingers or two so as to extend the liquid metal over the whole of the surface. any mercury in excess, forming liquid drops upon the zinc, should be wiped off[b]. [a] recent experimental researches, &c., , p. , &c. [b] the experiment may be made with pure zinc, which, as chemists well know, is but slightly acted upon by dilute sulphuric acid in comparison with ordinary zinc, which during the action is subject to an infinity of voltaic actions. see de la rive on this subject, bibliothèque universelle, , p. . . two plates of zinc thus amalgamated were dried and accurately weighed; one, which we will call a, weighed . grains; the other, to be called b, weighed . grains. they were about five inches long, and . of an inch wide. an earthenware pneumatic trough was filled with dilute sulphuric acid, of the strength just described ( .), and a gas jar, also filled with the acid, inverted in it[a]. a plate of platina of nearly the same length, but about three times as wide as the zinc plates, was put up into this jar. the zinc plate a was also introduced into the jar, and brought in contact with the platina, and at the same moment the plate b was put into the acid of the trough, but out of contact with other metallic matter. [a] the acid was left during a night with a small piece of unamalgamated zinc in it, for the purpose of evolving such air as might be inclined to separate, and bringing the whole into a constant state. . strong action immediately occurred in the jar upon the contact of the zinc and platina plates. hydrogen gas rose from the platina, and was collected in the jar, but no hydrogen or other gas rose from _either_ zinc plate. in about ten or twelve minutes, sufficient hydrogen having been collected, the experiment was stopped; during its progress a few small bubbles had appeared upon plate b, but none upon plate a. the plates were washed in distilled water, dried, and reweighed. plate b weighed . grains, as before, having lost nothing by the direct chemical action of the acid. plate a weighed . grains, . grains of it having been oxidized and dissolved during the experiment. . the hydrogen gas was next transferred to a water-trough and measured; it amounted to . cubic inches, the temperature being °, and the barometer . inches. this quantity, corrected for temperature, pressure, and moisture, becomes . cubic inches of dry hydrogen at mean temperature and pressure; which, increased by one half for the oxygen that must have gone to the _anode_, i.e. to the zinc, gives . cubic inches as the quantity of oxygen and hydrogen evolved from the water decomposed by the electric current. according to the estimate of the weight of the mixed gas before adopted ( .), this volume is equal to . grains, which therefore is the weight of water decomposed; and this quantity is to . , the quantity of zinc oxidized, as is to . . now taking as the equivalent number of water, the number . is given as the equivalent number of zinc; a coincidence sufficiently near to show, what indeed could not but happen, that for an equivalent of zinc oxidized an equivalent of water must be decomposed[a]. [a] the experiment was repeated several times with the same results. . but let us observe _how_ the water is decomposed. it is electrolyzed, i.e. is decomposed voltaically, and not in the ordinary manner (as to appearance) of chemical decompositions; for the oxygen appears at the _anode_ and the hydrogen at the _cathode_ of the body under decomposition, and these were in many parts of the experiment above an inch asunder. again, the ordinary chemical affinity was not enough under the circumstances to effect the decomposition of the water, as was abundantly proved by the inaction on plate b; the voltaic current was essential. and to prevent any idea that the chemical affinity was almost sufficient to decompose the water, and that a smaller current of electricity might, under the circumstances, cause the hydrogen to pass to the _cathode_, i need only refer to the results which i have given ( . .) to shew that the chemical action at the electrodes has not the slightest influence over the _quantities_ of water or other substances decomposed between them, but that they are entirely dependent upon the quantity of electricity which passes. . what, then, follows as a necessary consequence of the whole experiment? why, this: that the chemical action upon . parts, or one equivalent of zinc, in this simple voltaic circle, was able to evolve such quantity of electricity in the form of a current, as, passing through water, should decompose parts, or one equivalent of that substance: and considering the definite relations of electricity as developed in the preceding parts of the present paper, the results prove that the quantity of electricity which, being naturally associated with the particles of matter, gives them their combining power, is able, when thrown into a current, to separate those particles from their state of combination; or, in other words, that _the electricity which decomposes, and that which is evolved by the decomposition of a certain quantity of matter, are alike._ . the harmony which this theory of the definite evolution and the equivalent definite action of electricity introduces into the associated theories of definite proportions and electrochemical affinity, is very great. according to it, the equivalent weights of bodies are simply those quantities of them which contain equal quantities of electricity, or have naturally equal electric powers; it being the electricity which _determines_ the equivalent number, _because_ it determines the combining force. or, if we adopt the atomic theory or phraseology, then the atoms of bodies which are equivalents to each other in their ordinary chemical action, have equal quantities of electricity naturally associated with them. but i must confess i am jealous of the term _atom_; for though it is very easy to talk of atoms, it is very difficult to form a clear idea of their nature, especially when compound bodies are under consideration. . i cannot refrain from recalling here the beautiful idea put forth, i believe, by berzelius ( .) in his development of his views of the electro-chemical theory of affinity, that the heat and light evolved during cases of powerful combination are the consequence of the electric discharge which is at the moment taking place. the idea is in perfect accordance with the view i have taken of the _quantity_ of electricity associated with the particles of matter. . in this exposition of the law of the definite action of electricity, and its corresponding definite proportion in the particles of bodies, i do not pretend to have brought, as yet, every case of chemical or electro-chemical action under its dominion. there are numerous considerations of a theoretical nature, especially respecting the compound particles of matter and the resulting electrical forces which they ought to possess, which i hope will gradually receive their development; and there are numerous experimental cases, as, for instance, those of compounds formed by weak affinities, the simultaneous decomposition of water and salts, &c., which still require investigation. but whatever the results on these and numerous other points may be, i do not believe that the facts which i have advanced, or even the general laws deduced from them, will suffer any serious change; and they are of sufficient importance to justify their publication, though much may yet remain imperfect or undone. indeed, it is the great beauty of our science, chemistry, that advancement in it, whether in a degree great or small, instead of exhausting the subjects of research, opens the doors to further and more abundant knowledge, overflowing with beauty and utility, to those who will be at the easy personal pains of undertaking its experimental investigation. . the definite production of electricity ( .) in association with its definite action proves, i think, that the current of electricity in the voltaic pile: is sustained by chemical decomposition, or rather by chemical action, and not by contact only. but here, as elsewhere ( .), i beg to reserve my opinion as to the real action of contact, not having yet been able to make up my mind as to whether it is an exciting cause of the current, or merely necessary to allow of the conduction of electricity, otherwise generated, from one metal to the other. . but admitting that chemical action is the source of electricity, what an infinitely small fraction of that which is active do we obtain and employ in our voltaic batteries! zinc and platina wires, one-eighteenth of an inch in diameter and about half an inch long, dipped into dilute sulphuric acid, so weak that it is not sensibly sour to the tongue, or scarcely to our most delicate test-papers, will evolve more electricity in one-twentieth of a minute ( .) than any man would willingly allow to pass through his body at once. the chemical action of a grain of water upon four grains of zinc can evolve electricity equal in quantity to that of a powerful thunder-storm ( . .). nor is it merely true that the quantity is active; it can be directed and made to perform its full equivalent duty ( . &c.). is there not, then, great reason to hope and believe that, by a closer _experimental_ investigation of the principles which govern the development and action of this subtile agent, we shall be able to increase the power of our batteries, or invent new instruments which shall a thousandfold surpass in energy those which we at present possess? . here for a while i must leave the consideration of the _definite chemical action of electricity_. but before i dismiss this series of experimental researches, i would call to mind that, in a former series, i showed the current of electricity was also _definite in its magnetic action_ ( . . . . .); and, though this result was not pursued to any extent, i have no doubt that the success which has attended the development of the chemical effects is not more than would accompany an investigation of the magnetic phenomena. _royal institution, december st, ._ eighth series. § . _on the electricity of the voltaic pile; its source, quantity, intensity, and general characters._ ¶ i. _on simple voltaic circles._ ¶ ii. _on the intensity necessary for electrolyzation._ ¶ iii. _on associated voltaic circles, or the voltaic battery._ ¶ iv. _on the resistance of an electrolyte to electrolytic action._ ¶ v. _general remarks on the active voltaic battery._ received april ,--read june , . ¶ i. _on simple voltaic circles._ . the great question of the source of electricity, in the voltaic pile has engaged the attention of so many eminent philosophers, that a man of liberal mind and able to appreciate their powers would probably conclude, although he might not have studied the question, that the truth was somewhere revealed. but if in pursuance of this impression he were induced to enter upon the work of collating results and conclusions, he would find such contradictory evidence, such equilibrium of opinion, such variation and combination of theory, as would leave him in complete doubt respecting what he should accept as the true interpretation of nature: he would be forced to take upon himself the labour of repeating and examining the facts, and then use his own judgement on them in preference to that of others. . this state of the subject must, to those who have made up their minds on the matter, be my apology for entering upon its investigation. the views i have taken of the definite action of electricity in decomposing bodies ( .), and the identity of the power so used with the power to be overcome ( .), founded not on a mere opinion or general notion, but on facts which, being altogether new, were to my mind precise and conclusive, gave me, as i conceived, the power of examining the question with advantages not before possessed by any, and which might compensate, on my part, for the superior clearness and extent of intellect on theirs. such are the considerations which have induced me to suppose i might help in deciding the question, and be able to render assistance in that great service of removing _doubtful knowledge_. such knowledge is the early morning light of every advancing science, and is essential to its development; but the man who is engaged in dispelling that which is deceptive in it, and revealing more clearly that which is true, is as useful in his place, and as necessary to the general progress of the science, as he who first broke through the intellectual darkness, and opened a path into knowledge before unknown to man. . the identity of the force constituting the voltaic current or electrolytic agent, with that which holds the elements of electrolytes together ( .), or in other words with chemical affinity, seemed to indicate that the electricity of the pile itself was merely a mode of exertion, or exhibition, or existence of _true chemical action_, or rather of its cause; and i have consequently already said that i agree with those who believe that the _supply_ of electricity is due to chemical powers ( .). . but the great question of whether it is originally due to metallic contact or to chemical action, i.e. whether it is the first or the second which _originates_ and determines the current, was to me still doubtful; and the beautiful and simple experiment with amalgamated zinc and platina, which i have described minutely as to its results ( , &c.), did not decide the point; for in that experiment the chemical action does not take place without the contact of the metals, and the metallic contact is inefficient without the chemical action. hence either might be looked upon as the _determining_ cause of the current. . i thought it essential to decide this question by the simplest possible forms of apparatus and experiment, that no fallacy might be inadvertently admitted. the well-known difficulty of effecting decomposition by a single pair of plates, except in the fluid exciting them into action ( .), seemed to throw insurmountable obstruction in the way of such experiments; but i remembered the easy decomposability of the solution of iodide of potassium ( .), and seeing no theoretical reason, if metallic contact was not _essential_, why true electro-decomposition should not be obtained without it, even in a single circuit, i persevered and succeeded. . a plate of zinc, about eight inches long and half an inch wide, was cleaned and bent in the middle to a right angle, fig. _a_, plate vi. a plate of platina, about three inches long and half an inch wide, was fastened to a platina wire, and the latter bent as in the figure, _b_. these two pieces of metal were arranged together as delineated, but as yet without the vessel _c_, and its contents, which consisted of dilute sulphuric acid mingled with a little nitric acid. at _x_ a piece of folded bibulous paper, moistened in a solution of iodide of potassium, was placed on the zinc, and was pressed upon by the end of the platina wire. when under these circumstances the plates were dipped into the acid of the vessel _c_, there was an immediate effect at _x_, the iodide being decomposed, and iodine appearing at the _anode_ ( .), i.e. against the end of the platina wire. . as long as the lower ends of the plates remained in the acid the electric current continued, and the decomposition proceeded at _x_. on removing the end of the wire from place to place on the paper, the effect was evidently very powerful; and on placing a piece of turmeric paper between the white paper and zinc, both papers being moistened with the solution of iodide of potassium, alkali was evolved at the _cathode_ ( .) against the zinc, in proportion to the evolution of iodine at the _anode_. hence the decomposition was perfectly polar, and decidedly dependent upon a current of electricity passing from the zinc through the acid to the platina in the vessel _c_, and back from the platina through the solution to the zinc at the paper _x_. . that the decomposition at _x_ was a true electrolytic action, due to a current determined by the state of things in the vessel _c_, and not dependent upon any mere direct chemical action of the zinc and platina on the iodide, or even upon any _current_ which the solution of iodide might by its action on those metals tend to form at _x_, was shown, in the first place, by removing the vessel _c_ and its acid from the plates, when all decomposition at _x_ ceased, and in the next by connecting the metals, either in or out of the acid, together, when decomposition of the iodide at _x_ occurred, but in a _reverse order_; for now alkali appeared against the end of the platina wire, and the iodine passed to the zinc, the current being the contrary of what it was in the former instance, and produced directly by the difference of action of the solution in the paper on the two metals. the iodine of course _combined_ with the zinc. . when this experiment was made with pieces of zinc amalgamated over the whole surface ( .), the results were obtained with equal facility and in the same direction, even when only dilute sulphuric acid was contained in the vessel _c_ (fig. .). whichsoever end of the zinc was immersed in the acid, still the effects were the same: so that if, for a moment, the mercury might be supposed to supply the metallic contact, the inversion of the amalgamated piece destroys that objection. the use of _unamalgamated zinc_ ( .) removes all possibility of doubt[a]. [a] the following is a more striking mode of making the above elementary experiment. prepare a plate of zinc, ten or twelve inches long and two inches wide, and clean it thoroughly: provide also two discs of clean platina, about one inch and a half in diameter:--dip three or four folds of bibulous paper into a strong solution of iodide of potassium, place them on the clean zinc at one end of the plate, and put on them one of the platina discs: finally dip similar folds of paper or a piece of linen cloth into a mixture of equal parts nitric acid and water, and place it at the other end of the zinc plate with the second platina disc upon it. in this state of things no change at the solution of the iodide will be perceptible; but if the two discs be connected by a platina (or any other) wire for a second or two, and then that over the iodide be raised, it will be found that the _whole_ of the surface beneath is deeply stained with _evolved iodine_.--_dec. ._ when, in pursuance of other views ( .), the vessel _c_ was made to contain a solution of caustic potash in place of acid, still the same results occurred. decomposition of the iodide was effected freely, though there was no metallic contact of dissimilar metals, and the current of electricity was in the _same direction_ as when acid was used at the place of excitement. . even a solution of common salt in the glass _c_ could produce all these effects. . having made a galvanometer with platina wires, and introduced it into the course of the current between the platina plate and the place of decomposition _x_, it was affected, giving indications of currents in the same direction as those shown to exist by the chemical action. . if we consider these results generally, they lead to very important conclusions. in the first place, they prove, in the most decisive manner, that _metallic contact is not necessary for the production of the voltaic current._ in the next place, they show a most extraordinary mutual relation of the chemical affinities of the fluid which _excites_ the current, and the fluid which is _decomposed_ by it. . for the purpose of simplifying the consideration, let us take the experiment with amalgamated zinc. the metal so prepared exhibits no effect until the current can pass: it at the same time introduces no new action, but merely removes an influence which is extraneous to those belonging either to the production or the effect of the electric current under investigation ( .); an influence also which, when present, tends only to confuse the results. . let two plates, one of amalgamated zinc and the other of platina, be placed parallel to each other (fig. .), and introduce a drop of dilute sulphuric acid, _y_, between them at one end: there will be no sensible chemical action at that spot unless the two plates are connected somewhere else, as at pz, by a body capable of conducting electricity. if that body be a metal or certain forms of carbon, then the current passes, and, as it circulates through the fluid at _y_, decomposition ensues. . then remove the acid from _y_, and introduce a drop of the solution of iodide of potassium at _x_ (fig. .). exactly the same set of effects occur, except that when the metallic communication is made at pz, the electric current is in the opposite direction to what it was before, as is indicated by the arrows, which show the courses of the currents ( .). . now _both_ the solutions used are conductors, but the conduction in them is essentially connected with decomposition ( .) in a certain constant order, and therefore the appearance of the elements in certain places _shows_ in what direction a current has passed when the solutions are thus employed. moreover, we find that when they are used at opposite ends of the plates, as in the last two experiments ( . .), metallic contact being allowed at the other extremities, the currents are in opposite directions. we have evidently, therefore, the power of opposing the actions of the two fluids simultaneously to each other at the opposite ends of the plates, using each one as a conductor for the discharge of the current of electricity, which the other tends to generate; in fact, substituting them for metallic contact, and combining both experiments into one (fig. .). under these circumstances, there is an opposition of forces: the fluid, which brings into play the stronger set of chemical affinities for the zinc, (being the dilute acid,) overcomes the force of the other, and determines the formation and direction of the electric current; not merely making that current pass through the weaker liquid, but actually reversing the tendency which the elements of the latter have in relation to the zinc and platina if not thus counteracted, and forcing them in the contrary direction to that they are inclined to follow, that its own current may have free course. if the dominant action at _y_ be removed by making metallic contact there, then the liquid at _x_ resumes its power; or if the metals be not brought into contact at _y_ but the affinities of the solution there weakened, whilst those active _x_ are strengthened, then the latter gains the ascendency, and the decompositions are produced in a contrary order. . before drawing a _final_ conclusion from this mutual dependence and state of the chemical affinities of two distant portions of acting fluids ( .), i will proceed to examine more minutely the various circumstances under which the re-action of the body suffering decomposition is rendered evident upon the action of the body, also undergoing decomposition, which produces the voltaic current. . the use of _metallic contact_ in a single pair of plates, and the cause of its great superiority above contact made by other kinds of matter, become now very evident. when an amalgamated zinc plate is dipped into dilute sulphuric acid, the force of chemical affinity exerted between the metal and the fluid is not sufficiently powerful to cause sensible action at the surfaces of contact, and occasion the decomposition of water by the oxidation of the metal, although it _is_ sufficient to produce such a condition of the electricity (or the power upon which chemical affinity depends) as would produce a current if there were a path open for it ( . .); and that current would complete the conditions necessary, under the circumstances, for the decomposition of the water. . now the presence of a piece of platina touching both the zinc and the fluid to be decomposed, opens the path required for the electricity. its _direct communication_ with the zinc is effectual, far beyond any communication made between it and that metal, (i.e. between the platina and zinc,) by means of decomposable conducting bodies, or, in other words, _electrolytes_, as in the experiment already described ( .); because, when _they_ are used, the chemical affinities between them and the zinc produce a contrary and opposing action to that which is influential in the dilute sulphuric acid; or if that action be but small, still the affinity of their component parts for each other has to be overcome, for they cannot conduct without suffering decomposition; and this decomposition is found _experimentally_ to re-act back upon the forces which in the acid tend to produce the current ( . . &c.), and in numerous cases entirely to neutralize them. where direct contact of the zinc and platina occurs, these obstructing forces are not brought into action, and therefore the production and the circulation of the electric current and the concomitant action of decomposition are then highly favoured. . it is evident, however, that one of these opposing actions may be dismissed, and yet an electrolyte be used for the purpose of completing the circuit between the zinc and platina immersed separately into the dilute acid; for if, in fig. , the platina wire be retained in metallic contact with the zinc plate _a_, at _x_, and a division of the platina be made elsewhere, as at _s_, then the solution of iodide placed there, being in contact with platina at both surfaces, exerts no chemical affinities for that metal; or if it does, they are equal on both sides. its power, therefore, of forming a current in opposition to that dependent upon the action of the acid in the vessel _c_, is removed, and only its resistance to decomposition remains as the obstacle to be overcome by the affinities exerted in the dilute sulphuric acid. . this becomes the condition of a single pair of active plates where _metallic contact_ is allowed. in such cases, only one set of opposing affinities are to be overcome by those which are dominant in the vessel _c_; whereas, when metallic contact is not allowed, two sets of opposing affinities must be conquered ( .). . it has been considered a difficult, and by some an impossible thing, to decompose bodies by the current from a single pair of plates, even when it was so powerful as to heat bars of metal red-hot, as in the case of hare's calorimeter, arranged as a single voltaic circuit, or of wollaston's powerful single pair of metals. this difficulty has arisen altogether from the antagonism of the chemical affinity engaged in producing the current with the chemical affinity to be overcome, and depends entirely upon their relative intensity; for when the sum of forces in one has a certain degree of superiority over the sum of forces in the other, the former gain the ascendency, determine the current, and overcome the latter so as to make the substance exerting them yield up its elements in perfect accordance, both as to direction and quantity, with the course of those which are exerting the most intense and dominant action. . water has generally been the substance, the decomposition of which has been sought for as a chemical test of the passage of an electric current. but i now began to perceive a reason for its failure, and for a fact which i had observed long before ( . .) with regard to the iodide of potassium, namely, that bodies would differ in facility of decomposition by a given electric current, according to the condition and intensity of their ordinary chemical affinities. this reason appeared in their _re-action upon the affinities_ tending to cause the current; and it appeared probable, that many substances might be found which could be decomposed by the current of a single pair of zinc and platina plates immersed in dilute sulphuric acid, although water resisted its action. i soon found this to be the case, and as the experiments offer new and beautiful proofs of the direct relation and opposition of the chemical affinities concerned in producing and in resisting the stream of electricity, i shall briefly describe them. . the arrangement of the apparatus was as in fig. . the vessel _v_ contained dilute sulphuric acid; z and p are the zinc and platina plates; _a_, _b_, and _c_ are platina wires; the decompositions were effected at _x_, and occasionally, indeed generally, a galvanometer was introduced into the circuit at _g_: its place only is here given, the circle at _g_ having no reference to the size of the instrument. various arrangements were made at _x_, according to the kind of decomposition to be effected. if a drop of liquid was to be acted upon, the two ends were merely dipped into it; if a solution contained in the pores of paper was to be decomposed, one of the extremities was connected with a platina plate supporting the paper, whilst the other extremity rested on the paper, _e_, fig. : or sometimes, as with sulphate of soda, a plate of platina sustained two portions of paper, one of the ends of the wires resting upon each piece, _c_, fig. . the darts represent the direction of the electric current ( .). . solution of _iodide of potassium_, in moistened paper, being placed at the interruption of the circuit at _x_, was readily decomposed. iodine was evolved at the _anode_, and alkali at the _cathode_, of the decomposing body. . _protochloride of tin_, when fused and placed at _x_, was also readily decomposed, yielding perchloride of tin at the _anode_ ( .), and tin at the _cathode_. . fused chloride of silver, placed at _x_, was also easily decomposed; chlorine was evolved at the _anode_, and brilliant metallic silver, either in films upon the surface of the liquid, or in crystals beneath, evolved at the _cathode_. . water acidulated with sulphuric acid, solution of muriatic acid, solution of sulphate of soda, fused nitre, and the fused chloride and iodide of lead were not decomposed by this single pair of plates, excited only by dilute sulphuric acid. . these experiments give abundant proofs that a single pair of plates can electrolyze bodies and separate their elements. they also show in a beautiful manner the direct relation and opposition of the chemical affinities concerned at the two points of action. in those cases where the sum of the opposing affinities at _x_ was sufficiently beneath the sum of the acting affinities in _v_, decomposition took place; but in those cases where they rose higher, decomposition was effectually resisted and the current ceased to pass ( .). . it is however, evident, that the sum of acting affinities in _v_ may be increased by using other fluids than dilute sulphuric acid, in which latter case, as i believe, it is merely the affinity of the zinc for the oxygen already combined with hydrogen in the water that is exerted in producing the electric current ( .): and when the affinities are so increased, the view i am supporting leads to the conclusion, that bodies which resisted in the preceding experiments would then be decomposed, because of the increased difference between their affinities and the acting affinities thus exalted. this expectation was fully confirmed in the following manner. . a little nitric acid was added to the liquid in the vessel _r_, so as to make a mixture which i shall call diluted nitro-sulphuric acid. on repeating the experiments with this mixture, all the substances before decomposed again gave way, and much more readily. but, besides that, many which before resisted electrolyzation, now yielded up their elements. thus, solution of sulphate of soda, acted upon in the interstices of litmus and turmeric paper, yielded acid at the _anode_ and alkali at the _cathode_; solution of muriatic acid tinged by indigo yielded chlorine at the _anode_ and hydrogen at the _cathode_; solution of nitrate of silver yielded silver at the _cathode_. again, fused nitre and the fused iodide and chloride of lead were decomposable by the current of this single pair of plates, though they were not by the former ( .). . a solution of acetate of lead was apparently not decomposed by this pair, nor did water acidulated by sulphuric acid seem at first to give way ( .). . the increase of intensity or power of the current produced by a simple voltaic circle, with the increase of the force of the chemical action at the exciting place, is here sufficiently evident. but in order to place it in a clearer point of view, and to show that the decomposing effect was not at all dependent, in the latter cases, upon the mere capability of evolving _more_ electricity, experiments were made in which the quantity evolved could be increased without variation in the intensity of the exciting cause. thus the experiments in which dilute sulphuric acid was used ( .), were repeated, using large plates of zinc and platina in the acid; but still those bodies which resisted decomposition before, resisted it also under these new circumstances. then again, where nitro-sulphuric acid was used ( .), mere wires of platina and zinc were immersed in the exciting acid; yet, notwithstanding this change, those bodies were now decomposed which resisted any current tending to be formed by the dilute sulphuric acid. for instance, muriatic acid could not be decomposed by a single pair of plates when immersed in dilute sulphuric acid; nor did making the solution of sulphuric acid strong, nor enlarging the size of the zinc and platina plates immersed in it, increase the power; but if to a weak sulphuric acid a very little nitric acid was added, then the electricity evolved had power to decompose the muriatic acid, evolving chlorine at the _anode_ and hydrogen at the _cathode_, even when mere wires of metals were used. this mode of increasing the intensity of the electric current, as it excludes the effect dependent upon many pairs of plates, or even the effect of making any one acid stronger or weaker, is at once referable to the condition and force of the chemical affinities which are brought into action, and may, both in principle and practice, be considered as perfectly distinct from any other mode. . the direct reference which is thus experimentally made in the simple voltaic circle of the _intensity_ of the electric current to the _intensity_ of the chemical action going on at the place where the existence and direction of the current is determined, leads to the conclusion that by using selected bodies, as fused chlorides, salts, solutions of acids, &c., which may act upon the metals employed with different degrees of chemical force; and using also metals in association with platina, or with each other, which shall differ in the degree of chemical action exerted between them and the exciting fluid or electrolyte, we shall be able to obtain a series of comparatively constant effects due to electric currents of different intensities, which will serve to assist in the construction of a scale competent to supply the means of determining relative degrees of intensity with accuracy in future researches[a]. [a] in relation to this difference and its probable cause, see considerations on inductive polarization, , &c.--_dec. ._ . i have already expressed the view which i take of the decomposition in the experimental place, as being the direct consequence of the superior exertion at some other spot of the same kind of power as that to be overcome, and therefore as the result of an antagonism of forces of the _same_ nature ( . .). those at the place of decomposition have a re-action upon, and a power over, the exerting or determining set proportionate to what is needful to overcome their own power; and hence a curious result of _resistance_ offered by decompositions to the original determining force, and consequently to the current. this is well shown in the cases where such bodies as chloride of lead, iodide of lead, and water would not decompose with the current produced by a single pair of zinc and platina plates in sulphuric acid ( .), although they would with a current of higher intensity produced by stronger chemical powers. in such cases no sensible portion of the current passes ( .); the action is stopped; and i am now of opinion that in the case of the law of conduction which i described in the fourth series of these researches ( .), the bodies which are electrolytes in the fluid state cease to be such in the solid form, because the attractions of the particles by which they are retained in combination and in their relative position, are then too powerful for the electric current[a]. the particles retain their places; and as decomposition is prevented, the transmission of the electricity is prevented also; and although a battery of many plates may be used, yet if it be of that perfect kind which allows of no extraneous or indirect action ( .), the whole of the affinities concerned in the activity of that battery are at the same time also suspended and counteracted. [a] refer onwards to .--_dec. ._ . but referring to the _resistance_ of each single case of decomposition, it would appear that as these differ in force according to the affinities by which the elements in the substance tend to retain their places, they also would supply cases constituting a series of degrees by which to measure the initial intensities of simple voltaic or other currents of electricity, and which, combined with the scale of intensities determined by different degrees of _acting force_ ( .), would probably include a sufficient set of differences to meet almost every important case where a reference to intensity would be required. . according to the experiments i have already had occasion to make, i find that the following bodies are electrolytic in the order in which i have placed them, those which are first being decomposed by the current of lowest intensity. these currents were always from a single pair of plates, and may be considered as elementary _voltaic forces_. iodide of potassium (solution). chloride of silver (fused). protochloride of tin (fused). chloride of lead (fused). iodide of lead (fused). muriatic acid (solution). water, acidulated with sulphuric acid. . it is essential that, in all endeavours to obtain the relative electrolytic intensity necessary for the decomposition of different bodies, attention should be paid to the nature of the electrodes and the other bodies present which may favour secondary actions ( .). if in electro-decomposition one of the elements separated has an affinity for the electrode, or for bodies present in the surrounding fluid, then the affinity resisting decomposition is in part balanced by such power, and the true place of the electrolyte in a table of the above kind is not obtained: thus, chlorine combines with a positive platina electrode freely, but iodine scarcely at all, and therefore i believe it is that the fused chlorides stand first in the preceding table. again, if in the decomposition of water not merely sulphuric but also a little nitric acid be present, then the water is more freely decomposed, for the hydrogen at the _cathode_ is not ultimately expelled, but finds oxygen in the nitric acid, with which it can combine to produce a secondary result; the affinities opposing decomposition are in this way diminished, and the elements of the water can then be separated by a current of lower intensity. . advantage may be taken of this principle to interpolate more minute degrees into the scale of initial intensities already referred to ( . .) than is there spoken of; for by combining the force of a current _constant_ in its intensity, with the use of electrodes consisting of matter, having more or less affinity for the elements evolved from the decomposing electrolyte, various intermediate degrees may be obtained. * * * * * . returning to the consideration of the source of electricity ( . &c.), there is another proof of the most perfect kind that metallic contact has nothing to do with the _production_ of electricity in the voltaic circuit, and further, that electricity is only another mode of the exertion of chemical forces. it is, the production of the _electric spark_ before any contact of metals is made, and by the exertion of _pure and unmixed chemical forces_. the experiment, which will be described further on ( .), consists in obtaining the spark upon making contact between a plate of zinc and a plate of copper plunged into dilute sulphuric acid. in order to make the arrangement as elementary as possible, mercurial surfaces were dismissed, and the contact made by a copper wire connected with the copper plate, and then brought to touch a clean part of the zinc plate. the electric spark appeared, and it must of necessity have existed and passed _before the zinc and the copper were in contact_. . in order to render more distinct the principles which i have been endeavouring to establish, i will restate them in their simplest form, according to my present belief. the electricity of the voltaic pile ( . note) is not dependent either in its origin or its continuance upon the contact of the metals with each other ( . .). it is entirely due to chemical action ( .), and is proportionate in its intensity to the intensity of the affinities concerned in its production ( .); and in its quantity to the quantity of matter which has been chemically active during its evolution ( .). this definite production is again one of the strongest proofs that the electricity is of chemical origin. . as _volta-electro-generation_ is a case of mere chemical action, so _volta-electro-decomposition_ is simply a case of the preponderance of one set of chemical affinities more powerful in their nature, over another set which are less powerful: and if the instance of two opposing sets of such forces ( .) be considered, and their mutual relation and dependence borne in mind, there appears no necessity for using, in respect to such cases, any other term than chemical affinity, (though that of electricity may be very convenient,) or supposing any new agent to be concerned in producing the results; for we may consider that the powers at the two places of action are in direct communion and balanced against each other through the medium of the metals ( .), fig. , in a manner analogous to that in which mechanical forces are balanced against each other by the intervention of the lever ( .). . all the facts show us that that power commonly called chemical affinity, can be communicated to a distance through the metals and certain forms of carbon; that the electric current is only another form of the forces of chemical affinity; that its power is in proportion to the chemical affinities producing it; that when it is deficient in force it may be helped by calling in chemical aid, the want in the former being made up by an equivalent of the latter; that, in other words, _the forces termed chemical affinity and electricity are one and the same._ . when the circumstances connected with the production of electricity in the ordinary voltaic circuit are examined and compared, it appears that the source of that agent, always meaning the electricity which circulates and completes the current in the voltaic apparatus, and gives that apparatus power and character ( . .), exists in the chemical action which takes place directly between the metal and the body with which it combines, and not at all in the subsequent action of the substance so produced with the acid present[a]. thus, when zinc, platina, and dilute sulphuric acid are used, it is the union of the zinc with the oxygen of the water which determines the current; and though the acid is essential to the removal of the oxide so formed, in order that another portion of zinc may act on another portion of water, it does not, by combination with that oxide, produce any sensible portion of the current of electricity which circulates; for the quantity of electricity is dependent upon the quantity of zinc oxidized, and in definite proportion to it: its intensity is in proportion to the intensity of the chemical affinity of the zinc for the oxygen under the circumstances, and is scarcely, if at all, affected by the use of either strong or weak acid ( .). [a] wollaston, philosophical transactions, , p. . . again, if zinc, platina, and muriatic acid are used, the electricity appears to be dependent upon the affinity of the zinc for the chlorine, and to be circulated in exact proportion to the number of particles of zinc and chlorine which unite, being in fact an equivalent to them. . but in considering this oxidation, or other direct action upon the metal itself, as the cause and source of the electric current, it is of the utmost importance to observe that the oxygen or other body must be in a peculiar condition, namely, in the state of _combination_; and not only so, but limited still further to such a state of combination and in such proportions as will constitute an _electrolyte_ ( .). a pair of zinc and platina plates cannot be so arranged in oxygen gas as to produce a current of electricity, or act as a voltaic circle, even though the temperature may be raised so high as to cause oxidation of the zinc far more rapidly than if the pair of plates were plunged into dilute sulphuric acid; for the oxygen is not part of an electrolyte, and cannot therefore conduct the forces onwards by decomposition, or even as metals do by itself. or if its gaseous state embarrass the minds of some, then liquid chlorine may be taken. it does not excite a current of electricity through the two plates by combining with the zinc, for its particles cannot transfer the electricity active at the point of combination across to the platina. it is not a conductor of itself, like the metals; nor is it an electrolyte, so as to be capable of conduction during decomposition, and hence there is simple chemical action at the spot, and no electric current[a]. [a] i do not mean to affirm that no traces of electricity ever appear in such cases. what i mean is, that no electricity is evolved in any way, due or related to the causes which excite voltaic electricity, or proportionate to them. that which does appear occasionally is the smallest possible fraction of that which the acting matter could produce if arranged so as to act voltaically, probably not the one hundred thousandth, or even the millionth part, and is very probably altogether different in its source. . it might at first be supposed that a conducting body not electrolytic, might answer as the third substance between the zinc and the platina; and it is true that we have some such capable of exerting chemical action upon the metals. they must, however, be chosen from the metals themselves, for there are no bodies of this kind except those substances and charcoal. to decide the matter by experiment, i made the following arrangement. melted tin was put into a glass tube bent into the form of the letter v, fig. , so as to fill the half of each limb, and two pieces of thick platina wire, _p_, _w_, inserted, so as to have their ends immersed some depth in the tin: the whole was then allowed to cool, and the ends _p_ and _w_ connected with a delicate galvanometer. the part of the tube at _x_ was now reheated, whilst the portion _y_ was retained cool. the galvanometer was immediately influenced by the thermo-electric current produced. the heat was steadily increased at _x_, until at last the tin and platina combined there; an effect which is known to take place with strong chemical action and high ignition; but not the slightest additional effect occurred at the galvanometer. no other deflection than that due to the thermo-electric current was observable the whole time. hence, though a conductor, and one capable of exerting chemical action on the tin, was used, yet, not being an _electrolyte_, not the slightest effect of an electrical current could be observed ( .). . from this it seems apparent that the peculiar character and condition of an electrolyte is _essential_ in one part of the voltaic circuit; and its nature being considered, good reasons appear why it and it alone should be effectual. an electrolyte is always a compound body: it can conduct, but only whilst decomposing. its conduction depends upon its decomposition and the _transmission of its particles_ in directions parallel to the current; and so intimate is this connexion, that if their transition be stopped, the current is stopped also; if their course be changed, its course and direction change with them; if they proceed in one direction, it has no power to proceed in any other than a direction invariably dependent on them. the particles of an electrolytic body are all so mutually connected, are in such relation with each other through their whole extent in the direction of the current, that if the last is not disposed of, the first is not at liberty to take up its place in the new combination which the powerful affinity of the most active metal tends to produce; and then the current itself is stopped; for the dependencies of the current and the decomposition are so mutual, that whichsoever be originally determined, i.e. the motion of the particles or the motion of the current, the other is invariable in its concomitant production and its relation to it. . consider, then, water as an electrolyte and also as an oxidizing body. the attraction of the zinc for the oxygen is greater, under the circumstances, than that of the oxygen for the hydrogen; but in combining with it, it tends to throw into circulation a current of electricity in a certain direction. this direction is consistent (as is found by innumerable experiments) with the transfer of the hydrogen from the zinc towards the platina, and the transfer in the opposite direction of fresh oxygen from the platina towards the zinc; so that the current _can pass_ in that one line, and, whilst it passes, can consist with and favour the renewal of the conditions upon the surface of the zinc, which at first determined both the combination and circulation. hence the continuance of the action there, and the continuation of the current. it therefore appears quite as essential that there should be an electrolyte in the circuit, in order that the action may be transferred forward, in a _certain constant direction,_ as that there should be an oxidizing or other body capable of acting directly on the metal; and it also appears to be essential that these two should merge into one, or that the principle directly active on the metal by chemical action should be one of the _ions_ of the electrolyte used. whether the voltaic arrangement be excited by solution of acids, or alkalies, or sulphurets, or by fused substances ( .), this principle has always hitherto, as far as i am aware, been an _anion_ ( .); and i anticipate, from a consideration of the principles of electric action, that it must of necessity be one of that class of bodies. . if the action of the sulphuric acid used in the voltaic circuit be considered, it will be found incompetent to produce any sensible portion of the electricity of the current by its combination with the oxide formed, for this simple reason, it is deficient in a most essential condition: it forms no part of an electrolyte, nor is it in relation with any other body present in the solution which will permit of the mutual transfer of the particles and the consequent transfer of the electricity. it is true, that as the plane at which the acid is dissolving the oxide of zinc formed by the action of the water, is in contact with the metal zinc, there seems no difficulty in considering how the oxide there could communicate an electrical state, proportionate to its own chemical action on the acid, to the metal, which is a conductor without decomposition. but on the side of the acid there is no substance to complete the circuit: the water, as water, cannot conduct it, or at least only so small a proportion that it is merely an incidental and almost inappreciable effect ( .); and it cannot conduct it as an electrolyte, because an electrolyte conducts in consequence of the _mutual_ relation and action of its particles; and neither of the elements of the water, nor even the water itself, as far as we can perceive, are _ions_ with respect to the sulphuric acid ( .)[a]. [a] it will be seen that i here agree with sir humphry davy, who has experimentally supported the opinion that acids and alkalies in combining do not produce any current of electricity. philosophical transactions, , p. . . this view of the secondary character of the sulphuric acid as an agent in the production of the voltaic current, is further confirmed by the fact, that the current generated and transmitted is directly and exactly proportional to the quantity of water decomposed and the quantity of zinc oxidized ( . .), and is the same as that required to decompose the same quantity of water. as, therefore, the decomposition of the water shows that the electricity has passed by its means, there remains no other electricity to be accounted for or to be referred to any action other than that of the zinc and the water on each other. . the general case (for it includes the former one ( .),) of acids and bases, may theoretically be stated in the following manner. let _a_, fig. , be supposed to be a dry oxacid, and _b_ a dry base, in contact at _c_, and in electric communication at their extremities by plates of platina _pp_, and a platina wire _w_. if this acid and base were fluid, and combination took place at _c_, with an affinity ever so vigorous, and capable of originating an electric current, the current could not circulate in any important degree; because, according to the experimental results, neither _a_ nor _b_ could conduct without being decomposed, for they are either electrolytes or else insulators, under all circumstances, except to very feeble and unimportant currents ( . .). now the affinities at _c_ are not such as tend to cause the _elements_ either of _a_ or _b_ to separate, but only such as would make the two bodies combine together as a whole; the point of action is, therefore, insulated, the action itself local ( . .), and no current can be formed. . if the acid and base be dissolved in water, then it is possible that a small portion of the electricity due to chemical action may be conducted by the water without decomposition ( . .); but the quantity will be so small as to be utterly disproportionate to that due to the equivalents of chemical force; will be merely incidental; and, as it does not involve the essential principles of the voltaic pile, it forms no part of the phenomena at present under investigation[a]. [a] it will i trust be fully understood, that in these investigations i am not professing to take an account of every small, incidental, or barely possible effect, dependent upon slight disturbances of the electric fluid during chemical action, but am seeking to distinguish and identify those actions on which the power of the voltaic battery essentially depends. . if for the oxacid a hydracid be substituted ( .),--as one analogous to the muriatic, for instance,--then the state of things changes altogether, and a current due to the chemical action of the acid on the base is possible. but now both the bodies act as electrolytes, for it is only one principle of each which combine mutually,--as, for instance, the chlorine with the metal,--and the hydrogen of the acid and the oxygen of the base are ready to traverse with the chlorine of the acid and the metal of the base in conformity with the current and according to the general principles already so fully laid down. . this view of the oxidation of the metal, or other _direct_ chemical action upon it, being the sole cause of the production of the electric current in the ordinary voltaic pile, is supported by the effects which take place when alkaline or sulphuretted solutions ( . .) are used for the electrolytic conductor instead of dilute sulphuric acid. it was in elucidation of this point that the experiments without metallic contact, and with solution of alkali as the exciting fluid, already referred to ( .), were made. . advantage was then taken of the more favourable condition offered, when metallic contact is allowed ( .), and the experiments upon the decomposition of bodies by a single pair of plates ( .) were repeated, solution of caustic potassa being employed in the vessel _v_, fig. . in place of dilute sulphuric acid. all the effects occurred as before: the galvanometer was deflected; the decompositions of the solutions of iodide of potassium, nitrate of silver, muriatic acid, and sulphate of soda ensued at _x_; and the places where the evolved principles appeared, as well as the deflection of the galvanometer, indicated a current in the _same direction_ as when acid was in the vessel _v_; i.e. from the zinc through the solution to the platina, and back by the galvanometer and substance suffering decomposition to the zinc. . the similarity in the action of either dilute sulphuric acid or potassa goes indeed far beyond this, even to the proof of identity in _quantity_ as well as in _direction_ of the electricity produced. if a plate of amalgamated zinc be put into a solution of potassa, it is not sensibly acted upon; but if touched in the solution by a plate of platina, hydrogen is evolved on the surface of the latter metal, and the zinc is oxidized exactly as when immersed in dilute sulphuric acid ( .). i accordingly repeated the experiment before described with weighed plates of zinc ( . &c.), using however solution of potassa instead of dilute sulphuric acid. although the time required was much longer than when acid was used, amounting to three hours for the oxidizement of . grains of zinc, still i found that the hydrogen evolved at the platina plate was the equivalent of the metal oxidized at the surface of the zinc. hence the whole of the reasoning which was applicable in the former instance applies also here, the current being in the same direction, and its decomposing effect in the same degree, as if acid instead of alkali had been used ( .). . the proof, therefore, appears to me complete, that the combination of the acid with the oxide, in the former experiment, had nothing to do with the production of the electric current; for the same current is here produced when the action of the acid is absent, and the reverse action of an alkali is present. i think it cannot be supposed for a moment, that the alkali acted chemically as an acid to the oxide formed; on the contrary, our general chemical knowledge leads to the conclusion, that the ordinary metallic oxides act rather as acids to the alkalies; yet that kind of action would tend to give a reverse current in the present case, if any were due to the union of the oxide of the exciting metal with the body which combines with it. but instead of any variation of this sort, the direction of the electricity was constant, and its quantity also directly proportional to the water decomposed, or the zinc oxidized. there are reasons for believing that acids and alkalies, when in contact with metals upon which they cannot act directly, still have a power of influencing their attractions for oxygen ( .); but all the effects in these experiments prove, i think, that it is the oxidation of the metal necessarily dependent upon, and associated as it is with, the electrolyzation of the water ( . .) that produces the current; and that the acid or alkali merely acts as solvents, and by removing the oxidized zinc, allows other portions to decompose fresh water, and so continues the evolution or determination of the current. . the experiments were then varied by using solution of ammonia instead of solution of potassa; and as it, when pure, is like water, a bad conductor ( .), it was occasionally improved in that power by adding sulphate of ammonia to it. but in all the cases the results were the same as before; decompositions of the same kind were effected, and the electric current producing these was in the same direction as in the experiments just described. . in order to put the equal and similar action of acid and alkali to stronger proof, arrangements were made as in fig. .; the glass vessel a contained dilute sulphuric acid, the corresponding glass vessel b solution of potassa, pp was a plate of platina dipping into both solutions, and zz two plates of amalgamated zinc connected with a delicate galvanometer. when these were plunged at the same time into the two vessels, there was generally a first feeble effect, and that in favour of the alkali, i.e. the electric current tended to pass through the vessels in the direction of the arrow, being the reverse direction of that which the acid in a would have produced alone: but the effect instantly ceased, and the action of the plates in the vessels was so equal, that, being contrary because of the contrary position of the plates, no permanent current resulted. . occasionally a zinc plate was substituted for the plate pp, and platina plates for the plates zz; but this caused no difference in the results: nor did a further change of the middle plate to copper produce any alteration. . as the opposition of electro-motive pairs of plates produces results other than those due to the mere difference of their independent actions ( . .), i devised another form of apparatus, in which the action of acid and alkali might be more directly compared. a cylindrical glass cup, about two inches deep within, an inch in internal diameter, and at least a quarter of an inch in thickness, was cut down the middle into halves, fig. . a broad brass ring, larger in diameter than the cup, was supplied with a screw at one side; so that when the two halves of the cup were within the ring, and the screw was made to press tightly against the glass, the cup held any fluid put into it. bibulous paper of different degrees of permeability was then cut into pieces of such a size as to be easily introduced between the loosened halves of the cup, and served when the latter were tightened again to form a porous division down the middle of the cup, sufficient to keep any two fluids on opposite sides of the paper from mingling, except very slowly, and yet allowing them to act freely as one _electrolyte_. the two spaces thus produced i will call the cells a and b, fig. . this instrument i have found of most general application in the investigation of the relation of fluids and metals amongst themselves and to each other. by combining its use with that of the galvanometer, it is easy to ascertain the relation of one metal with two fluids, or of two metals with one fluid, or of two metals and two fluids upon each other. . dilute sulphuric acid, sp. gr. . , was put into the cell a, and a strong solution of caustic potassa into the cell b; they mingled slowly through the paper, and at last a thick crust of sulphate of potassa formed on the side of the paper next to the alkali. a plate of clean platina was put into each cell and connected with a delicate galvanometer, but no electric current could be observed. hence the _contact_ of acid with one platina plate, and alkali with the other, was unable to produce a current; nor was the combination of the acid with the alkali more effectual ( .). . when one of the platina plates was removed and a zinc plate substituted, either amalgamated or not, a strong electric current was produced. but, whether the zinc were in the acid whilst the platina was in the alkali, or whether the reverse order were chosen, the electric current was always from the zinc through the electrolyte to the platina, and back through the galvanometer to the zinc, the current seeming to be strongest when the zinc was in the alkali and the platina in the acid. . in these experiments, therefore, the acid seems to have no power over the alkali, but to be rather inferior to it in force. hence there is no reason to suppose that the combination of the oxide formed with the acid around it has any direct influence in producing the electricity evolved, the whole of which appears to be due to the oxidation of the metal ( .). . the alkali, in fact, is superior to the acid in bringing a metal into what is called the positive state; for if plates of the same metal, as zinc, tin, lead, or copper, be used both in the acid or alkali, the electric current is from the alkali across the cell to the acid, and back through the galvanometer to the alkali, as sir humphry davy formerly stated [a]. this current is so powerful, that if amalgamated zinc, or tin, or lead be used, the metal in the acid evolves hydrogen the moment it is placed in communication with that in the alkali, not from any direct action of the acid upon it, for if the contact be broken the action ceases, but because it is powerfully negative with regard to the metal in the alkali. [a] elements of chemical philosophy, p. ; or philosophical transactions, , p. . . the superiority of alkali is further proved by this, that if zinc and tin be used, or tin and lead, whichsoever metal is put into the alkali becomes positive, that in the acid being negative. whichsoever is in the alkali is oxidized, whilst that in the acid remains in the metallic state, as far as the electric current is concerned. . when sulphuretted solutions are used ( .) in illustration of the assertion, that it is the chemical action of the metal and one of the _ions_ of the associated electrolyte that produces all the electricity of the voltaic circuit, the proofs are still the same. thus, as sir humphry davy[a] has shown, if iron and copper be plunged into dilute acid, the current is from the iron through the liquid to the copper; in solution of potassa it is in the same direction, but in solution of sulphuret of potassa it is reversed. in the two first cases it is oxygen which combines with the iron, in the latter sulphur which combines with the copper, that produces the electric current; but both of these are _ions_, existing as such in the electrolyte, which is at the same moment suffering decomposition; and, what is more, both of these are _anions_, for they leave the electrolytes at their _anodes_, and act just as chlorine, iodine, or any other _anion_ would act which might have been previously chosen as that which should be used to throw the voltaic circle into activity. [a] elements of chemical philosophy, p. . . the following experiments complete the series of proofs of the origin of the electricity in the voltaic pile. a fluid amalgam of potassium, containing not more than a hundredth of that metal, was put into pure water, and connected, through the galvanometer with a plate of platina in the same water. there was immediately an electric current from the amalgam through the electrolyte to the platina. this must have been due to the oxidation only of the metal, for there was neither acid nor alkali to combine with, or in any way act on, the body produced. . again, a plate of clean lead and a plate of platina were put into _pure_ water. there was immediately a powerful current produced from the lead through the fluid to the platina: it was even intense enough to decompose solution of the iodide of potassium when introduced into the circuit in the form of apparatus already described ( .), fig. . here no action of acid or alkali on the oxide formed from the lead could supply the electricity: it was due solely to the oxidation of the metal. * * * * * . there is no point in electrical science which seems to me of more importance than the state of the metals and the electrolytic conductor in a simple voltaic circuit _before and at_ the moment when metallic contact is first completed. if clearly understood, i feel no doubt it would supply us with a direct key to the laws under which the great variety of voltaic excitements, direct and incidental, occur, and open out new fields of research for our investigation[a]. [a] in connexion with this part of the subject refer now to series xi. , series xii. - , and series xiii. . &c.--_dec. ._ . we seem to have the power of deciding to a certain extent in numerous cases of chemical affinity, (as of zinc with the oxygen of water, &c. &c.) which of _two modes of action of the attractive power_ shall be exerted ( .). in the one mode we can transfer the power onwards, and make it produce elsewhere its equivalent of action ( . .); in the other, it is not transferred, but exerted wholly at the spot. the first is the case of volta-electric excitation, the other ordinary chemical affinity: but both are chemical actions and due to one force or principle. . the general circumstances of the former mode occur in all instances of voltaic currents, but may be considered as in their perfect condition, and then free from those of the second mode, in some only of the cases; as in those of plates of zinc and platina in solution of potassa, or of amalgamated zinc and platina in dilute sulphuric acid. . assuming it sufficiently proved, by the preceding experiments and considerations, that the electro-motive action depends, when zinc, platina, and dilute sulphuric acid are used, upon the mutual affinity of the metal zinc and the oxygen of the water ( . .), it would appear that the metal, when alone, has not power enough, under the circumstances, to take the oxygen and expel the hydrogen from the water; for, in fact, no such action takes place. but it would also appear that it has power so far to act, by its attraction for the oxygen of the particles in contact with it, as to place the similar forces already active between these and the other particles of oxygen and the particles of hydrogen in the water, in a peculiar state of tension or polarity, and probably also at the same time to throw those of its own particles which are in contact with the water into a similar but opposed state. whilst this state is retained, no further change occurs; but when it is relieved, by completion of the circuit, in which case the forces determined in opposite directions, with respect to the zinc and the electrolyte, are found exactly competent to neutralize each other, then a series of decompositions and recompositions takes place amongst the particles of oxygen and hydrogen constituting the water, between the place of contact with the platina and the place where the zinc is active; these intervening particles being evidently in close dependence upon and relation to each other. the zinc forms a direct compound with those particles of oxygen which were, previously, in divided relation to both it and the hydrogen: the oxide is removed by the acid, and a fresh surface of zinc is presented to the water, to renew and repeat the action. . practically, the state of tension is best relieved by dipping a metal which has less attraction for oxygen than the zinc, into the dilute acid, and making it also touch the zinc. the force of chemical affinity, which has been influenced or polarized in the particles of the water by the dominant attraction of the zinc for the oxygen, is then transferred, in a most extraordinary manner, through the two metals, so as to re-enter upon the circuit in the electrolytic conductor, which, unlike the metals in that respect, cannot convey or transfer it without suffering decomposition; or rather, probably, it is exactly balanced and neutralized by the force which at the same moment completes the combination of the zinc with the oxygen of the water. the forces, in fact, of the two particles which are acting towards each other, and which are therefore in opposite directions, are the origin of the two opposite forces, or directions of force, in the current. they are of necessity equivalent to each other. being transferred forward in contrary directions, they produce what is called the voltaic current: and it seems to me impossible to resist the idea that it must be preceded by a _state of tension_ in the fluid, and between the fluid and the zinc; the _first consequence_ of the affinity of the zinc for the oxygen of the water. . i have sought carefully for indications of a state of tension in the electrolytic conductor; and conceiving that it might produce something like structure, either before or during its discharge, i endeavoured to make this evident by polarized light. a glass cell, seven inches long, one inch and a half wide, and six inches deep, had two sets of platina electrodes adapted to it, one set for the ends, and the other for the sides. those for the _sides_ were seven inches long by three inches high, and when in the cell were separated by a little frame of wood covered with calico; so that when made active by connexion with a battery upon any solution in the cell, the bubbles of gas rising from them did not obscure the central parts of the liquid. . a saturated solution of sulphate of soda was put into the cell, and the electrodes connected with a battery of pairs of -inch plates: the current of electricity was conducted across the cell so freely, that the discharge was as good as if a wire had been used. a ray of polarized light was then transmitted through this solution, directly across the course of the electric current, and examined by an analysing plate; but though it penetrated seven inches of solution thus subject to the action of the electricity, and though contact was sometimes made, sometimes broken, and occasionally reversed during the observations, not the slightest trace of action on the ray could be perceived. . the large electrodes were then removed, and others introduced which fitted the _ends_ of the cell. in each a slit was cut, so as to allow the light to pass. the course of the polarized ray was now parallel to the current, or in the direction of its axis ( .); but still no effect, under any circumstances of contact or disunion, could be perceived upon it. . a strong solution of nitrate of lead was employed instead of the sulphate of soda, but no effects could be detected. . thinking it possible that the discharge of the electric forces by the successive decompositions and recompositions of the particles of the electrolyte might neutralize and therefore destroy any effect which the first state of tension could by possibility produce, i took a substance which, being an excellent electrolyte when fluid, was a perfect insulator when solid, namely, borate of lead, in the form of a glass plate, and connecting the sides and the edges of this mass with the metallic plates, sometimes in contact with the poles of a voltaic battery, and sometimes even with the electric machine, for the advantage of the much higher intensity then obtained, i passed a polarized ray across it in various directions, as before, but could not obtain the slightest appearance of action upon the light. hence i conclude, that notwithstanding the new and extraordinary state which must be assumed by an electrolyte, either during decomposition (when a most enormous quantity of electricity must be traversing it), or in the state of tension which is assumed as preceding decomposition, and which might be supposed to be retained in the solid form of the electrolyte, still it has no power of affecting a polarized ray of light; for no kind of structure or tension can in this way be rendered evident. . there is, however, one beautiful experimental proof of a state of tension acquired by the metals and the electrolyte before the electric current is produced, and _before contact_ of the different metals is made ( .); in fact, at that moment when chemical forces only are efficient as a cause of action. i took a voltaic apparatus, consisting of a single pair of large plates, namely, a cylinder of amalgamated zinc, and a double cylinder of copper. these were put into a jar containing dilute sulphuric acid[a], and could at pleasure be placed in metallic communication by a copper wire adjusted so as to dip at the extremities into two cups of mercury connected with the two plates. [a] when nitro-sulphuric acid is used, the spark is more powerful, but local chemical action can then commence, and proceed without requiring metallic contact. . being thus arranged, there was no chemical action whilst the plates were not connected. on _making_ the connexion a spark was obtained[a], and the solution was immediately decomposed. on breaking it, the usual spark was obtained, and the decomposition ceased. in this case it is evident that the first spark must have occurred before metallic contact was made, for it passed through an interval of air; and also that it must have tended to pass before the electrolytic action began; for the latter could not take place until the current passed, and the current could not pass before the spark appeared. hence i think there is sufficient proof, that as it is the zinc and water which by their mutual action produce the electricity of this apparatus, so these, by their first contact with each other, were placed in a state of powerful tension ( .), which, though it could not produce the actual decomposition of the water, was able to make a spark of electricity pass between the zinc and a fit discharger as soon as the interval was rendered sufficiently small. the experiment demonstrates the direct production of the electric spark from pure chemical forces. [a] it has been universally supposed that no spark is produced on making the contact between a single pair of plates. i was led to expect one from the considerations already advanced in this paper. the wire of communication should be short; for with a long wire, circumstances strongly affecting the spark are introduced. . there are a few circumstances connected with the production of this spark by a single pair of plates, which should be known, to ensure success to the experiment[b]. when the amalgamated surfaces of contact are quite clean and dry, the spark, on making contact, is quite as brilliant as on breaking it, if not even more so. when a film of oxide or dirt was present at either mercurial surface, then the first spark was often feeble, and often failed, the breaking spark, however, continuing very constant and bright. when a little water was put over the mercury, the spark was greatly diminished in brilliancy, but very regular both on making and breaking contact. when the contact was made between clean platina, the spark was also very small, but regular both ways. the true electric spark is, in fact, very small, and when surfaces of mercury are used, it is the combustion of the metal which produces the greater part of the light. the circumstances connected with the burning of the mercury are most favourable on breaking contact; for the act of separation exposes clean surfaces of metal, whereas, on making contact, a thin film of oxide, or soiling matter, often interferes. hence the origin of the general opinion that it is only when the contact is broken that the spark passes. [b] see in relation to precautions respecting a spark, .--_dec. ._ . with reference to the other set of cases, namely, those of local action ( .) in which chemical affinity being exerted causes no transference of the power to a distance where no electric current is produced, it is evident that forces of the most intense kind must be active, and in some way balanced in their activity, during such combinations; these forces being directed so immediately and exclusively towards each other, that no signs of the powerful electric current they can produce become apparent, although the same final state of things is obtained as if that current had passed. it was berzelius, i believe, who considered the heat and light evolved in cases of combustion as the consequences of this mode of exertion of the electric powers of the combining particles. but it will require a much more exact and extensive knowledge of the nature of electricity, and the manner in which it is associated with the atoms of matter, before we can understand accurately the action of this power in thus causing their union, or comprehend the nature of the great difference which it presents in the two modes of action just distinguished. we may imagine, but such imaginations must for the time be classed with the great mass of _doubtful knowledge_ ( .) which we ought rather to strive to diminish than to increase; for the very extensive contradictions of this knowledge by itself shows that but a small portion of it can ultimately prove true[a]. [a] refer to , &c. series xiv.--_dec. ._ . of the two modes of action in which chemical affinity is exerted, it is important to remark, that that which produces the electric current is as _definite_ as that which causes ordinary chemical combination; so that in examining the _production_ or _evolution_ of electricity in cases of combination or decomposition, it will be necessary, not merely to observe certain effects dependent upon a current of electricity, but also their _quantity_: and though it may often happen that the forces concerned in any particular case of chemical action may be partly exerted in one mode and partly in the other, it is only those which are efficient in producing the current that have any relation to voltaic action. thus, in the combination of oxygen and hydrogen to produce water, electric powers to a most enormous amount are for the time active ( . .); but any mode of examining the flame which they form during energetic combination, which has as yet been devised, has given but the feeblest traces. these therefore may not, cannot, be taken as evidences of the nature of the action; but are merely incidental results, incomparably small in relation to the forces concerned, and supplying no information of the way in which the particles are active on each other, or in which their forces are finally arranged. . that such cases of chemical action produce no _current of electricity_, is perfectly consistent with what we know of the voltaic apparatus, in which it is essential that one of the combining elements shall form part of, or be in direct relation with, an electrolytic conductor ( . .). that such cases produce _no free electricity of tension_, and that when they are converted into cases of voltaic action they produce a current in which the opposite forces are so equal as to neutralize each other, prove the equality of the forces in the opposed acting particles of matter, and therefore the equality of electric power in those quantities of matter which are called _electro-chemical equivalents_ ( ). hence another proof of the definite nature of electro-chemical action ( . &c.), and that chemical affinity and electricity are forms of the same power ( . &c.). . the direct reference of the effects produced by the voltaic pile at the place of experimental decomposition to the chemical affinities active at the place of excitation ( . .), gives a very simple and natural view of the cause why the bodies (or _ions_) evolved pass in certain directions; for it is only when they pass in those directions that their forces can consist with and compensate (in direction at least) the superior forces which are dominant at the place where the action of the whole is determined. if, for instance, in a voltaic circuit, the activity of which is determined, by the attraction of zinc for the oxygen of water, the zinc move from right to left, then any other _cation_ included in the circuit, being part of an electrolyte, or forming part of it at the moment, will also move from right to left: and as the oxygen of the water, by its natural affinity for the zinc, moves from left to right, so any other body of the same class with it (i.e. any other _anion_), under its government for the time, will move from left to right. . this i may illustrate by reference to fig. , the double circle of which may represent a complete voltaic circuit, the direction of its forces being determined by supposing for a moment the zinc _b_ and the platina _c_ as representing plates of those metals acting upon water, _d, e_, and other substances, but having their energy exalted so as to effect several decompositions by the use of a battery at _a_ ( .). this supposition may be allowed, because the action in the battery will only consist of repetitions of what would take place between _b_ and _c_, if they really constituted but a single pair. the zinc _b_, and the oxygen _d_, by their mutual affinity, tend to unite; but as the oxygen is already in association with the hydrogen _e_, and has its inherent chemical or electric powers neutralized for the time by those of the latter, the hydrogen _e_ must leave the oxygen _d_, and advance in the direction of the arrow head, or else the zinc _b_ cannot move in the same direction to unite to the oxygen _d_, nor the oxygen _d_ move in the contrary direction to unite to the zinc _b_, the relation of the _similar_ forces of _b_ and _c_, in contrary directions, to the _opposite_ forces of _d_ being the preventive. as the hydrogen _e_ advances, it, on coming against the platina _c, f_, which forms a part of the circuit, communicates its electric or chemical forces through it to the next electrolyte in the circuit, fused chloride of lead, _g, h_, where the chlorine must move in conformity with the direction of the oxygen at _d_, for it has to compensate the forces disturbed in its part of the circuit by the superior influence of those between the oxygen and zinc at _d, b_, aided as they are by those of the battery _a_; and for a similar reason the lead must move in the direction pointed out by the arrow head, that it may be in right relation to the first moving body of its own class, namely, the zinc _b_. if copper intervene in the circuit from _i_ to _k_, it acts as the platina did before; and if another electrolyte, as the iodide of tin, occur at _l, m_, then the iodine _l_, being an _anion_, must move in conformity with the exciting _anion_, namely, the oxygen _d_, and the _cation_ tin _m_ move in correspondence with the other _cations b, e_, and _h_, that the chemical forces may be in equilibrium as to their direction and quantity throughout the circuit. should it so happen that the anions in their circulation can combine with the metals at the _anodes_ of the respective electrolytes, as would be the case at the platina _f_ and the copper _k_, then those bodies becoming parts of electrolytes, under the influence of the current, immediately travel; but considering their relation to the zinc _b_, it is evidently impossible that they can travel in any other direction than what will accord with its course, and therefore can never tend to pass otherwise than _from_ the anode and _to_ the cathode. . in such a circle as that delineated, therefore, all the known _anions_ may be grouped within, and all the _cations_ without. if any number of them enter as _ions_ into the constitution of _electrolytes_, and, forming one circuit, are simultaneously subject to one common current, the anions must move in accordance with each other in one direction, and the cations in the other. nay, more than that, equivalent portions of these bodies must so advance in opposite directions: for the advance of every . parts of the zinc _b_ must be accompanied by a motion in the opposite direction of parts of oxygen at _d_, of parts of chlorine at _g_, of parts of iodine at _l_; and in the same direction by electro-chemical equivalents of hydrogen, lead, copper and tin, at _e, h, k_. and _m_. . if the present paper be accepted as a correct expression of facts, it will still only prove a confirmation of certain general views put forth by sir humphry davy in his bakerian lecture for [a], and revised and re-stated by him in another bakerian lecture, on electrical and chemical changes, for the year [b]. his general statement is, that "_chemical and electrical attractions were produced by the same cause, acting in one case on particles, in the other on masses, of matter; and that the same property, under different modifications, was the cause of all the phenomena exhibited by different voltaic combinations_[c]." this statement i believe to be true; but in admitting and supporting it, i must guard myself from being supposed to assent to all that is associated with it in the two papers referred to, or as admitting the experiments which are there quoted as decided proofs of the truth of the principle. had i thought them so, there would have been no occasion for this investigation. it may be supposed by some that i ought to go through these papers, distinguishing what i admit from what i reject, and giving good experimental or philosophical reasons for the judgment in both cases. but then i should be equally bound to review, for the same purpose, all that has been written both for and against the necessity of metallic contact,--for and against the origin of voltaic electricity in chemical action,--a duty which i may not undertake in the present paper[d]. [a] philosophical transactions, . [b] ibid. , p. . [c] ibid. , p. . [d] i at one time intended to introduce here, in the form of a note, a table of reference to the papers of the different philosophers who have referred the origin of the electricity in the voltaic pile to contact, or to chemical action, or to both; but on the publication of the first volume of m. becquerel's highly important and valuable traité de l'electricité et du magnétisme, i thought it far better to refer to that work for these references, and the views held by the authors quoted. see pages , , , , , , , , , , , , , , , , , , , , &c.--july rd, . ¶ ii. _on the intensity necessary for electrolyzation._ . it became requisite, for the comprehension of many of the conditions attending voltaic action, to determine positively, if possible, whether electrolytes could resist the action of an electric current when beneath a certain intensity? whether the intensity at which the current ceased to act would be the same for all bodies? and also whether the electrolytes thus resisting decomposition would conduct the electric current as a metal does, after they ceased to conduct as electrolytes, or would act as perfect insulators? . it was evident from the experiments described ( . .) that different bodies were decomposed with very different facilities, and apparently that they required for their decomposition currents of different intensities, resisting some, but giving way to others. but it was needful, by very careful and express experiments, to determine whether a current could really pass through, and yet not decompose an electrolyte ( .). . an arrangement (fig. .) was made, in which two glass vessels contained the same dilute sulphuric acid, sp. gr. . . the plate _z_ was amalgamated zinc, in connexion, by a platina wire _a_, with the platina plate _e_; _b_ was a platina wire connecting the two platina plates pp'; _c_ was a platina wire connected with the platina plate p". on the plate _e_ was placed a piece of paper moistened in solution of iodide of potassium: the wire _c_ was so curved that its end could be made to rest at pleasure on this paper, and show, by the evolution of iodine there, whether a current was passing; or, being placed in the dotted position, it formed a direct communication with the platina plate _e_, and the electricity could pass without causing decomposition. the object was to produce a current by the action of the acid on the amalgamated zinc in the first vessel a; to pass it through the acid in the second vessel b by platina electrodes, that its power of decomposing water might, if existing, be observed; and to verify the existence of the current at pleasure, by decomposition at _e_, without involving the continual obstruction to the current which would arise from making the decomposition there constant. the experiment, being arranged, was examined and the existence of a current ascertained by the decomposition at _e_; the whole was then left with the end of the wire _c_ resting on the plate _e_, so as to form a constant metallic communication there. . after several hours, the end of the wire _c_ was replaced on the test-paper at _e_: decomposition occurred, and _the proof_ of a passing current was therefore complete. the current was very feeble compared to what it had been at the beginning of the experiment, because of a peculiar state acquired by the metal surfaces in the second vessel, which caused them to oppose the passing current by a force which they possess under these circumstances ( .). still it was proved, by the decomposition, that this state of the plates in the second vessel was not able entirely to stop the current determined in the first, and that was all that was needful to be ascertained in the present inquiry. . this apparatus was examined from time to time, and an electric current always found circulating through it, until twelve days had elapsed, during which the water in the second vessel had been constantly subject to its action. notwithstanding this lengthened period, not the slightest appearance of a bubble upon either of the plates in that vessel occurred. from the results of the experiment, i conclude that a current _had_ passed, but of so low an intensity as to fall beneath that degree at which the elements of water, unaided by any secondary force resulting from the capability of combination with the matter of the electrodes, or of the liquid surrounding them, separated from each other. . it may be supposed, that the oxygen and hydrogen had been evolved in such small quantities as to have entirely dissolved in the water, and finally to have escaped at the surface, or to have reunited into water. that the hydrogen can be so dissolved was shown in the first vessel; for after several days minute bubbles of gas gradually appeared upon a glass rod, inserted to retain the zinc and platina apart, and also upon the platina plate itself, and these were hydrogen. they resulted principally in this way:--notwithstanding the amalgamation of the zinc, the acid exerted a little direct action upon it, so that a small stream of hydrogen bubbles was continually rising from its surface; a little of this hydrogen gradually dissolved in the dilute acid, and was in part set free against the surfaces of the rod and the plate, according to the well-known action of such solid bodies in solutions of gases ( . &c.). . but if the gases had been evolved in the second vessel by the decomposition of water, and had tended to dissolve, still there would have been every reason to expect that a few bubbles should have appeared on the electrodes, especially on the negative one, if it were only because of its action as a nucleus on the solution supposed to be formed; but none appeared even after twelve days. . when a few drops only of nitric acid were added to the vessel a, fig. , then the results were altogether different. in less than five minutes bubbles of gas appeared on the plates p' and p" in the second vessel. to prove that this was the effect of the electric current (which by trial at _c_ was found at the same time to be passing,) the connexion at _c_ was broken, the plates p'p" cleared from bubbles and left in the acid of the vessel b, for fifteen minutes: during that time no bubbles appeared upon them; but on restoring the communication at _c_, a minute did not elapse before gas appeared in bubbles upon the plates. the proof, therefore, is most full and complete, that the current excited by dilute sulphuric acid with a little nitric acid in vessel a, has intensity enough to overcome the chemical affinity exerted between the oxygen and hydrogen of the water in the vessel b, whilst that excited by dilute sulphuric acid alone has _not_ sufficient intensity. . on using a strong solution of caustic potassa in the vessel a, to excite the current, it was found by the decomposing effects at _e_, that the current passed. but it had not intensity enough to decompose the water in the vessel b; for though left for fourteen days, during the whole of which time the current was found to be passing, still not the slightest appearance of gas appeared on the plates p'p", nor any other signs of the water having suffered decomposition. . sulphate of soda in solution was then experimented with, for the purpose of ascertaining with respect to it, whether a certain electrolytic intensity was also required for its decomposition in this state, in analogy with the result established with regard to water ( ). the apparatus was arranged as in fig. ; p and z are the platina and zinc plates dipping into a solution of common salt; _a_ and _b_ are platina plates connected by wires of platina (except in the galvanometer _g_) with p and z; _c_ is a connecting wire of platina, the ends of which can be made to rest either on the plates _a, b_, or on the papers moistened in solutions which are placed upon them; so that the passage of the current without decomposition, or with one or two decompositions, was under ready command, as far as arrangement was concerned. in order to change the _anodes_ and _cathodes_ at the places of decomposition, the form of apparatus fig. , was occasionally adopted. here only one platina plate, _c_, was used; both pieces of paper on which decomposition was to be effected were placed upon it, the wires from p and z resting upon these pieces of paper, or upon the plate _c_, according as the current with or without decomposition of the solutions was required. . on placing solution of iodide of potassium in paper at one of the decomposing localities, and solution of sulphate of soda at the other, so that the electric current should pass through both at once, the solution of iodide was slowly decomposed, yielding iodine at the _anode_ and alkali at the _cathode_; but the solution of sulphate of soda exhibited no signs of decomposition, neither acid nor alkali being evolved from it. on placing the wires so that the iodide alone was subject to the action of the current ( .), it was quickly and powerfully decomposed; but on arranging them so that the sulphate of soda alone was subject to action, it still refused to yield up its elements. finally, the apparatus was so arranged under a wet bell-glass, that it could be left for twelve hours, the current passing during the whole time through a solution of sulphate of soda, retained in its place by only two thicknesses of bibulous litmus and turmeric paper. at the end of that time it was ascertained by the decomposition of iodide of potassium at the second place of action, that the current was passing and had passed for the twelve hours, and yet no trace of acid or alkali from the sulphate of soda appeared. . from these experiments it may, i think, be concluded, that a solution of sulphate of soda can conduct a current of electricity, which is unable to decompose the neutral salt present; that this salt in the state of solution, like water, requires a certain electrolytic intensity for its decomposition; and that the necessary intensity is much higher for this substance than for the iodide of potassium in a similar state of solution. . i then experimented on bodies rendered decomposable by fusion, and first on _chloride of lead_. the current was excited by dilute sulphuric acid without any nitric acid between zinc and platina plates, fig. , and was then made to traverse a little chloride of lead fused upon glass at _a_, a paper moistened in solution of iodide of potassium at _b_, and a galvanometer at _g_. the metallic terminations at _a_ and _b_ were of platina. being thus arranged, the decomposition at _b_ and the deflection at _g_ showed that an electric current was passing, but there was no appearance of decomposition at _a_, not even after a _metallic_ communication at _b_ was established. the experiment was repeated several times, and i am led to conclude that in this case the current has not intensity sufficient to cause the decomposition of the chloride of lead; and further, that, like water ( .), fused chloride of lead can conduct an electric current having an intensity below that required to effect decomposition. . _chloride of silver_ was then placed at _a_, fig. , instead of chloride of lead. there was a very ready decomposition of the solution of iodide of potassium at _b_, and when metallic contact was made there, very considerable deflection of the galvanometer needle at _g_. platina also appeared to be dissolved at the anode of the fused chloride at _a_, and there was every appearance of a decomposition having been effected there. . a further proof of decomposition was obtained in the following manner. the platina wires in the fused chloride at _a_ were brought very near together (metallic contact having been established at _b_), and left so; the deflection at the galvanometer indicated the passage of a current, feeble in its force, but constant. after a minute or two, however, the needle would suddenly be violently affected, and indicate a current as strong as if metallic contact had taken place at _a_. this i actually found to be the case, for the silver reduced by the action of the current crystallized in long delicate spiculæ, and these at last completed the metallic communication; and at the same time that they transmitted a more powerful current than the fused chloride, they proved that electro-chemical decomposition of that chloride had been going on. hence it appears, that the current excited by dilute sulphuric acid between zinc and platina, has an intensity above that required to electrolyze the fused chloride of silver when placed between platina electrodes, although it has not intensity enough to decompose chloride of lead under the same circumstances. . a drop of _water_ placed at _a_ instead of the fused chlorides, showed as in the former case ( .), that it could conduct a current unable to decompose it, for decomposition of the solution of iodide at _b_ occurred after some time. but its conducting power was much below that of the fused chloride of lead ( .). . fused _nitre_ at _a_ conducted much better than water: i was unable to decide with certainty whether it was electrolyzed, but i incline to think not, for there was no discoloration against the platina at the _cathode_. if sulpho-nitric acid had been used in the exciting vessel, both the nitre and the chloride of lead would have suffered decomposition like the water ( .). . the results thus obtained of conduction without decomposition, and the necessity of a certain electrolytic intensity for the separation of the _ions_ of different electrolytes, are immediately connected with the experiments and results given in § . of the fourth series of these researches ( . . . .). but it will require a more exact knowledge of the nature of intensity, both as regards the first origin of the electric current, and also the manner in which it may be reduced, or lowered by the intervention of longer or shorter portions of bad conductors, whether decomposable or not, before their relation can be minutely and fully understood. . in the case of water, the experiments i have as yet made, appear to show, that, when the electric current is reduced in intensity below the point required for decomposition, then the degree of conduction is the same whether sulphuric acid, or any other of the many bodies which can affect its transferring power as an electrolyte, are present or not. or, in other words, that the necessary electrolytic intensity for water is the same whether it be pure, or rendered a better conductor by the addition of these substances; and that for currents of less intensity than this, the water, whether pure or acidulated, has equal conducting power. an apparatus, fig. , was arranged with dilute sulphuric acid in the vessel a, and pure distilled water in the vessel b. by the decomposition at _c_, it appeared as if water was a _better_ conductor than dilute sulphuric acid for a current of such low intensity as to cause no decomposition. i am inclined, however, to attribute this apparent superiority of water to variations in that peculiar condition of the platina electrodes which is referred to further on in this series ( .), and which is assumed, as far as i can judge, to a greater degree in dilute sulphuric acid than in pure water. the power therefore, of acids, alkalies, salts, and other bodies in solution, to increase conducting power, appears to hold good only in those cases where the electrolyte subject to the current suffers decomposition, and loses all influence when the current transmitted has too low an intensity to affect chemical change. it is probable that the ordinary conducting power of an electrolyte in the solid state ( .) is the same as that which it possesses in the fluid state for currents, the tension of which is beneath the due electrolytic intensity. . currents of electricity, produced by less than eight or ten series of voltaic elements, can be reduced to that intensity at which water can conduct them without suffering decomposition, by causing them to pass through three or four vessels in which water shall be successively interposed between platina surfaces. the principles of interference upon which this effect depends, will be described hereafter ( . .), but the effect may be useful in obtaining currents of standard intensity, and is probably applicable to batteries of any number of pairs of plates. . as there appears every reason to expect that all electrolytes will be found subject to the law which requires an electric current of a certain intensity for their decomposition, but that they will differ from each other in the degree of intensity required, it will be desirable hereafter to arrange them in a table, in the order of their electrolytic intensities. investigations on this point must, however, be very much extended, and include many more bodies than have been here mentioned before such a table can be constructed. it will be especially needful in such experiments, to describe the nature of the electrodes used, or, if possible, to select such as, like platina or plumbago in certain cases, shall have no power of assisting the separation of the _ions_ to be evolved ( ). . of the two modes in which bodies can transmit the electric forces, namely, that which is so characteristically exhibited by the metals, and usually called conduction, and that in which it is accompanied by decomposition, the first appears common to all bodies, although it occurs with almost infinite degrees of difference; the second is at present distinctive of the electrolytes. it is, however, just possible that it may hereafter be extended to the metals; for their power of conducting without decomposition may, perhaps justly, be ascribed to their requiring a very high electrolytic intensity for their decomposition. - / . the establishment of the principle that a certain electrolytic intensity is necessary before decomposition can be effected, is of great importance to all those considerations which arise regarding the probable effects of weak currents, such for instance as those produced by natural thermo-electricity, or natural voltaic arrangements in the earth. for to produce an effect of decomposition or of combination, a current must not only exist, but have a certain intensity before it can overcome the quiescent affinities opposed to it, otherwise it will be conducted, producing no permanent chemical effects. on the other hand, the principles are also now evident by which an opposing action can be so weakened by the juxtaposition of bodies not having quite affinity enough to cause direct action between them ( .), that a very weak current shall be able to raise the sum of actions sufficiently high, and cause chemical changes to occur. . in concluding this division _on the intensity necessary for electrolyzation_, i cannot resist pointing out the following remarkable conclusion in relation to intensity generally. it would appear that when a voltaic current is produced, having a certain intensity, dependent upon the strength of the chemical affinities by which that current is excited ( .), it can decompose a particular electrolyte without relation to the quantity of electricity passed, the _intensity_ deciding whether the electrolyte shall give way or not. if that conclusion be confirmed, then we may arrange circumstances so that the _same quantity_ of electricity may pass in the _same time_, in at the _same surface_, into the _same decomposing body in the same state_, and yet, differing in intensity, will _decompose in one case and in the other not_:--for taking a source of too low an intensity to decompose, and ascertaining the quantity passed in a given time, it is easy to take another source having a sufficient intensity, and reducing the quantity of electricity from it by the intervention of bad conductors to the same proportion as the former current, and then all the conditions will be fulfilled which are required to produce the result described. ¶ iii. _on associated voltaic circles, or the voltaic battery._ . passing from the consideration of single circles ( . &c.) to their association in the voltaic battery, it is a very evident consequence, that if matters are so arranged that two sets of affinities, in place of being opposed to each other as in figg. . . ( . .), are made to act in conformity, then, instead of either interfering with the other, it will rather assist it. this is simply the case of two voltaic pairs of metals arranged so as to form one circuit. in such arrangements the activity of the whole is known to be increased, and when ten, or a hundred, or any larger number of such alternations are placed in conformable association with each other, the power of the whole becomes proportionally exalted, and we obtain that magnificent instrument of philosophic research, the _voltaic battery_. . but it is evident from the principles of definite action already laid down, that the _quantity_ of electricity in the current cannot be increased with the increase of the _quantity of metal_ oxidized and dissolved at each new place of chemical action. a single pair of zinc and platina plates throws as much electricity into the form of a current, by the oxidation of . grains of the zinc ( .) as would be circulated by the same alteration of a thousand times that quantity, or nearly five pounds of metal oxidized at the surface of the zinc plates of a thousand pairs placed in regular battery order. for it is evident, that the electricity which passes across the acid from the zinc to the platina in the first cell, and which has been associated with, or even evolved by, the decomposition of a definite portion of water in that cell, cannot pass from the zinc to the platina across the acid in the second cell, without the decomposition of the same quantity of water there, and the oxidation of the same quantity of zinc by it ( . .). the same result recurs in every other cell; the electro-chemical equivalent of water must be decomposed in each, before the current can pass through it; for the quantity of electricity passed and the quantity of electrolyte decomposed, _must_ be the equivalents of each other. the action in each cell, therefore, is not to increase the quantity set in motion in any one cell, but to aid in urging forward that quantity, the passing of which is consistent with the oxidation of its own zinc; and in this way it exalts that peculiar property of the current which we endeavour to express by the term _intensity_, without increasing the _quantity_ beyond that which is proportionate to the quantity of zinc oxidized in any single cell of the series. . to prove this, i arranged ten pairs of amalgamated zinc and platina plates with dilute sulphuric acid in the form of a battery. on completing the circuit, all the pairs acted and evolved gas at the surfaces of the platina. this was collected and found to be alike in quantity for each plate; and the quantity of hydrogen evolved at any one platina plate was in the same proportion to the quantity of metal dissolved from any one zinc plate, as was given in the experiment with a single pair ( . &c.). it was therefore certain, that, just as much electricity and no more had passed through the series of ten pair of plates as had passed through, or would have been put into motion by, any single pair, notwithstanding that ten times the quantity of zinc had been consumed. . this truth has been proved also long ago in another way, by the action of the evolved current on a magnetic needle; the deflecting power of one pair of plates in a battery being equal to the deflecting power of the whole, provided the wires used be sufficiently large to carry the current of the single pair freely; but the _cause_ of this equality of action could not be understood whilst the definite action and evolution of electricity ( . .) remained unknown. . the superior decomposing power of a battery over a single pair of plates is rendered evident in two ways. electrolytes held together by an affinity so strong as to resist the action of the current from a single pair, yield up their elements to the current excited by many pairs; and that body which is decomposed by the action of one or of few pairs of metals, &c., is resolved into its _ions_ the more readily as it is acted upon by electricity urged forward by many alternations. . both these effects are, i think, easily understood. whatever _intensity_ may be, (and that must of course depend upon the nature of electricity, whether it consist of a fluid or fluids, or of vibrations of an ether, or any other kind or condition of matter,) there seems to be no difficulty in comprehending that the _degree_ of intensity at which a current of electricity is evolved by a first voltaic element, shall be increased when that current is subjected to the action of a second voltaic element, acting in conformity and possessing equal powers with the first: and as the decompositions are merely opposed actions, but exactly of the same kind as those which generate the current ( .), it seems to be a natural consequence, that the affinity which can resist the force of a single decomposing action may be unable to oppose the energies of many decomposing actions, operating conjointly, as in the voltaic battery. . that a body which can give way to a current of feeble intensity, should give way more freely to one of stronger force, and yet involve no contradiction to the law of definite electrolytic action, is perfectly consistent. all the facts and also the theory i have ventured to put forth, tend to show that the act of decomposition opposes a certain force to the passage of the electric current; and, that this obstruction should be overcome more or less readily, in proportion to the greater or less intensity of the decomposing current, is in perfect consistency with all our notions of the electric agent. . i have elsewhere ( .) distinguished the chemical action of zinc and dilute sulphuric acid into two portions; that which, acting effectually on the zinc, evolves hydrogen at once upon its surface, and that which, producing an arrangement of the chemical forces throughout the electrolyte present, (in this case water,) tends to take oxygen from it, but cannot do so unless the electric current consequent thereon can have free passage, and the hydrogen be delivered elsewhere than against the zinc. the electric current depends altogether upon the second of these; but when the current can pass, by favouring the electrolytic action it tends to diminish the former and increase the latter portion. . it is evident, therefore, that when ordinary zinc is used in a voltaic arrangement, there is an enormous waste of that power which it is the object to throw into the form of an electric current; a consequence which is put in its strongest point of view when it is considered that three ounces and a half of zinc, properly oxidized, can circulate enough electricity to decompose nearly one ounce of water, and cause the evolution of about cubic inches of hydrogen gas. this loss of power not only takes place during the time the electrodes of the battery are in communication, being then proportionate to the quantity of hydrogen evolved against the surface of any one of the zinc plates, but includes also _all_ the chemical action which goes on when the extremities of the pile are not in communication. . this loss is far greater with ordinary zinc than with the pure metal, as m. de la rive has shown[a]. the cause is, that when ordinary zinc is acted upon by dilute sulphuric acid, portions of copper, lead, cadmium, or other metals which it may contain, are set free upon its surface; and these, being in contact with the zinc, form small but very active voltaic circles, which cause great destruction of the zinc and evolution of hydrogen, apparently upon the zinc surface, but really upon the surface of these incidental metals. in the same proportion as they serve to discharge or convey the electricity back to the zinc, do they diminish its power of producing an electric current which shall extend to a greater distance across the acid, and be discharged only through the copper or platina plate which is associated with it for the purpose of forming a voltaic apparatus. [a] quarterly journal of science, , p. ; or bibliothèque universelle, , p. . . all these evils are removed by the employment of an amalgam of zinc in the manner recommended by mr. kemp[a], or the use of the amalgamated zinc plates of mr. sturgeon ( .), who has himself suggested and objected to their application in galvanic batteries; for he says, "were it not on account of the brittleness and other inconveniences occasioned by the incorporation of the mercury with the zinc, amalgamation of the zinc surfaces in galvanic batteries would become an important improvement; for the metal would last much longer, and remain bright for a considerable time, even for several successive hours; essential considerations in the employment of this apparatus[b]." [a] jameson's edinburgh journal, october . [b] recent experimental researches, p. , &c. mr. sturgeon is of course unaware of the definite production of electricity by chemical action, and is in fact quoting the experiment as the strongest argument _against_ the chemical theory of galvanism. . zinc so prepared, even though impure, does not sensibly decompose the water of dilute sulphuric acid, but still has such affinity for the oxygen, that the moment a metal which, like copper or platina, has little or no affinity, touches it in the acid, action ensues, and a powerful and abundant electric current is produced. it is probable that the mercury acts by bringing the surface, in consequence of its fluidity, into one uniform condition, and preventing those differences in character between one spot and another which are necessary for the formation of the minute voltaic circuits referred to ( .). if any difference does exist at the first moment, with regard to the proportion of zinc and mercury, at one spot on the _surface_, as compared with another, that spot having the least mercury is first acted on, and, by solution of the zinc, is soon placed in the same condition as the other parts, and the whole plate rendered superficially uniform. one part cannot, therefore, act as a discharger to another; and hence _all_ the chemical power upon the water at its surface is in that equable condition ( .), which, though it tends to produce an electric current through the liquid to another plate of metal which can act as a discharger ( .), presents no irregularities by which any one part, having weaker affinities for oxygen, can act as a discharger to another. two excellent and important consequences follow upon this state of the metal. the first is, that _the full equivalent_ of electricity is obtained for the oxidation of a certain quantity of zinc; the second, that a battery constructed with the zinc so prepared, and charged with dilute sulphuric acid, is active only whilst the electrodes are connected, and ceases to act or be acted upon by the acid the instant the communication is broken. . i have had a small battery of ten pairs of plates thus constructed, and am convinced that arrangements of this kind will be very important, especially in the development and illustration of the philosophical principles of the instrument. the metals i have used are amalgamated zinc and platina, connected together by being soldered to platina wires, the whole apparatus having the form of the couronne des tasses. the liquid used was dilute sulphuric acid of sp. gr. . . no action took place upon the metals except when the electrodes were in communication, and then the action upon the zinc was only in proportion to the decomposition in the experimental cell; for when the current was retarded there, it was retarded also in the battery, and no waste of the powers of the metal was incurred. . in consequence of this circumstance, the acid in the cells remained active for a very much longer time than usual. in fact, time did not tend to lower it in any sensible degree: for whilst the metal was preserved to be acted upon at the proper moment, the acid also was preserved almost at its first strength. hence a constancy of action far beyond what can be obtained by the use of common zinc. . another excellent consequence was the renewal, during the interval of rest, between two experiments of the first and most efficient state. when an amalgamated zinc and a platina plate, immersed in dilute sulphuric acid, are first connected, the current is very powerful, but instantly sinks very much in force, and in some cases actually falls to only an eighth or a tenth of that first produced ( .). this is due to the acid which is in contact with the zinc becoming neutralized by the oxide formed; the continued quick oxidation of the metal being thus prevented. with ordinary zinc, the evolution of gas at its surface tends to mingle all the liquid together, and thus bring fresh acid against the metal, by which the oxide formed there can be removed. with the amalgamated zinc battery, at every cessation of the current, the saline solution against the zinc is gradually diffused amongst the rest of the liquid; and upon the renewal of contact at the electrodes, the zinc plates are found most favourably circumstanced for the production of a ready and powerful current. . it might at first be imagined that amalgamated zinc would be much inferior in force to common zinc, because, of the lowering of its energy, which the mercury might be supposed to occasion over the whole of its surface; but this is not the case. when the electric currents of two pairs of platina and zinc plates were opposed, the difference being that one of the zincs was amalgamated and the other not, the current from the amalgamated zinc was most powerful, although no gas was evolved against it, and much was evolved at the surface of the unamalgamated metal. again, as davy has shown[a], if amalgamated and unamalgamated zinc be put in contact, and dipped into dilute sulphuric acid, or other exciting fluids, the former is positive to the latter, i.e. the current passes from the amalgamated zinc, through the fluid, to the unprepared zinc. this he accounts for by supposing that "there is not any inherent and specific property in each metal which gives it the electrical character, but that it depends upon its peculiar state--on that form of aggregation which fits it for chemical change." [a] philosophical transactions, , p. . . the superiority of the amalgamated zinc is not, however, due to any such cause, but is a very simple consequence of the state of the fluid in contact with it; for as the unprepared zinc acts directly and alone upon the fluid, whilst that which is amalgamated does not, the former (by the oxide it produces) quickly neutralizes the acid in contact with its surface, so that the progress of oxidation is retarded, whilst at the surface of the amalgamated zinc, any oxide formed is instantly removed by the free acid present, and the clean metallic surface is always ready to act with full energy upon the water. hence its superiority ( .). . the progress of improvement in the voltaic battery and its applications, is evidently in the contrary direction at present to what it was a few years ago; for in place of increasing the number of plates, the strength of acid, and the extent altogether of the instrument, the change is rather towards its first state of simplicity, but with a far more intimate knowledge and application of the principles which govern its force and action. effects of decomposition can now be obtained with ten pairs of plates ( .), which required five hundred or a thousand pairs for their production in the first instance. the capability of decomposing fused chlorides, iodides, and other compounds, according to the law before established ( . &c.), and the opportunity of collecting certain of the products, without any loss, by the use of apparatus of the nature of those already described ( . . &c.), render it probable that the voltaic battery may become a useful and even economical manufacturing instrument; for theory evidently indicates that an equivalent of a rare substance may be obtained at the expense of three or four equivalents of a very common body, namely, zinc: and practice seems thus far to justify the expectation. in this point of view i think it very likely that plates of platina or silver may be used instead of plates of copper with advantage, and that then the evil arising occasionally from solution of the copper, and its precipitation on the zinc, (by which the electromotive power of the zinc is so much injured,) will be avoided ( .). ¶ iv. _on the resistance of an electrolyte to electrolytic action, and on interpositions._ . i have already illustrated, in the simplest possible form of experiment ( . .), the resistance established at the place of decomposition to the force active at the exciting place. i purpose examining the effects of this resistance more generally; but it is rather with reference to their practical interference with the action and phenomena of the voltaic battery, than with any intention at this time to offer a strict and philosophical account of their nature. their general and principal cause is the resistance of the chemical affinities to be overcome; but there are numerous other circumstances which have a joint influence with these forces ( . . &c.), each of which would require a minute examination before a correct account of the whole could be given. . as it will be convenient to describe the experiments in a form different to that in which they were made, both forms shall first be explained. plates of platina, copper, zinc, and other metals, about three quarters of an inch wide and three inches long, were associated together in pairs by means of platina wires to which they were soldered, fig. , the plates of one pair being either alike or different, as might be required. these were arranged in glasses, fig. , so as to form volta's crown of cups. the acid or fluid in the cups never covered the whole of any plate; and occasionally small glass rods were put into the cups, between the plates, to prevent their contact. single plates were used to terminate the series and complete the connexion with a galvanometer, or with a decomposing apparatus ( . . &c.), or both. now if fig. be examined and compared with fig. , the latter may be admitted as representing the former in its simplest condition; for the cups i, ii, and iii of the former, with their contents, are represented by the cells i, ii, and iii of the latter, and the metal plates z and p of the former by the similar plates represented z and p in the latter. the only difference, in fact, between the apparatus, fig. , and the trough represented fig. , is that twice the quantity of surface of contact between the metal and acid is allowed in the first to what would occur in the second. . when the extreme plates of the arrangement just described, fig. , are connected metallically through the galvanometer _g_, then the whole represents a battery consisting of two pairs of zinc and platina plates urging a current forward, which has, however, to decompose water unassisted by any direct chemical affinity before it can be transmitted across the cell iii, and therefore before it can circulate. this decomposition of water, which is opposed to the passage of the current, may, as a matter of convenience, be considered as taking place either against the surfaces of the two platina plates which constitute the electrodes in the cell in, or against the two surfaces of that platina plate which separates the cells ii and iii, fig. , from each other. it is evident that if that plate were away, the battery would consist of two pairs of plates and two cells, arranged in the most favourable position for the production of a current. the platina plate therefore, which being introduced as at _x_, has oxygen evolved at one surface and hydrogen at the other (that is, if the decomposing current passes), may be considered as the cause of any obstruction arising from the decomposition of water by the electrolytic action of the current; and i have usually called it the interposed plate. . in order to simplify the conditions, dilute sulphuric acid was first used in all the cells, and platina for the interposed plates; for then the initial intensity of the current which tends to be formed is constant, being due to the power which zinc has of decomposing water; and the opposing force of decomposition is also constant, the elements of the water being unassisted in their separation at the interposed plates by any affinity or secondary action at the electrodes ( .), arising either from the nature of the plate itself or the surrounding fluid. . when only one voltaic pair of zinc and platina plates was used, the current of electricity was entirely stopped to all practical purposes by interposing one platina plate, fig. , i.e. by requiring of the current that it should decompose water, and evolve both its elements, before it should pass. this consequence is in perfect accordance with the views before given ( . . .). for as the whole result depends upon the opposition of forces at the places of electric excitement and electro-decomposition, and as water is the substance to be decomposed at both before the current can move, it is not to be expected that the zinc should have such powerful attraction for the oxygen, as not only to be able to take it from its associated hydrogen, but leave such a surplus of force as, passing to the second place of decomposition, should be there able to effect a second separation of the elements of water. such an effect would require that the force of attraction between zinc and oxygen should under the circumstances be _at least_ twice as great as the force of attraction between the oxygen and hydrogen. . when two pairs of zinc and platina exciting plates were used, the current was also practically stopped by one interposed platina plate, fig. . there was a very feeble effect of a current at first, but it ceased almost immediately. it will be referred to, with many other similar effects, hereafter ( .). . three pairs of zinc and platina plates, fig. , were able to produce a current which could pass an interposed platina plate, and effect the electrolyzation of water in cell iv. the current was evident, both by the continued deflection of the galvanometer, and the production of bubbles of oxygen and hydrogen at the electrodes in cell iv. hence the accumulated surplus force of three plates of zinc, which are active in decomposing water, is more than equal, when added together, to the force with which oxygen and hydrogen are combined in water, and is sufficient to cause the separation of these elements from each other. . the three pairs of zinc and platina plates were now opposed by two intervening platina plates, fig. . in this case the current was stopped. . four pairs of zinc and platina plates were also neutralized by two interposed platina plates, fig. . . five pairs of zinc and platina, with two interposed platina plates, fig. , gave a feeble current; there was permanent deflection at the galvanometer, and decomposition in the cells vi and vii. but the current was very feeble; very much less than when all the intermediate plates were removed and the two extreme ones only retained: for when they were placed six inches asunder in one cell, they gave a powerful current. hence five exciting pairs, with two interposed obstructing plates, do not give a current at all comparable to that of a single unobstructed pair. . i have already said that a _very feeble current_ passed when the series included one interposed platina and two pairs of zinc and platina plates ( .). a similarly feeble current passed in every case, and even when only one exciting pair and four intervening platina plates were used, fig. , a current passed which could be detected at _x_, both by chemical action on the solution of iodide of potassium, and by the galvanometer. this current i believe to be due to electricity reduced in intensity below the point requisite for the decomposition of water ( . .); for water can conduct electricity of such low intensity by the same kind of power which it possesses in common with metals and charcoal, though it cannot conduct electricity of higher intensity without suffering decomposition, and then opposing a new force consequent thereon. with an electric current of, or under this intensity, it is probable that increasing the number of interposed platina plates would not involve an increased difficulty of conduction. . in order to obtain an idea of the additional interfering power of each added platina plate, six voltaic pairs and four intervening platinas were arranged as in fig. ; a very feeble current then passed ( . .). when one of the platinas was removed so that three intervened, a current somewhat stronger passed. with two intervening platinas a still stronger current passed; and with only one intervening platina a very fair current was obtained. but the effect of the successive plates, taken in the order of their interposition, was very different, as might be expected; for the first retarded the current more powerfully than the second, and the second more than the third. . in these experiments both amalgamated and unamalgamated zinc were used, but the results generally were the same. . the effects of retardation just described were altered altogether when changes were made in the _nature of the liquid_ used between the plates, either in what may be called the _exciting_ or the _retarding_ cells. thus, retaining the exciting force the same, by still using pure dilute sulphuric acid for that purpose, if a little nitric acid were added to the liquid in the _retarding_ cells, then the transmission of the current was very much facilitated. for instance, in the experiment with one pair of exciting plates and one intervening plate ( .), fig. , when a few drops of nitric acid were added to the contents of cell ii, then the current of electricity passed with considerable strength (though it soon fell from other causes ( ; .),) and the same increased effect was produced by the nitric acid when many interposed plates were used. . this seems to be a consequence of the diminution of the difficulty of decomposing water when its hydrogen, instead of being absolutely expelled, as in the former cases, is transferred to the oxygen of the nitric acid, producing a secondary result at the _cathode_ ( .); for in accordance with the chemical views of the electric current and its action already advanced ( .), the water, instead of opposing a resistance to decomposition equal to the full amount of the force of mutual attraction between its oxygen and hydrogen, has that force counteracted in part, and therefore diminished by the attraction of the hydrogen at the _cathode_ for the oxygen of the nitric acid which surrounds it, and with which it ultimately combines instead of being evolved in its free state. . when a little nitric acid was put into the exciting cells, then again the circumstances favouring the transmission of the current were strengthened, for the _intensity_ of the current itself was increased by the addition ( .). when therefore a little nitric acid was added to both the _exciting_ and the _retarding_ cells, the current of electricity passed with very considerable freedom. . when dilute muriatic acid was used, it produced and transmitted a current more easily than pure dilute sulphuric acid, but not so readily as dilute nitric acid. as muriatic acid appears to be decomposed more freely than water ( .), and as the affinity of zinc for chlorine is very powerful, it might be expected to produce a current more intense than that from the use of dilute sulphuric acid; and also to transmit it more freely by undergoing decomposition at a lower intensity ( .). . in relation to the effect of these interpositions, it is necessary to state that they do not appear to be at all dependent upon the size of the electrodes, or their distance from each other in the acid, except that when a current _can pass_, changes in these facilitate or retard its passage. for on repeating the experiment with one intervening and one pair of exciting plates ( .), fig. , and in place of the interposed plate p using sometimes a mere wire, and sometimes very large plates ( .), and also changing the terminal exciting plates z and p, so that they were sometimes wires only and at others of great size, still the results were the same as those already obtained. . in illustration of the effect of distance, an experiment like that described with two exciting pairs and one intervening plate ( .), fig. , was arranged so that the distance between the plates in the third cell could be increased to six or eight inches, or diminished to the thickness of a piece of intervening bibulous paper. still the result was the same in both cases, the effect not being sensibly greater, when the plates were merely separated by the paper, than when a great way apart; so that the principal opposition to the current in this case does not depend upon the _quantity_ of intervening electrolytic conductor, but on the _relation of its elements to the intensity of the current_, or to the chemical nature of the electrodes and the surrounding fluids. . when the acid was sulphuric acid, _increasing its strength_ in any of the cells, caused no change in the effects; it did not produce a more intense current in the exciting cells ( .), or cause the current produced to traverse the decomposing cells more freely. but if to very weak sulphuric acid a few drops of nitric acid were added, then either one or other of those effects could be produced; and, as might be expected in a case like this, where the exciting or conducting action bore a _direct_ reference to the acid itself, increasing the strength of this (the nitric acid), also increased its powers. . the _nature of the interposed plate_ was now varied to show its relation to the phenomena either of excitation or retardation, and amalgamated zinc was first substituted for platina. on employing one voltaic pair and one interposed zinc plate, fig. , there was as powerful a current, apparently, as if the interposed zinc plate was away. hydrogen was evolved against p in cell ii, and against the side of the second zinc in cell i; but no gas appeared against the side of the zinc in cell ii, nor against the zinc in cell i. . on interposing two amalgamated zinc plates, fig. , instead of one, there was still a powerful current, but interference had taken place. on using three intermediate zinc plates, fig. , there was still further retardation, though a good current of electricity passed. . considering the retardation as due to the inaction of the amalgamated zinc upon the dilute acid, in consequence of the slight though general effect of diminished chemical power produced by the mercury on the surface, and viewing this inaction as the circumstance which rendered it necessary that each plate should have its tendency to decompose water assisted slightly by the electric current, it was expected that plates of the metal in the unamalgamated state would probably not require such assistance, and would offer no sensible impediment to the passing of the current. this expectation was fully realized in the use of two and three interposed unamalgamated plates. the electric current passed through them as freely as if there had been no such plates in the way. they offered no obstacle, because they could decompose water without the current; and the latter had only to give direction to a part of the forces, which would have been active whether it had passed or not. . interposed plates of copper were then employed. these seemed at first to occasion no obstruction, but after a few minutes the current almost entirely ceased. this effect appears due to the surfaces taking up that peculiar condition ( .) by which they tend to produce a reverse current; for when one or more of the plates were turned round, which could easily be effected with the couronne des tasses form of experiment, fig. , then the current was powerfully renewed for a few moments, and then again ceased. plates of platina and copper, arranged as a voltaic pile with dilute sulphuric acid, could not form a voltaic trough competent to act for more than a _few_ minutes, because of this peculiar counteracting effect. . all these effects of retardation, exhibited by decomposition against surfaces for which the evolved elements have more or less affinity, or are altogether deficient in attraction, show generally, though beautifully, the chemical relations and source of the current, and also the balanced state of the affinities at the places of excitation and decomposition. in this way they add to the mass of evidence in favour of the identity of the two; for they demonstrate, as it were, the antagonism of the _chemical powers_ at the electromotive part with the _chemical powers_ at the interposed parts; they show that the first are _producing_ electric effects, and the second _opposing_ them; they bring the two into direct relation; they prove that either can determine the other, thus making what appears to be cause and effect convertible, and thereby demonstrating that both chemical and electrical action are merely two exhibitions of one single agent or power ( . &c.). . it is quite evident, that as water and other electrolytes can conduct electricity without suffering decomposition ( .), when the electricity is of sufficiently low intensity, it may not be asserted as absolutely true in all cases, that whenever electricity passes through an electrolyte, it produces a definite effect of decomposition. but the quantity of electricity which can pass in a given time through an electrolyte without causing decomposition, is so small as to bear no comparison to that required in a case of very moderate decomposition, and with electricity above the intensity required for electrolyzation, i have found no sensible departure as yet from the law of _definite electrolytic action_ developed in the preceding series of these researches ( . &c.). . i cannot dismiss this division of the present paper without making a reference to the important experiments of m. aug. de la rive on the effects of interposed plates[a]. as i have had occasion to consider such plates merely as giving rise to new decompositions, and in that way only causing obstruction to the passage of the electric current, i was freed from the necessity of considering the peculiar effects described by that philosopher. i was the more willing to avoid for the present touching upon these, as i must at the same time have entered into the views of sir humphry davy upon the same subject[b] and also those of marianini[c] and hitter[d], which are connected with it. [a] annales de chimie, tom. xxviii. p ; and mémoires de génève. [b] philosophical transactions, , p. . [c] annales de chimie, tom. xxxiii. pp. , , &c. [d] journal de physique, tom. lvii. pp. , . ¶ v. _general remarks on the active voltaic battery._ . when the ordinary voltaic battery is brought into action, its very activity produces certain effects, which re-act upon it, and cause serious deterioration of its power. these render it an exceedingly inconstant instrument as to the _quantity_ of effect which it is capable of producing. they are already, in part, known and understood; but as their importance, and that of certain other coincident results, will be more evident by reference to the principles and experiments already stated and described, i have thought it would be useful, in this investigation of the voltaic pile, to notice them briefly here. . when the battery is in action, it causes such substances to be formed and arranged in contact with the plates as very much weaken its power, or even tend to produce a counter current. they are considered by sir humphry davy as sufficient to account for the phenomena of ritter's secondary piles, and also for the effects observed by m.a. de la rive with interposed platina plates[a]. [a] philosophical transactions, , p. . . i have already referred to this consequence ( .), as capable, in some cases, of lowering the force of the current to one-eighth or one-tenth of what it was at the first moment, and have met with instances in which its interference was very great. in an experiment in which one voltaic pair and one interposed platina plate were used with dilute sulphuric acid in the cells fig. , the wires of communication were so arranged, that the end of that marked could be placed at pleasure upon paper moistened in the solution of iodide of potassium at _x_, or directly upon the platina plate there. if, after an interval during which the circuit had not been complete, the wire were placed upon the paper, there was evidence of a current, decomposition ensued, and the galvanometer was affected. if the wire were made to touch the metal of _p_, a comparatively strong sudden current was produced, affecting the galvanometer, but lasting only for a moment; the effect at the galvanometer ceased, and if the wire were placed on the paper at _x_, no signs of decomposition occurred. on raising the wire , and breaking the circuit altogether for a while, the apparatus resumed its first power, requiring, however, from five to ten minutes for this purpose; and then, as before, on making contact between and _p_, there was again a momentary current, and immediately all the effects apparently ceased. . this effect i was ultimately able to refer to the state of the film of fluid in contact with the zinc plate in cell i. the acid of that film is instantly neutralized by the oxide formed; the oxidation of the zinc cannot, of course, go on with the same facility as before; and the chemical action being thus interrupted, the voltaic action diminishes with it. the time of the rest was required for the diffusion of the liquid, and its replacement by other acid. from the serious influence of this cause in experiments with single pairs of plates of different metals, in which i was at one time engaged, and the extreme care required to avoid it, i cannot help feeling a strong suspicion that it interferes more frequently and extensively than experimenters are aware of, and therefore direct their attention to it. . in considering the effect in delicate experiments of this source of irregularity of action, in the voltaic apparatus, it must be remembered that it is only that very small portion of matter which is directly in contact with the oxidizable metal which has to be considered with reference to the change of its nature; and this portion is not very readily displaced from its position upon the surface of the metal ( . .), especially if that metal be rough and irregular. in illustration of this effect, i will quote a remarkable experiment. a burnished platina plate ( .) was put into hot strong sulphuric acid for an instant only: it was then put into distilled water, moved about in it, taken out, and wiped dry: it was put into a second portion of distilled water, moved about in it, and again wiped: it was put into a third portion of distilled water, in which it was moved about for nearly eight seconds; it was then, without wiping, put into a fourth portion of distilled water, where it was allowed to remain five minutes. the two latter portions of water were then tested for sulphuric acid; the third gave no sensible appearance of that substance, but the fourth gave indications which were not merely evident, but abundant for the circumstances under which it had been introduced. the result sufficiently shows with what difficulty that portion of the substance which is in _contact_ with the metal leaves it; and as the contact of the fluid formed against the plate in the voltaic circuit must be as intimate and as perfect as possible, it is easy to see how quickly and greatly it must vary from the general fluid in the cells, and how influential in diminishing the force of the battery this effect must be. . in the ordinary voltaic pile, the influence of this effect will occur in all variety of degrees. the extremities of a trough of twenty pairs of plates of wollaston's construction were connected with the volta-electrometer, fig. . ( .), of the seventh series of these researches, and after five minutes the number of bubbles of gas issuing from the extremity of the tube, in consequence of the decomposition of the water, noted. without moving the plates, the acid between the copper and zinc was agitated by the introduction of a feather. the bubbles were immediately evolved more rapidly, above twice the number being produced in the same portion of time as before. in this instance it is very evident that agitation by a feather must have been a very imperfect mode of restoring the acid in the cells against the plates towards its first equal condition; and yet imperfect as the means were, they more than doubled the power of the battery. the _first effect_ of a battery which is known to be so superior to the degree of action which the battery can sustain, is almost entirely due to the favourable condition of the acid in contact with the plates. . a _second_ cause of diminution in the force of the voltaic battery, consequent upon its own action, is that extraordinary state of the surfaces of the metals ( .) which was first described, i believe, by ritter[a], to which he refers the powers of his secondary piles, and which has been so well experimented upon by marianini, and also by a. de la rive. if the apparatus, fig. . ( .), be left in action for an hour or two, with the wire in contact with the plate _p_, so as to allow a free passage for the current, then, though the contact be broken for ten or twelve minutes, still, upon its renewal, only a feeble current will pass, not at all equal in force to what might be expected. further, if p^{ } and p^{ } be connected by a metal wire, a powerful momentary current will pass from p^{ } to p^{ } through the acid, and therefore in the reverse direction to that produced by the action of the zinc in the arrangement; and after this has happened, the general current can pass through the whole of the system as at first, but by its passage again restores the plates p^{ } and p^{ } into the former opposing condition. this, generally, is the fact described by ritter, marianini, and de la rive. it has great opposing influence on the action of a pile, especially if the latter consist of but a small number of alternations, and has to pass its current through many interpositions. it varies with the solution in which the interposed plates are immersed, with the intensity of the current, the strength of the pile, the time of action, and especially with accidental discharges of the plates by inadvertent contacts or reversions of the plates during experiments, and must be carefully watched in every endeavour to trace the source, strength, and variations of the voltaic current. its effect was avoided in the experiments already described ( . &c.), by making contact between the plates p^{ } and p^{ } before the effect dependent upon the state of the solution in contact with the zinc plate was observed, and by other precautions. [a] journal de physique, lvii. p. . . when an apparatus like fig. . ( .) with several platina plates was used, being connected with a battery able to force a current through them, the power which they acquired, of producing a reversed current, was very considerable. . _weak and exhausted charges_ should never be used at the same time with _strong and fresh ones_ in the different cells of a trough, or the different troughs of a battery: the fluid in all the cells should be alike, else the plates in the weaker cells, in place of assisting, retard the passage of the electricity generated in, and transmitted across, the stronger cells. each zinc plate so circumstanced has to be assisted in decomposing power before the whole current can pass between it and the liquid. so, that, if in a battery of fifty pairs of plates, ten of the cells contain a weaker charge than the others, it is as if ten decomposing plates were opposed to the transit of the current of forty pairs of generating plates ( .). hence a serious loss of force, and hence the reason why, if the ten pairs of plates were removed, the remaining forty pairs would be much more powerful than the whole fifty. . five similar troughs, of ten pairs of plates each, were prepared, four of them with a good uniform charge of acid, and the fifth with the partially neutralized acid of a used battery. being arranged in right order, and connected with a volta-electrometer ( .), the whole fifty pairs of plates yielded . cubic inch of oxygen and hydrogen in one minute: but on moving one of the connecting wires so that only the four well-charged troughs should be included in the circuit, they produced with the same volta-electrometer . cubical inches of gas in the same time. nearly seven-eighths of the power of the four troughs had been lost, therefore, by their association with the fifth trough. . the same battery of fifty pairs of plates, after being thus used, was connected with a volta-electrometer ( .), so that by quickly shifting the wires of communication, the current of the whole of the battery, or of any portion of it, could be made to pass through the instrument for given portions of time in succession. the whole of the battery evolved . of a cubic inch of oxygen and hydrogen in half a minute; the forty plates evolved . cubic inches in the same time; the whole then evolved cubic inch in the half-minute; the ten weakly charged evolved . of a cubic inch in the time given: and finally the whole evolved . cubic inch in the standard time. the order of the observations was that given: the results sufficiently show the extremely injurious effect produced by the mixture of strong and weak charges in the same battery[a]. [a] the gradual increase in the action of the whole fifty pairs of plates was due to the elevation of temperature in the weakly charged trough by the passage of the current, in consequence of which the exciting energies of the fluid within were increased. . in the same manner associations of _strong and weak_ pairs of plates should be carefully avoided. a pair of copper and platina plates arranged in _accordance_ with a pair of zinc and platina plates in dilute sulphuric acid, were found to stop the action of the latter, or even of two pairs of the latter, as effectually almost as an interposed plate of platina ( .), or as if the copper itself had been platina. it, in fact, became an interposed decomposing plate, and therefore a retarding instead of an assisting pair. . the _reversal_, by accident or otherwise, of the plates in a battery has an exceedingly injurious effect. it is not merely the counteraction of the current which the reversed plates can produce, but their effect also in retarding even as indifferent plates, and requiring decomposition to be effected upon their surface, in _accordance_ with the course of the current, before the latter can pass. they oppose the current, therefore, in the first place, as interposed platina plates would do ( - .); and to this they add a force of opposition as counter-voltaic plates. i find that, in a series of four pairs of zinc and platina plates in dilute sulphuric acid, if one pair be reversed, it very nearly neutralizes the power of the whole. . there are many other causes of reaction, retardation, and irregularity in the voltaic battery. amongst them is the not unusual one of precipitation of copper upon the zinc in the cells, the injurious effect of which has before been adverted to ( .). but their interest is not perhaps sufficient to justify any increase of the length of this paper, which is rather intended to be an investigation of the theory of the voltaic pile than a particular account of its practical application[a]. [a] for further practical results relating to these points of the philosophy of the voltaic battery, see series x. § . .-- .--_dec. ._ _note_.--many of the views and experiments in this series of my experimental researches will be seen at once to be corrections and extensions of the theory of electro-chemical decomposition, given in the fifth and seventh series of these researches. the expressions i would now alter are those which concern the independence of the evolved elements in relation to the poles or electrodes, and the reference of their evolution to powers entirely internal ( . . .). the present paper fully shows my present views; and i would refer to paragraphs . . . . . . . . . &c., as stating what they are. i hope this note will be considered as sufficient in the way of correction at present; for i would rather defer revising the whole theory of electro-chemical decomposition until i can obtain clearer views of the way in which the power under consideration can appear at one time as associated with particles giving them their chemical attraction, and at another as free electricity ( . .).--m.f. _royal institution, march st, ._ ninth series. § . _on the influence by induction of an electric current on itself:--and on the inductive action of electric currents generally._ received december , ,--read january , . . the following investigations relate to a very remarkable inductive action of electric currents, or of the different parts of the same current ( .), and indicate an immediate connexion between such inductive action and the direct transmission of electricity through conducting bodies, or even that exhibited in the form of a spark. . the inquiry arose out of a fact communicated to me by mr. jenkin, which is as follows. if an ordinary wire of short length be used as the medium of communication between the two plates of an electromotor consisting of a single pair of metals, no management will enable the experimenter to obtain an electric shock from this wire; but if the wire which surrounds an electro-magnet be used, a shock is felt each time the contact with the electromotor is broken, provided the ends of the wire be grasped one in each hand. . another effect is observed at the same time, which has long been known to philosophers, namely, that a bright electric spark occurs at the place of disjunction. . a brief account of these results, with some of a corresponding character which i had observed in using long wires, was published in the philosophical magazine for [a]; and i added to them some observations on their nature. further investigations led me to perceive the inaccuracy of my first notions, and ended in identifying these effects with the phenomena of induction which i had been fortunate enough to develop in the first series of these experimental researches ( .- .)[b]. notwithstanding this identity, the extension and the results supply, lead me to believe that they will be found worthy of the attention of the royal society. [a] vol. v. pp. , . [b] philosophical transactions, , p. . . the _electromotor_ used consisted of a cylinder of zinc introduced between the two parts of a double cylinder of copper, and preserved from metallic contact in the usual way by corks. the zinc cylinder was eight inches high and four inches in diameter. both it and the copper cylinder were supplied with stiff wires, surmounted by cups containing mercury; and it was at these cups that the contacts of wires, helices, or electro-magnets, used to complete the circuit, were made or broken. these cups i will call g and e throughout the rest of this paper ( .). . certain _helices_ were constructed, some of which it will be necessary to describe. a pasteboard tube had four copper wires, one twenty-fourth of an inch in thickness, wound round it, each forming a helix in the same direction from end to end: the convolutions of each wire were separated by string, and the superposed helices prevented from touching by intervening calico. the lengths of the wires forming the helices were , . , , and feet. the first and third wires were united together so as to form one consistent helix of feet in length; and the second and fourth wires were similarly united to form a second helix, closely interwoven with the first, and . feet in length. these helices may be distinguished by the numbers i and ii. they were carefully examined by a powerful current of electricity and a galvanometer, and found to have no communication with each other. . another helix was constructed upon a similar pasteboard tube, two lengths of the same copper wire being used, each forty-six feet long. these were united into one consistent helix of ninety-two feet, which therefore was nearly equal in value to either of the former helices, but was not in close inductive association with them. it may be distinguished by the number iii. . a fourth helix was constructed of very thick copper wire, being one-fifth of an inch in diameter; the length of wire used was seventy-nine feet, independent of the straight terminal portions. . the principal _electro-magnet_ employed consisted of a cylindrical bar of soft iron twenty-five inches long, and one inch and three quarters in diameter, bent into a ring, so that the ends nearly touched, and surrounded by three coils of thick copper wire, the similar ends of which were fastened together; each of these terminations was soldered to a copper rod, serving as a conducting continuation of the wire. hence any electric current sent through the rods was divided in the helices surrounding the ring, into three parts, all of which, however, moved in the same direction. the three wires may therefore be considered as representing one wire, of thrice the thickness of the wire really used. . other electro-magnets could be made at pleasure by introducing a soft iron rod into any of the helices described ( , &c.). . the _galvanometer_ which i had occasion to use was rough in its construction, having but one magnetic needle, and not at all delicate in its indications. . the effects to be considered _depend on the conductor_ employed to complete the communication between the zinc and copper plates of the electromotor; and i shall have to consider this conductor under four different forms: as the helix of an electro-magnet ( ); as an ordinary helix ( , &c.); as a _long_ extended wire, having its course such that the parts can exert little or no mutual influence; and as a _short_ wire. in all cases the conductor was of copper. . the peculiar effects are best shown by the _electro-magnet_ ( .). when it was used to complete the communication at the electromotor, there was no sensible spark on _making_ contact, but on _breaking_ contact there was a very large and bright spark, with considerable combustion of the mercury. then, again, with respect to the shock: if the hands were moistened in salt and water, and good contact between them and the wires retained, no shock could be felt upon _making_ contact at the electromotor, but a powerful one on _breaking_ contact. . when the _helix_ i or iii ( , &c.) was used as the connecting conductor, there was also a good spark on breaking contact, but none (sensibly) on making contact. on trying to obtain the shock from these helices, i could not succeed at first. by joining the similar ends of i and ii so as to make the two helices equivalent to one helix, having wire of double thickness, i could just obtain the sensation. using the helix of thick wire ( .) the shock was distinctly obtained. on placing the tongue between two plates of silver connected by wires with the parts which the hands had heretofore touched ( .), there was a powerful shock on _breaking_ contact, but none on _making_ contact. . the power of producing these phenomena exists therefore in the simple helix, as in the electro-magnet, although by no means in the same high degree. . on putting a bar of soft iron into the helix, it became an electro-magnet ( .), and its power was instantly and greatly raised. on putting a bar of copper into the helix, no change was produced, the action being that of the helix alone. the two helices i and ii, made into one helix of twofold length of wire, produced a greater effect than either i or ii alone. . on descending from the helix to the mere _long wire_, the following effects were obtained, a copper wire, . of an inch in diameter, and feet in length, was laid out upon the floor of the laboratory, and used as the connecting conductor ( .); it gave no sensible spark on making contact, but produced a bright one on breaking contact, yet not so bright as that from the helix ( .) on endeavouring to obtain the electric shock at the moment contact was broken, i could not succeed so as to make it pass through the hands; but by using two silver plates fastened by small wires to the extremity of the principal wire used, and introducing the tongue between those plates, i succeeded in obtaining powerful shocks upon the tongue and gums, and could easily convulse a flounder, an eel, or a frog. none of these effects could be obtained directly from the electromotor, i.e. when the tongue, frog, or fish was in a similar, and therefore comparative manner, interposed in the course of the communication between the zinc and copper plates, separated everywhere else by the acid used to excite the combination, or by air. the bright spark and the shock, produced only on breaking contact, are therefore effects of the same kind as those produced in a higher degree by the helix, and in a still higher degree by the electro-magnet. . in order to compare an extended wire with a helix, the helix i, containing ninety-six feet, and ninety-six feet of the same-sized wire lying on the floor of the laboratory, were used alternately as conductors: the former gave a much brighter spark at the moment of disjunction than the latter. again, twenty-eight feet of copper wire were made up into a helix, and being used gave a good spark on disjunction at the electromotor; being then suddenly pulled out and again employed, it gave a much smaller spark than before, although nothing but its spiral arrangement had been changed. . as the superiority of a helix over a wire is important to the philosophy of the effect, i took particular pains to ascertain the fact with certainty. a wire of copper sixty-seven feet long was bent in the middle so as to form a double termination which could be communicated with the electromotor; one of the halves of this wire was made into a helix and the other remained in its extended condition. when these were used alternately as the connecting wire, the helix half gave by much the strongest spark. it even gave a stronger spark than when it and the extended wire were used conjointly as a double conductor. . when a _short wire_ is used, _all_ these effects disappear. if it be only two or three inches long, a spark can scarcely be perceived on breaking the junction. if it be ten or twelve inches long and moderately thick, a small spark may be more easily obtained. as the length is increased, the spark becomes proportionately brighter, until from extreme length the resistance offered by the metal as a conductor begins to interfere with the principal result. . the effect of elongation was well shown thus: feet of copper wire, one-eighteenth of an inch in diameter, were extended on the floor and used as a conductor; it remained cold, but gave a bright spark on breaking contact. being crossed so that the two terminations were in contact near the extremities, it was again used as a conductor, only twelve inches now being included in the circuit: the wire became very hot from the greater quantity of electricity passing through it, and yet the spark on breaking contact was scarcely visible. the experiment was repeated with a wire one-ninth of an inch in diameter and thirty-six feet long with the same results. . that the effects, and also the action, in all these forms of the experiment are identical, is evident from the manner in which the former can be gradually raised from that produced by the shortest wire to that of the most powerful electro-magnet: and this capability of examining what will happen by the most powerful apparatus, and then experimenting for the same results, or reasoning from them, with the weaker arrangements, is of great advantage in making out the true principles of the phenomena. . the action is evidently dependent upon the wire which serves as a conductor; for it varies as that wire varies in its length or arrangement. the shortest wire may be considered as exhibiting the full effect of spark or shock which the electromotor can produce by its own direct power; all the additional force which the arrangements described can excite being due to some affection of the current, either permanent or momentary, in the wire itself. that it is a _momentary_ effect, produced only at the instant of breaking contact, will be fully proved ( . .). . no change takes place in the quantity or intensity of the current during the time the latter is _continued_, from the moment after contact is made, up to that previous to disunion, except what depends upon the increased obstruction offered to the passage of the electricity by a long wire as compared to a short wire. to ascertain this point with regard to _quantity_, the helix i ( .) and the galvanometer ( .) were both made parts of the metallic circuit used to connect the plates of a small electromotor, and the deflection at the galvanometer was observed; then a soft iron core was put into the helix, and as soon as the momentary effect was over, and the needle had become stationary, it was again observed, and found to stand exactly at the same division as before. thus the quantity passing through the wire when the current was continued was the same either with or without the soft iron, although the peculiar effects occurring at the moment of disjunction were very different in degree under such variation of circumstances. . that the quality of _intensity_ belonging to the constant current did not vary with the circumstances favouring the peculiar results under consideration, so as to yield an explanation of those results, was ascertained in the following manner. the current excited by an electromotor was passed through short wires, and its intensity tried by subjecting different substances to its electrolyzing power ( . . &c.); it was then passed through the wires of the powerful electro-magnet ( .), and again examined with respect to its intensity by the same means and found unchanged. again, the constancy of the _quantity_ passed in the above experiment ( .) adds further proof that the intensity could not have varied; for had it been increased upon the introduction of the soft iron, there is every reason to believe that the quantity passed in a given time would also have increased. . the fact is, that under many variations of the experiments, the permanent current _loses_ in force as the effects upon breaking contact become _exalted_. this is abundantly evident in the comparative experiments with long and short wires ( .); and is still more strikingly shown by the following variation. solder an inch or two in length of fine platina wire (about one-hundredth of an inch in diameter) on to one end of the long communicating wire, and also a similar length of the same platina wire on to one end of the short communication; then, in comparing the effects of these two communications, make and break contact between the platina terminations and the mercury of the cup g or e ( .). when the short wire is used, the platina will be _ignited by the constant current_, because of the quantity of electricity, but the spark on breaking contact will be hardly visible; on using the longer communicating wire, which by obstructing will diminish the current, the platina will remain cold whilst the current passes, but give a bright spark at the moment it ceases: thus the strange result is obtained of a diminished spark and shock from the strong current, and increased effects from the weak one. hence the spark and shock at the moment of disjunction, although resulting from great intensity and quantity, of the current _at that moment_, are no direct indicators or measurers of the intensity or quantity of the constant current previously passing, and by which they are ultimately produced. * * * * * . it is highly important in using the spark as an indication, by its relative brightness, of these effects, to bear in mind certain circumstances connected with its production and appearance ( .). an ordinary electric spark is understood to be the bright appearance of electricity passing suddenly through an interval of air, or other badly conducting matter. a voltaic spark is sometimes of the same nature, but, generally, is due to the ignition and even combustion of a minute portion of a good conductor; and that is especially the case when the electromotor consists of but one or few pairs of plates. this can be very well observed if either or both of the metallic surfaces intended to touch be solid and pointed. the moment they come in contact the current passes; it heats, ignites, and even burns the touching points, and the appearance is as if the spark passed on making contact, whereas it is only a case of ignition by the current, contact being previously made, and is perfectly analogous to the ignition of a fine platina wire connecting the extremities of a voltaic battery. . when mercury constitutes one or both of the surfaces used, the brightness of the spark is greatly increased. but as this effect is due to the action on, and probable combustion of, the metal, such sparks must only be compared with other sparks also taken from mercurial surfaces, and not with such as may be taken, for instance, between surfaces of platina or gold, for then the appearances are far less bright, though the same quantity of electricity be passed. it is not at all unlikely that the commonly occurring circumstance of combustion may affect even the duration of the light; and that sparks taken between mercury, copper, or other combustible bodies, will continue for a period sensibly longer than those passing between platina or gold. . when the end of a short clean copper wire, attached to one plate of an electromotor, is brought down carefully upon a surface of mercury connected with the other plate, a spark, almost continuous, can be obtained. this i refer to a succession of effects of the following nature: first, contact,--then ignition of the touching points,--recession of the mercury from the mechanical results of the heat produced at the place of contact, and the electro-magnetic condition of the parts at the moment[a], --breaking of the contact and the production of the peculiar intense effect dependent thereon,--renewal of the contact by the returning surface of the undulating mercury,--and then a repetition of the same series of effects, and that with such rapidity as to present the appearance of a continued discharge. if a long wire or an electro-magnet be used as the connecting conductor instead of a short wire, a similar appearance may be produced by tapping the vessel containing the mercury and making it vibrate; but the sparks do not usually follow each other so rapidly as to produce an apparently continuous spark, because of the time required, when the long wire or electro-magnet is used, both for the full development of the current ( . .) and for its complete cessation. [a] quarterly journal of science, vol. xii, p. . . returning to the phenomena in question, the first thought that arises in the mind is, that the electricity circulates with something like _momentum or inertia_ in the wire, and that thus a long wire produces effects at the instant the current is stopped, which a short wire cannot produce. such an explanation is, however, at once set aside by the fact, that the same length of wire produces the effects in very different degrees, according as it is simply extended, or made into a helix, or forms the circuit of an electro-magnet ( .). the experiments to be adduced ( .) will still more strikingly show that the idea of momentum cannot apply. . the bright spark at the electromotor, and the shock in the arms, appeared evidently to be due to _one_ current in the long wire, divided into two parts by the double channel afforded through the body and through the electromotor; for that the spark was evolved at the place of disjunction with the electromotor, not by any direct action of the latter, but by a force immediately exerted in the wire of communication, seemed to be without doubt ( .). it followed, therefore, that by using a better conductor in place of the human body, the _whole_ of this extra current might be made to pass at that place; and thus be separated from that which the electromotor could produce by its immediate action, and its _direction_ be examined apart from any interference of the original and originating current. this was found to be true; for on connecting the ends of the principal wire together by a cross wire two or three feet in length, applied just where the hands had felt the shock, the whole of the extra current passed by the new channel, and then no better spark than one producible by a short wire was obtained on disjunction at the electromotor. . the _current_ thus separated was examined by galvanometers and decomposing apparatus introduced into the course of this wire. i will always speak of it as the current in the cross wire or wires, so that no mistake, as to its place or origin, may occur. in the wood-cut, z and c represent the zinc and copper plates of the electromotor; g and e the cups of mercury where contact is made or broken ( .); a and b the terminations of d, the long wire, the helix or the electro-magnet, used to complete the circuit; n and p are the cross wires, which can either be brought into contact at _x_, or else have a galvanometer ( .) or an electrolyzing apparatus ( . .) interposed there. [illustration] the production of the _shock_ from the current in the cross wire, whether d was a long extended wire, or a helix, or an electro-magnet, has been already described ( . . .). . the _spark_ of the cross-wire current could be produced at _x_ in the following manner: d was made an electro-magnet; the metallic extremities at _x_ were held close together, or rubbed lightly against each other, whilst contact was broken at g or e. when the communication was perfect at _x_, little or no spark appeared at g or e. when the condition of vicinity at _x_ was favourable for the result required, a bright spark would pass there at the moment of disjunction, _none_ occurring at g and e: this spark was the luminous passage of the extra current through the cross-wires. when there was no contact or passage of current at _x_, then the spark appeared at g or e, the extra current forcing its way through the electromotor itself. the same results were obtained by the use of the helix or the extended wire at d in place of the electro-magnet. . on introducing a fine platina wire at _x_, and employing the electro-magnet at d, no visible effects occurred as long as contact was continued; but on breaking contact at g or e, the fine wire was instantly ignited and fused. a longer or thicker wire could be so adjusted at _x_ as to show ignition, without fusion, every time the contact was broken at g or e. . it is rather difficult to obtain this effect with helices or wires, and for very simple reasons: with the helices i, ii, or iii, there was such retardation of the electric current, from the length of wire used, that a full inch of platina wire one-fiftieth of an inch in diameter could be retained ignited at the cross-wires during the _continuance of contact_, by the portion of electricity passing through it. hence it was impossible to distinguish the particular effects at the moments of making or breaking contact from this constant effect. on using the thick wire helix ( .), the same results ensued. . proceeding upon the known fact that electric currents of great quantity but low intensity, though able to ignite thick wires, cannot produce that effect upon thin ones, i used a very fine platina wire at _x_, reducing its diameter until a spark appeared at g or e, when contact was broken there. a quarter of an inch of such wire might be introduced at _x_ without being ignited by the _continuance_ of contact at g or e; but when contact was broken at either place, this wire became red-hot; proving, by this method, the production of the induced current at that moment. . _chemical decomposition_ was next effected by the cross-wire current, an electro-magnet being used at d, and a decomposing apparatus, with solution of iodide of potassium in paper ( .), employed at _x_. the conducting power of the connecting system a b d was sufficient to carry all the primary current, and consequently no chemical action took place at _x_ during the _continuance_ of contact at g and e; but when contact was broken, there was instantly decomposition at _x_. the iodine appeared against the wire n, and not against the wire p; thus demonstrating that the current through the cross-wires, when contact was broken, was in the _reverse direction_ to that marked by the arrow, or that which the electromotor would have sent through it. . in this experiment a bright spark occurs at the place of disjunction, indicating that only a small part of the extra current passed the apparatus at _x_, because of the small conducting power of the latter. . i found it difficult to obtain the chemical effects with the simple helices and wires, in consequence of the diminished inductive power of these arrangements, and because of the passage of a strong constant current at _x_ whenever a very active electromotor was used ( ). . the most instructive set of results was obtained, however, when the _galvanometer_ was introduced at _x_. using an electro-magnet at d, and continuing contact, a current was then indicated by the deflection, proceeding from p to n, in the direction of the arrow; the cross-wire serving to carry one part of the electricity excited by the electromotor, and that part of the arrangement marked a b d, the other and far greater part, as indicated by the arrows. the magnetic needle was then forced back, by pins applied upon opposite sides of its two extremities, to its natural position when uninfluenced by a current; after which, contact being _broken_ at g or e, it was deflected strongly in the opposite direction; thus showing, in accordance with the chemical effects ( ), that the extra current followed a course in the cross-wires _contrary_ to that indicated by the arrow, i. e. contrary to the one produced by the direct action of the electromotor[a]. [a] it was ascertained experimentally, that if a strong current was passed through the galvanometer only, and the needle restrained in one direction as above in its natural position, when the current was stopped, no vibration of the needle in the opposite direction took place. . with the _helix_ only ( .), these effects could scarcely be observed, in consequence of the smaller inductive force of this arrangement, the opposed action from induction in the galvanometer wire itself, the mechanical condition and tension of the needle from the effect of blocking ( .) whilst the current due to continuance of contact was passing round it; and because of other causes. with the _extended wire_ ( .) all these circumstances had still greater influence, and therefore allowed less chance of success. . these experiments, establishing as they did, by the quantity, intensity, and even direction, a distinction between the primary or generating current and the extra current, led me to conclude that the latter was identical with the induced current described ( . . .) in the first series of these researches; and this opinion i was soon able to bring to proof, and at the same times obtained not the partial ( .) but entire separation of one current from the other. . the double helix ( .) was arranged so that it should form the connecting wire between the plates of the electromotor, in being out of the current, and its ends unconnected. in this condition it acted very well, and gave a good spark at the time and place of disjunction. the opposite ends of ii were then connected together so as to form an endless wire, i remaining unchanged: but now _no spark_, or one scarcely sensible, could be obtained from the latter at the place of disjunction. then, again, the ends of ii were held so nearly together that any current running round that helix should be rendered visible as a spark; and in this manner a spark was obtained from ii when the junction of i with the electromotor was broken, in place of appearing at the disjoined extremity of i itself. . by introducing a galvanometer or decomposing apparatus into the circuit formed by the helix ii, i could easily obtain the deflections and decomposition occasioned by the induced current due to the breaking contact at helix i, or even to that occasioned by making contact of that helix with the electromotor; the results in both cases indicating the contrary directions of the two induced currents thus produced ( .). . all these effects, except those of decomposition, were reproduced by two extended long wires, not having the form of helices, but placed close to each other; and thus it was proved that the _extra current_ could be removed from the wire carrying the original current to a neighbouring wire, and was at the same time identified, in direction and every other respect, with the currents producible by induction ( .). the case, therefore, of the bright spark and shock on disjunction may now be stated thus: if a current be established in a wire, and another wire, forming a complete circuit, be placed parallel to the first, at the moment the current in the first is stopped it induces a current in the _same_ direction in the second, the first exhibiting then but a feeble spark; but if the second wire be away, disjunction of the first wire induces a current in itself in the same direction, producing a strong spark. the strong spark in the single long wire or helix, at the moment of disjunction, is therefore the equivalent of the current which would be produced in a neighbouring wire if such second current were permitted. . viewing the phenomena as the results of the induction of electrical currents, many of the principles of action, in the former experiments, become far more evident and precise. thus the different effects of short wires, long wires, helices, and electro-magnets ( .) may be comprehended. if the inductive action of a wire a foot long upon a collateral wire also a foot in length, be observed, it will be found very small; but if the same current be sent through a wire fifty feet long, it will induce in a neighbouring wire of fifty feet a far more powerful current at the moment of making or breaking contact, each successive foot of wire adding to the sum of action; and by parity of reasoning, a similar effect should take place when the conducting wire is also that in which the induced current is formed ( .): hence the reason why a long wire gives a brighter spark on breaking contact than a short one ( .), although it carries much less electricity. . if the long wire be made into a helix, it will then be still more effective in producing sparks and shocks on breaking contact; for by the mutual inductive action of the convolutions each aids its neighbour, and will be aided in turn, and the sum of effect will be very greatly increased. . if an electro-magnet be employed, the effect will be still more highly exalted; because the iron, magnetized by the power of the continuing current, will lose its magnetism at the moment the current ceases to pass, and in so doing will tend to produce an electric current in the wire around it ( . .), in conformity with that which the cessation of current in the helix itself also tends to produce. . by applying the laws of the induction of electric currents formerly developed ( . &c.), various new conditions of the experiments could be devised, which by their results should serve as tests of the accuracy of the view just given. thus, if a long wire be doubled, so that the current in the two halves shall have opposite actions, it ought not to give a sensible spark at the moment of disjunction: and this proved to be the case, for a wire forty feet long, covered with silk, being doubled and tied closely together to within four inches of the extremities, when used in that state, gave scarcely a perceptible spark; but being opened out and the parts separated, it gave a very good one. the two helices i and ii being joined at their similar ends, and then used at their other extremities to connect the plates of the electromotor, thus constituted one long helix, of which one half was opposed in direction to the other half: under these circumstances it gave scarcely a sensible spark, even when the soft iron core was within, although containing nearly two hundred feet of wire. when it was made into one consistent helix of the same length of wire it gave a very bright spark. . similar proofs can be drawn from the mutual inductive action of two separate currents ( .); and it is important for the general principles that the consistent action of two such currents should be established. thus, two currents going in the same direction should, if simultaneously stopped, aid each other by their relative influence; or if proceeding in contrary directions, should oppose each other under similar circumstances. i endeavoured at first to obtain two currents from two different electromotors, and passing them through the helices i and ii, tried to effect the disjunctions mechanically at the same moment. but in this i could not succeed; one was always separated before the other, and in that case produced little or no spark, its inductive power being employed in throwing a current round the remaining complete circuit ( .): the current which was stopped last always gave a bright spark. if it were ever to become needful to ascertain whether two junctions were accurately broken at the same moment, these sparks would afford a test for the purpose, having an infinitesimal degree of perfection. . i was able to prove the points by other expedients. two short thick wires were selected to serve as terminations, by which contact could be made or broken with the electromotor. the compound helix, consisting of i and ii ( .), was adjusted so that the extremities of the two helices could be placed in communication with the two terminal wires, in such a manner that the current moving through the thick wires should be divided into two equal portions in the two helices, these portions travelling, according to the mode of connexion, either in the same direction or in contrary directions at pleasure. in this manner two streams could be obtained, both of which could be stopped simultaneously, because the disjunction could be broken at g or f by removing a single wire. when the helices were in contrary directions, there was scarcely a sensible spark at the place of disjunction; but when they were in accordance there was a very bright one. . the helix i was now used constantly, being sometimes associated, as above, with helix ii in an according direction, and sometimes with helix iii, which was placed at a little distance. the association i and ii, which presented two currents able to affect each other by induction, because of their vicinity, gave a brighter spark than the association i and iii, where the two streams could not exert their mutual influence; but the difference was not so great as i expected. . thus all the phenomena tend to prove that the effects are due to an inductive action, occurring at the moment when the principal current is stopped. i at one time thought they were due to an action continued during the _whole time_ of the current, and expected that a steel magnet would have an influence according to its position in the helix, comparable to that of a soft iron bar, in assisting the effect. this, however, is not the case; for hard steel, or a magnet in the helix, is not so effectual as soft iron; nor does it make any difference how the magnet is placed in the helix, and for very simple reasons, namely, that the effect does not depend upon a permanent state of the core, but a _change of state_; and that the magnet or hard steel cannot sink through such a difference of state as soft iron, at the moment contact ceases, and therefore cannot produce an equal effect in generating a current of electricity by induction ( . .). * * * * * . as an electric current acts by induction with equal energy at the moment of its commencement as at the moment of its cessation ( . .), but in a contrary direction, the reference of the effects under examination to an inductive action, would lead to the conclusion that corresponding effects of an opposite nature must occur in a long wire, a helix, or an electro-magnet, every time that _contact is made with_ the electromotor. these effects will tend to establish a resistance for the first moment in the long conductor, producing a result equivalent to the reverse of a shock or a spark. now it is very difficult to devise means fit for the recognition of such negative results; but as it is probable that some positive effect is produced at the time, if we knew what to expect, i think the few facts bearing upon this subject with which i am acquainted are worth recording. . the electro-magnet was arranged with an electrolyzing apparatus at _x_, as before described ( .), except that the intensity of the chemical action at the electromotor was increased until the electric current was just able to produce the feeblest signs of decomposition whilst contact was continued at g and e ( .); (the iodine of course appearing against the end of the cross wire p;) the wire n was also separated from a at _r_, so that contact there could be made or broken at pleasure. under these circumstances the following set of actions was repeated several times: contact was broken at _r_, then broken at g, next made at _r_, and lastly renewed at g; thus any current from n to p due to _breaking_ of contact was avoided, but any additional force to the current from p to n due to _making_ contact could be observed. in this way it was found, that a much greater decomposing effect (causing the evolution of iodine against p) could be obtained by a few completions of contact than by the current which could pass in a much longer time if the contact was _continued_. this i attribute to the act of induction in the wire abd at the moment of contact rendering that wire a worse conductor, or rather retarding the passage of the electricity through it for the instant, and so throwing a greater quantity of the electricity which the electromotor could produce, through the cross wire passage np. the instant the induction ceased, abd resumed its full power of carrying a constant current of electricity, and could have it highly increased, as we know by the former experiments ( .) by the opposite inductive action brought into activity at the moment contact at z or c was _broken_. . a galvanometer was then introduced at _x_, and the deflection of the needle noted whilst contact was continued at g and e: the needle was then blocked as before in one direction ( .), so that it should not return when the current ceased, but remain in the position in which the current could retain it. contact at g or e was broken, producing of course no visible effect; it was then renewed, and the needle was instantly deflected, passing from the blocking pins to a position still further from its natural place than that which the constant current could give, and thus showing, by the temporary excess of current in this cross communication, the temporary retardation in the circuit abd. . on adjusting a platina wire at _x_ ( .) so that it should not be ignited by the current passing through it whilst contact at g and e was _continued_, and yet become red-hot by a current somewhat more powerful, i was readily able to produce its ignition upon _making contact_, and again upon _breaking contact_. thus the momentary retardation in abd on making contact was again shown by this result, as well also as the opposite result upon breaking contact. the two ignitions of the wire at _x_ were of course produced by electric currents moving in opposite directions. . using the _helix_ only, i could not obtain distinct deflections at _x_, due to the extra effect on making contact, for the reasons already mentioned ( .). by using a very fine platina wire there ( .), i did succeed in obtaining the igniting effect for making contact in the same manner, though by no means to the same degree, as with the electro-magnet ( ). . we may also consider and estimate the effect on _making contact_, by transferring the force of induction from the wire carrying the original current to a lateral wire, as in the cases described ( .); and we then are sure, both by the chemical and galvanometrical results ( .), that the forces upon making and breaking contact, like action and reaction, are equal in their strength but contrary in their direction. if, therefore, the effect on making contact resolves itself into a mere retardation of the current at the first moment of its existence, it must be, in its degree, equivalent to the high exaltation of that same current at the moment contact is broken. . thus the case, under the circumstances, is, that the intensity and quantity of electricity moving in a current are smaller when the current commences or is increased, and greater when it diminishes or ceases, than they would be if the inductive action occurring at these moments did not take place; or than they are in the original current wire if the inductive action be transferred from that wire to a collateral one ( .). . from the facility of transference to neighbouring wires, and from the effects generally, the inductive forces appear to be lateral, i.e. exerted in a direction perpendicular to the direction of the originating and produced currents: and they also appear to be accurately represented by the magnetic curves, and closely related to, if not identical with, magnetic forces. . there can be no doubt that the current in one part of a wire can act by induction upon other parts of the _same_ wire which are lateral to the first, i.e. in the same vertical section ( .), or in the parts which are more or less oblique to it ( .), just as it can act in producing a current in a neighbouring wire or in a neighbouring coil of the same wire. it is this which gives the appearance of the current acting upon itself: but all the experiments and all analogy tend to show that the elements (if i may so say) of the currents do not act upon themselves, and so cause the effect in question, but produce it by exciting currents in conducting matter which is lateral to them. . it is possible that some of the expressions i have used may seem to imply, that the inductive action is essentially the action of one current upon another, or of one element of a current upon another element of the same current. to avoid any such conclusion i must explain more distinctly my meaning. if an endless wire be taken, we have the means of generating a current in it which shall run round the circuit without adding any electricity to what was previously in the wire. as far as we can judge, the electricity which appears as a current is the same as that which before was quiescent in the wire; and though we cannot as yet point out the essential condition of difference of the electricity at such times, we can easily recognize the two states. now when a current acts by induction upon conducting matter lateral to it, it probably acts upon the electricity in that conducting matter whether it be in the form of a _current_ or _quiescent_, in the one case increasing or diminishing the current according to its direction, in the other producing a current, and the _amount_ of the inductive action is probably the same in both cases. hence, to say that the action of induction depended upon the mutual relation of two or more currents, would, according to the restricted sense in which the term current is understood at present ( . . .), be an error. . several of the effects, as, for instances, those with helices( .), with according or counter currents ( . .), and those on the production of lateral currents ( .), appeared to indicate that a current could produce an effect of induction in a neighbouring wire more readily than in its own carrying wire, in which case it might be expected that some variation of result would be produced if a bundle of wires were used as a conductor instead of a single wire. in consequence the following experiments were made. a copper wire one twenty-third of an inch in diameter was cut into lengths of five feet each, and six of these being laid side by side in one bundle, had their opposite extremities soldered to two terminal pieces of copper. this arrangement could be used as a discharging wire, but the general current could be divided into six parallel streams, which might be brought close together, or, by the separation of the wires, be taken more or less out of each other's influence. a somewhat brighter spark was, i think, obtained on breaking contact when the six wires were close together than when held asunder. . another bundle, containing twenty of these wires, was eighteen feet long: the terminal pieces were one-fifth of an inch in diameter, and each six inches long. this was compared with nineteen feet in length of copper wire one-fifth of an inch in diameter. the bundle gave a smaller spark on breaking contact than the latter, even when its strands were held together by string: when they were separated, it gave a still smaller spark. upon the whole, however, the diminution of effect was not such as i expected: and i doubt whether the results can be considered as any proof of the truth of the supposition which gave rise to them. . the inductive force by which two elements of one current ( . .) act upon each other, appears to diminish as the line joining them becomes oblique to the direction of the current and to vanish entirely when it is parallel. i am led by some results to suspect that it then even passes into the repulsive force noticed by ampère[a]; which is the cause of the elevations in mercury described by sir humphry davy[b], and which again is probably directly connected with the quality of intensity. [a] recueil d'observations electro-dynamiques, p. . [b] philosophical transactions, , p. . . notwithstanding that the effects appear only at the making and breaking of contact, (the current remaining unaffected, seemingly, in the interval,) i cannot resist the impression that there is some connected and correspondent effect produced by this lateral action of the elements of the electric stream during the time of its continuance ( . .). an action of this kind, in fact, is evident in the magnetic relations of the parts of the current. but admitting (as we may do for the moment) the magnetic forces to constitute the power which produces such striking and different results at the commencement and termination of a current, still there appears to be a link in the chain of effects, a wheel in the physical mechanism of the action, as yet unrecognised. if we endeavour to consider electricity and magnetism as the results of two forces of a physical agent, or a peculiar condition of matter, exerted in determinate directions perpendicular to each other, then, it appears to me, that we must consider these two states or forces as convertible into each other in a greater or smaller degree; i.e. that an element of an electric current has not a determinate electric force and a determinate magnetic force constantly existing in the same ratio, but that the two forces are, to a certain degree, convertible by a process or change of condition at present unknown to us. how else can a current of a given intensity and quantity be able, by its direct action, to sustain a state which, when allowed to react, (at the cessation of the original current,) shall produce a second current, having an intensity and quantity far greater than the generating one? this cannot result from a direct reaction of the electric force; and if it result from a change of electrical into magnetic force, and a reconversion back again, it will show that they differ in something more than mere direction, as regards _that agent_ in the conducting wire which constitutes their immediate cause. . with reference to the appearance, at different times, of the contrary effects produced by the making and breaking contact, and their separation by an intermediate and indifferent state, this separation is probably more apparent than real. if the conduction of electricity be effected by vibrations ( .), or by any other mode in which opposite forces are successively and rapidly excited and neutralized, then we might expect a peculiar and contrary development of force at the commencement and termination of the periods during which the conducting action should last (somewhat in analogy with the colours produced at the outside of an imperfectly developed solar spectrum): and the intermediate actions, although not sensible in the same way, may be very important and, for instance, perhaps constitute the very essence of conductibility. it is by views and reasons such as these, which seem to me connected with the fundamental laws and facts of electrical science, that i have been induced to enter, more minutely than i otherwise should have done, into the experimental examination of the phenomena described in this paper. . before concluding, i may briefly remark, that on using a voltaic battery of fifty pairs of plates instead of a single pair ( .), the effects were exactly of the same kind. the spark on making contact, for the reasons before given, was very small ( . .); that on breaking contact, very excellent and brilliant. the _continuous_ discharge did not seem altered in character, whether a short wire or the powerful electro-magnet were used as a connecting discharger. . the effects produced at the commencement and end of a current, (which are separated by an interval of time when that current is supplied from a voltaic apparatus,) must occur at the same moment when a common electric discharge is passed through a long wire. whether, if happening accurately at the same moment, they would entirely neutralize each other, or whether they would not still give some definite peculiarity to the discharge, is a matter remaining to be examined; but it is very probable that the peculiar character and pungency of sparks drawn from a long wire depend in part upon the increased intensity given at the termination of the discharge by the inductive action then occurring. . in the wire of the helix of magneto-electric machines, (as, for instance, in mr. saxton's beautiful arrangement,) an important influence of these principles of action is evidently shown. from the construction of the apparatus the current is permitted to move in a complete metallic circuit of great length during the first instants of its formation: it gradually rises in strength, and is then suddenly stopped by the breaking of the metallic circuit; and thus great intensity is given _by induction_ to the electricity, which at that moment passes ( . .). this intensity is not only shown by the brilliancy of the spark and the strength of the shock, but also by the necessity which has been experienced of well-insulating the convolutions of the helix, in which the current is formed: and it gives to the current a force at these moments very far above that which the apparatus could produce if the principle which forms the subject of this paper were not called into play. _royal institution, december th, ._ tenth series. § . _on an improved form of the voltaic battery._ § . _some practical results respecting the construction and use of the voltaic battery._ received june ,--read june , . . i have lately had occasion to examine the voltaic trough practically, with a view to improvements in its construction and use; and though i do not pretend that the results have anything like the importance which attaches to the discovery of a new law or principle, i still think they are valuable, and may therefore, if briefly told, and in connexion with former papers, be worthy the approbation of the royal society. § . _on an improved form of the voltaic battery._ . in a simple voltaic circuit (and the same is true of the battery) the chemical forces which, during their activity, give power to the instrument, are generally divided into two portions; one of these is exerted locally, whilst the other is transferred round the circle ( . .); the latter constitutes the electric current of the instrument, whilst the former is altogether lost or wasted. the ratio of these two portions of power may be varied to a great extent by the influence of circumstances: thus, in a battery not closed, _all_ the action is local; in one of the ordinary construction, _much_ is in circulation when the extremities are in communication: and in the perfect one, which i have described ( .), _all_ the chemical power circulates and becomes electricity. by referring to the quantity of zinc dissolved from the plates ( . .), and the quantity of decomposition effected in the volta-electrometer ( . ,) or elsewhere, the proportions of the local and transferred actions under any particular circumstances can be ascertained, and the efficacy of the voltaic arrangement, or the waste of chemical power at its zinc plates, be accurately determined. . if a voltaic battery were constructed of zinc and platina, the latter metal surrounding the former, as in the double copper arrangement, and the whole being excited by dilute sulphuric acid, then no insulating divisions of glass, porcelain or air would be required between the contiguous platina surfaces; and, provided these did not touch metallically, the same acid which, being between the zinc and platina, would excite the battery into powerful action, would, between the two surfaces of platina, produce no discharge of the electricity, nor cause any diminution of the power of the trough. this is a necessary consequence of the resistance to the passage of the current which i have shown occurs at the place of decomposition ( . .); for that resistance is fully able to stop the current, and therefore acts as insulation to the electricity of the contiguous plates, inasmuch as the current which tends to pass between them never has a higher intensity than that due to the action of a single pair. . if the metal surrounding the zinc be copper ( .), and if the acid be nitro-sulphuric acid ( .), then a slight discharge between the two contiguous coppers does take place, provided there be no other channel open by which the forces may circulate; but when such a channel is permitted, the return or back discharge of which i speak is exceedingly diminished, in accordance with the principles laid down in the eighth series of these researches. . guided by these principles i was led to the construction of a voltaic trough, in which the coppers, passing round both surfaces of the zincs, as in wollaston's construction, should not be separated from each other except by an intervening thickness of paper, or in some other way, so as to prevent metallic contact, and should thus constitute an instrument compact, powerful, economical, and easy of use. on examining, however, what had been done before, i found that the new trough was in all essential respects the same as that invented and described by dr. hare, professor in the university of pennsylvania, to whom i have great pleasure in referring it. . dr. hare has fully described his trough[a]. in it the contiguous copper plates are separated by thin veneers of wood, and the acid is poured on to, or off, the plates by a quarter revolution of an axis, to which both the trough containing the plates, and another trough to collect and hold the liquid, are fixed. this arrangement i have found the most convenient of any, and have therefore adopted it. my zinc plates were cut from rolled metal, and when soldered to the copper plates had the form delineated, fig. . these were then bent over a gauge into the form fig. , and when packed in the wooden box constructed to receive them, were arranged as in fig. [b], little plugs of cork being used to keep the zinc plates from touching the copper plates, and a single or double thickness of cartridge paper being interposed between the contiguous surfaces of copper to prevent them from coming in contact. such was the facility afforded by this arrangement, that a trough of forty pairs of plates could be unpacked in five minutes, and repacked again in half an hour; and the whole series was not more than fifteen inches in length. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] [a] philosophical magazine, , vol. lxiii. p. ; or silliman's journal, vol. vii. see also a previous paper by dr. hare, annals of philosophy, , vol. i. p. , in which he speaks of the non-necessity of insulation between the coppers. [b] the papers between the coppers are, for the sake of distinctness, omitted in the figure. . this trough, of forty pairs of plates three inches square, was compared, as to the ignition of a platina wire, the discharge between points of charcoal, the shock on the human frame, &c., with forty pairs of four-inch plates having double coppers, and used in porcelain troughs divided into insulating cells, the strength of the acid employed to excite both being the same. in all these effects the former appeared quite equal to the latter. on comparing a second trough of the new construction, containing twenty pairs of four-inch plates, with twenty pairs of four-inch plates in porcelain troughs, excited by acid of the same strength, the new trough appeared to surpass the old one in producing these effects, especially in the ignition of wire. . in these experiments the new trough diminished in its energy much more rapidly than the one on the old construction, and this was a necessary consequence of the smaller quantity of acid used to excite it, which in the case of the forty pairs of new construction was only one-seventh part of that used for the forty pairs in the porcelain troughs. to compare, therefore, both forms of the voltaic trough in their decomposing powers, and to obtain accurate data as to their relative values, experiments of the following kind were made. the troughs were charged with a known quantity of acid of a known strength; the electric current was passed through a volta-electrometer ( .) having electrodes inches long and . inches in width, so as to oppose as little obstruction as possible to the current; the gases evolved were collected and measured, and gave the quantity of water decomposed. then the whole of the charge used was mixed together, and a known part of it analyzed, by being precipitated and boiled with excess of carbonate of soda, and the precipitate well-washed, dried, ignited, and weighed. in this way the quantity of metal oxidized and dissolved by the acid was ascertained; and the part removed from each zinc plate, or from all the plates, could be estimated and compared with the water decomposed in the volta-electrometer. to bring these to one standard of comparison, i have reduced the results so as to express the loss at the plates in equivalents of zinc for the equivalent of water decomposed at the volta-electrometer: i have taken the equivalent number of water as , and of zinc as . , and have considered cubic inches of the mixed oxygen and hydrogen, as they were collected over a pneumatic trough, to result from the decomposition of . grains of water. . the acids used in these experiments were three,--sulphuric, nitric, and muriatic. the sulphuric acid was strong oil of vitriol; one cubical inch of it was equivalent to grains of marble. the nitric acid was very nearly pure; one cubical inch dissolved grains of marble. the muriatic acid was also nearly pure, and one cubical inch dissolved grains of marble. these were always mixed with water by volumes, the standard of volume being a cubical inch. . an acid was prepared consisting of parts water, - / parts sulphuric acid, and parts nitric acid; and with this both my trough containing forty pairs of three-inch plates, and four porcelain troughs, arranged in succession, each containing ten pairs of plates with double coppers four inches square, were charged. these two batteries were then used in succession, and the action of each was allowed to continue for twenty or thirty minutes, until the charge was nearly exhausted, the connexion with the volta-electrometer being carefully preserved during the whole time, and the acid in the troughs occasionally mixed together. in this way the former trough acted so well, that for each equivalent of water decomposed in the volta-electrometer only from to . equivalents of zinc were dissolved from each plate. in four experiments the average was . equivalents for each plate, or . for the whole battery. in the experiments with the porcelain troughs, the equivalents of consumption at each plate were . , or . for the whole battery. in a perfect voltaic battery of forty pairs of plates ( . .) the consumption would have been one equivalent for each zinc plate, or forty for the whole. . similar experiments were made with two voltaic batteries, one containing twenty pairs of four-inch plates, arranged as i have described ( .), and the other twenty pairs of four-inch plates in porcelain troughs. the average of five experiments with the former was a consumption of . equivalents of zinc from each plate, or from the whole: the average of three experiments with the latter was . equivalents from each plate, or from the whole: to obtain this conclusion two experiments were struck out, which were much against the porcelain troughs, and in which some unknown deteriorating influence was supposed to be accidentally active. in all the experiments, care was taken not to compare _new_ and _old_ plates together, as that would have introduced serious errors into the conclusions ( .). . when ten pairs of the new arrangement were used, the consumption of zinc at each plate was . equivalents, or . for the whole. with ten pairs of the common construction, in a porcelain trough, the zinc oxidized was, upon an average, . equivalents each plate, or for the entire trough. . no doubt, therefore, can remain of the equality or even the great superiority of this form of voltaic battery over the best previously in use, namely, that with double coppers, in which the cells are insulated. the insulation of the coppers may therefore be dispensed with; and it is that circumstance which principally permits of such other alterations in the construction of the trough as gives it its practical advantages. . the advantages of this form of trough are very numerous and great. i. it is exceedingly compact, for pairs of plates need not occupy a trough of more than three feet in length, ii. by dr. hare's plan of making the trough turn upon copper pivots which rest upon copper bearings, the latter afford _fixed_ terminations; and these i have found it very convenient to connect with two cups of mercury, fastened in the front of the stand of the instrument. these fixed terminations give the great advantage of arranging an apparatus to be used in connexion with the battery _before_ the latter is put into action, iii. the trough is put into readiness for use in an instant, a single jug of dilute acid being sufficient for the charge of pairs of four-inch plates, iv. on making the trough pass through a quarter of a revolution, it becomes active, and the great advantage is obtained of procuring for the experiment the effect of the _first contact_ of the zinc and acid, which is twice or sometimes even thrice that which the battery can produce a minute or two after ( . .). v. when the experiment is completed, the acid can be at once poured from between the plates, so that the battery is never left to waste during an unconnected state of its extremities; the acid is not unnecessarily exhausted; the zinc is not uselessly consumed; and, besides avoiding these evils, the charge is mixed and rendered uniform, which produces a great and good result ( .); and, upon proceeding to a second experiment, the important effect of _first contact_ is again obtained. vi. the saving of zinc is very great. it is not merely that, whilst in action, the zinc performs more voltaic duty ( . .), but _all_ the destruction which takes place with the ordinary forms of battery between the experiments is prevented. this saving is of such extent, that i estimate the zinc in the new form of battery to be thrice as effective as that in the ordinary form. vii. the importance of this saving of metal is not merely that the value of the zinc is saved, but that the battery is much lighter and more manageable; and also that the surfaces of the zinc and copper plates may be brought much nearer to each other when the battery is constructed, and remain so until it is worn out: the latter is a very important advantage ( .). viii. again, as, in consequence of the saving, thinner plates will perform the duty of thick ones, rolled zinc may be used; and i have found rolled zinc superior to cast zinc in action; a superiority which i incline to attribute to its greater purity ( .). ix. another advantage is obtained in the economy of the acid used, which is proportionate to the diminution of the zinc dissolved. x. the acid also is more easily exhausted, and is in such small quantity that there is never any occasion to return an old charge into use. the acid of old charges whilst out of use, often dissolves portions of copper from the black flocculi usually mingled with it, which are derived from the zinc; now any portion of copper in solution in the charge does great harm, because, by the _local_ action of the acid and zinc, it tends to precipitate upon the latter, and diminish its voltaic efficacy ( .). xi. by using a due mixture of nitric and sulphuric acid for the charge ( .), no gas is evolved from the troughs; so that a battery of several hundred pairs of plates may, without inconvenience, be close to the experimenter. xii. if, during a series of experiments, the acid becomes exhausted, it can be withdrawn, and replaced by other acid with the utmost facility; and after the experiments are concluded, the great advantage of easily washing the plates is at command. and it appears to me, that in place of making, under different circumstances, mutual sacrifices of comfort, power, and economy, to obtain a desired end, all are at once obtained by dr. hare's form of trough. . but there are some disadvantages which i have not yet had time to overcome, though i trust they will finally be conquered. one is the extreme difficulty of making a wooden trough constantly water-tight under the alternations of wet and dry to which the voltaic instrument is subject. to remedy this evil, mr. newman is now engaged in obtaining porcelain troughs. the other disadvantage is a precipitation of copper on the zinc plates. it appears to me to depend mainly on the circumstance that the papers between the coppers retain acid when the trough is emptied; and that this acid slowly acting on the copper, forms a salt, which gradually mingles with the next charge, and is reduced on the zinc plate by the local action ( .): the power of the whole battery is then reduced. i expect that by using slips of glass or wood to separate the coppers at their edges, their contact can be sufficiently prevented, and the space between them be left so open that the acid of a charge can be poured and washed out, and so be removed from _every part_ of the trough when the experiments in which the latter is used are completed. . the actual superiority of the troughs which i have constructed on this plan, i believe to depend, first and principally, on the closer approximation of the zinc and copper surfaces;--in my troughs they are only one-tenth of an inch apart ( .);--and, next, on the superior quality of the rolled zinc above the cast zinc used in the construction of the ordinary pile. it cannot be that insulation between the contiguous coppers is a disadvantage, but i do not find that it is any advantage; for when, with both the forty pairs of three-inch plates and the twenty pairs of four-inch plates, i used papers well-soaked in wax[a], these being so large that when folded at the edges they wrapped over each other, so as to make cells as insulating as those of the porcelain troughs, still no sensible advantage in the chemical action was obtained. [a] a single paper thus prepared could insulate the electricity of a trough of forty pairs of plates. . as, upon principle, there must be a discharge of part of the electricity from the edges of the zinc and copper plates at the sides of the trough, i should prefer, and intend having, troughs constructed with a plate or plates of crown glass at the sides of the trough: the bottom will need none, though to glaze that and the ends would be no disadvantage. the plates need not be fastened in, but only set in their places; nor need they be in large single pieces. § . _some practical results respecting the construction and use of the voltaic battery_ ( . &c.). . the electro-chemical philosopher is well acquainted with some practical results obtained from the voltaic battery by mm.. gay-lussac and thenard, and given in the first forty-five pages of their 'recherches physico-chimiques'. although the following results are generally of the same nature, yet the advancement made in this branch of science of late years, the knowledge of the definite action of electricity, and the more accurate and philosophical mode of estimating the results by the equivalents of zinc consumed, will be their sufficient justification. . _nature and strength of the acid._--my battery of forty pairs of three-inch plates was charged with acid consisting of parts water and oil of vitriol. each plate lost, in the average of the experiments, . equivalents of zinc for the equivalent of water decomposed in the volta-electrometer, or the whole battery . equivalents of zinc. being charged with a mixture of water and of the muriatic acid, each plate lost . , equivalents of zinc for the water decomposed, or the whole battery equivalents of zinc. being charged with a mixture of water and nitric acid, each plate lost . , equivalents of zinc for one equivalent of water decomposed, or the whole battery . equivalents of zinc. the sulphuric and muriatic acids evolved much hydrogen at the plates in the trough; the nitric acid no gas whatever. the relative strengths of the original acids have already been given ( .); but a difference in that respect makes no important difference in the results when thus expressed by equivalents ( .). . thus nitric acid proves to be the best for this purpose; its superiority appears to depend upon its favouring the electrolyzation of the liquid in the cells of the trough upon the principles already explained ( . , .), and consequently favouring the transmission of the electricity, and therefore the production of transferable power ( .). . the addition of nitric acid might, consequently, be expected to improve sulphuric and muriatic acids. accordingly, when the same trough was charged with a mixture of water, oil of vitriol, and nitric acid, the consumption of zinc was at each plate . , and for the whole battery . , equivalents. when the charge was water, oil of vitriol, and nitric acid, the loss per plate was . , or for the whole battery . , equivalents. when the trough was charged with a mixture of water, muriatic acid, and nitric acid, the loss per plate was . , or for the whole battery . , equivalents. similar results were obtained with my battery of twenty pairs of four-inch plates ( .). hence it is evident that the nitric acid was of great service when mingled with the sulphuric acid; and the charge generally used after this time for ordinary experiments consisted of water, - / oil of vitriol, and nitric acid. . it is not to be supposed that the different strengths of the acids produced the differences above; for within certain limits i found the electrolytic effects to be nearly as the strengths of the acids, so as to leave the expression of force, when given in equivalents, almost constant. thus, when the trough was charged with a mixture of water and nitric acid, each plate lost . equivalent of zinc. when the charge was water and nitric acid, the loss per plate was . equivalent. when it was water and nitric acid, the loss was . equivalents. the differences here are not greater than happen from unavoidable irregularities, depending on other causes than the strength of acid. . again, when a charge consisting of water, - / oil of vitriol, and nitric acid was used, each zinc plate lost . equivalents; when the charge with the same battery was water, oil of vitriol, and nitric acid, each zinc plate lost . equivalents. . i need hardly say that no copper is dissolved during the regular action of the voltaic trough. i have found that much ammonia is formed in the cells when nitric acid, either pure or mixed with sulphuric acid, is used. it is produced in part as a secondary result at the cathodes ( .) of the different portions of fluid constituting the necessary electrolyte, in the cells. . _uniformity of the charge._--this is a most important point, as i have already shown experimentally ( . &c.). hence one great advantage of dr. hare's mechanical arrangement of his trough. . _purity of the zinc._--if pure zinc could be obtained, it would be very advantageous in the construction of the voltaic apparatus ( .). most zincs, when put into dilute sulphuric acid, leave more or less of an insoluble matter upon the surface in the form of a crust, which contains various metals, as copper, lead, zinc, iron, cadmium, &c., in the metallic state. such particles, by discharging part of the transferable power, render it, as to the whole battery, local; and so diminish the effect. as an indication connected with the more or less perfect action of the battery, i may mention that no gas ought to rise from the zinc plates. the more gas which is generated upon these surfaces, the greater is the local action and the less the transferable force. the investing crust is also inconvenient, by preventing the displacement and renewal of the charge upon the surface of the zinc. such zinc as, dissolving in the cleanest manner in a dilute acid, dissolves also the slowest, is the best; zinc which contains much copper should especially be avoided. i have generally found rolled liege or mosselman's zinc the purest; and to the circumstance of having used such zinc in its construction attribute in part the advantage of the new battery ( .). . _foulness of the zinc plates._--after use, the plates of a battery should be cleaned from the metallic powder upon their surfaces, especially if they are employed to obtain the laws of action of the battery itself. this precaution was always attended to with the porcelain trough batteries in the experiments described ( , &c.). if a few foul plates are mingled with many clean ones, they make the action in the different cells irregular, and the transferable power is accordingly diminished, whilst the local and wasted power is increased. no old charge containing copper should be used to excite a battery. . _new and old plates._--i have found voltaic batteries far more powerful when the plates were new than when they have been used two or three times; so that a new and an used battery cannot be compared together, or even a battery with itself on the first and after times of use. my trough of twenty pairs of four-inch plates, charged with acid consisting of water, - / oil of vitriol, and nitric acid, lost, upon the first time of being used, . equivalents per plate. when used after the fourth time with the same charge, the loss was from . to . equivalents per plate; the average being . equivalents. the first time the forty pair of plates ( .) were used, the loss at each plate was only . equivalent; but afterwards it became . , . , . . the first time twenty pair of four-inch plates in porcelain troughs were used, they lost, per plate, only . equivalents; but after that, the loss was . , . , . equivalents. yet in all these cases the zincs had been well-cleaned from adhering copper, &c., before each trial of power. . with the rolled zinc the fall in force soon appeared to become constant, i.e. to proceed no further. but with the cast zinc plates belonging to the porcelain troughs, it appeared to continue, until at last, with the same charge, each plate lost above twice as much zinc for a given amount of action as at first. these troughs were, however, so irregular that i could not always determine the circumstances affecting the amount of electrolytic action. . _vicinity of the copper and zinc._--the importance of this point in the construction of voltaic arrangements, and the greater power, as to immediate action, which is obtained when the zinc and copper surfaces are near to each other than when removed further apart, are well known. i find that the power is not only greater on the instant, but also that the sum of transferable power, in relation to the whole sum of chemical action at the plates, is much increased. the cause of this gain is very evident. whatever tends to retard the circulation of the transferable force, (i.e. the electricity,) diminishes the proportion of such force, and increases the proportion of that which is local ( . .). now the liquid in the cells possesses this retarding power, and therefore acts injuriously, in greater or less proportion, according to the quantity of it between the zinc and copper plates, i.e. according to the distances between their surfaces. a trough, therefore, in which the plates are only half the distance asunder at which they are placed in another, will produce more transferable, and less local, force than the latter; and thus, because the electrolyte in the cells can transmit the current more readily; both the intensity and quantity of electricity is increased for a given consumption of zinc. to this circumstance mainly i attribute the superiority of the trough i have described ( .). . the superiority of _double coppers_ over single plates also depends in part upon diminishing the resistance offered by the electrolyte between the metals. for, in fact, with double coppers the sectional area of the interposed acid becomes nearly double that with single coppers, and therefore it more freely transfers the electricity. double coppers are, however, effective, mainly because they virtually double the acting surface of the zinc, or nearly so; for in a trough with single copper plates and the usual construction of cells, that surface of zinc which is not opposed to a copper surface is thrown almost entirely out of voltaic action, yet the acid continues to act upon it and the metal is dissolved, producing very little more than local effect ( . ). but when by doubling the copper, that metal is opposed to the second surface of the zinc plate, then a great part of the action upon the latter is converted into transferable force, and thus the power of the trough as to quantity of electricity is highly exalted. . _first immersion of the plates._--the great effect produced at the first immersion of the plates, (apart from their being new or used ( .),) i have attributed elsewhere to the unchanged condition of the acid in contact with the zinc plate ( . .): as the acid becomes neutralized, its exciting power is proportionally diminished. hare's form of trough secures much advantage of this kind, by mingling the liquid, and bringing what may be considered as a fresh surface of acid against the plates every time it is used immediately after a rest. . _number of plates._[a]--the most advantageous number of plates in a battery used for chemical decomposition, depends almost entirely upon the resistance to be overcome at the place of action; but whatever that resistance may be, there is a certain number which is more economical than either a greater or a less. ten pairs of four-inch plates in a porcelain trough of the ordinary construction, acting in the volta-electrometer ( .) upon dilute sulphuric acid of spec. grav. . , gave an average consumption of . equivalents per plate, or equivalents on the whole. twenty pairs of the same plates, with the same acid, gave only a consumption of . per plate, or equivalents upon the whole. when forty pairs of the same plates were used, the consumption was . equivalents per plate, or . upon the whole battery. thus the consumption of zinc arranged as _twenty_ plates was more advantageous than if arranged either as _ten_ or as _forty_. [a] gay-lussac and thenard, recherches physico-chimiques, tom. i. p. . . again, ten pairs of my four-inch plates ( .) lost . each, or the whole ten . equivalents of zinc, in effecting decomposition; whilst twenty pairs of the same plates, excited by the same acid, lost . equivalents each, or on the whole equivalents. in other comparative experiments of numbers, ten pairs of the three inch-plates, ( .) lost . , or . equivalents upon the whole; whilst twenty pairs lost . each, or . in all; and forty pairs lost on an average . , or . altogether. in both these cases, therefore, increase of numbers had not been advantageous as to the effective production of _transferable chemical power_ from the _whole quantity of chemical force_ active at the surfaces of excitation ( .). . but if i had used a weaker acid or a worse conductor in the volta-electrometer, then the number of plates which would produce the most advantageous effect would have risen; or if i had used a better conductor than that really employed in the volta-electrometer, i might have reduced the number even to one; as, for instance, when a thick wire is used to complete the circuit ( ., &c.). and the cause of these variations is very evident, when it is considered that each successive plate in the voltaic apparatus does not add anything to the _quantity_ of transferable power or electricity which the first plate can put into motion, provided a good conductor be present, but tends only to exalt the _intensity_ of that quantity, so as to make it more able to overcome the obstruction of bad conductors ( . .). . _large or small plates._[a]--the advantageous use of large or small plates for electrolyzations will evidently depend upon the facility with which the transferable power of electricity can pass. if in a particular case the most effectual number of plates is known ( .), then the addition of more zinc would be most advantageously made in increasing the _size_ of the plates, and not their _number_. at the same time, large increase in the size of the plates would raise in a small degree the most favourable number. [a] gay-lussac and thenard, recherches physico-chimiques, tom, i. p. . . large and small plates should not be used together in the same battery: the small ones occasion a loss of the power of the large ones, unless they be excited by an acid proportionably more powerful; for with a certain acid they cannot transmit the same portion of electricity in a given time which the same acid can evolve by action on the larger plates. . _simultaneous decompositions._--when the number of plates in a battery much surpasses the most favourable proportion ( -- .), two or more decompositions may be effected simultaneously with advantage. thus my forty pairs of plates ( .) produced in one volta-electrometer . cubic inches of gas. being recharged exactly in the same manner, they produced in each of two volta-electrometers cubical inches. in the first experiment the whole consumption of zinc was . equivalents, and in the second only . equivalents, for the whole of the water decomposed in both volta-electrometers. . but when the twenty pairs of four-inch plates ( .) were tried in a similar manner, the results were in the opposite direction. with one volta-electrometer cubic inches of gas were obtained; with two, only . cubic inches from each. the quantity of charge was not the same in both cases, though it was of the same strength; but on rendering the results comparative by reducing them to equivalents ( .), it was found that the consumption of metal in the first case was , and in the second case , equivalents for the _whole_ of the water decomposed. these results of course depend upon the same circumstances of retardation, &c., which have been referred to in speaking of the proper number of plates ( .). . that the _transferring_, or, as it is usually called, _conducting, power_ of an electrolyte which is to be decomposed, or other interposed body, should be rendered as good as possible[a], is very evident ( . .). with a perfectly good conductor and a good battery, nearly all the electricity is passed, i.e. _nearly all_ the chemical power becomes transferable, even with a single pair of plates ( .). with an interposed nonconductor none of the chemical power becomes transferable. with an imperfect conductor more or less of the chemical power becomes transferable as the circumstances favouring the transfer of forces across the imperfect conductor are exalted or diminished: these circumstances are, actual increase or improvement of the conducting power, enlargement of the electrodes, approximation of the electrodes, and increased intensity of the passing current. [a] gay-lussac and thenard, recherches physico-chimiques, tom. i. pp. , , . . the introduction of common spring water in place of one of the volta-electrometers used with twenty pairs of four-inch plates ( .) caused such obstruction as not to allow one-fifteenth of the transferable force to pass which would have circulated without it. thus fourteen-fifteenths of the available force of the battery were destroyed, local force, (which was rendered evident by the evolution of gas from the being converted into zincs,) and yet the platina electrodes in the water were three inches long, nearly an inch wide, and not a quarter of an inch apart. . these points, i.e. the increase of conducting power, the enlargement of the electrodes, and their approximation, should be especially attended to in _volta-electrometers_. the principles upon which their utility depend are so evident that there can be no occasion for further development of them here. _royal institution, october , ._ eleventh series. § . _on induction._ ¶ i. _induction an action of contiguous particles._ ¶ ii. _absolute charge of matter._ ¶ iii. _electrometer and inductive apparatus employed._ ¶ iv. _induction in curved lines._ ¶ v. _specific inductive capacity._ ¶ vi. _general results as to induction._ received november ,--read december , . ¶ i. _induction an action of contiguous particles._ . the science of electricity is in that state in which every part of it requires experimental investigation; not merely for the discovery of new effects, but what is just now of far more importance, the development of the means by which the old effects are produced, and the consequent more accurate determination of the first principles of action of the most extraordinary and universal power in nature:--and to those philosophers who pursue the inquiry zealously yet cautiously, combining experiment with analogy, suspicious of their preconceived notions, paying more respect to a fact than a theory, not too hasty to generalize, and above all things, willing at every step to cross-examine their own opinions, both by reasoning and experiment, no branch of knowledge can afford so fine and ready a field for discovery as this. such is most abundantly shown to be the case by the progress which electricity has made in the last thirty years: chemistry and magnetism have successively acknowledged its over-ruling influence; and it is probable that every effect depending upon the powers of inorganic matter, and perhaps most of those related to vegetable and animal life, will ultimately be found subordinate to it. . amongst the actions of different kinds into which electricity has conventionally been subdivided, there is, i think, none which excels, or even equals in importance, that called _induction_. it is of the most general influence in electrical phenomena, appearing to be concerned in every one of them, and has in reality the character of a first, essential, and fundamental principle. its comprehension is so important, that i think we cannot proceed much further in the investigation of the laws of electricity without a more thorough understanding of its nature; how otherwise can we hope to comprehend the harmony and even unity of action which doubtless governs electrical excitement by friction, by chemical means, by heat, by magnetic influence, by evaporation, and even by the living being? . in the long-continued course of experimental inquiry in which i have been engaged, this general result has pressed upon me constantly, namely, the necessity of admitting two forces, or two forms or directions of a force ( . .), combined with the impossibility of separating these two forces (or electricities) from each other, either in the phenomena of statical electricity or those of the current. in association with this, the impossibility under any circumstances, as yet, of absolutely charging matter of any kind with one or the other electricity only, dwelt on my mind, and made me wish and search for a clearer view than any that i was acquainted with, of the way in which electrical powers and the particles of matter are related; especially in inductive actions, upon which almost all others appeared to rest. . when i discovered the general fact that electrolytes refused to yield their elements to a current when in the solid state, though they gave them forth freely if in the liquid condition ( . . .), i thought i saw an opening to the elucidation of inductive action, and the possible subjugation of many dissimilar phenomena to one law. for let the electrolyte be water, a plate of ice being coated with platina foil on its two surfaces, and these coatings connected with any continued source of the two electrical powers, the ice will charge like a leyden arrangement, presenting a case of common induction, but no current will pass. if the ice be liquefied, the induction will fall to a certain degree, because a current can now pass; but its passing is dependent upon a _peculiar molecular arrangement_ of the particles consistent with the transfer of the elements of the electrolyte in opposite directions, the degree of discharge and the quantity of elements evolved being exactly proportioned to each other ( . .). whether the charging of the metallic coating be effected by a powerful electrical machine, a strong and large voltaic battery, or a single pair of plates, makes no difference in the principle, but only in the degree of action ( ). common induction takes place in each case if the electrolyte be solid, or if fluid, chemical action and decomposition ensue, provided opposing actions do not interfere; and it is of high importance occasionally thus to compare effects in their extreme degrees, for the purpose of enabling us to comprehend the nature of an action in its weak state, which may be only sufficiently evident to us in its stronger condition ( .). as, therefore, in the electrolytic action, _induction_ appeared to be the _first_ step, and _decomposition_ the _second_ (the power of separating these steps from each other by giving the solid or fluid condition to the electrolyte being in our hands); as the induction was the same in its nature as that through air, glass, wax, &c. produced by any of the ordinary means; and as the whole effect in the electrolyte appeared to be an action of the particles thrown into a peculiar or polarized state, i was led to suspect that common induction itself was in all cases an _action of contiguous particles_[a], and that electrical action at a distance (i.e. ordinary inductive action) never occurred except through the influence of the intervening matter. [a] the word _contiguous_ is perhaps not the best that might have been used here and elsewhere; for as particles do not touch each other it is not strictly correct. i was induced to employ it, because in its common acceptation it enabled me to state the theory plainly and with facility. by contiguous particles i mean those which are next.--_dec. ._ . the respect which i entertain towards the names of epinus, cavendish, poisson, and other most eminent men, all of whose theories i believe consider induction as an action at a distance and in straight lines, long indisposed me to the view i have just stated; and though i always watched for opportunities to prove the opposite opinion, and made such experiments occasionally as seemed to bear directly on the point, as, for instance, the examination of electrolytes, solid and fluid, whilst under induction by polarized light ( . .), it is only of late, and by degrees, that the extreme generality of the subject has urged me still further to extend my experiments and publish my view. at present i believe ordinary induction in all cases to be an action of contiguous particles consisting in a species of polarity, instead of being an action of either particles or masses at sensible distances; and if this be true, the distinction and establishment of such a truth must be of the greatest consequence to our further progress in the investigation of the nature of electric forces. the linked condition of electrical induction with chemical decomposition; of voltaic excitement with chemical action; the transfer of elements in an electrolyte; the original cause of excitement in all cases; the nature and relation of conduction and insulation of the direct and lateral or transverse action constituting electricity and magnetism; with many other things more or less incomprehensible at present, would all be affected by it, and perhaps receive a full explication in their reduction under one general law. . i searched for an unexceptionable test of my view, not merely in the accordance of known facts with it, but in the consequences which would flow from it if true; especially in those which would not be consistent with the theory of action at a distance. such a consequence seemed to me to present itself in the direction in which inductive action could be exerted. if in straight lines only, though not perhaps decisive, it would be against my view; but if in curved lines also, that would be a natural result of the action of contiguous particles, but, as i think, utterly incompatible with action at a distance, as assumed by the received theories, which, according to every fact and analogy we are acquainted with, is always in straight lines. . again, if induction be an action of contiguous particles, and also the first step in the process of electrolyzation ( . .), there seemed reason to expect some particular relation of it to the different kinds of matter through which it would be exerted, or something equivalent to a _specific electric induction_ for different bodies, which, if it existed, would unequivocally prove the dependence of induction on the particles; and though this, in the theory of poisson and others, has never been supposed to be the case, i was soon led to doubt the received opinion, and have taken great pains in subjecting this point to close experimental examination. . another ever-present question on my mind has been, whether electricity has an actual and independent existence as a fluid or fluids, or was a mere power of matter, like what we conceive of the attraction of gravitation. if determined either way it would be an enormous advance in our knowledge; and as having the most direct and influential bearing on my notions, i have always sought for experiments which would in any way tend to elucidate that great inquiry. it was in attempts to prove the existence of electricity separate from matter, by giving an independent charge of either positive or negative power only, to some one substance, and the utter failure of all such attempts, whatever substance was used or whatever means of exciting or _evolving_ electricity were employed, that first drove me to look upon induction as an action of the particles of matter, each having _both_ forces developed in it in exactly equal amount. it is this circumstance, in connection with others, which makes me desirous of placing the remarks on absolute charge first, in the order of proof and argument, which i am about to adduce in favour of my view, that electric induction is an action of the contiguous particles of the insulating medium or _dielectric_[a]. [a] i use the word _dielectric_ to express that substance through or across which the electric forces are acting.--_dec. ._ ¶ ii. _on the absolute charge of matter._ . can matter, either conducting or non-conducting, be charged with one electric force independently of the other, in any degree, either in a sensible or latent state? . the beautiful experiments of coulomb upon the equality of action of _conductors_, whatever their substance, and the residence of _all_ the electricity upon their surfaces[a], are sufficient, if properly viewed, to prove that _conductors cannot be bodily charged_; and as yet no means of communicating electricity to a conductor so as to place its particles in relation to one electricity, and not at the same time to the other in exactly equal amount, has been discovered. [a] mémoires de l'académie, , pp. . . ; , p. . . with regard to electrics or non-conductors, the conclusion does not at first seem so clear. they may easily be electrified bodily, either by communication ( .) or excitement; but being so charged, every case in succession, when examined, came out to be a case of induction, and not of absolute charge. thus, glass within conductors could easily have parts not in contact with the conductor brought into an excited state; but it was always found that a portion of the inner surface of the conductor was in an opposite and equivalent state, or that another part of the glass itself was in an equally opposite state, an _inductive_ charge and not an _absolute_ charge having been acquired. . well-purified oil of turpentine, which i find to be an excellent liquid insulator for most purposes, was put into a metallic vessel, and, being insulated, an endeavour was made to charge its particles, sometimes by contact of the metal with the electrical machine, and at others by a wire dipping into the fluid within; but whatever the mode of communication, no electricity of one kind only was retained by the arrangement, except what appeared on the exterior surface of the metal, that portion being present there only by an inductive action through the air to the surrounding conductors. when the oil of turpentine was confined in glass vessels, there were at first some appearances as if the fluid did receive an absolute charge of electricity from the charging wire, but these were quickly reduced to cases of common induction jointly through the fluid, the glass, and the surrounding air. . i carried these experiments on with air to a very great extent. i had a chamber built, being a cube of twelve feet. a slight cubical wooden frame was constructed, and copper wire passed along and across it in various directions, so as to make the sides a large net-work, and then all was covered in with paper, placed in close connexion with the wires, and supplied in every direction with bands of tin foil, that the whole might be brought into good metallic communication, and rendered a free conductor in every part. this chamber was insulated in the lecture-room of the royal institution; a glass tube about six feet in length was passed through its side, leaving about four feet within and two feet on the outside, and through this a wire passed from the large electrical machine ( .) to the air within. by working the machine, the air in this chamber could be brought into what is considered a highly electrified state (being, in fact, the same state as that of the air of a room in which a powerful machine is in operation), and at the same time the outside of the insulated cube was everywhere strongly charged. but putting the chamber in communication with the perfect discharging train described in a former series ( .), and working the machine so as to bring the air within to its utmost degree of charge if i quickly cut off the connexion with the machine, and at the same moment or instantly after insulated the cube, the air within had not the least power to communicate a further charge to it. if any portion of the air was electrified, as glass or other insulators may be charged ( .), it was accompanied by a corresponding opposite action _within_ the cube, the whole effect being merely a case of induction. every attempt to charge air bodily and independently with the least portion of either electricity failed. i put a delicate gold-leaf electrometer within the cube, and then charged the whole by an _outside_ communication, very strongly, for some time together; but neither during the charge or after the discharge did the electrometer or air within show the least signs of electricity. i charged and discharged the whole arrangement in various ways, but in no case could i obtain the least indication of an absolute charge; or of one by induction in which the electricity of one kind had the smallest superiority in quantity over the other. i went into the cube and lived in it, and using lighted candles, electrometers, and all other tests of electrical states, i could not find the least influence upon them, or indication of any thing particular given by them, though all the time the outside of the cube was powerfully charged, and large sparks and brushes were darting off from every part of its outer surface. the conclusion i have come to is, that non-conductors, as well as conductors, have never yet had an absolute and independent charge of one electricity communicated to them, and that to all appearance such a state of matter is impossible. . there is another view of this question which may be taken under the supposition of the existence of an electric fluid or fluids. it may be impossible to have one fluid or state in a free condition without its producing by induction the other, and yet possible to have cases in which an isolated portion of matter in one condition being uncharged, shall, by a change of state, evolve one electricity or the other: and though such evolved electricity might immediately induce the opposite state in its neighbourhood, yet the mere evolution of one electricity without the other in the _first instance_, would be a very important fact in the theories which assume a fluid or fluids; these theories as i understand them assigning not the slightest reason why such an effect should not occur. . but on searching for such cases i cannot find one. evolution by friction, as is well known, gives both powers in equal proportion. so does evolution by chemical action, notwithstanding the great diversity of bodies which may be employed, and the enormous quantity of electricity which can in this manner be evolved ( . . . . .). the more promising cases of change of state, whether by evaporation, fusion, or the reverse processes, still give both forms of the power in _equal_ proportion; and the cases of splitting of mica and other crystals, the breaking of sulphur, &c., are subject to the same law of limitation. . as far as experiment has proceeded, it appears, therefore, impossible either to evolve or make disappear one electric force without equal and corresponding change in the other. it is also equally impossible experimentally to charge a portion of matter with one electric force independently of the other. charge always implies _induction_, for it can in no instance be effected without; and also the presence of the _two_ forms of power, equally at the moment of the development and afterwards. there is no _absolute_ charge of matter with one fluid; no latency of a single electricity. this though a negative result is an exceedingly important one, being probably the consequence of a natural impossibility, which will become clear to us when we understand the true condition and theory of the electric power. . the preceding considerations already point to the following conclusions: bodies cannot be charged absolutely, but only relatively, and by a principle which is the same with that of _induction_. all _charge_ is sustained by induction. all phenomena of _intensity_ include the principle of induction. all _excitation_ is dependent on or directly related to induction. all _currents_ involve previous intensity and therefore previous induction. induction appears to be the essential function both the first development and the consequent phenomena of electricity. ¶ iii. _electrometer and inductive apparatus employed._ . leaving for a time the further consideration of the preceding facts until they can be collated with other results bearing directly on the great question of the nature of induction, i will now describe the apparatus i have had occasion to use; and in proportion to the importance of the principles sought to be established is the necessity of doing this so clearly, as to leave no doubt of the results behind. . _electrometer._--the measuring instrument i have employed has been the torsion balance electrometer of coulomb, constructed, generally, according to his directions[a], but with certain variations and additions, which i will briefly describe. the lower part was a glass cylinder eight inches in height and eight inches in diameter; the tube for the torsion thread was seventeen inches in length. the torsion thread itself was not of metal, but glass, according to the excellent suggestion of the late dr. ritchie[b]. it was twenty inches in length, and of such tenuity that when the shell-lac lever and attached ball, &c. were connected with it, they made about ten vibrations in a minute. it would bear torsion through four revolutions or °, and yet, when released, return accurately to its position; probably it would have borne considerably more than this without injury. the repelled ball was of pith, gilt, and was . of an inch in diameter. the horizontal stem or lever supporting it was of shell-lac, according to coulomb's direction, the arm carrying the ball being . inches long, and the other only . inches: to this was attached the vane, also described by coulomb, which i found to answer admirably its purpose of quickly destroying vibrations. that the inductive action within the electrometer might be uniform in all positions of the repelled ball and in all states of the apparatus, two bands of tin foil, about an inch wide each, were attached to the inner surface of the glass cylinder, going entirely round it, at the distance of . of an inch from each other, and at such a height that the intermediate clear surface was in the same horizontal plane with the lever and ball. these bands were connected with each other and with the earth, and, being perfect conductors, always exerted a uniform influence on the electrified balls within, which the glass surface, from its irregularity of condition at different times, i found, did not. for the purpose of keeping the air within the electrometer in a constant state as to dryness, a glass dish, of such size as to enter easily within the cylinder, had a layer of fused potash placed within it, and this being covered with a disc of fine wire-gauze to render its inductive action uniform at all parts, was placed within the instrument at the bottom and left there. [a] mémoires de l'académie, , p. . [b] philosophical transactions, . . the moveable ball used to take and measure the portion of electricity under examination, and which may be called the _repelling_, or the _carrier_, ball, was of soft alder wood, well and smoothly gilt. it was attached to a fine shell-lac stem, and introduced through a hole into the electrometer according to coulomb's method: the stem was fixed at its upper end in a block or vice, supported on three short feet; and on the surface of the glass cover above was a plate of lead with stops on it, so that when the carrier ball was adjusted in its right position, with the vice above bearing at the same time against these stops, it was perfectly easy to bring away the carrier-ball and restore it to its place again very accurately, without any loss of time. . it is quite necessary to attend to certain precautions respecting these balls. if of pith alone they are bad; for when very dry, that substance is so imperfect a conductor that it neither receives nor gives a charge freely, and so, after contact with a charged conductor, it is liable to be in an uncertain condition. again, it is difficult to turn pith so smooth as to leave the ball, even when gilt, so free from irregularities of form, as to retain its charge undiminished for a considerable length of time. when, therefore, the balls are finally prepared and gilt they should be examined; and being electrified, unless they can hold their charge with very little diminution for a considerable time, and yet be discharged instantly and perfectly by the touch of an uninsulated conductor, they should be dismissed. . it is, perhaps, unnecessary to refer to the graduation of the instrument, further than to explain how the observations were made. on a circle or ring of paper on the outside of the glass cylinder, fixed so as to cover the internal lower ring of tinfoil, were marked four points corresponding to angles of °; four other points exactly corresponding to these points being marked on the upper ring of tinfoil within. by these and the adjusting screws on which the whole instrument stands, the glass torsion thread could be brought accurately into the centre of the instrument and of the graduations on it. from one of the four points on the exterior of the cylinder a graduation of ° was set off, and a corresponding graduation was placed upon the upper tinfoil on the opposite side of the cylinder within; and a dot being marked on that point of the surface of the repelled ball nearest to the side of the electrometer, it was easy, by observing the line which this dot made with the lines of the two graduations just referred to, to ascertain accurately the position of the ball. the upper end of the glass thread was attached, as in coulomb's original electrometer, to an index, which had its appropriate graduated circle, upon which the degree of torsion was ultimately to be read off. . after the levelling of the instrument and adjustment of the glass thread, the blocks which determine the place of the _carrier ball_ are to be regulated ( .) so that, when the carrier arrangement is placed against them, the centre of the ball may be in the radius of the instrument corresponding to ° on the lower graduation or that on the side of the electrometer, and at the same level and distance from the centre as the _repelled ball_ on the suspended torsion lever. then the torsion index is to be turned until the ball connected with it (the repelled ball) is accurately at °, and finally the graduated arc belonging to the torsion index is to be adjusted so as to bring ° upon it to the index. this state of the instrument was adopted as that which gave the most direct expression of the experimental results, and in the form having fewest variable errors; the angular distance of ° being always retained as the standard distance to which the balls were in every case to be brought, and the whole of the torsion being read off at once on the graduated circle above. under these circumstances the distance of the balls from each other was not merely the same in degree, but their position in the instrument, and in relation to every part of it, was actually the same every time that a measurement was made; so that all irregularities arising from slight difference of form and action in the instrument and the bodies around were avoided. the only difference which could occur in the position of anything within, consisted in the deflexion of the torsion thread from a vertical position, more or less, according to the force of repulsion of the balls; but this was so slight as to cause no interfering difference in the symmetry of form within the instrument, and gave no error in the amount of torsion force indicated on the graduation above. . although the constant angular distance of ° between the centres of the balls was adopted, and found abundantly sensible, for all ordinary purposes, yet the facility of rendering the instrument far more sensible by diminishing this distance was at perfect command; the results at different distances being very easily compared with each other either by experiment, or, as they are inversely as the squares of the distances, by calculation. . the coulomb balance electrometer requires experience to be understood; but i think it a very valuable instrument in the hands of those who will take pains by practice and attention to learn the precautions needful in its use. its insulating condition varies with circumstances, and should be examined before it is employed in experiments. in an ordinary and fair condition, when the balls were so electrified as to give a repulsive torsion force of ° at the standard distance of °, it took nearly four hours to sink to ° at the same distance; the average loss from ° to ° being at the rate of °. per minute, from ° to ° of °. per minute, from ° to ° of °. per minute, and from ° to ° of °. per minute. as a complete measurement by the instrument may be made in much less than a minute, the amount of loss in that time is but small, and can easily be taken into account. . _the inductive apparatus._--my object was to examine inductive action carefully when taking place through different media, for which purpose it was necessary to subject these media to it in exactly similar circumstances, and in such quantities as should suffice to eliminate any variations they might present. the requisites of the apparatus to be constructed were, therefore, that the inducing surfaces of the conductors should have a constant form and state, and be at a constant distance from each other; and that either solids, fluids, or gases might be placed and retained between these surfaces with readiness and certainty, and for any length of time. . the apparatus used may be described in general terms as consisting of two metallic spheres of unequal diameter, placed, the smaller within the larger, and concentric with it; the interval between the two being the space through which the induction was to take place. a section of it is given (plate vii. fig. .) on a scale of one-half: _a, a_ are the two halves of a brass sphere, with an air-tight joint at _b_, like that of the magdeburg hemispheres, made perfectly flush and smooth inside so as to present no irregularity; _c_ is a connecting piece by which the apparatus is joined to a good stop-cock _d_, which is itself attached either to the metallic foot _e_, or to an air-pump. the aperture within the hemisphere at _f_ is very small: _g_ is a brass collar fitted to the upper hemisphere, through which the shell-lac support of the inner ball and its stem passes; _h_ is the inner ball, also of brass; it screws on to a brass stem _i_, terminated above by a brass ball b, _l, l_ is a mass of shell-lac, moulded carefully on to _i_, and serving both to support and insulate it and its balls _h_, b. the shell-lac stem _l_ is fitted into the socket _g_, by a little ordinary resinous cement, more fusible than shell-lac, applied at _mm_ in such a way as to give sufficient strength and render the apparatus air-tight there, yet leave as much as possible of the lower part of the shell-lac stem untouched, as an insulation between the ball _h_ and the surrounding sphere _a, a_. the ball _h_ has a small aperture at _n_, so that when the apparatus is exhausted of one gas and filled with another, the ball _h_ may itself also be exhausted and filled, that no variation of the gas in the interval _o_ may occur during the course of an experiment. . it will be unnecessary to give the dimensions of all the parts, since the drawing is to a scale of one-half: the inner ball has a diameter . inches, and the surrounding sphere an internal diameter of . inches. hence the width of the intervening space, through which the induction is to take place, is . of an inch; and the extent of this place or plate, i.e. the surface of a medium sphere, may be taken as twenty-seven square inches, a quantity considered as sufficiently large for the comparison of different substances. great care was taken in finishing well the inducing surfaces of the ball _h_ and sphere _a, a_; and no varnish or lacquer was applied to them, or to any part of the metal of the apparatus. . the attachment and adjustment of the shell-lac stem was a matter requiring considerable care, especially as, in consequence of its cracking, it had frequently to be renewed. the best lac was chosen and applied to the wire _i_, so as to be in good contact with it everywhere, and in perfect continuity throughout its own mass. it was not smaller than is given by scale in the drawing, for when less it frequently cracked within a few hours after it was cold. i think that very slow cooling or annealing improved its quality in this respect. the collar _g_ was made as thin as could be, that the lac might be as wide there as possible. in order that at every re-attachment of the stem to the upper hemisphere the ball _h_ might have the same relative position, a gauge _p_ (fig. .) was made of wood, and this being applied to the ball and hemisphere whilst the cement at _m_ was still soft, the bearings of the ball at _qq_, and the hemisphere at _rr_, were forced home, and the whole left until cold. thus all difficulty in the adjustment of the ball in the sphere was avoided. . i had occasion at first to attach the stem to the socket by other means, as a band of paper or a plugging of white silk thread; but these were very inferior to the cement, interfering much with the insulating power of the apparatus. . the retentive power of this apparatus was, when in good condition, better than that of the electrometer ( .), i.e. the proportion of loss of power was less. thus when the apparatus was electrified, and also the balls in the electrometer, to such a degree, that after the inner ball had been in contact with the top _k_ of the ball of the apparatus, it caused a repulsion indicated by ° of torsion force, then in falling from ° to ° the average loss was °. per minute; from ° to ° the average loss was °. per minute; from ° to ° it was °. per minute; from ° to ° it was ° per minute. this was after the apparatus had been charged for a short time; at the first instant of charging there is an apparent loss of electricity, which can only be comprehended hereafter ( . .). . when the apparatus loses its insulating power suddenly, it is almost always from a crack near to or within the brass socket. these cracks are usually transverse to the stem. if they occur at the part attached by common cement to the socket, the air cannot enter, and thus constituting vacua, they conduct away the electricity and lower the charge, as fast almost as if a piece of metal had been introduced there. occasionally stems in this state, being taken out and cleared from the common cement, may, by the careful application of the heat of a spirit-lamp, be so far softened and melted as to restore the perfect continuity of the parts; but if that does not succeed in replacing things in a good condition, the remedy is a new shell-lac stem. . the apparatus when in order could easily be exhausted of air and filled with any given gas; but when that gas was acid or alkaline, it could not properly be removed by the air-pump, and yet required to be perfectly cleared away. in such cases the apparatus was opened and emptied of gas; and with respect to the inner ball _h_, it was washed out two or three times with distilled water introduced at the screw-hole, and then being heated above °, air was blown through to render the interior perfectly dry. . the inductive apparatus described is evidently a leyden phial, with the advantage, however, of having the _dielectric_ or insulating medium changed at pleasure. the balls _h_ and b, with the connecting wire _i_, constitute the charged conductor, upon the surface of which all the electric force is resident by virtue of induction ( .). now though the largest portion of this induction is between the ball _h_ and the surrounding sphere _aa_, yet the wire _i_ and the ball b determine a part of the induction from their surfaces towards the external surrounding conductors. still, as all things in that respect remain the same, whilst the medium within at _oo_, may be varied, any changes exhibited by the whole apparatus will in such cases depend upon the variations made in the interior; and these were the changes i was in search of, the negation or establishment of such differences being the great object of my inquiry. i considered that these differences, if they existed, would be most distinctly set forth by having two apparatus of the kind described, precisely similar in every respect; and then, _different insulating media_ being within, to charge one and measure it, and after dividing the charge with the other, to observe what the ultimate conditions of both were. if insulating media really had any specific differences in favouring or opposing inductive action through them, such differences, i conceived, could not fail of being developed by such a process. . i will wind up this description of the apparatus, and explain the precautions necessary to their use, by describing the form and order of the experiments made to prove their equality when both contained common air. in order to facilitate reference i will distinguish the two by the terms app. i. and app. ii. . the electrometer is first to be adjusted and examined ( .), and the app. i. and ii. are to be perfectly discharged. a leyden phial is to be charged to such a degree that it would give a spark of about one-sixteenth or one-twentieth of an inch in length between two balls of half an inch diameter; and the carrier ball of the electrometer being charged by this phial, is to be introduced into the electrometer, and the lever ball brought by the motion of the torsion index against it; the charge is thus divided between the balls, and repulsion ensues. it is useful then to bring the repelled ball to the standard distance of ° by the motion of the torsion index, and observe the force in degrees required for this purpose; this force will in future experiments be called _repulsion of the balls_. . one of the inductive apparatus, as, for instance, app. i., is now to be charged from the leyden phial, the latter being in the state it was in when used to charge the balls; the carrier ball is to be brought into contact with the top of its upper ball (_k_, fig. .), then introduced into the electrometer, and the repulsive force (at the distance of °) measured. again, the carrier should be applied to the app. i. and the measurement repeated; the apparatus i. and ii. are then to be joined, so as to _divide_ the charge, and afterwards the force of each measured by the carrier ball, applied as before, and the results carefully noted. after this both i. and ii. are to be discharged; then app. ii. charged, measured, divided with app. i., and the force of each again measured and noted. if in each case the half charges of app. i. and ii. are equal, and are together equal to the whole charge before division, then it may be considered as proved that the two apparatus are precisely equal in power, and fit to be used in cases of comparison between different insulating media or _dielectrics_. . but the _precautions_ necessary to obtain accurate results are numerous. the apparatus i. and ii. must always be placed on a thoroughly uninsulating medium. a mahogany table, for instance, is far from satisfactory in this respect, and therefore a sheet of tinfoil, connected with an extensive discharging train ( .), is what i have used. they must be so placed also as not to be too near each other, and yet equally exposed to the inductive influence of surrounding objects; and these objects, again, should not be disturbed in their position during an experiment, or else variations of induction upon the external ball b of the apparatus may occur, and so errors be introduced into the results. the carrier ball, when receiving its portion of electricity from the apparatus, should always be applied at the same part of the ball, as, for instance, the summit _k_, and always in the same way; variable induction from the vicinity of the head, hands, &c. being avoided, and the ball after contact being withdrawn upwards in a regular and constant manner. . as the stem had occasionally to be changed ( .), and the change might occasion slight variations in the position of the ball within, i made such a variation purposely, to the amount of an eighth of an inch (which is far more than ever could occur in practice), but did not find that it sensibly altered the relation of the apparatus, or its inductive condition _as a whole_. another trial of the apparatus was made as to the effect of dampness in the air, one being filled with very dry air, and the other with air from over water. though this produced no change in the result, except an occasional tendency to more rapid dissipation, yet the precaution was always taken when working with gases ( .) to dry them perfectly. . it is essential that the interior of the apparatus should be perfectly free from _dust or small loose particles_, for these very rapidly lower the charge and interfere on occasions when their presence and action would hardly be expected. to breathe on the interior of the apparatus and wipe it out quietly with a clean silk handkerchief, is an effectual way of removing them; but then the intrusion of other particles should be carefully guarded against, and a dusty atmosphere should for this and several other reasons be avoided. . the shell-lac stem requires occasionally to be well-wiped, to remove, in the first instance, the film of wax and adhering matter which is upon it; and afterwards to displace dirt and dust which will gradually attach to it in the course of experiments. i have found much to depend upon this precaution, and a silk handkerchief is the best wiper. . but wiping and some other circumstances tend to give a charge to the surface of the shell-lac stem. this should be removed, for, if allowed to remain, it very seriously affects the degree of charge given to the carrier ball by the apparatus ( .). this condition of the stem is best observed by discharging the apparatus, applying the carrier ball to the stem, touching it with the finger, insulating and removing it, and examining whether it has received any charge (by induction) from the stem; if it has, the stem itself is in a charged state. the best method of removing the charge i have found to be, to cover the finger with a single fold of a silk handkerchief, and breathing on the stem, to wipe it immediately after with the finger; the ball b and its connected wire, &c. being at the same time _uninsulated_: the wiping place of the silk must not be changed; it then becomes sufficiently damp not to excite the stem, and is yet dry enough to leave it in a clean and excellent insulating condition. if the air be dusty, it will be found that a single charge of the apparatus will bring on an electric state of the outside of the stem, in consequence of the carrying power of the particles of dust; whereas in the morning, and in a room which has been left quiet, several experiments can be made in succession without the stem assuming the least degree of charge. . experiments should not be made by candle or lamp light except with much care, for flames have great and yet unsteady powers of affecting and dissipating electrical charges. . as a final observation on the state of the apparatus, they should retain their charges well and uniformly, and alike for both, and at the same time allow of a perfect and instantaneous discharge, giving afterwards no charge to the carrier ball, whatever part of the ball b it may be applied to ( .). . with respect to the balance electrometer, all the precautions that need be mentioned, are, that the carrier ball is to be preserved during the first part of an experiment in its electrified state, the loss of electricity which would follow upon its discharge being avoided; and that in introducing it into the electrometer through the hole in the glass plate above, care should be taken that it do not touch, or even come near to, the edge of the glass. . when the whole charge in one apparatus is divided between the two, the gradual fall, apparently from dissipation, in the apparatus which has _received_ the half charge is greater than in the one _originally_ charged. this is due to a peculiar effect to be described hereafter ( . .), the interfering influence of which may be avoided to a great extent by going through the steps of the process regularly and quickly; therefore, after the original charge has been measured, in app. i. for instance, i. and ii. are to be symmetrically joined by their balls b, the carrier touching one of these balls at the same time; it is first to be removed, and then the apparatus separated from each other; app. ii. is next quickly to be measured by the carrier, then app. i.; lastly, ii. is to be discharged, and the discharged carrier applied to it to ascertain whether any residual effect is present ( .), and app. i. being discharged is also to be examined in the same manner and for the same purpose. . the following is an example of the division of a charge by the two apparatus, air being the dielectric in both of them. the observations are set down one under the other in the order in which they were taken, the left-hand numbers representing the observations made on app. i., and the right-hand numbers those on app. ii. app. i. is that which was originally charged, and after two measurements, the charge was divided with app. ii. app. i. app. ii. balls ° . . . . ° ° . . . . . . . . divided and instantly taken . . . . . . . . . . . . after being discharged. . . . . after being discharged. . without endeavouring to allow for the loss which must have been gradually going on during the time of the experiment, let us observe the results of the numbers as they stand. as ° remained in app. i. in an undischargeable state, ° may be taken as the utmost amount of the transferable or divisible charge, the half of which is °. . as app. ii. was free of charge in the first instance, and immediately after the division was found with °, this amount _at least_ may be taken as what it had received. on the other hand ° minus °, or °, may be taken as the half of the transferable charge retained by app. i. now these do not differ much from each other, or from °. , the half of the full amount of transferable charge; and when the gradual loss of charge evident in the difference between ° and ° of app. i. is also taken into account, there is every reason to admit the result as showing an equal division of charge, _unattended by any disappearance of power_ except that due to dissipation. . i will give another result, in which app. ii. was first charged, and where the residual action of that apparatus was greater than in the former case. app. i. app. ii. balls ° . . . . ° . . . . divided and instantly taken ° . . . . . . . . . . . . immediately after discharge. . . . . immediately after discharge. . the transferable charge being ° - °, its half is °. , which is not far removed from °, the half charge of i.; or from °, the half charge of ii.: these half charges again making up the sum of °, or just the amount of the whole transferable charge. considering the errors of experiment, therefore, these results may again be received as showing that the apparatus were equal in inductive capacity, or in their powers of receiving charges. . the experiments were repeated with charges of negative electricity with the same general results. . that i might be sure of the sensibility and action of the apparatus, i made such a change in one as ought upon principle to increase its inductive force, i.e. i put a metallic lining into the lower hemisphere of app. i., so as to diminish the thickness of the intervening air in that part, from . to . of an inch: this lining was carefully shaped and rounded so that it should not present a sudden projection within at its edge, but a gradual transition from the reduced interval in the lower part of the sphere to the larger one in the upper. . this change immediately caused app. i. to produce effects indicating that it had a greater aptness or capacity for induction than app. ii. thus, when a transferable charge in app. ii. of ° was divided with app. i., the former retained a charge of °, whilst the latter showed one of °, i.e. the former had lost ° in communicating ° to the latter: on the other hand, when app. i. had a transferable charge in it of ° divided by contact with app. ii., it lost ° only, whilst it gave to app. ii. as many as :--the sum of the divided forces being in the first instance _less_, and in the second instance _greater_ than the original undivided charge. these results are the more striking, as only one-half of the interior of app. i. was modified, and they show that the instruments are capable of bringing out differences in inductive force from amongst the errors of experiment, when these differences are much less than that produced by the alteration made in the present instance. ¶ iv. _induction in curved lines._ . amongst those results deduced from the molecular view of induction ( .), which, being of a peculiar nature, are the best tests of the truth or error of the theory, the expected action in curved lines is, i think, the most important at present; for, if shown to take place in an unexceptionable manner, i do not see how the old theory of action at a distance and in straight lines can stand, or how the conclusion that ordinary induction is an action of contiguous particles can be resisted. . there are many forms of old experiments which might be quoted as favourable to, and consistent with the view i have adopted. such are most cases of electro-chemical decomposition, electrical brushes, auras, sparks, &c.; but as these might be considered equivocal evidence, inasmuch as they include a current and discharge, (though they have long been to me indications of prior molecular action ( .)) i endeavoured to devise such experiments for first proofs as should not include transfer, but relate altogether to the pure simple inductive action of statical electricity. . it was also of importance to make these experiments in the simplest possible manner, using not more than one insulating medium or dielectric at a time, lest differences of slow conduction should produce effects which might erroneously be supposed to result from induction in curved lines. it will be unnecessary to describe the steps of the investigation minutely; i will at once proceed to the simplest mode of proving the facts, first in air and then in other insulating media. . a cylinder of solid shell-lac, . of an inch in diameter and seven inches in length, was fixed upright in a wooden foot (fig. .): it was made concave or cupped at its upper extremity so that a brass ball or other small arrangement could stand upon it. the upper half of the stem having been excited _negatively_ by friction with warm flannel, a brass ball, b, inch in diameter, was placed on the top, and then the whole arrangement examined by the carrier ball and coulomb's electrometer ( . &c.). for this purpose the balls of the electrometer were charged _positively_ to about °, and then the carrier being applied to various parts of the ball b, the two were uninsulated whilst in contact or in position, then insulated[a], separated, and the charge of the carrier examined as to its nature and force. its electricity was always positive, and its force at the different positions _a, b, c, d,_ &c. (figs. . and .) observed in succession, was as follows: at _a_ above ° _b_ it was _c_ _d_ _b_ [a] it can hardly be necessary for me to say here, that whatever general state the carrier ball acquired in any place where it was uninsulated and then insulated, it retained on removal from that place, notwithstanding that it might pass through other places that would have given to it, if uninsulated, a different condition. . to comprehend the full force of these results, it must first be understood, that all the charges of the ball b and the carrier are charges by induction, from the action of the excited surface of the shell-lac cylinder; for whatever electricity the ball b received by _communication_ from the shell-lac, either in the first instance or afterwards, was removed by the uninsulating contacts, only that due to induction remaining; and this is shown by the charges taken from the ball in this its uninsulated state being always positive, or of the contrary character to the electricity of the shell-lac. in the next place, the charges at _a_, _c_, and _d_ were of such a nature as might be expected from an inductive action in straight lines, but that obtained at _b_ is _not so_: it is clearly a charge by induction, but _induction_ in _a curved line_; for the carrier ball whilst applied to _b_, and after its removal to a distance of six inches or more from b, could not, in consequence of the size of b, be connected by a straight line with any part of the excited and inducing shell-lac. . to suppose that the upper part of the _uninsulated_ ball b, should in some way be retained in an electrified state by that portion of the surface of the ball which is in sight of the shell-lac, would be in opposition to what we know already of the subject. electricity is retained upon the surface of conductors only by induction ( .); and though some persons may not be prepared as yet to admit this with respect to insulated conductors, all will as regards uninsulated conductors like the ball b; and to decide the matter we have only to place the carrier ball at _e_ (fig. .), so that it shall not come in contact with b, uninsulate it by a metallic rod descending perpendicularly, insulate it, remove it, and examine its state; it will be found charged with the same kind of electricity as, and even to a _higher degree_ ( .) than, if it had been in contact with the summit of b. . to suppose, again, that induction acts in some way _through or across_ the metal of the ball, is negatived by the simplest considerations; but a fact in proof will be better. if instead of the ball b a small disc of metal be used, the carrier may be charged at, or above the middle of its upper surface: but if the plate be enlarged to about - / or inches in diameter, c (fig. .), then no charge will be given to the carrier at _f_, though when applied nearer to the edge at _g_, or even _above the middle_ at _h_, a charge will be obtained; and this is true though the plate may be a mere thin film of gold-leaf. hence it is clear that the induction is not _through_ the metal, but through the surrounding air or _dielectric_, and that in curved lines. . i had another arrangement, in which a wire passing downwards through the middle of the shell-lac cylinder to the earth, was connected with the ball b (fig. .) so as to keep it in a constantly uninsulated state. this was a very convenient form of apparatus, and the results with it were the same as those just described. . in another case the ball b was supported by a shell-lac stem, independently of the excited cylinder of shell-lac, and at half an inch distance from it; but the effects were the same. then the brass ball of a charged leyden jar was used in place of the excited shell-lac to produce induction; but this caused no alteration of the phenomena. both positive and negative inducing charges were tried with the same general results. finally, the arrangement was inverted in the air for the purpose of removing every possible objection to the conclusions, but they came out exactly the same. . some results obtained with a brass hemisphere instead of the ball b were exceedingly interesting, it was . of an inch in diameter, (fig. .), and being placed on the top of the excited shell-lac cylinder, the carrier ball was applied, as in the former experiments ( .), at the respective positions delineated in the figure. at _i_ the force was °, at _k_ °, at _l_ °, at _m_ °; the inductive force gradually diminishing, as might have been expected, to this point. but on raising the carrier to the position _n_, the charge increased to °; and on raising it still higher to _o_, the charge still further increased to °: at a higher point still, _p_, the charge taken was smaller in amount, being °, and continued to diminish for more elevated positions. here the induction fairly turned a corner. nothing, in fact, can better show both the curved lines or courses of the inductive action, disturbed as they are from their rectilineal form by the shape, position, and condition of the metallic hemisphere; and also a _lateral tension,_ so to speak, of these lines on one another:--all depending, as i conceive, on induction being an action of the contiguous particles of the dielectric, which being thrown into a state of polarity and tension, are in mutual relation by their forces in all directions. . as another proof that the whole of these actions were inductive i may state a result which was exactly what might be expected, namely, that if uninsulated conducting matter was brought round and near to the excited shell-lac stem, then the inductive force was directed towards it, and could not be found on the top of the hemisphere. removing this matter the lines of force resumed their former direction. the experiment affords proofs of the lateral tension of these lines, and supplies a warning to remove such matter in repeating the above investigation. . after these results on curved inductive action in air i extended the experiments to other gases, using first carbonic acid and then hydrogen: the phenomena were precisely those already described. in these experiments i found that if the gases were confined in vessels they required to be very large, for whether of glass or earthenware, the conducting power of such materials is so great that the induction of the excited shell-lac cylinder towards them is as much as if they were metal; and if the vessels be small, so great a portion of the inductive force is determined towards them that the lateral tension or mutual repulsion of the lines of force before spoken of, ( .) by which their inflexion is caused, is so much relieved in other directions, that no inductive charge will be given to the carrier ball in the positions _k, l, m, n, o, p_ (fig. .). a very good mode of making the experiment is to let large currents of the gases ascend or descend through the air, and carry on the experiments in these currents. . these experiments were then varied by the substitution of a liquid dielectric, namely, _oil of turpentine_, in place of air and gases. a dish of thin glass well-covered with a film of shell-lac ( .), which was found by trial to insulate well, had some highly rectified oil of turpentine put into it to the depth of half an inch, and being then placed upon the top of the brass hemisphere (fig. .), observations were made with the carrier ball as before ( .). the results were the same, and the circumstance of some of the positions being within the fluid and some without, made no sensible difference. . lastly, i used a few solid dielectrics for the same purpose, and with the same results. these were shell-lac, sulphur, fused and cast borate of lead, flint glass well-covered with a film of lac, and spermaceti. the following was the form of experiment with sulphur, and all were of the same kind. a square plate of the substance, two inches in extent and . of an inch in thickness, was cast with a small hole or depression in the middle of one surface to receive the carrier ball. this was placed upon the surface of the metal hemisphere (fig. .) arranged on the excited lac as in former cases, and observations were made at _n, o, p_, and _q_. great care was required in these experiments to free the sulphur or other solid substance from any charge it might previously have received. this was done by breathing and wiping ( .), and the substance being found free from all electrical excitement, was then used in the experiment; after which it was removed and again examined, to ascertain that it had received no charge, but had acted really as a dielectric. with all these precautions the results were the same: and it is thus very satisfactory to obtain the curved inductive action through _solid bodies_, as any possible effect from the translation of charged particles in fluids or gases, which some persons might imagine to be the case, is here entirely negatived. . in these experiments with solid dielectrics, the degree of charge assumed by the carrier ball at the situations _n, o, p_ (fig. .), was decidedly greater than that given to the ball at the same places when air only intervened between it and the metal hemisphere. this effect is consistent with what will hereafter be found to be the respective relations of these bodies, as to their power of facilitating induction through them ( . . .). . i might quote _many_ other forms of experiment, some old and some new, in which induction in curved or contorted lines takes place, but think it unnecessary after the preceding results; i shall therefore mention but two. if a conductor a, (fig. .) be electrified, and an uninsulated metallic ball b, or even a plate, provided the edges be not too thin, be held before it, a small electrometer at _c_ or at _d_, uninsulated, will give signs of electricity, opposite in its nature to that of a, and therefore caused by induction, although the influencing and influenced bodies cannot be joined by a right line passing through the air. or if, the electrometers being removed, a point be fixed at the back of the ball in its uninsulated state as at c, this point will become luminous and discharge the conductor a. the latter experiment is described by nicholson[a], who, however, reasons erroneously upon it. as to its introduction here, though it is a case of discharge, the discharge is preceded by induction, and that induction must be in curved lines. [a] encyclopædia britannica, vol. vi. p. . . as argument against the received theory of induction and in favour of that which i have ventured to put forth, i cannot see how the preceding results can be avoided. the effects are clearly inductive effects produced by electricity, not in currents but in its statical state, and this induction is exerted in lines of force which, though in many experiments they may be straight, are here curved more or less according to circumstances. i use the term _line of inductive force_ merely as a temporary conventional mode of expressing the direction of the power in cases of induction; and in the experiments with the hemisphere ( .), it is curious to see how, when certain lines have terminated on the under surface and edge of the metal, those which were before lateral to them _expand and open out from each other_, some bending round and terminating their action on the upper surface of the hemisphere, and others meeting, as it were, above in their progress outwards, uniting their forces to give an increased charge to the carrier ball, at an _increased distance_ from the source of power, and influencing each other so as to cause a second flexure in the contrary direction from the first one. all this appears to me to prove that the whole action is one of contiguous particles, related to each other, not merely in the lines which they may be conceived to form through the dielectric, between the _inductric_ and the _inducteous_ surfaces ( .), but in other lateral directions also. it is this which gives an effect equivalent to a lateral repulsion or expansion in the lines of force i have spoken of, and enables induction to turn a corner ( .). the power, instead of being like that of gravity, which causes particles to act on each other through straight lines, whatever other particles may be between them, is more analogous to that of a series of magnetic needles, or to the condition of the particles considered as forming the whole of a straight or a curved magnet. so that in whatever way i view it, and with great suspicion of the influence of favourite notions over myself, i cannot perceive how the ordinary theory applied to explain induction can be a correct representation of that great natural principle of electrical action. . i have had occasion in describing the precautions necessary in the use of the inductive apparatus, to refer to one founded on induction in curved lines ( .); and after the experiments already described, it will easily be seen how great an influence the shell-lac stem may exert upon the charge of the carrier ball when applied to the apparatus ( .), unless that precaution be attended to. . i think it expedient, next in the course of these experimental researches, to describe some effects due to _conduction_, obtained with such bodies as glass, lac, sulphur, &c., which had not been anticipated. being understood, they will make us acquainted with certain precautions necessary in investigating the great question of specific inductive capacity. . one of the inductive apparatus already described ( , &c.) had a hemispherical cup of shell-lac introduced, which being in the interval between the inner bull and the lower hemisphere, nearly occupied the space there; consequently when the apparatus was charged, the lac was the dielectric or insulating medium through which the induction took place in that part. when this apparatus was first charged with electricity ( .) up to a certain intensity, as °, measured by the coulomb's electrometer ( .), it sank much faster from that degree than if it had been previously charged to a higher point, and had gradually fallen to °; or than it would do if the charge were, by a second application, raised up again to °; all other things remaining the same. again, if after having been charged for some time, as fifteen or twenty minutes, it was suddenly and perfectly discharged, even the stem having all electricity removed from it ( .), then the apparatus being left to itself, would gradually recover a charge, which in nine or ten minutes would rise up to ° or °, and in one instance to °. . the electricity, which in these cases returned from an apparently latent to a sensible state, was always of the same kind as that which had been given by the charge. the return took place at both the inducing surfaces; for if after the perfect discharge of the apparatus the whole was insulated, as the inner ball resumed a positive state the outer sphere acquired a negative condition. . this effect was at once distinguished from that produced by the excited stem acting in curved lines of induction ( . .), by the circumstance that all the returned electricity could be perfectly and instantly discharged. it appeared to depend upon the shell-lac within, and to be, in some way, due to electricity evolved from it in consequence of a previous condition into which it had been brought by the charge of the metallic coatings or balls. . to examine this state more accurately, the apparatus, with the hemispherical cup of shell-lac in it, was charged for about forty-five minutes to above ° with positive electricity at the balls _h_ and b. (fig. .) above and within. it was then discharged, opened, the shell-lac taken out, and its state examined; this was done by bringing the carrier ball near the shell-lac, uninsulating it, insulating it, and then observing what charge it had acquired. as it would be a charge by induction, the state of the ball would indicate the opposite state of electricity in that surface of the shell-lac which had produced it. at first the lac appeared quite free from any charge; but gradually its two surfaces assumed opposite states of electricity, the concave surface, which had been next the inner and positive ball; assuming a positive state, and the convex surface, which had been in contact with the negative coating, acquiring a negative state; these states gradually increased in intensity for some time. . as the return action was evidently greatest instantly after the discharge, i again put the apparatus together, and charged it for fifteen minutes as before, the inner ball positively. i then discharged it, instantly removing the upper hemisphere with the interior ball, and, leaving the shell-lac cup in the lower uninsulated hemisphere, examined its inner surface by the carrier ball as before ( .). in this way i found the surface of the shell-lac actually _negative_, or in the reverse state to the ball which had been in it; this state quickly disappeared, and was succeeded by a positive condition, gradually increasing in intensity for some time, in the same manner as before. the first negative condition of the surface opposite the positive charging ball is a natural consequence of the state of things, the charging ball being in contact with the shell-lac only in a few points. it does not interfere with the general result and peculiar state now under consideration, except that it assists in illustrating in a very marked manner the ultimate assumption by the surfaces of the shell-lac of an electrified condition, similar to that of the metallic surfaces opposed to or against them. . _glass_ was then examined with respect to its power of assuming this peculiar state. i had a thick flint-glass hemispherical cup formed, which would fit easily into the space _o_ of the lower hemisphere ( . .); it had been heated and varnished with a solution of shell-lac in alcohol, for the purpose of destroying the conducting power of the vitreous surface ( .). being then well-warmed and experimented with, i found it could also assume the _same state_, but not apparently to the same degree, the return action amounting in different cases to quantities from ° to °. . _spermaceti_ experimented with in the same manner gave striking results. when the original charge had been sustained for fifteen or twenty minutes at about °, the return charge was equal to ° or °, and was about fourteen minutes arriving at the maximum effect. a charge continued for not more than two or three seconds was here succeeded by a return charge of ° or °. the observations formerly made ( .) held good with this substance. spermaceti, though it will insulate a low charge for some time, is a better conductor than shell-lac, glass, and sulphur; and this conducting power is connected with the readiness with which it exhibits the particular effect under consideration. . _sulphur._--i was anxious to obtain the amount of effect with this substance, first, because it is an excellent insulator, and in that respect would illustrate the relation of the effect to the degree of conducting power possessed by the dielectric ( .); and in the next place, that i might obtain that body giving the smallest degree of the effect now under consideration for the investigation of the question of specific inductive capacity ( .). . with a good hemispherical cup of sulphur cast solid and sound, i obtained the return charge, but only to an amount of ° or °. thus glass and sulphur, which are bodily very bad conductors of electricity, and indeed almost perfect insulators, gave very little of this return charge. . i tried the same experiment having _air_ only in the inductive apparatus. after a continued high charge for some time i could obtain a little effect of return action, but it was ultimately traced to the shell-lac of the stem. . i sought to produce something like this state with one electric power and without induction; for upon the theory of an electric fluid or fluids, that did not seem impossible, and then i should have obtained an absolute charge ( . .), or something equivalent to it. in this i could not succeed. i excited the outside of a cylinder of shell-lac very highly for some time, and then quickly discharging it ( .), waited and watched whether any return charge would appear, but such was not the case. this is another fact in favour of the inseparability of the two electric forces ( .), and another argument for the view that induction and its concomitant phenomena depend upon a polarity of the particles of matter. . although inclined at first to refer these effects to a peculiar masked condition of a certain portion of the forces, i think i have since correctly traced them to known principles of electrical action. the effects appear to be due to an actual penetration of the charge to some distance within the electric, at each of its two surfaces, by what we call _conduction_; so that, to use the ordinary phrase, the electric forces sustaining the induction are not upon the metallic surfaces only, but upon and within the dielectric also, extending to a greater or smaller depth from the metal linings. let _c_ (fig. .) be the section of a plate of any dielectric, _a_ and _b_ being the metallic coatings; let _b_ be uninsulated, and _a_ be charged positively; after ten or fifteen minutes, if _a_ and _b_ be discharged, insulated, and immediately examined, no electricity will appear in them; but in a short time, upon a second examination, they will appear charged in the same way, though not to the same degree, as they were at first. now suppose that a portion of the positive force has, under the coercing influence of all the forces concerned, penetrated the dielectric and taken up its place at the line _p_, a corresponding portion of the negative force having also assumed its position at the line _n_; that in fact the electric at these two parts has become charged positive and negative; then it is clear that the induction of these two forces will be much greater one towards the other, and less in an external direction, now that they are at the small distance _np_ from each other, than when they were at the larger interval _ab_. then let _a_ and _b_ be discharged; the discharge destroys or neutralizes all external induction, and the coatings are therefore found by the carrier ball unelectrified; but it also removes almost the whole of the forces by which the electric charge was driven into the dielectric, and though probably a part of that charge goes forward in its passage and terminates in what we call discharge, the greater portion returns on its course to the surfaces of _c_, and consequently to the conductors _a_ and _b_, and constitutes the recharge observed. . the following is the experiment on which i rest for the truth of this view. two plates of spermaceti, _d_ and, _f_ (fig. .), were put together to form the dielectric, _a_ and _b_ being the metallic coatings of this compound plate, as before. the system was charged, then discharged, insulated, examined, and found to give no indications of electricity to the carrier ball. the plates _d_ and _f_were then separated from each other, and instantly _a_ with _d_ was found in a positive state, and _b_ with _f_ in a negative state, nearly all the electricity being in the linings _a_ and _b_. hence it is clear that, of the forces sought for, the positive was in one-half of the compound plate and the negative in the other half; for when removed bodily with the plates from each other's inductive influence, they appeared in separate places, and resumed of necessity their power of acting by induction on the electricity of surrounding bodies. had the effect depended upon a peculiar relation of the contiguous particles of matter only, then each half-plate, _d_ and _f_, should have shown positive force on one surface and negative on the other. . thus it would appear that the best solid insulators, such as shell-lac, glass, and sulphur, have conductive properties to such an extent, that electricity can penetrate them bodily, though always subject to the overruling condition of induction ( .). as to the depth to which the forces penetrate in this form of charge of the particles, theoretically, it should be throughout the mass, for what the charge of the metal does for the portion of dielectric next to it, should be close by the charged dielectric for the portion next beyond it again; but probably in the best insulators the sensible charge is to a very small depth only in the dielectric, for otherwise more would disappear in the first instance whilst the original charge is sustained, less time would be required for the assumption of the particular state, and more electricity would re-appear as return charge. . the condition of _time_ required for this penetration of the charge is important, both as respects the general relation of the cases to conduction, and also the removal of an objection that might otherwise properly be raised to certain results respecting specific inductive capacities, hereafter to be given ( . .) . it is the assumption for a time of this charged state of the glass between the coatings in the leyden jar, which gives origin to a well-known phenomenon, usually referred to the diffusion of electricity over the uncoated portion of the glass, namely, the _residual charge_. the extent of charge which can spontaneously be recovered by a large battery, after perfect uninsulation of both surfaces, is very considerable, and by far the largest portion of this is due to the return of electricity in the manner described. a plate of shell-lac six inches square, and half an inch thick, or a similar plate of spermaceti an inch thick, being coated on the sides with tinfoil as a leyden arrangement, will show this effect exceedingly well. * * * * * . the peculiar condition of dielectrics which has now been described, is evidently capable of producing an effect interfering with the results and conclusions drawn from the use of the two inductive apparatus, when shell-lac, glass, &c. is used in one or both of them ( . .), for upon dividing the charge in such cases according to the method described ( . .), it is evident that the apparatus just receiving its half charge must fall faster in its tension than the other. for suppose app. i. first charged, and app. ii. used to divide with it; though both may actually lose alike, yet app. i., which has been diminished one-half, will be sustained by a certain degree of return action or charge ( .), whilst app. ii. will sink the more rapidly from the coming on of the particular state. i have endeavoured to avoid this interference by performing the whole process of comparison as quickly as possible, and taking the force of app. ii. immediately after the division, before any sensible diminution of the tension arising from the assumption of the peculiar state could be produced; and i have assumed that as about three minutes pass between the first charge of app. i. and the division, and three minutes between the division and discharge, when the force of the non-transferable electricity is measured, the contrary tendencies for those periods would keep that apparatus in a moderately steady and uniform condition for the latter portion of time. . the particular action described occurs in the shell-lac of the stems, as well as in the _dielectric_ used within the apparatus. it therefore constitutes a cause by which the outside of the stems may in some operations become charged with electricity, independent of the action of dust or carrying particles ( .). ¶ v. _on specific induction, or specific inductive capacity._ . i now proceed to examine the great question of specific inductive capacity, i.e. whether different dielectric bodies actually do possess any influence over the degree of induction which takes place through them. if any such difference should exist, it appeared to me not only of high importance in the further comprehension of the laws and results of induction, but an additional and very powerful argument for the theory i have ventured to put forth, that the whole depends upon a molecular action, in contradistinction to one at sensible distances. the question may be stated thus: suppose a an electrified plate of metal suspended in the air, and b and c two exactly similar plates, placed parallel to and on each side of a at equal distances and uninsulated; a will then induce equally towards b and c. if in this position of the plates some other dielectric than air, as shell-lac, be introduced between a and c, will the induction between them remain the same? will the relation of c and b to a be unaltered, notwithstanding the difference of the dielectrics interposed between them?[a] [a] refer for the practical illustration of this statement to the supplementary note commencing , &c.--_dec. ._ . as far as i recollect, it is assumed that no change will occur under such variation of circumstances, and that the relations of b find c to a depend entirely upon their distance. i only remember one experimental illustration of the question, and that is by coulomb[a], in which he shows that a wire surrounded by shell-lac took exactly the same quantity of electricity from a charged body as the same wire in air. the experiment offered to me no proof of the truth of the supposition: for it is not the mere films of dielectric substances surrounding the charged body which have to be examined and compared, but the _whole mass_ between that body and the surrounding conductors at which the induction terminates. charge depends upon induction ( . .); and if induction is related to the particles of the surrounding dielectric, then it is related to _all_ the particles of that dielectric inclosed by the surrounding conductors, and not merely to the few situated next to the charged body. whether the difference i sought for existed or not, i soon found reason to doubt the conclusion that might be drawn from coulomb's result; and therefore had the apparatus made, which, with its use, has been already described ( , &c.), and which appears to me well-suited for the investigation of the question. [a] mémoires de l'académie, , pp. , . . glass, and many bodies which might at first be considered as very fit to test the principle, proved exceedingly unfit for that purpose. glass, principally in consequence of the alkali it contains, however well-warmed and dried it may be, has a certain degree of conducting power upon its surface, dependent upon the moisture of the atmosphere, which renders it unfit for a test experiment. resin, wax, naphtha, oil of turpentine, and many other substances were in turn rejected, because of a slight degree of conducting power possessed by them; and ultimately shell-lac and sulphur were chosen, after many experiments, as the dielectrics best fitted for the investigation. no difficulty can arise in perceiving how the possession of a feeble degree of conducting power tends to make a body produce effects, which would seem to indicate that it had a greater capability of allowing induction through it than another body perfect in its insulation. this source of error has been that which i have found most difficult to obviate in the proving experiments. * * * * * . _induction through shell-lac._--as a preparatory experiment, i first ascertained generally that when a part of the surface of a thick plate of shell-lac was excited or charged, there was no sensible difference in the character of the induction sustained by that charged part, whether exerted through the air in the one direction, or through the shell-lac of the plate in the other; provided the second surface of the plate had not, by contact with conductors, the action of dust, or any other means, become charged ( .). its solid condition enabled it to retain the excited particles in a permanent position, but that appeared to be all; for these particles acted just as freely through the shell-lac on one side as through the air on the other. the same general experiment was made by attaching a disc of tinfoil to one side of the shell-lac plate, and electrifying it, and the results were the same. scarcely any other solid substance than shell-lac and sulphur, and no liquid substance that i have tried, will bear this examination. glass in its ordinary state utterly fails; yet it was essentially necessary to obtain this prior degree of perfection in the dielectric used, before any further progress could be made in the principal investigation. . _shell-lac and air_ were compared in the first place. for this purpose a thick hemispherical cup of shell-lac was introduced into the lower hemisphere of one of the inductive apparatus ( , &c.), so as nearly to fill the lower half of the space _o, o_ (fig. .) between it and the inner ball; and then charges were divided in the manner already described ( . .), each apparatus being used in turn to receive the first charge before its division by the other. as the apparatus were known to have equal inductive power when air was in both ( . .), any differences resulting from the introduction of the shell-lac would show a peculiar action in it, and if unequivocally referable to a specific inductive influence, would establish the point sought to be sustained. i have already referred to the precautions necessary in making the experiments ( , &c.); and with respect to the error which might be introduced by the assumption of the peculiar state, it was guarded against, as far as possible, in the first place, by operating quickly ( ); and, afterwards, by using that dielectric as glass or sulphur, which assumed the peculiar state most slowly, and in the least degree ( . .). . the shell-lac hemisphere was put into app. i., and app. ii. left filled with air. the results of an experiment in which the charge through air was divided and reduced by the shell-lac app. were as follows: app. i. lac. app. ii. air. balls °. ° . . . . . . . . ° . . . . charge divided. . . . . . . . . . . . . after being discharged. . . . . after being discharged. . here °, minus °, or °, may be taken as the divisible charge of app. ii. (the ° being fixed stem action ( . .)), of which ° is the half. the lac app. i. gave ° as the power or tension it had acquired after division; and the air app. ii. gave °, minus °, or °, as the force it possessed from what it retained of the divisible charge of °. these two numbers should evidently be alike, and they are very nearly so, indeed far within the errors of experiment and observation, but these numbers differ very much from °, or the force which the half charge would have had if app. i. had contained air instead of shell-lac; and it appears that whilst in the division the induction through the air has lost ° of force, that through the lac has only gained °. . if this difference be assumed as depending entirely on the greater facility possessed by shell-lac of allowing or causing inductive action through its substance than that possessed by air, then this capacity for electric induction would be inversely as the respective loss and gain indicated above; and assuming the capacity of the air apparatus as , that of the shell-lac apparatus would be / or . . . this extraordinary difference was so unexpected in its amount, as to excite the greatest suspicion of the general accuracy of the experiment, though the perfect discharge of app. i. after the division, showed that the ° had been taken and given up readily. it was evident that, if it really existed, it ought to produce corresponding effects in the reverse order; and that when induction through shell-lac was converted into induction through air, the force or tension of the whole ought to be _increased_. the app. i. was therefore charged in the first place, and its force divided with app. ii. the following were the results: app. i. lac. app. ii. air. . . . . ° ° . . . . . . . . charge divided. . . . . . . . . . . . . after being discharged. . . . . after being discharged. . here ° must be the utmost of the divisible charge. the app. i. and app. ii. present ° as their respective forces; both now much _above_ the half of the first force, or °, whereas in the former case they were below it. the lac app. i. has lost only °, yet it has given to the air app. ii. °, so that the lac still appears much to surpass the air, the capacity of the lac app. i. to the air app. ii. being as . to . . the difference of . and . as the expression of the capacity for the induction of shell-lac seems considerable, but is in reality very admissible under the circumstances, for both are in error in _contrary directions_. thus in the last experiment the charge fell from ° to ° by the joint effects of dissipation and absorption ( . .), during the time which elapsed in the electrometer operations, between the applications of the carrier ball required to give those two results. nearly an equal time must have elapsed between the application of the carrier which gave the ° result, and the division of the charge between the two apparatus; and as the fall in force progressively decreases in amount ( .), if in this case it be taken at ° only, it will reduce the whole transferable charge at the time of division to ° instead of °; this diminishes the loss of the shell-lac charge to ° instead of °; and then the expression of specific capacity for it is increased, and, instead of . , is . times that of air. . applying the same correction to the former experiment in which air was _first_ charged, the result is of the _contrary_ kind. no shell-lac hemisphere was then in the apparatus, and therefore the loss would be principally from dissipation, and not from absorption: hence it would be nearer to the degree of loss shown by the numbers ° and °, and being assumed as ° would reduce the divisible charge to °. in that case the air would have lost °, and communicated only ° to the shell-lac; and the relative specific capacity of the latter would appear to be . , which is very little indeed removed from . , the expression given by the second experiment when corrected in the same way. . the shell-lac was then removed from app. i. and put into app. ii. and the experiments of division again made. i give the results, because i think the importance of the point justifies and even requires them. app. i. air. app. ii. lac. balls °. . . . . °. ° . . . . . . . . charge divided. . . . . . . . . . . . . . after discharge. trace . . . . after discharge. here app. i. retained °, having lost ° in communicating ° to app. ii.; and the capacity of the air app. is to the lac app., therefore, as to . . if the divided charge be corrected for an assumed loss of only °, being the amount of previous loss in the same time, it will make the capacity of the shell-lac app. . only. . then app. ii. was charged, and the charge divided thus: app. i. air. app. ii. lac, ° . . . . . . . . ° . . . . charge divided. . . . . . . . . a little . . . . after discharge. . . . . a little after discharge. here app. i. acquired a charge of °, while app. ii. lost only ° in communicating that amount of force; the capacities being, therefore, to each other as to . . if the whole transferable charge be corrected for a loss of ° previous to division, it gives the expression of l. for the capacity of the shell-lac apparatus. . these four expressions of . , . , . , and . for the power of the shell-lac apparatus, through the different variations of the experiment, are very near to each other; the average is close upon . , which may hereafter be used as the expression of the result. it is a very important result; and, showing for this particular piece of shell-lac a decided superiority over air in allowing or causing the act of induction, it proved the growing necessity of a more close and rigid examination of the whole question. . the shell-lac was of the best quality, and had been carefully selected and cleaned; but as the action of any conducting particles in it would tend, virtually, to diminish the quantity or thickness of the dielectric used, and produce effects as if the two inducing surfaces of the conductors in that apparatus were nearer together than in the one with air only, i prepared another shell-lac hemisphere, of which the material had been dissolved in strong spirit of wine, the solution filtered, and then carefully evaporated. this is not an easy operation, for it is difficult to drive off the last portions of alcohol without injuring the lac by the heat applied; and unless they be dissipated, the substance left conducts too well to be used in these experiments. i prepared two hemispheres this way, one of them unexceptionable; and with it i repeated the former experiments with all precautions. the results were exactly of the same kind; the following expressions for the capacity of the shell-lac apparatus, whether it were app. i. or ii., being given directly by the experiments, . , . , . , . ; the average of these and several others being very nearly . . . as a final check upon the general conclusion, i then actually brought the surfaces of the air apparatus, corresponding to the place of the shell-lac in its apparatus, nearer together, by putting a metallic lining into the lower hemisphere of the one not containing the lac ( .). the distance of the metal surface from the carrier ball was in this way diminished from . of an inch to . of an inch, whilst the interval occupied by the lac in the other apparatus remained o. of an inch as before. notwithstanding this change, the lac apparatus showed its former superiority; and whether it or the air apparatus was charged first, the capacity of the lac apparatus to the air apparatus was by the experimental results as . to . . from all the experiments i have made, and their constant results, i cannot resist the conclusion that shell-lac does exhibit a case of _specific inductive capacity_. i have tried to check the trials in every way, and if not remove, at least estimate, every source of error. that the final result is not due to common conduction is shown by the capability of the apparatus to retain the communicated charge; that it is not due to the conductive power of inclosed small particles, by which they could acquire a polarized condition as conductors, is shown by the effects of the shell-lac purified by alcohol; and, that it is not due to any influence of the charged state, formerly described ( .), first absorbing and then evolving electricity, is indicated by the _instantaneous_ assumption and discharge of those portions of the power which are concerned in the phenomena, that instantaneous effect occurring in these cases, as in all others of ordinary induction, by charged conductors. the latter argument is the more striking in the case where the air apparatus is employed to divide the charge with the lac apparatus, for it obtains its portion of electricity in an _instant_, and yet is charged far above the _mean_. . admitting for the present the general fact sought to be proved; then . , though it expresses the capacity of the apparatus containing the hemisphere of shell-lac, by no means expresses the relation of lac to air. the lac only occupies one-half of the space _o, o_, of the apparatus containing it, through which the induction is sustained; the rest is filled with air, as in the other apparatus; and if the effect of the two upper halves of the globes be abstracted, then the comparison of the shell-lac powers in the lower half of the one, with the power of the air in the lower half of the other, will be as : ; and even this must be less than the truth, for the induction of the upper part of the apparatus, i.e. of the wire and ball b. (fig. .) to external objects, must be the same in both, and considerably diminish the difference dependent upon, and really producible by, the influence of the shell-lac within. * * * * * . _glass._--i next worked with glass as the dielectric. it involved the possibility of conduction on its surface, but it excluded the idea of conducting particles within its substance ( .) other than those of its own mass. besides this it does not assume the charged state ( .) so readily, or to such an extent, as shell-lac. . a thin hemispherical cup of glass being made hot was covered with a coat of shell-lac dissolved in alcohol, and after being dried for many hours in a hot place, was put into the apparatus and experimented with. it exhibited effects so slight, that, though they were in the direction indicating a superiority of glass over air, they were allowed to pass as possible errors of experiment; and the glass was considered as producing no sensible effect. . i then procured a thick hemispherical flint glass cup resembling that of shell-lac ( .), but not filling up the space _o, o_, so well. its average thickness was . of an inch, there being an additional thickness of air, averaging . of an inch, to make up the whole space of . of an inch between the inductive metallic surfaces. it was covered with a film of shell-lac as the former was, ( .) and being made very warm, was introduced into the apparatus, also warmed, and experiments made with it as in the former instances ( . &c.). the general results were the same as with shell-lac, i.e. glass surpassed air in its power of favouring induction through it. the two best results as respected the state of the apparatus for retention of charge, &c., gave, when the air apparatus was charged first . , and when the glass apparatus was charged first . , as the specific inductive capacity for glass, both being without correction. the average of nine results, four with the glass apparatus first charged, and five with the air apparatus first charged, gave . as the power of the glass apparatus; . and . being the minimum and maximum numbers with all the errors of experiment upon them. in all the experiments the glass apparatus took up its inductive charge instantly, and lost it as readily ( .); and during the short time of each experiment, acquired the peculiar state in a small degree only, so that the influence of this state, and also of conduction upon the results, must have been small. . allowing specific inductive capacity to be proved and active in this case, and . as the expression for the glass apparatus, then the specific inductive capacity of flint glass will be above . , not forgetting that this expression is for a piece of glass of such thickness as to occupy not quite two-thirds of the space through which the induction is sustained ( . .). * * * * * . _sulphur._--the same hemisphere of this substance was used in app. ii. as was formerly referred to ( .). the experiments were well made, i.e. the sulphur itself was free from charge both before and after each experiment, and no action from the stem appeared ( . .), so that no correction was required on that account. the following are the results when the air apparatus was first charged and divided: app. i. air, app. ii. sulphur. balls °. ° . . . . . . . . ° . . . . . . . . charge divided. . . . . . . . . . . . . . . . . . . . . after discharge. . . . . after discharge. here app. i. retained °, having lost ° in communicating ° to app. ii., and the capacity of the air apparatus is to that of the sulphur apparatus as to . . . then the sulphur apparatus was charged first, thus: . . . . ° ° . . . . . . . . . . . . charge divided. . . . . . . . . . . . . after discharge. . . . . after discharge. here app. ii. retained °, and gave up ° in communicating a charge of ° to app. i., and the capacity of the air apparatus is to that of the sulphur apparatus as to . . these results are very near to each other, and we may take the mean . as representing the specific inductive capacity of the sulphur apparatus; in which case the specific inductive capacity of sulphur itself as compared to air = ( .) will be about or above . . . this result with sulphur i consider as one of the most unexceptionable. the substance when fused was perfectly clear, pellucid, and free from particles of dirt ( .), so that no interference of small conducting bodies confused the result. the substance when solid is an excellent insulator, and by experiment was found to take up, with great slowness, that state ( . .) which alone seemed likely to disturb the conclusion. the experiments themselves, also, were free from any need of correction. yet notwithstanding these circumstances, so favourable to the exclusion of error, the result is a higher specific inductive capacity for sulphur than for any other body as yet tried; and though this may in part be clue to the sulphur being in a better shape, i.e. filling up more completely the space _o, o_, (fig. .) than the cups of shell-lac and glass, still i feel satisfied that the experiments altogether fully prove the existence of a difference between dielectrics as to their power of favouring an inductive action through them; which difference may, for the present, be expressed by the term _specific inductive capacity_. . having thus established the point in the most favourable cases that i could anticipate, i proceeded to examine other bodies amongst solids, liquids, and gases. these results i shall give with all convenient brevity. * * * * * . _spermaceti._--a good hemisphere of spermaceti being tried as to conducting power whilst its two surfaces were still in contact with the tinfoil moulds used in forming it, was found to conduct sensibly even whilst warm. on removing it from the moulds and using it in one of the apparatus, it gave results indicating a specific inductive capacity between . and . for the apparatus containing it. but as the only mode of operation was to charge the air apparatus, and then after a quick contact with the spermaceti apparatus, ascertain what was left in the former ( .), no great confidence can be placed in the results. they are not in opposition to the general conclusion, but cannot be brought forward as argument in favour of it. * * * * * . i endeavoured to find some liquids which would insulate well, and could be obtained in sufficient quantity for these experiments. oil of turpentine, native naphtha rectified, and the condensed oil gas fluid, appeared by common experiments to promise best as to insulation. being left in contact with fused carbonate of potassa, chloride of lime, and quick lime for some days and then filtered, they were found much injured in insulating power; but after distillation acquired their best state, though even then they proved to be conductors when extensive metallic contact was made with them. . _oil of turpentine rectified._--i filled the lower half of app. i. with the fluid: and as it would not hold a charge sufficiently to enable me first to measure and then divide it, i charged app. ii. containing air, and dividing its charge with app. i. by a quick contact, measured that remaining in app. ii.: for, theoretically, if a quick contact would divide up to equal tension between the two apparatus, yet without sensible loss from the conducting power of app. i.; and app. ii. were left charged to a degree of tension above half the original charge, it would indicate that oil of turpentine had less specific inductive capacity than air; or, if left charged below that mean state of tension, it would imply that the fluid had the greater inductive capacity. in an experiment of this kind, app. ii. gave as its charge ° before division with app. i., and ° afterwards, which is less than the half of °. again, being at ° before division, it was ° after, which is also less than half the divided charge. being at °, it was a third time divided, and then fell to °, less than the half of °. such are the best results i could obtain; they are not inconsistent with the belief that oil of turpentine has a greater specific capacity than air, but they do not prove the fact, since the disappearance of more than half the charge may be due to the conducting power merely of the fluid. . _naphtha._--this liquid gave results similar in their nature and direction to those with oil of turpentine. * * * * * . a most interesting class of substances, in relation to specific inductive capacity, now came under review, namely, the gases or aëriform bodies. these are so peculiarly constituted, and are bound together by so many striking physical and chemical relations, that i expected some remarkable results from them: air in various states was selected for the first experiments. . _air, rare and dense._--some experiments of division ( .) seemed to show that dense and rare air were alike in the property under examination. a simple and better process was to attach one of the apparatus to an air-pump, to charge it, and then examine the tension of the charge when the air within was more or less rarefied. under these circumstances it was found, that commencing with a certain charge, that charge did not change in its tension or force as the air was rarefied, until the rarefaction was such that _discharge_ across the space _o_, _o_ (fig. .) occurred. this discharge was proportionate to the rarefaction; but having taken place, and lowered the tension to a certain degree, that degree was not at all affected by restoring the pressure and density of the air to their first quantities. inches of mercury. thus at a pressure of the charge was ° again the charge was again the charge was reduced to the charge was raised again to the charge was being now reduced to . the charge fell to raised again to the charge was still . the charges were low in these experiments, first that they might not pass off at low pressure, and next that little loss by dissipation might occur. i now reduced them still lower, that i might rarefy further, and for this purpose in the following experiment used a measuring interval in the electrometer of only ° ( .). the pressure of air within the apparatus being reduced to . inches of mercury, the charge was found to be °; then letting in air till the pressure was inches, the charge was still °. . these experiments were repeated with pure oxygen with the same consequences. . this result of _no variation_ in the electric tension being produced by variation in the density or pressure of the air, agrees perfectly with those obtained by mr. harris[a], and described in his beautiful and important investigations contained in the philosophical transactions; namely that induction is the same in rare and dense air, and that the divergence of an electrometer under such variations of the air continues the same, provided no electricity pass away from it. the effect is one entirely independent of that power which dense air has of causing a higher charge to be _retained_ upon the surface of conductors in it than can be retained by the same conductors in rare air; a point i propose considering hereafter. [a] philosophical transactions, , pp. , , , . . i then compared _hot and cold air_ together, by raising the temperature of one of the inductive apparatus as high as it could be without injury, and then dividing charges between it and the other apparatus containing cold air. the temperatures were about ° and °, still the power or capacity appeared to be unchanged; and when i endeavoured to vary the experiment, by charging a cold apparatus and then warming it by a spirit lamp, i could obtain no proof that the inductive capacity underwent any alteration. . i compared _damp and dry air_ together, but could find no difference in the results. * * * * * . _gases._--a very long series of experiments was then undertaken for the purpose of comparing _different gases_ one with another. they were all found to insulate well, except such as acted on the shell-lac of the supporting stem; these were chlorine, ammonia, and muriatic acid. they were all dried by appropriate means before being introduced into the apparatus. it would have been sufficient to have compared each with air; but, in consequence of the striking result which came out, namely, that _all had the same power of_ or _capacity for_, sustaining induction through them, (which perhaps might have been expected after it was found that no variation of density or pressure produced any effect,) i was induced to compare them, experimentally, two and two in various ways, that no difference might escape me, and that the sameness of result might stand in full opposition to the contrast of property, composition, and condition which the gases themselves presented. . the experiments were made upon the following pairs of gases. . nitrogen and oxygen. . oxygen air. . hydrogen air. . muriatic acid gas air. . oxygen hydrogen. . oxygen carbonic acid. . oxygen olefiant gas. . oxygen nitrous gas. . oxygen sulphurous acid. . oxygen ammonia. . hydrogen carbonic acid. hydrogen olefiant gas. . hydrogen sulphurous acid. . hydrogen fluo-silicic acid. . hydrogen ammonia. , hydrogen arseniuretted hydrogen. . hydrogen sulphuretted hydrogen. , nitrogen olefiant gas. . nitrogen nitrous gas. . nitrogen nitrous oxide. . nitrogen ammonia. . carbonic oxide carbonic acid. . carbonic oxide olefiant gas. . nitrous oxide nitrous gas. . ammonia sulphurous acid. . notwithstanding the striking contrasts of all kinds which these gases present of property, of density, whether simple or compound, anions or cations ( .), of high or low pressure ( . .), hot or cold ( .), not the least difference in their capacity to favour or admit electrical induction through them could be perceived. considering the point established, that in all these gases induction takes place by an action of contiguous particles, this is the more important, and adds one to the many striking relations which hold between bodies having the gaseous condition and form. another equally important electrical relation, which will be examined in the next paper[a], is that which the different gases have to each other at the _same pressure_ of causing the retention of the _same or different degrees of charge_ upon conductors in them. these two results appear to bear importantly upon the subject of electrochemical excitation and decomposition; for as _all_ these phenomena, different as they seem to be, must depend upon the electrical forces of the particles of matter, the very distance at which they seem to stand from each other will do much, if properly considered, to illustrate the principle by which they are held in one common bond, and subject, as they must be, to one common law. [a] see in relation to this point . &c.--_dec. ._ . it is just possible that the gases may differ from each other in their specific inductive capacity, and yet by quantities so small as not to be distinguished in the apparatus i have used. it must be remembered, however, that in the gaseous experiments the gases occupy all the space _o, o_, (fig. .) between the inner and the outer ball, except the small portion filled by the stem; and the results, therefore, are twice as delicate as those with solid dielectrics. . the insulation was good in all the experiments recorded, except nos. , , , and , being those in which ammonia was compared with other gases. when shell-lac is put into ammoniacal gas its surface gradually acquires conducting power, and in this way the lac part of the stem within was so altered, that the ammonia apparatus could not retain a charge with sufficient steadiness to allow of division. in these experiments, therefore, the other apparatus was charged; its charge measured and divided with the ammonia apparatus by a quick contact, and what remained untaken away by the division again measured ( .). it was so nearly one-half of the original charge, as to authorize, with this reservation, the insertion of ammoniacal gas amongst the other gases, as having equal power with them. ¶ vi. _general results as to induction._ . thus _induction_ appears to be essentially an action of contiguous particles, through the intermediation of which the electric force, originating or appearing at a certain place, is propagated to or sustained at a distance, appearing there as a force of the same kind exactly equal in amount, but opposite in its direction and tendencies ( .). induction requires no sensible thickness in the conductors which may be used to limit its extent; an uninsulated leaf of gold may be made very highly positive on one surface, and as highly negative on the other, without the least interference of the two states whilst the inductions continue. nor is it affected by the nature of the limiting conductors, provided time be allowed, in the case of those which conduct slowly, for them to assume their final state ( .). . but with regard to the _dielectrics_ or insulating media, matters are very different ( .). their thickness has an immediate and important influence on the degree of induction. as to their quality, though all gases and vapours are alike, whatever their state; yet amongst solid bodies, and between them and gases, there are differences which prove the existence of _specific inductive capacities_, these differences being in some cases very great. . the direct inductive force, which may be conceived to be exerted in lines between the two limiting and charged conducting surfaces, is accompanied by a lateral or transverse force equivalent to a dilatation or repulsion of these representative lines ( .); or the attractive force which exists amongst the particles of the dielectric in the direction of the induction is accompanied by a repulsive or a diverging force in the transverse direction ( .). . induction appears to consist in a certain polarized state of the particles, into which they are thrown by the electrified body sustaining the action, the particles assuming positive and negative points or parts, which are symmetrically arranged with respect to each other and the inducting surfaces or particles[a]. the state must be a forced one, for it is originated and sustained only by force, and sinks to the normal or quiescent state when that force is removed. it can be _continued_ only in insulators by the same portion of electricity, because they only can retain this state of the particles ( ). [a] the theory of induction which i am stating does not pretend to decide whether electricity be a fluid or fluids, or a mere power or condition of recognized matter. that is a question which i may be induced to consider in the next or following series of these researches. . the principle of induction is of the utmost generality in electric action. it constitutes charge in every ordinary case, and probably in every case; it appears to be the cause of all excitement, and to precede every current. the degree to which the particles are affected in this their forced state, before discharge of one kind or another supervenes, appears to constitute what we call _intensity_. . when a leyden jar is _charged_, the particles of the glass are forced into this polarized and constrained condition by the electricity of the charging apparatus. _discharge_ is the return of these particles to their natural state from their state of tension, whenever the two electric forces are allowed to be disposed of in some other direction. . all charge of conductors is on their surface, because being essentially inductive, it is there only that the medium capable of sustaining the necessary inductive state begins. if the conductors are hollow and contain air or any other dielectric, still no _charge_ can appear upon that internal surface, because the dielectric there cannot assume the polarized state throughout, in consequence of the opposing actions in different directions. . the known influence of _form_ is perfectly consistent with the corpuscular view of induction set forth. an electrified cylinder is more affected by the influence of the surrounding conductors (which complete the condition of charge) at the ends than at the middle, because the ends are exposed to a greater sum of inductive forces than the middle; and a point is brought to a higher condition than a ball, because by relation to the conductors around, more inductive force terminates on its surface than on an equal surface of the ball with which it is compared. here too, especially, can be perceived the influence of the lateral or transverse force ( .), which, being a power of the nature of or equivalent to repulsion, causes such a disposition of the lines of inductive force in their course across the dielectric, that they must accumulate upon the point, the end of the cylinder, or any projecting part. . the influence of _distance_ is also in harmony with the same view. there is perhaps no distance so great that induction cannot take place through it[a]; but with the same constraining force ( .) it takes place the more easily, according as the extent of dielectric through which it is exerted is lessened. and as it is assumed by the theory that the particles of the dielectric, though tending to remain in a normal state, are thrown into a forced condition during the induction; so it would seem to follow that the fewer there are of these intervening particles opposing their tendency to the assumption of the new state, the greater degree of change will they suffer, i.e. the higher will be the condition they assume, and the larger the amount of inductive action exerted through them. [a] i have traced it experimentally from a ball placed in the middle of the large cube formerly described ( .) to the sides of the cube six feet distant, and also from the same ball placed in the middle of our large lecture-room to the walls of the room at twenty-six feet distance, the charge sustained upon the ball in these cases being solely due to induction through these distances. . i have used the phrases _lines of inductive force_ and _curved lines_ of force ( . . . .) in a general sense only, just as we speak of the lines of magnetic force. the lines are imaginary, and the force in any part of them is of course the resultant of compound forces, every molecule being related to every other molecule in _all_ directions by the tension and reaction of those which are contiguous. the transverse force is merely this relation considered in a direction oblique to the lines of inductive force, and at present i mean no more than that by the phrase. with respect to the term _polarity_ also, i mean at present only a disposition of force by which the same molecule acquires opposite powers on different parts. the particular way in which this disposition is made will come into consideration hereafter, and probably varies in different bodies, and so produces variety of electrical relation[a]. all i am anxious about at present is, that a more particular meaning should not be attached to the expressions used than i contemplate. further inquiry, i trust, will enable us by degrees to restrict the sense more and more, and so render the explanation of electrical phenomena day by day more and more definite. [a] see now . &c.--_dec. ._ . as a test of the probable accuracy of my views, i have throughout this experimental examination compared them with the conclusions drawn by m. poisson from his beautiful mathematical inquiries[a]. i am quite unfit to form a judgment of these admirable papers; but as far as i can perceive, the theory i have set forth and the results i have obtained are not in opposition to such of those conclusions as represent the final disposition and state of the forces in the limited number of cases be has considered. his theory assumes a very different mode of action in induction to that which i have ventured to support, and would probably find its mathematical test in the endeavour to apply it to cases of induction in curved lines. to my feeling it is insufficient in accounting for the retention of electricity upon the surface of conductors by the pressure of the air, an effect which i hope to show is simple and consistent according to the present view[b]; and it does not touch voltaic electricity, or in any way associate it and what is called ordinary electricity under one common principle. [a] mémoires de l'institut, , tom. xii. the first page , and the second paging . [b] refer to , , , .--_dec. ._ i have also looked with some anxiety to the results which that indefatigable philosopher harris has obtained in his investigation of the laws of induction[a], knowing that they were experimental, and having a full conviction of their exactness; but i am happy in perceiving no collision at present between them and the views i have taken. [a] philosophical transactions, , p. . . finally, i beg to say that i put forth my particular view with doubt and fear, lest it should not bear the test of general examination, for unless true it will only embarrass the progress of electrical science. it has long been on my mind, but i hesitated to publish it until the increasing persuasion of its accordance with all known facts, and the manner in which it linked together effects apparently very different in kind, urged me to write the present paper. i as yet see no inconsistency between it and nature, but, on the contrary, think i perceive much new light thrown by it on her operations; and my next papers will be devoted to a review of the phenomena of conduction, electrolyzation, current, magnetism, retention, discharge, and some other points, with an application of the theory to these effects, and an examination of it by them. _royal institution, november , ._ * * * * * _supplementary note to experimental researches in electricity._ _eleventh series._ received march , . . i have recently put into an experimental form that general statement of the question of _specific inductive capacity_ which is given at no. of series xi., and the result is such as to lead me to hope the council of the royal society will authorize its addition to the paper in the form of a supplementary note. three circular brass plates, about five inches in diameter, were mounted side by side upon insulating pillars; the middle one, a, was a fixture, but the outer plates b and c were moveable on slides, so that all three could be brought with their sides almost into contact, or separated to any required distance. two gold leaves were suspended in a glass jar from insulated wires; one of the outer plates b was connected with one of the gold leaves, and the other outer plate with the other leaf. the outer plates b and c were adjusted at the distance of an inch and a quarter from the middle plate a, and the gold leaves were fixed at two inches apart; a was then slightly charged with electricity, and the plates b and c, with their gold leaves, thrown out of insulation _at the same time_, and then left insulated. in this state of things a was charged positive inductrically, and b and c negative inducteously; the same dielectric, air, being in the two intervals, and the gold leaves hanging, of course, parallel to each other in a relatively unelectrified state. . a plate of shell-lac three-quarters of an inch in thickness, and four inches square, suspended by clean white silk thread, was very carefully deprived of all charge ( .) (so that it produced no effect on the gold leaves if a were uncharged) and then introduced between plates a and b; the electric relation of the three plates was immediately altered, and the gold leaves attracted each other. on removing the shell-lac this attraction ceased; on introducing it between a and c it was renewed; on removing it the attraction again ceased; and the shell-lac when examined by a delicate coulomb electrometer was still without charge. . as a was positive, b and c were of course negative; but as the specific inductive capacity of shell-lac is about twice that of air ( .), it was expected that when the lac was introduced between a and b, a would induce more towards b than towards c; that therefore b would become more negative than before towards a, and consequently, because of its insulated condition, be positive externally, as at its back or at the gold leaves; whilst c would be less negative towards a, and therefore negative outwards or at the gold leaves. this was found to be the case; for on whichever side of a the shell-lac was introduced the external plate at that side was positive, and the external plate on the other side negative towards each other, and also to uninsulated external bodies. . on employing a plate of sulphur instead of shell-lac, the same results were obtained; consistent with the conclusions drawn regarding the high specific inductive capacity of that body already given ( .). . these effects of specific inductive capacity can be exalted in various ways, and it is this capability which makes the great value of the apparatus. thus i introduced the shell-lac between a and b, and then for a moment connected b and c, uninsulated them, and finally left them in the insulated state; the gold leaves were of course hanging parallel to each other. on removing the shell-lac the gold leaves attracted each other; on introducing the shell-lac between a and c this attraction was _increased_, (as had been anticipated from theory,) and the leaves came together, though not more than four inches long, and hanging three inches apart. . by simply bringing the gold leaves nearer to each other i was able to show the difference of specific inductive capacity when only thin plates of shell-lac were used, the rest of the dielectric space being filled with air. by bringing b and c nearer to a another great increase of sensibility was made. by enlarging the size of the plates still further power was gained. by diminishing the extent of the wires, &c. connected with the gold leaves, another improvement resulted. so that in fact the gold leaves became, in this manner, as delicate a test of _specific inductive action_ as they are, in bennet's and singer's electrometers, of ordinary electrical charge. . it is evident that by making the three plates the sides of cells, with proper precautions as regards insulation, &c., this apparatus may be used in the examination of gases, with far more effect than the former apparatus ( . ), and may, perhaps, bring out differences which have as yet escaped me ( . .) . it is also evident that two metal plates are quite sufficient to form the instrument; the state of the single inducteous plate when the dielectric is changed, being examined either by bringing a body excited in a known manner towards its gold leaves, or, what i think will be better, employing a carrier ball in place of the leaf, and examining that ball by the coulomb electrometer ( .). the inductive and inducteous surfaces may even be balls; the latter being itself the carrier ball of the coulomb's electrometer ( . .). . to increase the effect, a small condenser may be used with great advantage. thus if, when two inducteous plates are used, a little condenser were put in the place of the gold leaves, i have no doubt the three principal plates might be reduced to an inch or even half an inch in diameter. even the gold leaves act to each other for the time as the plates of a condenser. if only two plates were used, by the proper application of the condenser the same reduction might take place. this expectation is fully justified by an effect already observed and described ( .). . in that case the application of the instrument to very extensive research is evident. comparatively small masses of dielectrics could be examined, as diamonds and crystals. an expectation, that the specific inductive capacity of crystals will vary in different directions, according as the lines of inductive force ( .) are parallel to, or in other positions in relation to the axes of the crystals, can be tested[a]: i purpose that these and many other thoughts which arise respecting specific inductive action and the polarity of the particles of dielectric matter, shall be put to the proof as soon as i can find time. [a] refer for this investigation to - .--_dec. ._ . hoping that this apparatus will form an instrument of considerable use, i beg to propose for it (at the suggestion of a friend) the name of _differential inductometer_. _royal institution, march , ._ twelfth series. § . _on induction (continued)._ ¶ vii. _conduction, or conductive discharge._ ¶ viii. _electrolytic discharge._ ¶ ix. _disruptive discharge--insulation--spark--brush--difference of discharge at the positive and negative surfaces of conductors._ received january ,--read february , . . i proceed now, according to my promise, to examine, by the great facts of electrical science, that theory of induction which i have ventured to put forth ( . . &c.). the principle of induction is so universal that it pervades all electrical phenomena; but the general case which i purpose at present to go into consists of insulation traced into and terminating with discharge, with the accompanying effects. this case includes the various _modes_ of discharge, and also the condition and characters of a current; the elements of magnetic action being amongst the latter. i shall necessarily have occasion to speak theoretically, and even hypothetically; and though these papers profess to be experimental researches, i hope that, considering the facts and investigations contained in the last series in support of the particular view advanced, i shall not be considered as taking too much liberty on the present occasion, or as departing too far from the character which they ought to have, especially as i shall use every opportunity which presents itself of returning to that strong test of truth, experiment. . induction has as yet been considered in these papers only in cases of insulation; opposed to insulation is _discharge_. the action or effect which may be expressed by the general term _discharge_, may take place, as far as we are aware at present, in several modes. thus, that which is called simply _conduction_ involves no chemical action, and apparently no displacement of the particles concerned. a second mode may be called _electrolytic discharge_; in it chemical action does occur, and particles must, to a certain degree, be displaced. a third mode, namely, that by sparks or brushes, may, because of its violent displacement of the particles of the _dielectric_ in its course, be called the _disruptive discharge_; and a fourth may, perhaps, be conveniently distinguished for a time by the words _convection_, or _carrying discharge_, being that in which discharge is effected either by the carrying power of solid particles, or those of gases and liquids. hereafter, perhaps, all these modes may appear as the result of one common principle, but at present they require to be considered apart; and i will now speak of the _first_ mode, for amongst all the forms of discharge, that which we express by the term conduction appears the most simple and the most directly in contrast with insulation. ¶ vii. _conduction, or conductive discharge._ . though assumed to be essentially different, yet neither cavendish nor poisson attempt to explain by, or even state in, their theories, what the essential difference between insulation and conduction is. nor have i anything, perhaps, to offer in this respect, _except_ that, according to my view of induction, insulation and conduction depend upon the same molecular action of the dielectrics concerned; are only extreme degrees of _one common condition_ or effect; and in any sufficient mathematical theory of electricity must be taken as cases of the same kind. hence the importance of the endeavour to show the connection between them under my theory of the electrical relations of contiguous particles. . though the action of the insulating dielectric in the charged leyden jar, and that of the wire in discharging it, may seem very different, they may be associated by numerous intermediate links, which carry us on from one to the other, leaving, i think, no necessary connection unsupplied. we may observe some of these in succession for information respecting the whole case. . spermnceti has been examined and found to be a dielectric, through which induction can take place ( . .), its specific inductive capacity being about or above . ( .), and the inductive action has been considered in it, as in all other substances, an action of contiguous particles. . but spermaceti is also a _conductor_, though in so low a degree that we can trace the process of conduction, as it were, step by step through the mass ( .); and even when the electric force has travelled through it to a certain distance, we can, by removing the coercitive (which is at the same time the inductive) force, cause it to return upon its path and reappear in its first place ( . .). here induction appears to be a necessary preliminary to conduction. it of itself brings the contiguous particles of the dielectric into a certain condition, which, if retained by them, constitutes _insulation_, but if lowered by the communication of power from one particle to another, constitutes _conduction_. . if _glass_ or _shell-lac_ be the substances under consideration, the same capabilities of suffering either induction or conduction through them appear ( . . .), but not in the same degree. the conduction almost disappears ( . .); the induction therefore is sustained, i.e. the polarized state into which the inductive force has brought the contiguous particles is retained, there being little discharge action between them, and therefore the _insulation_ continues. but, what discharge there is, appears to be consequent upon that condition of the particles into which the induction throws them; and thus it is that ordinary insulation and conduction are closely associated together or rather are extreme cases of one common condition. . in ice or water we have a better conductor than spermaceti, and the phenomena of induction and insulation therefore rapidly disappear, because conduction quickly follows upon the assumption of the inductive state. but let a plate of cold ice have metallic coatings on its sides, and connect one of these with a good electrical machine in work, and the other with the ground, and it then becomes easy to observe the phenomena of induction through the ice, by the electrical tension which can be obtained and continued on both the coatings ( . .). for although that portion of power which at one moment gave the inductive condition to the particles is at the next lowered by the consequent discharge due to the conductive act, it is succeeded by another portion of force from the machine to restore the inductive state. if the ice be converted into water the same succession of actions can be just as easily proved, provided the water be distilled, and (if the machine be not powerful enough) a voltaic battery be employed. . all these considerations impress my mind strongly with the conviction, that insulation and ordinary conduction cannot be properly separated when we are examining into their nature; that is, into the general law or laws under which their phenomena are produced. they appear to me to consist in an action of contiguous particles dependent on the forces developed in electrical excitement; these forces bring the particles into a state of tension or polarity, which constitutes both _induction_ and _insulation_; and being in this state, the continuous particles have a power or capability of communicating their forces one to the other, by which they are lowered, and discharge occurs. every body appears to discharge ( . .); but the possession of this capability in a _greater or smaller degree_ in different bodies, makes them better or worse conductors, worse or better insulators; and both _induction_ and _conduction_ appear to be the same in their principle and action ( .), except that in the latter an effect common to both is raised to the highest degree, whereas in the former it occurs in the best cases, in only an almost insensible quantity. . that in our attempts to penetrate into the nature of electrical action, and to deduce laws more general than those we are at present acquainted with, we should endeavour to bring apparently opposite effects to stand side by side in harmonious arrangement, is an opinion of long standing, and sanctioned by the ablest philosophers. i hope, therefore, i may be excused the attempt to look at the highest cases of conduction as analogous to, or even the same in kind with, those of induction and insulation. . if we consider the slight penetration of sulphur ( . .) or shell-lac ( .) by electricity, or the feebler insulation sustained by spermaceti ( . .), as essential consequences and indications of their _conducting_ power, then may we look on the resistance of metallic wires to the passage of electricity through them as _insulating_ power. of the numerous well-known cases fitted to show this resistance in what are called the perfect conductors, the experiments of professor wheatstone best serve my present purpose, since they were carried to such an extent as to show that _time_ entered as an element into the conditions of conduction[a] even in metals. when discharge was made through a copper wire feet in length, and / th of an inch in diameter, so that the luminous sparks at each end of the wire, and at the middle, could be observed in the same place, the latter was found to be sensibly behind the two former in time, they being by the conditions of the experiment simultaneous. hence a proof of retardation; and what reason can be given why this retardation should not be of the same kind as that in spermaceti, or in lac, or sulphur? but as, in them, retardation is insulation, and insulation is induction, why should we refuse the same relation to the same exhibitions of force in the metals? [a] philosophical transactions, , p. . . we learn from the experiment, that if _time_ be allowed the retardation is gradually overcome; and the same thing obtains for the spermaceti, the lac, and glass ( .); give but time in proportion to the retardation, and the latter is at last vanquished. but if that be the case, and all the results are alike in kind, the only difference being in the length of time, why should we refuse to metals the previous inductive action, which is admitted to occur in the other bodies? the diminution of _time_ is no negation of the action; nor is the lower degree of tension requisite to cause the forces to traverse the metal, as compared to that necessary in the cases of water, spermaceti, or lac. these differences would only point to the conclusion, that in metals the particles under induction can transfer their forces when at a lower degree of tension or polarity, and with greater facility than in the instances of the other bodies. . let us look at mr. wheatstone's beautiful experiment in another point of view, if, leaving the arrangement at the middle and two ends of the long copper wire unaltered, we remove the two intervening portions and replace them by wires of iron or platina, we shall have a much greater retardation of the middle spark than before. if, removing the iron, we were to substitute for it only five or six feet of water in a cylinder of the same diameter as the metal, we should have still greater retardation. if from water we passed to spermaceti, either directly or by gradual steps through other bodies, (even though we might vastly enlarge the bulk, for the purpose of evading the occurrence of a spark elsewhere ( .) than at the three proper intervals,) we should have still greater retardation, until at last we might arrive, by degrees so small as to be inseparable from each other, at actual and permanent insulation. what, then, is to separate the principle of these two extremes, perfect conduction and perfect insulation, from each other; since the moment we leave in the smallest degree perfection at either extremity, we involve the element of perfection at the opposite end? especially too, as we have not in nature the case of perfection either at one extremity or the other, either of insulation or conduction. . again, to return to this beautiful experiment in the various forms which may be given to it: the forces are not all in the wire (after they have left the leyden jar) during the whole time ( .) occupied by the discharge; they are disposed in part through the surrounding dielectric under the well-known form of induction; and if that dielectric be air, induction takes place from the wire through the air to surrounding conductors, until the ends of the wire are electrically related through its length, and discharge has occurred, i.e. for the _time_ during which the middle spark is retarded beyond the others. this is well shown by the old experiment, in which a long wire is so bent that two parts (plate viii. fig. .), _a, b_, near its extremities shall approach within a short distance, as a quarter of an inch, of each other in the air. if the discharge of a leyden jar, charged to a sufficient degree, be sent through such a wire, by far the largest portion of the electricity will pass as a spark across the air at the interval, and not by the metal. does not the middle part of the wire, therefore, act here as an insulating medium, though it be of metal? and is not the spark through the air an indication of the tension (simultaneous with _induction_) of the electricity in the ends of this single wire? why should not the wire and the air both be regarded as dielectrics; and the action at its commencement, and whilst there is tension, as an inductive action? if it acts through the contorted lines of the wire, so it also does in curved and contorted lines through air ( , , .), and other insulating dielectrics ( ); and we can apparently go so far in the analogy, whilst limiting the case to the inductive action only, as to show that amongst insulating dielectrics some lead away the lines of force from others ( .), as the wire will do from worse conductors, though in it the principal effect is no doubt due to the ready discharge between the particles whilst in a low state of tension. the retardation is for the time insulation; and it seems to me we may just as fairly compare the air at the interval _a, b_ (fig. .) and the wire in the circuit, as two bodies of the same kind and acting upon the same principles, as far as the first inductive phenomena are concerned, notwithstanding the different forms of discharge which ultimately follow[a], as we may compare, according to coulomb's investigations[b] _different lengths_ of different insulating bodies required to produce the same amount of insulating effect. [a] these will be examined hereafter ( . &c.). [b] mémoires de l'académie, , p. . or ency. britann. first supp. vol. i. p. . . this comparison is still more striking when we take into consideration the experiment of mr. harris, in which he stretched a fine wire across a glass globe, the air within being rarefied[a]. on sending a charge through the joint arrangement of metal and rare air, as much, if not more, electricity passed by the latter as by the former. in the air, rarefied as it was, there can be no doubt the discharge was preceded by induction ( .); and to my mind all the circumstances indicate that the same was the case with the metal; that, in fact, both substances are dielectrics, exhibiting the same effects in consequence of the action of the same causes, the only variation being one of degree in the different substances employed. [a] philosophical transactions, , p, . . judging on these principles, velocity of discharge through the _same wire_ may be varied greatly by attending to the circumstances which cause variations of discharge through spermaceti or sulphur. thus, for instance, it must vary with the tension or intensity of the first urging force ( . .), which tension is charge and induction. so if the two ends of the wire, in professor wheatstone's experiment, were immediately connected with two large insulated metallic surfaces exposed to the air, so that the primary act of induction, after making the contact for discharge, might be in part removed from the internal portion of the wire at the first instant, and disposed for the moment on its surface jointly with the air and surrounding conductors, then i venture to anticipate that the middle spark would be more retarded than before; and if these two plates were the inner and outer coating of a large jar or a leyden battery, then the retardation of that spark would be still greater. . cavendish was perhaps the first to show distinctly that discharge was not always by one channel[a], but, if several are present, by many at once. we may make these different channels of different bodies, and by proportioning their thicknesses and lengths, may include such substances as air, lac, spermaceti, water, protoxide of iron, iron and silver, and by _one_ discharge make each convey its proportion of the electric force. perhaps the air ought to be excepted, as its discharge by conduction is questionable at present ( .); but the others may all be limited in their mode of discharge to pure conduction. yet several of them suffer previous induction, precisely like the induction through the air, it being a necessary preliminary to their discharging action. how can we therefore separate any one of these bodies from the others, as to the _principles and mode_ of insulating and conducting, except by mere degree? all seem to me to be dielectrics acting alike, and under the same common laws. [a] _philosophical transactions_, , p. . . i might draw another argument in favour of the general sameness, in nature and action, of good and bad conductors (and all the bodies i refer to are conductors more or less), from the perfect equipoise in action of very different bodies when opposed to each other in magneto-electric inductive action, as formerly described ( .), but am anxious to be as brief as is consistent with the clear examination of the probable truth of my views. . with regard to the possession by the gases of any conducting power of the simple kind now under consideration, the question is a very difficult one to determine at present. experiments seem to indicate that they do insulate certain low degrees of tension perfectly, and that the effects which may have appeared to be occasioned by _conduction_ have been the result of the carrying power of the charged particles, either of the air or of dust, in it. it is equally certain, however, that with higher degrees of tension or charge the particles discharge to one another, and that is conduction. if the gases possess the power of insulating a certain low degree of tension continuously and perfectly, such a result may be due to their peculiar physical state, and the condition of separation under which their particles are placed. but in that, or in any case, we must not forget the fine experiments of cagniard de la tour[a], in which he has shown that liquids and their vapours can be made to pass gradually into each other, to the entire removal of any marked distinction of the two states. thus, hot dry steam and cold water pass by insensible gradations into each other; yet the one is amongst the gases as an insulator, and the other a comparatively good conductor. as to conducting power, therefore, the transition from metals even up to gases is gradual; substances make but one series in this respect, and the various cases must come under one condition and law ( .). the specific differences of bodies as to conducting power only serves to strengthen the general argument, that conduction, like insulation, is a result of induction, and is an action of contiguous particles. [a] annales de chimie, xxi. pp. , , or quarterly journal of science, xv. . . i might go on now to consider induction and its concomitant, _conduction_, through mixed dielectrics, as, for instance, when a charged body, instead of acting across air to a distant uninsulated conductor, acts jointly through it and an interposed insulated conductor. in such a case, the air and the conducting body are the mixed dielectrics; and the latter assumes a polarized condition as a mass, like that which my theory assumes _each particle_ of the air to possess at the same time ( ). but i fear to be tedious in the present condition of the subject, and hasten to the consideration of other matter. . to sum up, in some degree, what has been said, i look upon the first effect of an excited body upon neighbouring matters to be the production of a polarized state of their particles, which constitutes _induction_; and this arises from its action upon the particles in immediate contact with it, which again act upon those contiguous to them, and thus the forces are transferred to a distance. if the induction remain undiminished, then perfect insulation is the consequence; and the higher the polarized condition which the particles can acquire or maintain, the higher is the intensity which may be given to the acting forces. if, on the contrary, the contiguous particles, upon acquiring the polarized state, have the power to communicate their forces, then conduction occurs, and the tension is lowered, conduction being a distinct act of discharge between neighbouring particles. the lower the state of tension at which this discharge between the particles of a body takes place, the better conductor is that body. in this view, insulators may be said to be bodies whose particles can retain the polarized state; whilst conductors are those whose particles cannot be permanently polarized. if i be right in my view of induction, then i consider the reduction of these two effects (which have been so long held distinct) to an action of contiguous particles obedient to one common law, as a very important result; and, on the other hand, the identity of character which the two acquire when viewed by the theory ( .), is additional presumptive proof in favour of the correctness of the latter. * * * * * . that heat has great influence over simple conduction is well known ( .), its effect being, in some cases, almost an entire change of the characters of the body ( . .). harris has, however, shown that it in no respect affects gaseous bodies, or at least air[a]; and davy has taught us that, as a class, metals have their conducting power _diminished_ by it[b]. [a] _philosophical transactions_, , p. [b] ibid. , p. . . i formerly described a substance, sulphuret of silver, whose conducting power was increased by heat ( . . .); and i have since then met with another as strongly affected in the same way: this is fluoride of lead. when a piece of that substance, which had been fused and cooled, was introduced into the circuit of a voltaic battery, it stopped the current. being heated, it acquired conducting powers before it was visibly red-hot in daylight; and even sparks could be taken against it whilst still solid. the current alone then raised its temperature (as in the case of sulphuret of silver) until it fused, after which it seemed to conduct as well as the metallic vessel containing it; for whether the wire used to complete the circuit touched the fused fluoride only, or was in contact with the platina on which it was supported, no sensible difference in the force of the current was observed. during all the time there was scarcely a trace of decomposing action of the fluoride, and what did occur, seemed referable to the air and moisture of the atmosphere, and not to electrolytic action. . i have now very little doubt that periodide of mercury ( . . .) is a case of the same kind, and also corrosive sublimate ( .). i am also inclined to think, since making the above experiments, that the anomalous action of the protoxide of antimony, formerly observed and described ( . .), may be referred in part to the same cause. . i have no intention at present of going into the particular relation of heat and electricity, but we may hope hereafter to discover by experiment the law which probably holds together all the above effects with those of the _evolution_ and the _disappearance_ of heat by the current, and the striking and beautiful results of thermo-electricity, in one common bond. ¶ viii. _electrolytic discharge._ . i have already expressed in a former paper ( .), the view by which i hope to associate ordinary induction and electrolyzation. under that view, the discharge of electric forces by electrolyzation is rather an effect superadded, in a certain class of bodies, to those already described as constituting induction and insulation, than one independent of and distinct from these phenomena. . electrolytes, as respects their insulating and conducting forces, belong to the general category of bodies ( . .); and if they are in the solid state (as nearly all can assume that state), they retain their place, presenting then no new phenomenon ( . &c.); or if one occur, being in so small a proportion as to be almost unimportant. when liquefied, they also belong to the same list whilst the electric intensity is below a certain degree; but at a given intensity ( . . .), fixed for each, and very low in all known cases, they play a new part, causing discharge in proportion ( .) to the development of certain chemical effects of combination and decomposition; and at this point, move out from the general class of insulators and conductors, to form a distinct one by themselves. the former phenomena have been considered ( . .); it is the latter which have now to be revised, and used as a test of the proposed theory of induction. . the theory assumes, that the particles of the dielectric (now an electrolyte) are in the first instance brought, by ordinary inductive action, into a polarized state, and raised to a certain degree of tension or intensity before discharge commences; the inductive state being, in fact, a _necessary preliminary_ to discharge. by taking advantage of those circumstances which bear upon the point, it is not difficult to increase the tension indicative of this state of induction, and so make the state itself more evident. thus, if distilled water be employed, and a long narrow portion of it placed between the electrodes of a powerful voltaic battery, we have at once indications of the intensity which can be sustained at these electrodes by the inductive action through the water as a dielectric, for sparks may be obtained, gold leaves diverged, and leyden bottles charged at their wires. the water is in the condition of the spermaceti ( . .) a bad conductor and a bad insulator; but what it does insulate is by virtue of inductive action, and that induction is the preparation for and precursor of discharge ( .). . the induction and tension which appear at the limits of the portion of water in the direction of the current, are only the sums of the induction and tension of the contiguous particles between those limits; and the limitation of the inductive tension, to a certain degree shows (time entering in each case as an important element of the result), that when the particles have acquired a certain relative state, _discharge_, or a transfer of forces equivalent to ordinary conduction, takes place. . in the inductive condition assumed by water before discharge comes on, the particles polarized are the particles of the _water_ that being the dielectric used[a]; but the discharge between particle and particle is not, as before, a mere interchange of their powers or forces at the polar parts, but an actual separation of them into their two elementary particles, the oxygen travelling in one direction, and carrying with it its amount of the force it had acquired during the polarization, and the hydrogen doing the same thing in the other direction, until they each meet the next approaching particle, which is in the same electrical state with that they have left, and by association of their forces with it, produce what constitutes discharge. this part of the action may be regarded as a carrying one ( . . .), performed by the constituent particles of the dielectric. the latter is always a compound body ( . .); and by those who have considered the subject and are acquainted with the philosophical view of transfer which was first put forth by grotthuss[b], its particles may easily be compared to a series of metallic conductors under inductive action, which, whilst in that state, are divisible into these elementary moveable halves. [a] see - .--_dec. _ [b] annales de chimie, lviii. . and lxiii, . . electrolytic discharge depends, of necessity, upon the non-conduction of the dielectric as a whole, and there are two steps or acts in the process: first a polarization of the molecules of the substance and then a lowering of the forces by the separation, advance in opposite directions, and recombination of the elements of the molecules, these being, as it were, the halves of the originally polarized conductors or particles. . these views of the decomposition of electrolytes and the consequent effect of discharge, which, as to the particular case, are the same with those of grotthuss ( .) and davy ( .), though they differ from those of biot ( .), de la rive ( .), and others, seem to me to be fully in accordance not merely with the theory i have given of induction generally ( .), but with all the known _facts_ of common induction, conduction, and electrolytic discharge; and in that respect help to confirm in my mind the truth of the theory set forth. the new mode of discharge which electrolyzation presents must surely be an evidence of the _action of contiguous particles_; and as this appears to depend directly upon a previous inductive state, which is the same with common induction, it greatly strengthens the argument which refers induction in all cases to an action of contiguous particles also ( , &c.). . as an illustration of the condition of the polarized particles in a dielectric under induction, i may describe an experiment. put into a glass vessel some clear rectified oil of turpentine, and introduce two wires passing through glass tubes where they coincide with the surface of the fluid, and terminating either in balls or points. cut some very clean dry white silk into small particles, and put these also into the liquid: then electrify one of the wires by an ordinary machine and discharge by the other. the silk will immediately gather from all parts of the liquid, and form a band of particles reaching from wire to wire, and if touched by a glass rod will show considerable tenacity; yet the moment the supply of electricity ceases, the band will fall away and disappear by the dispersion of its parts. the _conduction_ by the silk is in this case very small; and after the best examination i could give to the effects, the impression on my mind is, that the adhesion of the whole is due to the polarity which each filament acquires, exactly as the particles of iron between the poles of a horse-shoe magnet are held together in one mass by a similar disposition of forces. the particles of silk therefore represent to me the condition of the molecules of the dielectric itself, which i assume to be polar, just as that of the silk is. in all cases of conductive discharge the contiguous polarized particles of the body are able to effect a neutralization of their forces with greater or less facility, as the silk does also in a very slight degree. further we are not able to carry the parallel, except in imagination; but if we could divide each particle of silk into two halves, and let each half travel until it met and united with the next half in an opposite state, it would then exert its carrying power ( .), and so far represent electrolytic discharge. . admitting that electrolytic discharge is a consequence of previous induction, then how evidently do its numerous cases point to induction in curved lines ( . .), and to the divergence or lateral action of the lines of inductive force ( .), and so strengthen that part of the general argument in the former paper! if two balls of platina, forming the electrodes of a voltaic battery, are put into a large vessel of dilute sulphuric acid, the whole of the surfaces are covered with the respective gases in beautifully regulated proportions, and the mind has no difficulty in conceiving the direction of the curved lines of discharge, and even the intensity of force of the different lines, by the quantity of gas evolved upon the different parts of the surface. from this condition of the lines of inductive force arise the general effects of diffusion; the appearance of the anions or cathions round the edges and on the further side of the electrodes when in the form of plates; and the manner in which the current or discharge will follow all the forms of the electrolyte, however contorted. hence, also, the effects which nobili has so well examined and described[a] in his papers on the distribution of currents in conducting masses. all these effects indicate the curved direction of the currents or discharges which occur in and through the dielectrics, and these are in every case _preceded_ by equivalent inductive actions of the contiguous particles. [a] bibliothèque universelle, , lix. . . . hence also the advantage, when the exciting forces are weak or require assistance, of enlarging the mass of the electrolyte; of increasing the size of the electrodes; of making the coppers surround the zincs:--all is in harmony with the view of induction which i am endeavouring to examine; i do not perceive as yet one fact against it. . there are many points of _electrolytic discharge_ which ultimately will require to be very closely considered, though i can but slightly touch upon them. it is not that, as far as i have investigated them, they present any contradiction to the view taken (for i have carefully, though unsuccessfully, sought for such cases), but simply want of time as yet to pursue the inquiry, which prevents me from entering upon them here. . one point is, that different electrolytes or dielectrics require different initial intensities for their decomposition ( .). this may depend upon the degree of polarization which the particles require before electrolytic discharge commences. it is in direct relation to the chemical affinity of the substances concerned; and will probably be found to have a relation or analogy to the specific inductive capacity of different bodies ( . .). it thus promises to assist in causing the great truths of those extensive sciences, which are occupied in considering the forces of the particles of matter, to fall into much closer order and arrangement than they have heretofore presented. . another point is the facilitation of electrolytic conducting power or discharge by the addition of substances to the dielectric employed. this effect is strikingly shown where water is the body whose qualities are improved, but, as yet, no general law governing all the phenomena has been detected. thus some acids, as the sulphuric, phosphoric, oxalic, and nitric, increase the power of water enormously; whilst others, as the tartaric and citric acids, give but little power; and others, again, as the acetic and boracic acids, do not produce a change sensible to the voltameter ( .). ammonia produces no effect, but its carbonate does. the caustic alkalies and their carbonates produce a fair effect. sulphate of soda, nitre ( .), and many soluble salts produce much effect. percyanide of mercury and corrosive sublimate produce no effect; nor does iodine, gum, or sugar, the test being a voltameter. in many cases the added substance is acted on either directly or indirectly, and then the phenomena are more complicated; such substances are muriatic acid ( .), the soluble protochlorides ( .), and iodides ( .), nitric acid ( .), &c. in other cases the substance added is not, when alone, subject to or a conductor of the powers of the voltaic battery, and yet both gives and receives power when associated with water. m. de la rive has pointed this result out in sulphurous acid[a], iodine and bromine[b]; the chloride of arsenic produces the same effect. a far more striking case, however, is presented by that very influential body sulphuric acid ( .): and probably phosphoric acid also is in the same peculiar relation. [a] quarterly journal, xxvii. . or bibliothèque universelle, xl. . kemp says sulphurous acid is a very good conductor, quarterly journal, , p. . [b] quarterly journal, xxiv, . or annales de chimie, xxxv. . . it would seem in the cases of those bodies which suffer no change themselves, as sulphuric acid (and perhaps in all), that they affect water in its conducting power only as an electrolyte; for whether little or much improved, the decomposition is proportionate to the quantity of electricity passing ( . .), and the transfer is therefore due to electrolytic discharge. this is in accordance with the fact already stated as regards water ( .), that the conducting power is not improved for electricity of force below the electrolytic intensity of the substance acting as the dielectric; but both facts (and some others) are against the opinion which i formerly gave, that the power of salts, &c. might depend upon their assumption of the liquid state by solution in the water employed ( .). it occurs to me that the effect may perhaps be related to, and have its explanation in differences of specific inductive capacities. . i have described in the last paper, cases, where shell-lac was rendered a conductor by absorption of ammonia ( .). the same effect happens with muriatic acid; yet both these substances, when gaseous, are non-conductors; and the ammonia, also when in strong solution ( .). mr. harris has mentioned instances[a] in which the conducting power of metals is seriously altered by a very little alloy. these may have no relation to the former cases, but nevertheless should not be overlooked in the general investigation which the whole question requires. [a] philosophical transactions, , p. . . nothing is perhaps more striking in that class of dielectrics which we call electrolytes, than the extraordinary and almost complete suspension of their peculiar mode of effecting discharge when they are rendered _solid_ ( , &c.), even though the intensity of the induction acting through them may be increased a hundredfold or more ( .). it not only establishes a very general relation between the physical properties of these bodies and electricity acting by induction through them, but draws both their physical and chemical relations so near together, as to make us hope we shall shortly arrive at the full comprehension of the influence they mutually possess over each other. ¶ ix. _disruptive discharge and insulation._ . the next form of discharge has been distinguished by the adjective _disruptive_ ( .), as it in every case displaces more or less the particles amongst and across which it suddenly breaks. i include under it, discharge in the form of sparks, brushes, and glow ( .), but exclude the cases of currents of air, fluids, &c., which, though frequently accompanying the former, are essentially distinct in their nature. . the conditions requisite for the production of an electric spark in its simplest form are well-known. an insulating dielectric must be interposed between two conducting surfaces in opposite states of electricity, and then if the actions be continually increased in strength, or otherwise favoured, either by exalting the electric state of the two conductors, or bringing them nearer to each other, or diminishing the density of the dielectric, a _spark_ at last appears, and the two forces are for the time annihilated, for _discharge_ has occurred. . the conductors (which may be considered as the termini of the inductive action) are in ordinary cases most generally metals, whilst the dielectrics usually employed are common air and glass. in my view of induction, however, every dielectric becomes of importance, for as the results are considered essentially dependent on these bodies, it was to be expected that differences of action never before suspected would be evident upon close examination, and so at once give fresh confirmation of the theory, and open new doors of discovery into the extensive and varied fields of our science. this hope was especially entertained with respect to the gases, because of their high degree of insulation, their uniformity in physical condition, and great difference in chemical properties. . all the effects prior to the discharge are inductive; and the degree of tension which it is necessary to attain before the spark passes is therefore, in the examination i am now making of the new view of induction, a very important point. it is the limit of the influence which the dielectric exerts in resisting discharge; it is a measure, consequently, of the conservative power of the dielectric, which in its turn may be considered as becoming a measure, and therefore a representative of the intensity of the electric forces in activity. . many philosophers have examined the circumstances of this limiting action in air, but, as far as i know, none have come near mr. harris as to the accuracy with, and the extent to, which he has carried on his investigations[a]. some of his results i must very briefly notice, premising that they are all obtained with the use of air as the _dielectric_ between the conducting surfaces. [a] philosophical transactions, , p. . . first as to the _distance_ between the two balls used, or in other words, the _thickness_ of the dielectric across which the induction was sustained. the quantity of electricity, measured by a unit jar, or otherwise on the same principle with the unit jar, in the charged or inductive ball, necessary to produce spark discharge, was found to vary exactly with the distance between the balls, or between the discharging points, and that under very varied and exact forms of experiment[a]. [a] philosophical transactions, , p. . . then with respect to variation in the _pressure_ or _density_ of the air. the quantities of electricity required to produce discharge across a _constant_ interval varied exactly with variations of the density; the quantity of electricity and density of the air being in the same simple ratio. or, if the quantity was retained the same, whilst the interval and density of the air were varied, then these were found in the inverse simple ratio of each other, the same quantity passing across twice the distance with air rarefied to one-half[a]. [a] philosophical transactions, , p. . . it must be remembered that these effects take place without any variation of the _inductive_ force by condensation or rarefaction of the air. that force remains the same in air[a], and in all gases ( . .), whatever their rarefaction may be. [a] philosophical transactions, , p. , . . variation of the _temperature_ of the air produced no variation of the quantity of electricity required to cause discharge across a given interval[a]. [a] philosophical transactions, , p. such are the general results, which i have occasion for at present, obtained by mr. harris, and they appear to me to be unexceptionable. . in the theory of induction founded upon a molecular action of the dielectric, we have to look to the state of that body principally for the cause and determination of the above effects. whilst the induction continues, it is assumed that the particles of the dielectric are in a certain polarized state, the tension of this state rising higher in each particle as the induction is raised to a higher degree, either by approximation of the inducing surfaces, variation of form, increase of the original force, or other means; until at last, the tension of the particles having reached the utmost degree which they can sustain without subversion of the whole arrangement, discharge immediately after takes place. . the theory does not assume, however, that _all_ the particles of the dielectric subject to the inductive action are affected to the same amount, or acquire the same tension. what has been called the lateral action of the lines of inductive force ( . .), and the diverging and occasionally curved form of these lines, is against such a notion. the idea is, that any section taken through the dielectric across the lines of inductive force, and including _all of them,_ would be equal, in the sum of the forces, to the sum of the forces in any other section; and that, therefore, the whole amount of tension for each such section would be the same. . discharge probably occurs, not when all the particles have attained to a certain degree of tension, but when that particle which is most affected has been exalted to the subverting or turning point ( .). for though _all_ the particles in the line of induction resist charge, and are associated in their actions so as to give a sum of resisting force, yet when any one is brought up to the overturning point, _all_ must give way in the case of a spark between ball and ball. the breaking down of that one must of necessity cause the whole barrier to be overturned, for it was at its utmost degree of resistance when it possessed the aiding power of that one particle, in addition to the power of the rest, and the power of that one is now lost. hence _tension_ or _intensity_[a] may, according to the theory, be considered as represented by the particular condition of the particles, or the amount in them of forced variation from their normal state ( . .). [a] see harris on proposed particular meaning of these terms, philosophical transactions, , p. . . the whole effect produced by a charged conductor on a distant conductor, insulated or not, is by my theory assumed to be due to an action propagated from particle to particle of the intervening and insulating dielectric, all the particles being considered as thrown for the time into a forced condition, from which they endeavour to return to their normal or natural state. the theory, therefore, seems to supply an easy explanation of the influence of _distance_ in affecting induction ( . .). as the distance is diminished induction increases; for there are then fewer particles in the line of inductive force to oppose their united resistance to the assumption of the forced or polarized state, and _vice versa._ again, as the distance diminishes, discharge across happens with a lower charge of electricity; for if, as in harris's experiments ( ), the interval be diminished to one-half, then half the electricity required to discharge across the first interval is sufficient to strike across the second; and it is evident, also, that at that time there are only half the number of interposed molecules uniting their forces to resist the discharge. . the effect of enlarging the conducting surfaces which are opposed to each other in the act of induction, is, if the electricity be limited in its supply, to lower the intensity of action; and this follows as a very natural consequence from the increased area of the dielectric across which the induction is effected. for by diffusing the inductive action, which at first was exerted through one square inch of sectional area of the dielectric, over two or three square inches of such area, twice or three times the number of molecules of the dielectric are brought into the polarized condition, and employed in sustaining the inductive action, and consequently the tension belonging to the smaller number on which the limited force was originally accumulated, must fall in a proportionate degree. . for the same reason diminishing these opposing surfaces must increase the intensity, and the effect will increase until the surfaces become points. but in this case, the tension of the particles of the dielectric next the points is higher than that of particles midway, because of the lateral action and consequent bulging, as it were, of the lines of inductive force at the middle distance ( .). . the more exalted effects of induction on a point _p_, or any small surface, as the rounded end of a rod, when it is opposed to a large surface, as that of a ball or plate, rather than to another point or end, the distance being in both cases the same, fall into harmonious relation with my theory ( .). for in the latter case, the small surface _p_ is affected only by those particles which are brought into the inductive condition by the equally small surface of the opposed conductor, whereas when that is a ball or plate the lines of inductive force from the latter are concentrated, as it were, upon the end _p_. now though the molecules of the dielectric against the large surface may have a much lower state of tension than those against the corresponding smaller surface, yet they are also far more numerous, and, as the lines of inductive force converge towards a point, are able to communicate to the particles contained in any cross section ( .) nearer the small surface an amount of tension equal to their own, and consequently much higher for each individual particle; so that, at the surface of the smaller conductor, the tension of a particle rises much, and if that conductor were to terminate in a point, the tension would rise to an infinite degree, except that it is limited, as before ( .), by discharge. the nature of the discharge from small surfaces and points under induction will be resumed hereafter ( . &c.) . _rarefaction_ of the air does not alter the _intensity_ of inductive action ( . .); nor is there any reason, as far as i can perceive, why it should. if the quantity of electricity and the distance remain the same, and the air be rarefied one-half, then, though one-half of the particles of the dielectric are removed, the other half assume a double degree of tension in their polarity, and therefore the inductive forces are balanced, and the result remains unaltered as long as the induction and insulation are sustained. but the case of _discharge_ is very different; for as there are only half the number of dielectric particles in the rarefied atmosphere, so these are brought up to the discharging intensity by half the former quantity of electricity; discharge, therefore, ensues, and such a consequence of the theory is in perfect accordance with mr. harris's results ( .). . the _increase_ of electricity required to cause discharge over the same distance, when the pressure of the air or its density is increased, flows in a similar manner, and on the same principle ( .), from the molecular theory. . here i think my view of induction has a decided advantage over others, especially over that which refers the retention of electricity on the surface of conductors in air to the _pressure of the atmosphere_ ( .). the latter is the view which, being adopted by poisson and biot[a], is also, i believe, that generally received; and it associates two such dissimilar things, as the ponderous air and the subtile and even hypothetical fluid or fluids of electricity, by gross mechanical relations; by the bonds of mere static pressure. my theory, on the contrary, sets out at once by connecting the electric forces with the particles of matter; it derives all its proofs, and even its origin in the first instance, from experiment; and then, without any further assumption, seems to offer at once a full explanation of these and many other singular, peculiar, and, i think, heretofore unconnected effects. [a] encyclopædia britannica, supplement, vol. iv. article electricity, pp. , . &c. . an important assisting experimental argument may here be adduced, derived from the difference of specific inductive capacity of different dielectrics ( . . .). consider an insulated sphere electrified positively and placed in the centre of another and larger sphere uninsulated, a uniform dielectric, as air, intervening. the case is really that of my apparatus ( .), and also, in effect, that of any ball electrified in a room and removed to some distance from irregularly-formed conductors. whilst things remain in this state the electricity is distributed (so to speak) uniformly over the surface of the electrified sphere. but introduce such a dielectric as sulphur or lac, into the space between the two conductors on one side only, or opposite one part of the inner sphere, and immediately the electricity on the latter is diffused unequally ( . . .), although the form of the conducting surfaces, their distances, and the _pressure_ of the atmosphere remain perfectly unchanged. . fusinieri took a different view from that of poisson, biot, and others, of the reason why rarefaction of air caused easy diffusion of electricity. he considered the effect as due to the removal of the _obstacle_ which the air presented to the expansion of the substances from which the electricity passed[a]. but platina balls show the phenomena _in vacuo_ as well as volatile metals and other substances; besides which, when the rarefaction is very considerable, the electricity passes with scarcely any resistance, and the production of no sensible heat; so that i think fusinieri's view of the matter is likely to gain but few assents. [a] bib. univ. , xlviii. . . i have no need to remark upon the discharging or collecting power of flame or hot air. i believe, with harris, that the mere heat does nothing ( .), the rarefaction only being influential. the effect of rarefaction has been already considered generally ( .); and that caused by the heat of a burning light, with the pointed form of the wick, and the carrying power of the carbonaceous particles which for the time are associated with it, are fully sufficient to account for all the effects. . we have now arrived at the important question, how will the inductive tension requisite for insulation and disruptive discharge be sustained in gases, which, having the same physical state and also the _same pressure_ and the _same temperature_ as _air_, differ from it in specific gravity, in chemical qualities, and it may be in peculiar relations, which not being as yet recognized, are purely electrical ( .)? . into this question i can enter now only as far as is essential for the present argument, namely, that insulation and inductive tension do not depend merely upon the charged conductors employed, but also, and essentially, upon the interposed dielectric, in consequence of the molecular action of its particles ( .). . a glass vessel _a_ (fig. .)[a] was ground at the top and bottom so as to be closed by two ground brass plates, _b_ and _c_; _b_ carried a stuffing-box, with a sliding rod _d_ terminated by a brass ball _s_ below, and a ring above. the lower plate was connected with a foot, stop-cock, and socket, _e_, _f_ and _g_; and also with a brass ball _l_, which by means of a stem attached to it and entering the socket _g_, could be fixed at various heights. the metallic parts of this apparatus were not varnished, but the glass was well-covered with a coat of shell-lac previously dissolved in alcohol. on exhausting the vessel at the air-pump it could be filled with any other gas than air, and, in such cases, the gas so passed in was dried whilst entering by fused chloride of calcium. [a] the drawing is to a scale of / . . the other part of the apparatus consisted of two insulating pillars, _h_ and _i_, to which were fixed two brass balls, and through these passed two sliding rods, _k_ and _m_, terminated at each end by brass balls; _n_ is the end of an insulated conductor, which could be rendered either positive or negative from an electrical machine; _o_ and _p_ are wires connecting it with the two parts previously described, and _q_ is a wire which, connecting the two opposite sides of the collateral arrangements, also communicates with a good discharging train _r_ ( .). . it is evident that the discharge from the machine electricity may pass either between _s_ and _l_, or s and l. the regulation adopted in the first experiments was to keep _s_ and _l_ with their distance _unchanged_, but to introduce first one gas and then another into the vessel _a_, and then balance the discharge at the one place against that at the other; for by making the interval at _a_ sufficiently small, all the discharge would pass there, or making it sufficiently large it would all occur at the interval _v_ in the receiver. on principle it seemed evident, that in this way the varying interval _u_ might be taken as a measure, or rather indication of the resistance to discharge through the gas at the constant interval _v_. the following are the constant dimensions. ball _s_ . of an inch. ball s . of an inch. ball _l_ . of an inch. ball l . of an inch. interval _v_ . of an inch. . on proceeding to experiment it was found that when air or any gas was in the receiver _a_, the interval _u_ was not a fixed one; it might be altered through a certain range of distance, and yet sparks pass either there or at _v_ in the receiver. the extremes were therefore noted, i.e. the greatest distance short of that at which the discharge _always_ took place at _v_ in the gas, and the least distance short of that at which it _always_ took place at _u_ in the air. thus, with air in the receiver, the extremes at _u_ were . and . of an inch, the range of . between these distances including intervals at which sparks passed occasionally either at one place or the other. . the small balls _s_ and s could be rendered either positive or negative from the machine, and as gases were expected and were found to differ from each other in relation to this change ( .), the results obtained under these differences of charge were also noted. . the following is a table of results; the gas named is that in the vessel _a_. the smallest, greatest, and mean interval at _u_ in air is expressed in parts of an inch, the interval _v_ being constantly . of an inch. smallest. greatest. mean. _ | air, _s_ and s, pos. . . . |_air, _s_ and s, neg. . . . _ | oxygen, _s_ and s, pos. . . . |_oxygen, _s_ and s, neg. . . . _ | nitrogen, _s_ and s, pos. . . . |_nitrogen, _s_ and s, neg. . . . _ | hydrogen, _s_ and s, pos. . . . |_hydrogen, _s_ and s, neg. . . . _ | carbonic acid, _s_ and s, pos. . . . |_carbonic acid, _s_ and s, neg. . . . _ | olefiant gas, _s_ and s, pos. . . . |_olefiant gas, _s_ and s, neg. . . . _ | coal gas, _s_ and s, pos. . . . |_coal gas, _s_ and s, neg. . . . _ | muriatic acid gas, _s_ and s, pos. . . . |_muriatic acid gas, _s_ and s, neg. . . . . the above results were all obtained at one time. on other occasions other experiments were made, which gave generally the same results as to order, though not as to numbers. thus: hydrogen, _s_ and s, pos. . . . carbonic acid, _s_ and s, pos. . . . olefiant gas, _s_ and s, pos. . . . i did not notice the difference of the barometer on the days of experiment[a]. [a] similar experiments in different gases are described at . .--_dec. ._ . one would have expected only two distances, one for each interval, for which the discharge might happen either at one or the other; and that the least alteration of either would immediately cause one to predominate constantly over the other. but that under common circumstances is not the case. with air in the receiver, the variation amounted to . of an inch nearly on the smaller interval of . , and with muriatic acid gas, the variation was above . on the smaller interval of . . why is it that when a fixed interval (the one in the receiver) will pass a spark that cannot go across . of air at one time, it will immediately after, and apparently under exactly similar circumstances, not pass a spark that can go across . of air? . it is probable that part of this variation will be traced to particles of dust in the air drawn into and about the circuit ( .). i believe also that part depends upon a variable charged condition of the surface of the glass vessel _a_. that the whole of the effect is not traceable to the influence of circumstances in the vessel _a_, may be deduced from the fact, that when sparks occur between balls in free air they frequently are not straight, and often pass otherwise than by the shortest distance. these variations in air itself, and at different parts of the very same balls, show the presence and influence of circumstances which are calculated to produce effects of the kind now under consideration. . when a spark had passed at either interval, then, generally, more tended to appear at the _same_ interval, as if a preparation had been made for the passing of the latter sparks. so also on continuing to work the machine quickly the sparks generally followed at the same place. this effect is probably due in part to the warmth of the air heated by the preceding spark, in part to dust, and i suspect in part, to something unperceived as yet in the circumstances of discharge. . a very remarkable difference, which is _constant_ in its direction, occurs when the electricity communicated to the balls _s_ and s is changed from positive to negative, or in the contrary direction. it is that the range of variation is always greater when the small bulls are positive than when they are negative. this is exhibited in the following table, drawn from the former experiments. pos. neg. in air the range was . . oxygen . . nitrogen . . hydrogen . . carbonic acid . . olefiant gas . . coal gas . . muriatic acid . . i have no doubt these numbers require considerable correction, but the general result is striking, and the differences in several cases very great. * * * * * . though, in consequence of the variation of the striking distance ( .), the interval in air fails to be a measure, as yet, of the insulating or resisting power of the gas in the vessel, yet we may for present purposes take the mean interval as representing in some degree that power. on examining these mean intervals as they are given in the third column ( .), it will be very evident, that gases, when employed as dielectrics, have peculiar electrical relations to insulation, and therefore to induction, very distinct from such as might be supposed to depend upon their mere physical qualities of specific gravity or pressure. . first, it is clear that at the _same pressure_ they are not alike, the difference being as great as and . when the small balls are charged positively, and with the same surfaces and the same pressure, muriatic acid gas has three times the insulating or restraining power ( .) of hydrogen gas, and nearly twice that of oxygen, nitrogen, or air. . yet it is evident that the difference is not due to specific gravity, for though hydrogen is the lowest, and therefore lower than oxygen, oxygen is much beneath nitrogen, or olefiant gas; and carbonic acid gas, though considerably heavier than olefiant gas or muriatic acid gas, is lower than either. oxygen as a heavy, and olefiant as a light gas, are in strong contrast with each other; and if we may reason of olefiant gas from harris's results with air ( .), then it might be rarefied to two-thirds its usual density, or to a specific gravity of . (hydrogen being ), and having neither the same density nor pressure as oxygen, would have equal insulating powers with it, or equal tendency to resist discharge. . experiments have already been described ( . .) which show that the gases are sensibly alike in their inductive capacity. this result is not in contradiction with the existence of great differences in their restraining power. the same point has been observed already in regard to dense and rare air ( .). . hence arises a new argument proving that it cannot be mere pressure of the atmosphere which prevents or governs discharge ( . .), but a specific electric quality or relation of the gaseous medium. hence also additional argument for the theory of molecular inductive action. . other specific differences amongst the gases may be drawn from the preceding series of experiments, rough and hasty as they are. thus the positive and negative series of mean intervals do not give the same differences. it has been already noticed that the negative numbers are lower than the positive ( .), but, besides that, the _order_ of the positive and negative results is not the same. thus, on comparing the mean numbers (which represent for the present insulating tension,) it appears that in air, hydrogen, carbonic acid, olefiant gas and muriatic acid, the tension rose higher when the smaller ball was made positive than when rendered negative, whilst in oxygen, nitrogen, and coal gas, the reverse was the case. now though the numbers cannot be trusted as exact, and though air, oxygen, and nitrogen should probably be on the same side, yet some of the results, as, for instance, those with muriatic acid, fully show a peculiar relation and difference amongst gases in this respect. this was further proved by making the interval in air . of an inch whilst muriatic acid gas was in the vessel _a_; for on charging the small balls _s_ and s positively, _all_ the discharge took place through the _air_; but on charging them negatively, _all_ the discharge took place through the _muriatic acid gas_. . so also, when the conductor _n_ was connected _only_ with the muriatic acid gas apparatus, it was found that the discharge was more facile when the small ball _s_ was negative than when positive; for in the latter case, much of the electricity passed off as brush discharge through the air from the connecting wire _p_ but in the former case, it all seemed to go through the muriatic acid. . the consideration, however, of positive and negative discharge across air and other gases will be resumed in the further part of this, or in the next paper ( . .). . here for the present i must leave this part of the subject, which had for its object only to observe how far gases agreed or differed as to their power of retaining a charge on bodies acting by induction through them. all the results conspire to show that induction is an action of contiguous molecules ( . &c.); but besides confirming this, the first principle placed for proof in the present inquiry, they greatly assist in developing the specific properties of each gaseous dielectric, at the same time showing that further and extensive experimental investigation is necessary, and holding out the promise of new discovery as the reward of the labour required. * * * * * . when we pass from the consideration of dielectrics like the gases to that of bodies having the liquid and solid condition, then our reasonings in the present state of the subject assume much more of the character of mere supposition. still i do not perceive anything adverse to the theory, in the phenomena which such bodies present. if we take three insulating dielectrics, as air, oil of turpentine, and shell-lac, and use the same balls or conductors at the same intervals in these three substances, increasing the intensity of the induction until discharge take place, we shall find that it must be raised much higher in the fluid than for the gas, and higher still in the solid than for the fluid. nor is this inconsistent with the theory; for with the liquid, though its molecules are free to move almost as easily as those of the gas, there are many more particles introduced into the given interval; and such is also the case when the solid body is employed. besides that with the solid, the cohesive force of the body used will produce some effect; for though the production of the polarized states in the particle of a solid may not be obstructed, but, on the contrary, may in some cases be even favoured ( . .) by its solidity or other circumstances, yet solidity may well exert an influence on the point of final subversion, (just as it prevents discharge in an electrolyte,) and so enable inductive intensity to rise to a much higher degree. . in the cases of solids and liquids too, bodies may, and most probably do, possess specific differences as to their ability of assuming the polarized state, and also as to the extent to which that polarity must rise before discharge occurs. an analogous difference exists in the specific inductive capacities already pointed out in a few substances ( .) in the last paper. such a difference might even account for the various degrees of insulating and conducting power possessed by different bodies, and, if it should be found to exist, would add further strength to the argument in favour of the molecular theory of inductive action. * * * * * . having considered these various cases of sustained insulation in non-conducting dielectrics up to the highest point which they can attain, we find that they terminate at last in _disruptive discharge_; the peculiar condition of the molecules of the dielectric which was necessary to the continuous induction, being equally essential to the occurrence of that effect which closes all the phenomena. this discharge is not only in its appearance and condition different to the former modes by which the lowering of the powers was effected ( . .), but, whilst really the same in principle, varies much from itself in certain characters, and thus presents us with the forms of _spark_, _brush_, and _glow_ ( .). i will first consider _the spark_, limiting it for the present to the case of discharge between two oppositely electrified conducting surfaces. _the electric spark or flash._ . the _spark_ is consequent upon a discharge or lowering of the polarized inductive state of many dielectric particles, by a particular action of a few of the particles occupying a very small and limited space; all the previously polarized particles returning to their first or normal condition in the inverse order in which they left it, and uniting their powers meanwhile to produce, or rather to continue, ( .-- .) the discharge effect in the place where the subversion of force first occurred. my impression is, that the few particles situated where discharge occurs are not merely pushed apart, but assume a peculiar state, a highly exulted condition for the time, i.e. have thrown upon them all the surrounding forces in succession, and rising up to a proportionate intensity of condition, perhaps equal to that of chemically combining atoms, discharge the powers, possibly in the same manner as they do theirs, by some operation at present unknown to us; and so the end of the whole. the ultimate effect is exactly as if a metallic wire had been put into the place of the discharging particles; and it does not seem impossible that the principles of action in both cases, may, hereafter, prove to be the same. . the _path of the spark_, or of the discharge, depends on the degree of tension acquired by the particles in the line of discharge, circumstances, which in every common case are very evident and by the theory easy to understand, rendering it higher in them than in their neighbours, and, by exalting them first to the requisite condition, causing them to determine the course of the discharge. hence the selection of the path, and the solution of the wonder which harris has so well described[a] as existing under the old theory. all is prepared amongst the molecules beforehand, by the prior induction, for the path either of the electric spark or of lightning itself. [a] nautical magazine, , p . . the same difficulty is expressed as a principle by nobili for voltaic electricity, almost in mr. harris's words, namely[a], "electricity directs itself towards the point where it can most easily discharge itself," and the results of this as a principle he has well wrought out for the case of voltaic currents. but the _solution_ of the difficulty, or the proximate cause of the effects, is the same; induction brings the particles up to or towards a certain degree of tension ( .); and by those which first attain it, is the discharge first and most efficiently performed. [a] bibliothèque universelle, , lix. . . the _moment_ of discharge is probably determined by that molecule of the dielectric which, from the circumstances, has its tension most quickly raised up to the maximum intensity. in all cases where the discharge passes from conductor to conductor this molecule must be on the surface of one of them; but when it passes between a conductor and a nonconductor, it is, perhaps, not always so ( .). when this particle has acquired its maximum tension, then the whole barrier of resistance is broken down in the line or lines of inductive action originating at it, and disruptive discharge occurs ( .): and such an inference, drawn as it is from the theory, seems to me in accordance with mr. harris's facts and conclusions respecting the resistance of the atmosphere, namely, that it is not really greater at any one discharging distance than another[a]. [a] philosophical transactions, , pp. , . . it seems probable, that the tension of a particle of the same dielectric, as air, which is requisite to produce discharge, is a _constant quantity_, whatever the shape of the part of the conductor with which it is in contact, whether ball or point; whatever the thickness or depth of dielectric throughout which induction is exerted; perhaps, even, whatever the state, as to rarefaction or condensation of the dielectric; and whatever the nature of the conductor, good or bad, with which the particle is for the moment associated. in saying so much, i do not mean to exclude small differences which may be caused by the reaction of neighbouring particles on the deciding particle, and indeed, it is evident that the intensity required in a particle must be related to the condition of those which are contiguous. but if the expectation should be found to approximate to truth, what a generality of character it presents! and, in the definiteness of the power possessed by a particular molecule, may we not hope to find an immediate relation to the force which, being electrical, is equally definite and constitutes chemical affinity? . theoretically it would seem that, at the moment of discharge by the spark in one line of inductive force, not merely would all the other lines throw their forces into this one ( .), but the lateral effect, equivalent to a repulsion of these lines ( . .), would be relieved and, perhaps, followed by a contrary action, amounting to a collapse or attraction of these parts. having long sought for some transverse force in statical electricity, which should be the equivalent to magnetism or the transverse force of current electricity, and conceiving that it might be connected with the transverse action of the lines of inductive force, already described ( .), i was desirous, by various experiments, of bringing out the effect of such a force, and making it tell upon the phenomena of electro-magnetism and magneto-electricity[a]. [a] see further investigations of this subject, - . - .--_dec. ._ . amongst other results, i expected and sought for the mutual affection, or even the lateral coalition of two similar sparks, if they could be obtained simultaneously side by side, and sufficiently near to each other. for this purpose, two similar leyden jars were supplied with rods of copper projecting from their balls in a horizontal direction, the rods being about . of an inch thick, and rounded at the ends. the jars were placed upon a sheet of tinfoil, and so adjusted that their rods, _a_ and _b_, were near together, in the position represented in plan at fig. : _c_ and _d_ were two brass balls connected by a brass rod and insulated: _e_ was also a brass ball connected, by a wire, with the ground and with the tinfoil upon which the leyden jars were placed. by laying an insulated metal rod across from _a_ to _b_, charging the jars, and removing the rod, both the jars could be brought up to the same intensity of charge ( .). then, making the ball _e_ approach the ball _d_, at the moment the spark passed there, two sparks passed between the rods _n_, _o_, and the ball _c_; and as far as the eye could judge, or the conditions determine, they were simultaneous. . under these circumstances two modes of discharge took place; either each end had its own particular spark to the ball, or else one end only was associated by a spark with the ball, but was at the same time related to the other end by a spark between the two. . when the ball _c_ was about an inch in diameter, the ends _n_ and _o_, about half an inch from it, and about . of an inch from each other, the two sparks to the ball could be obtained. when for the purpose of bringing the sparks nearer together, the ends, _n_ and _o_, were brought closer to each other, then, unless very carefully adjusted, only one end had a spark with the ball, the other having a spark to it; and the least variation of position would cause either _n_ or _o_ to be the end which, giving the direct spark to the ball, was also the one through, or by means of which, the other discharged its electricity. . on making the ball _c_ smaller, i found that then it was needful to make the interval between the ends _n_ and _o_ larger in proportion to the distance between them and the ball _c_. on making _c_ larger, i found i could diminish the interval, and so bring the two simultaneous separate sparks closer together, until, at last, the distance between them was not more at the widest part than . of their whole length. . numerous sparks were then passed and carefully observed. they were very rarely straight, but either curved or bent irregularly. in the average of cases they were, i think, decidedly convex towards each other; perhaps two-thirds presented more or less of this effect, the rest bulging more or less outwards. i was never able, however, to obtain sparks which, separately leaving the ends of the wires _n_ and _o_, conjoined into one spark before they reached or communicated with the ball _c_. at present, therefore, though i think i saw a tendency in the sparks to unite, i cannot assert it as a fact. . but there is one very interesting effect here, analogous to, and it may be in part the same with, that i was searching for: i mean the increased facility of discharge where the spark passes. for instance, in the cases where one end, as _n_, discharged the electricity of both ends to the ball _c_, fig. , the electricity of the other end _o_, had to pass through an interval of air . times as great as that which it might have taken, by its direct passage between the end and the ball itself. in such cases, the eye could not distinguish, even by the use of wheatstone's means[a], that the spark from the end _n_, which contained both portions of electricity, was a double spark. it could not have consisted of two sparks taking separate courses, for such an effect would have been visible to the eye; but it is just possible, that the spark of the first end _n_ and its jar, passing at the smallest interval of time before that of the other _o_ had heated and expanded the air in its course, and made it so much more favourable to discharge, that the electricity of the end _o_ preferred leaping across to it and taking a very circuitous route, rather than the more direct one to the ball. it must, however, be remarked, in answer to this supposition, that the one spark between _d_ and _e_ would, by its influence, tend to produce simultaneous discharges at _n_ and _o_, and certainly did so, when no preponderance was given to one wire over the other, as to the previous inductive effect ( .). [a] philosophical transactions, , pp. , . . the fact, however, is, that disruptive discharge is favourable to itself. it is at the outset a case of tottering equilibrium: and if _time_ be an element in discharge, in however minute a proportion ( .), then the commencement of the act at any point favours its continuance and increase there, and portions of power will be discharged by a course which they would not otherwise have taken. . the mere heating and expansion of the air itself by the first portion of electricity which passes, must have a great influence in producing this result. . as to the result itself, we see its effect in every electric spark; for it is not the whole quantity which passes that determines the discharge, but merely that small portion of force which brings the deciding molecule ( .) up to its maximum tension; then, when its forces are subverted and discharge begins, all the rest passes by the same course, from the influence of the favouring circumstances just referred to; and whether it be the electricity on a square inch, or a thousand square inches of charged glass, the discharge is complete. hereafter we shall find the influence of this effect in the formation of brushes ( .); and it is not impossible that we may trace it producing the jagged spark and the forked lightning. * * * * * . the characters of the electric spark in _different gases_ vary, and the variation _may_ be due simply to the effect of the heat evolved at the moment. but it may also be due to that specific relation of the particles and the electric forces which i have assumed as the basis of a theory of induction; the facts do not oppose such a view; and in that view the variation strengthens the argument for molecular action, as it would seem to show the influence of the latter in every part of the electrical effect ( . .). . the appearances of the sparks in different gases have often been observed and recorded[a], but i think it not out of place to notice briefly the following results; they were obtained with balls of brass, (platina surfaces would have been better,) and at common pressures. in _air_, the sparks have that intense light and bluish colour which are so well known, and often have faint or dark parts in their course, when the quantity of electricity passing is not great. in _nitrogen_, they are very beautiful, having the same general appearance as in air, but have decidedly more colour of a bluish or purple character, and i thought were remarkably sonorous. in _oxygen_, the sparks were whiter than in air or nitrogen, and i think not so brilliant. in _hydrogen_, they had a very fine crimson colour, not due to its rarity, for the character passed away as the atmosphere was rarefied ( .)[b]. very little sound was produced in this gas; but that is a consequence of its physical condition[c]. in _carbonic acid gas_, the colour was similar to that of the spark in air, but with a little green in it: the sparks were remarkably irregular in form, more so than in common air: they could also, under similar circumstances as to size of ball, &c., be obtained much longer than in air, the gas showing a singular readiness to cause the discharge in the form of spark. in _muriatic acid gas_, the spark was nearly white: it was always bright throughout, never presenting those dark parts which happen in air, nitrogen, and some other gases. the gas was dry, and during the whole experiment the surface of the glass globe within remained quite dry and bright. in _coal gas_, the spark was sometimes green, sometimes red, and occasionally one part was green and another red: black parts also occur very suddenly in the line of the spark, i.e. they are not connected by any dull part with bright portions, but the two seem to join directly one with the other. [a] see van marum's description of the teylerian machine, vol. i. p. , and vol. ii. p. ; also ency. britan., vol. vi., article electricity, pp. , . [b] van marum says they are about four times as large in hydrogen as in air. vol. i. p. . [c] leslie. cambridge phil. transactions, . . these varieties of character impress my mind with a feeling, that they are due to a direct relation of the electric powers to the particles of the dielectric through which the discharge occurs, and are not the mere results of a casual ignition or a secondary kind of action of the electricity, upon the particles which it finds in its course and thrusts aside in its passage ( .). . the spark may be obtained in media which are far denser than air, as in oil of turpentine, olive oil, resin, glass, &c.: it may also be obtained in bodies which being denser likewise approximate to the condition of conductors, as spermaceti, water, &c. but in these cases, nothing occurs which, as far as i can perceive, is at all hostile to the general views i have endeavoured to advocate. _the electrical brush._ . the _brush_ is the next form of disruptive discharge which i shall consider. there are many ways of obtaining it, or rather of exalting its characters; and all these ways illustrate the principles upon which it is produced. if an insulated conductor, connected with the positive conductor of an electrical machine, have a metal rod . of an inch in diameter projecting from it outwards from the machine, and terminating by a rounded end or a small ball, it will generally give good brushes; or, if the machine be not in good action, then many ways of assisting the formation of the brush can be resorted to; thus, the hand or any _large_ conducting surface may be approached towards the termination to increase inductive force ( .): or the termination may be smaller and of badly conducting matter, as wood: or sparks may be taken between the prime conductor of the machine and the secondary conductor to which the termination giving brushes belongs: or, which gives to the brushes exceedingly fine characters and great magnitude, the air around the termination may be rarefied more or less, either by heat or the air-pump; the former favourable circumstances being also continued. . the brush when obtained by a powerful machine on a ball about . of an inch in diameter, at the end of a long brass rod attached to the positive prime conductor, had the general appearance as to form represented in fig. : a short conical bright part or root appeared at the middle part of the ball projecting directly from it, which, at a little distance from the ball, broke out suddenly into a wide brush of pale ramifications having a quivering motion, and being accompanied at the same time with a low dull chattering sound. . at first the brush seems continuous, but professor wheatstone has shown that the whole phenomenon consists of successive intermitting discharges[a]. if the eye be passed rapidly, not by a motion of the head, but of the eyeball itself, across the direction of the brush, by first looking steadfastly about ° or ° above, and then instantly as much below it, the general brush will be resolved into a number of individual brushes, standing in a row upon the line which the eye passed over; each elementary brush being the result of a single discharge, and the space between them representing both the time during which the eye was passing over that space, and that which elapsed between one discharge and another. [a] philosophical transactions, , p. . . the single brushes could easily be separated to eight or ten times their own width, but were not at the same time extended, i.e. they did not become more indefinite in shape, but, on the contrary, less so, each being more distinct in form, ramification, and character, because of its separation from the others, in its effects upon the eye. each, therefore, was instantaneous in its existence ( .). each had the conical root complete ( .). . on using a smaller ball, the general brush was smaller, and the sound, though weaker, more continuous. on resolving the brush into its elementary parts, as before, these were found to occur at much shorter intervals of time than in the former case, but still the discharge was intermitting. . employing a wire with a round end, the brush was still smaller, but, as before, separable into successive discharges. the sound, though feebler, was higher in pitch, being a distinct musical note. . the sound is, in fact, due to the recurrence of the noise of each separate discharge, and these, happening at intervals nearly equal under ordinary circumstances, cause a definite note to be heard, which, rising in pitch with the increased rapidity and regularity of the intermitting discharges, gives a ready and accurate measure of the intervals, and so may be used in any case when the discharge is heard, even though the appearances may not be seen, to determine the element of _time_. so when, by bringing the hand towards a projecting rod or ball, the pitch of the tone produced by a brushy discharge increases, the effect informs us that we have increased the induction ( .), and by that means increased the rapidity of the alternations of charge and discharge. . by using wires with finer terminations, smaller brushes were obtained, until they could hardly be distinguished as brushes; but as long as _sound_ was heard, the discharge could be ascertained by the eye to be intermitting; and when the sound ceased, the light became _continuous_ as a glow ( . . - .). . to those not accustomed to use the eye in the manner i have described, or, in cases where the recurrence is too quick for any unassisted eye, the beautiful revolving mirror of professor wheatstone[a] will be useful for such developments of condition as those mentioned above. another excellent process is to produce the brush or other luminous phenomenon on the end of a rod held in the hand opposite to a charged positive or negative conductor, and then move the rod rapidly from side to side whilst the eye remains still. the successive discharges occur of course in different places, and the state of things before, at, and after a single coruscation or brush can be exceedingly well separated. [a] philosophical transactions, , pp. , . . the _brush_ is in reality a discharge between a bad or a non-conductor and either a conductor or another non-conductor. under common circumstances, the brush is a discharge between a conductor and air, and i conceive it to take place in something like the following manner. when the end of an electrified rod projects into the middle of a room, induction takes place between it and the walls of the room, across the dielectric, air; and the lines of inductive force accumulate upon the end in greater quantity than elsewhere, or the particles of air at the end of the rod are more highly polarized than those at any other part of the rod, for the reasons already given ( .). the particles of air situated in sections across these lines of force are least polarized in the sections towards the walls and most polarized in those nearer to the end of the wires ( .): thus, it may well happen, that a particle at the end of the wire is at a tension that will immediately terminate in discharge, whilst in those even only a few inches off, the tension is still beneath that point. but suppose the rod to be charged positively, a particle of air a, fig. , next it, being polarized, and having of course its negative force directed towards the rod and its positive force outwards; the instant that discharge takes place between the positive force of the particle of the rod opposite the air and the negative force of the particle of air towards the rod, the whole particle of air becomes positively electrified; and when, the next instant, the discharged part of the rod resumes its positive state by conduction from the surface of metal behind, it not only acts on the particles beyond a, by throwing a into a polarized state again, but a itself, because of its charged state, exerts a distinct inductive act towards these further particles, and the tension is consequently so much exalted between a and b, that discharge takes place there also, as well as again between the metal and a. . in addition to this effect, it has been shown, that, the act of discharge having once commenced, the whole operation, like a case of unstable equilibrium, is hastened to a conclusion ( . .), the rest of the act being facilitated in its occurrence, and other electricity than that which caused the first necessary tension hurrying to the spot. when, therefore, disruptive discharge has once commenced at the root of a brush, the electric force which has been accumulating in the conductor attached to the rod, finds a more ready discharge there than elsewhere, and will at once follow the course marked out as it were for it, thus leaving the conductor in a partially discharged state, and the air about the end of the wire in a charged condition; and the time necessary for restoring the full charge of the conductor, and the dispersion of the charged air in a greater or smaller degree, by the joint forces of repulsion from the conductor and attraction towards the walls of the room, to which its inductive action is directed, is just that time which forms the interval between brush and brush ( . . . .). . the words of this description are long, but there is nothing in the act or the forces on which it depends to prevent the discharge being _instantaneous_, as far as we can estimate and measure it. the consideration of _time_ is, however, important in several points of view ( .), and in reference to disruptive discharge, it seemed from theory far more probable that it might be detected in a brush than in a spark; for in a brush, the particles in the line through which the discharge passes are in very different states as to intensity, and the discharge is already complete in its act at the root of the brush, before the particles at the extremity of the ramifications have yet attained their maximum intensity. . i consider _brush_ discharge as probably a successive effect in this way. discharge begins at the root ( . .), and, extending itself in succession to all parts of the single brush, continues to go on at the root and the previously formed parts until the whole brush is complete; then, by the fall in intensity and power at the conductor, it ceases at once in all parts, to be renewed, when that power has risen again to a sufficient degree. but in a _spark_, the particles in the line of discharge being, from the circumstances, nearly alike in their intensity of polarization, suffer discharge so nearly at the same moment as to make the time quite insensible to us. . mr. wheatstone has already made experiments which fully illustrate this point. he found that the brush generally had a sensible duration, but that with his highest capabilities he could not detect any such effect in the spark[a]. i repeated his experiment on the brush, though with more imperfect means, to ascertain whether i could distinguish a longer duration in the stem or root of the brush than in the extremities, and the appearances were such as to make me think an effect of this kind was produced. [a] philosophical transactions, , pp. , . . that the discharge breaks into several ramifications, and by them passes through portions of air alike, or nearly alike, as to polarization and the degree of tension the particles there have acquired, is a very natural result of the previous state of things, and rather to be expected than that the discharge should continue to go straight out into space in a single line amongst those particles which, being at a distance from the end of the rod, are in a lower state of tension than those which are near: and whilst we cannot but conclude, that those parts where the branches of a single brush appear, are more favourably circumstanced for discharge than the darker parts between the ramifications, we may also conclude, that in those parts where the light of concomitant discharge is equal, there the circumstances are nearly equal also. the single successive brushes are by no means of the same particular shape even when they are observed without displacement of the rod or surrounding objects ( . .), and the successive discharges may be considered as taking place into the mass of air around, through different roads at each brush, according as minute circumstances, such as dust, &c. ( . .), may have favoured the course by one set of particles rather than another. . brush discharge does not essentially require any current of the medium in which the brush appears: the current almost always occurs, but is a consequence of the brush, and will be considered hereafter ( - .). on holding a blunt point positively charged towards uninsulated water, a star or glow appeared on the point, a current of air passed from it, and the surface of the water was depressed; but on bringing the point so near that sonorous brushes passed, then the current of air instantly ceased, and the surface of the water became level. . the discharge by a brush is not to all the particles of air that are near the electrified conductor from which the brush issues; only those parts where the ramifications pass are electrified: the air in the central dark parts between them receives no charge, and, in fact, at the time of discharge, has its electric and inductive tension considerably lowered. for consider fig. to represent a single positive brush;--the induction before the discharge is from the end of the rod outwards, in diverging lines towards the distant conductors, as the walls of the room, &c., and a particle at _a_ has polarity of a certain degree of tension, and tends with a certain force to become charged; but at the moment of discharge, the air in the ramifications _b_ and _d_, acquiring also a positive state, opposes its influence to that of the positive conductor on _a_, and the tension of the particle at _a_ is therefore diminished rather than increased. the charged particles at _b_ and _d_ are now inductive bodies, but their lines of inductive action are still outwards towards the walls of the room; the direction of the polarity and the tendency of other particles to charge from these, being governed by, or in conformity with, these lines of force. . the particles that are charged are probably very highly charged, but, the medium being a non-conductor, they cannot communicate that state to their neighbours. they travel, therefore, under the influence of the repulsive and attractive forces, from the charged conductor towards the nearest uninsulated conductor, or the nearest body in a different state to themselves, just as charged particles of dust would travel, and are then discharged; each particle acting, in its course, as a centre of inductive force upon any bodies near which it may come. the travelling of these charged particles when they are numerous, causes wind and currents, but these will come into consideration under _carrying discharge_ ( . . &c.). . when air is said to be electrified, and it frequently assumes this state near electrical machines, it consists, according to my view, of a mixture of electrified and unelectrified particles, the latter being in very large proportion to the former. when we gather electricity from air, by a flame or by wires, it is either by the actual discharge of these particles, or by effects dependent on their inductive action, a case of either kind being produceable at pleasure. that the law of equality between the two forces or forms of force in inductive action is as strictly preserved in these as in other cases, is fully shown by the fact, formerly stated ( . .), that, however strongly air in a vessel might be charged positively, there was an exactly equal amount of negative force on the inner surface of the vessel itself, for no residual portion of either the one or the other electricity could be obtained. . i have nowhere said, nor does it follow, that the air is charged only where the luminous brush appears. the charging may extend beyond those parts which are visible, i.e. particles to the right or left of the lines of light may receive electricity, the parts which are luminous being so only because much electricity is passing by them to other parts ( .); just as in a spark discharge the light is greater as more electricity passes, though it has no necessary relation to the quantity required to commence discharge ( . .). hence the form we see in a brush may by no means represent the whole quantity of air electrified; for an invisible portion, clothing the visible form to a certain depth, may, at the same time, receive its charge ( .). . several effects which i have met with in muriatic acid gas tend to make me believe, that that gaseous body allows of a dark discharge. at the same time, it is quite clear from theory, that in some gases, the reverse of this may occur, i.e. that the charging of the air may not extend even so far as the light. we do not know as yet enough of the electric light to be able to state on what it depends, and it is very possible that, when electricity bursts forth into air, all the particles of which are in a state of tension, light may be evolved by such as, being very near to, are not of, those which actually receive a charge at the time. . the further a brush extends in a gas, the further no doubt is the charge or discharge carried forward; but this may vary between different gases, and yet the intensity required for the first moment of discharge not vary in the same, but in some other proportion. thus with respect to nitrogen and muriatic acid gases, the former, as far as my experiments have proceeded, produces far finer and larger brushes than the latter ( . .), but the intensity required to commence discharge is much higher for the muriatic acid than the nitrogen ( .). here again, therefore, as in many other qualities, specific differences are presented by different gaseous dielectrics, and so prove the special relation of the latter to the act and the phenomena of induction. . to sum up these considerations respecting the character and condition of the brush, i may state that it is a spark to air; a diffusion of electric force to matter, not by conduction, but disruptive discharge, a dilute spark which, passing to very badly conducting matter, frequently discharges but a small portion of the power stored up in the conductor; for as the air charged reacts on the conductor, whilst the conductor, by loss of electricity, sinks in its force ( .), the discharge quickly ceases, until by the dispersion of the charged air and the renewal of the excited conditions of the conductor, circumstances have risen up to their first effective condition, again to cause discharge, and again to fall and rise, . the brush and spark gradually pass into one another, making a small ball positive by a good electrical machine with a large prime conductor, and approaching a large uninsulated discharging ball towards it, very beautiful variations from the spark to the brush may be obtained. the drawings of long and powerful sparks, given by van marum[a], harris[b], and others, also indicate the same phenomena. as far as i have observed, whenever the spark has been brushy in air of common pressures, the whole of the electricity has not been discharged, but only portions of it, more or less according to circumstances; whereas, whenever the effect has been a distinct spark throughout the whole of its course, the discharge has been perfect, provided no interruption had been made to it elsewhere, in the discharging circuit, than where the spark occurred. [a] description of the teylerian machine, vol. i. pp. . .; vol. ii. p. , &c. [b] philosophical transactions, , p. . . when an electrical brush from an inch to six inches in length or more is issuing into free air, it has the form given, fig. . but if the hand, a ball, of any knobbed conductor be brought near, the extremities of the coruscations turn towards it and each other, and the whole assumes various forms according to circumstances, as in figs. , , and . the influence of the circumstances in each case is easily traced, and i might describe it here, but that i should be ashamed to occupy the time of the society in things so evident. but how beautifully does the curvature of the ramifications illustrate the curved form of the lines of inductive force existing previous to the discharge! for the former are consequences of the latter, and take their course, in each discharge, where the previous inductive tension had been raised to the proper degree. they represent these curves just as well as iron filings represent magnetic curves, the visible effects in both cases being the consequences of the action of the forces in _the places where_ the effects appear. the phenomena, therefore, constitute additional and powerful testimony ( . .) to that already given in favour both of induction through dielectrics in curved lines ( .), and of the lateral relation of these lines, by an effect equivalent to a repulsion producing divergence, or, as in the cases figured, the bulging form. . in reference to the theory of molecular inductive action, i may also add, the proof deducible from the long brushy ramifying spark which, may be obtained between a small ball on the positive conductor of an electrical machine, and a larger one at a distance ( . .). what a fine illustration that spark affords of the previous condition of _all_ the particles of the dielectric between the surfaces of discharge, and how unlike the appearances are to any which would be deduced from the theory which assumes inductive action to be action at a distance, in straight lines only; and charge, as being electricity retained upon the surface of conductors by the mere pressure of the atmosphere! * * * * * . when the brush is obtained in rarefied air, the appearances vary greatly, according to circumstances, and are exceedingly beautiful. sometimes a brush may be formed of only six or seven branches, these being broad and highly luminous, of a purple colour, and in some parts an inch or more apart: by a spark discharge at the prime conductor ( .) single brushes may be obtained at pleasure. discharge in the form of a brush is favoured by rarefaction of the air, in the same manner and for the same reason as discharge in the form of a spark ( .); but in every case there is previous induction and charge through the dielectric, and polarity of its particles ( .), the induction being, as in any other instance, alternately raised by the machine and lowered by the discharge. in certain experiments the rarefaction was increased to the utmost degree, and the opposed conducting surfaces brought as near together as possible without producing glow ( .): the brushes then contracted in their lateral dimensions, and recurred so rapidly as to form an apparently continuous arc of light from metal to metal. still the discharge could be observed to intermit ( .), so that even under these high conditions, induction preceded each single brush, and the tense polarized condition of the contiguous particles was a necessary preparation for the discharge itself. . the brush form of disruptive discharge may be obtained not only in air and gases, but also in much denser media. i procured it in _oil of turpentine_ from the end of a wire going through a glass tube into the fluid contained in a metal vessel. the brush was small and very difficult to obtain; the ramifications were simple, and stretched out from each other, diverging very much. the light was exceedingly feeble, a perfectly dark room being required for its observation. when a few solid particles, as of dust or silk, were in the liquid, the brush was produced with much greater facility. . the running together or coalescence of different lines of discharge ( .) is very beautifully shown in the brush in air. this point may present a little difficulty to those who are not accustomed to see in every discharge an equal exertion of power in opposite directions, a positive brush being considered by such (perhaps in consequence of the common phrase _direction of a current_) as indicating a breaking forth in different directions of the original force, rather than a tendency to convergence and union in one line of passage. but the ordinary case of the brush may be compared, for its illustration, with that in which, by holding the knuckle opposite to highly excited glass, a discharge occurs, the ramifications of a brush then leading from the glass and converging into a spark on the knuckle. though a difficult experiment to make, it is possible to obtain discharge between highly excited shell-lac and the excited glass of a machine: when the discharge passes, it is, from the nature of the charged bodies, brush at each end and spark in the middle, beautifully illustrating that tendency of discharge to facilitate like action, which i have described in a former page ( .). . the brush has _specific characters_ in different gases, indicating a relation to the particles of these bodies even in a stronger degree than the spark ( . .). this effect is in strong contrast with the non-variation caused by the use of different substances as _conductors_ from which the brushes are to originate. thus, using such bodies as wood, card, charcoal, nitre, citric acid, oxalic acid, oxide of lead, chloride of lead, carbonate of potassa, potassa fusa, strong solution of potash, oil of vitriol, sulphur, sulphuret of antimony, and hæmatite, no variation in the character of the brushes was obtained, except that (dependent upon their effect as better or worse conductors) of causing discharge with more or less readiness and quickness from the machine[a]. [a] exception must, of course, be made of those cases where the root of the brush, becoming a spark, causes a little diffusion or even decomposition of the matter there, and so gains more or less of a particular colour at that part. . the following are a few of the effects i observed in different gasses at the positively charged surfaces, and with atmospheres varying in their pressure. the general effect of rarefaction was the same for all the gases: at first, sparks passed; these gradually were converted into brushes, which became larger and more distinct in their ramifications, until, upon further rarefaction, the latter began to collapse and draw in upon each other, till they formed a stream across from conductor to conductor: then a few lateral streams shot out towards the glass of the vessel from the conductors; these became thick and soft in appearance, and were succeeded by the full constant glow which covered the discharging wire. the phenomena varied with the size of the vessel ( .), the degree of rarefaction, and the discharge of electricity from the machine. when the latter was in successive sparks, they were most beautiful, the effect of a spark from a small machine being equal to, and often surpassing, that produced by the _constant_ discharge of a far more powerful one. . _air._--fine positive brushes are easily obtained in air at common pressures, and possess the well-known purplish light. when the air is rarefied, the ramifications are very long, filling the globe ( .); the light is greatly increased, and is of a beautiful purple colour, with an occasional rose tint in it. . _oxygen._--at common pressures, the brush is very close and compressed, and of a dull whitish colour. in rarefied oxygen, the form and appearance are better, the colour somewhat purplish, but all the characters very poor compared to those in air. . _nitrogen_ gives brushes with great facility at the positive surface, far beyond any other gas i have tried: they are almost always fine in form, light, and colour, and in rarefied nitrogen, are magnificent. they surpass the discharges in any other gas as to the quantity of light evolved. . _hydrogen_, at common pressures, gave a better brush than oxygen, but did not equal nitrogen; the colour was greenish gray. in rarefied hydrogen, the ramifications were very fine in form and distinctness, but pale in colour, with a soft and velvety appearance, and not at all equal to those in nitrogen. in the rarest state of the gas, the colour of the light was a pale gray green. . _coal gas._--the brushes were rather difficult to produce, the contrast with nitrogen being great in this respect. they were short and strong, generally of a greenish colour, and possessing much of the spark character: for, occurring on both the positive and negative terminations, often when there was a dark interval of some length between the two brushes, still the quick, sharp sound of the spark was produced, as if the discharge had been sudden through this gas, and partaking, in that respect, of the character of a spark. in rare coal gas, the brush forms were better, but the light very poor and the colour gray. . _carbonic acid gas_ produces a very poor brush at common pressures, as regards either size, light, or colour; and this is probably connected with the tendency which this gas has to discharge the electricity as a spark ( .). in rarefied carbonic acid, the brush is better in form, but weak as to light, being of a dull greenish or purplish line, varying with the pressure and other circumstances. . _muriatic acid gas._--it is very difficult to obtain the brush in this gas at common pressures. on gradually increasing the distance of the rounded ends, the sparks suddenly ceased when the interval was about an inch, and the discharge, which was still through the gas in the globe, was silent and dark. occasionally a very short brush could for a few moments be obtained, but it quickly disappeared. even when the intermitting spark current ( .) from the machine was used, still i could only with difficulty obtain a brush, and that very short, though i used rods with rounded terminations (about . of an inch in diameter) which had before given them most freely in air and nitrogen. during the time of this difficulty with the muriatic gas, magnificent brushes were passing off from different parts of the machine into the surrounding air. on rarefying the gas, the formation of the brush was facilitated, but it was generally of a low squat form, very poor in light, and very similar on both the positive and negative surfaces. on rarefying the gas still more, a few large ramifications were obtained of a pale bluish colour, utterly unlike those in nitrogen. * * * * * . in all the gases, the different forms of disruptive discharge may be linked together and gradually traced from one extreme to the other, i.e. from the spark to the glow ( . .), or, it may be, to a still further condition to be called dark discharge ( - .); but it is, nevertheless, very surprising to see what a specific character each keeps whilst under the predominance of the general law. thus, in muriatic acid, the brush is very difficult to obtain, and there comes in its place almost a dark discharge, partaking of the readiness of the spark action. moreover, in muriatic acid, i have _never_ observed the spark with any dark interval in it. in nitrogen, the spark readily changes its character into that of brush. in carbonic acid gas, there seems to be a facility to occasion spark discharge, whilst yet that gas is unlike nitrogen in the facility of the latter to form brushes, and unlike muriatic acid in its own facility to continue the spark. these differences add further force, first to the observations already made respecting the spark in various gases ( . .), and then, to the proofs deducible from it, of the relation of the electrical forces to the particles of matter. . the peculiar characters of nitrogen in relation to the electric discharge ( . .) must, evidently, have an important influence over the form and even the occurrence of lightning. being that gas which most readily produces coruscations, and, by them, extends discharge to a greater distance than any other gas tried, it is also that which constitutes four-fifths of our atmosphere; and as, in atmospheric electrical phenomena, one, and sometimes both the inductive forces are resident on the particles of the air, which, though probably affected as to conducting power by the aqueous particles in it, cannot be considered as a good conductor; so the peculiar power possessed by nitrogen, to originate and effect discharge in the form of a brush or of ramifications, has, probably, an important relation to its electrical service in nature, as it most seriously affects the character and condition of the discharge when made. the whole subject of discharge from and through gases is of great interest, and, if only in reference to atmospheric electricity, deserves extensive and close experimental investigation. _difference of discharge at the positive and negative conducting surfaces._ . i have avoided speaking of this well-known phenomenon more than was quite necessary, that i might bring together here what i have to say on the subject. when the brush discharge is observed in air at the positive and negative surfaces, there is a very remarkable difference, the true and full comprehension of which would, no doubt, be of the utmost importance to the physics of electricity; it would throw great light on our present subject, i.e. the molecular action of dielectrics under induction, and its consequences; and seems very open to, and accessible by, experimental inquiry. . the difference in question used to be expressed in former times by saying, that a point charged positively gave brushes into the air, whilst the same point charged negatively gave a star. this is true only of bad conductors, or of metallic conductors charged intermittingly, or otherwise controlled by collateral induction. if metallic points project _freely_ into the air, the positive and negative light upon them differ very little in appearance, and the difference can be observed only upon close examination. . the effect varies exceedingly under different circumstances, but, as we must set out from some position, may perhaps be stated thus: if a metallic wire with a rounded termination in free air be used to produce the brushy discharge, then the brushes obtained when the wire is charged negatively are very poor and small, by comparison with those produced when the charge is positive. or if a large metal ball connected with the electrical machine be charged _positively_, and a fine uninsulated point be gradually brought towards it, a star appears on the point when at a considerable distance, which, though it becomes brighter, does not change its form of a star until it is close up to the ball: whereas, if the ball be charged negatively, the point at a considerable distance has a star on it as before; but when brought nearer, (in my case to the distance of - / inch,) a brush formed on it, extending to the negative ball; and when still nearer, (at / of an inch distance,) the brush ceased, and bright sparks passed. these variations, i believe, include the whole series of differences, and they seem to show at once, that the negative surface tends to retain its discharging character unchanged, whilst the positive surface, under similar circumstances, permits of great variation. . there are several points in the character of the negative discharge to air which it is important to observe. a metal rod, . of an inch in diameter, with a rounded end projecting into the air, was charged negatively, and gave a short noisy brush (fig. .). it was ascertained both by sight ( . .) and sound ( .), that the successive discharges were very rapid in their recurrence, being seven or eight times more numerous in the same period, than those produced when the rod was charged positively to an equal degree. when the rod was positive, it was easy, by working the machine a little quicker, to replace the brush by a glow ( . .), but when it was negative no efforts could produce this change. even by bringing the hand opposite the wire, the only effect was to increase the number of brush discharges in a given period, raising at the same time the sound to a higher pitch. . a point opposite the negative brush exhibited a star, and as it was approximated caused the size and sound of the negative brush to diminish, and, at last, to cease, leaving the negative end silent and dark, yet effective as to discharge. . when the round end of a smaller wire (fig. .) was advanced towards the negative brush, it (becoming positive by induction) exhibited the quiet glow at inches distance, the negative brush continuing. when nearer, the pitch of the sound of the negative brush rose, indicating quicker intermittences ( .); still nearer, the positive end threw off ramifications and distinct brushes; at the same time, the negative brush contracted in its lateral directions and collected together, giving a peculiar narrow longish brush, in shape like a hair pencil, the two brushes existing at once, but very different in their form and appearance, and especially in the more rapid recurrence of the negative discharges than of the positive. on using a smaller positive wire for the same experiment, the glow first appeared on it, and then the brush, the negative brush being affected at the same time; and the two at one distance became exceedingly alike in appearance, and the sounds, i thought, were in unison; at all events they were in harmony, so that the intermissions of discharge were either isochronous, or a simple ratio existed between the intervals. with a higher action of the machine, the wires being retained unaltered, the negative surface became dark and silent, and a glow appeared on the positive one. a still higher action changed the latter into a spark. finer positive wires gave other variations of these effects, the description of which i must not allow myself to go into here. . a thinner rod was now connected with the negative conductor in place of the larger one ( .), its termination being gradually diminished to a blunt point, as in fig. ; and it was beautiful to observe that, notwithstanding the variation of the brush, the same general order of effects was produced. the end gave a small sonorous negative brush, which the approach of the hand or a large conducting surface did not alter, until it was so near as to produce a spark. a fine point opposite to it was luminous at a distance; being nearer it did not destroy the light and sound of the negative brush, but only tended to have a brush produced on itself, which, at a still less distance, passed into a spark joining the two surfaces. . when the distinct negative and positive brushes are produced simultaneously in relation to each other in air, the former almost always has a contracted form, as in fig. , very much indeed resembling the figure which the positive brush itself has when influenced by the lateral vicinity of positive parts acting by induction. thus a brush issuing from a point in the re-entering angle of a positive conductor has the same compressed form (fig. .). . the character of the negative brush is not affected by the chemical nature of the substances of the conductors ( .), but only by their possession of the conducting power in a greater or smaller degree. . rarefaction of common air about a negative ball or blunt point facilitated the development of the negative brush, the effect being, i think, greater than on a positive brush, though great on both. extensive ramifications could be obtained from a ball or end electrified negatively to the plate of the air-pump on which the jar containing it stood. . a very important variation of the relative forms and conditions of the positive and negative brush takes place on varying the dielectric in which they are produced. the difference is so very great that it points to a specific relation of this form of discharge to the particular gas in which it takes place, and opposes the idea that gases are but obstructions to the discharge, acting one like another and merely in proportion to their pressure ( .). . in _air_, the superiority of the positive brush is well known ( . .). in _nitrogen_, it is as great or even greater than in air ( .). in _hydrogen_, the positive brush loses a part of its superiority, not being so good as in nitrogen or air; whilst the negative brush does not seem injured ( .). in _oxygen_, the positive brush is compressed and poor ( ); whilst the negative did not become less: the two were so alike that the eye frequently could not tell one from the other, and this similarity continued when the oxygen was gradually rarefied. in _coal gas_, the brushes are difficult of production as compared to nitrogen ( .), and the positive not much superior to the negative in its character, either at common or low pressures. in _carbonic acid gas_, this approximation of character also occurred. in _muriatic acid gas_, the positive brush was very little better than the negative, and both difficult to produce ( .) as compared with the facility in nitrogen or air. . these experiments were made with rods of brass about a quarter of an inch thick having rounded ends, these being opposed in a glass globe inches in diameter, containing the gas to be experimented with. the electric machine was used to communicate directly, sometimes the positive, and sometimes the negative state, to the rod in connection with it. . thus we see that, notwithstanding there is a general difference in favour of the superiority of the positive brush over the negative, that difference is at its maximum in nitrogen and air; whilst in carbonic acid, muriatic acid, coal gas, and oxygen, it diminishes, and at last almost disappears. so that in this particular effect, as in all others yet examined, the evidence is in favour of that view which refers the results to a direct relation of the electric forces with the molecules of the matter concerned in the action ( . . .). even when special phenomena arise under the operation of the general law, the theory adopted seems fully competent to meet the case. . before i proceed further in tracing the probable cause of the difference between the positive and negative brush discharge, i wish to know the results of a few experiments which are in course of preparation: and thinking this series of researches long enough, i shall here close it with the expectation of being able in a few weeks to renew the inquiry, and entirely redeem my pledge ( .). _royal institution, dec. rd, ._ thirteenth series. § . _on induction (continued)._ ¶ ix. _disruptive discharge (continued)--peculiarities of positive and negative discharge either as spark or brush--glow discharge--dark discharge._ ¶ x. _convection, or carrying discharge._ ¶ xi. _relation of a vacuum to electrical phenomena._ § . _nature of the electrical current._ received february ,--read march , . ¶ ix. _disruptive discharge (continued)._ . let us now direct our attention to the general difference of the positive and negative disruptive discharge, with the object of tracing, as far as possible, the cause of that difference, and whether it depends on the charged conductors principally, or on the interposed dielectric; and as it appears to be great in air and nitrogen ( .), let us observe the phenomena in air first. . the general case is best understood by a reference to surfaces of considerable size rather than to points, which involve (as a secondary effect) the formation of currents ( ). my investigation, therefore, was carried on with balls and terminations of different diameters, and the following are some of the principal results. . if two balls of very different dimensions, as for instance one-half an inch, and the other three inches in diameter, be arranged at the ends of rods so that either can be electrified by a machine and made to discharge by sparks to the other, which is at the same time uninsulated; then, as is well known, far longer sparks are obtained when the small ball is positive and the large ball negative, than when the small ball is negative and the large ball positive. in the former case, the sparks are or inches in length; in the latter, an inch or an inch and a half only. * * * * * . but previous to the description of further experiments, i will mention two words, for which with many others i am indebted to a friend, and which i think it would be expedient to introduce and use. it is important in ordinary inductive action, to distinguish at which charged surface the induction originates and is sustained: i.e. if two or more metallic balls, or other masses of matter, are in inductive relation, to express which are charged originally, and which are brought by them into the opposite electrical condition. i propose to call those bodies which are originally charged, _inductric_ bodies; and those which assume the opposite state, in consequence of the induction, _inducteous_ bodies. this distinction is not needful because there is any difference between the sums of the _inductric_ and the _inducteous_ forces; but principally because, when a ball a is inductric, it not merely brings a ball b, which is opposite to it, into an inducteous state, but also many other surrounding conductors, though some of them may be a considerable distance off, and the consequence is, that the balls do not bear the same precise relation to each other when, first one, and then the other, is made the inductric ball; though, in each case, the _same ball_ be made to assume the _same state._ , another liberty which i may also occasionally take in language i will explain and limit. it is that of calling a particular spark or brush, _positive_ or _negative_, according as it may be considered as _originating_ at a positive or a negative surface. we speak of the brush as positive or negative when it shoots out from surfaces previously in those states; and the experiments of mr. wheatstone go to prove that it _really begins_ at the charged surface, and from thence extends into the air ( . .) or other dielectric. according to my view, _sparks_ also originate or are determined at one particular spot ( .), namely, that where the tension first rises up to the maximum degree; and when this can be determined, as in the simultaneous use of large and small balls, in which case the discharge begins or is determined by the latter, i would call that discharge which passes _at once_, a positive spark, if it was at the positive surface that the maximum intensity was first obtained; or a negative spark, if that necessary intensity was first obtained at the negative surface. * * * * * . an apparatus was arranged, as in fig. . (plate viii.): a and b were brass balls of very different diameters attached to metal rods, moving through sockets on insulating pillars, so that the distance between the balls could be varied at pleasure. the large ball a, inches in diameter, was connected with an insulated brass conductor, which could be rendered positive or negative directly from a cylinder machine: the small ball b, . of an inch in diameter, was connected with a discharging train ( .) and perfectly uninsulated. the brass rods sustaining the balls were . of an inch in thickness. . when the large ball was _positive_ and inductric ( .), negative sparks occurred until the interval was . of an inch; then mixed brush and spark between that and . ; and from . and upwards, negative brush alone. when the large ball was made _negative_ and inductric, then positive spark alone occurred until the interval was as great as . inches; spark and brush from that up to . ; and to have the positive brush alone, it required an interval of at least . inches. . the balls a and b were now changed for each other. then making the small ball b inductric _positively_, the positive sparks alone continued only up to . ; spark and brush occurred from . up to . ; and positive brush alone from . and upwards. rendering the small ball b inductric and _negative_, negative sparks alone occurred up to . ; then spark and brush at . ; whilst from . and upwards the noisy negative brush alone took place. . we thus find a great difference as the balls are rendered inductric or inducteous; the small ball rendered _positive_ inducteously giving a spark nearly twice as long as that produced when it was charged positive inductrically, and a corresponding difference, though not, under the circumstances, to the same extent, was manifest, when it was rendered _negative_[a]. [a] for similar experiments on different gases, see .--_dec. ._ . other results are, that the small ball rendered positive gives a much longer spark than when it is rendered negative, and that the small ball rendered negative gives a brush more readily than when positive, in relation to the effect produced by increasing the distance between the two balls. . when the interval was below . of an inch, so that the small ball should give sparks, whether positive or negative, i could not observe that there was any constant difference, either in their ready occurrence or the number which passed in a given time. but when the interval was such that the small ball when negative gave a brush, then the discharges from it, as separate negative brushes, were far more numerous than the corresponding discharges from it when rendered positive, whether those positive discharges were as sparks or brushes. . it is, therefore, evident that, when a ball is discharging electricity in the form of brushes, the brushes are far more numerous, and each contains or carries off far less electric force when the electricity so discharged is negative, than when it is positive. . in all such experiments as those described, the point of change from spark to brush is very much governed by the working state of the electrical machine and the size of the conductor connected with the discharging ball. if the machine be in strong action and the conductor large, so that much power is accumulated quickly for each discharge, then the interval is greater at which the sparks are replaced by brushes; but the general effect is the same[a]. [a] for similar experiments in different gases, see - .--_dec. ._ . these results, though indicative of very striking and peculiar relations of the electric force or forces, do not show the relative degrees of charge which the small ball acquires before discharge occurs, i.e. they do not tell whether it acquires a higher condition in the negative, or in the positive state, immediately preceding that discharge. to illustrate this important point i arranged two places of discharge as represented, fig . a and d are brass balls inches diameter, b and c are smaller brass balls . of an inch in diameter; the forks l and r supporting them were of brass wire . of an inch in diameter; the space between the large and small ball on the same fork was inches, that the two places of discharge _n_ and _o_ might be sufficiently removed from each other's influence. the fork l was connected with a projecting cylindrical conductor, which could be rendered positive or negative at pleasure, by an electrical machine, and the fork r was attached to another conductor, but thrown into an uninsulated state by connection with a discharging train ( .). the two intervals or places of discharge _n_ and _o_ could be varied at pleasure, their extent being measured by the occasional introduction of a diagonal scale. it is evident, that, as the balls a and b connected with the same conductor are always charged at once, and that discharge may take place to either of the balls connected with the discharging train, the intervals of discharge _n_ and _o_ may be properly compared to each other, as respects the influence of large and small balls when charged positively and negatively in air. . when the intervals _n_ and _o_ were each made = . of an inch, and the balls a and b inductric _positively_, the discharge was all at _n_ from the small ball of the conductor to the large ball of the discharging train, and mostly by positive brush, though once by a spark. when the balls a and b were made inductric _negatively_, the discharge was still from the same small ball, at _n_, by a constant negative brush. . i diminished the intervals _n_ and _o_ to . of an inch. when a and b were inductric _positively_, all the discharge was at _n_ as a positive brush: when a and b were inductric _negatively_, still all the discharge was at _n_, as a negative brush. . the facility of discharge at the positive and negative small balls, therefore, did not appear to be very different. if a difference had existed, there were always two small balls, one in each state, that the discharge might happen at that most favourable to the effect. the only difference was, that one was in the inductric, and the other in the inducteous state, but whichsoever happened for the time to be in that state, whether positive or negative, had the advantage. . to counteract this interfering influence, i made the interval _n_ = . and interval _o_ = . of an inch. then, when the balls a and b were _inductric positive_, the discharge was about equal at both intervals. when, on the other hand, the balls a and b were inductric _negative_, there was discharge, still at both, but most at _n_, as if the small ball _negative_ could discharge a little easier than the same ball _positive_. . the small balls and terminations used in these and similar experiments may very correctly be compared, in their action, to the same balls and ends when electrified in free air at a much greater distance from conductors, than they were in those cases from each other. in the first place, the discharge, even when as a spark, is, according to my view, determined, and, so to speak, begins at a spot on the surface of the small ball ( .), occurring when the intensity there has risen up to a certain maximum degree ( .); this determination of discharge at a particular spot first, being easily traced from the spark into the brush, by increasing the distance, so as, at last, even to render the time evident which is necessary for the production of the effect ( . .). in the next place, the large balls which i have used might be replaced by larger balls at a still greater distance, and so, by successive degrees, may be considered as passing into the sides of the rooms; these being under general circumstances the inducteous bodies, whilst the small ball rendered either positive or negative is the inductric body. . but, as has long been recognised, the small ball is only a blunt end, and, electrically speaking, a point only a small ball; so that when a point or blunt end is throwing out its brushes into the air, it is acting exactly as the small balls have acted in the experiments already described, and by virtue of the same properties and relations. . it may very properly be said with respect to the experiments, that the large negative ball is as essential to the discharge as the small positive ball, and also that the large negative ball shows as much superiority over the large positive ball (which is inefficient in causing a spark from its opposed small negative ball) as the small positive ball does over the small negative ball; and probably when we understand the real cause of the difference, and refer it rather to the condition of the particles of the dielectric than to the sizes of the conducting balls, we may find much importance in such an observation. but for the present, and whilst engaged in investigating the point, we may admit, what is the fact, that the forces are of higher intensity at the surfaces of the smaller balls than at those of the larger ( . .); that the former, therefore, determine the discharge, by first rising up to that exalted condition which is necessary for it; and that, whether brought to this condition by induction towards the walls of a room or the large balls i have used, these may fairly be compared one with the other in their influence and actions. . the conclusions i arrive at are: first, that when two equal small conducting surfaces equally placed in air are electrified, one positively and the other negatively, that which is negative can discharge to the air at a tension a little lower than that required for the positive ball: second, that when discharge does take place, much more passes at each time from the positive than from the negative surface ( .). the last conclusion is very abundantly proved by the optical analysis of the positive and negative brushes already described ( .), the latter set of discharges being found to recur five or six times oftener than the former[a]. [a] a very excellent mode of examining the relation of small positive and negative surfaces would be by the use of drops of gum water, solutions, or other liquids. see onwards ( . .). . if, now, a small ball be made to give brushes or brushy sparks by a powerful machine, we can, in some measure, understand and relate the difference perceived when it is rendered positive or negative. it is known to give when positive a much larger and more powerful spark than when negative, and with greater facility ( .): in fact, the spark, although it takes away so much more electricity at once, commences at a tension higher only in a small degree, if at all. on the other hand, if rendered negative, though discharge may commence at a lower degree, it continues but for a very short period, very little electricity passing away each time. these circumstances are directly related; for the extent to which the positive spark can reach, and the size and extent of the positive brush, are consequences of the capability which exists of much electricity passing off at one discharge from the positive surface ( . .). . but to refer these effects only to the form and size of the conductor, would, according to my notion of induction, be a very imperfect mode of viewing the whole question ( . .). i apprehend that the effects are due altogether to the mode in which the particles of the interposed dielectric polarize, and i have already given some experimental indications of the differences presented by different electrics in this respect ( . .). the modes of polarization, as i shall have occasion hereafter to show, may be very diverse in different dielectrics. with respect to common air, what seems to be the consequence of a superiority in the positive force at the surface of the small ball, may be due to the more exalted condition of the negative polarity of the particles of air, or of the nitrogen in it (the negative part being, perhaps, more compressed, whilst the positive part is more diffuse, or _vice versa_ ( . &c.)); for such a condition could determine certain effects at the positive ball which would not take place to the same degree at the negative ball, just as well as if the positive ball had possessed some special and independent power of its own. . the opinion, that the effects are more likely to be dependent upon the dielectric than the ball, is supported by the character of the two discharges. if a small positive ball be throwing off brushes with ramifications ten inches long, how can the ball affect that part of a ramification which is five inches from it? yet the portion beyond that place has the same character as that preceding it, and no doubt has that character impressed by the same general principle and law. looking upon the action of the contiguous particles of a dielectric as fully proved, i see, in such a ramification, a propagation of discharge from particle to particle, each doing for the one next it what was done for it by the preceding particle, and what was done for the first particle by the charged metal against which it was situated. . with respect to the general condition and relations of the positive and negative brushes in dense or rare air, or in other media and gases, if they are produced at different times and places they are of course independent of each other. but when they are produced from opposed ends or balls at the same time, in the same vessel of gas ( . .), they are frequently related; and circumstances may be so arranged that they shall be isochronous, occurring in equal numbers in equal times; or shall occur in multiples, i.e. with two or three negatives to one positive; or shall alternate, or be quite irregular. all these variations i have witnessed; and when it is considered that the air in the vessel, and also the glass of the vessel, can take a momentary charge, it is easy to comprehend their general nature and cause. * * * * * . similar experiments to those in air ( . .) were made in different gases, the results of which i will describe as briefly as possible. the apparatus is represented fig. , consisting of a bell-glass eleven inches in diameter at the widest part, and ten and a half inches high up to the bottom of the neck. the balls are lettered, as in fig. , and are in the same relation to each other; but a and b were on separate sliding wires, which, however, were generally joined by a cross wire, _w_, above, and that connected with the brass conductor, which received its positive or negative charge from the machine. the rods of a and b were graduated at the part moving through the stuffing-box, so that the application of a diagonal scale applied there, told what was the distance between these balls and those beneath them. as to the position of the balls in the jar, and their relation to each other, c and d were three and a quarter inches apart, their height above the pump plate five inches, and the distance between any of the balls and the glass of the jar one inch and three quarters at least, and generally more. the balls a and d were two inches in diameter, as before ( .); the balls b and c only . of an inch in diameter. another apparatus was occasionally used in connection with that just described, being an open discharger (fig. .), by which a comparison of the discharge in air and that in gases could be obtained. the balls e and f, each . of an inch in diameter, were connected with sliding rods and other balls, and were insulated. when used for comparison, the brass conductor was associated at the same time with the balls a and b of figure and ball e of this apparatus (fig. .); whilst the balls c, d and f were connected with the discharging train. . i will first tabulate the results as to the _restraining power_ of the gases over discharge. the balls a and c (fig. .) were thrown out of action by distance, and the effects at b and d, or the interval _n_ in the gas, compared with those at the interval _p_ in the air, between e and f (fig. .). the table sufficiently explains itself. it will be understood that all discharge was in the air, when the interval there was less than that expressed in the first or third columns of figures; and all the discharge in the gas, when the interval in air was greater than that in the second or fourth column of figures. at intermediate distances the discharge was occasionally at both places, i.e. sometimes in the air, sometimes in the gas. _____________________________________________________________________ | | | | | interval _p_ in parts of an inch | |_________________|___________________________________________________| | | | | | | when the small ball b | when the small ball b | | constant inter- | was inductric and | was inductric and | | val _n_ between | _positive_ the | _negative_ the | | b and d = | discharge was all | discharge was all | | inch | at _p_ in at _n_ in | at _p_ in at _n_ in | | | air before the gas | air before the gas | | | after | after | |_________________|_________________________|_________________________| | | _p_ = | _p_ = | _p_ = | _p_ = | |in air | . | . | . | . | |in nitrogen | . | . | . | . | |in oxygen | . | . | . | . | |in hydrogen | . | . | . | . | |in coal gas | . | . | . | . | |in carbonic acid | . | . | . | . | |_________________|____________|____________|____________|____________| . these results are the same generally, as far as they go, as those of the like nature in the last series ( .), and confirm the conclusion that different gases restrain discharge in very different proportions. they are probably not so good as the former ones, for the glass jar not being varnished, acted irregularly, sometimes taking a certain degree of charge as a non-conductor, and at other times acting as a conductor in the conveyance and derangement of that charge. another cause of difference in the ratios is, no doubt, the relative sizes of the discharge balls in air; in the former case they were of very different size, here they were alike. . in future experiments intended to have the character of accuracy, the influence of these circumstances ought to be ascertained, and, above all things, the gases themselves ought to be contained in vessels of metal, and not of glass. * * * * * . the next set of results are those obtained when the intervals _n_ and _o_ (fig. .) were made equal to each other, and relate to the greater facility of discharge at the small ball, when rendered positive or negative ( .). . in _air_, with the intervals = . of an inch, a and b being inductric and positive, discharge was nearly equal at _n_ and _o_; when a and b were inductric and negative, the discharge was mostly at _n_ by negative brush. when the intervals were = . of an inch, with a and b inductric positively, all discharge was at _n_ by positive brush; with a and b inductric negatively, all the discharge was at _n_ by a negative brush. it is doubtful, therefore, from these results, whether the negative ball has any greater facility than the positive. . _nitrogen._--intervals _n_ and _o_ = . of an inch: a, b inductric positive, discharge at both intervals, most at _n_, by positive sparks; a, b inductric negative, discharge equal at _n_ and _o_. the intervals made = . of an inch: a, b inductric positive, discharge all at _n_ by positive brush; a, b inductric negative, discharge most at _o_ by positive brush. in this gas, therefore, though the difference is not decisive, it would seem that the positive small ball caused the most ready discharge. . _oxygen._--intervals _n_ and _o_ = . of an inch: a, b inductric positive, discharge nearly equal; inductric negative, discharge mostly at _n_ by negative brush. made the intervals = . of an inch: a, b inductric positive, discharge both at _n_ and _o_; inductric negative, discharge all at _o_ by negative brush. so here the negative small ball seems to give the most ready discharge. . _hydrogen._--intervals _n_ and _o_ = . of an inch: a, b inductric positive, discharge nearly equal: inductric negative, discharge mostly at _o_. intervals = . of an inch: a and b inductric positive, discharge mostly at _n_, as positive brush; inductric negative, discharge mostly at _o_, as positive brush. here the positive discharge seems most facile. . _coal gas._--_n_ and _o_ = . of an inch: a, b inductric positive, discharge nearly all at _o_ by negative spark: a, b inductric negative, discharge nearly all at _n_ by negative spark. intervals = . of an inch, and a, b inductric positive, discharge mostly at _o_ by negative brush: a, b inductric negative, discharge all at _n_ by negative brush. here the negative discharge most facile. . _carbonic acid gas._--_n_ and _o_ = . of an inch: a, b inductric positive, discharge nearly all at _o_, or negative: a, b inductric negative, discharge nearly all at _n_, or negative. intervals = . of an inch: a, b inductric positive, discharge mostly at _o_, or negative. a, b inductric negative, discharge all at _n_, or negative. in this case the negative had a decided advantage in facility of discharge. . thus, if we may trust this form of experiment, the negative small ball has a decided advantage in facilitating disruptive discharge over the positive small ball in some gases, as in carbonic acid gas and coal gas ( .), whilst in others that conclusion seems more doubtful; and in others, again, there seems a probability that the positive small ball may be superior. all these results were obtained at very nearly the same pressure of the atmosphere. * * * * * . i made some experiments in these gases whilst in the air jar (fig. .), as to the change from spark to brush, analogous to those in the open air already described ( . .). i will give, in a table, the results as to when brush began to appear mingled with the spark; but the after results were so varied, and the nature of the discharge in different gases so different, that to insert the results obtained without further investigation, would be of little use. at intervals less than those expressed the discharge was always by spark. _______________________________________________________________________ | | | | | | discharge between | discharge between | | | balls b and d. | balls a and c. | | |___________________________|___________________________| | | | | | | | | small ball | small ball | large ball | large ball | | | b inductric | b inductric | a inductric | a inductric | | | _pos_. | _neg_. | _pos_. | _neg_. | |_______________|_____________|_____________|_____________|_____________| | | | | | | | air | . | . | . | . | | nitrogen | . | . | . | . | | oxygen | . | . | . | . | | hydrogen | . | . | | | | coal gas | . | . | . | . | | carbonic acid | . | . | . | {above . ;| | | | | | had not | | | | | | space.) | |_______________|_____________|_____________|_____________|_____________| . it is to be understood that sparks occurred at much higher intervals than these; the table only expresses that distance beneath which all discharge was as spark. some curious relations of the different gases to discharge are already discernible, but it would be useless to consider them until illustrated by further experiments. * * * * * . i ought not to omit noticing here, that professor belli of milan has published a very valuable set of experiments on the relative dissipation of positive and negative electricity in the air[a]; he finds the latter far more ready, in this respect, than the former. [a] bibliothèque universelle, , september, p. . . i made some experiments of a similar kind, but with sustained high charges; the results were less striking than those of signore belli, and i did not consider them as satisfactory. i may be allowed to mention, in connexion with the subject, an interfering effect which embarrassed me for a long time. when i threw positive electricity from a given point into the air, a certain intensity was indicated by an electrometer on the conductor connected with the point, but as the operation continued this intensity rose several degrees; then making the conductor negative with the same point attached to it, and all other things remaining the same, a certain degree of tension was observed in the first instance, which also gradually rose as the operation proceeded. returning the conductor to the positive state, the tension was at first low, but rose as before; and so also when again made negative. . this result appeared to indicate that the point which had been giving off one electricity, was, by that, more fitted for a short time to give off the other. but on closer examination i found the whole depended upon the inductive reaction of that air, which being charged by the point, and gradually increasing in quantity before it, as the positive or negative issue was continued, diverted and removed a part of the inductive action of the surrounding wall, and thus apparently affected the powers of the point, whilst really it was the dielectric itself that was causing the change of tension. * * * * * . the results connected with the different conditions of positive and negative discharge will have a far greater influence on the philosophy of electrical science than we at present imagine, especially if, as i believe, they depend on the peculiarity and degree of polarized condition which the molecules of the dielectrics concerned acquire ( . .). thus, for instance, the relation of our atmosphere and the earth within it, to the occurrence of spark or brush, must be especial and not accidental ( .). it would not else consist with other meteorological phenomena, also of course dependent on the special properties of the air, and which being themselves in harmony the most perfect with the functions of animal and vegetable life, are yet restricted in their actions, not by loose regulations, but by laws the most precise. . even in the passage through air of the voltaic current we see the peculiarities of positive and negative discharge at the two charcoal points; and if these discharges are made to take place simultaneously to mercury, the distinction is still more remarkable, both as to the sound and the quantity of vapour produced. . it seems very possible that the remarkable difference recently observed and described by my friend professor daniell[a], namely, that when a zinc and a copper ball, the same in size, were placed respectively in copper and zinc spheres, also the same in size, and excited by electrolytes or dielectrics of the same strength and nature, the zinc ball far surpassed the zinc sphere in action, may also be connected with these phenomena; for it is not difficult to conceive how the polarity of the particles shall be affected by the circumstance of the positive surface, namely the zinc, being the larger or the smaller of the two inclosing the electrolyte. it is even possible, that with different electrolytes or dielectrics the ratio may be considerably varied, or in some cases even inverted. [a] philosophical transactions, , p. . * * * * * _glow discharge._ . that form of disruptive discharge which appears as a _glow_ ( . .), is very peculiar and beautiful: it seems to depend on a quick and almost continuous charging of the air close to, and in contact with, the conductor. . _diminution of the charging surface_ will produce it. thus, when a rod . of an inch in diameter, with a rounded termination, was rendered positive in free air, it gave fine brushes from the extremity, but occasionally these disappeared, and a quiet phosphorescent continuous glow took their place, covering the whole of the end of the wire, and extending a very small distance from the metal into the air. with a rod . of an inch in diameter the glow was more readily produced. with still smaller rods, and also with blunt conical points, it occurred still more readily; and with a fine point i could not obtain the brush in free air, but only this glow. the positive glow and the positive star are, in fact, the same. . _increase of power in the machine_ tends to produce the glow; for rounded terminations which will give only brushes when the machine is in weak action, will readily give the glow when it is in good order. . _rarefaction of the air_ wonderfully favours the glow phenomena. a brass ball, two and a half inches in diameter, being made positively inductric in an air-pump receiver, became covered with glow over an area of two inches in diameter, when the pressure was reduced to . inches of mercury. by a little adjustment the ball could be covered all over with this light. using a brass ball . inches in diameter, and making it inducteously positive by an inductric negative point, the phenomena, at high degrees of rarefaction, were exceedingly beautiful. the glow came over the positive ball, and gradually increased in brightness, until it was at last very luminous; and it also stood up like a low flame, half an inch or more in height. on touching the sides of the glass jar this lambent flame was affected, assumed a ring form, like a crown on the top of the ball, appeared flexible, and revolved with a comparatively slow motion, i.e. about four or five times in a second. this ring-shape and revolution are beautifully connected with the mechanical currents ( .) taking place within the receiver. these glows in rarefied air are often highly exalted in beauty by a spark discharge at the conductor ( . _note_.). . to obtain a _negative glow_ in air at common pressures is difficult. i did not procure it on the rod . of an inch in diameter by my machine, nor on much smaller rods; and it is questionable as yet, whether, even on fine points, what is called the negative star is a very reduced and minute, but still intermitting brush, or a glow similar to that obtained on a positive point. . in rarefied air the negative glow can easily be obtained. if the rounded ends of two metal rods, about o. of an inch in diameter, are introduced into a globe or jar (the air within being rarefied), and being opposite to each other, are about four inches apart, the glow can be obtained on both rods, covering not only the ends, but an inch or two of the part behind. on using _balls_ in the air-pump jar, and adjusting the distance and exhaustion, the negative ball could be covered with glow, whether it were the inductric or the inducteous surface. . when rods are used it is necessary to be aware that, if placed concentrically in the jar or globe, the light on one rod is often reflected by the sides of the vessel on to the other rod, and makes it apparently luminous, when really it is not so. this effect may be detected by shifting the eye at the time of observation, or avoided by using blackened rods. . it is curious to observe the relation _of glow, brush_, and _spark_ to each other, as produced by positive or negative surfaces; thus, beginning with spark discharge, it passes into brush much sooner when the surface at which the discharge commences ( .) is negative, than it does when positive; but proceeding onwards in the order of change, we find that the positive brush passes into _glow_ long before the negative brush does. so that, though each presents the three conditions in the same general order, the series are not precisely the same. it is probable, that, when these points are minutely examined, as they must be shortly, we shall find that each different gas or dielectric presents its own peculiar results, dependent upon the mode in which its particles assume polar electric condition. . the glow occurs in all gases in which i have looked for it. these are air, nitrogen, oxygen, hydrogen, coal gas, carbonic acid, muriatic acid, sulphurous acid and ammonia. i thought also that i obtained it in oil of turpentine, but if so it was very dull and small. . the glow is always accompanied by a wind proceeding either directly out from the glowing part, or directly towards it; the former being the most general case. this takes place even when the glow occurs upon a ball of considerable size: and if matters be so arranged that the ready and regular access of air to a part exhibiting the glow be interfered with or prevented, the glow then disappears. . i have never been able to analyse or separate the glow into visible elementary intermitting discharges ( . .), nor to obtain the other evidence of intermitting action, namely an audible sound ( .). the want of success, as respects trials made by ocular means, may depend upon the large size of the glow preventing the separation of the visible images: and, indeed, if it does intermit, it is not likely that all parts intermit at once with a simultaneous regularity. . all the effects tend to show, that _glow_ is due to a continuous charge or discharge of air; in the former case being accompanied by a current from, and in the latter by one to, the place of the glow. as the surrounding air comes up to the charged conductor, on attaining that spot at which the tension of the particles is raised to the sufficient degree ( . .), it becomes charged, and then moves off, by the joint action of the forces to which it is subject; and, at the same time that it makes way for other particles to come and be charged in turn, actually helps to form that current by which they are brought into the necessary position. thus, through the regularity of the forces, a constant and quiet result is produced; and that result is, the charging of successive portions of air, the production of a current, and of a continuous glow. . i have frequently been able to make the termination of a rod, which, when left to itself, would produce a brush, produce in preference a glow, simply by aiding the formation of a current of air at its extremity; and, on the other hand, it is not at all difficult to convert the glow into brushes, by affecting the current of air ( . .) or the inductive action near it. . the transition from glow, on the one hand, to brush and spark, on the other, and, therefore, their connexion, may be established in various ways. those circumstances which tend to facilitate the charge of the air by the excited conductor, and also those which tend to keep the tension at the same degree notwithstanding the discharge, assist in producing the glow; whereas those which tend to resist the charge of the air or other dielectric, and those which favour the accumulation of electric force prior to discharge, which, sinking by that act, has to be exalted before the tension can again acquire the requisite degree, favour intermitting discharge, and, therefore, the production of brush or spark. thus, rarefaction of the air, the removal of large conducting surfaces from the neighbourhood of the glowing termination, the presentation of a sharp point towards it, help to sustain or produce the glow: but the condensation of the air, the presentation of the hand or other large surface, the gradual approximation of a discharging ball, tend to convert the glow into brush or even spark. all these circumstances may be traced and reduced, in a manner easily comprehensible, to their relative power of assisting to produce, either a _continuous_ discharge to the air, which gives the glow; or an _interrupted_ one, which produces the brush, and, in a more exalted condition, the spark. . the rounded end of a brass rod, . of an inch in diameter, was covered with a positive glow by the working of an electrical machine: on stopping the machine, so that the charge of the connected conductor should fall, the glow changed for a moment into brushes just before the discharge ceased altogether, illustrating the necessity for a certain high continuous charge, for a certain sized termination. working the machine so that the intensity should be just low enough to give continual brushes from the end in free air, the approach of a fine point changed these brushes into a glow. working the machine so that the termination presented a continual glow in free air, the gradual approach of the hand caused the glow to contract at the very end of the wire, then to throw out a luminous point, which, becoming a foot stalk ( .), finally produced brushes with large ramifications. all these results are in accordance with what is stated above ( .). . greasing the end of a rounded wire will immediately make it produce brushes instead of glow. a ball having a blunt point which can be made to project more or less beyond its surface, at pleasure, can be made to produce every gradation from glow, through brush, to spark. . it is also very interesting and instructive to trace the transition from spark to glow, through the intermediate condition of stream, between ends in a vessel containing air more or less rarefied; but i fear to be prolix. . all the effects show, that the glow is in its nature exactly the same as the luminous part of a brush or ramification, namely a charging of air; the only difference being, that the glow has a continuous appearance from the constant renewal of the same action in the same place, whereas the ramification is due to a momentary, independent and intermitting action of the same kind. * * * * * _dark discharge._ . i will now notice a very remarkable circumstance in the luminous discharge accompanied by negative glow, which may, perhaps, be correctly traced hereafter into discharges of much higher intensity. two brass rods, . of an inch in diameter, entering a glass globe on opposite sides, had their ends brought into contact, and the air about them very much rarefied. a discharge of electricity from the machine was then made through them, and whilst that was continued the ends were separated from each other. at the moment of separation a continuous glow came over the end of the negative rod, the positive termination remaining quite dark. as the distance was increased, a purple stream or haze appeared on the end of the positive rod, and proceeded directly outwards towards the negative rod; elongating as the interval was enlarged, but never joining the negative glow, there being always a short dark space between. this space, of about / th or / th of an inch, was apparently invariable in its extent and its position, relative to the negative rod; nor did the negative glow vary. whether the negative end were inductric or inducteous, the same effect was produced. it was strange to see the positive purple haze diminish or lengthen as the ends were separated, and yet this dark space and the negative glow remain unaltered (fig. ). . two balls were then used in a large air-pump receiver, and the air rarefied. the usual transitions in the character of the discharge took place; but whenever the luminous stream, which appears after the spark and the brush have ceased, was itself changed into glow at the balls, the dark space occurred, and that whether the one or the other ball was made inductric, or positive, or negative. . sometimes when the negative ball was large, the machine in powerful action, and the rarefaction high, the ball would be covered over half its surface with glow, and then, upon a hasty observation, would seem to exhibit no dark space: but this was a deception, arising from the overlapping of the convex termination of the negative glow and the concave termination of the positive stream. more careful observation and experiment have convinced me, that when the negative glow occurs, it never visibly touches the luminous part of the positive discharge, but that the dark space is always there. . this singular separation of the positive and negative discharge, as far as concerns their luminous character, under circumstances which one would have thought very favourable to their coalescence, is probably connected with their differences when in the form of brush, and is perhaps even dependent on the same cause. further, there is every likelihood that the dark parts which occur in feeble sparks are also connected with these phenomena[a]. to understand them would be very important, for it is quite clear that in many of the experiments, indeed in all that i have quoted, discharge is taking place across the dark part of the dielectric to an extent quite equal to what occurs in the luminous part. this difference in the result would seem to imply a distinction in the modes by which the two electric forces are brought into equilibrium in the respective parts; and looking upon all the phenomena as giving additional proofs, that it is to the condition of the particles of the dielectric we must refer for the principles of induction and discharge, so it would be of great importance if we could know accurately in what the difference of action in the dark and the luminous parts consisted. [a] see professor johnson's experiments. silliman's journal, xxv. p. . . the dark discharge through air ( .), which in the case mentioned is very evident ( .), leads to the inquiry, whether the particles of air are generally capable of effecting discharge from one to another without becoming luminous; and the inquiry is important, because it is connected with that degree of tension which is necessary to originate discharge ( . .). discharge between _air and conductors_ without luminous appearances are very common; and non-luminous discharges by carrying currents of air and other fluids ( . .) are also common enough: but these are not cases in point, for they are not discharges between insulating particles. . an arrangement was made for discharge between two balls ( .) (fig. .) but, in place of connecting the inducteous ball directly with the discharging train, it was put in communication with the inside coating of a leyden jar, and the discharging train with the outside coating. then working the machine, it was found that whenever sonorous and luminous discharge occurred at the balls a b, the jar became charged; but that when these did not occur, the jar acquired no charge: and such was the case when small rounded terminations were used in place of the balls, and also in whatever manner they were arranged. under these circumstances, therefore, discharge even between the air and conductors was always luminous. . but in other cases, the phenomena are such as to make it almost certain, that dark discharge can take place across air. if the rounded end of a metal rod, . of an inch in diameter, be made to give a good negative brush, the approach of a smaller end or a blunt point opposite to it will, at a certain distance, cause a diminution of the brush, and a glow will appear on the positive inducteous wire, accompanied by a current of air passing from it. now, as the air is being charged both at the positive and negative surfaces, it seems a reasonable conclusion, that the charged portions meet somewhere in the interval, and there discharge to each other, without producing any luminous phenomena. it is possible, however, that the air electrified positively at the glowing end may travel on towards the negative surface, and actually form that atmosphere into which the visible negative brushes dart, in which case dark discharge need not, of necessity, occur. but i incline to the former opinion, and think, that the diminution in size of the negative brush, as the positive glow comes on to the end of the opposed wire, is in favour of that view. . using rarefied air as the dielectric, it is very easy to obtain luminous phenomena as brushes, or glow, upon both conducting balls or terminations, whilst the interval is dark, and that, when the action is so momentary that i think we cannot consider currents as effecting discharge across the dark part. thus if two balls, about an inch in diameter, and or more inches apart, have the air rarefied about them, and are then interposed in the course of discharge, an interrupted or spark current being produced at the machine[a], each termination may be made to show luminous phenomena, whilst more or less of the interval is quite dark. the discharge will pass as suddenly as a retarded spark ( . .), i.e. in an interval of time almost inappreciably small, and in such a case, i think it must have passed across the dark part as true disruptive discharge, and not by convection. [a] by spark current i mean one passing in a series of spark between the conductor of the machine and the apparatus: by a continuous current one that passes through metallic conductors, and in that respect without interruption at the same place. . hence i conclude that dark disruptive discharge may occur ( . .); and also, that, in the luminous brush, the visible ramifications may not show the full extent of the disruptive discharge ( . .), but that each may have a dark outside, enveloping, as it were, every part through which the discharge extends. it is probable, even, that there are such things as dark discharges analogous in form to the brush and the spark, but not luminous in any part ( .). . the occurrence of dark discharge in any case shows at how low a tension disruptive discharge may occur ( ,), and indicates that the light of the ultimate brush or spark is in no relation to the intensity required ( . .). so to speak, the discharge begins in darkness, and the light is a mere consequence of the quantity which, after discharge has commenced, flows to that spot and there finds its most facile passage ( . .). as an illustration of the growth generally of discharge, i may remark that, in the experiments on the transition in oxygen of the discharge from spark to brush ( .), every spark was immediately preceded by a short brush. . the phenomena relative to dark discharge in other gases, though differing in certain characters from those in air, confirm the conclusions drawn above. the two rounded terminations ( .) (fig. .), were placed in _muriatic acid gas_ ( . .) at the pressure of . inches of mercury, and a continuous machine current of electricity sent through the apparatus: bright sparks occurred until the interval was about or above an inch, when they were replaced by squat brushy intermitting glows upon both terminations, with a dark part between. when the current at the machine was in spark, then each spark caused a discharge across the muriatic acid gas, which, with a certain interval, was bright; with a larger interval, was straight across and flamy, like a very exhausted and sudden, but not a dense sharp spark; and with a still larger interval, produced a feeble brush on the inductric positive end, and a glow on the inducteous negative end, the dark part being between ( .); and at such times, the spark at the conductor, instead of being sudden and sonorous, was dull and quiet ( .). . on introducing more muriatic acid gas, until the pressure was . inches, the same terminations gave bright sparks within at small distances; but when they were about an inch or more apart, the discharge was generally with very small brushes and glow, and frequently with no light at all, though electricity had passed through the gas. whenever the bright spark did pass through the muriatic acid gas at this pressure, it was bright throughout, presenting no dark or dull space. . in _coal gas_, at common pressures, when the distance was about an inch, the discharge was accompanied by short brushes on the ends, and a dark interval of half an inch or more between them, notwithstanding the discharge had the sharp quick sound of a dull spark, and could not have depended in the dark part on _convection_ ( .). . this gas presents several curious points in relation to the bright and dark parts of spark discharge. when bright sparks passed between the rod ends . of an inch in diameter ( .), very sudden dark parts would occur next to the brightest portions of the spark. again with these ends and also with balls ( .), the bright sparks would be sometimes red, sometimes green, and occasionally green and red in different parts of the same spark. again, in the experiments described ( .), at certain intervals a very peculiar pale, dull, yet sudden discharge would pass, which, though apparently weak, was very direct in its course, and accompanied by a sharp snapping noise, as if quick in its occurrence. . _hydrogen_ frequently gave peculiar sparks, one part being bright red, whilst the other was a dull pale gray, or else the whole spark was dull and peculiar. . _nitrogen_ presents a very remarkable discharge, between two balls of the respective diameters of . and inches ( . .), the smaller one being rendered negative either directly inducteously. the peculiar discharge occurs at intervals between . and . , and even at . inches when the large ball was inductric positively; it consisted of a little brushy part on the small negative ball, then a dark space, and lastly a dull straight line on the large positive ball (fig. .). the position of the dark space was very constant, and is probably in direct relation to the dark space described when negative glow was produced ( .). when by any circumstance a bright spark was determined, the contrast with the peculiar spark described was very striking; for it always had a faint purple part, but the place of this part was constantly near the positive ball. . thus dark discharge appears to be decidedly established. but its establishment is accompanied by proofs that it occurs in different degrees and modes in different gases. hence then another specific action, added to the many ( . . . . . .) by which the electrical relations of insulating dielectrics are distinguished and established, and another argument in favour of that molecular theory of induction, which is at present under examination[a]. [a] i cannot resist referring here by a note to biot's philosophical view of the nature of the light of the electric discharge, annales de chimie, liii. p. . * * * * * . what i have had to say regarding disruptive discharge has extended to some length, but i hope will be excused in consequence of the importance of the subject. before concluding my remarks, i will again intimate in the form of a query, whether we have not reason to consider the tension or retention and after discharge in air or other insulating dielectrics, as the same thing with retardation and discharge in a metal wire, differing only, but almost infinitely, in degree ( . .). in other words, can we not, by a gradual chain of association, carry up discharge from its occurrence in air, through spermaceti and water, to solutions, and then on to chlorides, oxides and metals, without any essential change in its character; and, at the same time, connecting the insensible conduction of air, through muriatic acid gas and the dark discharge, with the better conduction of spermaceti, water, and the all but perfect conduction of the metals, associate the phenomena at both extremes? and may it not be, that the retardation and ignition of a wire are effects exactly correspondent in their nature to the retention of charge and spark in air? if so, here again the two extremes in property amongst dielectrics will be found to be in intimate relation, the whole difference probably depending upon the mode and degree in which their particles polarize under the influence of inductive actions ( . . .). * * * * * ¶ x. _convection, or carrying discharge._ . the last kind of discharge which i have to consider is that effected by the motion of charged particles from place to place. it is apparently very different in its nature to any of the former modes of discharge ( .), but, as the result is the same, may be of great importance in illustrating, not merely the nature of discharge itself, but also of what we call the electric current. it often, as before observed, in cases of brush and glow ( . .), joins its effect to that of disruptive discharge, to complete the act of neutralization amongst the electric forces. . the particles which being charged, then travel, may be either of insulating or conducting matter, large or small. the consideration in the first place of a large particle of conducting matter may perhaps help our conceptions. . a copper boiler feet in diameter was insulated and electrified, but so feebly, that dissipation by brushes or disruptive discharge did not occur at its edges or projecting parts in a sensible degree. a brass ball, inches in diameter, suspended by a clean white silk thread, was brought towards it, and it was found that, if the ball was held for a second or two near any part of the charged surface of the boiler, at such distance (two inches more or less) as not to receive any direct charge from it, it became itself charged, although insulated the whole time; and its electricity was the _reverse_ of that of the boiler. . this effect was the strongest opposite the edges and projecting parts of the boiler, and weaker opposite the sides, or those extended portions of the surface which, according to coulomb's results, have the weakest charge. it was very strong opposite a rod projecting a little way from the boiler. it occurred when the copper was charged negatively as well as positively: it was produced also with small balls down to . of an inch and less in diameter, and also with smaller charged conductors than the copper. it is, indeed, hardly possible in some cases to carry an insulated ball within an inch or two of a charged plane or convex surface without its receiving a charge of the contrary kind to that of the surface. . this effect is one of induction between the bodies, not of communication. the ball, when related to the positive charged surface by the intervening dielectric, has its opposite sides brought into contrary states, that side towards the boiler being negative and the outer side positive. more inductric action is directed towards it than would have passed across the same place if the ball had not been there, for several reasons; amongst others, because, being a conductor, the resistance of the particles of the dielectric, which otherwise would have been there, is removed ( .); and also, because the reacting positive surface of the ball being projected further out from the boiler than when there is no introduction of conducting matter, is more free therefore to act through the rest of the dielectric towards surrounding conductors, and so favours the exaltation of that inductric polarity which is directed in its course. it is, as to the exaltation of force upon its outer surface beyond that upon the inductric surface of the boiler, as if the latter were itself protuberant in that direction. thus it acquires a state like, but higher than, that of the surface of the boiler which causes it; and sufficiently exalted to discharge at its positive surface to the air, or to affect small particles, as it is itself affected by the boiler, and they flying to it, take a charge and pass off; and so the ball, as a whole, is brought into the contrary inducteous state. the consequence is, that, if free to move, its tendency, under the influence of all the forces, to approach the boiler is increased, whilst it at the same time becomes more and more exalted in its condition, both of polarity and charge, until, at a certain distance, discharge takes place, it acquires the same state as the boiler, is repelled, and passing to that conductor most favourably circumstanced to discharge it, there resumes its first indifferent condition. . it seems to me, that the manner in which inductric bodies affect uncharged floating or moveable conductors near them, is very frequently of this nature, and generally so when it ends in a carrying operation ( . .). the manner in which, whilst the dominant inductric body cannot give off its electricity to the air, the inducteous body _can_ effect the discharge of the same kind of force, is curious, and, in the case of elongated or irregularly shaped conductors, such as filaments or particles of dust, the effect will often be very ready, and the consequent attraction immediate. . the effect described is also probably influential in causing those variations in spark discharge referred to in the last series ( . . .): for if a particle of dust were drawn towards the axis of induction between the balls, it would tend, whilst at some distance from that axis, to commence discharge at itself, in the manner described ( .), and that commencement might so far facilitate the act ( . .) as to make the complete discharge, as spark, pass through the particle, though it might not be the shortest course from ball to ball. so also, with equal balls at equal distances, as in the experiments of comparison already described ( . .), a particle being between one pair of balls would cause discharge there in preference; or even if a particle were between each, difference of size or shape would give one for the time a predominance over the other. . the power of particles of dust to carry off electricity in cases of high tension is well known, and i have already mentioned some instances of the kind in the use of the inductive apparatus ( .). the general operation is very well shown by large light objects, as the toy called the electrical spider; or, if smaller ones are wanted for philosophical investigation, by the smoke of a glowing green wax taper, which, presenting a successive stream of such particles, makes their course visible. . on using oil of turpentine as the dielectric, the action and course of small conducting carrying particles in it can be well observed. a few short pieces of thread will supply the place of carriers, and their progressive action is exceedingly interesting. . a very striking effect was produced on oil of turpentine, which, whether it was due to the carrying power of the particles in it, or to any other action of them, is perhaps as yet doubtful. a portion of that fluid in a glass vessel had a large uninsulated silver dish at the bottom, and an electrified metal rod with a round termination dipping into it at the top. the insulation was very good, and the attraction and other phenomena striking. the rod end, with a drop of gum water attached to it, was then electrified in the fluid; the gum water soon spun off in fine threads, and was quickly dissipated through the oil of turpentine. by the time that four drops had in this way been commingled with a pint of the dielectric, the latter had lost by far the greatest portion of its insulating power; no sparks could be obtained in the fluid; and all the phenomena dependent upon insulation had sunk to a low degree. the fluid was very slightly turbid. upon being filtered through paper only, it resumed its first clearness, and now insulated as well as before. the water, therefore, was merely diffused through the oil of turpentine, not combined with or dissolved in it: but whether the minute particles acted as carriers, or whether they were not rather gathered together in the line of highest inductive tension ( .), and there, being drawn into elongated forms by the electric forces, combined their effects to produce a band of matter having considerable conducting power, as compared with the oil of turpentine, is as yet questionable. . the analogy between the action of solid conducting carrying particles and that of the charged particles of fluid insulating substances, acting as dielectrics, is very evident and simple; but in the latter case the result is, necessarily, currents in the mobile media. particles are brought by inductric action into a polar state; and the latter, after rising to a certain tension ( .), is followed by the communication of a part of the force originally on the conductor; the particles consequently become charged, and then, under the joint influence of the repellent and attractive forces, are urged towards a discharging place, or to that spot where these inductric forces are most easily compensated by the contrary inducteous forces. . why a point should be so exceedingly favourable to the production of currents in a fluid insulating dielectric, as air, is very evident. it is at the extremity of the point that the intensity necessary to charge the air is first acquired ( .); it is from thence that the charged particle recedes; and the mechanical force which it impresses on the air to form a current is in every way favoured by the shape and position of the rod, of which the point forms the termination. at the same time, the point, having become the origin of an active mechanical force, does, by the very act of causing that force, namely, by discharge, prevent any other part of the rod from acquiring the same necessary condition, and so preserves and sustains its own predominance. . the very varied and beautiful phenomena produced by sheltering or enclosing the point, illustrate the production of the current exceedingly well, and justify the same conclusions; it being remembered that in such cases the effect upon the discharge is of two kinds. for the current may be interfered with by stopping the access of fresh uncharged air, or retarding the removal of that which has been charged, as when a point is electrified in a tube of insulating matter closed at one extremity; or the _electric condition_ of the point itself may be altered by the relation of other parts in its neighbourhood, also rendered electric, as when the point is in a metal tube, by the metal itself, or when it is in the glass tube, by a similar action of the charged parts of the glass, or even by the surrounding air which has been charged, and which cannot escape. . whenever it is intended to observe such inductive phenomena in a fluid dielectric as have a direct relation to, and dependence upon, the fluidity of the medium, such, for instance, as discharge from points, or attractions and repulsions, &c., then the mass of the fluid should be great, and in such proportion to the distance between the inductric and inducteous surfaces as to include all the _lines of inductive force_ ( .) between them; otherwise, the effects of currents, attraction, &c., which are the resultants of all these forces, cannot be obtained. the phenomena, which occur in the open air, or in the middle of a globe filled with oil of turpentine, will not take place in the same media if confined in tubes of glass, shell-lac, sulphur, or other such substances, though they be excellent insulating dielectrics; nor can they be expected: for in such cases, the polar forces, instead of being all dispersed amongst fluid particles, which tend to move under their influence, are now associated in many parts with particles that, notwithstanding their tendency to motion, are constrained by their solidity to remain quiescent. . the varied circumstances under which, with conductors differently formed and constituted, currents can occur, all illustrate the same simplicity of production. a _ball_, if the intensity be raised sufficiently on its surface, and that intensity be greatest on a part consistent with the production of a current of air up to and off from it, will produce the effect like a point ( ); such is the case whenever the glow occurs upon a ball, the current being essential to that phenomenon. if as large a sphere as can well be employed with the production of glow be used, the glow will appear at the place where the current leaves the ball, and that will be the part directly opposite to the connection of the ball and rod which supports it; but by increasing the tension elsewhere, so as to raise it above the tension upon that spot, which can easily be effected inductively, then the place of the glow and the direction of the current will also change, and pass to that spot which for the time is most favourable for their production ( .). . for instance, approaching the hand towards the ball will tend to cause brush ( .), but by increasing the supply of electricity the condition of glow may be preserved; then on moving the hand about from side to side the position of the glow will very evidently move with it. . a point brought towards a glowing ball would at twelve or fourteen inches distance make the glow break into brush, but when still nearer, glow was reproduced, probably dependent upon the discharge of wind or air passing from the point to the ball, and this glow was very obedient to the motion of the point, following it in every direction. . even a current of wind could affect the place of the glow; for a varnished glass tube being directed sideways towards the ball, air was sometimes blown through it at the ball and sometimes not. in the former case, the place of the glow was changed a little, as if it were blown away by the current, and this is just the result which might have been anticipated. all these effects illustrate beautifully the general causes and relations, both of the glow and the current of air accompanying it ( .). . flame facilitates the production of a current in the dielectric surrounding it. thus, if a ball which would not occasion a current in the air have a flame, whether large or small, formed on its surface, the current is produced with the greatest ease; but not the least difficulty can occur in comprehending the effective action of the flame in this case, if its relation, as part of the surrounding dielectric, to the electrified ball, be but for a moment considered ( . .). . conducting fluid terminations, instead of rigid points, illustrate in a very beautiful manner the formation of the currents, with their effects and influence in exalting the conditions under which they were commenced. let the rounded end of a brass rod, . of an inch or thereabouts in diameter, point downwards in free air; let it be amalgamated, and have a drop of mercury suspended from it; and then let it be powerfully electrized. the mercury will present the phenomenon of _glow_; a current of air will rush along the rod, and set off from the mercury directly downwards; and the form of the metallic drop will be slightly affected, the convexity at a small part near the middle and lower part becoming greater, whilst it diminishes all round at places a little removed from this spot. the change is from the form of _a_ (fig. .) to that of _b_, and is due almost, if not entirely, to the mechanical force of the current of air sweeping over its surface. . as a comparative observation, let it be noticed, that a ball gradually brought towards it converts the glow into brushes, and ultimately sparks pass from the most projecting part of the mercury. a point does the same, but at much smaller distances. . take next a drop of strong solution of muriate of lime; being electrified, a part will probably be dissipated, but a considerable portion, if the electricity be not too powerful, will remain, forming a conical drop (fig. .), accompanied by a strong current. if glow be produced, the drop will be smooth on the surface. if a short low brush is formed, a minute tremulous motion of the liquid will be visible; but both effects coincide with the principal one to be observed, namely, the regular and successive charge of air, the formation of a wind or current, and the form given by that current to the fluid drop, if a discharge ball be gradually brought toward the cone, sparks will at last pass, and these will be from the apex of the cone to the approached ball, indicating a considerable degree of conducting power in this fluid. . with a drop of water, the effects were of the same kind, and were best obtained when a portion of gum water or of syrup hung from a ball (fig. .). when the machine was worked slowly, a fine large quiet conical drop, with concave lateral outline, and a small rounded end, was produced, on which the glow appeared, whilst a steady wind issued, in a direction from the point of the cone, of sufficient force to depress the surface of uninsulated water held opposite to the termination. when the machine was worked more rapidly some of the water was driven off; the smaller pointed portion left was roughish on the surface, and the sound of successive brush discharges was heard. with still more electricity, more water was dispersed; that which remained was elongated and contracted, with an alternating motion; a stronger brush discharge was heard, and the vibrations of the water and the successive discharges of the individual brushes were simultaneous. when water from beneath was brought towards the drop, it did not indicate the same regular strong contracted current of air as before; and when the distance was such that sparks passed, the water beneath was _attracted_ rather than driven away, and the current of air _ceased_. . when the discharging ball was brought near the drop in its first quiet glowing state ( .), it converted that glow into brushes, and caused the vibrating motion of the drop. when still nearer, sparks passed, but they were always from the metal of the rod, over the surface of the water, to the point, and then across the air to the ball. this is a natural consequence of the deficient conducting power of the fluid ( . .). . why the drop vibrated, changing its form between the periods of discharging brushes, so as to be more or less acute at particular instants, to be most acute when the brush issued forth, and to be isochronous in its action, and how the quiet glowing liquid drop, on assuming the conical form, facilitated, as it were, the first action, are points, as to theory, so evident, that i will not stop to speak of them. the principal thing to observe at present is, the formation of the carrying current of air, and the manner in which it exhibits its existence and influence by giving form to the drop. . that the drop, when of water, or a better conductor than water, is formed into a cone principally by the current of air, is shown amongst other ways ( .) thus. a sharp point being held opposite the conical drop, the latter soon lost its pointed form; was retraced and became round; the current of air from it ceased, and was replaced by one from the point beneath, which, if the latter were held near enough to the drop, actually blew it aside, and rendered it concave in form. . it is hardly necessary to say what happened with still worse conductors than water, as oil, or oil of turpentine; the fluid itself was then spun out into threads and carried off, not only because the air rushing over its surface helped to sweep it away, but also because its insulating particles assumed the same charged state as the particles of air, and, not being able to discharge to them in a much greater decree than the air particles themselves could do, were carried off by the same causes which urged those in their course. a similar effect with melted sealing-wax on a metal point forms an old and well-known experiment. . a drop of gum water in the exhausted receiver of the air-pump was not sensibly affected in its form when electrified. when air was let in, it begun to show change of shape when the pressure was ten inches of mercury. at the pressure of fourteen or fifteen inches the change was more sensible, and as the air increased in density the effects increased, until they were the same as those in the open atmosphere. the diminished effect in the rare air i refer to the relative diminished energy of its current; that diminution depending, in the first place, on the lower electric condition of the electrified ball in the rarefied medium, and in the next, on the attenuated condition of the dielectric, the cohesive force of water in relation to rarefied air being something like that of mercury to dense air ( .), whilst that of water in dense air may be compared to that of mercury in oil of turpentine ( .). . when a ball is covered with a thick conducting fluid, as treacle or syrup, it is easy by inductive action to determine the wind from almost any part of it ( .); the experiment, which before was of rather difficult performance, being rendered facile in consequence of the fluid enabling that part, which at first was feeble in its action, to rise into an exalted condition by assuming a pointed form. . to produce the current, the electric intensity must rise and continue at _one spot_, namely, at the origin of the current, higher than elsewhere, and then, air having a uniform and ready access, the current is produced. if no current be allowed ( .), then discharge may take place by brush or spark. but whether it be by brush or spark, or wind, it seems very probable that the initial intensity or tension at which a particle of a given gaseous dielectric charges, or commences discharge, is, under the conditions before expressed, always the same ( .). . it is not supposed that all the air which enters into motion is electrified; on the contrary, much that is not charged is carried on into the stream. the part which is really charged may be but a small proportion of that which is ultimately set in motion ( .). . when a drop of gum water ( .) is made _negative_, it presents a larger cone than when made positive; less of the fluid is thrown off, and yet, when a ball is approached, sparks can hardly be obtained, so pointed is the cone, and so free the discharge. a point held opposite to it did not cause the retraction of the cone to such an extent as when it was positive. all the effects are so different from those presented by the positive cone, that i have no doubt such drops would present a very instructive method of investigating the difference of positive and negative discharge in air and other dielectrics ( . .). . that i may not be misunderstood ( .), i must observe here that i do not consider the cones produced as the result _only_ of the current of air or other insulating dielectric over their surface. when the drop is of badly conducting matter, a part of the effect is due to the electrified state of the particles, and this part constitutes almost the whole when the matter is melted sealing-wax, oil of turpentine, and similar insulating bodies ( .). but even when the drop is of good conducting matter, as water, solutions, or mercury, though the effect above spoken of will then be insensible ( .), still it is not the mere current of air or other dielectric which produces all the change of form; for a part is due to those attractive forces by which the charged drop, if free to move, would travel along the line of strongest induction, and not being free to move, has its form elongated until the _sum_ of the different forces tending to produce this form is balanced by the cohesive attraction of the fluid. the effect of the attractive forces are well shown when treacle, gum water, or syrup is used; for the long threads which spin out, at the same time that they form the axes of the currents of air, which may still be considered as determined at their points, are like flexible conductors, and show by their directions in what way the attractive forces draw them. . when the phenomena of currents are observed in dense insulating dielectrics, they present us with extraordinary degrees of mechanical force. thus, if a pint of well-rectified and filtered ( .) oil of turpentine be put into a glass vessel, and two wires be dipped into it in different places, one leading to the electrical machine, and the other to the discharging train, on working the machine the fluid will be thrown into violent motion throughout its whole mass, whilst at the same time it will rise two, three or four inches up the machine wire, and dart off jets from it into the air. . if very clean uninsulated mercury be at the bottom of the fluid, and the wire from the machine be terminated either by a ball or a point, and also pass through a glass tube extending both above and below the surface of the oil of turpentine, the currents can be better observed, and will be seen to rush down the wire, proceeding directly from it towards the mercury, and there, diverging in all directions, will ripple its surface strongly, and mounting up at the sides of the vessel, will return to re-enter upon their course. . a drop of mercury being suspended from an amalgamated brass ball, preserved its form almost unchanged in air ( .); but when immersed in the oil of turpentine it became very pointed, and even particles of the metal could be spun out and carried off by the currents of the dielectric. the form of the liquid metal was just like that of the syrup in air ( .), the point of the cone being quite as fine, though not so long. by bringing a sharp uninsulated point towards it, it could also be effected in the same manner as the syrup drop in air ( .), though not so readily, because of the density and limited quantity of the dielectric. . if the mercury at the bottom of the fluid be connected with the electrical machine, whilst a rod is held in the hand terminating in a ball three quarters of an inch, less or more, in diameter, and the ball be dipped into the electrified fluid, very striking appearances ensue. when the ball is raised again so as to be at a level nearly out of the fluid, large portions of the latter will seem to cling to it (fig. .). if it be raised higher, a column of the oil of turpentine will still connect it with that in the basin below (fig. .). if the machine be excited into more powerful action, this will become more bulky, and may then also be raised higher, assuming the form (fig. ); and all the time that these effects continue, currents and counter-currents, sometimes running very close together, may be observed in the raised column of fluid. . it is very difficult to decide by sight the direction of the currents in such experiments as these. if particles of silk are introduced they cling about the conductors; but using drops of water and mercury the course of the fluid dielectric seems well indicated. thus, if a drop of water be placed at the end of a rod ( .) over the uninsulated mercury, it is soon swept away in particles streaming downwards towards the mercury. if another drop be placed on the mercury beneath the end of the rod, it is quickly dispersed in all directions in the form of streaming particles, the attractive forces drawing it into elongated portions, and the currents carrying them away. if a drop of mercury be hung from a ball used to raise a column of the fluid ( .), then the shape of the drop seems to show currents travelling in the fluid in the direction indicated by the arrows (fig. .). . a very remarkable effect is produced on these phenomena, connected with positive and negative charge and discharge, namely, that a ball charged positively raises a much higher and larger column of the oil of turpentine than when charged negatively. there can be no doubt that this is connected with the difference of positive and negative action already spoken of ( . .), and tends much to strengthen the idea that such difference is referable to the particles of the dielectric rather than to the charged conductors, and is dependent upon the mode in which these particles polarize ( . .). . whenever currents travel in insulating dielectrics they really effect discharge; and it is important to observe, though a very natural result, that it is indifferent which way the current or particles travel, as with reversed direction their state is reversed. the change is easily made, either in air or oil of turpentine, between two opposed rods, for an insulated ball being placed in connexion with either rod and brought near its extremity, will cause the current to set towards it from the opposite end. . the two currents often occur at once, as when both terminations present brushes, and frequently when they exhibit the glow ( .). in such cases, the charged particles, or many of them, meet and mutually discharge each other ( . .). if a smoking wax taper be held at the end of an insulating rod towards a charged prime conductor, it will very often happen that two currents will form, and be rendered visible by its vapour, one passing as a fine filament of smoky particles directly to the charged conductor, and the other passing as directly from the same taper wick outwards, and from the conductor: the principles of inductric action and charge, which were referred to in considering the relation of a carrier ball and a conductor ( .), being here also called into play. * * * * * . the general analogy and, i think i may say, identity of action found to exist as to insulation and conduction ( . .) when bodies, the best and the worst in the classes of insulators or conductors, were compared, led me to believe that the phenomena of _convection_ in badly conducting media were not without their parallel amongst the best conductors, such even as the metals. upon consideration, the cones produced by davy[a] in fluid metals, as mercury and tin, seemed to be cases in point, and probably also the elongation of the metallic medium through which a current of electricity was passing, described by ampère ( )[b]; for it is not difficult to conceive, that the diminution of convective effect, consequent upon the high conducting power of the metallic media used in these experiments, might be fully compensated for by the enormous quantity of electricity passing. in fact, it is impossible not to expect _some_ effect, whether sensible or not, of the kind in question, when such a current is passing through a fluid offering a sensible resistance to the passage of the electricity, and, thereby, giving proof of a certain degree of insulating power ( .). [a] philosophical transactions, , p. . [b] bibliothèque universelle, xxi, . . i endeavoured to connect the convective currents in air, oil of turpentine, &c. and those in metals, by intermediate cases, but found this not easy to do. on taking bodies, for instance, which, like water, adds, solutions, fused salts or chlorides, &c., have intermediate conducting powers, the minute quantity of electricity which the common machine can supply ( . .) is exhausted instantly, so that the cause of the phenomenon is kept either very low in intensity, or the instant of time during which the effect lasts is so small, that one cannot hope to observe the result sought for. if a voltaic battery be used, these bodies are all electrolytes, and the evolution of gas, or the production of other changes, interferes and prevents observation of the effect required. . there are, nevertheless, some experiments which illustrate the connection. two platina wires, forming the electrodes of a powerful voltaic battery, were placed side by side, near each other, in distilled water, hermetically sealed up in a strong glass tube, some minute vegetable fibres being present in the water. when, from the evolution of gas and the consequent increased pressure, the bubbles formed on the electrodes were so small as to produce but feebly ascending currents, then it could be observed that the filaments present were attracted and repelled between the two wires, as they would have been between two oppositely charged surfaces in air or oil of turpentine, moving so quickly as to displace and disturb the bubbles and the currents which these tended to form. now i think it cannot be doubted, that under similar circumstances, and with an abundant supply of electricity, of sufficient tension also, convective currents might have been formed; the attractions and repulsions of the filaments were, in fact, the elements of such currents ( .), and therefore water, though almost infinitely above air or oil of turpentine as a conductor, is a medium in which similar currents can take place. . i had an apparatus made (fig. .) in which _a_ is a plate of shell-lac, _b_ a fine platina wire passing through it, and having only the section of the wire exposed above; _c_ a ring of bibulous paper resting on the shell-lac, and _d_ distilled water retained by the paper in its place, and just sufficient in quantity to cover the end of the wire _b_; another wire, _e_, touched a piece of tinfoil lying in the water, and was also connected with a discharging train; in this way it was easy, by rendering _b_ either positive or negative, to send a current of electricity by its extremity into the fluid, and so away by the wire _e_. . on connecting _b_ with the conductor of a powerful electrical machine, not the least disturbance of the level of the fluid over the end of the wire during the working of the machine could be observed; but at the same time there was not the smallest indication of electrical charge about the conductor of the machine, so complete was the discharge. i conclude that the quantity of electricity passed in a _given time_ had been too small, when compared with the conducting power of the fluid to produce the desired effect. . i then charged a large leyden battery ( .), and discharged it through the wire _b_, interposing, however, a wet thread, two feet long, to prevent a spark in the water, and to reduce what would else have been a sudden violent discharge into one of more moderate character, enduring for a sensible length of time ( .). i now did obtain a very brief elevation of the water over the end of the wire; and though a few minute bubbles of gas were at the same time formed there, so as to prevent me from asserting that the effect was unequivocally the same as that obtained by davy in the metals, yet, according to my best judgement, it was partly, and i believe principally, of that nature. . i employed a voltaic battery of pair of four-inch plates for experiments of a similar nature with electrolytes. in these cases the shell-lac was cupped, and the wire _b_ . of an inch in diameter. sometimes i used a positive amalgamated zinc wire in contact with dilute sulphuric acid; at others, a negative copper wire in a solution of sulphate of copper; but, because of the evolution of gas, the precipitation of copper, &c., i was not able to obtain decided results. it is but right to mention, that when i made use of mercury, endeavouring to repeat davy's experiment, the battery of pair was not sufficient to produce the elevations[a]. [a] in the experiments at the royal institution, sir h. davy used, i think, or pairs of plates. those at the london institution were made with the apparatus of mr. pepys (consisting of an enormous single pair of plates), described in the philosophical transactions for , p. . . the latter experiments ( .) may therefore be considered as failing to give the hoped-for proof, but i have much confidence in the former ( . .), and in the considerations ( .) connected with them. if i have rightly viewed them, and we may be allowed to compare the currents at points and surfaces in such extremely different bodies as air and the metals, and admit that they are effects of the _same_ kind, differing only in degree and in proportion to the insulating or conducting power of the dielectric used, what great additional argument we obtain in favour of that theory, which in the phenomena of insulation and conduction also, as in these, would link _the same_ apparently dissimilar substances together ( . .); and how completely the general view, which refers all the phenomena to the direct action of the molecules of matter, seems to embrace the various isolated phenomena as they successively come under consideration! * * * * * . the connection of this convective or carrying effect, which depends upon a certain degree of insulation, with conduction; i.e. the occurrence of both in so many of the substances referred to, as, for instance, the metals, water, air, &c., would lead to many very curious theoretical generalizations, which i must not indulge in here. one point, however, i shall venture to refer to. conduction appears to be essentially an action of contiguous particles, and the considerations just stated, together with others formerly expressed ( , , &c.), lead to the conclusion, that all bodies conduct, and by the same process, air as well as metals; the only difference being in the necessary degree of force or tension between the particles which must exist before the act of conduction or transfer from one particle to another can take place. . the question then arises, what is this limiting condition which separates, as it were, conduction and insulation from each other? does it consist in a difference between the two contiguous particles, or the contiguous poles of these particles, in the nature and amount of positive and negative force, no communication or discharge occurring unless that difference rises up to a certain degree, variable for different bodies, but always the same for the same body? or is it true that, however small the difference between two such particles, if _time_ be allowed, equalization of force will take place, even with the particles of such bodies as air, sulphur or lac? in the first case, insulating power in any particular body would be proportionate to the degree of the assumed necessary difference of force; in the second, to the _time_ required to equalize equal degrees of difference in different bodies. with regard to airs, one is almost led to expect a permanent difference of force; but in all other bodies, time seems to be quite sufficient to ensure, ultimately, complete conduction. the difference in the modes by which insulation may be sustained, or conduction effected, is not a mere fanciful point, but one of great importance, as being essentially connected with the molecular theory of induction, and the manner in which the particles of bodies assume and retain their polarized state. * * * * * ¶ xi. _relation of a vacuum to electrical phenomena._ . it would seem strange, if a theory which refers all the phenomena of insulation and conduction, i.e. all electrical phenomena, to the action of contiguous particles, were to omit to notice the assumed possible case of a _vacuum_. admitting that a vacuum can be produced, it would be a very curious matter indeed to know what its relation to electrical phenomena would be; and as shell-lac and metal are directly opposed to each other, whether a vacuum would be opposed to them both, and allow neither of induction or conduction across it. mr. morgan[a] has said that a vacuum does not conduct. sir h. davy concluded from his investigations, that as perfect a vacuum as could be made[b] did conduct, but does not consider the prepared spaces which he used as absolute vacua. in such experiments i think i have observed the luminous discharge to be principally on the inner surface of the glass; and it does not appear at all unlikely, that, if the vacuum refused to conduct, still the surface of glass next it might carry on that action. [a] philosophical transactions, , p. [b] ibid. , p. . . at one time, when i thought inductive force was exerted in right lines, i hoped to illustrate this important question by making experiments on induction with metallic mirrors (used only as conducting vessels) exposed towards a very clear sky at night time, and of such concavity that nothing but the firmament could be visible from the lowest part of the concave _n_, fig. . such mirrors, when electrified, as by connexion with a leyden jar, and examined by a carrier ball, readily gave electricity at the lowest part of their concavity if in a room; but i was in hopes of finding that, circumstanced as before stated, they would give little or none at the same spot, if the atmosphere above really terminated in a vacuum. i was disappointed in the conclusion, for i obtained as much electricity there as before; but on discovering the action of induction in curved lines ( .), found a full and satisfactory explanation of the result. . my theory, as far as i have ventured it, does not pretend to decide upon the consequences of a vacuum. it is not at present limited sufficiently, or rendered precise enough, either by experiments relating to spaces void of matter, or those of other kinds, to indicate what would happen in the vacuum case. i have only as yet endeavoured to establish, what all the facts seem to prove, that when electrical phenomena, as those of induction, conduction, insulation and discharge occur, they depend on, and are produced by the action of _contiguous_ particles of matter, the next existing particle being considered as the contiguous one; and i have further assumed, that these particles are polarized; that each exhibits the two forces, or the force in two directions ( . .); and that they act at a distance, only by acting on the _contiguous_ and intermediate particles. . but assuming that a perfect vacuum were to intervene in the course of the lines of inductive action ( .), it does not follow from this theory, that the particles on opposite sides of such a vacuum could not act on each other. suppose it possible for a positively electrified particle to be in the centre of a vacuum an inch in diameter, nothing in my present views forbids that the particle should act at the distance of half an inch on all the particles forming the inner superficies of the bounding sphere, and with a force consistent with the well-known law of the squares of the distance. but suppose the sphere of an inch were full of insulating matter, the electrified particle would not then, according to my notion, act directly on the distant particles, but on those in immediate association with it, employing _all_ its power in polarizing them; producing in them negative force equal in amount to its own positive force and directed towards the latter, and positive force of equal amount directed outwards and acting in the same manner upon the layer of particles next in succession. so that ultimately, those particles in the surface of a sphere of half an inch radius, which were acted on _directly_ when that sphere was a vacuum, will now be acted on _indirectly_ as respects the central particle or source of action, i.e. they will be polarized in the same way, and with the same amount of force. § . _nature of the electric current._ . the word _current_ is so expressive in common language, that when applied in the consideration of electrical phenomena we can hardly divest it sufficiently of its meaning, or prevent our minds from being prejudiced by it ( . .). i shall use it in its common electrical sense, namely, to express generally a certain condition and relation of electrical forces supposed to be in progression. . a current is produced both by excitement and discharge; and whatsoever the variation of the two general causes may be, the effect remains the same. thus excitement may occur in many ways, as by friction, chemical action, influence of heat, change of condition, induction, &c.; and discharge has the forms of conduction, electrolyzation, disruptive discharge, and convection; yet the current connected with these actions, when it occurs, appears in all cases to be the same. this constancy in the character of the current, notwithstanding the particular and great variations which may be made in the mode of its occurrence, is exceedingly striking and important; and its investigation and development promise to supply the most open and advantageous road to a true and intimate understanding of the nature of electrical forces. . as yet the phenomena of the current have presented nothing in opposition to the view i have taken of the nature of induction as an action of contiguous particles. i have endeavoured to divest myself of prejudices and to look for contradictions, but i have not perceived any in conductive, electrolytic, convective, or disruptive discharge. . looking at the current as a _cause_, it exerts very extraordinary and diverse powers, not only in its course and on the bodies in which it exists, but collaterally, as in inductive or magnetic phenomena. . _electrolytic action._--one of its direct actions is the exertion of pure chemical force, this being a result which has now been examined to a considerable extent. the effect is found to be _constant_ and _definite_ for the quantity of electric force discharged ( . &c.); and beyond that, the _intensity_ required is in relation to the intensity of the affinity or forces to be overcome ( . . .). the current and its consequences are here proportionate; the one may be employed to represent the other; no part of the effect of either is lost or gained; so that the case is a strict one, and yet it is the very case which most strikingly illustrates the doctrine that induction is an action of contiguous particles ( . .). . the process of electrolytic discharge appears to me to be in close analogy, and perhaps in its nature identical with another process of discharge, which at first seems very different from it, i mean _convection_ ( . .). in the latter case the particles may travel for yards across a chamber; they may produce strong winds in the air, so as to move machinery; and in fluids, as oil of turpentine, may even shake the hand, and carry heavy metallic bodies about[a]; and yet i do not see that the force, either in kind or action, is at all different to that by which a particle of hydrogen leaves one particle of oxygen to go to another, or by which a particle of oxygen travels in the contrary direction. [a] if a metallic vessel three or four inches deep, containing oil of turpentine, be insulated and electrified, and a rod with a ball (an inch or more in diameter) at the end have the ball immersed in the fluid whilst the end is held in the hand, the mechanical force generated when the ball is moved to and from the sides of the vessel will soon be evident to the experimenter. . travelling particles of the air can effect chemical changes just as well as the contact of a fixed platina electrode, or that of a combining electrode, or the ions of a decomposing electrolyte ( . .); and in the experiment formerly described, where eight places of decomposition were rendered active by one current ( .), and where charged particles of air in motion were the only electrical means of connecting these parts of the current, it seems to me that the action of the particles of the electrolyte and of the air were essentially the same. a particle of air was rendered positive; it travelled in a certain determinate direction, and coming to an electrolyte, communicated its powers; an equal amount of positive force was accordingly acquired by another particle (the hydrogen), and the latter, so charged, travelled as the former did, and in the same direction, until it came to another particle, and transferred its power and motion, making that other particle active. now, though the particle of air travelled over a visible and occasionally a large space, whilst the particle of the electrolyte moved over an exceedingly small one; though the air particle might be oxygen, nitrogen, or hydrogen, receiving its charge from force of high intensity, whilst the electrolytic particle of hydrogen had a natural aptness to receive the positive condition with extreme facility; though the air particle might be charged with very little electricity at a very high intensity by one process, whilst the hydrogen particle might be charged with much electricity at a very low intensity by another process; these are not differences of kind, as relates to the final discharging action of these particles, but only of degree; not essential differences which make things unlike, but such differences as give to things, similar in their nature, that great variety which fits them for their office in the system of the universe. . so when a particle of air, or of dust in it, electrified at a negative point, moves on through the influence of the inductive forces ( .) to the next positive surface, and after discharge passes away, it seems to me to represent exactly that particle of oxygen which, having been rendered negative in the electrolyte, is urged by the same disposition of inductive forces, and going to the positive platina electrode, is there discharged, and then passes away, as the air or dust did before it. . _heat_ is another direct effect of the _current_ upon substances in which it occurs, and it becomes a very important question, as to the relation of the electric and heating forces, whether the latter is always definite in amount[a]. there are many cases, even amongst bodies which conduct without change, that at present are irreconcileable with the assumption that it is[b]; but there are also many which indicate that, when proper limitations are applied, the heat produced is definite. harris has shown this for a given length of current in a metallic wire, using common electricity[c]; and de la rive has proved the same point for voltaic electricity by his beautiful application of breguet's thermometer[d]. [a] see de la rive's researches, bib. universelle, , xl. p. . [b] amongst others, davy, philosophical transactions, , p. . pelletier's important results, annales de chimie, , lvi. p. . and becquerel's non-heating current, bib. universelle, , lx. . [c] philosophical transactions, , pp. . . [d] annales de chimie, , lxii. . . when the production of heat is observed in electrolytes under decomposition, the results are still more complicated. but important steps have been taken in the investigation of this branch of the subject by de la rive[a] and others; and it is more than probable that, when the right limitations are applied, constant and definite results will here also be obtained. [a] bib. universelle, , xl. ; and ritchie, phil. trans. . p. . * * * * * . it is a most important part of the character of the current, and essentially connected with its very nature, that it is always the same. the two forces are everywhere in it. there is never one current of force or one fluid only. any one part of the current may, as respects the presence of the two forces there, be considered as precisely the same with any other part; and the numerous experiments which imply their possible separation, as well as the theoretical expressions which, being used daily, assume it, are, i think, in contradiction with facts ( , &c.). it appears to me to be as impossible to assume a current of positive or a current of negative force alone, or of the two at once with any predominance of one over the other, as it is to give an absolute charge to matter ( . . .). . the establishment of this truth, if, as i think, it be a truth, or on the other hand the disproof of it, is of the greatest consequence. if, as a first principle, we can establish, that the centres of the two forces, or elements of force, never can be separated to any sensible distance, or at all events not further than the space between two contiguous particles ( .), or if we can establish the contrary conclusion, how much more clear is our view of what lies before us, and how much less embarrassed the ground over which we have to pass in attaining to it, than if we remain halting between two opinions! and if, with that feeling, we rigidly test every experiment which bears upon the point, as far as our prejudices will let us ( .), instead of permitting them with a theoretical expression to pass too easily away, are we not much more likely to attain the real truth, and from that proceed with safety to what is at present unknown? . i say these things, not, i hope, to advance a particular view, but to draw the strict attention of those who are able to investigate and judge of the matter, to what must be a turning point in the theory of electricity; to a separation of two roads, one only of which can be right: and i hope i may be allowed to go a little further into the facts which have driven me to the view i have just given. . when a wire in the voltaic circuit is heated, the temperature frequently rises first, or most at one end. if this effect were due to any relation of positive or negative as respects the current, it would be exceedingly important. i therefore examined several such cases; but when, keeping the contacts of the wire and its position to neighbouring things unchanged, i altered the direction of the current, i found that the effect remained unaltered, showing that it depended, not upon the direction of the current, but on other circumstances. so there is here no evidence of a difference between one part of the circuit and another. . the same point, i.e. uniformity in every part, may be illustrated by what may be considered as the inexhaustible nature of the current when producing particular effects; for these effects depend upon transfer only, and do not consume the power. thus a current which will heat one inch of platina wire will heat a hundred inches ( . note). if a current be sustained in a constant state, it will decompose the fluid in one voltameter only, or in twenty others if they be placed in the circuit, in each to an amount equal to that in the single one. . again, in cases of disruptive discharge, as in the spark, there is frequently a dark part ( .) which, by professor johnson, has been called the neutral point[a]; and this has given rise to the use of expressions implying that there are two electricities existing separately, which, passing to that spot, there combine and neutralize each other[b]. but if such expressions are understood as correctly indicating that positive electricity alone is moving between the positive ball and that spot, and negative electricity only between the negative ball and that spot, then what strange conditions these parts must be in; conditions, which to my mind are every way unlike those which really occur! in such a case, one part of a current would consist of positive electricity only, and that moving in one direction; another part would consist of negative electricity only, and that moving in the other direction; and a third part would consist of an accumulation of the two electricities, not moving in either direction, but mixing up together! and being in a relation to each other utterly unlike any relation which could be supposed to exist in the two former portions of the discharge. this does not seem to me to be natural. in a current, whatever form the discharge may take, or whatever part of the circuit or current is referred to, as much positive force as is there exerted in one direction, so much negative force is there exerted in the other. if it were not so we should have bodies electrified not merely positive and negative, but on occasions in a most extraordinary manner, one being charged with five, ten, or twenty times as much of both positive and negative electricity in equal quantities as another. at present, however, there is no known fact indicating such states. [a] silliman's journal, , xxv. p. . [b] thomson on heat and electricity, p. . . even in cases of convection, or carrying discharge, the statement that the current is everywhere the same must in effect be true ( .); for how, otherwise, could the results formerly described occur? when currents of air constituted the mode of discharge between the portions of paper moistened with iodide of potassium or sulphate of soda ( . .), decomposition occurred; and i have since ascertained that, whether a current of positive air issued from a spot, or one of negative air passed towards it, the effect of the evolution of iodine or of acid was the same, whilst the reversed currents produced alkali. so also in the magnetic experiments ( .) whether the discharge was effected by the introduction of a wire, or the occurrence of a spark, or the passage of convective currents either one way or the other (depending on the electrified state of the particles), the result was the same, being in all cases dependent upon the perfect current. . hence, the section of a current compared with other sections of the same current must be a constant quantity, if the actions exerted be of the same kind; or if of different kinds, then the forms under which the effects are produced are equivalent to each other, and experimentally convertible at pleasure. it is in sections, therefore, we must look for identity of electrical force, even to the sections of sparks and carrying actions, as well as those of wires and electrolytes. . in illustration of the utility and importance of establishing that which may be the true principle, i will refer to a few cases. the doctrine of unipolarity, as formerly stated, and i think generally understood[a], is evidently inconsistent with my view of a current ( .); and the later singular phenomena of poles and flames described by erman and others[b] partake of the same inconsistency of character. if a unipolar body could exist, i.e. one that could conduct the one electricity and not the other, what very new characters we should have a right to expect in the currents of single electricities passing through them, and how greatly ought they to differ, not only from the common current which is supposed to have both electricities travelling in opposite directions in equal amount at the same time, but also from each other! the facts, which are excellent, have, however, gradually been more correctly explained by becquerel[c], andrews[d], and others; and i understand that professor ohms[e] has perfected the work, in his close examination of all the phenomena; and after showing that similar phenomena can take place with good conductors, proves that with soap, &c. many of the effects are the mere consequences of the bodies evolved by electrolytic action. [a] erman, annales de chimie, . lxi. p. . davy's elements, p. . biot, ency. brit. supp, iv. p. . becquerel, traité, i. p. . de la rive, bib. univ. . vii. . [b] erman, annales de chimie, . xxv. . becquerel, ibid. xxxvi. p. [c] becquerel, annales de chimie, . xlvi. p. . [d] andrews, philosophical magazine, . ix. . [e] schweigger's jahrbuch de chimie, &c. . heft . not understanding german, it is with extreme regret i confess i have not access, and cannot do justice, to the many most valuable papers in experimental electricity published in that language. i take this opportunity also of stating another circumstance which occasions me great trouble, and, as i find by experience, may make, me seemingly regardless of the labours of others:--it is a gradual loss of memory for some years past; and now, often when i read a memoir, i remember that i have seen it before, and would have rejoiced if at the right time i could have recollected and referred to it in the progress of my own papers.--m.f. . i conclude, therefore, that the _facts_ upon which the doctrine of unipolarity was founded are not adverse to that unity and indivisibility of character which i have stated the current to possess, any more than the phenomena of the pile itself (which might well bear comparison with those of unipolar bodies,) are opposed to it. probably the effects which have been called effects of unipolarity, and the peculiar differences of the positive and negative surface when discharging into air, gases, or other dielectrics ( . .) which have been already referred to, may have considerable relation to each other[a]. [a] see also hare in silliman's journal, . xxiv. . * * * * * . m. de la rive has recently described a peculiar and remarkable effect of heat on a current when passing between electrodes and a fluid[a]. it is, that if platina electrodes dip into acidulated water, no change is produced in the passing current by making the positive electrode hotter or colder; whereas making the negative electrode hotter increased the deflexion of a galvanometer affected by the current, from ° to ° and even °, whilst making it colder diminished the current in the same high proportions. [a] bibliothèque universelle, , vii. . . that one electrode should have this striking relation to heat whilst the other remained absolutely without, seem to me as incompatible with what i conceived to be the character of a current as unipolarity ( . .), and it was therefore with some anxiety that i repeated the experiment. the electrodes which i used were platina; the electrolyte, water containing about one sixth of sulphuric acid by weight: the voltaic battery consisted of two pairs of amalgamated zinc and platina plates in dilute sulphuric acid, and the galvanometer in the circuit was one with two needles, and gave when the arrangement was complete a deflexion of ° or °. . under these circumstances heating either electrode increased the current; heating both produced still more effect. when both were heated, if either were cooled, the effect on the current fell in proportion. the proportion of effect due to heating this or that electrode varied, but on the whole heating the negative seemed to favour the passage of the current somewhat more than heating the positive. whether the application of heat were by a flame applied underneath, or one directed by a blowpipe from above, or by a hot iron or coal, the effect was the same. . having thus removed the difficulty out of the way of my views regarding a current, i did not pursue this curious experiment further. it is probable, that the difference between my results and those of m. de la rive may depend upon the relative values of the currents used; for i employed only a weak one resulting from two pairs of plates two inches long and half an inch wide, whilst m. de la rive used four pairs of plates of sixteen square inches in surface. * * * * * . electric discharges in the atmosphere in the form of balls of fire have occasionally been described. such phenomena appear to me to be incompatible with all that we know of electricity and its modes of discharge. as _time_ is an element in the effect ( . .) it is possible perhaps that an electric discharge might really pass as a ball from place to place; but as every thing shows that its velocity must be almost infinite, and the time of its duration exceedingly small, it is impossible that the eye should perceive it as anything else than a line of light. that phenomena of balls of fire may appear in the atmosphere, i do not mean to deny; but that they have anything to do with the discharge of ordinary electricity, or are at all related to lightning or atmospheric electricity, is much more than doubtful. * * * * * . all these considerations, and many others, help to confirm the conclusion, drawn over and over again, that the current is an indivisible thing; an axis of power, in every part of which both electric forces are present in equal amount[a] ( . .). with conduction and electrolyzation, and even discharge by spark, such a view will harmonize without hurting any of our preconceived notions; but as relates to convection, a more startling result appears, which must therefore be considered. [a] i am glad to refer here to the results obtained by mr. christie with magneto-electricity, philosophical transactions, , p. note. as regards the current in a wire, they confirm everything that i am contending for. . if two balls a and b be electrified in opposite states and held within each other's influence, the moment they move towards each other, a current, or those effects which are understood by the word current, will be produced. whether a move towards b, or b move in the opposite direction towards a, a current, and in both cases having the same _direction_, will result. if a and b move from each other, then a _current_ in the opposite direction, or equivalent effects, will be produced. . or, as charge exists only by induction ( . .), and a body when electrified is necessarily in relation to other bodies in the opposite state; so, if a ball be electrified positively in the middle of a room and be then moved in any direction, effects will be produced, as _current_ in the same direction (to use the conventional mode of expression) had existed: or, if the ball be negatively electrified, and then moved, effects as if a current in a direction contrary to that of the motion had been formed, will be produced. . i am saying of a single particle or of two what i have before said, in effect, of many ( .). if the former account of currents be true, then that just stated must be a necessary result. and, though the statement may seem startling at first, it is to be considered that, according to my theory of induction, the charged conductor or particle is related to the distant conductor in the opposite state, or that which terminates the extent of the induction, by all the intermediate particles ( , .), these becoming polarized exactly as the particles of a solid electrolyte do when interposed between the two electrodes. hence the conclusion regarding the unity and identity of the current in the case of convection, jointly with the former cases, is not so strange as it might at first appear. * * * * * . there is a very remarkable phenomenon or effect of the electrolytic discharge, first pointed out, i believe, by mr. porrett, of the accumulation of fluid under decomposing action in the current on one side of an interposed diaphragm[a]. it is a mechanical result; and as the liquid passes from the positive towards the negative electrode in all the known cases, it seems to establish a relation to the polar condition of the dielectric in which the current exists ( . .). it has not as yet been sufficiently investigated by experiment; for de la rive says[b], it requires that the water should be a bad conductor, as, for instance, distilled water, the effect not happening with strong solutions; whereas, dutrochet says[c] the contrary is the case, and that, the effect is not directly due to the electric current. [a] annals of philosophy, . viii. p. . [b] annales de chimie, . xxviii. p. . [c] annales de chimie, , xlix. p. . . becquerel, in his traité de l'electricité, has brought together the considerations which arise for and against the opinion, that the effect generally is an electric effect[a]. though i have no decisive fact to quote at present, i cannot refrain from venturing an opinion, that the effect is analogous both to combination and convection ( .), being a case of carrying due to the relation of the diaphragm and the fluid in contact with it, through which the electric discharge is jointly effected; and further, that the peculiar relation of positive and negative small and large surfaces already referred to ( . . .), may be the direct cause of the fluid and the diaphragm travelling in contrary but determinate directions. a very valuable experiment has been made by m. becquerel with particles of clay[b], which will probably bear importantly on this point. [a] vol. iv. p. , . [b] traité de l'electricité, i. p. . * * * * * . _as long as_ the terms _current_ and _electro-dynamic_ are used to express those relations of the electric forces in which progression of either fluids or effects are supposed to occur ( .), _so long_ will the idea of velocity be associated with them; and this will, perhaps, be more especially the case if the hypothesis of a fluid or fluids be adopted. . hence has arisen the desire of estimating this velocity either directly or by some effect dependent on it; and amongst the endeavours to do this correctly, may be mentioned especially those of dr. watson[a] in , and of professor wheatstone[b] in ; the electricity in the early trials being supposed to travel from end to end of the arrangement, but in the later investigations a distinction occasionally appearing to be made between the transmission of the effect and of the supposed fluid by the motion of whose particles that effect is produced. [a] philosophical transactions, . [b] ibid. , p. . . electrolytic action has a remarkable bearing upon this question of the velocity of the current, especially as connected with the theory of an electric fluid or fluids. in it there is an evident transfer of power with the transfer of each particle of the anion or cathion present, to the next particles of the cathion or anion; and as the amount of power is definite, we have in this way a means of localizing as it were the force, identifying it by the particle and dealing it out in successive portions, which leads, i think, to very striking results. . suppose, for instance, that water is undergoing decomposition by the powers of a voltaic battery. each particle of hydrogen as it moves one way, or of oxygen as it moves in the other direction, will transfer a certain amount of electrical force associated with it in the form of chemical affinity ( . . .) onwards through a distance, which is equal to that through which the particle itself has moved. this transfer will be accompanied by a corresponding movement in the electrical forces throughout every part of the circuit formed ( . .), and its effects may be estimated, as, for instance, by the heating of a wire ( .) at any particular section of the current however distant. if the water be a cube of an inch in the side, the electrodes touching, each by a surface of one square inch, and being an inch apart, then, by the time that a tenth of it, or . grs., is decomposed, the particles of oxygen and hydrogen throughout the mass may be considered as having moved relatively to each other in opposite directions, to the amount of the tenth of an inch; i.e. that two particles at first in combination will after the motion be the tenth of an inch apart. other motions which occur in the fluid will not at all interfere with this result; for they have no power of accelerating or retarding the electric discharge, and possess in fact no relation to it. . the quantity of electricity in . grains of water is, according to an estimate of the force which i formerly made ( .), equal to above millions of charges of a large leyden battery; or it would have kept any length of a platina wire / of an inch in diameter red-hot for an hour and a half ( .). this result, though given only as an approximation, i have seen no reason as yet to alter, and it is confirmed generally by the experiments and results of m. pouillet[a]. according to mr. wheatstone's experiments, the influence or effects of the current would appear at a distance of , miles in a second[b]. we have, therefore, in this view of the matter, on the one hand, an enormous quantity of power equal to a most destructive thunder-storm appearing instantly at the distance of , miles from its source, and on the other, a quiet effect, in producing which the power had taken an hour and a half to travel through the tenth of an inch: yet these are the equivalents to each other, being effects observed at the sections of one and the same current ( .). [a] becquerel, traité de l'electricité, v. p. . [b] philosophical transactions, , p. . * * * * * . it is time that i should call attention to the lateral or transverse forces of the _current_. the great things which have been achieved by oersted, arago, ampère, davy, de la rive, and others, and the high degree of simplification which has been introduced into their arrangement by the theory of ampère, have not only done their full service in advancing most rapidly this branch of knowledge, but have secured to it such attention that there is no necessity for urging on its pursuit. i refer of course to magnetic action and its relations; but though this is the only recognised lateral action of the current, there is great reason for believing that others exist and would by their discovery reward a close search for them ( .). . the magnetic or transverse action of the current seems to be in a most extraordinary degree independent of those variations or modes of action which it presents directly in its course; it consequently is of the more value to us, as it gives us a higher relation of the power than any that might have varied with each mode of discharge. this discharge, whether it be by conduction through a wire with infinite velocity ( .), or by electrolyzation with its corresponding and exceeding slow motion ( .), or by spark, and probably even by convection, produces a transverse magnetic action always the same in kind and direction. . it has been shown by several experimenters, that whilst the discharge is of the _same kind_ the amount of lateral or magnetic force is very constant ( . . . . .). but when we wish to compare discharge of different kinds, for the important purpose of ascertaining whether the same amount of current will in its _different forms_ produce the same amount of transverse action, we find the data very imperfect. davy noticed, that when the electric current was passing through an aqueous solution it affected a magnetic needle[a], and dr. ritchie says, that the current in the electrolyte is as magnetic as that in a metallic wire[b], and has caused water to revolve round a magnet as a wire carrying the current would revolve. [a] philosophical transactions, , p. . [b] ibid. , p. . . disruptive discharge produces its magnetic effects: a strong spark, passed transversely to a steel needle, will magnetise it as well as if the electricity of the spark were conducted by a metallic wire occupying the line of discharge; and sir h. davy has shown that the discharge of a voltaic battery in vacuo is affected and has motion given to it by approximated magnets[a]. [a] philosophical transactions, , p. . . thus the three very different modes of discharge, namely, conduction, electrolyzation, and disruptive discharge, agree in producing the important transverse phenomenon of magnetism. whether convection or carrying discharge will produce the same phenomenon has not been determined, and the few experiments i have as yet had time to make do not enable me to answer in the affirmative. * * * * * . having arrived at this point in the consideration of the current and in the endeavour to apply its phenomena as tests of the truth or fallacy of the theory of induction which i have ventured to set forth, i am now very much tempted to indulge in a few speculations respecting its lateral action and its possible connexion with the transverse condition of the lines of ordinary induction ( , .)[a]. i have long sought and still seek for an effect or condition which shall be to statical electricity what magnetic force is to current electricity ( .); for as the lines of discharge are associated with a certain transverse effect, so it appeared to me impossible but that the lines of tension or of inductive action, which of necessity precede that discharge, should also have their correspondent transverse condition or effect ( .). [a] refer for further investigations to .-- .--_dec. ._ . according to the beautiful theory of ampère, the transverse force of a current may be represented by its attraction for a similar current and its repulsion of a contrary current. may not then the equivalent transverse force of static electricity be represented by that lateral tension or repulsion which the lines of inductive action appear to possess ( .)? then again, when current or discharge occurs between two bodies, previously under inductrical relations to each other, the lines of inductive force will weaken and fade away, and, as their lateral repulsive tension diminishes, will contract and ultimately disappear in the line of discharge. may not this be an effect identical with the attractions of similar currents? i.e. may not the passage of static electricity into current electricity, and that of the lateral tension of the lines of inductive force into the lateral attraction of lines of similar discharge, have the same relation and dependences, and run parallel to each other? . the phenomena of induction amongst currents which i had the good fortune to discover some years ago ( . &c. .) may perchance here form a connecting link in the series of effects. when a current is first formed, it tends to produce a current in the contrary direction in all the matter around it; and if that matter have conducting properties and be fitly circumstanced, such a current is produced. on the contrary, when the original current is stopped, one in the same direction tends to form all around it, and, in conducting matter properly arranged, will be excited. . now though we perceive the effects only in that portion of matter which, being in the neighbourhood, has conducting properties, yet hypothetically it is probable, that the nonconducting matter has also its relations to, and is affected by, the disturbing cause, though we have not yet discovered them. again and again the relation of conductors and non-conductors has been shown to be one not of opposition in kind, but only of degree ( , .); and, therefore, for this, as well as for other reasons, it is probable, that what will affect a conductor will affect an insulator also; producing perhaps what may deserve the term of the electrotonic state ( . . .). . it is the feeling of the necessity of some lateral connexion between the lines of electric force ( .); of some link in the chain of effects as yet unrecognised, that urges me to the expression of these speculations. the same feeling has led me to make many experiments on the introduction of insulating dielectrics having different inductive capacities ( . .) between magnetic poles and wires carrying currents, so as to pass across the lines of magnetic force. i have employed such bodies both at rest and in motion, without, as yet, being able to detect any influence produced by them; but i do by no means consider the experiments as sufficiently delicate, and intend, very shortly, to render them more decisive[a]. [a] see onwards .-- .--_dec. ._ . i think the hypothetical question may at present be put thus: can such considerations as those already generally expressed ( .) account for the transverse effects of electrical currents? are two such currents in relation to each other merely by the inductive condition of the particles of matter between them, or are they in relation by some higher quality and condition ( .), which, acting at a distance and not by the intermediate particles, has, like the force of gravity, no relation to them? . if the latter be the case, then, when electricity is acting upon and in matter, its direct and its transverse action are essentially different in their nature; for the former, if i am correct, will depend upon the contiguous particles, and the latter will not. as i have said before, this may be so, and i incline to that view at present; but i am desirous of suggesting considerations why it may not, that the question may be thoroughly sifted. . the transverse power has a character of polarity impressed upon it. in the simplest forms it appears as attraction or repulsion, according as the currents are in the same or different directions: in the current and the magnet it takes up the condition of tangential forces; and in magnets and their particles produces poles. since the experiments have been made which have persuaded me that the polar forces of electricity, as in induction and electrolytic action ( . .), show effects at a distance only by means of the polarized contiguous and intervening particles, i have been led to expect that _all polar forces_ act in the same general manner; and the other kinds of phenomena which one can bring to bear upon the subject seem fitted to strengthen that expectation. thus in crystallizations the effect is transmitted from particle to particle; and in this manner, in acetic acid or freezing water a crystal a few inches or even a couple of feet in length will form in less than a second, but progressively and by a transmission of power from particle to particle. and, as far as i remember, no case of polar action, or partaking of polar action, except the one under discussion, can be found which does not act by contiguous particles[a]. it is apparently of the nature of polar forces that such should be the case, for the one force either finds or developed the contrary force near to it, and has, therefore, no occasion to seek for it at a distance. [a] i mean by contiguous particles those which are next to each other, not that there is _no_ space between them. see ( .). . but leaving these hypothetical notions respecting the nature of the lateral action out of sight, and returning to the direct effects, i think that the phenomena examined and reasoning employed in this and the two preceding papers tend to confirm the view first taken ( .), namely, that ordinary inductive action and the effects dependent upon it are due to an action of the contiguous particles of the dielectric interposed between the charged surfaces or parts which constitute, as it were, the terminations of the effect. the great point of distinction and power (if it have any) in the theory is, the making the dielectric of essential and specific importance, instead of leaving it as it were a mere accidental circumstance or the simple representative of space, having no more influence over the phenomena than the space occupied by it. i have still certain other results and views respecting the nature of the electrical forces and excitation, which are connected with the present theory; and, unless upon further consideration they sink in my estimation, i shall very shortly put them into form as another series of these electrical researches. _royal institution. february th, ._ fourteenth series. § . _nature of the electric force or forces._ § . _relation of the electric and magnetic forces._ § . _note on electrical excitation._ received june , .--read june , . § . _nature of the electric force or forces._ . the theory of induction set forth and illustrated in the three preceding series of experimental researches does not assume anything new as to the nature of the electric force or forces, but only as to their distribution. the effects may depend upon the association of one electric fluid with the particles of matter, as in the theory of franklin, epinus, cavendish, and mossotti; or they may depend upon the association of two electric fluids, as in the theory of dufay and poisson; or they may not depend upon anything which can properly be called the electric fluid, but on vibrations or other affections of the matter in which they appear. the theory is unaffected by such differences in the mode of viewing the nature of the forces; and though it professes to perform the important office of stating _how_ the powers are arranged (at least in inductive phenomena), it does not, as far as i can yet perceive, supply a single experiment which can be considered as a distinguishing test of the truth of any one of these various views, . but, to ascertain how the forces are arranged, to trace them in their various relations to the particles of matter, to determine their general laws, and also the specific differences which occur under these laws, is as important as, if not more so than, to know whether the forces reside in a fluid or not; and with the hope of assisting in this research, i shall offer some further developments, theoretical and experimental, of the conditions under which i suppose the particles of matter are placed when exhibiting inductive phenomena. . the theory assumes that all the _particles_, whether of insulating or conducting matter, are as wholes conductors. . that not being polar in their normal state, they can become so by the influence of neighbouring charged particles, the polar state being developed at the instant, exactly as in an insulated conducting _mass_ consisting of many particles. . that the particles when polarized are in a forced state, and tend to return to their normal or natural condition. . that being as wholes conductors, they can readily be charged, either _bodily_ or _polarly_. . that particles which being contiguous[a] are also in the line of inductive action can communicate or transfer their polar forces one to another _more_ or _less_ readily. [a] see note to .--_dec. ._ . that those doing so less readily require the polar forces to be raised to a higher degree before this transference or communication takes place. . that the _ready_ communication of forces between contiguous particles constitutes _conduction_, and the _difficult_ communication _insulation_; conductors and insulators being bodies whose particles naturally possess the property of communicating their respective forces easily or with difficulty; having these differences just as they have differences of any other natural property. . that ordinary induction is the effect resulting from the action of matter charged with excited or free electricity upon insulating matter, tending to produce in it an equal amount of the contrary state. . that it can do this only by polarizing the particles contiguous to it, which perform the same office to the next, and these again to those beyond; and that thus the action is propagated from the excited body to the next conducting mass, and there renders the contrary force evident in consequence of the effect of communication which supervenes in the conducting mass upon the polarization of the particles of that body ( .). . that therefore induction can only take place through or across insulators; that induction is insulation, it being the necessary consequence of the state of the particles and the mode in which the influence of electrical forces is transferred or transmitted through or across such insulating media. . the particles of an insulating dielectric whilst under induction may be compared to a series of small magnetic needles, or more correctly still to a series of small insulated conductors. if the space round a charged globe were filled with a mixture of an insulating dielectric, as oil of turpentine or air, and small globular conductors, as shot, the latter being at a little distance from each other so as to be insulated, then these would in their condition and action exactly resemble what i consider to be the condition and action of the particles of the insulating dielectric itself ( .). if the globe were charged, these little conductors would all be polar; if the globe were discharged, they would all return to their normal state, to be polarized again upon the recharging of the globe. the state developed by induction through such particles on a mass of conducting mutter at a distance would be of the contrary kind, and exactly equal in amount to the force in the inductric globe. there would be a lateral diffusion of force ( . .), because each polarized sphere would be in an active or tense relation to all those contiguous to it, just as one magnet can affect two or more magnetic needles near it, and these again a still greater number beyond them. hence would result the production of curved lines of inductive force if the inducteous body in such a mixed dielectric were an uninsulated metallic ball ( . &c.) or other properly shaped mass. such curved lines are the consequences of the two electric forces arranged as i have assumed them to be: and, that the inductive force can be directed in such curved lines is the strongest proof of the presence of the two powers and the polar condition of the dielectric particles. . i think it is evident, that in the case stated, action at a distance can only result through an action of the contiguous conducting particles. there is no reason why the inductive body should polarize or affect _distant_ conductors and leave those _near_ it, namely the particles of the dielectric, unaffected: and everything in the form of fact and experiment with conducting masses or particles of a sensible size contradicts such a supposition. . a striking character of the electric power is that it is limited and exclusive, and that the two forces being always present are exactly equal in amount. the forces are related in one of two ways, either as in the natural normal condition of an uncharged insulated conductor; or as in the charged state, the latter being a case of induction. . cases of induction are easily arranged so that the two forces being limited in their direction shall present no phenomena or indications external to the apparatus employed, thus, if a leyden jar, having its external coating a little higher than the internal, be charged and then its charging ball and rod removed, such jar will present no electrical appearances so long as its outside is uninsulated. the two forces which may be said to be in the coatings, or in the particles of the dielectric contiguous to them, are entirely engaged to each other by induction through the glass; and a carrier ball ( .) applied either to the inside or outside of the jar will show no signs of electricity. but if the jar be insulated, and the charging ball and rod, in an uncharged state and suspended by an insulating thread of white silk, be restored to their place, then the part projecting above the jar will give electrical indications and charge the carrier, and at the same time the _outside_ coating of the jar will be found in the opposite state and inductric towards external surrounding objects. . these are simple consequences of the theory. whilst the charge of the inner coating could induce only through the glass towards the outer coating, and the latter contained no more of the contrary force than was equivalent to it, no induction external to the jar could be perceived; but when the inner coating was extended by the rod and ball so that it could induce through the air towards external objects, then the tension of the polarized glass molecules would, by their tendency to return to the normal state, fall a little, and a portion of the charge passing to the surface of this new part of the inner conductor, would produce inductive action through the air towards distant objects, whilst at the same time a part of the force in the outer coating previously directed inwards would now be at liberty, and indeed be constrained to induct outwards through the air, producing in that outer coating what is sometimes called, though i think very improperly, free charge. if a small leyden jar be converted into that form of apparatus usually known by the name of the electric well, it will illustrate this action very completely. . the terms _free charge_ and _dissimulated electricity_ convey therefore erroneous notions if they are meant to imply any difference as to the mode or kind of action. the charge upon an insulated conductor in the middle of a room is in the same relation to the walls of that room as the charge upon the inner coating of a leyden jar is to the outer coating of the same jar. the one is not more _free_ or more _dissimulated_ than the other; and when sometimes we make electricity appear where it was not evident before, as upon the outside of a charged jar, when, after insulating it, we touch the inner coating, it is only because we divert more or less of the inductive force from one direction into another; for not the slightest change is in such circumstances impressed upon the character or action of the force. * * * * * . having given this general theoretical view, i will now notice particular points relating to the nature of the assumed electric polarity of the insulating dielectric particles. . the polar state may be considered in common induction as a forced state, the particles tending to return to their normal condition. it may probably be raised to a very high degree by approximation of the inductric and inducteous bodies or by other circumstances; and the phenomena of electrolyzation ( . . .) seem to imply that the quantity of power which can thus be accumulated on a single particle is enormous. hereafter we may be able to compare corpuscular forces, as those of gravity, cohesion, electricity, and chemical affinity, and in some way or other from their effects deduce their relative equivalents; at present we are not able to do so, but there seems no reason to doubt that their electrical, which are at the same time their chemical forces ( . .), will be by far the most energetic. . i do not consider the powers when developed by the polarization as limited to two distinct points or spots on the surface of each particle to be considered as the poles of an axis, but as resident on large portions of that surface, as they are upon the surface of a conductor of sensible size when it is thrown into a polar state. but it is very probable, notwithstanding, that the particles of different bodies may present specific differences in this respect, the powers not being equally diffused though equal in quantity; other circumstances also, as form and quality, giving to each a peculiar polar relation. it is perhaps to the existence of some such differences as these that we may attribute the specific actions of the different dielectrics in relation to discharge( . .). thus with respect to oxygen and nitrogen singular contrasts were presented when spark and brush discharge were made to take place in these gases, as may be seen by reference to the table in paragraph of the thirteenth series; for with nitrogen, when the small, negative or the large positive ball was rendered inductric, the effects corresponded with those which in oxygen were produced when the small positive or the large negative ball was rendered inductric. . in such solid bodies as glass, lac, sulphur, &c., the particles appear to be able to become polarized in all directions, for a mass when experimented upon so as to ascertain its inductive capacity in three or more directions ( .), gives no indication of a difference. now as the particles are fixed in the mass, and as the direction of the induction through them must change with its change relative to the mass, the constant effect indicates that they can be polarized electrically in any direction. this accords with the view already taken of each particle as a whole being a conductor ( .), and, as an experimental fact, helps to confirm that view. . but though particles may thus be polarized in _any_ direction under the influence of powers which are probably of extreme energy ( .), it does not follow that each particle may not tend to polarize to a greater degree, or with more facility, in one direction than another; or that different kinds may not have specific differences in this respect, as they have differences of conducting and other powers ( . . .). i sought with great anxiety for a relation of this nature; and selecting crystalline bodies as those in which all the particles are symmetrically placed, and therefore best fitted to indicate any result which might depend upon variation of the direction of the forces to the direction of the particles in which they were developed, experimented very carefully with them. i was the more strongly stimulated to this inquiry by the beautiful electrical condition of the crystalline bodies tourmaline and boracite, and hoped also to discover a relation between electric polarity and that of crystallization, or even of cohesion itself ( .). my experiments have not established any connexion of the kind sought for. but as i think it of equal importance to show either that there is or is not such a relation, i shall briefly describe the results. . the form of experiment was as follows. a brass ball . of an inch in diameter, fixed at the end of a horizontal brass rod, and that at the end of a brass cylinder, was by means of the latter connected with a large leyden battery ( .) by perfect metallic communications, the object being to keep that ball, by its connexion with the charged battery in an electrified state, very nearly uniform, for half an hour at a time. this was the inductric ball. the inducteous ball was the carrier of the torsion electrometer ( . .); and the dielectric between them was a cube cut from a crystal, so that two of its faces should be perpendicular to the optical axis, whilst the other four were parallel to it. a small projecting piece of shell-lac was fixed on the inductric ball at that part opposite to the attachment of the brass rod, for the purpose of preventing actual contact between the ball and the crystal cube. a coat of shell-lac was also attached to that side of the carrier ball which was to be towards the cube, being also that side which was furthest from the repelled ball in the electrometer when placed in its position in that instrument. the cube was covered with a thin coat of shell-lac dissolved in alcohol, to prevent the deposition of damp upon its surface from the air. it was supported upon a small table of shell-lac fixed on the top of a stem of the same substance, the latter being of sufficient strength to sustain the cube, and yet flexible enough from its length to act as a spring, and allow the cube to bear, when in its place, against the shell-lac on the inductric ball. [illustration:] . thus it was easy to bring the inducteous ball always to the same distance from the inductric bull, and to uninsulate and insulate it again in its place; and then, after measuring the force in the electrometer ( .), to return it to its place opposite to the inductric ball for a second observation. or it was easy by revolving the stand which supported the cube to bring four of its faces in succession towards the inductric ball, and so observe the force when the lines of inductive action ( .) coincided with, or were transverse to, the direction of the optical axis of the crystal. generally from twenty to twenty-eight observations were made in succession upon the four vertical faces of a cube, and then an average expression of the inductive force was obtained, and compared with similar averages obtained at other times, every precaution being taken to secure accurate results. . the first cube used was of _rock crystal_; it was . of an inch in the side. it presented a remarkable and constant difference, the average of not less than observations, giving for the specific inductive capacity in the direction coinciding with the optical axis of the cube, whilst . and . were the expressions for the two transverse directions. . but with a second cube of rock crystal corresponding results were not obtained. it was . of an inch in the side. the average of many experiments gave for the specific inductive capacity coinciding with the direction of the optical axis, and . and . for the two other directions. . lord ashley, whom i have found ever ready to advance the cause of science, obtained for me the loan of three globes of rock crystal belonging to her grace the duchess of sutherland for the purposes of this investigation. two had such fissures as to render them unfit for the experiments ( . .). the third, which was very superior, gave me no indications of any difference in the inductive force for different directions. . i then used cubes of iceland spar. one . of an inch in diameter gave for the axial direction, and . and . for the two cross directions. the other, . of an inch in the side, gave for the axial direction, whilst . and . were the numbers for the cross direction. . besides these differences there were others, which i do not think it needful to state, since the main point is not confirmed. for though the experiments with the first cube raised great expectation, they have not been generalized by those which followed. i have no doubt of the results as to that cube, but they cannot as yet be referred to crystallization. there are in the cube some faintly coloured layers parallel to the optical axis, and the matter which colours them may have an influence; but then the layers are also nearly parallel to a cross direction, and if at all influential should show some effect in that direction also, which they did not. . in some of the experiments one half or one part of a cube showed a superiority to another part, and this i could not trace to any charge the different parts had received. it was found that the varnishing of the cubes prevented any communication of charge to them, except (in a few experiments) a small degree of the negative state, or that which was contrary to the state of the inductric ball ( . .). . i think it right to say that, as far as i could perceive, the insulating character of the cubes used was perfect, or at least so nearly perfect, as to bear a comparison with shell-lac, glass, &c. ( ). as to the cause of the differences, other than regular crystalline structure, there may be several. thus minute fissures in the crystal insensible to the eye may be so disposed as to produce a sensible electrical difference ( .). or the crystallization may be irregular; or the substance may not be quite pure; and if we consider how minute a quantity of matter will alter greatly the conducting power of water, it will seem not unlikely that a little extraneous matter diffused through the whole or part of a cube, may produce effects sufficient to account for all the irregularities of action that have been observed. . an important inquiry regarding the electrical polarity of the particles of an insulating dielectric, is, whether it be the molecules of the particular substance acted on, or the component or ultimate particles, which thus act the part of insulated conducting polarizing portions ( .). . the conclusion i have arrived at is, that it is the molecules of the substance which polarize as wholes ( .); and that however complicated the composition of a body may be, all those particles or atoms which are held together by chemical affinity to form one molecule of the resulting body act as one conducting mass or particle when inductive phenomena and polarization are produced in the substance of which it is a part. . this conclusion is founded on several considerations. thus if we observe the insulating and conducting power of elements when they are used as dielectrics, we find some, as sulphur, phosphorus, chlorine, iodine, &c., whose particles insulate, and therefore polarize in a high degree; whereas others, as the metals, give scarcely any indication of possessing a sensible proportion of this power ( .), their particles freely conducting one to another. yet when these enter into combination they form substances having no direct relation apparently, in this respect, to their elements; for water, sulphuric acid, and such compounds formed of insulating elements, conduct by comparison freely; whilst oxide of lead, flint glass, borate of lead, and other metallic compounds containing very high proportions of conducting matter, insulate excellently well. taking oxide of lead therefore as the illustration, i conceive that it is not the particles of oxygen and lead which polarize separately under the act of induction, but the molecules of oxide of lead which exhibit this effect, all the elements of one particle of the resulting body, being held together as parts of one conducting individual by the bonds of chemical affinity; which is but another term for electrical force ( .). . in bodies which are electrolytes we have still further reason for believing in such a state of things. thus when water, chloride of tin, iodide of lead, &c. in the solid state are between the electrodes of the voltaic battery, their particles polarize as those of any other insulating dielectric do ( .); but when the liquid state is conferred on these substances, the polarized particles divide, the two halves, each in a highly charged state, travelling onwards until they meet other particles in an opposite and equally charged state, with which they combine, to the neutralization of their chemical, i.e. their electrical forces, and the reproduction of compound particles, which can again polarize as wholes, and again divide to repeat the same series of actions ( .). . but though electrolytic particles polarize as wholes, it would appear very evident that in them it is not a matter of entire indifference _how_ the particle polarizes ( .), since, when free to move ( , &c.) the polarities are ultimately distributed in reference to the elements; and sums of force equivalent to the polarities, and very definite in kind and amount, separate, as it were, from each other, and travel onwards with the elementary particles. and though i do not pretend to know what an atom is, or how it is associated or endowed with electrical force, or how this force is arranged in the cases of combination and decomposition, yet the strong belief i have in the electrical polarity of particles when under inductive action, and the hearing of such an opinion on the general effects of induction, whether ordinary or electrolytic, will be my excuse, i trust, for a few hypothetical considerations. in electrolyzation it appears that the polarized particles would (because of the gradual change which has been induced upon the chemical, i.e. the electrical forces of their elements ( .)) rather divide than discharge to each other without division ( .); for if their division, i.e. their decomposition and recombination, be prevented by giving them the solid state, then they will insulate electricity perhaps a hundredfold more intense than that necessary for their electrolyzation ( , &c.). hence the tension necessary for direct conduction in such bodies appears to be much higher than that for decomposition ( . . .). . the remarkable stoppage of electrolytic conduction by solidification ( . .), is quite consistent with these views of the dependence of that process on the polarity which is common to all insulating matter when under induction, though attended by such peculiar electro-chemical results in the case of electrolytes. thus it may be expected that the first effect of induction is so to polarize and arrange the particles of water that the positive or hydrogen pole of each shall be from the positive electrode and towards the negative electrode, whilst the negative or oxygen pole of each shall be in the contrary direction; and thus when the oxygen and hydrogen of a particle of water have separated, passing to and combining with other hydrogen and oxygen particles, unless these new particles of water could turn round they could not take up that position necessary for their successful electrolytic polarization. now solidification, by fixing the water particles and preventing them from assuming that essential preliminary position, prevents also their electrolysis ( .); and so the transfer of forces in that manner being prevented ( . .), the substance acts as an ordinary insulating dielectric (for it is evident by former experiments ( . .) that the insulating tension is higher than the electrolytic tension), induction through it rises to a higher degree, and the polar condition of the molecules as wholes, though greatly exalted, is still securely maintained. . when decomposition happens in a fluid electrolyte, i do not suppose that all the molecules in the same sectional plane ( .) part with and transfer their electrified particles or elements at once. probably the _discharge force_ for that plane is summed up on one or a few particles, which decomposing, travelling and recombining, restore the balance of forces, much as in the case of spark disruptive discharge ( .); for as those molecules resulting from particles which have just transferred power must by their position ( .) be less favourably circumstanced than others, so there must be some which are most favourably disposed, and these, by giving way first, will for the time lower the tension and produce discharge. . in former investigations of the action of electricity ( , &c.) it was shown, from many satisfactory cases, that the quantity of electric power transferred onwards was in proportion to and was definite for a given quantity of matter moving as anion or cathion onwards in the electrolytic line of action; and there was strong reason to believe that each of the particles of matter then dealt with, had associated with it a definite amount of electrical force, constituting its force of chemical affinity, the chemical equivalents and the electro-chemical equivalents being the same ( .). it was also found with few, and i may now perhaps say with no exceptions ( .), that only those compounds containing elements in single proportions could exhibit the characters and phenomena of electrolytes ( .); oxides, chlorides, and other bodies containing more than one proportion of the electro-negative element refusing to decompose under the influence of the electric current. . probable reasons for these conditions and limitations arise out of the molecular theory of induction. thus when a liquid dielectric, as chloride of tin, consists of molecules, each composed of a single particle of each of the elements, then as these can convey equivalent opposite forces by their separation in opposite directions, both decomposition and transfer can result. but when the molecules, as in the bichloride of tin, consist of one particle or atom of one element, and two of the other, then the simplicity with which the particles may be supposed to be arranged and to act, is destroyed. and, though it may be conceived that when the molecules of bichloride of tin are polarized as wholes by the induction across them, the positive polar force might accumulate on the one particle of tin whilst the negative polar force accumulated on the two particles of chlorine associated with it, and that these might respectively travel right and left to unite with other two of chlorine and one of tin, in analogy with what happens in cases of compounds consisting of single proportions, yet this is not altogether so evident or probable. for when a particle of tin combines with two of chlorine, it is difficult to conceive that there should not be some relation of the three in the resulting molecule analogous to fixed position, the one particle of metal being perhaps symmetrically placed in relation to the two of chlorine: and, it is not difficult to conceive of such particles that they could not assume that position dependent both on their polarity and the relation of their elements, which appears to be the first step in the process of electrolyzation ( . .). § . _relation of the electric and magnetic forces._ . i have already ventured a few speculations respecting the probable relation of magnetism, as the transverse force of the current, to the divergent or transverse force of the lines of inductive action belonging to static electricity ( , &c.). . in the further consideration of this subject it appeared to me to be of the utmost importance to ascertain, if possible, whether this lateral action which we call magnetism, or sometimes the induction of electrical currents ( . , &c.), is extended to a distance _by the action of the intermediate particles_ in analogy with the induction of static electricity, or the various effects, such as conduction, discharge, &c., which are dependent on that induction; or, whether its influence at a distance is altogether independent of such intermediate particles ( .). . i arranged two magneto-electric helices with iron cores end to end, but with an interval of an inch and three quarters between them, in which interval was placed the end or pole of a bar magnet. it is evident, that on moving the magnetic pole from one core towards the other, a current would tend to form in both helices, in the one because of the lowering, and in the other because of the strengthening of the magnetism induced in the respective soft iron cores. the helices were connected together, and also with a galvanometer, so that these two currents should coincide in direction, and tend by their joint force to deflect the needle of the instrument. the whole arrangement was so effective and delicate, that moving the magnetic pole about the eighth of an inch to and fro two or three times, in periods equal to those required for the vibrations of the galvanometer needle, was sufficient to cause considerable vibration in the latter; thus showing readily the consequence of strengthening the influence of the magnet on the one core and helix, and diminishing it on the other. . then without disturbing the distances of the magnet and cores, plates of substances were interposed. thus calling the two cores a and b, a plate of shell-lac was introduced between the magnetic pole and a for the time occupied by the needle in swinging one way; then it was withdrawn for the time occupied in the return swing; introduced again for another equal portion of time; withdrawn for another portion, and so on eight or nine times; but not the least effect was observed on the needle. in other cases the plate was alternated, i.e. it was introduced between the magnet and a for one period of time, withdrawn and introduced between the magnet and b for the second period, withdrawn and restored to its first place for the third period, and so on, but with no effect on the needle. . in these experiments _shell-lac_ in plates . of an inch in thickness, _sulphur_ in a plate . of an inch in thickness, and _copper_ in a plate . of an inch in thickness were used without any effect. and i conclude that bodies, contrasted by the extremes of conducting and insulating power, and opposed to each other as strongly as metals, air, and sulphur, show no difference with respect to magnetic forces when placed in their lines of action, at least under the circumstances described. . with a plate of iron, or even a small piece of that metal, as the head of a nail, a very different effect was produced, for then the galvanometer immediately showed its sensibility, and the perfection of the general arrangement. . i arranged matters so that a plate of _copper_ . of an inch in thickness, and ten inches in diameter, should have the part near the edge interposed between the magnet and the core, in which situation it was first rotated rapidly, and then held quiescent alternately, for periods according with that required for the swinging of the needle; but not the least effect upon the galvanometer was produced. . a plate of shell-lac . of an inch in thickness was applied in the same manner, but whether rotating or not it produced no effect. . occasionally the plane of rotation was directly across the magnetic curve: at other times it was made as oblique as possible; the direction of the rotation being also changed in different experiments, but not the least effect was produced. . i now removed the helices with their soft iron cores, and replaced them by two _flat helices_ wound upon card board, each containing forty-two feet of silked copper wire, and having no associated iron. otherwise the arrangement was as before, and exceedingly sensible; for a very slight motion of the magnet between the helices produced an abundant vibration of the galvanometer needle. . the introduction of plates of shell-lac, sulphur, or copper into the intervals between the magnet and these helices ( .), produced not the least effect, whether the former were quiescent or in rapid revolution ( .). so here no evidence of the influence of the intermediate particles could be obtained ( .). . the magnet was then removed and replaced by a flat helix, corresponding to the two former, the three being parallel to each other. the middle helix was so arranged that a voltaic current could be sent through it at pleasure. the former galvanometer was removed, and one with a double coil employed, one of the lateral helices being connected with one coil, and the other helix with the other coil, in such manner that when a voltaic current was sent through the middle helix its inductive action ( .) on the lateral helices should cause currents in them, having contrary directions in the coils of the galvanometer. by a little adjustment of the distances these induced currents were rendered exactly equal, and the galvanometer needle remained stationary notwithstanding their frequent production in the instrument. i will call the middle coil c, and the external coils a and b. . a plate of copper . of an inch thick and six inches square, was placed between coils c and b, their respective distances remaining unchanged; and then a voltaic current from twenty pairs of inch plates was sent through the coil c, and intermitted, in periods fitted to produce an effect on the galvanometer ( .). if any difference had been produced in the effect of c on a and b. but notwithstanding the presence of air in one interval and copper in the other, the inductive effect was exactly alike on the two coils, and as if air had occupied both intervals. so that notwithstanding the facility with which any induced currents might form in the thick copper plate, the coil outside of it was just as much affected by the central helix c as if no such conductor as the copper had been there ( .). . then, for the copper plate was substituted one of sulphur . of an inch thick; still the results were exactly the same, i.e. there was no action at the galvanometer. . thus it appears that when a voltaic current in one wire is exerting its inductive action to produce a contrary or a similar current in a neighbouring wire, according as the primary current is commencing or ceasing, it makes not the least difference whether the intervening space is occupied by such insulating bodies as air, sulphur and shell-lac, or such conducting bodies as copper, and the other non-magnetic metals. . a correspondent effect was obtained with the like forces when resident in a magnet thus. a single flat helix ( .) was connected with a galvanometer, and a magnetic pole placed near to it; then by moving the magnet to and from the helix, or the helix to and from the magnet, currents were produced indicated by the galvanometer. . the thick copper plate ( .) was afterwards interposed between the magnetic pole and the helix; nevertheless on moving these to and fro, effects, exactly the same in direction and amount, were obtained as if the copper had not been there. so also on introducing a plate of sulphur into the interval, not the least influence on the currents produced by motion of the magnet or coils could be obtained. . these results, with many others which i have not thought it needful to describe, would lead to the conclusion that (judging by the _amount_ of effect produced at a distance by forces transverse to the electric current, i.e. magnetic forces,) the intervening matter, and therefore the intervening particles, have nothing to do with the phenomena; or in other words, that though the inductive force of static electricity is transmitted to a distance by the action of the intermediate particles ( . .), the transverse inductive force of currents, which can also act at a distance, is not transmitted by the intermediate particles in a similar way. . it is however very evident that such a conclusion cannot be considered as proved. thus when the metal copper is between the pole and the helix ( . . .) or between the two helices ( .) we know that its particles are affected, and can by proper arrangements make their peculiar state for the time very evident by the production of either electrical or magnetical effects. it seems impossible to consider this effect on the particles of the intervening matter as independent of that produced by the inductric coil or magnet c, on the inducteous coil or core a ( . .); for since the inducteous body is equally affected by the inductric body whether these intervening and affected particles of copper are present or not ( . .), such a supposition would imply that the particles so affected had no reaction back on the original inductric forces. the more reasonable conclusion, as it appears to me, is, to consider these affected particles as efficient in continuing the action onwards from the inductric to the inducteous body, and by this very communication producing the effect of _no loss_ of induced power at the latter. . but then it may be asked what is the relation of the particles of insulating bodies, such as air, sulphur, or lac, when _they_ intervene in the line of magnetic action? the answer to this is at present merely conjectural. i have long thought there must be a particular condition of such bodies corresponding to the state which causes currents in metals and other conductors ( . . . . .); and considering that the bodies are insulators one would expect that state to be one of tension. i have by rotating non-conducting bodies near magnetic poles and poles near them, and also by causing powerful electric currents to be suddenly formed and to cease around and about insulators in various directions, endeavoured to make some such state sensible, but have not succeeded. nevertheless, as any such state must be of exceedingly low intensity, because of the feeble intensity of the currents which are used to induce it, it may well be that the state may exist, and may be discoverable by some more expert experimentalist, though i have not been able to make it sensible. . it appears to me possible, therefore, and even probable, that magnetic action may be communicated to a distance by the action of the intervening particles, in a manner having a relation to the way in which the inductive forces of static electricity are transferred to a distance ( .); the intervening particles assuming for the time more or less of a peculiar condition, which (though with a very imperfect idea) i have several times expressed by the term _electro-tonic state_ ( . . . .). i hope it will not be understood that i hold the settled opinion that such is the case. i would rather in fact have proved the contrary, namely, that magnetic forces are quite independent of the matter intervening between the inductric and the inductions bodies; but i cannot get over the difficulty presented by such substances as copper, silver, lead, gold, carbon, and even aqueous solutions ( . .), which though they are known to assume a peculiar state whilst intervening between the bodies acting and acted upon ( .), no more interfere with the final result than those which have as yet had no peculiarity of condition discovered in them. . a remark important to the whole of this investigation ought to be made here. although i think the galvanometer used as i have described it ( . .) is quite sufficient to prove that the final amount of action on each of the two coils or the two cores a and b ( . .) is equal, yet there is an effect which _may_ be consequent on the difference of action of two interposed bodies which it would not show. as time enters as an element into these actions[a] ( .), it is very possible that the induced actions on the helices or cores a, b, though they rise to the same degree when air and copper, or air and lac are contrasted as intervening substances, do not do so in the same time; and yet, because of the length of time occupied by a vibration of the needle, this difference may not be visible, both effects rising to their maximum in periods so short as to make no sensible portion of that required for a vibration of the needle, and so exert no visible influence upon it. [a] see annnles de chimie, , tom. li. pp. , . * * * * * . if the lateral or transverse force of electrical currents, or what appears to be the same thing, magnetic power, could be proved to be influential at a distance independently of the intervening contiguous particles, then, as it appears to me, a real distinction of a high and important kind, would be established between the natures of these two forces ( . .). i do not mean that the powers are independent of each other and might be rendered separately active, on the contrary they are probably essentially associated ( .), but it by no means follows that they are of the same nature. in common statical induction, in conduction, and in electrolyzation, the forces at the opposite extremities of the particles which coincide with the lines of action and have commonly been distinguished by the term electric, are polar, and in the cases of contiguous particles act only to insensible distances; whilst those which are transverse to the direction of these lines, and are called magnetic, are circumferential, act at a distance, and if not through the mediation of the intervening particles, have their relations to ordinary matter entirely unlike those of the electrical forces with which they are associated. . to decide this question of the identity or distinction of the two kinds of power, and establish their true relation, would be exceedingly important. the question seems fully within the reach of experiment, and offers a high reward to him who will attempt its settlement. . i have already expressed a hope of finding an effect or condition which shall be to statical electricity what magnetic force is to current electricity ( .). if i could have proved to my own satisfaction that magnetic forces extended their influence to a distance by the conjoined action of the intervening particles in a manner analogous to that of electrical forces, then i should have thought that the natural tension of the lines of inductive action ( .), or that state so often hinted at as the electro-tonic state ( . .), was this related condition of statical electricity. . it may be said that the state of _no lateral action_ is to static or inductive force the equivalent of _magnetism_ to current force; but that can only be upon the view that electric and magnetic action are in their nature essentially different ( .). if they are the same power, the whole difference in the results being the consequence of the difference of _direction_, then the normal or _undeveloped_ state of electric force will correspond with the state of _no lateral action_ of the magnetic state of the force; the electric current will correspond with the lateral effects commonly called magnetism; but the state of static induction which is between the normal condition and the current will still require a corresponding lateral condition in the magnetic series, presenting its own peculiar phenomena; for it can hardly be supposed that the normal electric, and the inductive or polarized electric, condition, can both have the same lateral relation. if magnetism be a separate and a higher relation of the powers developed, then perhaps the argument which presses for this third condition of that force would not be so strong. . i cannot conclude these general remarks upon the relation of the electric and magnetic forces without expressing my surprise at the results obtained with the copper plate ( . .). the experiments with the flat helices represent one of the simplest cases of the induction of electrical currents ( .); the effect, as is well known, consisting in the production of a momentary current in a wire at the instant when a current in the contrary direction begins to pass through a neighbouring parallel wire, and the production of an equally brief current in the reverse direction when the determining current is stopped ( .). such being the case, it seems very extraordinary that this induced current which takes place in the helix a when there is only air between a and c ( .). should be equally strong when that air is replaced by an enormous mass of that excellently conducting metal copper ( .). it might have been supposed that this mass would have allowed of the formation and discharge of almost any quantity of currents in it, which the helix c was competent to induce, and so in some degree have diminished if not altogether prevented the effect in a: instead of which, though we can hardly doubt that an infinity of currents are formed at the moment in the copper plate, still not the smallest diminution or alteration of the effect in a appears ( .). almost the only way of reconciling this effect with generally received notions is, as it appears to me, to admit that magnetic action is communicated by the action of the intervening particles ( . .). . this condition of things, which is very remarkable, accords perfectly with the effects observed in solid helices where wires are coiled over wires to the amount of five or six or more layers in succession, no diminution of effect on the outer ones being occasioned by those within. § _ . note on electrical excitation._ . that the different modes in which electrical excitement takes place will some day or other be reduced under one common law can hardly be doubted, though for the present we are bound to admit distinctions. it will be a great point gained when these distinctions are, not removed, but understood. . the strict relation of the electrical and chemical powers renders the chemical mode of excitement the most instructive of all, and the case of two isolated combining particles is probably the simplest that we possess. here however the action is local, and we still want such a test of electricity as shall apply to it, to cases of current electricity, and also to those of static induction. whenever by virtue of the previously combined condition of some of the acting particles ( .) we are enabled, as in the voltaic pile, to expand or convert the local action into a current, then chemical action can be traced through its variations to the production of _all_ the phenomena of tension and the static state, these being in every respect the same as if the electric forces producing them had been developed by friction. . it was berzelius, i believe, who first spoke of the aptness of certain particles to assume opposite states when in presence of each other ( .). hypothetically we may suppose these states to increase in intensity by increased approximation, or by heat, &c. until at a certain point combination occurs, accompanied by such an arrangement of the forces of the two particles between themselves as is equivalent to a discharge, producing at the same time a particle which is throughout a conductor ( .). . this aptness to assume an excited electrical state (which is probably polar in those forming non-conducting matter) appears to be a primary fact, and to partake of the nature of induction ( .), for the particles do not seem capable of retaining their particular state independently of each other ( .) or of matter in the opposite state. what appears to be definite about the particles of matter is their assumption of a _particular_ state, as the positive or negative, in relation to each other, and not of either one or other indifferently; and also the acquirement of force up to a certain amount. . it is easily conceivable that the same force which causes local action between two free particles shall produce current force if one of the particles is previously in combination, forming part of an electrolyte ( . .). thus a particle of zinc, and one of oxygen, when in presence of each other, exert their inductive forces ( .), and these at last rise up to the point of combination. if the oxygen be previously in union with hydrogen, it is held so combined by an analogous exertion and arrangement of the forces; and as the forces of the oxygen and hydrogen are for the time of combination mutually engaged and related, so when the superior relation of the forces between the oxygen and zinc come into play, the induction of the former or oxygen towards the metal cannot be brought on and increased without a corresponding deficiency in its induction towards the hydrogen with which it is in combination (for the amount of force in a particle is considered as definite), and the latter therefore has its force turned towards the oxygen of the next particle of water; thus the effect may be considered as extended to sensible distances, and thrown into the condition of static induction, which being discharged and then removed by the action of other particles produces currents. . in the common voltaic battery, the current is occasioned by the tendency of the zinc to take the oxygen of the water from the hydrogen, the effective action being at the place where the oxygen leaves the previously existing electrolyte. but schoenbein has arranged a battery in which the effective action is at the other extremity of this essential part of the arrangement, namely, where oxygen goes to the electrolyte[a]. the first may be considered as a case where the current is put into motion by the abstraction of oxygen from hydrogen, the latter by that of hydrogen from oxygen. the direction of the electric current is in both cases the same, when referred to the direction in which the elementary particles of the electrolyte are moving ( . .), and both are equally in accordance with the hypothetical view of the inductive action of the particles just described ( .). [a] philosophical magazine, , xii. , . also de la rive's results with peroxide of manganese. annales de chimie, , lxi. p. .--_dec. ._ . in such a view of voltaic excitement, the action of the particles may be divided into two parts, that which occurs whilst the force in a particle of oxygen is rising towards a particle of zinc acting on it, and falling towards the particle of hydrogen with which it is associated (this being the progressive period of the inductive action), and that which occurs when the change of association takes place, and the particle of oxygen leaves the hydrogen and combines with the zinc. the former appears to be that which produces the current, or if there be no current, produces the state of tension at the termination of the battery; whilst the latter, by terminating for the time the influence of the particles which have been active, allows of others coming into play, and so the effect of current is continued. . it seems highly probable, that excitement by friction may very frequently be of the same character. wollaston endeavoured to refer such excitement to chemical action[a]; but if by chemical action ultimate union of the acting particles is intended, then there are plenty of cases which are opposed to such a view. davy mentions some such, and for my own part i feel no difficulty in admitting other means of electrical excitement than chemical action, especially if by chemical action is meant a final combination of the particles. [a] philosophical transactions, , p. . . davy refers experimentally to the opposite states which two particles having opposite chemical relations can assume when they are brought into the close vicinity of each other, but _not_ allowed to combine[a]. this, i think, is the first part of the action already described ( .); but in my opinion it cannot give rise to a continuous current unless combination take place, so as to allow other particles to act successively in the same manner, and not even then unless one set of the particles be present as an element of an electrolyte ( . .); i.e. mere quiescent contact alone without chemical action does not in such cases produce a _current_. [a] philosophical transactions, , p. . . still it seems very possible that such a relation may produce a high charge, and thus give rise to excitement by friction. when two bodies are rubbed together to produce electricity in the usual way, one at least must be an insulator. during the act of rubbing, the particles of opposite kinds must be brought more or less closely together, the few which are most favourably circumstanced being in such close contact as to be short only of that which is consequent upon chemical combination. at such moments they may acquire by their mutual induction ( .) and partial discharge to each other, very exalted opposite states, and when, the moment after, they are by the progress of the rub removed from each other's vicinity, they will retain this state if both bodies be insulators, and exhibit them upon their complete separation. . all the circumstances attending friction seem to me to favour such a view. the irregularities of form and pressure will cause that the particles of the two rubbing surfaces will be at very variable distances, only a few at once being in that very close relation which is probably necessary for the development of the forces; further, those which are nearest at one time will be further removed at another, and others will become the nearest, and so by continuing the friction many will in succession be excited. finally, the lateral direction of the separation in rubbing seems to me the best fitted to bring many pairs of particles, first of all into that close vicinity necessary for their assuming the opposite states by relation to each other, and then to remove them from each other's influence whilst they retain that state. . it would be easy, on the same view, to explain hypothetically, how, if one of the rubbing bodies be a conductor, as the amalgam of an electrical machine, the state of the other when it comes from under the friction is (as a mass) exalted; but it would be folly to go far into such speculation before that already advanced has been confirmed or corrected by fit experimental evidence. i do not wish it to be supposed that i think all excitement by friction is of this kind; on the contrary, certain experiments lead me to believe, that in many cases, and perhaps in all, effects of a thermo-electric nature conduce to the ultimate effect; and there are very probably other causes of electric disturbance influential at the same time, which we have not as yet distinguished. _royal institution. june, ._ index. * * * * * n.b. a dash rule represents the _italics_ immediately preceding it. the references are sometimes to the individual paragraph, and sometimes to that in conjunction with those which follow. * * * * * _absolute_ charge of matter, . ---- quantity of electricity in matter, , , . acetate of potassa, its electrolysis, . acetates, their electrolysis, . acetic acid, its electrolysis, . _acid_, nitric, formed in air by a spark, . ----, or alkali, alike in exciting the pile, . ----, transference of, . ---- _for battery_, its nature and strength, , . ---- ----, nitric, the best, . ---- ----, effect of different strengths, . ---- _in voltaic pile_, does not evolve the electricity, , . ---- ----, its use, . acids and bases, their relation in the voltaic pile, , . active battery, general remarks on, , . adhesion of fluids to metals, . advantages of a new voltaic battery, . _affinities, chemical_, opposed voltaically, , , . ----, their relation in the active pile, . _air_, its attraction by surfaces, . ----, _charge of_, . ----, ----, by brush, , . ----, ----, by glow, , . ----, convective currents in, , , . ----, dark discharge in, . ----, disruptive discharge in, , , , . ----, induction in, , , , . ----, its insulating and conducting power, , , , . ----, its rarefaction facilitates discharge, . ----, electrified, . ----, electro-chemical decompositions in, , . ----, hot, discharges voltaic battery, , . ----, poles of, , , . ----, _positive and negative_ brush in, , , . ----, ---- glow in, , . ----, ---- spark in, . ----, rarefied, brush in, , . ----, retention of electricity on conductors by, , . ----, _specific inductive capacity of_, . ----, ----, not varied by temperature or pressure, , . _alkali_ has strong exciting power in voltaic pile, , , . ----, transference of, . _amalgamated zinc_, its condition, . ----, how prepared, . ----, its valuable use, , . ---- battery, . _ammonia_, nature of its electrolysis, . ----, solution of, a bad conductor, , . ampère's inductive results, , , _note_. _anions_ defined, , . ----, table of, . ---- related through the entire circuit, . ----, their action in the voltaic pile, . ----, their direction of transfer, , anode defined, . _antimony_, its relation to magneto-electric induction, . ----, chloride of, not an electrolyte, , . ----, oxide of, how affected by the electric current, . ---- _supposed new_ protoxide, . ---- ----, sulphuret, . _animal electricity_, its general characters considered, . ---- is identical with other electricities, , . ----, its chemical force, . ----, enormous amount, . ----, evolution of heat, , ----, magnetic force, . ----, physiological effects, . ----, spark, . ----, tension, . apparatus, inductive, . _see_ inductive apparatus. _arago's magnetic phenomena_, their nature, , . ----, reason why no effect if no motion, , ----, direction of motion accounted for, . ----, due to induced electric currents, , . ----, like electro-magnetic rotations in principle, . ----, not due to direct induction of magnetism, , , , , . ----, obtained with electro-magnets, . ----, produced by conductors only, , . ----, time an element in, . ----, babbage and hershel's results explained, . arago's experiment, sturgeon's form of, . associated voltaic circles, . _atmospheric_ balls of fire, . ----, electricity, its chemical action, . atomic number judged of from electrochemical equivalent, . _atoms of matter_, , . ----, their electric power, , . attraction of particles, its influence in döbereiner's phenomena, . _attractions_, electric, their force, _note_. ----, _chemic, produce_ current force, , , , , . ----, ---- local force, , , , , . ----, hygrometric, . aurora borealis referred to magneto-electric induction, . axis of power, the electric current on, , , . balls of fire, atmospheric, . barlow's revolving globe, magnetic effects explained, , . barry, decomposed bodies by atmospheric electricity, . bases and acids, their relation in the pile, . battery, leyden, that generally used, . _battery, voltaic_, its nature, , . ----, _origin of its power_, , . ----, ---- not in contact, , , ----, ---- chemical, , , , . ----, ----, oxidation of the zinc, , . ----, its circulating force, , . ----, its local force, . ----, quantity of electricity circulating, . ----, intensity of electricity circulating, , . ----, _intensity of its current_, , . ----, ---- increased, , . ----, _its diminution in power_, . ----, ---- _from_ adhesion of fluid, , . ----, ---- ---- peculiar state of metal, . ----, ---- ---- exhaustion of charge, . ----, ---- ---- irregularity of plates, , . ----, use of metallic contact in, , . ----, _electrolytes essential to it_, . ----, ----, why, , . ----, state of metal and electrolyte before contact, . ----, conspiring action of associated affinities, . ----, purity of its zinc, . ----, use of amalgamated zinc in, . ----, _plates, their_ number, . ----, ---- size, . ----, ---- vicinity, . ----, ---- immersion, . ----, ---- relative age, . ----, ---- foulness, . ----, _excited by_ acid, , , . ----, ---- alkali, , , . ----, ---- sulphuretted solutions, . ----, the acid, its use, , ----, acid for, , . ----, nitric acid best for, . ----, construction of, , , . ----, with numerous alternations, . ----, hare's, . ----, general remarks on, . . ----, simultaneous decompositions with, . ----, practical results with, . ----, _improved_, , , . ----, ----, its construction, . ----, ----, power, , . ----, ----, advantages, . ----, ----, disadvantages, . batteries, voltaic, compared, . becquerel, his important secondary results, , . berzelius, his view of combustion, , . biot's theory of electro-chemical decomposition, . bismuth, its relation to magneto-electric induction, . _bodies_ classed in relation to the electric current, . ---- classed in relation to magnetism, . bodies electrolyzable, . bonijol decomposed substances by atmospheric electricity, . boracic acid a bad conductor, . _brush, electric_, . ----, produced, . ----, not affected by nature of conductors, , . ----, is affected by the dielectrics, , , . ----, not dependent on current of air, . ----, proves molecular action of dielectric, , . ----, its analysis, , . ----, nature, , , . ----, form, , , . ----, _ramifications_, . ---- ----, their coalescence, . ----, sound, , . ----, requisite intensity for, . ---- has sensible duration, . ---- is intermitting, , , . ----, _light of_, , , . ----, ----, in different gases, , . ----, dark? , . ----, passes into spark, . ----, spark and glow relation of, , , . ----, in gases, , , . ----, oxygen, , . ----, nitrogen, , . ----, hydrogen, , . ----, coal-gas, , . ----, carbonic acid gas, , . ----, muriatic acid gas, , . ----, rare air, , , . ----, oil of turpentine, . ----, positive, , , . ----, _negative_, , , . ----, ----, of rapid recurrence, , . ----, positive and negative in different gases, , , . _capacity, specific inductive_, . ----. _see_ specific inductive capacity. _carbonic acid gas_ facilitates formation of spark, . ----, brush in, , . ----, glow in, . ----, spark in, , . ----, _positive and negative_ brush in, . ----, ---- discharge in, . ----, non-interference of, , . carbonic oxide gas, interference of, , . _carrying discharge_, . ----. _see_ discharge convective. cathode described, , . _cations_, or cathions, described, , . ----, table of, . ----, direction of their transfer, . cations, are in relation through the entire circuit, . _characters of_ electricity, table of, . ---- the electric current, constant, , . ---- voltaic electricity, . ---- ordinary electricity, . ---- magneto-electricity, . ---- thermo-electricity, . ---- animal electricity, . _charge_, free, . ---- is always induction, , , , . ---- on surface of conductors: why, . ----. _influence of_ form on, . ----, ---- distance on, . ----, loss of, by convection, . ----, removed from good insulators, . ---- of matter, absolute, . ---- _of air_, . ---- ---- by brush, , . ---- ---- by glow, , , . ---- of particles in air, . ---- of oil of turpentine, . ---- of inductive apparatus divided, . ----, residual, of a leyden jar, . ----, _chemical, for battery_, good, . -----, ----, weak and exhausted, , . _chemical action_, the, exciting the pile is oxidation, . ---- _superinduced by_ metals, . ---- ---- platina, , , . ---- tested by iodide of potassium, . chemical actions, distant, opposed to each other, , , . _chemical affinity_ influenced by mechanical forces, . ---- transferable through metals, . ---- statical or local, , , , . ---- current, , , , . _chemical decomposition by_ voltaic electricity, , , . ---- common electricity, , . ---- magneto-electricity, . ---- thermo-electricity, . ---- animal electricity, . ----. _see_ decomposition electro-chemical. chemical and electrical forces identical, , , , , , . _chloride of_ antimony not an electrolyte, . ---- _lead_, its electrolysis, , . ---- ----, electrolytic intensity for, . ---- _silver_, its electrolysis, , , . ---- ----, electrolytic intensity for, . ---- tin, its electrolysis, , . _chlorides in_ solution, their electrolysis, . ---- fusion, their electrolysis, , . circle of anions and cathions, . _circles_, simple voltaic, . ----, associated voltaic, . circuit, voltaic, relation of bodies in, . _classification of bodies in relation to_ magnetism, . ---- the electric current, , . cleanliness of metals and other solids, . _clean platina_, its characters, , . ----, _its power of effecting combination_, , , , . ----, ----. _see_ plates of platina. _coal gas_, brush in, . ----, dark discharge in, . ----, positive and negative brush in, . ----, positive and negative discharge in, . ----, spark in, . colladon on magnetic force of common electricity, . collectors, magneto-electric, . _combination effected by_ metals, , . ---- solids, , . ---- poles of platina, . ---- _platina_, , , , , . ---- ----, as plates, . ---- ----, as sponge, , . ---- ----, cause of, , , , . ---- ----, how, . ---- ----, interferences with, , , . ---- ---- _retarded by_ olefiant gas, . ---- ---- ---- carbonic oxide, , . ---- ---- ---- sulphuret of carbon, . ---- ---- ---- ether, . ---- ---- ---- other substances, , , . comparison of voltaic batteries, , . _conditions_, general, of voltaic decomposition, . ----, new, of electro-chemical decomposition, . _conducting power_ measured by a magnet, . ---- of solid electrolytes, . ---- of water, constant, . _conduction_, , . ----, its nature, , , . ----, of two kinds, . ----, preceded by induction, , , . ---- and insulation, cases of the same kind, , , , , . ----, its relation to the intensity of the current conducted, . ---- common to all bodies, , . ---- by a vacuum, . ---- by lac, , . ---- by sulphur, , . ---- by glass, , . ---- by spermaceti, , . ---- by gases, . ----, slow, , , . ---- affected by temperature, , . ---- by metals diminished by heat, , . ---- increased by heat, , , . ---- of electricity and heat, relation of, . ----, _simple, can occur in electrolytes_, , . ----, ---- with very feeble currents, . ---- by electrolytes without decomposition, , , . ---- and decomposition associated in electrolytes, , , . ---- facilitated in electrolytes, . ---- _by water_ bad, . ---- ---- improved by dissolved bodies, , . ----, electrolytic, stopped, , , . ---- of currents stopped by ice, . ---- conferred by liquefaction, , . ---- _taken away by solidification_, , . ---- ---- why, , . ----, _new law of_, , , . ----, ----, supposed exception to, , . ----, general results as to, . conductive discharge, . _conductors_, electrolytic, . ----, magneto-electric, . ----, their nature does not affect the electric brush, . ----, size of, affects discharge, . ----, form of, affects discharge, , . ----, _distribution of electricity on_, . ----, ----, _affected by_ form, . ----, ----, ---- distance, , . ----, ----, ---- air pressure, . ----, ----, irregular with equal pressure, . constancy of electric current, . _constitution of electrolytes as to_ proportions, , , , . ---- liquidity, , . _contact of metals_ not necessary for electrolyzation, . ----, its use in the voltaic battery, . ---- not necessary for spark, , . _contiguous particles_, their relation to induction, , . ---- active in electrolysis, , . _convection_, , . ---- or convective discharge. _see_ discharge convective. copper, iron, and sulphur circle, . coruscations of lightning, . _coulomb's electrometer_, . ----, precautions in its use, , , . crystals, induction through, . cube, large, electrified, . cubes of crystals, induction through, , . current chemical affinity, , , , . current, voltaic, without metallic contact, , . _current, electric_, . ----, defined, , . ----, nature of, , , , . ----, variously produced, . ----, _produced by_ chemical action, , , . ----, ---- animals, . ----, ---- friction, , , . ----, ---- heat, , ----, ---- discharge of static electricity, , , . ----, ---- _induction by_ other currents, , . ----, ---- ---- magnets, , , . ----, evolved in the moving earth, . ----, in the earth, . ----, natural standard of direction, . ----, none of one electricity, , , . ----, two forces everywhere in it, , , , . ----, one, and indivisible, . ----, an axis of power, , . ----, constant in its characters, , . ----, inexhaustibility of, . ----, _its velocity in_ conduction, . ----, ---- electrolyzation, . ----, regulated by a fine wire, , _note_. ----, affected by heat, . ----, stopped by solidification, . ----, _its section_, , , . ----, ---- presents a constant force, . ----, _produces_ chemical phenomena, . ----, ---- heat, . ----, its heating power uniform, . ----, produces magnetism, . ----, porrett's effects produced by, . ----, _induction of_, , , , , , . ----, ----, on itself, . ----, ----. _see_ induction of electric current. ----, its inductive force lateral, . ----, induced in different metals, , , , . ----, _its transverse effects_, . ----, ---- constant, . ----, _its transverse forces_, . ----, ---- are in relation to contiguous particles, . ----, ---- their polarity of character, . ---- and magnet, their relation remembered, , _note_. _currents_ in air by convection, , . ----, metals by convection, . ----, oil of turpentine by convection, , . curved lines, induction in, . curves, magnetic, their relation to dynamic induction, , . daniell on the size of the voltaic metals, . _dark discharge_, , . ----. _see_ discharge, dark. dates of some facts and publications, , _note after_. _davy's_ theory of electro-chemical decomposition, , . ---- electro-chemical views, . ---- mercurial cones, convective phenomena, . _decomposing force_ alike in every section of the current, , . ----, variation of, on each particle, . _decomposition_ and conduction associated in electrolytes, , . ----, primary and secondary results of, , . ---- _by common electricity_, , . ---- ----, precautions, . _decomposition, electro-chemical_, , . ----, nomenclature of, . ----, new terms relating to, . ----, its distinguishing character, . ----, by common electricity, , . ----, by a single pair of plates, , , , . ----, by the electric current, . ----, without metallic contact, , . ----, its cause, , , . ----, not due to direct attraction or repulsion of poles, , , , , . ----, _dependent on_ previous induction, . ----, ---- the electric current, , , , . ----, ---- intensity of current, . ----, ---- chemical affinity of particles, , , . ----, resistance to, , , . ----, intensity requisite for, , . ----, stopped by solidification, , , . ----, retarded by interpositions, . ----, assisted by dissolved bodies, . ----, division of the electrolyte, , , . ----, transference, , , , , , . ----, why elements appear at the poles, . ----, uncombined bodies do not travel, , . ----, circular series of effects, , . ----, simultaneous, , ----, _definite_, , , , , , , , , , , , , , . ----, ---- independent of variations of electrodes, , , , . ----, necessary intensity of current, , , , . ----, influence of water in, . ----, in air, , , . ----, some general conditions of, . ----, new conditions of, . ----, primary results, . ----, secondary results, , , , . ----, of acetates, . ----, acetic acid, . ----, ammonia, . ----, _chloride of_ antimony, , . ----, ---- lead, , . ----, ---- silver, , , . ----, _chlorides in_ solution, . ----, ---- fusion, , . ----, fused electrolytes, . ----, hydriodic acid and iodides, , . ----, hydrocyanic acid and cyanides, . ----, hydrofluoric acid and fluorides, . ----, _iodide of_ lead, , . ----, ---- potassium, . ----, muriatic acid, , . ----, nitre, . ----, nitric acid, . ----, _oxide_ antimony, . ----, ---- lead, . ----, protochloride of tin, , . ----, protiodide of tin, . ----, sugar, gum, &c., . ----, of sulphate of magnesia, . ----, sulphuric acid, . ----, sulphurous acid, . ----, tartaric acid, . ----, water, , , . ----, _theory of_, , . ----, ----, by a. de la rive, , , , . ----, ----, biot, . ----, ----, davy, , . ----, ----, grotthuss, , , . ----, ----, hachette, , , ----, ----, riffault and chompré, , , . ----, author's theory, , , , , , . _definite_ decomposing action of electricity, , , , , , , . ----, magnetic action of electricity, , , , . ----, _electro-chemical action_, , , . ----, ----, general principles of, , . ----, ----, _in chloride of lead_, . ----, ----, ---- silver, . ----, ----, in hydriodic acid, , . ----, ----, iodide of lead, , . ----, ----, muriatic acid, , , ----, ----, protochloride of tin, . ----, ----, water, , , . degree in measuring electricity, proposal for, . _de la rive_ on heat at the electrodes, . ----, his theory of electro-chemical decomposition, , , , . _dielectrics_, what, . ----, their importance in electrical actions, . ----, their relation to static induction, . ----, their condition under induction, , . ----, their nature affects the brush, . ----, their specific electric actions, , , , , , . difference of positive and negative discharge, , , . differential inductometer, . _direction of_ ions in the circuit, . ----, the electric current, . ----, the magneto-electric current, , . ----, the induced volta-electric current, , , . disruptive discharge, , . _see_ discharge, disruptive. _discharge, electric_, as balls of fire, . ----, of leyden jar, . ----, _of voltaic battery by_ hot air, , . ----, ---- points, . ----, velocity of, in metal, varied, . ----, varieties of, . ----, brush, . _see_ brush. ----, carrying, . _see_ discharge, convective. ----, conductive, . _see_ conduction. ----, dark, , . ----, disruptive, , . ----, electrolytic, , , . ----, glow, . _see_ glow. ----, positive and negative, . ----, spark, . _see_ spark, electric. _discharge, connective_, , , , , , . ----, in insulating media, , . ----, in good conductors, . ----, _with fluid terminations in_ air, , . ----, ---- liquids, . ----, from a ball, , . ----, influence of points in, . ----, _affected by_ mechanical causes, . ----, ---- flame, . ----, with glow, . ----, _charge of a particle in_ air, . ----, ---- oil of turpentine, . ----, charge of air by, , . ----, _currents produced in_ air, , , . ----, ---- oil of turpentine, , . ----, direction of the currents, , . ----, porrett's effects, , ----, positive and negative, , , . ----, related to electrolytic discharge, , . _discharge, dark_, , , . ----, with negative glow, . ----, between positive and negative glow, . ----, in air, . ----, muriatic acid gas, . ----, coal gas, . ----, hydrogen, . ----, nitrogen, . _discharge, disruptive_, . ----, preceded by induction, . ----, determined by one particle, , . ----, necessary intensity, , . ----, determining intensity constant, . ----, related to particular dielectric, . ----, facilitates like action, , , , . ----, its time, , , , . ----, _varied by_ form of conductors, , , . ----, ---- change in the dielectric, , , . ----, ---- rarefaction of air, , , . ----, ---- temperature, , . ----, ---- distance of conductors, , , . ----, ---- size of conductors, . ----, in liquids and solids, . ----, in _different gases_, , , . ----, ---- not alike, . ----, ---- specific differences, , , . ----, _positive and negative_, , , , . ----, ----, distinctions, , , . ----, ----, differences, , . ----, ----, relative facility, , . ----, ----, dependent on the dielectric, . ----, ----, in different gases, , , , . ----, ----, of voltaic current, . ----, brush, . ----, collateral, . ----, dark, , , . ----, glow, . ----, spark, . ----, theory of, , , . _discharge, electrolytic_, , , , , . ----, previous induction, , . ----, necessary intensity, , , , . ----, division of the electrolyte, , . ----, stopped by solidifying the electrolyte, , , . ----, facilitated by added bodies, . ----, in curved lines, , , . ----, proves action of contiguous particles, . ----, positive and negative, . ----, velocity of electric current in, . ----, related to convective discharge, . ----, theory of, , , . discharging train generally used, . disruptive discharge, . _see_ discharge, disruptive. dissimulated electricity, . _distance, its influence_ in induction, , , . ---- over disruptive discharge, , . distant chemical actions, connected and opposed, , . distinction of magnetic and magneto-electric action, , , , . division of a charge by inductive apparatus, . döbereiner on combination effected by platina, , . dulong and thenard on combination by platina and solids, , . dust, charge of its particles, . earth, natural magneto-electric induction in, , , . _elasticity of_ gases, . ---- gaseous particles, . _electric_ brush, . _see_ brush, electric. ---- condition of particles of matter, , . ---- conduction, . _see_ conduction. ---- _current_ defined, , . ---- ----, nature of, , , . ---- ----. _see_ current, electric. ---- ----, _induction of_, , , , , . _see_ induction of electric current. ---- ----, ----, on itself, . ---- discharge, . _see_ discharge. ---- force, nature of, . _see_ forces. ---- induction, . _see_ induction. ---- inductive capacity, . _see_ specific inductive capacity. ---- polarity, . _see_ polarity, electric. ---- spark, . _see_ spark, electric. ---- and magnetic forces, their relation, , , , , , . electrics, charge of, , . _electrical_ excitation, . _see_ excitation. ---- machine generally used, . ---- battery generally used, . ---- and chemical forces identical, , , , , , . _electricities_, their identity, however excited, , . ----, one or two, , . ----, _two_, . ----, ----, their independent existence, . ----, ----, their inseparability, , , . ----, ----, never separated in the current, . _electricity_, quantity of, in matter, , . ----, _its distribution on conductors_, . ----, ---- _influenced by_ form, , . ----, ---- ---- distance, , , . ----, ---- ---- air's pressure, . ----, relation of a vacuum to, . ----, dissimulated, . ----, common and voltaic, measured, , . ----, _its definite_ decomposing action, , , , . ----, ---- heating action, . ----, ---- magnetic action, , . ----, animal, its characters, . ----, magneto-, its characters, . ----, ordinary, its characters, . ----, thermo-, its characters, . ----, voltaic, its characters, . _electricity from magnetism_, , , , , , . ----, _on magnetisation of soft iron by_ currents, , , , . ---- ---- magnets, , . ----, _employing_ permanent magnets, , , . ----, ---- terrestrial magnetic force, , , . ----, ---- _moving conductors_, , , , , , , . ----, ---- ---- essential condition, . ---- _by revolving plate_, , , . ---- ---- a constant source of electricity, , , . ---- ----, law of evolution, . ---- ----, direction of the current evolved, , , , , . ---- ----, course of the currents in the plate, , . ---- by a revolving globe, , . ---- by plates, , . ---- by a wire, , , , , . ----, conductors and magnet may move together, . ----, _current produced_ in a single wire, , , . ----, ---- a ready source of electricity, , _note_. ----, ---- momentary, , , . ----, ---- permanent, , . ----, ---- deflects galvanometer, , , . ----, ---- makes magnets, . ----, ----, shock of, . ----, ----, spark of, . ----, ---- traverses fluids, , . ----, ----, its direction, , , , , , , , , , , , , , . ----, effect of approximation and recession, , , . ----, the essential condition, . ----, general expression of the effects, . ----, from magnets alone, . _electricity of the voltaic pile_, . ---- _its source_, . ---- ---- not metallic contact, , . ---- ---- is in chemical action, , , , , . _electro-chemical decomposition_, , . ----, nomenclature, . ----, general conditions of, . ----, new conditions of, , ----, influence of water in, . ----, primary and secondary results, . ----, definite, , . ----, theory of, . ----. _see_ also decomposition, electrochemical. _electro-chemical equivalents_, , , , . ----, table of, . ----, how ascertained, . ---- always consistent, . ---- same as chemical equivalents, , . ---- able to determine atomic number, . electro-chemical excitation, , , . electrode defined, . _electrodes_ affected by heat, . ----, _varied in_ size, , . ----, ---- nature, . ----. _see_ poles. electrolysis, resistance to, . _electrolyte_ defined, . ---- _exciting, solution of_ acid, , . ---- ---- alkali, , . ---- _exciting_, water, , . ---- ---- sulphuretted solution, . _electrolytes_, their necessary constitution, , , , , , , . ---- consist of single proportionals of elements, , , , . ---- _essential to voltaic pile_, . ---- ----, why, , . ---- conduct and decompose simultaneously, . ---- can conduct feeble currents without decomposition, . ----, as ordinary conductors, , , . ----, solid, their insulating and conducting power, . ----, real conductive power not affected by dissolved matters, . ----, needful conducting power, . ---- are good conductors when fluid, , . _electrolytes non-conductors when solid_ , . ----, why, , . ----, the exception, . _electrolytes_, their particles polarize as wholes, . ----, polarized light sent across, . ----, relation of their moving elements to the passing current, , . ----, their resistance to decomposition, , , . ----, and metal, their states in the voltaic pile, . ----, salts considered as, . ----, acids not of this class, . _electrolytic_ action of the current, , , . ---- conductors, . ---- discharge, . _see_ discharge, electrolytic. ---- induction, , . ---- _intensity_, , , . ---- ---- varies for different bodies, , , . ---- ---- of chloride of lead, . ---- ---- chloride of silver, . ---- ---- sulphate of soda, . ---- ---- water, , . ---- ---- its natural relation, . _electrolyzation_, , , , , . _see_ decomposition electro-chemical. ---- defined, . ---- facilitated, , , , . ---- in a single circuit, , . ----, intensity needful for, , , ---- of chloride of silver, , . ---- sulphate of magnesia, . ---- and conduction associated, , . electro-magnet, inductive effects in, . electro-magnetic induction definite, , . _electrometer, coulomb's_, described, . ----, how used, . _electro-tonic state_, , , , , , . ---- _considered common to all_ metals, . ---- ---- conductors, . ---- is a state of tension, . ---- is dependent on particles, . elementary bodies probably ions, . _elements evolved_ by force of the current, , , . ---- at the poles, why, . ---- determined to either pole, , , . ----, transference of, , . ----, if not combined, do not travel, , . _equivalents_, electro-chemical, , , . ----, chemical and electro-chemical, the same, , . ether, interference of, . _evolution_ of electricity, , . ---- of one electric force impossible, . ---- of elements at the poles, why, . _excitation_, electrical, . ---- by chemical action, , , . ---- by friction, . exclusive induction, . flame favours convectivc discharge, . flowing water, electric currents in, . fluid terminations for convection, . fluids, their adhesion to metals, . fluoride of lead, hot, conducts well, . _force, chemical_, local, , , . ----, circulating, , , , . _force_, electric, nature of, , . ----, inductive, of currents, its nature, , , . _forces, electric_, two, . ----, inseparable, , , , . ---- and chemical, are the same, , . ---- _and magnetic_, relation of, , , , . ---- ----, are they essentially different? , . _forces, exciting, of voltaic apparatus_, , . ----, exalted, , , , . _forces_, polar, . ---- _of the current_, direct, . ---- ----, lateral or transverse, , . _form, its influence on_ induction, , . ---- discharge, , . fox, his terrestrial electric currents, . _friction_ electricity, its characters, . ----, excitement by, . fusion, conduction consequent upon, , . fusinieri, on combination effected by platina, . _galvanometer_, affected by common electricity, , . ----, a correct measure of electricity, , _note_. _gases_, their elasticity, , . ----, conducting power, . ----, _insulating power_, , . ----, ---- not alike, , . ----, _specific inductive capacity_, , . ----, ---- alike in all, . ----, specific influence on brush and spark, , . ----, discharge, disruptive, through, . ----, brush in, . ----, spark in, . ----, _positive and negative brushes in_, . ----, ----, their differences, . ----, positive and negative discharge in, , , . ----, solubility of, in cases of electrolyzation, , . ----, from water, spontaneous recombination of, . ----, mixtures of, affected by platina plates, . ----, mixed, relation of their particles, . _general_ principles of definite electrolytic action, . ---- remarks on voltaic batteries, , . ---- _results as to_ conduction, . ---- ---- induction, . _glass_, its conducting power, . ----, its specific inductive capacity, . ----, _its attraction for_ air, . ----, ---- water, . _globe, revolving of barlow_, effects explained, , . ----, is magnetic, . _glow_, , . ----, produced, . ----, positive, . ----, negative, . ----, favoured by rarefaction of air, . ----, is a continuous charge of air, , , . ----, occurs in all gases, . ----, accompanied by a wind, . ----, its nature, , ----, discharge, . ----, brush and spark relation of, , , , . grotthuss' theory of electro-chemical decomposition, , , . _growth of a_ brush, . ---- spark, . hachette's view of electro-chemical decomposition, . hare's voltaic trough, , . harris on induction in air, . _heat_ affects the two electrodes, . ---- increases the conducting power of some bodies, , , . ----, its conduction related to that of electricity, . ----, as a result of the electric current, , _note_, , . ---- _evolved by_ animal electricity, . ---- ---- common electricity, . ---- ---- magneto-electricity, . ---- ---- thermo-electricity, . ---- ---- voltaic electricity, . helix, inductive effects in, , . hydriodic acid, its electrolyses, , . hydrocyanic acid, its electrolyses, , . hydrofluoric acid, not electrolysable, . _hydrogen_, brush in, . ----, _positive and negative_ brush in, . ----, ---- discharge in, . _hydrogen and oxygen combined by_ platina plates, , . ---- spongy platina, . _ice_, its conducting power, . ---- a non-conductor of voltaic currents, . iceland crystal, induction across, . _identity_, of electricities, , . ---- of chemical and electrical forces, , , , , . ignition of wire by electric current, , _note_, . improved voltaic battery, , . increase of cells in voltaic battery, effect of, . inducteous surfaces, . _induction apparatus_, . ----, fixing the stem, , , . ----, precautions, , , , , . ----, removal of charge, . ----, retention of charge, , . ----, a charge divided, . ----, peculiar effects with, . _induction, static_, . ----, an action of contiguous particles, , , , , , , . ----, consists in a polarity of particles, , , . ----, continues only in insulators, , , . ----, intensity of, sustained, . ----, _influenced by the_ form of conductors, . ----, ---- distance of conductors, . ----, ---- relation of the bounding surfaces, . ----, charge, a case of, , , . ----, exclusive action, . ----, towards space, . ----, across a vacuum, . ---- _through_ air, , . ---- ---- different gases, , . ---- ---- crystals, , ---- ---- lac, , , . ---- ---- metals, , . ---- ---- all bodies, , . ----, _its relation to_ other electrical actions, , . ----, ---- insulation, , , , . ----, ---- conduction, . ----, ---- discharge, , , . ----, ---- electrolyzation, , . ----, ---- intensity, , . ----, ---- excitation, , . ----, its relation to charge, , . ---- an essential general electric function, , . ----, general results as to, . ----, theory of, , , , , . ---- _in curved lines_, , , . ---- ----, _through_ air, , . ---- ----, ---- other gases, . ---- ----, ---- lac, . ---- ----, ---- sulphur, . ---- ----, ---- oil of turpentine, . _induction, specific_, , , . ----, _the problem_ stated, . ----, ---- solved, . ----, _of air_, . ----, ----, invariable, , . ----, _of gases_, , . ----, ---- alike in all, . ----, of shell-lac, , . ----, glass, . ----, sulphur, . ----, spermaceti, . ----, certain fluid insulators, . _induction of electric currents_, , , , , , , , , . ----, on aiming the principal current, , , . ----, on stopping the principal current, , , , , . ---- by approximation, , . ---- by increasing distance, , . ---- _effective through_ conductors, , , . ---- ---- insulators, , , . ---- in different metals, , , , . ---- in the moving earth, . ---- in flowing water, . ---- in revolving plates, , . ----, _the induced current, its_ direction, , . ----, ---- duration, , , . ----, ----, traverses fluids, , . ----, ----, its intensity in different conductors, , , , , . ----, ----, not obtained by leyden discharge, . ----, ampère's results, , , , _note_. _induction of a current on itself_, , . ----, apparatus used, . ----, _in a_ long wire, , , , . ----, ---- doubled wire, . ----, ---- helix, , . ---- in doubled helices, . ---- in an electro-magnet, , . ----, wire and helix compared, . ----, short wire, effects with, . ----, action momentary, , , . ----, causes no permanent change in the current, . ----, not due to momentum, . ----, induced current separated, , . ----, _effect at_ breaking contact, , , , . ----, ---- making contact, , . ----, _effects produced_, shock, , , . ----, ---- spark, , , . ----, ---- chemical decomposition, . ----, ---- ignition of wire, , . ----, cause is in the conductor, , . ----, general principles of the action, , . ----, direction of the forces lateral, . _induction, magnetic_, , , . ----, by intermediate particles, , , , . ----, _through_ quiescent bodies, , , , . ----, ---- moving bodies, , , . ---- and magneto-electric, distinguished, , , , . _induction_, magneto-electric, , , , , , . _see_ arago's magnetic phenomena. ----, magnelectric, . ----, electrolytic, , , , . ----, volta-electric, . inductive capacity, specific, , . _inductive force of currents_ lateral, , . ----, its nature, , , , . _inductive force, lines of_, , , . ----, often curved, , , . ----, exhibited by the brush, . ----, their lateral relation, , , . ----, their relation to magnetism, , , . inductometer, differential, , . inductric surfaces, . inexhaustible nature of the electric current, . inseparability of the two electric forces, , , , . insulating power of different gases, , , . _insulation_, , , . ----, its nature, . ---- is sustained induction, . ----, degree of induction sustained, . ---- _dependent on the_ dielectrics, . ---- ---- distance in air, , , . ---- ---- density of air, , . ---- ---- induction, . ---- ---- form of conductors, , . ----, as affected by temperature of air, , . ---- _in different gases_, , . ---- ---- differs, . ---- in liquids and solids, . ---- in metals, , , . ---- and conduction not essentially different, , , , , . ----, its relation to induction, , , , . _insulators_, liquid, good, . ----, solid, good, . ----, the best conduct, , , , , . ---- tested as to conduction, . ---- and conductors, relation of, , , . _intensity_, its influence in conduction, . ----, inductive, how represented, . ----, relative, of magneto-electric currents, , , , . ---- of disruptive discharge constant, . ----, electrolytic, , , , . ---- necessary for electrolyzation, , . ---- _of the current of single circles_, . ---- ---- increased, . ---- of electricity in the voltaic battery, , . ---- of voltaic current increased, , . _interference with combining power of platina_, , . ---- by olefiant gas, . ---- carbonic oxide, . ---- sulphuret of carbon, . ---- ether, . interpositions, their retarding effects, . _iodides in_ solution, their electrolysis, . ---- fusion, their electrolysis, , . _iodide_ of lead, electrolysed, , . ---- of potassium, test of chemical action, . _ions_, what, , , , , . ---- not transferable alone, , , . ----, table of, . _iron_, both magnetic and magneto-electric at once, , . ----, copper and sulphur circles, . jenkin, his shock by one pair of plates, . kemp, his amalgam of zinc, . knight, dr. gowin, his magnet, . _lac_, charge removed from, . ----, induction through, . ----, specific inductive capacity of, , . ----, effects of its conducting power, . ----, its relation to conduction and insulation, . _lateral_ direction of inductive forces of currents, , . ---- forces of the current, , . _law of_ conduction, new, , , . ---- magneto-electric induction, . ---- volta-electric induction, . _lead_, chloride of, electrolysed, , . ----, fluoride of, conducts well when heated, . ----, iodide of, electrolysed, , . ----, oxide of, electrolysed, . _leyden jar_, condition of its charge, . ----, its charge, nature of, . ----, its discharge, . ----, its residual charge, . _light_, polarized, passed across electrolytes, . ----, _electric_, , , , _note_. ----, ----, spark, , . ----, ----, brush, , , . ----, ----, glow, . lightning, , , . _lines of inductive force_, , , ---- often curved, , , . ----, as shown by the brush, . ----, their lateral relation, , , . ----, their relation to magnetism, , , . liquefaction, conduction consequent upon, , , . liquid bodies which are non-conductors, . local chemical affinity, , , , . _machine_, electric, evolution of electricity by, . ------, magneto-electric, , , , . _magnelectric_ induction, . ----, collectors or conductors, . _magnesia_, sulphate, decomposed against water, , . ----, transference of, . _magnet_, a measure of conducting power, . ---- _and_ current, their relation remembered, , _note_. ---- ---- plate revolved together, . ---- ---- cylinder revolved together, . ---- revolved alone, , . ---- and moving conductors, their general relation, . ---- made by induced current, , . ----, electricity from, , , . _magnetic_ bodies, but few, . ----, curves, their inductive relation, , . ---- _effects of_ voltaic electricity, . ---- ---- common electricity, , . ---- ---- magneto-electricity, , , . ---- ---- thermo-electricity, . ---- ---- animal electricity, . ---- and electric forces, their relation, , , , , , . ---- forces active through intermediate particles, , , , . ---- _forces of the current_, . ---- ---- very constant, . ---- deflection by common electricity, , . ---- phenomena of arago explained, . ---- induction. _see_ induction, magnetic. ---- _induction through_ quiescent bodies, , , , . ---- ---- moving bodies, , . ---- and magneto-electric action distinguished, , , , . _magnetism_, electricity evolved by, . ----, its relation to the lines of inductive force, , , . ---- bodies classed in relation to, . _magneto-electric currents_, their intensity, , , , . ----, their direction, , . ---- traverse fluids, . ---- momentary, . ---- permanent, . ---- in all conductors, , . _magneto-electric induction_, , . ----, terrestrial, , . ----, law of, . ----. _see_ arago's magnetic phenomena. _magneto-electric machines_, , , . ----, inductive effects in their wires, , _magneto-electricity_, its general characters considered, , &c. ---- identical with other electricities, . ----, its tension, . ----, evolution of heat, . ----, magnetic force, . ----, chemical force, . ----, spark, . ----, physiological effects, . ----. _see_ induction, magnetic. _matter_, atoms of, , . ----, new condition of, , , , , , . ----, quantity of electricity in, , , , . ----, absolute charge of, . _measures of electricity_, galvanometer, , _note_. ----, voltameter, , , . ----, metal precipitated, , . measure of specific inductive capacity, , . _measurement of_ common and voltaic electricities, , , . ---- _electricity_, degree, , . ---- ---- by voltameter, , , . ---- ---- by galvanometer, , _note_. ---- ---- by metal precipitated, , . mechanical forces affect chemical affinity, . mercurial terminations for convection, . _mercury_, periodide of, an exception to the law of conduction? , . ----, perchloride of, , . _metallic contact_ not necessary for electrolyzation, . ---- not essential to the voltaic current, , , . ---- its use in the pile, , . metallic poles, . metal and electrolyte, their state, . _metals_, adhesion of fluids to, . ----, _their power of inducing combination_ , . ----, ---- interfered with, . ----, static induction in, , . ----, different, currents induced in, , . ----, generally secondary results of electrolysis, . ---- transfer chemical force, . ----, transference of, , . ---- insulate in a certain degree, . ----, convective currents in, . ----, but few magnetic, . model of relation of magnetism and electricity, . molecular inductive action, , . _motion_ essential to magneto-electric induction, , , . ---- across magnetic curves, . ---- _of conductor and magnet, relative_, . ---- ---- not necessary, . moving magnet is electric, . _muriatic acid gas_, its high insulating power, . ----, brush in, . ----, dark discharge in, . ----, glow in, . ----, positive and negative brush in, . ----, _spark in_, , . ----, ----, has no dark interval, , . _muriatic acid_ decomposed by common electricity, . ----, its electrolysis (primary), , . nascent state, its relation to combination, , . _natural_ standard of direction for current, . ---- relation of electrolytic intensity, . _nature of the electric_ current, . ---- force or forces, . _negative_ current, none, , . ---- _discharge_, , . ---- ----, as spark, , . ---- ----, as brush, , . ---- spark or brush, , . _negative and positive discharge_, , , ---- in different gases, . _new_ electrical condition of matter, , , , , , . ---- law of conduction, , , . _nitric acid_ formed by spark in air, . ---- _favours_ excitation of current, , ---- ---- transmission of current, . ---- is best for excitation of battery, . ----, nature of its electrolysis, . _nitrogen_ determined to either pole, , , . ---- a secondary result of electrolysis, , . ----, brush in, . ----, dark discharge in, . ----, glow in, . ----, spark in, , . ----, _positive and negative_ brush in, . ----, ---- discharge in, . ----, its influence on lightning, . nomenclature, , . nonconduction by solid electrolytes, , , . note on electrical excitation, . nuclei, their action, , . olefiant gas, interference of, , . _ordinary electricity_, its tension, . ---- evolution of heat, . ---- magnetic force, , . ---- _chemical force_, , . ---- ----, precautions, . ---- spark, . ---- physiological effect, . ---- general characters considered, . ----, identity with other electricities, . origin of the force of the voltaic pile, , , . oxidation the origin of the electric current in the voltaic pile, , . oxide of lead electrolysed, . _oxygen_, brush in, . ----, _positive and negative_ brush in, , ----, ---- discharge in, . ----, solubility of, in cases of electrolyzation, , . ----, spark in, . ---- _and hydrogen combined by_ platina plates, , , . ---- ---- spongy platina, , . ---- ---- other metals, . _particles_, their nascent state, . ---- in air, how charged, . ----, neighbouring, their relation to each other, , , . ----, contiguous, active in induction, , . ---- of a dielectric, their inductive condition, , , . ----, polarity of, when under induction, , . ----, _how polarised_, , . ----, ----, in any direction, . ----, ----, as wholes or elements, . ----, ----, in electrolytes, . ----, crystalline, . ----, contiguous, active in electrolysis, , . ----, _their_ action in electrolyzation, , , . ----, ---- local chemical action, , . ----, ---- relation to electric action, . ----, ---- electric action, , , . path of the electric spark, . phosphoric acid not an electrolyte, . _physiological effects of_ voltaic-electricity, . ---- common electricity, . ---- magneto-electricity, , . ---- thermo-electricity, . ---- animal electricity, . _pile, voltaic_, electricity of, . ----. _see_ battery, voltaic. _plates of platina_ effect combination, , , , . ---- _prepared by_ electricity, , , . ---- ---- friction, . ---- ---- heat, . ---- ---- chemical cleansing, , , ----, clean, their general properties, , . ----, _their power preserved_, . ----, ---- in water, . ----, _their power diminished by_ action, . ----, ---- exposure to air, . ----, _their power affected by_ washing in water, . ----, ---- heat, , . ----, ---- presence of certain gases, , . ----, their power, cause of, , , . ----, _theory of their action_, döbereiner's, . ----, ----, dulong and thenard's, . ----, ----, fusinieri's, . ----, ----, author's, , , , . _plates of voltaic battery_ foul, . ----, new and old, . ----, vicinity of, . ----, immersion of, , . ----, number of, , . ----, large or small, . _platina_, clean, its characters, , . ---- attracts matter from the air, . ----, spongy, its state, . ----, _clean, its power of effecting combination_, , , , , . ----, ---- interfered with, . ----, _its action retarded by_ olefiant gas, , . ----, ----, carbonic oxide, , . ----. _see_ combination, plates of platina, and interference. ---- poles, recombination effected by, , . plumbago poles for chlorides, . poisson's theory of electric induction, . _points_, favour convective discharge, . ----, fluid for convection, . _polar_ forces, their character, . ---- decomposition by common electricity, , , . _polarity_, meaning intended, , . ---- of particles under induction, , . ----, _electric_, , . ----, ----, its direction, , , ----, ----, its variation, . ----, ----, its degree, . ----, ----, in crystals, . ----, ----, in molecules or atoms, . ----, ----, in electrolytes, . polarized light across electrolytes, . _poles, electric_, their nature, , , , . ----, appearance of evolved bodies at, accounted for, . ---- one element to either? , , . ----, of air, , , . ----, of water, , . ----, of metal, . ----, of platina, recombination effected by, , . ----, of plumbago, . poles, magnetic, distinguished, , _note_. porrett's peculiar effects, . _positive_ current none, , . ---- _discharge_, , . ---- ----, as spark, , . ---- ----, as brush, , . ---- spark or brush, , . ---- _and negative_, convective discharge, . ---- ---- _disruptive discharge_, , , , . ---- ---- ---- in different gases, . ---- ---- voltaic discharge, . ---- ---- electrolytic discharge, . potassa acetate, nature of its electrolysis, . potassium, iodide of, electrolysed, . power of voltaic batteries estimated, . powers, their state of tension in the pile, . practical results with the voltaic battery, . pressure of air retains electricity, explained, , . primary electrolytical results, . principles, general, of definite electrolytic action, . proportionals in electrolytes, single, , . _quantity of electricity in_ matter, , , , . ---- voltaic battery, . rarefaction of air facilitates discharge, why, . recombination, spontaneous, of gases from water, . _relation_, by measure, of electricities, . ---- of magnets and moving conductors, . ---- of magnetic induction to intervening bodies, , . ---- of a current and magnet, to remember, , _note_. ---- of electric and magnetic forces, , , , , , . ---- of conductors and insulators, , , , . ---- of conduction and induction, , . ---- _of induction and_ disruptive discharge, . ---- ---- electrolyzation, , . ---- ---- excitation, , . ---- ---- charge, , , . ---- of insulation and induction, , , , . ----, lateral, of lines of inductive force, , , . ---- of a vacuum to electricity, . ---- of spark, brush, and glow, , , . ---- of gases to positive and negative discharge, . ---- of neighbouring particles to each other, , . ---- _of elements in_ decomposing electrolytes, , . ---- ---- exciting electrolytes, . ---- of acids and bases voltaically, , . remarks on the active battery, , . residual charge of a leyden jar, . _resistance_ to electrolysis, , , , . ---- of an electrolyte to decomposition, . _results_ of electrolysis, primary or secondary, , . ----, practical, with the voltaic battery, . ----, general, as to induction, , . retention of electricity by pressure of the atmosphere explained, , . _revolving_ plate. _see_ arago's phenomena. ---- _globe, barlow's_, effect explained, , , . ---- ----, magnetic, . ---- ----, direction of currents in, , . riffault's and chompré's theory of electro-chemical decomposition, , , . rock crystal, induction across, . room, insulated and electrified, . rotation of the earth a cause of magneto-electric induction, . salts considered as electrolytes, . scale of electrolytic intensities, . _secondary electrolytical results_, , , , . ---- become measures of the electric current, . _sections of the current_, , . ----, decomposing force alike in all, , . _sections of lines of inductive action_, . ----, amount of force constant, . shock, strong, with one voltaic pair, . _silver, chloride of_, its electrolyzation, , , . ----, electrolytic intensity for, . silver, sulphuret of, hot, conducts well, . _simple voltaic circles_, . ----, decomposition effected by, , , . single and many pairs of plates, relation of, . _single voltaic circuits_, . ---- without metallic contact, . ---- with metallic contact, . ---- their force exalted, . ---- _give_ strong shocks, . ---- ---- a bright spark, . _solid electrolytes are non-conductors_, , , . ----, why, , . _solids, their power of inducing combination_, , . ---- interfered with, . solubility of gases in cases of electrolyzation, , . _source of electricity in the voltaic pile_, . ---- is chemical action, , , , . spark, , . _spark, electric, its_ conditions, , , . ---- path, . ---- light, . ---- insensible duration or time, . ---- accompanying dark parts, , . ---- determination, . . _spark is affected by the_ dielectrics, , . ---- size of conductor, . ---- form of conductor, , . ---- rarefaction of air, . _spark_, atmospheric or lightning, , . ----, negative, , , , , . ----, positive, , , , , , . ----, ragged, , . ----, when not straight, why, . ----, variation in its length, . ----, tendency to its repetition, . ----, facilitates discharge, , . ----, passes into brush, . ----, preceded by induction, . ----, forms nitric acid in air, . ----, in gases, , . ----, in air, . ----, in nitrogen, , . ----, in oxygen, . ----, in hydrogen, . ----, in carbonic acid, , . ----, in muriatic acid gas, , . ----, in coal-gas, . ----, in liquids, . ----, precautions, , . ----, voltaic, without metallic contact, , . ---- from single voltaic pair, . ---- from common and voltaic electricity assimilated, . ----, first magneto-electric, . ---- of voltaic electricity, . ---- of common electricity, . ---- of magneto-electricity, . ---- of thermo-electricity, . ---- of animal electricity, . ----, brush and glow related, , , . sparks, their expected coalition, . specific induction. _see_ induction, specific, . _specific inductive capacity_, . ----, apparatus for, . ---- of lac, , , . ---- of sulphur, , . ---- of air, . ---- of gases, , . ---- of glass, . _spermaceti_, its conducting power, , . ----, its relation to conduction and insulation, . standard of direction in the current, . state, electrotonic, , , , , , . static induction. _see_ induction, static. _sturgeon_, his form of arago's experiment, . ----, use of amalgamated zinc by, , . _sulphate of soda_, decomposed by common electricity, . ----, electrolytic intensity for, . _sulphur_ determined to either pole, , , . ----, its conducting power, , . ----, its specific inductive capacity, . ----, copper and iron, circle, . _sulphuret of_ carbon, interference of, . ---- silver, hot, conducts well, . sulphuretted solution excites the pile, . _sulphuric acid_, conduction by, , . ----, magneto-electric induction on, , . ---- in voltaic pile, its use, . ---- not an electrolyte, . ----, its transference, . ----, its decomposition, , . sulphurous acid, its decomposition, . _summary of_ conditions of conduction, . ---- molecular inductive theory, . _table of_ discharge in gases, . ---- electric effects, . ---- electro-chemical equivalents, . ---- electrolytes affected by fusion, . ---- insulation in gases, . ---- ions, anions, and cathions, . tartaric acid, nature of its electrolysis, . _tension_, inductive, how represented, . ---- of voltaic electricity, . ---- of common electricity, . ---- of thermo-electricity, . ---- of magneto-electricity, . ---- of animal electricity, . ---- of zinc and electrolyte in the voltaic pile, . terrestrial electric currents, . _terrestrial magneto-electric induction_, . ---- cause of aurora borealis, . ----, _electric currents produced by_, , . ----, ----, _in helices_ alone, . ----, ----, ---- with iron, , . ----, ----, ---- with a magnet, . ----, ---- a single wire, . ----, ---- a revolving plate, . ----, ---- a revolving ball, . ----, ---- the earth, . test between magnetic and magneto-electric action, , . _theory of_ combination of gases by clean platina, , , , . ---- electro-chemical decomposition, , , , . ---- the voltaic apparatus, , . ---- static induction, , , , , . ---- disruptive discharge, , , . ---- arago's phenomena, . _thermo-electricity_, its general characters, . ---- identical with other electricities, . ----, its evolution of heat, . ----, magnetic, force, . ----, physiological effects, . ----, spark, . time, , , , , , , , , , , , , . _tin_, iodide of, electrolysed, . ----, protochloride, electrolysis of, definite, , . _torpedo_, nature of its electric discharge, . ----, its enormous amount of electric force, . transfer of elements and the current, their relation, , . _transference_ is simultaneous in opposite directions, , . ----, uncombined bodies do not travel, , , . ---- _of elements_, , , , , . ---- ---- across great intervals, , . ---- ----, its nature, , , , . ---- of chemical force, . transverse forces of the current, , . travelling of charged particles, . trough, voltaic. _see_ battery, voltaic. _turpentine, oil of_, a good fluid insulator, . ----, its insulating power destroyed, . ---- charged, . ----, brush in, , ----, electric motions in, , , ----, convective currents in, , . unipolarity, . vacuum, its relation to electricity, . vaporization, . _velocity of_ conduction in metals varied, . ---- the electric discharge, , . ---- conductive and electrolytic discharge, difference of, . vicinity of plates in voltaic battery, . volta-electric induction, . _volta-electrometer_, , . ----, fluid decomposed in it, water, , , . ----, forms of, , . ---- _tested for variation of_ electrodes, , . ---- ---- fluid within, . ---- ---- intensity, . ----, strength of acid used in, , . ----, _its indications by_ oxygen and hydrogen, . ----, ---- hydrogen, . ----, ---- oxygen, . ----, how used, . voltameter, . _voltaic battery_, its nature, , . ----, remarks on, , . ----, improved, , . ----, practical results with, . ----. _see_ battery, voltaic. _voltaic circles, simple_, . ----, decomposition by, . voltaic circles associated, or battery, . _voltaic circuit_, relation of bodies in, . ----, defined, , . ----, origin of, , . ----, its direction, , , ----, intensity increased, , . ----, produced by oxidation of zinc, , . ---- not due to combination of oxide and acid, , . ----, _its relation to the_ combining oxygen, , . ----, ---- combining sulphur, . ----, ---- the transferred elements, , . ----, relation of bodies in, . voltaic current, . _see_ current, electric. voltaic discharge, positive and negative, . voltaic decomposition, , . _see_ decomposition, electro-chemical. _voltaic electricity_, identical with electricity, otherwise evolved, , . ----, _discharged by_ points, . ----, ---- hot air, , . ----, its tension, , . ----, evolution of heat by, . ----, its magnetic force, . ----, its chemical force, . ----, its spark, . ----, its physiological effects, . ----, its general characters considered, . _voltaic pile_ distinguished, , _note_. ----, electricity of, . ----, depends on chemical action, . ----, relation of acid and bases in the, . ----. _see_ battery, voltaic. _voltaic spark_ without contact, , . ----, precautions, , . voltaic trough, . _see_ battery, voltaic. _water_, flowing, electric currents in, . ----, retardation of current by, . ----, _its direct conducting power_, , , . ----, ---- constant, . ----, electro-chemical decomposition against, , . ----, poles of, , , . ----, its influence in electro-chemical decomposition, . ---- is the great electrolyte, . ----, _the exciting electrolyte when_ pure, . ----, ---- acidulated, , , . ----, ---- alkalized, , , . ----, electrolytic intensity for, , , . ---- electrolyzed in a single circuit, . ----, its electrolysis definite, , , . ----, decomposition of by fine wires, . ----, quantity of electricity in its elements, , . ----, determined to either pole, . _wheatstone's_ analysis of the electric brush, . ---- measurement of conductive velocity in metals, . _wire, ignition of, by the electric current_, , _note_, . ---- is uniform throughout, . _wire_ a regulator of the electric current, , _note_. ----, velocity of conduction in, varied, . ----, single, a current induced in, . ----, long, inductive effects in, , . _wollaston on_ decomposition by common electricity, . ---- decomposition of water by points, . _zinc, amalgamated_, its condition, , . ----, used in pile, . _zinc_, how amalgamated, . ----, of troughs, its purity, . ----, its relation to the electrolyte, . ----, its oxidation is the source of power in the pile, . ---- _plates of troughs_, foul, . ---- ----, new and old, . ----, waste of, in voltaic batteries, . the end. printed by richard and john e. taylor. red lion court, fleet street. works published by richard and john e. taylor, _printers and publishers to the university of london_, red lion court, fleet street. daubeny on active and extinct volcanos, earthquakes, and thermal springs; with remarks on their causes, products, and influence on the condition of the globe. by professor charles daubeny, m.d., f.r.s. second edition, greatly enlarged. s. cloth, with maps and plates. the london, edinburgh and dublin philosophical magazine and journal of science, being a continuation of tilloch's 'philosophical magazine,' nicholson's 'journal,' and thomson's 'annals of philosophy,' conducted by sir david brewster, k.r. ll.d. f.r.s. l.&e. &c. richard taylor, f.s.a. l.s. g.s. astr. s. &c. richard phillips, f.r.s. l.&e. f.g.s. &c. sir robert kane, m.d. m.r.i.a. in monthly numbers, price s. d. each. the second series, in eleven volumes, with a general index; and the third or present series, in thirty-one volumes (with a general index to the first twelve), may be had; also a complete set at a reduced price, from to june , in one hundred and eleven volumes. annals and magazine of natural history; including zoology, botany, and geology; being a continuation of the 'annals' combined with messrs. london and charlesworth's 'magazine of natural history,' conducted by sir w. jardine, bart., f.l.s. p.j. selby, esq., f.l.s. dr. johnston. charles c. babington, esq, m.a., f.l.s. dr. balfour, prof. bot. edinb., and richard taylor, f.l.s. new series.--in monthly numbers, price s. d. each. the first series may be had at reduced prices: viz. vol. i. to x., £ s. vol. xi. to xx., £ s. or the whole set of volumes for £ . scientific memoirs, selected from the transactions of foreign academies of science, and learned societies, end from foreign journals; edited by richard taylor, f.s.a. f.l.s. f.g.s. &c. &c. in parts, price s. each, the chemical gazette; or journal of practical chemistry, in all its applications to pharmacy, arts and manufactures. conducted by william francis, ph.d., f.i.s., &c.--published on the st and th of each month, price sixpence (_stamped, seven pence_). five volumes published. vol. i. s.; vols. ii., iii., iv., v., s. each. faraday's experimental researches in electricity. vol. i. with plates, cloth, s. vol. ii. with plates, cloth, s. the catalogue of stars of the british association for the advancement of science; containing the mean right ascensions and north polar distances of eight thousand three hundred and seventy-seven fixed stars, reduced to january , : together with their annual precessions, secular variations, and proper motions, as well as the logarithmic constants for computing precession, aberration and nutation: with a preface explanatory of their construction and application. by the late francis baily, esq., d.c.l. oxford and dublin; president of the royal astr. soc. &c. &c. &c.--price three guineas. a catalogue of , stars, for the beginning of the year , from the observations of lalande, in the histoire celeste franÇaise. reduced at the expense of the british association for the advancement of science, under the immediate superintendence of the late francis baily, esq. printed at the expense of her majesty's government.--price £ s. a catalogue of stars in the southern hemisphere, for the beginning of the year , from the observations of the abbe de lacaille, made at the cape of good hope in the years and . reduced at the expense of the british association for the advancement of science, under the immediate superintendence of the late professor henderson, director of the royal observatory, edinburgh. and printed at the expense of her majesty's government, under the direction of the late francis baily, esq. with a preface by sir j.f.w. herschel, bart., h.k., president of the royal astronomical society.--price s. hygrometrical tables, to be used with, and description of the dry- and wet-bulb thermometers. by james glaisher, esq., of the royal observatory, greenwich. royal vo, s. d. griffith's practical manual on urine, &c., containing a description of the general, chemical and microscopical characters of the urine and its deposits, both in health and disease. by john william griffith, m.d., f.l.s. &c. with two plates, price s. cloth. griffith's practical manual on blood, &c., containing a description of the general, chemical and microscopical characters of the other fluids of the body, viz. the blood, chyle, lymph, gastric juice, bile, &c.; with the best methods of analysing them qualitatively as well its quantitatively. by john william griffith, m.d., f.l.s. &c. with two plates, price s. cloth. the botanical gazette. edited by arthur henfrey, esq. f.l.s. &c. published on the first of each month. price s. human body are photographed.--xxiv. the electric motor and how it does work.--xxv. electric cars, boats and automobiles.--xxvi. a word about central stations.--xxvii. miscellaneous uses of electricity. this book explains, in simple, straightforward language, many things about electricity; things in which the american boy is intensely interested; things he wants to know; things he should know. it is free from technical language and rhetorical frills, but it tells how things work, and why they work. it is brimful of illustrations--the best that can be had--illustrations that are taken directly from apparatus and machinery, and that show what they are intended to show. this book does not contain experiments, or tell how to make apparatus; our other books do that. after explaining the simple principles of electricity, it shows how these principles are used and combined to make electricity do every-day work. * * * * * _everyone should know about electricity._ * * * * * a very appropriate present things a boy should know about wireless by thomas m. st. john, met. e. author of "things a boy should know about electricity," "fun with electricity," "the study of elementary electricity and magnetism by experiment," "the study of electric motors by experiment," "electrical handicraft," etc., etc. pages-- illustrations and diagrams bound in cloth--net $ . this book contains much practical and some theoretical information regarding the operation and explanation of wireless outfits. it discusses enough of the theoretical side to make the student sure of himself and to give a well-rounded knowledge of this most practical subject. the author has explained the various pieces of apparatus needed in a wireless station in such a clear manner that the student can not fail to understand how they work and why they work. the numerous drawings and diagrams simplify the discussions to such an extent that the reader will not want to skip a single paragraph. "things a boy should know about wireless" will be welcomed by thousands of enthusiasts and it should find its way into every library. from thomas m. st. john cascade ranch. east windham,--n.y. the study of elementary electricity and magnetism by experiment by thomas m. st. john, met. e. fourth edition price, postpaid, $ . . the book contains pages and illustrations. it measures × - / in., and it is bound in green cloth. contents: part i. magnetism.--chapter i. iron and steel.--ii. magnets.--iii. induced magnetism.--iv. the magnetic field.--v. terrestrial magnetism. part ii. static electricity.--vi. electrification.--vii. insulators and conductors--viii. charging and discharging conductors.--ix. induced electrification.--x. condensation of electrification.--xi. electroscopes.--xii. miscellaneous experiments.--xiii. atmospheric electricity. part iii. current electricity.--xiv. construction and use of apparatus.--xv. galvanic cells and batteries.--xvi. the electric circuit.--xvii. electromotive force.--xviii. electrical resistance.--xix. measurement of resistance.--xx. current strength.--xxi. chemical effects of the electric current.--xxii. electromagnetism.--xxiii. electromagnets.--xxiv. thermo electricity.--xxv. induced currents.--xxvi. the production of motion by currents.--xxvii. applications of electricity.--xxviii. wire tables.--apparatus list.--index. this is a text-book for amateurs, students, and others who want to take up a systematic course of electrical experiments at home or in school. it will give a practical and experimental knowledge of elementary electricity, and thoroughly prepare students for advanced work. full directions are given for two hundred experiments. the experiments and discussions are so planned that the student is always prepared for what follows. although the experiments may be performed with the apparatus that is usually found in school laboratories, the author has designed a complete set of apparatus for those who want to have their own outfit. * * * * * _if you want to take up a systematic course of experiments--experiments that will build a lasting foundation for your electrical knowledge--this book will serve as a valuable guide._ jan. , student's discount discontinued price to all, $ . owing to greatly increased costs of labor and materials the discount of c formerly allowed on this set has been discontinued. complete sets only now sold. shipping weight on improved sets lbs. securely packed in wooden box. sent by parcel post if proper postage is included in your remittance; otherwise by express charges collect. fun with magnetism and fun with electricity have started more young men upon electrical careers than any other scientific outfits ever placed before the public. the thousands upon thousands that have been sold in all parts of the world have furnished fun and science for people of all ages, and the mere fact that they are listed by the new york board of education, and recommend to the pupils and teachers of the new york public and private schools is a guarantee of their value. were it not for the fact that these are made in such large quantities and sold by stores, agents and mail-order houses, the price would be much higher. don't fail to get these. they have a national reputation. fun with magnetism this outfit contains a -page book of instructions, with illustrations, together with a complete set of apparatus for performing fascinating experiments. it will give you some new ideas about magnetism and start you at the right place in your study of electricity. think what that means--to start right! the book contains experiments with the horseshoe magnet, with bar magnets, with floating magnets, etc., etc., thus giving a practical knowledge of the subject; and it is all done in such an interesting way that one can't help remembering it. every experiment clinches some fact and every fact is important. amusing experiments.--something for nervous people to try.--the jersey mosquito.--the stampede.--the runaway.--the dog-fight.--the whirligig.--the naval battle.--a string of fish.--a magnetic gun.--a top upside down.--a magnetic windmill.--a compass upside down.--the magnetic acrobat.--the busy ant-hill.--the magnetic bridge.--the merry-go-round.--the tight-rope walker.--a magnetic motor using attractions and repulsions.--and others. no. r --"fun with magnetism," complete outfit, postpaid $ . [illustration] fun with electricity the author of this fun with science series has spent a great deal of time and money in experimenting to devise apparatus that will do the proper work and be, at the same time, simple and cheap, and in no outfit has he succeeded better than in fun with electricity. when you think of an outfit retailing for c. and covering the whole subject of "static electricity," giving scientific experiments upon its production, conduction and induction, with a -page book of instructions with drawings, and a complete set of apparatus of articles for performing these experiments, you will understand why the sales of this outfit have been enormous. as the subject is presented in a fascinating way--and not as mere dry science--every one likes to do the experiments. no wonder these sets are highly praised by parents and educators in every part of the country! there is fun in these experiments: chain lightning.--an electric whirligig.--the baby thunderstorm.--a race with electricity.--an electric frog pond.--an electric ding-dong.--the magic finger.--daddy long-legs.--jumping sally.--an electric kite.--very shocking.--condensed lightning.--an electric fly-trap.--the merry pendulum.--an electric ferry-boat.--a funny piece of paper.--a joke on the family cat.--electricity plays leap-frog.--lightning goes over a bridge.--electricity carries a lantern.--and others. there isn't an outfit anywhere at any price that gives better value for the money. an ideal present for a boy. no. r --"fun with electricity," complete outfit, postpaid $ . fun with puzzles here is an outfit that every boy and girl should have, for it is amusing, instructive and educational. it is real fun to do puzzles and to puzzle your friends, and this book contains some real brain-teasers that will make you think. the book contains chapters, pages, and illustrations, and measures × - / inches. if you can't do any particular puzzle you will find its solution in the "key," which is bound with the book. if you want to win prizes by doing the puzzles in the magazines, you will find this book of four hundred puzzles a regular school of puzzles that will give you a thorough training for this kind of work. the book alone is well worth the price, to say nothing of the outfit of numbers, counters, pictures, etc. contents of book: chapter ( ) secret writing. ( ) magic triangles, squares, rectangles, hexagons, crosses, circles, etc. ( ) dropped letter and dropped word puzzles. ( ) mixed proverbs, prose and rhyme. ( ) word diamonds, squares, triangles, and rhomboids. ( ) numerical enigmas. ( ) jumbled writing and magic proverbs. ( ) dissected puzzles. ( ) hidden and concealed words. ( ) divided cakes, pies, gardens, farms, etc. ( ) bicycle and boat puzzles. ( ) various word and letter puzzles. ( ) puzzles with counters. ( ) combination puzzles. ( ) mazes and labyrinths. secret writing is explained in this book, and it shows how you can write letters to your friends and be sure that no one can read them unless they are also in the secret. this one thing alone will give you a great deal of enjoyment. get this outfit and have some fun. no. r --"fun with puzzles," complete outfit, postpaid $ . * * * * * fun with soap-bubbles fancy bubbles and films are not easily blown without special apparatus, and even with the proper outfit one must "know how." that's why we furnish a -page book with every set to show just how to do it. with the aid of the illustrations and the directions you can produce remarkable results that will surprise and entertain your friends. a child can do it as well as a grown person. [illustration] soap-bubble parties using these outfits create real sensations. why not be the first in your town to give a "fun with soap-bubbles party?" just write and ask about the price for any special number of them--say six or a dozen. contents of book: twenty-one illustrations.--introduction.--the colors of soap-bubbles.--the outfit.--soap mixture.--useful hints.--bubbles blown with pipes.--bubbles blown with straws.--bubbles blown with the horn.--floating bubbles.--baby bubbles.--smoke bubbles.--bombshell bubbles.--dancing bubbles.--bubble games.--supported bubbles.--bubble cluster.--suspended bubbles.--bubble lamp chimney.--bubble lenses.--bubble basket.--bubble bellows.--to draw a bubble through a ring.--bubble acorn.--bubble bottle.--a bubble within a bubble.--another way.--bubble shade.--bubble hammock.--wrestling bubbles.--a smoking bubble.--soap films.--the tennis racket film.--fish-net film.--pan-shaped film.--bow and arrow film.--bubble dome.--double bubble dome.--pyramid bubbles.--turtle-back bubbles.--soap-bubbles and frictional electricity. "there is nothing more beautiful than the airy-fairy soap-bubble with its everchanging colors." this outfit gives the best possible amusement for old and young. no. r --"fun with soap-bubbles," complete outfit, postpaid $ . three extra packages of prepared soap, postpaid . fun with shadows no wonder shadow-making has been popular for several centuries! what could give keener delight than comical shadow-pictures, pantomimes, entertainments, etc.? professional shadowists use wires, forms, and various devices to aid them, and that is why they get such wonderful results on the stage. do you want to do the same thing right in your own home and entertain your friends with all kinds of fancy shadows? you can do it with this outfit, for the book contains illustrations and diagrams with directions for using the numerous articles included in the box. you will be surprised to see how easily you can make these funny shadows with the aid of the outfit. better get one now and make shadows like a professional. [illustration] the outfit contains everything necessary for all ordinary shadow pictures, shadow entertainments, shadow plays, etc. the following articles are included: one book of instructions called "fun with shadows"; shadow screen; sheets of tracing paper; coil of wire for movable figures; cardboard frame for circular screen; cardboard house for stage scenery; jointed wire fish-pole and line; bent wire scenery holders; clamps for screen; wire figure support; wire for oar; spring wire table clamps; wire candlestick holder; cardboard plates containing the following printed figures that should be cut out with shears; character hats; boat; oar-blade; fish; candlestick; cardboard plate containing printed parts for making movable figures. no. r --"fun with shadows," complete outfit, postpaid $ . * * * * * fun with photography popular pastimes are numerous, but to many there is nothing more fascinating than photography. the magic of sunshine, the wonders of nature, and the beauties of art are tools in the hands of the amateur photographer. if you want to get a start in this up-to-date hobby, this outfit will help you. you will enjoy the work and be delighted with the beautiful pictures you can make. the outfit contains everything necessary for making prints--together with other articles to be used in various ways. the following things are included: one illustrated book of instructions, called "fun with photography"; package of sensitized paper; printing frame, including glass, back, and spring; set of masks for printing frame; set of patterns for fancy shapes; book of negatives (patented) ready for use; sheets of blank negative paper; alphabet sheet; package of card mounts; package of folding mounts; package of "fixo." [illustration] contents of book: chapter i. introduction.--photography.--magic sunshine.--the outfit.--ii. general instructions.--the sensitized paper.--how the effects are produced.--negatives.--prints.--printing frames.--our printing frame.--putting negatives in printing frame.--printing.--developing.--fixing.--drying--- trimming.--fancy shapes.--mounting.--iii. negatives and how to make them.--the paper.--making transparent paper.--making the negatives.--printed negatives.--perforated negatives.--negatives made from magazine pictures.--ground glass negatives.--iv. nature photography.--aids to nature study.--ferns and leaves.--photographing leaves.--perforating leaves.--drying leaves, ferns, etc., for negatives.--flowers.--v. miscellaneous photographs.--magnetic photographs.--combination pictures.--initial pictures.--name plates.--christmas, easter and birthday cards. no. r --"fun with photography," complete outfit, postpaid $ . fun with chemistry [illustration: fun with chemistry] chemistry is universally considered to be an interesting subject, even in school, and it is certainly an important one in these days of scientific progress. this outfit starts you at the right place and presents the elements of the subject in a most interesting fashion. the experiments are so enjoyable that you will take pleasure in doing them over and over again, and you will want to do them for your friends. you can have a lot of fun with this set, and even if you have taken advanced courses in the subject you will find something new in these experiments. the more you know about chemistry the more you will enjoy it, for then you can more easily appreciate what a splendid outfit this is for the money. the outfit contains over different articles, including chemicals, test-tubes, adjustable ring-stand, litmus paper, filter paper, glass tubing, etc.; in fact, everything needed for the forty-one experiments. the book of instructions is fully illustrated, and measures × - / inches. fun found here: from white to black, or the phantom ship.--yellow tears.--smoke pearls.--an ocean of smoke.--a tiny whirlwind.--a smoke cascade.--an explosion in a teacup.--a gas factory in a test-tube.--making charcoal.--flame goes over a bridge.--a smoke toboggan-slide.--fountains of flame.--making an acid.--making an alkali.--a chemical fight.--through walls of flame.--an artificial gas well.--a lampblack factory.--steam from a flame.--the flame that committed suicide.--chemical soup.--a baby skating-rink.--a magic milk-shake.--the wizard's breath.--a chemical curtain.--scrambled chemicals.--and many other experiments. no. r --"fun with chemistry," complete outfit, postpaid $ . * * * * * electric shooting game shooting animals by electricity is certainly a most original game, and it will furnish a vast amount of amusement to all. the game is patented and copyrighted--because it is really a brand-new idea in games--and it brings into use that most mysterious something called electricity. while the electricity is perfectly harmless, there being no batteries, acids or liquids, it is very active and you will have plenty to laugh at. it is so simple that the smallest child can play it and so fascinating that grandpa will want to try it. the "game-preserve" is neatly printed in colors, and the birds and wild animals are well worth hunting. each has a fixed value--and some of them must not be shot at all--so there is ample chance for skill. tissue-paper bullets are actually shot from the "electric gun" by electricity, and it is truly a weird sight to see them shoot through the air impelled by this unseen force. the outfit contains the "game-preserve," the "electric gun," the "shooting-box," and the "electric bullets," together with complete illustrated directions, all placed in a neat box. no. r --"electric shooting game," complete, postpaid $ . * * * * * new idea tit-tat-toe splendid game for two, three, or four players; great improvement upon the good old game; fascinating game instantly learned; nothing better for children's parties and progressive birthday parties; box with game-board, men, directions; discount for party orders. no. r --new idea tit-tat-toe, sample, postpaid $ . electric air-ships and other games (patented) [illustration] this is the age of air-ships and electricity, so what could be more up-to-date than electric "air-ships" that will float and dive and race around at the will of the operator? in this game mr. st. john has again made use of a scientific principle, the "air-ships" being actually controlled by electricity. they are made to act in a most peculiar manner, with no wires, no fuss, no danger. they are under perfect control and can be made to ascend to the ceiling, drop to the floor or race across the room, as desired. you simply can't imagine how entertaining it is to see a lot of excited people managing these aerial racers, each eager to win. the outfit contains illustrated directions and materials for two players, including the apparatus for producing the electricity and the "repeller" for managing the "air-ships" in mid-air. the little "air-ships" are actually made of metal and they can be instantly formed. here's the latest sport for all ages, because the little ones can play too. get into the game and be an aviator. no. r --"electric air-ships and other games," postpaid $ . aviation tournaments at home real electric toy-making for boys _by thomas m. st. john. met. e._ this book contains pages and over one hundred original drawings, diagrams, and full-page plates. it measures × - / in., and is bound in cloth. second edition price, postpaid, $ . contents: chapter i. toys operated by permanent magnets.--ii. toys operated by static electricity.--iii. making electromagnets for toys.--iv. electric batteries.--v. circuits and connections.--vi. toys operated by electromagnets. vii. making solenoids for toys.--viii. toys operated by solenoids.--ix. electric motors.--x. power, speed, and gearing.--xi. shafting and bearings.--xii. pulleys and winding-drums.--xiii. belts and cables.--xiv. toys operated by electric motors.--xv. miscellaneous electric toys.--xvi. tools.--xvii. materials.--xviii. various aids to construction. while planning this book, mr. st. john definitely decided that he would not fill it with descriptions of complicated, machine-made instruments and apparatus, under the name of "toy-making," for it is just as impossible for most boys to get the parts for such things as it is for them to do the required machine work even after they have the raw materials. great care has been taken in designing the toys which are described in this book, in order to make them so simple that any boy of average ability can construct them out of ordinary materials. the author can personally guarantee the designs, for there is no guesswork about them. every toy was made, changed, and experimented with until it was as simple as possible; the drawings were then made from the perfected models. as the result of the enormous amount of work and experimenting which were required to originate and perfect so many new models, the author feels that this book may be truly called "real electric toy-making for boys." * * * * * every boy should make electrical toys. a motor that can do things the "st j. motor no. " (list no. ) is designed for students and others who want a small motor for experimental purposes as well as for all of the work that any small motor can do. we believe this to be the best small motor made, and we know that it can be used in more ways than any other motor of equal cost ever built. it has four binding-posts,--making it possible to energize the field or armature separately,--and so it can be used in circuits with reversers and rheostats for experiments. the speed and direction of rotation can be changed at will, thus adapting it for running toys, etc. as the binding-posts are mounted upon the frame, this motor can be taken from the base for remounting and using in many ways, and as it has a three-pole armature it will start promptly in any position. the shaft carries a pulley, and a fan can be added at any time. one cell will give a high speed, and more cells may be added, according to the work it has to do. motor no. stands - / inches high. it is finished in black enamel with nickel-plated trimmings,--strong and well made. with it are furnished three nickel-plated connecting-straps, which are to be used for connecting the field and armature in "series" or "shunt." so much can be done with this motor that it is simply impossible to tell it here; in fact, it is used as the basis for a whole book of experiments called "the study of electric motors by experiment," and, when used in connection with the other parts of the motor outfits, it will give a practical knowledge of motors that no other plan can give. [illustration: no. ] these motors and motor outfits have been highly praised by electrical experts and educators as being invaluable to students. they can do everything the big motors can do, and if used with the rheostats, reversers and other apparatus in the outfits, the student will have a whole motor laboratory. why not get a motor that has brains and that can do tricks and experiments? any good motor will go when you turn on the power; but that doesn't mean much when it comes to understanding things. no. --"st. j. motor no. ," with wiring-diagrams $ . if sent by mail, postage extra, shipping weight one pound. "st. j." electric motor outfit these outfits have been designed for students and others who want to do real experimental work with motors, so as to get right down to the bottom of the matter and thoroughly master the foundation principles of the subject. it is simply astonishing to see how much can be learned with one of these outfits, especially if the work be done as fully detailed in "the study of electric motors by experiment." every electrical laboratory should have one of these sets, and the more you know about motors the more you will appreciate an outfit of this kind. don't simply read about motors--get right down to the practical part of it and experiment for yourself. every experiment will settle an important point in your mind. electric motor outfit no. x contains everything needed for sixty interesting and profitable experiments. with the improved apparatus that we now give we feel that this is the most complete set ever sold for the money. the following articles are included, packed in a wooden box: the "st. j." motor, fully described on another page, is well called "a motor that can do things." the five-point rheostat is used as a "starting-box" in the armature-circuit and in various other ways to regulate speed. (see cut.) the eleven-point rheostat is used to regulate the "field-magnetism," as one method of speed-regulation, and for other purposes. (see cut.) the double-key current reverser is, really, a key, a two-point switch, and a current-reverser combined. on this account it can be used in many ways, shown in numerous wiring-diagrams. (see cut.) the handy current-detector is used as a current-detector and as a device for studying the counter-electromotive force of motor while running. the two-point switch is useful in quickly switching the current wherever it is needed, and for many other experiments. the strap key protects the batteries and closes the circuit. the miniature electric lamp and socket are used in the motor-circuit to prove certain things and form an attractive addition to the outfit. the magnetic needle in the new outfits is nickel-plated and serves as a compass for studying the magnetism of the poles, etc. in addition to the articles mentioned above, the outfit contains a set of wires for connections, a box of iron-filings for studying lines of force, an experimental package containing iron, steel, etc., three connecting-wires, and the book of instructions, called "the study of electric motors by experiment." this contains chapters, pages, and over illustrations and diagrams. bound in stiff paper. batteries are not included, unless ordered extra. three of our no. batteries cost c., and extra postage for lbs. no. x--complete motor outfit, as above (p. weight, lbs.) $ . * * * * * the study of electric motors by experiment contains sixty experiments that bear directly upon the construction, operation, and explanation of electric motors, together with much helpful information upon the experimental apparatus required. this book will be a great help to those who want to do real experimental work with motors. it contains chapters, pages, over illustrations and diagrams, and you can not afford to be without it. no. r p--"study of motors," bound in paper, postpaid $ . no. r c--"study of motors," bound in cloth, postpaid $ . fun with telegraphy (patented) two great outfits for students [illustration] these two outfits are similar in construction, although they differ in details, each being designed for its special work. the "keys," "sounders" and "binding-posts" are neatly mounted upon ebonized bases measuring - / × - / in., these also serving as sounding-boards. "fun with telegraphy" is the original low-price telegraph outfit for students that has sold by the thousands and given universal satisfaction. it is considered the best -cent outfit ever produced, and, although we have made several improvements lately, the price is the same as before. in connection with a peculiar oscillating electro-magnet and a queer anvil, the sounding-board aids in giving out a loud, clear click that is found elsewhere only in noisy railroad sounders. this outfit is best adapted for a learner's set of one instrument and a battery to be used on the table for practising, either with or without the "codegraph," and not for telegraphing over wires to other stations. outfit: illustrated book of instructions, called "fun with telegraphy"; telegraph "key"; telegraph "sounder"; nickel-plated "binding-posts"; insulated wires for connections. no. r --"fun with telegraphy," without battery, postpaid $ . no. r , r b--"fun with telegraphy," with one dry battery, postpaid, . "improved telegraphy no. ."--in answer to a number of requests for an improved outfit for regular line work between two stations a few hundred feet apart, we now offer this set, which is, in general, similar in plan to our first "telegraphy no. ." we have replaced the single electro-magnet of the old set, as shown in the cut, with two larger ones of superior construction, thus making the instrument much more sensitive. the key has also been greatly improved, and we now have a fine set at low cost. no expensive gravity batteries are needed with this ingenious arrangement, as it is designed to work with dry batteries which are clean and cheap. by means of a peculiar switch, either station may "call" the other at any time, even though the line is kept on "open circuit." there is absolutely no waste of current when the line is not in use--and, even then, only at the instant the dots and dashes are made. this is certainly a great advantage over the old-fashioned methods with gravity batteries which amateurs have heretofore been obliged to use. with this instrument you have a learner's set as well as one that can be used to send messages to another station. if you do not care for the superior advantages of "semi-wireless," this outfit will give entire satisfaction for ordinary work. outfit: illustrated book of instructions called "telegraphy number two"; improved telegraph "key"; telegraph "sounder" with double electromagnets: special "switch" for controlling the batteries; nickel-plated screw "binding-posts"; insulated wires for connections. no. --"improved telegraphy number two" (no batteries), postpaid, $ . no. b--same as no. , but with two dry batteries, postpaid, . the codegraph (patented) note--continental code sent unless otherwise ordered. [illustration] the codegraph is a brand-new scheme for thoroughly and rapidly learning the telegraphic code, and it has been worked out with the beginner in mind. this code-learning system really adapts itself to the beginner, and it gives a personal touch to each individual student according to his needs. no other system can do so much, for the student sees, hears and feels every letter and signal. the greatest trouble that every one has in learning by listening to regular messages is in separating the letters and words as they come in so fast. there is no time to think, and letters pile up in the mind. the codegraph avoids all confusion because every letter is under perfect control and may be repeated as many times as desired; hard things can be made easy; words and sentences can be built at will. we guarantee that any one of average ability can make rapid improvement with the codegraph. what it is. a complete codegraph outfit, as shown in the cut, has three main parts: ( ) the "plate and pen," ( ) some form of "key and sounder" and ( ) two batteries. while any key and sounder can be used with the plate, we wish to call especial attention to the duplex sounder shown, as this has been designed to do double work. if you already have "fun with telegraphy," for example, and want to order the "codegraph plate and pen," we will include, free of charge, an extra attachment for connecting up your instrument. the plate and pen. when the pen is lightly drawn over the plate, the sounder responds and shows exactly how every letter and signal should sound. the student can then practise each letter until perfect. the surface of the plate is covered with a special insulating enamel, bare spots corresponding to correct dots and dashes. the polished brass plate measures about × inches and has a most elegant appearance. the book tells all about practising, etc. duplex codegraph key and sounder, as shown, has a double action and is the latest thing in code-learning devices. by the mere turning of a switch you can have the ordinary telegraph clicks or the wireless buzzes, making two sounders in one and at the cost of one. the combination sounder and a substantial key are mounted upon a finely finished base with nickel-plated trimmings, binding-posts, switch, etc. if you want to become an operator in the shortest possible time, no matter whether you have ever tried before or not, get one of these outfits and begin at once. you will be pleased right from the start, because you will make rapid progress right from the start. no. --"codegraph plate and pen," with book of instructions $ . if sent by mail, postage extra . no. --"duplex codegraph key and sounder" (no batteries) . if sent by mail, postage extra . no. --two dry batteries, as shown . if sent by mail, postage extra . special--complete codegraph outfit, as in cut, postpaid . telegraph and telephone sets (list t) original outfits that are worthy of your attention and that give fine results; products of hundreds of experiments and models that give best value for least money. a complete line of outfits beginning with "fun with telegraphy" and ending with combined "semi-wireless telegraph, telephone and electric light signal sets," with endless possibilities. don't forget to add postage according to weight and zone. list no. list price --"new fun with telegraphy." a book, key, sounder, wires. nicely mounted, sensitive, adjustable, improved, practical. (p. wt. lb.) $ . b --same as no. , but with dry battery. (p. wt. lb.) $ . --"new telegraphy number two." for regular line-work: has ingenious switch; uses dry batteries. key, sounder, book, wires. (p. wt. lb.) $ . b --same as no. but with two dry batteries. (p. wt. lbs.) $ . --"clickerbuzz" two-station telegraph outfit. special value; loud, resonant, substantial, very neat and does several things. complete with two separate no. "wonderbuzz" instruments, morse code, continental code, wire for short line, pkg. small telegraph blanks, instructions and wiring diagrams. (p. wt. lbs.) $ . b --same as no. but with four dry batteries. (p. wt. lbs.) $ . --codegraph plate, pen and book. teaches continental wireless code, giving correct sounds on your buzzer or on ours. original, practical, solves home study. (weight pound.) price $ . --codegraph outfit mounted on ebonized base with high-pitch nickel-plated buzzer, binding-posts and key, books and wires. fine set for practice and study. continental code. (weight, lbs.) price $ . b --same as , but with batteries. (wt. lbs.) price $ . --"semi-wireless wonderbuzz," a real wonder that can actually be used in hookups. a basic instrument around which to build code-teaching devices, blinker signal systems, numerous click telegraphs, buzz telegraphs, semi-wireless telegraphs, several telephone plans, combined telegraph and telephone schemes over the same wire, actual room-to-room wireless, etc., etc. can't begin to tell it all here. an all-useful instrument with directions. (weight, lb.) price $ . --the "wonderphone" is a practical, inexpensive telephone set; sensitive, strong and well made. outfit for one station: receiver, carbon grain transmitter, both with flexible wires, combination binding-post and instrument support, battery box, wire for connections, ft. of line wire, directions. (weight, lb.) price $ . b --same as no. , but with batteries, (wt. lbs.) price $ . --"semi-wireless telegraph, telephone, and electric light signal set." a combination of the "wonderbuzz," the "wonderphone," night signal attachment, and a lot of extras, line wire, etc. a set that beats them all and does most. no other system does so much for the money and no other can do so much for ten times the money. a wonder combination of usefulness. please read about the "wonderbuzz" and the "wonderphone." the latest word in telegraphy from cascade ranch. (weight, lbs.) price $ . b --same as no. but with batteries. (wt. lbs.) $ . no. stjc--save-time-jiffy-code. learn to send and receive slowly in an hour or less. in a day you can telegraph in a jiffy, any message, punctuation, numbers, sentence-signals and the whole business. every boy a telegrapher. fun to make your own cipher codes on this as a basis. complete, postpaid c., two for $ . --"dandy handiphone." an inexpensive house-to-house telephone. sensitive, attractive, practical, efficient. rings bell or buzzer to call, using dry batteries. will work as far as any battery-phone, and farther than many of them. a dandy handiphone. in preparation. be sure to add postage according to weight and zone. thomas m. st. john, cascade ranch, east windham, n.y. online distributed proofreaders europe at http://dp.rastko.net the hurricane guide: being an attempt to connect the rotatory gale or revolving storm with atmospheric waves. including instructions for observing the phÆnomena of the waves and storms; with practical directions for avoiding the centres of the latter. by william radcliff birt. london: john murray, albemarle street. _publisher to the admiralty._ . printed by w. clowes and sons, stamford street. preface. in introducing the following pages to the notice of the public, it is the author's wish to exhibit in as clear a light as our present researches on the subjects treated of will allow, the connexion between one of the most terrific phænomena with which our globe is visited, and a phænomenon which, although but little known, appears to be intimately connected with revolving storms. how far he has succeeded, either in this particular object or in endeavouring to render the essential phænomena of storms familiar to the seaman, is left for the public to determine. should any advantage be found to result from the study of the atmospheric waves, as explained and recommended in this little work, or the seaman be induced by its perusal to attend more closely to the observations of those instruments that are calculated to warn him of his danger, an object will be attained strikingly illustrative of the baconian aphorism, "knowledge is power." _bethnal green_, april , . contents. page chap. i.--phÆnomena of revolving storms " ii.--phÆnomena of atmospheric waves " iii.--observations sect. i.--instruments " ii.--times of observation " iii.--localities for additional observations " iv.--storms, hurricanes, and typhoons " v.--seasons for extra observations " iv.--practical directions for avoiding the centres of storms notice. in the pocket accompanying this work are two rings of stiff cardboard, on which will be found all the information contained in figures and . when they are laid flatly upon a chart, the continuity of the lines on the chart is not materially interfered with, while the idea of a body of air rotating in the direction indicated by the arrows is conspicuously presented to the mind. these rings are more particularly referred to on page . the hurricane guide. chapter i. phÆnomena of revolving storms. it is the object of the following pages to exhibit, so far as observation may enable us, and in as brief a manner as possible, the connexion, if any, that exists between those terrific meteorological phænomena known as "revolving storms," and those more extensive and occult but not less important phænomena, "atmospheric waves." to the great body of our seamen, whether in her majesty's or the mercantile service, the subject can present none other than the most interesting features. the laws that govern the transmission of large bodies of air from one part of the oceanic surface to another, either in a state of rapid rotation or presenting a more or less rectilineal direction, must at all times form an important matter of inquiry, and bear very materially on the successful prosecution of the occupation of the voyager. in order to place the subjects above alluded to in such a point of view that the connexion between them may be readily seen, it will be important to notice the principal phænomena presented by each. without going over the ground so well occupied by those able writers on the subject of storms--redfield, reid, piddington, and thom--it will be quite sufficient for our present purpose simply to notice the essential phænomena of revolving storms as manifested by the barometer and vane. the usual indications of a storm in connexion with these instruments are the _falling_ of the barometer and the _freshening_ of the wind, and it is generally considered that a _rapid_ fall of the mercury in the hurricane regions invariably precedes the setting in of a storm. there are three classes of phænomena that present themselves to an observer, according as he is situated _on_ the line or axis of translation, or _in_ either the right or left hand semicircle of the storm. these will be rendered very apparent by a little attention to the annexed engraving, fig. . [illustration: compass rose] in this figure the arrow-head is supposed to be directed true north, and the hurricane--as is the case in the american storms north of the th parallel--to be moving towards the n.e. on the line n.e.--s.w. if the ship take the hurricane with the wind s.e.,--the letters within the two larger circles indicating _the direction of the wind in the storm_ according to the rotation as shown by the circle of arrow-heads, and which it is to be particularly noted is in the northern hemisphere _contrary to the direction in which the hands of a watch move_: in the southern hemisphere the rotation is reversed--the only phænomena presented by the storm are as follows:--the wind continues to blow from the s.e., increasing considerably in force with the barometer falling to a very great extent until the centre of the storm reaches the ship, when the fury of the winds is hushed, and a lull or calm takes place, generally for about half an hour, after which the wind springs up mostly with increased violence, but from the opposite quarter n.w., the barometer begins to rise, and as the storm passes off, the force of the wind abates. the point to which we wish particularly to direct attention in connexion with this exposition of the phænomena attending the transmission of a storm is this:--if the observer so place himself at the commencement that the wind passes _from his left hand towards his right_, his face will be directed towards the centre of the storm; and the wind undergoing no change in direction, but only in force, will acquaint him with this important fact that the _centre_ is not only gradually but surely approaching him: in other words, in the case before us, when he finds the wind from the s.e., and he places himself with his face to the s.w. he is looking towards the centre, and the wind rushes past him _from his left to his right hand_. now the connexion of the barometer with this phase of the storm is _falling with the wind from left to right, the observer facing the centre while the first half is transiting_.[ ] during the latter half these conditions are reversed, the observer still keeping his position, his face directed to the s.w., the barometer _rises_ with a n.w. wind, which rushes past him _from his right to his left hand_ with a decreasing force. we have therefore _a rising barometer with the wind from right to left during the latter half of the storm, the observer having his back to the centre_. the above _general_ enunciations of the barometric and anemonal phænomena of a rotating storm hold good with regard to the _northern_ hemisphere, whatever may be the direction in which the hurricanes advance. this may be placed in a clearer light, as well as the remaining classes of phænomena shown, by consulting the following tables, constructed for the basin of the northern atlantic, and comparing them with fig. . in this basin, with lower latitudes than °, the usual paths of the hurricanes are towards the north-west, in higher latitudes than ° towards the north-east. the tables exhibit the veering of the wind with the movements of the barometer, according as the ship is situated in the right or left hand semicircle of the hurricane. it must here be understood that the right and left hand semicircles are determined by the observer so placing himself that his face is directed towards the quarter to which the hurricane is advancing. lower latitudes. northern hemisphere. axis line, wind n.e., barometer falling, first half of storm. axis line, wind s.w., barometer rising, last half of storm. right-hand semicircle. wind e.n.e., e., e.s.e., s.e., barometer falling, storm increasing. wind s.s.w., s., s.s.e., s.e., barometer rising, storm passing off. left-hand semicircle. wind n.n.e., n., n.n.w., n.w., barometer falling, storm increasing. wind w.s.w., w., w.n.w., n.w., barometer rising, storm passing off. higher latitudes. northern hemisphere.[ ] axis line, wind s.e., barometer falling, first half of storm. axis line, wind n.w., barometer rising, last half of storm. right-hand semicircle. wind s.s.e., s., s.s.w., s.w., barometer falling, storm increasing. wind w.n.w., w., w.s.w., s.w., barometer rising, storm passing off. left-hand semicircle. wind e.s.e., e., e.n.e., n.e., barometer falling, storm increasing. wind n.n.w., n., n.n.e., n.e., barometer rising, storm passing off. n.b. the directions of the hurricane winds are so arranged as to show the points of commencement and termination. thus in the lower latitudes a storm commencing at e.n.e. passes off at s.s.w. after the wind has veered e., e.s.e., s.e., s.s.e., and s., being in the order of the letters in the upper line and contrary to their order in the lower. one commencing at e.s.e. passes off at s.s.e. right-hand semicircle. in the higher latitudes a ship taking the storm at e.n.e. will be in the left-hand semicircle, and the hurricane will pass off at n.n.e. these changes are rendered very apparent by moving the hurricane circle in the direction in which the storm is expected to proceed. fig. represents the whirl and hurricane winds in the south. [illustration: fig. ] chapter ii. phÆnomena of atmospheric waves. professor dove of berlin has suggested that in the temperate zones the compensating currents of the atmosphere necessary to preserve its equilibrium may be arranged as parallel currents on the _surface_, and not superposed as in or near the torrid zone. his views may be thus enunciated:--that in the parallels of central europe the n.e. current flowing towards the equator to feed the ascending column of heated air is not compensated by a current in the upper regions of the atmosphere flowing from the s.w. as in the border of the torrid zone, but there are also s.w. currents on each side the n.e., which to the various countries over which they pass appear as surface-winds, the winds in fact being disposed in alternate beds or layers, s.w., n.e., as in fig. . [illustration: fig. .] the professor also suggests that these parallel and oppositely directed winds are shifting, _i. e._ they gradually change their position with a lateral motion in the direction of the large arrow cutting them transversely. in the course of the author's researches on atmospheric waves he had an opportunity of testing the correctness of professor dove's suggestion, and in addition ascertained that there existed another set of oppositely directed winds at right angles to those supposed to exist by the professor. these currents were n.w. and s.e. with a lateral motion towards the n.e. he also carefully discussed the barometric phænomena with relation to both these sets of currents, and arrived at the following conclusions. the details will be found in the author's third report, presented to the british association for the advancement of science (reports, , pp. to ). during the period under examination the author found the barometer generally to rise with n.e. and n.w. winds, and fall with s.w. and s.e. winds, and that the phænomena might be thus illustrated:--let the strata _a a a' a', b' b' b b_, fig. , represent two parallel aërial currents or winds, _a a a' a'_ from s.w. or s.e., and _b' b' b b_ from n.e. or n.w. and conceive them both to advance from the n.w. in the first instance and from the s.w. in the second, in the direction of the large arrow. now conceive the barometer to commence rising just as the edge _b b_ passes any line of country, and to continue rising until the edge _b' b'_ arrives at that line, when the maximum is attained. it will be remarked that this rise is coincident with a n.e. or n.w. wind. the wind now changes and the barometer begins to fall, and continues falling until the edge _a a_ coincides with the line of country on which _b b_ first impinged. during this process we have all the phænomena exhibited by an atmospheric wave: when the edge _b b_ passes a line of country the barometer is at a _minimum_, and this minimum has been termed the _anterior trough_. during the period the stratum _b' b' b b_ transits, the barometer rises, and this rise has been called the _anterior slope_. when the conterminous edges of the strata _a' a' b' b'_ pass, a barometric _maximum_ extends along the line of country formerly occupied by the anterior trough, and this maximum has been designated the _crest_. during the transit of the stratum _a' a' a a_ the barometer _falls_, and this fall has been characterised as the _posterior slope_; and when the edge _a a_ occupies the place of _b b_, the descent of the mercurial column is completed, another _minimum_ extends in the direction of the former, and this minimum has been termed the _posterior trough_. it will be readily seen that the lateral passage of the n.w. and s.e. currents towards the n.e. presents precisely the same barometric and anemonal phænomena as the rotatory storms when moving in the same direction. if the observer, when the barometer is at a _maximum_ with a n.w. wind, place himself in the same position with regard to the laterally advancing current as he did with regard to the advancing storm, _i. e._ with his face _towards_ the quarter from which it is advancing--s.w., he will find that with a _falling barometer and s.e. wind the current passes him from the left to the right hand_; but if at a barometric _minimum_ he place himself in the same position with his face directed to the quarter from which the n.w. current is advancing laterally, also s.w., he will find that with a rising barometer _and n.w. wind the current passes him from right to left_. now the two classes of phænomena are identical, and it would not be difficult to show that, had we an instance of a rotatory storm in the northern hemisphere moving from n.w. to s.e., it would present precisely the same phænomena as to the direction of currents passing from left to right and from right to left with falling and rising barometers, increase and decrease in the force of the wind, &c., as the oppositely directed aërial currents do which pass over western central europe. in the absence of direct evidence of the production of a revolving storm from the crossing of two large waves, as suggested by sir john herschel, although it is not difficult to obtain such evidence, especially from the surface of the ocean, the identity of the two classes of phænomena exhibited by the storms and waves as above explained amounts to a strong presumption that there is a close connexion between them, and that a more minute investigation of the phænomena of atmospheric waves is greatly calculated to throw considerable light on the laws that govern the storm paths in both hemispheres. the localities in which these atmospheric movements, the waves, have been hitherto studied, have been confined to the northern and central parts of europe--the west of ireland, alten in the north of europe, lougan near the sea of azov, and geneva, being the angular points of the included area. it will be remarked that the greatest portion of this area is _inland_, but there is one important feature which the study of the barometer has brought to light, and which is by no means devoid of significance, viz. that the oscillations are much greater in the neighbourhood of _water_, and this appears to indicate that the junction lines of land and water form by far the most important portions of the globe in which to study both the phænomena of storms and waves. it is also very desirable that our knowledge of these phænomena should, with immediate reference to the surface of the ocean, be increased, and in this respect captains and masters of vessels may render essential service by observing and recording the state of the barometer, and direction and force of the wind, several times in the course of the day and night;[ ] and when it is considered that the immediate object in view is one in which the mariner is personally interested, and one in which, it may be, his own safety is concerned, it is hoped that the keeping of a meteorological register having especial reference to the indications of the barometer, and force and direction of the wind, will not be felt as irksome, but rather will be found an interesting occupation, the instruments standing in the place of faithful monitors, directing when and where to avoid danger, and the record furnishing important data whereby the knowledge of general laws may be arrived at, having an essential bearing on the interests of the service at large. chapter iii. observations. in sketching out a system of observation having especial reference to atmospheric waves and rotatory storms, regard has been had--_first_, to the instruments that should be used, the observations to be made with them, the corrections to be applied to such observations, and the form of registry most suitable for recording the results: _second_, to the times of observation: _third_, to the more important localities that should be submitted to additional observation: _fourth_, to peculiar phænomena requiring extraordinary observations for their elucidation: and _fifth_, to particular seasons, when the instruments should be watched with more than ordinary care. the more important objects of observation having especial reference to atmospheric waves are those points which have been termed _crests_ and _troughs_. these are simply the _highest_ and _lowest_ readings of the barometer, usually designated _maxima_ and _minima_, and should for the object in view receive particular attention. whenever there is reason to believe that the barometer is approaching either a _maximum_ or _minimum_, additional observations should be resorted to, so as to secure as nearly as possible _the precise time_ as reckoned at the ship, with her position, of its occurrence, as well as the altitude of the mercurial column at that time and place. by means of such observations as these on board several ships scattered over the surfaces of our great oceans, much valuable information may be accumulated of a character capable of throwing considerable light on the _direction_ in which the lines of barometric maxima and minima stretch, and also a tolerably accurate notion may be formed of their progress, both as regards direction and rate. in immediate connexion with such observations particular attention should be paid to the direction of the wind according to the season. section i.--instruments. _description and position of instruments._--the principal instrument requisite in these observations is the barometer, which should be of the marine construction, and as nearly alike as possible to those furnished to the antarctic expedition which sailed under the command of sir james clark ross. these instruments were similar to the ordinary portable barometers, and differed from them only in the mode of their suspension and the necessary contraction of the tubes to prevent oscillation from the motion of the ship. the barometer on shipboard should be suspended on a gimbal frame, which ought not to swing too freely, but rather so as to deaden oscillations by some degree of friction. to the upper portion of the tube in this construction of instrument light is alike accessible either in front or behind, and the vernier is furnished with a back and front edge, both being in precisely the same plane, nearly embracing the tube, and sliding up and down it by the motion of rack-work; by the graduation of the scale and vernier the altitude of the mercury can be read off to · inch. when the barometer is placed in the ship, its position should be as near midships as possible, out of the reach of sunshine, but in a good light for reading, and in a situation in which it will be but little liable to sudden gusts of wind and changes of temperature. great care should be taken to ascertain the exact height of its cistern above the water-line, and in order to facilitate night observations every possible arrangement should be made for placing behind it a light screened by white paper. _observations._--the first thing to be done is the reading off and recording the temperature indicated by the thermometer that in this construction of instrument dips into the mercury in the cistern. sir john herschel has suggested that "the bulb of the thermometer should be so situated as to afford the best chance of its indicating the exact mean of the whole barometric column, that is to say, fifteen inches above the cistern enclosed within the case of the barometer, nearly in contact with its tube, and with a stem so long as to be read off at the upper level." previous to making an observation with the barometer the instrument should be slightly tapped to free the mercury from any adhesion to the glass; any violent oscillation should, however, be carefully avoided. the vernier should then be adjusted to the upper surface of the mercury in the tube; for this purpose its back and front edges should be made to coincide, that is, the eye should be placed in exactly the same plane which passes through the edges; they should then be brought carefully down until they form a tangent with the curve produced by the convex surface of the mercury and the light is _just_ excluded from between them and the point of contact. it is desirable in making this adjustment that the eye should be assisted by a magnifying-glass. the reading of the scale should then be taken and entered in the column appropriated to it in the proper form. if the instrument have no tubular or double-edged index, the eye should be placed carefully at the level of the upper surface of the mercury and the index of the vernier brought gently down to the same level so as apparently just to touch the surface, great care being taken that the eye index and surface of the mercury are all in the same plane. each observation of the barometer should be accompanied by an observation of the direction of the wind, which should be noted in the usual manner in which it is observed at sea. in connexion with the _direction_ the _force_ of the wind should be recorded in accordance with the following scale, contrived by admiral sir francis beaufort:-- . calm . light air or just sufficient to give steerage way. . light breeze { or that in which a well- } to knots. . gentle breeze { conditioned man of war, } to knots. . moderate breeze { with all sail set, and } to knots. { clean full, would go in } { smooth water, from } . fresh breeze } { royals, &c. . strong breeze } { single-reefed top-sails } { and top-gallant } or that in which such a { sails. . moderate gale } ship could just carry in { double-reefed } chase full and by { topsails, jib, &c. . fresh gale } { triple-reefed } { topsails, &c. . strong gale } { close-reefed top-sails } { and courses. . whole gale or that with which she could scarcely bear close-reefed main topsail and reefed foresail. . storm or that which reduces her to storm staysails. . hurricane or that which no canvas could withstand. _corrections._--as soon after the observations have been made as circumstances will permit, the reading of the barometer should be _corrected_ for the relation existing between the capacities of the tube and cistern (if its construction be such as to require that correction), and for the capillary action of the tube; and then _reduced_ to the standard temperature of ° fahr., and to the sea-level, if on shipboard. for the first correction the _neutral point_ should be marked upon each instrument. it is that particular height which, in its construction, has been actually measured from the surface of the mercury in the cistern, and indicated by the scale. in general the mercury will stand either above or below the neutral point; if _above_, a portion of the mercury must have left the cistern, and consequently must have _lowered_ the surface in the cistern: in this case the altitude as measured by the scale will be _too short--vice versâ_, if below. the relation of the capacities of the tube and cistern should be experimentally ascertained, and marked upon the instrument by the maker. suppose the capacity to be / , marked thus on the instrument, "_capacity / :_" this indicates that for every inch of variation of the mercury in the tube, that in the cistern will vary contrariwise / th of an inch. when the mercury in the tube is _above_ the neutral point, the difference between it and the neutral point is to be reduced in the proportion expressed by the "capacity" (in the case supposed, divided by ), and the quotient _added_ to the observed height; if _below, subtracted_ from it. in barometers furnished with a fiducial point for adjusting the lower level, this correction is superfluous, and must not be applied. the second correction required is for the capillary action of the tube, the effect of which is always to depress the mercury in the tube by a certain quantity inversely proportioned to the diameter of the tube. this quantity should be experimentally determined during the construction of the instrument, and its amount marked upon it by the maker, and is always to be _added_ to the height of the mercurial column, previously corrected as before. for the convenience of those who may have barometers, the capillary action of which has not been determined, a table of corrections for tubes of different diameters is placed in the appendix, table i. the next correction, and in some respects the most important of all, is that due to the temperature of the mercury in the barometer tube at the time of observation, and to the expansion of the scale. table ii. of the appendix gives for every degree of the thermometer and every half-inch of the barometer, the proper quantity to be added or subtracted for the reduction of the observed height to the standard temperature of the mercury at ° fahr. after these the index correction should be applied. this is the amount of difference between the particular instrument and the readings of the royal society's flint-glass barometer when properly corrected, and is generally known as the _zero_. it is impossible to pay too much attention to the determination of this point. for this purpose, when practicable, the instrument should be immediately compared with the royal society's standard, and the difference of the readings of both instruments, when corrected as above, carefully noted and preserved. where, however, this is impracticable, the comparison should be effected by means either of some other standard previously so compared, or of an intermediate portable barometer, the zero point of which has been _well determined_. suspend the portable barometer as near as convenient to the ship's barometer, and after at least an hour's quiet exposure, take as many readings of both instruments as may be necessary to reduce the probable error of the mean of the differences below . inch. under these circumstances the mean difference of all the readings will be the _relative_ zero or index error, whence, if that of the intermediate barometer be known, that of the other may be found. as such comparisons will always be made when the vessel is in port, sufficient time can be allowed for making the requisite number of observations: hourly readings would perhaps be best, and they would have the advantage of forming part of the system when in operation, and might be accordingly used as such. it is not only desirable that the zero point of the barometer should be well determined in the first instance; it should also be carefully verified on every opportunity which presents itself; and in every instance, previous to sailing, it should be re-compared with the standard on shore by the intervention of a portable barometer, and no opportunity should be lost of comparing it on the voyage by means of such an intermediate instrument with the standard barometers at st. helena, the cape of good hope, bombay, madras, paramatta, van diemen's island, and with any other instruments likely to be referred to as standards, or employed in research elsewhere. any vessel having a portable barometer on board, the zero of which has been well determined, would do well, on touching at any of the ports above named, to take comparative readings with the standards at those ports, and record the differences between the standard, the portable, and the ship barometers. by such means the zero of one standard may be transported over the whole world, and those of others compared with it ascertained. to do so, however, with perfect effect, will require that the utmost care should be taken of the portable barometer; it should be guarded as much as possible from all accident, and should be kept safely in the "portable" state when not immediately used for comparison. to transport a well-authenticated zero from place to place is by no means a point of trifling importance. neither should it be executed hurriedly nor negligently. some of the greatest questions in meteorology depend on its due execution, and the objects for which these instructions have been prepared will be greatly advanced by the zero points of all barometers being referred to one common standard. upon the arrival of the vessel in england, at the termination of the voyage, the ship's barometer should be again compared with the same standard with which it was compared previous to sailing; and should any difference be found, it should be most carefully recorded. the correction for the height of the cistern _above_ or _below_ the water-line is _additive_ in the former case, _subtractive_ in the latter. its amount may be taken, nearly enough, by allowing · in. of the barometer for each foot of difference of level. an example of the application of these several corrections is subjoined:-- | _attached therm_. °· . |_data for the correction of | | | the instrument_. | +---------------------------------------+-------------------------------+ |barometer reading. · |neutral point · | |corr. for capacity - · |capacity / | | |capillary action + · | +---------------------------------------| | | · |zero to royal society + · | |corr. for capillarity + · |corr. for altitude above | | | water-line + · | +---------------------------------------| | | · | | |corr. for temperature - · | | +---------------------------------------| | | · | | |corr. for zero and water-line + · | | +---------------------------------------| | |aggregate = pressure at | | | sea-level · | | +---------------------------------------+-------------------------------+ it would greatly facilitate the comparison of the barometric observations by projecting them in curves when all the proper corrections have been applied. this may be accomplished by a much smaller expenditure of time than may at first be supposed. a paper of engraved squares on which the observations of twelve days may be laid down on double the natural scale, would be very suitable for the purpose.[ ] the projection of each day's observations would occupy but a short time; and should circumstances on any occasion prevent the execution of it, when the ship was becalmed or leisure otherwise afforded, it would form an interesting and useful occupation, and serve to beguile some of the tedium often experienced at such intervals. _registers._--for the particular object in view the register need not be very extensive. one kept in the annexed form will be amply sufficient. it should, however, be borne in mind that none but _uncorrected_ observations should find admission; in point of fact it should be strictly a register of phænomena as _observed_, and on no account whatever should any entry be made from recollection, or any attempt made to fill up a blank by the apparent course of the numbers before and after. the headings of the columns will, it is hoped, be sufficiently explicit. it is desirable in practice that the column for remarks should embrace an entire page opposite the other entries, in order that occasional observations, as well as several other circumstances continually coming under review in the course of keeping a journal, may find entry. meteorological register kept on board ______ during her voyage from ______ to ______ by ______. +---------+----+------+-------+------+------------------+--------+----------+ | | | | | | wind. | | | | | | | | att. |-----------+------| | | | date. |lat.| long.| barom.| ther.| direction.|force.| remarks| observer.| |---------|----|------|-------|------|-----------|------|--------|----------| | |h. m.| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | +---+-----+----+------+-------+------+-----------+------+--------+----------+ the only difference between the above form and one for the reception of _corrected_ readings will be the dispensing with the column for the attached thermometer, and placing under the word barom. "corrected." ii.--times of observation. there can be no question that the greatest amount of information, the accuracy of the data supplied, and in fact every meteorological element necessary to increase our knowledge of atmospheric waves, may be best obtained by an uninterrupted series of _hourly_ observations made on board vessels from their leaving england until their safe arrival again at the close of their respective voyages; but from a variety of circumstances--the nature of the service in which the vessels may be employed, particular states of the weather, &c.--such a course of unremitting labour cannot be expected; it is therefore necessary to fix on some stated hours at which the instruments before particularized should be regularly observed throughout the voyage, and their indications faithfully recorded. the hours of a.m., a.m., p.m., and p.m., are now so generally known as _meteorological hours_, that nothing should justify a departure from them; and it is the more essential that these hours should be adopted in the present inquiry, because the series of observations made at intervals terminated by these hours can the more readily be used in connexion with those made contemporaneously on land, and will also serve to carry on investigations previously instituted, and which have received considerable illustration by means of observations at the regular meteorological hours; we therefore recommend their general adoption in all observations conducted at sea. it is intended in the sequel to call attention to particular parts of the earth's surface where it is desirable that additional observations should be made, in order to furnish data of a more accurate character, and to mark more distinctly barometric changes than the four daily readings are capable of effecting. the best means of accomplishing this for the object in view appears to be the division of the interval of six hours into two equal portions, and to make the necessary observations eight times in the course of twenty-four hours. in the particular localities to which allusion has been made we recommend the following as the hours of observation:-- a.m. , , , noon. p.m. , , , midnight. in other localities besides those hereafter to be mentioned, when opportunities serve, readings at these hours would greatly enhance the value of the four daily readings. there are, however, portions of the surface of our planet, and probably also phænomena that occur in its atmosphere, which require still closer attention than the eight daily readings. one such portion would appear to exist off the western coast of africa, and we recommend the adoption of _hourly_ readings while sailing to the westward of this junction of aqueous and terrestrial surface; more attention will be directed to this point as we proceed. there are also phænomena the localities of which may be undetermined, and the times of their occurrence unknown, but so important a relation do they bear to the subject of our inquiries, that they demand the closest attention. they will be more particularly described under the head of accumulations of pressure preceding and succeeding storms, and minute directions given for the hourly observations of the necessary instruments. in the mean time we may here remark that hourly observations under the circumstances above alluded to are the more important when we consider that the barometer, the instrument employed in observing these moving atmospheric masses, is itself in motion. the ship may meet the accumulation of pressure and sail through it transversely; or she may sail along it, the course of the vessel being parallel to the line marking the highest pressure, the ridge or crest of the wave; or the ship may make any angle with this line: but whatever the circumstances may be under which she passes through or along with such an accumulation of pressure, it should ever be borne in mind that her position on the earth's surface is scarcely ever the same at any one observation as it was at the preceding, the barometer in the interval has changed _its_ position as well as the line of maximum pressure, the rate of progress of which it is desirable to observe. it will, therefore, be at once apparent that in order to obtain the most accurate data on this head hourly observations are indispensable. to these readings should of course be appended the places of the ship from hour to hour, especially if she alter her course much. there is another point to which we wish to call attention in immediate connexion with hourly readings--it is the observation of the instruments on the days fixed for that purpose: they were originally suggested by sir john herschel, whose directions should be strictly attended to: they are as follows:-- the days fixed upon for these observations are the st of march, the st of june, the st of september, and the st of december, being those, or immediately adjoining to those of the equinoxes and solstices, in which the _solar influence_ is either stationary or in a state of most rapid variation. _but should any one of those st days fall on a sunday, then it will be understood that the observations are to be deferred till the next day, the nd._ the series of observations on board each vessel should commence at o'clock a.m. of the appointed days, and terminate at a.m. of the days following, according to the usual reckoning of time adopted in the daily observations. in addition to the twenty-five hourly readings at the solstices and equinoxes as above recommended, it would be desirable to continue the observations until a complete elevation and depression of the barometer had been observed at these seasons. this plan is adopted at the royal observatory, greenwich, and would be attended with this advantage were it generally so--the progress of the elevation and depression would be more readily traced and their velocities more accurately determined than from the four or eight daily readings. iii.--localities for additional observations. in sketching out a system of barometric observation having especial reference to the acquisition of data from which the _barometric character_ of certain large areas of the surface of the globe may be determined--inasmuch as such areas are distinguished from each other, on the one hand by consisting of extensive spaces of the oceanic surface unbroken, or scarcely broken, by land; on the other by the proximity of such oceanic surface to large masses of land, and these masses presenting two essentially different features, the one consisting of land particularly characterized as continental, the other as insular, regard has been accordingly had to such distribution of land and water. as these instructions have especial reference to observations at sea, observations on land have not been alluded to; but in order that the data accumulated may possess that value which is essential for carrying on the inquiry in reference to atmospheric waves with success, provision is made to mark out more distinctly the barometric effects of the junction of large masses of land and water. it is well known that the oceanic surface, and even the smaller surfaces of inland seas, produce decided inflexions of the isothermal lines. they exercise an important influence on temperature. it has also been shown that the neighbourhood of water has a very considerable influence in increasing the oscillations of the mercurial column in the barometer, and in the great systems of european undulations it is well known that these oscillations increase especially towards the north-west. the converse of this, however, has not yet been subjected to observation; there has been no systematic co-operation of observers for the purpose of determining the barometric affections of large masses of water, such as the central portion of the basin of the northern atlantic, the portion of oceanic surface between the cape of good hope and cape horn, the indian and southern oceans, and the vast basin of the pacific. nor are we yet acquainted with the character of the oscillations, whether increasing or decreasing, as we recede from the central portions of the oceanic surfaces we have mentioned towards the land which forms their eastern, western, or northern boundaries. this influence of the junction line of land and water, so far as it is yet known, has been kept in view in framing these instructions, and, as it appears so prominently in europe, it is hoped the additional observations between the four daily readings to which probably many observers may habitually restrict themselves, making on certain occasions and in particular localities a series of observations at intervals of three hours, will not be considered too frequent when the great importance of the problem to be solved is fully apprehended. it need scarcely be said that the value of these observations at three-hourly intervals will be greatly increased by the number of observers co-operating in them. upon such an extensive system of co-operation a large space on the earth's surface, possessing peculiarities which distinguish it from others extremely unlike it in their general character, or assimilate it to such as possess with it many features in common, is marked out below for particular observation, occupying more than two-thirds of a zone in the northern hemisphere, having a breadth of °, and including every possible variety of terrestrial and aqueous surface, from the burning sands of the great african desert, situated about the centre, to the narrow strip of land connecting the two americas on the one side, and the chain of islands connecting china and hindostan with australia on the other. on each side of the african continent we have spaces of open sea between ° and ° west longitude north of the equator, and between ° and ° east longitude, in or to the south of the equator, admirably suited for contrasting the barometric affections, as manifested in these spaces of open water, with those occurring in situations where the influence of the terrestrial surface comes into more active operation. the localities where three-hourly readings are chiefly desirable may be specified under the heads of _northern atlantic, southern atlantic, indian_ and _southern oceans,_ and _pacific ocean_. _northern atlantic. homeward-bound voyages._--the discussion of observations made in the united kingdom and the western border of central europe, has indicated that off the north-west of scotland a centre of great barometric disturbance exists. this centre of disturbance appears to be considerably removed from the usual tracks of vessels crossing the atlantic; nevertheless some light may be thrown on the barometric phænomena resulting from this disturbance by observations during homeward-bound voyages, especially after the vessels have passed the meridian of ° west longitude. voyagers to or from baffin and hudson bays would do well during the whole of the voyage to read off the barometer every three hours, as their tracks would approach nearest the centre of disturbance in question. before crossing the th meridian, the undulations arising from the distribution of land and water in the neighbourhood of these vast inland seas would receive considerable elucidation from the shorter intervals of observation, and after passing the th meridian the extent of undulation, as compared with that observed by the more southerly vessels, would be more distinctly marked by the three-hourly series. surveying vessels stationed on the north-western coasts of ireland and scotland may contribute most important information on this head by a regular and, as far as circumstances will allow, an uninterrupted series either of six-hourly or three-hourly observations. the intervals of observation on board vessels stationed at the western isles, the orkneys, and the shetland isles, ought not to be longer than _three_ hours, principally on account of the great extent of oscillation observed in those localities. vessels arriving from all parts of the world as they approach the united kingdom should observe at shorter intervals than six hours. as a general instruction on this head the series of three-hourly observations may be commenced on board vessels from america and the pacific by the way of cape horn on their passing the th meridian, such three-hourly observations to be continued until the arrival of the vessels in port. ships by the way of the cape of good hope should commence the three-hourly series either on leaving or passing the colony, in order that the phænomena of the tropical depression hereafter to be noticed may be well observed. _northern atlantic. outward-bound voyages_.--vessels sailing to the united states, mexico, and the west indies, should observe at three hours' interval upon passing the th meridian. observations at this interval, on board vessels navigating the gulf of mexico and the caribbean sea, will be particularly valuable in determining the extent of oscillation as influenced by the masses of land and water in this portion of the torrid zone, as compared with the oscillation noticed off the western coast of africa, hereafter to be referred to. _southern atlantic. outward and homeward bound_.--without doubt the most interesting phænomenon, and one that lies at the root of the great atmospheric movements, especially those proceeding northwards in the northern hemisphere and southwards in the southern, is the equatorial depression first noticed by von humboldt and confirmed by many observers since. we shall find the general expression of this most important meteorological fact in the report of the committee of physics and meteorology, appointed by the royal society in , as follows: "the barometer, at the level of the sea, does not indicate a mean atmospheric pressure of equal amount in all parts of the earth; but, on the contrary, the equatorial pressure is uniformly less in its mean amount than at and beyond the tropics." vessels that are outward bound should, upon passing ° north latitude, commence the series of three-hourly observations, with an especial reference to the equatorial depression. these three-hourly observations should be continued until the latitude of ° south has been passed: the whole series will then include the minimum of the depression and the two maxima or apices forming its boundaries. (see daniell's 'meteorological essays,' rd edition.) in passages across the equator, should the ships be delayed by calms, opportunities should be embraced for observing this depression with greater precision by means of _hourly_ readings; and these readings will not only be valuable as respects the depression here spoken of, but will go far to indicate the character of any disturbance that may arise, and point out, as nearly as such observations will allow, the precise time when such disturbance produced its effects in the neighbourhood of the ships. in point of fact they will clearly illustrate the diversion of the tendency to rise, spoken of in the report before alluded to, as resulting in ascending columns and sheets, between which wind flaws, capricious in their direction and intensity, and often amounting to sharp squalls, mark out the course of their feeders and the indraft of cooler air from a distance to supply their void. hourly observations, with especial reference to this and the following head of inquiry, should also be made off the western coast of africa during the homeward-bound voyage. immediately connected with this part of the outward-bound voyage, hourly observations, as often as circumstances will permit, while the ships are sailing from the madeiras to the equator, will be extremely valuable in elucidating the origin of the great system of south-westerly atmospheric waves that traverse europe, and in furnishing data for comparison with the amount of oscillation and other barometric phænomena in the gulf of mexico and the caribbean sea, a portion of the torrid zone essentially different in its configuration and in the relations of its area to land and water, as contra-distinguished to the northern portion of the african continent; and these hourly observations are the more desirable as the vessels may approach the land. they may be discontinued on passing the equator, and the three-hourly series resumed. there are two points in the southern hemisphere, between ° west longitude and ° east longitude, that claim particular attention in a barometric point of view, viz., cape horn and the cape of good hope; the latter is within the area marked out for the three-hourly observations, and too much attention cannot be paid to the indications of the barometer as vessels are approaching or leaving the cape. the northern part of the south atlantic ocean has been termed the _true pacific ocean of the world_; and at st. helena a gale was scarcely ever known; it is also said to be entirely free from actual storms (col. reid's 'law of storms,' st edition, p. ). it may therefore be expected that the barometer will present in this locality but a small oscillation, and ships in sailing from st. helena to the cape will do well to ascertain, by means of the three-hourly observations, the increase of oscillation as they approach the cape. the same thing will hold good with regard to cape horn: it appears from previous observation that a permanent barometric depression exists in this locality, most probably in some way connected with the immense depression noticed by captain sir james clark ross, towards the antarctic circle. the general character of the atmosphere off cape horn is also extremely different from its character at st. helena. it would therefore be well for vessels sailing into the pacific by cape horn, to continue the three-hourly observations until the th meridian is passed. before quitting the atlantic ocean it may be well to notice the marine stations mentioned in my third report on atmospheric waves,[ ] as being particularly suitable for testing the views advanced in that report and for tracing a wave of the south-westerly system from the most western point of africa to the extreme north of europe. a series of hourly observations off the western coast of africa has already been suggested. vessels staying at cape verd islands should not omit to make observations at three hours' interval _during the whole of their stay_, and when circumstances will allow, hourly readings. at the canaries, madeiras, and the azores, similar observations should be made. vessels touching at cape cantin, tangier, gibraltar, cadiz, lisbon, oporto, corunna, and brest, should also make these observations while they are in the localities of these ports. at the scilly isles we have six-hourly observations, made under the superintendence of the honourable the corporation of the trinity house. ships in nearing these islands and making the observations already pointed out, will greatly assist in determining the increase of oscillation proceeding westward from the nodal point of the two great european systems. we have already mentioned the service surveying vessels employed on the coasts of ireland and scotland may render, and the remaining portion of the area marked out in the report may be occupied by vessels navigating the north sea and the coast of norway, as far as hammerfest. in connexion with these observations, having especial reference to the european system of south-westerly atmospheric waves, the mediterranean presents a surface of considerable interest, both as regards these particular waves, and the influence its waters exert in modifying the two great systems of central europe. the late professor daniell has shown from the manheim observations, that small undulations, having their origin on the northern borders of the mediterranean, have propagated themselves northward, and in this manner, but in a smaller degree, the waters of the mediterranean have contributed to increase the oscillation as well as the larger surface of the northern atlantic. in most of the localities of this great inland sea six-hourly observations may suffice for this immediate purpose; but in sailing from lisbon through the straits of gibraltar, in the neighbourhood of sicily and italy, and in the grecian archipelago, we should recommend the three-hourly series, as marking more distinctly the effects resulting from the proximity of land; this remark has especial reference to the passage through the straits of gibraltar, where, if possible, hourly observations should be made. _the indian and southern oceans. outward and homeward bound._--on sailing from the cape of good hope to the east indies, china, or australia, observations at intervals of three hours should be made until the th meridian east is passed (homeward-bound vessels should commence the three-hourly readings on arriving at this meridian). upon leaving the th meridian the six-hourly observations may be resumed on board vessels bound for the indies and china until they arrive at the equator, when the readings should again be made at intervals of three hours, and continued until the arrival of the vessels in port. with regard to vessels bound for australia and new zealand, the six-hourly readings may be continued from the th to the th meridian, and upon the vessels passing the latter, the three-hourly readings should be commenced and continued until the vessels arrive in port. vessels navigating the archipelago between china and new zealand, should make observations every three hours, in order that the undulations arising from the configuration of the terrestrial and oceanic surfaces may be more distinctly marked and more advantageously compared with the gulf of mexico, the caribbean sea, and the northern portion of the african continent. _the pacific ocean._--as this ocean presents so vast an aqueous surface, generally speaking observations at intervals of six hours will be amply sufficient to ascertain its leading barometric phænomena. vessels, however, on approaching the continents of north and south america, or sailing across the equator, should resort to the three-hourly readings, in order to ascertain more distinctly the effect of the neighbourhood of land on the oscillations of the barometer, as generally observed, over so immense a surface of water in the one case, and the phænomena of the equatorial depression in the other: the same remarks relative to the latter subject, which we offered under the head of south atlantic, will equally apply in the present instance. the configuration of the western shores of north america renders it difficult to determine the precise boundary where the three-hourly series should commence; the th meridian is recommended for the boundary as regards south america, and from this a judgment may be formed as to where the three-hourly observations should commence in reference to north america. in the previous sketch of the localities for the more important observations, it will be seen that within the tropics there are three which demand the greatest regard. i. the archipelago between the two americas, more particularly comprised within the th and th meridians west longitude, and the equator and the th degree of north latitude. as a general principle we should say that vessels within this area should observe the barometer every three hours. its eastern portion includes the lower branches of the storm paths, and on this account is peculiarly interesting, especially in a barometric point of view. ii. _the northern portion of the african continent, including the sahara or great desert._--this vast radiating surface must exert considerable influence on the waters on each side northern africa. vessels sailing within the area comprised between ° west and ° east, and the equator and the th parallel, should also make observations at intervals of three hours. iii. _the great eastern archipelago._--this presents a somewhat similar character to the western; like that, it is the region of terrific hurricanes, and it becomes a most interesting object to determine its barometric phænomena; the three-hourly system of observation may therefore be resorted to within an area comprised between the th and th meridians, and the equator and the th degree of north latitude. the southern hemisphere also presents three important localities, the prolongations of the three tropical areas. it is unnecessary to enlarge upon these, as ample instructions have been already given. we may, however, remark, with regard to australia, that three-hourly observations should be made within the area comprised between the th and th meridians east, and the equator and the th parallel south, and hourly ones in the immediate neighbourhood of all its coasts. iv.--storms, hurricanes, and typhoons. the solution of the question--how far and in what manner are storms connected with atmospheric waves?--must be extremely interesting to every one engaged in either the naval or merchant service. as we have in the former chapters directed attention to their connexion, our great object here will be to endeavour to mark out such a line of observation as appears most capable of throwing light, not only on the most important desiderata as connected with storms, but also their connexion or non-connexion with atmospheric waves. we shall accordingly arrange this portion of the instructions under the following heads:--_desiderata_; _localities_; _margins_; _preceding and succeeding accumulations of pressure._ _desiderata._--the most important desiderata appertaining to the subject of storms, are certainly their origin and termination. of these initial and terminal points in the course of great storms we absolutely know nothing, unless _the white appearance of a round form_ observed by mr. seymour on board the judith and esther, in lat. ° ' north and long. ° ' west (see col. reid's 'law of storms,' st edit. p. ), may be regarded as the commencement of the antigua hurricane of august , . this vessel was the most eastern of those from which observations had been obtained; and it is the absence of contemporaneous observations to the eastward of the th meridian that leaves the question as to the origin of the west indian revolving storms unsolved. not one of mr. redfield's storm routes extends eastward of the th meridian; this at once marks out, so far as storms are concerned, the entire space included between the th and th meridians, the equator and the th parallel, as a most suitable area for observations, under particular circumstances hereafter to be noticed, with especial reference either to the commencement or termination of storms, or the prolongation of mr. redfield's storm paths. _localities._--the three principal localities of storms are as follows:--i. the western portion of the basin of the north atlantic; ii. the china sea and bay of bengal; and iii. the indian ocean, more particularly in the neighbourhood of mauritius. the first two have already been marked out as areas for the three-hourly observations; to the latter, the remark as to extra observations under the head of desiderata will apply. _margins._--mr. redfield has shown that on some occasions storms have been preceded by an unusual pressure of the atmosphere; the barometer has stood remarkably _high_, and it has hence been inferred that there has existed _around_ the gale an accumulation of air forming a margin; barometers placed under this margin indicating a much greater pressure than the mean of the respective localities. with regard to the west indian and american hurricanes--any considerable increase of pressure, especially within the space marked out to the eastward of the th meridian, will demand immediate attention. upon the barometer ranging _very high_ within this space, three-hourly observations should be immediately resorted to; and if possible, _hourly_ readings taken, and this is the more important the nearer the vessel may be to the th meridian. each observation of the barometer should be accompanied by an observation of the wind--its direction should be most carefully noted, and the force estimated according to the scale in page , or by the anemometer. it would be as well _at the time_ to project the barometric readings in a curve even of a rough character, that the extent of fall after the mercury had passed its maximum might be readily discernible by the eye. a paper ruled in squares, the vertical lines representing the commencement of hours, and the horizontal tenths of an inch, would be quite sufficient for this purpose. the _force_ of the wind should be noted at, or as near to the time of the passage of the maximum as possible. during the fall of the mercury particular attention should be paid to the manner in which the wind changes, should any change be observed; and should the wind continue blowing steadily in _one_ direction, but gradually _increasing_ in force, then such increments of force should be most carefully noted. during the fall of the barometer, should the changes of the wind and its increasing force indicate the neighbourhood of a revolving storm, (independent of the obvious reasons for avoiding the focus of the storm,) it would contribute as much to increase our knowledge of these dangerous vortices to keep as near as possible to their margins as to approach their centres. the recess from the centre towards the margin of the storm, will probably be rendered apparent by the _rising_ of the mercury; and so far as the observations may be considered valuable for elucidating the connexion of atmospheric waves with rotatory storms (other motives being balanced), it might be desirable to keep the ship near the margin--provided she is not carried beyond the influence of the winds which characterize the latter half of the storm--until the barometer has nearly attained its usual elevation. by this means some notion might be formed of the general direction of the line of barometric pressure preceding or succeeding a storm. should a gale be observed commencing without its having been preceded by an unusual elevation of the mercurial column, and consequently no additional observation have been made; when the force of the wind is noted in the usual observations at or above , then the three-hourly series should be resorted to, and the same care taken in noting the direction, changes, and force of the wind as pointed out in the preceding paragraph. the foregoing remarks relate especially to the central and western portions of the north atlantic; they will however equally apply to the remaining localities of storms. under any circumstances, and in any locality, a _high_ barometer not less than a low one should demand particular attention, and if possible, _hourly_ readings taken some time before and after the passage of the maximum: this will be referred to more particularly under the next head. _preceding and succeeding accumulations of pressure._--mr. redfield has shown in his memoir of the cuba hurricane of october, , that two associated storms were immediately preceded by a barometric wave, or accumulation of pressure, the barometer rising above the usual or annual mean. we have just referred to the importance of _hourly_ observations on occasions of the readings being _high_ as capable of illustrating the marginal phænomena of storms, and in connexion with these accumulations of pressure in advance of storms we would reiterate the suggestion. these strips of accumulated pressure are doubtless crests of atmospheric waves rolling forwards. in some cases a ship in its progress may cut them transversely in a direction at right angles to their _length_, in others very obliquely; but in all cases, whatever section may be given by the curve representing the observations, too much attention cannot be bestowed on the barometer, the wet and dry bulb thermometer, the direction and force of the wind, the state of the sky, and the appearance of the ocean during the ship's passage _through_ such an accumulation of pressure. when the barometer attains its mean altitude, and is rapidly rising above it in any locality, then _hourly_ observations of the instruments and phænomena above noticed should be commenced and continued until after the mercury had attained its highest point and had sunk again to its mean state. in such observations particular attention should be paid to the direction and force of the wind preceding the barometric maximum--and the same phænomena succeeding it, and particular notice should be taken of the time when, and amount of any change either in the direction or force of the wind. it is by such observations as these, carried on with great care and made at every accessible portion of the oceanic surface, that we may be able to ascertain the continuity of these atmospheric waves, to determine somewhat respecting their length, to show the character of their connexion with the rotatory storm, and to deduce the direction and rate of their progress. v.--seasons for extra observations. in reference to certain desiderata that have presented themselves in the course of my researches on this subject (see report of the british association for the advancement of science, , p. ), the _phases_ of the larger barometric undulations, and the _types_ of the various seasons of the year, demand particular attention and call for extra observations at certain seasons: of these, three only have yet been ascertained--the type for the middle of november--the annual depression on or about the th of november--and the annual elevation on or about the th of december. the enunciation of the first is as under: "that during fourteen days in november, more or less equally disposed about the middle of the month, the oscillations of the barometer exhibit a remarkably symmetrical character, that is to say, the fall succeeding the transit of the maximum or the highest reading is to a great extent similar to the preceding rise. this rise and fall is not continuous or unbroken; in some cases it consists of _five_, in others of _three_ distinct elevations. the complete rise and fall has been termed the great symmetrical barometric wave of november. at its setting in the barometer is generally low, sometimes below twenty-nine inches. this depression is generally succeeded by _two_ well-marked undulations, varying from one to two days in duration. the central undulation, which also forms the apex of the great wave, is of larger extent, occupying from three to five days; when this has passed, two smaller undulations corresponding to those at the commencement of the wave make their appearance, and at the close of the last the wave terminates." with but slight exceptions, the observations of eight successive years have confirmed the general correctness of this type. on two occasions the central apex has not been the highest, and these deviations, with some of a minor character, form the exceptions alluded to. this type only has reference to london and the south-eastern parts of england; proceeding westward, north-westward, and northward, the symmetrical character of this type is considerably departed from; each locality possessing its own type of the barometric movements during november. the desiderata in immediate connexion with the november movements, as observed in the southern and south-eastern parts of england, that present themselves, are--the determination of the types for november, especially its middle portion, as exhibited on the oceanic surface within an area comprised between the th and th parallels, and the st and th meridians west. vessels sailing within this area may contribute greatly to the determination of these types by making observations at intervals of three hours from the st of november to the th or th of december. the entire period of the great symmetrical wave of november will most probably be embraced by such a series of observations, as well as the annual depression of the th. for the elevation of the th of december the three-hourly observations should be commenced on the st, and continued until the rd or th of the succeeding january. with respect to the great wave of november, our knowledge of it would be much increased by such a series of observations as mentioned above, being made on board surveying and other vessels employed off scotland and ireland; vessels navigating the north sea; vessels stationed off the coasts of france, spain, portugal, and the northern parts of africa, and at all our stations in the mediterranean. in this way the area of examination would be greatly enlarged, and the _differences_ of the curves more fully elucidated; and this extended area of observation is the more desirable, as there is some reason to believe that the line of greatest symmetry _revolves_ around a fixed point, most probably the nodal point of the great european systems. it is highly probable that movements of a somewhat similar character, although presenting very different curves, exist in the southern hemisphere. the november wave is more or less associated with storms. it has been generally preceded by a high barometer and succeeded by a low one, and this low state of the barometer has been accompanied by stormy weather. we are therefore prepared to seek for similar phænomena in the southern hemisphere, in those localities which present similar states of weather, and at seasons when such weather predominates. we have already marked out the two capes in the southern hemisphere for three-hourly observations: they must doubtless possess very peculiar barometric characters, stretching as they do into the vast area of the southern ocean. it is highly probable that the oscillations, especially at some seasons, are very considerable, and vessels visiting them at such seasons would do well to record with especial care the indications of the instruments already alluded to. at present we know but little of the barometric movements in the southern hemisphere, and every addition to our knowledge in this respect will open the way to more important conclusions. it has been observed in the south-east of england that the barometer has generally passed a maximum on or about the rd of every month, and this has been so frequently the case as to form the rule rather than the exception. the same fact during a more limited period has been observed at toronto. with especial reference to this subject the three-hourly series of observations may be resorted to in all localities, but especially north of the th parallel in the northern hemisphere. they should be commenced at midnight immediately preceding the st and continued to midnight succeeding the th. chapter iv. practical directions for avoiding the centres of rotating storms. figures and , enlarged and printed on narrow rings of stiff cardboard, are employed for this purpose. the letters outside the thick circle are intended to distinguish the points of the compass, and in use should always coincide with those points on the chart. the letters within the thick circle indicate the direction of the wind in a hurricane, the whirl being shown by the arrows between the letters. in the northern hemisphere the direction of the whirl is always contrary to that in which the hands of a watch move, and in the southern coincident thereto. the graduation is intended to assist the mariner in ascertaining the bearing of the centre of a storm from his ship. _use._ at any time when a severe gale or hurricane is expected, the seaman should at once find the position of his ship on the chart, and place upon it the graduated point which answers to the direction of the wind at the time, taking care that the needle is directed to the north, so that the exterior letters may point on the chart to the respective points of the compass: this is very essential. this simple process will at once acquaint the seaman with two important facts relative to the coming hurricane--his position in the storm, and the direction in which it is moving. _examples._ a captain of a ship in latitude ° ' n., longitude ° ' w., bound to the united states, observes the barometer to stand unusually high, say · inches: shortly after the mercury begins to fall, at first slowly and steadily; as the glass falls the wind freshens, and is noticed to blow with increasing force from the s. so as to threaten a gale. the position of the ship on the chart is now to be found, and the graduated point under the letters e. s. is to be placed thereon, taking care to direct the needle to the north. from these two circumstances, the falling barometer and the wind blowing from the south with increasing force, the mariner is aware of this simple fact, that he is situated in the advancing portion of a body of air which is proceeding towards the n.e.; and if he turn his face to the n.e. he will find he is on the right of the axis line, or line cutting the advancing body transversely. the hurricane circle as it lies on the chart reveals to him another important fact, which is, that if he pursue his course he will sail _towards_ the axis line of the hurricane, and may stand a chance of foundering in its centre. to avoid this he has one of two courses to adopt; either to lay-to on the _starboard tack_, according to col. reid's rules (see his 'law of storms,' st edit., pp. to ), the ship being in the right-hand semicircle of the hurricane, or so to alter his course as to keep without the influence of the storm. in the present case the adoption of the latter alternative would involve a reversal of his former course; nevertheless it is clear the more he bears to the s.e. the less he will experience the violence of the hurricane: should he heave his ship to, upon moving the hurricane circle from the ship's place on the chart towards the n.e., he will be able to judge of the changes of the wind he is likely to experience: thus it will first veer to s.s.w., the barometer still falling; then to s.w., the barometer at a minimum--this marks the position of the most violent portion of the storm he may be in, and by keeping the barometer as high as he can by bearing towards the s.e., the farther he will be from the centre--the barometer now begins to rise, the wind veering to w.s.w., and the hurricane finally passes off with the wind at w. it is to be particularly remarked that in this example the ship is in the _most dangerous quadrant_, as by scudding she would be driven in advance of the track of the storm's centre, which of course would be approaching her. assuming that the hurricane sets in at the ship's place with the wind at s.e., the proceeding will be altogether different. at first the wind is fair for the prosecution of the voyage, and it is desirable to take advantage of this fair wind to avoid as much as possible the track of the centre, which passes over the ship's place in this instance, and is always the most dangerous part of the storm. as the ship is able to make good distance from this track by bearing towards the n.w., provided she has plenty of sea-room, she will experience less of the violence of the hurricane; but as most of the atlantic storms sweep over the shore, it will be desirable to lay-to at some point on the _larboard tack_, the ship being now in the left-hand semicircle. by moving the circle as before directed it will be seen that the veering of the wind is now e.s.e., e., e.n.e., n.e., the lowest barometer n.n.e., n., and n.n.w., the ship experiencing more or less of these changes as it is nearer to or farther from the axis line. in latitudes lower than ° n. the atlantic hurricanes usually move towards the n.w. taking the same positions of our ship with regard to the storms as in the two former examples, if the storm set in with the wind e. the proper proceeding is to bear away for the n.e., the most dangerous quadrant of the hurricane having overtaken the ship, the veering of the wind if she is lying-to will be e., e.s.e., s.e., with the lowest barometer s.s.e. and s. should the storm set in at n.e., her position at the time will be some indication of the distance of the centre's track from the nearest land, and will greatly assist in determining the point at which the captain ought to lay-to after taking advantage of the n.e. wind, should he be able so to do, to bear away from the centre line, so as to avoid as much as possible the violence of the storm. from the proximity of the west indian islands to this locality of the storm-paths, the danger is proportionally increased. the above examples have reference only to the lower and upper branches of the storm paths of the northern atlantic in the neighbourhood of the west indies and the united states. in latitudes from about ° to ° these paths usually _re-curve_, and at some point will move towards the north. the veering of the wind will consequently be more or less complicated according as the ship may be nearer to or farther from the centre. the tables on page , combined with the first of those immediately following the next paragraph, will, it is hoped, prove advantageous in assisting the mariner as to the course to be adopted. as a general principle we should say it would be best to bear to the eastward, so as not only to avoid the greater fury of the storm, but to get into the s. and s.w. winds, which give the principal chances of making a westerly course. we have in page called attention to the fact that the storm paths traced by mr. redfield do not extend eastward of the th meridian. this by no means precludes the existence of severe storms and those of a rotatory character in the great basin of the northern atlantic, especially between the th and th parallels. a remarkable instance has come under the author's attention of the wind hauling _apparently_ contrary to the usual theory: it may be that the storm route was in a direction not generally observed. we are at the present moment destitute of any information that at all indicates a _reversion_ of the rotation in either hemisphere. the following tables constructed for the northern hemisphere, and for storm routes _not yet ascertained_, may probably be consulted with advantage on anomalous occasions. hurricane moving from south to north. axis line, wind e., barometer falling, first half of storm. axis line, wind w., barometer rising, last half of storm. right-hand semicircle. wind e.s.e., s.e., s.s.e., s., barometer falling, first half of storm. wind w.s.w., s.w., s.s.w., s., barometer rising, last half of storm. left-hand semicircle. wind e.n.e., n.e., n.n.e., n., barometer falling, first half of storm. wind w.n.w., n.w., n.n.w., n., barometer rising, last half of storm. hurricane moving from north to south. axis line, wind w., barometer falling, first half of storm. axis line, wind e., barometer rising, last half of storm. right-hand semicircle. wind w.n.w., n.w., n.n.w., n., barometer falling, first half of storm. wind e.n.e., n.e., n.n.e., n., barometer rising, last half of storm. left-hand semicircle. wind w.s.w., s.w., s.s.w., s., barometer falling, first half of storm. wind e.s.e., s.e., s.s.e., s,, barometer rising, last half of storm. hurricane moving prom west to east. axis line, wind s., barometer falling, first half of storm. axis line, wind n., barometer rising, last half of storm. right-hand semicircle. wind s.s.w., s.w., w.s.w., w., barometer falling, first half of storm. wind n.n.w., n.w., w.n.w., w., barometer rising, last half of storm. left-hand semicircle. wind s.s.e., s.e., e.s.e., e., barometer falling, first half of storm. wind n.n.e., n.e., e.n.e., e., barometer rising, last half of storm. hurricane moving from north-west to south-east. axis line, wind s.w., barometer falling, first half of storm. axis line, wind n.e., barometer rising, last half of storm. right-hand semicircle. wind w.s.w., w., w.n.w., n.w., barometer falling, first half of storm. wind n.n.e., n., n.n.w., n.w., barometer rising, last half of storm. left-hand semicircle. wind s.s.w., s., s.s.e., s.e., barometer falling, first half of storm. wind e.n.e., e., e.s.e., s.e., barometer rising, last half of storm. appendix. table i.--correction to be added to barometers for capillary action. +--------------------+---------------------------------+ | | correction for | | diameter of tube. |-----------------+---------------| | | unboiled tubes. | boiled tubes. | |--------------------|-----------------|---------------| | inch. | inch. | inch. | | · | · | · | | · | · | · | | · | · | · | | · | · | · | | · | · | · | | · | · | · | | · | · | · | | · | · | · | | · | · | · | | · | · | · | +--------------------+-----------------+---------------+ +---------------------------------------------------------------------+ |transcibers note: the following line table has been split into | |two, both vertically and horizontally, so that it can be accommodated| |on these pages. | +---------------------------------------------------------------------+ table ii.--correction to be applied to barometers with _brass scales_, extending from the cistern to the top of the mercurial column, to reduce the observation to ° fahrenheit. ---+------------------------------------------------------------+---- | i n c h e s. | t | -----+-------+------+-------+------+-------+------+--------| t e | | | | | | | | | e m | | · | | · | | · | | · | m p | | | | | | | | | p ---+------+-------+------+-------+------+-------+------+--------+---- ° | + | + | + | + | + | + | + | + | ° | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | | | | | | | | | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | | | | | | | | | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | -- | -- | -- | -- | -- | -- | -- | -- | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | | | | | | | | | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | | | | | | | | | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | ---+------+-------+------+-------+------+-------+------+--------+---- ---+-----------------------------------------------------+---- | i n c h e s. | t |-------+------+-------+------+-------+------+--------| t e | | | | | | | | e m | | · | | · | | · | | m p | | | | | | | | p ---+-------+------+-------+------+-------+------+--------+---- ° | + | + | + | + | + | + | + | ° | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | | | | | | | | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | | | | | | | | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | -- | -- | -- | -- | -- | -- | -- | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | | | | | | | | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | | | | | | | | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | ---+-------+------+-------+------+-------+------+--------+---- table ii.--_continued_ ----+------------------------------------------------------------+----- | i n c h e s. | t |------+-------+------+-------+------+-------+------+--------| t e | | | | | | | | | e m | | · | | · | | · | | · | m p | | | | | | | | | p ----+------+-------+------+-------+------+-------+------+--------+----- ° | + | + | + | + | + | + | + | + | ° | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | | | | | | | | | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | | | | | | | | | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | | | | | | | | | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | | | | | | | | | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | · | ----+------+-------+------+-------+------+-------+------+--------+----- ----+-----------------------------------------------------+----- | i n c h e s. | t |-------+------+-------+------+-------+------+--------| t e | | | | | | | | e m | | · | | · | | · | | m p | | | | | | | | p ----+-------+------+-------+------+-------+------+--------+----- ° | + | + | + | + | + | + | + | ° | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | | | | | | | | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | | · | · | · | · | · | · | · | 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[ ] this table is also applicable to the hurricanes in the neighbourhood of mauritius in the southern hemisphere, where all the phænomena are reversed; the motion of the hurricanes being towards the s.w., and the rotation in the direction of the hands of a watch, the same barometric and anemonal phænomena are experienced as in a hurricane in the northern hemisphere moving towards the n.e. [ ] by the officer of the watch being charged with this duty, and its being executed under his immediate superintendence, it is apprehended that a register may be kept with great regularity. [ ] these papers may be obtained from messrs. w. h. allen and co., booksellers to the honourable east india company, no. , leadenhall street, london. [ ] reports of the british association for the advancement of science, , p. . _the romance of science_ the machinery of the universe mechanical conceptions of physical phenomena by a. e. dolbear, a.b., a.m., m.e., ph.d. professor of physics and astronomy, tufts college, mass. published under general literature committee. london: society for promoting christian knowledge, northumberland avenue, w.c.; , queen victoria street, e.c. brighton: , north street. new york: e. & j. b. young & co. . preface for thirty years or more the expressions "correlation of the physical forces" and "the conservation of energy" have been common, yet few persons have taken the necessary pains to think out clearly what mechanical changes take place when one form of energy is transformed into another. since tyndall gave us his book called _heat as a mode of motion_ neither lecturers nor text-books have attempted to explain how all phenomena are the necessary outcome of the various forms of motion. in general, phenomena have been attributed to _forces_--a metaphysical term, which explains nothing and is merely a stop-gap, and is really not at all needful in these days, seeing that transformable modes of motion, easily perceived and understood, may be substituted in all cases for forces. in december the author gave a lecture before the franklin institute of philadelphia, on "mechanical conceptions of electrical phenomena," in which he undertook to make clear what happens when electrical phenomena appear. the publication of this lecture in _the journal of the franklin institute_ and in _nature_ brought an urgent request that it should be enlarged somewhat and published in a form more convenient for the public. the enlargement consists in the addition of a chapter on the "_contrasted properties of matter and the ether_," a chapter containing something which the author believes to be of philosophical importance in these days when electricity is so generally described as a phenomenon of the ether. a. e. dolbear. table of contents chapter i ideas of phenomena ancient and modern, metaphysical and mechanical--imponderables--forces, invented and discarded--explanations--energy, its factors, kinetic and potential--motions, kinds and transformations of--mechanical, molecular, and atomic--invention of ethers, faraday's conceptions p. chapter ii properties of matter and ether compared--discontinuity _versus_ continuity--size of atoms--astronomical distances--number of atoms in the universe--ether unlimited--kinds of matter, permanent qualities of--atomic structure; vortex-rings, their properties--ether structureless--matter gravitative, ether not--friction in matter, ether frictionless--chemical properties--energy in matter and in ether--matter as a transformer of energy--elasticity--vibratory rates and waves--density--heat--indestructibility of matter--inertia in matter and in ether--matter not inert--magnetism and ether waves--states of matter--cohesion and chemism affected by temperature--shearing stress in solids and in ether--ether pressure--sensation dependent upon matter--nervous system not affected by ether states--other stresses in ether--transformations of motion--terminology p. chapter iii antecedents of electricity--nature of what is transformed--series of transformations for the production of light--positive and negative electricity--positive and negative twists--rotations about a wire--rotation of an arc--ether a non-conductor--electro-magnetic waves--induction and inductive action--ether stress and atomic position--nature of an electric current--electricity a condition, not an entity p. chapter i ideas of phenomena ancient and modern, metaphysical and mechanical--imponderables--forces, invented and discarded--explanations--energy, its factors, kinetic and potential--motions, kinds and transformations of--mechanical, molecular, and atomic--invention of ethers, faraday's conceptions. 'and now we might add something concerning a most subtle spirit which pervades and lies hid in all gross bodies, by the force and action of which spirit the particles of bodies attract each other at near distances, and cohere if contiguous, and electric bodies operate at greater distances, as well repelling as attracting neighbouring corpuscles, and light is emitted, reflected, inflected, and heats bodies, and all sensation is excited, and members of animal bodies move at the command of the will.'--newton, _principia_. in newton's day the whole field of nature was practically lying fallow. no fundamental principles were known until the law of gravitation was discovered. this law was behind all the work of copernicus, kepler, and galileo, and what they had done needed interpretation. it was quite natural that the most obvious and mechanical phenomena should first be reduced, and so the _principia_ was concerned with mechanical principles applied to astronomical problems. to us, who have grown up familiar with the principles and conceptions underlying them, all varieties of mechanical phenomena seem so obvious, that it is difficult for us to understand how any one could be obtuse to them; but the records of newton's time, and immediately after this, show that they were not so easy of apprehension. it may be remembered that they were not adopted in france till long after newton's day. in spite of what is thought to be reasonable, it really requires something more than complete demonstration to convince most of us of the truth of an idea, should the truth happen to be of a kind not familiar, or should it chance to be opposed to our more or less well-defined notions of what it is or ought to be. if those who labour for and attain what they think to be the truth about any matter, were a little better informed concerning mental processes and the conditions under which ideas grow and displace others, they would be more patient with mankind; teachers of every rank might then discover that what is often called stupidity may be nothing else than mental inertia, which can no more be made active by simply willing than can the movement of a cannon ball by a like effort. we _grow_ into our beliefs and opinions upon all matters, and scientific ideas are no exceptions. whewell, in his _history of the inductive sciences_, says that the greeks made no headway in physical science because they lacked appropriate ideas. the evidence is overwhelming that they were as observing, as acute, as reasonable as any who live to-day. with this view, it would appear that the great discoverers must have been men who started out with appropriate ideas: were looking for what they found. if, then, one reflects upon the exceeding great difficulty there is in discovering one new truth, and the immense amount of work needed to disentangle it, it would appear as if even the most successful have but indistinct ideas of what is really appropriate, and that their mechanical conceptions become clarified by doing their work. this is not always the fact. in the statement of newton quoted at the head of this chapter, he speaks of a spirit which lies hid in all gross bodies, etc., by means of which all kinds of phenomena are to be explained; but he deliberately abandons that idea when he comes to the study of light, for he assumes the existence and activity of light corpuscles, for which he has no experimental evidence; and the probability is that he did this because the latter conception was one which he could handle mathematically, while he saw no way for thus dealing with the other. his mechanical instincts were more to be trusted than his carefully calculated results; for, as all know, what he called "spirits," is what to-day we call the ether, and the corpuscular theory of light has now no more than a historic interest. the corpuscular theory was a mechanical conception, but each such corpuscle was ideally endowed with qualities which were out of all relation with the ordinary matter with which it was classed. until the middle of the present century the reigning physical philosophy held to the existence of what were called imponderables. the phenomena of heat were explained as due to an imponderable substance called "caloric," which ordinary matter could absorb and emit. a hot body was one which had absorbed an imponderable substance. it was, therefore, no heavier than before, but it possessed ability to do work proportional to the amount absorbed. carnot's ideal engine was described by him in terms that imply the materiality of heat. light was another imponderable substance, the existence of which was maintained by sir david brewster as long as he lived. electricity and magnetism were imponderable fluids, which, when allied with ordinary matter, endowed the latter with their peculiar qualities. the conceptions in each case were properly mechanical ones _part_ (but not all) _of the time_; for when the immaterial substances were dissociated from matter, where they had manifested themselves, no one concerned himself to inquire as to their whereabouts. they were simply off duty, but could be summoned, like the genii in the story of aladdin's lamp. now, a mechanical conception of any phenomenon, or a mechanical explanation of any kind of action, must be mechanical all the time, in the antecedents as well as the consequents. nothing else will do except a miracle. during the fifty years, from about to , a somewhat different kind of explanation of physical events grew up. the interest that was aroused by the discoveries in all the fields of physical science--in heat, electricity, magnetism and chemistry--by faraday, joule, helmholtz, and others, compelled a change of conceptions; for it was noticed that each special kind of phenomenon was preceded by some other definite and known kind; as, for instance, that chemical action preceded electrical currents, that mechanical or electrical activity resulted from changing magnetism, and so on. as each kind of action was believed to be due to a special force, there were invented such terms as mechanical force, electrical force, magnetic, chemical and vital forces, and these were discovered to be convertible into one another, and the "doctrine of the correlation of the physical forces" became a common expression in philosophies of all sorts. by "convertible into one another," was meant, that whenever any given force appeared, it was at the expense of some other force; thus, in a battery chemical force was changed into electrical force; in a magnet, electrical force was changed into magnetic force, and so on. the idea here was the _transformation of forces_, and _forces_ were not so clearly defined that one could have a mechanical idea of just what had happened. that part of the philosophy was no clearer than that of the imponderables, which had largely dropped out of mind. the terminology represented an advance in knowledge, but was lacking in lucidity, for no one knew what a force of any kind was. the first to discover this and to repudiate the prevailing terminology were the physiologists, who early announced their disbelief in a vital force, and their belief that all physiological activities were of purely physical and chemical origin, and that there was no need to assume any such thing as a vital force. then came the discovery that chemical force, or affinity, had only an adventitious existence, and that, at absolute zero, there was no such activity. the discovery of, or rather the appreciation of, what is implied by the term _absolute zero_, and especially of the nature of heat itself, as expressed in the statement that heat is a mode of motion, dismissed another of the so-called forces as being a metaphysical agency having no real existence, though standing for phenomena needing further attention and explanation; and by explanation is meant _the presentation of the mechanical antecedents for a phenomenon, in so complete a way that no supplementary or unknown factors are necessary_. the train moves because the engine pulls it; the engine pulls because the steam pushes it. there is no more necessity for assuming a steam force between the steam and the engine, than for assuming an engine force between the engine and the train. all the processes are mechanical, and have to do only with ordinary matter and its conditions, from the coal-pile to the moving freight, though there are many transformations of the forms of motion and of energy between the two extremes. during the past thirty years there has come into common use another term, unknown in any technical sense before that time, namely, _energy_. what was once called the conservation of force is now called the conservation of energy, and we now often hear of forms of energy. thus, heat is said to be a form of energy, and the forms of energy are convertible into one another, as the so-called forces were formerly supposed to be transformable into one another. we are asked to consider gravitative energy, heat energy, mechanical energy, chemical energy, and electrical energy. when we inquire what is meant by energy, we are informed that it means ability to do work, and that work is measurable as a pressure into a distance, and is specified as foot-pounds. a mass of matter moves because energy has been spent upon it, and has acquired energy equal to the work done on it, and this is believed to hold true, no matter what the kind of energy was that moved it. if a body moves, it moves because another body has exerted pressure upon it, and its energy is called _kinetic energy_; but a body may be subject to pressure and not move appreciably, and then the body is said to possess potential energy. thus, a bent spring and a raised weight are said to possess potential energy. in either case, _an energized body receives its energy by pressure, and has ability to produce pressure on another body_. whether or not it does work on another body depends on the rigidity of the body it acts upon. in any case, it is simply a mechanical action--body a pushes upon body b (fig. ). there is no need to assume anything more mysterious than mechanical action. whether body b moves this way or that depends upon the direction of the push, the point of its application. whether the body be a mass as large as the earth or as small as a molecule, makes no difference in that particular. suppose, then, that _a_ (fig. ) spends its energy on _b_, _b_ on _c_, _c_ on _d_, and so on. the energy of _a_ gives translatory motion to _b_, _b_ sets _c_ vibrating, and _c_ makes _d_ spin on some axis. each of these has had energy spent on it, and each has some form of energy different from the other, but no new factor has been introduced between _a_ and _d_, and the only factor that has gone from _a_ to _d_ has been motion--motion that has had its direction and quality changed, but not its nature. if we agree that energy is neither created nor annihilated, by any physical process, and if we assume that _a_ gave to _b_ all its energy, that is, all its motion; that _b_ likewise gave its all to _c_, and so on; then the succession of phenomena from _a_ to _d_ has been simply the transference of a definite amount of motion, and therefore of energy, from the one to the other; for _motion has been the only variable factor_. if, furthermore, we should agree to call the translatory motion [alpha], the vibratory motion [beta], the rotary [gamma], then we should have had a conversion of [alpha] into [beta], of [beta] into [gamma]. if we should consider the amount of transfer motion instead of the kind of motion, we should have to say that the [alpha] energy had been transformed into [beta] and the [beta] into [gamma]. [illustration: fig. .] [illustration: fig. .] what a given amount of energy will do depends only upon its _form_, that is, the kind of motion that embodies it. the energy spent upon a stone thrown into the air, giving it translatory motion, would, if spent upon a tuning fork, make it sound, but not move it from its place; while if spent upon a top, would enable the latter to stand upon its point as easily as a person stands on his two feet, and to do other surprising things, which otherwise it could not do. one can, without difficulty, form a mechanical conception of the whole series without assuming imponderables, or fluids or forces. mechanical motion only, by pressure, has been transferred in certain directions at certain rates. suppose now that some one should suddenly come upon a spinning top (fig. ) while it was standing upon its point, and, as its motion might not be visible, should cautiously touch it. it would bound away with surprising promptness, and, if he were not instructed in the mechanical principles involved, he might fairly well draw the conclusion that it was actuated by other than simple mechanical principles, and, for that reason, it would be difficult to persuade him that there was nothing essentially different in the body that appeared and acted thus, than in a stone thrown into the air; nevertheless, that statement would be the simple truth. [illustration: fig. .] all our experience, without a single exception, enforces the proposition that no body moves in any direction, or in any way, except when some other body _in contact_ with it presses upon it. the action is direct. in newton's letter to his friend bentley, he says--"that one body should act upon another through empty space, without the mediation of anything else by and through which their action and pressure may be conveyed from one to another, is to me so great an absurdity that i believe no man who has in philosophical matters a competent faculty of thinking can ever fall into it." for mathematical purposes, it has sometimes been convenient to treat a problem as if one body could act upon another without any physical medium between them; but such a conception has no degree of rationality, and i know of no one who believes in it as a fact. if this be granted, then our philosophy agrees with our experience, and every body moves because it is pushed, and the mechanical antecedent of every kind of phenomenon is to be looked for in some adjacent body possessing energy--that is, the ability to push or produce pressure. it must not be forgotten that energy is not a simple factor, but is always a product of two factors--a mass with a velocity, a mass with a temperature, a quantity of electricity into a pressure, and so on. one may sometimes meet the statement that matter and energy are the two realities; both are spoken of as entities. it is much more philosophical to speak of matter and motion, for in the absence of motion there is no energy, and the energy varies with the amount of motion; and furthermore, to understand any manifestation of energy one must inquire what kind of motion is involved. this we do when we speak of mechanical energy as the energy involved in a body having a translatory motion; also, when we speak of heat as a vibratory, and of light as a wave motion. to speak of energy without stating or implying these distinctions, is to speak loosely and to keep far within the bounds of actual knowledge. to speak thus of a body possessing energy, or expending energy, is to imply that the body possesses some kind of motion, and produces pressure upon another body because it has motion. tait and others have pointed out the fact, that what is called potential energy must, in its nature, be kinetic. tait says--"now it is impossible to conceive of a truly dormant form of energy, whose magnitude should depend, in any way, upon the unit of time; and we are forced to conclude that potential energy, like kinetic energy, depends (even if unexplained or unimagined) upon motion." all this means that it is now too late to stop with energy as a final factor in any phenomenon, that the _form of motion_ which embodies the energy is the factor that determines _what_ happens, as distinguished from how _much_ happens. here, then, are to be found the distinctions which have heretofore been called forces; here is embodied the proof that direct pressure of one body upon another is what causes the latter to move, and that the direction of movement depends on the point of application, with reference to the centre of mass. it is needful now to look at the other term in the product we call energy, namely, the substance moving, sometimes called matter or mass. it has been mentioned that the idea of a medium filling space was present to newton, but his gravitation problem did not require that he should consider other factors than masses and distances. the law of gravitation as considered by him was--every particle of matter attracts every other particle of matter with a stress which is proportional to the product of their masses, and inversely to the squares of the distance between them. here we are concerned only with the statement that every particle of matter attracts every other particle of matter. everything then that possesses gravitative attraction is matter in the sense in which that term is used in this law. if there be any other substance in the universe that is not thus subject to gravitation, then it is improper to call it matter, otherwise the law should read, "some particles of matter attract," etc., which will never do. we are now assured that there is something else in the universe which has no gravitative property at all, namely, the ether. it was first imagined in order to account for the phenomena of light, which was observed to take about eight minutes to come from the sun to the earth. then young applied the wave theory to the explanation of polarization and other phenomena; and in foucault proved experimentally that the velocity of light was less in water than in air, as it should be if the wave theory be true, and this has been considered a crucial experiment which took away the last hope for the corpuscular theory, and demonstrated the existence of the ether as a space-filling medium capable of transmitting light-waves known to have a velocity of , miles per second. it was called the luminiferous ether, to distinguish it from other ethers which had also been imagined, such as electric ether for electrical phenomena, magnetic ether for magnetic phenomena, and so on--as many ethers, in fact, as there were different kinds of phenomena to be explained. it was faraday who put a stop to the invention of ethers, by suggesting that the so-called luminiferous ether might be the one concerned in all the different phenomena, and who pointed out that the arrangement of iron filings about a magnet was indicative of the direction of the stresses in the ether. this suggestion did not meet the approval of the mathematical physicists of his day, for it necessitated the abandonment of the conceptions they had worked with, as well as the terminology which had been employed, and made it needful to reconstruct all their work to make it intelligible--a labour which was the more distasteful as it was forced upon them by one who, although expert enough in experimentation, was not a mathematician, and who boasted that the most complicated mathematical work he ever did was to turn the crank of a calculating machine; who did all his work, formed his conclusions, and then said--"the work is done; hand it over to the computers." it has turned out that faraday's mechanical conceptions were right. every one now knows of maxwell's work, which was to start with faraday's conceptions as to magnetic phenomena, and follow them out to their logical conclusions, applying them to molecules and the reactions of the latter upon the ether. thus he was led to conclude that light was an electro-magnetic phenomenon; that is, that the waves which constitute light, and the waves produced by changing magnetism were identical in their nature, were in the same medium, travelled with the same velocity, were capable of refraction, and so on. now that all this is a matter of common knowledge to-day, it is curious to look back no further than ten years. maxwell's conclusions were adopted by scarcely a physicist in the world. although it was known that inductive action travelled with finite velocity in space, and that an electro-magnet would affect the space about it practically inversely as the square of the distance, and that such phenomena as are involved in telephonic induction between circuits could have no other meaning than the one assigned by maxwell, yet nearly all the physicists failed to form the only conception of it that was possible, and waited for hertz to devise apparatus for producing interference before they grasped it. it was even then so new, to some, that it was proclaimed to be a demonstration of the existence of the ether itself, as well as a method of producing waves short enough to enable one to notice interference phenomena. it is obvious that hertz himself must have had the mechanics of wave-motion plainly in mind, or he would not have planned such experiments. the outcome of it all is, that we now have experimental demonstration, as well as theoretical reason for believing, that the ether, once considered as only luminiferous, is concerned in all electric and magnetic phenomena, and that waves set up in it by electro-magnetic actions are capable of being reflected, refracted, polarized, and twisted, in the same way as ordinary light-waves can be, and that the laws of optics are applicable to both. chapter ii properties of matter and ether properties of matter and ether compared--discontinuity _versus_ continuity--size of atoms--astronomical distances--number of atoms in the universe--ether unlimited--kinds of matter, permanent qualities of--atomic structure; vortex-rings, their properties--ether structureless--matter gravitative, ether not--friction in matter, ether frictionless--chemical properties--energy in matter and in ether--matter as a transformer of energy--elasticity--vibratory rates and waves--density--heat--indestructibility of matter--inertia in matter and in ether--matter not inert--magnetism and ether waves--states of matter--cohesion and chemism affected by temperature--shearing stress in solids and in ether--ether pressure--sensation dependent upon matter--nervous system not affected by ether states--other stresses in ether--transformations of motion--terminology. a common conception of the ether has been that it is a finer-grained substance than ordinary matter, but otherwise so like the latter that the laws found to hold good with matter were equally applicable to the ether, and hence the mechanical conceptions formed from experience in regard to the one have been transferred to the other, and the properties belonging to one, such as density, elasticity, etc., have been asserted as properties of the other. there is so considerable a body of knowledge bearing upon the similarities and dissimilarities of these two entities that it will be well to compare them. after such comparison one will be better able to judge of the propriety of assuming them to be subject to identical laws. . matter is discontinuous. matter is made up of atoms having dimensions approximately determined to be in the neighbourhood of the one fifty-millionth of an inch in diameter. these atoms may have various degrees of aggregation;--they may be in practical contact, as in most solid bodies such as metals and rocks; in molecular groupings as in water, and in gases such as hydrogen, oxygen, and so forth, where two, three, or more atoms cohere so strongly as to enable the molecules to act under ordinary circumstances like simple particles. any or all of these molecules and atoms may be separated by any assignable distance from each other. thus, in common air the molecules, though rapidly changing their positions, are on the average about two hundred and fifty times their own diameter apart. this is a distance relatively greater than the distance apart of the earth and the moon, for two hundred and fifty times the diameter of the earth will be Ã� = , , miles, while the distance to the moon is but , miles. the sun is , , miles from the earth, and the most of the bodies of the solar system are still more widely separated, neptune being nearly millions of miles from the sun. as for the fixed stars, they are so far separated from us that, at the present rate of motion of the solar system in its drift through space-- millions of miles in a year--it would take not less than , years to reach the nearest star among its neighbours, while for the more remote ones millions of years must be reckoned. the huge space separating these masses is practically devoid of matter; it is a vacuum. the ether is continuous. the idea of continuity as distinguished from discontinuity may be gained by considering what would be made visible by magnification. water appears to the eye as if it were without pores, but if sugar or salt be put into it, either will be dissolved and quite disappear among the molecules of the water as steam does in the air, which shows that there are some unoccupied spaces between the molecules. if a microscope be employed to magnify a minute drop of water it still shows the same lack of structure as that looked at with the unaided eye. if the magnifying power be the highest it may reveal a speck as small as the hundred-thousandth part of an inch, yet the speck looks no different in character. we know that water is composed of two different kinds of atoms, hydrogen and oxygen, for they can be separated by chemical means and kept in separate bottles, and again made to combine to form water having all the qualities that belonged to it before it was decomposed. if a very much higher magnifying power were available, we should ultimately be able to see the individual water molecules, and recognize their hydrogen and oxygen constituents by their difference in size, rate of movements, and we might possibly separate them by mechanical methods. what one would see would be something very different in structure from the water as it appears to our eyes. if the ether were similarly to be examined through higher and still higher magnifying powers, even up to infinity, there is no reason for thinking that the last examination would show anything different in structure or quality from that which was examined with low power or with no microscope at all. this is all expressed by saying that the ether is a continuous substance, without interstices, that it fills space completely, and, unlike gases, liquids, and solids, is incapable of absorbing or dissolving anything. . matter is limited. there appears to be a definite amount of matter in the visible universe, a definite number of molecules and atoms. how many molecules there are in a cubic inch of air under ordinary pressure has been determined, and is represented approximately by a huge number, something like a thousand million million millions. when the diameter of a molecule has been measured, as it has been approximately, and found to be about one fifty-millionth of an inch, then fifty million in a row would reach an inch, and the cube of fifty million is , , , , , one hundred and twenty-five thousand million million millions. in a cubic foot there will of course be times that number. one may if one likes find how many there may be in the earth, and moon, sun and planets, for the dimensions of them are all very well known. only the multiplication table need be used, and the sum of all these will give how many molecules there are in the solar system. if one should feel that the number thus obtained was not very accurate, he might reflect that if there were ten times as many it would add but another cipher to a long line of similar ones and would not materially modify it. the point is that there is a definite, computable number. if one will then add to these the number of molecules in the more distant stars and nebulæ, of which there are visible about , , , making such estimate of their individual size as he thinks prudent, the sum of all will give the number of molecules in the visible universe. the number is not so large but it can be written down in a minute or two. those who have been to the pains to do the sum say it may be represented by seven followed by ninety-one ciphers. one could easily compute how many molecules so large a space would contain if it were full and as closely packed as they are in a drop of water, but there would be a finite and not an infinite number, and therefore there is a limited number of atoms in the visible universe. the ether is unlimited. the evidence for this comes to us from the phenomena of light. experimentally, ether waves of all lengths are found to have a velocity of , miles in a second. it takes about eight minutes to reach us from the sun, four hours from neptune the most distant planet, and from the nearest fixed star about three and a half years. astronomers tell us that some visible stars are so distant that their light requires not less than ten thousand years and probably more to reach us, though travelling at the enormous rate of , miles a second. this means that the whole of space is filled with this medium. if there were any vacant spaces, the light would fail to get through them, and stars beyond them would become invisible. there are no such vacant spaces, for any part of the heavens shows stars beaming continuously, and every increase in telescopic power shows stars still further removed than any seen before. the whole of this intervening space must therefore be filled with the ether. some of the waves that reach us are not more than the hundred-thousandth of an inch long, so there can be no crack or break or absence of ether from so small a section as the hundred-thousandth of an inch in all this great expanse. more than this. no one can think that the remotest visible stars are upon the boundary of space, that if one could get to the most distant star he would have on one side the whole of space while the opposite side would be devoid of it. space we know is of three dimensions, and a straight line may be prolonged in any direction to an infinite distance, and a ray of light may travel on for an infinite time and come to no end provided space be filled with ether. how long the sun and stars have been shining no one knows, but it is highly probable that the sun has existed for not less than million years, and has during that time been pouring its rays as radiant energy into space. if then in half that time, or millions of years, the light had somewhere reached a boundary to the ether, it could not have gone beyond but would have been reflected back into the ether-filled space, and such part of the sky would be lit up by this reflected light. there is no indication that anything like reflection comes to us from the sky. this is equivalent to saying that the ether fills space in every direction away from us to an unlimited distance, and so far is itself unlimited. . matter is heterogeneous. the various kinds of matter we are acquainted with are commonly called the elements. these when combined in various ways exhibit characteristic phenomena which depend upon the kinds of matter, the structure and motions which are involved. there are some seventy different kinds of this elemental matter which may be identified as constituents of the earth. many of the same elements have been identified in the sun and stars, such for instance as hydrogen, carbon, and iron. such phenomena lead us to conclude that the kinds of matter elsewhere in the universe are identical with such as we are familiar with, and that elsewhere the variety is as great. the qualities of the elements, within a certain range of temperature, are permanent; they are not subject to fluctuations, though the qualities of combinations of them may vary indefinitely. the elements therefore may be regarded as retaining their identity in all ordinary experience. the ether is homogeneous. one part of the ether is precisely like any other part everywhere and always, and there are no such distinctions in it as correspond with the elemental forms of matter. . matter is atomic. there is an ultimate particle of each one of the elements which is practically absolute and known as an atom. the atom retains its identity through all combinations and processes. it may be here or there, move fast or slow, but its atomic form persists. the ether is non-atomic. one might infer, from what has already been said about continuity, that the ether could not be constituted of separable particles like masses of matter; for no matter how minute they might be, there would be interspaces and unoccupied spaces which would present us with phenomena which have never been seen. it is the general consensus of opinion among those who have studied the subject that the ether is not atomic in structure. . matter has definite structure. every atom of every element is so like every other atom of the same element as to exhibit the same characteristics, size, weight, chemical activity, vibratory rate, etc., and it is thus shown conclusively that the structural form of the elemental particles is the same for each element, for such characteristic reactions as they exhibit could hardly be if they were mechanically unlike. of what form the atoms of an element may be is not very definitely known. the earlier philosophers assumed them to be hard round particles, but later thinkers have concluded that atoms of such a character are highly improbable, for they could not exhibit in this case the properties which the elements do exhibit. they have therefore dismissed such a conception from consideration. in place of this hypothesis has been substituted a very different idea, namely, that an atom is a vortex-ring[ ] of ether floating in the ether, as a smoke-ring puffed out by a locomotive in still air may float in the air and show various phenomena. [footnote : vortex-rings for illustration may be made by having a wooden box about a foot on a side, with a round orifice in the middle of one side, and the side opposite covered with stout cloth stretched tight over a framework. a saucer containing strong ammonia water, and another containing strong hydrochloric acid, will cause dense fumes in the box, and a tap with the hand upon the cloth back will force out a ring from the orifice. these may be made to follow and strike each other, rebounding and vibrating, apparently attracting each other and being attracted by neighbouring bodies. by filling the mouth with smoke, and pursing the lips as if to make the sound _o_, one may make fifteen or twenty small rings by snapping the cheek with the finger.] a vortex-ring produced in the air behaves in the most surprising manner. [illustration: fig. .--method of making vortex-rings and their behaviour.] . it retains its ring form and the same material rotating as it starts with. . it can travel through the air easily twenty or thirty feet in a second without disruption. . its line of motion when free is always at right angles to the plane of the ring. . it will not stand still unless compelled by some object. if stopped in the air it will start up itself to travel on without external help. . it possesses momentum and energy like a solid body. . it is capable of vibrating like an elastic body, making a definite number of such vibrations per second, the degree of elasticity depending upon the rate of vibration. the swifter the rotation, the more rigid and elastic it is. . it is capable of spinning on its own axis, and thus having rotary energy as well as translatory and vibratory. . it repels light bodies in front of it, and attracts into itself light bodies in its rear. . if projected along parallel with the top of a long table, it will fall upon it every time, just as a stone thrown horizontally will fall to the ground. . if two rings of the same size be travelling in the same line, and the rear one overtakes the other, the front one will enlarge its diameter, while the rear one will contract its own till it can go through the forward one, when each will recover its original diameter, and continue on in the same direction, but vibrating, expanding and contracting their diameters with regularity. . if two rings be moving in the same line, but in opposite directions, they will repel each other when near, and thus retard their speed. if one goes through the other, as in the former case, it may quite lose its velocity, and come to a standstill in the air till the other has moved on to a distance, when it will start up in its former direction. . if two rings be formed side by side, they will instantly collide at their edges, showing strong attraction. . if the collision does not destroy them, they may either break apart at the point of the collision, and then weld together into a single ring with twice the diameter, and then move on as if a single ring had been formed, or they may simply bounce away from each other, in which case they always rebound _in a plane_ at right angles to the plane of collision. that is, if they collided on their sides, they would rebound so that one went up and the other down. . three may in like manner collide and fuse into a single ring. such rings formed in air by a locomotive may rise wriggling in the air to the height of several hundred feet, but they are soon dissolved and disappear. this is because the friction and viscosity of the air robs the rings of their substance and energy. if the air were without friction this could not happen, and the rings would then be persistent, and would retain all their qualities. suppose then that such rings were produced in a medium without friction as the ether is believed to be, they would be permanent structures with a variety of properties. they would occupy space, have definite form and dimensions, momentum, energy, attraction and repulsion, elasticity; obey the laws of motion, and so far behave quite like such matter as we know. for such reasons it is thought by some persons to be not improbable that the atoms of matter are minute vortex-rings of ether in the ether. that which distinguishes the atom from the ether is the form of motion which is embodied in it, and if the motion were simply arrested, there would be nothing to distinguish the atom from the ether into which it dissolved. in other words, such a conception makes the atoms of matter a form of motion of the ether, and not a created something put into the ether. the ether is structureless. if the ether be the boundless substance described, it is clear it can have no form as a whole, and if it be continuous it can have no minute structure. if not constituted of atoms or molecules there is nothing descriptive that can be said about it. a molecule or a particular mass of matter could be identified by its form, and is thus in marked contrast with any portion of ether, for the latter could not be identified in a similar way. one may therefore say that the ether is formless. . matter is gravitative. the law of gravitation is held as being universal. according to it every particle of matter in the universe attracts every other particle. the evidence for this law in the solar system is complete. sun, planets, satellites, comets and meteors are all controlled by gravitation, and the movements of double stars testify to its activity among the more distant bodies of the universe. the attraction does not depend upon the kind of matter nor the arrangement of molecules or atoms, but upon the amount or mass of matter present, and if it be of a definite kind of matter, as of hydrogen or iron, the gravitative action is proportional to the number of atoms. the ether is gravitationless. one might infer already that if the ether were structureless, physical laws operative upon such material substances as atoms could not be applicable to it, and so indeed all the evidence we have shows that gravitation is not one of its properties. if it were, and it behaved in any degree like atomic structures, it would be found to be denser in the neighbourhood of large bodies like the earth, planets, and the sun. light would be turned from its straight path while travelling in such denser medium, or made to move with less velocity. there is not the slightest indication of any such effect anywhere within the range of astronomical vision. gravitation then is a property belonging to matter and not to ether. the impropriety of thinking or speaking of the ether as matter of any kind will be apparent if one reflects upon the significance of the law of gravitation as stated. every particle of matter in the universe attracts every other particle. if there be anything else in the universe which has no such quality, then it should not be called matter, else the law should read: some particles of matter attract some other particles, which would be no law at all, for a real physical law has no exceptions any more than the multiplication table has. physical laws are physical relations, and all such relations are quantitative. . matter is frictionable. a bullet shot into the air has its velocity continuously reduced by the air, to which its energy is imparted by making it move out of its way. a railway train is brought to rest by the friction brake upon the wheels. the translatory energy of the train is transformed into the molecular energy called heat. the steamship requires to propel it fast, a large amount of coal for its engines, because the water in which it moves offers great friction--resistance which must be overcome. whenever one surface of matter is moved in contact with another surface there is a resistance called friction, the moving body loses its rate of motion, and will presently be brought to rest unless energy be continuously supplied. this is true for masses of matter of all sizes and with all kinds of motion. friction is the condition for the transformation of all kinds of mechanical motions into heat. the test of the amount of friction is the rate of loss of motion. a top will spin some time in the air because its point is small. it will spin longer on a plate than on the carpet, and longer in a vacuum than in the air, for it does not have the air friction to resist it, and there is no kind or form of matter not subject to frictional resistance. the ether is frictionless. the earth is a mass of matter moving in the ether. in the equatorial region the velocity of a point is more than a thousand miles in an hour, for the circumference of the earth is , miles, and it turns once on its axis in hours, which is the length of the day. if the earth were thus spinning in the atmosphere, the latter not being in motion, the wind would blow with ten times hurricane velocity. the friction would be so great that nothing but the foundation rocks of the earth's crust could withstand it, and the velocity of rotation would be reduced appreciably in a relatively short time. the air moves along with the earth as a part of it, and consequently no such frictional destruction takes place, but the earth rotates in the ether with that same rate, and if the ether offered resistance it would react so as to retard the rotation and increase the length of the day. astronomical observations show that the length of the day has certainly not changed so much as the tenth of a second during the past years. the earth also revolves about the sun, having a speed of about miles in a second, or , miles an hour. this motion of the earth and the other planets about the sun is one of the most stable phenomena we know. the mean distance and period of revolution of every planet is unalterable in the long run. if the earth had been retarded by its friction in the ether the length of the year would have been changed, and astronomers would have discovered it. they assert that a change in the length of a year by so much as the hundredth part of a second has not happened during the past thousand years. this then is testimony, that a velocity of nineteen miles a second for a thousand years has produced no effect upon the earth's motion that is noticeable. nineteen miles a second is not a very swift astronomical motion, for comets have been known to have a velocity of miles a second when in the neighbourhood of the sun, and yet they have not seemed to suffer any retardation, for their orbits have not been shortened. some years ago a comet was noticed to have its periodic time shortened an hour or two, and the explanation offered at first was that the shortening was due to friction in the ether although no other comet was thus affected. the idea was soon abandoned, and to-day there is no astronomical evidence that bodies having translatory motion in the ether meet with any frictional resistance whatever. if a stone could be thrown in interstellar space with a velocity of fifty feet a second it would continue to move in a straight line with the same speed for any assignable time. as has been said, light moves with the velocity of , miles per second, and it may pursue its course for tens of thousands of years. there is no evidence that it ever loses either its wave-length or energy. it is not transformed as friction would transform it, else there would be some distance at which light of given wave-length and amplitude would be quite extinguished. the light from distant stars would be different in character from that coming from nearer stars. furthermore, as the whole solar system is drifting in space some , , of miles in a year, new stars would be coming into view in that direction, and faint stars would be dropping out of sight in the opposite direction--a phenomenon which has not been observed. altogether the testimony seems conclusive that the ether is a frictionless medium, and does not transform mechanical motion into heat. . matter is Ã�olotropic. that is, its properties are not alike in all directions. chemical phenomena, crystallization, magnetic and electrical phenomena show each in their way that the properties of atoms are not alike on opposite faces. atoms combine to form molecules, and molecules arrange themselves in certain definite geometric forms such as cubes, tetrahedra, hexagonal prisms and stellate forms, with properties emphasized on certain faces or ends. thus quartz will twist a ray of light in one direction or the other, depending upon the arrangement which may be known by the external form of the crystal. calc spar will break up a ray of light into two parts if the light be sent through it in certain directions, but not if in another. tourmaline polarizes light sent through its sides and becomes positively electrified at one end while being heated. some substances will conduct sound or light or heat or electricity better in one direction than in another. all matter is magnetic in some degree, and that implies polarity. if one will recall the structure of a vortex-ring, he will see how all the motion is inward on one side and outward on the other, which gives different properties to the two sides: a push away from it on one side and a pull toward it on the other. the ether is isotropic. that is, its properties are alike in every direction. there is no distinction due to position. a mass of matter will move as freely in one direction as in another; a ray of light of any wave-length will travel in it in one direction as freely as in any other; neither velocity nor direction are changed by the action of the ether alone. . matter is chemically selective. when the elements combine to form molecules they always combine in definite ways and in definite proportions. carbon will combine with hydrogen, but will drop it if it can get oxygen. oxygen will combine with iron or lead or sodium, but cannot be made to combine with fluorine. no more than two atoms of oxygen can be made to unite with one carbon atom, nor more than one hydrogen with one chlorine atom. there is thus an apparent choice for the kind and number of associates in molecular structure, and the instability of a molecule depends altogether upon the presence in its neighbourhood of other atoms for which some of the elements in the molecule have a stronger attraction or affinity than they have for the atoms they are now combined with. thus iron is not stable in the presence of water molecules, and it becomes iron oxide; iron oxide is not stable in the presence of hot sulphur, it becomes an iron sulphide. all the elements are thus selective, and it is by such means that they may be chemically identified. there is no phenomenon in the ether that is comparable with this. evidently there could not be unless there were atomic structures having in some degree different characteristics which we know the ether to be without. . the elements of matter are harmonically related. it is possible to arrange the elements in the order of their atomic weights in columns which will show communities of property. newlands, mendeléeff, meyer, and others have done this. the explanation for such an arrangement has not yet been forthcoming, but that it expresses a real fact is certain, for in the original scheme there were several gaps representing undiscovered elements, the properties of which were predicted from that of their associates in the table. some of these have since been discovered, and their atomic weight and physical properties accord with those predicted. with the ether such a scheme is quite impossible, for the very evident reason that there are no different things to have relation with each other. every part is just like every other part. where there are no differences and no distinctions there can be no relations. the ether is quite harmonic without relations. . matter embodies energy. so long as the atoms of matter were regarded as hard round particles, they were assumed to be inert and only active when acted upon by what were called forces, which were held to be entities of some sort, independent of matter. these could pull or push it here or there, but the matter was itself incapable of independent activity. all this is now changed, and we are called upon to consider every atom as being itself a form of energy in the same sense as heat or light are forms of energy, the energy being embodied in particular forms of motion. light, for instance, is a wave motion of the ether. an atom is a rotary ring of ether. stop the wave motion, and the light would be annihilated. stop the rotation, and the atom would be annihilated for the same reason. as the ray of light is a particular embodiment of energy, and has no existence apart from it, so an atom is to be regarded as an embodiment of energy. on a previous page it is said that energy is the ability of one body to act upon and move another in some degree. an atom of any kind is not the inert thing it has been supposed to be, for it can do something. even at absolute zero, when all its vibratory or heat energy would be absent, it would be still an elastic whirling body pulling upon every other atom in the universe with gravitational energy, twisting other atoms into conformity with its own position with its magnetic energy; and, if such ether rings are like the rings which are made in air, will not stand still in one place even if no others act upon it, but will start at once by its own inherent energy to move in a right line at right angles to its own plane and in the direction of the whirl inside the ring. two rings of wood or iron might remain in contact with each other for an indefinite time, but vortex-rings will not, but will beat each other away as two spinning tops will do if they touch ever so gently. if they do not thus separate it is because there are other forms of energy acting to press them together, but such external pressure will be lessened by the rings' own reactions. it is true that in a frictionless medium like the ether one cannot at present see how such vortex-rings could be produced in it. certainly not by any such mechanical methods as are employed to make smoke-rings in air, for the friction of the air is the condition for producing them. however they came to be, there is implied the previous existence of the ether and of energy in some form capable of acting upon it in a manner radically different from any known in physical science. there is good spectroscopic evidence that in some way elements of different kinds are now being formed in nebulæ, for the simplest show the presence of hydrogen alone. as they increase in complexity other elements are added, until the spectrum exhibits all the elements we know of. it has thus seemed likely either that most of what are called elements are composed of molecular groupings of some fundamental element, which by proper physical methods might be decomposed, as one can now decompose a molecule of ammonia or sulphuric acid, or that the elements are now being created by some extra-physical process in those far-off regions. in either case an atom is the embodiment of energy in such a form as to be permanent under ordinary physical circumstances, but of which, if in any manner it should be destroyed, only the form would be lost. the ether would remain, and the energy which was embodied would be distributed in other ways. the ether is endowed with energy. the distinction between energy in matter and energy in the ether will be apparent, on considering that both the ether and energy in some form must be conceived as existing independent of matter; though every atom were annihilated, the ether would remain and all the energy embodied in the atoms would be still in existence in the ether. the atomic energy would simply be dissolved. one can easily conceive the ether as the same space-filling, continuous, unlimited medium, without an atom in it. on this assumption it is clear that no form of energy with which we have to deal in physical science would have any existence in the ether; for every one of those forms, gravitational, thermal, electric, magnetic, or any other--all are the results of the forms of energy in matter. if there were no atoms, there would be no gravitation, for that is the attraction of atoms upon each other. if there were no atoms, there could be no atomic vibration, therefore no heat, and so on for each and all. nevertheless, if an atom be the embodiment of energy, there must have been energy in the ether before any atom existed. one of the properties of the ether is its ability to distribute energy in certain ways, but there is no evidence that of itself it ever transforms energy. once a given kind of energy is in it, it does not change; hence for the apparition of a form of energy, like the first vortex-ring, there must have been not only energy, but some other agency capable of transforming that energy into a permanent structure. to the best of our knowledge to-day, the ether would be absolutely helpless. such energy as was active in forming atoms must be called by another name than what is appropriate for such transformations as occur when, for instance, the mechanical energy of a bullet is transformed into heat when the target is struck. behind the ether must be assumed some agency, directing and controlling energy in a manner totally different from any agency, which is operative in what we call physical science. nothing short of what is called a miracle will do--an event without a physical antecedent in any way necessarily related to its factors, as is the fact of a stone related to gravity or heat to an electric current. ether energy is an endowment instead of being an embodiment, and implies antecedents of a super-physical kind. . matter is an energy transformer. as each different kind of energy represents some specific form of motion, and _vice versâ_, some sort of mechanism is needful for transforming one kind into another, therefore molecular structure of one kind or another is essential. the transformation is a mechanical process, and matter in some particular and appropriate form is the condition of its taking place. if heat appears, then its antecedent has been some other form of motion acting upon the substance heated. it may have been the mechanical motion of another mass of matter, as when a bullet strikes a target and becomes heated; or it may be friction, as when a car-axle heats when run without proper oiling to reduce friction; or it may be condensation, as when tinder is ignited by condensing the air about it; or chemical reactions, when molecular structure is changed as in combustion, or an electrical current, which implies a dynamo and steam-engine or water-power. if light appears, its antecedent has been impact or friction, condensation or chemical action, and if electricity appears the same sort of antecedents are present. whether the one or the other of these forms of energy is developed, depends upon what kind of a structure the antecedent energy has acted upon. if radiant energy, so-called, falls upon a mass of matter, what is absorbed is at once transformed into heat or into electric or magnetic effects; _which_ one of these depends upon the character of the mechanism upon which the radiant energy acts, but the radiant energy itself, which consists of ether-waves, is traceable back in every case to a mass of matter having definite characteristic motions. one may therefore say with certainty that every physical phenomenon is a change in the direction, or velocity, or character, of the energy present, and such change has been produced by matter acting as a transformer. the ether is a non-transformer. it has already been said that the absence of friction in the ether enables light-waves to maintain their identity for an indefinite time, and to an indefinitely great distance. in a uniform, homogeneous substance of any kind, any kind of energy which might be in it would continue in it without any change. uniformity and homogeneity imply similarity throughout, and the necessary condition for transformation is unlikeness. one might not look for any kind of physical phenomenon which was not due to the presence and activity of some heterogeneity. as a ray of light continues a ray of light so long as it exists in free ether, so all kinds of radiations, of whatever wave-length, continue identical until they fall upon some mechanical structure called matter. translatory motion continues translatory, rotary continues rotary, and vibratory continues to be vibratory, and no transforming change can take place in the absence of matter. the ether is helpless. . matter is elastic. it is commonly stated that certain substances, like putty and dough, are inelastic, while some other substances, like glass, steel, and wood, are elastic. this quality of elasticity, as manifested in such different degrees, depends upon molecular combinations; some of which, as in glass and steel, are favourable for exhibiting it, while others mask it, for the ultimate atoms of all kinds are certainly highly elastic. the measure of elasticity in a mass of matter is the velocity with which a wave-motion will be transmitted through it. thus the elasticity of the air determines the velocity of sound in it. if the air be heated, the elasticity is increased and the sound moves faster. the rates of such sound-conduction range from a few feet in a second to about , , five times swifter than a cannon ball. in such elastic bodies as vibrate to and fro like the prongs of a tuning-fork, or give sounds of a definite pitch, the rate of vibration is determined by the size and shape of the body as well as by their elementary composition. the smaller a body is, the higher its vibratory rate, if it be made of the same material and the form remains the same. thus a tuning-fork, that may be carried in the waistcoat-pocket, may vibrate times a second. if it were only the fifty-millionth of an inch in size, but of the same material and form, it would vibrate , , times a second; and if it were made of ether, instead of steel, it would vibrate as many times faster as the velocity of waves in the ether is greater than it is in steel, and would be as many as , , times per second. the amount of displacement, or the amplitude of vibration, with the pocket-fork might be no more than the hundredth of an inch, and this rate measured as translation velocity would be but five inches per second. if the fork were of atomic magnitude, and should swing its sides one half the diameter of the atom, or say the hundred-millionth of an inch, the translational velocity would be equivalent to about eighty miles a second, or a hundred and fifty times the velocity of a cannon ball, which may be reckoned at about feet. that atoms really vibrate at the above rate per second is very certain, for their vibrations produce ether-waves the length of which may be accurately measured. when a tuning-fork vibrates times a second, and the sound travels feet in the same interval, the length of each wave will be found by dividing the velocity in the air by the number of vibrations, or ÷ = . feet. in like manner, when one knows the velocity and wave-length, he may compute the number of vibrations by dividing the velocity by the wave-length. now the velocity of the waves called light is , miles a second, and a light-wave may be one forty thousandth of an inch long. the atom that produces the wave must be vibrating as many times per second as the fifth thousandth of an inch is contained in , miles. reducing this number to inches we have , Ã� Ã� ------------------- = , , , , , nearly. / , this shows that the atoms are minute elastic bodies that change their form rapidly when struck. as rapid as the change is, yet the rate of movement is only one-fifth that of a comet when near the sun, and is therefore easily comparable with other velocities observed in masses of matter. these vibratory motions, due to the elasticity of the atoms, is what constitutes heat. the ether is elastic. the elasticity of a mass of matter is its ability to recover its original form after that form has been distorted. there is implied that a stress changes its shape and dimensions, which in turn implies a limited mass and relative change of position of parts and some degree of discontinuity. from what has been said of the ether as being unlimited, continuous, and not made of atoms or molecules, it will be seen how difficult, if not impossible, it is to conceive how such a property as elasticity, as manifested in matter, can be attributed to the ether, which is incapable of deformation, either in structure or form, the latter being infinitely extended in every direction and therefore formless. nevertheless, certain forms of motion, such as light-waves, move in it with definite velocity, quite independent of how they originate. this velocity of , miles a second so much exceeds any movement of a mass of matter that the motions can hardly be compared. thus if miles per second be the swiftest speed of any mass of matter known--that of a comet near the sun--the ether-wave moves , ÷ = times faster than such comet, and , times faster than sound travels in air. it is clear that if this rate of motion depends upon elasticity, the elasticity must be of an entirely different type from that belonging to matter, and cannot be defined in any such terms as are employed for matter. if one considers gravitative phenomena, the difficulty is enormously increased. the orbit of a planet is never an exact ellipse, on account of the perturbations produced by the planetary attractions--perturbations which depend upon the direction and distance of the attracting bodies. these, however, are so well known that slight deviations are easily noticed. if gravitative attraction took any such appreciable time to go from one astronomical body to another as does light, it would make very considerable differences in the paths of the planets and the earth. indeed, if the velocity of gravitation were less than a million times greater than that of light, its effects would have been discovered long ago. it is therefore considered that the velocity of gravitation cannot be less than , miles per second. how much greater it may be no one can guess. seeing that gravitation is ether-pressure, it does not seem probable that its velocity can be infinite. however that may be, the ability of the ether to transmit pressure and various disturbances, evidently depends upon properties so different from those that enable matter to transmit disturbances that they deserve to be called by different names. to speak of the elasticity of the ether may serve to express the fact that energy may be transmitted at a finite rate in it, but it can only mislead one's thinking if he imagines the process to be similar to energy transmission in a mass of matter. the two processes are incomparable. no other word has been suggested, and perhaps it is not needful for most scientific purposes that another should be adopted, but the inappropriateness of the one word for the different phenomena has long been felt. . matter has density. this quality is exhibited in two ways in matter. in the first, the different elements in their atomic form have different masses or atomic weights. an atom of oxygen weighs sixteen times as much as an atom of hydrogen; that is, it has sixteen times as much matter, as determined by weight, as the hydrogen atom has, or it takes sixteen times as many hydrogen atoms to make a pound as it takes of oxygen atoms. this is generally expressed by saying that oxygen has sixteen times the density of hydrogen. in like manner, iron has fifty-six times the density, and gold one hundred and ninety-six. the difference is one in the structure of the atomic elements. if one imagines them to be vortex-rings, they may differ in size, thickness, and rate of rotation; either of these might make all the observed difference between the elements, including their density. in the second way, density implies compactness of molecules. thus if a cubic foot of air be compressed until it occupies but half a cubic foot, each cubic inch will have twice as many molecules in it as at first. the amount of air per unit volume will have been doubled, the weight will have been doubled, the amount of matter as determined by its weight will have been doubled, and consequently we say its density has been doubled. if a bullet or a piece of iron be hammered, the molecules are compacted closer together, and a greater number can be got into a cubic inch when so condensed. in this sense, then, density means the number of molecules in a unit of space, a cubic inch or cubic centimeter. there is implied in this latter case that the molecules do not occupy all the available space, that they may have varying degrees of closeness; in other words, matter is discontinuous, and therefore there may be degrees in density. the ether has density. it is common to have the degree of density of the ether spoken of in the same way, and for the same reason, that its elasticity is spoken of. the rate of transmission of a physical disturbance, as of a pressure or a wave-motion in matter, is conditioned by its degree of density; that is, the amount of matter per cubic inch as determined by its weight; the greater the density the slower the rate. so if rate of speed and elasticity be known, the density may be computed. in this way the density of the ether has been deduced by noting the velocity of light. the enormous velocity is supposed to prove that its density is very small, even when compared with hydrogen. this is stated to be about equal to that of the air at the height of two hundred and ten miles above the surface of the earth, where the air molecules are so few that a molecule might travel for , , miles without coming in collision with another molecule. in air of ordinary density, a molecule can on the average move no further than about the two-hundred-and-fifty-thousandth of an inch without such collision. it is plain the density of the ether is so far removed from the density of anything we can measure, that it is hardly comparable with such things. if, in addition, one recalls the fact that the ether is homogeneous, that is all of one kind, and also that it is not composed of atoms and molecules, then degree of compactness and number of particles per cubic inch have no meaning, and the term density, if used, can have no such meaning as it has when applied to matter. there is no physical conception gained from the study of matter that can be useful in thinking of it. as with elasticity, so density is inappropriately applied to the ether, but there is no substitute yet offered. . matter is heatable. so long as heat was thought to be some kind of an imponderable thing, which might retain its identity whether it were in or out of matter, its real nature was obscured by the name given to it. an imponderable was a mysterious something like a spirit, which was the cause of certain phenomena in matter. heat, light, electricity, magnetism, gravitation, were due to such various agencies, and no one concerned himself with the nature of one or the other. bacon thought that heat was a brisk agitation of the particles of substances, and count rumford and sir humphrey davy thought they proved that it could be nothing else, but they convinced nobody. mayer in germany and joule in england showed that quantitative relations existed between work done and heat developed, but not until the publication of the book called _heat as a mode of motion_, was there a change of opinion and terminology as to the nature of heat. for twenty years after that it was common to hear the expressions heat, and radiant heat, to distinguish between phenomena in matter and what is now called radiant energy radiations, or simply ether-waves. not until the necessity arose for distinguishing between different forms of energy, and the conditions for developing them, did it become clear to all that a change in the form of energy implied a change in the form of motion that embodied it. the energy called heat energy was proved to be a vibratory motion of molecules, and what happened in the ether as a result of such vibrations is no longer spoken of as heat, but as ether waves. when it is remembered that the ultimate atoms are elastic bodies, and that they will, if free, vibrate in a periodic manner when struck or shaken in any way, just as a ball will vibrate after it is struck, it is easy to keep in mind the distinction between the mechanical form of motion spent in striking and the vibratory form of the motion produced by it. the latter is called heat; no other form of motion than that is properly called heat. it is this alone that represents temperature, the rate and amplitude of such atomic and molecular vibrations as constitute change, of form. where molecules like those in a gas have some freedom of movement between impacts, they bound away from each other with varying velocities. the path of such motion may be long or short, depending upon the density or compactness of the molecules, but such changes in position are not heat for a molecule any more than the flight of a musket ball is heat, though it may be transformed into heat on striking the target. this conception of heat as the rapid change in the form of atoms and molecules, due to their elasticity, is a phenomenon peculiar to matter. it implies a body possessing form that may be changed; elasticity, that its changes may be periodic, and degrees of freedom that secure space for the changes. such a body may be heated. its temperature will depend upon the amplitude of such vibrations, and will be limited by the maximum amplitude. the ether is unheatable. the translatory motion of a mass of matter, big or little, through the ether, is not arrested in any degree so far as observed, but the internal vibratory motion sets up waves in the ether, the ether absorbs the energy, and the amplitude is continually lessened. the motion has been transferred and transformed; transferred from matter to the ether, and transformed from vibratory to waves travelling at the rate of , miles per second. the latter is not heat, but the result of heat. with the ether constituted as described, such vibratory motion as constitutes heat is impossible to it, and hence the characteristic of heat-motion in it is impossible; it cannot therefore be heated. the space between the earth and the sun may have any assignable amount of energy in the form of ether waves or light, but not any temperature. one might loosely say that the temperature of empty spaces was absolute zero, but that would not be quite correct, for the idea of temperature cannot properly be entertained as applicable to the ether. to say that its temperature was absolute zero, would serve to imply that it might be higher, which is inadmissible. when energy has been transformed, the old name by which the energy was called must be dropped. ether cannot be heated. . matter is indestructible. this is commonly said to be one of the essential properties of matter. all that is meant by it, however, is simply this: in no physical or chemical process to which it has been experimentally subjected has there been any apparent loss. the matter experimented upon may change from a solid or liquid to a gas, or the molecular change called chemical may result in new compounds, but the weight of the material and its atomic constituents have not appreciably changed. that matter cannot be annihilated is only the converse of the proposition that matter cannot be created, which ought always to be modified by adding, by physical or chemical processes at present known. a chemist may work with a few grains of a substance in a beaker, or test-tube, or crucible, and after several solutions, precipitations, fusions and dryings, may find by final weighing that he has not lost any appreciable amount, but how much is an appreciable amount? a fragment of matter the ten-thousandth of an inch in diameter has too small a weight to be noted in any balance, yet it would be made up of thousands of millions of atoms. hence if, in the processes to which the substance had been subjected, there had been the total annihilation of thousands of millions of atoms, such phenomenon would not have been discovered by weighing. neither would it have been discovered if there had been a similar creation or development of new matter. all that can be asserted concerning such events is, that they have not been discovered with our means of observation. the alchemists sought to transform one element into another, as lead into gold. they did not succeed. it was at length thought to be impossible, and the attempt to do it an absurdity. lately, however, telescopic observation of what is going on in nebulæ, which has already been referred to, has somewhat modified ideas of what is possible and impossible in that direction. it is certainly possible roughly to conceive how such a structure as a vortex-ring in the ether might be formed. with certain polarizing apparatus it is possible to produce rays of circularly polarized light. these are rays in which the motion is an advancing rotation like the wire in a spiral spring. if such a line of rotations in the ether were flexible, and the two ends should come together, there is reason for thinking they would weld together, in which case the structure would become a vortex-ring and be as durable as any other. there is reason for believing, also, that somewhat similar movements are always present in a magnetic field, and though we do not know how to make them close up in the proper way, it does not follow that it is impossible for them to do so. the bearing of all this upon the problem of the transmutation of elements is evident. no one now will venture to deny its possibility as strongly as it was denied a generation ago. it will also lead one to be less confident in the theory that matter is indestructible. assuming the vortex-ring theory of atoms to be true, if in any way such a ring could be cut or broken, there would not remain two or more fragments of a ring or atom. the whole would at once be dissolved into the ether. the ring and rotary energy that made it an atom would be destroyed, but not the substance it was made of, nor the energy which was embodied therein. for a long time philosophers have argued, and commonsense has agreed with them, that an atom which could not be ideally broken into two parts was impossible, that one could at any rate think of half an atom as a real objective possibility. this vortex-ring theory shows easily how possible it is to-day to think what once was philosophically incredible. it shows that metaphysical reasoning may be ever so clear and apparently irrefragable, yet for all that it may be very unsound. the trouble does not come so much from the logic as from the assumption upon which the logic is founded. in this particular case the assumption was that the ultimate particles of matter were hard, irrefragable somethings, without necessary relations to anything else, or to energy, and irrefragable only because no means had been found of breaking them. the destructibility or indestructibility of the ether cannot be considered from the same standpoint as that for matter, either ideally or really. not ideally, because we are utterly without any mechanical conceptions of the substance upon which one can base either reason or analogy; and not really, because we have no experimental evidence as to its nature or mode of operation. if it be continuous, there are no interspaces, and if it be illimitable there is no unfilled space anywhere. furthermore, one might infer that if in any way a portion of the ether could be annihilated, what was left would at once fill up the vacated space, so there would be no record left of what had happened. apparently, its destruction would be the destruction of a substance, which is a very different thing from the destruction of a mode of motion. in the latter, only the form of the motion need be destroyed to completely obliterate every trace of the atom. in the former, there would need to be the destruction of both substance and energy, for it is certain, for reasons yet to be attended to, that the ether is saturated with energy. one may, without mechanical difficulties, imagine a vortex-ring destroyed. it is quite different with the ether itself, for if it were destroyed in the same sense as the atom of matter, it would be changed into something else which is not ether, a proposition which assumes the existence of another entity, the existence for which is needed only as a mechanical antecedent for the other. the same assumption would be needed for this entity as for the ether, namely, something out of which it was made, and this process of assuming antecedents would be interminable. the last one considered would have the same difficulties to meet as the ether has now. the assumption that it was in some way and at some time created is more rational, and therefore more probable, than that it either created itself or that it always existed. considered as the underlying stratum of matter, it is clear that changes of any kind in matter can in no way affect the quantity of ether. . matter has inertia. the resistance that a mass of matter opposes to a change in its position or rate and direction of movement, is called inertia. that it should actively oppose anything has been already pointed out as reason for denying that matter is inert, but inertia is the measure of the reaction of a body when it is acted upon by pressure from any source tending to disturb its condition of either rest or motion. it is the equivalent of mass, or the amount of matter as measured by gravity, and is a fixed quantity; for inertia is as inherent as any other quality, and belongs to the ultimate atoms and every combination of them. it implies the ability to absorb energy, for it requires as much energy to bring a moving body to a standstill as was required to give it its forward motion. both rotary and vibratory movements are opposed by the same property. a grindstone, a tuning-fork, and an atom of hydrogen require, to move them in their appropriate ways, an amount of energy proportionate to their mass or inertia, which energy is again transformed through friction into heat and radiated away. one may say that inertia is the measure of the ability of a body to transfer or transform mechanical energy. the meteorite that falls upon the earth to-day gives, on its impact, the same amount of energy it would have given if it had struck the earth ten thousand years ago. the inertia of the meteor has persisted, not as energy, but as a factor of energy. we commonly express the energy of a mass of matter by _mv_^{ }/ , where _m_ stands for the mass and _v_ for its velocity. we might as well, if it were as convenient, substitute inertia for mass, and write the expression _iv_^{ }/ , for the mass, being measured by its inertia, is only the more common and less definitive word for the same thing. the energy of a mass of matter is, then, proportional to its inertia, because inertia is one of its factors. energy has often been treated as if it were an objective thing, an entity and a unity; but such a conception is evidently wrong, for, as has been said before, it is a product of two factors, either of which may be changed in any degree if the other be changed inversely in the same degree. a cannon ball weighing pounds, and moving feet per second, will have , foot-pounds of energy, but a musket ball weighing an ounce will have the same amount when its velocity is , feet per second. nevertheless, another body acting upon either bullet or cannon ball, tending to move either in some new direction, will be as efficient while those bodies are moving at any assignable rate as when they are quiescent, for the change in direction will depend upon the inertia of the bodies, and that is constant. the common theory of an inert body is one that is wholly passive, having no power of itself to move or do anything, except as some agency outside itself compels it to move in one way or another, and thus endows it with energy. thus a stone or an iron nail are thought to be inert bodies in that sense, and it is true that either of them will remain still in one place for an indefinite time and move from it only when some external agency gives them impulse and direction. still it is known that such bodies will roll down hill if they will not roll up, and each of them has itself as much to do with the down-hill movement as the earth has; that is, it attracts the earth as much as the earth attracts it. if one could magnify the structure of a body until the molecules became individually visible, every one of them would be seen to be in intense activity, changing its form and relative position an enormous number of times per second in undirected ways. no two such molecules move in the same way at the same time, and as all the molecules cohere together, their motions in different directions balance each other, so that the body as a whole does not change its position, not because there is no moving agency in itself, but because the individual movements are scattering, and not in a common direction. an army may remain in one place for a long time. to one at a distance it is quiescent, inert. to one in the camp there is abundant sign of activity, but the movements are individual movements, some in one direction and some in another, and often changing. the same army on the march has the same energy, the same rate of individual movement; but all have a common direction, it moves as a whole body into new territory. so with the molecules of matter. in large masses they appear to be inert, and to do nothing, and to be capable of doing nothing. that is only due to the fact that their energy is undirected, not that they can do nothing. the inference that if quiescent bodies do not act in particular ways they are inert, and cannot act in any kind of a way, is a wrong inference. an illustration may perhaps make this point plainer. a lump of coal will be still as long as anything if it be undisturbed. indeed, it has thus lain in a coal-bed for millions of years probably, but if coal be placed where it can combine with oxygen, it forthwith does so, and during the process yields a large amount of energy in the shape of heat. one pound of coal in this way gives out , heat units, which is the equivalent of , , foot-pounds of work, and if it could be all utilized would furnish a horse-power for five and a half hours. can any inert body weighing a pound furnish a horse-power for half a day? and can a body give out what it has not got? are gunpowder and nitro-glycerine inert? are bread and butter and foods in general inert because they will not push and pull as a man or a horse may? all have energy, which is available in certain ways and not in others, and whatever possesses energy available in any way is not an ideally inert body. lastly, how many inert bodies together will it take to make an active body? if the question be absurd, then all the phenomena witnessed in bodies, large or small, are due to the fact that the atoms are not inert, but are immensely energetic, and their inertia is the measure of their rates of exchanging energy. the ether is conditionally possessed of inertia. a moving mass of matter is brought to rest by friction, because it imparts its motion at some rate to the body it is in contact with. generally the energy is transformed into heat, but sometimes it appears as electrification. friction is only possible because one or both of the bodies possess inertia. that a body may move in the ether for an indefinite time without losing its velocity has been stated as a reason for believing the ether to be frictionless. if it be frictionless, then it is without inertia, else the energy of the earth and of a ray of light would be frittered away. a ray of light can only be transformed when it falls upon molecules which may be heated by it. as the ether cannot be heated and cannot transform translational energy, it is without inertia for _such_ a form of motion and its embodied energy. it is not thus with other forms of energy than the translational. atomic and molecular vibrations are so related to the ether that they are transformed into waves, which are conducted away at a definite rate. this shows that such property of inertia as is possessed by the ether is selective and not like that of matter, which is equally "inertiative" under all conditions. similarly with electric and magnetic phenomena, it is capable of transforming the energy which may reside as stress in the ether, and other bodies moving in the space so affected meet with frictional resistance, for they become heated if the motion be maintained. on the other hand, there is no evidence that the body which produced the electric or magnetic stress suffers any degree of friction on moving in precisely the same space. a bar magnet rotating on its longitudinal axis does not disturb its own field, but a piece of iron revolving near the magnet will not only become heated, but will heat the stationary magnet. much experimental work has been done to discover, if possible, the relation of a magnet to its ether field. as the latter is not disturbed by the rotation of the magnet, it has been concluded that the field does not rotate; but as every molecule in the magnet has its own field independent of all the rest, it is mechanically probable that each such field does vary in the rotation, but among the thousands of millions of such fields the average strength of the field does not vary within measurable limits. another consideration is that the magnetic field itself, when moved in space, suffers no frictional resistance. there is no magnetic energy wasted through ether inertia. these phenomena show that whether the ether exhibits the quality called inertia depends upon the kind of motion it has. . matter is magnetic. the ordinary phenomenon of magnetism is shown by bringing a piece of iron into the neighbourhood of a so-called magnet, where it is attracted by the latter, and if free to move will go to and cling to the magnet. a delicately suspended magnetic needle will be affected appreciably by a strong magnet at the distance of several hundred feet. as the strength of such action varies inversely as the square of the distance from the magnet, it is evident there can be no absolute boundary to it. at a distance from an ordinary magnet it becomes too weak to be detected by our methods, not that there is a limit to it. it is customary to think of iron as being peculiarly endowed with magnetic quality, but all kinds of matter possess it in some degree. wood, stone, paper, oats, sulphur, and all the rest, are attracted by a magnet, and will stick to it if the magnet be a strong one. whether a piece of iron itself exhibits the property depends upon its temperature, for near degrees it becomes as magnetically indifferent as a piece of copper at ordinary temperature. oxygen, too, at degrees below the zero of centigrade adheres to a magnet like iron. in this as in so many other particulars, how a piece of matter behaves depends upon its temperature, not that the essential qualities are modified in any degree, but temperature interferes with atomic arrangement and aggregation, and so disguises their phenomena. as every kind of matter is thus affected by a magnet, the manifestations differing but in degree, it follows that all kinds of atoms--all the elements--are magnetic. an inherent property in them, as much so as gravitation or inertia; apparently a quality depending upon the structure of the atoms themselves, in the same sense as gravitation is thus dependent, as it is not a quality of the ether. an atom must, then, be thought of as having polarity, different qualities on the two sides, and possessing a magnetic field as extensive as space itself. the magnetic field is the stress or pressure in the ether produced by the magnetic body. this ether pressure produced by a magnet may be as great as a ton per square inch. it is this pressure that holds an armature to the magnet. as heat is a molecular condition of vibration, and radiant energy the result of it, so is magnetism a property of molecules, and the magnetic field the temporary condition in the ether, which depends upon the presence of a magnetic body. we no longer speak of the wave-motion in the ether which results from heat, as heat, but call it radiation, or ether waves, and for a like reason the magnetic field ought not to be called magnetism. the ether is non-magnetic. a magnetic field manifests itself in a way that implies that the ether structure, if it may be said to have any, is deformed--deformed in such a sense that another magnet in it tends to set itself in the plane of the stress; that is, the magnet is twisted into a new position to accommodate itself to the condition of the medium about it. the new position is the result of the reaction of the ether upon the magnet and ether pressure acting at right angles to the body that produced the stress. such an action is so anomalous as to suggest the propriety of modifying the so-called third law of motion, viz., action and reaction are equal and opposite, adding that sometimes action and reaction are at right angles. there is no condition or property exhibited by the ether itself which shows it to have any such characteristic as attraction, repulsion, or differences in stress, except where its condition is modified by the activities of matter in some way. the ether itself is not attracted or repelled by a magnet; that is, it is not a magnetic body in any such sense as matter in any of its forms is, and therefore cannot properly be called magnetic. it has been a mechanical puzzle to understand how the vibratory motions called heat could set up light waves in the ether seeing that there is an absence of friction in the latter. in the endeavour to conceive it, the origin of sound-waves has been in mind, where longitudinal air-waves are produced by the vibrations of a sounding body, and molecular impact is the antecedent of the waves. the analogy does not apply. the following exposition may be helpful in grasping the idea of such transformation and change of energy from matter to the ether. consider a straight bar permanent magnet to be held in the hand. it has its north and south poles and its field, the latter extending in every direction to an indefinite distance. the field is to be considered as ether stress of such a sort as to tend to set other magnets in it in new positions. if at a distance of ten feet there were a delicately-poised magnet needle, every change in the position of the magnet held in the hand would bring about a change in the position of the needle. if the position of the hand magnet were completely reversed, so the south pole faced where the north pole faced before, the field would have been completely reversed, and the poised needle would have been pushed by the field into an opposite position. if the needle were a hundred feet away, the change would have been the same except in amount. the same might be said if the two were a mile apart, or the distance of the moon or any other distance, for there is no limit to an ether magnetic field. suppose the hand magnet to have its direction completely reversed once in a second. the whole field, and the direction of the stress, would necessarily be reversed as often. but this kind of change in stress is known by experiment to travel with the speed of light, , miles a second; the disturbance due to the change of position of the magnet will therefore be felt in some degree throughout space. in a second and a third of a second it will have reached the moon, and a magnet there will be in some measure affected by it. if there were an observer there with a delicate-enough magnet, he could be witness to its changes once a second for the same reason one in the room could. the only difference would be one of amount of swing. it is therefore theoretically possible to signal to the moon with a swinging magnet. suppose again that the magnet should be swung twice a second, there would be formed two waves, each one half as long as the first. if it should swing ten times a second, then the waves would be one-tenth of , miles long. if in some mechanical way it could be rotated , times a second, the wave would be but one mile long. artificial ways have been invented for changing this magnet field as many as million times a second, and the corresponding wave is less than a foot long. the shape of a magnet does not necessarily make it weaker or stronger as a magnet, but if the poles are near together the magnetic field is denser between them than when they are separated. the ether stress is differently distributed for every change in the relative positions of the poles. a common u-magnet, if struck, will vibrate like a tuning-fork, and gives out a definite pitch. its poles swing towards and away from each other at uniform rates, and the pitch of the magnet will depend upon its size, thickness, and the material it is made of. let ten or fifteen ohms of any convenient-sized wire be wound upon the bend of a commercial u-magnet. let this wire be connected to a telephone in its circuit. when the magnet is made to sound like a tuning-fork, the pitch will be reproduced in the telephone very loudly. if another magnet with a different pitch be allowed to vibrate near the former, the pitch of the vibrating body will be heard in the telephone, and these show that the changing magnetic field reacts upon the quiescent magnet, and compels the latter to vibrate at the same rate. the action is an ether action, the waves are ether waves, but they are relatively very long. if the magnet makes vibrations a second, the waves will be miles long, the number of times is contained in , miles. imagine the magnet to become smaller and smaller until it was the size of an atom, the one-fifty-millionth of an inch. its vibratory rate would be proportionally increased, and changes in its form will still bring about changes in its magnetic field. but its magnetic field is practically limitless, and the number of vibrations per second is to be reckoned as millions of millions; the waves are correspondingly short, small fractions of an inch. when they are as short as the one-thirty-seven-thousandth of an inch, they are capable of affecting the retina of the eye, and then are said to be visible as red light. if the vibratory rate be still higher, and the corresponding waves be no more than one-sixty-thousandth of an inch long, they affect the retina as violet light, and between these limits there are all the waves that produce a complete spectrum. the atoms, then, shake the ether in this way because they all have a magnetic hold upon the ether, so that any disturbance of their own magnetism, such as necessarily comes when they collide, reacts upon the ether for the same reason that a large magnet acts thus upon it when its poles approach and recede from each other. it is not a phenomenon of mechanical impact or frictional resistance, since neither are possible in the ether. . matter exists in several states. molecular cohesion exists between very wide ranges. when strong, so if one part of a body is moved the whole is moved in the same way, without breaking continuity or the relative positions of the molecules, we call the body a solid. in a liquid, cohesion is greatly reduced, and any part of it may be deformed without materially changing the form of the rest. the molecules are free to move about each other, and there is no definite position which any need assume or keep. with gases, the molecules are without any cohesion, each one is independent of every other one, collides with and bounds away from others as free elastic particles do. between impacts it moves in what is called its free path, which may be long or short as the density of the gas be less or greater. these differing degrees of cohesion depend upon temperature, for if the densest and hardest substances are sufficiently heated they will become gaseous. this is only another way of saying that the states of matter depend upon the amount of molecular energy present. solid ice becomes water by the application of heat. more heat reduces it to steam; still more decomposes the steam molecules into oxygen and hydrogen molecules; and lastly, still more heat will decompose these molecules into their atomic state, complete dissociation. on cooling, the process of reduction will be reversed until ice has been formed again. cohesive strength in solids is increased by reduction of temperature, and metallic rods become stronger the colder they are. no distinction is now made between cohesion and chemical affinity, and yet at low temperatures chemical action will not take place, which phenomenon shows there is a distinction between molecular cohesion and molecular structure. in molecular structure, as determined by chemical activity, the molecules and atoms are arranged in definite ways which depend upon the rate of vibrations of the components. the atoms are set in definite positions to constitute a given molecule. but atoms or molecules may cohere for other reasons, gravitative or magnetic, and relative positions would be immaterial. in the absence of temperature, a solid body would be solider and stronger than ever, while a gaseous mass would probably fall by gravity to the floor of the containing vessel like so much dust. the molecular structure might not be changed, for there would be no agency to act upon it in a disturbing way. the ether has no corresponding states. degrees of density have already been excluded, and the homogeneity and continuity of the ether would also exclude the possibility of different states at all comparable with such as belong to matter. as for cohesion, it is doubtful if the term ought to be applied to such a substance. the word itself seems to imply possible separateness, and if the ether be a single indivisible substance, its cohesion must be infinite and is therefore not a matter of degree. the ether has sometimes been considered as an elastic solid, but such solidity is comparable with nothing we call solid in matter, and the word has to be defined in a special sense in order that its use may be tolerated at all. in addition to this, some of the phenomena exhibited by it, such as diffraction and double refraction, are quite incompatible with the theory that the ether is an elastic solid. the reasons why it cannot be considered as a liquid or gas have been considered previously. the expression _states of matter_ cannot be applied to the ether in any such sense as it is applied to matter, but there is one sense when possibly it may be considered applicable. let it be granted that an atom is a vortex-ring of ether in the ether, then the state of being in ring rotation would suffice to differentiate that part of the ether from the rest, and give to it a degree of individuality not possessed by the rest; and such an atom might be called a state of ether. in like manner, if other forms of motion, such as transverse waves, circular and elliptical spirals, or others, exist in the ether, then such movements give special character to the part thus active, and it would be proper to speak of such states of the ether, but even thus the word would not be used in the same sense as it is used when one speaks of the states of matter as being solid, liquid, and gaseous. . solid matter can experience a shearing stress, liquids and gases cannot. a sliding stress applied to a solid deforms it to a degree which depends upon the stress and the degree of rigidity preserved by the body. thus if the hand be placed upon a closed book lying on the table, and pressure be so applied as to move the upper side of the book but not the lower, the book is said to be subject to a shearing stress. if the pressing hand has a twisting motion, the book will be warped. any solid may be thus sheared or warped, but neither liquids nor gases can be so affected. molecular cohesion makes it possible in the one, and the lack of it, impossible in the others. the solid can maintain such a deformation indefinitely long, if the pressure does not rupture its molecular structure. the ether can maintain a shearing stress. the phenomena in a magnetic field show that the stress is of such a sort as to twist into a new directional position the body upon which it acts as exhibited by a magnetic needle, also as indicated by the transverse vibrations of the ether waves, and again by the twist given to plane polarized light when moving through a magnetic field. these are all interpreted as indicative of the direction of ether stress, as being similar to a shearing stress in solid matter. the fact has been adduced to show the ether to be a solid, but such a phenomenon is certainly incompatible with a liquid or gaseous ether. this kind of stress is maintained indefinitely about a permanent magnet, and the mechanical pressure which may result from it is a measure of the strength of the magnetic field, and may exceed a thousand pounds per square inch. . other properties of matter. there are many secondary qualities exhibited by matter in some of its forms, such as hardness, brittleness, malleability, colour, etc., and the same ultimate element may exhibit itself in the most diverse ways, as is the case with carbon, which exists as lamp-black, charcoal, graphite, jet, anthracite and diamond, ranging from the softest to the hardest of known bodies. then it may be black or colourless. gold is yellow, copper red, silver white, chlorine green, iodine purple. the only significance any or all of such qualities have for us here is that the ether exhibits none of them. there is neither hardness nor brittleness, nor colour, nor any approach to any of the characteristics for the identification of elementary matter. . sensation depends upon matter. however great the mystery of the relation of body to mind, it is quite true that the nervous system is the mechanism by and through which all sensation comes, and that in our experience in the absence of nerves there is neither sensation nor consciousness. the nerves themselves are but complex chemical structures; their molecular constitution is said to embrace as many as , atoms, chiefly carbon, hydrogen, oxygen, and nitrogen. there must be continuity of this structure too, for to sever a nerve is to paralyze all beyond. if all knowledge comes through experience, and all experience comes through the nervous system, the possibilities depend upon the mechanism each one is provided with for absorbing from his environment, what energies there are that can act upon the nerves. touch, taste, and smell imply contact, sound has greater range, and sight has the immensity of the universe for its field. the most distant but visible star acts through the optic nerve to present itself to consciousness. it is not the ego that looks out through the eyes, but it is the universe that pours in upon the ego. again, all the known agencies that act upon the nerves, whether for touch or sound or sight, imply matter in some of its forms and activities, to adapt the energy to the nervous system. the mechanism for the perception of light is complicated. the light acts upon a sensitive surface where molecular structure is broken up, and this disturbance is in the presence of nerve terminals, and the sensation is not in the eye but in the sensorium. in like manner for all the rest; so one may fairly say that matter is the condition for sensation, and in its absence there would be nothing we call sensation. the ether is insensible to nerves. the ether is in great contrast with matter in this particular. there is no evidence that in any direct way it acts upon any part of the nervous system, or upon the mind. it is probable that this lack of relation between the ether and the nervous system was the chief reason why its discovery was so long delayed, as the mechanical necessities for it even now are felt only by such as recognize continuity as a condition for the transmission of energy of whatever kind it may be. action at a distance contradicts all experience, is philosophically incredible, and is repudiated by every one who once perceives that energy has two factors--substance and motion. the table given below presents a list of twenty-two of the known properties of matter contrasted with those exhibited by the ether. in none of them are the properties of the two identical, and in most of them what is true for one is not true for the other. they are not simply different, they are incomparable. from the necessities of the case, as knowledge has been acquired and terminology became essential for making distinctions, the ether has been described in terms applicable to matter, hence such terms as mass, solidity, elasticity, density, rigidity, etc., which have a definite meaning and convey definite mechanical conceptions when applied to matter, but have no corresponding meaning and convey no such mechanical conceptions when applied to the ether. it is certain that they are inappropriate, and that the ether and its properties cannot be described in terms applicable to matter. mathematical considerations derived from the study of matter have no advantage, and are not likely to lead us to a knowledge of the ether. only a few have perceived the inconsistency of thinking of the two in the same terms. in his _grammar of science_, prof. karl pearson says, "we find that our sense-impressions of hardness, weight, colour, temperature, cohesion, and chemical constitution, may all be described by the aid of the motions of a single medium, which itself is conceived to have no hardness, weight, colour, temperature, nor indeed elasticity of the ordinary conceptual type." none of the properties of the ether are such as one would or could have predicted if he had had all the knowledge possessed by mankind. every phenomenon in it is a surprise to us, because it does not follow the laws which experience has enabled us to formulate for matter. a substance which has none of the phenomenal properties of matter, and is not subject to the known laws of matter, ought not to be called matter. ether phenomena and matter phenomena belong to different categories, and the ends of science will not be conserved by confusing them, as is done when the same terminology is employed for both. there are other properties belonging to the ether more wonderful, if possible, than those already mentioned. its ability to maintain enormous stresses of various kinds without the slightest evidence of interference. there is the gravitational stress, a direct pull between two masses of matter. between two molecules it is immeasurably small even when close together, but the prodigious number of them in a bullet brings the action into the field of observation, while between such bodies as the earth and moon or sun, the quantity reaches an astonishing figure. thus if the gravitative tension due to the gravitative attraction of the earth and moon were to be replaced by steel wires connecting the two bodies to prevent the moon from leaving its orbit, there would be needed four number ten steel wires to every square inch upon the earth, and these would be strained nearly to the breaking point. yet this stress is not only endured continually by this pliant, impalpable, transparent medium, but other bodies can move through the same space apparently as freely as if it were entirely free. in addition to this, the stress from the sun and the more variable stresses from the planets are all endured by the same medium in the same space and apparently a thousand or a million times more would not make the slightest difference. rupture is impossible. electric and magnetic stresses, acting parallel or at right angles to the other, exist in the same space and to indefinite degrees, neither modifying the direction nor amount of either of the others. these various stresses have been computed to represent energy, which if it could be utilized, each cubic inch of space would yield five hundred horse-power. it shows what a store-house of energy the ether is. if every particle of matter were to be instantly annihilated, the universe of ether would still have an inexpressible amount of energy left. to draw at will directly from this inexhaustible supply, and utilize it for the needs of mankind, is not a forlorn hope. the accompanying table presents these contrasting properties for convenient inspection. contrasted properties of matter and the ether. matter. ether. . discontinuous continuous . limited unlimited . heterogeneous homogeneous . atomic non-atomic . definite structure structureless . gravitative gravitationless . frictionable frictionless . Ã�olotropic isotropic . chemically selective ---- . harmonically related ---- . energy embodied energy endowed . energy transformer non-transformer . elastic elastic? . density density? . heatable unheatable . indestructible? indestructible . inertiative inertiative conditionally . magnetic ---- . variable states ---- . subject to shearing stress in solid shearing stress maintained . has secondary qualities ---- . sensation depends upon insensible to nerves chapter iii antecedents of electricity--nature of what is transformed--series of transformations for the production of light--positive and negative electricity--positive and negative twists--rotations about a wire--rotation of an arc--ether a non-conductor--electro-magnetic waves--induction and inductive action--ether stress and atomic position--nature of an electric current--electricity a condition, not an entity. so far as we have knowledge to-day, the only factors we have to consider in explaining physical phenomena are: ( ) ordinary matter, such as constitutes the substance of the earth, and the heavenly bodies; ( ) the ether, which is omnipresent; and ( ) the various forms of motion, which are mutually transformable in matter, and some of which, but not all, are transformable into ether forms. for instance, the translatory motion of a mass of matter can be imparted to another mass by simple impact, but translatory motion cannot be imparted to the ether, and, for that reason, a body moving in it is not subject to friction, and continues to move on with velocity undiminished for an indefinite time; but the vibratory motion which constitutes heat is transformable into wave-motion in the ether, and is transmitted away with the speed of light. the kind of motion which is thus transformed is not even a to-and-fro swing of an atom, or molecule, like the swing of a pendulum bob, but that due to a change of form of the atoms within the molecule, otherwise there could be no such thing as spectrum analysis. vibratory motion of the matter becomes undulatory motion in the ether. the vibratory motion we call heat; the wave-motion we call sometimes radiant energy, sometimes light. neither of these terms is a good one, but we now have no others. it is conceded that it is not proper to speak of the wave-motion in the ether as _heat_; it is also admitted that the ether is not heated by the presence of the wave--or, in other words, the temperature of the ether is absolute zero. matter only can be heated. but the ether waves can heat other matter they may fall on; so there are three steps in the process and two transformations--( ) vibrating matter; ( ) waves in the ether; ( ) vibration in other matter. energy has been transferred indirectly. what is important to bear in mind is, that when a form of energy in matter is transformed in any manner so as to lose its characteristics, it is not proper to call it by the same name after as before, and this we do in all cases when the transformation is from one kind in matter to another kind in matter. thus, when a bullet is shot against a target, before it strikes it has what we call mechanical energy, and we measure that in foot-pounds; after it has struck the target, the transformation is into heat, and this has its mechanical equivalent, but is not called mechanical energy, nor are the motions which embody it similar. the mechanical ideas in these phenomena are easy to grasp. they apply to the phenomena of the mechanics of large and small bodies, to sound, to heat, and to light, as ordinarily considered, but they have not been applied to electric phenomena, as they evidently should be, unless it be held that such phenomena are not related to ordinary phenomena, as the latter are to one another. when we would give a complete explanation of the phenomena exhibited by, say, a heated body, we need to inquire as to the antecedents of the manifestation, and also its consequents. where and how did it get its heat? where and how did it lose it? when we know every step of those processes, we know all there is to learn about them. let us undertake the same thing for some electrical phenomena. first, under what circumstances do electrical phenomena arise? ( ) _mechanical_, as when two different kinds of matter are subject to friction. ( ) _thermal_, as when two substances in molecular contact are heated at the junction. ( ) _magnetic_, as when any conductor is in a changing magnetic field. ( ) _chemical_, as when a metal is being dissolved in any solution. ( ) _physiological_, as when a muscle contracts. [illustration: fig. .--frictional electrical machine.] each of these has several varieties, and changes may be rung on combinations of them, as when mechanical and magnetic conditions interact. ( ) in the first case, ordinary mechanical or translational energy is spent as friction, an amount measurable in foot-pounds, and the factors we know, a pressure into a distance. if the surface be of the same kind of molecules, the whole energy is spent as heat, and is presently radiated away. if the surfaces are of unlike molecules, the product is a compound one, part heat, part electrical. what we have turned into the machine we know to be a particular mode of motion. we have not changed the amount of matter involved; indeed, we assume, without specifying and without controversy, that matter is itself indestructible, and the product, whether it be of one kind or another, can only be some form of motion. whether we can describe it or not is immaterial; but if we agree that heat is vibratory molecular motion, and there be any other kind of a product than heat, it too must also be some other form of motion. so if one is to form a conception of the mechanical origin of electricity, this is the only one he can have--transformed motion. [illustration: fig. .--thermo-pile.] [illustration: fig. .--dynamo.] ( ) when heat is the antecedent of electricity, as in the thermo-pile, that which is turned into the pile we know to be molecular motion of a definite kind. that which comes out of it must be some equivalent motion, and if all that went in were transformed, then all that came out would be transformed, call it by what name we will and let its amount be what it may. ( ) when a conductor is moved in a magnetic field, the energy spent is measurable in foot-pounds, as before, a pressure into a distance. the energy appears in a new form, but the quantity of matter being unchanged, the only changeable factor is the kind of motion, and that the motion is molecular is evident, for the molecules are heated. mechanical or mass motion is the antecedent, molecular heat motion is the consequent, and the way we know there has been some intermediate form is, that heat is not conducted at the rate which is observed in such a case. call it by what name one will, some form of motion has been intermediate between the antecedent and the consequent, else we have some other factor of energy to reckon with than ether, matter and motion. ( ) in a galvanic battery, the source of electricity is chemical action; but what is chemical action? simply an exchange of the constituents of molecules--a change which involves exchange of energy. molecules capable of doing chemical work are loaded with energy. the chemical products of battery action are molecules of different constitution, with smaller amounts of energy as measured in calorics or heat units. if the results of the chemical reaction be prevented from escaping, by confining them to the cell itself, the whole energy appears as heat and raises the temperature of the cell. if a so-called circuit be provided, the energy is distributed through it, and less heat is spent in the cell, but whether it be in one place or another, the mass of matter involved is not changed, and the variable factor is the motion, the same as in the other cases. the mechanical conceptions appropriate are the transformation of one kind of motion into another kind by the mechanical conditions provided. [illustration: fig. .--galvanic battery.] ( ) physiological antecedents of electricity are exemplified by the structure and mode of operation of certain muscles (fig. , _a_) in the torpedo and other electrical animals. the mechanical contraction of them results in an electrical excitation, and, if a proper circuit be provided, in an electric current. the energy of a muscle is derived from food, which is itself but a molecular compound loaded with energy of a kind available for muscular transformation. bread-and-butter has more available energy, pound for pound, than has coal, and can be substituted for coal for running an engine. it is not used, because it costs so much more. there is nothing different, so far as the factors of energy go, between the food of an animal and the food of an engine. what becomes of the energy depends upon the kind of structure it acts on. it may be changed into translatory, and the whole body moves in one direction; or into molecular, and then appears as heat or electrical energy. if one confines his attention to the only variable factor in the energy in all these cases, and traces out in each just what happens, he will have only motions of one sort or another, at one rate or another, and there is nothing mysterious which enters into the processes. we will turn now to the mode in which electricity manifests itself, and what it can do. it may be well to point out at the outset what has occasionally been stated, but which has not received the philosophical attention it deserves--namely, that electrical phenomena are reversible; that is, any kind of a physical process which is capable of producing electricity, electricity is itself able to produce. thus to name a few: if mechanical motion develops electricity, electricity will produce mechanical motion; the movement of a pith ball and an electric motor are examples. if chemical action can produce it, it will produce chemical action, as in the decomposition of water and electro-plating. as heat may be its antecedent, so will it produce heat. if magnetism be an antecedent factor, magnetism may be its product. what is called induction may give rise to it in an adjacent conductor, and, likewise, induction may be its effect. [illustration: fig. .--torpedo.] [illustration: fig. .--dynamo and motor.] let us suppose ourselves to be in a building in which a steam-engine is at work. there is fuel, the furnace, the boiler, the pipes, the engine with its fly-wheel turning. the fuel burns in the furnace, the water is superheated in the boiler, the steam is directed by the pipes, the piston is moved by the steam pressure, and the fly-wheel rotates because of proper mechanism between it and the piston. no one who has given attention to the successive steps in the process is so puzzled as to feel the need of inventing a particular force, or a new kind of matter, or any agency, at any stage of the process, different from the simple mechanical ones represented by a push or a pull. even if he cannot see clearly how heat can produce a push, he does not venture to assume a genii to do the work, but for the time is content with saying that if he starts with motion in the furnace and stops with the motion of the fly-wheel, any assumption of any other factor than some form of motion between the two would be gratuitous. he can truthfully say that he understands the _nature_ of that which goes on between the furnace and the wheel; that it is some sort of motion, the particular kind of which he might make out at his leisure. suppose once more that, across the road from an engine-house, there was another building, where all sorts of machines--lathes, planers, drills, etc.--were running, but that the source of the power for all this was out of sight, and that one could see no connection between this and the engine on the other side of the street. would one need to suppose there was anything mysterious between the two--a force, a fluid, an immaterial something? this question is put on the supposition that one should not be aware of the shaft that might be between the two buildings, and that it was not obvious on simple inspection how the machines got their motions from the engine. no one would be puzzled because he did not know just what the intervening mechanism might be. if the boiler were in the one building, and the engine in the other with the machines, he could see nothing moving between them, even if the steam-pipes were of glass. if matter of any kind were moving, he could not see it there. he would say there _must_ be something moving, or pressure could not be transferred from one place to the other. substitute for the furnace and boiler a galvanic battery or a dynamo; for the machines of the shop, one or more motors with suitable wire connections. when the dynamo goes the motors go; when the dynamo stops the motors stop; nothing can be seen to be turning or moving in any way between them. is there any necessity for assuming a mysterious agency, or a force of a _nature_ different from the visible ones at the two ends of the line? is it not certain that the question is, how does the motion get from one to the other, whether there be a wire or not? if there be a wire, it is plain that there is motion in it, for it is heated its whole length, and heat is known to be a mode of motion, and every molecule which is thus heated must have had some antecedent motions. whether it be defined or not, and whether it be called by one name or another, are quite immaterial, if one is concerned only with the _nature_ of the action, whether it be matter or ether, or motion or abracadabra. once more: suppose we have a series of active machines. (fig. .) an arc lamp, radiating light-waves, gets its energy from the wire which is heated, which in turn gets its energy from the electric current; that from a dynamo, the dynamo from a steam-engine; that from a furnace and the chemical actions going on in it. let us call the chemical actions a, the furnace b, the engine c, the dynamo d, the electric lamp e, the ether waves f. (fig. .) [illustration: fig. .] the product of the chemical action of the coal is molecular motion, called heat in the furnace. the product of the heat is mechanical motion in the engine. the product of the mechanical motion is electricity in the dynamo. the product of the electric current in the lamp is light-waves in the ether. no one hesitates for an instant to speak of the heat as being molecular motion, nor of the motions of the engine as being mechanical; but when we come to the product of the dynamo, which we call electricity, behold, nearly every one says, not that he does not know what it is, but that no one knows! does any one venture to say he does not know what heat is, because he cannot describe in detail just what goes on in a heated body, as it might be described by one who saw with a microscope the movements of the molecules? let us go back for a moment to the proposition stated early in this book, namely, that if any body of any magnitude moves, it is because some other body in motion and in contact with it has imparted its motion by mechanical pressure. therefore, the ether waves at f (fig. ) imply continuous motions of some sort from a to f. that they are all motions of ordinary matter from a to e is obvious, because continuous matter is essential for the maintenance of the actions. at e the motions are handed over to the ether, and they are radiated away as light-waves. [illustration: fig. .] [illustration: fig. .] a puzzling electrical phenomenon has been what has been called its duality-states, which are spoken of as positive and negative. thus, we speak of the positive plate of a battery and the negative pole of a dynamo; and another troublesome condition to idealize has been, how it could be that, in an electric circuit, there could be as much energy at the most remote part as at the source. but, if one will take a limp rope, or feet long, tie its ends together, and then begin to twist it at any point, he will see the twist move in a right-handed spiral on the one hand, and in a left-handed spiral on the other, and each may be traced quite round the circuit; so there will be as much twist, as much motion, and as much energy in one part of the rope as in any other; and if one chooses to call the right-handed twist positive, and the left-handed twist negative, he will have the mechanical phenomenon of energy-distribution and the terminology, analogous to what they are in an electric conductor. (fig. .) are the cases more dissimilar than the mechanical analogy would make them seem to be? are there any phenomena which imply that rotation is going on in an electric conductor? there are. an electric arc, which is a current in the air, and is, therefore, less constrained than it is in a conductor, rotates. especially marked is this when in front of the pole of a magnet; but the rotation may be noticed in an ordinary arc by looking at it with a stroboscope disk, rotated so as to make the light to the eye intermittent at the rate of four or five hundred per second. a ray of plane polarized light, parallel with a wire conveying a current, has its plane of vibration twisted to the right or left, as the current goes one way or the other through the wire, and to a degree that depends upon the distance it travels; not only so, but if the ray be sent, by reflection, back through the same field, it is twisted as much more--a phenomenon which convinces one that rotation is going on in the space through which the ray travels. if the ether through which the ray be sent were simply warped or in some static stress, the ray, after reflection, would be brought back to its original plane, which is not the case. this rotation in the ether is produced by what is going on in the wire. the ether waves called light are interpreted to imply that molecules originate them by their vibrations, and that there are as many ether waves per second as of molecular vibrations per second. in like manner, the implication is the same, that if there be rotations in the ether they must be produced by molecular rotation, and there must be as many rotations per second in the ether as there are molecular rotations that produce them. the space about a wire carrying a current is often pictured as filled with whorls indicating this motion (fig. ), and one must picture to himself, not the wire as a whole rotating, but each individual molecule independently. but one is aware that the molecules of a conductor are practically in contact with each other, and that if one for any reason rotates, the next one to it would, from frictional action, cause the one it touched to rotate in the opposite direction, whereas, the evidence goes to show that all rotation is in the same direction. [illustration: fig. .] how can this be explained mechanically? recall the kind of action that constitutes heat, that it is not translatory action in any degree, but vibratory, in the sense of a change of form of an elastic body, and this, too, of the atoms that make up the molecule of whatever sort. each atom is so far independent of every other atom in the molecule that it can vibrate in this way, else it could not be heated. the greater the amplitude of vibration, the more free space to move in, and continuous contact of atoms is incompatible with the mechanics of heat. there must, therefore, be impact and freedom alternating with each other in all degrees in a heated body. if, in any way, the atoms themselves _were_ made to rotate, their heat impacts not only would restrain the rotations, but the energy also of the rotation motion would increase the vibrations; that is, the heat would be correspondingly increased, which is what happens always when an electric current is in a conductor. it appears that the cooler a body is the less electric resistance it has, and the indications are that at absolute zero there is no resistance; that is, impacts do not retard rotation, but it is also apparent that any current sent through a conductor at that temperature would at once heat it. this is the same as saying that an electric current could not be sent through a conductor at absolute zero. so far, mechanical conceptions are in accordance with electrical phenomena, but there are several others yet to be noted. electrical phenomena has been explained as molecular or atomic phenomena, and there is one more in that category which is well enough known, and which is so important and suggestive, that the wonder is its significance has not been seen by those who have sought to interpret electrical phenomena. the reference is to the fact that electricity cannot be transmitted through a vacuum. an electric arc begins to spread out as the density of the air decreases, and presently it is extinguished. an induction spark that will jump two or three feet in air cannot be made to bridge the tenth of an inch in an ordinary vacuum. a vacuum is a perfect non-conductor of electricity. is there more than one possible interpretation to this, namely, that electricity is fundamentally a molecular and atomic phenomenon, and in the absence of molecules cannot exist? one may say, "electrical _action_ is not hindered by a vacuum," which is true, but has quite another interpretation than the implication that electricity is an ether phenomenon. the heat of the sun in some way gets to the earth, but what takes place in the ether is not heat-transmission. there is no heat in space, and no one is at liberty to say, or think, that there can be heat in the absence of matter. when heat has been transformed into ether waves, it is no longer heat, call it by what name one will. formerly, such waves were called heat-waves; no one, properly informed, does so now. in like manner, if electrical motions or conditions in matter be transformed, no matter how, it is no longer proper to speak of such transformed motions or conditions as electricity. thus, if electrical energy be transformed into heat, no one thinks of speaking of the latter as electrical. if the electrical energy be transformed into mechanical of any sort, no one thinks of calling the latter electrical because of its antecedent. if electrical motions be transformed into ether actions of any kind, why should we continue to speak of the transformed motions or energy as being electrical? electricity may be the antecedent, in the same sense as the mechanical motion of a bullet may be the antecedent of the heat developed when the latter strikes the target; and if it be granted that a vacuum is a perfect non-conductor of electricity, then it is manifestly improper to speak of any phenomenon in the ether as an electrical phenomenon. it is from the failure to make this distinction that most of the trouble has come in thinking on this subject. some have given all their attention to what goes on in matter, and have called that electricity; others have given their attention to what goes on in the ether, and have called that electricity, and some have considered both as being the same thing, and have been confounded. let us consider what is the relation between an electrified body and the ether about it. when a body is electrified, the latter at the same time creates an ether stress about it, which is called an electric field. the ether stress may be considered as a warp in the distribution of the energy about the body (fig. ), by the new positions given to the molecules by the process of electrification. it has been already said that the evidence from other sources is that atoms, rather than molecules, in larger masses, are what affect the ether. one is inclined to inquire for the evidence we have as to the constitution of matter or of atoms. there is only one hypothesis to-day that has any degree of probability; that is, the vortex-ring theory, which describes an atom as being a vortex-ring of ether in the ether. it possesses a definite amount of energy in virtue of the motion which constitutes it, and this motion differentiates it from the surrounding ether, giving it dimensions, elasticity, momentum, and the possibility of translatory, rotary, vibratory motions, and combinations of them. without going further into this, it is sufficient, for a mechanical conception, that one should have so much in mind, as it will vastly help in forming a mechanical conception of reactions between atoms and the ether. an exchange of energy between such an atom and the ether is not an exchange between different kinds of things, but between different conditions of the same thing. next, it should be remembered that all the elements are magnetic in some degree. this means that they are themselves magnets, and every magnet has a magnetic field unlimited in extent, which can almost be regarded as a part of itself. if a magnet of any size be moved, its field is moved with it, and if in any way the magnetism be increased or diminished, the field changes correspondingly. [illustration: fig. .] assume a straight bar electro-magnet in circuit, so that a current can be made intermittent, say, once a second. when the circuit is closed and the magnet is made, the field at once is formed and travels outwards at the rate of , miles per second. when the current stops, the field adjacent is destroyed. another closure develops the field again, which, like the other, travels outwards; and so there may be formed a series of waves in the ether, each , miles long, with an electro-magnetic antecedent. if the circuit were closed ten times a second, the waves would be , miles long; if , times a second, they would be but one mile long. if million of millions times a second, they would be but the forty-thousandth of an inch long, and would then affect the eye, and we should call them light-waves, but the latter would not differ from the first wave in any particular except in length. as it is proved that such electro-magnetic waves have all the characteristics of light, it follows that they must originate with electro-magnetic action, that is, in the changing magnetism of a magnetic body. this makes it needful to assume that the atoms which originate waves are magnets, as they are experimentally found to be. but how can a magnet, not subject to a varying current, change its magnetic field? the strength or density of a magnetic field depends upon the form of the magnet. when the poles are near together, the field is densest; when the magnet is bent back to a straight bar, the field is rarest or weakest, and a change in the form of the magnet from a u-form to a straight bar would result in a change of the magnetic field within its greatest limits. a few turns of wire--as has been already said--wound about the poles of an ordinary u-magnet, and connected to an ordinary magnetic telephone, will enable one, listening to the latter, to hear the pitch of the former loudly reproduced when the magnet is struck like a tuning-fork, so as to vibrate. this shows that the field of the magnet changes at the same rate as the vibrations. assume that the magnet becomes smaller and smaller until it is of the dimensions of an atom, say for an approximation, the fifty-millionth of an inch. it would still have its field; it would still be elastic and capable of vibration, but at an enormously rapid rate; but its vibration would change its field in the same way, and so there would be formed those waves in the ether, which, because they are so short that they can affect the eye, we call light. the mechanical conceptions are legitimate, because based upon experiments having ranges through nearly the whole gamut as waves in ether. the idea implies that every atom has what may be loosely called an electro-magnetic grip upon the whole of the ether, and any change in the former brings some change in the latter. lastly, the phenomenon called induction may be mechanically conceived. it is well known that a current in a conductor makes a magnet of the wire, and gives it an electro-magnetic field, so that other magnets in its neighbourhood are twisted in a way tending to set them at right angles to the wire. also, if another wire be adjacent to the first, an electric current having an opposite direction is induced in it. thus: consider a permanent magnet a (fig. ), free to turn on an axis in the direction of the arrow. if there be other free magnets, b and c, in line, they will assume such positions that their similar poles all point one way. let a be twisted to a position at right angles, then b will turn, but in the opposite direction, and c in similar. that is, if a turn in the direction of the hands of a clock, b and c will turn in opposite directions. these are simply the observed movements of large magnets. imagine that these magnets be reduced to atomic dimensions, yet retaining their magnetic qualities, poles and fields. would they not evidently move in the same way and for the same reason? if it be true, that a magnet field always so acts upon another as to tend by rotation to set the latter into a certain position, with reference to the stress in that field, then, _wherever there is a changing magnetic field, there the atoms are being adjusted by it_. [illustration: fig. .] suppose we have a line of magnetic needles free to turn, hundreds or thousands of them, but disarranged. let a strong magnetic field be produced at one end of the line. the field would be strongest and best conducted along the magnet line, but every magnet in the line would be compelled to rotate, and if the first were kept rotating, the rotation would be kept up along the whole line. this would be a mechanical illustration of how an electric current travels in a conductor. the rotations are of the atomic sort, and are at right angles to the direction of the conductor. that which makes the magnets move is inductive magnetic ether stress, but the advancing motion represents mechanical energy of rotation, and it is this motion, with the resulting friction, which causes the heat in a conductor. what is important to note is, that the action in the ether is not electric action, but more properly the result of electro-magnetic action. whatever name be given to it, and however it comes about, there is no good reason for calling any kind of ether action electrical. electric action, like magnetic action, begins and ends in matter. it is subject to transformations into thermal and mechanical actions, also into ether stress--right-handed or left-handed--which, in turn, can similarly affect other matter, but with opposite polarities. in his _modern views of electricity_, prof. o. j. lodge warns us, quite rightly, that perhaps, after all, there is no such _thing_ as electricity--that electrification and electric energy may be terms to be kept for convenience; but if electricity as a term be held to imply a force, a fluid, an imponderable, or a thing which could be described by some one who knew enough, then it has no degree of probability, for spinning atomic magnets seem capable of developing all the electrical phenomena we meet. it must be thought of as a _condition_ and not as an entity. the end _richard clay & sons, limited, london & bungay._ publications of the society for promoting christian knowledge. the romance of science. a series of books which shows that science has for the masses as great interest as, and more edification than, the romances of the day. _small post vo, cloth boards._ =coal, and what we get from it.= by professor raphael meldola, f.r.s., f.i.c. with several illustrations. _s._ _d._ =colour measurement and mixture.= by captain w. de w. abney, c.b., r.e. with numerous illustrations. _s._ _d._ =the making of flowers.= by the rev. professor george henslow, m.a., f.l.s. with several illustrations. _s._ _d._ =the birth and growth of worlds.= a lecture by professor a. h. green, m.a., f.r.s. _s._ =soap-bubbles, and the forces which mould them.= a course of lectures by c. v. boys, a.r.s.m., f.r.s. with numerous diagrams. _s._ _d._ =spinning tops.= by professor j. perry, m.e., d.sc., f.r.s. with numerous diagrams. _s._ _d._ =our secret friends and foes.= by p. f. frankland, f.r.s. with numerous illustrations. new edition, _s._ =diseases of plants.= by professor marshall ward, m.a., f.r.s., f.l.s. with numerous illustrations. _s._ _d._ =the story of a tinder-box.= by the late charles meymott tidy, m.b., m.s. with numerous illustrations. _s._ =time and tide.= a romance of the moon. by sir robert s. ball, ll.d., royal astronomer of ireland. with illustrations. third edition, revised. _s._ _d._ =the splash of a drop.= by professor a. m. worthington, f.r.s. with numerous illustrations. _s._ _d._ natural history rambles. _fcap. vo., with numerous woodcuts, cloth boards, s. d. each._ =in search of minerals.= by the late d. t. anstead, m.a., f.r.s. =lakes and rivers.= by c. o. groom napier, f.g.s. =lane and field.= by the late rev. j. g. wood, m.a., author of "man and his handiwork," &c. =mountain and moor.= by j. e. taylor, f.l.s., f.g.s., editor of "science-gossip." =ponds and ditches.= by m. c. cooke, m.a., ll.d. =the sea-shore.= by professor p. martin duncan, m.b. 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in these lectures i have sought to render clear a difficult but profoundly interesting subject. my aim has been not only to describe and illustrate in a familiar manner the principal laws and phenomena of light, but to point out the origin, and show the application, of the theoretic conceptions which underlie and unite the whole, and without which no real interpretation is possible. the lectures, as stated on the title-page, were delivered in the united states in - . i still retain a vivid and grateful remembrance of the cordiality with which they were received. my scope and object are briefly indicated in the 'summary and conclusion,' which, as recommended in a former edition, might be, not unfitly, read as an introduction to the volume. j.t. alp lusgen: _october_ . contents. lecture i. introductory uses of experiment early scientific notions sciences of observation knowledge of the ancients regarding light defects of the eye our instruments rectilineal propagation of light law of incidence and reflection sterility of the middle ages refraction discovery of snell partial and total reflection velocity of light roemer, bradley, foucault, and fizeau principle of least action descartes and the rainbow newton's experiments on the composition of solar light his mistake regarding achromatism synthesis of white light yellow and blue lights produce white by their mixture colours of natural bodies absorption mixture of pigments contrasted with mixture of lights lecture ii. origin of physical theories scope of the imagination newton and the emission theory verification of physical theories the luminiferous ether wave-theory of light thomas young fresnel and arago conception of wave-motion interference of waves constitution of sound-waves analogies of sound and light illustrations of wave-motion interference of sound waves optical illustrations pitch and colour lengths of the waves of light and rates of vibration of the ether-particles interference of light phenomena which first suggested the undulatory theory boyle and hooke the colours of thin plates the soap-bubble newton's rings theory of 'fits' its explanation of the rings overthrow of the theory diffraction of light colours produced by diffraction colours of mother-of-pearl. lecture iii. relation of theories to experience origin of the notion of the attraction of gravitation notion of polarity, how generated atomic polarity structural arrangements due to polarity architecture of crystals considered as an introduction to their action upon light notion of atomic polarity applied to crystalline structure experimental illustrations crystallization of water expansion by heat and by cold deportment of water considered and explained bearings of crystallization on optical phenomena refraction double refraction polarization action of tourmaline character of the beams emergent from iceland spar polarization by ordinary refraction and reflection depolarization. lecture iv. chromatic phenomena produced by crystals in polarized light the nicol prism polarizer and analyzer action of thick and thin plates of selenite colours dependent on thickness resolution of polarized beam into two others by the selenite one of them more retarded than the other recompounding of the two systems of waves by the analyzer interference thus rendered possible consequent production of colours action of bodies mechanically strained or pressed action of sonorous vibrations action of glass strained or pressed by heat circular polarization chromatic phenomena produced by quartz the magnetization of light rings surrounding the axes of crystals biaxal and uniaxal crystals grasp of the undulatory theory the colour and polarization of sky-light generation of artificial skies. lecture v. range of vision not commensurate with range of radiation the ultra-violet rays fluorescence the rendering of invisible rays visible vision not the only sense appealed to by the solar and electric beam heat of beam combustion by total beam at the foci of mirrors and lenses combustion through ice-lens ignition of diamond search for the rays here effective sir william herschel's discovery of dark solar rays invisible rays the basis of the visible detachment by a ray-filter of the invisible rays from the visible combustion at dark foci conversion of heat-rays into light-rays calorescence part played in nature by dark rays identity of light and radiant heat invisible images reflection, refraction, plane polarization, depolarization, circular polarization, double refraction, and magnetization of radiant heat lecture vi. principles of spectrum analysis prismatic analysis of the light of incandescent vapours discontinuous spectra spectrum bands proved by bunsen and kirchhoff to be characteristic of the vapour discovery of rubidium, cæsium, and thallium relation of emission to absorption the lines of fraunhofer their explanation by kirchhoff solar chemistry involved in this explanation foucault's experiment principles of absorption analogy of sound and light experimental demonstration of this analogy recent applications of the spectroscope summary and conclusion appendix. on the spectra of polarized light measurement of the waves of light index on light lecture i. introductory uses of experiment early scientific notions sciences of observation knowledge of the ancients regarding light defects of the eye our instruments rectilineal propagation of light law of incidence and reflection sterility of the middle ages refraction discovery of snell partial and total reflection velocity of light roemer, bradley, foucault, and fizeau principle of least action descartes and the rainbow newton's experiments on the composition of solar light his mistake as regards achromatism synthesis of white light yellow and blue lights produce white by their mixture colours of natural bodies absorption mixture of pigments contrasted with mixture of lights. § . _introduction_. some twelve years ago i published, in england, a little book entitled the 'glaciers of the alps,' and, a couple of years subsequently, a second book, entitled 'heat a mode of motion.' these volumes were followed by others, written with equal plainness, and with a similar aim, that aim being to develop and deepen sympathy between science and the world outside of science. i agreed with thoughtful men[ ] who deemed it good for neither world to be isolated from the other, or unsympathetic towards the other, and, to lessen this isolation, at least in one department of science, i swerved, for a time, from those original researches which have been the real pursuit and pleasure of my life. the works here referred to were, for the most part, republished by the messrs. appleton of new york,[ ] under the auspices of a man who is untiring in his efforts to diffuse sound scientific knowledge among the people of the united states; whose energy, ability, and single-mindedness, in the prosecution of an arduous task, have won for him the sympathy and support of many of us in 'the old country.' i allude to professor youmans. quite as rapidly as in england, the aim of these works was understood and appreciated in the united states, and they brought me from this side of the atlantic innumerable evidences of good-will. year after year invitations reached me[ ] to visit america, and last year ( ) i was honoured with a request so cordial, signed by five-and-twenty names, so distinguished in science, in literature, and in administrative position, that i at once resolved to respond to it by braving not only the disquieting oscillations of the atlantic, but the far more disquieting ordeal of appearing in person before the people of the united states. this invitation, conveyed to me by my accomplished friend professor lesley, of philadelphia, and preceded by a letter of the same purport from your scientific nestor, the celebrated joseph henry, of washington, desired that i should lecture in some of the principal cities of the union. this i agreed to do, though much in the dark as to a suitable subject. in answer to my inquiries, however, i was given to understand that a course of lectures, showing the uses of experiment in the cultivation of natural knowledge, would materially promote scientific education in this country. and though such lectures involved the selection of weighty and delicate instruments, and their transfer from place to place, i determined to meet the wishes of my friends, as far as the time and means at my disposal would allow. § . _subject of the course. source of light employed._ experiments have two great uses--a use in discovery, and a use in tuition. they were long ago defined as the investigator's language addressed to nature, to which she sends intelligible replies. these replies, however, usually reach the questioner in whispers too feeble for the public ear. but after the investigator comes the teacher, whose function it is so to exalt and modify the experiments of his predecessor, as to render them fit for public presentation. this secondary function i shall endeavour, in the present instance, to fulfil. taking a single department of natural philosophy as my subject, i propose, by means of it, to illustrate the growth of scientific knowledge under the guidance of experiment. i wish, in the first place, to make you acquainted with certain elementary phenomena; then to point out to you how the theoretical principles by which phenomena are explained take root in the human mind, and finally to apply these principles to the whole body of knowledge covered by the lectures. the science of optics lends itself particularly well to this mode of treatment, and on it, therefore, i propose to draw for the materials of the present course. it will be best to begin with the few simple facts regarding light which were known to the ancients, and to pass from them, in historic gradation, to the more abstruse discoveries of modern times. all our notions of nature, however exalted or however grotesque, have their foundation in experience. the notion of personal volition in nature had this basis. in the fury and the serenity of natural phenomena the savage saw the transcript of his own varying moods, and he accordingly ascribed these phenomena to beings of like passions with himself, but vastly transcending him in power. thus the notion of _causality_--the assumption that natural things did not come of themselves, but had unseen antecedents--lay at the root of even the savage's interpretation of nature. out of this bias of the human mind to seek for the causes of phenomena all science has sprung. we will not now go back to man's first intellectual gropings; much less shall we enter upon the thorny discussion as to how the groping man arose. we will take him at that stage of his development, when he became possessed of the apparatus of thought and the power of using it. for a time--and that historically a long one--he was limited to mere observation, accepting what nature offered, and confining intellectual action to it alone. the apparent motions of sun and stars first drew towards them the questionings of the intellect, and accordingly astronomy was the first science developed. slowly, and with difficulty, the notion of natural forces took root in the human mind. slowly, and with difficulty, the science of mechanics had to grow out of this notion; and slowly at last came the full application of mechanical principles to the motions of the heavenly bodies. we trace the progress of astronomy through hipparchus and ptolemy; and, after a long halt, through copernicus, galileo, tycho brahe, and kepler; while from the high table-land of thought occupied by these men, newton shoots upwards like a peak, overlooking all others from his dominant elevation. but other objects than the motions of the stars attracted the attention of the ancient world. light was a familiar phenomenon, and from the earliest times we find men's minds busy with the attempt to render some account of it. but without _experiment_, which belongs to a later stage of scientific development, little progress could be here made. the ancients, accordingly, were far less successful in dealing with light than in dealing with solar and stellar motions. still they did make some progress. they satisfied themselves that light moved in straight lines; they knew also that light was reflected from polished surfaces, and that the angle of incidence was equal to the angle of reflection. these two results of ancient scientific curiosity constitute the starting-point of our present course of lectures. but in the first place it will be useful to say a few words regarding the source of light to be employed in our experiments. the rusting of iron is, to all intents and purposes, the slow burning of iron. it develops heat, and, if the heat be preserved, a high temperature may be thus attained. the destruction of the first atlantic cable was probably due to heat developed in this way. other metals are still more combustible than iron. you may ignite strips of zinc in a candle flame, and cause them to burn almost like strips of paper. but we must now expand our definition of combustion, and include under this term, not only combustion in air, but also combustion in liquids. water, for example, contains a store of oxygen, which may unite with, and consume, a metal immersed in it; it is from this kind of combustion that we are to derive the heat and light employed in our present course. the generation of this light and of this heat merits a moment's attention. before you is an instrument--a small voltaic battery--in which zinc is immersed in a suitable liquid. an attractive force is at this moment exerted between the metal and the oxygen of the liquid; actual combination, however, being in the first instance avoided. uniting the two ends of the battery by a thick wire, the attraction is satisfied, the oxygen unites with the metal, zinc is consumed, and heat, as usual, is the result of the combustion. a power which, for want of a better name, we call an electric current, passes at the same time through the wire. cutting the thick wire in two, let the severed ends be united by a thin one. it glows with a white heat. whence comes that heat? the question is well worthy of an answer. suppose in the first instance, when the thick wire is employed, that we permit the action to continue until grains of zinc are consumed, the amount of heat generated in the battery would be capable of accurate numerical expression. let the action then continue, with the thin wire glowing, until grains of zinc are consumed. will the amount of heat generated in the battery be the same as before? no; it will be less by the precise amount generated in the thin wire outside the battery. in fact, by adding the internal heat to the external, we obtain for the combustion of grains of zinc a total which never varies. we have here a beautiful example of that law of constancy as regards natural energies, the establishment of which is the greatest achievement of modern science. by this arrangement, then, we are able to burn our zinc at one place, and to exhibit the effects of its combustion at another. in new york, for example, we may have our grate and fuel; but the heat and light of our fire may be made to appear at san francisco. [illustration: fig. .] removing the thin wire and attaching to the severed ends of the thick one two rods of coke we obtain, on bringing the rods together (as in fig. ), a small star of light. now, the light to be employed in our lectures is a simple exaggeration of this star. instead of being produced by ten cells, it is produced by fifty. placed in a suitable camera, provided with a suitable lens, this powerful source will give us all the light necessary for our experiments. and here, in passing, i am reminded of the common delusion that the works of nature, the human eye included, are theoretically perfect. the eye has grown for ages _towards_ perfection; but ages of perfecting may be still before it. looking at the dazzling light from our large battery, i see a luminous globe, but entirely fail to see the shape of the coke-points whence the light issues. the cause may be thus made clear: on the screen before you is projected an image of the carbon points, the _whole_ of the glass lens in front of the camera being employed to form the image. it is not sharp, but surrounded by a halo which nearly obliterates the carbons. this arises from an imperfection of the glass lens, called its _spherical aberration_, which is due to the fact that the circumferential and central rays have not the same focus. the human eye labours under a similar defect, and from this, and other causes, it arises that when the naked light from fifty cells is looked at the blur of light upon the retina is sufficient to destroy the definition of the retinal image of the carbons. a long list of indictments might indeed be brought against the eye--its opacity, its want of symmetry, its lack of achromatism, its partial blindness. all these taken together caused helmholt to say that, if any optician sent him an instrument so defective, he would be justified in sending it back with the severest censure. but the eye is not to be judged from the standpoint of theory. it is not perfect, but is on its way to perfection. as a practical instrument, and taking the adjustments by which its defects are neutralized into account, it must ever remain a marvel to the reflecting mind. § . _rectilineal propagation of light. elementary experiments. law of reflection._ the ancients were aware of the rectilineal propagation of light. they knew that an opaque body, placed between the eye and a point of light, intercepted the light of the point. possibly the terms 'ray' and 'beam' may have been suggested by those straight spokes of light which, in certain states of the atmosphere, dart from the sun at his rising and his setting. the rectilineal propagation of light may be illustrated by permitting the solar light to enter, through a small aperture in a window-shutter, a dark room in which a little smoke has been diffused. in pure _air_ you cannot see the beam, but in smoky air you can, because the light, which passes unseen through the air, is scattered and revealed by the smoke particles, among which the beam pursues a straight course. the following instructive experiment depends on the rectilineal propagation of light. make a small hole in a closed window-shutter, before which stands a house or a tree, and place within the darkened room a white screen at some distance from the orifice. every straight ray proceeding from the house, or tree, stamps its colour upon the screen, and the sum of all the rays will, therefore, be an image of the object. but, as the rays cross each other at the orifice, the image is inverted. at present we may illustrate and expand the subject thus: in front of our camera is a large opening (l, fig. ), from which the lens has been removed, and which is closed at present by a sheet of tin-foil. pricking by means of a common sewing-needle a small aperture in the tin-foil, an inverted image of the carbon-points starts forth upon the screen. a dozen apertures will give a dozen images, a hundred a hundred, a thousand a thousand. but, as the apertures come closer to each other, that is to say, as the tin-foil between the apertures vanishes, the images overlap more and more. removing the tin-foil altogether, the screen becomes uniformly illuminated. hence the light upon the screen may be regarded as the overlapping of innumerable images of the carbon-points. in like manner the light upon every white wall, on a cloudless day, may be regarded as produced by the superposition of innumerable images of the sun. [illustration: fig. .] the law that the angle of incidence is equal to the angle of reflection has a bearing upon theory, to be subsequently mentioned, which renders its simple illustration here desirable. a straight lath (pointing to the figure on the arc in fig. ) is fixed as an index perpendicular to a small looking-glass (m), capable of rotation. we begin by receiving a beam of light upon the glass which is reflected back along the line of its incidence. the index being then turned, the mirror turns with it, and at each side of the index the incident and the reflected beams (l _o_, _o_ r) track themselves through the dust of the room. the mere inspection of the two angles enclosed between the index and the two beams suffices to show their equality; while if the graduated arc be consulted, the arc from to _m_ is found accurately equal to the arc from to _n_. the complete expression of the law of reflection is, not only that the angles of incidence and reflection are equal, but that the incident and reflected rays always lie in a plane perpendicular to the reflecting surface. [illustration: fig. .] this simple apparatus enables us to illustrate another law of great practical importance, namely, that when a mirror rotates, the angular velocity of a beam reflected from it is twice that of the reflecting mirror. a simple experiment will make this plain. the arc (_m n_, fig. ) before you is divided into ten equal parts, and when the incident beam and the index cross the zero of the graduation, both the incident and reflected beams are horizontal. moving the index of the mirror to , the reflected beam cuts the arc at ; moving the index to , the arc is cut at ; moving the index to , the arc is cut at ; moving the index at , the arc is cut at ; finally, moving the index to , the arc is cut at (as in the figure). in every case the reflected beam moves through twice the angle passed over by the mirror. one of the principal problems of science is to help the senses of man, by carrying them into regions which could never be attained without that help. thus we arm the eye with the telescope when we want to sound the depths of space, and with the microscope when we want to explore motion and structure in their infinitesimal dimensions. now, this law of angular reflection, coupled with the fact that a beam of light possesses no weight, gives us the means of magnifying small motions to an extraordinary degree. thus, by attaching mirrors to his suspended magnets, and by watching the images of divided scales reflected from the mirrors, the celebrated gauss was able to detect the slightest thrill of variation on the part of the earth's magnetic force. by a similar arrangement the feeble attractions and repulsions of the diamagnetic force have been made manifest. the minute elongation of a bar of metal, by the mere warmth of the hand, may be so magnified by this method, as to cause the index-beam to move through or feet. the lengthening of a bar of iron when it is magnetized may be also thus demonstrated. helmholtz long ago employed this method of rendering evident to his students the classical experiments of du bois raymond on animal electricity; while in sir william thomson's reflecting galvanometer the principle receives one of its latest and most important applications. § . _the refraction of light. total reflection._ for more than a thousand years no step was taken in optics beyond this law of reflection. the men of the middle ages, in fact, endeavoured, on the one hand, to develop the laws of the universe _à priori_ out of their own consciousness, while many of them were so occupied with the concerns of a future world that they looked with a lofty scorn on all things pertaining to this one. speaking of the natural philosophers of his time, eusebius says, 'it is not through ignorance of the things admired by them, but through contempt of their useless labour, that we think little of these matters, turning our souls to the exercise of better things.' so also lactantius--'to search for the causes of things; to inquire whether the sun be as large as he seems; whether the moon is convex or concave; whether the stars are fixed in the sky, or float freely in the air; of what size and of what material are the heavens; whether they be at rest or in motion; what is the magnitude of the earth; on what foundations is it suspended or balanced;--to dispute and conjecture upon such matters is just as if we chose to discuss what we think of a city in a remote country, of which we never heard but the name.' as regards the refraction of light, the course of real inquiry was resumed in by an arabian philosopher named alhazen. then it was taken up in succession by roger bacon, vitellio, and kepler. one of the most important occupations of science is the determination, by precise measurements, of the quantitative relations of phenomena; the value of such measurements depending greatly upon the skill and conscientiousness of the man who makes them. vitellio appears to have been both skilful and conscientious, while kepler's habit was to rummage through the observations of his predecessors, to look at them in all lights, and thus distil from them the principles which united them. he had done this with the astronomical measurements of tycho brahe, and had extracted from them the celebrated 'laws of kepler.' he did it also with vitellio's measurements of refraction. but in this case he was not successful. the principle, though a simple one, escaped him, and it was first discovered by willebrord snell, about the year . less with the view of dwelling upon the phenomenon itself than of introducing it in a form which will render subsequently intelligible to you the play of theoretic thought in newton's mind, the fact of refraction may be here demonstrated. i will not do this by drawing the course of the beam with chalk on a black board, but by causing it to mark its own white track before you. a shallow circular vessel (rig, fig. ), half filled with water, rendered slightly turbid by the admixture of a little milk, or the precipitation of a little mastic, is placed with its glass front vertical. by means of a small plane reflector (m), and through a slit (i) in the hoop surrounding the vessel, a beam of light is admitted in any required direction. it impinges upon the water (at o), enters it, and tracks itself through the liquid in a sharp bright band (o g). meanwhile the beam passes unseen through the air above the water, for the air is not competent to scatter the light. a puff of smoke into this space at once reveals the track of the incident-beam. if the incidence be vertical, the beam is unrefracted. if oblique, its refraction at the common surface of air and water (at o) is rendered clearly visible. it is also seen that _reflection_ (along o r) accompanies refraction, the beam dividing itself at the point of incidence into a refracted and a reflected portion.[ ] [illustration: fig. .] the law by which snell connected together all the measurements executed up to his time, is this: let a b c d (fig. ) represent the outline of our circular vessel, a c being the water-line. when the beam is incident along b e, which is perpendicular to a c, there is no refraction. when it is incident along _m_ e, there is refraction: it is bent at e and strikes the circle at _n_. when it is incident along _m'_ e there is also refraction at e, the beam striking the point _n'_. from the ends of the two incident beams, let the perpendiculars _m_ _o_, _m'_ _o'_ be drawn upon b d, and from the ends of the refracted beams let the perpendiculars _p_ _n_, _p'_ _n'_ be also drawn. measure the lengths of _o m_ and of _p_ _n_, and divide the one by the other. you obtain a certain quotient. in like manner divide _m'_ _o'_ by the corresponding perpendicular _p'_ _n'_; you obtain precisely the same quotient. snell, in fact, found this quotient to be _a constant quantity_ for each particular substance, though it varied in amount from one substance to another. he called the quotient the _index of refraction_. [illustration fig. ] in all cases where the light is incident from air upon the surface of a solid or a liquid, or, to speak more generally, when the incidence is from a less highly refracting to a more highly refracting medium, the reflection is _partial_. in this case the most powerfully reflecting substances either transmit or absorb a portion of the incident light. at a perpendicular incidence water reflects only rays out of every , ; glass reflects only rays, while mercury reflects when the rays strike the surface obliquely the reflection is augmented. at an incidence of °, for example, water reflects rays, at ° it reflects rays, at ° rays; while at an incidence of ½°, where the light almost grazes the surface, it reflects rays out of every , . thus, as the obliquity increases, the reflection from water approaches, and finally quite overtakes, the perpendicular reflection from mercury; but at no incidence, however great, when the incidence is from air, is the reflection from water, mercury, or any other substance, _total_. still, total reflection may occur, and with a view to understanding its subsequent application in the nicol's prism, it is necessary to state when it occurs. this leads me to the enunciation of a principle which underlies all optical phenomena--the principle of reversibility.[ ] in the case of refraction, for instance, when the ray passes obliquely from air into water, it is bent _towards_ the perpendicular; when it passes from water to air, it is bent _from_ the perpendicular, and accurately reverses its course. thus in fig. , if _m_ e _n_ be the track of a ray in passing from air into water, _n_ e _m_ will be its track in passing from water into air. let us push this principle to its consequences. supposing the light, instead of being incident along _m_ e or _m'_ e, were incident as close as possible along c e (fig. ); suppose, in other words, that it just grazes the surface before entering the water. after refraction it will pursue say the course e _n_''. conversely, if the light start from _n_'', and be incident at e, it will, on escaping into the air, just graze the surface of the water. the question now arises, what will occur supposing the ray from the water to follow the course _n_''' e, which lies beyond _n_'' e? the answer is, it will not quit the water at all, but will be _totally_ reflected (along e _x_). at the under surface of the water, moreover, the law is just the same as at its upper surface, the angle of incidence (d e _n_''') being equal to the angle of reflection (d e _x_). [illustration: fig. ] total reflection may be thus simply illustrated:--place a shilling in a drinking-glass, and tilt the glass so that the light from the shilling shall fall with the necessary obliquity upon the water surface above it. look upwards through the water towards that surface, and you see the image of the shilling shining there as brightly as the shilling itself. thrust the closed end of an empty test-tube into water, and incline the tube. when the inclination is sufficient, horizontal light falling upon the tube cannot enter the air within it, but is totally reflected upward: when looked down upon, such a tube looks quite as bright as burnished silver. pour a little water into the tube; as the liquid rises, total reflection is abolished, and with it the lustre, leaving a gradually diminishing shining zone, which disappears wholly when the level of the water within the tube reaches that without it. any glass tube, with its end stopped water-tight, will produce this effect, which is both beautiful and instructive. total reflection never occurs except in the attempted passage of a ray from a more refracting to a less refracting medium; but in this case, when the obliquity is sufficient, it always occurs. the mirage of the desert, and other phantasmal appearances in the atmosphere, are in part due to it. when, for example, the sun heats an expanse of sand, the layer of air in contact with the sand becomes lighter and less refracting than the air above it: consequently, the rays from a distant object, striking very obliquely on the surface of the heated stratum, are sometimes totally reflected upwards, thus producing images similar to those produced by water. i have seen the image of a rock called mont tombeline distinctly reflected from the heated air of the strand of normandy near avranches; and by such delusive appearances the thirsty soldiers of the french army in egypt were greatly tantalised. the angle which marks the limit beyond which total reflection takes place is called the _limiting angle_ (it is marked in fig. by the strong line e _n_''). it must evidently diminish as the refractive index increases. for water it is ½°, for flint glass ° ', and for diamond ° '. thus all the light incident from two complete quadrants, or °, in the case of diamond, is condensed into an angular space of ° ' (twice ° ') by refraction. coupled with its great refraction, are the great dispersive and great reflective powers of diamond; hence the extraordinary radiance of the gem, both as regards white light and prismatic light. § . _velocity of light. aberration. principle of least action._ in a great impulse was given to optics by astronomy. in that year olav roemer, a learned dane, was engaged at the observatory of paris in observing the eclipses of jupiter's moons. the planet, whose distance from the sun is , , miles, has four satellites. we are now only concerned with the one nearest to the planet. roemer watched this moon, saw it move round the planet, plunge into jupiter's shadow, behaving like a lamp suddenly extinguished: then at the other edge of the shadow he saw it reappear, like a lamp suddenly lighted. the moon thus acted the part of a signal light to the astronomer, and enabled him to tell exactly its time of revolution. the period between two successive lightings up of the lunar lamp he found to be hours, minutes, and seconds. this measurement of time was so accurate, that having determined the moment when the moon emerged from the shadow, the moment of its hundredth appearance could also be determined. in fact, it would be times hours, minutes, seconds, after the first observation. roemer's first observation was made when the earth was in the part of its orbit nearest jupiter. about six months afterwards, the earth being then at the opposite side of its orbit, when the little moon ought to have made its hundredth appearance, it was found unpunctual, being fully minutes behind its calculated time. its appearance, moreover, had been growing gradually later, as the earth retreated towards the part of its orbit most distant from jupiter. roemer reasoned thus: 'had i been able to remain at the other side of the earth's orbit, the moon might have appeared always at the proper instant; an observer placed there would probably have seen the moon minutes ago, the retardation in my case being due to the fact that the light requires minutes to travel from the place where my first observation was made to my present position.' this flash of genius was immediately succeeded by another. 'if this surmise be correct,' roemer reasoned, 'then as i approach jupiter along the other side of the earth's orbit, the retardation ought to become gradually less, and when i reach the place of my first observation, there ought to be no retardation at all.' he found this to be the case, and thus not only proved that light required time to pass through space, but also determined its rate of propagation. the velocity of light, as determined by roemer, is , miles in a second. for a time, however, the observations and reasonings of roemer failed to produce conviction. they were doubted by cassini, fontenelle, and hooke. subsequently came the unexpected corroboration of roemer by the english astronomer, bradley, who noticed that the fixed stars did not really appear to be fixed, but that they describe little orbits in the heavens every year. the result perplexed him, but bradley had a mind open to suggestion, and capable of seeing, in the smallest fact, a picture of the largest. he was one day upon the thames in a boat, and noticed that as long as his course remained unchanged, the vane upon his masthead showed the wind to be blowing constantly in the same direction, but that the wind appeared to vary with every change in the direction of his boat. 'here,' as whewell says, 'was the image of his case. the boat was the earth, moving in its orbit, and the wind was the light of a star.' we may ask, in passing, what, without the faculty which formed the 'image,' would bradley's wind and vane have been to him? a wind and vane, and nothing more. you will immediately understand the meaning of bradley's discovery. imagine yourself in a motionless railway-train, with a shower of rain descending vertically downwards. the moment the train begins to move, the rain-drops begin to slant, and the quicker the motion of the train the greater is the obliquity. in a precisely similar manner the rays from a star, vertically overhead, are caused to slant by the motion of the earth through space. knowing the speed of the train, and the obliquity of the falling rain, the velocity of the drops may be calculated; and knowing the speed of the earth in her orbit, and the obliquity of the rays due to this cause, we can calculate just as easily the velocity of light. bradley did this, and the 'aberration of light,' as his discovery is called, enabled him to assign to it a velocity almost identical with that deduced by roemer from a totally different method of observation. subsequently fizeau, and quite recently cornu, employing not planetary or stellar distances, but simply the breadth of the city of paris, determined the velocity of light: while foucault--a man of the rarest mechanical genius--solved the problem without quitting his private room. owing to an error in the determination of the earth's distance from the sun, the velocity assigned to light by both roemer and bradley is too great. with a close approximation to accuracy it may be regarded as , miles a second. by roemer's discovery, the notion entertained by descartes, and espoused by hooke, that light is propagated instantly through space, was overthrown. but the establishment of its motion through stellar space led to speculations regarding its velocity in transparent terrestrial substances. the 'index of refraction' of a ray passing from air into water is / . newton assumed these numbers to mean that the velocity of light in water being , its velocity in air is ; and he deduced the phenomena of refraction from this assumption. huyghens took the opposite and truer view. according to this great man, the velocity of light in water being , its velocity in air is ; but both in newton's time and ours the same great principle determined, and determines, the course of light in all cases. in passing from point to point, whatever be the media in its path, or however it may be refracted or reflected, light takes the course which occupies _least time_. thus in fig. , taking its velocity in air and in water into account, the light reaches g from i more rapidly by travelling first to o, and there changing its course, than if it proceeded straight from i to g. this is readily comprehended, because, in the latter case, it would pursue a greater distance through the water, which is the more retarding medium. § . _descartes' explanation of the rainbow_. snell's law of refraction is one of the corner-stones of optical science, and its applications to-day are million-fold. immediately after its discovery descartes applied it to the explanation of the rainbow. a beam of solar light falling obliquely upon a rain-drop is refracted on entering the drop. it is in part reflected at the back of the drop, and on emerging it is again refracted. by these two refractions, and this single reflection, the light is sent to the eye of an observer facing the drop, and with his back to the sun. conceive a line drawn from the sun, through the back of his head, to the observer's eye and prolonged beyond it. conceive a second line drawn from the shower to the eye, and enclosing an angle of ½° with the line drawn from the sun. along this second line a rain-drop when struck by a sunbeam will send red light to the eye. every other drop similarly situated, that is, every drop at an angular distance of ½° from the line through the sun and eye, will do the same. a circular band of red light is thus formed, which may be regarded as the boundary of the base of a cone, with its apex at the observer's eye. because of the magnitude of the sun, the angular width of this red band will be half a degree. from the eye of the observer conceive another line to be drawn, enclosing an angle, not of ½°, but of ½°, with the prolongation of the line drawn from the sun. along this other line a rain-drop, at its remote end, when struck by a solar beam, will send violet light to the eye. all drops at the same angular distance will do the same, and we shall therefore obtain a band of violet light of the same width as the red band. these two bands constitute the limiting colours of the rainbow, and between them the bands corresponding to the other colours lie. thus the line drawn from the eye to the _middle_ of the bow, and the line drawn through the eye to the sun, always enclose an angle of about °. to account for this was the great difficulty, which remained unsolved up to the time of descartes. taking a pen in hand, and calculating by means of snell's law the track of every ray through a raindrop, descartes found that, at one particular angle, the rays, reflected at its back, emerged from the drop _almost parallel to each other_. they were thus enabled to preserve their intensity through long atmospheric distances. at all other angles the rays quitted the drop _divergent_, and through this divergence became so enfeebled as to be practically lost to the eye. the angle of parallelism here referred to was that of forty-one degrees, which observation had proved to be invariably associated with the rainbow. from what has been said, it is clear that two observers standing beside each other, or one above the other, nay, that even the two eyes of the same observer, do not see exactly the same bow. the position of the base of the cone changes with that of its apex. and here we have no difficulty in answering a question often asked--namely, whether a rainbow is ever seen reflected in water. seeing two bows, the one in the heavens, the other in the water, you might be disposed to infer that the one bears the same relation to the other that a tree upon the water's edge bears to its reflected image. the rays, however, which reach an observer's eye after reflection from the water, and which form a bow in the water, would, were their course from the shower uninterrupted, converge to a point vertically under the observer, and as far below the level of the water as his eye is above it. but under no circumstances could an eye above the water-level and one below it see the same bow--in other words, the self-same drops of rain cannot form the reflected bow and the bow seen directly in the heavens. the reflected bow, therefore, is not, in the usual optical sense of the term, the _image_ of the bow seen in the sky. § . _analysis and synthesis of light. doctrine of colours_. in the rainbow a new phenomenon was introduced--the phenomenon of colour. and here we arrive at one of those points in the history of science, when great men's labours so intermingle that it is difficult to assign to each worker his precise meed of honour. descartes was at the threshold of the discovery of the composition of solar light; but for newton was reserved the enunciation of the true law. he went to work in this way: through the closed window-shutter of a room he pierced an orifice, and allowed a thin sunbeam to pass through it. the beam stamped a round white image of the sun on the opposite wall of the room. in the path of this beam newton placed a prism, expecting to see the beam refracted, but also expecting to see the image of the sun, after refraction, still round. to his astonishment, it was drawn out to an image with a length five times its breadth. it was, moreover, no longer white, but divided into bands of different colours. newton saw immediately that solar light was _composite_, not simple. his elongated image revealed to him the fact that some constituents of the light were more deflected by the prism than others, and he concluded, therefore, that white light was a mixture of lights of different colours, possessing different degrees of refrangibility. let us reproduce this celebrated experiment. on the screen is now stamped a luminous disk, which may stand for newton's image of the sun. causing the beam (from the aperture l, fig. ) which produces the disk to pass through a lens (e), we form a sharp image of the aperture. placing in the track of the beam a prism (p), we obtain newton's coloured image, with its red and violet ends, which he called a _spectrum_. newton divided the spectrum into seven parts--red, orange, yellow, green, blue, indigo, violet; which are commonly called the seven primary or prismatic colours. the drawing out of the white light into its constituent colours is called _dispersion_. [illustration: fig. .] this was the first _analysis_ of solar light by newton; but the scientific mind is fond of verification, and never neglects it where it is possible. newton completed his proof by _synthesis_ in this way: the spectrum now before you is produced by a glass prism. causing the decomposed beam to pass through a second similar prism, but so placed that the colours are refracted back and reblended, the perfectly white luminous disk is restored. [illustration: fig. .] in this case, refraction and dispersion are simultaneously abolished. are they always so? can we have the one without the other? it was newton's conclusion that we could not. here he erred, and his error, which he maintained to the end of his life, retarded the progress of optical discovery. dollond subsequently proved that by combining two different kinds of glass, the colours can be extinguished, still leaving a residue of refraction, and he employed this residue in the construction of achromatic lenses--lenses yielding no colour--which newton thought an impossibility. by setting a water-prism--water contained in a wedge-shaped vessel with glass sides (b, fig. )--in opposition to a wedge of glass (to the right of b), this point can be illustrated before you. we have first of all the position (dotted) of the unrefracted beam marked upon the screen; then we produce the narrow water-spectrum (w); finally, by introducing a flint-glass prism, we refract the beam back, until the colour disappears (at a). the image of the slit is now _white_; but though the dispersion is abolished, there remains a very sensible amount of refraction. this is the place to illustrate another point bearing upon the instrumental means employed in these lectures. bodies differ widely from each other as to their powers of refraction and dispersion. note the position of the water-spectrum upon the screen. altering in no particular the wedge-shaped vessel, but simply substituting for the water the transparent bisulphide of carbon, you notice how much higher the beam is thrown, and how much richer is the display of colour. to augment the size of our spectrum we here employ (at l) a slit, instead of a circular aperture.[ ] [illustration: fig. .] the synthesis of white light may be effected in three ways, all of which are worthy of attention: here, in the first instance, we have a rich spectrum produced by the decomposition of the beam (from l, fig. ). one face of the prism (p) is protected by a diaphragm (not shown in the figure), with a longitudinal slit, through which the beam passes into the prism. it emerges decomposed at the other side. i permit the colours to pass through a cylindrical lens (c), which so squeezes them together as to produce upon the screen a sharply defined rectangular image of the longitudinal slit. in that image the colours are reblended, and it is perfectly white. between the prism and the cylindrical lens may be seen the colours, tracking themselves through the dust of the room. cutting off the more refrangible fringe by a card, the rectangle is seen red: cutting off the less refrangible fringe, the rectangle is seen blue. by means of a thin glass prism (w), i deflect one portion of the colours, and leave the residual portion. on the screen are now two coloured rectangles produced in this way. these are _complementary_ colours--colours which, by their union, produce white. note, that by judicious management, one of these colours is rendered _yellow_, and the other _blue_. i withdraw the thin prism; yellow and blue immediately commingle, and we have _white_ as the result of their union. on our way, then, we remove the fallacy, first exposed by wünsch, and afterwards independently by helmholtz, that the mixture of blue and yellow lights produces green. restoring the circular aperture, we obtain once more a spectrum like that of newton. by means of a lens, we can gather up these colours, and build them together, not to an image of the aperture, but to an image of the carbon-points themselves. finally, by means of a rotating disk, on which are spread in sectors the colours of the spectrum, we blend together the prismatic colours in the eye itself, and thus produce the impression of whiteness. having unravelled the interwoven constituents of white light, we have next to inquire, what part the constitution so revealed enables this agent to play in nature? to it we owe all the phenomena of colour, and yet not to it alone; for there must be a certain relationship between the ultimate particles of natural bodies and white light, to enable them to extract from it the luxury of colour. but the function of natural bodies is here _selective_, not _creative_. there is no colour _generated_ by any natural body whatever. natural bodies have showered upon them, in the white light of the sun, the sum total of all possible colours; and their action is limited to the sifting of that total--the appropriating or absorbing of some of its constituents, and the rejecting of others. it will fix this subject in your minds if i say, that it is the portion of light which they reject, and not that which they appropriate or absorb, that gives bodies their colours. let us begin our experimental inquiries here by asking, what is the meaning of blackness? pass a black ribbon through the colours of the spectrum; it quenches all of them. the meaning of blackness is thus revealed--it is the result of the absorption of all the constituents of solar light. pass a red ribbon through the spectrum. in the red light the ribbon is a vivid red. why? because the light that enters the ribbon is not quenched or absorbed, but in great part sent back to the eye. place the same ribbon in the green of the spectrum; it is black as jet. it absorbs the green light, and renders the space on which that light falls a space of intense darkness. place a green ribbon in the green of the spectrum. it shines vividly with its proper colour; transfer it to the red, it is black as jet. here it absorbs all the light that falls upon it, and offers mere darkness to the eye. thus, when white light is employed, the red sifts it by quenching the green, and the green sifts it by quenching the red, both exhibiting the residual colour. the process through which natural bodies acquire their colours is therefore a _negative_ one. the colours are produced by subtraction, not by addition. this red glass is red because it destroys all the more refrangible rays of the spectrum. this blue liquid is blue because it destroys all the less refrangible rays. both together are opaque because the light transmitted by the one is quenched by the other. in this way, by the union of two transparent substances, we obtain a combination as dark as pitch to solar light. this other liquid, finally, is purple because it destroys the green and the yellow, and allows the terminal colours of the spectrum to pass unimpeded. from the blending of the blue and the red this gorgeous purple is produced. one step further for the sake of exactness. the light which falls upon a body is divided into two portions, one of which is reflected from the surface of the body; and this is of the same colour as the incident light. if the incident light be white, the superficially reflected light will also be white. solar light, for example, reflected from the surface of even a black body, is white. the blackest camphine smoke in a dark room, through which a sunbeam passes from an aperture in the window-shutter, renders the track of the beam white, by the light scattered from the surfaces of the soot particles. the moon appears to us as if 'clothed in white samite, mystic, wonderful;' but were it covered with the blackest velvet it would still hang as a white orb in the heavens, shining upon our world substantially as it does now. § . _colours of pigments as distinguished from colours of light_. the second portion of the incident light enters the body, and upon its treatment there the colour of the body depends. and here a moment may properly be given to the analysis of the action of pigments upon light. they are composed of fine particles mixed with a vehicle; but how intimately soever the particles may be blended, they still remain particles, separated, it may be, by exceedingly minute distances, but still separated. to use the scientific phrase, they are not optically continuous. now, wherever optical continuity is ruptured we have reflection of the incident light. it is the multitude of reflections at the limiting surfaces of the particles that prevents light from passing through snow, powdered glass, or common salt. the light here is exhausted in echoes, not extinguished by true absorption. it is the same kind of reflection that renders the thunder-cloud so impervious to light. such a cloud is composed of particles of water, mixed with particles of air, both separately transparent, but practically opaque when thus mixed together. in the case of pigments, then, the light is _reflected_ at the limiting surfaces of the particles, but it is in part _absorbed_ within the particles. the reflection is necessary to send the light back to the eye; the absorption is necessary to give the body its colour. the same remarks apply to flowers. the rose is red, in virtue, not of the light reflected from its surface, but of light which has entered its substance, which has been reflected from surfaces within, and which, in returning _through_ the substance, has had its green extinguished. a similar process in the case of hard green leaves extinguishes the red, and sends green light from the body of the leaves to the eye. all bodies, even the most transparent, are more or less absorbent of light. take the case of water. a glass cell of clear water interposed in the track of our beam does not perceptibly change any one of the colours of the spectrum. still absorption, though insensible, has here occurred, and to render it sensible we have only to increase the depth of the water through which the light passes. instead of a cell an inch thick, let us take a layer, ten or fifteen feet thick: the colour of the water is then very evident. by augmenting the thickness we absorb more of the light, and by making the thickness very great we absorb the light altogether. lampblack or pitch can do no more, and the only difference in this respect between them and water is that a very small depth in their case suffices to extinguish all the light. the difference between the highest known transparency and the highest known opacity is one of degree merely. if, then, we render water sufficiently deep to quench all the light; and if from the interior of the water no light reaches the eye, we have the condition necessary to produce blackness. looked properly down upon, there are portions of the atlantic ocean to which one would hardly ascribe a trace of colour: at the most a tint of dark indigo reaches the eye. the water, in fact, is practically _black_, and this is an indication both of its depth and purity. but the case is entirely changed when the ocean contains solid particles in a state of mechanical suspension, capable of sending the light impinging on them back to the eye. throw, for example, a white pebble, or a white dinner plate, into the blackest atlantic water; as it sinks it becomes greener and greener, and, before it disappears, it reaches a vivid blue green. break such a pebble, or plate, into fragments, these will behave like the unbroken mass: grind the pebble to powder, every particle will yield its modicum of green; and if the particles be so fine as to remain suspended in the water, the scattered light will be a uniform green. hence the greenness of shoal water. you go to bed with the black water of the atlantic around you. you rise in the morning, find it a vivid green, and correctly infer that you are crossing the bank of newfoundland. such water is found charged with fine matter in a state of mechanical suspension. the light from the bottom may sometimes come into play, but it is not necessary. the subaqueous foam, generated by the screw or paddle-wheels of a steamer, also sends forth a vivid green. the foam here furnishes a _reflecting surface_, the water between the eye and it the _absorbing medium_. nothing can be more superb than the green of the atlantic waves when the circumstances are favourable to the exhibition of the colour. as long as a wave remains unbroken no colour appears, but when the foam just doubles over the crest like an alpine snow-cornice, under the cornice we often see a display of the most exquisite green. it is metallic in its brilliancy. the foam is first illuminated, and it scatters the light in all directions; the light which passes through the higher portion of the wave alone reaches the eye, and gives to that portion its matchless colour. the folding of the wave, producing, as it does, a series of longitudinal protuberances and furrows which act like cylindrical lenses, introduces variations in the intensity of the light, and materially enhances its beauty. we are now prepared for the further consideration of a point already adverted to, and regarding which error long found currency. you will find it stated in many books that blue light and yellow light mixed together, produce green. but blue and yellow have been just proved to be complementary colours, producing white by their mixture. the mixture of blue and yellow _pigments_ undoubtedly produces green, but the mixture of pigments is a totally different thing from the mixture of lights. helmholtz has revealed the cause of the green produced by a mixture of blue and yellow pigments. no natural colour is _pure_. a blue liquid, or a blue powder, permits not only the blue to pass through it, but a portion of the adjacent green. a yellow powder is transparent not only to the yellow light, but also in part to the adjacent green. now, when blue and yellow are mixed together, the blue cuts off the yellow, the orange, and the red; the yellow, on the other hand, cuts off the violet, the indigo, and the blue. green is the only colour to which both are transparent, and the consequence is that, when white light falls upon a mixture of yellow and blue powders, the green alone is sent back to the eye. you have already seen that the fine blue ammonia-sulphate of copper transmits a large portion of green, while cutting off all the less refrangible light. a yellow solution of picric acid also allows the green to pass, but quenches all the more refrangible light. what must occur when we send a beam through both liquids? the experimental answer to this question is now before you: the green band of the spectrum alone remains upon the screen. the impurity of natural colours is strikingly illustrated by an observation recently communicated to me by mr. woodbury. on looking through a blue glass at green leaves in sunshine, he saw the superficially reflected light blue. the light, on the contrary, which came from the body of the leaves was crimson. on examination, i found that the glass employed in this observation transmitted both ends of the spectrum, the red as well as the blue, and that it quenched the middle. this furnished an easy explanation of the effect. in the delicate spring foliage the blue of the solar light is for the most part absorbed, and a light, mainly yellowish green, but containing a considerable quantity of red, escapes from the leaf to the eye. on looking at such foliage through the violet glass, the green and the yellow are stopped, and the red alone reaches the eye. thus regarded, therefore, the leaves appear like faintly blushing roses, and present a very beautiful appearance. with the blue ammonia-sulphate of copper, which transmits no red, this effect is not obtained. as the year advances the crimson gradually hardens to a coppery red; and in the dark green leaves of old ivy it is almost absent. permitting a beam of white light to fall upon fresh leaves in a dark room, the sudden change from green to red, and from red back to green, when the violet glass is alternately introduced and withdrawn, is very surprising. looked at through the same glass, the meadows in may appear of a warm purple. with a solution of permanganate of potash, which, while it quenches the centre of the spectrum, permits its ends to pass more freely than the violet glass, excellent effects are also obtained.[ ] this question of absorption, considered with reference to its molecular mechanism, is one of the most subtle and difficult in physics. we are not yet in a condition to grapple with it, but we shall be by-and-by. meanwhile we may profitably glance back on the web of relations which these experiments reveal to us. we have, firstly, in solar light an agent of exceeding complexity, composed of innumerable constituents, refrangible in different degrees. we find, secondly, the atoms and molecules of bodies gifted with the power of sifting solar light in the most various ways, and producing by this sifting the colours observed in nature and art. to do this they must possess a molecular structure commensurate in complexity with that of light itself. thirdly, we have the human eye and brain, so organized as to be able to take in and distinguish the multitude of impressions thus generated. the light, therefore, at starting is complex; to sift and select it as they do, natural bodies must be complex; while to take in the impressions thus generated, the human eye and brain, however we may simplify our conceptions of their action,[ ] must be highly complex. whence this triple complexity? if what are called material purposes were the only end to be served, a much simpler mechanism would be sufficient. but, instead of simplicity, we have prodigality of relation and adaptation--and this, apparently, for the sole purpose of enabling us to see things robed in the splendours of colour. would it not seem that nature harboured the intention of educating us for other enjoyments than those derivable from meat and drink? at all events, whatever nature meant--and it would be mere presumption to dogmatize as to what she meant--we find ourselves here, as the upshot of her operations, endowed, not only with capacities to enjoy the materially useful, but endowed with others of indefinite scope and application, which deal alone with the beautiful and the true. lecture ii. origin of physical theories scope of the imagination newton and the emission theory verification of physical theories the luminiferous ether wave theory of light thomas young fresnel and arago conception of wave-motion interference of waves constitution of sound-waves analogies of sound and light illustrations of wave-motion interference of sound-waves optical illustrations pitch and colour lengths of the waves of light and rates of vibration of the ether-particles interference of light phenomena which first suggested the undulatory theory boyle and hooke the colours of thin plates the soap-bubble newton's rings theory of 'fits' its explanation of the rings over-throw of the theory diffraction of light colours produced by diffraction colours of mother-of-pearl. § . _origin and scope of physical theories_. we might vary and extend our experiments on light indefinitely, and they certainly would prove us to possess a wonderful mastery over the phenomena. but the vesture of the agent only would thus be revealed, not the agent itself. the human mind, however, is so constituted that it can never rest satisfied with this outward view of natural things. brightness and freshness take possession of the mind when it is crossed by the light of principles, showing the facts of nature to be organically connected. let us, then, inquire what this thing is that we have been generating, reflecting, refracting and analyzing. in doing this, we shall learn that the life of the experimental philosopher is twofold. he lives, in his vocation, a life of the senses, using his hands, eyes, and ears in his experiments: but such a question as that now before us carries him beyond the margin of the senses. he cannot consider, much less answer, the question, 'what is light?' without transporting himself to a world which underlies the sensible one, and out of which all optical phenomena spring. to realise this subsensible world the mind must possess a certain pictorial power. it must be able to form definite images of the things which that world contains; and to say that, if such or such a state of things exist in the subsensible world, then the phenomena of the sensible one must, of necessity, grow out of this state of things. physical theories are thus formed, the truth of which is inferred from their power to explain the known and to predict the unknown. this conception of physical theory implies, as you perceive, the exercise of the imagination--a word which seems to render many respectable people, both in the ranks of science and out of them, uncomfortable. that men in the ranks of science should feel thus is, i think, a proof that they have suffered themselves to be misled by the popular definition of a great faculty, instead of observing its operation in their own minds. without imagination we cannot take a step beyond the bourne of the mere animal world, perhaps not even to the edge of this one. but, in speaking thus of imagination, i do not mean a riotous power which deals capriciously with facts, but a well-ordered and disciplined power, whose sole function is to form such conceptions as the intellect imperatively demands. imagination, thus exercised, never really severs itself from the world of fact. this is the storehouse from which its materials are derived; and the magic of its art consists, not in creating things anew, but in so changing the magnitude, position, grouping, and other relations of sensible things, as to render them fit for the requirements of the intellect in the subsensible world.[ ] descartes imagined space to be filled with something that transmitted light _instantaneously_. firstly, because, in his experience, no measurable interval was known to exist between the appearance of a flash of light, however distant, and its effect upon consciousness; and secondly, because, as far as his experience went, no physical power is conveyed from place to place without a vehicle. but his imagination helped itself farther by illustrations drawn from the world of fact. 'when,' he says,' one walks in darkness with staff in hand, the moment the distant end of the staff strikes an obstacle the hand feels it. this explains what might otherwise be thought strange, that the light reaches us instantaneously from the sun. i wish thee to believe that light in the bodies that we call luminous is nothing more than a very brisk and violent motion, which, by means of the air and other transparent media, is conveyed to the eye, exactly as the shock through the walking-stick reaches the hand of a blind man. this is instantaneous, and would be so even if the intervening distance were greater than that between earth and heaven. it is therefore no more necessary that anything material should reach the eye from the luminous object, than that something should be sent from the ground to the hand of the blind man when he is conscious of the shock of his staff.' the celebrated robert hooke at first threw doubt upon this notion of descartes, but he afterwards substantially espoused it. the belief in instantaneous transmission was destroyed by the discovery of roemer referred to in our last lecture. § . _the emission theory of light_. the case of newton still more forcibly illustrates the position, that in forming physical theories we draw for our materials upon the world of fact. before he began to deal with light, he was intimately acquainted with the laws of elastic collision, which all of you have seen more or less perfectly illustrated on a billiard-table. as regards the collision of sensible elastic masses, newton knew the angle of incidence to be equal to the angle of reflection, and he also knew that experiment, as shown in our last lecture (fig. ), had established the same law with regard to light. he thus found in his previous knowledge the material for theoretic images. he had only to change the magnitude of conceptions already in his mind to arrive at the emission theory of light. newton supposed light to consist of elastic particles of inconceivable minuteness, shot out with inconceivable rapidity by luminous bodies. optical reflection certainly occurred _as if_ light consisted of such particles, and this was newton's justification for introducing them. but this is not all. in another important particular, also, newton's conceptions regarding the nature of light were influenced by his previous knowledge. he had been pondering over the phenomena of gravitation, and had made himself at home amid the operations of this universal power. perhaps his mind at this time was too freshly and too deeply imbued with these notions to permit of his forming an unfettered judgment regarding the nature of light. be that as it may, newton saw in refraction the result of an attractive force exerted on the light-particles. he carried his conception out with the most severe consistency. dropping vertically downwards towards the earth's surface, the motion of a body is accelerated as it approaches the earth. dropping downwards towards a horizontal surface--say from air on to glass or water--the velocity of the light-particles, when they came close to the surface, is, according to newton, also accelerated. approaching such a surface obliquely, he supposed the particles, when close to it, to be drawn down upon it, as a projectile is deflected by gravity to the surface of the earth. this deflection was, according to newton, the refraction seen in our last lecture (fig. ). finally, it was supposed that differences of colour might be due to differences in the 'bigness' of the particles. this was the physical theory of light enunciated and defended by newton; and you will observe that it simply consists in the transference of conceptions, born in the world of the senses, to a subsensible world. but, though the region of physical theory lies thus behind the world of senses, the verifications of theory occur in that world. laying the theoretic conception at the root of matters, we determine by deduction what are the phenomena which must of necessity grow out of this root. if the phenomena thus deduced agree with those of the actual world, it is a presumption in favour of the theory. if, as new classes of phenomena arise, they also are found to harmonise with theoretic deduction, the presumption becomes still stronger. if, finally, the theory confers prophetic vision upon the investigator, enabling him to predict the occurrence of phenomena which have never yet been seen, and if those predictions be found on trial to be rigidly correct, the persuasion of the truth of the theory becomes overpowering. thus working backwards from a limited number of phenomena, the human mind, by its own expansive force, reaches a conception which covers them all. there is no more wonderful performance of the intellect than this; but we can render no account of it. like the scriptural gift of the spirit, no man can tell whence it cometh. the passage from fact to principle is sometimes slow, sometimes rapid, and at all times a source of intellectual joy. when rapid, the pleasure is concentrated, and becomes a kind of ecstasy or intoxication. to any one who has experienced this pleasure, even in a moderate degree, the action of archimedes when he quitted the bath, and ran naked, crying 'eureka!' through the streets of syracuse, becomes intelligible. how, then, did it fare with the emission theory when the deductions from it were brought face to face with natural phenomena? tested by experiment, it was found competent to explain many facts, and with transcendent ingenuity its author sought to make it account for all. he so far succeeded, that men so celebrated as laplace and malus, who lived till , and biot and brewster, who lived till our own time, were found among his disciples. § . _the undulatory theory of light_. still, even at an early period of the existence of the emission theory, one or two great men were found espousing a different one. they furnish another illustration of the law that, in forming theories, the scientific imagination must draw its materials from the world of fact and experience. it was known long ago that sound is conveyed in waves or pulses through the air; and no sooner was this truth well housed in the mind than it became the basis of a theoretic conception. it was supposed that light, like sound, might also be the product of wave-motion. but what, in this case, could be the material forming the waves? for the waves of sound we have the air of our atmosphere; but the stretch of imagination which filled all space with a _luminiferous ether_ trembling with the waves of light was so bold as to shock cautious minds. in one of my latest conversations with sir david brewster, he said to me that his chief objection to the undulatory theory of light was, that he could not think the creator capable of so clumsy a contrivance as the filling of space with ether to produce light. this, i may say, is very dangerous ground, and the quarrel of science with sir david, on this point as with many estimable persons on other points, is, that they profess to know too much about the mind of the creator. this conception of an ether was advocated, and successfully applied to various phenomena of optics, by the illustrious astronomer, huyghens. he deduced from it the laws of reflection and refraction, and applied it to explain the double refraction of iceland spar. the theory was espoused and defended by the celebrated mathematician, euler. they were, however, opposed by newton, whose authority at the time bore them down. or shall we say it was authority merely? not quite so. newton's preponderance was in some degree due to the fact that, though huyghens and euler were right in the main, they did not possess sufficient data to _prove_ themselves right. no human authority, however high, can maintain itself against the voice of nature speaking through experiment. but the voice of nature may be an uncertain voice, through the scantiness of data. this was the case at the period now referred to, and at such a period, by the authority of newton, all antagonists were naturally overborne. the march of mind is rhythmic, not uniform, and this great emission theory, which held its ground so long, resembled one of those circles which, according to your countryman emerson, the intermittent force of genius periodically draws round the operations of the intellect, but which are eventually broken through by pressure from behind. in the year was born, at milverton, in somersetshire, a circle-breaker of this kind. he was educated for the profession of a physician, but was too strong to be tied down to professional routine. he devoted himself to the study of natural philosophy, and became in all its departments a master. he was also a master of letters. languages, ancient and modern, were housed within his brain, and, to use the words of his epitaph, 'he first penetrated the obscurity which had veiled for ages the hieroglyphics of egypt.' it fell to the lot of this man to discover facts in optics which newton's theory was incompetent to explain, and his mind roamed in search of a sufficient theory. he had made himself acquainted with all the phenomena of wave-motion; with all the phenomena of sound; working successfully in this domain as an original discoverer. thus informed and disciplined, he was prepared to detect any resemblance which might reveal itself between the phenomena of light and those of wave-motion. such resemblances he did detect; and, spurred on by the discovery, he pursued his speculations and experiments, until he finally succeeded in placing on an immovable basis the undulatory theory of light. the founder of this great theory was thomas young, a name, perhaps, unfamiliar to many of you, but which ought to be familiar to you all. permit me, therefore, by a kind of geometrical construction which i once ventured to employ in london, to give you a notion of the magnitude of this man. let newton stand erect in his age, and young in his. draw a straight line from newton to young, tangent to the heads of both. this line would slope downwards from newton to young, because newton was certainly the taller man of the two. but the slope would not be steep, for the difference of stature was not excessive. the line would form what engineers call a gentle gradient from newton to young. place underneath this line the biggest man born in the interval between both. it may be doubted whether he would reach the line; for if he did he would be taller intellectually than young, and there was probably none taller. but i do not want you to rest on english estimates of young; the german, helmholtz, a kindred genius, thus speaks of him: "his was one of the most profound minds that the world has ever seen; but he had the misfortune to be too much in advance of his age. he excited the wonder of his contemporaries, who, however, were unable to follow him to the heights at which his daring intellect was accustomed to soar. his most important ideas lay, therefore, buried and forgotten in the folios of the royal society, until a new generation gradually and painfully made the same discoveries, and proved the exactness of his assertions and the truth of his demonstrations." it is quite true, as helmholtz says, that young was in advance of his age; but something is to be added which illustrates the responsibility of our public writers. for twenty years this man of genius was quenched--hidden from the appreciative intellect of his country-men--deemed in fact a dreamer, through the vigorous sarcasm of a writer who had then possession of the public ear, and who in the _edinburgh review_ poured ridicule upon young and his speculations. to the celebrated frenchmen fresnel and arago he was first indebted for the restitution of his rights; for they, especially fresnel, independently remade and vastly extended his discoveries. to the students of his works young has long since appeared in his true light, but these twenty blank years pushed him from the public mind, which became in time filled with the fame of young's colleague at the royal institution, davy, and afterwards with the fame of faraday. carlyle refers to a remark of novalis, that a man's self-trust is enormously increased the moment he finds that others believe in him. if the opposite remark be true--if it be a fact that public disbelief weakens a man's force--there is no calculating the amount of damage these twenty years of neglect may have done to young's productiveness as an investigator. it remains to be stated that his assailant was mr. henry brougham, afterwards lord chancellor of england. § . _wave-motion, interference of waves, 'whirlpool rapids' of niagara_. our hardest work is now before us. but the capacity for hard work depends in a great measure on the antecedent winding up of the will; i would call upon you, therefore, to gird up your loins for coming labours. in the earliest writings of the ancients we find the notion that sound is conveyed by the air. aristotle gives expression to this notion, and the great architect vitruvius compares the waves of sound to waves of water. but the real mechanism of wave-motion was hidden from the ancients, and indeed was not made clear until the time of newton. the central difficulty of the subject was, to distinguish between the motion of the wave itself, and the motion of the particles which at any moment constitute the wave. stand upon the seashore and observe the advancing rollers before they are distorted by the friction of the bottom. every wave has a back and a front, and, if you clearly seize the image of the moving wave, you will see that every particle of water along the front of the wave is in the act of rising, while every particle along its back is in the act of sinking. the particles in front reach in succession the crest of the wave, and as soon as the crest is past they begin to fall. they then reach the furrow or _sinus_ of the wave, and can sink no farther. immediately afterwards they become the front of the succeeding wave, rise again until they reach the crest, and then sink as before. thus, while the waves pass onwards horizontally, the individual particles are simply lifted up and down vertically. observe a sea-fowl, or, if you are a swimmer, abandon yourself to the action of the waves; you are not carried forward, but simply rocked up and down. the propagation of a wave is the propagation of a _form_, and not the transference of the substance which constitutes the wave. the _length_ of the wave is the distance from crest to crest, while the distance through which the individual particles oscillate is called the _amplitude_ of the oscillation. you will notice that in this description the particles of water are made to vibrate _across_ the line of propagation.[ ] and now we have to take a step forwards, and it is the most important step of all. you can picture two series of waves proceeding from different origins through the same water. when, for example, you throw two stones into still water, the ring-waves proceeding from the two centres of disturbance intersect each other. now, no matter how numerous these waves may be, the law holds good that the motion of every particle of the water is the algebraic sum of all the motions imparted to it. if crest coincide with crest and furrow with furrow, the wave is lifted to a double height above its sinus; if furrow coincide with crest, the motions are in opposition and their sum is zero. we have then _still_ water. this action of wave upon wave is technically called _interference_, a term, to be remembered. to the eye of a person conversant with these principles, nothing can be more interesting than the crossing of water ripples. through their interference the water-surface is sometimes shivered into the most beautiful mosaic, trembling rhythmically as if with a kind of visible music. when waves are skilfully generated in a dish of mercury, a strong light thrown upon the shining surface, and reflected on to a screen, reveals the motions of the liquid metal. the shape of the vessel determines the forms of the figures produced. in a circular dish, for example, a disturbance at the centre propagates itself as a series of circular waves, which, after reflection, again meet at the centre. if the point of disturbance be a little way removed from the centre, the interference of the direct and reflected waves produces the magnificent chasing shown in the annexed figure.[ ] the light reflected from such a surface yields a pattern of extraordinary beauty. when the mercury is slightly struck by a needle-point in a direction concentric with the surface of the vessel, the lines of light run round in mazy coils, interlacing and unravelling themselves in a wonderful manner. when the vessel is square, a splendid chequer-work is produced by the crossing of the direct and reflected waves. thus, in the case of wave-motion, the most ordinary causes give rise to most exquisite effects. the words of emerson are perfectly applicable here:-- [illustration: fig. .] 'thou can'st not wave thy staff in the air, or dip thy paddle in the lake, but it carves the brow of beauty there. and the ripples in rhymes the oars forsake.' the most impressive illustration of the action of waves on waves that i have ever seen occurs near niagara. for a distance of two miles, or thereabouts, below the falls, the river niagara flows unruffled through its excavated gorge. the bed subsequently narrows, and the water quickens its motion. at the place called the 'whirlpool rapids,' i estimated the width of the river at feet, an estimate confirmed by the dwellers on the spot. when it is remembered that the drainage of nearly half a continent is compressed into this space, the impetuosity of the river's escape through this gorge may be imagined. two kinds of motion are here obviously active, a motion of translation and a motion of undulation--the race of the river through its gorge, and the great waves generated by its collision with the obstacles in its way. in the middle of the stream, the rush and tossing are most violent; at all events, the impetuous force of the individual waves is here most strikingly displayed. vast pyramidal heaps leap incessantly from the river, some of them with such energy as to jerk their summits into the air, where they hang suspended as bundles of liquid pearls, which, when shone upon by the sun, are of indescribable beauty. the first impression, and, indeed, the current explanation of these rapids is, that the central bed of the river is cumbered with large boulders, and that the jostling, tossing, and wild leaping of the waters there are due to its impact against these obstacles. a very different explanation occurred to me upon the spot. boulders derived from the adjacent cliffs visibly cumber the _sides_ of the river. against these the water rises and sinks rhythmically but violently, large waves being thus produced. on the generation of each wave there is an immediate compounding of the wave-motion with the river-motion. the ridges, which in still water would proceed in circular curves round the centre of disturbance, cross the river obliquely, and the result is, that at the centre waves commingle which have really been generated at the sides. this crossing of waves may be seen on a small scale in any gutter after rain; it may also be seen on simply pouring water from a wide-lipped jug. where crest and furrow cross each other, the wave is annulled; where furrow and furrow cross, the river is ploughed to a greater depth; and where crest and crest aid each other, we have that astonishing leap of the water which breaks the cohesion of the crests, and tosses them shattered into the air. the phenomena observed at the whirlpool rapids constitute, in fact, one of the grandest illustrations of the principle of interference. § . _analogies of sound and light._ thomas young's fundamental discovery in optics was that the principle of interference was applicable to light. long prior to his time an italian philosopher, grimaldi, had stated that under certain circumstances two thin beams of light, each of which, acting singly, produced a luminous spot upon a white wall, when caused to act together, partially quenched each other and darkened the spot. this was a statement of fundamental significance, but it required the discoveries and the genius of young to give it meaning. how he did so will gradually become clear to you. you know that air is compressible: that by pressure it can be rendered more dense, and that by dilatation it can be rendered more rare. properly agitated, a tuning-fork now sounds in a manner audible to you all, and most of you know that the air through which the sound is passing is parcelled out into spaces in which the air is condensed, followed by other spaces in which the air is rarefied. these condensations and rarefactions constitute what we call _waves_ of sound. you can imagine the air of a room traversed by a series of such waves, and you can imagine a second series sent through the same air, and so related to the first that condensation coincides with condensation and rarefaction with rarefaction. the consequence of this coincidence would be a louder sound than that produced by either system of waves taken singly. but you can also imagine a state of things where the condensations of the one system fall upon the rarefactions of the other system. in this case (other things being equal) the two systems would completely neutralize each other. each of them taken singly produces sound; both of them taken together produce no sound. thus by adding sound to sound we produce silence, as grimaldi, in his experiment, produced darkness by adding light to light. through his investigations on sound, which were fruitful and profound, young approached the study of light. he put meaning into the observation of grimaldi, and immensely extended it. with splendid success he applied the undulatory theory to the explanation of the colours of thin plates, and to those of striated surfaces. he discovered and explained classes of colour which had been previously unnoticed or unknown. on the assumption that light was wave-motion, all his experiments on interference were accounted for; on the assumption that light was flying particles, nothing was explained. in the time of huyghens and euler a medium had been assumed for the transmission of the waves of light; but newton raised the objection that, if light consisted of the waves of such a medium, shadows could not exist. the waves, he contended, would bend round opaque bodies and produce the motion of light behind them, as sound turns a corner, or as waves of water wash round a rock. it was proved that the bending round referred to by newton actually occurs, but that the inflected waves abolish each other by their mutual interference. young also discerned a fundamental difference between the waves of light and those of sound. could you see the air through which sound-waves are passing, you would observe every individual particle of air oscillating to and fro, _in the direction of propagation_. could you see the luminiferous ether, you would also find every individual particle making a small excursion to and fro; but here the motion, like that assigned to the water-particles above referred to, would be _across_ the line of propagation. the vibrations of the air are _longitudinal_, those of the ether _transversal_. the most familiar illustration of the interference of sound-waves is furnished by the _beats_ produced by two musical sounds slightly out of unison. when two tuning-forks in perfect unison are agitated together the two sounds flow without roughness, as if they were but one. but, by attaching with wax to one of the forks a little weight, we cause it to vibrate more slowly than its neighbour. suppose that one of them performs vibrations in the time required by the other to perform , and suppose that at starting the condensations and rarefactions of both forks coincide. at the st vibration of the quicker fork they will again coincide, that fork at this point having gained one whole vibration, or one whole wavelength, upon the other. but a little reflection will make it clear that, at the th vibration, the two forks condensation where the other tends to produce a rarefaction; by the united action of the two forks, therefore, the sound is quenched, and we have a pause of silence. this occurs where one fork has gained _half a wavelength_ upon the other. at the st vibration, as already stated, we have coincidence, and, therefore, augmented sound; at the th vibration we have again a quenching of the sound. here the one fork is _three half-waves_ in advance of the other. in general terms, the waves conspire when the one series is an _even_ number of half-wave lengths, and they destroy each other when the one series is an _odd_ number of half-wave lengths in advance of the other. with two forks so circumstanced, we obtain those intermittent shocks of sound separated by pauses of silence, to which we give the name of beats. by a suitable arrangement, moreover, it is possible to make one sound wholly extinguish another. along four distinct lines, for example, the vibrations of the two prongs of a tuning-fork completely blot each other out.[ ] the _pitch_ of sound is wholly determined by the rapidity of the vibration, as the _intensity_ is by the amplitude. what pitch is to the ear in acoustics, colour is to the eye in the undulatory theory of light. though never seen, the lengths of the waves of light have been determined. their existence is proved _by their effects_, and from their effects also their lengths may be accurately deduced. this may, moreover, be done in many ways, and, when the different determinations are compared, the strictest harmony is found to exist between them. this consensus of evidence is one of the strongest points of the undulatory theory. the shortest waves of the visible spectrum are those of the extreme violet; the longest, those of the extreme red; while the other colours are of intermediate pitch or wavelength. the length of a wave of the extreme red is such, that it would require , such waves, placed end to end, to cover one inch, while , of the extreme violet waves would be required to span the same distance. now, the velocity of light, in round numbers, is , miles per second. reducing this to inches, and multiplying the number thus found by , , we find the number of waves of the extreme red, in , miles, to be four hundred and sixty millions of millions. _all these waves enter the eye, and strike the retina at the back of the eye in one second_. in a similar manner, it may be found that the number of shocks corresponding to the impression of violet is six hundred and seventy-eight millions of millions. all space is filled with matter oscillating at such rates. from every star waves of these dimensions move, with the velocity of light, like spherical shells in all directions. and in ether, just as in water, the motion of every particle is the algebraic sum of all the separate motions imparted to it. one motion does not blot out the other; or, if extinction occur at one point, it is strictly atoned for, by augmented motion, at some other point. every star declares by its light its undamaged individuality, as if it alone had sent its thrills through space. § . _interference of light_. [illustration: fig. .] the principle of interference, as just stated, applies to the waves of light as it does to the waves of water and the waves of sound. and the conditions of interference are the same in all three. if two series of light-waves of the same length start at the same moment from a common origin (say a, fig. ), crest coincides with crest, sinus with sinus, and the two systems blend together to a single system (a _m_ _n_) of double amplitude. if both series start at the same moment, one of them being, at starting, a whole wavelength in advance of the other, they also add themselves together, and we have an augmented luminous effect. the same occurs when the one system of waves is any _even_ number of semi-undulations in advance of the other. but if the one system be half a wave-length (as at a' _a_', fig. ), or any _odd_ number of half wavelengths, in advance, then the crests of the one fall upon the sinuses of the other; the one system, in fact, tends to _lift_ the particles of ether at the precise places where the other tends to _depress_ them; hence, through the joint action of these opposing forces (indicated by the arrows) the light-ether remains perfectly still. this stillness of the ether is what we call darkness, which corresponds with a dead level in the case of water. [illustration: fig. .] it was said in our first lecture, with reference to the colours produced by absorption, that the function of natural bodies is selective, not creative; that they extinguish certain constituents of the white solar light, and appear in the colours of the unextinguished light. it must at once occur to you that, inasmuch as we have in interference an agency by which light may be self-extinguished, we may have in it the conditions for the production of colour. but this would imply that certain constituents are quenched by interference, while others are permitted to remain. this is the fact; and it is entirely due to the difference in the lengths of the waves of light. § . _colours of thin films. observations of boyle and hooke_. this subject may be illustrated by the phenomena which first suggested the undulatory theory to the mind of hooke. these are the colours of thin transparent films of all kinds, known as the _colours of thin plates_. in this relation no object in the world possesses a deeper scientific interest than a common soap-bubble. and here let me say emerges one of the difficulties which the student of pure science encounters in the presence of 'practical' communities like those of america and england; it is not to be expected that such communities can entertain any profound sympathy with labours which seem so far removed from the domain of practice as are many of the labours of the man of science. imagine dr. draper spending his days in blowing soap-bubbles and in studying their colours! would you show him the necessary patience, or grant him the necessary support? and yet be it remembered it was thus that minds like those of boyle, newton and hooke were occupied; and that on such experiments has been founded a theory, the issues of which are incalculable. i see no other way for you, laymen, than to trust the scientific man with the choice of his inquiries; he stands before the tribunal of his peers, and by their verdict on his labours you ought to abide. whence, then, are derived the colours of the soap-bubble? imagine a beam of white light impinging on the bubble. when it reaches the first surface of the film, a known fraction of the light is reflected back. but a large portion of the beam enters the film, reaches its second surface, and is again in part reflected. the waves from the second surface thus turn back and hotly pursue the waves from the first surface. and, if the thickness of the film be such as to cause the necessary retardation, the two systems of waves interfere with each other, producing augmented or diminished light, as the case may be. but, inasmuch as the waves of light are of different lengths, it is plain that, to produce extinction in the case of the longer waves, a greater thickness of film is necessary than in the case of the shorter ones. different colours, therefore, must appear at different thicknesses of the film. take with you a little bottle of spirit of turpentine, and pour it into one of your country ponds. you will then see the glowing of those colours over the surface of the water. on a small scale we produce them thus: a common tea-tray is filled with water, beneath the surface of which dips the end of a pipette. a beam of light falls upon the water, and is reflected by it to the screen. spirit of turpentine is poured into the pipette; it descends, issues from the end in minute drops, which rise in succession to the surface. on reaching it, each drop spreads suddenly out as a film, and glowing colours immediately flash forth upon the screen. the colours change as the thickness of the film changes by evaporation. they are also arranged in zones, in consequence of the gradual diminution of thickness from the centre outwards. any film whatever will produce these colours. the film of air between two plates of glass squeezed together, exhibits, as shown by hooke, rich fringes of colour. a particularly fine example of these fringes is now before you. nor is even air necessary; the rupture of optical continuity suffices. smite with an axe the black, transparent ice--black, because it is pure and of great depth--under the moraine of a glacier; you readily produce in the interior flaws which no air can reach, and from these flaws the colours of thin plates sometimes break like fire. but the source of most historic interest is, as already stated, the soap-bubble. with one of the mixtures employed by the eminent blind philosopher, plateau, in his researches on the cohesion figures of thin films, we obtain in still air a bubble ten or twelve inches in diameter. you may look at the bubble itself, or you may look at its projection upon the screen; rich colours arranged in zones are, in both cases, exhibited. rendering the beam parallel, and permitting it to impinge upon the sides, bottom, and top of the bubble, gorgeous fans of colour, reflected from the bubble, overspread the screen, rotating as the beam is carried round. by this experiment the internal motions of the film are also strikingly displayed. not in a moment are great theories elaborated: the facts which demand them become first prominent; then, to the period of observation succeeds a period of pondering and of tentative explanation. by such efforts the human mind is gradually prepared for the final theoretic illumination. the colours of thin plates, for example, occupied the attention of robert boyle. in his 'experimental history of colours' he contends against the schools which affirmed that colour was 'a penetrative quality that reaches to the innermost parts of the object,' adducing opposing facts. 'to give you a first instance,' he says, 'i shall need but to remind you of what i told you a little after the beginning of this essay, touching the blue and red and yellow that may be produced upon a piece of tempered steel; for these colours, though they be very vivid, yet if you break the steel they adorn, they will appear to be but superficial.' he then describes, in phraseology which shows the delight he took in his work, the following beautiful experiment:-- 'we took a quantity of clean lead, and melted it with a strong fire, and then immediately pouring it out into a clean vessel of convenient shape and matter (we used one of iron, that the great and sudden heat might not injure it), and then carefully and nimbly taking off the scum that floated on the top, we perceived, as we expected, the smooth and glossy surface of the melted matter to be adorned with a very glorious colour, which, being as transitory as delightful, did almost immediately give place to another vivid colour, and that was as quickly succeeded by a third, and this, as it were, chased away by a fourth; and so these wonderfully vivid colours successively appeared and vanished till the metal ceasing to be hot enough to hold any longer this pleasing spectacle, the colours that chanced to adorn the surface when the lead thus began to cool remained upon it, but were so superficial that how little soever we scraped off the surface of the lead, we did, in such places, scrape off all the colour.' 'these things,' he adds, 'suggested to me some thoughts or ravings which i have not now time to acquaint you with.'[ ] he extends his observations to essential oils and spirits of wine, 'which being shaken till they have good store of bubbles, those bubbles will (if attentively considered) appear adorned with various and lovely colours, which all immediately vanish upon the retrogressing of the liquid which affords these bubbles their skins into the rest of the oil.' he also refers to the colour of glass films. 'i have seen one that was skilled in fashioning glasses by the help of a lamp blowing some of them so strongly as to burst them; whereupon it was found that the tenacity of the metal was such that before it broke it suffered itself to be reduced into films so extremely thin that they constantly showed upon their surface the varying colours of the rainbow.'[ ] subsequent to boyle the colours of thin plates occupied the attention of robert hooke, in whose writings we find a dawning of the undulatory theory of light. he describes with great distinctness the colours obtained with thin flakes of 'muscovy glass' (talc), also those surrounding flaws in crystals where optical continuity is destroyed. he shows very clearly the dependence of the colour upon the thickness of the film, and proves by microscopic observation that plates of a uniform thickness yield uniform colours. 'if,' he says, 'you take any small piece of the muscovy glass, and with a needle, or some other convenient instrument, cleave it oftentimes into thinner and thinner laminæ, you shall find that until you come to a determinate thinness of them they shall appear transparent and colourless; but if you continue to split and divide them further, you shall find at last that each plate shall appear most lovely tinged or imbued with a determinate colour. if, further, by any means you so flaw a pretty thick piece that one part begins to cleave a little from the other, and between these two there be gotten some pellucid medium, those laminated or pellucid bodies that fill that space shall exhibit several rainbows or coloured lines, the colours of which will be disposed and ranged according to the various thicknesses of the several parts of the plate.' he then describes fully and clearly the experiment with pressed glasses already referred to:-- 'take two small pieces of ground and polished looking-glass plate, each about the bigness of a shilling: take these two dry, and with your forefingers and thumbs press them very hard and close together, and you shall find that when they approach each other very near there will appear several irises or coloured lines, in the same manner almost as in the muscovy glass; and you may very easily change any of the colours of any part of the interposed body by pressing the plates closer and harder together, or leaving them more lax--that is, a part which appeared coloured with a red, may presently be tinged with a yellow, blue, green, purple, or the like. 'any substance,' he says, 'provided it be thin and transparent, will show these colours.' like boyle, he obtained them with glass films; he also procured them with bubbles of pitch, rosin, colophony, turpentine, solutions of several gums, as gum arabic in water, any glutinous liquor, as wort, wine, spirit of wine, oyl of turpentine, glare of snails, &c. hooke's writings show that even in his day the idea that both light and heat are modes of motion had taken possession of many minds. 'first,' he says, 'that all kind _of fiery burning bodies_ have their parts in motion i think will be easily granted me. that the spark struck from a flint and steel is in rapid agitation i have elsewhere made probable;... that heat argues a motion of the internal parts is (as i said before) generally granted;... and that in all extremely hot shining bodies there is a very quick motion that causes light, as well as a more robust that causes heat, may be argued from the celerity wherewith the bodies are dissolved. next, it must be _a vibrative motion.'_ his reference to the quick motion of light and the more robust motion of heat is a remarkable stroke of sagacity; but hooke's direct insight is better than his reasoning; for the proofs he adduces that light is 'a vibrating motion' have no particular bearing upon the question. still the undulatory theory had undoubtedly dawned upon the mind of this remarkable man. in endeavouring to account for the colours of thin plates, he again refers to the relation of colour to thickness: he dwells upon the fact that the film which shows these colours must be transparent, proving this by showing that however thin an opaque body was rendered no colours were produced. 'this,' he says, 'i have often tried by pressing a small globule of mercury between two smooth plates of glass, whereby i have reduced that body to a much greater thinness than was requisite to exhibit the colours with a transparent body.' then follows the sagacious remark that to produce the colours 'there must be a considerable reflecting body adjacent to the under or further side of the lamina or plate: for this i always found, that the greater that reflection was the more vivid were the appearing colours. from which observation,' he continues, 'it is most evident, _that the reflection from the further or under side of the body is the principal cause of the production of these colours._' he draws a diagram, correctly representing the reflection at the two surfaces of the film; but here his clearness ends. he ascribes the colours to a coalescence or confusion of the two reflecting pulses; the principal of interference being unknown to him, he could not go further in the way of explanation. § . _newton's rings. relation of colour to thickness of film_. [illustration: fig. ] in this way, then, by the active operation of different minds, facts are observed, examined, and the precise conditions of their appearance determined. all such work in science is the prelude to other work; and the efforts of boyle and hooke cleared the way for the optical career of newton. he conquered the difficulty which hooke had found insuperable, and determined by accurate measurements the relation of the thickness of the film to the colour it displays. in doing this his first care was to obtain a film of variable and calculable depth. on a plano-convex glass lens (d b e, fig. ) of very feeble curvature he laid a plate of glass (a c) with a plane surface, thus obtaining a film of air of gradually increasing depth from the point of contact (b) outwards. on looking at the film in monochromatic light he saw, with the delight attendant on fulfilled prevision, surrounding the place of contact, a series of bright rings separated from each other by dark ones, and becoming more closely packed together as the distance from the point of contact augmented (as in fig. ). when he employed red light, his rings had certain diameters; when he employed blue light, the diameters were less. in general terms, the more refrangible the light the smaller were the rings. causing his glasses to pass through the spectrum from red to blue, the rings gradually contracted; when the passage was from blue to red, the rings expanded. this is a beautiful experiment, and appears to have given newton the most lively satisfaction. when white light fell upon, the glasses, inasmuch as the colours were not superposed, a series _of iris-coloured_ circles was obtained. a magnified image of _newton's rings_ is now before you, and, by employing in succession red, blue, and white light, we obtain all the effects observed by newton. you notice that in monochromatic light the rings run closer and closer together as they recede from the centre. this is due to the fact that at a distance the film of air thickens more rapidly than near the centre. when white light is employed, this closing up of the rings causes the various colours to be superposed, so that after a certain thickness they are blended together to white light, the rings then ceasing altogether. it needs but a moment's reflection to understand that the colours of thin plates, produced by white light, are never unmixed or monochromatic. [illustration: fig. ] newton compared the tints obtained in this way with the tints of his soap-bubble, and he calculated the corresponding thickness. how he did this may be thus made plain to you: suppose the water of the ocean to be absolutely smooth; it would then accurately represent the earth's curved surface. let a perfectly horizontal plane touch the surface at any point. knowing the earth's diameter, any engineer or mathematician in this room could tell you how far the sea's surface will lie below this plane, at the distance of a yard, ten yards, a hundred yards, or a thousand yards from the point of contact of the plane and the sea. it is common, indeed, in levelling operations, to allow for the curvature of the earth. newton's calculation was precisely similar. his plane glass was a tangent to his curved one. from its refractive index and focal distance he determined the diameter of the sphere of which his curved glass formed a segment, he measured the distances of his rings from the place of contact, and he calculated the depth between the tangent plane and the curved surface, exactly as the engineer would calculate the distance between his tangent plane and the surface of the sea. the wonder is, that, where such infinitesimal distances are involved, newton, with the means at his disposal, could have worked with such marvellous exactitude. to account for these rings was the greatest optical difficulty that newton, ever encountered. he quite appreciated the difficulty. over his eagle eye there was no film--no vagueness in his conceptions. at the very outset his theory was confronted by the question, why, when a beam of light is incident on a transparent body, are some of the light-particles reflected and some transmitted? is it that there are two kinds of particles, the one specially fitted for transmission and the other for reflection? this cannot be the reason; for, if we allow a beam of light which has been reflected from one piece of glass to fall upon another, it, as a general rule, is also divided into a reflected and a transmitted portion. the particles once reflected are not always reflected, nor are the particles once transmitted always transmitted. newton saw all this; he knew he had to explain why it is that the self-same particle is at one moment reflected and at the next moment transmitted. it could only he through _some change in the condition of the particle itself_. the self-same particle, he affirmed, was affected by 'fits' of easy transmission and reflection. § . _theory of 'fits' applied to newton's rings_. if you are willing to follow me in an attempt to reveal the speculative groundwork of this theory of fits, the intellectual discipline will, i think, repay you for the necessary effort of attention. newton was chary of stating what he considered to be the cause of the fits, but there can hardly be a doubt that his mind rested on a physical cause. nor can there be a doubt that here, as in all attempts at theorising, he was compelled to fall back upon experience for the materials of his theory. let us attempt to restore his course of thought and observation. a magnet would furnish him with the notion of attracted and repelled poles; and he who habitually saw in the visible an image of the invisible would naturally endow his light-particles with such poles. turning their attracted poles towards a transparent substance, the particles would be sucked in and transmitted; turning their repelled poles, they would be driven away or reflected. thus, by the ascription of poles, the transmission and reflection of the self-same particle at different times might be accounted for. consider these rings of newton as seen in pure red light: they are alternately bright and dark. the film of air corresponding to the outermost of them is not thicker than an ordinary soap-bubble, and it becomes thinner on approaching the centre; still newton, as i have said, measured the thickness corresponding to every ring, and showed the difference of thickness between ring and ring. now, mark the result. for the sake of convenience, let us call the thickness of the film of air corresponding to the first dark ring _d_; then newton found the distance corresponding to the second dark ring _d_; the thickness corresponding to the third dark ring _d_; the thickness corresponding to the tenth dark ring _d_, and so on. surely there must be some hidden meaning in this little distance, _d_, which turns up so constantly? one can imagine the intense interest with which newton pondered its meaning. observe the probable outcome of his thought. he had endowed his light-particles with poles, but now he is forced to introduce the notion of _periodic recurrence_. here his power of transfer from the sensible to the subsensible would render it easy for him to suppose the light-particles animated, not only with a motion of translation, but also with a motion of rotation. newton's astronomical knowledge rendered all such conceptions familiar to him. the earth has such a double motion. in the time occupied in passing over a million and a half of miles of its orbit--that is, in twenty-four hours--our planet performs a complete rotation; and in the time required to pass over the distance _d_, newton's light-particle might be supposed to perform a complete rotation. true, the light-particle is smaller than the planet, and the distance _d_, instead of being a million and a half of miles, is a little over the ninety thousandth of an inch. but the two conceptions are, in point of intellectual quality, identical. imagine, then, a particle entering the film of air where it possesses this precise thickness. to enter the film, its attracted end must be presented. within the film it is able to turn _once_ completely round; at the other side of the film its attracted pole will be again presented; it will, therefore, enter the glass at the opposite side of the film _and be lost to the eye_. all round the place of contact, wherever the film possesses this precise thickness, the light will equally disappear--we shall therefore have a ring of darkness. and now observe how well this conception falls in with the law of proportionality discovered by newton. when the thickness of the film is _d_, the particle has time to perform, _two_ complete rotations within the film; when the thickness is _d, three_ complete rotations; when _d, ten_ complete rotations are performed. it is manifest that in each of these cases, on arriving at the second surface of the film, the attracted pole of the particle will be presented. it will, therefore, be transmitted; and, because no light is sent to the eye, we shall have a ring of darkness at each of these places. the bright rings follow immediately from the same conception. they occur between the dark rings, the thicknesses to which they correspond being also intermediate between those of the dark ones. take the case of the first bright ring. the thickness of the film is ½_d_; in this interval the rotating particle can perform only half a rotation. when, therefore, it reaches the second surface of the film, its repelled pole is presented; it is, therefore, driven back and reaches the eye. at all distances round the centre corresponding to this thickness the same effect is produced, and the consequence is a ring of brightness. the other bright rings are similarly accounted for. at the second one, where the thickness is ½_d_, a rotation and a half is performed; at the third, two rotations and a half; and at each of these places the particles present their repelled poles to the lower surface of the film. they are therefore sent back to the eye, and produce there the impression of brightness. this analysis, though involving difficulties when closely scrutinised, enables us to see how the theory of fits may have grown into consistency in the mind of newton. it has been already stated that the emission theory assigned a greater velocity to light in glass and water than in air or stellar space; and that on this point it was at direct issue with the theory of undulation, which makes the velocity in air or stellar space greater than in glass or water. by an experiment proposed by arago, and executed with consummate skill by foucault and fizeau, this question was brought to a crucial test, and decided in favour of the theory of undulation. in the present instance also the two theories are at variance. newton assumed that the action which produces the alternate bright and dark rings took place at a _single surface_; that is, the second surface of the film. the undulatory theory affirms that the rings are caused by the interference of waves reflected from both surfaces. this also has been demonstrated by experiment. by a proper arrangement, as we shall afterwards learn, we may abolish reflection from one of the surfaces of the film, and when this is done the rings vanish altogether. rings of feeble intensity are also formed by _transmitted_ light. these are referred by the undulatory theory to the interference of waves which have passed _directly_ through the film, with others which have suffered _two_ reflections within the film, and are thus completely accounted for. § . _the diffraction of light_. newton's espousal of the emission theory is said to have retarded scientific discovery. it might, however, be questioned whether, in the long run, the errors of great men have not really their effect in rendering intellectual progress rhythmical, instead of permitting it to remain uniform, the 'retardation' in each case being the prelude to a more impetuous advance. it is confusion and stagnation, rather than error, that we ought to avoid. thus, though the undulatory theory was held back for a time, it gathered strength in the interval, and its development within the last half century has been so rapid and triumphant as to leave no rival in the field. we have now to turn to the investigation of new classes of phenomena, of which it alone can render a satisfactory account. newton, who was familiar with the idea of an ether, and who introduced it in some of his speculations, objected, as already stated, that if light consisted of waves shadows could not exist; for that the waves would bend round the edges of opaque bodies and agitate the ether behind them. he was right in affirming that this bending ought to occur, but wrong in supposing that it does not occur. the bending is real, though in all ordinary cases it is masked by the action of interference. this inflection of the light receives the name of _diffraction_. to study the phenomena of diffraction it is necessary that our source of light should be a physical point, or a fine line; for when a luminous surface is employed, the waves issuing from different points of the surface obscure and neutralize each other. a _point_ of light of high intensity is obtained by admitting the parallel rays of the sun through an aperture in a window-shutter, and concentrating the beam by a lens of short focus. the small solar image at the focus constitutes a suitable point of light. the image of the sun formed on the convex surface of a glass bead, or of a watch-glass blackened within, though less intense, will also answer. an intense _line_ of light is obtained by admitting the sunlight through a slit and sending it through a strong cylindrical lens. the slice of light is contracted to a physical line at the focus of the lens. a glass tube blackened within and placed in the light, reflects from its surface a luminous line which, though less intense, also answers the purpose. in the experiment now to be described a vertical slit of variable width is placed in front of the electric lamp, and this slit is looked at from a distance through another vertical slit, also of variable aperture, and held in the hand. the light of the lamp being, in the first place, rendered monochromatic by placing a pure red glass in front of the slit, when the eye is placed in the straight line drawn through both slits an extraordinary appearance (shown in fig. ) is observed. firstly, the slit in front of the lamp is seen as a vivid rectangle of light; but right and left of it is a long series of rectangles, decreasing in vividness, and separated from each other by intervals of absolute darkness. the breadth of these bands is seen to vary with the width of the slit held before the eye. when the slit is widened the bands become narrower, and crowd more losely together; when the slit is narrowed, the individual bands widen and also retreat from each other, leaving between them wider spaces of darkness than before. [illustration: fig. .] leaving everything else unchanged, let a blue glass or a solution of ammonia-sulphate of copper, which gives a very pure blue, be placed in the path of the light. a series of blue bands is thus obtained, exactly like the former in all respects save one; the blue rectangles are _narrower_, and they are _closer together_ than the red ones. if we employ colours of intermediate refrangibilities, which we may do by causing the different colours of a spectrum to shine through the slit, we obtain bands of colour intermediate in width, and occupying intermediate positions, between those of the red and blue. the aspect of the bands in red, green, and violet light is represented in fig. . when _white light_, therefore, passes through the slit the various colours are not superposed, and instead of a series of monochromatic bands, separated from each other by intervals of darkness, we have a series of coloured spectra placed side by side. when the distant slit is illuminated by a candle flame, instead of the more intense electric light, or when a distant platinum wire raised to a white heat by an electric current is employed, substantially the same effects are observed. [illustration: fig. .] § . _application of the wave-theory to the phenomena of diffraction_. of these and of a multitude of similar effects the emission theory is incompetent to offer any satisfactory explanation. let us see how they are accounted for by the theory of undulation. and here, with the view of reaching absolute clearness, i must make an appeal to that faculty the importance of which i have dwelt upon so earnestly here and elsewhere--the faculty of imagination. figure yourself upon the sea-shore, with a well-formed wave advancing. take a line of particles along the front of the wave, all at the same distance below the crest; they are all rising in the same manner and at the same rate. take a similar line of particles on the back of the wave, they are all falling in the same manner and at the same rate. take a line of particles along the crest, they are all in the same condition as regards the motion of the wave. the same is true for a line of particles along the furrow of the wave. the particles referred to in each of these cases respectively, being in the same condition as regards the motion of the wave, are said to be in the same _phase_ of vibration. but if you compare a particle on the front of the wave with one at the back; or, more generally, if you compare together any two particles not occupying the same position in the wave, their conditions of motion not being the same, they are said to be in different phases of vibration. if one of the particles lie upon the crest, and the other on the furrow of the wave, then, as one is about to rise and the other about to fall, they are said to be in _opposite_ phases of vibration. there is still another point to be cleared up--and it is one of the utmost importance as regards our present subject. let o (fig. ) be a spot in still water which, when disturbed, produces a series of circular waves: the disturbance necessary to produce these waves is simply an oscillation up and down of the water at o. let _m_ _n_ be the position of the ridge of one of the waves at any moment, and _m'_ _n'_ its position a second or two afterwards. now every particle of water, as the wave passes it, oscillates, as we have learned, up and down. if, then, this oscillation be a sufficient origin of wave-motion, each distinct particle of the wave _m_ _n_ ought to give birth, to a series of circular waves. this is the important point up to which i wish to lead you. every particle of the wave _m_ _n_ _does_ act in this way. taking each particle as a centre, and surrounding it by a circular wave with a radius equal to the distance between _m_ _n_ and _m'_ _n'_, the coalescence of all these little waves would build up the large ridge _m'_ _n'_ exactly as we find it built up in nature. here, in fact, we resolve the wave-motion into its elements, and having succeeded in doing this we shall have no great difficulty in applying our knowledge to optical phenomena. [illustration: fig. .] now let us return to our slit, and, for the sake of simplicity, we will first consider the case of monochromatic light. conceive a series of waves of ether advancing from the first slit towards the second, and finally filling the second slit. when each wave passes through the latter it not only pursues its direct course to the retina, but diverges right and left, tending to throw into motion the entire mass of the ether behind the slit. in fact, as already explained, _every point of the wave which fills the slit is itself a centre of a new wave system which is transmitted in all directions through the ether behind the slit_. this is the celebrated principle of huyghens: we have now to examine how these secondary waves act upon each other. [illustration: fig. .] let us first regard the central band of the series. let ap (fig. ) be the width of the aperture held before the eye, grossly exaggerated of course, and let the dots across the aperture represent ether particles, all in the same phase of vibration. let e t represent a portion of the retina. from o, in the centre of the slit, let a perpendicular o r be imagined drawn upon the retina. the motion communicated to the point r will then be the sum of all the motions emanating in this direction from the ether particles in the slit. considering the extreme narrowness of the aperture, we may, without sensible error, regard all points of the wave a p as equally distant from r. no one of the partial waves lags sensibly behind the others: hence, at r, and in its immediate neighbourhood, we have no sensible reduction of the light by interference. this undiminished light produces the brilliant central band of the series. let us now consider those waves which diverge laterally behind the second slit. in this case the waves from the two sides of the slit have, in order to converge upon the retina, to pass over unequal distances. let a p (fig. ) represent, as before, the width of the second slit. we have now to consider the action of the various parts of the wave a p upon a point r' of the retina, not situated in the line joining the two slits. [illustration: fig. .] let us take the particular case in which the difference of path from the two marginal points a, p, to the retina is a whole wave-length of the red light; how must this difference affect the final illumination of the retina? let us fix our attention upon the particular oblique line that passes through the _centre_ o of the slit to the retina at r'. the difference of path between the waves which pass along this line and those from the two margins is, in the case here supposed, half a wavelength. make _e_ r' equal to p r', join p and _e_, and draw o _d_ parallel to p e. a e is then the length of a wave of light, while a _d_ is half a wave-length. now the least reflection will make it clear that not only is there discordance between the central and marginal waves, but that every line of waves such as _x_ r', on the one side of o r', finds a line _x_' r' upon the other side of o r', from which its path differs by half an undulation--with which, therefore, it is in complete discordance. the consequence is, that the light on the one side of the central line will completely abolish the light on the other side of that line, absolute darkness being the result of their coalescence. the first dark interval of our series of bands is thus accounted for. it is produced by an obliquity of direction which causes the paths of the marginal waves to be _a whole wave-length_ different from each other. when the difference between the paths of the marginal waves is _half a wave-length,_ a partial destruction of the light is effected. the luminous intensity corresponding to this obliquity is a little less than one-half--accurately . --that of the undiffracted light. if the paths of the marginal waves be three semi-undulations different from each other, and if the whole beam be divided into three equal parts, two of these parts will, for the reasons just given, completely neutralize each other, the third only being effective. corresponding, therefore, to an obliquity which produces a difference of three semi-undulations in the marginal waves, we have a luminous band, but one of considerably less intensity than the undiffracted central band. with a marginal difference of path of four semi-undulations we have a second extinction of the entire beam, because here the beam can be divided into four equal parts, every two of which quench each other. a second space of absolute darkness will therefore correspond to the obliquity producing this difference. in this way we might proceed further, the general result being that, whenever the direction of wave-motion is such as to produce a marginal difference of path of an _even_ number of semi-undulations, we have complete extinction; while, when the marginal difference is an _odd_ number of semi-undulations, we have only partial extinction, a portion of the beam remaining as a luminous band. a moment's reflection will make it plain that the wider the slit the less will be the obliquity of direction needed to produce the necessary difference of path. with a wide slit, therefore, the bands, as observed, will be closer together than with a narrow one. it is also plain that the shorter the wave, the less will be the obliquity required to produce the necessary retardation. the maxima and minima of violet light must therefore fall nearer to the centre than the maxima and minima of red light. the maxima and minima of the other colours fall between these extremes. in this simple way the undulatory theory completely accounts for the extraordinary appearance above referred to. when a slit and telescope are used, instead of the slit and naked eye, the effects are magnified and rendered more brilliant. looking, moreover, through a properly adjusted telescope with a small circular aperture in front of it, at a distant point of light, the point is seen encircled by a series of coloured bands. if monochromatic light be used, these bands are simply bright and dark, but with white light the circles display iris-colours. if a slit be shortened so as to form a square aperture, we have two series of spectra at right angles to each other. the effects, indeed, are capable of endless variation by varying the size, shape, and number of the apertures through which the point of light is observed. through two square apertures, with their corners touching each other as at a, schwerd observed the appearance shown in fig. . adding two others to them, as at b, he observed the appearance represented in fig. . the position of every band of light and shade in such figures has been calculated from theory by fresnel, fraunhofer, herschel, schwerd, and others, and completely verified by experiment. your eyes could not tell you with greater certainty of the existence of these bands than the theoretic calculation. [illustration: fig. .] the street-lamps at night, looked at through the meshes of a handkerchief, show diffraction phenomena. the diffraction effects obtained in looking through a bird's feathers are, as shown by schwerd, very brilliant. the iridescence of certain alpine clouds is also an effect of diffraction which may be imitated by the spores of lycopodium. when shaken over a glass plate these spores cause a point of light, looked at through the dusted plate, to be surrounded by coloured circles, which rise to actual splendour when the light becomes intense. shaken in the air the spores produce the same effect. the diffraction phenomena obtained during the artificial precipitation of clouds from the vapours of various liquids in an intensely illuminated tube are, as i have elsewhere shewn, exceedingly fine. [illustration: fig. .] one of the most interesting cases of diffraction by small particles that ever came before me was that of an artist whose vision was disturbed by vividly coloured circles. he was in great dread of losing his sight; assigning as a cause of his increased fear that the circles were becoming larger and the colours more vivid. i ascribed the colours to minute particles in the humours of the eye, and ventured to encourage him by the assurance that the increase of size and vividness on the part of the circles indicated that the diffracting particles were becoming _smaller_, and that they might finally be altogether absorbed. the prediction was verified. it is needless to say one word on the necessity of optical knowledge in the case of the practical oculist. without breaking ground on the chromatic phenomena presented by crystals, two other sources of colour may be mentioned here. by interference in the earth's atmosphere, the light of a star, as shown by arago, is self-extinguished, the twinkling of the star and the changes of colour which it undergoes being due to this cause. looking at such a star through an opera-glass, and shaking the glass so as to cause the image of the star to pass rapidly over the retina, you produce a row of coloured beads, the spaces between which correspond to the periods of extinction. fine scratches drawn upon glass or polished metal reflect the waves of light from their sides; and some, being reflected from the opposite sides of the same scratch, interfere with and quench each other. but the obliquity of reflection which extinguishes the shorter waves does not extinguish the longer ones, hence the phenomena of colours. these are called the colours of _striated surfaces_. they are beautifully illustrated by mother-of-pearl. this shell is composed of exceedingly thin layers, which, when cut across by the polishing of the shell, expose their edges and furnish the necessary small and regular grooves. the most conclusive proof that the colours are due to the mechanical state of the surface is to be found in the fact, established by brewster, that by stamping the shell carefully upon black sealing-wax, we transfer the grooves, and produce upon the wax the colours of mother-of-pearl. lecture iii. relation of theories to experience origin of the notion of the attraction of gravitation notion of polarity, how generated atomic polarity structural arrangements due to polarity architecture of crystals considered as an introduction to their action upon light notion of atomic polarity applied to crystalline structure experimental illustrations crystallization of water expansion by heat and by cold deportment of water considered and explained bearings of crystallization on optical phenomena refraction double refraction polarization action of tourmaline character of the beams emergent from iceland spar polarization by ordinary refraction and reflection depolarization § . _derivation of theoretic conceptions from experience._ one of the objects of our last lecture, and that not the least important, was to illustrate the manner in which scientific theories are formed. they, in the first place, take their rise in the desire of the mind to penetrate to the sources of phenomena. from its infinitesimal beginnings, in ages long past, this desire has grown and strengthened into an imperious demand of man's intellectual nature. it long ago prompted cæsar to say that he would exchange his victories for a glimpse of the sources of the nile; it wrought itself into the atomic theories of lucretius; it impelled darwin to those daring speculations which of late years have so agitated the public mind. but in no case, while framing theories, does the imagination _create_ its materials. it expands, diminishes, moulds, and refines, as the case may be, materials derived from the world of fact and observation. this is more evidently the case in a theory like that of light, where the motions of a subsensible medium, the ether, are presented to the mind. but no theory escapes the condition. newton took care not to encumber the idea of gravitation with unnecessary physical conceptions; but we know that he indulged in them, though he did not connect them with his theory. but even the theory, as it stands, did not enter the mind as a revelation dissevered from the world of experience. the germ of the conception that the sun and planets are held together by a force of attraction is to be found in the fact that a magnet had been previously seen to attract iron. the notion of matter attracting matter came thus from without, not from within. in our present lecture the magnetic force must serve as the portal into a new domain; but in the first place we must master its elementary phenomena. the general facts of magnetism are most simply illustrated by a magnetized bar of steel, commonly called a bar magnet. placing such a magnet upright upon a table, and bringing a magnetic needle near its bottom, one end of the needle is observed to retreat from the magnet, while the other as promptly approaches. the needle is held quivering there by some invisible influence exerted upon it. raising the needle along the magnet, but still avoiding contact, the rapidity of its oscillations decreases, because the force acting upon it becomes weaker. at the centre the oscillations cease. above the centre, the end of the needle which had been previously drawn towards the magnet retreats, and the opposite end approaches. as we ascend higher, the oscillations become more violent, because the force becomes stronger. at the upper end of the magnet, as at the lower, the force reaches a maximum; but all the lower half of the magnet, from e to s (fig. ), attracts one end of the needle, while all the upper half, from e to n, attracts the opposite end. this _doubleness_ of the magnetic force is called _polarity_, and the points near the ends of the magnet in which the forces seem concentrated are called its _poles_. [illustration: fig. .] what, then, will occur if we break this magnet in two at the centre e? shall we obtain two magnets, each with a single pole? no; each half is in itself a perfect magnet, possessing two poles. this may be proved by breaking something of less value than the magnet--the steel of a lady's stays, for example, hardened and magnetized. it acts like the magnet. when broken, each half acts like the whole; and when these parts are again broken, we have still the perfect magnet, possessing, as in the first instance, two poles. push your breaking to its utmost sensible limit--you cannot stop there. the bias derived from observation will infallibly carry you beyond the bourne of the senses, and compel you to regard this thing that we call magnetic polarity as resident in the ultimate particles of the steel. you come to the conclusion that each molecule of the magnet is endowed with this polar force. like all other forces, this force of magnetism is amenable to mechanical laws; and, knowing the direction and magnitude of the force, we can predict its action. placing a small magnetic needle near a bar magnet, it takes a determinate position. that position might be deduced theoretically from the mutual action of the poles. moving the needle round the magnet, for each point of the surrounding space there is a definite direction of the needle and no other. a needle of iron will answer as well as the magnetic needle; for the needle of iron is magnetized by the magnet, and acts exactly like a steel needle independently magnetized. if we place two or more needles of iron near the magnet, the action becomes more complex, for then the needles are not only acted on by the magnet, but they act upon each other. and if we pass to smaller masses of iron--to iron filings, for example--we find that they act substantially as the needles, arranging themselves in definite forms, in obedience to the magnetic action. placing a sheet of paper or glass over a bar magnet and showering iron filings upon the paper, i notice a tendency of the filings to arrange themselves in determinate lines. they cannot freely follow this tendency, for they are hampered by the friction against the paper. they are helped by tapping the paper; each tap releasing them for a moment, and enabling them to follow their tendencies. but this is an experiment which can only be seen by myself. to enable you all to see it, i take a pair of small magnets and by a simple optical arrangement throw the magnified images of the magnets upon the screen. scattering iron filings over the glass plate to which the small magnets are attached, and tapping the plate, you see the arrangement of the iron filings in those magnetic curves which have been so long familiar to scientific men (fig. ). [illustration: fig. . n is the nozzle of the lamp; m a plane mirror, reflecting the beam upwards. at p the magnets and iron filings are placed; l is a lens which forms an image of the magnets and filings; and r is a totally reflecting prism, which casts the image g upon the screen.] (by a very ingenious device, professor mayer, of hoboken, has succeeded in fixing and photographing the magnetic curves. i am indebted to his kindness for the annexed beautiful illustration, fig. .) the aspect of these curves so fascinated faraday that the greater portion of his intellectual life was devoted to pondering over them. he invested the space through which they run with a kind of materiality; and the probability is that the progress of science, by connecting the phenomena of magnetism with the luminiferous ether, will prove these 'lines of force,' as faraday loved to call them, to represent a condition of this mysterious substratum of all radiant action. it is not, however, the magnetic curves, as such, but their relationship to theoretic conceptions, that we have now to consider. by the action of the bar magnet upon the needle we obtain the notion of a polar force; by the breaking of the strip of magnetized steel we attain the notion that polarity can attach itself to the ultimate particles of matter. the experiment with the iron filings introduces a new idea into the mind; the idea, namely, of _structural arrangement_. every pair of filings possesses four poles, two of which are attractive and two repulsive. the attractive poles approach, the repulsive poles retreat; the consequence being a certain definite arrangement of the particles with reference to each other. § . _theory of crystallization._ now this idea of structure, as produced by polar force, opens a way for the intellect into an entirely new region, and the reason you are asked to accompany me into this region is, that our next inquiry relates to the action of crystals upon light. prior to speaking of this action, i wish you to realise intellectually the process of crystalline architecture. look then into a granite quarry, and spend a few minutes in examining the rock. it is not of perfectly uniform texture. it is rather an agglomeration of pieces, which, on examination, present curiously defined forms. you have there what mineralogists call quartz, you have felspar, you have mica. in a mineralogical cabinet, where these substances are preserved separately, you will obtain some notion of their forms. you will see there, also, specimens of beryl, topaz, emerald, tourmaline, heavy spar, fluor-spar, iceland spar--possibly a full-formed diamond, as it quitted the hand of nature, not yet having got into the hands of the lapidary. [illustration: fig. .] these crystals, you will observe, are put together according to law; they are not chance productions; and, if you care to examine them more minutely, you will find their architecture capable of being to some extent revealed. they often split in certain directions before a knife-edge, exposing smooth and shining surfaces, which are called planes of cleavage; and by following these planes you sometimes reach an internal form, disguised beneath the external form of the crystal. ponder these beautiful edifices of a hidden builder. you cannot help asking yourself how they were built; and familiar as you now are with the notion of a polar force, and the ability of that force to produce structural arrangement, your inevitable answer will be, that those crystals are built by the play of polar forces with which their molecules are endowed. in virtue of these forces, molecule lays itself to molecule in a perfectly definite way, the final visible form of the crystal depending upon this play of its ultimate particles. everywhere in nature we observe this tendency to run into definite forms, and nothing is easier than to give scope to this tendency by artificial arrangements. dissolve nitre in water, and allow the water slowly to evaporate; the nitre remains and the solution soon becomes so concentrated that the liquid condition can no longer be preserved. the nitre-molecules approach each other, and come at length within the range of their polar forces. they arrange themselves in obedience to these forces, a minute crystal of nitre being at first produced. on this crystal the molecules continue to deposit themselves from the surrounding liquid. the crystal grows, and finally we have large prisms of nitre, each of a perfectly definite shape. alum crystallizes with the utmost ease in this fashion. the resultant crystal is, however, different in shape from that of nitre, because the poles of the molecules are differently disposed. when they are _nursed_ with proper care, crystals of these substances may be caused to grow to a great size. the condition of perfect crystallization is, that the crystallizing force shall act with deliberation. there should be no hurry in its operations; but every molecule ought to be permitted, without disturbance from its neighbours, to exercise its own rights. if the crystallization be too sudden, the regularity disappears. water may be saturated with sulphate of soda, dissolved when the water is hot, and afterwards permitted to cool. when cold the solution is supersaturated; that is to say, more solid matter is contained in it than corresponds to its temperature. still the molecules show no sign of building themselves together. this is a very remarkable, though a very common fact. the molecules in the centre of the liquid are so hampered by the action of their neighbours that freedom to follow their own tendencies is denied to them. fix your mind's eye upon a molecule within the mass. it wishes to unite with its neighbour to the right, but it wishes equally to unite with its neighbour to the left; the one tendency neutralizes the other and it unites with neither. but, if a crystal of sulphate of soda be dropped into the solution, the molecular indecision ceases. on the crystal the adjacent molecules will immediately precipitate themselves; on these again others will be precipitated, and this act of precipitation will continue from the top of the flask to the bottom, until the solution has, as far as possible, assumed the solid form. the crystals here produced are small, and confusedly arranged. the process has been too hasty to admit of the pure and orderly action of the crystallizing force. it typifies the state of a nation in which natural and healthy change is resisted, until society becomes, as it were, supersaturated with the desire for change, the change being then effected through confusion and revolution. let me illustrate the action of the crystallizing force by two examples of it: nitre might be employed, but another well-known substance enables me to make the experiment in a better form. the substance is common sal-ammoniac, or chloride of ammonium, dissolved in water. cleansing perfectly a glass plate, the solution of the chloride is poured over the glass, to which when the plate is set on edge, a thin film of the liquid adheres. warming the glass slightly, evaporation is promoted, but by evaporation the water only is removed. the plate is then placed in a solar microscope, and an image of the film is thrown upon a white screen. the warmth of the illuminating beam adds itself to that already imparted to the glass plate, so that after a moment or two the dissolved salt can no longer exist in the liquid condition. molecule then closes with molecule, and you have a most impressive display of crystallizing energy overspreading the whole screen. you may produce something similar if you breathe upon the frost ferns which overspread your window-panes in winter, and then observe through a pocket lens the subsequent recongelation of the film. in this case the crystallizing force is hampered by the adhesion of the film to the glass; nevertheless, the play of power is strikingly beautiful. sometimes the crystals start from the edge of the film and run through it from that edge; for, the crystallization being once started, the molecules throw themselves by preference on the crystals already formed. sometimes the crystals start from definite nuclei in the centre of the film, every small crystalline particle which rests in the film furnishing a starting-point. throughout the process you notice one feature which is perfectly unalterable, and that is, angular magnitude. the spiculæ branch from the trunk, and from these branches others shoot; but the angles enclosed by the spiculæ are unalterable. in like manner you may find alum-crystals, quartz-crystals, and all other crystals, distorted in shape. they are thus far at the mercy of the accidents of crystallization; but in one particular they assert their superiority over all such accidents--_angular magnitude_ is always rigidly preserved. my second example of the action of crystallizing force is this: by sending a voltaic current through a liquid, you know that we decompose the liquid, and if it contains a metal, we liberate this metal by electrolysis. this small cell contains a solution of acetate of lead, which is chosen for our present purpose, because lead lends itself freely to this crystallizing power. into the cell are dipped two very thin platinum wires, and these are connected by other wires with a small voltaic battery. on sending the voltaic current through the solution, the lead will be slowly severed from the atoms with which it is now combined; it will be liberated upon one of the wires, and at the moment of its liberation it will obey the polar forces of its atoms, and produce crystalline forms of exquisite beauty. they are now before you, sprouting like ferns from the wire, appearing indeed like vegetable growths rendered so rapid as to be plainly visible to the naked eye. on reversing the current, these wonderful lead-fronds will dissolve, while from the other wire filaments of lead dart through the liquid. in a moment or two the growth of the lead-trees recommences, but they now cover the other wire. in the process of crystallization, nature first reveals herself as a builder. where do her operations stop? does she continue by the play of the same forces to form the vegetable, and afterwards the animal? whatever the answer to these questions may be, trust me that the notions of the coming generations regarding this mysterious thing, which some have called 'brute matter,' will be very different from those of the generations past. there is hardly a more beautiful and instructive example of this play of molecular force than that furnished by water. you have seen the exquisite fern-like forms produced by the crystallization of a film of water on a cold window-pane.[ ] you have also probably noticed the beautiful rosettes tied together by the crystallizing force during the descent of a snow-shower on a very calm day. the slopes and summits of the alps are loaded in winter with these blossoms of the frost. they vary infinitely in detail of beauty, but the same angular magnitude is preserved throughout: an inflexible power binding spears and spiculæ to the angle of degrees. the common ice of our lakes is also ruled in its formation by the same angle. you may sometimes see in freezing water small crystals of stellar shapes, each star consisting of six rays, with this angle of ° between every two of them. this structure may be revealed in ordinary ice. in a sunbeam, or, failing that, in our electric beam, we have an instrument delicate enough to unlock the frozen molecules, without disturbing the order of their architecture. cutting from clear, sound, regularly frozen ice, a slab parallel to the planes of freezing, and sending a sunbeam through such a slab, it liquefies internally at special points, round each point a six-petalled liquid flower of exquisite beauty being formed. crowds of such flowers are thus produced. from an ice-house we sometimes take blocks of ice presenting misty spaces in the otherwise continuous mass; and when we inquire into the cause of this mistiness, we find it to be due to myriads of small six-petalled flowers, into which the ice has been resolved by the mere heat of conduction. a moment's further devotion to the crystallization of water will be well repaid; for the sum of qualities which renders this substance fitted to play its part in nature may well excite wonder and stimulate thought. like almost all other substances, water is expanded by heat and contracted by cold. let this expansion and contraction be first illustrated:-- a small flask is filled with coloured water, and stopped with a cork. through the cork passes a glass tube water-tight, the liquid standing at a certain height in the tube. the flask and its tube resemble the bulb and stem of a thermometer. applying the heat of a spirit-lamp, the water rises in the tube, and finally trickles over the top. expansion by heat is thus illustrated. removing the lamp and piling a freezing mixture round the flask, the liquid column falls, thus showing the contraction of the water by the cold. but let the freezing mixture continue to act: the falling of the column continues to a certain point; it then ceases. the top of the column remains stationary for some seconds, and afterwards begins to rise. the contraction has ceased, and _expansion by cold_ sets in. let the expansion continue till the liquid trickles a second time over the top of the tube. the freezing mixture has here produced to all appearance the same effect as the flame. in the case of water, contraction by cold ceases, and expansion by cold sets in at the definite temperature of ° fahr. crystallization has virtually here commenced, the molecules preparing themselves for the subsequent act of solidification, which occurs at °, and in which the expansion suddenly culminates. in virtue of this expansion, ice, as you know, is lighter than water in the proportion of to .[ ] a molecular problem of great interest is here involved, and i wish now to place before you, for the satisfaction of your minds, a possible solution of the problem:-- consider, then, the ideal case of a number of magnets deprived of weight, but retaining their polar forces. if we had a mobile liquid of the specific gravity of steel, we might, by making the magnets float in it, realize this state of things, for in such a liquid the magnets would neither sink nor swim. now, the principle of gravitation enunciated by newton is that every particle of matter, of every kind, attracts every other particle with a force varying inversely as the square of the distance. in virtue of the attraction of gravity, then, the magnets, if perfectly free to move, would slowly approach each other. but besides the unpolar force of gravity, which belongs to matter in general, the magnets are endowed with the polar force of magnetism. for a time, however, the polar forces do not come sensibly into play. in this condition the magnets resemble our water-molecules at the temperature say of °. but the magnets come at length sufficiently near each other to enable their poles to interact. from this point the action ceases to be solely a general attraction of the masses. attractions of special points of the masses and repulsions of other points now come into play; and it is easy to see that the rearrangement of the magnets consequent upon the introduction of these new forces may be such as to require a greater amount of room. this, i take it, is the case with our water-molecules. like our ideal magnets, they approach each other for a time _as wholes_. previous to reaching the temperature ° fahr., the polar forces had doubtless begun to act, but it is at this temperature that their claim to more room exactly balances the contraction due to cold. at lower temperatures, as regards change of volume, the polar forces predominate. but they carry on a struggle with the force of contraction until the freezing temperature is attained. the molecules then close up to form solid crystals, a considerable augmentation of volume being the immediate consequence. § . _ordinary refraction of light explained by the wave theory_. we have now to exhibit the bearings of this act of crystallization upon optical phenomena. according to the undulatory theory, the velocity of light in water and glass is less than in air. consider, then, a small portion of a wave issuing from a point of light so distant that the minute area may be regarded as practically plane. moving vertically downwards, and impinging on a horizontal surface of glass or water, the wave would go through the medium without change of direction. as, however, the velocity in glass or water is less than the velocity in air, the wave would be retarded on passing into the denser medium. [illustration: fig. .] but suppose the wave, before reaching the glass, to be _oblique_ to the surface; that end of the wave which first reaches the medium will be the first retarded by it, the other portions as they enter the glass being retarded in succession. it is easy to see that this retardation of the one end of the wave must cause it to swing round and change its front, so that when the wave has fully entered the glass its course is oblique to its original direction. according to the undulatory theory, light is thus _refracted_. with these considerations to guide us, let us follow the course of a beam of monochromatic light through our glass prism. the velocity in air is to its velocity in glass as : . let a b c (fig. ) be the section of our prism, and _a_ _b_ the section of a plane wave approaching it in the direction of the arrow. when it reaches _c_ _d_, one end of the wave is on the point of entering the glass. following it still further, it is obvious that while the portion of the wave still in the air passes over the distance _c_ _e_, the wave in the glass will have passed over only two-thirds of this distance, or _d_ _f_. the line _e_ _f_ now marks the front of the wave. immersed wholly in the glass it pursues its way to _g_ _h_, where the end _g_ of the wave is on the point of escaping into the air. during the time required by the end _h_ of the wave to pass over the distance _h_ _k_ to the surface of the prism, the other end _g_, moving more rapidly, will have reached the point _i_. the wave, therefore, has again changed its front, so that after its emergence from the prism it will pass on to _l_ _m_, and subsequently in the direction of the arrow. the refraction of the beam is thus completely accounted for; and it is, moreover, based upon actual experiment, which proves that the ratio of the velocity of light in glass to its velocity in air is that here mentioned. it is plain that if the change of velocity on entering the glass were greater, the refraction also would be greater. § . _double refraction of light explained by the wave theory_. the two elements of rapidity of propagation, both of sound and light, in any substance whatever, are _elasticity_ and _density_, the speed increasing with the former and diminishing with the latter. the enormous velocity of light in stellar space is attainable because the ether is at the same time of infinitesimal density and of enormous elasticity. now the ether surrounds the atoms of all bodies, but it is not independent of them. in ponderable matter it acts as if its density were increased without a proportionate increase of elasticity; and this accounts for the diminished velocity of light in refracting bodies. we here reach a point of cardinal importance. in virtue of the crystalline architecture that we have been considering, the ether in many crystals possesses different densities, and different elasticities, in different directions; the consequence is, that in such crystals light is transmitted with different velocities. and as refraction depends wholly upon the change of velocity on entering the refracting medium, being greatest where the change of velocity is greatest, we have in many crystals two different refractions. by such crystals a beam of light is divided into two. this effect is called _double refraction_. in ordinary water, for example, there is nothing in the grouping of the molecules to interfere with the perfect homogeneity of the ether; but, when water crystallizes to ice, the case is different. in a plate of ice the elasticity of the ether in a direction perpendicular to the surface of freezing is different from what it is parallel to the surface of freezing; ice is, therefore, a double refracting substance. double refraction is displayed in a particularly impressive manner by iceland spar, which is crystallized carbonate of lime. the difference of ethereal density in two directions in this crystal is very great, the separation of the beam into the two halves being, therefore, particularly striking. i am unwilling to quit this subject before raising it to unmistakable clearness in your minds. the vibrations of light being transversal, the elasticity concerned in the propagation of any ray is the elasticity at right angles to the direction of propagation. in iceland spar there is one direction round which the crystalline molecules are symmetrically built. this direction is called the axis of the crystal. in consequence of this symmetry the elasticity is the same in all directions perpendicular to the axis, and hence a ray transmitted along the axis suffers no double refraction. but the elasticity along the axis is greater than the elasticity at right angles to it. consider, then, a system of waves crossing the crystal in a direction perpendicular to the axis. two directions of vibration are open to such waves: the ether particles can vibrate parallel to the axis or perpendicular to it. _they do both_, and hence immediately divide themselves into two systems propagated with different velocities. double refraction is the necessary consequence. [illustration: fig. .] by means of iceland spar cut in the proper direction, double refraction is capable of easy illustration. causing the beam which builds the image of our carbon-points to pass through the spar, the single image is instantly divided into two. projecting (by the lens e, fig. ) an image of the aperture (l) through which the light issues from the electric lamp, and introducing the spar (p), two luminous disks (e o) appear immediately upon the screen instead of one. the two beams into which the spar divides the single incident-beam have been subjected to the closest examination. they do not behave alike. one of them obeys the ordinary law of refraction discovered by snell, and is, therefore, called the _ordinary ray_: its index of refraction is . . the other does not obey this law. its index of refraction, for example, is not constant, but varies from a maximum of . to a minimum of . ; nor in this case do the incident and refracted rays always lie in the same plane. it is, therefore, called the _extraordinary ray_. in calc-spar, as just stated, the ordinary ray is the most refracted. one consequence of this merits a passing notice. pour water and bisulphide of carbon into two cups of the same depth; the cup that contains the more strongly refracting liquid will appear shallower than the other. place a piece of iceland spar over a dot of ink; two dots are seen, the one appearing nearer than the other to the eye. the nearest dot belongs to the most strongly refracted ray, exactly as the nearest cup-bottom belongs to the most highly refracting liquid. when you turn the spar round, the extraordinary image of the dot rotates round the ordinary one, which remains fixed. this is also the deportment of our two disks upon the screen. § . _polarization of light explained by the wave theory_. the double refraction of iceland spar was first treated in a work published by erasmus bartholinus, in . huyghens sought to account for this phenomenon on the principles of the wave theory, and he succeeded in doing so. he, moreover, made highly important observations on the distinctive character of the two beams transmitted by the spar, admitting, with resigned candour, that he had not solved the difficulty, and leaving the solution to future times. newton, reflecting on the observations of huyghens, came to the conclusion that each of the beams transmitted by iceland spar had two sides; and from the analogy of this _two-sidedness_ with the _two-endedness_ of a magnet, wherein consists its polarity, the two beams came subsequently to be described as _polarized_. we may begin the study of the polarization of light, with ease and profit, by means of a crystal of tourmaline. but we must start with a clear conception of an ordinary beam of light. it has been already explained that the vibrations of the individual ether-particles are executed _across_ the line of propagation. in the case of ordinary light we are to figure the ether-particles as vibrating in all directions, or azimuths, as it is sometimes expressed, across this line. now, in the case of a plate of tourmaline cut parallel to the axis of the crystal, a beam of light incident upon the plate is divided into two, the one vibrating parallel to the axis of the crystal, the other at right angles to the axis. the grouping of the molecules, and of the ether associated with the molecules, reduces all the vibrations incident upon the crystal to these two directions. one of these beams, namely, that whose vibrations are perpendicular to the axis, is quenched with exceeding rapidity by the tourmaline. to such vibrations many specimens of the crystal are highly opaque; so that, after having passed through a very small thickness of the tourmaline, the light emerges with all its vibrations reduced to a single plane. in this condition it is what we call _plane polarized light_. [illustration: fig. .] [illustration: fig. .] a moment's reflection will show that, if what is here stated be correct, on placing a second plate of tourmaline with its axis parallel to the first, the light will pass through both; but that, if the axes be crossed, the light that passes through the one plate will be quenched by the other, a total interception of the light being the consequence. let us test this conclusion by experiment. the image of a plate of tourmaline (_t_ _t_, fig. ) is now before you. i place parallel to it another plate (_t'_ _t'_): the green of the crystal is a little deepened, nothing more; this agrees with our conclusion. by means of an endless screw, i now turn one of the crystals gradually round, and you observe that as long as the two plates are oblique to each other, a certain portion of light gets through; but that when they are at right angles to each other, the space common to both is a space of darkness (fig. ). our conclusion, arrived at prior to experiment, is thus verified. let us now return to a single plate; and here let me say that it is on the green light transmitted by the tourmaline that you are to fix your attention. we have to illustrate the two-sidedness of that green light, in contrast to the all-sidedness of ordinary light. the white light surrounding the green image, being ordinary light, is reflected by a plane glass mirror in all directions; the green light, on the contrary, is not so reflected. the image of the tourmaline is now horizontal; reflected upwards, it is still green; reflected sideways, the image is reduced to blackness, because of the incompetency of the green light to be reflected in this direction. making the plate of tourmaline vertical, and reflecting it as before, it is the light of the upper image that is quenched; the side image now shows the green. this is a result of the greatest significance. if the vibrations of light were longitudinal, like those of sound, you could have no action of this kind; and this very action compels us to assume that the vibrations are transversal. picture the thing clearly. in the one case the mirror receives, as it were, the impact of the _edges_ of the waves, the green light being then quenched. in the other case the _sides_ of the waves strike the mirror, and the green light is reflected. to render the extinction complete, the light must be received upon the mirror at a special angle. what this angle is we shall learn presently. the quality of two-sidedness conferred upon light by bi-refracting crystals may also be conferred upon it by ordinary reflection. malus made this discovery in , while looking through iceland spar at the light of the sun reflected from the windows of the luxembourg palace in paris. i receive upon a plate of window-glass the beam from our lamp; a great portion of the light reflected from the glass is polarized. the vibrations of this reflected beam are executed, for the most part, parallel to the surface of the glass, and when the glass is held so that the beam shall make an angle of ° with the perpendicular to the glass, the _whole_ of the reflected beam is polarized. it was at this angle that the image of the tourmaline was completely quenched in our former experiment. it is called _the polarizing angle_. sir david brewster proved the angle of polarization of a medium to be that particular angle at which the refracted and reflected rays inclose a right angle.[ ] the polarizing angle augments with the index of refraction. for water it is ½°; for glass, as already stated, °; while for diamond it is °. and now let us try to make substantially the experiment of malus. the beam from the lamp is received at the proper angle upon a plate of glass and reflected through the spar. instead of two images, you see but one. so that the light, when polarized, as it now is by reflection, can only get through the spar in one direction, and consequently can produce but one image. why is this? in the iceland spar as in the tourmaline, all the vibrations of the ordinary light are reduced to two planes at right angles to each other; but, unlike the tourmaline, both beams are transmitted with equal facility by the spar. the two beams, in short, emergent from the spar, are polarized, their directions of vibration being at right angles to each other. when, therefore, the light is first polarized by reflection, the direction of vibration in the spar which coincides with the direction of vibration of the polarized beam, transmits the beam, and that direction only. only one image, therefore, is possible under the conditions. you will now observe that such logic as connects our experiments is simply a transcript of the logic of nature. on the screen before you are two disks of light produced by the double refraction of iceland spar. they are, as you know, two images of the aperture through which the light issues from the camera. placing the tourmaline in front of the aperture, two images of the crystal will also be obtained; but now let us reason out beforehand what is to be expected from this experiment. the light emergent from the tourmaline is polarized. placing the crystal with its axis horizontal, the vibrations of its transmitted light will be horizontal. now the spar, as already stated, has two directions of vibration, one of which at the present moment is vertical, the other horizontal. what are we to conclude? that the green light will be transmitted along the latter, which is parallel to the axis of the tourmaline, and not along the former, which is perpendicular to that axis. hence we may infer that one image of the tourmaline will show the ordinary green light of the crystal, while the other image will be black. tested by experiment, our reasoning is verified to the letter (fig. ). [illustration: fig. .] [illustration; fig. .] let us push our test still further. by means of an endless screw, the crystal can be turned ninety degrees round. the black image, as i turn, becomes gradually brighter, and the bright one gradually darker; at an angle of forty-five degrees both images are equally bright (fig. ); while, when ninety degrees have been obtained, the axis of the crystal being then vertical, the bright and black images have changed places, exactly as reasoning would have led us to suppose (fig. ). [illustration: fig. .] [illustration: fig. .] considering what has been already said (p. ) regarding the reflection of light polarized by transmission through tourmaline, you will readily foresee what must occur when we receive upon a plate of glass, held at the polarizing angle, the two beams emergent from our prism of iceland spar. i cause both beams to pass side by side through the air, catch them on a glass plate, and seek to reflect them upwards. at the polarizing angle one beam only is capable of being thus reflected. which? your prompt answer will be, the beam whose vibrations are horizontal (fig. ). i now turn the glass plate and try to reflect both beams laterally. one of them only is reflected; that, namely, the vibrations of which are vertical (fig. ). it is plain that, by means either of the tourmaline or the reflecting glass, we can determine in a moment the direction of vibration in any polarized beam. [illustration: fig. .] as already stated, the whole of a beam of ordinary light reflected from glass at the polarizing angle is polarized; a word must now be added regarding the far larger portion of the light which is _transmitted_ by the glass. the transmitted beam contains a quantity of polarized light equal to the reflected beam; but this is only a fraction of the whole transmitted light. by taking two plates of glass instead of one, we augment the quantity of the transmitted polarized light; and by taking _a bundle_ of plates, we so increase the quantity as to render the transmitted beam, for all practical purposes, _perfectly_ polarized. indeed, bundles of glass plates are often employed as a means of furnishing polarized light. it is important to note that the plane of vibration of this transmitted light is at right angles to that of the reflected light. one word more. when the tourmalines are crossed, the space where they cross each other is black. but we have seen that the least obliquity on the part of the crystals permits light to get through both. now suppose, when the two plates are crossed, that we interpose a third plate of tourmaline between them, with its axis oblique to both. a portion of the light transmitted by the first plate will get through this intermediate one. but, after it has got through, _its plane of vibration is changed_: it is no longer perpendicular to the axis of the crystal in front. hence it will, in part, get through that crystal. thus, by pure reasoning, we infer that the interposition of a third plate of tourmaline will in part abolish the darkness produced by the perpendicular crossing of the other two plates. i have not a third plate of tourmaline; but the talc or mica which you employ in your stoves is a more convenient substance, which acts in the same way. between the crossed tourmalines, i introduce a film of this crystal with its axis oblique to theirs. you see the edge of the film slowly descending, and, as it descends, light takes the place of darkness. the darkness, in fact, seems scraped away, as if it were something material. this effect has been called, naturally but improperly, _depolarization_. its proper meaning will be disclosed in our next lecture. these experiments and reasonings, if only thoroughly studied and understood, will form a solid groundwork for the analysis of the splendid optical phenomena next to be considered. lecture iv. chromatic phenomena produced by crystals in polarized light the nicol prism polarizer and analyzer action of thick and thin plates of selenite colours dependent on thickness resolution of polarized beam into two others by the selenite one of them more retarded than the other recompounding of the two systems of waves by the analyzer interference thus rendered possible consequent production of colours action of bodies mechanically strained or pressed action of sonorous vibrations action of glass strained or pressed by heat circular polarization chromatic phenomena produced by quartz the magnetization of light rings surrounding the axes of crystals biaxal and uniaxal crystals grasp of the undulatory theory the colour and polarization of sky-light generation of artificial skies. § . _action of crystals on polarized light: the nicol prism._ we have this evening to examine and illustrate the chromatic phenomena produced by the action of crystals, and double-refracting bodies generally, upon polarized light, and to apply the undulatory theory to their elucidation. for a long time investigators were compelled to employ plates of tourmaline for this purpose, and the progress they made with so defective a means of inquiry is astonishing. but these men had their hearts in their work, and were on this account enabled to extract great results from small instrumental appliances. for our present purpose we need far larger apparatus; and, happily, in these later times this need has been to a great extent satisfied. we have seen and examined the two beams emergent from iceland spar, and have proved them to be polarized. if, at the sacrifice of half the light, we could abolish one of these, the other would place at our disposal a beam of polarized light, incomparably stronger than any attainable from tourmaline. the beams, as you know, are refracted differently, and from this, as made plain in § , lecture i., we are able to infer that the one may be totally reflected, when the other is not. an able optician, named nicol, cut a crystal of iceland spar in two halves in a certain direction. he polished the severed surfaces, and reunited them by canada balsam, the surface of union being so inclined to the beam traversing the spar that the ordinary ray, which is the most highly refracted, was totally reflected by the balsam, while the extraordinary ray was permitted to pass on. let _b x, c y_ (fig. ) represent the section of an elongated rhomb of iceland spar cloven from the crystal. let this rhomb be cut along the plane _b c_; and the two severed surfaces, after having been polished, reunited by canada balsam. we learned, in our first lecture, that total reflection only takes place when a ray seeks to escape from a more refracting to a less refracting medium, and that it always, under these circumstances, takes place when the obliquity is sufficient. now the refractive index of iceland spar is, for the extraordinary ray less, and for the ordinary greater, than for canada balsam. hence, in passing from the spar to the balsam, the extraordinary ray passes from a less refracting to a more refracting medium, where total reflection cannot occur; while the ordinary ray passes from a more refracting to a less refracting medium, where total reflection can occur. the requisite obliquity is secured by making the rhomb of such a length that the plane of which _b c_ is the section shall be perpendicular, or nearly so, to the two end surfaces of the rhomb _b x, c y_. [illustration: fig. .] the invention of the nicol prism was a great step in practical optics, and quite recently such prisms have been constructed of a size and purity which enable audiences like the present to witness the chromatic phenomena of polarized light to a degree altogether unattainable a short time ago. (the two prisms employed in these experiments were lent to me by my lamented friend mr. william spottiswoode, and they were manufactured by mr. ahrens, an optician of consummate skill.) § . _colours of films of selenite in polarized light_. two nicol prisms play the same part as the two plates of tourmaline. placed with their directions of vibration parallel, the light passes through both; while when these directions are crossed the light is quenched. introducing a film of mica between the prisms, the light, as in the case of the tourmaline, is restored. but notice, when the film of mica is _thin_ you have sometimes not only light, but _coloured_ light. our work for some time to come will consist of the examination of such colours. with this view, i will take a representative crystal, one easily dealt with, because it cleaves with great facility--the crystal gypsum, or selenite, which is crystallized sulphate of lime. between the crossed nicols i place a thick plate of this crystal; like the mica, it restores the light, but it produces no colour. with my penknife i take a thin splinter from the crystal and place it between the prisms; the image of the splinter glows with the richest colours. turning the prism in front, these colours gradually fade and disappear, but, by continuing the rotation until the vibrating sections of the prisms are parallel to each other, vivid colours again arise, but these colours are complementary to the former ones. some patches of the splinter appear of one colour, some of another. these differences are due to the different thicknesses of the film. as in the case of hooke's thin plates, if the thickness be uniform the colour is uniform. here, for instance, is a stellar shape, every lozenge of the star being a film of gypsum of uniform thickness: each lozenge, you observe, shows a brilliant and uniform colour. it is easy, by shaping our films so as to represent flowers or other objects, to exhibit such objects in hues unattainable by art. here, for example, is a specimen of heart's-ease, the colours of which you might safely defy the artist to reproduce. by turning the front nicol degrees round, we pass through a colourless phase to a series of colours complementary to the former ones. this change is still more strikingly represented by a rose-tree, which is now presented in its natural hues--a red flower and green leaves; turning the prism degrees round, we obtain a green flower and red leaves. all these wonderful chromatic effects have definite mechanical causes in the motions of the ether. the principle of interference duly applied and interpreted explains them all. § . _colours of crystals in polarized light explained by the undulatory theory_. by this time you have learned that the word 'light' may be used in two different senses: it may mean the impression made upon consciousness, or it may mean the physical cause of the impression. it is with this cause that we have to occupy ourselves at present. the luminiferous ether is a substance which fills all space, and surrounds the atoms and molecules of bodies. to this inter-stellar and inter-atomic medium definite mechanical properties are ascribed, and we deal with it in our reasonings and calculations as a body possessed of these properties. in mechanics we have the composition and resolution of forces and of motions, extending to the composition and resolution of _vibrations_. we treat the luminiferous ether on mechanical principles, and, from the composition and resolution of its vibrations we deduce all the phenomena displayed by crystals in polarized light. [illustration: fig. .] let us take, as an example, the crystal of tourmaline, with which we are now so familiar. let a vibration cross this crystal oblique to its axis. experiment has assured us that a portion of the light will pass through. the quantity which passes we determine in this way. let a b (fig. ) be the axis of the tourmaline, and let _a_ _b_ represent the amplitude of an oblique ethereal vibration before it reaches a b. from _a_ and _b_ let the two perpendiculars _a_ _c_ and _b_ _d_ be drawn upon the axis: then _c_ _d_ will be the amplitude of the transmitted vibration. i shall immediately ask you to follow me while i endeavour to explain the effects observed when a film of gypsum is placed between the two nicol prisms. but, prior to this, it will be desirable to establish still further the analogy between the action of the prisms and that of the two plates of tourmaline. the magnified images of these plates, with their axes at right-angles to each other, are now before you. introducing between them a film of selenite, you observe that by turning the film round it may be placed in a position where it has no power to abolish the darkness of the superposed portions of the tourmalines. why is this? the answer is, that in the gypsum there are two directions, at right angles to each other, in which alone vibrations can take place, and that in our present experiment one of these directions is parallel to one of the axes of the tourmaline, and the other parallel to the other axis. when this is the case, the film exercises no sensible action upon the light. but now i turn the film so as to render its directions of vibration _oblique_ to the two tourmaline axes; then, you see it exercises the power, demonstrated in the last lecture, of partially restoring the light. [illustration: fig. .] let us now mount our nicol prisms, and cross them as we crossed the tourmaline. introducing our film of gypsum between them, you notice that in one particular position the film has no power whatever over the field of view. but, when the film is turned a little way round, the light passes. we have now to understand the mechanism by which this is effected. first, then, we have a prism which receives the light from the electric lamp, and which is called the _polarizer_. then we have the plate of gypsum (supposed to be placed at s, fig. ), and then the prism in front, which is called the _analyzer_. on its emergence from the first prism, the light is polarized; and, in the particular case now before us, its vibrations are executed in a horizontal plane. we have to examine what occurs when the two directions of vibration in the interposed gypsum are oblique to the horizon. draw a rectangular cross (a b, c d, fig. ) to represent these two directions. draw a line (_a_ _b_) to represent the amplitude of the horizontal vibration on the emergence of the light from the first nicol. let fall from each end of this line two perpendiculars (_a_ _c_, _a_ _f_, _b_ _d_, _b_ _e_) on the two arms of the cross; then the distances (_c_ _d_, _e_ _f_) between the feet of these perpendiculars represent the amplitudes of two rectangular vibrations, which are the _components_ of the first single vibration. thus the polarized ray, when it enters the gypsum, is resolved into its two equivalents, which vibrate at right angles to each other. [illustration; fig. .] in one of these two rectangular directions the ether within the gypsum is more sluggish than in the other; and, as a consequence, the waves that follow this direction are more retarded than the others. in both cases the undulations are shortened when they enter the gypsum, but in the one case they are more shortened than in the other. you can readily imagine that in this way the one system of waves may get half a wave-length, or indeed any number of half wavelengths, in advance of the other. the possibility of interference here at once flashes upon the mind. a little consideration, however, will render it evident that, as long as the vibrations are executed at right angles to each other, they cannot quench each other, no matter what the retardation may be. this brings us at once to the part played by the analyzer. its sole function is to recompound the two vibrations emergent from the gypsum. it reduces them to a single plane, where, if one of them be retarded by the proper amount, extinction will occur. but here, as in the case of thin films, the different lengths of the waves of light come into play. red will require a greater thickness to produce the retardation necessary for extinction than blue; consequently when the longer waves have been withdrawn by interference, the shorter ones remain, the film of gypsum shining with the colours which the short waves confer. conversely, when the shorter waves have been withdrawn, the thickness is such that the longer waves remain. an elementary consideration suffices to show, that when the directions of vibration of the prisms and the gypsum enclose an angle of forty-five degrees, the colours are at their maximum brilliancy. when the film is turned from this direction, the colours gradually fade, until, at the point where the directions of vibration in plate and prisms are parallel, they disappear altogether. (the best way of obtaining a knowledge of these phenomena is to construct a model of thin wood or pasteboard, representing the plate of gypsum, its planes of vibration, and also those of the polarizer and analyzer. two parallel pieces of the board are to be separated by an interval which shall represent the thickness of the film of gypsum. between them two other pieces, intersecting each other at a right angle, are to represent the planes of vibration within the film; while attached to the two parallel surfaces outside are two other pieces of board, which represent the planes of vibration of the polarizer and analyzer. on the two intersecting planes the waves are to be drawn, showing the resolution of the first polarized beam into two others, and then the subsequent reduction of the two systems of vibrations to a common plane by the analyzer. following out rigidly the interaction of the two systems of waves, we are taught by such a model that all the phenomena of colour obtained by the combination of the waves, when the planes of vibration of the two nicols are parallel, are displaced by the _complementary_ phenomena, when the planes of vibration are perpendicular to each other.) in considering the next point, we will operate, for the sake of simplicity, with monochromatic light--with red light, for example, which is easily obtained pure by red glass. supposing a certain thickness of the gypsum produces a retardation of half a wave-length, twice this thickness will produce a retardation of two half wave-lengths, three times this thickness a retardation of three half wave-lengths, and so on. now, when the nicols are parallel, the retardation of half a wave-length, or of any _odd_ number of half wave-lengths, produces extinction; at all thicknesses, on the other hand, which correspond to a retardation of an _even_ number of half wave-lengths, the two beams support each other, when they are brought to a common plane by the analyzer. supposing, then, that we take a plate of a wedge form, which grows gradually thicker from edge to back, we ought to expect, in red light, a series of recurrent bands of light and darkness; the dark bands occurring at thicknesses which produce retardations of one, three, five, etc., half wave-lengths, while the bright bands occur between the dark ones. experiment proves the wedge-shaped film to show these bands. they are also beautifully shown by a circular film, so worked as to be thinnest at the centre, and gradually increasing in thickness from the centre outwards. a splendid series of rings of light and darkness is thus produced. when, instead of employing red light, we employ blue, the rings are also seen: but as they occur at thinner portions of the film, they are smaller than the rings obtained with the red light. the consequence of employing white light may be now inferred; inasmuch as the red and the blue fall in different places, we have _iris-coloured_ rings produced by the white light. some of the chromatic effects of irregular crystallization are beautiful in the extreme. could i introduce between our two nicols a pane of glass covered by those frost-ferns which your cold weather renders now so frequent, rich colours would be the result. the beautiful effects of the irregular crystallization of tartaric acid and other substances on glass plates now presented to you, illustrate what you might expect from the frosted window-pane. and not only do crystalline bodies act thus upon light, but almost all bodies that possess a definite structure do the same. as a general rule, organic bodies act thus upon light; for their architecture implies an arrangement of the molecules, and of the ether associated with the molecules, which involves double refraction. a film of horn, or the section of a shell, for example, yields very beautiful colours in polarized light. in a tree, the ether certainly possesses different degrees of elasticity along and across the fibre; and, were wood transparent, this peculiarity of molecular structure would infallibly reveal itself by chromatic phenomena like those that you have seen. § . _colours produced by strain and pressure._ not only do natural bodies behave in this way, but it is possible, as shown by brewster, to confer, by artificial strain or pressure, a temporary double refracting structure upon non-crystalline bodies such as common glass. this is a point worthy of illustration. when i place a bar of wood across my knee and seek to break it, what is the mechanical condition of the bar? it bends, and its convex surface is _strained_ longitudinally; its concave surface, that next my knee, is longitudinally _pressed_. both in the strained portion and in the pressed portion of the wood the ether is thrown into a condition which would render the wood, were it transparent, double-refracting. for, in cases like the present, the drawing of the molecules asunder longitudinally is always accompanied by their approach to each other laterally; while the longitudinal squeezing is accompanied by lateral retreat. each half of the bar of wood exhibits this antithesis, and is therefore double-refracting. let us now repeat this experiment with a bar of glass. between the crossed nicols i introduce such a bar. by the dim residue of light lingering upon the screen, you see the image of the glass, but it has no effect upon the light. i simply bend the glass bar with my finger and thumb, keeping its length oblique to the directions of vibration in the nicols. instantly light flashes out upon the screen. the two sides of the bar are illuminated, the edges most, for here the strain and pressure are greatest. in passing from longitudinal strain to longitudinal pressure, we cross a portion of the glass where neither is exerted. this is the so-called neutral axis of the bar of glass, and along it you see a dark band, indicating that the glass along this axis exercises no action upon the light. by employing the force of a press, instead of the force of my finger and thumb, the brilliancy of the light is greatly augmented. again, i have here a square of glass which can be inserted into a press of another kind. introducing the uncompressed square between the prisms, its neutrality is declared; but it can hardly be held sufficiently loosely in the press to prevent its action from manifesting itself. already, though the pressure is infinitesimal, you see spots of light at the points where the press is in contact with the glass. on turning a screw, the image of the square of glass flashes out upon the screen. luminous spaces are seen separated from each other by dark bands. every two adjacent spaces are in opposite mechanical conditions. on one side of the dark band we have strain, on the other side pressure, the band marking the neutral axis between both. i now tighten the vice, and you see colour; tighten still more, and the colours appear as rich as those presented by crystals. releasing the vice, the colours suddenly vanish; tightening suddenly, they reappear. from the colours of a soap-bubble newton was able to infer the thickness of the bubble, thus uniting by the bond of thought apparently incongruous things. from the colours here presented to you, the magnitude of the pressure employed might be inferred. indeed, the late m. wertheim, of paris, invented an instrument for the determination of strains and pressures, by the colours of polarized light, which exceeded in accuracy all previous instruments of the kind. and now we have to push these considerations to a final illustration. polarized light may be turned to account in various ways as an analyzer of molecular condition. it may, for instance, be applied to reveal the condition of a solid body when it becomes sonorous. a strip of glass six feet long, two inches wide and a quarter of an inch thick, is held at the centre between the finger and thumb. on sweeping a wet woollen rag over one of its halves, you hear an acute sound due to the vibrations of the glass. what is the condition of the glass while the sound is heard? this: its two halves lengthen and shorten in quick succession. its two ends, therefore, are in a state of quick vibration; but at the centre the pulses from the two ends alternately meet and retreat from each other. between their opposing actions, the glass at the centre is kept motionless: but, on the other hand, it is alternately strained and compressed. in fig. , a b may be taken to represent the glass rectangle with its centre condensed; while a' b' represents the same rectangle with its centre rarefied. the ends of the strip suffer neither condensation nor rarefaction. [illustration: fig. ] if we introduce the strip of glass (_s_ _s'_, fig. ) between the crossed nicols, taking care to keep it oblique to the directions of vibration of the nicols, and sweep our wet rubber over the glass, this is what may be expected to occur: at every moment of compression the light will flash through; at every moment of strain the light will also flash through; and these states of strain and pressure will follow each other so rapidly, that we may expect a permanent luminous impression to be made upon the eye. by pure reasoning, therefore, we reach the conclusion that the light will be revived whenever the glass is sounded. that it is so, experiment testifies: at every sweep of the rubber (_h_, fig. ) a fine luminous disk (o) flashes out upon the screen. the experiment may be varied in this way: placing in front of the polarizer a plate of unannealed glass, you have a series of beautifully coloured rings, intersected by a black cross. every sweep of the rubber not only abolishes the rings, but introduces complementary ones, the black cross being, for the moment, supplanted by a white one. this is a modification of a beautiful experiment which we owe to biot. his apparatus, however, confined the observation of it to a single person at a time. [illustration: fig. .] § . _colours of unannealed glass_. bodies are usually expanded by heat and contracted by cold. if the heat be applied with perfect uniformity, no local strains or pressures come into play; but, if one portion of a solid be heated and another portion not, the expansion of the heated portion introduces strains and pressures which reveal themselves under the scrutiny of polarized light. when a square of common window-glass is placed between the nicols, you see its dim outline, but it exerts no action on the polarized light. held for a moment over the flame of a spirit-lamp, on reintroducing it between the nicols, light flashes out upon the screen. here, as in the case of mechanical action, you have luminous spaces of strain divided by dark neutral axes from spaces of pressure. [illustration: fig. .] [illustration: fig. .] let us apply the heat more symmetrically. a small square of glass is perforated at the centre, and into the orifice a bit of copper wire is introduced. placing the square between the prisms, and heating the wire, the heat passes by conduction to the glass, through which it spreads from the centre outwards. you immediately see four luminous quadrants and a dim cross, which becomes gradually blacker, by comparison with the adjacent brightness. and as, in the case of pressure, we produced colours, so here also, by the proper application of heat, gorgeous chromatic effects may be evoked. the condition necessary to the production of these colours may be rendered permanent by first heating the glass sufficiently, and then cooling it, so that the chilled mass shall remain in a state of permanent strain and pressure. two or three examples will illustrate this point. figs. and represent the figures obtained with two pieces of glass thus prepared; two rectangular pieces of unannealed glass, crossed and placed between the polarizer and analyzer, exhibit the beautiful iris fringes represented in fig. . [illustration: fig. .] § . _circular polarization._ but we have to follow the ether still further into its hiding-places. suspended before you is a pendulum, which, when drawn aside and liberated, oscillates to and fro. if, when the pendulum is passing the middle point of its excursion, i impart a shock to it tending to drive it at right angles to its present course, what occurs? the two impulses compound themselves to a vibration oblique in direction to the former one, but the pendulum still oscillates in _a plane_. but, if the rectangular shock be imparted to the pendulum when it is at the limit of its swing, then the compounding of the two impulses causes the suspended ball to describe, not a straight line, but an ellipse; and, if the shock be competent of itself to produce a vibration of the same amplitude as the first one, the ellipse becomes a circle. why do i dwell upon these things? simply to make known to you the resemblance of these gross mechanical vibrations to the vibrations of light. i hold in my hand a plate of quartz cut from the crystal perpendicular to its axis. the crystal thus cut possesses the extraordinary power of twisting the plane of vibration of a polarized ray to an extent dependent on the thickness of the crystal. and the more refrangible the light the greater is the amount of twisting; so that, when white light is employed, its constituent colours are thus drawn asunder. placing the quartz plate between the polarizer and analyzer, this vivid red appears; and, turning the analyzer in front from right to left, the other colours of the spectrum appear in succession. specimens of quartz have been found which require the analyzer to be turned from left to right to obtain the same succession of colours. crystals of the first class are therefore called right-handed, and of the second class, left-handed crystals. with profound sagacity, fresnel, to whose genius we mainly owe the expansion and final triumph of the undulatory theory of light, reproduced mentally the mechanism of these crystals, and showed their action to be due to the circumstance that, in them, the waves of ether so act upon each other as to produce the condition represented by our rotating pendulum. instead of being plane polarized, the light in rock crystal is _circularly polarized_. two such rays, transmitted along the axis of the crystal, and rotating in opposite directions, when brought to interference by the analyzer, are demonstrably competent to produce all the observed phenomena. § . _complementary colours of bi-refracting spar in circularly polarized light. proof that yellow and blue are complementary._ i now remove the analyzer, and put in its place the piece of iceland spar with which we have already illustrated double refraction. the two images of the carbon-points are now before you, produced, as you know, by two beams vibrating at right angles to each other. introducing a plate of quartz between the polarizer and the spar, the two images glow with complementary colours. employing the image of an aperture instead of that of the carbon-points, we have two coloured circles. as the analyzer is caused to rotate, the colours pass through various changes: but they are always complementary. when the one is red, the other is green; when the one is yellow, the other is blue. here we have it in our power to demonstrate afresh a statement made in our first lecture, that although the mixture of blue and yellow pigments produces green, the mixture of blue and yellow lights produces white. by enlarging our aperture, the two images produced by the spar are caused to approach each other, and finally to overlap. the one image is now a vivid yellow, the other a vivid blue, and you notice that where these colours are superposed we have a pure white. (see fig. , where n is the end of the polarizer, q the quartz plate, l a lens, and b the bi-refracting spar. the two images overlap at o, and produce white by their mixture.) [illustration: fig. .] § . _the magnetization of light._ this brings us to a point of our inquiries which, though rarely illustrated in lectures, is nevertheless so likely to affect profoundly the future course of scientific thought that i am unwilling to pass it over without reference. i refer to the experiment which faraday, its discoverer, called the 'magnetization of light.' the arrangement for this celebrated experiment is now before you. we have, first, our electric lamp, then a nicol prism, to polarize the beam emergent from the lamp; then an electro-magnet, then a second nicol, and finally our screen. at the present moment the prisms are crossed, and the screen is dark. i place from pole to pole of the electro-magnet a cylinder of a peculiar kind of glass, first made by faraday, and called faraday's heavy glass. through this glass the beam from the polarizer now passes, being intercepted by the nicol in front. on exciting the magnet light instantly appears upon the screen. by the action of the magnet upon the heavy glass the plane of vibration is caused to rotate, the light being thus enabled to get through the analyzer. the two classes into which quartz-crystals are divided have been already mentioned. in my hand i hold a compound plate, one half of it taken from a right-handed, and the other from a left-handed crystal. placing the plate in front of the polarizer, i turn one of the nicols until the two halves of the plate show a common puce colour. this yields an exceedingly sensitive means of rendering visible the action of a magnet upon light. by turning either the polarizer or the analyzer through the smallest angle, the uniformity of the colour disappears, and the two halves of the quartz show different colours. the magnet produces an effect equivalent to this rotation. the puce-coloured circle is now before you on the screen. (see fig. , where n is the nozzle of the lamp, h the first nicol, q the biquartz plate, l a lens, m the electro-magnet, with the heavy glass across its perforated poles, and p the second nicol.) exciting the magnet, one half of the image becomes suddenly red, the other half green. interrupting the current, the two colours fade away, and the primitive puce is restored. the action, moreover, depends upon the polarity of the magnet, or, in other words, on the direction of the current which surrounds the magnet. reversing the current, the red and green reappear, but they have changed places. the red was formerly to the right, and the green to the left; the green is now to the right, and the red to the left. with the most exquisite ingenuity, faraday analyzed all those actions and stated their laws. this experiment, however, long remained a scientific curiosity rather than a fruitful germ. that it would bear fruit of the highest importance, faraday felt profoundly convinced, and present researches are on the way to verify his conviction. [illustration: fig. ] § . _iris-rings surrounding the axes of crystals._ a few more words are necessary to complete our knowledge of the wonderful interaction between ponderable molecules and the ether interfused among them. symmetry of molecular arrangement implies symmetry on the part of the ether; atomic dissymmetry, on the other hand, involves the dissymmetry of the ether, and, as a consequence, double refraction. in a certain class of crystals the structure is homogeneous, and such crystals produce no double refraction. in certain other crystals the molecules are ranged symmetrically round a certain line, and not around others. along the former, therefore, the ray is undivided, while along all the others we have double refraction. ice is a familiar example: its molecules are built with perfect symmetry around the perpendiculars to the planes of freezing, and a ray sent through ice in this direction is not doubly refracted; whereas, in all other directions, it is. iceland spar is another example of the same kind: its molecules are built symmetrically round the line uniting the two blunt angles of the rhomb. in this direction a ray suffers no double refraction, in all others it does. this direction of no double refraction is called the _optic axis_ of the crystal. hence, if a plate be cut from a crystal of iceland spar perpendicular to the axis, all rays sent across this plate in the direction of the axis will produce but one image. but, the moment we deviate from the parallelism with the axis, double refraction sets in. if, therefore, a beam that has been rendered _conical_ by a converging lens be sent through the spar so that the central ray of the cone passes along the axis, this ray only will escape double refraction. each of the others will be divided into an ordinary and an extraordinary ray, the one moving more slowly through the crystal than the other; the one, therefore, retarded with reference to the other. here, then, we have the conditions for interference, when the waves are reduced by the analyzer to a common plane. placing the plate of iceland spar between the crossed nicol prisms, and employing the conical beam, we have upon the screen a beautiful system of iris-rings surrounding the end of the optic axis, the circular bands of colour being intersected by a black cross (fig. ). the arms of this cross are parallel to the two directions of vibration in the polarizer and analyzer. it is easy to see that those rays whose planes of vibration within the spar coincide with the plane of vibration of _either_ prism, cannot get through _both_. this complete interception produces the arms of the cross. [illustration: fig. .] with monochromatic light the rings would be simply bright and black--the bright rings occurring at those thicknesses of the spar which cause the rays to conspire; the black rings at those thicknesses which cause them to quench each other. turning the analyzer ° round, we obtain the complementary phenomena. the black cross gives place to a bright one, and every dark ring is supplanted also by a bright one (fig. ). here, as elsewhere, the different lengths of the light-waves give rise to iris-colours when white light is employed. [illustration: fig. .] [illustration: fig. .] besides the _regular_ crystals which produce double refraction in no direction, and the _uniaxal_ crystals which produce it in all directions but one, brewster discovered that in a large class of crystals there are _two_ directions in which double refraction does not take place. these are called _biaxal_ crystals. when plates of these crystals, suitably cut, are placed between the polarizer and analyzer, the axes (a a', fig. ) are seen surrounded, not by circles, but by curves of another order and of a perfectly definite mathematical character. each band, as proved experimentally by herschel, forms a _lemniscata_; but the experimental proof was here, as in numberless other cases, preceded by the deduction which showed that, according to the undulatory theory, the bands must possess this special character. § . _power of the wave theory_. i have taken this somewhat wide range over polarization itself, and over the phenomena exhibited by crystals in polarized light, in order to give you some notion of the firmness and completeness of the theory which grasps them all. starting from the single assumption of transverse undulations, we first of all determine the wave-lengths, and find that on them all the phenomena of colour are dependent. the wavelengths may be determined in many independent ways. newton virtually determined them when he measured the periods of his fits: the length of a fit, in fact, is that of a quarter of an undulation. the wave-lengths may be determined by diffraction at the edges of a slit (as in the appendix to these lectures); they may be deduced from the interference fringes produced by reflection; from the fringes produced by refraction; also by lines drawn with a diamond upon glass at measured distances asunder. and when the length determined by these independent methods are compared together, the strictest agreement is found to exist between them. with the wave-lengths once at our disposal, we follow the ether into the most complicated cases of interaction between it and ordinary matter, 'the theory is equal to them all. it makes not a single new physical hypothesis; but out of its original stock of principles it educes the counterparts of all that observation shows. it accounts for, explains, simplifies the most entangled cases; corrects known laws and facts; predicts and discloses unknown ones; becomes the guide of its former teacher observation; and, enlightened by mechanical conceptions, acquires an insight which pierces through shape and colour to force and cause.'[ ] but, while i have thus endeavoured to illustrate before you the power of the undulatory theory as a solver of all the difficulties of optics, do i therefore wish you to close your eyes to any evidence that may arise against it? by no means. you may urge, and justly urge, that a hundred years ago another theory was held by the most eminent men, and that, as the theory then held had to yield, the undulatory theory may have to yield also. this seems reasonable; but let us understand the precise value of the argument. in similar language a person in the time of newton, or even in our time, might reason thus: hipparchus and ptolemy, and numbers of great men after them, believed that the earth was the centre of the solar system. but this deep-set theoretic notion had to give way, and the helio-centric theory may, in its turn, have to give way also. this is just as reasonable as the first argument. wherein consists the strength of the present theory of gravitation? solely in its competence to account for all the phenomena of the solar system. wherein consists the strength of the theory of undulation? solely in its competence to disentangle and explain phenomena a hundred-fold more complex than those of the solar system. accept if you will the scepticism of mr. mill[ ] regarding the undulatory theory; but if your scepticism be philosophical, it will wrap the theory of gravitation in the same or in greater doubt.[ ] § . _the blue of the sky_. i am unwilling to quit these chromatic phenomena without referring to a source of colour which has often come before me of late in the blue of your skies at noon, and the deep crimson of your horizon after the set of sun. i will here summarize and extend what i have elsewhere said upon this subject. proofs of the most cogent description could be adduced to show that the blue light of the firmament is reflected light. that light comes to us across the direction of the solar rays, and even against the direction of the solar rays; and this lateral and opposing rush of wave-motion can only be due to the rebound of the waves from the air itself, or from something suspended in the air. the solar light, moreover, is not scattered by the sky in the proportions which produce white. the sky is blue, which indicates an excess of the smaller waves. the blueness of the air has been given as a reason for the blueness of the sky; but then the question arises, how, if the air be blue, can the light of sunrise and sunset, which travels through vast distances of air, be yellow, orange, or even red? the passage of the white solar light through a blue medium could by no possibility redden the light; the hypothesis of a blue atmosphere is therefore untenable. in fact, the agent, whatever it be, which sends us the light of the sky, exercises in so doing a dichroitic action. the light reflected is blue, the light transmitted is orange or red, a marked distinction is thus exhibited between reflection from the sky and that from an ordinary cloud, which exercises no such dichroitic action. the cloud, in fact, takes no note of size on the part of the waves of ether, but reflects them all alike. now the cause of this may be that the cloud-particles are so large in comparison with the size of the waves of ether as to scatter them all indifferently. a broad cliff reflects an atlantic roller as easily as it reflects a ripple produced by a sea-bird's wing; and, in the presence of large reflecting surfaces, the existing differences of magnitude among the waves of ether may also disappear. but supposing the reflecting particles, instead of being very large, to be very small, in comparison with the size of the waves. then, instead of the whole wave being fronted and in great part thrown back, a small portion only is shivered off by the obstacle. suppose, then, such minute foreign particles to be diffused in our atmosphere. waves of all sizes impinge upon them, and at every collision a portion of the impinging wave is struck off. all the waves of the spectrum, from the extreme red to the extreme violet, are thus acted upon; but in what proportions will they be scattered? largeness is a thing of relation; and the smaller the wave, the greater is the relative size of any particle on which the wave impinges, and the greater also the relative reflection. a small pebble, placed in the way of the ring-ripples produced by heavy rain-drops on a tranquil pond, will throw back a large fraction of each ripple incident upon it, while the fractional part of a larger wave thrown back by the same pebble might be infinitesimal. now to preserve the solar light white, its constituent proportions must not be altered; but in the scattering of the light by these very small particles we see that the proportions _are_ altered. the smaller waves are in excess, and, as a consequence, in the scattered light blue will be the predominant colour. the other colours of the spectrum must, to some extent, be associated with the blue: they are not absent, but deficient. we ought, in fact, to have them all, but in diminishing proportions, from the violet to the red. we have thus reasoned our way to the conclusion, that were particles, small in comparison to the size of the ether waves, sown in our atmosphere, the light scattered by those particles would be exactly such as we observe in our azure skies. and, indeed, when this light is analyzed, all the colours of the spectrum are found in the proportions indicated by our conclusion. by its successive collisions with the particles the white light is more and more robbed of its shorter waves; it therefore loses more and more of its due proportion of blue. the result may be anticipated. the transmitted light, where moderate distances are involved, will appear yellowish. but as the sun sinks towards the horizon the atmospheric distance increases, and consequently the number of the scattering particles. they weaken in succession the violet, the indigo, the blue, and even disturb the proportions of green. the transmitted light under such circumstances must pass from yellow through orange to red. this also is exactly what we find in nature. thus, while the reflected light gives us, at noon, the deep azure of the alpine skies, the transmitted light gives us, at sunset, the warm crimson of the alpine snows. but can small particles be really proved to act in the manner indicated? no doubt of it. each one of you can submit the question to an experimental test. water will not dissolve resin, but spirit will; and when spirit which holds resin in solution is dropped into water, the resin immediately separates in solid particles, which render the water milky. the coarseness of this precipitate depends on the quantity of the dissolved resin. professor brücke has given us the proportions which produce particles particularly suited to our present purpose. one gramme of clean mastic is dissolved in eighty-seven grammes of absolute alcohol, and the transparent solution is allowed to drop into a beaker containing clear water briskly stirred. an exceedingly fine precipitate is thus formed, which declares its presence by its action upon light. placing a dark surface behind the beaker, and permitting the light to fall into it from the top or front, the medium is seen to be of a very fair sky-blue. a trace of soap in water gives it a tint of blue. london milk makes an approximation to the same colour, through the operation of the same cause: and helmholtz has irreverently disclosed the fact that a blue eye is simply a turbid medium. § . _artificial sky_. but we have it in our power to imitate far more closely the natural conditions of this problem. we can generate in air artificial skies, and prove their perfect identity with the natural one, as regards the exhibition of a number of wholly unexpected phenomena. it has been recently shown in a great number of instances by myself that waves of ether issuing from a strong source, such as the sun or the electric light, are competent to shake asunder the atoms of gaseous molecules. the apparatus used to illustrate this consists of a glass tube about a yard in length, and from ½ to inches internal diameter. the gas or vapour to be examined is introduced into this tube, and upon it the condensed beam of the electric lamp is permitted to act. the vapour is so chosen that one, at least, of its products of decomposition, as soon as it is formed, shall be _precipitated_ to a kind of cloud. by graduating the quantity of the vapour, this precipitation may be rendered of any degree of fineness, forming particles distinguishable by the naked eye, or particles which are probably far beyond the reach of our highest microscopic powers. i have no reason to doubt that particles may be thus obtained whose diameters constitute but a very small fraction of the length of a wave of violet light. now, in all such cases when suitable vapours are employed in a sufficiently attenuated state, no matter what the vapour may be, the visible action commences with the formation of a _blue cloud_. let me guard myself at the outset against all misconception as to the use of this term. the blue cloud here referred to is totally invisible in ordinary daylight. to be seen, it requires to be surrounded by darkness, _it only_ being illuminated by a powerful beam of light. this cloud differs in many important particulars from the finest ordinary clouds, and might justly have assigned to it an intermediate position between these clouds and true cloudless vapour. it is possible to make the particles of this _actinic cloud_ grow from an infinitesimal and altogether ultra-microscopic size to particles of sensible magnitude; and by means of these in a certain stage of their growth, we produce a blue which rivals, if it does not transcend, that of the deepest and purest italian sky. introducing into our tube a quantity of mixed air and nitrite of butyl vapour sufficient to depress the mercurial column of an air-pump one-twentieth of an inch, adding a quantity of air and hydrochloric acid sufficient to depress the mercury half an inch further, and sending through this compound and highly attenuated atmosphere the beam of the electric light, within the tube arises gradually a splendid azure, which strengthens for a time, reaches a maximum of depth and purity, and then, as the particles grow larger, passes into whitish blue. this experiment is representative, and it illustrates a general principle. various other colourless substances of the most diverse properties, optical and chemical, might be employed for this experiment. the _incipient cloud_, in every case, would exhibit this superb blue; thus proving to demonstration that particles of infinitesimal size, without any colour of their own, and irrespective of those optical properties exhibited by the substance in a massive state, are competent to produce the blue colour of the sky. § . _polarization of skylight_. but there is another subject connected with our firmament, of a more subtle and recondite character than even its colour. i mean that 'mysterious and beautiful phenomenon,' as sir john herschel calls it, the polarization of the light of the sky. looking at various points of the blue firmament through a nicol prism, and turning the prism round its axis, we soon notice variations of brightness. in certain positions of the prism, and from certain points of the firmament, the light appears to be wholly transmitted, while it is only necessary to turn the prism round its axis through an angle of ninety degrees to materially diminish the intensity of the light. experiments of this kind prove that the blue light sent to us by the firmament is polarized, and on close scrutiny it is also found that the direction of most perfect polarization is perpendicular to the solar rays. were the heavenly azure like the ordinary light of the sun, the turning of the prism would have no effect upon it; it would be transmitted equally during the entire rotation of the prism. the light of the sky may be in great part quenched, because it is in great part polarized. the same phenomenon is exhibited in perfection by our actinic clouds, the only condition necessary to its production being the smallness of the particles. in all cases, and with all substances, the cloud formed at the commencement, when the precipitated particles are sufficiently fine, is _blue_. in all cases, moreover, this fine blue cloud polarizes _perfectly_ the beam which illuminates it, the direction of polarization enclosing an angle of ° with the axis of the illuminating beam. it is exceedingly interesting to observe both the growth and the decay of this polarization. for ten or fifteen minutes after its first appearance, the light from a vividly illuminated incipient cloud, looked at horizontally, is absolutely quenched by a nicol prism with its longer diagonal vertical. but as the sky-blue is gradually rendered impure by the introduction of particles of too large a size, in other words, as real clouds begin to be formed, the polarization begins to deteriorate, a portion of the light passing through the prism in all its positions, as it does in the case of skylight. it is worthy of note that for some time after the cessation of perfect polarization the _residual_ light which passes, when the nicol is in its position of minimum transmission, is of a gorgeous blue, the whiter light of the cloud being extinguished. when the cloud-texture has become sufficiently coarse to approximate to that of ordinary clouds, the rotation of the nicol ceases to have any sensible effect on the light discharged at right angles to the beam. the perfection of the polarization in a direction perpendicular to the illuminating beam may be also illustrated by the following experiment, which has been executed with many vapours. a nicol prism large enough to embrace the entire beam of the electric lamp was placed between the lamp and the experimental tube. sending the beam polarized by the nicol through the tube, i placed myself in front of it, the eyes being on a level with its axis, my assistant occupying a similar position behind the tube. the short diagonal of the large nicol was in the first instance vertical, the plane of vibration of the emergent beam being therefore also vertical. as the light continued to act, a superb blue cloud visible to both my assistant and myself was slowly formed. but this cloud, so deep and rich when looked at from the positions mentioned, utterly disappeared when looked at vertically downwards, or vertically upwards. reflection from the cloud was not possible in these directions. when the large nicol was slowly turned round its axis, the eye of the observer being on the level of the beam, and the line of vision perpendicular to it, entire extinction of the light emitted horizontally occurred when the longer diagonal of the large nicol was vertical. but a vivid blue cloud was seen when looked at downwards or upwards. this truly fine experiment, which i should certainly have made without suggestion, was, as a matter of fact, first definitely suggested by a remark addressed to me in a letter by professor stokes. all the phenomena of colour and of polarization observable in the case of skylight are manifested by those actinic clouds; and they exhibit additional phenomena which it would be neither convenient to pursue, nor perhaps possible to detect, in the actual firmament. they enable us, for example, to follow the polarization from its first appearance on the barely visible blue to its final extinction in the coarser cloud. these changes, as far as it is now necessary to refer to them, may be thus summed up:-- . the actinic cloud, as long as it continues blue, discharges polarized light in all directions, but the direction of maximum polarization, like that of skylight, is at right angles to the direction of the illuminating beam. . as long as the cloud remains distinctly blue, the light discharged from it at right angles to the illuminating beam is _perfectly_ polarized. it may be utterly quenched by a nicol prism, the cloud from which it issues being caused to disappear. any deviation from the perpendicular enables a portion of the light to get through the prism. . the direction of vibration of the polarized light is at right angles to the illuminating beam. hence a plate of tourmaline, with its axis parallel to the beam, stops the light, and with the axis perpendicular to the beam transmits the light. . a plate of selenite placed between the nicol and the actinic cloud shows the colours of polarized light; in fact, the cloud itself plays the part of a polarizing nicol. . the particles of the blue cloud are immeasurably small, but they increase gradually in size, and at a certain period of their growth cease to discharge perfectly polarized light. for some time afterwards the light that reaches the eye, through the nicol in its position of least transmission, is of a magnificent blue, far exceeding in depth and purity that of the purest sky; thus the waves that first feel the influence of size, at both limits of the polarization, are the shortest waves of the spectrum. these are the first to accept polarization, and they are the first to escape from it. lecture v. range of vision not commensurate with range of radiation the ultra-violet bays fluorescence the rendering of invisible rays visible vision not the only sense appealed to by the solar and electric beam heat of beam combustion by total beam at the foci of mirrors and lenses combustion through ice-lens ignition of diamond search for the rays here effective sir william herschel's discovery of dark solar rays invisible rays the basis of the visible detachment by a ray-filter of the invisible rays from the visible combustion at dark foci conversion of heat-rays into light-rays calorescence part played in nature by dark rays identity of light and radiant heat invisible images reflection, refraction, plane polarization, depolarization, circular polarization, double refraction, and magnetization of radiant heat. § . _range of vision and of radiation_. the first question that we have to consider to-night is this: is the eye, as an organ of vision, commensurate with the whole range of solar radiation--is it capable of receiving visual impressions from all the rays emitted by the sun? the answer is negative. if we allowed ourselves to accept for a moment that notion of gradual growth, amelioration, and ascension, implied by the term _evolution_, we might fairly conclude that there are stores of visual impressions awaiting man, far greater than those now in his possession. ritter discovered in that beyond the extreme violet of the spectrum there is a vast efflux of rays which are totally useless as regards our present powers of vision. these ultra-violet waves, however, though incompetent to awaken the optic nerve, can shake asunder the molecules of certain compound substances on which they impinge, thus producing chemical decomposition. but though the blue, violet, and ultra-violet rays can act thus upon certain substances, the fact is hardly sufficient to entitle them to the name of 'chemical rays,' which is usually applied to distinguish them from the other constituents of the spectrum. as regards their action upon the salts of silver, and many other substances, they may perhaps merit this title; but in the case of the grandest example of the chemical action of light--the decomposition of carbonic acid in the leaves of plants, with which my eminent friend dr. draper (now no more) has so indissolubly associated his name--the yellow rays are found to be the most active. there are substances, however, on which the violet and ultra-violet waves exert a special decomposing power; and, by permitting the invisible spectrum to fall upon surfaces prepared with such substances, we reveal both the existence and the extent of the ultraviolet spectrum. § . _ultra-violet rays: fluorescence_. the method of exhibiting the action of the ultraviolet rays by their chemical action has been long known; indeed, thomas young photographed the ultra-violet rings of newton. we have now to demonstrate their presence in another way. as a general rule, bodies either transmit light or absorb it; but there is a third case in which the light falling upon the body is neither transmitted nor absorbed, but converted into light of another kind. professor stokes, the occupant of the chair of newton in the university of cambridge, has demonstrated this change of one kind of light into another, and has pushed his experiments so far as to render the invisible rays visible. a large number of substances examined by stokes, when excited by the invisible ultra-violet waves, have been proved to emit light. you know the rate of vibration corresponding to the extreme violet of the spectrum; you are aware that to produce the impression of this colour, the retina is struck millions of millions of times in a second. at this point, the retina ceases to be useful as an organ of vision; for, though struck by waves of more rapid recurrence, they are incompetent to awaken the sensation of light. but when such non-visual waves are caused to impinge upon the molecules of certain substances--on those of sulphate of quinine, for example--they compel those molecules, or their constituent atoms, to vibrate; and the peculiarity is, that the vibrations thus set up are _of slower period_ than those of the exciting waves. by this lowering of the rate of vibration through the intermediation of the sulphate of quinine, the invisible rays are brought within the range of vision. we shall subsequently have abundant opportunity for learning that transparency to the visible by no means involves transparency to the invisible rays. our bisulphide of carbon, for example, which, employed in prisms, is so eminently suitable for experiments on the visual rays, is by no means so suitable for these ultra-violet rays. flint glass is better, and rock crystal is better than flint glass. a glass prism, however, will suit our present purpose. casting by means of such a prism a spectrum, not upon the white surface of our screen, but upon a sheet of paper which has been wetted with a saturated solution of the sulphate of quinine and afterwards dried, an obvious extension of the spectrum is revealed. we have, in the first instance, a portion of the violet rendered whiter and more brilliant; but, besides this, we have the gleaming of the colour where, in the case of unprepared paper, nothing is seen. other substances produce a similar effect. a substance, for example, recently discovered by president morton, and named by him _thallene_, produces a very striking elongation of the spectrum, the new light generated being of peculiar brilliancy. fluor spar, and some other substances, when raised to a temperature still under redness, emit light. during the ages which have elapsed since their formation, this capacity of shaking the ether into visual tremors appears to have been enjoyed by these substances. light has been potential within them all this time; and, as well explained by draper, the heat, though not itself of visual intensity, can unlock the molecules so as to enable them to exert their long-latent power of vibration. this deportment of fluor spar determined stokes in his choice of a name for his great discovery: he called this rendering visible of the ultra-violet rays _fluorescence_. by means of a deeply coloured violet glass, we cut off almost the whole of the light of our electric beam; but this glass is peculiarly transparent to the violet and ultra-violet rays. the violet beam now crosses a large jar filled with water, into which i pour a solution of sulphate of quinine. clouds, to all appearance opaque, instantly tumble downwards. fragments of horse-chestnut bark thrown upon the water also send down beautiful cloud-like strife. but these are not clouds: there is nothing precipitated here: the observed action is an action of _molecules_, not of _particles_. the medium before you is not a turbid medium, for when you look through it at a luminous surface it is perfectly clear. if we paint upon a piece of paper a flower or a bouquet with the sulphate of quinine, and expose it to the full beam, scarcely anything is seen. but on interposing the violet glass, the design instantly flashes forth in strong contrast with the deep surrounding violet. president morton has prepared for me a most beautiful example of such a design which, when placed in the violet light, exhibits a peculiarly brilliant fluorescence. from the experiments of drs. bence jones and dupré, it would seem that there is some substance in the human body resembling the sulphate of quinine, which causes all the tissues of the body to be more or less fluorescent. all animal infusions show this fluorescence. the crystalline lens of the eye exhibits the effect in a very striking manner. when, for example, i plunge my eye into this violet beam, i am conscious of a whitish-blue shimmer filling the space before me. this is caused by fluorescent light generated in the eye itself. looked at from without, the crystalline lens at the same time is seen to gleam vividly. long before its physical origin was understood this fluorescent light attracted attention. boyle describes it with great fulness and exactness. 'we have sometimes,' he says, 'found in the shops of our druggists certain wood which is there called _lignum nephriticum,_ because the inhabitants of the country where it grows are wont to use the infusion of it, made in fair water, against the stone in the kidneys. this wood may afford us an experiment which, besides the singularity of it, may give no small assistance to an attentive considerer towards the detection of the nature of colours. take _lignum, nephriticum_, and with a knife cut it into thin slices: put about a handful of these slices into two or three or four pounds of the purest spring water. decant this impregnated water into a glass phial; and if you hold it directly between the light and your eye, you shall see it wholly tinted with an almost golden colour. but if you hold this phial from the light, so that your eye be placed betwixt the window and the phial, the liquid will appear of a deep and lovely ceruleous colour.' 'these,' he continues, 'and other phenomena which i have observed in this delightful experiment, divers of my friends have looked upon, not without some wonder; and i remember an excellent oculist, finding by accident in a friend's chamber a phial full of this liquor, which i had given that friend, and having never heard anything of the experiment, nor having anybody near him who could tell him what this strange liquor might be, was a great while apprehensive, as he presently afterwards told me, that some strange new distemper was invading his eyes. and i confess that the unusualness of the phenomenon made me very solicitous to find out the cause of this experiment; and though i am far from pretending to have found it, yet my enquiries have, i suppose, enabled me to give such hints as may lead your greater sagacity to the discovery of the cause of this wonder.'[ ] goethe in his 'farbenlehre' thus describes the fluorescence of horse-chestnut bark:--'let a strip of fresh horse-chestnut bark be taken and clipped into a glass of water; the most perfect sky-blue will be immediately produced.'[ ] sir john herschel first noticed and described the fluorescence of the sulphate of quinine, and showed that the light proceeded from a thin stratum of the solution adjacent to the surface where the light enters it. he showed, moreover, that the incident beam, although not sensibly weakened in luminous intensity, lost, in its transmission through the solution of sulphate of quinine, the power of producing the blue fluorescent light. sir david brewster also worked at the subject; but to professor stokes we are indebted not only for its expansion, but for its full and final explanation. § . _the heat of the electric beam. ignition through a lens of ice. possible cometary temperature_. but the waves from our incandescent carbon-points appeal to another sense than that of vision. they not only produce light, but heat, as a sensation. the magnified image of the carbon-points is now upon the screen; and with a suitable instrument the heating power of the rays which form that image might be readily demonstrated. in this case, however, the heat is spread over too large an area to be very intense. drawing out the camera lens, and causing a movable screen to approach the lamp, the image is seen to become smaller and smaller; the rays at the same time becoming more and more concentrated, until finally they are able to pierce black paper with a burning ring. pushing back the lens so as to render the rays parallel, and receiving them upon a concave mirror, they are brought to a focus; paper placed at that focus is caused to smoke and burn. heat of this intensity may be obtained with our ordinary camera and lens, and a concave mirror of very moderate power. [illustration: fig. .] we will now adopt stronger measures with the radiation. in this larger camera of blackened tin is placed a lamp, in all particulars similar to those already employed. but instead of gathering up the rays from the carbon-points by a condensing lens, we gather them up by a concave mirror (_m_ _m'_, fig. ), silvered in front and placed behind the carbons (p). by this mirror we can cause the rays to issue through the orifice in front of the camera, either parallel or convergent. they are now parallel, and therefore to a certain extent diffused. we place a convex lens (l) in the path of the beam; the light is converged to a focus (c), and at that focus paper is not only pierced, but it is instantly set ablaze. many metals may be burned up in the same way. in our first lecture the combustibility of zinc was mentioned. placing a strip of sheet-zinc at this focus, it is instantly ignited, burning with its characteristic purple flame. and now i will substitute for our glass lens (l) one of a more novel character. in a smooth iron mould a lens of pellucid ice has been formed. placing it in the position occupied a moment ago by the glass lens, i can see the beam brought to a sharp focus. at the focus i place, a bit of black paper, with a little gun-cotton folded up within it. the paper immediately ignites and the cotton explodes. strange, is it not, that the beam should possess such heating power after having passed through so cold a substance? in his arctic expeditions dr. scoresby succeeded in exploding gunpowder by the sun's rays, converged by large lenses of ice; here we have succeeded in producing the effect with a small lens, and with a terrestrial source of heat. in this experiment, you observe that, before the beam reaches the ice-lens, it has passed through a glass cell containing water. the beam is thus sifted of constituents, which, if permitted to fall upon the lens, would injure its surface, and blur the focus. and this leads me to say an anticipatory word regarding transparency. in our first lecture we entered fully into the production of colours by absorption, and we spoke repeatedly of the quenching of the rays of light. did this mean that the light was altogether annihilated? by no means. it was simply so lowered in refrangibility as to escape the visual range. it was converted into heat. our red ribbon in the green of the spectrum quenched the green, but if suitably examined its temperature would have been found raised. our green ribbon in the red of the spectrum quenched the red, but its temperature at the same time was augmented to a degree exactly equivalent to the light extinguished. our black ribbon, when passed through the spectrum, was found competent to quench all its colours; but at every stage of its progress an amount of heat was generated in the ribbon exactly equivalent to the light lost. it is only when _absorption_ takes place that heat is thus produced: and heat is always a result of absorption. examine the water, then, in front of the lamp after the beam has passed through it: it is sensibly warm, and, if permitted to remain there long enough, it might be made to boil. this is due to the absorption, by the water, of a certain portion of the electric beam. but a portion passes through unabsorbed, and does not at all contribute to the heating of the water. now, ice is also in great part transparent to these latter rays, and therefore is but little melted by them. hence, by employing the portion of the beam transmitted by water, we are able to keep our lens intact, and to produce by means of it a sharply defined focus. placed at that focus, white paper is not ignited, because it fails to absorb the rays emergent from the ice-lens. at the same place, however, black paper instantly burns, because it absorbs the transmitted light. and here it may be useful to refer to an estimate by newton, based upon doubtful data, but repeated by various astronomers of eminence since his time. the comet of , when nearest to the sun, was only a sixth of the sun's diameter from his surface. newton estimated its temperature, in this position, to be more than two thousand times that of molted iron. now it is clear from the foregoing experiments that the temperature of the comet could not be inferred from its nearness to the sun. if its power of absorption were sufficiently low, the comet might carry into the sun's neighbourhood the chill of stellar space. § . _combustion of a diamond by radiant heat_. the experiment of burning a diamond in oxygen by the concentrated rays of the sun was repeated at florence, in presence of sir humphry davy, on tuesday, the th of march, . it is thus described by faraday:--'to-day we made the grand experiment of burning the diamond, and certainly the phenomena presented were extremely beautiful and interesting. a glass globe containing about cubical inches was exhausted of air, and filled with pure oxygen. the diamond was supported in the centre of this globe. the duke's burning-glass was the instrument used to apply heat to the diamond. it consists of two double convex lenses, distant from each other about ½ feet; the large lens is about or inches in diameter, the smaller one about inches in diameter. by means of the second lens the focus is very much reduced, and the heat, when the sun shines brightly, rendered very intense. the diamond was placed in the focus and anxiously watched. on a sudden sir h. davy observed the diamond to burn visibly, and when removed from the focus it was found to be in a state of active and rapid combustion.' the combustion of the diamond had never been effected by radiant heat from a terrestrial source. i tried to accomplish this before crossing the atlantic, and succeeded in doing so. the small diamond now in my hand is held by a loop of platinum wire. to protect it as far as possible from air currents, and also to concentrate the heat upon it, it is surrounded by a hood of sheet platinum. bringing a jar of oxygen underneath, i cause the focus of the electric beam to fall upon the diamond. a small fraction of the time expended in the experiment described by faraday suffices to raise the diamond to a brilliant red. plunging it then into the oxygen, it glows like a little white star; and it would continue to burn and glow until wholly consumed. the focus can also be made to fall upon the diamond in oxygen, as in the florentine experiment: the result is the same. it was simply to secure more complete mastery over the position of the focus, so as to cause it to fall accurately upon the diamond, that the mode of experiment here described was resorted to. § . _ultra-red rays: calorescence_. in the path of the beam issuing from our lamp i now place a cell with glass sides containing a solution of alum. all the _light_ of the beam passes through this solution. this light is received on a powerfully converging mirror silvered in front, and brought to a focus by the mirror. you can see the conical beam of reflected light tracking itself through the dust of the room. a scrap of white paper placed at the focus shines there with dazzling brightness, but it is not even charred. on removing the alum cell, however, the paper instantly inflames. there must, therefore, be something in this beam besides its light. the _light_ is not absorbed by the white paper, and therefore does not burn the paper; but there is something over and above the light which _is_ absorbed, and which provokes combustion. what is this something? in the year sir william herschel passed a thermometer through the various colours of the solar spectrum, and marked the rise of temperature corresponding to each colour. he found the heating effect to augment from the violet to the red; he did not, however, stop at the red, but pushed his thermometer into the dark space beyond it. here he found the temperature actually higher than in any part of the visible spectrum. by this important observation, he proved that the sun emitted heat-rays which are entirely unfit for the purposes of vision. the subject was subsequently taken up by seebeck, melloni, müller, and others, and within the last few years it has been found capable of unexpected expansions and applications. i have devised a method whereby the solar or electric beam can be so _filtered_ as to detach from it, and preserve intact, this invisible ultra-red emission, while the visible and ultra-violet emissions are wholly intercepted. we are thus enabled to operate at will upon the purely ultra-red waves. in the heating of solid bodies to incandescence, this non-visual emission is the necessary basis of the visual. a platinum wire is stretched in front of the table, and through it an electric current flows. it is warmed by the current, and may be felt to be warm by the hand. it emits waves of heat, but no light. augmenting the strength of the current, the wire becomes hotter; it finally glows with a sober red light. at this point dr. draper many years ago began an interesting investigation. he employed a voltaic current to heat his platinum, and he studied, by means of a prism, the successive introduction of the colours of the spectrum. his first colour, as here, was red; then came orange, then yellow, then green, and lastly all the shades of blue. as the temperature of the platinum was gradually augmented, the atoms were caused to vibrate more rapidly; shorter waves were thus introduced, until finally waves were obtained corresponding to the entire spectrum. as each successive colour was introduced, the colours preceding it became more vivid. now the vividness or intensity of light, like that of sound, depends not upon the length of the wave, but on the amplitude of the vibration. hence, as the less refrangible colours grew more intense when the more refrangible ones were introduced, we are forced to conclude that side by side with the introduction of the shorter waves we had an augmentation of the amplitude of the longer ones. these remarks apply not only to the visible emission examined by dr. draper, but to the invisible emission which precedes the appearance of any light. in the emission from the white-hot platinum wire now before you, the lightless waves exist with which we started, only their intensity has been increased a thousand-fold by the augmentation of temperature necessary to the production of this white light. both effects are bound up together: in an incandescent solid, or in a molten solid, you cannot have the shorter waves without this intensification of the longer ones. a sun is possible only on these conditions; hence sir william herschel's discovery of the invisible ultra-red solar emission. the invisible heat, emitted both by dark bodies and by luminous ones, flies through space with the velosity of light, and is called _radiant heat_. now, radiant heat may be made a subtle and powerful explorer of molecular condition, and, of late years, it has given a new significance to the act of chemical combination. take, for example, the air we breathe. it is a mixture of oxygen and nitrogen; and it behaves towards radiant heat like a vacuum, being incompetent to absorb it in any sensible degree. but permit the same two gases to unite chemically; then, without any augmentation of the quantity of matter, without altering the gaseous condition, without interfering in any way with the transparency of the gas, the act of chemical union is accompanied by an enormous diminution of its _diathermancy_, or perviousness to radiant heat. the researches which established this result also proved the elementary gases, generally, to be highly transparent to radiant heat. this, again, led to the proof of the diathermancy of elementary liquids, like bromine, and of solutions of the solid elements sulphur, phosphorus, and iodine. a spectrum is now before you, and you notice that the transparent bisulphide of carbon has no effect upon the colours. dropping into the liquid a few flakes of iodine, you see the middle of the spectrum cut away. by augmenting the quantity of iodine, we invade the entire spectrum, and finally cut it off altogether. now, the iodine, which proves itself thus hostile to the light, is perfectly transparent to the ultra-red emission with which we have now to deal. it, therefore, is to be our ray-filter. placing the alum-cell again in front of the electric lamp, we assure ourselves, as before, of the utter inability of the concentrated light to fire white paper-introducing a cell containing the solution of iodine, the light is entirely cut off; and then, on removing the alum-cell, the white paper at the dark focus is instantly set on fire. black paper is more absorbent than white for these rays; and the consequence is, that with it the suddenness and vigour of the combustion are augmented. zinc is burnt up at the same place, magnesium bursts into vivid combustion, while a sheet of platinized platinum, placed at the focus, is heated to whiteness. looked at through a prism, the white-hot platinum yields all the colours of the spectrum. before impinging upon the platinum, the waves were of too slow recurrence to awaken vision; by the atoms of the platinum, these long and sluggish waves are broken up into shorter ones, being thus brought within the visual range. at the other end of the spectrum, by the interposition of suitable substances, professor stokes _lowered_ the refrangibility, so as to render the non-visual rays visual, and to this change he gave the name of _fluorescence_. here, by the intervention of the platinum, the refrangibility is _raised_, so as to render the non-visual visual, and to this change i have given the name of _calorescence_. at the perfectly invisible focus where these effects are produced, the air may be as cold as ice. air, as already stated, does not absorb radiant heat, and is therefore not warmed by it. nothing could more forcibly illustrate the isolation, if i may use the term, of the luminiferous ether from the air. the wave-motion of the one is heaped up to an extraordinary degree of intensity, without producing any sensible effect upon the other. i may add that, with suitable precautions, the eye may be placed in a focus competent to heat platinum to vivid redness, without experiencing any damage, or the slightest sensation either of light or heat. the important part played by these ultra-red rays in nature may be thus illustrated: i remove the iodine filter, and concentrate the total beam upon a test tube containing water. it immediately begins to splutter, and in a minute or two it _boils_. what boils it? placing the alum solution in front of the lamp, the boiling instantly ceases. now, the alum is pervious to all the luminous rays; hence it cannot be these rays that caused the boiling. i now introduce the iodine, and remove the alum: vigorous ebullition immediately recommences at the invisible focus. so that we here fix upon the invisible ultra-red rays the heating of the water. we are thus enabled to understand the momentous part played by these rays in nature. it is to them that we owe the warming and the consequent evaporation of the tropical ocean; it is to them, therefore, that we owe our rains and snows. they are absorbed close to the surface of the ocean, and warm the superficial water, while the luminous rays plunge to great depths without producing any sensible effect. but we can proceed further than this. here is a large flask containing a freezing mixture, which has so chilled the flask, that the aqueous vapour of the air of this room has been condensed and frozen upon it to a white fur. introducing the alum-cell, and placing the coating of hoar-frost at the intensely luminous focus of the electric lamp, not a spicula of the dazzling frost is melted. introducing the iodine-cell, and removing the alum, a broad space of the frozen coating is instantly melted away. hence we infer that the snow and ice, which feed the rhone, the rhine, and other rivers with glaciers for their sources, are released from their imprisonment upon the mountains by the invisible ultra-red rays of the sun. § . _identity of light and radiant heat. reflection from plane and curved surfaces. total reflection of heat_. the growth of science is organic. that which today is an _end_ becomes to-morrow a _means_ to a remoter end. every new discovery in science is immediately made the basis of other discoveries, or of new methods of investigation. thus about fifty years ago oersted, of copenhagen, discovered the deflection of a magnetic needle by an electric current; and about the same time thomas seebeck, of berlin, discovered thermoelectricity. these great discoveries were soon afterwards turned to account, by nobili and melloni, in the construction of an instrument which has vastly augmented our knowledge of radiant heat. this instrument, which is called a _thermo-electric pile_, or more briefly a thermo-pile, consists of thin bars of bismuth and antimony, soldered alternately together at their ends, but separated from each other elsewhere. from the ends of this 'thermo-pile' wires pass to a galvanometer, which consists of a coil of covered wire, within and above which are suspended two magnetic needles, joined to a rigid system, and carefully defended from currents of air. the action of the arrangement is this: the heat, falling on the pile, produces an electric current; the current, passing through the coil, deflects the needles, and the magnitude of the deflection may be made a measure of the heat. the upper needle moves over a graduated dial far too small to be directly seen. it is now, however, strongly illuminated; and above it is a lens which, if permitted, would form an image of the needle and dial upon the ceiling. there, however, it could not be conveniently viewed. the beam is therefore received upon a looking-glass, placed at the proper angle, which throws the image upon a screen. in this way the motions of this small needle may be made visible to you all. the delicacy of this apparatus is such that in a room filled, as this room now is, with an audience physically warm, it is exceedingly difficult to work with it. my assistant stands several feet off. i turn the pile towards him: the heat radiated from his face, even at this distance, produces a deflection of °. i turn the instrument towards a distant wall, a little below the average temperature of the room. the needle descends and passes to the other side of zero, declaring by this negative deflection that the pile has lost its warmth by radiation against the cold wall. possessed of this instrument, of our ray-filter, and of our large nicol prisms, we are in a condition to investigate a subject of great philosophical interest; one which long engaged the attention of some of our foremost scientific workers--the substantial _identity of light and radiant heat_. that they are identical in _all_ respects cannot of course be the case, for if they were they would act in the same manner upon all instruments, the _eye_ included. the identity meant is such as subsists between one colour and another, causing them to behave alike as regards reflection, refraction, double refraction, and polarization. let us here run rapidly over the resemblances of light and heat. as regards reflection from plane surfaces, we may employ a looking-glass to reflect the light. marking any point in the track of the reflected beam, cutting off the light by the dissolved iodine, and placing the pile at the marked point, the needle immediately starts aside, showing that the heat is reflected in the same direction as the light. this is true for every position of the mirror. recurring, for example, to the simple apparatus employed in our first lecture (fig. , p. ); moving the index attached to the mirror along the divisions of our graduated arc (_m_ _n_), and determining by the pile the positions of the invisible reflected beam, we prove that the angular velocity of the heat-beam, like that of the light-beam, is twice that of the mirror. [illustration: fig. .] as regards reflection from curved surfaces, the identity also holds good. receiving the beam from our electric lamp on a concave mirror (_m_ _m_, fig. ), it is gathered up into a cone of reflected light rendered visible by the floating dust of the air; marking the apex of the cone by a pointer, and cutting off the light by the iodine solution (t), a moment's exposure of the pile (p) at the marked point produces a violent deflection of the needle. the common reflection and the total reflection of a beam of radiant heat may be simultaneously demonstrated. from the nozzle of the lamp (l, fig. ) a beam impinges upon a plane mirror (m n), is reflected upwards, and enters a right-angled prism, of which _a_ _b_ _c_ is the section. it meets the hypothenuse at an obliquity greater than the limiting angle,[ ] and is therefore totally reflected. quenching the light by the ray-filter at f, and placing the pile at p, the totally reflected heat-beam is immediately felt by the pile, and declared by the galvanometric deflection. [illustration: fig. .] § . _invisible images formed by radiant heat._ perhaps no experiment proves more conclusively the substantial identity of light and radiant heat, than the formation of invisible heat-images. employing the mirror already used to raise the beam to its highest state of concentration, we obtain, as is well known, an inverted image of the carbon points, formed by the light rays at the focus. cutting off the light by the ray-filter, and placing at the focus a thin sheet of platinized platinum, the invisible rays declare their presence and distribution, by stamping upon the platinum a white-hot image of the carbons. (see fig. .) [illustration: fig. .] § . _polarization of heat_. whether radiant heat be capable of polarization or not was for a long time a subject of discussion. bérard had announced affirmative results, but powell and lloyd failed to verify them. the doubts thus thrown upon the question were removed by the experiments of forbes, who first established the polarization and 'depolarization' of heat. the subject was subsequently followed up by melloni, an investigator of consummate ability, who sagaciously turned to account his own discovery, that the obscure rays of luminous sources are in part transmitted by black glass. intercepting by a plate of this glass the light from an oil flame, and operating upon the transmitted invisible heat, he obtained effects of polarization, far exceeding in magnitude those which could be obtained with non-luminous sources. at present the possession of our more perfect ray-filter, and more powerful source of heat, enables us to pursue this identity question to its utmost practical limits. [illustration: fig. .] mounting our two nicols (b and c, fig. ) in front of the electric lamp, with their principal sections crossed, no light reaches the screen. placing our thermo-electric pile (d) behind the prisms, with its face turned towards the source, no deflection of the galvanometer is observed. interposing between the lamp (a) and the first prism (b) our ray-filter, the light previously transmitted through the first nicol is quenched; and now the slightest turning of either nicol opens a way for the transmission of the heat, a very small rotation sufficing to send the needle up to °. when the nicol is turned back to its first position, the needle again sinks to zero, thus demonstrating, in the plainest manner, the polarization of the heat. when the nicols are crossed and the field is dark, you have seen, in the case of light, the effect of introducing a plate of mica between the polarizer and analyzer. in two positions the mica exerts no sensible influence; in all others it does. a precisely analogous deportment is observed as regards radiant heat. introducing our ray-filter, the thermo-pile, playing the part of an eye as regards the invisible radiation, receives no heat when the eye receives no light; but when the mica is so turned as to make its planes of vibration oblique to those of the polarizer and analyzer, the heat immediately passes through. so strong does the action become, that the momentary plunging of the film of mica into the dark space between the nicols suffices to send the needle up to °. this is the effect to which the term 'depolarization' has been applied; the experiment really proving that with both light and heat we have the same resolution by the plate of mica, and recompounding by the analyzer, of the ethereal vibrations. removing the mica and restoring the needle once more to °, i introduce between the nicols a plate of quartz cut perpendicular to the axis; the immediate deflection of the needle declares the transmission of the heat, and when the transmitted beam is properly examined, it is found to be circularly polarized, exactly as a beam of light is polarized under the same conditions. § . _double refraction of heat_. i will now abandon the nicols, and send through the piece of iceland spar (b, fig. ), already employed (in lecture iii.) to illustrate the double refraction of light, our sifted beam of invisible heat. to determine the positions of the two images, let us first operate upon the luminous beam. marking the places of the light-images, we introduce between n and l our ray-filter (not in the figure) and quench the light. causing the pile to approach one of the marked places, the needle remains unmoved until the place has been attained; here the pile at once detects the heat. pushing the pile across the interval separating the two marks, the needle first falls to °, and then rises again to ° in the second position. this proves the double refraction of the heat. [illustration: fig. .] i now turn the iceland spar: the needle remains fixed; there is no alteration of the deflection. passing the pile rapidly across to the other mark, the deflection is maintained. once more i turn the spar, but now the needle falls to °, rising, however, again to ° after a rotation of °. we know that in the case of light the extraordinary beam rotates round the ordinary one; and we have here been operating on the extraordinary heat-beam, which, as regards double refraction, behaves exactly like a beam of light. § . _magnetization of heat_. to render our series of comparisons complete, we must demonstrate the magnetization of heat. but here a slight modification of our arrangement will be necessary. in repeating faraday's experiment on the magnetization of light, we had, in the first instance, our nicols crossed and the field rendered dark, a flash of light appearing upon the screen when the magnet was excited. now the quantity of light transmitted in this case is really very small, its effect being rendered striking through contrast with the preceding darkness. when we so place the nicols that their principal sections enclose an angle of °, the excitement of the magnet causes a far greater positive augmentation of the light, though the augmentation is not so well _seen_ through lack of contrast, because here, at starting, the field is illuminated. in trying to magnetize our beam of heat, we will adopt this arrangement. here, however, at the outset, a considerable amount of heat falls upon one face of the pile. this it is necessary to neutralize, by permitting rays from another source to fall upon the opposite face of the pile. the needle is thus brought to zero. cutting off the light by our ray-filter, and exciting the magnet, the needle is instantly deflected, proving that the magnet has opened a door for the heat, exactly as in faraday's experiment it opened a door for the light. thus, in every case brought under our notice, the substantial identity of light and radiant heat has been demonstrated. by the refined experiments of knoblauch, who worked long and successfully at this question, the double refraction of heat, by iceland spar, was first demonstrated; but, though he employed the luminous heat of the sun, the observed deflections were exceedingly small. so, likewise, those eminent investigators de la povostaye and desains succeeded in magnetizing a beam of heat; but though, in their case also, the luminous solar heat was employed, the deflection obtained did not amount to more than two or three degrees. with _obscure_ radiant heat the effect, prior to the experiments now brought before you, had not been obtained; but, with the arrangement here described, we obtain deflections from purely invisible heat, equal to of the lower degrees of the galvanometer. § . _distribution of heat in the electric spectrum_. we have finally to determine the position and magnitude of the invisible radiation which produces these results. for this purpose we employ a particular form of the thermo-pile. its face is a rectangle, which by movable side-pieces can be rendered as narrow as desirable. throwing a small and concentrated spectrum upon a screen, by means of an endless screw we move the rectangular pile through the entire spectrum, and determine in succession the thermal power of all its colours. [illustration: spectrum of electric light.] when this instrument is brought to the violet end of the spectrum, the heat is found to be almost insensible. as the pile gradually moves from the violet towards the red, it encounters a gradually augmenting heat. the red itself possesses the highest heating power of all the colours of the spectrum. pushing the pile into the dark space beyond the red, the heat rises suddenly in intensity, and at some distance beyond the red it attains a maximum. from this point the heat falls somewhat more rapidly than it rose, and afterwards gradually fades away. drawing a horizontal line to represent the length of the spectrum, and erecting along it, at various points, perpendiculars proportional in length to the heat existing at those points, we obtain a curve which exhibits the distribution of heat in the prismatic spectrum. it is represented in the adjacent figure. beginning at the blue, the curve rises, at first very gradually; towards the red it rises more rapidly, the line c d (fig. , opposite page) representing the strength of the extreme red radiation. beyond the red it shoots upwards in a steep and massive peak to b; whence it falls, rapidly for a time, and afterwards gradually fades from the perception of the pile. this figure is the result of more than twelve careful series of measurements, from each of which the curve was constructed. on superposing all these curves, a satisfactory agreement was found to exist between them. so that it may safely be concluded that the areas of the dark and white spaces, respectively, represent the relative energies of the visible and invisible radiation. the one is . times the other. but in verification, as already stated, consists the strength of science. determining in the first place the total emission from the electric lamp, and then, by means of the iodine filter, determining the ultra-red emission; the difference between both gives the luminous emission. in this way, it is found that the energy of the invisible emission is eight times that of the visible. no two methods could be more opposed to each other, and hardly any two results could better harmonize. i think, therefore, you may rely upon the accuracy of the distribution of heat here assigned to the prismatic spectrum of the electric light. there is nothing vague in the mode of investigation, or doubtful in its conclusions. spectra are, however, formed by _diffraction_, wherein the distribution of both heat and light is different from that produced by the prism. these diffractive spectra have been examined with great skill by draper and langley. in the prismatic spectrum the less refrangible rays are compressed into a much smaller space than in the diffraction spectrum. lecture vi. principles of spectrum analysis prismatic analysis of the light of incandescent vapours discontinuous spectra spectrum bands proved by bunsen and kirchhoff to be characteristic of the vapour discovery of rubidium, cÆsium, and thallium relation of emission to absorption the lines of fraunhofer their explanation by kirchhoff solar chemistry involved in this explanation foucault's experiment principles of absorption analogy of sound and light experimental demonstration of this analogy recent applications of the spectroscope summary and conclusion. we have employed as our source of light in these lectures the ends of two rods of coke rendered incandescent by electricity. coke is particularly suitable for this purpose, because it can bear intense heat without fusion or vaporization. it is also black, which helps the light; for, other circumstances being equal, as shown experimentally by professor balfour stewart, the blacker the body the brighter will be its light when incandescent. still, refractory as carbon is, if we closely examined our voltaic arc, or stream of light between the carbon-points, we should find there incandescent carbon-vapour. and if we could detach the light of this vapour from the more dazzling light of the solid points, we should find its spectrum not only less brilliant, but of a totally different character from the spectra that we have already seen. instead of being an unbroken succession of colours from red to violet, the carbon-vapour would yield a few bands of colour with spaces of darkness between them. what is true of the carbon is true in a still more striking degree of the metals, the most refractory of which can be fused, boiled, and reduced to vapour by the electric current. from the incandescent vapour the light, as a general rule, flashes in groups of rays of definite degrees of refrangibility, spaces existing between group and group, which are unfilled by rays of any kind. but the contemplation of the facts will render this subject more intelligible than words can make it. within the camera is now placed a cylinder of carbon hollowed out at the top; in the hollow is placed a fragment of the metal thallium. down upon this we bring the upper carbon-point, and then separate the one from the other. a stream of incandescent thallium-vapour passes between them, the magnified image of which is now seen upon the screen. it is of a beautiful green colour. what is the meaning of that green? we answer the question by subjecting the light to prismatic analysis. sent through the prism, its spectrum is seen to consist of a single refracted band. light of one degree of refrangibility--that corresponding to this particular green--is emitted by the thallium-vapour. we will now remove the thallium and put a bit of silver in its place. the are of silver is not to be distinguished from that of thallium; it is not only green, but the same shade of green. are they then alike? prismatic analysis enables us to answer the question. however impossible it is to distinguish the one _colour_ from the other, it is equally impossible to confound the _spectrum_ of incandescent silver-vapour with that of thallium. in the case of silver, we have two green bands instead of one. if we add to the silver in our camera a bit of thallium, we shall obtain the light of both metals. after waiting a little, we see that the green of the thallium lies midway between the two greens of the silver. hence this similarity of colour. but why have we to 'wait a little' before we see this effect? the thallium band at first almost masks the silver bands by its superior brightness. indeed, the silver bands have wonderfully degenerated since the bit of thallium was put in, and for a reason worth knowing. it is the _resistance_ offered to the passage of the electric current from carbon to carbon, that calls forth the power of the current to produce heat. if the resistance were materially lessened, the heat would be materially lessened; and if all resistance were abolished, there would be no heat at all. now, thallium is a much more fusible and vaporizable metal than silver; and its vapour facilitates the passage of the electricity to such a degree, as to render the current almost incompetent to vaporize the more refractory silver. but the thallium is gradually consumed; its vapour diminishes, the resistance rises, until finally you see the two silver bands as brilliant as they were at first.[ ] we have in these bands a perfectly unalterable characteristic of the two metals. you never get other bands than these two green ones from the silver, never other than the single green band from the thallium, never other than the three green bands from the mixture of both metals. every known metal has its own particular bands, and in no known case are the bands of two different metals alike in refrangibility. it follows, therefore, that these spectra may be made a sure test for the presence or absence of any particular metal. if we pass from the metals to their alloys, we find no confusion. copper gives green bands; zinc gives blue and red bands; brass--an alloy of copper and zinc--gives the bands of both metals, perfectly unaltered in position or character. but we are not confined to the metals themselves; the _salts_ of these metals yield the bands of the metals. chemical union is ruptured by a sufficiently high heat; the vapour of the metal is set free, and it yields its characteristic bands. the chlorides of the metals are particularly suitable for experiments of this character. common salt, for example, is a compound of chlorine and sodium; in the electric lamp it yields the spectrum of the metal sodium. the chlorides of copper, lithium, and strontium yield, in like manner, the bands of these metals. when, therefore, bunsen and kirchhoff, the illustrious founders of _spectrum analysis_, after having established by an exhaustive examination the spectra of all known substances, discovered a spectrum containing bands different from any known bands, they immediately inferred the existence of a new metal. they were operating at the time upon a residue, obtained by evaporating one of the mineral waters of germany. in that water they knew the unknown metal was concealed, but vast quantities of it had to be evaporated before a residue could be obtained sufficiently large to enable ordinary chemistry to grapple with the metal. they, however, hunted it down, and it now stands among chemical substances as the metal _rubidium_. they subsequently discovered a second metal, which they called _cæsium_. thus, having first placed spectrum analysis on a sure foundation, they demonstrated its capacity as an agent of discovery. soon afterwards mr. crookes, pursuing the same method, discovered the bright green band of _thallium_, and obtained the salts of the metal which yielded it. the metal itself was first isolated in ingots by m. lamy, a french chemist. all this relates to chemical discovery upon earth, where the materials are in our own hands. but it was soon shown how spectrum analysis might be applied to the investigation of the sun and stars; and this result was reached through the solution of a problem which had been long an enigma to natural philosophers. the scope and conquest of this problem we must now endeavour to comprehend. a spectrum is _pure_ in which the colours do not overlap each other. we purify the spectrum by making our beam narrow, and by augmenting the number of our prisms. when a pure spectrum of the sun has been obtained in this way, it is found to be furrowed by innumerable dark lines. four of them were first seen by dr. wollaston, but they were afterwards multiplied and measured by fraunhofer with such masterly skill, that they are now universally known as fraunhofer's lines. to give an explanation of these lines was, as i have said, a problem which long challenged the attention of philosophers, and to professor kirchhoff belongs the honour of having first conquered this problem. (the positions of the principal lines, lettered according to fraunhofer, are shown in the annexed sketch (fig. ) of the solar spectrum. a is supposed to stand near the extreme red, and j near the extreme violet.) [illustration: fig. .] the brief memoir of two pages, in which this immortal discovery is recorded, was communicated to the berlin academy on october , . fraunhofer had remarked in the spectrum of a candle flame two bright lines, which coincide accurately, as to position, with the double dark line d of the solar spectrum. these bright lines are produced with particular intensity by the yellow flame derived from a mixture of salt and alcohol. they are in fact the lines of sodium vapour. kirchhoff produced a spectrum by permitting the sunlight to enter his telescope by a slit and prism, and in front of the slit he placed the yellow sodium flame. as long as the spectrum remained feeble, there always appeared two bright lines, derived from the flame, in the place of the two dark lines d of the spectrum. in this case, such absorption as the flame exerted upon the sunlight was more than atoned for by the radiation from the flame. when, however, the solar spectrum was rendered sufficiently intense, the bright bands vanished, and the two dark fraunhofer lines appeared with much greater sharpness and distinctness than when the flame was not employed. this result, be it noted, was not due to any real quenching of the bright lines of the flame, but to the augmentation of the intensity of the adjacent spectrum. the experiment proved to demonstration, that when the white light sent through the flame was sufficiently intense, the quantity which the flame absorbed was far in excess of that which it radiated. here then is a result of the utmost significance. kirchhoff immediately inferred from it that the salt flame, which could intensify so remarkably the dark lines of fraunhofer, ought also to be able to _produce_ them. the spectrum of the drummond light is known to exhibit the two bright lines of sodium, which, however, gradually disappear as the modicum of sodium, contained as an impurity in the incandescent lime, is exhausted. kirchhoff formed a spectrum of the limelight, and after the two bright lines had vanished, he placed his salt flame in front of the slit. the two dark lines immediately started forth. thus, in the continuous spectrum of the lime-light, he evoked, artificially, the lines d of fraunhofer. kirchhoff knew that this was an action not peculiar to the sodium flame, and he immediately extended his generalisation to all coloured flames which yield sharply defined bright bands in their spectra. white light, with all its constituents complete, sent through such flames, would, he inferred, have those precise constituents absorbed, whose refrangibilities are the same as those of the bright bands; so that after passing through such flames, the white light, if sufficiently intense, would have its spectrum furrowed by bands of darkness. on the occasion here referred to kirchhoff also succeeded in reversing a bright band of lithium. the long-standing difficulty of fraunhofer's lines fell to pieces in the presence of facts and reflections like these, which also carried with them an immeasurable extension of the chemist's power. kirchhoff saw that from the agreement of the lines in the spectra of terrestrial substances with fraunhofer's lines, the presence of these substances in the sun and fixed stars might be immediately inferred. thus the dark lines d in the solar spectrum proved the existence of sodium in the solar atmosphere; while the bright lines discovered by brewster in a nitre flame, which had been proved to coincide exactly with certain dark lines between a and b in the solar spectrum, proved the existence of potassium in the sun. all subsequent research verified the accuracy of these first daring conclusions. in his second paper, communicated to the berlin academy before the close of , kirchhoff proved the existence of iron in the sun. the bright lines of the spectrum of iron vapour are exceedingly numerous, and of them were subsequently proved by kirchhoff to be absolutely identical in position with dark fraunhofer's lines. Ångström and thalén pushed the coincidences to for iron, while, according to the same excellent investigators, the following numbers express the coincidences, in the case of the respective metals to which they are attached:-- calcium barium magnesium manganese titanium chromium nickel cobalt hydrogen aluminium zinc copper the probability is overwhelming that all these substances exist in the atmosphere of the sun. kirchhoff's discovery profoundly modified the conceptions previously entertained regarding the constitution of the sun, leading him to views which, though they may be modified in detail, will, i believe, remain substantially valid to the end of time. the sun, according to kirchhoff, consists of a molten nucleus which is surrounded by a flaming atmosphere of lower temperature. the nucleus may, in part, be _clouds_, mixed with, or underlying true vapour. the light of the nucleus would give us a continuous spectrum, like that of the drummond light; but having to pass through the photosphere, as kirchhoff's beam passed through the sodium flame, those rays of the nucleus which the photosphere emit are absorbed, and shaded lines, corresponding to the rays absorbed, occur in the spectrum. abolish the solar nucleus, and we should have a spectrum showing a bright line in the place of every dark line of fraunhofer, just as, in the case of kirchhoff's second experiment, we should have the bright sodium lines of the flame if the lime-light were withdrawn. these lines of fraunhofer are therefore not absolutely dark, but dark by an amount corresponding to the difference between the light intercepted and the light emitted by the photosphere. almost every great scientific discovery is approached contemporaneously by many minds, the fact that one mind usually confers upon it the distinctness of demonstration being an illustration, not of genius isolated, but of genius in advance. thus foucault, in , came to the verge of kirchhoff's discovery. by converging an image of the sun upon a voltaic arc, and thus obtaining the spectra of both sun and arc superposed, he found that the two bright lines which, owing to the presence of a little sodium in the carbons or in the air, are seen in the spectrum of the arc, coincide with the dark lines d of the solar spectrum. the lines d he found to he considerably strengthened by the passage of the solar light through the voltaic arc. instead of the image of the sun, foucault then projected upon the arc the image of one of the solid incandescent carbon points, which of itself would give a continuous spectrum; and he found that the lines d were thus _generated_ in that spectrum. foucault's conclusion from this admirable experiment was 'that the arc is a medium which emits the rays d on its own account, and at the same time absorbs them when they come from another quarter.' here he stopped. he did not extend his observations beyond the voltaic arc; he did not offer any explanation of the lines of fraunhofer; he did not arrive at any conception of solar chemistry, or of the constitution of the sun. his beautiful experiment remained a germ without fruit, until the discernment, ten years subsequently, of the whole class of phenomena to which it belongs, enabled kirchhoff to solve these great problems. soon after the publication of kirchhoff's discovery, professor stokes, who also, ten years prior to the discovery, had nearly anticipated it, borrowed an illustration from sound, to explain the reciprocity of radiation and absorption. a stretched string responds to aërial vibrations which synchronize with its own. a great number of such strings stretched in space would roughly represent a medium; and if the note common to them all were sounded at a distance they would take up or absorb its vibrations. when a violin-bow is drawn across this tuning-fork, the room is immediately filled with a musical sound, which may be regarded as the _radiation_ or _emission_ of sound from the fork. a few days ago, on sounding this fork, i noticed that when its vibrations were quenched, the sound seemed to be continued, though more feebly. it appeared, moreover, to come from under a distant table, where stood a number of tuning-forks of different sizes and rates of vibration. one of these, and one only, had been started by the sounding fork, and it was the one whose rate of vibration was the same as that of the fork which started it. this is an instance of the _absorption_ of the sound of one fork by another. placing two unisonant forks near each other, sweeping the bow over one of them, and then quenching the agitated fork, the other continues to sound; this other can re-excite the former, and several transfers of sound between the two forks can be thus effected. placing a cent-piece on each prong of one of the forks, we destroy its perfect synchronism with the other, and no such communication of sound from the one to the other is then possible. i have now to bring before you, on a suitable scale, the demonstration that we can do with _light_ what has been here done with sound. for several days in i endeavoured to accomplish this, with only partial success. in iron dishes a mixture of dilute alcohol and salt was placed, and warmed so as to promote vaporization. the vapour was ignited, and through the yellow flame thus produced the beam from the electric lamp was sent; but a faint darkening only of the yellow band of a projected spectrum could be obtained. a trough was then made which, when fed with the salt and alcohol, yielded a flame ten feet thick; but the result of sending the light through this depth of flame was still unsatisfactory. remembering that the direct combustion of sodium in a bunsen's flame produces a yellow far more intense than that of the salt flame, and inferring that the intensity of the colour indicated the copiousness of the incandescent vapour, i sent through the flame from metallic sodium the beam of the electric lamp. the success was complete; and this experiment i wish now to repeat in your presence.[ ] firstly then you notice, when a fragment of sodium is placed in a platinum spoon and introduced into a bunsen's flame, an intensely yellow light is produced. it corresponds in refrangibility with the yellow band of the spectrum. like our tuning-fork, it emits waves of a special period. when the white light from the electric lamp is sent through that flame, you will have ocular proof that the yellow flame intercepts the yellow of the spectrum; in other words, that it absorbs waves of the same period as its own, thus producing, to all intents and purposes, a dark fraunhofer's band in the place of the yellow. in front of the slit (at l, fig. ) through which the beam issues is placed a bunsen's burner (_b_) protected by a chimney (c). this beam, after passing through a lens, traverses the prism (p) (in the real experiment there was a pair of prisms), is there decomposed, and forms a vivid continuous spectrum (s s) upon the screen. introducing a platinum spoon with its pellet of sodium into the bunsen's flame, the pellet first fuses, colours the flame intensely yellow, and at length bursts into violent combustion. at the same moment the spectrum is furrowed by an intensely dark band (d), two inches wide and two feet long. introducing and withdrawing the sodium flame in rapid succession, the sudden appearance and disappearance of the band of darkness is shown in a most striking manner. in contrast with the adjacent brightness this band appears absolutely black, so vigorous is the absorption. the blackness, however, is but relative, for upon the dark space falls a portion of the light of the sodium flame. [illustration: fig. .] i have already referred to the experiment of foucault; but other workers also had been engaged on the borders of this subject before it was taken up by bunsen and kirchhoff. with some modification i have on a former occasion used the following words regarding the precursors of the discovery of spectrum analysis, and solar chemistry:--'mr. talbot had observed the bright lines in the spectra of coloured flames, and both he and sir john herschel pointed out the possibility of making prismatic analysis a chemical test of exceeding delicacy, though not of entire certainty. more than a quarter of a century ago dr. miller gave drawings and descriptions of the spectra of various coloured flames. wheatstone, with his accustomed acuteness, analyzed the light of the electric spark, and proved that the metals between which the spark passed determined the bright bands in its spectrum. in an investigation described by kirchhoff as "classical," swan had shown that / , , of a grain of sodium in a bunsen's flame could be detected by its spectrum. he also proved the constancy of the bright lines in the spectra of hydrocarbon flames. masson published a prize essay on the bands of the induction spark; while van der willigen, and more recently plücker, have also given us beautiful drawings of spectra obtained from the same source. 'but none of these distinguished men betrayed the least knowledge of the connexion between the bright bands of the metals and the dark lines of the solar spectrum; nor could spectrum analysis be said to be placed upon anything like a safe foundation prior to the researches of bunsen and kirchhoff. the man who, in a published paper, came nearest to the philosophy of the subject was Ångström. in that paper, translated by myself, and published in the "philosophical magazine" for , he indicates that the rays which a body absorbs are precisely those which, when luminous, it can emit. in another place, he speaks of one of his spectra giving the general impression of the _reversal_ of the solar spectrum. but his memoir, philosophical as it is, is distinctly marked by the uncertainty of his time. foucault, thomson, and balfour stewart have all been near the discovery, while, as already stated, it was almost hit by the acute but unpublished conjecture of stokes.' mentally, as well as physically, every year of the world's age is the outgrowth and offspring of all preceding years. science proves itself to be a genuine product of nature by growing according to this law. we have no solution of continuity here. all great discoveries are duly prepared for in two ways; first, by other discoveries which form their prelude; and, secondly, by the sharpening of the inquiring intellect. thus ptolemy grew out of hipparchus, copernicus out of both, kepler out of all three, and newton out of all the four. newton did not rise suddenly from the sea-level of the intellect to his amazing elevation. at the time that he appeared, the table-land of knowledge was already high. he juts, it is true, above the table-land, as a massive peak; still he is supported by the plateau, and a great part of his absolute height is the height of humanity in his time. it is thus with the discoveries of kirchhoff. much had been previously accomplished; this he mastered, and then by the force of individual genius went beyond it. he replaced uncertainty by certainty, vagueness by definiteness, confusion by order; and i do not think that newton has a surer claim to the discoveries that have made his name immortal, than kirchhoff has to the credit of gathering up the fragmentary knowledge of his time, of vastly extending it, and of infusing into it the life of great principles. with one additional point we will wind up our illustrations of the principles of solar chemistry. owing to the scattering of light by matter floating mechanically in the earth's atmosphere, the sun is seen not sharply defined, but surrounded by a luminous glare. now, a loud noise will drown a whisper, an intense light will overpower a feeble one, and so this circumsolar glare prevents us from seeing many striking appearances round the border of the sun. the glare is abolished in total eclipses, when the moon comes between the earth and the sun, and there are then seen a series of rose-coloured protuberances, stretching sometimes tens of thousands of miles beyond the dark edge of the moon. they are described by vassenius in the 'philosophical transactions' for ; and were probably observed even earlier than this. in they attracted great attention, and were then compared to alpine snow-peaks reddened by the evening sun. that these prominences are flaming gas, and principally hydrogen gas, was first proved by m. janssen during an eclipse observed in india, on the th of august, . but the prominences may be rendered visible in sunshine; and for a reason easily understood. you have seen in these lectures a single prism employed to produce a spectrum, and you have seen a pair of prisms employed. in the latter case, the dispersed white light, being diffused over about twice the area, had all its colours proportionately diluted. you have also seen one prism and a pair of prisms employed to produce the bands of incandescent vapours; but here the light of each band, being absolutely monochromatic, was incapable of further dispersion by the second prism, and could not therefore be weakened by such dispersion. apply these considerations to the circumsolar region. the glare of white light round the sun can be dispersed and weakened to any extent, by augmenting the number of prisms; while a monochromatic light, mixed with this glare, and masked by it, would retain its intensity unenfeebled by dispersion. upon this consideration has been founded a method of observation, applied independently by m. janssen in india and by mr. lockyer in england, by which the monochromatic bands of the prominences are caused to obtain the mastery, and to appear in broad daylight. by searching carefully and skilfully round the sun's rim, mr. lockyer has proved these prominences to be mere local juttings from a fiery envelope which entirely clasps the sun, and which he has called the _chromosphere_. it would lead us far beyond the object of these lectures to dwell upon the numerous interesting and important results obtained by secchi, respighi, young, and other distinguished men who have worked at the chemistry of the sun and its appendages. nor can i do more at present than make a passing reference to the excellent labours of dr. huggins in connexion with the fixed stars, nebulae, and comets. they, more than any others, illustrate the literal truth of the statement, that the establishment of spectrum analysis, and the explanation of fraunhofer's lines, carried with them an immeasurable extension of the chemist's range. the truly powerful experiments of professor dewar are daily adding to our knowledge, while the refined researches of capt. abney and others are opening new fields of inquiry. but my object here is to make principles plain, rather than to follow out the details of their illustration. summary and conclusion. my desire in these lectures has been to show you, with as little breach of continuity as possible, something of the past growth and present aspect of a department of science, in which have laboured some of the greatest intellects the world has ever seen. i have sought to confer upon each experiment a distinct intellectual value, for experiments ought to be the representatives and expositors of thought--a language addressed to the eye as spoken words are to the ear. in association with its context, nothing is more impressive or instructive than a fit experiment; but, apart from its context, it rather suits the conjurer's purpose of surprise, than the purpose of education which ought to be the ruling motive of the scientific man. and now a brief summary of our work will not be out of place. our present mastery over the laws and phenomena of light has its origin in the desire of man to _know_. we have seen the ancients busy with this problem, but, like a child who uses his arms aimlessly, for want of the necessary muscular training, so these early men speculated vaguely and confusedly regarding natural phenomena, not having had the discipline needed to give clearness to their insight, and firmness to their grasp of principles. they assured themselves of the rectilineal propagation of light, and that the angle of incidence was equal to the angle of reflection. for more than a thousand years--i might say, indeed, for more than fifteen hundred years--the scientific intellect appears as if smitten with paralysis, the fact being that, during this time, the mental force, which might have run in the direction of science, was diverted into other directions. the course of investigation, as regards light, was resumed in by an arabian philosopher named alhazen. then it was taken up in succession by roger bacon, vitellio, and kepler. these men, though failing to detect the principles which ruled the facts, kept the fire of investigation constantly burning. then came the fundamental discovery of snell, that cornerstone of optics, as i have already called it, and immediately afterwards we have the application, by descartes, of snell's discovery to the explanation of the rainbow. following this we have the overthrow, by roemer, of the notion of descartes, that light was transmitted instantaneously through space. then came newton's crowning experiments on the analysis and synthesis of white light, by which it was proved to be compounded of various kinds of light of different degrees of refrangibility. up to his demonstration of the composition of white light, newton had been everywhere triumphant--triumphant in the heavens, triumphant on the earth, and his subsequent experimental work is, for the most part, of immortal value. but infallibility is not an attribute of man, and, soon after his discovery of the nature of white light, newton proved himself human. he supposed that refraction and chromatic dispersion went hand in hand, and that you could not abolish the one without at the same time abolishing the other. here dollond corrected him. but newton committed a graver error than this. science, as i sought to make clear to you in our second lecture, is only in part a thing of the senses. the roots of phenomena are embedded in a region beyond the reach of the senses, and less than the root of the matter will never satisfy the scientific mind. we find, accordingly, in this career of optics the greatest minds constantly yearning to break the bounds of the senses, and to trace phenomena to their subsensible foundation. thus impelled, they entered the region of theory, and here newton, though drawn from time to time towards truth, was drawn still more strongly towards error; and he made error his substantial choice. his experiments are imperishable, but his theory has passed away. for a century it stood like a dam across the course of discovery; but, as with all barriers that rest upon authority, and not upon truth, the pressure from behind increased, and eventually swept the barrier away. in malus, looking through iceland spar at the sun, reflected from the window of the luxembourg palace in paris, discovered the polarization of light by reflection. as stated at the time, this discovery ushered in the darkest hour in the fortunes of the wave theory. but the darkness did not continue. in arago discovered the splendid chromatic phenomena which we have had illustrated by the deportment of plates of gypsum in polarized light; he also discovered the rotation of the plane of polarization by quartz-crystals. in seebeck discovered the polarization of light by tourmaline. that same year brewster discovered those magnificent bands of colour that surround the axes of biaxal crystals. in wollaston discovered the rings of iceland spar. all these effects, which, without a theoretic clue, would leave the human mind in a jungle of phenomena without harmony or relation, were organically connected by the theory of undulation. the wave theory was applied and verified in all directions, airy being especially conspicuous for the severity and conclusiveness of his proofs. a most remarkable verification fell to the lot of the late sir william hamilton, of dublin, who, taking up the theory where fresnel had left it, arrived at the conclusion that at four special points of the 'wave-surface' in double-refracting crystals, the ray was divided, not into two parts but into an infinite number of parts; forming at these points a continuous conical envelope instead of two images. no human eye had ever seen this envelope when sir william hamilton inferred its existence. he asked dr. lloyd to test experimentally the truth of his theoretic conclusion. lloyd, taking a crystal of arragonite, and following with the most scrupulous exactness the indications of theory, cutting the crystal where theory said it ought to be cut, observing it where theory said it ought to be observed, discovered the luminous envelope which had previously been a mere idea in the mind of the mathematician. nevertheless this great theory of undulation, like many another truth, which in the long run has proved a blessing to humanity, had to establish, by hot conflict, its right to existence. illustrious names were arrayed against it. it had been enunciated by hooke, it had been expounded and applied by huyghens, it had been defended by euler. but they made no impression. and, indeed, the theory in their hands lacked the strength of a demonstration. it first took the form of a demonstrated verity in the hands of thomas young. he brought the waves of light to bear upon each other, causing them to support each other, and to extinguish each other at will. from their mutual actions he determined their lengths, and applied his knowledge in all directions. he finally showed that the difficulty of polarization yielded to the grasp of theory. after him came fresnel, whose transcendent mathematical abilities enabled him to give the theory a generality unattained by young. he seized it in its entirety; followed the ether into the hearts of crystals of the most complicated structure, and into bodies subjected to strain and pressure. he showed that the facts discovered by malus, arago, brewster, and biot were so many ganglia, so to speak, of his theoretic organism, deriving from it sustenance and explanation. with a mind too strong for the body with which it was associated, that body became a wreck long before it had become old, and fresnel died, leaving, however, behind him a name immortal in the annals of science. one word more i should like to say regarding fresnel. there are things better even than science. character is higher than intellect, but it is especially pleasant to those who wish to think well of human nature when high intellect and upright character are found combined. they were combined in this young frenchman. in those hot conflicts of the undulatory theory, he stood forth as a man of integrity, claiming no more than his right, and ready to concede their rights to others. he at once recognized and acknowledged the merits of thomas young. indeed, it was he, and his fellow-countryman arago, who first startled england into the consciousness of the injustice done to young in the 'edinburgh review.' i should like to read to you a brief extract from a letter written by fresnel to young in , as it throws a pleasant light upon the character of the french philosopher. 'for a long time,' says fresnel, 'that sensibility, or that vanity, which people call love of glory has been much blunted in me. i labour much less to catch the suffrages of the public, than to obtain that inward approval which has always been the sweetest reward of my efforts. without doubt, in moments of disgust and discouragement, i have often needed the spur of vanity to excite me to pursue my researches. but all the compliments i have received from arago, de la place, and biot never gave me so much pleasure as the discovery of a theoretic truth or the confirmation of a calculation by experiment.' * * * * * this, then, is the core of the whole matter as regards science. it must be cultivated for its own sake, for the pure love of truth, rather than for the applause or profit that it brings. and now my occupation in america is well-nigh gone. still i will bespeak your tolerance for a few concluding remarks, in reference to the men who have bequeathed to us the vast body of knowledge of which i have sought to give you some faint idea in these lectures. what was the motive that spurred them on? what urged them to those battles and those victories over reticent nature, which have become the heritage of the human race? it is never to be forgotten that not one of those great investigators, from aristotle down to stokes and kirchhoff, had any practical end in view, according to the ordinary definition of the word 'practical.' they did not propose to themselves money as an end, and knowledge as a means of obtaining it. for the most part, they nobly reversed this process, made knowledge their end, and such money as they possessed the means of obtaining it. we see to-day the issues of their work in a thousand practical forms, and this may be thought sufficient to justify, if not ennoble, their efforts. but they did not work for such issues; their reward was of a totally different kind. in what way different? we love clothes, we love luxuries, we love fine equipages, we love money, and any man who can point to these as the result of his efforts in life, justifies these results before all the world. in america and england, more especially, he is a 'practical' man. but i would appeal confidently to this assembly whether such things exhaust the demands of human nature? the very presence here for six inclement nights of this great audience, embodying so much of the mental force and refinement of this vast city,[ ] is an answer to my question. i need not tell such an assembly that there are joys of the intellect as well as joys of the body, or that these pleasures of the spirit constituted the reward of our great investigators. led on by the whisperings of natural truth, through pain and self-denial, they often pursued their work. with the ruling passion strong in death, some of them, when no longer able to hold a pen, dictated to their friends the last results of their labours, and then rested from them for ever. could we have seen these men at work, without any knowledge of the consequences of their work, what should we have thought of them? to the uninitiated, in their day, they might often appear as big children playing with soap-bubbles and other trifles. it is so to this hour. could you watch the true investigator--your henry or your draper, for example--in his laboratory, unless animated by his spirit, you could hardly understand what keeps him there. many of the objects which rivet his attention might appear to you utterly trivial; and if you were to ask him what is the _use_ of his work, the chances are that you would confound him. he might not be able to express the use of it in intelligible terms. he might not be able to assure you that it will put a dollar into the pocket of any human being present or to come. that scientific discovery _may_ put not only dollars into the pockets of individuals, but millions into the exchequers of nations, the history of science amply proves; but the hope of its doing so never was, and it never can be, the motive power of the investigator. i know that some risk is run in speaking thus before practical men. i know what de tocqueville says of you. 'the man of the north,' he says, 'has not only experience, but knowledge. he, however, does not care for science as a pleasure, and only embraces it with avidity when it leads to useful applications.' but what, i would ask, are the hopes of useful applications which have caused you so many times to fill this place, in spite of snow-drifts and biting cold? what, i may ask, is the origin of that kindness which drew me from my work in london to address you here, and which, if i permitted it, would send me home a millionaire? not because i had taught you to make a single cent by science am i here to-night, but because i tried to the best of my ability to present science to the world as an intellectual good. surely no two terms were ever so distorted and misapplied with reference to man, in his higher relations, as these terms useful and practical. let us expand our definitions until they embrace all the needs of man, his highest intellectual needs inclusive. it is specially on this ground of its administering to the higher needs of the intellect; it is mainly because i believe it to be wholesome, not only as a source of knowledge but as a means of discipline, that i urge the claims of science upon your attention. but with reference to material needs and joys, surely pure science has also a word to say. people sometimes speak as if steam had not been studied before james watt, or electricity before wheatstone and morse; whereas, in point of fact, watt and wheatstone and morse, with all their practicality, were the mere outcome of antecedent forces, which acted without reference to practical ends. this also, i think, merits a moment's attention. you are delighted, and with good reason, with your electric telegraphs, proud of your steam-engines and your factories, and charmed with the productions of photography. you see daily, with just elation, the creation of new forms of industry--new powers of adding to the wealth and comfort of society. industrial england is heaving with forces tending to this end; and the pulse of industry beats still stronger in the united states. and yet, when analyzed, what are industrial america and industrial england? if you can tolerate freedom of speech on my part, i will answer this question by an illustration. strip a strong arm, and regard the knotted muscles when the hand is clenched and the arm bent. is this exhibition of energy the work of the muscle alone? by no means. the muscle is the channel of an influence, without which it would be as powerless as a lump of plastic dough. it is the delicate unseen nerve that unlocks the power of the muscle. and without those filaments of genius, which have been shot like nerves through the body of society by the original discoverer, industrial america, and industrial england, would be very much in the condition of that plastic dough. at the present time there is a cry in england for technical education, and it is a cry in which the most commonplace intellect can join, its necessity is so obvious. but there is no such cry for original investigation. still, without this, as surely as the stream dwindles when the spring dies, so surely will 'technical education' lose all force of growth, all power of reproduction. our great investigators have given us sufficient work for a time; but if their spirit die out, we shall find ourselves eventually in the condition of those chinese mentioned by de tocqueville, who, having forgotten the scientific origin of what they did, were at length compelled to copy without variation the inventions of an ancestry wiser than themselves, who had drawn their inspiration direct from nature. both england and america have reason to bear those things in mind, for the largeness and nearness of material results are only too likely to cause both countries to forget the small spiritual beginnings of such results, in the mind of the scientific discoverer. you multiply, but he creates. and if you starve him, or otherwise kill him--nay, if you fail to secure for him free scope and encouragement--you not only lose the motive power of intellectual progress, but infallibly sever yourselves from the springs of industrial life. what has been said of technical operations holds equally good for education, for here also the original investigator constitutes the fountain-head of knowledge. it belongs to the teacher to give this knowledge the requisite form; an honourable and often a difficult task. but it is a task which receives its final sanctification, when the teacher himself honestly tries to add a rill to the great stream of scientific discovery. indeed, it may be doubted whether the real life of science can be fully felt and communicated by the man who has not himself been taught by direct communion with nature. we may, it is true, have good and instructive lectures from men of ability, the whole of whose knowledge is second-hand, just as we may have good and instructive sermons from intellectually able and unregenerate men. but for that power of science, which corresponds to what the puritan fathers would call experimental religion in the heart, you must ascend to the original investigator. to keep society as regards science in healthy play, three classes of workers are necessary: firstly, the investigator of natural truth, whose vocation it is to pursue that truth, and extend the field of discovery for the truth's own sake and without reference to practical ends. secondly, the teacher of natural truth, whose vocation it is to give public diffusion to the knowledge already won by the discoverer. thirdly, the applier of natural truth, whose vocation it is to make scientific knowledge available for the needs, comforts, and luxuries of civilized life. these three classes ought to co-exist and interact. now, the popular notion of science, both in this country and in england, often relates not to science strictly so called, but to the applications of science. such applications, especially on this continent, are so astounding--they spread themselves so largely and umbrageously before the public eye--that they often shut out from view those workers who are engaged in the quieter and profounder business of original investigation. take the electric telegraph as an example, which has been repeatedly forced upon my attention of late. i am not here to attenuate in the slightest degree the services of those who, in england and america, have given the telegraph a form so wonderfully fitted for public use. they earned a great reward, and they have received it. but i should be untrue to you and to myself if i failed to tell you that, however high in particular respects their claims and qualities may be, your practical men did not discover the electric telegraph. the discovery of the electric telegraph implies the discovery of electricity itself, and the development of its laws and phenomena. such discoveries are not made by practical men, and they never will be made by them, because their minds are beset by ideas which, though of the highest value from one point of view, are not those which stimulate the original discoverer. the ancients discovered the electricity of amber; and gilbert, in the year , extended the discovery to other bodies. then followed boyle, von guericke, gray, canton, du fay, kleist, cunæus, and your own franklin. but their form of electricity, though tried, did not come into use for telegraphic purposes. then appeared the great italian volta, who discovered the source of electricity which bears his name, and applied the most profound insight, and the most delicate experimental skill to its development. then arose the man who added to the powers of his intellect all the graces of the human heart, michael faraday, the discoverer of the great domain of magneto-electricity. oersted discovered the deflection of the magnetic needle, and arago and sturgeon the magnetization of iron by the electric current. the voltaic circuit finally found its theoretic newton in ohm; while henry, of princeton, who had the sagacity to recognize the merits of ohm while they were still decried in his own country, was at this time in the van of experimental inquiry. in the works of these men you have all the materials employed at this hour, in all the forms of the electric telegraph. nay, more; gauss, the illustrious astronomer, and weber, the illustrious natural philosopher, both professors in the university of göttingen, wishing to establish a rapid mode of communication between the observatory and the physical cabinet of the university, did this by means of an electric telegraph. thus, before your practical men appeared upon the scene, the force had been discovered, its laws investigated and made sure, the most complete mastery of its phenomena had been attained--nay, its applicability to telegraphic purposes demonstrated--by men whose sole reward for their labours was the noble excitement of research, and the joy attendant on the discovery of natural truth. are we to ignore all this? we do so at our peril. for i say again that, behind all our practical applications, there is a region of intellectual action to which practical men have rarely contributed, but from which they draw all their supplies. cut them off from this region, and they become eventually helpless. in no case is the adage truer, 'other men laboured, but ye are entered into their labours,' than in the case of the discoverer and applier of natural truth. but now a word on the other side. while practical men are not the men to make the necessary antecedent discoveries, the cases are rare, though, in our day, not absent, in which the discoverer knows how to turn his labours to practical account. different qualities of mind and habits of thought are usually needed in the two cases; and while i wish to give emphatic utterance to the claims of those whose position, owing to the simple fact of their intellectual elevation, is often misunderstood, i am not here to exalt the one class of workers at the expense of the other. they are the necessary complements of each other. but remember that one class is sure to be taken care of. all the material rewards of society are already within their reach, while that same society habitually ascribes to them intellectual achievements which were never theirs. this cannot but act to the detriment of those studies out of which, not only our knowledge of nature, but our present industrial arts themselves, have sprung, and from which the rising genius of the country is incessantly tempted away. pasteur, one of the most illustrious members of the institute of france, in accounting for the disastrous overthrow of his country, and the predominance of germany in the late war, expresses himself thus: 'few persons comprehend the real origin of the marvels of industry and the wealth of nations. i need no further proof of this than the employment, more and more frequent, in official language, and in writings of all sorts, of the erroneous expression _applied science_. the abandonment of scientific careers by men capable of pursuing them with distinction, was recently deplored in the presence of a minister of the greatest talent. the statesman endeavoured to show that we ought not to be surprised at this result, because _in our day the reign of theoretic science yielded place to that of applied science_. nothing could be more erroneous than this opinion, nothing, i venture to say, more dangerous, even to practical life, than the consequences which might flow from these words. they have rested in my mind as a proof of the imperious necessity of reform in our superior education. there exists no category of the sciences, to which the name of applied science could be rightly given. _we have science, and the applications of science_, which are united together as the tree and its fruit.' and cuvier, the great comparative anatomist, writes thus upon the same theme: 'these grand practical innovations are the mere applications of truths of a higher order, not sought with a practical intent, but pursued for their own sake, and solely through an ardour for knowledge. those who applied them could not have discovered them; but those who discovered them had no inclination to pursue them to a practical end. engaged in the high regions whither their thoughts had carried them, they hardly perceived these practical issues though born of their own deeds. these rising workshops, these peopled colonies, those ships which furrow the seas--this abundance, this luxury, this tumult--all this comes from discoveries in science, and it all remains strange to the discoverers. at the point where science merges into practice they abandon it; it concerns them no more.' when the pilgrim fathers landed at plymouth rock, and when penn made his treaty with the indians, the new-comers had to build their houses, to cultivate the earth, and to take care of their souls. in such a community science, in its more abstract forms, was not to be thought of. and at the present hour, when your hardy western pioneers stand face to face with stubborn nature, piercing the mountains and subduing the forest and the prairie, the pursuit of science, for its own sake, is not to be expected. the first need of man is food and shelter; but a vast portion of this continent is already raised far beyond this need. the gentlemen of new york, brooklyn, boston, philadelphia, baltimore, and washington have already built their houses, and very beautiful they are; they have also secured their dinners, to the excellence of which i can also bear testimony. they have, in fact, reached that precise condition of well-being and independence when a culture, as high as humanity has yet reached, may be justly demanded at their hands. they have reached that maturity, as possessors of wealth and leisure, when the investigator of natural truth, for the truth's own sake, ought to find among them promoters and protectors. among the many problems before them they have this to solve, whether a republic is able to foster the highest forms of genius. you are familiar with the writings of de tocqueville, and must be aware of the intense sympathy which he felt for your institutions; and this sympathy is all the more valuable from the philosophic candour with which he points out not only your merits, but your defects and dangers. now if i come here to speak of science in america in a critical and captious spirit, an invisible radiation from my words and manner will enable you to find me out, and will guide your treatment of me to-night. but if i in no unfriendly spirit--in a spirit, indeed, the reverse of unfriendly--venture to repeat before you what this great historian and analyst of democratic institutions said of america, i am persuaded that you will hear me out. he wrote some three and twenty years ago, and, perhaps, would not write the same to-day; but it will do nobody any harm to have his words repeated, and, if necessary, laid to heart. in a work published in , de tocqueville says: 'it must be confessed that, among the civilized peoples of our age, there are few in which the highest sciences have made so little progress as in the united states.'[ ] he declares his conviction that, had you been alone in the universe, you would soon have discovered that you cannot long make progress in practical science without cultivating theoretic science at the same time. but, according to de tocqueville, you are not thus alone. he refuses to separate america from its ancestral home; and it is there, he contends, that you collect the treasures of the intellect, without taking the trouble to create them. de tocqueville evidently doubts the capacity of a democracy to foster genius as it was fostered in the ancient aristocracies. 'the future,' he says, 'will prove whether the passion for profound knowledge, so rare and so fruitful, can be born and developed as readily in democratic societies as in aristocracies. for my part,' he continues, 'i can hardly believe it.' he speaks of the unquiet feverishness of democratic communities, not in times of great excitement, for such times may give an extraordinary impetus to ideas, but in times of peace. there is then, he says, 'a small and uncomfortable agitation, a sort of incessant attrition of man against man, which troubles and distracts the mind without imparting to it either loftiness or animation.' it rests with you to prove whether these things are necessarily so--whether scientific genius cannot find, in the midst of you, a tranquil home. i should be loth to gainsay so keen an observer and so profound a political writer, but, since my arrival in this country, i have been unable to see anything in the constitution of society, to prevent a student, with the root of the matter in him, from bestowing the most steadfast devotion on pure science. if great scientific results are not achieved in america, it is not to the small agitations of society that i should be disposed to ascribe the defect, but to the fact that the men among you who possess the endowments necessary for profound scientific inquiry, are laden with duties of administration, or tuition, so heavy as to be utterly incompatible with the continuous and tranquil meditation which original investigation demands. it may well be asked whether henry would have been transformed into an administrator, or whether draper would have forsaken science to write history, if the original investigator had been honoured as he ought to be in this land. i hardly think they would. still i do not imagine this state of things likely to last. in america there is a willingness on the part of individuals to devote their fortunes, in the matter of education, to the service of the commonwealth, which is probably without a parallel elsewhere; and this willingness requires but wise direction to enable you effectually to wipe away the reproach of de tocqueville. your most difficult problem will be, not to build institutions, but to discover men. you may erect laboratories and endow them; you may furnish them with all the appliances needed for inquiry; in so doing you are but creating opportunity for the exercise of powers which come from sources entirely beyond your reach. you cannot create genius by bidding for it. in biblical language, it is the gift of god; and the most you could do, were your wealth, and your willingness to apply it, a million-fold what they are, would be to make sure that this glorious plant shall have the freedom, light, and warmth necessary for its development. we see from time to time a noble tree dragged down by parasitic runners. these the gardener can remove, though the vital force of the tree itself may lie beyond him: and so, in many a case you men of wealth can liberate genius from the hampering toils which the struggle for existence often casts around it. drawn by your kindness, i have come here to give these lectures, and now that my visit to america has become almost a thing of the past, i look back upon it as a memory without a single stain. no lecturer was ever rewarded as i have been. from this vantage-ground, however, let me remind you that the work of the lecturer is not the highest work; that in science, the lecturer is usually the distributor of intellectual wealth amassed by better men. and though lecturing and teaching, in moderation, will in general promote their moral health, it is not solely or even chiefly, as lecturers, but as investigators, that your highest men ought to be employed. you have scientific genius amongst you--not sown broadcast, believe me, it is sown thus nowhere--but still scattered here and there. take all unnecessary impediments out of its way. keep your sympathetic eye upon the originator of knowledge. give him the freedom necessary for his researches, not overloading him, either with the duties of tuition or of administration, nor demanding from him so-called practical results--above all things, avoiding that question which ignorance so often addresses to genius: 'what is the use of your work?' let him make truth his object, however unpractical for the time being it may appear. if you cast your bread thus upon the waters, be assured it will return to you, though it be after many days. appendix. on the spectra of polarized light. mr. william spottiswoode introduced some years ago to the members of the royal institution, in a very striking form, a series of experiments on the spectra of polarized light. with his large nicol prisms he in the first place repeated and explained the experiments of foucault and fizeau, and subsequently enriched the subject by very beautiful additions of his own. i here append a portion of the abstract of his discourse:-- 'it is well known that if a plate of selenite sufficiently thin be placed between two nicol's prisms, or, more technically speaking, between a polarizer and analyzer, colour will be produced. and the question proposed is, what is the nature of that colour? is it simply a pure colour of the spectrum, or is it a compound, and if so, what are its component parts? the answer given by the wave theory is in brief this: in its passage through the selenite plate the rays have been so separated in the direction of their vibrations and in the velocity of their transmission, that, when re-compounded by means of the analyzer, they have in some instances neutralized one another. if this be the case, the fact ought to be visible when the beam emerging from the analyzer is dispersed by the prism; for then we have the rays of all the different colours ranged side by side, and, if any be wanting, their absence will be shown by the appearance of a dark band in their place in the spectrum. but not only so; the spectrum ought also to give an account of the other phenomena exhibited by the selenite when the analyzer is turned round, viz. that when the angle of turning amounts to °, all trace of colour disappears; and also that when the angle amounts to °, colour reappears, not, however, the original colour, but one complementary to it. 'you see in the spectrum of the reddish light produced by the selenite a broad but dark band in the blue; when the analyzer is turned round the band becomes less and less dark, until when the angle of turning amounts to ° it has entirely disappeared. at this stage each part of the spectrum has its own proportional intensity, and the whole produces the colourless image seen without the spectroscope. lastly, as the turning of the analyzer is continued, a dark band appears in the red, the part of the spectrum complementary to that occupied by the first band; and the darkness is most complete when the turning amounts to °. thus we have from the spectroscope a complete account of what has taken place to produce the original colour and its changes. 'it is further well known that the colour produced by a selenite, or other crystal plate, is dependent upon the thickness of the plate. and, in fact, if a series of plates be taken, giving different colours, their spectra are found to show bands arranged in different positions. the thinner plates show bands in the parts of the spectrum nearest to the violet, where the waves are shorter, and consequently give rise to redder colours; while the thicker show bands nearer to the red, where the waves are longer and consequently supply bluer tints. 'when the thickness of the plate is continually increased, so that the colour produced has gone through the complete cycle of the spectrum, a further increase of thickness causes a reproduction of the colours in the same order; but it will be noticed that at each recurrence of the cycle the tints become paler, until when a number of cycles have been performed, and the thickness of the plate is considerable, all trace of colour is lost. let us now take a series of plates, the first two of which, as you see, give colours; with the others which are successively of greater thickness the tints are so feeble that they can scarcely be distinguished. the spectrum of the first shows a single band; that of the second, two; showing that the second series of tints is not identical with the first, but that it is produced by the extinction of two colours from the components of white light. the spectra of the others show series of bands more and more numerous in proportion to the thickness of the plate, an array which may be increased indefinitely. the total light, then, of which the spectrum is deprived by the thicker plates is taken from a greater number of its parts; or, in other words, the light which still remains is distributed more and more evenly over the spectrum; and in the same proportion the sum total of it approaches more and more nearly to white light. 'these experiments were made more than thirty years ago by the french philosophers, mm. foucault and fizeau. 'if instead of selenite, iceland spar, or other ordinary crystals, we use plates of quartz cut perpendicularly to the axis, and turn the analyzer round as before, the light, instead of exhibiting only one colour and its complementary with an intermediate stage in which colour is absent, changes continuously in tint; and the order of the colour depends partly upon the direction in which the analyzer is turned, and partly upon the character of the crystal, _i.e._ whether it is right-handed or left-handed. if we examine the spectrum in this case we find that the dark band never disappears, but marches from one end of the spectrum to another, or _vice versâ_, precisely in such a direction as to give rise to the tints seen by direct projection. 'the kind of polarization effected by the quartz plates is called circular, while that effected by the other class of crystals is called plane, on account of the form of the vibrations executed by the molecules of æther; and this leads us to examine a little more closely the nature of the polarization of different parts of these spectra of polarized light. 'now, two things are clear: first, that if the light be plane-polarized--that is, if all the vibrations throughout the entire ray are rectilinear and in one plane--they must in all their bearings have reference to a particular direction in space, so that they will be differently affected by different positions of the analyzer. secondly, that if the vibrations be circular, they will be affected in precisely the same way (whatever that may be) in all positions of the analyzer. this statement merely recapitulates a fundamental point in polarization. in fact, plane-polarized light is alternately transmitted and extinguished by the analyzer as it is turned through °; while circularly polarized light [if we could get a single ray] remains to all appearance unchanged. and if we examine carefully the spectrum of light which has passed through a selenite, or other ordinary crystal, we shall find that, commencing with two consecutive bands in position, the parts occupied by the bands and those midway between them are plane-polarized, for they become alternately dark and bright; while the intermediate parts, _i.e._ the parts at one-fourth of the distance from one band to the next, remain permanently bright. these are, in fact, circularly polarized. but it would be incorrect to conclude from this experiment alone that such is really the case, because the same appearance would be seen if those parts were unpolarized, _i.e._ in the condition of ordinary lights. and on such a supposition we should conclude with equal justice that the parts on either side of the parts last mentioned (e.g. the parts separated by eighth parts of the interval between two bands) were partially polarized. but there is an instrument of very simple construction, called a "quarter-undulation plate," a plate usually of mica, whose thickness is an odd multiple of a quarter of a wave-length, which enables us to discriminate between light unpolarized and circularly polarized. the exact mechanical effect produced upon the ray could hardly be explained in detail within our present limits of time; but suffice it for the present to say that, when placed in a proper position, the plate transforms plane into circular and circular into plane polarization. that being so, the parts which were originally banded ought to remain bright, and those which originally remained bright ought to become banded during the rotation of the analyzer. the general effect to the eye will consequently be a general shifting of the bands through one-fourth of the space which separates each pair. 'circular polarization, like circular motion generally, may of course be of two kinds, which differ only in the direction of the motion. and, in fact, to convert the circular polarization produced by this plate from one of these kinds to the other (say from right-handed to left-handed, or _vice versâ_), we have only to turn the plate round through °. conversely, right-handed circular polarization will be changed by the plate into plane-polarization in one direction, while left-handed will be changed into plane at right angles to the first. hence if the plate be turned round through ° we shall see that the bands are shifted in a direction opposite to that in which they were moved at first. in this therefore we have evidence not only that the polarization immediately on either side of a band is circular; but also that that immediately on the one side is right-handed, while that immediately on the other is left-handed[ ]. 'if time permitted, i might enter still further into detail, and show that the polarization between the plane and the circular is elliptical, and even the positions of the longer and shorter axes and the direction of motion in each case. but sufficient has, perhaps, been said for our present purpose. 'before proceeding to the more varied forms of spectral bands, which i hope presently to bring under your notice, i should like to ask your attention for a few minutes to the peculiar phenomena exhibited when two plates of selenite giving complementary colours are used. the appearance of the spectrum varies with the relative position of the plates. if they are similarly placed--that is, as if they were one plate of crystal--they will behave as a single plate, whose thickness is the sum of the thicknesses of each, and will produce double the number of bands which one alone would give; and when the analyzer is turned, the bands will disappear and re-appear in their complementary positions, as usual in the case of plane-polarization. if one of them be turned round through °, a single band will be seen at a particular position in the spectrum. this breaks into two, which recede from one another towards the red and violet ends respectively, or advance towards one another according to the direction in which the analyzer is turned. if the plate be turned through ° in the opposite direction, the effects will be reversed. the darkness of the bands is, however, not equally complete during their whole passage. lastly, if one of the plates be turned through °, no bands will be seen, and the spectrum will be alternately bright and dark, as if no plates were used, except only that the polarization is itself turned through °. 'if a wedge-shaped crystal be used, the bands, instead of being straight, will cross the spectrum diagonally, the direction of the diagonal (dexter or sinister) being determined by the position of the thicker end of the wedge. if two similar wedges be used with their thickest ends together, they will act as a wedge whose angle and whose thickness is double of the first. if they be placed in the reverse position they will act as a flat plate, and the bands will again cross the spectrum in straight lines at right angles to its length. 'if a concave plate be used the bands will dispose themselves in a fanlike arrangement, their divergence depending upon the distance of the slit from the centre of concavity. 'if two quartz wedges, one of which has the optic axis parallel to the edge of the refractory angle, and the other perpendicular to it, but in one of the planes containing the angle (babinet's compensator), the appearances of the bands are very various. 'the diagonal bands, besides sometimes doubling themselves as with ordinary wedges, sometimes combine so as to form longitudinal (instead of transverse) bands; and sometimes cross one another so as to form a diaper pattern with bright compartments in a dark framework, and _vice versâ_, according to the position of the plates. 'the effects of different dispositions of the interposed crystals might be varied indefinitely; but enough has perhaps been said to show the delicacy of the method of spectrum analysis as applied to the examination of polarized light.' * * * * * the singular and beautiful effect obtained with a circular plate of selenite, thin at the centre, and gradually thickening towards the circumference, is easily connected with a similar effect obtained with newton's rings. let a thin slice of light fall upon the glasses which show the rings, so as to cover a narrow central vertical zone passing through them all. the image of this zone upon the screen is crossed by portions of the iris-rings. subjecting the reflected beam to prismatic analysis, the resultant spectrum may be regarded as an indefinite number of images of the zone placed side by side. in the image before dispersion we have _iris-rings_, the extinction of the light being nowhere complete; but when the different colours are separated by dispersion, each colour is crossed transversely by its own system of dark interference bands, which become gradually closer with the increasing refrangibility of the light. the complete spectrum, therefore, appears furrowed by a system of continuous dark bands, crossing the colours transversely, and approaching each other as they pass from red to blue. in the case of the plate of selenite, a slit is placed in front of the polarizer, and the film of selenite is held close to the slit, so that the light passes through the central zone of the film. as in the case of newton's rings, the image of the zone is crossed by iris-coloured bands; but when subjected to prismatic dispersion, the light of the zone yields a spectrum furrowed by bands of complete darkness exactly as in the case of newton's rings and for a similar reason. this is the beautiful effect described by mr. spottiswoode as the fanlike arrangement of the bands--the fan opening out at the red end of the spectrum. * * * * * _measurement of the waves of light._ the diffraction fringes described in lecture ii., instead of being formed on the retina, may be formed on a screen, or upon ground glass, when they can be looked at through a magnifying lens from behind, or they can be observed in the air when the ground glass is removed. instead of permitting them to form on the retina, we will suppose them formed on a screen. this places us in a condition to understand, even without trigonometry, the solution of the important problem of measuring _the length_ of a wave of light. we will suppose the screen so distant that the rays falling upon it from the two margins of the slit are sensibly parallel. we have learned in lecture ii. that the first of the dark bands corresponds to a difference of marginal path of one undulation; the second dark band to a difference of path of two undulations; the third dark band to a difference of three undulations, and so on. now the angular distance of the bands from the centre is capable of exact measurement; this distance depending, as already stated, on the width of the slit. with a slit . millimeter wide,[ ] schwerd found the angular distance of the first dark band from the centre of the field to be ' "; the angular distances of the second, third, fourth dark bands being twice, three times, four times this quantity. [illustration: fig. .] let a b, fig. , be the plate in which the slit is cut, and c d the grossly exaggerated width of the slit, with the beam of red light proceeding from it at the obliquity corresponding to the first dark band. let fall a perpendicular from one edge, d, of the slit on the marginal ray of the other edge at _d_. the distance, c _d_, between the foot of this perpendicular and the other edge is the length of a wave of the light. the angle c d _d_, moreover, being equal to r c r', is, in the case now under consideration, ' ". from the centre d, with the width d c as radius, describe a semicircle; its radius d c being . millimeter, the length of this semicircle is found by an easy calculation to be . millimeters. the length c _d_ is so small that it sensibly coincides with the arc of the circle. hence the length of the semicircle is to the length c _d_ of the wave as ° to ' ", or, reducing all to seconds, as , " to ". thus, we have the proportion-- , : :: . to the wave-length c _d_. making the calculation, we find the wave-length for this particular kind of light to be . of a millimeter, or . of an inch. footnotes: [footnote : among whom may be especially mentioned the late sir edmund head, bart., with whom i had many conversations on this subject.] [footnote : at whose hands it gives me pleasure to state i have always experienced honourable and liberal treatment.] [footnote : one of the earliest of these came from mr. john amory lowell of boston.] [footnote : it will be subsequently shown how this simple apparatus may be employed to determine the 'polarizing angle' of a liquid.] [footnote : from this principle sir john herschel deduces in a simple and elegant manner the fundamental law of reflection.--see _familiar lectures_, p. .] [footnote : the low dispersive power of water masks, as helmholtz has remarked, the imperfect achromatism of the eye. with the naked eye i can see a distant blue disk sharply defined, but not a red one. i can also see the lines which mark the upper and lower boundaries of a horizontally refracted spectrum sharp at the blue end, but ill-defined at the red end. projecting a luminous disk upon a screen, and covering one semicircle of the aperture with a red and the other with a blue or green glass, the difference between the apparent sizes of the two semicircles is in my case, and in numerous other cases, extraordinary. many persons, however, see the apparent sizes of the two semicircles reversed. if with a spectacle glass i correct the dispersion of the red light over the retina, then the blue ceases to give a sharply defined image. thus examined, the departure of the eye from achromatism appears very gross indeed.] [footnote : both in foliage and in flowers there are striking differences of absorption. the copper beech and the green beech, for example, take in different rays. but the very growth of the tree is due to some of the rays thus taken in. are the chemical rays, then, the same in the copper and the green beech? in two such flowers as the primrose and the violet, where the absorptions, to judge by the colours, are almost complementary, are the chemically active rays the same? the general relation of colour to chemical action is worthy of the application of the method by which dr. draper proved so conclusively the chemical potency of the yellow rays of the sun.] [footnote : young, helmholtz, and maxwell reduce all differences of hue to combinations in different proportions of three primary colours. it is demonstrable by experiment that from the red, green, and violet _all_ the other colours of the spectrum may be obtained. some years ago sir charles wheatstone drew my attention to a work by christian ernst wünsch, leipzig , in which the author announces the proposition that there are neither five nor seven, but only three simple colours in white light. wünsch produced five spectra, with five prisms and five small apertures, and he mixed the colours first in pairs, and afterwards in other ways and proportions. his result is that red is a _simple_ colour incapable of being decomposed; that orange is compounded of intense red and weak green; that yellow is a mixture of intense red and intense green; that green is a _simple_ colour; that blue is compounded of saturated green and saturated violet; that indigo is a mixture of saturated violet and weak green; while violet is a pure _simple_ colour. he also finds that yellow and indigo blue produce _white_ by their mixture. yellow mixed with bright blue (hochblau) also produces white, which seems, however, to have a tinge of green, while the pigments of these two colours when mixed always give a more or less beautiful green, wünsch very emphatically distinguishes the mixture of pigments from that of lights. speaking of the generation of yellow, he says, 'i say expressly _red and green light_, because i am speaking about light-colours (lichtfarben), and not about pigments.' however faulty his theories may be, wünsch's experiments appear in the main to be precise and conclusive. nearly ten years subsequently, young adopted red, green, and violet as the three primary colours, each of them capable of producing three sensations, one of which, however, predominates over the two others. helmholtz adopts, elucidates, and enriches this notion. (_popular lectures_, p. . the paper of helmholtz on the mixture of colours, translated by myself, is published in the _philosophical magazine_ for . maxwell's memoir on the theory of compound colours is published in the _philosophical transactions_, vol. , p. .)] [footnote : the following charming extract, bearing upon this point, was discovered and written out for me by my deeply lamented friend dr. bence jones, when hon. secretary to the royal institution:-- 'in every kind of magnitude there is a degree or sort to which our sense is proportioned, the perception and knowledge of which is of the greatest use to mankind. the same is the groundwork of philosophy; for, though all sorts and degrees are equally the object of philosophical speculation, yet it is from those which are proportioned to sense that a philosopher must set out in his inquiries, ascending or descending afterwards as his pursuits may require. he does well indeed to take his views from many points of sight, and supply the defects of sense by a well-regulated imagination; nor is he to be confined by any limit in space or time; but, as his knowledge of nature is founded on the observation of sensible things, he must begin with these, and must often return to them to examine his progress by them. here is his secure hold: and as he sets out from thence, so if he likewise trace not often his steps backwards with caution, he will be in hazard of losing his way in the labyrinths of nature.'--(_maclaurin: an account of sir i. newton's philosophical discoveries. written ; second edition_, ; pp. , .) ] [footnote : i do not wish to encumber the conception here with the details of the motion, but i may draw attention to the beautiful model of prof. lyman, wherein waves are shown to be produced by the _circular_ motion of the particles. this, as proved by the brothers weber, is the real motion in the case of water-waves.] [footnote : copied from weber's _wellenlehre_.] [footnote : see _lectures on sound_, st and nd ed., lecture vii.; and rd ed., chap. viii. longmans.] [footnote : _boyle's works_, birch's edition, p. .] [footnote : page .] [footnote : the beautiful plumes produced by water-crystallization have been successfully photographed by professor lockett.] [footnote : in a little volume entitled 'forms of water,' i have mentioned that cold iron floats upon molten iron. in company with my friend sir william armstrong, i had repeated opportunities of witnessing this fact in his works at elswick, . faraday, i remember, spoke to me subsequently of the perfection of iron castings as probably due to the swelling of the metal on solidification. beyond this, i have given the subject no special attention; and i know that many intelligent iron-founders doubt the fact of expansion. it is quite possible that the solid floats because it is not _wetted_ by the molten iron, its volume being virtually augmented by capillary repulsion. certain flies walk freely upon water in virtue of an action of this kind. with bismuth, however, it is easy to burst iron bottles by the force of solidification.] [footnote : this beautiful law is usually thus expressed: _the index of refraction of any substance is the tangent of its polarizing angle_. with the aid of this law and an apparatus similar to that figured at page , we can readily determine the index of refraction of any liquid. the refracted and reflected beams being visible, they can readily be caused to inclose a right angle. the polarizing angle of the liquid may be thus found with the sharpest precision. it is then only necessary to seek out its natural tangent to obtain the index of refraction.] [footnote : whewell.] [footnote : removed from us since these words were written.] [footnote : the only essay known to me on the undulatory theory, from the pen of an american writer, is an excellent one by president barnard, published in the smithsonian report for .] [footnote : _boyle's works_, birch's edition, vol. i. pp, and .] [footnote : _werke_, b. xxix. p. .] [footnote : defined in lecture i.] [footnote : this circumstance ought not to be lost sight of in the examination of compound spectra. other similar instances might be cited.] [footnote : the dark band produced when the sodium is placed within the lamp was observed on the same occasion. then was also observed for the first time the magnificent blue band of lithium which the bunsen's flame fails to bring out.] [footnote : new york: for more than a decade no such weather had been experienced. the snow was so deep that the ordinary means of locomotion were for a time suspended.] [footnote : 'il faut reconnaître que parmi les peuples civilisés de nos jours il en est pen chez qui les hautes sciences aient fait moins de progrès qu'aux États-unis, ou qui aient fourni moins de grands artistes, de poëtes illustres et de célèbres écrivains.' (_de la démocratie en amérique_, etc. tome ii. p. .)] [footnote : at these points the two rectangular vibrations into which the original polarized ray is resolved by the plates of gypsum, act upon each other like the two rectangular impulses imparted to our pendulum in lecture iv., one being given when the pendulum is at the limit of its swing. vibration is thus converted into rotation.] [footnote : the millimeter is about / th of an inch.] index. absorption, principles of, airy, sir george, severity and conclusiveness of his proofs, alhazen, his inquiry respecting light, , analyzer, polarizer and, ----recompounding of the two systems of waves by the analyzer, Ångström, his paper on spectrum analysis, arago, françois, and dr. young, ----his discoveries respecting light, atomic polarity, - bacon, roger, his inquiry respecting light, , bartholinus, erasmus, on iceland spar, bérard on polarization of heat, blackness, meaning of, boyle, robert, his observations on colours, , ----his remarks on fluorescence, , bradley, james, discovers the aberration of light, , brewster, sir david, his chief objection to the undulatory theory of light, brewster, sir david, his discovery in biaxal crystals, brougham, mr. (afterwards lord), ridicules dr. t. young's speculations, , cæsium, discovery of, calorescence, clouds, actinic, - ----polarization of, colours of thin plates, ----boyle's observations on, , ----hooke on the colours of thin plates, ----of striated surfaces, , comet of , newton's estimate of the temperature of, crookes, mr., his discovery of thallium, crystals, action of, upon light, ----built by polar force, ----illustrations of crystallization, ----architecture of, considered as an introduction to their action upon light, ----bearings of crystallization upon optical phenomena, crystals, rings surrounding the axes of, uniaxal and biaxal, cuvier on ardour for knowledge, de tocqueville, writings of, , , descartes, his explanation of the rainbow, , ----his ideas respecting the transmission of light, ----his notion of light, diamond, ignition of a, in oxygen, diathermancy, diffraction of light, phenomena of, ----bands, , ----explanation of, ----colours produced by, dollond, his experiments on achromatism, draper, dr., his investigation on heat, drummond light, spectrum of, earth, daily orbit of, electric beam, heat of the, electricity, discoveries in, , emission theory of light, bases of the, ----newton espouses the theory, and the results of this espousal, ether, huyghens and euler advocate and defend the conception of an, , ----objected to by newton, euler espouses and defends the conception of an ether, , eusebius on the natural philosophers of his time, expansion by cold, experiment, uses of, eye, the, its imperfections, grown for ages towards perfection, ----imperfect achromatism of the, , _note_ faraday, michael, his discovery of magneto-electricity, 'fits,' theory of, ----its explanation of newton's rings, ----overthrow of the theory, fizeau determines the velocity of light, fluorescence, stokes's discovery of, ----the name, forbes, professor, polarizes and depolarizes heat, foucault, determines the velocity of light, ----his experiments on absorption, , fraunhofer, his theoretical calculations respecting diffraction, ----his lines, ------their explanation by kirchhoff, fresnel, and dr. young, ----his theoretical calculations respecting diffraction, ----his mathematical abilities and immortal name, goethe on fluorescence, gravitation, origin of the notion of the attraction of, ----strength of the theory of, grimaldi, his discovery with respect to light, ----young's generalizations of, hamilton, sir william, of dublin, his discovery of conical refraction, heat, generation of, ----dr. draper's investigation respecting, helmholtz, his estimate of the genius of young, ----on the imperfect achromatism of the eye, _note_, ----reveals the cause of green in the case of pigments, henry, professor joseph, his invitation, herschel, sir john, his theoretical calculations respecting diffraction, ----first notices and describes the fluorescence of sulphate of quinine, ----his experiments on spectra, herschel, sir william, his experiments on the heat of the various colours of the solar spectrum, hooke, robert, on the colours of thin plates, ----his remarks on the idea that light and heat are modes of motion, horse-chestnut bark, fluorescence of, huggins, dr., his labours, huyghens advocates the conception of ether, , ----his celebrated principle, huyghens on the double refraction of iceland spar, iceland spar, ----double refraction caused by, ----this double refraction first treated by erasmus bartholinus, ----character of the beams emergent from, ----tested by tourmaline, ----knoblauch's demonstration of the double refraction of, ice-lens, combustion through, imagination, scope of the, ----note by maclaurin on this point, _note_ janssen, m., on the rose-coloured solar prominences, jupiter, roemer's observations of the moons of, jupiter's distance from the sun, kepler, his investigations on the refraction of light, , kirchhoff, professor, his explanation of fraunhofer's lines, ----his precursors, ----his claims, knoblauch, his demonstration of the double refraction of heat of iceland spar, lactantius, on the natural philosophers of his time, lamy, m., isolates thallium in ingots, lesley, professor, his invitation, light familiar to the ancients, ----generation of, , ----spherical aberration of, ----the rectilineal propagation of, and mode of producing it, ----illustration showing that the angle of incidence is equal to the angle of reflection, , ----sterility of the middle ages, ----history of refraction, ----demonstration of the fact of refraction, ----partial and total reflection of, - ----velocity of, ----bradley's discovery of the aberration of light, , ----principle of least time, ----descartes and the rainbow, ----newton's analysis of, , ----synthesis of white light, ----complementary colours, ----yellow and blue lights produce white by their mixture, ----what is the meaning of blackness? ----analysis of the action of pigments upon, ----absorption, ----mixture of pigments contrasted with mixture of lights, ----wünsch on three simple colours in white light, _note_ ----newton arrives at the emission theory, ----young's discovery of the undulatory theory, ----illustrations of wave-motion, ----interference of sound-waves, ----velocity of, ----principle of interference of waves of, ----phenomena which first suggested the undulatory theory - ----soap-bubbles and their colours, - ----newton's rings, - ----his espousal of the emission theory, and the results of this espousal, ----transmitted light, ----diffraction, , ----origin of the notion of the attraction of gravitation, ----polarity, how generated, ----action of crystals upon, ----refraction of, ----elasticity and density, ----double refraction, ----chromatic phenomena produced by crystals in polarized, ----the nicol prism, ----mechanism of, ----vibrations, ----composition and resolution of vibrations, ----polarizer and analyzer, ----recompounding the two systems of waves by the analyzer, ----interference thus rendered possible, ----chromatic phenomena produced by quartz, ----magnetization, of, ----rings surrounding the axes of crystals, ----colour and polarization of sky, , ----range of vision incommensurate with range of radiation, ----effect of thallene on the spectrum, ----fluorescence, ----transparency, ----the ultra-red rays, ----part played in nature by these rays, ----conversion of heat-rays into light-rays, ----identity of radiant heat and, ----polarization of heat, ----principles of spectrum analysis, ----spectra of incandescent vapours, ----fraunhofer's lines, and kirchhoff's explanation of them, ----solar chemistry, - ----demonstration of analogy between sound and, , ----kirchhoff and his precursors, ----rose-coloured solar prominences, ----results obtained by various workers, ----summary and conclusion, ----polarized, the spectra of, ----measurement of the waves of, lignum nephriticum, fluorescence of, lloyd, dr., on polarization of heat, , lockyer, mr., on the rose-coloured solar prominences, lycopodium, diffraction effects caused by the spores of, magnetization of light, malus, his discovery respecting reflected light through iceland spar, ----discovers the polarization of light by reflection, masson, his essay on the bands of the induction spark, melloni, on the polarization of heat, metals, combustion of, , ----spectrum analysis of, ----spectrum bands proved by bunsen and kirchhoff to be characteristic of the vapour of, mill, john stuart, his scepticism regarding the undulatory theory, miller, dr., his drawings and descriptions of the spectra of various coloured flames, morton, professor, his discovery of thallene, mother-of-pearl, colours of, nature, a savage's interpretation of, newton, sir isaac, his experiments on the composition of solar light, ----his spectrum, ----dispersion, ----arrives at the emission theory of light, ----his objection to the conception of an ether espoused and defended by huyghens and euler, ----his optical career, ----his rings, - ----his rings explained by the theory of 'fits,' ----espouses the emission theory, ----effects of this espousal, ----his idea of gravitation, ----his errors, nicol prism, the, ocean, colour of the, oersted, discovers the deflection of a magnetic needle by an electric current, optics, science of, pasteur referred to, physical theories, origin of, - pigments, analysis of the action of, upon light, ----mixture of, contrasted with mixture of lights, ----helmholtz reveals the cause of the green in the case of mixed blue and yellow pigments, ----impurity of natural colours, pitch of sound, plücker, his drawings of spectra, polariscope, stained glass in the, , ----unannealed glass in the, polarity, notion of, how generated, ----atomic, - ----structural arrangements due to, ----polarization of light, ----tested by tourmaline, ----and by reflection and refraction, ----depolarization, polarization of light, ----circular, ----sky-light, , ----of artificial sky, ----of radiant heat, polarizer and analyzer, poles of a magnet, powell, professor, on polarization of heat, prism, the nicol, quartz, chromatic phenomena produced by, radiant heat, ----diathermancy, or perviousness to radiant heat, ----conversion of heat-rays into light rays, ----formation of invisible heat-images, ----polarization of, ----double refraction, ----magnetization of, rainbow, descartes' explanation of the, refraction, demonstration of, refraction of light, ----double, reflection, partial and total, - respighi, results obtained by, ritter, his discovery of the ultraviolet rays of the sun, roemer, olav, his observations of jupiter's moons, ----his determination of the velocity of light, rubidium, discovery of, rusting of iron, what it is, schwerd, his observations respecting diffraction, science, growth of, , scoresby, dr., succeeds in exploding gunpowder by the sun's rays conveyed by large lenses of ice, secchi, results obtained by, seebeck, thomas, discovers thermo-electricity, ----discovers the polarization of light by tourmaline, selenite, experiments with thick and thin plates of, silver spectrum, analysis of, , sky-light, colour and polarization of, , ----generation of artificial skies, snell, willebrord, his discovery, ----his law, , soap-bubbles and their colours, , sound, early notions of the ancients respecting, ----interference of waves of, ----pitch of, ----analogies of light and, ----demonstration of analogy between, and light, , sonorous vibrations, action of, spectrum analysis, principles of, spectra of incandescent vapours, ----discontinuous, , ----of polarized light, spectrum bands proved by bunsen and kirchhoff to be characteristic of the vapour, ----its capacity as an agent of discovery, ----analysis of the sun and stars, spottiswoode, mr. william, , stewart, professor balfour, stokes, professor, results of his examination of substances excited by the ultra-violet waves, ----his discovery of fluorescence, ----on fluorescence, ----nearly anticipates kirchhoff's discovery, , striated surfaces, colours of, sulphate of quinine first noticed and described by sir john herschel, sun, chemistry of the, sun, rose-coloured solar prominences, talbot, mr., his experiments, tartaric acid, irregular crystallization of, and its effects, thallene, its effect on the spectrum, thallium, spectrum analysis of, , ----discovery of, ----isolated in ingots by m. lamy, theory, relation of, to experience, thermo-electric pile, thermo-electricity, discovery of, tombeline, mont, inverted image of, tourmaline, polarization of light by means of, transmitted light, reason for, transparency, remarks on, ultra-violet sun-rays, discovered by ritter, ----effects of, ultra-red rays of the solar spectrum, ----part played by the, undulatory theory of light, bases of the, ----sir david brewster's chief objection to the, undulatory theory of light, young's foundation of the, ----phenomena which first suggested the, , ----mr. mill's scepticism regarding the, ----a demonstrated verity in the hands of young, vassenius describes the rose-coloured solar prominences in , vitellio, his skill and conscientiousness, ----his investigations respecting light, voltaic battery, use of, and its production of heat, , water, deportment of, considered and explained, , waves of water, ----length of a wave, ----interference of waves, - wertheim, m., his instrument for the determination of strains and pressures by the colours of polarized light, wheatstone, sir charles, his analysis of the light of the electric spark, whirlpool rapids, illustration of the principle of the interference of waves at the, willigen, van der, his drawings of spectra, wollaston, dr., first observes lines in solar spectrum, ----discovers the rings of iceland spar, woodbury, mr., on the impurity of natural colours, wünsch, christian ernst, on the three simple colours in white lights, _note_ ----his experiments, _note_ young, dr. thomas, his discovery of egyptian hieroglyphics, ; ----and the undulatory theory of light, ----helmholtz's estimate of him, ----ridiculed by brougham in the 'edinburgh review,' ----generalizes grimaldi's observation on light, , ----photographs the ultra-violet rings of newton, none the book of the damned a procession of the damned. by the damned, i mean the excluded. we shall have a procession of data that science has excluded. battalions of the accursed, captained by pallid data that i have exhumed, will march. you'll read them--or they'll march. some of them livid and some of them fiery and some of them rotten. some of them are corpses, skeletons, mummies, twitching, tottering, animated by companions that have been damned alive. there are giants that will walk by, though sound asleep. there are things that are theorems and things that are rags: they'll go by like euclid arm in arm with the spirit of anarchy. here and there will flit little harlots. many are clowns. but many are of the highest respectability. some are assassins. there are pale stenches and gaunt superstitions and mere shadows and lively malices: whims and amiabilities. the naïve and the pedantic and the bizarre and the grotesque and the sincere and the insincere, the profound and the puerile. a stab and a laugh and the patiently folded hands of hopeless propriety. the ultra-respectable, but the condemned, anyway. the aggregate appearance is of dignity and dissoluteness: the aggregate voice is a defiant prayer: but the spirit of the whole is processional. the power that has said to all these things that they are damned, is dogmatic science. but they'll march. the little harlots will caper, and freaks will distract attention, and the clowns will break the rhythm of the whole with their buffooneries--but the solidity of the procession as a whole: the impressiveness of things that pass and pass and pass, and keep on and keep on and keep on coming. the irresistibleness of things that neither threaten nor jeer nor defy, but arrange themselves in mass-formations that pass and pass and keep on passing. * * * * * so, by the damned, i mean the excluded. but by the excluded i mean that which will some day be the excluding. or everything that is, won't be. and everything that isn't, will be-- but, of course, will be that which won't be-- it is our expression that the flux between that which isn't and that which won't be, or the state that is commonly and absurdly called "existence," is a rhythm of heavens and hells: that the damned won't stay damned; that salvation only precedes perdition. the inference is that some day our accursed tatterdemalions will be sleek angels. then the sub-inference is that some later day, back they'll go whence they came. * * * * * it is our expression that nothing can attempt to be, except by attempting to exclude something else: that that which is commonly called "being" is a state that is wrought more or less definitely proportionately to the appearance of positive difference between that which is included and that which is excluded. but it is our expression that there are no positive differences: that all things are like a mouse and a bug in the heart of a cheese. mouse and a bug: no two things could seem more unlike. they're there a week, or they stay there a month: both are then only transmutations of cheese. i think we're all bugs and mice, and are only different expressions of an all-inclusive cheese. or that red is not positively different from yellow: is only another degree of whatever vibrancy yellow is a degree of: that red and yellow are continuous, or that they merge in orange. so then that, if, upon the basis of yellowness and redness, science should attempt to classify all phenomena, including all red things as veritable, and excluding all yellow things as false or illusory, the demarcation would have to be false and arbitrary, because things colored orange, constituting continuity, would belong on both sides of the attempted borderline. as we go along, we shall be impressed with this: that no basis for classification, or inclusion and exclusion, more reasonable than that of redness and yellowness has ever been conceived of. science has, by appeal to various bases, included a multitude of data. had it not done so, there would be nothing with which to seem to be. science has, by appeal to various bases, excluded a multitude of data. then, if redness is continuous with yellowness: if every basis of admission is continuous with every basis of exclusion, science must have excluded some things that are continuous with the accepted. in redness and yellowness, which merge in orangeness, we typify all tests, all standards, all means of forming an opinion-- or that any positive opinion upon any subject is illusion built upon the fallacy that there are positive differences to judge by-- that the quest of all intellection has been for something--a fact, a basis, a generalization, law, formula, a major premise that is positive: that the best that has ever been done has been to say that some things are self-evident--whereas, by evidence we mean the support of something else-- that this is the quest; but that it has never been attained; but that science has acted, ruled, pronounced, and condemned as if it had been attained. what is a house? it is not possible to say what anything is, as positively distinguished from anything else, if there are no positive differences. a barn is a house, if one lives in it. if residence constitutes houseness, because style of architecture does not, then a bird's nest is a house: and human occupancy is not the standard to judge by, because we speak of dogs' houses; nor material, because we speak of snow houses of eskimos--or a shell is a house to a hermit crab--or was to the mollusk that made it--or things seemingly so positively different as the white house at washington and a shell on the seashore are seen to be continuous. so no one has ever been able to say what electricity is, for instance. it isn't anything, as positively distinguished from heat or magnetism or life. metaphysicians and theologians and biologists have tried to define life. they have failed, because, in a positive sense, there is nothing to define: there is no phenomenon of life that is not, to some degree, manifest in chemism, magnetism, astronomic motions. white coral islands in a dark blue sea. their seeming of distinctness: the seeming of individuality, or of positive difference one from another--but all are only projections from the same sea bottom. the difference between sea and land is not positive. in all water there is some earth: in all earth there is some water. so then that all seeming things are not things at all, if all are inter-continuous, any more than is the leg of a table a thing in itself, if it is only a projection from something else: that not one of us is a real person, if, physically, we're continuous with environment; if, psychically, there is nothing to us but expression of relation to environment. our general expression has two aspects: conventional monism, or that all "things" that seem to have identity of their own are only islands that are projections from something underlying, and have no real outlines of their own. but that all "things," though only projections, are projections that are striving to break away from the underlying that denies them identity of their own. i conceive of one inter-continuous nexus, in which and of which all seeming things are only different expressions, but in which all things are localizations of one attempt to break away and become real things, or to establish entity or positive difference or final demarcation or unmodified independence--or personality or soul, as it is called in human phenomena-- that anything that tries to establish itself as a real, or positive, or absolute system, government, organization, self, soul, entity, individuality, can so attempt only by drawing a line about itself, or about the inclusions that constitute itself, and damning or excluding, or breaking away from, all other "things": that, if it does not so act, it cannot seem to be; that, if it does so act, it falsely and arbitrarily and futilely and disastrously acts, just as would one who draws a circle in the sea, including a few waves, saying that the other waves, with which the included are continuous, are positively different, and stakes his life upon maintaining that the admitted and the damned are positively different. our expression is that our whole existence is animation of the local by an ideal that is realizable only in the universal: that, if all exclusions are false, because always are included and excluded continuous: that if all seeming of existence perceptible to us is the product of exclusion, there is nothing that is perceptible to us that really is: that only the universal can really be. our especial interest is in modern science as a manifestation of this one ideal or purpose or process: that it has falsely excluded, because there are no positive standards to judge by: that it has excluded things that, by its own pseudo-standards, have as much right to come in as have the chosen. * * * * * our general expression: that the state that is commonly and absurdly called "existence," is a flow, or a current, or an attempt, from negativeness to positiveness, and is intermediate to both. by positiveness we mean: harmony, equilibrium, order, regularity, stability, consistency, unity, realness, system, government, organization, liberty, independence, soul, self, personality, entity, individuality, truth, beauty, justice, perfection, definiteness-- that all that is called development, progress, or evolution is movement toward, or attempt toward, this state for which, or for aspects of which, there are so many names, all of which are summed up in the one word "positiveness." at first this summing up may not be very readily acceptable. at first it may seem that all these words are not synonyms: that "harmony" may mean "order," but that by "independence," for instance, we do not mean "truth," or that by "stability" we do not mean "beauty," or "system," or "justice." i conceive of one inter-continuous nexus, which expresses itself in astronomic phenomena, and chemic, biologic, psychic, sociologic: that it is everywhere striving to localize positiveness: that to this attempt in various fields of phenomena--which are only quasi-different--we give different names. we speak of the "system" of the planets, and not of their "government": but in considering a store, for instance, and its management, we see that the words are interchangeable. it used to be customary to speak of chemic equilibrium, but not of social equilibrium: that false demarcation has been broken down. we shall see that by all these words we mean the same state. as every-day conveniences, or in terms of common illusions, of course, they are not synonyms. to a child an earth worm is not an animal. it is to the biologist. by "beauty," i mean that which seems complete. obversely, that the incomplete, or the mutilated, is the ugly. venus de milo. to a child she is ugly. when a mind adjusts to thinking of her as a completeness, even though, by physiologic standards, incomplete, she is beautiful. a hand thought of only as a hand, may seem beautiful. found on a battlefield--obviously a part--not beautiful. but everything in our experience is only a part of something else that in turn is only a part of still something else--or that there is nothing beautiful in our experience: only appearances that are intermediate to beauty and ugliness--that only universality is complete: that only the complete is the beautiful: that every attempt to achieve beauty is an attempt to give to the local the attribute of the universal. by stability, we mean the immovable and the unaffected. but all seeming things are only reactions to something else. stability, too, then, can be only the universal, or that besides which there is nothing else. though some things seem to have--or have--higher approximations to stability than have others, there are, in our experience, only various degrees of intermediateness to stability and instability. every man, then, who works for stability under its various names of "permanency," "survival," "duration," is striving to localize in something the state that is realizable only in the universal. by independence, entity, and individuality, i can mean only that besides which there is nothing else, if given only two things, they must be continuous and mutually affective, if everything is only a reaction to something else, and any two things would be destructive of each other's independence, entity, or individuality. all attempted organizations and systems and consistencies, some approximating far higher than others, but all only intermediate to order and disorder, fail eventually because of their relations with outside forces. all are attempted completenesses. if to all local phenomena there are always outside forces, these attempts, too, are realizable only in the state of completeness, or that to which there are no outside forces. or that all these words are synonyms, all meaning the state that we call the positive state-- that our whole "existence" is a striving for the positive state. the amazing paradox of it all: that all things are trying to become the universal by excluding other things. that there is only this one process, and that it does animate all expressions, in all fields of phenomena, of that which we think of as one inter-continuous nexus: the religious and their idea or ideal of the soul. they mean distinct, stable entity, or a state that is independent, and not a mere flux of vibrations or complex of reactions to environment, continuous with environment, merging away with an infinitude of other interdependent complexes. but the only thing that would not merge away into something else would be that besides which there is nothing else. that truth is only another name for the positive state, or that the quest for truth is the attempt to achieve positiveness: scientists who have thought that they were seeking truth, but who were trying to find out astronomic, or chemic, or biologic truths. but truth is that besides which there is nothing: nothing to modify it, nothing to question it, nothing to form an exception: the all-inclusive, the complete-- by truth i mean the universal. so chemists have sought the true, or the real, and have always failed in their endeavors, because of the outside relations of chemical phenomena: have failed in the sense that never has a chemical law, without exceptions, been discovered: because chemistry is continuous with astronomy, physics, biology--for instance, if the sun should greatly change its distance from this earth, and if human life could survive, the familiar chemic formulas would no longer work out: a new science of chemistry would have to be learned-- or that all attempts to find truth in the special are attempts to find the universal in the local. and artists and their striving for positiveness, under the name of "harmony"--but their pigments that are oxydizing, or are responding to a deranging environment--or the strings of musical instruments that are differently and disturbingly adjusting to outside chemic and thermal and gravitational forces--again and again this oneness of all ideals, and that it is the attempt to be, or to achieve, locally, that which is realizable only universally. in our experience there is only intermediateness to harmony and discord. harmony is that besides which there are no outside forces. and nations that have fought with only one motive: for individuality, or entity, or to be real, final nations, not subordinate to, or parts of, other nations. and that nothing but intermediateness has ever been attained, and that history is record of failures of this one attempt, because there always have been outside forces, or other nations contending for the same goal. as to physical things, chemic, mineralogic, astronomic, it is not customary to say that they act to achieve truth or entity, but it is understood that all motions are toward equilibrium: that there is no motion except toward equilibrium, of course always away from some other approximation to equilibrium. all biologic phenomena act to adjust: there are no biologic actions other than adjustments. adjustment is another name for equilibrium. equilibrium is the universal, or that which has nothing external to derange it. but that all that we call "being" is motion: and that all motion is the expression, not of equilibrium, but of equilibrating, or of equilibrium unattained: that life-motions are expressions of equilibrium unattained: that all thought relates to the unattained: that to have what is called being in our quasi-state, is not to be in the positive sense, or is to be intermediate to equilibrium and inequilibrium. so then: that all phenomena in our intermediate state, or quasi-state, represent this one attempt to organize, stabilize, harmonize, individualize--or to positivize, or to become real: that only to have seeming is to express failure or intermediateness to final failure and final success: that every attempt--that is observable--is defeated by continuity, or by outside forces--or by the excluded that are continuous with the included: that our whole "existence" is an attempt by the relative to be the absolute, or by the local to be the universal. in this book, my interest is in this attempt as manifested in modern science: that it has attempted to be real, true, final, complete, absolute: that, if the seeming of being, here, in our quasi-state, is the product of exclusion that is always false and arbitrary, if always are included and excluded continuous, the whole seeming system, or entity, of modern science is only quasi-system, or quasi-entity, wrought by the same false and arbitrary process as that by which the still less positive system that preceded it, or the theological system, wrought the illusion of its being. in this book, i assemble some of the data that i think are of the falsely and arbitrarily excluded. the data of the damned. i have gone into the outer darkness of scientific and philosophical transactions and proceedings, ultra-respectable, but covered with the dust of disregard. i have descended into journalism. i have come back with the quasi-souls of lost data. they will march. * * * * * as to the logic of our expressions to come-- that there is only quasi-logic in our mode of seeming: that nothing ever has been proved-- because there is nothing to prove. when i say that there is nothing to prove, i mean that to those who accept continuity, or the merging away of all phenomena into other phenomena, without positive demarcations one from another, there is, in a positive sense, no one thing. there is nothing to prove. for instance nothing can be proved to be an animal--because animalness and vegetableness are not positively different. there are some expressions of life that are as much vegetable as animal, or that represent the merging of animalness and vegetableness. there is then no positive test, standard, criterion, means of forming an opinion. as distinct from vegetables, animals do not exist. there is nothing to prove. nothing could be proved to be good, for instance. there is nothing in our "existence" that is good, in a positive sense, or as really outlined from evil. if to forgive be good in times of peace, it is evil in wartime. there is nothing to prove: good in our experience is continuous with, or is only another aspect of evil. as to what i'm trying to do now--i accept only. if i can't see universally, i only localize. so, of course then, that nothing ever has been proved: that theological pronouncements are as much open to doubt as ever they were, but that, by a hypnotizing process, they became dominant over the majority of minds in their era: that, in a succeeding era, the laws, dogmas, formulas, principles, of materialistic science never were proved, because they are only localizations simulating the universal; but that the leading minds of their era of dominance were hypnotized into more or less firmly believing them. newton's three laws, and that they are attempts to achieve positiveness, or to defy and break continuity, and are as unreal as are all other attempts to localize the universal: that, if every observable body is continuous, mediately or immediately, with all other bodies, it cannot be influenced only by its own inertia, so that there is no way of knowing what the phenomena of inertia may be; that, if all things are reacting to an infinitude of forces, there is no way of knowing what the effects of only one impressed force would be; that if every reaction is continuous with its action, it cannot be conceived of as a whole, and that there is no way of conceiving what it might be equal and opposite to-- or that newton's three laws are three articles of faith: or that demons and angels and inertias and reactions are all mythological characters: but that, in their eras of dominance, they were almost as firmly believed in as if they had been proved. * * * * * enormities and preposterousnesses will march. they will be "proved" as well as moses or darwin or lyell ever "proved" anything. * * * * * we substitute acceptance for belief. cells of an embryo take on different appearances in different eras. the more firmly established, the more difficult to change. that social organism is embryonic. that firmly to believe is to impede development. that only temporarily to accept is to facilitate. * * * * * but: except that we substitute acceptance for belief, our methods will be the conventional methods; the means by which every belief has been formulated and supported: or our methods will be the methods of theologians and savages and scientists and children. because, if all phenomena are continuous, there can be no positively different methods. by the inconclusive means and methods of cardinals and fortune tellers and evolutionists and peasants, methods which must be inconclusive, if they relate always to the local, and if there is nothing local to conclude, we shall write this book. if it function as an expression of its era, it will prevail. * * * * * all sciences begin with attempts to define. nothing ever has been defined. because there is nothing to define. darwin wrote _the origin of species_. he was never able to tell what he meant by a "species." it is not possible to define. nothing has ever been finally found out. because there is nothing final to find out. it's like looking for a needle that no one ever lost in a haystack that never was-- but that all scientific attempts really to find out something, whereas really there is nothing to find out, are attempts, themselves, really to be something. a seeker of truth. he will never find it. but the dimmest of possibilities--he may himself become truth. or that science is more than an inquiry: that it is a pseudo-construction, or a quasi-organization: that it is an attempt to break away and locally establish harmony, stability, equilibrium, consistency, entity-- dimmest of possibilities--that it may succeed. * * * * * that ours is a pseudo-existence, and that all appearances in it partake of its essential fictitiousness-- but that some appearances approximate far more highly to the positive state than do others. we conceive of all "things" as occupying gradations, or steps in series between positiveness and negativeness, or realness and unrealness: that some seeming things are more nearly consistent, just, beautiful, unified, individual, harmonious, stable--than others. we are not realists. we are not idealists. we are intermediatists--that nothing is real, but that nothing is unreal: that all phenomena are approximations one way or the other between realness and unrealness. so then: that our whole quasi-existence is an intermediate stage between positiveness and negativeness or realness and unrealness. like purgatory, i think. but in our summing up, which was very sketchily done, we omitted to make clear that realness is an aspect of the positive state. by realness, i mean that which does not merge away into something else, and that which is not partly something else: that which is not a reaction to, or an imitation of, something else. by a real hero, we mean one who is not partly a coward, or whose actions and motives do not merge away into cowardice. but, if in continuity, all things do merge, by realness, i mean the universal, besides which there is nothing with which to merge. that, though the local might be universalized, it is not conceivable that the universal can be localized: but that high approximations there may be, and that these approximate successes may be translated out of intermediateness into realness--quite as, in a relative sense, the industrial world recruits itself by translating out of unrealness, or out of the seemingly less real imaginings of inventors, machines which seem, when set up in factories, to have more of realness than they had when only imagined. that all progress, if all progress is toward stability, organization, harmony, consistency, or positiveness, is the attempt to become real. so, then, in general metaphysical terms, our expression is that, like a purgatory, all that is commonly called "existence," which we call intermediateness, is quasi-existence, neither real nor unreal, but expression of attempt to become real, or to generate for or recruit a real existence. our acceptance is that science, though usually thought of so specifically, or in its own local terms, usually supposed to be a prying into old bones, bugs, unsavory messes, is an expression of this one spirit animating all intermediateness: that, if science could absolutely exclude all data but its own present data, or that which is assimilable with the present quasi-organization, it would be a real system, with positively definite outlines--it would be real. its seeming approximation to consistency, stability, system--positiveness or realness--is sustained by damning the irreconcilable or the unassimilable-- all would be well. all would be heavenly-- if the damned would only stay damned. in the autumn of , and for years afterward, occurred brilliant-colored sunsets, such as had never been seen before within the memory of all observers. also there were blue moons. i think that one is likely to smile incredulously at the notion of blue moons. nevertheless they were as common as were green suns in . science had to account for these unconventionalities. such publications as _nature_ and _knowledge_ were besieged with inquiries. i suppose, in alaska and in the south sea islands, all the medicine men were similarly upon trial. something had to be thought of. upon the th of august, , the volcano of krakatoa, of the straits of sunda, had blown up. terrific. we're told that the sound was heard , miles, and that , persons were killed. seems just a little unscientific, or impositive, to me: marvel to me we're not told , miles and , persons. the volume of smoke that went up must have been visible to other planets--or, tormented with our crawlings and scurryings, the earth complained to mars; swore a vast black oath at us. in all text-books that mention this occurrence--no exception so far so i have read--it is said that the extraordinary atmospheric effects of were first noticed in the last of august or the first of september. that makes a difficulty for us. it is said that these phenomena were caused by particles of volcanic dust that were cast high in the air by krakatoa. this is the explanation that was agreed upon in -- but for seven years the atmospheric phenomena continued-- except that, in the seven, there was a lapse of several years--and where was the volcanic dust all that time? you'd think that such a question as that would make trouble? then you haven't studied hypnosis. you have never tried to demonstrate to a hypnotic that a table is not a hippopotamus. according to our general acceptance, it would be impossible to demonstrate such a thing. point out a hundred reasons for saying that a hippopotamus is not a table: you'll have to end up agreeing that neither is a table a table--it only seems to be a table. well, that's what the hippopotamus seems to be. so how can you prove that something is not something else, when neither is something else some other thing? there's nothing to prove. this is one of the profundities that we advertised in advance. you can oppose an absurdity only with some other absurdity. but science is established preposterousness. we divide all intellection: the obviously preposterousness and the established. but krakatoa: that's the explanation that the scientists gave. i don't know what whopper the medicine men told. we see, from the start, the very strong inclination of science to deny, as much as it can, external relations of this earth. this book is an assemblage of data of external relations of this earth. we take the position that our data have been damned, upon no consideration for individual merits or demerits, but in conformity with a general attempt to hold out for isolation of this earth. this is attempted positiveness. we take the position that science can no more succeed than, in a similar endeavor, could the chinese, or than could the united states. so then, with only pseudo-consideration of the phenomena of , or as an expression of positivism in its aspect of isolation, or unrelatedness, scientists have perpetrated such an enormity as suspension of volcanic dust seven years in the air--disregarding the lapse of several years--rather than to admit the arrival of dust from somewhere beyond this earth. not that scientists themselves have ever achieved positiveness, in its aspect of unitedness, among themselves--because nordenskiold, before , wrote a great deal upon his theory of cosmic dust, and prof. cleveland abbe contended against the krakatoan explanation--but that this is the orthodoxy of the main body of scientists. my own chief reason for indignation here: that this preposterous explanation interferes with some of my own enormities. it would cost me too much explaining, if i should have to admit that this earth's atmosphere has such sustaining power. later, we shall have data of things that have gone up in the air and that have stayed up--somewhere--weeks--months--but not by the sustaining power of this earth's atmosphere. for instance, the turtle of vicksburg. it seems to me that it would be ridiculous to think of a good-sized turtle hanging, for three or four months, upheld only by the air, over the town of vicksburg. when it comes to the horse and the barn--i think that they'll be classics some day, but i can never accept that a horse and a barn could float several months in this earth's atmosphere. the orthodox explanation: see the _report of the krakatoa committee of the royal society_. it comes out absolutely for the orthodox explanation--absolutely and beautifully, also expensively. there are pages in the "report," and plates, some of them marvelously colored. it was issued after an investigation that took five years. you couldn't think of anything done more efficiently, artistically, authoritatively. the mathematical parts are especially impressive: distribution of the dust of krakatoa; velocity of translation and rates of subsidence; altitudes and persistences-- _annual register_, - : that the atmospheric effects that have been attributed to krakatoa were seen in trinidad before the eruption occurred: _knowledge_, - : that they were seen in natal, south africa, six months before the eruption. * * * * * inertia and its inhospitality. or raw meat should not be fed to babies. we shall have a few data initiatorily. i fear me that the horse and the barn were a little extreme for our budding liberalities. the outrageous is the reasonable, if introduced politely. hailstones, for instance. one reads in the newspapers of hailstones the size of hens' eggs. one smiles. nevertheless i will engage to list one hundred instances, from the _monthly weather review_, of hailstones the size of hens' eggs. there is an account in _nature_, nov. , , of hailstones that weighed almost two pounds each. see chambers' encyclopedia for three-pounders. _report of the smithsonian institution_, - --two-pounders authenticated, and six-pounders reported. at seringapatam, india, about the year , fell a hailstone-- i fear me, i fear me: this is one of the profoundly damned. i blurt out something that should, perhaps, be withheld for several hundred pages--but that damned thing was the size of an elephant. we laugh. or snowflakes. size of saucers. said to have fallen at nashville, tenn., jan. , . one smiles. "in montana, in the winter of , fell snowflakes inches across, and inches thick." (_monthly weather review_, - .) in the topography of intellection, i should say that what we call knowledge is ignorance surrounded by laughter. * * * * * black rains--red rains--the fall of a thousand tons of butter. jet-black snow--pink snow--blue hailstones--hailstones flavored like oranges. punk and silk and charcoal. * * * * * about one hundred years ago, if anyone was so credulous as to think that stones had ever fallen from the sky, he was reasoned with: in the first place there are no stones in the sky: therefore no stones can fall from the sky. or nothing more reasonable or scientific or logical than that could be said upon any subject. the only trouble is the universal trouble: that the major premise is not real, or is intermediate somewhere between realness and unrealness. in , a committee, of whom lavoisier was a member, was appointed by the french academy, to investigate a report that a stone had fallen from the sky at luce, france. of all attempts at positiveness, in its aspect of isolation, i don't know of anything that has been fought harder for than the notion of this earth's unrelatedness. lavoisier analyzed the stone of luce. the exclusionists' explanation at that time was that stones do not fall from the sky: that luminous objects may seem to fall, and that hot stones may be picked up where a luminous object seemingly had landed--only lightning striking a stone, heating, even melting it. the stone of luce showed signs of fusion. lavoisier's analysis "absolutely proved" that this stone had not fallen: that it had been struck by lightning. so, authoritatively, falling stones were damned. the stock means of exclusion remained the explanation of lightning that was seen to strike something--that had been upon the ground in the first place. but positiveness and the fate of every positive statement. it is not customary to think of damned stones raising an outcry against a sentence of exclusion, but, subjectively, aerolites did--or data of them bombarded the walls raised against them-- _monthly review_, - "the phenomenon which is the subject of the remarks before us will seem to most persons as little worthy of credit as any that could be offered. the falling of large stones from the sky, without any assignable cause of their previous ascent, seems to partake so much of the marvelous as almost entirely to exclude the operation of known and natural agents. yet a body of evidence is here brought to prove that such events have actually taken place, and we ought not to withhold from it a proper degree of attention." the writer abandons the first, or absolute, exclusion, and modifies it with the explanation that the day before a reported fall of stones in tuscany, june , , there had been an eruption of vesuvius-- or that stones do fall from the sky, but that they are stones that have been raised to the sky from some other part of the earth's surface by whirlwinds or by volcanic action. it's more than one hundred and twenty years later. i know of no aerolite that has ever been acceptably traced to terrestrial origin. falling stones had to be undamned--though still with a reservation that held out for exclusion of outside forces. one may have the knowledge of a lavoisier, and still not be able to analyze, not be able even to see, except conformably with the hypnoses, or the conventional reactions against hypnoses, of one's era. we believe no more. we accept. little by little the whirlwind and volcano explanations had to be abandoned, but so powerful was this exclusion-hypnosis, sentence of damnation, or this attempt at positiveness, that far into our own times some scientists, notably prof. lawrence smith and sir robert ball, continued to hold out against all external origins, asserting that nothing could fall to this earth, unless it had been cast up or whirled up from some other part of this earth's surface. it's as commendable as anything ever has been--by which i mean it's intermediate to the commendable and the censurable. it's virginal. meteorites, data of which were once of the damned, have been admitted, but the common impression of them is only a retreat of attempted exclusion: that only two kinds of substance fall from the sky: metallic and stony: that the metallic objects are of iron and nickel-- butter and paper and wool and silk and resin. we see, to start with, that the virgins of science have fought and wept and screamed against external relations--upon two grounds: there in the first place; or up from one part of this earth's surface and down to another. as late as november, , in _nature notes_, - , a member of the selborne society still argued that meteorites do not fall from the sky; that they are masses of iron upon the ground "in the first place," that attract lightning; that the lightning is seen, and is mistaken for a falling, luminous object-- by progress we mean rape. butter and beef and blood and a stone with strange inscriptions upon it. so then, it is our expression that science relates to real knowledge no more than does the growth of a plant, or the organization of a department store, or the development of a nation: that all are assimilative, or organizing, or systematizing processes that represent different attempts to attain the positive state--the state commonly called heaven, i suppose i mean. there can be no real science where there are indeterminate variables, but every variable is, in finer terms, indeterminate, or irregular, if only to have the appearance of being in intermediateness is to express regularity unattained. the invariable, or the real and stable, would be nothing at all in intermediateness--rather as, but in relative terms, an undistorted interpretation of external sounds in the mind of a dreamer could not continue to exist in a dreaming mind, because that touch of relative realness would be of awakening and not of dreaming. science is the attempt to awaken to realness, wherein it is attempt to find regularity and uniformity. or the regular and uniform would be that which has nothing external to disturb it. by the universal we mean the real. or the notion is that the underlying super-attempt, as expressed in science, is indifferent to the subject-matter of science: that the attempt to regularize is the vital spirit. bugs and stars and chemical messes: that they are only quasi-real, and that of them there is nothing real to know; but that systematization of pseudo-data is approximation to realness or final awakening-- or a dreaming mind--and its centaurs and canary birds that turn into giraffes--there could be no real biology upon such subjects, but attempt, in a dreaming mind, to systematize such appearances would be movement toward awakening--if better mental co-ordination is all that we mean by the state of being awake--relatively awake. so it is, that having attempted to systematize, by ignoring externality to the greatest possible degree, the notion of things dropping in upon this earth, from externality, is as unsettling and as unwelcome to science as--tin horns blowing in upon a musician's relatively symmetric composition--flies alighting upon a painter's attempted harmony, and tracking colors one into another--suffragist getting up and making a political speech at a prayer meeting. if all things are of a oneness, which is a state intermediate to unrealness and realness, and if nothing has succeeded in breaking away and establishing entity for itself, and could not continue to "exist" in intermediateness, if it should succeed, any more than could the born still at the same time be the uterine, i of course know of no positive difference between science and christian science--and the attitude of both toward the unwelcome is the same--"it does not exist." a lord kelvin and a mrs. eddy, and something not to their liking--it does not exist. of course not, we intermediates say: but, also, that, in intermediateness, neither is there absolute non-existence. or a christian scientist and a toothache--neither exists in the final sense: also neither is absolutely non-existent, and, according to our therapeutics, the one that more highly approximates to realness will win. a secret of power-- i think it's another profundity. do you want power over something? be more nearly real than it. we'll begin with yellow substances that have fallen upon this earth: we'll see whether our data of them have a higher approximation to realness than have the dogmas of those who deny their existence--that is, as products from somewhere external to this earth. in mere impressionism we take our stand. we have no positive tests nor standards. realism in art: realism in science--they pass away. in , the thing to do was to accept darwinism; now many biologists are revolting and trying to conceive of something else. the thing to do was to accept it in its day, but darwinism of course was never proved: the fittest survive. what is meant by the fittest? not the strongest; not the cleverest-- weakness and stupidity everywhere survive. there is no way of determining fitness except in that a thing does survive. "fitness," then, is only another name for "survival." darwinism: that survivors survive. although darwinism, then, seems positively baseless, or absolutely irrational, its massing of supposed data, and its attempted coherence approximate more highly to organization and consistency than did the inchoate speculations that preceded it. or that columbus never proved that the earth is round. shadow of the earth on the moon? no one has ever seen it in its entirety. the earth's shadow is much larger than the moon. if the periphery of the shadow is curved--but the convex moon--a straight-edged object will cast a curved shadow upon a surface that is convex. all the other so-called proofs may be taken up in the same way. it was impossible for columbus to prove that the earth is round. it was not required: only that with a higher seeming of positiveness than that of his opponents, he should attempt. the thing to do, in , was nevertheless to accept that beyond europe, to the west, were other lands. i offer for acceptance, as something concordant with the spirit of this first quarter of the th century, the expression that beyond this earth are--other lands--from which come things as, from america, float things to europe. as to yellow substances that have fallen upon this earth, the endeavor to exclude extra-mundane origins is the dogma that all yellow rains and yellow snows are colored with pollen from this earth's pine trees. _symons' meteorological magazine_ is especially prudish in this respect and regards as highly improper all advances made by other explainers. nevertheless, the _monthly weather review_, may, , reports a golden-yellow fall, of feb. , , at peckloh, germany, in which four kinds of organisms, not pollen, were the coloring matter. there were minute things shaped like arrows, coffee beans, horns, and disks. they may have been symbols. they may have been objective hieroglyphics-- mere passing fancy--let it go-- in the _annales de chimie_, - , there is a list of rains said to have contained sulphur. i have thirty or forty other notes. i'll not use one of them. i'll admit that every one of them is upon a fall of pollen. i said, to begin with, that our methods would be the methods of theologians and scientists, and they always begin with an appearance of liberality. i grant thirty or forty points to start with. i'm as liberal as any of them--or that my liberality won't cost me anything--the enormousness of the data that we shall have. or just to look over a typical instance of this dogma, and the way it works out: in the _american journal of science_, - - , we are told of a yellow substance that fell by the bucketful upon a vessel, one "windless" night in june, in pictou harbor, nova scotia. the writer analyzed the substance, and it was found to "give off nitrogen and ammonia and an animal odor." now, one of our intermediatist principles, to start with, is that so far from positive, in the aspect of homogeneousness, are all substances, that, at least in what is called an elementary sense, anything can be found anywhere. mahogany logs on the coast of greenland; bugs of a valley on the top of mt. blanc; atheists at a prayer meeting; ice in india. for instance, chemical analysis can reveal that almost any dead man was poisoned with arsenic, we'll say, because there is no stomach without some iron, lead, tin, gold, arsenic in it and of it--which, of course, in a broader sense, doesn't matter much, because a certain number of persons must, as a restraining influence, be executed for murder every year; and, if detectives aren't able really to detect anything, illusion of their success is all that is necessary, and it is very honorable to give up one's life for society as a whole. the chemist who analyzed the substance of pictou sent a sample to the editor of the _journal_. the editor of course found pollen in it. my own acceptance is that there'd have to be some pollen in it: that nothing could very well fall through the air, in june, near the pine forests of nova scotia, and escape all floating spores of pollen. but the editor does not say that this substance "contained" pollen. he disregards "nitrogen, ammonia, and an animal odor," and says that the substance was pollen. for the sake of our thirty or forty tokens of liberality, or pseudo-liberality, if we can't be really liberal, we grant that the chemist of the first examination probably wouldn't know an animal odor if he were janitor of a menagerie. as we go along, however, there can be no such sweeping ignoring of this phenomenon: the fall of animal-matter from the sky. i'd suggest, to start with, that we'd put ourselves in the place of deep-sea fishes: how would they account for the fall of animal-matter from above? they wouldn't try-- or it's easy enough to think of most of us as deep-sea fishes of a kind. _jour. franklin inst._, - : that, upon the th of february, , there fell, at genoa, italy, according to director boccardo, of the technical institute of genoa, and prof. castellani, a yellow substance. but the microscope revealed numerous globules of cobalt blue, also corpuscles of a pearly color that resembled starch. see _nature_, - . _comptes rendus_, - : m. bouis says of a substance, reddish varying to yellowish, that fell enormously and successively, or upon april , may and may , in france and spain, that it carbonized and spread the odor of charred animal matter--that it was not pollen--that in alcohol it left a residue of resinous matter. hundreds of thousands of tons of this matter must have fallen. "odor of charred animal matter." or an aerial battle that occurred in inter-planetary space several hundred years ago--effect of time in making diverse remains uniform in appearance-- it's all very absurd because, even though we are told of a prodigious quantity of animal matter that fell from the sky--three days--france and spain--we're not ready yet: that's all. m. bouis says that this substance was not pollen; the vastness of the fall makes acceptable that it was not pollen; still, the resinous residue does suggest pollen of pine trees. we shall hear a great deal of a substance with a resinous residue that has fallen from the sky: finally we shall divorce it from all suggestion of pollen. _blackwood's magazine_, - : a yellow powder that fell at gerace, calabria, march , . some of this substance was collected by sig. simenini, professor of chemistry, at naples. it had an earthy, insipid taste, and is described as "unctuous." when heated, this matter turned brown, then black, then red. according to the _annals of philosophy_, - , one of the components was a greenish-yellow substance, which, when dried, was found to be resinous. but concomitants of this fall: loud noises were heard in the sky. stones fell from the sky. according to chladni, these concomitants occurred, and to me they seem--rather brutal?--or not associable with something so soft and gentle as a fall of pollen? * * * * * black rains and black snows--rains as black as a deluge of ink--jet-black snowflakes. such a rain as that which fell in ireland, may , , described in the _annals of scientific discovery_, , and the _annual register_, . it fell upon a district of square miles, and was the color of ink, and of a fetid odor and very disagreeable taste. the rain at castlecommon, ireland, april , --"thick, black rain." (_amer. met. jour._, - .) a black rain fell in ireland, oct. and , . (_symons' met. mag._ - .) "it left a most peculiar and disagreeable smell in the air." the orthodox explanation of this rain occurs in _nature_, march , --cloud of soot that had come from south wales, crossing the irish channel and all of ireland. so the black rain of ireland, of march, : ascribed in _symons' met. mag._ - , to clouds of soot from the manufacturing towns of north england and south scotland. our intermediatist principle of pseudo-logic, or our principle of continuity is, of course, that nothing is unique, or individual: that all phenomena merge away into all other phenomena: that, for instance--suppose there should be vast celestial super-oceanic, or inter-planetary vessels that come near this earth and discharge volumes of smoke at times. we're only supposing such a thing as that now, because, conventionally, we are beginning modestly and tentatively. but if it were so, there would necessarily be some phenomenon upon this earth, with which that phenomenon would merge. extra-mundane smoke and smoke from cities merge, or both would manifest in black precipitations in rain. in continuity, it is impossible to distinguish phenomena at their merging-points, so we look for them at their extremes. impossible to distinguish between animal and vegetable in some infusoria--but hippopotamus and violet. for all practical purposes they're distinguishable enough. no one but a barnum or a bailey would send one a bunch of hippopotami as a token of regard. so away from the great manufacturing centers: black rain in switzerland, jan. , . switzerland is so remote, and so ill at ease is the conventional explanation here, that _nature_, - , says of this rain that in certain conditions of weather, snow may take on an appearance of blackness that is quite deceptive. may be so. or at night, if dark enough, snow may look black. this is simply denying that a black rain fell in switzerland, jan. , . extreme remoteness from great manufacturing centers: _la nature_, , - : that aug. , , there fell at the cape of good hope, a rain so black as to be described as a "shower of ink." continuity dogs us. continuity rules us and pulls us back. we seemed to have a little hope that by the method of extremes we could get away from things that merge indistinguishably into other things. we find that every departure from one merger is entrance upon another. at the cape of good hope, vast volumes of smoke from great manufacturing centers, as an explanation, cannot very acceptably merge with the explanation of extra-mundane origin--but smoke from a terrestrial volcano can, and that is the suggestion that is made in _la nature_. there is, in human intellection, no real standard to judge by, but our acceptance, for the present, is that the more nearly positive will prevail. by the more nearly positive we mean the more nearly organized. everything merges away into everything else, but proportionately to its complexity, if unified, a thing seems strong, real, and distinct: so, in aesthetics, it is recognized that diversity in unity is higher beauty, or approximation to beauty, than is simpler unity; so the logicians feel that agreement of diverse data constitute greater convincingness, or strength, than that of mere parallel instances: so to herbert spencer the more highly differentiated and integrated is the more fully evolved. our opponents hold out for mundane origin of all black rains. our method will be the presenting of diverse phenomena in agreement with the notion of some other origin. we take up not only black rains but black rains and their accompanying phenomena. a correspondent to _knowledge_, - , writes of a black rain that fell in the clyde valley, march , : of another black rain that fell two days later. according to the correspondent, a black rain had fallen in the clyde valley, march , : then again march , . according to _nature_, - , a black rain fell at marlsford, england, sept. , ; more than twenty-four hours later another black rain fell in the same small town. the black rains of slains: according to rev. james rust (_scottish showers_): a black rain at slains, jan. , --another at carluke, miles from slains, may , --at slains, may , --slains, oct. , . but after two of these showers, vast quantities of a substance described sometimes as "pumice stone," but sometimes as "slag," were washed upon the sea coast near slains. a chemist's opinion is given that this substance was slag: that it was not a volcanic product: slag from smelting works. we now have, for black rains, a concomitant that is irreconcilable with origin from factory chimneys. whatever it may have been the quantity of this substance was so enormous that, in mr. rust's opinion, to have produced so much of it would have required the united output of all the smelting works in the world. if slag it were, we accept that an artificial product has, in enormous quantities, fallen from the sky. if you don't think that such occurrences are damned by science, read _scottish showers_ and see how impossible it was for the author to have this matter taken up by the scientific world. the first and second rains corresponded, in time, with ordinary ebullitions of vesuvius. the third and fourth, according to mr. rust, corresponded with no known volcanic activities upon this earth. _la science pour tous_, - : that, between october, , and january, , four more black rains fell at slains, scotland. the writer of this supplementary account tells us, with a better, or more unscrupulous, orthodoxy than mr. rust's, that of the eight black rains, five coincided with eruptions of vesuvius and three with eruptions of etna. the fate of all explanation is to close one door only to have another fly wide open. i should say that my own notions upon this subject will be considered irrational, but at least my gregariousness is satisfied in associating here with the preposterous--or this writer, and those who think in his rut, have to say that they can think of four discharges from one far-distant volcano, passing over a great part of europe, precipitating nowhere else, discharging precisely over one small northern parish-- but also of three other discharges, from another far-distant volcano, showing the same precise preference, if not marksmanship, for one small parish in scotland. nor would orthodoxy be any better off in thinking of exploding meteorites and their débris: preciseness and recurrence would be just as difficult to explain. my own notion is of an island near an oceanic trade-route: it might receive débris from passing vessels seven times in four years. other concomitants of black rains: in timb's _year book_, - , there is an account of "a sort of rumbling, as of wagons, heard for upward of an hour without ceasing," july , , bulwick rectory, northampton, england. on the th, a black rain fell. in _nature_, - , a correspondent writes of an intense darkness at preston, england, april , : page , another correspondent writes of black rain at crowle, near worcester, april : that a week later, or may , it had fallen again: another account of black rain, upon the th of april, near church shetton, so intense that the following day brooks were still dyed with it. according to four accounts by correspondents to _nature_ there were earthquakes in england at this time. or the black rain of canada, nov. , . this time it is orthodoxy to attribute the black precipitate to smoke of forest fires south of the ohio river-- zurcher, _meteors_, p. : that this black rain was accompanied by "shocks like those of an earthquake." _edinburgh philosophical journal_, - : that the earthquake had occurred at the climax of intense darkness and the fall of black rain. * * * * * red rains. orthodoxy: sand blown by the sirocco, from the sahara to europe. especially in the earthquake regions of europe, there have been many falls of red substance, usually, but not always, precipitated in rain. upon many occasions, these substances have been "absolutely identified" as sand from the sahara. when i first took this matter up, i came across assurance after assurance, so positive to this effect, that, had i not been an intermediatist, i'd have looked no further. samples collected from a rain at genoa--samples of sand forwarded from the sahara--"absolute agreement" some writers said: same color, same particles of quartz, even the same shells of diatoms mixed in. then the chemical analyses: not a disagreement worth mentioning. our intermediatist means of expression will be that, with proper exclusions, after the scientific or theological method, anything can be identified with anything else, if all things are only different expressions of an underlying oneness. to many minds there's rest and there's satisfaction in that expression "absolutely identified." absoluteness, or the illusion of it--the universal quest. if chemists have identified substances that have fallen in europe as sand from african deserts, swept up in african whirlwinds, that's assuasive to all the irritations that occur to those cloistered minds that must repose in the concept of a snug, isolated, little world, free from contact with cosmic wickednesses, safe from stellar guile, undisturbed by inter-planetary prowlings and invasions. the only trouble is that a chemist's analysis, which seems so final and authoritative to some minds, is no more nearly absolute than is identification by a child or description by an imbecile-- i take some of that back: i accept that the approximation is higher-- but that it's based upon delusion, because there is no definiteness, no homogeneity, no stability, only different stages somewhere between them and indefiniteness, heterogeneity, and instability. there are no chemical elements. it seems acceptable that ramsay and others have settled that. the chemical elements are only another disappointment in the quest for the positive, as the definite, the homogeneous, and the stable. if there were real elements, there could be a real science of chemistry. upon nov. and , , occurred the greatest fall of matter in the history of australia. upon the th of november, it "rained mud," in tasmania. it was of course attributed to australian whirlwinds, but, according to the _monthly weather review_, - , there was a haze all the way to the philippines, also as far as hong kong. it may be that this phenomenon had no especial relation with the even more tremendous fall of matter that occurred in europe, february, . for several days, the south of england was a dumping ground--from somewhere. if you'd like to have a chemist's opinion, even though it's only a chemist's opinion, see the report of the meeting of the royal chemical society, april , . mr. e.g. clayton read a paper upon some of the substance that had fallen from the sky, collected by him. the sahara explanation applies mostly to falls that occur in southern europe. farther away, the conventionalists are a little uneasy: for instance, the editor of the _monthly weather review_, - , says of a red rain that fell near the coast of newfoundland, early in : "it would be very remarkable if this was sahara dust." mr. clayton said that the matter examined by him was "merely wind-borne dust from the roads and lanes of wessex." this opinion is typical of all scientific opinion--or theological opinion--or feminine opinion--all very well except for what it disregards. the most charitable thing i can think of--because i think it gives us a broader tone to relieve our malices with occasional charities--is that mr. clayton had not heard of the astonishing extent of this fall--had covered the canary islands, on the th, for instance. i think, myself, that in , we passed through the remains of a powdered world--left over from an ancient inter-planetary dispute, brooding in space like a red resentment ever since. or, like every other opinion, the notion of dust from wessex turns into a provincial thing when we look it over. to think is to conceive incompletely, because all thought relates only to the local. we metaphysicians, of course, like to have the notion that we think of the unthinkable. as to opinions, or pronouncements, i should say, because they always have such an authoritative air, of other chemists, there is an analysis in _nature_, - , giving water and organic matter at . per cent. it's that carrying out of fractions that's so convincing. the substance is identified as sand from the sahara. the vastness of this fall. in _nature_, - , we are told that it had occurred in ireland, too. the sahara, of course--because, prior to february , there had been dust storms in the sahara--disregarding that in that great region there's always, in some part of it, a dust storm. however, just at present, it does look reasonable that dust had come from africa, via the canaries. the great difficulty that authoritativeness has to contend with is some other authoritativeness. when an infallibility clashes with a pontification-- they explain. _nature_, march , : another analysis-- per cent organic matter. such disagreements don't look very well, so, in _nature_, - , one of the differing chemists explains. he says that his analysis was of muddy rain, and the other was of sediment of rain-- we're quite ready to accept excuses from the most high, though i do wonder whether we're quite so damned as we were, if we find ourselves in a gracious and tolerant mood toward the powers that condemn--but the tax that now comes upon our good manners and unwillingness to be too severe-- _nature_, - : another chemist. he says it was . per cent water and organic matter. he "identifies" this matter as sand from an african desert--but after deducting organic matter-- but you and i could be "identified" as sand from an african desert, after deducting all there is to us except sand-- why we cannot accept that this fall was of sand from the sahara, omitting the obvious objection that in most parts the sahara is not red at all, but is usually described as "dazzling white"-- the enormousness of it: that a whirlwind might have carried it, but that, in that case it would be no supposititious, or doubtfully identified whirlwind, but the greatest atmospheric cataclysm in the history of this earth: _jour. roy. met. soc._, - : that, up to the th of february, this fall had continued in belgium, holland, germany and austria; that in some instances it was not sand, or that almost all the matter was organic: that a vessel had reported the fall as occurring in the atlantic ocean, midway between southampton and the barbados. the calculation is given that, in england alone, , , tons of matter had fallen. it had fallen in switzerland (_symons' met. mag._, march, ). it had fallen in russia (_bull. com. geolog._, - ). not only had a vast quantity of matter fallen several months before, in australia, but it was at this time falling in australia (_victorian naturalist_, june, )--enormously--red mud--fifty tons per square mile. the wessex explanation-- or that every explanation is a wessex explanation: by that i mean an attempt to interpret the enormous in terms of the minute--but that nothing can be finally explained, because by truth we mean the universal; and that even if we could think as wide as universality, that would not be requital to the cosmic quest--which is not for truth, but for the local that is true--not to universalize the local, but to localize the universal--or to give to a cosmic cloud absolute interpretation in terms of the little dusty roads and lanes of wessex. i cannot conceive that this can be done: i think of high approximation. our intermediatist concept is that, because of the continuity of all "things," which are not separate, positive, or real things, all pseudo-things partake of the underlying, or are only different expressions, degrees, or aspects of the underlying: so then that a sample from somewhere in anything must correspond with a sample from somewhere in anything else. that, by due care in selection, and disregard for everything else, or the scientific and theological method, the substance that fell, february, , could be identified with anything, or with some part or aspect of anything that could be conceived of-- with sand from the sahara, sand from a barrel of sugar, or dust of your great-great-grandfather. different samples are described and listed in the _journal of the royal meteorological society_, - --or we'll see whether my notion that a chemist could have identified some one of these samples as from anywhere conceivable, is extreme or not: "similar to brick dust," in one place; "buff or light brown," in another place; "chocolate-colored and silky to the touch and slightly iridescent"; "gray"; "red-rust color"; "reddish raindrops and gray sand"; "dirty gray"; "quite red"; "yellow-brown, with a tinge of pink"; "deep yellow-clay color." in _nature_, it is described as of a peculiar yellowish cast in one place, reddish somewhere else, and salmon-colored in another place. or there could be real science if there were really anything to be scientific about. or the science of chemistry is like a science of sociology, prejudiced in advance, because only to see is to see with a prejudice, setting out to "prove" that all inhabitants of new york came from africa. very easy matter. samples from one part of town. disregard for all the rest. there is no science but wessex-science. according to our acceptance, there should be no other, but that approximation should be higher: that metaphysics is super-evil: that the scientific spirit is of the cosmic quest. our notion is that, in a real existence, such a quasi-system of fables as the science of chemistry could not deceive for a moment: but that in an "existence" endeavoring to become real, it represents that endeavor, and will continue to impose its pseudo-positiveness until it be driven out by a higher approximation to realness: that the science of chemistry is as impositive as fortune-telling-- or no-- that, though it represents a higher approximation to realness than does alchemy, for instance, and so drove out alchemy, it is still only somewhere between myth and positiveness. the attempt at realness, or to state a real and unmodified fact here, is the statement: all red rains are colored by sands from the sahara desert. my own impositivist acceptances are: that some red rains are colored by sands from the sahara desert; some by sands from other terrestrial sources; some by sands from other worlds, or from their deserts--also from aerial regions too indefinite or amorphous to be thought of as "worlds" or planets-- that no supposititious whirlwind can account for the hundreds of millions of tons of matter that fell upon australia, pacific ocean and atlantic ocean and europe in and --that a whirlwind that could do that would not be supposititious. but now we shall cast off some of our own wessicality by accepting that there have been falls of red substance other than sand. we regard every science as an expression of the attempt to be real. but to be real is to localize the universal--or to make some one thing as wide as all things--successful accomplishment of which i cannot conceive of. the prime resistance to this endeavor is the refusal of the rest of the universe to be damned, excluded, disregarded, to receive christian science treatment, by something else so attempting. although all phenomena are striving for the absolute--or have surrendered to and have incorporated themselves in higher attempts, simply to be phenomenal, or to have seeming in intermediateness is to express relations. a river. it is water expressing the gravitational relation of different levels. the water of the river. expression of chemic relations of hydrogen and oxygen--which are not final. a city. manifestation of commercial and social relations. how could a mountain be without base in a greater body? storekeeper live without customers? the prime resistance to the positivist attempt by science is its relations with other phenomena, or that it only expresses those relations in the first place. or that a science can have seeming, or survive in intermediateness, as something pure, isolated, positively different, no more than could a river or a city or a mountain or a store. this intermediateness-wide attempt by parts to be wholes--which cannot be realized in our quasi-state, if we accept that in it the co-existence of two or more wholes or universals is impossible--high approximation to which, however, may be thinkable-- scientists and their dream of "pure science." artists and their dream of "art for art's sake." it is our notion that if they could almost realize, that would be almost realness: that they would instantly be translated into real existence. such thinkers are good positivists, but they are evil in an economic and sociologic sense, if, in that sense, nothing has justification for being, unless it serve, or function for, or express the relations of, some higher aggregate. so science functions for and serves society at large, and would, from society at large, receive no support, unless it did so divert itself or dissipate and prostitute itself. it seems that by prostitution i mean usefulness. there have been red rains that, in the middle ages, were called "rains of blood." such rains terrified many persons, and were so unsettling to large populations, that science, in its sociologic relations, has sought, by mrs. eddy's method, to remove an evil-- that "rains of blood" do not exist; that rains so called are only of water colored by sand from the sahara desert. my own acceptance is that such assurances, whether fictitious or not, whether the sahara is a "dazzling white" desert or not, have wrought such good effects, in a sociologic sense, even though prostitutional in the positivist sense, that, in the sociologic sense, they were well justified: but that we've gone on: that this is the twentieth century; that most of us have grown up so that such soporifics of the past are no longer necessary: that if gushes of blood should fall from the sky upon new york city, business would go on as usual. we began with rains that we accepted ourselves were, most likely, only of sand. in my own still immature hereticalness--and by heresy, or progress, i mean, very largely, a return, though with many modifications, to the superstitions of the past, i think i feel considerable aloofness to the idea of rains of blood. just at present, it is my conservative, or timid purpose, to express only that there have been red rains that very strongly suggest blood or finely divided animal matter-- débris from inter-planetary disasters. aerial battles. food-supplies from cargoes of super-vessels, wrecked in inter-planetary traffic. there was a red rain in the mediterranean region, march , . twelve days later, it fell again. whatever this substance may have been, when burned, the odor of animal matter from it was strong and persistent. (_l'astronomie_, - .) but--infinite heterogeneity--or débris from many different kinds of aerial cargoes--there have been red rains that have been colored by neither sand nor animal matter. _annals of philosophy_, - : that, nov. , --week before the black rain and earthquake of canada--there fell, at blankenberge, holland, a red rain. as to sand, two chemists of bruges concentrated ounces of the rain to ounces--"no precipitate fell." but the color was so marked that had there been sand, it would have been deposited, if the substance had been diluted instead of concentrated. experiments were made, and various reagents did cast precipitates, but other than sand. the chemists concluded that the rain-water contained muriate of cobalt--which is not very enlightening: that could be said of many substances carried in vessels upon the atlantic ocean. whatever it may have been, in the _annales de chimie_, - - , its color is said to have been red-violet. for various chemic reactions, see _quar. jour. roy. inst._, - , and _edin. phil. jour._, - . something that fell with dust said to have been meteoric, march , , , : described in the _chemical news_, - , as a "peculiar substance," consisted of red iron ocher, carbonate of lime, and organic matter. orange-red hail, march , , in tuscany. (notes and queries - - .) rain of lavender-colored substance, at oudon, france, dec. , . (_bull. soc. met. de france_, - .) _la nature_, - - : that, according to prof. schwedoff, there fell, in russia, june , , red hailstones, also blue hailstones, also gray hailstones. _nature_, - : a correspondent writes that he had been told by a resident of a small town in venezuela, that there, april , , had fallen hailstones, some red, some blue, some whitish: informant said to have been one unlikely ever to have heard of the russian phenomenon; described as an "honest, plain countryman." _nature_, july , , quotes a roman correspondent to the london _times_ who sent a translation from an italian newspaper: that a red rain had fallen in italy, june , , containing "microscopically small particles of sand." or, according to our acceptance, any other story would have been an evil thing, in the sociologic sense, in italy, in . but the english correspondent, from a land where terrifying red rains are uncommon, does not feel this necessity. he writes: "i am by no means satisfied that the rain was of sand and water." his observations are that drops of this rain left stains "such as sandy water could not leave." he notes that when the water evaporated, no sand was left behind. _l'année scientifique_, - : that, dec. , , there fell, in cochin china, a substance like blood, somewhat coagulated. _annales de chimie_, - : that a thick, viscous, red matter fell at ulm, in . we now have a datum with a factor that has been foreshadowed; which will recur and recur and recur throughout this book. it is a factor that makes for speculation so revolutionary that it will have to be reinforced many times before we can take it into full acceptance. _year book of facts_, - : quotation from a letter from prof. campini to prof. matteucci: that, upon dec. , , at about a.m., in the northwestern part of siena, a reddish rain fell copiously for two hours. a second red shower fell at o'clock. three days later, the red rain fell again. the next day another red rain fell. still more extraordinarily: each fall occurred in "exactly the same quarter of town." it is in the records of the french academy that, upon march , , in the town of châtillon-sur-seine, fell a reddish substance that was "thick, viscous, and putrid." _american journal of science_, - - : story of a highly unpleasant substance that had fallen from the sky, in wilson county, tennessee. we read that dr. troost visited the place and investigated. later we're going to investigate some investigations--but never mind that now. dr. troost reported that the substance was clear blood and portions of flesh scattered upon tobacco fields. he argued that a whirlwind might have taken an animal up from one place, mauled it around, and have precipitated its remains somewhere else. but, in volume , page , of the _journal_, there is an apology. the whole matter is, upon newspaper authority, said to have been a hoax by negroes, who had pretended to have seen the shower, for the sake of practicing upon the credulity of their masters: that they had scattered the decaying flesh of a dead hog over the tobacco fields. if we don't accept this datum, at least we see the sociologically necessary determination to have all falls accredited to earthly origins--even when they're falls that don't fall. _annual register_, - : that, upon the th of august, , something had fallen from the sky at amherst, mass. it had been examined and described by prof. graves, formerly lecturer at dartmouth college. it was an object that had upon it a nap, similar to that of milled cloth. upon removing this nap, a buff-colored, pulpy substance was found. it had an offensive odor, and, upon exposure to the air, turned to a vivid red. this thing was said to have fallen with a brilliant light. also see the _edinburgh philosophical journal_, - . in the _annales de chimie_, - , m. arago accepts the datum, and gives four instances of similar objects or substances said to have fallen from the sky, two of which we shall have with our data of gelatinous, or viscous matter, and two of which i omit, because it seems to me that the dates given are too far back. in the _american journal of science_, - - , is professor graves' account, communicated by professor dewey: that, upon the evening of august , , a light was seen in amherst--a falling object--sound as if of an explosion. in the home of prof. dewey, this light was reflected upon a wall of a room in which were several members of prof. dewey's family. the next morning, in prof. dewey's front yard, in what is said to have been the only position from which the light that had been seen in the room, the night before, could have been reflected, was found a substance "unlike anything before observed by anyone who saw it." it was a bowl-shaped object, about inches in diameter, and one inch thick. bright buff-colored, and having upon it a "fine nap." upon removing this covering, a buff-colored, pulpy substance of the consistency of soft-soap, was found--"of an offensive, suffocating smell." a few minutes of exposure to the air changed the buff color to "a livid color resembling venous blood." it absorbed moisture quickly from the air and liquefied. for some of the chemic reactions, see the _journal_. there's another lost quasi-soul of a datum that seems to me to belong here: london _times_, april , : fall of fish that had occurred in the neighborhood of allahabad, india. it is said that the fish were of the chalwa species, about a span in length and a seer in weight--you know. they were dead and dry. or they had been such a long time out of water that we can't accept that they had been scooped out of a pond, by a whirlwind--even though they were so definitely identified as of a known local species-- or they were not fish at all. i incline, myself, to the acceptance that they were not fish, but slender, fish-shaped objects of the same substance as that which fell at amherst--it is said that, whatever they were, they could not be eaten: that "in the pan, they turned to blood." for details of this story see the _journal of the asiatic society of bengal_, - . may or , , is the date given in the _journal_. in the _american journal of science_, - - , occurs the inevitable damnation of the amherst object: prof. edward hitchcock went to live in amherst. he says that years later, another object, like the one said to have fallen in , had been found at "nearly the same place." prof. hitchcock was invited by prof. graves to examine it. exactly like the first one. corresponded in size and color and consistency. the chemic reactions were the same. prof. hitchcock recognized it in a moment. it was a gelatinous fungus. he did not satisfy himself as to just the exact species it belonged to, but he predicted that similar fungi might spring up within twenty-four hours-- but, before evening, two others sprang up. or we've arrived at one of the oldest of the exclusionists' conventions--or nostoc. we shall have many data of gelatinous substance said to have fallen from the sky: almost always the exclusionists argue that it was only nostoc, an alga, or, in some respects, a fungous growth. the rival convention is "spawn of frogs or of fishes." these two conventions have made a strong combination. in instances where testimony was not convincing that gelatinous matter had been seen to fall, it was said that the gelatinous substance was nostoc, and had been upon the ground in the first place: when the testimony was too good that it had fallen, it was said to be spawn that had been carried from one place to another in a whirlwind. now, i can't say that nostoc is always greenish, any more than i can say that blackbirds are always black, having seen a white one: we shall quote a scientist who knew of flesh-colored nostoc, when so to know was convenient. when we come to reported falls of gelatinous substances, i'd like it to be noticed how often they are described as whitish or grayish. in looking up the subject, myself, i have read only of greenish nostoc. said to be greenish, in webster's dictionary--said to be "blue-green" in the new international encyclopedia--"from bright green to olive-green" (_science gossip_, - ); "green" (_science gossip_, - ); "greenish" (_notes and queries_, - - ). it would seem acceptable that, if many reports of white birds should occur, the birds are not blackbirds, even though there have been white blackbirds. or that, if often reported, grayish or whitish gelatinous substance is not nostoc, and is not spawn if occurring in times unseasonable for spawn. "the kentucky phenomenon." so it was called, in its day, and now we have an occurrence that attracted a great deal of attention in its own time. usually these things of the accursed have been hushed up or disregarded--suppressed like the seven black rains of slains--but, upon march , , something occurred, in bath county, kentucky, that brought many newspaper correspondents to the scene. the substance that looked like beef that fell from the sky. upon march , , at olympian springs, bath county, kentucky, flakes of a substance that looked like beef fell from the sky--"from a clear sky." we'd like to emphasize that it was said that nothing but this falling substance was visible in the sky. it fell in flakes of various sizes; some two inches square, one, three or four inches square. the flake-formation is interesting: later we shall think of it as signifying pressure--somewhere. it was a thick shower, on the ground, on trees, on fences, but it was narrowly localized: or upon a strip of land about yards long and about yards wide. for the first account, see the _scientific american_, - , and the _new york times_, march , . then the exclusionists. something that looked like beef: one flake of it the size of a square envelope. if we think of how hard the exclusionists have fought to reject the coming of ordinary-looking dust from this earth's externality, we can sympathize with them in this sensational instance, perhaps. newspaper correspondents wrote broadcast and witnesses were quoted, and this time there is no mention of a hoax, and, except by one scientist, there is no denial that the fall did take place. it seems to me that the exclusionists are still more emphatically conservators. it is not so much that they are inimical to all data of externally derived substances that fall upon this earth, as that they are inimical to all data discordant with a system that does not include such phenomena-- or the spirit or hope or ambition of the cosmos, which we call attempted positivism: not to find out the new; not to add to what is called knowledge, but to systematize. _scientific american supplement_, - : that the substance reported from kentucky had been examined by leopold brandeis. "at last we have a proper explanation of this much talked of phenomenon." "it has been comparatively easy to identify the substance and to fix its status. the kentucky 'wonder' is no more or less than nostoc." or that it had not fallen; that it had been upon the ground in the first place, and had swollen in rain, and, attracting attention by greatly increased volume, had been supposed by unscientific observers to have fallen in rain-- what rain, i don't know. also it is spoken of as "dried" several times. that's one of the most important of the details. but the relief of outraged propriety, expressed in the _supplement_, is amusing to some of us, who, i fear, may be a little improper at times. very spirit of the salvation army, when some third-rate scientist comes out with an explanation of the vermiform appendix or the os coccygis that would have been acceptable to moses. to give completeness to "the proper explanation," it is said that mr. brandeis had identified the substance as "flesh-colored" nostoc. prof. lawrence smith, of kentucky, one of the most resolute of the exclusionists: _new york times_, march , : that the substance had been examined and analyzed by prof. smith, according to whom it gave every indication of being the "dried" spawn of some reptile, "doubtless of the frog"--or up from one place and down in another. as to "dried," that may refer to condition when prof. smith received it. in the _scientific american supplement_, - , dr. a. mead edwards, president of the newark scientific association, writes that, when he saw mr. brandeis' communication, his feeling was of conviction that propriety had been re-established, or that the problem had been solved, as he expresses it: knowing mr. brandeis well, he had called upon that upholder of respectability, to see the substance that had been identified as nostoc. but he had also called upon dr. hamilton, who had a specimen, and dr. hamilton had declared it to be lung-tissue. dr. edwards writes of the substance that had so completely, or beautifully--if beauty is completeness--been identified as nostoc--"it turned out to be lung-tissue also." he wrote to other persons who had specimens, and identified other specimens as masses of cartilage or muscular fibers. "as to whence it came, i have no theory." nevertheless he endorses the local explanation--and a bizarre thing it is: a flock of gorged, heavy-weighted buzzards, but far up and invisible in the clear sky-- they had disgorged. prof. fassig lists the substance, in his "bibliography," as fish spawn. mcatee (_monthly weather review_, may, ) lists it as a jelly-like material, supposed to have been the "dried" spawn either of fishes or of some batrachian. or this is why, against the seemingly insuperable odds against all things new, there can be what is called progress-- that nothing is positive, in the aspects of homogeneity and unity: if the whole world should seem to combine against you, it is only unreal combination, or intermediateness to unity and disunity. every resistance is itself divided into parts resisting one another. the simplest strategy seems to be--never bother to fight a thing: set its own parts fighting one another. we are merging away from carnal to gelatinous substance, and here there is an abundance of instances or reports of instances. these data are so improper they're obscene to the science of today, but we shall see that science, before it became so rigorous, was not so prudish. chladni was not, and greg was not. i shall have to accept, myself, that gelatinous substance has often fallen from the sky-- or that, far up, or far away, the whole sky is gelatinous? that meteors tear through and detach fragments? that fragments are brought down by storms? that the twinkling of stars is penetration of light through something that quivers? i think, myself, that it would be absurd to say that the whole sky is gelatinous: it seems more acceptable that only certain areas are. humboldt (_cosmos_, - ) says that all our data in this respect must be "classed amongst the mythical fables of mythology." he is very sure, but just a little redundant. we shall be opposed by the standard resistances: there in the first place; up from one place, in a whirlwind, and down in another. we shall not bother to be very convincing one way or another, because of the over-shadowing of the datum with which we shall end up. it will mean that something had been in a stationary position for several days over a small part of a small town in england: this is the revolutionary thing that we have alluded to before; whether the substance were nostoc, or spawn, or some kind of a larval nexus, doesn't matter so much. if it stood in the sky for several days, we rank with moses as a chronicler of improprieties--or was that story, or datum, we mean, told by moses? then we shall have so many records of gelatinous substance said to have fallen with meteorites, that, between the two phenomena, some of us will have to accept connection--or that there are at least vast gelatinous areas aloft, and that meteorites tear through, carrying down some of the substance. _comptes rendus_, - : that, in , m. vallot, member of the french academy, placed before the academy some fragments of a gelatinous substance, said to have fallen from the sky, and asked that they be analyzed. there is no further allusion to this subject. _comptes rendus_, - : that, in wilna, lithuania, april , , in a rainstorm, fell nut-sized masses of a substance that is described as both resinous and gelatinous. it was odorless until burned: then it spread a very pronounced sweetish odor. it is described as like gelatine, but much firmer: but, having been in water hours, it swelled out, and looked altogether gelatinous-- it was grayish. we are told that, in and , a similar substance had fallen in asia minor. in _notes and queries_, - - , it is said that, early in august, , thousands of jellyfish, about the size of a shilling, had fallen at bath, england. i think it is not acceptable that they were jellyfish: but it does look as if this time frog spawn did fall from the sky, and may have been translated by a whirlwind--because, at the same time, small frogs fell at wigan, england. _nature_, - : that, june , , at eton, bucks, england, the ground was found covered with masses of jelly, the size of peas, after a heavy rainfall. we are not told of nostoc, this time: it is said that the object contained numerous eggs of "some species of chironomus, from which larvae soon emerged." i incline, then, to think that the objects that fell at bath were neither jellyfish nor masses of frog spawn, but something of a larval kind-- this is what had occurred at bath, england, years before. london _times_, april , : that, upon the nd of april, , a storm of glutinous drops neither jellyfish nor masses of frog spawn, but something of a [line missing here in original text. ed.] railroad station, at bath. "many soon developed into a worm-like chrysalis, about an inch in length." the account of this occurrence in the _zoologist_, - - , is more like the eton-datum: of minute forms, said to have been infusoria; not forms about an inch in length. _trans. ent. soc. of london_, -proc. xxii: that the phenomenon has been investigated by the rev. l. jenyns, of bath. his description is of minute worms in filmy envelopes. he tries to account for their segregation. the mystery of it is: what could have brought so many of them together? many other falls we shall have record of, and in most of them segregation is the great mystery. a whirlwind seems anything but a segregative force. segregation of things that have fallen from the sky has been avoided as most deep-dyed of the damned. mr. jenyns conceives of a large pool, in which were many of these spherical masses: of the pool drying up and concentrating all in a small area; of a whirlwind then scooping all up together-- but several days later, more of these objects fell in the same place. that such marksmanship is not attributable to whirlwinds seems to me to be what we think we mean by common sense: it may not look like common sense to say that these things had been stationary over the town of bath, several days-- the seven black rains of slains; the four red rains of siena. an interesting sidelight on the mechanics of orthodoxy is that mr. jenyns dutifully records the second fall, but ignores it in his explanation. r.p. greg, one of the most notable of cataloguers of meteoritic phenomena, records (_phil. mag._: - - ) falls of viscid substance in the years , , , , , , . he gives earlier dates, but i practice exclusions, myself. in the _report of the british association_, - , greg records a meteor that seemed to pass near the ground, between barsdorf and freiburg, germany: the next day a jelly-like mass was found in the snow-- unseasonableness for either spawn or nostoc. greg's comment in this instance is: "curious if true." but he records without modification the fall of a meteorite at gotha, germany, sept. , , "leaving a jelly-like mass on the ground." we are told that this substance fell only three feet away from an observer. in the _report of the british association_, - , according to a letter from greg to prof. baden-powell, at night, oct. , , near coblenz, a german, who was known to greg, and another person saw a luminous body fall close to them. they returned next morning and found a gelatinous mass of grayish color. according to chladni's account (_annals of philosophy_, n.s., - ) a viscous mass fell with a luminous meteorite between siena and rome, may, ; viscous matter found after the fall of a fire ball, in lusatia, march, ; fall of a gelatinous substance, after the explosion of a meteorite, near heidelberg, july, . in the _edinburgh philosophical journal_, - , the substance that fell at lusatia is said to have been of the "color and odor of dried, brown varnish." in the _amer. jour. sci._, - - , it is said that gelatinous matter fell with a globe of fire, upon the island of lethy, india, . in the _amer. jour. sci._, - - , in many observations upon the meteors of november, , are reports of falls of gelatinous substance: that, according to newspaper reports, "lumps of jelly" were found on the ground at rahway, n.j. the substance was whitish, or resembled the coagulated white of an egg: that mr. h.h. garland, of nelson county, virginia, had found a jelly-like substance of about the circumference of a twenty-five-cent piece: that, according to a communication from a.c. twining to prof. olmstead, a woman at west point, n.y., had seen a mass the size of a teacup. it looked like boiled starch: that, according to a newspaper, of newark, n.j., a mass of gelatinous substance, like soft soap, had been found. "it possessed little elasticity, and, on the application of heat, it evaporated as readily as water." it seems incredible that a scientist would have such hardihood, or infidelity, as to accept that these things had fallen from the sky: nevertheless, prof. olmstead, who collected these lost souls, says: "the fact that the supposed deposits were so uniformly described as gelatinous substance forms a presumption in favor of the supposition that they had the origin ascribed to them." in contemporaneous scientific publications considerable attention was given to prof. olmstead's series of papers upon this subject of the november meteors. you will not find one mention of the part that treats of gelatinous matter. i shall attempt not much of correlation of dates. a mathematic-minded positivist, with his delusion that in an intermediate state twice two are four, whereas, if we accept continuity, we cannot accept that there are anywhere two things to start with, would search our data for periodicities. it is so obvious to me that the mathematic, or the regular, is the attribute of the universal, that i have not much inclination to look for it in the local. still, in this solar system, "as a whole," there is considerable approximation to regularity; or the mathematic is so nearly localized that eclipses, for instance, can, with rather high approximation, be foretold, though i have notes that would deflate a little the astronomers' vainglory in this respect--or would if that were possible. an astronomer is poorly paid, uncheered by crowds, considerably isolated: he lives upon his own inflations: deflate a bear and it couldn't hibernate. this solar system is like every other phenomenon that can be regarded "as a whole"--or the affairs of a ward are interfered with by the affairs of the city of which it is a part; city by county; county by state; state by nation; nation by other nations; all nations by climatic conditions; climatic conditions by solar circumstances; sun by general planetary circumstances; solar system "as a whole" by other solar systems--so the hopelessness of finding the phenomena of entirety in the ward of a city. but positivists are those who try to find the unrelated in the ward of a city. in our acceptance this is the spirit of cosmic religion. objectively the state is not realizable in the ward of a city. but, if a positivist could bring himself to absolute belief that he had found it, that would be a subjective realization of that which is unrealizable objectively. of course we do not draw a positive line between the objective and the subjective--or that all phenomena called things or persons are subjective within one all-inclusive nexus, and that thoughts within those that are commonly called "persons" are sub-subjective. it is rather as if intermediateness strove for regularity in this solar system and failed: then generated the mentality of astronomers, and, in that secondary expression, strove for conviction that failure had been success. i have tabulated all the data of this book, and a great deal besides--card system--and several proximities, thus emphasized, have been revelations to me: nevertheless, it is only the method of theologians and scientists--worst of all, of statisticians. for instance, by the statistic method, i could "prove" that a black rain has fallen "regularly" every seven months, somewhere upon this earth. to do this, i'd have to include red rains and yellow rains, but, conventionally, i'd pick out the black particles in red substances and in yellow substances, and disregard the rest. then, too, if here and there a black rain should be a week early or a month late--that would be "acceleration" or "retardation." this is supposed to be legitimate in working out the periodicities of comets. if black rains, or red or yellow rains with black particles in them, should not appear at all near some dates--we have not read darwin in vain--"the records are not complete." as to other, interfering black rains, they'd be either gray or brown, or for them we'd find other periodicities. still, i have had to notice the year , for instance. i shall not note them all in this book, but i have records of extraordinary events in . someone should write a book upon the phenomena of this one year--that is, if books should be written. is notable for extraordinary falls, so far apart that a local explanation seems inadequate--not only the black rain of ireland, may, , but a red rain in sicily and a red rain in wales. also, it is said (timb's _year book_, - ) that, upon april or , , shepherds near mt. ararat, found a substance that was not indigenous, upon areas measuring to miles in circumference. presumably it had fallen there. we have already gone into the subject of science and its attempted positiveness, and its resistances in that it must have relations of service. it is very easy to see that most of the theoretic science of the th century was only a relation of reaction against theologic dogma, and has no more to do with truth than has a wave that bounds back from a shore. or, if a shop girl, or you or i, should pull out a piece of chewing gum about a yard long, that would be quite as scientific a performance as was the stretching of this earth's age several hundred millions of years. all "things" are not things, but only relations, or expressions of relations: but all relations are striving to be the unrelated, or have surrendered to, and subordinated to, higher attempts. so there is a positivist aspect to this reaction that is itself only a relation, and that is the attempt to assimilate all phenomena under the materialist explanation, or to formulate a final, all-inclusive system, upon the materialist basis. if this attempt could be realized, that would be the attaining of realness; but this attempt can be made only by disregarding psychic phenomena, for instance--or, if science shall eventually give in to the psychic, it would be no more legitimate to explain the immaterial in terms of the material than to explain the material in terms of the immaterial. our own acceptance is that material and immaterial are of a oneness, merging, for instance, in a thought that is continuous with a physical action: that oneness cannot be explained, because the process of explaining is the interpreting of something in terms of something else. all explanation is assimilation of something in terms of something else that has been taken as a basis: but, in continuity, there is nothing that is any more basic than anything else--unless we think that delusion built upon delusion is less real than its pseudo-foundation. in (timb's _year book_, - ) in persia fell a substance that the people said they had never seen before. as to what it was, they had not a notion, but they saw that the sheep ate it. they ground it into flour and made bread, said to have been passable enough, though insipid. that was a chance that science did not neglect. manna was placed upon a reasonable basis, or was assimilated and reconciled with the system that had ousted the older--and less nearly real--system. it was said that, likely enough, manna had fallen in ancient times--because it was still falling--but that there was no tutelary influence behind it--that it was a lichen from the steppes of asia minor--from one place in a whirlwind and down in another place. "in the _american almanac_, - , it is said that this substance--to the inhabitants of the region"--was "immediately recognized" by scientists who examined it: and that "the chemical analysis also identified it as a lichen." this was back in the days when chemical analysis was a god. since then his devotees have been shocked and disillusioned. just how a chemical analysis could so botanize, i don't know--but it was chemical analysis who spoke, and spoke dogmatically. it seems to me that the ignorance of inhabitants, contrasting with the local knowledge of foreign scientists, is overdone: if there's anything good to eat, within any distance conveniently covered by a whirlwind--inhabitants know it. i have data of other falls, in persia and asiatic turkey, of edible substances. they are all dogmatically said to be "manna"; and "manna" is dogmatically said to be a species of lichens from the steppes of asia minor. the position that i take is that this explanation was evolved in ignorance of the fall of vegetable substances, or edible substances, in other parts of the world: that it is the familiar attempt to explain the general in terms of the local; that, if we shall have data of falls of vegetable substance, in, say, canada or india, they were not of lichens from the steppes of asia minor; that, though all falls in asiatic turkey and persia are sweepingly and conveniently called showers of "manna," they have not been even all of the same substance. in one instance the particles are said to have been "seeds." though, in _comptes rendus_, the substance that fell in and is said to have been gelatinous, in the _bull. sci. nat. de neuchatel_, it is said to have been of something, in lumps the size of a filbert, that had been ground into flour; that of this flour had been made bread, very attractive-looking, but flavorless. the great difficulty is to explain segregation in these showers-- but deep-sea fishes and occasional falls, down to them, of edible substances; bags of grain, barrels of sugar; things that had not been whirled up from one part of the ocean-bottom, in storms or submarine disturbances, and dropped somewhere else-- i suppose one thinks--but grain in bags never has fallen-- object of amherst--its covering like "milled cloth"-- or barrels of corn lost from a vessel would not sink--but a host of them clashing together, after a wreck--they burst open; the corn sinks, or does when saturated; the barrel staves float longer-- if there be not an overhead traffic in commodities similar to our own commodities carried over this earth's oceans--i'm not the deep-sea fish i think i am. i have no data other than the mere suggestion of the amherst object of bags or barrels, but my notion is that bags and barrels from a wreck on one of this earth's oceans, would, by the time they reached the bottom, no longer be recognizable as bags or barrels; that, if we can have data of the fall of fibrous material that may have been cloth or paper or wood, we shall be satisfactory and grotesque enough. _proc. roy. irish acad._, - : "in the year , some workmen, who had been fetching water from a pond, seven german miles from memel, on returning to their work after dinner (during which there had been a snowstorm) found the flat ground around the pond covered with a coal-black, leafy mass; and a person who lived near said he had seen it fall like flakes with the snow." some of these flake-like formations were as large as a table-top. "the mass was damp and smelt disagreeably, like rotten seaweed, but, when dried, the smell went off." "it tore fibrously, like paper." classic explanation: "up from one place, and down in another." but what went up, from one place, in a whirlwind? of course, our intermediatist acceptance is that had this been the strangest substance conceivable, from the strangest other world that could be thought of; somewhere upon this earth there must be a substance similar to it, or from which it would, at least subjectively, or according to description, not be easily distinguishable. or that everything in new york city is only another degree or aspect of something, or combination of things, in a village of central africa. the novel is a challenge to vulgarization: write something that looks new to you: someone will point out that the thrice-accursed greeks said it long ago. existence is appetite: the gnaw of being; the one attempt of all things to assimilate all other things, if they have not surrendered and submitted to some higher attempt. it was cosmic that these scientists, who had surrendered to and submitted to the scientific system, should, consistently with the principles of that system, attempt to assimilate the substance that fell at memel with some known terrestrial product. at the meeting of the royal irish academy it was brought out that there is a substance, of rather rare occurrence, that has been known to form in thin sheets upon marsh land. it looks like greenish felt. the substance of memel: damp, coal-black, leafy mass. but, if broken up, the marsh-substance is flake-like, and it tears fibrously. an elephant can be identified as a sunflower--both have long stems. a camel is indistinguishable from a peanut--if only their humps be considered. trouble with this book is that we'll end up a lot of intellectual roués: we'll be incapable of being astonished with anything. we knew, to start with, that science and imbecility are continuous; nevertheless so many expressions of the merging-point are at first startling. we did think that prof. hitchcock's performance in identifying the amherst phenomenon as a fungus was rather notable as scientific vaudeville, if we acquit him of the charge of seriousness--or that, in a place where fungi were so common that, before a given evening two of them sprang up, only he, a stranger in this very fungiferous place, knew a fungus when he saw something like a fungus--if we disregard its quick liquefaction, for instance. it was only a monologue, however: now we have an all-star cast: and they're not only irish; they're royal irish. the royal irishmen excluded "coal-blackness" and included fibrousness: so then that this substance was "marsh paper," which "had been raised into the air by storms of wind, and had again fallen." second act: it was said that, according to m. ehrenberg, "the meteor-paper was found to consist partly of vegetable matter, chiefly of conifervæ." third act: meeting of the royal irishmen: chairs, tables, irishmen: some flakes of marsh-paper were exhibited. their composition was chiefly of conifervæ. this was a double inclusion: or it's the method of agreement that logicians make so much of. so no logician would be satisfied with identifying a peanut as a camel, because both have humps: he demands accessory agreement--that both can live a long time without water, for instance. now, it's not so very unreasonable, at least to the free and easy vaudeville standards that, throughout this book, we are considering, to think that a green substance could be snatched up from one place in a whirlwind, and fall as a black substance somewhere else: but the royal irishmen excluded something else, and it is a datum that was as accessible to them as it is to me: that, according to chladni, this was no little, local deposition that was seen to occur by some indefinite person living near a pond somewhere. it was a tremendous fall from a vast sky-area. likely enough all the marsh paper in the world could not have supplied it. at the same time, this substance was falling "in great quantities," in norway and pomerania. or see kirkwood, _meteoric astronomy_, p. : "substance like charred paper fell in norway and other parts of northern europe, jan. , ." or a whirlwind, with a distribution as wide as that, would not acceptably, i should say, have so specialized in the rare substance called "marsh paper." there'd have been falls of fence rails, roofs of houses, parts of trees. nothing is said of the occurrence of a tornado in northern europe, in january, . there is record only of this one substance having fallen in various places. time went on, but the conventional determination to exclude data of all falls to this earth, except of substances of this earth, and of ordinary meteoric matter, strengthened. _annals of philosophy_, - : the substance that fell in january, , is described as "a mass of black leaves, having the appearance of burnt paper, but harder, and cohering, and brittle." "marsh paper" is not mentioned, and there is nothing said of the "conifervæ," which seemed so convincing to the royal irishmen. vegetable composition is disregarded, quite as it might be by someone who might find it convenient to identify a crook-necked squash as a big fishhook. meteorites are usually covered with a black crust, more or less scale-like. the substance of is black and scale-like. if so be convenience, "leaf-likeness" is "scale-likeness." in this attempt to assimilate with the conventional, we are told that the substance is a mineral mass: that it is like the black scales that cover meteorites. the scientist who made this "identification" was von grotthus. he had appealed to the god chemical analysis. or the power and glory of mankind--with which we're not always so impressed--but the gods must tell us what we want them to tell us. we see again that, though nothing has identity of its own, anything can be "identified" as anything. or there's nothing that's not reasonable, if one snoopeth not into its exclusions. but here the conflict did not end. berzelius examined the substance. he could not find nickel in it. at that time, the presence of nickel was the "positive" test of meteoritic matter. whereupon, with a supposititious "positive" standard of judgment against him, von grotthus revoked his "identification." (_annals and mag. of nat. hist._, - - .) this equalization of eminences permits us to project with our own expression, which, otherwise, would be subdued into invisibility: that it's too bad that no one ever looked to see--hieroglyphics?--something written upon these sheets of paper? if we have no very great variety of substances that have fallen to this earth; if, upon this earth's surface there is infinite variety of substances detachable by whirlwinds, two falls of such a rare substance as marsh paper would be remarkable. a writer in the _edinburgh review_, - , says that, at the time of writing, he had before him a portion of a sheet of square feet, of a substance that had fallen at carolath, silesia, in --exactly similar to cotton-felt, of which clothing might have been made. the god microscopic examination had spoken. the substance consisted chiefly of conifervæ. _jour. asiatic soc. of bengal_, -pt. - : that march , --about the time of a fall of edible substance in asia minor--an olive-gray powder fell at shanghai. under the microscope, it was seen to be an aggregation of hairs of two kinds, black ones and rather thick white ones. they were supposed to be mineral fibers, but, when burned, they gave out "the common ammoniacal smell and smoke of burnt hair or feathers." the writer described the phenomenon as "a cloud of square miles of fibers, alkali, and sand." in a postscript, he says that other investigators, with more powerful microscopes, gave opinion that the fibers were not hairs; that the substance consisted chiefly of conifervæ. or the pathos of it, perhaps; or the dull and uninspired, but courageous persistence of the scientific: everything seemingly found out is doomed to be subverted--by more powerful microscopes and telescopes; by more refined, precise, searching means and methods--the new pronouncements irrepressibly bobbing up; their reception always as truth at last; always the illusion of the final; very little of the intermediatist spirit-- that the new that has displaced the old will itself some day be displaced; that it, too, will be recognized as myth-stuff-- but that if phantoms climb, spooks of ladders are good enough for them. _annual register_, - : that, according to a report by m. lainé, french consul at pernambuco, early in october, , there was a shower of a substance resembling silk. the quantity was as tremendous as might be a whole cargo, lost somewhere between jupiter and mars, having drifted around perhaps for centuries, the original fabrics slowly disintegrating. in _annales de chimie_, - - , it is said that samples of this substance were sent to france by m. lainé, and that they proved to have some resemblances to silky filaments which, at certain times of the year, are carried by the wind near paris. in the _annals of philosophy_, n.s., - , there is mention of a fibrous substance like blue silk that fell near naumberg, march , . according to chladni (_annales de chimie_, - - ), the quantity was great. he places a question mark before the date. one of the advantages of intermediatism is that, in the oneness of quasiness, there can be no mixed metaphors. whatever is acceptable of anything, is, in some degree or aspect, acceptable of everything. so it is quite proper to speak, for instance, of something that is as firm as a rock and that sails in a majestic march. the irish are good monists: they have of course been laughed at for their keener perceptions. so it's a book we're writing, or it's a procession, or it's a museum, with the chamber of horrors rather over-emphasized. a rather horrible correlation occurs in the _scientific american_, - . what interests us is that a correspondent saw a silky substance fall from the sky--there was an aurora borealis at the time--he attributes the substance to the aurora. since the time of darwin, the classic explanation has been that all silky substances that fall from the sky are spider webs. in , aboard the _beagle_, at the mouth of la plata river, miles from land, darwin saw an enormous number of spiders, of the kind usually known as "gossamer" spiders, little aeronauts that cast out filaments by which the wind carries them. it's difficult to express that silky substances that have fallen to this earth were not spider webs. my own acceptance is that spider webs are the merger; that there have been falls of an externally derived silky substance, and also of the webs, or strands, rather, of aeronautic spiders indigenous to this earth; that in some instances it is impossible to distinguish one from the other. of course, our expression upon silky substances will merge away into expressions upon other seeming textile substances, and i don't know how much better off we'll be-- except that, if fabricable materials have fallen from the sky-- simply to establish acceptance of that may be doing well enough in this book of first and tentative explorations. in _all the year round_, - , is described a fall that took place in england, sept. , , in the towns of bradly, selborne, and alresford, and in a triangular space included by these three towns. the substance is described as "cobwebs"--but it fell in flake-formation, or in "flakes or rags about one inch broad and five or six inches long." also these flakes were of a relatively heavy substance--"they fell with some velocity." the quantity was great--the shortest side of the triangular space is eight miles long. in the _wernerian nat. hist. soc. trans._, - , it is said that there were two falls--that they were some hours apart--a datum that is becoming familiar to us--a datum that cannot be taken into the fold, unless we find it repeated over and over and over again. it is said that the second fall lasted from nine o'clock in the morning until night. now the hypnosis of the classic--that what we call intelligence is only an expression of inequilibrium; that when mental adjustments are made, intelligence ceases--or, of course, that intelligence is the confession of ignorance. if you have intelligence upon any subject, that is something you're still learning--if we agree that that which is learned is always mechanically done--in quasi-terms, of course, because nothing is ever finally learned. it was decided that this substance was spiders' web. that was adjustment. but it's not adjustment to me; so i'm afraid i shall have some intelligence in this matter. if i ever arrive at adjustment upon this subject, then, upon this subject, i shall be able to have no thoughts, except routine-thoughts. i haven't yet quite decided absolutely everything, so i am able to point out: that this substance was of quantity so enormous that it attracted wide attention when it came down-- that it would have been equally noteworthy when it went up-- that there is no record of anyone, in england or elsewhere, having seen tons of "spider webs" going up, september, . further confession of intelligence upon my part: that, if it be contested, then, that the place of origin may have been far away, but still terrestrial-- then it's that other familiar matter of incredible "marksmanship" again--hitting a small, triangular space for hours--interval of hours--then from nine in the morning until night: same small triangular space. these are the disregards of the classic explanation. there is no mention of spiders having been seen to fall, but a good inclusion is that, though this substance fell in good-sized flakes of considerable weight, it was viscous. in this respect it was like cobwebs: dogs nosing it on grass, were blindfolded with it. this circumstance does strongly suggest cobwebs-- unless we can accept that, in regions aloft, there are vast viscous or gelatinous areas, and that things passing through become daubed. or perhaps we clear up the confusion in the descriptions of the substance that fell in and , in asia minor, described in one publication as gelatinous, and in another as a cereal--that it was a cereal that had passed through a gelatinous region. that the paper-like substance of memel may have had such an experience may be indicated in that ehrenberg found in it gelatinous matter, which he called "nostoc." (_annals and mag. of nat. hist._, - - .) _scientific american_, - : fall of a substance described as "cobwebs," latter part of october, , in milwaukee, wis., and other towns: other towns mentioned are green bay, vesburge, fort howard, sheboygan, and ozaukee. the aeronautic spiders are known as "gossamer" spiders, because of the extreme lightness of the filaments that they cast out to the wind. of the substance that fell in wisconsin, it is said: "in all instances the webs were strong in texture and very white." the editor says: "curiously enough, there is no mention in any of the reports that we have seen, of the presence of spiders." so our attempt to divorce a possible external product from its terrestrial merger: then our joy of the prospector who thinks he's found something: the _monthly weather review_, - , quotes the _montgomery_ (ala.) _advertiser_: that, upon nov. , , numerous batches of spider-web-like substance fell in montgomery, in strands and in occasional masses several inches long and several inches broad. according to the writer, it was not spiders' web, but something like asbestos; also that it was phosphorescent. the editor of the _review_ says that he sees no reason for doubting that these masses were cobwebs. _la nature_, - : a correspondent writes that he sends a sample of a substance said to have fallen at montussan (gironde), oct. , . according to a witness, quoted by the correspondent, a thick cloud, accompanied by rain and a violent wind, had appeared. this cloud was composed of a woolly substance in lumps the size of a fist, which fell to the ground. the editor (tissandier) says of this substance that it was white, but was something that had been burned. it was fibrous. m. tissandier astonishes us by saying that he cannot identify this substance. we thought that anything could be "identified" as anything. he can say only that the cloud in question must have been an extraordinary conglomeration. _annual register, - :_ that, march, , there fell, in the fields of kourianof, russia, a combustible yellowish substance, covering, at least two inches thick, an area of or square feet. it was resinous and yellowish: so one inclines to the conventional explanation that it was pollen from pine trees--but, when torn, it had the tenacity of cotton. when placed in water, it had the consistency of resin. "this resin had the color of amber, was elastic, like india rubber, and smelled like prepared oil mixed with wax." so in general our notion of cargoes--and our notion of cargoes of food supplies: in _philosophical transactions_, - , is an extract from a letter by mr. robert vans, of kilkenny, ireland, dated nov. , : that there had been "of late," in the counties of limerick and tipperary, showers of a sort of matter like butter or grease... having "a very stinking smell." there follows an extract from a letter by the bishop of cloyne, upon "a very odd phenomenon," which was observed in munster and leinster: that for a good part of the spring of there fell a substance which the country people called "butter"--"soft, clammy, and of a dark yellow"--that cattle fed "indifferently" in fields where this substance lay. "it fell in lumps as big as the end of one's finger." it had a "strong ill scent." his grace calls it a "stinking dew." in mr. vans' letter, it is said that the "butter" was supposed to have medicinal properties, and "was gathered in pots and other vessels by some of the inhabitants of this place." and: in all the following volumes of _philosophical transactions_ there is no speculation upon this extraordinary subject. ostracism. the fate of this datum is a good instance of damnation, not by denial, and not by explaining away, but by simple disregard. the fall is listed by chladni, and is mentioned in other catalogues, but, from the absence of all inquiry, and of all but formal mention, we see that it has been under excommunication as much as was ever anything by the preceding system. the datum has been buried alive. it is as irreconcilable with the modern system of dogmas as ever were geologic strata and vermiform appendix with the preceding system-- if, intermittently, or "for a good part of the spring," this substance fell in two irish provinces, and nowhere else, we have, stronger than before, a sense of a stationary region overhead, or a region that receives products like this earth's products, but from external sources, a region in which this earth's gravitational and meteorological forces are relatively inert--if for many weeks a good part of this substance did hover before finally falling. we suppose that, in , mr. vans and the bishop of cloyne could describe what they saw as well as could witnesses in : nevertheless, it is going far back; we shall have to have many modern instances before we can accept. as to other falls, or another fall, it is said in the _amer. jour. sci._, - - , that, april , --about a month after the fall of the substance of kourianof--fell a substance that was wine-yellow, transparent, soft, and smelling like rancid oil. m. herman, a chemist who examined it, named it "sky oil." for analysis and chemic reactions, see the _journal_. the _edinburgh new philosophical journal_, - , mentions an "unctuous" substance that fell near rotterdam, in . in _comptes rendus_, - , there is an account of an oily, reddish matter that fell at genoa, february, . whatever it may have been-- altogether, most of our difficulties are problems that we should leave to later developers of super-geography, i think. a discoverer of america should leave long island to someone else. if there be, plying back and forth from jupiter and mars and venus, super-constructions that are sometimes wrecked, we think of fuel as well as cargoes. of course the most convincing data would be of coal falling from the sky: nevertheless, one does suspect that oil-burning engines were discovered ages ago in more advanced worlds--but, as i say, we should leave something to our disciples--so we'll not especially wonder whether these butter-like or oily substances were food or fuel. so we merely note that in the _scientific american_, - , is an account of hail that fell, in the middle of april, , in mississippi, in which was a substance described as turpentine. something that tasted like orange water, in hailstones, about the first of june, , near nîmes, france; identified as nitric acid (_jour. de pharmacie_, - ). hail and ashes, in ireland, (_sci. amer._, - ). that, at elizabeth, n.j., june , , fell hail in which was a substance, said, by prof. leeds, of stevens institute, to be carbonate of soda (_sci. amer._, - ). we are getting a little away from the lines of our composition, but it will be an important point later that so many extraordinary falls have occurred with hail. or--if they were of substances that had had origin upon some other part of this earth's surface--had the hail, too, that origin? our acceptance here will depend upon the number of instances. reasonably enough, some of the things that fall to this earth should coincide with falls of hail. as to vegetable substances in quantities so great as to suggest lost cargoes, we have a note in the _intellectual observer_, - : that, upon the first of may, , a rain fell at perpignan, "bringing down with it a red substance, which proved on examination to be a red meal mixed with fine sand." at various points along the mediterranean, this substance fell. there is, in _philosophical transactions_, - , an account of a seeming cereal, said to have fallen in wiltshire, in --said that some of the "wheat" fell "enclosed in hailstones"--but the writer in _transactions_, says that he had examined the grains, and that they were nothing but seeds of ivy berries dislodged from holes and chinks where birds had hidden them. if birds still hide ivy seeds, and if winds still blow, i don't see why the phenomenon has not repeated in more than two hundred years since. or the red matter in rain, at siena, italy, may, ; said, by arago, to have been vegetable matter (arago, _oeuvres_, - ). somebody should collect data of falls at siena alone. in the _monthly weather review_, - , a correspondent writes that, upon feb. , , at pawpaw, michigan, upon a day that was so calm that his windmill did not run, fell a brown dust that looked like vegetable matter. the editor of the _review_ concludes that this was no widespread fall from a tornado, because it had been reported from nowhere else. rancidness--putridity--decomposition--a note that has been struck many times. in a positive sense, of course, nothing means anything, or every meaning is continuous with all other meanings: or that all evidences of guilt, for instance, are just as good evidences of innocence--but this condition seems to mean--things lying around among the stars a long time. horrible disaster in the time of julius caesar; remains from it not reaching this earth till the time of the bishop of cloyne: we leave to later research the discussion of bacterial action and decomposition, and whether bacteria could survive in what we call space, of which we know nothing-- _chemical news_, - : dr. a.t. machattie, f.c.s., writes that, at london, ontario, feb. , , in a violent storm, fell, with snow, a dark-colored substance, estimated at tons, over a belt miles by miles. it was examined under a microscope, by dr. machattie, who found it to consist mainly of vegetable matter "far advanced in decomposition." the substance was examined by dr. james adams, of glasgow, who gave his opinion that it was the remains of cereals. dr. machattie points out that for months before this fall the ground of canada had been frozen, so that in this case a more than ordinarily remote origin has to be thought of. dr. machattie thinks of origin to the south. "however," he says, "this is mere conjecture." _amer. jour. sci._, - : that, march , --during a thunderstorm--at rajkit, india, occurred a fall of grain. it was reported by col. sykes, of the british association. the natives were greatly excited--because it was grain of a kind unknown to them. usually comes forward a scientist who knows more of the things that natives know best than the natives know--but it so happens that the usual thing was not done definitely in this instance: "the grain was shown to some botanists, who did not immediately recognize it, but thought it to be either a spartium or a vicia." lead, silver, diamonds, glass. they sound like the accursed, but they're not: they're now of the chosen--that is, when they occur in metallic or stony masses that science has recognized as meteorites. we find that resistance is to substances not so mixed in or incorporated. of accursed data, it seems to me that punk is pretty damnable. in the _report of the british association_, - , there is mention of a light chocolate-brown substance that has fallen with meteorites. no particulars given; not another mention anywhere else that i can find. in this english publication, the word "punk" is not used; the substance is called "amadou." i suppose, if the datum has anywhere been admitted to french publications, the word "amadou" has been avoided, and "punk" used. or oneness of allness: scientific works and social registers: a goldstein who can't get in as goldstein, gets in as jackson. the fall of sulphur from the sky has been especially repulsive to the modern orthodoxy--largely because of its associations with the superstitions or principles of the preceding orthodoxy--stories of devils: sulphurous exhalations. several writers have said that they have had this feeling. so the scientific reactionists, who have rabidly fought the preceding, because it was the preceding: and the scientific prudes, who, in sheer exclusionism, have held lean hands over pale eyes, denying falls of sulphur. i have many notes upon the sulphurous odor of meteorites, and many notes upon phosphorescence of things that come from externality. some day i shall look over old stories of demons that have appeared sulphurously upon this earth, with the idea of expressing that we have often had undesirable visitors from other worlds; or that an indication of external derivation is sulphurousness. i expect some day to rationalize demonology, but just at present we are scarcely far enough advanced to go so far back. for a circumstantial account of a mass of burning sulphur, about the size of a man's fist, that fell at pultusk, poland, jan. , , upon a road, where it was stamped out by a crowd of villagers, see _rept. brit. assoc._, - . the power of the exclusionists lies in that in their stand are combined both modern and archaic systematists. falls of sandstone and limestone are repulsive to both theologians and scientists. sandstone and limestone suggest other worlds upon which occur processes like geological processes; but limestone, as a fossiliferous substance, is of course especially of the unchosen. in _science_, march , , we read of a block of limestone, said to have fallen near middleburg, florida. it was exhibited at the sub-tropical exposition, at jacksonville. the writer, in _science_, denies that it fell from the sky. his reasoning is: there is no limestone in the sky; therefore this limestone did not fall from the sky. better reasoning i cannot conceive of--because we see that a final major premise--universal--true--would include all things: that, then, would leave nothing to reason about--so then that all reasoning must be based upon "something" not universal, or only a phantom intermediate to the two finalities of nothingness and allness, or negativeness and positiveness. _la nature_, - - : fall, at pel-et-der (l'aube), france, june , , of limestone pebbles. identified with limestone at château-landon--or up and down in a whirlwind. but they fell with hail--which, in june, could not very well be identified with ice from château-landon. coincidence, perhaps. upon page , _science gossip_, , the editor says, of a stone that was reported to have fallen at little lever, england, that a sample had been sent to him. it was sandstone. therefore it had not fallen, but had been on the ground in the first place. but, upon page , _science gossip_, , is an account of "a large, smooth, water-worn, gritty sandstone pebble" that had been found in the wood of a full-grown beech tree. looks to me as if it had fallen red-hot, and had penetrated the tree with high velocity. but i have never heard of anything falling red-hot from a whirlwind-- the wood around this sandstone pebble was black, as if charred. dr. farrington, for instance, in his books, does not even mention sandstone. however, the british association, though reluctant, is less exclusive: _report_ of , p. : substance about the size of a duck's egg, that fell at raphoe, ireland, june , --date questioned. it is not definitely said that this substance was sandstone, but that it "resembled" friable sandstone. falls of salt have occurred often. they have been avoided by scientific writers, because of the dictum that only water and not substances held in solution, can be raised by evaporation. however, falls of salty water have received attention from dalton and others, and have been attributed to whirlwinds from the sea. this is so reasonably contested--quasi-reasonably--as to places not far from the sea-- but the fall of salt that occurred high in the mountains of switzerland-- we could have predicted that that datum could be found somewhere. let anything be explained in local terms of the coast of england--but also has it occurred high in the mountains of switzerland. large crystals of salt fell--in a hailstorm--aug. , , in switzerland. the orthodox explanation is a crime: whoever made it, should have had his finger-prints taken. we are told (_an. rec. sci._, ) that these objects of salt "came over the mediterranean from some part of africa." or the hypnosis of the conventional--provided it be glib. one reads such an assertion, and provided it be suave and brief and conventional, one seldom questions--or thinks "very strange" and then forgets. one has an impression from geography lessons: mediterranean not more than three inches wide, on the map; switzerland only a few more inches away. these sizable masses of salt are described in the _amer. jour. sci._, - - , as "essentially imperfect cubic crystals of common salt." as to occurrence with hail--that can in one, or ten, or twenty, instances be called a coincidence. another datum: extraordinary year : london _times_, dec. , : translation from a turkish newspaper; a substance that fell at scutari, dec. , ; described as an unknown substance, in particles--or flakes?--like snow. "it was found to be saltish to the taste, and to dissolve readily in water." miscellaneous: "black, capillary matter" that fell, nov. , , at charleston, s.c. (_amer. jour. sci._, - - ). fall of small, friable, vesicular masses, from size of a pea to size of a walnut, at lobau, jan. , (_rept. brit. assoc._, - ). objects that fell at peshawur, india, june, , during a storm: substance that looked like crystallized niter, and that tasted like sugar (_nature_, july , ). i suppose sometimes deep-sea fishes have their noses bumped by cinders. if their regions be subjacent to cunard or white star routes, they're especially likely to be bumped. i conceive of no inquiry: they're deep-sea fishes. or the slag of slains. that it was a furnace-product. the rev. james rust seemed to feel bumped. he tried in vain to arouse inquiry. as to a report, from chicago, april , , that slag had fallen from the sky, prof. e.s. bastian (_amer. jour. sci._, - - ) says that the slag "had been on the ground in the first place." it was furnace-slag. "a chemical examination of the specimens has shown that they possess none of the characteristics of true meteorites." over and over and over again, the universal delusion; hope and despair of attempted positivism; that there can be real criteria, or distinct characteristics of anything. if anybody can define--not merely suppose, like prof. bastian, that he can define--the true characteristics of anything, or so localize trueness anywhere, he makes the discovery for which the cosmos is laboring. he will be instantly translated, like elijah, into the positive absolute. my own notion is that, in a moment of super-concentration, elijah became so nearly a real prophet that he was translated to heaven, or to the positive absolute, with such velocity that he left an incandescent train behind him. as we go along, we shall find the "true test of meteoritic material," which in the past has been taken as an absolute, dissolving into almost utmost nebulosity. prof. bastian explains mechanically, or in terms of the usual reflexes to all reports of unwelcome substances: that near where the slag had been found, telegraph wires had been struck by lightning; that particles of melted wire had been seen to fall near the slag--which had been on the ground in the first place. but, according to the _new york times_, april , , about two bushels of this substance had fallen. something that was said to have fallen at darmstadt, june , ; listed by greg (_rept. brit. assoc._, - ) as "only slag." _philosophical magazine_, - - : that, in , a large stone was found far in the interior of a tree, in battersea fields. sometimes cannon balls are found embedded in trees. doesn't seem to be anything to discuss; doesn't seem discussable that any one would cut a hole in a tree and hide a cannon ball, which one could take to bed, and hide under one's pillow, just as easily. so with the stone of battersea fields. what is there to say, except that it fell with high velocity and embedded in the tree? nevertheless, there was a great deal of discussion-- because, at the foot of the tree, as if broken off the stone, fragments of slag were found. i have nine other instances. slag and cinders and ashes, and you won't believe, and neither will i, that they came from the furnaces of vast aerial super-constructions. we'll see what looks acceptable. as to ashes, the difficulties are great, because we'd expect many falls of terrestrially derived ashes--volcanoes and forest fires. in some of our acceptances, i have felt a little radical-- i suppose that one of our main motives is to show that there is, in quasi-existence, nothing but the preposterous--or something intermediate to absolute preposterousness and final reasonableness--that the new is the obviously preposterous; that it becomes the established and disguisedly preposterous; that it is displaced, after a while, and is again seen to be the preposterous. or that all progress is from the outrageous to the academic or sanctified, and back to the outrageous--modified, however, by a trend of higher and higher approximation to the impreposterous. sometimes i feel a little more uninspired than at other times, but i think we're pretty well accustomed now to the oneness of allness; or that the methods of science in maintaining its system are as outrageous as the attempts of the damned to break in. in the _annual record of science_, - , prof. daubrée is quoted: that ashes that had fallen in the azores had come from the chicago fire-- or the damned and the saved, and there's little to choose between them; and angels are beings that have not obviously barbed tails to them--or never have such bad manners as to stroke an angel below the waist-line. however this especial outrage was challenged: the editor of the _record_ returns to it, in the issue of : considers it "in the highest degree improper to say that the ashes of chicago were landed in the azores." _bull. soc. astro. de france_, - : account of a white substance, like ashes, that fell at annoy, france, march , : simply called a curious phenomenon; no attempt to trace to a terrestrial source. flake formations, which may signify passage through a region of pressure, are common; but spherical formations--as if of things that have rolled and rolled along planar regions somewhere--are commoner: _nature_, jan. , , quotes a kimberley newspaper: that, toward the close of november, , a thick shower of ashy matter fell at queenstown, south africa. the matter was in marble-sized balls, which were soft and pulpy, but which, upon drying, crumbled at touch. the shower was confined to one narrow streak of land. it would be only ordinarily preposterous to attribute this substance to krakatoa-- but, with the fall, loud noises were heard-- but i'll omit many notes upon ashes: if ashes should sift down upon deep-sea fishes, that is not to say that they came from steamships. data of falls of cinders have been especially damned by mr. symons, the meteorologist, some of whose investigations we'll investigate later--nevertheless-- notice of a fall, in victoria, australia, april , (_rept. brit. assoc._, - )--at least we are told, in the reluctant way, that someone "thought" he saw matter fall near him at night, and the next day found something that looked like cinders. in the _proc. of the london roy. soc._, - , there is an account of cinders that fell on the deck of a lightship, jan. , . in the _amer. jour. sci._, - - , there is a notice that the editor had received a specimen of cinders said to have fallen--in showery weather--upon a farm, near ottowa, ill., jan. , . but after all, ambiguous things they are, cinders or ashes or slag or clinkers, the high priest of the accursed that must speak aloud for us is--coal that has fallen from the sky. or coke: the person who thought he saw something like cinders, also thought he saw something like coke, we are told. _nature_, - : something that "looked exactly like coke" that fell--during a thunderstorm--in the orne, france, april , . or charcoal: dr. angus smith, in the _lit. and phil. soc. of manchester memoirs_, - - , says that, about --like a great deal in lyell's _principles_ and darwin's _origin_, this account is from hearsay--something fell from the sky, near allport, england. it fell luminously, with a loud report, and scattered in a field. a fragment that was seen by dr. smith, is described by him as having "the appearance of a piece of common wood charcoal." nevertheless, the reassured feeling of the faithful, upon reading this, is burdened with data of differences: the substance was so uncommonly heavy that it seemed as if it had iron in it; also there was "a sprinkling of sulphur." this material is said, by prof. baden-powell, to be "totally unlike that of any other meteorite." greg, in his catalogue (_rept. brit. assoc._, - ), calls it "a more than doubtful substance"--but again, against reassurance, that is not doubt of authenticity. greg says that it is like compact charcoal, with particles of sulphur and iron pyrites embedded. reassurance rises again: prof. baden-powell says: "it contains also charcoal, which might perhaps be acquired from matter among which it fell." this is a common reflex with the exclusionists: that substances not "truly meteoritic" did not fall from the sky, but were picked up by "truly meteoritic" things, of course only on their surfaces, by impact with this earth. rhythm of reassurances and their declines: according to dr. smith, this substance was not merely coated with charcoal; his analysis gives . per cent carbon. our acceptance that coal has fallen from the sky will be via data of resinous substances and bituminous substances, which merge so that they cannot be told apart. resinous substance said to have fallen at kaba, hungary, april , (_rept. brit. assoc._, - ). a resinous substance that fell after a fireball? at neuhaus, bohemia, dec. , (_rept. brit. assoc._, - ). fall, july , , at luchon, during a storm, of a brownish substance; very friable, carbonaceous matter; when burned it gave out a resinous odor (_comptes rendus_, - ). substance that fell, feb. , , , , at genoa, italy, said to have been resinous; said by arago (_oeuvres_, - ) to have been bituminous matter and sand. fall--during a thunderstorm--july, , near cape cod, upon the deck of an english vessel, the _albemarle_, of "burning, bituminous matter" (_edin. new phil. jour._, - ); a fall, at christiania, norway, june , , of bituminous matter, listed by greg as doubtful; fall of bituminous matter, in germany, march , , listed by greg. lockyer (_the meteoric hypothesis_, p. ) says that the substance that fell at the cape of good hope, oct. , --about five cubic feet of it: substance so soft that it was cuttable with a knife--"after being experimented upon, it left a residue, which gave out a very bituminous smell." and this inclusion of lockyer's--so far as findable in all books that i have read--is, in books, about as close as we can get to our desideratum--that coal has fallen from the sky. dr. farrington, except with a brief mention, ignores the whole subject of the fall of carbonaceous matter from the sky. proctor, in all of his books that i have read--is, in books, about as close as we can get to the admission that carbonaceous matter has been found in meteorites "in very minute quantities"--or my own suspicion is that it is possible to damn something else only by losing one's own soul--quasi-soul, of course. _sci. amer._, - : that the substance that fell at the cape of good hope "resembled a piece of anthracite coal more than anything else." it's a mistake, i think: the resemblance is to bituminous coal--but it is from the periodicals that we must get our data. to the writers of books upon meteorites, it would be as wicked--by which we mean departure from the characters of an established species--quasi-established, of course--to say that coal has fallen from the sky, as would be, to something in a barnyard, a temptation that it climb a tree and catch a bird. domestic things in a barnyard: and how wild things from forests outside seem to them. or the homeopathist--but we shall shovel data of coal. and, if over and over, we shall learn of masses of soft coal that have fallen upon this earth, if in no instance has it been asserted that the masses did not fall, but were upon the ground in the first place; if we have many instances, this time we turn down good and hard the mechanical reflex that these masses were carried from one place to another in whirlwinds, because we find it too difficult to accept that whirlwinds could so select, or so specialize in a peculiar substance. among writers of books, the only one i know of who makes more than brief mention is sir robert ball. he represents a still more antique orthodoxy, or is an exclusionist of the old type, still holding out against even meteorites. he cites several falls of carbonaceous matter, but with disregards that make for reasonableness that earthy matter may have been caught up by whirlwinds and flung down somewhere else. if he had given a full list, he would be called upon to explain the special affinity of whirlwinds for a special kind of coal. he does not give a full list. we shall have all that's findable, and we shall see that against this disease we're writing, the homeopathist's prescription availeth not. another exclusionist was prof. lawrence smith. his psycho-tropism was to respond to all reports of carbonaceous matter falling from the sky, by saying that this damned matter had been deposited upon things of the chosen by impact with this earth. most of our data antedate him, or were contemporaneous with him, or were as accessible to him as to us. in his attempted positivism it is simply--and beautifully--disregarded that, according to berthelot, berzelius, cloez, wohler and others these masses are not merely coated with carbonaceous matter, but are carbonaceous throughout, or are permeated throughout. how anyone could so resolutely and dogmatically and beautifully and blindly hold out would puzzle us were it not for our acceptance that only to think is to exclude and include; and to exclude some things that have as much right to come in as have the included--that to have an opinion upon any subject is to be a lawrence smith--because there is no definite subject. dr. walter flight (_eclectic magazine_, - ) says, of the substance that fell near alais, france, march , , that it "emits a faint bituminous substance" when heated, according to the observations of bergelius and a commission appointed by the french academy. this time we have not the reluctances expressed in such words as "like" and "resembling." we are told that this substance is "an earthy kind of coal." as to "minute quantities" we are told that the substance that fell at the cape of good hope has in it a little more than a quarter of organic matter, which, in alcohol, gives the familiar reaction of yellow, resinous matter. other instances given by dr. flight are: carbonaceous matter that fell in , in tennessee; cranbourne, australia, ; montauban, france, may , (twenty masses, some of them as large as a human head, of a substance that "resembled a dull-colored earthy lignite"); goalpara, india, about (about per cent of a hydrocarbon); at ornans, france, july , ; substance with "an organic, combustible ingredient," at hessle, sweden, jan. , . _knowledge_, - : that, according to m. daubrée, the substance that had fallen in the argentine republic, "resembled certain kinds of lignite and boghead coal." in _comptes rendus_, - , it is said that this mass fell, june , , in the province entre ríos, argentina: that it is "like" brown coal; that it resembles all the other carbonaceous masses that have fallen from the sky. something that fell at grazac, france, aug. , : when burned, it gave out a bituminous odor (_comptes rendus_, - ). carbonaceous substance that fell at rajpunta, india, jan. , : very friable: per cent of its soluble in water (_records geol. survey of india_, -pt. - ). a combustible carbonaceous substance that fell with sand at naples, march , (_amer. jour. sci._, - - ). _sci. amer. sup._, - : that, june , , a very friable substance, of a deep, greenish black, fell at mighei, russia. it contained per cent organic matter, which, when powdered and digested in alcohol, yielded, after evaporation, a bright yellow resin. in this mass was per cent of an unknown mineral. cinders and ashes and slag and coke and charcoal and coal. and the things that sometimes deep-sea fishes are bumped by. reluctances and the disguises or covered retreats of such words as "like" and "resemble"--or that conditions of intermediateness forbid abrupt transitions--but that the spirit animating all intermediateness is to achieve abrupt transitions--because, if anything could finally break away from its origin and environment, that would be a real thing--something not merging away indistinguishably with the surrounding. so all attempt to be original; all attempt to invent something that is more than mere extension or modification of the preceding, is positivism--or that if one could conceive of a device to catch flies, positively different from, or unrelated to, all other devices--up he'd shoot to heaven, or the positive absolute--leaving behind such an incandescent train that in one age it would be said that he had gone aloft in a fiery chariot, and in another age that he had been struck by lightning-- i'm collecting notes upon persons supposed to have been struck by lightning. i think that high approximation to positivism has often been achieved--instantaneous translation--residue of negativeness left behind, looking much like effects of a stroke of lightning. some day i shall tell the story of the _marie celeste_--"properly," as the _scientific american supplement_ would say--mysterious disappearance of a sea captain, his family, and the crew-- of positivists, by the route of abrupt transition, i think that manet was notable--but that his approximation was held down by his intense relativity to the public--or that it is quite as impositive to flout and insult and defy as it is to crawl and placate. of course, manet began with continuity with courbet and others, and then, between him and manet there were mutual influences--but the spirit of abrupt difference is the spirit of positivism, and manet's stand was against the dictum that all lights and shades must merge away suavely into one another and prepare for one another. so a biologist like de vries represents positivism, or the breaking of continuity, by trying to conceive of evolution by mutation--against the dogma of indistinguishable gradations by "minute variations." a copernicus conceives of helio-centricity. continuity is against him. he is not permitted to break abruptly with the past. he is permitted to publish his work, but only as "an interesting hypothesis." continuity--and that all that we call evolution or progress is attempt to break away from it-- that our whole solar system was at one time attempt by planets to break away from a parental nexus and set up as individualities, and, failing, move in quasi-regular orbits that are expressions of relations with the sun and with one another, all having surrendered, being now quasi-incorporated in a higher approximation to system: intermediateness in its mineralogic aspect of positivism--or iron that strove to break away from sulphur and oxygen, and be real, homogeneous iron--failing, inasmuch as elemental iron exists only in text-book chemistry: intermediateness in its biologic aspect of positivism--or the wild, fantastic, grotesque, monstrous things it conceived of, sometimes in a frenzy of effort to break away abruptly from all preceding types--but failing, in the giraffe-effort, for instance, or only caricaturing an antelope-- all things break one relation only by the establishing of some other relation-- all things cut an umbilical cord only to clutch a breast. so the fight of the exclusionists to maintain the traditional--or to prevent abrupt transition from the quasi-established--fighting so that here, more than a century after meteorites were included, no other notable inclusion has been made, except that of cosmic dust, data of which nordenskiold made more nearly real than data in opposition. so proctor, for instance, fought and expressed his feeling of the preposterous, against sir w.h. thomson's notions of arrival upon this earth of organisms on meteorites-- "i can only regard it as a jest" (_knowledge_, - ). or that there is nothing but jest--or something intermediate to jest and tragedy: that ours is not an existence but an utterance; that momus is imagining us for the amusement of the gods, often with such success that some of us seem almost alive--like characters in something a novelist is writing; which often to considerable degree take their affairs away from the novelist-- that momus is imagining us and our arts and sciences and religions, and is narrating or picturing us as a satire upon the gods' real existence. because--with many of our data of coal that has fallen from the sky as accessible then as they are now, and with the scientific pronouncement that coal is fossil, how, in a real existence, by which we mean a consistent existence, or a state in which there is real intelligence, or a form of thinking that does not indistinguishably merge away with imbecility, could there have been such a row as that which was raised about forty years ago over dr. hahn's announcement that he had found fossils in meteorites? accessible to anybody at that time: _philosophical magazine_, - - : that the substance that fell at kaba, hungary, april , , contained organic matter "analagous to fossil waxes." or limestone: of the block of limestone which was reported to have fallen at middleburg, florida, it is said (_science_, - ) that, though something had been seen to fall in "an old cultivated field," the witnesses who ran to it picked up something that "had been upon the ground in the first place." the writer who tells us this, with the usual exclusion-imagination known as stupidity, but unjustly, because there is no real stupidity, thinks he can think of a good-sized stone that had for many years been in a cultivated field, but that had never been seen before--had never interfered with plowing, for instance. he is earnest and unjarred when he writes that this stone weighs pounds. my own notion, founded upon my own experience in seeing, is that a block of stone weighing pounds might be in one's parlor twenty years, virtually unseen--but not in an old cultivated field, where it interfered with plowing--not anywhere--if it interfered. dr. hahn said that he had found fossils in meteorites. there is a description of the corals, sponges, shells, and crinoids, all of them microscopic, which he photographed, in _popular science_, - . dr. hahn was a well-known scientist. he was better known after that. anybody may theorize upon other worlds and conditions upon them that are similar to our own conditions: if his notions be presented undisguisedly as fiction, or only as an "interesting hypothesis," he'll stir up no prude rages. but dr. hahn said definitely that he had found fossils in specified meteorites: also he published photographs of them. his book is in the new york public library. in the reproductions every feature of some of the little shells is plainly marked. if they're not shells, neither are things under an oyster-counter. the striations are very plain: one sees even the hinges where bivalves are joined. prof. lawrence smith (_knowledge_, - ): "dr. hahn is a kind of half-insane man, whose imagination has run away with him." conservation of continuity. then dr. weinland examined dr. hahn's specimens. he gave his opinion that they are fossils and that they are not crystals of enstatite, as asserted by prof. smith, who had never seen them. the damnation of denial and the damnation of disregard: after the publication of dr. weinland's findings--silence. the living things that have come down to this earth: attempts to preserve the system: that small frogs and toads, for instance, never have fallen from the sky, but were--"on the ground, in the first place"; or that there have been such falls--"up from one place in a whirlwind, and down in another." were there some especially froggy place near europe, as there is an especially sandy place, the scientific explanation would of course be that all small frogs falling from the sky in europe come from that center of frogeity. to start with, i'd like to emphasize something that i am permitted to see because i am still primitive or intelligent or in a state of maladjustment: that there is not one report findable of a fall of tadpoles from the sky. as to "there in the first place": see _leisure hours_, - , for accounts of small frogs, or toads, said to have been seen to fall from the sky. the writer says that all observers were mistaken: that the frogs or toads must have fallen from trees or other places overhead. tremendous number of little toads, one or two months old, that were seen to fall from a great thick cloud that appeared suddenly in a sky that had been cloudless, august, , near toulouse, france, according to a letter from prof. pontus to m. arago. (_comptes rendus_, - .) many instances of frogs that were seen to fall from the sky. (_notes and queries_, - - ); accounts of such falls, signed by witnesses. (_notes and queries_, - - .) _scientific american_, july , : "a shower of frogs which darkened the air and covered the ground for a long distance is the reported result of a recent rainstorm at kansas city, mo." as to having been there "in the first place": little frogs found in london, after a heavy storm, july , . (_notes and queries_, - - ); little toads found in a desert, after a rainfall (_notes and queries_, - - ). to start with i do not deny--positively--the conventional explanation of "up and down." i think that there may have been such occurrences. i omit many notes that i have upon indistinguishables. in the london _times_, july , , there is an account of a shower of twigs and leaves and tiny toads in a storm upon the slopes of the apennines. these may have been the ejectamenta of a whirlwind. i add, however, that i have notes upon two other falls of tiny toads, in , one in france and one in tahiti; also of fish in scotland. but in the phenomenon of the apennines, the mixture seems to me to be typical of the products of a whirlwind. the other instances seem to me to be typical of--something like migration? their great numbers and their homogeneity. over and over in these annals of the damned occurs the datum of segregation. but a whirlwind is thought of as a condition of chaos--quasi-chaos: not final negativeness, of course-- _monthly weather review_, july, : "a small pond in the track of the cloud was sucked dry, the water being carried over the adjoining fields together with a large quantity of soft mud, which was scattered over the ground for half a mile around." it is so easy to say that small frogs that have fallen from the sky had been scooped up by a whirlwind; but here are the circumstances of a scoop; in the exclusionist-imagination there is no regard for mud, débris from the bottom of a pond, floating vegetation, loose things from the shores--but a precise picking out of frogs only. of all instances i have that attribute the fall of small frogs or toads to whirlwinds, only one definitely identifies or places the whirlwind. also, as has been said before, a pond going up would be quite as interesting as frogs coming down. whirlwinds we read of over and over--but where and what whirlwind? it seems to me that anybody who had lost a pond would be heard from. in _symons' meteorological magazine_, - , a fall of small frogs, near birmingham, england, june , , is attributed to a specific whirlwind--but not a word as to any special pond that had contributed. and something that strikes my attention here is that these frogs are described as almost white. i'm afraid there is no escape for us: we shall have to give to civilization upon this earth--some new worlds. places with white frogs in them. upon several occasions we have had data of unknown things that have fallen from--somewhere. but something not to be overlooked is that if living things have landed alive upon this earth--in spite of all we think we know of the accelerative velocity of falling bodies--and have propagated--why the exotic becomes the indigenous, or from the strangest of places we'd expect the familiar. or if hosts of living frogs have come here--from somewhere else--every living thing upon this earth may, ancestrally, have come from--somewhere else. i find that i have another note upon a specific hurricane: _annals and mag. of nat. hist._, - - : after one of the greatest hurricanes in the history of ireland, some fish were found "as far as yards from the edge of a lake." have another: this is a good one for the exclusionists: fall of fish in paris: said that a neighboring pond had been blown dry. (_living age_, - .) date not given, but i have seen it recorded somewhere else. the best-known fall of fishes from the sky is that which occurred at mountain ash, in the valley of abedare, glamorganshire, feb. , . the editor of the _zoologist_, - , having published a report of a fall of fishes, writes: "i am continually receiving similar accounts of frogs and fishes." but, in all the volumes of the _zoologist_, i can find only two reports of such falls. there is nothing to conclude other than that hosts of data have been lost because orthodoxy does not look favorably upon such reports. the _monthly weather review_ records several falls of fishes in the united states; but accounts of these reported occurrences are not findable in other american publications. nevertheless, the treatment by the _zoologist_ of the fall reported from mountain ash is fair. first appears, in the issue of - , a letter from the rev. john griffith, vicar of abedare, asserting that the fall had occurred, chiefly upon the property of mr. nixon, of mountain ash. upon page , dr. gray, of the british museum, bristling with exclusionism, writes that some of these fishes, which had been sent to him alive, were "very young minnows." he says: "on reading the evidence, it seems to me most probably only a practical joke: that one of mr. nixon's employees had thrown a pailful of water upon another, who had thought fish in it had fallen from the sky"--had dipped up a pailful from a brook. those fishes--still alive--were exhibited at the zoological gardens, regent's park. the editor says that one was a minnow and that the rest were sticklebacks. he says that dr. gray's explanation is no doubt right. but, upon page , he publishes a letter from another correspondent, who apologizes for opposing "so high an authority as dr. gray," but says that he had obtained some of these fishes from persons who lived at a considerable distance apart, or considerably out of range of the playful pail of water. according to the _annual register_, - , the fishes themselves had fallen by pailfuls. if these fishes were not upon the ground in the first place, we base our objections to the whirlwind explanation upon two data: that they fell in no such distribution as one could attribute to the discharge of a whirlwind, but upon a narrow strip of land: about yards long and yards wide-- the other datum is again the suggestion that at first seemed so incredible, but for which support is piling up, a suggestion of a stationary source overhead-- that ten minutes later another fall of fishes occurred upon this same narrow strip of land. even arguing that a whirlwind may stand still axially, it discharges tangentially. wherever the fishes came from it does not seem thinkable that some could have fallen and that others could have whirled even a tenth of a minute, then falling directly after the first to fall. because of these evil circumstances the best adaptation was to laugh the whole thing off and say that someone had soused someone else with a pailful of water in which a few "very young" minnows had been caught up. in the london _times_, march , , is a letter from mr. aaron roberts, curate of st. peter's, carmathon. in this letter the fishes are said to have been about four inches long, but there is some question of species. i think, myself, that they were minnows and sticklebacks. some persons, thinking them to be sea fishes, placed them in salt water, according to mr. roberts. "the effect is stated to have been almost instantaneous death." "some were placed in fresh water. these seemed to thrive well." as to narrow distribution, we are told that the fishes fell "in and about the premises of mr. nixon." "it was not observed at the time that any fish fell in any other part of the neighborhood, save in the particular spot mentioned." in the london _times_, march , , vicar griffith writes an account: "the roofs of some houses were covered with them." in this letter it is said that the largest fishes were five inches long, and that these did not survive the fall. _report of the british association_, - : "the evidence of the fall of fish on this occasion was very conclusive. a specimen of the fish was exhibited and was found to be the _gasterosteus leirus_." _gasterosteus_ is the stickleback. altogether i think we have not a sense of total perdition, when we're damned with the explanation that someone soused someone else with a pailful of water in which were thousands of fishes four or five inches long, some of which covered roofs of houses, and some of which remained ten minutes in the air. by way of contrast we offer our own acceptance: that the bottom of a super-geographical pond had dropped out. i have a great many notes upon the fall of fishes, despite the difficulty these records have in getting themselves published, but i pick out the instances that especially relate to our super-geographical acceptances, or to the principles of super-geography: or data of things that have been in the air longer than acceptably could a whirlwind carry them; that have fallen with a distribution narrower than is attributable to a whirlwind; that have fallen for a considerable length of time upon the same narrow area of land. these three factors indicate, somewhere not far aloft, a region of inertness to this earth's gravitation, of course, however, a region that, by the flux and variation of all things, must at times be susceptible--but, afterward, our heresy will bifurcate-- in amiable accommodation to the crucifixion it'll get, i think-- but so impressed are we with the datum that, though there have been many reports of small frogs that have fallen from the sky, not one report upon a fall of tadpoles is findable, that to these circumstances another adjustment must be made. apart from our three factors of indication, an extraordinary observation is the fall of living things without injury to them. the devotees of st. isaac explain that they fall upon thick grass and so survive: but sir james emerson tennant, in his _history of ceylon_, tells of a fall of fishes upon gravel, by which they were seemingly uninjured. something else apart from our three main interests is a phenomenon that looks like what one might call an alternating series of falls of fishes, whatever the significance may be: meerut, india, july, (_living age_, - ); fifeshire, scotland, summer of (_wernerian nat. hist. soc. trans._, - ); moradabad, india, july, (_living age_, - ); ross-shire, scotland, (_living age_, - ); moradabad, india, july , (_lin. soc. trans._, - ); perthshire, scotland (_living age_, - ); argyleshire, scotland, , march , (_recreative science_, - ); feridpoor, india, feb. , (_jour. asiatic soc. of bengal_, - ). a psycho-tropism that arises here--disregarding serial significance--or mechanical, unintelligent, repulsive reflex--is that the fishes of india did not fall from the sky; that they were found upon the ground after torrential rains, because streams had overflowed and had then receded. in the region of inertness that we think we can conceive of, or a zone that is to this earth's gravitation very much like the neutral zone of a magnet's attraction, we accept that there are bodies of water and also clear spaces--bottoms of ponds dropping out--very interesting ponds, having no earth at bottom--vast drops of water afloat in what is called space--fishes and deluges of water falling-- but also other areas, in which fishes--however they got there: a matter that we'll consider--remain and dry, or even putrefy, then sometimes falling by atmospheric dislodgment. after a "tremendous deluge of rain, one of the heaviest falls on record" (_all the year round_, - ) at rajkote, india, july , , "the ground was found literally covered with fishes." the word "found" is agreeable to the repulsions of the conventionalists and their concept of an overflowing stream--but, according to dr. buist, some of these fishes were "found" on the tops of haystacks. ferrel (_a popular treatise_, p. ) tells of a fall of living fishes--some of them having been placed in a tank, where they survived--that occurred in india, about miles south of calcutta, sept. , . a witness of this fall says: "the most strange thing which ever struck me was that the fish did not fall helter-skelter, or here and there, but they fell in a straight line, not more than a cubit in breadth." see _living age_, - . _amer. jour. sci._, - - : that, according to testimony taken before a magistrate, a fall occurred, feb. , , near feridpoor, india, of many fishes, of various sizes--some whole and fresh and others "mutilated and putrefying." our reflex to those who would say that, in the climate of india, it would not take long for fishes to putrefy, is--that high in the air, the climate of india is not torrid. another peculiarity of this fall is that some of the fishes were much larger than others. or to those who hold out for segregation in a whirlwind, or that objects, say, twice as heavy as others would be separated from the lighter, we point out that some of these fishes were twice as heavy as others. in the _journal of the asiatic society of bengal_, - , depositions of witnesses are given: "some of the fish were fresh, but others were rotten and without heads." "among the number which i got, five were fresh and the rest stinking and headless." they remind us of his grace's observation of some pages back. according to dr. buist, some of these fishes weighed one and a half pounds each and others three pounds. a fall of fishes at futtepoor, india, may , : "they were all dead and dry." (dr. buist, _living age_, - .) india is far away: about was long ago. _nature_, sept. , - : a correspondent writes, from the dove marine laboratory, cuttercoats, england, that, at hindon, a suburb of sunderland, aug. , , hundreds of small fishes, identified as sand eels, had fallen-- again the small area: about by yards. the fall occurred during a heavy rain that was accompanied by thunder--or indications of disturbance aloft--but by no visible lightning. the sea is close to hindon, but if you try to think of these fishes having described a trajectory in a whirlwind from the ocean, consider this remarkable datum: that, according to witnesses, the fall upon this small area occupied ten minutes. i cannot think of a clearer indication of a direct fall from a stationary source. and: "the fish were all dead, and indeed stiff and hard, when picked up, immediately after the occurrence." by all of which i mean that we have only begun to pile up our data of things that fall from a stationary source overhead: we'll have to take up the subject from many approaches before our acceptance, which seems quite as rigorously arrived at as ever has been a belief, can emerge from the accursed. i don't know how much the horse and the barn will help us to emerge: but, if ever anything did go up from this earth's surface and stay up--those damned things may have: _monthly weather review_, may, : in a tornado, in wisconsin, may , , "a barn and a horse were carried completely away, and neither horse nor barn, nor any portion of either have since been found." after that, which would be a little strong were it not for a steady improvement in our digestions that i note as we go along, there is little of the bizarre or the unassimilable in the turtle that hovered six months or so over a small town in mississippi: _monthly weather review_, may, : that, may , , at vicksburg, miss., fell a small piece of alabaster; that, at bovina, eight miles from vicksburg, fell a gopher turtle. they fell in a hailstorm. this item was widely copied at the time: for instance, _nature_, one of the volumes of , page , and _jour. roy. met. soc._, - . as to discussion--not a word. or science and its continuity with presbyterianism--data like this are damned at birth. the _weather review_ does sprinkle, or baptize, or attempt to save, this infant--but in all the meteorological literature that i have gone through, after that date--not a word, except mention once or twice. the editor of the _review_ says: "an examination of the weather map shows that these hailstorms occur on the south side of a region of cold northerly winds, and were but a small part of a series of similar storms; apparently some special local whirls or gusts carried heavy objects from this earth's surface up to the cloud regions." of all incredibilities that we have to choose from, i give first place to a notion of a whirlwind pouncing upon a region and scrupulously selecting a turtle and a piece of alabaster. this time, the other mechanical thing "there in the first place" cannot rise in response to its stimulus: it is resisted in that these objects were coated with ice--month of may in a southern state. if a whirlwind at all, there must have been very limited selection: there is no record of the fall of other objects. but there is no attempt in the _review_ to specify a whirlwind. these strangely associated things were remarkably separated. they fell eight miles apart. then--as if there were real reasoning--they must have been high to fall with such divergence, or one of them must have been carried partly horizontally eight miles farther than the other. but either supposition argues for power more than that of a local whirl or gust, or argues for a great, specific disturbance, of which there is no record--for the month of may, . nevertheless--as if i really were reasonable--i do feel that i have to accept that this turtle had been raised from this earth's surface, somewhere near vicksburg--because the gopher turtle is common in the southern states. then i think of a hurricane that occurred in the state of mississippi weeks or months before may , . no--i don't look for it--and inevitably find it. or that things can go up so high in hurricanes that they stay up indefinitely--but may, after a while, be shaken down by storms. over and over have we noted the occurrence of strange falls in storms. so then that the turtle and the piece of alabaster may have had far different origins--from different worlds, perhaps--have entered a region of suspension over this earth--wafting near each other--long duration--final precipitation by atmospheric disturbance--with hail--or that hailstones, too, when large, are phenomena of suspension of long duration: that it is highly unacceptable that the very large ones could become so great only in falling from the clouds. over and over has the note of disagreeableness, or of putrefaction, been struck--long duration. other indications of long duration. i think of a region somewhere above this earth's surface in which gravitation is inoperative and is not governed by the square of the distance--quite as magnetism is negligible at a very short distance from a magnet. theoretically the attraction of a magnet should decrease with the square of the distance, but the falling-off is found to be almost abrupt at a short distance. i think that things raised from this earth's surface to that region have been held there until shaken down by storms-- the super-sargasso sea. derelicts, rubbish, old cargoes from inter-planetary wrecks; things cast out into what is called space by convulsions of other planets, things from the times of the alexanders, caesars and napoleons of mars and jupiter and neptune; things raised by this earth's cyclones: horses and barns and elephants and flies and dodoes, moas, and pterodactyls; leaves from modern trees and leaves of the carboniferous era--all, however, tending to disintegrate into homogeneous-looking muds or dusts, red or black or yellow--treasure-troves for the palaeontologists and for the archaeologists--accumulations of centuries--cyclones of egypt, greece, and assyria--fishes dried and hard, there a short time: others there long enough to putrefy-- but the omnipresence of heterogeneity--or living fishes, also--ponds of fresh water: oceans of salt water. as to the law of gravitation, i prefer to take one simple stand: orthodoxy accepts the correlation and equivalence of forces: gravitation is one of these forces. all other forces have phenomena of repulsion and of inertness irrespective of distance, as well as of attraction. but newtonian gravitation admits attraction only: then newtonian gravitation can be only one-third acceptable even to the orthodox, or there is denial of the correlation and equivalence of forces. or still simpler: here are the data. make what you will, yourself, of them. in our intermediatist revolt against homogeneous, or positive, explanations, or our acceptance that the all-sufficing cannot be less than universality, besides which, however, there would be nothing to suffice, our expression upon the super-sargasso sea, though it harmonizes with data of fishes that fall as if from a stationary source--and, of course, with other data, too--is inadequate to account for two peculiarities of the falls of frogs: that never has a fall of tadpoles been reported; that never has a fall of full-grown frogs been reported-- always frogs a few months old. it sounds positive, but if there be such reports they are somewhere out of my range of reading. but tadpoles would be more likely to fall from the sky than would frogs, little or big, if such falls be attributed to whirlwinds; and more likely to fall from the super-sargasso sea if, though very tentatively and provisionally, we accept the super-sargasso sea. before we take up an especial expression upon the fall of immature and larval forms of life to this earth, and the necessity then of conceiving of some factor besides mere stationariness or suspension or stagnation, there are other data that are similar to data of falls of fishes. _science gossip_, - : that small snails, of a land species, had fallen near redruth, cornwall, july , , "during a heavy thunderstorm": roads and fields strewn with them, so that they were gathered up by the hatful: none seen to fall by the writer of this account: snails said to be "quite different to any previously known in this district." but, upon page , we have better orthodoxy. another correspondent writes that he had heard of the supposed fall of snails: that he had supposed that all such stories had gone the way of witch stories; that, to his astonishment, he had read an account of this absurd story in a local newspaper of "great and deserved repute." "i thought i should for once like to trace the origin of one of these fabulous tales." our own acceptance is that justice cannot be in an intermediate existence, in which there can be approximation only to justice or to injustice; that to be fair is to have no opinion at all; that to be honest is to be uninterested; that to investigate is to admit prejudice; that nobody has ever really investigated anything, but has always sought positively to prove or to disprove something that was conceived of, or suspected, in advance. "as i suspected," says this correspondent, "i found that the snails were of a familiar land-species"--that they had been upon the ground "in the first place." he found that the snails had appeared after the rain: that "astonished rustics had jumped to the conclusion that they had fallen." he met one person who said that he had seen the snails fall. "this was his error," says the investigator. in the _philosophical magazine_, - , there is an account of snails said to have fallen at bristol in a field of three acres, in such quantities that they were shoveled up. it is said that the snails "may be considered as a local species." upon page , another correspondent says that the numbers had been exaggerated, and that in his opinion they had been upon the ground in the first place. but that there had been some unusual condition aloft comes out in his observation upon "the curious azure-blue appearance of the sun, at the time." _nature_, - : that, according to _das wetter_, december, , upon aug. , , a yellow cloud appeared over paderborn, germany. from this cloud, fell a torrential rain, in which were hundreds of mussels. there is no mention of whatever may have been upon the ground in the first place, nor of a whirlwind. lizards--said to have fallen on the sidewalks of montreal, canada, dec. , . (_notes and queries_, - - .) in the _scientific american_, - , a correspondent writes, from south granville, n.y., that, during a heavy shower, july , , he heard a peculiar sound at his feet, and looking down, saw a snake lying as if stunned by a fall. it then came to life. gray snake, about a foot long. these data have any meaning or lack of meaning or degree of damnation you please: but, in the matter of the fall that occurred at memphis, tennessee, occur some strong significances. our quasi-reasoning upon this subject applies to all segregations so far considered. _monthly weather review_, jan. , : that, in memphis, tenn., jan. , , rather strictly localized, or "in a space of two blocks," and after a violent storm in which the rain "fell in torrents," snakes were found. they were crawling on sidewalks, in yards, and in streets, and in masses--but "none were found on roofs or any other elevation above ground" and "none were seen to fall." if you prefer to believe that the snakes had always been there, or had been upon the ground in the first place, and that it was only that something occurred to call special attention to them, in the streets of memphis, jan. , --why, that's sensible: that's the common sense that has been against us from the first. it is not said whether the snakes were of a known species or not, but that "when first seen, they were of a dark brown, almost black." blacksnakes, i suppose. if we accept that these snakes did fall, even though not seen to fall by all the persons who were out sight-seeing in a violent storm, and had not been in the streets crawling loose or in thick tangled masses, in the first place: if we try to accept that these snakes had been raised from some other part of this earth's surface in a whirlwind: if we try to accept that a whirlwind could segregate them-- we accept the segregation of other objects raised in that whirlwind. then, near the place of origin, there would have been a fall of heavier objects that had been snatched up with the snakes--stones, fence rails, limbs of trees. say that the snakes occupied the next gradation, and would be the next to fall. still farther would there have been separate falls of lightest objects: leaves, twigs, tufts of grass. in the _monthly weather review_ there is no mention of other falls said to have occurred anywhere in january, . again ours is the objection against such selectiveness by a whirlwind. conceivably a whirlwind could scoop out a den of hibernating snakes, with stones and earth and an infinitude of other débris, snatching up dozens of snakes--i don't know how many to a den--hundreds maybe--but, according to the account of this occurrence in the _new york times_, there were thousands of them; alive; from one foot to eighteen inches in length. the _scientific american_, - , records the fall, and says that there were thousands of them. the usual whirlwind-explanation is given--"but in what locality snakes exist in such abundance is yet a mystery." this matter of enormousness of numbers suggests to me something of a migratory nature--but that snakes in the united states do not migrate in the month of january, if ever. as to falls or flutterings of winged insects from the sky, prevailing notions of swarming would seem explanatory enough: nevertheless, in instances of ants, there are some peculiar circumstances. _l'astronomie_, - : fall of fishes, june , , in holland; ants, aug. , , strasbourg; little toads, aug. , , savoy. fall of ants, cambridge, england, summer of --"some were wingless." (_scientific american_, - .) enormous fall of ants, nancy, france, july , --"most of them were wingless." (_nature_, - .) fall of enormous, unknown ants--size of wasps--manitoba, june, . (_sci. amer._, - .) however, our expression will be: that wingless, larval forms of life, in numbers so enormous that migration from some place external to this earth is suggested, have fallen from the sky. that these "migrations"--if such can be our acceptance--have occurred at a time of hibernation and burial far in the ground of larvae in the northern latitudes of this earth; that there is significance in recurrence of these falls in the last of january--or that we have the square of an incredibility in such a notion as that of selection of larvae by whirlwinds, compounded with selection of the last of january. i accept that there are "snow worms" upon this earth--whatever their origin may have been. in the _proc. acad. nat. sci. of philadelphia_, - , there is a description of yellow worms and black worms that have been found together on glaciers in alaska. almost positively were there no other forms of insect-life upon these glaciers, and there was no vegetation to support insect-life, except microscopic organisms. nevertheless the description of this probably polymorphic species fits a description of larvae said to have fallen in switzerland, and less definitely fits another description. there is no opposition here, if our data of falls are clear. frogs of every-day ponds look like frogs said to have fallen from the sky--except the whitish frogs of birmingham. however, all falls of larvae have not positively occurred in the last of january: london _times_, april , : that, in the parish of bramford speke, devonshire, a large number of black worms, about three-quarters of an inch in length, had fallen in a snowstorm. in timb's _year book_, - , it is said that, in the winter of , at christiania, norway, worms were found crawling upon the ground. the occurrence is considered a great mystery, because the worms could not have come up from the ground, inasmuch as the ground was frozen at the time, and because they were reported from other places, also, in norway. immense number of black insects in a snowstorm, in , at pakroff, russia. (_scientific american_, - .) fall, with snow, at orenburg, russia, dec. , , of a multitude of small, black insects, said to have been gnats, but also said to have had flea-like motions. (_amer. jour. sci._, - - .) large number of worms found in a snowstorm, upon the surface of snow about four inches thick, near sangerfield, n.y., nov. , (_scientific american_, - ). the writer thinks that the worms had been brought to the surface of the ground by rain, which had fallen previously. _scientific american_, feb. , : "a puzzling phenomenon has been noted frequently in some parts of the valley bend district, randolph county, va., this winter. the crust of the snow has been covered two or three times with worms resembling the ordinary cut worms. where they come from, unless they fall with the snow is inexplicable." in the _scientific american_, march , , the editor says that similar worms had been seen upon the snow near utica, n.y., and in oneida and herkimer counties; that some of the worms had been sent to the department of agriculture at washington. again two species, or polymorphism. according to prof. riley, it was not polymorphism, "but two distinct species"--which, because of our data, we doubt. one kind was larger than the other: color-differences not distinctly stated. one is called the larvae of the common soldier beetle and the other "seems to be a variety of the bronze cut worm." no attempt to explain the occurrence in snow. fall of great numbers of larvae of beetles, near mortagne, france, may, . the larvae were inanimate as if with cold. (_annales société entomologique de france_, .) _trans. ent. soc. of london_, - , records "snowing of larvae," in silesia, ; "appearance of many larvae on the snow," in saxony, ; "larvae found alive on the snow," ; larvae and snow which "fell together," in the eifel, jan. , ; "fall of insects," jan. , , in lithuania; occurrence of larvae estimated at , on the snow in switzerland, in . the compiler says that most of these larvae live underground, or at the roots of trees; that whirlwinds uproot trees, and carry away the larvae--conceiving of them as not held in masses of frozen earth--all as neatly detachable as currants in something. in the _revue et magasin de zoologie_, - , there is an account of the fall in lithuania, jan. , --that black larvae had fallen in enormous numbers. larvae thought to have been of beetles, but described as "caterpillars," not seen to fall, but found crawling on the snow, after a snowstorm, at warsaw, jan. , . (_all the year round_, - .) flammarion (_the atmosphere_, p. ) tells of a fall of larvae that occurred jan. , , in a snowstorm, in upper savoy: "they could not have been hatched in the neighborhood, for, during the days preceding, the temperature had been very low"; said to have been of a species common in the south of france. in _la science pour tous_, - , it is said that with these larvae there were developed insects. _l'astronomie_, - : that, upon the last of january, , there fell, in a great tempest, in switzerland, incalculable numbers of larvae: some black and some yellow; numbers so great that hosts of birds were attracted. altogether we regard this as one of our neatest expressions for external origins and against the whirlwind explanation. if an exclusionist says that, in january, larvae were precisely and painstakingly picked out of frozen ground, in incalculable numbers, he thinks of a tremendous force--disregarding its refinements: then if origin and precipitation be not far apart, what becomes of an infinitude of other débris, conceiving of no time for segregation? if he thinks of a long translation--all the way from the south of france to upper savoy, he may think then of a very fine sorting over by differences of specific gravity--but in such a fine selection, larvae would be separated from developed insects. as to differences in specific gravity--the yellow larvae that fell in switzerland january, , were three times the size of the black larvae that fell with them. in accounts of this occurrence, there is no denial of the fall. or that a whirlwind never brought them together and held them together and precipitated them and only them together-- that they came from genesistrine. there's no escape from it. we'll be persecuted for it. take it or leave it-- genesistrine. the notion is that there is somewhere aloft a place of origin of life relatively to this earth. whether it's the planet genesistrine, or the moon, or a vast amorphous region super-jacent to this earth, or an island in the super-sargasso sea, should perhaps be left to the researches of other super--or extra--geographers. that the first unicellular organisms may have come here from genesistrine--or that men or anthropomorphic beings may have come here before amoebae: that, upon genesistrine, there may have been an evolution expressible in conventional biologic terms, but that evolution upon this earth has been--like evolution in modern japan--induced by external influences; that evolution, as a whole, upon this earth, has been a process of population by immigration or by bombardment. some notes i have upon remains of men and animals encysted, or covered with clay or stone, as if fired here as projectiles, i omit now, because it seems best to regard the whole phenomenon as a tropism--as a geotropism--probably atavistic, or vestigial, as it were, or something still continuing long after expiration of necessity; that, once upon a time, all kinds of things came here from genesistrine, but that now only a few kinds of bugs and things, at long intervals, feel the inspiration. not one instance have we of tadpoles that have fallen to this earth. it seems reasonable that a whirlwind could scoop up a pond, frogs and all, and cast down the frogs somewhere else: but, then, more reasonable that a whirlwind could scoop up a pond, tadpoles and all--because tadpoles are more numerous in their season than are the frogs in theirs: but the tadpole-season is earlier in the spring, or in a time that is more tempestuous. thinking in terms of causation--as if there were real causes--our notion is that, if x is likely to cause y, but is more likely to cause z, but does not cause z, x is not the cause of y. upon this quasi-sorites, we base our acceptance that the little frogs that have fallen to this earth are not products of whirlwinds: that they came from externality, or from genesistrine. i think of genesistrine in terms of biologic mechanics: not that somewhere there are persons who collect bugs in or about the last of january and frogs in july and august, and bombard this earth, any more than do persons go through northern regions, catching and collecting birds, every autumn, then casting them southward. but atavistic, or vestigial, geotropism in genesistrine--or a million larvae start crawling, and a million little frogs start hopping--knowing no more what it's all about than we do when we crawl to work in the morning and hop away at night. i should say, myself, that genesistrine is a region in the super-sargasso sea, and that parts of the super-sargasso sea have rhythms of susceptibility to this earth's attraction. i accept that, when there are storms, the damnedest of excluded, excommunicated things--things that are leprous to the faithful--are brought down--from the super-sargasso sea--or from what for convenience we call the super-sargasso sea--which by no means has been taken into full acceptance yet. that things are brought down by storms, just as, from the depths of the sea things are brought up by storms. to be sure it is orthodoxy that storms have little, if any, effect below the waves of the ocean--but--of course--only to have an opinion is to be ignorant of, or to disregard a contradiction, or something else that modifies an opinion out of distinguishability. _symons' meteorological magazine_, - : that, along the coast of new zealand, in regions not subject to submarine volcanic action, deep-sea fishes are often brought up by storms. iron and stones that fall from the sky; and atmospheric disturbances: "there is absolutely no connection between the two phenomena." (_symons._) the orthodox belief is that objects moving at planetary velocity would, upon entering this earth's atmosphere, be virtually unaffected by hurricanes; might as well think of a bullet swerved by someone fanning himself. the only trouble with the orthodox reasoning is the usual trouble--its phantom-dominant--its basing upon a myth--data we've had, and more we'll have, of things in the sky having no independent velocity. there are so many storms and so many meteors and meteorites that it would be extraordinary if there were no concurrences. nevertheless so many of these concurrences are listed by prof. baden-powell (_rept. brit. assoc._, - ) that one--notices. see _rept. brit. assoc._, --other instances. the famous fall of stones at siena, italy, --"in a violent storm." see _greg's catalogues_--many instances. one that stands out is--"bright ball of fire and light in a hurricane in england, sept. , ." the remarkable datum here is that this phenomenon was visible forty minutes. that's about times the duration that the orthodox give to meteors and meteorites. see the _annual register_--many instances. in _nature_, oct. , , and the london _times_, oct. , , something that fell in a gale of oct. , , is described as a "huge ball of green fire." this phenomenon is described by another correspondent, in _nature_, - , and an account of it by another correspondent was forwarded to _nature_ by w.f. denning. there are so many instances that some of us will revolt against the insistence of the faithful that it is only coincidence, and accept that there is connection of the kind called causal. if it is too difficult to think of stones and metallic masses swerved from their courses by storms, if they move at high velocity, we think of low velocity, or of things having no velocity at all, hovering a few miles above this earth, dislodged by storms, and falling luminously. but the resistance is so great here, and "coincidence" so insisted upon that we'd better have some more instances: aerolite in a storm at st. leonards-on-sea, england, sept. , --no trace of it found (_annual register_, ); meteorite in a gale, march , , described in the _monthly weather review_, march, ; meteorite in a thunderstorm, off coast of greece, nov. , (_nature_, - ); fall of a meteorite in a storm, july , , near lachine, quebec (_monthly weather review_, july, ); same phenomenon noted in _nature_, - ; meteorite in a whirlwind, sweden, sept. , (_nature_, - ). _london roy. soc. proc._, - : a triangular cloud that appeared in a storm, dec. , ; a red nucleus, about half the apparent diameter of the moon, and a long tail; visible minutes; explosion of the nucleus. nevertheless, in _science gossip_, n.s., - , it is said that, though meteorites have fallen in storms, no connection is supposed to exist between the two phenomena, except by the ignorant peasantry. but some of us peasants have gone through the _report of the british association_, . upon page , dr. buist, who had never heard of the super-sargasso sea, says that, though it is difficult to trace connection between the phenomena, three aerolites had fallen in five months, in india, during thunderstorms, in (may have been ). for accounts by witnesses, see page of the _report_. or--we are on our way to account for "thunderstones." it seems to me that, very strikingly here, is borne out the general acceptance that ours is only an intermediate existence, in which there is nothing fundamental, or nothing final to take as a positive standard to judge by. peasants believed in meteorites. scientists excluded meteorites. peasants believe in "thunderstones." scientists exclude "thunderstones." it is useless to argue that peasants are out in the fields, and that scientists are shut up in laboratories and lecture rooms. we cannot take for a real base that, as to phenomena with which they are more familiar, peasants are more likely to be right than are scientists: a host of biologic and meteorologic fallacies of peasants rises against us. i should say that our "existence" is like a bridge--except that that comparison is in static terms--but like the brooklyn bridge, upon which multitudes of bugs are seeking a fundamental--coming to a girder that seems firm and final--but the girder is built upon supports. a support then seems final. but it is built upon underlying structures. nothing final can be found in all the bridge, because the bridge itself is not a final thing in itself, but is a relationship between manhattan and brooklyn. if our "existence" is a relationship between the positive absolute and the negative absolute, the quest for finality in it is hopeless: everything in it must be relative, if the "whole" is not a whole, but is, itself, a relation. in the attitude of acceptance, our pseudo-base is: cells of an embryo are in the reptilian era of the embryo; some cells feel stimuli to take on new appearances. if it be of the design of the whole that the next era be mammalian, those cells that turn mammalian will be sustained against resistance, by inertia, of all the rest, and will be relatively right, though not finally right, because they, too, in time will have to give way to characters of other eras of higher development. if we are upon the verge of a new era, in which exclusionism must be overthrown, it will avail thee not to call us base-born and frowsy peasants. in our crude, bucolic way, we now offer an outrage upon common sense that we think will some day be an unquestioned commonplace: that manufactured objects of stone and iron have fallen from the sky: that they have been brought down from a state of suspension, in a region of inertness to this earth's attraction, by atmospheric disturbances. the "thunderstone" is usually "a beautifully polished, wedge-shaped piece of greenstone," says a writer in the _cornhill magazine_, - . it isn't: it's likely to be of almost any kind of stone, but we call attention to the skill with which some of them have been made. of course this writer says it's all superstition. otherwise he'd be one of us crude and simple sons of the soil. conventional damnation is that stone implements, already on the ground--"on the ground in the first place"--are found near where lightning was seen to strike: that are supposed by astonished rustics, or by intelligence of a low order, to have fallen in or with lightning. throughout this book, we class a great deal of science with bad fiction. when is fiction bad, cheap, low? if coincidence is overworked. that's one way of deciding. but with single writers coincidence seldom is overworked: we find the excess in the subject at large. such a writer as the one of the _cornhill magazine_ tells us vaguely of beliefs of peasants: there is no massing of instance after instance after instance. here ours will be the method of mass-formation. conceivably lightning may strike the ground near where there was a wedge-shaped object in the first place: again and again and again: lightning striking ground near wedge-shaped object in china; lightning striking ground near wedge-shaped object in scotland; lightning striking ground near wedge-shaped object in central africa: coincidence in france; coincidence in java; coincidence in south america-- we grant a great deal but note a tendency to restlessness. nevertheless this is the psycho-tropism of science to all "thunderstones" said to have fallen luminously. as to greenstone, it is in the island of jamaica, where the notion is general that axes of a hard greenstone fall from the sky--"during the rains." (_jour. inst. jamaica_, - .) some other time we shall inquire into this localization of objects of a specific material. "they are of a stone nowhere else to be found in jamaica." (_notes and queries_, - - .) in my own tendency to exclude, or in the attitude of one peasant or savage who thinks he is not to be classed with other peasants or savages, i am not very much impressed with what natives think. it would be hard to tell why. if the word of a lord kelvin carries no more weight, upon scientific subjects, than the word of a sitting bull, unless it be in agreement with conventional opinion--i think it must be because savages have bad table manners. however, my snobbishness, in this respect, loosens up somewhat before very widespread belief by savages and peasants. and the notion of "thunderstones" is as wide as geography itself. the natives of burma, china, japan, according to blinkenberg (_thunder weapons_, p. )--not, of course, that blinkenberg accepts one word of it--think that carved stone objects have fallen from the sky, because they think they have seen such objects fall from the sky. such objects are called "thunderbolts" in these countries. they are called "thunderstones" in moravia, holland, belgium, france, cambodia, sumatra, and siberia. they're called "storm stones" in lausitz; "sky arrows" in slavonia; "thunder axes" in england and scotland; "lightning stones" in spain and portugal; "sky axes" in greece; "lightning flashes" in brazil; "thunder teeth" in amboina. the belief is as widespread as is belief in ghosts and witches, which only the superstitious deny today. as to beliefs by north american indians, tyler gives a list of references (_primitive culture_, - ). as to south american indians--"certain stone hatchets are said to have fallen from the heavens." (_jour. amer. folk lore_, - .) if you, too, revolt against coincidence after coincidence after coincidence, but find our interpretation of "thunderstones" just a little too strong or rich for digestion, we recommend the explanation of one, tallius, written in : "the naturalists say they are generated in the sky by fulgurous exhalation conglobed in a cloud by the circumfused humor." of course the paper in the _cornhill magazine_ was written with no intention of trying really to investigate this subject, but to deride the notion that worked-stone objects have ever fallen from the sky. a writer in the _amer. jour. sci._, - - , read this paper and thinks it remarkable "that any man of ordinary reasoning powers should write a paper to prove that thunderbolts do not exist." i confess that we're a little flattered by that. over and over: "it is scarcely necessary to suggest to the intelligent reader that thunderstones are a myth." we contend that there is a misuse of a word here: we admit that only we are intelligent upon this subject, if by intelligence is meant the inquiry of inequilibrium, and that all other intellection is only mechanical reflex--of course that intelligence, too, is mechanical, but less orderly and confined: less obviously mechanical--that as an acceptance of ours becomes firmer and firmer-established, we pass from the state of intelligence to reflexes in ruts. an odd thing is that intelligence is usually supposed to be creditable. it may be in the sense that it is mental activity trying to find out, but it is confession of ignorance. the bees, the theologians, the dogmatic scientists are the intellectual aristocrats. the rest of us are plebeians, not yet graduated to nirvana, or to the instinctive and suave as differentiated from the intelligent and crude. blinkenberg gives many instances of the superstition of "thunderstones" which flourishes only where mentality is in a lamentable state--or universally. in malacca, sumatra, and java, natives say that stone axes have often been found under trees that have been struck by lightning. blinkenberg does not dispute this, but says it is coincidence: that the axes were of course upon the ground in the first place: that the natives jumped to the conclusion that these carved stones had fallen in or with lightning. in central africa, it is said that often have wedge-shaped, highly polished objects of stone, described as "axes," been found sticking in trees that have been struck by lightning--or by what seemed to be lightning. the natives, rather like the unscientific persons of memphis, tenn., when they saw snakes after a storm, jumped to the conclusion that the "axes" had not always been sticking in the trees. livingstone (_last journal_, pages , , , ) says that he had never heard of stone implements used by natives of africa. a writer in the _report of the smithsonian institution_, - , says that there are a few. that they are said, by the natives, to have fallen in thunderstorms. as to luminosity, it is my lamentable acceptance that bodies falling through this earth's atmosphere, if not warmed even, often fall with a brilliant light, looking like flashes of lightning. this matter seems important: we'll take it up later, with data. in prussia, two stone axes were found in the trunks of trees, one under the bark. (blinkenberg, _thunder weapons_, p. .) the finders jumped to the conclusion that the axes had fallen there. another stone ax--or wedge-shaped object of worked stone--said to have been found in a tree that had been struck by something that looked like lightning. (_thunder weapons_, p. .) the finder jumped to the conclusion. story told by blinkenberg, of a woman, who lived near kulsbjaergene, sweden, who found a flint near an old willow--"near her house." i emphasize "near her house" because that means familiar ground. the willow had been split by something. she jumped. cow killed by lightning, or by what looked like lightning (isle of sark, near guernsey). the peasant who owned the cow dug up the ground at the spot and found a small greenstone "ax." blinkenberg says that he jumped to the conclusion that it was this object that had fallen luminously, killing the cow. _reliquary_, - : a flint ax found by a farmer, after a severe storm--described as a "fearful storm"--by a signal staff, which had been split by something. i should say that nearness to a signal staff may be considered familiar ground. whether he jumped, or arrived at the conclusion by a more leisurely process, the farmer thought that the flint object had fallen in the storm. in this instance we have a lamentable scientist with us. it's impossible to have positive difference between orthodoxy and heresy: somewhere there must be a merging into each other, or an overlapping. nevertheless, upon such a subject as this, it does seem a little shocking. in most works upon meteorites, the peculiar, sulphurous odor of things that fall from the sky is mentioned. sir john evans (_stone implements_, p. ) says--with extraordinary reasoning powers, if he could never have thought such a thing with ordinary reasoning powers--that this flint object "proved to have been the bolt, by its peculiar smell when broken." if it did so prove to be, that settles the whole subject. if we prove that only one object of worked stone has fallen from the sky, all piling up of further reports is unnecessary. however, we have already taken the stand that nothing settles anything; that the disputes of ancient greece are no nearer solution now than they were several thousand years ago--all because, in a positive sense, there is nothing to prove or solve or settle. our object is to be more nearly real than our opponents. wideness is an aspect of the universal. we go on widely. according to us the fat man is nearer godliness than is the thin man. eat, drink, and approximate to the positive absolute. beware of negativeness, by which we mean indigestion. the vast majority of "thunderstones" are described as "axes," but meunier (_la nature_, - - ) tells of one that was in his possession; said to have fallen at ghardia, algeria, contrasting "profoundment" (pear-shaped) with the angular outlines of ordinary meteorites. the conventional explanation that it had been formed as a drop of molten matter from a larger body seems reasonable to me; but with less agreeableness i note its fall in a thunderstorm, the datum that turns the orthodox meteorologist pale with rage, or induces a slight elevation of his eyebrows, if you mention it to him. meunier tells of another "thunderstone" said to have fallen in north africa. meunier, too, is a little lamentable here: he quotes a soldier of experience that such objects fall most frequently in the deserts of africa. rather miscellaneous now: "thunderstone" said to have fallen in london, april, : weight about pounds: no particulars as to shape (timb's _year book_, - ). "thunderstone" said to have fallen at cardiff, sept. , (london _times_, sept. , ). according to _nature_, - , it was coincidence; only a lightning flash had been seen. stone that fell in a storm, near st. albans, england: accepted by the museum of st. albans; said, at the british museum, not to be of "true meteoritic material." (_nature_, - .) london _times_, april , : that, april , , near wolverhampton, fell a mass of meteoritic iron during a heavy fall of rain. an account of this phenomenon in _nature_, - , by h.s. maskelyne, who accepts it as authentic. also, see _nature_, - . for three other instances, see the _scientific american_, - ; - ; - . as to wedge-shape larger than could very well be called an "ax": _nature_, - : that, may , , at tysnas, norway, a meteorite had fallen: that the turf was torn up at the spot where the object had been supposed to have fallen; that two days later "a very peculiar stone" was found near by. the description is--"in shape and size very like the fourth part of a large stilton cheese." it is our acceptance that many objects and different substances have been brought down by atmospheric disturbance from what--only as a matter of convenience now, and until we have more data--we call the super-sargasso sea; however, our chief interest is in objects that have been shaped by means similar to human handicraft. description of the "thunderstones" of burma (_proc. asiatic soc. of bengal_, - ): said to be of a kind of stone unlike any other found in burma; called "thunderbolts" by the natives. i think there's a good deal of meaning in such expressions as "unlike any other found in burma"--but that if they had said anything more definite, there would have been unpleasant consequences to writers in the th century. more about the "thunderstones" of burma, in the _proc. soc. antiq. of london_, - - . one of them, described as an "adze," was exhibited by captain duff, who wrote that there was no stone like it in its neighborhood. of course it may not be very convincing to say that because a stone is unlike neighboring stones it had foreign origin--also we fear it is a kind of plagiarism: we got it from the geologists, who demonstrate by this reasoning the foreign origin of erratics. we fear we're a little gross and scientific at times. but it's my acceptance that a great deal of scientific literature must be read between the lines. it's not everyone who has the lamentableness of a sir john evans. just as a great deal of voltaire's meaning was inter-linear, we suspect that a captain duff merely hints rather than to risk having a prof. lawrence smith fly at him and call him "a half-insane man." whatever captain duff's meaning may have been, and whether he smiled like a voltaire when he wrote it, captain duff writes of "the extremely soft nature of the stone, rendering it equally useless as an offensive or defensive weapon." story, by a correspondent, in _nature_, - , of a malay, of "considerable social standing"--and one thing about our data is that, damned though they be, they do so often bring us into awful good company--who knew of a tree that had been struck, about a month before, by something in a thunderstorm. he searched among the roots of this tree and found a "thunderstone." not said whether he jumped or leaped to the conclusion that it had fallen: process likely to be more leisurely in tropical countries. also i'm afraid his way of reasoning was not very original: just so were fragments of the bath-furnace meteorite, accepted by orthodoxy, discovered. we shall now have an unusual experience. we shall read of some reports of extraordinary circumstances that were investigated by a man of science--not of course that they were really investigated by him, but that his phenomena occupied a position approximating higher to real investigation than to utter neglect. over and over we read of extraordinary occurrences--no discussion; not even a comment afterward findable; mere mention occasionally--burial and damnation. the extraordinary and how quickly it is hidden away. burial and damnation, or the obscurity of the conspicuous. we did read of a man who, in the matter of snails, did travel some distance to assure himself of something that he had suspected in advance; and we remember prof. hitchcock, who had only to smite amherst with the wand of his botanical knowledge, and lo! two fungi sprang up before night; and we did read of dr. gray and his thousands of fishes from one pailful of water--but these instances stand out; more frequently there was no "investigation." we now have a good many reported occurrences that were "investigated." of things said to have fallen from the sky, we make, in the usual scientific way, two divisions: miscellaneous objects and substances, and symmetric objects attributable to beings like human beings, sub-dividing into--wedges, spheres, and disks. _jour. roy. met. soc._, - : that, july , , a correspondent to a london newspaper wrote that something had fallen from the sky, during a thunderstorm of june , , at netting hill. mr. g.t. symons, of _symons' meteorological magazine_, investigated, about as fairly, and with about as unprejudiced a mind, as anything ever has been investigated. he says that the object was nothing but a lump of coal: that next door to the home of the correspondent coal had been unloaded the day before. with the uncanny wisdom of the stranger upon unfamiliar ground that we have noted before, mr. symons saw that the coal reported to have fallen from the sky, and the coal unloaded more prosaically the day before, were identical. persons in the neighborhood, unable to make this simple identification, had bought from the correspondent pieces of the object reported to have fallen from the sky. as to credulity, i know of no limits for it--but when it comes to paying out money for credulity--oh, no standards to judge by, of course--just the same-- the trouble with efficiency is that it will merge away into excess. with what seems to me to be super-abundance of convincingness, mr. symons then lugs another character into his little comedy: that it was all a hoax by a chemist's pupil, who had filled a capsule with an explosive, and "during the storm had thrown the burning mass into the gutter, so making an artificial thunderbolt." or even shakespeare, with all his inartistry, did not lug in king lear to make hamlet complete. whether i'm lugging in something that has no special meaning, myself, or not, i find that this storm of june , , was peculiar. it is described in the london _times_, july , : that "during the storm, the sky in many places remained partially clear while hail and rain were falling." that may have more meaning when we take up the possible extra-mundane origin of some hailstones, especially if they fall from a cloudless sky. mere suggestion, not worth much, that there may have been falls of extra-mundane substances, in london, june , . clinkers, said to have fallen, during a storm, at kilburn, july , : according to the _kilburn times_, july , , quoted by mr. symons, a street had been "literally strewn," during the storm, with a mass of clinkers, estimated at about two bushels: sizes from that of a walnut to that of a man's hand--"pieces of the clinkers can be seen at the _kilburn times_ office." if these clinkers, or cinders, were refuse from one of the super-mercantile constructions from which coke and coal and ashes occasionally fall to this earth, or, rather, to the super-sargasso sea, from which dislodgment by tempests occurs, it is intermediatistic to accept that they must merge away somewhere with local phenomena of the scene of precipitation. if a red-hot stove should drop from a cloud into broadway, someone would find that at about the time of the occurrence, a moving van had passed, and that the moving men had tired of the stove, or something--that it had not been really red-hot, but had been rouged instead of blacked, by some absent-minded housekeeper. compared with some of the scientific explanations that we have encountered, there's considerable restraint, i think, in that one. mr. symons learned that in the same street--he emphasizes that it was a short street--there was a fire-engine station. i had such an impression of him hustling and bustling around at notting hill, searching cellars until he found one with newly arrived coal in it; ringing door bells, exciting a whole neighborhood, calling up to second-story windows, stopping people in the streets, hotter and hotter on the trail of a wretched imposter of a chemist's pupil. after his efficiency at notting hill, we'd expect to hear that he went to the station, and--something like this: "it is said that clinkers fell, in your street, at about ten minutes past four o'clock, afternoon of july fifth. will you look over your records and tell me where your engine was at about ten minutes past four, july fifth?" mr. symons says: "i think that most probably they had been raked out of the steam fire-engine." june , , it was reported that a "thunderstone" had struck the house at oakley street, chelsea, falling down the chimney, into the kitchen grate. mr. symons investigated. he describes the "thunderstone" as an "agglomeration of brick, soot, unburned coal, and cinder." he says that, in his opinion, lightning had flashed down the chimney, and had fused some of the brick of it. he does think it remarkable that the lightning did not then scatter the contents of the grate, which were disturbed only as if a heavy body had fallen. if we admit that climbing up the chimney to find out is too rigorous a requirement for a man who may have been large, dignified and subject to expansions, the only unreasonableness we find in what he says--as judged by our more modern outlook, is: "i suppose that no one would suggest that bricks are manufactured in the atmosphere." sounds a little unreasonable to us, because it is so of the positivistic spirit of former times, when it was not so obvious that the highest incredibility and laughability must merge away with the "proper"--as the _sci. am. sup._ would say. the preposterous is always interpretable in terms of the "proper," with which it must be continuous--or--clay-like masses such as have fallen from the sky--tremendous heat generated by their velocity--they bake--bricks. we begin to suspect that mr. symons exhausted himself at notting hill. it's a warning to efficiency-fanatics. then the instance of three lumps of earthy matter, found upon a well-frequented path, after a thunderstorm, at reading, july , . there are so many records of the fall of earthy matter from the sky that it would seem almost uncanny to find resistance here, were we not so accustomed to the uncompromising stands of orthodoxy--which, in our metaphysics, represent good, as attempts, but evil in their insufficiency. if i thought it necessary, i'd list one hundred and fifty instances of earthy matter said to have fallen from the sky. it is his antagonism to atmospheric disturbance associated with the fall of things from the sky that blinds and hypnotizes a mr. symons here. this especial mr. symons rejects the reading substance because it was not "of true meteoritic material." it's uncanny--or it's not uncanny at all, but universal--if you don't take something for a standard of opinion, you can't have any opinion at all: but, if you do take a standard, in some of its applications it must be preposterous. the carbonaceous meteorites, which are unquestioned--though avoided, as we have seen--by orthodoxy, are more glaringly of untrue meteoritic material than was this substance of reading. mr. symons says that these three lumps were upon the ground "in the first place." whether these data are worth preserving or not, i think that the appeal that this especial mr. symons makes is worthy of a place in the museum we're writing. he argues against belief in all external origins "for our credit as englishmen." he is a patriot, but i think that these foreigners had a small chance "in the first place" for hospitality from him. then comes a "small lump of iron (two inches in diameter)" said to have fallen, during a thunderstorm, at brixton, aug. , . mr. symons says: "at present i cannot trace it." he was at his best at notting hill: there's been a marked falling off in his later manner: in the london _times_, feb. , , it is said that a roundish object of iron had been found, "after a violent thunderstorm," in a garden at brixton, aug. , . it was analyzed by a chemist, who could not identify it as true meteoritic material. whether a product of workmanship like human workmanship or not, this object is described as an oblate spheroid, about two inches across its major diameter. the chemist's name and address are given: mr. j. james morgan: ebbw vale. garden--familiar ground--i suppose that in mr. symons' opinion this symmetric object had been upon the ground "in the first place," though he neglects to say this. but we do note that he described this object as a "lump," which does not suggest the spheroidal or symmetric. it is our notion that the word "lump" was, because of its meaning of amorphousness, used purposely to have the next datum stand alone, remote, without similars. if mr. symons had said that there had been a report of another round object that had fallen from the sky, his readers would be attracted by an agreement. he distracts his readers by describing in terms of the unprecedented-- "iron cannon ball." it was found in a manure heap, in sussex, after a thunderstorm. however, mr. symons argues pretty reasonably, it seems to me, that, given a cannon ball in a manure heap, in the first place, lightning might be attracted by it, and, if seen to strike there, the untutored mind, or mentality below the average, would leap or jump, or proceed with less celerity, to the conclusion that the iron object had fallen. except that--if every farmer isn't upon very familiar ground--or if every farmer doesn't know his own manure heap as well as mr. symons knew his writing desk-- then comes the instance of a man, his wife, and his three daughters, at casterton, westmoreland, who were looking out at their lawn, during a thunderstorm, when they "considered," as mr. symons expresses it, that they saw a stone fall from the sky, kill a sheep, and bury itself in the ground. they dug. they found a stone ball. symons: coincidence. it had been there in the first place. this object was exhibited at a meeting of the royal meteorological society by mr. c. carus-wilson. it is described in the _journal's_ list of exhibits as a "sandstone" ball. it is described as "sandstone" by mr. symons. now a round piece of sandstone may be almost anywhere in the ground--in the first place--but, by our more or less discreditable habit of prying and snooping, we find that this object was rather more complex and of material less commonplace. in snooping through _knowledge_, oct. , , we read that this "thunderstone" was in the possession of mr. c. carus-wilson, who tells the story of the witness and his family--the sheep killed, the burial of something in the earth, the digging, and the finding. mr. c. carus-wilson describes the object as a ball of hard, ferruginous quartzite, about the size of a cocoanut, weight about twelve pounds. whether we're feeling around for significance or not, there is a suggestion not only of symmetry but of structure in this object: it had an external shell, separated from a loose nucleus. mr. carus-wilson attributes this cleavage to unequal cooling of the mass. my own notion is that there is very little deliberate misrepresentation in the writings of scientific men: that they are quite as guiltless in intent as are other hypnotic subjects. such a victim of induced belief reads of a stone ball said to have fallen from the sky. mechanically in his mind arise impressions of globular lumps, or nodules, of sandstone, which are common almost everywhere. he assimilates the reported fall with his impressions of objects in the ground, in the first place. to an intermediatist, the phenomena of intellection are only phenomena of universal process localized in human minds. the process called "explanation" is only a local aspect of universal assimilation. it looks like materialism: but the intermediatist holds that interpretation of the immaterial, as it is called, in terms of the material, as it is called, is no more rational than interpretation of the "material" in terms of the "immaterial": that there is in quasi-existence neither the material nor the immaterial, but approximations one way or the other. but so hypnotic quasi-reasons: that globular lumps of sandstone are common. whether he jumps or leaps, or whether only the frowsy and base-born are so athletic, his is the impression, by assimilation, that this especial object is a ball of sandstone. or human mentality: its inhabitants are conveniences. it may be that mr. symons' paper was written before this object was exhibited to the members of the society, and with the charity with which, for the sake of diversity, we intersperse our malices, we are willing to accept that he "investigated" something that he had never seen. but whoever listed this object was uncareful: it is listed as "sandstone." we're making excuses for them. really--as it were--you know, we're not quite so damned as we were. one does not apologize for the gods and at the same time feel quite utterly prostrate before them. if this were a real existence, and all of us real persons, with real standards to judge by, i'm afraid we'd have to be a little severe with some of these mr. symonses. as it is, of course, seriousness seems out of place. we note an amusing little touch in the indefinite allusion to "a man," who with his un-named family, had "considered" that he had seen a stone fall. the "man" was the rev. w. carus-wilson, who was well-known in his day. the next instance was reported by w.b. tripp, f.r.m.s.--that, during a thunderstorm, a farmer had seen the ground in front of him plowed up by something that was luminous. dug. bronze ax. my own notion is that an expedition to the north pole could not be so urgent as that representative scientists should have gone to that farmer and there spent a summer studying this one reported occurrence. as it is--un-named farmer--somewhere--no date. the thing must stay damned. another specimen for our museum is a comment in _nature_ upon these objects: that they are "of an amusing character, thus clearly showing that they were of terrestrial, and not a celestial, character." just why celestiality, or that of it which, too, is only of intermediateness should not be quite as amusing as terrestriality is beyond our reasoning powers, which we have agreed are not ordinary. of course there is nothing amusing about wedges and spheres at all--or archimedes and euclid are humorists. it is that they were described derisively. if you'd like a little specimen of the standardization of orthodox opinion-- _amer. met. jour._, - : "they are of an amusing character, thus clearly showing that they were of a terrestrial and not a celestial character." i'm sure--not positively, of course--that we've tried to be as easygoing and lenient with mr. symons as his obviously scientific performance would permit. of course it may be that sub-consciously we were prejudiced against him, instinctively classing him with st. augustine, darwin, st. jerome, and lyell. as to the "thunderstones," i think that he investigated them mostly "for the credit of englishmen," or in the spirit of the royal krakatoa committee, or about as the commission from the french academy investigated meteorites. according to a writer in _knowledge_, - , the krakatoa committee attempted not in the least to prove what had caused the atmospheric effects of , but to prove--that krakatoa did it. altogether i should think that the following quotation should be enlightening to anyone who still thinks that these occurrences were investigated not to support an opinion formed in advance: in opening his paper, mr. symons says that he undertook his investigation as to the existence of "thunderstones," or "thunderbolts" as he calls them--"feeling certain that there was a weak point somewhere, inasmuch as 'thunderbolts' have no existence." we have another instance of the reported fall of a "cannon ball." it occurred prior to mr. symons' investigations, but is not mentioned by him. it was investigated, however. in the _proc. roy. soc. edin._, - , is the report of a "thunderstone," "supposed to have fallen in hampshire, sept., ." it was an iron cannon ball, or it was a "large nodule of iron pyrites or bisulphuret of iron." no one had seen it fall. it had been noticed, upon a garden path, for the first time, after a thunderstorm. it was only a "supposed" thing, because--"it had not the character of any known meteorite." in the london _times_, sept. , , appears a letter from mr. george e. bailey, a chemist of andover, hants. he says that, in a very heavy thunderstorm, of the first week of september, , this iron object, had fallen in the garden of mr. robert dowling, of andover; that it had fallen upon a path "within six yards of the house." it had been picked up "immediately" after the storm by mrs. dowling. it was about the size of a cricket ball: weight four pounds. no one had seen it fall. in the _times_, sept. , , there is an account of this thunderstorm, which was of unusual violence. there are some other data relative to the ball of quartz of westmoreland. they're poor things. there's so little to them that they look like ghosts of the damned. however, ghosts, when multiplied, take on what is called substantiality--if the solidest thing conceivable, in quasi-existence, is only concentrated phantomosity. it is not only that there have been other reports of quartz that has fallen from the sky; there is another agreement. the round quartz object of westmoreland, if broken open and separated from its loose nucleus, would be a round, hollow, quartz object. my pseudo-position is that two reports of similar extraordinary occurrences, one from england and one from canada--are interesting. _proc. canadian institute_, - - : that, at the meeting of the institute, of dec. , , one of the members, mr. j.a. livingstone, exhibited a globular quartz body which he asserted had fallen from the sky. it had been split open. it was hollow. but the other members of the institute decided that the object was spurious, because it was not of "true meteoritic material." no date; no place mentioned; we note the suggestion that it was only a geode, which had been upon the ground in the first place. its crystalline lining was geode-like. quartz is upon the "index prohibitory" of science. a monk who would read darwin would sin no more than would a scientist who would admit that, except by the "up and down" process, quartz has ever fallen from the sky--but continuity: it is not excommunicated if part of or incorporated in a baptized meteorite--st. catherine's of mexico, i think. it's as epicurean a distinction as any ever made by theologians. fassig lists a quartz pebble, found in a hailstone (_bibliography_, part - ). "up and down," of course. another object of quartzite was reported to have fallen, in the autumn of , at schroon lake, n.y.--said in the _scientific american_, - to be a fraud--it was not--the usual. about the first of may, , the newspapers published a story of a "snow-white" meteorite that had fallen, at vincennes, indiana. the editor of the _monthly weather review_ (issue of april, ) requested the local observer, at vincennes, to investigate. the editor says that the thing was only a fragment of a quartz boulder. he says that anyone with at least a public school education should know better than to write that quartz has ever fallen from the sky. _notes and queries_, - - : that, in the leyden museum of antiquities, there is a disk of quartz: centimeters by millimeters by about centimeters; said to have fallen upon a plantation in the dutch west indies, after a meteoric explosion. bricks. i think this is a vice we're writing. i recommend it to those who have hankered for a new sin. at first some of our data were of so frightful or ridiculous mien as to be hated, or eyebrowed, was only to be seen. then some pity crept in? i think that we can now embrace bricks. the baked-clay-idea was all right in its place, but it rather lacks distinction, i think. with our minds upon the concrete boats that have been building terrestrially lately, and thinking of wrecks that may occur to some of them, and of a new material for the deep-sea fishes to disregard-- object that fell at richland, south carolina--yellow to gray--said to look like a piece of brick. (_amer. jour. sci._, - - .) pieces of "furnace-made brick" said to have fallen--in a hailstorm--at padua, august, . (_edin. new phil. jour._, - .) the writer offered an explanation that started another convention: that the fragments of brick had been knocked from buildings by the hailstones. but there is here a concomitant that will be disagreeable to anyone who may have been inclined to smile at the now digestible--enough notion that furnace-made bricks have fallen from the sky. it is that in some of the hailstones--two per cent of them--that were found with the pieces of brick, was a light grayish powder. _monthly notices of the royal astronomical society_, - : padre sechi explains that a stone said to have fallen, in a thunderstorm, at supino, italy, september, , had been knocked from a roof. _nature_, - : that it had been reported that a good-sized stone, of form clearly artificial, had fallen at naples, november, . the stone was described by two professors of naples, who had accepted it as inexplicable but veritable. they were visited by dr. h. johnstone-lavis, the correspondent to _nature_, whose investigations had convinced him that the object was a "shoemaker's lapstone." now to us of the initiated, or to us of the wider outlook, there is nothing incredible in the thought of shoemakers in other worlds--but i suspect that this characterization is tactical. this object of worked stone, or this shoemaker's lapstone, was made of vesuvian lava, dr. johnstone-lavis thinks: most probably of lava of the flow of , from the la scala quarries. we condemn "most probably" as bad positivism. as to the "men of position," who had accepted that this thing had fallen from the sky--"i have now obliged them to admit their mistake," says dr. johnstone-lavis--or it's always the stranger in naples who knows la scala lava better than the natives know it. explanation: that the thing had been knocked from, or thrown from, a roof. as to attempt to trace the occurrence to any special roof--nothing said upon that subject. or that dr. johnstone-lavis called a carved stone a "lapstone," quite as mr. symons called a spherical object a "cannon ball": bent upon a discrediting incongruity: shoemaking and celestiality. it is so easy to say that axes, or wedge-shaped stones found on the ground, were there in the first place, and that it is only coincidence that lightning should strike near one--but the credibility of coincidences decreases as the square root of their volume, i think. our massed instances speak too much of coincidences of coincidences. but the axes, or wedge-shaped objects that have been found in trees, are more difficult for orthodoxy. for instance, arago accepts that such finds have occurred, but he argues that, if wedge-shaped stones have been found in tree trunks, so have toads been found in tree trunks--did the toads fall there? not at all bad for a hypnotic. of course, in our acceptance, the irish are the chosen people. it's because they are characteristically best in accord with the underlying essence of quasi-existence. m. arago answers a question by asking another question. that's the only way a question can be answered in our hibernian kind of an existence. dr. bodding argued with the natives of the santal parganas, india, who said that cut and shaped stones had fallen from the sky, some of them lodging in tree trunks. dr. bodding, with orthodox notions of velocity of falling bodies, having missed, i suppose, some of the notes i have upon large hailstones, which, for size, have fallen with astonishingly low velocity, argued that anything falling from the sky would be "smashed to atoms." he accepts that objects of worked stone have been found in tree trunks, but he explains: that the santals often steal trees, but do not chop them down in the usual way, because that would be to make too much noise: they insert stone wedges, and hammer them instead: then, if they should be caught, wedges would not be the evidence against them that axes would be. or that a scientific man can't be desperate and reasonable too. or that a pickpocket, for instance, is safe, though caught with his hand in one's pocket, if he's gloved, say: because no court in the land would regard a gloved hand in the same way in which a bare hand would be regarded. that there's nothing but intermediateness to the rational and the preposterous: that this status of our own ratiocinations is perceptible wherein they are upon the unfamiliar. dr. bodding collected of these shaped stones, said to have fallen from the sky, in the course of many years. he says that the santals are a highly developed race, and for ages have not used stone implements--except in this one nefarious convenience to him. all explanations are localizations. they fade away before the universal. it is difficult to express that black rains in england do not originate in the smoke of factories--less difficult to express that black rains of south africa do not. we utter little stress upon the absurdity of dr. bedding's explanation, because, if anything's absurd everything's absurd, or, rather, has in it some degree or aspect of absurdity, and we've never had experience with any state except something somewhere between ultimate absurdity and final reasonableness. our acceptance is that dr. bedding's elaborate explanation does not apply to cut-stone objects found in tree trunks in other lands: we accept that for the general, a local explanation is inadequate. as to "thunderstones" not said to have fallen luminously, and not said to have been found sticking in trees, we are told by faithful hypnotics that astonished rustics come upon prehistoric axes that have been washed into sight by rains, and jump to the conclusion that the things have fallen from the sky. but simple rustics come upon many prehistoric things: scrapers, pottery, knives, hammers. we have no record of rusticity coming upon old pottery after a rain, reporting the fall of a bowl from the sky. just now, my own acceptance is that wedge-shaped stone objects, formed by means similar to human workmanship, have often fallen from the sky. maybe there are messages upon them. my acceptance is that they have been called "axes" to discredit them: or the more familiar a term, the higher the incongruity with vague concepts of the vast, remote, tremendous, unknown. in _notes and queries_, - - , a writer says that he had a "thunderstone," which he had brought from jamaica. the description is of a wedge-shaped object; not of an ax: "it shows no mark of having been attached to a handle." of ten "thunderstones," figured upon different pages in blinkenberg's book, nine show no sign of ever having been attached to a handle: one is perforated. but in a report by dr. c. leemans, director of the leyden museum of antiquities, objects, said by the japanese to have fallen from the sky, are alluded to throughout as "wedges." in the _archaeologic journal_, - , in a paper upon the "thunderstones" of java, the objects are called "wedges" and not "axes." our notion is that rustics and savages call wedge-shaped objects that fall from the sky, "axes": that scientific men, when it suits their purposes, can resist temptations to prolixity and pedantry, and adopt the simple: that they can be intelligible when derisive. all of which lands us in a confusion, worse, i think, than we were in before we so satisfactorily emerged from the distresses of--butter and blood and ink and paper and punk and silk. now it's cannon balls and axes and disks--if a "lapstone" be a disk--it's a flat stone, at any rate. a great many scientists are good impressionists: they snub the impertinences of details. had he been of a coarse, grubbing nature, i think dr. bodding could never have so simply and beautifully explained the occurrence of stone wedges in tree trunks. but to a realist, the story would be something like this: a man who needed a tree, in a land of jungles, where, for some unknown reason, everyone's very selfish with his trees, conceives that hammering stone wedges makes less noise than does the chopping of wood: he and his descendants, in a course of many years, cut down trees with wedges, and escape penalty, because it never occurs to a prosecutor that the head of an ax is a wedge. the story is like every other attempted positivism--beautiful and complete, until we see what it excludes or disregards; whereupon it becomes the ugly and incomplete--but not absolutely, because there is probably something of what is called foundation for it. perhaps a mentally incomplete santal did once do something of the kind. story told to dr. bodding: in the usual scientific way, he makes a dogma of an aberration. or we did have to utter a little stress upon this matter, after all. they're so hairy and attractive, these scientists of the th century. we feel the zeal of a sitting bull when we think of their scalps. we shall have to have an expression of our own upon this confusing subject. we have expressions: we don't call them explanations: we've discarded explanations with beliefs. though everyone who scalps is, in the oneness of allness, himself likely to be scalped, there is such a discourtesy to an enemy as the wearing of wigs. cannon balls and wedges, and what may they mean? bombardments of this earth-- attempts to communicate-- or visitors to this earth, long ago--explorers from the moon--taking back with them, as curiosities, perhaps, implements of this earth's prehistoric inhabitants--a wreck--a cargo of such things held for ages in suspension in the super-sargasso sea--falling, or shaken, down occasionally by storms-- but, by preponderance of description, we cannot accept that "thunderstones" ever were attached to handles, or are prehistoric axes-- as to attempts to communicate with this earth by means of wedge-shaped objects especially adapted to the penetration of vast, gelatinous areas spread around this earth-- in the _proc. roy. irish acad._, - , there is an account of a stone wedge that fell from the sky, near cashel, tipperary, aug. , . the phenomenon is not questioned, but the orthodox preference is to call it, not ax-like, nor wedge-shaped, but "pyramidal." for data of other pyramidal stones said to have fallen from the sky, see _rept. brit. assoc._, - . one fell at segowolee, india, march , . of the object that fell at cashel, dr. haughton says in the _proceedings_: "a singular feature is observable in this stone, that i have never seen in any other:--the rounded edges of the pyramid are sharply marked by lines on the black crust, as perfect as if made by a ruler." dr. haughton's idea is that the marks may have been made by "some peculiar tension in the cooling." it must have been very peculiar, if in all aerolites not wedge-shaped, no such phenomenon had ever been observed. it merges away with one or two instances known, after dr. haughton's time, of seeming stratification in meteorites. stratification in meteorites, however, is denied by the faithful. i begin to suspect something else. a whopper is coming. later it will be as reasonable, by familiarity, as anything else ever said. if someone should study the stone of cashel, as champollion studied the rosetta stone, he might--or, rather, would inevitably--find meaning in those lines, and translate them into english-- nevertheless i begin to suspect something else: something more subtle and esoteric than graven characters upon stones that have fallen from the sky, in attempts to communicate. the notion that other worlds are attempting to communicate with this world is widespread: my own notion is that it is not attempt at all--that it was achievement centuries ago. i should like to send out a report that a "thunderstone" had fallen, say, somewhere in new hampshire-- and keep track of every person who came to examine that stone--trace down his affiliations--keep track of him-- then send out a report that a "thunderstone" had fallen at stockholm, say-- would one of the persons who had gone to new hampshire, be met again in stockholm? but--what if he had no anthropological, lapidarian, or meteorological affiliations--but did belong to a secret society-- it is only a dawning credulity. of the three forms of symmetric objects that have, or haven't, fallen from the sky, it seems to me that the disk is the most striking. so far, in this respect, we have been at our worst--possibly that's pretty bad--but "lapstones" are likely to be of considerable variety of form, and something that is said to have fallen at sometime somewhere in the dutch west indies is profoundly of the unchosen. now we shall have something that is high up in the castes of the accursed: _comptes rendus_, - : that, upon june , , in a "violent storm"--two months before the reported fall of the symmetric iron object of brixton--a small stone had fallen from the sky at tarbes, france: millimeters in diameter; millimeters thick; weight grammes. reported to the french academy by m. sudre, professor of the normal school, tarbes. this time the old convenience "there in the first place" is too greatly resisted--the stone was covered with ice. this object had been cut and shaped by means similar to human hands and human mentality. it was a disk of worked stone--"tres regulier." "il a été assurement travaillé." there's not a word as to any known whirlwind anywhere: nothing of other objects or débris that fell at or near this date, in france. the thing had fallen alone. but as mechanically as any part of a machine responds to its stimulus, the explanation appears in _comptes rendus_ that this stone had been raised by a whirlwind and then flung down. it may be that in the whole nineteenth century no event more important than this occurred. in _la nature_, , and in _l'année scientifique_, , this occurrence is noted. it is mentioned in one of the summer numbers of _nature_, . fassig lists a paper upon it in the _annuaire de soc. met._, . not a word of discussion. not a subsequent mention can i find. our own expression: what matters it how we, the french academy, or the salvation army may explain? a disk of worked stone fell from the sky, at tarbes, france, june , . my own pseudo-conclusion: that we've been damned by giants sound asleep, or by great scientific principles and abstractions that cannot realize themselves: that little harlots have visited their caprices upon us; that clowns, with buckets of water from which they pretend to cast thousands of good-sized fishes have anathematized us for laughing disrespectfully, because, as with all clowns, underlying buffoonery is the desire to be taken seriously; that pale ignorances, presiding over microscopes by which they cannot distinguish flesh from nostoc or fishes' spawn or frogs' spawn, have visited upon us their wan solemnities. we've been damned by corpses and skeletons and mummies, which twitch and totter with pseudo-life derived from conveniences. or there is only hypnosis. the accursed are those who admit they're the accursed. if we be more nearly real we are reasons arraigned before a jury of dream-phantasms. of all meteorites in museums, very few were seen to fall. it is considered sufficient grounds for admission if specimens can't be accounted for in any way other than that they fell from the sky--as if in the haze of uncertainty that surrounds all things, or that is the essence of everything, or in the merging away of everything into something else, there could be anything that could be accounted for in only one way. the scientist and the theologian reason that if something can be accounted for in only one way, it is accounted for in that way--or logic would be logical, if the conditions that it imposes, but, of course, does not insist upon, could anywhere be found in quasi-existence. in our acceptance, logic, science, art, religion are, in our "existence," premonitions of a coming awakening, like dawning awarenesses of surroundings in the mind of a dreamer. any old chunk of metal that measures up to the standard of "true meteoritic material" is admitted by the museums. it may seem incredible that modern curators still have this delusion, but we suspect that the date on one's morning newspaper hasn't much to do with one's modernity all day long. in reading fletcher's catalogue, for instance, we learn that some of the best-known meteorites were "found in draining a field"--"found in making a road"--"turned up by the plow" occurs a dozen times. someone fishing in lake okeechobee, brought up an object in his fishing net. no meteorite had ever been seen to fall near it. the u.s. national museum accepts it. if we have accepted only one of the data of "untrue meteoritic material"--one instance of "carbonaceous" matter--if it be too difficult to utter the word "coal"--we see that in this inclusion-exclusion, as in every other means of forming an opinion, false inclusion and false exclusion have been practiced by curators of museums. there is something of ultra-pathos--of cosmic sadness--in this universal search for a standard, and in belief that one has been revealed by either inspiration or analysis, then the dogged clinging to a poor sham of a thing long after its insufficiency has been shown--or renewed hope and search for the special that can be true, or for something local that could also be universal. it's as if "true meteoritic material" were a "rock of ages" to some scientific men. they cling. but clingers cannot hold out welcoming arms. the only seemingly conclusive utterance, or seemingly substantial thing to cling to, is a product of dishonesty, ignorance, or fatigue. all sciences go back and back, until they're worn out with the process, or until mechanical reaction occurs: then they move forward--as it were. then they become dogmatic, and take for bases, positions that were only points of exhaustion. so chemistry divided and sub-divided down to atoms; then, in the essential insecurity of all quasi-constructions, it built up a system, which, to anyone so obsessed by his own hypnoses that he is exempt to the chemist's hypnoses, is perceptibly enough an intellectual anæmia built upon infinitesimal debilities. in _science_, n.s., - , e.d. hovey, of the american museum of natural history, asserts or confesses that often have objects of material such as fossiliferous limestone and slag been sent to him he says that these things have been accompanied by assurances that they have been seen to fall on lawns, on roads, in front of houses. they are all excluded. they are not of true meteoritic material. they were on the ground in the first place. it is only by coincidence that lightning has struck, or that a real meteorite, which was unfindable, has struck near objects of slag and limestone. mr. hovey says that the list might be extended indefinitely. that's a tantalizing suggestion of some very interesting stuff-- he says: "but it is not worth while." i'd like to know what strange, damned, excommunicated things have been sent to museums by persons who have felt convinced that they had seen what they may have seen, strongly enough to risk ridicule, to make up bundles, go to express offices, and write letters. i accept that over the door of every museum, into which such things enter, is written: "abandon hope." if a mr. symons mentions one instance of coal, or of slag or cinders, said to have fallen from the sky, we are not--except by association with the "carbonaceous" meteorites--strong in our impression that coal sometimes falls to this earth from coal-burning super-constructions up somewhere-- in _comptes rendus_, - , m. daubrée tells the same story. our acceptance, then, is that other curators could tell this same story. then the phantomosity of our impression substantiates proportionately to its multiplicity. m. daubrée says that often have strange damned things been sent to the french museums, accompanied by assurances that they had been seen to fall from the sky. especially to our interest, he mentions coal and slag. excluded. buried un-named and undated in science's potter's field. i do not say that the data of the damned should have the same rights as the data of the saved. that would be justice. that would be of the positive absolute, and, though the ideal of, a violation of, the very essence of quasi-existence, wherein only to have the appearance of being is to express a preponderance of force one way or another--or inequilibrium, or inconsistency, or injustice. our acceptance is that the passing away of exclusionism is a phenomenon of the twentieth century: that gods of the twentieth century will sustain our notions be they ever so unwashed and frowsy. but, in our own expressions, we are limited, by the oneness of quasiness, to the very same methods by which orthodoxy established and maintains its now sleek, suave preposterousnesses. at any rate, though we are inspired by an especial subtle essence--or imponderable, i think--that pervades the twentieth century, we have not the superstition that we are offering anything as a positive fact. rather often we have not the delusion that we're any less superstitious and credulous than any logician, savage, curator, or rustic. an orthodox demonstration, in terms of which we shall have some heresies, is that if things found in coal could have got there only by falling there--they fell there. so, in the _manchester lit. and phil. soc. mems._, - - , it is argued that certain roundish stones that have been found in coal are "fossil aerolites": that they had fallen from the sky, ages ago, when the coal was soft, because the coal had closed around them, showing no sign of entrance. _proc. soc. of antiq. of scotland_, - - : that, in a lump of coal, from a mine in scotland, an iron instrument had been found-- "the interest attaching to this singular relic arises from the fact of its having been found in the heart of a piece of coal, seven feet under the surface." if we accept that this object of iron was of workmanship beyond the means and skill of the primitive men who may have lived in scotland when coal was forming there-- "the instrument was considered to be modern." that our expression has more of realness, or higher approximation to realness, than has the attempt to explain that is made in the _proceedings_: that in modern times someone may have bored for coal, and that his drill may have broken off in the coal it had penetrated. why he should have abandoned such easily accessible coal, i don't know. the important point is that there was no sign of boring: that this instrument was in a lump of coal that had closed around it so that its presence was not suspected, until the lump of coal was broken. no mention can i find of this damned thing in any other publication. of course there is an alternative here: the thing may not have fallen from the sky: if in coal-forming times, in scotland, there were, indigenous to this earth, no men capable of making such an iron instrument, it may have been left behind by visitors from other worlds. in an extraordinary approximation to fairness and justice, which is permitted to us, because we are quite as desirous to make acceptable that nothing can be proved as we are to sustain our own expressions, we note: that in _notes and queries_, - - , there is an account of an ancient copper seal, about the size of a penny, found in chalk, at a depth of from five to six feet, near bredenstone, england. the design upon it is said to be of a monk kneeling before a virgin and child: a legend upon the margin is said to be: "st. jordanis monachi spaldingie." i don't know about that. it looks very desirable--undesirable to us. there's a wretch of an ultra-frowsy thing in the _scientific american_, - , which we condemn ourselves, if somewhere, because of the oneness of allness, the damned must also be the damning. it's a newspaper story: that about the first of june, , a powerful blast, near dorchester, mass., cast out from a bed of solid rock a bell-shaped vessel of an unknown metal: floral designs inlaid with silver; "art of some cunning workman." the opinion of the editor of the _scientific american_ is that the thing had been made by tubal cain, who was the first inhabitant of dorchester. though i fear that this is a little arbitrary, i am not disposed to fly rabidly at every scientific opinion. _nature_, - : a block of metal found in coal, in austria, . it is now in the salsburg museum. this time we have another expression. usually our intermediatist attack upon provincial positivism is: science, in its attempted positivism takes something such as "true meteoritic material" as a standard of judgment; but carbonaceous matter, except for its relative infrequency, is just as veritable a standard of judgment; carbonaceous matter merges away into such a variety of organic substances, that all standards are reduced to indistinguishability: if, then, there is no real standard against us, there is no real resistance to our own acceptances. now our intermediatism is: science takes "true meteoritic material" as a standard of admission; but now we have an instance that quite as truly makes "true meteoritic material" a standard of exclusion; or, then, a thing that denies itself is no real resistance to our own acceptances--this depending upon whether we have a datum of something of "true meteoritic material" that orthodoxy can never accept fell from the sky. we're a little involved here. our own acceptance is upon a carved, geometric thing that, if found in a very old deposit, antedates human life, except, perhaps, very primitive human life, as an indigenous product of this earth: but we're quite as much interested in the dilemma it made for the faithful. it is of "true meteoritic material." _l'astronomie_, - , it is said that, though so geometric, its phenomena so characteristic of meteorites exclude the idea that it was the work of man. as to the deposit--tertiary coal. composition--iron, carbon, and a small quantity of nickel. it has the pitted surface that is supposed by the faithful to be characteristic of meteorites. for a full account of this subject, see _comptes rendus_, - . the scientists who examined it could reach no agreement. they bifurcated: then a compromise was suggested; but the compromise is a product of disregard: that it was of true meteoritic material, and had not been shaped by man; that it was not of true meteoritic material, but telluric iron that had been shaped by man: that it was true meteoritic material that had fallen from the sky, but had been shaped by man, after its fall. the data, one or more of which must be disregarded by each of these three explanations, are: "true meteoritic material" and surface markings of meteorites; geometric form; presence in an ancient deposit; material as hard as steel; absence upon this earth, in tertiary times, of men who could work in material as hard as steel. it is said that, though of "true meteoritic material," this object is virtually a steel object. st. augustine, with his orthodoxy, was never in--well, very much worse--difficulties than are the faithful here. by due disregard of a datum or so, our own acceptance that it was a steel object that had fallen from the sky to this earth, in tertiary times, is not forced upon one. we offer ours as the only synthetic expression. for instance, in _science gossip_, - , it is described as a meteorite: in this account there is nothing alarming to the pious, because, though everything else is told, its geometric form is not mentioned. it's a cube. there is a deep incision all around it. of its faces, two that are opposite are rounded. though i accept that our own expression can only rather approximate to truth, by the wideness of its inclusions, and because it seems, of four attempts, to represent the only complete synthesis, and can be nullified or greatly modified by data that we, too, have somewhere disregarded, the only means of nullification that i can think of would be demonstration that this object is a mass of iron pyrites, which sometimes forms geometrically. but the analysis mentions not a trace of sulphur. of course our weakness, or impositiveness, lies in that, by anyone to whom it would be agreeable to find sulphur in this thing, sulphur would be found in it--by our own intermediatism there is some sulphur in everything, or sulphur is only a localization or emphasis of something that, unemphasized, is in all things. so there have, or haven't, been found upon this earth things that fell from the sky, or that were left behind by extra-mundane visitors to this earth-- a yarn in the london _times_, june , : that some workmen, quarrying rock, close to the tweed, about a quarter of a mile below rutherford mills, discovered a gold thread embedded in the stone at a depth of feet: that a piece of the gold thread had been sent to the office of the _kelso chronicle_. pretty little thing; not at all frowsy; rather damnable. london _times_, dec. , : that hiram de witt, of springfield, mass., returning from california, had brought with him a piece of auriferous quartz about the size of a man's fist. it was accidentally dropped--split open--nail in it. there was a cut-iron nail, size of a six-penny nail, slightly corroded. "it was entirely straight and had a perfect head." or--california--ages ago, when auriferous quartz was forming--super-carpenter, million of miles or so up in the air--drops a nail. to one not an intermediatist, it would seem incredible that this datum, not only of the damned, but of the lowest of the damned, or of the journalistic caste of the accursed, could merge away with something else damned only by disregard, and backed by what is called "highest scientific authority"-- communication by sir david brewster (_rept. brit. assoc._, - ): that a nail had been found in a block of stone from kingoodie quarry, north britain. the block in which the nail was found was nine inches thick, but as to what part of the quarry it had come from, there is no evidence--except that it could not have been from the surface. the quarry had been worked about twenty years. it consisted of alternate layers of hard stone and a substance called "till." the point of the nail, quite eaten with rust, projected into some "till," upon the surface of the block of stone. the rest of the nail lay upon the surface of the stone to within an inch of the head--that inch of it was embedded in the stone. although its caste is high, this is a thing profoundly of the damned--sort of a brahmin as regarded by a baptist. its case was stated fairly; brewster related all circumstances available to him--but there was no discussion at the meeting of the british association: no explanation was offered-- nevertheless the thing can be nullified-- but the nullification that we find is as much against orthodoxy in one respect as it is against our own expression that inclusion in quartz or sandstone indicates antiquity--or there would have to be a revision of prevailing dogmas upon quartz and sandstone and age indicated by them, if the opposing data should be accepted. of course it may be contended by both the orthodox and us heretics that the opposition is only a yarn from a newspaper. by an odd combination, we find our two lost souls that have tried to emerge, chucked back to perdition by one blow: _pop. sci. news_, - : that, according to the _carson appeal_, there had been found in a mine, quartz crystals that could have had only years in which to form: that, where a mill had been built, sandstone had been found, when the mill was torn down, that had hardened in years: that in this sandstone was a piece of wood "with a nail in it." _annals of scientific discovery_, - : that, at the meeting of the british association, , sir david brewster had announced that he had to bring before the meeting an object "of so incredible a nature that nothing short of the strongest evidence was necessary to render the statement at all probable." a crystal lens had been found in the treasure-house at nineveh. in many of the temples and treasure houses of old civilizations upon this earth have been preserved things that have fallen from the sky--or meteorites. again we have a brahmin. this thing is buried alive in the heart of propriety: it is in the british museum. carpenter, in _the microscope and its revelations_, gives two drawings of it. carpenter argues that it is impossible to accept that optical lenses had ever been made by the ancients. never occurred to him--someone a million miles or so up in the air--looking through his telescope--lens drops out. this does not appeal to carpenter: he says that this object must have been an ornament. according to brewster, it was not an ornament, but "a true optical lens." in that case, in ruins of an old civilization upon this earth, has been found an accursed thing that was, acceptably, not a product of any old civilization indigenous to this earth. early explorers have florida mixed up with newfoundland. but the confusion is worse than that still earlier. it arises from simplicity. very early explorers think that all land westward is one land, india: awareness of other lands as well as india comes as a slow process. i do not now think of things arriving upon this earth from some especial other world. that was my notion when i started to collect our data. or, as is a commonplace of observation, all intellection begins with the illusion of homogeneity. it's one of spencer's data: we see homogeneousness in all things distant, or with which we have small acquaintance. advance from the relatively homogeneous to the relatively heterogeneous is spencerian philosophy--like everything else, so-called: not that it was really spencer's discovery, but was taken from von baer, who, in turn, was continuous with preceding evolutionary speculation. our own expression is that all things are acting to advance to the homogeneous, or are trying to localize homogeneousness. homogeneousness is an aspect of the universal, wherein it is a state that does not merge away into something else. we regard homogeneousness as an aspect of positiveness, but it is our acceptance that infinite frustrations of attempts to positivize manifest themselves in infinite heterogeneity: so that though things try to localize homogeneousness they end up in heterogeneity so great that it amounts to infinite dispersion or indistinguishability. so all concepts are little attempted positivenesses, but soon have to give in to compromise, modification, nullification, merging away into indistinguishability--unless, here and there, in the world's history, there may have been a super-dogmatist, who, for only an infinitesimal of time, has been able to hold out against heterogeneity or modification or doubt or "listening to reason," or loss of identity--in which case--instant translation to heaven or the positive absolute. odd thing about spencer is that he never recognized that "homogeneity," "integration," and "definiteness" are all words for the same state, or the state that we call "positiveness." what we call his mistake is in that he regarded "homogeneousness" as negative. i began with a notion of some one other world, from which objects and substances have fallen to this earth; which had, or which, to less degree, has a tutelary interest in this earth; which is now attempting to communicate with this earth--modifying, because of data which will pile up later, into acceptance that some other world is not attempting but has been, for centuries, in communication with a sect, perhaps, or a secret society, or certain esoteric ones of this earth's inhabitants. i lose a great deal of hypnotic power in not being able to concentrate attention upon some one other world. as i have admitted before i'm intelligent, as contrasted with the orthodox. i haven't the aristocratic disregard of a new york curator or an eskimo medicine-man. i have to dissipate myself in acceptance of a host of other worlds: size of the moon, some of them: one of them, at least--tremendous thing: we'll take that up later. vast, amorphous aerial regions, to which such definite words as "worlds" and "planets" seem inapplicable. and artificial constructions that i have called "super-constructions": one of them about the size of brooklyn, i should say, offhand. and one or more of them wheel-shaped things a goodly number of square miles in area. i think that earlier in this book, before we liberalized into embracing everything that comes along, your indignation, or indigestion would have expressed in the notion that, if this were so, astronomers would have seen these other worlds and regions and vast geometric constructions. you'd have had that notion: you'd have stopped there. but the attempt to stop is saying "enough" to the insatiable. in cosmic punctuation there are no periods: illusion of periods is incomplete view of colons and semi-colons. we can't stop with the notion that if there were such phenomena, astronomers would have seen them. because of our experience with suppression and disregard, we suspect, before we go into the subject at all, that astronomers have seen them; that navigators and meteorologists have seen them; that individual scientists and other trained observers have seen them many times-- that it is the system that has excluded data of them. as to the law of gravitation, and astronomers' formulas, remember that these formulas worked out in the time of laplace as well as they do now. but there are hundreds of planetary bodies now known that were then not known. so a few hundred worlds more of ours won't make any difference. laplace knew of about only thirty bodies in this solar system: about six hundred are recognized now-- what are the discoveries of geology and biology to a theologian? his formulas still work out as well as they ever did. if the law of gravitation could be stated as a real utterance, it might be a real resistance to us. but we are told only that gravitation is gravitation. of course to an intermediatist, nothing can be defined except in terms of itself--but even the orthodox, in what seems to me to be the innate premonitions of realness, not founded upon experience, agree that to define a thing in terms of itself is not real definition. it is said that by gravitation is meant the attraction of all things proportionately to mass and inversely as the square of the distance. mass would mean inter-attraction holding together final particles, if there were final particles. then, until final particles be discovered, only one term of this expression survives, or mass is attraction. but distance is only extent of mass, unless one holds out for absolute vacuum among planets, a position against which we could bring a host of data. but there is no possible means of expressing that gravitation is anything other than attraction. so there is nothing to resist us but such a phantom as--that gravitation is the gravitation of all gravitations proportionately to gravitation and inversely as the square of gravitation. in a quasi-existence, nothing more sensible than this can be said upon any so-called subject--perhaps there are higher approximations to ultimate sensibleness. nevertheless we seem to have a feeling that with the system against us we have a kind of resistance here. we'd have felt so formerly, at any rate: i think the dr. grays and prof. hitchcocks have modified our trustfulness toward indistinguishability. as to the perfection of this system that quasi-opposes us and the infallibility of its mathematics--as if there could be real mathematics in a mode of seeming where twice two are not four--we've been told over and over of their vindication in the discovery of neptune. i'm afraid that the course we're taking will turn out like every other development. we began humbly, admitting that we're of the damned-- but our eyebrows-- just a faint flicker in them, or in one of them, every time we hear of the "triumphal discovery of neptune"--this "monumental achievement of theoretical astronomy," as the text-books call it. the whole trouble is that we've looked it up. the text-books omit this: that, instead of the orbit of neptune agreeing with the calculations of adams and leverrier, it was so different--that leverrier said that it was not the planet of his calculations. later it was thought best to say no more upon that subject. the text-books omit this: that, in , everyone who knew a sine from a cosine was out sining and cosining for a planet beyond uranus. two of them guessed right. to some minds, even after leverrier's own rejection of neptune, the word "guessed" may be objectionable--but, according to prof. peirce, of harvard, the calculations of adams and leverrier would have applied quite as well to positions many degrees from the position of neptune. or for prof. peirce's demonstration that the discovery of neptune was only a "happy accident," see _proc. amer. acad. sciences_, - . for references, see lowell's _evolution of worlds_. or comets: another nebulous resistance to our own notions. as to eclipses, i have notes upon several of them that did not occur upon scheduled time, though with differences only of seconds--and one delightful lost soul, deep-buried, but buried in the ultra-respectable records of the royal astronomical society, upon an eclipse that did not occur at all. that delightful, ultra-sponsored thing of perdition is too good and malicious to be dismissed with passing notice: we'll have him later. throughout the history of astronomy, every comet that has come back upon predicted time--not that, essentially, there was anything more abstruse about it than is a prediction that you can make of a postman's periodicities tomorrow--was advertised for all it was worth. it's the way reputations are worked up for fortune-tellers by the faithful. the comets that didn't come back--omitted or explained. or encke's comet. it came back slower and slower. but the astronomers explained. be almost absolutely sure of that: they explained. they had it all worked out and formulated and "proved" why that comet was coming back slower and slower--and there the damn thing began coming faster and faster. halley's comet. astronomy--"the perfect science, as we astronomers like to call it." (jacoby.) it's my own notion that if, in a real existence, an astronomer could not tell one longitude from another, he'd be sent back to this purgatory of ours until he could meet that simple requirement. halley was sent to the cape of good hope to determine its longitude. he got it degrees wrong. he gave to africa's noble roman promontory a retroussé twist that would take the pride out of any kaffir. we hear everlastingly of halley's comet. it came back--maybe. but, unless we look the matter up in contemporaneous records, we hear nothing of--the leonids, for instance. by the same methods as those by which halley's comet was predicted, the leonids were predicted. november, --no leonids. it was explained. they had been perturbed. they would appear in november, . november, --november, --no leonids. my notion of astronomic accuracy: who could not be a prize marksman, if only his hits be recorded? as to halley's comet, of --everybody now swears he saw it. he has to perjure himself: otherwise he'd be accused of having no interest in great, inspiring things that he's never given any attention to. regard this: that there never is a moment when there is not some comet in the sky. virtually there is no year in which several new comets are not discovered, so plentiful are they. luminous fleas on a vast black dog--in popular impressions, there is no realization of the extent to which this solar system is flea-bitten. if a comet have not the orbit that astronomers have predicted--perturbed. if--like halley's comet--it be late--even a year late--perturbed. when a train is an hour late, we have small opinion of the predictions of timetables. when a comet's a year late, all we ask is--that it be explained. we hear of the inflation and arrogance of astronomers. my own acceptance is not that they are imposing upon us: that they are requiting us. for many of us priests no longer function to give us seeming rapport with perfection, infallibility--the positive absolute. astronomers have stepped forward to fill a vacancy--with quasi-phantomosity--but, in our acceptance, with a higher approximation to substantiality than had the attenuations that preceded them. i should say, myself, that all that we call progress is not so much response to "urge" as it is response to a hiatus--or if you want something to grow somewhere, dig out everything else in its area. so i have to accept that the positive assurances of astronomers are necessary to us, or the blunderings, evasions and disguises of astronomers would never be tolerated: that, given such latitude as they are permitted to take, they could not be very disastrously mistaken. suppose the comet called halley's had not appeared-- early in , a far more important comet than the anæmic luminosity said to be halley's, appeared. it was so brilliant that it was visible in daylight. the astronomers would have been saved anyway. if this other comet did not have the predicted orbit--perturbation. if you're going to coney island, and predict there'll be a special kind of a pebble on the beach, i don't see how you can disgrace yourself, if some other pebble will do just as well--because the feeble thing said to have been seen in was no more in accord with the sensational descriptions given out by astronomers in advance than is a pale pebble with a brick-red boulder. i predict that next wednesday, a large chinaman, in evening clothes, will cross broadway, at nd street, at p.m. he doesn't, but a tubercular jap in a sailor's uniform does cross broadway, at th street, friday, at noon. well, a jap is a perturbed chinaman, and clothes are clothes. i remember the terrifying predictions made by the honest and credulous astronomers, who must have been themselves hypnotized, or they could not have hypnotized the rest of us, in . wills were made. human life might be swept from this planet. in quasi-existence, which is essentially hibernian, that would be no reason why wills should not be made. the less excitable of us did expect at least some pretty good fireworks. i have to admit that it is said that, in new york, a light was seen in the sky. it was about as terrifying as the scratch of a match on the seat of some breeches half a mile away. it was not on time. though i have heard that a faint nebulosity, which i did not see, myself, though i looked when i was told to look, was seen in the sky, it appeared several days after the time predicted. a hypnotized host of imbeciles of us: told to look up at the sky: we did--like a lot of pointers hypnotized by a partridge. the effect: almost everybody now swears that he saw halley's comet, and that it was a glorious spectacle. an interesting circumstance here is that seemingly we are trying to discredit astronomers because astronomers oppose us--that's not my impression. we shall be in the brahmin caste of the hell of the baptists. almost all our data, in some regiments of this procession, are observations by astronomers, few of them mere amateur astronomers. it is the system that opposes us. it is the system that is suppressing astronomers. i think we pity them in their captivity. ours is not malice--in a positive sense. it's chivalry--somewhat. unhappy astronomers looking out from high towers in which they are imprisoned--we appear upon the horizon. but, as i have said, our data do not relate to some especial other world. i mean very much what a savage upon an ocean island might vaguely think of in his speculations--not upon some other land, but complexes of continents and their phenomena: cities, factories in cities, means of communication-- now all the other savages would know of a few vessels sailing in their regular routes, passing this island in regularized periodicities. the tendency in these minds would be expression of the universal tendency toward positivism--or completeness--or conviction that these few regularized vessels constituted all. now i think of some especial savage who suspects otherwise--because he's very backward and unimaginative and insensible to the beautiful ideals of the others: not piously occupied, like the others, in bowing before impressive-looking sticks of wood; dishonestly taking time for his speculations, while the others are patriotically witch-finding. so the other higher and nobler savages know about the few regularized vessels: know when to expect them; have their periodicities all worked out; just about when vessels will pass, or eclipse each other--explaining that all vagaries were due to atmospheric conditions. they'd come out strong in explaining. you can't read a book upon savages without noting what resolute explainers they are. they'd say that all this mechanism was founded upon the mutual attraction of the vessels--deduced from the fall of a monkey from a palm tree--or, if not that, that devils were pushing the vessels--something of the kind. storms. débris, not from these vessels, cast up by the waves. disregarded. how can one think of something and something else, too? i'm in the state of mind of a savage who might find upon a shore, washed up by the same storm, buoyant parts of a piano and a paddle that was carved by cruder hands than his own: something light and summery from india, and a fur overcoat from russia--or all science, though approximating wider and wider, is attempt to conceive of india in terms of an ocean island, and of russia in terms of india so interpreted. though i am trying to think of russia and india in world-wide terms, i cannot think that that, or the universalizing of the local, is cosmic purpose. the higher idealist is the positivist who tries to localize the universal, and is in accord with cosmic purpose: the super-dogmatist of a local savage who can hold out, without a flurry of doubt, that a piano washed up on a beach is the trunk of a palm tree that a shark has bitten, leaving his teeth in it. so we fear for the soul of dr. gray, because he did not devote his whole life to that one stand that, whether possible or inconceivable, thousands of fishes had been cast from one bucket. so, unfortunately for myself, if salvation be desirable, i look out widely but amorphously, indefinitely and heterogeneously. if i say i conceive of another world that is now in secret communication with certain esoteric inhabitants of this earth, i say i conceive of still other worlds that are trying to establish communication with all the inhabitants of this earth. i fit my notions to the data i find. that is supposed to be the right and logical and scientific thing to do; but it is no way to approximate to form, system, organization. then i think i conceive of other worlds and vast structures that pass us by, within a few miles, without the slightest desire to communicate, quite as tramp vessels pass many islands without particularizing one from another. then i think i have data of a vast construction that has often come to this earth, dipped into an ocean, submerged there a while, then going away--why? i'm not absolutely sure. how would an eskimo explain a vessel, sending ashore for coal, which is plentiful upon some arctic beaches, though of unknown use to the natives, then sailing away, with no interest in the natives? a great difficulty in trying to understand vast constructions that show no interest in us: the notion that we must be interesting. i accept that, though we're usually avoided, probably for moral reasons, sometimes this earth has been visited by explorers. i think that the notion that there have been extra-mundane visitors to china, within what we call the historic period, will be only ordinarily absurd, when we come to that datum. i accept that some of the other worlds are of conditions very similar to our own. i think of others that are very different--so that visitors from them could not live here--without artificial adaptations. how some of them could breathe our attenuated air, if they came from a gelatinous atmosphere-- masks. the masks that have been found in ancient deposits. most of them are of stone, and are said to have been ceremonial regalia of savages-- but the mask that was found in sullivan county, missouri, in (_american antiquarian_, - ). it is made of iron and silver. one of the damnedest in our whole saturnalia of the accursed-- because it is hopeless to try to shake off an excommunication only by saying that we're damned by blacker things than ourselves; and that the damned are those who admit they're of the damned. inertia and hypnosis are too strong for us. we say that: then we go right on admitting we're of the damned. it is only by being more nearly real that we can sweep away the quasi-things that oppose us. of course, as a whole, we have considerable amorphousness, but we are thinking now of "individual" acceptances. wideness is an aspect of universalness or realness. if our syntheses disregard fewer data than do opposing syntheses--which are often not syntheses at all, but mere consideration of some one circumstance--less widely synthetic things fade away before us. harmony is an aspect of the universal, by which we mean realness. if we approximate more highly to harmony among the parts of an expression and to all available circumstances of an occurrence, the self-contradictors turn hazy. solidity is an aspect of realness. we pile them up, and we pile them up, or they pass and pass and pass: things that bulk large as they march by, supporting and solidifying one another-- and still, and for regiments to come, hypnosis and inertia rule us-- one of the damnedest of our data: in the _scientific american_, sept. , , charles f. holder writes: "many years ago, a strange stone resembling a meteorite, fell into the valley of the yaqui, mexico, and the sensational story went from one end to the other of the country that a stone bearing human inscriptions had descended to the earth." the bewildering observation here is mr. holder's assertion that this stone did fall. it seems to me that he must mean that it fell by dislodgment from a mountainside into a valley--but we shall see that it was such a marked stone that very unlikely would it have been unknown to dwellers in a valley, if it had been reposing upon a mountainside above them. it may have been carelessness: intent may have been to say that a sensational story of a strange stone said to have fallen, etc. this stone was reported by major frederick burnham, of the british army. later major burnham revisited it, and mr. holder accompanied him, their purpose to decipher the inscriptions upon it, if possible. "this stone was a brown, igneous rock, its longest axis about eight feet, and on the eastern face, which had an angle of about forty-five degrees, was the deep-cut inscription." mr. holder says that he recognized familiar mayan symbols in the inscription. his method was the usual method by which anything can be "identified" as anything else: that is to pick out whatever is agreeable and disregard the rest. he says that he has demonstrated that most of the symbols are mayan. one of our intermediatist pseudo-principles is that any way of demonstrating anything is just as good a way of demonstrating anything else. by mr. holder's method we could demonstrate that we're mayan--if that should be a source of pride to us. one of the characters upon this stone is a circle within a circle--similar character found by mr. holder is a mayan manuscript. there are two 's. 's can be found in mayan manuscripts. a double scroll. there are dots and there are dashes. well, then, we, in turn, disregard the circle within a circle and the double scroll and emphasize that 's occur in this book, and that dots are plentiful, and would be more plentiful if it were customary to use the small "i" for the first personal pronoun--that when it comes to dashes--that's demonstrated: we're mayan. i suppose the tendency is to feel that we're sneering at some valuable archaeologic work, and that mr. holder did make a veritable identification. he writes: "i submitted the photographs to the field museum and the smithsonian and one or two others, and, to my surprise, the reply was that they could make nothing out of it." our indefinite acceptance, by preponderance of three or four groups of museum-experts against one person, is that a stone bearing inscriptions unassimilable with any known language upon this earth, is said to have fallen from the sky. another poor wretch of an outcast belonging here is noted in the _scientific american_, - : that, of an object, or a meteorite, that fell feb. , , near brescia, italy, a false report was circulated that one of the fragments bore the impress of a hand. that's all that is findable by me upon this mere gasp of a thing. intermediatistically, my acceptance is that, though in the course of human history, there have been some notable approximations, there never has been a real liar: that he could not survive in intermediateness, where everything merges away or has its pseudo-base in something else--would be instantly translated to the negative absolute. so my acceptance is that, though curtly dismissed, there was something to base upon in this report; that there were unusual markings upon this object. of course that is not to jump to the conclusion that they were cuneiform characters that looked like finger-prints. altogether, i think that in some of our past expressions, we must have been very efficient, if the experience of mr. symons be typical, so indefinite are we becoming here. just here we are interested in many things that have been found, especially in the united states, which speak of a civilization, or of many civilizations not indigenous to this earth. one trouble is in trying to decide whether they fell here from the sky, or were left behind by visitors from other worlds. we have a notion that there have been disasters aloft, and that coins have dropped here: that inhabitants of this earth found them or saw them fall, and then made coins imitatively: it may be that coins were showered here by something of a tutelary nature that undertook to advance us from the stage of barter to the use of a medium. if coins should be identified as roman coins, we've had so much experience with "identifications" that we know a phantom when we see one--but, even so, how could roman coins have got to north america--far in the interior of north america--or buried under the accumulation of centuries of soil--unless they did drop from--wherever the first romans came from? ignatius donnelly, in _atlantis_, gives a list of objects that have been found in mounds that are supposed to antedate all european influence in america: lathe-made articles, such as traders--from somewhere--would supply to savages--marks of the lathe said to be unmistakable. said to be: of course we can't accept that anything is unmistakable. in the _rept. smithson. inst._, - , there is an account, by charles c. jones, of two silver crosses that were found in georgia. they are skillfully made, highly ornamented crosses, but are not conventional crucifixes: all arms of equal length. mr. jones is a good positivist--that de sota had halted at the "precise" spot where these crosses were found. but the spirit of negativeness that lurks in all things said to be "precise" shows itself in that upon one of these crosses is an inscription that has no meaning in spanish or any other known, terrestrial language: "iynkicidu," according to mr. jones. he thinks that this is a name, and that there is an aboriginal ring to it, though i should say, myself, that he was thinking of the far-distant incas: that the spanish donor cut on the cross the name of an indian to whom it was presented. but we look at the inscription ourselves and see that the letters said to be "c" and "d" are turned the wrong way, and that the letter said to be "k" is not only turned the wrong way, but is upside down. it is difficult to accept that the remarkable, the very extensive, copper mines in the region of lake superior were ever the works of american aborigines. despite the astonishing extent of these mines, nothing has ever been found to indicate that the region was ever inhabited by permanent dwellers-- "... not a vestige of a dwelling, a skeleton, or a bone has been found." the indians have no traditions relating to the mines. (_amer. antiquarian_, - .) i think that we've had visitors: that they have come here for copper, for instance. as to other relics of them--but we now come upon frequency of a merger that has not so often appeared before: fraudulency. hair called real hair--then there are wigs. teeth called real teeth--then there are false teeth. official money--counterfeit money. it's the bane of psychic research. if there be psychic phenomena, there must be fraudulent psychic phenomena. so desperate is the situation here that carrington argues that, even if palladino be caught cheating, that is not to say that all her phenomena are fraudulent. my own version is: that nothing indicates anything, in a positive sense, because, in a positive sense, there is nothing to be indicated. everything that is called true must merge away indistinguishably into something called false. both are expressions of the same underlying quasiness, and are continuous. fraudulent antiquarian relics are very common, but they are not more common than are fraudulent paintings. w.s. forest, _historical sketches of norfolk, virginia_: that, in september, , when some workmen, near norfolk, were boring for water, a coin was drawn up from a depth of about feet. it was about the size of an english shilling, but oval--an oval disk, if not a coin. the figures upon it were distinct, and represented "a warrior or hunter and other characters, apparently of roman origin." the means of exclusion would probably be--men digging a hole--no one else looking: one of them drops a coin into the hole--as to where he got a strange coin, remarkable in shape even--that's disregarded. up comes the coin--expressions of astonishment from the evil one who had dropped it. however, the antiquarians have missed this coin. i can find no other mention of it. another coin. also a little study in the genesis of a prophet. in the _american antiquarian_, - , is copied a story by a correspondent to the _detroit news_, of a copper coin about the size of a two-cent piece, said to have been found in a michigan mound. the editor says merely that he does not endorse the find. upon this slender basis, he buds out, in the next number of the _antiquarian_: "the coin turns out, as we predicted, to be a fraud." you can imagine the scorn of elijah, or any of the old more nearly real prophets. or all things are tried by the only kind of jurisprudence we have in quasi-existence: presumed to be innocent until convicted--but they're guilty. the editor's reasoning is as phantom-like as my own, or st. paul's, or darwin's. the coin is condemned because it came from the same region from which, a few years before, had come pottery that had been called fraudulent. the pottery had been condemned because it was condemnable. _scientific american_, june , : that a farmer, in cass co., ill., had picked up, on his farm, a bronze coin, which was sent to prof. f.f. hilder, of st. louis, who identified it as a coin of antiochus iv. inscription said to be in ancient greek characters: translated as "king antiochus epiphanes (illustrious) the victorius." sounds quite definite and convincing--but we have some more translations coming. in the _american pioneer_, - , are shown two faces of a copper coin, with characters very much like those upon the grave creek stone--which, with translations, we'll take up soon. this coin is said to have been found in connecticut, in . _records of the past_, - : that, early in , a coin, said to be a roman coin, was reported as discovered in an illinois mound. it was sent to dr. emerson, of the art institute, of chicago. his opinion was that the coin is "of the rare mintage of domitius domitianus, emperor in egypt." as to its discovery in an illinois mound, dr. emerson disclaims responsibility. but what strikes me here is that a joker should not have been satisfied with an ordinary roman coin. where did he get a rare coin, and why was it not missed from some collection? i have looked over numismatic journals enough to accept that the whereabouts of every rare coin in anyone's possession is known to coin-collectors. seems to me nothing left but to call this another "identification." _proc. amer. phil. soc._, - : that, in july, , a letter was received from mr. jacob w. moffit, of chillicothe, ill., enclosing a photograph of a coin, which he said had been brought up, by him, while boring, from a depth of feet. of course, by conventional scientific standards, such depth has some extraordinary meaning. palaeontologists, geologists, and archaeologists consider themselves reasonable in arguing ancient origin of the far-buried. we only accept: depth is a pseudo-standard with us; one earthquake could bury a coin of recent mintage feet below the surface. according to a writer in the _proceedings_, the coin is uniform in thickness, and had never been hammered out by savages--"there are other tokens of the machine shop." but, according to prof. leslie, it is an astrologic amulet. "there are upon it the signs of pisces and leo." or, with due disregard, you can find signs of your great-grand-mother, or of the crusades, or of the mayans, upon anything that ever came from chillicothe or from a five and ten cent store. anything that looks like a cat and a goldfish looks like leo and pisces: but, by due suppressions and distortions there's nothing that can't be made to look like a cat and a goldfish. i fear me we're turning a little irritable here. to be damned by slumbering giants and interesting little harlots and clowns who rank high in their profession is at least supportable to our vanity; but, we find that the anthropologists are of the slums of the divine, or of an archaic kindergarten of intellectuality, and it is very unflattering to find a mess of moldy infants sitting in judgment upon us. prof. leslie then finds, as arbitrarily as one might find that some joker put the brooklyn bridge where it is, that "the piece was placed there as a practical joke, though not by its present owner; and is a modern fabrication, perhaps of the sixteenth century, possibly hispano-american or french-american origin." it's sheer, brutal attempt to assimilate a thing that may or may not have fallen from the sky, with phenomena admitted by the anthropologic system: or with the early french or spanish explorers of illinois. though it is ridiculous in a positive sense to give reasons, it is more acceptable to attempt reasons more nearly real than opposing reasons. of course, in his favor, we note that prof. leslie qualifies his notions. but his disregards are that there is nothing either french or spanish about this coin. a legend upon it is said to be "somewhere between arabic and phoenician, without being either." prof. winchell (_sparks from a geologist's hammer_, p. ) says of the crude designs upon this coin, which was in his possession--scrawls of an animal and of a warrior, or of a cat and a goldfish, whichever be convenient--that they had been neither stamped nor engraved, but "looked as if etched with an acid." that is a method unknown in numismatics of this earth. as to the crudity of design upon this coin, and something else--that, though the "warrior" may be, by due disregards, either a cat or a goldfish, we have to note that his headdress is typical of the american indian--could be explained, of course, but for fear that we might be instantly translated to the positive absolute, which may not be absolutely desirable, we prefer to have some flaws or negativeness in our own expressions. data of more than the thrice-accursed: tablets of stone, with the ten commandments engraved upon them, in hebrew, said to have been found in mounds in the united states: masonic emblems said to have been found in mounds in the united states. we're upon the borderline of our acceptances, and we're amorphous in the uncertainties and mergings of our outline. conventionally, or, with no real reason for so doing, we exclude these things, and then, as grossly and arbitrarily and irrationally--though our attempt is always to approximate away from these negative states--as ever a kepler, newton, or darwin made his selections, without which he could not have seemed to be, at all, because every one of them is now seen to be an illusion, we accept that other lettered things have been found in mounds in the united states. of course we do what we can to make the selection seem not gross and arbitrary and irrational. then, if we accept that inscribed things of ancient origin have been found in the united states; that cannot be attributed to any race indigenous to the western hemisphere; that are not in any language ever heard of in the eastern hemisphere--there's nothing to it but to turn non-euclidian and try to conceive of a third "hemisphere," or to accept that there has been intercourse between the western hemisphere and some other world. but there is a peculiarity to these inscribed objects. they remind me of the records left, by sir john franklin, in the arctic; but, also, of attempts made by relief expeditions to communicate with the franklin expedition. the lost explorers cached their records--or concealed them conspicuously in mounds. the relief expeditions sent up balloons, from which messages were dropped broadcast. our data are of things that have been cached, and of things that seem to have been dropped-- or a lost expedition from--somewhere. explorers from somewhere, and their inability to return--then, a long, sentimental, persistent attempt, in the spirit of our own arctic relief-expeditions--at least to establish communication-- what if it may have succeeded? we think of india--the millions of natives who are ruled by a small band of esoterics--only because they receive support and direction from--somewhere else--or from england. in , mr. a.b. tomlinson, owner of the great mound at grave creek, west virginia, excavated the mound. he said that, in the presence of witnesses, he had found a small, flat, oval stone--or disk--upon which were engraved alphabetic characters. col. whittelsey, an expert in these matters, says that the stone is now "universally regarded by archaeologists as a fraud": that, in his opinion, mr. tomlinson had been imposed upon. avebury, _prehistoric times_, p. : "i mention it because it has been the subject of much discussion, but it is now generally admitted to be a fraud. it is inscribed with hebrew characters, but the forger has copied the modern instead of the ancient form of the letters." as i have said, we're as irritable here, under the oppressions of the anthropologists as ever were slaves in the south toward superiorities from "poor white trash." when we finally reverse our relative positions we shall give lowest place to the anthropologists. a dr. gray does at least look at a fish before he conceives of a miraculous origin for it. we shall have to submerge lord avebury far below him--if we accept that the stone from grave creek is generally regarded as a fraud by eminent authorities who did not know it from some other object--or, in general, that so decided an opinion must be the product of either deliberate disregard or ignorance or fatigue. the stone belongs to a class of phenomena that is repulsive to the system. it will not assimilate with the system. let such an object be heard of by such a systematist as avebury, and the mere mention of it is as nearly certainly the stimulus to a conventional reaction as is a charged body to an electroscope or a glass of beer to a prohibitionist. it is of the ideals of science to know one object from another before expressing an opinion upon a thing, but that is not the spirit of universal mechanics: a thing. it is attractive or repulsive. its conventional reaction follows. because it is not the stone from grave creek that is in hebrew characters, either ancient or modern: it is a stone from newark, ohio, of which the story is told that a forger made this mistake of using modern instead of ancient hebrew characters. we shall see that the inscription upon the grave creek stone is not in hebrew. or all things are presumed to be innocent, but are supposed to be guilty--unless they assimilate. col. whittelsey (_western reserve historical tracts, no. _) says that the grave creek stone was considered a fraud by wilson, squires, and davis. then he comes to the congress of archaeologists at nancy, france, . it is hard for col. whittelsey to admit that, at this meeting, which sounds important, the stone was endorsed. he reminds us of mr. symons, and "the man" who "considered" that he saw something. col. whittelsey's somewhat tortuous expression is that the finder of the stone "so imposed his views" upon the congress that it pronounced the stone genuine. also the stone was examined by schoolcraft. he gave his opinion for genuineness. or there's only one process, and "see-saw" is one of its aspects. three or four fat experts on the side against us. we find four or five plump ones on our side. or all that we call logic and reasoning ends up as sheer preponderance of avoirdupois. then several philologists came out in favor of genuineness. some of them translated the inscription. of course, as we have said, it is our method--or the method of orthodoxy--way in which all conclusions are reached--to have some awfully eminent, or preponderantly plump, authorities with us whenever we can--in this case, however, we feel just a little apprehensive in being caught in such excellently obese, but somewhat negativized, company: translation by m. jombard: "thy orders are laws: thou shinest in impetuous élan and rapid chamois." m. maurice schwab: "the chief of emigration who reached these places (or this island) has fixed these characters forever." m. oppert: "the grave of one who was assassinated here. may god, to revenge him, strike his murderer, cutting off the hand of his existence." i like the first one best. i have such a vivid impression from it of someone polishing up brass or something, and in an awful hurry. of course the third is more dramatic--still they're all very good. they are perturbations of one another, i suppose. in tract , col. whittelsey returns to the subject. he gives the conclusion of major de helward, at the congress of luxembourg, : "if prof. read and myself are right in the conclusion that the figures are neither of the runic, phoenician, canaanite, hebrew, lybian, celtic, or any other alphabet-language, its importance has been greatly over-rated." obvious to a child; obvious to any mentality not helplessly subjected to a system: that just therein lies the importance of this object. it is said that an ideal of science is to find out the new--but, unless a thing be of the old, it is "unimportant." "it is not worth while." (hovey.) then the inscribed ax, or wedge, which, according to dr. john c. evans, in a communication to the american ethnological society, was plowed up, near pemberton, n.j., . the characters upon this ax, or wedge, are strikingly similar to the characters on the grave creek stone. also, with a little disregard here and a little more there, they look like tracks in the snow by someone who's been out celebrating, or like your handwriting, or mine, when we think there's a certain distinction in illegibility. method of disregard: anything's anything. dr. abbott describes this object in the _report of the smithsonian institution_, - . he says he has no faith in it. all progress is from the outrageous to the commonplace. or quasi-existence proceeds from rape to the crooning of lullabies. it's been interesting to me to go over various long-established periodicals and note controversies between attempting positivists and then intermediatistic issues. bold, bad intruders of theories; ruffians with dishonorable intentions--the alarms of science; her attempts to preserve that which is dearer than life itself--submission--then a fidelity like mrs. micawber's. so many of these ruffians, or wandering comedians that were hated, or scorned, pitied, embraced, conventionalized. there's not a notion in this book that has a more frightful, or ridiculous, mien than had the notion of human footprints in rocks, when that now respectabilized ruffian, or clown, was first heard from. it seems bewildering to one whose interests are not scientific that such rows should be raised over such trifles: but the feeling of a systematist toward such an intruder is just about what anyone's would be if a tramp from the street should come in, sit at one's dinner table, and say he belonged there. we know what hypnosis can do: let him insist with all his might that he does belong there, and one begins to suspect that he may be right; that he may have higher perceptions of what's right. the prohibitionists had this worked out very skillfully. so the row that was raised over the stone from grave creek--but time and cumulativeness, and the very factor we make so much of--or the power of massed data. there were other reports of inscribed stones, and then, half a century later, some mounds--or caches, as we call them--were opened by the rev. mr. gass, near the city of davenport. (_american antiquarian_, - .) several stone tablets were found. upon one of them, the letters "tftowns" may easily be made out. in this instance we hear nothing of fraudulency--time, cumulativeness, the power of massed data. the attempt to assimilate this datum is: that the tablet was probably of mormon origin. why? because, at mendon, ill., was found a brass plate, upon which were similar characters. why that? because that was found "near a house once occupied by a mormon." in a real existence, a real meteorologist, suspecting that cinders had come from a fire engine--would have asked a fireman. tablets of davenport--there's not a record findable that it ever occurred to any antiquarian--to ask a mormon. other tablets were found. upon one of them are two "f's" and two " 's." also a large tablet, twelve inches by eight to ten inches "with roman numerals and arabic." it is said that the figure " " occurs three times, and the figure or letter "o" seven times. "with these familiar characters are others that resemble ancient alphabets, either phoenecian or hebrew." it may be that the discovery of australia, for instance, will turn out to be less important than the discovery and the meaning of these tablets-- but where will you read of them in anything subsequently published; what antiquarian has ever since tried to understand them, and their presence, and indications of antiquity, in a land that we're told was inhabited only by unlettered savages? these things that are exhumed only to be buried in some other way. another tablet was found, at davenport, by mr. charles harrison, president of the american antiquarian society. "... and other hieroglyphics are upon this tablet." this time, also, fraud is not mentioned. my own notion is that it is very unsportsmanlike ever to mention fraud. accept anything. then explain it your way. anything that assimilates with one explanation, must have assimilable relations, to some degree, with all other explanations, if all explanations are somewhere continuous. mormons are lugged in again, but the attempt is faint and helpless--"because general circumstances make it difficult to explain the presence of these tablets." altogether our phantom resistance is mere attribution to the mormons, without the slightest attempt to find base for the attribution. we think of messages that were showered upon this earth, and of messages that were cached in mounds upon this earth. the similarity to the franklin situation is striking. conceivably centuries from now, objects dropped from relief-expedition-balloons may be found in the arctic, and conceivably there are still undiscovered caches left by franklin, in the hope that relief expeditions would find them. it would be as incongruous to attribute these things to the eskimos as to attribute tablets and lettered stones to the aborigines of america. some time i shall take up an expression that the queer-shaped mounds upon this earth were built by explorers from somewhere, unable to get back, designed to attract attention from some other world, and that a vast sword-shaped mound has been discovered upon the moon--just now we think of lettered things and their two possible significances. a bizarre little lost soul, rescued from one of the morgues of the _american journal of science_: an account, sent by a correspondent, to prof. silliman, of something that was found in a block of marble, taken november, , from a quarry, near philadelphia (_am. j. sci._, - - ). the block was cut into slabs. by this process, it is said, was exposed an indentation in the stone, about one and a half inches by five-eighths of an inch. a geometric indentation: in it were two definite-looking raised letters, like "i u": only difference is that the corners of the "u" are not rounded, but are right angles. we are told that this block of stone came from a depth of seventy or eighty feet--or that, if acceptable, this lettering was done long, long ago. to some persons, not sated with the commonness of the incredible that has to be accepted, it may seem grotesque to think that an indentation in sand could have tons of other sand piled upon it and hardening into stone, without being pressed out--but the famous nicaraguan footprints were found in a quarry under eleven strata of solid rock. there was no discussion of this datum. we only take it out for an airing. as to lettered stones that may once upon a time have been showered upon europe, if we cannot accept that the stones were inscribed by indigenous inhabitants of europe, many have been found in caves--whence they were carried as curiosities by prehistoric men, or as ornaments, i suppose. about the size and shape of the grave creek stone, or disk: "flat and oval and about two inches wide." (sollas.) characters painted upon them: found first by m. piette, in the cave of mas d'azil, ariége. according to sollas, they are marked in various directions with red and black lines. "but on not a few of them, more complex characters occur, which in a few instances simulate some of the capital letters of the roman alphabet." in one instance the letters "f e i" accompanied by no other markings to modify them, are as plain as they could be. according to sollas (_ancient hunters_, p. ) m. cartailhac has confirmed the observations of piette, and m. boule has found additional examples. "they offer one of the darkest problems of prehistoric times." (sollas.) as to caches in general, i should say that they are made with two purposes: to proclaim and to conceal; or that caches documents are hidden, or covered over, in conspicuous structures; at least, so are designed the cairns in the arctic. _trans. n.y. acad. of sciences_, - : that mr. j.h. hooper, bradley co., tenn., having come upon a curious stone, in some woods upon his farm, investigated. he dug. he unearthed a long wall. upon this wall were inscribed many alphabetic characters. " characters have been examined, many of them duplicates, and a few imitations of animal forms, the moon, and other objects. accidental imitations of oriental alphabets are numerous." the part that seems significant: that these letters had been hidden under a layer of cement. and still, in our own heterogeneity, or unwillingness, or inability, to concentrate upon single concepts, we shall--or we sha'n't--accept that, though there may have been a lost colony or lost expedition from somewhere, upon this earth, and extra-mundane visitors who could never get back, there have been other extra-mundane visitors, who have gone away again--altogether quite in analogy with the franklin expedition and peary's flittings in the arctic-- and a wreck that occurred to one group of them-- and the loot that was lost overboard-- the chinese seals of ireland. not the things with the big, wistful eyes that lie on ice, and that are taught to balance objects on their noses--but inscribed stamps, with which to make impressions. _proc. roy. irish acad._, - : a paper was read by mr. j. huband smith, descriptive of about a dozen chinese seals that had been found in ireland. they are all alike: each a cube with an animal seated upon it. "it is said that the inscriptions upon them are of a very ancient class of chinese characters." the three points that have made a leper and an outcast of this datum--but only in the sense of disregard, because nowhere that i know of is it questioned: agreement among archaeologists that there were no relations, in the remote past, between china and ireland: that no other objects, from ancient china--virtually, i suppose--have ever been found in ireland: the great distances at which these seals have been found apart. after mr. smith's investigations--if he did investigate, or do more than record--many more chinese seals were found in ireland, and, with one exception, only in ireland. in , about had been found. of all archaeologic finds in ireland, "none is enveloped in greater mystery." (_chambers' journal_, - .) according to the writer in _chambers' journal_, one of these seals was found in a curiosity shop in london. when questioned, the shopkeeper said that it had come from ireland. in this instance, if you don't take instinctively to our expression, there is no orthodox explanation for your preference. it is the astonishing scattering of them, over field and forest, that has hushed the explainers. in the _proceedings of the royal irish academy_, - , dr. frazer says that they "appear to have been sown broadcast over the country in some strange way that i cannot offer solution of." the struggle for expression of a notion that did not belong to dr. frazer's era: "the invariable story of their find is what we might expect if they had been accidentally dropped...." three were found in tipperary; six in cork; three in down; four in waterford; all the rest--one or two to a county. but one of these chinese seals was found in the bed of the river boyne, near clonard, meath, when workmen were raising gravel. that one, at least, had been dropped there. astronomy. and a watchman looking at half a dozen lanterns, where a street's been torn up. there are gas lights and kerosene lamps and electric lights in the neighborhood: matches flaring, fires in stoves, bonfires, house afire somewhere; lights of automobiles, illuminated signs-- the watchman and his one little system. ethics. and some young ladies and the dear old professor of a very "select" seminary. drugs and divorce and rape: venereal diseases, drunkenness, murder-- excluded. the prim and the precise, or the exact, the homogeneous, the single, the puritanic, the mathematic, the pure, the perfect. we can have illusion of this state--but only by disregarding its infinite denials. it's a drop of milk afloat in acid that's eating it. the positive swamped by the negative. so it is in intermediateness, where only to "be" positive is to generate corresponding and, perhaps, equal negativeness. in our acceptance, it is, in quasi-existence, premonitory, or pre-natal, or pre-awakening consciousness of a real existence. but this consciousness of realness is the greatest resistance to efforts to realize or to become real--because it is feeling that realness has been attained. our antagonism is not to science, but to the attitude of the sciences that they have finally realized; or to belief, instead of acceptance; to the insufficiency, which, as we have seen over and over, amounts to paltriness and puerility of scientific dogmas and standards. or, if several persons start out to chicago, and get to buffalo, and one be under the delusion that buffalo is chicago, that one will be a resistance to the progress of the others. so astronomy and its seemingly exact, little system-- but data we shall have of round worlds and spindle-shaped worlds, and worlds shaped like a wheel; worlds like titanic pruning hooks; worlds linked together by streaming filaments; solitary worlds, and worlds in hordes: tremendous worlds and tiny worlds: some of them made of material like the material of this earth; and worlds that are geometric super-constructions made of iron and steel-- or not only fall from the sky of ashes and cinders and coke and charcoal and oily substances that suggest fuel--but the masses of iron that have fallen upon this earth. wrecks and flotsam and fragments of vast iron constructions-- or steel. sooner or later we shall have to take up an expression that fragments of steel have fallen from the sky. if fragments not of iron, but of steel have fallen upon this earth-- but what would a deep-sea fish learn even if a steel plate of a wrecked vessel above him should drop and bump him on the nose? our submergence in a sea of conventionality of almost impenetrable density. sometimes i'm a savage who has found something on the beach of his island. sometimes i'm a deep-sea fish with a sore nose. the greatest of mysteries: why don't they ever come here, or send here, openly? of course there's nothing to that mystery if we don't take so seriously the notion--that we must be interesting. it's probably for moral reasons that they stay away--but even so, there must be some degraded ones among them. or physical reasons: when we can specially take up that subject, one of our leading ideas, or credulities, will be that near approach by another world to this world would be catastrophic: that navigable worlds would avoid proximity; that others that have survived have organized into protective remotenesses, or orbits which approximate to regularity, though by no means to the degree of popular supposition. but the persistence of the notion that we must be interesting. bugs and germs and things like that: they're interesting to us: some of them are too interesting. dangers of near approach--nevertheless our own ships that dare not venture close to a rocky shore can send rowboats ashore-- why not diplomatic relations established between the united states and cyclorea--which, in our advanced astronomy, is the name of a remarkable wheel-shaped world or super-construction? why not missionaries sent here openly to convert us from our barbarous prohibitions and other taboos, and to prepare the way for a good trade in ultra-bibles and super-whiskeys; fortunes made in selling us cast-off super-fineries, which we'd take to like an african chief to someone's old silk hat from new york or london? the answer that occurs to me is so simple that it seems immediately acceptable, if we accept that the obvious is the solution of all problems, or if most of our perplexities consist in laboriously and painfully conceiving of the unanswerable, and then looking for answers--using such words as "obvious" and "solution" conventionally-- or: would we, if we could, educate and sophisticate pigs, geese, cattle? would it be wise to establish diplomatic relation with the hen that now functions, satisfied with mere sense of achievement by way of compensation? i think we're property. i should say we belong to something: that once upon a time, this earth was no-man's land, that other worlds explored and colonized here, and fought among themselves for possession, but that now it's owned by something: that something owns this earth--all others warned off. nothing in our own times--perhaps--because i am thinking of certain notes i have--has ever appeared upon this earth, from somewhere else, so openly as columbus landed upon san salvador, or as hudson sailed up his river. but as to surreptitious visits to this earth, in recent times, or as to emissaries, perhaps, from other worlds, or voyagers who have shown every indication of intent to evade and avoid, we shall have data as convincing as our data of oil or coal-burning aerial super-constructions. but, in this vast subject, i shall have to do considerable neglecting or disregarding, myself. i don't see how i can, in this book, take up at all the subject of possible use of humanity to some other mode of existence, or the flattering notion that we can possibly be worth something. pigs, geese, and cattle. first find out that they are owned. then find out the whyness of it. i suspect that, after all, we're useful--that among contesting claimants, adjustment has occurred, or that something now has a legal right to us, by force, or by having paid out analogues of beads for us to former, more primitive, owners of us--all others warned off--that all this has been known, perhaps for ages, to certain ones upon this earth, a cult or order, members of which function like bellwethers to the rest of us, or as superior slaves or overseers, directing us in accordance with instructions received--from somewhere else--in our mysterious usefulness. but i accept that, in the past, before proprietorship was established, inhabitants of a host of other worlds have--dropped here, hopped here, wafted, sailed, flown, motored--walked here, for all i know--been pulled here, been pushed; have come singly, have come in enormous numbers; have visited occasionally, have visited periodically for hunting, trading, replenishing harems, mining: have been unable to stay here, have established colonies here, have been lost here; far-advanced peoples, or things, and primitive peoples or whatever they were: white ones, black ones, yellow ones-- i have a very convincing datum that the ancient britons were blue ones. of course we are told by conventional anthropologists that they only painted themselves blue, but in our own advanced anthropology, they were veritable blue ones-- _annals of philosophy_, - : note of a blue child born in england. that's atavism. giants and fairies. we accept them, of course. or, if we pride ourselves upon being awfully far-advanced, i don't know how to sustain our conceit except by very largely going far back. science of today--the superstition of tomorrow. science of tomorrow--the superstition of today. notice of a stone ax, inches long: inches across broad end. (_proc. soc. of ants. of scotland_, - - .) _amer. antiquarian_, - : copper ax from an ohio mound: inches long; weight pounds. _amer. anthropologist_, n.s., - : stone ax found at birchwood, wisconsin--exhibited in the collection of the missouri historical society--found with "the pointed end embedded in the soil"--for all i know, may have dropped there-- inches long, wide, thick--weight pounds. or the footprints, in sandstone, near carson, nevada--each print to inches long. (_amer. jour. sci._, - - .) these footprints are very clear and well-defined: reproduction of them in the _journal_--but they assimilate with the system, like sour apples to other systems: so prof. marsh, a loyal and unscrupulous systematist, argues: "the size of these footprints and specially the width between the right and left series, are strong evidence that they were not made by men, as has been so generally supposed." so these excluders. stranglers of minerva. desperadoes of disregard. above all, or below all, the anthropologists. i'm inspired with a new insult--someone offends me: i wish to express almost absolute contempt for him--he's a systematistic anthropologist. simply to read something of this kind is not so impressive as to see for one's self: if anyone will take the trouble to look up these footprints, as pictured in the _journal_, he will either agree with prof. marsh or feel that to deny them is to indicate a mind as profoundly enslaved by a system as was ever the humble intellect of a medieval monk. the reasoning of this representative phantom of the chosen, or of the spectral appearances who sit in judgment, or condemnation, upon us of the more nearly real: that there never were giants upon this earth, because gigantic footprints are more gigantic than prints made by men who are not giants. we think of giants as occasional visitors to this earth. of course--stonehenge, for instance. it may be that, as time goes on, we shall have to admit that there are remains of many tremendous habitations of giants upon this earth, and that their appearances here were more than casual--but their bones--or the absence of their bones-- except--that, no matter how cheerful and unsuspicious my disposition may be, when i go to the american museum of natural history, dark cynicisms arise the moment i come to the fossils--or old bones that have been found upon this earth--gigantic things--that have been reconstructed into terrifying but "proper" dinosaurs--but my uncheerfulness-- the dodo did it. on one of the floors below the fossils, they have a reconstructed dodo. it's frankly a fiction: it's labeled as such--but it's been reconstructed so cleverly and so convincingly-- fairies. "fairy crosses." _harper's weekly_, - : that, near the point where the blue ridge and the allegheny mountains unite, north of patrick county, virginia, many little stone crosses have been found. a race of tiny beings. they crucified cockroaches. exquisite beings--but the cruelty of the exquisite. in their diminutive way they were human beings. they crucified. the "fairy crosses," we are told in _harper's weekly_, range in weight from one-quarter of an ounce to an ounce: but it is said, in the _scientific american_, - , that some of them are no larger than the head of a pin. they have been found in two other states, but all in virginia are strictly localized on and along bull mountain. we are reminded of the chinese seals in ireland. i suppose they fell there. some are roman crosses, some st. andrew's, some maltese. this time we are spared contact with the anthropologists and have geologists instead, but i am afraid that the relief to our finer, or more nearly real, sensibilities will not be very great. the geologists were called upon to explain the "fairy crosses." their response was the usual scientific tropism--"geologists say that they are crystals." the writer in _harper's weekly_ points out that this "hold up," or this anæsthetic, if theoretic science be little but attempt to assuage pangs of the unexplained, fails to account for the localized distributions of these objects--which make me think of both aggregation and separation at the bottom of the sea, if from a wrecked ship, similar objects should fall in large numbers but at different times. but some are roman crosses, some st. andrew's, some maltese. conceivably there might be a mineral that would have a diversity of geometric forms, at the same time restricted to some expression of the cross, because snowflakes, for instance, have diversity but restriction to the hexagon, but the guilty geologists, cold-blooded as astronomers and chemists and all the other deep-sea fishes--though less profoundly of the pseudo-saved than the wretched anthropologists--disregarded the very datum--that it was wise to disregard: that the "fairy crosses" are not all made of the same material. it's the same old disregard, or it's the same old psycho-tropism, or process of assimilation. crystals are geometric forms. crystals are included in the system. so then "fairy crosses" are crystals. but that different minerals should, in a few different regions, be inspired to turn into different forms of the cross--is the kind of resistance that we call less nearly real than our own acceptances. we now come to some "cursed" little things that are of the "lost," but for the "salvation" of which scientific missionaries have done their damnedest. "pigmy flints." they can't very well be denied. they're lost and well known. "pigmy flints" are tiny, prehistoric implements. some of them are a quarter of an inch in size. england, india, france, south africa--they've been found in many parts of the world--whether showered there or not. they belong high up in the froth of the accursed: they are not denied, and they have not been disregarded; there is an abundant literature upon this subject. one attempt to rationalize them, or assimilate them, or take them into the scientific fold, has been the notion that they were toys of prehistoric children. it sounds reasonable. but, of course, by the reasonable we mean that for which the equally reasonable, but opposing, has not been found out--except that we modify that by saying that, though nothing's finally reasonable, some phenomena have higher approximations to reasonableness than have others. against the notion of toys, the higher approximation is that where "pygmy flints" are found, all flints are pygmies--at least so in india, where, when larger implements have been found in the same place, there are separations by strata. (wilson.) the datum that, just at present, leads me to accept that these flints were made by beings about the size of pickles, is a point brought out by prof. wilson (_rept. national museum_, - ): not only that the flints are tiny but that the chipping upon them is "minute." struggle for expression, in the mind of a th-century-ite, of an idea that did not belong to his era: in _science gossip_, - , r.a. galty says: "so fine is the chipping that to see the workmanship a magnifying glass is necessary." i think that would be absolutely convincing, if there were anything--absolutely anything--either that tiny beings, from pickle to cucumber-stature, made these things, or that ordinary savages made them under magnifying glasses. the idea that we are now going to develop, or perpetrate, is rather intensely of the accursed, or the advanced. it's a lost soul, i admit--or boast--but it fits in. or, as conventional as ever, our own method is the scientific method of assimilating. it assimilates, if we think of the inhabitants of elvera-- by the way, i forgot to tell the name of the giant's world: monstrator. spindle-shaped world--about , miles along its major axis--more details to be published later. but our coming inspiration fits in, if we think of the inhabitants of elvera as having only visited here: having, in hordes as dense as clouds of bats, come here, upon hunting excursions--for mice, i should say: for bees, very likely--or most likely of all, or inevitably, to convert the heathen here--horrified with anyone who would gorge himself with more than a bean at a time; fearful for the souls of beings who would guzzle more than a dewdrop at a time--hordes of tiny missionaries, determined that right should prevail, determining right by their own minutenesses. they must have been missionaries. only to be is motion to convert or assimilate something else. the idea now is that tiny creatures coming here from their own little world, which may be eros, though i call it elvera, would flit from the exquisite to the enormous--gulp of a fair-sized terrestrial animal--half a dozen of them gone and soon digested. one falls into a brook--torn away in a mighty torrent-- or never anything but conventional, we adopt from darwin: "the geological records are incomplete." their flints would survive, but, as to their fragile bodies--one might as well search for prehistoric frost-traceries. a little whirlwind--elverean carried away a hundred yards--body never found by his companions. they'd mourn for the departed. conventional emotion to have: they'd mourn. there'd have to be a funeral: there's no getting away from funerals. so i adopt an explanation that i take from the anthropologists: burial in effigy. perhaps the elvereans would not come to this earth again until many years later--another distressing occurrence--one little mausoleum for all burials in effigy. london _times_, july , : that, early in july, , some boys were searching for rabbits' burrows in the rocky formation, near edinburgh, known as arthur's seat. in the side of a cliff, they came upon some thin sheets of slate, which they pulled out. little cave. seventeen tiny coffins. three or four inches long. in the coffins were miniature wooden figures. they were dressed differently both in style and material. there were two tiers of eight coffins each, and a third tier begun, with one coffin. the extraordinary datum, which has especially made mystery here: that the coffins had been deposited singly, in the little cave, and at intervals of many years. in the first tier, the coffins were quite decayed, and the wrappings had moldered away. in the second tier, the effects of age had not advanced so far. and the top coffin was quite recent-looking. in the _proceedings of the society of antiquarians of scotland_, - - , there is a full account of this find. three of the coffins and three of the figures are pictured. so elvera with its downy forests and its microscopic oyster shells--and if the elvereans be not very far-advanced, they take baths--with sponges the size of pin heads-- or that catastrophes have occurred: that fragments of elvera have fallen to this earth: in _popular science_, - , francis bingham, writing of the corals and sponges and shells and crinoids that dr. hahn had asserted that he had found in meteorites, says, judging by the photographs of them, that their "notable peculiarity" is their "extreme smallness." the corals, for instance, are about one-twentieth the size of terrestrial corals. "they represent a veritable pygmy animal world," says bingham. the inhabitants of monstrator and elvera were primitives, i think, at the time of their occasional visits to this earth--though, of course, in a quasi-existence, anything that we semi-phantoms call evidence of anything may be just as good evidence of anything else. logicians and detectives and jurymen and suspicious wives and members of the royal astronomic society recognize this indeterminateness, but have the delusion that in the method of agreement there is final, or real evidence. the method is good enough for an "existence" that is only semi-real, but also it is the method of reasoning by which witches were burned, and by which ghosts have been feared. i'd not like to be so unadvanced as to deny witches and ghosts, but i do think that there never have been witches and ghosts like those of popular supposition. but stories of them have been supported by astonishing fabrications of details and of different accounts in agreement. so, if a giant left impressions of his bare feet in the ground, that is not to say that he was a primitive--bulk of culture out taking the kneipp cure. so, if stonehenge is a large, but only roughly geometric construction, the inattention to details by its builders--signifies anything you please--ambitious dwarfs or giants--if giants, that they were little more than cave men, or that they were post-impressionist architects from a very far-advanced civilization. if there are other worlds, there are tutelary worlds--or that kepler, for instance, could not have been absolutely wrong: that his notion of an angel assigned to push along and guide each planet may not be very acceptable, but that, abstractedly, or in the notion of a tutelary relation, we may find acceptance. only to be is to be tutelary. our general expression: that "everything" in intermediateness is not a thing, but is an endeavor to become something--by breaking away from its continuity, or merging away, with all other phenomena--is an attempt to break away from the very essence of a relative existence and become absolute--if it have not surrendered to, or become part of, some higher attempt: that to this process there are two aspects: attraction, or the spirit of everything to assimilate all other things--if it have not given in and subordinated to--or have not been assimilated by--some higher attempted system, unity, organization, entity, harmony, equilibrium-- and repulsion, or the attempt of everything to exclude or disregard the unassimilable. universality of the process: anything conceivable: a tree. it is doing all it can to assimilate substances of the soil and substances of the air, and sunshine, too, into tree-substance: obversely it is rejecting or excluding or disregarding that which it cannot assimilate. cow grazing, pig rooting, tiger stalking: planets trying, or acting, to capture comets; rag pickers and the christian religion, and a cat down headfirst in a garbage can; nations fighting for more territory, sciences correlating the data they can, trust magnates organizing, chorus girl out for a little late supper--all of them stopped somewhere by the unassimilable. chorus girl and the broiled lobster. if she eats not shell and all she represents universal failure to positivize. also, if she does she represents universal failure to positivize: her ensuing disorders will translate her to the negative absolute. or science and some of our cursed hard-shelled data. one speaks of the tutelarian as if it were something distinct in itself. so one speaks of a tree, a saint, a barrel of pork, the rocky mountains. one speaks of missionaries, as if they were positively different, or had identity of their own, or were a species by themselves. to the intermediatist, everything that seems to have identity is only attempted identity, and every species is continuous with all other species, or that which is called the specific is only emphasis upon some aspect of the general. if there are cats, they're only emphasis upon universal felinity. there is nothing that does not partake of that of which the missionary, or the tutelary, is the special. every conversation is a conflict of missionaries, each trying to convert the other, to assimilate, or to make the other similar to himself. if no progress be made, mutual repulsion will follow. if other worlds have ever in the past had relations with this earth, they were attempted positivizations: to extend themselves, by colonies, upon this earth; to convert, or assimilate, indigenous inhabitants of this earth. or parent-worlds and their colonies here-- super-romanimus-- or where the first romans came from. it's as good as the romulus and remus story. super-israelimus-- or that, despite modern reasoning upon this subject, there was once something that was super-parental or tutelary to early orientals. azuria, which was tutelary to the early britons: azuria, whence came the blue britons, whose descendants gradually diluting, like blueing in a wash-tub, where a faucet's turned on, have been most emphasized of sub-tutelarians, or assimilators ever since. worlds that were once tutelarian worlds--before this earth became sole property of one of them--their attempts to convert or assimilate--but then the state that comes to all things in their missionary-frustrations--unacceptance by all stomachs of some things; rejection by all societies of some units; glaciers that sort over and cast out stones-- repulsion. wrath of the baffled missionary. there is no other wrath. all repulsion is reaction to the unassimilable. so then the wrath of azuria-- because surrounding peoples of this earth would not assimilate with her own colonists in the part of the earth that we now call england. i don't know that there has ever been more nearly just, reasonable, or logical wrath, in this earth's history--if there is no other wrath. the wrath of azuria, because the other peoples of this earth would not turn blue to suit her. history is a department of human delusion that interests us. we are able to give a little advancement to history. in the vitrified forts of a few parts of europe, we find data that the humes and gibbons have disregarded. the vitrified forts surrounding england, but not in england. the vitrified forts of scotland, ireland, brittany, and bohemia. or that, once upon a time, with electric blasts, azuria tried to swipe this earth clear of the peoples who resisted her. the vast blue bulk of azuria appeared in the sky. clouds turned green. the sun was formless and purple in the vibrations of wrath that were emanating from azuria. the whitish, or yellowish, or brownish peoples of scotland, ireland, brittany, and bohemia fled to hilltops and built forts. in a real existence, hilltops, or easiest accessibility to an aerial enemy, would be the last choice in refuges. but here, in quasi-existence, if we're accustomed to run to hilltops, in times of danger, we run to them just the same, even with danger closest to hilltops. very common in quasi-existence: attempt to escape by running closer to the pursuing. they built forts, or already had forts, on hilltops. something poured electricity upon them. the stones of these forts exist to this day, vitrified, or melted and turned to glass. the archaeologists have jumped from one conclusion to another, like the "rapid chamois" we read of a while ago, to account for vitrified forts, always restricted by the commandment that unless their conclusions conformed to such tenets as exclusionism, of the system, they would be excommunicated. so archaeologists, in their medieval dread of excommunication, have tried to explain vitrified forts in terms of terrestrial experience. we find in their insufficiencies the same old assimilating of all that could be assimilated, and disregard for the unassimilable, conventionalizing into the explanation that vitrified forts were made by prehistoric peoples who built vast fires--often remote from wood-supply--to melt externally, and to cement together, the stones of their constructions. but negativeness always: so within itself a science can never be homogeneous or unified or harmonious. so miss russel, in the _journal of the b.a.a._, has pointed out that it is seldom that single stones, to say nothing of long walls, of large houses that are burned to the ground, are vitrified. if we pay a little attention to this subject, ourselves, before starting to write upon it, which is one of the ways of being more nearly real than oppositions so far encountered by us, we find: that the stones of these forts are vitrified in no reference to cementing them: that they are cemented here and there, in streaks, as if special blasts had struck, or played, upon them. then one thinks of lightning? once upon a time something melted, in streaks, the stones of forts on the tops of hills in scotland, ireland, brittany, and bohemia. lightning selects the isolated and conspicuous. but some of the vitrified forts are not upon tops of hills: some are very inconspicuous: their walls too are vitrified in streaks. something once had effect, similar to lightning, upon forts, mostly on hills, in scotland, ireland, brittany, and bohemia. but upon hills, all over the rest of the world, are remains of forts that are not vitrified. there is only one crime, in the local sense, and that is not to turn blue, if the gods are blue: but, in the universal sense, the one crime is not to turn the gods themselves green, if you're green. one of the most extraordinary of phenomena, or alleged phenomena, of psychic research, or alleged research--if in quasi-existence there never has been real research, but only approximations to research that merge away, or that are continuous with, prejudice and convenience-- "stone-throwing." it's attributed to poltergeists. they're mischievous spirits. poltergeists do not assimilate with our own present quasi-system, which is an attempt to correlate denied or disregarded data as phenomena of extra-telluric forces, expressed in physical terms. therefore i regard poltergeists as evil or false or discordant or absurd--names that we give to various degrees or aspects of the unassimilable, or that which resists attempts to organize, harmonize, systematize, or, in short, to positivize--names that we give to our recognitions of the negative state. i don't care to deny poltergeists, because i suspect that later, when we're more enlightened, or when we widen the range of our credulities, or take on more of that increase of ignorance that is called knowledge, poltergeists may become assimilable. then they'll be as reasonable as trees. by reasonableness i mean that which assimilates with a dominant force, or system, or a major body of thought--which is, itself, of course, hypnosis and delusion--developing, however, in our acceptance, to higher and higher approximations to realness. the poltergeists are now evil or absurd to me, proportionately to their present unassimilableness, compounded, however, with the factor of their possible future assimilableness. we lug in the poltergeists, because some of our own data, or alleged data, merge away indistinguishably with data, or alleged data, of them: instances of stones that have been thrown, or that have fallen, upon a small area, from an unseen and undetectable source. london _times_, april , : "from o'clock, thursday afternoon, until half past eleven, thursday night, the houses, and reverdy road, bermondsey, were assailed with stones and other missiles coming from an unseen quarter. two children were injured, every window broken, and several articles of furniture were destroyed. although there was a strong body of policemen scattered in the neighborhood, they could not trace the direction whence the stones were thrown." "other missiles" make a complication here. but if the expression means tin cans and old shoes, and if we accept that the direction could not be traced because it never occurred to anyone to look upward--why, we've lost a good deal of our provincialism by this time. london _times_, sept. , : that, in the home of mrs. charton, at sutton courthouse, sutton lane, chiswick, windows had been broken "by some unseen agent." every attempt to detect the perpetrator failed. the mansion was detached and surrounded by high walls. no other building was near it. the police were called. two constables, assisted by members of the household, guarded the house, but the windows continued to be broken "both in front and behind the house." or the floating islands that are often stationary in the super-sargasso sea; and atmospheric disturbances that sometimes affect them, and bring things down within small areas, upon this earth, from temporarily stationary sources. super-sargasso sea and the beaches of its floating islands from which i think, or at least accept, pebbles have fallen: wolverhampton, england, june, --violent storm--fall of so many little black pebbles that they were cleared away by shoveling (_la sci. pour tous_, - ); great number of small black stones that fell at birmingham, england, august, --violent storm--said to be similar to some basalt a few leagues from birmingham (_rept. brit. assoc._, - ); pebbles described as "common water-worn pebbles" that fell at palestine, texas, july , --"of a formation not found near palestine" (w.h. perry, sergeant, signal corps, _monthly weather review_, july, ); round, smooth pebbles at kandahor, (_am. j. sci._, - - ); "a number of stones of peculiar formation and shapes, unknown in this neighborhood, fell in a tornado at hillsboro, ill., may , ." (_monthly weather review_, may, .) pebbles from aerial beaches and terrestrial pebbles as products of whirlwinds, so merge in these instances that, though it's interesting to hear of things of peculiar shape that have fallen from the sky, it seems best to pay little attention here, and to find phenomena of the super-sargasso sea remote from the merger: to this requirement we have three adaptations: pebbles that fell where no whirlwind to which to attribute them could be learned of: pebbles which fell in hail so large that incredibly could that hail have been formed in this earth's atmosphere: pebbles which fell and were, long afterward, followed by more pebbles, as if from some aerial, stationary source, in the same place. in september, , there was a story in a new york newspaper, of lightning--or an appearance of luminosity?--in jamaica--something had struck a tree: near the tree were found some small pebbles. it was said that the pebbles had fallen from the sky, with the lightning. but the insult to orthodoxy was that they were not angular fragments such as might have been broken from a stony meteorite: that they were "water-worn pebbles." in the geographical vagueness of a mainland, the explanation "up from one place and down in another" is always good, and is never overworked, until the instances are massed as they are in this book: but, upon this occasion, in the relatively small area of jamaica, there was no whirlwind findable--however "there in the first place" bobs up. _monthly weather review_, august, - : that the government meteorologist had investigated: had reported that a tree had been struck by lightning, and that small water-worn pebbles had been found near the tree: but that similar pebbles could be found all over jamaica. _monthly weather review_, september, - : prof. fassig gives an account of a fall of hail that occurred in maryland, june , : hailstones the size of baseballs "not at all uncommon." "an interesting, but unconfirmed, account stated that small pebbles were found at the center of some of the larger hail gathered at annapolis. the young man who related the story offered to produce the pebbles, but has not done so." a footnote: "since writing this, the author states that he has received some of the pebbles." when a young man "produces" pebbles, that's as convincing as anything else i've ever heard of, though no more convincing than, if having told of ham sandwiches falling from the sky, he should "produce" ham sandwiches. if this "reluctance" be admitted by us, we correlate it with a datum reported by a weather bureau observer, signifying that, whether the pebbles had been somewhere aloft a long time or not, some of the hailstones that fell with them, had been. the datum is that some of these hailstones were composed of from twenty to twenty-five layers alternately of clear ice and snow-ice. in orthodox terms i argue that a fair-sized hailstone falls from the clouds with velocity sufficient to warm it so that it would not take on even one layer of ice. to put on twenty layers of ice, i conceive of something that had not fallen at all, but had rolled somewhere, at a leisurely rate, for a long time. we now have a commonplace datum that is familiar in two respects: little, symmetric objects of metal that fell at orenburg, russia, september, (_phil. mag._, - - ). a second fall of these objects, at orenburg, russia, jan. , (_quar. jour. roy. inst._, - - ). i now think of the disk of tarbes, but when first i came upon these data i was impressed only with recurrence, because the objects of orenburg were described as crystals of pyrites, or sulphate of iron. i had no notion of metallic objects that might have been shaped or molded by means other than crystallization, until i came to arago's account of these occurrences (_oeuvres_, - ). here the analysis gives per cent. red oxide of iron, and sulphur and loss by ignition per cent. it seems to me acceptable that iron with considerably less than per cent. sulphur in it is not iron pyrites--then little, rusty iron objects, shaped by some other means, have fallen, four months apart, at the same place. m. arago expresses astonishment at this phenomenon of recurrence so familiar to us. altogether, i find opening before us, vistas of heresies to which i, for one, must shut my eyes. i have always been in sympathy with the dogmatists and exclusionists: that is plain in our opening lines: that to seem to be is falsely and arbitrarily and dogmatically to exclude. it is only that exclusionists who are good in the nineteenth century are evil in the twentieth century. constantly we feel a merging away into infinitude; but that this book shall approximate to form, or that our data shall approximate to organization, or that we shall approximate to intelligibility, we have to call ourselves back constantly from wandering off into infinitude. the thing that we do, however, is to make our own outline, or the difference between what we include and what we exclude, vague. the crux here, and the limit beyond which we may not go--very much--is: acceptance that there is a region that we call the super-sargasso sea--not yet fully accepted, but a provisional position that has received a great deal of support-- but is it a part of this earth, and does it revolve with and over this earth-- or does it flatly overlie this earth, not revolving with and over this earth-- that this earth does not revolve, and is not round, or roundish, at all, but is continuous with the rest of its system, so that, if one could break away from the traditions of the geographers, one might walk and walk, and come to mars, and then find mars continuous with jupiter? i suppose some day such queries will sound absurd--the thing will be so obvious-- because it is very difficult for me to conceive of little metallic objects hanging precisely over a small town in russia, for four months, if revolving, unattached, with a revolving earth-- it may be that something aimed at that town, and then later took another shot. these are speculations that seem to me to be evil relatively to these early years in the twentieth century-- just now, i accept that this earth is--not round, of course: that is very old-fashioned--but roundish, or, at least, that it has what is called form of its own, and does revolve upon its axis, and in an orbit around the sun. i only accept these old traditional notions-- and that above it are regions of suspension that revolve with it: from which objects fall, by disturbances of various kinds, and then, later, fall again, in the same place: _monthly weather review_, may, - : report from the signal service observer, at bismarck, dakota: that, at o'clock, in the evening of may , , sharp sounds were heard throughout the city, caused by a fall of flinty stones striking against windows. fifteen hours later another fall of flinty stones occurred at bismarck. there is no report of stones having fallen anywhere else. this is a thing of the ultra-damned. all editors of scientific publications read the _monthly weather review_ and frequently copy from it. the noise made by the stones of bismarck, rattling against those windows, may be in a language that aviators will some day interpret: but it was a noise entirely surrounded by silences. of this ultra-damned thing, there is no mention, findable by me, in any other publication. the size of some hailstones has worried many meteorologists--but not text-book meteorologists. i know of no more serene occupation than that of writing text-books--though writing for the _war cry_, of the salvation army, may be equally unadventurous. in the drowsy tranquillity of a text-book, we easily and unintelligently read of dust particles around which icy rain forms, hailstones, in their fall, then increasing by accretion--but in the meteorological journals, we read often of air-spaces nucleating hailstones-- but it's the size of the things. dip a marble in icy water. dip and dip and dip it. if you're a resolute dipper, you will, after a while, have an object the size of a baseball--but i think a thing could fall from the moon in that length of time. also the strata of them. the maryland hailstones are unusual, but a dozen strata have often been counted. ferrel gives an instance of thirteen strata. such considerations led prof. schwedoff to argue that some hailstones are not, and cannot, be generated in this earth's atmosphere--that they come from somewhere else. now, in a relative existence, nothing can of itself be either attractive or repulsive: its effects are functions of its associations or implications. many of our data have been taken from very conservative scientific sources: it was not until their discordant implications, or irreconcilabilities with the system, were perceived, that excommunication was pronounced against them. prof. schwedoff's paper was read before the british association (_rept. of _, p. ). the implication, and the repulsiveness of the implication to the snug and tight little exclusionists of --though we hold out that they were functioning well and ably relatively to -- that there is water--oceans or lakes and ponds, or rivers of it--that there is water away from, and yet not far-remote from, this earth's atmosphere and gravitation-- the pain of it: that the snug little system of would be ousted from its reposefulness-- a whole new science to learn: the science of super-geography-- and science is a turtle that says that its own shell encloses all things. so the members of the british association. to some of them prof. schwedoff's ideas were like slaps on the back of an environment-denying turtle: to some of them his heresy was like an offering of meat, raw and dripping, to milk-fed lambs. some of them bleated like lambs, and some of them turled like turtles. we used to crucify, but now we ridicule: or, in the loss of vigor of all progress, the spike has etherealized into the laugh. sir william thomson ridiculed the heresy, with the phantomosities of his era: that all bodies, such as hailstones, if away from this earth's atmosphere, would have to move at planetary velocity--which would be positively reasonable if the pronouncements of st. isaac were anything but articles of faith--that a hailstone falling through this earth's atmosphere, with planetary velocity, would perform , times as much work as would raise an equal weight of water one degree centigrade, and therefore never fall as a hailstone at all; be more than melted--super-volatalized-- these turls and these bleats of pedantry--though we insist that, relatively to , these turls and bleats should be regarded as respectfully as we regard rag dolls that keep infants occupied and noiseless--it is the survival of rag dolls into maturity that we object to--so these pious and naïve ones who believed that , times something could have--that is, in quasi-existence--an exact and calculable resultant, whereas there is--in quasi-existence--nothing that can, except by delusion and convenience, be called a unit, in the first place--whose devotions to st. isaac required blind belief in formulas of falling bodies-- against data that were piling up, in their own time, of slow-falling meteorites; "milk warm" ones admitted even by farrington and merrill; at least one icy meteorite nowhere denied by the present orthodoxy, a datum as accessible to thomson, in , as it is now to us, because it was an occurrence of . beans and needles and tacks and a magnet. needles and tacks adhere to and systematize relatively to a magnet, but, if some beans, too, be caught up, they are irreconcilables to this system and drop right out of it. a member of the salvation army may hear over and over data that seem so memorable to an evolutionist. it seems remarkable that they do not influence him--one finds that he cannot remember them. it is incredible that sir william thomson had never heard of slow-falling, cold meteorites. it is simply that he had no power to remember such irreconcilabilities. and then mr. symons again. mr. symons was a man who probably did more for the science of meteorology than did any other man of his time: therefore he probably did more to hold back the science of meteorology than did any other man of his time. in _nature_, - , mr. symons says that prof. schwedoff's ideas are "very droll." i think that even more amusing is our own acceptance that, not very far above this earth's surface, is a region that will be the subject of a whole new science--super-geography--with which we shall immortalize ourselves in the resentments of the schoolboys of the future-- pebbles and fragments of meteors and things from mars and jupiter and azuria: wedges, delayed messages, cannon balls, bricks, nails, coal and coke and charcoal and offensive old cargoes--things that coat in ice in some regions and things that get into areas so warm that they putrefy--or that there are all the climates of geography in super-geography. i shall have to accept that, floating in the sky of this earth, there often are fields of ice as extensive as those on the arctic ocean--volumes of water in which are many fishes and frogs--tracts of land covered with caterpillars-- aviators of the future. they fly up and up. then they get out and walk. the fishing's good: the bait's right there. they find messages from other worlds--and within three weeks there's a big trade worked up in forged messages. sometime i shall write a guide book to the super-sargasso sea, for aviators, but just at present there wouldn't be much call for it. we now have more of our expression upon hail as a concomitant, or more data of things that have fallen from the sky, with hail. in general, the expression is: these things may have been raised from some other part of the earth's surface, in whirlwinds, or may not have fallen, and may have been upon the ground, in the first place--but were the hailstones found with them, raised from some other part of the earth's surface, or were the hailstones upon the ground, in the first place? as i said before, this expression is meaningless as to a few instances; it is reasonable to think of some coincidence between the fall of hail and the fall of other things: but, inasmuch as there have been a good many instances,--we begin to suspect that this is not so much a book we're writing as a sanitarium for overworked coincidences. if not conceivably could very large hailstones and lumps of ice form in this earth's atmosphere, and so then had to come from external regions, then other things in or accompanying very large hailstones and lumps of ice came from external regions--which worries us a little: we may be instantly translated to the positive absolute. _cosmos_, - , quotes a virginia newspaper, that fishes said to have been catfishes, a foot long, some of them, had fallen, in , at norfolk, virginia, with hail. vegetable débris, not only nuclear, but frozen upon the surfaces of large hailstones, at toulouse, france, july , . (_la science pour tous_, - .) description of a storm, at pontiac, canada, july , , in which it is said that it was not hailstones that fell, but "pieces of ice, from half an inch to over two inches in diameter" (_canadian naturalist_, - - ): "but the most extraordinary thing is that a respectable farmer, of undoubted veracity, says he picked up a piece of hail, or ice, in the center of which was a small green frog." storm at dubuque, iowa, june , , in which fell hailstones and pieces of ice (_monthly weather review_, june, ): "the foreman of the novelty iron works, of this city, states that in two large hailstones melted by him were found small living frogs." but the pieces of ice that fell upon this occasion had a peculiarity that indicates--though by as bizarre an indication as any we've had yet--that they had been for a long time motionless or floating somewhere. we'll take that up soon. _living age_, - : that, june , , fishes, one of which was ten inches long, fell at boston; that, eight days later, fishes and ice fell at derby. in timb's _year book_, - , it is said that, at derby, the fishes had fallen in enormous numbers; from half an inch to two inches long, and some considerably larger. in the _athenæum_, - , copied from the sheffield _patriot_, it is said that one of the fishes weighed three ounces. in several accounts, it is said that, with the fishes, fell many small frogs and "pieces of half-melted ice." we are told that the frogs and the fishes had been raised from some other part of the earth's surface, in a whirlwind; no whirlwind specified; nothing said as to what part of the earth's surface comes ice, in the month of july--interests us that the ice is described as "half-melted." in the london _times_, july , , it is said that the fishes were sticklebacks; that they had fallen with ice and small frogs, many of which had survived the fall. we note that, at dunfermline, three months later (oct. , ) fell many fishes, several inches in length, in a thunderstorm. (london _times_, oct. , .) hailstones, we don't care so much about. the matter of stratification seems significant, but we think more of the fall of lumps of ice from the sky, as possible data of the super-sargasso sea: lumps of ice, a foot in circumference, derbyshire, england, may , (_annual register_, - ); cuboidal mass, six inches in diameter, that fell at birmingham, days later (thomson, _intro. to meteorology_, p. ); size of pumpkins, bangalore, india, may , (_rept. brit. assoc._, - ); masses of ice of a pound and a half each, new hampshire, aug. , (lummis, _meteorology_, p. ); masses of ice, size of a man's head, in the delphos tornado (ferrel, _popular treatise_, p. ); large as a man's hand, killing thousands of sheep, texas, may , (_monthly weather review_, may, ); "pieces of ice so large that they could not be grasped in one hand," in a tornado, in colorado, june , (_monthly weather review_, june, ); lumps of ice four and a half inches long, richmond, england, aug. , (_symons' met. mag._, - ); mass of ice, inches in circumference that fell with hail, iowa, june, (_monthly weather review_, june, ); "pieces of ice" eight inches long, and an inch and a half thick, davenport, iowa, aug. , (_monthly weather review_, aug., ); lump of ice size of a brick; weight two pounds, chicago, july , (_monthly weather review_, july, ); lumps of ice that weighed one pound and a half each, india, may (?), (_nature_, - ); lump of ice weighing four pounds, texas, dec. , (_sc. am._, - ); lumps of ice one pound in weight, nov. , , in a tornado, victoria (_meteorology of australia_, p. ). of course it is our acceptance that these masses not only accompanied tornadoes, but were brought down to this earth by tornadoes. flammarion, _the atmosphere_, p. : block of ice, weighing four and a half pounds that fell at cazorta, spain, june , ; block of ice, weighing eleven pounds, at cette, france, october, ; mass of ice three feet long, three feet wide, and more than two feet thick, that fell, in a storm, in hungary, may , . _scientific american_, - : that, according to the _salina journal_, a mass of ice weighing about pounds had fallen from the sky, near salina, kansas, august, . we are told that mr. w.j. hagler, the north santa fé merchant became possessor of it, and packed it in sawdust in his store. london _times_, april , : that, upon the th of march, , in a snowstorm, in upper wasdale, blocks of ice, so large that at a distance they looked like a flock of sheep, had fallen. _rept. brit. assoc._, - : that a mass of ice about a cubic yard in size had fallen at candeish, india, . against these data, though, so far as i know, so many of them have never been assembled together before, there is a silence upon the part of scientific men that is unusual. our super-sargasso sea may not be an unavoidable conclusion, but arrival upon this earth of ice from external regions does seem to be--except that there must be, be it ever so faint, a merger. it is in the notion that these masses of ice are only congealed hailstones. we have data against this notion, as applied to all our instances, but the explanation has been offered, and, it seems to me, may apply in some instances. in the _bull. soc. astro. de france_, - , it is said of blocks of ice the size of decanters that had fallen at tunis that they were only masses of congealed hailstones. london _times_, aug. , . that a block of ice, described as "pure" ice, weighing pounds, had been found in the meadow of mr. warner, of cricklewood. there had been a storm the day before. as in some of our other instances, no one had seen this object fall from the sky. it was found after the storm: that's all that can be said about it. letter from capt. blakiston, communicated by gen. sabine, to the royal society (_london roy. soc. proc._, - ): that, jan. , , in a thunderstorm, pieces of ice had fallen upon capt. blakiston's vessel--that it was not hail. "it was not hail, but irregular-shaped pieces of solid ice of different dimensions, up to the size of half a brick." according to the _advertiser-scotsman_, quoted by the edinburgh _new philosophical magazine_, - , an irregular-shaped mass of ice fell at ord, scotland, august, , after "an extraordinary peal of thunder." it is said that this was homogeneous ice, except in a small part, which looked like congealed hailstones. the mass was about feet in circumference. the story, as told in the london _times_, aug. , , is that, upon the evening of the th of august, , after a loud peal of thunder, a mass of ice said to have been feet in circumference, had fallen upon the estate of mr. moffat, of balvullich, ross-shire. it is said that this object fell alone, or without hailstones. altogether, though it is not so strong for the super-sargasso sea, i think this is one of our best expressions upon external origins. that large blocks of ice could form in the moisture of this earth's atmosphere is about as likely as that blocks of stone could form in a dust whirl. of course, if ice or water comes to this earth from external sources, we think of at least minute organisms in it, and on, with our data, to frogs, fishes; on to anything that's thinkable, coming from external sources. it's of great importance to us to accept that large lumps of ice have fallen from the sky, but what we desire most--perhaps because of our interest in its archaeologic and palaeontologic treasures--is now to be through with tentativeness and probation, and to take the super-sargasso sea into full acceptance in our more advanced fold of the chosen of this twentieth century. in the _report of the british association_, - , it is said that, at poorhundur, india, dec. , , flat pieces of ice, many of them weighing several pounds--each, i suppose--had fallen from the sky. they are described as "large ice-flakes." vast fields of ice in the super-arctic regions, or strata, of the super-sargasso sea. when they break up, their fragments are flake-like. in our acceptance, there are aerial ice-fields that are remote from this earth; that break up, fragments grinding against one another, rolling in vapor and water, of different constituency in different regions, forming slowly as stratified hailstones--but that there are ice-fields near this earth, that break up into just such flat pieces of ice as cover any pond or river when ice of a pond or river is broken, and are sometimes soon precipitated to the earth, in this familiar flat formation. _symons' met. mag._, - : a correspondent writes that, at braemar, july , , when the sky was clear overhead, and the sun shining, flat pieces of ice fell--from somewhere. the sun was shining, but something was going on somewhere: thunder was heard. until i saw the reproduction of a photograph in the _scientific american_, feb. , , i had supposed that these ice-fields must be, say, at least ten or twenty miles away from this earth, and invisible, to terrestrial observers, except as the blurs that have so often been reported by astronomers and meteorologists. the photograph published by the _scientific american_ is of an aggregation supposed to be clouds, presumably not very high, so clearly detailed are they. the writer says that they looked to him like "a field of broken ice." beneath is a picture of a conventional field of ice, floating ordinarily in water. the resemblance between the two pictures is striking--nevertheless, it seems to me incredible that the first of the photographs could be of an aerial ice-field, or that gravitation could cease to act at only a mile or so from this earth's surface-- unless: the exceptional: the flux and vagary of all things. or that normally this earth's gravitation extends, say, ten or fifteen miles outward--but that gravitation must be rhythmic. of course, in the pseudo-formulas of astronomers, gravitation as a fixed quantity is essential. accept that gravitation is a variable force, and astronomers deflate, with a perceptible hissing sound, into the punctured condition of economists, biologists, meteorologists, and all the others of the humbler divinities, who can admittedly offer only insecure approximations. we refer all who would not like to hear the hiss of escaping arrogance, to herbert spencer's chapters upon the rhythm of all phenomena. if everything else--light from the stars, heat from the sun, the winds and the tides; forms and colors and sizes of animals; demands and supplies and prices; political opinions and chemic reactions and religious doctrines and magnetic intensities and the ticking of clocks; and arrival and departure of the seasons--if everything else is variable, we accept that the notion of gravitation as fixed and formulable is only another attempted positivism, doomed, like all other illusions of realness in quasi-existence. so it is intermediatism to accept that, though gravitation may approximate higher to invariability than do the winds, for instance, it must be somewhere between the absolutes of stability and instability. here then we are not much impressed with the opposition of physicists and astronomers, fearing, a little mournfully, that their language is of expiring sibilations. so then the fields of ice in the sky, and that, though usually so far away as to be mere blurs, at times they come close enough to be seen in detail. for description of what i call a "blur," see _pop. sci. news_, february, --sky, in general, unusually clear, but, near the sun, "a white, slightly curdled haze, which was dazzlingly bright." we accept that sometimes fields of ice pass between the sun and the earth: that many strata of ice, or very thick fields of ice, or superimposed fields would obscure the sun--that there have been occasions when the sun was eclipsed by fields of ice: flammarion, _the atmosphere_, p. : that a profound darkness came upon the city of brussels, june , : there fell flat pieces of ice, an inch long. intense darkness at aitkin, minn., april , : sand and "solid chunks of ice" reported to have fallen (_science_, april , ). in _symons' meteorological magazine_, - , are outlined rough-edged but smooth-surfaced pieces of ice that fell at manassas, virginia, aug. , . they look as much like the roughly broken fragments of a smooth sheet of ice--as ever have roughly broken fragments of a smooth sheet of ice looked. about two inches across, and one inch thick. in _cosmos_, - , it is said that, at rouen, july , , fell irregular-shaped pieces of ice, about the size of a hand, described as looking as if all had been broken from one enormous block of ice. that, i think, was an aerial iceberg. in the awful density, or almost absolute stupidity of the th century, it never occurred to anybody to look for traces of polar bears or of seals upon these fragments. of course, seeing what we want to see, having been able to gather these data only because they are in agreement with notions formed in advance, we are not so respectful to our own notions as to a similar impression forced upon an observer who had no theory or acceptance to support. in general, our prejudices see and our prejudices investigate, but this should not be taken as an absolute. _monthly weather review_, july, : that, from the weather bureau, of portland, oregon, a tornado, of june , , was reported. fragments of ice fell from the sky. they averaged three to four inches square, and about an inch thick. in length and breadth they had the smooth surfaces required by our acceptance: and, according to the writer in the _review_, "gave the impression of a vast field of ice suspended in the atmosphere, and suddenly broken into fragments about the size of the palm of the hand." this datum, profoundly of what we used to call the "damned," or before we could no longer accept judgment, or cut and dried condemnation by infants, turtles, and lambs, was copied--but without comment--in the _scientific american_, - . our theology is something like this: of course we ought to be damned--but we revolt against adjudication by infants, turtles, and lambs. we now come to some remarkable data in a rather difficult department of super-geography. vast fields of aerial ice. there's a lesson to me in the treachery of the imaginable. most of our opposition is in the clearness with which the conventional, but impossible, becomes the imaginable, and then the resistant to modifications. after it had become the conventional with me, i conceived clearly of vast sheets of ice, a few miles above this earth--then the shining of the sun, and the ice partly melting--that note upon the ice that fell at derby--water trickling and forming icicles upon the lower surface of the ice sheet. i seemed to look up and so clearly visualized those icicles hanging like stalactites from a flat-roofed cave, in white calcite. or i looked up at the under side of an aerial ice-lump, and seemed to see a papillation similar to that observed by a calf at times. but then--but then--if icicles should form upon the under side of a sheet of aerial ice, that would be by the falling of water toward this earth; an icicle is of course an expression of gravitation--and, if water melting from ice should fall toward this earth, why not the ice itself fall before an icicle could have time to form? of course, in quasi-existence, where everything is a paradox, one might argue that the water falls, but the ice does not, because the ice is heavier--that is, in masses. that notion, i think, belongs in a more advanced course than we are taking at present. our expression upon icicles: a vast field of aerial ice--it is inert to this earth's gravitation--but by universal flux and variation, part of it sags closer to this earth, and is susceptible to gravitation--by cohesion with the main mass, this part does not fall, but water melting from it does fall, and forms icicles--then, by various disturbances, this part sometimes falls in fragments that are protrusive with icicles. of the ice that fell, some of it enclosing living frogs, at dubuque, iowa, june , , it is said (_monthly weather review_, june, ) that there were pieces from one to seventeen inches in circumference, the largest weighing one pound and three-quarters--that upon some of them were icicles half an inch in length. we emphasize that these objects were not hailstones. the only merger is that of knobby hailstones, or of large hailstones with protuberances wrought by crystallization: but that is no merger with terrestrial phenomena, and such formations are unaccountable to orthodoxy; or it is incredible that hail could so crystallize--not forming by accretion--in the fall of a few seconds. for an account of such hailstones, see _nature_, - . note the size--"some of them the size of turkeys' eggs." it is our expression that sometimes the icicles themselves have fallen, as if by concussion, or as if something had swept against the under side of an aerial ice floe, detaching its papillations. _monthly weather review_, june, : that, at oswego, n.y., june , , according to the turin (n.y.) _leader_, there fell, in a thunderstorm, pieces of ice that "resembled the fragments of icicles." _monthly weather review_, - : that on florence island, st. lawrence river, aug. , , with ordinary hail, fell pieces of ice "formed like icicles, the size and shape of lead pencils that had been cut into sections about three-eighths of an inch in length." so our data of the super-sargasso sea, and its arctic region: and, for weeks at a time, an ice field may hang motionless over a part of this earth's surface--the sun has some effect upon it, but not much until late in the afternoon, i should say--part of it has sagged, but is held up by cohesion with the main mass--whereupon we have such an occurrence as would have been a little uncanny to us once upon a time--or fall of water from a cloudless sky, day after day, in one small part of this earth's surface, late in the afternoon, when the sun's rays had had time for their effects: _monthly weather review_, october, : that, according to the charlotte _chronicle_, oct. , , for three weeks there had been a fall of water from the sky, in charlotte, n.c., localized in one particular spot, every afternoon, about three o'clock; that, whether the sky was cloudy or cloudless, the water or rain fell upon a small patch of land between two trees and nowhere else. this is the newspaper account, and, as such, it seems in the depths of the unchosen, either by me or any other expression of the salvation army. the account by the signal service observer, at charlotte, published in the _review_, follows: "an unusual phenomenon was witnessed on the st: having been informed that, for some weeks prior to date, rain had been falling daily, after p.m., on a particular spot, near two trees, corner of th and d streets, i visited the place, and saw precipitation in the form of rain drops at : and : p.m., while the sun was shining brightly. on the nd, i again visited the place, and from : to : p.m., a light shower of rain fell from a cloudless sky.... sometimes the precipitation falls over an area of half an acre, but always appears to center at these two trees, and when lightest occurs there only." we see conventionally. it is not only that we think and act and speak and dress alike, because of our surrender to social attempt at entity, in which we are only super-cellular. we see what it is "proper" that we should see. it is orthodox enough to say that a horse is not a horse, to an infant--any more than is an orange an orange to the unsophisticated. it's interesting to walk along a street sometimes and look at things and wonder what they'd look like, if we hadn't been taught to see horses and trees and houses as horses and trees and houses. i think that to super-sight they are local stresses merging indistinguishably into one another, in an all-inclusive nexus. i think that it would be credible enough to say that many times have monstrator and elvera and azuria crossed telescopic fields of vision, and were not even seen--because it wouldn't be proper to see them; it wouldn't be respectable, and it wouldn't be respectful: it would be insulting to old bones to see them: it would bring on evil influences from the relics of st. isaac to see them. but our data: of vast worlds that are orbitless, or that are navigable, or that are adrift in inter-planetary tides and currents: the data that we shall have of their approach, in modern times, within five or six miles of this earth-- but then their visits, or approaches, to other planets, or to other of the few regularized bodies that have surrendered to the attempted entity of this solar system as a whole-- the question that we can't very well evade: have these other worlds, or super-constructions, ever been seen by astronomers? i think there would not be much approximation to realness in taking refuge in the notion of astronomers who stare and squint and see only that which it is respectable and respectful to see. it is all very well to say that astronomers are hypnotics, and that an astronomer looking at the moon is hypnotized by the moon, but our acceptance is that the bodies of this present expression often visit the moon, or cross it, or are held in temporary suspension near it--then some of them must often have been within the diameter of an astronomer's hypnosis. our general expression: that, upon the oceans of this earth, there are regularized vessels, but also that there are tramp vessels: that, upon the super-ocean, there are regularized planets, but also that there are tramp worlds: that astronomers are like mercantile purists who would deny commercial vagabondage. our acceptance is that vast celestial vagabonds have been excluded by astronomers, primarily because their irresponsibilities are an affront to the pure and the precise, or to attempted positivism; and secondarily because they have not been seen so very often. the planets steadily reflect the light of the sun: upon this uniformity a system that we call primary astronomy has been built up; but now the subject-matter of advanced astronomy is data of celestial phenomena that are sometimes light and sometimes dark, varying like some of the satellites of jupiter, but with a wider range. however, light or dark, they have been seen and reported so often that the only important reason for their exclusion is--that they don't fit in. with dark bodies that are probably external to our own solar system, i have, in the provincialism that no one can escape, not much concern. dark bodies afloat in outer space would have been damned a few years ago, but now they're sanctioned by prof. barnard--and, if he says they're all right, you may think of them without the fear of doing something wrong or ridiculous--the close kinship we note so often between the evil and the absurd--i suppose by the ridiculous i mean the froth of evil. the dark companion of algol, for instance. though that's a clear case of celestial miscegenation, the purists, or positivists, admit that's so. in the _proceedings of the national academy of science_, - , prof. barnard writes of an object--he calls it an "object"--in cephus. his idea is that there are dark, opaque bodies outside this solar system. but in the _astrophysical journal_, - , he modifies into regarding them as "dark nebulæ." that's not so interesting. we accept that venus, for instance, has often been visited by other worlds, or by super-constructions, from which come ciders and coke and coal; that sometimes these things have reflected light and have been seen from this earth--by professional astronomers. it will be noted that throughout this chapter our data are accursed brahmins--as, by hypnosis and inertia, we keep on and keep on saying, just as a good many of the scientists of the th century kept on and kept on admitting the power of the system that preceded them--or continuity would be smashed. there's a big chance here for us to be instantaneously translated to the positive absolute--oh, well-- what i emphasize here is that our damned data are observations by astronomers of the highest standing, excommunicated by astronomers of similar standing--but backed up by the dominant spirit of their era--to which all minds had to equilibrate or be negligible, unheard, submerged. it would seem sometimes, in this book, as if our revolts were against the dogmatisms and pontifications of single scientists of eminence. this is only a convenience, because it seems necessary to personify. if we look over _philosophical transactions_, or the publications of the royal astronomical society, for instance, we see that herschel, for instance, was as powerless as any boy stargazer, to enforce acceptance of any observation of his that did not harmonize with the system that was growing up as independently of him and all other astronomers, as a phase in the development of an embryo compels all cells to take on appearances concordantly with the design and the predetermined progress and schedule of the whole. visitors to venus: evans, _ways of the planets_, p. : that, in , a body large enough to look like a satellite was seen near venus. four times in the first half of the th century, a similar observation was reported. the last report occurred in . a large body has been seen--seven times, according to _science gossip_, - --near venus. at least one astronomer, houzeau, accepted these observations and named the--world, planet, super-construction--"neith." his views are mentioned "in passing, but without endorsement," in the _trans. n.y. acad._, - . houzeau or someone writing for the magazine-section of a sunday newspaper--outer darkness for both alike. a new satellite in this solar system might be a little disturbing--though the formulas of laplace, which were considered final in his day, have survived the admittance of five or six hundred bodies not included in those formulas--a satellite to venus might be a little disturbing, but would be explained--but a large body approaching a planet--staying awhile--going away--coming back some other time--anchoring, as it were-- azuria is pretty bad, but azuria is no worse than neith. _astrophysical journal_, - : a light-reflecting body, or a bright spot near mars: seen nov. , , by prof. pickering and others, at the lowell observatory, above an unilluminated part of mars--self-luminous, it would seem--thought to have been a cloud--but estimated to have been about twenty miles away from the planet. luminous spot seen moving across the disk of mercury, in , by harding and schroeter. (_monthly notices of the r.a.s._, - .) in the first bulletin issued by the lowell observatory, in , prof. lowell describes a body that was seen on the terminator of mars, may , . on may , it was "suspected." if still there, it had moved, we are told, about miles--"probably a dust cloud." very conspicuous and brilliant spots seen on the disk of mars, october and november, . (_popular astronomy_, vol. , no. .) so one of them accepted six or seven observations that were in agreement, except that they could not be regularized, upon a world--planet--satellite--and he gave it a name. he named it "neith." monstrator and elvera and azuria and super-romanimus-- or heresy and orthodoxy and the oneness of all quasiness, and our ways and means and methods are the very same. or, if we name things that may not be, we are not of lonely guilt in the nomenclature of absences-- but now leverrier and "vulcan." leverrier again. or to demonstrate the collapsibility of a froth, stick a pin in the largest bubble of it. astronomy and inflation: and by inflation we mean expansion of the attenuated. or that the science of astronomy is a phantom-film distended with myth-stuff--but always our acceptance that it approximates higher to substantiality than did the system that preceded it. so leverrier and the "planet vulcan." and we repeat, and it will do us small good to repeat. if you be of the masses that the astronomers have hypnotized--being themselves hypnotized, or they could not hypnotize others--or that the hypnotist's control is not the masterful power that it is popularly supposed to be, but only transference of state from one hypnotic to another-- if you be of the masses that the astronomers have hypnotized, you will not be able even to remember. ten pages from here, and leverrier and the "planet vulcan" will have fallen from your mind, like beans from a magnet, or like data of cold meteorites from the mind of a thomson. leverrier and the "planet vulcan." and much the good it will do us to repeat. but at least temporarily we shall have an impression of a historic fiasco, such as, in our acceptance, could occur only in a quasi-existence. in , dr. lescarbault, an amateur astronomer, of orgères, france, announced that, upon march , of that year, he had seen a body of planetary size cross the sun. we are in a subject that is now as unholy to the present system as ever were its own subjects to the system that preceded it, or as ever were slanders against miracles to the preceding system. nevertheless few text-books go so far as quite to disregard this tragedy. the method of the systematists is slightingly to give a few instances of the unholy, and dispose of the few. if it were desirable to them to deny that there are mountains upon this earth, they would record a few observations upon some slight eminences near orange, n.j., but say that commuters, though estimable persons in several ways, are likely to have their observations mixed. the text-books casually mention a few of the "supposed" observations upon "vulcan," and then pass on. dr. lescarbault wrote to leverrier, who hastened to orgères-- because this announcement assimilated with his own calculations upon a planet between mercury and the sun-- because this solar system itself has never attained positiveness in the aspect of regularity: there are to mercury, as there are to neptune, phenomena irreconcilable with the formulas, or motions that betray influence by something else. we are told that leverrier "satisfied himself as to the substantial accuracy of the reported observation." the story of this investigation is told in _monthly notices_, - . it seems too bad to threaten the naïve little thing with our rude sophistications, but it is amusingly of the ingenuousness of the age from which present dogmas have survived. lescarbault wrote to leverrier. leverrier hastened to orgères. but he was careful not to tell lescarbault who he was. went right in and "subjected dr. lescarbault to a very severe cross-examination"--just the way you or i may feel at liberty to go into anybody's home and be severe with people--"pressing him hard step by step"--just as anyone might go into someone else's house and press him hard, though unknown to the hard-pressed one. not until he was satisfied, did leverrier reveal his identity. i suppose dr. lescarbault expressed astonishment. i think there's something utopian about this: it's so unlike the stand-offishness of new york life. leverrier gave the name "vulcan" to the object that dr. lescarbault had reported. by the same means by which he is, even to this day, supposed--by the faithful--to have discovered neptune, he had already announced the probable existence of an intra-mercurial body, or group of bodies. he had five observations besides lescarbault's upon something that had been seen to cross the sun. in accordance with the mathematical hypnoses of his era, he studied these six transits. out of them he computed elements giving "vulcan" a period of about days, or a formula for heliocentric longitude at any time. but he placed the time of best observation away up in . but even so, or considering that he still had probably a good many years to live, it may strike one that he was a little rash--that is if one has not gone very deep into the study of hypnoses--that, having "discovered" neptune by a method which, in our acceptance, had no more to recommend it than had once equally well-thought-of methods of witch-finding, he should not have taken such chances: that if he was right as to neptune, but should be wrong as to "vulcan," his average would be away below that of most fortune-tellers, who could scarcely hope to do business upon a fifty per cent. basis--all that the reasoning of a tyro in hypnoses. the date: march , . the scientific world was up on its hind legs nosing the sky. the thing had been done so authoritatively. never a pope had said a thing with more of the seeming of finality. if six observations correlated, what more could be asked? the editor of _nature_, a week before the predicted event, though cautious, said that it is difficult to explain how six observers, unknown to one another, could have data that could be formulated, if they were not related phenomena. in a way, at this point occurs the crisis of our whole book. formulas are against us. but can astronomic formulas, backed up by observations in agreement, taken many years apart, calculated by a leverrier, be as meaningless, in a positive sense, as all other quasi-things that we have encountered so far? the preparations they made, before march , . in england, the astronomer royal made it the expectation of his life: notified observers at madras, melbourne, sydney, and new zealand, and arranged with observers in chili and the united states. m. struve had prepared for observations in siberia and japan-- march , -- not absolutely, hypocritically, i think it's pathetic, myself. if anyone should doubt the sincerity of leverrier, in this matter, we note, whether it has meaning or not, that a few months later he died. i think we'll take up monstrator, though there's so much to this subject that we'll have to come back. according to the _annual register_, - , upon the th of august, , m. de rostan, of basle, france, was taking altitudes of the sun, at lausanne. he saw a vast, spindle-shaped body, about three of the sun's digits in breadth and nine in length, advancing slowly across the disk of the sun, or "at no more than half the velocity with which the ordinary solar spots move." it did not disappear until the th of september, when it reached the sun's limb. because of the spindle-like form, i incline to think of a super-zeppelin, but another observation, which seems to indicate that it was a world, is that, though it was opaque, and "eclipsed the sun," it had around it a kind of nebulosity--or atmosphere? a penumbra would ordinarily be a datum of a sun spot, but there are observations that indicate that this object was at a considerable distance from the sun: it is recorded that another observer, at paris, watching the sun, at this time, had not seen this object: but that m. croste, at sole, about forty-five german leagues northward from lausanne, had seen it, describing the same spindle-form, but disagreeing a little as to breadth. then comes the important point: that he and m. de rostan did not see it upon the same part of the sun. this, then, is parallax, and, compounded with invisibility at paris, is great parallax--or that, in the course of a month, in the summer of , a large, opaque, spindle-shaped body traversed the disk of the sun, but at a great distance from the sun. the writer in the _register_ says: "in a word, we know of nothing to have recourse to, in the heavens, by which to explain this phenomenon." i suppose he was not a hopeless addict to explaining. extraordinary--we fear he must have been a man of loose habits in some other respects. as to us-- monstrator. in the _monthly notices of the r.a.s._, february, , leverrier, who never lost faith, up to the last day, gives the six observations upon an unknown body of planetary size, that he had formulated: fritsche, oct. , ; stark, oct. , ; de cuppis, oct. , ; sidebotham, nov. , ; lescarbault, march , ; lummis, march , . if we weren't so accustomed to science in its essential aspect of disregard, we'd be mystified and impressed, like the editor of _nature_, with the formulation of these data: agreement of so many instances would seem incredible as a coincidence: but our acceptance is that, with just enough disregard, astronomers and fortune-tellers can formulate anything--or we'd engage, ourselves, to formulate periodicities in the crowds in broadway--say that every wednesday morning, a tall man, with one leg and a black eye, carrying a rubber plant, passes the singer building, at quarter past ten o'clock. of course it couldn't really be done, unless such a man did have such periodicity, but if some wednesday mornings it should be a small child lugging a barrel, or a fat negress with a week's wash, by ordinary disregard that would be prediction good enough for the kind of quasi-existence we're in. so whether we accuse, or whether we think that the word "accuse" over-dignifies an attitude toward a quasi-astronomer, or mere figment in a super-dream, our acceptance is that leverrier never did formulate observations-- that he picked out observations that could be formulated-- that of this type are all formulas-- that, if leverrier had not been himself helplessly hypnotized, or if he had had in him more than a tincture of realness, never could he have been beguiled by such a quasi-process: but that he was hypnotized, and so extended, or transferred, his condition to others, that upon march , , he had this earth bristling with telescopes, with the rigid and almost inanimate forms of astronomers behind them-- and not a blessed thing of any unusuality was seen upon that day or succeeding days. but that the science of astronomy suffered the slightest in prestige? it couldn't. the spirit of was behind it. if, in an embryo, some cells should not live up to the phenomena of their era, the others will sustain the scheduled appearances. not until an embryo enters the mammalian stage are cells of the reptilian stage false cells. it is our acceptance that there were many equally authentic reports upon large planetary bodies that had been seen near the sun; that, of many, leverrier picked out six; not then deciding that all the other observations related to still other large, planetary bodies, but arbitrarily, or hypnotically, disregarding--or heroically disregarding--every one of them--that to formulate at all he had to exclude falsely. the dénouement killed him, i think. i'm not at all inclined to place him with the grays and hitchcocks and symonses. i'm not, because, though it was rather unsportsmanlike to put the date so far ahead, he did give a date, and he did stick to it with such a high approximation-- i think leverrier was translated to the positive absolute. the disregarded: observation, of july , , by gruthinson--but that was of two bodies that crossed the sun together-- _nature_, - : that, according to the astronomer, j.r. hind, benjamin scott, city chamberlain of london, and mr. wray, had, in , seen a body similar to "vulcan" cross the sun. similar observation by hind and lowe, march , (_l'année scientifique_, - ). _nature_, - : body of apparent size of mercury, seen, jan. , , by f.a.r. russell and four other observers, crossing the sun. de vico's observation of july , (_observatory_, - ). _l'année scientifique_, - : that another amateur astronomer, m. coumbray, of constantinople, had written to leverrier, that, upon the th of march, , he had seen a black point, sharply outlined, traverse the disk of the sun. it detached itself from a group of sun spots near the limb of the sun, and took minutes to reach the other limb. figuring upon the diagram sent by m. coumbray, a central passage would have taken a little more than an hour. this observation was disregarded by leverrier, because his formula required about four times that velocity. the point here is that these other observations are as authentic as those that leverrier included; that, then, upon data as good as the data of "vulcan," there must be other "vulcans"--the heroic and defiant disregard, then, of trying to formulate one, omitting the others, which, by orthodox doctrine, must have influenced it greatly, if all were in the relatively narrow space between mercury and the sun. observation upon another such body, of april , , by m. weber, of berlin. as to this observation, leverrier was informed by wolf, in august, (_l'année scientifique_, - ). it made no difference, so far as can be known, to this notable positivist. two other observations noted by hind and denning--london _times_, nov. , , and march , . _monthly notices of the r.a.s._, - : standacher, february, ; lichtenberg, nov. , ; hoffman, may, ; dangos, jan. , ; stark, feb. , . an observation by schmidt, oct. , , is said to be doubtful: but, upon page , it is said that this doubt had arisen because of a mistaken translation, and two other observations by schmidt are given: oct. , , and feb. , --also an observation by lofft, jan. , . observation by steinheibel, at vienna, april , (_monthly notices_, ). haase had collected reports of twenty observations like lescarbault's. the list was published in , by wolf. also there are other instances like gruthinsen's: _amer. jour. sci._, - - : report by pastorff that he had seen twice in , and once in , two round spots of unequal size moving across the sun, changing position relatively to each other, and taking a different course, if not orbit, each time: that, in , he had seen similar bodies pass six times across the disk of the sun, looking very much like mercury in his transits. march , -- but to point out leverrier's poverty-stricken average--or discovering planets upon a fifty per cent. basis--would be to point out the low percentage of realness in the quasi-myth-stuff of which the whole system is composed. we do not accuse the text-books of omitting this fiasco, but we do note that theirs is the conventional adaptation here of all beguilers who are in difficulties-- the diverting of attention. it wouldn't be possible in a real existence, with real mentality, to deal with, but i suppose it's good enough for the quasi-intellects that stupefy themselves with text-books. the trick here is to gloss over leverrier's mistake, and blame lescarbault--he was only an amateur--had delusions. the reader's attention is led against lescarbault by a report from m. lias, director of the brazilian coast survey, who, at the time of lescarbault's "supposed" observation had been watching the sun in brazil, and, instead of seeing even ordinary sun spots, had noted that the region of the "supposed transit" was of "uniform intensity." but the meaninglessness of all utterances in quasi-existence-- "uniform intensity" turns our way as much as against us--or some day some brain will conceive a way of beating newton's third law--if every reaction, or resistance, is, or can be, interpretable as stimulus instead of resistance--if this could be done in mechanics, there's a way open here for someone to own the world--specifically in this matter, "uniform intensity" means that lescarbault saw no ordinary sun spot, just as much as it means that no spot at all was seen upon the sun. continuing the interpretation of a resistance as an assistance, which can always be done with mental forces--making us wonder what applications could be made with steam and electric forces--we point out that invisibility in brazil means parallax quite as truly as it means absence, and, inasmuch as "vulcan" was supposed to be distant from the sun, we interpret denial as corroboration--method of course of every scientist, politician, theologian, high-school debater. so the text-books, with no especial cleverness, because no especial cleverness is needed, lead the reader into contempt for the amateur of orgères, and forgetfulness of leverrier--and some other subject is taken up. but our own acceptance: that these data are as good as ever they were; that, if someone of eminence should predict an earthquake, and if there should be no earthquake at the predicted time, that would discredit the prophet, but data of past earthquakes would remain as good as ever they had been. it is easy enough to smile at the illusion of a single amateur-- the mass-formation: fritsche, stark, de cuppis, sidebotham, lescarbault, lummis, gruthinson, de vico, scott, wray, russell, hind, lowe, coumbray, weber, standacher, lichtenberg, dangos, hoffman, schmidt, lofft, steinheibel, pastorff-- these are only the observations conventionally listed relatively to an intra-mercurial planet. they are formidable enough to prevent our being diverted, as if it were all the dream of a lonely amateur--but they're a mere advance-guard. from now on other data of large celestial bodies, some dark and some reflecting light, will pass and pass and keep on passing-- so that some of us will remember a thing or two, after the procession's over--possibly. taking up only one of the listed observations-- or our impression that the discrediting of leverrier has nothing to do with the acceptability of these data: in the london _times_, jan. , , is benjamin scott's account of his observation: that, in the summer of , he had seen a body that had seemed to be the size of venus, crossing the sun. he says that, hardly believing the evidence of his sense of sight, he had looked for someone, whose hopes or ambitions would not make him so subject to illusion. he had told his little son, aged five years, to look through the telescope. the child had exclaimed that he had seen "a little balloon" crossing the sun. scott says that he had not had sufficient self-reliance to make public announcement of his remarkable observation at the time, but that, in the evening of the same day, he had told dr. dick, f.r.a.s., who had cited other instances. in the _times_, jan. , , is published a letter from richard abbott, f.r.a.s.: that he remembered mr. scott's letter to him upon this observation, at the time of the occurrence. i suppose that, at the beginning of this chapter, one had the notion that, by hard scratching through musty old records we might rake up vague, more than doubtful data, distortable into what's called evidence of unrecognized worlds or constructions of planetary size-- but the high authenticity and the support and the modernity of these of the accursed that we are now considering-- and our acceptance that ours is a quasi-existence, in which above all other things, hopes, ambitions, emotions, motivations, stands attempt to positivize: that we are here considering an attempt to systematize that is sheer fanaticism in its disregard of the unsystematizable--that it represented the highest good in the th century--that it is mono-mania, but heroic mono-mania that was quasi-divine in the th century-- but that this isn't the th century. as a doubly sponsored brahmin--in the regard of baptists--the objects of july , , stand out and proclaim themselves so that nothing but disregard of the intensity of mono-mania can account for their reception by the system: or the total eclipse of july , , and the reports by prof. watson, from rawlins, wyoming, and by prof. swift, from denver, colorado: that they had seen two shining objects at a considerable distance from the sun. it's quite in accord with our general expression: not that there is an intra-mercurial planet, but that there are different bodies, many vast things; near this earth sometimes, near the sun sometimes; orbitless worlds, which, because of scarcely any data of collisions, we think of as under navigable control--or dirigible super-constructions. prof. watson and prof. swift published their observations. then the disregard that we cannot think of in terms of ordinary, sane exclusions. the text-book systematists begin by telling us that the trouble with these observations is that they disagree widely: there is considerable respectfulness, especially for prof. swift, but we are told that by coincidence these two astronomers, hundreds of miles apart, were illuded: their observations were so different-- prof. swift (_nature_, sept. , ): that his own observation was "in close approximation to that given by prof. watson." in the _observatory_, - , swift says that his observations and watson's were "confirmatory of each other." the faithful try again: that watson and swift mistook stars for other bodies. in the _observatory_, - , prof. watson says that he had previously committed to memory all stars near the sun, down to the seventh magnitude-- and he's damned anyway. how such exclusions work out is shown by lockyer (_nature_, aug. , ). he says: "there is little doubt that an intra-mercurial planet has been discovered by prof. watson." that was before excommunication was pronounced. he says: "if it will fit one of leverrier's orbits"-- it didn't fit. in _nature_, - , prof. swift says: "i have never made a more valid observation, nor one more free from doubt." he's damned anyway. we shall have some data that will not live up to most rigorous requirements, but, if anyone would like to read how carefully and minutely these two sets of observations were made, see prof. swift's detailed description in the _am. jour. sci._, - ; and the technicalities of prof. watson's observations in _monthly notices_, - . our own acceptance upon dirigible worlds, which is assuredly enough, more nearly real than attempted concepts of large planets relatively near this earth, moving in orbits, but visible only occasionally; which more nearly approximates to reasonableness than does wholesale slaughter of swift and watson and fritsche and stark and de cuppis--but our own acceptance is so painful to so many minds that, in another of the charitable moments that we have now and then for the sake of contrast, we offer relief: the things seen high in the sky by swift and watson-- well, only two months before--the horse and the barn-- we go on with more observations by astronomers, recognizing that it is the very thing that has given them life, sustained them, held them together, that has crushed all but the quasi-gleam of independent life out of them. were they not systematized, they could not be at all, except sporadically and without sustenance. they are systematized: they must not vary from the conditions of the system: they must not break away for themselves. the two great commandments: thou shalt not break continuity; thou shalt try. we go on with these disregarded data, some of which, many of which, are of the highest degree of acceptability. it is the system that pulls back its variations, as this earth is pulling back the matterhorn. it is the system that nourishes and rewards, and also freezes out life with the chill of disregard. we do note that, before excommunication is pronounced, orthodox journals do liberally enough record unassimilable observations. all things merge away into everything else. that is continuity. so the system merges away and evades us when we try to focus against it. we have complained a great deal. at least we are not so dull as to have the delusion that we know just exactly what it is that we are complaining about. we speak seemingly definitely enough of "the system," but we're building upon observations by members of that very system. or what we are doing--gathering up the loose heresies of the orthodox. of course "the system" fringes and ravels away, having no real outline. a swift will antagonize "the system," and a lockyer will call him back; but, then, a lockyer will vary with a "meteoric hypothesis," and a swift will, in turn, represent "the system." this state is to us typical of all intermediatist phenomena; or that not conceivably is anything really anything, if its parts are likely to be their own opposites at any time. we speak of astronomers--as if there were real astronomers--but who have lost their identity in a system--as if it were a real system--but behind that system is plainly a rapport, or loss of identity in the spirit of an era. bodies that have looked like dark bodies, and lights that may have been sunlight reflected from inter-planetary--objects, masses, constructions-- lights that have been seen upon--or near?--the moon: in _philosophical transactions_, - , is herschel's report upon many luminous points, which he saw upon--or near?--the moon, during an eclipse. why they should be luminous, whereas the moon itself was dark, would get us into a lot of trouble--except that later we shall, or we sha'n't, accept that many times have luminous objects been seen close to this earth--at night. but numerousness is a new factor, or new disturbance, to our explorations-- a new aspect of inter-planetary inhabitancy or occupancy-- worlds in hordes--or beings--winged beings perhaps--wouldn't astonish me if we should end up by discovering angels--or beings in machines--argosies of celestial voyagers-- in and , herschel reported more lights on or near the moon, which he supposed were volcanic. the word of a herschel has had no more weight, in divergences from the orthodox, than has had the word of a lescarbault. these observations are of the disregarded. bright spots seen on the moon, november, (_proc. london roy. soc._, - ). for four other instances, see loomis (_treatise on astronomy_, p. ). a moving light is reported in _phil. trans._, - . to the writer, it looked like a star passing over the moon--"which, on the next moment's consideration i knew to be impossible." "it was a fixed, steady light upon the dark part of the moon." i suppose "fixed" applies to luster. in the _report of the brit. assoc._, - , there is an observation by rankin, upon luminous points seen on the shaded part of the moon, during an eclipse. they seemed to this observer like reflections of stars. that's not very reasonable: however, we have, in the _annual register_, - , a light not referable to a star--because it moved with the moon: was seen three nights in succession; reported by capt. kater. see _quart. jour. roy. inst._, - . _phil. trans._, - : report from the cape town observatory: a whitish spot on the dark part of the moon's limb. three smaller lights were seen. the call of positiveness, in its aspects of singleness, or homogeneity, or oneness, or completeness. in data now coming, i feel it myself. a leverrier studies more than twenty observations. the inclination is irresistible to think that they all relate to one phenomenon. it is an expression of cosmic inclination. most of the observations are so irreconcilable with any acceptance other than of orbitless, dirigible worlds that he shuts his eyes to more than two-thirds of them; he picks out six that can give him the illusion of completeness, or of all relating to one planet. or let it be that we have data of many dark bodies--still do we incline almost irresistibly to think of one of them as the dark-body-in-chief. dark bodies, floating, or navigating, in inter-planetary space--and i conceive of one that's the prince of dark bodies: melanicus. vast dark thing with the wings of a super-bat, or jet-black super-construction; most likely one of the spores of the evil one. the extraordinary year, : london _times_, dec. , : extract from a letter by hicks pashaw: that, in egypt, sept. , , he had seen, through glasses, "an immense black spot upon the lower part of the sun." sun spot, maybe. one night an astronomer was looking up at the sky, when something obscured a star, for three and a half seconds. a meteor had been seen nearby, but its train had been only momentarily visible. dr. wolf was the astronomer (_nature_, - ). the next datum is one of the most sensational we have, except that there is very little to it. a dark object that was seen by prof. heis, for eleven degrees of arc, moving slowly across the milky way. (greg's catalogue, _rept. brit. assoc._, - .) one of our quasi-reasons for accepting that orbitless worlds are dirigible is the almost complete absence of data of collisions: of course, though in defiance of gravitation, they may, without direction like human direction, adjust to one another in the way of vortex rings of smoke--a very human-like way, that is. but in _knowledge_, february, , are two photographs of brooks' comet that are shown as evidence of its seeming collision with a dark object, october, . our own wording is that it "struck against something": prof. barnard's is that it had "entered some dense medium, which shattered it." for all i know it had knocked against merely a field of ice. melanicus. that upon the wings of a super-bat, he broods over this earth and over other worlds, perhaps deriving something from them: hovers on wings, or wing-like appendages, or planes that are hundreds of miles from tip to tip--a super-evil thing that is exploiting us. by evil i mean that which makes us useful. he obscures a star. he shoves a comet. i think he's a vast, black, brooding vampire. _science_, july , : that, according to a newspaper account, mr. w.r. brooks, director of the smith observatory, had seen a dark round object pass rather slowly across the moon, in a horizontal direction. in mr. brooks' opinion it was a dark meteor. in _science_, sept. , , a correspondent writes that, in his opinion, it may have been a bird. we shall have no trouble with the meteor and bird mergers, if we have observations of long duration and estimates of size up to hundreds of miles. as to the body that was seen by brooks, there is a note from the dutch astronomer, muller, in the _scientific american_, - , that, upon april , , he had seen a similar phenomenon. in _science gossip_, n.s., - , are more details of the brooks object--apparent diameter about one-thirtieth of the moon's--moon's disk crossed in three or four seconds. the writer, in _science gossip_, says that, on june , , at one o'clock in the morning, he was looking at the moon with a -inch achromatic, power , when a long black object sailed past, from west to east, the transit occupying or seconds. he believed this object to be a bird--there was, however, no fluttering motion observable in it. in the _astronomische nachrichten_, no. , dr. brendel, of griefswald, pomerania, writes that postmaster ziegler and other observers had seen a body about feet in diameter crossing the sun's disk. the duration here indicates something far from the earth, and also far from the sun. this thing was seen a quarter of an hour before it reached the sun. time in crossing the sun was about an hour. after leaving the sun it was visible an hour. i think he's a vast, black vampire that sometimes broods over this earth and other bodies. communication from dr. f.b. harris (_popular astronomy_, - ): that, upon the evening of jan. , , dr. harris saw, upon the moon, "an intensely black object." he estimated it to be miles long and miles wide. "the object resembled a crow poised, as near as anything." clouds then cut off observation. dr. harris writes: "i cannot but think that a very interesting and curious phenomenon happened." short chapter coming now, and it's the worst of them all. i think it's speculative. it's a lapse from our usual pseudo-standards. i think it must mean that the preceding chapter was very efficiently done, and that now by the rhythm of all quasi-things--which can't be real things, if they're rhythms, because a rhythm is an appearance that turns into its own opposite and then back again--but now, to pay up, we're what we weren't. short chapter, and i think we'll fill in with several points in intermediatism. a puzzle: if it is our acceptance that, out of the negative absolute, the positive absolute is generating itself, recruiting, or maintaining, itself, via a third state, or our own quasi-state, it would seem that we're trying to conceive of universalness manufacturing more universalness from nothingness. take that up yourself, if you're willing to run the risk of disappearing with such velocity that you'll leave an incandescent train behind, and risk being infinitely happy forever, whereas you probably don't want to be happy--i'll sidestep that myself, and try to be intelligible by regarding the positive absolute from the aspect of realness instead of universalness, recalling that by both realness and universalness we mean the same state, or that which does not merge away into something else, because there is nothing else. so the idea is that out of unrealness, instead of nothingness, realness, instead of universalness, is, via our own quasi-state, manufacturing more realness. just so, but in relative terms, of course, all imaginings that materialize into machines or statues, buildings, dollars, paintings or books in paper and ink are graduations from unrealness to realness--in relative terms. it would seem then that intermediateness is a relation between the positive absolute and the negative absolute. but the absolute cannot be the related--of course a confession that we can't really think of it at all, if here we think of a limit to the unlimited. doing the best we can, and encouraged by the reflection that we can't do worse than has been done by metaphysicians in the past, we accept that the absolute can't be the related. so then that our quasi-state is not a real relation, if nothing in it is real. on the other hand, it is not an unreal relation, if nothing in it is unreal. it seems thinkable that the positive absolute can, by means of intermediateness, have a quasi-relation, or be only quasi-related, or be the unrelated, in final terms, or, at least, not be the related, in final terms. as to free will and intermediatism--same answer as to everything else. by free will we mean independence--or that which does not merge away into something else--so, in intermediateness, neither free-will nor slave-will--but a different approximation for every so-called person toward one or the other of the extremes. the hackneyed way of expressing this seems to me to be the acceptable way, if in intermediateness, there is only the paradoxical: that we're free to do what we have to do. i am not convinced that we make a fetish of the preposterous. i think our feeling is that in first gropings there's no knowing what will afterward be the acceptable. i think that if an early biologist heard of birds that grow on trees, he should record that he had heard of birds that grow on trees: then let sorting over of data occur afterward. the one thing that we try to tone down but that is to a great degree unavoidable is having our data all mixed up like long island and florida in the minds of early american explorers. my own notion is that this whole book is very much like a map of north america in which the hudson river is set down as a passage leading to siberia. we think of monstrator and melanicus and of a world that is now in communication with this earth: if so, secretly, with certain esoteric ones upon this earth. whether that world's monstrator and monstrator's melanicus--must be the subject of later inquiry. it would be a gross thing to do: solve up everything now and leave nothing to our disciples. i have been very much struck with phenomena of "cup marks." they look to me like symbols of communication. but they do not look to me like means of communication between some of the inhabitants of this earth and other inhabitants of this earth. my own impression is that some external force has marked, with symbols, rocks of this earth, from far away. i do not think that cup marks are inscribed communications among different inhabitants of this earth, because it seems too unacceptable that inhabitants of china, scotland, and america should all have conceived of the same system. cup marks are strings of cup-like impressions in rocks. sometimes there are rings around them, and sometimes they have only semi-circles. great britain, america, france, algeria, circassia, palestine: they're virtually everywhere--except in the far north, i think. in china, cliffs are dotted with them. upon a cliff near lake como, there is a maze of these markings. in italy and spain and india they occur in enormous numbers. given that a force, say, like electric force, could, from a distance, mark such a substance as rocks, as, from a distance of hundreds of miles, selenium can be marked by telephotographers--but i am of two minds-- the lost explorers from somewhere, and an attempt, from somewhere, to communicate with them: so a frenzy of showering of messages toward this earth, in the hope that some of them would mark rocks near the lost explorers-- or that somewhere upon this earth, there is an especial rocky surface, or receptor, or polar construction, or a steep, conical hill, upon which for ages have been received messages from some other world; but that at times messages go astray and mark substances perhaps thousands of miles from the receptor: that perhaps forces behind the history of this earth have left upon the rocks of palestine and england and india and china records that may some day be deciphered, of their misdirected instructions to certain esoteric ones--order of the freemasons--the jesuits-- i emphasize the row-formation of cup marks: prof. douglas (_saturday review_, nov. , ): "whatever may have been their motive, the cup-markers showed a decided liking for arranging their sculpturings in regularly spaced rows." that cup marks are an archaic form of inscription was first suggested by canon greenwell many years ago. but more specifically adumbratory to our own expression are the observations of rivett-carnac (_jour. roy. asiatic soc._, - ): that the braille system of raised dots is an inverted arrangement of cup marks: also that there are strong resemblances to the morse code. but no tame and systematized archaeologist can do more than casually point out resemblances, and merely suggest that strings of cup marks look like messages, because--china, switzerland, algeria, america--if messages they be, there seems to be no escape from attributing one origin to them--then, if messages they be, i accept one external origin, to which the whole surface of this earth was accessible, for them. something else that we emphasize: that rows of cup marks have often been likened to footprints. but, in this similitude, their unilinear arrangement must be disregarded--of course often they're mixed up in every way, but arrangement in single lines is very common. it is odd that they should so often be likened to footprints: i suppose there are exceptional cases, but unless it's something that hops on one foot, or a cat going along a narrow fence-top, i don't think of anything that makes footprints one directly ahead of another--cop, in a station house, walking a chalk line, perhaps. upon the witch's stone, near ratho, scotland, there are twenty-four cups, varying in size from one and a half to three inches in diameter, arranged in approximately straight lines. locally it is explained that these are tracks of dogs' feet (_proc. soc. antiq. scotland_, - - ). similar marks are scattered bewilderingly all around the witch's stone--like a frenzy of telegraphing, or like messages repeating and repeating, trying to localize differently. in inverness-shire, cup marks are called "fairies' footmarks." at valna's church, norway, and st. peter's, ambleteuse, there are such marks, said to be horses' hoofprints. the rocks of clare, ireland, are marked with prints supposed to have been made by a mythical cow (_folklore_, - ). we now have such a ghost of a thing that i'd not like to be interpreted as offering it as a datum: it simply illustrates what i mean by the notion of symbols, like cups, or like footprints, which, if like those of horses or cows, are the reverse of, or the negatives of, cups--of symbols that are regularly received somewhere upon this earth--steep, conical hill, somewhere, i think--but that have often alighted in wrong places--considerably to the mystification of persons waking up some morning to find them upon formerly blank spaces. an ancient record--still worse, an ancient chinese record--of a courtyard of a palace--dwellers of the palace waking up one morning, finding the courtyard marked with tracks like the footprints of an ox--supposed that the devil did it. (_notes and queries_, - - .) . angels. hordes upon hordes of them. beings massed like the clouds of souls, or the commingling whiffs of spirituality, or the exhalations of souls that doré pictured so often. it may be that the milky way is a composition of stiff, frozen, finally-static, absolute angels. we shall have data of little milky ways, moving swiftly; or data of hosts of angels, not absolute, or still dynamic. i suspect, myself, that the fixed stars are really fixed, and that the minute motions said to have been detected in them are illusions. i think that the fixed stars are absolutes. their twinkling is only the interpretation by an intermediatist state of them. i think that soon after leverrier died, a new fixed star was discovered--that, if dr. gray had stuck to his story of the thousands of fishes from one pail of water, had written upon it, lectured upon it, taken to street corners, to convince the world that, whether conceivable or not, his explanation was the only true explanation: had thought of nothing but this last thing at night and first thing in the morning--his obituary--another "nova" reported in _monthly notices_. i think that milky ways, of an inferior, or dynamic, order, have often been seen by astronomers. of course it may be that the phenomena that we shall now consider are not angels at all. we are simply feeling around, trying to find out what we can accept. some of our data indicate hosts of rotund and complacent tourists in inter-planetary space--but then data of long, lean, hungry ones. i think that there are, out in inter-planetary space, super tamerlanes at the head of hosts of celestial ravagers--which have come here and pounced upon civilizations of the past, cleaning them up all but their bones, or temples and monuments--for which later historians have invented exclusionist histories. but if something now has a legal right to us, and can enforce its proprietorship, they've been warned off. it's the way of all exploitation. i should say that we're now under cultivation: that we're conscious of it, but have the impertinence to attribute it all to our own nobler and higher instincts. against these notions is the same sense of finality that opposes all advance. it's why we rate acceptance as a better adaptation than belief. opposing us is the strong belief that, as to inter-planetary phenomena, virtually everything has been found out. sense of finality and illusion of homogeneity. but that what is called advancing knowledge is violation of the sense of blankness. a drop of water. once upon a time water was considered so homogeneous that it was thought of as an element. the microscope--and not only that the supposititiously elementary was seen to be of infinite diversity, but that in its protoplasmic life there were new orders of beings. or the year --and a european looking westward over the ocean--his feeling that that suave western droop was unbreakable; that gods of regularity would not permit that smooth horizon to be disturbed by coasts or spotted with islands. the unpleasantness of even contemplating such a state--wide, smooth west, so clean against the sky--spotted with islands--geographic leprosy. but coasts and islands and indians and bison, in the seemingly vacant west: lakes, mountains, rivers-- one looks up at the sky: the relative homogeneity of the relatively unexplored: one thinks of only a few kinds of phenomena. but the acceptance is forced upon me that there are modes and modes and modes of inter-planetary existence: things as different from planets and comets and meteors as indians are from bison and prairie dogs: a super-geography--or celestiography--of vast stagnant regions, but also of super-niagaras and ultra-mississippis: and a super-sociology--voyagers and tourists and ravagers: the hunted and the hunting: the super-mercantile, the super-piratic, the super-evangelical. sense of homogeneity, or our positivist illusion of the unknown--and the fate of all positivism. astronomy and the academic. ethics and the abstract. the universal attempt to formulate or to regularize--an attempt that can be made only by disregarding or denying. or all things disregard or deny that which will eventually invade and destroy them-- until comes the day when some one thing shall say, and enforce upon infinitude: "thus far shalt thou go: here is absolute demarcation." the final utterance: "there is only i." in the _monthly notices of the r.a.s._, - , there is a letter from the rev. w. read: that, upon the th of september, , at : a.m., he had seen a host of self-luminous bodies, passing the field of his telescope, some slowly and some rapidly. they appeared to occupy a zone several degrees in breadth. the direction of most of them was due east to west, but some moved from north to south. the numbers were tremendous. they were observed for six hours. editor's note: "may not these appearances be attributed to an abnormal state of the optic nerves of the observer?" in _monthly notices_, - , mr. read answers that he had been a diligent observer, with instruments of a superior order, for about years--"but i have never witnessed such an appearance before." as to illusion he says that two other members of his family had seen the objects. the editor withdraws his suggestion. we know what to expect. almost absolutely--in an existence that is essentially hibernian--we can predict the past--that is, look over something of this kind, written in , and know what to expect from the exclusionists later. if mr. read saw a migration of dissatisfied angels, numbering millions, they must merge away, at least subjectively, with commonplace terrestrial phenomena--of course disregarding mr. read's probable familiarity, of years' duration, with the commonplaces of terrestrial phenomena. _monthly notices_, - : letter from rev. w.r. dawes: that he had seen similar objects--and in the month of september--that they were nothing but seeds floating in the air. in the _report of the british association_, - , there is a communication from mr. read to prof. baden-powell: that the objects that had been seen by him and by mr. dawes were not similar. he denies that he had seen seeds floating in the air. there had been little wind, and that had come from the sea, where seeds would not be likely to have origin. the objects that he had seen were round and sharply defined, and with none of the feathery appearance of thistledown. he then quotes from a letter from c.b. chalmers, f.r.a.s., who had seen a similar stream, a procession, or migration, except that some of the bodies were more elongated--or lean and hungry--than globular. he might have argued for sixty-five years. he'd have impressed nobody--of importance. the super-motif, or dominant, of his era, was exclusionism, and the notion of seeds in the air assimilates--with due disregards--with that dominant. or pageantries here upon our earth, and things looking down upon us--and the crusades were only dust clouds, and glints of the sun on shining armor were only particles of mica in dust clouds. i think it was a crusade that read saw--but that it was right, relatively to the year , to say that it was only seeds in the wind, whether the wind blew from the sea or not. i think of things that were luminous with religious zeal, mixed up, like everything else in intermediateness, with black marauders and from gray to brown beings of little personal ambitions. there may have been a richard coeur de lion, on his way to right wrongs in jupiter. it was right, relatively to , to say that he was a seed of a cabbage. prof. coffin, u.s.n. (_jour. frank. inst._, - ): that, during the eclipse of august, , he had noted the passage, across his telescope, of several bright flakes resembling thistleblows, floating in the sunlight. but the telescope was so focused that, if these things were distinct, they must have been so far away from this earth that the difficulties of orthodoxy remain as great, one way or another, no matter what we think they were-- they were "well-defined," says prof. coffin. henry waldner (_nature_, - ): that, april , , he had seen great numbers of small, shining bodies passing from west to east. he had notified dr. wolf, of the observatory of zurich, who "had convinced himself of this strange phenomenon." dr. wolf had told him that similar bodies had been seen by sig. capocci, of the capodimonte observatory, at naples, may , . the shapes were of great diversity--or different aspects of similar shapes? appendages were seen upon some of them. we are told that some were star-shaped, with transparent appendages. i think, myself, it was a mohammed and his hegira. may have been only his harem. astonishing sensation: afloat in space with ten million wives around one. anyway, it would seem that we have considerable advantage here, inasmuch as seeds are not in season in april--but the pulling back to earth, the bedraggling by those sincere but dull ones of some time ago. we have the same stupidity--necessary, functioning stupidity--of attribution of something that was so rare that an astronomer notes only one instance between and , to an every-day occurrence-- or mr. waldner's assimilative opinion that he had seen only ice crystals. whether they were not very exclusive veils of a super-harem, or planes of a very light material, we have an impression of star-shaped things with transparent appendages that have been seen in the sky. hosts of small bodies--black, this time--that were seen by the astronomers herrick, buys-ballot, and de cuppis (_l'année scientifique_, - ); vast numbers of bodies that were seen by m. lamey, to cross the moon (_l'année scientifique_, - ); another instance of dark ones; prodigious number of dark, spherical bodies reported by messier, june , (arago, _oeuvres_, - ); considerable number of luminous bodies which appeared to move out from the sun, in diverse directions; seen at havana, during eclipse of the sun, may , , by prof. auber (poey); m. poey cites a similar instance, of aug. , ; m. lotard's opinion that they were birds (_l'astronomie_, - ); large number of small bodies crossing disk of the sun, some swiftly, some slowly; most of them globular, but some seemingly triangular, and some of more complicated structure; seen by m. trouvelet, who, whether seeds, insects, birds, or other commonplace things, had never seen anything resembling these forms (_l'année scientifique_, - ); report from the rio de janeiro observatory, of vast numbers of bodies crossing the sun, some of them luminous and some of them dark, from some time in december, , until jan. , (_la nature_, - ). of course, at a distance, any form is likely to look round or roundish: but we point out that we have notes upon the seeming of more complex forms. in _l'astronomie_, - , is recorded m. briguiere's observation, at marseilles, april and april , , upon the crossing of the sun by bodies that were irregular in form. some of them moved as if in alignment. letter from sir robert inglis to col. sabine (_rept. brit. assoc._, - ): that, at p.m., aug. , , at gais, switzerland, inglis had seen thousands and thousands of brilliant white objects, like snowflakes in a cloudless sky. though this display lasted about twenty-five minutes, not one of these seeming snowflakes was seen to fall. inglis says that his servant "fancied" that he had seen something like wings on these--whatever they were. upon page , of the _report_, sir john herschel says that, in or , his attention had been attracted by objects of considerable size, in the air, seemingly not far away. he had looked at them through a telescope. he says that they were masses of hay, not less than a yard or two in diameter. still there are some circumstances that interest me. he says that, though no less than a whirlwind could have sustained these masses, the air about him was calm. "no doubt wind prevailed at the spot, but there was no roaring noise." none of these masses fell within his observation or knowledge. to walk a few fields away and find out more would seem not much to expect from a man of science, but it is one of our superstitions, that such a seeming trifle is just what--by the spirit of an era, we'll call it--one is not permitted to do. if those things were not masses of hay, and if herschel had walked a little and found out, and had reported that he had seen strange objects in the air--that report, in , would have been as misplaced as the appearance of a tail upon an embryo still in its gastrula era. i have noticed this inhibition in my own case many times. looking back--why didn't i do this or that little thing that would have cost so little and have meant so much? didn't belong to that era of my own development. _nature_, - : that, at kattenau, germany, about half an hour before sunrise, march , , "an enormous number of luminous bodies rose from the horizon, and passed in a horizontal direction from east to west." they are described as having appeared in a zone or belt. "they shone with a remarkably brilliant light." so they've thrown lassos over our data to bring them back to earth. but they're lassos that cannot tighten. we can't pull out of them: we may step out of them, or lift them off. some of us used to have an impression of science sitting in calm, just judgment: some of us now feel that a good many of our data have been lynched. if a crusade, perhaps from mars to jupiter, occur in the autumn--"seeds." if a crusade or outpouring of celestial vandals is seen from this earth in the spring--"ice crystals." if we have record of a race of aerial beings, perhaps with no substantial habitat, seen by someone in india--"locusts." this will be disregarded: if locusts fly high, they freeze and fall in thousands. _nature_, - : locusts that were seen in the mountains of india, at a height of , feet--"in swarms and dying by thousands." but no matter whether they fly high or fly low, no one ever wonders what's in the air when locusts are passing overhead, because of the falling of stragglers. i have especially looked this matter up--no mystery when locusts are flying overhead--constant falling of stragglers. _monthly notices_, - : "an unusual phenomenon noticed by lieut. herschel, oct. and , , while observing the sun, at bangalore, india." lieut. herschel had noticed dark shadows crossing the sun--but away from the sun there were luminous, moving images. for two days bodies passed in a continuous stream, varying in size and velocity. the lieutenant tries to explain, as we shall see, but he says: "as it was, the continuous flight, for two whole days, in such numbers, in the upper regions of the air, of beasts that left no stragglers, is a wonder of natural history, if not of astronomy." he tried different focusing--he saw wings--perhaps he saw planes. he says that he saw upon the objects either wings or phantom-like appendages. then he saw something that was so bizarre that, in the fullness of his nineteenth-centuriness, he writes: "there was no longer doubt: they were locusts or flies of some sort." one of them had paused. it had hovered. then it had whisked off. the editor says that at that time "countless locusts had descended upon certain parts of india." we now have an instance that is extraordinary in several respects--super-voyagers or super-ravagers; angels, ragamuffins, crusaders, emigrants, aeronauts, or aerial elephants, or bison or dinosaurs--except that i think the thing had planes or wings--one of them has been photographed. it may be that in the history of photography no more extraordinary picture than this has ever been taken. _l'astronomie_, - : that, at the observatory of zacatecas, mexico, aug. , , about , meters above sea level, were seen a large number of small luminous bodies, entering upon the disk of the sun. m. bonilla telegraphed to the observatories of the city of mexico and of puebla. word came back that the bodies were not visible there. because of this parallax, m. bonilla placed the bodies "relatively near the earth." but when we find out what he called "relatively near the earth"--birds or bugs or hosts of a super-tamerlane or army of a celestial richard coeur de lion--our heresies rejoice anyway. his estimate is "less distance than the moon." one of them was photographed. see _l'astronomie_, - . the photograph shows a long body surrounded by indefinite structures, or by the haze of wings or planes in motion. _l'astronomie_, - ; signer ricco, of the observatory of palermo, writes that, nov. , , at : o'clock in the morning, he was watching the sun, when he saw, slowly traversing its disk, bodies in two long, parallel lines, and a shorter, parallel line. the bodies looked winged to him. but so large were they that he had to think of large birds. he thought of cranes. he consulted ornithologists, and learned that the configuration of parallel lines agrees with the flight-formation of cranes. this was in : anybody now living in new york city, for instance, would tell him that also it is a familiar formation of aeroplanes. but, because of data of focus and subtended angles, these beings or objects must have been high. sig. ricco argues that condors have been known to fly three or four miles high, and that heights reached by other birds have been estimated at two or three miles. he says that cranes have been known to fly so high that they have been lost to view. our own acceptance, in conventional terms, is that there is not a bird of this earth that would not freeze to death at a height of more than four miles: that if condors fly three or four miles high, they are birds that are especially adapted to such altitudes. sig. ricco's estimate is that these objects or beings or cranes must have been at least five and a half miles high. the vast dark thing that looked like a poised crow of unholy dimensions. assuming that i shall ever have any readers, let him, or both of them, if i shall ever have such popularity as that, note how dim that bold black datum is at the distance of only two chapters. the question: was it a thing or the shadow of a thing? acceptance either way calls not for mere revision but revolution in the science of astronomy. but the dimness of the datum of only two chapters ago. the carved stone disk of tarbes, and the rain that fell every afternoon for twenty--if i haven't forgotten, myself, whether it was twenty-three or twenty-five days!--upon one small area. we are all thomsons, with brains that have smooth and slippery, though corrugated, surfaces--or that all intellection is associative--or that we remember that which correlates with a dominant--and a few chapters go by, and there's scarcely an impression that hasn't slid off our smooth and slippery brains, of leverrier and the "planet vulcan." there are two ways by which irreconcilables can be remembered--if they can be correlated in a system more nearly real than the system that rejects them--and by repetition and repetition and repetition. vast black thing like a crow poised over the moon. the datum is so important to us, because it enforces, in another field, our acceptance that dark bodies of planetary size traverse this solar system. our position: that the things have been seen: also that their shadows have been seen. vast black thing poised like a crow over the moon. so far it is a single instance. by a single instance, we mean the negligible. in _popular science_, - , serviss tells of a shadow that schroeter saw, in , in the lunar alps. first he saw a light. but then, when this region was illuminated, he saw a round shadow where the light had been. our own expression: that he saw a luminous object near the moon: that that part of the moon became illuminated, and the object was lost to view; but that then its shadow underneath was seen. serviss explains, of course. otherwise he'd not be prof. serviss. it's a little contest in relative approximations to realness. prof. serviss thinks that what schroeter saw was the "round" shadow of a mountain--in the region that had become lighted. he assumes that schroeter never looked again to see whether the shadow could be attributed to a mountain. that's the crux: conceivably a mountain could cast a round--and that means detached--shadow, in the lighted part of the moon. prof. serviss could, of course, explain why he disregards the light in the first place--maybe it had always been there "in the first place." if he couldn't explain, he'd still be an amateur. we have another datum. i think it is more extraordinary than-- vast thing, black and poised, like a crow, over the moon. but only because it's more circumstantial, and because it has corroboration, do i think it more extraordinary than-- vast poised thing, black as a crow, over the moon. mr. h.c. russell, who was usually as orthodox as anybody, i suppose--at least, he wrote "f.r.a.s." after his name--tells in the _observatory_, - , one of the wickedest, or most preposterous, stories that we have so far exhumed: that he and another astronomer, g.d. hirst, were in the blue fountains, near sydney, n.s.w., and mr. hirst was looking at the moon-- he saw on the moon what russell calls "one of those remarkable facts, which being seen should be recorded, although no explanation can at present be offered." that may be so. it is very rarely done. our own expression upon evolution by successive dominants and their correlates is against it. on the other hand, we express that every era records a few observations out of harmony with it, but adumbratory or preparatory to the spirit of eras still to come. it's very rarely done. lashed by the phantom-scourge of a now passing era, the world of astronomers is in a state of terrorism, though of a highly attenuated, modernized, devitalized kind. let an astronomer see something that is not of the conventional, celestial sights, or something that it is "improper" to see--his very dignity is in danger. some one of the corralled and scourged may stick a smile into his back. he'll be thought of unkindly. with a hardihood that is unusual in his world of ethereal sensitivenesses, russell says, of hirst's observation: "he found a large part of it covered with a dark shade, quite as dark as the shadow of the earth during an eclipse of the moon." but the climax of hardihood or impropriety or wickedness, preposterousness or enlightenment: "one could hardly resist the conviction that it was a shadow, yet it could not be the shadow of any known body." richard proctor was a man of some liberality. after a while we shall have a letter, which once upon a time we'd have called delirious--don't know that we could read such a thing now, for the first time, without incredulous laughter--which mr. proctor permitted to be published in _knowledge_. but a dark, unknown world that could cast a shadow upon a large part of the moon, perhaps extending far beyond the limb of the moon; a shadow as deep as the shadow of this earth-- too much for mr. proctor's politeness. i haven't read what he said, but it seems to have been a little coarse. russell says that proctor "freely used" his name in the _echo_, of march , , ridiculing this observation which had been made by russell as well as hirst. if it hadn't been proctor, it would have been someone else--but one notes that the attack came out in a newspaper. there is no discussion of this remarkable subject, no mention in any other astronomic journal. the disregard was almost complete--but we do note that the columns of the _observatory_ were open to russell to answer proctor. in the answer, i note considerable intermediateness. far back in , it would have been a beautiful positivism, if russell had said-- "there was a shadow on the moon. absolutely it was cast by an unknown body." according to our religion, if he had then given all his time to the maintaining of this one stand, of course breaking all friendships, all ties with his fellow astronomers, his apotheosis would have occurred, greatly assisted by means well known to quasi-existence when its compromises and evasions, and phenomena that are partly this and partly that, are flouted by the definite and uncompromising. it would be impossible in a real existence, but mr. russell, of quasi-existence, says that he did resist the conviction; that he had said that one could "hardly resist"; and most of his resentment is against mr. proctor's thinking that he had not resisted. it seems too bad--if apotheosis be desirable. the point in intermediatism here is: not that to adapt to the conditions of quasi-existence is to have what is called success in quasi-existence, but is to lose one's soul-- but is to lose "one's" chance of attaining soul, self, or entity. one indignation quoted from proctor interests us: "what happens on the moon may at any time happen to this earth." or: that is just the teaching of this department of advanced astronomy: that russell and hirst saw the sun eclipsed relatively to the moon by a vast dark body: that many times have eclipses occurred relatively to this earth, by vast, dark bodies: that there have been many eclipses that have not been recognized as eclipses by scientific kindergartens. there is a merger, of course. we'll take a look at it first--that, after all, it may have been a shadow that hirst and russell saw, but the only significance is that the sun was eclipsed relatively to the moon by a cosmic haze of some kind, or a swarm of meteors close together, or a gaseous discharge left behind by a comet. my own acceptance is that vagueness of shadow is a function of vagueness of intervention; that a shadow as dense as the shadow of this earth is cast by a body denser than hazes and swarms. the information seems definite enough in this respect--"quite as dark as the shadow of this earth during the eclipse of the moon." though we may not always be as patient toward them as we should be, it is our acceptance that the astronomic primitives have done a great deal of good work: for instance, in the allaying of fears upon this earth. sometimes it may seem as if all science were to us very much like what a red flag is to bulls and anti-socialists. it's not that: it's more like what unsquare meals are to bulls and anti-socialists--not the scientific, but the insufficient. our acceptance is that evil is the negative state, by which we mean the state of maladjustment, discord, ugliness, disorganization, inconsistency, injustice, and so on--as determined in intermediateness, not by real standards, but only by higher approximations to adjustment, harmony, beauty, organization, consistency, justice, and so on. evil is outlived virtue, or incipient virtue that has not yet established itself, or any other phenomenon that is not in seeming adjustment, harmony, consistency with a dominant. the astronomers have functioned bravely in the past. they've been good for business: the big interests think kindly, if at all, of them. it's bad for trade to have an intense darkness come upon an unaware community and frighten people out of their purchasing values. but if an obscuration be foretold, and if it then occur--may seem a little uncanny--only a shadow--and no one who was about to buy a pair of shoes runs home panic-stricken and saves the money. upon general principles we accept that astronomers have quasi-systematized data of eclipses--or have included some and disregarded others. they have done well. they have functioned. but now they're negatives, or they're out of harmony-- if we are in harmony with a new dominant, or the spirit of a new era, in which exclusionism must be overthrown; if we have data of many obscurations that have occurred, not only upon the moon, but upon our own earth, as convincing of vast intervening bodies, usually invisible, as is any regularized, predicted eclipse. one looks up at the sky. it seems incredible that, say, at the distance of the moon, there could be, but be invisible, a solid body, say, the size of the moon. one looks up at the moon, at a time when only a crescent of it is visible. the tendency is to build up the rest of it in one's mind; but the unillumined part looks as vacant as the rest of the sky, and it's of the same blueness as the rest of the sky. there's a vast area of solid substance before one's eyes. it's indistinguishable from the sky. in some of our little lessons upon the beauties of modesty and humility, we have picked out basic arrogances--tail of a peacock, horns of a stag, dollars of a capitalist--eclipses of astronomers. though i have no desire for the job, i'd engage to list hundreds of instances in which the report upon an expected eclipse has been "sky overcast" or "weather unfavorable." in our super-hibernia, the unfavorable has been construed as the favorable. some time ago, when we were lost, because we had not recognized our own dominant, when we were still of the unchosen and likely to be more malicious than we now are--because we have noted a steady tolerance creeping into our attitude--if astronomers are not to blame, but are only correlates to a dominant--we advertised a predicted eclipse that did not occur at all. now, without any especial feeling, except that of recognition of the fate of all attempted absolutism, we give the instance, noting that, though such an evil thing to orthodoxy, it was orthodoxy that recorded the non-event. _monthly notices of the r.a.s._, - : "remarkable appearances during the total eclipse of the moon on march , ": in an extract from a letter from mr. forster, of bruges, it is said that, according to the writer's observations at the time of the predicted total eclipse, the moon shone with about three times the intensity of the mean illumination of an eclipsed lunar disk: that the british consul, at ghent, who did not know of the predicted eclipse, had written enquiring as to the "blood-red" color of the moon. this is not very satisfactory to what used to be our malices. but there follows another letter, from another astronomer, walkey, who had made observations at clyst st. lawrence: that, instead of an eclipse, the moon became--as is printed in italics--"most beautifully illuminated" ... "rather tinged with a deep red"... "the moon being as perfect with light as if there had been no eclipse whatever." i note that chambers, in his work upon eclipses, gives forster's letter in full--and not a mention of walkey's letter. there is no attempt in _monthly notices_ to explain upon the notion of greater distance of the moon, and the earth's shadow falling short, which would make as much trouble for astronomers, if that were not foreseen, as no eclipse at all. also there is no refuge in saying that virtually never, even in total eclipses, is the moon totally dark--"as perfect with light as if there had been no eclipse whatever." it is said that at the time there had been an aurora borealis, which might have caused the luminosity, without a datum that such an effect, by an aurora, had ever been observed upon the moon. but single instances--so an observation by scott, in the antarctic. the force of this datum lies in my own acceptance, based upon especially looking up this point, that an eclipse nine-tenths of totality has great effect, even though the sky be clouded. scott (_voyage of the discovery_, vol. ii, p. ): "there may have been an eclipse of the sun, sept. , , as the almanac said, but we should, none of us, have liked to swear to the fact." this eclipse had been set down at nine-tenths of totality. the sky was overcast at the time. so it is not only that many eclipses unrecognized by astronomers as eclipses have occurred, but that intermediatism, or impositivism, breaks into their own seemingly regularized eclipses. our data of unregularized eclipses, as profound as those that are conventionally--or officially?--recognized, that have occurred relatively to this earth: in _notes and queries_ there are several allusions to intense darknesses that have occurred upon this earth, quite as eclipses occur, but that are not referable to any known eclipsing body. of course there is no suggestion here that these darknesses may have been eclipses. my own acceptance is that if in the nineteenth century anyone had uttered such a thought as that, he'd have felt the blight of a dominant; that materialistic science was a jealous god, excluding, as works of the devil, all utterances against the seemingly uniform, regular, periodic; that to defy him would have brought on--withering by ridicule--shrinking away by publishers--contempt of friends and family--justifiable grounds for divorce--that one who would so defy would feel what unbelievers in relics of saints felt in an earlier age; what befell virgins who forgot to keep fires burning, in a still earlier age--but that, if he'd almost absolutely hold out, just the same--new fixed star reported in _monthly notices_. altogether, the point in positivism here is that by dominants and their correlates, quasi-existence strives for the positive state, aggregating, around a nucleus, or dominant, systematized members of a religion, a science, a society--but that "individuals" who do not surrender and submerge may of themselves highly approximate to positiveness--the fixed, the real, the absolute. in _notes and queries_, - - , there is an account of a darkness in holland, in the midst of a bright day, so intense and terrifying that many panic-stricken persons lost their lives stumbling into the canals. _gentleman's magazine_, - : a darkness that came upon london, aug. , , "greater than at the great eclipse of ." however, our preference is not to go so far back for data. for a list of historic "dark days," see humboldt, _cosmos_, - . _monthly weather review_, march, - : that, according to the _la crosse daily republican_, of march , , darkness suddenly settled upon the city of oshkosh, wis., at p.m., march . in five minutes the darkness equaled that of midnight. consternation. i think that some of us are likely to overdo our own superiority and the absurd fears of the middle ages-- oshkosh. people in the streets rushing in all directions--horses running away--women and children running into cellars--little modern touch after all: gas meters instead of images and relics of saints. this darkness, which lasted from eight to ten minutes, occurred in a day that had been "light but cloudy." it passed from west to east, and brightness followed: then came reports from towns to the west of oshkosh: that the same phenomenon had already occurred there. a "wave of total darkness" had passed from west to east. other instances are recorded in the _monthly weather review_, but, as to all of them, we have a sense of being pretty well-eclipsed, ourselves, by the conventional explanation that the obscuring body was only a very dense mass of clouds. but some of the instances are interesting--intense darkness at memphis, tenn., for about fifteen minutes, at a.m., dec. , --"we are told that in some quarters a panic prevailed, and that some were shouting and praying and imagining that the end of the world had come." (_m.w.r._, - .) at louisville, ky., march , , at about a.m.: duration about half an hour; had been raining moderately, and then hail had fallen. "the intense blackness and general ominous appearance of the storm spread terror throughout the city." (_m.w.r._, - .) however, this merger between possible eclipses by unknown dark bodies and commonplace terrestrial phenomena is formidable. as to darknesses that have fallen upon vast areas, conventionality is--smoke from forest fires. in the _u.s. forest service bulletin_, no. , f.g. plummer gives a list of eighteen darknesses that have occurred in the united states and canada. he is one of the primitives, but i should say that his dogmatism is shaken by vibrations from the new dominant. his difficulty, which he acknowledges, but which he would have disregarded had he written a decade or so earlier, is the profundity of some of these obscurations. he says that mere smokiness cannot account for such "awe-inspiring dark days." so he conceives of eddies in the air, concentrating the smoke from forest fires. then, in the inconsistency or discord of all quasi-intellection that is striving for consistency or harmony, he tells of the vastness of some of these darknesses. of course mr. plummer did not really think upon this subject, but one does feel that he might have approximated higher to real thinking than by speaking of concentration and then listing data of enormous area, or the opposite of circumstances of concentration--because, of his nineteen instances, nine are set down as covering all new england. in quasi-existence, everything generates or is part of its own opposite. every attempt at peace prepares the way for war; all attempts at justice result in injustice in some other respect: so mr. plummer's attempt to bring order into his data, with the explanation of darkness caused by smoke from forest fires, results in such confusion that he ends up by saying that these daytime darknesses have occurred "often with little or no turbidity of the air near the earth's surface"--or with no evidence at all of smoke--except that there is almost always a forest fire somewhere. however, of the eighteen instances, the only one that i'd bother to contest is the profound darkness in canada and northern parts of the united states, nov. , --which we have already considered. its concomitants: lights in the sky; fall of a black substance; shocks like those of an earthquake. in this instance, the only available forest fire was one to the south of the ohio river. for all i know, soot from a very great fire south of the ohio might fall in montreal, canada, and conceivably, by some freak of reflection, light from it might be seen in montreal, but the earthquake is not assimilable with a forest fire. on the other hand, it will soon be our expression that profound darkness, fall of matter from the sky, lights in the sky, and earthquakes are phenomena of the near approach of other worlds to this world. it is such comprehensiveness, as contrasted with inclusion of a few factors and disregard for the rest, that we call higher approximation to realness--or universalness. a darkness, of april , , at wimbledon, england (_symons' met. mag._, - ). it came from a smokeless region: no rain, no thunder; lasted minutes; too dark to go "even out in the open." as to darknesses in great britain, one thinks of fogs--but in _nature_, - , there are some observations by major j. herschel, upon an obscuration in london, jan. , , at : a.m., so great that he could hear persons upon the opposite side of the street, but could not see them--"it was obvious that there was no fog to speak of." _annual register_, - : an account by charles a. murray, british envoy to persia, of a darkness of may , , that came upon bagdad--"a darkness more intense than ordinary midnight, when neither stars nor moon are visible...." "after a short time the black darkness was succeeded by a red, lurid gloom, such as i never saw in any part of the world." "panic seized the whole city." "a dense volume of red sand fell." this matter of sand falling seems to suggest conventional explanation enough, or that a simoon, heavily charged with terrestrial sand, had obscured the sun, but mr. murray, who says that he had had experience with simoons, gives his opinion that "it cannot have been a simoon." it is our comprehensiveness now, or this matter of concomitants of darknesses that we are going to capitalize. it is all very complicated and tremendous, and our own treatment can be but impressionistic, but a few of the rudiments of advanced seismology we shall now take up--or the four principal phenomena of another world's close approach to this world. if a large substantial mass, or super-construction, should enter this earth's atmosphere, it is our acceptance that it would sometimes--depending upon velocity--appear luminous or look like a cloud, or like a cloud with a luminous nucleus. later we shall have an expression upon luminosity--different from the luminosity of incandescence--that comes upon objects falling from the sky, or entering this earth's atmosphere. now our expression is that worlds have often come close to this earth, and that smaller objects--size of a haystack or size of several dozen skyscrapers lumped, have often hurtled through this earth's atmosphere, and have been mistaken for clouds, because they were enveloped in clouds-- or that around something coming from the intense cold of inter-planetary space--that is of some regions: our own suspicion is that other regions are tropical--the moisture of this earth's atmosphere would condense into a cloud-like appearance around it. in _nature_, - , there is an account by mr. s.w. clifton, collector of customs, at freemantle, western australia, sent to the melbourne observatory--a clear day--appearance of a small black cloud, moving not very swiftly--bursting into a ball of fire, of the apparent size of the moon-- or that something with the velocity of an ordinary meteorite could not collect vapor around it, but that slower-moving objects--speed of a railway train, say--may. the clouds of tornadoes have so often been described as if they were solid objects that i now accept that sometimes they are: that some so-called tornadoes are objects hurtling through this earth's atmosphere, not only generating disturbances by their suctions, but crushing, with their bulk, all things in their way, rising and falling and finally disappearing, demonstrating that gravitation is not the power that the primitives think it is, if an object moving at relatively low velocity be not pulled to this earth, or being so momentarily affected, bounds away. in finley's _reports on the character of tornadoes_ very suggestive bits of description occur: "cloud bounded along the earth like a ball"-- or that it was no meteorological phenomenon, but something very much like a huge solid ball that was bounding along, crushing and carrying with it everything within its field-- "cloud bounded along, coming to the earth every eight hundred or one thousand yards." here's an interesting bit that i got somewhere else. i offer it as a datum in super-biology, which, however, is a branch of advanced science that i'll not take up, restricting to things indefinitely called "objects"-- "the tornado came wriggling, jumping, whirling like a great green snake, darting out a score of glistening fangs." though it's interesting, i think that's sensational, myself. it may be that vast green snakes sometimes rush past this earth, taking a swift bite wherever they can, but, as i say, that's a super-biologic phenomenon. finley gives dozens of instances of tornado clouds that seem to me more like solid things swathed in clouds, than clouds. he notes that, in the tornado at americus, georgia, july , , "a strange sulphurous vapor was emitted from the cloud." in many instances, objects, or meteoritic stones, that have come from this earth's externality, have had a sulphurous odor. why a wind effect should be sulphurous is not clear. that a vast object from external regions should be sulphurous is in line with many data. this phenomenon is described in the _monthly weather review_, july, , as "a strange sulphurous vapor ... burning and sickening all who approached close enough to breathe it." the conventional explanation of tornadoes as wind-effects--which we do not deny in some instances--is so strong in the united states that it is better to look elsewhere for an account of an object that has hurtled through this earth's atmosphere, rising and falling and defying this earth's gravitation. _nature_, - : that, according to a correspondent to the _birmingham morning news_, the people living near king's sutton, banbury, saw, about one o'clock, dec. , , something like a haycock hurtling through the air. like a meteor it was accompanied by fire and a dense smoke and made a noise like that of a railway train. "it was sometimes high in the air and sometimes near the ground." the effect was tornado-like: trees and walls were knocked down. it's a late day now to try to verify this story, but a list is given of persons whose property was injured. we are told that this thing then disappeared "all at once." these are the smaller objects, which may be derailed railway trains or big green snakes, for all i know--but our expression upon approach to this earth by vast dark bodies-- that likely they'd be made luminous: would envelop in clouds, perhaps, or would have their own clouds-- but that they'd quake, and that they'd affect this earth with quakes-- and that then would occur a fall of matter from such a world, or rise of matter from this earth to a nearby world, or both fall and rise, or exchange of matter--process known to advanced seismology as celestio-metathesis-- except that--if matter from some other world--and it would be like someone to get it into his head that we absolutely deny gravitation, just because we cannot accept orthodox dogmas--except that, if matter from another world, filling the sky of this earth, generally, as to a hemisphere, or locally, should be attracted to this earth, it would seem thinkable that the whole thing should drop here, and not merely its surface-materials. objects upon a ship's bottom. from time to time they drop to the bottom of the ocean. the ship does not. or, like our acceptance upon dripping from aerial ice-fields, we think of only a part of a nearby world succumbing, except in being caught in suspension, to this earth's gravitation, and surface-materials falling from that part-- explain or express or accept, and what does it matter? our attitude is: here are the data. see for yourself. what does it matter what my notions may be? here are the data. but think for yourself, or think for myself, all mixed up we must be. a long time must go by before we can know florida from long island. so we've had data of fishes that have fallen from our now established and respectabilized super-sargasso sea--which we've almost forgotten, it's now so respectable--but we shall have data of fishes that have fallen during earthquakes. these we accept were dragged down from ponds or other worlds that have been quaked, when only a few miles away, by this earth, some other world also quaking this earth. in a way, or in its principle, our subject is orthodox enough. only grant proximity of other worlds--which, however, will not be a matter of granting, but will be a matter of data--and one conventionally conceives of their surfaces quaked--even of a whole lake full of fishes being quaked and dragged down from one of them. the lake full of fishes may cause a little pain to some minds, but the fall of sand and stones is pleasantly enough thought of. more scientific persons, or more faithful hypnotics than we, have taken up this subject, unpainfully, relatively to the moon. for instance, perrey has gone over , records of earthquakes, and he has correlated many with proximities of the moon, or has attributed many to the pull of the moon when nearest this earth. also there is a paper upon this subject in the _proc. roy. soc. of cornwall_, . or, theoretically, when at its closest to this earth, the moon quakes the face of this earth, and is itself quaked--but does not itself fall to this earth. as to showers of matter that may have come from the moon at such times--one can go over old records and find what one pleases. that is what we now shall do. our expressions are for acceptance only. our data: we take them from four classes of phenomena that have preceded or accompanied earthquakes: unusual clouds, darkness profound, luminous appearances in the sky, and falls of substances and objects whether commonly called meteoritic or not. not one of these occurrences fits in with principles of primitive, or primary, seismology, and every one of them is a datum of a quaked body passing close to this earth or suspended over it. to the primitives there is not a reason in the world why a convulsion of this earth's surface should be accompanied by unusual sights in the sky, by darkness, or by the fall of substances or objects from the sky. as to phenomena like these, or storms, preceding earthquakes, the irreconcilability is still greater. it was before that perrey made his great compilation. we take most of our data from lists compiled long ago. only the safe and unpainful have been published in recent years--at least in ambitious, voluminous form. the restraining hand of the "system"--as we call it, whether it has any real existence or not--is tight upon the sciences of today. the uncanniest aspect of our quasi-existence that i know of is that everything that seems to have one identity has also as high a seeming of everything else. in this oneness of allness, or continuity, the protecting hand strangles; the parental stifles; love is inseparable from phenomena of hate. there is only continuity--that is in quasi-existence. _nature_, at least in its correspondents' columns, still evades this protective strangulation, and the _monthly weather review_ is still a rich field of unfaithful observation: but, in looking over other long-established periodicals, i have noted their glimmers of quasi-individuality fade gradually, after about , and the surrender of their attempted identities to a higher attempted organization. some of them, expressing intermediateness-wide endeavor to localize the universal, or to localize self, soul, identity, entity--or positiveness or realness--held out until as far as ; traces findable up to --and then, expressing the universal process--except that here and there in the world's history there may have been successful approximations to positiveness by "individuals"--who only then became individuals and attained to selves or souls of their own--surrendered, submitted, became parts of a higher organization's attempt to individualize or systematize into a complete thing, or to localize the universal or the attributes of the universal. after the death of richard proctor, whose occasional illiberalities i'd not like to emphasize too much, all succeeding volumes of _knowledge_ have yielded scarcely an unconventionality. note the great number of times that the _american journal of science_ and the _report of the british association_ are quoted: note that, after, say, , they're scarcely mentioned in these inspired but illicit pages--as by hypnosis and inertia, we keep on saying. about . throttle and disregard. but the coercion could not be positive, and many of the excommunicated continued to creep in; or, even to this day, some of the strangled are faintly breathing. some of our data have been hard to find. we could tell stories of great labor and fruitless quests that would, though perhaps imperceptibly, stir the sympathy of a mr. symons. but, in this matter of concurrence of earthquakes with aerial phenomena, which are as unassociable with earthquakes, if internally caused, as falls of sand on convulsed small boys full of sour apples, the abundance of so-called evidence is so great that we can only sketchily go over the data, beginning with robert mallet's catalogue (_rept. brit. assoc._, ), omitting some extraordinary instances, because they occurred before the eighteenth century: earthquake "preceded" by a violent tempest, england, jan. , --"preceded" by a brilliant meteor, switzerland, nov. , --"luminous cloud, moving at high velocity, disappearing behind the horizon," florence, dec. , --"thick mists in the air, through which a dim light was seen: several weeks before the shock, globes of light had been seen in the air," swabia, may , --rain of earth, carpentras, france, oct. , --a black cloud, london, march , --violent storm and a strange star of octagonal shape, slavange, norway, april , --balls of fire from a streak in the sky, augermannland, --numerous meteorites, lisbon, oct. , --"terrible tempests" over and over--"falls of hail" and "brilliant meteors," instance after instance--"an immense globe," switzerland, nov. , --oblong, sulphurous cloud, germany, april, --extraordinary mass of vapor, boulogne, april, --heavens obscured by a dark mist, grenada, aug. , --"strange, howling noises in the air, and large spots obscuring the sun," palermo, italy, april , --"luminous meteor moving in the same direction as the shock," naples, nov. , --fire ball appearing in the sky: apparent size of the moon, thuringerwald, nov. , . and, unless you be polarized by the new dominant, which is calling for recognition of multiplicities of external things, as a dominant, dawning new over europe in , called for recognition of terrestrial externality to europe--unless you have this contact with the new, you have no affinity for these data--beans that drop from a magnet--irreconcilables that glide from the mind of a thomson-- or my own acceptance that we do not really think at all; that we correlate around super-magnets that i call dominants--a spiritual dominant in one age, and responsively to it up spring monasteries, and the stake and the cross are its symbols: a materialist dominant, and up spring laboratories, and microscopes and telescopes and crucibles are its ikons--that we're nothing but iron filings relatively to a succession of magnets that displace preceding magnets. with no soul of your own, and with no soul of my own--except that some day some of us may no longer be intermediatisms, but may hold out against the cosmos that once upon a time thousands of fishes were cast from one pail of water--we have psycho-valency for these data, if we're obedient slaves to the new dominant, and repulsion to them, if we're mere correlates to the old dominant. i'm a soulless and selfless correlate to the new dominant, myself: i see what i have to see. the only inducement i can hold out, in my attempt to rake up disciples, is that some day the new will be fashionable: the new correlates will sneer at the old correlates. after all, there is some inducement to that--and i'm not altogether sure it's desirable to end up as a fixed star. as a correlate to the new dominant, i am very much impressed with some of these data--the luminous object that moved in the same direction as an earthquake--it seems very acceptable that a quake followed this thing as it passed near this earth's surface. the streak that was seen in the sky--or only a streak that was visible of another world--and objects, or meteorites, that were shaken down from it. the quake at carpentras, france: and that, above carpentras, was a smaller world, more violently quaked, so that earth was shaken down from it. but i like best the super-wolves that were seen to cross the sun during the earthquake at palermo. they howled. or the loves of the worlds. the call they feel for one another. they try to move closer and howl when they get there. the howls of the planets. i have discovered a new unintelligibility. in the _edinburgh new philosophical journal_--have to go away back to --days of less efficient strangulation--sir david milne lists phenomena of quakes in great britain. i pick out a few that indicate to me that other worlds were near this earth's surface: violent storm before a shock of --ball of fire "preceding," --a large ball of fire seen upon day following a quake, --"uncommon phenomenon in the air: a large luminous body, bent like a crescent, which stretched itself over the heavens, --vast ball of fire, --black rains and black snows, --numerous instances of upward projection--or upward attraction?--during quakes--preceded by a cloud, very black and lowering," --fall of black powder, preceding a quake, by six hours, . some of these instances seem to me to be very striking--a smaller world: it is greatly racked by the attraction of this earth--black substance is torn down from it--not until six hours later, after an approach still closer, does this earth suffer perturbation. as to the extraordinary spectacle of a thing, world, super-construction, that was seen in the sky, in , i have not yet been able to find out more. i think that here our acceptance is relatively sound: that this occurrence was tremendously of more importance than such occurrence as, say, transits of venus, upon which hundreds of papers have been written--that not another mention have i found, though i have not looked so especially as i shall look for more data--that all but undetailed record of this occurrence was suppressed. altogether we have considerable agreement here between data of vast masses that do not fall to this earth, but from which substances fall, and data of fields of ice from which ice may not fall, but from which water may drip. i'm beginning to modify: that, at a distance from this earth, gravitation has more effect than we have supposed, though less effect than the dogmatists suppose and "prove." i'm coming out stronger for the acceptance of a neutral zone--that this earth, like other magnets, has a neutral zone, in which is the super-sargasso sea, and in which other worlds may be buoyed up, though projecting parts may be subject to this earth's attraction-- but my preference: here are the data. i now have one of the most interesting of the new correlates. i think i should have brought it in before, but, whether out of place here, because not accompanied by earthquake, or not, we'll have it. i offer it as an instance of an eclipse, by a vast, dark body, that has been seen and reported by an astronomer. the astronomer is m. lias: the phenomenon was seen by him, at pernambuco, april , . _comptes rendus_, - : it was about noon--sky cloudless--suddenly the light of the sun was diminished. the darkness increased, and, to illustrate its intensity, we are told that the planet venus shone brilliant. but venus was of low visibility at this time. the observation that burns incense to the new dominant is: that around the sun appeared a corona. there are many other instances that indicate proximity of other world's during earthquakes. i note a few--quake and an object in the sky, called "a large, luminous meteor" (_quar. jour. roy. inst._, - ); luminous body in the sky, earthquake, and fall of sand, italy, feb. and , (_la science pour tous_, - ); many reports upon luminous object in the sky and earthquake, connecticut, feb. , (_monthly weather review_, february, ); luminous object, or meteor, in the sky, fall of stones from the sky, and earthquake, italy, jan. , (_l'astronomie_, - ); earthquake and prodigious number of luminous bodies, or globes, in the air, boulogne, france, june , (sestier, "_la foudre_," - ); earthquake at manila, , and "curious luminous appearance in the sky" (ponton, _earthquakes_, p. ). the most notable appearance of fishes during an earthquake is that of riobamba. humboldt sketched one of them, and it's an uncanny-looking thing. thousands of them appeared upon the ground during this tremendous earthquake. humboldt says that they were cast up from subterranean sources. i think not myself, and have data for thinking not, but there'd be such a row arguing back and forth that it's simpler to consider a clearer instance of the fall of living fishes from the sky, during an earthquake. i can't quite accept, myself, whether a large lake, and all the fishes in it, was torn down from some other world, or a lake in the super-sargasso sea, distracted between two pulling worlds, was dragged down to this earth-- here are the data: _la science pour tous_, - : feb. , . an earthquake at singapore. then came an extraordinary downpour of rain--or as much water as any good-sized lake would consist of. for three days this rain or this fall of water came down in torrents. in pools on the ground, formed by this deluge, great numbers of fishes were found. the writer says that he had, himself, seen nothing but water fall from the sky. whether i'm emphasizing what a deluge it was or not, he says that so terrific had been the downpour that he had not been able to see three steps away from him. the natives said that the fishes had fallen from the sky. three days later the pools dried up and many dead fishes were found, but, in the first place--though that's an expression for which we have an instinctive dislike--the fishes had been active and uninjured. then follows material for another of our little studies in the phenomena of disregard. a psycho-tropism here is mechanically to take pen in hand and mechanically write that fishes found on the ground after a heavy rainfall came from overflowing streams. the writer of the account says that some of the fishes had been found in his courtyard, which was surrounded by high walls--paying no attention to this, a correspondent (_la science pour tous_, - ) explains that in the heavy rain a body of water had probably overflowed, carrying fishes with it. we are told by the first writer that these fishes of singapore were of a species that was very abundant near singapore. so i think, myself, that a whole lakeful of them had been shaken down from the super-sargasso sea, under the circumstances we have thought of. however, if appearance of strange fishes after an earthquake be more pleasing in the sight, or to the nostrils, of the new dominant, we faithfully and piously supply that incense--an account of the occurrence at singapore was read by m. de castelnau, before the french academy. m. de castelnau recalled that, upon a former occasion, he had submitted to the academy the circumstance that fishes of a new species had appeared at the cape of good hope, after an earthquake. it seems proper, and it will give luster to the new orthodoxy, now to have an instance in which, not merely quake and fall of rocks or meteorites, or quake and either eclipse or luminous appearances in the sky have occurred, but in which are combined all the phenomena, one or more of which, when accompanying earthquake, indicate, in our acceptance, the proximity of another world. this time a longer duration is indicated than in other instances. in the _canadian institute proceedings_, - - , there is an account, by the deputy commissioner at dhurmsalla, of the extraordinary dhurmsalla meteorite--coated with ice. but the combination of events related by him is still more extraordinary: that within a few months of the fall of this meteorite there had been a fall of live fishes at benares, a shower of red substance at furruckabad, a dark spot observed on the disk of the sun, an earthquake, "an unnatural darkness of some duration," and a luminous appearance in the sky that looked like an aurora borealis-- but there's more to this climax: we are introduced to a new order of phenomena: visitors. the deputy commissioner writes that, in the evening, after the fall of the dhurmsalla meteorite, or mass of stone covered with ice, he saw lights. some of them were not very high. they appeared and went out and reappeared. i have read many accounts of the dhurmsalla meteorite--july , --but never in any other of them a mention of this new correlate--something as out of place in the nineteenth century as would have been an aeroplane--the invention of which would not, in our acceptance, have been permitted, in the nineteenth century, though adumbrations to it were permitted. this writer says that the lights moved like fire balloons, but: "i am sure that they were neither fire balloons, lanterns, nor bonfires, or any other thing of that sort, but bona fide lights in the heavens." it's a subject for which we shall have to have a separate expression--trespassers upon territory to which something else has a legal right--perhaps someone lost a rock, and he and his friends came down looking for it, in the evening--or secret agents, or emissaries, who had an appointment with certain esoteric ones near dhurmsalla--things or beings coming down to explore, and unable to stay down long-- in a way, another strange occurrence during an earthquake is suggested. the ancient chinese tradition--the marks like hoof marks in the ground. we have thought--with a low degree of acceptance--of another world that may be in secret communication with certain esoteric ones of this earth's inhabitants--and of messages in symbols like hoof marks that are sent to some receptor, or special hill, upon this earth--and of messages that at times miscarry. this other world comes close to this world--there are quakes--but advantage of proximity is taken to send a message--the message, designed for a receptor in india, perhaps, or in central europe, miscarries all the way to england--marks like the marks of the chinese tradition are found upon a beach, in cornwall, after an earthquake-- _phil. trans._, - : after the quake of july , , upon the sands of penzance, cornwall, in an area of more than square yards, were found marks like hoof prints, except that they were not crescentic. we feel a similarity, but note an arbitrary disregard of our own, this time. it seems to us that marks described as "little cones surrounded by basins of equal diameter" would be like hoof prints, if hoofs printed complete circles. other disregards are that there were black specks on the tops of cones, as if something, perhaps gaseous, had issued from them; that from one of these formations came a gush of water as thick as a man's wrist. of course the opening of springs is common in earthquakes--but we suspect, myself, that the negative absolute is compelling us to put in this datum and its disorders. there's another matter in which the negative absolute seems to work against us. though to super-chemistry, we have introduced the principle of celestio-metathesis, we have no good data of exchange of substances during proximities. the data are all of falls and not of upward translations. of course upward impulses are common during earthquakes, but i haven't a datum upon a tree or a fish or a brick or a man that ever did go up and stay up and that never did come down again. our classic of the horse and barn occurred in what was called a whirlwind. it is said that, in an earthquake in calabria, paving stones shot up far in the air. the writer doesn't specifically say that they came down again, but something seems to tell me they did. the corpses of riobamba. humboldt reported that, in the quake of riobamba, "bodies were torn upward from graves"; that "the vertical motion was so strong that bodies were tossed several hundred feet in the air." i explain. i explain that, if in the center of greatest violence of an earthquake, anything ever has gone up, and has kept on going up, the thoughts of the nearest observers were very likely upon other subjects. the quay of lisbon. we are told that it went down. a vast throng of persons ran to the quay for refuge. the city of lisbon was in profound darkness. the quay and all the people on it disappeared. if it and they went down--not a single corpse, not a shred of clothing, not a plank of the quay, nor so much as a splinter of it ever floated to the surface. the new dominant. i mean "primarily" all that opposes exclusionism-- that development or progress or evolution is attempt to positivize, and is a mechanism by which a positive existence is recruited--that what we call existence is a womb of infinitude, and is itself only incubatory--that eventually all attempts are broken down by the falsely excluded. subjectively, the breaking down is aided by our own sense of false and narrow limitations. so the classic and academic artists wrought positivist paintings, and expressed the only ideal that i am conscious of, though we so often hear of "ideals" instead of different manifestations, artistically, scientifically, theologically, politically, of the one ideal. they sought to satisfy, in its artistic aspect, cosmic craving for unity or completeness, sometimes called harmony, called beauty in some aspects. by disregard they sought completeness. but the light-effects that they disregarded, and their narrow confinement to standardized subjects brought on the revolt of the impressionists. so the puritans tried to systematize, and they disregarded physical needs, or vices, or relaxations: they were invaded and overthrown when their narrowness became obvious and intolerable. all things strive for positiveness, for themselves, or for quasi-systems of which they are parts. formality and the mathematic, the regular and the uniform are aspects of the positive state--but the positive is the universal--so all attempted positiveness that seems to satisfy in the aspects of formality and regularity, sooner or later disqualifies in the aspect of wideness or universalness. so there is revolt against the science of today, because the formulated utterances that were regarded as final truths in a past generation, are now seen to be insufficiencies. every pronouncement that has opposed our own acceptances has been found to be a composition like any academic painting: something that is arbitrarily cut off from relations with environment, or framed off from interfering and disturbing data, or outlined with disregards. our own attempt has been to take in the included, but also to take in the excluded into wider expressions. we accept, however, that for every one of our expressions there are irreconcilables somewhere--that final utterance would include all things. however, of such is the gossip of angels. the final is unutterable in quasi-existence, where to think is to include but also to exclude, or be not final. if we admit that for every opinion we have expressed, there must somewhere be an irreconcilable, we are intermediatists and not positivists; not even higher positivists. of course it may be that some day we shall systematize and dogmatize and refuse to think of anything that we may be accused of disregarding, and believe instead of merely accepting: then, if we could have a wider system, which would acknowledge no irreconcilables we'd be higher positivists. so long as we only accept, we are not higher positivists, but our feeling is that the new dominant, even though we have thought of it only as another enslavement, will be the nucleus for higher positivism--and that it will be the means of elevating into infinitude a new batch of fixed stars--until, as a recruiting instrument, it, too, will play out, and will give way to some new medium for generating absoluteness. it is our acceptance that all astronomers of today have lost their souls, or, rather, all chance of attaining entity, but that copernicus and kepler and galileo and newton, and, conceivably, leverrier are now fixed stars. some day i shall attempt to identify them. in all this, i think we're quite a moses. we point out the promised land, but, unless we be cured of our intermediatism, will never be reported in _monthly notices_, ourself. in our acceptance, dominants, in their succession, displace preceding dominants not only because they are more nearly positive, but because the old dominants, as recruiting mediums, play out. our expression is that the new dominant, of wider inclusions, is now manifesting throughout the world, and that the old exclusionism is everywhere breaking down. in physics exclusionism is breaking down by its own researches in radium, for instance, and in its speculations upon electrons, or its merging away into metaphysics, and by the desertion that has been going on for many years, by such men as gurney, crookes, wallace, flammarion, lodge, to formerly disregarded phenomena--no longer called "spiritualism" but now "psychic research." biology is in chaos: conventional darwinites mixed up with mutationists and orthogenesists and followers of wisemann, who take from darwinism one of its pseudo-bases, and nevertheless try to reconcile their heresies with orthodoxy. the painters are metaphysicians and psychologists. the breaking down of exclusionism in china and japan and in the united states has astonished history. the science of astronomy is going downward so that, though pickering, for instance, did speculate upon a trans-neptunian planet, and lowell did try to have accepted heretical ideas as to marks on mars, attention is now minutely focused upon such technicalities as variations in shades of jupiter's fourth satellite. i think that, in general acceptance, over-refinement indicates decadence. i think that the stronghold of inclusionism is in aeronautics. i think that the stronghold of the old dominant, when it was new, was in the invention of the telescope. or that coincidentally with the breakdown of exclusionism appears the means of finding out--whether there are vast aerial fields of ice and floating lakes full of frogs and fishes or not--where carved stones and black substances and great quantities of vegetable matter and flesh, which may be dragons' flesh, come from--whether there are inter-planetary trade routes and vast areas devastated by super-tamerlanes--whether sometimes there are visitors to this earth--who might be pursued and captured and questioned. i have industriously sought data for an expression upon birds, but the prospecting has not been very quasi-satisfactory. i think i rather emphasize our industriousness, because a charge likely to be brought against the attitude of acceptance is that one who only accepts must be one of languid interest and little application of energy. it doesn't seem to work out: we are very industrious. i suggest to some of our disciples that they look into the matter of messages upon pigeons, of course attributed to earthly owners, but said to be undecipherable. i'd do it, ourselves, only that would be selfish. that's more of the intermediatism that will keep us out of the firmament: positivism is absolute egoism. but look back in the time of andrée's polar expedition. pigeons that would have no publicity ordinarily, were often reported at that time. in the _zoologist_, - - , is recorded an instance of a bird (puffin) that had fallen to the ground with a fractured head. interesting, but mere speculation--but what solid object, high in the air, had that bird struck against? tremendous red rain in france, oct. and , ; great storm at the time, and red rain supposed to have been colored by matter swept up from this earth's surface, and then precipitated (_comptes rendus_, - ). but in _comptes rendus_, - , the description of this red rain differs from one's impression of red, sandy or muddy water. it is said that this rain was so vividly red and so blood-like that many persons in france were terrified. two analyses are given (_comptes rendus_, - ). one chemist notes a great quantity of corpuscles--whether blood-like corpuscles or not--in the matter. the other chemist sets down organic matter at per cent. it may be that an inter-planetary dragon had been slain somewhere, or that this red fluid, in which were many corpuscles, came from something not altogether pleasant to contemplate, about the size of the catskill mountains, perhaps--but the present datum is that with this substance, larks, quail, ducks, and water hens, some of them alive, fell at lyons and grenoble and other places. i have notes upon other birds that have fallen from the sky, but unaccompanied by the red rain that makes the fall of birds in france peculiar, and very peculiar, if it be accepted that the red substance was extra-mundane. the other notes are upon birds that have fallen from the sky, in the midst of storms, or of exhausted, but living, birds, falling not far from a storm-area. but now we shall have an instance for which i can find no parallel: fall of dead birds, from a clear sky, far-distant from any storm to which they could be attributed--so remote from any discoverable storm that-- my own notion is that, in the summer of , something, or some beings, came as near to this earth as they could, upon a hunting expedition; that, in the summer of , an expedition of super-scientists passed over this earth, and let down a dragnet--and what would it catch, sweeping through the air, supposing it to have reached not quite to this earth? in the _monthly weather review_, may, , w.l. mcatee quotes from the baton rouge correspondence to the _philadelphia times_: that, in the summer of , into the streets of baton rouge, la., and from a "clear sky," fell hundreds of dead birds. there were wild ducks and cat birds, woodpeckers, and "many birds of strange plumage," some of them resembling canaries. usually one does not have to look very far from any place to learn of a storm. but the best that could be done in this instance was to say: "there had been a storm on the coast of florida." and, unless he have psycho-chemic repulsion for the explanation, the reader feels only momentary astonishment that dead birds from a storm in florida should fall from an unstormy sky in louisiana, and with his intellect greased like the plumage of a wild duck, the datum then drops off. our greasy, shiny brains. that they may be of some use after all: that other modes of existence place a high value upon them as lubricants; that we're hunted for them; a hunting expedition to this earth--the newspapers report a tornado. if from a clear sky, or a sky in which there were no driven clouds, or other evidences of still-continuing wind-power--or, if from a storm in florida, it could be accepted that hundreds of birds had fallen far away, in louisiana, i conceive, conventionally, of heavier objects having fallen in alabama, say, and of the fall of still heavier objects still nearer the origin in florida. the sources of information of the weather bureau are widespread. it has no records of such falls. so a dragnet that was let down from above somewhere-- or something that i learned from the more scientific of the investigators of psychic phenomena: the reader begins their works with prejudice against telepathy and everything else of psychic phenomena. the writers deny spirit-communication, and say that the seeming data are data of "only telepathy." astonishing instances of seeming clairvoyance--"only telepathy." after a while the reader finds himself agreeing that it's only telepathy--which, at first, had been intolerable to him. so maybe, in , a super-dragnet did not sweep through this earth's atmosphere, gathering up all the birds within its field, the meshes then suddenly breaking-- or that the birds of baton rouge were only from the super-sargasso sea-- upon which we shall have another expression. we thought we'd settled that, and we thought we'd establish that, but nothing's ever settled, and nothing's ever established, in a real sense, if, in a real sense, there is nothing in quasiness. i suppose there had been a storm somewhere, the storm in florida, perhaps, and many birds had been swept upward into the super-sargasso sea. it has frigid regions and it has tropical regions--that birds of diverse species had been swept upward, into an icy region, where, huddling together for warmth, they had died. then, later, they had been dislodged--meteor coming along--boat--bicycle--dragon--don't know what did come along--something dislodged them. so leaves of trees, carried up there in whirlwinds, staying there years, ages, perhaps only a few months, but then falling to this earth at an unseasonable time for dead leaves--fishes carried up there, some of them dying and drying, some of them living in volumes of water that are in abundance up there, or that fall sometimes in the deluges that we call "cloudbursts." the astronomers won't think kindly of us, and we haven't done anything to endear ourselves to the meteorologists--but we're weak and mawkish intermediatists--several times we've tried to get the aeronauts with us--extraordinary things up there: things that curators of museums would give up all hope of ever being fixed stars, to obtain: things left over from whirlwinds of the time of the pharaohs, perhaps: or that elijah did go up in the sky in something like a chariot, and may not be vega, after all, and that there may be a wheel or so left of whatever he went up in. we basely suggest that it would bring a high price--but sell soon, because after a while there'd be thousands of them hawked around-- we weakly drop a hint to the aeronauts. in the _scientific american_, - , there is an account of some hay that fell from the sky. from the circumstances we incline to accept that this hay went up, in a whirlwind, from this earth, in the first place, reached the super-sargasso sea, and remained there a long time before falling. an interesting point in this expression is the usual attribution to a local and coinciding whirlwind, and identification of it--and then data that make that local whirlwind unacceptable-- that, upon july , , small masses of damp hay had fallen at monkstown, ireland. in the _dublin daily express_, dr. j.w. moore had explained: he had found a nearby whirlwind, to the south of monkstown, that coincided. but, according to the _scientific american_, a similar fall had occurred near wrexham, england, two days before. in november, , i made some studies upon light objects thrown into the air. armistice-day. i suppose i should have been more emotionally occupied, but i made notes upon torn-up papers thrown high in the air from windows of office buildings. scraps of paper did stay together for a while. several minutes, sometimes. _cosmos_, - - : that, upon the th of april, , at autriche (indre-et-loire) a great number of oak leaves--enormous segregation of them--fell from the sky. very calm day. so little wind that the leaves fell almost vertically. fall lasted about ten minutes. flammarion, in _the atmosphere_, p. , tells this story. he has to find a storm. he does find a squall--but it had occurred upon april rd. flammarion's two incredibilities are--that leaves could remain a week in the air: that they could stay together a week in the air. think of some of your own observations upon papers thrown from an aeroplane. our one incredibility: that these leaves had been whirled up six months before, when they were common on the ground, and had been sustained, of course not in the air, but in a region gravitationally inert; and had been precipitated by the disturbances of april rains. i have no records of leaves that have so fallen from the sky in october or november, the season when one might expect dead leaves to be raised from one place and precipitated somewhere else. i emphasize that this occurred in april. _la nature_, - - : that, upon april , , dried leaves, of different species, oak, elm, etc., fell from the sky. this day, too, was a calm day. the fall was tremendous. the leaves were seen to fall fifteen minutes, but, judging from the quantity on the ground, it is the writer's opinion that they had already been falling half an hour. i think that the geyser of corpses that sprang from riobamba toward the sky must have been an interesting sight. if i were a painter, i'd like that subject. but this cataract of dried leaves, too, is a study in the rhythms of the dead. in this datum, the point most agreeable to us is the very point that the writer in _la nature_ emphasizes. windlessness. he says that the surface of the loire was "absolutely smooth." the river was strewn with leaves as far as he could see. _l'astronomie_, - : that, upon the th of april, , dried leaves fell at clairvaux and outre-aube, france. the fall is described as prodigious. half an hour. then, upon the th, a fall of dried leaves occurred at pontcarré. it is in this recurrence that we found some of our opposition to the conventional explanation. the editor (flammarion) explains. he says that the leaves had been caught up in a cyclone which had expended its force; that the heavier leaves had fallen first. we think that that was all right for , and that it was quite good enough for . but, in these more exacting days, we want to know how wind-power insufficient to hold some leaves in the air could sustain others four days. the factors in this expression are unseasonableness, not for dried leaves, but for prodigious numbers of dried leaves; direct fall, windlessness, month of april, and localization in france. the factor of localization is interesting. not a note have i upon fall of leaves from the sky, except these notes. were the conventional explanation, or "old correlate" acceptable, it would seem that similar occurrences in other regions should be as frequent as in france. the indication is that there may be quasi-permanent undulations in the super-sargasso sea, or a pronounced inclination toward france-- inspiration: that there may be a nearby world complementary to this world, where autumn occurs at the time that is springtime here. let some disciple have that. but there may be a dip toward france, so that leaves that are borne high there, are more likely to be held in suspension than highflying leaves elsewhere. some other time i shall take up super-geography, and be guilty of charts. i think, now, that the super-sargasso sea is an oblique belt, with changing ramifications, over great britain, france, italy, and on to india. relatively to the united states i am not very clear, but think especially of the southern states. the preponderance of our data indicates frigid regions aloft. nevertheless such phenomena as putrefaction have occurred often enough to make super-tropical regions, also, acceptable. we shall have one more datum upon the super-sargasso sea. it seems to me that, by this time, our requirements of support and reinforcement and agreement have been quite as rigorous for acceptance as ever for belief: at least for full acceptance. by virtue of mere acceptance, we may, in some later book, deny the super-sargasso sea, and find that our data relate to some other complementary world instead--or the moon--and have abundant data for accepting that the moon is not more than twenty or thirty miles away. however, the super-sargasso sea functions very well as a nucleus around which to gather data that oppose exclusionism. that is our main motive: to oppose exclusionism. or our agreement with cosmic processes. the climax of our general expression upon the super-sargasso sea. coincidentally appears something else that may overthrow it later. _notes and queries_, - - : that in the province of macerata, italy (summer of ?) an immense number of small, blood-colored clouds covered the sky. about an hour later a storm broke, and myriad seeds fell to the ground. it is said that they were identified as products of a tree found only in central africa and the antilles. if--in terms of conventional reasoning--these seeds had been high in the air, they had been in a cold region. but it is our acceptance that these seeds had, for a considerable time, been in a warm region, and for a time longer than is attributable to suspension by wind-power: "it is said that a great number of the seeds were in the first stage of germination." the new dominant. inclusionism. in it we have a pseudo-standard. we have a datum, and we give it an interpretation, in accordance with our pseudo-standard. at present we have not the delusions of absolutism that may have translated some of the positivists of the nineteenth century to heaven. we are intermediatists--but feel a lurking suspicion that we may some day solidify and dogmatize and illiberalize into higher positivists. at present we do not ask whether something be reasonable or preposterous, because we recognize that by reasonableness and preposterousness are meant agreement and disagreement with a standard--which must be a delusion--though not absolutely, of course--and must some day be displaced by a more advanced quasi-delusion. scientists in the past have taken the positivist attitude--is this or that reasonable or unreasonable? analyze them and we find that they meant relatively to a standard, such as newtonism, daltonism, darwinism, or lyellism. but they have written and spoken and thought as if they could mean real reasonableness and real unreasonableness. so our pseudo-standard is inclusionism, and, if a datum be a correlate to a more widely inclusive outlook as to this earth and its externality and relations with externality, its harmony with inclusionism admits it. such was the process, and such was the requirement for admission in the days of the old dominant: our difference is in underlying intermediatism, or consciousness that though we're more nearly real, we and our standards are only quasi-- or that all things--in our intermediate state--are phantoms in a super-mind in a dreaming state--but striving to awaken to realness. though in some respects our own intermediatism is unsatisfactory, our underlying feeling is-- that in a dreaming mind awakening is accelerated--if phantoms in that mind know that they're only phantoms in a dream. of course, they too are quasi, or--but in a relative sense--they have an essence of what is called realness. they are derived from experience or from senes-relations, even though grotesque distortions. it seems acceptable that a table that is seen when one is awake is more nearly real than a dreamed table, which, with fifteen or twenty legs, chases one. so now, in the twentieth century, with a change of terms, and a change in underlying consciousness, our attitude toward the new dominant is the attitude of the scientists of the nineteenth century to the old dominant. we do not insist that our data and interpretations shall be as shocking, grotesque, evil, ridiculous, childish, insincere, laughable, ignorant to nineteenth-centuryites as were their data and interpretations to the medieval-minded. we ask only whether data and interpretations correlate. if they do, they are acceptable, perhaps only for a short time, or as nuclei, or scaffolding, or preliminary sketches, or as gropings and tentativenesses. later, of course, when we cool off and harden and radiate into space most of our present mobility, which expresses in modesty and plasticity, we shall acknowledge no scaffoldings, gropings or tentativenesses, but think we utter absolute facts. a point in intermediatism here is opposed to most current speculations upon development. usually one thinks of the spiritual as higher than the material, but, in our acceptance, quasi-existence is a means by which the absolutely immaterial materializes absolutely, and, being intermediate, is a state in which nothing is finally either immaterial or material, all objects, substances, thoughts, occupying some grade of approximation one way or the other. final solidification of the ethereal is, to us, the goal of cosmic ambition. positivism is puritanism. heat is evil. final good is absolute frigidity. an arctic winter is very beautiful, but i think that an interest in monkeys chattering in palm trees accounts for our own intermediatism. visitors. our confusion here, out of which we are attempting to make quasi-order, is as great as it has been throughout this book, because we have not the positivist's delusion of homogeneity. a positivist would gather all data that seem to relate to one kind of visitors and coldly disregard all other data. i think of as many different kinds of visitors to this earth as there are visitors to new york, to a jail, to a church--some persons go to church to pick pockets, for instance. my own acceptance is that either a world or a vast super-construction--or a world, if red substances and fishes fell from it--hovered over india in the summer of . something then fell from somewhere, july , , at dhurmsalla. whatever "it" was, "it" is so persistently alluded to as "a meteorite" that i look back and see that i adopted this convention myself. but in the london _times_, dec. , , syed abdoolah, professor of hindustani, university college, london, writes that he had sent to a friend in dhurmsalla, for an account of the stones that had fallen at that place. the answer: "... divers forms and sizes, many of which bore great resemblance to ordinary cannon balls just discharged from engines of war." it's an addition to our data of spherical objects that have arrived upon this earth. note that they are spherical stone objects. and, in the evening of this same day that something--took a shot at dhurmsalla--or sent objects upon which there may be decipherable markings--lights were seen in the air-- i think, myself, of a number of things, beings, whatever they were, trying to get down, but resisted, like balloonists, at a certain altitude, trying to get farther up, but resisted. not in the least except to good positivists, or the homogeneous-minded, does this speculation interfere with the concept of some other world that is in successful communication with certain esoteric ones upon this earth, by a code of symbols that print in rock, like symbols of telephotographers in selenium. i think that sometimes, in favorable circumstances, emissaries have come to this earth--secret meetings-- of course it sounds-- but: secret meetings--emissaries--esoteric ones in europe, before the war broke out-- and those who suggested that such phenomena could be. however, as to most of our data, i think of super-things that have passed close to this earth with no more interest in this earth than have passengers upon a steamship in the bottom of the sea--or passengers may have a keen interest, but circumstances of schedules and commercial requirements forbid investigation of the bottom of the sea. then, on the other hand, we may have data of super-scientific attempts to investigate phenomena of this earth from above--perhaps by beings from so far away that they had never even heard that something, somewhere, asserts a legal right to this earth. altogether, we're good intermediatists, but we can't be very good hypnotists. still another source of the merging away of our data: that, upon general principles of continuity, if super-vessels, or super-vehicles, have traversed this earth's atmosphere, there must be mergers between them and terrestrial phenomena: observations upon them must merge away into observations upon clouds and balloons and meteors. we shall begin with data that we cannot distinguish ourselves and work our way out of mergers into extremes. in the _observatory_, - , it is said that, according to a newspaper, march , , residents of warmley, england, were greatly excited by something that was supposed to be "a splendidly illuminated aeroplane, passing over the village." "the machine was apparently traveling at a tremendous rate, and came from the direction of bath, and went on toward gloucester." the editor says that it was a large, triple-headed fireball. "tremendous indeed!" he says. "but we are prepared for anything nowadays." that is satisfactory. we'd not like to creep up stealthily and then jump out of a corner with our data. this editor, at least, is prepared to read-- _nature_, oct. , : a correspondent writes that, in the county wicklow, ireland, at about o'clock in the evening, he had seen, in the sky, an object that looked like the moon in its three-quarter aspect. we note the shape which approximates to triangularity, and we note that in color it is said to have been golden yellow. it moved slowly, and in about five minutes disappeared behind a mountain. the editor gives his opinion that the object may have been an escaped balloon. in _nature_, aug. , , there is a story, taken from the july number of the _canadian weather review_, by the meteorologist, f.f. payne: that he had seen, in the canadian sky, a large, pear-shaped object, sailing rapidly. at first he supposed that the object was a balloon, "its outline being sharply defined." "but, as no cage was seen, it was concluded that it must be a mass of cloud." in about six minutes this object became less definite--whether because of increasing distance or not--"the mass became less dense, and finally it disappeared." as to cyclonic formation--"no whirling motion could be seen." _nature_, - : that, upon july , , a correspondent had seen, at kiel, an object in the sky, colored red by the sun, which had set. it was about as broad as a rainbow, and about twelve degrees high. "it remained in its original brightness about five minutes, and then faded rapidly, and then remained almost stationary again, finally disappearing about eight minutes after i first saw it." in an intermediate existence, we quasi-persons have nothing to judge by because everything is its own opposite. if a hundred dollars a week be a standard of luxurious living to some persons, it is poverty to others. we have instances of three objects that were seen in the sky in a space of three months, and this concurrence seems to me to be something to judge by. science has been built upon concurrence: so have been most of the fallacies and fanaticisms. i feel the positivism of a leverrier, or instinctively take to the notion that all three of these observations relate to the same object. however, i don't formulate them and predict the next transit. here's another chance for me to become a fixed star--but as usual--oh, well-- a point in intermediatism: that the intermediatist is likely to be a flaccid compromiser. our own attitude: ours is a partly positive and partly negative state, or a state in which nothing is finally positive or finally negative-- but, if positivism attract you, go ahead and try: you will be in harmony with cosmic endeavor--but continuity will resist you. only to have appearance in quasiness is to be proportionately positive, but beyond a degree of attempted positivism, continuity will rise to pull you back. success, as it is called--though there is only success-failure in intermediateness--will, in intermediateness, be yours proportionately as you are in adjustment with its own state, or some positivism mixed with compromise and retreat. to be very positive is to be a napoleon bonaparte, against whom the rest of civilization will sooner or later combine. for interesting data, see newspaper accounts of fate of one dowie, of chicago. intermediatism, then, is recognition that our state is only a quasi-state: it is no bar to one who desires to be positive: it is recognition that he cannot be positive and remain in a state that is positive-negative. or that a great positivist--isolated--with no system to support him--will be crucified, or will starve to death, or will be put in jail and beaten to death--that these are the birth-pangs of translation to the positive absolute. so, though positive-negative, myself, i feel the attraction of the positive pole of our intermediate state, and attempt to correlate these three data: to see them homogeneously; to think that they relate to one object. in the aeronautic journals and in the london _times_ there is no mention of escaped balloons, in the summer or fall of . in the _new york times_ there is no mention of ballooning in canada or the united states, in the summer of . london _times_, sept. , : a clipping from the _royal gazette_, of bermuda, of sept. , , sent to the _times_ by general lefroy: that, upon aug. , , at about : a.m., there was observed by mrs. adelina d. bassett, "a strange object in the clouds, coming from the north." she called the attention of mrs. l. lowell to it, and they were both somewhat alarmed. however, they continued to watch the object steadily for some time. it drew nearer. it was of triangular shape, and seemed to be about the size of a pilot-boat mainsail, with chains attached to the bottom of it. while crossing the land it had appeared to descend, but, as it went out to sea, it ascended, and continued to ascend, until it was lost to sight high in the clouds. or with such power to ascend, i don't think much myself of the notion that it was an escaped balloon, partly deflated. nevertheless, general lefroy, correlating with exclusionism, attempts to give a terrestrial interpretation to this occurrence. he argues that the thing may have been a balloon that had escaped from france or england--or the only aerial thing of terrestrial origin that, even to this date of about thirty-five years later, has been thought to have crossed the atlantic ocean. he accounts for the triangular form by deflation--"a shapeless bag, barely able to float." my own acceptance is that great deflation does not accord with observations upon its power to ascend. in the _times_, oct. , , charles harding, of the r.m.s., argues that if it had been a balloon from europe, surely it would have been seen and reported by many vessels. whether he was as good a briton as the general or not, he shows awareness of the united states--or that the thing may have been a partly collapsed balloon that had escaped from the united states. general lefroy wrote to _nature_ about it (_nature_, - ), saying--whatever his sensitivenesses may have been--that the columns of the _times_ were "hardly suitable" for such a discussion. if, in the past, there had been more persons like general lefroy, we'd have better than the mere fragments of data that in most cases are too broken up very well to piece together. he took the trouble to write to a friend of his, w.h. gosling, of bermuda--who also was an extraordinary person. he went to the trouble of interviewing mrs. bassett and mrs. lowell. their description to him was somewhat different: an object from which nets were suspended-- deflated balloon, with its network hanging from it-- a super-dragnet? that something was trawling overhead? the birds of baton rouge. mr. gosling wrote that the item of chains, or suggestion of a basket that had been attached, had originated with mr. bassett, who had not seen the object. mr. gosling mentioned a balloon that had escaped from paris in july. he tells of a balloon that fell in chicago, september , or three weeks later than the bermuda object. it's one incredibility against another, with disregards and convictions governed by whichever of the two dominants looms stronger in each reader's mind. that he can't think for himself any more than i can is understood. my own correlates: i think that we're fished for. it may be that we're highly esteemed by super-epicures somewhere. it makes me more cheerful when i think that we may be of some use after all. i think that dragnets have often come down and have been mistaken for whirlwinds and waterspouts. some accounts of seeming structure in whirlwinds and waterspouts are astonishing. and i have data that, in this book, i can't take up at all--mysterious disappearances. i think we're fished for. but this is a little expression on the side: relates to trespassers; has nothing to do with the subject that i shall take up at some other time--or our use to some other mode of seeming that has a legal right to us. _nature_, - : "our paris correspondent writes that in relation to the balloon which is said to have been seen over bermuda, in september, no ascent took place in france which can account for it." last of august: not september. in the london _times_ there is no mention of balloon ascents in great britain, in the summer of , but mention of two ascents in france. both balloons had escaped. in _l'aéronaute_, august, , it is said that these balloons had been sent up from fêtes of the fourteenth of july-- days before the observation at bermuda. the aeronauts were gower and eloy. gower's balloon was found floating on the ocean, but eloy's balloon was not found. upon the th of july it was reported by a sea captain: still in the air; still inflated. but this balloon of eloy's was a small exhibition balloon, made for short ascents from fêtes and fair grounds. in _la nature_, - - , it is said that it was a very small balloon, incapable of remaining long in the air. as to contemporaneous ballooning in the united states, i find only one account: an ascent in connecticut, july , . upon leaving this balloon, the aeronauts had pulled the "rip cord," "turning it inside out." (_new york times_, aug. , .) to the intermediatist, the accusation of "anthropomorphism" is meaningless. there is nothing in anything that is unique or positively different. we'd be materialists were it not quite as rational to express the material in terms of the immaterial as to express the immaterial in terms of the material. oneness of allness in quasiness. i will engage to write the formula of any novel in psycho-chemic terms, or draw its graph in psycho-mechanic terms: or write, in romantic terms, the circumstances and sequences of any chemic or electric or magnetic reaction: or express any historic event in algebraic terms--or see boole and jevons for economic situations expressed algebraically. i think of the dominants as i think of persons--not meaning that they are real persons--not meaning that we are real persons-- or the old dominant and its jealousy, and its suppression of all things and thoughts that endangered its supremacy. in reading discussions of papers, by scientific societies, i have often noted how, when they approached forbidden--or irreconcilable--subjects, the discussions were thrown into confusion and ramification. it's as if scientific discussions have often been led astray--as if purposefully--as if by something directive, hovering over them. of course i mean only the spirit of all development. just so, in any embryo, cells that would tend to vary from the appearances of their era are compelled to correlate. in _nature_, - , charles tilden smith writes that, at chisbury, wiltshire, england, april , , he saw something in the sky-- "--unlike anything that i had ever seen before." "although i have studied the skies for many years, i have never seen anything like it." he saw two stationary dark patches upon clouds. the extraordinary part: they were stationary upon clouds that were rapidly moving. they were fan-shaped--or triangular--and varied in size, but kept the same position upon different clouds as cloud after cloud came along. for more than half an hour mr. smith watched these dark patches-- his impression as to the one that appeared first: that it was "really a heavy shadow cast upon a thin veil of clouds by some unseen object away in the west, which was intercepting the sun's rays." upon page , of this volume of _nature_, is a letter from another correspondent, to the effect that similar shadows are cast by mountains upon clouds, and that no doubt mr. smith was right in attributing the appearance to "some unseen object, which was intercepting the sun's rays." but the old dominant that was a jealous dominant, and the wrath of the old dominant against such an irreconcilability as large, opaque objects in the sky, casting down shadows upon clouds. still the dominants are suave very often, or are not absolute gods, and the way attention was led away from this subject is an interesting study in quasi-divine bamboozlement. upon page , charles j.p. cave, the meteorologist, writes that, upon april and , at ditcham park, petersfield, he had observed a similar appearance, while watching some pilot balloons--but he describes something not in the least like a shadow on clouds, but a stationary cloud--the inference seems to be that the shadows at chisbury may have been shadows of pilot balloons. upon page , another correspondent writes upon shadows cast by mountains; upon page someone else carries on the divergence by discussing this third letter: then someone takes up the third letter mathematically; and then there is a correction of error in this mathematic demonstration--i think it looks very much like what i think it looks like. but the mystery here: that the dark patches at chisbury could not have been cast by stationary pilot balloons that were to the west, or that were between clouds and the setting sun. if, to the west of chisbury, a stationary object were high in the air, intercepting the sun's rays, the shadow of the stationary object would not have been stationary, but would have moved higher and higher with the setting of the sun. i have to think of something that is in accord with no other data whatsoever: a luminous body--not the sun--in the sky--but, because of some unknown principle or atmospheric condition, its light extended down only about to the clouds; that from it were suspended two triangular objects, like the object that was seen in bermuda; that it was this light that fell short of the earth that these objects intercepted; that the objects were drawn up and lowered from something overhead, so that, in its light, their shadows changed size. if my grope seem to have no grasp in it, and, if a stationary balloon will, in half an hour, not cast a stationary shadow from the setting sun, we have to think of two triangular objects that accurately maintained positions in a line between sun and clouds, and at the same time approached and receded from clouds. whatever it may have been, it's enough to make the devout make the sign of the crucible, or whatever the devotees of the old dominant do in the presence of a new correlate. vast, black thing poised like a crow over the moon. it is our acceptance that these two shadows of chisbury looked, from the moon, like vast things, black as crows, poised over the earth. it is our acceptance that two triangular luminosities and then two triangular patches, like vast black things, poised like crows over the moon, and, like the triangularities at chisbury, have been seen upon, or over, the moon: _scientific american_, - : two triangular, luminous appearances reported by several observers in lebanon, conn., evening of july , , on the moon's upper limb. they disappeared, and two dark triangular appearances that looked like notches were seen three minutes later upon the lower limb. they approached each other, met and instantly disappeared. the merger here is notches that have at times been seen upon the moon's limb: thought to be cross sections of craters (_monthly notices, r.a.s._, - ). but these appearances of july , , were vast upon the moon--"seemed to be cutting off or obliterating nearly a quarter of its surface." something else that may have looked like a vast black crow poised over this earth from the moon: _monthly weather review_, - : description of a shadow in the sky, of some unseen body, april , , fort worth, texas--supposed to have been cast by an unseen cloud--this patch of shade moved with the declining sun. _rept. brit. assoc._, - : account by two observers of a faint but distinctly triangular object, visible for six nights in the sky. it was observed from two stations that were not far apart. but the parallax was considerable. whatever it was, it was, acceptably, relatively close to this earth. i should say that relatively to phenomena of light we are in confusion as great as some of the discords that orthodoxy is in relatively to light. broadly and intermediatistically, our position is: that light is not really and necessarily light--any more than is anything else really and necessarily anything--but an interpretation of a mode of force, as i suppose we have to call it, as light. at sea level, the earth's atmosphere interprets sunlight as red or orange or yellow. high up on mountains the sun is blue. very high up on mountains the zenith is black. or it is orthodoxy to say that in inter-planetary space, where there is no air, there is no light. so then the sun and comets are black, but this earth's atmosphere, or, rather, dust particles in it, interpret radiations from these black objects as light. we look up at the moon. the jet-black moon is so silvery white. i have about fifty notes indicating that the moon has atmosphere: nevertheless most astronomers hold out that the moon has no atmosphere. they have to: the theory of eclipses would not work out otherwise. so, arguing in conventional terms, the moon is black. rather astonishing--explorers upon the moon--stumbling and groping in intense darkness--with telescopes powerful enough, we could see them stumbling and groping in brilliant light. or, just because of familiarity, it is not now obvious to us how the preposterousnesses of the old system must have seemed to the correlates of the system preceding it. ye jet-black silvery moon. altogether, then, it may be conceivable that there are phenomena of force that are interpretable as light as far down as the clouds, but not in denser strata of air, or just the opposite of familiar interpretations. i now have some notes upon an occurrence that suggests a force not interpreted by air as light, but interpreted, or reflected by the ground as light. i think of something that, for a week, was suspended over london: of an emanation that was not interpreted as light until it reached the ground. _lancet_, june , : that every night for a week, a light had appeared in woburn square, london, upon the grass of a small park, enclosed by railings. crowds gathering--police called out "for the special service of maintaining order and making the populace move on." the editor of the _lancet_ went to the square. he says that he saw nothing but a patch of light falling upon an arbor at the northeast corner of the enclosure. seems to me that that was interesting enough. in this editor we have a companion for mr. symons and dr. gray. he suggests that the light came from a street lamp--does not say that he could trace it to any such origin himself--but recommends that the police investigate neighboring street lamps. i'd not say that such a commonplace as light from a street lamp would not attract and excite and deceive great crowds for a week--but i do accept that any cop who was called upon for extra work would have needed nobody's suggestion to settle that point the very first thing. or that something in the sky hung suspended over a london square for a week. _knowledge_, dec. , : "seeing so many meteorological phenomena in your excellent paper, _knowledge_, i am tempted to ask for an explanation of the following, which i saw when on board the british india company's steamer _patna_, while on a voyage up the persian gulf. in may, , on a dark night, about : p.m., there suddenly appeared on each side of the ship an enormous luminous wheel, whirling around, the spokes of which seemed to brush the ship along. the spokes would be or yards long, and resembled the birch rods of the dames' schools. each wheel contained about sixteen spokes, and, although the wheels must have been some or yards in diameter, the spokes could be distinctly seen all the way round. the phosphorescent gleam seemed to glide along flat on the surface of the sea, no light being visible in the air above the water. the appearance of the spokes could be almost exactly represented by standing in a boat and flashing a bull's eye lantern horizontally along the surface of the water, round and round. i may mention that the phenomenon was also seen by captain avern, of the _patna_, and mr. manning, third officer. "lee fore brace. "p.s.--the wheels advanced along with the ship for about twenty minutes.--l.f.b." _knowledge_, jan. , : letter from "a. mc. d.": that "lee fore brace," "who sees 'so many meteorological phenomena in your excellent paper,' should have signed himself 'the modern ezekiel,' for his vision of wheels is quite as wonderful as the prophet's." the writer then takes up the measurements that were given, and calculates a velocity at the circumference of a wheel, of about yards per second, apparently considering that especially incredible. he then says: "from the nom de plume he assumes, it might be inferred that your correspondent is in the habit of 'sailing close to the wind.'" he asks permission to suggest an explanation of his own. it is that before : p.m. there had been numerous accidents to the "main brace," and that it had required splicing so often that almost any ray of light would have taken on a rotary motion. in _knowledge_, jan. , , mr. "brace" answers and signs himself "j.w. robertson": "i don't suppose a. mc. d. means any harm, but i do think it's rather unjust to say a man is drunk because he sees something out of the common. if there's one thing i pride myself upon, it's being able to say that never in my life have i indulged in anything stronger than water." from this curiosity of pride, he goes on to say that he had not intended to be exact, but to give his impressions of dimensions and velocity. he ends amiably: "however, 'no offense taken, where i suppose none is meant.'" to this letter mr. proctor adds a note, apologizing for the publication of "a. mc. d's." letter, which had come about by a misunderstood instruction. then mr. proctor wrote disagreeable letters, himself, about other persons--what else would you expect in a quasi-existence? the obvious explanation of this phenomenon is that, under the surface of the sea, in the persian gulf, was a vast luminous wheel: that it was the light from its submerged spokes that mr. robertson saw, shining upward. it seems clear that this light did shine upward from origin below the surface of the sea. but at first it is not so clear how vast luminous wheels, each the size of a village, ever got under the surface of the persian gulf: also there may be some misunderstanding as to what they were doing there. a deep-sea fish, and its adaptation to a dense medium-- that, at least in some regions aloft, there is a medium dense even to gelatinousness-- a deep-sea fish, brought to the surface of the ocean: in a relatively attenuated medium, it disintegrates-- super-constructions adapted to a dense medium in inter-planetary space--sometimes, by stresses of various kinds, they are driven into this earth's thin atmosphere-- later we shall have data to support just this: that things entering this earth's atmosphere disintegrate and shine with a light that is not the light of incandescence: shine brilliantly, even if cold-- vast wheel-like super-constructions--they enter this earth's atmosphere, and, threatened with disintegration, plunge for relief into an ocean, or into a denser medium. of course the requirements now facing us are: not only data of vast wheel-like super-constructions that have relieved their distresses in the ocean, but data of enormous wheels that have been seen in the air, or entering the ocean, or rising from the ocean and continuing their voyages. very largely we shall concern ourselves with enormous fiery objects that have either plunged into the ocean or risen from the ocean. our acceptance is that, though disruption may intensify into incandescence, apart from disruption and its probable fieriness, things that enter this earth's atmosphere have a cold light which would not, like light from molten matter, be instantly quenched by water. also it seems acceptable that a revolving wheel would, from a distance, look like a globe; that a revolving wheel, seen relatively close by, looks like a wheel in few aspects. the mergers of ball-lightning and meteorites are not resistances to us: our data are of enormous bodies. so we shall interpret--and what does it matter? our attitude throughout this book: that here are extraordinary data--that they never would be exhumed, and never would be massed together, unless-- here are the data: our first datum is of something that was once seen to enter an ocean. it's from the puritanic publication, _science_, which has yielded us little material, or which, like most puritans, does not go upon a spree very often. whatever the thing could have been, my impression is of tremendousness, or of bulk many times that of all meteorites in all museums combined: also of relative slowness, or of long warning of approach. the story, in _science_, - , is from an account sent to the hydrographic office, at washington, from the branch office, at san francisco: that, at midnight, feb. , , lat. ° n., and long. ° e., or somewhere between yokohama and victoria, the captain of the bark _innerwich_ was aroused by his mate, who had seen something unusual in the sky. this must have taken appreciable time. the captain went on deck and saw the sky turning fiery red. "all at once, a large mass of fire appeared over the vessel, completely blinding the spectators." the fiery mass fell into the sea. its size may be judged by the volume of water cast up by it, said to have rushed toward the vessel with a noise that was "deafening." the bark was struck flat aback, and "a roaring, white sea passed ahead." "the master, an old, experienced mariner, declared that the awfulness of the sight was beyond description." in _nature_, - , and _l'astronomie_; - , we are told that an object, described as "a large ball of fire," was seen to rise from the sea, near cape race. we are told that it rose to a height of fifty feet, and then advanced close to the ship, then moving away, remaining visible about five minutes. the supposition in _nature_ is that it was "ball lightning," but flammarion, _thunder and lightning_, p. , says that it was enormous. details in the american _meteorological journal_, - --nov. , --british steamer _siberian_--that the object had moved "against the wind" before retreating--that captain moore said that at about the same place he had seen such appearances before. _report of the british association_, - : that, upon june , , according to the _malta times_, from the brig _victoria_, about miles east of adalia, asia minor ( ° ' ", n. lat.: ° ' " e. long.), three luminous bodies were seen to issue from the sea, at about half a mile from the vessel. they were visible about ten minutes. the story was never investigated, but other accounts that seem acceptably to be other observations upon this same sensational spectacle came in, as if of their own accord, and were published by prof. baden-powell. one is a letter from a correspondent at mt. lebanon. he describes only two luminous bodies. apparently they were five times the size of the moon: each had appendages, or they were connected by parts that are described as "sail-like or streamer-like," looking like "large flags blown out by a gentle breeze." the important point here is not only suggestion of structure, but duration. the duration of meteors is a few seconds: duration of fifteen seconds is remarkable, but i think there are records up to half a minute. this object, if it were all one object, was visible at mt. lebanon about one hour. an interesting circumstance is that the appendages did not look like trains of meteors, which shine by their own light, but "seemed to shine by light from the main bodies." about miles west of the position of the _victoria_ is the town of adalia, asia minor. at about the time of the observation reported by the captain of the _victoria_, the rev. f. hawlett, f.r.a.s., was in adalia. he, too, saw this spectacle, and sent an account to prof. baden-powell. in his view it was a body that appeared and then broke up. he places duration at twenty minutes to half an hour. in the _report of the british association_, - , the phenomenon was reported from syria and malta, as two very large bodies "nearly joined." _rept. brit. assoc._, - : that, at cherbourg, france, jan. , , was seen a luminous body, seemingly two-thirds the size of the moon. it seemed to rotate on an axis. central to it there seemed to be a dark cavity. for other accounts, all indefinite, but distortable into data of wheel-like objects in the sky, see _nature_, - ; london _times_, oct. , ; _nature_, - ; _monthly weather review_, - . _l'astronomie_, - : that, upon the morning of dec. , , an appearance in the sky was seen by many persons in virginia, north carolina, and south carolina. a luminous body passed overhead, from west to east, until at about degrees in the eastern horizon, it appeared to stand still for fifteen or twenty minutes. according to some descriptions it was the size of a table. to some observers it looked like an enormous wheel. the light was a brilliant white. acceptably it was not an optical illusion--the noise of its passage through the air was heard. having been stationary, or having seemed to stand still fifteen or twenty minutes, it disappeared, or exploded. no sound of explosion was heard. vast wheel-like constructions. they're especially adapted to roll through a gelatinous medium from planet to planet. sometimes, because of miscalculations, or because of stresses of various kinds, they enter this earth's atmosphere. they're likely to explode. they have to submerge in the sea. they stay in the sea awhile, revolving with relative leisureliness, until relieved, and then emerge, sometimes close to vessels. seamen tell of what they see: their reports are interred in scientific morgues. i should say that the general route of these constructions is along latitudes not far from the latitudes of the persian gulf. _journal of the royal meteorological society_, - : that, upon april , , about : , in the persian gulf, captain hoseason, of the steamship _kilwa_, according to a paper read before the society by captain hoseason, was sailing in a sea in which there was no phosphorescence--"there being no phosphorescence in the water." i suppose i'll have to repeat that: "... there being no phosphorescence in the water." vast shafts of light--though the captain uses the word "ripples"--suddenly appeared. shaft followed shaft, upon the surface of the sea. but it was only a faint light, and, in about fifteen minutes, died out: having appeared suddenly, having died out gradually. the shafts revolved at a velocity of about miles an hour. phosphorescent jellyfish correlate with the old dominant: in one of the most heroic compositions of disregards in our experience, it was agreed, in the discussion of capt. hoseason's paper, that the phenomenon was probably pulsations of long strings of jellyfish. _nature_, - : reprint of a letter from r.e. harris, commander of the a.h.n. co.'s steamship _shahjehan_, to the calcutta _englishman_, jan. , : that upon the th of june, , off the coast of malabar, at p.m., water calm, sky cloudless, he had seen something that was so foreign to anything that he had ever seen before, that he had stopped his ship. he saw what he describes as waves of brilliant light, with spaces between. upon the water were floating patches of a substance that was not identified. thinking in terms of the conventional explanation of all phosphorescence at sea, the captain at first suspected this substance. however, he gives his opinion that it did no illuminating but was, with the rest of the sea, illuminated by tremendous shafts of light. whether it was a thick and oily discharge from the engine of a submerged construction or not, i think that i shall have to accept this substance as a concomitant, because of another note. "as wave succeeded wave, one of the most grand and brilliant, yet solemn, spectacles that one could think of, was here witnessed." _jour. roy. met. soc._, - : extract from a letter from mr. douglas carnegie, blackheath, england. date some time in -- "this last voyage we witnessed a weird and most extraordinary electric display." in the gulf of oman, he saw a bank of apparently quiescent phosphorescence: but, when within twenty yards of it, "shafts of brilliant light came sweeping across the ship's bows at a prodigious speed, which might be put down as anything between and miles an hour." "these light bars were about feet apart and most regular." as to phosphorescence--"i collected a bucketful of water, and examined it under the microscope, but could not detect anything abnormal." that the shafts of light came up from something beneath the surface--"they first struck us on our broadside, and i noticed that an intervening ship had no effect on the light beams: they started away from the lee side of the ship, just as if they had traveled right through it." the gulf of oman is at the entrance to the persian gulf. _jour. roy. met. soc._, - : extract from a letter by mr. s.c. patterson, second officer of the p. and o. steamship _delta_: a spectacle which the _journal_ continues to call phosphorescent: malacca strait, a.m., march , : "... shafts which seemed to move round a center--like the spokes of a wheel--and appeared to be about yards long. the phenomenon lasted about half an hour, during which time the ship had traveled six or seven miles. it stopped suddenly." _l'astronomie_, - : a correspondent writes that, in october, , in the china sea, he had seen shafts or lances of light that had had the appearance of rays of a searchlight, and that had moved like such rays. _nature_, - : report to the admiralty by capt. evans, the hydrographer of the british navy: that commander j.e. pringle, of h.m.s. _vulture_, had reported that, at lat. ° ' n., and long. ° ' e.--in the persian gulf--may , , he had noticed luminous waves or pulsations in the water, moving at great speed. this time we have a definite datum upon origin somewhere below the surface. it is said that these waves of light passed under the _vulture_. "on looking toward the east, the appearance was that of a revolving wheel with a center on that bearing, and whose spokes were illuminated, and, looking toward the west, a similar wheel appeared to be revolving, but in the opposite direction." or finally as to submergence--"these waves of light extended from the surface well under the water." it is commander pringle's opinion that the shafts constituted one wheel, and that doubling was an illusion. he judges the shafts to have been about feet broad, and the spaces about . velocity about miles an hour. duration about minutes. time : p.m. before and after this display the ship had passed through patches of floating substance described as "oily-looking fish spawn." upon page of this number of _nature_, e.l. moss says that, in april, , when upon h.m.s. _bulldog_, a few miles north of vera cruz, he had seen a series of swift lines of light. he had dipped up some of the water, finding in it animalcule, which would, however, not account for phenomena of geometric formation and high velocity. if he means vera cruz, mexico, this is the only instance we have out of oriental waters. _scientific american_, - : that, in the _nautical meteorological annual_, published by the danish meteorological institute, appears a report upon a "singular phenomenon" that was seen by capt. gabe, of the danish east asiatic co.'s steamship _bintang_. at a.m., june , , while sailing through the straits of malacca, captain gabe saw a vast revolving wheel of light, flat upon the water--"long arms issuing from a center around which the whole system appeared to rotate." so vast was the appearance that only half of it could be seen at a time, the center lying near the horizon. this display lasted about fifteen minutes. heretofore we have not been clear upon the important point that forward motions of these wheels do not synchronize with a vessel's motions, and freaks of disregard, or, rather, commonplaces of disregard, might attempt to assimilate with lights of a vessel. this time we are told that the vast wheel moved forward, decreasing in brilliancy, and also in speed of rotation, disappearing when the center was right ahead of the vessel--or my own interpretation would be that the source of light was submerging deeper and deeper and slowing down because meeting more and more resistance. the danish meteorological institute reports another instance: that, when capt. breyer, of the dutch steamer _valentijn_, was in the south china sea, midnight, aug. , , he saw a rotation of flashes. "it looked like a horizontal wheel, turning rapidly." this time it is said that the appearance was above water. "the phenomenon was observed by the captain, the first and second mates, and the first engineer, and upon all of them it made a somewhat uncomfortable impression." in general, if our expression be not immediately acceptable, we recommend to rival interpreters that they consider the localization--with one exception--of this phenomenon, to the indian ocean and adjacent waters, or persian gulf on one side and china sea on the other side. though we're intermediatists, the call of attempted positivism, in the aspect of completeness, is irresistible. we have expressed that from few aspects would wheels of fire in the air look like wheels of fire, but, if we can get it, we must have observation upon vast luminous wheels, not interpretable as optical illusions, but enormous, substantial things that have smashed down material resistances, and have been seen to plunge into the ocean: _athenæum_, - : that at the meeting of the british association, , sir w.s. harris said that he had recorded an account sent to him of a vessel toward which had whirled "two wheels of fire, which the men described as rolling millstones of fire." "when they came near, an awful crash took place: the topmasts were shivered to pieces." it is said that there was a strong sulphurous odor. _journal of the royal meteorological society_, - : extract from the log of the bark _lady of the lake_, by capt. f.w. banner: communicated by r.h. scott, f.r.s.: that, upon the nd of march, , at lat. ° ' n., long. ° ' w., the sailors of the _lady of the lake_ saw a remarkable object, or "cloud," in the sky. they reported to the captain. according to capt. banner, it was a cloud of circular form, with an included semi-circle divided into four parts, the central dividing shaft beginning at the center of the circle and extending far outward, and then curving backward. geometricity and complexity and stability of form: and the small likelihood of a cloud maintaining such diversity of features, to say nothing of appearance of organic form. the thing traveled from a point at about degrees above the horizon to a point about degrees above. then it settled down to the northeast, having appeared from the south, southeast. light gray in color, or it was cloud-color. "it was much lower than the other clouds." and this datum stands out: that, whatever it may have been, it traveled against the wind. "it came up obliquely against the wind, and finally settled down right in the wind's eye." for half an hour this form was visible. when it did finally disappear that was not because it disintegrated like a cloud, but because it was lost to sight in the evening darkness. capt. banner draws the following diagram: [illustration] text-books tell us that the dhurmsalla meteorites were picked up "soon," or "within half an hour." given a little time the conventionalists may argue that these stones were hot when they fell, but that their great interior coldness had overcome the molten state of their surfaces. according to the deputy commissioner of dhurmsalla, these stones had been picked up "immediately" by passing coolies. these stones were so cold that they benumbed the fingers. but they had fallen with a great light. it is described as "a flame of fire about two feet in depth and nine feet in length." acceptably this light was not the light of molten matter. in this chapter we are very intermediatistic--and unsatisfactory. to the intermediatist there is but one answer to all questions: sometimes and sometimes not. another form of this intermediatist "solution" of all problems is: yes and no. everything that is, also isn't. a positivist attempts to formulate: so does the intermediatist, but with less rigorousness: he accepts but also denies: he may seem to accept in one respect and deny in some other respect, but no real line can be drawn between any two aspects of anything. the intermediatist accepts that which seems to correlate with something that he has accepted as a dominant. the positivist correlates with a belief. in the dhurmsalla meteorites we have support for our expression that things entering this earth's atmosphere sometimes shine with a light that is not the light of incandescence--or so we account, or offer an expression upon, "thunderstones," or carved stones that have fallen luminously to this earth, in streaks that have looked like strokes of lightning--but we accept, also, that some things that have entered this earth's atmosphere, disintegrate with the intensity of flame and molten matter--but some things, we accept, enter this earth's atmosphere and collapse non-luminously, quite like deep-sea fishes brought to the surface of the ocean. whatever agreement we have is an indication that somewhere aloft there is a medium denser than this earth's atmosphere. i suppose our stronghold is in that such is not popular belief-- or the rhythm of all phenomena: air dense at sea level upon this earth--less and less dense as one ascends--then denser and denser. a good many bothersome questions arise-- our attitude: here are the data: luminous rains sometimes fall (_nature_, march , ; _nature_, - ). this is light that is not the light of incandescence, but no one can say that these occasional, or rare, rains come from this earth's externality. we simply note cold light of falling bodies. for luminous rain, snow, and dust, see hartwig, _aerial world_, p. . as to luminous clouds, we have more nearly definite observations and opinions: they mark transition between the old dominant and the new dominant. we have already noted the transition in prof. schwedoffs theory of external origin of some hailstones--and the implications that, to a former generation, seemed so preposterous--"droll" was the word--that there are in inter-planetary regions volumes of water--whether they have fishes and frogs in them or not. now our acceptance is that clouds sometimes come from external regions, having had origin from super-geographical lakes and oceans that we shall not attempt to chart, just at present--only suggesting to enterprising aviators--and we note that we put it all up to them, and show no inclination to go columbusing on our own account--that they take bathing suits, or, rather, deep-sea diving-suits along. so then that some clouds come from inter-planetary oceans--of the super-sargasso sea--if we still accept the super-sargasso sea--and shine, upon entering this earth's atmosphere. in _himmel und erde_, february, --a phenomenon of transition of thirty years ago--herr o. jesse, in his observations upon luminous night-clouds, notes the great height of them, and drolly or sensibly suggests that some of them may have come from regions external to this earth. i suppose he means only from other planets. but it's a very droll and sensible idea either way. in general i am accounting for a great deal of this earth's isolation: that it is relatively isolated by circumstances that are similar to the circumstances that make for relative isolation of the bottom of the ocean--except that there is a clumsiness of analogy now. to call ourselves deep-sea fishes has been convenient, but, in a quasi-existence, there is no convenience that will not sooner or later turn awkward--so, if there be denser regions aloft, these regions should now be regarded as analogues of far-submerged oceanic regions, and things coming to this earth would be like things rising to an attenuated medium--and exploding--sometimes incandescently, sometimes with cold light--sometimes non-luminously, like deep-sea fishes brought to the surface--altogether conditions of inhospitality. i have a suspicion that, in their own depths, deep-sea fishes are not luminous. if they are, darwinism is mere jesuitism, in attempting to correlate them. such advertising would so attract attention that all advantages would be more than offset. darwinism is largely a doctrine of concealment: here we have brazen proclamation--if accepted. fishes in the mammoth cave need no light to see by. we might have an expression that deep-sea fishes turn luminous upon entering a less dense medium--but models in the american museum of natural history: specialized organs of luminosity upon these models. of course we do remember that awfully convincing "dodo," and some of our sophistications we trace to him--at any rate disruption is regarded as a phenomenon of coming from a dense to a less dense medium. an account by m. acharius, in the _transactions of the swedish academy of sciences_, - , translated for the _north american review_, - : that m. acharius, having heard of "an extraordinary and probably hitherto unseen phenomenon," reported from near the town of skeninge, sweden, investigated: that, upon the th of may, , at about p.m., the sun suddenly turned dull brick-red. at the same time there appeared, upon the western horizon, a great number of round bodies, dark brown, and seemingly the size of a hat crown. they passed overhead and disappeared in the eastern horizon. tremendous procession. it lasted two hours. occasionally one fell to the ground. when the place of a fall was examined, there was found a film, which soon dried and vanished. often, when approaching the sun, these bodies seemed to link together, or were then seen to be linked together, in groups not exceeding eight, and, under the sun, they were seen to have tails three or four fathoms long. away from the sun the tails were invisible. whatever their substance may have been, it is described as gelatinous--"soapy and jellied." i place this datum here for several reasons. it would have been a good climax to our expression upon hordes of small bodies that, in our acceptance, were not seeds, nor birds, nor ice-crystals: but the tendency would have been to jump to the homogeneous conclusion that all our data in that expression related to this one kind of phenomena, whereas we conceive of infinite heterogeneity of the external: of crusaders and rabbles and emigrants and tourists and dragons and things like gelatinous hat crowns. or that all things, here, upon this earth, that flock together, are not necessarily sheep, presbyterians, gangsters, or porpoises. the datum is important to us, here, as indication of disruption in this earth's atmosphere--dangers in entering this earth's atmosphere. i think, myself, that thousands of objects have been seen to fall from aloft, and have exploded luminously, and have been called "ball lightning." "as to what ball lightning is, we have not yet begun to make intelligent guesses." (_monthly weather review_, - .) in general, it seems to me that when we encounter the opposition "ball lightning" we should pay little attention, but confine ourselves to guesses that are at least intelligent, that stand phantom-like in our way. we note here that in some of our acceptances upon intelligence we should more clearly have pointed out that they were upon the intelligent as opposed to the instinctive. in the _monthly weather review_, - , there is an account of "ball lightning" that struck a tree. it made a dent such as a falling object would make. some other time i shall collect instances of "ball lightning," to express that they are instances of objects that have fallen from the sky, luminously, exploding terrifically. so bewildered is the old orthodoxy by these phenomena that many scientists have either denied "ball lightning" or have considered it very doubtful. i refer to dr. sestier's list of one hundred and fifty instances, which he considered authentic. in accord with our disaccord is an instance related in the _monthly weather review_, march, --something that fell luminously from the sky, accompanied by something that was not so affected, or that was dark: that, according to capt. c.d. sweet, of the dutch bark, _j.p.a._, upon march , , n. ° ', w. ° ', he encountered a severe storm. he saw two objects in the air above the ship. one was luminous, and might be explained in several ways, but the other was dark. one or both fell into the sea, with a roar and the casting up of billows. it is our acceptance that these things had entered this earth's atmosphere, having first crashed through a field of ice--"immediately afterward lumps of ice fell." one of the most astonishing of the phenomena of "ball lightning" is a phenomenon of many meteorites: violence of explosion out of all proportion to size and velocity. we accept that the icy meteorites of dhurmsalla could have fallen with no great velocity, but the sound from them was tremendous. the soft substance that fell at the cape of good hope was carbonaceous, but was unburned, or had fallen with velocity insufficient to ignite it. the tremendous report that it made was heard over an area more than seventy miles in diameter. that some hailstones have been formed in a dense medium, and violently disintegrate in this earth's relatively thin atmosphere: _nature_, - : large hailstones noted at the university of missouri, nov. , : they exploded with sounds like pistol shots. the writer says that he had noticed a similar phenomenon, eighteen years before, at lexington, kentucky. hailstones that seemed to have been formed in a denser medium: when melted under water they gave out bubbles larger than their central air spaces. (_monthly weather review_, - .) our acceptance is that many objects have fallen from the sky, but that many of them have disintegrated violently. this acceptance will co-ordinate with data still to come, but, also, we make it easy for ourselves in our expressions upon super-constructions, if we're asked why, from thinkable wrecks of them, girders, plates, or parts recognizably of manufactured metal have not fallen from the sky. however, as to composition, we have not this refuge, so it is our expression that there have been reported instances of the fall of manufactured metal from the sky. the meteorite of rutherford, north carolina, is of artificial material: mass of pig iron. it is said to be fraudulent. (_amer. jour. sci._, - - .) the object that was said to have fallen at marblehead, mass., in , is described in the _amer. jour. sci._, - - , as "a furnace product, formed in smelting copper ores, or iron ores containing copper." it is said to be fraudulent. according to ehrenberg, the substance reported by capt. callam to have fallen upon his vessel, near java, "offered complete resemblance to the residue resulting from combustion of a steel wire in a flask of oxygen." (zurcher, _meteors_, p. .) _nature_, nov. , , publishes a notice that, according to the _yuma sentinel_, a meteorite that "resembles steel" had been found in the mohave desert. in _nature_, feb. , , we read that one of the meteorites brought to the united states by peary, from greenland, is of tempered steel. the opinion is that meteoric iron had fallen in water or snow, quickly cooling and hardening. this does not apply to composition. nov. , , _nature_ publishes a notice of a paper by prof. berwerth, of vienna, upon "the close connection between meteoric iron and steel-works' steel." at the meeting of nov. , , of the essex field club, was exhibited a piece of metal said to have fallen from the sky, oct. , , at braintree. according to the _essex naturalist_, dr. fletcher, of the british museum, had declared this metal to be smelted iron--"so that the mystery of its reported 'fall' remained unexplained." we shall have an outcry of silences. if a single instance of anything be disregarded by a system--our own attitude is that a single instance is a powerless thing. of course our own method of agreement of many instances is not a real method. in continuity, all things must have resemblances with all other things. anything has any quasi-identity you please. some time ago conscription was assimilated with either autocracy or democracy with equal facility. note the need for a dominant to correlate to. scarcely anybody said simply that we must have conscription: but that we must have conscription, which correlates with democracy, which was taken as a base, or something basically desirable. of course between autocracy and democracy nothing but false demarcation can be drawn. so i can conceive of no subject upon which there should be such poverty as a single instance, if anything one pleases can be whipped into line. however, we shall try to be more nearly real than the darwinites who advance concealing coloration as darwinism, and then drag in proclaiming luminosity, too, as darwinism. i think the darwinites had better come in with us as to the deep-sea fishes--and be sorry later, i suppose. it will be amazing or negligible to read all the instances now to come of things that have been seen in the sky, and to think that all have been disregarded. my own opinion is that it is not possible, or very easy, to disregard them, now that they have been brought together--but that, if prior to about this time we had attempted such an assemblage, the old dominant would have withered our typewriter--as it is the letter "e" has gone back on us, and the "s" is temperamental. "most extraordinary and singular phenomenon," north wales, aug. , ; a disk from which projected an orange-colored body that looked like "an elongated flatfish," reported by admiral ommanney (_nature_, - ); disk from which projected a hook-like form, india, about ; diagram of it given; disk about size of the moon, but brighter than the moon; visible about twenty minutes; by g. pettit, in prof. baden-powell's catalogue (_rept. brit. assoc._, ); very brilliant hook-like form, seen in the sky at poland, trumbull co., ohio, during the stream of meteors, of ; visible more than an hour: large luminous body, almost stationary "for a time"; shaped like a square table; niagara falls, nov. , (_amer. jour. sci._, - - ); something described as a bright white cloud, at night, nov. , , at hamar, norway; from it were emitted brilliant rays of light; drifted across the sky; "retained throughout its original form" (_nature_, dec. , - ); thing with an oval nucleus, and streamers with dark bands and lines very suggestive of structure; new zealand, may , (_nature_, - ); luminous object, size of full moon, visible an hour and a half, chili, nov. , (_comptes rendus_, - ); bright object near sun, dec. , (_knowledge_, - ); light that looked like a great flame, far out at sea, off ryook phyoo, dec. , (_london roy. soc. proc._, - ); something like a gigantic trumpet, suspended, vertical, oscillating gently, visible five or six minutes, length estimated at feet, at oaxaca, mexico, july , (_sci. am. sup._, - ); two luminous bodies, seemingly united, visible five or six minutes, june , (_la nature_, - - ); thing with a tail, crossing moon, transit half a minute, sept. , (london _times_, sept. , ); object four or five times size of moon, moving slowly across sky, nov. , , near adrianople (_l'astronomie_, - ); large body, colored red, moving slowly, visible minutes, reported by coggia, marseilles, aug. , (_chem. news_, - ); details of this observation, and similar observation by guillemin, and other instances by de fonville (_comptes rendus_, - , ); thing that was large and that was stationary twice in seven minutes, oxford, nov. , ; listed by lowe (_rec. sci._, - ); grayish object that looked to be about three and a half feet long, rapidly approaching the earth at saarbruck, april , ; sound like thunder; object expanding like a sheet (_am. jour. sci._, - - ; _quar. jour. roy. inst._, - ); report by an astronomer, n.s. drayton, upon an object duration of which seemed to him extraordinary; duration three-quarters of a minute, jersey city, july , (_sci. amer._, - ); object like a comet, but with proper motion of degrees an hour; visible one hour; reported by purine and glancy from the cordoba observatory, argentina, march , (_sci. amer._, - ); something like a signal light, reported by glaisher, oct. , ; bright as jupiter, "sending out quick flickering waves of light" (_year book of facts_, - ). i think that with the object known as eddie's "comet" passes away the last of our susceptibility to the common fallacy of personifying. it is one of the most deep-rooted of positivist illusions--that people are persons. we have been guilty too often of spleens and spites and ridicules against astronomers, as if they were persons, or final unities, individuals, completenesses, or selves--instead of indeterminate parts. but, so long as we remain in quasi-existence, we can cast out illusion only with some other illusion, though the other illusion may approximate higher to reality. so we personify no more--but we super-personify. we now take into full acceptance our expression that development is an autocracy of successive dominants--which are not final--but which approximate higher to individuality or self-ness, than do the human tropisms that irresponsibly correlate to them. eddie reported a celestial object, from the observatory at grahamstown, south africa. it was in . the new dominant was only heir presumptive then, or heir apparent but not obvious. the thing that eddie reported might as well have been reported by a night watchman, who had looked up through an unplaced sewer pipe. it did not correlate. the thing was not admitted to _monthly notices_. i think myself that if the editor had attempted to let it in--earthquake--or a mysterious fire in his publishing house. the dominants are jealous gods. in _nature_, presumably a vassal of the new god, though of course also plausibly rendering homage to the old, is reported a comet-like body, of oct. , , observed at grahamstown, by eddie. it may have looked comet-like, but it moved degrees while visible, or one hundred degrees in three-quarters of an hour. see _nature_, - , . in _nature_, - , prof. copeland describes a similar appearance that he had seen, sept. , . dreyer says (_nature_, - ) that he had seen this object at the armagh observatory. he likens it to the object that was reported by eddie. it was seen by dr. alexander graham bell, sept. , , in nova scotia. but the old dominant was a jealous god. so there were different observations upon something that was seen in november, . these observations were philistines in . in the _amer. met. jour._, - , a correspondent reports having seen an object like a comet, with two tails, one up and one down, nov. or , . very likely this phenomenon should be placed in our expression upon torpedo-shaped bodies that have been seen in the sky--our data upon dirigibles, or super-zeppelins--but our attempted classifications are far from rigorous--or are mere gropes. in the _scientific american_, - , a correspondent writes from humacao, porto rico, that, nov. , , he and several other--persons--or persons, as it were--had seen a majestic appearance, like a comet. visible three successive nights: disappeared then. the editor says that he can offer no explanation. if accepted, this thing must have been close to the earth. if it had been a comet, it would have been seen widely, and the news would have been telegraphed over the world, says the editor. upon page of this volume of the _scientific american_, a correspondent writes that, at sulphur springs, ohio, he had seen "a wonder in the sky," at about the same date. it was torpedo-shaped, or something with a nucleus, at each end of which was a tail. again the editor says that he can offer no explanation: that the object was not a comet. he associates it with the atmospheric effects general in . but it will be our expression that, in england and holland, a similar object was seen in november, . in the _scientific american_, - , is published a letter from henry harrison, of jersey city, copied from the _new york tribune_: that upon the evening of april , , mr. harrison was searching for brorsen's comet, when he saw an object that was moving so rapidly that it could not have been a comet. he called a friend to look, and his observation was confirmed. at two o'clock in the morning this object was still visible. in the _scientific american supplement_, - , mr. harrison disclaims sensationalism, which he seems to think unworthy, and gives technical details: he says that the object was seen by mr. j. spencer devoe, of manhattanville. "a formation having the shape of a dirigible." it was reported from huntington, west virginia (_sci. amer._, - ). luminous object that was seen july , , at about p.m. observed through "rather powerful field glasses," it looked to be about two degrees long and half a degree wide. it gradually dimmed, disappeared, reappeared, and then faded out of sight. another person--as we say: it would be too inconvenient to hold to our intermediatist recognitions--another person who observed this phenomenon suggested to the writer of the account that the object was a dirigible, but the writer says that faint stars could be seen behind it. this would seem really to oppose our notion of a dirigible visitor to this earth--except for the inconclusiveness of all things in a mode of seeming that is not final--or we suggest that behind some parts of the object, thing, construction, faint stars were seen. we find a slight discussion here. prof. h.m. russell thinks that the phenomenon was a detached cloud of aurora borealis. upon page of this volume of the _scientific american_, another correlator suggests that it was a light from a blast furnace--disregarding that, if there be blast furnaces in or near huntington, their reflections would be commonplaces there. we now have several observations upon cylindrical-shaped bodies that have appeared in this earth's atmosphere: cylindrical, but pointed at both ends, or torpedo-shaped. some of the accounts are not very detailed, but out of the bits of description my own acceptance is that super-geographical routes are traversed by torpedo-shaped super-constructions that have occasionally visited, or that have occasionally been driven into this earth's atmosphere. from data, the acceptance is that upon entering this earth's atmosphere, these vessels have been so racked that had they not sailed away, disintegration would have occurred: that, before leaving this earth, they have, whether in attempted communication or not, or in mere wantonness or not, dropped objects, which did almost immediately violently disintegrate or explode. upon general principles we think that explosives have not been purposely dropped, but that parts have been racked off, and have fallen, exploding like the things called "ball lightning." many have been objects of stone or metal with inscriptions upon them, for all we know, at present. in all instances, estimates of dimensions are valueless, but ratios of dimensions are more acceptable. a thing said to have been six feet long may have been six hundred feet long; but shape is not so subject to the illusions of distance. _nature_, - : that, aug. , , during a violent storm, an object that looked to be about inches long and inches wide, fell, rather slowly, at east twickenham, england. it exploded. no substance from it was found. _l'année scientifique_, - : that, oct. , , m. leverrier had sent to the academy three letters from witnesses of a long luminous body, tapering at both ends, that had been seen in the sky. in _thunder and lightning_, p. , flammarion says that on aug. , , during a rather violent storm, m.a. trécul, of the french academy, saw a very brilliant yellowish-white body, apparently to centimeters long, and about centimeters wide. torpedo-shaped. or a cylindrical body, "with slightly conical ends." it dropped something, and disappeared in the clouds. whatever it may have been that was dropped, it fell vertically, like a heavy object, and left a luminous train. the scene of this occurrence may have been far from the observer. no sound was heard. for m. trécul's account, see _comptes rendus_, - . _monthly weather review_, - : that, july , , in the town of burlington, vermont, a terrific explosion had been heard throughout the city. a ball of light, or a luminous object, had been seen to fall from the sky--or from a torpedo-shaped thing, or construction, in the sky. no one had seen this thing that had exploded fall from a larger body that was in the sky--but if we accept that at the same time there was a larger body in the sky-- my own acceptance is that a dirigible in the sky, or a construction that showed every sign of disrupting, had barely time to drop--whatever it did drop--and to speed away to safety above. the following story is told, in the _review_, by bishop john s. michaud: "i was standing on the corner of church and college streets, just in front of the howard bank, and facing east, engaged in conversation with ex-governor woodbury and mr. a.a. buell, when, without the slightest indication, or warning, we were startled by what sounded like a most unusual and terrific explosion, evidently very nearby. raising my eyes, and looking eastward along college street, i observed a torpedo-shaped body, some feet away, stationary in appearance, and suspended in the air, about feet above the tops of the buildings. in size it was about feet long by inches in diameter, the shell, or covering, having a dark appearance, with here and there tongues of fire issuing from spots on the surface, resembling red-hot, unburnished copper. although stationary when first noticed, this object soon began to move, rather slowly, and disappeared over dolan brothers' store, southward. as it moved, the covering seemed rupturing in places, and through these the intensely red flames issued." bishop michaud attempts to correlate it with meteorological observations. because of the nearby view this is perhaps the most remarkable of the new correlates, but the correlate now coming is extraordinary because of the great number of recorded observations upon it. my own acceptance is that, upon nov. , , a vast dirigible crossed england, but by the definiteness-indefiniteness of all things quasi-real, some observations upon it can be correlated with anything one pleases. e.w. maunder, invited by the editors of the _observatory_ to write some reminiscences for the th number of their magazine, gives one that he says stands out (_observatory_, - ). it is upon something that he terms "a strange celestial visitor." maunder was at the royal observatory, greenwich, nov. , , at night. there was an aurora, without features of special interest. in the midst of the aurora, a great circular disk of greenish light appeared and moved smoothly across the sky. but the circularity was evidently the effect of foreshortening. the thing passed above the moon, and was, by other observers, described as "cigar-shaped," "like a torpedo," "a spindle," "a shuttle." the idea of foreshortening is not mine: maunder says this. he says: "had the incident occurred a third of a century later, beyond doubt everyone would have selected the same simile--it would have been 'just like a zeppelin.'" the duration was about two minutes. color said to have been the same as that of the auroral glow in the north. nevertheless, maunder says that this thing had no relation to auroral phenomena. "it appeared to be a definite body." motion too fast for a cloud, but "nothing could be more unlike the rush of a meteor." in the _philosophical magazine_, - - , j. rand capron, in a lengthy paper, alludes throughout to this phenomenon as an "auroral beam," but he lists many observations upon its "torpedo-shape," and one observation upon a "dark nucleus" in it--host of most confusing observations--estimates of height between and miles--observations in holland and belgium. we are told that according to capron's spectroscopic observations the phenomenon was nothing but a beam of auroral light. in the _observatory_, - , is maunder's contemporaneous account. he gives apparent approximate length and breadth at twenty-seven degrees and three degrees and a half. he gives other observations seeming to indicate structure--"remarkable dark marking down the center." in _nature_, - , capron says that because of the moonlight he had been able to do little with the spectroscope. color white, but aurora rosy (_nature_, - ). bright stars seen through it, but not at the zenith, where it looked opaque. this is the only assertion of transparency (_nature_, - ). too slow for a meteor, but too fast for a cloud (_nature_, - ). "surface had a mottled appearance" (_nature_, - ). "very definite in form, like a torpedo" (_nature_, - ). "probably a meteoric object" (dr. groneman, _nature_, - ). technical demonstration by dr. groneman, that it was a cloud of meteoric matter (_nature_, - ). see _nature_, - , , , , , . "very little doubt it was an electric phenomenon" (proctor, _knowledge_, - ). in the london _times_, nov. , , the editor says that he had received a great number of letters upon this phenomenon. he publishes two. one correspondent describes it as "well-defined and shaped like a fish... extraordinary and alarming." the other correspondent writes of it as "a most magnificent luminous mass, shaped somewhat like a torpedo." _notes and queries_, - - : about lights that were seen in wales, over an area of about miles, all keeping their own ground, whether moving together perpendicularly, horizontally, or over a zigzag course. they looked like electric lights--disappearing, reappearing dimly, then shining as bright as ever. "we have seen them three or four at a time afterward, on four or five occasions." london _times_, oct. , : "from time to time the west coast of wales seems to have been the scene of mysterious lights.... and now we have a statement from towyn that within the last few weeks lights of various colors have been seen moving over the estuary of the dysynni river, and out to sea. they are generally in a northerly direction, but sometimes they hug the shore, and move at high velocity for miles toward aberdovey, and suddenly disappear." _l'année scientifique_, - : lights that appeared in the sky, above vence, france, march , ; described as balls of fire of dazzling brightness; appeared from a cloud about a degree in diameter; moved relatively slowly. they were visible more than an hour, moving northward. it is said that eight or ten years before similar lights or objects had been seen in the sky, at vence. london _times_, sept. , : that, at inverness, scotland, two large, bright lights that looked like stars had been seen in the sky: sometimes stationary, but occasionally moving at high velocity. _l'année scientifique_, - : observed near st. petersburg, july , , in the evening: a large spherical light and two smaller ones, moving along a ravine: visible three minutes; disappearing without noise. _nature_, - : that, at yloilo, sept. , , was seen a luminous object the size of the full moon. it "floated" slowly "northward," followed by smaller ones close to it. "the false lights of durham." every now and then in the english newspapers, in the middle of the nineteenth century, there is something about lights that were seen against the sky, but as if not far above land, oftenest upon the coast of durham. they were mistaken for beacons by sailors. wreck after wreck occurred. the fishermen were accused of displaying false lights and profiting by wreckage. the fishermen answered that mostly only old vessels, worthless except for insurance, were so wrecked. in (london _times_, jan. , ) popular excitement became intense. there was an investigation. before a commission, headed by admiral collinson, testimony was taken. one witness described the light that had deceived him as "considerably elevated above ground." no conclusion was reached: the lights were called "the mysterious lights." but whatever the "false lights of durham" may have been, they were unaffected by the investigation. in , the tyne pilotage board took the matter up. opinion of the mayor of tyne--"a mysterious affair." in the _report of the british association_, - , there is a description of a group of "meteors" that traveled with "remarkable slowness." they were in sight about three minutes. "remarkable," it seems, is scarcely strong enough: one reads of "remarkable" as applied to a duration of three seconds. these "meteors" had another peculiarity; they left no train. they are described as "seemingly huddled together like a flock of wild geese, and moving with the same velocity and grace of regularity." _jour. roy. astro. soc. of canada_, november and december, : that, according to many observations collected by prof. chant, of toronto, there appeared, upon the night of feb. , , a spectacle that was seen in canada, the united states, and at sea, and in bermuda. a luminous body was seen. to it there was a long tail. the body grew rapidly larger. "observers differ as to whether the body was single, or was composed of three or four parts, with a tail to each part." the group, or complex structure, moved with "a peculiar, majestic deliberation." "it disappeared in the distance, and another group emerged from its place of origin. onward they moved, at the same deliberate pace, in twos or threes or fours." they disappeared. a third group, or a third structure, followed. some observers compared the spectacle to a fleet of airships: others to battleships attended by cruisers and destroyers. according to one writer: "there were probably or bodies, and the peculiar thing about them was their moving in fours and threes and twos, abreast of one another; and so perfect was the lining up that you would have thought it was an aerial fleet maneuvering after rigid drilling." _nature_, may , : a letter from capt. charles j. norcock, of h.m.s. _caroline_: that, upon the th of february, , at p.m., between shanghai and japan, the officer of the watch had reported "some unusual lights." they were between the ship and a mountain. the mountain was about , feet high. the lights seemed to be globular. they moved sometimes massed, but sometimes strung out in an irregular line. they bore "northward," until lost to sight. duration two hours. the next night the lights were seen again. they were, for a time, eclipsed by a small island. they bore north at about the same speed and in about the same direction as speed and direction of the _caroline_. but they were lights that cast a reflection: there was a glare upon the horizon under them. a telescope brought out but few details: that they were reddish, and seemed to emit a faint smoke. this time the duration was seven and a half hours. then capt. norcock says that, in the same general locality, and at about the same time, capt. castle, of h.m.s. _leander_, had seen lights. he had altered his course and had made toward them. the lights had fled from him. at least, they had moved higher in the sky. _monthly weather review_, march, - : report from the observations of three members of his crew by lieut. frank h. schofield, u.s.n, of the u.s.s. _supply_: feb. , . three luminous objects, of different sizes, the largest having an apparent area of about six suns. when first sighted, they were not very high. they were below clouds of an estimated height of about one mile. they fled, or they evaded, or they turned. they went up into the clouds below which they had, at first, been sighted. their unison of movement. but they were of different sizes, and of different susceptibilities to all forces of this earth and of the air. _monthly weather review_, august, - : two letters from c.n. crotsenburg, crow agency, montana: that, in the summer of , when this writer was a railroad postal clerk--or one who was experienced in train-phenomena--while his train was going "northward," from trenton, mo., he and another clerk saw, in the darkness of a heavy rain, a light that appeared to be round, and of a dull-rose color, and seemed to be about a foot in diameter. it seemed to float within a hundred feet of the earth, but soon rose high, or "midway between horizon and zenith." the wind was quite strong from the east, but the light held a course almost due north. its speed varied. sometimes it seemed to outrun the train "considerably." at other times it seemed to fall behind. the mail-clerks watched until the town of linville, iowa, was reached. behind the depot of this town, the light disappeared, and was not seen again. all this time there had been rain, but very little lightning, but mr. crotsenburg offers the explanation that it was "ball lightning." the editor of the _review_ disagrees. he thinks that the light may have been a reflection from the rain, or fog, or from leaves of trees, glistening with rain, or the train's light--not lights. in the december number of the _review_ is a letter from edward m. boggs--that the light was a reflection, perhaps, from the glare--one light, this time--from the locomotive's fire-box, upon wet telegraph wires--an appearance that might not be striated by the wires, but consolidated into one rotundity--that it had seemed to oscillate with the undulations of the wires, and had seemed to change horizontal distance with the varying angles of reflection, and had seemed to advance or fall behind, when the train had rounded curves. all of which is typical of the best of quasi-reasoning. it includes and assimilates diverse data: but it excludes that which will destroy it: that, acceptably, the telegraph wires were alongside the track beyond, as well as leading to linville. mr. crotsenburg thinks of "ball lightning," which, though a sore bewilderment to most speculation, is usually supposed to be a correlate with the old system of thought: but his awareness of "something else" is expressed in other parts of his letters, when he says that he has something to tell that is "so strange that i should never have mentioned it, even to my friends, had it not been corroborated... so unreal that i hesitated to speak of it, fearing that it was some freak of the imagination." vast and black. the thing that was poised, like a crow over the moon. round and smooth. cannon balls. things that have fallen from the sky to this earth. our slippery brains. things like cannon balls have fallen, in storms, upon this earth. like cannon balls are things that, in storms, have fallen to this earth. showers of blood. showers of blood. showers of blood. whatever it may have been, something like red-brick dust, or a red substance in a dried state, fell at piedmont, italy, oct. , (_electric magazine_, - ). a red powder fell, in switzerland, winter of (_pop. sci. rev._, - )-- that something, far from this earth, had bled--super-dragon that had rammed a comet-- or that there are oceans of blood somewhere in the sky--substance that dries, and falls in a powder--wafts for ages in powdered form--that there is a vast area that will some day be known to aviators as the desert of blood. we attempt little of super-topography, at present, but ocean of blood, or desert of blood--or both--italy is nearest to it--or to them. i suspect that there were corpuscles in the substance that fell in switzerland, but all that could be published in was that in this substance there was a high proportion of "variously shaped organic matter." at giessen, germany, in , according to the _report of the british association_, - , fell a rain of a peach-red color. in this rain were flakes of a hyacinthine tint. it is said that this substance was organic: we are told that it was pyrrhine. but distinctly enough, we are told of one red rain that it was of corpuscular composition--red snow, rather. it fell, march , , near the crystal palace, london (_year book of facts_, - ; _nature_, - ). as to the "red snow" of polar and mountainous regions, we have no opposition, because that "snow" has never been seen to fall from the sky: it is a growth of micro-organisms, or of a "protococcus," that spreads over snow that is on the ground. this time nothing is said of "sand from the sahara." it is said of the red matter that fell in london, march , , that it was composed of corpuscles-- of course: that they looked like "vegetable cells." a note: that nine days before had fallen the red substance--flesh--whatever it may have been--of bath county, kentucky. i think that a super-egotist, vast, but not so vast as it had supposed, had refused to move to one side for a comet. we summarize our general super-geographical expressions: gelatinous regions, sulphurous regions, frigid and tropical regions: a region that has been source of life relatively to this earth: regions wherein there is density so great that things from them, entering this earth's thin atmosphere, explode. we have had a datum of explosive hailstones. we now have support to the acceptance that they had been formed in a medium far denser than air of this earth at sea-level. in the _popular science news_, - , is an account of ice that had been formed, under great pressure, in the laboratory of the university of virginia. when released and brought into contact with ordinary air, this ice exploded. and again the flesh-like substance that fell in kentucky: its flake-like formation. here is a phenomenon that is familiar to us: it suggests flattening, under pressure. but the extraordinary inference is--pressure not equal on all sides. in the _annual record of science_, - , it is said that, in , after a heavy thunderstorm in louisiana, a tremendous number of fish scales were found, for a distance of forty miles, along the banks of the mississippi river: bushels of them picked up in single places: large scales that were said to be of the gar fish, a fish that weighs from five to fifty pounds. it seems impossible to accept this identification: one thinks of a substance that had been pressed into flakes or scales. and round hailstones with wide thin margins of ice irregularly around them--still, such hailstones seem to me more like things that had been stationary: had been held in a field of thin ice. in the _illustrated london news_, - , are drawings of hailstones so margined, as if they had been held in a sheet of ice. some day we shall have an expression which will be, to our advanced primitiveness, a great joy: that devils have visited this earth: foreign devils: human-like beings, with pointed beards: good singers; one shoe ill-fitting--but with sulphurous exhalations, at any rate. i have been impressed with the frequent occurrence of sulphurousness with things that come from the sky. a fall of jagged pieces of ice, orkney, july , (_trans. roy. soc. edin._, - ). they had a strong sulphurous odor. and the coke--or the substance that looked like coke--that fell at mortrée, france, april , : with it fell a sulphurous substance. the enormous round things that rose from the ocean, near the _victoria_. whether we still accept that they were super-constructions that had come from a denser atmosphere and, in danger of disruption, had plunged into the ocean for relief, then rising and continuing on their way to jupiter or uranus--it was reported that they spread a "stench of sulphur." at any rate, this datum of proximity is against the conventional explanation that these things did not rise from the ocean, but rose far away above the horizon, with illusion of nearness. and the things that were seen in the sky july, : i have another note. in _nature_, - , a correspondent writes that, upon july , , at sedberg, he had seen in the sky--a red object--or, in his own wording, something that looked like the red part of a rainbow, about degrees long. but the sky was dark at the time. the sun had set. a heavy rain was falling. throughout this book, the datum that we are most impressed with: successive falls. or that, if upon one small area, things fall from the sky, and then, later, fall again upon the same small area, they are not products of a whirlwind, which though sometimes axially stationary, discharges tangentially-- so the frogs that fell at wigan. i have looked that matter up again. later more frogs fell. as to our data of gelatinous substance said to have fallen to this earth with meteorites, it is our expression that meteorites, tearing through the shaky, protoplasmic seas of genesistrine--against which we warn aviators, or they may find themselves suffocating in a reservoir of life, or stuck like currants in a blanc mange--that meteorites detach gelatinous, or protoplasmic, lumps that fall with them. now the element of positiveness in our composition yearns for the appearance of completeness. super-geographical lakes with fishes in them. meteorites that plunge through these lakes, on their way to this earth. the positiveness in our make-up must have expression in at least one record of a meteorite that has brought down a lot of fishes with it-- _nature_, - : that, near the bank of a river, in peru, feb. , , a meteorite fell. "on the spot, it is reported, several dead fishes were found, of different species." the attempt to correlate is--that the fishes "are supposed to have been lifted out of the river and dashed against the stones." whether this be imaginable or not depends upon each one's own hypnoses. _nature_, - : that the fishes had fallen among the fragments of the meteorite. _popular science review_, - : that one day, mr. le gould, an australian scientist, was traveling in queensland. he saw a tree that had been broken off close to the ground. where the tree had been broken was a great bruise. near by was an object that "resembled a ten-inch shot." a good many pages back there was an instance of over-shadowing, i think. the little carved stone that fell at tarbes is my own choice as the most impressive of our new correlates. it was coated with ice, remember. suppose we should sift and sift and discard half the data in this book--suppose only that one datum should survive. to call attention to the stone of tarbes would, in my opinion, be doing well enough, for whatever the spirit of this book is trying to do. nevertheless, it seems to me that a datum that preceded it was slightingly treated. the disk of quartz, said to have fallen from the sky, after a meteoric explosion: said to have fallen at the plantation bleijendal, dutch guiana: sent to the museum of leyden by m. van sypesteyn, adjutant to the governor of dutch guiana (_notes and queries_, - - ). and the fragments that fall from super-geographic ice fields: flat pieces of ice with icicles on them. i think that we did not emphasize enough that, if these structures were not icicles, but crystalline protuberances, such crystalline formations indicate long suspension quite as notably as would icicles. in the _popular science news_, - , it is said that in , near tiflis, fell large hailstones with long protuberances. "the most remarkable point in connection with the hailstones is the fact that, judging from our present knowledge, a very long time must have been occupied in their formation." according to the _geological magazine_, - , this fall occurred may , . the writer in the _geological magazine_ says that of all theories that he had ever heard of, not one could give him light as to this occurrence--"these growing crystalline forms must have been suspended a long time"-- again and again this phenomenon: fourteen days later, at about the same place, more of these hailstones fell. rivers of blood that vein albuminous seas, or an egg-like composition in the incubation of which this earth is a local center of development--that there are super-arteries of blood in genesistrine: that sunsets are consciousness of them: that they flush the skies with northern lights sometimes: super-embryonic reservoirs from which life-forms emanate-- or that our whole solar system is a living thing: that showers of blood upon this earth are its internal hemorrhages-- or vast living things in the sky, as there are vast living things in the oceans-- or some one especial thing: an especial time: an especial place. a thing the size of the brooklyn bridge. it's alive in outer space--something the size of central park kills it-- it drips. we think of the ice fields above this earth: which do not, themselves, fall to this earth, but from which water does fall-- _popular science news_, - : that, according to prof. luigi palazzo, head of the italian meteorological bureau, upon may , , at messignadi, calabria, something the color of fresh blood fell from the sky. this substance was examined in the public-health laboratories of rome. it was found to be blood. "the most probable explanation of this terrifying phenomenon is that migratory birds (quails or swallows) were caught and torn in a violent wind." so the substance was identified as birds' blood-- what matters it what the microscopists of rome said--or had to say--and what matters it that we point out that there is no assertion that there was a violent wind at the time--and that such a substance would be almost infinitely dispersed in a violent wind--that no bird was said to have fallen from the sky--or said to have been seen in the sky--that not a feather of a bird is said to have been seen-- this one datum: the fall of blood from the sky-- but later, in the same place, blood again fell from the sky. _notes and queries_, - - : a correspondent who had been to devonshire writes for information as to a story that he had heard there: of an occurrence of about thirty-five years before the date of writing: of snow upon the ground--of all south devonshire waking up one morning to find such tracks in the snow as had never before been heard of--"clawed footmarks" of "an unclassifiable form"--alternating at huge but regular intervals with what seemed to be the impression of the point of a stick--but the scattering of the prints--amazing expanse of territory covered--obstacles, such as hedges, walls, houses, seemingly surmounted-- intense excitement--that the track had been followed by huntsmen and hounds, until they had come to a forest--from which the hounds had retreated, baying and terrified, so that no one had dared to enter the forest. _notes and queries_, - - : whole occurrence well-remembered by a correspondent: a badger had left marks in the snow: this was determined, and the excitement had "dropped to a dead calm in a single day." _notes and queries_, - - : that for years a correspondent had had a tracing of the prints, which his mother had taken from those in the snow in her garden, in exmouth: that they were hoof-like marks--but had been made by a biped. _notes and queries_, - - : well remembered by another correspondent, who writes of the excitement and consternation of "some classes." he says that a kangaroo had escaped from a menagerie--"the footprints being so peculiar and far apart gave rise to a scare that the devil was loose." we have had a story, and now we shall tell it over from contemporaneous sources. we have had the later accounts first very largely for an impression of the correlating effect that time brings about, by addition, disregard and distortion. for instance, the "dead calm in a single day." if i had found that the excitement did die out rather soon, i'd incline to accept that nothing extraordinary had occurred. i found that the excitement had continued for weeks. i recognize this as a well-adapted thing to say, to divert attention from a discorrelate. all phenomena are "explained" in the terms of the dominant of their era. this is why we give up trying really to explain, and content ourselves with expressing. devils that might print marks in snow are correlates to the third dominant back from this era. so it was an adjustment by nineteenth-century correlates, or human tropisms, to say that the marks in the snow were clawed. hoof-like marks are not only horsey but devilish. it had to be said in the nineteenth century that those prints showed claw-marks. we shall see that this was stated by prof. owen, one of the greatest biologists of his day--except that darwin didn't think so. but i shall give reference to two representations of them that can be seen in the new york public library. in neither representation is there the faintest suggestion of a claw-mark. there never has been a prof. owen who has explained: he has correlated. another adaptation, in the later accounts, is that of leading this discorrelate to the old dominant into the familiar scenery of a fairy story, and discredit it by assimilation to the conventionally fictitious--so the idea of the baying, terrified hounds, and forest like enchanted forests, which no one dared to enter. hunting parties were organized, but the baying, terrified hounds do not appear in contemporaneous accounts. the story of the kangaroo looks like adaptation to needs for an animal that could spring far, because marks were found in the snow on roofs of houses. but so astonishing is the extent of snow that was marked that after a while another kangaroo was added. but the marks were in single lines. my own acceptance is that not less than a thousand one-legged kangaroos, each shod with a very small horseshoe, could have marked that snow of devonshire. london _times_, feb , : "considerable sensation has been caused in the towns of topsham, lymphstone, exmouth, teignmouth, and dawlish, in devonshire, in consequence of the discovery of a vast number of foot tracks of a most strange and mysterious description." the story is of an incredible multiplicity of marks discovered in the morning of feb. , , in the snow, by the inhabitants of many towns and regions between towns. this great area must of course be disregarded by prof. owen and the other correlators. the tracks were in all kinds of unaccountable places: in gardens enclosed by high walls, and up on the tops of houses, as well as in the open fields. there was in lymphstone scarcely one unmarked garden. we've had heroic disregards but i think that here disregard was titanic. and, because they occurred in single lines, the marks are said to have been "more like those of a biped than of a quadruped"--as if a biped would place one foot precisely ahead of another--unless it hopped--but then we have to think of a thousand, or of thousands. it is said that the marks were "generally inches in advance of each other." "the impression of the foot closely resembles that of a donkey's shoe, and measured from an inch and a half, in some instances, to two and a half inches across." or the impressions were cones in incomplete, or crescentic basins. the diameters equaled diameters of very young colts' hoofs: too small to be compared with marks of donkey's hoofs. "on sunday last the rev. mr. musgrave alluded to the subject in his sermon and suggested the possibility of the footprints being those of a kangaroo, but this could scarcely have been the case, as they were found on both sides of the este. at present it remains a mystery, and many superstitious people in the above-named towns are actually afraid to go outside their doors after night." the este is a body of water two miles wide. london _times_, march , : "the interest in this matter has scarcely yet subsided, many inquiries still being made into the origin of the footprints, which caused so much consternation upon the morning of the th ult. in addition to the circumstances mentioned in the _times_ a little while ago, it may be stated that at dawlish a number of persons sallied out, armed with guns and other weapons, for the purpose, if possible, of discovering and destroying the animal which was supposed to have been so busy in multiplying its footprints. as might have been expected, the party returned as they went. various speculations have been made as to the cause of the footprints. some have asserted that they are those of a kangaroo, while others affirm that they are the impressions of claws of large birds driven ashore by stress of weather. on more than one occasion reports have been circulated that an animal from a menagerie had been caught, but the matter at present is as much involved in mystery as ever it was." in the _illustrated london news_, the occurrence is given a great deal of space. in the issue of feb. , , a sketch is given of the prints. i call them cones in incomplete basins. except that they're a little longish, they look like prints of hoofs of horses--or, rather, of colts. but they're in a single line. it is said that the marks from which the sketch was made were inches apart, and that this spacing was regular and invariable "in every parish." also other towns besides those named in the _times_ are mentioned. the writer, who had spent a winter in canada, and was familiar with tracks in snow, says that he had never seen "a more clearly defined track." also he brings out the point that was so persistently disregarded by prof. owen and the other correlators--that "no known animal walks in a line of single footsteps, not even man." with these wider inclusions, this writer concludes with us that the marks were not footprints. it may be that his following observation hits upon the crux of the whole occurrence: that whatever it may have been that had made the marks, it had removed, rather than pressed, the snow. according to his observations the snow looked "as if branded with a hot iron." _illustrated london news_, march , - : prof. owen, to whom a friend had sent drawings of the prints, writes that there were claw-marks. he says that the "track" was made by "a" badger. six other witnesses sent letters to this number of the _news_. one mentioned, but not published, is a notion of a strayed swan. always this homogeneous-seeing--"a" badger--"a" swan--"a" track. i should have listed the other towns as well as those mentioned in the _times_. a letter from mr. musgrave is published. he, too, sends a sketch of the prints. it, too, shows a single line. there are four prints, of which the third is a little out of line. there is no sign of a claw-mark. the prints look like prints of longish hoofs of a very young colt, but they are not so definitely outlined as in the sketch of february th, as if drawn after disturbance by wind, or after thawing had set in. measurements at places a mile and a half apart, gave the same inter-spacing--"exactly eight inches and a half apart." we now have a little study in the psychology and genesis of an attempted correlation. mr. musgrave says: "i found a very apt opportunity to mention the name 'kangaroo' in allusion to the report then current." he says that he had no faith in the kangaroo-story himself, but was glad "that a kangaroo was in the wind," because it opposed "a dangerous, degrading, and false impression that it was the devil." "mine was a word in season and did good." whether it's jesuitical or not, and no matter what it is or isn't, that is our own acceptance: that, though we've often been carried away from this attitude controversially, that is our acceptance as to every correlate of the past that has been considered in this book--relatively to the dominant of its era. another correspondent writes that, though the prints in all cases resembled hoof marks, there were indistinct traces of claws--that "an" otter had made the marks. after that many other witnesses wrote to the _news_. the correspondence was so great that, in the issue of march th, only a selection could be given. there's "a" jumping-rat solution and "a" hopping-toad inspiration, and then someone came out strong with an idea of "a" hare that had galloped with pairs of feet held close together, so as to make impressions in a single line. london _times_, march , : "among the high mountains of that elevated district where glenorchy, glenlyon and glenochay are contiguous, there have been met with several times, during this and also the former winter, upon the snow, the tracks of an animal seemingly unknown at present in scotland. the print, in every respect, is an exact resemblance to that of a foal of considerable size, with this small difference, perhaps, that the sole seems a little longer, or not so round; but as no one has had the good fortune as yet to have obtained a glimpse of this creature, nothing more can be said of its shape or dimensions; only it has been remarked, from the depth to which the feet sank in the snow, that it must be a beast of considerable size. it has been observed also that its walk is not like that of the generality of quadrupeds, but that it is more like the bounding or leaping of a horse when scared or pursued. it is not in one locality that its tracks have been met with, but through a range of at least twelve miles." in the _illustrated london news_, march , , a correspondent from heidelberg writes, "upon the authority of a polish doctor of medicine," that on the piashowa-gora (sand hill) a small elevation on the border of galicia, but in russian poland, such marks are to be seen in the snow every year, and sometimes in the sand of this hill, and "are attributed by the inhabitants to supernatural influences." none proofreading team experiments and considerations touching colours. first occasionally written, among some other _essays_, to a friend; and now suffer'd to come abroad as the beginning of an experimental history of colours. by the honourable robert boyle, fellow of the royal society. _non fingendum, aut excogitandum, sed inveniendum, quid natura faciat, aut ferat._ bacon. _london._ printed for _henry herringman_ at the _anchor_ on the lower walk of the _new exchange._ mdclxiv. * * * * * the preface. having in convenient places of the following treatise, mention'd the motives, that induc'd me to write it, and the scope i propos'd to my self in it; i think it superfluous to entertain the reader now, with what he will meet with hereafter. and i should judge it needless, to trouble others, or my self, with any thing of preface: were it not that i can scarce doubt, but this book will fall into the hands of some readers, who being unacquainted with the difficulty of attempts of this nature, will think itn strange that i should publish any thing about colours, without a particular theory of them. but i dare expect that intelligent and equitable readers will consider on my behalf: that the professed design of this treatise is to deliver things rather _historical_ than _dogmatical_, and consequently if i have added divers new _speculative_ considerations and hints, which perhaps may afford no despicable assistance, towards the framing of a solid and comprehensive hypothesis, i have done at least as much as i promis'd, or as the nature of my undertaking exacted. but another thing there is, which if it should be objected, i fear i should not be able so easily to answer it, and that is; that in the following treatise (especially in the third part of it) the experiments might have been better marshall'd, and some of them deliver'd in fewer words. for i must confess that this essay was written to a private friend, and that too, by snatches, at several times, and places, and (after my manner) in loose sheets, of which i oftentimes had not all by me that i had already written, when i was writing more, so that it needs be no wonder if all the experiments be not rang'd to the best advantage, and if some connections and consecutions of them might easily have been mended. especially since having carelessly laid by the loose papers, for several years after they were written, when i came to put them together to dispatch them to the press, i found some of those i reckon'd upon, to be very unseasonably wanting. and to make any great change in the order of the rest, was more than the printers importunity, and that, of my own avocations (and perhaps also considerabler solicitations) would permit. but though some few preambles of the particular experiments might have (perchance) been spar'd, or shorten'd, if i had had all my papers under my view at once; yet in the most of those introductory passages, the reader will (i hope) find hints, or advertisements, as well as transitions. if i sometimes seem to insist long upon the circumstances of a tryall, i hope i shall be easily excused by those that both know, how nice divers experiments of colours are, and consider that i was not barely to _relate_ them, but so as to teach a young gentleman to make them. and if i was not sollicitous, to make a nicer division of the whole treatise, than into three parts, whereof the one contains some considerations about colours in general. the other exhibits a specimen of an account of particular colours, exemplifi'd in whiteness and blackness. and the third promiscuous experiments about the remaining colours (especially red) in order to a theory of them. if, i say, i contented my self with this easie division of my discourse, it was perhaps because i did not think it so necessary to be curious about the method or contrivance of a treatise, wherein i do not pretend to present my reader with a compleat fabrick, or so much as modell; but only to bring in materials proper for the building; and if i did not well know how ingenious the curiosity and civility of friends makes them, to perswade men by specious allegations, to gratifie their desires; i should have been made to believe by persons very well qualify'd to judge of matters of this nature, that the following experiments will not need the addition of accurate method and speculative notions to procure acceptance for the treatise that contains them: for it hath been represented, that in most of them, as the novelty will make them surprizing, and the quickness of performance, keep them from being tedious; so the sensible changes, that are effected by them, are so manifest, so great, and so sudden, that scarce any will be displeased to see them, and those that are any thing curious will scarce be able to see them, without finding themselves excited, to make reflexions upon them. but though with me, who love to measure physical things by their _use_, not their _strangeness_, or _prettiness_, the partiality of others prevails not to make me over value these, or look upon them in themselves as other than trifles: yet i confess, that ever since i did divers years ago shew some of them to a learned company of _virtuosi_: so many persons of differing conditions, and ev'n sexes, have been curious to see them, and pleas'd not to dislike them, that i cannot despair, but that by complying with those that urge the publication of them, i may both gratifie and excite the curious, and lay perhaps a foundation whereon either others or my self may in time superstruct a substantial theory of colours. and if _aristotle_, after his master _plato_, have rightly observ'd admiration to be the _parent of philosophy_, the wonder, some of these trifles have been wont to produce in all sorts of beholders, and the access they have sometimes gain'd ev'n to the closets of ladies, seem to promise, that since the subject is so pleasing, that the speculation appears as delightful! as difficult, such easie and recreative experiments, which require but little time, or charge, or trouble in the making, and when made are sensible and surprizing enough, may contribute more than others, (far more important but as much more difficult) to recommend those parts of learning (chymistry and corpuscular philosophy) by which they have been produc'd, and to which they give testimony ev'n to such kind of persons, as value a pretty trick more than a true notion, and would scarce admit philosophy, if it approach'd them in another dress: without the strangeness or endearments of pleasantness to recommend it. i know that i do but ill consult my own advantage in the consenting to the publication of the following treatise: for those things, which, whilst men knew not how they were perform'd, appear'd so strange, will, when the way of making them, and the grounds on which i devis'd them, shall be publick, quickly lose all that their being _rarityes_, and their _being thought mysteries_, contributed to recommend them. but 'tis fitter for mountebancks than naturalis to desire to have their discoverys rather admir'd than understood, and for my part i had much rather deserve the thanks of the ingenious, than enjoy the applause of the ignorant. and if i can so farr contribute to the discovery of the nature of colours, as to help the curious to it, i shall have reach'd my end, and sav'd my self some labour which else i may chance be tempted to undergo in prosecuting that subect, and adding to this treatise, which i therefore call a _history_, because it chiefly contains matters of fact, and which history the title declares me to look upon but as _begun_: because though that above a hundred, not to say a hundred and fifty experiments, (some loose, and others interwoven amongst the discourses themselves) may suffice to give a _beginning_ to a history not hitherto, that i know, begun, by any; yet the subject is so fruitfull, and so worthy, that those that are curious of these matters will be farr more wanting to themselves than i can suspect, if what i now publish prove any more than a _beginning_. for, as i hope my endeavours may afford them some assistance towards this work, so those endeavours are much too vnfinish'd to give them any discouragement, as if there were little left for others to do towards the history of colours. for (first) i have been willing to leave unmention'd the _most part_ of those phænomena of colours, that nature presents us of her own accord, (that is, without being guided or over-ruld by man) such as the different colours that several sorts of fruites pass through before they are perfectly ripe, and those that appear upon the fading of flowers and leaves, and the putrifaction (and its several degrees) of fruits, &c. together with a thousand other obvious instances of the changes of colours. nor have i _much_ medled with those familiar phænomena wherein man is not an idle spectator; such as the greenness produc'd by salt in beef much powder'd, and the redness produc'd in the shells of lobsters upon the boyling of those fishes; for i was willing to leave the _gathering_ of _observations_ to those that have not the opportunity to _make experiments_. and for the same reasons, among others, i did purposly omit the lucriferous practise of trades-men about colours; as the ways of making pigments, of bleanching wax, of dying scarlet, &c. though to divers of them i be not a stranger, and of some i have myself made tryall. next; i did purposely pass by divers experiments of other writers that i had made tryall of (and that not without registring some of their events) unless i could some way or other improve them, because i wanted leasure to insert them, and had thoughts of prosecuting the work once begun of laying together those i had examin'd by themselves in case of my not being prevented by others diligence. so that there remains not a little, among the things that are already published, to imploy those that have a mind to exercise themselves in repeating and examining them. and i will not undertake, that _none_ of the things deliver'd, ev'n in this treatise, though never so faithfully set down, may not prove to be thus farr of this sort, as to afford the curious somewhat to add about them. for i remember that i have somewhere in the book it self acknowledged, that having written it by snatches, partly in the counntrey, and partly at unseasonable times of the year, when the want of fit instruments, and of a competent variety of flowers, salts, pigments, and other materials made me leave some of the following experiments, (especialy those about emphatical colours) far more unfinish'd than they should have been, if it had been as easie for me to _supply_ what was wanting to compleat them, as to _discern_. thirdly to avoyd discouraging the young gentleman i call pyrophilus, whom the less familiar, and more laborious operations of chymistry would probably have frighted, i purposely declin'd in what i writ to him, the setting down any number of such chymicall experiments, as, by being very elaborate or tedious, would either require much skill, or exercise his patience. and yet that this sort of experiments is exceedingly numerous, and might more than a little inrich the history of colours, those that are vers'd in chymical processes, will, i presume, easily allow me. and (lastly) for as much as i have occasion more than once in my several writings to treat either porposely or incidentally of matters relating to colours; i did not, perhaps, conceive my self oblig'd, to deliver in one treatise _all_ that i would say concerning that subject. but to conclude, by summing up what i would say concerning what i _have_ and what i _have not_ done, in the following papers; i shall not (_on the one side_) deny, that considering that i pretended not to write an accurate treatise of colours, but an occasional essay to acquaint a private friend with what then occurrd to me of the things i had thought or try'd concerning them; i might presume i did enough for once, if i did clearly and faithfully set down, though not _all_ the experiments i could, yet at least such a variety of them, that an attentive reader that shall consider the grounds on which they have been made, and the hints that are purposely (though dispersedly) couched in them, may easily _compound_ them, and otherwise _vary_ them, so as very much to increase their number. and yet (_on the other side_) i am so sensible both of how much i have, either out of necessity or choice, left undone, and of the fruitfullness of the subject i have begun to handle; that though i had performed far more then 'tis like many readers will judge i have, i should yet be very free to let them apply to my attempts that of _seneca_, where having spoken of the study of natures mysteries, and particularly of the cause of earth-quakes, he subjoins.[ ] _nulla res consummata est dum incipit. nec in hac tantum re omnium maxima ac involutissimá, in quâ etiam cum multum actum erit, omnis ætas, quod agat inveniet; sed in omni alio negotio, longè semper à perfecto fuere principia._ [ ] l. annæ senecæ natur. quest. l. . c. . * * * * * _the publisher to the_ reader. _friendly reader,_ here is presented to thy view one of the abstrusest as well as the gentilest subjects of natural philosophy, the _experimentall history of colours_; which though the noble author be pleased to think but _begun_, yet i must take leave to say, that i think it so well begun, that the work is more than half dispatcht. concerning which i cannot but give this advertisement to the reader, that i have heard the author express himself, that it would not surprise him, if it should happen to be objected, that some of these experiments have been already published, partly by chymists, and partly by two or three very fresh writers upon other subjects. and though the number of these experiments be but very small, and though they be none of the considerablest, yet it may on this occasion be further represented, that it is easie for our author to name several men, (of whose number i can truly name my self) who remember either their having seen him make, or their having read, his accounts of the experiments delivered in the following tract several years since, and long before the publication of the books, wherein they are mentioned. nay in divers passages (where he could do it without any great inconvenience) he hath struck out experiments, which he had tryed many years ago, because he since found them divulged by persons from whom he had not the least hint of them; which yet is not touched, with design to reflect upon any ingenious man, as if he were a plagiary: for, though our generous author were not reserved enough in showing his experiments to those that expressed a curiosity to see them (amongst whom a very learned man hath been pleased publickly to acknowledge it several years ago[ ]; yet the same thing may be well enough lighted on by persons that know nothing of one another. and especially chymical laboratories may many times afford the same _phænomenon_ about colours to several persons at the same or differing times. and as for the few _phænomena_ mentioned in the same chymical writers, as well as in the following treatise, our author hath given an account, why he did not decline rejecting them, in the anotations upon the th experiment of the third part. not here to mention, what he elsewhere saith, to shew what use may be justifiably made of experiments not of his own devising by a writer of natural history, if, what he employes of others mens, be well examined or verified by himself. [ ] he that desires more instances of this kind and matter, that according to this doctrine may much help the theory of colours, and particularly the force both of sulphureous and volatile, is likewise of alcalizate and acid salts, and in what particulars, colours likely depend not in the causation from any salt at all, may beg his information from m. boyle who hath some while since honoured me with the sight of his papers concerning this subject, containing many excellent experiments, made by him for the elucidation of this doctrine, &c dr. r. sharrock in his ingenious and usefull history of the propagation and improvement of vegetables, published in the yeare . in the mean time, this treatise is such, that there needs no other invitation to peruse it, but that tis composed by one of the deepest & most indefatigable searchers of nature, which, i think the world, as far as i know it, affords. for mine own part, i feel a secret joy within me, to see such beginings upon such _themes_, it being demonstratively true, _mota facilius moveri_, which causeth me to entertain strong hopes, that this illustrious _virtuoso_ and restless inquirer into nature's secrets will not stop here, but go on and prosper in the disquisition or the other principal colours, _green, red_, and _yellow_. the reasoning faculty set once afloat, will be carried on, and that with ease, especially, when the productions thereof meet, as they do here, with so greedy an entertainment at home and abroad. i am confident, that the royal society, lately constituted by his most excellent majesty _for improving natural knowledge_, will judge it their interest to exhort our author to the prosecution of this argument, considering, how much it is their design and business to accumulate a good stock of such accurate observations and experiments, as may afford them and their offpring genuine matter to raise a masculine philosophy upon, whereby the mind of man may be enobled with the knowledge of solid truths, and the life of man benefited with ampler accommodations, than it hath been hitherto. our great author, one of the pillars of that illustrious corporation, is constantly furnishing large _symbola_'s to this work, and is now falln, as you see, upon so comprehensive and important a theme, as will, if insisted on and compleated, prove one of the considerablest peeces of that structure. to which, if he shall please to add his treatise of _heat_ and _flame_, as he is ready to publish his experimental accounts of _cold_, i esteem, the world will be obliged to him for having shewed them both the _right_ and _left hand_ of nature, and the operations thereof. the considering reader will by this very treatise see abundant cause to sollicit the author for more; sure i am, that of whatever of the productions of his ingeny comes into _forein parts_ (where i am happy in the acquaintance of many intelligent friends) is highly valued; and to my knowledge, there are those among the french, that have lately begun to learn english, on purpose to enable themselves to read his books, being impatient of their traduction into latin. if i durst say all, i know of the elogies received by me from abroad concerning him, i should perhaps make this preamble too prolix, and certainly offend the modesty of our author. wherefore i shall leave this, and conclude with desiring the reader, that if he meet with other faults besides those, that the errata take notice of (as i believe he may) he will please to consider both the weakness of the authors eyes, for not reviewing, and the manifold avocations of the publisher for not doing his part; who taketh his leave with inviting those, that have also considered this nice subject experimentally, to follow the example of our noble author, and impart such and the like performances to the now very inquisitive world. _farewell._ _h. o._ * * * * * the contents. * * * * * chap. i. _the author shews the reason, first of his writing on this subject_ ( .) _next of his present manner of handling it, and why he partly declines a methodical way_ ( .) _and why he has partly made use of it in the history of_ whiteness _and_ blackness. ( .) chap. . _some general considerations are premis'd, first of the insignificancy of the observasion of colours in many bodies_ ( , .) _and the importance of it in others_ ( .) _as particularly in the tempering of steel_ ( , , .) _the reason why other particular instances are in that place omitted_ ( ) _a necessary distinction about colour premis'd_ ( , .) _that colour is not inherent in the object_ ( .) _prov'd first by the phantasms of colours to_ dreaming _men, and_ lunaticks; _secondly by the sensation or apparition of light upon a blow given the eye or the distemper of the brain from internal vapours_ ( .) _the author recites a particular instance in himself; another that hapn'd to an excellent person related to him_ ( .) _and a third told him by an ingenious physician_ ( , .) _thirdly, from the change of colours made by the sensory disaffected_ ( , .) _some instances of this are related by the author, observ'd in himself_ ( , .) _others told him by a lady of known veracity_ ( .) _and others told him by a very eminent man_ ( .) _but the strange instances afforded by such as are bit by the_ tarantula _are omitted, as more properly deliver'd in another place_. ( .) chap. . _that the colour of bodies depends chiefly on the disposition of the superficial parts, and partly upon the variety of the texture of the object_ ( .) _the former of these are confirm'd by several persons_ ( .) _and two instances, the first of the steel mention'd before, the second of melted lead_ ( , .) _of which last several observables are noted_ ( .) _a third instance is added of the porousness of the appearing smooth surface of cork_ ( , .) _and that the same kind of porousness may be also in the other colour'd bodies; and of what kind of figures, the superficial reflecting particles of them may be_ ( .) _and of what bulks, and closeness of position_ ( .) _how much these may conduce to the generation of colour instanc'd in the whiteness of froth, and in the mixtures of dry colour'd powders_ ( .) _a further explication of the variety that may be in the superficial parts of colour'd bodies, that may cause that effect, by an example drawn from the surface of the earth_ ( .) _an apology for that gross comparison_ ( .) _that the appearances of the superficial asperities may be varied from the position of the eye, and several instances given of such appearances_ ( , , .) _that the appearance of the superficial particles may be varied also by their motion, confirm'd by an instance of the smoaking liquor_ ( .) _especially if the superficial parts be of such a nature as to appear divers in several postures, explain'd by the variety of colours exhibited by the shaken leaves of some plants_ ( .) _and by changeable taffities_ ( , , .) _the authors wish that the variety of colours in mother of pearl were examin'd with a_ microscope ( .) _and his conjectures, that possibly good_ microscopes _might discover those superficial inequalities to be real, which we now only imagine with his reasons drawn partly from the discoveries of the_ telescope, _and_ microscope ( .) _and partly also from the prodigiously strange example of a blind man that could feel colours_ ( .) _whose history is related_ ( , , .) _the authors conjecture and thoughts of it_ ( , , , .) _and several conclusions and corollaries drawn from it about the nature of blackness and black bodies_ ( , , .) _and about the asperities of several other colour'd bodies_ ( .) _and from these, and some premis'd considerations, are propos'd some conjectures; that the reason of the several phænomena of colours, afterwards to be met with, depends upon the disposition of the seen parts of the object_ ( .) _that liquors may alter the colours of each other, and of other bodies, first by their insinuating themselves into the pores, and filling them, whence the asperity of the surface of a body becomes alter'd, explicated with some instances_ ( , .) _next by removing those bodies, which before hindred the appearance of the genuine colour, confirm'd by several examples_ ( ) _thirdly, by making a fissure or separation either in the contiguous or continued particles of a body_ ( .) _fourthly, by a union or conjunction of the formerly separated particles; illustrated with divers instances of precipitated bodies_ ( .) _fifthly, by dislocating the parts, and putting them both into other orders and postures, which is illustrated with instances_ ( , .) _sixthly, by motion, which is explain'd_ ( .) _and lastly, and chiefly, by the union of the saline bodies, with the superficial parts of another body, whereby both their bigness and shape must necessarily be alter'd_ ( , .) _explain'd by experiments_ ( , .) _that the colour of bodies may be chang'd by the concurrence of two or more of these ways_ ( .) _and besides all these, eight reflective causes of colours, there may be in transparent bodies several refractive_ ( , ) _why the author thinks the nature of colours deserves yet a further inquiry_ ( .) _first for that the little motes of dust exhibited very lovely colours in a darkned room, whilst in a convenient posture to the eye, which in other postures and lights they did not_ ( .) _and that though the smaller parts of some colour'd bodies are transparent, yet of others they are not, so that the first doubt's, whether the superficial parts create those colours, and the second, whether there be any refraction at all in the later_ ( , , .) _a famous controversie among philosophers, about the nature of colour decided_. ( . .) chap. . _the controversie stated about real and emphatical colours_ ( , .) _that the great disparity between them seems to be, partly their duration in the same state, and partly, that genuine colours are produc'd in opacous bodies by reflection, and emphatical in transparent by refraction_ ( .) _but that this is not to be taken in too large a sense, the cautionary instance of froth is alleged and insisted on_ ( , .) _that the duration is not a sufficient characteristick, exemplify'd by the duration of froth, and other emphatical colours, and the suddain fading of flowers, and other bodies of real ones_ ( .) _that the position of the eye is not necessary to the discerning emphatical colours, shew'd by the seeing white froth, or an iris cast on the wall by a prism, in what place of the room soever the eye be_ ( .) _which proceeds from the specular reflection of the wall_ ( .) _that emphatical colours may be compounded, and that the present discourse is not much concern'd, whether there be, or be not made a distinction between real and emphatical colours_. ( .) chap. . _six hypotheses about colour recited_ ( , ) _why the author cannot more fully speak of any of these_ ( .) _nor acquiesce in them_ ( , .) _what_ pyrophilus _is to expect in this treatise_ ( , .) _what hypothesis of light and colour the author most inclines too_ ( .) _why he thinks neither that nor any other sufficient; and what his difficulties are, that make him decline all hypotheses, and to think it very difficult to stick to any._ ( , .) * * * * * part the second. _of the nature of whiteness and blackness,_ chap. i. _the reason why the author chose the explication of whiteness and blackness_ ( .) _wherein_ democritus _thought amiss of these_ ( .) gassendus _his opinion about them_ ( .) _what the author approves, and a more full explication of white, makinig it a multiplicity of light or reflections_ ( , .) _confirm'd first by the whiteness of the_ meridian _sun, observ'd in water_ ( .) _and of a piece of iron glowing hot_ ( .) _secondly, by the offensiveness of snow to the travellers eyes, confirm'd by an example of a person that has travell'd much in russia_ ( .) _and by an observation out of_ olaus magnus ( .) _and that the snow does inlighten and clear the air in the night, confirm'd by the mosco physician, and captain_ james ( .) _but that snow has no inherent light, prov'd by experience_ ( .) _thirdly, by the great store of reflections, from white bodies observ'd in a darkned room, and by their unaptness to be kindled by a burning-glass_ ( .) _fourthly, the specularness of white bodies is confirm'd by the reflections in a dark room from other bodies_ ( .) _and by the appearance of a river, which both to the eye and in a darkned room appear'd white_ ( , .) _fifthly, by the whiteness of distill'd_ mercury, _and that of the_ galaxie ( , .) _and by the whiteness of froth, rais'd from whites of eggs beaten; that this whiteness comes not from the air, shew'd by experiments_ ( , .) _where occasionally the whiteness of distill'd oyls, hot water, &c. are shew'd_ ( .) _that it seems not necessary the reflecting surfaces should be sphærical, confirm'd by experiments_ ( , .) _sixthly, by the whiteness of the powders of transparent bodies_ ( .) _seventhly, by the experiment of whitening and burnishing silver._ ( , .) chap. . _a recital of some opinions about blackness, and which the author inclines to_ ( .) _which he further insists on and explicates_ ( , .) _and shews for what reasons he imbrac'd that hypothesis_ ( .) _first, from the contrary nature of whiteness and blackness, white reflecting most beams outwards, black should reflect most inward_ ( .) _next, from the black appearance of all bodies, when shadow'd; and the manner how this paucity of reflection outwards is caus'd, is further explicated, by shewing that the superficial parts may be conical and pyramical_ ( .) _this and other considerations formerly deliver'd, illustrated by experiments with black and white marble_ ( , .) _thirdly, from the black appearance of holes in white linnen, and from the appearance of velvet stroak'd several ways, and from an observation of carrots_ ( , .) _fourthly, from the small reflection from black in a darkned room_ ( , .) _fifthly, from the experiment of a checker'd tile expos'd to the sun-beams_ ( .) _which is to be preferr'd before a similar experiment try'd in_ italy, _with black and white marble_ ( .) _some other congruous observations_ ( .) _sixthly, from the roasting black'd eggs in the sun_ ( .) _seventhly, by the observation of the blind man lately mention'd, and of another mention'd by_ bartholine ( .) _that notwithstanding all these reasons, the author is not absolutely positive, but remains yet a seeker after the true nature of whiteness and blackness._ ( , .) experiments _in consort, touching_ whiteness _and_ blackness. _the first_ experiment, _with a solution of sublimate, made white with spirit of urine_, &c. ( , .) _the second_ experiment, _with an infusion of galls, made black with vitriol_, &c. ( , .) _further discours'd of_ ( .) _the third_ experiment, _of the blacking of hartshorn, and ivory, and tartar, and by a further calcination making them white_ ( , .) _the fourth_ experiment, _limiting the_ chymist's _principle_, adusta nigra sed perusta alba, _by several instances of calcin'd alabaster, lead, antimony, vitriol, and by the testimony of_ bellonius, _about the white charcoles of_ oxy-cædar, _and by that of_ camphire. ( , , .) _that which follows about inks was misplac'd by an errour of the printer, for it belongs to what has been formerly said of galls_ ( , .) _the fifth_ experiment, _of the black smoak of camphire_ ( .) _the sixth_ experiment, _of a black_ caput mortuum, _of oyl of vitriol, with oyl of worm-word, and also with oyl of winter-savory_ ( .) _the seventh_ experiment, _of whitening wax_ ( .) _the eighth_ experiment, _with tin-glass, and sublimate_ ( , .) _the ninth_ experiment, _of a black powder of gold in the bottom of_ aqua-fortis, _and of the blacking of refin'd gold and silver_ ( , .) _the tenth_ experiment, _of the staining hair, skin, ivory_, &c. _black, with crystals of silver_ ( , .) _the eleventh_ experiment, _about the blackness of the skin, and hair of_ negroes, _and inhabitants of hot climates. several objections are made, and the whole matter more fully discours'd and stated from several notable histories and observations_ (from the to the .) _the twelfth_ experiment, _of the white powders, afforded by precipitating several bodies, as crabs eyes, minium, coral, silver, lead, tin, quick-silver, tin-glass, antimony, benzoin, and resinous gumms out of spirit of wine_, &c. _but this is not universal, since other bodies, as gold, antimony, quick-silver_, &c. _may be precipitated of other colours_ ( , , .) _the thirteenth_ experiment, _of changing the blackness of some bodies into other colours_ ( , .) _and of whitening what would be minium, and copper, with tin, and of copper with arsnick, which with coppilling again vanishes; of covering the colour of that of_ / _of gold with_ / _of silver melted in a mass together_ ( , ) _the fourteenth_ experiment, _of turning the black body of horn into a white immediately with scraping, without changing the substantial form, or without the intervention of salt, sulphur, or mercury_ ( .) _the fifteenth_ experiment, _contains several instances against the opinion of the_ chymists _that sulphur_ adust _is the cause of blackness, and the whole matter is fully discuss'd and stated_ (from to ) part the third. _concerning promiscuous experiments about colours_. experiment the first. _in confirmation of a former conjecture about the generation of colours from diversity of reflections are set down several observations made in a darkned room_ ( , .) experiment _the second, that white linnen seem'd ting'd with the red of silk plac'd near it in a light room_ ( , .) experiment _the third, of the trajection of light through colour'd papers_ ( , .) experiment _the fourth, observations of a prism in a dark room_ ( , .) experiment _the fifth, of the refracting and reflecting prismatical colours in a light room_ ( .) experiment _the sixth, on the vanishing of the_ iris _of the prism, upon the access of a greater adventitious light_ ( .) experiment _the seventh, of the appearances of the same colour'd papers by candle-light_ ( , ). experiment _the eighth, of the yellowness of the flame of a candle_ ( ). experiment _the ninth, of the greenish blew transparency of leaf gold_ ( ). experiment _the tenth, of the curious tinctures afforded by_ lignum nephriticum (from to ). _several trials for the investigation of the nature of it_ (from to .) kircher's _relation of this wood set down, and examin'd_ (from to ). _a corollary on this tenth_ experiment, _shewing how it may be applicable for the discovering, whether any salt be of an acid, or a sulphureous, and alcalizate nature_ (from to ). _the eleventh_ experiment, _of certain pieces of glass that afforded this variety of colours; and of the way of so tinging any plate of glass with silver_ (from to ). _the twelfth_ experiment, _of the mixing and tempering of painters pigments_ ( , , ). _the thirteenth_ experiment, _of compounding several colours by trajecting the sun-beams through ting'd glasses_ (from to ). _the fourteenth_ experiment, _of the compounding of real and phantastical colours, and the results_ ( , , .) _as also the same of phantastical colours_ ( , .) _the fifteenth_ experiment, _of varying the trajected_ iris _by a colour'd prism_ ( , .) _the sixteenth_ experiment, _of the red fumes of spirit of_ nitre, _and, the resembling redness of the horizontal sun-beams_ ( , .) _the seventeenth_ experiment, _of making a green by nine kinds of compositions_ (from to .) _and some deductions from them against the necessity of recurring to substantial forms and hypostatical principles for the production of colours_ (from to .) _the eighteenth_ experiment, _of several compositions of blew and yellow which produce not a green, and of the production of a green by other colours_ ( , .) _the nineteenth_ experiment, _contains several instances of producing colours, without the alteration of any hypostatical principle, by the prism, bubbles, and feathers_ ( from to .) _the twentieth_ experiment _of turning the blew of violets into a red by acid salts, and to a green by alcalizate ( , .) and the use of it for investigating the nature of salts_ ( , .) _the one and twentieth_ experiment, _of the same changes effected by the same means on the blew tinctures of corn-flowers_ ( , .) _and some restrictions to shew it not to be so general a propriety as one might imagine_ ( .) _the twenty second_ experiment, _of turning a solution of verdigrease into a blew, with alcalizate and urinous salts_ ( , , .) _the twenty third_ experiment, _of taking away the colour of roses with the steams of sulphur, and heightning them with the steams condens'd into oyl of sulphur_ per campanam ( , .) _the twenty fourth_ experiment, _of tinging a great quantity of liquor with a very little ting'd substance, instanced in_ cochineel (from to .) _the twenty fifth_ experiment, _of the more general use of alcalizate and sulphureous salts in the tinctures of vegetables, further instanced in the tincture of privet berries, and of the flowers of mesereon and pease_ (from to .) _an_ annotation, _shewing that of the three hypostatical principles, salt according to_ paracelsus _is the most active about colours_ (from to .) _some things precursory premis'd to three several instances next following, against the fore-mention'd operations of salts_ ( , .) _the twenty sixth_ experiment, _containing trials with acid and sulphureous salts on the red tinctures of clove-july-flowers, buckthorn berries, red-roses, brasil_, &c. ( , .) _the twenty seventh_ experiment, _of the changes of the colour of jasmin flowers, and snow drops, by alcalizate and sulphureous salts_ ( , .) _the twenty eighth_ experiment, _of other differing effects on mary-golds, prim-roses, and fresh madder_ ( .) _with an admonition, that these salts may have differing effects in the changing of the tinctures of divers other vegetables_ ( , .) _the twenty ninth_ experiment, _of the differing effects of these salts on ripe and unripe juices, instanced in black-berries, and the juices of roses_ (from to .) _two reasons, why the author added this twenty ninth_ experiment, _the last of which is confirm'd by an instance of mr._ parkinson, _consonant to the confession of the makers of such colours_ ( .) _the thirtieth_ experiment, _of several changes in colours by digestion, exemplify'd by an_ amalgam _of_ gold _and_ mercury _and by spirit of harts-horn. and (to such as believe it) by the changes of the_ elixir. _the thirty first_ experiment, _shewing that most tinctures drawn by digestion incline to a red, instanc'd in_ jalap, guaicum, _amber, benzoin, sulphur, antimony_, &c. ( , .) _the thirty second_ experiment, _that some reds with diluting turn yellow, others not, exemplify'd by the tincture of_ cochineel, _and by balsam of_ sulphur, _tinctures of_ amber, &c. ( , , .) _the thirty third_ experiment, _of a red tincture of_ saccarum saturni _and oyl of_ turpentine _made by digestion_ ( .) _the thirty fourth_ experiment, _of drawing a volatile red tincture of mercury_, _whose steams were white, but it would tinge the skin black_ ( , .) _the thirty fifth_ experiment, _of a suddain way of making a blood red colour with oyl of_ vitriol, _and oyl of_ anniseeds, _two transparent liquors_ ( , .) _the thirty sixth_ experiment, _of the degenerating of several colours exemplify'd in the last mention'd blood red, and by mr._ parkinsons _relation of_ turnsol, _by some trials with the juice of buck-thorn berries, and other vegetables, to which several notable considerations and advertisements back'd with_ experiments _are adjoyn'd_ (from to .) _the thirty seventh_ experiment, _of varying the colour of the tinctures of_ cochineel, _red-cherries, and brasil, with acid and sulphureous salts, and divers considerations thereon_ (from to .) _the thirty eighth_ experiment, _about the red fumes of some, and white of other distill'd bodies, and of their coalition for the most part into a transparent liquor_ ( , .) _and of the various colours of dry sublimations, exemplify'd with several_ experiments ( , , .) _the thirty ninth_ experiment, _of varying the decoction of_ balaustiums _with acid and urinous salts_ ( , .) _some_ annotations _wherein two_ experiments _of_ gassendus _are related, examined, and improv'd_ (from to .) _the fortieth_ experiment, _of the no less strange than pleasant changes made with a solution of sublimate_ (from to .) _the difference between a chymical axd philosophical solution of a_ phænomenon ( , .) _the authors chymical explication of the_ phænomena, _confirm d by several_ experiments _made on_ mercury, _with several saline liquors_ (from to .) _an improvement of the fortieth_ experiment, _by a fresh decoction of_ antimony _in a_ lixivium ( , , .) _reflections on the tenth, twentieth, and fortieth_ experiments, _compar'd together, shewing a way with this tincture of sublimate to distinguish whether any saline body to be examin'd be of a urinous or alcalizate nature_ (from to .) _the examination of spirit of_ sal-armoniack, _and spirit of_ oak _by these principles_ (from to .) _that the author knows ways of making highly operative saline bodies, that produce none of the before mention'd effects_ ( , .) _some notable_ experiments _about solutions and precipitations of gold and silver_ ( , .) _the one and fortieth_ experiment, _of depriving a deep blew solution of copper of its colour_ ( .) _to which is adjoyn'd the discolouring or making transparent a solution of verdigrease, &c. and another of restoring or increasing it_ ( , .) _the forty second_ experiment, _of changing a milk white precipitate of_ mercury _into a yellow, by affusion of fair water, with several considerations thereon_ (from to .) _the forty third_ experiment, _of extracting a green solution with fair water out of imperfectly calcin'd vitriol_ ( .) _the forty fourth_ experiment, _of the deepning and diluting of several tinctures, by the affusions of liquors, and by conical glasses that contain'd them, exemplify'd in the tinctures of_ cochineel, brasil, verdigrease, glass, litmus, _of which last on this occasion several pleasant_ phænomena _are related_ (from to .) _to which are adjoyn'd certain cautional corollaries_ ( , .) _the waterdrinker and some of his legerdemain tricks related._( .) _the forty fifth_ experiment, _of the turning rhenish and white wine into a lovely green, with a preparation of steel _( , .) _some further trial made about these tinctures, and a similar_ experiment _of_ olaus wormius ( .) _the forty sixth_ experiment, _of the internal colour of metalls exhibited by calcination_ ( , , .) annotation _the first, that several degrees of fire may disclose a differing colour_ ( .) annotation _the second, that the glasses of metalls may exhibit also other kinds of colours_ ( .) annotation _the third, that minerals by several degrees of fire may disclose several colours_( ). experiment _the forty seventh, of the internal colours of metalls disclos'd by their dissolutions in several_ menstruums (from to .) annotation _the first, the authors apology for recording some already known_ experiments, _without mentioning their authors_ (from to .) annotation _the second, that some minerals also by dissolutions in_ menstruums _may exhibit divers colours_. annotation _the third, that metalls disclose other colours by precipitations, instanc'd in_ mercury (from to .) _the forty eighth_ experiment, _of tinging glass blew with leaf silver, and with calcin'd copper, and white with putty_ (from to .) annotation _the first, that this white glass is the basis of ammels_ ( .) annotion _the second, that colour'd glasses may be compounded like colour'd liquors in dying fats_ ( .) annotation _the third, of tinging glass with minerel substances, and of trying what metalls they contain by this means_ (from to .) annotation _the fourth, that metalls may be ting'd by mineralls_ ( , .) annotation _the fifth, of making several kinds of amauses or counterfeit stones_ (from to .) annotation _the sixth, of the scarlet dye, of the stains of dissolv'd gold and silver_ ( , ) _of the greenness of salt beef, and redness of neats tongues from salts; of gilding silver with bathe water_ ( , .) _and tinging the nails and skin with_ alcanna ( ) _the forty ninth_ experiment, _of making lakes_ ( .) _a particular example in turmerick_ ( , .) annotation _the first, that in precipitations wherein allum is a coefficient, a great part of them may consist of the stony particles of that compound body_ (from to .) annotation _the second, that lakes may be made of other substances, as madder, rue,_ &c. _but that alcalizate salts do not always extract the same colour of which the vegetable appears_ (from to .) annotation _the third, that the_ experiments _related may hint divers others_ ( ) annotation _the fourth, that alum is usefull for the preparing other than vegetable pigments_ ( .) _the fiftieth_ experiment, _of the similar effects of_ saccarum saturni _and_ alkalies, _of precipitating with oyl of_ vitriol _out of_ aqua-fortis, _and spirit of_ vinegar; _and of divers varyings of the colours, with these compounded_ (from to .) _another very pretty_ experiment, _with a solution of_ minium ( , .) _that these_ experiments _skilfully digested may hint divers matters about colours_ ( .) _the authors apologetick conclusion, in which is cursorily hinted the bow or scarlet dye_ ( .) _the authors letter to sir_ robert moray, _concerning his observations on the shining diamond_ ( . &c.) _and the observations themselves_. * * * * * errata. pag. . l. . these words, _and to manifest_, with the rest of what is by a mistake further printed in this fourth experiment, belongeth, and is to be referred to the end of the second eperiment, p. . pag. . l. . leg. _matter_. . l. . leg. _bolts-head_. pag . in the marginal note l. . dele _de_ ib. l. . lege lib . p . l. ult. insert _where_ between the words _places_ and _the_. p. l. . dele _that_. ibid, l. . leg _epidermis_. ibid. l. leg. . for . p. . l. . leg. _into it_. p. . l. . & . leg. _some solutions hereafter to be mentioned_, for _the solutions of potashes_, and other _lixiviate salts_. p. . l. . insert _part of_ between the words _most_ and _dissolved_ p. . l. ult. insert the participle _it_ between the words _judged_ and _not_ p. . l. . leg. _woud-wax_ or _wood-wax_. p. l. . leg. _urine_ for _urne_. * * * * * _the_ _experimental history_ _of colours begun._ the first part. chap. i. i have seen you so passionately addicted, _pyrophilus_ to the delightful art of limning and painting, that i cannot but think my self obliged to acquaint you with some of those things that have occurred to mee concerning the changes of colours. and i may expect that i shall as well serve the _virtuosi_ in general, as gratifie you in particular, by furnishing a person, who, i hope, will both improve my communications, and communicate his improvements, with such experiments and observations as may both invite you to enquire seriously into the nature of colours, and assist you in the investigation of it. this being the principal scope of the following tract, i should do that which might prevent my own design, if i should here attempt to deliver you an accurate and particular theory of colours; for that were to present you with what i desire to receive from you; and, as farr as in mee lay, to make that study needless, to which i would engage you. wherefore my present work shall be but to divert and recreate, as well as excite you by the delivery of matters of fact, such as you may for the most part try with much _ease_, and possibly not without some _delight_: and lest you should expect any thing of elaborate or methodical in what you will meet with here, i must confess to you before-hand, that the seasons i was wont to chuse to devise and try experiments about colours, were those daies, wherein having taken physick, and finding my self as unfit to speculate, as unwilling to be altogether idle, i chose this diversion, as a kind of mean betwixt the one and the other. and i have the less scrupled to set down the following experiments, as some of them came to my mind, and as the notes wherein i had set down the rest, occurr'd to my hands, that by declining a methodical way of delivering them, i might leave you and my self the greater liberty and convenience to add to them, and transpose them as shall appear expedient. yea, that you may not think mee too reserv'd, or look upon an enquiry made up of meer narratives, as somewhat jejune, am content to _premise_ a few considerations, that now offer themselves to my thoughts, which relate in a more general way, either to the nature of colours, or to the study of it. and i shall _insert_ an _essay_, as well speculative as historical, of the nature of whiteness and blackness, that you may have a _specimen_ of the history of colours, i have sometimes had thoughts of; and if you dislike not the method i have made use of, i hope, you, and some of the _virtuosi_, your friends, may be thereby invited to go thorow with _red, blew, yellow_, and the rest of the particular colours, as i have done with _white_ and _black_, but with farr more sagacity and success. and if i can invite ingenious men to undertake such tasks, i doubt not but the curious will quickly obtain a better account of colours, than as yet we have, since in our method the theorical part of the enquiry being attended, and as it were interwoven with the historical, whatever becomes of the disputable conjectures, the philosophy of colours will be promoted by the indisputable experiments. * * * * * chap. ii. to come then in the first place to our more general considerations, i shall begin with saying something as to the importance of examining the colours of bodies. for there are some, especially _chymists_, who think, that a considerable diversity of colours does constantly argue an equal diversity of nature, in the bodies wherein it is conspicuous; but i confess i am not altogether of their mind; for not to mention changeable taffaties, the blew and golden necks of pidgeons, and divers water-fowl, rainbows natural and artificial, and other bodies, whose colours the philosophers have been pleased to call not real, but apparent and phantastical; not to insist on these, i say, (for fear of needlesly engaging in a controversie) we see in parrots, goldfinches, and divers other birds, not only that the contiguous feathers which are probably as near in properties as place, are some of them red, and others white, some of them blew, & others yellow, _&c._ but that in the several parts of the self-same feather there may often be seen the greatest disparity of colours; and so in the leaves of tulips, july-flowers, and some other vegetables the several leaves, and even the several parts of the same leaf, although no difference have been observed in their other properties, are frequently found painted with very different colours. and such a variety we have much more admired in that lovely plant which is commonly, and not unjustly call'd the _marvayl of peru_; for of divers scores of fine flowers, which in its season that gaudy plant does almost daily produce, i have scarce taken notice of any two that were dyed perfectly alike. but though _pyro_: such things as these, among others, keep mee from daring to affirm, that the diversity and change of colours does _alwaies_ argue any great difference or alteration, betwixt, or in, the bodies, wherein it is to be discerned, yet that _oftentimes_ the alteration of colours does signifie considerable alterations in the disposition of parts of bodies, may appear in the extraction of tinctures, and divers other chymical operations, wherein the change of colours is the chief, and sometimes the only thing, by which the artist regulates his proceeding, and is taught to know when 'tis seasonable for him to leave off. instances of this sort are more obvious in divers sorts of fruits, as cherries, plums, &c. wherein, according as the vegetable sap is sweetned, or otherwise ripened, by passing from one degree to another of maturation, the external part of the fruit passes likewise from one to another colour. but one of the noblest instances i have met with of this kind, is not so obvious; and that is the way of tempering steel to make gravers, drills, springs, and other mechanical instruments, which we have divers times both made artificers practise in our presence, and tryed our selves, after the following manner, first, the slender steel to be tempered is to be hardened by heating as much of it as is requisite among glowing coals, till it be glowing hot, but it must not be quenched assoon as it is taken from the fire (for that would make it too brittle, and spoil it) but must be held over a bason of water, till it descend from a white heat to a red one, which assoon as ever you perceive, you must immediately quench as much as you desire to harden in the cold water. the steel thus hardened, will, if it be good, look somewhat white and must be made bright at the end, that its change of colours may be there conspicuous; and then holding it so in the flame of a candle, that the bright end may be, for about half an inch, or more, out of the flame, that the smoak do not stain or sully the brightness of it, you shall after a while see that clean end, which is almost contiguous to the flame, pass very nimbly from one colour to another, as from a brighter yellow, to a deeper and reddish yellow, which artificers call a _sanguine_, and from that to a fainter first, and then a a deeper blew. and to bring home this experiment to our present purpose, it is found by daily experience, that each of these succeeding colours argue such a change made in the texture of the steel, that if it be taken from the flame, and immediately quenched in the tallow (whereby it is setled in whatever temper it had before) when it is yellow, it is of such a hardness as makes it fit for gravers drills, and such like tools; but if it be kept a few minutes longer in the flame till it grow blew, it becomes much softer, and unfit to make gravers for metalls, but fit to make springs for watches, and such like instruments, which are therefore commonly of that colour; and if the steel be kept in the flame, after that this deep blew hath disclosed it self, it will grow so soft, as to need to be new hardened again, before it can be brought to a temper, fit for drills or penknives. and i confess _pyro._ i have taken much pleasure to see the colours run along from the parts of the steel contiguous to the flame, to the end of the instrument, and succeed one another so fast, that if a man be not vigilant, to thrust the steel into the tallow at the very nick of time, at which it has attain'd its due colour, he shall miss of giving his tool the right temper. but because the flame of a candle is offensive to my weak eyes, and because it is apt to either black or sully the contiguous part of the steel which is held in it, and thereby hinder the change of colours from being so long and clearly discern'd, i have sometimes made this experiment by laying the steel to be tempered upon a heated bar of iron, which we finde also to be employ'd by some artificers in the tempering of such great instruments, as are too big to be soon heated sufficiently by the flame of a candle. and you may easily satisfie your self _pyro_: of the differing hardness and toughness, which is ascribed to steel temper'd at different colours, if you break but some slender wires of steel so temper'd, and observe how they differ in brittleness, and if with a file you also make tryal of their various degrees of hardness. but _pyrophilus_, i must not at present any further prosecute the consideration of the importance of experiments about colours, not only because you will in the following papers finde some instances, that would here be presented you out of their due place, of the use that may be made of such experiments, in discovering in divers bodies, what kind the salt is, that is predominant in them; but also because a speculative naturalist might justly enough allege, that as light is so pleasing an object, as to be well worth our looking on, though it discover'd to us nothing but its self; so modifi'd light called colour, were worth our contemplation, though by understanding its nature we should be taught nothing else. and however, i need not make either you or my self excuses for entertaining you on the subject i am now about to treat of, since the pleasure _pyro_: takes in mixing and laying on of colours, will i presume keep him, and will (i am sure) keep mee from thinking it troublesome to set down, especially after the tedious processes (about other matters) wherewith i fear i may have tyr'd him, some easie, and not unpleasant experiments relating to that subject. but, before we descend to the more particular considerations, we are to present you concerning colours, i presume it will be seasonable to propose at the very entrance a distinction; the ignorance or neglect of which, seems to mee to have frequently enough occasioned either mistakes or confusion in the writings of divers modern philosophers; for colour may be considered, either as it is a quality residing in the body that is said to be coloured, or to modifie the light after such or such a manner; or else as the light it self, which so modifi'd, strikes upon the organ of sight, and so causes that sensation which we call colour; and that this latter may be look'd upon as the more proper, though not the usual acception of the word colour, will be made probable by divers passages in the insuing part of our discourse; and indeed it is the light it self, which after a certain manner, either mingled with shades, or some other waies troubled, strikes our eyes, that does more immediately produce that motion in the organ, upon whose account men say they see such or such a colour in the object; yet, because there is in the body that is said to be coloured, a certain disposition of the superficial particles, whereby it sends the light reflected, or refracted, to our eyes thus and thus alter'd, and not otherwise, it may also in some sense be said, that colour depends upon the visible body; and therefore we shall not be against that way of speaking of colours that is most used among the modern naturalists, provided we be allowed to have recourse when occasion shall require to the premis'd distinction, and to take the more immediate cause of colour to be the modifi'd light it self, as it affects the sensory; though the disposition also of the colour'd body, as that modifies the light, may be call'd by that name metonimically (to borrow a school term) or efficiently, that is in regard of its turning the light, that rebounds from it, or passes thorow it, into this or that particular colour. i know not whether i may not on this occasion add, that colour is so far from being an inherent quality of the object in the sense that is wont to be declar'd by the schools, or even in the sense of some modern atomists, that, if we consider the matter more attentively, we shall see cause to suspect, if not to conclude, that though light do more immediately affect the organ of sight, than do the bodies that send it thither, yet light it self produces the sensation of a colour, but as it produces such a determinate kind of local motion in some part of the brain; which, though it happen most commonly from the motion whereinto the slender strings of the _retina_ are put, by the appulse of light, yet if the like motion happen to be produc'd by any other cause, wherein the light concurrs not at all, a man shall think he sees the same colour. for proof of this, i might put you in mind, that 'tis usual for dreaming men to think they see the images that appear to them in their sleep, adorn'd some with this, and some with that lively colour, whilst yet, both the curtains of their bed, and those of their eyes are close drawn. and i might add the confidence with which distracted persons do oftentimes, when they are awake, think, they see black fiends in places, where there is no black object in sight without them. but i will rather observe, that not only when a man receives a great stroak upon his eye, or a very great one upon some other part of his head, he is wont to see, as it were, flashes of lightning, and little vivid, but vanishing flames, though perhaps his eyes be shut: but the like apparitions may happen, when the motion proceeds not from something without, but from something within the body, provided the unwonted fumes that wander up and down in the head, or the propagated concussion of any internal part in the body, do cause about the inward extremities of the optick nerve, such a motion as is wont to be there produc'd, when the stroak of the light upon the _retina_ makes us conclude, that we see either light, or such and such a colour: this the most ingenious _des cartes_ hath very well observ'd, but because he seems not to have exemplifi'd it by any unobvious or peculiar observation, i shall indeavour to illustrate this doctrine by a few instances. and first, i remember, that having, through gods goodness, been free for several years, from troublesome coughs, being afterwards, by an accident, suddenly cast into a violent one, i did often, when i was awaked in the night by my distempers, observe, that upon coughing strongly, it would seem to mee, that i saw very vivid, but immediately disappearing flames, which i took particular notice of, because of the conjecture i am now mentioning. an excellent and very discreet person, very near ally'd both to you and mee, was relating to mee, that some time since, whilst she was talking with some other ladies, upon a sudden, all the objects, she looked upon, appeared to her dyed with unusual colours, some of one kind, and some of another, but all so bright and vivid, that she should have been as much delighted, as surpriz'd with them, but that finding the apparition to continue, she fear'd it portended some very great alteration as to her health: as indeed the day after she was assaulted with such violence by hysterical and hypocondrical distempers, as both made her rave for some daies, and gave her, during that time, a bastard palsey. being a while since in a town, where the plague had made great havock, and inquiring of an ingenious man, that was so bold, as without much scruple to visit those that were sick of it, about the odd symptomes of a disease that had swept away so many there; he told mee, among other things, that he was able to tell divers patients, to whom he was called, before they took their beds, or had any evident symptomes of the plague, that they were indeed infected upon peculiar observations, that being asked, they would tell him that the neighbouring objects, and particularly his cloths, appear'd to them beautifi'd with most glorious colours, like those of the rainbow, oftentimes succeeding one another; and this he affirm'd to be one of the most usual, as well as the most early symptomes, by which this odd pestilence disclos'd it self: and when i asked how long the patients were wont to be thus affected, he answered, that it was most commonly for about a day; and when i further inquired whether or no vomits, which in that pestilence were usually given, did not remove this symptome (for some used the taking of a vomit, when they came ashore, to cure themselves of the obstinate and troublesome giddiness caus'd by the motion of the ship) reply'd, that generally, upon the evacuation made by the vomit, that strange apparition of colours ceased, though the other symptomes were not so soon abated, yet he added (to take notice of that upon the by, because the observation may perchance do good) that an excellent physician, in whose company he was wont to visit the sick, did give to almost all those to whom he was called, in the beginning before nature was much weakened, a pretty odd vomit consisting of eight or ten dramms of infusion of _crocus metallorum_, and about half a dramm, or much more, of white vitriol, with such success, that scarce one of ten to whom it was seasonably administred, miscarried. but to return to the consideration of colours: as an apparition of them may be produced by motions from within, without the assistance of an outward object, so i have observed, that 'tis sometimes possible that the colour that would otherwise be produced by an outward object, may be chang'd by some motion, or new texture already produced in the sensory, as long as that unusual motion, or new disposition lasts; for i have divers times try'd, that after i have through a telescope look'd upon the sun, though thorow a thick, red, or blew glass, to make its splendor supportable to the eye, the impression upon the _retina_, would be not only so vivid, but so permanent, that if afterwards i turned my eye towards a flame, it would appear to mee of a colour very differing from its usual one. and if i did divers times successively shut and open the same eye, i should see the adventitious colour, (if i may so call it) changed or impair'd by degrees, till at length (for this unusual motion of the eye would not presently cease) the flame would appear to mee, of the same hew that it did to other beholders; a not unlike effect i found by looking upon the moon, when she was near full, thorow an excellent telescope, without colour'd glass to screen my eye with; but that which i desire may be taken notice of, because we may elsewhere have occasion to reflect upon it, and because it seems not agreeable to what anatomists and optical writers deliver, touching the relation of the two eyes to each other, is this circumstance, that though my right eye, with which i looked thorow the telescope, were thus affected by the over-strong impression of the light, yet when the flame of a candle, or some other bright object appear'd to me of a very unusual colour, whilst look'd upon with the discompos'd eye, or (though not so notably) with both eyes at once; yet if i shut that eye, and looked upon the same object with the other, it would appear with no other than its usual colour, though if i again opened, and made use of the dazled eye, the vivid adventitious colour would again appear. and on this occasion i must not pretermit an observation which may perswade us, that an over-vehement stroak upon the sensory, especially if it be naturally of a weak constitution, may make a more lasting impression than one would imagine, which impression may in some cases, as it were, mingle with, and vitiate the action of vivid objects for a long time after. for i know a lady of unquestionable veracity, who having lately, by a desperate fall, receiv'd several hurts, and particularly a considerable one upon a part of her face near her eye, had her sight so troubl'd and disorder'd, that, as she hath more than once related to me, not only when the next morning one of her servants came to her bed side, to ask how she did, his cloaths appear'd adorn'd with such variety of dazling colours, that she was fain presently to command him to withdraw, but the images in her hangings, did, for many daies after, appear to her, if the room were not extraordinarily darken'd, embellish'd with several offensively vivid colours, which no body else could see in them; and when i enquir'd whether or no white objects did not appear to her adorn'd with more luminous colours than others, and whether she saw not some which she could not now well describe to any, whose eyes had never been distemper'd, she answer'd mee, that sometimes she thought she saw colours so new and glorious, that they were of a peculiar kind, and such as she could not describe by their likeness to any she had beheld either before or since, and that white objects did so much disorder her sight, that if several daies after her fall, she look'd upon the inside of a book, she fanci'd she saw there colours like those of the rain-bow, and even when she thought her self pretty well recover'd, and made bold to leave her chamber, the coming into a place where the walls and ceeling were whited over, made those objects appear to her cloath'd with such glorious and dazling colours, as much offended her sight, and made her repent her venturousness, and she added, that this distemper of her eyes lasted no less than five or six weeks, though, since that, she hath been able to read and write much without finding the least inconvenience in doing so. i would gladly have known, whether if she had shut the injur'd eye, the _phænomena_ would have been the same, when she employ'd only the other, but i heard not of this accident early enough to satisfie that enquiry. wherefore, i shall now add, that some years before, a person exceedingly eminent for his profound skil in almost all kinds of philological learning, coming to advise with mee about a distemper in his eyes, told me, among other circumstances of it, that, having upon a time looked too fixedly upon the sun, thorow a telescope, without any coloured glass, to take off from the dazling splendour of the object, the excess of light did so strongly affect his eye, that ever since, when he turns it towards a window, or any white object, he fancies, he seeth a globe of light, of about the bigness the sun then appeared of to him, to pass before his eyes: and having inquir'd of him, how long he had been troubled with this indisposition, he reply'd, that it was already nine or ten years, since the accident, that occasioned it, first befel him. i could here subjoyn, _pyrophilus_, some memorable relations that i have met with in the account given us by the experienc'd _epiphanius ferdinandus_, of the symptomes he observ'd to be incident to those that are bitten with the tarantula, by which (relations) i could probably shew, that without any change in the object, a change in the instruments of vision may for a great while make some colours appear charming, and make others provoking, and both to a high degree, though neither of them produc'd any such effects before. these things, i say, i could here subjoyn in confirmation of what i have been saying, to shew, that the disposition of the organ is of great importance in the dijudications we make of colours, were it not that these strange stories belonging more properly to another discourse, i had rather, (contenting my self to have given you an intimation of them here) that you should meet with them fully deliver'd there. * * * * * chap. iii. but, _pyrophilus_, i would not by all that i have hitherto discours'd, be thought to have forgotten the distinction (of colour) that i mentioned to you about the beginning of the third section of the former chapter; and therefore, after all i have said of colour, as it is modifi'd light, and immediately affects the sensory, i shall now re-mind you, that i did not deny, but that colour might in some sense be consider'd as a quality residing in the body that is said to be colour'd, and indeed the greatest part of the following experiments referr to colour principally under that notion, for there is in the bodyes we call colour'd, and chiefly in their superficial parts, a certain disposition, whereby they do so trouble the light that comes from them to our eye, as that it there makes that distinct impression, upon whose account we say, that the seen body is either white or black, or red or yellow, or of any one determinate colour. but because we shall (god permiting) by the experiments that are to follow some pages hence, more fully and particularly shew, that the changes, and consequently in divers places the production and the appearance of colours depends upon the continuing or alter'd texture of the object, we shall in this place intimate (and that too but as by the way) two or three things about this matter. . and first it is not without some reason, that i ascribe colour (in the sense formerly explan'd) _chiefly_ to the superficial parts of bodies, for not to question how much opacous corpuscles may abound even in those bodies we call diaphanous, it seems plain that of opacous bodies we do indeed see little else than the superficies, for if we found the beams of light that rebound from the object to the eye, to peirce deep into the colour'd body, we should not judge it opacous, but either translucid, or at least semi-diaphanous, and though the schools seem to teach us that colour is a penetrative quality, that reaches to the innermost parts of the object, as if a piece of sealing-wax be broken into never so many pieces, the internal fragments will be as red as the external surface did appear, yet that is but a particular example that will not overthrow the reason lately offer'd, especially since i can alleage other examples of a contrary import, and two or three negative instances are sufficient to overthrow the generality of a positive rule, especially if that be built but upon one or a few examples. not (then) to mention cherries, plums, and i know not how many other bodies, wherein the skin is of one colour, and what it hides of another, i shall name a couple of instances drawn from the colours of durable bodies that are thought far more homogeneous, and have not parts that are either organical, or of a nature approaching thereunto. to give you the first instance, i shall need but to remind you of what i told you a little after the beginning of this essay, touching the blew and red and yellow, that may be produc'd upon a piece of temper'd steel, for these colours though they be very vivid, yet if you break the steel they adorn, they will appear to be but superficial; not only the innermost parts of the metall, but those that are within a hairs breadth of the superficies, having not any of these colours, but retaining that of the steel it self. besides that, we may as well confirm this observation, as some other particulars we elsewhere deliver concerning colours, by the following experiment which we purposely made. we took a good quantity of clean lead, and melted it with a strong fire, and then immediately pouring it out into a clean vessel of a convenient shape and matter, (we us'd one of iron, that the great and sudden heat might not injure it) and then carefully and nimbly taking off the scum that floated on the top, we perceiv'd, as we expected, the smooth and glossie surface of the melted matter, to be adorn'd with a very glorious colour, which being as transitory as delightfull, did almost immediately give place to another vivid colour, and that was as quickly succeeded by a third, and this as it were chas'd away by a fourth, and so these wonderfully vivid colours successively appear'd and vanish'd, (yet the same now and then appearing the second time) till the metall ceasing to be hot enough to afford any longer this pleasing spectacle, the colours that chanc'd to adorn the surface, when the lead thus began to cool, remain'd upon it; but were so superficial, that how little soever we scrap'd off the surface of the lead, we did in such places scrape off all the colour, and discover only that which is natural to the metall it self, which receiving its adventitious colours, only when the heat was very intense, and in that part which was expos'd to the comparatively very cold air, (which by other experiments seems to abound with subtil saline parts, perhaps not uncapable of working upon lead so dispos'd:) these things i say, together with my observing that whatever parts of the so strongly melted lead were expos'd a while to the air, turn'd into a kind of scum or litharge, how bright and clean soever they appear'd before, suggested to me some thoughts or ravings, which i have not now time to acquaint you with. one that did not know me, _pyrophilus_, would perchance think i endeavour'd to impose upon you by relating this experiment, which i have several times try'd, but the reason why the _phænomena_ mention'd have not been taken notice of, may be, that unless lead be brought to a much higher degree of fusion or fluidity than is usual, or than is indeed requisite to make it melt, the _phænomena_ i mention'd will scarce at all disclose themselves; and we have also observ'd that this successive appearing and vanishing of vivid colours, was wont to be impair'd or determin'd whilst the metal expos'd to the air remain'd yet hotter than one would readily suspect. and one thing i must further note, of which i leave you to search after the reason, namely, that the same colours did not always and regularly succeed one another, as is usually in steel, but in the diversify'd order mention'd in this following note, which i was scarce able to write down, the succession of the colours was so very quick, whether that proceeded from the differing degrees of heat in the lead expos'd to the cool air, or from some other reason, i leave you to examine. [_blew, yellow, purple, blew; green, purple, blew, yellow, red; purple, blew, yellow and blew, yellow, blew, purple, green mixt, yellow, red, blew, green, yellow, red, purple, green_.] . the _atomists_ of old, and some learned men of late, have attempted to explicate the variety of colours in opacous bodies from the various figures of their superficial parts; the attempt is ingenious, and the doctrine seems partly true, but i confess i think there are divers other things that must be taken in as concurrent to produce those differing forms of asperity, whereon the colours of opacous bodies seem to depend. to declare this a little, we must assume, that the surfaces of all such bodies how smooth or polite soever they may appear to our dull sight and touch, are exactly smooth only in a popular, or at most in a physical sense, but not in a strict and rigid sense. . this, excellent _microscopes_ shew us in many bodies, that seem smooth to our naked eyes; and this not only as to the little hillocks or protuberancies that swell above that which may be conceiv'd to be the plain or level of the consider'd surface, for it is obvious enough to those that are any thing conversant with such glasses, but as to numerous depressions beneath that level, of which sort of cavities by the help of a _microscope_, which the greatest artificer that makes them, judges to be the greatest magnifying glass in _europe_, except one that equals it, we have on the surface of a thin piece of cork that appear'd smooth to the eye, observ'd about sixty in a row, within the length of less then an and part of an inch, (for the glass takes in no longer a space at one view) and these cavities (which made that little piece of cork look almost like an empty honey-comb) were not only very distinct, and figur'd like one another, but of a considerable bigness, and a scarce credible depth; insomuch that their distinct shadows as well as sides were plainly discern'd and easiy to be reckon'd, and might have been well distinguish'd, though they had been ten times lesser than they were; which i thought it not amiss to mention to you _pyrophilus_ upon the by, that you may thence make some estimate, what a strange inequality, and what a multitude of little shades, there may really be, in a scarce sensible part of the physical superficies, though the naked eye sees no such matter. and as excellent _microscopes_ shew us this ruggedness in many bodies that pass for smooth, so there are divers experiments, though we must not now stay to urge them, which seem to perswade us of the same thing as to the rest of such bodies as we are now treating off; so, that there is no sensible part of an opacous body, that may not be conceiv'd to be made up of a multitude of singly insensible corpuscles, but in the giving these surfaces that disposition, which makes them alter the light that reflects thence to the eye after the manner requisite to make the object appear green, blew, &c. the figures of these particles have _a great_, but not _the only_ stroak. 'tis true indeed that the protuberant particles may be of very great variety of figures, sphærical, elliptical, conical, cylindrical, polyedrical, and some very irregular, and that according to the nature of these, and the situation of the lucid body, the light must be variously affected, after one manner from surfaces (i now speak of physical surfaces) consisting of sphaerical, and in another from those that are made up of conical or cylindrical corpuscles; some being fitted to reflect more of the incident beams of light, others less, and some towards one part, others towards another. but besides this difference of shape, there may be divers other things that may eminently concurr to vary the forms of asperity that colours so much depend on. for, willingly allowing the figure of the particles in the first place, i consider secondly, that the superficial corpuscles, if i may so call them, may be bigger in one body, and less in another, and consequently fitted to allay the light falling on them with greater shades. next, the protuberant particles may be set more or less close together, that is, there may be a greater or a smaller number of them within the compass of one, than within the compass of another small part of the surface of the same extent, and how much these qualities may serve to produce colour may be somewhat guess'd at, by that which happens in the agitation of water; for if the bubbles that are thereby made be great, and but few, the water will scarce acquire a sensible colour, but if it be reduc'd to a froth, consisting of bubbles, which being very minute and contiguous to each other, are a multitude of them crowded into a narrow room, the water (turned to froth) does then exhibit a very manifest white colour,[ ] (to which these last nam'd conditions of the bubbles do as well as their convex figure contribute) and that for reasons to be mention'd anon. besides, it is not necessary that the superficial particles that exhibit one colour, should be all of them round, or all conical, or all of any one shape, but corpuscles of differing figures may be mingled on the surface of the opacous body, as when the corpuscles that make a blew colour, and those that make a yellow, come to be accurately and skilfully mix'd, they make up a green, which though it seem one simple colour, yet in this case appears to be made by corpuscles of very differing kinds, duely commix'd. moreover the figure and bigness of the little depressions, cavities, furrows or pores intercepted betwixt these protuberant corpuscles, are as well to be consider'd as the sizes and shapes of the corpuscles themselves: for we may conceive the physical superficies of a body, where (as we said) its colour does as it were reside, to be cut transversly by a mathematical plain, which you know is conceiv'd to be without any depth or thickness at all, and then as some parts of the physical superficies will be protuberant; or swell above this last plain, so others may be depress'd beneath it; as (to explane my self by a gross comparison) in divers places of the surface of the earth, there are not only neighbouring hills, trees, &c. that are rais'd above the horizontal level of the valley, but rivers, wells, pits and other cavities that are depress'd beneath it, and that such protuberant and concave parts of a surface may remit the light so differingly, as much to vary a colour, some examples and other things, that we shall hereafter have occasion to take notice off in this tract, will sufficiently declare, till when, it may suffice to put you in mind, that of two flat-sides of the same piece of, for example, red marble, the one being diligently polished, and the other left to its former roughness, the differing degrees or sorts of asperity, for the side that is smooth to the touch wants not its roughness, will so diversifie the light reflected from the several plains to the eye, that a painter would employ two differing colours to represent them. [ ] _see the discourse of the nature of whiteness and blackness._ . and i hope, _pyrophilus_, you will not think it strange or impertinent, that i employ in divers passages of these papers, examples drawn from bodies and shadows far more gross, than those minute protuberances and shady pores on which in most cases the colour of a body as 'tis an inherent quality or disposition of its surface, seems to depend. for sometimes i employ such examples, rather to declare my meaning, than prove my conjecture; things, whom their smallness makes insensible, being better represented to the imagination by such familiar objects, as being like them enough in other respects, are of a visible bulk. and next, though the beams of light are such subtil bodies, that in respect of them, even surfaces that are sensibly smooth, are not exactly so, but have their own degree of roughness, consisting of little protuberances and depressions; and though consequently such inequalities may suffice to give bodies differing colours, as we see in marble that appears white or black, or red or blew, even when the most carefully polish'd, yet 'tis plain by the late instance of red marble, and many others, that even bigger protuberances and greater shades may likewise so diversifie the roughness of a bodies superficies, as manifestly to concurr to the varying of its colour, whereby such examples appear to be proper enough to be employ'd in such a subject as we have now in hand. and having hinted thus much on this occasion, i now proceed. . the situation also of the superficial particles is considerable, which i distinguish into the posture of the single corpuscles, in respect of the light, and of the eye, and the order of them in reference also to one another; for a body may otherwise reflect the light, when its superficial particles are more erected upon the plain that may be conceiv'd to pass along their basis, and when the points or extremes of such particles are obverted to the eye, than when those particles are so inclin'd, that their sides are in great part discernable, as the colour of plush or velvet will appear vary'd to you, if you carefully stroak part of it one way, and part of it another, the posture of the particular thrids, in reference to the light, or the eye, becoming thereby different. and you may observe in a field of ripe corn blown upon by the wind, that there will appear as it were waves of a colour (at least gradually) differing from that of the rest of the field, the wind by depressing some of the ears, and not at the same time others, making the one reflect more from the lateral and strawy parts, than do the rest. and so, when doggs are so angry, as to erect the hairs upon their necks, and upon some other parts of their bodies, those parts seem to acquire a colour vary'd from that which the same hairs made, when in their usual posture they did farr more stoop. and that the order wherein the superficial corpuscles are rang'd is not to be neglected, we may guess by turning of water into froth, the beating of glass, and the scraping of horns, in which cases the corpuscles that were before so marshall'd as to be perspicuous, do by the troubling of that order become dispos'd to terminate and reflect more light, and thereby to appear whitish. and there are other ways in which the order of the protuberant parts, in reference to the eye, may much contribute to the appearing of a particular colour, for i have often observ'd, that when pease are planted, or set in parallel lines, and are shot up about half a foot above the surface of the ground, by looking on the field or plot of ground from that part towards which the parallel lines tended, the greater part of the ground by farr would appear of its own dirty colour, but if i look'd upon it transversly, the plot would appear very green, the upper parts of the pease hindering the intercepted parts of the ground, which as i said retain'd their wonted colour, from being discover'd by the eye. and i know not, _pyrophilus_, whether i might not add, that even the motion of the small parts of a visible object may in some cases contribute, though it be not so easie to say how, to the producing or the varying of a colour; for i have several times made a liquor, which when it has well settled in a close vial, is transparent and colourless, but as soon as the glass is unstopp'd, begins to fly away very plentifully in a white and opacous fume; and there are other bodies, whose fumes, when they fill a receiver, would make one suspect it contains milk, and yet when these fumes settle into a liquor, that liquor is not white, but transparent; and such white fumes i have seen afforded by unstopping a liquor i know, which yet is it self diaphanous and red; nor are these the only instances of this kind, that our tryals can supply us with. and if the superficial corpuscles be of the grosser sort, and be so framed, that their differing sides or faces may exhibit differing colours, then the motion or rest of those corpuscles may be considerable, as to the colour of the superficies they compose, upon this account, that sometimes more, sometimes fewer of the sides dispos'd to exhibit such a colour may by this means become or continue more obverted to the eye than the rest, and compose a physical surface, that will be more or less sensibly interrupted; as, to explane my meaning, by proposing a gross example, i remember, that in some sorts of leavy plants thick set by one another, the two sides of whose leaves were of somewhat differing colours, there would be a notable disparity as to colour, if you look'd upon them both when the leaves being at rest had their upper and commonly expos'd sides obverted to the eye, and when a breath of wind passing thorow them, made great numbers of the usually hidden sides of the leaves become conspicuous. and though the little bodies, we were lately speaking of, may singly and apart seem almost colourless, yet when many of them are plac'd by one another, so near, that the eye does not easily discern an interruption, within a sensible space, they may exhibit a colour; as we see, that though a slenderest thrid of dy'd silk do's, whilst look'd on single, seem almost quite devoyd of redness, (for instance) yet when numbers of these thrids are brought together into one skein, their colour becomes notorious. . but the same occasion that invited me to say what i have mention'd concerning the leaves of trees, invites me also to give you some account of what happens in changeable taffities, where we see differing colours, as it were, emerge and vanish upon the ruffling of the same piece of silk: as i have divers times with pleasure observ'd, by the help of such a _microscope_, as, though it do not very much magnifie the object, has in recompence this great conveniency, that you may easily, as fast as you please, remove it from one part to another of a large object, of which the glass taking a great part at once, you may thereby presently survey the whole. now by the help of such a _microscope_ i could easily (as i began to say) discern, that in a piece of changeable taffity, (that appear'd, for instance, sometimes red, and sometimes green) the stuff was compos'd of red thrids and green, passing under and over each other, and crossing one another in almost innumerable points; and if i look'd through the glass upon any considerable portion of the stuff, that (for example sake) to the naked eye appear'd to be red, i could plainly see, that in that position, the red thrids were conspicuous, and reflected a vivid light; and though i could also perceive, that there were green ones, yet by reason of their disadvantagious position in the _physical surface_ of the taffity, they were in part hid by the more protuberant thrids of the other colour; and for the same cause, the reflection from as much of the green as was discover'd, was comparatively but dim and faint. and if, on the contrary, i look'd through the _microscope_ upon any part that appear'd green, i could plainly see that the red thrids were less fully expos'd to the eye, and obscur'd by the green ones, which therefore made up the predominant colour. and by observing the texture of the silken stuff, i could easisy so expose the thrids either of the one colour or of the other to my eye, as at pleasure to exhibit an apparition of red or green, or make those colours succeed one another: so that, when i observ'd their succession by the help of the glass, i could mark how the predominant colour did as it were start out, when the thrids that exhibited it came to be advanagiously plac'd; and by making little folds in the stuff after a certain manner, the sides that met and terminated in those folds, would appear to the naked eye, one of them red, and the other green. when thrids of more than two differing colours chance to be interwoven, the resulting changeableness of the taffity may be also somewhat different. but i choose to give an instance in the stuff i have been speaking off, because the mixture being more simple, the way whereby the changeableness is produc'd, may be the more easily apprehended: and though reason alone might readily enough lead a considering man to guess at the explication, in case he knew how changeable taffities are made: yet i thought it not impertinent to mention it, because both scholars and gentlemen are wont to look upon the inquiry into manufactures, as a _mechanick_ imployment, and consequently below them; and because also with such a _microscope_ as i have been mentioning, the discovery is as well pleasant as satisfactory, and may afford hints of the solution of other _phænomena_ of colours. and it were not amiss, that some diligent inquiry were made, whether the _microscope_ would give us an account of the variableness of colour, that is so conspicuous and so delightfull in mother of pearl, in opalls, and some other resembling bodies: for though i remember i did formerly attempt something of that kind (fruitlesly enough) upon mother of pearl, yet not having then the advantage of my best _microscope_, nor some conveniences that might have been wish'd, i leave it to you, who have better eyes, to try what you can do further; since 'twill be _some_ discovery to find, that, in this case, the best eyes and _microscopes_ themselves can make _none_. . i confess, _pyrophilus_, that a great part of what i have deliver'd, (or propos'd rather) concerning the differing forms of asperity in bodies, by which differences the incident light either comes to be reflected with more or less of shade, and with that shade more or less interrupted, or else happens to be also otherwise modify'd or troubl'd, is but conjectural. but i am not sure, that if it were not for the dullness of our senses, either these or some other notions of kin to them, might be better countenanc'd; for i am apt to suspect, that if we were sharp sighted enough, or had such perfect _microscopes_, as i fear are more to be wish'd than hop'd for, our promoted sense might discern in the physical surfaces of bodies, both a great many latent ruggidnesses, and the particular sizes, shapes, and situations of the extremely little bodies that cause them, and perhaps might perceive among other varieties that we now can but imagine, how those little protuberances and cavities do interrupt and dilate the light, by mingling with it a multitude of little and singly undiscernable shades, though some of them more, and some of them less minute, some less, and some more numerous; according to the nature and degree of the particular colour we attribute to the visible object; as we see, that in the moon we can with excellent _telescopes_ discern many hills and vallies, and as it were pits and other parts, whereof some are more, and some less vividly illustrated, and others have a fainter, others a deeper shade, though the naked eye can discern no such matter in that planet. and with an excellent _microscope_, where the _naked_ eye did see but a green powder, the _assisted_ eye as we noted above, could discern particular granules, some of them of a blew, and some of them of a yellow colour, which corpuscles we had beforehand caus'd to be exquisitly mix'd to compound the green. . and, _pyrophilus_, that you may not think me altogether extravagant in what i have said of the possibility, (for i speak of no more) of discerning the differing forms of asperity in the surfaces of bodies of several colours, i'l here set down a memorable particular that chanc'd to come to my knowledge, since i writ a good part of this _essay_; and it is this. meeting casually the other day with the deservedly famous[ ] dr. _j. finch_, extraordinary _anatomist_ to that great patron of the _virtuosi_, the now great duke of _toscany_, and enquiring of this ingenious person, what might be the chief rarity he had seen in his late return out of _italy_ into _england_, he told me, it was a man at _maestricht_ in the low-countrys, who at certain times can discern and _distinguish colours by the touch_ with his fingers. you'l easily conclude, that this is farr more strange, than what i propos'd but as _not impossible_; since the sense of the _retina_ seeming to be much more tender and quick than that of those grosser filaments, nerves or membranes of our fingers, wherewith we use to handle gross and hard bodies, it seems scarce credible, that any accustomance, or diet, or peculiarity of constitution, should enable a man to distinguish with such gross and unsuitable organs, such nice and subtile differences as those of the forms of asperity, that belong to differing colours, to receive whose languid and delicate impressions by the intervention of light, nature seems to have appointed and contexed into the _retina_ the tender and delicate pith of the optick nerve. wherefore i confess, i propos'd divers scruples, and particularly whether the doctor had taken care to bind a napkin or hankerchief over his eyes so carefully, as to be sure he could make no use of his sight, though he had but counterfeited the want of it, to which i added divers other questions, to satisfie my self, whether there were any likelihood of collusion or other tricks. but i found that the judicious doctor having gone farr out of his way, purposely to satisfie himself and his learned prince about this wonder, had been very watchfull and circumspect to keep _himself_ from being impos'd upon. and that he might not through any mistake in point of memory mis-inform _me_, he did me the favour at my request, to look out the notes he had written for his own and his princes information, the summ of which memorials, as far as we shall mention them here, was this, that the doctor having been inform'd at _utrecht_, that there lived one at some miles distance from _maestricht_, who could distinguish colours by the touch, when he came to the last nam'd town, he sent a messenger for him, and having examin'd him, was told upon enquiry these particulars: [ ] since for his eminent qualities and loyalty grac'd, by his majesty, with the honour of knighthood. that the man's name was _john vermaasen_, at that time about years of age; that when he was but two years old, he had the small pox, which rendred him absolutely blind: that at this present he is an _organist_, and serves that office in a publick quire. that the doctor discoursing with him over night, the blind man affirm'd, that he could distinguish colours by the touch, but that he could not do it, unless he were fasting; any quantity of drink taking from him that exquisitness of touch, which is requisite to so nice a sensation. that hereupon the doctor provided against the next morning seven pieces of ribbon, of these seven colours, black, white, red, blew, green, yellow, and gray, but as for _mingled_ colours, this _vermaasen_ would not undertake to discern them, though if offer'd, he would tell that they were _mix'd_. that to discern the colour of the ribbon, he places it betwixt the thumb and the fore-finger, but his most exquisite perception was in his thumb, and much better in the right thumb than in the left. that after the blind man had four or five times told the doctor the several colours, (though blinded with a napkin for fear he might have some sight) the doctor found he was twice mistaken, for he call'd the white black, and the red blew, but still, he, before his errour, would lay them by in pairs, saying, that though he could easily distinguish them from all others, yet those two pairs were not easily distinguish'd amongst themselves, whereupon the doctor desir'd to be told by him what kind of discrimination he had of colours by his touch, to which he gave a reply, for whose sake chiefly i insert all this narrative in this place, namely, that all the difference was more or less asperity, for says he, (i give you the doctor's own words) black feels as if you were feeling needles points, or some harsh sand, and red feels very smooth. that the doctor having desir'd him to tell in order the difference of colours to his touch, he did as follows; black and white are the most asperous or unequal of all colours, and so like, that 'tis very hard to distinguish them, but black is the most rough of the two, green is next in asperity, gray next to green in asperity, yellow is the fifth in degree of asperity, red and blew are so like, that they are as hard to distinguish as black and white, but red is somewhat more asperous than blew, so that red has the sixth place, and blew the seventh in asperity. . to these informations the obliging doctor was pleas'd to add the welcome present of three of those very pieces of ribbon, whose colours in his presence the blind man had distinguished, pronouncing the one gray, the other red, and the third green, which i keep by me as rarities, and the rather, because he fear'd the rest were miscarry'd. . before i saw the notes that afforded me the precedent narrative, i confess i suspected this man might have thus discriminated colours, rather by the smell than by the touch; for some of the ingredients imployed by dyers to colour things, have sents, that are not so languid, nor so near of kin, but that i thought it not impossible that a very critical nose might distinguish them, and this i the rather suspected, because he requir'd, that the ribbons, whose colours he was to name, should be offer'd him fasting in the morning; for i have observ'd in setting doggs, that the feeding of them (especially with some sorts of aliments) does very much impair the exquisite sent of their noses. and though some of the foregoing particulars would have prevented that conjecture, yet i confess to you (_pyrophilus_) that i would gladly have had the opportunity of examining this man my self, and of questioning him about divers particulars which i do not find to have been yet thought upon. and though it be not incredible to me, that since the liquors that dyers imploy to tinge, are qualifi'd to do so by multitudes of little corpuscles of the pigment or dying stuff, which are dissolved and extracted by the liquor, and swim to and fro in it, those corpuscles of colour (as the _atomists_ call them) insinuating themselves into, and filling all the pores of the body to be dyed, may asperate its superficies more or less according to the bigness and texture of the corpuscles of the pigment; yet i can scarce believe, that our blind man could distinguish all the colours he did, meerly by the ribbons having more or less of asperity, so that i cannot but think, notwithstanding this history, that the blind man distinguish'd colours not only by the _degrees_ of asperity in the bodies offer'd to him, but by _forms_ of it, though this (latter) would perhaps have been very difficult for him to make an intelligible mention of, because those minute disparities having not been taken notice of by men for want of touch as exquisite as our blind mans, are things he could not have intelligibly express'd, which will easily seem probable, if you consider, that under the name of sharp, and sweet, and sour, there are abundance of, as it were, immediate peculiar relishes or tasts in differing sorts of wine, which though critical and experienc'd palats can easily discern themselves cannot make them be understood by others, such minute differences not having hitherto any distinct names assign'd them. and it seems that there was somthing in the forms of asperity that was requisite to the distinction of colours, besides the degree of it, since he found it so difficult to distingush black and white from one another, though not from other colours. for i might urge, that he seems not consonant to himself about the _red_, which as you have seen in one place, he represents as somewhat more asperous than the _blew_; and in another, very smooth: but because he speaks of this smoothness in that place, where he mentions the roughness of _black_, we may favourably presume that he might mean but a _comparative smoothness_; and therefore i shall not insist on this, but rather countenance my conjecture by this, that he found it so difficult, not only, to discriminate red and blew, (though the first of our promiscuous experiments will inform you, that the red reflects by great odds more light than the other) but also to distinguish black and white from one another, though not from other colours. and indeed, though in the ribbonds that were offer'd him, they might be almost equally rough, yet in such slender corpuscles as those of colour, there may easily enough be conceiv'd, not only a greater closeness of parts, or else paucity of protuberant corpuscles, and the little extant particles may be otherwise figur'd, and rang'd in the white than in the black, but the cavities may be much deeper in the one than the other. . and perhaps, (_pyrophilus_) it may prove some _illustration of what i mean_, and help you to conceive how _this may_ be, if i represent, that where the particles are so exceeding slender, we may allow the parts expos'd to the sight and touch to be a little convex in comparison of the erected particle of black bodies, as if there were wyres i know not how many times slenderer than a hair: whether you suppose them to be figur'd like needles, or cylindrically, like the hairs of a brush, with hemisphærical (or at least convex) tops, they will be so very slender, and consequently the points both of the one sort and the other so very sharp, that even an exquisite touch will be able to distinguish no greater difference between them, than that which our blind man allow'd, when comparing black and white bodies, he said, that the latter was the less rough of the two. nor is every kind of roughness, though sensible enough, inconsistent with whiteness, there being cases, wherein the physical superficies of a body is made by the same operation both _rough_ and _white_, as when the level surface of clear water being by agitation asperated with a multitude of unequal bubbles, do's thereby acquire a whiteness; and as a smooth piece of glass, by being scratch'd with a diamond, do's in the asperated part of its surface disclose the same colour. but more (perchance) of this elsewhere. . and therefore, we shall here pass by the question, whether any thing might be consider'd about the opacity of the corpuscles of black pigments, and the _comparative_ diaphaneity of those of many white bodies, apply'd to our present case; and proceed, to represent, that the newly mention'd exiguity and shape of the extant particles being suppos'd, it will then be considerable what we lately but hinted, (and therefore must now somewhat explane) that the depth of the little cavities, intercepted between the extant particles, without being so much greater in black bodies than in white ones, as to be perceptibly so to the gross organs of touch, may be very much greater in reference to their disposition of reflecting the imaginary subtile beams of light. for in black bodies, those little intercepted cavities, and other depressions, may be so figur'd, so narrow and so deep, that the incident beams of light, which the more extant parts of the physical superficies are dispos'd to reflect inwards, may be detain'd there, and prove unable to emerge; whilst in a white body, the slender particles may not only by their figure be fitted to reflect the light copiously outwards, but the intercepted cavities being not deep, nor perhaps very narrow, the bottoms of them may be so constituted, as to be fit to reflect outwards much of the light that falls even upon them; as you may possibly better apprehend, when we shall come to treat of whiteness and blackness. in the mean time it may suffice, that you take notice with me, that the blind mans relations import no necessity of concluding, that, though, because, according to the judgment of his touch, black was the roughest, as it is the darkest of colours, therefore white, which (according to us) is the lightest, should be also the smoothest: since i observe, that he makes yellow to be two degrees more asperous than blew, and as much less asperous than green; whereas indeed, yellow do's not only appear to the eye a lighter colour than blew, but (by our first experiment hereafter to be mention'd) it will appear, that yellow reflected much more light than blew, and manifestly more than green, (which we need not much wonder at, since in this colour and the two others (blew and yellow) 'tis not _only_ the _reflected light_ that is to be considered, since to produce both these, _refraction_ seems to intervene, which by its varieties may much alter the case:) which both seems to strengthen the conjecture i was formerly proposing, that there was something else in the _kinds_ of asperity, as well as in the _degrees_ of it, which enabled our blind man to discriminate colours, and do's at least show, that we cannot in all cases from the bare difference in the degrees of asperity betwixt colours, safely conclude, that the rougher of any two always reflects the least light. . but this notwithstanding, (_pyrophilus_) and what ever curiosity i may have had to move some questions to our sagacious blind man, yet thus much i think you will admit us to have gain'd by his testimony, that since many colours may be felt with the circumstances above related, the surfaces of such coloured bodies must certainly have differing _degrees_, and in all probability have differing _forms_ or kinds of asperity belonging to them, which is all the use that my present attempt obliges me to make of the history above deliver'd, that being sufficient to prove, _that_ colour do's much depend upon the disposition of the superficial parts of bodies, and to shew in general, _wherein_ 'tis probable that such a disposition do's (principally at least) consist. . but to return to what i was saying before i began to make mention of our blind _organist_, what we have deliver'd touching the causes of the several forms or asperity that may diversifie the surfaces of colour'd bodies, may perchance somewhat assist us to make some conjectures in the general, at several of the ways whereby 'tis possible for the experiments hereafter to be mention'd, to produce the suddain changes of colours that are wont to be consequent upon them; for most of these _phænomena_ being produc'd by the intervention of liquors, and these for the most part abounding with very minute, active, and variously figur'd saline corpuscles, liquors so qualify'd may well enough very nimbly after the texture of the body they are imploy'd to work upon, and so may change the form of asperity, and thereby make them remit to the eye the light that falls on them, after another manner than they did before, and by that means vary the colour, so farr forth as it depends upon the texture or disposition of the seen parts of the object, which i say, _pyrophilus_, that you may not think i would absolutely exclude all other ways of modifying the beams of light between their parting from the lucid body, and their reception into the common sensory. . now there seem to me divers ways, by which we may conceive that liquors may nimbly alter the colour of one another, and of other bodies, upon which they act, but my present haste will allow me to mention but some of them, without insisting so much as upon those i shall name. . and first, the minute corpuscles that compose a liquor may early insinuate themselves into those pores of bodies, whereto their size and figure makes them congruous, and these pores they may either exactly fill, or but inadequately, and in this latter case they will for the most part alter the number and figure, and always the bigness of the former pores. and in what capacity soever these corpuscles of a liquor come to be lodg'd or harbour'd in the pores that admit them, the surface of the body will for the most part have its asperity alter'd, and the incident light that meets with a grosser liquor in the little cavities that before contain'd nothing but air, or some yet subtiler fluid, will have its beams either refracted, or imbib'd, or else reflected more or less interruptedly, than they would be, if the body had been unmoistned, as we see, that even fair water falling on white paper, or linnen, and divers other bodies apt to soak it in, will for some such reasons as those newly mention'd, immediately alter the colour of them, and for the most part make it sadder than that of the unwetted parts of the same bodies. and so you may see, that when in the summer the high-ways are dry and dusty, if there falls store of rain, they will quickly appear of a much darker colour than they did before, and if a drop of oyl be let fall upon a sheet of white paper, that part of it, which by the imbibition of the liquor acquires a greater continuity, and some transparency, will appear much darker than the rest, many of the incident beams of light being now transmitted, that otherwise would be reflected towards the beholders eyes. . secondly, a liquor may alter the colour of a body by freeing it from those things that hindred it from appearing in its genuine colour; and though this may be said to be rather a restauration of a body to its own colour, or a retection of its native colour, than a change, yet still there intervenes in it a change of the colour which the body appear'd to be of before this operation. and such a change a liquor may work, either by dissolving, or corroding, or by some such way of carrying off that matter, which either veil'd or disguis'd the colour that afterwards appears. thus we restore old pieces of dirty gold to a clean and nitid yellow, by putting them into the fire, and into _aqua-fortis_, which take off the adventitious filth that made that pure metall look of a dirty colour. and there is also an easie way to restore silver coyns to their due lustre, by fetching off that which discolour'd them. and i know a _chymical_ liquor, which i employ'd to restore pieces of cloath spotted with grease to their proper colour, by imbibing the spotted part with this liquor, which incorporating with the grease, and yet being of a very volatile nature, does easily carry it away with it self. and i have sometimes try'd, that by rubbing upon a good touch-stone a certain _metalline_ mixture so compounded, that the impression it left upon the stone appear'd of a very differing colour from that of gold, yet a little of _aqua-fortis_ would in a trice make the golden colour disclose it self, by dissolving the other _metalline_ corpuscles that conceal'd those of the gold, which you know that _menstruum_ will leave untouch'd. . thirdly, a liquor may alter the colour of a body by making a comminution of its parts, and that principally two ways, the first by disjoyning and dissipating those clusters of particles, if i may so call them, which stuck more loosely together, being fastned only by some more easily dissoluble ciment, which seems to be the case of some of the following experiments, where you'l find the colour of many corpuscles brought to cohere by having been precipitated together, destroy'd by the affusion of very peircing and incisive liquors. the other of the two ways i was speaking of, is, by dividing the grosser and more solid particles into minute ones, which will be always lesser, and for the most part otherwise shap'd than the entire corpuscle so divided, as it will happen in a piece of wood reduc'd into splinters or chips, or as when a piece of chrystal heated red hot and quench'd in cold water is crack'd into a multitude of little fragments, which though they fall not asunder, alter the disposition of the body of the chrystal, as to its manner of reflecting the light, as we shall have occasion to shew hereafter. . there is a fourth way contrary to the third, whereby a liquor may change the colour of another body, especially of another fluid, and that is, by procuring the coalition of several particles that before lay too scatter'd and dispers'd to exhibit the colour that afterwards appears. thus sometimes when i have had a solution of gold so dilated, that i doubted whether the liquor had really imbib'd any true gold or no, by pouring in a little _mercury_, i have been quickly able to satisfie my self, that the liquor contain'd gold, that mettall after a little while cloathing the surface of the _quick-silver_, with a thin film of its own livery. and chiefly, though not only by this way of bringing the minute parts of bodies together in such numbers as to make them become notorious to the eye, many of these colours seem to be generated which are produc'd by precipitations, especially by such as are wont to be made with fair water, as when resinous gumms dissolv'd in spirit of wine, are let fall again, if the spirit be copiously diluted with that weakning liquor. and so out of the rectify'd and transparent butter of _antimony_, by the bare mixture of fair water, there will be plentifully precipitated that milk-white substance, which by having its looser salts well wash'd off, is turn'd into that medicine, which vulgar _chymists_ are pleas'd to call _mercurius vitæ._ . a fifth way, by which a liquor may change the colour of a body, is, by dislocating the parts, and putting them out of their former order into another, and perhaps also altering the posture of the single corpuscles as well as their order or situation in respect of one another. what certain kinds of commotion or dislocation of the parts of a body may do towards the changing its colour, is not only evident in the mutations of colour observable in _quick-silver_, and some other concretes long kept by _chymists_ in a convenient heat, though in close vessels, but in the obvious degenerations of colour, which every body may take notice of in bruis'd cherries, and other fruit, by comparing after a while the colour of the injur'd with that of the sound part of the same fruit. and that also such liquors, as we have been speaking of, may greatly discompose the textures of many bodies, and thereby alter the disposition of their superficial parts, the great commotion made in metalls, and several other bodies by _aqua-fortis_, oyl of _vitriol_, and other saline _menstruums_, may easily perswade us, and what such vary'd situations of parts may do towards the diversifying of the manner of their reflecting the light, may be guess'd in some measure by the beating of transparent glass into a white powder, but farr better by the experiments lately pointed at, and hereafter deliver'd, as the producing and destroying colours by the means of subtil saline liquors, by whose affusion the parts of other liquors are manifestly both agitated, and likewise dispos'd after another manner than they were before such affusion. and in some _chymical_ oyls, as particularly that of lemmon pills, by barely shaking the glass, that holds it, into bubbles, that transposition of the parts which is consequent to the shaking, will shew you on the surfaces of the bubbles exceeding orient and lively colours, which when the bubbles relapse into the rest of the oyl, do immediately vanish. . i know not, _pyrophilus_, whether i should mention as a distinct way, because it is of a somewhat more general nature, that power, whereby a liquor may alter the colour of another body, by putting the parts of it into motion; for though possibly the motion so produc'd, does, as such, seldome suddenly change the colour of the body whose parts are agitated, yet this seems to be one of the most general, however not immediate causes of the quick change of colours in bodies. for the parts being put into motion by the adventitious liquor, divers of them that were before united, may become thereby disjoyn'd, and when that motion ceases or decays others of them may stick together, and that in a new order, by which means the motion may sometimes produce permanent changes of colours, as in the experiment you will meet with hereafter, of presently turning a snowy white body into a yellow, by the bare affusion of fair water, which probably so dissolves the saline corpuscles that remain'd in the _calx_, and sets them at liberty to act upon one another, and the metall, far more powerfully than the water without the assistance of such saline corpuscles could do. and though you rubb blew _vitriol_, how venereal and unsophisticated soever it be, upon the whetted blade of a knife, it will not impart to the iron its latent colour, but if you moisten the _vitriol_ with your spittle, or common water, the particles of the liquor disjoyning those of the _vitriol_, and thereby giving them the various agitation requisite to fluid bodies, the metalline corpuscles of the thus dissolv'd _vitriol_ will lodge themselves in throngs in the small and congruous pores of the iron they are rubb'd on, and so give the surface of it the genuine colour of the copper. . there remains yet a way, _pyrophilus_ to be mention'd, by which a liquor may alter the colour of another body, and this seems the most important of all, because though it be nam'd but as one, yet it may indeed comprehend many, and that is, by associating the saline corpuscles, or any other sort of the more rigid ones of the liquor, with the particles of the body that it is employ'd to work upon. for these adventitious corpuscles associating themselves with the protuberant particles of the surface of a colour'd body, must necessarily alter their bigness, and will most commonly alter their shape. and how much the colours of bodies depend upon the bulk and figure of their superficial particles, you may guess by this, that eminent antient _philosophers_ and divers _moderns_, have thought that all colours might in a general way be made out by these two; whose being diversify'd, will in our case be attended with these two circumstances, the one, that the protuberant particles being increas'd in bulk, they will oftentimes be vary'd as to the closness or laxity of their order, fewer of them being contain'd within the same sensible (though minute) space than before; or else by approaching to one another, they must straighten the pores, and it may be too, they will by their manner of associating themselves with the protuberant particles, intercept new pores. and this invites me to consider farther, that the adventitious corpuscles, i have been speaking of, may likewise produce a great change as well in the little cavities or pores as in the protuberances of a colour'd body; for besides what we have just now taken notice of, they may by lodging themselves in those little cavities, fill them up, and it may well happen, that they may not only fill the pores they insinuate themselves into, but likewise have their upper parts extant above them; and partly by these new protuberances, partly by increasing the bulk of the former, these extraneous corpuscles may much alter the number and bigness of the surfaces pores, changing the old and intercepting new ones. and then 'tis odds, but the order of the little extancies, and consequently that of the little depressions in point of situation will be alter'd likewise: as if you dissolve _quick-silver_ in some kind of _aqua-fortis_, the saline particles of the _menstruum_ associating themselves with the mercurial corpuscles, will make a green solution, which afterwards easily enough degenerates. and red lead or _minium_ being dissolv'd in spirit of vinegar, yields not a red, but a clear solution, the redness of the lead being by the liquor destroy'd. but a better instance may be taken from copper, for i have try'd, that if upon a copper-plate you let some drops of weak _aqua-fortis_ rest for a while, the corpuscles of the _menstruum_, joyning with those of the metall, will produce a very sensible asperity upon the surface of the plate, and will concoagulate that way into very minute grains of a pale blew _vitriol_; whereas if upon another part of the same plate you suffer a little strong spirit of urine to rest a competent time, you shall find the asperated surface adorn'd with a deeper and richer blew. and the same _aqua-fortis_, that will quickly change the redness of red lead into a darker colour, will, being put upon crude lead, produce a whitish substance, as with copper it did a blewish. and as with iron it will produce a reddish, and on white quills a yellowish, so much may the coalition of the parts of the same liquor, with the differingly figur'd particles of stable bodies, divers ways asperate the differingly dispos'd surfaces, and to diversifie the colour of those bodies. and you'l easily believe, that in many changes of colour, that happen upon the dissolutions of metalls, and precipitations made with oyl of _tartar_, and the like fix'd salts, there may intervene a coalition of saline corpuscles with the particles of the body dissolv'd or precipitated, if you examine how much the _vitriol_ of a metall may be heavier than the metalline part of it alone, upon the score of the saline parts concoagulated therewith, and, that in several precipitations the weight of the _calx_ does for the same reason much exceed that of the metall, when it was first put in to be dissolv'd. . but, _pyrophilus_, to consider these matters more particularly would be to forget that i declar'd against adventuring, at least for this time, at particular theories of colours, and that accordingly you may justly expect from me rather experiments than speculations, and therefore i shall dismiss this subject of the forms of superficial asperity in colour'd bodies, as soon as i shall but have nam'd to you by way of supplement to what we have hitherto discours'd in this section, a couple of particulars, (which you'l easily grant me) the one, that there are divers other ways for the speedy production even of true and permanent colours in bodies, besides those practicable by the help of liquors; for proof of which advertisement, though several examples might be alleged, yet i shall need but re-mind you of what i mention'd to you above, touching the change of colours suddenly made on temper'd steel, and on lead, by the operation of heat, without the intervention of a liquor. but the other particular i am to observe to you is of more importance to our present subject and it is, that though nature and art may in some cases so change the asperity of the superficial parts of a body, as to change its colour by either of the ways i have propos'd single or unassisted, yet for the most part 'tis by two or three, or perhaps by more of the fore-mention'd ways associated together, that the effect is produc'd, and if you consider how variously those several ways and some others ally'd unto them, which i have left unmention'd, may be compounded and apply'd, you will not much wonder that such fruitfull, whether principles (or manners of diversification) should be fitted to change or generate no small store of differing colours. . hitherto, _pyrophilus_, we have in discoursing of the asperity of bodies consider'd the little protuberances of other superficial particles which make up that roughness, as if we took it for granted, that they must be perfectly opacous and impenetrable by the beams of light, and so, must contribute to the variety of colours as they terminate more or less light, and reflect it to the eye mix'd with more or less of thus or thus mingl'd shades. but to deal ingenuously with you, _pyrophilus_, before i proceed any further, i must not conceal from you, that i have often thought it worth a serious enquiry, whether or no particles of matter, each of them sing'y insensible, and therefore small enough to be capable of being such minute particles as the _atomists_ both of old and of late have (not absurdly) called _corpuscula coloris_, may not yet consist each of them of divers yet minuter particles, betwixt which we may conceive little commissures where they adhere to one another, and, however, may not be porous enough to be, at least in some degree, pervious to the unimaginably subtile corpuscles that make up the beams of light, and consequently to be in such a degree diaphanous. for, _pyrophilus_, that the proposed enquiry may be of moment to him that searches after the nature of colour, you'l easily grant, if you consider, that whereas perfectly opacous bodies can but reflect the incident beams of light, those that are diaphanous are qualified to refract them too, and that refraction has such a stroak in the production of colours, as you cannot but have taken notice of, and perhaps admir'd in the colours generated by the trajection of light through drops of water that exhibit a rain-bow, through prismatical glasses, and through divers other transparent bodies. but 'tis like, _pyrophilus_, you'l more easily allow that about this matter 'tis rather important to have a certainty, than that 'tis rational to entertain a doubt; wherefore i must mention to you some of the reasons that make me think it may need a further enquiry, for i find that in a darkned room, where the light is permitted to enter but at one hole, the little wandering particles of dust, that are commonly called motes, and, unless in the sunbeams, are not taken notice of by the unassisted sight, i have, i say, often observ'd, that these roving corpuscles being look'd on by an eye plac'd on one side of the beams that enter'd the little hole, and by the darkness having its pupill much enlarg'd, i could discern that these motes as soon as they came within the compass of the luminous, whether cylinder or inverted cone, if i may so call it, that was made up by the unclouded beams of the sun, did in certain positions appear adorn'd with very vivid colours, like those of the rain-bow, or rather like those of very minute, but sparkling fragments of diamonds; and as soon as the continuance of their motion had brought them to an inconvenient position in reference to the light and the eye, they were only visible without darting any lively colours as before, which seems to argue that these little motes, or minute fragments, of several sorts of bodies reputed opacous, and only crumbled as to their exteriour and looser parts into dust, did not barely reflect the beams that fell upon them, but remit them to the eye refracted too. we may also observe, that several bodies, (as well some of a vegetable, as others of an animal nature) which are wont to pass for opacous, appear in great part transparent, when they are reduc'd into thin parts, and held against a powerful light. this i have not only taken notice of in pieces of ivory reduc'd but into thick leaves, as also in divers considerable thick shells of fishes, and in shaving of wood, but i have also found that a piece of deal, far thicker than one would easily imagine, being purposly interposed betwixt my eye plac'd in a room, and the clear daylight, was not only somewhat transparent, but (perhaps by reason of its gummous nature) appear'd quite through of a lovely red. and in the darkned room above mention'd, bodies held against the hole at which the light enter'd, appear'd far less opacous then they would elsewhere have done, insomuch that i could easily and plainly see through the whole thickness of my hand, the motions of a body plac'd (at a very near distance indeed, but yet) beyond it. and even in minerals, the opacity is not always so great as many think, if the body be made thin, for white marble though of a pretty thickness, being within a due distance plac'd betwixt the eye and a convenient light, will suffer the motions of ones finger to be well discern'd through it, and so will pieces, thick enough, of many common flints. but above all, that instance is remarkable, that is afforded us by _muscovie_ glass, (which some call _selenites_, others _lapis specularis_) for though plates of this mineral, though but of a moderate thickness, do often appear opacous, yet if one of these be dextrously split into the thinnest leaves 'tis made up of, it will yield such a number of them, as scarce any thing but experience could have perswaded me, and these leaves will afford the most transparent sort of consistent bodies, that, for ought i have observ'd, are yet known; and a single leaf or plate will be so far from being opacous, that 'twill scarce be so much as visible. and multitudes of bodies there are, whose fragments seem opacous to the naked eye, which yet, when i have included them in good _microscopes_, appear'd transparent; but, _pyrophilus_, on the other side i am not yet sure that there are no bodies, whose minute particles even in such a _microscope_ as that of mine, which i was lately mentioning, will not appear diaphanous. for having consider'd _mercury_ precipitated _per se_, the little granules that made up the powder, look'd like little fragments of coral beheld by the naked eye at a distance (for very near at hand coral will sometimes, especially if it be good, shew some transparency.) filings likewise of steel and copper, though in an excellent _microscope_, and a fair day, they show'd like pretty big fragments of those metalls, and had considerable brightness on some of their surfaces, yet i was not satisfi'd, that i perceiv'd any reflection from the inner parts of any of the filings. nay, having look'd in my best _microscope_ upon the red _calx_ of lead, (commonly call'd _minium_) neither i, nor any i shew'd it to, could discern it to be other than opacous, though the day were clear, and the object strongly enlightned. and the deeply red colour of _vitriol_ appear'd in the same _microscope_ (notwithstanding the great comminution effected by the fire) but like grossy beaten brick. so that, _pyrophilus_, i shall willingly resign you the care of making some further enquiries into the subject we have now been considering; for i confess, as i told you before, that i think that the matter may need a further scrutiny, nor would i be forward to determine how far or in what cases the transparency or semi-diaphaniety of the superficial corpuscles of bigger bodies, may have an interest in the production of their colours, especially because that even in divers white bodies, as beaten glass, snow and froth, where it seems manifest that the superficial parts are singly diaphanous, (being either water, or air, or glass) we see not that such variety of colours are produc'd as usually are by the refraction of light, even in those bodies, when by their bigness, shape, &c. they are conveniently qualify'd to exhibit such various and lively colours as those of the rain-bow, and of prismatical glasses. . by what has been hitherto discours'd, _pyrophilus_, we may be assisted to judge of that famous controversie which was of old disputed betwixt the _epicureans_ and other _atomists_ on the one side, and most other _philosophers_ on the other side. the former denying bodies to be colour'd in the dark, and the latter making colour to be an inherent quality, as well as figure, hardness; weight, or the like. for though this controversie be reviv'd, and hotly agitated among the _moderns_, yet i doubt whether it be not in great part a nominal dispute, and therefore let us, according to the doctrine formerly deliver'd, distinguish the acceptions of the word colour, and say, that if it be taken in the stricter sense, the _epicureans_ seem to be in the right, for if colour be indeed, though not according to them, but light modify'd, how can we conceive that it can subsist in the dark, that is, where it must be suppos'd there is no light; but on the other side, if colour be consider'd as a certain constant disposition of the superficial parts of the object to trouble the light they reflect after such and such a determinate manner, this constant, and, if i may so speak, modifying disposition persevering in the object, whether it be shin'd upon or no, there seems no just reason to deny, but that in this sense, bodies retain their colour as well in the night as day; or, to speak a little otherwise, it may be said, that bodies are potentially colour'd in the dark, and actually in the light. but of this matter discoursing more fully elsewhere, as 'tis a difficulty that concerns qualities in general, i shall forbear to insist on it here. * * * * * chap. iv . of greater moment in the investigation of the nature of colours is the controversie, whether those of the rain-bow, and those that are often seen in clouds, before the rising, or after the setting of the sun; and in a word, whether those other colours, that are wont to be call'd emphatical, ought or ought not to be accounted true colours. i need not tell you that the negative is the common opinion, especially in the schools, as may appear by that vulgar distinction of colours, whereby these under consideration are term'd apparent, by way of opposition to those that in the other member of the distinction are call'd true or genuine. this question i say seems to me of importance, upon this account, that it being commonly granted, (or however, easie enough to be prov'd) that emphatical colours are light it self modify'd by refractions chiefly, with a concurrence sometimes of reflections, and perhaps some other accidents depending on these two; if these emphatical colours be resolv'd to be genuine, it will seem consequent, that colours, or at least divers of them, are but diversify'd light, and not such real and inherent qualities as they are commonly thought to be. . now since we are wont to esteem the echoes and other sounds of bodies, to be true sounds, all their odours to be true odours, and (to be short) since we judge other sensible qualities to be true ones, because they are the proper objects of some or other of our senses, i see not why emphatical colours, being the proper and peculiar objects of the organ of sight, and capable to affect it as truly and as powerfully as other colours, should be reputed but imaginary ones. and if we have (which perchance you'l allow) formerly evinc'd colour, (when the word is taken in its more proper sense) to be but modify'd light, there will be small reason to deny these to be true colours, which more manifestly than others disclose themselves to be produc'd by diversifications of the light. . there is indeed taken notice of a difference betwixt these apparent colours, and those that are wont to be esteem'd genuine, as to the duration, which has induc'd some learned men to call the former rather evanid than fantastical. but as the ingenious _gassendus_ does somewhere judiciously observe, if this way of arguing were good, the greeness of a leaf ought to pass for apparent, because, soon fading into a yellow, it scarce lasts at all, in comparison of the greeness of an emerauld. i shall add, that if the sun-beams be in a convenient manner trajected through a glass-prism, and thrown upon some well-shaded object within a room, the rain-bow thereby painted on the surface of the body that terminates the beams, may oftentimes last longer than some colours i have produc'd in certain bodies, which would justly, and without scruple be accounted genuine colours, and yet suddenly degenerate, and lose their nature. . a greater disparity betwixt emphatical colours, and others, may perhaps be taken from this, that genuine colours seem to be produc'd in opacous bodies by reflection, but apparent ones in diaphanous bodies, and principally by refraction, i say principally rather than solely, because in some cases reflection also may concurr, but still this seems not to conclude these latter colours not to be true ones. nor must what has been newly said of the differences of true and apparent colours, be interpreted in too unlimited a sense, and therefore it may perhaps somewhat assist you, both to reflect upon the two fore-going objections, and to judge of some other passages which you'l meet with in this tract, if i take this occasion to observe to you, that if water be agitated into froth, it exhibits you know a white colour, which soon after it loses upon the resolution of the bubbles into air and water, now in this case either the whiteness of the froth is a true colour or not, if it be, then true colours, supposing the water pure and free from mixtures of any thing tenacious, may be as short-liv'd as those of the rain-bow; also the matter, wherein the whiteness did reside, may in a few moments perfectly lose all foot-steps or remains of it. and besides, even diaphanous bodies may be capable of exhibiting true colours by reflection, for that whiteness is so produc'd, we shall anon make it probable. but if on the other side it be said, that the whiteness of froth is an emphatical colour, then it must no longer be said, that fantastical colours require a certain position of the luminary and the eye, and must be vary'd or destroy'd by the change thereof, since froth appears white, whether the sun be rising or setting, or in the meridian, or any where between it and the horizon, and from what (neighbouring) place soever the beholders eye looks upon it. and since by making a liquor tenacious enough, yet without destroying its transparency, or staining it with any colour, you may give the little films, whereof the bubbles consist, such a texture, as may make the froth last very many hours, if not some days, or even weeks, it will render it somewhat improper to assign duration for the distinguishing character to discriminate genuine from fantastical colours. for such froth may much outlast the undoubtedly true colours of some of nature's productions, as in that gaudy plant not undeservedly call'd the mervail of _peru_, the flowers do often fade, the same day they are blown; and i have often seen a _virginian_ flower, which usually withers within the compass of a day; and i am credibly inform'd, that not far from hence a curious herborist has a plant, whose flowers perish in about an hour. but if the whiteness of water turn'd into froth must therefore be reputed emphatical, because it appears not that the nature of the body is alter'd, but only that the disposition of its parts in reference to the incident light is chang'd, why may not the whiteness be accounted emphatical too, which i shall shew anon to be producible, barely by such another change in black horn? and yet this so easily acquir'd whiteness seems to be as truly its colour as the blackness was before, and at least is more permanent than the greenness of leaves, the redness of roses, and, in short, than the genuine colours of the most part of nature's productions. it may indeed be further objected, that according as the sun or other luminous body changes place, these emphatical colours alter or vanish. but not to repeat what i have just now said, i shall add, that if a piece of cloath in a drapers shop (in such the light being seldome primary) be variously folded, it will appear of differing colours, as the parts happen to be more illuminated or more shaded, and if you stretch it flat, it will commonly exhibit some one uniform colour, and yet these are not wont to be reputed emphatical, so that the difference seems to be chiefly this, that in the case of the rain-bow, and the like, the position of the luminary varies the colour, and in the cloath i have been mentioning, the position of the object does it. nor am i forward to allow that in all cases the apparition of emphatical colours requires a determinate position of the eye, for if men will have the whiteness of froth emphatical, you know what we have already inferr'd from thence. besides, the sun-beams trajected through a triangular glass, after the manner lately mention'd, will, upon the body that terminates them, paint a rain-bow, that may be seen whether the eye be plac'd on the right hand of it or the left, or above or beneath it, or before or behind it; and though there may appear some little variation in the colours of the rain-bow, beheld from differing parts of the room, yet such a diversity may be also observ'd by an attentive eye in real colours, look'd upon under the like circumstances, nor will it follow, that because there remains no footsteps of the colour upon the object, when the prism is remov'd, that therefore the colour was not real, since the light was truly modify'd by the refraction and reflection it suffer'd in its trajection through the prism; and the object in our case serv'd for a specular body, to reflect that colour to the eye. and that you may not be startled, _pyrophilus_, that i should venture to say, that a rough and coiour'd object may serve for a _speculum_ to reflect the artificial rain-bow i have been mentioning, consider what usually happens in darkned rooms, where a wall, or other body conveniently situated within, may so reflect the colours of bodies, without the room, that they may very clearly be discern'd and distinguish'd, and yet 'tis taken for granted, that the colours seen in a darkned room, though they leave no traces of themselves upon the wall or body that receives them, are the true colours of the external objects, together with which the colours of the images are mov'd or do rest. and the errour is not in the eye, whose office is only to perceive the appearances of things, and which does truly so, but in the judging or estimative faculty, which mistakingly concludes that colour to belong to the wall, which does indeed belong to the object, because the wall is that from whence the beams of light that carry the visible _species_, do come in straight lines directly to the eye, as for the same reason we are wont at a certain distance from concave sphærical glasses, to perswade our selves that we see the image come forth to meet us, and hang in the air betwixt the glass and us, because the reflected beams that compose the image cross in that place, where the image seems to be, and thence, and not from the glass, do in direct lines take their course to the eye, and upon the like cause it is, that divers deceptions in sounds and other sensible objects do depend, as we elsewhere declare. . i know not, whether i need add, that i have purposely try'd, (as you'l find some pages hence, and will perhaps think somewhat strange) that colours that are call'd emphatical, because not inherent in, the bodies in which they appear, may be compounded with one another, as those that are confessedly genuine may. but when all this is said, _pyrophilus_, i must advertise you, that it is but problematically spoken, and that though i think the opinion i have endeavour'd to fortifie probable, yet a great part of our discourse concerning colours may be true, whether that opinion be so or not. * * * * * chap. v. . there are you know, _pyrophilus_, besides those obsolete opinions about colours which have been long since rejected, very various theories that have each of them, even at this day, eminent men for its abetters; for the peripatetick schools, though they dispute amongst themselves divers particulars concerning colours, yet in this they seem unanimously enough to agree, that colours are inherent and real qualities, which the light doth but disclose, and not concurr to produce. besides there are _moderns_, who with a slight variation adopt the opinion of _plato_, and as he would have colour to be nothing but a kind of flame consisting of minute corpuscles as it were darted by the object against the eye, to whose pores their littleness and figure made them congruous, so these would have colour to be an internal light of the more lucid parts of the object, darkned and consequently alter'd by the various mixtures of the less luminous parts. there are also others, who in imitation of some of the ancient _atomists_, make colour not to be lucid steam, but yet a corporeal _effluvium_ issuing out of the colour'd body, but the knowingst of these have of late reform'd their hypothesis, by acknowledging and adding that some external light is necessary to excite, and as _they_ speak, sollicit these corpuscles of colour as _they_ call them, and bring them to the eye. another and more principal opinion of the _modern_ philosophers, to which this last nam'd may by a favourable explication be reconcil'd, is that which derives colours from the mixture of light and darkness, or rather light and shadows. and as for the _chymists_ 'tis known, that the generality of them ascribes the origine of colours to the sulphureous principle in bodies, though i find, as i elsewhere largely shew, that some of the chiefest of them derive colours rather from salt than sulphur, and others, from the third hypostatical principle, _mercury_. and as for the _cartesians_ i need not tell you, that they, supposing the sensation of light to bee produc'd by the impulse made upon the organs of sight, by certain extremely minute and solid globules, to which the pores of the air and other diaphanous bodies are pervious, endeavour to derive the varieties of colours from the various proportion of the direct progress or motion of these globules to their circumvolution or motion about their own centre, by which varying proportion they are by this hypothesis suppos'd qualify'd to strike the optick nerve after several distinct manners, so to produce the perception of differing colours. . besides these six principal hypotheses, _pyrophilus_, there may be some others, which though less known, may perhaps as well as thesc deserve to be taken into consideration by you; but that i should copiously debate any of them at present, i presume you will not expect, if you consider the scope of these papers, and the brevity i have design'd in them, and therefore i shall at this time only take notice to you in the general of two or three things that do more peculiarly concern the treatise you have now in your hands. . and first, though the embracers of the several hypotheses i have been naming to you, by undertaking each sect of them to explicate colours indefinitely, by the particular hypotheses they maintain, seem to hold it forth as the only needful theory about that subject, yet for my part i doubt whether any one of all these hypotheses have a right to be admitted exclusively to all others, for i think it probable, that whiteness and blackness may be explicated by reflection alone without refraction, as you'l find endeavour'd in the discourse you'l meet with e're long of the origine of whiteness and blackness, and on the other side, since i have not found that by any mixture of white and true black, (for there is a blewish black which many mistake for a genuine) there can be a blew, a yellow, or a red, to name no other colours, produced, and since we do find that these colours may be produc'd in the glass-prism and other transparent bodies, by the help of refractions, it seems that refraction is to be taken in into the explication of some colours, to whose generation they seem to concurr, either by making a further or other commixture of shades with the refracted light, or by some other way not now to be discours'd. and as it seems not improbable, that in case the pores of the air, and other diaphanous bodies be every where almost fill'd with such _globuli_ as the _cartesians_ suppose, the various kind of motion of these _globuli_, may in many cases have no small stroak in varying our perception of colour, so without the supposition of these _globuli_, which 'tis not so easie to evince, i think we may probably enough conceive in general, that the eye may be variously affected, not only by the entire beams of light that fall upon it as they are such, but by the order, and by the degree of swiftness, and in a word by the manner according to which the particles that compose each particular beam arrive at the sensory, so that whatever be the figure of the little corpuscles, of which the beams of light consist, not only the celerity or slowness of their revolution or rotation in reference to their progressive motion, but their more absolute celerity, their direct or undulating motion, and other accidents, which may attend their appulse to the eye, may fit them to make differing impressions on it. . secondly, for these and the like considerations, _pyrophilus_, i must desire that you would look upon this little treatise, not as a discourse written principally to maintain any of the fore-mention'd theories, exclusively to all others, or substitute a new one of my own, but as the beginning of a history of colours, upon which, when you and your ingenious friends shall have enrich'd it, a solid theory may be safely built. but yet because this history is not meant barely for a register of the things recorded in it, but for an _apparatus_ to a sound and comprehensitive hypothesis, i thought fit, so to temper the whole discourse, as to make it as conducible, as conveniently i can to that end, and therefore i have not scrupled to let you see that i was willing, as to save you the labour of cultivating some theories that i thought would never enable you to reach the ends you aim at, so to contract your enquiries into a narrow compass, for both which purposes i thought it requisite to do these two things, the _one_, to set down some experiments which by the help of the reflections and insinuations that attend them, may assist you to discover the infirmness and insufficiency both of the common peripatetick doctrine, and of the now more applauded theory of the _chymists_ about colour, because those two doctrines having possess'd themselves, the one of the most part of the schools, and the other of the esteem of the generality ef physicians and other learned men, whose professions and ways of study do not exact that they should scrupulously examine the very first and simplest principles of nature, i fear'd it would be to little purpose, without doing something to discover the insufficiency of these hypotheses, that i should, (which was the _other_ thing i thought requisite for me to do) set down among my other experiments those in the greatest number, that may let you see, that, till i shall be better inform'd, i encline to take colour to be a modification of light, and would invite you chiefly to cultivate that hypothesis, and improve it to the making out of the generation of particular colours, as i have endeavour'd to apply it to the explication of whiteness and blackness. . thirdly. but, _pyrophilus_, though this be at present the hypothesis i preferr, yet i propose it but in a general sense, teaching only that the beams of light, modify'd by the bodies whence they are sent (reflected or refracted) to the eye, produce there that kind of sensation, men commonly call colour; but whether i think this modification of the light to be perform'd by mixing it with shades, or by varying the proportion of the progress and rotation of the _cartesian globuli cælestes_, or by some other way which i am not now to mention, i pretend not here to declare. much less do i pretend to determine, or scarce so much as to hope to know all that were requisite to be known, to give you, or even my self, a perfect account of the theory of vision and colours, for in order to such an undertaking i would first know what light is, and if it be a body (as a body or the motion of a body it seems to be) what kind of corpuscles for size and shape it consists of, with what swiftness they move forwards, and whirl about their own centres. then i would know the nature of refraction, which i take to be one of the abstrusest things (not to explicate plausibly, but to explicate satisfactorily) that i have met with in physicks; i would further know what kind and what degree of commixture of darkness or shades is made by refractions or reflections, or both, in the superficial particles of those bodies, that being shin'd upon, constantly exhibit the one, for instance, a blew, the other a yellow, the third a red colour; i would further know why this contemperation of light and shade, that is made, for example, by the skin of a ripe cherry, should exhibit a red, and not a green, and the leaf of the same tree should exhibit a green rather than a red; and indeed, lastly, why since the light that is modify'd into these colours consists but of corpuscles moved against the _retina_ or pith of the optick nerve, it should there not barely give a stroak, but produce a colour, whereas a needle wounding likewise the eye, would not produce colour but pain. these, and perhaps other things i should think requisite to be known, before i should judge my self to have fully comprehended the true and whole nature of colours; and therefore, though by making the experiments and reflections deliver'd in this paper, i have endeavour'd somewhat to lessen my ignorance in this matter, and think it far more desireable to discover a little, than to discover nothing, yet i pretend but to make it probable by the experiments i mention, that some colours may be plausibly enough explicated in the general by the doctrine here propos'd; for whensoever i would descend to the minute and accurate explication of particulars, i find my self very sensible of the great obscurity of things, without excepting those which we never see but when they are enlightned, and confess with _scaliger_[ ], _latet natura hæc_, (says he, speaking of that of colour) _& sicut aliarum rerum species in profundissima caligine inscitiæ humanæ._ [ ] exercitat. parag. * * * * * _the_ _experimental history_ _of colours._ * * * * * part. ii. _of the nature of whiteness and_ _blackness._ chap. i. . though after what i have acknowledged, _pyrophilus_, of the abstruse nature of colours in _particular_, you will easily believe, that i pretend not to give you a satisfactory account of whiteness and blackness; yet not wholly to frustrate your expectation of my offering something by way of specimen towards the explication of some colours in particular, i shall make choice of these as the most simple ones, (and by reason of their mutual opposition the least hardly explicable) about which to present you my thoughts, upon condition you will take them at most to be my conjectures, not my opinions. . when i apply'd my self to consider, how the cause of whiteness might be explan'd by intelligible and mechanical principles, i remembred not to have met with any thing among the antient _corpuscularian_ philosophers, touching the quality we call whiteness, save that _democritus_ is by _aristotle_ said to have ascrib'd the whiteness of bodies to their smoothness, and on the contrary their blackness to their asperity.[ ] but though about the latter of those qualities his opinion be allowable, as we shall see anon, yet that he heeds a favourable interpretation in what is deliver'd concerning the first, (at least if his doctrine be not mis-represented in this point, as it has been in many others) we shall quickly have occasion to manifest. but amongst the _moderns_, the most learned _gassendus_ in his ingenious epistle publish'd in the year . _de apparente magnitudine solis humilis & sublimis_, reviving the _atomical_ philosophy, has, though but incidentally, deliver'd something towards the explication of whiteness upon mechanical principles: and because no man that i know of, has done so before him, i shall, to be sure to do him right, give you his sense in his own words:[ ] _cogites velim_ (says he) _lucem quidem in diaphano nullius coloris videri, sed in opaco tamen terminante candicare, ac tantò magis, quantò densior seu collectior fuerit. deinde aquam non esse quidem coloris ex se candidi & radium tamen ex eâ reflexum versus oculum candicare. rursus cum plana aquæ superficies non nisi ex una parte eam reflexionem faciat: si contigerit tamen illam in aliquot bullas intumescere, bullam unamquamque reflectionem facere, & candoris speciem creare certa superficiei parte. ad hæc spumam ex aqua pura non alia ratione videri candescere & albescerere quam quod sit congeries confertissima minutissimarum bullarum, quarum unaquæque suum radium reflectit, unde continens candor alborve apparet. denique nivem nihil aliud videri quam speciem purissimæ spumæ ex bullulis quam minutissimis & confertissimis cohærentis. sed ridiculam me exhibeam, si tales meas nugas uberius proponem._ [ ] _album quippe & agrum, hoc quidem asperum esse dicit, hoc vero læve. de sensu & sensib. . ._ [ ] epist. . pag. . . but though in this passage, that very ingenous person has anticipated part of what i should say; yet i presume you will for all that expect, that i should give you a fuller account of that notion of whiteness, which i have the least exceptions to, and of the particulars whence i deduce it, which to do, i must mention to you the following experiments and observations. whiteness then consider'd as a quality in the object, seems chiefly to depend upon this, that the superficies of the body that is call'd white, is asperated by almost innumerable small surfaces, which being of an almost specular nature, are also so plac'd, that some looking this way, and some that way, they yet reflect the rays of light that fall on them, not towards one another, but outwards towards the spectators eye. in this rude and general account of whiteness, it seems that besides those qualities, which are common to bodies of other colours, as for instance the minuteness and number of the superficial parts, the two chief things attributed to bodies as white are made to be, first, that its little protuberances and superficial parts be of somewhat a specular nature, that they may as little looking-glasses each of them reflect the beams it receives, (or the little picture of the sun made on it) without otherwise considerably altering them; whereas in most other colours, they are wont to be much chang'd, by being also refracted, or by being return'd to the eye, mixt with shades or otherwise. and next, that its superficial parts be so situated, that they retain not the incident rays of light by reflecting them inwards, but send them almost all back, so that the outermost corpuscles of a white body, having their various little surfaces of a specular nature, a man can from no place behold the body, but that there will be among those innumerable _superficieculæ_, that look some one way, and some another, enough of them obverted to his eye, to afford like a broken looking-glass, a confused idæa, or representation of light, and make such an impression on the organ, as that for which men are wont to call a body white. but this notion will perhaps be best explan'd by the same experiments and observations, on which it is built, and therefore i shall now advance to _them_. . and in the first place i consider, that the sun and other powerfully lucid bodies, are not only wont to offend, which we call to dazle our eyes, but that if any colour be to be ascrib'd to them as they are lucid, it seems it should be whiteness: for the sun at noon-day, and in clear weather, and when his face is less troubled, and as it were stained by the steams of sublunary bodies, and when his beams have much less of the atmosphere to traject in their passage to our eyes, appears of a colour more approaching to white, than when nearer the horizon, the interposition of certain sorts of fumes and vapours make him oftentimes appear either red, or at least more yellow. and when the sun shines upon that natural looking-glass, a smooth water, that part of it, which appears to this or that particular beholder, the most shin'd on, does to his eye seem far whiter than the rest. and here i shall add, that i have sometimes had the opportunity to observe a thing, that may make to my present purpose, namely, that when the sun was veil'd over as it were, with a thin white cloud, and yet was too bright to be look'd upon directly without dazling, by casting my eyes upon a smooth water, as we sometimes do to observe eclipses without prejudice to our eyes, the sun then not far from the meridian, appear'd to me not red, but so white, that 'twas not without some wonder, that i made the observation. besides, though we in _english_ are wont to say, a thing is red hot, as an expression of its being superlatively _ignitum_, (if i may so speak for want of a proper _english_ word) yet in the forges of smiths, and the furnaces of other artificers, by that which they call a white heat, they mean a further degree of _ignition_, than by that which both they and we call a red heat. . secondly, i consider, that common experience informs us, that as much light over-powers the eye, so when the ground is covered with snow, (a body extremely white) those that have weak eyes are wont to complain of too much light: and even those that have not, are generally sensible of an extraordinary measure of light in the air; and if they are fain to look very long upon the snow, find their sight offended by it. on which occasion we may call to mind what _xenophon_ relates, that his _cyrus_ marching his army for divers days through mountains covered with snow, the dazling splendor of its whiteness prejudic'd the sight of very many of his souldiers, and blinded some of them; and other stories of that nature be met with in writers of good note. and the like has been affirm'd to me by credible persons of my own acquaintance, and especially by one who though skill'd in physick and not ancient confess'd to me when i purposely ask'd him, that not only during his stay in _muscovy_, he found his eyes much impair'd, by being reduc'd frequently to travel in the snow, but that the weakness of his eyes did not leave him when he left that country, but has follow'd him into these parts, and yet continues to trouble him. and to this doth agree what i as well as others have observ'd, namely, that when i travell'd by night, when the ground was all cover'd with snow, though the night otherwise would not have been lightsome, yet i could very well see to choose my way. but much more remarkable to my present purpose is that, which i have met with in _olaus magnus_,[ ] concerning the way of travelling in winter in the _northern_ regions, where the days of that season are so very short; for after other things not needfull to be here transcribed: _iter_, says he, _diurnum duo scilicet montana milliaria (quæ italica sunt) consiciunt. nocte verò sub splendissima luna, duplatum iter consumunt aut triplatum. neque id incommodè fit, cum nivium reverberatione lunaris splendoris sublimes & declives campos illustret, ac etiam montium præcipitia ac noxias feras à lorgè prospiciant evitandas_. which testimony i the less scruple to allege, because that it agrees very well with what has been affirm'd to me by a physician of _mosco_, whom the notion i have been treating of concerning whiteness invited me to ask whether he could not see much farther when he travell'd by night in _russia_ than he could do in _england_, or elsewhere, when there was no snow upon the ground; for this ingenious person inform'd me, that he could see things at a farr greater distance, and with more clearness, when he travell'd by night on the _russian_ snow, though without the assistance of moon-shine, than we in these parts would easily be perswaded. though it seems not unlikely to me, that the intenseness of the cold may contribute something to the considerableness of the effect, by much clearing the air of darkish steams, which in these more temperate climates are wont to thicken it in snowy weather: for having purposely inquir'd of this doctor, and consulted that ingenious navigator captain _james_'s voyage hereafter to be further mention'd, i find both their relations agree in this, that in dark frosty nights they could discover more stars, and see the rest clearer than we in _england_ are wont to do. [ ] gent. septen. histor. lib. cap. . . i know indeed that divers learned men think, that snow so strongly affects our eye, not by a borrow'd, but a native light; but i venture to give it as a proof, that white bodies reflect more light than others, because having once purposely plac'd a parcel of snow in a room carefully darkned, that no celestial light might come to fall upon it; neither i, nor an ingenous person, (skill'd in opticks) whom i desir'd for a witness, could find, that it had any other light than what it receiv'd. and however, 'tis usual among those that travel in dark nights, that the guides wear something of white to be discern'd by, there being scarce any night so dark, but that in the free air there remains some light, though broken and debilitated perhaps by a thousand reflections from the opacous corpuscles that swim in the air, and lend it to one another before it comes to arrive at the eye. . thirdly, and the better to shew that white bodies reflect store of light, in comparson of those that are otherwise colour'd, i did in the darkn'd room, formerly mention'd, hold not far from the hole, at which the light was admitted, a sheet only of white paper, from whence casting the sun-beams upon a white wall, whereunto it was obverted, it manifestly appear'd both to me, and to the person i took for a witness of the experiment, that it reflected a far greater light, than any of the other colours formerly mention'd, the light so thrown upon one wall notably enlightning it, and by it a good part of the room. and yet further to show you, that white bodies reflect the beams from them, and not towards themselves, let me add, that ordinary burning-glasses, such as are wont to be employ'd to light tobacco, will not in a great while burn, or so much as discolour a sheet of white paper. insomuch that even when i was a boy, and lov'd to make tryals with burning-glasses, i could not but wonder at this odd _phænomenon_, which set me very early upon guessing at the nature of whiteness, especially because i took notice, that the image of the sun upon a white paper was not so well defin'd (the light seeming too diffus'd) as upon black, and because i try'd, that blacking over the paper with ink, not only the ink would be quickly dry'd up, but the paper that i could not burn before, would be quickly set on fire. i have also try'd, that by exposing my hand with a thin black glove over it to the warm sun, it was thereby very quickly and considerably more heated, than if i took off the glove, and held my hand naked, or put on it another glove of thin but white leather. and having thus shewn you, _pyrophilus_, that white bodies reflect the most light of any, let us now proceed, to consider what is further to be taken notice of in them, in order to our present enquiry. . and fourthly, whereas among the dispositions we attributed to white bodies, we also intimated this, that such bodies are apt, like _speculums_, though but imperfect ones, to reflect the light that falls on them untroubled or unstain'd, we shall besides other particulars to be met with in these papers, offer you this in favour of the conjecture; that in the darkned room several times mention'd in this treatse, we try'd that the sun-beams being cast from a coloured body upon a neighbouring white wall, the determinate colour of the body was from the wall reflected to the eye; whereas we could in divers cases manifestly alter the colour arriving at the eye, by substituting at a convenient distance, a (conveniently) colour'd (and glossy) body instead of the white wall. as by throwing the beams from a yellow body upon a blew, there would be exhibited a kind of green, as in the experiments about colours is more fully declar'd. . i know not whether i should on this occasion take notice, that when, as when looking upon the calm and smooth surface of a river betwixt my eye and the sun, it appear'd to be a natural _speculum_, wherein that part which reflected to my eye the entire and defin'd image of the sun, and the beams less remote from those which exhibited that image, appear'd indeed of a great and whitish brightness, but the rest comparatively dark enough: if afterwards the superficies chanc'd to be a little, but not much troubled, by a gentle breath of wind, and thereby reduc'd into a multitude of small and smooth _speculums_, the surface of the river would suitably to the doctrine lately deliver'd, at a distance appear very much of kin to white, though it would lose that brightness or whiteness upon the return of the surface to calmness and an uniform level. and i have sometimes for tryals sake brought in by a lenticular glass, the image of a river, shin'd upon by the sun, into an upper room darkn'd, and distant about a quarter of a mile from the river, by which means the numerous declining surfaces of the water appear'd so contracted, that upon the body that receiv'd the images, the whole river appear'd a very white object at two or three paces distance. but if we drew near it, this whiteness appear'd to proceed from an innumerable company of lucid reflections, from the several gently wav'd superficies of the water, which look'd near at hand like a multitude of very little, but shining scales of fish, of which many did every moment disappear, and as many were by the sun, wind and river generated anew. but though this observation seem'd sufficiently to discover, how the appearing whiteness in that case was produc'd, yet in some other cases water may have the same, though not so vivid a colour upon other accounts; for oftentimes it happens that the smooth surface of the water does appear bright or whitish, by reason of the reflection not immediatly of the images of the sun, but of the brightness of the sky; and in such cases a convenient wind may where it passes along make the surface look black, by causing many such furrows and cavities, as may make the inflected superficies of the water reflect the brightness of the sky rather inward than outward. and again if the wind increase into a storm, the water may appear white, especially near the shore and the ship, namely because the rude agitation breaks it into fome or froth. so much do whiteness and blackness depend upon the disposition of the superficial parts of a body to reflect the beams of light inward or outward. but that as white bodies reflect the most light of any, so there superficial particles are, in the sense newly deliver'd, of a specular nature, i shall now further endeavour to shew both by the making of specular bodies white, and the making of a white body specular. . in the fifth place then, i will inform you, that (not to repeat what _gassendus_ observes concerning water) i have for curiosity sake distill'd quicksilver in a cucurbit, fitted with a capacious glass-head, and observ'd that when the operation was perform'd by the degrees of fire requisite for my purpose, there would stick to the inside of the alembick a multitude of little round drops of _mercury_. and as you know that _mercury_ is a specular body, so each of these little drops was a small round looking-glass, and a multitude of them lying thick and near one another, they did both in my judgment, and that of those i invited to see it, make the glass they were fastened to, appear manifestly a white body. and yet as i said, this whiteness depended upon the minuteness and nearness of the little mercurial _globuli_, the convexity of whose surfaces fitted them to represent in a narrow compass a multitude of little lucid images to differingly situated beholders. and here let me observe a thing that seems much to countenance the notion i have been recommending: namely, that whereas divers parts of the sky, and especially the milky-way, do to the naked eye appear white, (as the name it self imports) yet the galaxie look'd upon through the telescope, does not shew white, but appears to be made up of a vast multitude of little starrs; so that a multitude of lucid bodies, if they be so small that they cannot singly or apart be discern'd by the eye, and if they be sufficiently thick set by one another, may by their confus'd beams appear to the eye one white body. and why it is not possible, that the like may be done, when a multitude of bright and little corpuscles being crowded together, are made to send together vivid beams to the eye, though they shine but as the planets by a borrow'd light? . but to return to our experiments. we may take notice, that the white of an egg, though in part transparent, yet by its power of reflecting some incident rays of light, is in some measure a natural _speculum_, being long agitated with a whisk or spoon, loses its transparency, and becomes very white, by being turn'd into froth, that is into an aggregate of numerous small bubbles, whose convex superficies fits them to reflect the light every way outwards. and 'tis worth noting, that when water, for instance, is agitated into froth, if the bubbles be great and few, the whiteness will be but faint, because the number of _specula_ within a narrow compass is but small, and they are not thick set enough to reflect so many little images or beams of the lucid body, as are requisite to produce a vigorous sensation of whiteness: and partly least it should be said, that the whiteness of such globulous particles proceeds from the air included in the froth; (which to make good, it should be prov'd that the air it self is white) and partly to illustrate the better the notion we have propos'd of whiteness, i shall add, that i purposely made this experiment, i took a quantity fair water, & put to it in a clear glass phial, a convenient quantity of oyl or spirit of turpentine, because that liquor will not incorporate with water, and yet is almost as clear and colourless as it; these being gently shaken together, the agitation breaks the oyl (which as i said, is indispos'd to mix like wine or milk _per minima_ with the water) into a multitude of little globes, which each of them reflecting outwards a lucid image, make the imperfect mixture of the two liquors appear whitish; but if by vehemently shaking the glass for a competent time you make a further comminution of the oyl into far more numerous and smaller _globuli_, and thereby confound it also better with the water, the mixture will appear of a much greater whiteness, and almost like milk; whereas if the glass be a while let alone, the colour will by degrees impair, as the oyly globes grow fewer and bigger, and at length will quite vanish, leaving both the liquors distinct and diaphanous as before. and such a tryal hath not ill succeeded, when insteed of the colourless oyl of turpentine i took a yellow mixture made of a good proportion of crude turpentine dissolv'd in that liquor; and (if i mis-remember not) it also succeeded better than one would expect, when i employ'd an oyl brought by filings of copper infused in it, to a deep green. and this (by the way) may be the reason, why often times when the oyls of some spices and of anniseeds &c. are distilled in a limbec with water, the water (as i have several times observ'd) comes over whitish, and will perhaps continue so for a good while, because if the fire be made too strong, the subtile chymical oyl is thereby much agitated and broken, and blended with the water in such numerous and minute globules, as cannot easily in a short time emerge to the top of the water, and whilst they remain in it, make it, for the reason newly intimated, look whitish; and perhaps upon the same ground a cause may be rendred, why hot water is observ'd to be usually more opacous and whitish, than the same water cold, the agitation turning the more spirituous or otherwise conveniently dispos'd particles of the water into vapours, thereby producing in the body of the liquor a multitude of small bubbles, which interrupt the free passage, that the beams of light would else have every way, and from the innermost parts of the water reflect many of them outwards. these and the like examples, _pyrophilus_, have induc'd me to suspect, that the superficial particles of white bodies, may for the most part be as well convex as smooth; i content my self to say _suspect_ and _for the most part_, because it seems not easie to prove, that when diaphanous bodies, as we shall see by and by, are reduc'd into white powders, each corpuscle must needs be of a convex superficies, since perhaps it may suffice that specular surfaces look severally ways. for (as we have seen) when a diaphanous body comes to be reduc'd to very minute parts, it thereby requires a multitude of little surfaces within a narrow compass. and though each of these should not be of a figure convenient to reflect a round image of the sun, yet even from such an inconveniently figur'd body, there may be reflected some (either streight or crooked) physical line of light, which line i call physical, because it has some breadth in it, and in which line in many cases some refraction of the light falling upon the body it depends on, may contribute to the brightness, as if a slender wire, or solid cylinder of glass be expos'd to the light, you shall see in some part of it a vivid line of light, and if we were able to draw out and lay together a multitude of these little wires or thrids of glass, so slender, that the eye could not discern a distance betwixt the luminous lines, there is little doubt (as far as i can guess by a tryal purposely made with very slender, but far less slender thrids of glass, whose aggregate was look'd upon one way white) but the whole physical superficies compos'd of them, would to the eye appear white, and if so, it will not be always necessary that the figure of those corpuscles, that make a body appear white, should be _globulous_. and as for snow it self, though the learned _gassendus_ (as we have seen above) makes it to seem nothing else but a pure frozen froth, consisting of exceedingly minute and thickset bubbles; yet i see no necessity of admitting that, since not only by the variously and curiously figur'd snow, that i have divers times had the opportunity with pleasure to observe, but also by the common snow, it rather doth appear both to the naked eye, and in a _microscope_, often, if not most commonly, to consist principally of little slender icicles of several shapes, which afford such numerous lines of light, as we have been newly speaking of. . sixthly, if you take a diaphanous body, as for instance a piece of glass, and reduce it to powder, the same body, which when it was entire, freely transmitted the beams of light, acquiring by contusion a multitude of minute surfaces, each of which is as it were a little, but imperfect _speculum_, is qualify'd to reflect in a confus'd manner, so many either beams, or little and singly unobservable images of the lucid body, that from a diaphanous it degenerates into a white body. and i remember, i have for trials sake taken lumps of rock crystal, and heating them red hot in a crucible, i found according to my expectation, that being quench'd in fair water, even those that remain'd in seemingly entire lumps exchang'd their translucency for whiteness, the ignition and extinction having as it were crack'd each lump into a multitude of minute bodies, and thereby given it a great multitude of new surfaces. and ev'n with diaphanous bodies, that are colour'd, there may be this way a greater degree of whiteness produced, than one would lightly think; as i remember, i have by contusion obtain'd whitish powders of _granates_, glass of _antimony_, and _emeralds_ finely beaten, and you may more easily make the experiment, by taking good venereal _vitriol_ of a deep blew, and comparing with some of the entire crystalls purposely reserv'd, some of the subtile powder of the same salt, which will comparatively exhibit a very considerable degree of whitishness. . seventhly, and as by a change of position in the parts, a body that is not white, may be made white, so by a slight change of the texture of its surface, a white body may be depriv'd of its whiteness. for if, (as i have try'd in gold-smiths shops) you take a piece of silver that has been freshly boyl'd, as the artificers call it, (which is done by, first brushing, and then decocting it with salt and tartar, and perhaps some other ingredients) you shall find it to be of a lovely white. but if you take a piece of smooth steel, and therewith burnish a part of it, which may be presently done, you shall find that part will lose its whiteness, and turn a _speculum_, looking almost every where dark, as other looking-glasses do, which may not a little confirm our doctrine. for by this we may guess, what it is chiefly that made the body white before, by considering that all that was done to deprive it of that whiteness, was only to depress the little protuberances that were before on the surface of the silver into one continu'd superficies, and thereby effect this, that now the image of the lucid body, and consequently a kind of whiteness shall appear to your eye, but in some place of the greater silver looking-glass (whence the beams reflected at an angle equal to that wherewith they fall on it, may reach your eye) whilst the asperity remain'd undestroy'd, the light falling on innumerable little _specula_ obverted some one way, and some another, did from all sensibly distinguishable parts of the superficies reflect confus'd beams or representations of light to the beholders eye, from whence soever he chance to look upon it. and among the experiments annex'd to this discourse, you will find one, wherein by the change of texture in bodies, whiteness is in a trice both generated and destroy'd. * * * * * chap. ii. . what we have discours'd of whiteness, may somewhat assist us to form a notion of blackness, those two qualities being contrary enough to illustrate each other. yet among the antient _philosophers_ i find less assistance to form a notion of blackness than of whiteness, only _democritus_ in the passage above recited out of _aristotle_ has given a general hint of the cause of this colour, by referring the blackness of bodies to their asperity. but this i call but a general hint, because those bodies that are green, and purple, and blew, seem to be so as well as black ones, upon the account of their superficial asperity. but among the _moderns_, the formerly mention'd _gassendus_, perhaps invited by this hint of _democritus_, has incidentally in another epistle given us, though a very short, yet a somewhat clearer account of the nature of blackness in these words: _existimare par est corpora suâpte naturâ nigra constare ex particulis, quarum superficieculæ scabræ sint, nec facilè lucem extrorsum reflectant._ i wish this ingenious man had enlarg'd himself upon this subject; for indeed it seems, that as that which makes a body white, is chiefly such a disposition of its parts, that it reflects (i mean without much interruption) more of the light that falls on it, than bodies of any other colour do, so that which makes a body black is principally a peculiar kind of texture, chiefly of its superficial particle, whereby it does as it were dead the light that falls on it, so that very little is reflected outwards to the eye. . and this texture may be explicated two, and perhaps more than two several ways, whereof the first is by supposing in the superficies of the black body a particular kind of asperity, whereby the superficial particles reflect but few of the incident beams outwards, and the rest inwards towards the body it self. as if for instance, we should conceive the surface of a black body to be asperated by an almost numberless throng of little cylinders, pyramids, cones, and other such corpuscles, which by their being thick set and _erected_, reflect the beams of light from one to another inwards, and send them too and fro so often, that at length they are lost before they can come to rebound out again to the eye. and this is the first of the two mention'd ways of explicating blackness. the other way is by supposing the texture of black bodies to be such, that either by their yielding to the beams of light, or upon some other account, they do as it were dead the beams of light, and keep them from being reflected in any plenty, or with any considerable vigour of motion, outwards. according to this notion it may be said, that the corpuscles that make up the beams of light, whether they be solary _effluviums_, or minute particles of some Ætherial substance, thrusting on one another from the lucid body, do, falling on black bodies, meet with such a texture, that such bodies receive into themselves, and retain almost all the motion communicated to them by the corpuscles that make up the beams of light, and consequently reflect but few of them, or those but languidly, towards the eye, it happening here almost in like manner as to a ball, which thrown against a stone or floor, would rebound a great way upwards, but rebounds very little or not at all, when it is thrown against water, or mud, or a loose net, because the parts yield, and receive into themselves the motion, on whose account the ball should be reflected outwards. but this last way of explicating blackness, i shall content my self to have propos'd, without either adopting it, or absolutely rejecting it. for the hardness of touchstones, black marble and other bodies, that being black are solid, seem to make it somewhat improbable, that such bodies should be of so yielding a texture, unless we should say, that some bodies may be more dispos'd to yield to the impulses of the corpuscles of light by reason of a peculiar texture, than other bodies, that in other tryals appear to be softer than they. but though the former of these two explications of blackness be that, by which we shall endeavour to give an account of it, yet as we said, we shall not absolutely reject this latter, partly because they both agree in this, that black bodies reflect but little of the light that falls on them, and partly because it is not impossible, that in some cases both the disposition of the superficial particles, as to figure and position, and the yielding of the body, or some of its parts, may joyntly, though not in an equal measure concurr to the rendring of a body black. the considerations that induc'd me to propose this notion of blackness, as i explan'd it, are principally these: . first, that as i lately said, whiteness and blackness being generally reputed to be contrary qualities, whiteness depending as i said upon the disposition of the parts of a body to reflect much light, it seems likely, that blackness may depend upon a contrary disposition of the black bodies surface; but upon this i shall not insist. . next then we see, that if a body of one and the same colour be plac'd, part in the sun-beams, and part in the shade, that part which is not shin'd on will appear more of kin to blackness than the other, from which more light rebounds to the eye; and dark colours seem the blacker, the less light they are look'd upon in, and we think all things black in the dark, when they send no beams to make impressions on our organs of sight, so that shadows and darkness are near of kin, and shaddow we know is but a privation of light; and accordingly blackness seems to proceed from the paucity of beams reflected from the black body to the eye, i say the paucity of beams, because those bodies that we call black, as marble, jeat, &c. are short of being perfectly so, else we should not see them at all. but though the beams that fall on the sides of those erected particles that we have been mentioning, do few of them return outwards, yet those that fall upon the points of those cylinders, cones, or pyramids, may thence rebound to the eye, though they make there but a faint impression, because they arrive not there, but mingl'd with a great proportion of little shades. this may be confirm'd by my having procur'd a large piece of black marble well polish'd, and brought to the form of a large sphærical and concave _speculum_; for on the inside this marble being well polish'd, was a kind of dark looking-glass, wherein i could plainly see a little image of the sun, when that shin'd upon it. but this image was very far from offending and dazling my eyes, as it would have done from another _speculum_; nor, though the _speculum_ were large, could i in a long time, or in a hot sun set a piece of wood on fire, though a far less _speculum_ of the same form, and of a more reflecting matter, would have made it flame in a trice. . and on this occasion we may as well in reference to something formerly deliver'd concerning whiteness, as in reference to what has been newly said, subjoyn what we further observ'd touching the differing reflections of light from white and black marble, namely, that having taking a pretty large mortar of white marble, new and polish'd in the inside, and expos'd it to the sun, we found that it reflected a great deal of glaring light, but so dispers'd, that we could not make the reflected beams concurr in any such conspicuous _focus_, as that newly taken notice of in the black marble, though perhaps there may enough of them be made to meet near the bottom, to make some kind of _focus_, especially since by holding in the night-time a candle at a convenient distance, we were able to procure a concourse of some, though not many of the reflected beams, at about two inches distant from the bottom of the mortar: but we found the heat even of the sunbeams so dispersedly reflected to be very languid, even in comparison of the black marbles _focus_. and the little picture of the sun, that appear'd upon the white marble as a _speculum_, was but very faint and exceeding ill defin'd. secondly, that taking two pieces of plain and polish'd surfaces, and casting on them successively the beams of the same candle, in such manner, as that the neighbouring superficies being shaded by an opacous and perforated body, the incident beams were permitted to pass but through a round hole of about half an inch diameter, the circle of light that appear'd on the white marble was in comparison very bright, but very ill defin'd; whereas that on the black marble was far less luminous, but much more precisely defin'd. . thirdly, when you look upon a piece of linnen that has small holes in it, those holes appear very black, and men are often deceiv'd in taking holes for spots of ink; and painters to represent holes, make use of black, the reason of which seems to be, that the beams that fall on those holes, fall into them so deep, that none of them is reflected back to the eye. and in narrow wells part of the mouth seems black, because the incident beams are reflected downwards from one side to another, till they can no more rebound to the eye. we may consider too, that if differing parts of the same piece of black velvet be stroak'd opposite ways, the piece of velvet will appear of two distinct kinds of blackness, the one far darker than the other, of which disparity the reason seems to be, that in the less obscure part of the velvet, the little silken piles whereof 'tis made up, being inclin'd, there is a greater part of each of them obverted to the eye, whereas in the other part the piles of silk being more erected, there are far fewer beams reflected outwards from the lateral parts of each pile, so that most of those that rebound to the eye, come from the tops of the piles, which make but a small part of the whole superficies, that may be cover'd by the piece of velvet. which explication i propose, not that i think the blackness of the velvet proceeds from the cause assign'd, since each single pile of silk is black by reason of its texture, in what position soever you look upon it; but that the greater blackness of one of these tuffts seems to proceed from the greater paucity of beams reflected from it, and that from the fewness of those parts of a surface that reflect beams, and the multitude of those shaded parts that reflect none. and i remember, that i have oftentimes observ'd, that the position of particular bodies far greater than piles of silk in reference to the eye, may notwithstanding their having each of them a colour of its own, make one part of their aggregate appear far darker than the other; for i have near great towns often taken notice, that a cart-load of carrots pack'd up, appear'd of a much darker colour when look'd upon, where the points of the carrots were obverted to the eye, than where the sides of them were so. . fourthly, in a darkned room, i purposely observ'd, that if the sun-beams, which came in at the hole were receiv'd upon white or any other colour, and directed to a convenient place of the room, they would manifestly, though not all equally, encrease the light of that part; whereas if we substituted, either a piece of black cloth or black velvet, it would so dead the incident beams, that the place (newly mention'd) whereto i obverted the black body, would be less enlightned than it was before, when it received its light but from the weak and oblique reflections of the floor and walls of a pretty large room, through which the beams that came in at the hole were confusedly and brokenly dispers'd. . fifthly, and to shew that the beams that fall on black bodies, as they do not rebound outwards to the eye, so they are reflected towards the body it self, as the nature of those erected particles to which we have imputed blackness, requires, we will add an experiment that will also confirm our doctrine touching whiteness; namely, that we took a broad and large tile, and having whitened over one half of the superficies of it, and black'd the other, we expos'd it to the summer sun; and having let it lye there a convenient time (for the difference is more apparent, if it have not lain there too long) we found, as we expected, that whilst the whited part of the tile remained cool enough, the black'd part of the same tile was grown not only sensible, but very hot, (sometimes to a strong degree.) and to satisfie some of our friends the more, we have sometimes left upon the surface of the tile, besides the white and black parts thereof, a part that retain'd the native red of the tile it self, and exposing them to the sun, we observ'd this last mention'd to have contracted a heat in comparison of the white, but a heat inferiour to that of the black, of which the reason seems to be, that the superficial particles of black bodies, being, as we said, more erected, than those of white or red ones, the corpuscles of light falling on their sides, being for the most part reflected inwards from one particle to another, and thereby engag'd as it were and kept from rebounding upwards, they communicate their brisk motion, wherewith they were impell'd against the black body, (upon whose account had they fallen upon a white body, they would have been reflected outwards) to the small parts of the black body, and thereby produce in those small parts such an agitation, as (when we feel it) we are wont to call heat. i have been lately inform'd, that an observation near of kin to ours, has been made by some learned men in _france_ and _italy_, by long exposing to a very hot sun, two pieces of marble, the one white, the other black; but though the observation be worthy of them, and may confirm the same truth with our experiment, yet besides that our tryal needs not the summer, nor any great heat to succeed, it seems to have this advantage above the other, that whereas bodies more solid, and of a closer texture, though they use to be more slowly heated, are wont to receive a greater degree of heat from the sun or fire, than (_cæteris paribus_) bodies of a slightest texture; i have found by the information of stone-cutters, and by other ways of enquiry, that black marble is much solider and harder than white, so that possibly the difference betwixt the degrees of heat they receive from the sunbeams will by many be ascrib'd to the difference of their texture, rather than to that of their colour, though i think our experiment will make it probable enough that the greater part of that difference may well be ascrib'd to that disposition of parts, which makes the one reflect the sunbeams inward; and the other outwards. and with this doctrine accords very well, that rooms hung with black, are not only darker than else they would be, but are wont to be warmer too; insomuch that i have known a great lady, whose constitution was somewhat tender, complain that she was wont to catch cold, when she went out into the air, after having made any long visits to persons, whose rooms were hung with black. and this is not the only lady i have heard complain of the warmth of such rooms, which though perhaps it may be partly imputed to the _effluvia_ of those materials wherewith the hangings were dy'd, yet probably the warmth of such rooms depends chiefly upon the same cause that the darkness does; as (not to repeat what i formerly noted touching my gloves,) to satisfie some curious persons of that sex, i have convinc'd them, by tryall, that of two pieces of silken stuff given me by themselves, and expos'd in their presence, to the same window, shin'd on by that sun, the white was _considerably_ heated, when the black was not so much as _sensibly_ so. . sixthly, i remember, that acquainting one day a _virtuoso_ of unsuspected credit, that had visited hot countries, with part of what i have here deliver'd concerning blackness, he related to me by way of confirmation of it, a very notable experiment, which he had both others make, and made himself in a warm climate, namely, that having carefully black'd over eggs, and expos'd them to the hot sun, they were thereby in no very long time well roasted, to which effect i conceive the heat of the climate must have concurr'd with the disposition of the black surface to reflect the sunbeams inward, for i remember, that having made that among other tryals in _england_, though in summer-time, the eggs i expos'd, acquir'd indeed a considerable degree of heat, but yet not so intense a one, as prov'd sufficient to roast them. . seventhly, and lastly, our conjectures at the nature of blackness may be somewhat confirm'd by the (formerly mention'd) observation of the blind _dutch-man_, that discerns colours with his fingers; for he says, that he feels a greater roughness upon the surfaces of black bodies, than upon those of red, or yellow, or green. and i remember, that the diligent _bartholinus_ says,[ ] that a blind earl of _mansfield_ could distinguish white from black only by the touch, which would sufficiently argue a great disparity in the asperities, or other superficial textures of bodies of those two colours, if the learn'd relator had affirm'd the matter upon his own knowledge. [ ] hist. anatom. cent. . hist. . ii. these, _pyrophilus_, are the chief things that occurr to me at present, about the nature of whiteness and blackness, which it they have rendred it so much as probable, that in _most_; or at least _many_ cases, the causes of these qualities may be such as i have adventur'd to deliver, it is as much as i pretend to; for till i have opportunity to examine the matter by some further tryals, i am not sure, but that in some white and black bodies, there may concurr to the colour some peculiar texture or disposition of the body, whereby the motion of the small corpuscles that make up the incident beams of light, may be differingly modify'd, before they reach the eye, especially in this, that white bodies do not only copiously reflect those incident corpuscles outwards, but reflect them briskly, and do not otherwise alter them in the manner of their motion. nor shall i now stay to enquire, whether some of those other ways, (as a disposition to alter the velocity, the rotation, or the order and manner of appulse so the eye of the reflected corpuscles that compos'd the incident beams of light) which we mention'd when we consider'd the production of colours in general, may not in some cases be applicable to those of white and black bodies: for i am yet so much a _seeker_ in this matter, and so little wedded to the opinions i have propos'd, that what i am to add shall be but the beginning of a collection of experiments and observation towards the history of whiteness and blackness, without at present interposing my explications of them, that so, i may assist your enquires without much fore-stalling or biassing your judgment. * * * * * experiment in consort, touching whiteness & blackness. * * * * * experiment i. having promis'd in the , and . pages of the foregoing discourse of whiteness and blackness, to shew, that those two colours may by a change of texture in bodies, each of them apart diaphanous and colourless, be at pleasure and in a trice as well generated as destroy'd, we shall begin with experiments that may acquit us of that promise. take then what quantity you please of fair water, and having heated it, put into it as much good common sublimate, as it is able to dissolve, and (to be sure of having it well glutted:) continue putting in the sublimate, till some of it lye untouch'd in the bottom of the liquor, filter this solution through cap-paper, to have it cleer and limpid, and into a spoonfull or two thereof, (put into a clean glass vessel,) shake about four or five drops (according as you took more or less of this solution) of good limpid spirits of urine, and immediately the whole mixture will appear white like milk, to which mixture if you presently add a convenient proportion of rectifi'd _aqua fortis_ (for the number of drops is hard to determine, because of the differing strength of the liquor, but easily found by tryal) the whiteness will presently disappear, and the whole mixture become transparent, which you may, if you please, again reduce to a good degree of whiteness (though inferiour to the first) onely by a more copious affusion of fresh spirit of urine. _n_. first, that it is not so necessary to employ either _aqua fortis_ or spirit of urine about this experiment, but that we have made it with other liquors instead of these, of which perhaps more elsewhere. secondly, that this experiment, though not made with the same _menstruums_, nor producing the same colour is yet much of kin to that other to be mentioned in this tract among our other experiments of colours, about turning a solution of præcipitate into an orange-colour, and the chymical reason being much alike in both, the annexing it to one of them may suffice for both. _experiment ii._ make a strong infusion of broken galls in fair water, and having filtred it into a clean vial, add more of the same liquor to it, till you have made it somewhat transparent, and sufficiently diluted the colour, for the credit of the experiment, lest otherwise the darkness of the liquor might make it be objected, that 'twas already almost ink; into this infusion shake a convenient quantity of a cleer, but very strong solution of vitriol, and you shall immediately see the mixture turn black almost like ink, and such a way of producing blackness is vulgar enough; but if presently after you doe upon this mixture drop a small quantity of good oyl of vitriol, and, by shaking the vial disperse it nimbly through the two other liquors, you shall (if you perform your part well, and have employ'd oyl of vitriol cleer and strong enough) see the darkness of the liquor presently begin to be discuss'd, and grow pretty cleer and transparent, losing its inky blackness, which you may again restore to it by the affusion of a small quantity of a very strong solution of salt of tartar. and though neither of these atramentous liquors will seem other than very pale ink, if you write with a clean pen dipt in them, yet that is common to them with some sorts of ink that prove very good when dry, as i have also found, that when i made these carefully, what i wrote with either of them, especially with the former, would when throughly dry grow black enough not to appear bad ink. this experiment of taking away and restoring blackness from and to the liquors, we have likewise tryed in common ink; but there it succeeds not so well, and but very slowly, by reason that the gum wont to be employed in the making it, does by its tenacity oppose the operations of the above mention'd saline liquors. but to consider gum no more, what some kind of præcipitation may have to do in the producing and destroying of inks without it, i have elsewhere given you some occasion and assistance to enquire; but i must not now stay to do so my self, only i shall take notice to you, that though it be taken for granted that bodies will not be præcipitated by alcalizat salts, that have not first been dissolved in some acid _menstruums_, yet i have found upon tryals, which my conjectures lead me to make on purpose, that divers vegetables _barely infus'd_, or, _but slightly decocted in common water_, would, upon the affusion of a strong and cleer _lixivium_ of potashes, and much more of some other præcipitating liquors that i sometimes employ, afford good store of a crudled matter, such as i have had in the præcipitations of vegetable substances, by the intervention of acid things, and that this matter was easily separable from the rest of the liquor, being left behind by it in the filtre; and in making the first ink mention'd in this experiment, i found that i could by filtration separate pretty store of a very black pulverable substance, that remain'd in the filtre, and when the ink was made cleer again by the oyl of vitriol, the affusion of dissolv'd _sal tartari_ seem'd but to præcipitate, and thereby to unite and render conspicuous the particles of the black mixture that had before been dispers'd into very minute and singly invisible particles by the incisive and resolving power of the highly corrosive oyl of vitriol. and to manifest, _pyrophilus_, that galls are not so requisite as many suppose to the making atramentous liquors, we have sometimes made the following experiment, we took dryed rose leaves and decocted them for a while in fair water, into two or three spoonfulls of this decoction we shook a few drops of a strong and well filtrated solution of vitriol (which perhaps had it been green would have done as well) and immediately the mixture did turn black, and when into this mixture presently after it was made, we shook a just proportion of _aqua fortis_, we turn'd it from a black ink to a deep red one, which by the affusion of a little spirit of urine may be reduc'd immediately to an opacous and blackish colour. and in regard, _pyrophilus_, that in the former experiments, both the infusion of galls, and the decoction of roses, and the solution of copperis employ'd about them, are endow'd each of them with its own colour, there may be a more noble experiment of the sudden production of blackness made by the way mention'd in the second section of the second part of our essays, for though upon the confusion of the two liquors there mention'd, there do immediately emerge a very black mixture, yet both the infusion of _orpiment_ and the solution of _minium_ were before their being joyn'd together, limpid and colourless. _experiment iii._ if pieces of white harts-horn be with a competent degree of fire distill'd in a glass-retort, they will, after the avolation of the flegm, spirit, volatile salt, and the looser and lighter parts of the oleagenous substance, remain behind of a cole-black colour. and even ivory it self being skilfully burnt (how i am wont to do it, i have elsewhere set down) affords painters one of the best and deepest blacks they have, and yet in the instance of distill'd harts-horn, the operation being made in glass-vessels carefully clos'd, it appears there is no extraneous black substance that insinuates it self into white harts-horn, and thereby makes it turn black; but that the whiteness is destroy'd, and the blackness generated, only by a change of texture, made in the burnt body, by the recess of some parts and the transposition of others. and though i remember not that in many distillations of harts-horn i ever sound the _cap. mort_. to pass from black to a true whiteness, whilst it continu'd in clos'd vessels, yet having taken out the cole-black fragments, and calcin'd them in open vessels, i could in few hours quite destroy that blackness, & without sensibly changing their bulk or figure, reduce them to great whiteness. so much do these two colours depend upon the disposition of the little parts, that the bodies wherein they are to be met with do consist of. and we find, that if whitewine tartar, or even the white crystalls of such tartar be burnt without being truly calcin'd, the _cap. mortuum_ (as the chymists call the more fixt part) will be black. but if you further continue the calcination till you have perfectly incinerated the tartar, & kept it long enough in a strong fire, the remaining _calx_ will be white. and so we see that not only other vegetable substances, but even white woods, as the hazel, will yield a black charcoal, and afterwards whitish ashes; and so animal substances naturally white, as bones and eggshels, will grow black upon the being burnt, and white again when they are perfectly calcin'd. _experiment iv._ but yet i much question whether that rule delivered by divers, as well philosophers as chymists, _adusta nigra, sed perusta alba_, will hold as universally as is presum'd, since i have several examples to allege against it: for i have found that by burning alablaster, so as both to make it appear to boyl almost like milk, and to reduce it to a very fine powder, it would not at all grow black, but retain its pure and native whiteness, and though by keeping it longer than is usual in the fire, i produced but a faint yellow, even in that part of the powder that lay nearest the top of the crucible, yet having purposely enquired of an experienced stone-cutter, who is curious enough in tryng conclusions in his own trade, he told me he had found that if alabaster or plaster of paris be very long kept in a strong fire, the whole heap of burnt powder would exchange its whiteness for a much deeper colour than the yellow i observ'd. lead being calcin'd with a strong fire turns (after having purhaps run thorough divers other colour) into _minium_, whose colour we know is a deep red; and if you urge this _minium_, as i have purposely done with a strong fire, you may much easier find a glassie and brittle body darker than _minium_, than any white _calx_ or glass. 'tis known among chymists, that the white _calx_ of antimony, by the further and more vehement operation of the fire, may be melted into glass, which we have obtain'd of a red colour, which is far deeper than that of the _calx_ of burnt antimony, and though common glafs of antimony being usually adulterated with _borax_, have its colour thereby diluted, oftentimes to a very pale yellow; yet not onely ours made more sincerily, was, as we said, of a colour less remote from black, than was the _calx_; but we observ'd, that by melting it once or twice more, and so exposing it to the further operation of the fire, we had, as we expected, the colour heightned. to which we shall add but this one instance, (which is worth the taking notice of in reference to colours:) that, if you take blew, but unsophisticated, vitriol, and burn it very slowly, and with a gentle degree of heat, you may observe, that when it has burnt but a little, and yet so far as that you may rub it to powder betwixt your fingers, it will be of a white or whitish colour; but if you prosecute the calcination, this body which by a light adustion was made white, will pass through other colours, as gray, yellowish, and red; and if you further burn it with a long and vehement fire, by that time it comes to be _perustum_, it will be of a dark purple, nearer to black, not only than the first _calx_, but than the vitriol before it at all felt the fire. i might add that _crocus_ _martis_ (_per se_ as they call it) made by the lasting violence of the reverberated flames is not so near a kin to white, as the iron or steel that afforded it was before its calcinations; but that i suppose, these instances may suffice to satisfie you, that minerals are to be excepted out of the forementioned rule, which perhaps, though it seldome fail in substances belonging to the vegetable or animal kingdome, may yet be question'd even in some of these, if that be true, which the judicious traveller _bellonius_ affirms, that charcoales made out of the wood of _oxycæder_ are white; and i could not find that though in retorts hartshorn and other white bodies will be denigrated by heat, yet camphire would not at all lose its whiteness, though i have purposely kept it in such a heat, as made it melt and boyl. _experiment v._ and now i speak of camphire, it puts me in mind of adding this experiment, that, though as i said in clos'd glasses, i could not denigrate it by heat, but it would sublime to the sides and top of the glass, as it was before, yet not only it will, being set on fire in the free air, send forth a copious smoak, but having purposely upon some of it that was flaming, clapt a large glass, almost in the form of a hive, (but more slender only) with a hole at the top, (which i caus'd to be made to trye experiments of fire and flame in) it continued so long burning that it lin'd all the inside of the glass with a soot as black as ink, and so copious, that the closeness of the vessel consider'd, almost all that part of the white camphire that did take fire, seem'd to have been chang'd into that deep black substance. _experiment vi_ and this also brings into my mind another experiment that i made about the production of blackness, whereof, for reasons too long to be here deduced, i expected and found a good success, an it was this: i took rectifi'd oyl of vitriol (that i might have the liquor clean as well as strong) and by degrees mixt with it a convenient proportion of the essential oyl, as chymists call it, of wormwood, drawn over with store of water in a limbec, and warily distilling the mixture in a retort, there remain'd a scarce credible quantity of dry matter, black as a coal. and because the oyl of wormwood, though a chymical oyl drawn by a _virtuoso_, seem'd to have somewhat in it of the colour of the plant, i substituted in its room, the pure and subtile essential oyl of winter-savory, and mixing little by little this liquor, with (if i mis-remember not) an equal weight of the formerly mention'd rectifi'd oyl of vitriol, and distilling them as before in a retort, besides what there pass'd over into the receiver, even these two clear liquors left me a considerable proportion, (though not so great as the two former) of a substance black as pitch, which i yet keep by me as a rarity. _experiment vii._ a way of whiting wax cheaply and in great quantity may be a thing of good oeconomical use, and we have elsewhere set down the practice of trades-men that blanch it; but here treating of whiteness only in order to the philosophy of colours, i shall not examine which of the slow wayes may be best employ'd, to free wax from the yellow melleous parts, but shall rather set down a quick way of making it white, though but in very small quantities. take then a little yellow wax, scraped or thinly sliced, and putting it into a bolts-head or some other convenient glass, pour to it a pretty deal of spirit of wine, and placing the vessel in warm sand, encrease the heat by degrees, till the spirit of wine begin to simper or to boyl a little; and continuing that degree of fire, if you have put liquor enough, you will quickly have the wax dissolv'd, then taking it off the fire, you may either suffer it to cool as hastily as with safety to the glass you can, or pour it whilst 'tis yet hot into a filtre of paper, and either in the glass where it cools, or in the filtre, you will soon find the wax and _menstruum_ together reduc'd into a white substance, almost like butter, which by letting the spirit exhale will shrink into a much lesser bulk, but still retaining its whiteness. and that which is pretty in the working of this magistery of wax, is, that the yellowness vanishes, neither appearing in the spirit of wine that passes limpid through the filtre, nor in the butter of wax, if i may so call it, that, as i said, is white. _experiment viii._ there is an experiment, _pyrophilus_, which though i do not so exactly remember, and though it be somewhat nice to make, yet i am willing to acquaint you with, because the thing produc'd, though it be but a curiosity, is wont not a little to please the beholders, and it is a way of turning by the help of a dry substance, an almost golden-colour'd concrete, into a white one, the several tryals are not at present so fresh in my memory to enable me to tell you certainly, whether an equal onely or a double weight of common sublimate must be taken in reference to the tinglass, but if i mistake not, there was in the experiment that succeeded best, two parts of the former taken to one of the latter. these ingredients being finely powdred and exactly mix'd, we sublim'd together by degrees of fire (the due gradation of which is in this experiment a thing of main importance) there ascended a matter of a very peculiar texture, for it was for the most part made up of very thin, smooth, soft and slippery plates, almost like the finest sort of the scales of fishes, but of so lovely a white inclining to pearl-colour, and of so curious and shining a gloss, that they appear'd in some respect little inferiour to orient pearls, and in other regards, they seem'd to surpass them, and were applauded for a sort of the prettiest trifles that we had ever prepar'd to amuse the eye. i will not undertake that though you'l hardly miss changing the colour of your shining tinglass, yet you will the first or perhaps the second time hit right upon the way of making the glistring sublimate i have been mentioning. _experiment ix._ when we dissolve in _aqua fortis_ a mixture of gold and silver melted into one lump, it usually happens that the powder of gold that falls to the bottom, as not being dissoluble by that _menstruum_, will not have its own yellow, but appear of a black colour, though neither the gold, nor the silver, nor the _aqua fortis_ did before manifest any blackness. and divers alchymists, when they make solutions of minerals they would examine, are very glad, if they see a black powder præcipitated to the bottom, taking it for a hopefull sign, that those particles are of a golden nature, which appear in a colour so ordinary to gold parted from other metalls by _aqua fortis_, that it is a trouble to the refiner to reduce the præcipitated _calx_ to its native colour. for though, (as we have try'd,) that may be quickly enough done by fire, which will make this gold look very gloriously (as indeed 'tis at least one of the best wayes that is practis'd for the refining of gold,) yet it requires both watchfulness and skill, to give it such a degree of fire as will serve to restore it to its lustre, without giving it such a one, as may bring it to fusion, to which the minuteness of the _corpuseles_ it consists of makes the powder very apt. and this brings into my mind, that having taken a flat and bright piece of gold, that was refin'd by a curious and skilfull person on purpose to trye to what height of purity gold could be brought by art, i found that this very piece, as glorious as it look'd, being rubb'd a little upon a piece of fine clean linnen, did sully it with a kind of black; and the like i have observ'd in refin'd silver, which i therefore mention, because i formerly suspected that the impurity of the metall might have been the only cause of what i have divers times obferv'd in wearing silver-hilted swords, namely, that where they rubb'd upon my clothes, if they were of a light-colour'd cloath, the affriction would quickly black them; and congruously hereunto i have found pens blackt almost all over, when i had a while carri'd them about me in a silver ink-case. to which i shall only add, that whereas in these several instances of denigration, the metalls are worn off, or otherwise reduc'd into very minute parts, that circumstance may prove not unworthy your notice. _experiment x._ that a solution of silver does dye hair of a black colour, is a known experiment, which some persons more curious than dextrous, have so unluckily made upon themselves as to make their friends very merry. and i remember that the other day, i made my self some sport by an improvement of this observation, for having dissolv'd some pure silver in _aqua fortis_, and evaporated the _menstruum ad siccitatem_, as they speak, i caus'd a quantity of fair water to be pour'd upon the _calx_ two or three several times, and to be at each evaporated, till the _calx_ was very drye, and all the greenish blewness that is wont to appear in common crystals of silver, was quite carry'd away. then i made those i meant to deceive, moisten some part of their skin with their own spittle, and slightly rub the moistned parts with a little of this prepar'd silver, whereupon they admir'd to see, that a snow-white body laid upon the white skin should presently produce a deep blackness, as if the stains had been made with ink, especially considering that this blackness could not, like that produc'd by ordinary ink, be readily wash'd off, but requir'd many hours, and part of it some dayes to its obliteration. and with the same white _calx_ and a little fair water we likewise stain'd the white hafts of knives, with a lasting black in those parts where the _calx_ was plentifully enough laid on, for where it was laid on but very thinly, the stain was not quite of so deep a colour. _experiment xi_ the cause of the blackness of those many nations, which by one common name we are wont to call _negroes_, has been long since disputed of by learned men, who possibly had not done amiss, if they had also taken into consideration, why some whole races of other animals besides men, as foxes and hares, are distinguish'd by a blackness not familiar to the generality of animals of the same species; the general opinion (to be mention'd a little lower) has been rejected even by some of the antient geographers, and among our moderns _ortelius_ and divers other learned men have question'd it. but this is no place to mention what thoughts i have had to and fro about these matters: only as i shall freely acknowledge, that to me the inquiry seems more abstruse than it does to many others, and that because consulting with authors, and with books of voyages, and with travellers, to satisfie my self in matters of fact, i have met with some things among them, which seem not to agree very well with the notions of the most classick authors concerning these things; for it being my present work to deliver rather matters historical than theorys, i shall annex some few of my collections, instead of a solemn disputation. it is commonly presum'd that the heat of the climate wherein they live, is the reason, why so many inhabitants of the scorching regions of _africa_ are black; and there is this familiar observation to countenance this conjecture, that we plainly see that mowers, reapers, and other countrey-people, who spend the most part of the hot summer dayes expos'd to the sun, have the skin of their hands and faces, which are the parts immediately expos'd to the sun and air, made of a darker colour than before, and consequently tending to blackness; and contrarywise we observe that the _danes_ and some other people that inhabit cold climates, and even the _english_ who feel not so rigorous a cold, have usually whiter faces than the _spaniards_, _portugalls_ and other european inhabitants of hotter climates. but this argument i take to be far more specious than convincing; for though the heat of the sun may darken the colour of the skin, by that operation, which we in _english_ call sun-burning, yet experience doth not evince, that i remember, that that heat alone can produce a discolouring that shall amount to a true blackness, like that of _negroes_, and we shall see by and by that even the children of some _negroes_ not yet . dayes old (perhaps not so much by three quarters of that time) will notwithstanding their infancy be of the same hue with their parents. besides, there is this strong argument to be alleg'd against the vulgar opinion, that in divers places in _asia_ under the same parallel, or even of the same degree of latitude with the _african_ regions inhabited by blacks, the people are at most but tawny;[ ] and in _africa_ it self divers nations in the empire of _ethiopia_ are not _negroes_, though situated in the torrid zone, and as neer the Æquinoctial, as other nations that are so (as the black inhabitants of _zeylan_ and _malabar_ are not in our globes plac'd so near the line as _amara_ the famousest place in _ethiopia_.) moreover, (that which is of no small moment in our present disquisition) i find not by the best navigators and travellers to the _west-indies_, whose books or themselves i have consulted on this subject, that excepting perhaps one place or two of small extent, there are any blacks originally natives of any part of _america_ (for the blacks now there have been by the _europeans_ long transplanted thither) though the new world contain in it so great a variety of climates, and particularly reach quite cross the torri'd zone from one tropick to another. and enough it be true that the _danes_ be a whiter people than the _spaniards_, yet that may proceed rather from other causes (not here to be enquired into) than from the coldness of the climate, since not onely the _swedes_ and other inhabitants of those cold countreys, are not usually so white as the _danes_, nor whiter than other nations in proportion to their vicinity to the pole. [and since the writing of the former part of this essay, having an opportunity on a solemn occasion to take notice of the numerous train of some extraordinary embassadours sent from the _russian_ emperour to a great monarch, observ'd, that (though it were then winter) the colour of their hair and skin was far less whitish than the _danes_ who inhabit a milder region is wont to be, but rather for the most part of a darkish brown; and the physician to the embassadour with whom those _russes_ came, being ask'd by me whether in _muscovy_ it self the generality of the people were more inclin'd to have dark-colour'd hair than flaxen, he answer'd affirmatively; but seem'd to suspect that the true and antient _russians_, a sept of whom he told me he had met with in one of the provinces of that vast empire, were rather white like the _danes_, than any thing near so brown as the present _muscovites_ whom he guesses to be descended of the _tartars_, and to have inherited their colour from them.] but to prosecute our former discourse, i shall add for further proof of the conjecture i was countenancing that good authors inform us that there are _negroes_ in _africa_ not far from the _cape of good hope_, and consequently beyond the southern tropick, and without the torrid zone, much about the same northern latitude (or very little more) wherein there are divers _american_ nations that are not _negroes_, and wherein the inhabitants of _candia_, some parts of _sicily_, and even of _spain_ are not so much as tawny-mores. but (which is a fresh and strong argument against the common opinion,) i find by our recent relations of _greenland_ (our accounts whereof we owe to the curiosity of that royal _virtuoso_ the present king of _denmark_,) that the inhabitants are olive-colour'd, or rather of a darker hiew. but if the case were the same with men, and those other kinds of animals i formerly nam'd, i should offer something as a considerable proof, that, cold may do much towards the making men white or black, and however i shall let down the observation as i have met with it, as worthy to come into the history of whiteness and blackness, and it is, that in some parts of _russia_ and of _livonia_ it is affirm'd by _olaus magnus_ and others, that hares and foxes (some add partridges) which before were black, or red, or gray, do in the depth of winter become white by reason of the great cold; (for that it should be, as some conceive, by looking upon the snow, seems improbable upon divers accounts) and i remember that having purposely enquir'd of a _virtuoso_ who lately travell'd through _livonia_ to _mosco_ concerning the truth of this tradition, he both told me, he believ'd it, and added, that he saw divers of those lately nam'd animals either in _russia_ or _livonia_, (for i do not very well remember whether of the two) which, though white when he saw them in winter, they assur'd him had been black, or of other colours before the winter began, and would be so again when it was over. but for further satisfaction, i also consulted one that had for some years been an eminent physician in _russia_, who though he rejected some other traditions that are generally enough believ'd concerning that countrey, told me nevertheless, that he saw no cause to doubt of this tradition of _olaus magnus_ as to foxes and hares, not onely because 'tis the common and uncontroul'd assertion of the natives, but also because he himself in the winter could never that he remember'd see foxes and hares of any other colour than white; and i my self having seen a small white fox brought out of _russia_ into _england_ towards the latter end of winter, foretold those that shew'd him me, that he would change colour in summer, and accordingly coming to look upon him again in _july_, i found that the back and sides, together with the upper part of the head and tayl were already grown of a dark colour, the lower part of the head and belly containing as yet a whiteness. let me add, that were it not for some scruple i have, i should think more than what _olaus_ relates, confirm'd by the judicious _olearius_, who was twice employ'd into those parts as a publick minister, who in his account of _moscovy_ has this passage: _the hares there are gray; but in some provinces they grow white in the winter_. and within some few lines after: _it is not very difficult to find the cause of this change, which certainly proceeds only from the outward cold, since i know that even in summer, hares will change colour, if they be kept a competent time in a cellar_; i say, were it not for some scruple, because i take notice, that in the same page the author affirms, that the like change of colour that happens to hares in some provinces of _muscovy_, happens to them also in _livonia_, and yet immediately subjoyns, that in _curland_ the hares vary not their colour in winter, though these two last named countries be contiguous, (that is) sever'd only by the river of _dugna_; for it is scarce conceivable how cold alone should have, in countries so near, so strangely differing an operation, though no less strange a thing is confess'd by many, that ascribe the complexion of _negroes_ to the heat of the sun, when they would have the river of _cenega_ so to bound the _moors_, that though on the north-side they are but tawny, on the other side they are black. [ ] olearius voyage de mosco. et de perse _liv_. . there is another opinion concerning the complexion of _negroes_, that is not only embrac'd by many of the more vulgar writers, but likewise by that ingenious traveller mr. _sandys_, and by a late most learned critick, besides other men of note, and these would have the blackness of _negroes_ an effect of _noah's_ curse ratify'd by god's, upon _cham_; but though i think that even a naturalist may without disparagement believe all the miracles attested by the holy scriptures, yet in this case to flye to a supernatural cause, will, i fear, look like shifting off the difficulty, instead of resolving it; for we enquire not the first and universal, but the proper, immediate, and physical cause of the jetty colour of _negroes_; and not only we do not find expressed in the scripture, that the curse meant by _noah_ to _cham_, was the blackness of his posterity, but we do find plainly enough there that the curse was quite another thing, namely that he should be a servant of servants, that is by an ebraism, a very abject servant to his brethren, which accordingly did in part come to pass, when the _israelites_ of the posterity of _sem_, subdued the _canaanites_, that descended from _cham_, and kept them in great subjection. nor is it evident that blackness is a curse, for navigators tell us of black nations, who think so much otherwise of their own condition, that they paint the devil white. nor is blackness inconsistent with beauty, which even to our european eyes consists not so much in colour, as an advantageous stature, a comely symmetry of the parts of the body, and good features in the face. so that i see not why blackness should be thought such a curse to the _negroes_, unless perhaps it be, that being wont to go naked in those hot climates, the colour of their skin does probably, according to the doctrine above deliver'd, make the sun-beams more scorching to them, than they would prove to a people of a white complexion. greater probability there is, that the principal cause (for i would not exclude all concurrent ones) of the blackness of _negroes_ is some peculiar and seminal impression, for not onely we see that _blackmore_ boyes brought over into these colder climates lose not their colour; but good authors inform us, that the off-spring of _negroes_ transplanted out of _africa_, above a hundred years ago, retain still the complexion of their progenitors, though possibly in tract of time it will decay; as on the other side, the white people removing into very hot climates, have their skins by the heat of the sun scorch'd into dark colours; yet neither they, nor their children have been observ'd, even in the countreys of _negroes_, to descend to a colour amounting to that of the natives; whereas i remember i have read in _pisos_[ ] excellent account of _brasile_, that betwixt the _americans_ and _negroes_ are generated a distinct sort of men, which they call _cabocles_, and betwixt _portugalls_ and _Æthiopian_ women, he tells us, he has sometimes seen twins, whereof one had a white skin, the other a black; not to mention here some other instances, he gives, that the productions of the mixtures of differing people, that is (indeed,) the effects of seminal impressions which they consequently argue to have been their causes; and we shall not much scruple at this, if we consider, that even organical parts may receive great differences from such peculiar impressions, upon what account soever they came to be setled in the first individual persons, from whom they are propogated to posterity, as we see in the blobber-lips and flat-noses of most nations of _negroes_. and if we may credit what learned men deliver concerning the little feet of the _chinesses_, the _macrocephali_ taken notice of by _hippocrates_, will not be the only instance we might apply to our present purpose. and on this occasion it will not perchance be impertinent to add something of what i have observ'd in other animals, as that there is a sort of hens that want rumps; and that (not to mention that in several places there is a sort of crows or daws that are not cole-black as ours, but partly of a whitish colour) in spight of _porphyries_ examples of inseparable accidents, i have seen a perfectly white raven, as to bill as well as feathers, which i attentively considered, for fear of being impos'd upon. and this recalls into my memory, what a very ingenious physician has divers times related to me of a young lady, to whom being call'd, he found that though she much complain'd of want of health, yet there appear'd so little cause either in her body, or her condition to guess that she did any more than fancy her self sick, that scrupling to give her physick, he perswaded her friends rather to divert her mind by little journeys of pleasure, in one of which going to visit st. _winifrids_ well, this lady, who was a _catholick_, and devout in her religion, and a pretty while in the water to perform some devotions, and had occasion to fix her eyes very attentively upon the red pipple-stones, which in a scatter'd order made up a good part of those that appear'd through the water, and a while after growing bigg, she was deliver'd of a child, whose white skin was copiously speckl'd with spots of the colour and bignesss of those stones, and though now this child have already liv'd several years, yet she still retains them. i have but two things to add concerning the blackness of _negroes_, the one is, that the seat of that colour seems to be but the thin _epidermes_, or outward skin, for i knew a young _negroe_, who having been lightly sick of the small pox or measles, (for it was doubted which of the two was his disease) i found by enquiry of a person that was concern'd for him, that in those places where the little tumors had broke their passage through the skin, when they were gone, they left within specks behind them; and the lately commended _piso_ assures us, that having the opportunity in _brasil_ to dissect many _negroes_, he cleerly found that their blackness went no deeper than the very outward skin, which _cuticula_ or _epidermis_ being remov'd, the undermost skin or _cutis_ appear'd just as white as that of _europæan_ bodyes. and the like has been affirmed to me by a physician of our own, whom, hearing he had dissectcd a _negroe_ here in _england_, i consulted about this particular. the other thing to be here taken notice of concerning _negroes_ is, that having enquir'd of an intelligent acquaintance of mine (who keeps in the _indies_ about . of them as well women as men to work in his plantations,) whether their children come black into the world; he answer'd, that they did not, but were brought forth of almost the like reddish colour with our _european_ children; and having further enquir'd, how long it was before these infants appear'd black, be reply'd, that 'twas not wont to be many daies. and agreeable to this account i find that, given us in a freshly publish'd french book written by a _jesuit_, that had good opportunity of knowing the truth of what he delivers, for being one of the missionaries of his order into the southern _america_ upon the laudable design of converting infidels to christianity, he baptiz'd several infants, which when newly born, were much of the same colour with _european_ babes, but within about a week began to appear of the hue of their parents. but more pregnant is the testimony of our countrey-man _andrew battel_, who being sent prisoner by the _portugalls_ to _angola_, liv'd there, and in the adjoyning regions, partly as a prisoner, partly as a pilot, and partly as a souldier, near . years, and he mentioning the _african_ kingdom of _longo_, peopl'd with blacks, has this passage:[ ] _the children in this countrey are born white, and change their colour in two dayes to a perfect black_. as for example, _the_ portugalls _which dwell in the kingdome of_ longo _have sometimes children by the_ negroe_-women, and many times the fathers are deceived, thinking, when the child is born, that it is theirs, and within two dayes it proves the son or daughter of a_ negroe,_ which the_ portugalls _greatly grieve at_; and the same person has elsewhere a relation, which, if he have made no use at all of the liberty of a traveller, is very well worth our notice, since this, together with that we have formerly mention'd of seminal impressions, shews a possibility, that a race of _negroes_ might be begun, though none of the sons of _adam_, for many precedent generations were of that complexion. for i see not why it should not be at least as possible, that white parents may sometimes have black children, as that _african negroes_ should sometimes have lastingly white ones, especially since concurrent causes may easily more befriend the productions of the former kind, than under the scorching heat of _africa_ those of the latter. and i remember on the occasion of what he delivers, that of the white raven formerly mention'd, the possessor affirm'd to me, that in the nest out of which he was taken white, they found with him but one other young one, and that he was of as jetty a black as any common raven. but let us hear our author himself[ ]; _here are_ (sayes he, speaking of the formerly mention'd regions) _born in this countrey white children, which is very rare among them, for their parents are_ negroes; _and when any of them are born, they are presented to the king, and are call'd_ dondos; _these are as white as any white men. these are the kings witches, and are brought up in witchcraft, and alwayes wait on the king: there is no man that dare meddle with these_ dondos, _if they go to the market they may take what they lift, for all men stand in awe of them. the king of_ longo _hath four of them_. and yet this countrey in our globes is plac'd almost in the midst of the torrid zone (four or five degrees southward of the line.) and our author elsewhere tells us of the inhabitants, that they are so fond of their blackness, that they will not suffer any that is not of that colour (as the _portugalls_ that come to trade thither) to be so much as buri'd in their land, of which he annexes a particular example,[ ] that may be seen in his voyage preserv'd by our industrious countreyman mr. _purchas_. but it is high time for me to dismiss observations, and go on with experiments. [ ] _piso_ nat. & med. hist. _brasil. lib_ . in fine. [ ] _purchas_ pilgrim. second part, seventh book . chap. sect . [ ] _purchas_. ibid. [ ] _purchas_ ibid. in fin _experiment xii._ the way, _pyrophilus,_ of producing whiteness by chymical præcipitations is very well worth our observing, for thereby bodyes of very differing colours as well as natures, though dissolv'd in several liquors, are all brought into _calces_ or powders that are white. thus we find that not only crabs-eyes, that are of themselves white, and pearls that are almost so, but _coral_ and _minium_ that are red, being dissolv'd in spirit of vinegar, may be uniformly præcipitated by oyl of _tartar_ into white powders. thus silver and tin separately dissolv'd in _aqua fortis_, will the one præcipitate it self, and the other be præcipitated by common salt-water into a white _calx_, and so will crude lead and quicksilver first dissolv'd likewise in _aqua fortis_. the like _calx_ will be afforded as i have try'd by a solution of that shining mineral tinglass dissolv'd in _aqua fortis_, and præcipitated out of it; and divers of these _calces_ may be made at least as fair and white, if not better colour'd, if instead of oyl of _tartar_ they were præcipitated with oyl of _vitriol_, or with another liquor i could name. nay, that black mineral _antimony_ it self, being reduc'd by and with the salts that concurr to the composition of common sublimate, into that cleer though unctuous liquor that chymists commonly call rectifi'd butter of _antimony_, will by the bare affusion of store of fair water be struck down into that snow-white powder, which when the adhering saltness is well wash'd off, chymists are pleas'd to call _mercurius vitæ_, though the like powder may be made of _antimony_, without the addition of any _mercury_ at all. and this lactescence if i may so call it, does also commonly ensue when spirit of wine, being impregnated with those parts of gums or other vegetable concretions, that are suppos'd to abound with sulphureous corpuscles, fair water is suddenly pour'd upon the tincture or solution. and i remember that very lately i did, for tryal sake, on a tincture of _benjamin_ drawn with spirit of wine, and brought to be as red as blood, pour some fair water, which presently mingling with the liquor, immediately turn'd the whole mixture white. but if such seeming milks be suffer'd to stand unstirr'd for a convenient while, they are wont to let fall to the bottome a resinous substance, which the spirit of wine diluted and weakned by the water pour'd into it was unable to support any longer. and something of kin to this change of colour in vegetables is that, which chymists are wont to observe upon the pouring of acid spirits upon the red solution of _sulphur_, dissolv'd in an infusion of pot-ashes, or in some other sharp _lixivium_, the præcipitated _sulphur_ before it subsides, immediately turning the red liquor into a white one. and other examples might be added of this way of producing whiteness in bodyes by præcipitating them out of the liquors wherein they have been dissolv'd; but i think it may be more usefull to admonish you, _pyrophilus_, that this observation admits of restrictions, and is not so universal, as by this time perhaps you have begun to think it; for though most præcipitated bodyes are white, yet i know some that are not; for gold dissolv'd in _aqua regis_, whether you præcipitate it with oyl of _tartar_, or with spirit of _sal armoniack_, will not afford a white but a yellow _calx_. _mercury_ also though reduc'd into sublimate, and præcipitated with liquors abounding with volatile salts, as the spirits drawn from urine, harts-horn, and other animal substances, yet will afford, as we noted in our first experiment about whiteness and blackness, a white præcipitate, yet with some solutions hereafter to be mentioned, it will let fall an orange-tawny powder. and so will crude _antimony_, if, being dissolv'd in a strong lye, you pour (as farr as i remember) any acid liquor upon the solution newly filtrated, whilst it is yet warm. and if upon the filtrated solution of _vitriol_, you pour a solution of one of these fix'd salts, there will subside a copious substance, very farr from having any whiteness, which the chymists are pleas'd to call, how properly i have elsewhere examin'd, the _sulphur of vitriol_. so that most part of dissolv'd bodyes being by præcipitation brought to white powders, and yet some affording præcipitates of other colours, the reason of both the phænomena may deserve to be enquir'd into. _experiment xiii._ some learned modern writers[ ] are of opinion, that the account upon which whiteness and blackness ought to be call'd, as they commonly are, the two extreme colours, is, that blackness (by which i presume is meant the bodyes endow'd with it) receives no other colours; but whiteness very easily receives them all; whence some of them compare whiteness to the _aristotelian materia prima_, that being capable of any sort of forms, as they suppose white bodyes to be of every kind of colour. but not to dispute about names or expressions, the thing it self that is affirm'd as matter of fact, seems to be true enough in most cases, not in all, or so, as to hold universally. for though it be a common observation among dyers, that clothes, which have once been throughly imbu'd with black, cannot so well afterwards be dy'd into lighter colours, the præexistent dark colour infecting the ingredients, that carry the lighter colour to be introduc'd, and making it degenerate into some more sad one; yet the experiments lately mention'd may shew us, that where the change of colour in black bodies is attempted, not by mingling bodyes of lighter colours with them, but by addition of such things as are proper to alter the texture of those corpuscles that contain the black colour, 'tis no such difficult matter, as the lately mention'd learned men imagine, to alter the colour of black bodyes. for we saw that inks of several kinds might in a trice be depriv'd of all their blackness; and those made with logwood and red-roses might also be chang'd, the one into a red, the other into a reddish liquor; and with oyl of _vitriol_ i have sometimes turn'd black pieces of silk into a kind of yellow, and though the taffaty were thereby made rotten, yet the spoyling of that does no way prejudice the experiment, the change of black silk into yellow, being never the less true, because the yellow silk is the less good. and as for whiteness, i think the general affirmation of its being so easily destroy'd or transmuted by any other colour, ought not to be receiv'd without some cautions and restrictions. for whereas, according to what i formerly noted, lead is by calcination turned into that red powder we call _minium_; and tin by calcination reduc'd to a white _calx_, the common putty that is sold and us'd so much in shops, instead of being, as it is pretended and ought to be, only the _calx_ of tin, is, by the artificers that make it, to save the charge of tin, made, (as some, of themselves have confess'd, and as i long suspected by the cheap rate it may be bought for) but of half tin and half lead, if not far more lead than tin, and yet the putty in spight of so much lead is a very white powder, without disclosing any mixture of _minium_. and so if you take two parts of copper, which is a high-colour'd metall, to but one of tin, you may by fusion bring them into one mass, wherein the whiteness of the tin is much more conspicuous and predominant than the reddishness of the copper. and on this occasion it may not be impertinent to mention an experiment, which i relate upon the credit of a very honest man, whom i purposely enquir'd of about it, being my self not very fond of making tryals with _arsenick_, the experiment is this, that if you colliquate _arsenick_ and copper in a due proportion, the _arsenick_ will blanch the copper both within and without, which is an experiment well enough known; but when i enquir'd, whether or no this white mixture being skilfully kept a while upon the cupel would not let go its _arsenick_, which made whiteness its prædominant colour, and return to the reddishness of copper, i was assur'd of the affirmative; so that among mineral bodyes, some of those that are white, may be far more capable, than those i am reasoning with seem to have known, of eclipsing others, and of making their colour prædominant in mixtures. in further confirmation of which may be added, that i remember that i also took a lump of silver and gold melted together, wherein by the Æstimate of a very experienced refiner, there might be about a fourth or third part of gold, and yet the yellow colour of the gold was so hid by the white of the silver, that the whole mass appear'd to be but silver, and when it was rubb'd upon the touchstone, an ordinary beholder could scarce have distinguish'd it from the touch of common silver; though if i put a little _aqua fortis_ upon any part of the white surface it had given the touchstone, the silver in the moistned part being immediately taken up and conceal'd by the liquor, the golden particles would presently disclose that native yellow, and look rather as if gold, than if the above mention'd mixture, had been rubb'd upon the stone. [ ] see _scaliger_ exercit. . sect. . _experiment xiv._ i took a piece of black-horn, (polish'd as being part of a comb) this with a piece of broken glass i scrap'd into many thin and curdled flakes, some shorter and some longer, and having laid a pretty quantity of these scrapings together, i found, as i look'd for, that the heap they compos'd was white, and though, if i laid it upon a clean piece of white paper, its colour seem'd somewhat eclips'd by the greater whiteness of the body it was compar'd with, looking somewhat like linnen that had been sulli'd by a little wearing, yet if i laid it upon a very black body, as upon a beaver hatt, it then appear'd to be of a good white, which experiment, that you may in a trice make when you please, seems very much to disfavour both their doctrine that would have colours to flow from the substantial forms of bodyes, and that of the chymists also, who ascribe them to one or other of their three hypostatical principles; for though in our case there was so great a change made, that the same body without being substantially either increas'd or lessened, passes immediately from one extreme colour to another (and that too from black to white) yet this so great and sudden change is effected by a slight mechanical transposition of parts, there being no salt or _sulphur_ or _mercury_ that can be pretended to be added or taken away, nor yet any substantial form that can reasonably be suppos'd to be generated and destroy'd, the effect proceeding only from a local motion of the parts which so vary'd their position as to multiply their distinct surfaces, and to qualifie them to reflect far more light to the eye, than they could before they were scrap'd off from the entire piece of black horn. _experiment xv._ and now, _pyrophilus_, it will not be improper for us to take some notice of an opinion touching the cause of blackness, which i judged it not so seasonable to question, till i i had set down some of the experiments, that might justifie my dissent from it. you know that of late divers learned men, having adopted the three hypostatical principles, besides other notions of the chymists, are very inclinable to reduce all qualities of bodies to one or other of those three principles, and particularly assign for the cause of blackness the sootie steam of _adust_ or _torrifi'd sulphur_. but i hope that what we have deliver'd above to countenance the opinion we have propos'd about the cause of blackness, will so easily supply you with several particulars that may be made use of against this opinion, that i shall now represent to you but two things concerning it. and first it seems that the favourers of the chymicall theories might have pitcht upon some more proper term, to express the efficient of blackness than _sulphur adust_; for we know that _common sulphur_, not only when melted, but even when sublim'd, does not grow black by suffering the action of the fire, but continues and ascends yellow, and rather more than less white, than it was before its being expos'd to the fire. and if it be set on fire, as when we make that acid liquor, that chymists call _oleum sulphuris per campanam_, it affords very little soot, and indeed the flame yeelds so little, that it will scarce in any degree black a sheet of white paper, held a pretty while over the flame and smoak of it, which is observed rather to whiten than infect linnen, and which does plainly make red roses grow very pale, but not at all black, as far as the smoak is permitted to reach the leaves. and i can shew you of a sort of fixt sulphur made by an industrious laborant of your acquaintance, who assur'd me that he was wont to keep it for divers weeks together night and day in a naked and violent fire, almost like that of the glass-house, and when, to satisfie my curiosity, i made him take out a lump of it, though it were glowing hot (and yet not melted,) it did not, when i had suffered it to cool, appear black, the true colour of it being a true red. i know it may be said, that _chymists_ in the opinion above recited mean the _principle of sulphur_, and not _common sulphur_ which receives its name, not from its being _all_ perfectly of a sulphureous nature, but for that _plenty_ and _predominancy_ of the sulphureous principle in it. but allowing this, 'tis easie to reply, that still according to this very reason, torrifi'd sulphur should afford more blackness, than most other concretes, wherein that principle is confess'd to be far less copious. also when i have expos'd camphire to the fire in close vessels, as inflamable, and consequenly (according to the chymists) as sulphureous a body as it is, i could not by such a degree of heat, as brought it to fusion, and made it boyl in the glass, impress any thing of blackness, or of any other colour, than its own pure white, upon this vegetable concrete. but what shall we say to spirit of wine, which being made by a chymical analysis of the liquor that affords it, and being totally inflamable, seems to have a full right to the title they give it of _sulphur vegetabile_, & yet this fluid sulphur not only contracts not any degree of blackness by being often so heated, as to be made to boyl, but when it burns away with an actual flame, i have not found that it would discolour a piece of white paper held over it, with any discernable soot. tin also, that wants not, according to the chymists, a _sulphur joviale_, when throughly burned by the fire into a _calx_, is not black, but eminently white. and i lately noted to you out of _bellonius_, that the charcoals of oxy-cedar are not of the former of these two colours, but of the latter. and the smoak of our tinby coals here in _england_, has been usually observ'd, rather to blanch linnen then to black it. to all which, other particulars of the like nature might be added, but i rather choose to put you in mind of the third experiment, about making black liquors, or inks, of bodies that were non of them black before. for how can it be said, that when those liquors are put together actually cold, and continue so after their mixture, there intervenes any new _adustion of sulphur_ to produce the emergent blackness? (and the same question will be appliable to the blackness produc'd upon the blade of a knife, that has cut lemmons and some kind of sowr apples, if the juyce (though both actually and potentially cold) be not quickly wip'd of) and when by the instilling either of a few drops of oyl of vitriol as in the second experiment, or of a little of the liquor mention'd in the passage pointed at in the fourth experiment, (where i teach at once to destroy one black ink, and make another) the blackness produc'd by those experiments is presently destroy'd; if the colour proceeded only from the plenty of sulphurous parts, torrify'd in the black bodies, i demand, what becomes of them, when the colour so suddenly dissappears? for it cannot reasonably be said, that all those that suffic'd to make so great a quantity of black matter, should resort to so very small a proportion of the clarifying liquor, (if i may so call it) as to be deluted by it, with out at all denigrating it. and if it be said that the instill'd liquor dispers'd those black corpuscles, i demand, how that dispersion comes to destroy their blackness, but by making such a local motion of their parts, as destroys their former texture? which may be a matter of such moment in cases like ours, that i remember that i have in few houres, without addition, from soot it self, attain'd pretty store of crystalline salt, and good store of transparent liquor, and (which i have on another occasion noted as remarkable) this so black substance had its colour so alter'd, by the change of texture it receiv'd from the fire, wherewith it was distill'd, that it did for a great while afford such plenty of very white exhalations, that the receiver, though large, seem'd to be almost fill'd with milk. secondly, but were it granted, as it is in some cases not improbable, that divers bodies may receive a blackness from a sootie exhalation, occasion'd by the adustion of their sulphur, which (for the reasons lately mention'd i should rather call their oyly parts;) yet still this account is applicable but to some particular bodies, and will afford us no general theory of blackness. for if, for example, white harts-horn, being, in vessels well luted to each other, expos'd to the fire, be said to turn black by the infection of its own smoak, i think i may justly demand, what it is that makes the smoak or soot it self black, since no such colour, but its contrary, appear'd before in the harts-horn? and with the same reason, when we are told, that torrify'd sulphur makes bodies black, i desire to be told also, why torrefaction makes sulphur it self black? nor will there be any satisfactory reason assign'd of these quæries, without taking in those fertile as well as intelligible mechanical principles of the position and texture of the minute parts of the body in reference to the light and the eye; and these applicable principles may serve the turn in many cases, where the adustion of sulphur cannot be pretended; as in the appearing blackness of an open window, lookt upon at a somewhat remote distance from the house, as also in the blackness men think they see in the holes that happen to be in white linnen, or paper of the like colour; and in the increasing blackness immediatly produc'd barely by so rubbing velvet, whose piles were inclin'd before, as to reduce them to a more erected posture, in which and in many other cases formerly alleg'd, there appears nothing requisite to the production of _the_ blackness, but the hindering of the incident beams of light from rebounding plentifully enough to the eye. to be short, those i reason with, do concerning blackness, what the chymists are wont also to do concerning other qualities, namely to content themselves to tell us, in what ingredient of a mixt body, the quality enquir'd after, does reside, instead of explicating the nature of it, which (to borrow a comparison from their own laboratories) is much as if in an enquiry after the cause of salivation, they should think it enough to tell us, that the several kinds of præcipitates of gold and _mercury_) as likewise of quick-silver and silver (for i know that make and use of such precipitates also) do salivate upon the account of the _mercury_, which though disguis'd abounds in them, whereas the difficulty is as much to know upon what account _mercury_ it self, rather than other bodies, has that power of working by salivation. which i say not, as though it were not _something_ (and too often the most we can arrive at) to discover in which of the ingredients of a compounded body, the quality, whose nature is sought, resides, but because, though this discovery it self may pass for _something_, and is oftentimes more than what is taught us about the same subjects in the schools, yet we ought not to think it _enough_, when more clear and particular accounts are to be had. * * * * * the experimental history of colours begun. * * * * * the third part. * * * * * containing promiscuous experiments about colours. * * * * * experiment i. because that, according to the conjectures i have above propos'd, one of the most general causes of the diversity of colours in opacous bodyes, is, that some reflect the light mingl'd with more, others with less of shade (either as to quantity, or as to interruption) i hold it not unfit to mention in the first place, the experiments that i thought upon to examine this conjecture. and though coming to transcribe them out of some physiological _adversaria_ i had written in loose papers, i cannot find one of the chief records i had of my tryals of this nature, yet the papers that scap'd miscarrying, will, i presume, suffice to manifest the main thing for which i now allege them; i find then among my _adversaria_, the following narrative. _october_ the . about ten in the morning in sun-shiny weather, (but not without fleeting clouds) we took several sorts of paper stain'd, some of one colour, and some of another; and in a darken'd room whose window look'd southward, we cast the beams that came in at a hole about three inches and a half in diameter, upon a white wall that was plac'd on one side, about five foot distance from them. the white gave much the brightest reflection. the green, red, and blew being compar'd together, the red gave much the strongest reflection, and manifestly enough also threw its _colour_ upon the wall; the green and blew were scarce discernable by their colours, and seem'd to reflect an almost equal light. the yellow compar'd with the two last nam'd, reflected somewhat more light. the red and purple being compar'd together, the former manifestly reflected a good deal more light. the blew and purple compar'd together, the former seem'd to reflect a little more light, though the purple colour were more manifestly seen. a sheet of very well fleck'd marbl'd paper being apply'd as the others, did not cast any or its distinct colours upon the wall; nor throw its light upon it with an equal diffusion, but threw the beams unstain'd and bright to this and that part of the wall, as if it's polish had given it the nature of a specular body. but comparing it with a sheet of white paper, we found the reflection of the latter to be much stronger, it diffusing almost as much light to a _good extent_ as the marble paper did to _one part_ of the wall. the green and purple left us somewhat in suspence which reflected the most light; only the purple seem'd to have some little advantage over the green, which was dark in its kind. thus much i find in our above mention'd _collections_, among which there are also some notes concerning the production of _compounded colours_, _by reflection_ from bodyes differingly colour'd. and these notes we intended should supply us with what we should mention as our second experiment: but having lost the paper that contain'd the particulars, and remembring onely in general, that if the objects which reflected the light were not strongly colour'd and somewhat glossy, the reflected beams would not manifestly make a compounded colour upon the wall, and even then but very faintly, we shall now say no more of that matter, only reserving our selves to mention hereafter the composition of a green, which we still retain in memory. _experiment ii._ we may add, _pyrophilus_, on this occasion, that though a darken'd room be generally thought requisite to make the colour of a body appear by reflection from another body, that is not one of those that are commonly agreed upon to be specular (as polish'd metall, quick silver, glass, water, &c.) yet i have often observ'd that when i wore doublets lin'd with some silken stuff that was very glossy and vividly colour'd, especially red, i could in an inlightned room plainly enough discern the colour, upon the pure white linnen that came out at my sleeve and reach'd to my cufs; as if that fine white body were more specular, than colour'd and unpolish'd bodyes are thought capable of being. _experiment iii._ whilst we were making the newly mention'd experiments, we thought fit to try also what composition of colours might be made by altering the light in its passage to the eye by the interposition not of perfectly diaphanous bodies, (that having been already try'd by others as well as by us (as we shall soon have occasion to take notice) but of semi-opacous bodyes, and those such as look'd upon in an ordinary light, and not held betwixt it and the eye, are not wont to be discriminated from the rest of opacous bodyes; of this tryal, our mention'd _adversaria_ present us the following account. holding these sheets, sometimes one sometimes the other of them, before the hole betwixt the sun and the eye, with the colour'd sides obverted to the sun; we found them _single_ to be somewhat transparent, and appear of the same colour as before, onely a little alter'd by the great light they were plac'd in; but laying _two_ of them one over another and applying them so to the hole, the colours were compounded as follows. the blew and yellow scarce exhibited any thing but a darker yellow, which we ascrib'd to the coarseness of the blew papers, and its darkness in its kind. for applying the blew parts of the marbl'd paper with the yellow paper after the same manner, they exhibited a good green. the yellow and red look'd upon together gave us but a dark red, somewhat (and but a little,) inclining to an orange colour. the purple and red look'd on together appear'd more scarlet. the purple and yellow made an orange. the green and red made a dark orange tawny. the green and purple made the purple appear more dirty. the blew and purple made the purple more lovely, and far more deep. the red parts of the marbl'd paper look'd upon with the yellow appear'd of a red far more like scarlet than without it. [page ] but the fineness or coarseness of the papers, their being carefully or slightly colour'd, and divers other circumstances, may so vary the events of such experiments as these, that if, _pyrophilus_, you would build much on them, you must carefully repeat them. _experiment iv._ the triangular prismatical glass being the instrument upon whose effects we may the most commodiously speculate the nature of emphatical colours, (and perhaps that of others too;) we thought it might be usefull to observe the several reflections and refractions which the incident beams of light suffer in rebounding from it, and passing through it. and this we thought might be best done, not (as is usual,) in an ordinary inlightn'd room, where (by reason of the difficulty of doing otherwise) ev'n the curious have left particulars unheeded, which may in a convenient place be easily taken notice of; but in a darken'd room, where by placing the glass in a convenient posture, the various reflections and refractions may be distinctly observ'd; and where it may appear _what_ beams are unting'd; and _which_ they are, that upon the bodyes that terminate them, do paint either the primary or secondary iris. in pursuance of this we did in the above mention'd darken'd room, make observation of no less than four reflections, and three refractions that were afforded us by the same prism, and thought that notwithstanding what was taught us by the rules of catoptricks and dioptricks, it would not be amiss to find also, by hiding sometimes one part of the prism, and sometimes another, and observing where the light or colour vanish'd thereupon, by which reflection and by which refraction each of the several places whereon the light rebounding from, or passing through, the prism appear'd either sincere or tincted, was produc'd. but because it would be tedious and not so intelligible to deliver this in words, i have thought fit to referr you to the annexed scheme where the newly mention'd particulars may be at one view taken notice of. _experiment v._ [illustration: _the explication of the scheme._] _ppp_. an aequilaterotriangular crystalline prism, one of whose edges _p_. is placed directly towards the sun. _a b_ & [alpha] [beta] two rays from the sun falling on the prism at _b_ [beta]. and thence partly reflected towards _c_ & [gamma]. and partly refracted towards _d_ & [delta]. _b c_ & [beta] [gamma]. those reflected rays. _b d_ & [beta] [delta]. those refracted rays which are partly refracted towards _e_ & [epsilon]. and there paint an iris . denoting the five consecutions of colours red, yellow, green, blew, and purple; and are partly reflected towards _f_ & [zeta]. _d f_ & [delta] [zeta]. those reflected rays which are partly refracted towards _g_ & [eta]. colourless, and partly reflected, towards _h_ & [theta]. _f h_ & [zeta] [theta]. those reflected rays which are refracted towards _i_ & [iota]. and there paint an other fainter iris, the colours of which are contrary to the former . signifying purple, blew, green, yellow, red, so that the prism in this posture exhibits four rainbows. i know not whether you will think it inconsiderable to annex to this experiment, that we observ'd in a room not darken'd, that the prismatical iris (if i may so call it) might be reflected without losing any of its several _colours_ (for we now consider not their _order_) not onely from a plain looking-glass and from the calm surface of fair water, but also from a concave looking-glass; and that refraction did as little destroy those colours as reflection. for by the help of a large (double convex) burning-glass through which we refracted the suns beams, we found that one part of the iris might be made to appear either beyond, or on this side of the other parts of the same iris; but yet the same vivid colours would appear in the displac'd part (if i may so term it) as in the other. to which i shall add, that having, by hiding the side of the prism, obverted to the sun with an opacous body, wherein only one small hole was left for the light to pass through, reduc'd the prismatical iris (cast upon white paper) into a very narrow compass, and look'd upon it througn a microscope; the colours appear'd the same as to kind that they did to the naked eye. _experiment vi._ it may afford matter of speculation to the inquisitive, such as you, _prophilus_, that as the colours of outward objects brought into a darken'd room, do so much depend for their visibility upon the dimness of the light they are there beheld by; that the ordinary light of the day being freely let in upon them, they immediately disappear: so our tryals have inform'd us, that as to the prismatical iris painted on the floor by the beams of the sun trajected through a triangular-glass; though the colours of it appear very vivid ev'n at noon-day, and in sun shiny weather, yet by a more powerfull light they may be made to disappear. for having sometimes, (in prosecution of some conjectures of mine not now to be insisted on,) taken a large metalline concave _speculum_, and with it cast the converging beams of the sun upon a prismatical iris which i had caus'd to be projected upon the floor, i found that the over-powerfull light made the colours of the iris disappear. and if i so reflected the light as that it cross'd but the middle of the iris, in that part only the colours vanish'd or were made invisible; those parts of the iris that were on the right and left hand of the reflected light (which seem'd to divide them, and cut the iris asunder) continuing to exhibit the same colours as before. but upon this we must not now stay to speculate. _experiment vii._ i have sometimes thought it worth while to take notice, whether or no the colours of opacous bodies might not appear to the eye somewhat diversify'd, not only by the disposition of the superficial parts of the bodyes themselves and by the position of the eye in reference to the object and the light, (for these things are notorious enough;) but according also to the nature of the lucid body that shines upon them. and i remember that in prosecution of this curiosity, i observ'd a manifest difference in some kinds of colour'd bodyes look'd on by day-light, and afterwards by the light of the moon; either directly falling on them or reflected upon them from a concave looking-glass. but not finding at present in my collections about colours any thing set down of this kind, i shall, till i have opportunity to repeat them, content my self to add what i find register'd concerning colours look'd on by candle-light, in regard that not only the experiment is more easie to be repeated, but the objects being the same sorts of colour'd paper lastly mention'd, the collation of the two experiments may help to make the conjectures they will suggest somewhat the less uncertain. within a few dayes of the time above mention'd, divers sheets of colour'd paper that had been look'd upon before in the sunshine were look'd upon at night by the light of a pretty big candle, (snuff'd) and the changes that were observ'd were these. the yellow seem'd much fainter than in the day, and inclinable to a pale straw colour. the red seem'd little chang'd; but seem'd to reflect light more strongly than any other colour (for white was none of them.) a fair deep green look'd upon by it self seem'd to be a dark blew: but being look'd upon together with a dark blew, appear'd greenish; and beheld together with a yellow appear'd more blew than at first. the blew look'd more like a deep purple or murray than it had done in the daylight. the purple seem'd very little alter'd. the red look'd upon with the yellow made the yellow look almost like brown cap-paper. _n_. the caution subjoyned to the third experiments is also applicable to this. _experiment viii._ but here i must not omit to subjoyn, that to satisfie our selves, whether or no the light of a candle were not made unsincere, and as it were ting'd with a yellow colour by the admixtion of the corpuscles it assumes from its fuel; we did not content our selves with what appears to the naked eye, but taking a pretty thick rod or cylinder (for thin peeces would not serve the turn) of deep blew glass, and looking upon the candles flame at a convenient distance througn it, we perceiv'd as we expected, the flame to look green; which as we often note, is the colour wont to emerge from the composition of opacous bodies, which were apart one of them blew, and the other yellow. and this perchance may be the main reason of that which some observe, that a sheet of very white paper being look'd upon by candle light, 'tis not easie at first to discern it from a light yellow or lemon colour; white bodyes (as we have elsewhere observ'd) having more than those that are otherwise colour'd, of a specular nature; in regard that though they exhibit not, (unless they be polish'd,) the shape of the luminary that shines on them, yet they reflect its light more sincere and untroubl'd, by either shades or refractions, than bodyes of other colours (as blew, or green, or yellow or the like.) _experiment ix._ we took a leaf of such foliated gold as apothecaries are wont to gild their pills with; and with the edge of a knife, (lightly moysten'd by drawing it over the surface of the tongue, and afterwards) laid upon the edge of the gold leaf; we so fasten'd it to the knife, that being held against the light, it conctinu'd extended like a little flagg. this leaf being held very near the eye, and obverted to the light, appear'd so full of pores, that it seem'd to have such a kind of transparency as that of a sive, or a piece of cyprus, or a love-hood; but the light that pass'd by these pores was in its passages so temper'd with shadow, and modify'd, that the eye discern'd no more a golden colour, but a greenish blew. and for other's satisfaction, we did in the night look upon a candle through such a leaf of gold; and by trying the effect of several proportions of distance betwixt the leaf, the eye and the light, we quickly hit upon such a position for the leaf of gold, as that the flame, look'd on through it, appear'd of a greenish blew, as we have seen in the day time. the like experiment try'd with a leaf of silver succeeded not well. * * * * * _experiment x._ we have sometimes found in the shops of our druggists, a certain wood, which is there called _lignum nephriticum_, because the inhabitants of the country where it grows, are wont to use the infusion of it made in fair water against the stone of the kidneys, and indeed an eminent physician of our acquaintance, who has very particularly enquir'd into that disease, assures me, that he has found such an infusion one of the most effectual remedyes, which he has ever tried against that formidable disease. the ancientest account i have met with of this simple, is given us by the experienc'd _monardes_ in these words. _nobis,_ says he,[ ] _nova hispania mittit quoddam ligni genus crassum & enode, cujus usus jam diu receptus fuit in his regionibus ad renum vitia & urinæ difficultates ac arenulas pellendas. fit autem hac ratione, lignum assulatim & minutim concisum in limpidissima aqua fontana maceratur, inque ea relinquitur, donec aqua à bibentibus absumpta sit, dimidia hora post injectum lignum aqua cæruleum colorem contrabit, qui sensim intenditur pro temporis diuturnitate, tametsi lignum candidum fit_. this wood, _pyrophilus_, may afford us an experiment, which besides the singularity of it, may give no small assistance to an attentive considerer towards the detection of the nature of colours. the experiment as we made it is this. take _lignum nephriticum_, and with a knife cut it into thin slices, put about a handfull of these slices into two three or four pound of the purest spring-water, let them infuse there a night, but if you be in hast, a much shorter time may suffice; _decant_ this impregnated water into a clear glass vial, and if you hold it directly between the light and your eye, you shall see it wholly tincted (excepting the very top of the liquor, wherein you will some times discern a sky-colour'd circle) with an almost golden colour, unless your infusion have been made too strong of the wood, for in that case it will against the light appear somewhat dark and reddish, and requires to be diluted by the addition of a convenient quantity of fair water. but if you hold this vial from the light, so that your eye be plac'd betwixt the window and the vial, the liquor will appear of a deep and lovely cæruleous colour, of which also the drops, if any be lying on the outside of the glass, will seem to be very perfectly; and thus far we have try'd the experiment, and found it to succeed even by the light of candles of the larger size. if you so hold the vial over against your eyes, that it may have a window on one side of it, and a dark part of the room both before it and on the other side, you shall see the liquor partly of a blewish and partly of a golden colour. if turning your back to the window, you powr out some of the liquor towards the light and towards your eyes, it will seem at the comming out of the glass to be perfectly cæruleous, but when it is fallen down a little way, the drops may seem particolour'd, according as the beams of light do more or less fully penetrate and illustrate them. if you take a bason about half full of water, and having plac'd it so in the sun-beams shining into a room, that one part of the water may be freely illustrated by the beams of light, and the other part of it darkned by the shadow of the brim of the bason, if then i say you drop of our tincture, made somewhat strong, both into the shaded and illuminated parts of the water, you may by looking upon it from several places, and by a little agitation of the water, observe divers pleasing phænomena which were tedious to particularize. if you powr a little of this tincture upon a sheet of white paper, so as the liquor may remain of some depth upon it, you may perceive the neighbouring drops to be partly of one colour, and partly of the other, according to the position of your eye in reference to the light when it looks upon them, but if you powr off all the liquor, the paper will seem dy'd of an almost yellow colour. and if a sheet of paper with some of this liquor in it be plac'd in a window where the sunbeams may shine freely on it, then if you turn your back to the sun and take a pen or some such slender body, and hold it over-thwart betwixt the sun and the liquor, you may perceive that the shadow projected by the pen upon the liquor, will not all of it be a vulgar and dark, but in part a curiously colour'd shadow, that edge of it, which is next the body that makes it, being almost of a lively golden colour, and the remoter verge of a cæruleous one. [ ] _nicolaus monardes_ lib _simplic. ex india allatis_, cap. . these and other phænomena, which i have observ'd in this delightfull experiment, divers of my friends have look'd upon not without some wonder, and i remember an excellent oculist finding by accident in a friends chamber a fine vial full of this liquor, which i had given that friend, and having never heard any thing of the experiment, nor having any body near him that could tell him what this strange liquor might be, was a great while apprehensive, as he presently after told me, that some strange new distemper was invading his eyes. and i confess that the unusualness of the phænomena made me very sollicitous to find out the cause of this experiment, and though i am far from pretending to have found it, yet my enquiries have, i suppose, enabled me to give such hints, as may lead your greater sagacity to the discovery of the cause of this wonder. and first finding that this tincture, if it were too copious in the water, kept the colours from being so lively, and their change from being so discernable, and finding also that the impregnating virtue of this wood did by its being frequently infus'd in new water by degrees decay, i conjectur'd that the tincture afforded by the wood must proceed from some subtiler parts of it drawn forth by the water, which swimming too and fro in it did so modifie the light, as to exhibit such and such colours; and because these subtile parts were so easily soluble even in cold water, i concluded that they must abound with salts, and perhaps contain much of the essential salt, as the _chymists_ call it, of the wood. and to try whether these subtile parts were volatile enough to be distill'd, without the dissolution of their texture, i carefully distill'd some of the tincted liquor in very low vessels, and the gentle heat of a lamp furnace; but found all that came over to be as limpid and colourless as rock-water, and the liquor remaining in the vessel to be so deeply cæruleous, that it requir'd to be oppos'd to a very strong light to appear of any other colour. i took likewise a vial with spirit of wine, and a little salt of harts-horn, and found that there was a certain proportion to be met with betwixt the liquor and the salt, which made the mixture fit to exhibit some little variety of colours not observable in ordinary liquors, as it was variously directed in reference to the light and the eye, but this change of colour was very far short from that which we had admir'd in our tincture. but however, i suspected that the tinging particles did abound with such salts, whose texture, and the colour springing from it, would probably be alter'd by peircing acid salts, which would in likelihood either make some dissipation of their parts, or associate themselves to the like bodies, and either way alter the colour exhibited by them; whereupon pouring into a small vial full of impregnated water, a very little spirit of vinegar, i found that according to my expectation, the cæruleous colour immediately vanish'd, but was deceiv'd in the expectation i had, that the golden colour would do so too; for, which way soever i turned the vial, either to or from the light, i found the liquor to appear always of a yellowish colour and no other: upon this i imagin'd that the acid salts of the vinegar having been able to deprive the liquor of its cæruleous colour, a sulphureous salt being of a contrary nature, would be able to mortifie the saline particles of vinegar, and destroy their effects; and accordingly having plac'd my self betwixt the window, and the vial, and into the same liquor dropt a few drops of oyl of tartar _per deliquium_, (as _chymists_ call it) i observ'd with pleasure, that immediately upon the diffusion of this liquor, the impregnated water was restor'd to its former cæruleous colour; and this liquor of _tartar_ being very ponderous, and falling at first to the bottom of the vial, it was easie to observe that for a little while the lower part of the liquor appear'd deeply cæruleous; whilst all the upper part retain'd its former yellowness, which it immediately lost as soon as either agitation or time had made a competent diffusion of the liquor of _tartar_ through the body of the former tincture; and this restored liquor did, as it was look'd upon against or from the light, exhibit the same _phænomena_ as the tincted water did, before either of the adventitious liquors was pour'd into it. having made, _pyrophilus_, divers tryals upon this nephritick wood, we found mention made of it by the industrious jesuit _kircherus_, who having received a cup turned of it from the _mexican_ procurator of his society, has probably receiv'd also from him the information he gives us concerning that _exotick_ plant, and therefore partly for that reason, and partly because what he writes concerning it, does not perfectly agree with what we have deliver'd, we shall not scruple to acquaint you in his own words, with as much of what he writes concerning our wood, as is requisite to our present purpose. _hoc loco_ (says he)[ ] _neutiquam omittendum duximus quoddam ligni candidi mexicani genus, quod indigenæ coalle & tlapazatli vocant, quod etsi experientia hucusque non nisi cæruleo aquam colore tingere docuerit, nos tamen continua experientia invenimus id aquam in omne colorum genus transformare, quod merito cuipiam paradoxum videri posset; ligni frutex grandis, ut aiunt, non rarò in molem arboris excrescit, truncus illius eft crassus, enodis, instar piri arboris, folia ciceris foliis, aut rutæ haud absimilia, flores exigui, oblongi, lutei & spicatim digesti; est frigida & humida planta, licet parum recedat à medio temperamento. hujus itaque descriptæ arboris lignum in poculum efformatum, aquam eidem infusam primo in aquam intense cæruleam, colore floris buglossæ; tingit, & quo diutius in eo steterit, tanto intensiorem colorem acquirit. hanc igitur aquam si vitreæ sphæræ infuderis, lucique exposueris, ne ullum quidem cærulei coloris vestigium apparebit, sed instar aquæ puræ putæ fontanæ limpidam claramque aspicientibus se præbebit. porro si hanc phialam vitream versus locum magis umbrosum direxeris, totus humor gratissimum virorem referet; si adhuc umbrosioribus locis, subrubrum, & sic pro rerum objectarum conditione, mirum dictu, colorem mutabit; in tenebris verò vel in vase opaco posita, cæruleum colorem suum resumet._ [ ] kircher. art. mag. lucis & umbræ, _lib. . part. ._ in this passage we may take notice of the following particulars. and first, he calls it a white _mexican_ wood, whereas (not to mention that _mornardes_ informs us that it is brought out of _nova hispania_) the wood that we have met with in several places, and employ'd as _lignum nephriticum_, was not white, but for the most part of a much darker colour, not unlike that of the sadder colour'd wood of juniper. 'tis true, that _monardes_ himself also says, that the wood is white; and it is affirm'd, that the wood which is of a sadder colour is adulterated by being imbu'd with the tincture of a vegetable, in whose decoction it is steep'd. but having purposely enquir'd of the eminentest of our _english_ druggists, he peremptorily deny'd it. and indeed, having consider'd some of the fairest round pieces of this wood that i could meet with in these parts, i had opportunity to take notice that in one or two of them it was the external part of the wood that was white, and the more inward part that was of the other colour, the contrary of which would probably have appear'd, if the wood had been adulterated after the afore-mention'd manner. and i have at present by me a piece of such wood, which for about an inch next the bark is white, and then as it were abruptly passes to the above-mention'd colour, and yet this wood by the tincture, it afforded us in water, appears to have its colour'd part genuine enough; for as for the white part, it appears upon tryal of both at once, much less enrich'd with the tingent property. next, whereas our author tells us, that the infusion of this wood expos'd in a vial to the light, looks like spring-water, in which he afterwards adds, that there is no tincture to be seen in it, our observation and his agree not, for the liquor, which opposed to the darker part of a room exhibits a sky-colour, did constantly, when held against the light, appear yellowish or reddish, according as its tincture was more dilute or deep; and then, whereas it has been already said, that the cæruleous colour was by acid salts abolished, this yellowish one surviv'd without any considerable alteration, so that unless our author's words be taken in a very limited sense, we must conclude, that either his memory mis-inform'd him, or that his white _nephritick_ wood, and the sadder colour'd one which we employ'd, were not altogether of the same nature: what he mentions of the cup made of _lignum nephriticum_, we have not had opportunity to try, not having been able to procure pieces of that wood great enough, and otherwise fit to be turned into cups; but as for what he says in the title of his experiment, that this wood tinges the water with all sorts of colours, that is much more than any of those pieces of nephritick wood that we have hitherto employ'd, was able to make good; the change of colours discernable in a vial full of water, impregnated by any of them, as it is directed towards a place more lightsome or obscure, being far from affording a variety answerable to so promising a title. and as for what he tells us, that in the dark the infusion of our wood will resume a cæruleous colour, i wish he had inform'd us how he try'd it. but this brings into my mind, that having sometimes for curiosity sake, brought a round vial with a long neck fill'd with the tincture of _lignum nephriticum_ into the darken'd room already often mention'd, and holding it sometimes in, sometimes near the sun-beams that enter'd at the hole, and sometimes partly in them, and partly out of them, the glass being held in several postures, and look'd upon from several neighbouring parts of the room, disclos'd a much greater variety of colours than in ordinary inlightn'd rooms it is wont to do; exhibiting, besides the usual colours, a red in some parts, and a green in others, besides intermediate colours produc'd by the differing degrees, and odd mixtures of light and shade. by all this you may see, _pyrophilus_, the reasonableness of what we elsewhere had occasion to mention, when we have divers times told you, that it is usefull to have new experiments try'd over again, though they were, at first, made by knowing and candid men, such reiterations of experiments commonly exhibiting some new phænomena, detecting some mistake or hinting some truth, in reference to them, that was not formerly taken notice of. and some of our friends have been pleas'd to think, that we have made no unusefull addition to this experiment, by shewing a way, how in a moment our liquor may be depriv'd of its blewness, and restor'd to it again by the affusion of a very few drops of liquors, which have neither of them any colour at all of their own. and that which deserves some particular wonder, is, that the cæruleous tincture of our wood is subject by the former method to be destroy'd or restor'd, the yellowish or reddish tincture continuing what it was. and that you may see, that salts are of a considerable use in the striking of colours, let me add to the many experiments which may be afforded us to this purpose by the dyers trade, this observation; that as far as we have hitherto try'd, those liquors in general that are strong of acid salts have the power of destroying the blewness of the infusion of our wood, and those liquors indiscriminatly that abound with sulphureous salts, (under which i comprehend the urinous and volatile salts of animal substances, and the alcalisate or fixed salts that are made by incineration) have the vertue of restoring it. _a corollary of the tenth experiment._ that this experiment, _pyrophilus_, may be as well usefull as delightfull to you, i must mind you, _pyrophilus_, that in the newly mention'd observation, i have hinted to you a new and easie way of discovering in many liquors (for i dare not say in all) whether it be an acid or sulphureous salt, that is predominant; and that such a discovery is oftentimes of great difficulty, and may frequently be of great use, he that is not a stranger to the various properties and effects of salts, and of how great moment it is to be able to distinguish their tribes, may readily conceive. but to proceed to the way of trying other liquors by an infusion of our wood, take it briefly thus. suppose i have a mind to try whether i conjecture aright, when i imagine that allom, though it be plainly a mixt body, does abound rather with acid than sulphureous salt. to satisfie my self herein, i turn my back to the light, and holding a small vial full of the tincture of _lignum nephriticum_, which look'd upon in that position, appears cæruleous, i drop into it a little of a strong solution of allom made in fair water, and finding upon the affusion and shaking of this new liquor, that the blewness formerly conspicuous in our tincture does presently vanish, i am thereby incited to suppose, that the salt prædominant in allom belongs to the family of sour salts; but if on the other side i have a mind to examine whether or no i rightly conceive that salt of urine, or of harts-horn is rather of a saline sulphureous (if i may so speak) than of an acid nature, i drop a little of the saline spirit of either into the nephritick tincture, and finding that the cæruleous colour is rather thereby deepned than destroy'd, i collect that the salts, which constitute these spirits, are rather sulphureous than acid. and to satisfie my self yet farther in this particular, i take a small vial of fresh tincture, and placing both it and my self in reference to the light as formerly, i drop into the infusion just as much distill'd vinegar, or other acid liquor as will serve to deprive it of its blewness (which a few drops, if the sour liquor be strong, and the vial small will suffice to do) then without changing my posture, i drop and shake into the same vial a small proportion of spirit of hartshorn or urine, and finding that upon this affusion, the tincture immediately recovers its cæruleous colour, i am thereby confirm'd firm'd in my former opinion, of the sulphureous nature of these salts. and so, whereas it is much doubted by some modern chymists to what sort of salt, that which is prædominant in quick-lime belongs, we have been perswaded to referr it rather to lixiviate than acid salts, by having observ'd, that though an evaporated infusion of it will scarce yield such a salt, as ashes and other alcalizate bodyes are wont to do, yet if we deprive our nephritick tincture of its blewness by just so much distill'd vinegar as is requisite to make that colour vanish, the _lixivium_ of quick-lime will immediately upon its affusion recall the banished colour; but not so powerfully as either of the sulphureous liquors formerly mention'd. and therefore i allow my self to guess at the _strength_ of the liquors examin'd by this experiment, by the _quantity_ of them which is sufficient to destroy or restore the cæruleous colour of our tincture. but whether concerning liquors, wherein neither acid nor alcalisate salts are eminently prædominant, our tincture will enable us to conjecture any thing more than that such salts are not prædominant in them, i take not upon me to determine here, but leave to further tryal; for i find not that spirit of wine, spirit of tartar freed from acidity, or chymical oyl of turpentine, (although liquors which must be conceiv'd very saline, if chymists have, which is here no place to dispute, rightly ascrib'd tasts to the saline principle of bodyes,) have any remarkable power either to deprive our tincture of its cæruleous colour, or restore it, when upon the affusion of spirit of vinegar it has disappear'd. _experiment xi._ and here i must not omit, _pyrophilus_, to inform you, that we can shew you even in a mineral body something that may seem very near of kin to the changeable quality of the tincture of _lignum nephriticum_, for we have several flat pieces of glass, of the thickness of ordinary panes for windows one of which being interposed betwixt the eye and a clear light, appears of a golden colour, not much unlike that of the moderate tincture of our wood, but being so look'd upon as that the beams of light are not so much trajected thorough it as reflected from it to the eye, that yellow seems to degenerate into a pale blew, somewhat like that of a turquoise. and what which may also appear strange, is this, that if in a certain posture you hold one of these plates perpendicular to the horizon, so that the sun-beams shine upon half of it, the other half being shaded, you may see that the part shin'd upon will be of a much diluter yellow than the shaded part which will appear much more richly colour'd; and if you alter the posture of the glass, so that it be not held perpendicular, but parallel in reference to the horizon, you may see, (which perhaps you will admire) the shaded part look of a golden colour, but the other that the sun shines freely on, will appear considerably blew, and as you remove any part of the glass thus held horizontally into the sun-beams or shade, it will in the twinkling of an eye seem to pass from one of the above mention'd colours to the other, the sun-beams trajected through it upon a sheet of white paper held near it, do colour it with a yellow, somewhat bordering upon a red, but yet the glass may be so oppos'd to the sun, that it may upon paper project a mix'd colour here and there more inclin'd to yellow, and here and there more to blew. the other phænomena of this odd glass, i fear it would be scarce worth while to record, and therefore i shall rather advertise you, _first_ that in the trying of these experiments with it, you must take notice that one of the sides has either alone, or at least principally its superficial parts dispos'd to the reflection of the blew colour above nam'd, and that therefore you must have a care to keep that side nearest to the eye. and next, that we have our selves made glasses not unfit to exhibit an experiment not unlike that i have been speaking of, by laying upon pieces of glass some very finely foliated silver, and giving it by degrees a much stronger fire than is requisite or usual for the tinging of glasses of other colours. and this experiment, not to mention that it was made without a furnace in which artificers that paint glass are wont to be very curious, is the more considerable, because, that though a skilfull painter could not deny to me that 'twas with silver he colour'd his glasses yellow; yet he told me, that when to burn them (as they speak) he layes on the plates of glass nothing but a _calx_ of silver calcin'd without corrosive liquors, and temper'd with fair water, the plates are ting'd of a fine yellow that looks of a golden colour, which part soever of it you turn to or from the light; whereas (whether it be what an artificer would call over-doing, or burning, or else the imploying the silver crude that makes the difference,) we have found more than once, that some pieces of glass prepar'd as we have related, though held against the light they appear'd of a transparent yellow, yet look'd on with ones back turn'd to the light they exhibited an untransparent blew. _experiment xii._ if you will allow me, _pyrophilus_, for the avoiding of ambiguity, to imploy the word pigments, to signifie such prepared materials (as cochinele, vermilion, orpiment,) as painters, dyers and other artificers make use of to impart or imitate particular colours, i shall be the better understood in divers passages of the following papers, and particularly when i tell you, that the mixing of pigments being no inconsiderable part of the painters art, it may seem an incroachment in me to meddle with it. but i think i may easily be excus'd (though i do not altogether pass it by) if i restrain my self to the making of a transient mention of some few of their practices about this matter; and that only so far forth, as may warrant me to observe to you, that there are but few simple and primary colours (if i may so call them) from whose various compositions all the rest do as it were result. for though painters can imitate the hues (though not always the splendor) of those almost numberless differing colours that are to be met with in the works of nature, and of art, i have not yet found, that to exhibit this strange variety they need imploy any more than _white_, and _black_, and _red_, and _blew_, and _yellow_; these _five_, variously _compounded_, and (if i may so speak) _decompounded_, being sufficient to exhibit a variety and number of colours, such, as those that are altogether strangers to the painters pallets, can hardly imagine. thus (for instance) black and white differingly mix'd, make a vast company of lighter and darker grays. blew and yellow make a huge variety of greens. red and yellow make orange tawny. red with a little white makes a carnation. red with an eye of blew, makes a purple; and by these simple compositions again compounded among themselves, the skilfull painter can produce what kind of colour he pleases, and a great many more than we have yet names for. but, as i intimated above, 'tis not my design to prosecute this subject, though i thought it not unfit to take some notice of it, because we may hereafter have occasion to make use of what has been now deliver'd, to illustrate the generation of intermediate colours; concerning which we must yet subjoyn this caution, that to make the rules about the emergency of colours, fit to be relied upon, the corpuscles whereof the pigments consist must be such as do not destroy one anothers texture, for in case they do, the produced colour may be very different from that which would result from the mixture of other harmless pigments of the same colours, as i shall have occasion to shew ere long. _experiment xiii._ it may also give much light to an enquirer into the nature of colours, to know that not only in green, but in many (if not all) other colours, the light of the sun passing through diaphanous bodies of differing hues may be tinged of the same compound colour, as if it came from some painters colours of the same denomination, though this later be exhibited by reflection, and be (as the former experiment declares) manifestly compounded of material pigments. wherefore to try the composition of colours by trajection, we provided several plates of tinged glass, which being laid two at a time one on the top of another, the object look'd upon through them both, appear'd of a compounded colour, which agrees well with what we have observ'd in the second experiment, of looking against the light through differingly colour'd papers. but we thought the experiment would be more satisfactory, if we procur'd the sun-beams to be so ting'd in their passage through plates of glass, as to exhibit the compounded colour upon a sheet of white paper. and though by reason of the thickness of the glasses, the effect was but faint, even when the sun was high and shin'd forth clear, yet, we easily remedied that by contracting the beams we cast on them by means of a convex burning-glass, which where it made the beams much converge increas'd the light enough to make the compounded colour very manifest upon the paper. by this means we observ'd, that the beams trajected through blew and yellow compos'd a green, that an intense and moderate red did with yellow make differing degrees of saffron, and orange tawny colours, that green and blew made a colour partaking of both, such as that which some latin writers call _pavonaceus_, that red and blew made a purple, to which we might add other colours, that we produc'd by the combinations of glasses differingly ting'd, but that i want proper words to express them in our language, and had not when we made the tryals, the opportunity of consulting with a painter, who perchance might have suppli'd me with some of the terms i wanted. i know not whether it will be requisite to subjoyn on this occasion, what i tried concerning reflections from colour'd glasses, and other transparent bodies, namely, that having expos'd four or five sorts of them to the sun, and cast the reflected beams upon white paper held near at hand, the light appear'd not manifestly ting'd, but as if it had been reflected from the impervious parts of a colourless glass, only that reflected from the yellow was here and there stain'd with the same colour, as if those beams were not all reflected from the superficial, but some from the internal parts of the glass; upon which occasion you may take notice, that a skilfull tradesman, who makes such colour'd glass told me, that where as the red pigment was but superficial, the yellow penetrated to the very midst of the plate. but for further satisfaction, not having the opportunity to foliate those plates, and so turn them into looking-glasses, we foliated a plate of _muscovy_ glass, and then laying on it a little transparent varnish of a gold colour, we expos'd it to the sun-beams, so as to cast them upon a body fit to receive them, on which the reflected light, appearing, as we expected, yellow, manifested that rebounding from the specular part of the _selenitis_, it was ting'd in its return with the colour of the transparent varnish through which it pass'd. _experiment xiv._ after what we have said of the composition of colours, it will now be seasonable to annex some experiments that we made in favour of those colours, that are taught in the schools not to be real, but only apparent and phantastical; for we found by tryals, that these colours might be compounded, both with true and stable colours, and with one another, as well as unquestionably genuine and lasting colours, and that the colours resulting from such compositions, would respectively deserve the same denominations. for first, having by the trajection of the sun-beams through a glass-prism thrown an iris on the floor, i found that by placing a blew glass at a convenient distance betwixt the prism and the iris, that part of the iris that was before yellow, might be made to appear green, though not of a grass green, but of one more dilute and yellowish. and it seems not improbable, that the narrow greenish list (if i may so call it) that is wont to be seen between the yellow and blew parts of the iris, is made by the confusion of those two bordering colours. next, i found, that though the want of a sufficient liveliness in either of the compounding colours, or a light error in the manner of making the following tryals, was enough to render some of them unsuccessfull, yet when all necessary circumstances were duely observ'd, the event was answerable to our expectation and desire. and (as i formerly noted) that red and blew compound a purple, so i could produce this last nam'd colour, by casting at some distance from the glass the blew part of the prismatical iris (as i think it may be call'd for distinction sake) upon a lively red, (for else the experiment succeeds not so well.) and i remember, that sometimes when i try'd this upon a piece of red cloath, _that_ part of the iris which would have been blew, (as i try'd by covering that part of the cloath with a piece of white paper) and compounded with the red, wherewith the cloath was imbued before, appear'd of a fair purple, did, when i came to view it near at hand, look very odly, as if there were some strange reflection or refraction or both made in the hairs of which that cloath was composed. calling likewise the prismatical iris upon a very vivid blew, i found that part of it, which would else have been the yellow, appear green. (another somewhat differing tryal, and yet fit to confirm this, you will find in the fifteenth experiment.) but it may seem somewhat more strange, that though the prismatical iris being made by the refraction of light through a body that has no colour at all, must according to the doctrine of the schools consist of as purely emphatical colours, as may be, yet even these may be compounded with one another, as well as real colours in the grossest pigments. for i took at once two triangular glasses, and one of them being kept fixt in the same posture, that the iris it projected on the floor might not waver, i cast on the same floor another iris with the other prism, and moving it too and fro to bring what part of the second iris i pleas'd, to fall upon what part of the first i thought fit, we did sometimes (for a small errour suffices to hinder the success) obtain by this means a green colour in that part of the more stable iris, that before was yellow, or blew, and frequently by casting those beams that in one of the iris's made the blew upon the red parts of the other iris, we were able to produce a lovely purple, which we can destroy or recompose at pleasure, by severing and reapproaching the edges of the two iris's. _experiment xv._ on this occasion, _pyrophilus_, i shall add, that finding the glass-prism to be the usefullest instrument men have yet imploy'd about the contemplation of colours, and considering that prisms hitherto in use are made of glass, transparent and colourless, i thought it would not be amiss to try, what change the superinduction of a colour, without the destruction of the diaphaneity, would produce in the colours exhibited by the prism. but being unable to procure one to be made of colour'd glass, and fearing also that if it were not carefully made, the thickness of it would render it too opacous, i endeavoured to substitute one made of clarify'd rosin, or of turpentine brought (as i elsewhere teach) to the consistence of a transparent gum. but though these endeavours were not wholly lost, yet we found it so difficult to give these materials their true shape, that we chose rather to varnish over an ordinary prism with some of these few pigments that are to be had transparent; as accordingly we did first with yellow, and then with red, or rather crimson, made with lake temper'd with a convenient oyl, and the event was, that for want of good transparent colours, (of which you know there are but very few) both the yellow and the red made the glass so opacous, (though the pigment were laid on but upon two sides of the glass, no more being absolutely necessary) that unless i look'd upon an inlightned window, or the flame of a candle, or some other luminous or very vivid object, i could scarce discern any colours at all, especially when the glass was cover'd with red. but when i did look on such objects, it appear'd (as i expected) that the colour of the pigment had vitiated or drown'd some of those which the prism would according to its wont have exhibited, and mingling with others, alter'd them: as i remember, that both to my eyes, and others to whom i show'd it, when the prism was cover'd with yellow, it made those parts of bright objects, where the blew would else have been conspicuous, appear of a light green. but, _pyrophilus_, both the nature of the colours, and the degree of transparency, or of darkness in the pigment, besides divers other circumstances, did so vary the _phænomena_ of these tryals, that till i can procure small colour'd prisms, or hollow ones that may be filled with tincted liquor, or obtain some better pigments than those i was reduc'd to imploy, i shall forbear to build any thing upon what has been delivered, and shall make no other use of it, than to invite you to prosecute the inquiry further. _experiment xvi._ and here, _pyrophilus_, since we are treating of emphatical colours, we shall add what we think not unworthy your observation, and not unfit to afford some exercise to the speculative. for there are some liquors, which though colourless themselves, when they come to be elevated, and dispers'd into exhalations, exhibit a conspicuous colour, which they lose again, when they come to be reconjoyn'd into a liquor, as good spirit of _nitre_; or upon its account strong _aqua-fortis_, though devoid of all appearance of redness whilst they continue in the form of a liquor, if a little heat chance to turn the minute parts of them into vapour, the steam will appear of a reddish or deep yellow colour, which will vanish when those exhalations come to resume the form of liquor. and not only if you look upon a glass half full of _aqua-fortis_, or spirit of _nitre_, and half full of _nitrous_ steams proceeding from it, you will see the upper part of the glass of the colour freshly mention'd, if through it you look upon the light. but which is much more considerable, i have tried, that putting _aqua-fortis_ in a long clear glass, and adding a little copper or some such open metall to it, to excite heat and fumes, the light trajected through those fumes, and cast upon a sheet of white paper, did upon that appear of the colour that the fumes did, when directly look'd upon, as if the light were as well ting'd in its passage through these fumes, as it would have been by passing through some glass or liquor in which the same colour was inherent. to which i shall further add, that having sometimes had the curiosity to observe whether the beams of the sun near the horizon trajected through a very red sky, would not (though such rednesses are taken to be but emphatical colours) exhibit the like colour, i found that the beams falling within a room upon a very white object, plac'd directly opposite to the sun, disclos'd a manifest redness, as if they had pass'd through a colour'd _medium_. _experiment xvii._ the emergency, _pyrophilus_, of colours upon the coalition of the particles of such bodies as were neither of them of the colour of that mixture whereof they are the ingredients, is very well worth our attentive observation, as being of good use both speculative and practical; for much of the mechanical use of colours among painters and dyers, doth depend upon the knowledge of what colours may be produc'd by the mixtures of pigments so and so colour'd. and (as we lately intimated) 'tis of advantage to the contemplative naturalist, to know how many and which colours are primitive (if i may so call them) and simple, because it both eases his labour by confining his most sollicitous enquiry to a small number of colours upon which the rest depend, and assists him to judge of the nature of particular compounded colours, by shewing him from the mixture of what more simple ones, and of what proportions of them to one another, the particular colour to be consider'd does result. but because to insist on the proportions, the manner and the effects of such mixtures would oblige me to consider a greater part of the painters art and dyers trade, than i am well acquainted with, i confin'd my self to make trial of _several ways to produce green_, by the composition of blew and yellow. and shall in this place both recapitulate most of the things i have dispersedly deliver'd already concerning that subject, and recruit them. and first, whereas painters (as i noted above) are wont to make green by tempering blew and yellow, both of them made into a soft consistence, with either water or oyl, or some liquor of kin to one of those two, according as the picture is to be drawn with those they call _water colours_, or those they term _oyl colours_, i found that by choosing fit ingredients, and mixing them in the form of dry powders, i could do, what i could not if the ingredients were temper'd up with a liquor; but the blew and yellow powders must not only be finely ground, but such as that the corpuscles of the one may not be too unequal to those of the other, lest by their disproportionate minuteness the smaller cover and hide the greater. we us'd with good success a slight mixture of the fine powder of bise, with that of orpiment, or that of good yellow oker, i say a _slight_ mixture, because we found that an _exquisite_ mixture did not do so well, but by lightly mingling the two pigments in several little parcels, those of them in which the proportion and manner of mixture was more lucky, afforded us a good green. . we also learn'd in the dye-houses, that cloth being dy'd blew with woad, is afterwards by the yellow decoction of _luteola_ or woud-wax or wood-wax dy'd into a green colour. . you may also remember what we above related, where we intimated, that having in a darkn'd room taken two bodies, a blew and a yellow, and cast the light reflected from the one upon the other, we likewise obtain'd a green. . and you may remember, that we observ'd a green to be produc'd, when in the same darkn'd room we look'd at the hole at which alone the light enter'd, through the green and yellow parts of a sheet of marbl'd paper laid over one another. . we found too, that the beams of the sun being trajected through two pieces of glass, the one blew and the other yellow, laid over one another, did upon a sheet of white paper on which they were made to fall, exhibit a lovely green. . i hope also, that you have not already forgot, what was so lately deliver'd, concerning the composition of a green, with a blew and yellow; of which most authors would call the one a _real_, and the other an _emphatical_. . and i presume, you may have yet fresh in your memory, what the fourteenth experiment informs you, concerning the exhibiting of a green, by the help of a blew and yellow, that were both of them emphatical. . wherefore we will proceed to take notice, that we also devis'd a way of trying whether or no metalline solutions though one of them at least had its colour adventitious, by the mixture of the _menstruum_ employ'd to dissolve it, might not be made to compound a green after the manner of other bodies. and though this seem'd not easie to be perform'd by reason of the difficulty of finding metalline solutions of the colour requisite, that would mix without præcipitating each other; yet after a while having consider'd the matter, the first tryal afforded me the following experiment. i took a high yellow solution of good gold in _aqua-regis_, (made of _aqua-fortis_, and as i remember half its weight of spirit of salt) to this i put a due proportion of a deep and lovely blew solution of crude copper, (which i have elsewhere taught to be readily dissoluble in strong spirit of urine) and these two liquors though at first they seem'd a little to curdle one another, yet being throughly mingl'd by shaking, they presently, as had been conjectur'd, united into a transparent green liquor, which continu'd so for divers days that i kept it in a small glass wherein 'twas made, only letting fall a little blackish powder to the bottom. the other _phænomena_ of this experiment belong not to this place, where it may suffice to take notice of the production of a green, and that the experiment was more than once repeated with success. . and lastly, to try whether this way of compounding colours would hold ev'n in ingredients actually melted by the violence of the fire, provided their texture were capable of safely induring fusion, we caus'd some blew and yellow ammel to be long and well wrought together in the flame of a lamp, which being strongly and incessantly blown on them kept them in some degree of fusion, and at length (for the experiment requires some patience as well as skil) we obtain'd the expected ammel of a green colour. i know not, _pyrophilus_, whether it be worth while to acquaint you with the ways that came into my thoughts, whereby in some measure to explicate the first of the mention'd ways of making a green; for i have sometimes conjectur'd, that the mixture of the bise and the orpiment produc'd a green by so altering the superficial asperity, which each of those ingredients had apart, that the light incident on the mixture was reflected with differing shades, as to quantity, or order, or both, from those of either of the ingredients, and such as the light is wont to be modify'd with, when it reflects from grass, or leaves, or some of those other bodies that we are wont to call green. and sometimes too i have doubted, whether the produced green might not be partly at least deriv'd from this, that the beams that rebound from the corpuscles of the orpiment, giving one kind of stroak upon the _retina_, whose perception we call yellow, and the beams reflected from the corpuscles of the bise, giving another stroak upon the same _retina_, like to objects that are blew, the contiguity and minuteness of these corpuscles may make the appulse of the reflected light fall upon the _retina_ within so narrow a compass, that the part they beat upon being but as it were a physical point, they may give a compounded stroak, which may consequently exhibit a compounded and new kind of sensation, as we see that two strings of a musical instrument being struck together, making two noises that arrive at the ear at the same time as to sense, yield a sound differing from either of them, and as it were compounded of both; insomuch that if they be discordantly ton'd, though each of them struck apart would yield a pleasing sound, yet being struck together they make but a harsh and troublesome noise. but this not being so fit a place to prosecute speculations, i shall not insist, neither upon these conjectures nor any others, which the experiment we have been mentioning may have suggested to me. and i shall leave it to you, _pyrophilus_, to derive what instruction you can from comparing together the various ways whereby a yellow and a blew can be made to compound a green. that which i now pretend to, being only to shew that the first of those mention'd ways, (not to take at present notice of the rest) does far better agree with our conjectures about colours, than either with the doctrine of the schools, or with that of the _chymists_, both which seem to be very much disfavour'd by it. for first, since in the mixture of the two mention'd powders i could by the help of a very excellent _microscope_ (for ordinary ones will scarce serve the turn) discover that which seem'd to the naked eye a green body, to be but a heap of distinct, though very small grains of yellow orpiment and blew bise confusedly enough blended together, it appears that the colour'd corpuscles of either kind did each retain its own nature and colour; by which it may be guess'd, what meer transposition and juxtaposition of minute and singly unchang'd particles of matter can do to produce a new colour; for that this local motion and new disposition of the small parts of the orpiment did intervene is much more manifest than it is easie to explicate how they should produce this new green otherwise than by the new manner of their being put together, and consequently by their new disposition to modifie the incident light by reflecting it otherwise than they did before they were mingl'd together. secondly, the green thus made being (if i may so speak) mechanically produc'd, there is no pretence to derive it from i know not what incomprehensible substantial form, from which yet many would have us believe that colours must flow; nor does this green, though a real and permanent, not a phantastical and vanid colour, seem to be such an inherent quality as they would have it, since not only each part of the mixture remains unalter'd in colour, and consequently of a differing colour from the heap they compose, but if the eye be assisted by a _microscope_ to discern things better and more distinctly than before it could, it sees not a green body, but a heap of blew and yellow corpuscles. and in the third place, i demand what either sulphur, or salt, or mercury has to do in the production of this green; for neither the bise nor the orpiment were indu'd with that colour before, and the bare juxtaposition of the corpuscles of the two powders that work not upon each other, but might if we had convenient instruments be separated, unalter'd, cannot with any probability be imagin'd either to increase or diminish any of the three hypostatical principles, (to which of them soever the _chymists_ are pleas'd to ascribe colours) nor does there here intervene so much as heat to afford them any colour to pretend, that at least there is made an extraversion (as the _helmontians_ speak) of the sulphur or of any of the two other supposed principles; but upon this experiment we have already reflected enough, if not more than enough for once. _experiment xviii._ but here, _pyrophilus_, i must advertise you, that 'tis not every yellow and every blew that being mingl'd will afford a green; for in case one of the ingredients do not act only as endow'd with such a colour, but as having a power to alter the texture of the corpuscles of the other, so as to indispose them to reflect the light, as corpuscles that exhibit a blew or a yellow are wont to reflect it, the emergent colour may be not green, but such as the change of texture in the corpuscles of one or both of the ingredients qualifies them to shew forth; as for instance, if you let fall a few drops of syrrup of violets upon a piece of white paper, though the syrrup being spread will appear blew, yet mingling with it two or three drops of the lately mention'd solution of gold, i obtain'd not a green but a reddish mixture, which i expected from the remaining power of the acid salts abounding in the solution, such salts or saline spirits being wont, as we shall see anon, though weakn'd, so to work upon that syrrup as to change it into a red or reddish colour. and to confirm that for which i allege the former experiment, i shall add this other, that having made a very strong and high-colour'd solution of filings of copper with spirit of urine, though the _menstruum_ seem'd glutted with the metall, because i put in so much filings that many of them remain'd for divers days undissolv'd at the bottom, yet having put three or four drops of syrrup of violets upon white paper, i found that the deep blew solution proportionably mingl'd with this other blew liquor did not make a blew mixture, but, as i expected, a fair green, upon the account of the urinous salt that was in the _menstruum_. _experiment xix._ to shew the _chymists_, that colours may be made to appear or vanish, where there intervenes no accession or change either of the sulphureous, or the saline, or the mercurial principle (as they speak) of bodies: i shall not make use of the iris afforded by the glass-prism, nor of the colours to be seen in a fair morning in those drops of dew that do in a convenient manner reflect and refract the beams of light to the eye; but i will rather mind them of what they may observe in their own laboratories, namely, that divers, if not all, chymical essential oyls, as also good spirit of wine, being shaken till they have good store of bubbles, those bubbles will (if attentively consider'd) appear adorn'd with various and lovely colours, which all immediately vanish, upon the relapsing of the liquor that affords those bubbles their skins, into the rest of the oyl, or spirit of wine, so that a colourless liquor may be made in a trice to exhibit variety of colours, and may lose them in a moment without the accession or diminution of any of its hypostatical principles. and, by the way, 'tis not unworthy our notice, that some bodies, as well colourless, as colour'd, by being brought to a great thinness of parts, acquire colours though they had none before, or colours differing from them they were before endued with: for, not to insist on the variety of colours, that water, made somewhat glutinous by sope, acquires, when 'tis blown into such sphærical bubbles as boys are wont to make and play with; turpentine (though it have a colour deep enough of its own) may (by being blown into after a certain manner) be brought to afford bubbles adorn'd with variety of orient colours, which though they vanish after some while upon the breaking of the bubbles, yet they would in likelihood always exhibit colours upon their _superfices_, (though not always the same in the same parts of them, but vary'd according to the incidence of the sight, and the position of the eye) if their texture were durable enough: for i have seen one that was skill'd at fashioning glasses by the help of a lamp, blowing some of them so strongly as to burst them, whereupon it was found, that the tenacity of the metall was such, that before it broke it suffer'd it self to be reduc'd into films so extremely thin, that being kept clean they constantly shew'd on their surfaces (but after the manner newly mention'd) the varying colours of the rain-bow, which were exceedingly vivid, as i had often opportunity to observe in some, that i caus'd purposely to be made, to keep by me. but lest it should be objected, that the above mentioned instances are drawn from transparent liquors, it may possibly appear, not impertinent to add, what i have sometimes thought upon, and several times tried, when i was considering the opinions of the _chymists_ about colours, i took then a feather of a convenient bigness and shape, and holding it at a fit distance betwixt my eye and the sun when he was near the horizon, me thought there appear'd to me a variety of little rain-bows, with differing and very vivid colours, of which none was constantly to be seen in the feather; the like _phænomenon_ i have at other times (though not with altogether so good success) produc'd, by interposing at a due distance a piece of black ribband betwixt the almost setting sun and my eye, not to mention the trials i have made to the same purpose, with other bodies. _experiment xx._ take good syrrup of violets, imprægnated with the tincture of the flowers, drop a little of it upon a white paper (for by that means the change of colour will be more conspicuous, and the experiment may be practis'd in smaller quantities) and on this liquor let fall two or three drops of spirit either of salt or vinegar, or almost any other eminently acid liquor, and upon the mixture of these you shall find the syrrup immediatly turn'd red, and the way of effecting such a change has not been unknown to divers persons who have produc'd the like, by spirit of vitriol, or juice of limmons, but have groundlessly ascrib'd the effect to some peculiar quality of those two liquors, whereas, (as we have already intimated) almost any acid salt will turn syrrup of violets red. but to improve the experiment, let me add what has not (that i know of) been hitherto observ'd, and has, when we first shew'd it them, appear'd something strange, even to those that have been inquisitive into the nature of colours; namely, that if instead of spirit of salt, or that of vinegar, you drop upon the syrrup of violets a little oyl of tartar _per deliquium_, or the like quantity of solution of potashes, and rubb them together with your finger, you shall find the blew colour of the syrrup turn'd in a moment into a perfect green, and the like may be perform'd by divers other liquors, as we may have occasion elsewhere to inform you. _annotation upon the twentieth experiment_. the use of what we lately deliver'd concerning the way of turning syrrup of violets, red or green, may be this; that, though it be a far more common and procurable liquor than the infusion of _lignum nephriticum_, it may yet be easily substituted in its room, when we have a mind to examine, whether or no the salt predominant in a liquor or other body, wherein 'tis loose and abundant, belong to the tribe of _acid_ salts or not. for if such a body turn the syrrup of a red or reddish purple colour, it does for the most part argue the body (especially if it be a distill'd liquor) to abound with acid salt. but if the syrrup be made green, that argues the predominant salt to be of a nature repugnant to that of the tribe of acids. for, as i find that either spirit of salt, or oyl of vitriol, or _aqua-fortis_, or spirit of vinegar, or juice of lemmons, or any of the acid liquors i have yet had occasion to try, will turn syrrup of violets, of a _red_, (or at least, of a _reddish_ colour, so i have found, that not only the volatile salts of all animal substances i have us'd, as spirit of harts-horn, of urine, of sal-armoniack, of blood, &c. but also all the alcalizate salts i have imploy'd, as the solution of salt of tartar, of pot-ashes, of common wood-ashes, lime-water, &c. will immediately change the blew syrrup, into a perfect green. and by the same way (to hint that upon the by) i elsewhere show you, both the changes that nature and time produce, in the more saline parts of some bodies, may be discover'd, and also how ev'n such chymically prepar'd bodies, as belong not either to the animal kingdome, or to the tribe of _alcali's_, may have their new and superinduc'd nature successfully examin'd. in this place i shall only add, that not alone the changing the colour of the syrrup, requires, that the changing body be more strong, of the acid, or other sort of salt that is predominant in it, than is requisite for the working upon the tincture of _lignum nephriticum_; but that in this is also, the operation of the formerly mention'd salts upon our syrrup, differs from their operation upon our tinctures, that in this liquor, if the cæruleous colour be _destroy'd_ by an acid salt, it may be _restor'd_ by one that is either volatile, or lixiviate; whereas in syrrup of violets, though one of these contrary salts will _destroy_ the action of the other, yet neither of them will _restore_ the syrrup to its native blew; but each of them will change it into the colour which it self doth (if i may so speak) affect, as we shall have occasion to show in the notes on the twenty fifth experiment. _experiment xxi._ there is a weed, more known to plowmen than belov'd by them, whose flowers from their colour are commonly call'd _blew-bottles_, and _corn-weed_ from their growing among corn[ ]. these flowers some ladies do, upon the account of their lovely colour, think worth the being candied, which when they are, they will long retain so fair a colour, as makes them a very fine sallad in the winter. but i have try'd, that when they are freshly gather'd, they will afford a juice, which when newly express'd, (for in some cases 'twill soon enough degenerate) affords a very deep and pleasant blew. now, (to draw this to our present scope) by dropping on this fresh juice, a little spirit of salt, (that being the acid spirit i had then at hand) it immediately turn'd (as i predicted) into a red. and if instead of the sowr spirit i mingled with it a little strong solution of an alcalizate salt, it did presently disclose a lovely green; the same changes being by those differing sorts of saline liquors, producible in this _natural juice_, that we lately mention'd to have happen'd to that _factitious mixture_, the syrrup of violets. and i remember, that finding this blew liquor, when freshly made, to be capable of serving in a pen for an ink of that colour, i attempted by moistning one part of a piece of white paper with the spirit of salt i have been mentioning, and another with some alcalizate or volatile liquor, to draw a line on the leisurely dry'd paper, that should, e'vn before the ink was dry, appear partly blew, partly red, and partly green: but though the latter part of the experiment succeeded not well, (whether because volatile salts are too fugitive to be retain'd in the paper, and alcalizate ones are too unctuous, or so apt to draw moisture from the air, that they keep the paper from drying well) yet the former part succeeded well enough; the blew and red being conspicuous enough to afford a surprizing spectacle to those, i acquaint not with (what i willingly allow you to call) the _trick_. [ ] _herbarists_ are wont to call this plant _cyanus vulgaris minor_. _annotation upon the one and twentieth experiment._ but lest you should be tempted to think (_pyrophilus_) that volatile or alcalizate salts change blews into green, rather upon the score of the easie transition of the former colour into the latter, than upon the account of the texture, wherein most vegetables, that afford a blew, seem, though otherwise differing, to be allied, i will add, that when i purposely dissolv'd blew vitriol in fair water, and thereby imbu'd sufficiently that liquor with that colour, a lixiviate liquor, and a urinous salt being copiously pour'd upon distinct parcels of it, did each of them, though perhaps with some difference, turn the liquor not green, but of a deep yellowish colour, almost like that of yellow oker, which colour the precipitated corpuscles retain'd, when they had leisurely subsided to the bottom. what this precipitated substance is, it is not needfull now to enquire in this place, and in another, i have shown you, that notwithstanding its colour, and its being obtainable from an acid _menstruum_ by the help of salt of tartar, it is yet far enough from being the true sulphur of vitriol. _experiment xxii._ our next experiment (_pyrophilus_) will perhaps seem to be of a contrary nature to the two former, made upon syrrup of violets, and juice of blew-bottles. for as in them by the affusion of oyl of tartar, a blewish liquor is made green, so in this, by the sole mixture of the same oyl, a greenish liquor becomes blew. the hint of this experiment was given us by the practice of some _italian_ painters, who being wont to counterfeit _ultra-marine azure_ (as they call it) by grinding verdigrease with sal-armoniack, and some other saline ingredients, and letting them rot (as they imagine) for a good while together in a dunghill, we suppos'd, that the change of colour wrought in the verdigrease by this way of preparation, must proceed from the action of certain volatile and alcalizate salts, abounding in some of the mingled concretes, and brought to make a further dissolution of the copper abounding in the verdigrease, and therefore we conjectur'd, that if both the verdigrease, and such salts were dissolv'd in fair water, the small parts of both being therein more subdivided, and set at liberty, would have better access to each other, and thereby incorporate much the more suddenly; and accordingly we found, that if upon a strong solution of good french verdigrease (for 'tis that we are wont to imploy, as the best) you pour a just quantity of oyl of tartar, and shake them well together, you shall immediately see a notable change of colour, and the mixture will grow thick, and not transparent, but if you stay a while, till the grosser part be precipitated to, and setled in the bottom, you may obtain a clear liquor of a very lovely colour, and exceeding delightfull to the eye. but, you must have a care to drop in a competent quantity of oyl of tartar, for else the colour will not be so deep, and rich; and if instead of this oyl you imploy a clear _lixivium_ of pot-ashes, you may have an azure somewhat lighter or paler than, and therefore differing from, the former. and if instead of either of these liquors, you make use of spirit of urine, or of harts-horn, you may according to the quantity and quality of the spirit you pour in, obtain some further variety (though scarce considerable) of cæruleous liquors. and yet lately by the help of this urinous spirit we made a blew liquor, which not a few ingenious persons, and among them, some, whose profession makes them very conversant with colours, have looked upon with some wonder. but these azure colour'd liquors should be freed from the subsiding matter, which the salts of tartar or urine precipitate out of them, rather by being decanted, than by filtration. for by the latter of these ways we have sometimes found, the colour of them very much impair'd, and little superiour to that of the grosser substance, that it left in the filtre. _experiment xxiii._ that roses held over the fume of sulphur, may quickly by it be depriv'd of their colour, and have as much of their leaves, as the fume works upon, burn'd pale, is an experiment, that divers others have tried, as well as i. but (_pyrophilus_) it may seem somewhat strange to one that has never consider'd the compounded nature of brimstone, that, whereas the fume of sulphur will, as we have said, whiten the leaves of roses; that liquor, which is commonly call'd oyl of sulphur _per campanam_, because it is suppos'd to be made by the condensation of these fumes in glasses shap't like bells, into a liquor, does powerfully heighten the tincture of red roses, and make it more red and vivid, as we have easily tried by putting some red-rose leaves, that had been long dried, (and so had lost much of their colour) into a vial of fair water. for a while after the affusion of a convenient quantity of the liquor we are speaking of, both the leaves themselves, and the water they were steep'd in, discover'd a very fresh and lovely colour. _experiment xxiv._ it may (_pyrophilus_) somewhat serve to illustrate, not only the doctrine of _pigments_, and of _colours_, but divers other parts of the _corpuscular philosophy_; as that explicates odours, and many other things, not as the schools by aery qualities, but by real, though extremely minute bodies; to examine, how much of a colourless liquor, a very small parcel of a pigment may imbue with a _discernable_ colour. and though there be scarce any thing of preciseness to be expected from such trials, yet i presum'd, that (at least) i should be able to show a much further subdivision of the parts of matter into _visible_ particles, than i have hitherto found taken notice of, and than most men would imagine; no body, that i know of, having yet attempted to reduce this matter to any measure. the bodies, the most promising for such a purpose, might seem to be the metalls, especially gold, because of the multitude, and minuteness of its parts, which might be argu'd from the incomparable closeness of its texture: but though we tried a solution of gold made in _aqua regia_ first, and then in fair water; yet in regard we were to determine the pigment we imploy'd, not by _bulk_ but _weight_, and because also, that the yellow colour of gold is but a faint one in comparison of the deep colour of _cochineel_, we rather chose this to make our trials with. but among divers of these it will suffice to set down one, which was carefully made in vessels conveniently shap'd; (and that in the presence of a witness, and an assistant) the sum whereof i find among my _adversaria_, registred in the following words. to which i shall only premise, (to lessen the wonder of so strange a diffusion of the pigment) that _cochineel_ will be better dissolv'd, and have its colour far more heightn'd by spirit of urine, than (i say not by common water, but) by rectify'd spirit of wine it self. the note i spoke off is this. [one grain of _cochineel_ dissolv'd in a pretty quantity of spirit of urine, and then dissolv'd further by degrees in fair water, imparted a discernable, though but a very faint colour, to about six glass-fulls of water, each of them containing about forty three ounces and an half, which amounts to above a hundred twenty five thousand times its own weight.] _experiment xxv._ it may afford a considerable hint (_pyrophilus_) to him, that would improve the art of dying, to know what change of colours may be produc'd by the three several sorts of salts already often mention'd, (some or other of which may be procur'd in quantity at reasonable rates) in the juices, decoctions, infusions, and (in a word) the more soluble parts of vegetables. and, though the design of this discourse be the improvement of knowledge, not of trades: yet thus much i shall not scruple to intimate here, that the blew liquors, mention'd in the twentieth and one and twentieth experiments, are far from being the only vegetable substances, upon which acid, urinous, and alcalizate salts have the like operations to those recited in those two experiments. for ripe _privet berries_ (for instance) being crush'd upon white paper, though they stain it with a purplish colour, yet if we let fall on some part of it two or three drops of spirit of salt, and on the other part a little more of the strong solution of pot-ashes, the former liquor immediately turn'd that part of the thick juice or pulp, on which it fell, into a lovely red, and the latter turn'd the other part of it into a delightfull green. though i will not undertake, that those colours in that substance shall not be much more orient, than lasting; and though (_pyrophilus_) this experiment may seem to be almost the same with those already deliver'd concerning syrrup of violets, and the juice of blew-bottles, yet i think it not amiss to take this occasion to inform you, that this experiment reaches much farther, than perhaps you yet imagine, and may be of good use to those, whom it concerns to know, how dying stuffs may be wrought upon by saline liquors. for, i have found this experiment to succeed in so many various berries, flowers, blossoms, and other finer parts of vegetables, that neither my memory, nor my leisure serves me to enumerate them. and it is somewhat surprizing to see, by how differingly-colour'd flowers, or blossoms, (for example) the paper being stain'd, will by an acid spirit be immediately turn'd red, and by any _alcaly_ or any urinous spirit turn'd green; insomuch that ev'n the crush'd blossoms of _meserion_, (which i gather'd in winter and frosty weather) and those of pease, crush'd upon white paper, how remote soever their colours be from green, would in a moment pass into a deep degree of that colour, upon the touch of an alcalizate liquor. to which let us add, that either of those new pigments (if i may so call them) may by the affusion of enough of a contrary liquor, be presently chang'd from red into green, and from green into red, which observation will hold also in syrrup of violets, juices of blew-bottles, &c. _annotation._ after what i have formerly deliver'd to evince, that there are many instances, wherein new colours are produc'd or acquir'd by bodies, which _chymists_ are wont to think destitute of salt, or to whose change of colours no new accession of saline particles does appear to contribute, i think we may safely enough acknowledge, that we have taken notice of so many changes made by the intervention of salts in the colours of mix'd bodies, that it has lessen'd our wonder, that though _many chymists_ are wont to ascribe the colours of such bodies to their sulphureous, and _the rest_ to their mercurial principle; yet _paracelsus_ himself directs us in the indagation of colours, to have an eye principally upon salts, as we find in that passage of his, wherein he takes upon him to oblige his readers much by instructing them, of what things they are to expect the knowledge from each of the three distinct principles of bodies. _alias_ (says he) _colorum similis ratio est: de quibus brevem institutionem hanc attendite, quod scilicet colores omnes ex sale prodeant. sal enim dat colorem, dat balsamum._[ ] and a little beneath. _iam natura ipsa colores protrathit ex sale, cuique speciei dans illum, qui ipsi competit_, &c. after which he concludes; _itaque qui rerum omnium corpora cognoscere vult, huic opus est, ut ante omnia cognoscat sulphur, ab hoc, qui desiderat novisse colores is scientiam istorum petat à sale, qui scire vult virtutes, is scrutetur arcana mercurii. sic nimirum fundamentum hauserit mysteriorum, in quolibet crescenti indagandorum, prout natura cuilibet speciei ea ingessit_. but though _paracelsus_ ascribes to each of his belov'd hypostatical principles, much more than i fear will be found to belong to it; yet if we please to consider colours, not as _philosophers_, but as _dyers_, the concurrence of salts to the striking and change of colours, and their efficacy, will, i suppose, appear so considerable, that we shall not need to quarrel much with _paracelsus_, for ascribing in this place (for i dare not affirm that he uses to be still of one mind) the colours of bodies to their salts, if by salts he here understood, not only elementary salts, but such also as are commonly taken for salts, as allom, crystals of tartar, vitriol, &c. because the saline principle does chiefly abound in them, though indeed they be, as we elsewhere declare, mix'd bodies, and have most of them, besides what is saline, both sulphureous, aqueous, and gross or earthy parts. [ ] paracelsus de mineral. tract. . pag. m. but though (_pyrophilus_) i have observ'd a red and green to be produc'd, the former, by acid salts, the later by salts not acid, in the express juices of so many differing vegetable substances, that the observation, if persued, may prove (as i said) of good use: yet to show you how much e'vn these effects depend upon the particular texture of bodies, i must subjoyn some cases wherein i (who am somewhat backwards to admit observations for universal) had the curiosity to discover, that the experiments would not uniformly succeed, and of these exceptions, the chief that i now remember, are reducible to the following three. _experiment xxvi._ and, (first) i thought fit to try the operation of acid salts upon vegetable substances, that are already and by their own nature red. and accordingly i made trial upon syrrup of clove-july-flowers, the clear express'd juice of the succulent berries of _spina cervina_, or buckthorn (which i had long kept by me for the sake of its deep colour) upon red roses, infusion of brazil, and divers other vegetable substances, on some of which crush'd (as is often mention'd) upon white paper, (which is also to be understood in most of these experiments, if no circumstance of them argue otherwise) spirit of salt either made no considerable change, or alter'd the colour but from a darker to a lighter red. how it will succeed in many other vegetable juices, and infusions of the same colour, i have at present so few at hand, that i must leave you to find it out your self. but as for the operation of the other sorts of salts upon these red substances, i found it not very uniform, some red, or reddish infusions, as of roses, being turn'd thereby into a dirty colour, but yet inclining to green. nor was the syrrup of clove-july-flowers turn'd by the solution of pot-ashes to a much better, though somewhat a greener, colour. another sort of red infusions was by an _alcaly_ not turn'd into a green, but advanc'd into a crimson, as i shall have occasion to note ere long. but there were other sorts, as particularly the lovely colour'd juice of buckthorn berries, that readily pass'd into a lovely green. _experiment xxvii._ among other vegetables, which we thought likely to afford exceptions to the general observation about the differing changes of colours produc'd by acid and sulphureous salts, we thought fit to make trial upon the flowers of _jasmin_, they being both white as to colour, and esteem'd to be of a more oyly nature than other flowers. whereupon having taken the white parts only of the flowers, and rubb'd them somewhat hard with my finger upon a piece of clean paper, it appear'd very little discolour'd. nor had spirit of salt, wherewith i moisten'd one part of it, any considerable operation upon it. but spirit of urine, and somewhat more effectually a strong alcalizate solution, did immediately turn the almost colourless paper moisten'd by the juice of the _jasmin_, not as those liquors are wont to do, when put upon the juices of other flowers, of a good green, but of a deep, though somewhat greenish yellow, which experiment i did afterwards at several times repeat with the like success. but it seems not that a great degree of unctuousness is necessary to the production of the like effects, for when we try'd the experiment with the leaves of those purely white flowers that appear about the end of winter, and are commonly call'd _snow drops_, the event, was not much unlike that, which, we have been newly mentioning. _experiment xxviii._ another sort of instances to show, how much changes of colour effected by salts, depend upon the particular texture of the colour'd bodies, has been afforded me by several _yellow_ flowers, and other vegetables, as mary-gold leaves, early prim-roses, fresh madder, &c. for being rubb'd upon white paper, till they imbued it with their colour, i found not, that by the addition of alcalizate liquors, nor yet by that of an urinous spirit, they would be turn'd either green or red: nor did so acid a spirit, as that of salt, considerably alter their colour, save that it seem'd a little to dilute it. only in some early prim-roses it destroy'd the greatest part of the colour, and made the paper almost white agen. and madder also afforded some thing peculiar, and very differing from what we have newly mention'd: for having gather'd some roots of it, and, (whilst they were recent) express'd upon white paper the yellow juice, an alcalizate solution drop'd upon it did not turn it either green or white, but red. and the bruis'd madder it self being drench'd with the like alcalizate solution, exchang'd also its yellowishness for a redness. _an admonition touching the four preceding experiments._ having thus (_pyrophilus_) given you divers instances, to countenance the general observation deliver'd in the twenty fifth experiment, and divers exceptions whereby it ought to be limited; i must leave the further inquiry into these matters to your own industry. for not remembring at present many of those other trials, long since made to satisfie my self about particulars, and not having now the opportunity to repeat them, i must content my self to have given you the hint, and the ways of prosecuting the search your self; and only declare to you in general, that, as i have made many trials, unmention'd in this treatise, whose events were agreeable to those mention'd in the twenty fifth experiment, so (to name now no other instances) what i have try'd with acid and sulphureous salts upon the pulp of juniper berries, rubb'd upon white paper, inclines me to think, that among that vast multitude, and strange variety of plants that adorn the face of the earth, perhaps many other vegetables may be found, on which such _menstruums_ may not have such operations, as upon the juice of violets, pease-blossoms, &c. no nor upon any of those three other sorts of vegetables, that i have taken notice of in the three fore-going experiments. it sufficiently appearing ev'n by these, that the effects of a salt upon the juices of particular vegetables do very much depend upon their particular textures. _experiment xxix._ it may be of some use towards the discovery of the nature of these changes, which the alimental juice receives in some vegetables, according to the differing degrees of their maturity, and according to the differing kinds of plants of the same denomination, to observe what operation acid, urinous, and alcalizate salts will have upon the juices of the several sorts of the vegetable substances i have been mentioning. to declare my meaning by an example, i took from the same cluster, one blackberry full ripe, and another that had not yet gone beyond a redness, and rubbing apiece of white paper, with the former, i observ'd, that the juice adhering to it was of adark reddish colour, full of little black specks; and that this juice by a drop of a strong _lixivium_, was immediately turn'd into a greenish colour deep enough, by as much urinous spirit into a colour much of kin to the former, though somewhat differing, and fainter; and by a drop of spirit of salt into a fine and lightsome red: where as the red berry being in like manner rubb'd upon paper, left on it a red colour, which was very little alter'd by the acid spirit newly nam'd, and by the urinous and lixiviate salts receiv'd changes of colour differing from those that had been just before produc'd in the dark juice of the ripe blackberry. i remember also, that though the infusion of damask-roses would as well, though not so much, as that of red, be heightned by acid spirits to an intense degree of redness, and by lixiviate salts be brought to a darkish green; yet having for trials sake taken a rose, whose leaves, which were large and numerous, like those of a province rose, were perfectly yellow, though in a solution of salt of tartar, they afforded a green blewish tincture, yet i did not by an acid liquor obtain a red one; all that the saline spirit i imploy'd, perform'd, being (if i much misremember not) to dilute somewhat the yellowness of the leaves. i would also have tried the tincture of yellow violets, but could procure none. and if i were in those islands of _banda_, which are made famous as well as rich, by being the almost only places, where cloves will prosper, i should think it worth my curiosity to try, what operation the three differing kinds of salts, i have so often mention'd, would have upon the juice of this spice, (express'd at the several seasons of it) as it grows upon the tree. since good authors inform us, (of what is remarkable) that these whether fruits, or rudiments of fruits, are at first _white_, afterward _green_, and then _reddish_, before they be beaten off the tree, after which being dry'd before they are put up, they grow _blackish_ as we see them. and one of the recentest _herbarists_ informs us, that the flower grows upon the top of the clove it self, consisting of four small leaves, like a cherry blossom, but of an excellent _blew_. but (_pyrophilus_) to return to our own observations, i shall add, that i the rather choose, to mention to you an example drawn from roses, because that though i am apt to think, as i elsewhere advertise, that something may be guess'd at about some of the qualities of the juices of vegetables, by the resemblance or disparity that we meet with in the changes made of their colours, by the operation of the same kinds of salts; yet that those conjectures should be very warily made, may appear among other things, by the instance i have chosen to give in roses. for though, (as i formerly told you) the dry'd leaves, both of the damask, and of red ones, give a red tincture to water sharpen'd with acid salts, yet the one sort of leaves is known to have a purgative faculty,[ ] and the other are often, and divers ways, imploy'd for binding. [ ] see _parkinson_ th. boran. trib. . cap. . and i also choose (_pyrophilus_) to subjoyn this twenty ninth experiment to those that precede it, about the change of the colours of vegetables by salts, for these two reasons: the first, that you may not easily entertain suspitions, if in the trials of an experiment of some of the kinds formerly mention'd, you should meet with an event somewhat differing from what my relations may have made you expect. and the second, that you may hereby be invited to discern, that it may not be amiss to take notice of the particular seasons wherein you gather the vegetables which in nicer experiments you make use of. for, it i were not hindred both by haste and some justifiable considerations, i could perhaps add considerable instances, to those lately deliver'd, for the making out of this observation; but for certain reasons i shall at present substitute a remarkable passage to be met with in that laborious herbarist mr. _parkinson_, where treating of the virtues of the (already divers times mention'd) buckthorn berries, he subjoyns the following account of several pigments that are made of them, not only according to the several ways of handling them, but according to the differing seasons of maturity, at which they are gather'd; _of these berries_, (says he) _are made three several sorts of colours as they shall be gather'd, that is, being gather'd while they are green, and kept dry, are call'd sapberries, which being steep'd into some allom-water, or fresh bruis'd into allom-water, they give a reasonable fair yellow colour which painters use for their work, and book-binders to colour the edges of books, and leather-dressers to colour leather, as they use also to make a green colour, call'd sap-green, taken from the berries when they are black, being bruis'd and put into a brass or copper kettle or pan, and there suffer'd to abide three or four_ _days, or a little heated upon the fire, and some beaten allom put unto them, and afterwards press'd forth, the juice or liquor is usually put in great bladders tied with strong thred at the head and hung up untill it be dry, which is dissolv'd in water or wine, but sack_ (he affirms) _is the best to preserve the colour from starving, (as they call it) that is, from decaying, and make it hold fresh the longer. the third colour (where of none_ (says he) _that i can find have made mention but only_ tragus_) is a purplish colour, which is made of the berries suffer'd to grow upon the bushes untill the middle or end of_ november, _that they are ready to drop from the trees._ and, i remember (_pyrophilus_) that i try'd, with a success that pleas'd me well enough, to make such a kind of pigment, as painters call sap-green, by a way not unlike that, deliver'd here by our author, but i cannot now find any thing relating to that matter among my loose papers. and my trials were made so many years ago, that i dare not trust my memory for circumstances, but will rather tell you, that in a noted colour-shop, i brought them by questions to confess to me, that they made their sap-green much after the ways by our _botanist_ here mention'd. and on this occasion i shall add an observation, which though it does not strictly belong to this place, may well enough be mention'd here, namely, that i find by an account given us by the learned _clusius_, of _alaternus_, that ev'n the grosser parts of the same plant, are some of them one colour, and some another; for speaking of that plant, he tells us, that the _portugalls_ use the bark to dye their nets into a red colour, and with the chips of the wood, which are whitish, they dye a blackish blew. _experiment xxx._ among the experiments that tend to shew that the change of colours in bodies may proceed from the vary'd texture of their parts, and the consequent change of their disposition to reflect or refract the light, that sort of experiments must not be left unmention'd, which is afforded us by chymical digestions. for, if _chymists_ will believe several famous writers about what they call the philosophers stone, they must acknowledge that the same matter, seald up hermetically in a philosophical egg, will by the continuance of digestion, or if they will have it so (for it is not material in our case which of the two it be) of decoction, run through a great variety of differing colours, before it come to that of the noblest _elixir_; whether that be scarlet, or purple, or what ever other kind of red. but without building any thing on so obtruse and questionable an operation, (which yet may be pertinently represented to those that believe the thing) we may observe, that divers bodies digested in carefully-clos'd vessels, will in tract of time, change their colour: as i have elsewhere mention'd my having observ'd ev'n in rectify'd spirit of harts-horn, and as is evident in the precipitations of amalgams of gold, and mercury, without addition, where by the continuance of a due heat the silver-colour'd amalgam is reduc'd into a shining red powder. further instances of this kind you may find here and there in divers places of my other essays. and indeed it has been a thing, that has much contributed to deceive many _chymists_, that there are more bodies than one, which by digestion will be brought to exhibit that variety and succession of colours, which they imagine to be peculiar to what they call the _true matter of the philosophers_. but concerning this, i shall referr you to what you may elsewhere find in the discourse written touching the passive deceptions of _chymists_, and more about the production of colours by digestion you will meet with presently. wherefore i shall now make only this observation from what has been deliver'd, that in these operations there appears not any cause to attribute the new colours emergent to the action of a new substantial form, nor to any increase or decrement of either the salt, sulphur, or mercury of the matter that acquires new colours: for the vessels are clos'd, and these principles according to the _chymists_ are ingenerable and incorruptible; so that the effect seems to proceed from hence, that the heat agitating and shuffling the corpuscles of the body expos'd to it, does in process of time so change its texture, as that the transposed parts do modifie the incident light otherwise, than they did when the matter appear'd of another colour. _experiment xxxi._ among the several changes of colour, which bodies acquire or disclose by digestion, it it very remarkable, that _chymists_ find a redness rather than any other colour in most of the tinctures they draw, and ev'n in the more gross solutions they make of almost all concretes, that abound either with mineral or vegetable sulphur, though the _menstruum_ imploy'd about these solutions or tinctures be never so limpid or colourless. this we have observ'd in i know not how many tinctures drawn with spirit of wine from _jalap_, _guaicum_, and several other vegetables; and not only in the solutions of _amber_, _benzoin_, and divers other concretes made with the same _menstruum_, but also in divers mineral tinctures. and, not to urge that familiar instance of the ruby of sulphur, as _chymists_ upon the score of its colour, call the solution of flowers of brimstone, made with the spirit of turpentine, nor to take notice of other more known examples of the aptness of chymical oyls, to produce a red colour with the sulphur they extract, or dissolve; not to insist (i say) upon instances of this nature, i shall further represent to you, as a thing remarkable, that, both acid and alcalizate salts, though in most other cases of such contrary operations, in reference to colours, will with many bodies that abound with sulphureous, or with oyly parts, produce a red; as is manifest partly in the more vulgar instances of the tinctures, or solutions of sulphur made with _lixiviums_, either of calcin'd tartar or pot-ashes, and other obvious examples, partly by this, that the true glass of antimony extracted with some acid spirits, with or without wine, will yield a red tincture, and that i know an acid liquor, which in a moment will turn oyl of turpentine into a deep red. but among the many instances i could give you of the easie production of redness by the operation of saline spirit, as well as of spirit of wine; i remember two or three of those i have tried, which seem remarkable enough to deserve to be mention'd to you apart. _experiment xxxii._ but before we set them down, it will not perhaps appear impertinent to premise; that there seems to be a manifest disparity betwixt red liquors, so that some of them may be said to have a genuine redness in comparison of others, that have a yellowish redness: for if you take (for example) a good tincture of _chochineel_, dilute it never so much with fair water, you will not (as far as i can judge by what i have tried) be able to make it a yellow liquor. insomuch that a single drop of a rich solution of _cochineel_ in spirit of urine, being diluted with above an ounce of fair water, exhibited no yellowishness at all, but a fair (though somewhat faint) pinck or carnation; and even when _cochineel_ was by degrees diluted much beyond the newly mention'd colour, by the way formerly related to you in the twenty fourth experiment, i remember not, that there appear'd in the whole trial any yellow. but if you take balsom of sulphur (for instance) though it may appear in a glass, where it has a good thickness, to be of a deep red, yet if you shake the glass, or pour a few drops on a sheet of white paper, spreading them on it with your finger, the balsom that falls back along the sides of the glass, and that which stains the paper, will appear yellow, not red. and there are divers tinctures, such as that of amber made with spirit of wine, (to name now no more) that will appear either yellow or red, according as the vessels that they fill, are slender or broad. _experiment xxxiii._ but to proceed to the experiments i was about to deliver; _first_; oyl or spirit of turpentine, though clear as fair water, being digested upon the purely white sugar of lead, has, in a short time, afforded us a high red tincture, that some artists are pleas'd to call the balsom of _saturn_, which they very much (and probably not altogether without cause) extoll as an excellent medicine in divers outward affections. _experiment xxxiv._ _next_, take of common brimstone finely powdred five ounces, of sal-armoniack likewise pulveriz'd an equal weight, of beaten quick-lime six ounces, mix these powders exquisitely, and distill them through a retort plac'd in sand by degrees of fire, giving at length as intense a heat as you well can in sand, there will come over (if you have wrought well) a volatile tincture of sulphur, which may probably prove an excellent medicine, and should have been mention'd among the other preparations of sulphur, which we have elsewhere imparted to you, but that it is very pertinent to our present subject, the change of colours. for though none of the ingredients be red, the distill'd liquor will be so: and this liquor if it be well drawn, will upon a little agitation of the vial first unstop'd (especially if it be held in a warmer hand) lend forth a copious fume, not red, like that of nitre, but white; and sometimes this liquor may be so drawn, that i remember, not long since, i took pleasure to observe in a parcel of it, that ingredients not red, did not only yield by distillation a volatile spirit that was red, but though that liquor did upon the bare opening of the bottle it was kept in, drive us away with the plenty and sulphureous sent of a white steam which it sent forth, yet the liquor it self being touch'd by our fingers, did immediately dye them black. _experiment xxxv._ the third and _last_ experiment i shall now mention to shew, how prone bodies abounding in sulphureous parts are to afford a red colour, is one, wherein by the operation of a saline spirit upon a white or whitish body, which according to the _chymists_ should be altogether sulphureous, a redness may be produc'd, not (as in the former experiments) slowly, but in the twinkling of an eye. we took then of the essential oyl of anniseeds, which has this peculiarity, that in cold weather it loses its fluidity and the greatest part of its transparency, and looks like a white or whitish oyntment, and near at hand seems to consist of a multitude of little soft scales: of this coagulated stuff we spread a little with a knife upon a piece of white paper, and letting fall on it, and mixing with it a drop or two of oyl of vitriol, immediately (as we fore-saw) there emerg'd together with some heat and smoak, a blood-red colour, which therefore was in a trice produc'd by two bodies, whereof the one had but a whitish colour, and the other (if carefully rectify'd) had no colour at all. _experiment xxxvi._ but on this occasion (_pyrophilus_) we must add once for all, that in many of the above-recited experiments, though the changes of colour happen'd as we have mention'd them: yet the emergent or produc'd colour is oft times very subject to degenerate, both quickly and much. notwithstanding which, since the changes, we have set down, do happen presently upon the operation of the bodies upon each other, or at the times by us specify'd; _that_ is sufficient both to justifie our veracity, and to shew what we intend; it not being essential to the genuineness of a colour to be durable. for a fading leaf, that is ready to rot, and moulder into dust, may have as true a yellow, as a wedge of gold, which so obstinately resists both time and fire. and the reason, why i take occasion from the former experiment to subjoyn this general advertisement, is, that i have several times observ'd, that the mixture resulting from the oyls of vitriol, and of anniseeds, though it acquire a thicker consistence than either of the ingredients had, has quickly lost its colour, turning in a very short time into a dirty gray, at least in the superficial parts, where 'tis expos'd to the air; which last circumstance i therefore mention, because that, though it seem probable, that this degeneration of colours may oft times and in divers cases proceed from the further action of the saline corpuscles, and the other ingredients upon one another, yet in many cases much of the quick change of colours seems ascribeable to the air, as may be made probable by several reasons: the first whereof may be fetcht from the newly recited example of the two oyls; the next may be, that we have sometimes observ'd long window-curtains of light colours, to have that part of them, which was expos'd to the air, when the window was open, of one colour, and the lower part, that was sheltred from the air by the wall, of another colour: and the third argument may be fetch'd from divers observations, both of others, and our own; for of that pigment so well known in painters shops, by the name of _turnsol_, our industrious _parkinson_, in the particular account he gives of the plant that bears it, tells us also, that _the berries when they are at their full maturity, have within them between the outer skin and the inward kirnel or seed, a certain juice or moisture, which being rubb'd upon paper or cloath, at the first appears of a fresh and lovely green colour, but presently changeth into a kind of blewish purple, upon the cloath or paper, and the same cloath afterwards wet in water, and wrung forth, will colour the water into a claret wine colour, and these_ (concludes he) _are those raggs of cloath, which are usually call'd_ turnsol _in the druggists or grocers shops_[ ]. and to this observation of our _botanist_ we will add an experiment of our own, (made before we met with that) which, though in many circumstances, very differing, serves to prove the same thing; for having taken of the deeply red juice of _buckthorn_ berries, which i bought of the man that uses to sell it to the apothecaries, to make their syrrup _de spina cervina_, i let some of it drop upon a piece of white paper, and having left it there for many hours, till the paper was grown dry again, i found what i was inclin'd to suspect, namely, that this juice was degenerated from a deep red to a dirty kind of greyish colour, which, in a great part of the stain'd paper seem'd not to have so much as an eye of red: though a little spirit of salt or dissolv'd _alcaly_ would turn this unpleasant colour (as formerly i told you it would change the not yet alter'd juice) into a red or green. and to satisfie my self, that this degeneration of colour did not proceed from the paper, i drop'd some of the deep red or crimson juice upon a white glaz'd tile, and suffering it to dry on there, i found that ev'n in that body, on which it could not soak, and by which it could not be wrought, it nevertheless lost its colour. and these instances (_pyrophilus_) i am the more carefull to mention to you, that you may not be much surpris'd or discourag'd, if you should sometimes miss of performing punctually what i affirm my self to have done in point of changing colours; since in these experiments the over-sight or neglect of such little circumstances, as in many others would not be perhaps considerable, may occasion the mis-carrying of a trial. and i was willing also to take this occasion of advertising you in the repeating of the experiments mention'd in this treatise, to make use of the juices of vegetables, and other things prepar'd for your trials, as soon as ever they are ready, lest one or other of them grow less fit, if not quite unfit by delay; and to estimate the event of the trials by the change, that is produc'd presently upon the due and sufficient application of actives to passives, (as they speak) because in many cases the effects of such mixtures may not be lasting, and the newly produc'd colour may in a little time degenerate. but, (_pyrophilus_) i forgot to add to the two former observations lately made about vegetables, a third of the same import, made in mineral substances, by telling you, that the better to satisfie a friend or two in this particular, i sometimes made, according to some conjectures of mine, this experiment; that having dissolv'd good silver in _aqua-fortis_, and precipitated it with spirit of salt, upon the first decanting of the liquor, the remaining matter would be purely white; but after it had lain a while uncover'd, that part of it, that was contiguous to the air, would not only lose its whiteness, but appear of a very dark and almost blackish colour, i say that part that was contiguous to the air, because if that were gently taken off, the subjacent part of the same mass would appear very white, till that also, having continu'd a while expos'd to the air, would likewise degenerate. now whether the air perform these things by the means of a subtile salt, which we elsewhere show it not to be destitute of, or by a peircing moisture, that is apt easily to insinuate it self into the pores of some bodies, and thereby change their texture, and so their colour; or by solliciting the avolation of certain parts of the bodies, to which 'tis contiguous; or by some other way, (which possibly i may elsewhere propose and consider) i have not now the leisure to discourse. and for the same reason, though i could add many other instances, of what i formerly noted touching the emergency of redness upon the digestion of many bodies, insomuch that i have often seen upon the borders of _france_ (and probably we may have the like in _england_) a sort of pears, which digested for some time with a little wine, in a vessel exactly clos'd, will in not many hours appear throughout of a deep red colour, (as also that of the juice, wherein they are stew'd, becomes) but ev'n on pure and white salt of tartar, pure spirit of wine, as clear as rock-water, will (as we elsewhere declare) by long digestion acquire a redness; though i say such instances might be multiply'd, and though there be some other obvious changes of colours, which happen so frequently, that they cannot but be as well considerable as notorious; such as is the blackness of almost all bodies burn'd in the open air: yet our haste invites us to resign you the exercise of enquiring into the causes of these changes. and certainly, the reason both _why_ the soots of such differing bodies are almost all of them all black, _why_ so much the greater part of vegetables should be rather green than of any other colour, and particularly (which more directly concerns this place) _why_ gentle heats do so frequently in chymical operations produce rather a redness than another colour in digested _menstruums_, not only sulphureous, as spirit of wine, but saline, as spirit of vinegar, may be very well worth a serious inquiry; which i shall therefore recommend to _pyrophilus_ and his ingenious friends. [ ] _parkinson_, thea. bot. trib. cap. . _experiment xxxvii._ it may seem somewhat strange, that if you take the crimson solution of _cochineel_, or the juice of black cherries, and of some other vegetables that afford the like colour, (which because many take but for a deep red, we do with them sometimes call it so) and let some of it fall upon a piece of paper, a drop or two of an acid spirit, such as spirit of salt, or _aqua-fortis_, will immediately turn it into a fair red. whereas if you make an infusion of brazil in fair water, and drop a little spirit of salt or _aqua-fortis_ into it, that will destroy its redness, and leave the liquor of a yellow, (sometimes pale) i might perhaps plausibly enough say on this occasion, that if we consider the case a little more attentively, we may take notice, that the action of the acid spirit seems in both cases, but to weaken the colour of the liquor on which it falls. and so though it destroy redness in the tincture of brazil, as well as produce red in the tincture of _chochineel_, its operations may be uniform enough, since as crimson seems to be little else than a very deep red, with (perhaps) an eye of blew, so some kinds of red seem (as i have lately noted) to be little else than heightned yellow. and consequently in such bodies, the yellow seems to be but a diluted red. and accordingly alcalizate solutions and urinous spirits, which seem dispos'd to deepen the colours of the juices and liquors of most vegetables, will not only restore the solution of _cochineel_ and the infusion of brazil to the crimson, whence the spirit of salt had chang'd them into a truer red; but will also (as i lately told you) not only heighthen the yellow juice of madder into red, but advance the red infusion of brazil to a crimson. but i know not whether it will not be much safer to derive these changes from vary'd textures, than certain kinds of bodies; and you will perhaps think it worth while, that i should add on this occasion, that it may deserve some speculation, why, notwithstanding what we have been observing, though blew and purple seem to be deeper colours than red, and therefore the juices of plants of either of the two former colours may (congruously enough to what has been just now noted) be turn'd red by spirit of salt or _aqua-fortis_, yet blew syrrup of violets and some purples should both by oyl of tartar and spirit of urine be chang'd into green, which seems to be not a deeper but a more diluted colour than blew, if not also than purple. _experiment xxxviii._ it would much contribute to the history of colours, if _chymists_ would in their laboratories take a heedfull notice, and give us a faithfull account of the colours observ'd in the steams of bodies either sublim'd or distill'd, and of the colours of those productions of the fire, that are made up by the coalition of those steams. as (for instance) we observe in the distilling of pure salt peter, that at a certain season of the operation, the body, though it seem either crystalline, or white, affords very red fumes: whereas though vitriol be green or blew, the spirit of it is observ'd to come over in whitish fumes. the like colour i have taken notice of in the fumes of several other concretes of differing colours, and natures, especially when distill'd with strong fires. and we elsewhere note, that ev'n soot, as black as it is, has fill'd our receivers with such copious white fumes, that they seem'd to have had their in-sides wash'd with milk. and no less observable may be, the distill'd liqours, into which such fumes convene, (for though we will not deny, that by skill and care a reddish liqour may be obtain'd from nitre) yet the common spirit of it, in the making ev'n of which store of these red fumes are wont to pass over into the receiver, appears not to be at all red. and besides, that neither the spirit of vitriol, nor that of soot is any thing white; and, besides also, that as far as i have observ'd, most (for i say not all) of the empyreumatical oyls of woods, and other concretes, are either of a deep red, or of a colour between red and black; besides this, i say, 'tis very remarkable that notwithstanding that great variety of colours to be met with in the herbs, flowers, and other bodies wont to be distill'd in _balneo_: yet (as far at least as our common distillers experience reacheth) all the waters and spirits that first come over by that way of distillation, leave the colours of their concretes behind them, though indeed there be one or two vegetables not commonly taken notice of, whose distill'd liqours i elsewhere observe to carry over the tincture of the concrete with them. and as in distillations, so in sublimations, it were worth while to take notice of what comes up, in reference to our present scope, by purposely performing them (as i have in some cafes done) in conveniently shap'd glasses, that the colour of the ascending fumes may be discern'd; for it may afford a naturalist good information to observe the congruities or the differences betwixt the colours of the ascending fumes, and those of the _flowers_, they compose by their convention. for it is evident, that these _flowers_, do many of them in point of colour, much differ, not only from one another, but oft times from the concretes that afforded them. thus, (not here to repeat what i formerly noted of the black soots of very differingly colour'd bodies) though camphire and brimstone afford _flowers_ much of their own colour, save that those of brimstone are wont to be a little paler, than the lumps that yielded them; yet ev'n of red _benzoin_, that sublim'd substance, which _chymists_ call its _flowers_, is wont to be white or whitish. and to omit other instances, ev'n one and the same black mineral, antimony, may be made to afford _flowers_, some of them red, and some grey, and, which is more strange, some of them purely white. and 'tis the prescription of some glass-men by exquisitely mingling a convenient proportion of brimstone, sal-armoniack, and quicksilver, and subliming them, together, to make a sublimate of an excellent blew; and though having caus'd the experiment to be made, we found the produc'd sublimate to be far from being of a lovely colour, (as was promis'd) that there and there, it seem'd blewish, and at least was of a colour differing enough from either of the ingredients, which is sufficient for our present purpose. but a much finer colour is promis'd by some of the empiricks, that pretend to secrets, who tell us, that orpiment, being sublim'd, will afford among the parts of it that fly upward, some little masses, which, though the mineral it self be of a good yellow, will be red enough to emulate rubies, both in colour and translucency. and this experiment may, for ought i know, sometimes succeed; for i remember, that having in a small bolt-head purposely sublim'd some powder'd orpiment, we could in the lower part of the sublimate discern here and there some reddish lines, though much of the upper part of the sublimate consisted of a matter, which was not alone purely yellow, but transparent almost like a powder. and we have also this way obtain'd a sublimate, the lower part whereof though it consisted not of rubies, yet the small pieces of it, which were numerous enough, were of a pleasant reddish colour, and glitter'd very prettily. but to insist on such kind of trials and observations (where the ascending fumes of bodies differ in colour from the bodies themselves) though it might indeed inrich the history of colours, would robb me of too much of the little time i have to dispatch what i have further to tell you concerning them. _experiment xxxix_ take the dry'd buds (or blossoms) of the pomegranate tree, (which are commonly call'd in the shops _balaustiums_) pull off the reddish leaves, and by a gentle ebullition of them in fair water, or by a competent infusion of them in like water well heated, extract a faint reddish tincture, which if the liquor be turbid, you may clarifie it by filtrating it into this, if you pour a little good spirit of urine, or some other spirit abounding in the like sort of volatile salts, the mixture will presently turn of a dark greenish colour, but if instead of the fore-mention'd liquor, you drop into the simple infusion a little rectify'd spirit of sea-salt, the pale and almost colourless liquor will immediately not only grow more transparent, but acquire a high redness, like that of rich claret wine, which so suddenly acquir'd colour, may as quickly be destroy'd and turn'd into a dirty blewish green, by the affusion of a competent quantity of the above-mention'd spirit of urine. _annotation._ this experiment may bring some light to, and receive some from a couple of other experiments, that i remember i have met with in the ingenious _gassendus_'s animadversions upon _epicurus_'s philosophy, whilst i was turning over the leaves of those learned commentaries; (my eyes being too weak to let me read such voluminous books quite thorough) and i the less scruple (notwithstanding my contrary custom in this treatise) to set down these experiments of another, because i shall a little improve the latter of them, and because by comparing there with that which i have last recited, we may be assisted to conjecture upon what account it is, that oyl of vitriol heightens the tincture of red-rose leaves, since spirit of salt, which is a highly acid _menstruum_, but otherwise differing enough from oyl of vitriol, does the same thing. our authors experiments then, as we made them, are these; we took about a glass-full of luke-warm water, and in it immerg'd a quantity of the leaves of _senna_, and presently upon the immersion there did not appear any redness in the water, but dropping into it a little oyl of tartar, the liquor soon discover'd a redness to the watchfull eye, whereas by a little of that acid liquor of vitriol, which is like the former, undeservedly called oyl, such a colour would not be extracted from the infused _senna_. on the other side we took some red-rose leaves dry'd, and having shaken them into a glass of fair water, they imparted to it no redness, but upon the affusion of a little oyl of vitriol the water was immediately turn'd red, which it would not have been, if instead of oyl of vitriol, we had imployed oyl of tartar to produce that colour: that these were _gassendus_ his experiments, i partly remember, and was assur'd by a friend, who lately transcribed them out of _gassendus_ his book, which i therefore add, because i have not now that book at hand. and the design of _gassendus_ in these experiments our friend affirms to be, to prove, that of things not red a redness may be made only by mixture, and the varied position of parts, wherein the doctrine of that subtil philosopher doth not a little authorize, what we have formerly delivered concerning the emergency and change of colours. but the instances, that we have out of him set down, seem not to be the most eminent, that may be produced of this truth: for our next experiment will shew the production of several colours out of liquors, which have not any of them any such colour, nor indeed any discernable one at all; and whereas though our author tells us, that there was no redness either in the water, or the leaves of _senna_, or the oyl of tartar; and though it be true, that the predominant colour of the leaves of _senna_ be another than red, yet we have try'd, that by steeping that plant a night even in cold water, it would afford a very deep yellow or reddish tincture without the help of the oyl of tartar, which seems to do little more than assist the water to extract more nimbly a plenty of that red tincture, wherewith the leaves of _senna_ do of themselves abound, and having taken off the tincture of _senna_, made only with fair water, before it grew to be reddish, and decanted it from the leaves, we could not perceive, that by dropping some oyl of tartar into it, that colour was considerable, though it were a little heightned into a redness; which might have been expected, if the particles of the oyl did eminently co-operate, otherwise than we have expressed, to the production of this redness. and as for the experiment with red-rose leaves, the same thing may be alleged, for we found that such leaves by bare infusion for a night and day in fair water, did afford us a tincture bordering at least upon redness, and that colour being conspicuous in the leaves themselves, would not by some seem so much to be produc'd as to be extracted by the affusion of oyl of vitriol. and the experiment try'd with the dry'd leaves of damask-roses succeeded but imperfectly, but that is indeed observable to our authors purpose, that oyl of tartar will not perform in this experiment what oyl of vitriol doth; but because this last named liquor is not so easily to be had, give me leave to advertise you, that the experiment will succeed, if instead of it you imploy _aqua-fortis_. and though some trials of our own formerly made, and others easily deducible from what we have already deliver'd, about the different families and operations of salt, might enable us to present you an experiment upon red-rose leaves, more accommodated to our authors purpose, than that which he hath given us; yet our reverence to so candid a philosopher, invites us rather to improve his experiment, than substitute another in its place. take therefore of the tincture of red-rose leaves, (for with damask-rose leaves the experiment succeedeth not well) made as before hath been taught with a little oyl of vitriol, and a good quantity of fair water, pour off this liquor into a clear vial, half fill'd with limpid water; till the water held against the light have acquir'd a competent redness, without losing its transparency, into this tincture drop leisurely a little good spirit of urine, and shaking the vial, which you must still hold against the light, you shall see the red liquor immediately turn'd into a fine greenish blew, which colour was not to be found in any of the bodies, upon whose mixture it emerg'd, and this change is the more observable, because in many bodies the degenerating of blew into red is usual enough, but the turning of red into blew is very unfrequent. if at every drop of spirit of urine you shake the vial containing the red tincture, you may delightfully observe a pretty variety of colours in the passage of that tincture from a red to a blew, and sometimes we have this way hit upon such a liquor, as being look't upon against and from the light, did seem faintly to emulate the above-mention'd tincture of _lignum nephriticum_. and if you make the tincture of red-roses very high, and without diluting it with fair water, pour on the spirit of urine, you may have a blew so deep, as to make the liquor opacous, but being dropt upon white paper the colour will soon disclose it self. also having made the red, and consequently the blew tincture very transparent, and suffer'd it to rest in a small open vial for a day or two, we found according to our conjecture, that not only the blew but the red colour also was vanish'd; the clear liquor being of a bright amber colour, at the bottom of which subsided a light, but copious feculency of almost the same colour, which seems to be nothing but the tincted parts of the rose leaves drawn out by the acid spirits of the oyl of vitriol, and precipitated by the volatile salt of the spirit of urine, which makes it the more probable, that the redness drawn by the oyl of vitriol, was at least as well an extraction of the tinging parts of the roses, as a production of redness; and lastly, if you be destitute of spirit of urine, you may change the colour of the tincture of roses with many other sulphureous salts, as a strong solution of pot-ashes, oyl of tartar, &c. which yet are seldome so free from feculency, as the spirituous parts of urine becomes by repeated distillation. _annotation_. on this, occasion, i call to mind, that i found, a way of producing, though not the same kind of blew, as i have been mentioning, yet a colour near of kin to it, namely, a fair purple, by imploying a liquor not made red by art, instead of the tincture of red-roses, made with an acid spirit; and my way was only to take log-wood, (a wood very well known to dyers) having by infusion the powder of it a while in fair water made that liquor red, i dropt into it a _tantillum_ of an urinous spirit, as that of sal-armoniack, (and i have done the same thing with an _alcali_) by which the colour was in a moment turn'd into a rich, and lovely purple. but care must be had, that you let not fall into a spoonfull above two or three drops, lest the colour become so deep, as to make the liquor too opacous. and (to answer the other part of _gassendus_ his experiment) if instead of fair water, i infus'd the log-wood in water made somewhat sowr by the acid spirit of salt, i should obtain neither a purple liquor, nor a red, but only a yellow one. _experiment xl._ the experiment i am now to mention to you, _pyrophilus_, is that which both you, and all the other _virtuosi_ that have seen it, have been pleas'd to think very strange; and indeed of all the experiments of colours, i have yet met with, it seems to be the fittest to recommend the doctrine propos'd in this treatise, and to shew that we need not suppose, that all colours must necessarily be inherent qualities, flowing from the substantial forms of the bodies they are said to belong to, since by a bare mechanical change of texture in the minute parts of bodies; two colours may in a moment be generated quite _de novo,_ and utterly destroy'd. for there is this difference betwixt the following experiment, and most of the others deliver'd in these papers, that in this, the colour that a body already had, is not chang'd into another, but betwixt two bodies, each of them apart devoid of colour, there is in a moment generated a very deep colour, and which if it were let alone, would be permanent; and yet by a very small parcel of a third body, that has no colour of its own, (lest some may pretend i know not what antipathy betwixt colours) this otherwise permanent colour will be in another trice so quite destroy'd, that there will remain no foot-stepts either of it or of any other colour in the whole mixture. the experiment is very easie, and it is thus perform'd: take good common sublimate, and fully satiate with it what quantity of water you please, filtre the solution carefully through clean and close paper, that it may drop down as clear and colourless as fountain water. then when you'l shew the experiment, put of it about a spoonfull into a small wine-glass, or any other convenient vessel made of clear glass, and droping in three or four drops of good oyl of tartar, _per deliquium_; well filtred that it may likewise be without colour, these two limpid liquors will in the twinkling of an eye turn into an opacous mixture of a deep orange colour, which by keeping the glass continually shaking in your hand, you must preserve from setling too soon to the bottom; and when the spectators have a little beheld this first change, then you must presently drop in about four or five drops of oyl of vitriol, and continuing to shake the glass pretty strongly, that it may the nimbler diffuse it self, the whole colour, if you have gone skilfully to work, will immediately disappear, and all the liquor in the glass will be clear and colourless as before, without so much as a sediment at the bottom. but for the more gracefull trial of this experiment, 'twill not be amiss to observe, first, that there should not be taken too much of the solution of sublimate, nor too much of the oyl of tartar drop'd in, to avoid the necessity of putting in so much oyl of vitriol as may make an ebullition, and perhaps run over the glass. secondly, that 'tis convenient to keep the glass always a little shaking, both for the better mixing of the liquors, and to keep the yellow substance from subsiding, which else it would in a short time do, though when 'tis subsided it will retain its colour, and also be capable of being depriv'd of it by the oyl newly mention'd. thirdly, that if any yellow matter stick at the sides of the glass, 'tis but inclining the glass, till the clarify'd liquor can wash alongst it, and the liquor will presently imbibe it, and deprive it of its colour. many have somewhat wondred, how i came to light upon this experiment, but the notions or conjectures i have about the differing natures of the several tribes of salts, having led me to devise the experiment, it will not be difficult for me to give you the chymical reason, if i may so speak, of the _phænomenon_. having then observ'd, that _mercury_ being dissolv'd in some _menstruums_, would yield a dark yellow precipitate, and supposing that, as to this, common water, and the salts that stick to the _mercury_ would be equivalent to those acid _menstruums_, which work upon the _quick-silver_, upon the account of their saline particles, i substituted a solution of sublimate in fair water, instead of a solution of _mercury_ in _aqua-fortis_, or spirit of _nitre_, that simple solution being both clearer and free from that very offensive smell, which accompanies the solutions of _mercury_ made with those other corrosive liquors; then i consider'd, that that, which makes the yellow colour, is indeed but a precipitate made by the means of the oyl of tartar, which we drop in, and which, as _chymists_ know, does generally precipitate metalline bodies corroded by acid salts; so that the colour in our case results from the coalition of the mercurial particles with the saline ones, wherewith they were formerly associated, and with the alcalizate particles of the salt of tartar that swim up and down in the oyl. wherefore considering also, that very many of the effects of lixiviate liquors, upon the solutions of other bodies, may be destroy'd by acid _menstruums_, as i elsewhere more particularly declare, i concluded, that if i chose a very potently acid liquor, which by its incisive power might undo the work of the oyl of tartar, and disperse again those particles, which the other had by precipitation associated, into such minute corpuscles as were before singly inconspicuous, they would become inconspicuous again, and consequently leave the liquor as colourless as before the precipitation was made. this, as i said, _pyrophilus_, seems to be the chymical reason of this experiment, that is such a reason, as, supposing the truth of those chymical notions i have elsewhere i hope evinc'd, may give such an account of the _phænomena_ as chymical notions can supply us with; but i both here and elsewhere make use of this way of speaking, to intimate that i am sufficiently aware of the difference betwixt a chymical explication of a _phænomenon_, and one that is truly philosophical or mechanical; as in our present case, i tell you something, when i tell you that the yellowness of the mercurial solution and the oyl of tartar is produc'd by the precipitation occasion'd by the affusion of the latter of those liquors, and that the destruction of the colour proceeds from the dissipation of that curdl'd matter, whose texture is destroy'd, and which is dissolv'd into minute and invisible particles by the potently acid _menstruum_, which is the reason, why there remains no sediment in the bottom, because the infused oyl takes it up, and resolves it into hidden or invisible parts, as water does salt or sugar. but when i have told you all this, i am far from thinking i have told all that such an inquisitive person as your self would know, for i presume you would desire as well as i to learn (at least) why the particles of the _mercury_, of the tartar, and of the acid salts convening together, should make rather an orange colour than a red, or a blew, or a green, for 'tis not enough to say what i related a little before, that divers mercurial solutions, though otherwise made, would yield a yellow precipitate, because the question will recurr concerning them; and to give it a satisfactory answer, is, i freely acknowledge, more than i dare as yet pretend to. but to confirm my conjecture about the chymical reason of our experiment, i may add, that as i have (_viz._ pag. th. of this treatise) elsewhere (on another occasion) told you, with saline liquors of another kind and nature than salt of tartar, (namely, with spirit of urine, and liquors of kin to that) i can make the _mercury_ precipitate out of the first simple solution quite of another colour than that hitherto mention'd; nay, if instead of altering the precipitating liquor, i alter'd the texture of the sublimate in such a way as my notions about salt requir'd, i could produce the same _phænomenon_. for having purposely sublim'd together equal parts (or thereabout) of sal-armoniack and sublimate, first diligently mix'd, the ascending flowers being diffolv'd in fair water, and filtred, gave a solution limpid and colourless, like that of the other sublimates, and yet an _akaly_ drop'd into this liquor did not turn it yellow but white. and upon the same grounds we may with _quick-silver_, without the help of common sublimate, prepare another sort of flowers dissoluble in water without discolouring it, with which i could likewise do what i newly mention'd; to which i shall add, (what possibly you'l somewhat wonder at) that so much does the colour depend upon the texture resulting from the convention of the several sorts of corpuscles, that though in out experiment, oyl of vitriol destroys the yellow colour, yet with _quick-silver_ and fair water, by the help of oyl of vitriol alone, we may easily make a kind of precipitate of a fair and permanent yellow, as you will e're long (in the forty second expement of this third part) be taught. and i may further add, that i chose oyl of vitriol, not so much for any other or peculiar quality, as for its being, when 'tis well rectify'd, (which 'tis somewhat hazardous to bring it to be) not only devoid of colour and in smells, but extremely strong and incisive; for though common and undephlegmated _aqua-fortis_ will not perform the same thing well, yet that which is made exceeding strong by being carefully dephlegm'd, will do it pretty well, though not so well as oyl of vitriol which is so strong, that even without rectification it may for a need be made use of. i will not here tell you what i have try'd, that i may be able to deprive at pleasure the precipitate that one of the sulphureous liquors had made, by the copious affusion of the other: because i found, though this experiment is too ticklish to let me give a full account of it in few words, i shall therefore tell you, that it is not only for once, that the other above-mention'd experiment may be made, the same numerical parcels of liquor being still imploy'd in it; for after i have clarify'd the orange colour'd liquor, by the addition of as little of the oyl of viriol as will suffice to perform the effect, i can again at pleasure re-produce the opacous colour, by the dropping in of fresh oyl of tartar, and destroy it again by the re-affusion of more of the acid _menstruum_; and yet oftner if i please, can i with these two contrariant liquors recall and disperse the colour, though by reason of the addition of so much new liquor, in reference to the mercurial particles, the colour will at length appear more dilute and faint. _an improvement of the fortieth experiment_. and, _pyrophilus_, to confirm yet further the notions that led me to think on the propos'd experiment, i shall acquaint you with another, which when i had conveniency i have sometimes added to it, and which has to the spectators appear'd little less odd than the first; and though because the liquor, requisite to make the trial succeed well, must be on purpose prepar'd anew a while before, because it will not long retain its fitness for this work, i do but seldome annex this experiment to the other, yet i shall tell you how i devis'd it, and how i make it. if you boyl crude antimony in a strong and clear _lixivium_, you shall separate a substance from it, which some modern _chymists_ are pleas'd to call its sulphur, but how deservedly i shall not here examine, having elsewhere done it in an opportune place; wherefore i shall now but need to take notice, that when this suppos'd sulphur (not now to call it rather a kind of _crocus_) is let fall by the liquor upon its refrigeration, it often settles in flakes, or such like parcels of a yellow substance, (which being by the precedent dissolution reduc'd into minute parts, may peradventure be made to take fire much more easily than the grosser powder of unprepar'd antimony would have done.) considering therefore, that common sulphur boyl'd in a _lixivium_ may be precipitated out of it by rhenish-wine or white-wine, which are sowrish liquors, and have in them, as i elsewhere shew, an acid salt; and having found also by trial, that with other acid liquors i could precipitate out of lixiviate solvents some other mineral concretions abounding with sulphureous parts, of which sort is crude antimony, i concluded it to be easie to precipitate the antimony dissolv'd, as was lately mention'd, with the acid oyl of vitriol; and though common sulphur yields a white precipitate, which the _chymists_ call _lac sulphuris_, yet i suppos'd the precipitated antimony would be of a deep yellow colour, as well, if made with oyl of vitriol, as if made only by refrigeration and length of time. from this 'twas easie to deduce this experiment, that if you put into one glass some of the freshly impregnated and filtrated solution of antimony, and into another some of the orange-colour'd mixture, (which i formerly shew'd you how to make with a mercurial solution and oyl of tartar) a few drops of oyl of vitriol dropp'd into the last mention'd glass, would, as i told you before, turn the deep yellow mixture into a cleer liquor; whereas a little of the same oyl dropp'd out of the same viol into the other glass would presently (but not without some ill sent) turn the moderately cleer solution into a deep yellow substance, but this, as i said, succeeds not well, unless you employ a _lixivium_ that has but newly dissolv'd antimony, and has not yet let it fall. but yet in summer time, if your _lixivium_ have been duly impregnated and well filtred after it is quite cold, it will for some dayes (perhaps much longer than i had occasion to try) retain antimony enough to exhibit, upon the affusion of the corrosive oyl, as much of a good yellow substance as is necessary to satisfie the beholders of the possibility of the experiment. _reflections upon the xl. experiment compared with the x. and xx._ the knowledge of the distinction of salts which we have propos'd, whereby they are discriminated into _acid, volatile,_ or _salfuginous_ (if i may for distinction sake so call the fugitive salts of animal substances) and _fix'd_ or _alcalizate_, may possibly (by that little part which we have already deliver'd, of what we could say of its applicableness) appear of so much use in natural philosophy (especially in the practick part of it) that i doubt not but it will be no unwelcome corollary of the preceding experiment, if by the help of it i teach you to distinguish, which of those salts is predominant in chymical liquors, as well as whether any of them be so or not. for though in our notes upon the x. and xx. experiments i have shown you a way by means of the tincture of _lignum nephriticum_, or of syrrup of violets, to discover whether a propounded salt be acid or not, yet you can thereby only find in general that such and such salts belong not to the tribe of acids, but cannot determine whether they belong to the tribe of urinous salts (under which for distinction sake i comprehend all those volatile salts of animal or other substances that are contrary to acids) or to that of alcalies. for as well the one as the other of these salino-sulphurous salts will restore the cæruleous colour to the tincture of _lignum nephriticum_, and turn that of syrrup of violets into green. wherefore this xl. experiment does opportunely supply the deficiency of those. for being sollicitous to find out some ready wayes of discriminating the tribes of chymical salts, i found that all those i thought fit to make tryal of, would, if they were of a lixiviate nature, make with sublimate dissolv'd in fair water an _orange tawny_ precipitate; whereas if they were of an urinous nature the precipitate would be _white_ and milky. so that having alwayes by me some syrrup of violets and some solution of sublimate, i can by the help of the first of those liquors discover in a trice, whether the propounded salt or saline body be of an acid nature or no, if it be i need (you know) inquire no further; but if it be not, i can very easily, and as readily distinguish between the other two kinds of salts, by the white or orange-colour that is immediately produc'd, by letting fall a few drops or grains of the salt to be examin'd, into a spoonfull of the cleer solution of sublimate. for example, it has been suppos'd by some eminently learned, that when sal armoniack being mingled with an alcaly is forc'd from it by the fire in close vessels, the volatile salt that will thereby be obtain'd (if the operation be skilfully perform'd,) is but a more fine and subtile sort of sal armoniack, which, 'tis presum'd, this operation do's but more exquisitely purifie, than common solutions, filtrations, and coagulations. but this opinion may be easily shown to be erroneous, as by other arguments, so particularly by the lately deliver'd method of distinguishing the tribes of salts. for the saline spirit of sal armoniack, as it is in many other manifest qualities very like the spirit of urine, so like, that it will in a trice make syrrup of violets of a lovely green, turn a solution of good verdigrease into an excellent azure, and make the solution of a sublimate yield a white precipitate, insomuch that in most (for i say not all of the experiments) where i aim onely at producing a sudden change of colour, i scruple not to use spirit of sal armoniack when it is at hand, instead of spirit of urine, as indeed it seems chiefly to consist (besides the flegm that helps to make it fluid) of the volatile urinous salt (yet not excluding that of soot) that abounds in the sal armoniack and is set at liberty from the sea salt wherewith it was formerly associated, and clogg'd, by the operation of the alcaly, that divides the ingredients of sal armoniack, and retains that sea salt with it self. what use may be made of the like way of exploration in that inquiry which puzzles so many modern naturalists, whether the rich pigment (which we have often had occasion to mention) belongs to the vegetable or animal kingdome, you may find in another place where i give you some account of what i try'd about cocheneel. but i think it needless to exemplifie here our method by any other instances, many such being to be met with in divers parts of this treatise; but i will rather advertise you, that, by this way of examining chymical liquors, you may not onely in most cases conclude _affirmatively_, but in some cases _negatively_. as since spirit of wine, and as far as i have try'd, those chymical oyles which artists call essential, did not (when i us'd them as i had us'd the several families of salts upon that syrrup) turn syrrup of violets red or green, nor the solution of sublimate white or yellow, i inferr'd it may thence be probably argued, that either they are destitute of salt, or have such as belongs not to either of the three grand families already often mention'd. when i went to examine the spirit of oak or of such like concretes forced over through a retort, i found by this means amongst others, that (as i elsewhere show) these chymists are much mistaken in it, that account it a simple liquor, and one of their hypostatical principles: for not to mention what flegm it may have, i found that with a few drops of one of this sort of spirits mix'd with a good proportion of syrrup of violets, i could change the colour and make it purplish, by the affinity of which colour to redness, i conjectur'd that this spirit had some acid corpuscles in it, and accordingly i found that as it would destroy the blewness of a tincture of _lignum nephriticum_, so being put upon corals it would corrode them, as common spirit of vinegar, and other acid liquors are wont to do. and farther to examine whether there were not a great part of the liquor that was not of an acid nature, having separated the sour or vinegar-like part from the rest, which (if i mistake not) is far the more copious, we concluded as we had conjectured, the other or remaining part, though it had a strong taste as well as smell, to be of a nature differing from that of either of the three sorts of salts above mention'd, since it did as little as spirit of wine, and chymical oyls, alter the colour either of syrrup of violets or solution of sublimate, whence we also inferr'd that the change that had been made of that syrrup into a purple colour, was effected by the vinegar, that was one of the two ingredients of the liquor, which was wont to pass for a simple or uncompounded spirit. and, upon this account, 'twas of the spirit of oak (and the like concretes) freed from it's vinegar that i elsewhere told you, that i had not then observ'd it, (and i have repeated the tryal but very lately) to destroy the cæruleous tincture of _lignum nephriticum_. but this onely, _en passant_; for the chief thing i had to add was this, that by the same way may be examin'd and discover'd, divers changes that are produc'd in bodies either by nature only, or by art; either of them being able by changing the texture of some concretes i could name, to qualifie them to operate after a new manner upon the above mention'd syrrup, or solution, or both. and by this means, to tell you that upon the by, i have been able to discover, that there may be made bodies, which though they run _per deliquium_, as readily as salt of tartar, belong in other respects, not to the family of alcaliz, much less to that of salfuginous, or that of acid salts. perhaps too, i may know a way of making a highly operative saline body that shall neither change the colour of syrrup of violets, nor precipitate the solution of sublimate; and, i can likewise if i please conceal by what liquors i perform such changes of colour, as i have been mentioning to you, by quite altering the texture of some ordinary chymical productions, the exploration of which is the main use of the fortieth experiment, which i think teaches not a little, if it teach us to discover the nature of those things (in reference to salt) that are obtain'd by the ordinary chymical analysis of mix'd bodyes, though perhaps there may be other bodyes prepar'd by chymistry which may have the same effects in the change of colours; and yet be produc'd not from what chymists call the resolution of bodies, but from their composition. but the discoursing of things of this nature is more proper for another place. i shall now onely add, what might perhaps have been more seasonably told you before; that the reason why the way of exploration of salts hitherto deliver'd, succeeds in the solution of sublimate, depends upon the particular texture of that solution, as well as upon the differing natures of the saline liquors imploy'd to precipitate it. for gold dissolv'd in _aqua regia_, whether you precipitate it with oyl of tartar which is an alcaly, or with spirit of urine, or sal armoniack which belongs to the family of volatile salts, will either way afford a yellow substance: though with such an acid liquor, as, i say not spirit of salt, the body that yields it, being upon the matter an ingredient of _aqua regis_, but oyl of vitriol it self, i did not find that i could precipitate the metall out of the solution, or destroy the colour of it, though the same oyl of vitriol would readily precipitate silver dissolv'd in _aqua-fortis_. and if you dissolve pure silver in _aqua-fortis_, and suffer it to shoot into crystals, the cleer solution of these made in fair water, will afford a very white precipitate, whether it be made with an alcaly, or an acid spirit, as that of salt, whereas, which may seem somewhat strange, with spirit of sal armoniack (that i us'd was made of quicklime) i could obtain no such white precipitate; that volatile spirit, nor (as i remember) that of urine, scarce doing any more than striking down a very small quantity of matter, which was neither white nor whitish, so that the remaining liquor being suffer'd to evaporate till the superfluous moisture was gone, the greatest part of the metalline corpuscles with the saline ones that had imbib'd them, concoagulated into salt, as is usual in such solutions, wherein the metall has not been precipitated. _experiment xli._ of kin to the last or fortieth experiment is another which i remember i have sometimes shewn to _virtuosi_ that were pleas'd not to dislike it. i took spirit of urine made by fermentation, and with a due proportion of copper brought into small parts, i obtain'd a very lovely azure solution, and when i saw the colour was such as was requisite, pouring into a clean glass, about a spoonfull of this tincted liquor, (of which i us'd to keep a quantity by me,) i could by shaking into it some drops of strong oyl of vitriol, deprive it in a trice of its deep colour, and make it look like common-water. _annotation_. this experiment brings into my mind this other, which oftentimes succceds well enough, though not quite so well as the former; namely, that if into about a small spoonfull of a solution of good french verdigrease made in fair water, i drop't and shak'd some strong spirit of salt, or rather deflegm'd _aqua fortis_, the greenness of the solution would be made in a trice almost totally to disappear, & the liquor held against the light would scarce seeme other than cleer or limpid, to any but an attentive eye, which is therefore remarkable; because we know that _aqua-fortis_ corroding copper, which is it that gives the colour to verdigrease, is wont to reduce it to a green blew solution. but if into the other altogether or almost colourless liquor i was speaking of, you drop a just quantity either of oyl of tartar or spirit of urine, you shall find that after the ebullition is ceas'd, the mixture will disclose a lively colour, though somewhat differing from that which the solution of verdigrease had at first. _experiment xlii._ that the colour (_pyrophilus_) of a body may be chang'd by a liquor which of it self is of no colour, provided it be saline, we have already manifested by a multitude of instances. nor doth it seem so strange, because saline particles swimming up and down in liquors, have been by many observ'd to be very operative in the production and change of colours. but divers of our friends that are not acquainted with chymical operations have thought it very strange that a white body, and a dry one too, should immediately acquire a rich new colour upon the bare affusion of spring-water destitute as well of adventitious salt as of tincture. and yet (_pyrophilus_) the way of producing such a change of colours may be easily enough lighted on by those that are conversant in the solutions of mercury. for we have try'd, that though by evaporating a solution of quick-silver in _aqua-fortis_, and abstracting the liquor till the remaining matter began to be well, but not too strongly dryed, fair water pour'd on the remaining _calx_ made it but somewhat yellowish; yet when we took good quick-silver, and three or four times its weight of oyl of vitriol, in case we in a glass retort plac'd in sand drew off the saline _menstruum_ from the metalline liquor, till there remain'd a dry _calx_ at the bottome, though this precipitate were a snow white body, yet upon pouring on it a large quantity of fair water, we did almost in a moment perceive it to pass from a milky colour to one of the loveliest light yellows that ever we had beheld. nor is the turbith mineral, that chymists extol for its power to salivate, and for other vertues, of a colour much inferiour to this, though it be often made with a differing proportion of the ingredients, a more troublesome way. for _beguinus_,[ ] who calls it _mercurius præcipitatus optimus_, takes to one part of quick-silver, but two of liquor, and that is rectifi'd oyl of sulphur, which is (in _england_ at least) far more scarce and dear than oyl of vitriol; he also requires a previous digestion, two or three cohobations, and frequent ablutions with hot distill'd water, with other prescriptions, which though they may conduce to the goodness of the medicine, which is that he aims at, are troublesome, and, our tryals have inform'd you unneccessary to the _obtaining the lemmon colour_ which he regards not. but though we have very rarely seen either in painters shops, or elsewhere a finer yellow than that which we have divers times this way produc'd (which is the more considerable, because durable and pleasant yellows are very hard to be met with, as may appear by the great use which painters are for its colours sake fain to make of that pernicious and heavy mineral, orpiment) yet i fear our yellow is too costly, to be like to be imploy'd by painters, unless about choice pieces of work, nor do i know how well it will agree with every pigment, especially, wich oyl'd colours. and whether this experiment, though it have seem'd somewhat strange to most we have shown it to, be really of another nature than those wherein saline liquors are imploy'd, may, as we formerly also hinted, be so plausibly doubted, that whether the water pour'd on the _calx_, do barely by imbibing some of its saline parts alter its colour by altering its texture, or whether by dissolving the concoagulated salts, it does become a saline _menstruum_, and, as such, work upon the mercury, i freely leave to you (_pyrophilus_) to consider. and that i may give you some assistance in your enquiry, i will not only tell you, that i have several times with fair water wash'd from this _calx_, good store of strongly tasted corpuscles, which by the abstraction of the _menstruum_, i could reduce into salt; but i will also subjoyn an experiment, which i devis'd, to shew among other things, how much a real and permanent colour may be as it were drawn forth by a liquor that has neither colour, nor so much as saline or other active parts, provided it can but bring the parts of the body it imbibes to convene into clusters dispos'd after the manner requisite to the exhibiting of the emergent colour. the experiment was this. [ ] _beguinus_, tyr. chy. lib. º. cap. º. _experiment xliii._ we took good common vitriol, and having beaten it to powder, and put it into a crucible, we kept it melted in a gentle heat, till by the evaporation of some parts, and the shuffling of the rest, it had quite lost its former colour, what remain'd we took out, and found it to be a friable _calx_, of a dirty gray. on this we pour'd fair water, which it did not colour green or blew, but only seem'd to make a muddy mixture with it, then stopping the vial wherein the ingredients were put, we let it stand in a quiet place for some dayes, and after many hours the water having dissolv'd a good part of the imperfectly calcin'd body, the vitriolate corpuscles swiming to and fro in the liquor, had time by their opportune occursions to constitute many little masses of vitriol, which gave the water they impregnated a fair vitriolate colour; and this liquor being pour'd off, the remaining dirty powder did in process of time communicate the like colour, but not so deep, to a second parcel of cleer water that we pour'd on it. but this experiment _pyrophilus_ is, (to give you that hint by the way) of too luciferous a nature to be fit to be fully prosecuted, now that i am in haste, and willing to dispatch what remains. and we have already said of it, as much as is requisite to our present purpose. _experiment xliv._ it may (_pyrophilus_) somewhat contribute towards the shewing how much some colours depend upon the less or greater mixture, and (as it were,) contemperation of the light with shades, to observe, how that sometimes the number of particles, of the same colour, receiv'd into the pores of a liquor, or swiming up and down in it, do seem much to vary the colour of it. i could here present you with particular instances to show, how in many (if not most) consistent bodyes, if the colour be not a light one, as white, yellow, or the like, the closeness of parts in the pigments makes it look blackish, though when it is display'd and laid on thinly, it will perhaps appear to be either blew, or green, or red. but the colours of consistent pigments, not being those which the preamble of this experiment has lead you to expect examples in, i shall take the instances i am now to give you, rather from liquors than dry bodyes. if then you put a little fair water into a cleer and slender vial, (or rather into one of those pipes of glass, which we shall by and by mention;) and let fall into it a few drops of a strong decoction or infusion of _cochineel_, or (for want of that) of _brazil_; you may see the tincted drops descend like little clouds into the liquor; through which, if, by shaking the vial, you diffuse them, they will turn the water either of a pinck colour, or like that which is wont to be made by the washing of raw flesh in fair water; by dropping a little more of the decoction, you may heighten the colour into a fine red, almost like that which ennobles rubies; by continuing the affusion, you may bring the liquor to a kind of a crimson, and afterwards to a dark and opacous redness, somewhat like that of clotted blood. and in the passage of the liquor from one of these colours to the other, you may observe, if you consider it attentively, divers other less noted colours belonging to red, to which it is not easie to give names; especially considering how much the proportion of the decoction to the fair water, and the strength of that decoction, together with that of the trajected light and other circumstances, may vary the phænomena of this experiment. for the convenienter making whereof, we use instead of a vial, any slender pipe of glass of about a foot or more in length, and about the thickness of a mans little finger; for, if leaving one end of this pipe open, you seal up the other hermetically, (or at least stop it exquisitely with a cork well fitted to it, and over-laid with hard sealing wax melted, and rubb'd upon it;) you shall have a glass, wherein may be observ'd the variations of the colours of liquors much better than in large vials, and wherein experiments of this nature may be well made with very small quantities of liquor. and if you please, you may in this pipe produce variety of colours in the various parts of the liquor, and keep them swimming upon one another unmix'd for a good while. and some have marveil'd to see, what variety of colours we have sometimes (but i confess rather by chance than skill) produc'd in those glasses, by the bare infusion of brazil, variously diluted with fair water, and alter'd by the infusion of several chymical spirits and other saline liquors devoid themselves of colour, and when the whole liquor is reduc'd to an uniform degree of colour, i have taken pleasure to make that very liquor seem to be of colours gradually differing, by filling with it glasses of a conical figure, (whether the glass have its basis in the ordinary position, or turn'd upwards.) and yet you need not glasses of an extraordinary shape to see an instance of what the vari'd mixture of light and shadow can do in the diversifying of the colour. for if you take but a large round vial, with a somewhat long and slender neck, and filling it with our red infusion of brazil, hold it against the light, you will discern a notable disparity betwixt the colour of that part of the liquor which is in the body of the vial, and that which is more pervious to the light in the neck. nay, i remember, that i once had a glass and a blew liquor (consisting chiefly (or only, if my memory deceive me not,) of a certain solution of verdigrease) so fitted for my purpose, that though in other glasses the experiment would not succeed, yet when that particular glass was fill'd with that solution, in the body of the vial it appear'd of a lovely blew, and in the neck, (where the light did more dilute the colour,) of a manifest green; and though i suspected there might be some latent yellowness in the substance of the neck of the glass, which might with the blew compose that green, yet was i not satisfi'd my self with my conjecture, but the thing seem'd odd to me, as well as to divers curious persons to whom it was shown. and i lately had a broad piece of glass, which being look'd on against the light seem'd clear enough, and held from the light appear'd very lightly discolour'd, and yet it was a piece knock'd off from a great lump of glass, to which if we rejoyn'd it, where it had been broken off, the whole mass was as green as grass. and i have several times us'd bottles and stopples that were both made (as those, i had them from assur'd me) of the very same metall, and yet whilst the bottle appear'd but inclining towards a green, the stopple (by reason of its great thickness) was of so deep a colour that you would hardly believe they could possibly be made of the same materials. but to satisfie some ingenious men, on another occasion, i provided my self of a flat glass (which i yet have by me,) with which if i look against the light with the broad side obverted to the eye, it appeares like a good ordinary window glass; but if i turn the edge of it to my eye, and place my eye in a convenient posture in reference to the light, it may contend for deepness of colour with an emerald. and this greeness puts me in mind of a certain thickish, but not consistent pigment i have sometimes made, and can show you when you please, which being dropp'd on a piece of white paper appears, where any quantity of it is fallen, of a somewhat crimson colour, but being with ones finger spread thinly on the paper does presently exhibit a fair green, which seems to proceed only from its disclosing its colour upon the extenuation of its depth into superficies, if the change be not somewhat help'd by the colours degenerating upon one or other of the accounts formerly mention'd. let me add, that having made divers tryals with that blew substance, which in painters shops is call'd _litmase_, we have sometimes taken pleasure to observe, that being dissolv'd in a due proportion of fair water, the solution either oppos'd to the light, or dropp'd upon white paper, did appear of a deep colour betwixt crimson and purple; and yet that being spread very thin on the paper and suffer'd to dry on there, the paper was wont to appear stain'd of a fine blew. and to satisfie my selfe, that the diversity came not from the paper, which one might suspect capable of inbibing the liquor, and altering the colour, i made the tryal upon a flat piece of purely white glass'd earth, (which i sometimes make use of about experiments of colours) with an event not unlike the former. and now i speak of _litmass_, i will add, that having this very day taken a piece of it, that i had kept by me these several years, to make tryals about colours, and having let fall a few drops of the strong infusion of it in fair water, into a fine crystal glass, shap'd like an inverted cone, and almost full of fair water, i had now (as formerly) the pleasure to see, and to show others, how these few tincted drops variously dispersing themselves through the limpid water, exhibited divers colours, or varieties of purple and crimson. and when the corpuscles of the pigment seem'd to have equally diffus'd themselves through the whole liquor, i then by putting two or three drops of spirit of salt, first made an odd change in the colour of the liquor, as well as a visible commotion among its small parts, and in a short time chang'd it wholly into a very glorious yellow, like that of a topaz. after which if i let fall a few drops of the strong and heavy solution of pot-ashes, whose weight would quickly carry it to the sharp bottome of the glass, there would soon appear four very pleasant and distinct colours; namely, a bright, but dilute colour at the picked bottome of the glass; a purple, a little higher; a deep and glorious crimson, (which crimson seem'd to terminate the operation of the salt upward) in the confines betwixt the purple and the yellow; and an excellent yellow, the same that before enobled the whole liquor, reaching from thence to the top of the glass. and if i pleas'd to pour very gently a little spirit of sal armoniack, upon the upper part of this yellow, there would also be a purple or a crimson, or both, generated there, so that the unalter'd part of the yellow liquor appear'd intercepted betwixt the two neighbouring colours. my scope in this d. experiment (_pyrophilus_) is manifold, as first to invite you to be wary in judging of the colour of liquors in such glasses as are therein recommended to you, and consequently as much, if not more, when you imploy other glasses. secondly, that you may not think it strange, that i often content my self to rub upon a piece of white paper, the juice of bodies i would examine, since not onely i could not easily procure a sufficient quantity of the juices of divers of them; but in several cases the tryals of the quantities of such juices in glasses would make us more lyable to mistakes, than the way that in those cases i have made use of. thirdly, i hope you will by these and divers other particulars deliver'd in this treatise, be easily induc'd to think that i may have set down many phænomena very faithfully, and just as they appear'd to me, and yet by reason of some unheeded circumstance in the conditions of the matter, and in the degree of light, or the manner of trying the experiment, you may find some things to vary from the relations i make of them. lastly, i design'd to give you an opportunity to free your self from the amazement which possesses most men, at the tricks of those mountebancks that are commonly call'd water-drinkers. for though not only the vulgar, but ev'n many persons that are far above that rank, have so much admir'd to see, a man after having drunk a great deal of fair water, to spurt it out again in the form of claret wine, sack, and milk, that they have suspected the intervening of magick, or some forbidden means to effect what they conceived above the power of art; yet having once by chance had occasion to oblige a wanderer that made profession of that and other jugling tricks, i was easily confirm'd by his ingenious confession to me, that this so much admir'd art, indeed consisted rather in a few tricks, than in any great skill, in altering the nature and colours of things. and i am easy to be perswaded; that there may be a great deal of truth in a little pamphlet printed divers years ago in english, wherein the author undertakes to discover, and that (if i mistake not) by the confession of some of the complices themselves, that a famous water-drinker then much admir'd in _england_, perform'd his pretended transmutations of liquors by the help of two or three inconsiderable preparations and mixtures of not unobvious liquors, and chiefly of an infusion of brazil variously diluted and made pale or yellowish, (and otherwise alter'd) with vinegar, the rest of their work being perform'd by the shape of the glasses, by craft and legerdemane. and for my part, that which i marvel at in this business, is, the drinkers being able to take down so much water, and spout it out with that violence; though custome and a vomit seasonably taken before hand, may in some of them much facilitate the work. but as for the changes made in the liquors, they were but few and slight in comparison of those, that the being conversant in chymical experiments, and dextrous in applying them to the transmuting of colours, may easily enough enable a man to make, as ev'n what has been newly deliver'd in this, and the foregoing experiment; especially if we add to it the things contained in the xx, the xxxix and the xl. experiments, may perhaps have already perswaded you. _experiment xlv._ you may i presume (_pyrophilus_) have taken notice, that in this whole treatise, i purposely decline (as far as i well can) the mentioning of elaborate chymical experiments, for fear of frighting you by their tediousness and difficulty; but yet in confirmation of what i have been newly telling you about the possibility of varying the colours of liquors, better than the water-drinkers are wont to do, i shall add, that _helmont_ used to make a preparation of steel, which a very ingenious chymist, his sons friend, whom you know, sometimes employes for a succedaneum to the spaw-waters, by diluting this _essentia martis liquida_ (as he calls it) with a due proportion of water. now that for which i mention to you this preparation, (which as he communicated to me, i know he will not refuse to _pyrophilus_) is this, that though the liquor (as i can shew you when you please) be almost of the colour of a german (not an oriental) amethyst, and consequently remote enough from green, yet a very few drops being let fall into a large proportion of good rhenish, or (in want of that) white wine (which yet do's not quite so well) immediately turn'd the liquor into a lovely green, as i have not without delight shown several curious persons. by which _phænomenon_ you may learn, among other things, how requisite it is in experiments about the changes of colours heedfully to mind the circumstances of them; for water will not, as i have purposely try'd, concurr to the production of any such green, nor did it give that colour to moderate spirit of wine, wherein i purposely dissolv'd it, and wine it self is a liquor that few would suspect of being able to work suddenly any such change in a metalline preparation of this nature; and to satisfie my self that this new colour proceeds rather from the peculiar texture of the wine, than from any greater acidity, that rhenish or white-wine (for that may not absurdly be suspected) has in comparison of water; i purposely sharpen'd the solution of this essence in fair water, with a good quantity of spirit of salt, notwithstanding which, the mixture acquir'd no greenness. and to vary the experiment a little, i try'd, that if into a glass of rhenish wine made green by this essence, i dropp'd an alcalizate solution, or urinous spirit, the wine would presently grow turbid, and of an odd dirty colour; but if instead of dissolving the essence in wine, i dissolv'd it in fair water sharpen'd perhaps with a little spirit of salt, then either the urinous spirit of sal armoniack, or the solution of the fix'd salt of pot-ashes would immediately turn it of a yellowish colour, the fix'd or urinous salt precipitating the vitriolate substance contain'd in the essence. but here i must not forget to take notice of a circumstance that deserves to be compar'd with some part of the foregoing experiment, for whereas our essence imparts a greenness to wine, but not to water, the industrious _olaus wormius_[ ] in his late _musæum_ tells us of a rare kind of turn-sole which he calls _bezetta rubra_ given him by an apothecary that knew not how it was made, whose lovely redness would be easily communicated to water, if it were immers'd in it; but scarce to wine, and not at all to spirit of wine, in which last circumstance it agrees with what i lately told you of our essence, notwithstanding their disagreement in other particulars. [ ] libr. do cap. . _experiment xlvi._ we have often taken notice, as of a remarkable thing, that metalls as they appear to the eye, before they come to be farther alter'd by other bodyes, do exhibit colours very different from those which the fire and the _menstruum_, either apart, or both together, do produce in them; especially considering that these metalline bodyes are after all these disguises reducible not only to their former metalline consistence and other more radical properties, but to their colour too, as if nature had given divers metalls to each of them a double colour, an _external_, and an _internal_; but though upon a more attentive consideration of this difference of colours, it seem'd probable to me, that divers (for i say not all) of those colours which we have just now call'd _internal_, are rather produc'd by the coalition of metalline particles with those of the salts, or other bodyes employ'd to work on them, than by the bare alteration of the parts of the metalls themselves: and though therefore we may call the obvious colours, natural or common, & the others adventitious, yet because such changes of colours, from whatsoever cause they be resolv'd to proceed may be properly enough taken in to illustrate our present subject, we shall not scruple to take notice of some of them, especially because there are among them such as are produc'd without the intervention of saline _menstruums_. of the adventitious colours of metalline bodies the chief sorts seem to be these three. the first, such colours as are produc'd without other additaments by the action of the fire upon metalls. the next such as emerge from the coalition of metalline particles with those of some _menstruum_ imploy'd to corrode a metall or precipitate it; and the last, the colours afforded by metalline bodyes either colliquated with, or otherwise penetrating into, other bodies, especially fusible ones. but these (_pyrophilus,_) are only as i told you, the _chief_ sorts of the adventitious colours of metalls, for there may others belong to them, of which i shall hereafter have occasion to take notice of some, and of which also there possibly may be others that i never took notice of. and to begin with the first sort of colours, 'tis well enough known to chymists, that tin being calcin'd by fire alone is wont to afford a white _calx_, and lead calcin'd by fire alone affords that most common red-powder we call _minium:_ copper also calcin'd _per se_, by a long or violent fire, is wont to yield (as far as i have had occasion to take notice of it) a very dark or blackish powder; that iron likewise may by the action of reverberated flames be turn'd into a colour almost like that of saffron, may be easily deduc'd from the preparation of that powder, which by reason of its colour and of the metall 'tis made of is by chymists call'd, _crocus martis per se_. and that _mercury_ made by the stress of fire, may be turn'd into a red powder, which chymists call precipitate _per se_, i elsewhere more particularly declare. _annotation i._ it is not unworthy the admonishing you, (_pyrophilus_,) and it agrees very well with our conjectures about the dependence of the change of a body's colour upon that of its texture, that the same metall may by the successive operation of the fire receive divers adventitious colours, as is evident in lead, which before it come to so deep a colour as that of _minium_, may pass through divers others. _annotation ii_. not only the _calces_, but the glasses of metalls, vitrify'd _per se_, may be of colours differing from the natural or obvious colour of the metall; as i have observ'd in the glass of lead, made by long exposing crude lead to a violent fire, and what i have observ'd about the glass or slagg of copper, (of which i can show you some of an odd kind of texture,) may be elsewhere more conveniently related. i have likewise seen a piece of very dark glass, which an ingenious artificer that show'd it me profess'd himself to have made of silver alone by an extreme _violence_ (which seems to be no more than is needfull) of the fire. _annotation iii_. minerals also by the action of the fire may be brought to afford colours very differing from their own, as i not long since noted to you about the variously colour'd flowers of antimony, to which we may add the whitish grey-colour of its _calx_, and the yellow or reddish colour of the glass, where into that _calx_ may be flux'd. and i remember, that i elsewhere told you, that vitriol calcin'd with a very gentle heat, and afterwards with higher and higher degrees of it, may be made to pass through several colours before it descends to a dark purplish colour, whereto a strong fire is wont at length to reduce it. but to insist on the colours produc'd by the operation of fire upon several minerals would take up farr more time than i have now to spare. _experiment xlvii._ the adventitious colours produc'd upon metalls, or rather with them, by saline liquors, are many of them so well known to chymists, that i would not here mention them, but that besides a not un-needed testimony, i can add something of my own, to what i shall repeat about them, and divers experiments which are familiar to chymists, are as yet unknown to the greatest part of ingenious men. that gold dissolv'd in _aqua regia_ ennobles the _menstruum_ with its own colour, is a thing that you cannot (_pyrophilus_,) but have often seen. the solutions of mercury in _aqua-fortis_ are not generally taken notice of, to give any notable tincture to the _menstruum_; but sometimes when the liquor first falls upon the quick silver, i have observ'd a very remarkable, though not durable, greenness, or blewness to be produc'd, which is a _phænomenon_ not unfit for you to consider, though i have not now the leisure to discourse upon it. tin corroded by _aqua-fortis_ till the _menstruum_ will work no farther on it, becomes exceeding white, but as we elsewhere note, does very easily of it self acquire the consistence, not of a metalline _calx_, but of a coagulated matter, which we have observ'd with pleasure to look so like, either to curdled milk, or curdled whites of eggs, that a person unacquainted with such solutions may easily be mistaken in it. but when i purposely prepar'd a _menstruum_ that would dissolve it as _aqua-fortis_ dissolves silver, and not barely corrode it, and quickly let it fall again, i remember not that i took notice of any particular colour in the solution, as if the more whitish metalls did not much tinge their _menstruums_, though the conspicuously colour'd metalls as gold, and copper, do. for lead dissolv'd in spirit of vinegar or _aqua-fortis_ gives a solution cleer enough, and if the _menstruum_ be abstracted appears either diaphanous or white. of the colour of iron we have elsewhere said something: and 'tis worth noting, that though if that metall be dissolv'd in oyl of vitriol diluted with water, it affords a salt or magistery so like in colour, as well as some other qualities, to other green vitriol, that chymists do not improperly call it _vitriolum martis_; yet i have purposely try'd, that, by changing the _menstruum_, and pouring upon the filings of steel, instead of oyl of vitriol, _aqua fortis_, (whereof as i remember, i us'd parts to one of the metall) i obtain'd not a green, but a saffron colour solution; or rather a thick liquor of a deep but yellowish red. common silver, such as is to be met with in coines, being dissolv'd in _aqua fortis_, yields a solution tincted like that of copper, which is not to be wondred at, because in the coining of silver, they are wont (as we elsewhere particularly inform you) to give it an allay of copper, and that which is sold in shops for refined silver, is not (so far as we have tryed) so perfectly free from that ignobler metall, but that a solution of it in _aqua fortis_, will give a venereal tincture to the _menstruum_. but we could not observe upon the solution of some silver, which was perfectly refin'd, (such as some that we have, from which or times its weight of lead has been blown off) that the _menstruum_ though held against the light in a crystal vial did manifestly disclose any tincture, only it seem'd sometimes not to be quite destitute of a little, but very faint blewishness. but here i must take notice, that of all the metalls, there is not any which doth so easily and constantly disclose its unobvious colour as copper doth. for not only in acid _menstruums_ as _aqua fortis_ and spirit of vinegar, it gives a blewish green solution, but if it be almost any way corroded, it _appears of one of those_ two colours, as may be observ'd in verdigreese made several wayes, in that odd preparation of _venus_, which we elsewhere teach you to make with sublimate, and in the common vitriols of _venus_ deliver'd by chymists; and so constant is the disposition of copper, notwithstanding the disguise artists put upon it, to disclose the colour we have been mentioning, that we have by forcing it up with _sal armoniack_ obtain'd a sublimate of a blewish colour. nay a famous spagyrist affirms, that the very mercury of it is green, but till he teach us an intelligible way of making such a mercury, we must content ourselves to inform you, that we have had a cupreous body, that was præcipitated out of a distill'd liquor, that seem'd to be the the sulphur of _venus_, and seem'd even when flaming, of a greenish colour. and indeed copper is a metall so easily wrought upon by liquors of several kinds, that i should tell you, i know not any mineral, that will concurr to the production of such a variety of colours as copper dissol'd in several _menstruums_, as spirit of vinegar, _aqua fortis_, _aqua regis_, spirit of nitre, of urine, of soot, oyls of several kinds, and i know not how many other liquors, if the variety of somewhat differing colours (that copper will be made to assume, as it is wrought upon by several liquors) were not comprehended within the limits of greenish blew, or blewish green. and yet i must advertise you (_pyrophilus_) that being desirous to try if i could not make with crude copper a green solution without the blewishness that is wont to accompany its vulgar solutions, i bethought my self of using two _menstruums_, which i had not known imploy'd to work on this metall, and which i had certain reasons to make tryal of, as i successfully did. the one of these liquors (if i much misremember not) was spirit of sugar distill'd in a retort, which must be warily done, (if you will avoid breaking your glasses) and the other, oyl or spirit of turpentine, which affords a fine green solution that is useful to me on several occasions. and yet to shew that the adventitious colour may result, as well from the true and permanent copper it self, as the salts wherewith 'tis corroded, i shall add, that if you take a piece of good _dantzick_ copperis, or any other vitriol wherein _venus_ is prædominant, and having moistened it in your mouth, or with fair water, rubb it upon a whetted knife, or any other bright piece of steel or iron, it will (as we have formerly told you) present'y stain the steel with a reddish colour, like that of copper, the reason of which, we must not now stay to inquire. _annotation i._ i presume you may have taken notice (_pyrophilus_) that i have borrowed some of the instances mention'd in this th experiment, from the laboratories of chymists, and because in some (though very few) other passages of this essay, i have likewise made use of experiments mention'd also by some spagyrical writers, i think it not amiss to represent to you on this occasion once for all, some things besides those which i intimated in the præamble of this present experiment; for besides, that 'tis very allowable for a writer to repeat an experiment which he invented not, in case he improve it; and besides that many experiments familiar to chymists are unknown to the generality of learned men, who either never read chymical processes, or never understood their meaning, or never durst believe them; besides these things, i say, i shall represent, that, as to the few experiments i have borrowed from the chymists, if they be very vulgar, 'twould perhaps be difficult to ascribe each of them its own author, and 'tis more than the generality of chymists themselves can do: and if they be not of very known and familiar practise among them, unless the authors wherein i found them had given me cause to believe, themselves had try'd them, i know not why i might not set them down, as a part of the _phænomena_ of colours which i present you; many things unanimously enough deliver'd as matters of fact by (i know not how many chymical writers) being not to be rely'd on, upon the single authority of such authors: for instance, as some spagyrists deliver (perhaps amongst several deceitful processes) that _saccarum saturni_ with spirit of turpentine will afford a balsom, so _beguinus_ and many more tell us, that the same concrete (_saccarum saturni_) will yield an incomparably fragrant spirit, and a pretty quantity of two several oyles, and yet since many have complain'd, as well as i have done, that they could find no such odoriferous, but rather an ill-sented liquor, and scarce any oyl in their distillation of that sweet vitriol, a wary person would as little build any thing on what they say of the former experiment, as upon what they averr of the later, and therefore i scrupled not to mention this red balsom of which i have not seen any, (but what i made) among my other experiments about redness. _annot. ii._ we have sometimes had the curiosity to try what colours minerals, as tinglass, antimony, spelter, &c. would yield in several _menstruums_, nor have we forborn to try the colours of stones, of which that famous one, (which _helmont_ calls _paracelsus's ludus_) though it be digg'd out of the earth and seem a true stone, has afforded in _menstruums_ capable to dissolve so solid a stone, sometimes a yellowish, sometimes a red solution of both which i can show you. but though i have from minerals obtain'd with several _menstruums_ very differing colours, and some such as perhaps you would be surpriz'd to see drawn from such bodies: yet i must now pass by the particulars, being desirous to put an end to this treatise, before i put an end to your patience and my own. _annotation iii._ and yet before i pass to the next experiment, i must put you in mind, that the colours of metals may in many cases be further alter'd by imploying, either præcipitating salts, or other convenient substances to act upon their solutions. of this you may remember, that i have given you several instances already, to which may be added such as these, that if quicksilver be dissolv'd in _aqua fortis_, and præcipitated out of the solution, either with water impregnated with sea salt, or with the spirit of that concrete, it falls to the bottom in the form of a white powder, whereas if it be præcipitated with an alcaly, it will afford a yellowish or tawny powder, and if there be no præcipitation made, and the _menstruum_ be drawn off with a convenient fire, the corroded mercury will remain in the bottom, in the form of a substance that may be made to appear of differing colours by differing degrees of heat; as i remember that lately having purposely abstracted _aqua fortis_ from some quicksilver that we had dissolv'd in it, so that there remain'd a white _calx_, exposing that to several degrees of fire, and afterwards to a naked one, we obtain'd some new colours, and at length the greatest part of the _calx_ lying at the bottome of the vial, and being brought partly to a deep yellow, and partly to a red colour, the rest appear'd elevated to the upper part and neck of the vial, some in the form of a reddish, and some of an ash-colour sublimate. but of the differing colours which by differing wayes and working of quick silver with fire, and saline bodies, may be produc'd in precipitates, i may elsewhere have occasion to take further notice. i also told you not long since, that if you corrode quick-silver with oyl of vitriol instead of _aqua-fortis_, and abstract the _menstruum_, there will remain a white _calx_ which by the affusion of fair water presently turns into a lemmon colour. and ev'n the _succedaneum_ to a _menstruum_ may sometimes serve the turn to change the colours of a metal. the lovely red which painters call vermillion, is made of mercury, which is of the colour of silver, and of brimstone which is of kin to that of gold, sublim'd up together in a certain proportion, as is vulgarly known to spagyrists. _experiment xlviii._ the third chief sort of the adventitious colours of metals, is, that which is produc'd by associating them (especially when calcin'd) with other fusible bodies, and principally venice, and other fine glass devoid of colour. i have formerly given you an example, whereby it may appear, that a metal may impart to glass a colour much differing from its own, when i told you, how with silver, i had given glass a lovely golden colour. and i shall now add, that i have learn'd from one of the chief artificers that sells painted glass, that those of his trade colour it yellow with a preparation of the _calx_ of silver. though having lately had occasion among other tryals to mingle a few grains of shell-silver (such as is imploy'd with the pensil and pen) with a convenient proportion of powder'd crystal glass, having kept them two or three hours in fusion, i was surpriz'd to find the colliquated mass to appear upon breaking the crucible of a lovely saphirine blew, which made me suspect my servant might have brought me a wrong crucible, but he constantly affirm'd it to be the same wherein the silver was put, and considerable circumstances countenanc'd his assertion, so that till i have opportunity to make farther tryal, i cannot but suspect, either that silver which is not (which is not very probable) brought to a perfect fusion and colliquation with glass, may impart to it other colours than when neal'd upon it, or else (which is less unlikely) that though silver beaters usually chuse the finest coyn they can get, as that which is most extensive under the hammer, yet the silver-leaves of which this shel-silver was made, might retain so much copper as to enable it to give the predominant tincture to the glass. for, i must proceed to tell you (_pyrophilus_) as another instance of the adventitious colours of metals, that which is something strange, namely, that though copper calcin'd _per se_ affords but a dark and basely colour'd _calx_, yet the glassmen do with it, as themselves inform me, tinge their glass green. and i remember, that when once we took some crude copper, and by frequent ignition quenching it in water had reduc'd it to a dark and ill-colour'd powder, and afterward kept it in fusion in about a . times its weight of fine glass, we had, though not a green, yet a blew colour'd mass, which would perhaps have been green, if we had hit right upon the proportion of the materials, and the degree of fire, and the time wherein it ought to be kept in fusion, so plentifully does that metal abound in a venerial tincture, as artists call it, and in so many wayes does it disclose that richness. but though copper do as we have said give somewhat near the like colour to glass, which it does to _aqua-fortis_, yet it seems worth inquiry, whether those new colours which mineral bodies disclose in melted glass, proceed from the coalition of the corpuscles of the mineral with the particles of the glass as such, or from the action (excited or actuated by fire) of the alcalizate salt (which is a main ingredient of glass,) upon the mineral body, or from the concurrence of both these causes, or else from any other. but to return to that which we were saying, we may observe that _putty_ made by calcining together a proportion of tin and lead, as it is it self a white _calx_, so does it turn the _pitta di crystallo_ (as the glassmen call the matter of the purer sort of glass, wherewith it is colliquated into a white mass, which if it be opacous enough is employ'd, as we elsewhere declare, for white amel. but of the colours which the other metals may be made to produce in colourless glass, and other vitrifiable bodies, that have native colours of their own, i must leave you to inform your self upon tryal, or at least must forbear to do it till another time, considering how many annotations are to follow, upon what has in this and the two former experiments been said already. _annotation i._ when the materials of glass being melted with calcin'd tin, have compos'd a mass undiaphanous and white, this white amel is as it were the basis of all those fine concretes that goldsmiths and several artificers imploy in the curious art of enamelling. for this white and fusible substance will receive into it self, without spoyling them, the colours of divers other mineral substances, which like it will indure the fire. _annotation ii._ so that as by the present (xlviii.) experiment it appears, that divers minerals will impart to fusible masses, colours differing from their own; so by the making and compounding of amels, it may appear, that divers bodies will both retain their colour in the fire, and impart the _same_ to some others wherewith they were vitrifi'd, and in such tryals as that mention'd in the . experiment, where i told you, that ev'n in amels a blew and yellow will compound a green. 'tis pretty to behold, not only that some colours are of so fix'd a nature, as to be capable of mixture without receiving any detriment by the fire, that do's so easily destroy or spoyl those of other bodies; but mineral pigments may be mingled by fire little less regularly and successfully, than in ordinary dyeing fatts, the vulgar colours are wont to be mingled by the help of water. _annotation iii._ 'tis not only metalline, but other mineral bodies, that may be imploy'd, to give tinctures unto glass (and 'tis worth noting how small a quantity of some mineral substances, will tinge a comparatively vast proportion of glass, and we have sometimes attempted to colour glass, ev'n with pretious stones, and had cause to think the experiment not cast away. and 'tis known by them that have look'd into the art of glass, that the artificers use to tinge their glass blew, with that dark mineral _zaffora_, (some of my tryals on which i elsewhere acquaint you) which some would have to be a mineral earth, others a stone, and others neither the one, nor the other, but which is confessedly of a dark, but not a blew colour, though it be not agreed of what particular colour it is. 'tis likewise though a familiar yet a remarkable practise among those that deal in the making of glass, to imploy (as some of themselves have inform'd me) what they call manganess, and some authors call _magnesia_ (of which i make particular mention in another treatise) to exhibit in glass not only other colours than its own, (which is so like in darkness or blackishness to the load stone, that 'tis given by mineralists, for one of the reasons of its latine name) but colours differing from one another. for though they use it, (which is somewhat strange) to clarifye their glass, and free it from that blewish greenish colour, which else it would too often be subject to, yet they also imploy it in certain proportions, to tinge their glass both with a red colour, and with a purplish or murry, and putting in a greater quantity, they also make with it that deep obscure glass which is wont to pass for black, which agrees very well with, and may serve to confirm what we noted near the beginning of the th experiment, of the seeming blackness of those bodies that are overcharg'd with the corpuscles of such colours, as red, or blew, or green, &c. and as by several metals and other minerals we can give various colours to glass, so on the other side, by the differing colours that mineral oars, or other mineral powders being melted with glass disclose in it, a good conjecture may be oftentimes made of the metall or known mineral, that the oar propos'd, either holds, or is most of kin to. and this easie way of examining oars, may be in some cases of good use, and is not ill deliver'd by _glauber_, to whom i shall at present refer you, for a more particular account of it: unless your curiosity command also what i have observ'd about these matters; only i must here advertise you, that great circumspection is requisite to keep this way from proving fallacious, upon the account of the variations of colour that may be produc'd by the differing proportions that may be us'd betwixt the oar and the glass, by the richness or poorness of the oar it self, by the degree of fire, and (especially) by the length of time, during which the matter is kept in fusion; as you will easily gather from what you will quickly meet with in the following annotation upon this present th experiment. _annotation iv._ there is another way and differing enough from those already mention'd, by which metalls may be brought to exhibit adventitious colours: for by this, the metall do's not so much impart a colour to another body, as receive a colour from it, or rather both bodies do by the new texture resulting from their mistion produce a new colour. i will not insist to this purpose upon the examples afforded us by yellow orpiment, and common sea salt, from which, sublim'd together, chymists unanimously affirm their white or crystalline arsenick to be made: but 'tis not unworthy our noting, that though yellow orpiment be acknowledg'd to be the copiousest by far of the two ingredients of arsenick, yet this last nam'd body being duely added to the highest colour'd metall copper, when 'tis in fusion, gives it a whiteness both within and without. thus _lapis calaminaris_ changes and improves the colour of copper by turning it into brass. and i have sometimes by the help of zinck duely mix'd after a certain manner, given copper one of the richest golden colours that ever i have seen the best true gold ennobled with. but pray have a care that such hints fall not into any hands that may mis-imploy them. _annotation v._ upon the knowledge of the differing wayes of making minerals and metalls produce their adventitious colours in bodies capable of vitrification, depends the pretty art of making what chymists by a barbarous word are pleas'd to call _amanses_, that is counterfeit, or factitious gemms, as emeralds, rubies, saphires, topazes, and the like. for in the making of these, though pure sand or calcin'd crystal give the body, yet 'tis for the most part some metalline or mineral _calx_, mingled in a small proportion that gives the colour. but though i have many years since taken delight, to divert my self with this pleasing art, and have seen very pretty productions of it, yet besides that i fear i have now forgot most of the little skill i had in it, this is no place to entertain you with what would rather take up an intire discourse, than be comprehended in an annotation; wherefore the few things which i shall here take notice of to you, are only what belong to the present argument, namely, first, that i have often observ'd that calcin'd lead colliquated with fine white sand or crystal, reduc'd by ignitions and subsequent extinctions in water to a subtile powder, will of it self be brought by a due decoction to give a cleer mass colour'd like a _german_ amethyst. for though this glass of lead, is look'd upon by them that know no better way of making _amanses_, as the grand work of them all, yet which is an inconvenience that much blemishes this way, the calcin'd lead it self does not only afford matter to the _amanses_, but has also as well as other metals a colour of its own, which as i was saying, i have often found to be like that of _german_ (as many call them) not eastern amethysts. secondly, that nevertheless this colour may be easily over-powr'd by those of divers other mineral pigments (if i may so call them) so that with a glass of lead, you may emulate (for instance) the fresh and lovely greenness of an emerald, though in divers cases the colour which the lead it self upon vitrification tends to, may vitiate that of the pigment, which you would introduce into the mass. thirdly, that so much ev'n these colours depend upon texture, that in the glass of lead it self made of about three parts of _lytharge_ or _minium_ colliquated with one of very finely powder'd crystal or sand, we have taken pleasure to make the mixture pass through differing colours, as we kept it more or less in the fusion. for it was not usually till after a pretty long decoction that the mass attain'd to the amethystin colour. fourthly and lastly, that the degrees of coction and other circumstances may so vary the colour produc'd in the same mass, that in a crucible that was not great i have had fragments of the same mass, in some of which perhaps not so big as a hazel-nut, you may discern four distinct colours. _annotation vi._ you may remember (_pyrophilus_) that when i mention'd the three sorts of adventitious colours of metals, i mention'd them but as the chief, not the only. for there may be other wayes, which though they do not in so strict a sense belong to the adventitious colours of metals, may not inconveniently be reduc'd to them. and of these i shall name now a couple, without denying that there may be more. the first may be drawn from the practise of those that dye scarlet. for the famousest master in that art, either in _england_ or _holland_, has confess'd to me, that neither others, nor he can strike that lovely colour which is now wont to be call'd the _bow-dye_, without their materials be boyl'd in vessels, either made of, or lin'd with a particular metall. but of what i have known attempted in this kind, i must not as yet for fear of prejudicing or displeasing others give you any particular account.[ ] the other way (_pyrophilus_) of making metals afford unobvious colours, is by imbuing divers bodies with solutions of them made in their proper _menstruum's_, as (for instance) though copper plentifully dissolv'd in _aqua fortis_, will imbue several bodies with the colour of the solution; yet some other metalls will not (as i elsewhere tell you) and have often try'd. gold dissolv'd in _aqua regia_, will, (which is not commonly known) dye the nails and skin, and hafts of knives, and other things made of ivory, not with a golden, but a purple colour, which though it manifest it self but slowly, is very durable, and scarce ever to be wash'd out. and if i misremember not, i have already told you in this treatise, that the purer crystals of fine silver made with _aqua fortis_, though they appear white, will presently dye the skin and nails, with a black, or at least a very dark colour, which water will not wash off, as it will ordinary ink from the same parts. and divers other bodies may the same way be dy'd, some of a black, and others of a blackish colour. [ ] see the latter end of the fiftieth experiment. and as metalline, so likewise mineral solutions may produce colours differing enough from those of the liquors themselves. i shall not fetch an example of this, from what we daily see happen in the powdring of beef, which by the brine imploy'd about it (especially if the flesh be over salted) do's oftentimes appear at our tables of a green, and sometimes of a reddish colour, (deep enough) nor shall i insist on the practise of some that deal in salt petre, who, (as i suspected, and as themselves acknowledg'd to me) do, with the mixture of a certain proportion of that; and common salt, give a fine redness, not only to neats tongues, but which is more pretty as well as difficult, to such flesh, as would otherwise be purely white; these examples, i say, i shall decline insisting on, as chusing rather to tell you, that i have several times try'd, that a solution of the sulphur of vitriol, or ev'n of common sulphur, though the liquor appear'd clear enough, would immediately tinge a piece of new coin, or other clean silver, sometimes with a golden, sometimes with a deeper, and more reddish colour, according to the strength of the solution, and the quantity of it, that chanc'd to adhere to the metall; which may take off your wonder that the water of the hot spring at _bath_, abounding with dissolv'd substances of a very sulphureous nature, should for a while, as it were gild, the new or clean pieces of silver coyn, that are for a due time immers'd in it. and to these may be added those formerly mention'd examples of the adventitious colours of mineral bodies; which brings into my mind, that, ev'n vegetable liquors, whether by degeneration, or by altering the texture of the body that imbibes them, may stain other bodies with colours differing enough, from their own, of which very good herbarists have afforded us a notable example, by affirming that the juice of _alcanna_ being green (in which state i could never here procure it) do's yet dye the skin and nails of a lasting red. but i see this treatise is like to prove too bulky without the addition of further instances of this nature. _experiment xlix._ meeting the other day, _pyrophilus_, in an _italian_ book, that treats of other matters, with a way of preparing what the author calls a _lacca_ of vegetables, by which the _italians_ mean a kind of extract fit for painting, like that rich _lacca_ in english commonly call'd _lake_, which is imploy'd by painters as a glorious red. and finding the experiment not to be inconsiderable, and very defectively set down, it will not be amiss to acquaint you with what some tryals have inform'd us, in reference to this experiment, which both by our italian author, and by divers of his countrymen, is look'd upon as no trifling secret. take then the root call'd in latin _curcuma_, and in english turmerick, (which i made use of, because it was then at hand, and is among vegetables fit for that purpose one of the most easiest to be had) and when it is beaten, put what quantity of it you please into fair water, adding to every pound of water about a spoonfull or better of as strong a _lixivium_ or solution of potashes as you can well make, clarifying it by filtration before you put it to the decocting water. let these things boyl, or rather simper over a soft fire in a clean glaz'd earthen vessel, till you find by the immersion of a sheet of white paper (or by some other way of tryal) that the liquor is sufficiently impregnated with the golden tincture of the turmerick, then take the decoction off the fire, and filter or strain it that it may be clean, and leisurely dropping into it a strong solution of roch allum, you shall find the decoction as it were curdl'd, and the tincted part of it either to emerge, to subside, or to swim up and down, like little yellow flakes; and if you pour this mixture into a tunnel lin'd with cap paper, the liquor that filtred formerly so yellow, will now pass clean thorow the filtre, leaving its tincted, and as it were curdled parts in the filtre, upon which fair water must be so often pour'd, till you have dulcifi'd the matter therein contain'd, the sign of which dulcification is (you know) when the water that has pass'd through it, comes from it as tasteless as it was pour'd on it. and if without filtration you would gather together the flakes of this vegetable lake, you must pour a great quantity of fair water upon the decoction after the affusion of the alluminous solution, and you shall find the liquor to grow clearer, and the lake to settle together at the bottom, or emerge to the top of the water, though sometimes having not pour'd out a sufficient quantity of fair water, we have observ'd the lake partly to subside, and partly to emerge, leaving all the middle of the liquor clear. but to make this lake fit for use, it must by repeated affusions of fresh water, be dulcifi'd from the adhering salts, as well as that separated by filtration, and be spread and suffer'd to dry leisurely upon pieces of cloth, with brown paper, or chalk, or bricks under them to imbibe the moisture[ ]. [page ] _annotation i._ whereas it is presum'd that the magistery of vegetables obtain'd this way consists but of the more soluble and coloured parts of the plants that afford it, i must take the liberty to question the supposition. and for my so doing, i shall give you this account. according to the notions (such as they were) that i had concerning salts; allom, though to sense a homogeneous body, ought not to be reckon'd among true salts, but to be it self look'd upon as a kind of magistery, in regard that as native vitriol (for such i have had) contains both a saline substance and a metall, whether copper, or iron, corroded by it, and associated with it; so allom which may be of so near a kin to vitriol, that in some places of _england_ (as we are assur'd by good authority the same stone will sometimes afford both) seems manifestly to contain a peculiar kind of acid spirit, generated in the bowels of the earth, and some kind of stony matter dissolv'd by it. and though in making our ordinary allom, the workmen use the ashes of a sea weed (vulgarly call'd kelp) and urine: yet those that should know, inform us, that, here in _england_, there is besides the factitious allom, allom made by nature without the help of those additaments. now (_pyrophilus_) when i consider'd this composition of allom, and that alcalizate salts are wont to præcipitate what acid salts have dissolv'd, i could not but be prone to suspect that the curdled matter, which is call'd the magistery of vegetables, may have in it no inconsiderable proportion of a stony substance præcipitated out of the allom by the _lixivium_, wherein the vegetable had been decocted, and to shew you, that there is no necessity, that all the curdl'd substance must belong to the vegetable, i shall add, that i took a strong solution of allom, and having filtred it, by pouring in a convenient quantity of a strong solution of potashes, i presently, as i expected, turn'd the mixture into a kind of white curds, which being put to filtre, the paper retain'd a stony _calx_, copious enough, very white, and which seem'd to be of a mineral nature, both by some other signes, and this, that little bits of it being put upon a live coal, which was gently blown whilst they were on it, they did neither melt nor fly away, and you may keep a quantity of this white substance for a good while, (nay for ought i can guess for a very long one) in a red hot crucible without losing or spoiling it; nor did hot water wherein i purposely kept another parcel of such _calx_, seem to do any more than wash away the looser adhering salts from the stony substance, which therefore seem'd unlikely to be separable by ablutions (though reiterated) from the præcipitated parts of the vegetable, whose lake is intended. and to shew you, that there is likewise in allom a body, with which the fix'd salt of the alcalizate solution will concoagulate into a saline substance differing from either of them, i shall add, that i have taken pleasure to recover out of the slowly exhal'd liquor, that pass'd through the filtre, and left the foremention'd _calx_ behind, a body that at least seem'd a salt very pretty to look on, as being very white, and consisting of an innumerable company of exceeding slender, and shining particles, which would in part easily melt at the flame of a candle, and in part flye away with some little noise. but of this substance, and its odd qualities more perhaps elsewhere; for now i shall only take notice to you, that i have likewise with urinous salts, such as the spirit of sal armoniack, as well as with the spirit of urine it self, nay, (if i much mistake not) ev'n with stale urine undistil'd, easily precipitated such a white _calx_ as i was formerly speaking of, out of a limpid solution of allom, so that there is need of circumspection in judging of the natures of liquors by precipitations wherein allom intervenes, else we may sometimes mistakingly imagine that to be precipitated out of a liquor by allom, which is rather precipitated out of allom by the liquor: and this puts me in mind to tell you, that 'tis not unpleasant to behold how quickly the solution of allom (or injected lumps of allom) do's occasion the severing of the colour'd parts of the decoction from the liquor that seem'd to have so perfectly imbib'd them. [ ] _the curious reader that desires further information concerning lakes, may resort to the th book of_ neri's _art of glass, englished ( or years since the writing of this th experiment) and illustrated with learned observations, by the inquisitive and experienc'd dr._ charles merret. _annot. ii._ the above mention'd way of making lakes we have tryed not only with turmerick, but also with madder, which yielded us a red lake; and with rue, which afforded us an extract, of (almost if not altogether) the same colour with that of the leaves. but in regard that 'tis principally the alcalizate salt of the pot-ashes, which enables the water to extract so powerfully the tincture of the decocted vegetables, i fear that our author may be mistaken by supposing that the decoction will alwayes be of the very same colour with the vegetable it is made off. for lixiviate salts, to which pot-ashes eminently belong, though by peircing and opening the bodies of vegetables, they prepare and dispose them to part readily with their tincture, yet some tinctures they do not only draw out, but likewise alter them, as may be easily made appear by many of the experiments already set down in this treatise, and though allom being of an acid nature, its solutions may in some cases destroy the adventitious colours produc'd by the alcaly, and restore the former: yet besides that allom is not, as i have lately shown, a meer acid salt, but a mixt body, and besides, that its operations are languid in comparison of the activity of salts freed by distillation, or by incineration and dissolution, from the most of their earthy parts, we have seen already examples, that in divers cases an acid salt will not restore a vegetable substance to the colour of which an alcalizate one had depriv'd it, but makes it assume a third very differing from both, as we formerly told you, that if syrrup of violets were by an alcaly turn'd green, (which colour, as i have try'd, may be the same way produc'd in the violet-leaves themselves without any relation to a syrrup) an acid salt would not make it blew again, but red. and though i have by this way of making lakes, made magisteries (for such they seem to be) of brazil, and as i remember of cochinele it self, and of other things, red, yellow or green which lakes were enobled with a rich colour, and others had no bad one; yet in some the colour of the lake seem'd rather inferiour than otherwise to that of the plant, and in others it seem'd both very differing, and much worse; but writing this in a time and place where i cannot provide my self of flowres and other vegetables to prosecute such tryals in a competent variety of subjects, i am content not to be positive in delivering a judgment of this way of lakes, till experience, or you, _pyrophilus_, shall have afforded me a fuller and more particular information. _annotation iii._ and on this occasion (_pyrophilus_) i must here (having forgot to do it sooner) advertise you once for all, that having written several of the foregoing experiments, not only in haste but at seasons of the year, and in places wherein i could not furnish my self with such instruments, and such a variety of materials, as the design of giving you an introduction into the history of colours requir'd, it can scarce be otherwise but that divers of the experiments, that i have set down, may afford you some matter of new tryals, if you think fit to supply the deficiencies of some of them (especially the freshly mention'd about lakes, and those that concern emphatical colours) which deficiencies for want of being befriended with accommodations i could better discern than avoid. _annotation iv._ the use of allom is very great as well as familiar in the dyers trade, and i have not been ill pleas'd with the use i have been able to make of it in preparing other pigments than those they imploy with vegetable juices. but the lucriferous practises of dyers and other tradesmen, i do, for reasons that you may know when you please, purposely forbear in this essay, though not strictly from pointing at, yet from making it a part of my present work explicitly and circumstantially to deliver, especially since i now find (though late and not without some blushes at my prolixity) that what i intended but for a short essay, is already swell'd into almost a volume. _experiment l._ yet here, _pyrophilus_, i must take leave to insert an experiment, though perhaps you'l think its coming in here an intrusion, for i confess its more proper place would have been among those experiments, that were brought as proofs and applications of our notions concerning the differences of salts; but not having remembred to insert it in its fittest place, i had rather take notice of it in this, than leave it quite unmention'd: partly because it doth somewhat differ from the rest of our experiments about colours, in the way whereby 'tis made; and partly because the grounds upon which i devis'd it, may hint to you somewhat of the method i use in designing and varying experiments about colours, and upon this account i shall inform you, not only what i did, but why i did it. i consider'd then that the work of the former experiments was either to change the colour of a body into another, or quite to destroy it, without giving it a successor, but i had a mind to give you also a way, whereby to turn a body endued with one colour into two bodies, of colours, as well as consistencies, very distinct from each other, and that by the help of a body that had it self no colour at all. in order to this, i remembred, that finding the acidity of spirit of vinegar to be wholly destroy'd by its working upon _minium_ (or calcin'd lead) whereby the saline particles of the _menstruum_ have their taste and nature quite alter'd, i had, among other conjectures i had built upon that change, rightly concluded, that the solution of lead in spirit of vinegar would alter the colour of the juices and infusions of several plants, much after the like manner that i had found oyl of tartar to do; and accordingly i was quickly satisfied upon tryal, that the infusion of rose-leaves would by a small quantity of this solution well mingl'd with it, be immediately turn'd into a somewhat sad green. and further, i had often found, that oyl of vitriol, though a potently acid _menstruum_, will yet præcipitate many bodies, both mineral and others, dissolv'd not onely in _aqua fortis_ (as some chymists have observ'd) but particularly in spirit of vinegar, and i have further found, that the _calces_ or powders præcipitated by this liquor were usually fair and white. laying these things together, 'twas not difficult to conclude, that if upon a good tincture of red rose-leaves made with fair water, i dropp'd a pretty quantity of a strong and sweet solution of _minium_, the liquor would be turn'd into the like muddy green substance, as i have formerly intimated to you, that oyl of tartar would reduce it to, and that if then i added a convenient quantity of good oyl of vitriol, this last nam'd liquor would have two distinct operations upon the mixture, the one, that it would præcipitate that resolv'd lead in the form of a white powder; the other, that it would clarifie the muddy mixture, and both restore, and exceedingly heighten the redness of the infusion of roses, which was the most copious ingredient of the green composition, and accordingly trying the experiment in a wine glass sharp at the bottom (like an inverted cone) that the subsiding powder might seem to take up the more room, and be the more conspicuous, i found that when i had shaken the green mixture, that the colour'd liquor might be the more equally dispersed, a few drops of the rectifi'd oyl of vitriol did presently turn the opacous liquor into one that was cleer and red, almost like a rubie, and threw down good store of a powder, which when 'twas settl'd, would have appear'd very white, if some interspers'd particles of the red liquor had not a little allay'd the purity, though not blemish'd the beauty of the colour. and to shew you, _pyrophilus_, that these effects do not flow from the oyl of vitriol, as it is such, but as it is a strongly acid _menstruum_, that has the property both to præcipitate lead, as well as some other concretes out of spirit of vinegar, and to heighten the colour of red rose-leaves, i add, that i have done the same thing, though perhaps not quite so well with spirit of salt, and that i could not do it with _aqua-fortis_, because though that potent _menstruum_ does as well as the others heighthen the redness of roses, yet it would not like them precipitate lead out of spirit of vinegar, but would rather have dissolv'd it, if it had not found it dissolv'd already. and as by this way we have produc'd a red liquor, and a white precipitate out of a dirty green magistery of rose-leaves, so by the same method, you may produce a fair yellow, and sometimes a red liquor, and the like precipitate, out of an infusion of a curious purple colour. for you may call to mind, that in the annotation upon the th. experiment i intimated to you, that i had with a few drops of an alcaly turn'd the infusion of logg-wood into a lovely purple. now if instead of this alcaly i substituted a very strong and well filtrated solution of _minium_, made with spirit of vinegar, and put about half as much of this liquor as there was of the infusion of logg-wood, (that the mixture might afford a pretty deal of precipitate,) the affusion of a convenient proportion of spirit of salt, would (if the liquors were well and nimbly stirr'd together) presently strike down a precipitate like that formerly mention'd, and turn the liquor that swam above it, for the most part into a lovely yellow. but for the advancing of this experiment a little further, i consider'd, that in case i first turn'd a spoonfull of the infusion of logg-wood purple, by a convenient proportion of the solution of _minium_, the affusion of spirit of sal armnoniack, would precipitate the corpuscles of lead conceal'd in the solution of _minium_, and yet not destroy the purple colour of the liquor; whereupon i thus proceeded; i took about a spoonfull of the _fresh_ tincture of logg-wood, (for i found that if it were _stale_ the experiment would not alwayes succeed,) and having put to it a convenient proportion of the solution of _minium_ to turn it into a deep and almost opacous purple, i then drop'd in as much spirit of sal armoniack, as i guess'd would precipitate about half or more (but not all) of the lead, and immediately stirring the mixture well together, i mingled the precipitated parts with the others, so that they fell to the bottom, partly in the form of a powder, and partly in the form of a curdled substance, that (by reason of the predominancy of the ting'd corpuscles over the white) retain'd as well as the supernatant liquor; a blewish purple colour sufficiently deep, and then instantly (but yet warily,) pouring on a pretty quantity of spirit of salt, the matter first precipitated, was, by the above specified figure of the bottome of the glass preserv'd from being reach'd by the spirituous salt; which hastily precipitated upon it a new bed (if i may so call it) of white powder, being the remaining corpuscles of the lead, that the urinous spirit had not struck down: so that there appear'd in the glass three distinct and very differingly colour'd substances; a purple or violet-colour'd precipitate at the bottom, a white and carnation (sometimes a variously colour'd) precipitate over that, and at the top of all a transparent liquor of a lovely yellow, or red. thus you see, _pyrophilus_, that though to some i may have seem'd to have lighted on this ( th.) experiment by chance, and though others may imagine, that to have excogitated it, must have proceeded from some extraordinary insight into the nature of colours, yet indeed, the devising of it need not be look'd upon as any great matter, especially to one that is a little vers'd in the notions, i have in these, and other papers hinted concerning the differences of salts. and perhaps i might add upon more than conjecture, that these very notions and some particulars scatteringly deliver'd in this treatise, being skilfully put together, may suggest divers matters (at least,) about colours, that will not be altogether despicable. but those hinted, _pyrophilus_, i must now leave such as you to prosecute, having already spent farr more time than i intended to allow my self in acquainting you with particular experiments and observations concerning the changes of colour, to which i might have added many more, but that i hope i may have presented you with a competent number to make out in some measure what i have at the beginning of this essay either propos'd as my design in this tract, or deliver'd as my conjectures concerning these matters. and it not being my present designe, as i have more than once declar'd, to deliver any positive hypothesis or solemn theory of colours, but only to furnish you with some experiments towards the framing of such a theory; i shall add nothing to what i have said already, but a request that you would not be forward to think i have been mistaken in any thing i have deliver'd as matter of fact concerning the changes of colours, in case you should not every time you trye it, find it exactly to succeed. for besides the contingencies to which we have elsewhere shewn some other experiments to be obnoxious, the omission or variation of a seemingly unconsiderable circumstance, may hinder the success of an experiment, wherein no other fault has been committed. of which truth i shall only give you that single and almost obvious, but yet illustrious instance of the art of dying scarlets, for though you should see every ingredient that is us'd about it, though i should particularly inform you of the weight of each, and though you should be present at the kindling of the fire, and at the increasing and remitting of it, when ever the degree of heat is to be alter'd, and though (in a word) you should see every thing done so particularly that you would scarce harbour the least doubt of your comprehending the whole art: yet if i should not disclose to you, that the vessels, that immediately contain the tinging ingredients, are to be made of or to be lin'd with tin, you would never be able by all that i could tell you else (at-least, if the famousest and candidest artificers do not strangely delude themselves) to bring your tincture of chochinele to dye a perfect scarlet. so much depends upon the very vessel, wherein the tinging matters are boyl'd, and so great an influence may an unheeded circumstance have on the success of experiments concerning colours. * * * * * _finis._ * * * * * a short account of some observations made by mr. _boyle_ about a _diamond_ that _shines_ in the dark. first enclosed in a letter written to a friend, and now together with it annexed to the foregoing treatise, upon the score of the affinity betwixt _light_ and _colours_. * * * * * _london,_ printed for _henry herringman_. * * * * * a copy of the letter that mr. _boyle_ wrote to sir _robert morray_, to accompany the _observations_ touching the _shining diamond_. _sir,_ though sir _robert morray_ and monsieur _zulichem_ be persons that have deserv'd so well of the commonwealth of learning, that i should think my self unworthy to be look'd upon as a member of it, if i declin'd to obey them, or to serve them; yet i should not without reluctancy send you the notes, you desire for him, if i did not hope that you will transmit together with them, some account why they are not less unworthy of his perusal; which, that you may do; i must inform you, how the writing of them was occasion'd, which in short was thus. as i was just going out of town, hearing that an ingenious gentleman of my acquaintance, lately return'd from _italy_, had a diamond, that being rubb'd, would shine in the dark, and that he was not far off, i snatch'd time from my occasions to make him a visit, but finding him ready to go abroad, and having in vain try'd to make the stone yield any light in the day time, i borrow'd it of him for that night, upon condition to restore it him within a day or two at furthest, at _gresham_ college, where we appointed to attend the meeting of the society, that was then to be at that place. and hereupon i hasted that evening out of town, and finding after supper that the stone which in the day time would afford no discernable light, was really conspicuous in the dark, i was so taken with the novelty, and so desirous to make some use of an opportunity that was like to last so little a while, that though at that time i had no body to assist me but a foot-boy, yet sitting up late, i made a shift that night to try a pretty number of such of the things that then came into my thoughts, as were not in that place and time unpracticable. and the next day being otherwise imploy'd, i was fain to make use of a drowsie part of the night to set down hastily in writing what i had observ'd, and without having the time in the morning, to stay the transcribing of it, i order'd the observations to be brought after me to _gresham_ college, where you may remember, that they were together with the stone it self shown to the royal society, by which they had the good fortune not to be dislik'd, though several things were through hast omitted, some of which you will find in the margin of the inclosed paper. the substance of this short narrative i hope you will let monsieur _zulichem_ know, that he may be kept from expecting any thing of finish'd in the observations, and be dispos'd to excuse the want of it. but such as they are, i hope they will prove (without a clinch) luciferous experiments, by setting the speculations of the curious on work, in a diligent inquiry after the nature of light, towards the discovery of which, perhaps they have not yet met with so considerable an experiment, since here we see light produc'd in a dead and opacous body, and that not as in rotten wood, or in fishes, or as in the _bolonian_ stone, by a natural corruption, or by a violent destruction of the texture of the body, but by so slight a mechanical operation upon its texture, as we seem to know what it is, and as is immediately perform'd, and that several wayes without at all prejudicing the body, or making any sensible alterations in its manifest qualities. and i am the more willing to expose my hasty tryals to monsieur _zulichem_, and to you, because, he being upon the consideration of dioptricks, so odd a _phænomenon_ relateing to the subject, as probably he treats of, light will, i hope, excite a person to consider it, that is wont to consider things he treats of very well. and for you sir, i hope you will both recrute and perfect the observations you receive, for you know that i cannot add to them, having a good while since restor'd to mr. _clayton_ the stone, which though it be now in the hands of a prince that so highly deserves, by understanding them, the greatest curiosities; yet he vouchsafes you that access to him as keeps me from doubting, you may easily obtain leave to make further tryals with it, of such a monarch as ours, that is not more inquisitive himself, than a favourer of them that are so. i doubt not but these notes will put you in mind of the motion you made to the society, to impose upon me the task of bringing in, what i had on other occasions observ'd concerning shining bodies. but though i deny not, that i sometimes made observations about the _bolonian_ stone, and try'd some experiments about some other shining bodies; yet the same reasons that reduc'd me then to be unwilling to receive ev'n their commands, must now be my apology for not answering your expectations, namely the abstruse nature of light, and my being already over-burden'd, and but too much kept imploy'd by the urgency of the press, as well as by more concerning and distracting occasions. but yet i will tell you some part of what i have met with in reference to the stone, of which i send you an account. because i find on the one side, that a great many think it no rarity upon a mistaken perswasion, that not only there are store of carbuncles, of which this is one; but that all diamonds and other glistering jewels shine in the dark. whereas on the other side there are very learn'd men, who (plausibly enough) deny that there are any carbuncles or shining stones at all. and certainly, those judicious men have much more to say for themselves, than the others commonly plead, and therefore did deservedly look upon mr. _clayton_'s diamond as a great rarity. for not only _boetius de boot_, who is judg'd the best author on this subject, ascribes no such virtue to diamonds, but begins what he delivers of carbuncles, with this passage.[ ] _magna fama est carbunculi. is vulgo putatur in tenebris carbonis instar lucere; fortassis quia pyropus seu anthrax appellatus a veteribus fuit. verum hactenus nemo nunquam verè asserere ausus fuit, se gemmam noctu lucentem vidisse. garcias ab horto proregis indiæ medicus, refert se allocutum fuisse, qui se vidisse affirmarent. sed iis fidem non habuit._ and a later author, the diligent and judicious _johannes de laet_ in his chapter of carbuncles and of rubies, has this passage. _quia autem carbunculi, pyropi & anthraces a veteribus nominantur, vulgo creditum fuit, carbonis instar in tenebris lucere, quod tamen nullâ gemmâ hastenus deprehensum, licet à quibusdam temerè jactetur._ and the recentest writer i have met with on this subject, _olaus wormius_, in his account of his well furnish'd _musæum_, do's, where he treats of rubies, concurr with the former writers by these words.[ ] _sunt qui rubinum veterum carbunculum esse existimant, sed deest una illa nota, quod in tenebris instar anthracis non luceat: ast talem carbunculum in rerum naturâ non inveniri major pars authoram existimant. licet unum aut alterum in india apud magnates quosdam reperiri scribant, cum tamen ex aliorum relatione id habeant saltem, sed ipsi non viderint._ in confirmation of which i shall only add, that hearing of a rubie, so very vivid, that the jewellers themselves have several times begg'd leave of the fair lady to whom it belong'd, that they might try their choicest rubies by comparing them with that, i had the opportunity by the favour of this lady and her husband, (both which i have the honour to be acquainted with) to make a trial of this famous rubie in the night, and in a room well darkn'd, but not only could not discern any thing of light, by looking on the stone before any thing had been done to it, but could not by all my rubbing bring it to afford the least glimmering of light. [ ] boetius de boot. gem. & lapid. histor. lib. . cap. . [ ] musæi wormiani. cap. . but, sir, though i be very backward to admit strange things for truths, yet i am not very forward to reject them as impossibilities, and therefore i would not discourage any from making further inquiry, whether or no there be really in _rerum natura_, any such thing as a true carbuncle or stone that without rubbing will shine in the dark. for if such a thing can be found, it may afford no small assistance to the curious in the investigation of light, besides the nobleness and rarity of the thing it selfe. and though _vartomannus_ was not an eye witness of what he relates, that the king of _pegu_, one of the chief kings of the _east-indies_, had a true carbuncle of that bigness and splendour, that it shin'd very gloriously in the dark, and though _garcias ab horto_, the _indian_ vice-roys physician, speaks of another carbuncle, only upon the report of one, that he discours'd with, who affirmed himself to have seen it; yet as we are not sure that these men that gave themselves out to be eye-witnesses speak true, yet they may have done so for ought we know to the contrary. and i could present you with a much considerabler testimony to the same purpose, if i had the permission of a person concern'd, without whose leave i must not do it. i might tell you that _marcus paulus venetus_[ ] (whose suppos'd fables, divers of our later travellours and navigatours have since found to be truths) speaking of the king of _zeilan_ that then was, tells us, that he was said to have the best rubie in the world, a palm long and as big as a mans arm, without spot, shining like a fire, and he subjoyns, that the great _cham_, under whom _paulus_ was a considerable officer, sent and offer'd the value of a city for it; but the king answer'd, he would not give it for the treasure of the world, nor part with it, having been his ancestours. and i could add, that in the relation made by two _russian_ cossacks of their journey into _catay_[ ], written to their emperour, they mention'd their having been told by the people of those parts, that their king had a stone, which lights as the sun both day and night, call'd in their language _sarra_, which those cossacks interpret a ruby. but these relations are too uncertain for me to build any thing upon, and therefore i shall proceed to tell you, that there came hither about two years since out of _america_, the governour of one of the principal colonies there, an ancient _virtuoso_, and one that has the honour to be a member of the royal society; this gentleman finding some of the chief affairs of his country committed to another and me, made me divers visits, and in one of them when i enquir'd what rare stones they had in those parts of the _indies_ he belong'd to, he told me, that the _indians_ had a tradition that in a certain hardly accessible hill, a pretty way up in the country, there was a stone which in the night time shin'd very vividly, and to a great distance, and he assur'd me, that though he thought it not fit to venture himself so far among those savages, yet he purposely sent thither a bold _englishman_, with some natives to be his guides, and that this messenger brought him back word, that at a distance from the hillock he had plainly perceiv'd such a shining substance as the _indians_ tradition mention'd, and being stimulated by curiosity, had slighted those superstitious fears of the inhabitants, and with much ado by reason of the difficulty of the way, had made a shift to clamber up to that part of the hill, where, by a very heedful observation, he suppos'd himself to have seen the light: but whether 'twere that he had mistaken the place, or for some other reason, he could not find it there, though when he was return'd to his former station, he did agen see the light shining in the same place where it shone before. a further account of this light i expect from the gentleman that gave me this, who lately sent me the news of his being landed in that country. and though i reserve to my self a full liberty of believing no more than i see cause; yet i do the less scruple to relate this, because a good part of it agrees well enough with another story that i shall in the next place have occasion to subjoyn, in order whereunto i shall tell you, that though the learned authors i formerly mention'd, tell us, that no writer has affirm'd his having himself seen a real carbuncle, yet, considering the light of mr. _claytons_ diamond, it recall'd into my mind, that some years before, when i was inquisitive about stones, i had met with an old _italian_ book highly extoll'd to me by very competent judges, and that though the book were very scarce, i had purchas'd it at a dear rate, for the sake of a few considerable passages i met with in it, and particularly one, which being very remarkable in it self, and pertinent to our present argument, i shall put it for you, though not word for word, which i fear i have forgot to do, yet as to the sense, into _english_. [ ] _purchas_'s pilgrim. lib. . cap. . pag. . [ ] in the year . _having promis'd_ (says our author)[ ] _to say something of that most precious sort of jewels,_ carbuncles, _because they are very rarely to be met with, we shall briefly deliver what we know of them. in_ clement _the seventh's time, i happen'd to see one of_ _them at a certain_ ragusian _merchants, nam'd_ beigoio di bona, _this was a carbuncle white, of that kind of whiteness which we said was to be found in those rubies of which we made mention a little above,_ (where he had said that those rubies had a kind of livid whiteness or paleness like that of a calcidonian) _but it had in it a lustre so pleasing and so marveilous, that it shin'd in the dark, but not as much as colour'd carbuncles, though it be true, that in an exceeding dark place i saw it shine in the manner of fire almost gone out. but as for colour'd carbuncles, it has not been my fortune to have seen any, wherefore i will onely set down what i learn'd about them discoursing in my youth with a_ roman _gentleman of antient experience in matters of jewels, who told me, that one_ jacopo cola _being by night in a vineyard of his, and espying something in the midst of it, that shin'd like a little_ glowing coal, at the foot of a vine, went near towards the place where he thought himself to have seen that fire, but not finding it, he said, that being return'd to the same place, whence he had first descry'd it, and perceiving there the same splendor as before, he mark'd it so heedfully, that he came at length to it, where he took up a very little stone, which he carry'd away with transports and joy. and the next day carrying it about to show it divers of his friends, whilst he was relating after what manner he found it, there casually interven'd a _venetian_ embassadour, exceedingly expert in jewels, who presently knowing it to be a carbuncle, did craftily before he and the said _jacopo_ parted (so that there was no body present that understood the worth of so precious a gemm) purchase it for the value of . crowns, and the next day left _rome_ to shun the being necessitated to restore it, and (as he affirm'd) it was known within some while after that the said _venetian_ gentleman did in _constantinople_ sell that carbuncle to the then grand seignior, newly come to the empire, for a hundred thousand crowns. _and this is what i can say_ concerning _carbuncles_, and this is not a little at least as to the first part of this account, where our _cellini_ affirms himself to have seen a real carbuncle with his own eyes, especially since this author appears wary in what he delivers, and is inclin'd rather to lessen, than increase the wonder of it. and his testimony is the more considerable, because though he were born a subject neither to the pope nor the then king of _france_ (that royal _virtuoso_ _francis_ the first) yet both the one and the other of those princes imploy'd him much about making of their noblest jewels. what is now reported concerning a shining substance to be seen in one of the islands about _scotland_, were very improper for me to mention to sr. _robert morray_, to whom the first information was originally brought, and from whom i expect a farther (for i scarce dare expect a convincing) account of it. but i must not omit that some _virtuoso_ questioning me the other day at _white-hall_ about mr. _claytons_ diamond, and meeting amongst them an ingenious _dutch_ gentleman, whose father was long embassador for the netherlands in _england_, i learn'd of him, that, he is acquainted with a person, whose name he told (but i do not well remember it) who was admiral of the _dutch_ in the _east-indies_, and who assur'd this gentleman _monsieur boreel_, that at his return from thence he brought back with him into _holland_ a stone, which though it look'd but like a pale dull diamond, such as he saw mr. _claytons_ to be, yet was it a real carbuncle, and did without rubbing shine so much, that when the admiral had occasion to open a chest which he kept under deck in a dark place, where 'twas forbidden to bring candles for fear of mischances, as soon as he open'd the trunck, the stone would by its native light, shine so as to illustrate a great part of it, and this gentleman having very civilly and readily granted me the request i made him, to write to the admiral, who is yet alive in _holland_, (and probably may still have the jewel by him,) for a particular account of this stone, i hope ere long to receive it, which will be the more welcome to me, not onely because so unlikely a thing needs a cleer evidence, but because i have had some suspition of that (supposing the truth of the thing) what may be a shining stone in a very hot countrey as the _east-indies_, may perhaps cease to be so (at least in certain seasons,) in one as cold as _holland_. for i observ'd in the diamond i send you an account of, that not onely rubbing but a very moderate degree of warmth, though excited by other wayes, would make it shine a little. and 'tis not impossible that there may be stones as much more susceptible than that, of the alterations requisite to make a diamond shine, as that appeares to be more susceptible of them, than ordinary diamonds. and i confess to you, that this is not the only odd suspition (for they are not so much as conjectures) that what i try'd upon this diamond suggested to me. for not here to entertain you with the changes i think may be effected ev'n in harder sorts of stones, by wayes not vulgar, nor very promising, because i may elsewhere have occasion to speak of them, and this letter is but too prolix already, that which i shall now acknowledge to you is, that i began to doubt whether there may not in some cases be some truth in what is said of the right turquois, that it often changes colour as the wearer is sick or well, and manifestly loses its splendor at his death. for when i found that ev'n the warmth of an affriction that lasted not above a quarter of a minute, nay, that of my body, (whose constitution you know is none of the hottest) would make a manifest change in the solidest of stones a diamond, it seem'd not impossible, that certain warm and saline steams issuing from the body of a living man, may by their plenty or paucity, or by their peculiar nature, or by the total absence of them, diversifie the colour, and the splendor of so soft a stone as the turquois. and though i admir'd to see, that i know not how many men otherwise learn'd, should confidently ascribe to jewels such virtues as seem no way competible to inanimate agents, if to any corporeal ones at all, yet as to what is affirm'd concerning the turquois's changing colour, i know not well how to reject the affirmation of so learned (and which in this case is much more considerable) so judicious a lapidary as _boetius de boot_[ ], who upon his own particular and repeated experience delivers so memorable a narrative of the turquois's changing colour, that i cannot but think it worth your perusal, especially since a much later and very experienc'd author, _olaus wormius_,[ ] where he treats of that stone, confirms it with this testimony. _imprimis memorandum exemplum quod anshelmus boetius de seipso refert, tam mutati coloris, quam à casu preservationis. cui & ipse haud dissimile adferre possum, nisi ex anshelmo petitum quis putaret._ i remember that i saw two or three years since a _turcois_ (worn in a ring) wherein there were some small spots, which the _virtuoso_ whose it was asur'd me he had observ'd to grow sometimes greater sometimes less, and to be sometimes in one part of the stone, sometimes in another. and i having encourag'd to make pictures from time to time of the stone, and of the situation of the cloudy parts, thatso their motion may be more indisputable, and better observ'd, he came to me about the midle of this very week, and assur'd me that he had, as i wish'd, made from time to time schemes or pictures of the differing parts of the stone, whereby the several removes and motions of the above mentioned clouds are very manifest, though the cause seem'd to him very occult: these pictures he has promis'd to show me, and is very ready to put the stone it self into my hands. but the ring having been the other day casually broken upon his finger, unless it can be taken out, and set again without any considerable heat, he is loath to have it medled with, for fear its peculiarity should be thereby destroy'd. and possibly his apprehension would have been strengthen'd, if i had had opportunity to tell him what is related by the learned _wormius_[ ] of an acquaintance of his, that had a _nephritick_ stone, of whose eminent virtues he had often experience ev'n in himself, and for that cause wore it still about his wrist; and yet going upon a time into a bath of fair water only, wherein certain herbs had been boyl'd, the stone by being wetted with this decoction, was depriv'd of all his virtue, whence _wormius_ takes occasion to advertise the sick, to lay by such stones whensoever they make use of a bath. and we might expect to find _turcos_ likewise, easily to be wrought upon in point of colour, if that were true, which the curious _antonio neri_, in his ingenious _arte vetraria_[ ] teaches of it, namely, that _turcois's discolour'd_ and grown white, will regain and acquire an excellent colour, if you but keep them two or three days at most cover'd with oyl of sweet almonds kept in a temperate heat by warm ashes, i say if it were true, because i doubt whether it be so, and have not as yet had opportunity to satisfie my self by tryals, because i find by the confession of the most skilfull persons among whom i have laid out for _turcoises_, that the true ones are great rarities, though others be not at all so. and therefore i shall now only mind you of one thing that you know as well as i, namely, that the rare stone which is called _oculus mundi_, if it be good in its kind, will have so great a change made in its texture by being barely left a while in the languidest of liquors, common waters, that from opacous it will become transparent, and acquire a lustre of which it will again be depriv'd, without using any other art or violence, by leaving it a while in the air. and before experience had satisfy'd us of the truth of this, it seem'd as unlikely that common water or air, should work such great changes in that gemm, as it now seems that the effluviums of a human body should effect lesser changes in a _turcois_, especially if more susceptible of them, than other stones of the same kind. but both my watch and my eyes tell me that 'tis now high time to think of going to sleep, matters of this nature, will be better, as well as more easily, clear'd by conference, than writing. and therefore since i think you know me too well to make it needfull for me to disclame credulity, notwithstanding my having entertain'd you with all these extravagancies; for you know well, how wide a difference i am wont to put betwixt things that barely _may be_, and things that _are_, and between those relations that are but not unworthy to be inquir'd into, and those that are not worthy to be actually believ'd; without making apologies for my ravings, i shall readily comply with the drowsiness that calls upon me to release you, and the rather, because monsieur _zulichem_ being concern'd in your desire to know the few things i have observed about the shining stone. to entertain those with suspicions that are accustomed not to acquiesce but in demonstrations, were a thing that cannot be look'd upon as other than very improper by, sir, _your most affectionate_ and _most faithfull servant,_ ro. boyle. [ ] benvonuto cellini _nell arte del_ gioiellare, _lib._ . _pag._ . [ ] the narrative in the authors own words, is this. _ego_ (sayes he) _sanctè affirmare possum me unam aureo annulo inclusam perpetuo gestare, cujus facultatem (si gemmæ est) nunquam satis admirari potui. gestaverat enim ante triginta annos hispanus quidam non procula puternis ædibus habitans. is cum vitâ functus esset, & ipsius suspellex (ut moris apud nos est) venum exposita esset, inter cætera etiam turcois exponebatur. verum nemo (licet complures eo concurrissent, ut eam propter coloris elegantiam, quam vivo domino habuerat emerent) sibi emptam voluit, pristinum enim nitorem & colorem prorsus amiserat, ut potius malachites, quam turcois videretur. aderat tum temporis gemmæ habendæ desiderio etiam parens & frater meus, qui antea sæpius gratiam & elegantiam ipsius viderant, mirabundi eam nunc tam esse deformem, emit eam nihilominus pater, satisque vili pretio, qua omnibus contemptui erat, ac presentes non eam esse quam hispanus gestarat, arbitrarentur. domum reversus pater, qui tam turpem gemmam gestare sibi indecorum putabat, eam mihi dono dat, inquiens; quandoquidem, fili mi, vulgi fama est, turcoidem, ut facultates suas exercere possit, dono dari debere tibi eam devoveo, ego acceptam gemmam sculptori trado, at gentilitia mea insignia illi, quamadmodum fieri solet, in jaspide chalcedono, aliisque ignobilioribus gemmis, insculperat. turpe enim existimabam, hujusmodi gemmâ ornatus gratia, dum gratiam nullam haberet, uti. paret sculptor redditque gemmam, quam gesto pro annulo signatorio. vix per mensem gestaram, redit illi pristinus color, sed non ita nitens propter sculpturam, ac inæqualem superficiem. miramur omnes gemmam, atque id præcipuè quod color indies pulchrior fieret. id quià observabam, nunquam fere eam à manu deposui, ita ut nunc adhuc candem gestem._ [ ] _olaus wormius, in musæ. º pag. ._ [ ] _musæ. worm._ pag. . [ ] arte vetraria, lib. cap. . * * * * * observations made this th.[ ] of _october_ . about mr. _clayton's_ diamond.[ ] being look'd on in the day time, though in a bed, whose curtains were carefully drawn, i could not discern it to shine at all, though well rubb'd, but about a little after sun-set, whilst the twilight yet lasted, nay, this morning[ ] a pretty while after sun-rising, (but before i had been abroad in the more freely inlightned air of the chamber) i could upon a light affriction easily perceive the stone to shine. [ ] these were brought in and read before the royal society, (the day following) _oct._ . . [ ] _the stone it self being to be shown to the royal society, when the observations were deliver'd, i was willing (being in haste) to omit the description of it, which is in short, that it was a flat or table diamond, of about a third part of an inch in length, and somewhat less in breadth, that it was a dull stone, and of a very bad water, having in the day time very little of the vividness of ev'n ordinary diamonds, and being blemished with a whitish cloud about the middle of it, which covered near a third part of the stone._ [ ] _hast made me forget to take notice that i went abroad the same morning, the sun shining forth clear enough, to look upon the diamond though a_ microscope, _that i might try whether by that magnifying glass any thing of peculiar could be discern'd in the texture of the stone, and especially of the whitish cloud that possest a good part of it. but for all my attention i could not discover any peculiarity worth mentioning._ secondly, the candles being removed, i could not in a dark place discern the stone to have any light, when i looked on it, without having rubb'd or otherwise prepar'd it. thirdly, by two white pibbles though hard rubb'd one against another, nor by the long and vehement affriction of rock crystal against a piece of red cloath, nor yet by rubbing two diamonds set in ring, as i had rubb'd this stone, i could produce any sensible degree of light. fourthly, i found this diamond hard enough, not only to enable me to write readily with it upon glass, but to grave on rock crystal it self. fifthly, i found this to have like other diamonds, an electrical faculty.[ ] [ ] v. _for it drew light bodies like amber, jet, and other concretes that are noted to do so; but its attractive power seem'd inferiour to theirs._ sixthly, being rubb'd upon my cloaths, as is usual for the exciting of amber, wax, and other electrical bodies, it did in the dark manifestly shine like rotten wood, or the scales of whitings, or other putrified fish. seventhly, but this conspicuousness was fainter than that of the scales, and slabber (if i may so call it) of whitings, and much fainter than the light of a glow-worm, by which i have been sometimes able to read a short word, whereas after an ordinary affriction of this diamond i was not able to discern distinctly by the light of it any of the nearest bodies: and this glimmering also did very manifestly and considerably decay presently upon the ceasing of the affriction, though the stone continued visible some while after. eighthly, but if it were rubb'd upon a convenient body for a pretty while, and briskly enough, i found the light would be for some moments much more considerable, almost like the light of a glow-worm, insomuch after i ceased rubbing, i could with the chaf'd stone exhibit a little luminous circle, like that, but not so bright as that which children make by moving a stick fir'd at the end, and in this case it would continue visible about seven or eight times as long as i had been in rubbing it. ninthly, i found that holding it a while near[ ] the flame of a candle, (from which yet i was carefull to avert my eyes) and being immediately remov'd into the dark, it disclosed some faint glimmering, but inferiour to that, it was wont to acquire by rubbing. and afterward holding it near a fire that had but little flame, i found the stone to be rather less than more excited, than it had been by the candle. [ ] ix. _we durst not hold it in the flame of a candle, no more than put it into a naked fire; for fear too violent a heat (which has been observ'd to spoil many other precious stones) should vitiate and impair a jewel, that was but borrow'd, and was suppos'd to be the only one of its kind._ tenthly, i likewise indeavour'd to make it shine, by holding it a pretty while in a very dark place, over a thick piece of iron, that was well heated, but not to that degree as to be visibly so. and though at length i found, that by this way also, the stone acquired some glimmering, yet it was less than by either of the other ways above mention'd. eleventhly, i also brought it to some kind of glimmering light, by taking it into bed with me, and holding it a good while upon a warm part of my naked body. twelfthly, to satisfie my self, whether the motion introduc'd into the stone did generate the light upon the account of its producing heat there, i held it near the flame of a candle, till it was qualify'd to shine pretty well in the dark, and then immediately i apply'd a slender hair to try whether it would attract it, but found not that it did so; though if it were made to shine by rubbing, it was as i formerly noted electrical. and for further confirmation, though i once purposedly kept it so near the hot iron i just now mention'd, as to make it sensibly warm, yet it shin'd more dimly than it had done by affriction or the flame of a candle, though by both those ways it had not acquir'd any warmth that was sensible. thirteenthly, having purposely rubb'd it upon several bodies differing as to colour, and as to texture, there seem'd to be some little disparity in the excitation (if i may so call it) of light. upon white and red cloths it seem'd to succeed best, especially in comparison of black ones. fourteenthly, but to try what it would do rubb'd upon bodies more hard, and less apt to yield heat upon a light affriction, than cloath, i first rubb'd it upon a white wooden box, by which it was excited, and afterwards upon a piece of purely glazed earth, which seem'd during the attrition to make it shine better than any of the other bodies had done, without excepting the white ones, which i add, lest the effect should be wholly ascrib'd to the disposition white bodies are wont to have to reflect much light. fifteenthly, having well excited the stone, i nimbly plung'd it under water[ ], that i had provided for that purpose, and perceiv'd it to shine whilst it was beneath the surface of that liquor, and this i did divers times. but when i indeavour'd to produce a light by rubbing it upon the lately mentioned cover of the box, the stone and it being both held beneath the surface of the water, i did not well satisfie my self in the event of the trial; but this i found, if i took the stone out, and rubb'd it upon a piece of cloath, it would not as else it was wont to do, presently acquire a luminousness, but needed to be rubb'd manifestly much longer before the desired effect was found. [ ] xv. _we likewise plung'd it as soon as we had excited it, under liquors of several sorts, as spirit of wine, oyl both chymical and express'd, an acid spirit, and as i remember an alcalizate solution, and found not any of those various liquors to destroy its shining property._ sixteenthly, i also try'd several times, that by covering it with my warm spittle (having no warm water at hand) it did not lose his light.[ ] [ ] xvi. _having found by this observation, that a warm liquor would not extinguish light in the diamond, i thought fit to try, whether by reason of its warmth it would not excite it, and divers times i found, that if it were kept therein, till the water had leisure to communicate some of its heat to it, it would often shine as soon as it was taken out, and probably we should have seen it shine more, whilst it was in the water, if some degree of opacity which heated water is wont to acquire, upon the score of the numerous little bubbles generated in it, had not kept us from discerning the lustre of the stone._ seventeenthly, finding that by rubbing the stone with the flat side downwards, i did by reason of the opacity of the ring; and the sudden decay of light upon the ceasing of the attrition, probably lose the sight of the stones greatest vividness; and supposing that the commotion made in one part of the stone will be easily propagated all over, i sometimes held the piece of cloath upon which i rubb'd it, so, that one side of the stone was exposed to my eye, whilst i was rubbing the other, whereby it appear'd more vivid than formerly, and to make luminous tracts by its motions too and fro. and sometimes holding the stone upwards, i rubb'd its broad side with a fine smooth piece of transparent horn, by which means the light through that diaphanous substance, did whilst i was actually rubbing the stone, appear so brisk that sometimes and in some places it seem'd to have little sparks of fire. eighteenthly, i took also a piece of flat blew glass, and having rubb'd the diamond well upon a cloath, and nimbly clapt the glass upon it, to try whether in case the light could peirce it, it would by appearing green, or of some other colour than blew, assist me to guess whether it self were sincere or no. but finding the glass impervious to so faint a light, i then thought it fit to try whether that hard bodies would not by attrition increase the diamonds light so as to become penetrable thereby, and accordingly when i rubb'd the glass briskly upon the stone, i found the light to be conspicuous enough, and somewhat dy'd in its passage, but found it not easie to give a name to the colour it exhibited. lastly, to comply with the suspition i had upon the whole matter, that the chief manifest change wrought in the stone, was by compression of its parts, rather than incalescence, i took a piece of white tile well glaz'd, and if i press'd the stone hard against it, it seem'd though i did not rub it to and fro, to shine at the sides: and however it did both very manifestly and vigorously shine, if whilst i so press'd it, i mov'd it any way upon the surface of the tile, though i did not make it draw a line of above a quarter of an inch long, or thereabouts. and though i made it not move to and fro, but only from one end of the short line to the other, without any return or lateral motion. nay, after it had been often rubb'd, and suffer'd to lose its light again, not only it seem'd more easie to be excited than at the beginning of the night; but if i did press hard upon it with my finger, at the very instant that i drew it briskly off, it would disclose a very vivid but exceeding short liv'd splendour, not to call it a little coruscation.[ ] so that a _cartesian_ would scarce scruple to think he had found in this stone no slight confirmation of his ingenious masters _hypothesis_, touching the generation of light in sublunary bodies, not sensibly hot. [ ] _i after bethought my self of imploying a way, which produc'd the desir'd effect both sooner and better. for holding betwixt my fingers a steel bodkin, near the lower part of it, i press'd the point hard against the surface of the diamond, and much more if i struck the point against it, the coruscation would be extremely suddain, and very vivid, though very vanishing too, and this way which commonly much surpris'd and pleas'd the spectators, seem'd far more proper than the other, to show that pressure alone, if forcible enough, though it were so suddain, and short, that it could not well be suppos'd to give the stone any thing near a sensible degree of warmth, as may be suspected of rubbing, yet 'tis sufficient to generate a very vivid light._ * * * * * a postscript. annexed some hours after the observations were written. _so many particulars taken notice of in one night, may make this stone appear a kind of prodigie, and the rather, because having try'd as i formerly noted, not only a fine artificial crystal, and some also that is natural, but a ruby and two diamonds, i did not find that any of these disclos'd the like glimmering of light;[ ] yet after all, perceiving by the hardness, and the testimony of a skilfull goldsmith, that this was rather a natural than artificial stone; for fear lest there might be some difference in the way of setting, or in the shape of the diamonds i made use of, neither of which was like this, a flat table-stone, i thought fit to make a farther trial of my own diamonds, by such a brisk and assiduous affriction as might make amends for the disadvantages above-mention'd, in case they were the cause of the unsuccessfulness of the former attempts: and accordingly i found, that by this way i could easily bring a diamond i wore on my finger to disclose a light, that was sensible enough, and continued so though i cover'd it with spittle, and us'd some other trials about it. and this will much lessen the wonder of all the formerly mention'd observations, by shewing that the properties that are so strange are not peculiar to one diamond, but may be found in others also, and perhaps in divers other hard and_ diaphanous _stones. yet i hope that what this discovery takes away from the wonder of these observations, it will add to the instructiveness of them, by affording pregnants hints, towards the investigation of the nature of light._ [ ] we afterwards, try'd precious stones, as diamonds, rubies, saphires, and emeralls, &c. but found not any of them to shine except some diamonds, and of these we were not upon so little practice, able to fore-tell before hand, which would be brought to shine, and which would not; for several very good diamonds, either would not shine at all, or much less than others that were farr inferiour to them. and yet those ingenious men are mistaken, that think a diamond must be foul and cloudy, as mr. _claytons_ was, to be fit for shining; for as we could bring some such to afford a glimmering light, so with some clear and excellent diamonds, we could do the like. but none of those many that we try'd of all kinds, were equal to the diamond on which the observations were made, not only considering the degree of light it afforded, but the easiness wherewith it was excited, and the comparatively great duration of its shining. finis. * * * * * transcriber's notes. the errata of the printed book have all been corrected. they were as follows: pag. . l. . these words, and to manifest, with the rest of what is by a mistake further printed in this fourth experiment, belongeth, and is to be referred to the end of the second eperiment, p. . pag. . l. . leg. matter. . l. . leg. bolts-head. pag . in the marginal note l. . dele de ib. l. . lege lib . p . l. ult. insert where between the words places and the. p. l. . dele that. ibid, l. . leg epidermis. ibid. l. leg. . for . p. . l. . leg. into it. p. . l. . & . leg. some solutions hereafter to be mentioned, for the solutions of potashes, and other lixiviate salts. p. . l. . insert part of between the words most and dissolved p. . l. ult. insert the participle it between the words judged and not p. . l. . leg. woud-wax or wood-wax. p. l. . leg. urine for urne. in addition i have corrected the following original typos: the preface: i devis'd tbem -> i devis'd them the preface: make expements -> make experiments the publisher to the reader: made of eperiments -> made of experiments i. ch. iii. divers expements -> divers experiments i. ch. iii. epecially with some sorts -> especially with some sorts ii. ch. ii. slightet texture -> slightest texture ii. exp. i two colonrs -> two colours ii. exp. xiii were the change of colour -> where the change iii. exp. xii avoiding of ambignity -> avoiding of ambiguity iii. exp. xxix juice of this sipce -> juice of this spice iii. exp. xl forty second expement -> forty second experiment iii. exp. xliv keep them swimning -> keep them swimming iii. exp. xlvi it seem'd propable to me -> it seem'd probable to me iii. exp. xlvii where not comprehended -> were not comprehended iii. exp. xlviii frequent igintion -> frequent ignition iii. exp. l i could tell yon -> i could tell you a copy of the letter: nemo unqnam vere -> nemo nunquam vere (ib.): what is reladed -> what is related observations: carefulsy drawn -> carefully drawn - and emended phoenomenon/a to phaenomenon/a times and coeruleous etc. -> caeruleous times this ebook includes papers or speeches by james clerk maxwell. the contents are: foramen centrale theory of compound colours poinsot's theory address to the mathematical introductory lecture on the unequal sensibility of the foramen centrale to light of different colours. james clerk maxwell [from the _report of the british association_, .] when observing the spectrum formed by looking at a long ve rtical slit through a simple prism, i noticed an elongated dark spot running up and down in the blue, and following the motion of the eye as it moved _up and down_ the spectrum, but refusing to pass out of the blue into the other colours. it was plain that the spot belonged both to the eye and to the blue part of the spectrum. the result to which i have come is, that the appearance is due to the yellow spot on the retina, commonly called the _foramen centrale_ of soemmering. the most convenient method of observing the spot is by presenting to the eye in not too rapid succession, blue and yellow glasses, or, still better, allowing blue and yellow papers to revolve slowly before the eye. in this way the spot is seen in the blue. it fades rapidly, but is renewed every time the yellow comes in to relieve the effect of the blue. by using a nicol's prism along with this apparatus, the brushes of haidinger are well seen in connexion with the spot, and the fact of the brushes being the spot analysed by polarized light becomes evident. if we look steadily at an object behind a series of bright bars which move in front of it, we shall see a curious bending of the bars as they come up to the place of the yellow spot. the part which comes over the spot seems to start in advance of the rest of the bar, and this would seem to indicate a greater rapidity of sensation at the yellow spot than in the surrounding retina. but i find the experiment difficult, and i hope for better results from more accurate observers. on the theory of compound colours with reference to mixtures of blue and yellow light. james clerk maxwell [from the _report of the british association_, .] when we mix together blue and yellow paint, we obtain green paint. this fact is well known to all who have handled colours; and it is universally admitted that blue and yellow make green. red, yellow, and blue, being the primary colours among painters, green is regarded as a secondary colour, arising from the mixture of blue and yellow. newton, however, found that the green of the spectrum was not the same thing as the mixture of two colours of the spectrum, for such a mixture could be separated by the prism, while the green of the spectrum resisted further decomposition. but still it was believed that yellow and blue would make a green, though not that of the spectrum. as far as i am aware, the first experiment on the subject is that of m. plateau, who, before , made a disc with alternate sectors of prussian blue and gamboge, and observed that, when spinning, the resultant tint was not green, but a neutral gray, inclining sometimes to yellow or blue, but never to green. prof. j. d. forbes of edinburgh made similar experiments in , with the same result. prof. helmholtz of konigsberg, to whom we owe the most complete investigation on visible colour, has given the true explanation of this phenomenon. the result of mixing two coloured powders is not by any means the same as mixing the beams of light which flow from each separately. in the latter case we receive all the light which comes either from the one powder or the other. in the former, much of the light coming from one powder falls on particles of the other, and we receive only that portion which has escaped absorption by one or other. thus the light coming from a mixture of blue and yellow powder, consists partly of light coming directly from blue particles or yellow particles, and partly of light acted on by both blue and yellow particles. this latter light is green, since the blue stops the red, yellow, and orange, and the yellow stops the blue and violet. i have made experiments on the mixture of blue and yellow light--by rapid rotation, by combined reflexion and transmission, by viewing them out of focus, in stripes, at a great distance, by throwing the colours of the spectrum on a screen, and by receiving them into the eye directly; and i have arranged a portable apparatus by which any one may see the result of this or any other mixture of the colours of the spectrum. in all these cases blue and yellow do not make green. i have also made experiments on the mixture of coloured powders. those which i used principally were "mineral blue" (from copper) and "chrome-yellow." other blue and yellow pigments gave curious results, but it was more difficult to make the mixtures, and the greens were less uniform in tint. the mixtures of these colours were made by weight, and were painted on discs of paper, which were afterwards treated in the manner described in my paper "on colour as perceived by the eye," in the _transactions of the royal society of edinburgh_, vol. xxi. part . the visible effect of the colour is estimated in terms of the standard-coloured papers:--vermilion (v), ultramarine (u), and emerald-green (e). the accuracy of the results, and their significance, can be best understood by referring to the paper before mentioned. i shall denote mineral blue by b, and chrome-yellow by y; and b y means a mixture of three parts blue and five parts yellow. given colour. standard colours. coefficient v. u. e. of brightness. b , = ............ 		 b y , = ............ 		 b y , = ............ 		 b y , = ............ 		 b y , = ............ 		 b y , = - ............ 		 b y , = - ............ 		 b y , = - ............ 		 y , = - ............ the columns v, u, e give the proportions of the standard colours which are equivalent to of the given colour; and the sum of v, u, e gives a coefficient, which gives a general idea of the brightness. it will be seen that the first admixture of yellow _diminishes_ the brightness of the blue. the negative values of u indicate that a mixture of v, u, and e cannot be made equivalent to the given colour. the experiments from which these results were taken had the negative values transferred to the other side of the equation. they were all made by means of the colour-top, and were verified by repetition at different times. it may be necessary to remark, in conclusion, with reference to the mode of registering visible colours in terms of three arbitrary standard colours, that it proceeds upon that theory of three primary elements in the sensation of colour, which treats the investigation of the laws of visible colour as a branch of human physiology, incapable of being deduced from the laws of light itself, as set forth in physical optics. it takes advantage of the methods of optics to study vision itself; and its appeal is not to physical principles, but to our consciousness of our own sensations. on an instrument to illustrate poinsot's theory of rotation. james clerk maxwell [from the _report of the british association_, .] in studying the rotation of a solid body according to poinsot's method, we have to consider the successive positions of the instantaneous axis of rotation with reference both to directions fixed in space and axes assumed in the moving body. the paths traced out by the pole of this axis on the _invariable plane_ and on the _central ellipsoid_ form interesting subjects of mathematical investigation. but when we attempt to follow with our eye the motion of a rotating body, we find it difficult to determine through what point of the _body_ the instantaneous axis passes at any time,--and to determine its path must be still more difficult. i have endeavoured to render visible the path of the instantaneous axis, and to vary the circumstances of motion, by means of a top of the same kind as that used by mr elliot, to illustrate precession*. the body of the instrument is a hollow cone of wood, rising from a ring, inches in diameter and inch thick. an iron axis, inches long, screws into the vertex of the cone. the lower extremity has a point of hard steel, which rests in an agate cup, and forms the support of the instrument. an iron nut, three ounces in weight, is made to screw on the axis, and to be fixed at any point; and in the wooden ring are screwed four bolts, of three ounces, working horizontally, and four bolts, of one ounce, working vertically. on the upper part of the axis is placed a disc of card, on which are drawn four concentric rings. each ring is divided into four quadrants, which are coloured red, yellow, green, and blue. the spaces between the rings are white. when the top is in motion, it is easy to see in which quadrant the instantaneous axis is at any moment and the distance between it and the axis of the instrument; and we observe,-- st. that the instantaneous axis travels in a closed curve, and returns to its original position in the body. ndly. that by working the vertical bolts, we can make the axis of the instrument the centre of this closed curve. it will then be one of the principal axes of inertia. rdly. that, by working the nut on the axis, we can make the order of colours either red, yellow, green, blue, or the reverse. when the order of colours is in the same direction as the rotation, it indicates that the axis of the instrument is that of greatest moment of inertia. thly. that if we screw the two pairs of opposite horizontal bolts to different distances from the axis, the path of the instantaneous pole will no longer be equidistant from the axis, but will describe an ellipse, whose longer axis is in the direction of the mean axis of the instrument. thly. that if we now make one of the two horizontal axes less and the other greater than the vertical axis, the instantaneous pole will separate from the axis of the instrument, and the axis will incline more and more till the spinning can no longer go on, on account of the obliquity. it is easy to see that, by attending to the laws of motion, we may produce any of the above effects at pleasure, and illustrate many different propositions by means of the same instrument. * _transactions of the royal scottish society of arts_, . address to the mathematical and physical sections of the british association. james clerk maxwell [from the _british association report_, vol. xl.] [liverpool, _september_ , .] at several of the recent meetings of the british association the varied and important business of the mathematical and physical section has been introduced by an address, the subject of which has been left to the selection of the president for the time being. the perplexing duty of choosing a subject has not, however, fallen to me. professor sylvester, the president of section a at the exeter meeting, gave us a noble vindication of pure mathematics by laying bare, as it were, the very working of the mathematical mind, and setting before us, not the array of symbols and brackets which form the armoury of the mathematician, or the dry results which are only the monuments of his conquests, but the mathematician himself, with all his human faculties directed by his professional sagacity to the pursuit, apprehension, and exhibition of that ideal harmony which he feels to be the root of all knowledge, the fountain of all pleasure, and the condition of all action. the mathematician has, above all things, an eye for symmetry; and professor sylvester has not only recognized the symmetry formed by the combination of his own subject with those of the former presidents, but has pointed out the duties of his successor in the following characteristic note:-- "mr spottiswoode favoured the section, in his opening address, with a combined history of the progress of mathematics and physics; dr. tyndall's address was virtually on the limits of physical philosophy; the one here in print," says prof. sylvester, "is an attempted faint adumbration of the nature of mathematical science in the abstract. what is wanting (like a fourth sphere resting on three others in contact) to build up the ideal pyramid is a discourse on the relation of the two branches (mathematics and physics) to, their action and reaction upon, one another, a magnificent theme, with which it is to be hoped that some future president of section a will crown the edifice and make the tetralogy (symbolizable by _a+a'_, _a_, _a'_, _aa'_) complete." the theme thus distinctly laid down for his successor by our late president is indeed a magnificent one, far too magnificent for any efforts of mine to realize. i have endeavoured to follow mr spottiswoode, as with far-reaching vision he distinguishes the systems of science into which phenomena, our knowledge of which is still in the nebulous stage, are growing. i have been carried by the penetrating insight and forcible expression of dr tyndall into that sanctuary of minuteness and of power where molecules obey the laws of their existence, clash together in fierce collision, or grapple in yet more fierce embrace, building up in secret the forms of visible things. i have been guided by prof. sylvester towards those serene heights "where never creeps a cloud, or moves a wind, nor ever falls the least white star of snow, nor ever lowest roll of thunder moans, nor sound of human sorrow mounts to mar their sacred everlasting calm." but who will lead me into that still more hidden and dimmer region where thought weds fact, where the mental operation of the mathematician and the physical action of the molecules are seen in their true relation? does not the way to it pass through the very den of the metaphysician, strewed with the remains of former explorers, and abhorred by every man of science? it would indeed be a foolhardy adventure for me to take up the valuable time of the section by leading you into those speculations which require, as we know, thousands of years even to shape themselves intelligibly. but we are met as cultivators of mathematics and physics. in our daily work we are led up to questions the same in kind with those of metaphysics; and we approach them, not trusting to the native penetrating power of our own minds, but trained by a long-continued adjustment of our modes of thought to the facts of external nature. as mathematicians, we perform certain mental operations on the symbols of number or of quantity, and, by proceeding step by step from more simple to more complex operations, we are enabled to express the same thing in many different forms. the equivalence of these different forms, though a necessary consequence of self-evident axioms, is not always, to our minds, self-evident; but the mathematician, who by long practice has acquired a familiarity with many of these forms, and has become expert in the processes which lead from one to another, can often transform a perplexing expression into another which explains its meaning in more intelligible language. as students of physics we observe phenomena under varied circumstances, and endeavour to deduce the laws of their relations. every natural phenomenon is, to our minds, the result of an infinitely complex system of conditions. what we set ourselves to do is to unravel these conditions, and by viewing the phenomenon in a way which is in itself partial and imperfect, to piece out its features one by one, beginning with that which strikes us first, and thus gradually learning how to look at the whole phenomenon so as to obtain a continually greater degree of clearness and distinctness. in this process, the feature which presents itself most forcibly to the untrained inquirer may not be that which is considered most fundamental by the experienced man of science; for the success of any physical investigation depends on the judicious selection of what is to be observed as of primary importance, combined with a voluntary abstraction of the mind from those features which, however attractive they appear, we are not yet sufficiently advanced in science to investigate with profit. intellectual processes of this kind have been going on since the first formation of language, and are going on still. no doubt the feature which strikes us first and most forcibly in any phenomenon, is the pleasure or the pain which accompanies it, and the agreeable or disagreeable results which follow after it. a theory of nature from this point of view is embodied in many of our words and phrases, and is by no means extinct even in our deliberate opinions. it was a great step in science when men became convinced that, in order to understand the nature of things, they must begin by asking, not whether a thing is good or bad, noxious or beneficial, but of what kind is it? and how much is there of it? quality and quantity were then first recognized as the primary features to be observed in scientific inquiry. as science has been developed, the domain of quantity has everywhere encroached on that of quality, till the process of scientific inquiry seems to have become simply the measurement and registration of quantities, combined with a mathematical discussion of the numbers thus obtained. it is this scientific method of directing our attention to those features of phenomena which may be regarded as quantities which brings physical research under the influence of mathematical reasoning. in the work of the section we shall have abundant examples of the successful application of this method to the most recent conquests of science; but i wish at present to direct your attention to some of the reciprocal effects of the progress of science on those elementary conceptions which are sometimes thought to be beyond the reach of change. if the skill of the mathematician has enabled the experimentalist to see that the quantities which he has measured are connected by necessary relations, the discoveries of physics have revealed to the mathematician new forms of quantities which he could never have imagined for himself. of the methods by which the mathematician may make his labours most useful to the student of nature, that which i think is at present most important is the systematic classification of quantities. the quantities which we study in mathematics and physics may be classified in two different ways. the student who wishes to master any particular science must make himself familiar with the various kinds of quantities which belong to that science. when he understands all the relations between these quantities, he regards them as forming a connected system, and he classes the whole system of quantities together as belonging to that particular science. this classification is the most natural from a physical point of view, and it is generally the first in order of time. but when the student has become acquainted with several different sciences, he finds that the mathematical processes and trains of reasoning in one science resemble those in another so much that his knowledge of the one science may be made a most useful help in the study of the other. when he examines into the reason of this, he finds that in the two sciences he has been dealing with systems of quantities, in which the mathematical forms of the relations of the quantities are the same in both systems, though the physical nature of the quantities may be utterly different. he is thus led to recognize a classification of quantities on a new principle, according to which the physical nature of the quantity is subordinated to its mathematical form. this is the point of view which is characteristic of the mathematician; but it stands second to the physical aspect in order of time, because the human mind, in order to conceive of different kinds of quantities, must have them presented to it by nature. i do not here refer to the fact that all quantities, as such, are subject to the rules of arithmetic and algebra, and are therefore capable of being submitted to those dry calculations which represent, to so many minds, their only idea of mathematics. the human mind is seldom satisfied, and is certainly never exercising its highest functions, when it is doing the work of a calculating machine. what the man of science, whether he is a mathematician or a physical inquirer, aims at is, to acquire and develope clear ideas of the things he deals with. for this purpose he is willing to enter on long calculations, and to be for a season a calculating machine, if he can only at last make his ideas clearer. but if he finds that clear ideas are not to be obtained by means of processes the steps of which he is sure to forget before he has reached the conclusion, it is much better that he should turn to another method, and try to understand the subject by means of well-chosen illustrations derived from subjects with which he is more familiar. we all know how much more popular the illustrative method of exposition is found, than that in which bare processes of reasoning and calculation form the principal subject of discourse. now a truly scientific illustration is a method to enable the mind to grasp some conception or law in one branch of science, by placing before it a conception or a law in a different branch of science, and directing the mind to lay hold of that mathematical form which is common to the corresponding ideas in the two sciences, leaving out of account for the present the difference between the physical nature of the real phenomena. the correctness of such an illustration depends on whether the two systems of ideas which are compared together are really analogous in form, or whether, in other words, the corresponding physical quantities really belong to the same mathematical class. when this condition is fulfilled, the illustration is not only convenient for teaching science in a pleasant and easy manner, but the recognition of the formal analogy between the two systems of ideas leads to a knowledge of both, more profound than could be obtained by studying each system separately. there are men who, when any relation or law, however complex, is put before them in a symbolical form, can grasp its full meaning as a relation among abstract quantities. such men sometimes treat with indifference the further statement that quantities actually exist in nature which fulfil this relation. the mental image of the concrete reality seems rather to disturb than to assist their contemplations. but the great majority of mankind are utterly unable, without long training, to retain in their minds the unembodied symbols of the pure mathematician, so that, if science is ever to become popular, and yet remain scientific, it must be by a profound study and a copious application of those principles of the mathematical classification of quantities which, as we have seen, lie at the root of every truly scientific illustration. there are, as i have said, some minds which can go on contemplating with satisfaction pure quantities presented to the eye by symbols, and to the mind in a form which none but mathematicians can conceive. there are others who feel more enjoyment in following geometrical forms, which they draw on paper, or build up in the empty space before them. others, again, are not content unless they can project their whole physical energies into the scene which they conjure up. they learn at what a rate the planets rush through space, and they experience a delightful feeling of exhilaration. they calculate the forces with which the heavenly bodies pull at one another, and they feel their own muscles straining with the effort. to such men momentum, energy, mass are not mere abstract expressions of the results of scientific inquiry. they are words of power, which stir their souls like the memories of childhood. for the sake of persons of these different types, scientific truth should be presented in different forms, and should be regarded as equally scientific whether it appears in the robust form and the vivid colouring of a physical illustration, or in the tenuity and paleness of a symbolical expression. time would fail me if i were to attempt to illustrate by examples the scientific value of the classification of quantities. i shall only mention the name of that important class of magnitudes having direction in space which hamilton has called vectors, and which form the subject-matter of the calculus of quaternions, a branch of mathematics which, when it shall have been thoroughly understood by men of the illustrative type, and clothed by them with physical imagery, will become, perhaps under some new name, a most powerful method of communicating truly scientific knowledge to persons apparently devoid of the calculating spirit. the mutual action and reaction between the different departments of human thought is so interesting to the student of scientific progress, that, at the risk of still further encroaching on the valuable time of the section, i shall say a few words on a branch of physics which not very long ago would have been considered rather a branch of metaphysics. i mean the atomic theory, or, as it is now called, the molecular theory of the constitution of bodies. not many years ago if we had been asked in what regions of physical science the advance of discovery was least apparent, we should have pointed to the hopelessly distant fixed stars on the one hand, and to the inscrutable delicacy of the texture of material bodies on the other. indeed, if we are to regard comte as in any degree representing the scientific opinion of his time, the research into what takes place beyond our own solar system seemed then to be exceedingly unpromising, if not altogether illusory. the opinion that the bodies which we see and handle, which we can set in motion or leave at rest, which we can break in pieces and destroy, are composed of smaller bodies which we cannot see or handle, which are always in motion, and which can neither be stopped nor broken in pieces, nor in any way destroyed or deprived of the least of their properties, was known by the name of the atomic theory. it was associated with the names of democritus, epicurus, and lucretius, and was commonly supposed to admit the existence only of atoms and void, to the exclusion of any other basis of things from the universe. in many physical reasonings and mathematical calculations we are accustomed to argue as if such substances as air, water, or metal, which appear to our senses uniform and continuous, were strictly and mathematically uniform and continuous. we know that we can divide a pint of water into many millions of portions, each of which is as fully endowed with all the properties of water as the whole pint was; and it seems only natural to conclude that we might go on subdividing the water for ever, just as we can never come to a limit in subdividing the space in which it is contained. we have heard how faraday divided a grain of gold into an inconceivable number of separate particles, and we may see dr tyndall produce from a mere suspicion of nitrite of butyle an immense cloud, the minute visible portion of which is still cloud, and therefore must contain many molecules of nitrite of butyle. but evidence from different and independent sources is now crowding in upon us which compels us to admit that if we could push the process of subdivision still further we should come to a limit, because each portion would then contain only one molecule, an individual body, one and indivisible, unalterable by any power in nature. even in our ordinary experiments on very finely divided matter we find that the substance is beginning to lose the properties which it exhibits when in a large mass, and that effects depending on the individual action of molecules are beginning to become prominent. the study of these phenomena is at present the path which leads to the development of molecular science. that superficial tension of liquids which is called capillary attraction is one of these phenomena. another important class of phenomena are those which are due to that motion of agitation by which the molecules of a liquid or gas are continually working their way from one place to another, and continually changing their course, like people hustled in a crowd. on this depends the rate of diffusion of gases and liquids through each other, to the study of which, as one of the keys of molecular science, that unwearied inquirer into nature's secrets, the late prof. graham, devoted such arduous labour. the rate of electrolytic conduction is, according to wiedemann's theory, influenced by the same cause; and the conduction of heat in fluids depends probably on the same kind of action. in the case of gases, a molecular theory has been developed by clausius and others, capable of mathematical treatment, and subjected to experimental investigation; and by this theory nearly every known mechanical property of gases has been explained on dynamical principles; so that the properties of individual gaseous molecules are in a fair way to become objects of scientific research. now mr stoney has pointed out[ ] that the numerical results of experiments on gases render it probable that the mean distance of their particles at the ordinary temperature and pressure is a quantity of the same order of magnitude as a millionth of a millimetre, and sir william thomson has since[ ] shewn, by several independent lines of argument, drawn from phenomena so different in themselves as the electrification of metals by contact, the tension of soap-bubbles, and the friction of air, that in ordinary solids and liquids the average distance between contiguous molecules is less than the hundred-millionth, and greater than the two-thousand-millionth of a centimetre. [ ] _phil. mag._, aug. . [ ] _nature_, march , . these, of course, are exceedingly rough estimates, for they are derived from measurements some of which are still confessedly very rough; but if at the present time, we can form even a rough plan for arriving at results of this kind, we may hope that, as our means of experimental inquiry become more accurate and more varied, our conception of a molecule will become more definite, so that we may be able at no distant period to estimate its weight with a greater degree of precision. a theory, which sir w. thomson has founded on helmholtz's splendid hydrodynamical theorems, seeks for the properties of molecules in the ring vortices of a uniform, frictionless, incompressible fluid. such whirling rings may be seen when an experienced smoker sends out a dexterous puff of smoke into the still air, but a more evanescent phenomenon it is difficult to conceive. this evanescence is owing to the viscosity of the air; but helmholtz has shewn that in a perfect fluid such a whirling ring, if once generated, would go on whirling for ever, would always consist of the very same portion of the fluid which was first set whirling, and could never be cut in two by any natural cause. the generation of a ring-vortex is of course equally beyond the power of natural causes, but once generated, it has the properties of individuality, permanence in quantity, and indestructibility. it is also the recipient of impulse and of energy, which is all we can affirm of matter; and these ring-vortices are capable of such varied connexions and knotted self-involutions, that the properties of differently knotted vortices must be as different as those of different kinds of molecules can be. if a theory of this kind should be found, after conquering the enormous mathematical difficulties of the subject, to represent in any degree the actual properties of molecules, it will stand in a very different scientific position from those theories of molecular action which are formed by investing the molecule with an arbitrary system of central forces invented expressly to account for the observed phenomena. in the vortex theory we have nothing arbitrary, no central forces or occult properties of any other kind. we have nothing but matter and motion, and when the vortex is once started its properties are all determined from the original impetus, and no further assumptions are possible. even in the present undeveloped state of the theory, the contemplation of the individuality and indestructibility of a ring-vortex in a perfect fluid cannot fail to disturb the commonly received opinion that a molecule, in order to be permanent, must be a very hard body. in fact one of the first conditions which a molecule must fulfil is, apparently, inconsistent with its being a single hard body. we know from those spectroscopic researches which have thrown so much light on different branches of science, that a molecule can be set into a state of internal vibration, in which it gives off to the surrounding medium light of definite refrangibility--light, that is, of definite wave-length and definite period of vibration. the fact that all the molecules (say, of hydrogen) which we can procure for our experiments, when agitated by heat or by the passage of an electric spark, vibrate precisely in the same periodic time, or, to speak more accurately, that their vibrations are composed of a system of simple vibrations having always the same periods, is a very remarkable fact. i must leave it to others to describe the progress of that splendid series of spectroscopic discoveries by which the chemistry of the heavenly bodies has been brought within the range of human inquiry. i wish rather to direct your attention to the fact that, not only has every molecule of terrestrial hydrogen the same system of periods of free vibration, but that the spectroscopic examination of the light of the sun and stars shews that, in regions the distance of which we can only feebly imagine, there are molecules vibrating in as exact unison with the molecules of terrestrial hydrogen as two tuning-forks tuned to concert pitch, or two watches regulated to solar time. now this absolute equality in the magnitude of quantities, occurring in all parts of the universe, is worth our consideration. the dimensions of individual natural bodies are either quite indeterminate, as in the case of planets, stones, trees, &c., or they vary within moderate limits, as in the case of seeds, eggs, &c.; but even in these cases small quantitative differences are met with which do not interfere with the essential properties of the body. even crystals, which are so definite in geometrical form, are variable with respect to their absolute dimensions. among the works of man we sometimes find a certain degree of uniformity. there is a uniformity among the different bullets which are cast in the same mould, and the different copies of a book printed from the same type. if we examine the coins, or the weights and measures, of a civilized country, we find a uniformity, which is produced by careful adjustment to standards made and provided by the state. the degree of uniformity of these national standards is a measure of that spirit of justice in the nation which has enacted laws to regulate them and appointed officers to test them. this subject is one in which we, as a scientific body, take a warm interest; and you are all aware of the vast amount of scientific work which has been expended, and profitably expended, in providing weights and measures for commercial and scientific purposes. the earth has been measured as a basis for a permanent standard of length, and every property of metals has been investigated to guard against any alteration of the material standards when made. to weigh or measure any thing with modern accuracy, requires a course of experiment and calculation in which almost every branch of physics and mathematics is brought into requisition. yet, after all, the dimensions of our earth and its time of rotation, though, relatively to our present means of comparison, very permanent, are not so by any physical necessity. the earth might contract by cooling, or it might be enlarged by a layer of meteorites falling on it, or its rate of revolution might slowly slacken, and yet it would continue to be as much a planet as before. but a molecule, say of hydrogen, if either its mass or its time of vibration were to be altered in the least, would no longer be a molecule of hydrogen. if, then, we wish to obtain standards of length, time, and mass which shall be absolutely permanent, we must seek them not in the dimensions, or the motion, or the mass of our planet, but in the wave-length, the period of vibration, and the absolute mass of these imperishable and unalterable and perfectly similar molecules. when we find that here, and in the starry heavens, there are innumerable multitudes of little bodies of exactly the same mass, so many, and no more, to the grain, and vibrating in exactly the same time, so many times, and no more, in a second, and when we reflect that no power in nature can now alter in the least either the mass or the period of any one of them, we seem to have advanced along the path of natural knowledge to one of those points at which we must accept the guidance of that faith by which we understand that "that which is seen was not made of things which do appear." one of the most remarkable results of the progress of molecular science is the light it has thrown on the nature of irreversible processes--processes, that is, which always tend towards and never away from a certain limiting state. thus, if two gases be put into the same vessel, they become mixed, and the mixture tends continually to become more uniform. if two unequally heated portions of the same gas are put into the vessel, something of the kind takes place, and the whole tends to become of the same temperature. if two unequally heated solid bodies be placed in contact, a continual approximation of both to an intermediate temperature takes place. in the case of the two gases, a separation may be effected by chemical means; but in the other two cases the former state of things cannot be restored by any natural process. in the case of the conduction or diffusion of heat the process is not only irreversible, but it involves the irreversible diminution of that part of the whole stock of thermal energy which is capable of being converted into mechanical work. this is thomson's theory of the irreversible dissipation of energy, and it is equivalent to the doctrine of clausius concerning the growth of what he calls entropy. the irreversible character of this process is strikingly embodied in fourier's theory of the conduction of heat, where the formulae themselves indicate, for all positive values of the time, a possible solution which continually tends to the form of a uniform diffusion of heat. but if we attempt to ascend the stream of time by giving to its symbol continually diminishing values, we are led up to a state of things in which the formula has what is called a critical value; and if we inquire into the state of things the instant before, we find that the formula becomes absurd. we thus arrive at the conception of a state of things which cannot be conceived as the physical result of a previous state of things, and we find that this critical condition actually existed at an epoch not in the utmost depths of a past eternity, but separated from the present time by a finite interval. this idea of a beginning is one which the physical researches of recent times have brought home to us, more than any observer of the course of scientific thought in former times would have had reason to expect. but the mind of man is not, like fourier's heated body, continually settling down into an ultimate state of quiet uniformity, the character of which we can already predict; it is rather like a tree, shooting out branches which adapt themselves to the new aspects of the sky towards which they climb, and roots which contort themselves among the strange strata of the earth into which they delve. to us who breathe only the spirit of our own age, and know only the characteristics of contemporary thought, it is as impossible to predict the general tone of the science of the future as it is to anticipate the particular discoveries which it will make. physical research is continually revealing to us new features of natural processes, and we are thus compelled to search for new forms of thought appropriate to these features. hence the importance of a careful study of those relations between mathematics and physics which determine the conditions under which the ideas derived from one department of physics may be safely used in forming ideas to be employed in a new department. the figure of speech or of thought by which we transfer the language and ideas of a familiar science to one with which we are less acquainted may be called scientific metaphor. thus the words velocity, momentum, force, &c. have acquired certain precise meanings in elementary dynamics. they are also employed in the dynamics of a connected system in a sense which, though perfectly analogous to the elementary sense, is wider and more general. these generalized forms of elementary ideas may be called metaphorical terms in the sense in which every abstract term is metaphorical. the characteristic of a truly scientific system of metaphors is that each term in its metaphorical use retains all the formal relations to the other terms of the system which it had in its original use. the method is then truly scientific--that is, not only a legitimate product of science, but capable of generating science in its turn. there are certain electrical phenomena, again, which are connected together by relations of the same form as those which connect dynamical phenomena. to apply to these the phrases of dynamics with proper distinctions and provisional reservations is an example of a metaphor of a bolder kind; but it is a legitimate metaphor if it conveys a true idea of the electrical relations to those who have been already trained in dynamics. suppose, then, that we have successfully introduced certain ideas belonging to an elementary science by applying them metaphorically to some new class of phenomena. it becomes an important philosophical question to determine in what degree the applicability of the old ideas to the new subject may be taken as evidence that the new phenomena are physically similar to the old. the best instances for the determination of this question are those in which two different explanations have been given of the same thing. the most celebrated case of this kind is that of the corpuscular and the undulatory theories of light. up to a certain point the phenomena of light are equally well explained by both; beyond this point, one of them fails. to understand the true relation of these theories in that part of the field where they seem equally applicable we must look at them in the light which hamilton has thrown upon them by his discovery that to every brachistochrone problem there corresponds a problem of free motion, involving different velocities and times, but resulting in the same geometrical path. professor tait has written a very interesting paper on this subject. according to a theory of electricity which is making great progress in germany, two electrical particles act on one another directly at a distance, but with a force which, according to weber, depends on their relative velocity, and according to a theory hinted at by gauss, and developed by riemann, lorenz, and neumann, acts not instantaneously, but after a time depending on the distance. the power with which this theory, in the hands of these eminent men, explains every kind of electrical phenomena must be studied in order to be appreciated. another theory of electricity, which i prefer, denies action at a distance and attributes electric action to tensions and pressures in an all-pervading medium, these stresses being the same in kind with those familiar to engineers, and the medium being identical with that in which light is supposed to be propagated. both these theories are found to explain not only the phenomena by the aid of which they were originally constructed, but other phenomena, which were not thought of or perhaps not known at the time; and both have independently arrived at the same numerical result, which gives the absolute velocity of light in terms of electrical quantities. that theories apparently so fundamentally opposed should have so large a field of truth common to both is a fact the philosophical importance of which we cannot fully appreciate till we have reached a scientific altitude from which the true relation between hypotheses so different can be seen. i shall only make one more remark on the relation between mathematics and physics. in themselves, one is an operation of the mind, the other is a dance of molecules. the molecules have laws of their own, some of which we select as most intelligible to us and most amenable to our calculation. we form a theory from these partial data, and we ascribe any deviation of the actual phenomena from this theory to disturbing causes. at the same time we confess that what we call disturbing causes are simply those parts of the true circumstances which we do not know or have neglected, and we endeavour in future to take account of them. we thus acknowledge that the so-called disturbance is a mere figment of the mind, not a fact of nature, and that in natural action there is no disturbance. but this is not the only way in which the harmony of the material with the mental operation may be disturbed. the mind of the mathematician is subject to many disturbing causes, such as fatigue, loss of memory, and hasty conclusions; and it is found that, from these and other causes, mathematicians make mistakes. i am not prepared to deny that, to some mind of a higher order than ours, each of these errors might be traced to the regular operation of the laws of actual thinking; in fact we ourselves often do detect, not only errors of calculation, but the causes of these errors. this, however, by no means alters our conviction that they are errors, and that one process of thought is right and another process wrong. i one of the most profound mathematicians and thinkers of our time, the late george boole, when reflecting on the precise and almost mathematical character of the laws of right thinking as compared with the exceedingly perplexing though perhaps equally determinate laws of actual and fallible thinking, was led to another of those points of view from which science seems to look out into a region beyond her own domain. "we must admit," he says, "that there exist laws" (of thought) "which even the rigour of their mathematical forms does not preserve from violation. we must ascribe to them an authority, the essence of which does not consist in power, a supremacy which the analogy of the inviolable order of the natural world in no way assists us to comprehend." introductory lecture on experimental physics. james clerk maxwell the university of cambridge, in accordance with that law of its evolution, by which, while maintaining the strictest continuity between the successive phases of its history, it adapts itself with more or less promptness to the requirements of the times, has lately instituted a course of experimental physics. this course of study, while it requires us to maintain in action all those powers of attention and analysis which have been so long cultivated in the university, calls on us to exercise our senses in observation, and our hands in manipulation. the familiar apparatus of pen, ink, and paper will no longer be sufficient for us, and we shall require more room than that afforded by a seat at a desk, and a wider area than that of the black board. we owe it to the munificence of our chancellor, that, whatever be the character in other respects of the experiments which we hope hereafter to conduct, the material facilities for their full development will be upon a scale which has not hitherto been surpassed. the main feature, therefore, of experimental physics at cambridge is the devonshire physical laboratory, and i think it desirable that on the present occasion, before we enter on the details of any special study, we should consider by what means we, the university of cambridge, may, as a living body, appropriate and vitalise this new organ, the outward shell of which we expect soon to rise before us. the course of study at this university has always included natural philosophy, as well as pure mathematics. to diffuse a sound knowledge of physics, and to imbue the minds of our students with correct dynamical principles, have been long regarded as among our highest functions, and very few of us can now place ourselves in the mental condition in which even such philosophers as the great descartes were involved in the days before newton had announced the true laws of the motion of bodies. indeed the cultivation and diffusion of sound dynamical ideas has already effected a great change in the language and thoughts even of those who make no pretensions to science, and we are daily receiving fresh proofs that the popularisation of scientific doctrines is producing as great an alteration in the mental state of society as the material applications of science are effecting in its outward life. such indeed is the respect paid to science, that the most absurd opinions may become current, provided they are expressed in language, the sound of which recals some well-known scientific phrase. if society is thus prepared to receive all kinds of scientific doctrines, it is our part to provide for the diffusion and cultivation, not only of true scientific principles, but of a spirit of sound criticism, founded on an examination of the evidences on which statements apparently scientific depend. when we shall be able to employ in scientific education, not only the trained attention of the student, and his familiarity with symbols, but the keenness of his eye, the quickness of his ear, the delicacy of his touch, and the adroitness of his fingers, we shall not only extend our influence over a class of men who are not fond of cold abstractions, but, by opening at once all the gateways of knowledge, we shall ensure the association of the doctrines of science with those elementary sensations which form the obscure background of all our conscious thoughts, and which lend a vividness and relief to ideas, which, when presented as mere abstract terms, are apt to fade entirely from the memory. in a course of experimental physics we may consider either the physics or the experiments as the leading feature. we may either employ the experiments to illustrate the phenomena of a particular branch of physics, or we may make some physical research in order to exemplify a particular experimental method. in the order of time, we should begin, in the lecture room, with a course of lectures on some branch of physics aided by experiments of illustration, and conclude, in the laboratory, with a course of experiments of research. let me say a few words on these two classes of experiments,--experiments of illustration and experiments of research. the aim of an experiment of illustration is to throw light upon some scientific idea so that the student may be enabled to grasp it. the circumstances of the experiment are so arranged that the phenomenon which we wish to observe or to exhibit is brought into prominence, instead of being obscured and entangled among other phenomena, as it is when it occurs in the ordinary course of nature. to exhibit illustrative experiments, to encourage others to make them, and to cultivate in every way the ideas on which they throw light, forms an important part of our duty. the simpler the materials of an illustrative experiment, and the more familiar they are to the student, the more thoroughly is he likely to acquire the idea which it is meant to illustrate. the educational value of such experiments is often inversely proportional to the complexity of the apparatus. the student who uses home-made apparatus, which is always going wrong, often learns more than one who has the use of carefully adjusted instruments, to which he is apt to trust, and which he dares not take to pieces. it is very necessary that those who are trying to learn from books the facts of physical science should be enabled by the help of a few illustrative experiments to recognise these facts when they meet with them out of doors. science appears to us with a very different aspect after we have found out that it is not in lecture rooms only, and by means of the electric light projected on a screen, that we may witness physical phenomena, but that we may find illustrations of the highest doctrines of science in games and gymnastics, in travelling by land and by water, in storms of the air and of the sea, and wherever there is matter in motion. this habit of recognising principles amid the endless variety of their action can never degrade our sense of the sublimity of nature, or mar our enjoyment of its beauty. on the contrary, it tends to rescue our scientific ideas from that vague condition in which we too often leave them, buried among the other products of a lazy credulity, and to raise them into their proper position among the doctrines in which our faith is so assured, that we are ready at all times to act on them. experiments of illustration may be of very different kinds. some may be adaptations of the commonest operations of ordinary life, others may be carefully arranged exhibitions of some phenomenon which occurs only under peculiar conditions. they all, however, agree in this, that their aim is to present some phenomenon to the senses of the student in such a way that he may associate with it the appropriate scientific idea. when he has grasped this idea, the experiment which illustrates it has served its purpose. in an experiment of research, on the other hand, this is not the principal aim. it is true that an experiment, in which the principal aim is to see what happens under certain conditions, may be regarded as an experiment of research by those who are not yet familiar with the result, but in experimental researches, strictly so called, the ultimate object is to measure something which we have already seen--to obtain a numerical estimate of some magnitude. experiments of this class--those in which measurement of some kind is involved, are the proper work of a physical laboratory. in every experiment we have first to make our senses familiar with the phenomenon, but we must not stop here, we must find out which of its features are capable of measurement, and what measurements are required in order to make a complete specification of the phenomenon. we must then make these measurements, and deduce from them the result which we require to find. this characteristic of modern experiments--that they consist principally of measurements,--is so prominent, that the opinion seems to have got abroad, that in a few years all the great physical constants will have been approximately estimated, and that the only occupation which will then be left to men of science will be to carry on these measurements to another place of decimals. if this is really the state of things to which we are approaching, our laboratory may perhaps become celebrated as a place of conscientious labour and consummate skill, but it will be out of place in the university, and ought rather to be classed with the other great workshops of our country, where equal ability is directed to more useful ends. but we have no right to think thus of the unsearchable riches of creation, or of the untried fertility of those fresh minds into which these riches will continue to be poured. it may possibly be true that, in some of those fields of discovery which lie open to such rough observations as can be made without artificial methods, the great explorers of former times have appropriated most of what is valuable, and that the gleanings which remain are sought after, rather for their abstruseness, than for their intrinsic worth. but the history of science shews that even during that phase of her progress in which she devotes herself to improving the accuracy of the numerical measurement of quantities with which she has long been familiar, she is preparing the materials for the subjugation of new regions, which would have remained unknown if she had been contented with the rough methods of her early pioneers. i might bring forward instances gathered from every branch of science, shewing how the labour of careful measurement has been rewarded by the discovery of new fields of research, and by the development of new scientific ideas. but the history of the science of terrestrial magnetism affords us a sufficient example of what may be done by experiments in concert, such as we hope some day to perform in our laboratory. that celebrated traveller, humboldt, was profoundly impressed with the scientific value of a combined effort to be made by the observers of all nations, to obtain accurate measurements of the magnetism of the earth; and we owe it mainly to his enthusiasm for science, his great reputation and his wide-spread influence, that not only private men of science, but the governments of most of the civilised nations, our own among the number, were induced to take part in the enterprise. but the actual working out of the scheme, and the arrangements by which the labours of the observers were so directed as to obtain the best results, we owe to the great mathematician gauss, working along with weber, the future founder of the science of electro-magnetic measurement, in the magnetic observatory of gottingen, and aided by the skill of the instrument-maker leyser. these men, however, did not work alone. numbers of scientific men joined the magnetic union, learned the use of the new instruments and the new methods of reducing the observations; and in every city of europe you might see them, at certain stated times, sitting, each in his cold wooden shed, with his eye fixed at the telescope, his ear attentive to the clock, and his pencil recording in his note-book the instantaneous position of the suspended magnet. bacon's conception of "experiments in concert" was thus realised, the scattered forces of science were converted into a regular army, and emulation and jealousy became out of place, for the results obtained by any one observer were of no value till they were combined with those of the others. the increase in the accuracy and completeness of magnetic observations which was obtained by the new method, opened up fields of research which were hardly suspected to exist by those whose observations of the magnetic needle had been conducted in a more primitive manner. we must reserve for its proper place in our course any detailed description of the disturbances to which the magnetism of our planet is found to be subject. some of these disturbances are periodic, following the regular courses of the sun and moon. others are sudden, and are called magnetic storms, but, like the storms of the atmosphere, they have their known seasons of frequency. the last and the most mysterious of these magnetic changes is that secular variation by which the whole character of the earth, as a great magnet, is being slowly modified, while the magnetic poles creep on, from century to century, along their winding track in the polar regions. we have thus learned that the interior of the earth is subject to the influences of the heavenly bodies, but that besides this there is a constantly progressive change going on, the cause of which is entirely unknown. in each of the magnetic observatories throughout the world an arrangement is at work, by means of which a suspended magnet directs a ray of light on a preparred sheet of paper moved by clockwork. on that paper the never-resting heart of the earth is now tracing, in telegraphic symbols which will one day be interpreted, a record of its pulsations and its flutterings, as well as of that slow but mighty working which warns us that we must not suppose that the inner history of our planet is ended. but this great experimental research on terrestrial magnetism produced lasting effects on the progress of science in general. i need only mention one or two instances. the new methods of measuring forces were successfully applied by weber to the numerical determination of all the phenomena of electricity, and very soon afterwards the electric telegraph, by conferring a commercial value on exact numerical measurements, contributed largely to the advancement, as well as to the diffusion of scientific knowledge. but it is not in these more modern branches of science alone that this influence is felt. it is to gauss, to the magnetic union, and to magnetic observers in general, that we owe our deliverance from that absurd method of estimating forces by a variable standard which prevailed so long even among men of science. it was gauss who first based the practical measurement of magnetic force (and therefore of every other force) on those long established principles, which, though they are embodied in every dynamical equation, have been so generally set aside, that these very equations, though correctly given in our cambridge textbooks, are usually explained there by assuming, in addition to the variable standard of force, a variable, and therefore illegal, standard of mass. such, then, were some of the scientific results which followed in this case from bringing together mathematical power, experimental sagacity, and manipulative skill, to direct and assist the labours of a body of zealous observers. if therefore we desire, for our own advantage and for the honour of our university, that the devonshire laboratory should be successful, we must endeavour to maintain it in living union with the other organs and faculties of our learned body. we shall therefore first consider the relation in which we stand to those mathematical studies which have so long flourished among us, which deal with our own subjects, and which differ from our experimental studies only in the mode in which they are presented to the mind. there is no more powerful method for introducing knowledge into the mind than that of presenting it in as many different ways as we can. when the ideas, after entering through different gateways, effect a junction in the citadel of the mind, the position they occupy becomes impregnable. opticians tell us that the mental combination of the views of an object which we obtain from stations no further apart than our two eyes is sufficient to produce in our minds an impression of the solidity of the object seen; and we find that this impression is produced even when we are aware that we are really looking at two flat pictures placed in a stereoscope. it is therefore natural to expect that the knowledge of physical science obtained by the combined use of mathematical analysis and experimental research will be of a more solid, available, and enduring kind than that possessed by the mere mathematician or the mere experimenter. but what will be the effect on the university, if men pursuing that course of reading which has produced so many distinguished wranglers, turn aside to work experiments? will not their attendance at the laboratory count not merely as time withdrawn from their more legitimate studies, but as the introduction of a disturbing element, tainting their mathematical conceptions with material imagery, and sapping their faith in the formulae of the textbook? besides this, we have already heard complaints of the undue extension of our studies, and of the strain put upon our questionists by the weight of learning which they try to carry with them into the senate-house. if we now ask them to get up their subjects not only by books and writing, but at the same time by observation and manipulation, will they not break down altogether? the physical laboratory, we are told, may perhaps be useful to those who are going out in natural science, and who do not take in mathematics, but to attempt to combine both kinds of study during the time of residence at the university is more than one mind can bear. no doubt there is some reason for this feeling. many of us have already overcome the initial difficulties of mathematical training. when we now go on with our study, we feel that it requires exertion and involves fatigue, but we are confident that if we only work hard our progress will be certain. some of us, on the other hand, may have had some experience of the routine of experimental work. as soon as we can read scales, observe times, focus telescopes, and so on, this kind of work ceases to require any great mental effort. we may perhaps tire our eyes and weary our backs, but we do not greatly fatigue our minds. it is not till we attempt to bring the theoretical part of our training into contact with the practical that we begin to experience the full effect of what faraday has called "mental inertia"--not only the difficulty of recognising, among the concrete objects before us, the abstract relation which we have learned from books, but the distracting pain of wrenching the mind away from the symbols to the objects, and from the objects back to the symbols. this however is the price we have to pay for new ideas. but when we have overcome these difficulties, and successfully bridged over the gulph between the abstract and the concrete, it is not a mere piece of knowledge that we have obtained: we have acquired the rudiment of a permanent mental endowment. when, by a repetition of efforts of this kind, we have more fully developed the scientific faculty, the exercise of this faculty in detecting scientific principles in nature, and in directing practice by theory, is no longer irksome, but becomes an unfailing source of enjoyment, to which we return so often, that at last even our careless thoughts begin to run in a scientific channel. i quite admit that our mental energy is limited in quantity, and i know that many zealous students try to do more than is good for them. but the question about the introduction of experimental study is not entirely one of quantity. it is to a great extent a question of distribution of energy. some distributions of energy, we know, are more useful than others, because they are more available for those purposes which we desire to accomplish. now in the case of study, a great part of our fatigue often arises, not from those mental efforts by which we obtain the mastery of the subject, but from those which are spent in recalling our wandering thoughts; and these efforts of attention would be much less fatiguing if the disturbing force of mental distraction could be removed. this is the reason why a man whose soul is in his work always makes more progress than one whose aim is something not immediately connected with his occupation. in the latter case the very motive of which he makes use to stimulate his flagging powers becomes the means of distracting his mind from the work before him. there may be some mathematicians who pursue their studies entirely for their own sake. most men, however, think that the chief use of mathematics is found in the interpretation of nature. now a man who studies a piece of mathematics in order to understand some natural phenomenon which he has seen, or to calculate the best arrangement of some experiment which he means to make, is likely to meet with far less distraction of mind than if his sole aim had been to sharpen his mind for the successful practice of the law, or to obtain a high place in the mathematical tripos. i have known men, who when they were at school, never could see the good of mathematics, but who, when in after life they made this discovery, not only became eminent as scientific engineers, but made considerable progress in the study of abstract mathematics. if our experimental course should help any of you to see the good of mathematics, it will relieve us of much anxiety, for it will not only ensure the success of your future studies, but it will make it much less likely that they will prove injurious to your health. but why should we labour to prove the advantage of practical science to the university? let us rather speak of the help which the university may give to science, when men well trained in mathematics and enjoying the advantages of a well-appointed laboratory, shall unite their efforts to carry out some experimental research which no solitary worker could attempt. at first it is probable that our principal experimental work must be the illustration of particular branches of science, but as we go on we must add to this the study of scientific methods, the same method being sometimes illustrated by its application to researches belonging to different branches of science. we might even imagine a course of experimental study the arrangement of which should be founded on a classification of methods, and not on that of the objects of investigation. a combination of the two plans seems to me better than either, and while we take every opportunity of studying methods, we shall take care not to dissociate the method from the scientific research to which it is applied, and to which it owes its value. we shall therefore arrange our lectures according to the classification of the principal natural phenomena, such as heat, electricity, magnetism and so on. in the laboratory, on the other hand, the place of the different instruments will be determined by a classification according to methods, such as weighing and measuring, observations of time, optical and electrical methods of observation, and so on. the determination of the experiments to be performed at a particular time must often depend upon the means we have at command, and in the case of the more elaborate experiments, this may imply a long time of preparation, during which the instruments, the methods, and the observers themselves, are being gradually fitted for their work. when we have thus brought together the requisites, both material and intellectual, for a particular experiment, it may sometimes be desirable that before the instruments are dismounted and the observers dispersed, we should make some other experiment, requiring the same method, but dealing perhaps with an entirely different class of physical phenomena. our principal work, however, in the laboratory must be to acquaint ourselves with all kinds of scientific methods, to compare them, and to estimate their value. it will, i think, be a result worthy of our university, and more likely to be accomplished here than in any private laboratory, if, by the free and full discussion of the relative value of different scientific procedures, we succeed in forming a school of scientific criticism, and in assisting the development of the doctrine of method. but admitting that a practical acquaintance with the methods of physical science is an essential part of a mathematical and scientific education, we may be asked whether we are not attributing too much importance to science altogether as part of a liberal education. fortunately, there is no question here whether the university should continue to be a place of liberal education, or should devote itself to preparing young men for particular professions. hence though some of us may, i hope, see reason to make the pursuit of science the main business of our lives, it must be one of our most constant aims to maintain a living connexion between our work and the other liberal studies of cambridge, whether literary, philological, historical or philosophical. there is a narrow professional spirit which may grow up among men of science, just as it does among men who practise any other special business. but surely a university is the very place where we should be able to overcome this tendency of men to become, as it were, granulated into small worlds, which are all the more worldly for their very smallness. we lose the advantage of having men of varied pursuits collected into one body, if we do not endeavour to imbibe some of the spirit even of those whose special branch of learning is different from our own. it is not so long ago since any man who devoted himself to geometry, or to any science requiring continued application, was looked upon as necessarily a misanthrope, who must have abandoned all human interests, and betaken himself to abstractions so far removed from the world of life and action that he has become insensible alike to the attractions of pleasure and to the claims of duty. in the present day, men of science are not looked upon with the same awe or with the same suspicion. they are supposed to be in league with the material spirit of the age, and to form a kind of advanced radical party among men of learning. we are not here to defend literary and historical studies. we admit that the proper study of mankind is man. but is the student of science to be withdrawn from the study of man, or cut off from every noble feeling, so long as he lives in intellectual fellowship with men who have devoted their lives to the discovery of truth, and the results of whose enquiries have impressed themselves on the ordinary speech and way of thinking of men who never heard their names? or is the student of history and of man to omit from his consideration the history of the origin and diffusion of those ideas which have produced so great a difference between one age of the world and another? it is true that the history of science is very different from the science of history. we are not studying or attempting to study the working of those blind forces which, we are told, are operating on crowds of obscure people, shaking principalities and powers, and compelling reasonable men to bring events to pass in an order laid down by philosophers. the men whose names are found in the history of science are not mere hypothetical constituents of a crowd, to be reasoned upon only in masses. we recognise them as men like ourselves, and their actions and thoughts, being more free from the influence of passion, and recorded more accurately than those of other men, are all the better materials for the study of the calmer parts of human nature. but the history of science is not restricted to the enumeration of successful investigations. it has to tell of unsuccessful inquiries, and to explain why some of the ablest men have failed to find the key of knowledge, and how the reputation of others has only given a firmer footing to the errors into which they fell. the history of the development, whether normal or abnormal, of ideas is of all subjects that in which we, as thinking men, take the deepest interest. but when the action of the mind passes out of the intellectual stage, in which truth and error are the alternatives, into the more violently emotional states of anger and passion, malice and envy, fury and madness; the student of science, though he is obliged to recognise the powerful influence which these wild forces have exercised on mankind, is perhaps in some measure disqualified from pursuing the study of this part of human nature. but then how few of us are capable of deriving profit from such studies. we cannot enter into full sympathy with these lower phases of our nature without losing some of that antipathy to them which is our surest safeguard against a reversion to a meaner type, and we gladly return to the company of those illustrious men who by aspiring to noble ends, whether intellectual or practical, have risen above the region of storms into a clearer atmosphere, where there is no misrepresentation of opinion, nor ambiguity of expression, but where one mind comes into closest contact with another at the point where both approach nearest to the truth. i propose to lecture during this term on heat, and, as our facilities for experimental work are not yet fully developed, i shall endeavour to place before you the relative position and scientific connexion of the different branches of the science, rather than to discuss the details of experimental methods. we shall begin with thermometry, or the registration of temperatures, and calorimetry, or the measurement of quantities of heat. we shall then go on to thermodynamics, which investigates the relations between the thermal properties of bodies and their other dynamical properties, in so far as these relations may be traced without any assumption as to the particular constitution of these bodies. the principles of thermodynamics throw great light on all the phenomena of nature, and it is probable that many valuable applications of these principles have yet to be made; but we shall have to point out the limits of this science, and to shew that many problems in nature, especially those in which the dissipation of energy comes into play, are not capable of solution by the principles of thermodynamics alone, but that in order to understand them, we are obliged to form some more definite theory of the constitution of bodies. two theories of the constitution of bodies have struggled for victory with various fortunes since the earliest ages of speculation: one is the theory of a universal plenum, the other is that of atoms and void. the theory of the plenum is associated with the doctrine of mathematical continuity, and its mathematical methods are those of the differential calculus, which is the appropriate expression of the relations of continuous quantity. the theory of atoms and void leads us to attach more importance to the doctrines of integral numbers and definite proportions; but, in applying dynamical principles to the motion of immense numbers of atoms, the limitation of our faculties forces us to abandon the attempt to express the exact history of each atom, and to be content with estimating the average condition of a group of atoms large enough to be visible. this method of dealing with groups of atoms, which i may call the statistical method, and which in the present state of our knowledge is the only available method of studying the properties of real bodies, involves an abandonment of strict dynamical principles, and an adoption of the mathematical methods belonging to the theory of probability. it is probable that important results will be obtained by the application of this method, which is as yet little known and is not familiar to our minds. if the actual history of science had been different, and if the scientific doctrines most familiar to us had been those which must be expressed in this way, it is possible that we might have considered the existence of a certain kind of contingency a self-evident truth, and treated the doctrine of philosophical necessity as a mere sophism. about the beginning of this century, the properties of bodies were investigated by several distinguished french mathematicians on the hypothesis that they are systems of molecules in equilibrium. the somewhat unsatisfactory nature of the results of these investigations produced, especially in this country, a reaction in favour of the opposite method of treating bodies as if they were, so far at least as our experiments are concerned, truly continuous. this method, in the hands of green, stokes, and others, has led to results, the value of which does not at all depend on what theory we adopt as to the ultimate constitution of bodies. one very important result of the investigation of the properties of bodies on the hypothesis that they are truly continuous is that it furnishes us with a test by which we can ascertain, by experiments on a real body, to what degree of tenuity it must be reduced before it begins to give evidence that its properties are no longer the same as those of the body in mass. investigations of this kind, combined with a study of various phenomena of diffusion and of dissipation of energy, have recently added greatly to the evidence in favour of the hypothesis that bodies are systems of molecules in motion. i hope to be able to lay before you in the course of the term some of the evidence for the existence of molecules, considered as individual bodies having definite properties. the molecule, as it is presented to the scientific imagination, is a very different body from any of those with which experience has hitherto made us acquainted. in the first place its mass, and the other constants which define its properties, are absolutely invariable; the individual molecule can neither grow nor decay, but remains unchanged amid all the changes of the bodies of which it may form a constituent. in the second place it is not the only molecule of its kind, for there are innumerable other molecules, whose constants are not approximately, but absolutely identical with those of the first molecule, and this whether they are found on the earth, in the sun, or in the fixed stars. by what process of evolution the philosophers of the future will attempt to account for this identity in the properties of such a multitude of bodies, each of them unchangeable in magnitude, and some of them separated from others by distances which astronomy attempts in vain to measure, i cannot conjecture. my mind is limited in its power of speculation, and i am forced to believe that these molecules must have been made as they are from the beginning of their existence. i also conclude that since none of the processes of nature, during their varied action on different individual molecules, have produced, in the course of ages, the slightest difference between the properties of one molecule and those of another, the history of whose combinations has been different, we cannot ascribe either their existence or the identity of their properties to the operation of any of those causes which we call natural. is it true then that our scientific speculations have really penetrated beneath the visible appearance of things, which seem to be subject to generation and corruption, and reached the entrance of that world of order and perfection, which continues this day as it was created, perfect in number and measure and weight? we may be mistaken. no one has as yet seen or handled an individual molecule, and our molecular hypothesis may, in its turn, be supplanted by some new theory of the constitution of matter; but the idea of the existence of unnumbered individual things, all alike and all unchangeable, is one which cannot enter the human mind and remain without fruit. but what if these molecules, indestructible as they are, turn out to be not substances themselves, but mere affections of some other substance? according to sir w. thomson's theory of vortex atoms, the substance of which the molecule consists is a uniformly dense _plenum_, the properties of which are those of a perfect fluid, the molecule itself being nothing but a certain motion impressed on a portion of this fluid, and this motion is shewn, by a theorem due to helmholtz, to be as indestructible as we believe a portion of matter to be. if a theory of this kind is true, or even if it is conceivable, our idea of matter may have been introduced into our minds through our experience of those systems of vortices which we call bodies, but which are not substances, but motions of a substance; and yet the idea which we have thus acquired of matter, as a substance possessing inertia, may be truly applicable to that fluid of which the vortices are the motion, but of whose existence, apart from the vortical motion of some of its parts, our experience gives us no evidence whatever. it has been asserted that metaphysical speculation is a thing of the past, and that physical science has extirpated it. the discussion of the categories of existence, however, does not appear to be in danger of coming to an end in our time, and the exercise of speculation continues as fascinating to every fresh mind as it was in the days of thales. note: project gutenberg also has an html version of this file which includes the original illustrations. see -h.htm or -h.zip: (http://www.gutenberg.net/dirs/ / / / / / -h/ -h.htm) or (http://www.gutenberg.net/dirs/ / / / / / -h.zip) treatise on light in which are explained the causes of that which occurs in reflexion, & in refraction and particularly in the strange refraction of iceland crystal by christiaan huygens rendered into english by silvanus p. thompson university of chicago press preface i wrote this treatise during my sojourn in france twelve years ago, and i communicated it in the year to the learned persons who then composed the royal academy of science, to the membership of which the king had done me the honour of calling, me. several of that body who are still alive will remember having been present when i read it, and above the rest those amongst them who applied themselves particularly to the study of mathematics; of whom i cannot cite more than the celebrated gentlemen cassini, römer, and de la hire. and, although i have since corrected and changed some parts, the copies which i had made of it at that time may serve for proof that i have yet added nothing to it save some conjectures touching the formation of iceland crystal, and a novel observation on the refraction of rock crystal. i have desired to relate these particulars to make known how long i have meditated the things which now i publish, and not for the purpose of detracting from the merit of those who, without having seen anything that i have written, may be found to have treated of like matters: as has in fact occurred to two eminent geometricians, messieurs newton and leibnitz, with respect to the problem of the figure of glasses for collecting rays when one of the surfaces is given. one may ask why i have so long delayed to bring this work to the light. the reason is that i wrote it rather carelessly in the language in which it appears, with the intention of translating it into latin, so doing in order to obtain greater attention to the thing. after which i proposed to myself to give it out along with another treatise on dioptrics, in which i explain the effects of telescopes and those things which belong more to that science. but the pleasure of novelty being past, i have put off from time to time the execution of this design, and i know not when i shall ever come to an end if it, being often turned aside either by business or by some new study. considering which i have finally judged that it was better worth while to publish this writing, such as it is, than to let it run the risk, by waiting longer, of remaining lost. there will be seen in it demonstrations of those kinds which do not produce as great a certitude as those of geometry, and which even differ much therefrom, since whereas the geometers prove their propositions by fixed and incontestable principles, here the principles are verified by the conclusions to be drawn from them; the nature of these things not allowing of this being done otherwise. it is always possible to attain thereby to a degree of probability which very often is scarcely less than complete proof. to wit, when things which have been demonstrated by the principles that have been assumed correspond perfectly to the phenomena which experiment has brought under observation; especially when there are a great number of them, and further, principally, when one can imagine and foresee new phenomena which ought to follow from the hypotheses which one employs, and when one finds that therein the fact corresponds to our prevision. but if all these proofs of probability are met with in that which i propose to discuss, as it seems to me they are, this ought to be a very strong confirmation of the success of my inquiry; and it must be ill if the facts are not pretty much as i represent them. i would believe then that those who love to know the causes of things and who are able to admire the marvels of light, will find some satisfaction in these various speculations regarding it, and in the new explanation of its famous property which is the main foundation of the construction of our eyes and of those great inventions which extend so vastly the use of them. i hope also that there will be some who by following these beginnings will penetrate much further into this question than i have been able to do, since the subject must be far from being exhausted. this appears from the passages which i have indicated where i leave certain difficulties without having resolved them, and still more from matters which i have not touched at all, such as luminous bodies of several sorts, and all that concerns colours; in which no one until now can boast of having succeeded. finally, there remains much more to be investigated touching the nature of light which i do not pretend to have disclosed, and i shall owe much in return to him who shall be able to supplement that which is here lacking to me in knowledge. the hague. the january . note by the translator considering the great influence which this treatise has exercised in the development of the science of optics, it seems strange that two centuries should have passed before an english edition of the work appeared. perhaps the circumstance is due to the mistaken zeal with which formerly everything that conflicted with the cherished ideas of newton was denounced by his followers. the treatise on light of huygens has, however, withstood the test of time: and even now the exquisite skill with which he applied his conception of the propagation of waves of light to unravel the intricacies of the phenomena of the double refraction of crystals, and of the refraction of the atmosphere, will excite the admiration of the student of optics. it is true that his wave theory was far from the complete doctrine as subsequently developed by thomas young and augustin fresnel, and belonged rather to geometrical than to physical optics. if huygens had no conception of transverse vibrations, of the principle of interference, or of the existence of the ordered sequence of waves in trains, he nevertheless attained to a remarkably clear understanding of the principles of wave-propagation; and his exposition of the subject marks an epoch in the treatment of optical problems. it has been needful in preparing this translation to exercise care lest one should import into the author's text ideas of subsequent date, by using words that have come to imply modern conceptions. hence the adoption of as literal a rendering as possible. a few of the author's terms need explanation. he uses the word "refraction," for example, both for the phenomenon or process usually so denoted, and for the result of that process: thus the refracted ray he habitually terms "the refraction" of the incident ray. when a wave-front, or, as he terms it, a "wave," has passed from some initial position to a subsequent one, he terms the wave-front in its subsequent position "the continuation" of the wave. he also speaks of the envelope of a set of elementary waves, formed by coalescence of those elementary wave-fronts, as "the termination" of the wave; and the elementary wave-fronts he terms "particular" waves. owing to the circumstance that the french word _rayon_ possesses the double signification of ray of light and radius of a circle, he avoids its use in the latter sense and speaks always of the semi-diameter, not of the radius. his speculations as to the ether, his suggestive views of the structure of crystalline bodies, and his explanation of opacity, slight as they are, will possibly surprise the reader by their seeming modernness. and none can read his investigation of the phenomena found in iceland spar without marvelling at his insight and sagacity. s.p.t. june, . table of matters contained in this treatise chapter i. on rays propagated in straight lines. that light is produced by a certain movement. that no substance passes from the luminous object to the eyes. that light spreads spherically, almost as sound does. whether light takes time to spread. experience seeming to prove that it passes instantaneously. experience proving that it takes time. how much its speed is greater than that of sound. in what the emission of light differs from that of sound. that it is not the same medium which serves for light and sound. how sound is propagated. how light is propagated. detailed remarks on the propagation of light. why rays are propagated only in straight lines. how light coming in different directions can cross itself. chapter ii. on reflexion. demonstration of equality of angles of incidence and reflexion. why the incident and reflected rays are in the same plane perpendicular to the reflecting surface. that it is not needful for the reflecting surface to be perfectly flat to attain equality of the angles of incidence and reflexion. chapter iii. on refraction. that bodies may be transparent without any substance passing through them. proof that the ethereal matter passes through transparent bodies. how this matter passing through can render them transparent. that the most solid bodies in appearance are of a very loose texture. that light spreads more slowly in water and in glass than in air. third hypothesis to explain transparency, and the retardation which light suffers. on that which makes bodies opaque. demonstration why refraction obeys the known proportion of sines. why the incident and refracted rays produce one another reciprocally. why reflexion within a triangular glass prism is suddenly augmented when the light can no longer penetrate. that bodies which cause greater refraction also cause stronger reflexion. demonstration of the theorem of mr. fermat. chapter iv. on the refraction of the air. that the emanations of light in the air are not spherical. how consequently some objects appear higher than they are. how the sun may appear on the horizon before he has risen. that the rays of light become curved in the air of the atmosphere, and what effects this produces. chapter v. on the strange refraction of iceland crystal. that this crystal grows also in other countries. who first-wrote about it. description of iceland crystal; its substance, shape, and properties. that it has two different refractions. that the ray perpendicular to the surface suffers refraction, and that some rays inclined to the surface pass without suffering refraction. observation of the refractions in this crystal. that there is a regular and an irregular refraction. the way of measuring the two refractions of iceland crystal. remarkable properties of the irregular refraction. hypothesis to explain the double refraction. that rock crystal has also a double refraction. hypothesis of emanations of light, within iceland crystal, of spheroidal form, for the irregular refraction. how a perpendicular ray can suffer refraction. how the position and form of the spheroidal emanations in this crystal can be defined. explanation of the irregular refraction by these spheroidal emanations. easy way to find the irregular refraction of each incident ray. demonstration of the oblique ray which traverses the crystal without being refracted. other irregularities of refraction explained. that an object placed beneath the crystal appears double, in two images of different heights. why the apparent heights of one of the images change on changing the position of the eyes above the crystal. of the different sections of this crystal which produce yet other refractions, and confirm all this theory. particular way of polishing the surfaces after it has been cut. surprising phenomenon touching the rays which pass through two separated pieces; the cause of which is not explained. probable conjecture on the internal composition of iceland crystal, and of what figure its particles are. tests to confirm this conjecture. calculations which have been supposed in this chapter. chapter vi. on the figures of transparent bodies which serve for refraction and for reflexion. general and easy rule to find these figures. invention of the ovals of mr. des cartes for dioptrics. how he was able to find these lines. way of finding the surface of a glass for perfect refraction, when the other surface is given. remark on what happens to rays refracted at a spherical surface. remark on the curved line which is formed by reflexion in a spherical concave mirror. chapter i on rays propagated in straight lines as happens in all the sciences in which geometry is applied to matter, the demonstrations concerning optics are founded on truths drawn from experience. such are that the rays of light are propagated in straight lines; that the angles of reflexion and of incidence are equal; and that in refraction the ray is bent according to the law of sines, now so well known, and which is no less certain than the preceding laws. the majority of those who have written touching the various parts of optics have contented themselves with presuming these truths. but some, more inquiring, have desired to investigate the origin and the causes, considering these to be in themselves wonderful effects of nature. in which they advanced some ingenious things, but not however such that the most intelligent folk do not wish for better and more satisfactory explanations. wherefore i here desire to propound what i have meditated on the subject, so as to contribute as much as i can to the explanation of this department of natural science, which, not without reason, is reputed to be one of its most difficult parts. i recognize myself to be much indebted to those who were the first to begin to dissipate the strange obscurity in which these things were enveloped, and to give us hope that they might be explained by intelligible reasoning. but, on the other hand i am astonished also that even here these have often been willing to offer, as assured and demonstrative, reasonings which were far from conclusive. for i do not find that any one has yet given a probable explanation of the first and most notable phenomena of light, namely why it is not propagated except in straight lines, and how visible rays, coming from an infinitude of diverse places, cross one another without hindering one another in any way. i shall therefore essay in this book, to give, in accordance with the principles accepted in the philosophy of the present day, some clearer and more probable reasons, firstly of these properties of light propagated rectilinearly; secondly of light which is reflected on meeting other bodies. then i shall explain the phenomena of those rays which are said to suffer refraction on passing through transparent bodies of different sorts; and in this part i shall also explain the effects of the refraction of the air by the different densities of the atmosphere. thereafter i shall examine the causes of the strange refraction of a certain kind of crystal which is brought from iceland. and finally i shall treat of the various shapes of transparent and reflecting bodies by which rays are collected at a point or are turned aside in various ways. from this it will be seen with what facility, following our new theory, we find not only the ellipses, hyperbolas, and other curves which mr. des cartes has ingeniously invented for this purpose; but also those which the surface of a glass lens ought to possess when its other surface is given as spherical or plane, or of any other figure that may be. it is inconceivable to doubt that light consists in the motion of some sort of matter. for whether one considers its production, one sees that here upon the earth it is chiefly engendered by fire and flame which contain without doubt bodies that are in rapid motion, since they dissolve and melt many other bodies, even the most solid; or whether one considers its effects, one sees that when light is collected, as by concave mirrors, it has the property of burning as a fire does, that is to say it disunites the particles of bodies. this is assuredly the mark of motion, at least in the true philosophy, in which one conceives the causes of all natural effects in terms of mechanical motions. this, in my opinion, we must necessarily do, or else renounce all hopes of ever comprehending anything in physics. and as, according to this philosophy, one holds as certain that the sensation of sight is excited only by the impression of some movement of a kind of matter which acts on the nerves at the back of our eyes, there is here yet one reason more for believing that light consists in a movement of the matter which exists between us and the luminous body. further, when one considers the extreme speed with which light spreads on every side, and how, when it comes from different regions, even from those directly opposite, the rays traverse one another without hindrance, one may well understand that when we see a luminous object, it cannot be by any transport of matter coming to us from this object, in the way in which a shot or an arrow traverses the air; for assuredly that would too greatly impugn these two properties of light, especially the second of them. it is then in some other way that light spreads; and that which can lead us to comprehend it is the knowledge which we have of the spreading of sound in the air. we know that by means of the air, which is an invisible and impalpable body, sound spreads around the spot where it has been produced, by a movement which is passed on successively from one part of the air to another; and that the spreading of this movement, taking place equally rapidly on all sides, ought to form spherical surfaces ever enlarging and which strike our ears. now there is no doubt at all that light also comes from the luminous body to our eyes by some movement impressed on the matter which is between the two; since, as we have already seen, it cannot be by the transport of a body which passes from one to the other. if, in addition, light takes time for its passage--which we are now going to examine--it will follow that this movement, impressed on the intervening matter, is successive; and consequently it spreads, as sound does, by spherical surfaces and waves: for i call them waves from their resemblance to those which are seen to be formed in water when a stone is thrown into it, and which present a successive spreading as circles, though these arise from another cause, and are only in a flat surface. to see then whether the spreading of light takes time, let us consider first whether there are any facts of experience which can convince us to the contrary. as to those which can be made here on the earth, by striking lights at great distances, although they prove that light takes no sensible time to pass over these distances, one may say with good reason that they are too small, and that the only conclusion to be drawn from them is that the passage of light is extremely rapid. mr. des cartes, who was of opinion that it is instantaneous, founded his views, not without reason, upon a better basis of experience, drawn from the eclipses of the moon; which, nevertheless, as i shall show, is not at all convincing. i will set it forth, in a way a little different from his, in order to make the conclusion more comprehensible. [illustration] let a be the place of the sun, bd a part of the orbit or annual path of the earth: abc a straight line which i suppose to meet the orbit of the moon, which is represented by the circle cd, at c. now if light requires time, for example one hour, to traverse the space which is between the earth and the moon, it will follow that the earth having arrived at b, the shadow which it casts, or the interruption of the light, will not yet have arrived at the point c, but will only arrive there an hour after. it will then be one hour after, reckoning from the moment when the earth was at b, that the moon, arriving at c, will be obscured: but this obscuration or interruption of the light will not reach the earth till after another hour. let us suppose that the earth in these two hours will have arrived at e. the earth then, being at e, will see the eclipsed moon at c, which it left an hour before, and at the same time will see the sun at a. for it being immovable, as i suppose with copernicus, and the light moving always in straight lines, it must always appear where it is. but one has always observed, we are told, that the eclipsed moon appears at the point of the ecliptic opposite to the sun; and yet here it would appear in arrear of that point by an amount equal to the angle gec, the supplement of aec. this, however, is contrary to experience, since the angle gec would be very sensible, and about degrees. now according to our computation, which is given in the treatise on the causes of the phenomena of saturn, the distance ba between the earth and the sun is about twelve thousand diameters of the earth, and hence four hundred times greater than bc the distance of the moon, which is diameters. then the angle ecb will be nearly four hundred times greater than bae, which is five minutes; namely, the path which the earth travels in two hours along its orbit; and thus the angle bce will be nearly degrees; and likewise the angle ceg, which is greater by five minutes. but it must be noted that the speed of light in this argument has been assumed such that it takes a time of one hour to make the passage from here to the moon. if one supposes that for this it requires only one minute of time, then it is manifest that the angle ceg will only be minutes; and if it requires only ten seconds of time, the angle will be less than six minutes. and then it will not be easy to perceive anything of it in observations of the eclipse; nor, consequently, will it be permissible to deduce from it that the movement of light is instantaneous. it is true that we are here supposing a strange velocity that would be a hundred thousand times greater than that of sound. for sound, according to what i have observed, travels about toises in the time of one second, or in about one beat of the pulse. but this supposition ought not to seem to be an impossibility; since it is not a question of the transport of a body with so great a speed, but of a successive movement which is passed on from some bodies to others. i have then made no difficulty, in meditating on these things, in supposing that the emanation of light is accomplished with time, seeing that in this way all its phenomena can be explained, and that in following the contrary opinion everything is incomprehensible. for it has always seemed tome that even mr. des cartes, whose aim has been to treat all the subjects of physics intelligibly, and who assuredly has succeeded in this better than any one before him, has said nothing that is not full of difficulties, or even inconceivable, in dealing with light and its properties. but that which i employed only as a hypothesis, has recently received great seemingness as an established truth by the ingenious proof of mr. römer which i am going here to relate, expecting him himself to give all that is needed for its confirmation. it is founded as is the preceding argument upon celestial observations, and proves not only that light takes time for its passage, but also demonstrates how much time it takes, and that its velocity is even at least six times greater than that which i have just stated. for this he makes use of the eclipses suffered by the little planets which revolve around jupiter, and which often enter his shadow: and see what is his reasoning. let a be the sun, bcde the annual orbit of the earth, f jupiter, gn the orbit of the nearest of his satellites, for it is this one which is more apt for this investigation than any of the other three, because of the quickness of its revolution. let g be this satellite entering into the shadow of jupiter, h the same satellite emerging from the shadow. [illustration] let it be then supposed, the earth being at b some time before the last quadrature, that one has seen the said satellite emerge from the shadow; it must needs be, if the earth remains at the same place, that, after - / hours, one would again see a similar emergence, because that is the time in which it makes the round of its orbit, and when it would come again into opposition to the sun. and if the earth, for instance, were to remain always at b during revolutions of this satellite, one would see it again emerge from the shadow after times - / hours. but the earth having been carried along during this time to c, increasing thus its distance from jupiter, it follows that if light requires time for its passage the illumination of the little planet will be perceived later at c than it would have been at b, and that there must be added to this time of times - / hours that which the light has required to traverse the space mc, the difference of the spaces ch, bh. similarly at the other quadrature when the earth has come to e from d while approaching toward jupiter, the immersions of the satellite ought to be observed at e earlier than they would have been seen if the earth had remained at d. now in quantities of observations of these eclipses, made during ten consecutive years, these differences have been found to be very considerable, such as ten minutes and more; and from them it has been concluded that in order to traverse the whole diameter of the annual orbit kl, which is double the distance from here to the sun, light requires about minutes of time. the movement of jupiter in his orbit while the earth passed from b to c, or from d to e, is included in this calculation; and this makes it evident that one cannot attribute the retardation of these illuminations or the anticipation of the eclipses, either to any irregularity occurring in the movement of the little planet or to its eccentricity. if one considers the vast size of the diameter kl, which according to me is some thousand diameters of the earth, one will acknowledge the extreme velocity of light. for, supposing that kl is no more than thousand of these diameters, it appears that being traversed in minutes this makes the speed a thousand diameters in one minute, that is - / diameters in one second or in one beat of the pulse, which makes more than hundred times a hundred thousand toises; since the diameter of the earth contains , leagues, reckoned at to the degree, and each each league is , toises, according to the exact measurement which mr. picard made by order of the king in . but sound, as i have said above, only travels toises in the same time of one second: hence the velocity of light is more than six hundred thousand times greater than that of sound. this, however, is quite another thing from being instantaneous, since there is all the difference between a finite thing and an infinite. now the successive movement of light being confirmed in this way, it follows, as i have said, that it spreads by spherical waves, like the movement of sound. but if the one resembles the other in this respect, they differ in many other things; to wit, in the first production of the movement which causes them; in the matter in which the movement spreads; and in the manner in which it is propagated. as to that which occurs in the production of sound, one knows that it is occasioned by the agitation undergone by an entire body, or by a considerable part of one, which shakes all the contiguous air. but the movement of the light must originate as from each point of the luminous object, else we should not be able to perceive all the different parts of that object, as will be more evident in that which follows. and i do not believe that this movement can be better explained than by supposing that all those of the luminous bodies which are liquid, such as flames, and apparently the sun and the stars, are composed of particles which float in a much more subtle medium which agitates them with great rapidity, and makes them strike against the particles of the ether which surrounds them, and which are much smaller than they. but i hold also that in luminous solids such as charcoal or metal made red hot in the fire, this same movement is caused by the violent agitation of the particles of the metal or of the wood; those of them which are on the surface striking similarly against the ethereal matter. the agitation, moreover, of the particles which engender the light ought to be much more prompt and more rapid than is that of the bodies which cause sound, since we do not see that the tremors of a body which is giving out a sound are capable of giving rise to light, even as the movement of the hand in the air is not capable of producing sound. now if one examines what this matter may be in which the movement coming from the luminous body is propagated, which i call ethereal matter, one will see that it is not the same that serves for the propagation of sound. for one finds that the latter is really that which we feel and which we breathe, and which being removed from any place still leaves there the other kind of matter that serves to convey light. this may be proved by shutting up a sounding body in a glass vessel from which the air is withdrawn by the machine which mr. boyle has given us, and with which he has performed so many beautiful experiments. but in doing this of which i speak, care must be taken to place the sounding body on cotton or on feathers, in such a way that it cannot communicate its tremors either to the glass vessel which encloses it, or to the machine; a precaution which has hitherto been neglected. for then after having exhausted all the air one hears no sound from the metal, though it is struck. one sees here not only that our air, which does not penetrate through glass, is the matter by which sound spreads; but also that it is not the same air but another kind of matter in which light spreads; since if the air is removed from the vessel the light does not cease to traverse it as before. and this last point is demonstrated even more clearly by the celebrated experiment of torricelli, in which the tube of glass from which the quicksilver has withdrawn itself, remaining void of air, transmits light just the same as when air is in it. for this proves that a matter different from air exists in this tube, and that this matter must have penetrated the glass or the quicksilver, either one or the other, though they are both impenetrable to the air. and when, in the same experiment, one makes the vacuum after putting a little water above the quicksilver, one concludes equally that the said matter passes through glass or water, or through both. as regards the different modes in which i have said the movements of sound and of light are communicated, one may sufficiently comprehend how this occurs in the case of sound if one considers that the air is of such a nature that it can be compressed and reduced to a much smaller space than that which it ordinarily occupies. and in proportion as it is compressed the more does it exert an effort to regain its volume; for this property along with its penetrability, which remains notwithstanding its compression, seems to prove that it is made up of small bodies which float about and which are agitated very rapidly in the ethereal matter composed of much smaller parts. so that the cause of the spreading of sound is the effort which these little bodies make in collisions with one another, to regain freedom when they are a little more squeezed together in the circuit of these waves than elsewhere. but the extreme velocity of light, and other properties which it has, cannot admit of such a propagation of motion, and i am about to show here the way in which i conceive it must occur. for this, it is needful to explain the property which hard bodies must possess to transmit movement from one to another. when one takes a number of spheres of equal size, made of some very hard substance, and arranges them in a straight line, so that they touch one another, one finds, on striking with a similar sphere against the first of these spheres, that the motion passes as in an instant to the last of them, which separates itself from the row, without one's being able to perceive that the others have been stirred. and even that one which was used to strike remains motionless with them. whence one sees that the movement passes with an extreme velocity which is the greater, the greater the hardness of the substance of the spheres. but it is still certain that this progression of motion is not instantaneous, but successive, and therefore must take time. for if the movement, or the disposition to movement, if you will have it so, did not pass successively through all these spheres, they would all acquire the movement at the same time, and hence would all advance together; which does not happen. for the last one leaves the whole row and acquires the speed of the one which was pushed. moreover there are experiments which demonstrate that all the bodies which we reckon of the hardest kind, such as quenched steel, glass, and agate, act as springs and bend somehow, not only when extended as rods but also when they are in the form of spheres or of other shapes. that is to say they yield a little in themselves at the place where they are struck, and immediately regain their former figure. for i have found that on striking with a ball of glass or of agate against a large and quite thick thick piece of the same substance which had a flat surface, slightly soiled with breath or in some other way, there remained round marks, of smaller or larger size according as the blow had been weak or strong. this makes it evident that these substances yield where they meet, and spring back: and for this time must be required. now in applying this kind of movement to that which produces light there is nothing to hinder us from estimating the particles of the ether to be of a substance as nearly approaching to perfect hardness and possessing a springiness as prompt as we choose. it is not necessary to examine here the causes of this hardness, or of that springiness, the consideration of which would lead us too far from our subject. i will say, however, in passing that we may conceive that the particles of the ether, notwithstanding their smallness, are in turn composed of other parts and that their springiness consists in the very rapid movement of a subtle matter which penetrates them from every side and constrains their structure to assume such a disposition as to give to this fluid matter the most overt and easy passage possible. this accords with the explanation which mr. des cartes gives for the spring, though i do not, like him, suppose the pores to be in the form of round hollow canals. and it must not be thought that in this there is anything absurd or impossible, it being on the contrary quite credible that it is this infinite series of different sizes of corpuscles, having different degrees of velocity, of which nature makes use to produce so many marvellous effects. but though we shall ignore the true cause of springiness we still see that there are many bodies which possess this property; and thus there is nothing strange in supposing that it exists also in little invisible bodies like the particles of the ether. also if one wishes to seek for any other way in which the movement of light is successively communicated, one will find none which agrees better, with uniform progression, as seems to be necessary, than the property of springiness; because if this movement should grow slower in proportion as it is shared over a greater quantity of matter, in moving away from the source of the light, it could not conserve this great velocity over great distances. but by supposing springiness in the ethereal matter, its particles will have the property of equally rapid restitution whether they are pushed strongly or feebly; and thus the propagation of light will always go on with an equal velocity. [illustration] and it must be known that although the particles of the ether are not ranged thus in straight lines, as in our row of spheres, but confusedly, so that one of them touches several others, this does not hinder them from transmitting their movement and from spreading it always forward. as to this it is to be remarked that there is a law of motion serving for this propagation, and verifiable by experiment. it is that when a sphere, such as a here, touches several other similar spheres ccc, if it is struck by another sphere b in such a way as to exert an impulse against all the spheres ccc which touch it, it transmits to them the whole of its movement, and remains after that motionless like the sphere b. and without supposing that the ethereal particles are of spherical form (for i see indeed no need to suppose them so) one may well understand that this property of communicating an impulse does not fail to contribute to the aforesaid propagation of movement. equality of size seems to be more necessary, because otherwise there ought to be some reflexion of movement backwards when it passes from a smaller particle to a larger one, according to the laws of percussion which i published some years ago. however, one will see hereafter that we have to suppose such an equality not so much as a necessity for the propagation of light as for rendering that propagation easier and more powerful; for it is not beyond the limits of probability that the particles of the ether have been made equal for a purpose so important as that of light, at least in that vast space which is beyond the region of atmosphere and which seems to serve only to transmit the light of the sun and the stars. i have then shown in what manner one may conceive light to spread successively, by spherical waves, and how it is possible that this spreading is accomplished with as great a velocity as that which experiments and celestial observations demand. whence it may be further remarked that although the particles are supposed to be in continual movement (for there are many reasons for this) the successive propagation of the waves cannot be hindered by this; because the propagation consists nowise in the transport of those particles but merely in a small agitation which they cannot help communicating to those surrounding, notwithstanding any movement which may act on them causing them to be changing positions amongst themselves. but we must consider still more particularly the origin of these waves, and the manner in which they spread. and, first, it follows from what has been said on the production of light, that each little region of a luminous body, such as the sun, a candle, or a burning coal, generates its own waves of which that region is the centre. thus in the flame of a candle, having distinguished the points a, b, c, concentric circles described about each of these points represent the waves which come from them. and one must imagine the same about every point of the surface and of the part within the flame. [illustration] but as the percussions at the centres of these waves possess no regular succession, it must not be supposed that the waves themselves follow one another at equal distances: and if the distances marked in the figure appear to be such, it is rather to mark the progression of one and the same wave at equal intervals of time than to represent several of them issuing from one and the same centre. after all, this prodigious quantity of waves which traverse one another without confusion and without effacing one another must not be deemed inconceivable; it being certain that one and the same particle of matter can serve for many waves coming from different sides or even from contrary directions, not only if it is struck by blows which follow one another closely but even for those which act on it at the same instant. it can do so because the spreading of the movement is successive. this may be proved by the row of equal spheres of hard matter, spoken of above. if against this row there are pushed from two opposite sides at the same time two similar spheres a and d, one will see each of them rebound with the same velocity which it had in striking, yet the whole row will remain in its place, although the movement has passed along its whole length twice over. and if these contrary movements happen to meet one another at the middle sphere, b, or at some other such as c, that sphere will yield and act as a spring at both sides, and so will serve at the same instant to transmit these two movements. [illustration] but what may at first appear full strange and even incredible is that the undulations produced by such small movements and corpuscles, should spread to such immense distances; as for example from the sun or from the stars to us. for the force of these waves must grow feeble in proportion as they move away from their origin, so that the action of each one in particular will without doubt become incapable of making itself felt to our sight. but one will cease to be astonished by considering how at a great distance from the luminous body an infinitude of waves, though they have issued from different points of this body, unite together in such a way that they sensibly compose one single wave only, which, consequently, ought to have enough force to make itself felt. thus this infinite number of waves which originate at the same instant from all points of a fixed star, big it may be as the sun, make practically only one single wave which may well have force enough to produce an impression on our eyes. moreover from each luminous point there may come many thousands of waves in the smallest imaginable time, by the frequent percussion of the corpuscles which strike the ether at these points: which further contributes to rendering their action more sensible. [illustration] there is the further consideration in the emanation of these waves, that each particle of matter in which a wave spreads, ought not to communicate its motion only to the next particle which is in the straight line drawn from the luminous point, but that it also imparts some of it necessarily to all the others which touch it and which oppose themselves to its movement. so it arises that around each particle there is made a wave of which that particle is the centre. thus if dcf is a wave emanating from the luminous point a, which is its centre, the particle b, one of those comprised within the sphere dcf, will have made its particular or partial wave kcl, which will touch the wave dcf at c at the same moment that the principal wave emanating from the point a has arrived at dcf; and it is clear that it will be only the region c of the wave kcl which will touch the wave dcf, to wit, that which is in the straight line drawn through ab. similarly the other particles of the sphere dcf, such as _bb_, _dd_, etc., will each make its own wave. but each of these waves can be infinitely feeble only as compared with the wave dcf, to the composition of which all the others contribute by the part of their surface which is most distant from the centre a. one sees, in addition, that the wave dcf is determined by the distance attained in a certain space of time by the movement which started from the point a; there being no movement beyond this wave, though there will be in the space which it encloses, namely in parts of the particular waves, those parts which do not touch the sphere dcf. and all this ought not to seem fraught with too much minuteness or subtlety, since we shall see in the sequel that all the properties of light, and everything pertaining to its reflexion and its refraction, can be explained in principle by this means. this is a matter which has been quite unknown to those who hitherto have begun to consider the waves of light, amongst whom are mr. hooke in his _micrographia_, and father pardies, who, in a treatise of which he let me see a portion, and which he was unable to complete as he died shortly afterward, had undertaken to prove by these waves the effects of reflexion and refraction. but the chief foundation, which consists in the remark i have just made, was lacking in his demonstrations; and for the rest he had opinions very different from mine, as may be will appear some day if his writing has been preserved. to come to the properties of light. we remark first that each portion of a wave ought to spread in such a way that its extremities lie always between the same straight lines drawn from the luminous point. thus the portion bg of the wave, having the luminous point a as its centre, will spread into the arc ce bounded by the straight lines abc, age. for although the particular waves produced by the particles comprised within the space cae spread also outside this space, they yet do not concur at the same instant to compose a wave which terminates the movement, as they do precisely at the circumference ce, which is their common tangent. and hence one sees the reason why light, at least if its rays are not reflected or broken, spreads only by straight lines, so that it illuminates no object except when the path from its source to that object is open along such lines. for if, for example, there were an opening bg, limited by opaque bodies bh, gi, the wave of light which issues from the point a will always be terminated by the straight lines ac, ae, as has just been shown; the parts of the partial waves which spread outside the space ace being too feeble to produce light there. now, however small we make the opening bg, there is always the same reason causing the light there to pass between straight lines; since this opening is always large enough to contain a great number of particles of the ethereal matter, which are of an inconceivable smallness; so that it appears that each little portion of the wave necessarily advances following the straight line which comes from the luminous point. thus then we may take the rays of light as if they were straight lines. it appears, moreover, by what has been remarked touching the feebleness of the particular waves, that it is not needful that all the particles of the ether should be equal amongst themselves, though equality is more apt for the propagation of the movement. for it is true that inequality will cause a particle by pushing against another larger one to strive to recoil with a part of its movement; but it will thereby merely generate backwards towards the luminous point some partial waves incapable of causing light, and not a wave compounded of many as ce was. another property of waves of light, and one of the most marvellous, is that when some of them come from different or even from opposing sides, they produce their effect across one another without any hindrance. whence also it comes about that a number of spectators may view different objects at the same time through the same opening, and that two persons can at the same time see one another's eyes. now according to the explanation which has been given of the action of light, how the waves do not destroy nor interrupt one another when they cross one another, these effects which i have just mentioned are easily conceived. but in my judgement they are not at all easy to explain according to the views of mr. des cartes, who makes light to consist in a continuous pressure merely tending to movement. for this pressure not being able to act from two opposite sides at the same time, against bodies which have no inclination to approach one another, it is impossible so to understand what i have been saying about two persons mutually seeing one another's eyes, or how two torches can illuminate one another. chapter ii on reflexion having explained the effects of waves of light which spread in a homogeneous matter, we will examine next that which happens to them on encountering other bodies. we will first make evident how the reflexion of light is explained by these same waves, and why it preserves equality of angles. let there be a surface ab; plane and polished, of some metal, glass, or other body, which at first i will consider as perfectly uniform (reserving to myself to deal at the end of this demonstration with the inequalities from which it cannot be exempt), and let a line ac, inclined to ad, represent a portion of a wave of light, the centre of which is so distant that this portion ac may be considered as a straight line; for i consider all this as in one plane, imagining to myself that the plane in which this figure is, cuts the sphere of the wave through its centre and intersects the plane ab at right angles. this explanation will suffice once for all. [illustration] the piece c of the wave ac, will in a certain space of time advance as far as the plane ab at b, following the straight line cb, which may be supposed to come from the luminous centre, and which in consequence is perpendicular to ac. now in this same space of time the portion a of the same wave, which has been hindered from communicating its movement beyond the plane ab, or at least partly so, ought to have continued its movement in the matter which is above this plane, and this along a distance equal to cb, making its own partial spherical wave, according to what has been said above. which wave is here represented by the circumference snr, the centre of which is a, and its semi-diameter an equal to cb. if one considers further the other pieces h of the wave ac, it appears that they will not only have reached the surface ab by straight lines hk parallel to cb, but that in addition they will have generated in the transparent air, from the centres k, k, k, particular spherical waves, represented here by circumferences the semi-diameters of which are equal to km, that is to say to the continuations of hk as far as the line bg parallel to ac. but all these circumferences have as a common tangent the straight line bn, namely the same which is drawn from b as a tangent to the first of the circles, of which a is the centre, and an the semi-diameter equal to bc, as is easy to see. it is then the line bn (comprised between b and the point n where the perpendicular from the point a falls) which is as it were formed by all these circumferences, and which terminates the movement which is made by the reflexion of the wave ac; and it is also the place where the movement occurs in much greater quantity than anywhere else. wherefore, according to that which has been explained, bn is the propagation of the wave ac at the moment when the piece c of it has arrived at b. for there is no other line which like bn is a common tangent to all the aforesaid circles, except bg below the plane ab; which line bg would be the propagation of the wave if the movement could have spread in a medium homogeneous with that which is above the plane. and if one wishes to see how the wave ac has come successively to bn, one has only to draw in the same figure the straight lines ko parallel to bn, and the straight lines kl parallel to ac. thus one will see that the straight wave ac has become broken up into all the okl parts successively, and that it has become straight again at nb. now it is apparent here that the angle of reflexion is made equal to the angle of incidence. for the triangles acb, bna being rectangular and having the side ab common, and the side cb equal to na, it follows that the angles opposite to these sides will be equal, and therefore also the angles cba, nab. but as cb, perpendicular to ca, marks the direction of the incident ray, so an, perpendicular to the wave bn, marks the direction of the reflected ray; hence these rays are equally inclined to the plane ab. but in considering the preceding demonstration, one might aver that it is indeed true that bn is the common tangent of the circular waves in the plane of this figure, but that these waves, being in truth spherical, have still an infinitude of similar tangents, namely all the straight lines which are drawn from the point b in the surface generated by the straight line bn about the axis ba. it remains, therefore, to demonstrate that there is no difficulty herein: and by the same argument one will see why the incident ray and the reflected ray are always in one and the same plane perpendicular to the reflecting plane. i say then that the wave ac, being regarded only as a line, produces no light. for a visible ray of light, however narrow it may be, has always some width, and consequently it is necessary, in representing the wave whose progression constitutes the ray, to put instead of a line ac some plane figure such as the circle hc in the following figure, by supposing, as we have done, the luminous point to be infinitely distant. now it is easy to see, following the preceding demonstration, that each small piece of this wave hc having arrived at the plane ab, and there generating each one its particular wave, these will all have, when c arrives at b, a common plane which will touch them, namely a circle bn similar to ch; and this will be intersected at its middle and at right angles by the same plane which likewise intersects the circle ch and the ellipse ab. [illustration] one sees also that the said spheres of the partial waves cannot have any common tangent plane other than the circle bn; so that it will be this plane where there will be more reflected movement than anywhere else, and which will therefore carry on the light in continuance from the wave ch. i have also stated in the preceding demonstration that the movement of the piece a of the incident wave is not able to communicate itself beyond the plane ab, or at least not wholly. whence it is to be remarked that though the movement of the ethereal matter might communicate itself partly to that of the reflecting body, this could in nothing alter the velocity of progression of the waves, on which the angle of reflexion depends. for a slight percussion ought to generate waves as rapid as strong percussion in the same matter. this comes about from the property of bodies which act as springs, of which we have spoken above; namely that whether compressed little or much they recoil in equal times. equally so in every reflexion of the light, against whatever body it may be, the angles of reflexion and incidence ought to be equal notwithstanding that the body might be of such a nature that it takes away a portion of the movement made by the incident light. and experiment shows that in fact there is no polished body the reflexion of which does not follow this rule. but the thing to be above all remarked in our demonstration is that it does not require that the reflecting surface should be considered as a uniform plane, as has been supposed by all those who have tried to explain the effects of reflexion; but only an evenness such as may be attained by the particles of the matter of the reflecting body being set near to one another; which particles are larger than those of the ethereal matter, as will appear by what we shall say in treating of the transparency and opacity of bodies. for the surface consisting thus of particles put together, and the ethereal particles being above, and smaller, it is evident that one could not demonstrate the equality of the angles of incidence and reflexion by similitude to that which happens to a ball thrown against a wall, of which writers have always made use. in our way, on the other hand, the thing is explained without difficulty. for the smallness of the particles of quicksilver, for example, being such that one must conceive millions of them, in the smallest visible surface proposed, arranged like a heap of grains of sand which has been flattened as much as it is capable of being, this surface then becomes for our purpose as even as a polished glass is: and, although it always remains rough with respect to the particles of the ether it is evident that the centres of all the particular spheres of reflexion, of which we have spoken, are almost in one uniform plane, and that thus the common tangent can fit to them as perfectly as is requisite for the production of light. and this alone is requisite, in our method of demonstration, to cause equality of the said angles without the remainder of the movement reflected from all parts being able to produce any contrary effect. chapter iii on refraction in the same way as the effects of reflexion have been explained by waves of light reflected at the surface of polished bodies, we will explain transparency and the phenomena of refraction by waves which spread within and across diaphanous bodies, both solids, such as glass, and liquids, such as water, oils, etc. but in order that it may not seem strange to suppose this passage of waves in the interior of these bodies, i will first show that one may conceive it possible in more than one mode. first, then, if the ethereal matter cannot penetrate transparent bodies at all, their own particles would be able to communicate successively the movement of the waves, the same as do those of the ether, supposing that, like those, they are of a nature to act as a spring. and this is easy to conceive as regards water and other transparent liquids, they being composed of detached particles. but it may seem more difficult as regards glass and other transparent and hard bodies, because their solidity does not seem to permit them to receive movement except in their whole mass at the same time. this, however, is not necessary because this solidity is not such as it appears to us, it being probable rather that these bodies are composed of particles merely placed close to one another and held together by some pressure from without of some other matter, and by the irregularity of their shapes. for primarily their rarity is shown by the facility with which there passes through them the matter of the vortices of the magnet, and that which causes gravity. further, one cannot say that these bodies are of a texture similar to that of a sponge or of light bread, because the heat of the fire makes them flow and thereby changes the situation of the particles amongst themselves. it remains then that they are, as has been said, assemblages of particles which touch one another without constituting a continuous solid. this being so, the movement which these particles receive to carry on the waves of light, being merely communicated from some of them to others, without their going for that purpose out of their places or without derangement, it may very well produce its effect without prejudicing in any way the apparent solidity of the compound. by pressure from without, of which i have spoken, must not be understood that of the air, which would not be sufficient, but that of some other more subtle matter, a pressure which i chanced upon by experiment long ago, namely in the case of water freed from air, which remains suspended in a tube open at its lower end, notwithstanding that the air has been removed from the vessel in which this tube is enclosed. one can then in this way conceive of transparency in a solid without any necessity that the ethereal matter which serves for light should pass through it, or that it should find pores in which to insinuate itself. but the truth is that this matter not only passes through solids, but does so even with great facility; of which the experiment of torricelli, above cited, is already a proof. because on the quicksilver and the water quitting the upper part of the glass tube, it appears that it is immediately filled with ethereal matter, since light passes across it. but here is another argument which proves this ready penetrability, not only in transparent bodies but also in all others. when light passes across a hollow sphere of glass, closed on all sides, it is certain that it is full of ethereal matter, as much as the spaces outside the sphere. and this ethereal matter, as has been shown above, consists of particles which just touch one another. if then it were enclosed in the sphere in such a way that it could not get out through the pores of the glass, it would be obliged to follow the movement of the sphere when one changes its place: and it would require consequently almost the same force to impress a certain velocity on this sphere, when placed on a horizontal plane, as if it were full of water or perhaps of quicksilver: because every body resists the velocity of the motion which one would give to it, in proportion to the quantity of matter which it contains, and which is obliged to follow this motion. but on the contrary one finds that the sphere resists the impress of movement only in proportion to the quantity of matter of the glass of which it is made. then it must be that the ethereal matter which is inside is not shut up, but flows through it with very great freedom. we shall demonstrate hereafter that by this process the same penetrability may be inferred also as relating to opaque bodies. the second mode then of explaining transparency, and one which appears more probably true, is by saying that the waves of light are carried on in the ethereal matter, which continuously occupies the interstices or pores of transparent bodies. for since it passes through them continuously and freely, it follows that they are always full of it. and one may even show that these interstices occupy much more space than the coherent particles which constitute the bodies. for if what we have just said is true: that force is required to impress a certain horizontal velocity on bodies in proportion as they contain coherent matter; and if the proportion of this force follows the law of weights, as is confirmed by experiment, then the quantity of the constituent matter of bodies also follows the proportion of their weights. now we see that water weighs only one fourteenth part as much as an equal portion of quicksilver: therefore the matter of the water does not occupy the fourteenth part of the space which its mass obtains. it must even occupy much less of it, since quicksilver is less heavy than gold, and the matter of gold is by no means dense, as follows from the fact that the matter of the vortices of the magnet and of that which is the cause of gravity pass very freely through it. but it may be objected here that if water is a body of so great rarity, and if its particles occupy so small a portion of the space of its apparent bulk, it is very strange how it yet resists compression so strongly without permitting itself to be condensed by any force which one has hitherto essayed to employ, preserving even its entire liquidity while subjected to this pressure. this is no small difficulty. it may, however, be resolved by saying that the very violent and rapid motion of the subtle matter which renders water liquid, by agitating the particles of which it is composed, maintains this liquidity in spite of the pressure which hitherto any one has been minded to apply to it. the rarity of transparent bodies being then such as we have said, one easily conceives that the waves might be carried on in the ethereal matter which fills the interstices of the particles. and, moreover, one may believe that the progression of these waves ought to be a little slower in the interior of bodies, by reason of the small detours which the same particles cause. in which different velocity of light i shall show the cause of refraction to consist. before doing so, i will indicate the third and last mode in which transparency may be conceived; which is by supposing that the movement of the waves of light is transmitted indifferently both in the particles of the ethereal matter which occupy the interstices of bodies, and in the particles which compose them, so that the movement passes from one to the other. and it will be seen hereafter that this hypothesis serves excellently to explain the double refraction of certain transparent bodies. should it be objected that if the particles of the ether are smaller than those of transparent bodies (since they pass through their intervals), it would follow that they can communicate to them but little of their movement, it may be replied that the particles of these bodies are in turn composed of still smaller particles, and so it will be these secondary particles which will receive the movement from those of the ether. furthermore, if the particles of transparent bodies have a recoil a little less prompt than that of the ethereal particles, which nothing hinders us from supposing, it will again follow that the progression of the waves of light will be slower in the interior of such bodies than it is outside in the ethereal matter. all this i have found as most probable for the mode in which the waves of light pass across transparent bodies. to which it must further be added in what respect these bodies differ from those which are opaque; and the more so since it might seem because of the easy penetration of bodies by the ethereal matter, of which mention has been made, that there would not be any body that was not transparent. for by the same reasoning about the hollow sphere which i have employed to prove the smallness of the density of glass and its easy penetrability by the ethereal matter, one might also prove that the same penetrability obtains for metals and for every other sort of body. for this sphere being for example of silver, it is certain that it contains some of the ethereal matter which serves for light, since this was there as well as in the air when the opening of the sphere was closed. yet, being closed and placed upon a horizontal plane, it resists the movement which one wishes to give to it, merely according to the quantity of silver of which it is made; so that one must conclude, as above, that the ethereal matter which is enclosed does not follow the movement of the sphere; and that therefore silver, as well as glass, is very easily penetrated by this matter. some of it is therefore present continuously and in quantities between the particles of silver and of all other opaque bodies: and since it serves for the propagation of light it would seem that these bodies ought also to be transparent, which however is not the case. whence then, one will say, does their opacity come? is it because the particles which compose them are soft; that is to say, these particles being composed of others that are smaller, are they capable of changing their figure on receiving the pressure of the ethereal particles, the motion of which they thereby damp, and so hinder the continuance of the waves of light? that cannot be: for if the particles of the metals are soft, how is it that polished silver and mercury reflect light so strongly? what i find to be most probable herein, is to say that metallic bodies, which are almost the only really opaque ones, have mixed amongst their hard particles some soft ones; so that some serve to cause reflexion and the others to hinder transparency; while, on the other hand, transparent bodies contain only hard particles which have the faculty of recoil, and serve together with those of the ethereal matter for the propagation of the waves of light, as has been said. [illustration] let us pass now to the explanation of the effects of refraction, assuming, as we have done, the passage of waves of light through transparent bodies, and the diminution of velocity which these same waves suffer in them. the chief property of refraction is that a ray of light, such as ab, being in the air, and falling obliquely upon the polished surface of a transparent body, such as fg, is broken at the point of incidence b, in such a way that with the straight line dbe which cuts the surface perpendicularly it makes an angle cbe less than abd which it made with the same perpendicular when in the air. and the measure of these angles is found by describing, about the point b, a circle which cuts the radii ab, bc. for the perpendiculars ad, ce, let fall from the points of intersection upon the straight line de, which are called the sines of the angles abd, cbe, have a certain ratio between themselves; which ratio is always the same for all inclinations of the incident ray, at least for a given transparent body. this ratio is, in glass, very nearly as to ; and in water very nearly as to ; and is likewise different in other diaphanous bodies. another property, similar to this, is that the refractions are reciprocal between the rays entering into a transparent body and those which are leaving it. that is to say that if the ray ab in entering the transparent body is refracted into bc, then likewise cb being taken as a ray in the interior of this body will be refracted, on passing out, into ba. [illustration] to explain then the reasons of these phenomena according to our principles, let ab be the straight line which represents a plane surface bounding the transparent substances which lie towards c and towards n. when i say plane, that does not signify a perfect evenness, but such as has been understood in treating of reflexion, and for the same reason. let the line ac represent a portion of a wave of light, the centre of which is supposed so distant that this portion may be considered as a straight line. the piece c, then, of the wave ac, will in a certain space of time have advanced as far as the plane ab following the straight line cb, which may be imagined as coming from the luminous centre, and which consequently will cut ac at right angles. now in the same time the piece a would have come to g along the straight line ag, equal and parallel to cb; and all the portion of wave ac would be at gb if the matter of the transparent body transmitted the movement of the wave as quickly as the matter of the ether. but let us suppose that it transmits this movement less quickly, by one-third, for instance. movement will then be spread from the point a, in the matter of the transparent body through a distance equal to two-thirds of cb, making its own particular spherical wave according to what has been said before. this wave is then represented by the circumference snr, the centre of which is a, and its semi-diameter equal to two-thirds of cb. then if one considers in order the other pieces h of the wave ac, it appears that in the same time that the piece c reaches b they will not only have arrived at the surface ab along the straight lines hk parallel to cb, but that, in addition, they will have generated in the diaphanous substance from the centres k, partial waves, represented here by circumferences the semi-diameters of which are equal to two-thirds of the lines km, that is to say, to two-thirds of the prolongations of hk down to the straight line bg; for these semi-diameters would have been equal to entire lengths of km if the two transparent substances had been of the same penetrability. now all these circumferences have for a common tangent the straight line bn; namely the same line which is drawn as a tangent from the point b to the circumference snr which we considered first. for it is easy to see that all the other circumferences will touch the same bn, from b up to the point of contact n, which is the same point where an falls perpendicularly on bn. it is then bn, which is formed by small arcs of these circumferences, which terminates the movement that the wave ac has communicated within the transparent body, and where this movement occurs in much greater amount than anywhere else. and for that reason this line, in accordance with what has been said more than once, is the propagation of the wave ac at the moment when its piece c has reached b. for there is no other line below the plane ab which is, like bn, a common tangent to all these partial waves. and if one would know how the wave ac has come progressively to bn, it is necessary only to draw in the same figure the straight lines ko parallel to bn, and all the lines kl parallel to ac. thus one will see that the wave ca, from being a straight line, has become broken in all the positions lko successively, and that it has again become a straight line at bn. this being evident by what has already been demonstrated, there is no need to explain it further. now, in the same figure, if one draws eaf, which cuts the plane ab at right angles at the point a, since ad is perpendicular to the wave ac, it will be da which will mark the ray of incident light, and an which was perpendicular to bn, the refracted ray: since the rays are nothing else than the straight lines along which the portions of the waves advance. whence it is easy to recognize this chief property of refraction, namely that the sine of the angle dae has always the same ratio to the sine of the angle naf, whatever be the inclination of the ray da: and that this ratio is the same as that of the velocity of the waves in the transparent substance which is towards ae to their velocity in the transparent substance towards af. for, considering ab as the radius of a circle, the sine of the angle bac is bc, and the sine of the angle abn is an. but the angle bac is equal to dae, since each of them added to cae makes a right angle. and the angle abn is equal to naf, since each of them with ban makes a right angle. then also the sine of the angle dae is to the sine of naf as bc is to an. but the ratio of bc to an was the same as that of the velocities of light in the substance which is towards ae and in that which is towards af; therefore also the sine of the angle dae will be to the sine of the angle naf the same as the said velocities of light. to see, consequently, what the refraction will be when the waves of light pass into a substance in which the movement travels more quickly than in that from which they emerge (let us again assume the ratio of to ), it is only necessary to repeat all the same construction and demonstration which we have just used, merely substituting everywhere / instead of / . and it will be found by the same reasoning, in this other figure, that when the piece c of the wave ac shall have reached the surface ab at b, all the portions of the wave ac will have advanced as far as bn, so that bc the perpendicular on ac is to an the perpendicular on bn as to . and there will finally be this same ratio of to between the sine of the angle bad and the sine of the angle fan. hence one sees the reciprocal relation of the refractions of the ray on entering and on leaving one and the same transparent body: namely that if na falling on the external surface ab is refracted into the direction ad, so the ray ad will be refracted on leaving the transparent body into the direction an. [illustration] one sees also the reason for a noteworthy accident which happens in this refraction: which is this, that after a certain obliquity of the incident ray da, it begins to be quite unable to penetrate into the other transparent substance. for if the angle daq or cba is such that in the triangle acb, cb is equal to / of ab, or is greater, then an cannot form one side of the triangle anb, since it becomes equal to or greater than ab: so that the portion of wave bn cannot be found anywhere, neither consequently can an, which ought to be perpendicular to it. and thus the incident ray da does not then pierce the surface ab. when the ratio of the velocities of the waves is as two to three, as in our example, which is that which obtains for glass and air, the angle daq must be more than degrees minutes in order that the ray da may be able to pass by refraction. and when the ratio of the velocities is as to , as it is very nearly in water and air, this angle daq must exceed degrees minutes. and this accords perfectly with experiment. but it might here be asked: since the meeting of the wave ac against the surface ab ought to produce movement in the matter which is on the other side, why does no light pass there? to which the reply is easy if one remembers what has been said before. for although it generates an infinitude of partial waves in the matter which is at the other side of ab, these waves never have a common tangent line (either straight or curved) at the same moment; and so there is no line terminating the propagation of the wave ac beyond the plane ab, nor any place where the movement is gathered together in sufficiently great quantity to produce light. and one will easily see the truth of this, namely that cb being larger than / of ab, the waves excited beyond the plane ab will have no common tangent if about the centres k one then draws circles having radii equal to / of the lengths lb to which they correspond. for all these circles will be enclosed in one another and will all pass beyond the point b. now it is to be remarked that from the moment when the angle daq is smaller than is requisite to permit the refracted ray da to pass into the other transparent substance, one finds that the interior reflexion which occurs at the surface ab is much augmented in brightness, as is easy to realize by experiment with a triangular prism; and for this our theory can afford this reason. when the angle daq is still large enough to enable the ray da to pass, it is evident that the light from the portion ac of the wave is collected in a minimum space when it reaches bn. it appears also that the wave bn becomes so much the smaller as the angle cba or daq is made less; until when the latter is diminished to the limit indicated a little previously, this wave bn is collected together always at one point. that is to say, that when the piece c of the wave ac has then arrived at b, the wave bn which is the propagation of ac is entirely reduced to the same point b. similarly when the piece h has reached k, the part ah is entirely reduced to the same point k. this makes it evident that in proportion as the wave ca comes to meet the surface ab, there occurs a great quantity of movement along that surface; which movement ought also to spread within the transparent body and ought to have much re-enforced the partial waves which produce the interior reflexion against the surface ab, according to the laws of reflexion previously explained. and because a slight diminution of the angle of incidence daq causes the wave bn, however great it was, to be reduced to zero, (for this angle being degrees minutes in the glass, the angle ban is still degrees minutes, and the same angle being reduced by one degree only the angle ban is reduced to zero, and so the wave bn reduced to a point) thence it comes about that the interior reflexion from being obscure becomes suddenly bright, so soon as the angle of incidence is such that it no longer gives passage to the refraction. now as concerns ordinary external reflexion, that is to say which occurs when the angle of incidence daq is still large enough to enable the refracted ray to penetrate beyond the surface ab, this reflexion should occur against the particles of the substance which touches the transparent body on its outside. and it apparently occurs against the particles of the air or others mingled with the ethereal particles and larger than they. so on the other hand the external reflexion of these bodies occurs against the particles which compose them, and which are also larger than those of the ethereal matter, since the latter flows in their interstices. it is true that there remains here some difficulty in those experiments in which this interior reflexion occurs without the particles of air being able to contribute to it, as in vessels or tubes from which the air has been extracted. experience, moreover, teaches us that these two reflexions are of nearly equal force, and that in different transparent bodies they are so much the stronger as the refraction of these bodies is the greater. thus one sees manifestly that the reflexion of glass is stronger than that of water, and that of diamond stronger than that of glass. i will finish this theory of refraction by demonstrating a remarkable proposition which depends on it; namely, that a ray of light in order to go from one point to another, when these points are in different media, is refracted in such wise at the plane surface which joins these two media that it employs the least possible time: and exactly the same happens in the case of reflexion against a plane surface. mr. fermat was the first to propound this property of refraction, holding with us, and directly counter to the opinion of mr. des cartes, that light passes more slowly through glass and water than through air. but he assumed besides this a constant ratio of sines, which we have just proved by these different degrees of velocity alone: or rather, what is equivalent, he assumed not only that the velocities were different but that the light took the least time possible for its passage, and thence deduced the constant ratio of the sines. his demonstration, which may be seen in his printed works, and in the volume of letters of mr. des cartes, is very long; wherefore i give here another which is simpler and easier. [illustration] let kf be the plane surface; a the point in the medium which the light traverses more easily, as the air; c the point in the other which is more difficult to penetrate, as water. and suppose that a ray has come from a, by b, to c, having been refracted at b according to the law demonstrated a little before; that is to say that, having drawn pbq, which cuts the plane at right angles, let the sine of the angle abp have to the sine of the angle cbq the same ratio as the velocity of light in the medium where a is to the velocity of light in the medium where c is. it is to be shown that the time of passage of light along ab and bc taken together, is the shortest that can be. let us assume that it may have come by other lines, and, in the first place, along af, fc, so that the point of refraction f may be further from b than the point a; and let ao be a line perpendicular to ab, and fo parallel to ab; bh perpendicular to fo, and fg to bc. since then the angle hbf is equal to pba, and the angle bfg equal to qbc, it follows that the sine of the angle hbf will also have the same ratio to the sine of bfg, as the velocity of light in the medium a is to its velocity in the medium c. but these sines are the straight lines hf, bg, if we take bf as the semi-diameter of a circle. then these lines hf, bg, will bear to one another the said ratio of the velocities. and, therefore, the time of the light along hf, supposing that the ray had been of, would be equal to the time along bg in the interior of the medium c. but the time along ab is equal to the time along oh; therefore the time along of is equal to the time along ab, bg. again the time along fc is greater than that along gc; then the time along ofc will be longer than that along abc. but af is longer than of, then the time along afc will by just so much more exceed the time along abc. now let us assume that the ray has come from a to c along ak, kc; the point of refraction k being nearer to a than the point b is; and let cn be the perpendicular upon bc, kn parallel to bc: bm perpendicular upon kn, and kl upon ba. here bl and km are the sines of angles bkl, kbm; that is to say, of the angles pba, qbc; and therefore they are to one another as the velocity of light in the medium a is to the velocity in the medium c. then the time along lb is equal to the time along km; and since the time along bc is equal to the time along mn, the time along lbc will be equal to the time along kmn. but the time along ak is longer than that along al: hence the time along akn is longer than that along abc. and kc being longer than kn, the time along akc will exceed, by as much more, the time along abc. hence it appears that the time along abc is the shortest possible; which was to be proven. chapter iv on the refraction of the air we have shown how the movement which constitutes light spreads by spherical waves in any homogeneous matter. and it is evident that when the matter is not homogeneous, but of such a constitution that the movement is communicated in it more rapidly toward one side than toward another, these waves cannot be spherical: but that they must acquire their figure according to the different distances over which the successive movement passes in equal times. it is thus that we shall in the first place explain the refractions which occur in the air, which extends from here to the clouds and beyond. the effects of which refractions are very remarkable; for by them we often see objects which the rotundity of the earth ought otherwise to hide; such as islands, and the tops of mountains when one is at sea. because also of them the sun and the moon appear as risen before in fact they have, and appear to set later: so that at times the moon has been seen eclipsed while the sun appeared still above the horizon. and so also the heights of the sun and of the moon, and those of all the stars always appear a little greater than they are in reality, because of these same refractions, as astronomers know. but there is one experiment which renders this refraction very evident; which is that of fixing a telescope on some spot so that it views an object, such as a steeple or a house, at a distance of half a league or more. if then you look through it at different hours of the day, leaving it always fixed in the same way, you will see that the same spots of the object will not always appear at the middle of the aperture of the telescope, but that generally in the morning and in the evening, when there are more vapours near the earth, these objects seem to rise higher, so that the half or more of them will no longer be visible; and so that they seem lower toward mid-day when these vapours are dissipated. those who consider refraction to occur only in the surfaces which separate transparent bodies of different nature, would find it difficult to give a reason for all that i have just related; but according to our theory the thing is quite easy. it is known that the air which surrounds us, besides the particles which are proper to it and which float in the ethereal matter as has been explained, is full also of particles of water which are raised by the action of heat; and it has been ascertained further by some very definite experiments that as one mounts up higher the density of air diminishes in proportion. now whether the particles of water and those of air take part, by means of the particles of ethereal matter, in the movement which constitutes light, but have a less prompt recoil than these, or whether the encounter and hindrance which these particles of air and water offer to the propagation of movement of the ethereal progress, retard the progression, it follows that both kinds of particles flying amidst the ethereal particles, must render the air, from a great height down to the earth, gradually less easy for the spreading of the waves of light. [illustration] whence the configuration of the waves ought to become nearly such as this figure represents: namely, if a is a light, or the visible point of a steeple, the waves which start from it ought to spread more widely upwards and less widely downwards, but in other directions more or less as they approximate to these two extremes. this being so, it necessarily follows that every line intersecting one of these waves at right angles will pass above the point a, always excepting the one line which is perpendicular to the horizon. [illustration] let bc be the wave which brings the light to the spectator who is at b, and let bd be the straight line which intersects this wave at right angles. now because the ray or straight line by which we judge the spot where the object appears to us is nothing else than the perpendicular to the wave that reaches our eye, as will be understood by what was said above, it is manifest that the point a will be perceived as being in the line bd, and therefore higher than in fact it is. similarly if the earth be ab, and the top of the atmosphere cd, which probably is not a well defined spherical surface (since we know that the air becomes rare in proportion as one ascends, for above there is so much less of it to press down upon it), the waves of light from the sun coming, for instance, in such a way that so long as they have not reached the atmosphere cd the straight line ae intersects them perpendicularly, they ought, when they enter the atmosphere, to advance more quickly in elevated regions than in regions nearer to the earth. so that if ca is the wave which brings the light to the spectator at a, its region c will be the furthest advanced; and the straight line af, which intersects this wave at right angles, and which determines the apparent place of the sun, will pass above the real sun, which will be seen along the line ae. and so it may occur that when it ought not to be visible in the absence of vapours, because the line ae encounters the rotundity of the earth, it will be perceived in the line af by refraction. but this angle eaf is scarcely ever more than half a degree because the attenuation of the vapours alters the waves of light but little. furthermore these refractions are not altogether constant in all weathers, particularly at small elevations of or degrees; which results from the different quantity of aqueous vapours rising above the earth. and this same thing is the cause why at certain times a distant object will be hidden behind another less distant one, and yet may at another time be able to be seen, although the spot from which it is viewed is always the same. but the reason for this effect will be still more evident from what we are going to remark touching the curvature of rays. it appears from the things explained above that the progression or propagation of a small part of a wave of light is properly what one calls a ray. now these rays, instead of being straight as they are in homogeneous media, ought to be curved in an atmosphere of unequal penetrability. for they necessarily follow from the object to the eye the line which intersects at right angles all the progressions of the waves, as in the first figure the line aeb does, as will be shown hereafter; and it is this line which determines what interposed bodies would or would not hinder us from seeing the object. for although the point of the steeple a appears raised to d, it would yet not appear to the eye b if the tower h was between the two, because it crosses the curve aeb. but the tower e, which is beneath this curve, does not hinder the point a from being seen. now according as the air near the earth exceeds in density that which is higher, the curvature of the ray aeb becomes greater: so that at certain times it passes above the summit e, which allows the point a to be perceived by the eye at b; and at other times it is intercepted by the same tower e which hides a from this same eye. [illustration] but to demonstrate this curvature of the rays conformably to all our preceding theory, let us imagine that ab is a small portion of a wave of light coming from the side c, which we may consider as a straight line. let us also suppose that it is perpendicular to the horizon, the portion b being nearer to the earth than the portion a; and that because the vapours are less hindering at a than at b, the particular wave which comes from the point a spreads through a certain space ad while the particular wave which starts from the point b spreads through a shorter space be; ad and be being parallel to the horizon. further, supposing the straight lines fg, hi, etc., to be drawn from an infinitude of points in the straight line ab and to terminate on the line de (which is straight or may be considered as such), let the different penetrabilities at the different heights in the air between a and b be represented by all these lines; so that the particular wave, originating from the point f, will spread across the space fg, and that from the point h across the space hi, while that from the point a spreads across the space ad. now if about the centres a, b, one describes the circles dk, el, which represent the spreading of the waves which originate from these two points, and if one draws the straight line kl which touches these two circles, it is easy to see that this same line will be the common tangent to all the other circles drawn about the centres f, h, etc.; and that all the points of contact will fall within that part of this line which is comprised between the perpendiculars ak, bl. then it will be the line kl which will terminate the movement of the particular waves originating from the points of the wave ab; and this movement will be stronger between the points kl, than anywhere else at the same instant, since an infinitude of circumferences concur to form this straight line; and consequently kl will be the propagation of the portion of wave ab, as has been said in explaining reflexion and ordinary refraction. now it appears that ak and bl dip down toward the side where the air is less easy to penetrate: for ak being longer than bl, and parallel to it, it follows that the lines ab and kl, being prolonged, would meet at the side l. but the angle k is a right angle: hence kab is necessarily acute, and consequently less than dab. if one investigates in the same way the progression of the portion of the wave kl, one will find that after a further time it has arrived at mn in such a manner that the perpendiculars km, ln, dip down even more than do ak, bl. and this suffices to show that the ray will continue along the curved line which intersects all the waves at right angles, as has been said. chapter v on the strange refraction of iceland crystal . there is brought from iceland, which is an island in the north sea, in the latitude of degrees, a kind of crystal or transparent stone, very remarkable for its figure and other qualities, but above all for its strange refractions. the causes of this have seemed to me to be worthy of being carefully investigated, the more so because amongst transparent bodies this one alone does not follow the ordinary rules with respect to rays of light. i have even been under some necessity to make this research, because the refractions of this crystal seemed to overturn our preceding explanation of regular refraction; which explanation, on the contrary, they strongly confirm, as will be seen after they have been brought under the same principle. in iceland are found great lumps of this crystal, some of which i have seen of or pounds. but it occurs also in other countries, for i have had some of the same sort which had been found in france near the town of troyes in champagne, and some others which came from the island of corsica, though both were less clear and only in little bits, scarcely capable of letting any effect of refraction be observed. . the first knowledge which the public has had about it is due to mr. erasmus bartholinus, who has given a description of iceland crystal and of its chief phenomena. but here i shall not desist from giving my own, both for the instruction of those who may not have seen his book, and because as respects some of these phenomena there is a slight difference between his observations and those which i have made: for i have applied myself with great exactitude to examine these properties of refraction, in order to be quite sure before undertaking to explain the causes of them. . as regards the hardness of this stone, and the property which it has of being easily split, it must be considered rather as a species of talc than of crystal. for an iron spike effects an entrance into it as easily as into any other talc or alabaster, to which it is equal in gravity. [illustration] . the pieces of it which are found have the figure of an oblique parallelepiped; each of the six faces being a parallelogram; and it admits of being split in three directions parallel to two of these opposed faces. even in such wise, if you will, that all the six faces are equal and similar rhombuses. the figure here added represents a piece of this crystal. the obtuse angles of all the parallelograms, as c, d, here, are angles of degrees minutes, and consequently the acute angles, such as a and b, are of degrees minutes. . of the solid angles there are two opposite to one another, such as c and e, which are each composed of three equal obtuse plane angles. the other six are composed of two acute angles and one obtuse. all that i have just said has been likewise remarked by mr. bartholinus in the aforesaid treatise; if we differ it is only slightly about the values of the angles. he recounts moreover some other properties of this crystal; to wit, that when rubbed against cloth it attracts straws and other light things as do amber, diamond, glass, and spanish wax. let a piece be covered with water for a day or more, the surface loses its natural polish. when aquafortis is poured on it it produces ebullition, especially, as i have found, if the crystal has been pulverized. i have also found by experiment that it may be heated to redness in the fire without being in anywise altered or rendered less transparent; but a very violent fire calcines it nevertheless. its transparency is scarcely less than that of water or of rock crystal, and devoid of colour. but rays of light pass through it in another fashion and produce those marvellous refractions the causes of which i am now going to try to explain; reserving for the end of this treatise the statement of my conjectures touching the formation and extraordinary configuration of this crystal. . in all other transparent bodies that we know there is but one sole and simple refraction; but in this substance there are two different ones. the effect is that objects seen through it, especially such as are placed right against it, appear double; and that a ray of sunlight, falling on one of its surfaces, parts itself into two rays and traverses the crystal thus. . it is again a general law in all other transparent bodies that the ray which falls perpendicularly on their surface passes straight on without suffering refraction, and that an oblique ray is always refracted. but in this crystal the perpendicular ray suffers refraction, and there are oblique rays which pass through it quite straight. [illustration] . but in order to explain these phenomena more particularly, let there be, in the first place, a piece abfe of the same crystal, and let the obtuse angle acb, one of the three which constitute the equilateral solid angle c, be divided into two equal parts by the straight line cg, and let it be conceived that the crystal is intersected by a plane which passes through this line and through the side cf, which plane will necessarily be perpendicular to the surface ab; and its section in the crystal will form a parallelogram gcfh. we will call this section the principal section of the crystal. . now if one covers the surface ab, leaving there only a small aperture at the point k, situated in the straight line cg, and if one exposes it to the sun, so that his rays face it perpendicularly above, then the ray ik will divide itself at the point k into two, one of which will continue to go on straight by kl, and the other will separate itself along the straight line km, which is in the plane gcfh, and which makes with kl an angle of about degrees minutes, tending from the side of the solid angle c; and on emerging from the other side of the crystal it will turn again parallel to jk, along mz. and as, in this extraordinary refraction, the point m is seen by the refracted ray mki, which i consider as going to the eye at i, it necessarily follows that the point l, by virtue of the same refraction, will be seen by the refracted ray lri, so that lr will be parallel to mk if the distance from the eye ki is supposed very great. the point l appears then as being in the straight line irs; but the same point appears also, by ordinary refraction, to be in the straight line ik, hence it is necessarily judged to be double. and similarly if l be a small hole in a sheet of paper or other substance which is laid against the crystal, it will appear when turned towards daylight as if there were two holes, which will seem the wider apart from one another the greater the thickness of the crystal. . again, if one turns the crystal in such wise that an incident ray no, of sunlight, which i suppose to be in the plane continued from gcfh, makes with gc an angle of degrees and minutes, and is consequently nearly parallel to the edge cf, which makes with fh an angle of degrees minutes, according to the calculation which i shall put at the end, it will divide itself at the point o into two rays, one of which will continue along op in a straight line with no, and will similarly pass out of the other side of the crystal without any refraction; but the other will be refracted and will go along oq. and it must be noted that it is special to the plane through gcf and to those which are parallel to it, that all incident rays which are in one of these planes continue to be in it after they have entered the crystal and have become double; for it is quite otherwise for rays in all other planes which intersect the crystal, as we shall see afterwards. . i recognized at first by these experiments and by some others that of the two refractions which the ray suffers in this crystal, there is one which follows the ordinary rules; and it is this to which the rays kl and oq belong. this is why i have distinguished this ordinary refraction from the other; and having measured it by exact observation, i found that its proportion, considered as to the sines of the angles which the incident and refracted rays make with the perpendicular, was very precisely that of to , as was found also by mr. bartholinus, and consequently much greater than that of rock crystal, or of glass, which is nearly to . [illustration] . the mode of making these observations exactly is as follows. upon a leaf of paper fixed on a thoroughly flat table there is traced a black line ab, and two others, ced and kml, which cut it at right angles and are more or less distant from one another according as it is desired to examine a ray that is more or less oblique. then place the crystal upon the intersection e so that the line ab concurs with that which bisects the obtuse angle of the lower surface, or with some line parallel to it. then by placing the eye directly above the line ab it will appear single only; and one will see that the portion viewed through the crystal and the portions which appear outside it, meet together in a straight line: but the line cd will appear double, and one can distinguish the image which is due to regular refraction by the circumstance that when one views it with both eyes it seems raised up more than the other, or again by the circumstance that, when the crystal is turned around on the paper, this image remains stationary, whereas the other image shifts and moves entirely around. afterwards let the eye be placed at i (remaining always in the plane perpendicular through ab) so that it views the image which is formed by regular refraction of the line cd making a straight line with the remainder of that line which is outside the crystal. and then, marking on the surface of the crystal the point h where the intersection e appears, this point will be directly above e. then draw back the eye towards o, keeping always in the plane perpendicular through ab, so that the image of the line cd, which is formed by ordinary refraction, may appear in a straight line with the line kl viewed without refraction; and then mark on the crystal the point n where the point of intersection e appears. . then one will know the length and position of the lines nh, em, and of he, which is the thickness of the crystal: which lines being traced separately upon a plan, and then joining ne and nm which cuts he at p, the proportion of the refraction will be that of en to np, because these lines are to one another as the sines of the angles nph, nep, which are equal to those which the incident ray on and its refraction ne make with the perpendicular to the surface. this proportion, as i have said, is sufficiently precisely as to , and is always the same for all inclinations of the incident ray. . the same mode of observation has also served me for examining the extraordinary or irregular refraction of this crystal. for, the point h having been found and marked, as aforesaid, directly above the point e, i observed the appearance of the line cd, which is made by the extraordinary refraction; and having placed the eye at q, so that this appearance made a straight line with the line kl viewed without refraction, i ascertained the triangles reh, res, and consequently the angles rsh, res, which the incident and the refracted ray make with the perpendicular. . but i found in this refraction that the ratio of fr to rs was not constant, like the ordinary refraction, but that it varied with the varying obliquity of the incident ray. . i found also that when qre made a straight line, that is, when the incident ray entered the crystal without being refracted (as i ascertained by the circumstance that then the point e viewed by the extraordinary refraction appeared in the line cd, as seen without refraction) i found, i say, then that the angle qrg was degrees minutes, as has been already remarked; and so it is not the ray parallel to the edge of the crystal, which crosses it in a straight line without being refracted, as mr. bartholinus believed, since that inclination is only degrees minutes, as was stated above. and this is to be noted, in order that no one may search in vain for the cause of the singular property of this ray in its parallelism to the edges mentioned. [illustration] . finally, continuing my observations to discover the nature of this refraction, i learned that it obeyed the following remarkable rule. let the parallelogram gcfh, made by the principal section of the crystal, as previously determined, be traced separately. i found then that always, when the inclinations of two rays which come from opposite sides, as vk, sk here, are equal, their refractions kx and kt meet the bottom line hf in such wise that points x and t are equally distant from the point m, where the refraction of the perpendicular ray ik falls; and this occurs also for refractions in other sections of this crystal. but before speaking of those, which have also other particular properties, we will investigate the causes of the phenomena which i have already reported. it was after having explained the refraction of ordinary transparent bodies by means of the spherical emanations of light, as above, that i resumed my examination of the nature of this crystal, wherein i had previously been unable to discover anything. . as there were two different refractions, i conceived that there were also two different emanations of waves of light, and that one could occur in the ethereal matter extending through the body of the crystal. which matter, being present in much larger quantity than is that of the particles which compose it, was alone capable of causing transparency, according to what has been explained heretofore. i attributed to this emanation of waves the regular refraction which is observed in this stone, by supposing these waves to be ordinarily of spherical form, and having a slower progression within the crystal than they have outside it; whence proceeds refraction as i have demonstrated. . as to the other emanation which should produce the irregular refraction, i wished to try what elliptical waves, or rather spheroidal waves, would do; and these i supposed would spread indifferently both in the ethereal matter diffused throughout the crystal and in the particles of which it is composed, according to the last mode in which i have explained transparency. it seemed to me that the disposition or regular arrangement of these particles could contribute to form spheroidal waves (nothing more being required for this than that the successive movement of light should spread a little more quickly in one direction than in the other) and i scarcely doubted that there were in this crystal such an arrangement of equal and similar particles, because of its figure and of its angles with their determinate and invariable measure. touching which particles, and their form and disposition, i shall, at the end of this treatise, propound my conjectures and some experiments which confirm them. . the double emission of waves of light, which i had imagined, became more probable to me after i had observed a certain phenomenon in the ordinary [rock] crystal, which occurs in hexagonal form, and which, because of this regularity, seems also to be composed of particles, of definite figure, and ranged in order. this was, that this crystal, as well as that from iceland, has a double refraction, though less evident. for having had cut from it some well polished prisms of different sections, i remarked in all, in viewing through them the flame of a candle or the lead of window panes, that everything appeared double, though with images not very distant from one another. whence i understood the reason why this substance, though so transparent, is useless for telescopes, when they have ever so little length. . now this double refraction, according to my theory hereinbefore established, seemed to demand a double emission of waves of light, both of them spherical (for both the refractions are regular) and those of one series a little slower only than the others. for thus the phenomenon is quite naturally explained, by postulating substances which serve as vehicle for these waves, as i have done in the case of iceland crystal. i had then less trouble after that in admitting two emissions of waves in one and the same body. and since it might have been objected that in composing these two kinds of crystal of equal particles of a certain figure, regularly piled, the interstices which these particles leave and which contain the ethereal matter would scarcely suffice to transmit the waves of light which i have localized there, i removed this difficulty by regarding these particles as being of a very rare texture, or rather as composed of other much smaller particles, between which the ethereal matter passes quite freely. this, moreover, necessarily follows from that which has been already demonstrated touching the small quantity of matter of which the bodies are built up. . supposing then these spheroidal waves besides the spherical ones, i began to examine whether they could serve to explain the phenomena of the irregular refraction, and how by these same phenomena i could determine the figure and position of the spheroids: as to which i obtained at last the desired success, by proceeding as follows. [illustration] . i considered first the effect of waves so formed, as respects the ray which falls perpendicularly on the flat surface of a transparent body in which they should spread in this manner. i took ab for the exposed region of the surface. and, since a ray perpendicular to a plane, and coming from a very distant source of light, is nothing else, according to the precedent theory, than the incidence of a portion of the wave parallel to that plane, i supposed the straight line rc, parallel and equal to ab, to be a portion of a wave of light, in which an infinitude of points such as rh_h_c come to meet the surface ab at the points ak_k_b. then instead of the hemispherical partial waves which in a body of ordinary refraction would spread from each of these last points, as we have above explained in treating of refraction, these must here be hemi-spheroids. the axes (or rather the major diameters) of these i supposed to be oblique to the plane ab, as is av the semi-axis or semi-major diameter of the spheroid svt, which represents the partial wave coming from the point a, after the wave rc has reached ab. i say axis or major diameter, because the same ellipse svt may be considered as the section of a spheroid of which the axis is az perpendicular to av. but, for the present, without yet deciding one or other, we will consider these spheroids only in those sections of them which make ellipses in the plane of this figure. now taking a certain space of time during which the wave svt has spread from a, it would needs be that from all the other points k_k_b there should proceed, in the same time, waves similar to svt and similarly situated. and the common tangent nq of all these semi-ellipses would be the propagation of the wave rc which fell on ab, and would be the place where this movement occurs in much greater amount than anywhere else, being made up of arcs of an infinity of ellipses, the centres of which are along the line ab. . now it appeared that this common tangent nq was parallel to ab, and of the same length, but that it was not directly opposite to it, since it was comprised between the lines an, bq, which are diameters of ellipses having a and b for centres, conjugate with respect to diameters which are not in the straight line ab. and in this way i comprehended, a matter which had seemed to me very difficult, how a ray perpendicular to a surface could suffer refraction on entering a transparent body; seeing that the wave rc, having come to the aperture ab, went on forward thence, spreading between the parallel lines an, bq, yet itself remaining always parallel to ab, so that here the light does not spread along lines perpendicular to its waves, as in ordinary refraction, but along lines cutting the waves obliquely. [illustration] . inquiring subsequently what might be the position and form of these spheroids in the crystal, i considered that all the six faces produced precisely the same refractions. taking, then, the parallelopiped afb, of which the obtuse solid angle c is contained between the three equal plane angles, and imagining in it the three principal sections, one of which is perpendicular to the face dc and passes through the edge cf, another perpendicular to the face bf passing through the edge ca, and the third perpendicular to the face af passing through the edge bc; i knew that the refractions of the incident rays belonging to these three planes were all similar. but there could be no position of the spheroid which would have the same relation to these three sections except that in which the axis was also the axis of the solid angle c. consequently i saw that the axis of this angle, that is to say the straight line which traversed the crystal from the point c with equal inclination to the edges cf, ca, cb was the line which determined the position of the axis of all the spheroidal waves which one imagined to originate from some point, taken within or on the surface of the crystal, since all these spheroids ought to be alike, and have their axes parallel to one another. . considering after this the plane of one of these three sections, namely that through gcf, the angle of which is degrees minutes, since the angle f was shown above to be degrees minutes; and, imagining a spheroidal wave about the centre c, i knew, because i have just explained it, that its axis must be in the same plane, the half of which axis i have marked cs in the next figure: and seeking by calculation (which will be given with others at the end of this discourse) the value of the angle cgs, i found it degrees minutes. [illustration] . to know from this the form of this spheroid, that is to say the proportion of the semi-diameters cs, cp, of its elliptical section, which are perpendicular to one another, i considered that the point m where the ellipse is touched by the straight line fh, parallel to cg, ought to be so situated that cm makes with the perpendicular cl an angle of degrees minutes; since, this being so, this ellipse satisfies what has been said about the refraction of the ray perpendicular to the surface cg, which is inclined to the perpendicular cl by the same angle. this, then, being thus disposed, and taking cm at , parts, i found by the calculation which will be given at the end, the semi-major diameter cp to be , , and the semi-axis cs to be , , the ratio of which numbers is very nearly to ; so that the spheroid was of the kind which resembles a compressed sphere, being generated by the revolution of an ellipse about its smaller diameter. i found also the value of cg the semi-diameter parallel to the tangent ml to be , . [illustration] . now passing to the investigation of the refractions which obliquely incident rays must undergo, according to our hypothesis of spheroidal waves, i saw that these refractions depended on the ratio between the velocity of movement of the light outside the crystal in the ether, and that within the crystal. for supposing, for example, this proportion to be such that while the light in the crystal forms the spheroid gsp, as i have just said, it forms outside a sphere the semi-diameter of which is equal to the line n which will be determined hereafter, the following is the way of finding the refraction of the incident rays. let there be such a ray rc falling upon the surface ck. make co perpendicular to rc, and across the angle kco adjust ok, equal to n and perpendicular to co; then draw ki, which touches the ellipse gsp, and from the point of contact i join ic, which will be the required refraction of the ray rc. the demonstration of this is, it will be seen, entirely similar to that of which we made use in explaining ordinary refraction. for the refraction of the ray rc is nothing else than the progression of the portion c of the wave co, continued in the crystal. now the portions h of this wave, during the time that o came to k, will have arrived at the surface ck along the straight lines h_x_, and will moreover have produced in the crystal around the centres _x_ some hemi-spheroidal partial waves similar to the hemi-spheroidal gsp_g_, and similarly disposed, and of which the major and minor diameters will bear the same proportions to the lines _xv_ (the continuations of the lines h_x_ up to kb parallel to co) that the diameters of the spheroid gsp_g_ bear to the line cb, or n. and it is quite easy to see that the common tangent of all these spheroids, which are here represented by ellipses, will be the straight line ik, which consequently will be the propagation of the wave co; and the point i will be that of the point c, conformably with that which has been demonstrated in ordinary refraction. now as to finding the point of contact i, it is known that one must find cd a third proportional to the lines ck, cg, and draw di parallel to cm, previously determined, which is the conjugate diameter to cg; for then, by drawing ki it touches the ellipse at i. . now as we have found ci the refraction of the ray rc, similarly one will find c_i_ the refraction of the ray _r_c, which comes from the opposite side, by making c_o_ perpendicular to _r_c and following out the rest of the construction as before. whence one sees that if the ray _r_c is inclined equally with rc, the line c_d_ will necessarily be equal to cd, because c_k_ is equal to ck, and c_g_ to cg. and in consequence i_i_ will be cut at e into equal parts by the line cm, to which di and _di_ are parallel. and because cm is the conjugate diameter to cg, it follows that _i_i will be parallel to _g_g. therefore if one prolongs the refracted rays ci, c_i_, until they meet the tangent ml at t and _t_, the distances mt, m_t_, will also be equal. and so, by our hypothesis, we explain perfectly the phenomenon mentioned above; to wit, that when there are two rays equally inclined, but coming from opposite sides, as here the rays rc, _rc_, their refractions diverge equally from the line followed by the refraction of the ray perpendicular to the surface, by considering these divergences in the direction parallel to the surface of the crystal. . to find the length of the line n, in proportion to cp, cs, cg, it must be determined by observations of the irregular refraction which occurs in this section of the crystal; and i find thus that the ratio of n to gc is just a little less than to . and having regard to some other observations and phenomena of which i shall speak afterwards, i put n at , parts, of which the semi-diameter cg is found to contain , , making this ratio to - / . now this proportion, which there is between the line n and cg, may be called the proportion of the refraction; similarly as in glass that of to , as will be manifest when i shall have explained a short process in the preceding way to find the irregular refractions. . supposing then, in the next figure, as previously, the surface of the crystal _g_g, the ellipse gp_g_, and the line n; and cm the refraction of the perpendicular ray fc, from which it diverges by degrees minutes. now let there be some other ray rc, the refraction of which must be found. about the centre c, with semi-diameter cg, let the circumference _g_rg be described, cutting the ray rc at r; and let rv be the perpendicular on cg. then as the line n is to cg let cv be to cd, and let di be drawn parallel to cm, cutting the ellipse _g_mg at i; then joining ci, this will be the required refraction of the ray rc. which is demonstrated thus. [illustration] let co be perpendicular to cr, and across the angle ocg let ok be adjusted, equal to n and perpendicular to co, and let there be drawn the straight line ki, which if it is demonstrated to be a tangent to the ellipse at i, it will be evident by the things heretofore explained that ci is the refraction of the ray rc. now since the angle rco is a right angle, it is easy to see that the right-angled triangles rcv, kco, are similar. as then, ck is to ko, so also is rc to cv. but ko is equal to n, and rc to cg: then as ck is to n so will cg be to cv. but as n is to cg, so, by construction, is cv to cd. then as ck is to cg so is cg to cd. and because di is parallel to cm, the conjugate diameter to cg, it follows that ki touches the ellipse at i; which remained to be shown. . one sees then that as there is in the refraction of ordinary media a certain constant proportion between the sines of the angles which the incident ray and the refracted ray make with the perpendicular, so here there is such a proportion between cv and cd or ie; that is to say between the sine of the angle which the incident ray makes with the perpendicular, and the horizontal intercept, in the ellipse, between the refraction of this ray and the diameter cm. for the ratio of cv to cd is, as has been said, the same as that of n to the semi-diameter cg. . i will add here, before passing away, that in comparing together the regular and irregular refraction of this crystal, there is this remarkable fact, that if abps be the spheroid by which light spreads in the crystal in a certain space of time (which spreading, as has been said, serves for the irregular refraction), then the inscribed sphere bvst is the extension in the same space of time of the light which serves for the regular refraction. [illustration] for we have stated before this, that the line n being the radius of a spherical wave of light in air, while in the crystal it spread through the spheroid abps, the ratio of n to cs will be , to , . but it has also been stated that the proportion of the regular refraction was to ; that is to say, that n being the radius of a spherical wave of light in air, its extension in the crystal would, in the same space of time, form a sphere the radius of which would be to n as to . now , is to , as to less / . so that it is sufficiently nearly, and may be exactly, the sphere bvst, which the light describes for the regular refraction in the crystal, while it describes the spheroid bpsa for the irregular refraction, and while it describes the sphere of radius n in air outside the crystal. although then there are, according to what we have supposed, two different propagations of light within the crystal, it appears that it is only in directions perpendicular to the axis bs of the spheroid that one of these propagations occurs more rapidly than the other; but that they have an equal velocity in the other direction, namely, in that parallel to the same axis bs, which is also the axis of the obtuse angle of the crystal. [illustration] . the proportion of the refraction being what we have just seen, i will now show that there necessarily follows thence that notable property of the ray which falling obliquely on the surface of the crystal enters it without suffering refraction. for supposing the same things as before, and that the ray makes with the same surface _g_g the angle rcg of degrees minutes, inclining to the same side as the crystal (of which ray mention has been made above); if one investigates, by the process above explained, the refraction ci, one will find that it makes exactly a straight line with rc, and that thus this ray is not deviated at all, conformably with experiment. this is proved as follows by calculation. cg or cr being, as precedently, , ; cm being , ; and the angle rcv degrees minutes, cv will be , . but because ci is the refraction of the ray rc, the proportion of cv to cd is , to , , namely, that of n to cg; then cd is , . now the rectangle _g_dc is to the square of di as the square of cg is to the square of cm; hence di or ce will be , . but as ce is to ei, so will cm be to mt, which will then be , . and being added to ml, which is , (namely the sine of the angle lcm, which is degrees minutes, taking cm , as radius) we get lt , ; and this is to lc , as cv to vr, that is to say, as , , the tangent of the complement of the angle rcv, which is degrees minutes, is to the radius of the tables. whence it appears that rcit is a straight line; which was to be proved. . further it will be seen that the ray ci in emerging through the opposite surface of the crystal, ought to pass out quite straight, according to the following demonstration, which proves that the reciprocal relation of refraction obtains in this crystal the same as in other transparent bodies; that is to say, that if a ray rc in meeting the surface of the crystal cg is refracted as ci, the ray ci emerging through the opposite parallel surface of the crystal, which i suppose to be ib, will have its refraction ia parallel to the ray rc. [illustration] let the same things be supposed as before; that is to say, let co, perpendicular to cr, represent a portion of a wave the continuation of which in the crystal is ik, so that the piece c will be continued on along the straight line ci, while o comes to k. now if one takes a second period of time equal to the first, the piece k of the wave ik will, in this second period, have advanced along the straight line kb, equal and parallel to ci, because every piece of the wave co, on arriving at the surface ck, ought to go on in the crystal the same as the piece c; and in this same time there will be formed in the air from the point i a partial spherical wave having a semi-diameter ia equal to ko, since ko has been traversed in an equal time. similarly, if one considers some other point of the wave ik, such as _h_, it will go along _hm_, parallel to ci, to meet the surface ib, while the point k traverses k_l_ equal to _hm_; and while this accomplishes the remainder _l_b, there will start from the point _m_ a partial wave the semi-diameter of which, _mn_, will have the same ratio to _l_b as ia to kb. whence it is evident that this wave of semi-diameter _mn_, and the other of semi-diameter ia will have the same tangent ba. and similarly for all the partial spherical waves which will be formed outside the crystal by the impact of all the points of the wave ik against the surface of the ether ib. it is then precisely the tangent ba which will be the continuation of the wave ik, outside the crystal, when the piece k has reached b. and in consequence ia, which is perpendicular to ba, will be the refraction of the ray ci on emerging from the crystal. now it is clear that ia is parallel to the incident ray rc, since ib is equal to ck, and ia equal to ko, and the angles a and o are right angles. it is seen then that, according to our hypothesis, the reciprocal relation of refraction holds good in this crystal as well as in ordinary transparent bodies; as is thus in fact found by observation. . i pass now to the consideration of other sections of the crystal, and of the refractions there produced, on which, as will be seen, some other very remarkable phenomena depend. let abh be a parallelepiped of crystal, and let the top surface aehf be a perfect rhombus, the obtuse angles of which are equally divided by the straight line ef, and the acute angles by the straight line ah perpendicular to fe. the section which we have hitherto considered is that which passes through the lines ef, eb, and which at the same time cuts the plane aehf at right angles. refractions in this section have this in common with the refractions in ordinary media that the plane which is drawn through the incident ray and which also intersects the surface of the crystal at right angles, is that in which the refracted ray also is found. but the refractions which appertain to every other section of this crystal have this strange property that the refracted ray always quits the plane of the incident ray perpendicular to the surface, and turns away towards the side of the slope of the crystal. for which fact we shall show the reason, in the first place, for the section through ah; and we shall show at the same time how one can determine the refraction, according to our hypothesis. let there be, then, in the plane which passes through ah, and which is perpendicular to the plane afhe, the incident ray rc; it is required to find its refraction in the crystal. [illustration] . about the centre c, which i suppose to be in the intersection of ah and fe, let there be imagined a hemi-spheroid qg_qg_m, such as the light would form in spreading in the crystal, and let its section by the plane aehf form the ellipse qg_qg_, the major diameter of which q_q_, which is in the line ah, will necessarily be one of the major diameters of the spheroid; because the axis of the spheroid being in the plane through feb, to which qc is perpendicular, it follows that qc is also perpendicular to the axis of the spheroid, and consequently qc_q_ one of its major diameters. but the minor diameter of this ellipse, g_g_, will bear to q_q_ the proportion which has been defined previously, article , between cg and the major semi-diameter of the spheroid, cp, namely, that of , to , . let the line n be the length of the travel of light in air during the time in which, within the crystal, it makes, from the centre c, the spheroid qc_qg_m. then having drawn co perpendicular to the ray cr and situate in the plane through cr and ah, let there be adjusted, across the angle aco, the straight line ok equal to n and perpendicular to co, and let it meet the straight line ah at k. supposing consequently that cl is perpendicular to the surface of the crystal aehf, and that cm is the refraction of the ray which falls perpendicularly on this same surface, let there be drawn a plane through the line cm and through kch, making in the spheroid the semi-ellipse qm_q_, which will be given, since the angle mcl is given of value degrees minutes. and it is certain, according to what has been explained above, article , that a plane which would touch the spheroid at the point m, where i suppose the straight line cm to meet the surface, would be parallel to the plane qg_q_. if then through the point k one now draws ks parallel to g_g_, which will be parallel also to qx, the tangent to the ellipse qg_q_ at q; and if one conceives a plane passing through ks and touching the spheroid, the point of contact will necessarily be in the ellipse qm_q_, because this plane through ks, as well as the plane which touches the spheroid at the point m, are parallel to qx, the tangent of the spheroid: for this consequence will be demonstrated at the end of this treatise. let this point of contact be at i, then making kc, qc, dc proportionals, draw di parallel to cm; also join ci. i say that ci will be the required refraction of the ray rc. this will be manifest if, in considering co, which is perpendicular to the ray rc, as a portion of the wave of light, we can demonstrate that the continuation of its piece c will be found in the crystal at i, when o has arrived at k. . now as in the chapter on reflexion, in demonstrating that the incident and reflected rays are always in the same plane perpendicular to the reflecting surface, we considered the breadth of the wave of light, so, similarly, we must here consider the breadth of the wave co in the diameter g_g_. taking then the breadth c_c_ on the side toward the angle e, let the parallelogram co_oc_ be taken as a portion of a wave, and let us complete the parallelograms ck_kc_, ci_ic_, kl_ik_, ok_ko_. in the time then that the line o_o_ arrives at the surface of the crystal at k_k_, all the points of the wave co_oc_ will have arrived at the rectangle k_c_ along lines parallel to ok; and from the points of their incidences there will originate, beyond that, in the crystal partial hemi-spheroids, similar to the hemi-spheroid qm_q_, and similarly disposed. these hemi-spheroids will necessarily all touch the plane of the parallelogram ki_ik_ at the same instant that o_o_ has reached k_k_. which is easy to comprehend, since, of these hemi-spheroids, all those which have their centres along the line ck, touch this plane in the line ki (for this is to be shown in the same way as we have demonstrated the refraction of the oblique ray in the principal section through ef) and all those which have their centres in the line c_c_ will touch the same plane ki in the line i_i_; all these being similar to the hemi-spheroid qm_q_. since then the parallelogram k_i_ is that which touches all these spheroids, this same parallelogram will be precisely the continuation of the wave co_oc_ in the crystal, when o_o_ has arrived at k_k_, because it forms the termination of the movement and because of the quantity of movement which occurs more there than anywhere else: and thus it appears that the piece c of the wave co_oc_ has its continuation at i; that is to say, that the ray rc is refracted as ci. from this it is to be noted that the proportion of the refraction for this section of the crystal is that of the line n to the semi-diameter cq; by which one will easily find the refractions of all incident rays, in the same way as we have shown previously for the case of the section through fe; and the demonstration will be the same. but it appears that the said proportion of the refraction is less here than in the section through feb; for it was there the same as the ratio of n to cg, that is to say, as , to , , very nearly as to ; and here it is the ratio of n to cq the major semi-diameter of the spheroid, that is to say, as , to , , very nearly as to , but just a little less. which still agrees perfectly with what one finds by observation. . for the rest, this diversity of proportion of refraction produces a very singular effect in this crystal; which is that when it is placed upon a sheet of paper on which there are letters or anything else marked, if one views it from above with the two eyes situated in the plane of the section through ef, one sees the letters raised up by this irregular refraction more than when one puts one's eyes in the plane of section through ah: and the difference of these elevations appears by comparison with the other ordinary refraction of the crystal, the proportion of which is as to , and which always raises the letters equally, and higher than the irregular refraction does. for one sees the letters and the paper on which they are written, as on two different stages at the same time; and in the first position of the eyes, namely, when they are in the plane through ah these two stages are four times more distant from one another than when the eyes are in the plane through ef. we will show that this effect follows from the refractions; and it will enable us at the same time to ascertain the apparent place of a point of an object placed immediately under the crystal, according to the different situation of the eyes. . let us see first by how much the irregular refraction of the plane through ah ought to lift the bottom of the crystal. let the plane of this figure represent separately the section through q_q_ and cl, in which section there is also the ray rc, and let the semi-elliptic plane through q_q_ and cm be inclined to the former, as previously, by an angle of degrees minutes; and in this plane ci is then the refraction of the ray rc. [illustration] if now one considers the point i as at the bottom of the crystal, and that it is viewed by the rays icr, _icr_, refracted equally at the points c_c_, which should be equally distant from d, and that these rays meet the two eyes at r_r_; it is certain that the point i will appear raised to s where the straight lines rc, _rc_, meet; which point s is in dp, perpendicular to q_q_. and if upon dp there is drawn the perpendicular ip, which will lie at the bottom of the crystal, the length sp will be the apparent elevation of the point i above the bottom. let there be described on q_q_ a semicircle cutting the ray cr at b, from which bv is drawn perpendicular to q_q_; and let the proportion of the refraction for this section be, as before, that of the line n to the semi-diameter cq. then as n is to cq so is vc to cd, as appears by the method of finding the refraction which we have shown above, article ; but as vc is to cd, so is vb to ds. then as n is to cq, so is vb to ds. let ml be perpendicular to cl. and because i suppose the eyes r_r_ to be distant about a foot or so from the crystal, and consequently the angle rs_r_ very small, vb may be considered as equal to the semi-diameter cq, and dp as equal to cl; then as n is to cq so is cq to ds. but n is valued at , parts, of which cm contains , and cq , . then ds will have , . but cl is , , being the sine of the complement of the angle mcl which is degrees minutes; cm being supposed as radius. then dp, considered as equal to cl, will be to ds as , to , . and so the elevation of the point i by the refraction of this section is known. [illustration] . now let there be represented the other section through ef in the figure before the preceding one; and let cm_g_ be the semi-ellipse, considered in articles and , which is made by cutting a spheroidal wave having centre c. let the point i, taken in this ellipse, be imagined again at the bottom of the crystal; and let it be viewed by the refracted rays icr, i_cr_, which go to the two eyes; cr and _cr_ being equally inclined to the surface of the crystal g_g_. this being so, if one draws id parallel to cm, which i suppose to be the refraction of the perpendicular ray incident at the point c, the distances dc, d_c_, will be equal, as is easy to see by that which has been demonstrated in article . now it is certain that the point i should appear at s where the straight lines rc, _rc_, meet when prolonged; and that this point will fall in the line dp perpendicular to g_g_. if one draws ip perpendicular to this dp, it will be the distance ps which will mark the apparent elevation of the point i. let there be described on g_g_ a semicircle cutting cr at b, from which let bv be drawn perpendicular to g_g_; and let n to gc be the proportion of the refraction in this section, as in article . since then ci is the refraction of the radius bc, and di is parallel to cm, vc must be to cd as n to gc, according to what has been demonstrated in article . but as vc is to cd so is bv to ds. let ml be drawn perpendicular to cl. and because i consider, again, the eyes to be distant above the crystal, bv is deemed equal to the semi-diameter cg; and hence ds will be a third proportional to the lines n and cg: also dp will be deemed equal to cl. now cg consisting of , parts, of which cm contains , , n is taken as , . then ds will be , . but cl is also determined, and contains , parts, as has been said in articles and . then the ratio of pd to ds will be as , to , . and thus one knows the elevation of the point at the bottom i by the refraction of this section; and it appears that this elevation is greater than that by the refraction of the preceding section, since the ratio of pd to ds was there as , to , . [illustration] but by the regular refraction of the crystal, of which we have above said that the proportion is to , the elevation of the point i, or p, from the bottom, will be / of the height dp; as appears by this figure, where the point p being viewed by the rays pcr, p_cr_, refracted equally at the surface c_c_, this point must needs appear to be at s, in the perpendicular pd where the lines rc, _rc_, meet when prolonged: and one knows that the line pc is to cs as to , since they are to one another as the sine of the angle csp or dsc is to the sine of the angle spc. and because the ratio of pd to ds is deemed the same as that of pc to cs, the two eyes rr being supposed very far above the crystal, the elevation ps will thus be / of pd. [illustration] . if one takes a straight line ab for the thickness of the crystal, its point b being at the bottom, and if one divides it at the points c, d, e, according to the proportions of the elevations found, making ae / of ab, ab to ac as , to , , and ab to ad as , to , , these points will divide ab as in this figure. and it will be found that this agrees perfectly with experiment; that is to say by placing the eyes above in the plane which cuts the crystal according to the shorter diameter of the rhombus, the regular refraction will lift up the letters to e; and one will see the bottom, and the letters over which it is placed, lifted up to d by the irregular refraction. but by placing the eyes above in the plane which cuts the crystal according to the longer diameter of the rhombus, the regular refraction will lift the letters to e as before; but the irregular refraction will make them, at the same time, appear lifted up only to c; and in such a way that the interval ce will be quadruple the interval ed, which one previously saw. . i have only to make the remark here that in both the positions of the eyes the images caused by the irregular refraction do not appear directly below those which proceed from the regular refraction, but they are separated from them by being more distant from the equilateral solid angle of the crystal. that follows, indeed, from all that has been hitherto demonstrated about the irregular refraction; and it is particularly shown by these last demonstrations, from which one sees that the point i appears by irregular refraction at s in the perpendicular line dp, in which line also the image of the point p ought to appear by regular refraction, but not the image of the point i, which will be almost directly above the same point, and higher than s. but as to the apparent elevation of the point i in other positions of the eyes above the crystal, besides the two positions which we have just examined, the image of that point by the irregular refraction will always appear between the two heights of d and c, passing from one to the other as one turns one's self around about the immovable crystal, while looking down from above. and all this is still found conformable to our hypothesis, as any one can assure himself after i shall have shown here the way of finding the irregular refractions which appear in all other sections of the crystal, besides the two which we have considered. let us suppose one of the faces of the crystal, in which let there be the ellipse hde, the centre c of which is also the centre of the spheroid hme in which the light spreads, and of which the said ellipse is the section. and let the incident ray be rc, the refraction of which it is required to find. let there be taken a plane passing through the ray rc and which is perpendicular to the plane of the ellipse hde, cutting it along the straight line bck; and having in the same plane through rc made co perpendicular to cr, let ok be adjusted across the angle ock, so as to be perpendicular to oc and equal to the line n, which i suppose to measure the travel of the light in air during the time that it spreads in the crystal through the spheroid hdem. then in the plane of the ellipse hde let kt be drawn, through the point k, perpendicular to bck. now if one conceives a plane drawn through the straight line kt and touching the spheroid hme at i, the straight line ci will be the refraction of the ray rc, as is easy to deduce from that which has been demonstrated in article . [illustration] but it must be shown how one can determine the point of contact i. let there be drawn parallel to the line kt a line hf which touches the ellipse hde, and let this point of contact be at h. and having drawn a straight line along ch to meet kt at t, let there be imagined a plane passing through the same ch and through cm (which i suppose to be the refraction of the perpendicular ray), which makes in the spheroid the elliptical section hme. it is certain that the plane which will pass through the straight line kt, and which will touch the spheroid, will touch it at a point in the ellipse hme, according to the lemma which will be demonstrated at the end of the chapter. now this point is necessarily the point i which is sought, since the plane drawn through tk can touch the spheroid at one point only. and this point i is easy to determine, since it is needful only to draw from the point t, which is in the plane of this ellipse, the tangent ti, in the way shown previously. for the ellipse hme is given, and its conjugate semi-diameters are ch and cm; because a straight line drawn through m, parallel to he, touches the ellipse hme, as follows from the fact that a plane taken through m, and parallel to the plane hde, touches the spheroid at that point m, as is seen from articles and . for the rest, the position of this ellipse, with respect to the plane through the ray rc and through ck, is also given; from which it will be easy to find the position of ci, the refraction corresponding to the ray rc. now it must be noted that the same ellipse hme serves to find the refractions of any other ray which may be in the plane through rc and ck. because every plane, parallel to the straight line hf, or tk, which will touch the spheroid, will touch it in this ellipse, according to the lemma quoted a little before. i have investigated thus, in minute detail, the properties of the irregular refraction of this crystal, in order to see whether each phenomenon that is deduced from our hypothesis accords with that which is observed in fact. and this being so it affords no slight proof of the truth of our suppositions and principles. but what i am going to add here confirms them again marvellously. it is this: that there are different sections of this crystal, the surfaces of which, thereby produced, give rise to refractions precisely such as they ought to be, and as i had foreseen them, according to the preceding theory. in order to explain what these sections are, let abkf _be_ the principal section through the axis of the crystal ack, in which there will also be the axis ss of a spheroidal wave of light spreading in the crystal from the centre c; and the straight line which cuts ss through the middle and at right angles, namely pp, will be one of the major diameters. [illustration: {section abkf}] now as in the natural section of the crystal, made by a plane parallel to two opposite faces, which plane is here represented by the line gg, the refraction of the surfaces which are produced by it will be governed by the hemi-spheroids gng, according to what has been explained in the preceding theory. similarly, cutting the crystal through nn, by a plane perpendicular to the parallelogram abkf, the refraction of the surfaces will be governed by the hemi-spheroids ngn. and if one cuts it through pp, perpendicularly to the said parallelogram, the refraction of the surfaces ought to be governed by the hemi-spheroids psp, and so for others. but i saw that if the plane nn was almost perpendicular to the plane gg, making the angle ncg, which is on the side a, an angle of degrees minutes, the hemi-spheroids ngn would become similar to the hemi-spheroids gng, since the planes nn and gg were equally inclined by an angle of degrees minutes to the axis ss. in consequence it must needs be, if our theory is true, that the surfaces which the section through nn produces should effect the same refractions as the surfaces of the section through gg. and not only the surfaces of the section nn but all other sections produced by planes which might be inclined to the axis at an angle equal to degrees minutes. so that there are an infinitude of planes which ought to produce precisely the same refractions as the natural surfaces of the crystal, or as the section parallel to any one of those surfaces which are made by cleavage. i saw also that by cutting it by a plane taken through pp, and perpendicular to the axis ss, the refraction of the surfaces ought to be such that the perpendicular ray should suffer thereby no deviation; and that for oblique rays there would always be an irregular refraction, differing from the regular, and by which objects placed beneath the crystal would be less elevated than by that other refraction. that, similarly, by cutting the crystal by any plane through the axis ss, such as the plane of the figure is, the perpendicular ray ought to suffer no refraction; and that for oblique rays there were different measures for the irregular refraction according to the situation of the plane in which the incident ray was. now these things were found in fact so; and, after that, i could not doubt that a similar success could be met with everywhere. whence i concluded that one might form from this crystal solids similar to those which are its natural forms, which should produce, at all their surfaces, the same regular and irregular refractions as the natural surfaces, and which nevertheless would cleave in quite other ways, and not in directions parallel to any of their faces. that out of it one would be able to fashion pyramids, having their base square, pentagonal, hexagonal, or with as many sides as one desired, all the surfaces of which should have the same refractions as the natural surfaces of the crystal, except the base, which will not refract the perpendicular ray. these surfaces will each make an angle of degrees minutes with the axis of the crystal, and the base will be the section perpendicular to the axis. that, finally, one could also fashion out of it triangular prisms, or prisms with as many sides as one would, of which neither the sides nor the bases would refract the perpendicular ray, although they would yet all cause double refraction for oblique rays. the cube is included amongst these prisms, the bases of which are sections perpendicular to the axis of the crystal, and the sides are sections parallel to the same axis. from all this it further appears that it is not at all in the disposition of the layers of which this crystal seems to be composed, and according to which it splits in three different senses, that the cause resides of its irregular refraction; and that it would be in vain to wish to seek it there. but in order that any one who has some of this stone may be able to find, by his own experience, the truth of what i have just advanced, i will state here the process of which i have made use to cut it, and to polish it. cutting is easy by the slicing wheels of lapidaries, or in the way in which marble is sawn: but polishing is very difficult, and by employing the ordinary means one more often depolishes the surfaces than makes them lucent. after many trials, i have at last found that for this service no plate of metal must be used, but a piece of mirror glass made matt and depolished. upon this, with fine sand and water, one smoothes the crystal little by little, in the same way as spectacle glasses, and polishes it simply by continuing the work, but ever reducing the material. i have not, however, been able to give it perfect clarity and transparency; but the evenness which the surfaces acquire enables one to observe in them the effects of refraction better than in those made by cleaving the stone, which always have some inequality. even when the surface is only moderately smoothed, if one rubs it over with a little oil or white of egg, it becomes quite transparent, so that the refraction is discerned in it quite distinctly. and this aid is specially necessary when it is wished to polish the natural surfaces to remove the inequalities; because one cannot render them lucent equally with the surfaces of other sections, which take a polish so much the better the less nearly they approximate to these natural planes. before finishing the treatise on this crystal, i will add one more marvellous phenomenon which i discovered after having written all the foregoing. for though i have not been able till now to find its cause, i do not for that reason wish to desist from describing it, in order to give opportunity to others to investigate it. it seems that it will be necessary to make still further suppositions besides those which i have made; but these will not for all that cease to keep their probability after having been confirmed by so many tests. [illustration] the phenomenon is, that by taking two pieces of this crystal and applying them one over the other, or rather holding them with a space between the two, if all the sides of one are parallel to those of the other, then a ray of light, such as ab, is divided into two in the first piece, namely into bd and bc, following the two refractions, regular and irregular. on penetrating thence into the other piece each ray will pass there without further dividing itself in two; but that one which underwent the regular refraction, as here dg, will undergo again only a regular refraction at gh; and the other, ce, an irregular refraction at ef. and the same thing occurs not only in this disposition, but also in all those cases in which the principal section of each of the pieces is situated in one and the same plane, without it being needful for the two neighbouring surfaces to be parallel. now it is marvellous why the rays ce and dg, incident from the air on the lower crystal, do not divide themselves the same as the first ray ab. one would say that it must be that the ray dg in passing through the upper piece has lost something which is necessary to move the matter which serves for the irregular refraction; and that likewise ce has lost that which was necessary to move the matter which serves for regular refraction: but there is yet another thing which upsets this reasoning. it is that when one disposes the two crystals in such a way that the planes which constitute the principal sections intersect one another at right angles, whether the neighbouring surfaces are parallel or not, then the ray which has come by the regular refraction, as dg, undergoes only an irregular refraction in the lower piece; and on the contrary the ray which has come by the irregular refraction, as ce, undergoes only a regular refraction. but in all the infinite other positions, besides those which i have just stated, the rays dg, ce, divide themselves anew each one into two, by refraction in the lower crystal so that from the single ray ab there are four, sometimes of equal brightness, sometimes some much less bright than others, according to the varying agreement in the positions of the crystals: but they do not appear to have all together more light than the single ray ab. when one considers here how, while the rays ce, dg, remain the same, it depends on the position that one gives to the lower piece, whether it divides them both in two, or whether it does not divide them, and yet how the ray ab above is always divided, it seems that one is obliged to conclude that the waves of light, after having passed through the first crystal, acquire a certain form or disposition in virtue of which, when meeting the texture of the second crystal, in certain positions, they can move the two different kinds of matter which serve for the two species of refraction; and when meeting the second crystal in another position are able to move only one of these kinds of matter. but to tell how this occurs, i have hitherto found nothing which satisfies me. leaving then to others this research, i pass to what i have to say touching the cause of the extraordinary figure of this crystal, and why it cleaves easily in three different senses, parallel to any one of its surfaces. there are many bodies, vegetable, mineral, and congealed salts, which are formed with certain regular angles and figures. thus among flowers there are many which have their leaves disposed in ordered polygons, to the number of , , , or sides, but not more. this well deserves to be investigated, both as to the polygonal figure, and as to why it does not exceed the number . rock crystal grows ordinarily in hexagonal bars, and diamonds are found which occur with a square point and polished surfaces. there is a species of small flat stones, piled up directly upon one another, which are all of pentagonal figure with rounded angles, and the sides a little folded inwards. the grains of gray salt which are formed from sea water affect the figure, or at least the angle, of the cube; and in the congelations of other salts, and in that of sugar, there are found other solid angles with perfectly flat faces. small snowflakes almost always fall in little stars with points, and sometimes in hexagons with straight sides. and i have often observed, in water which is beginning to freeze, a kind of flat and thin foliage of ice, the middle ray of which throws out branches inclined at an angle of degrees. all these things are worthy of being carefully investigated to ascertain how and by what artifice nature there operates. but it is not now my intention to treat fully of this matter. it seems that in general the regularity which occurs in these productions comes from the arrangement of the small invisible equal particles of which they are composed. and, coming to our iceland crystal, i say that if there were a pyramid such as abcd, composed of small rounded corpuscles, not spherical but flattened spheroids, such as would be made by the rotation of the ellipse gh around its lesser diameter ef (of which the ratio to the greater diameter is very nearly that of to the square root of )--i say that then the solid angle of the point d would be equal to the obtuse and equilateral angle of this crystal. i say, further, that if these corpuscles were lightly stuck together, on breaking this pyramid it would break along faces parallel to those that make its point: and by this means, as it is easy to see, it would produce prisms similar to those of the same crystal as this other figure represents. the reason is that when broken in this fashion a whole layer separates easily from its neighbouring layer since each spheroid has to be detached only from the three spheroids of the next layer; of which three there is but one which touches it on its flattened surface, and the other two at the edges. and the reason why the surfaces separate sharp and polished is that if any spheroid of the neighbouring surface would come out by attaching itself to the surface which is being separated, it would be needful for it to detach itself from six other spheroids which hold it locked, and four of which press it by these flattened surfaces. since then not only the angles of our crystal but also the manner in which it splits agree precisely with what is observed in the assemblage composed of such spheroids, there is great reason to believe that the particles are shaped and ranged in the same way. [illustration: {pyramid and section of spheroids}] there is even probability enough that the prisms of this crystal are produced by the breaking up of pyramids, since mr. bartholinus relates that he occasionally found some pieces of triangularly pyramidal figure. but when a mass is composed interiorly only of these little spheroids thus piled up, whatever form it may have exteriorly, it is certain, by the same reasoning which i have just explained, that if broken it would produce similar prisms. it remains to be seen whether there are other reasons which confirm our conjecture, and whether there are none which are repugnant to it. [illustration: {paralleloid arrangement of spheroids with planes of potential cleavage}] it may be objected that this crystal, being so composed, might be capable of cleavage in yet two more fashions; one of which would be along planes parallel to the base of the pyramid, that is to say to the triangle abc; the other would be parallel to a plane the trace of which is marked by the lines gh, hk, kl. to which i say that both the one and the other, though practicable, are more difficult than those which were parallel to any one of the three planes of the pyramid; and that therefore, when striking on the crystal in order to break it, it ought always to split rather along these three planes than along the two others. when one has a number of spheroids of the form above described, and ranges them in a pyramid, one sees why the two methods of division are more difficult. for in the case of that division which would be parallel to the base, each spheroid would be obliged to detach itself from three others which it touches upon their flattened surfaces, which hold more strongly than the contacts at the edges. and besides that, this division will not occur along entire layers, because each of the spheroids of a layer is scarcely held at all by the of the same layer that surround it, since they only touch it at the edges; so that it adheres readily to the neighbouring layer, and the others to it, for the same reason; and this causes uneven surfaces. also one sees by experiment that when grinding down the crystal on a rather rough stone, directly on the equilateral solid angle, one verily finds much facility in reducing it in this direction, but much difficulty afterwards in polishing the surface which has been flattened in this manner. as for the other method of division along the plane ghkl, it will be seen that each spheroid would have to detach itself from four of the neighbouring layer, two of which touch it on the flattened surfaces, and two at the edges. so that this division is likewise more difficult than that which is made parallel to one of the surfaces of the crystal; where, as we have said, each spheroid is detached from only three of the neighbouring layer: of which three there is one only which touches it on the flattened surface, and the other two at the edges only. however, that which has made me know that in the crystal there are layers in this last fashion, is that in a piece weighing half a pound which i possess, one sees that it is split along its length, as is the above-mentioned prism by the plane ghkl; as appears by colours of the iris extending throughout this whole plane although the two pieces still hold together. all this proves then that the composition of the crystal is such as we have stated. to which i again add this experiment; that if one passes a knife scraping along any one of the natural surfaces, and downwards as it were from the equilateral obtuse angle, that is to say from the apex of the pyramid, one finds it quite hard; but by scraping in the opposite sense an incision is easily made. this follows manifestly from the situation of the small spheroids; over which, in the first manner, the knife glides; but in the other manner it seizes them from beneath almost as if they were the scales of a fish. i will not undertake to say anything touching the way in which so many corpuscles all equal and similar are generated, nor how they are set in such beautiful order; whether they are formed first and then assembled, or whether they arrange themselves thus in coming into being and as fast as they are produced, which seems to me more probable. to develop truths so recondite there would be needed a knowledge of nature much greater than that which we have. i will add only that these little spheroids could well contribute to form the spheroids of the waves of light, here above supposed, these as well as those being similarly situated, and with their axes parallel. _calculations which have been supposed in this chapter_. mr. bartholinus, in his treatise of this crystal, puts at degrees the obtuse angles of the faces, which i have stated to be degrees minutes. he states that he measured these angles directly on the crystal, which is difficult to do with ultimate exactitude, because the edges such as ca, cb, in this figure, are generally worn, and not quite straight. for more certainty, therefore, i preferred to measure actually the obtuse angle by which the faces cbda, cbvf, are inclined to one another, namely the angle ocn formed by drawing cn perpendicular to fv, and co perpendicular to da. this angle ocn i found to be degrees; and its supplement cnp, to be degrees, as it should be. [illustration] to find from this the obtuse angle bca, i imagined a sphere having its centre at c, and on its surface a spherical triangle, formed by the intersection of three planes which enclose the solid angle c. in this equilateral triangle, which is abf in this other figure, i see that each of the angles should be degrees, namely equal to the angle ocn; and that each of the sides should be of as many degrees as the angle acb, or acf, or bcf. having then drawn the arc fq perpendicular to the side ab, which it divides equally at q, the triangle fqa has a right angle at q, the angle a degrees, and f half as much, namely degrees minutes; whence the hypotenuse af is found to be degrees minutes. and this arc af is the measure of the angle acf in the figure of the crystal. [illustration] in the same figure, if the plane cghf cuts the crystal so that it divides the obtuse angles acb, mhv, in the middle, it is stated, in article , that the angle cfh is degrees minutes. this again is easily shown in the same spherical triangle abf, in which it appears that the arc fq is as many degrees as the angle gcf in the crystal, the supplement of which is the angle cfh. now the arc fq is found to be degrees minutes. then its supplement, degrees minutes, is the angle cfh. it was stated, in article , that the straight line cs, which in the preceding figure is ch, being the axis of the crystal, that is to say being equally inclined to the three sides ca, cb, cf, the angle gch is degrees minutes. this is also easily calculated by the same spherical triangle. for by drawing the other arc ad which cuts bf equally, and intersects fq at s, this point will be the centre of the triangle. and it is easy to see that the arc sq is the measure of the angle gch in the figure which represents the crystal. now in the triangle qas, which is right-angled, one knows also the angle a, which is degrees minutes, and the side aq degrees minutes; whence the side sq is found to be degrees minutes. in article it was required to show that pms being an ellipse the centre of which is c, and which touches the straight line md at m so that the angle mcl which cm makes with cl, perpendicular on dm, is degrees minutes, and its semi-minor axis cs making with cg (which is parallel to md) an angle gcs of degrees minutes, it was required to show, i say, that, cm being , parts, pc the semi-major diameter of this ellipse is , parts, and cs, the semi-minor diameter, , . let cp and cs be prolonged and meet the tangent dm at d and z; and from the point of contact m let mn and mo be drawn as perpendiculars to cp and cs. now because the angles scp, gcl, are right angles, the angle pcl will be equal to gcs which was degrees minutes. and deducting the angle lcm, which is degrees minutes, from lcp, which is degrees minutes, there remains mcp, degrees minutes. considering then cm as a radius of , parts, mn, the sine of degrees minutes, will be , . and in the right-angled triangle mnd, mn will be to nd as the radius of the tables is to the tangent of degrees minutes (because the angle nmd is equal to dcl, or gcs); that is to say as , to , : whence results nd , . but nc is , of the same parts, cm being , , because nc is the sine of the complement of the angle mcp, which was degrees minutes. then the whole line dc is , ; and cp, which is a mean proportional between dc and cn, since md touches the ellipse, will be , . [illustration] similarly, because the angle omz is equal to cdz, or lcz, which is degrees minutes, being the complement of gcs, it follows that, as the radius of the tables is to the tangent of degrees minutes, so will om , be to oz , . but oc is , of these same parts of which cm is , , because it is equal to mn, the sine of the angle mcp, which is degrees minutes. then the whole line cz is , ; and cs, which is a mean proportional between cz and co will be , . at the same place it was stated that gc was found to be , parts. to prove this, let pe be drawn in the same figure parallel to dm, and meeting cm at e. in the right-angled triangle cld the side cl is , (cm being , ), because cl is the sine of the complement of the angle lcm, which is degrees minutes. and since the angle lcd is degrees minutes, being equal to gcs, the side ld is found to be , : whence deducting ml , there will remain md , . now as cd (which was , ) is to dm , , so will cp , be to pe , . but as the rectangle meh (or rather the difference of the squares on cm and ce) is to the square on mc, so is the square on pe to the square on c_g_; then also as the difference of the squares on dc and cp to the square on cd, so also is the square on pe to the square on _g_c. but dp, cp, and pe are known; hence also one knows gc, which is , . _lemma which has been supposed_. if a spheroid is touched by a straight line, and also by two or more planes which are parallel to this line, though not parallel to one another, all the points of contact of the line, as well as of the planes, will be in one and the same ellipse made by a plane which passes through the centre of the spheroid. let led be the spheroid touched by the line bm at the point b, and also by the planes parallel to this line at the points o and a. it is required to demonstrate that the points b, o, and a are in one and the same ellipse made in the spheroid by a plane which passes through its centre. [illustration] through the line bm, and through the points o and a, let there be drawn planes parallel to one another, which, in cutting the spheroid make the ellipses lbd, pop, qaq; which will all be similar and similarly disposed, and will have their centres k, n, r, in one and the same diameter of the spheroid, which will also be the diameter of the ellipse made by the section of the plane that passes through the centre of the spheroid, and which cuts the planes of the three said ellipses at right angles: for all this is manifest by proposition of the book of conoids and spheroids of archimedes. further, the two latter planes, which are drawn through the points o and a, will also, by cutting the planes which touch the spheroid in these same points, generate straight lines, as oh and as, which will, as is easy to see, be parallel to bm; and all three, bm, oh, as, will touch the ellipses lbd, pop, qaq in these points, b, o, a; since they are in the planes of these ellipses, and at the same time in the planes which touch the spheroid. if now from these points b, o, a, there are drawn the straight lines bk, on, ar, through the centres of the same ellipses, and if through these centres there are drawn also the diameters ld, pp, qq, parallel to the tangents bm, oh, as; these will be conjugate to the aforesaid bk, on, ar. and because the three ellipses are similar and similarly disposed, and have their diameters ld, pp, qq parallel, it is certain that their conjugate diameters bk, on, ar, will also be parallel. and the centres k, n, r being, as has been stated, in one and the same diameter of the spheroid, these parallels bk, on, ar will necessarily be in one and the same plane, which passes through this diameter of the spheroid, and, in consequence, the points r, o, a are in one and the same ellipse made by the intersection of this plane. which was to be proved. and it is manifest that the demonstration would be the same if, besides the points o, a, there had been others in which the spheroid had been touched by planes parallel to the straight line bm. chapter vi on the figures of the transparent bodies which serve for refraction and for reflexion after having explained how the properties of reflexion and refraction follow from what we have supposed concerning the nature of light, and of opaque bodies, and of transparent media, i will here set forth a very easy and natural way of deducing, from the same principles, the true figures which serve, either by reflexion or by refraction, to collect or disperse the rays of light, as may be desired. for though i do not see yet that there are means of making use of these figures, so far as relates to refraction, not only because of the difficulty of shaping the glasses of telescopes with the requisite exactitude according to these figures, but also because there exists in refraction itself a property which hinders the perfect concurrence of the rays, as mr. newton has very well proved by experiment, i will yet not desist from relating the invention, since it offers itself, so to speak, of itself, and because it further confirms our theory of refraction, by the agreement which here is found between the refracted ray and the reflected ray. besides, it may occur that some one in the future will discover in it utilities which at present are not seen. [illustration] to proceed then to these figures, let us suppose first that it is desired to find a surface cde which shall reassemble at a point b rays coming from another point a; and that the summit of the surface shall be the given point d in the straight line ab. i say that, whether by reflexion or by refraction, it is only necessary to make this surface such that the path of the light from the point a to all points of the curved line cde, and from these to the point of concurrence (as here the path along the straight lines ac, cb, along al, lb, and along ad, db), shall be everywhere traversed in equal times: by which principle the finding of these curves becomes very easy. [illustration] so far as relates to the reflecting surface, since the sum of the lines ac, cb ought to be equal to that of ad, db, it appears that dce ought to be an ellipse; and for refraction, the ratio of the velocities of waves of light in the media a and b being supposed to be known, for example that of to (which is the same, as we have shown, as the ratio of the sines in the refraction), it is only necessary to make dh equal to / of db; and having after that described from the centre a some arc fc, cutting db at f, then describe another from centre b with its semi-diameter bx equal to / of fh; and the point of intersection of the two arcs will be one of the points required, through which the curve should pass. for this point, having been found in this fashion, it is easy forthwith to demonstrate that the time along ac, cb, will be equal to the time along ad, db. for assuming that the line ad represents the time which the light takes to traverse this same distance ad in air, it is evident that dh, equal to / of db, will represent the time of the light along db in the medium, because it needs here more time in proportion as its speed is slower. therefore the whole line ah will represent the time along ad, db. similarly the line ac or af will represent the time along ac; and fh being by construction equal to / of cb, it will represent the time along cb in the medium; and in consequence the whole line ah will represent also the time along ac, cb. whence it appears that the time along ac, cb, is equal to the time along ad, db. and similarly it can be shown if l and k are other points in the curve cde, that the times along al, lb, and along ak, kb, are always represented by the line ah, and therefore equal to the said time along ad, db. in order to show further that the surfaces, which these curves will generate by revolution, will direct all the rays which reach them from the point a in such wise that they tend towards b, let there be supposed a point k in the curve, farther from d than c is, but such that the straight line ak falls from outside upon the curve which serves for the refraction; and from the centre b let the arc ks be described, cutting bd at s, and the straight line cb at r; and from the centre a describe the arc dn meeting ak at n. since the sums of the times along ak, kb, and along ac, cb are equal, if from the former sum one deducts the time along kb, and if from the other one deducts the time along rb, there will remain the time along ak as equal to the time along the two parts ac, cr. consequently in the time that the light has come along ak it will also have come along ac and will in addition have made, in the medium from the centre c, a partial spherical wave, having a semi-diameter equal to cr. and this wave will necessarily touch the circumference ks at r, since cb cuts this circumference at right angles. similarly, having taken any other point l in the curve, one can show that in the same time as the light passes along al it will also have come along al and in addition will have made a partial wave, from the centre l, which will touch the same circumference ks. and so with all other points of the curve cde. then at the moment that the light reaches k the arc krs will be the termination of the movement, which has spread from a through dck. and thus this same arc will constitute in the medium the propagation of the wave emanating from a; which wave may be represented by the arc dn, or by any other nearer the centre a. but all the pieces of the arc krs are propagated successively along straight lines which are perpendicular to them, that is to say, which tend to the centre b (for that can be demonstrated in the same way as we have proved above that the pieces of spherical waves are propagated along the straight lines coming from their centre), and these progressions of the pieces of the waves constitute the rays themselves of light. it appears then that all these rays tend here towards the point b. one might also determine the point c, and all the others, in this curve which serves for the refraction, by dividing da at g in such a way that dg is / of da, and describing from the centre b any arc cx which cuts bd at n, and another from the centre a with its semi-diameter af equal to / of gx; or rather, having described, as before, the arc cx, it is only necessary to make df equal to / of dx, and from-the centre a to strike the arc fc; for these two constructions, as may be easily known, come back to the first one which was shown before. and it is manifest by the last method that this curve is the same that mr. des cartes has given in his geometry, and which he calls the first of his ovals. it is only a part of this oval which serves for the refraction, namely, the part dk, ending at k, if ak is the tangent. as to the, other part, des cartes has remarked that it could serve for reflexions, if there were some material of a mirror of such a nature that by its means the force of the rays (or, as we should say, the velocity of the light, which he could not say, since he held that the movement of light was instantaneous) could be augmented in the proportion of to . but we have shown that in our way of explaining reflexion, such a thing could not arise from the matter of the mirror, and it is entirely impossible. [illustration] [illustration] from what has been demonstrated about this oval, it will be easy to find the figure which serves to collect to a point incident parallel rays. for by supposing just the same construction, but the point a infinitely distant, giving parallel rays, our oval becomes a true ellipse, the construction of which differs in no way from that of the oval, except that fc, which previously was an arc of a circle, is here a straight line, perpendicular to db. for the wave of light dn, being likewise represented by a straight line, it will be seen that all the points of this wave, travelling as far as the surface kd along lines parallel to db, will advance subsequently towards the point b, and will arrive there at the same time. as for the ellipse which served for reflexion, it is evident that it will here become a parabola, since its focus a may be regarded as infinitely distant from the other, b, which is here the focus of the parabola, towards which all the reflexions of rays parallel to ab tend. and the demonstration of these effects is just the same as the preceding. but that this curved line cde which serves for refraction is an ellipse, and is such that its major diameter is to the distance between its foci as to , which is the proportion of the refraction, can be easily found by the calculus of algebra. for db, which is given, being called _a_; its undetermined perpendicular dt being called _x_; and tc _y_; fb will be _a - y_; cb will be sqrt(_xx + aa - ay + yy_). but the nature of the curve is such that / of tc together with cb is equal to db, as was stated in the last construction: then the equation will be between _( / )y + sqrt(xx + aa - ay + yy)_ and _a_; which being reduced, gives _( / )ay - yy_ equal to _( / )xx_; that is to say that having made do equal to / of db, the rectangle dfo is equal to / of the square on fc. whence it is seen that dc is an ellipse, of which the axis do is to the parameter as to ; and therefore the square on do is to the square of the distance between the foci as to - , that is to say ; and finally the line do will be to this distance as to . [illustration] again, if one supposes the point b to be infinitely distant, in lieu of our first oval we shall find that cde is a true hyperbola; which will make those rays become parallel which come from the point a. and in consequence also those which are parallel within the transparent body will be collected outside at the point a. now it must be remarked that cx and ks become straight lines perpendicular to ba, because they represent arcs of circles the centre of which is infinitely distant. and the intersection of the perpendicular cx with the arc fc will give the point c, one of those through which the curve ought to pass. and this operates so that all the parts of the wave of light dn, coming to meet the surface kde, will advance thence along parallels to ks and will arrive at this straight line at the same time; of which the proof is again the same as that which served for the first oval. besides one finds by a calculation as easy as the preceding one, that cde is here a hyperbola of which the axis do is / of ad, and the parameter equal to ad. whence it is easily proved that do is to the distance between the foci as to . [illustration] these are the two cases in which conic sections serve for refraction, and are the same which are explained, in his _dioptrique_, by des cartes, who first found out the use of these lines in relation to refraction, as also that of the ovals the first of which we have already set forth. the second oval is that which serves for rays that tend to a given point; in which oval, if the apex of the surface which receives the rays is d, it will happen that the other apex will be situated between b and a, or beyond a, according as the ratio of ad to db is given of greater or lesser value. and in this latter case it is the same as that which des cartes calls his rd oval. now the finding and construction of this second oval is the same as that of the first, and the demonstration of its effect likewise. but it is worthy of remark that in one case this oval becomes a perfect circle, namely when the ratio of ad to db is the same as the ratio of the refractions, here as to , as i observed a long time ago. the th oval, serving only for impossible reflexions, there is no need to set it forth. [illustration] as for the manner in which mr. des cartes discovered these lines, since he has given no explanation of it, nor any one else since that i know of, i will say here, in passing, what it seems to me it must have been. let it be proposed to find the surface generated by the revolution of the curve kde, which, receiving the incident rays coming to it from the point a, shall deviate them toward the point b. then considering this other curve as already known, and that its apex d is in the straight line ab, let us divide it up into an infinitude of small pieces by the points g, c, f; and having drawn from each of these points, straight lines towards a to represent the incident rays, and other straight lines towards b, let there also be described with centre a the arcs gl, cm, fn, do, cutting the rays that come from a at l, m, n, o; and from the points k, g, c, f, let there be described the arcs kq, gr, cs, ft cutting the rays towards b at q, r, s, t; and let us suppose that the straight line hkz cuts the curve at k at right-angles. [illustration] then ak being an incident ray, and kb its refraction within the medium, it needs must be, according to the law of refraction which was known to mr. des cartes, that the sine of the angle zka should be to the sine of the angle hkb as to , supposing that this is the proportion of the refraction of glass; or rather, that the sine of the angle kgl should have this same ratio to the sine of the angle gkq, considering kg, gl, kq as straight lines because of their smallness. but these sines are the lines kl and gq, if gk is taken as the radius of the circle. then lk ought to be to gq as to ; and in the same ratio mg to cr, nc to fs, of to dt. then also the sum of all the antecedents to all the consequents would be as to . now by prolonging the arc do until it meets ak at x, kx is the sum of the antecedents. and by prolonging the arc kq till it meets ad at y, the sum of the consequents is dy. then kx ought to be to dy as to . whence it would appear that the curve kde was of such a nature that having drawn from some point which had been assumed, such as k, the straight lines ka, kb, the excess by which ak surpasses ad should be to the excess of db over kb, as to . for it can similarly be demonstrated, by taking any other point in the curve, such as g, that the excess of ag over ad, namely vg, is to the excess of bd over dg, namely dp, in this same ratio of to . and following this principle mr. des cartes constructed these curves in his _geometric_; and he easily recognized that in the case of parallel rays, these curves became hyperbolas and ellipses. let us now return to our method and let us see how it leads without difficulty to the finding of the curves which one side of the glass requires when the other side is of a given figure; a figure not only plane or spherical, or made by one of the conic sections (which is the restriction with which des cartes proposed this problem, leaving the solution to those who should come after him) but generally any figure whatever: that is to say, one made by the revolution of any given curved line to which one must merely know how to draw straight lines as tangents. let the given figure be that made by the revolution of some curve such as ak about the axis av, and that this side of the glass receives rays coming from the point l. furthermore, let the thickness ab of the middle of the glass be given, and the point f at which one desires the rays to be all perfectly reunited, whatever be the first refraction occurring at the surface ak. i say that for this the sole requirement is that the outline bdk which constitutes the other surface shall be such that the path of the light from the point l to the surface ak, and from thence to the surface bdk, and from thence to the point f, shall be traversed everywhere in equal times, and in each case in a time equal to that which the light employs, to pass along the straight line lf of which the part ab is within the glass. [illustration] let lg be a ray falling on the arc ak. its refraction gv will be given by means of the tangent which will be drawn at the point g. now in gv the point d must be found such that fd together with / of dg and the straight line gl, may be equal to fb together with / of ba and the straight line al; which, as is clear, make up a given length. or rather, by deducting from each the length of lg, which is also given, it will merely be needful to adjust fd up to the straight line vg in such a way that fd together with / of dg is equal to a given straight line, which is a quite easy plane problem: and the point d will be one of those through which the curve bdk ought to pass. and similarly, having drawn another ray lm, and found its refraction mo, the point n will be found in this line, and so on as many times as one desires. to demonstrate the effect of the curve, let there be described about the centre l the circular arc ah, cutting lg at h; and about the centre f the arc bp; and in ab let as be taken equal to / of hg; and se equal to gd. then considering ah as a wave of light emanating from the point l, it is certain that during the time in which its piece h arrives at g the piece a will have advanced within the transparent body only along as; for i suppose, as above, the proportion of the refraction to be as to . now we know that the piece of wave which is incident on g, advances thence along the line gd, since gv is the refraction of the ray lg. then during the time that this piece of wave has taken from g to d, the other piece which was at s has reached e, since gd, se are equal. but while the latter will advance from e to b, the piece of wave which was at d will have spread into the air its partial wave, the semi-diameter of which, dc (supposing this wave to cut the line df at c), will be / of eb, since the velocity of light outside the medium is to that inside as to . now it is easy to show that this wave will touch the arc bp at this point c. for since, by construction, fd + / dg + gl are equal to fb + / ba + al; on deducting the equals lh, la, there will remain fd + / dg + gh equal to fb + / ba. and, again, deducting from one side gh, and from the other side / of as, which are equal, there will remain fd with / dg equal to fb with / of bs. but / of dg are equal to / of es; then fd is equal to fb with / of be. but dc was equal to / of eb; then deducting these equal lengths from one side and from the other, there will remain cf equal to fb. and thus it appears that the wave, the semi-diameter of which is dc, touches the arc bp at the moment when the light coming from the point l has arrived at b along the line lb. it can be demonstrated similarly that at this same moment the light that has come along any other ray, such as lm, mn, will have propagated the movement which is terminated at the arc bp. whence it follows, as has been often said, that the propagation of the wave ah, after it has passed through the thickness of the glass, will be the spherical wave bp, all the pieces of which ought to advance along straight lines, which are the rays of light, to the centre f. which was to be proved. similarly these curved lines can be found in all the cases which can be proposed, as will be sufficiently shown by one or two examples which i will add. let there be given the surface of the glass ak, made by the revolution about the axis ba of the line ak, which may be straight or curved. let there be also given in the axis the point l and the thickness ba of the glass; and let it be required to find the other surface kdb, which receiving rays that are parallel to ab will direct them in such wise that after being again refracted at the given surface ak they will all be reassembled at the point l. [illustration] from the point l let there be drawn to some point of the given line ak the straight line lg, which, being considered as a ray of light, its refraction gd will then be found. and this line being then prolonged at one side or the other will meet the straight line bl, as here at v. let there then be erected on ab the perpendicular bc, which will represent a wave of light coming from the infinitely distant point f, since we have supposed the rays to be parallel. then all the parts of this wave bc must arrive at the same time at the point l; or rather all the parts of a wave emanating from the point l must arrive at the same time at the straight line bc. and for that, it is necessary to find in the line vgd the point d such that having drawn dc parallel to ab, the sum of cd, plus / of dg, plus gl may be equal to / of ab, plus al: or rather, on deducting from both sides gl, which is given, cd plus / of dg must be equal to a given length; which is a still easier problem than the preceding construction. the point d thus found will be one of those through which the curve ought to pass; and the proof will be the same as before. and by this it will be proved that the waves which come from the point l, after having passed through the glass kakb, will take the form of straight lines, as bc; which is the same thing as saying that the rays will become parallel. whence it follows reciprocally that parallel rays falling on the surface kdb will be reassembled at the point l. [illustration] again, let there be given the surface ak, of any desired form, generated by revolution about the axis ab, and let the thickness of the glass at the middle be ab. also let the point l be given in the axis behind the glass; and let it be supposed that the rays which fall on the surface ak tend to this point, and that it is required to find the surface bd, which on their emergence from the glass turns them as if they came from the point f in front of the glass. having taken any point g in the line ak, and drawing the straight line igl, its part gi will represent one of the incident rays, the refraction of which, gv, will then be found: and it is in this line that we must find the point d, one of those through which the curve dg ought to pass. let us suppose that it has been found: and about l as centre let there be described gt, the arc of a circle cutting the straight line ab at t, in case the distance lg is greater than la; for otherwise the arc ah must be described about the same centre, cutting the straight line lg at h. this arc gt (or ah, in the other case) will represent an incident wave of light, the rays of which tend towards l. similarly, about the centre f let there be described the circular arc dq, which will represent a wave emanating from the point f. then the wave tg, after having passed through the glass, must form the wave qd; and for this i observe that the time taken by the light along gd in the glass must be equal to that taken along the three, ta, ab, and bq, of which ab alone is within the glass. or rather, having taken as equal to / of at, i observe that / of gd ought to be equal to / of sb, plus bq; and, deducting both of them from fd or fq, that fd less / of gd ought to be equal to fb less / of sb. and this last difference is a given length: and all that is required is to draw the straight line fd from the given point f to meet vg so that it may be thus. which is a problem quite similar to that which served for the first of these constructions, where fd plus / of gd had to be equal to a given length. in the demonstration it is to be observed that, since the arc bc falls within the glass, there must be conceived an arc rx, concentric with it and on the other side of qd. then after it shall have been shown that the piece g of the wave gt arrives at d at the same time that the piece t arrives at q, which is easily deduced from the construction, it will be evident as a consequence that the partial wave generated at the point d will touch the arc rx at the moment when the piece q shall have come to r, and that thus this arc will at the same moment be the termination of the movement that comes from the wave tg; whence all the rest may be concluded. having shown the method of finding these curved lines which serve for the perfect concurrence of the rays, there remains to be explained a notable thing touching the uncoordinated refraction of spherical, plane, and other surfaces: an effect which if ignored might cause some doubt concerning what we have several times said, that rays of light are straight lines which intersect at right angles the waves which travel along them. [illustration] for in the case of rays which, for example, fall parallel upon a spherical surface afe, intersecting one another, after refraction, at different points, as this figure represents; what can the waves of light be, in this transparent body, which are cut at right angles by the converging rays? for they can not be spherical. and what will these waves become after the said rays begin to intersect one another? it will be seen in the solution of this difficulty that something very remarkable comes to pass herein, and that the waves do not cease to persist though they do not continue entire, as when they cross the glasses designed according to the construction we have seen. according to what has been shown above, the straight line ad, which has been drawn at the summit of the sphere, at right angles to the axis parallel to which the rays come, represents the wave of light; and in the time taken by its piece d to reach the spherical surface age at e, its other parts will have met the same surface at f, g, h, etc., and will have also formed spherical partial waves of which these points are the centres. and the surface ek which all those waves will touch, will be the continuation of the wave ad in the sphere at the moment when the piece d has reached e. now the line ek is not an arc of a circle, but is a curved line formed as the evolute of another curve enc, which touches all the rays hl, gm, fo, etc., that are the refractions of the parallel rays, if we imagine laid over the convexity enc a thread which in unwinding describes at its end e the said curve ek. for, supposing that this curve has been thus described, we will show that the said waves formed from the centres f, g, h, etc., will all touch it. it is certain that the curve ek and all the others described by the evolution of the curve enc, with different lengths of thread, will cut all the rays hl, gm, fo, etc., at right angles, and in such wise that the parts of them intercepted between two such curves will all be equal; for this follows from what has been demonstrated in our treatise _de motu pendulorum_. now imagining the incident rays as being infinitely near to one another, if we consider two of them, as rg, tf, and draw gq perpendicular to rg, and if we suppose the curve fs which intersects gm at p to have been described by evolution from the curve nc, beginning at f, as far as which the thread is supposed to extend, we may assume the small piece fp as a straight line perpendicular to the ray gm, and similarly the arc gf as a straight line. but gm being the refraction of the ray rg, and fp being perpendicular to it, qf must be to gp as to , that is to say in the proportion of the refraction; as was shown above in explaining the discovery of des cartes. and the same thing occurs in all the small arcs gh, ha, etc., namely that in the quadrilaterals which enclose them the side parallel to the axis is to the opposite side as to . then also as to will the sum of the one set be to the sum of the other; that is to say, tf to as, and de to ak, and be to sk or dv, supposing v to be the intersection of the curve ek and the ray fo. but, making fb perpendicular to de, the ratio of to is also that of be to the semi-diameter of the spherical wave which emanated from the point f while the light outside the transparent body traversed the space be. then it appears that this wave will intersect the ray fm at the same point v where it is intersected at right angles by the curve ek, and consequently that the wave will touch this curve. in the same way it can be proved that the same will apply to all the other waves above mentioned, originating at the points g, h, etc.; to wit, that they will touch the curve ek at the moment when the piece d of the wave ed shall have reached e. now to say what these waves become after the rays have begun to cross one another: it is that from thence they fold back and are composed of two contiguous parts, one being a curve formed as evolute of the curve enc in one sense, and the other as evolute of the same curve in the opposite sense. thus the wave ke, while advancing toward the meeting place becomes _abc_, whereof the part _ab_ is made by the evolute _b_c, a portion of the curve enc, while the end c remains attached; and the part _bc_ by the evolute of the portion _b_e while the end e remains attached. consequently the same wave becomes _def_, then _ghk_, and finally cy, from whence it subsequently spreads without any fold, but always along curved lines which are evolutes of the curve enc, increased by some straight line at the end c. there is even, in this curve, a part en which is straight, n being the point where the perpendicular from the centre x of the sphere falls upon the refraction of the ray de, which i now suppose to touch the sphere. the folding of the waves of light begins from the point n up to the end of the curve c, which point is formed by taking ac to cx in the proportion of the refraction, as here to . as many other points as may be desired in the curve nc are found by a theorem which mr. barrow has demonstrated in section of his _lectiones opticae_, though for another purpose. and it is to be noted that a straight line equal in length to this curve can be given. for since it together with the line ne is equal to the line ck, which is known, since de is to ak in the proportion of the refraction, it appears that by deducting en from ck the remainder will be equal to the curve nc. similarly the waves that are folded back in reflexion by a concave spherical mirror can be found. let abc be the section, through the axis, of a hollow hemisphere, the centre of which is d, its axis being db, parallel to which i suppose the rays of light to come. all the reflexions of those rays which fall upon the quarter-circle ab will touch a curved line afe, of which line the end e is at the focus of the hemisphere, that is to say, at the point which divides the semi-diameter bd into two equal parts. the points through which this curve ought to pass are found by taking, beyond a, some arc ao, and making the arc op double the length of it; then dividing the chord op at f in such wise that the part fp is three times the part fo; for then f is one of the required points. [illustration] and as the parallel rays are merely perpendiculars to the waves which fall on the concave surface, which waves are parallel to ad, it will be found that as they come successively to encounter the surface ab, they form on reflexion folded waves composed of two curves which originate from two opposite evolutions of the parts of the curve afe. so, taking ad as an incident wave, when the part ag shall have met the surface ai, that is to say when the piece g shall have reached i, it will be the curves hf, fi, generated as evolutes of the curves fa, fe, both beginning at f, which together constitute the propagation of the part ag. and a little afterwards, when the part ak has met the surface am, the piece k having come to m, then the curves ln, nm, will together constitute the propagation of that part. and thus this folded wave will continue to advance until the point n has reached the focus e. the curve afe can be seen in smoke, or in flying dust, when a concave mirror is held opposite the sun. and it should be known that it is none other than that curve which is described by the point e on the circumference of the circle eb, when that circle is made to roll within another whose semi-diameter is ed and whose centre is d. so that it is a kind of cycloid, of which, however, the points can be found geometrically. its length is exactly equal to / of the diameter of the sphere, as can be found and demonstrated by means of these waves, nearly in the same way as the mensuration of the preceding curve; though it may also be demonstrated in other ways, which i omit as outside the subject. the area aobefa, comprised between the arc of the quarter-circle, the straight line be, and the curve efa, is equal to the fourth part of the quadrant dab. index archimedes, . atmospheric refraction, . barrow, isaac, . bartholinus, erasmus, , , , , , . boyle, hon. robert, . cassini, jacques, iii. caustic curves, . crystals, see iceland crystal, rock crystal. crystals, configuration of, . descartes, rénê, , , , , , , , , . double refraction, discovery of, , , . elasticity, , . ether, the, or ethereal matter, , , , . extraordinary refraction, , . fermat, principle of, . figures of transparent bodies, . hooke, robert, . iceland crystal, , sqq. iceland crystal, cutting and polishing of, , , . leibnitz, g.w., vi. light, nature of, . light, velocity of, , . molecular texture of bodies, , . newton, sir isaac, vi, . opacity, . ovals, cartesian, , . pardies, rev. father, . rays, definition of, , . reflexion, . refraction, , . rock crystal, , , , . römer, olaf, v, . roughness of surfaces, . sines, law of, , , , . spheres, elasticity of, . spheroidal waves in crystals, . spheroids, lemma about, . sound, speed of, , , . telescopes, lenses for, , . torricelli's experiment, , . transparency, explanation of, , , . waves, no regular succession of, . waves, principle of wave envelopes, , . waves, principle of elementary wave fronts, . waves, propagation of light as, , . _the romance of science_ the splash of a drop by prof. a.m. worthington, m.a., f.r.s. _being the reprint of a discourse delivered at the royal institution of great britain, may , ._ published under the direction of the general literature committee. london: society for promoting christian knowledge, northumberland avenue, charing cross, w.c.; , queen victoria street, e.c. brighton: , north street. new york: e. & j.b. young & co. . the splash of a drop instantaneous photographs of the splash of a water-drop falling about inches into milk. [illustration: time after contact = · sec.] [illustration: time after contact = · sec.] [illustration: time after contact = · sec.] the splash of a drop the splash of a drop is a transaction which is accomplished in the twinkling of an eye, and it may seem to some that a man who proposes to discourse on the matter for an hour must have lost all sense of proportion. if that opinion exists, i hope this evening to be able to remove it, and to convince you that we have to deal with an exquisitely regulated phenomenon, and one which very happily illustrates some of the fundamental properties of fluids. it may be mentioned also that the recent researches of lenard in germany and j.j. thomson at cambridge, on the curious development of electrical charges that accompanies certain kinds of splashes, have invested with a new interest any examination of the mechanics of the phenomenon. it is to the mechanical and not to the electrical side of the question that i shall call your attention this evening. the first well-directed and deliberate observations on the subject that i am acquainted with were made by a school-boy at rugby some twenty years ago, and were reported by him to the rugby natural history society. he had observed that the marks of accidental splashes of ink-drops that had fallen on some smoked glasses with which he was experimenting, presented an appearance not easy to account for. drops of the same size falling from the same height had made always the same kind of mark, which, when carefully examined with a lens, showed that the smoke had been swept away in a system of minute concentric rings and fine striæ. specimens of such patterns, obtained by letting drops of mercury, alcohol, and water fall on to smoked glass, are thrown on the screen, and the main characteristics are easily recognized. such a pattern corresponds to the footprints of the dance that has been performed on the surface, and though the drop may be lying unbroken on the plate, it has evidently been taking violent exercise, and were our vision acute enough we might observe that it was still palpitating after its exertions. a careful examination of a large number of such footprints showed that any opinion that could be formed therefrom of the nature of the motion of the drop must be largely conjectural, and it occurred to me about eighteen years ago to endeavour by means of the illumination of a suitably-timed electric spark to watch a drop through its various changes on impact. the reason that with ordinary continuous light nothing can be satisfactorily seen of the splash, is not that the phenomenon is of such short duration, but because the changes are so rapid that before the image of one stage has faded from the eye the image of a later and quite different stage is superposed upon it. thus the resulting impression is a confused assemblage of all the stages, as in the photograph of a person who has not sat still while the camera was looking at him. the problem to be solved experimentally was therefore this: to let a drop of definite size fall from a definite height in comparative darkness on to a surface, and to illuminate it by a flash of exceedingly short duration at any desired stage, so as to exclude all the stages previous and subsequent to the one thus picked out. the flash must be bright enough for the image of what is seen to remain long enough on the eye for the observer to be able to attend to it, and even to shift his attention from one part to another, and thus to make a drawing of what is seen. if necessary the experiment must be capable of repetition, with an exactly similar drop falling from exactly the same height, and illuminated at exactly the same stage. then, when this stage has been sufficiently studied, we must be able to arrange with another similar drop to illuminate it at a rather later stage, say / second later, and in this way to follow step by step the whole course of the phenomenon. the apparatus by which this has been accomplished is on the table before you. time will not suffice to explain how it grew out of earlier arrangements very different in appearance, but its action is very simple and easy to follow by reference to the diagram (fig. ). aa´ is a light wooden rod rather longer and thicker than an ordinary lead pencil, and pivoted on a horizontal axle o. the rod bears at the end a a small deep watch-glass, or segment of a watch-glass, whose surface has been smoked, so that a drop even of water will lie on it without adhesion. the end a´ carries a small strip of tinned iron, which can be pressed against and held down by an electro-magnet cc´. when the current of the electro-magnet is cut off the iron is released, and the end a´ of the rod is tossed up by the action of a piece of india-rubber stretched catapult-wise across two pegs at e, and by this means the drop resting on the watch-glass is left in mid-air free to fall from rest. [illustration: fig. .] bb´ is a precisely similar rod worked in just the same way, but carrying at b a small horizontal metal ring, on which an ivory timing sphere of the size of a child's marble can be supported. on cutting off the current of the electro-magnet the ends a´ and b´ of the two levers are simultaneously tossed up by the catapults, and thus drop and sphere begin to fall at the same moment. before, however, the drop reaches the surface on which it is to impinge, the timing sphere strikes a plate d attached to one end of a third lever pivoted at q, and thus breaks the contact between a platinum wire bound to the underside of this lever and another wire crossing the first at right angles. this action breaks an electric current which has traversed a second electro-magnet f (fig. ), and releases the iron armature n of the lever np, pivoted at p, thus enabling a strong spiral spring g to lift a stout brass wire l out of mercury, and to break at the surface of the mercury a strong current that has circulated round the primary circuit of a ruhmkorff's induction coil; this produces at the surface of the mercury a bright self-induction spark in the neighbourhood of the splash, and it is by this flash that the splash is viewed. the illumination is greatly helped by surrounding the place where the splash and flash are produced by a white cardboard enclosure, seen in fig. , from whose walls the light is diffused. [illustration: fig. .] it will be observed that the time at which the spark is made will depend upon the distance that the sphere has to fall before striking the plate d, for the subsequent action of demagnetizing f and pulling the wire l out of the mercury in the cup h is the same on each occasion. the modus operandi is consequently as follows:--the observer, sitting in comparative but by no means complete darkness, faces the apparatus as it appears in fig. , presses down the ends a´b´ of the levers first described, so that they are held by the electro-magnet c (fig. ); then he presses the lever np down on the electro-magnet f, sets the timing sphere and drop in place, and then by means of a bridge between two mercury cups, short-circuits and thus cuts off the current of the electro-magnet c. this lets off drop and sphere, and produces the flash. the stage of the phenomenon that is thus revealed having been sufficiently studied by repetition of the experiment as often as may be necessary, he lowers the plate d a fraction of an inch and thus obtains a later stage. not only is any desired stage of the phenomenon thus easily brought under examination, but the apparatus also affords the means of measuring the time interval between any two stages. all that is necessary is to know the distance that the timing sphere falls in the two cases. elementary dynamics then give us the interval required. thus, if the sphere falls one foot and we then lower d / inch, the interval between the corresponding stages will be about · second. having thus described the apparatus, which i hope shortly to show you in action, i pass to the information that has been obtained by it. this is contained in a long series of drawings, of which a selection will be presented on the screen. the first series that i have to show represents the splash of a drop of mercury · inch in diameter that has fallen inches on to a smooth glass plate. it will be noticed that very soon after the first moment of impact, minute rays are shot out in all directions on the surface. these are afterwards overflowed or united, until, as in fig. , the outline is only slightly rippled. then (fig. ) main rays shoot out, from the ends of which in some cases minute droplets of liquid would split off, to be left lying in a circle on the plate, and visible in all subsequent stages. by counting these droplets when they were thus left, the number of rays was ascertained to have been generally about . this exquisite shell-like configuration, shown in fig. , marks about the maximum spread of the liquid, which, subsiding in the middle, afterwards flows into an annulus or rim with a very thin central film, so thin, in fact, as often to tear more or less irregularly. this annular rim then divides or segments (figs. , , ) in such a manner as to join up the rays in pairs, and thus passes into the -lobed annulus of fig. . then the whole contracts, but contracts most rapidly between the lobes, the liquid then being driven into and feeding the arms, which follow more slowly. in fig. the end of this stage is reached, and now the arms continuing to come in, the liquid rises in the centre; this is, in fact, the beginning of the rebound of the drop from the plate. in the case before us the drops at the ends of the arms now break off (fig. ), while the central mass rises in a column which just fails itself to break up into drops, and falls back into the middle of the circle of satellites which, it will be understood, may in some cases again be surrounded by a second circle of the still smaller and more numerous droplets that split off the ends of the rays in fig. . the whole of the stages described are accomplished in about / second, so that the average interval between them is about / second. first series. [illustration: ] [illustration: ] [illustration: ] [illustration: ] [illustration: ] [illustration: ] [illustration: ] [illustration: ] [illustration: ] [illustration: ] [illustration: ] [illustration: ] [illustration: ] [illustration: ] [illustration: ] [illustration: ] [illustration: ] [illustration: ] [illustration: ] [illustration: ] [illustration: ] [illustration: ] [illustration: ] [illustration: ] [illustration: ] [illustration: ] [illustration: ] [illustration: ] [illustration: ] [illustration: ] it should be mentioned that it is only in rare cases that the subordinate drops seen in the last six figures, are found lying in a very complete circle after all is over, for there is generally some slight disturbing lateral velocity which causes many to mingle again with the central drop, or with each other. but even if only half or a quarter of the circle is left, it is easy to estimate how many drops, and therefore how many arms there have been. it may be mentioned that sometimes the surface of the central lake of liquid (figs. , , , ) was seen to be covered with beautiful concentric ripples, not shown in the figures. the question now naturally presents itself, why should the drop behave in this manner? in seeking the answer it will be useful to ask ourselves another question. what should we have expected the drop to do? well, to this i suppose most people would be inclined, arguing from analogy with a solid, to reply that it would be reasonable to expect the drop to flatten itself, and even very considerably flatten itself, and then, collecting itself together again, to rebound, perhaps as a column such as we have seen, but not to form this regular system of rays and arms and subordinate drops. now this argument from analogy with a solid is rather misleading, for the forces that operate in the case of a solid sphere that flattens itself and rebounds, are due to the bodily elasticity which enables it not only to resist, but also to recover from any distortion of shape or shearing of its internal parts past each other. but a liquid has no power of recovering from such internal shear, and the only force that checks the spread, and ultimately causes the recovery of shape, is the _surface tension_, which arises from the fact that the surface layers are always in a state of extension and always endeavouring to contract. thus we are at liberty when dealing with the motions of the drop to think of the interior liquid as not coherent, provided we furnish it with a suitable elastic skin. where the surface skin is sharply curved outwards, as it is at the sharp edge of the flattened disc, there the interior liquid will be strongly pressed back. in fact the process of flattening and recoil is one in which energy of motion is first expended in creating fresh liquid surface, and subsequently recovered as the surface contracts. the transformation is, however, at all moments accompanied by a great loss of energy as heat. moreover, it must be remembered that the energy expended in creating the surface of the satellite drops is not restored if these remain permanently separate. thus the surface tension explains the recoil, and it is also closely connected with the formation of the subordinate rays and arms. to explain this it is only necessary to remind you that a liquid cylinder is an unstable configuration. as you know, any fine jet becomes beaded and breaks into drops, but it is not necessary that there should be any flow of liquid along the jet; if, for example, we could realize a rod of liquid of the shape and size of this cylindrical ruler that i hold in my hand, and liberate it in the air, it would not retain its cylindrical shape, but would segment or divide itself up into a row of drops regularly disposed according to a definite and very simple numerical law, viz. that the distances between the centres of contiguous drops would be equal to the circumference of the cylinder. this can be shown by calculation to be a consequence of the surface tension, and the calculation has been closely verified by experiment. if the liquid cylinder were liberated on a plate, it would still topple into a regular row of drops, but they would be further apart; this was shown by plateau. now imagine the cylinder bent into an annulus. it will still follow the same law,[ ] _i.e._ it will topple into drops just as if it were straight. this i can show you by a direct experiment. i have here a small thick disc of iron, with an accurately planed face and a handle at the back. in the face is cut a circular groove, whose cross section is a semi-circle. i now lay this disc face downwards on the horizontal face of the lantern condenser, and through one of two small holes bored through to the back of the disc i fill the groove with quicksilver. now, suddenly lifting the disc from the plate, i release an annulus of liquid, which splits into the circle of very equal drops which you see projected on the screen. you will notice that the main drops have between them still smaller ones, which have come from the splitting up of the thin cylindrical necks of liquid which connected the larger drops at the last moment. now this tendency to segment or topple into drops, whether of a straight cylinder or of an annulus, is the key to the formation of the arms and satellites, and indeed to much that happens in all the splashes that we shall examine. thus in fig. we have an annular rim, which in figs. and is seen to topple into lobes by which the rays are united in pairs, and even the special rays that are seen in fig. owe their origin to the segmentation of the rim of the thin disc into which the liquid has spread. the proceeding is probably exactly analogous to what takes place in a sea wave that curls over in calm weather on a slightly sloping shore. any one may notice how, as it curls over, the wave presents a long smooth edge, from which at a given instant a multitude of jets suddenly shoot out, and at once the back of the wave, hitherto smooth, is seen to be furrowed or "combed." there can be no doubt that the cylindrical edge topples into alternate convexities and concavities; at the former the flow is helped, at the latter hindered, and thus the jets begin, and special lines of flow are determined. in precisely the same way the previously smooth circular edge of fig. topples, and determines the rays and lines of flow of fig. . before going on to other splashes i will now endeavour to reproduce a mercury splash of the kind i have described, in a manner that shall be visible to all. for this purpose i have reduplicated the apparatus which you have seen, and have it here so arranged that i can let the drop fall on to the horizontal condenser plate of the lantern, through which the light passes upwards, to be afterwards thrown upon this screen. the illuminating flash will be made inside the lantern, where the arc light would ordinarily be placed. i have now set a drop of mercury in readiness and put the timing sphere in place, and now if you will look intently at the middle of the screen i will darken the room and let off the splash. (the experiment was repeated four or five times, and the figures seen were like those of series x.) of course all that can be shown in this way is the outline, or rather a horizontal section of the splash; but you are able to recognize some of the configurations already described, and will be the more willing to believe that a momentary view is after all sufficient to give much information if one is on the alert and has acquired skill by practice. the general features of the splash that we have examined are not merely characteristic of the liquid mercury, but belong to all splashes of a liquid falling on to a surface which it does not wet, provided the height of fall or size of the drop are not so great as to cause complete disruption,[ ] in which case there is no recovery and rebound. thus a drop of milk falling on to smoked glass will, if the height of fall and size of drop are properly adjusted, give forms very similar to those presented by a drop of mercury. the whole course of the phenomenon depends, in fact, mainly on four quantities only: ( ) the size of the drop; ( ) the height of fall; ( ) the value of the surface tension; ( ) the viscosity of the liquid. the next series of drawings illustrates the splash of a drop of water falling into water. in order the better to distinguish the liquid of the original drop from that into which it falls, the latter was coloured with ink or with an aniline dye, and the drop itself was of water rendered turbid with finely-divided matter in suspension. finally drops of milk were found to be very suitable for the purpose, the substitution of milk for water not producing any observable change in the phenomenon. in series ii. the drop fell inches, and was / inch in diameter. [in most of the figures of this and of succeeding series the central white patch represents the original drop, and the white parts round it represent those raised portions of the liquid which catch the light. the numbers under each figure give the time interval in seconds from the occurrence of the first figure, or of the figure marked [tau] = .] series ii. _the splash of a drop, followed in detail by instantaneous illumination._ diameter of drop, / inch. height of fall, - / inches. [illustration: [tau] = sec.] [illustration: [tau] = sec.] [illustration: [tau] = · sec.] [illustration: [tau] = · sec.] [illustration: [tau] = · sec.] [illustration: ] [illustration: [tau] = · sec.] [illustration: [tau] = · sec.] [illustration: [tau] = · sec.] it will be observed that the drop flattens itself out somewhat, and descends at the bottom of a hollow with a raised beaded edge (fig. ). this edge would be smooth and circular but for the instability which causes it to topple into drops. as the drop descends the hollow becomes wider and deeper, and finally closes over the drop (fig. ), which, however, soon again emerges as the hollow flattens out, appearing first near, but still below the surface (fig. ), in a flattened, lobed form, afterwards rising as a column somewhat mixed with adherent water, in which traces of the lobes are at first very visible. the rising column, which is nearly cylindrical, breaks up into drops before or during its subsequent descent into the liquid. as it disappears below the surface the outward and downward flow causes a hollow to be again formed, up the sides of which an annulus of milk is carried, while the remainder descends to be torn again a second time into a vortex ring, which, however, is liable to disturbance from the falling in of the drops which once formed the upper part of the rebounding column. it is not difficult to recognize some features of this splash without any apparatus beyond a cup of tea and a spoonful of milk. any drinker of afternoon tea, after the tea is poured out and before the milk is put in, may let the milk fall into it drop by drop from one or two inches above it. the rebounding column will be seen to consist almost entirely of milk, and to break up into drops in the manner described, while the vortex ring, whose core is of milk, may be seen to shoot down into the liquid. but this is better observed by dropping ink into a tumbler of clear water. let us now increase the height of fall to inches. series iii. exhibits the result. all the characteristics of the last splash are more strongly marked. in fig. we have caught sight of the little raised rim of the hollow before it was headed, but in fig. special channels of easiest flow have been already determined. the number of ribs and rays in this basket-shaped hollow seemed to vary a good deal with different drops, as also did the number of arms and lobes seen in later figures, in a somewhat puzzling manner, and i made no attempt to select drawings which are in agreement in this respect. it will be understood that these rays contain little or none of the liquid of the drop, which remains collected together in the middle. drops from these rays or from the larger arms and lobes of subsequent figures are often thrown off high into the air. in figs. and the drop is clean gone below the surface of the hollow, which is now deeper and larger than before. the beautiful beaded annular edge then subsides, and in fig. we see the drop again, and in fig. it begins to emerge. but although the drop has fallen from a greater height than in the previous splash, the energy of the impact, instead of being expended in raising the same amount of liquid to a greater height, is now spent in lifting a much thicker adherent column to about the same height as in the last splash. there was sometimes noticed, as seen in fig. , a tendency in the water to flow up past the milk, which, still comparatively unmixed with water, rides triumphant on the top of the emergent column. the greater relative thickness of this column prevents it splitting into drops, and figs. and show it descending below the surface to form the hollow of fig. , up the sides of which an annular film of milk is carried (figs. and ), having been detached from the central mass, which descends to be torn again, this time centrally into a well-marked vortex ring. series iii. _the splash of a drop, followed in detail by instantaneous illumination._ diameter of drop, / inch. height of fall, ft. in. [illustration: [tau] = sec.] [illustration: [tau] = · sec.] [illustration: [tau] = · sec.] [illustration: [tau] = · sec.] [illustration: [tau] = · sec.] [illustration: [tau] = · sec.] [illustration: [tau] = · sec.] [illustration: [tau] = · sec.] [illustration: ] [illustration: ] [illustration: ] [illustration: [tau] = · sec.] [illustration: ] [illustration: ] if we keep to the same size of drop and increase the fall to something over a yard, no great change occurs in the nature of the splash, but the emergent column is rather higher and thinner and shows a tendency to split into drops. when, however, we double the volume of the drop and raise the height of fall to inches, the splash of series iv. is obtained, which is beginning to assume quite a different character. the raised rim of the previous series is now developed into a hollow shell of considerable height, which tends to close over the drop. this shell or dome is a characteristic feature of all splashes made by large drops falling from a considerable height, and is extremely beautiful. in the splash at present under consideration it does not always succeed in closing permanently, but opens out as it subsides, and is followed by the emergence of the drop (fig. ). in fig. the return wave overwhelms the drop for an instant, but it is again seen at the summit of the column in fig. . series iv. _the splash of a drop, followed in detail by instantaneous illumination._ diameter of drop, / inch. height of fall, ft. in. [illustration: [tau] = sec.] [illustration: [tau] = · sec.] [illustration: [tau] = · sec.] [illustration: [tau] = · sec.] [illustration: [tau] = · sec.] [illustration: [tau] = · sec.] [illustration: [tau] = · sec.] [illustration: [tau] = · sec.] [illustration: [tau] = · sec.] [illustration: ] [illustration: ] but on other occasions the shell or dome of figs. and closes permanently over the imprisoned air, the liquid then flowing down the sides, which become thinner and thinner, till at length we are left with a large bubble floating on the water (see series v.). it will be observed that the flow of liquid down the sides is chiefly along definite channels, which are probably determined by the arms thrown up at an earlier stage. the bubble is generally creased by the weight of the liquid along these channels. it must be remembered that the base of the bubble is in a state of oscillation, and that the whole is liable to burst at any moment, when such figures as and of the previous series will be seen. [illustration: series v. _the splash of a drop, followed in detail by instantaneous illumination._ the size of drop and height of fall are the same as before, but the hollow shell (see figs. and of the previous series) does not succeed in opening, but is left as a bubble on the surface. this explains the formation of bubbles when _big_ rain-drops fall into a pool of water.] such is the history of the building of the bubbles which big rain-drops leave on the smooth water of a lake, or pond, or puddle. only the bigger drops can do it, and reference to the number at the side of fig. of series iv. shows that the dome is raised in about two-hundredths of a second. should the domes fail to close, or should they open again, we have the emergent columns which any attentive observer will readily recognize, and which have never been better described than by mr. r.l. stevenson, who, in his delightful _inland voyage_, speaks of the surface of the belgian canals along which he was canoeing, as thrown up by the rain into "an infinity of little crystal fountains." very beautiful forms of the same type indeed, but different in detail, are those produced by a drop of water falling into the lighter and more mobile liquid, petroleum. it will now be interesting to turn to the splash that is produced when a solid sphere, such as a child's marble, falls into water. i found to my great surprise that the character of the splash, at any rate up to a height of or feet, depends entirely on the state of the surface of the sphere. a polished sphere of marble about · of an inch in diameter, rubbed very dry with a cloth just beforehand and dropped from a height of feet into water, gave the figures of series vi., in which it is seen that the water spreads over the sphere so rapidly, that it is sheathed with the liquid even before it has passed below the general level of the surface. the splash is insignificantly small and of very short duration.[ ] if the drying and polishing be not so perfect, the configurations of series vii. are produced; while if the sphere be roughened with sandpaper, or _left wet_, series viii. is obtained, in which it will be perceived that, as was the case with the liquid drop, the water is driven away laterally, forming the ribbed basket-shaped hollow, which, however, is now prolonged to a great depth, the drop being followed by a cone of air, while the water seems to find great difficulty in wetting the surface completely. part of this column of air was carried down at least inches, and then only detached when the sphere struck the bottom of the vessel. series vi., vii. _splash of a solid sphere (a marble / inch in diameter falling feet into water)._ [illustration: series vi. when the sphere is _dry_ and _polished_.] [illustration: series vii. when the sphere is _not_ well _dried_ and _polished_.] [illustration: series viii. _the splash of a solid sphere_--(continued.) when the sphere is _rough_ or _wet_.] [illustration: series ix. _the splash of a solid sphere_--(continued.) when the sphere is rough or wet, and falls above feet.] figs. and show the crater falling in, but this did not always happen, for the walls often closed over the hollow exactly as in figs. and of series iv. meanwhile the long and nearly cylindrical portion below breaks up into bubbles which rise quickly to the surface. by increasing the fall to feet we obtain the figures of series ix. the tube of fig. corresponds to the dome of series iv. and v., and is not only elevated to a surprising height, but is also in the act of cleaving (the outline being approximately that of the unduloid of m. plateau). figs. and show the bubble formed by the closing up of this tube, weighed down in the centre as in figs. and of series v. similar results were obtained with other liquids, such as petroleum and alcohol. it is easy to show in a very striking manner the paramount influence of the condition of the solid surface. i have here a number of similar marbles; this set has been well polished by rubbing with wash leather. i drop them one by one through a space of about foot into this deep, wide, cylindrical glass vessel, lighted up by a lamp placed behind it. you see each marble enters noiselessly and with hardly a visible trace of splash. now i pick them out and drop them in again (or to save trouble, i drop in the place of these other wet ones), everything is changed. you see how the air is carried to the very bottom of the vessel, and you hear the "phloisbos" of the bubbles as they rise to the surface and burst. these dry but rough marbles behave in much the same way. such are the main features of the natural history of splashes, as i made it out between thirteen and eighteen years ago. before passing on to the photographs that i have since obtained, i desire to add a few words of comment. i have not till now alluded to any imperfections in the timing apparatus. but no apparatus of the kind can be absolutely perfect, and, as a matter of fact, when everything is adjusted so as to display a particular stage, it will happen that in a succession of observations there is a certain variation in what is seen. thus the configuration viewed may be said to oscillate slightly about the mean for which the apparatus is adjusted. now this is due both to small imperfections in the timing apparatus and to the fact that the splashes themselves do actually vary within certain limits. the reasons are not very far to seek. in the first place the rate of demagnetization of the electro-magnets varies slightly, being partly dependent on the varying resistance of the contacts of crossed wires, partly on the temperature of the magnet, which is affected by the length of time for which the current has been running. but a much more important reason is the variation of the slight adhesion of the drop to the smoked watch-glass that has supported it, and consequently of the oscillations to which, as we shall see, the drop is subjected as it descends. thus the drop will sometimes strike the surface in a flattened form, at others in an elongated form, and there will be a difference, not only in the time of impact, but in the nature of the ensuing splash; consequently some judgment is required in selecting a consecutive series of drawings. the only way is to make a considerable number of drawings of each stage, and then to pick out a consecutive series. now, whenever judgment has to be used, there is room for error of judgment, and moreover, it is impossible to put together the drawings so as to tell a consecutive story, without being guided by some theory, such as i have already sketched, as to the nature of the motion and the conditions that govern it. you will therefore be good enough to remember that this chronicle of the events of a tenth of a second is not a mechanical record but is presented by a fallible human historian, whose account, like that of any other contemporary observer, will be none the worse for independent confirmation. that confirmation is fortunately obtainable. in an attempt made eighteen years ago to photograph the splash of a drop of mercury, i was unable to procure plates sufficiently sensitive to respond to the very short exposures that were required, and consequently abandoned the endeavour. but in recent years plates of exquisite sensitiveness have been produced, and such photographs as those taken by mr. boys of a flying rifle bullet have shown that difficulties on the score of sensitiveness have been practically overcome. within the last few weeks, with the valuable assistance of my colleague at devonport, mr. r.s. cole, i have succeeded in obtaining photographs of various splashes. following prof. boys' suggestion, we employed thomas's cyclist plates, or occasionally the less sensitive "extra-rapid" plates of the same makers, and as a developer, eikonogen solution of triple strength, in which the plates were kept for about minutes, the development being conducted in complete darkness. a few preliminary trials with the self-induction spark produced at the surface of mercury by the apparatus that you have seen at work, showed that the illumination, though ample for direct vision, was not sufficient for photography. when the current strength was increased, so as to make the illumination bright enough for the camera, then the spark became of too great duration, for it lasted for between and thousandths of a second, within which time there was very perceptible motion of the drop and consequent blurring. it was therefore necessary to modify the apparatus so as to employ a leyden-jar spark whose duration was probably less than -millionths of a second. a very slight change in the apparatus rendered it suitable for the new conditions, but time does not permit me to describe the arrangements in detail. it is, however, less necessary to do so as the method is in all essentials the same as that described in this room two years ago by lord rayleigh in connection with the photography of a breaking soap-film.[ ] i therefore pass at once to the photographs themselves. the first two series (x. and xi.) may be described as shadow photographs; they were obtained by allowing a drop of mercury to fall on to the naked photographic plate itself, the illuminating spark being produced vertically above it, and they give only a horizontal section of the drop in various stages, revealing the form of the outline of the part in contact with the plate, but of course telling nothing about the shape of the parts above. the first series corresponds to a mercury splash very similar to that first described, and the second to the splash of a larger drop such as was not described. in each series, the tearing of the thin central film to which allusion was made is well illustrated. i think the first comment that any one would make is that the photographs, while they bear out the drawings in many details, show greater irregularity than the drawings would have led one to expect. on this point i shall presently have something to say. series x. ( ) _instantaneous shadow photographs (life size) of the splash of a drop of mercury falling cm. on to the photographic plate._ [illustration: actual size of the drop, · mm.] [illustration: [tau] = ] [illustration: ] [illustration: ] [illustration: ] [illustration: ] [illustration: [tau] = · sec.] series xi. ( ) _instantaneous shadow photographs (life size) of the splash of a drop of mercury falling cm. on to glass._ [illustration: actual size, · mm. in diameter.] [illustration: [tau] = sec.] [illustration: ] [illustration: a [tau] = · sec.] [illustration: [tau] = · sec.] [illustration: a [tau] = · sec.] [illustration: [tau] = · sec.] comparing the first set of drawings (pp. - ) with the photographs of series x., it will be seen that photograph corresponds to drawing or " " " " " " " " " " " " but the irregularity of the last photograph almost masks the resemblance. series xii. _engravings from instantaneous photographs ( / of the real size) of the splash of a drop of mercury, · mm. in diameter, falling · cm. on to a hard polished surface._ [illustration: ] [illustration: [tau] = sec.] [illustration: ] [illustration: ] [illustration: ] [illustration: [tau] = · sec.] series xii. gives an objective view of a mercury splash as taken by the camera. only the first of this series shows any detail in the interior. the polished surface of the mercury is, in fact, very troublesome to illuminate, and this splash proved the most difficult of all to photograph. series xiii. shows the splash of a drop of milk falling on to a smoked glass plate, on which it runs about without adhesion just as mercury would. here there is more of detail. in fig. the central film is so thin in the middle that the black plate beneath it is seen through the liquid. in fig. this film has been torn. series xiv. exhibits the splash of a water drop falling into milk. the first four photographs show the oscillations of the drop about a mean spherical figure as it approaches the surface. in the subsequent figures it will be noticed that the arms which are thrown up at first, afterwards segment into drops which fly off and subside (see fig. ), to be followed by a second series which again subside (fig. ), to be again succeeded by a third set. in fact, so long as there is any downward momentum the drop and the air behind it are penetrating the liquid, and so long must there be an upward flow of displaced liquid. much of this flow is seen to be directed into the arms along the channels determined by the segmentation of the annular rim. this reproduction of the lobes and arms time after time on a varying scale goes far to explain the puzzling variations in their number which i mentioned in connection with the drawings. i had not, indeed, suspected this, which is one of the few new points that the photographs have so far revealed.[ ] series xiii. _engravings of instantaneous photographs ( / of the real size) of the splash of a drop of milk falling cm. on to smoked glass._ [illustration: ] [illustration: [tau] = sec.] [illustration: [tau] = · sec.] (it was not found possible to reproduce satisfactorily the missing figures of this series.) [illustration: [tau] = · sec.] [illustration: [tau] = · sec.] [illustration: [tau] = · sec.] series xiv. _engravings of instantaneous photographs of the splash of a drop of water falling cm. into milk._ scale about / of actual size. [illustration: ] [illustration: ] [illustration: ] [illustration: [tau] = sec.] [illustration: ] [illustration: [tau] = · sec.] [illustration: [tau] = · sec.] [illustration: ] [illustration: [tau] = · sec.] [illustration: [tau] = · sec.] [illustration: [tau] = · sec.] [illustration: [tau] = · sec.] [illustration: [tau] = · sec.] [illustration: [tau] = · sec.] [illustration: ] [illustration: [tau] = · sec.] [illustration: ] [illustration: [tau] = · sec.] with respect to these photographs,[ ] the credit of which i hope you will attribute firstly to the inventors of the sensitive plates, and secondly to the skill and experience of mr. cole, i desire to add that they are, as far as we know, the first really detailed objective views that have been obtained with anything approaching so short an exposure. even mr. boys' wonderful photographs of flying bullets were after all but shadow-photographs, and did not so strikingly illustrate the extreme sensitiveness of the plates, and i want you to distinguish between such and what (to borrow mr. f.j. smith's phrase) i call an "objective view." it remains only to speak of the greater irregularity in the arms and rays as shown by the photographs. the point is a curious and interesting one. in the first place i have to confess that in looking over my original drawings i find records of many irregular or unsymmetrical figures, yet in compiling the history it has been inevitable that these should be rejected, if only because identical irregularities never recur. thus the mind of the observer is filled with an ideal splash--an "auto-splash"--whose perfection may never be actually realized. but in the second place, when the splash is nearly regular it is very difficult to detect irregularity. this is easily proved by projecting on the screen with instantaneous illumination such a photograph as that of series x., fig. . my experience is that most persons pronounce what they have seen to be a regular and symmetrical star-shaped figure, and they are surprised when they come to examine it by detail in continuous light to find how far this is from the truth. especially is this the case if no irregularity is suspected beforehand. i believe that the observer, usually finding himself unable to attend to more than a portion of the rays in the system, is liable instinctively to pick out for attention a part of the circumference where they are regularly spaced, and to fill up the rest in imagination, and that where a ray may be really absent he prefers to consider that it has been imperfectly viewed. this opinion is confirmed by the fact that in several cases, i have been able to observe with the naked eye a splash that was also simultaneously photographed, and have made the memorandum "quite regular," though the photograph subsequently showed irregularity. it must, however, be observed that the absolute darkness and other conditions necessary for photography are not very favourable for direct vision. and now my tale is told, or rather as much of it as the limits of the time allowed me will permit. i think you will agree that the phenomena are very beautiful, and that the subject, commonplace and familiar though it is, has yet proved worthy of an hour's attention. the end. _richard clay & sons, limited, london & bungay._ footnotes: [ ] see worthington on the "spontaneous segmentation of a liquid annulus," _proceedings royal society_, no. , p. ( ). [ ] readers who wish a more detailed account of a greater variety of splashes are referred to papers by the author. _proceedings royal society_, vol. xxv. pp. and ( ); and vol. xxxiv. p. ( ). [ ] photographs obtained since this was written show that much may happen after the stages here traced. [ ] a detailed account of the optical, mechanical, and electrical arrangements employed, written by mr. cole, will be found in _nature_, vol. i., p. (july , ). [ ] the black streaks, seen especially in figs. , , and , are due to particles of lamp-black carried down by the drop from the surface of the smoked watch-glass on which it rested. [ ] three of these photographs, viz. nos. , , and , are reproduced full size, as a frontispiece, by a _photographic_ process, to enable the reader to form a more correct idea than can be gathered from the engravings, of the amount of detail actually obtained, though even in these reproductions much is inevitably lost. the romance of science. _small post vo. illustrated. cloth boards._ _s. d._ coal; and what we get from it. by professor raphael meldola, f.r.s., f.i.c. colour measurement and mixture. by captain w. de w. abney, c.b., r.e., f.r.s. diseases of plants. by h. marshall ward, m.a., f.r.s., f.l.s. our secret friends and foes. by percy faraday frankland, ph.d., b.sc. 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"i have applied this new idea to every kind of difform motion and have thus developed mathematical formulas which i am convinced give more precise results than those based on newton's theory. newton's formulas, however, are such close approximations that it was difficult to find by observation any obvious disagreement with experience." dr. einstein, it must be remembered, is a physicist and not an astronomer. he developed his theory as a mathematical formula. the confirmation of it came from the astronomers. as he himself says, the crucial test was supplied by the last total solar eclipse. observations then proved that the rays of fixed stars, having to pass close to the sun to reach the earth, were deflected the exact amount demanded by einstein's formulas. the deflection was also in the direction predicted by him. the question must have occurred to many, what has all this to do with relativity? when this query was propounded by the times correspondent to dr. einstein he replied as follows: "the term relativity refers to time and space. according to galileo and newton, time and space were absolute entities, and the moving systems of the universe were dependent on this absolute time and space. on this conception was built the science of mechanics. the resulting formulas sufficed for all motions of a slow nature; it was found, however, that they would not conform to the rapid motions apparent in electrodynamics. "this led the dutch professor, lorentz, and myself to develop the theory of special relativity. briefly, it discards absolute time and space and makes them in every instance relative to moving systems. by this theory all phenomena in electrodynamics, as well as mechanics, hitherto irreducible by the old formulae--and there are multitudes--were satisfactorily explained. "till now it was believed that time and space existed by themselves, even if there was nothing else--no sun, no earth, no stars--while now we know that time and space are not the vessel for the universe, but could not exist at all if there were no contents, namely, no sun, earth and other celestial bodies. "this special relativity, forming the first part of my theory, relates to all systems moving with uniform motion; that is, moving in a straight line with equal velocity. "gradually i was led to the idea, seeming a very paradox in science, that it might apply equally to all moving systems, even of difform motion, and thus i developed the conception of general relativity which forms the second part of my theory." as summarized by an american astronomer, professor henry norris russell, of princeton, in the scientific american for november , einstein's contribution amounts to this: "the central fact which has been proved--and which is of great interest and importance--is that the natural phenomena involving gravitation and inertia (such as the motions of the planets) and the phenomena involving electricity and magnetism (including the motion of light) are not independent of one another, but are intimately related, so that both sets of phenomena should be regarded as parts of one vast system, embracing all nature. the relation of the two is, however, of such a character that it is perceptible only in a very few instances, and then only to refined observations." already before the war, einstein had immense fame among physicists, and among all who are interested in the philosophy of science, because of his principle of relativity. clerk maxwell had shown that light is electro-magnetic, and had reduced the whole theory of electro-magnetism to a small number of equations, which are fundamental in all subsequent work. but these equations were entangled with the hypothesis of the ether, and with the notion of motion relative to the ether. since the ether was supposed to be at rest, such motion was indistinguishable from absolute motion. the motion of the earth relatively to the ether should have been different at different points of its orbit, and measurable phenomena should have resulted from this difference. but none did, and all attempts to detect effects of motions relative to the ether failed. the theory of relativity succeeded in accounting for this fact. but it was necessary incidentally to throw over the one universal time, and substitute local times attached to moving bodies and varying according to their motion. the equations on which the theory of relativity is based are due to lorentz, but einstein connected them with his general principle, namely, that there must be nothing, in observable phenomena, which could be attributed to absolute motion of the observer. in orthodox newtonian dynamics the principle of relativity had a simpler form, which did not require the substitution of local time for general time. but it now appeared that newtonian dynamics is only valid when we confine ourselves to velocities much less than that of light. the whole galileo-newton system thus sank to the level of a first approximation, becoming progressively less exact as the velocities concerned approached that of light. einstein's extension of his principle so as to account for gravitation was made during the war, and for a considerable period our astronomers were unable to become acquainted with it, owing to the difficulty of obtaining german printed matter. however, copies of his work ultimately reached the outside world and enabled people to learn more about it. gravitation, ever since newton, had remained isolated from other forces in nature; various attempts had been made to account for it, but without success. the immense unification effected by electro-magnetism apparently left gravitation out of its scope. it seemed that nature had presented a challenge to the physicists which none of them were able to meet. at this point einstein intervened with a hypothesis which, apart altogether from subsequent verification, deserves to rank as one of the great monuments of human genius. after correcting newton, it remained to correct euclid, and it was in terms of non-euclidean geometry that he stated his new theory. non-euclidean geometry is a study of which the primary motive was logical and philosophical; few of its promoters ever dreamed that it would come to be applied in physics. some of euclid's axioms were felt to be not "necessary truths," but mere empirical laws; in order to establish this view, self-consistent geometries were constructed upon assumptions other than those of euclid. in these geometries the sum of the angles of a triangle is not two right angles, and the departure from two right angles increases as the size of the triangle increases. it is often said that in non-euclidean geometry space has a curvature, but this way of stating the matter is misleading, since it seems to imply a fourth dimension, which is not implied by these systems. einstein supposes that space is euclidean where it is sufficiently remote from matter, but that the presence of matter causes it to become slightly non-euclidean--the more matter there is in the neighborhood, the more space will depart from euclid. by the help of this hypothesis, together with his previous theory of relativity, he deduces gravitation--very approximately, but not exactly, according to the newtonian law of the inverse square. the minute differences between the effects deduced from his theory and those deduced from newton are measurable in certain cases. there are, so far, three crucial tests of the relative accuracy of the new theory and the old. ( ) the perihelion of mercury shows a discrepancy which has long puzzled astronomers. this discrepancy is fully accounted for by einstein. at the time when he published his theory, this was its only experimental verification. ( ) modern physicists were willing to suppose that light might be subject to gravitation--i.e., that a ray of light passing near a great mass like the sun might be deflected to the extent to which a particle moving with the same velocity would be deflected according to the orthodox theory of gravitation. but einstein's theory required that the light should be deflected just twice as much as this. the matter could only be tested during an eclipse among a number of bright stars. fortunately a peculiarly favourable eclipse occurred last year. the results of the observations have now been published, and are found to verify einstein's prediction. the verification is not, of course, quite exact; with such delicate observations that was not to be expected. in some cases the departure is considerable. but taking the average of the best series of observations, the deflection at the sun's limb is found to be . '', with a probable error of about per cent., whereas the deflection calculated by einstein's theory should be . ''. it will be noticed that einstein's theory gave a deflection twice as large as that predicted by the orthodox theory, and that the observed deflection is slightly larger than einstein predicted. the discrepancy is well within what might be expected in view of the minuteness of the measurements. it is therefore generally acknowledged by astronomers that the outcome is a triumph for einstein. ( ) in the excitement of this sensational verification, there has been a tendency to overlook the third experimental test to which einstein's theory was to be subjected. if his theory is correct as it stands, there ought, in a gravitational field, to be a displacement of the lines of the spectrum towards the red. no such effect has been discovered. spectroscopists maintain that, so far as can be seen at present, there is no way of accounting for this failure if einstein's theory in its present form is assumed. they admit that some compensating cause may be discovered to explain the discrepancy, but they think it far more probable that einstein's theory requires some essential modification. meanwhile, a certain suspense of judgment is called for. the new law has been so amazingly successful in two of the three tests that there must be some thing valid about it, even if it is not exactly right as yet. einstein's theory has the very highest degree of aesthetic merit: every lover of the beautiful must wish it to be true. it gives a vast unified survey of the operations of nature, with a technical simplicity in the critical assumptions which makes the wealth of deductions astonishing. it is a case of an advance arrived at by pure theory: the whole effect of einstein's work is to make physics more philosophical (in a good sense), and to restore some of that intellectual unity which belonged to the great scientific systems of the seventeenth and eighteenth centuries, but which was lost through increasing specialization and the overwhelming mass of detailed knowledge. in some ways our age is not a good one to live in, but for those who are interested in physics there are great compensations. the einstein theory of relativity a concise statement by prof. h. a. lorentz, of the university of leyden the total eclipse of the sun of may , resulted in a striking confirmation of the new theory of the universal attractive power of gravitation developed by albert einstein, and thus reinforced the conviction that the defining of this theory is one of the most important steps ever taken in the domain of natural science. in response to a request by the editor, i will attempt to contribute something to its general appreciation in the following lines. for centuries newton's doctrine of the attraction of gravitation has been the most prominent example of a theory of natural science. through the simplicity of its basic idea, an attraction between two bodies proportionate to their mass and also proportionate to the square of the distance; through the completeness with which it explained so many of the peculiarities in the movement of the bodies making up the solar system; and, finally, through its universal validity, even in the case of the far-distant planetary systems, it compelled the admiration of all. but, while the skill of the mathematicians was devoted to making more exact calculations of the consequences to which it led, no real progress was made in the science of gravitation. it is true that the inquiry was transferred to the field of physics, following cavendish's success in demonstrating the common attraction between bodies with which laboratory work can be done, but it always was evident that natural philosophy had no grip on the universal power of attraction. while in electric effects an influence exercised by the matter placed between bodies was speedily observed--the starting-point of a new and fertile doctrine of electricity--in the case of gravitation not a trace of an influence exercised by intermediate matter could ever be discovered. it was, and remained, inaccessible and unchangeable, without any connection, apparently, with other phenomena of natural philosophy. einstein has put an end to this isolation; it is now well established that gravitation affects not only matter, but also light. thus strengthened in the faith that his theory already has inspired, we may assume with him that there is not a single physical or chemical phenomenon--which does not feel, although very probably in an unnoticeable degree, the influence of gravitation, and that, on the other side, the attraction exercised by a body is limited in the first place by the quantity of matter it contains and also, to some degree, by motion and by the physical and chemical condition in which it moves. it is comprehensible that a person could not have arrived at such a far-reaching change of view by continuing to follow the old beaten paths, but only by introducing some sort of new idea. indeed, einstein arrived at his theory through a train of thought of great originality. let me try to restate it in concise terms. the earth as a moving car everyone knows that a person may be sitting in any kind of a vehicle without noticing its progress, so long as the movement does not vary in direction or speed; in a car of a fast express train objects fall in just the same way as in a coach that is standing still. only when we look at objects outside the train, or when the air can enter the car, do we notice indications of the motion. we may compare the earth with such a moving vehicle, which in its course around the sun has a remarkable speed, of which the direction and velocity during a considerable period of time may be regarded as constant. in place of the air now comes, so it was reasoned formerly, the ether which fills the spaces of the universe and is the carrier of light and of electro-magnetic phenomena; there were good reasons to assume that the earth was entirely permeable for the ether and could travel through it without setting it in motion. so here was a case comparable with that of a railroad coach open on all sides. there certainly should have been a powerful "ether wind" blowing through the earth and all our instruments, and it was to have been expected that some signs of it would be noticed in connection with some experiment or other. every attempt along that line, however, has remained fruitless; all the phenomena examined were evidently independent of the motion of the earth. that this is the way they do function was brought to the front by einstein in his first or "special" theory of relativity. for him the ether does not function and in the sketch that he draws of natural phenomena there is no mention of that intermediate matter. if the spaces of the universe are filled with an ether, let us suppose with a substance, in which, aside from eventual vibrations and other slight movements, there is never any crowding or flowing of one part alongside of another, then we can imagine fixed points existing in it; for example, points in a straight line, located one meter apart, points in a level plain, like the angles or squares on a chess board extending out into infinity, and finally, points in space as they are obtained by repeatedly shifting that level spot a distance of a meter in the direction perpendicular to it. if, consequently, one of the points is chosen as an "original point" we can, proceeding from that point, reach any other point through three steps in the common perpendicular directions in which the points are arranged. the figures showing how many meters are comprized in each of the steps may serve to indicate the place reached and to distinguish it from any other; these are, as is said, the "co-ordinates" of these places, comparable, for example, with the numbers on a map giving the longitude and latitude. let us imagine that each point has noted upon it the three numbers that give its position, then we have something comparable with a measure with numbered subdivisions; only we now have to do, one might say, with a good many imaginary measures in three common perpendicular directions. in this "system of co-ordinates" the numbers that fix the position of one or the other of the bodies may now be read off at any moment. this is the means which the astronomers and their mathematical assistants have always used in dealing with the movement of the heavenly bodies. at a determined moment the position of each body is fixed by its three co-ordinates. if these are given, then one knows also the common distances, as well as the angles formed by the connecting lines, and the movement of a planet is to be known as soon as one knows how its co-ordinates are changing from one moment to the other. thus the picture that one forms of the phenomena stands there as if it were sketched on the canvas of the motionless ether. einstein's departure since einstein has cut loose from the ether, he lacks this canvas, and therewith, at the first glance, also loses the possibility of fixing the positions of the heavenly bodies and mathematically describing their movement--i.e., by giving comparisons that define the positions at every moment. how einstein has overcome this difficulty may be somewhat elucidated through a simple illustration. on the surface of the earth the attraction of gravitation causes all bodies to fall along vertical lines, and, indeed, when one omits the resistance of the air, with an equally accelerated movement; the velocity increases in equal degrees in equal consecutive divisions of time at a rate that in this country gives the velocity attained at the end of a second as centimeters ( . feet) per second. the number defines the "acceleration in the field of gravitation," and this field is fully characterized by that single number; with its help we can also calculate the movement of an object hurled out in an arbitrary direction. in order to measure the acceleration we let the body drop alongside of a vertical measure set solidly on the ground; on this scale we read at every moment the figure that indicates the height, the only co-ordinate that is of importance in this rectilinear movement. now we ask what would we be able to see if the measure were not bound solidly to the earth, if it, let us suppose, moved down or up with the place where it is located and where we are ourselves. if in this case the speed were constant, then, and this is in accord with the special theory of relativity, there would be no motion observed at all; we should again find an acceleration of for a falling body. it would be different if the measure moved with changeable velocity. if it went down with a constant acceleration of itself, then an object could remain permanently at the same point on the measure, or could move up or down itself alongside of it, with constant speed. the relative movement of the body with regard to the measure should be without acceleration, and if we had to judge only by what we observed in the spot where we were and which was falling itself, then we should get the impression that there was no gravitation at all. if the measure goes down with an acceleration equal to a half or a third of what it just was, then the relative motion of the body will, of course, be accelerated, but we should find the increase in velocity per second one-half or two-thirds of . if, finally, we let the measure rise with a uniformly accelerated movement, then we shall find a greater acceleration than for the body itself. thus we see that we, also when the measure is not attached to the earth, disregarding its displacement, may describe the motion of the body in respect to the measure always in the same way--i.e., as one uniformly accelerated, as we ascribe now and again a fixed value to the acceleration of the sphere of gravitation, in a particular case the value of zero. of course, in the case here under consideration the use of a measure fixed immovably upon the earth should merit all recommendation. but in the spaces of the solar system we have, now that we have abandoned the ether, no such support. we can no longer establish a system of co-ordinates, like the one just mentioned, in a universal intermediate matter, and if we were to arrive in one way or another at a definite system of lines crossing each other in three directions, then we should be able to use just as well another similar system that in respect to the first moves this or that way. we should also be able to remodel the system of co-ordinates in all kinds of ways, for example by extension or compression. that in all these cases for fixed bodies that do not participate in the movement or the remodelling of the system other co-ordinates will be read off again and again is clear. new system or co-ordinates what way einstein had to follow is now apparent. he must--this hardly needs to be said--in calculating definite, particular cases make use of a chosen system of co-ordinates, but as he had no means of limiting his choice beforehand and in general, he had to reserve full liberty of action in this respect. therefore he made it his aim so to arrange the theory that, no matter how the choice was made, the phenomena of gravitation, so far as its effects and its stimulation by the attracting bodies are concerned, may always be described in the same way--i.e., through comparisons of the same general form, as we again and again give certain values to the numbers that mark the sphere of gravitation. (for the sake of simplification i here disregard the fact that einstein desires that also the way in which time is measured and represented by figures shall have no influence upon the central value of the comparisons.) whether this aim could be attained was a question of mathematical inquiry. it really was attained, remarkably enough, and, we may say, to the surprise of einstein himself, although at the cost of considerable simplicity in the mathematical form; it appeared necessary for the fixation of the field of gravitation in one or the other point in space to introduce no fewer than ten quantities in the place of the one that occurred in the example mentioned above. in this connection it is of importance to note that when we exclude certain possibilities that would give rise to still greater intricacy, the form of comparison used by einstein to present the theory is the only possible one; the principle of the freedom of choice in co-ordinates was the only one by which he needed to allow himself to be guided. although thus there was no special effort made to reach a connection with the theory of newton, it was evident, fortunately, at the end of the experiment that the connection existed. if we avail ourselves of the simplifying circumstance that the velocities of the heavenly bodies are slight in comparison with that of light, then we can deduce the theory of newton from the new theory, the "universal" relativity theory, as it is called by einstein. thus all the conclusions based upon the newtonian theory hold good, as must naturally be required. but now we have got further along. the newtonian theory can no longer be regarded as absolutely correct in all cases; there are slight deviations from it, which, although as a rule unnoticeable, once in a while fall within the range of observation. now, there was a difficulty in the movement of the planet mercury which could not be solved. even after all the disturbances caused by the attraction of other planets had been taken into account, there remained an inexplicable phenomenon--i.e., an extremely slow turning of the ellipsis described by mercury on its own plane; leverrier had found that it amounted to forty-three seconds a century. einstein found that, according to his formulas, this movement must really amount to just that much. thus with a single blow he solved one of the greatest puzzles of astronomy. still more remarkable, because it has a bearing upon a phenomenon which formerly could not be imagined, is the confirmation of einstein's prediction regarding the influence of gravitation upon the course of the rays of light. that such an influence must exist is taught by a simple examination; we have only to turn back for a moment to the following comparison in which we were just imagining ourselves to make our observations. it was noted that when the compartment is falling with the acceleration of the phenomena therein will occur just as if there were no attraction of gravitation. we can then see an object, a, stand still somewhere in open space. a projectile, b, can travel with constant speed along a horizontal line, without varying from it in the slightest. a ray of light can do the same; everybody will admit that in each case, if there is no gravitation, light will certainly extend itself in a rectilinear way. if we limit the light to a flicker of the slightest duration, so that only a little bit, c, of a ray of light arises, or if we fix our attention upon a single vibration of light, c, while we on the other hand give to the projectile, b, a speed equal to that of light, then we can conclude that b and c in their continued motion can always remain next to each other. now if we watch all this, not from the movable compartment, but from a place on the earth, then we shall note the usual falling movement of object a, which shows us that we have to deal with a sphere of gravitation. the projectile b will, in a bent path, vary more and more from a horizontal straight line, and the light will do the same, because if we observe the movements from another standpoint this can have no effect upon the remaining next to each other of b and c. deflection of light the bending of a ray of light thus described is much too light on the surface of the earth to be observed. but the attraction of gravitation exercised by the sun on its surface is, because of its great mass, more than twenty-seven times stronger, and a ray of light that goes close by the superficies of the sun must surely be noticeably bent. the rays of a star that are seen at a short distance from the edge of the sun will, going along the sun, deviate so much from the original direction that they strike the eye of an observer as if they came in a straight line from a point somewhat further removed than the real position of the star from the sun. it is at that point that we think we see the star; so here is a seeming displacement from the sun, which increases in the measure in which the star is observed closer to the sun. the einstein theory teaches that the displacement is in inverse proportion to the apparent distance of the star from the centre of the sun, and that for a star just on its edge it will amount to '. ( . seconds). this is approximately the thousandth part of the apparent diameter of the sun. naturally, the phenomenon can only be observed when there is a total eclipse of the sun; then one can take photographs of neighboring stars and through comparing the plate with a picture of the same part of the heavens taken at a time when the sun was far removed from that point the sought-for movement to one side may become apparent. thus to put the einstein theory to the test was the principal aim of the english expeditions sent out to observe the eclipse of may , one to prince's island, off the coast of guinea, and the other to sobral, brazil. the first-named expedition's observers were eddington and cottingham, those of the second, crommelin and davidson. the conditions were especially favorable, for a very large number of bright stars were shown on the photographic plate; the observers at sobral being particularly lucky in having good weather. the total eclipse lasted five minutes, during four of which it was perfectly clear, so that good photographs could be taken. in the report issued regarding the results the following figures, which are the average of the measurements made from the seven plates, are given for the displacements of seven stars: ''. , ''. , ''. , ''. , ''. , ''. , ''. , whereas, according to the theory, the displacements should have amounted to: ''. , ''. , ''. , ''. , ''. , ''. , ''. . if we consider that, according to the theory the displacements must be in inverse ratio to the distance from the centre of the sun, then we may deduce from each observed displacement how great the sideways movement for a star at the edge of the sun should have been. as the most probable result, therefore, the number ''. was found from all the observations together. as the last of the displacements given above--i.e., ''. is about one-eighth of this, we may say that the influence of the attraction of the sun upon light made itself felt upon the ray at a distance eight times removed from its centre. the displacements calculated according to the theory are, just because of the way in which they are calculated, in inverse proportion to the distance to the centre. now that the observed deviations also accord with the same rule, it follows that they are surely proportionate with the calculated displacements. the proportion of the first and the last observed sidewise movements is . , and that of the two most extreme of the calculated numbers is . . this result is of importance, because thereby the theory is excluded, or at least made extremely improbable, that the phenomenon of refraction is to be ascribed to, a ring of vapor surrounding the sun for a great distance. indeed, such a refraction should cause a deviation in the observed direction, and, in order to produce the displacement of one of the stars under observation itself a slight proximity of the vapor ring should be sufficient, but we have every reason to expect that if it were merely a question of a mass of gas around the sun the diminishing effect accompanying a removal from the sun should manifest itself much faster than is really the case. we cannot speak with perfect certainty here, as all the factors that might be of influence upon the distribution of density in a sun atmosphere are not well enough known, but we can surely demonstrate that in case one of the gasses with which we are acquainted were held in equilibrium solely by the influence of attraction of the sun the phenomenon should become much less as soon as we got somewhat further from the edge of the sun. if the displacement of the first star, which amounts to . -seconds were to be ascribed to such a mass of gas, then the displacement of the second must already be entirely inappreciable. so far as the absolute extent of the displacements is concerned, it was found somewhat too great, as has been shown by the figures given above; it also appears from the final result to be . for the edge of the sun--i.e., per cent, greater than the theoretical value of . . it indeed seems that the discrepancies may be ascribed to faults in observations, which supposition is supported by the fact that the observations at prince's island, which, it is true, did not turn out quite as well as those mentioned above, gave the result, of . , somewhat lower than einstein's figure. (the observations made with a second instrument at sobral gave a result of . , but the observers are of the opinion that because of the shifting of the mirror which reflected the rays no value is to be attached to it.) difficulty exaggerated during a discussion of the results obtained at a joint meeting of the royal society and the royal astronomical society held especially for that purpose recently in london, it was the general opinion that einstein's prediction might be regarded as justified, and warm tributes to his genius were made on all sides. nevertheless, i cannot refrain, while i am mentioning it, from expressing my surprise that, according to the report in the times there should be so much complaint about the difficulty of understanding the new theory. it is evident that einstein's little book "about the special and the general theory of relativity in plain terms," did not find its way into england during wartime. any one reading it will, in my opinion, come to the conclusion that the basic ideas of the theory are really clear and simple; it is only to be regretted that it was impossible to avoid clothing them in pretty involved mathematical terms, but we must not worry about that. i allow myself to add that, as we follow einstein, we may retain much of what has been formerly gained. the newtonian theory remains in its full value as the first great step, without which one cannot imagine the development of astronomy and without which the second step, that has now been made, would hardly have been possible. it remains, moreover, as the first, and in most cases, sufficient, approximation. it is true that, according to einstein's theory, because it leaves us entirely free as to the way in which we wish to represent the phenomena, we can imagine an idea of the solar system in which the planets follow paths of peculiar form and the rays of light shine along sharply bent lines--think of a twisted and distorted planetarium--but in every case where we apply it to concrete questions we shall so arrange it that the planets describe almost exact ellipses and the rays of light almost straight lines. it is not necessary to give up entirely even the ether. many natural philosophers find satisfaction in the idea of a material intermediate substance in which the vibrations of light take place, and they will very probably be all the more inclined to imagine such a medium when they learn that, according to the einstein theory, gravitation itself does not spread instantaneously, but with a velocity that at the first estimate may be compared with that of light. especially in former years were such interpretations current and repeated attempts were made by speculations about the nature of the ether and about the mutations and movements that might take place in it to arrive at a clear presentation of electro-magnetic phenomena, and also of the functioning of gravitation. in my opinion it is not impossible that in the future this road, indeed abandoned at present, will once more be followed with good results, if only because it can lead to the thinking out of new experimental tests. einstein's theory need not keep us from so doing; only the ideas about the ether must accord with it. nevertheless, even without the color and clearness that the ether theories and the other models may be able to give, and even, we can feel it this way, just because of the soberness induced by their absence, einstein's work, we may now positively expect, will remain a monument of science; his theory entirely fulfills the first and principal demand that we may make, that of deducing the course of phenomena from certain principles exactly and to the smallest details. it was certainly fortunate that he himself put the ether in the background; if he had not done so, he probably would never have come upon the idea that has been the foundation of all his examinations. thanks to his indefatigable exertions and perseverance, for he had great difficulties to overcome in his attempts, einstein has attained the results, which i have tried to sketch, while still young; he is now years old. he completed his first investigations in switzerland, where he first was engaged in the patent bureau at berne and later as a professor at the polytechnic in zurich. after having been a professor for a short time at the university of prague, he settled in berlin, where the kaiser wilhelm institute afforded him the opportunity to devote himself exclusively to his scientific work. he repeatedly visited our country and made his netherland colleagues, among whom he counts many good friends, partners in his studies and his results. he attended the last meeting of the department of natural philosophy of the royal academy of sciences, and the members then had the privilege of hearing him explain, in his own fascinating, clear and simple way, his interpretations of the fundamental questions to which his theory gives rise. note: project gutenberg also has an html version of this file which includes the original illustrations. see -h.htm or -h.zip: (http://www.gutenberg.net/dirs/ / / / / / -h/ -h.htm) or (http://www.gutenberg.net/dirs/ / / / / / -h.zip) experiments with alternate currents of high potential and high frequency a lecture delivered before the institution of electrical engineers, london by nikola tesla with a portrait and biographical sketch of the author new york biographical sketch of nikola tesla. while a large portion of the european family has been surging westward during the last three or four hundred years, settling the vast continents of america, another, but smaller, portion has been doing frontier work in the old world, protecting the rear by beating back the "unspeakable turk" and reclaiming gradually the fair lands that endure the curse of mohammedan rule. for a long time the slav people--who, after the battle of kosovopjolje, in which the turks defeated the servians, retired to the confines of the present montenegro, dalmatia, herzegovina and bosnia, and "borderland" of austria--knew what it was to deal, as our western pioneers did, with foes ceaselessly fretting against their frontier; and the races of these countries, through their strenuous struggle against the armies of the crescent, have developed notable qualities of bravery and sagacity, while maintaining a patriotism and independence unsurpassed in any other nation. it was in this interesting border region, and from among these valiant eastern folk, that nikola tesla was born in the year , and the fact that he, to-day, finds himself in america and one of our foremost electricians, is striking evidence of the extraordinary attractiveness alike of electrical pursuits and of the country where electricity enjoys its widest application. mr. tesla's native place was smiljan, lika, where his father was an eloquent clergyman of the greek church, in which, by the way, his family is still prominently represented. his mother enjoyed great fame throughout the countryside for her skill and originality in needlework, and doubtless transmitted her ingenuity to nikola; though it naturally took another and more masculine direction. the boy was early put to his books, and upon his father's removal to gospic he spent four years in the public school, and later, three years in the real school, as it is called. his escapades were such as most quick witted boys go through, although he varied the programme on one occasion by getting imprisoned in a remote mountain chapel rarely visited for service; and on another occasion by falling headlong into a huge kettle of boiling milk, just drawn from the paternal herds. a third curious episode was that connected with his efforts to fly when, attempting to navigate the air with the aid of an old umbrella, he had, as might be expected, a very bad fall, and was laid up for six weeks. about this period he began to take delight in arithmetic and physics. one queer notion he had was to work out everything by three or the power of three. he was now sent to an aunt at cartstatt, croatia, to finish his studies in what is known as the higher real school. it was there that, coming from the rural fastnesses, he saw a steam engine for the first time with a pleasure that he remembers to this day. at cartstatt he was so diligent as to compress the four years' course into three, and graduated in . returning home during an epidemic of cholera, he was stricken down by the disease and suffered so seriously from the consequences that his studies were interrupted for fully two years. but the time was not wasted, for he had become passionately fond of experimenting, and as much as his means and leisure permitted devoted his energies to electrical study and investigation. up to this period it had been his father's intention to make a priest of him, and the idea hung over the young physicist like a very sword of damocles. finally he prevailed upon his worthy but reluctant sire to send him to gratz in austria to finish his studies at the polytechnic school, and to prepare for work as professor of mathematics and physics. at gratz he saw and operated a gramme machine for the first time, and was so struck with the objections to the use of commutators and brushes that he made up his mind there and then to remedy that defect in dynamo-electric machines. in the second year of his course he abandoned the intention of becoming a teacher and took up the engineering curriculum. after three years of absence he returned home, sadly, to see his father die; but, having resolved to settle down in austria, and recognizing the value of linguistic acquirements, he went to prague and then to buda-pesth with the view of mastering the languages he deemed necessary. up to this time he had never realized the enormous sacrifices that his parents had made in promoting his education, but he now began to feel the pinch and to grow unfamiliar with the image of francis joseph i. there was considerable lag between his dispatches and the corresponding remittance from home; and when the mathematical expression for the value of the lag assumed the shape of an eight laid flat on its back, mr. tesla became a very fair example of high thinking and plain living, but he made up his mind to the struggle and determined to go through depending solely on his own resources. not desiring the fame of a faster, he cast about for a livelihood, and through the help of friends he secured a berth as assistant in the engineering department of the government telegraphs. the salary was five dollars a week. this brought him into direct contact with practical electrical work and ideas, but it is needless to say that his means did not admit of much experimenting. by the time he had extracted several hundred thousand square and cube roots for the public benefit, the limitations, financial and otherwise, of the position had become painfully apparent, and he concluded that the best thing to do was to make a valuable invention. he proceeded at once to make inventions, but their value was visible only to the eye of faith, and they brought no grist to the mill. just at this time the telephone made its appearance in hungary, and the success of that great invention determined his career, hopeless as the profession had thus far seemed to him. he associated himself at once with telephonic work, and made various telephonic inventions, including an operative repeater; but it did not take him long to discover that, being so remote from the scenes of electrical activity, he was apt to spend time on aims and results already reached by others, and to lose touch. longing for new opportunities and anxious for the development of which he felt himself possible, if once he could place himself within the genial and direct influences of the gulf streams of electrical thought, he broke away from the ties and traditions of the past, and in made his way to paris. arriving in that city, the ardent young likan obtained employment as an electrical engineer with one of the largest electric lighting companies. the next year he went to strasburg to install a plant, and on returning to paris sought to carry out a number of ideas that had now ripened into inventions. about this time, however, the remarkable progress of america in electrical industry attracted his attention, and once again staking everything on a single throw, he crossed the atlantic. mr. tesla buckled down to work as soon as he landed on these shores, put his best thought and skill into it, and soon saw openings for his talent. in a short while a proposition was made to him to start his own company, and, accepting the terms, he at once worked up a practical system of arc lighting, as well as a potential method of dynamo regulation, which in one form is now known as the "third brush regulation." he also devised a thermo-magnetic motor and other kindred devices, about which little was published, owing to legal complications. early in the tesla electric company of new york was formed, and not long after that mr. tesla produced his admirable and epoch-marking motors for multiphase alternating currents, in which, going back to his ideas of long ago, he evolved machines having neither commutator nor brushes. it will be remembered that about the time that mr. tesla brought out his motors, and read his thoughtful paper before the american institute of electrical engineers, professor ferraris, in europe, published his discovery of principles analogous to those enunciated by mr. tesla. there is no doubt, however, that mr. tesla was an independent inventor of this rotary field motor, for although anticipated in dates by ferraris, he could not have known about ferraris' work as it had not been published. professor ferraris stated himself, with becoming modesty, that he did not think tesla could have known of his (ferraris') experiments at that time, and adds that he thinks tesla was an independent and original inventor of this principle. with such an acknowledgment from ferraris there can be little doubt about tesla's originality in this matter. mr. tesla's work in this field was wonderfully timely, and its worth was promptly appreciated in various quarters. the tesla patents were acquired by the westinghouse electric company, who undertook to develop his motor and to apply it to work of different kinds. its use in mining, and its employment in printing, ventilation, etc., was described and illustrated in _the electrical world_ some years ago. the immense stimulus that the announcement of mr. tesla's work gave to the study of alternating current motors would, in itself, be enough to stamp him as a leader. mr. tesla is only years of age. he is tall and spare with a clean-cut, thin, refined face, and eyes that recall all the stories one has read of keenness of vision and phenomenal ability to see through things. he is an omnivorous reader, who never forgets; and he possesses the peculiar facility in languages that enables the least educated native of eastern europe to talk and write in at least half a dozen tongues. a more congenial companion cannot be desired for the hours when one "pours out heart affluence in discursive talk," and when the conversation, dealing at first with things near at hand and next to us, reaches out and rises to the greater questions of life, duty and destiny. in the year he severed his connection with the westinghouse company, since which time he has devoted himself entirely to the study of alternating currents of high frequencies and very high potentials, with which study he is at present engaged. no comment is necessary on his interesting achievements in this field; the famous london lecture published in this volume is a proof in itself. his first lecture on his researches in this new branch of electricity, which he may be said to have created, was delivered before the american institute of electrical engineers on may , , and remains one of the most interesting papers read before that society. it will be found reprinted in full in _the electrical world_, july , . its publication excited such interest abroad that he received numerous requests from english and french electrical engineers and scientists to repeat it in those countries, the result of which has been the interesting lecture published in this volume. the present lecture presupposes a knowledge of the former, but it may be read and understood by any one even though he has not read the earlier one. it forms a sort of continuation of the latter, and includes chiefly the results of his researches since that time. experiments with alternate currents of high potential and high frequency i cannot find words to express how deeply i feel the honor of addressing some of the foremost thinkers of the present time, and so many able scientific men, engineers and electricians, of the country greatest in scientific achievements. the results which i have the honor to present before such a gathering i cannot call my own. there are among you not a few who can lay better claim than myself on any feature of merit which this work may contain. i need not mention many names which are world-known--names of those among you who are recognized as the leaders in this enchanting science; but one, at least, i must mention--a name which could not be omitted in a demonstration of this kind. it is a name associated with the most beautiful invention ever made: it is crookes! when i was at college, a good time ago, i read, in a translation (for then i was not familiar with your magnificent language), the description of his experiments on radiant matter. i read it only once in my life--that time--yet every detail about that charming work i can remember this day. few are the books, let me say, which can make such an impression upon the mind of a student. but if, on the present occasion, i mention this name as one of many your institution can boast of, it is because i have more than one reason to do so. for what i have to tell you and to show you this evening concerns, in a large measure, that same vague world which professor crookes has so ably explored; and, more than this, when i trace back the mental process which led me to these advances--which even by myself cannot be considered trifling, since they are so appreciated by you--i believe that their real origin, that which started me to work in this direction, and brought me to them, after a long period of constant thought, was that fascinating little book which i read many years ago. and now that i have made a feeble effort to express my homage and acknowledge my indebtedness to him and others among you, i will make a second effort, which i hope you will not find so feeble as the first, to entertain you. give me leave to introduce the subject in a few words. a short time ago i had the honor to bring before our american institute of electrical engineers[a] some results then arrived at by me in a novel line of work. i need not assure you that the many evidences which i have received that english scientific men and engineers were interested in this work have been for me a great reward and encouragement. i will not dwell upon the experiments already described, except with the view of completing, or more clearly expressing, some ideas advanced by me before, and also with the view of rendering the study here presented self-contained, and my remarks on the subject of this evening's lecture consistent. [footnote a: for mr. tesla's american lecture on this subject see the electrical world of july , , and for a report of his french lecture see the electrical world of march , .] this investigation, then, it goes without saying, deals with alternating currents, and, to be more precise, with alternating currents of high potential and high frequency. just in how much a very high frequency is essential for the production of the results presented is a question which even with my present experience, would embarrass me to answer. some of the experiments may be performed with low frequencies; but very high frequencies are desirable, not only on account of the many effects secured by their use, but also as a convenient means of obtaining, in the induction apparatus employed, the high potentials, which in their turn are necessary to the demonstration of most of the experiments here contemplated. of the various branches of electrical investigation, perhaps the most interesting and immediately the most promising is that dealing with alternating currents. the progress in this branch of applied science has been so great in recent years that it justifies the most sanguine hopes. hardly have we become familiar with one fact, when novel experiences are met with and new avenues of research are opened. even at this hour possibilities not dreamed of before are, by the use of these currents, partly realized. as in nature all is ebb and tide, all is wave motion, so it seems that; in all branches of industry alternating currents--electric wave motion--will have the sway. one reason, perhaps, why this branch of science is being so rapidly developed is to be found in the interest which is attached to its experimental study. we wind a simple ring of iron with coils; we establish the connections to the generator, and with wonder and delight we note the effects of strange forces which we bring into play, which allow us to transform, to transmit and direct energy at will. we arrange the circuits properly, and we see the mass of iron and wires behave as though it were endowed with life, spinning a heavy armature, through invisible connections, with great speed and power--with the energy possibly conveyed from a great distance. we observe how the energy of an alternating current traversing the wire manifests itself--not so much in the wire as in the surrounding space--in the most surprising manner, taking the forms of heat, light, mechanical energy, and, most surprising of all, even chemical affinity. all these observations fascinate us, and fill us with an intense desire to know more about the nature of these phenomena. each day we go to our work in the hope of discovering,--in the hope that some one, no matter who, may find a solution of one of the pending great problems,--and each succeeding day we return to our task with renewed ardor; and even if we _are_ unsuccessful, our work has not been in vain, for in these strivings, in these efforts, we have found hours of untold pleasure, and we have directed our energies to the benefit of mankind. we may take--at random, if you choose--any of the many experiments which may be performed with alternating currents; a few of which only, and by no means the most striking, form the subject of this evening's demonstration: they are all equally interesting, equally inciting to thought. here is a simple glass tube from which the air has been partially exhausted. i take hold of it; i bring my body in contact with a wire conveying alternating currents of high potential, and the tube in my hand is brilliantly lighted. in whatever position i may put it, wherever i may move it in space, as far as i can reach, its soft, pleasing light persists with undiminished brightness. here is an exhausted bulb suspended from a single wire. standing on an insulated support. i grasp it, and a platinum button mounted in it is brought to vivid incandescence. here, attached to a leading wire, is another bulb, which, as i touch its metallic socket, is filled with magnificent colors of phosphorescent light. here still another, which by my fingers' touch casts a shadow--the crookes shadow, of the stem inside of it. here, again, insulated as i stand on this platform, i bring my body in contact with one of the terminals of the secondary of this induction coil--with the end of a wire many miles long--and you see streams of light break forth from its distant end, which is set in violent vibration. here, once more, i attach these two plates of wire gauze to the terminals of the coil. i set them a distance apart, and i set the coil to work. you may see a small spark pass between the plates. i insert a thick plate of one of the best dielectrics between them, and instead of rendering altogether impossible, as we are used to expect, i _aid_ the passage of the discharge, which, as i insert the plate, merely changes in appearance and assumes the form of luminous streams. is there, i ask, can there be, a more interesting study than that of alternating currents? in all these investigations, in all these experiments, which are so very, very interesting, for many years past--ever since the greatest experimenter who lectured in this hall discovered its principle--we have had a steady companion, an appliance familiar to every one, a plaything once, a thing of momentous importance now--the induction coil. there is no dearer appliance to the electrician. from the ablest among you, i dare say, down to the inexperienced student, to your lecturer, we all have passed many delightful hours in experimenting with the induction coil. we have watched its play, and thought and pondered over the beautiful phenomena which it disclosed to our ravished eyes. so well known is this apparatus, so familiar are these phenomena to every one, that my courage nearly fails me when i think that i have ventured to address so able an audience, that i have ventured to entertain you with that same old subject. here in reality is the same apparatus, and here are the same phenomena, only the apparatus is operated somewhat differently, the phenomena are presented in a different aspect. some of the results we find as expected, others surprise us, but all captivate our attention, for in scientific investigation each novel result achieved may be the centre of a new departure, each novel fact learned may lead to important developments. usually in operating an induction coil we have set up a vibration of moderate frequency in the primary, either by means of an interrupter or break, or by the use of an alternator. earlier english investigators, to mention only spottiswoode and j.e.h. gordon, have used a rapid break in connection with the coil. our knowledge and experience of to-day enables us to see clearly why these coils under the conditions of the tests did not disclose any remarkable phenomena, and why able experimenters failed to perceive many of the curious effects which have since been observed. in the experiments such as performed this evening, we operate the coil either from a specially constructed alternator capable of giving many thousands of reversals of current per second, or, by disruptively discharging a condenser through the primary, we set up a vibration in the secondary circuit of a frequency of many hundred thousand or millions per second, if we so desire; and in using either of these means we enter a field as yet unexplored. it is impossible to pursue an investigation in any novel line without finally making some interesting observation or learning some useful fact. that this statement is applicable to the subject of this lecture the many curious and unexpected phenomena which we observe afford a convincing proof. by way of illustration, take for instance the most obvious phenomena, those of the discharge of the induction coil. here is a coil which is operated by currents vibrating with extreme rapidity, obtained by disruptively discharging a leyden jar. it would not surprise a student were the lecturer to say that the secondary of this coil consists of a small length of comparatively stout wire; it would not surprise him were the lecturer to state that, in spite of this, the coil is capable of giving any potential which the best insulation of the turns is able to withstand: but although he may be prepared, and even be indifferent as to the anticipated result, yet the aspect of the discharge of the coil will surprise and interest him. every one is familiar with the discharge of an ordinary coil; it need not be reproduced here. but, by way of contrast, here is a form of discharge of a coil, the primary current of which is vibrating several hundred thousand times per second. the discharge of an ordinary coil appears as a simple line or band of light. the discharge of this coil appears in the form of powerful brushes and luminous streams issuing from all points of the two straight wires attached to the terminals of the secondary. (fig. .) [illustration: fig. .--discharge between two wires with frequencies of a few hundred thousand per second.] now compare this phenomenon which you have just witnessed with the discharge of a holtz or wimshurst machine--that other interesting appliance so dear to the experimenter. what a difference there is between these phenomena! and yet, had i made the necessary arrangements--which could have been made easily, were it not that they would interfere with other experiments--i could have produced with this coil sparks which, had i the coil hidden from your view and only two knobs exposed, even the keenest observer among you would find it difficult, if not impossible, to distinguish from those of an influence or friction machine. this may be done in many ways--for instance, by operating the induction coil which charges the condenser from an alternating-current machine of very low frequency, and preferably adjusting the discharge circuit so that there are no oscillations set up in it. we then obtain in the secondary circuit, if the knobs are of the required size and properly set, a more or less rapid succession of sparks of great intensity and small quantity, which possess the same brilliancy, and are accompanied by the same sharp crackling sound, as those obtained from a friction or influence machine. another way is to pass through two primary circuits, having a common secondary, two currents of a slightly different period, which produce in the secondary circuit sparks occurring at comparatively long intervals. but, even with the means at hand this evening, i may succeed in imitating the spark of a holtz machine. for this purpose i establish between the terminals of the coil which charges the condenser a long, unsteady arc, which is periodically interrupted by the upward current of air produced by it. to increase the current of air i place on each side of the arc, and close to it, a large plate of mica. the condenser charged from this coil discharges into the primary circuit of a second coil through a small air gap, which is necessary to produce a sudden rush of current through the primary. the scheme of connections in the present experiment is indicated in fig. . [illustration: fig. .--imitating the spark of a holtz machine.] g is an ordinarily constructed alternator, supplying the primary p of an induction coil, the secondary s of which charges the condensers or jars cc. the terminals of the secondary are connected to the inside coatings of the jars, the outer coatings being connected to the ends of the primary pp of a second induction coil. this primary pp has a small air gap ab. the secondary s of this coil is provided with knobs or spheres kk of the proper size and set at a distance suitable for the experiment. a long arc is established between the terminals ab of the first induction coil. mm are the mica plates. each time the arc is broken between a and b the jars are quickly charged and discharged through the primary pp, producing a snapping spark between the knobs kk. upon the arc forming between a and b the potential falls, and the jars cannot be charged to such high potential as to break through the air gap ab until the arc is again broken by the draught. in this manner sudden impulses, at long intervals, are produced in the primary pp, which in the secondary s give a corresponding number of impulses of great intensity. if the secondary knobs or spheres, kk, are of the proper size, the sparks show much resemblance to those of a holtz machine. but these two effects, which to the eye appear so very different, are only two of the many discharge phenomena. we only need to change the conditions of the test, and again we make other observations of interest. when, instead of operating the induction coil as in the last two experiments, we operate it from a high frequency alternator, as in the next experiment, a systematic study of the phenomena is rendered much more easy. in such case, in varying the strength and frequency of the currents through the primary, we may observe five distinct forms of discharge, which i have described in my former paper on the subject[a] before the american institute of electrical engineers, may , . [footnote a: see the electrical world, july , .] it would take too much time, and it would lead us too far from the subject presented this evening, to reproduce all these forms, but it seems to me desirable to show you one of them. it is a brush discharge, which is interesting in more than one respect. viewed from a near position it resembles much a jet of gas escaping under great pressure. we know that the phenomenon is due to the agitation of the molecules near the terminal, and we anticipate that some heat must be developed by the impact of the molecules against the terminal or against each other. indeed, we find that the brush is hot, and only a little thought leads us to the conclusion that, could we but reach sufficiently high frequencies, we could produce a brush which would give intense light and heat, and which would resemble in every particular an ordinary flame, save, perhaps, that both phenomena might not be due to the same agent--save, perhaps, that chemical affinity might not be _electrical_ in its nature. as the production of heat and light is here due to the impact of the molecules, or atoms of air, or something else besides, and, as we can augment the energy simply by raising the potential, we might, even with frequencies obtained from a dynamo machine, intensify the action to such a degree as to bring the terminal to melting heat. but with such low frequencies we would have to deal always with something of the nature of an electric current. if i approach a conducting object to the brush, a thin little spark passes, yet, even with the frequencies used this evening, the tendency to spark is not very great. so, for instance, if i hold a metallic sphere at some distance above the terminal you may see the whole space between the terminal and sphere illuminated by the streams without the spark passing; and with the much higher frequencies obtainable by the disruptive discharge of a condenser, were it not for the sudden impulses, which are comparatively few in number, sparking would not occur even at very small distances. however, with incomparably higher frequencies, which we may yet find means to produce efficiently, and provided that electric impulses of such high frequencies could be transmitted through a conductor, the electrical characteristics of the brush discharge would completely vanish--no spark would pass, no shock would be felt--yet we would still have to deal with an _electric_ phenomenon, but in the broad, modern interpretation of the word. in my first paper before referred to i have pointed out the curious properties of the brush, and described the best manner of producing it, but i have thought it worth while to endeavor to express myself more clearly in regard to this phenomenon, because of its absorbing interest. when a coil is operated with currents of very high frequency, beautiful brush effects may be produced, even if the coil be of comparatively small dimensions. the experimenter may vary them in many ways, and, if it were nothing else, they afford a pleasing sight. what adds to their interest is that they may be produced with one single terminal as well as with two--in fact, often better with one than with two. but of all the discharge phenomena observed, the most pleasing to the eye, and the most instructive, are those observed with a coil which is operated by means of the disruptive discharge of a condenser. the power of the brushes, the abundance of the sparks, when the conditions are patiently adjusted, is often amazing. with even a very small coil, if it be so well insulated as to stand a difference of potential of several thousand volts per turn, the sparks may be so abundant that the whole coil may appear a complete mass of fire. curiously enough the sparks, when the terminals of the coil are set at a considerable distance, seem to dart in every possible direction as though the terminals were perfectly independent of each other. as the sparks would soon destroy the insulation it is necessary to prevent them. this is best done by immersing the coil in a good liquid insulator, such as boiled-out oil. immersion in a liquid may be considered almost an absolute necessity for the continued and successful working of such a coil. it is of course out of the question, in an experimental lecture, with only a few minutes at disposal for the performance of each experiment, to show these discharge phenomena to advantage, as to produce each phenomenon at its best a very careful adjustment is required. but even if imperfectly produced, as they are likely to be this evening, they are sufficiently striking to interest an intelligent audience. before showing some of these curious effects i must, for the sake of completeness, give a short description of the coil and other apparatus used in the experiments with the disruptive discharge this evening. [illustration: fig. .--disruptive discharge coil.] it is contained in a box b (fig. ) of thick boards of hard wood, covered on the outside with zinc sheet z, which is carefully soldered all around. it might be advisable, in a strictly scientific investigation, when accuracy is of great importance, to do away with the metal cover, as it might introduce many errors, principally on account of its complex action upon the coil, as a condenser of very small capacity and as an electrostatic and electromagnetic screen. when the coil is used for such experiments as are here contemplated, the employment of the metal cover offers some practical advantages, but these are not of sufficient importance to be dwelt upon. the coil should be placed symmetrically to the metal cover, and the space between should, of course, not be too small, certainly not less than, say, five centimetres, but much more if possible; especially the two sides of the zinc box, which are at right angles to the axis of the coil, should be sufficiently remote from the latter, as otherwise they might impair its action and be a source of loss. the coil consists of two spools of hard rubber rr, held apart at a distance of centimetres by bolts c and nuts n, likewise of hard rubber. each spool comprises a tube t of approximately centimetres inside diameter, and millimetres thick, upon which are screwed two flanges ff, centimetres square, the space between the flanges being about centimetres. the secondary, ss, of the best gutta percha-covered wire, has layers, turns in each, giving for each half a total of turns. the two halves are wound oppositely and connected in series, the connection between both being made over the primary. this disposition, besides being convenient, has the advantage that when the coil is well balanced--that is, when both of its terminals t_ t_ are connected to bodies or devices of equal capacity--there is not much danger of breaking through to the primary, and the insulation between the primary and the secondary need not be thick. in using the coil it is advisable to attach to _both_ terminals devices of nearly equal capacity, as, when the capacity of the terminals is not equal, sparks will be apt to pass to the primary. to avoid this, the middle point of the secondary may be connected to the primary, but this is not always practicable. the primary pp is wound in two parts, and oppositely, upon a wooden spool w, and the four ends are led out of the oil through hard rubber tubes tt. the ends of the secondary t_ t_ are also led out of the oil through rubber tubes t_ t_ of great thickness. the primary and secondary layers are insulated by cotton cloth, the thickness of the insulation, of course, bearing some proportion to the difference of potential between the turns of the different layers. each half of the primary has four layers, turns in each, this giving a total of turns. when both the parts are connected in series, this gives a ratio of conversion of about : . , and with the primaries in multiple, : . ; but in operating with very rapidly alternating currents this ratio does not convey even an approximate idea of the ratio of the e.m.fs. in the primary and secondary circuits. the coil is held in position in the oil on wooden supports, there being about centimetres thickness of oil all round. where the oil is not specially needed, the space is filled with pieces of wood, and for this purpose principally the wooden box b surrounding the whole is used. the construction here shown is, of course, not the best on general principles, but i believe it is a good and convenient one for the production of effects in which an excessive potential and a very small current are needed. in connection with the coil i use either the ordinary form of discharger or a modified form. in the former i have introduced two changes which secure some advantages, and which are obvious. if they are mentioned, it is only in the hope that some experimenter may find them of use. [illustration: fig. .--arrangement of improved discharger and magnet.] one of the changes is that the adjustable knobs a and b (fig. ), of the discharger are held in jaws of brass, jj, by spring pressure, this allowing of turning them successively into different positions, and so doing away with the tedious process of frequent polishing up. the other change consists in the employment of a strong electromagnet ns, which is placed with its axis at right angles to the line joining the knobs a and b, and produces a strong magnetic field between them. the pole pieces of the magnet are movable and properly formed so as to protrude between the brass knobs, in order to make the field as intense as possible; but to prevent the discharge from jumping to the magnet the pole pieces are protected by a layer of mica, mm, of sufficient thickness. s_ s_ and s_ s_ are screws for fastening the wires. on each side one of the screws is for large and the other for small wires. ll are screws for fixing in position the rods rr, which support the knobs. in another arrangement with the magnet i take the discharge between the rounded pole pieces themselves, which in such case are insulated and preferably provided with polished brass caps. the employment of an intense magnetic field is of advantage principally when the induction coil or transformer which charges the condenser is operated by currents of very low frequency. in such a case the number of the fundamental discharges between the knobs may be so small as to render the currents produced in the secondary unsuitable for many experiments. the intense magnetic field then serves to blow out the arc between the knobs as soon as it is formed, and the fundamental discharges occur in quicker succession. instead of the magnet, a draught or blast of air may be employed with some advantage. in this case the arc is preferably established between the knobs ab, in fig. (the knobs ab being generally joined, or entirely done away with), as in this disposition the arc is long and unsteady, and is easily affected by the draught. when a magnet is employed to break the arc, it is better to choose the connection indicated diagrammatically in fig. , as in this case the currents forming the arc are much more powerful, and the magnetic field exercises a greater influence. the use of the magnet permits, however, of the arc being replaced by a vacuum tube, but i have encountered great difficulties in working with an exhausted tube. [illustration: fig. .--arrangement with low-frequency alternator and improved discharger.] [illustration: fig. .--discharger with multiple gaps.] the other form of discharger used in these and similar experiments is indicated in figs. and . it consists of a number of brass pieces cc (fig. ), each of which comprises a spherical middle portion m with an extension e below--which is merely used to fasten the piece in a lathe when polishing up the discharging surface--and a column above, which consists of a knurled flange f surmounted by a threaded stem l carrying a nut n, by means of which a wire is fastened to the column. the flange f conveniently serves for holding the brass piece when fastening the wire, and also for turning it in any position when it becomes necessary to present a fresh discharging surface. two stout strips of hard rubber rr, with planed grooves gg (fig. ) to fit the middle portion of the pieces cc, serve to clamp the latter and hold them firmly in position by means of two bolts cc (of which only one is shown) passing through the ends of the strips. [illustration: fig. .--discharger with multiple gaps.] in the use of this kind of discharger i have found three principal advantages over the ordinary form. first, the dielectric strength of a given total width of air space is greater when a great many small air gaps are used instead of one, which permits of working with a smaller length of air gap, and that means smaller loss and less deterioration of the metal; secondly by reason of splitting the arc up into smaller arcs, the polished surfaces are made to last much longer; and, thirdly, the apparatus affords some gauge in the experiments. i usually set the pieces by putting between them sheets of uniform thickness at a certain very small distance which is known from the experiments of sir william thomson to require a certain electromotive force to be bridged by the spark. it should, of course, be remembered that the sparking distance is much diminished as the frequency is increased. by taking any number of spaces the experimenter has a rough idea of the electromotive force, and he finds it easier to repeat an experiment, as he has not the trouble of setting the knobs again and again. with this kind of discharger i have been able to maintain an oscillating motion without any spark being visible with the naked eye between the knobs, and they would not show a very appreciable rise in temperature. this form of discharge also lends itself to many arrangements of condensers and circuits which are often very convenient and time-saving. i have used it preferably in a disposition similar to that indicated in fig. , when the currents forming the arc are small. i may here mention that i have also used dischargers with single or multiple air gaps, in which the discharge surfaces were rotated with great speed. no particular advantage was, however, gained by this method, except in cases where the currents from the condenser were large and the keeping cool of the surfaces was necessary, and in cases when, the discharge not being oscillating of itself, the arc as soon as established was broken by the air current, thus starting the vibration at intervals in rapid succession. i have also used mechanical interrupters in many ways. to avoid the difficulties with frictional contacts, the preferred plan adopted was to establish the arc and rotate through it at great speed a rim of mica provided with many holes and fastened to a steel plate. it is understood, of course, that the employment of a magnet, air current, or other interrupter, produces no effect worth noticing, unless the self-induction, capacity and resistance are so related that there are oscillations set up upon each interruption. i will now endeavor to show you some of the most noteworthy of these discharge phenomena. i have stretched across the room two ordinary cotton covered wires, each about metres in length. they are supported on insulating cords at a distance of about centimetres. i attach now to each of the terminals of the coil one of the wires and set the coil in action. upon turning the lights off in the room you see the wires strongly illuminated by the streams issuing abundantly from their whole surface in spite of the cotton covering, which may even be very thick. when the experiment is performed under good conditions, the light from the wires is sufficiently intense to allow distinguishing the objects in a room. to produce the best result it is, of course, necessary to adjust carefully the capacity of the jars, the arc between the knobs and the length of the wires. my experience is that calculation of the length of the wires leads, in such case, to no result whatever. the experimenter will do best to take the wires at the start very long, and then adjust by cutting off first long pieces, and then smaller and smaller ones as he approaches the right length. a convenient way is to use an oil condenser of very small capacity, consisting of two small adjustable metal plates, in connection with this and similar experiments. in such case i take wires rather short and set at the beginning the condenser plates at maximum distance. if the streams for the wires increase by approach of the plates, the length of the wires is about right; if they diminish the wires are too long for that frequency and potential. when a condenser is used in connection with experiments with such a coil, it should be an oil condenser by all means, as in using an air condenser considerable energy might be wasted. the wires leading to the plates in the oil should be very thin, heavily coated with some insulating compound, and provided with a conducting covering--this preferably extending under the surface of the oil. the conducting cover should not be too near the terminals, or ends, of the wire, as a spark would be apt to jump from the wire to it. the conducting coating is used to diminish the air losses, in virtue of its action as an electrostatic screen. as to the size of the vessel containing the oil, and the size of the plates, the experimenter gains at once an idea from a rough trial. the size of the plates _in oil_ is, however, calculable, as the dielectric losses are very small. in the preceding experiment it is of considerable interest to know what relation the quantity of the light emitted bears to the frequency and potential of the electric impulses. my opinion is that the heat as well as light effects produced should be proportionate, under otherwise equal conditions of test, to the product of frequency and square of potential, but the experimental verification of the law, whatever it may be, would be exceedingly difficult. one thing is certain, at any rate, and that is, that in augmenting the potential and frequency we rapidly intensify the streams; and, though it may be very sanguine, it is surely not altogether hopeless to expect that we may succeed in producing a practical illuminant on these lines. we would then be simply using burners or flames, in which there would be no chemical process, no consumption of material, but merely a transfer of energy, and which would, in all probability emit more light and less heat than ordinary flames. [illustration: fig. .--effect produced by concentrating streams.] the luminous intensity of the streams is, of course, considerably increased when they are focused upon a small surface. this may be shown by the following experiment: i attach to one of the terminals of the coil a wire w (fig. ), bent in a circle of about centimetres in diameter, and to the other terminal i fasten a small brass sphere s, the surface of the wire being preferably equal to the surface of the sphere, and the centre of the latter being in a line at right angles to the plane of the wire circle and passing through its centre. when the discharge is established under proper conditions, a luminous hollow cone is formed, and in the dark one-half of the brass sphere is strongly illuminated, as shown in the cut. by some artifice or other, it is easy to concentrate the streams upon small surfaces and to produce very strong light effects. two thin wires may thus be rendered intensely luminous. in order to intensify the streams the wires should be very thin and short; but as in this case their capacity would be generally too small for the coil--at least, for such a one as the present--it is necessary to augment the capacity to the required value, while, at the same time, the surface of the wires remains very small. this may be done in many ways. here, for instance, i have two plates, rr, of hard rubber (fig. ), upon which i have glued two very thin wires ww, so as to form a name. the wires may be bare or covered with the best insulation--it is immaterial for the success of the experiment. well insulated wires, if anything, are preferable. on the back of each plate, indicated by the shaded portion, is a tinfoil coating tt. the plates are placed in line at a sufficient distance to prevent a spark passing from one to the other wire. the two tinfoil coatings i have joined by a conductor c, and the two wires i presently connect to the terminals of the coil. it is now easy, by varying the strength and frequency of the currents through the primary, to find a point at which, the capacity of the system is best suited to the conditions, and the wires become so strongly luminous that, when the light in the room is turned off the name formed by them appears in brilliant letters. [illustration: fig. .--wires rendered intensely luminous.] it is perhaps preferable to perform this experiment with a coil operated from an alternator of high frequency, as then, owing to the harmonic rise and fall, the streams are very uniform, though they are less abundant then when produced with such a coil as the present. this experiment, however, may be performed with low frequencies, but much less satisfactorily. [illustration: fig. .--luminous discs.] when two wires, attached to the terminals of the coil, are set at the proper distance, the streams between them may be so intense as to produce a continuous luminous sheet. to show this phenomenon i have here two circles, c and c (fig. ), of rather stout wire, one being about centimetres and the other centimetres in diameter. to each of the terminals of the coil i attach one of the circles. the supporting wires are so bent that the circles may be placed in the same plane, coinciding as nearly as possible. when the light in the room is turned off and the coil set to work, you see the whole space between the wires uniformly filled with streams, forming a luminous disc, which could be seen from a considerable distance, such is the intensity of the streams. the outer circle could have been much larger than the present one; in fact, with this coil i have used much larger circles, and i have been able to produce a strongly luminous sheet, covering an area of more than one square metre, which is a remarkable effect with this very small coil. to avoid uncertainty, the circle has been taken smaller, and the area is now about . square metre. the frequency of the vibration, and the quickness of succession of the sparks between the knobs, affect to a marked degree the appearance of the streams. when the frequency is very low, the air gives way in more or less the same manner, as by a steady difference of potential, and the streams consist of distinct threads, generally mingled with thin sparks, which probably correspond to the successive discharges occurring between the knobs. but when the frequency is extremely high, and the arc of the discharge produces a very _loud_ but _smooth_ sound--showing both that oscillation takes place and that the sparks succeed each other with great rapidity--then the luminous streams formed are perfectly uniform. to reach this result very small coils and jars of small capacity should be used. i take two tubes of thick bohemian glass, about centimetres in diameter and centimetres long. in each of the tubes i slip a primary of very thick copper wire. on the top of each tube i wind a secondary of much thinner gutta-percha covered wire. the two secondaries i connect in series, the primaries preferably in multiple arc. the tubes are then placed in a large glass vessel, at a distance of to centimetres from each other, on insulating supports, and the vessel is filled with boiled out oil, the oil reaching about an inch above the tubes. the free ends of the secondary are lifted out of the oil and placed parallel to each other at a distance of about centimetres. the ends which are scraped should be dipped in the oil. two four-pint jars joined in series may be used to discharge through the primary. when the necessary adjustments in the length and distance of the wires above the oil and in the arc of discharge are made, a luminous sheet is produced between the wires which is perfectly smooth and textureless, like the ordinary discharge through a moderately exhausted tube. i have purposely dwelt upon this apparently insignificant experiment. in trials of this kind the experimenter arrives at the startling conclusion that, to pass ordinary luminous discharges through gases, no particular degree of exhaustion is needed, but that the gas may be at ordinary or even greater pressure. to accomplish this, a very high frequency is essential; a high potential is likewise required, but this is a merely incidental necessity. these experiments teach us that, in endeavoring to discover novel methods of producing light by the agitation of atoms, or molecules, of a gas, we need not limit our research to the vacuum tube, but may look forward quite seriously to the possibility of obtaining the light effects without the use of any vessel whatever, with air at ordinary pressure. such discharges of very high frequency, which render luminous the air at ordinary pressures, we have probably often occasion to witness in nature. i have no doubt that if, as many believe, the aurora borealis is produced by sudden cosmic disturbances, such as eruptions at the sun's surface, which set the electrostatic charge of the earth in an extremely rapid vibration, the red glow observed is not confined to the upper rarefied strata of the air, but the discharge traverses, by reason of its very high frequency, also the dense atmosphere in the form of a _glow_, such as we ordinarily produce in a slightly exhausted tube. if the frequency were very low, or even more so, if the charge were not at all vibrating, the dense air would break down as in a lightning discharge. indications of such breaking down of the lower dense strata of the air have been repeatedly observed at the occurrence of this marvelous phenomenon; but if it does occur, it can only be attributed to the fundamental disturbances, which are few in number, for the vibration produced by them would be far too rapid to allow a disruptive break. it is the original and irregular impulses which affect the instruments; the superimposed vibrations probably pass unnoticed. when an ordinary low frequency discharge is passed through moderately rarefied air, the air assumes a purplish hue. if by some means or other we increase the intensity of the molecular, or atomic, vibration, the gas changes to a white color. a similar change occurs at ordinary pressures with electric impulses of very high frequency. if the molecules of the air around a wire are moderately agitated, the brush formed is reddish or violet; if the vibration is rendered sufficiently intense, the streams become white. we may accomplish this in various ways. in the experiment before shown with the two wires across the room, i have endeavored to secure the result by pushing to a high value both the frequency and potential: in the experiment with the thin wires glued on the rubber plate i have concentrated the action upon a very small surface--in other words, i have worked with a great electric density. a most curious form of discharge is observed with such a coil when the frequency and potential are pushed to the extreme limit. to perform the experiment, every part of the coil should be heavily insulated, and only two small spheres--or, better still, two sharp-edged metal discs (dd, fig. ) of no more than a few centimetres in diameter--should be exposed to the air. the coil here used is immersed in oil, and the ends of the secondary reaching out of the oil are covered with an air-tight cover of hard rubber of great thickness. all cracks, if there are any, should be carefully stopped up, so that the brush discharge cannot form anywhere except on the small spheres or plates which are exposed to the air. in this case, since there are no large plates or other bodies of capacity attached to the terminals, the coil is capable of an extremely rapid vibration. the potential may be raised by increasing, as far as the experimenter judges proper, the rate of change of the primary current. with a coil not widely differing from the present, it is best to connect the two primaries in multiple arc; but if the secondary should have a much greater number of turns the primaries should preferably be used in series, as otherwise the vibration might be too fast for the secondary. it occurs under these conditions that misty white streams break forth from the edges of the discs and spread out phantom-like into space. with this coil, when fairly well produced, they are about to centimetres long. when the hand is held against them no sensation is produced, and a spark, causing a shock, jumps from the terminal only upon the hand being brought much nearer. if the oscillation of the primary current is rendered intermittent by some means or other, there is a corresponding throbbing of the streams, and now the hand or other conducting object may be brought in still greater proximity to the terminal without a spark being caused to jump. [illustration: fig. .--phantom streams.] among the many beautiful phenomena which may be produced with such a coil i have here selected only those which appear to possess some features of novelty, and lead us to some conclusions of interest. one will not find it at all difficult to produce in the laboratory, by means of it, many other phenomena which appeal to the eye even more than these here shown, but present no particular feature of novelty. early experimenters describe the display of sparks produced by an ordinary large induction coil upon an insulating plate separating the terminals. quite recently siemens performed some experiments in which fine effects were obtained, which were seen by many with interest. no doubt large coils, even if operated with currents of low frequencies, are capable of producing beautiful effects. but the largest coil ever made could not, by far, equal the magnificent display of streams and sparks obtained from such a disruptive discharge coil when properly adjusted. to give an idea, a coil such as the present one will cover easily a plate of metre in diameter completely with the streams. the best way to perform such experiments is to take a very thin rubber or a glass plate and glue on one side of it a narrow ring of tinfoil of very large diameter, and on the other a circular washer, the centre of the latter coinciding with that of the ring, and the surfaces of both being preferably equal, so as to keep the coil well balanced. the washer and ring should be connected to the terminals by heavily insulated thin wires. it is easy in observing the effect of the capacity to produce a sheet of uniform streams, or a fine network of thin silvery threads, or a mass of loud brilliant sparks, which completely cover the plate. since i have advanced the idea of the conversion by means of the disruptive discharge, in my paper before the american institute of electrical engineers at the beginning of the past year, the interest excited in it has been considerable. it affords us a means for producing any potentials by the aid of inexpensive coils operated from ordinary systems of distribution, and--what is perhaps more appreciated--it enables us to convert currents of any frequency into currents of any other lower or higher frequency. but its chief value will perhaps be found in the help which it will afford us in the investigations of the phenomena of phosphorescence, which a disruptive discharge coil is capable of exciting in innumerable cases where ordinary coils, even the largest, would utterly fail. considering its probable uses for many practical purposes, and its possible introduction into laboratories for scientific research, a few additional remarks as to the construction of such a coil will perhaps not be found superfluous. it is, of course, absolutely necessary to employ in such a coil wires provided with the best insulation. good coils may be produced by employing wires covered with several layers of cotton, boiling the coil a long time in pure wax, and cooling under moderate pressure. the advantage of such a coil is that it can be easily handled, but it cannot probably give as satisfactory results as a coil immersed in pure oil. besides, it seems that the presence of a large body of wax affects the coil disadvantageously, whereas this does not seem to be the case with oil. perhaps it is because the dielectric losses in the liquid are smaller. i have tried at first silk and cotton covered wires with oil immersion, but i have been gradually led to use gutta-percha covered wires, which proved most satisfactory. gutta-percha insulation adds, of course, to the capacity of the coil, and this, especially if the coil be large, is a great disadvantage when extreme frequencies are desired; but on the other hand, gutta-percha will withstand much more than an equal thickness of oil, and this advantage should be secured at any price. once the coil has been immersed, it should never be taken out of the oil for more than a few hours, else the gutta-percha will crack up and the coil will not be worth half as much as before. gutta-percha is probably slowly attacked by the oil, but after an immersion of eight to nine months i have found no ill effects. i have obtained in commerce two kinds of gutta-percha wire: in one the insulation sticks tightly to the metal, in the other it does not. unless a special method is followed to expel all air, it is much safer to use the first kind. i wind the coil within an oil tank so that all interstices are filled up with the oil. between the layers i use cloth boiled out thoroughly in oil, calculating the thickness according to the difference of potential between the turns. there seems not to be a very great difference whatever kind of oil is used; i use paraffine or linseed oil. to exclude more perfectly the air, an excellent way to proceed, and easily practicable with small coils, is the following: construct a box of hard wood of very thick boards which have been for a long time boiled in oil. the boards should be so joined as to safely withstand the external air pressure. the coil being placed and fastened in position within the box, the latter is closed with a strong lid, and covered with closely fitting metal sheets, the joints of which are soldered very carefully. on the top two small holes are drilled, passing through the metal sheet and the wood, and in these holes two small glass tubes are inserted and the joints made air-tight. one of the tubes is connected to a vacuum pump, and the other with a vessel containing a sufficient quantity of boiled-out oil. the latter tube has a very small hole at the bottom, and is provided with a stopcock. when a fairly good vacuum has been obtained, the stopcock is opened and the oil slowly fed in. proceeding in this manner, it is impossible that any big bubbles, which are the principal danger, should remain between the turns. the air is most completely excluded, probably better than by boiling out, which, however, when gutta-percha coated wires are used, is not practicable. for the primaries i use ordinary line wire with a thick cotton coating. strands of very thin insulated wires properly interlaced would, of course, be the best to employ for the primaries, but they are not to be had. in an experimental coil the size of the wires is not of great importance. in the coil here used the primary is no. and the secondary no. brown & sharpe gauge wire; but the sections may be varied considerably. it would only imply different adjustments; the results aimed at would not be materially affected. i have dwelt at some length upon the various forms of brush discharge because, in studying them, we not only observe phenomena which please our eye, but also afford us food for thought, and lead us to conclusions of practical importance. in the use of alternating currents of very high tension, too much precaution cannot be taken to prevent the brush discharge. in a main conveying such currents, in an induction coil or transformer, or in a condenser, the brush discharge is a source of great danger to the insulation. in a condenser especially the gaseous matter must be most carefully expelled, for in it the charged surfaces are near each other, and if the potentials are high, just as sure as a weight will fall if let go, so the insulation will give way if a single gaseous bubble of some size be present, whereas, if all gaseous matter were carefully excluded, the condenser would safely withstand a much higher difference of potential. a main conveying alternating currents of very high tension may be injured merely by a blow hole or small crack in the insulation, the more so as a blowhole is apt to contain gas at low pressure; and as it appears almost impossible to completely obviate such little imperfections, i am led to believe that in our future distribution of electrical energy by currents of very high tension liquid insulation will be used. the cost is a great drawback, but if we employ an oil as an insulator the distribution of electrical energy with something like , volts, and even more, become, at least with higher frequencies, so easy that they could be hardly called engineering feats. with oil insulation and alternate current motors transmissions of power can be effected with safety and upon an industrial basis at distances of as much as a thousand miles. a peculiar property of oils, and liquid insulation in general, when subjected to rapidly changing electric stresses, is to disperse any gaseous bubbles which may be present, and diffuse them through its mass, generally long before any injurious break can occur. this feature may be easily observed with an ordinary induction coil by taking the primary out, plugging up the end of the tube upon which the secondary is wound, and filling it with some fairly transparent insulator, such as paraffine oil. a primary of a diameter something like six millimetres smaller than the inside of the tube may be inserted in the oil. when the coil is set to work one may see, looking from the top through the oil, many luminous points--air bubbles which are caught by inserting the primary, and which are rendered luminous in consequence of the violent bombardment. the occluded air, by its impact against the oil, heats it; the oil begins to circulate, carrying some of the air along with it, until the bubbles are dispersed and the luminous points disappear. in this manner, unless large bubbles are occluded in such way that circulation is rendered impossible, a damaging break is averted, the only effect being a moderate warming up of the oil. if, instead of the liquid, a solid insulation, no matter how thick, were used, a breaking through and injury of the apparatus would be inevitable. the exclusion of gaseous matter from any apparatus in which the dielectric is subjected to more or less rapidly changing electric forces is, however, not only desirable in order to avoid a possible injury of the apparatus, but also on account of economy. in a condenser, for instance, as long as only a solid or only a liquid dielectric is used, the loss is small; but if a gas under ordinary or small pressure be present the loss may be very great. whatever the nature of the force acting in the dielectric may be, it seems that in a solid or liquid the molecular displacement produced by the force is small; hence the product of force and displacement is insignificant, unless the force be very great; but in a gas the displacement, and therefore this product, is considerable; the molecules are free to move, they reach high speeds, and the energy of their impact is lost in heat or otherwise. if the gas be strongly compressed, the displacement due to the force is made smaller, and the losses are reduced. in most of the succeeding experiments i prefer, chiefly on account of the regular and positive action, to employ the alternator before referred to. this is one of the several machines constructed by me for the purposes of these investigations. it has pole projections, and is capable of giving currents of a frequency of about , per second. this machine has been illustrated and briefly described in my first paper before the american institute of electrical engineers, may , , to which i have already referred. a more detailed description, sufficient to enable any engineer to build a similar machine, will be found in several electrical journals of that period. the induction coils operated from the machine are rather small, containing from , to , turns in the secondary. they are immersed in boiled-out linseed oil, contained in wooden boxes covered with zinc sheet. i have found it advantageous to reverse the usual position of the wires, and to wind, in these coils, the primaries on the top; this allowing the use of a much bigger primary, which, of course, reduces the danger of overheating and increases the output of the coil. i make the primary on each side at least one centimetre shorter than the secondary, to prevent the breaking through on the ends, which would surely occur unless the insulation on the top of the secondary be very thick, and this, of course, would be disadvantageous. when the primary is made movable, which is necessary in some experiments, and many times convenient for the purposes of adjustment, i cover the secondary with wax, and turn it off in a lathe to a diameter slightly smaller than the inside of the primary coil. the latter i provide with a handle reaching out of the oil, which serves to shift it in any position along the secondary. i will now venture to make, in regard to the general manipulation of induction coils, a few observations bearing upon points which have not been fully appreciated in earlier experiments with such coils, and are even now often overlooked. the secondary of the coil possesses usually such a high self-induction that the current through the wire is inappreciable, and may be so even when the terminals are joined by a conductor of small resistance. if capacity is added to the terminals, the self-induction is counteracted, and a stronger current is made to flow through the secondary, though its terminals are insulated from each other. to one entirely unacquainted with the properties of alternating currents nothing will look more puzzling. this feature was illustrated in the experiment performed at the beginning with the top plates of wire gauze attached to the terminals and the rubber plate. when the plates of wire gauze were close together, and a small arc passed between them, the arc _prevented_ a strong current from passing through the secondary, because it did away with the capacity on the terminals; when the rubber plate was inserted between, the capacity of the condenser formed counteracted the self-induction of the secondary, a stronger current passed now, the coil performed more work, and the discharge was by far more powerful. the first thing, then, in operating the induction coil is to combine capacity with the secondary to overcome the self-induction. if the frequencies and potentials are very high gaseous matter should be carefully kept away from the charged surfaces. if leyden jars are used, they should be immersed in oil, as otherwise considerable dissipation may occur if the jars are greatly strained. when high frequencies are used, it is of equal importance to combine a condenser with the primary. one may use a condenser connected to the ends of the primary or to the terminals of the alternator, but the latter is not to be recommended, as the machine might be injured. the best way is undoubtedly to use the condenser in series with the primary and with the alternator, and to adjust its capacity so as to annul the self-induction of both the latter. the condenser should be adjustable by very small steps, and for a finer adjustment a small oil condenser with movable plates may be used conveniently. i think it best at this juncture to bring before you a phenomenon, observed by me some time ago, which to the purely scientific investigator may perhaps appear more interesting than any of the results which i have the privilege to present to you this evening. it may be quite properly ranked among the brush phenomena--in fact, it is a brush, formed at, or near, a single terminal in high vacuum. in bulbs provided with a conducting terminal, though it be of aluminium, the brush has but an ephemeral existence, and cannot, unfortunately, be indefinitely preserved in its most sensitive state, even in a bulb devoid of any conducting electrode. in studying the phenomenon, by all means a bulb having no leading-in wire should be used. i have found it best to use bulbs constructed as indicated in figs. and . in fig. the bulb comprises an incandescent lamp globe l, in the neck of which is sealed a barometer tube b, the end of which is blown out to form a small sphere s. this sphere should be sealed as closely as possible in the centre of the large globe. before sealing, a thin tube t, of aluminium sheet, may be slipped in the barometer tube, but it is not important to employ it. the small hollow sphere s is filled with some conducting powder, and a wire w is cemented in the neck for the purpose of connecting the conducting powder with the generator. [illustration: fig. . fig. . bulbs for producing rotating brush.] the construction shown in fig. was chosen in order to remove from the brush any conducting body which might possibly affect it. the bulb consists in this case of a lamp globe l, which has a neck n, provided with a tube b and small sphere s, sealed to it, so that two entirely independent compartments are formed, as indicated in the drawing. when the bulb is in use, the neck n is provided with a tinfoil coating, which is connected to the generator and acts inductively upon the moderately rarefied and highly conducting gas inclosed in the neck. from there the current passes through the tube b into the small sphere s to act by induction upon the gas contained in the globe l. it is of advantage to make the tube t very thick, the hole through it very small, and to blow the sphere s very thin. it is of the greatest importance that the sphere s be placed in the centre of the globe l. [illustration: fig. .--forms and phases of the rotating brush.] figs. , and indicate different forms, or stages, of the brush. fig. shows the brush as it first appears in a bulb provided with a conducting terminal; but, as in such a bulb it very soon disappears--often after a few minutes--i will confine myself to the description of the phenomenon as seen in a bulb without conducting electrode. it is observed under the following conditions: when the globe l (figs. and ) is exhausted to a very high degree, generally the bulb is not excited upon connecting the wire w (fig. ) or the tinfoil coating of the bulb (fig. ) to the terminal of the induction coil. to excite it, it is usually sufficient to grasp the globe l with the hand. an intense phosphorescence then spreads at first over the globe, but soon gives place to a white, misty light. shortly afterward one may notice that the luminosity is unevenly distributed in the globe, and after passing the current for some time the bulb appears as in fig. . from this stage the phenomenon will gradually pass to that indicated in fig. , after some minutes, hours, days or weeks, according as the bulb is worked. warming the bulb or increasing the potential hastens the transit. [illustration: fig. . fig. . forms and phases of the rotating brush.] when the brush assumes the form indicated in fig. , it maybe brought to a state of extreme sensitiveness to electrostatic and magnetic influence. the bulb hanging straight down from a wire, and all objects being remote from it, the approach of the observer at a few paces from the bulb will cause the brush to fly to the opposite side, and if he walks around the bulb it will always keep on the opposite side. it may begin to spin around the terminal long before it reaches that sensitive stage. when it begins to turn around principally, but also before, it is affected by a magnet, and at a certain stage it is susceptible to magnetic influence to an astonishing degree. a small permanent magnet, with its poles at a distance of no more than two centimetres, will affect it visibly at a distance of two metres, slowing down or accelerating the rotation according to how it is held relatively to the brush. i think i have observed that at the stage when it is most sensitive to magnetic, it is not most sensitive to electrostatic, influence. my explanation is, that the electrostatic attraction between the brush and the glass of the bulb, which retards the rotation, grows much quicker than the magnetic influence when the intensity of the stream is increased. when the bulb hangs with the globe l down, the rotation is always clockwise. in the southern hemisphere it would occur in the opposite direction and on the equator the brush should not turn at all. the rotation may be reversed by a magnet kept at some distance. the brush rotates best, seemingly, when it is at right angles to the lines of force of the earth. it very likely rotates, when at its maximum speed, in synchronism with the alternations, say , times a second. the rotation can be slowed down or accelerated by the approach or receding of the observer, or any conducting body, but it cannot be reversed by putting the bulb in any position. when it is in the state of the highest sensitiveness and the potential or frequency be varied the sensitiveness is rapidly diminished. changing either of these but little will generally stop the rotation. the sensitiveness is likewise affected by the variations of temperature. to attain great sensitiveness it is necessary to have the small sphere s in the centre of the globe l, as otherwise the electrostatic action of the glass of the globe will tend to stop the rotation. the sphere s should be small and of uniform thickness; any dissymmetry of course has the effect to diminish the sensitiveness. the fact that the brush rotates in a definite direction in a permanent magnetic field seems to show that in alternating currents of very high frequency the positive and negative impulses are not equal, but that one always preponderates over the other. of course, this rotation in one direction may be due to the action of two elements of the same current upon each other, or to the action of the field produced by one of the elements upon the other, as in a series motor, without necessarily one impulse being stronger than the other. the fact that the brush turns, as far as i could observe, in any position, would speak for this view. in such case it would turn at any point of the earth's surface. but, on the other hand, it is then hard to explain why a permanent magnet should reverse the rotation, and one must assume the preponderance of impulses of one kind. as to the causes of the formation of the brush or stream, i think it is due to the electrostatic action of the globe and the dissymmetry of the parts. if the small bulb s and the globe l were perfect concentric spheres, and the glass throughout of the same thickness and quality, i think the brush would not form, as the tendency to pass would be equal on all sides. that the formation of the stream is due to an irregularity is apparent from the fact that it has the tendency to remain in one position, and rotation occurs most generally only when it is brought out of this position by electrostatic or magnetic influence. when in an extremely sensitive state it rests in one position, most curious experiments may be performed with it. for instance, the experimenter may, by selecting a proper position, approach the hand at a certain considerable distance to the bulb, and he may cause the brush to pass off by merely stiffening the muscles of the arm. when it begins to rotate slowly, and the hands are held at a proper distance, it is impossible to make even the slightest motion without producing a visible effect upon the brush. a metal plate connected to the other terminal of the coil affects it at a great distance, slowing down the rotation often to one turn a second. i am firmly convinced that such a brush, when we learn how to produce it properly, will prove a valuable aid in the investigation of the nature of the forces acting in an electrostatic or magnetic field. if there is any motion which is measurable going on in the space, such a brush ought to reveal it. it is, so to speak, a beam of light, frictionless, devoid of inertia. i think that it may find practical applications in telegraphy. with such a brush it would be possible to send dispatches across the atlantic, for instance, with any speed, since its sensitiveness may be so great that the slightest changes will affect it. if it were possible to make the stream more intense and very narrow, its deflections could be easily photographed. i have been interested to find whether there is a rotation of the stream itself, or whether there is simply a stress traveling around in the bulb. for this purpose i mounted a light mica fan so that its vanes were in the path of the brush. if the stream itself was rotating the fan would be spun around. i could produce no distinct rotation of the fan, although i tried the experiment repeatedly; but as the fan exerted a noticeable influence on the stream, and the apparent rotation of the latter was, in this case, never quite satisfactory, the experiment did not appear to be conclusive. i have been unable to produce the phenomenon with the disruptive discharge coil, although every other of these phenomena can be well produced by it--many, in fact, much better than with coils operated from an alternator. it may be possible to produce the brush by impulses of one direction, or even by a steady potential, in which case it would be still more sensitive to magnetic influence. in operating an induction coil with rapidly alternating currents, we realize with astonishment, for the first time, the great importance of the relation of capacity, self-induction and frequency as regards the general result. the effects of capacity are the most striking, for in these experiments, since the self-induction and frequency both are high, the critical capacity is very small, and need be but slightly varied to produce a very considerable change. the experimenter may bring his body in contact with the terminals of the secondary of the coil, or attach to one or both terminals insulated bodies of very small bulk, such as bulbs, and he may produce a considerable rise or fall of potential, and greatly affect the flow of the current through the primary. in the experiment before shown, in which a brush appears at a wire attached to one terminal, and the wire is vibrated when the experimenter brings his insulated body in contact with the other terminal of the coil, the sudden rise of potential was made evident. i may show you the behavior of the coil in another manner which possesses a feature of some interest. i have here a little light fan of aluminium sheet, fastened to a needle and arranged to rotate freely in a metal piece screwed to one of the terminals of the coil. when the coil is set to work, the molecules of the air are rhythmically attracted and repelled. as the force with which they are repelled is greater than that with which they are attracted, it results that there is a repulsion exerted on the surfaces of the fan. if the fan were made simply of a metal sheet, the repulsion would be equal on the opposite sides, and would produce no effect. but if one of the opposing surfaces is screened, or if, generally speaking, the bombardment on this side is weakened in some way or other, there remains the repulsion exerted upon the other, and the fan is set in rotation. the screening is best effected by fastening upon one of the opposing sides of the fan insulated conducting coatings, or, if the fan is made in the shape of an ordinary propeller screw, by fastening on one side, and close to it, an insulated metal plate. the static screen may, however, be omitted, and simply a thickness of insulating material fastened to one of the sides of the fan. to show the behavior of the coil, the fan may be placed upon the terminal and it will readily rotate when the coil is operated by currents of very high frequency. with a steady potential, of course, and even with alternating currents of very low frequency, it would not turn, because of the very slow exchange of air and, consequently, smaller bombardment; but in the latter case it might turn if the potential were excessive. with a pin wheel, quite the opposite rule holds good; it rotates best with a steady potential, and the effort is the smaller the higher the frequency. now, it is very easy to adjust the conditions so that the potential is normally not sufficient to turn the fan, but that by connecting the other terminal of the coil with an insulated body it rises to a much greater value, so as to rotate the fan, and it is likewise possible to stop the rotation by connecting to the terminal a body of different size, thereby diminishing the potential. instead of using the fan in this experiment, we may use the "electric" radiometer with similar effect. but in this case it will be found that the vanes will rotate only at high exhaustion or at ordinary pressures; they will not rotate at moderate pressures, when the air is highly conducting. this curious observation was made conjointly by professor crookes and myself. i attribute the result to the high conductivity of the air, the molecules of which then do not act as independent carriers of electric charges, but act all together as a single conducting body. in such case, of course, if there is any repulsion at all of the molecules from the vanes, it must be very small. it is possible, however, that the result is in part due to the fact that the greater part of the discharge passes from the leading-in wire through the highly conducting gas, instead of passing off from the conducting vanes. in trying the preceding experiment with the electric radiometer the potential should not exceed a certain limit, as then the electrostatic attraction between the vanes and the glass of the bulb may be so great as to stop the rotation. a most curious feature of alternate currents of high frequencies and potentials is that they enable us to perform many experiments by the use of one wire only. in many respects this feature is of great interest. in a type of alternate current motor invented by me some years ago i produced rotation by inducing, by means of a single alternating current passed through a motor circuit, in the mass or other circuits of the motor, secondary currents, which, jointly with the primary or inducing current, created a moving field of force. a simple but crude form of such a motor is obtained by winding upon an iron core a primary, and close to it a secondary coil, joining the ends of the latter and placing a freely movable metal disc within the influence of the field produced by both. the iron core is employed for obvious reasons, but it is not essential to the operation. to improve the motor, the iron core is made to encircle the armature. again to improve, the secondary coil is made to overlap partly the primary, so that it cannot free itself from a strong inductive action of the latter, repel its lines as it may. once more to improve, the proper difference of phase is obtained between the primary and secondary currents by a condenser, self-induction, resistance or equivalent windings. i had discovered, however, that rotation is produced by means of a single coil and core; my explanation of the phenomenon, and leading thought in trying the experiment, being that there must be a true time lag in the magnetization of the core. i remember the pleasure i had when, in the writings of professor ayrton, which came later to my hand, i found the idea of the time lag advocated. whether there is a true time lag, or whether the retardation is due to eddy currents circulating in minute paths, must remain an open question, but the fact is that a coil wound upon an iron core and traversed by an alternating current creates a moving field of force, capable of setting an armature in rotation. it is of some interest, in conjunction with the historical arago experiment, to mention that in lag or phase motors i have produced rotation in the opposite direction to the moving field, which means that in that experiment the magnet may not rotate, or may even rotate in the opposite direction to the moving disc. here, then, is a motor (diagrammatically illustrated in fig. ), comprising a coil and iron core, and a freely movable copper disc in proximity to the latter. [illustration: fig. .--single wire and "no-wire" motor.] to demonstrate a novel and interesting feature, i have, for a reason which i will explain, selected this type of motor. when the ends of the coil are connected to the terminals of an alternator the disc is set in rotation. but it is not this experiment, now well known, which i desire to perform. what i wish to show you is that this motor rotates with _one single_ connection between it and the generator; that is to say, one terminal of the motor is connected to one terminal of the generator--in this case the secondary of a high-tension induction coil--the other terminals of motor and generator being insulated in space. to produce rotation it is generally (but not absolutely) necessary to connect the free end of the motor coil to an insulated body of some size. the experimenter's body is more than sufficient. if he touches the free terminal with an object held in the hand, a current passes through the coil and the copper disc is set in rotation. if an exhausted tube is put in series with the coil, the tube lights brilliantly, showing the passage of a strong current. instead of the experimenter's body, a small metal sheet suspended on a cord may be used with the same result. in this case the plate acts as a condenser in series with the coil. it counteracts the self-induction of the latter and allows a strong current to pass. in such a combination, the greater the self-induction of the coil the smaller need be the plate, and this means that a lower frequency, or eventually a lower potential, is required to operate the motor. a single coil wound upon a core has a high self-induction; for this reason principally, this type of motor was chosen to perform the experiment. were a secondary closed coil wound upon the core, it would tend to diminish the self-induction, and then it would be necessary to employ a much higher frequency and potential. neither would be advisable, for a higher potential would endanger the insulation of the small primary coil, and a higher frequency would result in a materially diminished torque. it should be remarked that when such a motor with a closed secondary is used, it is not at all easy to obtain rotation with excessive frequencies, as the secondary cuts off almost completely the lines of the primary--and this, of course, the more, the higher the frequency--and allows the passage of but a minute current. in such a case, unless the secondary is closed through a condenser, it is almost essential, in order to produce rotation, to make the primary and secondary coils overlap each other more or less. but there is an additional feature of interest about this motor, namely, it is not necessary to have even a single connection between the motor and generator, except, perhaps, through the ground: for not only is an insulated plate capable of giving off energy into space, but it is likewise capable of deriving it from an alternating electrostatic field, though in the latter case the available energy is much smaller. in this instance one of the motor terminals is connected to the insulated plate or body located within the alternating electrostatic field, and the other terminal preferably to the ground. it is quite possible, however, that such "no-wire" motors, as they might be called, could be operated by conduction through the rarefied air at considerable distances. alternate currents, especially of high frequencies, pass with astonishing freedom through even slightly rarefied gases. the upper strata of the air are rarefied. to reach a number of miles out into space requires the overcoming of difficulties of a merely mechanical nature. there is no doubt that with the enormous potentials obtainable by the use of high frequencies and oil insulation luminous discharges might be passed through many miles of rarefied air, and that, by thus directing the energy of many hundreds or thousands of horse-power, motors or lamps might be operated at considerable distances from stationary sources. but such schemes are mentioned merely as possibilities. we shall have no need to transmit power in this way. we shall have no need to _transmit_ power at all. ere many generations pass, our machinery will be driven by a power obtainable at any point of the universe. this idea is not novel. men have been led to it long ago by instinct or reason. it has been expressed in many ways, and in many places, in the history of old and new. we find it in the delightful myth of antheus, who derives power from the earth; we find it among the subtile speculations of one of your splendid mathematicians, and in many hints and statements of thinkers of the present time. throughout space there is energy. is this energy static or kinetic? if static our hopes are in vain; if kinetic--and this we know it is, for certain--then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature. of all, living or dead, crookes came nearest to doing it. his radiometer will turn in the light of day and in the darkness of the night; it will turn everywhere where there is heat, and heat is everywhere. but, unfortunately, this beautiful little machine, while it goes down to posterity as the most interesting, must likewise be put on record as the most inefficient machine ever invented! the preceding experiment is only one of many equally interesting experiments which may be performed by the use of only one wire with alternate currents of high potential and frequency. we may connect an insulated line to a source of such currents, we may pass an inappreciable current over the line, and on any point of the same we are able to obtain a heavy current, capable of fusing a thick copper wire. or we may, by the help of some artifice, decompose a solution in any electrolytic cell by connecting only one pole of the cell to the line or source of energy. or we may, by attaching to the line, or only bringing into its vicinity, light up an incandescent lamp, an exhausted tube, or a phosphorescent bulb. however impracticable this plan of working may appear in many cases, it certainly seems practicable, and even recommendable, in the production of light. a perfected lamp would require but little energy, and if wires were used at all we ought to be able to supply that energy without a return wire. it is now a fact that a body may be rendered incandescent or phosphorescent by bringing it either in single contact or merely in the vicinity of a source of electric impulses of the proper character, and that in this manner a quantity of light sufficient to afford a practical illuminant may be produced. it is, therefore, to say the least, worth while to attempt to determine the best conditions and to invent the best appliances for attaining this object. some experiences have already been gained in this direction, and i will dwell on them briefly, in the hope that they might prove useful. the heating of a conducting body inclosed in a bulb, and connected to a source of rapidly alternating electric impulses, is dependent on so many things of a different nature, that it would be difficult to give a generally applicable rule under which the maximum heating occurs. as regards the size of the vessel, i have lately found that at ordinary or only slightly differing atmospheric pressures, when air is a good insulator, and hence practically the same amount of energy by a certain potential and frequency is given off from the body, whether the bulb be small or large, the body is brought to a higher temperature if inclosed in a small bulb, because of the better confinement of heat in this case. at lower pressures, when air becomes more or less conducting, or if the air be sufficiently warmed as to become conducting, the body is rendered more intensely incandescent in a large bulb, obviously because, under otherwise equal conditions of test, more energy may be given off from the body when the bulb is large. at very high degrees of exhaustion, when the matter in the bulb becomes "radiant," a large bulb has still an advantage, but a comparatively slight one, over the small bulb. finally, at excessively high degrees of exhaustion, which cannot be reached except by the employment of special means, there seems to be, beyond a certain and rather small size of vessel, no perceptible difference in the heating. these observations were the result of a number of experiments, of which one, showing the effect of the size of the bulb at a high degree of exhaustion, may be described and shown here, as it presents a feature of interest. three spherical bulbs of inches, inches and inches diameter were taken, and in the centre of each was mounted an equal length of an ordinary incandescent lamp filament of uniform thickness. in each bulb the piece of filament was fastened to the leading-in wire of platinum, contained in a glass stem sealed in the bulb; care being taken, of course, to make everything as nearly alike as possible. on each glass stem in the inside of the bulb was slipped a highly polished tube made of aluminium sheet, which fitted the stem and was held on it by spring pressure. the function of this aluminium tube will be explained subsequently. in each bulb an equal length of filament protruded above the metal tube. it is sufficient to say now that under these conditions equal lengths of filament of the same thickness--in other words, bodies of equal bulk--were brought to incandescence. the three bulbs were sealed to a glass tube, which was connected to a sprengel pump. when a high vacuum had been reached, the glass tube carrying the bulbs was sealed off. a current was then turned on successively on each bulb, and it was found that the filaments came to about the same brightness, and, if anything, the smallest bulb, which was placed midway between the two larger ones, may have been slightly brighter. this result was expected, for when either of the bulbs was connected to the coil the luminosity spread through the other two, hence the three bulbs constituted really one vessel. when all the three bulbs were connected in multiple arc to the coil, in the largest of them the filament glowed brightest, in the next smaller it was a little less bright, and in the smallest it only came to redness. the bulbs were then sealed off and separately tried. the brightness of the filaments was now such as would have been expected on the supposition that the energy given off was proportionate to the surface of the bulb, this surface in each case representing one of the coatings of a condenser. accordingly, time was less difference between the largest and the middle sized than between the latter and the smallest bulb. an interesting observation was made in this experiment. the three bulbs were suspended from a straight bare wire connected to a terminal of the coil, the largest bulb being placed at the end of the wire, at some distance from it the smallest bulb, and an equal distance from the latter the middle-sized one. the carbons glowed then in both the larger bulbs about as expected, but the smallest did not get its share by far. this observation led me to exchange the position of the bulbs, and i then observed that whichever of the bulbs was in the middle it was by far less bright than it was in any other position. this mystifying result was, of course, found to be due to the electrostatic action between the bulbs. when they were placed at a considerable distance, or when they were attached to the corners of an equilateral triangle of copper wire, they glowed about in the order determined by their surfaces. as to the shape of the vessel, it is also of some importance, especially at high degrees of exhaustion. of all the possible constructions, it seems that a spherical globe with the refractory body mounted in its centre is the best to employ. in experience it has been demonstrated that in such a globe a refractory body of a given bulk is more easily brought to incandescence than when otherwise shaped bulbs are used. there is also an advantage in giving to the incandescent body the shape of a sphere, for self-evident reasons. in any case the body should be mounted in the centre, where the atoms rebounding from the glass collide. this object is best attained in the spherical bulb; but it is also attained in a cylindrical vessel with one or two straight filaments coinciding with its axis, and possibly also in parabolical or spherical bulbs with the refractory body or bodies placed in the focus or foci of the same; though the latter is not probable, as the electrified atoms should in all cases rebound normally from the surface they strike, unless the speed were excessive, in which case they _would_ probably follow the general law of reflection. no matter what shape the vessel may have, if the exhaustion be low, a filament mounted in the globe is brought to the same degree of incandescence in all parts; but if the exhaustion be high and the bulb be spherical or pear-shaped, as usual, focal points form and the filament is heated to a higher degree at or near such points. to illustrate the effect, i have here two small bulbs which are alike, only one is exhausted to a low and the other to a very high degree. when connected to the coil, the filament in the former glows uniformly throughout all its length; whereas in the latter, that portion of the filament which is in the centre of the bulb glows far more intensely than the rest. a curious point is that the phenomenon occurs even if two filaments are mounted in a bulb, each being connected to one terminal of the coil, and, what is still more curious, if they be very near together, provided the vacuum be very high. i noted in experiments with such bulbs that the filaments would give way usually at a certain point, and in the first trials i attributed it to a defect in the carbon. but when the phenomenon occurred many times in succession i recognized its real cause. in order to bring a refractory body inclosed in a bulb to incandescence, it is desirable, on account of economy, that all the energy supplied to the bulb from the source should reach without loss the body to be heated; from there, and from nowhere else, it should be radiated. it is, of course, out of the question to reach this theoretical result, but it is possible by a proper construction of the illuminating device to approximate it more or less. for many reasons, the refractory body is placed in the centre of the bulb, and it is usually supported on a glass stem containing the leading-in wire. as the potential of this wire is alternated, the rarefied gas surrounding the stem is acted upon inductively, and the glass stem is violently bombarded and heated. in this manner by far the greater portion of the energy supplied to the bulb--especially when exceedingly high frequencies are used--may be lost for the purpose contemplated. to obviate this loss, or at least to reduce it to a minimum, i usually screen the rarefied gas surrounding the stem from the inductive action of the leading-in wire by providing the stem with a tube or coating of conducting material. it seems beyond doubt that the best among metals to employ for this purpose is aluminium, on account of its many remarkable properties. its only fault is that it is easily fusible, and, therefore, its distance from the incandescing body should be properly estimated. usually, a thin tube, of a diameter somewhat smaller than that of the glass stem, is made of the finest aluminium sheet, and slipped on the stem. the tube is conveniently prepared by wrapping around a rod fastened in a lathe a piece of aluminium sheet of the proper size, grasping the sheet firmly with clean chamois leather or blotting paper, and spinning the rod very fast. the sheet is wound tightly around the rod, and a highly polished tube of one or three layers of the sheet is obtained. when slipped on the stem, the pressure is generally sufficient to prevent it from slipping off, but, for safety, the lower edge of the sheet may be turned inside. the upper inside corner of the sheet--that is, the one which is nearest to the refractory incandescent body--should be cut out diagonally, as it often happens that, in consequence of the intense heat, this corner turns toward the inside and comes very near to, or in contact with, the wire, or filament, supporting the refractory body. the greater part of the energy supplied to the bulb is then used up in heating the metal tube, and the bulb is rendered useless for the purpose. the aluminium sheet should project above the glass stem more or less--one inch or so--or else, if the glass be too close to the incandescing body, it may be strongly heated and become more or less conducting, whereupon it may be ruptured, or may, by its conductivity, establish a good electrical connection between the metal tube and the leading-in wire, in which case, again, most of the energy will be lost in heating the former. perhaps the best way is to make the top of the glass tube, for about an inch, of a much smaller diameter. to still further reduce the danger arising from the heating of the glass stem, and also with the view of preventing an electrical connection between the metal tube and the electrode, i preferably wrap the stem with several layers of thin mica, which extends at least as far as the metal tube. in some bulbs i have also used an outside insulating cover. the preceding remarks are only made to aid the experimenter in the first trials, for the difficulties which he encounters he may soon find means to overcome in his own way. to illustrate the effect of the screen, and the advantage of using it, i have here two bulbs of the same size, with their stems, leading-in wires and incandescent lamp filaments tied to the latter, as nearly alike as possible. the stem of one bulb is provided with an aluminium tube, the stem of the other has none. originally the two bulbs were joined by a tube which was connected to a sprengel pump. when a high vacuum had been reached, first the connecting tube, and then the bulbs, were sealed off; they are therefore of the same degree of exhaustion. when they are separately connected to the coil giving a certain potential, the carbon filament in the bulb provided with the aluminium screen is rendered highly incandescent, while the filament in the other bulb may, with the same potential, not even come to redness, although in reality the latter bulb takes generally more energy than the former. when they are both connected together to the terminal, the difference is even more apparent, showing the importance of the screening. the metal tube placed on the stem containing the leading-in wire performs really two distinct functions: first: it acts more or less as an electrostatic screen, thus economizing the energy supplied to the bulb; and, second, to whatever extent it may fail to act electrostatically, it acts mechanically, preventing the bombardment, and consequently intense heating and possible deterioration of the slender support of the refractory incandescent body, or of the glass stem containing the leading-in wire. i say _slender_ support, for it is evident that in order to confine the heat more completely to the incandescing body its support should be very thin, so as to carry away the smallest possible amount of heat by conduction. of all the supports used i have found an ordinary incandescent lamp filament to be the best, principally because among conductors it can withstand the highest degrees of heat. the effectiveness of the metal tube as an electrostatic screen depends largely on the degree of exhaustion. at excessively high degrees of exhaustion--which are reached by using great care and special means in connection with the sprengel pump--when the matter in the globe is in the ultra-radiant state, it acts most perfectly. the shadow of the upper edge of the tube is then sharply defined upon the bulb. at a somewhat lower degree of exhaustion, which is about the ordinary "non-striking" vacuum, and generally as long as the matter moves predominantly in straight lines, the screen still does well. in elucidation of the preceding remark it is necessary to state that what is a "non-striking" vacuum for a coil operated, as ordinarily, by impulses, or currents, of low-frequency, is not, by far, so when the coil is operated by currents of very high frequency. in such case the discharge may pass with great freedom through the rarefied gas through which a low-frequency discharge may not pass, even though the potential be much higher. at ordinary atmospheric pressures just the reverse rule holds good: the higher the frequency, the less the spark discharge is able to jump between the terminals, especially if they are knobs or spheres of some size. finally, at very low degrees of exhaustion, when the gas is well conducting, the metal tube not only does not act as an electrostatic screen, but even is a drawback, aiding to a considerable extent the dissipation of the energy laterally from the leading-in wire. this, of course, is to be expected. in this case, namely, the metal tube is in good electrical connection with the leading-in wire, and most of the bombardment is directed upon the tube. as long as the electrical connection is not good, the conducting tube is always of some advantage, for although it may not greatly economize energy, still it protects the support of the refractory button, and is a means for concentrating more energy upon the same. to whatever extent the aluminium tube performs the function of a screen, its usefulness is therefore limited to very high degrees of exhaustion when it is insulated from the electrode--that is, when the gas as a whole is non-conducting, and the molecules, or atoms, act as independent carriers of electric charges. in addition to acting as a more or less effective screen, in the true meaning of the word, the conducting tube or coating may also act, by reason of its conductivity, as a sort of equalizer or dampener of the bombardment against the stem. to be explicit, i assume the action as follows: suppose a rhythmical bombardment to occur against the conducting tube by reason of its imperfect action as a screen, it certainly must happen that some molecules, or atoms, strike the tube sooner than others. those which come first in contact with it give up their superfluous charge, and the tube is electrified, the electrification instantly spreading over its surface. but this must diminish the energy lost in the bombardment for two reasons: first, the charge given up by the atoms spreads over a great area, and hence the electric density at any point is small, and the atoms are repelled with less energy than they would be if they would strike against a good insulator: secondly, as the tube is electrified by the atoms which first come in contact with it, the progress of the following atoms against the tube is more or less checked by the repulsion which the electrified tube must exert upon the similarly electrified atoms. this repulsion may perhaps be sufficient to prevent a large portion of the atoms from striking the tube, but at any rate it must diminish the energy of their impact. it is clear that when the exhaustion is very low, and the rarefied gas well conducting, neither of the above effects can occur, and, on the other hand, the fewer the atoms, with the greater freedom they move; in other words, the higher the degree of exhaustion, up to a limit, the more telling will be both the effects. what i have just said may afford an explanation of the phenomenon observed by prof. crookes, namely, that a discharge through a bulb is established with much greater facility when an insulator than when a conductor is present in the same. in my opinion, the conductor acts as a dampener of the motion of the atoms in the two ways pointed out; hence, to cause a visible discharge to pass through the bulb, a much higher potential is needed if a conductor, especially of much surface, be present. for the sake of clearness of some of the remarks before made, i must now refer to figs. , and , which illustrate various arrangements with a type of bulb most generally used. [illustration: fig. .--bulb with mica tube and aluminium screen.] [illustration: fig. .--improved bulb with socket and screen.] fig. is a section through a spherical bulb l, with the glass stem s, containing the leading-in wire w; which has a lamp filament l fastened to it, serving to support the refractory button m in the centre. m is a sheet of thin mica wound in several layers around the stem s, and a is the aluminium tube. fig. illustrates such a bulb in a somewhat more advanced stage of perfection. a metallic tube s is fastened by means of some cement to the neck of the tube. in the tube is screwed a plug p, of insulating material, in the centre of which is fastened a metallic terminal t, for the connection to the leading-in wire w. this terminal must be well insulated from the metal tube s, therefore, if the cement used is conducting--and most generally it is sufficiently so--the space between the plug p and the neck of the bulb should be filled with some good insulating material, as mica powder. [illustration: fig. .--bulb for experiments with conducting tube.] fig. shows a bulb made for experimental purposes. in this bulb the aluminium tube is provided with an external connection, which serves to investigate the effect of the tube under various conditions. it is referred to chiefly to suggest a line of experiment followed. since the bombardment against the stem containing the leading-in wire is due to the inductive action of the latter upon the rarefied gas, it is of advantage to reduce this action as far as practicable by employing a very thin wire, surrounded by a very thick insulation of glass or other material, and by making the wire passing through the rarefied gas as short as practicable. to combine these features i employ a large tube t (fig. ), which protrudes into the bulb to some distance, and carries on the top a very short glass stem s, into which is sealed the leading-in wire w, and i protect the top of the glass stem against the heat by a small, aluminium tube a and a layer of mica underneath the same, as usual. the wire w, passing through the large tube to the outside of the bulb, should be well insulated--with a glass tube, for instance--and the space between ought to be filled out with some excellent insulator. among many insulating powders i have tried, i have found that mica powder is the best to employ. if this precaution is not taken, the tube t, protruding into the bulb, will surely be cracked in consequence of the heating by the brushes which are apt to form in the upper part of the tube, near the exhausted globe, especially if the vacuum be excellent, and therefore the potential necessary to operate the lamp very high. fig. illustrates a similar arrangement, with a large tube t protruding in to the part of the bulb containing the refractors button m. in this case the wire leading from the outside into the bulb is omitted, the energy required being supplied through condenser coatings cc. the insulating packing p should in this construction be tightly fitting to the glass, and rather wide, or otherwise the discharge might avoid passing through the wire w, which connects the inside condenser coating to the incandescent button m. the molecular bombardment against the glass stem in the bulb is a source of great trouble. as illustration i will cite a phenomenon only too frequently and unwillingly observed. a bulb, preferably a large one, may be taken, and a good conducting body, such as a piece of carbon, may be mounted in it upon a platinum wire sealed in the glass stem. the bulb may be exhausted to a fairly high degree, nearly to the point when phosphorescence begins to appear. [illustration: fig. .--improved bulb with non-conducting button.] [illustration: fig. .--type of bulb without leading-in wire.] when the bulb is connected with the coil, the piece of carbon, if small, may become highly incandescent at first, but its brightness immediately diminishes, and then the discharge may break through the glass somewhere in the middle of the stem, in the form of bright sparks, in spite of the fact that the platinum wire is in good electrical connection with the rarefied gas through the piece of carbon or metal at the top. the first sparks are singularly bright, recalling those drawn from a clear surface of mercury. but, as they heat the glass rapidly, they, of course, lose their brightness, and cease when the glass at the ruptured place becomes incandescent, or generally sufficiently hot to conduct. when observed for the first time the phenomenon must appear very curious, and shows in a striking manner how radically different alternate currents, or impulses, of high frequency behave, as compared with steady currents, or currents of low frequency. with such currents--namely, the latter--the phenomenon would of course not occur. when frequencies such as are obtained by mechanical means are used, i think that the rupture of the glass is more or less the consequence of the bombardment, which warms it up and impairs its insulating power; but with frequencies obtainable with condensers i have no doubt that the glass may give way without previous heating. although this appears most singular at first, it is in reality what we might expect to occur. the energy supplied to the wire leading into the bulb is given off partly by direct action through the carbon button, and partly by inductive action through the glass surrounding the wire. the case is thus analogous to that in which a condenser shunted by a conductor of low resistance is connected to a source of alternating currents. as long as the frequencies are low, the conductor gets the most, and the condenser is perfectly safe: but when the frequency becomes excessive, the _role_ of the conductor may become quite insignificant. in the latter case the difference of potential at the terminals of the condenser may become so great as to rupture the dielectric, notwithstanding the fact that the terminals are joined by a conductor of low resistance. [illustration: fig. .--effect produced by a ruby drop.] it is, of course, not necessary, when it is desired to produce the incandescence of a body inclosed in a bulb by means of these currents, that the body should be a conductor, for even a perfect non-conductor may be quite as readily heated. for this purpose it is sufficient to surround a conducting electrode with a non-conducting material, as, for instance, in the bulb described before in fig. , in which a thin incandescent lamp filament is coated with a non-conductor, and supports a button of the same material on the top. at the start the bombardment goes on by inductive action through the non-conductor, until the same is sufficiently heated to become conducting, when the bombardment continues in the ordinary way. a different arrangement used in some of the bulbs constructed is illustrated in fig. . in this instance a non-conductor m is mounted in a piece of common arc light carbon so as to project some small distance above the latter. the carbon piece is connected to the leading-in wire passing through a glass stem, which is wrapped with several layers of mica. an aluminium tube a is employed as usual for screening. it is so arranged that it reaches very nearly as high as the carbon and only the non-conductor m projects a little above it. the bombardment goes at first against the upper surface of carbon, the lower parts being protected by the aluminium tube. as soon, however, as the non-conductor m is heated it is rendered good conducting, and then it becomes the centre of the bombardment, being most exposed to the same. i have also constructed during these experiments many such single-wire bulbs with or without internal electrode, in which the radiant matter was projected against, or focused upon, the body to be rendered incandescent. fig. illustrates one of the bulbs used. it consists of a spherical globe l, provided with a long neck n, on the top, for increasing the action in some cases by the application of an external conducting coating. the globe l is blown out on the bottom into a very small bulb b, which serves to hold it firmly in a socket s of insulating material into which it is cemented. a fine lamp filament f, supported on a wire w, passes through the centre of the globe l. the filament is rendered incandescent in the middle portion, where the bombardment proceeding from the lower inside surface of the globe is most intense. the lower portion of the globe, as far as the socket s reaches, is rendered conducting, either by a tinfoil coating or otherwise, and the external electrode is connected to a terminal of the coil. the arrangement diagrammatically indicated in fig. was found to be an inferior one when it was desired to render incandescent a filament or button supported in the centre of the globe, but it was convenient when the object was to excite phosphorescence. in many experiments in which bodies of a different kind were mounted in the bulb as, for instance, indicated in fig. , some observations of interest were made. it was found, among other things, that in such cases, no matter where the bombardment began, just as soon as a high temperature was reached there was generally one of the bodies which seemed to take most of the bombardment upon itself, the other, or others, being thereby relieved. this quality appeared to depend principally on the point of fusion, and on the facility with which the body was "evaporated," or, generally speaking, disintegrated--meaning by the latter term not only the throwing off of atoms, but likewise of larger lumps. the observation made was in accordance with generally accepted notions. in a highly exhausted bulb electricity is carried off from the electrode by independent carriers, which are partly the atoms, or molecules, of the residual atmosphere, and partly the atoms, molecules, or lumps thrown off from the electrode. if the electrode is composed of bodies of different character, and if one of these is more easily disintegrated than the others, most of the electricity supplied is carried off from that body, which is then brought to a higher temperature than the others, and this the more, as upon an increase of the temperature the body is still more easily disintegrated. it seems to me quite probable that a similar process takes place in the bulb even with a homogeneous electrode, and i think it to be the principal cause of the disintegration. there is bound to be some irregularity, even if the surface is highly polished, which, of course, is impossible with most of the refractory bodies employed as electrodes. assume that a point of the electrode gets hotter, instantly most of the discharge passes through that point, and a minute patch is probably fused and evaporated. it is now possible that in consequence of the violent disintegration the spot attacked sinks in temperature, or that a counter force is created, as in an arc; at any rate, the local tearing off meets with the limitations incident to the experiment, whereupon the same process occurs on another place. to the eye the electrode appears uniformly brilliant, but there are upon it points constantly shifting and wandering around, of a temperature far above the mean, and this materially hastens the process of deterioration. that some such thing occurs, at least when the electrode is at a lower temperature, sufficient experimental evidence can be obtained in the following manner: exhaust a bulb to a very high degree, so that with a fairly high potential the discharge cannot pass--that is, not a _luminous_ one, for a weak invisible discharge occurs always, in all probability. now raise slowly and carefully the potential, leaving the primary current on no more than for an instant. at a certain point, two, three, or half a dozen phosphorescent spots will appear on the globe. these places of the glass are evidently more violently bombarded than others, this being due to the unevenly distributed electric density, necessitated, of course, by sharp projections, or, generally speaking, irregularities of the electrode. but the luminous patches are constantly changing in position, which is especially well observable if one manages to produce very few, and this indicates that the configuration of the electrode is rapidly changing. from experiences of this kind i am led to infer that, in order to be most durable, the refractory button in the bulb should be in the form of a sphere with a highly polished surface. such a small sphere could be manufactured from a diamond or some other crystal, but a better way would be to fuse, by the employment of extreme degrees of temperature, some oxide--as, for instance, zirconia--into a small drop, and then keep it in the bulb at a temperature somewhat below its point of fusion. interesting and useful results can no doubt be reached in the direction of extreme degrees of heat. how can such high temperatures be arrived at? how are the highest degrees of heat reached in nature? by the impact of stars, by high speeds and collisions. in a collision any rate of heat generation may be attained. in a chemical process we are limited. when oxygen and hydrogen combine, they fall, metaphorically speaking, from a definite height. we cannot go very far with a blast, nor by confining heat in a furnace, but in an exhausted bulb we can concentrate any amount of energy upon a minute button. leaving practicability out of consideration, this, then, would be the means which, in my opinion, would enable us to reach the highest temperature. but a great difficulty when proceeding in this way is encountered, namely, in most cases the body is carried off before it can fuse and form a drop. this difficulty exists principally with an oxide such as zirconia, because it cannot be compressed in so hard a cake that it would not be carried off quickly. i endeavored repeatedly to fuse zirconia, placing it in a cup or arc light carbon as indicated in fig. . it glowed with a most intense light, and the stream of the particles projected out of the carbon cup was of a vivid white: but whether it was compressed in a cake or made into a paste with carbon, it was carried off before it could be fused. the carbon cup containing the zirconia had to be mounted very low in the neck of a large bulb, as the heating of the glass by the projected particles of the oxide was so rapid that in the first trial the bulb was cracked almost in an instant when the current was turned on. the heating of the glass by the projected particles was found to be always greater when the carbon cup contained a body which was rapidly carried off--i presume because in such cases, with the same potential, higher speeds were reached, and also because, per unit of time, more matter was projected--that is, more particles would strike the glass. the before mentioned difficulty did not exist, however, when the body mounted in the carbon cup offered great resistance to deterioration. for instance, when an oxide was first fused in an oxygen blast and then mounted in the bulb, it melted very readily into a drop. generally during the process of fusion magnificent light effects were noted, of which it would be difficult to give an adequate idea. fig. is intended to illustrate the effect observed with a ruby drop. at first one may see a narrow funnel of white light projected against the top of the globe, where it produces an irregularly outlined phosphorescent patch. when the point of the ruby fuses the phosphorescence becomes very powerful; but as the atoms are projected with much greater speed from the surface of the drop, soon the glass gets hot and "tired," and now only the outer edge of the patch glows. in this manner an intensely phosphorescent, sharply defined line, _l_, corresponding to the outline of the drop, is produced, which spreads slowly over the globe as the drop gets larger. when the mass begins to boil, small bubbles and cavities are formed, which cause dark colored spots to sweep across the globe. the bulb may be turned downward without fear of the drop falling off, as the mass possesses considerable viscosity. i may mention here another feature of some interest, which i believe to have noted in the course of these experiments, though the observations do not amount to a certitude. it _appeared_ that under the molecular impact caused by the rapidly alternating potential the body was fused and maintained in that state at a lower temperature in a highly exhausted bulb than was the case at normal pressure and application of heat in the ordinary way--that is, at least, judging from the quantity of the light emitted. one of the experiments performed may be mentioned here by way of illustration. a small piece of pumice stone was stuck on a platinum wire, and first melted to it in a gas burner. the wire was next placed between two pieces of charcoal and a burner applied so as to produce an intense heat, sufficient to melt down the pumice stone into a small glass-like button. the platinum wire had to be taken of sufficient thickness to prevent its melting in the fire. while in the charcoal fire, or when held in a burner to get a better idea of the degree of heat, the button glowed with great brilliancy. the wire with the button was then mounted in a bulb, and upon exhausting the same to a high degree, the current was turned on slowly so as to prevent the cracking of the button. the button was heated to the point of fusion, and when it melted it did not, apparently, glow with the same brilliancy as before, and this would indicate a lower temperature. leaving out of consideration the observer's possible, and even probable, error, the question is, can a body under these conditions be brought from a solid to a liquid state with evolution of _less_ light? when the potential of a body is rapidly alternated it is certain that the structure is jarred. when the potential is very high, although the vibrations may be few--say , per second--the effect upon the structure may be considerable. suppose, for example, that a ruby is melted into a drop by a steady application of energy. when it forms a drop it will emit visible and invisible waves, which will be in a definite ratio, and to the eye the drop will appear to be of a certain brilliancy. next, suppose we diminish to any degree we choose the energy steadily supplied, and, instead, supply energy which rises and falls according to a certain law. now, when the drop is formed, there will be emitted from it three different kinds of vibrations--the ordinary visible, and two kinds of invisible waves: that is, the ordinary dark waves of all lengths, and, in addition, waves of a well defined character. the latter would not exist by a steady supply of the energy; still they help to jar and loosen the structure. if this really be the case, then the ruby drop will emit relatively less visible and more invisible waves than before. thus it would seem that when a platinum wire, for instance, is fused by currents alternating with extreme rapidity, it emits at the point of fusion less light and more invisible radiation than it does when melted by a steady current, though the total energy used up in the process of fusion is the same in both cases. or, to cite another example, a lamp filament is not capable of withstanding as long with currents of extreme frequency as it does with steady currents, assuming that it be worked at the same luminous intensity. this means that for rapidly alternating currents the filament should be shorter and thicker. the higher the frequency--that is, the greater the departure from the steady flow--the worse it would be for the filament. but if the truth of this remark were demonstrated, it would be erroneous to conclude that such a refractory button as used in these bulbs would be deteriorated quicker by currents of extremely high frequency than by steady or low frequency currents. from experience i may say that just the opposite holds good: the button withstands the bombardment better with currents of very high frequency. but this is due to the fact that a high frequency discharge passes through a rarefied gas with much greater freedom than a steady or low frequency discharge, and this will say that with the former we can work with a lower potential or with a less violent impact. as long, then, as the gas is of no consequence, a steady or low frequency current is better; but as soon as the action of the gas is desired and important, high frequencies are preferable. in the course of these experiments a great many trials were made with all kinds of carbon buttons. electrodes made of ordinary carbon buttons were decidedly more durable when the buttons were obtained by the application of enormous pressure. electrodes prepared by depositing carbon in well known ways did not show up well; they blackened the globe very quickly. from many experiences i conclude that lamp filaments obtained in this manner can be advantageously used only with low potentials and low frequency currents. some kinds of carbon withstand so well that, in order to bring them to the point of fusion, it is necessary to employ very small buttons. in this case the observation is rendered very difficult on account of the intense heat produced. nevertheless there can be no doubt that all kinds of carbon are fused under the molecular bombardment, but the liquid state must be one of great instability. of all the bodies tried there were two which withstood best--diamond and carborundum. these two showed up about equally, but the latter was preferable, for many reasons. as it is more than likely that this body is not yet generally known, i will venture to call your attention to it. it has been recently produced by mr. e.g. acheson, of monongahela city, pa., u.s.a. it is intended to replace ordinary diamond powder for polishing precious stones, etc., and i have been informed that it accomplishes this object quite successfully. i do not know why the name "carborundum" has been given to it, unless there is something in the process of its manufacture which justifies this selection. through the kindness of the inventor, i obtained a short while ago some samples which i desired to test in regard to their qualities of phosphorescence and capability of withstanding high degrees of heat. carborundum can be obtained in two forms--in the form of "crystals" and of powder. the former appear to the naked eye dark colored, but are very brilliant; the latter is of nearly the same color as ordinary diamond powder, but very much finer. when viewed under a microscope the samples of crystals given to me did not appear to have any definite form, but rather resembled pieces of broken up egg coal of fine quality. the majority were opaque, but there were some which were transparent and colored. the crystals are a kind of carbon containing some impurities; they are extremely hard, and withstand for a long time even an oxygen blast. when the blast is directed against them they at first form a cake of some compactness, probably in consequence of the fusion of impurities they contain. the mass withstands for a very long time the blast without further fusion; but a slow carrying off, or burning, occurs, and, finally, a small quantity of a glass-like residue is left, which, i suppose, is melted alumina. when compressed strongly they conduct very well, but not as well as ordinary carbon. the powder, which is obtained from the crystals in some way, is practically non-conducting. it affords a magnificent polishing material for stones. the time has been too short to make a satisfactory study of the properties of this product, but enough experience has been gained in a few weeks i have experimented upon it to say that it does possess some remarkable properties in many respects. it withstands excessively high degrees of heat, it is little deteriorated by molecular bombardment, and it does not blacken the globe as ordinary carbon does. the only difficulty which i have found in its use in connection with these experiments was to find some binding material which would resist the heat and the effect of the bombardment as successfully as carborundum itself does. i have here a number of bulbs which i have provided with buttons of carborundum. to make such a button of carborundum crystals i proceed in the following manner: i take an ordinary lamp filament and dip its point in tar, or some other thick substance or paint which may be readily carbonized. i next pass the point of the filament through the crystals, and then hold it vertically over a hot plate. the tar softens and forms a drop on the point of the filament, the crystals adhering to the surface of the drop. by regulating the distance from the plate the tar is slowly dried out and the button becomes solid. i then once more dip the button in tar and hold it again over a plate until the tar is evaporated, leaving only a hard mass which firmly binds the crystals. when a larger button is required i repeat the process several times, and i generally also cover the filament a certain distance below the button with crystals. the button being mounted in a bulb, when a good vacuum has been reached, first a weak and then a strong discharge is passed through the bulb to carbonize the tar and expel all gases, and later it is brought to a very intense incandescence. when the powder is used i have found it best to proceed as follows: i make a thick paint of carborundum and tar, and pass a lamp filament through the paint. taking then most of the paint off by rubbing the filament against a piece of chamois leather, i hold it over a hot plate until the tar evaporates and the coating becomes firm. i repeat this process as many times as it is necessary to obtain a certain thickness of coating. on the point of the coated filament i form a button in the same manner. there is no doubt that such a button--properly prepared under great pressure--of carborundum, especially of powder of the best quality, will withstand the effect of the bombardment fully as well as anything we know. the difficulty is that the binding material gives way, and the carborundum is slowly thrown off after some time. as it does not seem to blacken the globe in the least, it might be found useful for coating the filaments of ordinary incandescent lamps, and i think that it is even possible to produce thin threads or sticks of carborundum which will replace the ordinary filaments in an incandescent lamp. a carborundum coating seems to be more durable than other coatings, not only because the carborundum can withstand high degrees of heat, but also because it seems to unite with the carbon better than any other material i have tried. a coating of zirconia or any other oxide, for instance, is far more quickly destroyed. i prepared buttons of diamond dust in the same manner as of carborundum, and these came in durability nearest to those prepared of carborundum, but the binding paste gave way much more quickly in the diamond buttons: this, however, i attributed to the size and irregularity of the grains of the diamond. it was of interest to find whether carborundum possesses the quality of phosphorescence. one is, of course, prepared to encounter two difficulties: first, as regards the rough product, the "crystals," they are good conducting, and it is a fact that conductors do not phosphoresce; second, the powder, being exceedingly fine, would not be apt to exhibit very prominently this quality, since we know that when crystals, even such as diamond or ruby, are finely powdered, they lose the property of phosphorescence to a considerable degree. the question presents itself here, can a conductor phosphoresce? what is there in such a body as a metal, for instance, that would deprive it of the quality of phosphorescence, unless it is that property which characterizes it as a conductor? for it is a fact that most of the phosphorescent bodies lose that quality when they are sufficiently heated to become more or less conducting. then, if a metal be in a large measure, or perhaps entirely, deprived of that property, it should be capable of phosphorescence. therefore it is quite possible that at some extremely high frequency, when behaving practically as a non-conductor, a metal or any other conductor might exhibit the quality of phosphorescence, even though it be entirely incapable of phosphorescing under the impact of a low-frequency discharge. there is, however, another possible way how a conductor might at least _appear_ to phosphoresce. considerable doubt still exists as to what really is phosphorescence, and as to whether the various phenomena comprised under this head are due to the same causes. suppose that in an exhausted bulb, under the molecular impact, the surface of a piece of metal or other conductor is rendered strongly luminous, but at the same time it is found that it remains comparatively cool, would not this luminosity be called phosphorescence? now such a result, theoretically at least, is possible, for it is a mere question of potential or speed. assume the potential of the electrode, and consequently the speed of the projected atoms, to be sufficiently high, the surface of the metal piece against which the atoms are projected would be rendered highly incandescent, since the process of heat generation would be incomparably faster than that of radiating or conducting away from the surface of the collision. in the eye of the observer a single impact of the atoms would cause an instantaneous flash, but if the impacts were repeated with sufficient rapidity they would produce a continuous impression upon his retina. to him then the surface of the metal would appear continuously incandescent and of constant luminous intensity, while in reality the light would be either intermittent or at least changing periodically in intensity. the metal piece would rise in temperature until equilibrium was attained--that is until the energy continuously radiated would equal that intermittently supplied. but the supplied energy might under such conditions not be sufficient to bring the body to any more than a very moderate mean temperature, especially if the frequency of the atomic impacts be very low--just enough that the fluctuation of the intensity of the light emitted could not be detected by the eye. the body would now, owing to the manner in which the energy is supplied, emit a strong light, and yet be at a comparatively very low mean temperature. how could the observer call the luminosity thus produced? even if the analysis of the light would teach him something definite, still he would probably rank it under the phenomena of phosphorescence. it is conceivable that in such a way both conducting and non-conducting bodies may be maintained at a certain luminous intensity, but the energy required would very greatly vary with the nature and properties of the bodies. these and some foregoing remarks of a speculative nature were made merely to bring out curious features of alternate currents or electric impulses. by their help we may cause a body to emit _more_ light, while at a certain mean temperature, than it would emit if brought to that temperature by a steady supply; and, again, we may bring a body to the point of fusion, and cause it to emit _less_ light than when fused by the application of energy in ordinary ways. it all depends on how we supply the energy, and what kind of vibrations we set up: in one case the vibrations are more, in the other less, adapted to affect our sense of vision. some effects, which i had not observed before, obtained with carborundum in the first trials, i attributed to phosphorescence, but in subsequent experiments it appeared that it was devoid of that quality. the crystals possess a noteworthy feature. in a bulb provided with a single electrode in the shape of a small circular metal disc, for instance, at a certain degree of exhaustion the electrode is covered with a milky film, which is separated by a dark space from the glow filling the bulb. when the metal disc is covered with carborundum crystals, the film is far more intense, and snow-white. this i found later to be merely an effect of the bright surface of the crystals, for when an aluminium electrode was highly polished it exhibited more or less the same phenomenon. i made a number of experiments with the samples of crystals obtained, principally because it would have been of special interest to find that they are capable of phosphorescence, on account of their being conducting. i could not produce phosphorescence distinctly, but i must remark that a decisive opinion cannot be formed until other experimenters have gone over the same ground. the powder behaved in some experiments as though it contained alumina, but it did not exhibit with sufficient distinctness the red of the latter. its dead color brightens considerably under the molecular impact, but i am now convinced it does not phosphoresce. still, the tests with the powder are not conclusive, because powdered carborundum probably does not behave like a phosphorescent sulphide, for example, which could be finely powdered without impairing the phosphorescence, but rather like powdered ruby or diamond, and therefore it would be necessary, in order to make a decisive test, to obtain it in a large lump and polish up the surface. if the carborundum proves useful in connection with these and similar experiments, its chief value will be found in the production of coatings, thin conductors, buttons, or other electrodes capable of withstanding extremely high degrees of heat. the production of a small electrode capable of withstanding enormous temperatures i regard as of the greatest importance in the manufacture of light. it would enable us to obtain, by means of currents of very high frequencies, certainly times, if not more, the quantity of light which is obtained in the present incandescent lamp by the same expenditure of energy. this estimate may appear to many exaggerated, but in reality i think it is far from being so. as this statement might be misunderstood i think it necessary to expose clearly the problem with which in this line of work we are confronted, and the manner in which, in my opinion, a solution will be arrived at. any one who begins a study of the problem will be apt to think that what is wanted in a lamp with an electrode is a very high degree of incandescence of the electrode. there he will be mistaken. the high incandescence of the button is a necessary evil, but what is really wanted is the high incandescence of the gas surrounding the button. in other words, the problem in such a lamp is to bring a mass of gas to the highest possible incandescence. the higher the incandescence, the quicker the mean vibration, the greater is the economy of the light production. but to maintain a mass of gas at a high degree of incandescence in a glass vessel, it will always be necessary to keep the incandescent mass away from the glass; that is, to confine it as much as possible to the central portion of the globe. in one of the experiments this evening a brush was produced at the end of a wire. this brush was a flame, a source of heat and light. it did not emit much perceptible heat, nor did it glow with an intense light; but is it the less a flame because it does not scorch my hand? is it the less a flame because it does not hurt my eye by its brilliancy? the problem is precisely to produce in the bulb such a flame, much smaller in size, but incomparably more powerful. were there means at hand for producing electric impulses of a sufficiently high frequency, and for transmitting them, the bulb could be done away with, unless it were used to protect the electrode, or to economize the energy by confining the heat. but as such means are not at disposal, it becomes necessary to place the terminal in a bulb and rarefy the air in the same. this is done merely to enable the apparatus to perform the work which it is not capable of performing at ordinary air pressure. in the bulb we are able to intensify the action to any degree--so far that the brush emits a powerful light. the intensity of the light emitted depends principally on the frequency and potential of the impulses, and on the electric density of the surface of the electrode. it is of the greatest importance to employ the smallest possible button, in order to push the density very far. under the violent impact of the molecules of the gas surrounding it, the small electrode is of course brought to an extremely high temperature, but around it is a mass of highly incandescent gas, a flame photosphere, many hundred times the volume of the electrode. with a diamond, carborundum or zirconia button the photosphere can be as much as one thousand times the volume of the button. without much reflecting one would think that in pushing so far the incandescence of the electrode it would be instantly volatilized. but after a careful consideration he would find that, theoretically, it should not occur, and in this fact--which, however, is experimentally demonstrated--lies principally the future value of such a lamp. at first, when the bombardment begins, most of the work is performed on the surface of the button, but when a highly conducting photosphere is formed the button is comparatively relieved. the higher the incandescence of the photosphere the more it approaches in conductivity to that of the electrode, and the more, therefore, the solid and the gas form one conducting body. the consequence is that the further is forced the incandescence the more work, comparatively, is performed on the gas, and the less on the electrode. the formation of a powerful photosphere is consequently the very means for protecting the electrode. this protection, of course, is a relative one, and it should not be thought that by pushing the incandescence higher the electrode is actually less deteriorated. still, theoretically, with extreme frequencies, this result must be reached, but probably at a temperature too high for most of the refractory bodies known. given, then, an electrode which can withstand to a very high limit the effect of the bombardment and outward strain, it would be safe no matter how much it is forced beyond that limit. in an incandescent lamp quite different considerations apply. there the gas is not at all concerned: the whole of the work is performed on the filament; and the life of the lamp diminishes so rapidly with the increase of the degree of incandescence that economical reasons compel us to work it at a low incandescence. but if an incandescent lamp is operated with currents of very high frequency, the action of the gas cannot be neglected, and the rules for the most economical working must be considerably modified. in order to bring such a lamp with one or two electrodes to a great perfection, it is necessary to employ impulses of very high frequency. the high frequency secures, among others, two chief advantages, which have a most important bearing upon the economy of the light production. first, the deterioration of the electrode is reduced by reason of the fact that we employ a great many small impacts, instead of a few violent ones, which shatter quickly the structure; secondly, the formation of a large photosphere is facilitated. in order to reduce the deterioration of the electrode to the minimum, it is desirable that the vibration be harmonic, for any suddenness hastens the process of destruction. an electrode lasts much longer when kept at incandescence by currents, or impulses, obtained from a high-frequency alternator, which rise and fall more or less harmonically, than by impulses obtained from a disruptive discharge coil. in the latter case there is no doubt that most of the damage is done by the fundamental sudden discharges. one of the elements of loss in such a lamp is the bombardment of the globe. as the potential is very high, the molecules are projected with great speed; they strike the glass, and usually excite a strong phosphorescence. the effect produced is very pretty, but for economical reasons it would be perhaps preferable to prevent, or at least reduce to the minimum, the bombardment against the globe, as in such case it is, as a rule, not the object to excite phosphorescence, and as some loss of energy results from the bombardment. this loss in the bulb is principally dependent on the potential of the impulses and on the electric density on the surface of the electrode. in employing very high frequencies the loss of energy by the bombardment is greatly reduced, for, first, the potential needed to perform a given amount of work is much smaller; and, secondly, by producing a highly conducting photosphere around the electrode, the same result is obtained as though the electrode were much larger, which is equivalent to a smaller electric density. but be it by the diminution of the maximum potential or of the density, the gain is effected in the same manner, namely, by avoiding violent shocks, which strain the glass much beyond its limit of elasticity. if the frequency could be brought high enough, the loss due to the imperfect elasticity of the glass would be entirely negligible. the loss due to bombardment of the globe may, however, be reduced by using two electrodes instead of one. in such case each of the electrodes may be connected to one of the terminals; or else, if it is preferable to use only one wire, one electrode may be connected to one terminal and the other to the ground or to an insulated body of some surface, as, for instance, a shade on the lamp. in the latter case, unless some judgment is used, one of the electrodes might glow more intensely than the other. but on the whole i find it preferable when using such high frequencies to employ only one electrode and one connecting wire. i am convinced that the illuminating device of the near future will not require for its operation more than one lead, and, at any rate, it will have no leading-in wire, since the energy required can be as well transmitted through the glass. in experimental bulbs the leading-in wire is most generally used on account of convenience, as in employing condenser coatings in the manner indicated in fig. , for example, there is some difficulty in fitting the parts, but these difficulties would not exist if a great many bulbs were manufactured; otherwise the energy can be conveyed through the glass as well as through a wire, and with these high frequencies the losses are very small. such illuminating devices will necessarily involve the use of very high potentials, and this, in the eyes of practical men, might be an objectionable feature. yet, in reality, high potentials are not objectionable--certainly not in the least as far as the safety of the devices is concerned. there are two ways of rendering an electric appliance safe. one is to use low potentials, the other is to determine the dimensions of the apparatus so that it is safe no matter how high a potential is used. of the two the latter seems to me the better way, for then the safety is absolute, unaffected by any possible combination of circumstances which might render even a low-potential appliance dangerous to life and property. but the practical conditions require not only the judicious determination of the dimensions of the apparatus; they likewise necessitate the employment of energy of the proper kind. it is easy, for instance, to construct a transformer capable of giving, when operated from an ordinary alternate current machine of low tension, say , volts, which might be required to light a highly exhausted phosphorescent tube, so that, in spite of the high potential, it is perfectly safe, the shock from it producing no inconvenience. still, such a transformer would be expensive, and in itself inefficient; and, besides, what energy was obtained from it would not be economically used for the production of light. the economy demands the employment of energy in the form of extremely rapid vibrations. the problem of producing light has been likened to that of maintaining a certain high-pitch note by means of a bell. it should be said a _barely audible_ note; and even these words would not express it, so wonderful is the sensitiveness of the eye. we may deliver powerful blows at long intervals, waste a good deal of energy, and still not get what we want; or we may keep up the note by delivering frequent gentle taps, and get nearer to the object sought by the expenditure of much less energy. in the production of light, as far as the illuminating device is concerned, there can be only one rule--that is, to use as high frequencies as can be obtained; but the means for the production and conveyance of impulses of such character impose, at present at least, great limitations. once it is decided to use very high frequencies, the return wire becomes unnecessary, and all the appliances are simplified. by the use of obvious means the same result is obtained as though the return wire were used. it is sufficient for this purpose to bring in contact with the bulb, or merely in the vicinity of the same, an insulated body of some surface. the surface need, of course, be the smaller, the higher the frequency and potential used, and necessarily, also, the higher the economy of the lamp or other device. this plan of working has been resorted to on several occasions this evening. so, for instance, when the incandescence of a button was produced by grasping the bulb with the hand, the body of the experimenter merely served to intensify the action. the bulb used was similar to that illustrated in fig. , and the coil was excited to a small potential, not sufficient to bring the button to incandescence when the bulb was hanging from the wire; and incidentally, in order to perform the experiment in a more suitable manner, the button was taken so large that a perceptible time had to elapse before, upon grasping the bulb, it could be rendered incandescent. the contact with the bulb was, of course, quite unnecessary. it is easy, by using a rather large bulb with an exceedingly small electrode, to adjust the conditions so that the latter is brought to bright incandescence by the mere approach of the experimenter within a few feet of the bulb, and that the incandescence subsides upon his receding. [illustration: fig. .--bulb without leading-in wire, showing effect of projected matter.] in another experiment, when phosphorescence was excited, a similar bulb was used. here again, originally, the potential was not sufficient to excite phosphorescence until the action was intensified--in this case, however, to present a different feature, by touching the socket with a metallic object held in the hand. the electrode in the bulb was a carbon button so large that it could not be brought to incandescence, and thereby spoil the effect produced by phosphorescence. [illustration: fig. .--improved experimental bulb.] again, in another of the early experiments, a bulb was used as illustrated in fig. . in this instance, by touching the bulb with one or two fingers, one or two shadows of the stem inside were projected against the glass, the touch of the finger producing the same result as the application of an external negative electrode under ordinary circumstances. in all these experiments the action was intensified by augmenting the capacity at the end of the lead connected to the terminal. as a rule, it is not necessary to resort to such means, and would be quite unnecessary with still higher frequencies; but when it _is_ desired, the bulb, or tube, can be easily adapted to the purpose. [illustration: fig. .--improved bulb with intensifying reflector.] in fig. , for example, an experimental bulb l is shown, which is provided with a neck n on the top for the application of an external tinfoil coating, which may be connected to a body of larger surface. such a lamp as illustrated in fig. may also be lighted by connecting the tinfoil coating on the neck n to the terminal, and the leading-in wire w to an insulated plate. if the bulb stands in a socket upright, as shown in the cut, a shade of conducting material may be slipped in the neck n, and the action thus magnified. a more perfected arrangement used in some of these bulbs is illustrated in fig. . in this case the construction of the bulb is as shown and described before, when reference was made to fig. . a zinc sheet z, with a tubular extension t, is slipped over the metallic socket s. the bulb hangs downward from the terminal t, the zinc sheet z, performing the double office of intensifier and reflector. the reflector is separated from the terminal t by an extension of the insulating plug p. [illustration: fig. .--phosphorescent tube with intensifying reflector.] a similar disposition with a phosphorescent tube is illustrated in fig. . the tube t is prepared from two short tubes of a different diameter, which are sealed on the ends. on the lower end is placed an outside conducting coating c, which connects to the wire w. the wire has a hook on the upper end for suspension, and passes through the centre of the inside tube, which is filled with some good and tightly packed insulator. on the outside of the upper end of the tube t is another conducting coating c_ upon which is slipped a metallic reflector z, which should be separated by a thick insulation from the end of wire w. the economical use of such a reflector or intensifier would require that all energy supplied to an air condenser should be recoverable, or, in other words, that there should not be any losses, neither in the gaseous medium nor through its action elsewhere. this is far from being so, but, fortunately, the losses may be reduced to anything desired. a few remarks are necessary on this subject, in order to make the experiences gathered in the course of these investigations perfectly clear. suppose a small helix with many well insulated turns, as in experiment fig. , has one of its ends connected to one of the terminals of the induction coil, and the other to a metal plate, or, for the sake of simplicity, a sphere, insulated in space. when the coil is set to work, the potential of the sphere is alternated, and the small helix now behaves as though its free end were connected to the other terminal of the induction coil. if an iron rod be held within the small helix it is quickly brought to a high temperature, indicating the passage of a strong current through the helix. how does the insulated sphere act in this case? it can be a condenser, storing and returning the energy supplied to it, or it can be a mere sink of energy, and the conditions of the experiment determine whether it is more one or the other. the sphere being charged to a high potential, it acts inductively upon the surrounding air, or whatever gaseous medium there might be. the molecules, or atoms, which are near the sphere are of course more attracted, and move through a greater distance than the farther ones. when the nearest molecules strike the sphere they are repelled, and collisions occur at all distances within the inductive action of the sphere. it is now clear that, if the potential be steady, but little loss of energy can be caused in this way, for the molecules which are nearest to the sphere, having had an additional charge imparted to them by contact, are not attracted until they have parted, if not with all, at least with most of the additional charge, which can be accomplished only after a great many collisions. from the fact that with a steady potential there is but little loss in dry air, one must come to such a conclusion. when the potential of the sphere, instead of being steady, is alternating, the conditions are entirely different. in this case a rhythmical bombardment occurs, no matter whether the molecules after coming in contact with the sphere lose the imparted charge or not; what is more, if the charge is not lost, the impacts are only the more violent. still if the frequency of the impulses be very small, the loss caused by the impacts and collisions would not be serious unless the potential were excessive. but when extremely high frequencies and more or less high potentials are used, the loss may be very great. the total energy lost per unit of time is proportionate to the product of the number of impacts per second, or the frequency and the energy lost in each impact. but the energy of an impact must be proportionate to the square of the electric density of the sphere, since the charge imparted to the molecule is proportionate to that density. i conclude from this that the total energy lost must be proportionate to the product of the frequency and the square of the electric density; but this law needs experimental confirmation. assuming the preceding considerations to be true, then, by rapidly alternating the potential of a body immersed in an insulating gaseous medium, any amount of energy may be dissipated into space. most of that energy then, i believe, is not dissipated in the form of long ether waves, propagated to considerable distance, as is thought most generally, but is consumed--in the case of an insulated sphere, for example--in impact and collisional losses--that is, heat vibrations--on the surface and in the vicinity of the sphere. to reduce the dissipation it is necessary to work with a small electric density--the smaller the higher the frequency. but since, on the assumption before made, the loss is diminished with the square of the density, and since currents of very high frequencies involve considerable waste when transmitted through conductors, it follows that, on the whole, it is better to employ one wire than two. therefore, if motors, lamps, or devices of any kind are perfected, capable of being advantageously operated by currents of extremely high frequency, economical reasons will make it advisable to use only one wire, especially if the distances are great. when energy is absorbed in a condenser the same behaves as though its capacity were increased. absorption always exists more or less, but generally it is small and of no consequence as long as the frequencies are not very great. in using extremely high frequencies, and, necessarily in such case, also high potentials, the absorption--or, what is here meant more particularly by this term, the loss of energy due to the presence of a gaseous medium--is an important factor to be considered, as the energy absorbed in the air condenser may be any fraction of the supplied energy. this would seem to make it very difficult to tell from the measured or computed capacity of an air condenser its actual capacity or vibration period, especially if the condenser is of very small surface and is charged to a very high potential. as many important results are dependent upon the correctness of the estimation of the vibration period, this subject demands the most careful scrutiny of other investigators. to reduce the probable error as much as possible in experiments of the kind alluded to, it is advisable to use spheres or plates of large surface, so as to make the density exceedingly small. otherwise, when it is practicable, an oil condenser should be used in preference. in oil or other liquid dielectrics there are seemingly no such losses as in gaseous media. it being impossible to exclude entirely the gas in condensers with solid dielectrics, such condensers should be immersed in oil, for economical reasons if nothing else; they can then be strained to the utmost and will remain cool. in leyden jars the loss due to air is comparatively small, as the tinfoil coatings are large, close together, and the charged surfaces not directly exposed; but when the potentials are very high, the loss may be more or less considerable at, or near, the upper edge of the foil, where the air is principally acted upon. if the jar be immersed in boiled-out oil, it will be capable of performing four times the amount of work which it can for any length of time when used in the ordinary way, and the loss will be inappreciable. it should not be thought that the loss in heat in an air condenser is necessarily associated with the formation of _visible_ streams or brushes. if a small electrode, inclosed in an unexhausted bulb, is connected to one of the terminals of the coil, streams can be seen to issue from the electrode and the air in the bulb is heated; if, instead of a small electrode, a large sphere is inclosed in the bulb, no streams are observed, still the air is heated. nor should it be thought that the temperature of an air condenser would give even an approximate idea of the loss in heat incurred, as in such case heat must be given off much more quickly, since there is, in addition to the ordinary radiation, a very active carrying away of heat by independent carriers going on, and since not only the apparatus, but the air at some distance from it is heated in consequence of the collisions which must occur. owing to this, in experiments with such a coil, a rise of temperature can be distinctly observed only when the body connected to the coil is very small. but with apparatus on a larger scale, even a body of considerable bulk would be heated, as, for instance, the body of a person; and i think that skilled physicians might make observations of utility in such experiments, which, if the apparatus were judiciously designed, would not present the slightest danger. a question of some interest, principally to meteorologists, presents itself here. how does the earth behave? the earth is an air condenser, but is it a perfect or a very imperfect one--a mere sink of energy? there can be little doubt that to such small disturbance as might be caused in an experiment the earth behaves as an almost perfect condenser. but it might be different when its charge is set in vibration by some sudden disturbance occurring in the heavens. in such case, as before stated, probably only little of the energy of the vibrations set up would be lost into space in the form of long ether radiations, but most of the energy, i think, would spend itself in molecular impacts and collisions, and pass off into space in the form of short heat, and possibly light, waves. as both the frequency of the vibrations of the charge and the potential are in all probability excessive, the energy converted into heat may be considerable. since the density must be unevenly distributed, either in consequence of the irregularity of the earth's surface, or on account of the condition of the atmosphere in various places, the effect produced would accordingly vary from place to place. considerable variations in the temperature and pressure of the atmosphere may in this manner be caused at any point of the surface of the earth. the variations may be gradual or very sudden, according to the nature of the general disturbance, and may produce rain and storms, or locally modify the weather in any way. from the remarks before made one may see what an important factor of loss the air in the neighborhood of a charged surface becomes when the electric density is great and the frequency of the impulses excessive. but the action as explained implies that the air is insulating--that is, that it is composed of independent carriers immersed in an insulating medium. this is the case only when the air is at something like ordinary or greater, or at extremely small, pressure. when the air is slightly rarefied and conducting, then true conduction losses occur also. in such case, of course, considerable energy may be dissipated into space even with a steady potential, or with impulses of low frequency, if the density is very great. when the gas is at very low pressure, an electrode is heated more because higher speeds can be reached. if the gas around the electrode is strongly compressed, the displacements, and consequently the speeds, are very small, and the heating is insignificant. but if in such case the frequency could be sufficiently increased, the electrode would be brought to a high temperature as well as if the gas were at very low pressure; in fact, exhausting the bulb is only necessary because we cannot produce (and possibly not convey) currents of the required frequency. returning to the subject of electrode lamps, it is obviously of advantage in such a lamp to confine as much as possible the heat to the electrode by preventing the circulation of the gas in the bulb. if a very small bulb be taken, it would confine the heat better than a large one, but it might not be of sufficient capacity to be operated from the coil, or, if so, the glass might get too hot. a simple way to improve in this direction is to employ a globe of the required size, but to place a small bulb, the diameter of which is properly estimated, over the refractory button contained in the globe. this arrangement is illustrated in fig. . [illustration: fig. .--lamp with auxiliary bulb for confining the action to the centre.] the globe l has in this case a large neck n, allowing the small bulb b to slip through. otherwise the construction is the same as shown in fig. , for example. the small bulb is conveniently supported upon the stem s, carrying the refractory button m. it is separated from the aluminium tube a by several layers of mica m, in order to prevent the cracking of the neck by the rapid heating of the aluminium tube upon a sudden turning on of the current. the inside bulb should be as small as possible when it is desired to obtain light only by incandescence of the electrode. if it is desired to produce phosphorescence, the bulb should be larger, else it would be apt to get too hot, and the phosphorescence would cease. in this arrangement usually only the small bulb shows phosphorescence, as there is practically no bombardment against the outer globe. in some of these bulbs constructed as illustrated in fig. the small tube was coated with phosphorescent paint, and beautiful effects were obtained. instead of making the inside bulb large, in order to avoid undue heating, it answers the purpose to make the electrode m larger. in this case the bombardment is weakened by reason of the smaller electric density. many bulbs were constructed on the plan illustrated in fig. . here a small bulb b, containing the refractory button m, upon being exhausted to a very high degree was sealed in a large globe l, which was then moderately exhausted and sealed off. the principal advantage of this construction was that it allowed of reaching extremely high vacua, and, at the same time use a large bulb. it was found, in the course of experiences with bulbs such as illustrated in fig. , that it was well to make the stem s near the seal at e very thick, and the leading-in wire w thin, as it occurred sometimes that the stem at e was heated and the bulb was cracked. often the outer globe l was exhausted only just enough to allow the discharge to pass through, and the space between the bulbs appeared crimson, producing a curious effect. in some cases, when the exhaustion in globe l was very low, and the air good conducting, it was found necessary, in order to bring the button m to high incandescence, to place, preferably on the upper part of the neck of the globe, a tinfoil coating which was connected to an insulated body, to the ground, or to the other terminal of the coil, as the highly conducting air weakened the effect somewhat, probably by being acted upon inductively from the wire w, where it entered the bulb at e. another difficulty--which, however, is always present when the refractory button is mounted in a very small bulb--existed in the construction illustrated in fig. , namely, the vacuum in the bulb b would be impaired in a comparatively short time. [illustration: fig. .--lamp with independent auxiliary bulb.] the chief idea in the two last described constructions was to confine the heat to the central portion of the globe by preventing the exchange of air. an advantage is secured, but owing to the heating of the inside bulb and slow evaporation of the glass the vacuum is hard to maintain, even if the construction illustrated in fig. be chosen, in which both bulbs communicate. but by far the better way--the ideal way--would be to reach sufficiently high frequencies. the higher the frequency the slower would be the exchange of the air, and i think that a frequency may be reached at which there would be no exchange whatever of the air molecules around the terminal. we would then produce a flame in which there would be no carrying away of material, and a queer flame it would be, for it would be rigid! with such high frequencies the inertia of the particles would come into play. as the brush, or flame, would gain rigidity in virtue of the inertia of the particles, the exchange of the latter would be prevented. this would necessarily occur, for, the number of the impulses being augmented, the potential energy of each would diminish, so that finally only atomic vibrations could be set up, and the motion of translation through measurable space would cease. thus an ordinary gas burner connected to a source of rapidly alternating potential might have its efficiency augmented to a certain limit, and this for two reasons--because of the additional vibration imparted, and because of a slowing down of the process of carrying off. but the renewal being rendered difficult, and renewal being necessary to maintain the _burner_, a continued increase of the frequency of the impulses, assuming they could be transmitted to and impressed upon the flame, would result in the "extinction" of the latter, meaning by this term only the cessation of the chemical process. i think, however, that in the case of an electrode immersed in a fluid insulating medium, and surrounded by independent carriers of electric charges, which can be acted upon inductively, a sufficiently high frequency of the impulses would probably result in a gravitation of the gas all around toward the electrode. for this it would be only necessary to assume that the independent bodies are irregularly shaped; they would then turn toward the electrode their side of the greatest electric density, and this would be a position in which the fluid resistance to approach would be smaller than that offered to the receding. the general opinion, i do not doubt, is that it is out of the question to reach any such frequencies as might--assuming some of the views before expressed to be true--produce any of the results which i have pointed out as mere possibilities. this may be so, but in the course of these investigations, from the observation of many phenomena i have gained the conviction that these frequencies would be much lower than one is apt to estimate at first. in a flame we set up light vibrations by causing molecules, or atoms, to collide. but what is the ratio of the frequency of the collisions and that of the vibrations set up? certainly it must be incomparably smaller than that of the knocks of the bell and the sound vibrations, or that of the discharges and the oscillations of the condenser. we may cause the molecules of the gas to collide by the use of alternate electric impulses of high frequency, and so we may imitate the process in a flame; and from experiments with frequencies which we are now able to obtain, i think that the result is producible with impulses which are transmissible through a conductor. in connection with thoughts of a similar nature, it appeared to me of great interest to demonstrate the rigidity of a vibrating gaseous column. although with such low frequencies as, say , per second, which i was able to obtain without difficulty from a specially constructed alternator, the task looked discouraging at first, i made a series of experiments. the trials with air at ordinary pressure led to no result, but with air moderately rarefied i obtain what i think to be an unmistakable experimental evidence of the property sought for. as a result of this kind might lead able investigators to conclusions of importance i will describe one of the experiments performed. it is well known that when a tube is slightly exhausted the discharge may be passed through it in the form of a thin luminous thread. when produced with currents of low frequency, obtained from a coil operated as usual, this thread is inert. if a magnet be approached to it, the part near the same is attracted or repelled, according to the direction of the lines of force of the magnet. it occurred to me that if such a thread would be produced with currents of very high frequency, it should be more or less rigid, and as it was visible it could be easily studied. accordingly i prepared a tube about inch in diameter and metre long, with outside coating at each end. the tube was exhausted to a point at which by a little working the thread discharge could be obtained. it must be remarked here that the general aspect of the tube, and the degree of exhaustion, are quite different than when ordinary low frequency currents are used. as it was found preferable to work with one terminal, the tube prepared was suspended from the end of a wire connected to the terminal, the tinfoil coating being connected to the wire, and to the lower coating sometimes a small insulated plate was attached. when the thread was formed it extended through the upper part of the tube and lost itself in the lower end. if it possessed rigidity it resembled, not exactly an elastic cord stretched tight between two supports, but a cord suspended from a height with a small weight attached at the end. when the finger or a magnet was approached to the upper end of the luminous thread, it could be brought locally out of position by electrostatic or magnetic action; and when the disturbing object was very quickly removed, an analogous result was produced, as though a suspended cord would be displaced and quickly released near the point of suspension. in doing this the luminous thread was set in vibration, and two very sharply marked nodes, and a third indistinct one, were formed. the vibration, once set up, continued for fully eight minutes, dying gradually out. the speed of the vibration often varied perceptibly, and it could be observed that the electrostatic attraction of the glass affected the vibrating thread; but it was clear that the electrostatic action was not the cause of the vibration, for the thread was most generally stationary, and could always be set in vibration by passing the finger quickly near the upper part of the tube. with a magnet the thread could be split in two and both parts vibrated. by approaching the hand to the lower coating of the tube, or insulated plate if attached, the vibration was quickened; also, as far as i could see, by raising the potential or frequency. thus, either increasing the frequency or passing a stronger discharge of the same frequency corresponded to a tightening of the cord. i did not obtain any experimental evidence with condenser discharges. a luminous band excited in a bulb by repeated discharges of a leyden jar must possess rigidity, and if deformed and suddenly released should vibrate. but probably the amount of vibrating matter is so small that in spite of the extreme speed the inertia cannot prominently assert itself. besides, the observation in such a case is rendered extremely difficult on account of the fundamental vibration. the demonstration of the fact--which still needs better experimental confirmation--that a vibrating gaseous column possesses rigidity, might greatly modify the views of thinkers. when with low frequencies and insignificant potentials indications of that property may be noted, how must a gaseous medium behave under the influence of enormous electrostatic stresses which may be active in the interstellar space, and which may alternate with inconceivable rapidity? the existence of such an electrostatic, rhythmically throbbing force--of a vibrating electrostatic field--would show a possible way how solids might have formed from the ultra-gaseous uterus, and how transverse and all kinds of vibrations may be transmitted through a gaseous medium filling all space. then, ether might be a true fluid, devoid of rigidity, and at rest, it being merely necessary as a connecting link to enable interaction. what determines the rigidity of a body? it must be the speed and the amount of moving matter. in a gas the speed may be considerable, but the density is exceedingly small; in a liquid the speed would be likely to be small, though the density may be considerable; and in both cases the inertia resistance offered to displacement is practically _nil_. but place a gaseous (or liquid) column in an intense, rapidly alternating electrostatic field, set the particles vibrating with enormous speeds, then the inertia resistance asserts itself. a body might move with more or less freedom through the vibrating mass, but as a whole it would be rigid. there is a subject which i must mention in connection with these experiments: it is that of high vacua. this is a subject the study of which is not only interesting, but useful, for it may lead to results of great practical importance. in commercial apparatus, such as incandescent lamps, operated from ordinary systems of distribution, a much higher vacuum than obtained at present would not secure a very great advantage. in such a case the work is performed on the filament and the gas is little concerned; the improvement, therefore, would be but trifling. but when we begin to use very high frequencies and potentials, the action of the gas becomes all important, and the degree of exhaustion materially modifies the results. as long as ordinary coils, even very large ones, were used, the study of the subject was limited, because just at a point when it became most interesting it had to be interrupted on account of the "non-striking" vacuum being reached. but presently we are able to obtain from a small disruptive discharge coil potentials much higher than even the largest coil was capable of giving, and, what is more, we can make the potential alternate with great rapidity. both of these results enable us now to pass a luminous discharge through almost any vacua obtainable, and the field of our investigations is greatly extended. think we as we may, of all the possible directions to develop a practical illuminant, the line of high vacua seems to be the most promising at present. but to reach extreme vacua the appliances must be much more improved, and ultimate perfection will not be attained until we shall have discarded the mechanical and perfected an _electrical_ vacuum pump. molecules and atoms can be thrown out of a bulb under the action of an enormous potential: _this_ will be the principle of the vacuum pump of the future. for the present, we must secure the best results we can with mechanical appliances. in this respect, it might not be out of the way to say a few words about the method of, and apparatus for, producing excessively high degrees of exhaustion of which i have availed myself in the course of these investigations. it is very probable that other experimenters have used similar arrangements; but as it is possible that there may be an item of interest in their description, a few remarks, which will render this investigation more complete, might be permitted. [illustration: fig. .--apparatus used for obtaining high degrees of exhaustion.] the apparatus is illustrated in a drawing shown in fig. . s represents a sprengel pump, which has been specially constructed to better suit the work required. the stop-cock which is usually employed has been omitted, and instead of it a hollow stopper s has been fitted in the neck of the reservoir r. this stopper has a small hole h, through which the mercury descends; the size of the outlet o being properly determined with respect to the section of the fall tube t, which is sealed to the reservoir instead of being connected to it in the usual manner. this arrangement overcomes the imperfections and troubles which often arise from the use of the stopcock on the reservoir and the connection of the latter with the fall tube. the pump is connected through a u-shaped tube t to a very large reservoir r_ . especial care was taken in fitting the grinding surfaces of the stoppers p and p_ , and both of these and the mercury caps above them were made exceptionally long. after the u-shaped tube was fitted and put in place, it was heated, so as to soften and take off the strain resulting from imperfect fitting. the u-shaped tube was provided with a stopcock c, and two ground connections g and g_ --one for a small bulb b, usually containing caustic potash, and the other for the receiver r, to be exhausted. the reservoir r_ was connected by means of a rubber tube to a slightly larger reservoir r_ , each of the two reservoirs being provided with a stopcock c_ and c_ , respectively. the reservoir r_ could be raised and lowered by a wheel and rack, and the range of its motion was so determined that when it was filled with mercury and the stopcock c_ closed, so as to form a torricellian vacuum in it when raised, it could be lifted so high that the mercury in reservoir r_ would stand a little above stopcock c_ ; and when this stopcock was closed and the reservoir r_ descended, so as to form a torricellian vacuum in reservoir r_ , it could be lowered so far as to completely empty the latter, the mercury filling the reservoir r_ up to a little above stopcock c_ . the capacity of the pump and of the connections was taken as small as possible relatively to the volume of reservoir r_ , since, of course, the degree of exhaustion depended upon the ratio of these quantities. with this apparatus i combined the usual means indicated by former experiments for the production of very high vacua. in most of the experiments it was convenient to use caustic potash. i may venture to say, in regard to its use, that much time is saved and a more perfect action of the pump insured by fusing and boiling the potash as soon as, or even before, the pump settles down. if this course is not followed the sticks, as ordinarily employed, may give moisture off at a certain very slow rate, and the pump may work for many hours without reaching a very high vacuum. the potash was heated either by a spirit lamp or by passing a discharge through it, or by passing a current through a wire contained in it. the advantage in the latter case was that the heating could be more rapidly repeated. generally the process of exhaustion was the following:--at the start, the stop-cocks c and c_ being open, and all other connections closed, the reservoir r_ was raised so far that the mercury filled the reservoir r_ and a part of the narrow connecting u-shaped tube. when the pump was set to work, the mercury would, of course, quickly rise in the tube, and reservoir r_ was lowered, the experimenter keeping the mercury at about the same level. the reservoir r_ was balanced by a long spring which facilitated the operation, and the friction of the parts was generally sufficient to keep it almost in any position. when the sprengel pump had done its work, the reservoir r_ was further lowered and the mercury descended in r_ and filled r_ , whereupon stopcock c_ was closed. the air adhering to the walls of r_ and that absorbed by the mercury was carried off, and to free the mercury of all air the reservoir r_ was for a long time worked up and down. during this process some air, which would gather below stopcock c_ , was expelled from r_ by lowering it far enough and opening the stopcock, closing the latter again before raising the reservoir. when all the air had been expelled from the mercury, and no air would gather in r_ when it was lowered, the caustic potash was resorted to. the reservoir r_ was now again raised until the mercury in r_ stood above stopcock c_ . the caustic potash was fused and boiled, and the moisture partly carried off by the pump and partly re-absorbed; and this process of heating and cooling was repeated many times, and each time, upon the moisture being absorbed or carried off, the reservoir r_ was for a long time raised and lowered. in this manner all the moisture was carried off from the mercury, and both the reservoirs were in proper condition to be used. the reservoir r_ was then again raised to the top, and the pump was kept working for a long time. when the highest vacuum obtainable with the pump had been reached the potash bulb was usually wrapped with cotton which was sprinkled with ether so as to keep the potash at a very low temperature, then the reservoir r_ was lowered, and upon reservoir r_ being emptied the receiver r was quickly sealed up. when a new bulb was put on, the mercury was always raised above stopcock c_ which was closed, so as to always keep the mercury and both the reservoirs in fine condition, and the mercury was never withdrawn from r_ except when the pump had reached the highest degree of exhaustion. it is necessary to observe this rule if it is desired to use the apparatus to advantage. by means of this arrangement i was able to proceed very quickly, and when the apparatus was in perfect order it was possible to reach the phosphorescent stage in a small bulb in less than minutes, which is certainly very quick work for a small laboratory arrangement requiring all in all about pounds of mercury. with ordinary small bulbs the ratio of the capacity of the pump, receiver, and connections, and that of reservoir r was about - , and the degrees of exhaustion reached were necessarily very high, though i am unable to make a precise and reliable statement how far the exhaustion was carried. what impresses the investigator most in the course of these experiences is the behavior of gases when subjected to great rapidly alternating electrostatic stresses. but he must remain in doubt as to whether the effects observed are due wholly to the molecules, or atoms, of the gas which chemical analysis discloses to us, or whether there enters into play another medium of a gaseous nature, comprising atoms, or molecules, immersed in a fluid pervading the space. such a medium surely must exist, and i am convinced that, for instance, even if air were absent, the surface and neighborhood of a body in space would be heated by rapidly alternating the potential of the body; but no such heating of the surface or neighborhood could occur if all free atoms were removed and only a homogeneous, incompressible, and elastic fluid--such as ether is supposed to be--would remain, for then there would be no impacts, no collisions. in such a case, as far as the body itself is concerned, only frictional losses in the inside could occur. it is a striking fact that the discharge through a gas is established with ever increasing freedom as the frequency of the impulses is augmented. it behaves in this respect quite contrarily to a metallic conductor. in the latter the impedance enters prominently into play as the frequency is increased, but the gas acts much as a series of condensers would: the facility with which the discharge passes through seems to depend on the rate of change of potential. if it act so, then in a vacuum tube even of great length, and no matter how strong the current, self-induction could not assert itself to any appreciable degree. we have, then, as far as we can now see, in the gas a conductor which is capable of transmitting electric impulses of any frequency which we may be able to produce. could the frequency be brought high enough, then a queer system of electric distribution, which would be likely to interest gas companies, might be realized: metal pipes filled with gas--the metal being the insulator, the gas the conductor--supplying phosphorescent bulbs, or perhaps devices as yet uninvented. it is certainly possible to take a hollow core of copper, rarefy the gas in the same, and by passing impulses of sufficiently high frequency through a circuit around it, bring the gas inside to a high degree of incandescence; but as to the nature of the forces there would be considerable uncertainty, for it would be doubtful whether with such impulses the copper core would act as a static screen. such paradoxes and apparent impossibilities we encounter at every step in this line of work, and therein lies, to a great extent, the claim of the study. i have here a short and wide tube which is exhausted to a high degree and covered with a substantial coating of bronze, the coating allowing barely the light to shine through. a metallic clasp, with a hook for suspending the tube, is fastened around the middle portion of the latter, the clasp being in contact with the bronze coating. i now want to light the gas inside by suspending the tube on a wire connected to the coil. any one who would try the experiment for the first time, not having any previous experience, would probably take care to be quite alone when making the trial, for fear that he might become the joke of his assistants. still, the bulb lights in spite of the metal coating, and the light can be distinctly perceived through the latter. a long tube covered with aluminium bronze lights when held in one hand--the other touching the terminal of the coil--quite powerfully. it might be objected that the coatings are not sufficiently conducting; still, even if they were highly resistant, they ought to screen the gas. they certainly screen it perfectly in a condition of rest, but not by far perfectly when the charge is surging in the coating. but the loss of energy which occurs within the tube, notwithstanding the screen, is occasioned principally by the presence of the gas. were we to take a large hollow metallic sphere and fill it with a perfect incompressible fluid dielectric, there would be no loss inside of the sphere, and consequently the inside might be considered as perfectly screened, though the potential be very rapidly alternating. even were the sphere filled with oil, the loss would be incomparably smaller than when the fluid is replaced by a gas, for in the latter case the force produces displacements; that means impact and collisions in the inside. no matter what the pressure of the gas may be, it becomes an important factor in the heating of a conductor when the electric density is great and the frequency very high. that in the heating of conductors by lightning discharges air is an element of great importance, is almost as certain as an experimental fact. i may illustrate the action of the air by the following experiment: i take a short tube which is exhausted to a moderate degree and has a platinum wire running through the middle from one end to the other. i pass a steady or low frequency current through the wire, and it is heated uniformly in all parts. the heating here is due to conduction, or frictional losses, and the gas around the wire has--as far as we can see--no function to perform. but now let me pass sudden discharges, or a high frequency current, through the wire. again the wire is heated, this time principally on the ends and least in the middle portion; and if the frequency of the impulses, or the rate of change, is high enough, the wire might as well be cut in the middle as not, for practically all the heating is due to the rarefied gas. here the gas might only act as a conductor of no impedance diverting the current from the wire as the impedance of the latter is enormously increased, and merely heating the ends of the wire by reason of their resistance to the passage of the discharge. but it is not at all necessary that the gas in the tube should he conducting; it might be at an extremely low pressure, still the ends of the wire would be heated--as, however, is ascertained by experience--only the two ends would in such, case not be electrically connected through the gaseous medium. now what with these frequencies and potentials occurs in an exhausted tube occurs in the lightning discharges at ordinary pressure. we only need remember one of the facts arrived at in the course of these investigations, namely, that to impulses of very high frequency the gas at ordinary pressure behaves much in the same manner as though it were at moderately low pressure. i think that in lightning discharges frequently wires or conducting objects are volatilized merely because air is present and that, were the conductor immersed in an insulating liquid, it would be safe, for then the energy would have to spend itself somewhere else. from the behavior of gases to sudden impulses of high potential i am led to conclude that there can be no surer way of diverting a lightning discharge than by affording it a passage through a volume of gas, if such a thing can be done in a practical manner. there are two more features upon which i think it necessary to dwell in connection with these experiments--the "radiant state" and the "non-striking vacuum." any one who has studied crookes' work must have received the impression that the "radiant state" is a property of the gas inseparably connected with an extremely high degree of exhaustion. but it should be remembered that the phenomena observed in an exhausted vessel are limited to the character and capacity of the apparatus which is made use of. i think that in a bulb a molecule, or atom, does not precisely move in a straight line because it meets no obstacle, but because the velocity imparted to it is sufficient to propel it in a sensibly straight line. the mean free path is one thing, but the velocity--the energy associated with the moving body--is another, and under ordinary circumstances i believe that it is a mere question of potential or speed. a disruptive discharge coil, when the potential is pushed very far, excites phosphorescence and projects shadows, at comparatively low degrees of exhaustion. in a lightning discharge, matter moves in straight lines as ordinary pressure when the mean free path is exceedingly small, and frequently images of wires or other metallic objects have been produced by the particles thrown off in straight lines. [illustration: fig. .--bulb showing radiant lime stream at low exhaustion.] i have prepared a bulb to illustrate by an experiment the correctness of these assertions. in a globe l (fig. ) i have mounted upon a lamp filament f a piece of lime l. the lamp filament is connected with a wire which leads into the bulb, and the general construction of the latter is as indicated in fig. , before described. the bulb being suspended from a wire connected to the terminal of the coil, and the latter being set to work, the lime piece l and the projecting parts of the filament f are bombarded. the degree of exhaustion is just such that with the potential the coil is capable of giving phosphorescence of the glass is produced, but disappears as soon as the vacuum is impaired. the lime containing moisture, and moisture being given off as soon as heating occurs, the phosphorescence lasts only for a few moments. when the lime has been sufficiently heated, enough moisture has been given off to impair materially the vacuum of the bulb. as the bombardment goes on, one point of the lime piece is more heated than other points, and the result is that finally practically all the discharge passes through that point which is intensely heated, and a white stream of lime particles (fig. ) then breaks forth from that point. this stream is composed of "radiant" matter, yet the degree of exhaustion is low. but the particles move in straight lines because the velocity imparted to them is great, and this is due to three causes--to the great electric density, the high temperature of the small point, and the fact that the particles of the lime are easily torn and thrown off--far more easily than those of carbon. with frequencies such as we are able to obtain, the particles are bodily thrown off and projected to a considerable distance; but with sufficiently high frequencies no such thing would occur: in such case only a stress would spread or a vibration would be propagated through the bulb. it would be out of the question to reach any such frequency on the assumption that the atoms move with the speed of light; but i believe that such a thing is impossible; for this an enormous potential would be required. with potentials which we are able to obtain, even with a disruptive discharge coil, the speed must be quite insignificant. as to the "non-striking vacuum," the point to be noted is that it can occur only with low frequency impulses, and it is necessitated by the impossibility of carrying off enough energy with such impulses in high vacuum since the few atoms which are around the terminal upon coming in contact with the same are repelled and kept at a distance for a comparatively long period of time, and not enough work can be performed to render the effect perceptible to the eye. if the difference of potential between the terminals is raised, the dielectric breaks down. but with very high frequency impulses there is no necessity for such breaking down, since any amount of work can be performed by continually agitating the atoms in the exhausted vessel, provided the frequency is high enough. it is easy to reach--even with frequencies obtained from an alternator as here used--a stage at which the discharge does not pass between two electrodes in a narrow tube, each of these being connected to one of the terminals of the coil, but it is difficult to reach a point at which a luminous discharge would not occur around each electrode. a thought which naturally presents itself in connection with high frequency currents, is to make use of their powerful electro-dynamic inductive action to produce light effects in a sealed glass globe. the leading-in wire is one of the defects of the present incandescent lamp, and if no other improvement were made, that imperfection at least should be done away with. following this thought, i have carried on experiments in various directions, of which some were indicated in my former paper. i may here mention one or two more lines of experiment which have been followed up. many bulbs were constructed as shown in fig. and fig. . in fig. a wide tube t was sealed to a smaller w-shaped tube u, of phosphorescent glass. in the tube t was placed a coil c of aluminium wire, the ends of which were provided with small spheres t and t_ of aluminium, and reached into the u tube. the tube t was slipped into a socket containing a primary coil through which usually the discharges of leyden jars were directed, and the rarefied gas in the small u tube was excited to strong luminosity by the high-tension currents induced in the coil c. when leyden jar discharges were used to induce currents in the coil c, it was found necessary to pack the tube t tightly with insulating powder, as a discharge would occur frequently between the turns of the coil, especially when the primary was thick and the air gap, through which the jars discharged, large, and no little trouble was experienced in this way. [illustration: fig. .--electro-dynamic induction tube.] [illustration: fig. --electro-dynamic induction lamp.] in fig. is illustrated another form of the bulb constructed. in this case a tube t is sealed to a globe l. the tube contains a coil c, the ends of which pass through two small glass tubes t and t_ , which are sealed to the tube t. two refractory buttons m and m_ are mounted on lamp filaments which are fastened to the ends of the wires passing through the glass tubes t and t_ . generally in bulbs made on this plan the globe l communicated with the tube t. for this purpose the ends of the small tubes t and t_ were just a trifle heated in the burner, merely to hold the wires, but not to interfere with the communication. the tube t, with the small tubes, wires through the same, and the refractory buttons m and m_ , was first prepared, and then sealed to globe l, whereupon the coil c was slipped in and the connections made to its ends. the tube was then packed with insulating powder, jamming the latter as tight as possible up to very nearly the end, then it was closed and only a small hole left through which the remainder of the powder was introduced, and finally the end of the tube was closed. usually in bulbs constructed as shown in fig. an aluminium tube a was fastened to the upper end s of each of the tubes t and t_ , in order to protect that end against the heat. the buttons m and m_ could be brought to any degree of incandescence by passing the discharges of leyden jars around the coil c. in such bulbs with two buttons a very curious effect is produced by the formation of the shadows of each of the two buttons. another line of experiment, which has been assiduously followed, was to induce by electro-dynamic induction a current or luminous discharge in an exhausted tube or bulb. this matter has received such able treatment at the hands of prof. j.j. thomson that i could add but little to what he has made known, even had i made it the special subject of this lecture. still, since experiences in this line have gradually led me to the present views and results, a few words must be devoted here to this subject. it has occurred, no doubt, to many that as a vacuum tube is made longer the electromotive force per unit length of the tube, necessary to pass a luminous discharge through the latter, gets continually smaller; therefore, if the exhausted tube be made long enough, even with low frequencies a luminous discharge could be induced in such a tube closed upon itself. such a tube might be placed around a ball or on a ceiling, and at once a simple appliance capable of giving considerable light would be obtained. but this would be an appliance hard to manufacture and extremely unmanageable. it would not do to make the tube up of small lengths, because there would be with ordinary frequencies considerable loss in the coatings, and besides, if coatings were used, it would be better to supply the current directly to the tube by connecting the coatings to a transformer. but even if all objections of such nature were removed, still, with low frequencies the light conversion itself would be inefficient, as i have before stated. in using extremely high frequencies the length of the secondary--in other words, the size of the vessel--can be reduced as far as desired, and the efficiency of the light conversion is increased, provided that means are invented for efficiently obtaining such high frequencies. thus one is led, from theoretical and practical considerations, to the use of high frequencies, and this means high electromotive forces and small currents in the primary. when he works with condenser charges--and they are the only means up to the present known for reaching these extreme frequencies--he gets to electromotive forces of several thousands of volts per turn of the primary. he cannot multiply the electro-dynamic inductive effect by taking more turns in the primary, for he arrives at the conclusion that the best way is to work with one single turn--though he must sometimes depart from this rule--and he must get along with whatever inductive effect he can obtain with one turn. but before he has long experimented with the extreme frequencies required to set up in a small bulb an electromotive force of several thousands of volts he realizes the great importance of electrostatic effects, and these effects grow relatively to the electro-dynamic in significance as the frequency is increased. now, if anything is desirable in this case, it is to increase the frequency, and this would make it still worse for the electro-dynamic effects. on the other hand, it is easy to exalt the electrostatic action as far as one likes by taking more turns on the secondary, or combining self-induction and capacity to raise the potential. it should also be remembered that, in reducing the current to the smallest value and increasing the potential, the electric impulses of high frequency can be more easily transmitted through a conductor. these and similar thoughts determined me to devote more attention to the electrostatic phenomena, and to endeavor to produce potentials as high as possible, and alternating as fast as they could be made to alternate. i then found that i could excite vacuum tubes at considerable distance from a conductor connected to a properly constructed coil, and that i could, by converting the oscillatory current of a condenser to a higher potential, establish electrostatic alternating fields which acted through the whole extent of a room, lighting up a tube no matter where it was held in space. i thought i recognized that i had made a step in advance, and i have persevered in this line; but i wish to say that i share with all lovers of science and progress the one and only desire--to reach a result of utility to men in any direction to which thought or experiment may lead me. i think that this departure is the right one, for i cannot see, from the observation of the phenomena which manifest themselves as the frequency is increased, what there would remain to act between two circuits conveying, for instance, impulses of several hundred millions per second, except electrostatic forces. even with such trifling frequencies the energy would be practically all potential, and my conviction has grown strong that, to whatever kind of motion light may be due, it is produced by tremendous electrostatic stresses vibrating with extreme rapidity. of all these phenomena observed with currents, or electric impulses, of high frequency, the most fascinating for an audience are certainly those which are noted in an electrostatic field acting through considerable distance, and the best an unskilled lecturer can do is to begin and finish with the exhibition of these singular effects. i take a tube in the hand and move it about, and it is lighted wherever i may hold it; throughout space the invisible forces act. but i may take another tube and it might not light, the vacuum being very high. i excite it by means of a disruptive discharge coil, and now it will light in the electrostatic field. i may put it away for a few weeks or months, still it retains the faculty of being excited. what change have i produced in the tube in the act of exciting it? if a motion imparted to the atoms, it is difficult to perceive how it can persist so long without being arrested by frictional losses; and if a strain exerted in the dielectric, such as a simple electrification would produce, it is easy to see how it may persist indefinitely, but very difficult to understand why such a condition should aid the excitation when we have to deal with potentials which are rapidly alternating. since i have exhibited these phenomena for the first time, i have obtained some other interesting effects. for instance, i have produced the incandescence of a button, filament, or wire enclosed in a tube. to get to this result it was necessary to economize the energy which is obtained from the field and direct most of it on the small body to be rendered incandescent. at the beginning the task appeared difficult, but the experiences gathered permitted me to reach the result easily. in fig. and fig. two such tubes are illustrated which are prepared for the occasion. in fig. a short tube t_ , sealed to another long tube t, is provided with a stem s, with a platinum wire sealed in the latter. a very thin lamp filament l is fastened to this wire, and connection to the outside is made through a thin copper wire w. the tube is provided with outside and inside coatings, c and c_ respectively, and is filled as far as the coatings reach with conducting, and the space above with insulating powder. these coatings are merely used to enable me to perform two experiments with the tube--namely, to produce the effect desired either by direct connection of the body of the experimenter or of another body to the wire w, or by acting inductively through the glass. the stem s is provided with an aluminium tube a, for purposes before explained, and only a small part of the filament reaches out of this tube. by holding the tube t_ anywhere in the electrostatic field the filament is rendered incandescent. [illustration: fig. .--tube with filament rendered incandescent in an electrostatic field.] [illustration: fig. .--crookes' experiment in electrostatic field.] a more interesting piece of apparatus is illustrated in fig. . the construction is the same as before, only instead of the lamp filament a small platinum wire p, sealed in a stem s, and bent above it in a circle, is connected to the copper wire w, which is joined to an inside coating c. a small stem s_ is provided with a needle, on the point of which is arranged to rotate very freely a very light fan of mica v. to prevent the fan from falling out, a thin stem of glass g is bent properly and fastened to the aluminium tube. when the glass tube is held anywhere in the electrostatic field the platinum wire becomes incandescent, and the mica vanes are rotated very fast. intense phosphorescence may be excited in a bulb by merely connecting it to a plate within the field, and the plate need not be any larger than an ordinary lamp shade. the phosphorescence excited with these currents is incomparably more powerful than with ordinary apparatus. a small phosphorescent bulb, when attached to a wire connected to a coil, emits sufficient light to allow reading ordinary print at a distance of five to six paces. it was of interest to see how some of the phosphorescent bulbs of professor crookes would behave with these currents, and he has had the kindness to lend me a few for the occasion. the effects produced are magnificent, especially by the sulphide of calcium and sulphide of zinc. from the disruptive discharge coil they glow intensely merely by holding them in the hand and connecting the body to the terminal of the coil. to whatever results investigations of this kind may lead, their chief interest lies for the present in the possibilities they offer for the production of an efficient illuminating device. in no branch of electric industry is an advance more desired than in the manufacture of light. every thinker, when considering the barbarous methods employed, the deplorable losses incurred in our best systems of light production, must have asked himself, what is likely to be the light of the future? is it to be an incandescent solid, as in the present lamp, or an incandescent gas, or a phosphorescent body, or something like a burner, but incomparably more efficient? there is little chance to perfect a gas burner; not, perhaps, because human ingenuity has been bent upon that problem for centuries without a radical departure having been made--though this argument is not devoid of force-but because in a burner the higher vibrations can never be reached except by passing through all the low ones. for how is a flame produced unless by a fall of lifted weights? such process cannot be maintained without renewal, and renewal is repeated passing from low to high vibrations. one way only seems to be open to improve a burner, and that is by trying to reach higher degrees of incandescence. higher incandescence is equivalent to a quicker vibration; that means more light from the same material, and that, again, means more economy. in this direction some improvements have been made, but the progress is hampered by many limitations. discarding, then, the burner, there remain the three ways first mentioned, which are essentially electrical. suppose the light of the immediate future to be a solid rendered incandescent by electricity. would it not seem that it is better to employ a small button than a frail filament? from many considerations it certainly must be concluded that a button is capable of a higher economy, assuming, of course, the difficulties connected with the operation of such a lamp to be effectively overcome. but to light such a lamp we require a high potential; and to get this economically we must use high frequencies. such considerations apply even more to the production of light by the incandescence of a gas, or by phosphorescence. in all cases we require high frequencies and high potentials. these thoughts occurred to me a long time ago. incidentally we gain, by the use of very high frequencies, many advantages, such as a higher economy in the light production, the possibility of working with one lead, the possibility of doing away with the leading-in wire, etc. the question is, how far can we go with frequencies? ordinary conductors rapidly lose the facility of transmitting electric impulses when the frequency is greatly increased. assume the means for the production of impulses of very great frequency brought to the utmost perfection, every one will naturally ask how to transmit them when the necessity arises. in transmitting such impulses through conductors we must remember that we have to deal with _pressure_ and _flow_, in the ordinary interpretation of these terms. let the pressure increase to an enormous value, and let the flow correspondingly diminish, then such impulses--variations merely of pressure, as it were--can no doubt be transmitted through a wire even if their frequency be many hundreds of millions per second. it would, of course, be out of question to transmit such impulses through a wire immersed in a gaseous medium, even if the wire were provided with a thick and excellent insulation for most of the energy would be lost in molecular bombardment and consequent heating. the end of the wire connected to the source would be heated, and the remote end would receive but a trifling part of the energy supplied. the prime necessity, then, if such electric impulses are to be used, is to find means to reduce as much as possible the dissipation. the first thought is, employ the thinnest possible wire surrounded by the thickest practicable insulation. the next thought is to employ electrostatic screens. the insulation of the wire may be covered with a thin conducting coating and the latter connected to the ground. but this would not do, as then all the energy would pass through the conducting coating to the ground and nothing would get to the end of the wire. if a ground connection is made it can only be made through a conductor offering an enormous impedance, or though a condenser of extremely small capacity. this, however, does not do away with other difficulties. if the wave length of the impulses is much smaller than the length of the wire, then corresponding short waves will be sent up in the conducting coating, and it will be more or less the same as though the coating were directly connected to earth. it is therefore necessary to cut up the coating in sections much shorter than the wave length. such an arrangement does not still afford a perfect screen, but it is ten thousand times better than none. i think it preferable to cut up the conducting coating in small sections, even if the current waves be much longer than the coating. if a wire were provided with a perfect electrostatic screen, it would be the same as though all objects were removed from it at infinite distance. the capacity would then be reduced to the capacity of the wire itself, which would be very small. it would then be possible to send over the wire current vibrations of very high frequencies at enormous distance without affecting greatly the character of the vibrations. a perfect screen is of course out of the question, but i believe that with a screen such as i have just described telephony could be rendered practicable across the atlantic. according to my ideas, the gutta-percha covered wire should be provided with a third conducting coating subdivided in sections. on the top of this should be again placed a layer of gutta-percha and other insulation, and on the top of the whole the armor. but such cables will not be constructed, for ere long intelligence--transmitted without wires--will throb through the earth like a pulse through a living organism. the wonder is that, with the present state of knowledge and the experiences gained, no attempt is being made to disturb the electrostatic or magnetic condition of the earth, and transmit, if nothing else, intelligence. it has been my chief aim in presenting these results to point out phenomena or features of novelty, and to advance ideas which i am hopeful will serve as starting points of new departures. it has been my chief desire this evening to entertain you with some novel experiments. your applause, so frequently and generously accorded, has told me that i have succeeded. in conclusion, let me thank you most heartily for your kindness and attention, and assure you that the honor i have had in addressing such a distinguished audience, the pleasure i have had in presenting these results to a gathering of so many able men--and among them also some of those in whose work for many years past i have found enlightenment and constant pleasure--i shall never forget. [transcriber's note: corrected the following typesetting errors: ) 'preceived' to 'perceived', page . ) 'disharging' to 'discharging', page . ) 'park' to 'spark', page . ) 'pssition' to 'position', page . ) 'to th opposite side' to 'to the opposite side', page . ) 's resses' to 'stresses', page .] none none http://www.archive.org/details/autobiographyofe gibsrich the autobiography of an electron [illustration: a well-known phenomenon produced by electrons _photo_ _the fleet agency_ a sudden discharge of electrons from cloud to cloud, or from cloud to the earth, constitutes what we call "lightning."] the autobiography of an electron. wherein the scientific ideas of the present time are explained in an interesting and novel fashion by charles r. gibson, f.r.s.e. author of "scientific ideas of to-day," "electricity of to-day" "the romance of modern electricity," _&c. &c._ illustrated philadelphia j. b. lippincott company london: seeley & co. limited preface although text-books of science may appear to the general reader to be "very dry" material, there is no doubt that, when scientific facts and theories are put into everyday language, the general reader is genuinely interested. the reception accorded to the present author's _scientific ideas of to-day_ bears out this fact. while that volume explains, in non-technical language, the latest scientific theories, it aims at giving a fairly full account, which, of course, necessitates going into a great deal of detail. that the book has been appreciated by very varied classes of readers is evident from the large numbers of appreciative letters received from different quarters. but the author believes that if the story of modern science were told in a still more popular style, it would serve a further useful purpose. for there are readers who do not care to go into details, and yet would like to take an intelligent interest in the scientific progress of the present day. some of those readers do not wish to trouble about names and dates, while the mere mention of rates of vibration and such-like is a worry to them. they wish a book which they may read with the same ease as an interesting novel. hence the form of the present volume. * * * * * the author is indebted to professor james muir, m.a., d.sc., of the glasgow and west of scotland technical college, and to h. stanley allen, m.a., d.sc., senior lecturer in physics at king's college, university of london, for very kindly reading the proof-sheets. the author is indebted further to professor muir in connection with some of the illustrations, and for others to dixon and corbitt and r. s. newall, ltd., glasgow; siemens schuckert werke, berlin. contents page chapter i what the story is about the scribe introduces the electron to the reader. he has something to say also about the mysterious æther which pervades all space. he emphasises the fact that the electron is a real existing thing chapter ii the electron's preface the electron explains the reason why it has written its autobiography chapter iii the new arrival the electron points out who the new arrival is really. it relates an amusing experience. it tells how man disturbed electrons before he discovered their existence. an ancient experiment, and what the wise men of the east thought about it. how electrons are responsible for the electrification of any object. handled by a new experimenter, they surprise man. man becomes of special interest to the electrons chapter iv some good sport the electron explains how man succeeded in crowding them together, with some rather exciting results from the overcrowding. one historical incident. man's fear of the consequences. how a party of electrons wrecked a church steeple. an unfortunate accident chapter v my earliest recollections the electron's story begins at a very far distant period, before this world had taken shape. the electron was present when the atoms of matter were being formed. the birth of the moon. something still to be discovered. the moulding of the planet. boiling oceans. the electrons took an active part in making sea-water salt. the electron explains why it has been chosen to write the story of itself and its fellows chapter vi man pays us some attention the electrons are encouraged by one of the experiments made by man. they hope it may lead to their discovery, so that their services may be recognised. the electron's experience in a vacuum tube. a disappointment and a revival of hope. a great declaration by one individual man. the electron misjudges man. mention of a great discovery. the christening of the electrons chapter vii a steady march the electron explains how they produce the electric current. how man discovered means of making the electrons march. a simple explanation of how a complete electric circuit is always necessary. how an "earth circuit" works. how the marching electrons can do work chapter viii a useful dance a perpetual dance. a responsible position. how the safety of the mariner depends upon the electrons' dance. how electrons produce a magnet. a convenient kind of magnet, which gains and loses its attractive power when desired. how a permanent magnet is made. the great service of electrons in modern life chapter ix how we carry man's news the method of sending the news. the electron's personal experience. a series of forced marches. how man controls the electrons. how the electrons reproduce the signals chapter x how we communicate with distant ships an entirely different means of communication. a surprise to man, but not to the electrons. how the electrons produce waves in the surrounding æther. how these waves disturb distant electrons. the electron's personal experience. its description of its actions in a wireless telegraph station chapter xi how we reproduce speech why it is not correct to speak of the electrons as carriers of speech. the action of electrons in the working of telephones. the electron's own experience in wireless telephony chapter xii our heaviest duties a roving commission. how electrons can move gigantic cars and trains. the action of electrons in dynamos and motors. how the electrons transmit the energy. what makes the motor go chapter xiii a boon to man a simple explanation of how the electrons produce light. how the electron provides a connecting link between matter and the æther. how light reaches the earth from the sun. how the electrons produce that beautiful luminous effect which man calls an "aurora." how the earth has become a negatively charged body. how electrons produce radiant heat. the difference between light and heat chapter xiv how we produce colour what colour is really. how the different colour sensations are stimulated by the electrons. the electron as a faithful satellite to the atom. how electrons can produce the different æther waves. how the electrons respond to the different waves. the production of artificial light. co-operation of the electrons. man's ridiculously wasteful processes. the electrons' secret chapter xv we send messages from the stars the kind of messages referred to. how the electrons have informed man of what the stars are made. how man reads the electrons' wireless messages. how it is other electrons that enable man to read the messages. the real explanation of reflection of light. how light is absorbed by some objects. how some substances are transparent. why objects appear coloured. what makes the lines in the spectra of stars. the spectroscope chapter xvi how man proved our existence how man reasoned out a plan for detecting the electron. how the electrons altered some lines in the spectrum. the curious manner in which the electron informed man that certain stars are approaching this planet, while others are receding from it chapter xvii my x-ray experience x-rays are an old story to some electrons. the electron's personal experience. a very sudden stop. how electrons made a fluorescent screen send out light. the electrons assist the surgeon. a curious find. detecting imitation diamonds. the electron and the mummy chapter xviii our relationship to the atoms how the atoms of matter attract one another. what constitutes the temperature of a body. what the atoms are made of. an important thing still to discover about the atom. the elements. how the electrons produce compound substances. the real explanation of chemical changes chapter xix how we made the world talk it was nothing new on the part of the electrons. exaggerated rumours. the electrons and radium. fast-flying electrons. atomic explosions chapter xx conclusion the electron is made to sum up a few of the wonders which it has related, in order to emphasise the great services which electrons render to man appendix index list of illustrations page a well-known phenomenon produced by electrons _frontispiece_ damage done by a party of electrons a tobacco tin defying gravitation a motor-car with wireless telegraph a train impelled by moving electrons protection against a discharge of electrons the spectroscope and the electrons' wireless messages how electrons produce x-ray images chapter i what the story is about the reason for writing this story is given in the preface, but the title is so strange that the reader will wish naturally to know what the story is about. what is an electron? is it an imaginary thing, or is it a reality? one of the reasons for writing this story in its present form is to help the reader to realise that electrons are not mythical, but real existing things, and by far the most interesting things we know anything about. the discovery of electrons has shed a new light upon the meaning of very many things which have been puzzles until now. they give us a reasonable explanation of the cause of light and colour. they provide a new idea of the constitution of matter. they enable us to picture an electric current, and they give us definite, though by no means final, answers to the why and wherefore of magnetism, chemical union, and radio-activity. the story is imaginary only in so far that one of the electrons itself is supposed to tell the tale. but in the endeavour to make the story interesting, there has been no sacrifice of accuracy in the statements of fact. while all names and dates, and many other details, have been kept out rigidly from the story, a note of the more important of these has been added in an appendix for the sake of those readers who may wish to refer to them. it will be well to introduce the electron to the reader before leaving it to speak for itself. we have definite experimental proof of the existence of electrons, and yet it is very difficult to realise their existence, for two reasons. in the first place, they are so infinitesimally small. we count a microbe a small thing; we can see it only with the aid of a very powerful microscope. yet that little speck of matter contains myriads of particles or _atoms_. an atom of matter is therefore an inconceivably little thing, but even that is a great giant compared to an electron. our second difficulty in realising the existence of an electron is that it is not any form of what we call _matter_; it is a particle of _electricity_, whatever that may be. from the earliest experiments it became evident that there were two distinct kinds of electricity. these were described by the pioneer workers as _positive_ and _negative_ electricities. to-day we have definite experimental proof that negative electricity is composed of separate particles or units. just as matter is composed of invisible atoms, so also is negative electricity of an atomic nature. these particles of negative electricity have been christened electrons, _electron_ being the greek word for _amber_, from which man first obtained electricity. of course no one can ever hope to see an electron, but physicists have been able to determine its size and _mass_, its electric charge, and the speeds at which it moves. while it has been known for more than a century that _light_ is merely waves in the all-pervading æther of space, set up by incandescent bodies, it has been a puzzle always how matter could cause waves in the æther, as it offers no resistance to the movement of matter through it. here we are on the back of a great planet, flying through space at the enormous rate of one thousand miles per minute, and yet our flimsy atmospheric blanket is in no way disturbed by the æther through which we are flying. in the following story we shall see that these electrons help us towards a solution of this and many other problems; they provide the missing link between matter and the æther. but what is this _æther_ of which one hears so much in these days? the truth is we know nothing of its nature. we cannot say whether it is lighter than the lightest gas or denser than the densest solid. the æther, whatever it may be, is as real as the air we breathe. it is the medium which brings us light and heat from the sun, and which carries our wireless telegraph and telephone messages. the whole universe is moving in this great æther ocean. in order to make the electron's story perfectly intelligible to every reader, i have added a short explanatory note at the beginning of each chapter. these notes merely state the facts about which the electron is speaking. to make the electron's story as realistic as possible, it has been necessary to give the imaginary electron perfect freedom of knowledge concerning itself and its surroundings. in our schooldays we had to write the autobiographies of steel pens, and such-like, but these inanimate things had to be endowed with powers of thought, feeling, and desire. it is very important, however, to remember that an electron is a particle of negative electricity--_a real existing thing_. chapter ii the electron's preface while many scientific men now understand our place in the universe, we electrons are anxious that every person should know the very important part which we play in the workaday world. it was for this reason that my fellow-electrons urged me to write my own biography. my difficulty has been to find a scribe who would put down my story in the way i desired. the first man with whom i opened negotiations wished me to give him dates and names of which i knew nothing. and he asked such stupid questions about where i was born and who my parents were, as if i were flesh and blood. i am pleased to say that my relationship with the scribe who has put down my story in the following pages has been of the most friendly description. apart from a little tiff which we had at the outset, there has been no difference of opinion. he complained that i related things in too abstract a form. however, we got over the difficulty by a compromise; i have allowed him to place what he calls "the scribe's note" at the beginning of each chapter, but it will be understood clearly that these are merely convenient embellishments, and that i am responsible for the story of my own experiences. chapter iii the new arrival _the scribe's note on chapter three_ it will be well to keep clearly in mind that an electron is a real particle of negative electricity. electrons have been discovered only within recent years. no matter from what substances we take them, they are always identical in every respect. some electrons are attached to the atoms of matter in such a way that they may be removed easily from one object to another. when a surplus of these detachable electrons is crowded on to any object, we say that it is charged with negative electricity. we speak of the other object, which has lost these same electrons, as being charged with positive electricity. in this chapter the electron refers to the old-world experiment in which a piece of amber when rubbed attracts any light object to it. for many ages man believed this to be a special property belonging to amber alone. one of queen elizabeth's physicians discovered that this property was common to all substances. chapter iii the new arrival it is most amusing to me and my fellow-electrons to hear intelligent people speak of us as though we were new arrivals on this planet. dear me! we were here for countless ages before man put in an appearance. i wonder if any man can realise that we have been on the move ever since the foundations of this world were laid. it is man himself who is the new arrival. it does seem strange to us that men should be so distinctly different from one another. we electrons are at a decided disadvantage, for we are all identical in every respect. i have no individual name--it would serve no purpose. even if you could see me, you could not distinguish me from any other electron. i wonder sometimes if men appreciate the great advantage they have in possessing individual names. i was impressed with this thought one fine summer morning. while i was riding on the back of a particle of gas in the atmosphere, i was carried through the open window of a nursery just as the under-nurse was putting the room in order. a little later there was some commotion in the nursery, for the young mother and her mother had come to see the twin daughters being bathed by the nurses. the grandmother happened to remark how very much alike the two little infants were. she said laughingly to the head nurse that she must be careful not to get the children mixed. but the big brother, aged five years, remarked that it would not matter really how much they were mixed until they got their names. sometimes i wish we electrons did differ from one another, so that we might each possess an individual name, but no doubt it is necessary for us all to be exactly alike. long before man had discovered us, he caused us deliberately to do certain things. he was mystified by the results of his experiments, for he was not aware of our presence. a few of my fellow-electrons have rather hazy recollections of being disturbed while clinging to a piece of amber. they had been disturbed often before in a similar way, by being rubbed against a piece of woollen cloth, and the result had been always that a number of electrons let go their hold upon the cloth and crowded on to the amber. the overcrowding was uncomfortable, but it happened usually that the surplus electrons found some means of escape to the earth, where there is no need of excessive crowding. on the occasion to which i refer, it so happened that the rubbing had been unusually vigorous and prolonged, so that the electrons were crowded on to the amber in great numbers. in their endeavour to escape they produced a strain or stress in the surrounding æther, and this caused a small piece of straw, which was lying within the disturbed area, to be forced towards the amber. what attracted the attention of the electrons was that the man who was holding the piece of amber removed the clinging straw and replaced it exactly where it had been lying. in the meantime he had been handling the amber, and many of the crowded electrons had managed to make a bolt for the earth by way of the man's body. they did this so very quietly that the man did not feel any sensation. however, as soon as the amber was rubbed again, a similar crowd provided the same attractive property. we electrons became impatient to hear what man would say of our work, for it was apparent that he had noticed the movements of the straw. you will hardly believe me when i tell you to what decision these wise men of the east came. they declared that, in rubbing the amber, it had received heat and life. as if life could be originated in any such simple manner! you can picture our disappointment when we found that man was going to ignore our presence. occasionally we were given opportunities of displaying our abilities in drawing light objects towards pieces of rubbed amber. but the funny thing was that man got hold of the stupid idea that this attractive property belonged to the amber instead of to us. if he had only tried pieces of sulphur, resin, or glass, he would have found that these substances would have acted just as well. you see it was not really the substance, but we electrons who were the active agents. we had given up all hope of being discovered, when news came along that a learned man was on the hunt for us. he was crowding us on to all sorts of substances. he rubbed a piece of glass with some silk, and at first he was surprised greatly to see light objects jump towards the excited glass. of course, we were not surprised in the very least. the only thing that amused us was to find that he was making out a list of the different substances which showed attractive properties when rubbed. he could not, evidently, get away from the idea that it was the substances themselves that became attractive. we were sorry that the poor experimenter wasted so much time and energy in trying to crowd us on to a piece of metal rod. he rubbed and he rubbed that metal, but it would attract nothing, and i shall tell you the reason. you know that we electrons hate overcrowding; indeed we always separate from one another as far as possible when there is no force pulling us together. we only crowded on to the amber because we could not help ourselves; we had no way of escape, for amber is a substance we cannot pass through. but we have no difficulty whatever in making our way along a piece of metal, and as soon as the rubbing began, some electrons moved off the metal by way of the man's arm and body to make room for those being crowded on to the metal from the rubber. and so there never was any overcrowding, and consequently no straining of the æther. but it was not long before we found that man had succeeded in cutting off our way of escape. he had attached a glass handle to the metal rod, and we were compelled to overcrowd upon the metal as we could not pass through the glass handle. neighbouring light objects were attracted by the excited or "electrified" metal. even this demonstration did not put man upon our track. perhaps i should explain in passing, that when a glass rod is rubbed with a silk handkerchief we crowd on to the silk, and not on to the glass. this leaves the glass rod short of electrons, and the æther is strained so that light objects are attracted. man did notice that there was some difference between a piece of amber and a piece of glass when these were excited. what the difference was he could not imagine, but to distinguish the two different conditions he said that the amber was charged with _negative_ electricity and the glass with _positive_ electricity. from that time forward man became of special interest to us. we felt sure that sooner or later he was bound to recognise that we were at work behind the scenes. it seemed to us, however, that man was desperately slow in turning his attention towards us, and we tried to waken him up in a rather alarming fashion, as i shall relate in the succeeding chapter. chapter iv some good sport _the scribe's note on chapter four_ men began to make glass plate machines for producing electrification on a larger scale. the electric spark is produced. the electron tells the story of the first attempt to store electricity in a glass jar. this is what we do now by means of a leyden jar. a sudden expulsion of electrons from one object to another is called a discharge of electricity. lightning is a discharge of electrons from a cloud to the earth or from cloud to cloud. in repeating franklin's experiment of drawing electricity from thunder-clouds, a russian professor received a fatal shock. chapter iv some good sport now i must tell you of a surprise in which i took an active part. some man thought he would separate a great crowd of us from our friends. of course, he did not think really of _us_, but whatever he may have supposed he was doing, he succeeded in accumulating greater crowds of us together than he had done previously. he managed this by making simple machines to do the rubbing for him on a larger scale. the result was really too much for us; we were kept crowding on to a sort of brass comb arrangement from which we could not escape, as the metal was attached to a glass support. talk about overcrowding! i had never experienced the like before, and i felt sure some catastrophe would happen. suddenly there was a stampede, during which a great crowd of electrons forced their way across to a neighbouring object and thence to the earth. i can assure you it was no joke getting through the air. we all tried to leap together, but some of the crowd were forced back upon us; then bang forward we went again, back once more, and so on till we settled down to our normal condition. of course all this surging to and fro occupied far less time than it takes to tell. indeed, i could not tell you what a very small fraction of a second it took. i wish you had seen the experimenter's surprise as we made this jump. we caused such a bombardment in the air that there was a bright spark accompanied by a regular explosion. some men ran away with the idea that electricity was a mysterious fire, which only showed itself when it mixed with the atmosphere. nothing delighted us more, after our own surprise was over, than to have a chance of repeating these explosions, to the alarm of the experimenters. but the best sport of all was to come, and when i heard of it i was so disappointed that i had not been one of the sporting party. it came about in the following way. [illustration: damage done by a party of electrons _by permission of dixon and corbitt and r. s. newall, ltd._ _glasgow_ when a myriad of electrons is discharged suddenly from a cloud to the earth, it happens sometimes that considerable damage is done. the above photograph is of a church steeple damaged by lightning in . no lightning-conductor was provided, so the electrons had to get to earth by way of the steeple itself, with the disastrous result as shown.] one learned man thought he had hit upon a good idea. he tried to crowd a tremendous number of us into some water contained in a glass jar. without condescending to think of us, he crowded an enormous number of electrons from one of his rubbing machines along a piece of chain which led them into water. the overcrowding was appalling, for it was impossible to escape through the glass vessel. things had reached a terrible state, when the experimenter stopped the machine and put forward his hand to lift the chain out of the water. now was the chance of escape, so the whole excited crowd made one wild rush to earth by way of the experimenter's body. the rapid surging to and fro of the crowd racked the man's muscles. i wish i had been there to see him jump; they say it was something grand. you can imagine how the little sinners enjoyed the joke; they knew they were safe, as man had no idea of their existence at that time. another man was foolhardy enough to try a similar experiment, and they say that his alarm was even greater; indeed, he swore he would not take another shock even for the crown of france. we were all eager to get opportunities of alarming man, not that we wished him any harm, but we thought he might pay us a little more attention. i remember one occasion upon which some of us were boasting of what we had done in the way of alarming men, whereupon one fellow-electron rather belittled our doings. he maintained that he had jumped all the way from a cloud to the earth, along with a crowd of other electrons. in doing so they had scared the inhabitants of a whole village, for they alighted upon the steeple of a church, and in their wild rush they played such havoc among the atoms composing the steeple that they did considerable outward damage to the great structure. i may as well confess that we are not free agents in performing these gigantic jumps; we are compelled to go with the crowd when things are in such a state of stress. we simply cannot hold on to the atoms of matter upon which we happen to be located. it is only under very considerable pressure that we can perform this class of jump, and i beg to assure you that we are perfectly helpless in those cases where we have been dashed upon some poor creature with a message of death. alas! on one occasion i was one of a party who killed a very learned man. it was most distasteful to us; we could not possibly prevent it. he had erected a long rod which extended up into the air, and terminated at the lower end in his laboratory. some of us who were in the upper atmosphere were forced on to this iron rod, and from past experience we quite expected that we should be subjected to a sudden expulsion to earth. indeed we were waiting for the experimenter to provide us with a means of escape, when suddenly he brought his head too near to the end of the rod, and in a moment we were dashed to earth through his body. we learned with deep regret that the poor man had been robbed of his life. to turn to something of a happier nature, i shall proceed to tell you of some of my earliest recollections. remember i shall be speaking of a time long before man existed--even before this great planet was a solid ball. chapter v my earliest recollections _the scribe's note on chapter five_ this great globe upon which we live was once a glowing mass of flaming gas. it is possible that the whole solar system was once one great mass. in any case, we have no doubt that the moon is simply the result of a part of our glowing mass having become detached. in the hottest stars we find only the lightest atoms of matter, such as hydrogen gas, the atoms of heavier substances being found in stars which have begun to cool down. the electrons have been present from the very beginning, and it is they who go to make up the atoms of matter. we picture an atom of matter as a miniature solar system of revolving electrons. there is doubtless a corresponding amount of positive electricity, but so far we have no evidence of its nature. chapter v my earliest recollections before giving an account of the everyday duties which we perform, it may interest you to hear something of our early history. not only have we been on the move ever since the beginning of this world, but some of us have clear recollections of this planet long before it was a solid body. the whole world was a great ball of flaming gas. i have heard some fellow-electrons say that we were attached to a greater mass of incandescent gas before the beginning of this world, but i have no personal recollections of it. but one thing i do remember is a great upheaval which caused a large mass of gas to become detached from our habitation. without any warning a great myriad of our fellow-electrons were carried away on this smaller mass. at first this detached mass circled around our greater mass at very close quarters, but we soon found that our friends were being carried farther and farther away, until they are now circling around this solid planet at a comparatively great distance. man calls this detached mass _the moon_, and when i have heard children say in fun that they wish they could visit the man in the moon, i have longed to go and see how it fares with those fellow-electrons who seem to be separated from us in such a permanent manner. after this exciting event, which i have heard described as "the birth of the moon," our great ball of flaming gas began to cool gradually. but you will be interested in what happened before the moon's birth. i saw a crowd of electrons suddenly congregate together along with _something_ else which man has not discovered. never mind the other part, but picture a number of electrons forming a little world of their own. there they went whirling around in a giddy dance. i saw these little worlds or "atoms" being formed all around, and i feel truly thankful now that i was not caught in the mad whirl, for these fellow-electrons have been kept hard at it ever since, imprisoned within a single atom. i have met a very few electrons who have escaped from within an atom, but i shall tell you about them later on. the first thing i noticed was that each of the atoms had practically the same number of electrons in it. at that time i thought only in an abstract way, but since then i have learned that these were _hydrogen_ atoms; hydrogen being the lightest substance known to man. exactly what happened next i cannot recollect, but my attention was attracted later to larger congregations of electrons forming other little worlds of their own. these atoms were, of course, heavier than the hydrogen atoms. i saw quite a variety of different systems, of which i thought then in an abstract fashion, but which i know now to be atoms of _oxygen_, _nitrogen_, _carbon_, _iron_, _copper_, and so on. while man has given the atoms these distinguishing names, you will understand that the incidents which i am relating took place long before there was any appearance of solidity about our planet; these substances were all in a gaseous state. after this, i recollect that there was a great envelope of water-vapour condensed around the planet. some condensed into liquid water upon the surface of the globe, while part was suspended in the form of clouds. some of my fellow-electrons acted as _nuclei_ or foundations for the formation of the cloud particles. the water which condensed upon the earth settled down in the hollows, which had been produced previously by the immense pressure of the water-vapour envelope. we can hardly believe it is the same world. you cannot imagine how strange it was to see the great oceans boiling and steaming; of course, they were fresh water then. i need hardly tell you that they have become salt only because the rivers have brought down sodium into them, and when these sodium atoms unite with chlorine atoms they form particles of common salt. i know all about this because we electrons play a very important part in all such combinations. one very memorable recollection is that of life originating in the oceans. i wish i could let you into the secret of _the origin of life_, but, according to the creator's plan, man must find out for himself. your guesses are all wide of the mark. by the way, perhaps i should explain why i have been selected to write this biography. the first reason is that i am a free or detachable electron, and the second point in my favour is that i have had exceptional opportunities of seeing about me. i have heard men say that lookers-on see most of the game, and as i have witnessed the gradual evolution of things, you will understand that i have views of my own. a casual observer might think that things had deteriorated, for long ago there were immense monsters upon this planet, and these would put all modern creatures in the shade as far as size and strength are concerned. but one of the most interesting things to me has been to watch the evolution of man, and more especially the gradual development of his brain. indeed, sometimes i have wished that i had happened to be an electron in the brain of a man; but, on the other hand, my career would not have been of the varied kind which it has been. chapter vi man pays us some attention _the scribe's note on chapter six_ men found that by exhausting the air from glass globes or tubes it was possible to pass electric discharges through them, and in so doing some very beautiful luminous effects were produced within the vacuum tubes. it was when experimenting with one of these tubes that a scientist suggested that radiant particles were being shot across the tube. these particles were really electrons, but it was thought at that time that they were atoms of matter. another scientist declared, from certain mathematical calculations, that there existed extremely small particles of something around the atoms of matter, and that it was the motion of these in the æther which produced _light_. people were not willing to accept this theory. some time later another scientist was able to prove by experiment that these particles did exist. this was done by means of the spectroscope, as will be related by the electron in a later chapter. chapter vi man pays us some attention from the little i have told you already of our experiences, you will see that men had been making many experiments in which we electrons took a very active part. it was disappointing that even although we had surprised man in so many different ways, he had never become suspicious of our presence. one day, however, we did begin to hope for recognition. i was present, with a great crowd of electrons, imprisoned within a glass globe from which the air had been extracted. we were very pleased to find that the surrounding space had been cleared of air, for it was apparent that the experimenter was going to make us jump across from one end of the glass tube to the other. a crowd of us had collected on the extremity of a wire, or "electrode," at the one end of the tube, while another similar crowd was present on the other electrode at the opposite end of the tube. while i speak of a crowd, meaning that there were millions of us, i do not suggest that we were overcrowded, for we had plenty of elbow-room to move about on the atoms to which we were attached. all in a moment the scene was changed. we felt a crowd of electrons pressing us forward and forcing us right up to the very end of the electrode. we found that the crowd was approaching by a wire leading into the tube. soon the crowding had reached such a condition that we became alarmed; we could see no way of escape. we were imprisoned by the glass walls, but we soon discovered that many of the electrons who had been stationed on the other electrode had deserted their posts and fled along a wire leading out of the tube. if we could only follow them. it would be a tremendous jump to get over to the other wire, but the way was fairly clear of air. when the overcrowding reached a certain point we were literally shot across from the one electrode to the other. this was the first time i had ever experienced anything of the kind, but many fellow-electrons had gone through similar performances for years at the hands of other experimenters. however, it was somewhat alarming to be fired off like a rocket across the tube. what happened after that i cannot recollect, but some time later i was present in that or a similar tube when i heard the experimenter say to a friend that he believed there were particles flying across his tube. we sent news all along the line stating that at last we had been discovered, and i can assure you that we felt proud. but our joy was not long-lived, for it turned out that we were considered to be particles or atoms of matter; the experimenter spoke of us as "radiant matter." this was a real disappointment. it took us some time to recover from our disappointment at being mistaken for clumsy atoms of matter. we are of a higher order of things altogether. no atom of matter can travel at speeds such as we can. we cross these vacuum tubes with speeds equal to millions of miles per minute. a great many of us were kept busy within vacuum tubes by other experimenters, but nothing very exciting happened. indeed, we had lost all hope of attracting man's attention to ourselves as long as we were imprisoned within these tubes. in the meantime our hopes were revived by news which reached us from another quarter. we heard that a very learned man had declared boldly that there did exist little particles which revolved around the atoms of matter, and that it was the motion of these tiny particles in the æther which produced the well-known waves of _light_. there was considerable rejoicing among us, for we were anxious to have our services recognised by man. this great man was not guessing merely; he was willing to prove by mathematical calculations that we did exist in reality. of course, we ourselves required no proof of our existence, but we believed that man would be convinced. our high hopes were soon laid low; news reached us that people were shaking their heads and saying that figures could be made to prove anything. after we had settled down to our ordinary duties, we got word that at last man had really detected us in a flame of gas. this seemed quite reasonable, for, as i shall relate to you in another chapter, we have a very lively time of it in a flame of gas. however, when we were informed that man had discovered us by means of a sort of telescope arrangement, i, for one, began to doubt the truth of the discovery. some time before this i had heard that men were spying at gas flames in the hope of finding us, and this seemed most ridiculous, for if man could not see the large congregations of us called _atoms_, how could he expect to see individual electrons? my ignorance was dispelled when it was explained that man had not been looking for us directly, but for the æther waves which we produce. but i have not had an opportunity of explaining to you how some of us produce waves in the æther; i shall have to wait till a later chapter. in the meantime i may say that since this important discovery i have taken some part in an experiment similar to the historic one wherein we were detected, but of that too i shall have more to say again. the rejoicing at this discovery was not confined to us, for men of science were quick to grasp the importance which was attached to this new knowledge. we felt that man was bound to acknowledge our services from that day. the next event was our christening, and this was not all plain sailing. indeed, we have been rather annoyed with one name which some good friends persist in giving us. i refer to the name _corpuscle_, which we feel to be a sort of nickname, although it may have been suggested in all kindness. it may be difficult for you to appreciate our dislike to this name, but it seems to us to savour too much of material things. it is not dignified; you must remember we are not matter. we are delighted with what we prefer to call our real name--electron--for that speaks of electricity. as you know, we are units of particles of negative electricity, and so this seems a most sensible and suitable name. but i must hasten to tell of some of our everyday duties in which we serve man. chapter vii a steady march _the scribe's note on chapter seven_ the steady motion of electrons from atom to atom along a wire, or other conductor, constitutes the well-known "electric current." the moving electrons disturb the æther around the wire and produce what we know as a "magnetic field." the electron explains why it is necessary to have a complete circuit before any electric current can take place. also how one length of wire may be used to connect two distant places provided the two extremities of the wire are buried in the earth. chapter vii a steady march personally i knew nothing about marching until quite recently. indeed, none of my fellow-electrons seem to have had definite ideas of regular marches previous to last century. that century is prominent in our history as well as in man's. there is no doubt that before then we must have made more or less regular marches through the crust of the earth and elsewhere; but for myself i have no such recollection previous to the following occasion. the experience was not a very exciting one. i found myself passing along from atom to atom in a copper wire. but what was of special interest to us was that it became evident that these enforced marches were being deliberately controlled by man. of course you will understand that man knew nothing of our existence at that time. all he knew was that when he placed a piece of zinc and a piece of copper in a chemical solution, there were certain effects produced in some mysterious fashion. for instance, when he connected the top of the two metals in this chemical cell or "battery" by a piece of wire, he got what he described as an _electric current_. now all that happened really was this. the chemical action in this battery which man had devised caused a rearrangement among the atoms composing the metals and the solution, with the result that we poor electrons had to rearrange our domiciles. as an accumulation of electrons gathered on the zinc, some of us were forced along the connecting wire towards the copper. as long as the chemical action in the battery was kept up, so long were we kept on the march from the zinc to the copper by way of the wire. man tried increasing the length of this wire bridge across which we had to pass, but we had no difficulty in making our way along. but you must not run away with the idea that we rush along the wire with lightning speed. although we can fly through the æther at a prodigious speed, our progress from atom to atom in a wire is more like a snail-pace. as a matter of fact, our rate of march is much less than the walking pace of a man; indeed it may be stated conveniently as so many yards per hour. some people may find it difficult to believe that our rate of march is so very slow. their front door is a good many yards away from their electric bell, but it does not take us an hour, or any appreciable part of a minute, to summon the maid. the secret is that there is a whole regiment of us along the wire, and before one of us moves on to a neighbouring atom, another electron must move off that atom and on to its neighbour, and so on. in this way the electrons at the far end of the wire commence to move at practically the same moment as those near the battery. it has been a source of amusement to me to see people perfectly mystified by the fact that they can get no electric current unless they have a complete circuit. what else could they expect? how could man march if he had no road to march on? you see, the reason for our march is that we wish to escape from the overcrowding on the zinc, and we are forced towards the copper. the atoms composing the wire are our stepping-stones, and if there is not a complete chain of atoms we are helpless. you have already heard how we can jump an air-space under very great pressure, but that condition does not exist in the present case. when we are disturbed by the chemical action of the battery, we should prefer to have a short-cut from the zinc to the copper, but if the only path man gives us is by way of a long wire, then we must be content to travel that road, in order to reach the copper. it is a matter of little moment to us what arrangement man makes as long as he gives us a complete path. for instance, he may lead us out from the zinc to a distant telegraph instrument, and then, instead of providing a second wire to take us back to the battery, he may conduct us by a short wire to the earth. we are quite content to lose ourselves in this great reservoir, provided man places another short wire from the earth to the copper of the battery at the other end of the line. then as we slip off at the one end of the line, an equal number of electrons can climb up at the other end, and thus enable all our friends in the long wire to keep up a steady march. this march of ours is not merely a means of transporting ourselves from one place to another; it is to enable us to do work. it is only when we are in motion that we can do useful work, for we must move before we can disturb the æther, and it is by means of the æther that we transmit energy. if you place a magnetic needle or mariner's compass near a wire along which we are making a steady march, you will find that we can affect our fellow-electrons who are stationed within the magnetic needle. we cause the needle to swing round and take up a position at right angles to our line of march. we succeed in doing this because these electrons in the magnetic needle are on the move also. but this reminds me that i have never told you how we produce that æther disturbance which you call _magnetism_. when, as children, you played with toy magnets in the nursery, little did you think that there was a host of tiny electrons amusing you. and yet we electrons are responsible entirely for all magnetic effects, as i shall proceed to explain. chapter viii a useful dance _the scribe's note on chapter eight_ we believe magnetism to be due to electrons revolving around atoms of iron and other magnetic substances, as related by the electron in this chapter. we have seen that the steady motion of electrons along a wire produces a magnetic field around the wire. therefore if we have electrons revolving round and round the atoms in a piece of iron, there will be a miniature magnetic field around each atom. the electron explains why a piece of iron does not show the magnetic power locked up within it until it is "magnetised." the electron refers to electro-magnets; an electro-magnet is simply a piece of soft iron with a coil of insulated wire wound around it. the iron only shows its magnetic power as long as a current of electricity is kept passing through the surrounding coil of wire, for reasons which the electron explains. chapter viii a useful dance i may tell you quite frankly that i have never taken part in the perpetual dance of which i am about to tell you. i am of a free and roaming disposition, but i have often watched some of my fellow-electrons at this work. of course, it is pleasant work, as all our duties are, now that man acknowledges our services. we are responsible for the behaviour of the mariner's compass needle. it is we who cause it to point continually in one definite direction. if we ceased to dance around the iron atoms in the compass needle aboard a ship, the man at the helm could not tell in what direction he was going, and sooner or later he would be almost certain to wreck his vessel. for this service alone man ought to be grateful to us, but before i have finished my story, you will find that even this important duty is but a small affair when compared with many of our other tasks. there is one matter i should like to make quite clear to you. although we electrons are all identical, we have different stations to fill. you have doubtless become familiar with my roving disposition, and you probably think of me as a detachable electron. then there are our friends who are locked up within the atoms of matter--part and parcel of the atom. and now i am introducing you to those electrons who act as satellites to the atoms, revolving around them at a comparatively great distance, just as the moon revolves around the earth. these are the electrons which give rise to the magnetism in a piece of iron. there are other electrons which perform very rapid revolutions around all classes of atoms, but i shall introduce these friends later on. [illustration: a tobacco-tin defying gravitation that phenomenon known as "magnetism" is due to the steady locomotion of electrons, as explained in the text. here we see a large magnet attracting a tinned iron box which is tethered to the table by two cords. the result is that the box is supported in the air. the spiral wires are connected to the electro-magnet, an explanation of which is given in chapter viii.] i need hardly remark that a piece of ordinary iron does not behave like a magnet. indeed, it is fortunate that it does not. if it did, man could not get along with his work very well. the hammer would stick to the head of the nail it had struck, the fire-irons would stick to the fender, while the cook's pots and pans would hold on to the kitchen range. that would be a very stupid arrangement, but we electrons have really no say in the matter of arrangement. we are always on the move, performing a perpetual dance around the iron atoms, but the atoms arrange themselves in a higgledy-piggledy fashion, so that the electrons on one atom pull the æther in one direction while others pull the æther in an opposite direction. in this way the outward effect is not perceptible. when, however, man places a coil of wire around the iron, and makes a crowd of electrons march along the wire, these marching electrons affect the æther, which in turn influences the satellite electrons which are revolving around the atoms of iron. you may be somewhat surprised when i tell you that, owing to this æther disturbance, these satellite electrons are able to produce a rearrangement among the atoms. if you doubt my word, you may easily prove the truth of the statement. if you magnetise a long bar of iron you will find that its length is actually altered. this is due to our having disturbed the arrangement of the atoms. perhaps i should explain that when we force the atoms into their new condition, we can do so only under the æther stress set up by our fellow-electrons who are marching in the neighbouring wire. whenever their march ceases the æther stress is withdrawn, and the atoms are able to fall back into their old higgledy-piggledy condition. in this way man is able to make a piece of iron a magnet and to unmake it as often as he cares by simply switching on and off the electric current from the wire surrounding the iron. if a piece of hard steel is used in place of soft iron, then we find that the atoms are not so easily disturbed, but when they are once brought into line with one another, they will remain in their new condition after the æther disturbance has been withdrawn. it may seem strange to you that quite a small percentage of carbon atoms added to the pure soft iron should cause such a marked difference, but the matter seems plain enough to us. man was so impressed with the manner in which the atoms were evidently fixed in their new condition that he spoke of _permanent magnets_. it is especially fortunate for man that these pieces of steel do retain their magnetism, and give us a reliable mariner's compass. but i shall tell you how you may disturb even these sedate atoms. if you hammer the metal very vigorously, or if you heat it to redness, you will find that the atoms have been freed from what appeared to be their permanent position, and they are back to their old higgledy-piggledy condition, so that we electrons are all opposing one another. remember we are hard at work all the time although we may be giving no outward sign of our activity. while we render an important aid to man by providing this permanent magnet for his compass, you will find that a very great deal of our assistance to man in his everyday life depends upon our behaviour in soft iron electro-magnets. it is in these that man can control our behaviour at will. it is through this simple piece of apparatus--the electro-magnet--that man has been able to accomplish so much in signalling to his friends at a distance. it is also by means of these electro-magnets that man can get us to turn an electric motor, and so on. but i must tell you, first of all, how we enable man to signal to a distance, or, in other words, how we carry man's news. chapter ix how we carry man's news _the scribe's note on chapter nine_ the electron explains wherein its method differs from all other methods. it is well known that within recent years the old iron telegraph wires have been replaced by much lighter copper wires; the electron explains the reason for this change. it describes how the electrons manage to work the most widely used form of telegraph instrument, which is called the "morse," after its inventor. here we find one of the practical applications of the electro-magnet described in the preceding chapter. chapter ix how we carry man's news it is we electrons who have so very far outdistanced all material carriers of news. you must acknowledge that the best runner, the swiftest horse, the fastest express train, and the prize carrier pigeon, are all nowhere when compared with us electrons. but i do not wish to mislead you in any way, and i can speak from personal experience in this case. we do not race off with man's messages in the same sense as these other messengers do. our swiftness of communication depends upon the simple fact that man provides a whole connecting regiment of us between the two distant places. and when the order to march is given we all move off at practically the same moment. in this way the electrons at the far end of the connecting wire are able to cause signals there immediately. this is the secret of man's success in being able to hold immediate communication with his distant friends. his success is due entirely to the co-operation of us electrons. my personal experience has been in connection with a very simple telegraphic arrangement. indeed, the most of our duties in transmitting messages are performed with this particular kind of instrument, known as a "morse sounder." at the time of which i speak, i had become attached to an atom of iron in the end of a long telegraph wire. from this you will probably guess that my experience was gained some time ago, for man does not use iron wires nowadays in fitting up telegraph lines. he used iron at first, and some of these lines still exist, but when he discovered that a very much lighter copper wire would serve the same purpose, he discarded the heavy iron wires. man explained the matter by saying that the copper offered less resistance to the electric current, and the majority of people were quite satisfied with this kind of explanation. of course these are merely convenient phrases which give man no real reason for the difference. the real reason is that we electrons are able to move about from one copper atom to another with very much greater ease than we can among the iron atoms. that is the reason why man made the change from iron to copper wires, although he had no idea of the reason at the time. to return to my experience in connection with a telegraph instrument, i found that we were being subjected to a series of forced marches. the whole regiment of electrons along the line made a forward move. the line of march ended in a short length of fine wire wound around a piece of soft iron to form an electro-magnet. the end of the wire dipped into the earth, as i have explained in an earlier chapter. now all that we electrons had to do was to make a forward move, halt, forward again, another halt, and so on. sometimes the signal to halt was longer in being given than at other times, but we found that this was intentional, and that there were two definite lengths of march. i have explained already how we marching electrons cause an electro-magnet to attract a piece of iron and let it go again as soon as we cease marching. it only remains for me to give you a general statement of how we work the morse telegraph. man has arranged a little lever with an iron end-piece immediately above the electro-magnet, so that the magnet may attract it. of course you are aware that it is the electrons within the soft-iron core of the electro-magnet who produce the magnetic effect. every time we electrons in the surrounding wire make a forward move, the electro-magnet pulls down the end of the little lever referred to. as long as we keep marching, so long will the end of the lever remain down, but the moment we halt, the lever is free to be pulled up by a spring attached to it. the movements of the lever indicate the length of our long and short marches, and it is by means of these that man sends signals. all that he does is to control our march, by means of an electric push and a battery at one end of the wire, and it is we who produce the signals at the distant end of the wire. each time man presses the push we move the distant lever. when we pull the lever down it is so arranged that it makes a sound like "click," and when we let it spring up against a stop it makes another sound not unlike "clack." our long and short marches are therefore converted into long and short "click-clacks." man has made a simple code of signals representing his alphabet, and right merrily do we rap out the signals for which we receive orders at the distant end of the wire, while some one at the other end listens to the sounds we cause to be made. i have told you enough of our duties to let you see how we are able to carry man's news from one part of the earth to any other part. by far the greatest part of our signalling work is done with this simple morse sounder. it may interest you to note that we can produce those signals far faster than man can read them. when man found this out he took advantage of our powers. he made an automatic transmitter which could manipulate the make-and-break of the battery current far more rapidly than any human fingers could do. then as we rapped off the signals with lightning speed at the distant end, he attached a little ink-wheel to the end of the moving lever, so that it could mark short and long strokes on a ribbon of paper passing close to it. although man could not distinguish the signals by his ear he was able to read the record of those we caused to be left upon the paper ribbon. we have been made to work many other forms of telegraph instruments. in some of these we control type-letters, while in others we imitate handwriting, but all these are merely adaptations of our powers of marching. we are proud of our achievements in rapid signalling, which all right-thinking people have not been slow to acknowledge. chapter x how we communicate with distant ships _the scribe's note on chapter ten_ in this chapter the electron deals with that modern marvel--_wireless telegraphy_. here the æther of space plays a very prominent part. the author has given some particulars about the æther in the first chapter (_what the story is about_). in conjunction with that, the electron may be left to tell its own story. chapter x how we communicate with distant ships our duties in this case are totally different from those of which i have been telling you. while we electrons can do many wonderful things, we cannot march through space. we may be fired off like bullets from the sun to the earth, but that is quite another matter. i shall have something to say about that fact later on. you have seen already that man can make us jump only a very short distance, even when he has cleared our path of the obstructing air, as he does in a vacuum tube. if men were to provide us with a complete path of metal atoms from the shore to the ship, we could set to work upon the simple plan which i have described in the preceding chapter. but, needless to say, man has more sense than to attempt to keep up metallic connection with a ship going away out to sea. even the wisest men were surprised when they heard that we electrons could signal through space to great distances without any connecting wires. we ourselves were not surprised. had we not been doing this very thing from the foundation of the world? our fellow-electrons in the sun have never ceased to communicate with those of us upon the earth. of course i am referring at present to those æther waves which man calls _heat_ and _light_. but the waves which we make to carry man's messages through space are of the very same nature, the only difference being that they are much longer, or, in other words, much farther apart. they do not follow each other so closely, and they do not affect the eye or the sense of touch. however, these long waves are able to bestir some of us electrons who are situated at a great distance from the sending electrons. our method of producing such waves in the æther is by surging to and fro from atom to atom in an upright wire. when we make a rapid to-and-fro motion we send out great waves in the æther. the original plan adopted by man was to make us jump across a spark-gap, but in this case also it was our rapid oscillation to and fro that produced the waves. if we wish the waves to carry to a great distance, we must club together in considerable force to supply the necessary energy. the energy which we can get from a battery and induction coil is not sufficient for any very long distances. in such cases we require the aid of a _dynamo_, a machine about which i shall have some experience to relate in another chapter. in communicating through space, our position is very similar to that of two men shouting to one another over a distance. the one man disturbs the air, thus sending air-waves (sound) over to his friend, and these waves produce certain sensations which he can interpret. i should like you to understand that we electrons are upon a higher plane than atoms of matter. we cause waves in the all-pervading æther, not among clumsy particles of air. after these æther waves have travelled enormous distances they retain sufficient energy to disturb electrons situated at the distant place. i shall tell you of the first experience i had in this connection. i found myself attached to an atom of _nickel_, a kind of atom which looks to us electrons very much like an iron atom, because it has nearly the same number of electrons composing it, only they are arranged differently. but i was telling you that i found myself on this nickel atom sealed up in a small glass tube. of course there were myriads of similar atoms all around me, but i did not feel very happy. i was being urged forward, and yet i could not get across from some atoms to others, for the nickel was in the form of loose filings. from past experience i knew that there was a battery along the line somewhere; i could feel the strain. all of a sudden i was startled to find that i could move forward. exactly what happened, i am not at liberty to tell, but this much i may say, that it was the arrival of some æther waves which altered the condition of things among the filings in the tube. [illustration: a motor-car with wireless telegraph it has become quite a fashion in america to have motor-cars fitted up for wireless telegraphy. that the electrons play an important part in telegraphing through space is explained fully in chapter x.] we had just started out on our march forward when we received such a shaking that we found ourselves in the same isolated positions as at first; we could not get across from one particle to another. more æther waves arrived, we made a fresh start, then came another rude shaking, and so on we went starting and stopping. indeed, it was the regularity of these long and short marches that gave me the first idea that we were being controlled by some telegraph operator. we were amused to find that the rude shaking, of which i have been telling you, was caused by the action of some of our fellow-electrons. some of them in their march around an electro-magnet in the receiving instrument caused a little lever to knock against our tube and give us a sudden jolt. i should like you to notice that the energy with which we moved the telegraph instrument did not come from the distant station. it was a local battery which worked the receiving instrument, but this battery was controlled by the incoming æther waves affecting the tube of filings. there is really no mystery about the matter, but i am anxious not to take credit for anything more wonderful than we have actually accomplished. we electrons have rendered a very great service to man by enabling him to communicate with his friends who are far out on the ocean, and cut off from all possible chance of material communication. we are willing to serve man on land also, though we very much prefer the ordinary marching arrangement if he will provide a connecting wire. the fact is that we find it very much more difficult to send æther waves over land than we do over water. i have heard some men ask how many different telegraph instruments may be worked at one place simultaneously without confusion. that is a question for man himself to answer. we electrons are able to produce any variety of waves of different frequency or length; it remains only for man to construct apparatus that will respond only to a definite rate of waves. i hear that man has made considerable progress in tuning the wireless instruments. some men are eager to get us to carry messages through space across the great oceans from shore to shore. we shall not refuse, provided man supplies sufficient energy, but i must admit that we electrons prefer the submarine cable. of course man may put this down to our laziness; we certainly prefer as little severe straining as possible. i have been telling you of my earliest and only personal experience in connection with space telegraphy. i understand that greatly improved methods have been adopted since that time, but i have never happened to drift in their direction. chapter xi how we reproduce speech _the scribe's note on chapter eleven_ in the first part of this chapter the electron explains the part it plays in ordinary telephony. the reader will picture the transmitting instrument at the one end of the line influencing the receiving instrument at the distant end. towards the end of the chapter the electron turns its attention to the newer subject of _wireless telephony_, which has been accomplished now over a distance of several hundred miles. chapter xi how we reproduce speech my scribe suggested a rather clumsy title for this chapter--"electrons _versus_ atoms as carriers of speech." i expect he made this suggestion without much thought, for there are two serious objections to such a title. in the first place, we are not carriers of speech. we are controlled by speech at one end of the telephone line, and we make a reproduction of the speech at the distant end of the line. no sound passes between the two places; there is only a movement of electrons along the connecting line. my second objection to the hurriedly suggested title is that it is hardly fair to make any comparison between the achievements of atoms of matter and those of ourselves. we are not in the same category as atoms. besides, we electrons are dependent entirely upon the material atoms for making our work useful to man. for instance, we might keep on making waves in the æther for all time, and yet if the atoms of matter were to pay no heed to those imperceptible waves, man would never be aware of their presence. indeed we electrons act solely as go-betweens. on the other hand, it is only fair to ourselves to point out that a group of atoms in one town could never communicate with a group of atoms in a distant town unless we electrons came to their aid. it is true that over a very short distance the atoms may communicate directly. for instance, if a heavy blow is given to a large gong, the atoms of metal may vibrate so energetically that they succeed in disturbing the atoms of gas of the surrounding atmosphere for some considerable distance. but in the case of speech, the speaker cannot supply any great energy, so that he can disturb the atmosphere only to a very limited distance. we electrons, however, can do yeoman service in this respect. we have enabled men to speak to one another over immense distances. the whole affair is very simple. man speaks and causes the atmospheric atoms to vibrate and impinge upon a light disc or diaphragm in a simple instrument which man has named the _telephone_. this vibrating disc presses upon a myriad of carbon particles contained in a small case or box, the disc forming one side of the box. when these carbon particles are pressed together we electrons can get across more easily from atom to atom. there is a battery urging us forward, but our motion is dependent entirely upon the manner in which the vibrating disc presses upon the carbon particles. i cannot describe our movement in the line-wire as a march; it is in reality a surging to and fro. you will understand that this to-and-fro motion of the electrons in the line-wire varies according to the vibrations of the sending disc, which is controlled by the speaker's voice. at the distant end of the line we electrons bring our magnetic powers into action. we keep varying the attractive powers of an electro-magnet, according to the motion of the electrons in the wire. this ever-changing magnet produces vibrations in an iron disc which is fixed close to the magnet. this disc is set vibrating in exact sympathy with the sending disc. when the listener places this receiving disc close to his ear, the vibrations are carried by the atmospheric atoms to his hearing apparatus. all that we electrons have done is to cause one disc to vibrate in exact synchrony with another distant disc. but that is all that is required, for the receiving disc will reproduce similar air-vibrations to those set up by the man's voice at the distant place. i have pointed out already that we do not attempt to carry the sound. it is true that the atoms of matter do the hard work, but it is we electrons who enable a group of atoms in one town to communicate with a group of atoms in a distant town. it was natural that as soon as man found that he could work his telegraph instruments without the aid of connecting wires, he should try to do the same with his telephone instruments. we were sorry when we found men trying to use the original spark-telegraphy methods for telephones. while we had no difficulty in operating a telegraph instrument by means of æther waves and the tube of filings, it was quite impossible for us to produce telephone vibrations on the same principle. this spark method was a too rough-and-ready plan. the waves we produced were like sudden splashes in the æther ocean, whereas we knew that we must produce regular trains of continuous waves in order to reproduce telephone vibrations. however, you may be aware that we have succeeded by a different arrangement of apparatus. indeed it may interest you to know that one of my most recent experiences has been in connection with some wireless-telephone experiments. unfortunately i was not in a very favourable position to learn all that was going on, but it was quite exciting work. i happened to be attached to an atom of copper in a length of wire which had been run up into the air on a sort of flag-pole arrangement. i need hardly say that i was not alone, for by this time you will have become accustomed to picture myriads of electrons occupying a very small space. we were set vibrating to and fro with tremendous energy, but what bothered me most was the great variation in our movements. it was the nature of these variations which gave me the clue that we were being controlled by the vibrations of a telephone disc. i can tell you we did make a complex series of waves in the surrounding æther! these waves went out through space and influenced some electrons stationed at a great distance. when these electrons at the receiving station were set in motion they controlled the electric current from a local battery which set a second telephone disc vibrating in synchrony with the one at the sending station. on questioning some of my fellow-electrons who happened to have been nearer the transmitting part of the instrument than i had been, i got some interesting information. they tell me that there was a dynamo and an arc lamp in our circuit, while the telephone instrument was in a neighbouring circuit. the electrons surging to and fro in the telephone circuit influenced those energetic electrons in the arc-lamp circuit to which the ærial wire was attached. you see that my position in the ærial wire was not a very advantageous one for observing what was taking place. this was truly a great achievement--to enable one man to speak to another distant hundreds of miles, and without the aid of any connecting wire. i think you will agree with me that we have excelled all past records in the world of wonders. chapter xii our heaviest duties _the scribe's note on chapter twelve_ here the electron explains its behaviour in a dynamo at work. the principle of the dynamo was discovered by faraday in the thirties of last century. he found that when a coil of wire was moved through a magnetic field, there was a current of electricity induced in the moving coil. experimental machines were constructed, and after a while a practical dynamo was evolved. wires are attached to a dynamo and the electric current is led out. this current may be conducted to a distant tramway car, and, by sending the current through an electric motor, mechanical motion is produced and the car propelled along. an electric motor is practically the same as a dynamo, but instead of turning its coil round in order to produce an electric current, we pass a current into the coil and it moves round. it will be sufficient to leave the electron to tell its own story. chapter xii our heaviest duties this is another of those roving commissions in which i have been privileged to take part on more than one occasion. if you think of the giant size of an electric tramway car or a railway train, and try to compare one of these with an electron, such as your humble servant, it will seem quite ridiculous that i should suggest that it is we electrons who move those huge vehicles. yet such is the actual case. of course we require the application of very considerable power to urge us to so heavy a task. all the energy which we can get from a few electric batteries might enable us to drive a toy car, but when it comes to turning the wheels of a real car or train, we require a correspondingly greater amount of energy. i may as well tell you quite frankly that we electrons are only the intermediaries or go-betweens. indeed, you must have noticed that in every case we act merely as a connecting link between matter and the æther, and between the æther and matter. but what i want to tell you of, is the part we play in moving an electric car or railway train. it is really all very simple if you could only see it from our standpoint. picture a host of us attached to copper atoms in a coil of wire which is being moved through that disturbed æther called a _magnetic field_. we are set in motion immediately. it is true that when we are moved forward into the field we march off in one direction, only to be arrested and made to move off in the opposite direction as we leave the field, but it really makes no difference in our working capabilities as long as we are kept on the move. this is what is actually taking place in the armature of a dynamo as it revolves between the poles of the electro-magnet. there is no peace for us so long as the coil is kept revolving; we are kept in a constant state of rapid to-and-fro motion. [illustration: a train impelled by moving electrons _by permission of siemens schuckert werke_ _berlin_ it is remarkable that the motion of electrons in an electric conductor can result in the movement of heavy vehicles. how this comes about is explained in chapter xii.] this is all we electrons do in a dynamo, but when the ends of the outer circuit or mains are brought into contact with the ends of our revolving coil, we set the electrons in the mains surging to and fro in step with ourselves. man describes this motion of the electrons in the mains as an _alternating electric current_, but by a simple commutator on the dynamo he may arrange that we set the electrons marching in one direction in the mains. this he describes as a _direct electric current_. it is a matter of indifference to us whether man drives our coil round by means of a steam-engine, a water-wheel, or a wind-mill; all that we electrons want is to be kept surging or vibrating to and fro. now you will be able to appreciate how we electrons get up sufficient motion to enable us to perform what i have described as _our heaviest duties_. perhaps you will find it difficult to believe me when i tell you that as we march along the connecting wire to a distant tramway car we transmit the energy through the surrounding æther, and not through the wire. this is our mode of working in every case, whether it be an electric bell, a telegraph, or telephone. that is to say, while we electrons move from atom to atom in the connecting wire, it is the disturbed æther surrounding us which transmits the energy. you must have realised by this time how very intimate is the relationship between ourselves and the æther. to return to the tale of our tramway work, you will picture my fellow-electrons aboard the car being energised by the incoming current. those electrons present in the armature coil of the motor are set into motion, as also are those in the wire of the neighbouring electro-magnet. the result is that these two sets of electrons so disturb the æther and affect one another that the coil is moved round into a different position. you will remember the experiment of which i told you, in which a magnetic needle would insist always in taking up a position at right angles to a wire in which an electric current is passing. well, when the motor coil has turned into its new position, we electrons receive an impulse from our friends in the line-wire which causes us to retrace our steps in the coil. this action of ours causes the coil to make a further movement in the same direction as at first. again we change our direction of march, and again the coil changes its position towards the electro-magnet. the sole duty of these electrons in the armature coil is to keep surging to and fro, while those electrons in the electro-magnet keep up a steady march in one direction. this arrangement necessitates the armature coil to keep changing its position continually, and when we have the armature coil spinning round at a steady pace, it is easy for man to connect the armature to the axles of the tramway car and cause us to drive the wheels round. i need hardly say that it makes no difference to us whether we are asked to drive a tramway car, a railway train, or a host of machines in a factory or workshop. all that we electrons in the motor require is to have sufficient energy passed along to us from our fellows in the distant dynamo. again i admit frankly that the atoms of matter play a very important part in these our heaviest duties, but you will see that without our active assistance they could not transmit the necessary energy to a distant car or train. chapter xiii a boon to man _the scribe's note on chapter thirteen_ while it has been known for a long time that _light_ and _radiant heat_ are merely waves in the æther, it was not known until recently how these waves were produced. the discovery of electrons has given us a reasonable solution of our difficulty. the electron explains the actions of its fellows in this great work of producing light and heat. incidentally the electron explains how they produce an aurora in the heavens, and how it is that the earth has become a negatively electrified body. chapter xiii a boon to man every living thing is dependent upon our activities. it is we electrons who send out heat and light from the sun, and it is we who receive these on their arrival upon this planet. our action in the matter is really very simple, but until man discovered our existence, he was mystified considerably. we were amused to hear man say that the atoms of incandescent matter in the sun produced waves in the æther, and that when these æther waves fell upon other atoms on this planet, these were set into a state of vibration, thus producing heat and light. now if man had only stopped to think, he would have seen how ridiculous it was to speak of atoms of matter producing waves in the æther. he ought to have known that atoms of matter cannot affect the æther, for it offers no resistance to matter moving through it. man might have pictured himself riding on the back of this great planet, flying through space at a speed very similar to that of a rifle bullet, and yet even the flimsy blanket of air surrounding the planet is not disturbed by the æther through which it is rushing. it is true that the atoms of matter play an important part in the origin of heat, but the atoms in the sun could no more affect the atoms on the earth than could a man on the earth push the moon about. it is the very intimate connection between us electrons and the all-pervading æther which enables our fellows in the sun to communicate with those of us upon this planet. where would man be without us? [illustration: protection against a discharge of electrons _by permission of siemens schuckert werke_ _berlin_ when a man is encased completely in an over-all made of flexible metallic gauze he is proof against shock due to a discharge of high-tension electricity. the part played by electrons in the case of electric shock is explained in chapter iv.] i cannot understand wherein man should find any mystery in connection with this very simple action of ours. you will picture our distant fellow-electrons making very rapid revolutions around the atoms of matter to which they are attached as satellites. just as the moon circles around the earth, so do we circle around our atoms, but at an enormously greater speed. of course the whole length of our orbit is inconceivably small, and the speed of our revolutions is inconceivably great. it is our rapid motion through the æther which produces those waves known to man as radiant heat and light. some one may ask how it is that we electrons can disturb the æther while the giant atoms cannot. the obvious answer is that we are not matter, but electricity; we are not in the same category as atoms of matter. to complete the picture which i was drawing, you have only to think of the æther waves arriving upon this planet and disturbing sympathetic electrons, causing them to revolve around their atoms in similar fashion to our distant fellows who are producing the æther waves. it may be that some people get confused between this action and that of those electrons who are shot off bodily from the sun towards the earth. believe me, there is no connection between the two things. the stream of electrons shot off from the sun is deflected towards the magnetic poles of the earth, and as the electrons enter the upper layers of the atmosphere they produce that beautiful luminous effect which man describes as an _aurora_. i have never taken part in one of these great displays, for, as far as my recollection goes, i have never been in the sun, although some fellow-electrons declare that at one time we were all in the same great glowing mass of which the sun, and every member of the solar system, formed a part. however that may be, i certainly have no experience of auroræ, but i have assisted in producing the very same effect upon a small scale within a vacuum tube. the air remaining in these so-called vacuum tubes is just as rarified as the air in the upper layers of the atmosphere, and when we are shot across the tube we act in the same way as those electrons arriving upon this planet from the sun. you will observe that as a surplus of electrons arrives upon the earth from the sun, the earth is naturally a negatively electrified body, but i need hardly say that the earth does not keep all the electrons which arrive upon it. my scribe points out that i am wandering from the story which i set out to tell in this chapter, so i shall try and please him. the direct cause of light, whether it be natural or artificial, is the rapid motion of electrons around atoms of matter. if they revolve at a comparatively slow speed they produce those æther waves which man calls _radiant heat_. if these satellite electrons, however, desire to affect the eye of man, they have to move around at a very much greater speed. if we travel at too fast a speed, then we cease to cause the sensation of light. but, believe me, all the waves we make are of the same nature, no matter what names man has given them. the only difference we can make in the waves is the rate at which they follow one another. of course we can also make them larger or smaller in height, or, in other words, of greater or less amplitude, but that does not affect their properties. in the following chapter i shall tell you of some remarkable phenomena which our different æther waves produce in the brain of man. chapter xiv how we produce colour _the scribe's note on chapter fourteen_ colour is merely a sensation in the brain. what the electrons really produce are æther waves, and these give rise to the sensations of colour. however, the electrons may claim to produce colour in the same sense as we savages produce pain in fellow-men by firing rifle-bullets at them. the electron explains how some objects appear white, while others are red, and so forth. it explains also how electrons produce artificial light. the electron twits man upon his ridiculously wasteful processes of obtaining artificial light. chapter xiv how we produce colour in the preceding chapter i have been telling you how we electrons produce waves in the æther ocean. i pointed out that if we make the waves follow each other at too slow or too fast a rate they fail to affect man's eyes. it may seem strange to you that only a very small range of our æther waves should affect man's visionary apparatus. of course this limitation lies beyond our province; we can produce endless variety of æther waves--it is man's organs which fail to appreciate the bulk of these. however, there is plenty of variety in the sensations which we can produce in man. if we make the waves follow each other at a certain speed, man says he has the sensation of _red_. if we move faster, he speaks of _orange-colour_, and as we increase our speed he names his further sensations as _yellow_, _green_, _blue_, and _violet_. then if we combine all these waves--that is, if we produce them all at one time--he says he has the sensation of _white_. if we produce none of these waves, he calls the result _black_. while we electrons are very versatile, our actions are dependent in a great measure upon circumstances. for instance, if an electron is acting as a satellite to one particular kind of atom, its rate of revolution around that atom may be very different from that of an electron similarly attached to another kind of atom. we electrons are all identical, but the speed of revolution is determined by the kind of atom. the reason is very simple; electrons revolve around some atoms at a much greater distance than they would around other atoms. those making only the smaller orbits not only get around their atoms in less time, but they are also travelling at a greater pace. it is this fact which enables the electrons to produce the various wave-lengths which stimulate the different colour sensations in the brain of man. i think you will have no difficulty in seeing how it is that we come to produce such a variety of wave-lengths--in other words, how we are able to make the waves follow each other more or less rapidly. you will understand that we do not produce colours; we merely make various waves in the æther, and these waves excite the colour sensations in man. i mention this simple fact, because i hear many people speaking of our æther waves as "coloured rays," which, of course, is quite a ridiculous description. suppose some of those waves which give rise to the red sensation happen to fall upon a lump of matter which contains only electrons capable of producing waves that affect the green sensation. what will happen? there will be no response, and the object, although viewed by "red light," will appear black. if an object, such as the white paper upon which my scribe is recording my story, contains a variety of atoms with electrons capable of revolving at all the different rates which produce colour sensations, then when "white light" falls upon the object it appears white (all the colour sensations combined). if, on the other hand, a "red light" only falls upon it, then only the electrons capable of responding to that rate of wave will be set in motion, and the object will appear red, and so on with the other rates of æther waves. so far i have been telling you what happens when different waves of light fall upon us. now i shall endeavour to explain how man has caused us to produce artificial light. at present all man's methods in this direction are dependent upon making some substance so hot that it becomes incandescent. even his most modern methods seem to us to be ridiculously wasteful and most roundabout. i shall speak only of the electric glow lamp, as i have had some experience in connection with this. on one occasion i had been taking part in a regular forward march from copper atom to copper atom in a conducting wire. i had no idea of the purpose of our march till i suddenly found myself handed over to some carbon atoms, who were in a very lively state of vibration. we had much more difficulty in making our way through this substance, and it was the passive resistance offered to the advance of the electrons who had preceded me that had driven the carbon atoms into this state of great excitement. in our march through the copper conductor we had been offered very little resistance, so that we had left the copper atoms in peace--at least man could not detect easily any excitement (heat). but so long as our forced march was maintained among the carbon atoms, so long did the high temperature exist. you will understand i and the other marching electrons did not produce the waves of light sent out by the glow lamp. what we did was to set the atoms of carbon into a rapid vibratory state, and they in turn caused their satellite electrons to hasten their pace. some electrons produced one rate of waves, and some another rate, but by the time the carbon was incandescent there were electrons sending out all the variety of wave-lengths, the combination of which produces the sensation of white. i have accused man of adopting very wasteful processes, so i had better explain the matter. in the preceding description of what is occurring in an electric glow lamp, i have spoken only of those æther waves which constitute light. but there are myriads of electrons in the carbon of the glow lamp that never attain the requisite speed to produce those waves; they revolve around their atoms at too slow a rate. they certainly disturb the æther, but the crests of the waves are so far apart that they do not affect the eyes of man. the business of these waves is to set up heat in the bodies upon which they fall. you may be surprised to know that in this contrivance of man, called an electric glow lamp, and, indeed, in all his other artificial light-producers, he causes far more electrons to produce radiant heat than the desired light waves. a most wasteful process! man has a long way to travel yet before he succeeds in producing artificial light by a reasonable process. indeed i doubt if any of you can realise, as we do, how exceedingly stupid the existing methods are. think for a moment of the glow-worm, in which we electrons produce light without setting up any wasteful heat waves. there is a strong contrast between this peaceful plan and that of the excited carbon atoms. when will man succeed in discovering this secret of ours? chapter xv we send messages from the stars _the scribe's note on chapter fifteen_ it is remarkable that man has been able to discover what the distant stars are made of. our knowledge concerning the chemistry of the stars has been obtained by means of the spectroscope, in which a beam of light from the star is passed through a glass prism. the result is the well-known image of the coloured spectrum, in which certain well-defined lines appear, according to the distant elements originating the æther waves. the electron explains the whole subject from its own point of view. chapter xv we send messages from the stars it is only within recent times that man has observed that we send messages from the distant stars to this planet. but there is nothing new to us in this proceeding; we have been busy sending these messages ever since the solar system was formed. through all those ages we have kept on sending these messages, knowing that in time man must come to take notice of them. if the subject should happen to be new to you, you will be anxious to know to what kind of messages i refer. needless to say, they are wireless messages--waves in the great æther ocean. the waves, to which i refer specially, fall within that small range of which i told you something in the preceding chapter. in other words, they are those waves to which man has given the name _light_. but what special information do these waves, coming from the stars, convey to man? they tell him of what materials these distant stars are made. needless to say, it is we electrons who produce those informative waves. you are familiar with our method of producing waves. you know that we whirl around the atoms of matter at prodigious speeds, and that according to the number of revolutions we make per second, we produce waves of corresponding frequencies. in an earlier chapter i have hinted that the speed of the revolving electron is determined by the kind of atom to which it acts as a satellite. for instance, when electrons revolve around iron atoms they produce certain wave-lengths, while those moving around hydrogen atoms produce an entirely different series of waves. but how is man to recognise these? it is quite evident that man may gaze at a distant star and be little the wiser concerning the different lengths of the waves which impinge upon his eyes. he may observe that the sensation is inclined to red, from which he may infer that the waves are long ones--that they are farther apart than some of the waves produced by a white-hot body. but had man been content to try and decipher our wireless messages in this rough-and-ready manner, he would never have gained the interesting information which we have now placed in his hands. how, then, did we enable man to read our messages? our plan may seem to be somewhat mysterious, but i assure you that it is really very simple. when these æther waves of light fall upon a triangular prism of glass, the waves are bent out of their normally straight path. but the point that may seem strange to you, is that those waves which produce the sensation of red are not bent so much as the others. the more rapidly the waves follow one another, the greater is the bending of such a ray from its original direction. in this way the various wave-lengths are all spread out, so that they form an image like a coloured ribbon, red at one end, being followed by orange, yellow, green, blue, and violet. every man must be familiar with this coloured spectrum. when some of my fellows are enclosed in drops of water in the air they produce a great rainbow spectrum across the heavens. but i must tell you how we electrons succeed in bending these rays of light. i have told you already how we either absorb or reflect the æther waves which happen to fall upon us. in most substances it is only those electrons very near the surface that are disturbed. they succeed in stopping the waves. they may do this in either of two different ways. if the satellite electrons are attracted strongly by their atoms, the electrons will spin around the atoms keeping time to the movements of the incoming waves, and in this way the electrons take up the energy of the waves. in doing this, the electrons send out fresh waves in the æther. this is the real explanation of what man calls _reflection_ of light. [illustration: the spectroscope and the electrons' wireless messages the spectroscope is seen in the extreme left of no. photograph. the instrument is explained at page . the operator is passing an electric current through a glass tube containing a rarefied gas, causing the gas to become luminous. when he examines its light through the spectroscope he sees bright lines as shown in photograph no. , and from the position of these lines he can tell what substance is producing the light. no. is the spectrum of mercury vapour. no. is part of the spectrum of the sun. note the dark lines, as explained in the text.] in the second case, the electrons are not so firmly attached to their atoms, so that the incoming waves dislodge them, and they are knocked about from atom to atom, and in this way the energy of the waves is frittered away. man speaks of the light having been _absorbed_ by the substance upon which it fell. in both cases the only electrons which take part in these actions are those electrons who can move in sympathy with the incoming waves. it will be clear to you that only those of us who are near the surface of a substance know anything about these incoming waves. the electrons attached to atoms in the interior of the substance are left in peace, owing to the defensive actions of our fellows on the outside. but this is not the case with all substances. there are some congregations of atoms through which the æther waves can make their way. man calls such materials _transparent_; for example, glass and water are transparent substances. the fact of the matter is that in such substances none of us are able to respond to the incoming waves, and so we cannot stop them. i should say almost none of us, for there are always a few electrons present who happen to be in sympathy with the incoming waves. that is why no substance is perfectly transparent. the point concerning which i wish to speak in particular is this. although we allow the æther waves to pass through such substances, we do offer some slight resistance to the passage of the waves; the faster the to-and-fro motion of the waves, the more resistance do we offer. that is why the waves of highest frequency are bent farthest from the straight line when passed through a glass prism. we actually force the æther waves to travel slower through a piece of glass than through the air. now there should be no mystery concerning our action in a triangular piece of glass. whatever combination of æther waves falls upon it, the different trains of waves are sorted out according to their frequencies. suppose, for instance, that æther waves emitted from some incandescent sodium are passed through a glass prism. the bulk of the electrons attached to the sodium atoms are capable of revolving at speeds which produce waves causing the sensation of yellow. hence there will appear a very distinct line of yellow light in the spectrum. but why should the light be in the form of a line? simply because our æther waves are passed through a narrow slit in a shutter. but i need not trouble you with further details of our actions, which, although very simple to us, may seem somewhat strange to you. you will understand, however, that we form bright lines in different parts of the spectrum, according to the kinds of atoms to which we are attached. it was this fact which attracted man's attention to our wireless messages. he soon discovered the meaning of these lines, for he commenced to take exact notes of the different positions in which we placed these lines. he saw that when we were attached to hydrogen atoms we always produced three prominent lines; a very distinct line in the red section, another in the blue part, and a third one somewhat fainter and farther along in the blue. on the other hand, when attached to sodium atoms, we produced two very distinct lines in the yellow. when attached to iron atoms we produced a great variety of lines in the spectrum. of course these substances have to be incandescent to enable us to produce the æther waves. now it will be clear to you how we send wireless messages from the distant stars. these stars are great masses of flaming gases, so that the satellite electrons are kept busy dancing attendance to excited atoms. the electrons are constantly sending out æther waves, which reach this planet. we sort out these waves when man passes them through a glass prism, mounted in a telescope arrangement which he calls a _spectroscope_. he then examines the positions of the lines we produce in the resulting spectrum, and from these he knows what kinds of atoms are present in the distant star. it is we who have informed man that there are forty different materials in the sun, the most common of which are hydrogen, sodium, iron, copper, nickel, and zinc. of course these all exist in a gaseous form. there is one point about which i need hardly trouble you, although it is worth mentioning in passing. while we produce bright lines in the spectrum of any incandescent substance on this planet, our messages from the stars appear as dark lines. the reason for this is that there are cooler masses of the gases surrounding the incandescent masses forming the stars, and these cooler gases completely absorb the waves we produce. so completely are these waves absorbed that blank spaces are left in the spectrum, and these are the dark lines to which i refer. as they are in the same positions that the bright lines would have occupied had the waves reached the earth, it makes no difference to the reading of our messages. curiously enough, some of our actions in forming lines in the spectrum led to our actual discovery by man; but i shall tell you of this in the following chapter. chapter xvi how man proved our existence _the scribe's note on chapter sixteen_ several men of note declared that "little particles" revolved around the atoms of matter, and that it was the motion of these particles which produced the well-known æther waves of light. this idea was suggested by the result of certain mathematical calculations. it was some time before real experimental proof was obtained. the electron tells its own tale of this great discovery. when the electron speaks of a spectrum line being shifted up or down the scale, it means towards the violet or the red end respectively. we may picture the spectrum as analogous to the keyboard of a piano. in the second part of this chapter, the electron explains how it has enabled man to discover that certain stars are approaching the earth, while others are receding from it. chapter xvi how man proved our existence we electrons had waited long ages for man to acknowledge our services, but we did not despise the acknowledgment which a few men accorded us upon the basis of their mathematical calculations. it was natural, however, that we should want something more definite than this. you can imagine our joy when real experimental proof of our existence was established. perhaps you think that we should have been satisfied with this. but even this did not bring acknowledgment from many outside scientific circles, and not even from all within those circles. as our services to man are universal, we feel that all men should become acquainted with our doings. indeed that was the chief argument used by my fellow-electrons, who urged me to write this autobiography. the story of our actual discovery by man is an interesting one. it all came about in a very simple manner, but in quite a different way from what most electrons expected. man reasoned within himself that if we electrons really did revolve around atoms and thus produce waves in the æther, as had been suggested, he ought to be able to affect our movements by disturbing the æther in which we were revolving. of course man cannot disturb the æther directly; he must employ some of us to do this for him. he caused us to produce a very powerful magnetic field, which, as you know, is a disturbance of the æther. man did not bother thinking about _us_ in this connection; he simply sent an electric current around an electro-magnet, but i have explained to you the very active part we play in electric and magnetic actions. from my story in the preceding chapter, you are aware that man had observed the meaning of the bright lines in the spectrum of any incandescent body. when he examined the æther waves we send out from sodium atoms, he found two very distinct lines in the yellow. because of the brightness of these lines, man selected a sodium flame to experiment with in the present case. you will picture a great host of my fellow-electrons revolving around the atoms in a sodium flame. the flame was placed between the poles of a very powerful electro-magnet, and a beam of æther waves (light) produced by us was directed into the spectroscope. the experimenter focussed all his attention upon one of the bright yellow lines. he noted very carefully the exact position in which we placed it. he then produced the magnetic field around the flame, in which my fellow-electrons were revolving at a steady pace, and, behold, the line which he was watching split up into two lines, one taking up a position a little higher up the spectrum scale, and the other going a little lower down towards the red end. what could this mean? man had no difficulty in knowing the cause of this alteration; indeed, it was exactly what he had hoped would take place. of the two new lines, one represented waves a little shorter, while the other line indicated waves a little longer or farther apart, than the original waves forming the single line. this could only come about by some of the electrons having had their rate of revolution increased, while that of others had been reduced. these alterations were due to the æther disturbance (the magnetic field). those electrons whose orbits happened to lie in one position had their rate of revolution increased, while those whose orbits lay in another position had their speed reduced. man was convinced at last that we "particles" were real existing things. whenever man withdrew the æther disturbance, the electrons fell back into their natural rate of revolution, and the original single line appeared in the spectrum. i took no part in the original experiment which gave absolute proof of our existence, but since then i have been present in a laboratory when the same experiment has been repeated. this is not the only case in which we alter the positions of definite lines in the spectrum. indeed, we have given man some interesting information about the motions of distant stars--information which he could not have obtained in any other way. we have sent wireless messages from distant stars, indicating that they were approaching the earth, while electrons aboard other stars have signalled that they are receding from the earth. all this may seem mysterious to you, and yet our actions in the matter are very simple. indeed, we do nothing but what i have told you of in the preceding chapters. we send out definite wave-lengths in the manner described already. but if we are on board a star which is travelling towards the earth, our waves will naturally follow a little closer at each other's heels. on the other hand, if the star is receding from the earth, the waves must be a little farther apart than they would be if the star were at rest. you will understand that the electrons are revolving at the same speeds in both cases, but the forward movement of the star crowds the waves together, while a receding star stretches them out a little farther apart. the result at the receiving end is that the crowded waves are just as though they had come from electrons revolving at a greater speed than is actually the case. hence the line appears farther along the spectrum, up the scale of frequencies, than would have been the case had the star not been moving forward in the line of sight. thus if the hydrogen lines, of which i have spoken elsewhere, should appear higher up the spectrum than usual, then man knows that the star from which these waves are coming is approaching the earth. it will be evident that when known lines in the spectrum are shifted down the scale (towards the red end of the spectrum), then the rate of the waves has been decreased, and man knows that the star carrying these stimulating electrons is receding from him. you will observe that we electrons perform no new duty in connection with this matter; it is entirely the motion of the body carrying us that alters the positions of the lines. but i must hasten on to tell you of some personal experiences. chapter xvii my x-ray experiences _the scribe's note on chapter seventeen_ the present generation were all very much interested in the discovery of x-rays. with the aid of a battery and an induction coil, man causes an energetic electrical discharge to pass through a vacuum tube. when the flying electrons strike upon a little metal target placed in their path, they produce the well-known roentgen rays. we have all become familiar with the great penetrating powers of these rays. the electron may be left to tell its own story. chapter xvii my x-ray experiences it was no surprise to us that we could produce what man calls x-rays, but we were very much surprised at the use to which man put these splashes which we made in the æther. a limited number of us had been producing x-rays on our own account for many ages, but i shall tell you of that in a later chapter, when you will hear how we made the world talk. i must tell you of my own experiences in connection with these x-rays, which i hear some men describe also as _roentgen rays_. i found myself once more within a large vacuum tube, and as soon as i felt a crowd of my fellows pushing me forward, i was quite prepared to be shot across the tube, as on previous occasions. personally, i was not prepared for what was to come. just as we reached the centre of the tube we collided with a metal plate or target. it was no joke to be pulled up so suddenly when travelling at a terrific speed. i noticed at the time that our very sudden stoppage had a peculiar effect upon the æther. of course we never bothered about a name for this disturbance; it is man who requires to have names for everything. he was quite right to call this æther disturbance "x-rays," for even now he does not know the real nature of these. i have heard him describe them as thin pulses in the æther, but there is something more. i may as well confess that although we observed this æther disturbance arising from our sudden stoppage, we paid little attention to it, until it became apparent that man was continuing to produce these rays for some special purpose. he had discovered that we could shoot these rays right through many solid substances which were not transparent to light. but i have not told you how man came to know that we could produce these penetrating rays. on one occasion we were sending out these rays, which, by the way, do not cause any sensation in man's visionary apparatus. the room was in darkness. some of the invisible rays fell upon a collection of small chemical crystals which were fixed on the surface of a screen. our fellow-electrons, who were attached to the atoms of the crystals, were bestirred into action. they could not reflect the x-rays, but they set up regular trains of waves in the æther, some of which came within the range that affects man's vision. man knew that this chemical screen could not produce light on its own account, and it became apparent that the vacuum tube must be sending some æther waves towards the chemical screen. as the electrons on the screen produced an æther disturbance different from that which fell upon it, man called this a _fluorescent screen_. at first we took merely a passing interest in the experiments which man made with these x-rays of ours, for it seemed to us as though man thought them only good enough for amusing his friends. indeed, we paid little heed to what he was doing, until we observed that the rays were being used by surgeons. we were interested at once, for here we could serve man. my first experience in this connection was quite interesting. a young girl had got a needle into her hand while she was playing about, and the surgeons were at a loss to know where the needle had lodged. we lost no time in producing x-rays which could penetrate the flesh of the hand, and reach the fluorescent screen on the other side. the bones of the hand blocked the way of our rays, but not so completely as the needle did. hence we produced upon the screen a faint shadow of the flesh of the hand, a much deeper image of the bones, and a black shadow of the needle. this enabled the surgeon to see where the needle was hiding. sometimes we were called upon to produce rays for detecting bullets in the flesh, or for showing the nature of a fractured bone. we were never surprised to find that our call was to detect a coin in the throat of a child, but in this connection a big surprise awaited some of us. i was not one of the party, but i have the information from some fellow-electrons. [illustration: how electrons produce x-ray images the upper photograph shows the x-ray apparatus in use. the operator is examining the bones of the lady's hand, which she places between the x-ray tube and the fluorescent screen. the rays pass through the flesh, but are obstructed by the bones, the rings, and the bangle, so that a shadowgraph or image is formed upon the screen, which becomes luminous where the rays succeed in reaching it. the actual examination is made in a dark room. owing to the way x-ray photos are taken (by contact) the image is reversed in a photograph, so that a left looks like a right hand.] a party of electrons were present within an x-ray tube at a large hospital, when they were called upon to produce rays for examining the throat of a little girl. they had become so used to this call that they did not doubt there would be a coin in the child's throat. however, they lost no time in producing the penetrating rays, and you can imagine their surprise when they produced the image of a toy bicycle upon the screen. it seemed ridiculous that such a toy could have entered a child's throat. when we had shown the surgeons exactly where the toy was, they set to work to remove it. the electrons heard later that the operation was successful in every way. every one was interested, and we were proud. i do not wish to appear boastful, but i wonder how many operations owe their success to these rays which we produce for man. it was natural that man should try if these searching rays could affect the chemicals upon a photographic plate, and we soon proved that they could. it made no difference to us whether man kept the plate sealed up in its light-proof envelope, or whether he placed the plate within a wooden box. these protecting covers offered no barrier to our rays. we produced shadowgraphs of any objects placed between our tube and the photographic plate. two of my early experiences may be of interest to you. the first of these seemed to me a rather tame affair. our x-ray tube appeared to be arranged for the amusement of fashionable folk. one grand lady placed her hand behind the fluorescent screen, whereupon we produced an image of the bones of her hand and very dark images of all the many rings upon her fingers. several of the rings had enormous diamonds, but it was after she had gone away that i overheard two gentlemen speaking about the rings. one asked the other if he had observed the beautiful diamonds, whereupon the other roared with laughter. it seems that we proved them to be imitation diamonds, for our rays could not penetrate them, whereas they have no difficulty in passing through real diamonds. we therefore produced black shadows of the imitation diamonds. little did the grand lady know how we had exposed her sham jewels. my second experience was a very curious one. i learned that our tube was being carried to some distance. after a while we were placed beside a peculiar-looking object, which the men referred to as the "mummy." one of the men suggested that they should photograph its feet, but before doing so they darkened the room and set us to work upon the fluorescent screen. the owner of the mummy got rather nervous as to what we might disclose, and as the force urging us into action was somewhat erratic at first, we produced only a very indistinct image. we were greatly amused at the nervous excitement of the owner; he seemed to think our verdict was that there were no bones. however, the man with the apparatus soon got things into better condition, and this enabled us to produce x-rays satisfactorily. the result was that they secured some excellent photographs of the hidden bones of the mummy. before telling you how we made the world talk, i should like to give you a clear idea of our relationship to the atoms of matter. chapter xviii our relationship to the atoms _the scribe's note on chapter eighteen_ we have no doubt that an atom of matter is a miniature solar system of revolving electrons. these electrons, being negative particles of electricity, would repel each other just as any two similarly electrified bodies do. there must therefore be some equivalent of positive electricity, but whether this exists in the form of a sphere or in separate particles we have no definite knowledge. one atom differs from another in the number of electrons which go to make up the atom. the electron explains how the atoms of matter are united to one another, how different compound substances are formed, and how chemical changes take place. chapter xviii our relationship to the atoms i am sorry that this part of my story must remain incomplete for the present. i am not free to tell you all i know; you must try and get behind the scenes on your own account. one thing i am at liberty to tell you is that my fellow-electrons who are locked up within the atoms are not without hope that they may gain their freedom once more at some future time. i know this first-hand, for i have met some fellow-electrons who have escaped from within an atom, but i shall delay telling you about these fellows till the succeeding chapter. my object in mentioning this fact now is to give you confidence in what i am about to say regarding the nature of the atom. on one occasion i overheard a conversation between two men who were discussing the construction of matter. one remarked that the atoms were the bricks of the universe, whereupon the other asked how the little bricks were cemented together. i wish that man could have seen a lump of matter as we see it. he would have been surprised to learn that the atoms never really touch each other. they are always surging to and fro, or _vibrating_, and it is this motion which constitutes the _temperature_ of the body which they compose. it must be clear, however, that in a solid body one atom attracts another atom across the intervening atomic spaces. this is another duty devolving upon us; what we do, really, is to upset the electric balance between the different atoms, and thus produce electrical attraction. first of all, perhaps, i should explain that the different kinds of atoms are simply congregations of different numbers of electrons. of course there is the other part, of which i am forbidden to speak--the part which man vaguely describes as _positive electricity_. however, you may take it from me that while it is true that the main difference between an atom of gold and an atom of iron, or of oxygen, is in the number of electrons it contains, there is a very important difference in the arrangement of the electrons. you know that they form rings outside one another, all of which revolve at enormous speeds. the number of electrons in the different rings varies according to the kind of atom. it is quite correct for man to speak of the atoms containing certain definite numbers of electrons, but i should like you to understand clearly that the exact number of electrons is not permanently fixed; one or more electrons can slip off one atom and become attached to a neighbouring atom which happens to be capable of accepting it or them. it is the interchange of these few detachable electrons that causes one atom to attract another. in other words, it is the differently charged atoms which attract each other, just as man crowds a surplus of electrons on to one object and finds it attracted bodily towards another object having a deficiency of electrons. it is this electrical attraction between the atoms which enables us to build up the particles, or _molecules_, of matter in such a variety of forms. first of all, we play the most important part within the atoms. we have formed only a limited number of such atoms. i am not free to tell you exactly how many, for man has discovered only about eighty of these different congregations of electrons, each kind of which he calls an _element_. the way in which we have coupled these different elementary atoms together must appear remarkable to all thinking men; there seems to be no end to the possible variety of combinations. in one case we unite an atom of _chlorine_ to an atom of _sodium_ and thereby produce a molecule of common salt. in another case we unite an atom of _oxygen_ to two atoms of _hydrogen_, and the resulting combination is an invisible molecule of ordinary water. it has always seemed to me very strange how some men have difficulty in regard to these combinations. i have heard a man ask how two different gases, hydrogen and oxygen, when united, should form a liquid, and not a gas. i wish you could see things as we see them. the atoms are neither gaseous, liquid, nor solid; they are little worlds of revolving electrons. i have spoken of the attraction between atoms, and again between molecules, in forming a solid body. it will be clear that there is less of this _cohesive force_ in the case of a liquid, whereas it is absent entirely in the case of a gas. in this case the molecules have become so far separated from one another that they cease to attract each other, and if left free they will soon part company, and spread themselves broadcast over the face of the earth. whether a substance passes into a solid, a liquid, or a gaseous state, the atoms remain constant, but their vibratory motion is altered very considerably. however, i was about to tell you that we electrons can make some very interesting combinations of atoms. those i have mentioned so far are of a very simple nature, but we have built up individual molecules containing hundreds of atoms. we link about a hundred atoms together and produce a molecule of what man calls _alum_, and we require to unite about a thousand atoms together to make one molecule of _albumen_ (the white of an egg). when man speaks of a chemical change having taken place in a substance, it is simply the electrons who have made a friendly interchange of detachable electrons, thereby causing a different assemblage of the same atoms. during these changes we never alter the nature of the atom. that little world of revolving electrons known as an atom of gold, remains always an atom of gold. but you must not run away with the idea that the atoms will never change. indeed, man has discovered that the atoms are not eternal, as i shall explain in the following chapter. chapter xix how we made the world talk _the scribe's note on chapter nineteen_ the discovery of radium is within the memory of all. many exaggerated statements went abroad at the outset, but the real facts are full of interest, and they have shed much new light on many subjects. three different kinds of radiation were found to be emitted by radium. at first man could not tell what these were, so he named them after the first three letters of the greek alphabet--alpha, beta, and gamma, rays. the electron tells the interesting story of these rays, and relates the experiences of some fellow-electrons who escaped from within a radium atom. chapter xix how we made the world talk we electrons were amused at the stir which we unconsciously caused throughout the civilised world. we had done nothing different from what we had been doing for ages, but a few men had been taking note of what we were about, and when the phenomena to which i refer became known to the world, many wild rumours were circulated. one of these rumours was to the effect that steam-engines and their expensive furnaces were to disappear very quickly. if the two last words had been omitted--i should not say that the prophecy is untrue, but man has a long way to travel yet before reaching that goal. my fellows within the atoms have sufficient energy to supply all mankind with power if he could but unlock even a small fraction of it. another statement was that this newly discovered substance, _radium_, could cure some diseases which man had believed to be incurable. all i shall say about this is that the statement was an exaggerated one. then it was said that radium disproved much of man's scientific knowledge, but instead of that being so, we electrons have greatly extended man's knowledge by our radio-active actions. if any man believed the atoms of matter to be eternal, we certainly disproved that. here, in radium, man could see atoms going to pieces. i have questioned a fellow-electron who escaped from a radium atom as to what upset their equilibrium, but i find that he does not know, or he pretends not to know. all he has told me is that he was flung off suddenly from within the atom with great energy, for he had been revolving at a tremendous speed. in his sudden flight he passed some newly formed _helium_ atoms, which contained many of those electrons who had been his co-partners in the former radium atom. being an electron, he was travelling at a far greater speed than these flying atoms of matter, but he assures me that these helium atoms were going faster than atoms can travel under any other circumstances. another thing that this escaped electron told me was that when he and his fellow-electrons made a sudden start on leaving the atom of radium they caused a proper splash in the surrounding æther, just such as we electrons produce when we are suddenly stopped in an x-ray tube. man observed these rays proceeding from radium, but, not knowing the cause of them, he called them _gamma rays_. we can, of course, produce radiographs when these rays fall upon photographic plates. indeed, some of my fellow-electrons, when escaping from radium, have produced rays sufficient to penetrate a six-inch boulder and affect a photographic plate lying beneath the boulder. in time man recognised these rays as x-rays. man did not find only these rays--he discovered that electrons were escaping, but before he had recognised what we were, he had named us _beta rays_. these fast-flying electrons have had experiences which never fall to electrons except when escaping from an atom. their velocity is so great that they can be shot right through a sheet of aluminium foil. if these escaped electrons are allowed to settle on any object, they will necessarily cause an overcrowding, or, in other words, the object will become negatively electrified. the one thing that puzzled man most was to find out what the helium atoms were. he had named them _alpha_ rays, but as he found he could not get them to penetrate even a thin sheet of paper, he was confident that they must be atoms of matter. it was only when he had gathered sufficient to examine the spectrum that he found these to be helium atoms. i think what really made the world talk was the fact that electrons were escaping from what had been supposed to be an eternal habitation. in other words, this material radium was actually going to pieces. that is to say, _gradually_, as far as man is concerned, for, looking at it from our point of view, the word _gradual_ seems out of place entirely. the breaking up of an atom is really of the nature of an explosion. it is a continual bombardment that is proceeding in radium. why man is apt to think of it as a gradual effect is that there is such an enormous number of atoms in a tiny speck of radium, that even the incessant series of explosions will take a very long time to break down the whole of the small particle. electrons differ in their opinions as to whether man will succeed in drawing upon this internal energy of the atom. my own difficulty is that, having been a roaming electron at all times, i have no idea regarding the cause of the atomic explosions. i have remarked already that the electrons locked up within the atoms possess more energy than man could ever use. if all these electrons were deprived of their energy, the atoms of matter would cease to exist, and man, where would he be? chapter xx conclusion _the scribe's note on chapter twenty_ not many of us have realised the true importance of electrons in the creator's plans. in the following short chapter the electron is made to sum up a few of the wonders which it has related, in order to focus our attention upon the grand place which the electrons occupy in the universe. chapter xx conclusion from what i have told you of myself and my fellow-electrons, it must be apparent that we are of tremendous importance to man. i have told you something of the part we played in building up this world--how we not only form the atoms of matter, but also hold these bricks of the universe together. i have given you a rough sketch of the composition of these bricks. you must have realised also that without us the whole universe would be in darkness. there would be no light, no heat, and consequently no life. indeed, there could be no material existence without us. where would man be if we failed to perform our mission? he could not exist if we even neglected a few of our duties. not only do we form the atoms of which his body is composed, also holding these together, but we produce all those chemical changes within his body which are absolutely necessary to maintain life. his very thoughts are dependent upon our activities. i have told you how we send man's messages across the earth, and how we transmit power from place to place. also how we have enabled man to gain knowledge of the distant stars, and to examine the bones of his living body. if man could cross-examine me or any of my fellows, i expect the first question would be--what are you electrons made of? but man must find this out for himself. the creator has placed man in a world full of activity, and it is of intense interest to man to discover the meaning of all that lies around him. that is why i have been bound over by my fellows to tell you only so much of our history as man has discovered. but i am disclosing no secret when i admit that our very existence as electrons is dependent upon the æther. if i can find another scribe to write a revised biography for me a few hundred years hence, i shall have a much more interesting tale to tell, for many of our doings, of which man knows nothing at present, will be secrets no longer by that time. appendix _the scribe's note on appendix_ as explained by the author in chapter i., this appendix has been added for the sake of those readers who may wish further details than have been given in the electron's story. it is only necessary to give a brief notice of the more important particulars, as the author has written recently upon this subject in a popular form.[ ] [footnote : "scientific ideas of to-day." by chas. r. gibson, f.r.s.e. (london: seeley & co., ltd. five shillings net.)] appendix it was known two thousand years ago that when a piece of amber was rubbed with a woollen cloth, the amber would attract light objects towards it. amber was considered to be unique in this respect. about the year , one of queen elizabeth's physicians, dr. william gilbert, inquired into this attractive property of amber. he found that many other substances possessed the same property. indeed it is common to all substances in some degree. we say the amber or other object is "electrified." it was observed by the early experimenters that there were two kinds of electrification. to one of these they gave the name _positive electricity_, and to the other _negative electricity_. every electrified object will attract an object which is not electrified, and two objects which are oppositely electrified will attract one another also. but two objects which are similarly electrified will repel each other. man got tired of rubbing objects by hand, so he fitted up simple machines in which glass cylinders or plates were rubbed against leather cushions. the electricity was then collected by little metal points supported on an insulated metal sphere. the experiment of attempting to store electricity in a glass vessel filled with water was made at the university of leyden (netherlands). the water was replaced later by a coating of tin-foil on the inner surface, while a similar metallic coating on the outside took the place of the experimenter's hand. these jars are called _leyden jars_, after the place in which the discovery was made. about , professor galvani, of italy, observed that the legs of a freshly killed frog twitched at each discharge of an electrical machine. later he found that the same twitching occurred when he connected certain parts with a piece of copper and zinc. he believed this to be due to "animal electricity" secreted within the frog. professor volta, also of italy, proved that galvani's idea was wrong, and that the electricity resided in the metals rather than in the frog. he showed that when two pieces of dissimilar metal were put in contact with one another, there was a slight transference of electricity between them. he constructed a pile of copper and zinc discs, with a moist cloth between each pair or couple, and by connecting wires from the top copper disc to the lowest zinc disc he was able to show that an appreciable current of electricity was produced. later he placed a piece of copper and a piece of zinc in a vessel containing acidulated water, whereupon he found that a steady current of electricity was obtained. this was the invention of electric batteries. the phenomena of _magnetism_ were known to the ancients, but it was not until the nineteenth century that we found any real connection between electricity and magnetism. in , a danish philosopher, hans christian oersted, discovered that an electric current passing in a wire affected a magnet in its neighbourhood. if the magnet was supported on a pivot, after the manner of a compass needle, it would turn round and take up a position at right angles to the wire carrying the electric current. the molecular theory of magnetism presumes that every molecule of iron is a tiny magnet, having a north and south pole. in a piece of unmagnetised iron, these tiny magnets are all lying so that they neutralise one another. when they are turned round so that their north poles are all lying in one direction, then the iron is said to be magnetised. the electron theory of magnetism does not do away with the older molecular theory just referred to. the electron theory goes a step farther, and tells us that these molecules are magnets because of a steady motion of electrons around the atoms of iron. it was discovered in that when an electric current was sent through an insulated wire wound around a piece of soft iron, the iron became a magnet; when the current was stopped the magnetism disappeared. such magnets are called _electro-magnets_. if a piece of hard steel is treated in the same way it becomes a _permanent magnet_. it was this intimate connection between electricity and magnetism, or, in other words, the invention of these electro-magnets, which brought us electric bells, telegraphs, telephones, dynamos, and electric motors. it should be noted that while iron is attracted by either pole of a magnet, there is such a thing as magnetic repulsion. this, however, takes place only between two magnets, and then only between like poles. * * * * * some german physicists made a number of electrical experiments with vacuum tubes. when sir william crookes (england) was experimenting with similar vacuum tubes he suggested that matter was in a "radiant" state during the electric discharge within the tubes. in , h. a. lorentz, of amsterdam, declared that light was due to the motion of small particles revolving around the atoms of matter. professor zeeman, of holland, produced experimental proof of lorentz's theory. he showed that the revolving "particles" were influenced by a powerful magnetic field, in the manner explained in the electron's story. this discovery was made in , or sixteen years after lorentz's declaration. it was dr. johnstone stoney, of dublin university (ireland), who christened these particles "electrons." the x-rays were observed for the first time by professor roentgen, of germany, in . the screens used for viewing the luminous effects produced by the x-rays are coated with very fine crystals of _barium platinocyanide_. these screens were in use for another purpose previous to the discovery of x-rays. we know now that _chemical affinity_ is merely electrical attraction between the atoms of matter. the spectroscope consists of a glass prism, or series of prisms, mounted between two metal tubes. one tube is provided at one end with a vertical slit, through which the light that is to be examined is passed. at the other end of the tube is a lens, so that the beam of light from the slit emerges through the lens as a pencil of parallel rays. the pencil of light then falls upon the glass prism, striking it at an angle. in passing through the prism, the light is bent round so that it enters the second tube, which is simply a small telescope. the prism separates the æther waves according to their wave-lengths, and produces the well-known coloured spectrum, which is magnified by the telescope. the reason for the bending of the different waves is explained in the electron's story. index absorption of light, Æther, the, Æther waves, , , , , , , alpha rays from radium, alternating electric current, amber electrified, , to , artificial light, , atoms breaking up, , atoms co-operating with electrons, , atom's internal energy, , atoms of matter, , , , , , attraction between atoms, attraction, electrical, , attraction, magnetic, , aurora, automatic telegraph transmitter, battery, electric, , beginning of the world, beta rays from radium, birth of the moon, , bricks of the universe, , chemical affinity, chemical combinations, , chemistry of the stars, , , , chlorine atoms, , cloud formation, circuit, earth, coherer, tube, cohesive force, colour, compass needle, complete electric circuit, conductors, , connecting link between æther and matter, , corpuscles, crookes, sir william, current of electricity, dark lines in spectrum, detachable electrons, , detecting imitation diamonds, direct electric current, discharge of electricity, discharge through a vacuum, discovery of electrons, , discovery of x-rays, dynamo, , earth circuit, electrical discharge, electricity, positive, , , , , electricity, negative, , , electric battery, electric current, , electric motor, , electric shock, electrified objects, , , electro-magnets, , , , , electrodes, electrocution, electron as a go-between, electron, derivation of the word, electron, discovery of, , electrons, , , , , , , energy transmission through the æther, , energy within the atom, , field, magnetic, , , fluorescent screen, , galvani's discovery, gamma rays from radium, gilbert's discovery, glass, electrified, , glass prism, , glow-lamp, electric, , glow-worm, heat, radiant, , , , helium atoms, , hydrogen atoms, , insulators (non-conductors), , iron atoms, iron wires discarded, lamp, electric, leyden jar, , light, , , , light absorbed, light, artificial, , light, reflected, lightning, , lines in the spectrum, , , , lorentz's declaration, magnetic attraction, , magnetic field, , , magnetic repulsion, magnetism, , , , magnetism and electricity, magnets, electro-, , , , magnets, permanent, mariner's compass, matter, , metal electrified, , molecules of matter, , moon's birth, , morse telegraph, motion in line of sight, motor, electric, , negative electricity, , , oersted's discovery, oxygen atoms, permanent magnets, positive electricity, , , , , prism of glass, , radiant heat, , , radiant matter, , radium, rainbow, rays from radium, , reflection of light, repulsion, electrical, repulsion, magnetic, roentgen rays, roentgen's discovery, , sea, cause of saltness, shock, electric, silk, electrified, sodium atoms, , spark, electric, spectroscope, , , spectrum, , , , speed of electrons in conductor, stars approaching the earth, stars, constituents of the, , , stoney, dr. johnstone, sun, constituents of the, sun's heat, , telegraph signals, telegraphy, wireless, telephone, telephony, wireless, temperature, tramway, electric, , , transparent substances, vacuum tubes, , , , velocity of electrons, volta's discovery, waves in the æther, , , , , , wireless messages from the stars, wireless telegraphy, wireless telephony, x-rays, , x-rays from radium, x-ray photography, zeeman proves existence of electrons, , printed by ballantyne, hanson & co. edinburgh & london * * * * * transcriber's note the following changes have been made to the original text: page xi: "always necessary, how" changed to "always necessary. how" page : "vacuum tubes, when" changed to "vacuum tubes. when" page : "negative electricity, , , " changed to "negative 	 electricity, , , " none note: project gutenberg also has an html version of this file which includes the original illustrations. see -h.htm or -h.zip: (http://www.gutenberg.org/files/ / -h/ -h.htm) or (http://www.gutenberg.org/files/ / -h.zip) idaho agricultural extension service bulletin june, t- electricity for the -h scientist safety uses economy division i -h electric university of idaho college of agriculture how to use this book in fulfilling the goals of the -h electric project for the first and succeeding years the minimum goals for credit in the -h electric project vary according to the -h member's age and the number of years he or she has taken the electric project. for example, if you are a -h member beginning the -h electric project at the age of , you will not be required to earn as many credit points as a -year-old -h member beginning the -h electric project. however, if you are a -year-old in your second year of electricity you must earn as many credit points in that year as a -year-old does in his or her first year. each lesson or goal has been designated a certain number of credit points. these are shown near the title of each lesson or goal. you decide on the lessons you want to study, list them, and add up the credit points. for a full year's -h project credit, the total of your credit points should be at least as many as shown in the following table: examples of reading the table below are as follows: (a) an -year-old member is required to complete credit points the first year, (b) a -year-old is required to complete credit points his first year, (c) a -year-old taking the electric project for the third year must complete credit points that year. we recommend that, if you are taking the -h electric project, you start with the first lesson in the book and go on through to the back of the book in advanced years. but you may skip the less important or less interesting parts so long as you learn the basic lessons. a way to find out whether you know the basic lessons is to read them through and try to answer all questions under the heading "what did you learn." if you can answer these questions you may not wish to spend the time doing the things listed under "what to do." minimum number of credit points required for each year's work in the -h electric project -h member's| -h member's year in -h electric project age | | st year | nd year | rd year | th or | | | | later years - | | | | - | | | | - | | | | & over | | | | this system of credit points makes it possible for you to do the things you want to do with electricity and get credit for them in the -h electric project. -h electric, division i table of contents lesson credit page number title points number how to use this book b- getting acquainted with electricity b- tools for electricians b- rewire a lamp--be a lamp detective b- make a trouble light b- what makes motors run b- taking care of electric motors b- reading the electric meter b- ironing is fun b- let's be friends with electricity b- how electric bells work--for you b- first aid for electrical injuries b- how electricity heats b- mysterious magnetism b- give your appliances and lights a square meal b- you can measure electricity university of idaho college of agriculture agricultural extension service eric b. wilson, extension agricultural engineer published and distributed in furtherance of the acts of may and june , , by the university of idaho extension service, james e. kraus, director; and the u. s. department of agriculture, co-operating. lesson no. b-l credit points getting acquainted with electricity electricity serves you best when you understand how it works and use it properly. as a -h member, you should know about electricity and help to show others the way to obtain its tremendous work-saving benefits as well as how to use it with safety. a good way to think of electricity is to compare it with water. it acts a lot like water. however it is made of tiny parts of atoms called electrons. when there are more than the normal number of electrons in anything, it is said to be negatively charged; when there is a shortage of electrons, it is positively charged. as water flows downhill, "seeking it's level," electrons flow from negative to positive, seeking to "balance" the charge. electrical conductors even if you're never going to repair a lamp or make a chick brooder, you should know about conductors and insulators. this is because you happen to be a fairly good conductor of electricity. electricity will pass easily through you to other conductors--the ground, for instance. when this happens you may get a shock, burn, or serious injury. but it doesn't ever have to happen, if you learn to understand your friend, electricity. silver, copper, iron, aluminum and many other metals are very good conductors. water, acids, and salts are too. electricity passes over or through them very easily. like water pipes, the larger the conductor, the more electricity it can carry. when conductors are too small for the amount of electrons trying to move over them, they get hot, melt, may start fires. that's why wire size is important. electrical insulators insulators are the opposite of conductors. electricity has trouble passing through some materials. rubber, most plastics, dry wood, oils and glass are some of the good insulators. it's the amount and kind of insulation that counts. if it has enough force, electricity can pass through just about anything--even jump gaps! electricity, like water, flows along the easiest paths. it is always trying to get to the ground. the earth attracts it. it stays on the wires unless a person, a wet branch, or some other conductor gives it a path to the ground. do not touch any wire which might be carrying electricity. play it safe if you should touch a "hot" wire accidentally and are standing on a dry piece of wood, the conducting pathway to the ground is not good and the electricity may keep running along its wire. but do not touch some other conductor with another part of your body. this would complete a circuit through your body and would be very dangerous. always make sure there is plenty of good insulation material or plenty of distance between you and anything which might be carrying electricity. remember, too, insulation is of little use when it is wet. dew, mist, rain, condensation, a damp floor can change the whole picture. if you understand electricity and how it acts, you'll be safe enough, because you won't take chances or expose yourself to injury. electrical terms _alternating current_--usually referred to as "ac," alternating current is current which reverses its direction of flow at regular intervals, times a second. _direct current_--"dc" current flows only in one direction. battery current is dc. _ampere_--amperes are units by which the rate of flow of electrical current (electrons) is measured. an ampere is . billion electrons passing one point in a circuit, in one second. this compares with the way the flow of water is measured in gallons per second. _volts_--a volt is a unit to measure the tendency of electrons to move when they are shoved. voltage is the amount of "push" behind the electrons. it's like water pressure in a pipe. home power lines carry volts ( to volts). for appliances such as electric stoves, washers and driers, a second -volt line should be added, giving volts ( to volts). _watts_--watts equal volts times amperes. light bulbs, electric irons and other appliances are usually marked with the voltage they require and the number of watts. _kilowatts_--your electric bill usually reads in kilowatt hours. a kilowatt is watts. a kilowatt hour equals watts used for hour. one kilowatt equals about - / horsepower. a kilowatt is usually indicated by "kw" and a kilowatt hour by "kwh." _circuits_--a closed circuit is one in which the electricity is flowing, lighting a light, running a motor, or some other appliance. the circuit runs all the way from the place the electricity is being generated to your home, through the appliance or light bulb, and back to the generator. circuits are opened and closed by switches. when the circuit is opened, the electricity stops at the switch. before working on a switch, socket, fuse, or any part of the wiring be sure to open the main switch. the main switch is usually at the fuse box or near it. appliances should be disconnected when you work on them. everyone in the family should know where the main switch is so it can be pulled in case of accidents, fire, flood, or windstorm damage. _fuses and circuit breakers_--these are the safety valves of your electrical system. the different electrical circuits in your home are meant to carry only certain amounts of electricity. some carry only amps, others can carry or more. they are marked to show capacity. when a fuse burns out or a circuit breaker opens, look for an overload of lights and appliances on the circuit before you try to replace the fuse or close the circuit breaker. without these safeguards, the overloaded electric line will heat up and may start a fire. even if no fire starts, electricity will be wasted and the homeowner will be paying for electricity that's doing no good. remember: if you ever have to replace a fuse, pull the main switch first. keep a flashlight handy in your house. it seems that fuses usually blow at night, and it doesn't pay to stumble or fumble around electric wires in the dark. what to do: make a circuit board so that you can show others how electricity travels from here to there, and how it behaves under different conditions, make an electric circuit board. _materials needed:_ piece of / " board about " x " l-l/ -volt no. dry cell battery two pieces of bell wire, each " long, one black, one white two -penny box nails ( ") three -penny box nails ( ") two small screws or carpet tacks two -inch rubber bands two miniature sockets with solder terminals two l-l/ -volt flashlight bulbs _tools needed:_ ruler, pencils, hammer, pliers or vise. _making the board:_ . lay out the board with a pencil and ruler as indicated in figure . . bend the three-inch nail as shown in figure , using pliers, vise and hammer. . pound the one-inch nails into the board for a half-inch at points a, c, and d. use the three-inch nail to make a hole a half-inch deep at b. put the crank nail in this hole and pound in a little farther. attach the lamp socket brackets at e and f. stretch the rubber band as in figure . . lay out the electricity path, the circuit (figure ). use the black wire for the positive side of the circuit (the center pole of battery). twist it around the switch crank b, and the center pole of battery. run another piece to the outside terminal of bulb socket at e. run white piece to negative pole of battery from the other terminal at e. [illustration: figure (circuit board)] [illustration: figure (switch)] . close the switch. the rubber band should hold the switch nail tightly against nail at c. does the bulb light? __________ if it doesn't, check the connections. now you have a circuit--a closed circuit when the electricity runs all the way from the positive pole to the negative pole. the black wire is the hot side, the live wire, because it carries the full load of the battery up to the bulb. remember, battery current is direct current, dc. in the case of alternating current, ac, such as most homes and buildings use, the electricity flows in first one direction and then the other. [illustration: figure (closed circuit)] parallel wiring to make this circuit hookup, attach another white wire to the negative pole of battery and a terminal of the second flashlight bulb. run a black wire from the other terminal to the switch terminal at c (figure ). close switch. both bulbs will light. trace the circuit. electricity is going equally to each bulb, the same amount that went to the single bulb. the difference is that the battery will last only half as long. it's like a pail of water with two open spigots. the pail empties twice as fast as it would with just one spigot open. this type of wiring is called parallel wiring. if one bulb is unscrewed, the other will stay lit. [figure (parallel wiring)] series wiring to do this, run the negative wire to one terminal of the second bulb and attach a wire from the other terminal to a terminal of the first bulb. the other terminal connects with the switch at c (figure ). this is series wiring. if one bulb is unscrewed, the other will fail to light because the circuit is broken for both. anything that breaks the circuit has the effect of opening the switch. [illustration: figure (series wiring)] show there is a circuit through the bulb by screwing and unscrewing it. also, "jump" the socket by running the wire from c to the other terminal of the bulb at e while it is unscrewed. bulb at f will light. trace this circuit. suggested demonstrations using the circuit board, you can give many demonstrations of the way electricity flows, works and behaves. water and electricity to help others understand electricity better, draw a water system on an electric circuit board paralleling the circuit. for the battery show a water tank, pipes instead of wires, faucets instead of switches. somewhere on the board paste a comparison of electrical terms with terms used in describing water, such as the following: wire equals pipe volts equal pressure amperes equal rate of flow - gallons per second watts equal pressure times rate of flow switch equals faucet current equals flowing water show how to figure the wattage that a circuit protected by a ampere fuse can handle. do it with actual things or cut-out pictures of light bulbs, irons, toasters, coffee-makers, etc. you know that amperes times volts equal watts. if the voltage is , a amp circuit can handle volts times amps, or watts. the name plates on electric motors indicate the amperage at full load. you can convert this to watts, of course, by multiplying amperage by the line voltage. motors require an additional amount of electricity when they start. you need to allow for this fact, so fuses will not blow or circuits trip when a motor is turned on. you will learn more about this when you study electric motors. for more information your leader has many other sources of information about electricity and demonstrations you can perform. ask him. also, libraries have many books about electricity and its history, which are very interesting and useful. maybe you can find an electrician, someone from your power supplier, or an equipment dealer who will talk to your club on electricity or electrical safety. what did you learn? (underline the correct answers then discuss in the group.) . in a water pipe system water flows. in an electrical circuit (electrons) (atoms) (charges) flow. . electricity or electrons flow (easier) (harder) (about the same) in a conductor than in an insulator. . rubber is a good (conductor) (insulator) (ground). . the most common material used as an electrical conductor is (glass) (silver) (copper). . the unit of electrical pressure or push is the (ampere) (volt) (watt). . the rate of flow of electricity is measured in (gallons) (amperes per minute) (amperes). . volts times amperes equals (watts) (kilowatt hours) (alternating current). . a dry cell battery (stores) (makes) (uses) electrical energy. . in a parallel circuit the electricity has (one) (two or more) (no) paths to travel. . in a series circuit with two bulbs and a switch the bulbs are (brighter) (dimmer) (the same) as when they were in the parallel circuit. lesson no. b- credit points tools for electricians who goeth a borrowing goeth a sorrowing few lend (but fools) their working tools tusser - whenever a job comes up, it saves time and trouble when you have the right tools and they are all where you can find them. electrical work takes some special tools and some everyday tools. if you have ever watched a good electrician at work, you've seen how neatly he stores his tools in a box so every one of them is handy. when a lineman climbs a pole, he has his regular tools in a holster on his belt. special tools are kept in a box in racks in the repair truck, all ready for instant use. wouldn't you like to have electrician's tools all handy, ready for use, and know how to use them properly? basic tools for electrical work _knife_ a good knife with a sharp blade is one of the most useful tools. a camper's or electrician's type knife is probably best because it has other useful parts besides the cutting blades--a screwdriver or punch, for instance. of course, you'll never use the cutting blades as a screwdriver. this knife should be kept clean, dry, sharp, and free from rust. put a little oil on the joints from time to time. remember, "never whittle toward you and you'll never cut yourself." _pliers_ a pair of electrician's pliers should be part of your kit. wrap the handles with plastic insulating tape. even though you're not going to work on "hot" electric lines, it pays to play safe. later on, as you learn more about electricity, you'll want a pair of needle-nose pliers for the fine work. _screwdrivers_ you'll want a screwdriver which has true corners. a to inch plastic handled screwdriver with a narrow blade is best. you'll probably need more than one size to fit the various size screws you'll be turning. screwdrivers are easily damaged if you try to use them as chisels and pry bars, or use them in screw slots which are too large for the blade. you can be hurt by the screwdriver if you try to screw or unscrew things you are holding in your hand. keep your free hand away from the end of the screwdriver. place the work on a bench or where it can be handled easily. _soldering iron_ a good to -watt electric soldering iron will be useful. later on you may want to buy a soldering gun, but unless you are doing a lot of soldering it won't be necessary. a supply of resin-core electrician's solder will be needed. acid-core solder reacts with copper and in time causes a bad splice. _tape_ once it was necessary to use two types of tape on splices--rubber tape with friction tape over it. now there is a plastic tape on the market which takes the place of both and has good insulating quality. it is called electrical tape, or plastic tape, and resists water, oils (which would damage rubber tape), and acids. you'll need a lot of tape in your electrical work, so keep a roll on hand. _other tools and equipment_ as you go along in electrical work, you'll be adding tools and other equipment, such as a trouble light and maybe an ammeter or voltmeter. other tools you'll want to add will be a phillips screwdriver, open end wrenches, a crescent wrench, small hack saw, hand drill and bits. you'll also be using some regular carpenter's tools such as hammers, saws, and so on. unless you use them frequently, you don't need to keep them in your electrical kit. it's a good idea to start acquiring a supply of electrical parts--lengths of wire, fuses, switches, sockets, plugs, and other items that will come in handy. there are parts you can salvage from old lamps, motors, and other equipment. such a collection can be a real treasure chest when you need a part in a hurry. but be sure to throw away all faulty parts. [illustration: figure . completed tool chest.] what to do: build a tool chest to keep your tools always ready for use, a tool chest will be very handy. it's the -h way to work. you'll be surprised how much easier it makes a job when you have your tools, various parts and repair equipment all in one place. you can make the chest (figure ) with a saw, plane, screwdriver, pencil, ruler or carpenter's square, and hammer. _materials you'll need:_ a piece of lumber " by " by feet long. ( " lumber is actually only / " thick--this is the thickness you'll be working with.) small hinges, with wood screws small hasp, with wood screws small handles with wood screws, or one large handle small chain, " to " long some no. penny finishing nails or wood screws about the same length _making the chest:_ . cut your lumber into the following pieces: piece " x " for top piece - / " x - / " for bottom pieces " x - / " for two ends pieces " x " for front and back . lay out pieces as shown in figure . [illustration: figure ] then, set up the two end pieces and nail to bottom section. refer back to figure as you go along to see that box is shaping up as shown. nail the front and back sections to the ends along the bottom. wood screws can be used instead of nails. . lay the top in place and attach hinges to the back side, about two inches in from each end. . attach one part of hasp to the top, and the other part to front board in center. fasten the handles to each end. . attach chain to the top and front so the top will stay open when chain is fully extended. now you can invent your own improvements for your chest. you can paint it, put your name on it, and your club emblem and name if you wish. you can put a rack on the inside of the cover to hold your work sheets and other booklets and materials. you can install special slots or straps to hold each tool in its place along the sides of the box. maybe you will want to put some partitions in the box to separate various electrical equipment such as wires, fuses, switches, and plugs. _a working kit_ an accessory which you may want to add to your tool chest is an apron or holster to wear when you are moving around on the job. an apron can be made of a size of cloth about by inches. it should be folded up from the bottom, and sewn to fit the number and size of tools you have. figure shows such an apron. [illustration: figure . apron.] you can make a lineman's holster in the same way, using plastic or soft leather. merely make belt loops by cutting on the dotted lines. a snap fastener will hold the flap over the tools so they won't fall out. [illustration: figure . lineman's holster.] demonstrations you can give show and tell others the proper handling, care and use of tools. show and tell how to build an electrician's tool kit. for further information ask your power supplier or an electrician to tell the club about the various tools of the electrician's trade and demonstrate them. ask your leader how to get exhibit material or information about electrical tools and their use and then tell the club about them. lesson no. b- credit points rewire a lamp--be a lamp detective [illustration: the line-up of lamp suspects] one of the duties of a law officer is to prevent crime. it's that way with the lamp detective. you can become one. in the average home there are lamps about to commit the crime of shocking people, starting fires, and stealing electricity. some are refusing to do their job well and some are no-goods, sitting in closets or attics, doing nothing. you can put these lamps to working again safely and well. become the lamp expert in your family. what's in a lamp? a lamp gives light for comfortable and convenient use in the home. it consists normally of a stand, switch, cord, lampshade holder, and shade. some lamps have diffusing bowls which reduce glare and shadows. the most common fault found in an old lamp is in the cord, but sometimes the switch or the wiring in the lamp is bad. look over all the lamps in your home and find the ones needing to be fixed. what to do--rewire a lamp somewhere around your house you can probably find a lamp that is no longer used or needs repairing. you can make it useful again and at the same time learn how to wire a lamp. _materials needed_: tools: pocket knife, small or medium screwdriver, and pliers (electrician type is best). _new lamp cord_: for each lamp to be rewired, you'll need feet of cord plus the length of wire within the lamp stand. lamp cord wire comes in two sizes, no. and no. awg (american wire gauge). no. is smaller than no. , but is adequate for most lamps. cords are made with surface coverings of several different materials: braided cotton, rayon or silk, and molded rubber or plastic. braided cord is decorative, but rubber or plastic is easier to work with and is usually more desirable. _switch_: if the switch is bad, get a new one. socket switches are made with push-through, turn-knob, or pull-chain controls. the pull-chain type is seldom used on modern table or floor lamps. your lamp may have a separate push-switch in the base. in this case, get the same kind for replacement. some switches are " -circuit" switches for use with high, medium, and low-light bulbs. _plug_: plugs are made of various materials, mostly hard rubber or molded plastic. some have a shank or handle for better grasping. this type is more desirable. the plug on the old cord may be good, and if so, may be used on the new cord. how to do it: . if the plug on the old cord is good and you plan to use it, remove it from the old cord. . measure and cut a new lamp cord equal to the length of the cord within the lamp, plus feet. . pass one end of the new cord through the center of the plug. strip inches of the fabric insulation off cord, or in case of a rubber cord, split cord back two inches. be sure no bare wire shows in long split section (figure ). . use knife to strip insulation off wire for / " on end of each cord. be careful. don't cut yourself. don't cut wires. use a light touch, slope the knifeblade and slice with knife edge away from you (figure ). [illustration: figure (ready to wire plug)] . twist exposed strands of each wire tightly to make a good conductor, and place each conductor around its proper terminal in the direction in which the screw tightens (figure ). . tighten screws on terminal posts. pull cord until slack is out. lay aside until ready to attach to lamp. [illustration: figure (attaching cord to plug)] . remove lamp shade, shade-holder, bulb, and diffusing bowl, if there is one. . separate the metal shell of socket from its cap by pressing on shell at place marked "press," and pull socket from cap. . pull on socket body to get some slack in lamp cord. loosen screws and detach cord. pull cord out through base of lamp. you can splice new cord to the old one and use the latter to "string" the new wire. . pass the new cord up through the lamp base and socket cap, tie a simple half-hitch knot in the cord to prevent strain on the terminals, and attach wires to the terminals on the socket (figure ). if there is likely to be any strain on cord, use an underwriters' knot. twist strands and attach wire in direction in which screw tightens. . pull slack out of cord in lamp so that socket rests in socket cap, replace shell and reconnect cap. be sure the fiber insulator is in the shell. you'll feel or hear a click when the notches in shell are locked to the projections in the cap. . replace bulb, inspect carefully, and test. (in floor lamps where the cord runs through the center post and out under the base, the cord will last longer if it is fastened with tape so it doesn't rub edge of lamp base when lamp is moved.) . if the lamp has a porcelain socket, simply disconnect the wires at the terminals, remove the old wire and connect the new one. [illustration: figure (socket and switch assembly)] what did you learn? underline correct answers then discuss in the group. (there may be more than one correct answer.) . the part of the lamp that usually wears out first is (the socket) (the cord) (the plug). . lamps that waste electricity are those which have (bad wiring) (frayed cords) (dirty shades or bulb). . to unplug a lamp you should grasp (cord) (plug) firmly and pull. . wire in lamp cord usually comes in sizes or . size is the smaller (true) (false). . in fastening wire around a terminal post it should go around in a (clockwise) (counter-clockwise) direction. . when the switch on a lamp is turned off, the electricity only goes as far as (the wall plug) (the switch). . an underwriters' knot should be used (only when there is room for it in the plug) (whenever there is likely to be strain on the cord, even if you have to replace the plug with a larger one). suggested demonstrations show how to inspect a lamp and its cord. you might tie tags on the cord and lamp at points of danger or failure--at the plug, wear points next to lamp base, bad sockets. demonstrate the process of repairing a lamp cord, socket and plug. make a board display of the parts of the lamp socket showing cord attached. make a display of the types of lamp cords and plugs in common use. using two lamps, one with clean bulb and shade, the other dusty, show how the former gives more light. for more information lamps have an interesting history. look it up in your local library. ask someone from your power supplier or electric dealer to talk to the club about the different kinds of lamps. your leader has or can get additional information on lamps, if you wish. [illustration] what did you exhibit what did you demonstrate lesson no. b- credit points make a trouble light. a handy piece of equipment in the home and on the farm is a heavy-duty extension cord with a shielded light and a side outlet on it. when you want to work on the car or tractor in the yard at night, the trouble light is better than a flashlight. you can use it both for light and as an extension cord. it is safer than matches or a lantern, especially around the garage or barn. it is easy to make a trouble light, and it gives you good practice in electrical work. of course you can buy one, but you wouldn't have the fun of making it nor would it suit your needs. trouble lights are not for permanent use--they're for emergency use and to provide light or electricity in places where they are seldom needed. when you find a trouble light being used as permanent wiring, that's the place to install an outlet. what size cord? choose the right kind of cord. what length will be best for your various uses? a cord too long may be bothersome to use and store. what will be the heaviest load you are likely to put on the cord, in amperes? check appliances you may want to connect to it. no. wire can carry amperes safely for a distance of feet, while no. can carry only up to amperes for a distance of feet. you'll want a "hard service" cord, called s, st, or so-type cord by electricians. junior hard service cords, known as sj, sjt, or sjo, are fine for lighter duty. cord, plug and guard a rubber-handled socket should be used for safety and to withstand hard knocks. it should have a switch on it, preferably a push switch in a recess in the handle. the connector or attachment plug should be of rubber or solid plastic and have a metal cord grip fastened to it. this grip will hold the cord firmly and prevent strain on the terminal connections. [illustration: finished trouble light] get a good lamp guard. if the wire is too light, it may bend and break the bulb when hit or dropped. for the lamp itself, get a rough service lamp. an ordinary lamp won't last long with rough usage. how to make the trouble light _tools needed:_ your -h electrician's kit or screwdriver, knife and soldering iron _materials needed:_ . about feet of -wire, no. heavy duty (hard service) . a rubber-handled socket with switch and a side outlet . a shielded lamp guard [illustration: materials needed] . a good connector plug cap, preferably with a clamp-type grip for the cord . a rough service lamp bulb . solder and flux _steps to take:_ . remove about inches of the outer covering of cord at one end. . separate the wires and cut away the filler material. . remove / inch of the conductor insulation from the end of each wire and tightly twist the strands together to form a firm conductor. be careful not to cut any of the fine wires. ends may be soldered. . slide the plug in position on the cord. . if there is no cord grip, tie the underwriters' knot (figure ). if there isn't room enough, make an "s" loop by passing the wires around the prongs before fastening them to the terminal screws as explained in the next step. . loop the bare part of the wire around the screw in the direction the screw is turned to tighten (clockwise direction). this will prevent the wires from being forced out from under the head of the screw as it is tightened. now repeat with the second wire, wrapping it around the other prong of the plug. _connecting the socket._ . separate the parts of rubber-handled socket (figure ). . prepare the other end of the cord as in steps , , and above. . insert the cord through the rubber handle and socket guard. . tie the holding knot (underwriters' knot) as explained in step . . connect wires to terminal screws and assemble the rubber-handled socket. . screw in the rough service lamp and test your cord. . put the shielded lamp guard on the socket and tighten the holding clamp until it is firmly in place. you are now ready to use or demonstrate your trouble light. . after you've made your trouble light, decide on a good place to keep it where it will be handy for use. loop it carefully and hang it over a wooden dowel rather than a nail. it will last longer. [illustration: figure tying an underwriter's knot] [illustration: figure disassembled light] what did you learn? (underline correct answer) . a junior hard service cord is known as an (so-type) (sjo-type) cord. . you disconnect a cord by (jerking it from the socket) (grasping plug and pulling it out). . brass sockets are unsafe because (they break too easily) (the exposed metal can cause short circuits). . rubber-covered cord is safer for emergency cords than fabric because (it will stretch) (it will insulate and protect the wires inside). . in a trouble light (any kind of bulb will do) (a rough service bulb is best). ideas for demonstrations and exhibits . show how to make your trouble light and a method of storing it. . show a safe trouble light, and an unsafe trouble light with danger points marked. . show cutaway pieces of different types of cord. for more information ask your power supplier, county highway engineer, police official or leader to tell you about various types of portable emergency lights and their uses. lesson no. b- credit points what makes motors run what makes an electric motor run? can you make an electric motor that will run? certainly you can, and by doing so you'll learn why it runs. it won't be mysterious any more and you'll be ahead of all the millions of people who use motors every day and never know why or how the motor converts electrical energy into useful power. [illustration] motors are magnets you know how one end of a compass needle always points to north. no matter how you turn the compass, the same end of the needle always swings to the north. the earth itself and that small compass are both magnets (figure ). each has a north pole and a south pole. around the poles of each there are magnetic fields, invisible lines of force that attract and repel. [illustration: figure . the same end of the compass needle always points to the earth's magnetic north pole.] the n poles _repel_ each other and so do the s poles. the n and s poles _attract_ each other. in other words, opposite poles attract; poles that are alike repel each other. lay bar magnets on a table side-by-side. if both n poles are at one end, they'll repel each other and almost flip around until there's a n pole lying next to a s pole (figure ). [illustration: figure . small bar magnets laid side by side move so that the north pole of one is near the south pole of the other.] now suppose we place one of the bar magnets on the table. the other, we'll fix on a pivot so it can spin around. this one we'll move so its n pole almost touches the fixed magnet's n pole. as soon as we release it, the movable magnet will spin around so its s pole will be near the n pole of the stationary magnet. that's an electric motor--almost. [illustration: figure . a movable bar magnet pivots so its south pole is near the north pole of a stationary magnet.] it's not quite a motor because the rotating magnet will just move as far as it has to in order to get the opposite poles together. you might be able to cause the movable bar magnet to make turn after turn. you could do this by turning the fixed magnet quickly end for end. this wouldn't be very practical as a motor. we can improve it if we could change the pole on one end of the rotating magnet just as soon as it reaches the attracting pole, it could make a complete circle. in doing that, the pole at the near end of the rotating magnet would be repelled by the stationary magnet and pushed away. as soon as the opposite end of the rotating magnet would come into the magnetic field, it would be drawn to the stationary magnet. in order to keep the "motor" running, we would have to constantly change the poles at each end on every half revolution. we need an electromagnet we can't reverse the poles on simple bar magnets, but we can on _electromagnets_. we can make one by wrapping a wire several times around an iron core to form a coil. this magnet will also have a n and a s pole when connected to electrical current. the big difference is that the poles can be changed instantly by reversing the current in the wire. switching poles automatically the rotating electromagnet will have to be connected to the wires through which we pass the current. since it's rotating on a center shaft, we can't have a solid connection. instead we have to extend the wires from the coil out along the shaft and let the electric contact be made with brushes which touch the wires along the shaft. [illustration: figure . a rotating electromagnet changes poles as contacts are made first one way, then the other.] this is a simple way to reverse the current in the coil of the electromagnet. increasing efficiency instead of using only one pole of a stationary magnet, we can use both. this is done by shaping the stationary magnet around the path of the rotating electromagnet. this way we have the benefit of the attracting and repelling forces from both poles. the effect is doubled. we can also wrap wires around this circular iron and make an electromagnet of it. but when we wire this magnet we use no brushes because we want the current to flow in one direction only. the stationary electromagnet is called the _field_. the rotating electromagnet is the _armature_. what to do: make a motor _tools needed:_ pocket knife, hammer, vise (or pairs of pliers). _materials needed_: roll of no. enameled wire roll of electrician's tape - " ( -penny) nails - - / " ( -penny) nails - " brads ( penny) wood board for motor base staples or small brads tacks - volt dry cell batteries (or a volt transformer). step no. -armature wrap about - / " of a " nail with two layers of tape. this will be the shaft. the iron core will be made of two pairs of - / " nails. wrap tape around each pair with heads and points alternated. center both pairs on each side of the shaft. place them about " from the head of the shaft nail. wrap them together with two layers of tape from tip to tip. start at the shaft and wind no. enameled wire to one end and back. then do the same on the other end. always wind in the same direction. leave " of spare wire at start and finish. step no. -commutator scrape all insulation off the ends of the wire. bend the bare ends back and forth as shown. lay them flat over the taped shaft-one on each side of the shaft. hold the commutator down with narrow strips of tape. wrap tightly near the core and at the opposite end. step no. -field make the core by bending two " nails in the middle at right angles. space the heads about " apart to form a horseshoe. wrap together with two layers of tape. wind about turns of wire around the center. leave " of spare wire at start and finish. attach to wood base with staples at each end of the wire. small brads, bent over, will do just as well. step no. --armature supports and brushes scrape the insulation from the ends of two " pieces of wire. tack them to the base and bend them as shown to make brushes. drive two pairs of " brads into the base about - / " apart and in a line midway between the field poles. wrap wire around the supports to form armature bearings. scrape insulation off ends of wire from the field. connect one end to a brush wire. _assemble as shown_ adjust the position of commutator and tension of brushes against it for best operation. take the armature off the motor and connect the commutator wires to a dry cell battery. test the polarity of each end of the armature with a compass. switch the connections on the commutator and test again. see how the compass needle changes direction? with the armature still off, connect the field coil directly to the dry cell. test the polarity of each end of the field with the compass. how can you reverse the polarity? try it. it's easy. reassemble the motor again and start it. push the field poles slightly out of alignment with the turning armature. what happens to the motor's speed? can you tell why? this time, push the field poles completely out of the way. test the polarity of the armature as you slowly turn it by hand. do you see what happens and why it does? try to reverse the direction of rotation of your motor by reversing the connections at the battery. what happens? can you explain why? demonstrations you can give make a display board showing the parts of the toy motor and explain how each part works compared with the parts of a commercial motor. for further information there are several other types of toy motors you can build. your club leader or power supplier can help you find information about them. . did your toy motor run? . did your motor speed up or slow down when you pushed the field poles out of line? why? . what happens to the magnetic polarity of the armature when you turn it slowly by hand and check it with a compass? . how can you reverse the direction of rotation of your toy motor? is there another way too? what is it? lesson no. b- credit points taking care of electric motors through the magic of electric motors, much of our work is done faster and better at lower cost than we could do it without the help of the electric motor. people who use motors and treat them properly have much more time for other work and for leisure time activities. a / -horsepower motor running quietly and steadily hour after hour will do the work of one man, and operate all day for about cents without tiring. on many jobs it will work without "supervision", turning on and off automatically, as required. it does this on water pumps, in heating and cooling units, and on fans and similar appliances. all that a motor needs to do its work is electricity and a little care. let's see what you can do to give proper care to motors in your home and on your farm. you'll need a light oil (sae ) for motors of less than one horsepower and a slightly heavier oil (sae ) for larger motors. see if you need grease for cups which may be on large motors. if so, be sure you use ball-bearing grease and not ordinary cup grease. cotton waste or clean rags will be needed for wiping off the motors, and a tire pump or vacuum cleaner for blowing out the dust or dirt. [illustration: some motors have instructions for oiling on the name-plate.] what to do . first, make a list of all the electric motors that work for your home. you may wish to make a separate list for your farm buildings. you'll probably be surprised at how many there are. don't forget the sewing machine, the refrigerator, the freezer, the vacuum cleaner and other small but important motors. don't touch any motor that is running. disconnect them before you touch them. . make a motor service chart with columns headed: use, location, horsepower, volts, amperes, service required, date serviced and what was done. (see sample) then list all the motors that require any servicing. some will have the instructions on the motor or appliance; the instruction booklet that came with the motor or appliance will also tell what servicing is required. _step ._ plan the job. start with the motors in the home. then you can care for the motors on the farm. _step ._ be sure that any motor on which you are going to work is disconnected. then wipe the outside case clean with a cloth. if the motor has openings in the end, use a vacuum cleaner to suck out dust, dirt or chaff. a tire pump may also be used to blow out this dirt. if you use compressed air, be sure the pressure is not high as it may damage wiring inside the motor. dust-proof motors should be used in dusty or dirty places. _step ._ [illustration: if there are oil holes, oil according to the manufacturer's instructions.] if there are no instructions, remember a little oil goes a long way as far as motors are concerned. motors of less than one horsepower require only or drops (not squirts) of oil every or months if the motor is used frequently. too much oil can damage the motor. it spoils the insulation. if there are no oil holes or grease cups on the motor, it is probably lubricated by means of grease sealed in the bearings at the factory, or it may use greaseless bearings, and does not need to be oiled or greased periodically. indicate on your chart all motors which need periodic care and see that it is given according to schedule. wipe away any excess oil or grease. be sure oil holes are capped or covered. _step ._ reconnect motor and run for a moment. _step ._ record on the chart the date you serviced the motor and what was done. what did you learn? how many motors are there in your home? ______ on the farm? ______ how many motors need regular oiling or grease? ______ how many are less than one-horsepower? ______ sae oil ______ is used to oil motors up to / horsepower. how much oil?______ sae oil______ is used for larger motors. demonstrations you can give . show how to clean a small motor. . explain proper lubrication of motors. . using the chart prepared in this work sheet, give a talk about the motors that work for you-the job each one does, which ones need oil or grease, which need no attention, and why, etc. . use a homemade toy motor to explain "what makes motors run." . show proper way to replace worn cord on a small motor. for further information ask your county extension agent or -h leader for more literature on motors. they can help you obtain a film or a speaker such as a power supplier, a local electric dealer, or electrical contractor to discuss motors. also visit your public library and see a science teacher for more information on motors. electric motors service chart sample use a table like the following to list the motors around your farm and home. ---------------------------------------------------------------------- motor | location |h.p.|volt |amp |service |date serviced and use | | | | |needed |what was done ---------------------------------------------------------------------- food | kitchen | / | | . |clean & oil; | / -cleaned mixer | | | | |cord needs |w/cloth. repair | | | | |repair |oiled w/# oil; | | | | | |repaired cord ---------------------------------------------------------------------- tool | farm | / | | . |clear, oiling; | / -cleaned grinder | shop | | | |have switch |w/vacuum oiled # | | | | |have switch |oil. / -had | | | | |repaired |switch repaired ---------------------------------------------------------------------- pump |pump | / | | . |oiling, | / -cleaned |house | | | |cleaning |w/tire pump; | | | | | |oiled w/ oil ---------------------------------------------------------------------- lesson no. b- credit points reading the electric meter there is no question but what electricity is one of the lowest cost services in the home and on the farm. a few pennies worth of electricity will provide the power to run machines that take the place of a man or of several men working all day. however, we all like to know what things cost. sometime you may have to decide between different methods--man, horse, gasoline engine or electric motor power. then you'll want to know how to figure the cost of electricity, as well as the cost of the original equipment. first of all, you should know how to read an electric meter. reading a meter electric meters read in kilowatt hours, just as a water meter reads in gallons and a gas meter in cubic feet. a kilowatt hour is the electrical energy consumed by watts of electricity used for one hour. ten -watt light bulbs burning for one hour would use one kilowatt-hour--one kwh. [illustration: figure . some meters give the reading directly, like the mileage total on a speedometer.] some meters are read directly, as shown in figure . the more common type has four dials which are read from right to left--just the opposite from the way things are usually read. the hand on the extreme right turns clockwise, the next hand turns counter-clockwise, the next clockwise; the last hand on the left turns counter-clockwise. the first dial on the right can register up to kilowatt-hours; the second up to kwh; the third, to kwh; the fourth, to , kwh. after that, the meter starts over again. to take a reading you must read all four dials of the meter, from right to left. [illustration: figure . meter dials are read from right to left.] to read each dial, you use the number last passed by the dial hand. this may not be nearest the hand. for instance, if the pointer has passed and is almost on , you read it as . write down the figures in the same order you read the dial, from right to left. practice reading the meters shown in figure on the following page. what's your electric bill? meters aren't set back each month when the meter reader comes around. the difference in the readings from one month to the next shows how many kilowatt-hours have been used. if you know your electric rates, you can figure your bill by yourself. your power supplier will furnish you with a rate schedule on request. [illustration: figure . see if you can read the above correctly. the answers are shown in a box on the next page.] it will be interesting to you to find out how much it costs to operate the various electric appliances in your home. a sample rate schedule is shown in figure . [illustration: figure . sample rate schedule. note that as the use of electricity increases, the average cost per kwh is reduced.] estimating operating costs to find the cost of operating any single appliance, three steps are necessary: . learn the wattage of the appliance. . estimate how many hours the appliance is used. . find its operating cost. _to find wattage:_ watts, you know, are the measure of electrical power. they are the product of voltage (pressure) times amperes (rate of flow). volts times amps equals watts. the nameplate on the appliance will give the voltage required for proper operation as well as either amperage or watts. if it gives wattage, you have the information you want. otherwise you must multiply volts times amps to get the wattage. when voltage is given as - , use as your voltage. volts is nominal today. _how much will you use?_ now that you know the wattage of the appliance, multiply this figure by number of hours the equipment operates in one day. divide this by to get the kwh. now multiply the result by the number of days the appliance is used each month. this tells you the number of kwh used by the appliance during the month. |---------------------------------------------| | | |example no. | |_yard light:_ -watt lamp | | | |amount of use: hours per night. | | | |multiply lamp wattage times hours of use | |per night to get watt-hours per night. | | | | times = watt-hours per night. | | | |divide watt-hours by to get kwh per | |night. | | | | divided by = . kwh per night. | | | |multiply kwh per night times to get kwh | |per month. | | | |. times = kwh per month. | | | |if the yard light is used hours per night, | |it consumes kwh per month. | |---------------------------------------------| example no. _coffee maker_: volts, watts (from nameplate) amount of use: / hour per day. multiply wattage of coffee maker times hours of use per day to get watt-hours per day. times / hour = watt-hours per day. divide watt-hours by to get kwh per day. divided by = . kwh per day. multiply kwh per day times to get kwh per month. . times = . kwh per month. if the coffee maker is used l/ hour daily, it consumes . kwh per month. _calculate operating cost per month_ now that you know the number of kilowatt hours an appliance uses, go to your rate schedule and your electric bill to see what the average kwh costs. find the average cost of kwh by dividing the amount of your bill by the total number of kwh used in a month. _example_: kwh used. $ . total monthly bill average cost per kwh equals $ . divided by kwh- - / cents per kwh. therefore, the cost of operating the coffee maker for a month would be - / cents times . kwh-- . or cents. cost of operating the yard light would have been . or cents a month. (a) (b) (c) correct answers to the meter readings shown on the preceding page. adding low cost helpers you can see, by looking at your rate schedule, that the average cost per kwh gets lower as you use more electricity. to find the cost of operating additional electrical equipment, the cost per kilowatt hour is found from the last "step" in the bill--the lowest cost per kwh of the electricity you're now using. sometimes power suppliers give special rates for such equipment as electric water heaters. what to do: find the cost of operating electrical equipment make and fill in the blanks of a chart showing the electrical equipment you have and the operating costs per month. make a chart for the home (refer to chart one). show the probable operating cost of equipment you might add to what you now have. demonstrations you can give show how to read a meter, making one with plywood or cardboard. dials can be painted on the main board. arrows can be attached so they will revolve to give different readings. show how to find the wattage of various types of equipment. show how to figure the cost of the average kwh in a home. for further information your leader can get additional material for you or you may want to have someone from your power supplier talk to your club, telling about meters, how they work and how they are regularly checked for accuracy. chart one-the home column no. . . . . item wattage hours kwh per cost per rating used month month per (col. x )/ (col. x av. remarks month kwh cost) electric iron . stove . . (special rate) lesson no. b- credit points ironing is fun with the modern hand iron when you are getting ready to go to school or to a party, it probably gives you a good feeling to put on a clean, freshly-ironed skirt, blouse or dress. but did you ever think about the electric iron that helps so much to give you that well-dressed feeling? when you were younger, you may have had a play iron and pretended to iron your doll's dresses. now you are old enough to learn about real irons--the different kinds of irons, how the iron heats, the kind of cord needed, the type of outlet necessary, how to use safety rules when you iron, and even how to help with the ironing. important things to know there are many different irons, but the two kinds most important for you to know about now are the regular dry iron and the combination steam-and-dry iron. [illustration: the thermostat keeps the iron at an even temperature.] it isn't weight alone that makes an iron do its job, but the heat of the iron. the heat is given off in the sole plate. the automatic iron has what is called a _thermostatic_ control which holds the temperature of the iron at the heat you want. some clothes need to be ironed with a very hot iron, while others need only to be pressed lightly with a cool iron. the thermostat keeps the iron at an even temperature after you set it for the heat you want. the thermostat is the heart of the iron. take a look at the iron used in your home. it isn't heavy to lift, and has a handle that fits your hand easily. it looks graceful and has a smooth bottom, called the sole plate. and it may have a narrow, pointed tip which is helpful in ironing pleats, corners and gathers. [illustration: your iron has a smooth bottom called the sole plate.] the iron and safety if you are going to learn to do some ironing yourself, the most important thing for you to remember is safety. you should read all about the iron first in the instructions which came with it. never use an iron carelessly. remember the safety rules: . an iron should never be left even for a few minutes without being disconnected. turn off by removing the plug from the outlet, or by turning the control lever to "off." [illustration: take hold of the plug--not the cord--when you disconnect it from the outlet.] . let the iron cool before putting it away. . wrap the cord carefully around the iron after it is cold. . always stand the iron where it will not fall off on a child or pet or your own toes. what to do: learn about your iron materials needed: an automatic iron, some old play clothes, towels, napkins or handkerchiefs, and an ironing board. steps to take: . watch an experienced person iron. . ask questions about what clothes need to be sprinkled. . study the thermostat settings on the dial or indicator. [illustration: most irons have a dial to set for the proper heat for different fabrics.] . ask about the kind of fabric each piece of clothing is--cotton, linen, silk, nylon, etc.--and why the iron should be at high heat for some, cooler for others. . set the thermostat for the amount of heat needed, and with an older person watching you, iron some handkerchiefs, napkins, bath towels, and a pair of play shorts or blue jeans. . during a month iron some of these articles for your family, keeping a record of how many you do and what they were. . take care of your iron. be responsible for storing it. +--------+-----------------+-------------------+---------------------+ | | no. | | store iron properly | | date | articles ironed | type of article | (check) | +--------+-----------------+-------------------+---------------------+ | | | | | +--------+-----------------+-------------------+---------------------+ | | | | | +--------+-----------------+-------------------+---------------------+ ironing is fun . i (use) (do not use) an adjustable ironing board at home. if i do, i adjust it to the height that just clears my knees easily as i sit in a comfortable chair. yes no . there are three kinds of irons usually used--dry iron, steam iron or a combination steam or dry iron. i use a ---- iron. . i (have) (do not have) the instruction book. (if you do, read about the iron.) i know the iron's parts by their correct names. they are----. . i disconnect the iron if i leave it even for only a minute. this is a safety measure as fires have been known to start from irons left connected. yes no . i take hold of the plug--not the cord--when disconnecting the iron. yes no . i wait until the iron is cold before wrapping the cord around the handle and storing the iron because----. . most irons have a thermostatic control. the iron i am using has settings for----. . the purpose of the thermostat is----. . these fabrics need high temperature.---- these fabrics need medium temperature.---- these fabrics need low temperature.---- . these fabrics need sprinkling.---- . the heat and smoothness of the sole plate smoothes the wrinkles. pushing down on the handle or moving the iron rapidly only makes ironing hard work. i will iron slowly and steadily arranging and moving the garment with the left hand while guiding the iron with the right hand. (or the other way for the left handed.) yes no . i have watched an experienced person iron. yes no . i have practiced on handkerchiefs, napkins and pillow cases. . here is my record of ironing for one month. month ---- your name ---- date i have ironed: ---------+------------------------------------------------ | | demonstrations you can give . show a dry iron and a steam-and-dry iron. tell the difference between them and when each is to be used. . display garments that look nice because they have been ironed properly, and those that have been ironed improperly. explain about the heat, thermostat, type of iron and why results differ. for more information at a club meeting ask a parent to give a demonstration of ironing different articles. some power suppliers or dealers have people who will demonstrate the proper way to iron, and how to care for irons. lesson no. b- credit points let's be friends with electricity plan a hazard hunt electricity can be your important lifelong friend and helper, so you will want to know all you can about it and how to treat it properly. however, careless and improper use of electricity can do a lot of harm. used properly, and treated with respect, electricity can do wonderful things to help you every day in many ways. for safe and proper use of electricity, all wiring, fittings, insulation, cords and plugs must be in good condition. you can be a detective and track down defects in any such type of electrical equipment that you may be using in your home or on your farm. when you find anything that is wrong, and know where it is, and know what to do about it, you can very likely correct the condition yourself, such as replacing a worn extension cord with a new one. if you find defects in permanent wiring, or some places where wires are bare or terminals are needed, you should tell your parents about them. safety first, remember, should always be on your mind when working with anything electrical. what to do: _ . have a hazard hunt_ go on a hazard hunt to see how many electrical hazards you can find. look for defects such as broken insulation, worn cords, splices that are not properly soldered and taped, loose connections, or switches that aren't working properly. there are many ways to have a hazard hunt. choose the method that will be the most fun. use the hazard hunt guide in this outline to check your home, and other buildings. maybe you'll want to have a friend help check your home, then you help him check his. or, why not give each member of your family a hazard hunt guide and have a contest? parents may want to team up against you and other younger members of your family to see which team can find the most electrical hazards in some set time--say minutes. have a hazard hunt committee in your club check all member's homes and buildings and report its findings at the next club meeting. _to make it more fun_ . put a hazard tag, like the one shown, (figure ) by each hazard that is found. leave it until the hazard is corrected. have another contest to see which member of the family corrects the most hazards. [illustration: figure ] . report on your hazard hunt at the next club meeting. tell about the hazards found, and what you have done or plan to do about them. . suggest that the entire club have an electric hazard hunt at your club meeting places or any community building. this could be part of one meeting. . have a contest between two teams in the club to see which team can get the most homes in your community checked by the hazard hunt guide. losers could give a party for the winners. _ . get others interested_ promote a community electric hazard hunt. enlist the support of power suppliers, electric supply and equipment dealers, schools, newspapers, radio and television stations. _what to look for_ make a complete tour of your home and other buildings and see how many hazards you can locate. when you find a hazard, put a tag near it to mark it. safety tips put hazard tags _near_ the hazard but _not_ directly on broken or frayed wires, insulators, fittings, or other wiring equipment. do not touch them either. badly-frayed wires should be disconnected immediately from the power supply. in this way, you will not expose yourself to shock by accidentally touching an exposed live wire that may be carrying current. -h electric hazard hunt guide _wiring and protective devices_ . cable or conduit splices not in boxes---- . cable or conduit not securely clamped in boxes---- . conduit or armored cable not properly grounded---- . cracked or broken insulators (figure )---- . wire not completely covered with insulation---- . worn insulation on wire---- [illustration: figure ] . old unused wiring not yet removed---- . outlets, junction and switch boxes not securely fastened and covers not in place---- . switches not working properly (sparks fly as switch is flipped) (figure )---- . fuses not of proper ampere rating for circuit---- . extension cord used in place of permanent wiring---- . pull chain socket without an insulating link in the chain---- . pull chain socket near plumbing fixtures or where hands may be wet or one may stand in water---- [illustration: figure ] . no moisture-proof cords for outside weather conditions or heavy rubber cords for motors and motor driven appliances _lighting_ . fixtures in farm buildings installed so that they might be easily damaged . lights in haymows and other dusty locations not protected by dustproof globes . outside sockets not waterproof . heat lamps not properly supported by non-current carrying wire, chains, or brackets (figure ) . light bulbs not frosted, shaded, or placed so that light is diffused to prevent glare [illustration: figure ] _auxiliary wiring_ . outlets overloaded--in other words, "octopus wiring" . extension cords placed under rugs . extension cords run through doorways (figure ) [illustration: figure ] . extension cords or lamp cords should use underwriters' knot (figure ) [illustration: figure ] . plug connections fuzzy (figure ) [illustration: figure ] . extension cords run over heaters or radiators . extension cords, or appliance or lamp cords, worn or frayed . heating appliances without regular asbestos covered wire . open sockets or outlets where a baby or small child might stick a finger or metal toy . broken plugs (figure )---- . loose prongs on appliance or lamps plugs---- [illustration: figure ] how many hazards did you find? make a chart listing the hazards, their locations and what you did about them. make your own chart and list what you find. demonstrations you can give show and tell others how to have a hazard hunt. for further information check with your leader, then ask your power supplier or a local electrician to tell you about safe electrical wiring, connections and fixtures. +-------------------------+-------------+------------------------+ |hazard | location |what i did | +-------------------------+-------------+------------------------+ |_loose prong on lamp plug|living room |replaced with new plug_ | +-------------------------+-------------+------------------------+ |_cracked insultor on |back of house|notified power _ | |_service wire in house | |supplier_ | +-------------------------+-------------+------------------------+ |_conduit not securely |basement by |notified parents_ | |_clamped to box |fuse box_ | | +-------------------------+-------------+------------------------+ |_extension cord, old and |basement, by |replaced with new_ | |_worn |washing |rubber-covered one and_ | | |_machine |protected it from _ | | | |_water_ | +-------------------------+-------------+------------------------+ lesson no. b- credit points how electric bells work--for you when was the last time you wanted to get a simple message like "you're wanted on the telephone," "there's someone here to see you," or "there's a car in the driveway," to someone around your place? did you have to walk or run some distance and perhaps shout, too, to be heard by the other person? perhaps you had to stop some other work, or interrupt your favorite kind of fun, to do this bit of messenger work. if the nature of the message is like one of those mentioned, and the number of people in hearing is not too great, then perhaps you can use bells or buzzers or both to do some of your messenger work for you. even though a bell or a buzzer can't talk, it can convey a message. what to do . learn how bells and buzzers work, and learn about the many different kinds. . plan and install a bell system for your home or farm. bells and buzzers can tell a lot electric bells and buzzers use the same basic principle as the telegraph system, invented by samuel morse in . although not as important today as it was before radio, telephone, and teletype became common, the telegraph is still in use. bells and buzzers, however, are very common and have many uses. they are most often seen in the form of doorbells, and rare is the new home that does not have one or more. service stations have bell systems to let the operator know that a car is waiting at the gas pumps. a clock signal reminds the homemaker when the cooking time is completed. children are called to and released from school classes by means of bells and buzzers. also, various alarms employing bells and buzzers warn us when it's time to get up, or even that the place is on fire, or that a burglar is trying to break in! let's find out how bells and buzzers work, what different kinds there are, the different ways you can control them, and how you can put them to work for you. you'll find that buzzers and bells can help you with your -h projects, and with the proper controls, can be your eyes and voice in a dozen places at once. why they buzz or ring--electromagnetism if we were to look at an electric bell with the cover off, we'd find that it would be very much like figure . a push on the button, which is just a switch that is normally held "open" or off by means of a spring, sends the current from the battery or transformer through the circuit. [illustration: figure ] you will see that the current passes first through two small coils of wire, and each coil has at its center a piece of soft iron called the core. when the current is on, the core becomes magnetized and attracts another piece of iron called the armature with its clapper attached. this action rings the bell, but it also breaks the current by pulling the spring away from the screw on its return to the power supply. with the power off, the electromagnet lets the spring return the armature to its normal position, contact is made again, and the cycle starts all over again--just as long as you continue to push on the button. buzzers work exactly the same way, except that they do not have a bell and depend instead on the vibration of the armature for a noise that's not as loud or as musical. gongs or chimes, that strike only once when the button is pushed, are made by connecting the armature with the screw by means of a flexible wire. a special kind of electricity most buzzers and bells work on a much lower voltage than you normally find in the wires in your house. some are made to work at volts, others at volts, and still others at slightly higher voltages. you can get these low voltages by using one or more batteries, or by using a transformer connected to your house current. most bells and buzzers are now powered through transformers. how to control them the push button is the most common means of control. you can use one button to control several bells, or several buttons to control one bell, or have several buttons control several bells. because low voltage is used, adding extra buttons is simple, inexpensive, and safe. buzzers and bells can also be controlled by: _clocks_, as in the interval timer on an electric range or in a school class bell system; _temperature detectors_, as in a fire alarm or freezer alarm; _door and window trips_, as in a one-man repair shop or in a burglar alarm; and _treadles_, as in the driveway of a service station. [illustration: figure ] pick the right bell or buzzer some of the many different types of bells, and various ways of controlling them are suggested in the table below. just remember that no matter what the job or conditions, you can probably find a bell or buzzer and controls that suit your need. some typical jobs for bells & buzzers -------------------------------------------------------------------- number and type of location number and bell or of bells type of location job buzzer and buzzers control of controls --------------------------------------------------------------------- summon others in the house-- enough to push- one at the to the small to cover all buttons telephone telephone medium buzzers usual work and each in locations extension outbuildings-- phone medium to large bells outdoors-- large weatherproof bell all transformer- powered --------------------------------------------------------------------- notify club medium to large one may be hose one--in member that bell-- enough--if diaphragm the car is at his transformer- mounted on driveway produce stand powered the back of ----------------------- the stand (complete driveway including control, are available, ready to plug in.) -------------------------------------------------------------------- warn of power battery-powered one near relay, one, at failure to buzzer, medium the held open main incubator or size poultryman's as long as switch of brooder bedroom power is on, hatchery closed by or spring if brooder interruption house occurs -------------------------------------------------------------------- warn of battery-powered one, in or temperature one, with dangerously buzzer, medium near the detector bulb warm size kitchen (sensitive inside temperature thermostat) freezer in freezer --------------------------------------------------------------------- how to plan your system to save your time and steps when the telephone rings for someone else in your family who is some distance away, you can install a simple bell or buzzer system to summon that person. first, you must plan what you are going to do. on a large sheet of paper, draw to scale (roughly) a plan of your house and grounds, including those places where phones are located. it will help if you rule off your paper in / " or / " squares and let each square equal one foot. show the location of poles supporting your wiring. next, pick out those areas where you or others would likely be when someone else would answer the phone and want to call you to it. after you have thought about this, and talked it over with members of your family, show locations on your plan where you think you would like to have buzzers or bells, and show a button beside each telephone. (generally, you should have a bell or buzzer near each phone, also.) figure shows diagrams of various types of systems, and will help you determine the number of wires you will have to install to connect the buttons and bells that you have planned. inside, you will connect your transformer and the various buttons and bells with ordinary indoor bell wire. outdoors, however, you should use weatherproof -wire or -wire telephone twist. show on your plan the distances that must be traversed by each type of wire, and show the number of conductors in each. don't overlook the vertical distances (one floor to another). [illustration: figure ] materials you'll need because no two situations are just alike, it will be necessary for you to make your own list of materials. as a guide, however, here is a list of typical materials, with the quantities left blank, for you to fill in as your own requirements and measurements dictate. -volt transformer --- door buzzers --- doorbells --- weatherproof outdoor type bells --- ft. indoor bell wire --- ft. -wire weatherproof telephone twist --- ft. -wire weatherproof telephone twist --- lbs. staples (insulated) --- entrance insulators (for attaching weatherproof to buildings and poles) because your transformer must be wired into your regular house current, you should have some help on this from an electrician or other qualified person. also, you should get that person to review your plans and materials list before you place an order. install according to your plan with the aid of an electrician or other qualified person, install your transformer, and test it. you may then go ahead and complete your signal system, checking carefully with your plan, and making sure that your installations are both electrically and mechanically secure. test your system in all possible ways that it might be used. demonstrations you can give build a demonstration board incorporating a farm or home layout, with pushbuttons or other controls and bells and buzzers appropriately located. show and tell how the system would save time and energy. show and tell how some of these work, and their value: power-off alarm, freezer alarm, fire alarm, driveway alarm. for more information ask your power supplier or your nearest electrical supply house for catalogs or literature on various types of signal systems, or ask a dealer to show you equipment he has in stock. lesson no. b- credit points first aid for electrical injuries what would you do if you saw someone who had been hurt by electricity? did you know that you could save his life, if you had taken the time to learn and practice a few simple rules of electrical first aid? first aid training equips you to know what to do and what not to do for the injured until medical help can be obtained. while the main benefits are for you and your family, no one can call himself a good citizen if he fails to help a stranger who has been hurt. the information given here is only for electrical injuries. perhaps what you learn will inspire you to take a complete course in first aid. what to do learn how to prevent electrical accidents, and what to do if an electrical accident occurs. . make an electrical hazard hunt in your home or on your farm. point out to your parents everything that should be repaired or replaced for safety's sake. . read the first aid suggestions that follow. learn them. . get to know the six steps that are outlined for mouth-to-mouth rescue breathing. practice them on your brother, sister, or parents. teach the entire family how to do it. electricity can kill in this day of hundreds of uses of electricity, you should know about electrical dangers. electrocution can occur from either low voltage (household type) or high voltage currents. sometimes household voltages are more hazardous because people underestimate the dangers involved. a fraction of an ampere passing through your heart muscles can be fatal. your body offers some resistance to the flow of electricity to ground. if you are standing on wet ground or in water, or if your skin is damp, this resistance is greatly reduced. wire cables within walls and cords on appliances are all insulated with a shock proof covering. continued use, age, or damage may expose a bare wire and create a hazard. the point of exposure need be only a fraction of an inch. cords are often used and abused. exposed wires and signs of wear are danger signals. always be wary of overhead wires. people have been injured or killed when kite strings, model plane control lines, irrigation pipe, and water well equipment have come in contact with the power supplier's or their own overhead wiring. prevent accidents underwriters' laboratories (ul) have taken steps to see that minimum safety standards are met in the manufacture of electrical equipment. look for the ul label when you buy cords or appliances. never place cords under carpets or furniture, or drape them over a nail. replace or repair worn cords without delay. be especially careful when operating electric devices in the bathroom. keep in mind the dangers of a wet floor, grounded metal pipes, and wet skin. turning on an ac radio while you are taking a bath is asking for real trouble. there may be shorts in electric devices. keep your hands dry when using them, and do not touch them along with grounded metal objects. if you ever get a slight shock, sound the danger signal and do something about it. think, then act your first thought in rescuing a victim from an electrical accident should be your own safety. speed is also important, because a few seconds or minutes may save a life. the first question you should ask yourself is "can i quickly turn off the power?" this would be easier to do in the home than outside. in the case of a victim trapped in a bathtub from a radio accidentally knocked into the water, it might mean simply removing the plug from the wall outlet. if a victim is found grasping shorted, permanently installed equipment and cannot let go, the main switch might be used for quick release of the current. outdoors, especially with high tension wires, your danger in rescue is much greater. to handle the victim, touch him only with a long dry stick, dry rope, or a long length of dry cloth. be sure your hands are dry and that you are standing on a dry board. a broom might be a good lever to pry a victim from a high tension wire but never use a green stick containing sap. first aid once the rescue has been made and the victim is free of further danger, check to see if breathing has stopped. if so, start artificial respiration _immediately_ and send someone for a doctor. artificial respiration must be started as soon as possible after normal breathing ceases. _most persons will die within minutes or less if breathing stops completely unless they are given artificial respiration._ precious minutes may have passed before you get to the victim. since the victim may be within seconds of death by the time you are able to touch his body, you should seek to obtain an air flow to and from the lungs _immediately_. the victim may seem stiff as an effect of the current, so don't give up easily. continue the procedure for several hours. if transportation is necessary, remember that there may be internal injury, fractures, or severe burns. mouth-to-mouth rescue breathing there are various effective ways to give artificial respiration, each with its advantages and disadvantages. the mouth-to-mouth method is recommended as a good one to master. it can be used on victims of drowning, suffocation, and asphyxiation, too. people have been known to save lives with less exposure to the correct procedure than you are getting by reading this. so, pay attention and remember what you read. step . turn the victim on his back. wipe out victim's mouth quickly. turn his head to the side. use your fingers to get rid of mucus, food, sand, and other matter. [illustration: head position] step . straighten victim's head and tilt back so that chin points up. push or pull his jaw up into jutting out position to keep his tongue from blocking air passage. this position is essential for keeping the air passage open throughout the procedure. [illustration: push jaw up] [illustration: pinch nostrils] step . take a deep breath, place your mouth tightly over victim's mouth, and pinch nostrils closed to prevent air leakage. for a baby, cover both nose and mouth tightly with your mouth. (breathing through handkerchief or cloth placed over victim's mouth or nose will not greatly affect the exchange of air.) [illustration: breathe] step . breathe into victim's mouth or nose until you see his chest rise. (air may be blown through victim's teeth, even though they may be clenched.) step . remove your mouth and listen for the sound of returning air. if there is no air exchange, recheck jaw and head position. if you still do not get air exchange, turn victim on side and slap him on back between shoulder blades to dislodge matter that may be in throat. again, wipe his mouth to remove foreign matter. step . repeat breathing, removing mouth each time to allow air to escape. for an adult, breathe about times per minute. for a child, take relatively shallow breaths, about per minute. continue until victim breathes for himself. what did you learn? true or false . a broken arm should be splinted before artificial respiration is applied to a victim who is not breathing. . a person who has been severely shocked with an electric current should lie down. . a doctor should be called even though you successfully have revived a victim's breathing. . a fraction of an ampere through the human heart muscles can be fatal. . a copper wire would provide a better path than your body for stray currents, therefore all appliances should be grounded if possible. . outside wires are never a hazard because they are covered with insulation when they are installed. . cords need not be repaired until you can see bare wires. . tuning in an ac radio while you are bathing is always dangerous, even though your hands are dry. . in an emergency, a broom is an acceptable tool for prying a victim off a high tension wire. . in mouth-to-mouth breathing, an adult's lungs should be filled times per minute and a child's . demonstrations you can give show how to deal with an electrical first aid "problem" given to you by your leader. for more information ask your leader to have a first aid expert put on a demonstration. (many industrial plants and power suppliers have such people.) lesson no. b- credit points how electricity heats in ancient times, people thought that heat was a material just as air is. they called it "caloric". when something got warm, they said, caloric flowed into it. when something cooled off, caloric flowed out of it. it did not bother them that they could not see caloric. they could not see air either! now we know that heat is not a material. it does not take up space. it does not weigh anything. instead, it is a form of energy. and when we say that heat is a form of energy, we mean that it can be used to do work. what to do . make a simple resistance heater. . make some popcorn by: (a) conduction (b) convection (c) radiation "resistance" makes heat there are at least four ways that electricity can make heat. the one that we'll cover here is _resistance_ heating. (the others are: _dielectric_ heating, where the lines of force of an electrostatic field pass through a non-conductive material and heat it; the _heat pump_, which is a refrigerator in reverse; and _electronic_ heating, which uses high frequency waves similar to radio waves to create high speed movement of the molecules or tiny particles which rub together to make heat.) _resistance_ heating occurs because every conductor of electricity opposes the flow of current through it. some conductors resist more than others. when they do, a certain amount of warming takes place. the more resistance that is offered, the more heating there is. some materials, like silver, copper, and aluminum, offer little resistance. we say they are good conductors. other materials, like iron, offer more resistance. they are still conductors, but not as good as the others mentioned. the _size_ of the conductor, and its _length_ are the other two things that affect its resistance. the _smaller_ it is, the greater its resistance. also, the _longer_ it is, the greater its resistance. therefore, when we only want to _move_ electricity from place to place, we want relatively large, "good" conductors. here, we do not want to make heat. in fact, we want to avoid it, because too much heat in the wrong place can cause a fire. but when we want heat, we choose relatively small, "poor" conductors, and the more heat we want, the longer they must be. if you will think of the filament inside a lamp bulb; you may recall that it is a very fine wire, coiled so as to get a maximum length, and made of tungsten which has a high resistance. because of all these factors, this filament glows at a white heat, and is a source of both light and heat. make a simple resistance heater _materials you will need_: dry cell battery foot iron picture wire pliers use a short strand of iron picture wire and hook the ends to the terminals of a dry cell battery. use pliers so that you do not burn your fingers. disconnect the wires as soon as they become hot. tell why the wires heat. conduction is "touching" heat conduction occurs when you set a pan containing food right on a heating element. an egg cooking in a hot frying pan is a good example of conduction at work. this method is the most efficient single way of using electric heat for cooking. convection depends on air convection warms food in pans that are not actually touching the heating element. it uses the hot air around the element to carry heat to the pan. your oven in your range works by convection. most houses are warmed in winter in the same way. the heat produced in a furnace warms the air as it circulates through. this air in turn keeps your body warm. radiation is like the sun radiation heating is more difficult to explain. it results when heat or energy waves strike an object and are converted into heat. the energy we receive from the sun is a good example. when you are wearing dark clothes on a chilly day, you may become uncomfortably hot. the sunshine warms you even though the air around you has not been heated. radiant energy has a way of being absorbed by dark objects and reflected by light colored or shiny surfaces. did you ever notice how snow melts faster on a black top road than it does on a concrete road? the electric heat lamp is one of the most familiar sources of radiant heat. other examples are panels and cables that are built into the walls and ceilings of homes to provide heat. make popcorn ways how do you make popcorn? did you know that you can do this kind of a heating job three different ways? _materials needed_ popcorn cooking oil or shortening salt and butter -qt. saucepan, with cover. (a glass cover is preferred.) potholder electric range -watt heatlamps spring clamp type lampholders wire mesh corn popping basket or wire mesh kitchen strainer (improvise a screen wire cover) _first_, make popcorn the way you usually do. set a front surface unit control on the range at "medium high". pour enough oil to very lightly cover the bottom of the pan. when the pan is hot, pour in enough popcorn to cover the bottom with one layer of kernels. use the potholder in one hand to hold the cover on, and with the other move the pan back and forth across the unit. when the popping stops, remove from the heat. how did the heat get to the popcorn? _second_, make popcorn in the oven. add the oil to the pan, cover it and put it in the oven. turn the oven on, with the automatic control set at °. when the oven indicator light goes off, this means that the proper temperature has been reached. with the potholder, remove the pan and add one layer of popcorn kernels. replace the pan in the oven. when the popping stops (listen for it) remove the pan. what kind of heating took place here? _third_, make popcorn with the heat lamps. clamp the lampholders to the back of a chair or other vertical support. they should be to inches apart and pointed directly at each other. put about tablespoonfuls of popcorn in the wire basket or strainer. do not add oil. hold the basket midway between the two lamps. when the popping stops, turn off the lamps. what kind of heating was this? now, butter and salt the popcorn you have made and share it with others. what did you learn? . how is heat transferred from one body to another? . could chicks or pigs receive warmth from a heat lamp without the air in the pens becoming warm? explain. . how does a broiler unit in a range cook meat? . how does an oven bake food? . tell why iron picture wire was used instead of copper wire for your heating demonstration. lesson no. b- credit points mysterious magnetism in ancient times, people found certain rocks that clung together in bunches. these rocks were very mysterious. people didn't understand them and many superstitions grew up about lodestones, as these rocks were called. lodestone (sometimes spelled loadstone) means leading stone. people even told columbus not to sail out of sight of land because a giant lodestone was just over the horizon waiting to pull all the nails out of his ships. the chinese were the first to use magnets. they found that if you hung a lodestone by a string, one end of the stone would always point in the direction of the north star. they had the first magnetic compasses. an artificial magnet can be made by stroking or gently rubbing a piece of steel with a lodestone. this piece of steel then can be used to magnetize another piece of steel. this can be continued on and on. lodestones are not always available but you can get the same results with an electric current. so, magnetism and electricity are very closely related. what to do learn about magnetism by doing the experiments that follow. seeing is believing! materials you will need dry cell batteries (# ) a few feet of no. bell wire steel knitting needles or similar hard steel ft. of light thread sheet of light cardboard or stiff paper permanent magnet (bar or horseshoe) compass or more large nails or spikes red and black china-marking pencils or crayons iron filings wire cutters carpet tacks (iron filings usually can be found under the grinding wheel in a shop. if you can't find any, rub some steel wool pads together to produce bits of metal that will do.) "see" a magnetic field cover the permanent magnet with the cardboard or paper. sprinkle iron filings on the paper. tap the paper and note the pattern formed. strings or lines of filings pass from one pole of the magnet to the other. the area covered by the filings is the center of the magnetic field. to remember this, you might compare the magnetic lines of force that arrange the iron filings to the contour strips in a farmer's field. this magnetic field is one of the important things in our everyday life with electricity. if it were not for the magnetic field, we would not have electric motors. telephones, radios, television, and many other things we use every day also depend on this magnetic field. [illustration: figure ] make an electro-magnet you can make magnetism work for you by winding several turns of insulated wire around one or more large nails or spikes (soft iron). connect one end of the wire to the battery. touch the other end of the wire to the other terminal for a few seconds and see how many tacks you can pick up. repeat the experiment using as many turns as possible. how many more tacks were you able to pick up? [illustration: figure ] you have made what we call an electromagnet. when you disconnect the wire, the nails fall off. this is one of the advantages of an electromagnet. we can turn magnetism on and off as we wish. picture a crane operator throwing the switch and picking up scrap iron and steel. then he opens the switch to drop the scrap metals. soft iron can be magnetized easily as you have just seen, but loses its magnetism in a short time. steel is harder to magnetize but holds its magnetism almost indefinitely. make a permanent magnet wrap the insulated bell wire around the steel knitting needle. the wire should be wrapped the full length of the needle. one end of the wire is connected to the battery. the other end of the wire is then touched for just a few seconds to the other terminal. this should make the needle into a permanent bar magnet. if you did not get results, try two batteries in series, wind more turns of wire on the needle, and leave it connected a little longer. do the same thing with the second knitting needle. in the same way, you can magnetize a screwdriver, so that you can use it to pick up and hold steel screws. don't do it unless you want your screwdriver to be magnetized. [illustration: figure ] see how they attract and repel take one of the magnetized needles and hang it with a thread. a thread stirrup (figure ) will help keep it level. be sure it is not near other large pieces of steel. watch the needle. does it settle down, pointing in one direction? (check to see if this is the same direction as your compass). if it does, you have made a compass. the tip of the needle pointing north is called the north pole (north-seeking pole). the other end is called the south pole. mark the north pole with a stroke of the red marking pencil. mark the south pole black. do the same thing with the second needle. you can show this with a sewing needle, and a notched cork, and a bowl of water. rest the needle in the notched cork, and float it on the water. [illustration: figure ] hold the compass near the north pole of the needle. what happens? does the south pole of the needle attract the north or south pole of the compass? try this with the second magnetized needle. see if you can prove the rule that like poles repel (drive away) and unlike poles attract. [illustration: figure ] connect one end of a wire loop to the battery and run the wire directly over the compass. touch the other end of the wire to the battery. which way does the compass point now? if you get some motion out of the compass needle, this proves there is a magnetic field around the wire when current is flowing. this relation between electricity and magnetism is the thing that makes electric motors and generators work. [illustration: figure ] make many from one lay the third needle (unmagnetized) on a table and stroke it with one of the magnetized needles. (see diagram) always stroke it in the same direction. raise the magnetized needle at least two inches on each return stroke. thus you can magnetize the needle by using the other needle. [illustration: figure ] use the wire cutters to cut the first magnetized needle in short lengths. (cover the needle with a cloth to keep the pieces from flying.) can you show by using the compass that each piece is a complete magnet? hold one end, then the other, of each piece to a compass. does each piece have both a north pole and a south pole? magnetism and animals the things you have done show that electricity and magnetism are related in many ways. magnetism is mysterious, and there are still things to discover about it. it is thought that animals and birds are aided in their sense of direction by magnetism. it is commonly known that when a person gets lost in the woods, he tends to go around in circles. possibly this is caused by the earth's magnetic field. what did you learn? . where are natural magnets obtained? . how can artificial magnets be made? . what material is needed for a permanent magnet? for a temporary magnet? . how can you find out which is the north pole of an unmarked magnet? . how many poles does a magnet have? . which magnetic poles attract each other? . why couldn't you make a compass out of a strip of plastic? . what causes the compass to change direction when a wire carrying battery current is held over the needle? . list the materials you would need and tell how you would build a homemade compass. . tell what you enjoyed most about becoming acquainted with mysterious magnetism. lesson no. b- credit points give your appliances and lights a square meal would you say that having enough to eat was pretty important in the home that you know? the "food" for your appliances and lights is electricity, and like you they must be "fed" enough. what to do . list the appliances and lights in your home. . see if any of them are "starving" for the electricity they need. . learn how the electricity gets to where it's used. . make a chart of the electrical circuits in your home. . make sure that each circuit is protected with the right fuse or circuit breaker. count your electrical blessings many people in much of the rest of the world wish that they could trade places with us, because we have so many electrical appliances in our homes. of course, we have not always had as many appliances as there are today. when electricity first came along, people used it only for lights. then, they began to add flatirons, washing machines, refrigerators, coffee percolators, and radios. then more and more electrical things were made for people to use and enjoy. now we have dozens and dozens of uses for electricity in our homes. how many different uses for electricity are there in your home today? ask your parents how many there were when your home was built or first wired. how many were _common_ when your parents began to keep house? some homes are behind times many older homes were built before electricity was available, and were wired later. and like them, some older homes that were wired as they were built had only enough wiring for lights and a few other appliances, because those were the only uses that were known at that time. but people kept on living in these homes, and kept adding to the uses they made of electricity without adding to their wiring. what has this meant? well, if electricity were like cars and trucks, you could say that some people are trying to put turnpike traffic through a back-country dirt road! watch for signs of starvation of course, as your state has done with its highways, some people have expanded and modernized their wiring. but many others have not yet seen this need, or if they have, they may have to do it again. here's why: your power supplier delivers current to you at the right voltage or electrical pressure. if the wires in your house are large enough, they will pass this full voltage on to the appliances. but if your wiring is too small, the electricity arrives at the appliances so weak that they can't work properly, and much of what you pay for is wasted. here are some things you can watch for in your own home. they will tell you whether your appliances are getting enough electrical "food" or not. . _a shrinking tv picture_--if it draws in from the sides of the screen, fades, loses contrast, or if the sound becomes distorted, you may have low voltage. . _too much fuse blowing or circuit breaker tripping._ . _heating appliances are slow to do their jobs._ . _lights dimming_, when motors or other appliances are turned on. there should be enough ways to get "appliance-food" around if appliances in your home show these starvation signs, then you may not have enough ways for the electricity to get to where it's used. there are three kinds of these electrical highways or circuits, and your home should have enough of each: . _general purpose circuits_--these serve lights all over the house, and convenience outlets everywhere except in the kitchen, laundry, and dining areas. a rule-of-thumb is: there should be at least one general purpose circuit for each sq. ft. of floor space. . _small appliance circuits_--these are not used for lights, but instead they supply convenience outlets in the kitchen, laundry, and dining areas where portable appliances are most used. every home should have at least two small-appliance circuits. . _individual or special-purpose circuits_--one of these is needed for each: electric range, dishwasher, water heater, freezer, automatic washer, clothes dryer, air conditioner, pump, and house heating equipment. +----------+------+------+------+------+-------+ | | | | | | | | actual | | | | | | | size | | | | | | +----------+------+------+------+------+-------+ | gauge | | | | | | | size | | | | | | +----------+------+------+------+------+-------+ | fuse or | | | | | | | breaker | | | | | | +----------+------+------+------+------+-------+ |max. watts| | | | | | |at v. | | | | | | +----------+------+------+------+------+-------+ |max. watts| | | | | | |at v. | | | | | | +----------+------+------+------+------+-------+ [illustration: wire sizes commonly used in homes] each circuit big enough the capacity of each circuit is limited by the size of its wires. the chart above shows you the actual sizes of wires commonly used in permanent home wiring, and what each will carry. notice that each size is given a number, and the smaller the number, the bigger the wire. also notice that a given size of wire will carry twice as many watts at volts as it will at volts. (watts are figured by multiplying amps times volts.) general purpose circuits usually are either number or number wire, at volts. what is the capacity of each, in watts? (number wire is recommended for all new general purpose circuits.) small appliance circuits are required to be at least number wire. individual circuits are always sized according to the appliance they serve. find the size wire that should be used for a , -watt, -volt range; a -watt, -volt dishwasher; a -watt, -volt clothes dryer. ________ ________ ________ only one fuse size right a fuse in an electrical circuit is like an alert traffic policeman--stopping everything if there's danger. a circuit breaker serves the same purpose, and the right size is installed when the wiring is done. a policeman uses his brain to tell him when to blow his whistle, but a fuse depends on the size of the little fusible (meltable) metal link that you see under the glass. if too great an electrical load is added to a circuit, this link will melt and prevent a dangerous overload. if you put in a fuse with too heavy a link, it will not melt in time, and the wiring and equipment may be damaged. therefore the right size of fuse is very important, and is something that you should check in your own home. see the chart above for the right fuse for each size wire. make a circuit chart at one or more places in your home there is a box or panel containing the fuses or breakers for the various circuits. attached to the inside of the door of each such panel should be a chart something like this: [illustration] +-----+---------------------+-----------+ | no. | description | fuse size | +-----+---------------------+-----------+ | | range | | +-----+---------------------+-----------+ | | kitchen outlets | | +-----+---------------------+-----------+ | | dining room outlets | | +-----+---------------------+-----------+ | | living room outlets | | +-----+---------------------+-----------+ | | | | +-----+---------------------+-----------+ | | | | +-----+---------------------+-----------+ notice that in our chart we have made columns for a description of what each circuit serves, its number or position in the panel, and the proper size fuse for it. because most such charts leave out this last very important bit of information, you should make a complete new chart, like the one shown. provide as many lines as there are fuse positions. paste or tape it to the inside of the panel door. then, ask permission of your parents to disconnect all the circuits by unscrewing the fuses or flipping the circuit breakers. _do not touch anything but the fuse rim._ then reconnect them, one at a time, to find out what each circuit serves. turn on as many lights as you can, to help you in your detective work. use a test lamp at those outlets that do not have a light connected to them. write two or three words describing each circuit on the proper line on your chart. on a separate sheet, keep track of the appliances and lights that are on each circuit, and add up the watts. (if the name-plate of any appliance gives "amperes", "amps", or "a" instead of watts, just remember that amps times volts equals watts.) this will tell you if any of them are overloaded. show this sheet to your parents. check the wire sizes _disconnect the main switch_, and determine the size of the wires in each circuit. don't include the insulation in your measurement. _be careful! even though you have disconnected the main switch, the wires coming into it are still "live". so, do not touch any wires. instead hold the wire size chart near them so that you can tell which gauge each one is._ write in the proper size fuse for each circuit on your chart. replace any wrong-size fuses do the fuse sizes you have written on your chart agree with the ones that are in place in the panel? get the right size fuses and replace any that are wrong. make sure that you have a reserve supply of the right sizes, and that they are handy for future use. talk it over with your parents do you think that your home has enough of the proper size circuits? if not, talk it over with your parents. they may want to ask an electrician to go over the wiring and make the necessary changes. what did you learn? (underline the right answer.) . a (television set, radio) is very sensitive to changes in voltage. . dimming lights mean (static in the wires, an electrical overload). . wires that become warm from overload make it (more expensive, cheaper) to operate the equipment. . a home of , sq. ft. should have at least (three, four) general purpose circuits. . one solution to low voltage symptoms is (heavier fuses, more circuits). . full capacity for a number wire circuit at volts is ( watts, watts). . a room air conditioner should be on (a general purpose, an individual) circuit. . the purpose of a fuse is to (let you disconnect the circuit, automatically prevent overloading the circuit). . the right size fuse is determined by (wire size, the store where you buy it). . a circuit chart should give (circuit description and fuse size, the maker's name). demonstrations you can give ask your leader to help you plan a demonstration. you can show how lights dim when too many other appliances are connected, how a fuse protects against overloading, and the danger of using too large a fuse. for more information ask your extension agent, power supplier, or electrician for additional help. lesson no. b- credit points you can measure electricity instruments that can detect or measure the flow of electricity have helped to make possible the wonders of electricity as we know them today. scientists in laboratories must have measuring devices for experiments leading to new uses of electricity. power suppliers must have instruments that tell what the generating equipment is doing and to measure the amount of electricity being sold to users. factories need instruments that keep tab on electrical equipment to make sure electricity is being used efficiently. in fact, almost anywhere you find electric power at work you'll find electrical instruments--even in your home. the one you know best measures the amount of electricity used. another, in the family car, shows whether the generator is charging the battery or if the battery is discharging. what to do . make a simple kind of direct-current meter that will show you that there's a magnetic field around a wire carrying an electric current and that will detect a very tiny current. . make a more refined d.c. instrument (galvanoscope) and measure the voltage of different sizes of dry batteries, and show how an electric current can be induced. tools and materials you'll need: pair of pliers, knife, small hammer feet of no. bell or magnet wire compass two coins--a penny and a dime fine sandpaper blotting paper plastic or cellophane tape wooden blocks (see figure ) glue small nails one # dry cell, a penlight battery, and two regular flashlight batteries table salt drinking glass paper clips two machine bolts how they work like many electrical things, most electrical instruments depend on the action of magnetism created by an electric current. there is a magnetic _field_ or lines of force around any wire carrying an electric current. if this field is controlled and made to react on a sensitive device, like an easily moved pointer, we have an electrical instrument. detect a magnetic field first, let's prove that there is a magnetic field around any wire carrying an electric current. take a piece of wire about two feet long and scrape off about an inch of insulation from each end. connect one end to a battery terminal. make a loop of wire that crosses the face of your compass, north to south. now touch the other end of the wire to the other battery terminal. (do not attempt to substitute alternating current, as from a model railroad transformer because its alternating current will cause the compass needle to swing rapidly from one side to the other.) [illustration: figure . put your right hand beneath the wire so that your fingers point the way the needle deflects, and your thumb will point in the direction that the current is flowing.] what happens? your compass needle should move to one side because it is very sensitive to magnetic influences. this proved that the wire created a magnetic field or lines of force when we passed electricity through it. (figure ) detect a tiny current how sensitive is your simple electric meter? take about five feet of wire and wrap it around your compass as in figure , keeping the turns bunched together as much as you can. leave about six inches at both ends of the wire extended for leads. scrape the insulation off the last inch of both. rotate the coil and compass until the needle and coil are parallel, both pointing north and south. [illustration: figure ] take a copper penny and a dime, and clean off any corrosion or film on the coin faces with a bit of fine sandpaper. now take a piece of blotting paper about the size of the penny and dip it into strong salt water. place the damp blotting paper between the penny and the dime. place one of your compass coil leads against the dime, and the other against the penny as shown in figure . be sure you have good metal-to-metal contact between the wires and the coins. [illustration: figure ] at the instant that you squeeze the leads against the coins, watch what is happening to the compass needle. it should move for an instant from the north position each time you press the leads against the two coins. obviously, the little coin battery you have just made produces a very weak electrical current. even so, your instrument should be able to detect it. make a simple galvanoscope now let's make a meter that is a little more practical to use. broadly speaking, a galvanoscope is an instrument that detects the presence of electric currents. it sounds complicated but it is really quite simple. it is named in honor of an italian professor named galvani who made important early experiments with electricity. a refinement of the galvanoscope is today's galvanometer. other related instruments are the voltmeter and ammeter. these are very important instruments to the electrical engineer. using a glass or anything three to four inches in diameter, wind about turns of wire in a "bunched" coil as in figure . wrap the coil at several points with cellophane or plastic tape to keep it from unwinding. [illustration: figure ] make a wood base for your coil as shown in figure . the compass support blocks can be thin wood slats. do not attach them with steel nails or tacks. use glue instead. hold the coil in the slot between the blocks with glue or melted wax or use copper staples. place the compass on the supports and rotate the base so that the compass needle and coil are parallel, pointing north and south. measure the voltage of batteries do you know what difference the size of dry cell battery makes in the voltage it supplies? your meter can tell you. to test the voltage of batteries we must be able to control our galvanoscope. to do this, connect a glass of strong salt water in series with the battery as shown in figure . make sure the wire ends immersed in the salt water are scraped free of enamel. [illustration: figure ] with one of the batteries connected, move the wires in the salt water first closer, then farther apart (keeping them parallel to each other) while watching your compass needle. when the needle stays to degrees off north, lock the wires in the salt solution in place with paper clips. now disconnect the battery you have been using and connect a smaller battery. if both batteries are fresh, the compass needle should return to almost the same spot. this proves that both batteries regardless of size put out the very same voltage. the larger ones, however, are designed to last longer. measure the difference between series and parallel using the salt solution as in the previous experiment, connect two flashlight batteries in series as shown in figure . the compass needle should move about twice as far as it did with one battery connected. this shows that when you connect batteries this way you double their voltage. [illustration: figure ] now place your batteries side by side and connect the two top terminals and the two bases as shown in figure . the compass needle should move only as much as it did for one battery. this is called a parallel connection. you can see that this arrangement does not double the voltage, even though you used two batteries. [illustration: figure ] while you have this hookup, try reversing the position of the leads connected to your batteries. notice that reversing the direction of current flow in the coil causes the compass needle to swing in the opposite direction. test for induced current make a simple coil by winding about turns of wire around a machine bolt core. the bolt should be / to / " in diameter and about two inches long. connect the coil to your galvanoscope as shown in figure . pass the coil back and forth close to the end of a permanent magnet. [illustration: figure ] notice a slight deflection of the compass needle with each pass. you have shown that electricity can be induced in a wire coil by moving it through a magnetic field. currents generated in this way are called induced currents. [illustration: figure ] now make another coil and core just like the first one and arrange them and a connection as shown in figure . if you make and break the current to the second coil, you will build up and collapse a magnetic field around the first coil and again induce a current in it. you will see the compass needle swing back and forth again. these last two experiments give you a crude idea of how an electric generator works, producing electric current by induction as a coil-wound rotor revolves within a magnetic field. what did you learn? what does every current-carrying wire have around it? how does this help us to measure electricity? how sensitive are electrical instruments? what is the difference in voltage between (a) a large and a small dry cell? (b) batteries connected in series and in parallel? (c) your original connection and the reverse of it? what similarity does the test for induced current show between movement through a magnetic field and the making and breaking of a direct current? demonstrations you can give show others how your galvanoscope can detect: whether a battery is producing current, which way the current is flowing, and whether a current is strong or weak. demonstrate how a current can be generated using magnetism. for more information ask your power supplier representative to show you some of the instruments used by his organization, and to give you a brief explanation of how they work. ask him or an electrician to give you a demonstration of a split-core ammeter. opticks: or, a treatise of the _reflections_, _refractions_, _inflections_ and _colours_ of light. _the_ fourth edition, _corrected_. by sir _isaac newton_, knt. london: printed for william innys at the west-end of st. _paul's_. mdccxxx. title page of the edition sir isaac newton's advertisements advertisement i _part of the ensuing discourse about light was written at the desire of some gentlemen of the_ royal-society, _in the year , and then sent to their secretary, and read at their meetings, and the rest was added about twelve years after to complete the theory; except the third book, and the last proposition of the second, which were since put together out of scatter'd papers. to avoid being engaged in disputes about these matters, i have hitherto delayed the printing, and should still have delayed it, had not the importunity of friends prevailed upon me. if any other papers writ on this subject are got out of my hands they are imperfect, and were perhaps written before i had tried all the experiments here set down, and fully satisfied my self about the laws of refractions and composition of colours. i have here publish'd what i think proper to come abroad, wishing that it may not be translated into another language without my consent._ _the crowns of colours, which sometimes appear about the sun and moon, i have endeavoured to give an account of; but for want of sufficient observations leave that matter to be farther examined. the subject of the third book i have also left imperfect, not having tried all the experiments which i intended when i was about these matters, nor repeated some of those which i did try, until i had satisfied my self about all their circumstances. to communicate what i have tried, and leave the rest to others for farther enquiry, is all my design in publishing these papers._ _in a letter written to mr._ leibnitz _in the year , and published by dr._ wallis, _i mention'd a method by which i had found some general theorems about squaring curvilinear figures, or comparing them with the conic sections, or other the simplest figures with which they may be compared. and some years ago i lent out a manuscript containing such theorems, and having since met with some things copied out of it, i have on this occasion made it publick, prefixing to it an_ introduction, _and subjoining a_ scholium _concerning that method. and i have joined with it another small tract concerning the curvilinear figures of the second kind, which was also written many years ago, and made known to some friends, who have solicited the making it publick._ _i. n._ april , . advertisement ii _in this second edition of these opticks i have omitted the mathematical tracts publish'd at the end of the former edition, as not belonging to the subject. and at the end of the third book i have added some questions. and to shew that i do not take gravity for an essential property of bodies, i have added one question concerning its cause, chusing to propose it by way of a question, because i am not yet satisfied about it for want of experiments._ _i. n._ july , . advertisement to this fourth edition _this new edition of sir_ isaac newton's opticks _is carefully printed from the third edition, as it was corrected by the author's own hand, and left before his death with the bookseller. since sir_ isaac's lectiones opticæ, _which he publickly read in the university of_ cambridge _in the years , , and , are lately printed, it has been thought proper to make at the bottom of the pages several citations from thence, where may be found the demonstrations, which the author omitted in these_ opticks. * * * * * transcriber's note: there are several greek letters used in the descriptions of the illustrations. they are signified by [greek: letter]. square roots are noted by the letters sqrt before the equation. * * * * * the first book of opticks _part i._ my design in this book is not to explain the properties of light by hypotheses, but to propose and prove them by reason and experiments: in order to which i shall premise the following definitions and axioms. _definitions_ defin. i. _by the rays of light i understand its least parts, and those as well successive in the same lines, as contemporary in several lines._ for it is manifest that light consists of parts, both successive and contemporary; because in the same place you may stop that which comes one moment, and let pass that which comes presently after; and in the same time you may stop it in any one place, and let it pass in any other. for that part of light which is stopp'd cannot be the same with that which is let pass. the least light or part of light, which may be stopp'd alone without the rest of the light, or propagated alone, or do or suffer any thing alone, which the rest of the light doth not or suffers not, i call a ray of light. defin. ii. _refrangibility of the rays of light, is their disposition to be refracted or turned out of their way in passing out of one transparent body or medium into another. and a greater or less refrangibility of rays, is their disposition to be turned more or less out of their way in like incidences on the same medium._ mathematicians usually consider the rays of light to be lines reaching from the luminous body to the body illuminated, and the refraction of those rays to be the bending or breaking of those lines in their passing out of one medium into another. and thus may rays and refractions be considered, if light be propagated in an instant. but by an argument taken from the Æquations of the times of the eclipses of _jupiter's satellites_, it seems that light is propagated in time, spending in its passage from the sun to us about seven minutes of time: and therefore i have chosen to define rays and refractions in such general terms as may agree to light in both cases. defin. iii. _reflexibility of rays, is their disposition to be reflected or turned back into the same medium from any other medium upon whose surface they fall. and rays are more or less reflexible, which are turned back more or less easily._ as if light pass out of a glass into air, and by being inclined more and more to the common surface of the glass and air, begins at length to be totally reflected by that surface; those sorts of rays which at like incidences are reflected most copiously, or by inclining the rays begin soonest to be totally reflected, are most reflexible. defin. iv. _the angle of incidence is that angle, which the line described by the incident ray contains with the perpendicular to the reflecting or refracting surface at the point of incidence._ defin. v. _the angle of reflexion or refraction, is the angle which the line described by the reflected or refracted ray containeth with the perpendicular to the reflecting or refracting surface at the point of incidence._ defin. vi. _the sines of incidence, reflexion, and refraction, are the sines of the angles of incidence, reflexion, and refraction._ defin. vii _the light whose rays are all alike refrangible, i call simple, homogeneal and similar; and that whose rays are some more refrangible than others, i call compound, heterogeneal and dissimilar._ the former light i call homogeneal, not because i would affirm it so in all respects, but because the rays which agree in refrangibility, agree at least in all those their other properties which i consider in the following discourse. defin. viii. _the colours of homogeneal lights, i call primary, homogeneal and simple; and those of heterogeneal lights, heterogeneal and compound._ for these are always compounded of the colours of homogeneal lights; as will appear in the following discourse. _axioms._ ax. i. _the angles of reflexion and refraction, lie in one and the same plane with the angle of incidence._ ax. ii. _the angle of reflexion is equal to the angle of incidence._ ax. iii. _if the refracted ray be returned directly back to the point of incidence, it shall be refracted into the line before described by the incident ray._ ax. iv. _refraction out of the rarer medium into the denser, is made towards the perpendicular; that is, so that the angle of refraction be less than the angle of incidence._ ax. v. _the sine of incidence is either accurately or very nearly in a given ratio to the sine of refraction._ whence if that proportion be known in any one inclination of the incident ray, 'tis known in all the inclinations, and thereby the refraction in all cases of incidence on the same refracting body may be determined. thus if the refraction be made out of air into water, the sine of incidence of the red light is to the sine of its refraction as to . if out of air into glass, the sines are as to . in light of other colours the sines have other proportions: but the difference is so little that it need seldom be considered. [illustration: fig. ] suppose therefore, that rs [in _fig._ .] represents the surface of stagnating water, and that c is the point of incidence in which any ray coming in the air from a in the line ac is reflected or refracted, and i would know whither this ray shall go after reflexion or refraction: i erect upon the surface of the water from the point of incidence the perpendicular cp and produce it downwards to q, and conclude by the first axiom, that the ray after reflexion and refraction, shall be found somewhere in the plane of the angle of incidence acp produced. i let fall therefore upon the perpendicular cp the sine of incidence ad; and if the reflected ray be desired, i produce ad to b so that db be equal to ad, and draw cb. for this line cb shall be the reflected ray; the angle of reflexion bcp and its sine bd being equal to the angle and sine of incidence, as they ought to be by the second axiom, but if the refracted ray be desired, i produce ad to h, so that dh may be to ad as the sine of refraction to the sine of incidence, that is, (if the light be red) as to ; and about the center c and in the plane acp with the radius ca describing a circle abe, i draw a parallel to the perpendicular cpq, the line he cutting the circumference in e, and joining ce, this line ce shall be the line of the refracted ray. for if ef be let fall perpendicularly on the line pq, this line ef shall be the sine of refraction of the ray ce, the angle of refraction being ecq; and this sine ef is equal to dh, and consequently in proportion to the sine of incidence ad as to . in like manner, if there be a prism of glass (that is, a glass bounded with two equal and parallel triangular ends, and three plain and well polished sides, which meet in three parallel lines running from the three angles of one end to the three angles of the other end) and if the refraction of the light in passing cross this prism be desired: let acb [in _fig._ .] represent a plane cutting this prism transversly to its three parallel lines or edges there where the light passeth through it, and let de be the ray incident upon the first side of the prism ac where the light goes into the glass; and by putting the proportion of the sine of incidence to the sine of refraction as to find ef the first refracted ray. then taking this ray for the incident ray upon the second side of the glass bc where the light goes out, find the next refracted ray fg by putting the proportion of the sine of incidence to the sine of refraction as to . for if the sine of incidence out of air into glass be to the sine of refraction as to , the sine of incidence out of glass into air must on the contrary be to the sine of refraction as to , by the third axiom. [illustration: fig. .] much after the same manner, if acbd [in _fig._ .] represent a glass spherically convex on both sides (usually called a _lens_, such as is a burning-glass, or spectacle-glass, or an object-glass of a telescope) and it be required to know how light falling upon it from any lucid point q shall be refracted, let qm represent a ray falling upon any point m of its first spherical surface acb, and by erecting a perpendicular to the glass at the point m, find the first refracted ray mn by the proportion of the sines to . let that ray in going out of the glass be incident upon n, and then find the second refracted ray n_q_ by the proportion of the sines to . and after the same manner may the refraction be found when the lens is convex on one side and plane or concave on the other, or concave on both sides. [illustration: fig. .] ax. vi. _homogeneal rays which flow from several points of any object, and fall perpendicularly or almost perpendicularly on any reflecting or refracting plane or spherical surface, shall afterwards diverge from so many other points, or be parallel to so many other lines, or converge to so many other points, either accurately or without any sensible error. and the same thing will happen, if the rays be reflected or refracted successively by two or three or more plane or spherical surfaces._ the point from which rays diverge or to which they converge may be called their _focus_. and the focus of the incident rays being given, that of the reflected or refracted ones may be found by finding the refraction of any two rays, as above; or more readily thus. _cas._ . let acb [in _fig._ .] be a reflecting or refracting plane, and q the focus of the incident rays, and q_q_c a perpendicular to that plane. and if this perpendicular be produced to _q_, so that _q_c be equal to qc, the point _q_ shall be the focus of the reflected rays: or if _q_c be taken on the same side of the plane with qc, and in proportion to qc as the sine of incidence to the sine of refraction, the point _q_ shall be the focus of the refracted rays. [illustration: fig. .] _cas._ . let acb [in _fig._ .] be the reflecting surface of any sphere whose centre is e. bisect any radius thereof, (suppose ec) in t, and if in that radius on the same side the point t you take the points q and _q_, so that tq, te, and t_q_, be continual proportionals, and the point q be the focus of the incident rays, the point _q_ shall be the focus of the reflected ones. [illustration: fig. .] _cas._ . let acb [in _fig._ .] be the refracting surface of any sphere whose centre is e. in any radius thereof ec produced both ways take et and c_t_ equal to one another and severally in such proportion to that radius as the lesser of the sines of incidence and refraction hath to the difference of those sines. and then if in the same line you find any two points q and _q_, so that tq be to et as e_t_ to _tq_, taking _tq_ the contrary way from _t_ which tq lieth from t, and if the point q be the focus of any incident rays, the point _q_ shall be the focus of the refracted ones. [illustration: fig. .] and by the same means the focus of the rays after two or more reflexions or refractions may be found. [illustration: fig. .] _cas._ . let acbd [in _fig._ .] be any refracting lens, spherically convex or concave or plane on either side, and let cd be its axis (that is, the line which cuts both its surfaces perpendicularly, and passes through the centres of the spheres,) and in this axis produced let f and _f_ be the foci of the refracted rays found as above, when the incident rays on both sides the lens are parallel to the same axis; and upon the diameter f_f_ bisected in e, describe a circle. suppose now that any point q be the focus of any incident rays. draw qe cutting the said circle in t and _t_, and therein take _tq_ in such proportion to _t_e as _t_e or te hath to tq. let _tq_ lie the contrary way from _t_ which tq doth from t, and _q_ shall be the focus of the refracted rays without any sensible error, provided the point q be not so remote from the axis, nor the lens so broad as to make any of the rays fall too obliquely on the refracting surfaces.[a] and by the like operations may the reflecting or refracting surfaces be found when the two foci are given, and thereby a lens be formed, which shall make the rays flow towards or from what place you please.[b] so then the meaning of this axiom is, that if rays fall upon any plane or spherical surface or lens, and before their incidence flow from or towards any point q, they shall after reflexion or refraction flow from or towards the point _q_ found by the foregoing rules. and if the incident rays flow from or towards several points q, the reflected or refracted rays shall flow from or towards so many other points _q_ found by the same rules. whether the reflected and refracted rays flow from or towards the point _q_ is easily known by the situation of that point. for if that point be on the same side of the reflecting or refracting surface or lens with the point q, and the incident rays flow from the point q, the reflected flow towards the point _q_ and the refracted from it; and if the incident rays flow towards q, the reflected flow from _q_, and the refracted towards it. and the contrary happens when _q_ is on the other side of the surface. ax. vii. _wherever the rays which come from all the points of any object meet again in so many points after they have been made to converge by reflection or refraction, there they will make a picture of the object upon any white body on which they fall._ so if pr [in _fig._ .] represent any object without doors, and ab be a lens placed at a hole in the window-shut of a dark chamber, whereby the rays that come from any point q of that object are made to converge and meet again in the point _q_; and if a sheet of white paper be held at _q_ for the light there to fall upon it, the picture of that object pr will appear upon the paper in its proper shape and colours. for as the light which comes from the point q goes to the point _q_, so the light which comes from other points p and r of the object, will go to so many other correspondent points _p_ and _r_ (as is manifest by the sixth axiom;) so that every point of the object shall illuminate a correspondent point of the picture, and thereby make a picture like the object in shape and colour, this only excepted, that the picture shall be inverted. and this is the reason of that vulgar experiment of casting the species of objects from abroad upon a wall or sheet of white paper in a dark room. in like manner, when a man views any object pqr, [in _fig._ .] the light which comes from the several points of the object is so refracted by the transparent skins and humours of the eye, (that is, by the outward coat efg, called the _tunica cornea_, and by the crystalline humour ab which is beyond the pupil _mk_) as to converge and meet again in so many points in the bottom of the eye, and there to paint the picture of the object upon that skin (called the _tunica retina_) with which the bottom of the eye is covered. for anatomists, when they have taken off from the bottom of the eye that outward and most thick coat called the _dura mater_, can then see through the thinner coats, the pictures of objects lively painted thereon. and these pictures, propagated by motion along the fibres of the optick nerves into the brain, are the cause of vision. for accordingly as these pictures are perfect or imperfect, the object is seen perfectly or imperfectly. if the eye be tinged with any colour (as in the disease of the _jaundice_) so as to tinge the pictures in the bottom of the eye with that colour, then all objects appear tinged with the same colour. if the humours of the eye by old age decay, so as by shrinking to make the _cornea_ and coat of the _crystalline humour_ grow flatter than before, the light will not be refracted enough, and for want of a sufficient refraction will not converge to the bottom of the eye but to some place beyond it, and by consequence paint in the bottom of the eye a confused picture, and according to the indistinctness of this picture the object will appear confused. this is the reason of the decay of sight in old men, and shews why their sight is mended by spectacles. for those convex glasses supply the defect of plumpness in the eye, and by increasing the refraction make the rays converge sooner, so as to convene distinctly at the bottom of the eye if the glass have a due degree of convexity. and the contrary happens in short-sighted men whose eyes are too plump. for the refraction being now too great, the rays converge and convene in the eyes before they come at the bottom; and therefore the picture made in the bottom and the vision caused thereby will not be distinct, unless the object be brought so near the eye as that the place where the converging rays convene may be removed to the bottom, or that the plumpness of the eye be taken off and the refractions diminished by a concave-glass of a due degree of concavity, or lastly that by age the eye grow flatter till it come to a due figure: for short-sighted men see remote objects best in old age, and therefore they are accounted to have the most lasting eyes. [illustration: fig. .] ax. viii. _an object seen by reflexion or refraction, appears in that place from whence the rays after their last reflexion or refraction diverge in falling on the spectator's eye._ [illustration: fig. .] if the object a [in fig. .] be seen by reflexion of a looking-glass _mn_, it shall appear, not in its proper place a, but behind the glass at _a_, from whence any rays ab, ac, ad, which flow from one and the same point of the object, do after their reflexion made in the points b, c, d, diverge in going from the glass to e, f, g, where they are incident on the spectator's eyes. for these rays do make the same picture in the bottom of the eyes as if they had come from the object really placed at _a_ without the interposition of the looking-glass; and all vision is made according to the place and shape of that picture. in like manner the object d [in fig. .] seen through a prism, appears not in its proper place d, but is thence translated to some other place _d_ situated in the last refracted ray fg drawn backward from f to _d_. [illustration: fig. .] and so the object q [in fig. .] seen through the lens ab, appears at the place _q_ from whence the rays diverge in passing from the lens to the eye. now it is to be noted, that the image of the object at _q_ is so much bigger or lesser than the object it self at q, as the distance of the image at _q_ from the lens ab is bigger or less than the distance of the object at q from the same lens. and if the object be seen through two or more such convex or concave-glasses, every glass shall make a new image, and the object shall appear in the place of the bigness of the last image. which consideration unfolds the theory of microscopes and telescopes. for that theory consists in almost nothing else than the describing such glasses as shall make the last image of any object as distinct and large and luminous as it can conveniently be made. i have now given in axioms and their explications the sum of what hath hitherto been treated of in opticks. for what hath been generally agreed on i content my self to assume under the notion of principles, in order to what i have farther to write. and this may suffice for an introduction to readers of quick wit and good understanding not yet versed in opticks: although those who are already acquainted with this science, and have handled glasses, will more readily apprehend what followeth. footnotes: [a] in our author's _lectiones opticæ_, part i. sect. iv. prop , , there is an elegant method of determining these _foci_; not only in spherical surfaces, but likewise in any other curved figure whatever: and in prop. , , the same thing is done for any ray lying out of the axis. [b] _ibid._ prop. . _propositions._ _prop._ i. theor. i. _lights which differ in colour, differ also in degrees of refrangibility._ the proof by experiments. _exper._ . i took a black oblong stiff paper terminated by parallel sides, and with a perpendicular right line drawn cross from one side to the other, distinguished it into two equal parts. one of these parts i painted with a red colour and the other with a blue. the paper was very black, and the colours intense and thickly laid on, that the phænomenon might be more conspicuous. this paper i view'd through a prism of solid glass, whose two sides through which the light passed to the eye were plane and well polished, and contained an angle of about sixty degrees; which angle i call the refracting angle of the prism. and whilst i view'd it, i held it and the prism before a window in such manner that the sides of the paper were parallel to the prism, and both those sides and the prism were parallel to the horizon, and the cross line was also parallel to it: and that the light which fell from the window upon the paper made an angle with the paper, equal to that angle which was made with the same paper by the light reflected from it to the eye. beyond the prism was the wall of the chamber under the window covered over with black cloth, and the cloth was involved in darkness that no light might be reflected from thence, which in passing by the edges of the paper to the eye, might mingle itself with the light of the paper, and obscure the phænomenon thereof. these things being thus ordered, i found that if the refracting angle of the prism be turned upwards, so that the paper may seem to be lifted upwards by the refraction, its blue half will be lifted higher by the refraction than its red half. but if the refracting angle of the prism be turned downward, so that the paper may seem to be carried lower by the refraction, its blue half will be carried something lower thereby than its red half. wherefore in both cases the light which comes from the blue half of the paper through the prism to the eye, does in like circumstances suffer a greater refraction than the light which comes from the red half, and by consequence is more refrangible. _illustration._ in the eleventh figure, mn represents the window, and de the paper terminated with parallel sides dj and he, and by the transverse line fg distinguished into two halfs, the one dg of an intensely blue colour, the other fe of an intensely red. and bac_cab_ represents the prism whose refracting planes ab_ba_ and ac_ca_ meet in the edge of the refracting angle a_a_. this edge a_a_ being upward, is parallel both to the horizon, and to the parallel-edges of the paper dj and he, and the transverse line fg is perpendicular to the plane of the window. and _de_ represents the image of the paper seen by refraction upwards in such manner, that the blue half dg is carried higher to _dg_ than the red half fe is to _fe_, and therefore suffers a greater refraction. if the edge of the refracting angle be turned downward, the image of the paper will be refracted downward; suppose to [greek: de], and the blue half will be refracted lower to [greek: dg] than the red half is to [greek: pe]. [illustration: fig. .] _exper._ . about the aforesaid paper, whose two halfs were painted over with red and blue, and which was stiff like thin pasteboard, i lapped several times a slender thred of very black silk, in such manner that the several parts of the thred might appear upon the colours like so many black lines drawn over them, or like long and slender dark shadows cast upon them. i might have drawn black lines with a pen, but the threds were smaller and better defined. this paper thus coloured and lined i set against a wall perpendicularly to the horizon, so that one of the colours might stand to the right hand, and the other to the left. close before the paper, at the confine of the colours below, i placed a candle to illuminate the paper strongly: for the experiment was tried in the night. the flame of the candle reached up to the lower edge of the paper, or a very little higher. then at the distance of six feet, and one or two inches from the paper upon the floor i erected a glass lens four inches and a quarter broad, which might collect the rays coming from the several points of the paper, and make them converge towards so many other points at the same distance of six feet, and one or two inches on the other side of the lens, and so form the image of the coloured paper upon a white paper placed there, after the same manner that a lens at a hole in a window casts the images of objects abroad upon a sheet of white paper in a dark room. the aforesaid white paper, erected perpendicular to the horizon, and to the rays which fell upon it from the lens, i moved sometimes towards the lens, sometimes from it, to find the places where the images of the blue and red parts of the coloured paper appeared most distinct. those places i easily knew by the images of the black lines which i had made by winding the silk about the paper. for the images of those fine and slender lines (which by reason of their blackness were like shadows on the colours) were confused and scarce visible, unless when the colours on either side of each line were terminated most distinctly, noting therefore, as diligently as i could, the places where the images of the red and blue halfs of the coloured paper appeared most distinct, i found that where the red half of the paper appeared distinct, the blue half appeared confused, so that the black lines drawn upon it could scarce be seen; and on the contrary, where the blue half appeared most distinct, the red half appeared confused, so that the black lines upon it were scarce visible. and between the two places where these images appeared distinct there was the distance of an inch and a half; the distance of the white paper from the lens, when the image of the red half of the coloured paper appeared most distinct, being greater by an inch and an half than the distance of the same white paper from the lens, when the image of the blue half appeared most distinct. in like incidences therefore of the blue and red upon the lens, the blue was refracted more by the lens than the red, so as to converge sooner by an inch and a half, and therefore is more refrangible. _illustration._ in the twelfth figure (p. ), de signifies the coloured paper, dg the blue half, fe the red half, mn the lens, hj the white paper in that place where the red half with its black lines appeared distinct, and _hi_ the same paper in that place where the blue half appeared distinct. the place _hi_ was nearer to the lens mn than the place hj by an inch and an half. _scholium._ the same things succeed, notwithstanding that some of the circumstances be varied; as in the first experiment when the prism and paper are any ways inclined to the horizon, and in both when coloured lines are drawn upon very black paper. but in the description of these experiments, i have set down such circumstances, by which either the phænomenon might be render'd more conspicuous, or a novice might more easily try them, or by which i did try them only. the same thing, i have often done in the following experiments: concerning all which, this one admonition may suffice. now from these experiments it follows not, that all the light of the blue is more refrangible than all the light of the red: for both lights are mixed of rays differently refrangible, so that in the red there are some rays not less refrangible than those of the blue, and in the blue there are some rays not more refrangible than those of the red: but these rays, in proportion to the whole light, are but few, and serve to diminish the event of the experiment, but are not able to destroy it. for, if the red and blue colours were more dilute and weak, the distance of the images would be less than an inch and a half; and if they were more intense and full, that distance would be greater, as will appear hereafter. these experiments may suffice for the colours of natural bodies. for in the colours made by the refraction of prisms, this proposition will appear by the experiments which are now to follow in the next proposition. _prop._ ii. theor. ii. _the light of the sun consists of rays differently refrangible._ the proof by experiments. [illustration: fig. .] [illustration: fig. .] _exper._ . in a very dark chamber, at a round hole, about one third part of an inch broad, made in the shut of a window, i placed a glass prism, whereby the beam of the sun's light, which came in at that hole, might be refracted upwards toward the opposite wall of the chamber, and there form a colour'd image of the sun. the axis of the prism (that is, the line passing through the middle of the prism from one end of it to the other end parallel to the edge of the refracting angle) was in this and the following experiments perpendicular to the incident rays. about this axis i turned the prism slowly, and saw the refracted light on the wall, or coloured image of the sun, first to descend, and then to ascend. between the descent and ascent, when the image seemed stationary, i stopp'd the prism, and fix'd it in that posture, that it should be moved no more. for in that posture the refractions of the light at the two sides of the refracting angle, that is, at the entrance of the rays into the prism, and at their going out of it, were equal to one another.[c] so also in other experiments, as often as i would have the refractions on both sides the prism to be equal to one another, i noted the place where the image of the sun formed by the refracted light stood still between its two contrary motions, in the common period of its progress and regress; and when the image fell upon that place, i made fast the prism. and in this posture, as the most convenient, it is to be understood that all the prisms are placed in the following experiments, unless where some other posture is described. the prism therefore being placed in this posture, i let the refracted light fall perpendicularly upon a sheet of white paper at the opposite wall of the chamber, and observed the figure and dimensions of the solar image formed on the paper by that light. this image was oblong and not oval, but terminated with two rectilinear and parallel sides, and two semicircular ends. on its sides it was bounded pretty distinctly, but on its ends very confusedly and indistinctly, the light there decaying and vanishing by degrees. the breadth of this image answered to the sun's diameter, and was about two inches and the eighth part of an inch, including the penumbra. for the image was eighteen feet and an half distant from the prism, and at this distance that breadth, if diminished by the diameter of the hole in the window-shut, that is by a quarter of an inch, subtended an angle at the prism of about half a degree, which is the sun's apparent diameter. but the length of the image was about ten inches and a quarter, and the length of the rectilinear sides about eight inches; and the refracting angle of the prism, whereby so great a length was made, was degrees. with a less angle the length of the image was less, the breadth remaining the same. if the prism was turned about its axis that way which made the rays emerge more obliquely out of the second refracting surface of the prism, the image soon became an inch or two longer, or more; and if the prism was turned about the contrary way, so as to make the rays fall more obliquely on the first refracting surface, the image soon became an inch or two shorter. and therefore in trying this experiment, i was as curious as i could be in placing the prism by the above-mention'd rule exactly in such a posture, that the refractions of the rays at their emergence out of the prism might be equal to that at their incidence on it. this prism had some veins running along within the glass from one end to the other, which scattered some of the sun's light irregularly, but had no sensible effect in increasing the length of the coloured spectrum. for i tried the same experiment with other prisms with the same success. and particularly with a prism which seemed free from such veins, and whose refracting angle was - / degrees, i found the length of the image - / or inches at the distance of - / feet from the prism, the breadth of the hole in the window-shut being / of an inch, as before. and because it is easy to commit a mistake in placing the prism in its due posture, i repeated the experiment four or five times, and always found the length of the image that which is set down above. with another prism of clearer glass and better polish, which seemed free from veins, and whose refracting angle was - / degrees, the length of this image at the same distance of - / feet was also about inches, or - / . beyond these measures for about a / or / of an inch at either end of the spectrum the light of the clouds seemed to be a little tinged with red and violet, but so very faintly, that i suspected that tincture might either wholly, or in great measure arise from some rays of the spectrum scattered irregularly by some inequalities in the substance and polish of the glass, and therefore i did not include it in these measures. now the different magnitude of the hole in the window-shut, and different thickness of the prism where the rays passed through it, and different inclinations of the prism to the horizon, made no sensible changes in the length of the image. neither did the different matter of the prisms make any: for in a vessel made of polished plates of glass cemented together in the shape of a prism and filled with water, there is the like success of the experiment according to the quantity of the refraction. it is farther to be observed, that the rays went on in right lines from the prism to the image, and therefore at their very going out of the prism had all that inclination to one another from which the length of the image proceeded, that is, the inclination of more than two degrees and an half. and yet according to the laws of opticks vulgarly received, they could not possibly be so much inclined to one another.[d] for let eg [_fig._ . (p. )] represent the window-shut, f the hole made therein through which a beam of the sun's light was transmitted into the darkened chamber, and abc a triangular imaginary plane whereby the prism is feigned to be cut transversely through the middle of the light. or if you please, let abc represent the prism it self, looking directly towards the spectator's eye with its nearer end: and let xy be the sun, mn the paper upon which the solar image or spectrum is cast, and pt the image it self whose sides towards _v_ and _w_ are rectilinear and parallel, and ends towards p and t semicircular. ykhp and xljt are two rays, the first of which comes from the lower part of the sun to the higher part of the image, and is refracted in the prism at k and h, and the latter comes from the higher part of the sun to the lower part of the image, and is refracted at l and j. since the refractions on both sides the prism are equal to one another, that is, the refraction at k equal to the refraction at j, and the refraction at l equal to the refraction at h, so that the refractions of the incident rays at k and l taken together, are equal to the refractions of the emergent rays at h and j taken together: it follows by adding equal things to equal things, that the refractions at k and h taken together, are equal to the refractions at j and l taken together, and therefore the two rays being equally refracted, have the same inclination to one another after refraction which they had before; that is, the inclination of half a degree answering to the sun's diameter. for so great was the inclination of the rays to one another before refraction. so then, the length of the image pt would by the rules of vulgar opticks subtend an angle of half a degree at the prism, and by consequence be equal to the breadth _vw_; and therefore the image would be round. thus it would be were the two rays xljt and ykhp, and all the rest which form the image p_w_t_v_, alike refrangible. and therefore seeing by experience it is found that the image is not round, but about five times longer than broad, the rays which going to the upper end p of the image suffer the greatest refraction, must be more refrangible than those which go to the lower end t, unless the inequality of refraction be casual. this image or spectrum pt was coloured, being red at its least refracted end t, and violet at its most refracted end p, and yellow green and blue in the intermediate spaces. which agrees with the first proposition, that lights which differ in colour, do also differ in refrangibility. the length of the image in the foregoing experiments, i measured from the faintest and outmost red at one end, to the faintest and outmost blue at the other end, excepting only a little penumbra, whose breadth scarce exceeded a quarter of an inch, as was said above. _exper._ . in the sun's beam which was propagated into the room through the hole in the window-shut, at the distance of some feet from the hole, i held the prism in such a posture, that its axis might be perpendicular to that beam. then i looked through the prism upon the hole, and turning the prism to and fro about its axis, to make the image of the hole ascend and descend, when between its two contrary motions it seemed stationary, i stopp'd the prism, that the refractions of both sides of the refracting angle might be equal to each other, as in the former experiment. in this situation of the prism viewing through it the said hole, i observed the length of its refracted image to be many times greater than its breadth, and that the most refracted part thereof appeared violet, the least refracted red, the middle parts blue, green and yellow in order. the same thing happen'd when i removed the prism out of the sun's light, and looked through it upon the hole shining by the light of the clouds beyond it. and yet if the refraction were done regularly according to one certain proportion of the sines of incidence and refraction as is vulgarly supposed, the refracted image ought to have appeared round. so then, by these two experiments it appears, that in equal incidences there is a considerable inequality of refractions. but whence this inequality arises, whether it be that some of the incident rays are refracted more, and others less, constantly, or by chance, or that one and the same ray is by refraction disturbed, shatter'd, dilated, and as it were split and spread into many diverging rays, as _grimaldo_ supposes, does not yet appear by these experiments, but will appear by those that follow. _exper._ . considering therefore, that if in the third experiment the image of the sun should be drawn out into an oblong form, either by a dilatation of every ray, or by any other casual inequality of the refractions, the same oblong image would by a second refraction made sideways be drawn out as much in breadth by the like dilatation of the rays, or other casual inequality of the refractions sideways, i tried what would be the effects of such a second refraction. for this end i ordered all things as in the third experiment, and then placed a second prism immediately after the first in a cross position to it, that it might again refract the beam of the sun's light which came to it through the first prism. in the first prism this beam was refracted upwards, and in the second sideways. and i found that by the refraction of the second prism, the breadth of the image was not increased, but its superior part, which in the first prism suffered the greater refraction, and appeared violet and blue, did again in the second prism suffer a greater refraction than its inferior part, which appeared red and yellow, and this without any dilatation of the image in breadth. [illustration: fig. ] _illustration._ let s [_fig._ , .] represent the sun, f the hole in the window, abc the first prism, dh the second prism, y the round image of the sun made by a direct beam of light when the prisms are taken away, pt the oblong image of the sun made by that beam passing through the first prism alone, when the second prism is taken away, and _pt_ the image made by the cross refractions of both prisms together. now if the rays which tend towards the several points of the round image y were dilated and spread by the refraction of the first prism, so that they should not any longer go in single lines to single points, but that every ray being split, shattered, and changed from a linear ray to a superficies of rays diverging from the point of refraction, and lying in the plane of the angles of incidence and refraction, they should go in those planes to so many lines reaching almost from one end of the image pt to the other, and if that image should thence become oblong: those rays and their several parts tending towards the several points of the image pt ought to be again dilated and spread sideways by the transverse refraction of the second prism, so as to compose a four square image, such as is represented at [greek: pt]. for the better understanding of which, let the image pt be distinguished into five equal parts pqk, kqrl, lrsm, msvn, nvt. and by the same irregularity that the orbicular light y is by the refraction of the first prism dilated and drawn out into a long image pt, the light pqk which takes up a space of the same length and breadth with the light y ought to be by the refraction of the second prism dilated and drawn out into the long image _[greek: p]qkp_, and the light kqrl into the long image _kqrl_, and the lights lrsm, msvn, nvt, into so many other long images _lrsm_, _msvn_, _nvt[greek: t]_; and all these long images would compose the four square images _[greek: pt]_. thus it ought to be were every ray dilated by refraction, and spread into a triangular superficies of rays diverging from the point of refraction. for the second refraction would spread the rays one way as much as the first doth another, and so dilate the image in breadth as much as the first doth in length. and the same thing ought to happen, were some rays casually refracted more than others. but the event is otherwise. the image pt was not made broader by the refraction of the second prism, but only became oblique, as 'tis represented at _pt_, its upper end p being by the refraction translated to a greater distance than its lower end t. so then the light which went towards the upper end p of the image, was (at equal incidences) more refracted in the second prism, than the light which tended towards the lower end t, that is the blue and violet, than the red and yellow; and therefore was more refrangible. the same light was by the refraction of the first prism translated farther from the place y to which it tended before refraction; and therefore suffered as well in the first prism as in the second a greater refraction than the rest of the light, and by consequence was more refrangible than the rest, even before its incidence on the first prism. sometimes i placed a third prism after the second, and sometimes also a fourth after the third, by all which the image might be often refracted sideways: but the rays which were more refracted than the rest in the first prism were also more refracted in all the rest, and that without any dilatation of the image sideways: and therefore those rays for their constancy of a greater refraction are deservedly reputed more refrangible. [illustration: fig. ] but that the meaning of this experiment may more clearly appear, it is to be considered that the rays which are equally refrangible do fall upon a circle answering to the sun's disque. for this was proved in the third experiment. by a circle i understand not here a perfect geometrical circle, but any orbicular figure whose length is equal to its breadth, and which, as to sense, may seem circular. let therefore ag [in _fig._ .] represent the circle which all the most refrangible rays propagated from the whole disque of the sun, would illuminate and paint upon the opposite wall if they were alone; el the circle which all the least refrangible rays would in like manner illuminate and paint if they were alone; bh, cj, dk, the circles which so many intermediate sorts of rays would successively paint upon the wall, if they were singly propagated from the sun in successive order, the rest being always intercepted; and conceive that there are other intermediate circles without number, which innumerable other intermediate sorts of rays would successively paint upon the wall if the sun should successively emit every sort apart. and seeing the sun emits all these sorts at once, they must all together illuminate and paint innumerable equal circles, of all which, being according to their degrees of refrangibility placed in order in a continual series, that oblong spectrum pt is composed which i described in the third experiment. now if the sun's circular image y [in _fig._ .] which is made by an unrefracted beam of light was by any dilation of the single rays, or by any other irregularity in the refraction of the first prism, converted into the oblong spectrum, pt: then ought every circle ag, bh, cj, &c. in that spectrum, by the cross refraction of the second prism again dilating or otherwise scattering the rays as before, to be in like manner drawn out and transformed into an oblong figure, and thereby the breadth of the image pt would be now as much augmented as the length of the image y was before by the refraction of the first prism; and thus by the refractions of both prisms together would be formed a four square figure _p[greek: p]t[greek: t]_, as i described above. wherefore since the breadth of the spectrum pt is not increased by the refraction sideways, it is certain that the rays are not split or dilated, or otherways irregularly scatter'd by that refraction, but that every circle is by a regular and uniform refraction translated entire into another place, as the circle ag by the greatest refraction into the place _ag_, the circle bh by a less refraction into the place _bh_, the circle cj by a refraction still less into the place _ci_, and so of the rest; by which means a new spectrum _pt_ inclined to the former pt is in like manner composed of circles lying in a right line; and these circles must be of the same bigness with the former, because the breadths of all the spectrums y, pt and _pt_ at equal distances from the prisms are equal. i considered farther, that by the breadth of the hole f through which the light enters into the dark chamber, there is a penumbra made in the circuit of the spectrum y, and that penumbra remains in the rectilinear sides of the spectrums pt and _pt_. i placed therefore at that hole a lens or object-glass of a telescope which might cast the image of the sun distinctly on y without any penumbra at all, and found that the penumbra of the rectilinear sides of the oblong spectrums pt and _pt_ was also thereby taken away, so that those sides appeared as distinctly defined as did the circumference of the first image y. thus it happens if the glass of the prisms be free from veins, and their sides be accurately plane and well polished without those numberless waves or curles which usually arise from sand-holes a little smoothed in polishing with putty. if the glass be only well polished and free from veins, and the sides not accurately plane, but a little convex or concave, as it frequently happens; yet may the three spectrums y, pt and _pt_ want penumbras, but not in equal distances from the prisms. now from this want of penumbras, i knew more certainly that every one of the circles was refracted according to some most regular, uniform and constant law. for if there were any irregularity in the refraction, the right lines ae and gl, which all the circles in the spectrum pt do touch, could not by that refraction be translated into the lines _ae_ and _gl_ as distinct and straight as they were before, but there would arise in those translated lines some penumbra or crookedness or undulation, or other sensible perturbation contrary to what is found by experience. whatsoever penumbra or perturbation should be made in the circles by the cross refraction of the second prism, all that penumbra or perturbation would be conspicuous in the right lines _ae_ and _gl_ which touch those circles. and therefore since there is no such penumbra or perturbation in those right lines, there must be none in the circles. since the distance between those tangents or breadth of the spectrum is not increased by the refractions, the diameters of the circles are not increased thereby. since those tangents continue to be right lines, every circle which in the first prism is more or less refracted, is exactly in the same proportion more or less refracted in the second. and seeing all these things continue to succeed after the same manner when the rays are again in a third prism, and again in a fourth refracted sideways, it is evident that the rays of one and the same circle, as to their degree of refrangibility, continue always uniform and homogeneal to one another, and that those of several circles do differ in degree of refrangibility, and that in some certain and constant proportion. which is the thing i was to prove. there is yet another circumstance or two of this experiment by which it becomes still more plain and convincing. let the second prism dh [in _fig._ .] be placed not immediately after the first, but at some distance from it; suppose in the mid-way between it and the wall on which the oblong spectrum pt is cast, so that the light from the first prism may fall upon it in the form of an oblong spectrum [greek: pt] parallel to this second prism, and be refracted sideways to form the oblong spectrum _pt_ upon the wall. and you will find as before, that this spectrum _pt_ is inclined to that spectrum pt, which the first prism forms alone without the second; the blue ends p and _p_ being farther distant from one another than the red ones t and _t_, and by consequence that the rays which go to the blue end [greek: p] of the image [greek: pt], and which therefore suffer the greatest refraction in the first prism, are again in the second prism more refracted than the rest. [illustration: fig. .] [illustration: fig. .] the same thing i try'd also by letting the sun's light into a dark room through two little round holes f and [greek: ph] [in _fig._ .] made in the window, and with two parallel prisms abc and [greek: abg] placed at those holes (one at each) refracting those two beams of light to the opposite wall of the chamber, in such manner that the two colour'd images pt and mn which they there painted were joined end to end and lay in one straight line, the red end t of the one touching the blue end m of the other. for if these two refracted beams were again by a third prism dh placed cross to the two first, refracted sideways, and the spectrums thereby translated to some other part of the wall of the chamber, suppose the spectrum pt to _pt_ and the spectrum mn to _mn_, these translated spectrums _pt_ and _mn_ would not lie in one straight line with their ends contiguous as before, but be broken off from one another and become parallel, the blue end _m_ of the image _mn_ being by a greater refraction translated farther from its former place mt, than the red end _t_ of the other image _pt_ from the same place mt; which puts the proposition past dispute. and this happens whether the third prism dh be placed immediately after the two first, or at a great distance from them, so that the light refracted in the two first prisms be either white and circular, or coloured and oblong when it falls on the third. _exper._ . in the middle of two thin boards i made round holes a third part of an inch in diameter, and in the window-shut a much broader hole being made to let into my darkned chamber a large beam of the sun's light; i placed a prism behind the shut in that beam to refract it towards the opposite wall, and close behind the prism i fixed one of the boards, in such manner that the middle of the refracted light might pass through the hole made in it, and the rest be intercepted by the board. then at the distance of about twelve feet from the first board i fixed the other board in such manner that the middle of the refracted light which came through the hole in the first board, and fell upon the opposite wall, might pass through the hole in this other board, and the rest being intercepted by the board might paint upon it the coloured spectrum of the sun. and close behind this board i fixed another prism to refract the light which came through the hole. then i returned speedily to the first prism, and by turning it slowly to and fro about its axis, i caused the image which fell upon the second board to move up and down upon that board, that all its parts might successively pass through the hole in that board and fall upon the prism behind it. and in the mean time, i noted the places on the opposite wall to which that light after its refraction in the second prism did pass; and by the difference of the places i found that the light which being most refracted in the first prism did go to the blue end of the image, was again more refracted in the second prism than the light which went to the red end of that image, which proves as well the first proposition as the second. and this happened whether the axis of the two prisms were parallel, or inclined to one another, and to the horizon in any given angles. _illustration._ let f [in _fig._ .] be the wide hole in the window-shut, through which the sun shines upon the first prism abc, and let the refracted light fall upon the middle of the board de, and the middle part of that light upon the hole g made in the middle part of that board. let this trajected part of that light fall again upon the middle of the second board _de_, and there paint such an oblong coloured image of the sun as was described in the third experiment. by turning the prism abc slowly to and fro about its axis, this image will be made to move up and down the board _de_, and by this means all its parts from one end to the other may be made to pass successively through the hole _g_ which is made in the middle of that board. in the mean while another prism _abc_ is to be fixed next after that hole _g_, to refract the trajected light a second time. and these things being thus ordered, i marked the places m and n of the opposite wall upon which the refracted light fell, and found that whilst the two boards and second prism remained unmoved, those places by turning the first prism about its axis were changed perpetually. for when the lower part of the light which fell upon the second board _de_ was cast through the hole _g_, it went to a lower place m on the wall and when the higher part of that light was cast through the same hole _g_, it went to a higher place n on the wall, and when any intermediate part of the light was cast through that hole, it went to some place on the wall between m and n. the unchanged position of the holes in the boards, made the incidence of the rays upon the second prism to be the same in all cases. and yet in that common incidence some of the rays were more refracted, and others less. and those were more refracted in this prism, which by a greater refraction in the first prism were more turned out of the way, and therefore for their constancy of being more refracted are deservedly called more refrangible. [illustration: fig. .] [illustration: fig. .] _exper._ . at two holes made near one another in my window-shut i placed two prisms, one at each, which might cast upon the opposite wall (after the manner of the third experiment) two oblong coloured images of the sun. and at a little distance from the wall i placed a long slender paper with straight and parallel edges, and ordered the prisms and paper so, that the red colour of one image might fall directly upon one half of the paper, and the violet colour of the other image upon the other half of the same paper; so that the paper appeared of two colours, red and violet, much after the manner of the painted paper in the first and second experiments. then with a black cloth i covered the wall behind the paper, that no light might be reflected from it to disturb the experiment, and viewing the paper through a third prism held parallel to it, i saw that half of it which was illuminated by the violet light to be divided from the other half by a greater refraction, especially when i went a good way off from the paper. for when i viewed it too near at hand, the two halfs of the paper did not appear fully divided from one another, but seemed contiguous at one of their angles like the painted paper in the first experiment. which also happened when the paper was too broad. [illustration: fig. .] sometimes instead of the paper i used a white thred, and this appeared through the prism divided into two parallel threds as is represented in the nineteenth figure, where dg denotes the thred illuminated with violet light from d to e and with red light from f to g, and _defg_ are the parts of the thred seen by refraction. if one half of the thred be constantly illuminated with red, and the other half be illuminated with all the colours successively, (which may be done by causing one of the prisms to be turned about its axis whilst the other remains unmoved) this other half in viewing the thred through the prism, will appear in a continual right line with the first half when illuminated with red, and begin to be a little divided from it when illuminated with orange, and remove farther from it when illuminated with yellow, and still farther when with green, and farther when with blue, and go yet farther off when illuminated with indigo, and farthest when with deep violet. which plainly shews, that the lights of several colours are more and more refrangible one than another, in this order of their colours, red, orange, yellow, green, blue, indigo, deep violet; and so proves as well the first proposition as the second. i caused also the coloured spectrums pt [in _fig._ .] and mn made in a dark chamber by the refractions of two prisms to lie in a right line end to end, as was described above in the fifth experiment, and viewing them through a third prism held parallel to their length, they appeared no longer in a right line, but became broken from one another, as they are represented at _pt_ and _mn_, the violet end _m_ of the spectrum _mn_ being by a greater refraction translated farther from its former place mt than the red end _t_ of the other spectrum _pt_. i farther caused those two spectrums pt [in _fig._ .] and mn to become co-incident in an inverted order of their colours, the red end of each falling on the violet end of the other, as they are represented in the oblong figure ptmn; and then viewing them through a prism dh held parallel to their length, they appeared not co-incident, as when view'd with the naked eye, but in the form of two distinct spectrums _pt_ and _mn_ crossing one another in the middle after the manner of the letter x. which shews that the red of the one spectrum and violet of the other, which were co-incident at pn and mt, being parted from one another by a greater refraction of the violet to _p_ and _m_ than of the red to _n_ and _t_, do differ in degrees of refrangibility. i illuminated also a little circular piece of white paper all over with the lights of both prisms intermixed, and when it was illuminated with the red of one spectrum, and deep violet of the other, so as by the mixture of those colours to appear all over purple, i viewed the paper, first at a less distance, and then at a greater, through a third prism; and as i went from the paper, the refracted image thereof became more and more divided by the unequal refraction of the two mixed colours, and at length parted into two distinct images, a red one and a violet one, whereof the violet was farthest from the paper, and therefore suffered the greatest refraction. and when that prism at the window, which cast the violet on the paper was taken away, the violet image disappeared; but when the other prism was taken away the red vanished; which shews, that these two images were nothing else than the lights of the two prisms, which had been intermixed on the purple paper, but were parted again by their unequal refractions made in the third prism, through which the paper was view'd. this also was observable, that if one of the prisms at the window, suppose that which cast the violet on the paper, was turned about its axis to make all the colours in this order, violet, indigo, blue, green, yellow, orange, red, fall successively on the paper from that prism, the violet image changed colour accordingly, turning successively to indigo, blue, green, yellow and red, and in changing colour came nearer and nearer to the red image made by the other prism, until when it was also red both images became fully co-incident. i placed also two paper circles very near one another, the one in the red light of one prism, and the other in the violet light of the other. the circles were each of them an inch in diameter, and behind them the wall was dark, that the experiment might not be disturbed by any light coming from thence. these circles thus illuminated, i viewed through a prism, so held, that the refraction might be made towards the red circle, and as i went from them they came nearer and nearer together, and at length became co-incident; and afterwards when i went still farther off, they parted again in a contrary order, the violet by a greater refraction being carried beyond the red. _exper._ . in summer, when the sun's light uses to be strongest, i placed a prism at the hole of the window-shut, as in the third experiment, yet so that its axis might be parallel to the axis of the world, and at the opposite wall in the sun's refracted light, i placed an open book. then going six feet and two inches from the book, i placed there the above-mentioned lens, by which the light reflected from the book might be made to converge and meet again at the distance of six feet and two inches behind the lens, and there paint the species of the book upon a sheet of white paper much after the manner of the second experiment. the book and lens being made fast, i noted the place where the paper was, when the letters of the book, illuminated by the fullest red light of the solar image falling upon it, did cast their species on that paper most distinctly: and then i stay'd till by the motion of the sun, and consequent motion of his image on the book, all the colours from that red to the middle of the blue pass'd over those letters; and when those letters were illuminated by that blue, i noted again the place of the paper when they cast their species most distinctly upon it: and i found that this last place of the paper was nearer to the lens than its former place by about two inches and an half, or two and three quarters. so much sooner therefore did the light in the violet end of the image by a greater refraction converge and meet, than the light in the red end. but in trying this, the chamber was as dark as i could make it. for, if these colours be diluted and weakned by the mixture of any adventitious light, the distance between the places of the paper will not be so great. this distance in the second experiment, where the colours of natural bodies were made use of, was but an inch and an half, by reason of the imperfection of those colours. here in the colours of the prism, which are manifestly more full, intense, and lively than those of natural bodies, the distance is two inches and three quarters. and were the colours still more full, i question not but that the distance would be considerably greater. for the coloured light of the prism, by the interfering of the circles described in the second figure of the fifth experiment, and also by the light of the very bright clouds next the sun's body intermixing with these colours, and by the light scattered by the inequalities in the polish of the prism, was so very much compounded, that the species which those faint and dark colours, the indigo and violet, cast upon the paper were not distinct enough to be well observed. _exper._ . a prism, whose two angles at its base were equal to one another, and half right ones, and the third a right one, i placed in a beam of the sun's light let into a dark chamber through a hole in the window-shut, as in the third experiment. and turning the prism slowly about its axis, until all the light which went through one of its angles, and was refracted by it began to be reflected by its base, at which till then it went out of the glass, i observed that those rays which had suffered the greatest refraction were sooner reflected than the rest. i conceived therefore, that those rays of the reflected light, which were most refrangible, did first of all by a total reflexion become more copious in that light than the rest, and that afterwards the rest also, by a total reflexion, became as copious as these. to try this, i made the reflected light pass through another prism, and being refracted by it to fall afterwards upon a sheet of white paper placed at some distance behind it, and there by that refraction to paint the usual colours of the prism. and then causing the first prism to be turned about its axis as above, i observed that when those rays, which in this prism had suffered the greatest refraction, and appeared of a blue and violet colour began to be totally reflected, the blue and violet light on the paper, which was most refracted in the second prism, received a sensible increase above that of the red and yellow, which was least refracted; and afterwards, when the rest of the light which was green, yellow, and red, began to be totally reflected in the first prism, the light of those colours on the paper received as great an increase as the violet and blue had done before. whence 'tis manifest, that the beam of light reflected by the base of the prism, being augmented first by the more refrangible rays, and afterwards by the less refrangible ones, is compounded of rays differently refrangible. and that all such reflected light is of the same nature with the sun's light before its incidence on the base of the prism, no man ever doubted; it being generally allowed, that light by such reflexions suffers no alteration in its modifications and properties. i do not here take notice of any refractions made in the sides of the first prism, because the light enters it perpendicularly at the first side, and goes out perpendicularly at the second side, and therefore suffers none. so then, the sun's incident light being of the same temper and constitution with his emergent light, and the last being compounded of rays differently refrangible, the first must be in like manner compounded. [illustration: fig. .] _illustration._ in the twenty-first figure, abc is the first prism, bc its base, b and c its equal angles at the base, each of degrees, a its rectangular vertex, fm a beam of the sun's light let into a dark room through a hole f one third part of an inch broad, m its incidence on the base of the prism, mg a less refracted ray, mh a more refracted ray, mn the beam of light reflected from the base, vxy the second prism by which this beam in passing through it is refracted, n_t_ the less refracted light of this beam, and n_p_ the more refracted part thereof. when the first prism abc is turned about its axis according to the order of the letters abc, the rays mh emerge more and more obliquely out of that prism, and at length after their most oblique emergence are reflected towards n, and going on to _p_ do increase the number of the rays n_p_. afterwards by continuing the motion of the first prism, the rays mg are also reflected to n and increase the number of the rays n_t_. and therefore the light mn admits into its composition, first the more refrangible rays, and then the less refrangible rays, and yet after this composition is of the same nature with the sun's immediate light fm, the reflexion of the specular base bc causing no alteration therein. _exper._ . two prisms, which were alike in shape, i tied so together, that their axis and opposite sides being parallel, they composed a parallelopiped. and, the sun shining into my dark chamber through a little hole in the window-shut, i placed that parallelopiped in his beam at some distance from the hole, in such a posture, that the axes of the prisms might be perpendicular to the incident rays, and that those rays being incident upon the first side of one prism, might go on through the two contiguous sides of both prisms, and emerge out of the last side of the second prism. this side being parallel to the first side of the first prism, caused the emerging light to be parallel to the incident. then, beyond these two prisms i placed a third, which might refract that emergent light, and by that refraction cast the usual colours of the prism upon the opposite wall, or upon a sheet of white paper held at a convenient distance behind the prism for that refracted light to fall upon it. after this i turned the parallelopiped about its axis, and found that when the contiguous sides of the two prisms became so oblique to the incident rays, that those rays began all of them to be reflected, those rays which in the third prism had suffered the greatest refraction, and painted the paper with violet and blue, were first of all by a total reflexion taken out of the transmitted light, the rest remaining and on the paper painting their colours of green, yellow, orange and red, as before; and afterwards by continuing the motion of the two prisms, the rest of the rays also by a total reflexion vanished in order, according to their degrees of refrangibility. the light therefore which emerged out of the two prisms is compounded of rays differently refrangible, seeing the more refrangible rays may be taken out of it, while the less refrangible remain. but this light being trajected only through the parallel superficies of the two prisms, if it suffer'd any change by the refraction of one superficies it lost that impression by the contrary refraction of the other superficies, and so being restor'd to its pristine constitution, became of the same nature and condition as at first before its incidence on those prisms; and therefore, before its incidence, was as much compounded of rays differently refrangible, as afterwards. [illustration: fig. .] _illustration._ in the twenty second figure abc and bcd are the two prisms tied together in the form of a parallelopiped, their sides bc and cb being contiguous, and their sides ab and cd parallel. and hjk is the third prism, by which the sun's light propagated through the hole f into the dark chamber, and there passing through those sides of the prisms ab, bc, cb and cd, is refracted at o to the white paper pt, falling there partly upon p by a greater refraction, partly upon t by a less refraction, and partly upon r and other intermediate places by intermediate refractions. by turning the parallelopiped acbd about its axis, according to the order of the letters a, c, d, b, at length when the contiguous planes bc and cb become sufficiently oblique to the rays fm, which are incident upon them at m, there will vanish totally out of the refracted light opt, first of all the most refracted rays op, (the rest or and ot remaining as before) then the rays or and other intermediate ones, and lastly, the least refracted rays ot. for when the plane bc becomes sufficiently oblique to the rays incident upon it, those rays will begin to be totally reflected by it towards n; and first the most refrangible rays will be totally reflected (as was explained in the preceding experiment) and by consequence must first disappear at p, and afterwards the rest as they are in order totally reflected to n, they must disappear in the same order at r and t. so then the rays which at o suffer the greatest refraction, may be taken out of the light mo whilst the rest of the rays remain in it, and therefore that light mo is compounded of rays differently refrangible. and because the planes ab and cd are parallel, and therefore by equal and contrary refractions destroy one anothers effects, the incident light fm must be of the same kind and nature with the emergent light mo, and therefore doth also consist of rays differently refrangible. these two lights fm and mo, before the most refrangible rays are separated out of the emergent light mo, agree in colour, and in all other properties so far as my observation reaches, and therefore are deservedly reputed of the same nature and constitution, and by consequence the one is compounded as well as the other. but after the most refrangible rays begin to be totally reflected, and thereby separated out of the emergent light mo, that light changes its colour from white to a dilute and faint yellow, a pretty good orange, a very full red successively, and then totally vanishes. for after the most refrangible rays which paint the paper at p with a purple colour, are by a total reflexion taken out of the beam of light mo, the rest of the colours which appear on the paper at r and t being mix'd in the light mo compound there a faint yellow, and after the blue and part of the green which appear on the paper between p and r are taken away, the rest which appear between r and t (that is the yellow, orange, red and a little green) being mixed in the beam mo compound there an orange; and when all the rays are by reflexion taken out of the beam mo, except the least refrangible, which at t appear of a full red, their colour is the same in that beam mo as afterwards at t, the refraction of the prism hjk serving only to separate the differently refrangible rays, without making any alteration in their colours, as shall be more fully proved hereafter. all which confirms as well the first proposition as the second. _scholium._ if this experiment and the former be conjoined and made one by applying a fourth prism vxy [in _fig._ .] to refract the reflected beam mn towards _tp_, the conclusion will be clearer. for then the light n_p_ which in the fourth prism is more refracted, will become fuller and stronger when the light op, which in the third prism hjk is more refracted, vanishes at p; and afterwards when the less refracted light ot vanishes at t, the less refracted light n_t_ will become increased whilst the more refracted light at _p_ receives no farther increase. and as the trajected beam mo in vanishing is always of such a colour as ought to result from the mixture of the colours which fall upon the paper pt, so is the reflected beam mn always of such a colour as ought to result from the mixture of the colours which fall upon the paper _pt_. for when the most refrangible rays are by a total reflexion taken out of the beam mo, and leave that beam of an orange colour, the excess of those rays in the reflected light, does not only make the violet, indigo and blue at _p_ more full, but also makes the beam mn change from the yellowish colour of the sun's light, to a pale white inclining to blue, and afterward recover its yellowish colour again, so soon as all the rest of the transmitted light mot is reflected. now seeing that in all this variety of experiments, whether the trial be made in light reflected, and that either from natural bodies, as in the first and second experiment, or specular, as in the ninth; or in light refracted, and that either before the unequally refracted rays are by diverging separated from one another, and losing their whiteness which they have altogether, appear severally of several colours, as in the fifth experiment; or after they are separated from one another, and appear colour'd as in the sixth, seventh, and eighth experiments; or in light trajected through parallel superficies, destroying each others effects, as in the tenth experiment; there are always found rays, which at equal incidences on the same medium suffer unequal refractions, and that without any splitting or dilating of single rays, or contingence in the inequality of the refractions, as is proved in the fifth and sixth experiments. and seeing the rays which differ in refrangibility may be parted and sorted from one another, and that either by refraction as in the third experiment, or by reflexion as in the tenth, and then the several sorts apart at equal incidences suffer unequal refractions, and those sorts are more refracted than others after separation, which were more refracted before it, as in the sixth and following experiments, and if the sun's light be trajected through three or more cross prisms successively, those rays which in the first prism are refracted more than others, are in all the following prisms refracted more than others in the same rate and proportion, as appears by the fifth experiment; it's manifest that the sun's light is an heterogeneous mixture of rays, some of which are constantly more refrangible than others, as was proposed. _prop._ iii. theor. iii. _the sun's light consists of rays differing in reflexibility, and those rays are more reflexible than others which are more refrangible._ this is manifest by the ninth and tenth experiments: for in the ninth experiment, by turning the prism about its axis, until the rays within it which in going out into the air were refracted by its base, became so oblique to that base, as to begin to be totally reflected thereby; those rays became first of all totally reflected, which before at equal incidences with the rest had suffered the greatest refraction. and the same thing happens in the reflexion made by the common base of the two prisms in the tenth experiment. _prop._ iv. prob. i. _to separate from one another the heterogeneous rays of compound light._ [illustration: fig. .] the heterogeneous rays are in some measure separated from one another by the refraction of the prism in the third experiment, and in the fifth experiment, by taking away the penumbra from the rectilinear sides of the coloured image, that separation in those very rectilinear sides or straight edges of the image becomes perfect. but in all places between those rectilinear edges, those innumerable circles there described, which are severally illuminated by homogeneal rays, by interfering with one another, and being every where commix'd, do render the light sufficiently compound. but if these circles, whilst their centers keep their distances and positions, could be made less in diameter, their interfering one with another, and by consequence the mixture of the heterogeneous rays would be proportionally diminish'd. in the twenty third figure let ag, bh, cj, dk, el, fm be the circles which so many sorts of rays flowing from the same disque of the sun, do in the third experiment illuminate; of all which and innumerable other intermediate ones lying in a continual series between the two rectilinear and parallel edges of the sun's oblong image pt, that image is compos'd, as was explained in the fifth experiment. and let _ag_, _bh_, _ci_, _dk_, _el_, _fm_ be so many less circles lying in a like continual series between two parallel right lines _af_ and _gm_ with the same distances between their centers, and illuminated by the same sorts of rays, that is the circle _ag_ with the same sort by which the corresponding circle ag was illuminated, and the circle _bh_ with the same sort by which the corresponding circle bh was illuminated, and the rest of the circles _ci_, _dk_, _el_, _fm_ respectively, with the same sorts of rays by which the several corresponding circles cj, dk, el, fm were illuminated. in the figure pt composed of the greater circles, three of those circles ag, bh, cj, are so expanded into one another, that the three sorts of rays by which those circles are illuminated, together with other innumerable sorts of intermediate rays, are mixed at qr in the middle of the circle bh. and the like mixture happens throughout almost the whole length of the figure pt. but in the figure _pt_ composed of the less circles, the three less circles _ag_, _bh_, _ci_, which answer to those three greater, do not extend into one another; nor are there any where mingled so much as any two of the three sorts of rays by which those circles are illuminated, and which in the figure pt are all of them intermingled at bh. now he that shall thus consider it, will easily understand that the mixture is diminished in the same proportion with the diameters of the circles. if the diameters of the circles whilst their centers remain the same, be made three times less than before, the mixture will be also three times less; if ten times less, the mixture will be ten times less, and so of other proportions. that is, the mixture of the rays in the greater figure pt will be to their mixture in the less _pt_, as the latitude of the greater figure is to the latitude of the less. for the latitudes of these figures are equal to the diameters of their circles. and hence it easily follows, that the mixture of the rays in the refracted spectrum _pt_ is to the mixture of the rays in the direct and immediate light of the sun, as the breadth of that spectrum is to the difference between the length and breadth of the same spectrum. so then, if we would diminish the mixture of the rays, we are to diminish the diameters of the circles. now these would be diminished if the sun's diameter to which they answer could be made less than it is, or (which comes to the same purpose) if without doors, at a great distance from the prism towards the sun, some opake body were placed, with a round hole in the middle of it, to intercept all the sun's light, excepting so much as coming from the middle of his body could pass through that hole to the prism. for so the circles ag, bh, and the rest, would not any longer answer to the whole disque of the sun, but only to that part of it which could be seen from the prism through that hole, that it is to the apparent magnitude of that hole view'd from the prism. but that these circles may answer more distinctly to that hole, a lens is to be placed by the prism to cast the image of the hole, (that is, every one of the circles ag, bh, &c.) distinctly upon the paper at pt, after such a manner, as by a lens placed at a window, the species of objects abroad are cast distinctly upon a paper within the room, and the rectilinear sides of the oblong solar image in the fifth experiment became distinct without any penumbra. if this be done, it will not be necessary to place that hole very far off, no not beyond the window. and therefore instead of that hole, i used the hole in the window-shut, as follows. _exper._ . in the sun's light let into my darken'd chamber through a small round hole in my window-shut, at about ten or twelve feet from the window, i placed a lens, by which the image of the hole might be distinctly cast upon a sheet of white paper, placed at the distance of six, eight, ten, or twelve feet from the lens. for, according to the difference of the lenses i used various distances, which i think not worth the while to describe. then immediately after the lens i placed a prism, by which the trajected light might be refracted either upwards or sideways, and thereby the round image, which the lens alone did cast upon the paper might be drawn out into a long one with parallel sides, as in the third experiment. this oblong image i let fall upon another paper at about the same distance from the prism as before, moving the paper either towards the prism or from it, until i found the just distance where the rectilinear sides of the image became most distinct. for in this case, the circular images of the hole, which compose that image after the same manner that the circles _ag_, _bh_, _ci_, &c. do the figure _pt_ [in _fig._ .] were terminated most distinctly without any penumbra, and therefore extended into one another the least that they could, and by consequence the mixture of the heterogeneous rays was now the least of all. by this means i used to form an oblong image (such as is _pt_) [in _fig._ , and .] of circular images of the hole, (such as are _ag_, _bh_, _ci_, &c.) and by using a greater or less hole in the window-shut, i made the circular images _ag_, _bh_, _ci_, &c. of which it was formed, to become greater or less at pleasure, and thereby the mixture of the rays in the image _pt_ to be as much, or as little as i desired. [illustration: fig. .] _illustration._ in the twenty-fourth figure, f represents the circular hole in the window-shut, mn the lens, whereby the image or species of that hole is cast distinctly upon a paper at j, abc the prism, whereby the rays are at their emerging out of the lens refracted from j towards another paper at _pt_, and the round image at j is turned into an oblong image _pt_ falling on that other paper. this image _pt_ consists of circles placed one after another in a rectilinear order, as was sufficiently explained in the fifth experiment; and these circles are equal to the circle j, and consequently answer in magnitude to the hole f; and therefore by diminishing that hole they may be at pleasure diminished, whilst their centers remain in their places. by this means i made the breadth of the image _pt_ to be forty times, and sometimes sixty or seventy times less than its length. as for instance, if the breadth of the hole f be one tenth of an inch, and mf the distance of the lens from the hole be feet; and if _p_b or _p_m the distance of the image _pt_ from the prism or lens be feet, and the refracting angle of the prism be degrees, the breadth of the image _pt_ will be one twelfth of an inch, and the length about six inches, and therefore the length to the breadth as to , and by consequence the light of this image times less compound than the sun's direct light. and light thus far simple and homogeneal, is sufficient for trying all the experiments in this book about simple light. for the composition of heterogeneal rays is in this light so little, that it is scarce to be discovered and perceiv'd by sense, except perhaps in the indigo and violet. for these being dark colours do easily suffer a sensible allay by that little scattering light which uses to be refracted irregularly by the inequalities of the prism. yet instead of the circular hole f, 'tis better to substitute an oblong hole shaped like a long parallelogram with its length parallel to the prism abc. for if this hole be an inch or two long, and but a tenth or twentieth part of an inch broad, or narrower; the light of the image _pt_ will be as simple as before, or simpler, and the image will become much broader, and therefore more fit to have experiments try'd in its light than before. instead of this parallelogram hole may be substituted a triangular one of equal sides, whose base, for instance, is about the tenth part of an inch, and its height an inch or more. for by this means, if the axis of the prism be parallel to the perpendicular of the triangle, the image _pt_ [in _fig._ .] will now be form'd of equicrural triangles _ag_, _bh_, _ci_, _dk_, _el_, _fm_, &c. and innumerable other intermediate ones answering to the triangular hole in shape and bigness, and lying one after another in a continual series between two parallel lines _af_ and _gm_. these triangles are a little intermingled at their bases, but not at their vertices; and therefore the light on the brighter side _af_ of the image, where the bases of the triangles are, is a little compounded, but on the darker side _gm_ is altogether uncompounded, and in all places between the sides the composition is proportional to the distances of the places from that obscurer side _gm_. and having a spectrum _pt_ of such a composition, we may try experiments either in its stronger and less simple light near the side _af_, or in its weaker and simpler light near the other side _gm_, as it shall seem most convenient. [illustration: fig. .] but in making experiments of this kind, the chamber ought to be made as dark as can be, lest any foreign light mingle it self with the light of the spectrum _pt_, and render it compound; especially if we would try experiments in the more simple light next the side _gm_ of the spectrum; which being fainter, will have a less proportion to the foreign light; and so by the mixture of that light be more troubled, and made more compound. the lens also ought to be good, such as may serve for optical uses, and the prism ought to have a large angle, suppose of or degrees, and to be well wrought, being made of glass free from bubbles and veins, with its sides not a little convex or concave, as usually happens, but truly plane, and its polish elaborate, as in working optick-glasses, and not such as is usually wrought with putty, whereby the edges of the sand-holes being worn away, there are left all over the glass a numberless company of very little convex polite risings like waves. the edges also of the prism and lens, so far as they may make any irregular refraction, must be covered with a black paper glewed on. and all the light of the sun's beam let into the chamber, which is useless and unprofitable to the experiment, ought to be intercepted with black paper, or other black obstacles. for otherwise the useless light being reflected every way in the chamber, will mix with the oblong spectrum, and help to disturb it. in trying these things, so much diligence is not altogether necessary, but it will promote the success of the experiments, and by a very scrupulous examiner of things deserves to be apply'd. it's difficult to get glass prisms fit for this purpose, and therefore i used sometimes prismatick vessels made with pieces of broken looking-glasses, and filled with rain water. and to increase the refraction, i sometimes impregnated the water strongly with _saccharum saturni_. _prop._ v. theor. iv. _homogeneal light is refracted regularly without any dilatation splitting or shattering of the rays, and the confused vision of objects seen through refracting bodies by heterogeneal light arises from the different refrangibility of several sorts of rays._ the first part of this proposition has been already sufficiently proved in the fifth experiment, and will farther appear by the experiments which follow. _exper._ . in the middle of a black paper i made a round hole about a fifth or sixth part of an inch in diameter. upon this paper i caused the spectrum of homogeneal light described in the former proposition, so to fall, that some part of the light might pass through the hole of the paper. this transmitted part of the light i refracted with a prism placed behind the paper, and letting this refracted light fall perpendicularly upon a white paper two or three feet distant from the prism, i found that the spectrum formed on the paper by this light was not oblong, as when 'tis made (in the third experiment) by refracting the sun's compound light, but was (so far as i could judge by my eye) perfectly circular, the length being no greater than the breadth. which shews, that this light is refracted regularly without any dilatation of the rays. _exper._ . in the homogeneal light i placed a paper circle of a quarter of an inch in diameter, and in the sun's unrefracted heterogeneal white light i placed another paper circle of the same bigness. and going from the papers to the distance of some feet, i viewed both circles through a prism. the circle illuminated by the sun's heterogeneal light appeared very oblong, as in the fourth experiment, the length being many times greater than the breadth; but the other circle, illuminated with homogeneal light, appeared circular and distinctly defined, as when 'tis view'd with the naked eye. which proves the whole proposition. _exper._ . in the homogeneal light i placed flies, and such-like minute objects, and viewing them through a prism, i saw their parts as distinctly defined, as if i had viewed them with the naked eye. the same objects placed in the sun's unrefracted heterogeneal light, which was white, i viewed also through a prism, and saw them most confusedly defined, so that i could not distinguish their smaller parts from one another. i placed also the letters of a small print, one while in the homogeneal light, and then in the heterogeneal, and viewing them through a prism, they appeared in the latter case so confused and indistinct, that i could not read them; but in the former they appeared so distinct, that i could read readily, and thought i saw them as distinct, as when i view'd them with my naked eye. in both cases i view'd the same objects, through the same prism at the same distance from me, and in the same situation. there was no difference, but in the light by which the objects were illuminated, and which in one case was simple, and in the other compound; and therefore, the distinct vision in the former case, and confused in the latter, could arise from nothing else than from that difference of the lights. which proves the whole proposition. and in these three experiments it is farther very remarkable, that the colour of homogeneal light was never changed by the refraction. _prop._ vi. theor. v. _the sine of incidence of every ray considered apart, is to its sine of refraction in a given ratio._ that every ray consider'd apart, is constant to it self in some degree of refrangibility, is sufficiently manifest out of what has been said. those rays, which in the first refraction, are at equal incidences most refracted, are also in the following refractions at equal incidences most refracted; and so of the least refrangible, and the rest which have any mean degree of refrangibility, as is manifest by the fifth, sixth, seventh, eighth, and ninth experiments. and those which the first time at like incidences are equally refracted, are again at like incidences equally and uniformly refracted, and that whether they be refracted before they be separated from one another, as in the fifth experiment, or whether they be refracted apart, as in the twelfth, thirteenth and fourteenth experiments. the refraction therefore of every ray apart is regular, and what rule that refraction observes we are now to shew.[e] the late writers in opticks teach, that the sines of incidence are in a given proportion to the sines of refraction, as was explained in the fifth axiom, and some by instruments fitted for measuring of refractions, or otherwise experimentally examining this proportion, do acquaint us that they have found it accurate. but whilst they, not understanding the different refrangibility of several rays, conceived them all to be refracted according to one and the same proportion, 'tis to be presumed that they adapted their measures only to the middle of the refracted light; so that from their measures we may conclude only that the rays which have a mean degree of refrangibility, that is, those which when separated from the rest appear green, are refracted according to a given proportion of their sines. and therefore we are now to shew, that the like given proportions obtain in all the rest. that it should be so is very reasonable, nature being ever conformable to her self; but an experimental proof is desired. and such a proof will be had, if we can shew that the sines of refraction of rays differently refrangible are one to another in a given proportion when their sines of incidence are equal. for, if the sines of refraction of all the rays are in given proportions to the sine of refractions of a ray which has a mean degree of refrangibility, and this sine is in a given proportion to the equal sines of incidence, those other sines of refraction will also be in given proportions to the equal sines of incidence. now, when the sines of incidence are equal, it will appear by the following experiment, that the sines of refraction are in a given proportion to one another. [illustration: fig. .] _exper._ . the sun shining into a dark chamber through a little round hole in the window-shut, let s [in _fig._ .] represent his round white image painted on the opposite wall by his direct light, pt his oblong coloured image made by refracting that light with a prism placed at the window; and _pt_, or _ p t_, _ p t_, his oblong colour'd image made by refracting again the same light sideways with a second prism placed immediately after the first in a cross position to it, as was explained in the fifth experiment; that is to say, _pt_ when the refraction of the second prism is small, _ p t_ when its refraction is greater, and _ p t_ when it is greatest. for such will be the diversity of the refractions, if the refracting angle of the second prism be of various magnitudes; suppose of fifteen or twenty degrees to make the image _pt_, of thirty or forty to make the image _ p t_, and of sixty to make the image _ p t_. but for want of solid glass prisms with angles of convenient bignesses, there may be vessels made of polished plates of glass cemented together in the form of prisms and filled with water. these things being thus ordered, i observed that all the solar images or coloured spectrums pt, _pt_, _ p t_, _ p t_ did very nearly converge to the place s on which the direct light of the sun fell and painted his white round image when the prisms were taken away. the axis of the spectrum pt, that is the line drawn through the middle of it parallel to its rectilinear sides, did when produced pass exactly through the middle of that white round image s. and when the refraction of the second prism was equal to the refraction of the first, the refracting angles of them both being about degrees, the axis of the spectrum _ p t_ made by that refraction, did when produced pass also through the middle of the same white round image s. but when the refraction of the second prism was less than that of the first, the produced axes of the spectrums _tp_ or _ t p_ made by that refraction did cut the produced axis of the spectrum tp in the points _m_ and _n_, a little beyond the center of that white round image s. whence the proportion of the line _t_t to the line _p_p was a little greater than the proportion of _t_t or _p_p, and this proportion a little greater than that of _t_t to _p_p. now when the light of the spectrum pt falls perpendicularly upon the wall, those lines _t_t, _p_p, and _t_t, and _p_p, and _t_t, _p_p, are the tangents of the refractions, and therefore by this experiment the proportions of the tangents of the refractions are obtained, from whence the proportions of the sines being derived, they come out equal, so far as by viewing the spectrums, and using some mathematical reasoning i could estimate. for i did not make an accurate computation. so then the proposition holds true in every ray apart, so far as appears by experiment. and that it is accurately true, may be demonstrated upon this supposition. _that bodies refract light by acting upon its rays in lines perpendicular to their surfaces._ but in order to this demonstration, i must distinguish the motion of every ray into two motions, the one perpendicular to the refracting surface, the other parallel to it, and concerning the perpendicular motion lay down the following proposition. if any motion or moving thing whatsoever be incident with any velocity on any broad and thin space terminated on both sides by two parallel planes, and in its passage through that space be urged perpendicularly towards the farther plane by any force which at given distances from the plane is of given quantities; the perpendicular velocity of that motion or thing, at its emerging out of that space, shall be always equal to the square root of the sum of the square of the perpendicular velocity of that motion or thing at its incidence on that space; and of the square of the perpendicular velocity which that motion or thing would have at its emergence, if at its incidence its perpendicular velocity was infinitely little. and the same proposition holds true of any motion or thing perpendicularly retarded in its passage through that space, if instead of the sum of the two squares you take their difference. the demonstration mathematicians will easily find out, and therefore i shall not trouble the reader with it. suppose now that a ray coming most obliquely in the line mc [in _fig._ .] be refracted at c by the plane rs into the line cn, and if it be required to find the line ce, into which any other ray ac shall be refracted; let mc, ad, be the sines of incidence of the two rays, and ng, ef, their sines of refraction, and let the equal motions of the incident rays be represented by the equal lines mc and ac, and the motion mc being considered as parallel to the refracting plane, let the other motion ac be distinguished into two motions ad and dc, one of which ad is parallel, and the other dc perpendicular to the refracting surface. in like manner, let the motions of the emerging rays be distinguish'd into two, whereof the perpendicular ones are mc/ng × cg and ad/ef × cf. and if the force of the refracting plane begins to act upon the rays either in that plane or at a certain distance from it on the one side, and ends at a certain distance from it on the other side, and in all places between those two limits acts upon the rays in lines perpendicular to that refracting plane, and the actions upon the rays at equal distances from the refracting plane be equal, and at unequal ones either equal or unequal according to any rate whatever; that motion of the ray which is parallel to the refracting plane, will suffer no alteration by that force; and that motion which is perpendicular to it will be altered according to the rule of the foregoing proposition. if therefore for the perpendicular velocity of the emerging ray cn you write mc/ng × cg as above, then the perpendicular velocity of any other emerging ray ce which was ad/ef × cf, will be equal to the square root of cd_q_ + (_mcq/ngq_ × cg_q_). and by squaring these equals, and adding to them the equals ad_q_ and mc_q_ - cd_q_, and dividing the sums by the equals cf_q_ + ef_q_ and cg_q_ + ng_q_, you will have _mcq/ngq_ equal to _adq/efq_. whence ad, the sine of incidence, is to ef the sine of refraction, as mc to ng, that is, in a given _ratio_. and this demonstration being general, without determining what light is, or by what kind of force it is refracted, or assuming any thing farther than that the refracting body acts upon the rays in lines perpendicular to its surface; i take it to be a very convincing argument of the full truth of this proposition. so then, if the _ratio_ of the sines of incidence and refraction of any sort of rays be found in any one case, 'tis given in all cases; and this may be readily found by the method in the following proposition. _prop._ vii. theor. vi. _the perfection of telescopes is impeded by the different refrangibility of the rays of light._ the imperfection of telescopes is vulgarly attributed to the spherical figures of the glasses, and therefore mathematicians have propounded to figure them by the conical sections. to shew that they are mistaken, i have inserted this proposition; the truth of which will appear by the measure of the refractions of the several sorts of rays; and these measures i thus determine. in the third experiment of this first part, where the refracting angle of the prism was - / degrees, the half of that angle deg. min. is the angle of incidence of the rays at their going out of the glass into the air[f]; and the sine of this angle is , the radius being . when the axis of this prism was parallel to the horizon, and the refraction of the rays at their incidence on this prism equal to that at their emergence out of it, i observed with a quadrant the angle which the mean refrangible rays, (that is those which went to the middle of the sun's coloured image) made with the horizon, and by this angle and the sun's altitude observed at the same time, i found the angle which the emergent rays contained with the incident to be deg. and min. and the half of this angle added to the angle of incidence deg. min. makes the angle of refraction, which is therefore deg. min. and its sine . these are the sines of incidence and refraction of the mean refrangible rays, and their proportion in round numbers is to . this glass was of a colour inclining to green. the last of the prisms mentioned in the third experiment was of clear white glass. its refracting angle - / degrees. the angle which the emergent rays contained, with the incident deg. min. the sine of half the first angle . the sine of half the sum of the angles . and their proportion in round numbers to , as before. from the length of the image, which was about - / or inches, subduct its breadth, which was - / inches, and the remainder - / inches would be the length of the image were the sun but a point, and therefore subtends the angle which the most and least refrangible rays, when incident on the prism in the same lines, do contain with one another after their emergence. whence this angle is deg. ´. ´´. for the distance between the image and the prism where this angle is made, was - / feet, and at that distance the chord - / inches subtends an angle of deg. ´. ´´. now half this angle is the angle which these emergent rays contain with the emergent mean refrangible rays, and a quarter thereof, that is ´. ´´. may be accounted the angle which they would contain with the same emergent mean refrangible rays, were they co-incident to them within the glass, and suffered no other refraction than that at their emergence. for, if two equal refractions, the one at the incidence of the rays on the prism, the other at their emergence, make half the angle deg. ´. ´´. then one of those refractions will make about a quarter of that angle, and this quarter added to, and subducted from the angle of refraction of the mean refrangible rays, which was deg. ´, gives the angles of refraction of the most and least refrangible rays deg. ´ ´´, and deg. ´ ´´, whose sines are and , the common angle of incidence being deg. ´, and its sine ; and these sines in the least round numbers are in proportion to one another, as and to . now, if you subduct the common sine of incidence from the sines of refraction and , the remainders and shew, that in small refractions the refraction of the least refrangible rays is to the refraction of the most refrangible ones, as to very nearly, and that the difference of the refractions of the least refrangible and most refrangible rays is about the - / th part of the whole refraction of the mean refrangible rays. whence they that are skilled in opticks will easily understand,[g] that the breadth of the least circular space, into which object-glasses of telescopes can collect all sorts of parallel rays, is about the - / th part of half the aperture of the glass, or th part of the whole aperture; and that the focus of the most refrangible rays is nearer to the object-glass than the focus of the least refrangible ones, by about the - / th part of the distance between the object-glass and the focus of the mean refrangible ones. and if rays of all sorts, flowing from any one lucid point in the axis of any convex lens, be made by the refraction of the lens to converge to points not too remote from the lens, the focus of the most refrangible rays shall be nearer to the lens than the focus of the least refrangible ones, by a distance which is to the - / th part of the distance of the focus of the mean refrangible rays from the lens, as the distance between that focus and the lucid point, from whence the rays flow, is to the distance between that lucid point and the lens very nearly. now to examine whether the difference between the refractions, which the most refrangible and the least refrangible rays flowing from the same point suffer in the object-glasses of telescopes and such-like glasses, be so great as is here described, i contrived the following experiment. _exper._ . the lens which i used in the second and eighth experiments, being placed six feet and an inch distant from any object, collected the species of that object by the mean refrangible rays at the distance of six feet and an inch from the lens on the other side. and therefore by the foregoing rule, it ought to collect the species of that object by the least refrangible rays at the distance of six feet and - / inches from the lens, and by the most refrangible ones at the distance of five feet and - / inches from it: so that between the two places, where these least and most refrangible rays collect the species, there may be the distance of about - / inches. for by that rule, as six feet and an inch (the distance of the lens from the lucid object) is to twelve feet and two inches (the distance of the lucid object from the focus of the mean refrangible rays) that is, as one is to two; so is the - / th part of six feet and an inch (the distance between the lens and the same focus) to the distance between the focus of the most refrangible rays and the focus of the least refrangible ones, which is therefore - / inches, that is very nearly - / inches. now to know whether this measure was true, i repeated the second and eighth experiment with coloured light, which was less compounded than that i there made use of: for i now separated the heterogeneous rays from one another by the method i described in the eleventh experiment, so as to make a coloured spectrum about twelve or fifteen times longer than broad. this spectrum i cast on a printed book, and placing the above-mentioned lens at the distance of six feet and an inch from this spectrum to collect the species of the illuminated letters at the same distance on the other side, i found that the species of the letters illuminated with blue were nearer to the lens than those illuminated with deep red by about three inches, or three and a quarter; but the species of the letters illuminated with indigo and violet appeared so confused and indistinct, that i could not read them: whereupon viewing the prism, i found it was full of veins running from one end of the glass to the other; so that the refraction could not be regular. i took another prism therefore which was free from veins, and instead of the letters i used two or three parallel black lines a little broader than the strokes of the letters, and casting the colours upon these lines in such manner, that the lines ran along the colours from one end of the spectrum to the other, i found that the focus where the indigo, or confine of this colour and violet cast the species of the black lines most distinctly, to be about four inches, or - / nearer to the lens than the focus, where the deepest red cast the species of the same black lines most distinctly. the violet was so faint and dark, that i could not discern the species of the lines distinctly by that colour; and therefore considering that the prism was made of a dark coloured glass inclining to green, i took another prism of clear white glass; but the spectrum of colours which this prism made had long white streams of faint light shooting out from both ends of the colours, which made me conclude that something was amiss; and viewing the prism, i found two or three little bubbles in the glass, which refracted the light irregularly. wherefore i covered that part of the glass with black paper, and letting the light pass through another part of it which was free from such bubbles, the spectrum of colours became free from those irregular streams of light, and was now such as i desired. but still i found the violet so dark and faint, that i could scarce see the species of the lines by the violet, and not at all by the deepest part of it, which was next the end of the spectrum. i suspected therefore, that this faint and dark colour might be allayed by that scattering light which was refracted, and reflected irregularly, partly by some very small bubbles in the glasses, and partly by the inequalities of their polish; which light, tho' it was but little, yet it being of a white colour, might suffice to affect the sense so strongly as to disturb the phænomena of that weak and dark colour the violet, and therefore i tried, as in the th, th, and th experiments, whether the light of this colour did not consist of a sensible mixture of heterogeneous rays, but found it did not. nor did the refractions cause any other sensible colour than violet to emerge out of this light, as they would have done out of white light, and by consequence out of this violet light had it been sensibly compounded with white light. and therefore i concluded, that the reason why i could not see the species of the lines distinctly by this colour, was only the darkness of this colour, and thinness of its light, and its distance from the axis of the lens; i divided therefore those parallel black lines into equal parts, by which i might readily know the distances of the colours in the spectrum from one another, and noted the distances of the lens from the foci of such colours, as cast the species of the lines distinctly, and then considered whether the difference of those distances bear such proportion to - / inches, the greatest difference of the distances, which the foci of the deepest red and violet ought to have from the lens, as the distance of the observed colours from one another in the spectrum bear to the greatest distance of the deepest red and violet measured in the rectilinear sides of the spectrum, that is, to the length of those sides, or excess of the length of the spectrum above its breadth. and my observations were as follows. when i observed and compared the deepest sensible red, and the colour in the confine of green and blue, which at the rectilinear sides of the spectrum was distant from it half the length of those sides, the focus where the confine of green and blue cast the species of the lines distinctly on the paper, was nearer to the lens than the focus, where the red cast those lines distinctly on it by about - / or - / inches. for sometimes the measures were a little greater, sometimes a little less, but seldom varied from one another above / of an inch. for it was very difficult to define the places of the foci, without some little errors. now, if the colours distant half the length of the image, (measured at its rectilinear sides) give - / or - / difference of the distances of their foci from the lens, then the colours distant the whole length ought to give or - / inches difference of those distances. but here it's to be noted, that i could not see the red to the full end of the spectrum, but only to the center of the semicircle which bounded that end, or a little farther; and therefore i compared this red not with that colour which was exactly in the middle of the spectrum, or confine of green and blue, but with that which verged a little more to the blue than to the green: and as i reckoned the whole length of the colours not to be the whole length of the spectrum, but the length of its rectilinear sides, so compleating the semicircular ends into circles, when either of the observed colours fell within those circles, i measured the distance of that colour from the semicircular end of the spectrum, and subducting half this distance from the measured distance of the two colours, i took the remainder for their corrected distance; and in these observations set down this corrected distance for the difference of the distances of their foci from the lens. for, as the length of the rectilinear sides of the spectrum would be the whole length of all the colours, were the circles of which (as we shewed) that spectrum consists contracted and reduced to physical points, so in that case this corrected distance would be the real distance of the two observed colours. when therefore i farther observed the deepest sensible red, and that blue whose corrected distance from it was / parts of the length of the rectilinear sides of the spectrum, the difference of the distances of their foci from the lens was about - / inches, and as to , so is - / to - / . when i observed the deepest sensible red, and that indigo whose corrected distance was / or / of the length of the rectilinear sides of the spectrum, the difference of the distances of their foci from the lens, was about - / inches, and as to , so is - / to - / . when i observed the deepest sensible red, and that deep indigo whose corrected distance from one another was / or / of the length of the rectilinear sides of the spectrum, the difference of the distances of their foci from the lens was about inches; and as to , so is to - / . when i observed the deepest sensible red, and that part of the violet next the indigo, whose corrected distance from the red was / or / of the length of the rectilinear sides of the spectrum, the difference of the distances of their foci from the lens was about - / inches, and as to , so is - / to - / . for sometimes, when the lens was advantageously placed, so that its axis respected the blue, and all things else were well ordered, and the sun shone clear, and i held my eye very near to the paper on which the lens cast the species of the lines, i could see pretty distinctly the species of those lines by that part of the violet which was next the indigo; and sometimes i could see them by above half the violet, for in making these experiments i had observed, that the species of those colours only appear distinct, which were in or near the axis of the lens: so that if the blue or indigo were in the axis, i could see their species distinctly; and then the red appeared much less distinct than before. wherefore i contrived to make the spectrum of colours shorter than before, so that both its ends might be nearer to the axis of the lens. and now its length was about - / inches, and breadth about / or / of an inch. also instead of the black lines on which the spectrum was cast, i made one black line broader than those, that i might see its species more easily; and this line i divided by short cross lines into equal parts, for measuring the distances of the observed colours. and now i could sometimes see the species of this line with its divisions almost as far as the center of the semicircular violet end of the spectrum, and made these farther observations. when i observed the deepest sensible red, and that part of the violet, whose corrected distance from it was about / parts of the rectilinear sides of the spectrum, the difference of the distances of the foci of those colours from the lens, was one time - / , another time - / , another time - / inches; and as to , so are - / , - / , - / , to - / , - / , - / respectively. when i observed the deepest sensible red, and deepest sensible violet, (the corrected distance of which colours, when all things were ordered to the best advantage, and the sun shone very clear, was about / or / parts of the length of the rectilinear sides of the coloured spectrum) i found the difference of the distances of their foci from the lens sometimes - / sometimes - / , and for the most part inches or thereabouts; and as to , or to , so is five inches to - / or - / inches. and by this progression of experiments i satisfied my self, that had the light at the very ends of the spectrum been strong enough to make the species of the black lines appear plainly on the paper, the focus of the deepest violet would have been found nearer to the lens, than the focus of the deepest red, by about - / inches at least. and this is a farther evidence, that the sines of incidence and refraction of the several sorts of rays, hold the same proportion to one another in the smallest refractions which they do in the greatest. my progress in making this nice and troublesome experiment i have set down more at large, that they that shall try it after me may be aware of the circumspection requisite to make it succeed well. and if they cannot make it succeed so well as i did, they may notwithstanding collect by the proportion of the distance of the colours of the spectrum, to the difference of the distances of their foci from the lens, what would be the success in the more distant colours by a better trial. and yet, if they use a broader lens than i did, and fix it to a long strait staff, by means of which it may be readily and truly directed to the colour whose focus is desired, i question not but the experiment will succeed better with them than it did with me. for i directed the axis as nearly as i could to the middle of the colours, and then the faint ends of the spectrum being remote from the axis, cast their species less distinctly on the paper than they would have done, had the axis been successively directed to them. now by what has been said, it's certain that the rays which differ in refrangibility do not converge to the same focus; but if they flow from a lucid point, as far from the lens on one side as their foci are on the other, the focus of the most refrangible rays shall be nearer to the lens than that of the least refrangible, by above the fourteenth part of the whole distance; and if they flow from a lucid point, so very remote from the lens, that before their incidence they may be accounted parallel, the focus of the most refrangible rays shall be nearer to the lens than the focus of the least refrangible, by about the th or th part of their whole distance from it. and the diameter of the circle in the middle space between those two foci which they illuminate, when they fall there on any plane, perpendicular to the axis (which circle is the least into which they can all be gathered) is about the th part of the diameter of the aperture of the glass. so that 'tis a wonder, that telescopes represent objects so distinct as they do. but were all the rays of light equally refrangible, the error arising only from the sphericalness of the figures of glasses would be many hundred times less. for, if the object-glass of a telescope be plano-convex, and the plane side be turned towards the object, and the diameter of the sphere, whereof this glass is a segment, be called d, and the semi-diameter of the aperture of the glass be called s, and the sine of incidence out of glass into air, be to the sine of refraction as i to r; the rays which come parallel to the axis of the glass, shall in the place where the image of the object is most distinctly made, be scattered all over a little circle, whose diameter is _(rq/iq) × (s cub./d quad.)_ very nearly,[h] as i gather by computing the errors of the rays by the method of infinite series, and rejecting the terms, whose quantities are inconsiderable. as for instance, if the sine of incidence i, be to the sine of refraction r, as to , and if d the diameter of the sphere, to which the convex-side of the glass is ground, be feet or inches, and s the semi-diameter of the aperture be two inches, the diameter of the little circle, (that is (_rq × s cub.)/(iq × d quad._)) will be ( × × )/( × × × ) (or / ) parts of an inch. but the diameter of the little circle, through which these rays are scattered by unequal refrangibility, will be about the th part of the aperture of the object-glass, which here is four inches. and therefore, the error arising from the spherical figure of the glass, is to the error arising from the different refrangibility of the rays, as / to / , that is as to ; and therefore being in comparison so very little, deserves not to be considered. [illustration: fig. .] but you will say, if the errors caused by the different refrangibility be so very great, how comes it to pass, that objects appear through telescopes so distinct as they do? i answer, 'tis because the erring rays are not scattered uniformly over all that circular space, but collected infinitely more densely in the center than in any other part of the circle, and in the way from the center to the circumference, grow continually rarer and rarer, so as at the circumference to become infinitely rare; and by reason of their rarity are not strong enough to be visible, unless in the center and very near it. let ade [in _fig._ .] represent one of those circles described with the center c, and semi-diameter ac, and let bfg be a smaller circle concentrick to the former, cutting with its circumference the diameter ac in b, and bisect ac in n; and by my reckoning, the density of the light in any place b, will be to its density in n, as ab to bc; and the whole light within the lesser circle bfg, will be to the whole light within the greater aed, as the excess of the square of ac above the square of ab, is to the square of ac. as if bc be the fifth part of ac, the light will be four times denser in b than in n, and the whole light within the less circle, will be to the whole light within the greater, as nine to twenty-five. whence it's evident, that the light within the less circle, must strike the sense much more strongly, than that faint and dilated light round about between it and the circumference of the greater. but it's farther to be noted, that the most luminous of the prismatick colours are the yellow and orange. these affect the senses more strongly than all the rest together, and next to these in strength are the red and green. the blue compared with these is a faint and dark colour, and the indigo and violet are much darker and fainter, so that these compared with the stronger colours are little to be regarded. the images of objects are therefore to be placed, not in the focus of the mean refrangible rays, which are in the confine of green and blue, but in the focus of those rays which are in the middle of the orange and yellow; there where the colour is most luminous and fulgent, that is in the brightest yellow, that yellow which inclines more to orange than to green. and by the refraction of these rays (whose sines of incidence and refraction in glass are as and ) the refraction of glass and crystal for optical uses is to be measured. let us therefore place the image of the object in the focus of these rays, and all the yellow and orange will fall within a circle, whose diameter is about the th part of the diameter of the aperture of the glass. and if you add the brighter half of the red, (that half which is next the orange) and the brighter half of the green, (that half which is next the yellow) about three fifth parts of the light of these two colours will fall within the same circle, and two fifth parts will fall without it round about; and that which falls without will be spread through almost as much more space as that which falls within, and so in the gross be almost three times rarer. of the other half of the red and green, (that is of the deep dark red and willow green) about one quarter will fall within this circle, and three quarters without, and that which falls without will be spread through about four or five times more space than that which falls within; and so in the gross be rarer, and if compared with the whole light within it, will be about times rarer than all that taken in the gross; or rather more than or times rarer, because the deep red in the end of the spectrum of colours made by a prism is very thin and rare, and the willow green is something rarer than the orange and yellow. the light of these colours therefore being so very much rarer than that within the circle, will scarce affect the sense, especially since the deep red and willow green of this light, are much darker colours than the rest. and for the same reason the blue and violet being much darker colours than these, and much more rarified, may be neglected. for the dense and bright light of the circle, will obscure the rare and weak light of these dark colours round about it, and render them almost insensible. the sensible image of a lucid point is therefore scarce broader than a circle, whose diameter is the th part of the diameter of the aperture of the object-glass of a good telescope, or not much broader, if you except a faint and dark misty light round about it, which a spectator will scarce regard. and therefore in a telescope, whose aperture is four inches, and length an hundred feet, it exceeds not ´´ ´´´, or ´´. and in a telescope whose aperture is two inches, and length or feet, it may be ´´ or ´´, and scarce above. and this answers well to experience: for some astronomers have found the diameters of the fix'd stars, in telescopes of between and feet in length, to be about ´´ or ´´, or at most ´´ or ´´ in diameter. but if the eye-glass be tincted faintly with the smoak of a lamp or torch, to obscure the light of the star, the fainter light in the circumference of the star ceases to be visible, and the star (if the glass be sufficiently soiled with smoak) appears something more like a mathematical point. and for the same reason, the enormous part of the light in the circumference of every lucid point ought to be less discernible in shorter telescopes than in longer, because the shorter transmit less light to the eye. now, that the fix'd stars, by reason of their immense distance, appear like points, unless so far as their light is dilated by refraction, may appear from hence; that when the moon passes over them and eclipses them, their light vanishes, not gradually like that of the planets, but all at once; and in the end of the eclipse it returns into sight all at once, or certainly in less time than the second of a minute; the refraction of the moon's atmosphere a little protracting the time in which the light of the star first vanishes, and afterwards returns into sight. now, if we suppose the sensible image of a lucid point, to be even times narrower than the aperture of the glass; yet this image would be still much greater than if it were only from the spherical figure of the glass. for were it not for the different refrangibility of the rays, its breadth in an foot telescope whose aperture is inches, would be but / parts of an inch, as is manifest by the foregoing computation. and therefore in this case the greatest errors arising from the spherical figure of the glass, would be to the greatest sensible errors arising from the different refrangibility of the rays as / to / at most, that is only as to . and this sufficiently shews that it is not the spherical figures of glasses, but the different refrangibility of the rays which hinders the perfection of telescopes. there is another argument by which it may appear that the different refrangibility of rays, is the true cause of the imperfection of telescopes. for the errors of the rays arising from the spherical figures of object-glasses, are as the cubes of the apertures of the object glasses; and thence to make telescopes of various lengths magnify with equal distinctness, the apertures of the object-glasses, and the charges or magnifying powers ought to be as the cubes of the square roots of their lengths; which doth not answer to experience. but the errors of the rays arising from the different refrangibility, are as the apertures of the object-glasses; and thence to make telescopes of various lengths, magnify with equal distinctness, their apertures and charges ought to be as the square roots of their lengths; and this answers to experience, as is well known. for instance, a telescope of feet in length, with an aperture of - / inches, magnifies about times, with as much distinctness as one of a foot in length, with / of an inch aperture, magnifies times. [illustration: fig. .] now were it not for this different refrangibility of rays, telescopes might be brought to a greater perfection than we have yet describ'd, by composing the object-glass of two glasses with water between them. let adfc [in _fig._ .] represent the object-glass composed of two glasses abed and befc, alike convex on the outsides agd and chf, and alike concave on the insides bme, bne, with water in the concavity bmen. let the sine of incidence out of glass into air be as i to r, and out of water into air, as k to r, and by consequence out of glass into water, as i to k: and let the diameter of the sphere to which the convex sides agd and chf are ground be d, and the diameter of the sphere to which the concave sides bme and bne, are ground be to d, as the cube root of kk--ki to the cube root of rk--ri: and the refractions on the concave sides of the glasses, will very much correct the errors of the refractions on the convex sides, so far as they arise from the sphericalness of the figure. and by this means might telescopes be brought to sufficient perfection, were it not for the different refrangibility of several sorts of rays. but by reason of this different refrangibility, i do not yet see any other means of improving telescopes by refractions alone, than that of increasing their lengths, for which end the late contrivance of _hugenius_ seems well accommodated. for very long tubes are cumbersome, and scarce to be readily managed, and by reason of their length are very apt to bend, and shake by bending, so as to cause a continual trembling in the objects, whereby it becomes difficult to see them distinctly: whereas by his contrivance the glasses are readily manageable, and the object-glass being fix'd upon a strong upright pole becomes more steady. seeing therefore the improvement of telescopes of given lengths by refractions is desperate; i contrived heretofore a perspective by reflexion, using instead of an object-glass a concave metal. the diameter of the sphere to which the metal was ground concave was about _english_ inches, and by consequence the length of the instrument about six inches and a quarter. the eye-glass was plano-convex, and the diameter of the sphere to which the convex side was ground was about / of an inch, or a little less, and by consequence it magnified between and times. by another way of measuring i found that it magnified about times. the concave metal bore an aperture of an inch and a third part; but the aperture was limited not by an opake circle, covering the limb of the metal round about, but by an opake circle placed between the eyeglass and the eye, and perforated in the middle with a little round hole for the rays to pass through to the eye. for this circle by being placed here, stopp'd much of the erroneous light, which otherwise would have disturbed the vision. by comparing it with a pretty good perspective of four feet in length, made with a concave eye-glass, i could read at a greater distance with my own instrument than with the glass. yet objects appeared much darker in it than in the glass, and that partly because more light was lost by reflexion in the metal, than by refraction in the glass, and partly because my instrument was overcharged. had it magnified but or times, it would have made the object appear more brisk and pleasant. two of these i made about years ago, and have one of them still by me, by which i can prove the truth of what i write. yet it is not so good as at the first. for the concave has been divers times tarnished and cleared again, by rubbing it with very soft leather. when i made these an artist in _london_ undertook to imitate it; but using another way of polishing them than i did, he fell much short of what i had attained to, as i afterwards understood by discoursing the under-workman he had employed. the polish i used was in this manner. i had two round copper plates, each six inches in diameter, the one convex, the other concave, ground very true to one another. on the convex i ground the object-metal or concave which was to be polish'd, 'till it had taken the figure of the convex and was ready for a polish. then i pitched over the convex very thinly, by dropping melted pitch upon it, and warming it to keep the pitch soft, whilst i ground it with the concave copper wetted to make it spread eavenly all over the convex. thus by working it well i made it as thin as a groat, and after the convex was cold i ground it again to give it as true a figure as i could. then i took putty which i had made very fine by washing it from all its grosser particles, and laying a little of this upon the pitch, i ground it upon the pitch with the concave copper, till it had done making a noise; and then upon the pitch i ground the object-metal with a brisk motion, for about two or three minutes of time, leaning hard upon it. then i put fresh putty upon the pitch, and ground it again till it had done making a noise, and afterwards ground the object-metal upon it as before. and this work i repeated till the metal was polished, grinding it the last time with all my strength for a good while together, and frequently breathing upon the pitch, to keep it moist without laying on any more fresh putty. the object-metal was two inches broad, and about one third part of an inch thick, to keep it from bending. i had two of these metals, and when i had polished them both, i tried which was best, and ground the other again, to see if i could make it better than that which i kept. and thus by many trials i learn'd the way of polishing, till i made those two reflecting perspectives i spake of above. for this art of polishing will be better learn'd by repeated practice than by my description. before i ground the object-metal on the pitch, i always ground the putty on it with the concave copper, till it had done making a noise, because if the particles of the putty were not by this means made to stick fast in the pitch, they would by rolling up and down grate and fret the object-metal and fill it full of little holes. but because metal is more difficult to polish than glass, and is afterwards very apt to be spoiled by tarnishing, and reflects not so much light as glass quick-silver'd over does: i would propound to use instead of the metal, a glass ground concave on the foreside, and as much convex on the backside, and quick-silver'd over on the convex side. the glass must be every where of the same thickness exactly. otherwise it will make objects look colour'd and indistinct. by such a glass i tried about five or six years ago to make a reflecting telescope of four feet in length to magnify about times, and i satisfied my self that there wants nothing but a good artist to bring the design to perfection. for the glass being wrought by one of our _london_ artists after such a manner as they grind glasses for telescopes, though it seemed as well wrought as the object-glasses use to be, yet when it was quick-silver'd, the reflexion discovered innumerable inequalities all over the glass. and by reason of these inequalities, objects appeared indistinct in this instrument. for the errors of reflected rays caused by any inequality of the glass, are about six times greater than the errors of refracted rays caused by the like inequalities. yet by this experiment i satisfied my self that the reflexion on the concave side of the glass, which i feared would disturb the vision, did no sensible prejudice to it, and by consequence that nothing is wanting to perfect these telescopes, but good workmen who can grind and polish glasses truly spherical. an object-glass of a fourteen foot telescope, made by an artificer at _london_, i once mended considerably, by grinding it on pitch with putty, and leaning very easily on it in the grinding, lest the putty should scratch it. whether this way may not do well enough for polishing these reflecting glasses, i have not yet tried. but he that shall try either this or any other way of polishing which he may think better, may do well to make his glasses ready for polishing, by grinding them without that violence, wherewith our _london_ workmen press their glasses in grinding. for by such violent pressure, glasses are apt to bend a little in the grinding, and such bending will certainly spoil their figure. to recommend therefore the consideration of these reflecting glasses to such artists as are curious in figuring glasses, i shall describe this optical instrument in the following proposition. _prop._ viii. prob. ii. _to shorten telescopes._ let abcd [in _fig._ .] represent a glass spherically concave on the foreside ab, and as much convex on the backside cd, so that it be every where of an equal thickness. let it not be thicker on one side than on the other, lest it make objects appear colour'd and indistinct, and let it be very truly wrought and quick-silver'd over on the backside; and set in the tube vxyz which must be very black within. let efg represent a prism of glass or crystal placed near the other end of the tube, in the middle of it, by means of a handle of brass or iron fgk, to the end of which made flat it is cemented. let this prism be rectangular at e, and let the other two angles at f and g be accurately equal to each other, and by consequence equal to half right ones, and let the plane sides fe and ge be square, and by consequence the third side fg a rectangular parallelogram, whose length is to its breadth in a subduplicate proportion of two to one. let it be so placed in the tube, that the axis of the speculum may pass through the middle of the square side ef perpendicularly and by consequence through the middle of the side fg at an angle of degrees, and let the side ef be turned towards the speculum, and the distance of this prism from the speculum be such that the rays of the light pq, rs, &c. which are incident upon the speculum in lines parallel to the axis thereof, may enter the prism at the side ef, and be reflected by the side fg, and thence go out of it through the side ge, to the point t, which must be the common focus of the speculum abdc, and of a plano-convex eye-glass h, through which those rays must pass to the eye. and let the rays at their coming out of the glass pass through a small round hole, or aperture made in a little plate of lead, brass, or silver, wherewith the glass is to be covered, which hole must be no bigger than is necessary for light enough to pass through. for so it will render the object distinct, the plate in which 'tis made intercepting all the erroneous part of the light which comes from the verges of the speculum ab. such an instrument well made, if it be six foot long, (reckoning the length from the speculum to the prism, and thence to the focus t) will bear an aperture of six inches at the speculum, and magnify between two and three hundred times. but the hole h here limits the aperture with more advantage, than if the aperture was placed at the speculum. if the instrument be made longer or shorter, the aperture must be in proportion as the cube of the square-square root of the length, and the magnifying as the aperture. but it's convenient that the speculum be an inch or two broader than the aperture at the least, and that the glass of the speculum be thick, that it bend not in the working. the prism efg must be no bigger than is necessary, and its back side fg must not be quick-silver'd over. for without quicksilver it will reflect all the light incident on it from the speculum. [illustration: fig. .] in this instrument the object will be inverted, but may be erected by making the square sides ff and eg of the prism efg not plane but spherically convex, that the rays may cross as well before they come at it as afterwards between it and the eye-glass. if it be desired that the instrument bear a larger aperture, that may be also done by composing the speculum of two glasses with water between them. if the theory of making telescopes could at length be fully brought into practice, yet there would be certain bounds beyond which telescopes could not perform. for the air through which we look upon the stars, is in a perpetual tremor; as may be seen by the tremulous motion of shadows cast from high towers, and by the twinkling of the fix'd stars. but these stars do not twinkle when viewed through telescopes which have large apertures. for the rays of light which pass through divers parts of the aperture, tremble each of them apart, and by means of their various and sometimes contrary tremors, fall at one and the same time upon different points in the bottom of the eye, and their trembling motions are too quick and confused to be perceived severally. and all these illuminated points constitute one broad lucid point, composed of those many trembling points confusedly and insensibly mixed with one another by very short and swift tremors, and thereby cause the star to appear broader than it is, and without any trembling of the whole. long telescopes may cause objects to appear brighter and larger than short ones can do, but they cannot be so formed as to take away that confusion of the rays which arises from the tremors of the atmosphere. the only remedy is a most serene and quiet air, such as may perhaps be found on the tops of the highest mountains above the grosser clouds. footnotes: [c] _see our_ author's lectiones opticæ § . _sect. ii. § . and sect. iii. prop. ._ [d] see our author's _lectiones opticæ_, part. i. sect. . § . [e] _this is very fully treated of in our_ author's lect. optic. _part_ i. _sect._ ii. [f] _see our_ author's lect. optic. part i. sect. ii. § . [g] _this is demonstrated in our_ author's lect. optic. _part_ i. _sect._ iv. _prop._ . [h] _how to do this, is shewn in our_ author's lect. optic. _part_ i. _sect._ iv. _prop._ . the first book of opticks _part ii._ _prop._ i. theor. i. _the phænomena of colours in refracted or reflected light are not caused by new modifications of the light variously impress'd, according to the various terminations of the light and shadow_. the proof by experiments. _exper._ . for if the sun shine into a very dark chamber through an oblong hole f, [in _fig._ .] whose breadth is the sixth or eighth part of an inch, or something less; and his beam fh do afterwards pass first through a very large prism abc, distant about feet from the hole, and parallel to it, and then (with its white part) through an oblong hole h, whose breadth is about the fortieth or sixtieth part of an inch, and which is made in a black opake body gi, and placed at the distance of two or three feet from the prism, in a parallel situation both to the prism and to the former hole, and if this white light thus transmitted through the hole h, fall afterwards upon a white paper _pt_, placed after that hole h, at the distance of three or four feet from it, and there paint the usual colours of the prism, suppose red at _t_, yellow at _s_, green at _r_, blue at _q_, and violet at _p_; you may with an iron wire, or any such like slender opake body, whose breadth is about the tenth part of an inch, by intercepting the rays at _k_, _l_, _m_, _n_ or _o_, take away any one of the colours at _t_, _s_, _r_, _q_ or _p_, whilst the other colours remain upon the paper as before; or with an obstacle something bigger you may take away any two, or three, or four colours together, the rest remaining: so that any one of the colours as well as violet may become outmost in the confine of the shadow towards _p_, and any one of them as well as red may become outmost in the confine of the shadow towards _t_, and any one of them may also border upon the shadow made within the colours by the obstacle r intercepting some intermediate part of the light; and, lastly, any one of them by being left alone, may border upon the shadow on either hand. all the colours have themselves indifferently to any confines of shadow, and therefore the differences of these colours from one another, do not arise from the different confines of shadow, whereby light is variously modified, as has hitherto been the opinion of philosophers. in trying these things 'tis to be observed, that by how much the holes f and h are narrower, and the intervals between them and the prism greater, and the chamber darker, by so much the better doth the experiment succeed; provided the light be not so far diminished, but that the colours at _pt_ be sufficiently visible. to procure a prism of solid glass large enough for this experiment will be difficult, and therefore a prismatick vessel must be made of polish'd glass plates cemented together, and filled with salt water or clear oil. [illustration: fig. .] _exper._ . the sun's light let into a dark chamber through the round hole f, [in _fig._ .] half an inch wide, passed first through the prism abc placed at the hole, and then through a lens pt something more than four inches broad, and about eight feet distant from the prism, and thence converged to o the focus of the lens distant from it about three feet, and there fell upon a white paper de. if that paper was perpendicular to that light incident upon it, as 'tis represented in the posture de, all the colours upon it at o appeared white. but if the paper being turned about an axis parallel to the prism, became very much inclined to the light, as 'tis represented in the positions _de_ and _[greek: de]_; the same light in the one case appeared yellow and red, in the other blue. here one and the same part of the light in one and the same place, according to the various inclinations of the paper, appeared in one case white, in another yellow or red, in a third blue, whilst the confine of light and shadow, and the refractions of the prism in all these cases remained the same. [illustration: fig. .] [illustration: fig. .] _exper._ . such another experiment may be more easily tried as follows. let a broad beam of the sun's light coming into a dark chamber through a hole in the window-shut be refracted by a large prism abc, [in _fig._ .] whose refracting angle c is more than degrees, and so soon as it comes out of the prism, let it fall upon the white paper de glewed upon a stiff plane; and this light, when the paper is perpendicular to it, as 'tis represented in de, will appear perfectly white upon the paper; but when the paper is very much inclin'd to it in such a manner as to keep always parallel to the axis of the prism, the whiteness of the whole light upon the paper will according to the inclination of the paper this way or that way, change either into yellow and red, as in the posture _de_, or into blue and violet, as in the posture [greek: de]. and if the light before it fall upon the paper be twice refracted the same way by two parallel prisms, these colours will become the more conspicuous. here all the middle parts of the broad beam of white light which fell upon the paper, did without any confine of shadow to modify it, become colour'd all over with one uniform colour, the colour being always the same in the middle of the paper as at the edges, and this colour changed according to the various obliquity of the reflecting paper, without any change in the refractions or shadow, or in the light which fell upon the paper. and therefore these colours are to be derived from some other cause than the new modifications of light by refractions and shadows. if it be asked, what then is their cause? i answer, that the paper in the posture _de_, being more oblique to the more refrangible rays than to the less refrangible ones, is more strongly illuminated by the latter than by the former, and therefore the less refrangible rays are predominant in the reflected light. and where-ever they are predominant in any light, they tinge it with red or yellow, as may in some measure appear by the first proposition of the first part of this book, and will more fully appear hereafter. and the contrary happens in the posture of the paper [greek: de], the more refrangible rays being then predominant which always tinge light with blues and violets. _exper._ . the colours of bubbles with which children play are various, and change their situation variously, without any respect to any confine or shadow. if such a bubble be cover'd with a concave glass, to keep it from being agitated by any wind or motion of the air, the colours will slowly and regularly change their situation, even whilst the eye and the bubble, and all bodies which emit any light, or cast any shadow, remain unmoved. and therefore their colours arise from some regular cause which depends not on any confine of shadow. what this cause is will be shewed in the next book. to these experiments may be added the tenth experiment of the first part of this first book, where the sun's light in a dark room being trajected through the parallel superficies of two prisms tied together in the form of a parallelopipede, became totally of one uniform yellow or red colour, at its emerging out of the prisms. here, in the production of these colours, the confine of shadow can have nothing to do. for the light changes from white to yellow, orange and red successively, without any alteration of the confine of shadow: and at both edges of the emerging light where the contrary confines of shadow ought to produce different effects, the colour is one and the same, whether it be white, yellow, orange or red: and in the middle of the emerging light, where there is no confine of shadow at all, the colour is the very same as at the edges, the whole light at its very first emergence being of one uniform colour, whether white, yellow, orange or red, and going on thence perpetually without any change of colour, such as the confine of shadow is vulgarly supposed to work in refracted light after its emergence. neither can these colours arise from any new modifications of the light by refractions, because they change successively from white to yellow, orange and red, while the refractions remain the same, and also because the refractions are made contrary ways by parallel superficies which destroy one another's effects. they arise not therefore from any modifications of light made by refractions and shadows, but have some other cause. what that cause is we shewed above in this tenth experiment, and need not here repeat it. there is yet another material circumstance of this experiment. for this emerging light being by a third prism hik [in _fig._ . _part_ i.][i] refracted towards the paper pt, and there painting the usual colours of the prism, red, yellow, green, blue, violet: if these colours arose from the refractions of that prism modifying the light, they would not be in the light before its incidence on that prism. and yet in that experiment we found, that when by turning the two first prisms about their common axis all the colours were made to vanish but the red; the light which makes that red being left alone, appeared of the very same red colour before its incidence on the third prism. and in general we find by other experiments, that when the rays which differ in refrangibility are separated from one another, and any one sort of them is considered apart, the colour of the light which they compose cannot be changed by any refraction or reflexion whatever, as it ought to be were colours nothing else than modifications of light caused by refractions, and reflexions, and shadows. this unchangeableness of colour i am now to describe in the following proposition. _prop._ ii. theor. ii. _all homogeneal light has its proper colour answering to its degree of refrangibility, and that colour cannot be changed by reflexions and refractions._ in the experiments of the fourth proposition of the first part of this first book, when i had separated the heterogeneous rays from one another, the spectrum _pt_ formed by the separated rays, did in the progress from its end _p_, on which the most refrangible rays fell, unto its other end _t_, on which the least refrangible rays fell, appear tinged with this series of colours, violet, indigo, blue, green, yellow, orange, red, together with all their intermediate degrees in a continual succession perpetually varying. so that there appeared as many degrees of colours, as there were sorts of rays differing in refrangibility. _exper._ . now, that these colours could not be changed by refraction, i knew by refracting with a prism sometimes one very little part of this light, sometimes another very little part, as is described in the twelfth experiment of the first part of this book. for by this refraction the colour of the light was never changed in the least. if any part of the red light was refracted, it remained totally of the same red colour as before. no orange, no yellow, no green or blue, no other new colour was produced by that refraction. neither did the colour any ways change by repeated refractions, but continued always the same red entirely as at first. the like constancy and immutability i found also in the blue, green, and other colours. so also, if i looked through a prism upon any body illuminated with any part of this homogeneal light, as in the fourteenth experiment of the first part of this book is described; i could not perceive any new colour generated this way. all bodies illuminated with compound light appear through prisms confused, (as was said above) and tinged with various new colours, but those illuminated with homogeneal light appeared through prisms neither less distinct, nor otherwise colour'd, than when viewed with the naked eyes. their colours were not in the least changed by the refraction of the interposed prism. i speak here of a sensible change of colour: for the light which i here call homogeneal, being not absolutely homogeneal, there ought to arise some little change of colour from its heterogeneity. but, if that heterogeneity was so little as it might be made by the said experiments of the fourth proposition, that change was not sensible, and therefore in experiments, where sense is judge, ought to be accounted none at all. _exper._ . and as these colours were not changeable by refractions, so neither were they by reflexions. for all white, grey, red, yellow, green, blue, violet bodies, as paper, ashes, red lead, orpiment, indico bise, gold, silver, copper, grass, blue flowers, violets, bubbles of water tinged with various colours, peacock's feathers, the tincture of _lignum nephriticum_, and such-like, in red homogeneal light appeared totally red, in blue light totally blue, in green light totally green, and so of other colours. in the homogeneal light of any colour they all appeared totally of that same colour, with this only difference, that some of them reflected that light more strongly, others more faintly. i never yet found any body, which by reflecting homogeneal light could sensibly change its colour. from all which it is manifest, that if the sun's light consisted of but one sort of rays, there would be but one colour in the whole world, nor would it be possible to produce any new colour by reflexions and refractions, and by consequence that the variety of colours depends upon the composition of light. _definition._ the homogeneal light and rays which appear red, or rather make objects appear so, i call rubrifick or red-making; those which make objects appear yellow, green, blue, and violet, i call yellow-making, green-making, blue-making, violet-making, and so of the rest. and if at any time i speak of light and rays as coloured or endued with colours, i would be understood to speak not philosophically and properly, but grossly, and accordingly to such conceptions as vulgar people in seeing all these experiments would be apt to frame. for the rays to speak properly are not coloured. in them there is nothing else than a certain power and disposition to stir up a sensation of this or that colour. for as sound in a bell or musical string, or other sounding body, is nothing but a trembling motion, and in the air nothing but that motion propagated from the object, and in the sensorium 'tis a sense of that motion under the form of sound; so colours in the object are nothing but a disposition to reflect this or that sort of rays more copiously than the rest; in the rays they are nothing but their dispositions to propagate this or that motion into the sensorium, and in the sensorium they are sensations of those motions under the forms of colours. _prop._ iii. prob. i. _to define the refrangibility of the several sorts of homogeneal light answering to the several colours._ for determining this problem i made the following experiment.[j] _exper._ . when i had caused the rectilinear sides af, gm, [in _fig._ .] of the spectrum of colours made by the prism to be distinctly defined, as in the fifth experiment of the first part of this book is described, there were found in it all the homogeneal colours in the same order and situation one among another as in the spectrum of simple light, described in the fourth proposition of that part. for the circles of which the spectrum of compound light pt is composed, and which in the middle parts of the spectrum interfere, and are intermix'd with one another, are not intermix'd in their outmost parts where they touch those rectilinear sides af and gm. and therefore, in those rectilinear sides when distinctly defined, there is no new colour generated by refraction. i observed also, that if any where between the two outmost circles tmf and pga a right line, as [greek: gd], was cross to the spectrum, so as both ends to fall perpendicularly upon its rectilinear sides, there appeared one and the same colour, and degree of colour from one end of this line to the other. i delineated therefore in a paper the perimeter of the spectrum fap gmt, and in trying the third experiment of the first part of this book, i held the paper so that the spectrum might fall upon this delineated figure, and agree with it exactly, whilst an assistant, whose eyes for distinguishing colours were more critical than mine, did by right lines [greek: ab, gd, ez,] &c. drawn cross the spectrum, note the confines of the colours, that is of the red m[greek: ab]f, of the orange [greek: agdb], of the yellow [greek: gezd], of the green [greek: eêthz], of the blue [greek: êikth], of the indico [greek: ilmk], and of the violet [greek: l]ga[greek: m]. and this operation being divers times repeated both in the same, and in several papers, i found that the observations agreed well enough with one another, and that the rectilinear sides mg and fa were by the said cross lines divided after the manner of a musical chord. let gm be produced to x, that mx may be equal to gm, and conceive gx, [greek: l]x, [greek: i]x, [greek: ê]x, [greek: e]x, [greek: g]x, [greek: a]x, mx, to be in proportion to one another, as the numbers, , / , / , / , / , / , / , / , and so to represent the chords of the key, and of a tone, a third minor, a fourth, a fifth, a sixth major, a seventh and an eighth above that key: and the intervals m[greek: a], [greek: ag], [greek: ge], [greek: eê], [greek: êi], [greek: il], and [greek: l]g, will be the spaces which the several colours (red, orange, yellow, green, blue, indigo, violet) take up. [illustration: fig. .] [illustration: fig. .] now these intervals or spaces subtending the differences of the refractions of the rays going to the limits of those colours, that is, to the points m, [greek: a], [greek: g], [greek: e], [greek: ê], [greek: i], [greek: l], g, may without any sensible error be accounted proportional to the differences of the sines of refraction of those rays having one common sine of incidence, and therefore since the common sine of incidence of the most and least refrangible rays out of glass into air was (by a method described above) found in proportion to their sines of refraction, as to and , divide the difference between the sines of refraction and , as the line gm is divided by those intervals, and you will have , - / , - / , - / , - / , - / , - / , , the sines of refraction of those rays out of glass into air, their common sine of incidence being . so then the sines of the incidences of all the red-making rays out of glass into air, were to the sines of their refractions, not greater than to , nor less than to - / , but they varied from one another according to all intermediate proportions. and the sines of the incidences of the green-making rays were to the sines of their refractions in all proportions from that of to - / , unto that of to - / . and by the like limits above-mentioned were the refractions of the rays belonging to the rest of the colours defined, the sines of the red-making rays extending from to - / , those of the orange-making from - / to - / , those of the yellow-making from - / to - / , those of the green-making from - / to - / , those of the blue-making from - / to - / , those of the indigo-making from - / to - / , and those of the violet from - / , to . these are the laws of the refractions made out of glass into air, and thence by the third axiom of the first part of this book, the laws of the refractions made out of air into glass are easily derived. _exper._ . i found moreover, that when light goes out of air through several contiguous refracting mediums as through water and glass, and thence goes out again into air, whether the refracting superficies be parallel or inclin'd to one another, that light as often as by contrary refractions 'tis so corrected, that it emergeth in lines parallel to those in which it was incident, continues ever after to be white. but if the emergent rays be inclined to the incident, the whiteness of the emerging light will by degrees in passing on from the place of emergence, become tinged in its edges with colours. this i try'd by refracting light with prisms of glass placed within a prismatick vessel of water. now those colours argue a diverging and separation of the heterogeneous rays from one another by means of their unequal refractions, as in what follows will more fully appear. and, on the contrary, the permanent whiteness argues, that in like incidences of the rays there is no such separation of the emerging rays, and by consequence no inequality of their whole refractions. whence i seem to gather the two following theorems. . the excesses of the sines of refraction of several sorts of rays above their common sine of incidence when the refractions are made out of divers denser mediums immediately into one and the same rarer medium, suppose of air, are to one another in a given proportion. . the proportion of the sine of incidence to the sine of refraction of one and the same sort of rays out of one medium into another, is composed of the proportion of the sine of incidence to the sine of refraction out of the first medium into any third medium, and of the proportion of the sine of incidence to the sine of refraction out of that third medium into the second medium. by the first theorem the refractions of the rays of every sort made out of any medium into air are known by having the refraction of the rays of any one sort. as for instance, if the refractions of the rays of every sort out of rain-water into air be desired, let the common sine of incidence out of glass into air be subducted from the sines of refraction, and the excesses will be , - / , - / , - / , - / , - / , - / , . suppose now that the sine of incidence of the least refrangible rays be to their sine of refraction out of rain-water into air as to , and say as the difference of those sines is to the sine of incidence, so is the least of the excesses above-mentioned to a fourth number ; and will be the common sine of incidence out of rain-water into air, to which sine if you add all the above-mentioned excesses, you will have the desired sines of the refractions , - / , - / , - / , - / , - / , - / , . by the latter theorem the refraction out of one medium into another is gathered as often as you have the refractions out of them both into any third medium. as if the sine of incidence of any ray out of glass into air be to its sine of refraction, as to , and the sine of incidence of the same ray out of air into water, be to its sine of refraction as to ; the sine of incidence of that ray out of glass into water will be to its sine of refraction as to and to jointly, that is, as the factum of and to the factum of and , or as to . and these theorems being admitted into opticks, there would be scope enough of handling that science voluminously after a new manner,[k] not only by teaching those things which tend to the perfection of vision, but also by determining mathematically all kinds of phænomena of colours which could be produced by refractions. for to do this, there is nothing else requisite than to find out the separations of heterogeneous rays, and their various mixtures and proportions in every mixture. by this way of arguing i invented almost all the phænomena described in these books, beside some others less necessary to the argument; and by the successes i met with in the trials, i dare promise, that to him who shall argue truly, and then try all things with good glasses and sufficient circumspection, the expected event will not be wanting. but he is first to know what colours will arise from any others mix'd in any assigned proportion. _prop._ iv. theor. iii. _colours may be produced by composition which shall be like to the colours of homogeneal light as to the appearance of colour, but not as to the immutability of colour and constitution of light. and those colours by how much they are more compounded by so much are they less full and intense, and by too much composition they maybe diluted and weaken'd till they cease, and the mixture becomes white or grey. there may be also colours produced by composition, which are not fully like any of the colours of homogeneal light._ for a mixture of homogeneal red and yellow compounds an orange, like in appearance of colour to that orange which in the series of unmixed prismatick colours lies between them; but the light of one orange is homogeneal as to refrangibility, and that of the other is heterogeneal, and the colour of the one, if viewed through a prism, remains unchanged, that of the other is changed and resolved into its component colours red and yellow. and after the same manner other neighbouring homogeneal colours may compound new colours, like the intermediate homogeneal ones, as yellow and green, the colour between them both, and afterwards, if blue be added, there will be made a green the middle colour of the three which enter the composition. for the yellow and blue on either hand, if they are equal in quantity they draw the intermediate green equally towards themselves in composition, and so keep it as it were in Æquilibrion, that it verge not more to the yellow on the one hand, and to the blue on the other, but by their mix'd actions remain still a middle colour. to this mix'd green there may be farther added some red and violet, and yet the green will not presently cease, but only grow less full and vivid, and by increasing the red and violet, it will grow more and more dilute, until by the prevalence of the added colours it be overcome and turned into whiteness, or some other colour. so if to the colour of any homogeneal light, the sun's white light composed of all sorts of rays be added, that colour will not vanish or change its species, but be diluted, and by adding more and more white it will be diluted more and more perpetually. lastly, if red and violet be mingled, there will be generated according to their various proportions various purples, such as are not like in appearance to the colour of any homogeneal light, and of these purples mix'd with yellow and blue may be made other new colours. _prop._ v. theor. iv. _whiteness and all grey colours between white and black, may be compounded of colours, and the whiteness of the sun's light is compounded of all the primary colours mix'd in a due proportion._ the proof by experiments. _exper._ . the sun shining into a dark chamber through a little round hole in the window-shut, and his light being there refracted by a prism to cast his coloured image pt [in _fig._ .] upon the opposite wall: i held a white paper v to that image in such manner that it might be illuminated by the colour'd light reflected from thence, and yet not intercept any part of that light in its passage from the prism to the spectrum. and i found that when the paper was held nearer to any colour than to the rest, it appeared of that colour to which it approached nearest; but when it was equally or almost equally distant from all the colours, so that it might be equally illuminated by them all it appeared white. and in this last situation of the paper, if some colours were intercepted, the paper lost its white colour, and appeared of the colour of the rest of the light which was not intercepted. so then the paper was illuminated with lights of various colours, namely, red, yellow, green, blue and violet, and every part of the light retained its proper colour, until it was incident on the paper, and became reflected thence to the eye; so that if it had been either alone (the rest of the light being intercepted) or if it had abounded most, and been predominant in the light reflected from the paper, it would have tinged the paper with its own colour; and yet being mixed with the rest of the colours in a due proportion, it made the paper look white, and therefore by a composition with the rest produced that colour. the several parts of the coloured light reflected from the spectrum, whilst they are propagated from thence through the air, do perpetually retain their proper colours, because wherever they fall upon the eyes of any spectator, they make the several parts of the spectrum to appear under their proper colours. they retain therefore their proper colours when they fall upon the paper v, and so by the confusion and perfect mixture of those colours compound the whiteness of the light reflected from thence. _exper._ . let that spectrum or solar image pt [in _fig._ .] fall now upon the lens mn above four inches broad, and about six feet distant from the prism abc and so figured that it may cause the coloured light which divergeth from the prism to converge and meet again at its focus g, about six or eight feet distant from the lens, and there to fall perpendicularly upon a white paper de. and if you move this paper to and fro, you will perceive that near the lens, as at _de_, the whole solar image (suppose at _pt_) will appear upon it intensely coloured after the manner above-explained, and that by receding from the lens those colours will perpetually come towards one another, and by mixing more and more dilute one another continually, until at length the paper come to the focus g, where by a perfect mixture they will wholly vanish and be converted into whiteness, the whole light appearing now upon the paper like a little white circle. and afterwards by receding farther from the lens, the rays which before converged will now cross one another in the focus g, and diverge from thence, and thereby make the colours to appear again, but yet in a contrary order; suppose at [greek: de], where the red _t_ is now above which before was below, and the violet _p_ is below which before was above. let us now stop the paper at the focus g, where the light appears totally white and circular, and let us consider its whiteness. i say, that this is composed of the converging colours. for if any of those colours be intercepted at the lens, the whiteness will cease and degenerate into that colour which ariseth from the composition of the other colours which are not intercepted. and then if the intercepted colours be let pass and fall upon that compound colour, they mix with it, and by their mixture restore the whiteness. so if the violet, blue and green be intercepted, the remaining yellow, orange and red will compound upon the paper an orange, and then if the intercepted colours be let pass, they will fall upon this compounded orange, and together with it decompound a white. so also if the red and violet be intercepted, the remaining yellow, green and blue, will compound a green upon the paper, and then the red and violet being let pass will fall upon this green, and together with it decompound a white. and that in this composition of white the several rays do not suffer any change in their colorific qualities by acting upon one another, but are only mixed, and by a mixture of their colours produce white, may farther appear by these arguments. [illustration: fig. .] if the paper be placed beyond the focus g, suppose at [greek: de], and then the red colour at the lens be alternately intercepted, and let pass again, the violet colour on the paper will not suffer any change thereby, as it ought to do if the several sorts of rays acted upon one another in the focus g, where they cross. neither will the red upon the paper be changed by any alternate stopping, and letting pass the violet which crosseth it. and if the paper be placed at the focus g, and the white round image at g be viewed through the prism hik, and by the refraction of that prism be translated to the place _rv_, and there appear tinged with various colours, namely, the violet at _v_ and red at _r_, and others between, and then the red colours at the lens be often stopp'd and let pass by turns, the red at _r_ will accordingly disappear, and return as often, but the violet at _v_ will not thereby suffer any change. and so by stopping and letting pass alternately the blue at the lens, the blue at _v_ will accordingly disappear and return, without any change made in the red at _r_. the red therefore depends on one sort of rays, and the blue on another sort, which in the focus g where they are commix'd, do not act on one another. and there is the same reason of the other colours. i considered farther, that when the most refrangible rays p_p_, and the least refrangible ones t_t_, are by converging inclined to one another, the paper, if held very oblique to those rays in the focus g, might reflect one sort of them more copiously than the other sort, and by that means the reflected light would be tinged in that focus with the colour of the predominant rays, provided those rays severally retained their colours, or colorific qualities in the composition of white made by them in that focus. but if they did not retain them in that white, but became all of them severally endued there with a disposition to strike the sense with the perception of white, then they could never lose their whiteness by such reflexions. i inclined therefore the paper to the rays very obliquely, as in the second experiment of this second part of the first book, that the most refrangible rays, might be more copiously reflected than the rest, and the whiteness at length changed successively into blue, indigo, and violet. then i inclined it the contrary way, that the least refrangible rays might be more copious in the reflected light than the rest, and the whiteness turned successively to yellow, orange, and red. lastly, i made an instrument xy in fashion of a comb, whose teeth being in number sixteen, were about an inch and a half broad, and the intervals of the teeth about two inches wide. then by interposing successively the teeth of this instrument near the lens, i intercepted part of the colours by the interposed tooth, whilst the rest of them went on through the interval of the teeth to the paper de, and there painted a round solar image. but the paper i had first placed so, that the image might appear white as often as the comb was taken away; and then the comb being as was said interposed, that whiteness by reason of the intercepted part of the colours at the lens did always change into the colour compounded of those colours which were not intercepted, and that colour was by the motion of the comb perpetually varied so, that in the passing of every tooth over the lens all these colours, red, yellow, green, blue, and purple, did always succeed one another. i caused therefore all the teeth to pass successively over the lens, and when the motion was slow, there appeared a perpetual succession of the colours upon the paper: but if i so much accelerated the motion, that the colours by reason of their quick succession could not be distinguished from one another, the appearance of the single colours ceased. there was no red, no yellow, no green, no blue, nor purple to be seen any longer, but from a confusion of them all there arose one uniform white colour. of the light which now by the mixture of all the colours appeared white, there was no part really white. one part was red, another yellow, a third green, a fourth blue, a fifth purple, and every part retains its proper colour till it strike the sensorium. if the impressions follow one another slowly, so that they may be severally perceived, there is made a distinct sensation of all the colours one after another in a continual succession. but if the impressions follow one another so quickly, that they cannot be severally perceived, there ariseth out of them all one common sensation, which is neither of this colour alone nor of that alone, but hath it self indifferently to 'em all, and this is a sensation of whiteness. by the quickness of the successions, the impressions of the several colours are confounded in the sensorium, and out of that confusion ariseth a mix'd sensation. if a burning coal be nimbly moved round in a circle with gyrations continually repeated, the whole circle will appear like fire; the reason of which is, that the sensation of the coal in the several places of that circle remains impress'd on the sensorium, until the coal return again to the same place. and so in a quick consecution of the colours the impression of every colour remains in the sensorium, until a revolution of all the colours be compleated, and that first colour return again. the impressions therefore of all the successive colours are at once in the sensorium, and jointly stir up a sensation of them all; and so it is manifest by this experiment, that the commix'd impressions of all the colours do stir up and beget a sensation of white, that is, that whiteness is compounded of all the colours. and if the comb be now taken away, that all the colours may at once pass from the lens to the paper, and be there intermixed, and together reflected thence to the spectator's eyes; their impressions on the sensorium being now more subtilly and perfectly commixed there, ought much more to stir up a sensation of whiteness. you may instead of the lens use two prisms hik and lmn, which by refracting the coloured light the contrary way to that of the first refraction, may make the diverging rays converge and meet again in g, as you see represented in the seventh figure. for where they meet and mix, they will compose a white light, as when a lens is used. _exper._ . let the sun's coloured image pt [in _fig._ .] fall upon the wall of a dark chamber, as in the third experiment of the first book, and let the same be viewed through a prism _abc_, held parallel to the prism abc, by whose refraction that image was made, and let it now appear lower than before, suppose in the place s over-against the red colour t. and if you go near to the image pt, the spectrum s will appear oblong and coloured like the image pt; but if you recede from it, the colours of the spectrum s will be contracted more and more, and at length vanish, that spectrum s becoming perfectly round and white; and if you recede yet farther, the colours will emerge again, but in a contrary order. now that spectrum s appears white in that case, when the rays of several sorts which converge from the several parts of the image pt, to the prism _abc_, are so refracted unequally by it, that in their passage from the prism to the eye they may diverge from one and the same point of the spectrum s, and so fall afterwards upon one and the same point in the bottom of the eye, and there be mingled. [illustration: fig. .] [illustration: fig. .] and farther, if the comb be here made use of, by whose teeth the colours at the image pt may be successively intercepted; the spectrum s, when the comb is moved slowly, will be perpetually tinged with successive colours: but when by accelerating the motion of the comb, the succession of the colours is so quick that they cannot be severally seen, that spectrum s, by a confused and mix'd sensation of them all, will appear white. _exper._ . the sun shining through a large prism abc [in _fig._ .] upon a comb xy, placed immediately behind the prism, his light which passed through the interstices of the teeth fell upon a white paper de. the breadths of the teeth were equal to their interstices, and seven teeth together with their interstices took up an inch in breadth. now, when the paper was about two or three inches distant from the comb, the light which passed through its several interstices painted so many ranges of colours, _kl_, _mn_, _op_, _qr_, &c. which were parallel to one another, and contiguous, and without any mixture of white. and these ranges of colours, if the comb was moved continually up and down with a reciprocal motion, ascended and descended in the paper, and when the motion of the comb was so quick, that the colours could not be distinguished from one another, the whole paper by their confusion and mixture in the sensorium appeared white. [illustration: fig. .] let the comb now rest, and let the paper be removed farther from the prism, and the several ranges of colours will be dilated and expanded into one another more and more, and by mixing their colours will dilute one another, and at length, when the distance of the paper from the comb is about a foot, or a little more (suppose in the place d e) they will so far dilute one another, as to become white. with any obstacle, let all the light be now stopp'd which passes through any one interval of the teeth, so that the range of colours which comes from thence may be taken away, and you will see the light of the rest of the ranges to be expanded into the place of the range taken away, and there to be coloured. let the intercepted range pass on as before, and its colours falling upon the colours of the other ranges, and mixing with them, will restore the whiteness. let the paper d e be now very much inclined to the rays, so that the most refrangible rays may be more copiously reflected than the rest, and the white colour of the paper through the excess of those rays will be changed into blue and violet. let the paper be as much inclined the contrary way, that the least refrangible rays may be now more copiously reflected than the rest, and by their excess the whiteness will be changed into yellow and red. the several rays therefore in that white light do retain their colorific qualities, by which those of any sort, whenever they become more copious than the rest, do by their excess and predominance cause their proper colour to appear. and by the same way of arguing, applied to the third experiment of this second part of the first book, it may be concluded, that the white colour of all refracted light at its very first emergence, where it appears as white as before its incidence, is compounded of various colours. [illustration: fig. .] _exper._ . in the foregoing experiment the several intervals of the teeth of the comb do the office of so many prisms, every interval producing the phænomenon of one prism. whence instead of those intervals using several prisms, i try'd to compound whiteness by mixing their colours, and did it by using only three prisms, as also by using only two as follows. let two prisms abc and _abc_, [in _fig._ .] whose refracting angles b and _b_ are equal, be so placed parallel to one another, that the refracting angle b of the one may touch the angle _c_ at the base of the other, and their planes cb and _cb_, at which the rays emerge, may lie in directum. then let the light trajected through them fall upon the paper mn, distant about or inches from the prisms. and the colours generated by the interior limits b and _c_ of the two prisms, will be mingled at pt, and there compound white. for if either prism be taken away, the colours made by the other will appear in that place pt, and when the prism is restored to its place again, so that its colours may there fall upon the colours of the other, the mixture of them both will restore the whiteness. this experiment succeeds also, as i have tried, when the angle _b_ of the lower prism, is a little greater than the angle b of the upper, and between the interior angles b and _c_, there intercedes some space b_c_, as is represented in the figure, and the refracting planes bc and _bc_, are neither in directum, nor parallel to one another. for there is nothing more requisite to the success of this experiment, than that the rays of all sorts may be uniformly mixed upon the paper in the place pt. if the most refrangible rays coming from the superior prism take up all the space from m to p, the rays of the same sort which come from the inferior prism ought to begin at p, and take up all the rest of the space from thence towards n. if the least refrangible rays coming from the superior prism take up the space mt, the rays of the same kind which come from the other prism ought to begin at t, and take up the remaining space tn. if one sort of the rays which have intermediate degrees of refrangibility, and come from the superior prism be extended through the space mq, and another sort of those rays through the space mr, and a third sort of them through the space ms, the same sorts of rays coming from the lower prism, ought to illuminate the remaining spaces qn, rn, sn, respectively. and the same is to be understood of all the other sorts of rays. for thus the rays of every sort will be scattered uniformly and evenly through the whole space mn, and so being every where mix'd in the same proportion, they must every where produce the same colour. and therefore, since by this mixture they produce white in the exterior spaces mp and tn, they must also produce white in the interior space pt. this is the reason of the composition by which whiteness was produced in this experiment, and by what other way soever i made the like composition, the result was whiteness. lastly, if with the teeth of a comb of a due size, the coloured lights of the two prisms which fall upon the space pt be alternately intercepted, that space pt, when the motion of the comb is slow, will always appear coloured, but by accelerating the motion of the comb so much that the successive colours cannot be distinguished from one another, it will appear white. _exper._ . hitherto i have produced whiteness by mixing the colours of prisms. if now the colours of natural bodies are to be mingled, let water a little thicken'd with soap be agitated to raise a froth, and after that froth has stood a little, there will appear to one that shall view it intently various colours every where in the surfaces of the several bubbles; but to one that shall go so far off, that he cannot distinguish the colours from one another, the whole froth will grow white with a perfect whiteness. _exper._ . lastly, in attempting to compound a white, by mixing the coloured powders which painters use, i consider'd that all colour'd powders do suppress and stop in them a very considerable part of the light by which they are illuminated. for they become colour'd by reflecting the light of their own colours more copiously, and that of all other colours more sparingly, and yet they do not reflect the light of their own colours so copiously as white bodies do. if red lead, for instance, and a white paper, be placed in the red light of the colour'd spectrum made in a dark chamber by the refraction of a prism, as is described in the third experiment of the first part of this book; the paper will appear more lucid than the red lead, and therefore reflects the red-making rays more copiously than red lead doth. and if they be held in the light of any other colour, the light reflected by the paper will exceed the light reflected by the red lead in a much greater proportion. and the like happens in powders of other colours. and therefore by mixing such powders, we are not to expect a strong and full white, such as is that of paper, but some dusky obscure one, such as might arise from a mixture of light and darkness, or from white and black, that is, a grey, or dun, or russet brown, such as are the colours of a man's nail, of a mouse, of ashes, of ordinary stones, of mortar, of dust and dirt in high-ways, and the like. and such a dark white i have often produced by mixing colour'd powders. for thus one part of red lead, and five parts of _viride Æris_, composed a dun colour like that of a mouse. for these two colours were severally so compounded of others, that in both together were a mixture of all colours; and there was less red lead used than _viride Æris_, because of the fulness of its colour. again, one part of red lead, and four parts of blue bise, composed a dun colour verging a little to purple, and by adding to this a certain mixture of orpiment and _viride Æris_ in a due proportion, the mixture lost its purple tincture, and became perfectly dun. but the experiment succeeded best without minium thus. to orpiment i added by little and little a certain full bright purple, which painters use, until the orpiment ceased to be yellow, and became of a pale red. then i diluted that red by adding a little _viride Æris_, and a little more blue bise than _viride Æris_, until it became of such a grey or pale white, as verged to no one of the colours more than to another. for thus it became of a colour equal in whiteness to that of ashes, or of wood newly cut, or of a man's skin. the orpiment reflected more light than did any other of the powders, and therefore conduced more to the whiteness of the compounded colour than they. to assign the proportions accurately may be difficult, by reason of the different goodness of powders of the same kind. accordingly, as the colour of any powder is more or less full and luminous, it ought to be used in a less or greater proportion. now, considering that these grey and dun colours may be also produced by mixing whites and blacks, and by consequence differ from perfect whites, not in species of colours, but only in degree of luminousness, it is manifest that there is nothing more requisite to make them perfectly white than to increase their light sufficiently; and, on the contrary, if by increasing their light they can be brought to perfect whiteness, it will thence also follow, that they are of the same species of colour with the best whites, and differ from them only in the quantity of light. and this i tried as follows. i took the third of the above-mention'd grey mixtures, (that which was compounded of orpiment, purple, bise, and _viride Æris_) and rubbed it thickly upon the floor of my chamber, where the sun shone upon it through the opened casement; and by it, in the shadow, i laid a piece of white paper of the same bigness. then going from them to the distance of or feet, so that i could not discern the unevenness of the surface of the powder, nor the little shadows let fall from the gritty particles thereof; the powder appeared intensely white, so as to transcend even the paper it self in whiteness, especially if the paper were a little shaded from the light of the clouds, and then the paper compared with the powder appeared of such a grey colour as the powder had done before. but by laying the paper where the sun shines through the glass of the window, or by shutting the window that the sun might shine through the glass upon the powder, and by such other fit means of increasing or decreasing the lights wherewith the powder and paper were illuminated, the light wherewith the powder is illuminated may be made stronger in such a due proportion than the light wherewith the paper is illuminated, that they shall both appear exactly alike in whiteness. for when i was trying this, a friend coming to visit me, i stopp'd him at the door, and before i told him what the colours were, or what i was doing; i asked him, which of the two whites were the best, and wherein they differed? and after he had at that distance viewed them well, he answer'd, that they were both good whites, and that he could not say which was best, nor wherein their colours differed. now, if you consider, that this white of the powder in the sun-shine was compounded of the colours which the component powders (orpiment, purple, bise, and _viride Æris_) have in the same sun-shine, you must acknowledge by this experiment, as well as by the former, that perfect whiteness may be compounded of colours. from what has been said it is also evident, that the whiteness of the sun's light is compounded of all the colours wherewith the several sorts of rays whereof that light consists, when by their several refrangibilities they are separated from one another, do tinge paper or any other white body whereon they fall. for those colours (by _prop._ ii. _part_ .) are unchangeable, and whenever all those rays with those their colours are mix'd again, they reproduce the same white light as before. _prop._ vi. prob. ii. _in a mixture of primary colours, the quantity and quality of each being given, to know the colour of the compound._ [illustration: fig. .] with the center o [in _fig._ .] and radius od describe a circle adf, and distinguish its circumference into seven parts de, ef, fg, ga, ab, bc, cd, proportional to the seven musical tones or intervals of the eight sounds, _sol_, _la_, _fa_, _sol_, _la_, _mi_, _fa_, _sol_, contained in an eight, that is, proportional to the number / , / , / , / , / , / , / . let the first part de represent a red colour, the second ef orange, the third fg yellow, the fourth ca green, the fifth ab blue, the sixth bc indigo, and the seventh cd violet. and conceive that these are all the colours of uncompounded light gradually passing into one another, as they do when made by prisms; the circumference defgabcd, representing the whole series of colours from one end of the sun's colour'd image to the other, so that from d to e be all degrees of red, at e the mean colour between red and orange, from e to f all degrees of orange, at f the mean between orange and yellow, from f to g all degrees of yellow, and so on. let _p_ be the center of gravity of the arch de, and _q_, _r_, _s_, _t_, _u_, _x_, the centers of gravity of the arches ef, fg, ga, ab, bc, and cd respectively, and about those centers of gravity let circles proportional to the number of rays of each colour in the given mixture be describ'd: that is, the circle _p_ proportional to the number of the red-making rays in the mixture, the circle _q_ proportional to the number of the orange-making rays in the mixture, and so of the rest. find the common center of gravity of all those circles, _p_, _q_, _r_, _s_, _t_, _u_, _x_. let that center be z; and from the center of the circle adf, through z to the circumference, drawing the right line oy, the place of the point y in the circumference shall shew the colour arising from the composition of all the colours in the given mixture, and the line oz shall be proportional to the fulness or intenseness of the colour, that is, to its distance from whiteness. as if y fall in the middle between f and g, the compounded colour shall be the best yellow; if y verge from the middle towards f or g, the compound colour shall accordingly be a yellow, verging towards orange or green. if z fall upon the circumference, the colour shall be intense and florid in the highest degree; if it fall in the mid-way between the circumference and center, it shall be but half so intense, that is, it shall be such a colour as would be made by diluting the intensest yellow with an equal quantity of whiteness; and if it fall upon the center o, the colour shall have lost all its intenseness, and become a white. but it is to be noted, that if the point z fall in or near the line od, the main ingredients being the red and violet, the colour compounded shall not be any of the prismatick colours, but a purple, inclining to red or violet, accordingly as the point z lieth on the side of the line do towards e or towards c, and in general the compounded violet is more bright and more fiery than the uncompounded. also if only two of the primary colours which in the circle are opposite to one another be mixed in an equal proportion, the point z shall fall upon the center o, and yet the colour compounded of those two shall not be perfectly white, but some faint anonymous colour. for i could never yet by mixing only two primary colours produce a perfect white. whether it may be compounded of a mixture of three taken at equal distances in the circumference i do not know, but of four or five i do not much question but it may. but these are curiosities of little or no moment to the understanding the phænomena of nature. for in all whites produced by nature, there uses to be a mixture of all sorts of rays, and by consequence a composition of all colours. to give an instance of this rule; suppose a colour is compounded of these homogeneal colours, of violet one part, of indigo one part, of blue two parts, of green three parts, of yellow five parts, of orange six parts, and of red ten parts. proportional to these parts describe the circles _x_, _v_, _t_, _s_, _r_, _q_, _p_, respectively, that is, so that if the circle _x_ be one, the circle _v_ may be one, the circle _t_ two, the circle _s_ three, and the circles _r_, _q_ and _p_, five, six and ten. then i find z the common center of gravity of these circles, and through z drawing the line oy, the point y falls upon the circumference between e and f, something nearer to e than to f, and thence i conclude, that the colour compounded of these ingredients will be an orange, verging a little more to red than to yellow. also i find that oz is a little less than one half of oy, and thence i conclude, that this orange hath a little less than half the fulness or intenseness of an uncompounded orange; that is to say, that it is such an orange as may be made by mixing an homogeneal orange with a good white in the proportion of the line oz to the line zy, this proportion being not of the quantities of mixed orange and white powders, but of the quantities of the lights reflected from them. this rule i conceive accurate enough for practice, though not mathematically accurate; and the truth of it may be sufficiently proved to sense, by stopping any of the colours at the lens in the tenth experiment of this book. for the rest of the colours which are not stopp'd, but pass on to the focus of the lens, will there compound either accurately or very nearly such a colour, as by this rule ought to result from their mixture. _prop._ vii. theor. v. _all the colours in the universe which are made by light, and depend not on the power of imagination, are either the colours of homogeneal lights, or compounded of these, and that either accurately or very nearly, according to the rule of the foregoing problem._ for it has been proved (in _prop. . part ._) that the changes of colours made by refractions do not arise from any new modifications of the rays impress'd by those refractions, and by the various terminations of light and shadow, as has been the constant and general opinion of philosophers. it has also been proved that the several colours of the homogeneal rays do constantly answer to their degrees of refrangibility, (_prop._ . _part_ . and _prop._ . _part_ .) and that their degrees of refrangibility cannot be changed by refractions and reflexions (_prop._ . _part_ .) and by consequence that those their colours are likewise immutable. it has also been proved directly by refracting and reflecting homogeneal lights apart, that their colours cannot be changed, (_prop._ . _part_ .) it has been proved also, that when the several sorts of rays are mixed, and in crossing pass through the same space, they do not act on one another so as to change each others colorific qualities. (_exper._ . _part_ .) but by mixing their actions in the sensorium beget a sensation differing from what either would do apart, that is a sensation of a mean colour between their proper colours; and particularly when by the concourse and mixtures of all sorts of rays, a white colour is produced, the white is a mixture of all the colours which the rays would have apart, (_prop._ . _part_ .) the rays in that mixture do not lose or alter their several colorific qualities, but by all their various kinds of actions mix'd in the sensorium, beget a sensation of a middling colour between all their colours, which is whiteness. for whiteness is a mean between all colours, having it self indifferently to them all, so as with equal facility to be tinged with any of them. a red powder mixed with a little blue, or a blue with a little red, doth not presently lose its colour, but a white powder mix'd with any colour is presently tinged with that colour, and is equally capable of being tinged with any colour whatever. it has been shewed also, that as the sun's light is mix'd of all sorts of rays, so its whiteness is a mixture of the colours of all sorts of rays; those rays having from the beginning their several colorific qualities as well as their several refrangibilities, and retaining them perpetually unchanged notwithstanding any refractions or reflexions they may at any time suffer, and that whenever any sort of the sun's rays is by any means (as by reflexion in _exper._ , and . _part_ . or by refraction as happens in all refractions) separated from the rest, they then manifest their proper colours. these things have been prov'd, and the sum of all this amounts to the proposition here to be proved. for if the sun's light is mix'd of several sorts of rays, each of which have originally their several refrangibilities and colorific qualities, and notwithstanding their refractions and reflexions, and their various separations or mixtures, keep those their original properties perpetually the same without alteration; then all the colours in the world must be such as constantly ought to arise from the original colorific qualities of the rays whereof the lights consist by which those colours are seen. and therefore if the reason of any colour whatever be required, we have nothing else to do than to consider how the rays in the sun's light have by reflexions or refractions, or other causes, been parted from one another, or mixed together; or otherwise to find out what sorts of rays are in the light by which that colour is made, and in what proportion; and then by the last problem to learn the colour which ought to arise by mixing those rays (or their colours) in that proportion. i speak here of colours so far as they arise from light. for they appear sometimes by other causes, as when by the power of phantasy we see colours in a dream, or a mad-man sees things before him which are not there; or when we see fire by striking the eye, or see colours like the eye of a peacock's feather, by pressing our eyes in either corner whilst we look the other way. where these and such like causes interpose not, the colour always answers to the sort or sorts of the rays whereof the light consists, as i have constantly found in whatever phænomena of colours i have hitherto been able to examine. i shall in the following propositions give instances of this in the phænomena of chiefest note. _prop._ viii. prob. iii. _by the discovered properties of light to explain the colours made by prisms._ let abc [in _fig._ .] represent a prism refracting the light of the sun, which comes into a dark chamber through a hole f[greek: ph] almost as broad as the prism, and let mn represent a white paper on which the refracted light is cast, and suppose the most refrangible or deepest violet-making rays fall upon the space p[greek: p], the least refrangible or deepest red-making rays upon the space t[greek: t], the middle sort between the indigo-making and blue-making rays upon the space q[greek: ch], the middle sort of the green-making rays upon the space r, the middle sort between the yellow-making and orange-making rays upon the space s[greek: s], and other intermediate sorts upon intermediate spaces. for so the spaces upon which the several sorts adequately fall will by reason of the different refrangibility of those sorts be one lower than another. now if the paper mn be so near the prism that the spaces pt and [greek: pt] do not interfere with one another, the distance between them t[greek: p] will be illuminated by all the sorts of rays in that proportion to one another which they have at their very first coming out of the prism, and consequently be white. but the spaces pt and [greek: pt] on either hand, will not be illuminated by them all, and therefore will appear coloured. and particularly at p, where the outmost violet-making rays fall alone, the colour must be the deepest violet. at q where the violet-making and indigo-making rays are mixed, it must be a violet inclining much to indigo. at r where the violet-making, indigo-making, blue-making, and one half of the green-making rays are mixed, their colours must (by the construction of the second problem) compound a middle colour between indigo and blue. at s where all the rays are mixed, except the red-making and orange-making, their colours ought by the same rule to compound a faint blue, verging more to green than indigo. and in the progress from s to t, this blue will grow more and more faint and dilute, till at t, where all the colours begin to be mixed, it ends in whiteness. [illustration: fig. .] so again, on the other side of the white at [greek: t], where the least refrangible or utmost red-making rays are alone, the colour must be the deepest red. at [greek: s] the mixture of red and orange will compound a red inclining to orange. at [greek: r] the mixture of red, orange, yellow, and one half of the green must compound a middle colour between orange and yellow. at [greek: ch] the mixture of all colours but violet and indigo will compound a faint yellow, verging more to green than to orange. and this yellow will grow more faint and dilute continually in its progress from [greek: ch] to [greek: p], where by a mixture of all sorts of rays it will become white. these colours ought to appear were the sun's light perfectly white: but because it inclines to yellow, the excess of the yellow-making rays whereby 'tis tinged with that colour, being mixed with the faint blue between s and t, will draw it to a faint green. and so the colours in order from p to [greek: t] ought to be violet, indigo, blue, very faint green, white, faint yellow, orange, red. thus it is by the computation: and they that please to view the colours made by a prism will find it so in nature. these are the colours on both sides the white when the paper is held between the prism and the point x where the colours meet, and the interjacent white vanishes. for if the paper be held still farther off from the prism, the most refrangible and least refrangible rays will be wanting in the middle of the light, and the rest of the rays which are found there, will by mixture produce a fuller green than before. also the yellow and blue will now become less compounded, and by consequence more intense than before. and this also agrees with experience. and if one look through a prism upon a white object encompassed with blackness or darkness, the reason of the colours arising on the edges is much the same, as will appear to one that shall a little consider it. if a black object be encompassed with a white one, the colours which appear through the prism are to be derived from the light of the white one, spreading into the regions of the black, and therefore they appear in a contrary order to that, when a white object is surrounded with black. and the same is to be understood when an object is viewed, whose parts are some of them less luminous than others. for in the borders of the more and less luminous parts, colours ought always by the same principles to arise from the excess of the light of the more luminous, and to be of the same kind as if the darker parts were black, but yet to be more faint and dilute. what is said of colours made by prisms may be easily applied to colours made by the glasses of telescopes or microscopes, or by the humours of the eye. for if the object-glass of a telescope be thicker on one side than on the other, or if one half of the glass, or one half of the pupil of the eye be cover'd with any opake substance; the object-glass, or that part of it or of the eye which is not cover'd, may be consider'd as a wedge with crooked sides, and every wedge of glass or other pellucid substance has the effect of a prism in refracting the light which passes through it.[l] how the colours in the ninth and tenth experiments of the first part arise from the different reflexibility of light, is evident by what was there said. but it is observable in the ninth experiment, that whilst the sun's direct light is yellow, the excess of the blue-making rays in the reflected beam of light mn, suffices only to bring that yellow to a pale white inclining to blue, and not to tinge it with a manifestly blue colour. to obtain therefore a better blue, i used instead of the yellow light of the sun the white light of the clouds, by varying a little the experiment, as follows. [illustration: fig. .] _exper._ let hfg [in _fig._ .] represent a prism in the open air, and s the eye of the spectator, viewing the clouds by their light coming into the prism at the plane side figk, and reflected in it by its base heig, and thence going out through its plane side hefk to the eye. and when the prism and eye are conveniently placed, so that the angles of incidence and reflexion at the base may be about degrees, the spectator will see a bow mn of a blue colour, running from one end of the base to the other, with the concave side towards him, and the part of the base imng beyond this bow will be brighter than the other part emnh on the other side of it. this blue colour mn being made by nothing else than by reflexion of a specular superficies, seems so odd a phænomenon, and so difficult to be explained by the vulgar hypothesis of philosophers, that i could not but think it deserved to be taken notice of. now for understanding the reason of it, suppose the plane abc to cut the plane sides and base of the prism perpendicularly. from the eye to the line bc, wherein that plane cuts the base, draw the lines s_p_ and s_t_, in the angles s_pc_ degr. / , and s_tc_ degr. / , and the point _p_ will be the limit beyond which none of the most refrangible rays can pass through the base of the prism, and be refracted, whose incidence is such that they may be reflected to the eye; and the point _t_ will be the like limit for the least refrangible rays, that is, beyond which none of them can pass through the base, whose incidence is such that by reflexion they may come to the eye. and the point _r_ taken in the middle way between _p_ and _t_, will be the like limit for the meanly refrangible rays. and therefore all the least refrangible rays which fall upon the base beyond _t_, that is, between _t_ and b, and can come from thence to the eye, will be reflected thither: but on this side _t_, that is, between _t_ and _c_, many of these rays will be transmitted through the base. and all the most refrangible rays which fall upon the base beyond _p_, that is, between, _p_ and b, and can by reflexion come from thence to the eye, will be reflected thither, but every where between _p_ and _c_, many of these rays will get through the base, and be refracted; and the same is to be understood of the meanly refrangible rays on either side of the point _r_. whence it follows, that the base of the prism must every where between _t_ and b, by a total reflexion of all sorts of rays to the eye, look white and bright. and every where between _p_ and c, by reason of the transmission of many rays of every sort, look more pale, obscure, and dark. but at _r_, and in other places between _p_ and _t_, where all the more refrangible rays are reflected to the eye, and many of the less refrangible are transmitted, the excess of the most refrangible in the reflected light will tinge that light with their colour, which is violet and blue. and this happens by taking the line c _prt_ b any where between the ends of the prism hg and ei. _prop._ ix. prob. iv. _by the discovered properties of light to explain the colours of the rain-bow._ [illustration: fig. .] this bow never appears, but where it rains in the sun-shine, and may be made artificially by spouting up water which may break aloft, and scatter into drops, and fall down like rain. for the sun shining upon these drops certainly causes the bow to appear to a spectator standing in a due position to the rain and sun. and hence it is now agreed upon, that this bow is made by refraction of the sun's light in drops of falling rain. this was understood by some of the antients, and of late more fully discover'd and explain'd by the famous _antonius de dominis_ archbishop of _spalato_, in his book _de radiis visûs & lucis_, published by his friend _bartolus_ at _venice_, in the year , and written above years before. for he teaches there how the interior bow is made in round drops of rain by two refractions of the sun's light, and one reflexion between them, and the exterior by two refractions, and two sorts of reflexions between them in each drop of water, and proves his explications by experiments made with a phial full of water, and with globes of glass filled with water, and placed in the sun to make the colours of the two bows appear in them. the same explication _des-cartes_ hath pursued in his meteors, and mended that of the exterior bow. but whilst they understood not the true origin of colours, it's necessary to pursue it here a little farther. for understanding therefore how the bow is made, let a drop of rain, or any other spherical transparent body be represented by the sphere bnfg, [in _fig._ .] described with the center c, and semi-diameter cn. and let an be one of the sun's rays incident upon it at n, and thence refracted to f, where let it either go out of the sphere by refraction towards v, or be reflected to g; and at g let it either go out by refraction to r, or be reflected to h; and at h let it go out by refraction towards s, cutting the incident ray in y. produce an and rg, till they meet in x, and upon ax and nf, let fall the perpendiculars cd and ce, and produce cd till it fall upon the circumference at l. parallel to the incident ray an draw the diameter bq, and let the sine of incidence out of air into water be to the sine of refraction as i to r. now, if you suppose the point of incidence n to move from the point b, continually till it come to l, the arch qf will first increase and then decrease, and so will the angle axr which the rays an and gr contain; and the arch qf and angle axr will be biggest when nd is to cn as sqrt(ii - rr) to sqrt( )rr, in which case ne will be to nd as r to i. also the angle ays, which the rays an and hs contain will first decrease, and then increase and grow least when nd is to cn as sqrt(ii - rr) to sqrt( )rr, in which case ne will be to nd, as r to i. and so the angle which the next emergent ray (that is, the emergent ray after three reflexions) contains with the incident ray an will come to its limit when nd is to cn as sqrt(ii - rr) to sqrt( )rr, in which case ne will be to nd as r to i. and the angle which the ray next after that emergent, that is, the ray emergent after four reflexions, contains with the incident, will come to its limit, when nd is to cn as sqrt(ii - rr) to sqrt( )rr, in which case ne will be to nd as r to i; and so on infinitely, the numbers , , , , &c. being gather'd by continual addition of the terms of the arithmetical progression , , , , &c. the truth of all this mathematicians will easily examine.[m] now it is to be observed, that as when the sun comes to his tropicks, days increase and decrease but a very little for a great while together; so when by increasing the distance cd, these angles come to their limits, they vary their quantity but very little for some time together, and therefore a far greater number of the rays which fall upon all the points n in the quadrant bl, shall emerge in the limits of these angles, than in any other inclinations. and farther it is to be observed, that the rays which differ in refrangibility will have different limits of their angles of emergence, and by consequence according to their different degrees of refrangibility emerge most copiously in different angles, and being separated from one another appear each in their proper colours. and what those angles are may be easily gather'd from the foregoing theorem by computation. for in the least refrangible rays the sines i and r (as was found above) are and , and thence by computation the greatest angle axr will be found degrees and minutes, and the least angle ays, degrees and minutes. and in the most refrangible rays the sines i and r are and , and thence by computation the greatest angle axr will be found degrees and minutes, and the least angle ays degrees and minutes. suppose now that o [in _fig._ .] is the spectator's eye, and op a line drawn parallel to the sun's rays and let poe, pof, pog, poh, be angles of degr. min. degr. min. degr. min. and degr. min. respectively, and these angles turned about their common side op, shall with their other sides oe, of; og, oh, describe the verges of two rain-bows af, be and chdg. for if e, f, g, h, be drops placed any where in the conical superficies described by oe, of, og, oh, and be illuminated by the sun's rays se, sf, sg, sh; the angle seo being equal to the angle poe, or degr. min. shall be the greatest angle in which the most refrangible rays can after one reflexion be refracted to the eye, and therefore all the drops in the line oe shall send the most refrangible rays most copiously to the eye, and thereby strike the senses with the deepest violet colour in that region. and in like manner the angle sfo being equal to the angle pof, or degr. min. shall be the greatest in which the least refrangible rays after one reflexion can emerge out of the drops, and therefore those rays shall come most copiously to the eye from the drops in the line of, and strike the senses with the deepest red colour in that region. and by the same argument, the rays which have intermediate degrees of refrangibility shall come most copiously from drops between e and f, and strike the senses with the intermediate colours, in the order which their degrees of refrangibility require, that is in the progress from e to f, or from the inside of the bow to the outside in this order, violet, indigo, blue, green, yellow, orange, red. but the violet, by the mixture of the white light of the clouds, will appear faint and incline to purple. [illustration: fig. .] again, the angle sgo being equal to the angle pog, or gr. min. shall be the least angle in which the least refrangible rays can after two reflexions emerge out of the drops, and therefore the least refrangible rays shall come most copiously to the eye from the drops in the line og, and strike the sense with the deepest red in that region. and the angle sho being equal to the angle poh, or gr. min. shall be the least angle, in which the most refrangible rays after two reflexions can emerge out of the drops; and therefore those rays shall come most copiously to the eye from the drops in the line oh, and strike the senses with the deepest violet in that region. and by the same argument, the drops in the regions between g and h shall strike the sense with the intermediate colours in the order which their degrees of refrangibility require, that is, in the progress from g to h, or from the inside of the bow to the outside in this order, red, orange, yellow, green, blue, indigo, violet. and since these four lines oe, of, og, oh, may be situated any where in the above-mention'd conical superficies; what is said of the drops and colours in these lines is to be understood of the drops and colours every where in those superficies. thus shall there be made two bows of colours, an interior and stronger, by one reflexion in the drops, and an exterior and fainter by two; for the light becomes fainter by every reflexion. and their colours shall lie in a contrary order to one another, the red of both bows bordering upon the space gf, which is between the bows. the breadth of the interior bow eof measured cross the colours shall be degr. min. and the breadth of the exterior goh shall be degr. min. and the distance between them gof shall be gr. min. the greatest semi-diameter of the innermost, that is, the angle pof being gr. min. and the least semi-diameter of the outermost pog, being gr. min. these are the measures of the bows, as they would be were the sun but a point; for by the breadth of his body, the breadth of the bows will be increased, and their distance decreased by half a degree, and so the breadth of the interior iris will be degr. min. that of the exterior degr. min. their distance degr. min. the greatest semi-diameter of the interior bow degr. min. and the least of the exterior degr. min. and such are the dimensions of the bows in the heavens found to be very nearly, when their colours appear strong and perfect. for once, by such means as i then had, i measured the greatest semi-diameter of the interior iris about degrees, and the breadth of the red, yellow and green in that iris or minutes, besides the outmost faint red obscured by the brightness of the clouds, for which we may allow or minutes more. the breadth of the blue was about minutes more besides the violet, which was so much obscured by the brightness of the clouds, that i could not measure its breadth. but supposing the breadth of the blue and violet together to equal that of the red, yellow and green together, the whole breadth of this iris will be about - / degrees, as above. the least distance between this iris and the exterior iris was about degrees and minutes. the exterior iris was broader than the interior, but so faint, especially on the blue side, that i could not measure its breadth distinctly. at another time when both bows appeared more distinct, i measured the breadth of the interior iris gr. ´, and the breadth of the red, yellow and green in the exterior iris, was to the breadth of the same colours in the interior as to . this explication of the rain-bow is yet farther confirmed by the known experiment (made by _antonius de dominis_ and _des-cartes_) of hanging up any where in the sun-shine a glass globe filled with water, and viewing it in such a posture, that the rays which come from the globe to the eye may contain with the sun's rays an angle of either or degrees. for if the angle be about or degrees, the spectator (suppose at o) shall see a full red colour in that side of the globe opposed to the sun as 'tis represented at f, and if that angle become less (suppose by depressing the globe to e) there will appear other colours, yellow, green and blue successive in the same side of the globe. but if the angle be made about degrees (suppose by lifting up the globe to g) there will appear a red colour in that side of the globe towards the sun, and if the angle be made greater (suppose by lifting up the globe to h) the red will turn successively to the other colours, yellow, green and blue. the same thing i have tried, by letting a globe rest, and raising or depressing the eye, or otherwise moving it to make the angle of a just magnitude. i have heard it represented, that if the light of a candle be refracted by a prism to the eye; when the blue colour falls upon the eye, the spectator shall see red in the prism, and when the red falls upon the eye he shall see blue; and if this were certain, the colours of the globe and rain-bow ought to appear in a contrary order to what we find. but the colours of the candle being very faint, the mistake seems to arise from the difficulty of discerning what colours fall on the eye. for, on the contrary, i have sometimes had occasion to observe in the sun's light refracted by a prism, that the spectator always sees that colour in the prism which falls upon his eye. and the same i have found true also in candle-light. for when the prism is moved slowly from the line which is drawn directly from the candle to the eye, the red appears first in the prism and then the blue, and therefore each of them is seen when it falls upon the eye. for the red passes over the eye first, and then the blue. the light which comes through drops of rain by two refractions without any reflexion, ought to appear strongest at the distance of about degrees from the sun, and to decay gradually both ways as the distance from him increases and decreases. and the same is to be understood of light transmitted through spherical hail-stones. and if the hail be a little flatted, as it often is, the light transmitted may grow so strong at a little less distance than that of degrees, as to form a halo about the sun or moon; which halo, as often as the hail-stones are duly figured may be colour'd, and then it must be red within by the least refrangible rays, and blue without by the most refrangible ones, especially if the hail-stones have opake globules of snow in their center to intercept the light within the halo (as _hugenius_ has observ'd) and make the inside thereof more distinctly defined than it would otherwise be. for such hail-stones, though spherical, by terminating the light by the snow, may make a halo red within and colourless without, and darker in the red than without, as halos used to be. for of those rays which pass close by the snow the rubriform will be least refracted, and so come to the eye in the directest lines. the light which passes through a drop of rain after two refractions, and three or more reflexions, is scarce strong enough to cause a sensible bow; but in those cylinders of ice by which _hugenius_ explains the _parhelia_, it may perhaps be sensible. _prop._ x. prob. v. _by the discovered properties of light to explain the permanent colours of natural bodies._ these colours arise from hence, that some natural bodies reflect some sorts of rays, others other sorts more copiously than the rest. minium reflects the least refrangible or red-making rays most copiously, and thence appears red. violets reflect the most refrangible most copiously, and thence have their colour, and so of other bodies. every body reflects the rays of its own colour more copiously than the rest, and from their excess and predominance in the reflected light has its colour. _exper._ . for if in the homogeneal lights obtained by the solution of the problem proposed in the fourth proposition of the first part of this book, you place bodies of several colours, you will find, as i have done, that every body looks most splendid and luminous in the light of its own colour. cinnaber in the homogeneal red light is most resplendent, in the green light it is manifestly less resplendent, and in the blue light still less. indigo in the violet blue light is most resplendent, and its splendor is gradually diminish'd, as it is removed thence by degrees through the green and yellow light to the red. by a leek the green light, and next that the blue and yellow which compound green, are more strongly reflected than the other colours red and violet, and so of the rest. but to make these experiments the more manifest, such bodies ought to be chosen as have the fullest and most vivid colours, and two of those bodies are to be compared together. thus, for instance, if cinnaber and _ultra_-marine blue, or some other full blue be held together in the red homogeneal light, they will both appear red, but the cinnaber will appear of a strongly luminous and resplendent red, and the _ultra_-marine blue of a faint obscure and dark red; and if they be held together in the blue homogeneal light, they will both appear blue, but the _ultra_-marine will appear of a strongly luminous and resplendent blue, and the cinnaber of a faint and dark blue. which puts it out of dispute that the cinnaber reflects the red light much more copiously than the _ultra_-marine doth, and the _ultra_-marine reflects the blue light much more copiously than the cinnaber doth. the same experiment may be tried successfully with red lead and indigo, or with any other two colour'd bodies, if due allowance be made for the different strength or weakness of their colour and light. and as the reason of the colours of natural bodies is evident by these experiments, so it is farther confirmed and put past dispute by the two first experiments of the first part, whereby 'twas proved in such bodies that the reflected lights which differ in colours do differ also in degrees of refrangibility. for thence it's certain, that some bodies reflect the more refrangible, others the less refrangible rays more copiously. and that this is not only a true reason of these colours, but even the only reason, may appear farther from this consideration, that the colour of homogeneal light cannot be changed by the reflexion of natural bodies. for if bodies by reflexion cannot in the least change the colour of any one sort of rays, they cannot appear colour'd by any other means than by reflecting those which either are of their own colour, or which by mixture must produce it. but in trying experiments of this kind care must be had that the light be sufficiently homogeneal. for if bodies be illuminated by the ordinary prismatick colours, they will appear neither of their own day-light colours, nor of the colour of the light cast on them, but of some middle colour between both, as i have found by experience. thus red lead (for instance) illuminated with the ordinary prismatick green will not appear either red or green, but orange or yellow, or between yellow and green, accordingly as the green light by which 'tis illuminated is more or less compounded. for because red lead appears red when illuminated with white light, wherein all sorts of rays are equally mix'd, and in the green light all sorts of rays are not equally mix'd, the excess of the yellow-making, green-making and blue-making rays in the incident green light, will cause those rays to abound so much in the reflected light, as to draw the colour from red towards their colour. and because the red lead reflects the red-making rays most copiously in proportion to their number, and next after them the orange-making and yellow-making rays; these rays in the reflected light will be more in proportion to the light than they were in the incident green light, and thereby will draw the reflected light from green towards their colour. and therefore the red lead will appear neither red nor green, but of a colour between both. in transparently colour'd liquors 'tis observable, that their colour uses to vary with their thickness. thus, for instance, a red liquor in a conical glass held between the light and the eye, looks of a pale and dilute yellow at the bottom where 'tis thin, and a little higher where 'tis thicker grows orange, and where 'tis still thicker becomes red, and where 'tis thickest the red is deepest and darkest. for it is to be conceiv'd that such a liquor stops the indigo-making and violet-making rays most easily, the blue-making rays more difficultly, the green-making rays still more difficultly, and the red-making most difficultly: and that if the thickness of the liquor be only so much as suffices to stop a competent number of the violet-making and indigo-making rays, without diminishing much the number of the rest, the rest must (by _prop._ . _part_ .) compound a pale yellow. but if the liquor be so much thicker as to stop also a great number of the blue-making rays, and some of the green-making, the rest must compound an orange; and where it is so thick as to stop also a great number of the green-making and a considerable number of the yellow-making, the rest must begin to compound a red, and this red must grow deeper and darker as the yellow-making and orange-making rays are more and more stopp'd by increasing the thickness of the liquor, so that few rays besides the red-making can get through. of this kind is an experiment lately related to me by mr. _halley_, who, in diving deep into the sea in a diving vessel, found in a clear sun-shine day, that when he was sunk many fathoms deep into the water the upper part of his hand on which the sun shone directly through the water and through a small glass window in the vessel appeared of a red colour, like that of a damask rose, and the water below and the under part of his hand illuminated by light reflected from the water below look'd green. for thence it may be gather'd, that the sea-water reflects back the violet and blue-making rays most easily, and lets the red-making rays pass most freely and copiously to great depths. for thereby the sun's direct light at all great depths, by reason of the predominating red-making rays, must appear red; and the greater the depth is, the fuller and intenser must that red be. and at such depths as the violet-making rays scarce penetrate unto, the blue-making, green-making, and yellow-making rays being reflected from below more copiously than the red-making ones, must compound a green. now, if there be two liquors of full colours, suppose a red and blue, and both of them so thick as suffices to make their colours sufficiently full; though either liquor be sufficiently transparent apart, yet will you not be able to see through both together. for, if only the red-making rays pass through one liquor, and only the blue-making through the other, no rays can pass through both. this mr. _hook_ tried casually with glass wedges filled with red and blue liquors, and was surprized at the unexpected event, the reason of it being then unknown; which makes me trust the more to his experiment, though i have not tried it my self. but he that would repeat it, must take care the liquors be of very good and full colours. now, whilst bodies become coloured by reflecting or transmitting this or that sort of rays more copiously than the rest, it is to be conceived that they stop and stifle in themselves the rays which they do not reflect or transmit. for, if gold be foliated and held between your eye and the light, the light looks of a greenish blue, and therefore massy gold lets into its body the blue-making rays to be reflected to and fro within it till they be stopp'd and stifled, whilst it reflects the yellow-making outwards, and thereby looks yellow. and much after the same manner that leaf gold is yellow by reflected, and blue by transmitted light, and massy gold is yellow in all positions of the eye; there are some liquors, as the tincture of _lignum nephriticum_, and some sorts of glass which transmit one sort of light most copiously, and reflect another sort, and thereby look of several colours, according to the position of the eye to the light. but, if these liquors or glasses were so thick and massy that no light could get through them, i question not but they would like all other opake bodies appear of one and the same colour in all positions of the eye, though this i cannot yet affirm by experience. for all colour'd bodies, so far as my observation reaches, may be seen through if made sufficiently thin, and therefore are in some measure transparent, and differ only in degrees of transparency from tinged transparent liquors; these liquors, as well as those bodies, by a sufficient thickness becoming opake. a transparent body which looks of any colour by transmitted light, may also look of the same colour by reflected light, the light of that colour being reflected by the farther surface of the body, or by the air beyond it. and then the reflected colour will be diminished, and perhaps cease, by making the body very thick, and pitching it on the backside to diminish the reflexion of its farther surface, so that the light reflected from the tinging particles may predominate. in such cases, the colour of the reflected light will be apt to vary from that of the light transmitted. but whence it is that tinged bodies and liquors reflect some sort of rays, and intromit or transmit other sorts, shall be said in the next book. in this proposition i content my self to have put it past dispute, that bodies have such properties, and thence appear colour'd. _prop._ xi. prob. vi. _by mixing colour'd lights to compound a beam of light of the same colour and nature with a beam of the sun's direct light, and therein to experience the truth of the foregoing propositions._ [illustration: fig. .] let abc _abc_ [in _fig._ .] represent a prism, by which the sun's light let into a dark chamber through the hole f, may be refracted towards the lens mn, and paint upon it at _p_, _q_, _r_, _s_, and _t_, the usual colours violet, blue, green, yellow, and red, and let the diverging rays by the refraction of this lens converge again towards x, and there, by the mixture of all those their colours, compound a white according to what was shewn above. then let another prism deg _deg_, parallel to the former, be placed at x, to refract that white light upwards towards y. let the refracting angles of the prisms, and their distances from the lens be equal, so that the rays which converged from the lens towards x, and without refraction, would there have crossed and diverged again, may by the refraction of the second prism be reduced into parallelism and diverge no more. for then those rays will recompose a beam of white light xy. if the refracting angle of either prism be the bigger, that prism must be so much the nearer to the lens. you will know when the prisms and the lens are well set together, by observing if the beam of light xy, which comes out of the second prism be perfectly white to the very edges of the light, and at all distances from the prism continue perfectly and totally white like a beam of the sun's light. for till this happens, the position of the prisms and lens to one another must be corrected; and then if by the help of a long beam of wood, as is represented in the figure, or by a tube, or some other such instrument, made for that purpose, they be made fast in that situation, you may try all the same experiments in this compounded beam of light xy, which have been made in the sun's direct light. for this compounded beam of light has the same appearance, and is endow'd with all the same properties with a direct beam of the sun's light, so far as my observation reaches. and in trying experiments in this beam you may by stopping any of the colours, _p_, _q_, _r_, _s_, and _t_, at the lens, see how the colours produced in the experiments are no other than those which the rays had at the lens before they entered the composition of this beam: and by consequence, that they arise not from any new modifications of the light by refractions and reflexions, but from the various separations and mixtures of the rays originally endow'd with their colour-making qualities. so, for instance, having with a lens - / inches broad, and two prisms on either hand - / feet distant from the lens, made such a beam of compounded light; to examine the reason of the colours made by prisms, i refracted this compounded beam of light xy with another prism hik _kh_, and thereby cast the usual prismatick colours pqrst upon the paper lv placed behind. and then by stopping any of the colours _p_, _q_, _r_, _s_, _t_, at the lens, i found that the same colour would vanish at the paper. so if the purple _p_ was stopp'd at the lens, the purple p upon the paper would vanish, and the rest of the colours would remain unalter'd, unless perhaps the blue, so far as some purple latent in it at the lens might be separated from it by the following refractions. and so by intercepting the green upon the lens, the green r upon the paper would vanish, and so of the rest; which plainly shews, that as the white beam of light xy was compounded of several lights variously colour'd at the lens, so the colours which afterwards emerge out of it by new refractions are no other than those of which its whiteness was compounded. the refraction of the prism hik _kh_ generates the colours pqrst upon the paper, not by changing the colorific qualities of the rays, but by separating the rays which had the very same colorific qualities before they enter'd the composition of the refracted beam of white light xy. for otherwise the rays which were of one colour at the lens might be of another upon the paper, contrary to what we find. so again, to examine the reason of the colours of natural bodies, i placed such bodies in the beam of light xy, and found that they all appeared there of those their own colours which they have in day-light, and that those colours depend upon the rays which had the same colours at the lens before they enter'd the composition of that beam. thus, for instance, cinnaber illuminated by this beam appears of the same red colour as in day-light; and if at the lens you intercept the green-making and blue-making rays, its redness will become more full and lively: but if you there intercept the red-making rays, it will not any longer appear red, but become yellow or green, or of some other colour, according to the sorts of rays which you do not intercept. so gold in this light xy appears of the same yellow colour as in day-light, but by intercepting at the lens a due quantity of the yellow-making rays it will appear white like silver (as i have tried) which shews that its yellowness arises from the excess of the intercepted rays tinging that whiteness with their colour when they are let pass. so the infusion of _lignum nephriticum_ (as i have also tried) when held in this beam of light xy, looks blue by the reflected part of the light, and red by the transmitted part of it, as when 'tis view'd in day-light; but if you intercept the blue at the lens the infusion will lose its reflected blue colour, whilst its transmitted red remains perfect, and by the loss of some blue-making rays, wherewith it was allay'd, becomes more intense and full. and, on the contrary, if the red and orange-making rays be intercepted at the lens, the infusion will lose its transmitted red, whilst its blue will remain and become more full and perfect. which shews, that the infusion does not tinge the rays with blue and red, but only transmits those most copiously which were red-making before, and reflects those most copiously which were blue-making before. and after the same manner may the reasons of other phænomena be examined, by trying them in this artificial beam of light xy. footnotes: [i] see p. . [j] _see our_ author's lect. optic. _part_ ii. _sect._ ii. _p._ . [k] _as is done in our_ author's lect. optic. _part_ i. _sect._ iii. _and_ iv. _and part_ ii. _sect._ ii. [l] _see our_ author's lect. optic. _part_ ii. _sect._ ii. _pag._ , &c. [m] _this is demonstrated in our_ author's lect. optic. _part_ i. _sect._ iv. _prop._ _and_ . the second book of opticks _part i._ _observations concerning the reflexions, refractions, and colours of thin transparent bodies._ it has been observed by others, that transparent substances, as glass, water, air, &c. when made very thin by being blown into bubbles, or otherwise formed into plates, do exhibit various colours according to their various thinness, altho' at a greater thickness they appear very clear and colourless. in the former book i forbore to treat of these colours, because they seemed of a more difficult consideration, and were not necessary for establishing the properties of light there discoursed of. but because they may conduce to farther discoveries for compleating the theory of light, especially as to the constitution of the parts of natural bodies, on which their colours or transparency depend; i have here set down an account of them. to render this discourse short and distinct, i have first described the principal of my observations, and then consider'd and made use of them. the observations are these. _obs._ . compressing two prisms hard together that their sides (which by chance were a very little convex) might somewhere touch one another: i found the place in which they touched to become absolutely transparent, as if they had there been one continued piece of glass. for when the light fell so obliquely on the air, which in other places was between them, as to be all reflected; it seemed in that place of contact to be wholly transmitted, insomuch that when look'd upon, it appeared like a black or dark spot, by reason that little or no sensible light was reflected from thence, as from other places; and when looked through it seemed (as it were) a hole in that air which was formed into a thin plate, by being compress'd between the glasses. and through this hole objects that were beyond might be seen distinctly, which could not at all be seen through other parts of the glasses where the air was interjacent. although the glasses were a little convex, yet this transparent spot was of a considerable breadth, which breadth seemed principally to proceed from the yielding inwards of the parts of the glasses, by reason of their mutual pressure. for by pressing them very hard together it would become much broader than otherwise. _obs._ . when the plate of air, by turning the prisms about their common axis, became so little inclined to the incident rays, that some of them began to be transmitted, there arose in it many slender arcs of colours which at first were shaped almost like the conchoid, as you see them delineated in the first figure. and by continuing the motion of the prisms, these arcs increased and bended more and more about the said transparent spot, till they were compleated into circles or rings incompassing it, and afterwards continually grew more and more contracted. [illustration: fig. .] these arcs at their first appearance were of a violet and blue colour, and between them were white arcs of circles, which presently by continuing the motion of the prisms became a little tinged in their inward limbs with red and yellow, and to their outward limbs the blue was adjacent. so that the order of these colours from the central dark spot, was at that time white, blue, violet; black, red, orange, yellow, white, blue, violet, &c. but the yellow and red were much fainter than the blue and violet. the motion of the prisms about their axis being continued, these colours contracted more and more, shrinking towards the whiteness on either side of it, until they totally vanished into it. and then the circles in those parts appear'd black and white, without any other colours intermix'd. but by farther moving the prisms about, the colours again emerged out of the whiteness, the violet and blue at its inward limb, and at its outward limb the red and yellow. so that now their order from the central spot was white, yellow, red; black; violet, blue, white, yellow, red, &c. contrary to what it was before. _obs._ . when the rings or some parts of them appeared only black and white, they were very distinct and well defined, and the blackness seemed as intense as that of the central spot. also in the borders of the rings, where the colours began to emerge out of the whiteness, they were pretty distinct, which made them visible to a very great multitude. i have sometimes number'd above thirty successions (reckoning every black and white ring for one succession) and seen more of them, which by reason of their smalness i could not number. but in other positions of the prisms, at which the rings appeared of many colours, i could not distinguish above eight or nine of them, and the exterior of those were very confused and dilute. in these two observations to see the rings distinct, and without any other colour than black and white, i found it necessary to hold my eye at a good distance from them. for by approaching nearer, although in the same inclination of my eye to the plane of the rings, there emerged a bluish colour out of the white, which by dilating it self more and more into the black, render'd the circles less distinct, and left the white a little tinged with red and yellow. i found also by looking through a slit or oblong hole, which was narrower than the pupil of my eye, and held close to it parallel to the prisms, i could see the circles much distincter and visible to a far greater number than otherwise. _obs._ . to observe more nicely the order of the colours which arose out of the white circles as the rays became less and less inclined to the plate of air; i took two object-glasses, the one a plano-convex for a fourteen foot telescope, and the other a large double convex for one of about fifty foot; and upon this, laying the other with its plane side downwards, i pressed them slowly together, to make the colours successively emerge in the middle of the circles, and then slowly lifted the upper glass from the lower to make them successively vanish again in the same place. the colour, which by pressing the glasses together, emerged last in the middle of the other colours, would upon its first appearance look like a circle of a colour almost uniform from the circumference to the center and by compressing the glasses still more, grow continually broader until a new colour emerged in its center, and thereby it became a ring encompassing that new colour. and by compressing the glasses still more, the diameter of this ring would increase, and the breadth of its orbit or perimeter decrease until another new colour emerged in the center of the last: and so on until a third, a fourth, a fifth, and other following new colours successively emerged there, and became rings encompassing the innermost colour, the last of which was the black spot. and, on the contrary, by lifting up the upper glass from the lower, the diameter of the rings would decrease, and the breadth of their orbit increase, until their colours reached successively to the center; and then they being of a considerable breadth, i could more easily discern and distinguish their species than before. and by this means i observ'd their succession and quantity to be as followeth. next to the pellucid central spot made by the contact of the glasses succeeded blue, white, yellow, and red. the blue was so little in quantity, that i could not discern it in the circles made by the prisms, nor could i well distinguish any violet in it, but the yellow and red were pretty copious, and seemed about as much in extent as the white, and four or five times more than the blue. the next circuit in order of colours immediately encompassing these were violet, blue, green, yellow, and red: and these were all of them copious and vivid, excepting the green, which was very little in quantity, and seemed much more faint and dilute than the other colours. of the other four, the violet was the least in extent, and the blue less than the yellow or red. the third circuit or order was purple, blue, green, yellow, and red; in which the purple seemed more reddish than the violet in the former circuit, and the green was much more conspicuous, being as brisk and copious as any of the other colours, except the yellow, but the red began to be a little faded, inclining very much to purple. after this succeeded the fourth circuit of green and red. the green was very copious and lively, inclining on the one side to blue, and on the other side to yellow. but in this fourth circuit there was neither violet, blue, nor yellow, and the red was very imperfect and dirty. also the succeeding colours became more and more imperfect and dilute, till after three or four revolutions they ended in perfect whiteness. their form, when the glasses were most compress'd so as to make the black spot appear in the center, is delineated in the second figure; where _a_, _b_, _c_, _d_, _e_: _f_, _g_, _h_, _i_, _k_: _l_, _m_, _n_, _o_, _p_: _q_, _r_: _s_, _t_: _v_, _x_: _y_, _z_, denote the colours reckon'd in order from the center, black, blue, white, yellow, red: violet, blue, green, yellow, red: purple, blue, green, yellow, red: green, red: greenish blue, red: greenish blue, pale red: greenish blue, reddish white. [illustration: fig. .] _obs._ . to determine the interval of the glasses, or thickness of the interjacent air, by which each colour was produced, i measured the diameters of the first six rings at the most lucid part of their orbits, and squaring them, i found their squares to be in the arithmetical progression of the odd numbers, , , , , , . and since one of these glasses was plane, and the other spherical, their intervals at those rings must be in the same progression. i measured also the diameters of the dark or faint rings between the more lucid colours, and found their squares to be in the arithmetical progression of the even numbers, , , , , , . and it being very nice and difficult to take these measures exactly; i repeated them divers times at divers parts of the glasses, that by their agreement i might be confirmed in them. and the same method i used in determining some others of the following observations. _obs._ . the diameter of the sixth ring at the most lucid part of its orbit was / parts of an inch, and the diameter of the sphere on which the double convex object-glass was ground was about feet, and hence i gathered the thickness of the air or aereal interval of the glasses at that ring. but some time after, suspecting that in making this observation i had not determined the diameter of the sphere with sufficient accurateness, and being uncertain whether the plano-convex glass was truly plane, and not something concave or convex on that side which i accounted plane; and whether i had not pressed the glasses together, as i often did, to make them touch; (for by pressing such glasses together their parts easily yield inwards, and the rings thereby become sensibly broader than they would be, did the glasses keep their figures.) i repeated the experiment, and found the diameter of the sixth lucid ring about / parts of an inch. i repeated the experiment also with such an object-glass of another telescope as i had at hand. this was a double convex ground on both sides to one and the same sphere, and its focus was distant from it - / inches. and thence, if the sines of incidence and refraction of the bright yellow light be assumed in proportion as to , the diameter of the sphere to which the glass was figured will by computation be found inches. this glass i laid upon a flat one, so that the black spot appeared in the middle of the rings of colours without any other pressure than that of the weight of the glass. and now measuring the diameter of the fifth dark circle as accurately as i could, i found it the fifth part of an inch precisely. this measure was taken with the points of a pair of compasses on the upper surface on the upper glass, and my eye was about eight or nine inches distance from the glass, almost perpendicularly over it, and the glass was / of an inch thick, and thence it is easy to collect that the true diameter of the ring between the glasses was greater than its measur'd diameter above the glasses in the proportion of to , or thereabouts, and by consequence equal to / parts of an inch, and its true semi-diameter equal to / parts. now as the diameter of the sphere ( inches) is to the semi-diameter of this fifth dark ring ( / parts of an inch) so is this semi-diameter to the thickness of the air at this fifth dark ring; which is therefore / or / . parts of an inch; and the fifth part thereof, _viz._ the / part of an inch, is the thickness of the air at the first of these dark rings. the same experiment i repeated with another double convex object-glass ground on both sides to one and the same sphere. its focus was distant from it - / inches, and therefore the diameter of that sphere was inches. this glass being laid upon the same plain glass, the diameter of the fifth of the dark rings, when the black spot in their center appear'd plainly without pressing the glasses, was by the measure of the compasses upon the upper glass / parts of an inch, and by consequence between the glasses it was / : for the upper glass was / of an inch thick, and my eye was distant from it inches. and a third proportional to half this from the diameter of the sphere is / parts of an inch. this is therefore the thickness of the air at this ring, and a fifth part thereof, _viz._ the / th part of an inch is the thickness thereof at the first of the rings, as above. i tried the same thing, by laying these object-glasses upon flat pieces of a broken looking-glass, and found the same measures of the rings: which makes me rely upon them till they can be determin'd more accurately by glasses ground to larger spheres, though in such glasses greater care must be taken of a true plane. these dimensions were taken, when my eye was placed almost perpendicularly over the glasses, being about an inch, or an inch and a quarter, distant from the incident rays, and eight inches distant from the glass; so that the rays were inclined to the glass in an angle of about four degrees. whence by the following observation you will understand, that had the rays been perpendicular to the glasses, the thickness of the air at these rings would have been less in the proportion of the radius to the secant of four degrees, that is, of to . let the thicknesses found be therefore diminish'd in this proportion, and they will become / and / , or (to use the nearest round number) the / th part of an inch. this is the thickness of the air at the darkest part of the first dark ring made by perpendicular rays; and half this thickness multiplied by the progression, , , , , , , &c. gives the thicknesses of the air at the most luminous parts of all the brightest rings, _viz._ / , / , / , / , &c. their arithmetical means / , / , / , &c. being its thicknesses at the darkest parts of all the dark ones. _obs._ . the rings were least, when my eye was placed perpendicularly over the glasses in the axis of the rings: and when i view'd them obliquely they became bigger, continually swelling as i removed my eye farther from the axis. and partly by measuring the diameter of the same circle at several obliquities of my eye, partly by other means, as also by making use of the two prisms for very great obliquities, i found its diameter, and consequently the thickness of the air at its perimeter in all those obliquities to be very nearly in the proportions express'd in this table. -------------------+--------------------+----------+---------- angle of incidence |angle of refraction |diameter |thickness on | into | of the | of the the air. | the air. | ring. | air. -------------------+--------------------+----------+---------- deg. min. | | | | | | | | | | | | | | - / | - / | | | | | - / | - / | | | | | - / | - / | | | | | - / | | | | | | - / | - / | | | | | | | | | | | - / | - / | | | | | - / | - / | | | | | - / | | | | | | - / | - / | | | | | | - / | | | | | | - / -------------------+--------------------+----------+---------- in the two first columns are express'd the obliquities of the incident and emergent rays to the plate of the air, that is, their angles of incidence and refraction. in the third column the diameter of any colour'd ring at those obliquities is expressed in parts, of which ten constitute that diameter when the rays are perpendicular. and in the fourth column the thickness of the air at the circumference of that ring is expressed in parts, of which also ten constitute its thickness when the rays are perpendicular. and from these measures i seem to gather this rule: that the thickness of the air is proportional to the secant of an angle, whose sine is a certain mean proportional between the sines of incidence and refraction. and that mean proportional, so far as by these measures i can determine it, is the first of an hundred and six arithmetical mean proportionals between those sines counted from the bigger sine, that is, from the sine of refraction when the refraction is made out of the glass into the plate of air, or from the sine of incidence when the refraction is made out of the plate of air into the glass. _obs._ . the dark spot in the middle of the rings increased also by the obliquation of the eye, although almost insensibly. but, if instead of the object-glasses the prisms were made use of, its increase was more manifest when viewed so obliquely that no colours appear'd about it. it was least when the rays were incident most obliquely on the interjacent air, and as the obliquity decreased it increased more and more until the colour'd rings appear'd, and then decreased again, but not so much as it increased before. and hence it is evident, that the transparency was not only at the absolute contact of the glasses, but also where they had some little interval. i have sometimes observed the diameter of that spot to be between half and two fifth parts of the diameter of the exterior circumference of the red in the first circuit or revolution of colours when view'd almost perpendicularly; whereas when view'd obliquely it hath wholly vanish'd and become opake and white like the other parts of the glass; whence it may be collected that the glasses did then scarcely, or not at all, touch one another, and that their interval at the perimeter of that spot when view'd perpendicularly was about a fifth or sixth part of their interval at the circumference of the said red. _obs._ . by looking through the two contiguous object-glasses, i found that the interjacent air exhibited rings of colours, as well by transmitting light as by reflecting it. the central spot was now white, and from it the order of the colours were yellowish red; black, violet, blue, white, yellow, red; violet, blue, green, yellow, red, &c. but these colours were very faint and dilute, unless when the light was trajected very obliquely through the glasses: for by that means they became pretty vivid. only the first yellowish red, like the blue in the fourth observation, was so little and faint as scarcely to be discern'd. comparing the colour'd rings made by reflexion, with these made by transmission of the light; i found that white was opposite to black, red to blue, yellow to violet, and green to a compound of red and violet. that is, those parts of the glass were black when looked through, which when looked upon appeared white, and on the contrary. and so those which in one case exhibited blue, did in the other case exhibit red. and the like of the other colours. the manner you have represented in the third figure, where ab, cd, are the surfaces of the glasses contiguous at e, and the black lines between them are their distances in arithmetical progression, and the colours written above are seen by reflected light, and those below by light transmitted (p. ). _obs._ . wetting the object-glasses a little at their edges, the water crept in slowly between them, and the circles thereby became less and the colours more faint: insomuch that as the water crept along, one half of them at which it first arrived would appear broken off from the other half, and contracted into a less room. by measuring them i found the proportions of their diameters to the diameters of the like circles made by air to be about seven to eight, and consequently the intervals of the glasses at like circles, caused by those two mediums water and air, are as about three to four. perhaps it may be a general rule, that if any other medium more or less dense than water be compress'd between the glasses, their intervals at the rings caused thereby will be to their intervals caused by interjacent air, as the sines are which measure the refraction made out of that medium into air. _obs._ . when the water was between the glasses, if i pressed the upper glass variously at its edges to make the rings move nimbly from one place to another, a little white spot would immediately follow the center of them, which upon creeping in of the ambient water into that place would presently vanish. its appearance was such as interjacent air would have caused, and it exhibited the same colours. but it was not air, for where any bubbles of air were in the water they would not vanish. the reflexion must have rather been caused by a subtiler medium, which could recede through the glasses at the creeping in of the water. _obs._ . these observations were made in the open air. but farther to examine the effects of colour'd light falling on the glasses, i darken'd the room, and view'd them by reflexion of the colours of a prism cast on a sheet of white paper, my eye being so placed that i could see the colour'd paper by reflexion in the glasses, as in a looking-glass. and by this means the rings became distincter and visible to a far greater number than in the open air. i have sometimes seen more than twenty of them, whereas in the open air i could not discern above eight or nine. [illustration: fig. .] _obs._ . appointing an assistant to move the prism to and fro about its axis, that all the colours might successively fall on that part of the paper which i saw by reflexion from that part of the glasses, where the circles appear'd, so that all the colours might be successively reflected from the circles to my eye, whilst i held it immovable, i found the circles which the red light made to be manifestly bigger than those which were made by the blue and violet. and it was very pleasant to see them gradually swell or contract accordingly as the colour of the light was changed. the interval of the glasses at any of the rings when they were made by the utmost red light, was to their interval at the same ring when made by the utmost violet, greater than as to , and less than as to . by the most of my observations it was as to . and this proportion seem'd very nearly the same in all obliquities of my eye; unless when two prisms were made use of instead of the object-glasses. for then at a certain great obliquity of my eye, the rings made by the several colours seem'd equal, and at a greater obliquity those made by the violet would be greater than the same rings made by the red: the refraction of the prism in this case causing the most refrangible rays to fall more obliquely on that plate of the air than the least refrangible ones. thus the experiment succeeded in the colour'd light, which was sufficiently strong and copious to make the rings sensible. and thence it may be gather'd, that if the most refrangible and least refrangible rays had been copious enough to make the rings sensible without the mixture of other rays, the proportion which here was to would have been a little greater, suppose - / or - / to . _obs._ . whilst the prism was turn'd about its axis with an uniform motion, to make all the several colours fall successively upon the object-glasses, and thereby to make the rings contract and dilate: the contraction or dilatation of each ring thus made by the variation of its colour was swiftest in the red, and slowest in the violet, and in the intermediate colours it had intermediate degrees of celerity. comparing the quantity of contraction and dilatation made by all the degrees of each colour, i found that it was greatest in the red; less in the yellow, still less in the blue, and least in the violet. and to make as just an estimation as i could of the proportions of their contractions or dilatations, i observ'd that the whole contraction or dilatation of the diameter of any ring made by all the degrees of red, was to that of the diameter of the same ring made by all the degrees of violet, as about four to three, or five to four, and that when the light was of the middle colour between yellow and green, the diameter of the ring was very nearly an arithmetical mean between the greatest diameter of the same ring made by the outmost red, and the least diameter thereof made by the outmost violet: contrary to what happens in the colours of the oblong spectrum made by the refraction of a prism, where the red is most contracted, the violet most expanded, and in the midst of all the colours is the confine of green and blue. and hence i seem to collect that the thicknesses of the air between the glasses there, where the ring is successively made by the limits of the five principal colours (red, yellow, green, blue, violet) in order (that is, by the extreme red, by the limit of red and yellow in the middle of the orange, by the limit of yellow and green, by the limit of green and blue, by the limit of blue and violet in the middle of the indigo, and by the extreme violet) are to one another very nearly as the sixth lengths of a chord which found the notes in a sixth major, _sol_, _la_, _mi_, _fa_, _sol_, _la_. but it agrees something better with the observation to say, that the thicknesses of the air between the glasses there, where the rings are successively made by the limits of the seven colours, red, orange, yellow, green, blue, indigo, violet in order, are to one another as the cube roots of the squares of the eight lengths of a chord, which found the notes in an eighth, _sol_, _la_, _fa_, _sol_, _la_, _mi_, _fa_, _sol_; that is, as the cube roots of the squares of the numbers, , / , / , / , / , / , / , / . _obs._ . these rings were not of various colours like those made in the open air, but appeared all over of that prismatick colour only with which they were illuminated. and by projecting the prismatick colours immediately upon the glasses, i found that the light which fell on the dark spaces which were between the colour'd rings was transmitted through the glasses without any variation of colour. for on a white paper placed behind, it would paint rings of the same colour with those which were reflected, and of the bigness of their immediate spaces. and from thence the origin of these rings is manifest; namely, that the air between the glasses, according to its various thickness, is disposed in some places to reflect, and in others to transmit the light of any one colour (as you may see represented in the fourth figure) and in the same place to reflect that of one colour where it transmits that of another. [illustration: fig. .] _obs._ . the squares of the diameters of these rings made by any prismatick colour were in arithmetical progression, as in the fifth observation. and the diameter of the sixth circle, when made by the citrine yellow, and viewed almost perpendicularly was about / parts of an inch, or a little less, agreeable to the sixth observation. the precedent observations were made with a rarer thin medium, terminated by a denser, such as was air or water compress'd between two glasses. in those that follow are set down the appearances of a denser medium thin'd within a rarer, such as are plates of muscovy glass, bubbles of water, and some other thin substances terminated on all sides with air. _obs._ . if a bubble be blown with water first made tenacious by dissolving a little soap in it, 'tis a common observation, that after a while it will appear tinged with a great variety of colours. to defend these bubbles from being agitated by the external air (whereby their colours are irregularly moved one among another, so that no accurate observation can be made of them,) as soon as i had blown any of them i cover'd it with a clear glass, and by that means its colours emerged in a very regular order, like so many concentrick rings encompassing the top of the bubble. and as the bubble grew thinner by the continual subsiding of the water, these rings dilated slowly and overspread the whole bubble, descending in order to the bottom of it, where they vanish'd successively. in the mean while, after all the colours were emerged at the top, there grew in the center of the rings a small round black spot, like that in the first observation, which continually dilated it self till it became sometimes more than / or / of an inch in breadth before the bubble broke. at first i thought there had been no light reflected from the water in that place, but observing it more curiously, i saw within it several smaller round spots, which appeared much blacker and darker than the rest, whereby i knew that there was some reflexion at the other places which were not so dark as those spots. and by farther tryal i found that i could see the images of some things (as of a candle or the sun) very faintly reflected, not only from the great black spot, but also from the little darker spots which were within it. besides the aforesaid colour'd rings there would often appear small spots of colours, ascending and descending up and down the sides of the bubble, by reason of some inequalities in the subsiding of the water. and sometimes small black spots generated at the sides would ascend up to the larger black spot at the top of the bubble, and unite with it. _obs._ . because the colours of these bubbles were more extended and lively than those of the air thinn'd between two glasses, and so more easy to be distinguish'd, i shall here give you a farther description of their order, as they were observ'd in viewing them by reflexion of the skies when of a white colour, whilst a black substance was placed behind the bubble. and they were these, red, blue; red, blue; red, blue; red, green; red, yellow, green, blue, purple; red, yellow, green, blue, violet; red, yellow, white, blue, black. the three first successions of red and blue were very dilute and dirty, especially the first, where the red seem'd in a manner to be white. among these there was scarce any other colour sensible besides red and blue, only the blues (and principally the second blue) inclined a little to green. the fourth red was also dilute and dirty, but not so much as the former three; after that succeeded little or no yellow, but a copious green, which at first inclined a little to yellow, and then became a pretty brisk and good willow green, and afterwards changed to a bluish colour; but there succeeded neither blue nor violet. the fifth red at first inclined very much to purple, and afterwards became more bright and brisk, but yet not very pure. this was succeeded with a very bright and intense yellow, which was but little in quantity, and soon chang'd to green: but that green was copious and something more pure, deep and lively, than the former green. after that follow'd an excellent blue of a bright sky-colour, and then a purple, which was less in quantity than the blue, and much inclined to red. the sixth red was at first of a very fair and lively scarlet, and soon after of a brighter colour, being very pure and brisk, and the best of all the reds. then after a lively orange follow'd an intense bright and copious yellow, which was also the best of all the yellows, and this changed first to a greenish yellow, and then to a greenish blue; but the green between the yellow and the blue, was very little and dilute, seeming rather a greenish white than a green. the blue which succeeded became very good, and of a very bright sky-colour, but yet something inferior to the former blue; and the violet was intense and deep with little or no redness in it. and less in quantity than the blue. in the last red appeared a tincture of scarlet next to violet, which soon changed to a brighter colour, inclining to an orange; and the yellow which follow'd was at first pretty good and lively, but afterwards it grew more dilute until by degrees it ended in perfect whiteness. and this whiteness, if the water was very tenacious and well-temper'd, would slowly spread and dilate it self over the greater part of the bubble; continually growing paler at the top, where at length it would crack in many places, and those cracks, as they dilated, would appear of a pretty good, but yet obscure and dark sky-colour; the white between the blue spots diminishing, until it resembled the threds of an irregular net-work, and soon after vanish'd, and left all the upper part of the bubble of the said dark blue colour. and this colour, after the aforesaid manner, dilated it self downwards, until sometimes it hath overspread the whole bubble. in the mean while at the top, which was of a darker blue than the bottom, and appear'd also full of many round blue spots, something darker than the rest, there would emerge one or more very black spots, and within those, other spots of an intenser blackness, which i mention'd in the former observation; and these continually dilated themselves until the bubble broke. if the water was not very tenacious, the black spots would break forth in the white, without any sensible intervention of the blue. and sometimes they would break forth within the precedent yellow, or red, or perhaps within the blue of the second order, before the intermediate colours had time to display themselves. by this description you may perceive how great an affinity these colours have with those of air described in the fourth observation, although set down in a contrary order, by reason that they begin to appear when the bubble is thickest, and are most conveniently reckon'd from the lowest and thickest part of the bubble upwards. _obs._ . viewing in several oblique positions of my eye the rings of colours emerging on the top of the bubble, i found that they were sensibly dilated by increasing the obliquity, but yet not so much by far as those made by thinn'd air in the seventh observation. for there they were dilated so much as, when view'd most obliquely, to arrive at a part of the plate more than twelve times thicker than that where they appear'd when viewed perpendicularly; whereas in this case the thickness of the water, at which they arrived when viewed most obliquely, was to that thickness which exhibited them by perpendicular rays, something less than as to . by the best of my observations it was between and - / to ; an increase about times less than in the other case. sometimes the bubble would become of an uniform thickness all over, except at the top of it near the black spot, as i knew, because it would exhibit the same appearance of colours in all positions of the eye. and then the colours which were seen at its apparent circumference by the obliquest rays, would be different from those that were seen in other places, by rays less oblique to it. and divers spectators might see the same part of it of differing colours, by viewing it at very differing obliquities. now observing how much the colours at the same places of the bubble, or at divers places of equal thickness, were varied by the several obliquities of the rays; by the assistance of the th, th, th and th observations, as they are hereafter explain'd, i collect the thickness of the water requisite to exhibit any one and the same colour, at several obliquities, to be very nearly in the proportion expressed in this table. -----------------+------------------+---------------- incidence on | refraction into | thickness of the water. | the water. | the water. -----------------+------------------+---------------- deg. min. | deg. min. | | | | | | | | | - / | | | | - / | | | | - / | | | | | | | | - / | | | | - / -----------------+------------------+---------------- in the two first columns are express'd the obliquities of the rays to the superficies of the water, that is, their angles of incidence and refraction. where i suppose, that the sines which measure them are in round numbers, as to , though probably the dissolution of soap in the water, may a little alter its refractive virtue. in the third column, the thickness of the bubble, at which any one colour is exhibited in those several obliquities, is express'd in parts, of which ten constitute its thickness when the rays are perpendicular. and the rule found by the seventh observation agrees well with these measures, if duly apply'd; namely, that the thickness of a plate of water requisite to exhibit one and the same colour at several obliquities of the eye, is proportional to the secant of an angle, whose sine is the first of an hundred and six arithmetical mean proportionals between the sines of incidence and refraction counted from the lesser sine, that is, from the sine of refraction when the refraction is made out of air into water, otherwise from the sine of incidence. i have sometimes observ'd, that the colours which arise on polish'd steel by heating it, or on bell-metal, and some other metalline substances, when melted and pour'd on the ground, where they may cool in the open air, have, like the colours of water-bubbles, been a little changed by viewing them at divers obliquities, and particularly that a deep blue, or violet, when view'd very obliquely, hath been changed to a deep red. but the changes of these colours are not so great and sensible as of those made by water. for the scoria, or vitrified part of the metal, which most metals when heated or melted do continually protrude, and send out to their surface, and which by covering the metals in form of a thin glassy skin, causes these colours, is much denser than water; and i find that the change made by the obliquation of the eye is least in colours of the densest thin substances. _obs._ . as in the ninth observation, so here, the bubble, by transmitted light, appear'd of a contrary colour to that, which it exhibited by reflexion. thus when the bubble being look'd on by the light of the clouds reflected from it, seemed red at its apparent circumference, if the clouds at the same time, or immediately after, were view'd through it, the colour at its circumference would be blue. and, on the contrary, when by reflected light it appeared blue, it would appear red by transmitted light. _obs._ . by wetting very thin plates of _muscovy_ glass, whose thinness made the like colours appear, the colours became more faint and languid, especially by wetting the plates on that side opposite to the eye: but i could not perceive any variation of their species. so then the thickness of a plate requisite to produce any colour, depends only on the density of the plate, and not on that of the ambient medium. and hence, by the th and th observations, may be known the thickness which bubbles of water, or plates of _muscovy_ glass, or other substances, have at any colour produced by them. _obs._ . a thin transparent body, which is denser than its ambient medium, exhibits more brisk and vivid colours than that which is so much rarer; as i have particularly observed in the air and glass. for blowing glass very thin at a lamp furnace, those plates encompassed with air did exhibit colours much more vivid than those of air made thin between two glasses. _obs._ . comparing the quantity of light reflected from the several rings, i found that it was most copious from the first or inmost, and in the exterior rings became gradually less and less. also the whiteness of the first ring was stronger than that reflected from those parts of the thin medium or plate which were without the rings; as i could manifestly perceive by viewing at a distance the rings made by the two object-glasses; or by comparing two bubbles of water blown at distant times, in the first of which the whiteness appear'd, which succeeded all the colours, and in the other, the whiteness which preceded them all. _obs._ . when the two object-glasses were lay'd upon one another, so as to make the rings of the colours appear, though with my naked eye i could not discern above eight or nine of those rings, yet by viewing them through a prism i have seen a far greater multitude, insomuch that i could number more than forty, besides many others, that were so very small and close together, that i could not keep my eye steady on them severally so as to number them, but by their extent i have sometimes estimated them to be more than an hundred. and i believe the experiment may be improved to the discovery of far greater numbers. for they seem to be really unlimited, though visible only so far as they can be separated by the refraction of the prism, as i shall hereafter explain. [illustration: fig. .] but it was but one side of these rings, namely, that towards which the refraction was made, which by that refraction was render'd distinct, and the other side became more confused than when view'd by the naked eye, insomuch that there i could not discern above one or two, and sometimes none of those rings, of which i could discern eight or nine with my naked eye. and their segments or arcs, which on the other side appear'd so numerous, for the most part exceeded not the third part of a circle. if the refraction was very great, or the prism very distant from the object-glasses, the middle part of those arcs became also confused, so as to disappear and constitute an even whiteness, whilst on either side their ends, as also the whole arcs farthest from the center, became distincter than before, appearing in the form as you see them design'd in the fifth figure. the arcs, where they seem'd distinctest, were only white and black successively, without any other colours intermix'd. but in other places there appeared colours, whose order was inverted by the refraction in such manner, that if i first held the prism very near the object-glasses, and then gradually removed it farther off towards my eye, the colours of the d, d, th, and following rings, shrunk towards the white that emerged between them, until they wholly vanish'd into it at the middle of the arcs, and afterwards emerged again in a contrary order. but at the ends of the arcs they retain'd their order unchanged. i have sometimes so lay'd one object-glass upon the other, that to the naked eye they have all over seem'd uniformly white, without the least appearance of any of the colour'd rings; and yet by viewing them through a prism, great multitudes of those rings have discover'd themselves. and in like manner plates of _muscovy_ glass, and bubbles of glass blown at a lamp-furnace, which were not so thin as to exhibit any colours to the naked eye, have through the prism exhibited a great variety of them ranged irregularly up and down in the form of waves. and so bubbles of water, before they began to exhibit their colours to the naked eye of a bystander, have appeared through a prism, girded about with many parallel and horizontal rings; to produce which effect, it was necessary to hold the prism parallel, or very nearly parallel to the horizon, and to dispose it so that the rays might be refracted upwards. the second book of opticks _part ii._ _remarks upon the foregoing observations._ having given my observations of these colours, before i make use of them to unfold the causes of the colours of natural bodies, it is convenient that by the simplest of them, such as are the d, d, th, th, th, th, th, and th, i first explain the more compounded. and first to shew how the colours in the fourth and eighteenth observations are produced, let there be taken in any right line from the point y, [in _fig._ .] the lengths ya, yb, yc, yd, ye, yf, yg, yh, in proportion to one another, as the cube-roots of the squares of the numbers, / , / , / , / , / , / , / , , whereby the lengths of a musical chord to sound all the notes in an eighth are represented; that is, in the proportion of the numbers , , , , , , , . and at the points a, b, c, d, e, f, g, h, let perpendiculars a[greek: a], b[greek: b], &c. be erected, by whose intervals the extent of the several colours set underneath against them, is to be represented. then divide the line _a[greek: a]_ in such proportion as the numbers , , , , , , , , , &c. set at the points of division denote. and through those divisions from y draw lines i, k, l, m, n, o, &c. now, if a be supposed to represent the thickness of any thin transparent body, at which the outmost violet is most copiously reflected in the first ring, or series of colours, then by the th observation, hk will represent its thickness, at which the utmost red is most copiously reflected in the same series. also by the th and th observations, a and hn will denote the thicknesses at which those extreme colours are most copiously reflected in the second series, and a and hq the thicknesses at which they are most copiously reflected in the third series, and so on. and the thickness at which any of the intermediate colours are reflected most copiously, will, according to the th observation, be defined by the distance of the line ah from the intermediate parts of the lines k, n, q, &c. against which the names of those colours are written below. [illustration: fig. .] but farther, to define the latitude of these colours in each ring or series, let a design the least thickness, and a the greatest thickness, at which the extreme violet in the first series is reflected, and let hi, and hl, design the like limits for the extreme red, and let the intermediate colours be limited by the intermediate parts of the lines i, and l, against which the names of those colours are written, and so on: but yet with this caution, that the reflexions be supposed strongest at the intermediate spaces, k, n, q, &c. and from thence to decrease gradually towards these limits, i, l, m, o, &c. on either side; where you must not conceive them to be precisely limited, but to decay indefinitely. and whereas i have assign'd the same latitude to every series, i did it, because although the colours in the first series seem to be a little broader than the rest, by reason of a stronger reflexion there, yet that inequality is so insensible as scarcely to be determin'd by observation. now according to this description, conceiving that the rays originally of several colours are by turns reflected at the spaces i, l , m, o , pr , &c. and transmitted at the spaces ahi , lm , op , &c. it is easy to know what colour must in the open air be exhibited at any thickness of a transparent thin body. for if a ruler be applied parallel to ah, at that distance from it by which the thickness of the body is represented, the alternate spaces il , mo , &c. which it crosseth will denote the reflected original colours, of which the colour exhibited in the open air is compounded. thus if the constitution of the green in the third series of colours be desired, apply the ruler as you see at [greek: prsph], and by its passing through some of the blue at [greek: p] and yellow at [greek: s], as well as through the green at [greek: r], you may conclude that the green exhibited at that thickness of the body is principally constituted of original green, but not without a mixture of some blue and yellow. by this means you may know how the colours from the center of the rings outward ought to succeed in order as they were described in the th and th observations. for if you move the ruler gradually from ah through all distances, having pass'd over the first space which denotes little or no reflexion to be made by thinnest substances, it will first arrive at the violet, and then very quickly at the blue and green, which together with that violet compound blue, and then at the yellow and red, by whose farther addition that blue is converted into whiteness, which whiteness continues during the transit of the edge of the ruler from i to , and after that by the successive deficience of its component colours, turns first to compound yellow, and then to red, and last of all the red ceaseth at l. then begin the colours of the second series, which succeed in order during the transit of the edge of the ruler from to o, and are more lively than before, because more expanded and severed. and for the same reason instead of the former white there intercedes between the blue and yellow a mixture of orange, yellow, green, blue and indigo, all which together ought to exhibit a dilute and imperfect green. so the colours of the third series all succeed in order; first, the violet, which a little interferes with the red of the second order, and is thereby inclined to a reddish purple; then the blue and green, which are less mix'd with other colours, and consequently more lively than before, especially the green: then follows the yellow, some of which towards the green is distinct and good, but that part of it towards the succeeding red, as also that red is mix'd with the violet and blue of the fourth series, whereby various degrees of red very much inclining to purple are compounded. this violet and blue, which should succeed this red, being mixed with, and hidden in it, there succeeds a green. and this at first is much inclined to blue, but soon becomes a good green, the only unmix'd and lively colour in this fourth series. for as it verges towards the yellow, it begins to interfere with the colours of the fifth series, by whose mixture the succeeding yellow and red are very much diluted and made dirty, especially the yellow, which being the weaker colour is scarce able to shew it self. after this the several series interfere more and more, and their colours become more and more intermix'd, till after three or four more revolutions (in which the red and blue predominate by turns) all sorts of colours are in all places pretty equally blended, and compound an even whiteness. and since by the th observation the rays endued with one colour are transmitted, where those of another colour are reflected, the reason of the colours made by the transmitted light in the th and th observations is from hence evident. if not only the order and species of these colours, but also the precise thickness of the plate, or thin body at which they are exhibited, be desired in parts of an inch, that may be also obtained by assistance of the th or th observations. for according to those observations the thickness of the thinned air, which between two glasses exhibited the most luminous parts of the first six rings were / , / , / , / , / , / parts of an inch. suppose the light reflected most copiously at these thicknesses be the bright citrine yellow, or confine of yellow and orange, and these thicknesses will be f[greek: l], f[greek: m], f[greek: u], f[greek: x], f[greek: o], f[greek: t]. and this being known, it is easy to determine what thickness of air is represented by g[greek: ph], or by any other distance of the ruler from ah. but farther, since by the th observation the thickness of air was to the thickness of water, which between the same glasses exhibited the same colour, as to , and by the st observation the colours of thin bodies are not varied by varying the ambient medium; the thickness of a bubble of water, exhibiting any colour, will be / of the thickness of air producing the same colour. and so according to the same th and st observations, the thickness of a plate of glass, whose refraction of the mean refrangible ray, is measured by the proportion of the sines to , may be / of the thickness of air producing the same colours; and the like of other mediums. i do not affirm, that this proportion of to , holds in all the rays; for the sines of other sorts of rays have other proportions. but the differences of those proportions are so little that i do not here consider them. on these grounds i have composed the following table, wherein the thickness of air, water, and glass, at which each colour is most intense and specifick, is expressed in parts of an inch divided into ten hundred thousand equal parts. now if this table be compared with the th scheme, you will there see the constitution of each colour, as to its ingredients, or the original colours of which it is compounded, and thence be enabled to judge of its intenseness or imperfection; which may suffice in explication of the th and th observations, unless it be farther desired to delineate the manner how the colours appear, when the two object-glasses are laid upon one another. to do which, let there be described a large arc of a circle, and a streight line which may touch that arc, and parallel to that tangent several occult lines, at such distances from it, as the numbers set against the several colours in the table denote. for the arc, and its tangent, will represent the superficies of the glasses terminating the interjacent air; and the places where the occult lines cut the arc will show at what distances from the center, or point of contact, each colour is reflected. _the thickness of colour'd plates and particles of_ _____________|_______________ / \ air. water. glass. |---------+----------+----------+ {very black | / | / | / | {black | | / | / | {beginning of | | | | { black | | - / | - / | their colours of the {blue | - / | - / | - / | first order, {white | - / | - / | - / | {yellow | - / | - / | - / | {orange | | | - / | {red | | - / | - / | |---------+----------+----------| {violet | - / | - / | - / | {indigo | - / | - / | - / | {blue | | - / | | {green | - / | - / | - / | of the second order, {yellow | - / | - / | - / | {orange | - / | | - / | {bright red | - / | - / | - / | {scarlet | - / | - / | - / | |---------+----------+----------| {purple | | - / | - / | {indigo | - / | - / | - / | {blue | - / | - / | - / | of the third order, {green | - / | - / | - / | {yellow | - / | - / | - / | {red | | - / | - / | {bluish red | | | - / | |---------+----------+----------| {bluish green | | - / | | {green | - / | - / | - / | of the fourth order, {yellowish green | | | - / | {red | - / | - / | | |---------+----------+----------| {greenish blue | | - / | - / | of the fifth order, {red | - / | - / | | |---------+----------+----------| {greenish blue | - / | | | of the sixth order, {red | | - / | | |---------+----------+----------| of the seventh order, {greenish blue | | - / | - / | {ruddy white | | - / | - / | |---------+----------+----------| there are also other uses of this table: for by its assistance the thickness of the bubble in the th observation was determin'd by the colours which it exhibited. and so the bigness of the parts of natural bodies may be conjectured by their colours, as shall be hereafter shewn. also, if two or more very thin plates be laid one upon another, so as to compose one plate equalling them all in thickness, the resulting colour may be hereby determin'd. for instance, mr. _hook_ observed, as is mentioned in his _micrographia_, that a faint yellow plate of _muscovy_ glass laid upon a blue one, constituted a very deep purple. the yellow of the first order is a faint one, and the thickness of the plate exhibiting it, according to the table is - / , to which add , the thickness exhibiting blue of the second order, and the sum will be - / , which is the thickness exhibiting the purple of the third order. to explain, in the next place, the circumstances of the d and d observations; that is, how the rings of the colours may (by turning the prisms about their common axis the contrary way to that expressed in those observations) be converted into white and black rings, and afterwards into rings of colours again, the colours of each ring lying now in an inverted order; it must be remember'd, that those rings of colours are dilated by the obliquation of the rays to the air which intercedes the glasses, and that according to the table in the th observation, their dilatation or increase of their diameter is most manifest and speedy when they are obliquest. now the rays of yellow being more refracted by the first superficies of the said air than those of red, are thereby made more oblique to the second superficies, at which they are reflected to produce the colour'd rings, and consequently the yellow circle in each ring will be more dilated than the red; and the excess of its dilatation will be so much the greater, by how much the greater is the obliquity of the rays, until at last it become of equal extent with the red of the same ring. and for the same reason the green, blue and violet, will be also so much dilated by the still greater obliquity of their rays, as to become all very nearly of equal extent with the red, that is, equally distant from the center of the rings. and then all the colours of the same ring must be co-incident, and by their mixture exhibit a white ring. and these white rings must have black and dark rings between them, because they do not spread and interfere with one another, as before. and for that reason also they must become distincter, and visible to far greater numbers. but yet the violet being obliquest will be something more dilated, in proportion to its extent, than the other colours, and so very apt to appear at the exterior verges of the white. afterwards, by a greater obliquity of the rays, the violet and blue become more sensibly dilated than the red and yellow, and so being farther removed from the center of the rings, the colours must emerge out of the white in an order contrary to that which they had before; the violet and blue at the exterior limbs of each ring, and the red and yellow at the interior. and the violet, by reason of the greatest obliquity of its rays, being in proportion most of all expanded, will soonest appear at the exterior limb of each white ring, and become more conspicuous than the rest. and the several series of colours belonging to the several rings, will, by their unfolding and spreading, begin again to interfere, and thereby render the rings less distinct, and not visible to so great numbers. if instead of the prisms the object-glasses be made use of, the rings which they exhibit become not white and distinct by the obliquity of the eye, by reason that the rays in their passage through that air which intercedes the glasses are very nearly parallel to those lines in which they were first incident on the glasses, and consequently the rays endued with several colours are not inclined one more than another to that air, as it happens in the prisms. there is yet another circumstance of these experiments to be consider'd, and that is why the black and white rings which when view'd at a distance appear distinct, should not only become confused by viewing them near at hand, but also yield a violet colour at both the edges of every white ring. and the reason is, that the rays which enter the eye at several parts of the pupil, have several obliquities to the glasses, and those which are most oblique, if consider'd apart, would represent the rings bigger than those which are the least oblique. whence the breadth of the perimeter of every white ring is expanded outwards by the obliquest rays, and inwards by the least oblique. and this expansion is so much the greater by how much the greater is the difference of the obliquity; that is, by how much the pupil is wider, or the eye nearer to the glasses. and the breadth of the violet must be most expanded, because the rays apt to excite a sensation of that colour are most oblique to a second or farther superficies of the thinn'd air at which they are reflected, and have also the greatest variation of obliquity, which makes that colour soonest emerge out of the edges of the white. and as the breadth of every ring is thus augmented, the dark intervals must be diminish'd, until the neighbouring rings become continuous, and are blended, the exterior first, and then those nearer the center; so that they can no longer be distinguish'd apart, but seem to constitute an even and uniform whiteness. among all the observations there is none accompanied with so odd circumstances as the twenty-fourth. of those the principal are, that in thin plates, which to the naked eye seem of an even and uniform transparent whiteness, without any terminations of shadows, the refraction of a prism should make rings of colours appear, whereas it usually makes objects appear colour'd only there where they are terminated with shadows, or have parts unequally luminous; and that it should make those rings exceedingly distinct and white, although it usually renders objects confused and coloured. the cause of these things you will understand by considering, that all the rings of colours are really in the plate, when view'd with the naked eye, although by reason of the great breadth of their circumferences they so much interfere and are blended together, that they seem to constitute an uniform whiteness. but when the rays pass through the prism to the eye, the orbits of the several colours in every ring are refracted, some more than others, according to their degrees of refrangibility: by which means the colours on one side of the ring (that is in the circumference on one side of its center), become more unfolded and dilated, and those on the other side more complicated and contracted. and where by a due refraction they are so much contracted, that the several rings become narrower than to interfere with one another, they must appear distinct, and also white, if the constituent colours be so much contracted as to be wholly co-incident. but on the other side, where the orbit of every ring is made broader by the farther unfolding of its colours, it must interfere more with other rings than before, and so become less distinct. [illustration: fig. .] to explain this a little farther, suppose the concentrick circles av, and bx, [in _fig._ .] represent the red and violet of any order, which, together with the intermediate colours, constitute any one of these rings. now these being view'd through a prism, the violet circle bx, will, by a greater refraction, be farther translated from its place than the red av, and so approach nearer to it on that side of the circles, towards which the refractions are made. for instance, if the red be translated to _av_, the violet may be translated to _bx_, so as to approach nearer to it at _x_ than before; and if the red be farther translated to av, the violet may be so much farther translated to bx as to convene with it at x; and if the red be yet farther translated to [greek: ay], the violet may be still so much farther translated to [greek: bx] as to pass beyond it at [greek: x], and convene with it at _e_ and _f_. and this being understood not only of the red and violet, but of all the other intermediate colours, and also of every revolution of those colours, you will easily perceive how those of the same revolution or order, by their nearness at _xv_ and [greek: yx], and their coincidence at xv, _e_ and _f_, ought to constitute pretty distinct arcs of circles, especially at xv, or at _e_ and _f_; and that they will appear severally at _x_[greek: u] and at xv exhibit whiteness by their coincidence, and again appear severally at [greek: yx], but yet in a contrary order to that which they had before, and still retain beyond _e_ and _f_. but on the other side, at _ab_, ab, or [greek: ab], these colours must become much more confused by being dilated and spread so as to interfere with those of other orders. and the same confusion will happen at [greek: ux] between _e_ and _f_, if the refraction be very great, or the prism very distant from the object-glasses: in which case no parts of the rings will be seen, save only two little arcs at _e_ and _f_, whose distance from one another will be augmented by removing the prism still farther from the object-glasses: and these little arcs must be distinctest and whitest at their middle, and at their ends, where they begin to grow confused, they must be colour'd. and the colours at one end of every arc must be in a contrary order to those at the other end, by reason that they cross in the intermediate white; namely, their ends, which verge towards [greek: ux], will be red and yellow on that side next the center, and blue and violet on the other side. but their other ends which verge from [greek: ux], will on the contrary be blue and violet on that side towards the center, and on the other side red and yellow. now as all these things follow from the properties of light by a mathematical way of reasoning, so the truth of them may be manifested by experiments. for in a dark room, by viewing these rings through a prism, by reflexion of the several prismatick colours, which an assistant causes to move to and fro upon a wall or paper from whence they are reflected, whilst the spectator's eye, the prism, and the object-glasses, (as in the th observation,) are placed steady; the position of the circles made successively by the several colours, will be found such, in respect of one another, as i have described in the figures _abxv_, or abxv, or _[greek: abxu]_. and by the same method the truth of the explications of other observations may be examined. by what hath been said, the like phænomena of water and thin plates of glass may be understood. but in small fragments of those plates there is this farther observable, that where they lie flat upon a table, and are turned about their centers whilst they are view'd through a prism, they will in some postures exhibit waves of various colours; and some of them exhibit these waves in one or two positions only, but the most of them do in all positions exhibit them, and make them for the most part appear almost all over the plates. the reason is, that the superficies of such plates are not even, but have many cavities and swellings, which, how shallow soever, do a little vary the thickness of the plate. for at the several sides of those cavities, for the reasons newly described, there ought to be produced waves in several postures of the prism. now though it be but some very small and narrower parts of the glass, by which these waves for the most part are caused, yet they may seem to extend themselves over the whole glass, because from the narrowest of those parts there are colours of several orders, that is, of several rings, confusedly reflected, which by refraction of the prism are unfolded, separated, and, according to their degrees of refraction, dispersed to several places, so as to constitute so many several waves, as there were divers orders of colours promiscuously reflected from that part of the glass. these are the principal phænomena of thin plates or bubbles, whose explications depend on the properties of light, which i have heretofore deliver'd. and these you see do necessarily follow from them, and agree with them, even to their very least circumstances; and not only so, but do very much tend to their proof. thus, by the th observation it appears, that the rays of several colours, made as well by thin plates or bubbles, as by refractions of a prism, have several degrees of refrangibility; whereby those of each order, which at the reflexion from the plate or bubble are intermix'd with those of other orders, are separated from them by refraction, and associated together so as to become visible by themselves like arcs of circles. for if the rays were all alike refrangible, 'tis impossible that the whiteness, which to the naked sense appears uniform, should by refraction have its parts transposed and ranged into those black and white arcs. it appears also that the unequal refractions of difform rays proceed not from any contingent irregularities; such as are veins, an uneven polish, or fortuitous position of the pores of glass; unequal and casual motions in the air or Æther, the spreading, breaking, or dividing the same ray into many diverging parts; or the like. for, admitting any such irregularities, it would be impossible for refractions to render those rings so very distinct, and well defined, as they do in the th observation. it is necessary therefore that every ray have its proper and constant degree of refrangibility connate with it, according to which its refraction is ever justly and regularly perform'd; and that several rays have several of those degrees. and what is said of their refrangibility may be also understood of their reflexibility, that is, of their dispositions to be reflected, some at a greater, and others at a less thickness of thin plates or bubbles; namely, that those dispositions are also connate with the rays, and immutable; as may appear by the th, th, and th observations, compared with the fourth and eighteenth. by the precedent observations it appears also, that whiteness is a dissimilar mixture of all colours, and that light is a mixture of rays endued with all those colours. for, considering the multitude of the rings of colours in the d, th, and th observations, it is manifest, that although in the th and th observations there appear no more than eight or nine of those rings, yet there are really a far greater number, which so much interfere and mingle with one another, as after those eight or nine revolutions to dilute one another wholly, and constitute an even and sensibly uniform whiteness. and consequently that whiteness must be allow'd a mixture of all colours, and the light which conveys it to the eye must be a mixture of rays endued with all those colours. but farther; by the th observation it appears, that there is a constant relation between colours and refrangibility; the most refrangible rays being violet, the least refrangible red, and those of intermediate colours having proportionably intermediate degrees of refrangibility. and by the th, th, and th observations, compared with the th or th there appears to be the same constant relation between colour and reflexibility; the violet being in like circumstances reflected at least thicknesses of any thin plate or bubble, the red at greatest thicknesses, and the intermediate colours at intermediate thicknesses. whence it follows, that the colorifick dispositions of rays are also connate with them, and immutable; and by consequence, that all the productions and appearances of colours in the world are derived, not from any physical change caused in light by refraction or reflexion, but only from the various mixtures or separations of rays, by virtue of their different refrangibility or reflexibility. and in this respect the science of colours becomes a speculation as truly mathematical as any other part of opticks. i mean, so far as they depend on the nature of light, and are not produced or alter'd by the power of imagination, or by striking or pressing the eye. the second book of opticks _part iii._ _of the permanent colours of natural bodies, and the analogy between them and the colours of thin transparent plates._ i am now come to another part of this design, which is to consider how the phænomena of thin transparent plates stand related to those of all other natural bodies. of these bodies i have already told you that they appear of divers colours, accordingly as they are disposed to reflect most copiously the rays originally endued with those colours. but their constitutions, whereby they reflect some rays more copiously than others, remain to be discover'd; and these i shall endeavour to manifest in the following propositions. prop. i. _those superficies of transparent bodies reflect the greatest quantity of light, which have the greatest refracting power; that is, which intercede mediums that differ most in their refractive densities. and in the confines of equally refracting mediums there is no reflexion._ the analogy between reflexion and refraction will appear by considering, that when light passeth obliquely out of one medium into another which refracts from the perpendicular, the greater is the difference of their refractive density, the less obliquity of incidence is requisite to cause a total reflexion. for as the sines are which measure the refraction, so is the sine of incidence at which the total reflexion begins, to the radius of the circle; and consequently that angle of incidence is least where there is the greatest difference of the sines. thus in the passing of light out of water into air, where the refraction is measured by the ratio of the sines to , the total reflexion begins when the angle of incidence is about degrees minutes. in passing out of glass into air, where the refraction is measured by the ratio of the sines to , the total reflexion begins when the angle of incidence is degrees minutes; and so in passing out of crystal, or more strongly refracting mediums into air, there is still a less obliquity requisite to cause a total reflexion. superficies therefore which refract most do soonest reflect all the light which is incident on them, and so must be allowed most strongly reflexive. but the truth of this proposition will farther appear by observing, that in the superficies interceding two transparent mediums, (such as are air, water, oil, common glass, crystal, metalline glasses, island glasses, white transparent arsenick, diamonds, &c.) the reflexion is stronger or weaker accordingly, as the superficies hath a greater or less refracting power. for in the confine of air and sal-gem 'tis stronger than in the confine of air and water, and still stronger in the confine of air and common glass or crystal, and stronger in the confine of air and a diamond. if any of these, and such like transparent solids, be immerged in water, its reflexion becomes, much weaker than before; and still weaker if they be immerged in the more strongly refracting liquors of well rectified oil of vitriol or spirit of turpentine. if water be distinguish'd into two parts by any imaginary surface, the reflexion in the confine of those two parts is none at all. in the confine of water and ice 'tis very little; in that of water and oil 'tis something greater; in that of water and sal-gem still greater; and in that of water and glass, or crystal or other denser substances still greater, accordingly as those mediums differ more or less in their refracting powers. hence in the confine of common glass and crystal, there ought to be a weak reflexion, and a stronger reflexion in the confine of common and metalline glass; though i have not yet tried this. but in the confine of two glasses of equal density, there is not any sensible reflexion; as was shewn in the first observation. and the same may be understood of the superficies interceding two crystals, or two liquors, or any other substances in which no refraction is caused. so then the reason why uniform pellucid mediums (such as water, glass, or crystal,) have no sensible reflexion but in their external superficies, where they are adjacent to other mediums of a different density, is because all their contiguous parts have one and the same degree of density. prop. ii. _the least parts of almost all natural bodies are in some measure transparent: and the opacity of those bodies ariseth from the multitude of reflexions caused in their internal parts._ that this is so has been observed by others, and will easily be granted by them that have been conversant with microscopes. and it may be also tried by applying any substance to a hole through which some light is immitted into a dark room. for how opake soever that substance may seem in the open air, it will by that means appear very manifestly transparent, if it be of a sufficient thinness. only white metalline bodies must be excepted, which by reason of their excessive density seem to reflect almost all the light incident on their first superficies; unless by solution in menstruums they be reduced into very small particles, and then they become transparent. prop. iii. _between the parts of opake and colour'd bodies are many spaces, either empty, or replenish'd with mediums of other densities; as water between the tinging corpuscles wherewith any liquor is impregnated, air between the aqueous globules that constitute clouds or mists; and for the most part spaces void of both air and water, but yet perhaps not wholly void of all substance, between the parts of hard bodies._ the truth of this is evinced by the two precedent propositions: for by the second proposition there are many reflexions made by the internal parts of bodies, which, by the first proposition, would not happen if the parts of those bodies were continued without any such interstices between them; because reflexions are caused only in superficies, which intercede mediums of a differing density, by _prop._ . but farther, that this discontinuity of parts is the principal cause of the opacity of bodies, will appear by considering, that opake substances become transparent by filling their pores with any substance of equal or almost equal density with their parts. thus paper dipped in water or oil, the _oculus mundi_ stone steep'd in water, linnen cloth oiled or varnish'd, and many other substances soaked in such liquors as will intimately pervade their little pores, become by that means more transparent than otherwise; so, on the contrary, the most transparent substances, may, by evacuating their pores, or separating their parts, be render'd sufficiently opake; as salts or wet paper, or the _oculus mundi_ stone by being dried, horn by being scraped, glass by being reduced to powder, or otherwise flawed; turpentine by being stirred about with water till they mix imperfectly, and water by being form'd into many small bubbles, either alone in the form of froth, or by shaking it together with oil of turpentine, or oil olive, or with some other convenient liquor, with which it will not perfectly incorporate. and to the increase of the opacity of these bodies, it conduces something, that by the d observation the reflexions of very thin transparent substances are considerably stronger than those made by the same substances of a greater thickness. prop. iv. _the parts of bodies and their interstices must not be less than of some definite bigness, to render them opake and colour'd._ for the opakest bodies, if their parts be subtilly divided, (as metals, by being dissolved in acid menstruums, &c.) become perfectly transparent. and you may also remember, that in the eighth observation there was no sensible reflexion at the superficies of the object-glasses, where they were very near one another, though they did not absolutely touch. and in the th observation the reflexion of the water-bubble where it became thinnest was almost insensible, so as to cause very black spots to appear on the top of the bubble, by the want of reflected light. on these grounds i perceive it is that water, salt, glass, stones, and such like substances, are transparent. for, upon divers considerations, they seem to be as full of pores or interstices between their parts as other bodies are, but yet their parts and interstices to be too small to cause reflexions in their common surfaces. prop. v. _the transparent parts of bodies, according to their several sizes, reflect rays of one colour, and transmit those of another, on the same grounds that thin plates or bubbles do reflect or transmit those rays. and this i take to be the ground of all their colours._ for if a thinn'd or plated body, which being of an even thickness, appears all over of one uniform colour, should be slit into threads, or broken into fragments, of the same thickness with the plate; i see no reason why every thread or fragment should not keep its colour, and by consequence why a heap of those threads or fragments should not constitute a mass or powder of the same colour, which the plate exhibited before it was broken. and the parts of all natural bodies being like so many fragments of a plate, must on the same grounds exhibit the same colours. now, that they do so will appear by the affinity of their properties. the finely colour'd feathers of some birds, and particularly those of peacocks tails, do, in the very same part of the feather, appear of several colours in several positions of the eye, after the very same manner that thin plates were found to do in the th and th observations, and therefore their colours arise from the thinness of the transparent parts of the feathers; that is, from the slenderness of the very fine hairs, or _capillamenta_, which grow out of the sides of the grosser lateral branches or fibres of those feathers. and to the same purpose it is, that the webs of some spiders, by being spun very fine, have appeared colour'd, as some have observ'd, and that the colour'd fibres of some silks, by varying the position of the eye, do vary their colour. also the colours of silks, cloths, and other substances, which water or oil can intimately penetrate, become more faint and obscure by being immerged in those liquors, and recover their vigor again by being dried; much after the manner declared of thin bodies in the th and st observations. leaf-gold, some sorts of painted glass, the infusion of _lignum nephriticum_, and some other substances, reflect one colour, and transmit another; like thin bodies in the th and th observations. and some of those colour'd powders which painters use, may have their colours a little changed, by being very elaborately and finely ground. where i see not what can be justly pretended for those changes, besides the breaking of their parts into less parts by that contrition, after the same manner that the colour of a thin plate is changed by varying its thickness. for which reason also it is that the colour'd flowers of plants and vegetables, by being bruised, usually become more transparent than before, or at least in some degree or other change their colours. nor is it much less to my purpose, that, by mixing divers liquors, very odd and remarkable productions and changes of colours may be effected, of which no cause can be more obvious and rational than that the saline corpuscles of one liquor do variously act upon or unite with the tinging corpuscles of another, so as to make them swell, or shrink, (whereby not only their bulk but their density also may be changed,) or to divide them into smaller corpuscles, (whereby a colour'd liquor may become transparent,) or to make many of them associate into one cluster, whereby two transparent liquors may compose a colour'd one. for we see how apt those saline menstruums are to penetrate and dissolve substances to which they are applied, and some of them to precipitate what others dissolve. in like manner, if we consider the various phænomena of the atmosphere, we may observe, that when vapours are first raised, they hinder not the transparency of the air, being divided into parts too small to cause any reflexion in their superficies. but when in order to compose drops of rain they begin to coalesce and constitute globules of all intermediate sizes, those globules, when they become of convenient size to reflect some colours and transmit others, may constitute clouds of various colours according to their sizes. and i see not what can be rationally conceived in so transparent a substance as water for the production of these colours, besides the various sizes of its fluid and globular parcels. prop. vi. _the parts of bodies on which their colours depend, are denser than the medium which pervades their interstices._ this will appear by considering, that the colour of a body depends not only on the rays which are incident perpendicularly on its parts, but on those also which are incident at all other angles. and that according to the th observation, a very little variation of obliquity will change the reflected colour, where the thin body or small particles is rarer than the ambient medium, insomuch that such a small particle will at diversly oblique incidences reflect all sorts of colours, in so great a variety that the colour resulting from them all, confusedly reflected from a heap of such particles, must rather be a white or grey than any other colour, or at best it must be but a very imperfect and dirty colour. whereas if the thin body or small particle be much denser than the ambient medium, the colours, according to the th observation, are so little changed by the variation of obliquity, that the rays which are reflected least obliquely may predominate over the rest, so much as to cause a heap of such particles to appear very intensely of their colour. it conduces also something to the confirmation of this proposition, that, according to the d observation, the colours exhibited by the denser thin body within the rarer, are more brisk than those exhibited by the rarer within the denser. prop. vii. _the bigness of the component parts of natural bodies may be conjectured by their colours._ for since the parts of these bodies, by _prop._ . do most probably exhibit the same colours with a plate of equal thickness, provided they have the same refractive density; and since their parts seem for the most part to have much the same density with water or glass, as by many circumstances is obvious to collect; to determine the sizes of those parts, you need only have recourse to the precedent tables, in which the thickness of water or glass exhibiting any colour is expressed. thus if it be desired to know the diameter of a corpuscle, which being of equal density with glass shall reflect green of the third order; the number - / shews it to be ( - / )/ parts of an inch. the greatest difficulty is here to know of what order the colour of any body is. and for this end we must have recourse to the th and th observations; from whence may be collected these particulars. _scarlets_, and other _reds_, _oranges_, and _yellows_, if they be pure and intense, are most probably of the second order. those of the first and third order also may be pretty good; only the yellow of the first order is faint, and the orange and red of the third order have a great mixture of violet and blue. there may be good _greens_ of the fourth order, but the purest are of the third. and of this order the green of all vegetables seems to be, partly by reason of the intenseness of their colours, and partly because when they wither some of them turn to a greenish yellow, and others to a more perfect yellow or orange, or perhaps to red, passing first through all the aforesaid intermediate colours. which changes seem to be effected by the exhaling of the moisture which may leave the tinging corpuscles more dense, and something augmented by the accretion of the oily and earthy part of that moisture. now the green, without doubt, is of the same order with those colours into which it changeth, because the changes are gradual, and those colours, though usually not very full, yet are often too full and lively to be of the fourth order. _blues_ and _purples_ may be either of the second or third order, but the best are of the third. thus the colour of violets seems to be of that order, because their syrup by acid liquors turns red, and by urinous and alcalizate turns green. for since it is of the nature of acids to dissolve or attenuate, and of alcalies to precipitate or incrassate, if the purple colour of the syrup was of the second order, an acid liquor by attenuating its tinging corpuscles would change it to a red of the first order, and an alcali by incrassating them would change it to a green of the second order; which red and green, especially the green, seem too imperfect to be the colours produced by these changes. but if the said purple be supposed of the third order, its change to red of the second, and green of the third, may without any inconvenience be allow'd. if there be found any body of a deeper and less reddish purple than that of the violets, its colour most probably is of the second order. but yet there being no body commonly known whose colour is constantly more deep than theirs, i have made use of their name to denote the deepest and least reddish purples, such as manifestly transcend their colour in purity. the _blue_ of the first order, though very faint and little, may possibly be the colour of some substances; and particularly the azure colour of the skies seems to be of this order. for all vapours when they begin to condense and coalesce into small parcels, become first of that bigness, whereby such an azure must be reflected before they can constitute clouds of other colours. and so this being the first colour which vapours begin to reflect, it ought to be the colour of the finest and most transparent skies, in which vapours are not arrived to that grossness requisite to reflect other colours, as we find it is by experience. _whiteness_, if most intense and luminous, is that of the first order, if less strong and luminous, a mixture of the colours of several orders. of this last kind is the whiteness of froth, paper, linnen, and most white substances; of the former i reckon that of white metals to be. for whilst the densest of metals, gold, if foliated, is transparent, and all metals become transparent if dissolved in menstruums or vitrified, the opacity of white metals ariseth not from their density alone. they being less dense than gold would be more transparent than it, did not some other cause concur with their density to make them opake. and this cause i take to be such a bigness of their particles as fits them to reflect the white of the first order. for, if they be of other thicknesses they may reflect other colours, as is manifest by the colours which appear upon hot steel in tempering it, and sometimes upon the surface of melted metals in the skin or scoria which arises upon them in their cooling. and as the white of the first order is the strongest which can be made by plates of transparent substances, so it ought to be stronger in the denser substances of metals than in the rarer of air, water, and glass. nor do i see but that metallick substances of such a thickness as may fit them to reflect the white of the first order, may, by reason of their great density (according to the tenor of the first of these propositions) reflect all the light incident upon them, and so be as opake and splendent as it's possible for any body to be. gold, or copper mix'd with less than half their weight of silver, or tin, or regulus of antimony, in fusion, or amalgamed with a very little mercury, become white; which shews both that the particles of white metals have much more superficies, and so are smaller, than those of gold and copper, and also that they are so opake as not to suffer the particles of gold or copper to shine through them. now it is scarce to be doubted but that the colours of gold and copper are of the second and third order, and therefore the particles of white metals cannot be much bigger than is requisite to make them reflect the white of the first order. the volatility of mercury argues that they are not much bigger, nor may they be much less, lest they lose their opacity, and become either transparent as they do when attenuated by vitrification, or by solution in menstruums, or black as they do when ground smaller, by rubbing silver, or tin, or lead, upon other substances to draw black lines. the first and only colour which white metals take by grinding their particles smaller, is black, and therefore their white ought to be that which borders upon the black spot in the center of the rings of colours, that is, the white of the first order. but, if you would hence gather the bigness of metallick particles, you must allow for their density. for were mercury transparent, its density is such that the sine of incidence upon it (by my computation) would be to the sine of its refraction, as to , or to . and therefore the thickness of its particles, that they may exhibit the same colours with those of bubbles of water, ought to be less than the thickness of the skin of those bubbles in the proportion of to . whence it's possible, that the particles of mercury may be as little as the particles of some transparent and volatile fluids, and yet reflect the white of the first order. lastly, for the production of _black_, the corpuscles must be less than any of those which exhibit colours. for at all greater sizes there is too much light reflected to constitute this colour. but if they be supposed a little less than is requisite to reflect the white and very faint blue of the first order, they will, according to the th, th, th and th observations, reflect so very little light as to appear intensely black, and yet may perhaps variously refract it to and fro within themselves so long, until it happen to be stifled and lost, by which means they will appear black in all positions of the eye without any transparency. and from hence may be understood why fire, and the more subtile dissolver putrefaction, by dividing the particles of substances, turn them to black, why small quantities of black substances impart their colour very freely and intensely to other substances to which they are applied; the minute particles of these, by reason of their very great number, easily overspreading the gross particles of others; why glass ground very elaborately with sand on a copper plate, 'till it be well polish'd, makes the sand, together with what is worn off from the glass and copper, become very black: why black substances do soonest of all others become hot in the sun's light and burn, (which effect may proceed partly from the multitude of refractions in a little room, and partly from the easy commotion of so very small corpuscles;) and why blacks are usually a little inclined to a bluish colour. for that they are so may be seen by illuminating white paper by light reflected from black substances. for the paper will usually appear of a bluish white; and the reason is, that black borders in the obscure blue of the order described in the th observation, and therefore reflects more rays of that colour than of any other. in these descriptions i have been the more particular, because it is not impossible but that microscopes may at length be improved to the discovery of the particles of bodies on which their colours depend, if they are not already in some measure arrived to that degree of perfection. for if those instruments are or can be so far improved as with sufficient distinctness to represent objects five or six hundred times bigger than at a foot distance they appear to our naked eyes, i should hope that we might be able to discover some of the greatest of those corpuscles. and by one that would magnify three or four thousand times perhaps they might all be discover'd, but those which produce blackness. in the mean while i see nothing material in this discourse that may rationally be doubted of, excepting this position: that transparent corpuscles of the same thickness and density with a plate, do exhibit the same colour. and this i would have understood not without some latitude, as well because those corpuscles may be of irregular figures, and many rays must be obliquely incident on them, and so have a shorter way through them than the length of their diameters, as because the straitness of the medium put in on all sides within such corpuscles may a little alter its motions or other qualities on which the reflexion depends. but yet i cannot much suspect the last, because i have observed of some small plates of muscovy glass which were of an even thickness, that through a microscope they have appeared of the same colour at their edges and corners where the included medium was terminated, which they appeared of in other places. however it will add much to our satisfaction, if those corpuscles can be discover'd with microscopes; which if we shall at length attain to, i fear it will be the utmost improvement of this sense. for it seems impossible to see the more secret and noble works of nature within the corpuscles by reason of their transparency. prop. viii. _the cause of reflexion is not the impinging of light on the solid or impervious parts of bodies, as is commonly believed._ this will appear by the following considerations. first, that in the passage of light out of glass into air there is a reflexion as strong as in its passage out of air into glass, or rather a little stronger, and by many degrees stronger than in its passage out of glass into water. and it seems not probable that air should have more strongly reflecting parts than water or glass. but if that should possibly be supposed, yet it will avail nothing; for the reflexion is as strong or stronger when the air is drawn away from the glass, (suppose by the air-pump invented by _otto gueriet_, and improved and made useful by mr. _boyle_) as when it is adjacent to it. secondly, if light in its passage out of glass into air be incident more obliquely than at an angle of or degrees it is wholly reflected, if less obliquely it is in great measure transmitted. now it is not to be imagined that light at one degree of obliquity should meet with pores enough in the air to transmit the greater part of it, and at another degree of obliquity should meet with nothing but parts to reflect it wholly, especially considering that in its passage out of air into glass, how oblique soever be its incidence, it finds pores enough in the glass to transmit a great part of it. if any man suppose that it is not reflected by the air, but by the outmost superficial parts of the glass, there is still the same difficulty: besides, that such a supposition is unintelligible, and will also appear to be false by applying water behind some part of the glass instead of air. for so in a convenient obliquity of the rays, suppose of or degrees, at which they are all reflected where the air is adjacent to the glass, they shall be in great measure transmitted where the water is adjacent to it; which argues, that their reflexion or transmission depends on the constitution of the air and water behind the glass, and not on the striking of the rays upon the parts of the glass. thirdly, if the colours made by a prism placed at the entrance of a beam of light into a darken'd room be successively cast on a second prism placed at a greater distance from the former, in such manner that they are all alike incident upon it, the second prism may be so inclined to the incident rays, that those which are of a blue colour shall be all reflected by it, and yet those of a red colour pretty copiously transmitted. now if the reflexion be caused by the parts of air or glass, i would ask, why at the same obliquity of incidence the blue should wholly impinge on those parts, so as to be all reflected, and yet the red find pores enough to be in a great measure transmitted. fourthly, where two glasses touch one another, there is no sensible reflexion, as was declared in the first observation; and yet i see no reason why the rays should not impinge on the parts of glass, as much when contiguous to other glass as when contiguous to air. fifthly, when the top of a water-bubble (in the th observation,) by the continual subsiding and exhaling of the water grew very thin, there was such a little and almost insensible quantity of light reflected from it, that it appeared intensely black; whereas round about that black spot, where the water was thicker, the reflexion was so strong as to make the water seem very white. nor is it only at the least thickness of thin plates or bubbles, that there is no manifest reflexion, but at many other thicknesses continually greater and greater. for in the th observation the rays of the same colour were by turns transmitted at one thickness, and reflected at another thickness, for an indeterminate number of successions. and yet in the superficies of the thinned body, where it is of any one thickness, there are as many parts for the rays to impinge on, as where it is of any other thickness. sixthly, if reflexion were caused by the parts of reflecting bodies, it would be impossible for thin plates or bubbles, at one and the same place, to reflect the rays of one colour, and transmit those of another, as they do according to the th and th observations. for it is not to be imagined that at one place the rays which, for instance, exhibit a blue colour, should have the fortune to dash upon the parts, and those which exhibit a red to hit upon the pores of the body; and then at another place, where the body is either a little thicker or a little thinner, that on the contrary the blue should hit upon its pores, and the red upon its parts. lastly, were the rays of light reflected by impinging on the solid parts of bodies, their reflexions from polish'd bodies could not be so regular as they are. for in polishing glass with sand, putty, or tripoly, it is not to be imagined that those substances can, by grating and fretting the glass, bring all its least particles to an accurate polish; so that all their surfaces shall be truly plain or truly spherical, and look all the same way, so as together to compose one even surface. the smaller the particles of those substances are, the smaller will be the scratches by which they continually fret and wear away the glass until it be polish'd; but be they never so small they can wear away the glass no otherwise than by grating and scratching it, and breaking the protuberances; and therefore polish it no otherwise than by bringing its roughness to a very fine grain, so that the scratches and frettings of the surface become too small to be visible. and therefore if light were reflected by impinging upon the solid parts of the glass, it would be scatter'd as much by the most polish'd glass as by the roughest. so then it remains a problem, how glass polish'd by fretting substances can reflect light so regularly as it does. and this problem is scarce otherwise to be solved, than by saying, that the reflexion of a ray is effected, not by a single point of the reflecting body, but by some power of the body which is evenly diffused all over its surface, and by which it acts upon the ray without immediate contact. for that the parts of bodies do act upon light at a distance shall be shewn hereafter. now if light be reflected, not by impinging on the solid parts of bodies, but by some other principle; it's probable that as many of its rays as impinge on the solid parts of bodies are not reflected but stifled and lost in the bodies. for otherwise we must allow two sorts of reflexions. should all the rays be reflected which impinge on the internal parts of clear water or crystal, those substances would rather have a cloudy colour than a clear transparency. to make bodies look black, it's necessary that many rays be stopp'd, retained, and lost in them; and it seems not probable that any rays can be stopp'd and stifled in them which do not impinge on their parts. and hence we may understand that bodies are much more rare and porous than is commonly believed. water is nineteen times lighter, and by consequence nineteen times rarer than gold; and gold is so rare as very readily and without the least opposition to transmit the magnetick effluvia, and easily to admit quicksilver into its pores, and to let water pass through it. for a concave sphere of gold filled with water, and solder'd up, has, upon pressing the sphere with great force, let the water squeeze through it, and stand all over its outside in multitudes of small drops, like dew, without bursting or cracking the body of the gold, as i have been inform'd by an eye witness. from all which we may conclude, that gold has more pores than solid parts, and by consequence that water has above forty times more pores than parts. and he that shall find out an hypothesis, by which water may be so rare, and yet not be capable of compression by force, may doubtless by the same hypothesis make gold, and water, and all other bodies, as much rarer as he pleases; so that light may find a ready passage through transparent substances. the magnet acts upon iron through all dense bodies not magnetick nor red hot, without any diminution of its virtue; as for instance, through gold, silver, lead, glass, water. the gravitating power of the sun is transmitted through the vast bodies of the planets without any diminution, so as to act upon all their parts to their very centers with the same force and according to the same laws, as if the part upon which it acts were not surrounded with the body of the planet, the rays of light, whether they be very small bodies projected, or only motion or force propagated, are moved in right lines; and whenever a ray of light is by any obstacle turned out of its rectilinear way, it will never return into the same rectilinear way, unless perhaps by very great accident. and yet light is transmitted through pellucid solid bodies in right lines to very great distances. how bodies can have a sufficient quantity of pores for producing these effects is very difficult to conceive, but perhaps not altogether impossible. for the colours of bodies arise from the magnitudes of the particles which reflect them, as was explained above. now if we conceive these particles of bodies to be so disposed amongst themselves, that the intervals or empty spaces between them may be equal in magnitude to them all; and that these particles may be composed of other particles much smaller, which have as much empty space between them as equals all the magnitudes of these smaller particles: and that in like manner these smaller particles are again composed of others much smaller, all which together are equal to all the pores or empty spaces between them; and so on perpetually till you come to solid particles, such as have no pores or empty spaces within them: and if in any gross body there be, for instance, three such degrees of particles, the least of which are solid; this body will have seven times more pores than solid parts. but if there be four such degrees of particles, the least of which are solid, the body will have fifteen times more pores than solid parts. if there be five degrees, the body will have one and thirty times more pores than solid parts. if six degrees, the body will have sixty and three times more pores than solid parts. and so on perpetually. and there are other ways of conceiving how bodies may be exceeding porous. but what is really their inward frame is not yet known to us. prop. ix. _bodies reflect and refract light by one and the same power, variously exercised in various circumstances._ this appears by several considerations. first, because when light goes out of glass into air, as obliquely as it can possibly do. if its incidence be made still more oblique, it becomes totally reflected. for the power of the glass after it has refracted the light as obliquely as is possible, if the incidence be still made more oblique, becomes too strong to let any of its rays go through, and by consequence causes total reflexions. secondly, because light is alternately reflected and transmitted by thin plates of glass for many successions, accordingly as the thickness of the plate increases in an arithmetical progression. for here the thickness of the glass determines whether that power by which glass acts upon light shall cause it to be reflected, or suffer it to be transmitted. and, thirdly, because those surfaces of transparent bodies which have the greatest refracting power, reflect the greatest quantity of light, as was shewn in the first proposition. prop. x. _if light be swifter in bodies than in vacuo, in the proportion of the sines which measure the refraction of the bodies, the forces of the bodies to reflect and refract light, are very nearly proportional to the densities of the same bodies; excepting that unctuous and sulphureous bodies refract more than others of this same density._ [illustration: fig. .] let ab represent the refracting plane surface of any body, and ic a ray incident very obliquely upon the body in c, so that the angle aci may be infinitely little, and let cr be the refracted ray. from a given point b perpendicular to the refracting surface erect br meeting with the refracting ray cr in r, and if cr represent the motion of the refracted ray, and this motion be distinguish'd into two motions cb and br, whereof cb is parallel to the refracting plane, and br perpendicular to it: cb shall represent the motion of the incident ray, and br the motion generated by the refraction, as opticians have of late explain'd. now if any body or thing, in moving through any space of a given breadth terminated on both sides by two parallel planes, be urged forward in all parts of that space by forces tending directly forwards towards the last plane, and before its incidence on the first plane, had no motion towards it, or but an infinitely little one; and if the forces in all parts of that space, between the planes, be at equal distances from the planes equal to one another, but at several distances be bigger or less in any given proportion, the motion generated by the forces in the whole passage of the body or thing through that space shall be in a subduplicate proportion of the forces, as mathematicians will easily understand. and therefore, if the space of activity of the refracting superficies of the body be consider'd as such a space, the motion of the ray generated by the refracting force of the body, during its passage through that space, that is, the motion br, must be in subduplicate proportion of that refracting force. i say therefore, that the square of the line br, and by consequence the refracting force of the body, is very nearly as the density of the same body. for this will appear by the following table, wherein the proportion of the sines which measure the refractions of several bodies, the square of br, supposing cb an unite, the densities of the bodies estimated by their specifick gravities, and their refractive power in respect of their densities are set down in several columns. ---------------------+----------------+----------------+----------+----------- | | | | | | the square | the | the | | of br, to | density | refractive | the proportion | which the | and | power of | of the sines of| refracting | specifick| the body | incidence and | force of the | gravity | in respect the refracting | refraction of | body is | of the | of its bodies. | yellow light. | proportionate. | body. | density. ---------------------+----------------+----------------+----------+----------- a pseudo-topazius, | | | | being a natural, | | | | pellucid, brittle, | to | ' | ' | hairy stone, of a | | | | yellow colour. | | | | air. | to | ' | ' | glass of antimony. | to | ' | ' | a selenitis. | to | ' | ' | glass vulgar. | to | ' | ' | crystal of the rock. | to | ' | ' | island crystal. | to | ' | ' | sal gemmæ. | to | ' | ' | alume. | to | ' | ' | borax. | to | ' | ' | niter. | to | ' | ' | dantzick vitriol. | to | ' | ' | oil of vitriol. | to | ' | ' | rain water. | to | ' | ' | gum arabick. | to | ' | ' | spirit of wine well | | | | rectified. | to | ' | ' | camphire. | to | ' | ' | oil olive. | to | ' | ' | linseed oil. | to | ' | ' | spirit of turpentine.| to | ' | ' | amber. | to | ' | ' | a diamond. | to | ' | ' | ---------------------+----------------+----------------+----------+----------- the refraction of the air in this table is determin'd by that of the atmosphere observed by astronomers. for, if light pass through many refracting substances or mediums gradually denser and denser, and terminated with parallel surfaces, the sum of all the refractions will be equal to the single refraction which it would have suffer'd in passing immediately out of the first medium into the last. and this holds true, though the number of the refracting substances be increased to infinity, and the distances from one another as much decreased, so that the light may be refracted in every point of its passage, and by continual refractions bent into a curve-line. and therefore the whole refraction of light in passing through the atmosphere from the highest and rarest part thereof down to the lowest and densest part, must be equal to the refraction which it would suffer in passing at like obliquity out of a vacuum immediately into air of equal density with that in the lowest part of the atmosphere. now, although a pseudo-topaz, a selenitis, rock crystal, island crystal, vulgar glass (that is, sand melted together) and glass of antimony, which are terrestrial stony alcalizate concretes, and air which probably arises from such substances by fermentation, be substances very differing from one another in density, yet by this table, they have their refractive powers almost in the same proportion to one another as their densities are, excepting that the refraction of that strange substance, island crystal is a little bigger than the rest. and particularly air, which is times rarer than the pseudo-topaz, and times rarer than glass of antimony, and times rarer than the selenitis, glass vulgar, or crystal of the rock, has notwithstanding its rarity the same refractive power in respect of its density which those very dense substances have in respect of theirs, excepting so far as those differ from one another. again, the refraction of camphire, oil olive, linseed oil, spirit of turpentine and amber, which are fat sulphureous unctuous bodies, and a diamond, which probably is an unctuous substance coagulated, have their refractive powers in proportion to one another as their densities without any considerable variation. but the refractive powers of these unctuous substances are two or three times greater in respect of their densities than the refractive powers of the former substances in respect of theirs. water has a refractive power in a middle degree between those two sorts of substances, and probably is of a middle nature. for out of it grow all vegetable and animal substances, which consist as well of sulphureous fat and inflamable parts, as of earthy lean and alcalizate ones. salts and vitriols have refractive powers in a middle degree between those of earthy substances and water, and accordingly are composed of those two sorts of substances. for by distillation and rectification of their spirits a great part of them goes into water, and a great part remains behind in the form of a dry fix'd earth capable of vitrification. spirit of wine has a refractive power in a middle degree between those of water and oily substances, and accordingly seems to be composed of both, united by fermentation; the water, by means of some saline spirits with which 'tis impregnated, dissolving the oil, and volatizing it by the action. for spirit of wine is inflamable by means of its oily parts, and being distilled often from salt of tartar, grow by every distillation more and more aqueous and phlegmatick. and chymists observe, that vegetables (as lavender, rue, marjoram, &c.) distilled _per se_, before fermentation yield oils without any burning spirits, but after fermentation yield ardent spirits without oils: which shews, that their oil is by fermentation converted into spirit. they find also, that if oils be poured in a small quantity upon fermentating vegetables, they distil over after fermentation in the form of spirits. so then, by the foregoing table, all bodies seem to have their refractive powers proportional to their densities, (or very nearly;) excepting so far as they partake more or less of sulphureous oily particles, and thereby have their refractive power made greater or less. whence it seems rational to attribute the refractive power of all bodies chiefly, if not wholly, to the sulphureous parts with which they abound. for it's probable that all bodies abound more or less with sulphurs. and as light congregated by a burning-glass acts most upon sulphureous bodies, to turn them into fire and flame; so, since all action is mutual, sulphurs ought to act most upon light. for that the action between light and bodies is mutual, may appear from this consideration; that the densest bodies which refract and reflect light most strongly, grow hottest in the summer sun, by the action of the refracted or reflected light. i have hitherto explain'd the power of bodies to reflect and refract, and shew'd, that thin transparent plates, fibres, and particles, do, according to their several thicknesses and densities, reflect several sorts of rays, and thereby appear of several colours; and by consequence that nothing more is requisite for producing all the colours of natural bodies, than the several sizes and densities of their transparent particles. but whence it is that these plates, fibres, and particles, do, according to their several thicknesses and densities, reflect several sorts of rays, i have not yet explain'd. to give some insight into this matter, and make way for understanding the next part of this book, i shall conclude this part with a few more propositions. those which preceded respect the nature of bodies, these the nature of light: for both must be understood, before the reason of their actions upon one another can be known. and because the last proposition depended upon the velocity of light, i will begin with a proposition of that kind. prop. xi. _light is propagated from luminous bodies in time, and spends about seven or eight minutes of an hour in passing from the sun to the earth._ this was observed first by _roemer_, and then by others, by means of the eclipses of the satellites of _jupiter_. for these eclipses, when the earth is between the sun and _jupiter_, happen about seven or eight minutes sooner than they ought to do by the tables, and when the earth is beyond the sun they happen about seven or eight minutes later than they ought to do; the reason being, that the light of the satellites has farther to go in the latter case than in the former by the diameter of the earth's orbit. some inequalities of time may arise from the excentricities of the orbs of the satellites; but those cannot answer in all the satellites, and at all times to the position and distance of the earth from the sun. the mean motions of _jupiter_'s satellites is also swifter in his descent from his aphelium to his perihelium, than in his ascent in the other half of his orb. but this inequality has no respect to the position of the earth, and in the three interior satellites is insensible, as i find by computation from the theory of their gravity. prop. xii. _every ray of light in its passage through any refracting surface is put into a certain transient constitution or state, which in the progress of the ray returns at equal intervals, and disposes the ray at every return to be easily transmitted through the next refracting surface, and between the returns to be easily reflected by it._ this is manifest by the th, th, th, and th observations. for by those observations it appears, that one and the same sort of rays at equal angles of incidence on any thin transparent plate, is alternately reflected and transmitted for many successions accordingly as the thickness of the plate increases in arithmetical progression of the numbers, , , , , , , , , , &c. so that if the first reflexion (that which makes the first or innermost of the rings of colours there described) be made at the thickness , the rays shall be transmitted at the thicknesses , , , , , , , &c. and thereby make the central spot and rings of light, which appear by transmission, and be reflected at the thickness , , , , , , &c. and thereby make the rings which appear by reflexion. and this alternate reflexion and transmission, as i gather by the th observation, continues for above an hundred vicissitudes, and by the observations in the next part of this book, for many thousands, being propagated from one surface of a glass plate to the other, though the thickness of the plate be a quarter of an inch or above: so that this alternation seems to be propagated from every refracting surface to all distances without end or limitation. this alternate reflexion and refraction depends on both the surfaces of every thin plate, because it depends on their distance. by the st observation, if either surface of a thin plate of _muscovy_ glass be wetted, the colours caused by the alternate reflexion and refraction grow faint, and therefore it depends on them both. it is therefore perform'd at the second surface; for if it were perform'd at the first, before the rays arrive at the second, it would not depend on the second. it is also influenced by some action or disposition, propagated from the first to the second, because otherwise at the second it would not depend on the first. and this action or disposition, in its propagation, intermits and returns by equal intervals, because in all its progress it inclines the ray at one distance from the first surface to be reflected by the second, at another to be transmitted by it, and that by equal intervals for innumerable vicissitudes. and because the ray is disposed to reflexion at the distances , , , , , &c. and to transmission at the distances , , , , , , &c. (for its transmission through the first surface, is at the distance , and it is transmitted through both together, if their distance be infinitely little or much less than ) the disposition to be transmitted at the distances , , , , , &c. is to be accounted a return of the same disposition which the ray first had at the distance , that is at its transmission through the first refracting surface. all which is the thing i would prove. what kind of action or disposition this is; whether it consists in a circulating or a vibrating motion of the ray, or of the medium, or something else, i do not here enquire. those that are averse from assenting to any new discoveries, but such as they can explain by an hypothesis, may for the present suppose, that as stones by falling upon water put the water into an undulating motion, and all bodies by percussion excite vibrations in the air; so the rays of light, by impinging on any refracting or reflecting surface, excite vibrations in the refracting or reflecting medium or substance, and by exciting them agitate the solid parts of the refracting or reflecting body, and by agitating them cause the body to grow warm or hot; that the vibrations thus excited are propagated in the refracting or reflecting medium or substance, much after the manner that vibrations are propagated in the air for causing sound, and move faster than the rays so as to overtake them; and that when any ray is in that part of the vibration which conspires with its motion, it easily breaks through a refracting surface, but when it is in the contrary part of the vibration which impedes its motion, it is easily reflected; and, by consequence, that every ray is successively disposed to be easily reflected, or easily transmitted, by every vibration which overtakes it. but whether this hypothesis be true or false i do not here consider. i content my self with the bare discovery, that the rays of light are by some cause or other alternately disposed to be reflected or refracted for many vicissitudes. definition. _the returns of the disposition of any ray to be reflected i will call its_ fits of easy reflexion, _and those of its disposition to be transmitted its_ fits of easy transmission, _and the space it passes between every return and the next return, the_ interval of its fits. prop. xiii. _the reason why the surfaces of all thick transparent bodies reflect part of the light incident on them, and refract the rest, is, that some rays at their incidence are in fits of easy reflexion, and others in fits of easy transmission._ this may be gather'd from the th observation, where the light reflected by thin plates of air and glass, which to the naked eye appear'd evenly white all over the plate, did through a prism appear waved with many successions of light and darkness made by alternate fits of easy reflexion and easy transmission, the prism severing and distinguishing the waves of which the white reflected light was composed, as was explain'd above. and hence light is in fits of easy reflexion and easy transmission, before its incidence on transparent bodies. and probably it is put into such fits at its first emission from luminous bodies, and continues in them during all its progress. for these fits are of a lasting nature, as will appear by the next part of this book. in this proposition i suppose the transparent bodies to be thick; because if the thickness of the body be much less than the interval of the fits of easy reflexion and transmission of the rays, the body loseth its reflecting power. for if the rays, which at their entering into the body are put into fits of easy transmission, arrive at the farthest surface of the body before they be out of those fits, they must be transmitted. and this is the reason why bubbles of water lose their reflecting power when they grow very thin; and why all opake bodies, when reduced into very small parts, become transparent. prop. xiv. _those surfaces of transparent bodies, which if the ray be in a fit of refraction do refract it most strongly, if the ray be in a fit of reflexion do reflect it most easily._ for we shewed above, in _prop._ . that the cause of reflexion is not the impinging of light on the solid impervious parts of bodies, but some other power by which those solid parts act on light at a distance. we shewed also in _prop._ . that bodies reflect and refract light by one and the same power, variously exercised in various circumstances; and in _prop._ . that the most strongly refracting surfaces reflect the most light: all which compared together evince and rarify both this and the last proposition. prop. xv. _in any one and the same sort of rays, emerging in any angle out of any refracting surface into one and the same medium, the interval of the following fits of easy reflexion and transmission are either accurately or very nearly, as the rectangle of the secant of the angle of refraction, and of the secant of another angle, whose sine is the first of arithmetical mean proportionals, between the sines of incidence and refraction, counted from the sine of refraction._ this is manifest by the th and th observations. prop. xvi. _in several sorts of rays emerging in equal angles out of any refracting surface into the same medium, the intervals of the following fits of easy reflexion and easy transmission are either accurately, or very nearly, as the cube-roots of the squares of the lengths of a chord, which found the notes in an eight_, sol, la, fa, sol, la, mi, fa, sol, _with all their intermediate degrees answering to the colours of those rays, according to the analogy described in the seventh experiment of the second part of the first book._ this is manifest by the th and th observations. prop. xvii. _if rays of any sort pass perpendicularly into several mediums, the intervals of the fits of easy reflexion and transmission in any one medium, are to those intervals in any other, as the sine of incidence to the sine of refraction, when the rays pass out of the first of those two mediums into the second._ this is manifest by the th observation. prop. xviii. _if the rays which paint the colour in the confine of yellow and orange pass perpendicularly out of any medium into air, the intervals of their fits of easy reflexion are the / th part of an inch. and of the same length are the intervals of their fits of easy transmission._ this is manifest by the th observation. from these propositions it is easy to collect the intervals of the fits of easy reflexion and easy transmission of any sort of rays refracted in any angle into any medium; and thence to know, whether the rays shall be reflected or transmitted at their subsequent incidence upon any other pellucid medium. which thing, being useful for understanding the next part of this book, was here to be set down. and for the same reason i add the two following propositions. prop. xix. _if any sort of rays falling on the polite surface of any pellucid medium be reflected back, the fits of easy reflexion, which they have at the point of reflexion, shall still continue to return; and the returns shall be at distances from the point of reflexion in the arithmetical progression of the numbers , , , , , , &c. and between these fits the rays shall be in fits of easy transmission._ for since the fits of easy reflexion and easy transmission are of a returning nature, there is no reason why these fits, which continued till the ray arrived at the reflecting medium, and there inclined the ray to reflexion, should there cease. and if the ray at the point of reflexion was in a fit of easy reflexion, the progression of the distances of these fits from that point must begin from , and so be of the numbers , , , , , &c. and therefore the progression of the distances of the intermediate fits of easy transmission, reckon'd from the same point, must be in the progression of the odd numbers , , , , , &c. contrary to what happens when the fits are propagated from points of refraction. prop. xx. _the intervals of the fits of easy reflexion and easy transmission, propagated from points of reflexion into any medium, are equal to the intervals of the like fits, which the same rays would have, if refracted into the same medium in angles of refraction equal to their angles of reflexion._ for when light is reflected by the second surface of thin plates, it goes out afterwards freely at the first surface to make the rings of colours which appear by reflexion; and, by the freedom of its egress, makes the colours of these rings more vivid and strong than those which appear on the other side of the plates by the transmitted light. the reflected rays are therefore in fits of easy transmission at their egress; which would not always happen, if the intervals of the fits within the plate after reflexion were not equal, both in length and number, to their intervals before it. and this confirms also the proportions set down in the former proposition. for if the rays both in going in and out at the first surface be in fits of easy transmission, and the intervals and numbers of those fits between the first and second surface, before and after reflexion, be equal, the distances of the fits of easy transmission from either surface, must be in the same progression after reflexion as before; that is, from the first surface which transmitted them in the progression of the even numbers , , , , , &c. and from the second which reflected them, in that of the odd numbers , , , , &c. but these two propositions will become much more evident by the observations in the following part of this book. the second book of opticks _part iv._ _observations concerning the reflexions and colours of thick transparent polish'd plates._ there is no glass or speculum how well soever polished, but, besides the light which it refracts or reflects regularly, scatters every way irregularly a faint light, by means of which the polish'd surface, when illuminated in a dark room by a beam of the sun's light, may be easily seen in all positions of the eye. there are certain phænomena of this scatter'd light, which when i first observed them, seem'd very strange and surprizing to me. my observations were as follows. _obs._ . the sun shining into my darken'd chamber through a hole one third of an inch wide, i let the intromitted beam of light fall perpendicularly upon a glass speculum ground concave on one side and convex on the other, to a sphere of five feet and eleven inches radius, and quick-silver'd over on the convex side. and holding a white opake chart, or a quire of paper at the center of the spheres to which the speculum was ground, that is, at the distance of about five feet and eleven inches from the speculum, in such manner, that the beam of light might pass through a little hole made in the middle of the chart to the speculum, and thence be reflected back to the same hole: i observed upon the chart four or five concentric irises or rings of colours, like rain-bows, encompassing the hole much after the manner that those, which in the fourth and following observations of the first part of this book appear'd between the object-glasses, encompassed the black spot, but yet larger and fainter than those. these rings as they grew larger and larger became diluter and fainter, so that the fifth was scarce visible. yet sometimes, when the sun shone very clear, there appear'd faint lineaments of a sixth and seventh. if the distance of the chart from the speculum was much greater or much less than that of six feet, the rings became dilute and vanish'd. and if the distance of the speculum from the window was much greater than that of six feet, the reflected beam of light would be so broad at the distance of six feet from the speculum where the rings appear'd, as to obscure one or two of the innermost rings. and therefore i usually placed the speculum at about six feet from the window; so that its focus might there fall in with the center of its concavity at the rings upon the chart. and this posture is always to be understood in the following observations where no other is express'd. _obs._ . the colours of these rain-bows succeeded one another from the center outwards, in the same form and order with those which were made in the ninth observation of the first part of this book by light not reflected, but transmitted through the two object-glasses. for, first, there was in their common center a white round spot of faint light, something broader than the reflected beam of light, which beam sometimes fell upon the middle of the spot, and sometimes by a little inclination of the speculum receded from the middle, and left the spot white to the center. this white spot was immediately encompassed with a dark grey or russet, and that dark grey with the colours of the first iris; which colours on the inside next the dark grey were a little violet and indigo, and next to that a blue, which on the outside grew pale, and then succeeded a little greenish yellow, and after that a brighter yellow, and then on the outward edge of the iris a red which on the outside inclined to purple. this iris was immediately encompassed with a second, whose colours were in order from the inside outwards, purple, blue, green, yellow, light red, a red mix'd with purple. then immediately follow'd the colours of the third iris, which were in order outwards a green inclining to purple, a good green, and a red more bright than that of the former iris. the fourth and fifth iris seem'd of a bluish green within, and red without, but so faintly that it was difficult to discern the colours. _obs._ . measuring the diameters of these rings upon the chart as accurately as i could, i found them also in the same proportion to one another with the rings made by light transmitted through the two object-glasses. for the diameters of the four first of the bright rings measured between the brightest parts of their orbits, at the distance of six feet from the speculum were - / , - / , - / , - / inches, whose squares are in arithmetical progression of the numbers , , , . if the white circular spot in the middle be reckon'd amongst the rings, and its central light, where it seems to be most luminous, be put equipollent to an infinitely little ring; the squares of the diameters of the rings will be in the progression , , , , , &c. i measured also the diameters of the dark circles between these luminous ones, and found their squares in the progression of the numbers / , - / , - / , - / , &c. the diameters of the first four at the distance of six feet from the speculum, being - / , - / , - / , - / inches. if the distance of the chart from the speculum was increased or diminished, the diameters of the circles were increased or diminished proportionally. _obs._ . by the analogy between these rings and those described in the observations of the first part of this book, i suspected that there were many more of them which spread into one another, and by interfering mix'd their colours, and diluted one another so that they could not be seen apart. i viewed them therefore through a prism, as i did those in the th observation of the first part of this book. and when the prism was so placed as by refracting the light of their mix'd colours to separate them, and distinguish the rings from one another, as it did those in that observation, i could then see them distincter than before, and easily number eight or nine of them, and sometimes twelve or thirteen. and had not their light been so very faint, i question not but that i might have seen many more. _obs._ . placing a prism at the window to refract the intromitted beam of light, and cast the oblong spectrum of colours on the speculum: i covered the speculum with a black paper which had in the middle of it a hole to let any one of the colours pass through to the speculum, whilst the rest were intercepted by the paper. and now i found rings of that colour only which fell upon the speculum. if the speculum was illuminated with red, the rings were totally red with dark intervals, if with blue they were totally blue, and so of the other colours. and when they were illuminated with any one colour, the squares of their diameters measured between their most luminous parts, were in the arithmetical progression of the numbers, , , , , and the squares of the diameters of their dark intervals in the progression of the intermediate numbers / , - / , - / , - / . but if the colour was varied, they varied their magnitude. in the red they were largest, in the indigo and violet least, and in the intermediate colours yellow, green, and blue, they were of several intermediate bignesses answering to the colour, that is, greater in yellow than in green, and greater in green than in blue. and hence i knew, that when the speculum was illuminated with white light, the red and yellow on the outside of the rings were produced by the least refrangible rays, and the blue and violet by the most refrangible, and that the colours of each ring spread into the colours of the neighbouring rings on either side, after the manner explain'd in the first and second part of this book, and by mixing diluted one another so that they could not be distinguish'd, unless near the center where they were least mix'd. for in this observation i could see the rings more distinctly, and to a greater number than before, being able in the yellow light to number eight or nine of them, besides a faint shadow of a tenth. to satisfy my self how much the colours of the several rings spread into one another, i measured the diameters of the second and third rings, and found them when made by the confine of the red and orange to be to the same diameters when made by the confine of blue and indigo, as to , or thereabouts. for it was hard to determine this proportion accurately. also the circles made successively by the red, yellow, and green, differ'd more from one another than those made successively by the green, blue, and indigo. for the circle made by the violet was too dark to be seen. to carry on the computation, let us therefore suppose that the differences of the diameters of the circles made by the outmost red, the confine of red and orange, the confine of orange and yellow, the confine of yellow and green, the confine of green and blue, the confine of blue and indigo, the confine of indigo and violet, and outmost violet, are in proportion as the differences of the lengths of a monochord which sound the tones in an eight; _sol_, _la_, _fa_, _sol_, _la_, _mi_, _fa_, _sol_, that is, as the numbers / , / , / , / , / , / , / . and if the diameter of the circle made by the confine of red and orange be a, and that of the circle made by the confine of blue and indigo be a as above; their difference a- a will be to the difference of the diameters of the circles made by the outmost red, and by the confine of red and orange, as / + / + / + / to / , that is as / to / , or to , and to the difference of the circles made by the outmost violet, and by the confine of blue and indigo, as / + / + / + / to / + / , that is, as / to / , or as to . and therefore these differences will be / a and / a. add the first to a and subduct the last from a, and you will have the diameters of the circles made by the least and most refrangible rays / a and (( - / )/ )a. these diameters are therefore to one another as to - / or to , and their squares as to , that is, as to very nearly. which proportion differs not much from the proportion of the diameters of the circles made by the outmost red and outmost violet, in the th observation of the first part of this book. _obs._ . placing my eye where these rings appear'd plainest, i saw the speculum tinged all over with waves of colours, (red, yellow, green, blue;) like those which in the observations of the first part of this book appeared between the object-glasses, and upon bubbles of water, but much larger. and after the manner of those, they were of various magnitudes in various positions of the eye, swelling and shrinking as i moved my eye this way and that way. they were formed like arcs of concentrick circles, as those were; and when my eye was over against the center of the concavity of the speculum, (that is, feet and inches distant from the speculum,) their common center was in a right line with that center of concavity, and with the hole in the window. but in other postures of my eye their center had other positions. they appear'd by the light of the clouds propagated to the speculum through the hole in the window; and when the sun shone through that hole upon the speculum, his light upon it was of the colour of the ring whereon it fell, but by its splendor obscured the rings made by the light of the clouds, unless when the speculum was removed to a great distance from the window, so that his light upon it might be broad and faint. by varying the position of my eye, and moving it nearer to or farther from the direct beam of the sun's light, the colour of the sun's reflected light constantly varied upon the speculum, as it did upon my eye, the same colour always appearing to a bystander upon my eye which to me appear'd upon the speculum. and thence i knew that the rings of colours upon the chart were made by these reflected colours, propagated thither from the speculum in several angles, and that their production depended not upon the termination of light and shadow. _obs._ . by the analogy of all these phænomena with those of the like rings of colours described in the first part of this book, it seemed to me that these colours were produced by this thick plate of glass, much after the manner that those were produced by very thin plates. for, upon trial, i found that if the quick-silver were rubb'd off from the backside of the speculum, the glass alone would cause the same rings of colours, but much more faint than before; and therefore the phænomenon depends not upon the quick-silver, unless so far as the quick-silver by increasing the reflexion of the backside of the glass increases the light of the rings of colours. i found also that a speculum of metal without glass made some years since for optical uses, and very well wrought, produced none of those rings; and thence i understood that these rings arise not from one specular surface alone, but depend upon the two surfaces of the plate of glass whereof the speculum was made, and upon the thickness of the glass between them. for as in the th and th observations of the first part of this book a thin plate of air, water, or glass of an even thickness appeared of one colour when the rays were perpendicular to it, of another when they were a little oblique, of another when more oblique, of another when still more oblique, and so on; so here, in the sixth observation, the light which emerged out of the glass in several obliquities, made the glass appear of several colours, and being propagated in those obliquities to the chart, there painted rings of those colours. and as the reason why a thin plate appeared of several colours in several obliquities of the rays, was, that the rays of one and the same sort are reflected by the thin plate at one obliquity and transmitted at another, and those of other sorts transmitted where these are reflected, and reflected where these are transmitted: so the reason why the thick plate of glass whereof the speculum was made did appear of various colours in various obliquities, and in those obliquities propagated those colours to the chart, was, that the rays of one and the same sort did at one obliquity emerge out of the glass, at another did not emerge, but were reflected back towards the quick-silver by the hither surface of the glass, and accordingly as the obliquity became greater and greater, emerged and were reflected alternately for many successions; and that in one and the same obliquity the rays of one sort were reflected, and those of another transmitted. this is manifest by the fifth observation of this part of this book. for in that observation, when the speculum was illuminated by any one of the prismatick colours, that light made many rings of the same colour upon the chart with dark intervals, and therefore at its emergence out of the speculum was alternately transmitted and not transmitted from the speculum to the chart for many successions, according to the various obliquities of its emergence. and when the colour cast on the speculum by the prism was varied, the rings became of the colour cast on it, and varied their bigness with their colour, and therefore the light was now alternately transmitted and not transmitted from the speculum to the chart at other obliquities than before. it seemed to me therefore that these rings were of one and the same original with those of thin plates, but yet with this difference, that those of thin plates are made by the alternate reflexions and transmissions of the rays at the second surface of the plate, after one passage through it; but here the rays go twice through the plate before they are alternately reflected and transmitted. first, they go through it from the first surface to the quick-silver, and then return through it from the quick-silver to the first surface, and there are either transmitted to the chart or reflected back to the quick-silver, accordingly as they are in their fits of easy reflexion or transmission when they arrive at that surface. for the intervals of the fits of the rays which fall perpendicularly on the speculum, and are reflected back in the same perpendicular lines, by reason of the equality of these angles and lines, are of the same length and number within the glass after reflexion as before, by the th proposition of the third part of this book. and therefore since all the rays that enter through the first surface are in their fits of easy transmission at their entrance, and as many of these as are reflected by the second are in their fits of easy reflexion there, all these must be again in their fits of easy transmission at their return to the first, and by consequence there go out of the glass to the chart, and form upon it the white spot of light in the center of the rings. for the reason holds good in all sorts of rays, and therefore all sorts must go out promiscuously to that spot, and by their mixture cause it to be white. but the intervals of the fits of those rays which are reflected more obliquely than they enter, must be greater after reflexion than before, by the th and th propositions. and thence it may happen that the rays at their return to the first surface, may in certain obliquities be in fits of easy reflexion, and return back to the quick-silver, and in other intermediate obliquities be again in fits of easy transmission, and so go out to the chart, and paint on it the rings of colours about the white spot. and because the intervals of the fits at equal obliquities are greater and fewer in the less refrangible rays, and less and more numerous in the more refrangible, therefore the less refrangible at equal obliquities shall make fewer rings than the more refrangible, and the rings made by those shall be larger than the like number of rings made by these; that is, the red rings shall be larger than the yellow, the yellow than the green, the green than the blue, and the blue than the violet, as they were really found to be in the fifth observation. and therefore the first ring of all colours encompassing the white spot of light shall be red without any violet within, and yellow, and green, and blue in the middle, as it was found in the second observation; and these colours in the second ring, and those that follow, shall be more expanded, till they spread into one another, and blend one another by interfering. these seem to be the reasons of these rings in general; and this put me upon observing the thickness of the glass, and considering whether the dimensions and proportions of the rings may be truly derived from it by computation. _obs._ . i measured therefore the thickness of this concavo-convex plate of glass, and found it every where / of an inch precisely. now, by the sixth observation of the first part of this book, a thin plate of air transmits the brightest light of the first ring, that is, the bright yellow, when its thickness is the / th part of an inch; and by the tenth observation of the same part, a thin plate of glass transmits the same light of the same ring, when its thickness is less in proportion of the sine of refraction to the sine of incidence, that is, when its thickness is the / th or / th part of an inch, supposing the sines are as to . and if this thickness be doubled, it transmits the same bright light of the second ring; if tripled, it transmits that of the third, and so on; the bright yellow light in all these cases being in its fits of transmission. and therefore if its thickness be multiplied times, so as to become / of an inch, it transmits the same bright light of the th ring. suppose this be the bright yellow light transmitted perpendicularly from the reflecting convex side of the glass through the concave side to the white spot in the center of the rings of colours on the chart: and by a rule in the th and th observations in the first part of this book, and by the th and th propositions of the third part of this book, if the rays be made oblique to the glass, the thickness of the glass requisite to transmit the same bright light of the same ring in any obliquity, is to this thickness of / of an inch, as the secant of a certain angle to the radius, the sine of which angle is the first of an hundred and six arithmetical means between the sines of incidence and refraction, counted from the sine of incidence when the refraction is made out of any plated body into any medium encompassing it; that is, in this case, out of glass into air. now if the thickness of the glass be increased by degrees, so as to bear to its first thickness, (_viz._ that of a quarter of an inch,) the proportions which (the number of fits of the perpendicular rays in going through the glass towards the white spot in the center of the rings,) hath to , , , and , (the numbers of the fits of the oblique rays in going through the glass towards the first, second, third, and fourth rings of colours,) and if the first thickness be divided into equal parts, the increased thicknesses will be , , , and , and the angles of which these thicknesses are secants will be ´ ´´, ´ ´´, ´ ´´, and ´ ´´, the radius being ; and the sines of these angles are , , , and , and the proportional sines of refraction , , , and , the radius being . for since the sines of incidence out of glass into air are to the sines of refraction as to , and to the above-mentioned secants as to the first of arithmetical means between and , that is, as to - / , those secants will be to the sines of refraction as - / , to , and by this analogy will give these sines. so then, if the obliquities of the rays to the concave surface of the glass be such that the sines of their refraction in passing out of the glass through that surface into the air be , , , , the bright light of the th ring shall emerge at the thicknesses of the glass, which are to / of an inch as to , , , , respectively. and therefore, if the thickness in all these cases be / of an inch (as it is in the glass of which the speculum was made) the bright light of the th ring shall emerge where the sine of refraction is , and that of the th, th, and th ring where the sine is , , and respectively. and in these angles of refraction the light of these rings shall be propagated from the speculum to the chart, and there paint rings about the white central round spot of light which we said was the light of the th ring. and the semidiameters of these rings shall subtend the angles of refraction made at the concave-surface of the speculum, and by consequence their diameters shall be to the distance of the chart from the speculum as those sines of refraction doubled are to the radius, that is, as , , , and , doubled are to . and therefore, if the distance of the chart from the concave-surface of the speculum be six feet (as it was in the third of these observations) the diameters of the rings of this bright yellow light upon the chart shall be ' , ' , ' , ' inches: for these diameters are to six feet, as the above-mention'd sines doubled are to the radius. now, these diameters of the bright yellow rings, thus found by computation are the very same with those found in the third of these observations by measuring them, _viz._ with - / , - / , - / , and - / inches, and therefore the theory of deriving these rings from the thickness of the plate of glass of which the speculum was made, and from the obliquity of the emerging rays agrees with the observation. in this computation i have equalled the diameters of the bright rings made by light of all colours, to the diameters of the rings made by the bright yellow. for this yellow makes the brightest part of the rings of all colours. if you desire the diameters of the rings made by the light of any other unmix'd colour, you may find them readily by putting them to the diameters of the bright yellow ones in a subduplicate proportion of the intervals of the fits of the rays of those colours when equally inclined to the refracting or reflecting surface which caused those fits, that is, by putting the diameters of the rings made by the rays in the extremities and limits of the seven colours, red, orange, yellow, green, blue, indigo, violet, proportional to the cube-roots of the numbers, , / , / , / , / , / , / , / , which express the lengths of a monochord sounding the notes in an eighth: for by this means the diameters of the rings of these colours will be found pretty nearly in the same proportion to one another, which they ought to have by the fifth of these observations. and thus i satisfy'd my self, that these rings were of the same kind and original with those of thin plates, and by consequence that the fits or alternate dispositions of the rays to be reflected and transmitted are propagated to great distances from every reflecting and refracting surface. but yet to put the matter out of doubt, i added the following observation. _obs._ . if these rings thus depend on the thickness of the plate of glass, their diameters at equal distances from several speculums made of such concavo-convex plates of glass as are ground on the same sphere, ought to be reciprocally in a subduplicate proportion of the thicknesses of the plates of glass. and if this proportion be found true by experience it will amount to a demonstration that these rings (like those formed in thin plates) do depend on the thickness of the glass. i procured therefore another concavo-convex plate of glass ground on both sides to the same sphere with the former plate. its thickness was / parts of an inch; and the diameters of the three first bright rings measured between the brightest parts of their orbits at the distance of six feet from the glass were · - / · - / · inches. now, the thickness of the other glass being / of an inch was to the thickness of this glass as / to / , that is as to , or to , and the roots of these numbers are and , and in the proportion of the first of these roots to the second are the diameters of the bright rings made in this observation by the thinner glass, · - / · - / , to the diameters of the same rings made in the third of these observations by the thicker glass - / , - / . - / , that is, the diameters of the rings are reciprocally in a subduplicate proportion of the thicknesses of the plates of glass. so then in plates of glass which are alike concave on one side, and alike convex on the other side, and alike quick-silver'd on the convex sides, and differ in nothing but their thickness, the diameters of the rings are reciprocally in a subduplicate proportion of the thicknesses of the plates. and this shews sufficiently that the rings depend on both the surfaces of the glass. they depend on the convex surface, because they are more luminous when that surface is quick-silver'd over than when it is without quick-silver. they depend also upon the concave surface, because without that surface a speculum makes them not. they depend on both surfaces, and on the distances between them, because their bigness is varied by varying only that distance. and this dependence is of the same kind with that which the colours of thin plates have on the distance of the surfaces of those plates, because the bigness of the rings, and their proportion to one another, and the variation of their bigness arising from the variation of the thickness of the glass, and the orders of their colours, is such as ought to result from the propositions in the end of the third part of this book, derived from the phænomena of the colours of thin plates set down in the first part. there are yet other phænomena of these rings of colours, but such as follow from the same propositions, and therefore confirm both the truth of those propositions, and the analogy between these rings and the rings of colours made by very thin plates. i shall subjoin some of them. _obs._ . when the beam of the sun's light was reflected back from the speculum not directly to the hole in the window, but to a place a little distant from it, the common center of that spot, and of all the rings of colours fell in the middle way between the beam of the incident light, and the beam of the reflected light, and by consequence in the center of the spherical concavity of the speculum, whenever the chart on which the rings of colours fell was placed at that center. and as the beam of reflected light by inclining the speculum receded more and more from the beam of incident light and from the common center of the colour'd rings between them, those rings grew bigger and bigger, and so also did the white round spot, and new rings of colours emerged successively out of their common center, and the white spot became a white ring encompassing them; and the incident and reflected beams of light always fell upon the opposite parts of this white ring, illuminating its perimeter like two mock suns in the opposite parts of an iris. so then the diameter of this ring, measured from the middle of its light on one side to the middle of its light on the other side, was always equal to the distance between the middle of the incident beam of light, and the middle of the reflected beam measured at the chart on which the rings appeared: and the rays which form'd this ring were reflected by the speculum in angles equal to their angles of incidence, and by consequence to their angles of refraction at their entrance into the glass, but yet their angles of reflexion were not in the same planes with their angles of incidence. _obs._ . the colours of the new rings were in a contrary order to those of the former, and arose after this manner. the white round spot of light in the middle of the rings continued white to the center till the distance of the incident and reflected beams at the chart was about / parts of an inch, and then it began to grow dark in the middle. and when that distance was about - / of an inch, the white spot was become a ring encompassing a dark round spot which in the middle inclined to violet and indigo. and the luminous rings encompassing it were grown equal to those dark ones which in the four first observations encompassed them, that is to say, the white spot was grown a white ring equal to the first of those dark rings, and the first of those luminous rings was now grown equal to the second of those dark ones, and the second of those luminous ones to the third of those dark ones, and so on. for the diameters of the luminous rings were now - / , - / , - / , - / , &c. inches. when the distance between the incident and reflected beams of light became a little bigger, there emerged out of the middle of the dark spot after the indigo a blue, and then out of that blue a pale green, and soon after a yellow and red. and when the colour at the center was brightest, being between yellow and red, the bright rings were grown equal to those rings which in the four first observations next encompassed them; that is to say, the white spot in the middle of those rings was now become a white ring equal to the first of those bright rings, and the first of those bright ones was now become equal to the second of those, and so on. for the diameters of the white ring, and of the other luminous rings encompassing it, were now - / , - / , - / , - / , &c. or thereabouts. when the distance of the two beams of light at the chart was a little more increased, there emerged out of the middle in order after the red, a purple, a blue, a green, a yellow, and a red inclining much to purple, and when the colour was brightest being between yellow and red, the former indigo, blue, green, yellow and red, were become an iris or ring of colours equal to the first of those luminous rings which appeared in the four first observations, and the white ring which was now become the second of the luminous rings was grown equal to the second of those, and the first of those which was now become the third ring was become equal to the third of those, and so on. for their diameters were - / , - / , - / , - / inches, the distance of the two beams of light, and the diameter of the white ring being - / inches. when these two beams became more distant there emerged out of the middle of the purplish red, first a darker round spot, and then out of the middle of that spot a brighter. and now the former colours (purple, blue, green, yellow, and purplish red) were become a ring equal to the first of the bright rings mentioned in the four first observations, and the rings about this ring were grown equal to the rings about that respectively; the distance between the two beams of light and the diameter of the white ring (which was now become the third ring) being about inches. the colours of the rings in the middle began now to grow very dilute, and if the distance between the two beams was increased half an inch, or an inch more, they vanish'd whilst the white ring, with one or two of the rings next it on either side, continued still visible. but if the distance of the two beams of light was still more increased, these also vanished: for the light which coming from several parts of the hole in the window fell upon the speculum in several angles of incidence, made rings of several bignesses, which diluted and blotted out one another, as i knew by intercepting some part of that light. for if i intercepted that part which was nearest to the axis of the speculum the rings would be less, if the other part which was remotest from it they would be bigger. _obs._ . when the colours of the prism were cast successively on the speculum, that ring which in the two last observations was white, was of the same bigness in all the colours, but the rings without it were greater in the green than in the blue, and still greater in the yellow, and greatest in the red. and, on the contrary, the rings within that white circle were less in the green than in the blue, and still less in the yellow, and least in the red. for the angles of reflexion of those rays which made this ring, being equal to their angles of incidence, the fits of every reflected ray within the glass after reflexion are equal in length and number to the fits of the same ray within the glass before its incidence on the reflecting surface. and therefore since all the rays of all sorts at their entrance into the glass were in a fit of transmission, they were also in a fit of transmission at their returning to the same surface after reflexion; and by consequence were transmitted, and went out to the white ring on the chart. this is the reason why that ring was of the same bigness in all the colours, and why in a mixture of all it appears white. but in rays which are reflected in other angles, the intervals of the fits of the least refrangible being greatest, make the rings of their colour in their progress from this white ring, either outwards or inwards, increase or decrease by the greatest steps; so that the rings of this colour without are greatest, and within least. and this is the reason why in the last observation, when the speculum was illuminated with white light, the exterior rings made by all colours appeared red without and blue within, and the interior blue without and red within. these are the phænomena of thick convexo-concave plates of glass, which are every where of the same thickness. there are yet other phænomena when these plates are a little thicker on one side than on the other, and others when the plates are more or less concave than convex, or plano-convex, or double-convex. for in all these cases the plates make rings of colours, but after various manners; all which, so far as i have yet observed, follow from the propositions in the end of the third part of this book, and so conspire to confirm the truth of those propositions. but the phænomena are too various, and the calculations whereby they follow from those propositions too intricate to be here prosecuted. i content my self with having prosecuted this kind of phænomena so far as to discover their cause, and by discovering it to ratify the propositions in the third part of this book. _obs._ . as light reflected by a lens quick-silver'd on the backside makes the rings of colours above described, so it ought to make the like rings of colours in passing through a drop of water. at the first reflexion of the rays within the drop, some colours ought to be transmitted, as in the case of a lens, and others to be reflected back to the eye. for instance, if the diameter of a small drop or globule of water be about the th part of an inch, so that a red-making ray in passing through the middle of this globule has fits of easy transmission within the globule, and that all the red-making rays which are at a certain distance from this middle ray round about it have fits within the globule, and all the like rays at a certain farther distance round about it have fits, and all those at a certain farther distance fits, and so on; these concentrick circles of rays after their transmission, falling on a white paper, will make concentrick rings of red upon the paper, supposing the light which passes through one single globule, strong enough to be sensible. and, in like manner, the rays of other colours will make rings of other colours. suppose now that in a fair day the sun shines through a thin cloud of such globules of water or hail, and that the globules are all of the same bigness; and the sun seen through this cloud shall appear encompassed with the like concentrick rings of colours, and the diameter of the first ring of red shall be - / degrees, that of the second - / degrees, that of the third degrees minutes. and accordingly as the globules of water are bigger or less, the rings shall be less or bigger. this is the theory, and experience answers it. for in _june_ , i saw by reflexion in a vessel of stagnating water three halos, crowns, or rings of colours about the sun, like three little rain-bows, concentrick to his body. the colours of the first or innermost crown were blue next the sun, red without, and white in the middle between the blue and red. those of the second crown were purple and blue within, and pale red without, and green in the middle. and those of the third were pale blue within, and pale red without; these crowns enclosed one another immediately, so that their colours proceeded in this continual order from the sun outward: blue, white, red; purple, blue, green, pale yellow and red; pale blue, pale red. the diameter of the second crown measured from the middle of the yellow and red on one side of the sun, to the middle of the same colour on the other side was - / degrees, or thereabouts. the diameters of the first and third i had not time to measure, but that of the first seemed to be about five or six degrees, and that of the third about twelve. the like crowns appear sometimes about the moon; for in the beginning of the year , _febr._ th at night, i saw two such crowns about her. the diameter of the first or innermost was about three degrees, and that of the second about five degrees and an half. next about the moon was a circle of white, and next about that the inner crown, which was of a bluish green within next the white, and of a yellow and red without, and next about these colours were blue and green on the inside of the outward crown, and red on the outside of it. at the same time there appear'd a halo about degrees ´ distant from the center of the moon. it was elliptical, and its long diameter was perpendicular to the horizon, verging below farthest from the moon. i am told that the moon has sometimes three or more concentrick crowns of colours encompassing one another next about her body. the more equal the globules of water or ice are to one another, the more crowns of colours will appear, and the colours will be the more lively. the halo at the distance of - / degrees from the moon is of another sort. by its being oval and remoter from the moon below than above, i conclude, that it was made by refraction in some sort of hail or snow floating in the air in an horizontal posture, the refracting angle being about or degrees. the third book of opticks _part i._ _observations concerning the inflexions of the rays of light, and the colours made thereby._ grimaldo has inform'd us, that if a beam of the sun's light be let into a dark room through a very small hole, the shadows of things in this light will be larger than they ought to be if the rays went on by the bodies in straight lines, and that these shadows have three parallel fringes, bands or ranks of colour'd light adjacent to them. but if the hole be enlarged the fringes grow broad and run into one another, so that they cannot be distinguish'd. these broad shadows and fringes have been reckon'd by some to proceed from the ordinary refraction of the air, but without due examination of the matter. for the circumstances of the phænomenon, so far as i have observed them, are as follows. _obs._ . i made in a piece of lead a small hole with a pin, whose breadth was the d part of an inch. for of those pins laid together took up the breadth of half an inch. through this hole i let into my darken'd chamber a beam of the sun's light, and found that the shadows of hairs, thred, pins, straws, and such like slender substances placed in this beam of light, were considerably broader than they ought to be, if the rays of light passed on by these bodies in right lines. and particularly a hair of a man's head, whose breadth was but the th part of an inch, being held in this light, at the distance of about twelve feet from the hole, did cast a shadow which at the distance of four inches from the hair was the sixtieth part of an inch broad, that is, above four times broader than the hair, and at the distance of two feet from the hair was about the eight and twentieth part of an inch broad, that is, ten times broader than the hair, and at the distance of ten feet was the eighth part of an inch broad, that is times broader. nor is it material whether the hair be encompassed with air, or with any other pellucid substance. for i wetted a polish'd plate of glass, and laid the hair in the water upon the glass, and then laying another polish'd plate of glass upon it, so that the water might fill up the space between the glasses, i held them in the aforesaid beam of light, so that the light might pass through them perpendicularly, and the shadow of the hair was at the same distances as big as before. the shadows of scratches made in polish'd plates of glass were also much broader than they ought to be, and the veins in polish'd plates of glass did also cast the like broad shadows. and therefore the great breadth of these shadows proceeds from some other cause than the refraction of the air. let the circle x [in _fig._ .] represent the middle of the hair; adg, beh, cfi, three rays passing by one side of the hair at several distances; knq, lor, mps, three other rays passing by the other side of the hair at the like distances; d, e, f, and n, o, p, the places where the rays are bent in their passage by the hair; g, h, i, and q, r, s, the places where the rays fall on a paper gq; is the breadth of the shadow of the hair cast on the paper, and ti, vs, two rays passing to the points i and s without bending when the hair is taken away. and it's manifest that all the light between these two rays ti and vs is bent in passing by the hair, and turned aside from the shadow is, because if any part of this light were not bent it would fall on the paper within the shadow, and there illuminate the paper, contrary to experience. and because when the paper is at a great distance from the hair, the shadow is broad, and therefore the rays ti and vs are at a great distance from one another, it follows that the hair acts upon the rays of light at a good distance in their passing by it. but the action is strongest on the rays which pass by at least distances, and grows weaker and weaker accordingly as the rays pass by at distances greater and greater, as is represented in the scheme: for thence it comes to pass, that the shadow of the hair is much broader in proportion to the distance of the paper from the hair, when the paper is nearer the hair, than when it is at a great distance from it. _obs._ . the shadows of all bodies (metals, stones, glass, wood, horn, ice, &c.) in this light were border'd with three parallel fringes or bands of colour'd light, whereof that which was contiguous to the shadow was broadest and most luminous, and that which was remotest from it was narrowest, and so faint, as not easily to be visible. it was difficult to distinguish the colours, unless when the light fell very obliquely upon a smooth paper, or some other smooth white body, so as to make them appear much broader than they would otherwise do. and then the colours were plainly visible in this order: the first or innermost fringe was violet and deep blue next the shadow, and then light blue, green, and yellow in the middle, and red without. the second fringe was almost contiguous to the first, and the third to the second, and both were blue within, and yellow and red without, but their colours were very faint, especially those of the third. the colours therefore proceeded in this order from the shadow; violet, indigo, pale blue, green, yellow, red; blue, yellow, red; pale blue, pale yellow and red. the shadows made by scratches and bubbles in polish'd plates of glass were border'd with the like fringes of colour'd light. and if plates of looking-glass sloop'd off near the edges with a diamond-cut, be held in the same beam of light, the light which passes through the parallel planes of the glass will be border'd with the like fringes of colours where those planes meet with the diamond-cut, and by this means there will sometimes appear four or five fringes of colours. let ab, cd [in _fig._ .] represent the parallel planes of a looking-glass, and bd the plane of the diamond-cut, making at b a very obtuse angle with the plane ab. and let all the light between the rays eni and fbm pass directly through the parallel planes of the glass, and fall upon the paper between i and m, and all the light between the rays go and hd be refracted by the oblique plane of the diamond-cut bd, and fall upon the paper between k and l; and the light which passes directly through the parallel planes of the glass, and falls upon the paper between i and m, will be border'd with three or more fringes at m. [illustration: fig. .] [illustration: fig. .] so by looking on the sun through a feather or black ribband held close to the eye, several rain-bows will appear; the shadows which the fibres or threds cast on the _tunica retina_, being border'd with the like fringes of colours. _obs._ . when the hair was twelve feet distant from this hole, and its shadow fell obliquely upon a flat white scale of inches and parts of an inch placed half a foot beyond it, and also when the shadow fell perpendicularly upon the same scale placed nine feet beyond it; i measured the breadth of the shadow and fringes as accurately as i could, and found them in parts of an inch as follows. -------------------------------------------+-----------+-------- | half a | nine at the distance of | foot | feet -------------------------------------------+-----------+-------- the breadth of the shadow | / | / -------------------------------------------+-----------+-------- the breadth between the middles of the | / | brightest light of the innermost fringes | or | on either side the shadow | / | / -------------------------------------------+-----------+-------- the breadth between the middles of the | | brightest light of the middlemost fringes| | on either side the shadow | / - / | / -------------------------------------------+-----------+-------- the breadth between the middles of the | / | brightest light of the outmost fringes | or | on either side the shadow | / - / | / -------------------------------------------+-----------+-------- the distance between the middles of the | | brightest light of the first and second | | fringes | / | / -------------------------------------------+-----------+-------- the distance between the middles of the | | brightest light of the second and third | | fringes | / | / -------------------------------------------+-----------+-------- the breadth of the luminous part (green, | | white, yellow, and red) of the first | | fringe | / | / -------------------------------------------+-----------+-------- the breadth of the darker space between | | the first and second fringes | / | / -------------------------------------------+-----------+-------- the breadth of the luminous part of the | | second fringe | / | / -------------------------------------------+-----------+-------- the breadth of the darker space between | | the second and third fringes | / | / -------------------------------------------+-----------+-------- these measures i took by letting the shadow of the hair, at half a foot distance, fall so obliquely on the scale, as to appear twelve times broader than when it fell perpendicularly on it at the same distance, and setting down in this table the twelfth part of the measures i then took. _obs._ . when the shadow and fringes were cast obliquely upon a smooth white body, and that body was removed farther and farther from the hair, the first fringe began to appear and look brighter than the rest of the light at the distance of less than a quarter of an inch from the hair, and the dark line or shadow between that and the second fringe began to appear at a less distance from the hair than that of the third part of an inch. the second fringe began to appear at a distance from the hair of less than half an inch, and the shadow between that and the third fringe at a distance less than an inch, and the third fringe at a distance less than three inches. at greater distances they became much more sensible, but kept very nearly the same proportion of their breadths and intervals which they had at their first appearing. for the distance between the middle of the first, and middle of the second fringe, was to the distance between the middle of the second and middle of the third fringe, as three to two, or ten to seven. and the last of these two distances was equal to the breadth of the bright light or luminous part of the first fringe. and this breadth was to the breadth of the bright light of the second fringe as seven to four, and to the dark interval of the first and second fringe as three to two, and to the like dark interval between the second and third as two to one. for the breadths of the fringes seem'd to be in the progression of the numbers , sqrt( / ), sqrt( / ), and their intervals to be in the same progression with them; that is, the fringes and their intervals together to be in the continual progression of the numbers , sqrt( / ), sqrt( / ), sqrt( / ), sqrt( / ), or thereabouts. and these proportions held the same very nearly at all distances from the hair; the dark intervals of the fringes being as broad in proportion to the breadth of the fringes at their first appearance as afterwards at great distances from the hair, though not so dark and distinct. _obs._ . the sun shining into my darken'd chamber through a hole a quarter of an inch broad, i placed at the distance of two or three feet from the hole a sheet of pasteboard, which was black'd all over on both sides, and in the middle of it had a hole about three quarters of an inch square for the light to pass through. and behind the hole i fasten'd to the pasteboard with pitch the blade of a sharp knife, to intercept some part of the light which passed through the hole. the planes of the pasteboard and blade of the knife were parallel to one another, and perpendicular to the rays. and when they were so placed that none of the sun's light fell on the pasteboard, but all of it passed through the hole to the knife, and there part of it fell upon the blade of the knife, and part of it passed by its edge; i let this part of the light which passed by, fall on a white paper two or three feet beyond the knife, and there saw two streams of faint light shoot out both ways from the beam of light into the shadow, like the tails of comets. but because the sun's direct light by its brightness upon the paper obscured these faint streams, so that i could scarce see them, i made a little hole in the midst of the paper for that light to pass through and fall on a black cloth behind it; and then i saw the two streams plainly. they were like one another, and pretty nearly equal in length, and breadth, and quantity of light. their light at that end next the sun's direct light was pretty strong for the space of about a quarter of an inch, or half an inch, and in all its progress from that direct light decreased gradually till it became insensible. the whole length of either of these streams measured upon the paper at the distance of three feet from the knife was about six or eight inches; so that it subtended an angle at the edge of the knife of about or , or at most degrees. yet sometimes i thought i saw it shoot three or four degrees farther, but with a light so very faint that i could scarce perceive it, and suspected it might (in some measure at least) arise from some other cause than the two streams did. for placing my eye in that light beyond the end of that stream which was behind the knife, and looking towards the knife, i could see a line of light upon its edge, and that not only when my eye was in the line of the streams, but also when it was without that line either towards the point of the knife, or towards the handle. this line of light appear'd contiguous to the edge of the knife, and was narrower than the light of the innermost fringe, and narrowest when my eye was farthest from the direct light, and therefore seem'd to pass between the light of that fringe and the edge of the knife, and that which passed nearest the edge to be most bent, though not all of it. _obs._ . i placed another knife by this, so that their edges might be parallel, and look towards one another, and that the beam of light might fall upon both the knives, and some part of it pass between their edges. and when the distance of their edges was about the th part of an inch, the stream parted in the middle, and left a shadow between the two parts. this shadow was so black and dark that all the light which passed between the knives seem'd to be bent, and turn'd aside to the one hand or to the other. and as the knives still approach'd one another the shadow grew broader, and the streams shorter at their inward ends which were next the shadow, until upon the contact of the knives the whole light vanish'd, leaving its place to the shadow. and hence i gather that the light which is least bent, and goes to the inward ends of the streams, passes by the edges of the knives at the greatest distance, and this distance when the shadow begins to appear between the streams, is about the th part of an inch. and the light which passes by the edges of the knives at distances still less and less, is more and more bent, and goes to those parts of the streams which are farther and farther from the direct light; because when the knives approach one another till they touch, those parts of the streams vanish last which are farthest from the direct light. _obs._ . in the fifth observation the fringes did not appear, but by reason of the breadth of the hole in the window became so broad as to run into one another, and by joining, to make one continued light in the beginning of the streams. but in the sixth, as the knives approached one another, a little before the shadow appeared between the two streams, the fringes began to appear on the inner ends of the streams on either side of the direct light; three on one side made by the edge of one knife, and three on the other side made by the edge of the other knife. they were distinctest when the knives were placed at the greatest distance from the hole in the window, and still became more distinct by making the hole less, insomuch that i could sometimes see a faint lineament of a fourth fringe beyond the three above mention'd. and as the knives continually approach'd one another, the fringes grew distincter and larger, until they vanish'd. the outmost fringe vanish'd first, and the middlemost next, and the innermost last. and after they were all vanish'd, and the line of light which was in the middle between them was grown very broad, enlarging it self on both sides into the streams of light described in the fifth observation, the above-mention'd shadow began to appear in the middle of this line, and divide it along the middle into two lines of light, and increased until the whole light vanish'd. this enlargement of the fringes was so great that the rays which go to the innermost fringe seem'd to be bent above twenty times more when this fringe was ready to vanish, than when one of the knives was taken away. and from this and the former observation compared, i gather, that the light of the first fringe passed by the edge of the knife at a distance greater than the th part of an inch, and the light of the second fringe passed by the edge of the knife at a greater distance than the light of the first fringe did, and that of the third at a greater distance than that of the second, and that of the streams of light described in the fifth and sixth observations passed by the edges of the knives at less distances than that of any of the fringes. _obs._ . i caused the edges of two knives to be ground truly strait, and pricking their points into a board so that their edges might look towards one another, and meeting near their points contain a rectilinear angle, i fasten'd their handles together with pitch to make this angle invariable. the distance of the edges of the knives from one another at the distance of four inches from the angular point, where the edges of the knives met, was the eighth part of an inch; and therefore the angle contain'd by the edges was about one degree : the knives thus fix'd together i placed in a beam of the sun's light, let into my darken'd chamber through a hole the d part of an inch wide, at the distance of or feet from the hole, and let the light which passed between their edges fall very obliquely upon a smooth white ruler at the distance of half an inch, or an inch from the knives, and there saw the fringes by the two edges of the knives run along the edges of the shadows of the knives in lines parallel to those edges without growing sensibly broader, till they met in angles equal to the angle contained by the edges of the knives, and where they met and joined they ended without crossing one another. but if the ruler was held at a much greater distance from the knives, the fringes where they were farther from the place of their meeting, were a little narrower, and became something broader and broader as they approach'd nearer and nearer to one another, and after they met they cross'd one another, and then became much broader than before. whence i gather that the distances at which the fringes pass by the knives are not increased nor alter'd by the approach of the knives, but the angles in which the rays are there bent are much increased by that approach; and that the knife which is nearest any ray determines which way the ray shall be bent, and the other knife increases the bent. _obs._ . when the rays fell very obliquely upon the ruler at the distance of the third part of an inch from the knives, the dark line between the first and second fringe of the shadow of one knife, and the dark line between the first and second fringe of the shadow of the other knife met with one another, at the distance of the fifth part of an inch from the end of the light which passed between the knives at the concourse of their edges. and therefore the distance of the edges of the knives at the meeting of these dark lines was the th part of an inch. for as four inches to the eighth part of an inch, so is any length of the edges of the knives measured from the point of their concourse to the distance of the edges of the knives at the end of that length, and so is the fifth part of an inch to the th part. so then the dark lines above-mention'd meet in the middle of the light which passes between the knives where they are distant the th part of an inch, and the one half of that light passes by the edge of one knife at a distance not greater than the th part of an inch, and falling upon the paper makes the fringes of the shadow of that knife, and the other half passes by the edge of the other knife, at a distance not greater than the th part of an inch, and falling upon the paper makes the fringes of the shadow of the other knife. but if the paper be held at a distance from the knives greater than the third part of an inch, the dark lines above-mention'd meet at a greater distance than the fifth part of an inch from the end of the light which passed between the knives at the concourse of their edges; and therefore the light which falls upon the paper where those dark lines meet passes between the knives where the edges are distant above the th part of an inch. for at another time, when the two knives were distant eight feet and five inches from the little hole in the window, made with a small pin as above, the light which fell upon the paper where the aforesaid dark lines met, passed between the knives, where the distance between their edges was as in the following table, when the distance of the paper from the knives was also as follows. -----------------------------+------------------------------ | distances between the edges distances of the paper | of the knives in millesimal from the knives in inches. | parts of an inch. -----------------------------+------------------------------ - / . | ' - / . | ' - / . | ' . | ' . | ' . | ' _____________________________|______________________________ and hence i gather, that the light which makes the fringes upon the paper is not the same light at all distances of the paper from the knives, but when the paper is held near the knives, the fringes are made by light which passes by the edges of the knives at a less distance, and is more bent than when the paper is held at a greater distance from the knives. [illustration: fig. .] _obs._ . when the fringes of the shadows of the knives fell perpendicularly upon a paper at a great distance from the knives, they were in the form of hyperbola's, and their dimensions were as follows. let ca, cb [in _fig._ .] represent lines drawn upon the paper parallel to the edges of the knives, and between which all the light would fall, if it passed between the edges of the knives without inflexion; de a right line drawn through c making the angles acd, bce, equal to one another, and terminating all the light which falls upon the paper from the point where the edges of the knives meet; _eis_, _fkt_, and _glv_, three hyperbolical lines representing the terminus of the shadow of one of the knives, the dark line between the first and second fringes of that shadow, and the dark line between the second and third fringes of the same shadow; _xip_, _ykq_, and _zlr_, three other hyperbolical lines representing the terminus of the shadow of the other knife, the dark line between the first and second fringes of that shadow, and the dark line between the second and third fringes of the same shadow. and conceive that these three hyperbola's are like and equal to the former three, and cross them in the points _i_, _k_, and _l_, and that the shadows of the knives are terminated and distinguish'd from the first luminous fringes by the lines _eis_ and _xip_, until the meeting and crossing of the fringes, and then those lines cross the fringes in the form of dark lines, terminating the first luminous fringes within side, and distinguishing them from another light which begins to appear at _i_, and illuminates all the triangular space _ip_de_s_ comprehended by these dark lines, and the right line de. of these hyperbola's one asymptote is the line de, and their other asymptotes are parallel to the lines ca and cb. let _rv_ represent a line drawn any where upon the paper parallel to the asymptote de, and let this line cross the right lines ac in _m_, and bc in _n_, and the six dark hyperbolical lines in _p_, _q_, _r_; _s_, _t_, _v_; and by measuring the distances _ps_, _qt_, _rv_, and thence collecting the lengths of the ordinates _np_, _nq_, _nr_ or _ms_, _mt_, _mv_, and doing this at several distances of the line _rv_ from the asymptote dd, you may find as many points of these hyperbola's as you please, and thereby know that these curve lines are hyperbola's differing little from the conical hyperbola. and by measuring the lines c_i_, c_k_, c_l_, you may find other points of these curves. for instance; when the knives were distant from the hole in the window ten feet, and the paper from the knives nine feet, and the angle contained by the edges of the knives to which the angle acb is equal, was subtended by a chord which was to the radius as to , and the distance of the line _rv_ from the asymptote de was half an inch: i measured the lines _ps_, _qt_, _rv_, and found them ' , ' , ' inches respectively; and by adding to their halfs the line / _mn_, (which here was the th part of an inch, or ' inches,) the sums _np_, _nq_, _nr_, were ' , ' , ' inches. i measured also the distances of the brightest parts of the fringes which run between _pq_ and _st_, _qr_ and _tv_, and next beyond _r_ and _v_, and found them ' , ' , and ' inches. _obs._ . the sun shining into my darken'd room through a small round hole made in a plate of lead with a slender pin, as above; i placed at the hole a prism to refract the light, and form on the opposite wall the spectrum of colours, described in the third experiment of the first book. and then i found that the shadows of all bodies held in the colour'd light between the prism and the wall, were border'd with fringes of the colour of that light in which they were held. in the full red light they were totally red without any sensible blue or violet, and in the deep blue light they were totally blue without any sensible red or yellow; and so in the green light they were totally green, excepting a little yellow and blue, which were mixed in the green light of the prism. and comparing the fringes made in the several colour'd lights, i found that those made in the red light were largest, those made in the violet were least, and those made in the green were of a middle bigness. for the fringes with which the shadow of a man's hair were bordered, being measured cross the shadow at the distance of six inches from the hair, the distance between the middle and most luminous part of the first or innermost fringe on one side of the shadow, and that of the like fringe on the other side of the shadow, was in the full red light / - / of an inch, and in the full violet / . and the like distance between the middle and most luminous parts of the second fringes on either side the shadow was in the full red light / , and in the violet / of an inch. and these distances of the fringes held the same proportion at all distances from the hair without any sensible variation. so then the rays which made these fringes in the red light passed by the hair at a greater distance than those did which made the like fringes in the violet; and therefore the hair in causing these fringes acted alike upon the red light or least refrangible rays at a greater distance, and upon the violet or most refrangible rays at a less distance, and by those actions disposed the red light into larger fringes, and the violet into smaller, and the lights of intermediate colours into fringes of intermediate bignesses without changing the colour of any sort of light. when therefore the hair in the first and second of these observations was held in the white beam of the sun's light, and cast a shadow which was border'd with three fringes of coloured light, those colours arose not from any new modifications impress'd upon the rays of light by the hair, but only from the various inflexions whereby the several sorts of rays were separated from one another, which before separation, by the mixture of all their colours, composed the white beam of the sun's light, but whenever separated compose lights of the several colours which they are originally disposed to exhibit. in this th observation, where the colours are separated before the light passes by the hair, the least refrangible rays, which when separated from the rest make red, were inflected at a greater distance from the hair, so as to make three red fringes at a greater distance from the middle of the shadow of the hair; and the most refrangible rays which when separated make violet, were inflected at a less distance from the hair, so as to make three violet fringes at a less distance from the middle of the shadow of the hair. and other rays of intermediate degrees of refrangibility were inflected at intermediate distances from the hair, so as to make fringes of intermediate colours at intermediate distances from the middle of the shadow of the hair. and in the second observation, where all the colours are mix'd in the white light which passes by the hair, these colours are separated by the various inflexions of the rays, and the fringes which they make appear all together, and the innermost fringes being contiguous make one broad fringe composed of all the colours in due order, the violet lying on the inside of the fringe next the shadow, the red on the outside farthest from the shadow, and the blue, green, and yellow, in the middle. and, in like manner, the middlemost fringes of all the colours lying in order, and being contiguous, make another broad fringe composed of all the colours; and the outmost fringes of all the colours lying in order, and being contiguous, make a third broad fringe composed of all the colours. these are the three fringes of colour'd light with which the shadows of all bodies are border'd in the second observation. when i made the foregoing observations, i design'd to repeat most of them with more care and exactness, and to make some new ones for determining the manner how the rays of light are bent in their passage by bodies, for making the fringes of colours with the dark lines between them. but i was then interrupted, and cannot now think of taking these things into farther consideration. and since i have not finish'd this part of my design, i shall conclude with proposing only some queries, in order to a farther search to be made by others. _query_ . do not bodies act upon light at a distance, and by their action bend its rays; and is not this action (_cæteris paribus_) strongest at the least distance? _qu._ . do not the rays which differ in refrangibility differ also in flexibity; and are they not by their different inflexions separated from one another, so as after separation to make the colours in the three fringes above described? and after what manner are they inflected to make those fringes? _qu._ . are not the rays of light in passing by the edges and sides of bodies, bent several times backwards and forwards, with a motion like that of an eel? and do not the three fringes of colour'd light above-mention'd arise from three such bendings? _qu._ . do not the rays of light which fall upon bodies, and are reflected or refracted, begin to bend before they arrive at the bodies; and are they not reflected, refracted, and inflected, by one and the same principle, acting variously in various circumstances? _qu._ . do not bodies and light act mutually upon one another; that is to say, bodies upon light in emitting, reflecting, refracting and inflecting it, and light upon bodies for heating them, and putting their parts into a vibrating motion wherein heat consists? _qu._ . do not black bodies conceive heat more easily from light than those of other colours do, by reason that the light falling on them is not reflected outwards, but enters the bodies, and is often reflected and refracted within them, until it be stifled and lost? _qu._ . is not the strength and vigor of the action between light and sulphureous bodies observed above, one reason why sulphureous bodies take fire more readily, and burn more vehemently than other bodies do? _qu._ . do not all fix'd bodies, when heated beyond a certain degree, emit light and shine; and is not this emission perform'd by the vibrating motions of their parts? and do not all bodies which abound with terrestrial parts, and especially with sulphureous ones, emit light as often as those parts are sufficiently agitated; whether that agitation be made by heat, or by friction, or percussion, or putrefaction, or by any vital motion, or any other cause? as for instance; sea-water in a raging storm; quick-silver agitated in _vacuo_; the back of a cat, or neck of a horse, obliquely struck or rubbed in a dark place; wood, flesh and fish while they putrefy; vapours arising from putrefy'd waters, usually call'd _ignes fatui_; stacks of moist hay or corn growing hot by fermentation; glow-worms and the eyes of some animals by vital motions; the vulgar _phosphorus_ agitated by the attrition of any body, or by the acid particles of the air; amber and some diamonds by striking, pressing or rubbing them; scrapings of steel struck off with a flint; iron hammer'd very nimbly till it become so hot as to kindle sulphur thrown upon it; the axletrees of chariots taking fire by the rapid rotation of the wheels; and some liquors mix'd with one another whose particles come together with an impetus, as oil of vitriol distilled from its weight of nitre, and then mix'd with twice its weight of oil of anniseeds. so also a globe of glass about or inches in diameter, being put into a frame where it may be swiftly turn'd round its axis, will in turning shine where it rubs against the palm of ones hand apply'd to it: and if at the same time a piece of white paper or white cloth, or the end of ones finger be held at the distance of about a quarter of an inch or half an inch from that part of the glass where it is most in motion, the electrick vapour which is excited by the friction of the glass against the hand, will by dashing against the white paper, cloth or finger, be put into such an agitation as to emit light, and make the white paper, cloth or finger, appear lucid like a glowworm; and in rushing out of the glass will sometimes push against the finger so as to be felt. and the same things have been found by rubbing a long and large cylinder or glass or amber with a paper held in ones hand, and continuing the friction till the glass grew warm. _qu._ . is not fire a body heated so hot as to emit light copiously? for what else is a red hot iron than fire? and what else is a burning coal than red hot wood? _qu._ . is not flame a vapour, fume or exhalation heated red hot, that is, so hot as to shine? for bodies do not flame without emitting a copious fume, and this fume burns in the flame. the _ignis fatuus_ is a vapour shining without heat, and is there not the same difference between this vapour and flame, as between rotten wood shining without heat and burning coals of fire? in distilling hot spirits, if the head of the still be taken off, the vapour which ascends out of the still will take fire at the flame of a candle, and turn into flame, and the flame will run along the vapour from the candle to the still. some bodies heated by motion, or fermentation, if the heat grow intense, fume copiously, and if the heat be great enough the fumes will shine and become flame. metals in fusion do not flame for want of a copious fume, except spelter, which fumes copiously, and thereby flames. all flaming bodies, as oil, tallow, wax, wood, fossil coals, pitch, sulphur, by flaming waste and vanish into burning smoke, which smoke, if the flame be put out, is very thick and visible, and sometimes smells strongly, but in the flame loses its smell by burning, and according to the nature of the smoke the flame is of several colours, as that of sulphur blue, that of copper open'd with sublimate green, that of tallow yellow, that of camphire white. smoke passing through flame cannot but grow red hot, and red hot smoke can have no other appearance than that of flame. when gun-powder takes fire, it goes away into flaming smoke. for the charcoal and sulphur easily take fire, and set fire to the nitre, and the spirit of the nitre being thereby rarified into vapour, rushes out with explosion much after the manner that the vapour of water rushes out of an Æolipile; the sulphur also being volatile is converted into vapour, and augments the explosion. and the acid vapour of the sulphur (namely that which distils under a bell into oil of sulphur,) entring violently into the fix'd body of the nitre, sets loose the spirit of the nitre, and excites a great fermentation, whereby the heat is farther augmented, and the fix'd body of the nitre is also rarified into fume, and the explosion is thereby made more vehement and quick. for if salt of tartar be mix'd with gun-powder, and that mixture be warm'd till it takes fire, the explosion will be more violent and quick than that of gun-powder alone; which cannot proceed from any other cause than the action of the vapour of the gun-powder upon the salt of tartar, whereby that salt is rarified. the explosion of gun-powder arises therefore from the violent action whereby all the mixture being quickly and vehemently heated, is rarified and converted into fume and vapour: which vapour, by the violence of that action, becoming so hot as to shine, appears in the form of flame. _qu._ . do not great bodies conserve their heat the longest, their parts heating one another, and may not great dense and fix'd bodies, when heated beyond a certain degree, emit light so copiously, as by the emission and re-action of its light, and the reflexions and refractions of its rays within its pores to grow still hotter, till it comes to a certain period of heat, such as is that of the sun? and are not the sun and fix'd stars great earths vehemently hot, whose heat is conserved by the greatness of the bodies, and the mutual action and reaction between them, and the light which they emit, and whose parts are kept from fuming away, not only by their fixity, but also by the vast weight and density of the atmospheres incumbent upon them; and very strongly compressing them, and condensing the vapours and exhalations which arise from them? for if water be made warm in any pellucid vessel emptied of air, that water in the _vacuum_ will bubble and boil as vehemently as it would in the open air in a vessel set upon the fire till it conceives a much greater heat. for the weight of the incumbent atmosphere keeps down the vapours, and hinders the water from boiling, until it grow much hotter than is requisite to make it boil _in vacuo_. also a mixture of tin and lead being put upon a red hot iron _in vacuo_ emits a fume and flame, but the same mixture in the open air, by reason of the incumbent atmosphere, does not so much as emit any fume which can be perceived by sight. in like manner the great weight of the atmosphere which lies upon the globe of the sun may hinder bodies there from rising up and going away from the sun in the form of vapours and fumes, unless by means of a far greater heat than that which on the surface of our earth would very easily turn them into vapours and fumes. and the same great weight may condense those vapours and exhalations as soon as they shall at any time begin to ascend from the sun, and make them presently fall back again into him, and by that action increase his heat much after the manner that in our earth the air increases the heat of a culinary fire. and the same weight may hinder the globe of the sun from being diminish'd, unless by the emission of light, and a very small quantity of vapours and exhalations. _qu._ . do not the rays of light in falling upon the bottom of the eye excite vibrations in the _tunica retina_? which vibrations, being propagated along the solid fibres of the optick nerves into the brain, cause the sense of seeing. for because dense bodies conserve their heat a long time, and the densest bodies conserve their heat the longest, the vibrations of their parts are of a lasting nature, and therefore may be propagated along solid fibres of uniform dense matter to a great distance, for conveying into the brain the impressions made upon all the organs of sense. for that motion which can continue long in one and the same part of a body, can be propagated a long way from one part to another, supposing the body homogeneal, so that the motion may not be reflected, refracted, interrupted or disorder'd by any unevenness of the body. _qu._ . do not several sorts of rays make vibrations of several bignesses, which according to their bignesses excite sensations of several colours, much after the manner that the vibrations of the air, according to their several bignesses excite sensations of several sounds? and particularly do not the most refrangible rays excite the shortest vibrations for making a sensation of deep violet, the least refrangible the largest for making a sensation of deep red, and the several intermediate sorts of rays, vibrations of several intermediate bignesses to make sensations of the several intermediate colours? _qu._ . may not the harmony and discord of colours arise from the proportions of the vibrations propagated through the fibres of the optick nerves into the brain, as the harmony and discord of sounds arise from the proportions of the vibrations of the air? for some colours, if they be view'd together, are agreeable to one another, as those of gold and indigo, and others disagree. _qu._ . are not the species of objects seen with both eyes united where the optick nerves meet before they come into the brain, the fibres on the right side of both nerves uniting there, and after union going thence into the brain in the nerve which is on the right side of the head, and the fibres on the left side of both nerves uniting in the same place, and after union going into the brain in the nerve which is on the left side of the head, and these two nerves meeting in the brain in such a manner that their fibres make but one entire species or picture, half of which on the right side of the sensorium comes from the right side of both eyes through the right side of both optick nerves to the place where the nerves meet, and from thence on the right side of the head into the brain, and the other half on the left side of the sensorium comes in like manner from the left side of both eyes. for the optick nerves of such animals as look the same way with both eyes (as of men, dogs, sheep, oxen, &c.) meet before they come into the brain, but the optick nerves of such animals as do not look the same way with both eyes (as of fishes, and of the chameleon,) do not meet, if i am rightly inform'd. _qu._ . when a man in the dark presses either corner of his eye with his finger, and turns his eye away from his finger, he will see a circle of colours like those in the feather of a peacock's tail. if the eye and the finger remain quiet these colours vanish in a second minute of time, but if the finger be moved with a quavering motion they appear again. do not these colours arise from such motions excited in the bottom of the eye by the pressure and motion of the finger, as, at other times are excited there by light for causing vision? and do not the motions once excited continue about a second of time before they cease? and when a man by a stroke upon his eye sees a flash of light, are not the like motions excited in the _retina_ by the stroke? and when a coal of fire moved nimbly in the circumference of a circle, makes the whole circumference appear like a circle of fire; is it not because the motions excited in the bottom of the eye by the rays of light are of a lasting nature, and continue till the coal of fire in going round returns to its former place? and considering the lastingness of the motions excited in the bottom of the eye by light, are they not of a vibrating nature? _qu._ . if a stone be thrown into stagnating water, the waves excited thereby continue some time to arise in the place where the stone fell into the water, and are propagated from thence in concentrick circles upon the surface of the water to great distances. and the vibrations or tremors excited in the air by percussion, continue a little time to move from the place of percussion in concentrick spheres to great distances. and in like manner, when a ray of light falls upon the surface of any pellucid body, and is there refracted or reflected, may not waves of vibrations, or tremors, be thereby excited in the refracting or reflecting medium at the point of incidence, and continue to arise there, and to be propagated from thence as long as they continue to arise and be propagated, when they are excited in the bottom of the eye by the pressure or motion of the finger, or by the light which comes from the coal of fire in the experiments above-mention'd? and are not these vibrations propagated from the point of incidence to great distances? and do they not overtake the rays of light, and by overtaking them successively, do they not put them into the fits of easy reflexion and easy transmission described above? for if the rays endeavour to recede from the densest part of the vibration, they may be alternately accelerated and retarded by the vibrations overtaking them. _qu._ . if in two large tall cylindrical vessels of glass inverted, two little thermometers be suspended so as not to touch the vessels, and the air be drawn out of one of these vessels, and these vessels thus prepared be carried out of a cold place into a warm one; the thermometer _in vacuo_ will grow warm as much, and almost as soon as the thermometer which is not _in vacuo_. and when the vessels are carried back into the cold place, the thermometer _in vacuo_ will grow cold almost as soon as the other thermometer. is not the heat of the warm room convey'd through the _vacuum_ by the vibrations of a much subtiler medium than air, which after the air was drawn out remained in the _vacuum_? and is not this medium the same with that medium by which light is refracted and reflected, and by whose vibrations light communicates heat to bodies, and is put into fits of easy reflexion and easy transmission? and do not the vibrations of this medium in hot bodies contribute to the intenseness and duration of their heat? and do not hot bodies communicate their heat to contiguous cold ones, by the vibrations of this medium propagated from them into the cold ones? and is not this medium exceedingly more rare and subtile than the air, and exceedingly more elastick and active? and doth it not readily pervade all bodies? and is it not (by its elastick force) expanded through all the heavens? _qu._ . doth not the refraction of light proceed from the different density of this Æthereal medium in different places, the light receding always from the denser parts of the medium? and is not the density thereof greater in free and open spaces void of air and other grosser bodies, than within the pores of water, glass, crystal, gems, and other compact bodies? for when light passes through glass or crystal, and falling very obliquely upon the farther surface thereof is totally reflected, the total reflexion ought to proceed rather from the density and vigour of the medium without and beyond the glass, than from the rarity and weakness thereof. _qu._ . doth not this Æthereal medium in passing out of water, glass, crystal, and other compact and dense bodies into empty spaces, grow denser and denser by degrees, and by that means refract the rays of light not in a point, but by bending them gradually in curve lines? and doth not the gradual condensation of this medium extend to some distance from the bodies, and thereby cause the inflexions of the rays of light, which pass by the edges of dense bodies, at some distance from the bodies? _qu._ . is not this medium much rarer within the dense bodies of the sun, stars, planets and comets, than in the empty celestial spaces between them? and in passing from them to great distances, doth it not grow denser and denser perpetually, and thereby cause the gravity of those great bodies towards one another, and of their parts towards the bodies; every body endeavouring to go from the denser parts of the medium towards the rarer? for if this medium be rarer within the sun's body than at its surface, and rarer there than at the hundredth part of an inch from its body, and rarer there than at the fiftieth part of an inch from its body, and rarer there than at the orb of _saturn_; i see no reason why the increase of density should stop any where, and not rather be continued through all distances from the sun to _saturn_, and beyond. and though this increase of density may at great distances be exceeding slow, yet if the elastick force of this medium be exceeding great, it may suffice to impel bodies from the denser parts of the medium towards the rarer, with all that power which we call gravity. and that the elastick force of this medium is exceeding great, may be gather'd from the swiftness of its vibrations. sounds move about _english_ feet in a second minute of time, and in seven or eight minutes of time they move about one hundred _english_ miles. light moves from the sun to us in about seven or eight minutes of time, which distance is about , , _english_ miles, supposing the horizontal parallax of the sun to be about ´´. and the vibrations or pulses of this medium, that they may cause the alternate fits of easy transmission and easy reflexion, must be swifter than light, and by consequence above , times swifter than sounds. and therefore the elastick force of this medium, in proportion to its density, must be above x (that is, above , , , ) times greater than the elastick force of the air is in proportion to its density. for the velocities of the pulses of elastick mediums are in a subduplicate _ratio_ of the elasticities and the rarities of the mediums taken together. as attraction is stronger in small magnets than in great ones in proportion to their bulk, and gravity is greater in the surfaces of small planets than in those of great ones in proportion to their bulk, and small bodies are agitated much more by electric attraction than great ones; so the smallness of the rays of light may contribute very much to the power of the agent by which they are refracted. and so if any one should suppose that _Æther_ (like our air) may contain particles which endeavour to recede from one another (for i do not know what this _Æther_ is) and that its particles are exceedingly smaller than those of air, or even than those of light: the exceeding smallness of its particles may contribute to the greatness of the force by which those particles may recede from one another, and thereby make that medium exceedingly more rare and elastick than air, and by consequence exceedingly less able to resist the motions of projectiles, and exceedingly more able to press upon gross bodies, by endeavouring to expand it self. _qu._ . may not planets and comets, and all gross bodies, perform their motions more freely, and with less resistance in this Æthereal medium than in any fluid, which fills all space adequately without leaving any pores, and by consequence is much denser than quick-silver or gold? and may not its resistance be so small, as to be inconsiderable? for instance; if this _Æther_ (for so i will call it) should be supposed times more elastick than our air, and above times more rare; its resistance would be above , , times less than that of water. and so small a resistance would scarce make any sensible alteration in the motions of the planets in ten thousand years. if any one would ask how a medium can be so rare, let him tell me how the air, in the upper parts of the atmosphere, can be above an hundred thousand thousand times rarer than gold. let him also tell me, how an electrick body can by friction emit an exhalation so rare and subtile, and yet so potent, as by its emission to cause no sensible diminution of the weight of the electrick body, and to be expanded through a sphere, whose diameter is above two feet, and yet to be able to agitate and carry up leaf copper, or leaf gold, at the distance of above a foot from the electrick body? and how the effluvia of a magnet can be so rare and subtile, as to pass through a plate of glass without any resistance or diminution of their force, and yet so potent as to turn a magnetick needle beyond the glass? _qu._ . is not vision perform'd chiefly by the vibrations of this medium, excited in the bottom of the eye by the rays of light, and propagated through the solid, pellucid and uniform capillamenta of the optick nerves into the place of sensation? and is not hearing perform'd by the vibrations either of this or some other medium, excited in the auditory nerves by the tremors of the air, and propagated through the solid, pellucid and uniform capillamenta of those nerves into the place of sensation? and so of the other senses. _qu._ . is not animal motion perform'd by the vibrations of this medium, excited in the brain by the power of the will, and propagated from thence through the solid, pellucid and uniform capillamenta of the nerves into the muscles, for contracting and dilating them? i suppose that the capillamenta of the nerves are each of them solid and uniform, that the vibrating motion of the Æthereal medium may be propagated along them from one end to the other uniformly, and without interruption: for obstructions in the nerves create palsies. and that they may be sufficiently uniform, i suppose them to be pellucid when view'd singly, tho' the reflexions in their cylindrical surfaces may make the whole nerve (composed of many capillamenta) appear opake and white. for opacity arises from reflecting surfaces, such as may disturb and interrupt the motions of this medium. [sidenote: _see the following scheme, p. ._] _qu._ . are there not other original properties of the rays of light, besides those already described? an instance of another original property we have in the refraction of island crystal, described first by _erasmus bartholine_, and afterwards more exactly by _hugenius_, in his book _de la lumiere_. this crystal is a pellucid fissile stone, clear as water or crystal of the rock, and without colour; enduring a red heat without losing its transparency, and in a very strong heat calcining without fusion. steep'd a day or two in water, it loses its natural polish. being rubb'd on cloth, it attracts pieces of straws and other light things, like ambar or glass; and with _aqua fortis_ it makes an ebullition. it seems to be a sort of talk, and is found in form of an oblique parallelopiped, with six parallelogram sides and eight solid angles. the obtuse angles of the parallelograms are each of them degrees and minutes; the acute ones degrees and minutes. two of the solid angles opposite to one another, as c and e, are compassed each of them with three of these obtuse angles, and each of the other six with one obtuse and two acute ones. it cleaves easily in planes parallel to any of its sides, and not in any other planes. it cleaves with a glossy polite surface not perfectly plane, but with some little unevenness. it is easily scratch'd, and by reason of its softness it takes a polish very difficultly. it polishes better upon polish'd looking-glass than upon metal, and perhaps better upon pitch, leather or parchment. afterwards it must be rubb'd with a little oil or white of an egg, to fill up its scratches; whereby it will become very transparent and polite. but for several experiments, it is not necessary to polish it. if a piece of this crystalline stone be laid upon a book, every letter of the book seen through it will appear double, by means of a double refraction. and if any beam of light falls either perpendicularly, or in any oblique angle upon any surface of this crystal, it becomes divided into two beams by means of the same double refraction. which beams are of the same colour with the incident beam of light, and seem equal to one another in the quantity of their light, or very nearly equal. one of these refractions is perform'd by the usual rule of opticks, the sine of incidence out of air into this crystal being to the sine of refraction, as five to three. the other refraction, which may be called the unusual refraction, is perform'd by the following rule. [illustration: fig. .] let adbc represent the refracting surface of the crystal, c the biggest solid angle at that surface, gehf the opposite surface, and ck a perpendicular on that surface. this perpendicular makes with the edge of the crystal cf, an angle of degr. '. join kf, and in it take kl, so that the angle kcl be degr. '. and the angle lcf degr. '. and if st represent any beam of light incident at t in any angle upon the refracting surface adbc, let tv be the refracted beam determin'd by the given portion of the sines to , according to the usual rule of opticks. draw vx parallel and equal to kl. draw it the same way from v in which l lieth from k; and joining tx, this line tx shall be the other refracted beam carried from t to x, by the unusual refraction. if therefore the incident beam st be perpendicular to the refracting surface, the two beams tv and tx, into which it shall become divided, shall be parallel to the lines ck and cl; one of those beams going through the crystal perpendicularly, as it ought to do by the usual laws of opticks, and the other tx by an unusual refraction diverging from the perpendicular, and making with it an angle vtx of about - / degrees, as is found by experience. and hence, the plane vtx, and such like planes which are parallel to the plane cfk, may be called the planes of perpendicular refraction. and the coast towards which the lines kl and vx are drawn, may be call'd the coast of unusual refraction. in like manner crystal of the rock has a double refraction: but the difference of the two refractions is not so great and manifest as in island crystal. when the beam st incident on island crystal is divided into two beams tv and tx, and these two beams arrive at the farther surface of the glass; the beam tv, which was refracted at the first surface after the usual manner, shall be again refracted entirely after the usual manner at the second surface; and the beam tx, which was refracted after the unusual manner in the first surface, shall be again refracted entirely after the unusual manner in the second surface; so that both these beams shall emerge out of the second surface in lines parallel to the first incident beam st. and if two pieces of island crystal be placed one after another, in such manner that all the surfaces of the latter be parallel to all the corresponding surfaces of the former: the rays which are refracted after the usual manner in the first surface of the first crystal, shall be refracted after the usual manner in all the following surfaces; and the rays which are refracted after the unusual manner in the first surface, shall be refracted after the unusual manner in all the following surfaces. and the same thing happens, though the surfaces of the crystals be any ways inclined to one another, provided that their planes of perpendicular refraction be parallel to one another. and therefore there is an original difference in the rays of light, by means of which some rays are in this experiment constantly refracted after the usual manner, and others constantly after the unusual manner: for if the difference be not original, but arises from new modifications impress'd on the rays at their first refraction, it would be alter'd by new modifications in the three following refractions; whereas it suffers no alteration, but is constant, and has the same effect upon the rays in all the refractions. the unusual refraction is therefore perform'd by an original property of the rays. and it remains to be enquired, whether the rays have not more original properties than are yet discover'd. _qu._ . have not the rays of light several sides, endued with several original properties? for if the planes of perpendicular refraction of the second crystal be at right angles with the planes of perpendicular refraction of the first crystal, the rays which are refracted after the usual manner in passing through the first crystal, will be all of them refracted after the unusual manner in passing through the second crystal; and the rays which are refracted after the unusual manner in passing through the first crystal, will be all of them refracted after the usual manner in passing through the second crystal. and therefore there are not two sorts of rays differing in their nature from one another, one of which is constantly and in all positions refracted after the usual manner, and the other constantly and in all positions after the unusual manner. the difference between the two sorts of rays in the experiment mention'd in the th question, was only in the positions of the sides of the rays to the planes of perpendicular refraction. for one and the same ray is here refracted sometimes after the usual, and sometimes after the unusual manner, according to the position which its sides have to the crystals. if the sides of the ray are posited the same way to both crystals, it is refracted after the same manner in them both: but if that side of the ray which looks towards the coast of the unusual refraction of the first crystal, be degrees from that side of the same ray which looks toward the coast of the unusual refraction of the second crystal, (which may be effected by varying the position of the second crystal to the first, and by consequence to the rays of light,) the ray shall be refracted after several manners in the several crystals. there is nothing more required to determine whether the rays of light which fall upon the second crystal shall be refracted after the usual or after the unusual manner, but to turn about this crystal, so that the coast of this crystal's unusual refraction may be on this or on that side of the ray. and therefore every ray may be consider'd as having four sides or quarters, two of which opposite to one another incline the ray to be refracted after the unusual manner, as often as either of them are turn'd towards the coast of unusual refraction; and the other two, whenever either of them are turn'd towards the coast of unusual refraction, do not incline it to be otherwise refracted than after the usual manner. the two first may therefore be call'd the sides of unusual refraction. and since these dispositions were in the rays before their incidence on the second, third, and fourth surfaces of the two crystals, and suffered no alteration (so far as appears,) by the refraction of the rays in their passage through those surfaces, and the rays were refracted by the same laws in all the four surfaces; it appears that those dispositions were in the rays originally, and suffer'd no alteration by the first refraction, and that by means of those dispositions the rays were refracted at their incidence on the first surface of the first crystal, some of them after the usual, and some of them after the unusual manner, accordingly as their sides of unusual refraction were then turn'd towards the coast of the unusual refraction of that crystal, or sideways from it. every ray of light has therefore two opposite sides, originally endued with a property on which the unusual refraction depends, and the other two opposite sides not endued with that property. and it remains to be enquired, whether there are not more properties of light by which the sides of the rays differ, and are distinguished from one another. in explaining the difference of the sides of the rays above mention'd, i have supposed that the rays fall perpendicularly on the first crystal. but if they fall obliquely on it, the success is the same. those rays which are refracted after the usual manner in the first crystal, will be refracted after the unusual manner in the second crystal, supposing the planes of perpendicular refraction to be at right angles with one another, as above; and on the contrary. if the planes of the perpendicular refraction of the two crystals be neither parallel nor perpendicular to one another, but contain an acute angle: the two beams of light which emerge out of the first crystal, will be each of them divided into two more at their incidence on the second crystal. for in this case the rays in each of the two beams will some of them have their sides of unusual refraction, and some of them their other sides turn'd towards the coast of the unusual refraction of the second crystal. _qu._ . are not all hypotheses erroneous which have hitherto been invented for explaining the phænomena of light, by new modifications of the rays? for those phænomena depend not upon new modifications, as has been supposed, but upon the original and unchangeable properties of the rays. _qu._ . are not all hypotheses erroneous, in which light is supposed to consist in pression or motion, propagated through a fluid medium? for in all these hypotheses the phænomena of light have been hitherto explain'd by supposing that they arise from new modifications of the rays; which is an erroneous supposition. if light consisted only in pression propagated without actual motion, it would not be able to agitate and heat the bodies which refract and reflect it. if it consisted in motion propagated to all distances in an instant, it would require an infinite force every moment, in every shining particle, to generate that motion. and if it consisted in pression or motion, propagated either in an instant or in time, it would bend into the shadow. for pression or motion cannot be propagated in a fluid in right lines, beyond an obstacle which stops part of the motion, but will bend and spread every way into the quiescent medium which lies beyond the obstacle. gravity tends downwards, but the pressure of water arising from gravity tends every way with equal force, and is propagated as readily, and with as much force sideways as downwards, and through crooked passages as through strait ones. the waves on the surface of stagnating water, passing by the sides of a broad obstacle which stops part of them, bend afterwards and dilate themselves gradually into the quiet water behind the obstacle. the waves, pulses or vibrations of the air, wherein sounds consist, bend manifestly, though not so much as the waves of water. for a bell or a cannon may be heard beyond a hill which intercepts the sight of the sounding body, and sounds are propagated as readily through crooked pipes as through streight ones. but light is never known to follow crooked passages nor to bend into the shadow. for the fix'd stars by the interposition of any of the planets cease to be seen. and so do the parts of the sun by the interposition of the moon, _mercury_ or _venus_. the rays which pass very near to the edges of any body, are bent a little by the action of the body, as we shew'd above; but this bending is not towards but from the shadow, and is perform'd only in the passage of the ray by the body, and at a very small distance from it. so soon as the ray is past the body, it goes right on. [sidenote: _mais pour dire comment cela se fait, je n'ay rien trove jusqu' ici qui me satisfasse._ c. h. de la lumiere, c. , p. .] to explain the unusual refraction of island crystal by pression or motion propagated, has not hitherto been attempted (to my knowledge) except by _huygens_, who for that end supposed two several vibrating mediums within that crystal. but when he tried the refractions in two successive pieces of that crystal, and found them such as is mention'd above; he confessed himself at a loss for explaining them. for pressions or motions, propagated from a shining body through an uniform medium, must be on all sides alike; whereas by those experiments it appears, that the rays of light have different properties in their different sides. he suspected that the pulses of _Æther_ in passing through the first crystal might receive certain new modifications, which might determine them to be propagated in this or that medium within the second crystal, according to the position of that crystal. but what modifications those might be he could not say, nor think of any thing satisfactory in that point. and if he had known that the unusual refraction depends not on new modifications, but on the original and unchangeable dispositions of the rays, he would have found it as difficult to explain how those dispositions which he supposed to be impress'd on the rays by the first crystal, could be in them before their incidence on that crystal, and in general, how all rays emitted by shining bodies, can have those dispositions in them from the beginning. to me, at least, this seems inexplicable, if light be nothing else than pression or motion propagated through _Æther_. and it is as difficult to explain by these hypotheses, how rays can be alternately in fits of easy reflexion and easy transmission; unless perhaps one might suppose that there are in all space two Æthereal vibrating mediums, and that the vibrations of one of them constitute light, and the vibrations of the other are swifter, and as often as they overtake the vibrations of the first, put them into those fits. but how two _Æthers_ can be diffused through all space, one of which acts upon the other, and by consequence is re-acted upon, without retarding, shattering, dispersing and confounding one anothers motions, is inconceivable. and against filling the heavens with fluid mediums, unless they be exceeding rare, a great objection arises from the regular and very lasting motions of the planets and comets in all manner of courses through the heavens. for thence it is manifest, that the heavens are void of all sensible resistance, and by consequence of all sensible matter. for the resisting power of fluid mediums arises partly from the attrition of the parts of the medium, and partly from the _vis inertiæ_ of the matter. that part of the resistance of a spherical body which arises from the attrition of the parts of the medium is very nearly as the diameter, or, at the most, as the _factum_ of the diameter, and the velocity of the spherical body together. and that part of the resistance which arises from the _vis inertiæ_ of the matter, is as the square of that _factum_. and by this difference the two sorts of resistance may be distinguish'd from one another in any medium; and these being distinguish'd, it will be found that almost all the resistance of bodies of a competent magnitude moving in air, water, quick-silver, and such like fluids with a competent velocity, arises from the _vis inertiæ_ of the parts of the fluid. now that part of the resisting power of any medium which arises from the tenacity, friction or attrition of the parts of the medium, may be diminish'd by dividing the matter into smaller parts, and making the parts more smooth and slippery: but that part of the resistance which arises from the _vis inertiæ_, is proportional to the density of the matter, and cannot be diminish'd by dividing the matter into smaller parts, nor by any other means than by decreasing the density of the medium. and for these reasons the density of fluid mediums is very nearly proportional to their resistance. liquors which differ not much in density, as water, spirit of wine, spirit of turpentine, hot oil, differ not much in resistance. water is thirteen or fourteen times lighter than quick-silver and by consequence thirteen or fourteen times rarer, and its resistance is less than that of quick-silver in the same proportion, or thereabouts, as i have found by experiments made with pendulums. the open air in which we breathe is eight or nine hundred times lighter than water, and by consequence eight or nine hundred times rarer, and accordingly its resistance is less than that of water in the same proportion, or thereabouts; as i have also found by experiments made with pendulums. and in thinner air the resistance is still less, and at length, by ratifying the air, becomes insensible. for small feathers falling in the open air meet with great resistance, but in a tall glass well emptied of air, they fall as fast as lead or gold, as i have seen tried several times. whence the resistance seems still to decrease in proportion to the density of the fluid. for i do not find by any experiments, that bodies moving in quick-silver, water or air, meet with any other sensible resistance than what arises from the density and tenacity of those sensible fluids, as they would do if the pores of those fluids, and all other spaces, were filled with a dense and subtile fluid. now if the resistance in a vessel well emptied of air, was but an hundred times less than in the open air, it would be about a million of times less than in quick-silver. but it seems to be much less in such a vessel, and still much less in the heavens, at the height of three or four hundred miles from the earth, or above. for mr. _boyle_ has shew'd that air may be rarified above ten thousand times in vessels of glass; and the heavens are much emptier of air than any _vacuum_ we can make below. for since the air is compress'd by the weight of the incumbent atmosphere, and the density of air is proportional to the force compressing it, it follows by computation, that at the height of about seven and a half _english_ miles from the earth, the air is four times rarer than at the surface of the earth; and at the height of miles it is sixteen times rarer than that at the surface of the earth; and at the height of - / , , or miles, it is respectively , , or times rarer, or thereabouts; and at the height of , , miles, it is about , , or times rarer; and so on. heat promotes fluidity very much by diminishing the tenacity of bodies. it makes many bodies fluid which are not fluid in cold, and increases the fluidity of tenacious liquids, as of oil, balsam, and honey, and thereby decreases their resistance. but it decreases not the resistance of water considerably, as it would do if any considerable part of the resistance of water arose from the attrition or tenacity of its parts. and therefore the resistance of water arises principally and almost entirely from the _vis inertiæ_ of its matter; and by consequence, if the heavens were as dense as water, they would not have much less resistance than water; if as dense as quick-silver, they would not have much less resistance than quick-silver; if absolutely dense, or full of matter without any _vacuum_, let the matter be never so subtil and fluid, they would have a greater resistance than quick-silver. a solid globe in such a medium would lose above half its motion in moving three times the length of its diameter, and a globe not solid (such as are the planets,) would be retarded sooner. and therefore to make way for the regular and lasting motions of the planets and comets, it's necessary to empty the heavens of all matter, except perhaps some very thin vapours, steams, or effluvia, arising from the atmospheres of the earth, planets, and comets, and from such an exceedingly rare Æthereal medium as we described above. a dense fluid can be of no use for explaining the phænomena of nature, the motions of the planets and comets being better explain'd without it. it serves only to disturb and retard the motions of those great bodies, and make the frame of nature languish: and in the pores of bodies, it serves only to stop the vibrating motions of their parts, wherein their heat and activity consists. and as it is of no use, and hinders the operations of nature, and makes her languish, so there is no evidence for its existence, and therefore it ought to be rejected. and if it be rejected, the hypotheses that light consists in pression or motion, propagated through such a medium, are rejected with it. and for rejecting such a medium, we have the authority of those the oldest and most celebrated philosophers of _greece_ and _phoenicia_, who made a _vacuum_, and atoms, and the gravity of atoms, the first principles of their philosophy; tacitly attributing gravity to some other cause than dense matter. later philosophers banish the consideration of such a cause out of natural philosophy, feigning hypotheses for explaining all things mechanically, and referring other causes to metaphysicks: whereas the main business of natural philosophy is to argue from phænomena without feigning hypotheses, and to deduce causes from effects, till we come to the very first cause, which certainly is not mechanical; and not only to unfold the mechanism of the world, but chiefly to resolve these and such like questions. what is there in places almost empty of matter, and whence is it that the sun and planets gravitate towards one another, without dense matter between them? whence is it that nature doth nothing in vain; and whence arises all that order and beauty which we see in the world? to what end are comets, and whence is it that planets move all one and the same way in orbs concentrick, while comets move all manner of ways in orbs very excentrick; and what hinders the fix'd stars from falling upon one another? how came the bodies of animals to be contrived with so much art, and for what ends were their several parts? was the eye contrived without skill in opticks, and the ear without knowledge of sounds? how do the motions of the body follow from the will, and whence is the instinct in animals? is not the sensory of animals that place to which the sensitive substance is present, and into which the sensible species of things are carried through the nerves and brain, that there they may be perceived by their immediate presence to that substance? and these things being rightly dispatch'd, does it not appear from phænomena that there is a being incorporeal, living, intelligent, omnipresent, who in infinite space, as it were in his sensory, sees the things themselves intimately, and throughly perceives them, and comprehends them wholly by their immediate presence to himself: of which things the images only carried through the organs of sense into our little sensoriums, are there seen and beheld by that which in us perceives and thinks. and though every true step made in this philosophy brings us not immediately to the knowledge of the first cause, yet it brings us nearer to it, and on that account is to be highly valued. _qu._ . are not the rays of light very small bodies emitted from shining substances? for such bodies will pass through uniform mediums in right lines without bending into the shadow, which is the nature of the rays of light. they will also be capable of several properties, and be able to conserve their properties unchanged in passing through several mediums, which is another condition of the rays of light. pellucid substances act upon the rays of light at a distance in refracting, reflecting, and inflecting them, and the rays mutually agitate the parts of those substances at a distance for heating them; and this action and re-action at a distance very much resembles an attractive force between bodies. if refraction be perform'd by attraction of the rays, the sines of incidence must be to the sines of refraction in a given proportion, as we shew'd in our principles of philosophy: and this rule is true by experience. the rays of light in going out of glass into a _vacuum_, are bent towards the glass; and if they fall too obliquely on the _vacuum_, they are bent backwards into the glass, and totally reflected; and this reflexion cannot be ascribed to the resistance of an absolute _vacuum_, but must be caused by the power of the glass attracting the rays at their going out of it into the _vacuum_, and bringing them back. for if the farther surface of the glass be moisten'd with water or clear oil, or liquid and clear honey, the rays which would otherwise be reflected will go into the water, oil, or honey; and therefore are not reflected before they arrive at the farther surface of the glass, and begin to go out of it. if they go out of it into the water, oil, or honey, they go on, because the attraction of the glass is almost balanced and rendered ineffectual by the contrary attraction of the liquor. but if they go out of it into a _vacuum_ which has no attraction to balance that of the glass, the attraction of the glass either bends and refracts them, or brings them back and reflects them. and this is still more evident by laying together two prisms of glass, or two object-glasses of very long telescopes, the one plane, the other a little convex, and so compressing them that they do not fully touch, nor are too far asunder. for the light which falls upon the farther surface of the first glass where the interval between the glasses is not above the ten hundred thousandth part of an inch, will go through that surface, and through the air or _vacuum_ between the glasses, and enter into the second glass, as was explain'd in the first, fourth, and eighth observations of the first part of the second book. but, if the second glass be taken away, the light which goes out of the second surface of the first glass into the air or _vacuum_, will not go on forwards, but turns back into the first glass, and is reflected; and therefore it is drawn back by the power of the first glass, there being nothing else to turn it back. nothing more is requisite for producing all the variety of colours, and degrees of refrangibility, than that the rays of light be bodies of different sizes, the least of which may take violet the weakest and darkest of the colours, and be more easily diverted by refracting surfaces from the right course; and the rest as they are bigger and bigger, may make the stronger and more lucid colours, blue, green, yellow, and red, and be more and more difficultly diverted. nothing more is requisite for putting the rays of light into fits of easy reflexion and easy transmission, than that they be small bodies which by their attractive powers, or some other force, stir up vibrations in what they act upon, which vibrations being swifter than the rays, overtake them successively, and agitate them so as by turns to increase and decrease their velocities, and thereby put them into those fits. and lastly, the unusual refraction of island-crystal looks very much as if it were perform'd by some kind of attractive virtue lodged in certain sides both of the rays, and of the particles of the crystal. for were it not for some kind of disposition or virtue lodged in some sides of the particles of the crystal, and not in their other sides, and which inclines and bends the rays towards the coast of unusual refraction, the rays which fall perpendicularly on the crystal, would not be refracted towards that coast rather than towards any other coast, both at their incidence and at their emergence, so as to emerge perpendicularly by a contrary situation of the coast of unusual refraction at the second surface; the crystal acting upon the rays after they have pass'd through it, and are emerging into the air; or, if you please, into a _vacuum_. and since the crystal by this disposition or virtue does not act upon the rays, unless when one of their sides of unusual refraction looks towards that coast, this argues a virtue or disposition in those sides of the rays, which answers to, and sympathizes with that virtue or disposition of the crystal, as the poles of two magnets answer to one another. and as magnetism may be intended and remitted, and is found only in the magnet and in iron: so this virtue of refracting the perpendicular rays is greater in island-crystal, less in crystal of the rock, and is not yet found in other bodies. i do not say that this virtue is magnetical: it seems to be of another kind. i only say, that whatever it be, it's difficult to conceive how the rays of light, unless they be bodies, can have a permanent virtue in two of their sides which is not in their other sides, and this without any regard to their position to the space or medium through which they pass. what i mean in this question by a _vacuum_, and by the attractions of the rays of light towards glass or crystal, may be understood by what was said in the th, th, and th questions. _quest._ . are not gross bodies and light convertible into one another, and may not bodies receive much of their activity from the particles of light which enter their composition? for all fix'd bodies being heated emit light so long as they continue sufficiently hot, and light mutually stops in bodies as often as its rays strike upon their parts, as we shew'd above. i know no body less apt to shine than water; and yet water by frequent distillations changes into fix'd earth, as mr. _boyle_ has try'd; and then this earth being enabled to endure a sufficient heat, shines by heat like other bodies. the changing of bodies into light, and light into bodies, is very conformable to the course of nature, which seems delighted with transmutations. water, which is a very fluid tasteless salt, she changes by heat into vapour, which is a sort of air, and by cold into ice, which is a hard, pellucid, brittle, fusible stone; and this stone returns into water by heat, and vapour returns into water by cold. earth by heat becomes fire, and by cold returns into earth. dense bodies by fermentation rarify into several sorts of air, and this air by fermentation, and sometimes without it, returns into dense bodies. mercury appears sometimes in the form of a fluid metal, sometimes in the form of a hard brittle metal, sometimes in the form of a corrosive pellucid salt call'd sublimate, sometimes in the form of a tasteless, pellucid, volatile white earth, call'd _mercurius dulcis_; or in that of a red opake volatile earth, call'd cinnaber; or in that of a red or white precipitate, or in that of a fluid salt; and in distillation it turns into vapour, and being agitated _in vacuo_, it shines like fire. and after all these changes it returns again into its first form of mercury. eggs grow from insensible magnitudes, and change into animals; tadpoles into frogs; and worms into flies. all birds, beasts and fishes, insects, trees, and other vegetables, with their several parts, grow out of water and watry tinctures and salts, and by putrefaction return again into watry substances. and water standing a few days in the open air, yields a tincture, which (like that of malt) by standing longer yields a sediment and a spirit, but before putrefaction is fit nourishment for animals and vegetables. and among such various and strange transmutations, why may not nature change bodies into light, and light into bodies? _quest._ . have not the small particles of bodies certain powers, virtues, or forces, by which they act at a distance, not only upon the rays of light for reflecting, refracting, and inflecting them, but also upon one another for producing a great part of the phænomena of nature? for it's well known, that bodies act one upon another by the attractions of gravity, magnetism, and electricity; and these instances shew the tenor and course of nature, and make it not improbable but that there may be more attractive powers than these. for nature is very consonant and conformable to her self. how these attractions may be perform'd, i do not here consider. what i call attraction may be perform'd by impulse, or by some other means unknown to me. i use that word here to signify only in general any force by which bodies tend towards one another, whatsoever be the cause. for we must learn from the phænomena of nature what bodies attract one another, and what are the laws and properties of the attraction, before we enquire the cause by which the attraction is perform'd. the attractions of gravity, magnetism, and electricity, reach to very sensible distances, and so have been observed by vulgar eyes, and there may be others which reach to so small distances as hitherto escape observation; and perhaps electrical attraction may reach to such small distances, even without being excited by friction. for when salt of tartar runs _per deliquium_, is not this done by an attraction between the particles of the salt of tartar, and the particles of the water which float in the air in the form of vapours? and why does not common salt, or salt-petre, or vitriol, run _per deliquium_, but for want of such an attraction? or why does not salt of tartar draw more water out of the air than in a certain proportion to its quantity, but for want of an attractive force after it is satiated with water? and whence is it but from this attractive power that water which alone distils with a gentle luke-warm heat, will not distil from salt of tartar without a great heat? and is it not from the like attractive power between the particles of oil of vitriol and the particles of water, that oil of vitriol draws to it a good quantity of water out of the air, and after it is satiated draws no more, and in distillation lets go the water very difficultly? and when water and oil of vitriol poured successively into the same vessel grow very hot in the mixing, does not this heat argue a great motion in the parts of the liquors? and does not this motion argue, that the parts of the two liquors in mixing coalesce with violence, and by consequence rush towards one another with an accelerated motion? and when _aqua fortis_, or spirit of vitriol poured upon filings of iron dissolves the filings with a great heat and ebullition, is not this heat and ebullition effected by a violent motion of the parts, and does not that motion argue that the acid parts of the liquor rush towards the parts of the metal with violence, and run forcibly into its pores till they get between its outmost particles, and the main mass of the metal, and surrounding those particles loosen them from the main mass, and set them at liberty to float off into the water? and when the acid particles, which alone would distil with an easy heat, will not separate from the particles of the metal without a very violent heat, does not this confirm the attraction between them? when spirit of vitriol poured upon common salt or salt-petre makes an ebullition with the salt, and unites with it, and in distillation the spirit of the common salt or salt-petre comes over much easier than it would do before, and the acid part of the spirit of vitriol stays behind; does not this argue that the fix'd alcaly of the salt attracts the acid spirit of the vitriol more strongly than its own spirit, and not being able to hold them both, lets go its own? and when oil of vitriol is drawn off from its weight of nitre, and from both the ingredients a compound spirit of nitre is distilled, and two parts of this spirit are poured on one part of oil of cloves or carraway seeds, or of any ponderous oil of vegetable or animal substances, or oil of turpentine thicken'd with a little balsam of sulphur, and the liquors grow so very hot in mixing, as presently to send up a burning flame; does not this very great and sudden heat argue that the two liquors mix with violence, and that their parts in mixing run towards one another with an accelerated motion, and clash with the greatest force? and is it not for the same reason that well rectified spirit of wine poured on the same compound spirit flashes; and that the _pulvis fulminans_, composed of sulphur, nitre, and salt of tartar, goes off with a more sudden and violent explosion than gun-powder, the acid spirits of the sulphur and nitre rushing towards one another, and towards the salt of tartar, with so great a violence, as by the shock to turn the whole at once into vapour and flame? where the dissolution is slow, it makes a slow ebullition and a gentle heat; and where it is quicker, it makes a greater ebullition with more heat; and where it is done at once, the ebullition is contracted into a sudden blast or violent explosion, with a heat equal to that of fire and flame. so when a drachm of the above-mention'd compound spirit of nitre was poured upon half a drachm of oil of carraway seeds _in vacuo_, the mixture immediately made a flash like gun-powder, and burst the exhausted receiver, which was a glass six inches wide, and eight inches deep. and even the gross body of sulphur powder'd, and with an equal weight of iron filings and a little water made into paste, acts upon the iron, and in five or six hours grows too hot to be touch'd, and emits a flame. and by these experiments compared with the great quantity of sulphur with which the earth abounds, and the warmth of the interior parts of the earth, and hot springs, and burning mountains, and with damps, mineral coruscations, earthquakes, hot suffocating exhalations, hurricanes, and spouts; we may learn that sulphureous steams abound in the bowels of the earth and ferment with minerals, and sometimes take fire with a sudden coruscation and explosion; and if pent up in subterraneous caverns, burst the caverns with a great shaking of the earth, as in springing of a mine. and then the vapour generated by the explosion, expiring through the pores of the earth, feels hot and suffocates, and makes tempests and hurricanes, and sometimes causes the land to slide, or the sea to boil, and carries up the water thereof in drops, which by their weight fall down again in spouts. also some sulphureous steams, at all times when the earth is dry, ascending into the air, ferment there with nitrous acids, and sometimes taking fire cause lightning and thunder, and fiery meteors. for the air abounds with acid vapours fit to promote fermentations, as appears by the rusting of iron and copper in it, the kindling of fire by blowing, and the beating of the heart by means of respiration. now the above-mention'd motions are so great and violent as to shew that in fermentations the particles of bodies which almost rest, are put into new motions by a very potent principle, which acts upon them only when they approach one another, and causes them to meet and clash with great violence, and grow hot with the motion, and dash one another into pieces, and vanish into air, and vapour, and flame. when salt of tartar _per deliquium_, being poured into the solution of any metal, precipitates the metal and makes it fall down to the bottom of the liquor in the form of mud: does not this argue that the acid particles are attracted more strongly by the salt of tartar than by the metal, and by the stronger attraction go from the metal to the salt of tartar? and so when a solution of iron in _aqua fortis_ dissolves the _lapis calaminaris_, and lets go the iron, or a solution of copper dissolves iron immersed in it and lets go the copper, or a solution of silver dissolves copper and lets go the silver, or a solution of mercury in _aqua fortis_ being poured upon iron, copper, tin, or lead, dissolves the metal and lets go the mercury; does not this argue that the acid particles of the _aqua fortis_ are attracted more strongly by the _lapis calaminaris_ than by iron, and more strongly by iron than by copper, and more strongly by copper than by silver, and more strongly by iron, copper, tin, and lead, than by mercury? and is it not for the same reason that iron requires more _aqua fortis_ to dissolve it than copper, and copper more than the other metals; and that of all metals, iron is dissolved most easily, and is most apt to rust; and next after iron, copper? when oil of vitriol is mix'd with a little water, or is run _per deliquium_, and in distillation the water ascends difficultly, and brings over with it some part of the oil of vitriol in the form of spirit of vitriol, and this spirit being poured upon iron, copper, or salt of tartar, unites with the body and lets go the water; doth not this shew that the acid spirit is attracted by the water, and more attracted by the fix'd body than by the water, and therefore lets go the water to close with the fix'd body? and is it not for the same reason that the water and acid spirits which are mix'd together in vinegar, _aqua fortis_, and spirit of salt, cohere and rise together in distillation; but if the _menstruum_ be poured on salt of tartar, or on lead, or iron, or any fix'd body which it can dissolve, the acid by a stronger attraction adheres to the body, and lets go the water? and is it not also from a mutual attraction that the spirits of soot and sea-salt unite and compose the particles of sal-armoniac, which are less volatile than before, because grosser and freer from water; and that the particles of sal-armoniac in sublimation carry up the particles of antimony, which will not sublime alone; and that the particles of mercury uniting with the acid particles of spirit of salt compose mercury sublimate, and with the particles of sulphur, compose cinnaber; and that the particles of spirit of wine and spirit of urine well rectified unite, and letting go the water which dissolved them, compose a consistent body; and that in subliming cinnaber from salt of tartar, or from quick lime, the sulphur by a stronger attraction of the salt or lime lets go the mercury, and stays with the fix'd body; and that when mercury sublimate is sublimed from antimony, or from regulus of antimony, the spirit of salt lets go the mercury, and unites with the antimonial metal which attracts it more strongly, and stays with it till the heat be great enough to make them both ascend together, and then carries up the metal with it in the form of a very fusible salt, called butter of antimony, although the spirit of salt alone be almost as volatile as water, and the antimony alone as fix'd as lead? when _aqua fortis_ dissolves silver and not gold, and _aqua regia_ dissolves gold and not silver, may it not be said that _aqua fortis_ is subtil enough to penetrate gold as well as silver, but wants the attractive force to give it entrance; and that _aqua regia_ is subtil enough to penetrate silver as well as gold, but wants the attractive force to give it entrance? for _aqua regia_ is nothing else than _aqua fortis_ mix'd with some spirit of salt, or with sal-armoniac; and even common salt dissolved in _aqua fortis_, enables the _menstruum_ to dissolve gold, though the salt be a gross body. when therefore spirit of salt precipitates silver out of _aqua fortis_, is it not done by attracting and mixing with the _aqua fortis_, and not attracting, or perhaps repelling silver? and when water precipitates antimony out of the sublimate of antimony and sal-armoniac, or out of butter of antimony, is it not done by its dissolving, mixing with, and weakening the sal-armoniac or spirit of salt, and its not attracting, or perhaps repelling the antimony? and is it not for want of an attractive virtue between the parts of water and oil, of quick-silver and antimony, of lead and iron, that these substances do not mix; and by a weak attraction, that quick-silver and copper mix difficultly; and from a strong one, that quick-silver and tin, antimony and iron, water and salts, mix readily? and in general, is it not from the same principle that heat congregates homogeneal bodies, and separates heterogeneal ones? when arsenick with soap gives a regulus, and with mercury sublimate a volatile fusible salt, like butter of antimony, doth not this shew that arsenick, which is a substance totally volatile, is compounded of fix'd and volatile parts, strongly cohering by a mutual attraction, so that the volatile will not ascend without carrying up the fixed? and so, when an equal weight of spirit of wine and oil of vitriol are digested together, and in distillation yield two fragrant and volatile spirits which will not mix with one another, and a fix'd black earth remains behind; doth not this shew that oil of vitriol is composed of volatile and fix'd parts strongly united by attraction, so as to ascend together in form of a volatile, acid, fluid salt, until the spirit of wine attracts and separates the volatile parts from the fixed? and therefore, since oil of sulphur _per campanam_ is of the same nature with oil of vitriol, may it not be inferred, that sulphur is also a mixture of volatile and fix'd parts so strongly cohering by attraction, as to ascend together in sublimation. by dissolving flowers of sulphur in oil of turpentine, and distilling the solution, it is found that sulphur is composed of an inflamable thick oil or fat bitumen, an acid salt, a very fix'd earth, and a little metal. the three first were found not much unequal to one another, the fourth in so small a quantity as scarce to be worth considering. the acid salt dissolved in water, is the same with oil of sulphur _per campanam_, and abounding much in the bowels of the earth, and particularly in markasites, unites it self to the other ingredients of the markasite, which are, bitumen, iron, copper, and earth, and with them compounds allum, vitriol, and sulphur. with the earth alone it compounds allum; with the metal alone, or metal and earth together, it compounds vitriol; and with the bitumen and earth it compounds sulphur. whence it comes to pass that markasites abound with those three minerals. and is it not from the mutual attraction of the ingredients that they stick together for compounding these minerals, and that the bitumen carries up the other ingredients of the sulphur, which without it would not sublime? and the same question may be put concerning all, or almost all the gross bodies in nature. for all the parts of animals and vegetables are composed of substances volatile and fix'd, fluid and solid, as appears by their analysis; and so are salts and minerals, so far as chymists have been hitherto able to examine their composition. when mercury sublimate is re-sublimed with fresh mercury, and becomes _mercurius dulcis_, which is a white tasteless earth scarce dissolvable in water, and _mercurius dulcis_ re-sublimed with spirit of salt returns into mercury sublimate; and when metals corroded with a little acid turn into rust, which is an earth tasteless and indissolvable in water, and this earth imbibed with more acid becomes a metallick salt; and when some stones, as spar of lead, dissolved in proper _menstruums_ become salts; do not these things shew that salts are dry earth and watry acid united by attraction, and that the earth will not become a salt without so much acid as makes it dissolvable in water? do not the sharp and pungent tastes of acids arise from the strong attraction whereby the acid particles rush upon and agitate the particles of the tongue? and when metals are dissolved in acid _menstruums_, and the acids in conjunction with the metal act after a different manner, so that the compound has a different taste much milder than before, and sometimes a sweet one; is it not because the acids adhere to the metallick particles, and thereby lose much of their activity? and if the acid be in too small a proportion to make the compound dissolvable in water, will it not by adhering strongly to the metal become unactive and lose its taste, and the compound be a tasteless earth? for such things as are not dissolvable by the moisture of the tongue, act not upon the taste. as gravity makes the sea flow round the denser and weightier parts of the globe of the earth, so the attraction may make the watry acid flow round the denser and compacter particles of earth for composing the particles of salt. for otherwise the acid would not do the office of a medium between the earth and common water, for making salts dissolvable in the water; nor would salt of tartar readily draw off the acid from dissolved metals, nor metals the acid from mercury. now, as in the great globe of the earth and sea, the densest bodies by their gravity sink down in water, and always endeavour to go towards the center of the globe; so in particles of salt, the densest matter may always endeavour to approach the center of the particle: so that a particle of salt may be compared to a chaos; being dense, hard, dry, and earthy in the center; and rare, soft, moist, and watry in the circumference. and hence it seems to be that salts are of a lasting nature, being scarce destroy'd, unless by drawing away their watry parts by violence, or by letting them soak into the pores of the central earth by a gentle heat in putrefaction, until the earth be dissolved by the water, and separated into smaller particles, which by reason of their smallness make the rotten compound appear of a black colour. hence also it may be, that the parts of animals and vegetables preserve their several forms, and assimilate their nourishment; the soft and moist nourishment easily changing its texture by a gentle heat and motion, till it becomes like the dense, hard, dry, and durable earth in the center of each particle. but when the nourishment grows unfit to be assimilated, or the central earth grows too feeble to assimilate it, the motion ends in confusion, putrefaction, and death. if a very small quantity of any salt or vitriol be dissolved in a great quantity of water, the particles of the salt or vitriol will not sink to the bottom, though they be heavier in specie than the water, but will evenly diffuse themselves into all the water, so as to make it as saline at the top as at the bottom. and does not this imply that the parts of the salt or vitriol recede from one another, and endeavour to expand themselves, and get as far asunder as the quantity of water in which they float, will allow? and does not this endeavour imply that they have a repulsive force by which they fly from one another, or at least, that they attract the water more strongly than they do one another? for as all things ascend in water which are less attracted than water, by the gravitating power of the earth; so all the particles of salt which float in water, and are less attracted than water by any one particle of salt, must recede from that particle, and give way to the more attracted water. when any saline liquor is evaporated to a cuticle and let cool, the salt concretes in regular figures; which argues, that the particles of the salt before they concreted, floated in the liquor at equal distances in rank and file, and by consequence that they acted upon one another by some power which at equal distances is equal, at unequal distances unequal. for by such a power they will range themselves uniformly, and without it they will float irregularly, and come together as irregularly. and since the particles of island-crystal act all the same way upon the rays of light for causing the unusual refraction, may it not be supposed that in the formation of this crystal, the particles not only ranged themselves in rank and file for concreting in regular figures, but also by some kind of polar virtue turned their homogeneal sides the same way. the parts of all homogeneal hard bodies which fully touch one another, stick together very strongly. and for explaining how this may be, some have invented hooked atoms, which is begging the question; and others tell us that bodies are glued together by rest, that is, by an occult quality, or rather by nothing; and others, that they stick together by conspiring motions, that is, by relative rest amongst themselves. i had rather infer from their cohesion, that their particles attract one another by some force, which in immediate contact is exceeding strong, at small distances performs the chymical operations above-mention'd, and reaches not far from the particles with any sensible effect. all bodies seem to be composed of hard particles: for otherwise fluids would not congeal; as water, oils, vinegar, and spirit or oil of vitriol do by freezing; mercury by fumes of lead; spirit of nitre and mercury, by dissolving the mercury and evaporating the flegm; spirit of wine and spirit of urine, by deflegming and mixing them; and spirit of urine and spirit of salt, by subliming them together to make sal-armoniac. even the rays of light seem to be hard bodies; for otherwise they would not retain different properties in their different sides. and therefore hardness may be reckon'd the property of all uncompounded matter. at least, this seems to be as evident as the universal impenetrability of matter. for all bodies, so far as experience reaches, are either hard, or may be harden'd; and we have no other evidence of universal impenetrability, besides a large experience without an experimental exception. now if compound bodies are so very hard as we find some of them to be, and yet are very porous, and consist of parts which are only laid together; the simple particles which are void of pores, and were never yet divided, must be much harder. for such hard particles being heaped up together, can scarce touch one another in more than a few points, and therefore must be separable by much less force than is requisite to break a solid particle, whose parts touch in all the space between them, without any pores or interstices to weaken their cohesion. and how such very hard particles which are only laid together and touch only in a few points, can stick together, and that so firmly as they do, without the assistance of something which causes them to be attracted or press'd towards one another, is very difficult to conceive. the same thing i infer also from the cohering of two polish'd marbles _in vacuo_, and from the standing of quick-silver in the barometer at the height of , or inches, or above, when ever it is well-purged of air and carefully poured in, so that its parts be every where contiguous both to one another and to the glass. the atmosphere by its weight presses the quick-silver into the glass, to the height of or inches. and some other agent raises it higher, not by pressing it into the glass, but by making its parts stick to the glass, and to one another. for upon any discontinuation of parts, made either by bubbles or by shaking the glass, the whole mercury falls down to the height of or inches. and of the same kind with these experiments are those that follow. if two plane polish'd plates of glass (suppose two pieces of a polish'd looking-glass) be laid together, so that their sides be parallel and at a very small distance from one another, and then their lower edges be dipped into water, the water will rise up between them. and the less the distance of the glasses is, the greater will be the height to which the water will rise. if the distance be about the hundredth part of an inch, the water will rise to the height of about an inch; and if the distance be greater or less in any proportion, the height will be reciprocally proportional to the distance very nearly. for the attractive force of the glasses is the same, whether the distance between them be greater or less; and the weight of the water drawn up is the same, if the height of it be reciprocally proportional to the distance of the glasses. and in like manner, water ascends between two marbles polish'd plane, when their polish'd sides are parallel, and at a very little distance from one another, and if slender pipes of glass be dipped at one end into stagnating water, the water will rise up within the pipe, and the height to which it rises will be reciprocally proportional to the diameter of the cavity of the pipe, and will equal the height to which it rises between two planes of glass, if the semi-diameter of the cavity of the pipe be equal to the distance between the planes, or thereabouts. and these experiments succeed after the same manner _in vacuo_ as in the open air, (as hath been tried before the royal society,) and therefore are not influenced by the weight or pressure of the atmosphere. and if a large pipe of glass be filled with sifted ashes well pressed together in the glass, and one end of the pipe be dipped into stagnating water, the water will rise up slowly in the ashes, so as in the space of a week or fortnight to reach up within the glass, to the height of or inches above the stagnating water. and the water rises up to this height by the action only of those particles of the ashes which are upon the surface of the elevated water; the particles which are within the water, attracting or repelling it as much downwards as upwards. and therefore the action of the particles is very strong. but the particles of the ashes being not so dense and close together as those of glass, their action is not so strong as that of glass, which keeps quick-silver suspended to the height of or inches, and therefore acts with a force which would keep water suspended to the height of above feet. by the same principle, a sponge sucks in water, and the glands in the bodies of animals, according to their several natures and dispositions, suck in various juices from the blood. if two plane polish'd plates of glass three or four inches broad, and twenty or twenty five long, be laid one of them parallel to the horizon, the other upon the first, so as at one of their ends to touch one another, and contain an angle of about or minutes, and the same be first moisten'd on their inward sides with a clean cloth dipp'd into oil of oranges or spirit of turpentine, and a drop or two of the oil or spirit be let fall upon the lower glass at the other; so soon as the upper glass is laid down upon the lower, so as to touch it at one end as above, and to touch the drop at the other end, making with the lower glass an angle of about or minutes; the drop will begin to move towards the concourse of the glasses, and will continue to move with an accelerated motion, till it arrives at that concourse of the glasses. for the two glasses attract the drop, and make it run that way towards which the attractions incline. and if when the drop is in motion you lift up that end of the glasses where they meet, and towards which the drop moves, the drop will ascend between the glasses, and therefore is attracted. and as you lift up the glasses more and more, the drop will ascend slower and slower, and at length rest, being then carried downward by its weight, as much as upwards by the attraction. and by this means you may know the force by which the drop is attracted at all distances from the concourse of the glasses. now by some experiments of this kind, (made by mr. _hauksbee_) it has been found that the attraction is almost reciprocally in a duplicate proportion of the distance of the middle of the drop from the concourse of the glasses, _viz._ reciprocally in a simple proportion, by reason of the spreading of the drop, and its touching each glass in a larger surface; and again reciprocally in a simple proportion, by reason of the attractions growing stronger within the same quantity of attracting surface. the attraction therefore within the same quantity of attracting surface, is reciprocally as the distance between the glasses. and therefore where the distance is exceeding small, the attraction must be exceeding great. by the table in the second part of the second book, wherein the thicknesses of colour'd plates of water between two glasses are set down, the thickness of the plate where it appears very black, is three eighths of the ten hundred thousandth part of an inch. and where the oil of oranges between the glasses is of this thickness, the attraction collected by the foregoing rule, seems to be so strong, as within a circle of an inch in diameter, to suffice to hold up a weight equal to that of a cylinder of water of an inch in diameter, and two or three furlongs in length. and where it is of a less thickness the attraction may be proportionally greater, and continue to increase, until the thickness do not exceed that of a single particle of the oil. there are therefore agents in nature able to make the particles of bodies stick together by very strong attractions. and it is the business of experimental philosophy to find them out. now the smallest particles of matter may cohere by the strongest attractions, and compose bigger particles of weaker virtue; and many of these may cohere and compose bigger particles whose virtue is still weaker, and so on for divers successions, until the progression end in the biggest particles on which the operations in chymistry, and the colours of natural bodies depend, and which by cohering compose bodies of a sensible magnitude. if the body is compact, and bends or yields inward to pression without any sliding of its parts, it is hard and elastick, returning to its figure with a force rising from the mutual attraction of its parts. if the parts slide upon one another, the body is malleable or soft. if they slip easily, and are of a fit size to be agitated by heat, and the heat is big enough to keep them in agitation, the body is fluid; and if it be apt to stick to things, it is humid; and the drops of every fluid affect a round figure by the mutual attraction of their parts, as the globe of the earth and sea affects a round figure by the mutual attraction of its parts by gravity. since metals dissolved in acids attract but a small quantity of the acid, their attractive force can reach but to a small distance from them. and as in algebra, where affirmative quantities vanish and cease, there negative ones begin; so in mechanicks, where attraction ceases, there a repulsive virtue ought to succeed. and that there is such a virtue, seems to follow from the reflexions and inflexions of the rays of light. for the rays are repelled by bodies in both these cases, without the immediate contact of the reflecting or inflecting body. it seems also to follow from the emission of light; the ray so soon as it is shaken off from a shining body by the vibrating motion of the parts of the body, and gets beyond the reach of attraction, being driven away with exceeding great velocity. for that force which is sufficient to turn it back in reflexion, may be sufficient to emit it. it seems also to follow from the production of air and vapour. the particles when they are shaken off from bodies by heat or fermentation, so soon as they are beyond the reach of the attraction of the body, receding from it, and also from one another with great strength, and keeping at a distance, so as sometimes to take up above a million of times more space than they did before in the form of a dense body. which vast contraction and expansion seems unintelligible, by feigning the particles of air to be springy and ramous, or rolled up like hoops, or by any other means than a repulsive power. the particles of fluids which do not cohere too strongly, and are of such a smallness as renders them most susceptible of those agitations which keep liquors in a fluor, are most easily separated and rarified into vapour, and in the language of the chymists, they are volatile, rarifying with an easy heat, and condensing with cold. but those which are grosser, and so less susceptible of agitation, or cohere by a stronger attraction, are not separated without a stronger heat, or perhaps not without fermentation. and these last are the bodies which chymists call fix'd, and being rarified by fermentation, become true permanent air; those particles receding from one another with the greatest force, and being most difficultly brought together, which upon contact cohere most strongly. and because the particles of permanent air are grosser, and arise from denser substances than those of vapours, thence it is that true air is more ponderous than vapour, and that a moist atmosphere is lighter than a dry one, quantity for quantity. from the same repelling power it seems to be that flies walk upon the water without wetting their feet; and that the object-glasses of long telescopes lie upon one another without touching; and that dry powders are difficultly made to touch one another so as to stick together, unless by melting them, or wetting them with water, which by exhaling may bring them together; and that two polish'd marbles, which by immediate contact stick together, are difficultly brought so close together as to stick. and thus nature will be very conformable to her self and very simple, performing all the great motions of the heavenly bodies by the attraction of gravity which intercedes those bodies, and almost all the small ones of their particles by some other attractive and repelling powers which intercede the particles. the _vis inertiæ_ is a passive principle by which bodies persist in their motion or rest, receive motion in proportion to the force impressing it, and resist as much as they are resisted. by this principle alone there never could have been any motion in the world. some other principle was necessary for putting bodies into motion; and now they are in motion, some other principle is necessary for conserving the motion. for from the various composition of two motions, 'tis very certain that there is not always the same quantity of motion in the world. for if two globes joined by a slender rod, revolve about their common center of gravity with an uniform motion, while that center moves on uniformly in a right line drawn in the plane of their circular motion; the sum of the motions of the two globes, as often as the globes are in the right line described by their common center of gravity, will be bigger than the sum of their motions, when they are in a line perpendicular to that right line. by this instance it appears that motion may be got or lost. but by reason of the tenacity of fluids, and attrition of their parts, and the weakness of elasticity in solids, motion is much more apt to be lost than got, and is always upon the decay. for bodies which are either absolutely hard, or so soft as to be void of elasticity, will not rebound from one another. impenetrability makes them only stop. if two equal bodies meet directly _in vacuo_, they will by the laws of motion stop where they meet, and lose all their motion, and remain in rest, unless they be elastick, and receive new motion from their spring. if they have so much elasticity as suffices to make them re-bound with a quarter, or half, or three quarters of the force with which they come together, they will lose three quarters, or half, or a quarter of their motion. and this may be try'd, by letting two equal pendulums fall against one another from equal heights. if the pendulums be of lead or soft clay, they will lose all or almost all their motions: if of elastick bodies they will lose all but what they recover from their elasticity. if it be said, that they can lose no motion but what they communicate to other bodies, the consequence is, that _in vacuo_ they can lose no motion, but when they meet they must go on and penetrate one another's dimensions. if three equal round vessels be filled, the one with water, the other with oil, the third with molten pitch, and the liquors be stirred about alike to give them a vortical motion; the pitch by its tenacity will lose its motion quickly, the oil being less tenacious will keep it longer, and the water being less tenacious will keep it longest, but yet will lose it in a short time. whence it is easy to understand, that if many contiguous vortices of molten pitch were each of them as large as those which some suppose to revolve about the sun and fix'd stars, yet these and all their parts would, by their tenacity and stiffness, communicate their motion to one another till they all rested among themselves. vortices of oil or water, or some fluider matter, might continue longer in motion; but unless the matter were void of all tenacity and attrition of parts, and communication of motion, (which is not to be supposed,) the motion would constantly decay. seeing therefore the variety of motion which we find in the world is always decreasing, there is a necessity of conserving and recruiting it by active principles, such as are the cause of gravity, by which planets and comets keep their motions in their orbs, and bodies acquire great motion in falling; and the cause of fermentation, by which the heart and blood of animals are kept in perpetual motion and heat; the inward parts of the earth are constantly warm'd, and in some places grow very hot; bodies burn and shine, mountains take fire, the caverns of the earth are blown up, and the sun continues violently hot and lucid, and warms all things by his light. for we meet with very little motion in the world, besides what is owing to these active principles. and if it were not for these principles, the bodies of the earth, planets, comets, sun, and all things in them, would grow cold and freeze, and become inactive masses; and all putrefaction, generation, vegetation and life would cease, and the planets and comets would not remain in their orbs. all these things being consider'd, it seems probable to me, that god in the beginning form'd matter in solid, massy, hard, impenetrable, moveable particles, of such sizes and figures, and with such other properties, and in such proportion to space, as most conduced to the end for which he form'd them; and that these primitive particles being solids, are incomparably harder than any porous bodies compounded of them; even so very hard, as never to wear or break in pieces; no ordinary power being able to divide what god himself made one in the first creation. while the particles continue entire, they may compose bodies of one and the same nature and texture in all ages: but should they wear away, or break in pieces, the nature of things depending on them, would be changed. water and earth, composed of old worn particles and fragments of particles, would not be of the same nature and texture now, with water and earth composed of entire particles in the beginning. and therefore, that nature may be lasting, the changes of corporeal things are to be placed only in the various separations and new associations and motions of these permanent particles; compound bodies being apt to break, not in the midst of solid particles, but where those particles are laid together, and only touch in a few points. it seems to me farther, that these particles have not only a _vis inertiæ_, accompanied with such passive laws of motion as naturally result from that force, but also that they are moved by certain active principles, such as is that of gravity, and that which causes fermentation, and the cohesion of bodies. these principles i consider, not as occult qualities, supposed to result from the specifick forms of things, but as general laws of nature, by which the things themselves are form'd; their truth appearing to us by phænomena, though their causes be not yet discover'd. for these are manifest qualities, and their causes only are occult. and the _aristotelians_ gave the name of occult qualities, not to manifest qualities, but to such qualities only as they supposed to lie hid in bodies, and to be the unknown causes of manifest effects: such as would be the causes of gravity, and of magnetick and electrick attractions, and of fermentations, if we should suppose that these forces or actions arose from qualities unknown to us, and uncapable of being discovered and made manifest. such occult qualities put a stop to the improvement of natural philosophy, and therefore of late years have been rejected. to tell us that every species of things is endow'd with an occult specifick quality by which it acts and produces manifest effects, is to tell us nothing: but to derive two or three general principles of motion from phænomena, and afterwards to tell us how the properties and actions of all corporeal things follow from those manifest principles, would be a very great step in philosophy, though the causes of those principles were not yet discover'd: and therefore i scruple not to propose the principles of motion above-mention'd, they being of very general extent, and leave their causes to be found out. now by the help of these principles, all material things seem to have been composed of the hard and solid particles above-mention'd, variously associated in the first creation by the counsel of an intelligent agent. for it became him who created them to set them in order. and if he did so, it's unphilosophical to seek for any other origin of the world, or to pretend that it might arise out of a chaos by the mere laws of nature; though being once form'd, it may continue by those laws for many ages. for while comets move in very excentrick orbs in all manner of positions, blind fate could never make all the planets move one and the same way in orbs concentrick, some inconsiderable irregularities excepted, which may have risen from the mutual actions of comets and planets upon one another, and which will be apt to increase, till this system wants a reformation. such a wonderful uniformity in the planetary system must be allowed the effect of choice. and so must the uniformity in the bodies of animals, they having generally a right and a left side shaped alike, and on either side of their bodies two legs behind, and either two arms, or two legs, or two wings before upon their shoulders, and between their shoulders a neck running down into a back-bone, and a head upon it; and in the head two ears, two eyes, a nose, a mouth, and a tongue, alike situated. also the first contrivance of those very artificial parts of animals, the eyes, ears, brain, muscles, heart, lungs, midriff, glands, larynx, hands, wings, swimming bladders, natural spectacles, and other organs of sense and motion; and the instinct of brutes and insects, can be the effect of nothing else than the wisdom and skill of a powerful ever-living agent, who being in all places, is more able by his will to move the bodies within his boundless uniform sensorium, and thereby to form and reform the parts of the universe, than we are by our will to move the parts of our own bodies. and yet we are not to consider the world as the body of god, or the several parts thereof, as the parts of god. he is an uniform being, void of organs, members or parts, and they are his creatures subordinate to him, and subservient to his will; and he is no more the soul of them, than the soul of man is the soul of the species of things carried through the organs of sense into the place of its sensation, where it perceives them by means of its immediate presence, without the intervention of any third thing. the organs of sense are not for enabling the soul to perceive the species of things in its sensorium, but only for conveying them thither; and god has no need of such organs, he being every where present to the things themselves. and since space is divisible _in infinitum_, and matter is not necessarily in all places, it may be also allow'd that god is able to create particles of matter of several sizes and figures, and in several proportions to space, and perhaps of different densities and forces, and thereby to vary the laws of nature, and make worlds of several sorts in several parts of the universe. at least, i see nothing of contradiction in all this. as in mathematicks, so in natural philosophy, the investigation of difficult things by the method of analysis, ought ever to precede the method of composition. this analysis consists in making experiments and observations, and in drawing general conclusions from them by induction, and admitting of no objections against the conclusions, but such as are taken from experiments, or other certain truths. for hypotheses are not to be regarded in experimental philosophy. and although the arguing from experiments and observations by induction be no demonstration of general conclusions; yet it is the best way of arguing which the nature of things admits of, and may be looked upon as so much the stronger, by how much the induction is more general. and if no exception occur from phænomena, the conclusion may be pronounced generally. but if at any time afterwards any exception shall occur from experiments, it may then begin to be pronounced with such exceptions as occur. by this way of analysis we may proceed from compounds to ingredients, and from motions to the forces producing them; and in general, from effects to their causes, and from particular causes to more general ones, till the argument end in the most general. this is the method of analysis: and the synthesis consists in assuming the causes discover'd, and establish'd as principles, and by them explaining the phænomena proceeding from them, and proving the explanations. in the two first books of these opticks, i proceeded by this analysis to discover and prove the original differences of the rays of light in respect of refrangibility, reflexibility, and colour, and their alternate fits of easy reflexion and easy transmission, and the properties of bodies, both opake and pellucid, on which their reflexions and colours depend. and these discoveries being proved, may be assumed in the method of composition for explaining the phænomena arising from them: an instance of which method i gave in the end of the first book. in this third book i have only begun the analysis of what remains to be discover'd about light and its effects upon the frame of nature, hinting several things about it, and leaving the hints to be examin'd and improv'd by the farther experiments and observations of such as are inquisitive. and if natural philosophy in all its parts, by pursuing this method, shall at length be perfected, the bounds of moral philosophy will be also enlarged. for so far as we can know by natural philosophy what is the first cause, what power he has over us, and what benefits we receive from him, so far our duty towards him, as well as that towards one another, will appear to us by the light of nature. and no doubt, if the worship of false gods had not blinded the heathen, their moral philosophy would have gone farther than to the four cardinal virtues; and instead of teaching the transmigration of souls, and to worship the sun and moon, and dead heroes, they would have taught us to worship our true author and benefactor, as their ancestors did under the government of _noah_ and his sons before they corrupted themselves. ancient and modern physics by thomas e. willson contents preface i. physical basis of metaphysics ii. the two kinds of perception iii. matter and ether iv. what a teacher should teach v. the four manifested planes vi. one place on earth vii. the four globes viii. the battle ground ix. the dual man x. the septenary world xi. stumbling blocks in eastern physics preface the editor of the theosophical forum in april, , noted the death of mr. thomas e. willson in the previous month in an article which we reproduce for the reason that we believe many readers who have been following the chapters of "ancient and modern physics" during the last year will like to know something of the author. in these paragraphs is said all that need be said of one of our most devoted and understanding theosophists. in march, , the theosophical forum lost one of its most willing and unfailing contributors. mr. t.e. willson died suddenly, and the news of his death reached me when i actually was in the act of preparing the concluding chapter of his "ancient and modern physics" for the april number. like the swan, who sings his one song, when feeling that death is near, mr. willson gave his brother co-workers in the theosophical field all that was best, ripest and most suggestive in his thought in the series of articles the last of which is to come out in the same number with this. the last time i had a long talk with t.e. willson, he said" "for twenty years and more i was without a hearing, yet my interest and my faith in what i had to say never flagged, the eagerness of my love for my subject never diminished." this needs no comment. the quiet and sustained resistance to indifference and lack of appreciation, is truly the steady ballast which has prevented our theosophical ship from aimless and fatal wanderings, though of inclement weather and adverse winds we had plenty. for many long years mr. willson was the librarian of the new york "world." in the afternoons he was too busy to see outsiders, but, beginning with five o'clock in the afternoon until he went home somewhere in the neighbourhood of midnight, he always was glad to see his friends. he had a tiny little room of his own, very near the top of the tremendous building, his one window looking far above the roofs of the tallest houses in the district. there he sat at his desk, generally in his shirt sleeves, if the weather was at all warm, always busy with some matter already printed, or going to be, a quiet, yet impressive and dignified figure. the elevated isolation, both figuratively and literally speaking, in which t.e. willson lived and worked, in the midst of the most crowded thoroughfares of new york, always made me think of professor teufelsdrockh on the attic floor of "the highest house in the wahngasse." the two had more than one point of resemblance. they shared the loftiness of their point of view, their sympathetic understanding of other folks, their loneliness, and, above all, their patient, even humorous resignation to the fact of this loneliness. yet in his appearance mr. willson was not like the great weissnichtwo philosopher. in fact, in the cast of his features and in his ways, mr. willson never looked to me like a white man. in british india i have known brahmans of the better type exactly with the same sallow complexion, same quick and observant brown eye, same portly figure and same wide-awakeness and agility of manner. last summer i heard, on good authority, that mr. willson had thought himself into a most suggestive way of dealing with the problems of matter and spirit, a way which, besides being suggestive, bore a great resemblance to some theories of the same nature, current in ancient india. consequently mr. willson was offered, for the first time in his life, a chance of expressing his views on matter and spirit in as many articles and in as extensive a shape as he chose. the way he received this tardy recognition of the fact that he had something to say was highly instructive. he did not put on airs of unrecognized greatness, though, i own, the occasion was propitious; he did not say, "i told you so;" he simply and frankly was glad, in, the most childlike way. and now that i have used the word, it occurs to me that "childlike" is an adjective the best applied to this man, in spite of his portliness, and his three score and more winters. many a pleasant hour i have spent in the small bookroom of the great "world" building. with mr. willson talk never flagged. we discussed the past and the future of our planetary chain, we built plans for the true and wholesome relation of sexes, we tried to find out--and needless to say never did--the exact limit where matter stopped being matter and became spirit; we also read the latest comic poems and also, from time to time, we took a header into the stormy sea of american literature in order to find out what various wise heads had to say, consciously or unconsciously, in favour of our beloved theosophical views. and all this, being interrupted every three minutes or so by some weary apparition from some workroom in the "world" with some such question: "mr. willson, how am i to find out the present whereabouts of this or that russian man-of-war? mr. willson, what is the melting point of iron? mr. willson, when was `h.m.s. pinafore' produced for the first time?" etc., etc. and every time, mr. willson got up in the leisurely manner peculiar to him, reached for some book from the shelves that lined the room, gave the desired information, and as leisurely returned to the "pranic atom," or to "come and talk man talk, willy," or to whatever our subject chanced to be at the time. mr. willson's gratitude to the theosophical forum for its recognition was disproportionately great. as he wrote to the editor: "give me any kind of work, writing for you, reviewing, manuscript or proof reading, i shall do anything, i shall undertake any job, even to taking editorial scoldings in all good nature, only give me work." his devotion to theosophical thought and work in all their ramifications was just as great, as was his freedom from vanity, his perfectly natural and unaffected modesty. at the news of his death many a heart was sincerely sad, but none so sad as the heart of the editor of the theosophical forum. for a friend and co-worker like t.e. willson, ever ready to give material help and moral encouragement, is not easily replaced. for a soul so pure of any kind of selfishness the transition from the turmoil of life to the bright dreams of death must have been both easy and enviable. -------------- chapter one the physical basis if metaphysics the hindu system of physics, on which the metaphysical thought of the east is based, does not in its beginnings differ widely from the latest physics of the west; but it goes so much farther that our physics is soon lost sight of and forgotten. the hindu conception of the material universe, taken from the upanishads and some open teaching, will serve for an illustration. they divide physical matter into four kinds--prakriti, ether, prana, and manasa--which they call "planes." these differ only in the rate of vibration, each plane vibrating through one great octave, with gulfs of "lost" octaves between. the highest rate of vibration of prakriti is measured by the thousand, the lowest of the ether by trillions, and the lowest of prana by--never mind; they have, and we have not, the nomenclature. the earth, they teach, is a globe of prakriti, floating in an ocean of ether, which, as it has the sun for its center of gravity, must necessarily be a globe. this etheric sun-globe has a diameter of over , , , miles. all the planets revolve around the sun far within its atmosphere. the etheric sun-globe revolves on its axis once in about , years, and this revolution causes the precession of the equinoxes. this etheric sun-globe is revolving around alcyone with other etheric globes having suns for their centers and solar systems of prakritic globes within them in a great year of , , , of our common years. its orbit has a diameter of , , , , , miles. beyond the etheric globes, and between them, is a third form of matter called prana, as much rarer and finer than the ether as the ether is rarer and finer than prakriti. as this prana has alcyone for a center of gravity, it is necessarily a globe; and there are many of these pranic globes floating in a vast ocean of manasa--a form of matter as much finer than prana as prana is finer than ether, or ether than prakriti. with this manasa (which is a globe) the material, or physical, universe ends; but there are spiritual globes beyond. the material universe is created from manasa, downward, but it does not respond to or chord with the vibration of the globes above, except in a special instance and in a special way, which does not touch this inquiry. the physical universe of the ancient (and modern) hindu physicist was made up of these four kinds or planes of matter, distributed in space as "globes within globes." professor lodge in put forth the theory that prakriti (physical matter) as we call it, was in its atoms but "whirls" of ether. since then speculative science has generally accepted the idea that the physical atom is made up of many cubic feet of ether in chemical union, as many quarts of oxygen and hydrogen unite chemically to make a drop of water. this is an old story to the hindu sage. he tells his pupils that the great globe of manasa once filled all space, and there was nothing else. precisely as on this earth we have our elementary substances that change from liquids into solids and gases, so on this manasic globe there were elementary substances that took the form of liquids, solids and gases. its manasic matter was differentiated and vibrated through one octave, as the prakritic matter does on the earth. its substances combined as that does. one combination produced prana. the prana collected, and formed globes. on these pranic globes the process was repeated, with ether as the result, and the etheric globes formed. then the process was repeated on the etheric globes, as the modern scientists have discovered, and prakriti and prakritic globes came into being. the true diameter of the earth, the ancient hindu books say, is about , miles. that is to say, the true surface of the earth is the line of twenty-four-hour axial rotation; the line where gravity and apergy exactly balance; where a moon would have to be placed to revolve once in , seconds. within that is prakriti; without is ether. it is also the line of no friction, which does exist between matter of different planes. there is friction between prakriti, between ether, between prana; but not between ether and prana, or ether and prakriti. friction is a phenomenon confined to the matter of each plane separately. we live at the bottom of this gaseous ocean--on its floor -- , miles from the surface and only , miles from the center. here, in a narrow "skin" limited to a few miles above and below us, is the realm of phenomena, where solid turns into liquid and liquid into gas, or vice versa. the lesson impressed upon the pupil's mind by hindu physics is that he lives far within the earth, not on it. there is a comparatively narrow "skin" of and for phenomena within the etheric sun-globe, say the eastern teachers, where the etheric solids, liquids, and gases meet and mingle and interchange. within this "skin" are all the planets--the "gaseous" atmosphere of the etheric globe stretching millions of miles beyond the outermost planetary orbit. the earth is in this skin or belt of etheric phenomena, and its ether is in touch with the ether "in manifestation" on the etheric globe. the sun and other etheric globes are within the corresponding "skin" of phenomena of the pranic globes. the prana, manifesting as solid, liquid, and gas, or in combination and in forms, is in perfect touch with that of the etheric globe, and through that with the prana of the earth. that our prana is in touch with that on the pranic globe in all its manifestations means much in metaphysics. the same is true of the manasic globe, and of our manasa. the great lesson the eastern physics burns into the pupil is that we are living not only within the prakritic earth, but within each of the other globes as well in identically the same way and subject to the same laws. our lives are not passed on one globe, but in four globes. it is as if one said he lived in buffalo, erie county, new york, united states; that he was a citizen of each and subject to the laws of each. this question of the four globes, of the four planes of matter, of the four skins, and of the four conditions or states of all matter and necessarily of all persons, from the purely material standpoint, is not only the foundation of oriental physics, but the very essence of oriental metaphysics--its starting-point and corner-stone. to one who carries with him, consciously or unconsciously, the concrete knowledge of the physics, the abstract teaching of the metaphysics presents no difficulty; it is as clear as crystal. but without the physical teaching the metaphysical is not translatable. our western physics teaches that physical matter is divided into two kinds prakriti (commonly called "physical matter") and ether; that the differences of each of the elementary prakritic substances (iron, copper, sulphur, oxygen) are in their molecules, the fundamental atom being the same; that each of these elementary substances vibrates only through one octave, though on different keys; that it changes from solid to liquid and gas as the rate of vibration is increased and from gas to liquid and solid as its vibration is decreased within its octave; that the ether obeys identical laws; that it has elementary substances vibrating through one octave only, and that these are solids, liquids, or gases on the etheric plane as prakriti is on this; that these etheric substances change and combine in every way that prakriti does; and that while all our prakritic substances vibrate within (say) fifty simply octaves, the lowest vibration of etheric matter begins over one thousand octaves beyond our highest, making a gulf to leap. the eastern physics presents this with a wealth of detail that dazes the western student, and then adds: "but beyond the etheric plane (or octave) of vibration for matter there is a third plane (or octave) of vibration called prana and beyond that a fourth called manasa. what is true of one plane is true of the other three. one law governs the four. as above so below. there is no real gulf; there is perfect continuity." the western scientist teaches as the foundation of modern physics that "each and every atom of prakritic matter is the center of an etheric molecule of many atoms;" that "no two prakritic atoms touch," although their etheric envelopes or atmospheres do touch; and that "all physical phenomena are caused by the chording vibration of the prakritic atom and its envelope of ether," each "sounding the same note hundreds of octaves apart." the "solid earth" with its atmosphere represents the atom with its ether. as all the oxygen and hydrogen do not combine to make the drop of water, some remaining in mechanical union to give it an atmosphere, and about one-fourth of its bulk being gas, so the atom formed of the ether does not use all the ether in its chemical union, retaining some in mechanical union for its envelope or atmosphere. the hindu physics goes much farther along this road. it says that, when the pranic globes were formed, each atom of prana had its manasic envelope--was the center of a manasic molecule. when the etheric globes formed, each atom of ether was the center of a pranic molecule, each atom of which was surrounded with manasa. when the prakriti was formed from the ether, each and every atom of prakriti had the triple etheric-pranic-manasic envelope. "each and every prakritic atom is the center of an etheric molecule," says our western science; but that of the east adds this: "and each atom of that etheric molecule is the center of a pranic molecule, and each atom of prana in that pranic molecule is the center of a manasic molecule." the four great globes of matter in the material universe are represented and reproduced in each and every atom of prakriti, which is in touch with each one of the four globes and a part of it. the same is true of any aggregation of prakriti--of the earth itself and of all things in it, including man. as there are four atoms in each one, so there are four earths, four globes, consubstantial, one for each of the four elements, and in touch with it. one is formed of prakritic atoms--the globe we know; another, of the ether forming their envelopes; another, of the prana envelopes of ether, and a fourth of the manasa around the pranic atom. they are not "skins"; they are consubstantial. and what is true of atoms or globes is true of animals. each has four "material" bodies, with each body on the corresponding globe --whether of the earth or of the universe. this is the physical basis of the famous "chain of seven globes" that is such a stumbling-block in hindu metaphysics. the spirit passes through four to get in and three to get out--seven in all. the hindu understands without explanation. he understands his physics. the hindu physics teaches, with ours, that "the ether is the source of all energy," but, it adds, "as prana is the source of all life, and manasa of all mind." "when the prakritic atom is vibrating in chord with its etheric envelope," say our textbooks, "we have physical phenomena --light, heat, electricity." "yes," says the hindu teacher; "but when the atom and its ether and its prana are vibrating in chord, we have life and vital phenomena added to the energy. when the atom and its ether, prana, and manasa are vibrating in chord, we have mind and mental phenomena added to the life and energy." each atom has energy, life, and mind in posse. in the living leaf the prakriti, ether, and prana are sounding the threefold silver chord of life. in the animal, the manasa is sounding the same note with them, making the fourfold golden chord of mind. even in the plant there may be a faint manasic overtone, for the potentiality of life and mind is in everything. this unity of the physical universe with the physical atom, and with all things created--earth, animal, or crystal--is the physical backbone of oriental metaphysics. prakriti, ether, prana, and manasa are in our vernacular the earth, air, fire, and water of the old philosophers--the "four elements." the oriental physics has been guarded most jealously. for many thousands of years it has been the real occult and esoteric teaching, while the oriental metaphysics has been open and exoteric. it could not be understood without the key, and the key was in the physics known only to "the tried and approved disciple." a little has leaked out--enough to whet the appetite of the true student and make him ask for more. chapter two the two kinds of perception to the savage, matter appears in two forms--solid and liquid. as he advances a step he learns it has three forms--solid, liquid and gas. he cannot see the gas, but he knows it is there. a little further on he learns that matter as he knows it is only a minute portion of the great universe of matter--the few chords that can be struck on the five strings of his senses, and limited to one octave or key. whether the particular matter he investigates has a solid, a liquid, or a gaseous form depends upon its rate of vibration. if it is a liquid, by raising its rate of vibration one third it becomes a gas; by reducing it one third it becomes a solid. each kind of matter has vibration only through one octave. it is known to us only by its vibration in that octave. each kind of matter has a different octave--is set on a higher or lower key, so to speak, but all octaves of vibration are between the highest of hydrogen gas and the lowest of carbon. in mechanical compounds, such as air or brass, the rate of vibration of the compound is the least common multiple of the two or more rates. in chemical compounds, such as water or alcohol, the rate is that of the highest, the others uniting in harmonic fractions. all matter as we know it through our senses--prakriti, as it is called in the secret doctrine to distinguish it from non-sensual matter--is the vibration of an universal something, we do not know what, through these different octaves. the elementary substances (so-called) are one and the same thing--this something--in different keys and chords of vibration; keys that run into one another, producing all sorts of beautiful harmonies. taking any one of these elements, or any of their compounds, all we know of it is limited strictly to its changes during vibration through one octave. what happens when the vibration goes above or below the octave has not yet been treated hypothetically. while some elements are vibrating on higher and some on lower keys, we can consider them all as vibrating within one great octave, that octave of the universal something which produces sensual matter, or prakriti. but matter is not confined, we know, to this great octave, although our sensual knowledge of it is strictly confined to it. how do we know it? knowledge comes to us in two ways, and there are two kinds of knowledge. . that which comes through our senses, by observation and experience. this includes reasoning from relation. . that which comes through intuition--or, as some writers inaccurately say, "through the formal laws of thought." all the observation and experience of the rising and the setting of the sun for a thousand centuries could only have confirmed the first natural belief that it revolved daily around the earth; nor by joining this experience with other experiences could any deduction have come from our reason that would have opposed it. not our reason but our intuition said that the sun stood still and the earth revolved daily. the oldest books in existence tell us that this axial revolution of the earth was not only known in the very dawn of time but that it has been known to every race (except our own of european savages) from before the time thought was first transmitted by writing. ask the ablest living geographer or physicist to prove to you that the earth revolves daily and he will reply that it would be the job of his life. it can be done at great expense and great labor, but that is because we know the answer and can invent a way of showing it, not because there are any observations from which a deduction would naturally follow. nearly if not all our great discoveries have come to us through intuition and not from observation and experience. when we know the lines on which to work, when intuition has given us the key, then the observation and experience men prize so highly, and the reason they worship so devoutly, will fill in the details. the knowledge that flows from observation and the reasoning from the facts it records, is never more than relatively true, it is always limited by the facts, and any addition to the facts requires the whole thing to be restated. we never know all the facts; seldom even the more important; and reason grasps only details. lamarck's theory of evolution, known to all asiatic races from time immemorial, was the intuitional and absolute knowledge that comes to all men when they reach a certain stage of development. reason could never have furnished it from the facts, as cuvier proved in the great debate in the french academy in , when he knocked lamarck out, for the time being, because "it did not conform to the facts, and did not follow from any relation of the facts." darwin's theory of the survival of the fittest in the struggle for existence, as an explanation of the origin of species, was from observation and experience. it was based on observed facts. but darwin was an evolutionist--a disciple of lamarck. he held the key. he used the key. the value of darwin's work does not lie in his discovering that some bugs have been derived from other bugs and that the intermediate bugs have died off. its overwhelming value to mankind was in showing that work on the theory of evolution was correct work and that the theory was true. when the intuition of man points out the way the reason of man can follow the path and macadam the road. it usually does and claims all the credit for itself as the original discoverer. this knowledge through intuition is absolute and exact. it is not relatively true. it is absolutely and invariably true. no additional facts will ever modify it, or require a restatement. when sir william hamilton based his logic on the dictum that "all knowledge is relative, and only relatively true," the proposition was self-evidently false. it was in itself a statement of absolute knowledge about a certain thing. it was in itself knowledge that was not relative. all knowledge could not be relative if this knowledge was not. this knowledge could not be either absolute or relative without upsetting his whole proposition, for, if relative, then it was not always true; and if absolute, then it was never true. sir william did not know the distinction between the two kinds of knowledge, and what he meant to say was that "all knowledge obtained by observation and experience is relative, and only relatively true." his knowledge of this relativity was not obtained by observation or from reason. it could not possibly have been obtained in that way. it came from intuition, and it was absolute and exact. a man may have absolute and exact knowledge and yet not be able to put it into words that exactly express it to another. hamilton had this knowledge. but it was not clearly formulated even in his own mind. he had two separate and distinct meanings for the word "knowledge," without being conscious of it. we have yet to coin a proper word to express what comes to us through intuition. the old english word "wisdom" originally did. the old verb "wis" was meant what a man knew without being told it, as "ken" meant knowledge by experience. try and prove by reason that a straight line is the shortest distance between two points, or that a part can never be greater than the whole, and your reason has an impossible task. "you must take them for axioms," it says. you must take them because you wis them, not because you know (ken) them. intuitional knowledge must not be confounded with the relative knowledge that flows through the reason: that "if the sum of two numbers is one and their difference is five," the numbers are minus two and plus three. the point cannot be too strongly enforced that there is a distinction between the sources of what we know, and that while all we know through our sensations is only relatively true, that which we know from intuition is invariably and absolutely true. this is seen through a glass darkly, in theology, where intuition is called inspiration and not differentiated from reason. the false notion that we can only learn by observation and experience, that the concept can never transcend the observation, that we can only know what we can prove to our senses, has wrought incalculable injury to progress in philosophy. because our sensual knowledge of matter begins and ends with vibration in one octave it does not follow that this ends our knowledge of it. we may have intuitional knowledge, and this intuitional knowledge is as susceptible to reason as if we had obtained it by observation. the knowledge that comes through intuition tells us of matter vibrating in another great octave just beyond our own, which science has chosen to name the etheric octave, or plane. the instant our intuition reveals the cause of phenomena our reason drops in and tells us it is the chording vibration of the matter of the two planes--the physical and etheric--that produces all physical phenomena. it goes further and explains its variations. this knowledge of another octave or plane of matter comes from the logical relations of matter and its physical phenomena; but there was nothing in the observation or experience of mankind that would have led us to infer from reason an etheric plane of matter. it was "revealed" truth. but the flash of revelation having once made the path apparent, the light of reason carries us through all the winding ways. our knowledge of the ether is not guess-work or fancy, any more than our geometry is, because it is based on axioms our reason cannot prove. in both cases the basic axioms are obtained from intuition; the structural work from reason. our knowledge of the ether may be as absolute and exact as our knowledge of prakriti, working on physical as we work on geometrical axioms. the recognition of the two sources of knowledge, the work of the spirit within us and of the mind within us, is absolutely necessary to correctly comprehend the true significance of the results of modern science and to accept the ancient. chapter three matter and ether it is not worthwhile translating homer into english unless the readers of the translation understand english. it is not worthwhile attempting to translate the occult eastern physics into the language of our western and modern physics, unless those who are to read the translation understand generally and broadly what our own modern physics teach. it is not necessary that they should know all branches of our modern physics in all their minute ramifications; but it is necessary that they should understand clearly the fundamental principles upon which our scientific and technical knowledge of today rests. these fundamental principles have been discovered and applied in the past fifty years--in the memory of the living. they have revolutionized science in all its departments. our textbooks on chemistry, light, heat, electricity and sound have had to be entirely re-written; and in many other departments, notably in medicine and psychology, they have yet to be re-written. our textbooks are in a transition state, each new one going a step farther, to make the change gradual from the old forms of belief to the new, so that even tyndall's textbook on "sound" is now so antedated, or antiquated, that it might have been written in darkest africa before the pyramids were built, instead of twenty years ago. all this change has flowed from the discovery of faraday that there are two states or conditions of matter. in one it is revealed by one of our five senses, visible, tangible, smellable, tastable, or ponderable matter. this is matter as we know it. it may be a lump of metal or a flask of gas. the second condition or state of matter is not revealed by either of our five senses, but by the sixth sense, or intuition of man. this is the ether--supposed to be "matter in a very rarefied form, which permeates all space." so rare and fine is this matter that it interpenetrates carbon or steel as water interpenetrates a sponge, or ink a blotting pad. in fact, each atom of "physical" matter--by which is meant matter in the first condition--floats in an atmosphere of ether as the solid earth floats in its atmosphere of air. "no two physical atoms touch," said faraday. "each physical atom is the centre of an etheric molecule, and as far apart from every other atom as the stars in heaven from one another." this is true of every form of physical matter, whether it is a lump of metal, a cup of liquid, or a flask of gas; whether it is a bronze statue or a living man; a leaf, a cloud, or the earth itself. each and every physical atom is the centre of an etheric molecule made up of many atoms of the ether. this duality of matter was a wonderful discovery, revolutionizing every department of science. it placed man in actual touch with the whole visible universe. the ether in a man's eye (and in his whole body) reaches in one unbroken line--like a telegraph wire --from him to the sun, or the outermost planet. he is not separate and apart from "space," but a part of it. each physical atom of his physical body is the centre of an etheric molecule, and he has two bodies, as st. paul said, a visible physical and an invisible etheric body; the latter in actual touch with the whole universe. faraday went one step further. he demonstrated that all physical phenomena come from the chording vibration of the physical atom with the surrounding etheric atoms, and that the latter exercise the impelling force on the former. step into the sunshine. the line of ether from the sun is vibrating faster than the ether in the body, but the higher impels the lower, the greater controls the lesser, and soon both ethers are in unison. the physical atoms must coincide in vibration with their etheric envelopes, and the "note" is "heat." step into the shade, where the ocean of ether is vibrating more slowly, and the ether of the body reduces its vibration. "the ether is the origin of all force and of all phenomena." this etheric matter follows identical laws with prakritic matter, or, accurately, the laws of our matter flow from the etheric matter from which it is made. the ether has two hundred or more elementary substances, each atom of our eighty or ninety "elements" being the chemical union of great masses of two or more of the etheric elements or their combinations. these etheric elementary substances combine and unite; our elementary substances simply following in their combinations the law which they inherit from their parents. they take form and shape. they vibrate through one octave, and take solid liquid or gaseous form in ether, as their types here in our world take it in prakriti, as their vibrations are increased or diminished. in short, the ether is the prototype of our physical or prakritic world, out of which it is made and a product of which it is. as this ether is "physical" matter, the same as prakriti, one harmonic law covering both, and as this ether fills all space, modern science divides physical matter into two kinds, which, for convenience in differentiation, are here called prakritic and etheric. matter is something--science does not know or care to know what --in vibration. a very low octave of vibration produces prakriti; a very high octave of vibration produces ether. the vibration of prakriti ends in thousands; that of ether begins in billions. between them there is a gulf of vibrations that has not yet been bridged. for that reason science divides matter into two "planes," or octaves, of vibration--the matter of this visible and tangible plane being called prakriti and that of the invisible and intangible plane being called etheric. across this gulf the two planes respond to each other, note for note, the note in trillions chording when the note in thousands is struck. note for note, chord for chord, they answer one another, and the minutest and the most complex phenomena are alike the result of this harmonic vibration, that of the ether supplying force and that of the prakriti a medium in which it can manifest. this knowledge of ether is not guesswork or fancy, and, while it is as impossible of proof as the axioms of geometry, it is worthy the same credence and honor. we are working on physical axioms exactly as we work on geometrical axioms. modern science represents each and every prakritic atom as a globe like the earth, floating in space and surrounded by an atmosphere of ether. "the subdivision of prakritic matter until we reach etheric atoms chemically united to make the physical unit" is the correct definition of an atom. the prakritic physical atom has length, breadth and thickness. and it has an atmosphere of ether which not only interpenetrates the atom as oxygen and hydrogen interpenetrate the drop of water, but furnishes it with an envelope as the oxygen and hydrogen furnish the drop of water with one. each physical atom is the centre of an etheric molecule composed of many etheric atoms vibrating at a greater or lesser speed and interpenetrating the atom. each may be considered a miniature earth, with its aerial envelope, the air, penetrating all parts of it. the etheric plane of matter not only unites with this prakritic plane through the atom but it interpenetrates all combinations of it; beside the atom as well as through the atom. the grain of sand composed of many prakritic atoms is also composed of many times that number of etheric atoms. the grain of sand is etheric matter as well as prakritic matter. it exists on the etheric plane exactly the same as it exists on the prakritic, and it has etheric form as well as prakritic form. as each atom of this physical world of ours--whether of land, or water, or air; whether of solid, liquid or gas--is the centre of an etheric molecule, we have two worlds, not one: a physical world and an etheric one; a visible world and an invisible world; a tangible world and an intangible world; a world of effect and a world of cause. and each animal, including man, is made in the same way. he has a prakritic body and an etheric body; a visible body and an invisible body; an earthly body and one "not made with hands," in common touch with the whole universe. chapter four what a teacher should teach let us suppose that a certain wise teacher of physics places a row of bunsen burners under a long steel bar having a daniell's pyrometer at one end, and addresses his class (substantially) as follows: "at our last lecture we found that the matter of the universe permeated all space, but in two conditions, which we agreed to call physical and etheric, or tangible and intangible. it is all the same matter, subject to the same laws, but differing in the rate of vibration, the physical matter vibrating through one great octave or plane, and the etheric vibrating through another great octave or plane one degree higher--the chording vibration of the matter of the two planes in one note producing what we call energy or force, and with it phenomena. "this is a bar of steel inches long. it is composed of physical atoms but no two physical atoms touch. each physical atom is as far apart from every other atom as the stars in heaven from one another--in proportion to their size. the atoms and the spaces between them are so small to our sight that they seem to touch. if we had a microscope of sufficient power to reveal the atom, you would see that no two atoms touch, and that the spaces between them are, as faraday says, very great in proportion to their size. i showed you last term that what appeared to be a solid stream of water, when magnified and thrown upon a screen, was merely a succession of independent drops that did not touch. i can not yet give you proof of the bar of iron being composed of independent atoms, but that is the fault of our instruments, and you must take my word for it until the proof is simplified and made easy of application. "each one of these physical atoms is a miniature world. it is the center of an ocean of ether, composed of many atoms; and while no two physical atoms touch, their etheric atmospheres do touch, and any change in the vibration of the etheric atmosphere of one will be imparted to that of the next. as the vibration of the physical atom must be in harmony with that of its etheric atmosphere, any change coming to one will be imparted to the next, and the next, through the ether surrounding them. "you can see that the index at the end of the bar has moved, showing that it is now longer. that means the etheric atoms are now vibrating faster, taking more space, and have necessarily forced each physical atom farther apart. the bar is not only longer, but softer, and as the vibrations increase in rapidity the time will come when it will bend by its own weight, and even when it will become a liquid and a gas. "if you put your hand anywhere near the bar you will feel a sensation called heat, and say it has become hot. the reason for that is that you are in actual and literal touch with the bar or iron through the ether. it is not alone each atom of the bar of iron that is surrounded by the ether, but each atom of the air, and each atom of your body. their etheric atmospheres are all touching, and the increase in the vibration of the ether surrounding the atoms of iron is imparted to those of the air surrounding it, and these in turn raise the rate of vibration in the etheric atoms surrounding the physical atoms of your hand. this rate of vibration in your nerves causes a sensation, or mental impression, you call "heat." consciousness of it comes through your sense of touch; but after all it is merely a "rate of vibration" which your brain recognizes and names. "the bar has now reached a temperature of about degrees, and has become a dull red. why do you say the color has changed, and why do you say red? "because the rate of vibration of the etheric atoms in the bar is now about trillions per second, and this rate of vibration having been imparted to the ether of the air, has in turn been imparted to the ether of your eye, and this rate of vibration in the ether of the nerves of your eye your brain recognizes and calls 'red.' "the heat still continues and increases. you now have both heat and light. so you see that the ether is not vibrating in a single note, but in two chording notes, producing light and heat. there are two kinds of ether around the iron atom. there is sound also, but the note is too high for one's ears. it is a chord of three notes. "professor silliman, of yale, discovered over twenty years ago, that the ether could be differentiated into the luminiferous, or light ether, and the sonoriferous, or sound ether. "other great scientists since then have found a third ether--the heat ether. "their discoveries show that the atmospheric etheric envelope of each etheric atom is made up of etheric atoms of different vibratory powers. as the atmosphere of the earth is made up of atoms of oxygen and nitrogen and argon, so that of an atom is made up of three kinds of ethers, corresponding to three of our senses. that it consists of five ethers, corresponding to our five senses, as the ancient hindus assert--who can say? "i mention this subject of the differentiation of the ether merely that you may not suppose that the ether is a simple substance. for the present we will treat it as a simple substance, but next year we will take it up as a compound one. "this steel bar before you is not one bar, but two bars. there is a visible bar and an invisible bar, the visible bar being made of physical atoms, and the invisible bar of etheric atoms. the etheric bar is invisible, but it is made of matter, the same as the visible bar, and it is just as real, just as truly a bar as the one we see. "more than this. the etheric, invisible bar is the source and cause of all phenomena connected with the bar. it is the real bar, and the one we see is merely the shadow in physical matter of a real bar. in shape, strength, color, in short, in everything, it depends on the invisible one. the invisible dominates, governs, disposes. the visible is merely its attendant shadow, changing as the invisible, etheric bar changes, and recording for our senses these invisible changes. "the invisible change always comes first; the invisible phenomena invariably precede the visible. "in all this physical world--in all this universe--there is nothing, not even a grain of sand or an atom of hydrogen, that is not as this bar of iron is--the shadow cast on a visible world by the unknown and mysterious work of an invisible world. "land or water, mountain or lake, man or beast, bird or reptile, cold or heat, light or darkness, all are the reflection in physical matter of the true and real thing in the invisible and intangible world about us. "if we have a visible body we have an invisible one also," said saint paul. modern science has proven he was right, and that it is the invisible body which is the real body. "if this earth and all that it is composed of--land or ocean or air; man or beast; pyramid or pavement--could be resolved into the physical atoms composing everything in it or on it created by god or man, each atom of this dust would be identical physically. there would not be one kind of atom for iron and another for oxygen. "the differentiation between what are called elementary substances is first made apparent in the molecule or first combination of the atoms. it is not in the atom itself, unless it be in the size, as may not be improbable. the atoms combine in different numbers to make differently shaped molecules, and it is from this difference in the shape of the molecule that we get the difference between gold and silver, copper and tin, or oxygen and hydrogen. "in all chemical compounds, such as water and alcohol, the molecules at the base of the two or more substances break up into their original atoms and form a new molecule composed of all the atoms in the two or more things combined. to make this chemical combination we must change the rate of vibration of one or the other or both until they strike a common chord. as we saw last term, oxygen and hydrogen have different specific heats, and no two other elements have the same specific heat, while heat raises the rate of vibration. any given amount of heat raises the vibration of one more than another. apply heat, and the rate of one will rise faster than that of the other until they reach a common chord. then they fall apart and recombine. "if we pass a current of electricity through this sealed jar containing oxygen and hydrogen in mechanical union, the spark that leaps across the points furnishes the heat, and a drop of water appears and falls to the bottom. a large portion of the gases has disappeared. it has been converted into water. what is left of the gases will expand and fill the bottle. "the drop of water but for local causes, but for a certain attraction of the earth, would float in the centre of the jar at the centre of gravity, as the earth does in space. but the centre of gravity of the two bodies is far within the earth, and the drop gets as close to it as it can. the earth's 'pull' takes it to the bottom. if the jar were far enough away in space the drop would float, as the earth floats, at a point where all pulls balance, and the drop of water would have enough pull of its own, enough gravity within itself to hold all the gas left in the jar to itself as an atmosphere. it would be a centre of energy, a minature world. "the drop of water is not a homogenous mass. about one third of the bulk of the drop of water is made up of independent oxygen and hydrogen atoms interspersed through it, as any liquid is through this piece of blotting paper. and it has, and keeps, by its own attraction, an atmosphere of the gas. each molecule of water has a thin layer, or skin, of the gas; even as it comes from this faucet. "let us return again to the physical dust, the atom. why should it form by fives for iron, by nines for hydrogen? where did the atom come from? what is it? we know that like the drop of water, it is a miniature world with an atmosphere of ether; and the natural inference is that it is made from ether as the drop of water was made from gas. many things confirm this inference, and it may be accepted as 'a working hypothesis' that it is made from ether as the drop of water is made from gas, by the chemical union of a large amount of ether of different kinds, the etheric molecules of which consist of and or and etheric atoms, and that the tendency to combine in this or that number in physical matter is an inherited tendency brought with it from the etheric world of matter on which, or in which, each element of this world is two or more. there is no kind of matter in this physical world, that has not its prototype in the etheric, and the laws of its action and reaction here are laws which it inherits and brings with it. they are not laws made here. they are laws of the other world--even as the matter itself is matter of the other world. "in , professor lodge, in a lecture before the royal institution on 'the luminiferous ether' defined it as: "'one continuous substance, filling all space, which can vibrate as light, which can be sheared into positive and negative electricity, which in whirls constitutes matter, and which transmits by continuity and not impact every action and reaction of which matter is capable.' "this reads today like baby-talk but at that time (eighteen years ago), it was considered by many timid conservative scientists as 'a daring movement.' it is noteworthy in that it was the first public scientific announcement that the physical matter is a manifestation or form of the ether. and it was made before general acceptance of the division of the ether into soniferous, luminiferous and tangiferous. "'which in whirls constitutes matter.' professor lodge believed that 'some etheric molecules revolved so rapidly on their axis that they could not be penetrated.' watch the soap-bubbles that i am blowing. each and every one is revolving as the earth revolves, from west to east. what i wish to call your attention to is the fact that can be proven, both mathematically and theoretically, that at a certain rate of speed in the revolution they could not be penetrated by any rifle-ball. at a higher rate of speed they would be harder than globes of solid chilled steel, harder even than carbon. professor lodge believed that the etheric molecule revolved so rapidly that, thin as it was in its shell, it gave us the dust out of which worlds were made. there is one fatal error in this idea, although it is held even now by many. it is based entirely on gravity, and gravity is alone considered in its problems. there are two great forces in the universe, not one, as many scientific people fail to remember --gravity and apergy, or the centrifugal and centripetal forces. the pull in is and must be always balanced by the pull out. there is in the universe as much repulsion as attraction, and the former is a force quite as important as the latter. the bubble's speed kept increasing until apergy, the tendency to fly off, overcame gravity, and it ruptured. "professor lodge failed to take into account this apergic force, this tendency to fly off, when he gave such high revolutionary speed to the etheric molecules, a speed in which apergy would necessarily exceed gravity. the failure to take apergy into consideration has been the undoing of many physicists. "today we know that the ether is matter, the same as our own, only finer and rarer and in much more rapid vibration. we know that this ether has its solids, liquids and gases formed from molecules of its atoms, even as our own are formed. we know that its atoms combine as ours do, and while we have but eighty elementary combinations, it must have more than double the number. we know that every form and shape and combination of these elements from this plane flows from inherited tendencies having their root in the etheric world. "the two worlds are one world--as much at one with ours as the world of gas about us is at one with our liquids and solids. it is 'continuity, not impact.' they not only touch everywhere and in everything, but they are one and the same in action and reaction." thus spake a certain wise teacher of physics. to his wise utterances, we can only add that such as we are today "we see through a glass, darkly." yet there will come a day when the physical bandages will be removed from our eyes, and we shall see face to face the beauty and grandeur and glory of this invisible world, and that in truth it 'transmits by continuity and not impact every action and reaction of which matter is capable,' forming one continuous chain of cause and effect, without a link missing. there are no gulfs to cross; no bridges to be made. it is here; not there. it is at one with us. and we are at one with it. one and the same law controls and guides the etheric atom and the physical atom made from its molecules, whether the latter are made in "whirls," as at first supposed, or by orderly combination as now believed. in fact, this visible world of ours is the perfect product of the other invisible one, having in it its root and foundation, the very sap of its life. chapter five the four manifested planes the oriental idea of the universe does not differ fundamentally in its general conception, from that of modern science, but it goes farther and explains more. the physics of the secret doctrine are based upon a material universe of four planes of vibration and a spiritual universe of three planes of vibration beyond matter. this something in vibration may be given the english name, consciousness--without entering upon its nature. spirit is consciousness in vibration and undifferentiated. matter is consciousness in vibration and differentiated. as we divide the seven octaves of a piano into treble and bass for clearness of thought and writing, so the hidden knowledge divides the seven octaves of vibration, or planes, into spirit and matter. in their ultimate analysis they are one and the same thing, as ice and water are the same thing; but for study they must be differentiated. the material and physical universe consists of four planes of matter, or four great octaves of vibration, each differentiated from the other as in our physics prakriti is differentiated from ether. the material universe, the ancient physics teach, was originally pure thought, manasa, the product of the spiritual planes above. this manasic world was differentiated, a real world. that is to say it was given elementary substances by the union of its atoms in different sized molecules. some of its elements combined and formed prana. the prana gathered and formed other worlds, pranic worlds. then in the pranic world etheric worlds were formed; and finally in the etheric worlds, prakritic globes like the earth were formed. the earth is the centre of a prakritic globe, revolving in ether around the sun. the sun is the centre of a solar globe of ether, revolving in prana around alcyone. alcyone is the centre of a stellar globe of prana revolving in manasa around the central and hidden sun of the great manasic globe. these four conditions of matter prakriti, ether, prana, and manasa are the earth, water, fire, air of the ancient metaphysics, the four elements of matter, and are present in every atom of prakriti. when the atom of prana was formed, it had an envelope of manasa. when the atom of ether was formed it had an envelope of pranic-manasic atoms. when the prakritic atom was formed it had an envelope of etheric-pranic- manasic atoms, each of its encircling etheric atoms being the centre of a pranic molecule, and each pranic atom of that molecule being the centre of a manasic molecule. each atom of prakriti was the material universe in miniature. it held the potentialities of mind, life, and phenomena. in every aggregation of atoms, there were the four planes, each in touch through the cosmic mind, its manasa, with other atoms in the universe, with every other globe of whatever kind. "as above, so below," was the secret key-word. the unity of all the material universe in its prakriti, ether, prana and manasa, was the corner stone of this knowledge. the three planes above prakriti were called astral, and in common speech there was the ordinary division into two planes, visible and invisible, or "spirit," as the invisible was called, and "matter," as the visible was called. only in the hidden secret doctrine of physics, and in the open metaphysics which were a "stumbling block" and "foolishness" to those who had not the "inner light" of the physics, were the three divisions of the "astral" made known, and the true distinction between the spirit of the three higher planes and the matter of the four lower was kept out of the metaphysics, or only vaguely alluded to. there is no "oriental science" because the oriental does not attach the same value to merely physical knowledge that we do. but that must not be understood to imply that there is no oriental physics. in all the matters that interest us now, as far as principles are concerned, the oriental knew all that we know. he knew it thousands of years ago, when our ancestors were sleeping with the cave bears. "that is all the good it did him," the scientist says. no. that is not true. it is perfectly true that the oriental, the babylonian who carved on the black stone now in the british museum the five moons of jupiter, exposing himself to the derision of our astronomers prior to their own discovery of the fifth moon in , did not care particularly whether there were four moons or five, and had no sale for any telescopes he might make, for no one else cared particularly. but it was not true that he did not care for any and all knowledge that would improve his spiritual condition by giving him correct ideas of the universe and of his own part in it. to him life was more than meat and the body more than raiment. he was more afraid of sin than of ignorance. we are more afraid of ignorance than of sin. he preferred to better men's moral condition; we prefer to better their physical condition. if one of the sages of the east could be called up and put on the stand to be questioned, he would say, substantially: "you are right in regard to your ether, and to prakriti being ether that has been dropped a great octave in vibration. your physical atom is surrounded by a molecule of ether, this molecule containing many atoms of ether. the chording vibration does produce all physical phenomena. "but where did the ether atom come from? how can you explain how and whence life comes, or what it is? this explains physical, but how do you explain vital phenomena? "you are wrong in assuming that all the matter of the universe apart from the earth or planets is ether and only ether. the etheric world in which you are interested ends with your solar system. it ends with each solar system, to the people of that system. between each solar system and another there is another form of matter that is not ether. "this etheric solar world of ours is very large, many billions of miles in diameter; but it is not the whole universe. you know that the sun and all its planets are revolving around the star alcyone. your astronomers told you that years ago, and they have recently given you the rate of speed as , miles per hour. "did you not see and know that if they had this revolution around a central sun it must be within a solar globe? "did you think that the sun and its planets, and other suns and their planets, were tearing their way through the ether like so many fish on a dipsy-hook from a marblehead fishing smack running before the wind? "did it never occur to you that the ether of this solar system must be revolving around this central sun? the whole solar system, ether and planets, are revolving around alcyone, and the reason why their minor revolution around the sun is not affected by it is because the solar system is a vast globe of ether, having a thinner and rarer medium to revolve in, the same as our earth has. it is the motion of a fly in a moving car. "now fix your attention on this globe of ether, this solar globe. you must do it to get the concept before you. you have known of it all your life without once really apprehending it, for you have never learned to think, or to utilize the knowledge that was given you. the idea is as new and as strange as if you had never known it. "what lies beyond the surface of the solar globe? something must; something as much rarer and thinner than the ether as the ether is rarer and thinner than prakriti. can you not guess? "it is prana, the life force of the universe. as prakriti is made from ether, so ether is made from prana. it is made in the same way. each atom of the ether is the centre of a molecule of prana, surrounded by an atmosphere of pranic atoms, exactly as your prakritic atom is surrounded by an atmosphere of etheric atoms. you say that each atom of prakriti is the centre of a molecule of ether. so it is. but each atom of that etheric molecule is the centre of a pranic molecule. each atom of your physical matter is triple, not double. "you say that all physical phenomena come from the chording vibration of the etheric and prakritic atoms of the two planes of matter. yes. but do you not see that all vital phenomena come from the chording vibration of the pranic, etheric, and prakritic atom of the three planes of matter which are in each atom? "in the living leaf the three planes are sounding in chord in each atom of it. in the dead leaf, drying up and falling to pieces, only the lower two are sounding in chord. the silver chord has been broken. "each atom of prakriti you say has the potentiality of some kind of phenomenon. we add 'and of life also.' the potentialities of life are in every atom of prakriti. even the atom of iron may live in the blood. it cannot become a part of any living organism until its prana is sounding the chord of life in unison with the ether and prakriti--the threefold silver chord. "what is the centre of this prana? it is alcyone. there are other solar globes beside ours circling around alcyone, and we have been considering only our own solar globe of ether. alcyone is the centre of the prana in which they revolve as the sun is the centre of our ether in which the planets revolve. as this prana has a centre around which we revolve with other solar systems, then it must have a center of gravity. "then this prana is a globe. "the prana does not then fill this material universe. there must be yet another form of matter rarer and finer than prana, from which prana is made, as ether is made from prana and prakriti from ether. have we any other class of phenomena to explain, except vital and physical? yes, there is a very important class, mental. and here we have the explanation, if we exercise our reason. "these pranic globes are floating in an ocean of manasa, matter in its rarest form. "each atom of prana is formed from manasa, exactly as ether was formed from prana, and each pranic atom in the universe is the centre of a manasic molecule, having an atmosphere of manasic atoms. "so we are not exact in giving the prakritic atom three planes or octaves of vibration. it has four. you merely surround it with etheric atoms, and this is correct so far as it goes. you only wish to explain physical problems. but there are other problems to be explained, problems of life and mind, and the same knowledge you have explains them as well as the others, if you simply avail yourself of it. that you do not consider the atom as four-fold instead of two-fold is your own fault. i have not told you anything you did not already know. i have only asked you to apply your present knowledge of physics to these problems of life and mind, and apply your reasoning powers. "the chording vibration in an atom of matter of "the two planes produces force, or phenomena "the three planes produces life--the silver chord "the four planes produces mind--the golden chord. "you say there is no gulf between the prakritic and etheric worlds; that it is one continuous world; and all its phenomena are by continuity and not impact. that is true, but it is not the whole truth. "there is no gulf to cross between the prakritic and etheric worlds; none to cross between that and the manasic. the four worlds are one great world, continuous, interchangeable. through the four as well as through the two, there is continuity and not impact. whether it is an atom or a world, the four are there. nothing, no combination of atoms, no matter of any kind, however small or large, can exist in this prakritic world unless it has the four elements, which from time immemorial our philosophers have called earth, water, fire, air, meaning the four globes or forms of matter in the universe. we do not have to leave the earth to live in the etheric globe. it is here. nor do we have to go millions of miles to reach the pranic globe. it is here. the problems of light and heat are no easier than the problems of birth and death. the pranic globe is within us; within everything. so is the manasic. "it is here on these higher planes that the chances for worthy study are greatest. at least we think so, though you may not. we live on the manasic--pranic--etheric globe on precisely the same terms that we live on this of prakriti, and the problems of the three are equally open to us. "if there are any who care to follow up the line of thought i have opened, who care for the questions that interest us of the east, i will talk as long as they care to listen, provided they will not ask for knowledge that will give them power over others, which cannot fail to be used for evil." this is but a glimpse of hindu physics, yet it has helped us in the metaphysics. we now understand the chain of globes--in part. the earth is fourfold. as each atom of the earth is fourfold, so their aggregations give us prakritic earth, an etheric earth, a pranic earth, and a manasic earth--in coadunition and not like the skin of an onion. they are separate and distinct globes, each on its own plane. it is four down and three up for the angel entering matter, whether from the outmost boundary of manasic matter, or the surface of the earth, or the cover of a baseball. the "chain of globes" in the secret doctrine represents the unity of the material universe. the three-fold nature of the astral model is revealed, and the unity of all prakritic things. but more than that, to many minds, will be the explanation it gives of why there are but four planes of vibration in matter; that the highest form of development in prakriti shows only four elements, prakriti or body, sensation or force, life, and mind, and that these last three, present in all things in esse, become present in posse when they work together harmonically. chapter six our place on earth the next time our wise man from the east was asked to "say a few words and make his own topic," he spoke, perhaps, as follows: "how large do you think the earth is? you answer promptly, , miles in diameter. you are as far out of the way as you were in supposing that our sun could be a centre of gravity of a lot of planets revolving around it and around alcyone without being a globe of ether. now that it has been mentioned, you see very clearly for yourself that it must be a solar globe of ether. it follows from one of your physical axioms. when i tell you why the earth is and must be about fifty thousand miles in diameter, you will see that it must be so, and that you knew it all the time, but never stopped to formulate your knowledge. you have had the knowledge for three centuries without applying it. "it was in that your greatest astronomer, john kepler, announced as one of three harmonic laws by which the universe was governed, that the squares of the times of the planets were proportional to the cubes of their distances from the sun; and that this law was true in physics and everywhere. no one of your scientists has had the wisdom to study out what it meant, and for three centuries, for years, you have repeated his words like so many parrots, instead of using the key he gave you to unlock the mysteries of the universe. a corollary of his law is that the planets move in their orbits because they are impelled thereto between the two forces, and move in a mean curve between them; but it was not until that you discovered that the mean between two forces is always a curve and never a straight line. you have not a text book in a school today that does not repeat this fundamental and absurd error--which you have known for three centuries to be an error--that the motion resulting from a mean between forces is "in a straight line." the curves resulting here are not to be measured easily, and are so large that small segments appear straight lines; and it was not until carpenter demonstrated it mathematically that any one could believe it true. "there are two great forces in this universe. your grandfathers called them centripetal and centrifugal forces; your fathers called them gravity and apergy, names which still cling to them; and you call them attraction and repulsion. "it was kepler, not newton, who discovered that attraction or gravity was in inverse proportion to the square of the distance. "you know the meaning of this mystic phrase, 'as the squares of the distance.' you understand that it means the attraction at two feet is only one-fourth the attraction at one foot; at four feet only one-sixteenth; at eight feet, only one sixty-fourth. "but who knows or cares for kepler's great law of repulsion, or apergy? that was that the 'square of the times are as the cubes of the distance.' it has lain fallow for centuries. no one of your western physicists has ever studied it, or tried to explain it. it remains just where kepler left it, as the mere law of orbital revolution of the planets only. "it is the key to the proper understanding of the universe. "'the squares of the times are as the cubes of the distance' means that all motion is the result of two forces acting upon prakriti, and that where the two forces are balanced, or equal, the result in motion is a circle or ellipse, the square of the repulsion being equal to the cube of the attraction to make them equal and produce a circle. in other cases they produce hyperbola and parabola. "this is a little dry--nearly all fundamental knowledge is--but the reward of patience is great. "the orbital speed of the earth is about , miles per hour. the attraction of the sun exactly equals the repulsion created by the motion; more accurately, the speed created by the repulsion. the result of the two forces working together at exact balance is a circle. an ellipse is a circle bent a little, and the ellipse in which the earth actually moves comes from varying attraction and repulsion. kepler's second law covers that. "if the orbital speed of the earth were a mile less per hour, or even a foot less, then the earth would wind up around the sun as a dog gets wound up with his chain around a tree. if this speed were a mile more per hour, then the earth would wind out, each year getting farther and farther away, until finally it would be lost. when the speed is exactly proportional to the pull--that is, when it is as . is to ,--the result is a circular orbit, the eccentricity of which is caused by certain fluctuations in the attraction and repulsion. "suppose a planet were to be placed so that it would have a time of two years. its distance from the sun would be . that of the earth. why? because to get the time doubled we would have to take the square root of ; and to get the distance the cube root of the same number, . if you wish to be very exact the cube root is . , but . is near enough for all ordinary work. "if you wanted to find out the distance of a planet revolving in six months you would divide the earth's distance by . . "in proportion you get any time or distance you may desire with absolute accuracy. the distance of any planet from the sun gives its time, or its time gives its distance--when that of any of the others is known. this law applies throughout the universe; in everything and everywhere. it is not a law of orbital revolution alone, but a law of all motion. "our moon has a time of days and a speed of about , miles per day. if the speed were greater it would leave us, if less it would wind up, falling to the earth in the form of a spiral. "at what distance would it have to be to have a time of fourteen days? divide , miles by . . a seven-day moon, would be . that distance. and the exact distance for a one-day moon, for a moon that would always be in the same place in the heavens, moving as the earth revolved on its axis, would be about , miles. "this gives us the line of -hour axial rotation, the true surface of the earth, and the sheer-line of prakritic matter. beyond that line is the ether; within that line is prakriti. "it is the line of no weight, where gravity and apergy exactly balance. inside that line gravity exceeds apergy and everything revolving in less time, or that time, must fall to the centre. it is the true surface of any -hour globe of this size and weight. a moon to revolve around the earth in less than one day must move faster than the earth to develop enough apergy to overcome the attraction. that phenomenon we see in the moons of mars, which are within its atmosphere; within the planet itself. "we of the east learned this true size of the earth over six thousand years ago, from observing the moons of jupiter. the times of the first three are doubled. we asked ourselves what this meant and found that their distance was increased by the cube root of when their times were increased by the square root of ; that time was to distance as . was to . then we applied the key, and found it unlocked many mysteries. "the first lesson this taught us was that we did not live on the earth, but within the earth, at the line of liquid and gaseous changes, where the three forms of matter meet and mingle and interchange with each other. we lived at the bottom of a gaseous ocean , miles above us, and , miles from the centre of the globe. it gave us an entirely new conception of the earth, and of our place in it. "we saw that we lived in a narrow belt, or skin, of the earth, not more than miles thick, perhaps not more than ten miles. within this belt the prakritic elementary substances varied their condition, combined, and made forms by increasing or decreasing vibration. it was the creative and destructive zone, the evolutionary "mother"--the liquid level of the prakriti--the seat of all physical phenomena. fifty miles above, the masses of nitrogen and oxygen and argon were too cold to change their rate of vibration. fifty miles below the surface of the earth all things were too hot for changes in vibration. in this kinetic belt, between two static masses our bodies had been made, and also, in all probability, all combinations of the elementary substances. it was four thousand miles to the centre of the static prakritic mass beneath us; twenty-one thousand miles to the surface of the static prakritic mass above us, and the small kinetic belt between was only one hundred miles thick. but we had one consolation, the prakriti we had was all kinetic, and the best in the whole mass. "the second lesson it taught us was that as the earth had been made in the etheric globe, in a corresponding skin or plane of kinetic etheric energy, with our ether the best of the solar output, that we ourselves were subject through our ether to the phenomena of that kinetic solar plane in precisely the same way we now are to the phenomena of the kinetic prakritic plane. once rid of the fallacious notion that we were creatures of the surface of the earth, once clearly conscious that we were creatures of the interior, of the bottom of this gaseous ocean, then we could understand not only how the earth could be created in this etheric globe, but how we could be creatures of the solar globe living on it. "when we learned that lesson, and learned it well, it dawned upon us that we were living in the pranic globe at the same kinetic level or plane of that globe, the line where its solids and liquids and gases mingled and passed from one state to another, the kinetic belt in which our solar globe has been made, and that we were living as truly on that globe as we were on this prakritic globe. our position on each globe was the same. "and then the great truth came that we lived in the manasic globe, at the same kinetic level; and that we lived our lives on the four globes simultaneously. our bodies are fourfold. every atom is fourfold, ready to respond in our minds to the vibrations of the manasic world, in our vitality to the pranic vibrations of the pranic world, in our nerves to the etheric vibrations of the etheric world, and in our prakriti to vibrations of the prakritic world. each one of our bodies lived on its own earth globe, for there were four globes of this earth--in coadunition--in its corresponding kind of globe. "the four earth globes became one globe, as our four bodies were one body; and the chain of four kinds of globes in matter became one globe, as the manasic with the others on it. "these four kinds of globes were the beginning and the end of matter, as we distinguish and know matter. they were not the end of vibration; or of planes of vibration; or of realms beyond this material universe; but they were the limits of all that is common to each and every atom of this lower plane of vibration. "it is upon this solid and perfect foundation of physics, that accounts for and explains every kind of phenomena, we have constructed our metaphysics. all that belongs to these four lower planes we consider and treat as physics. all that relates to the planes beyond we consider metaphysics. can you teach a child equation of payments before he knows the first four rules? you would not attempt such a task. the first four rules are the physics of arithmetic; all beyond is the metaphysics of arithmetic. it flows out of them. can you comprehend our system of metaphysics until you have clearly and completely mastered our physics? would you not get into a fog at the very start? "there can be no system of metaphysics without a solid foundation of physics. the idea is unthinkable. the one grows out of the other. it is its life; its fruit, its flower. "you have no western system of physics. your physics are without form and void; patchwork, constantly changing. there is no substantial foundation for any system of metaphysics. what you say or do in physics is fragmentary or chaotic. "it is perfectly true, so far as you have gone through the first invisible world of ether, you are much more masters of detail than we are. "we have not cared particularly for the minor details by which explosives are made, or metals obtained from oxides. we have preferred to push on into realms beyond as fast as we could, seeking first the kingdom of heaven and its righteousness, knowing that when it was found all these things would be added unto us." chapter seven the four globes that we live in the earth, not on the earth, is one of the most important of the facts of eastern physics in the study of its metaphysics. the mathematical and physical proof that the physical earth is , miles in diameter should not be passed over lightly in our haste to get on, for the perfect understanding of all this fact implies makes easy the comprehension of how we live etherically in the solar etheric globe, of how we live pranically in the stellar pranic globe, and how we live manasically in the manasic globe. as we live within the narrow "skin" of phenomena, not more than miles thick, of this prakritic globe, with the whole earth within the corresponding skin of phenomena of the solar etheric globe, within the kinetic belt in which it was made, the ether which surrounds each prakritic molecule is not merely any and every kind of ether, but that particular kind of kinetic ether, which, by changing its rate of vibration through an octave, creates phenomena. the ether of all prakritic matter belongs to the kinetic or creative belt of the solar etheric globe. it is not static ether. the ether in our prakriti is in touch with all the prakritic kinetic ether of the solar globe, subject to all solar laws of change; and all our prak-solar laws of change; and all our prakritic matter, a mere detail of it, is a part of the solar phenomena. "our father, the sun," or "dyaus pitar" ("heavenly father"--latin, jupiter) meant more once than it does now. then the solar globe was the first heaven, and to live under its laws, puttings off the coat of skin, was an object which men believed to be worth striving for. they recognized, as we do not, that our prakritic laws were not all they had to obey; that the higher law of the solar globe on which they lived, of which the lower prakritic laws were merely an outcome and detail, was worthy of the closest study. and they recognized that these higher laws of the etheric globe were metaphysical as well as physical; that our moral law flows out of the moral law of the solar etheric world, as our physics flow from and out of solar physics. religion is correct in its assumption of this higher law of morals; incorrect only in its grasp and explanation. science is correct in holding only in its assumption that it is physical science; incorrect only in its assumption that it is physical science of this plane and globe only. there is no quarrel between science and religion when the full knowledge of one stands beside the full knowledge of the other. they are twin-sisters. this solar-etheric globe in which we are interested revolves around alcyone within that kinetic belt or skin of prana which is subject to phenomena or vibration through one octave--else it would never have been formed. all prana in the solar-etheric globe is of this particular kind of kinetic prana, which creates life of all kinds--which is subject to vibration through one octave. the solar globe is a detail of kinetic prana only, one of its phenomena. necessarily, all our prana is of this kinetic kind, and our earth a minor detail of it in the alcyone globe. all the changes and combinations possible in kinetic prana on the pranic globe are possible here, in our kinetic prana, as all the phenomena of the etheric world are possible here in our kinetic ether. as our earth is a globe of ether and a globe of prana as well as a globe of prakriti; we are actually living on a small "cabbage" of that pranic globe, and subject to all its laws. in the vast manasic globe that includes this whole material universe there is the same kinetic belt or skin of "phenomena" or vibration similar to that kinetic belt in which we live on the earth, and the manasa which permeates the alcyonic globe, the solar globe, and the earth is that kinetic manasa which is involving and evolving. this involving and evolving kinetic manasa of the alcyonic globe is that which surrounds every atom of ether of the solar globe and every atom of prakriti of this earth globe. in the great manasic globe this earth of ours is a minute village of helios (sun) county, in the state of alcyone. we are actually and literally living in this manasic globe precisely as we live in this earth, and as in the village we are subject to all the laws of the manasic world, we can study them here in this village as well as we could elsewhere. we can study them as easily as we study our prakritic village laws, or our etheric county laws, for all the forms of manasa subject to them anywhere are here with us. we are not limited to a study of the prakritic laws of the village fathers, nor yet to the etheric laws of the supervisors of helios county, as scientists say, nor even to the state laws of alcyone; only the manasic laws of the universe limit our material studies in that direction. as some men on this earth never leave their native village and never know or care for any matters outside of it, so in this little earth village, in the kinetic belt of the manasic globe, there are men who do not care to know anything which relate to matters outside its boundaries. as some men may pass the boundaries of their village, but not of their county, caring only for the matters concerning it, so the western scientists of this earth village on the manasic globe do not pass the boundaries of helios county, caring only for etheric matters. the philosophers and wise men of the east are broader minded and from time immemorial have taken greater interest in the pranic affairs of alcyone and the manasic condition of the universe in which alcyone is a state than in the rustic murmur of their village or the gossip of their county. there is nothing lacking in our manasic earth-village, nothing that is in more abundant measure in our county, state, and nation. we are of the best. we of this village may imagine, if we like, that there is nothing beyond the village limits, and nothing in it but that which relates to the village. we have the right to be silly, if we wish to be. and it is no sign of wisdom to say that there is a county beyond, but that the county boundaries end all, and only village and county politics may be studied. the european who believed--no asiatic or african or american could have believed --that the earth rested on an elephant and the elephant on a turtle was wise, in comparison. nor is it any sign of intelligence to say that we may learn something of the village and county while we live, but that to learn anything about the state and nation we must wait until we are dead. there are too many in the village who are familiar with both state and nation, and who have studied their laws, for this to be anything but idiotic. chapter eight the battle ground each and every one of our eighty-odd elementary substances owe their condition--whether solid, liquid, or gas--to their rate of vibration. we have reduced all gases to a liquid and nearly all to a solid form. conversely, we have raised all solids to a liquid and nearly all to a gaseous condition. this has been done by reducing or raising the vibration of each within one octave --each one of the eighty odd having a special octave, a tone or half-tone different from any other. normally, the solids, vibrating in the lower notes, gather together under attraction; while the gases, vibrating in the higher notes, diffuse under repulsion. between them, created by the interchange of these two forces, is our "skin" of phenomena, or kinetics. broadly, the attraction of the universe comes from its vibration at certain centres in the three higher notes; the repulsion comes from its vibration everywhere else in the three higher notes. the central note, d of the scale, represents the battle ground between the field of kinetics. this in simple illustration is water turning into gas. this is the great battle ground, the only one worth considering in a general view. there are minor "critical stages" which the chemist studies, but for us, in this broad sketch of the universe, the important battle-ground is that between solid and liquid on one side representing gravity, and gas on the other, representing apergy. all the solids and liquids of this earth of ours gather at the centre, in a core, each of the elements (or their combinations) in this core vibrating in their three lower notes, producing the attraction, which is "in proportion to the mass" and which decreases from the surface of the core "as the square of the substance." around this central core gather all the elements vibrating in the three higher notes of their octave as gases, producing repulsion which increases by . for each doubled time. it is worth while making this clear. it has never before appeared in print. let the amount of apergy, or repulsion, or centrifugal force at the surface of the earth be represented by x. this is the result of motion at the rate of , miles per hour. make this motion , miles per hour, and the apergy is increased . . four thousand miles above the surface of this earth the rotation is at the rate of , . it is the globe of , miles in circumference revolving in hours, and the speed is doubled. this apergy has increased by . . as the apergy increases at this rate every time the speed is doubled, at a distance of , miles the speed is , miles per hour and the centrifugal force has been increased nearly four times what it was at the surface of the ocean. the attraction has been decreased to about one-thirtieth. at the surface it is equal to x. at , miles to one-quarter, or x; at , miles to one-sixteenth, or x; and at , miles to x. if "equatorial gravity is about times that of the equatorial apergy," at the ocean level, then at the distance of , miles from it, in a revolving globe, the two forces would be equal; the "pull" of each being x, and an anchor will weigh no more than a feather, for weight is the excess of gravity or apergy. if the pyramids had been built of the heaviest known material on the gases , miles above us, and so that they should revolve in the same time, , miles per hour, they would remain there. all the attraction of the solid core of the earth that could be exerted on them at that distance would not be enough to pull them an inch nearer to it through our gaseous envelope. their gaseous foundation there would be as firm as igneous rock here. the force of repulsion created by the three higher notes of an octave means just as much at the attraction created by the three lower notes, whether it is in a chemical retort, within this earth, or within this universe. the two forces balance, and are exactly equal. they fight only within kinetic zones. given the vast manasic globe of differentiated matter, its atoms uniting in different numbers to form molecules as the bases of elementary substances, manasic substances, of course. the thrill of vibration is sweeping through it from the spiritual plane above, and the elements (and their combinations) which answer in the lower notes gather and form a core, the invisible central sun, with its attraction. the elements answering in the higher notes gather around it with their repulsion. so the two opposing forces were born, with a vast kinetic skin for a battle-ground between them. the attraction of the invisible central sun manifests itself to us in prakriti as light. the repulsion of its covering, or the higher static vibration of manasa, manifests itself to us as darkness. the first creative act in or on matter was the creation of light and its separation from the darkness. the next creative act was the establishment of a kinetic skin or zone between them, a firmament in which the two forces of light and darkness could strive for mastery. "and god called the firmament heaven." the third creative act was the gathering of the solids and liquids together, and the beginning of the kinetic work in the creation of forms and shapes, by the cross play of the two forces in their combinations of solid with gases. all this had to happen before the manasa combined and dropped in vibration to prana--and before the pranic globes were formed and the light could be manifested to us through them. it may be well to read the first chapter of genesis over and ask forgiveness for our ignorance, from the writer who records this creation of the pranic globes as the fourth act of creation, and the creation of the etheric sun and prakritic moon to follow that. that record is mutilated, fragmentary; but the writer of it knew the facts. if we had the full story, instead of a sentence here and there, taken from an older story not to tell of creation but to hide another tale for the priest, the writer of genesis would laugh last. but let us return to the kinetic skin of energy between light and the darkness--the firmament which god calls heaven--the battle ground for gravity and apergy, or attraction and repulsion, or good and evil, or the powers of light and darkness. this skin is like that of an onion, thickest at the equator and thinnest at the poles--not only on this earth but in the solar, alcyonic, and manasic globes. the equatorial belt, where phenomena are richest in the manasic globes, we call the milky way; in the solar globe we call it the plane of the ecliptic; and on the earth, the tropics. modern science has not yet found it in alcyonic globe--because it has never thought of looking for it. this division of the light from the darkness was all that was required for evolution on the manasic globe within the kinetic belt. this evolution was not confined to the making of a few alcyonic or pranic globes. it was (and is) a great and wonderful evolution beyond words and almost beyond imagination. it is the heaven which mankind has longed to see and know. the writer of genesis mixed it with the creation of this earth, using earthly metaphors. before finding fault, we should better his language. we have not the words in physics to do it, and must wait for our metaphysics. but of one thing we may be sure, that the pranic- alcyonic globes here and there at the "sea level" of the manasic globe--in what god calls heaven--amount to no more on that globe, or in heaven, than so many balls of thistle-down blown across a meadow do on this earth of ours. everything that can be created in thought must be there. it is in thought only, but in thought it is differentiated as sharply as anything in prakriti. the manasic world, the heaven of the bible, is as real as our own world can possibly be; in fact, more real, for when ours is resolved back into its final elements, it will be but "the dust of the ground" of the manasic world. the pranic globes created in this manasic skin by sound, or the logos, or vibration, evolved in identically the same way--with a central static core and an outer static envelope, of low and high vibration in prana, creating attraction and repulsion, or gravity and apergy. the kinetic skin between, in which these forces play in the pranic world, makes a real, not an imaginary pranic world, though but a faint reflection of the manasic. when our father, the central invisible sun, transfers his attraction to these alcyonic suns, the light has something in which to manifest itself, and we "see" this manifesting core and call it alcyone, and its manifestation light; but light in its last material analysis is but the static mind or thought vibrating in the three lower notes of the octave. chapter nine the dual man within the alcyonic globes of differentiated pranic-manasic atoms the vibration divided them also into solid-liquid cores and gaseous envelopes, and a kinetic skin of phenomena. and then a new world--a world of life, came into material existence. all the atoms of thought or manasa, surrounding each and every pranic atom, and making its molecule of energy, so to speak, were that particular kind of kinetic manasa ready to change its rate of vibration within an octave, and the forms prana assumes from the action of thought within the kinetic belt were living and thinking. each pranic globe, which was a small state of product of the manasic, consisted of two globes in coadunition--two in one. each pranic atom was the centre of a manasic molecule and represented the universe. all things were two in one, created by harmonic vibration between them, and existence by the greater strength of the lower notes, or attraction. it was at once less and more wonderful than the manasic world--a specialized form of it. when within this kinetic belt of the prana the etheric solar globes formed here and there, they were three fold, each atom of the new plane of matter having its surrounding envelope of prana- manasa--a specialization of the pranic world in which (what we call) force had been added to life and mind. the static ether, vibrating in each of its elements through one octave, divided into central core (our sun, and other suns) and outer covering, with a skin or belt of kinetic energy, "as above" which developed an etheric world. all things on this etheric world were caused by the harmonic vibration between the etheric atoms and their surrounding envelopes, except that while all things in this etheric world must have life, not all need have mind. the chord of three was not necessary to create; the chord of two was enough, and the manasic atoms might cease to vibrate in chord with the prana and ether without affecting the creation. only in the etheric world (and below it) could there be living mindless ones. to the etheric globes the stellar pranic cores transferred their light, which manifested itself in the solid static ether as attraction and in the gaseous static ether as repulsion, within the kinetic skin of each etheric world more specialized and less varied than the pranic. our sun is not of prakriti, but of static ether, composed of the separate and individual elementary substances of the ether, and their compounds vibrating in the lower notes of their octaves. it is our father, not our elder brother. its envelope of static ether in which the planet revolves is composed of the elementary substances and combinations vibrating in the higher notes of their octave. the light transferred to this etheric globe from its mother, alcyone, manifests itself in the lower vibration of the sun as attraction; in the higher vibrations of its envelope as repulsion, and within the kinetic skin wherein these forces play, the prakritic globes, planets, were born. take our earth. each atom is fourfold--whether of the static core or of the static gaseous envelope. creation on it is limited to the kinetic skin, wherein the attraction of the lower and repulsion of the higher notes in each octave of vibration have full play. all things on it must have come from the chording vibrations of the atoms of the prakritic elementary substances and their envelope of ether. they may or may not have life or mind the ether atom may have lost its chord with its pranic envelope, or the pranic envelope may have lost its chord with the manasic; but the combination must have force or energy within it. it may have lost mind and life in acquiring it, or after acquiring it; but it had to have life before it could become prakriti. all things in the prakritic world flow from the life of the etheric and the mind of the pranic worlds. everything in the etheric world has life, and our unconscious personification or "vivification" of etheric life transferred into fauna or flora, or into force of any kind, has a natural explanation. the thrill of vibration in one octave through the differentiated consciousness of the universe by which the light was separated from the darkness, the lower from the higher, was all that was required to create each star and sun, and world, and all that in them is. and it was all good. each thing on every lower world was but the translation into form of the type of the next world (or plane) above. as each element on this prakritic type, so each combination of those elements into crystal or tree or animal is but the translation. the normal earth from the crystal to (the animal) man was pure, and clean, and holy. sin had not entered. how did it come? on the vast manasic world there was "a special creation"--that of the angel man. the three planes of spirit above were undifferentiated consciousness, but they were in different octaves of vibration, and these working on the three highest forms of differentiated consciousness (manasic matter) brought them to chording vibration so that when they combined and reached their highest point in evolution they "created" the angel (or manasic) man. he was the product in kinetic manasa of the three spiritual planes above him, precisely as the animal man was the product in kinetic prakriti of the three material planes above him. the latter was the "shadow" of the other. the angel-man had a material (manasic) body, but his energy life, and mind were spiritual. the animal man had a prakritic body, with energy, life and mind that were material. so far all was good. the animal man has four bodies--one of prakriti, one of ether, one of prana, and one of manasa. it may be true, and probably is, that his manasic body is not sounding in chord with his prakritic body, but only with those atoms of it which are in his brain and nerves; but that is immaterial--for future consideration. the angel man had but one body, of manasa, in which the spirit dwelt; but that body was identical in substance with the body that made the mind of the animal man. his manasic body joined the manasic body of the animal man, joined with it by entering into the animal man's mind, as easily as water from one glass is added to water in another glass, and the animal "man became a living soul," endowed with speech, while the angel-man was given "a skin coat." the prakritic body of the animal man was the result in prakriti of an etheric-pranic-manasic, or "astral" body, formed in accordance with the universal law. for what he was by nature, he could not be blamed. he stood naked and not ashamed before the radiance. he did not make his astral body; he was the mere translation of it into prakriti, as all other created things were, and that invisible astral self (figuratively) stood at his right hand, moulding and shaping him. but when the angel-man entered his mind, all this was changed. he "knew good from evil." to his mind of manasa had been added the spirit--the atma-buddhi's consciousness of the three spiritual planes. he has become "as one of us," said the angel- men of the firmament, of heaven. he now held the seven planes and was a creator. each thought and desire that, when an animal only, fell harmless, now created on the pranic and etheric world. soon beside him, at his left hand (figuratively) there grew up a second etheric or astral body, that of his desires; and his prakritic body was no longer the product of the astral body on his right hand. it was the joint product of the left-hand kamic astral body he had created, and the right hand normal astral body. he was no longer in harmony with the radiance. he could no longer face it. he had created discord--sin. the pretty legend of the two "angels," one on the right hand and one on the left, has its physical basis in this truth, but, of course, as a matter of actual fact, the normal and abnormal astral bodies are in mechanical union. it is the kamic self-made astral body that remains from one incarnation to another, producing in joint action with a new normal astral body, a new physical body for the inner-self, or angel taking the pilgrimage through the lower world. all the angel-men did not enter the animal men on the pranic etheric-prakritic globes; only a few. it was a pilgrimage through matter in which those who make it are meeting many adventures, but the legends are many, and have no place in the physics, although the legends are all founded on the facts of the physics. of the number of monads, willing to undertake the pilgrimage, only a few of those within the kinetic belt of the manasic globe have reached the pranic. only a few of those within the pranic kinetic belts reached the etheric. and of all who have reached this earth, only a few may win their way back before the great day be-with-us. the problem of man, and his relations to the universe, are an entirely different line of study from that of the spiritual monad, the over-soul of every prakritic atom. each prakritic atom has what may be called a soul, its three-fold astral cause; and an over-soul, or the three-fold spiritual archetype, or causeless cause. every combination of these atoms, whether a knife, a leaf, an animal, an earth, a sun, or a star, has this soul and oversoul. once the idea of what is meant by these terms becomes clear, the difficulty in understanding them vanishes. the study of man is physical in its lower branches; metaphysical only in its highest and last analysis. the study of the monad is metaphysical from start to finish. the two studies are apt to be confused, because metaphysically they are often joined for study, the teacher taking it for granted that the pupil fully understands the simple and easy physics of the problem of humanity. this, in crude and bold outline, is the story of creation to the fall of man according to the ancient physics, translated into the words and phrases of modern physics. the latter, in the latest discoveries of modern science, seem to have stolen a shive from the ancient loaf in the expectation that it would not be detected. each and every step forward that modern science has made in the past twenty years, each and every discovery of every kind in the physical field, has been but the affirmative of some ancient doctrine taught in the temples of the east before "cain took unto himself a wife." chapter ten the septenary world in the physical universe we have the four informing physical globes, so that as a whole or in its parts, it is "a string of seven globes," reaching from the highest spirit to the lowest matter. the awakened universal consciousness in vibration --undifferentiated in the three globes above, differentiated in the four globes below--in its last analysis is all one. but there is a gulf between matter and spirit, radically dividing them, and in the physical universe we are concerned only with physics and physical laws, until we reach its outmost boundaries and come in touch with the spiritual planes beyond. this is the view of the universe at first glance, as in the smaller universe of this earth we at first see only its solid and liquid globes. and even after the discovery of the gas, we do not apprehend its important work in and behind the others until it has been pointed out to us. nor do we at first apprehend the work of the spiritual in the material, and the object of metaphysics is to show, through the physics, the connection between them that the spirit works through matter; that where we can see but four there are seven beads on each material string; and that the last bead of each string is itself a chain of beads, the "chain of seven" applying only to the seventh manifestation, or prakriti, while the "strings" apply to the way in which they come. on each unraveled string leading from our central sun down to a planet there are seven beads corresponding to the seven globes in the chain of each planet, each to each, yet not the same. there is a distinction, and it is no wonder there should have been confusion at first and a mixing of "strings" with "chains." the physics as they progress will clear this confusion away. in the manasic globe, which is the first differentiation of that which forms the spiritual globes above, the resulting mind or manasa is mainly the differentiated divine mind of the highest. it has a "chain" of two globes only, itself and the divine mind globe, although its "string" of globes is four. it is the perfected differentiation of the buddhi in manasa that causes the formation of the pranic globes, which have chains of four and strings of five, and the full and perfect differentiation of the atma in manasa-prana that causes the formation of the etheric globes, which have chains of six and strings of six. consciousness, buddhi and atma are practically the same as the manasa, prana, and ether, each to each, only the latter are differentiated and the former are not. each of the three astral globes is the reflection in matter of the three spiritual globes beyond, each to each, and all to all. the difference between matter and spirit is a difference in motion only. both are vibrating, so that both are in mechanical motion, from force without, like the waves of the ocean, but only the matter has what we may properly call motion of its own, or that produced from within--from the atom and each organism of it up to the all, as the vibration is from the all down to the atom. it is this centre of force in an atom, this motion outside of vibration, or rather beside it, which we call "differentiation." brinton's "daring psychological speculation" that "mind was coextensive with motion" (from organization) was but a repetition of one of the most ancient axioms. take our solar etheric globe. it has two other globes of matter, consubstantial; a globe of prana and a globe of manasa. they are not beyond it, or beside it, but one with it, atom for atom. but what are they in reality? globes of atma, buddhi, and consciousness in which the atoms, having organized, are in motion, are they not? let this motion in this material universe cease, and matter would melt away and resolve into spirit. from spirit it came, to spirit it belongs, and to spirit it returns. behind each and every astral globe, whether the globe be but an astral atom, or an astral planet, or an astral world; beyond its physics there is a meta-physical globe, its cause, and that is the real globe, of which the astral is but a temporary phenomenon. take a spiritual globe and differentiate it. the motion resulting produces a material astral globe. stop the motion; bring it to a state of rest. the astral shadow disappears. it was merely spiritual phenomena. each and every astral atom is a model in miniature of the material and spiritual universe. each and every prakritic atom is the joint result of spirit and matter united and working together--of physics and metaphysics; and in its last analysis pure spirit; pure metaphysics. behind each and every prakritic atom of our earth there are six other atoms (or globes), three material shadows and three spiritual realities, so that it is a string of seven--the whole universe in miniature--material and spiritual. and all things combined and formed on a prakritic base are a chain of seven --whether a peach or a planet. the "chain" belongs to the prakritic plane. the lines of descent from the light through the star and sun to planet are "strings." the "chains" are beads of the same size strung on a thread. the strings are beads of different sizes strung on a thread. the beads of the chain are in coadunition--in the same space, as gas in water and the water in a sponge. in metaphysics this earth can only be regarded as a chain of seven globes, its three astral globes in coadunition having their three spiritual doubles. of course no one of the higher globes can be seen by the prakritic eye, but that is not to say the astral world cannot be seen by the astral eye in sleep, or by the person who qualifies himself for the astral world, through the development of his astral body. "no upper globes of any chain in the solar system can be seen," says h.p. blavatsky in the secret doctrine (vol. i, p. ), yet she means by astronomers, not by sages. and she does not mean the upper globes in the stellar system of alcyone and its companions. in pure physics the earth can only be regarded as a chain of four globes consubstantial and in coadunition--four in and three out. this makes seven, and the metaphysician when talking physics uses the metaphysical terms interchangeably and speaks of "the chain of seven globes" meaning in one sentence the four material globes making this earth; in another meaning the line of descent or string of beads of different sizes reaching down from the divine consciousness; and in still another the seven beads or globes of the same size in coadunition to form this earth chain. to the student who is thoroughly grounded in the eastern physics this interweaving of the physical and metaphysical presents no difficulties; but to the western mind just beginning the study it is a tangle. we can now see what is meant by illusion, or maya, and understand why such stress is laid upon it by every teacher. take the physical side first. the motion of a top gives it bands of color to our eyes that it does not have at rest. they are temporary and not permanent, a result of motion merely; illusion and not reality. the motion of the material atoms of the four planes, in harmony with their vibration, a motion the spiritual world does not have, produces all material phenomena. this is of course within the kinetic belts, for above or below them there is no change, and its phenomena are the mere change in relation of one atom to another caused by motion. the changes are not real. they disappear when the motion stops. they have no existence in matter above or below the belt. all phenomena of every kind are as much an illusion as the supposed bands of colour around the top. the illusion is the result of changes of relation in differentiated atoms caused by their motion. without this motion the four material globes would dissolve into the atomic dust of the manasic world, with all that is within them. the whole material universe is all illusion; a mere temporary relation of its atoms through motion, without reality or permanence. what then is real? what is not illusion? that which is beyond the physical, that which is its cause and root; broadly, the metaphysical, which is not the result of differentiated atoms through relation. what was real in the top is real here. what was illusion in the top is illusion here. the meta-physical or spiritual (the terms are interchangeable) does not have to pass beyond the manasic globe to get on the solid ground of reality. the spiritual world is here in every physical atom and in every aggregation of them; in every planet, sun, and star; for they are seven, each and every one, not four. behind the illusion of one atom or many, whether here or on alcyone, there is reality and permanency in the undifferentiated cause, the spiritual archetype, the three higher beads on the string which are the proper study of metaphysics. chapter eleven stumbling blocks in eastern physics the western student of the ancient eastern physics soon meets serious stumbling-blocks; and one at the very threshold has in the last half century turned many back. in beginning his study of the solar system, the pupil is told: the first three planets--mercury, venus, and the moon--are dead and disintegrating. evolution on them has ceased. the proof of this is found in the fact, that they have no axial rotation, mercury and venus always presenting the same surface to their father, the sun, and the moon the same surface to its daughter, the earth. this is a concrete statement of physical fact at which the western student protests. if in the whole range of western astronomical science there is any one fact that he has accepted as absolutely proved, it is that mercury revolves once in h., m., . s., and venus once in h., m., s. he would as soon credit a statement that the earth has no axial rotation as that mercury or venus has none; and if he continues his study of eastern physics it is with no confidence in its accuracy, and as a matter of curiosity. the statement that mercury, venus, and the moon "are dead and disintegrating," the former two "always presenting the same surface" to the sun, is the basis for an elaborate superstructure, both in the physics and the metaphysics of the east. it is used in physics to explain how the "evolutionary wave" came to an end at the perfection of the mineral on mercury with the loss of its axial rotation; how the "wave" then passed on to venus with the seed of the vegetable kingdom, where the vegetable evolution ended with the loss of axial rotation; how from venus it leaped to the moon, mother of animals and controller of animal life, with the seed of animal life in the vegetable; and how finally it came to the earth, when the moon ceased to revolve, bringing in the animal the seed of man. here man will be evolved and perfected. man has not yet been "born" on this earth, they say. he is still in a prenatal or embryonic condition within the animal. the lunar pitris, the men-seed, have a physical reason for being, if this evolutionary theory be true; none if it is not. axial rotation is necessary in evolution, the ancient physics teaches, which must cease with it. the reasons for this are too lengthy to give here. briefly, the rotation makes the electrical flow and a thermopilic dynamo of each planet. the ancient astronomical teaching is absolutely true. there will not be a work on astronomy published in europe or the united states this year, or hereafter, that will not state that "mercury and venus revolve on their axes in the same time that they revolve around the sun," which is another way of saying that "they have no axial rotation, always presenting the same face to the sun," and an inaccurate way of presenting the truth. the screw that holds the tire at the outer end of the spoke does not revolve "once on its axis" each time the wheel revolves. run a cane through an orange and swing it around; the orange has not revolved "once on its axis." nor does the stone in a sling revolve "once on its axis" for each revolution around the hand. the motion of mercury is identically that of the impaled orange or the stone in the sling. it has no axis and no axial rotation. the modern astronomers, detected in pretenses to knowledge they never possessed, let themselves down easy. this "discovery," of no axial rotation by the interior planets, made by schiaparelli and confirmed by flammarion in , has since been fully verified by our western astronomers. all the new astronomies accept it. but the admission of astronomical "error," to speak politely, comes too late for the student it turned back from his study of eastern physics. he cannot regain his lost faith and lost ground. thirty years ago proctor made it clear to western students that the orbit of the moon was a cycloidal curve (a drawn-out spring) around the sun, the earth's orbit being coincident with its axis; and that the moon was, astronomically and correctly, a satellite of the sun, not a satellite of the earth. this has been the eastern view and teaching from time immemorial. the eastern distinction between father sun and mother moon, and the classification of the latter as a planet, did not disturb the western student. he understood that. it was the "absolute accuracy" of modern astronomers in regard to the length of the day on mercury or venus, which the astronomers declared had been corrected down to the fraction of a second, that made it impossible for him to accept the eastern physics when the latter squarely contradicted his own. this was but the first of many similar stumbling-blocks in the path of the student of eastern physics. "few were the followers, straggling far, that reached the lake of vennachar;" and when they did, this was what they had to face: "the planets absorb and use nearly all the solar energy--all except the very small amount the minor specks of cosmic dust may receive. there is not the least particle of the sun's light, or heat, or any one of the seven conditions of the solar energy, wasted. except for the planets, it is not manifested; it is not. there is no light, no heat, no form of solar energy, except on the planets as it is transferred from the laya center of each in the sun to them. the etheric globe is cold and dark, except along the lines to them--the "paths of fohat" [solar energy]. six laya centers are manifested in the sun; one is laid aside, though the wheels [planets] around the one eye be seven. [this alludes to the moon, whose laya center in the sun is now also that of the earth; but it is considered as a planet]. what each receives, that it also gives back. there is nothing lost." "that settles it," says one student; and the others agree. of the hundred who started, "the foremost horseman rode alone," before the next step was won. in the light of the tardy but perfect justification of the first stumbling-block, this statement may be worth following out, "to see what it means," and how "absurd" it can be. an etheric globe; cold as absolute zero, dark as erebus, with here and there small pencils of light and heat from the sun to the planets --just rays, and nothing more--is a very different one from the fiery furnace at absolute zero of the modern physicist. on a line drawn from the center of the earth to the center of the moon there is a point where the "weights" of the two bodies are said in our physics exactly to balance, and it lies, says our physics, " , miles from the center of the earth, and , miles from the surface." this is the earth's "laya center" of the eastern physics. it is of great importance in problems of life; but it may be passed over for the present. between the earth and the sun--precisely speaking, between this laya center and the sun--there is a "point of balance," which falls within the photosphere of the sun. this point in the sun is the earth's solar laya, the occult or hidden earth of the metaphysics. a diagram will make this clearer. draw a line from the laya center in the sun to that in the earth. draw a narrow ellipse, with this line as its major axis, and shade it. at each end of the axis strike the beginning of an ellipse that will be tangent. if positive energy is along the shaded ellipse, negative energy is in each field beyond--earth and sun. this is a very crude illustration of a fundamental statement elaborated to the most minute detail in explanation of all astronomical phenomena; but for the moment it will do. the point is that along this axial line connecting the laya centers play all the seven solar forces--light, heat, electricity, etc.--that affect the earth, and on every side of this line is the "electric field" of these forces. to this line any escaping solar energy is drawn, as the electricity of the air is drawn to a live wire or magnet. but there is little or none to escape. from the laya point in the sun to the laya point in the earth, the solar energy is transferred as sound is carried along a beam of light (photophone), or electricity from one point to another without a wire. to the advanced student of electricity the ancient teaching is easily apprehended; to others it is difficult to make clear. these laya centers, it says, are "the transforming points of energy." from the earth laya to the solar laya centre, the energy, we may say, is positive; beyond both the solar and the earth laya centre, in the fields touching at them, it is negative --or vice versa. the line connecting the layas is the "path of fohat"--the personification of solar energy. this is a very crude and brief way of putting many pages of teaching, but the important point is that this line between the layas is one of solar energy, with a dynamic "field" of solar energy, elliptical in shape, connecting with the reverse fields at the laya points. these "dead points" are the limits of each electric field, which "create", we say in electrical work, opposing fields beyond them. each one of these planets has its laya centre inside the sun's photosphere. each planet has a line of solar energy with its "field" of solar energy--not only a wireless telegraph, but a wireless lighting, heating, and life-giving system. these six solar laya points are the six "hidden planets," the earth and moon being one, of the ancient metaphysics. the moon is the one "laid aside." in their reception of energy from the sun, it is as if the planet were at the solar laya point, or connected with it by a special pipe-line. the position of these six planetary laya points in the sun is indicated by the position of the planets in the heavens, and they may often influence or modify one another. if mars, jupiter, or saturn is anywhere near conjunction with the earth, not only will a part of their "fields" be joined, but their laya points in the sun will be modified. the physical basis of the old astrology was the physical interferences of these fields of solar energy; and what it depended on mainly in its work was the position of the six hidden planets, or laya centers, which was shown by the position of the planet with reference to the earth. that the planets themselves affected any one or anything on this earth, no real astrologer ever believed; that their position in the heavens indicated certain changes and modifications of the flow of solar energy to the earth, they knew from their knowledge of physics. "the twelve houses are in the sun," says hermes, "six in the north and six in the south." connect them with the zodiac, and the position of the planets shows the interferences of the solar currents. the one objection to this ancient theory is that it does not present enough difficulties. the present value to science of the many theories in relation to the sun is the impossibility of reconciling any two of them, and the fact that no two theorists can unite to pummel a third. this ancient theory does not call for any great amount of heat, light, or energy in any condition to keep the cosmos in order--not even enough for two persons to quarrel over. it merely turns the sun into a large dynamo connected with smaller dynamos, and these with one another with return currents by which "there is nothing lost." in its details, it accounts for all facts--neatly, simply, and without exclamation points. it is so simple and homespun, so lacking in the gaudiness that makes (for example) our light and heat less than the billionth part wasted on space always at absolute zero, that we may have to wait many centuries to have it "verified" and "confirmed" by our western science. that it will be "verified" in time, even as the first stumbling-block has been removed at the end of the nineteenth century, its students may at least hope. the lesson, if there is one, is that the western student of eastern physics does not ride an auto along asphalted roads. he must own himself and not be owned by another man, or even by "modern science." faraday as a discoverer by john tyndall contents. preface. chapter . parentage: introduction to the royal institution: earliest experiments: first royal society paper: marriage. chapter . early researches: magnetic rotations: liquefaction of gases: heavy glass: charles anderson: contributions to physics. chapter . discovery of magneto-electricity: explanation of argo's magnetism of rotation: terrestrial magneto-electric induction: the extra current. chapter . points of character. chapter . identity of electricities; first researches on electro-chemistry. chapter . laws of electro-chemical decomposition. chapter . origin of power in the voltaic pile. chapter . researches on frictional electricity: induction: conduction: specific inductive capacity: theory of contiguous particles. chapter . rest needed--visit to switzerland. chapter . magnetization of light. chapter . discovery of diamagnetism--researches on magne-crystallic action. chapter . magnetism of flame and gases--atmospheric magnetism. chapter . speculations: nature of matter: lines of force. chapter . unity and convertibility of natural forces: theory of the electric current. chapter . summary. chapter . illustrations of character. preface to the fifth edition. daily and weekly, from all parts of the world, i receive publications bearing upon the practical applications of electricity. this great movement, the ultimate outcome of which is not to be foreseen, had its origin in the discoveries made by michael faraday, sixty-two years ago. from these discoveries have sprung applications of the telephone order, together with various forms of the electric telegraph. from them have sprung the extraordinary advances made in electrical illumination. faraday could have had but an imperfect notion of the expansions of which his discoveries were capable. still he had a vivid and strong imagination, and i do not doubt that he saw possibilities which did not disclose themselves to the general scientific mind. he knew that his discoveries had their practical side, but he steadfastly resisted the seductions of this side, applying himself to the development of principles; being well aware that the practical question would receive due development hereafter. during my sojourn in switzerland this year, i read through the proofs of this new edition, and by my reading was confirmed in the conviction that the book ought not to be suffered to go out of print. the memoir was written under great pressure, but i am not ashamed of it as it stands. glimpses of faraday's character and gleams of his discoveries are there to be found which will be of interest to humanity to the end of time. john tyndall. hind head, december, . [note.--it was, i believe, my husband's intention to substitute this preface, written a few days before his death, for all former prefaces. as, however, he had not the opportunity of revising the old prefatory pages himself, they have been allowed to remain just as they stood in the last edition. louisa c. tyndall.] preface to the fourth edition. when consulted a short time ago as to the republication of 'faraday as a discoverer,' it seemed to me that the labours, and points of character, of so great a worker and so good a man should not be allowed to vanish from the public eye. i therefore willingly fell in with the proposal of my publishers to issue a new edition of the little book. royal institution, february, . preface to the second edition. the experimental researches of faraday are so voluminous, their descriptions are so detailed, and their wealth of illustration is so great, as to render it a heavy labour to master them. the multiplication of proofs, necessary and interesting when the new truths had to be established, are however less needful now when these truths have become household words in science. i have therefore tried in the following pages to compress the body, without injury to the spirit, of these imperishable investigations, and to present them in a form which should be convenient and useful to the student of the present day. while i write, the volumes of the life of faraday by dr. bence jones have reached my hands. to them the reader must refer for an account of faraday's private relations. a hasty glance at the work shows me that the reverent devotion of the biographer has turned to admirable account the materials at his command. the work of dr. bence jones enables me to correct a statement regarding wollaston's and faraday's respective relations to the discovery of magnetic rotation. wollaston's idea was to make the wire carrying a current rotate round its own axis: an idea afterwards realised by the celebrated ampere. faraday's discovery was to make the wire carrying the current revolve round the pole of a magnet and the reverse. john tyndall. royal institution: december, . faraday as a discoverer. chapter . parentage: introduction to the royal institution: earliest experiments: first royal society paper: marriage. it has been thought desirable to give you and the world some image of michael faraday, as a scientific investigator and discoverer. the attempt to respond to this desire has been to me a labour of difficulty, if also a labour of love. for however well acquainted i may be with the researches and discoveries of that great master--however numerous the illustrations which occur to me of the loftiness of faraday's character and the beauty of his life--still to grasp him and his researches as a whole; to seize upon the ideas which guided him, and connected them; to gain entrance into that strong and active brain, and read from it the riddle of the world--this is a work not easy of performance, and all but impossible amid the distraction of duties of another kind. that i should at one period or another speak to you regarding faraday and his work is natural, if not inevitable; but i did not expect to be called upon to speak so soon. still the bare suggestion that this is the fit and proper time for speech sent me immediately to my task: from it i have returned with such results as i could gather, and also with the wish that those results were more worthy than they are of the greatness of my theme. it is not my intention to lay before you a life of faraday in the ordinary acceptation of the term. the duty i have to perform is to give you some notion of what he has done in the world; dwelling incidentally on the spirit in which his work was executed, and introducing such personal traits as may be necessary to the completion of your picture of the philosopher, though by no means adequate to give you a complete idea of the man. the newspapers have already informed you that michael faraday was born at newington butts, on september , , and that he died at hampton court, on august , . believing, as i do, in the general truth of the doctrine of hereditary transmission--sharing the opinion of mr. carlyle, that 'a really able man never proceeded from entirely stupid parents'--i once used the privilege of my intimacy with mr. faraday to ask him whether his parents showed any signs of unusual ability. he could remember none. his father, i believe, was a great sufferer during the latter years of his life, and this might have masked whatever intellectual power he possessed. when thirteen years old, that is to say in , faraday was apprenticed to a bookseller and bookbinder in blandford street, manchester square: here he spent eight years of his life, after which he worked as a journeyman elsewhere. you have also heard the account of faraday's first contact with the royal institution; that he was introduced by one of the members to sir humphry davy's last lectures, that he took notes of those lectures; wrote them fairly out, and sent them to davy, entreating him at the same time to enable him to quit trade, which he detested, and to pursue science, which he loved. davy was helpful to the young man, and this should never be forgotten: he at once wrote to faraday, and afterwards, when an opportunity occurred, made him his assistant.[ ] mr. gassiot has lately favoured me with the following reminiscence of this time:-- 'clapham common, surrey, 'november , . 'my dear tyndall,--sir h. davy was accustomed to call on the late mr. pepys, in the poultry, on his way to the london institution, of which pepys was one of the original managers; the latter told me that on one occasion sir h. davy, showing him a letter, said: "pepys, what am i to do, here is a letter from a young man named faraday; he has been attending my lectures, and wants me to give him employment at the royal institution--what can i do?" "do?" replied pepys, "put him to wash bottles; if he is good for anything he will do it directly, if he refuses he is good for nothing." "no, no," replied davy; "we must try him with something better than that." the result was, that davy engaged him to assist in the laboratory at weekly wages. 'davy held the joint office of professor of chemistry and director of the laboratory; he ultimately gave up the former to the late professor brande, but he insisted that faraday should be appointed director of the laboratory, and, as faraday told me, this enabled him on subsequent occasions to hold a definite position in the institution, in which he was always supported by davy. i believe he held that office to the last. 'believe me, my dear tyndall, yours truly, 'j. p. gassiot. 'dr. tyndall.' from a letter written by faraday himself soon after his appointment as davy's assistant, i extract the following account of his introduction to the royal institution:-- 'london, sept. , . 'as for myself, i am absent (from home) nearly day and night, except occasional calls, and it is likely shall shortly be absent entirely, but this (having nothing more to say, and at the request of my mother) i will explain to you. i was formerly a bookseller and binder, but am now turned philosopher,[ ] which happened thus:--whilst an apprentice, i, for amusement, learnt a little chemistry and other parts of philosophy, and felt an eager desire to proceed in that way further. after being a journeyman for six months, under a disagreeable master, i gave up my business, and through the interest of a sir h. davy, filled the situation of chemical assistant to the royal institution of great britain, in which office i now remain; and where i am constantly employed in observing the works of nature, and tracing the manner in which she directs the order and arrangement of the world. i have lately had proposals made to me by sir humphry davy to accompany him in his travels through europe and asia, as philosophical assistant. if i go at all i expect it will be in october next--about the end; and my absence from home will perhaps be as long as three years. but as yet all is uncertain.' this account is supplemented by the following letter, written by faraday to his friend de la rive,[ ] on the occasion of the death of mrs. marcet. the letter is dated september , :-- 'my dear friend,--your subject interested me deeply every way; for mrs. marcet was a good friend to me, as she must have been to many of the human race. i entered the shop of a bookseller and bookbinder at the age of thirteen, in the year , remained there eight years, and during the chief part of my time bound books. now it was in those books, in the hours after work, that i found the beginning of my philosophy. there were two that especially helped me, the "encyclopaedia britannica," from which i gained my first notions of electricity, and mrs. marcet's "conversation on chemistry," which gave me my foundation in that science. 'do not suppose that i was a very deep thinker, or was marked as a precocious person. i was a very lively imaginative person, and could believe in the "arabian nights" as easily as in the "encyclopaedia." but facts were important to me, and saved me. i could trust a fact, and always cross-examined an assertion. so when i questioned mrs. marcet's book by such little experiments as i could find means to perform, and found it true to the facts as i could understand them, i felt that i had got hold of an anchor in chemical knowledge, and clung fast to it. thence my deep veneration for mrs. marcet--first as one who had conferred great personal good and pleasure on me; and then as one able to convey the truth and principle of those boundless fields of knowledge which concern natural things to the young, untaught, and inquiring mind. 'you may imagine my delight when i came to know mrs. marcet personally; how often i cast my thoughts backward, delighting to connect the past and the present; how often, when sending a paper to her as a thank-offering, i thought of my first instructress, and such like thoughts will remain with me. 'i have some such thoughts even as regards your own father; who was, i may say, the first who personally at geneva, and afterwards by correspondence, encouraged, and by that sustained me.' twelve or thirteen years ago mr. faraday and myself quitted the institution one evening together, to pay a visit to our friend grove in baker street. he took my arm at the door, and, pressing it to his side in his warm genial way, said, 'come, tyndall, i will now show you something that will interest you.' we walked northwards, passed the house of mr. babbage, which drew forth a reference to the famous evening parties once assembled there. we reached blandford street, and after a little looking about he paused before a stationer's shop, and then went in. on entering the shop, his usual animation seemed doubled; he looked rapidly at everything it contained. to the left on entering was a door, through which he looked down into a little room, with a window in front facing blandford street. drawing me towards him, he said eagerly, 'look there, tyndall, that was my working-place. i bound books in that little nook.' a respectable-looking woman stood behind the counter: his conversation with me was too low to be heard by her, and he now turned to the counter to buy some cards as an excuse for our being there. he asked the woman her name--her predecessor's name--his predecessor's name. 'that won't do,' he said, with good-humoured impatience; 'who was his predecessor?' 'mr. riebau,' she replied, and immediately added, as if suddenly recollecting herself, 'he, sir, was the master of sir charles faraday.' 'nonsense!' he responded, 'there is no such person.' great was her delight when i told her the name of her visitor; but she assured me that as soon as she saw him running about the shop, she felt-though she did not know why--that it must be 'sir charles faraday.' faraday did, as you know, accompany davy to rome: he was re-engaged by the managers of the royal institution on may , . here he made rapid progress in chemistry, and after a time was entrusted with easy analyses by davy. in those days the royal institution published 'the quarterly journal of science,' the precursor of our own 'proceedings.' faraday's first contribution to science appeared in that journal in . it was an analysis of some caustic lime from tuscany, which had been sent to davy by the duchess of montrose. between this period and various notes and short papers were published by faraday. in he experimented upon 'sounding flames.' professor auguste de la rive had investigated those sounding flames, and had applied to them an explanation which completely accounted for a class of sounds discovered by himself, but did not account for those known to his predecessors. by a few simple and conclusive experiments, faraday proved the explanation insufficient. it is an epoch in the life of a young man when he finds himself correcting a person of eminence, and in faraday's case, where its effect was to develop a modest self-trust, such an event could not fail to act profitably. from time to time between and faraday published scientific notes and notices of minor weight. at this time he was acquiring, not producing; working hard for his master and storing and strengthening his own mind. he assisted mr. brande in his lectures, and so quietly, skilfully, and modestly was his work done, that mr. brande's vocation at the time was pronounced 'lecturing on velvet.' in faraday published a chemical paper 'on two new compounds of chlorine and carbon, and on a new compound of iodine, carbon, and hydrogen.' this paper was read before the royal society on december , , and it was the first of his that was honoured with a place in the 'philosophical transactions.' on june , , he married, and obtained leave to bring his young wife into his rooms at the royal institution. there for forty-six years they lived together, occupying the suite of apartments which had been previously in the successive occupancy of young, davy, and brande. at the time of her marriage mrs. faraday was twenty-one years of age, he being nearly thirty. regarding this marriage i will at present limit myself to quoting an entry written in faraday's own hand in his book of diplomas, which caught my eye while in his company some years ago. it ran thus:-- ' th january, . 'amongst these records and events, i here insert the date of one which, as a source of honour and happiness, far exceeds all the rest. we were married on june , . 'm. faraday.' then follows the copy of the minutes, dated may , , which gave him additional rooms, and thus enabled him to bring his wife to the royal institution. a feature of faraday's character which i have often noticed makes itself apparent in this entry. in his relations to his wife he added chivalry to affection. footnotes to chapter [ ] here is davy's recommendation of faraday, presented to the managers of the royal institution, at a meeting on the th of march, , charles hatchett, esq., in the chair:-- 'sir humphry davy has the honour to inform the managers that he has found a person who is desirous to occupy the situation in the institution lately filled by william payne. his name is michael faraday. he is a youth of twenty-two years of age. as far as sir h. davy has been able to observe or ascertain, he appears well fitted for the situation. his habits seem good; his disposition active and cheerful, and his manner intelligent. he is willing to engage himself on the same terms as given to mr. payne at the time of quitting the institution. 'resolved,--that michael faraday be engaged to fill the situation lately occupied by mr. payne, on the same terms.' [ ] faraday loved this word and employed it to the last; he had an intense dislike to the modern term physicist. [ ] to whom i am indebted for a copy of the original letter. chapter . early researches: magnetic rotations: liquefaction of gases: heavy glass: charles anderson: contributions to physics. oersted, in , discovered the action of a voltaic current on a magnetic needle; and immediately afterwards the splendid intellect of ampere succeeded in showing that every magnetic phenomenon then known might be reduced to the mutual action of electric currents. the subject occupied all men's thoughts: and in this country dr. wollaston sought to convert the deflection of the needle by the current into a permanent rotation of the needle round the current. he also hoped to produce the reciprocal effect of causing a current to rotate round a magnet. in the early part of , wollaston attempted to realise this idea in the presence of sir humphry davy in the laboratory of the royal institution.[ ] this was well calculated to attract faraday's attention to the subject. he read much about it; and in the months of july, august, and september he wrote a 'history of the progress of electro-magnetism,' which he published in thomson's 'annals of philosophy.' soon afterwards he took up the subject of 'magnetic rotations,' and on the morning of christmas-day, , he called his wife to witness, for the first time, the revolution of a magnetic needle round an electric current. incidental to the 'historic sketch,' he repeated almost all the experiments there referred to; and these, added to his own subsequent work, made him practical master of all that was then known regarding the voltaic current. in , he also touched upon a subject which subsequently received his closer attention--the vaporization of mercury at common temperatures; and immediately afterwards conducted, in company with mr. stodart, experiments on the alloys of steel. he was accustomed in after years to present to his friends razors formed from one of the alloys then discovered. during faraday's hours of liberty from other duties, he took up subjects of inquiry for himself; and in the spring of , thus self-prompted, he began the examination of a substance which had long been regarded as the chemical element chlorine, in a solid form, but which sir humphry davy, in , had proved to be a hydrate of chlorine, that is, a compound of chlorine and water. faraday first analysed this hydrate, and wrote out an account of its composition. this account was looked over by davy, who suggested the heating of the hydrate under pressure in a sealed glass tube. this was done. the hydrate fused at a blood-heat, the tube became filled with a yellow atmosphere, and was afterwards found to contain two liquid substances. dr. paris happened to enter the laboratory while faraday was at work. seeing the oily liquid in his tube, he rallied the young chemist for his carelessness in employing soiled vessels. on filing off the end of the tube, its contents exploded and the oily matter vanished. early next morning, dr. paris received the following note:-- 'dear sir,--the oil you noticed yesterday turns out to be liquid chlorine. 'yours faithfully, 'm. faraday.'[ ] the gas had been liquefied by its own pressure. faraday then tried compression with a syringe, and succeeded thus in liquefying the gas. to the published account of this experiment davy added the following note:--'in desiring mr. faraday to expose the hydrate of chlorine in a closed glass tube, it occurred to me that one of three things would happen: that decomposition of water would occur;... or that the chlorine would separate in a fluid state.' davy, moreover, immediately applied the method of self-compressing atmosphere to the liquefaction of muriatic gas. faraday continued the experiments, and succeeded in reducing a number of gases till then deemed permanent to the liquid condition. in he returned to the subject, and considerably expanded its limits. these important investigations established the fact that gases are but the vapours of liquids possessing a very low boiling-point, and gave a sure basis to our views of molecular aggregation. the account of the first investigation was read before the royal society on april , , and was published, in faraday's name, in the 'philosophical transactions.' the second memoir was sent to the royal society on december , . i may add that while he was conducting his first experiments on the liquefaction of gases, thirteen pieces of glass were on one occasion driven by an explosion into faraday's eye. some small notices and papers, including the observation that glass readily changes colour in sunlight, follow here. in and faraday published papers in the 'philosophical transactions' on 'new compounds of carbon and hydrogen,' and on 'sulphonaphthalic acid.' in the former of these papers he announced the discovery of benzol, which, in the hands of modern chemists, has become the foundation of our splendid aniline dyes. but he swerved incessantly from chemistry into physics; and in we find him engaged in investigating the limits of vaporization, and showing, by exceedingly strong and apparently conclusive arguments, that even in the case of mercury such a limit exists; much more he conceived it to be certain that our atmosphere does not contain the vapour of the fixed constituents of the earth's crust. this question, i may say, is likely to remain an open one. dr. rankine, for example, has lately drawn attention to the odour of certain metals; whence comes this odour, if it be not from the vapour of the metal? in faraday became a member of a committee, to which sir john herschel and mr. dollond also belonged, appointed by the royal society to examine, and if possible improve, the manufacture of glass for optical purposes. their experiments continued till , when the account of them constituted the subject of a 'bakerian lecture.' this lectureship, founded in by henry baker, esq., of the strand, london, provides that every year a lecture shall be given before the royal society, the sum of four pounds being paid to the lecturer. the bakerian lecture, however, has long since passed from the region of pay to that of honour, papers of mark only being chosen for it by the council of the society. faraday's first bakerian lecture, 'on the manufacture of glass for optical purposes,' was delivered at the close of . it is a most elaborate and conscientious description of processes, precautions, and results: the details were so exact and so minute, and the paper consequently so long, that three successive sittings of the royal society were taken up by the delivery of the lecture.[ ] this glass did not turn out to be of important practical use, but it happened afterwards to be the foundation of two of faraday's greatest discoveries.[ ] the experiments here referred to were commenced at the falcon glass works, on the premises of messrs. green and pellatt, but faraday could not conveniently attend to them there. in , therefore, a furnace was erected in the yard of the royal institution; and it was at this time, and with a view of assisting him at the furnace, that faraday engaged sergeant anderson, of the royal artillery, the respectable, truthful, and altogether trustworthy man whose appearance here is so fresh in our memories. anderson continued to be the reverential helper of faraday and the faithful servant of this institution for nearly forty years.[ ] in faraday published a paper, 'on a peculiar class of optical deceptions,' to which i believe the beautiful optical toy called the chromatrope owes its origin. in the same year he published a paper on vibrating surfaces, in which he solved an acoustical problem which, though of extreme simplicity when solved, appears to have baffled many eminent men. the problem was to account for the fact that light bodies, such as the seed of lycopodium, collected at the vibrating parts of sounding plates, while sand ran to the nodal lines. faraday showed that the light bodies were entangled in the little whirlwinds formed in the air over the places of vibration, and through which the heavier sand was readily projected. faraday's resources as an experimentalist were so wonderful, and his delight in experiment was so great, that he sometimes almost ran into excess in this direction. i have heard him say that this paper on vibrating surfaces was too heavily laden with experiments. footnotes to chapter [ ] the reader's attention is directed to the concluding paragraph of the 'preface to the second edition written in december, . also to the life of faraday by dr. bence jones, vol. i. p. et seq. [ ] paris: life of davy, p. . [ ] viz., november , december and . [ ] i make the following extract from a letter from sir john herschel, written to me from collingwood, on the rd of november, :--'i will take this opportunity to mention that i believe myself to have originated the suggestion of the employment of borate of lead for optical purposes. it was somewhere in the year , as well as i can recollect, that i mentioned it to sir james (then mr.) south; and, in consequence, the trial was made in his laboratory in blackman street, by precipitating and working a large quantity of borate of lead, and fusing it under a muffle in a porcelain evaporating dish. a very limpid (though slightly yellow) glass resulted, the refractive index . ! (which you will find set down in my table of refractive indices in my article "light," encyclopaedia metropolitana). it was, however, too soft for optical use as an object- glass. this faraday overcame, at least to a considerable degree, by the introduction of silica.' [ ] regarding anderson, faraday writes thus in :--'i cannot resist the occasion that is thus offered to me of mentioning the name of mr. anderson, who came to me as an assistant in the glass experiments, and has remained ever since in the laboratory of the royal institution. he assisted me in all the researches into which i have entered since that time; and to his care, steadiness, exactitude, and faithfulness in the performance of all that has been committed to his charge, i am much indebted.--m. f.' (exp. researches, vol. iii. p. , footnote.) chapter . discovery of magneto-electricity: explanation of argo's magnetism of rotation: terrestrial magneto-electric induction: the extra current. the work thus referred to, though sufficient of itself to secure no mean scientific reputation, forms but the vestibule of faraday's achievements. he had been engaged within these walls for eighteen years. during part of the time he had drunk in knowledge from davy, and during the remainder he continually exercised his capacity for independent inquiry. in we have him at the climax of his intellectual strength, forty years of age, stored with knowledge and full of original power. through reading, lecturing, and experimenting, he had become thoroughly familiar with electrical science: he saw where light was needed and expansion possible. the phenomena of ordinary electric induction belonged, as it were, to the alphabet of his knowledge: he knew that under ordinary circumstances the presence of an electrified body was sufficient to excite, by induction, an unelectrified body. he knew that the wire which carried an electric current was an electrified body, and still that all attempts had failed to make it excite in other wires a state similar to its own. what was the reason of this failure? faraday never could work from the experiments of others, however clearly described. he knew well that from every experiment issues a kind of radiation, luminous in different degrees to different minds, and he hardly trusted himself to reason upon an experiment that he had not seen. in the autumn of he began to repeat the experiments with electric currents, which, up to that time, had produced no positive result. and here, for the sake of younger inquirers, if not for the sake of us all, it is worth while to dwell for a moment on a power which faraday possessed in an extraordinary degree. he united vast strength with perfect flexibility. his momentum was that of a river, which combines weight and directness with the ability to yield to the flexures of its bed. the intentness of his vision in any direction did not apparently diminish his power of perception in other directions; and when he attacked a subject, expecting results he had the faculty of keeping his mind alert, so that results different from those which he expected should not escape him through preoccupation. he began his experiments 'on the induction of electric currents' by composing a helix of two insulated wires which were wound side by side round the same wooden cylinder. one of these wires he connected with a voltaic battery of ten cells, and the other with a sensitive galvanometer. when connection with the battery was made, and while the current flowed, no effect whatever was observed at the galvanometer. but he never accepted an experimental result, until he had applied to it the utmost power at his command. he raised his battery from cells to cells, but without avail. the current flowed calmly through the battery wire without producing, during its flow, any sensible result upon the galvanometer. 'during its flow,' and this was the time when an effect was expected--but here faraday's power of lateral vision, separating, as it were, from the line of expectation, came into play--he noticed that a feeble movement of the needle always occurred at the moment when he made contact with the battery; that the needle would afterwards return to its former position and remain quietly there unaffected by the flowing current. at the moment, however, when the circuit was interrupted the needle again moved, and in a direction opposed to that observed on the completion of the circuit. this result, and others of a similar kind, led him to the conclusion 'that the battery current through the one wire did in reality induce a similar current through the other; but that it continued for an instant only, and partook more of the nature of the electric wave from a common leyden jar than of the current from a voltaic battery.' the momentary currents thus generated were called induced currents, while the current which generated them was called the inducing current. it was immediately proved that the current generated at making the circuit was always opposed in direction to its generator, while that developed on the rupture of the circuit coincided in direction with the inducing current. it appeared as if the current on its first rush through the primary wire sought a purchase in the secondary one, and, by a kind of kick, impelled backward through the latter an electric wave, which subsided as soon as the primary current was fully established. faraday, for a time, believed that the secondary wire, though quiescent when the primary current had been once established, was not in its natural condition, its return to that condition being declared by the current observed at breaking the circuit. he called this hypothetical state of the wire the electro-tonic state: he afterwards abandoned this hypothesis, but seemed to return to it in later life. the term electro-tonic is also preserved by professor du bois reymond to express a certain electric condition of the nerves, and professor clerk maxwell has ably defined and illustrated the hypothesis in the tenth volume of the 'transactions of the cambridge philosophical society.' the mere approach of a wire forming a closed curve to a second wire through which a voltaic current flowed was then shown by faraday to be sufficient to arouse in the neutral wire an induced current, opposed in direction to the inducing current; the withdrawal of the wire also generated a current having the same direction as the inducing current; those currents existed only during the time of approach or withdrawal, and when neither the primary nor the secondary wire was in motion, no matter how close their proximity might be, no induced current was generated. faraday has been called a purely inductive philosopher. a great deal of nonsense is, i fear, uttered in this land of england about induction and deduction. some profess to befriend the one, some the other, while the real vocation of an investigator, like faraday, consists in the incessant marriage of both. he was at this time full of the theory of ampere, and it cannot be doubted that numbers of his experiments were executed merely to test his deductions from that theory. starting from the discovery of oersted, the illustrious french philosopher had shown that all the phenomena of magnetism then known might be reduced to the mutual attractions and repulsions of electric currents. magnetism had been produced from electricity, and faraday, who all his life long entertained a strong belief in such reciprocal actions, now attempted to effect the evolution of electricity from magnetism. round a welded iron ring he placed two distinct coils of covered wire, causing the coils to occupy opposite halves of the ring. connecting the ends of one of the coils with a galvanometer, he found that the moment the ring was magnetised, by sending a current through the other coil, the galvanometer needle whirled round four or five times in succession. the action, as before, was that of a pulse, which vanished immediately. on interrupting the circuit, a whirl of the needle in the opposite direction occurred. it was only during the time of magnetization or demagnetization that these effects were produced. the induced currents declared a change of condition only, and they vanished the moment the act of magnetization or demagnetization was complete. the effects obtained with the welded ring were also obtained with straight bars of iron. whether the bars were magnetised by the electric current, or were excited by the contact of permanent steel magnets, induced currents were always generated during the rise, and during the subsidence of the magnetism. the use of iron was then abandoned, and the same effects were obtained by merely thrusting a permanent steel magnet into a coil of wire. a rush of electricity through the coil accompanied the insertion of the magnet; an equal rush in the opposite direction accompanied its withdrawal. the precision with which faraday describes these results, and the completeness with which he defines the boundaries of his facts, are wonderful. the magnet, for example, must not be passed quite through the coil, but only half through; for if passed wholly through, the needle is stopped as by a blow, and then he shows how this blow results from a reversal of the electric wave in the helix. he next operated with the powerful permanent magnet of the royal society, and obtained with it, in an exalted degree, all the foregoing phenomena. and now he turned the light of these discoveries upon the darkest physical phenomenon of that day. arago had discovered, in , that a disk of non-magnetic metal had the power of bringing a vibrating magnetic needle suspended over it rapidly to rest; and that on causing the disk to rotate the magnetic needle rotated along with it. when both were quiescent, there was not the slightest measurable attraction or repulsion exerted between the needle and the disk; still when in motion the disk was competent to drag after it, not only a light needle, but a heavy magnet. the question had been probed and investigated with admirable skill both by arago and ampere, and poisson had published a theoretic memoir on the subject; but no cause could be assigned for so extraordinary an action. it had also been examined in this country by two celebrated men, mr. babbage and sir john herschel; but it still remained a mystery. faraday always recommended the suspension of judgment in cases of doubt. 'i have always admired,' he says, 'the prudence and philosophical reserve shown by m. arago in resisting the temptation to give a theory of the effect he had discovered, so long as he could not devise one which was perfect in its application, and in refusing to assent to the imperfect theories of others.' now, however, the time for theory had come. faraday saw mentally the rotating disk, under the operation of the magnet, flooded with his induced currents, and from the known laws of interaction between currents and magnets he hoped to deduce the motion observed by arago. that hope he realised, showing by actual experiment that when his disk rotated currents passed through it, their position and direction being such as must, in accordance with the established laws of electro-magnetic action, produce the observed rotation. introducing the edge of his disk between the poles of the large horseshoe magnet of the royal society, and connecting the axis and the edge of the disk, each by a wire with a galvanometer, he obtained, when the disk was turned round, a constant flow of electricity. the direction of the current was determined by the direction of the motion, the current being reversed when the rotation was reversed. he now states the law which rules the production of currents in both disks and wires, and in so doing uses, for the first time, a phrase which has since become famous. when iron filings are scattered over a magnet, the particles of iron arrange themselves in certain determinate lines called magnetic curves. in , faraday for the first time called these curves 'lines of magnetic force'; and he showed that to produce induced currents neither approach to nor withdrawal from a magnetic source, or centre, or pole, was essential, but that it was only necessary to cut appropriately the lines of magnetic force. faraday's first paper on magneto-electric induction, which i have here endeavoured to condense, was read before the royal society on the th of november, . on january , , he communicated to the royal society a second paper on terrestrial magneto-electric induction, which was chosen as the bakerian lecture for the year. he placed a bar of iron in a coil of wire, and lifting the bar into the direction of the dipping needle, he excited by this action a current in the coil. on reversing the bar, a current in the opposite direction rushed through the wire. the same effect was produced when, on holding the helix in the line of dip, a bar of iron was thrust into it. here, however, the earth acted on the coil through the intermediation of the bar of iron. he abandoned the bar and simply set a copper plate spinning in a horizontal plane; he knew that the earth's lines of magnetic force then crossed the plate at an angle of about degrees. when the plate spun round, the lines of force were intersected and induced currents generated, which produced their proper effect when carried from the plate to the galvanometer. 'when the plate was in the magnetic meridian, or in any other plane coinciding with the magnetic dip, then its rotation produced no effect upon the galvanometer.' at the suggestion of a mind fruitful in suggestions of a profound and philosophic character--i mean that of sir john herschel--mr. barlow, of woolwich, had experimented with a rotating iron shell. mr. christie had also performed an elaborate series of experiments on a rotating iron disk. both of them had found that when in rotation the body exercised a peculiar action upon the magnetic needle, deflecting it in a manner which was not observed during quiescence; but neither of them was aware at the time of the agent which produced this extraordinary deflection. they ascribed it to some change in the magnetism of the iron shell and disk. but faraday at once saw that his induced currents must come into play here, and he immediately obtained them from an iron disk. with a hollow brass ball, moreover, he produced the effects obtained by mr. barlow. iron was in no way necessary: the only condition of success was that the rotating body should be of a character to admit of the formation of currents in its substance: it must, in other words, be a conductor of electricity. the higher the conducting power the more copious were the currents. he now passes from his little brass globe to the globe of the earth. he plays like a magician with the earth's magnetism. he sees the invisible lines along which its magnetic action is exerted, and sweeping his wand across these lines evokes this new power. placing a simple loop of wire round a magnetic needle he bends its upper portion to the west: the north pole of the needle immediately swerves to the east: he bends his loop to the east, and the north pole moves to the west. suspending a common bar magnet in a vertical position, he causes it to spin round its own axis. its pole being connected with one end of a galvanometer wire, and its equator with the other end, electricity rushes round the galvanometer from the rotating magnet. he remarks upon the 'singular independence' of the magnetism and the body of the magnet which carries it. the steel behaves as if it were isolated from its own magnetism. and then his thoughts suddenly widen, and he asks himself whether the rotating earth does not generate induced currents as it turns round its axis from west to east. in his experiment with the twirling magnet the galvanometer wire remained at rest; one portion of the circuit was in motion relatively to another portion. but in the case of the twirling planet the galvanometer wire would necessarily be carried along with the earth; there would be no relative motion. what must be the consequence? take the case of a telegraph wire with its two terminal plates dipped into the earth, and suppose the wire to lie in the magnetic meridian. the ground underneath the wire is influenced like the wire itself by the earth's rotation; if a current from south to north be generated in the wire, a similar current from south to north would be generated in the earth under the wire; these currents would run against the same terminal plate, and thus neutralise each other. this inference appears inevitable, but his profound vision perceived its possible invalidity. he saw that it was at least possible that the difference of conducting power between the earth and the wire might give one an advantage over the other, and that thus a residual or differential current might be obtained. he combined wires of different materials, and caused them to act in opposition to each other, but found the combination ineffectual. the more copious flow in the better conductor was exactly counterbalanced by the resistance of the worse. still, though experiment was thus emphatic, he would clear his mind of all discomfort by operating on the earth itself. he went to the round lake near kensington palace, and stretched feet of copper wire, north and south, over the lake, causing plates soldered to the wire at its ends to dip into the water. the copper wire was severed at the middle, and the severed ends connected with a galvanometer. no effect whatever was observed. but though quiescent water gave no effect, moving water might. he therefore worked at london bridge for three days during the ebb and flow of the tide, but without any satisfactory result. still he urges, 'theoretically it seems a necessary consequence, that where water is flowing there electric currents should be formed. if a line be imagined passing from dover to calais through the sea, and returning through the land, beneath the water, to dover, it traces out a circuit of conducting matter one part of which, when the water moves up or down the channel, is cutting the magnetic curves of the earth, whilst the other is relatively at rest.... there is every reason to believe that currents do run in the general direction of the circuit described, either one way or the other, according as the passage of the waters is up or down the channel.' this was written before the submarine cable was thought of, and he once informed me that actual observation upon that cable had been found to be in accordance with his theoretic deduction.[ ] three years subsequent to the publication of these researches--that is to say, on january , --faraday read before the royal society a paper 'on the influence by induction of an electric current upon itself.' a shock and spark of a peculiar character had been observed by a young man named william jenkin, who must have been a youth of some scientific promise, but who, as faraday once informed me, was dissuaded by his own father from having anything to do with science. the investigation of the fact noticed by mr. jenkin led faraday to the discovery of the extra current, or the current induced in the primary wire itself at the moments of making and breaking contact, the phenomena of which he described and illustrated in the beautiful and exhaustive paper referred to. seven-and-thirty years have passed since the discovery of magneto-electricity; but, if we except the extra current, until quite recently nothing of moment was added to the subject. faraday entertained the opinion that the discoverer of a great law or principle had a right to the 'spoils'--this was his term--arising from its illustration; and guided by the principle he had discovered, his wonderful mind, aided by his wonderful ten fingers, overran in a single autumn this vast domain, and hardly left behind him the shred of a fact to be gathered by his successors. and here the question may arise in some minds, what is the use of it all? the answer is, that if man's intellectual nature thirsts for knowledge, then knowledge is useful because it satisfies this thirst. if you demand practical ends, you must, i think, expand your definition of the term practical, and make it include all that elevates and enlightens the intellect, as well as all that ministers to the bodily health and comfort of men. still, if needed, an answer of another kind might be given to the question 'what is its use?' as far as electricity has been applied for medical purposes, it has been almost exclusively faraday's electricity. you have noticed those lines of wire which cross the streets of london. it is faraday's currents that speed from place to place through these wires. approaching the point of dungeness, the mariner sees an unusually brilliant light, and from the noble phares of la heve the same light flashes across the sea. these are faraday's sparks exalted by suitable machinery to sunlike splendour. at the present moment the board of trade and the brethren of the trinity house, as well as the commissioners of northern lights, are contemplating the introduction of the magneto-electric light at numerous points upon our coasts; and future generations will be able to refer to those guiding stars in answer to the question. what has been the practical use of the labours of faraday? but i would again emphatically say, that his work needs no such justification, and that if he had allowed his vision to be disturbed by considerations regarding the practical use of his discoveries, those discoveries would never have been made by him. 'i have rather,' he writes in , 'been desirous of discovering new facts and new relations dependent on magneto-electric induction, than of exalting the force of those already obtained; being assured that the latter would find their full development hereafter.' in , when lecturing before a private society in london on the element chlorine, faraday thus expressed himself with reference to this question of utility. 'before leaving this subject, i will point out the history of this substance, as an answer to those who are in the habit of saying to every new fact. "what is its use?" dr. franklin says to such, "what is the use of an infant?" the answer of the experimentalist is, "endeavour to make it useful." when scheele discovered this substance, it appeared to have no use; it was in its infancy and useless state, but having grown up to maturity, witness its powers, and see what endeavours to make it useful have done.' footnote to chapter [ ] i am indebted to a friend for the following exquisite morsel:--'a short time after the publication of faraday's first researches in magneto-electricity, he attended the meeting of the british association at oxford, in . on this occasion he was requested by some of the authorities to repeat the celebrated experiment of eliciting a spark from a magnet, employing for this purpose the large magnet in the ashmolean museum. to this he consented, and a large party assembled to witness the experiments, which, i need not say, were perfectly successful. whilst he was repeating them a dignitary of the university entered the room, and addressing himself to professor daniell, who was standing near faraday, inquired what was going on. the professor explained to him as popularly as possible this striking result of faraday's great discovery. the dean listened with attention and looked earnestly at the brilliant spark, but a moment after he assumed a serious countenance and shook his head; "i am sorry for it," said he, as he walked away; in the middle of the room he stopped for a moment and repeated, "i am sorry for it:" then walking towards the door, when the handle was in his hand he turned round and said, "indeed i am sorry for it; it is putting new arms into the hands of the incendiary." this occurred a short time after the papers had been filled with the doings of the hayrick burners. an erroneous statement of what fell from the dean's mouth was printed at the time in one of the oxford papers. he is there wrongly stated to have said, "it is putting new arms into the hands of the infidel."' chapter . points of character. a point highly illustrative of the character of faraday now comes into view. he gave an account of his discovery of magneto-electricity in a letter to his friend m. hachette, of paris, who communicated the letter to the academy of sciences. the letter was translated and published; and immediately afterwards two distinguished italian philosophers took up the subject, made numerous experiments, and published their results before the complete memoirs of faraday had met the public eye. this evidently irritated him. he reprinted the paper of the learned italians in the 'philosophical magazine,' accompanied by sharp critical notes from himself. he also wrote a letter dated dec. , , to gay lussac, who was then one of the editors of the 'annales de chimie,' in which he analysed the results of the italian philosophers, pointing out their errors, and defending himself from what he regarded as imputations on his character. the style of this letter is unexceptionable, for faraday could not write otherwise than as a gentleman; but the letter shows that had he willed it he could have hit hard. we have heard much of faraday's gentleness and sweetness and tenderness. it is all true, but it is very incomplete. you cannot resolve a powerful nature into these elements, and faraday's character would have been less admirable than it was had it not embraced forces and tendencies to which the silky adjectives 'gentle' and 'tender' would by no means apply. underneath his sweetness and gentleness was the heat of a volcano. he was a man of excitable and fiery nature; but through high self-discipline he had converted the fire into a central glow and motive power of life, instead of permitting it to waste itself in useless passion. 'he that is slow to anger,' saith the sage, 'is greater than the mighty, and he that ruleth his own spirit than he that taketh a city.' faraday was not slow to anger, but he completely ruled his own spirit, and thus, though he took no cities, he captivated all hearts. as already intimated, faraday had contributed many of his minor papers--including his first analysis of caustic lime--to the 'quarterly journal of science.' in , he collected those papers and others together in a small octavo volume, labelled them, and prefaced them thus:-- 'papers, notes, notices, &c., &c.,published in octavo, up to . m. faraday.' 'papers of mine, published in octavo, in the "quarterly journal of science," and elsewhere, since the time that sir h. davy encouraged me to write the analysis of caustic lime. 'some, i think (at this date), are good; others moderate; and some bad. but i have put all into the volume, because of the utility they have been of to me--and none more than the bad--in pointing out to me in future, or rather, after times, the faults it became me to watch and to avoid. 'as i never looked over one of my papers a year after it was written without believing both in philosophy and manner it could have been much better done, i still hope the collection may be of great use to me. 'm. faraday. 'aug. , .' 'none more than the bad!' this is a bit of faraday's innermost nature; and as i read these words i am almost constrained to retract what i have said regarding the fire and excitability of his character. but is he not all the more admirable, through his ability to tone down and subdue that fire and that excitability, so as to render himself able to write thus as a little child? i once took the liberty of censuring the conclusion of a letter of his to the dean of st. paul's. he subscribed himself 'humbly yours,' and i objected to the adverb. 'well, but, tyndall,' he said, 'i am humble; and still it would be a great mistake to think that i am not also proud.' this duality ran through his character. a democrat in his defiance of all authority which unfairly limited his freedom of thought, and still ready to stoop in reverence to all that was really worthy of reverence, in the customs of the world or the characters of men. and here, as well as elsewhere, may be introduced a letter which bears upon this question of self-control, written long years subsequent to the period at which we have now arrived. i had been at glasgow in , at a meeting of the british association. on a certain day, i communicated a paper to the physical section, which was followed by a brisk discussion. men of great distinction took part in it, the late dr. whewell among the number, and it waxed warm on both sides. i was by no means content with this discussion; and least of all, with my own part in it. this discontent affected me for some days, during which i wrote to faraday, giving him no details, but expressing, in a general way, my dissatisfaction. i give the following extract from his reply:-- 'sydenham, oct. , . 'my dear tyndall,--these great meetings, of which i think very well altogether, advance science chiefly by bringing scientific men together and making them to know and be friends with each other; and i am sorry when that is not the effect in every part of their course. i know nothing except from what you tell me, for i have not yet looked at the reports of the proceedings; but let me, as an old man, who ought by this time to have profited by experience, say that when i was younger i found i often misinterpreted the intentions of people, and found they did not mean what at the time i supposed they meant; and, further, that as a general rule, it was better to be a little dull of apprehension where phrases seemed to imply pique, and quick in perception when, on the contrary, they seemed to imply kindly feeling. the real truth never fails ultimately to appear; and opposing parties, if wrong, are sooner convinced when replied to forbearingly, than when overwhelmed. all i mean to say is, that it is better to be blind to the results of partisanship, and quick to see good will. one has more happiness in oneself in endeavouring to follow the things that make for peace. you can hardly imagine how often i have been heated in private when opposed, as i have thought, unjustly and superciliously, and yet i have striven, and succeeded, i hope, in keeping down replies of the like kind. and i know i have never lost by it. i would not say all this to you did i not esteem you as a true philosopher and friend.[ ] 'yours, very truly, 'm. faraday.' footnote to chapter [ ] faraday would have been rejoiced to learn that, during its last meeting at dundee, the british association illustrated in a striking manner the function which he here describes as its principal one. in my own case, a brotherly welcome was everywhere manifested. in fact, the differences of really honourable and sane men are never beyond healing. chapter . identity of electricities; first researches on electro-chemistry. i have already once used the word 'discomfort' in reference to the occasional state of faraday's mind when experimenting. it was to him a discomfort to reason upon data which admitted of doubt. he hated what he called 'doubtful knowledge,' and ever tended either to transfer it into the region of undoubtful knowledge, or of certain and definite ignorance. pretence of all kinds, whether in life or in philosophy, was hateful to him. he wished to know the reality of our nescience as well as of our science. 'be one thing or the other,' he seemed to say to an unproved hypothesis; 'come out as a solid truth, or disappear as a convicted lie.' after making the great discovery which i have attempted to describe, a doubt seemed to beset him as regards the identity of electricities. 'is it right,' he seemed to ask, 'to call this agency which i have discovered electricity at all? are there perfectly conclusive grounds for believing that the electricity of the machine, the pile, the gymnotus and torpedo, magneto-electricity and thermo-electricity, are merely different manifestations of one and the same agent?' to answer this question to his own satisfaction he formally reviewed the knowledge of that day. he added to it new experiments of his own, and finally decided in favour of the 'identity of electricities.' his paper upon this subject was read before the royal society on january and , . after he had proved to his own satisfaction the identity of electricities, he tried to compare them quantitatively together. the terms quantity and intensity, which faraday constantly used, need a word of explanation here. he might charge a single leyden jar by twenty turns of his machine, or he might charge a battery of ten jars by the same number of turns. the quantity in both cases would be sensibly the same, but the intensity of the single jar would be the greatest, for here the electricity would be less diffused. faraday first satisfied himself that the needle of his galvanometer was caused to swing through the same arc by the same quantity of machine electricity, whether it was condensed in a small battery or diffused over a large one. thus the electricity developed by thirty turns of his machine produced, under very variable conditions of battery surface, the same deflection. hence he inferred the possibility of comparing, as regards quantity, electricities which differ greatly from each other in intensity. his object now is to compare frictional with voltaic electricity. moistening bibulous paper with the iodide of potassium--a favourite test of his--and subjecting it to the action of machine electricity, he decomposed the iodide, and formed a brown spot where the iodine was liberated. then he immersed two wires, one of zinc, the other of platinum, each / th of an inch in diameter, to a depth of / ths of an inch in acidulated water during eight beats of his watch, or / ths of a second; and found that the needle of his galvanometer swung through the same arc, and coloured his moistened paper to the same extent, as thirty turns of his large electrical machine. twenty-eight turns of the machine produced an effect distinctly less than that produced by his two wires. now, the quantity of water decomposed by the wires in this experiment totally eluded observation; it was immeasurably small; and still that amount of decomposition involved the development of a quantity of electric force which, if applied in a proper form, would kill a rat, and no man would like to bear it. in his subsequent researches 'on the absolute quantity of electricity associated with the particles or atoms of matter,' he endeavours to give an idea of the amount of electrical force involved in the decomposition of a single grain of water. he is almost afraid to mention it, for he estimates it at , discharges of his large leyden battery. this, if concentrated in a single discharge, would be equal to a very great flash of lightning; while the chemical action of a single grain of water on four grains of zinc would yield electricity equal in quantity to a powerful thunderstorm. thus his mind rises from the minute to the vast, expanding involuntarily from the smallest laboratory fact till it embraces the largest and grandest natural phenomena.[ ] in reality, however, he is at this time only clearing his way, and he continues laboriously to clear it for some time afterwards. he is digging the shaft, guided by that instinct towards the mineral lode which was to him a rod of divination. 'er riecht die wahrheit,' said the lamented kohlrausch, an eminent german, once in my hearing:--'he smells the truth.' his eyes are now steadily fixed on this wonderful voltaic current, and he must learn more of its mode of transmission. on may , , he read a paper before the royal society 'on a new law of electric conduction.' he found that, though the current passed through water, it did not pass through ice:--why not, since they are one and the same substance? some years subsequently he answered this question by saying that the liquid condition enables the molecule of water to turn round so as to place itself in the proper line of polarization, while the rigidity of the solid condition prevents this arrangement. this polar arrangement must precede decomposition, and decomposition is an accompaniment of conduction. he then passed on to other substances; to oxides and chlorides, and iodides, and salts, and sulphurets, and found them all insulators when solid, and conductors when fused. in all cases, moreover, except one--and this exception he thought might be apparent only--he found the passage of the current across the fused compound to be accompanied by its decomposition. is then the act of decomposition essential to the act of conduction in these bodies? even recently this question was warmly contested. faraday was very cautious latterly in expressing himself upon this subject; but as a matter of fact he held that an infinitesimal quantity of electricity might pass through a compound liquid without producing its decomposition. de la rive, who has been a great worker on the chemical phenomena of the pile, is very emphatic on the other side. experiment, according to him and others, establishes in the most conclusive manner that no trace of electricity can pass through a liquid compound without producing its equivalent decomposition.[ ] faraday has now got fairly entangled amid the chemical phenomena of the pile, and here his previous training under davy must have been of the most important service to him. why, he asks, should decomposition thus take place?--what force is it that wrenches the locked constituents of these compounds asunder? on the th of june, , he read a paper before the royal society 'on electro-chemical decomposition,' in which he seeks to answer these questions. the notion had been entertained that the poles, as they are called, of the decomposing cell, or in other words the surfaces by which the current enters and quits the liquid, exercised electric attractions upon the constituents of the liquid and tore them asunder. faraday combats this notion with extreme vigour. litmus reveals, as you know, the action of an acid by turning red, turmeric reveals the action of an alkali by turning brown. sulphate of soda, you know, is a salt compounded of the alkali soda and sulphuric acid. the voltaic current passing through a solution of this salt so decomposes it, that sulphuric acid appears at one pole of the decomposing cell and alkali at the other. faraday steeped a piece of litmus paper and a piece of turmeric paper in a solution of sulphate of soda: placing each of them upon a separate plate of glass, he connected them together by means of a string moistened with the same solution. he then attached one of them to the positive conductor of an electric machine, and the other to the gas-pipes of this building. these he called his 'discharging train.' on turning the machine the electricity passed from paper to paper through the string, which might be varied in length from a few inches to seventy feet without changing the result. the first paper was reddened, declaring the presence of sulphuric acid; the second was browned, declaring the presence of the alkali soda. the dissolved salt, therefore, arranged in this fashion, was decomposed by the machine, exactly as it would have been by the voltaic current. when instead of using the positive conductor he used the negative, the positions of the acid and alkali were reversed. thus he satisfied himself that chemical decomposition by the machine is obedient to the laws which rule decomposition by the pile. and now he gradually abolishes those so-called poles, to the attraction of which electric decomposition had been ascribed. he connected a piece of turmeric paper moistened with the sulphate of soda with the positive conductor of his machine; then he placed a metallic point in connection with his discharging train opposite the moist paper, so that the electricity should discharge through the air towards the point. the turning of the machine caused the corners of the piece of turmeric paper opposite to the point to turn brown, thus declaring the presence of alkali. he changed the turmeric for litmus paper, and placed it, not in connection with his conductor, but with his discharging train, a metallic point connected with the conductor being fixed at a couple of inches from the paper; on turning the machine, acid was liberated at the edges and corners of the litmus. he then placed a series of pointed pieces of paper, each separate piece being composed of two halves, one of litmus and the other of turmeric paper, and all moistened with sulphate of soda, in the line of the current from the machine. the pieces of paper were separated from each other by spaces of air. the machine was turned; and it was always found that at the point where the electricity entered the paper, litmus was reddened, and at the point where it quitted the paper, turmeric was browned. 'here,' he urges, 'the poles are entirely abandoned, but we have still electrochemical decomposition.' it is evident to him that instead of being attracted by the poles, the bodies separated are ejected by the current. the effects thus obtained with poles of air he also succeeded in obtaining with poles of water. the advance in faraday's own ideas made at this time is indicated by the word 'ejected.' he afterwards reiterates this view: the evolved substances are expelled from the decomposing body, and 'not drawn out by an attraction. having abolished this idea of polar attraction, he proceeds to enunciate and develop a theory of his own. he refers to davy's celebrated bakerian lecture, given in , which he says 'is almost entirely occupied in the consideration of electrochemical decompositions.' the facts recorded in that lecture faraday regards as of the utmost value. but 'the mode of action by which the effects take place is stated very generally; so generally, indeed, that probably a dozen precise schemes of electrochemical action might be drawn up, differing essentially from each other, yet all agreeing with the statement there given.' it appears to me that these words might with justice be applied to faraday's own researches at this time. they furnish us with results of permanent value; but little help can be found in the theory advanced to account for them. it would, perhaps, be more correct to say that the theory itself is hardly presentable in any tangible form to the intellect. faraday looks, and rightly looks, into the heart of the decomposing body itself; he sees, and rightly sees, active within it the forces which produce the decomposition, and he rejects, and rightly rejects, the notion of external attraction; but beyond the hypothesis of decompositions and recompositions, enunciated and developed by grothuss and davy, he does not, i think, help us to any definite conception as to how the force reaches the decomposing mass and acts within it. nor, indeed, can this be done, until we know the true physical process which underlies what we call an electric current. faraday conceives of that current as 'an axis of power having contrary forces exactly equal in amount in opposite directions'; but this definition, though much quoted and circulated, teaches us nothing regarding the current. an 'axis' here can only mean a direction; and what we want to be able to conceive of is, not the axis along which the power acts, but the nature and mode of action of the power itself. he objects to the vagueness of de la rive; but the fact is, that both he and de la rive labour under the same difficulty. neither wishes to commit himself to the notion of a current compounded of two electricities flowing in two opposite directions: but the time had not come, nor is it yet come, for the displacement of this provisional fiction by the true mechanical conception. still, however indistinct the theoretic notions of faraday at this time may be, the facts which are rising before him and around him are leading him gradually, but surely, to results of incalculable importance in relation to the philosophy of the voltaic pile. he had always some great object of research in view, but in the pursuit of it he frequently alighted on facts of collateral interest, to examine which he sometimes turned aside from his direct course. thus we find the series of his researches on electrochemical decomposition interrupted by an inquiry into 'the power of metals and other solids, to induce the combination of gaseous bodies.' this inquiry, which was received by the royal society on nov. , , though not so important as those which precede and follow it, illustrates throughout his strength as an experimenter. the power of spongy platinum to cause the combination of oxygen and hydrogen had been discovered by dobereiner in , and had been applied by him in the construction of his well-known philosophic lamp. it was shown subsequently by dulong and thenard that even a platinum wire, when perfectly cleansed, may be raised to incandescence by its action on a jet of cold hydrogen. in his experiments on the decomposition of water, faraday found that the positive platinum plate of the decomposing cell possessed in an extraordinary degree the power of causing oxygen and hydrogen to combine. he traced the cause of this to the perfect cleanness of the positive plate. against it was liberated oxygen, which, with the powerful affinity of the 'nascent state,' swept away all impurity from the surface against which it was liberated. the bubbles of gas liberated on one of the platinum plates or wires of a decomposing cell are always much smaller, and they rise in much more rapid succession than those from the other. knowing that oxygen is sixteen times heavier than hydrogen, i have more than once concluded, and, i fear, led others into the error of concluding, that the smaller and more quickly rising bubbles must belong to the lighter gas. the thing appeared so obvious that i did not give myself the trouble of looking at the battery, which would at once have told me the nature of the gas. but faraday would never have been satisfied with a deduction if he could have reduced it to a fact. and he has taught me that the fact here is the direct reverse of what i supposed it to be. the small bubbles are oxygen, and their smallness is due to the perfect cleanness of the surface on which they are liberated. the hydrogen adhering to the other electrode swells into large bubbles, which rise in much slower succession; but when the current is reversed, the hydrogen is liberated upon the cleansed wire, and then its bubbles also become small. footnotes to chapter [ ] buff finds the quantity of electricity associated with one milligramme of hydrogen in water to be equal to , charges of a leyden jar, with a height of millimetres, and a diameter of millimetres. weber and kohlrausch have calculated that, if the quantity of electricity associated with one milligramme of hydrogen in water were diffused over a cloud at a height of metres above the earth, it would exert upon an equal quantity of the opposite electricity at the earth's surface an attractive force of , , kilogrammes. (electrolytische maasbestimmungen, , p. .) [ ] faraday, sa vie et ses travaux, p. . chapter . laws of electro-chemical decomposition. in our conceptions and reasonings regarding the forces of nature, we perpetually make use of symbols which, when they possess a high representative value, we dignify with the name of theories. thus, prompted by certain analogies, we ascribe electrical phenomena to the action of a peculiar fluid, sometimes flowing, sometimes at rest. such conceptions have their advantages and their disadvantages; they afford peaceful lodging to the intellect for a time, but they also circumscribe it, and by-and-by, when the mind has grown too large for its lodging, it often finds difficulty in breaking down the walls of what has become its prison instead of its home.[ ] no man ever felt this tyranny of symbols more deeply than faraday, and no man was ever more assiduous than he to liberate himself from them, and the terms which suggested them. calling dr. whewell to his aid in , he endeavoured to displace by others all terms tainted by a foregone conclusion. his paper on electro-chemical decomposition, received by the royal society on january , , opens with the proposal of a new terminology. he would avoid the word 'current' if he could.[ ] he does abandon the word 'poles' as applied to the ends of a decomposing cell, because it suggests the idea of attraction, substituting for it the perfectly natural term electrodes. he applied the term electrolyte to every substance which can be decomposed by the current, and the act of decomposition he called electrolysis. all these terms have become current in science. he called the positive electrode the anode, and the negative one the cathode, but these terms, though frequently used, have not enjoyed the same currency as the others. the terms anion and cation, which he applied to the constituents of the decomposed electrolyte, and the term ion, which included both anions and cations, are still less frequently employed. faraday now passes from terminology to research; he sees the necessity of quantitative determinations, and seeks to supply himself with a measure of voltaic electricity. this he finds in the quantity of water decomposed by the current. he tests this measure in all possible ways, to assure himself that no error can arise from its employment. he places in the course of one and the same current a series of cells with electrodes of different sizes, some of them plates of platinum, others merely platinum wires, and collects the gas liberated on each distinct pair of electrodes. he finds the quantity of gas to be the same for all. thus he concludes that when the same quantity of electricity is caused to pass through a series of cells containing acidulated water, the electro-chemical action is independent of the size of the electrodes.[ ] he next proves that variations in intensity do not interfere with this equality of action. whether his battery is charged with strong acid or with weak; whether it consists of five pairs or of fifty pairs; in short, whatever be its source, when the same current is sent through his series of cells the same amount of decomposition takes place in all. he next assures himself that the strength or weakness of his dilute acid does not interfere with this law. sending the same current through a series of cells containing mixtures of sulphuric acid and water of different strengths, he finds, however the proportion of acid to water might vary, the same amount of gas to be collected in all the cells. a crowd of facts of this character forced upon faraday's mind the conclusion that the amount of electro-chemical decomposition depends, not upon the size of the electrodes, not upon the intensity of the current, not upon the strength of the solution, but solely upon the quantity of electricity which passes through the cell. the quantity of electricity he concludes is proportional to the amount of chemical action. on this law faraday based the construction of his celebrated voltameter, or measure of voltaic electricity. but before he can apply this measure he must clear his ground of numerous possible sources of error. the decomposition of his acidulated water is certainly a direct result of the current; but as the varied and important researches of mm. becquerel, de la rive, and others had shown, there are also secondary actions which may materially interfere with and complicate the pure action of the current. these actions may occur in two ways: either the liberated ion may seize upon the electrode against which it is set free, forming a chemical compound with that electrode; or it may seize upon the substance of the electrolyte itself, and thus introduce into the circuit chemical actions over and above those due to the current. faraday subjected these secondary actions to an exhaustive examination. instructed by his experiments, and rendered competent by them to distinguish between primary and secondary results, he proceeds to establish the doctrine of 'definite electro-chemical decomposition.' into the same circuit he introduced his voltameter, which consisted of a graduated tube filled with acidulated water and provided with platinum plates for the decomposition of the water, and also a cell containing chloride of tin. experiments already referred to had taught him that this substance, though an insulator when solid, is a conductor when fused, the passage of the current being always accompanied by the decomposition of the chloride. he wished to ascertain what relation this decomposition bore to that of the water in his voltameter. completing his circuit, he permitted the current to continue until 'a reasonable quantity of gas' was collected in the voltameter. the circuit was then broken, and the quantity of tin liberated compared with the quantity of gas. the weight of the former was . grains, that of the latter . of a grain. oxygen, as you know, unites with hydrogen in the proportion of to , to form water. calling the equivalent, or as it is sometimes called, the atomic weight of hydrogen , that of oxygen is ; that of water is consequently + or . now if the quantity of water decomposed in faraday's experiment be represented by the number , or in other words by the equivalent of water, then the quantity of tin liberated from the fused chloride is found by an easy calculation to be . , which is almost exactly the chemical equivalent of tin. thus both the water and the chloride were broken up in proportions expressed by their respective equivalents. the amount of electric force which wrenched asunder the constituents of the molecule of water was competent, and neither more nor less than competent, to wrench asunder the constituents of the molecules of the chloride of tin. the fact is typical. with the indications of his voltameter he compared the decompositions of other substances, both singly and in series. he submitted his conclusions to numberless tests. he purposely introduced secondary actions. he endeavoured to hamper the fulfilment of those laws which it was the intense desire of his mind to see established. but from all these difficulties emerged the golden truth, that under every variety of circumstances the decompositions of the voltaic current are as definite in their character as those chemical combinations which gave birth to the atomic theory. this law of electro-chemical decomposition ranks, in point of importance, with that of definite combining proportions in chemistry. footnotes to chapter [ ] i copy these words from the printed abstract of a friday evening lecture, given by myself, because they remind me of faraday's voice, responding to the utterance by an emphatic 'hear! hear!'--proceedings of the royal institution, vol. ii. p. . [ ] in he expresses himself thus:--'the word current is so expressive in common language that when applied in the consideration of electrical phenomena, we can hardly divest it sufficiently of its meaning, or prevent our minds from being prejudiced by it.'--exp. resear., vol. i. p. . ($ .) [ ] this conclusion needs qualification. faraday overlooked the part played by ozone. chapter . origin of power in the voltaic pile. in one of the public areas of the town of como stands a statue with no inscription on its pedestal, save that of a single name, 'volta.' the bearer of that name occupies a place for ever memorable in the history of science. to him we owe the discovery of the voltaic pile, to which for a brief interval we must now turn our attention. the objects of scientific thought being the passionless laws and phenomena of external nature, one might suppose that their investigation and discussion would be completely withdrawn from the region of the feelings, and pursued by the cold dry light of the intellect alone. this, however, is not always the case. man carries his heart with him into all his works. you cannot separate the moral and emotional from the intellectual; and thus it is that the discussion of a point of science may rise to the heat of a battle-field. the fight between the rival optical theories of emission and undulation was of this fierce character; and scarcely less fierce for many years was the contest as to the origin and maintenance of the power of the voltaic pile. volta himself supposed it to reside in the contact of different metals. here was exerted his 'electro-motive force,' which tore the combined electricities asunder and drove them as currents in opposite directions. to render the circulation of the current possible, it was necessary to connect the metals by a moist conductor; for when any two metals were connected by a third, their relation to each other was such that a complete neutralisation of the electric motion was the result. volta's theory of metallic contact was so clear, so beautiful, and apparently so complete, that the best intellects of europe accepted it as the expression of natural law. volta himself knew nothing of the chemical phenomena of the pile; but as soon as these became known, suggestions and intimations appeared that chemical action, and not metallic contact, might be the real source of voltaic electricity. this idea was expressed by fabroni in italy, and by wollaston in england. it was developed and maintained by those 'admirable electricians,' becquerel, of paris, and de la rive, of geneva. the contact theory, on the other hand, received its chief development and illustration in germany. it was long the scientific creed of the great chemists and natural philosophers of that country, and to the present hour there may be some of them unable to liberate themselves from the fascination of their first-love. after the researches which i have endeavoured to place before you, it was impossible for faraday to avoid taking a side in this controversy. he did so in a paper 'on the electricity of the voltaic pile,' received by the royal society on the th of april, . his position in the controversy might have been predicted. he saw chemical effects going hand in hand with electrical effects, the one being proportional to the other; and, in the paper now before us, he proved that when the former was excluded, the latter were sought for in vain. he produced a current without metallic contact; he discovered liquids which, though competent to transmit the feeblest currents--competent therefore to allow the electricity of contact to flow through them if it were able to form a current--were absolutely powerless when chemically inactive. one of the very few experimental mistakes of faraday occurred in this investigation. he thought that with a single voltaic cell he had obtained the spark before the metals touched, but he subsequently discovered his error. to enable the voltaic spark to pass through air before the terminals of the battery were united, it was necessary to exalt the electro-motive force of the battery by multiplying its elements; but all the elements faraday possessed were unequal to the task of urging the spark across the shortest measurable space of air. nor, indeed, could the action of the battery, the different metals of which were in contact with each other, decide the point in question. still, as regards the identity of electricities from various sources, it was at that day of great importance to determine whether or not the voltaic current could jump, as a spark, across an interval before contact. faraday's friend, mr. gassiot, solved this problem. he erected a battery of cells, and with it urged a stream of sparks from terminal to terminal, when separated from each other by a measurable space of air. the memoir on the 'electricity of the voltaic pile,' published in , appears to have produced but little impression upon the supporters of the contact theory. these indeed were men of too great intellectual weight and insight lightly to take up, or lightly to abandon a theory. faraday therefore resumed the attack in a paper, communicated to the royal society on the th of february, . in this paper he hampered his antagonists by a crowd of adverse experiments. he hung difficulty after difficulty about the neck of the contact theory, until in its efforts to escape from his assaults it so changed its character as to become a thing totally different from the theory proposed by volta. the more persistently it was defended, however, the more clearly did it show itself to be a congeries of devices, bearing the stamp of dialectic skill rather than of natural truth. in conclusion, faraday brought to bear upon it an argument which, had its full weight and purport been understood at the time, would have instantly decided the controversy. 'the contact theory,' he urged, 'assumed that a force which is able to overcome powerful resistance, as for instance that of the conductors, good or bad, through which the current passes, and that again of the electrolytic action where bodies are decomposed by it, can arise out of nothing; that, without any change in the acting matter, or the consumption of any generating force, a current shall be produced which shall go on for ever against a constant resistance, or only be stopped, as in the voltaic trough, by the ruins which its exertion has heaped up in its own course. this would indeed be a creation of power, and is like no other force in nature. we have many processes by which the form of the power may be so changed, that an apparent conversion of one into the other takes place. so we can change chemical force into the electric current, or the current into chemical force. the beautiful experiments of seebeck and peltier show the convertibility of heat and electricity; and others by oersted and myself show the convertibility of electricity and magnetism. but in no case, not even in those of the gymnotus and torpedo, is there a pure creation or a production of power without a corresponding exhaustion of something to supply it.' these words were published more than two years before either mayer printed his brief but celebrated essay on the forces of inorganic nature, or mr. joule published his first famous experiments on the mechanical value of heat. they illustrate the fact that before any great scientific principle receives distinct enunciation by individuals, it dwells more or less clearly in the general scientific mind. the intellectual plateau is already high, and our discoverers are those who, like peaks above the plateau, rise a little above the general level of thought at the time. but many years prior even to the foregoing utterance of faraday, a similar argument had been employed. i quote here with equal pleasure and admiration the following passage written by dr. roget so far back as . speaking of the contact theory, he says:--'if there could exist a power having the property ascribed to it by the hypothesis, namely, that of giving continual impulse to a fluid in one constant direction, without being exhausted by its own action, it would differ essentially from all the known powers in nature. all the powers and sources of motion with the operation of which we are acquainted, when producing these peculiar effects, are expended in the same proportion as those effects are produced; and hence arises the impossibility of obtaining by their agency a perpetual effect; or in other words a perpetual motion. but the electro-motive force, ascribed by volta to the metals, when in contact, is a force which, as long as a free course is allowed to the electricity it sets in motion, is never expended, and continues to be excited with undiminished power in the production of a never-ceasing effect. against the truth of such a supposition the probabilities are all but infinite.' when this argument, which he employed independently, had clearly fixed itself in his mind, faraday never cared to experiment further on the source of electricity in the voltaic pile. the argument appeared to him 'to remove the foundation itself of the contact theory,' and he afterwards let it crumble down in peace.[ ] footnote to chapter [ ] to account for the electric current, which was really the core of the whole discussion, faraday demonstrated the impotence of the contact theory as then enunciated and defended. still, it is certain that two different metals, when brought into contact, charge themselves, the one with positive and the other with negative electricity. i had the pleasure of going over this ground with kohlrausch in , and his experiments left no doubt upon my mind that the contact electricity of volta was a reality, though it could produce no current. with one of the beautiful instruments devised by himself, sir william thomson has rendered this point capable of sure and easy demonstration; and he and others now hold what may be called a contact theory, which, while it takes into account the action of the metals, also embraces the chemical phenomena of the circuit. helmholtz, i believe, was the first to give the contact theory this new form, in his celebrated essay, ueber die erhaltung der kraft, p. . chapter . researches on frictional electricity: induction: conduction: specific inductive capacity: theory of contiguous particles. the burst of power which had filled the four preceding years with an amount of experimental work unparalleled in the history of science partially subsided in , and the only scientific paper contributed by faraday in that year was a comparatively unimportant one, 'on an improved form of the voltaic battery.' he brooded for a time: his experiments on electrolysis had long filled his mind; he looked, as already stated, into the very heart of the electrolyte, endeavouring to render the play of its atoms visible to his mental eye. he had no doubt that in this case what is called 'the electric current' was propagated from particle to particle of the electrolyte; he accepted the doctrine of decomposition and recomposition which, according to grothuss and davy, ran from electrode to electrode. and the thought impressed him more and more that ordinary electric induction was also transmitted and sustained by the action of 'contiguous particles.' his first great paper on frictional electricity was sent to the royal society on november , . we here find him face to face with an idea which beset his mind throughout his whole subsequent life,--the idea of action at a distance. it perplexed and bewildered him. in his attempts to get rid of this perplexity, he was often unconsciously rebelling against the limitations of the intellect itself. he loved to quote newton upon this point; over and over again he introduces his memorable words, 'that gravity should be innate, inherent, and essential to matter, so that one body may act upon another at a distance through a vacuum and without the mediation of anything else, by and through which this action and force may be conveyed from one to another, is to me so great an absurdity, that i believe no man who has in philosophical matters a competent faculty of thinking, can ever fall into it. gravity must be caused by an agent acting constantly according to certain laws; but whether this agent be material or immaterial, i have left to the consideration of my readers.'[ ] faraday does not see the same difficulty in his contiguous particles. and yet, by transferring the conception from masses to particles, we simply lessen size and distance, but we do not alter the quality of the conception. whatever difficulty the mind experiences in conceiving of action at sensible distances, besets it also when it attempts to conceive of action at insensible distances. still the investigation of the point whether electric and magnetic effects were wrought out through the intervention of contiguous particles or not, had a physical interest altogether apart from the metaphysical difficulty. faraday grapples with the subject experimentally. by simple intuition he sees that action at a distance must be exerted in straight lines. gravity, he knows, will not turn a corner, but exerts its pull along a right line; hence his aim and effort to ascertain whether electric action ever takes place in curved lines. this once proved, it would follow that the action is carried on by means of a medium surrounding the electrified bodies. his experiments in reduced, in his opinion, this point of demonstration. he then found that he could electrify, by induction, an insulated sphere placed completely in the shadow of a body which screened it from direct action. he pictured the lines of electric force bending round the edges of the screen, and reuniting on the other side of it; and he proved that in many cases the augmentation of the distance between his insulated sphere and the inducing body, instead of lessening, increased the charge of the sphere. this he ascribed to the coalescence of the lines of electric force at some distance behind the screen. faraday's theoretic views on this subject have not received general acceptance, but they drove him to experiment, and experiment with him was always prolific of results. by suitable arrangements he placed a metallic sphere in the middle of a large hollow sphere, leaving a space of something more than half an inch between them. the interior sphere was insulated, the external one uninsulated. to the former he communicated a definite charge of electricity. it acted by induction upon the concave surface of the latter, and he examined how this act of induction was effected by placing insulators of various kinds between the two spheres. he tried gases, liquids, and solids, but the solids alone gave him positive results. he constructed two instruments of the foregoing description, equal in size and similar in form. the interior sphere of each communicated with the external air by a brass stem ending in a knob. the apparatus was virtually a leyden jar, the two coatings of which were the two spheres, with a thick and variable insulator between them. the amount of charge in each jar was determined by bringing a proof-plane into contact with its knob and measuring by a torsion balance the charge taken away. he first charged one of his instruments, and then dividing the charge with the other, found that when air intervened in both cases the charge was equally divided. but when shellac, sulphur, or spermaceti was interposed between the two spheres of one jar, while air occupied this interval in the other, then he found that the instrument occupied by the 'solid dielectric' takes more than half the original charge. a portion of the charge was absorbed by the dielectric itself. the electricity took time to penetrate the dielectric. immediately after the discharge of the apparatus, no trace of electricity was found upon its knob. but after a time electricity was found there, the charge having gradually returned from the dielectric in which it had been lodged. different insulators possess this power of permitting the charge to enter them in different degrees. faraday figured their particles as polarized, and he concluded that the force of induction is propagated from particle to particle of the dielectric from the inner sphere to the outer one. this power of propagation possessed by insulators he called their 'specific inductive capacity.' faraday visualizes with the utmost clearness the state of his contiguous particles; one after another they become charged, each succeeding particle depending for its charge upon its predecessor. and now he seeks to break down the wall of partition between conductors and insulators. 'can we not,' he says, 'by a gradual chain of association carry up discharge from its occurrence in air through spermaceti and water, to solutions, and then on to chlorides, oxides, and metals, without any essential change in its character?' even copper, he urges, offers a resistance to the transmission of electricity. the action of its particles differs from those of an insulator only in degree. they are charged like the particles of the insulator, but they discharge with greater ease and rapidity; and this rapidity of molecular discharge is what we call conduction. conduction then is always preceded by atomic induction; and when, through some quality of the body which faraday does not define, the atomic discharge is rendered slow and difficult, conduction passes into insulation. though they are often obscure, a fine vein of philosophic thought runs through those investigations. the mind of the philosopher dwells amid those agencies which underlie the visible phenomena of induction and conduction; and he tries by the strong light of his imagination to see the very molecules of his dielectrics. it would, however, be easy to criticise these researches, easy to show the looseness, and sometimes the inaccuracy, of the phraseology employed; but this critical spirit will get little good out of faraday. rather let those who ponder his works seek to realise the object he set before him, not permitting his occasional vagueness to interfere with their appreciation of his speculations. we may see the ripples, and eddies, and vortices of a flowing stream, without being able to resolve all these motions into their constituent elements; and so it sometimes strikes me that faraday clearly saw the play of fluids and ethers and atoms, though his previous training did not enable him to resolve what he saw into its constituents, or describe it in a manner satisfactory to a mind versed in mechanics. and then again occur, i confess, dark sayings, difficult to be understood, which disturb my confidence in this conclusion. it must, however, always be remembered that he works at the very boundaries of our knowledge, and that his mind habitually dwells in the 'boundless contiguity of shade' by which that knowledge is surrounded. in the researches now under review the ratio of speculation and reasoning to experiment is far higher than in any of faraday's previous works. amid much that is entangled and dark we have flashes of wondrous insight and utterances which seem less the product of reasoning than of revelation. i will confine myself here to one example of this divining power. by his most ingenious device of a rapidly rotating mirror, wheatstone had proved that electricity required time to pass through a wire, the current reaching the middle of the wire later than its two ends. 'if,' says faraday, 'the two ends of the wire in professor wheatstone's experiments were immediately connected with two large insulated metallic surfaces exposed to the air, so that the primary act of induction, after making the contact for discharge, might be in part removed from the internal portion of the wire at the first instance, and disposed for the moment on its surface jointly with the air and surrounding conductors, then i venture to anticipate that the middle spark would be more retarded than before. and if those two plates were the inner and outer coatings of a large jar or leyden battery, then the retardation of the spark would be much greater.' this was only a prediction, for the experiment was not made.[ ] sixteen years subsequently, however, the proper conditions came into play, and faraday was able to show that the observations of werner siemens, and latimer clark, on subterraneous and submarine wires were illustrations, on a grand scale, of the principle which he had enunciated in . the wires and the surrounding water act as a leyden jar, and the retardation of the current predicted by faraday manifests itself in every message sent by such cables. the meaning of faraday in these memoirs on induction and conduction is, as i have said, by no means always clear; and the difficulty will be most felt by those who are best trained in ordinary theoretic conceptions. he does not know the reader's needs, and he therefore does not meet them. for instance he speaks over and over again of the impossibility of charging a body with one electricity, though the impossibility is by no means evident. the key to the difficulty is this. he looks upon every insulated conductor as the inner coating of a leyden jar. an insulated sphere in the middle of a room is to his mind such a coating; the walls are the outer coating, while the air between both is the insulator, across which the charge acts by induction. without this reaction of the walls upon the sphere you could no more, according to faraday, charge it with electricity than you could charge a leyden jar, if its outer coating were removed. distance with him is immaterial. his strength as a generalizer enables him to dissolve the idea of magnitude; and if you abolish the walls of the room--even the earth itself--he would make the sun and planets the outer coating of his jar. i dare not contend that faraday in these memoirs made all his theoretic positions good. but a pure vein of philosophy runs through these writings; while his experiments and reasonings on the forms and phenomena of electrical discharge are of imperishable importance. footnotes to chapter [ ] newton's third letter to bentley. [ ] had sir charles wheatstone been induced to resume his measurements, varying the substances through which, and the conditions under which, the current is propagated, he might have rendered great service to science, both theoretic and experimental. chapter . rest needed--visit to switzerland. the last of these memoirs was dated from the royal institution in june, . it concludes the first volume of his 'experimental researches on electricity.' in , as already stated, he made his final assault on the contact theory, from which it never recovered.[ ] he was now feeling the effects of the mental strain to which he had been subjected for so many years. during these years he repeatedly broke down. his wife alone witnessed the extent of his prostration, and to her loving care we, and the world, are indebted for the enjoyment of his presence here so long. he found occasional relief in a theatre. he frequently quitted london and went to brighton and elsewhere, always choosing a situation which commanded a view of the sea, or of some other pleasant horizon, where he could sit and gaze and feel the gradual revival of the faith that 'nature never did betray the heart that loved her.' but very often for some days after his removal to the country, he would be unable to do more than sit at a window and look out upon the sea and sky. in , his state became more serious than it had ever been before. a published letter to mr. richard taylor, dated march , , contains an allusion to his previous condition. 'you are aware,' he says, 'that considerations regarding health have prevented me from working or reading on science for the last two years.' this, at one period or another of their lives, seems to be the fate of most great investigators. they do not know the limits of their constitutional strength until they have transgressed them. it is, perhaps, right that they should transgress them, in order to ascertain where they lie. faraday, however, though he went far towards it, did not push his transgression beyond his power of restitution. in mrs. faraday and he went to switzerland, under the affectionate charge of her brother, mr. george barnard, the artist. this time of suffering throws fresh light upon his character. i have said that sweetness and gentleness were not its only constituents; that he was also fiery and strong. at the time now referred to, his fire was low and his strength distilled away; but the residue of his life was neither irritability nor discontent. he was unfit to mingle in society, for conversation was a pain to him; but let us observe the great man-child when alone. he is at the village of interlaken, enjoying jungfrau sunsets, and at times watching the swiss nailers making their nails. he keeps a little journal, in which he describes the process of nailmaking, and incidentally throws a luminous beam upon himself. 'august , .--clout nailmaking goes on here rather considerably, and is a very neat and pretty operation to observe. i love a smith's shop and anything relating to smithery. my father was a smith.' from interlaken he went to the falls of the giessbach, on the pleasant lake of brientz. and here we have him watching the shoot of the cataract down its series of precipices. it is shattered into foam at the base of each, and tossed by its own recoil as water-dust through the air. the sun is at his back, shining on the drifting spray, and he thus describes and muses on what he sees:-- 'august , .--to-day every fall was foaming from the abundance of water, and the current of wind brought down by it was in some places too strong to stand against. the sun shone brightly, and the rainbows seen from various points were very beautiful. one at the bottom of a fine but furious fall was very pleasant,--there it remained motionless, whilst the gusts and clouds of spray swept furiously across its place and were dashed against the rock. it looked like a spirit strong in faith and steadfast in the midst of the storm of passions sweeping across it, and though it might fade and revive, still it held on to the rock as in hope and giving hope. and the very drops, which in the whirlwind of their fury seemed as if they would carry all away, were made to revive it and give it greater beauty.' footnote to chapter [ ] see note, p. . chapter . magnetization of light. but we must quit the man and go on to the discoverer: we shall return for a brief space to his company by-and-by. carry your thoughts back to his last experiments, and see him endeavouring to prove that induction is due to the action of contiguous particles. he knew that polarized light was a most subtle and delicate investigator of molecular condition. he used it in in exploring his electrolytes, and he tried it in upon his dielectrics. at that time he coated two opposite faces of a glass cube with tinfoil, connected one coating with his powerful electric machine and the other with the earth, and examined by polarized light the condition of the glass when thus subjected to strong electric influence. he failed to obtain any effect; still he was persuaded an action existed, and required only suitable means to call it forth. after his return from switzerland he was beset by these thoughts; they were more inspired than logical: but he resorted to magnets and proved his inspiration true. his dislike of 'doubtful knowledge' and his efforts to liberate his mind from the thraldom of hypotheses have been already referred to. still this rebel against theory was incessantly theorising himself. his principal researches are all connected by an undercurrent of speculation. theoretic ideas were the very sap of his intellect--the source from which all his strength as an experimenter was derived. while once sauntering with him through the crystal palace, at sydenham, i asked him what directed his attention to the magnetization of light. it was his theoretic notions. he had certain views regarding the unity and convertibility of natural forces; certain ideas regarding the vibrations of light and their relations to the lines of magnetic force; these views and ideas drove him to investigation. and so it must always be: the great experimentalist must ever be the habitual theorist, whether or not he gives to his theories formal enunciation. faraday, you have been informed, endeavoured to improve the manufacture of glass for optical purposes. but though he produced a heavy glass of great refractive power, its value to optics did not repay him for the pains and labour bestowed on it. now, however, we reach a result established by means of this same heavy glass, which made ample amends for all. in november, , he announced his discovery of the 'magnetization of light and the illumination of the lines of magnetic force.' this title provoked comment at the time, and caused misapprehension. he therefore added an explanatory note; but the note left his meaning as entangled as before. in fact faraday had notions regarding the magnetization of light which were peculiar to himself, and untranslatable into the scientific language of the time. probably no other philosopher of his day would have employed the phrases just quoted as appropriate to the discovery announced in . but faraday was more than a philosopher; he was a prophet, and often wrought by an inspiration to be understood by sympathy alone. the prophetic element in his character occasionally coloured, and even injured, the utterance of the man of science; but subtracting that element, though you might have conferred on him intellectual symmetry, you would have destroyed his motive force. but let us pass from the label of this casket to the jewel it contains. 'i have long,' he says, 'held an opinion, almost amounting to conviction, in common, i believe, with many other lovers of natural knowledge, that the various forms under which the forces of matter are made manifest have one common origin; in other words, are so directly related and mutually dependent, that they are convertible, as it were, into one another, and possess equivalents of power in their action.... this strong persuasion,' he adds, 'extended to the powers of light.' and then he examines the action of magnets upon light. from conversation with him and anderson, i should infer that the labour preceding this discovery was very great. the world knows little of the toil of the discoverer. it sees the climber jubilant on the mountain top, but does not know the labour expended in reaching it. probably hundreds of experiments had been made on transparent crystals before he thought of testing his heavy glass. here is his own clear and simple description of the result of his first experiment with this substance:--'a piece of this glass, about two inches square, and . of an inch thick, having flat and polished edges, was placed as a diamagnetic[ ] between the poles (not as yet magnetized by the electric current), so that the polarized ray should pass through its length; the glass acted as air, water, or any other transparent substance would do; and if the eye-piece were previously turned into such a position that the polarized ray was extinguished, or rather the image produced by it rendered invisible, then the introduction of the glass made no alteration in this respect. in this state of circumstances, the force of the electro-magnet was developed by sending an electric current through its coils, and immediately the image of the lamp-flame became visible and continued so as long as the arrangement continued magnetic. on stopping the electric current, and so causing the magnetic force to cease, the light instantly disappeared. these phenomena could be renewed at pleasure, at any instant of time, and upon any occasion, showing a perfect dependence of cause and effect.' in a beam of ordinary light the particles of the luminiferous ether vibrate in all directions perpendicular to the line of progression; by the act of polarization, performed here by faraday, all oscillations but those parallel to a certain plane are eliminated. when the plane of vibration of the polarizer coincides with that of the analyzer, a portion of the beam passes through both; but when these two planes are at right angles to each other, the beam is extinguished. if by any means, while the polarizer and analyzer remain thus crossed, the plane of vibration of the polarized beam between them could be changed, then the light would be, in part at least, transmitted. in faraday's experiment this was accomplished. his magnet turned the plane of polarization of the beam through a certain angle, and thus enabled it to get through the analyzer; so that 'the magnetization of light and the illumination of the magnetic lines of force' becomes, when expressed in the language of modern theory, the rotation of the plane of polarization. to him, as to all true philosophers, the main value of a fact was its position and suggestiveness in the general sequence of scientific truth. hence, having established the existence of a phenomenon, his habit was to look at it from all possible points of view, and to develop its relationship to other phenomena. he proved that the direction of the rotation depends upon the polarity of his magnet; being reversed when the magnetic poles are reversed. he showed that when a polarized ray passed through his heavy glass in a direction parallel to the magnetic lines of force, the rotation is a maximum, and that when the direction of the ray is at right angles to the lines of force, there is no rotation at all. he also proved that the amount of the rotation is proportional to the length of the diamagnetic through which the ray passes. he operated with liquids and solutions. of aqueous solutions he tried and more, and found the power in all of them. he then examined gases; but here all his efforts to produce any sensible action upon the polarized beam were ineffectual. he then passed from magnets to currents, enclosing bars of heavy glass, and tubes containing liquids and aqueous solutions within an electro-magnetic helix. a current sent through the helix caused the plane of polarization to rotate, and always in the direction of the current. the rotation was reversed when the current was reversed. in the case of magnets, he observed a gradual, though quick, ascent of the transmitted beam from a state of darkness to its maximum brilliancy, when the magnet was excited. in the case of currents, the beam attained at once its maximum. this he showed to be due to the time required by the iron of the electro-magnet to assume its full magnetic power, which time vanishes when a current, without iron, is employed. 'in this experiment,' he says, 'we may, i think, justly say that a ray of light is electrified, and the electric forces illuminated.' in the helix, as with the magnets, he submitted air to magnetic influence 'carefully and anxiously,' but could not discover any trace of action on the polarized ray. many substances possess the power of turning the plane of polarization without the intervention of magnetism. oil of turpentine and quartz are examples; but faraday showed that, while in one direction, that is, across the lines of magnetic force, his rotation is zero, augmenting gradually from this until it attains its maximum, when the direction of the ray is parallel to the lines of force; in the oil of turpentine the rotation is independent of the direction of the ray. but he showed that a still more profound distinction exists between the magnetic rotation and the natural one. i will try to explain how. suppose a tube with glass ends containing oil of turpentine to be placed north and south. fixing the eye at the south end of the tube, let a polarized beam be sent through it from the north. to the observer in this position the rotation of the plane of polarization, by the turpentine, is right-handed. let the eye be placed at the north end of the tube, and a beam be sent through it from the south; the rotation is still right-handed. not so, however, when a bar of heavy glass is subjected to the action of an electric current. in this case if, in the first position of the eye, the rotation be right-handed, in the second position it is left-handed. these considerations make it manifest that if a polarized beam, after having passed through the oil of turpentine in its natural state, could by any means be reflected back through the liquid, the rotation impressed upon the direct beam would be exactly neutralized by that impressed upon the reflected one. not so with the induced magnetic effect. here it is manifest that the rotation would be doubled by the act of reflection. hence faraday concludes that the particles of the oil of turpentine which rotate by virtue of their natural force, and those which rotate in virtue of the induced force, cannot be in the same condition. the same remark applies to all bodies which possess a natural power of rotating the plane of polarization. and then he proceeded with exquisite skill and insight to take advantage of this conclusion. he silvered the ends of his piece of heavy glass, leaving, however, a narrow portion parallel to two edges diagonally opposed to each other unsilvered. he then sent his beam through this uncovered portion, and by suitably inclining his glass caused the beam within it to reach his eye first direct, and then after two, four, and six reflections. these corresponded to the passage of the ray once, three times, five times, and seven times through the glass. he thus established with numerical accuracy the exact proportionality of the rotation to the distance traversed by the polarized beam. thus in one series of experiments where the rotation required by the direct beam was degrees, that acquired by three passages through the glass was degrees, while that acquired by five passages was degrees. but even when this method of magnifying was applied, he failed with various solid substances to obtain any effect; and in the case of air, though he employed to the utmost the power which these repeated reflections placed in his hands, he failed to produce the slightest sensible rotation. these failures of faraday to obtain the effect with gases seem to indicate the true seat of the phenomenon. the luminiferous ether surrounds and is influenced by the ultimate particles of matter. the symmetry of the one involves that of the other. thus, if the molecules of a crystal be perfectly symmetrical round any line through the crystal, we may safely conclude that a ray will pass along this line as through ordinary glass. it will not be doubly refracted. from the symmetry of the liquid figures, known to be produced in the planes of freezing, when radiant heat is sent through ice, we may safely infer symmetry of aggregation, and hence conclude that the line perpendicular to the planes of freezing is a line of no double refraction; that it is, in fact, the optic axis of the crystal. the same remark applies to the line joining the opposite blunt angles of a crystal of iceland spar. the arrangement of the molecules round this line being symmetrical, the condition of the ether depending upon these molecules shares their symmetry; and there is, therefore, no reason why the wavelength should alter with the alteration of the azimuth round this line. annealed glass has its molecules symmetrically arranged round every line that can be drawn through it; hence it is not doubly refractive. but let the substance be either squeezed or strained in one direction, the molecular symmetry, and with it the symmetry of the ether, is immediately destroyed and the glass becomes doubly refractive. unequal heating produces the same effect. thus mechanical strains reveal themselves by optical effects; and there is little doubt that in faraday's experiment it is the magnetic strain that produces the rotation of the plane of polarization.[ ] footnotes to chapter [ ] 'by a diamagnetic,' says faraday, 'i mean a body through which lines of magnetic force are passing, and which does not by their action assume the usual magnetic state of iron or loadstone.' faraday subsequently used this term in a different sense from that here given, as will immediately appear. [ ] the power of double refraction conferred on the centre of a glass rod, when it is caused to sound the fundamental note due to its longitudinal vibration, and the absence of the same power in the case of vibrating air (enclosed in a glass organ-pipe), seems to be analogous to the presence and absence of faraday's effect in the same two substances. faraday never, to my knowledge, attempted to give, even in conversation, a picture of the molecular condition of his heavy glass when subjected to magnetic influence. in a mathematical investigation of the subject, published in the proceedings of the royal society for , sir william thomson arrives at the conclusion that the 'diamagnetic' is in a state of molecular rotation. chapter . discovery of diamagnetism--researches on magne-crystallic action. faraday's next great step in discovery was announced in a memoir on the 'magnetic condition of all matter,' communicated to the royal society on december , . one great source of his success was the employment of extraordinary power. as already stated, he never accepted a negative answer to an experiment until he had brought to bear upon it all the force at his command. he had over and over again tried steel magnets and ordinary electro-magnets on various substances, but without detecting anything different from the ordinary attraction exhibited by a few of them. stronger coercion, however, developed a new action. before the pole of an electro-magnet, he suspended a fragment of his famous heavy glass; and observed that when the magnet was powerfully excited the glass fairly retreated from the pole. it was a clear case of magnetic repulsion. he then suspended a bar of the glass between two poles; the bar retreated when the poles were excited, and set its length equatorially or at right angles to the line joining them. when an ordinary magnetic body was similarly suspended, it always set axially, that is, from pole to pole. faraday called those bodies which were repelled by the poles of a magnet, diamagnetic bodies; using this term in a sense different from that in which he employed it in his memoir on the magnetization of light. the term magnetic he reserved for bodies which exhibited the ordinary attraction. he afterwards employed the term magnetic to cover the whole phenomena of attraction and repulsion, and used the word paramagnetic to designate such magnetic action as is exhibited by iron. isolated observations by brugmanns, becquerel, le baillif, saigy, and seebeck had indicated the existence of a repulsive force exercised by the magnet on two or three substances; but these observations, which were unknown to faraday, had been permitted to remain without extension or examination. having laid hold of the fact of repulsion, faraday immediately expanded and multiplied it. he subjected bodies of the most varied qualities to the action of his magnet:--mineral salts, acids, alkalis, ethers, alcohols, aqueous solutions, glass, phosphorus, resins, oils, essences, vegetable and animal tissues, and found them all amenable to magnetic influence. no known solid or liquid proved insensible to the magnetic power when developed in sufficient strength. all the tissues of the human body, the blood--though it contains iron--included, were proved to be diamagnetic. so that if you could suspend a man between the poles of a magnet, his extremities would retreat from the poles until his length became equatorial. soon after he had commenced his researches on diamagnetism, faraday noticed a remarkable phenomenon which first crossed my own path in the following way: in the year , while working in the cabinet of my friend, professor knoblauch, of marburg, i suspended a small copper coin between the poles of an electro-magnet. on exciting the magnet, the coin moved towards the poles and then suddenly stopped, as if it had struck against a cushion. on breaking the circuit, the coin was repelled, the revulsion being so violent as to cause it to spin several times round its axis of suspension. a silber-groschen similarly suspended exhibited the same deportment. for a moment i thought this a new discovery; but on looking over the literature of the subject, it appeared that faraday had observed, multiplied, and explained the same effect during his researches on diamagnetism. his explanation was based upon his own great discovery of magneto-electric currents. the effect is a most singular one. a weight of several pounds of copper may be set spinning between the electro-magnetic poles; the excitement of the magnet instantly stops the rotation. though nothing is apparent to the eye, the copper, if moved in the excited magnetic field, appears to move through a viscous fluid; while, when a flat piece of the metal is caused to pass to and fro like a saw between the poles, the sawing of the magnetic field resembles the cutting through of cheese or butter.[ ] this virtual friction of the magnetic field is so strong, that copper, by its rapid rotation between the poles, might probably be fused. we may easily dismiss this experiment by saying that the heat is due to the electric currents excited in the copper. but so long as we are unable to reply to the question, 'what is an electric current?' the explanation is only provisional. for my own part, i look with profound interest and hope on the strange action here referred to. faraday's thoughts ran intuitively into experimental combinations, so that subjects whose capacity for experimental treatment would, to ordinary minds, seem to be exhausted in a moment, were shown by him to be all but inexhaustible. he has now an object in view, the first step towards which is the proof that the principle of archimedes is true of magnetism. he forms magnetic solutions of various degrees of strength, places them between the poles of his magnet, and suspends in the solutions various magnetic bodies. he proves that when the solution is stronger than the body plunged in it, the body, though magnetic, is repelled; and when an elongated piece of it is surrounded by the solution, it sets, like a diamagnetic body, equatorially between the excited poles. the same body when suspended in a solution of weaker magnetic power than itself, is attracted as a whole, while an elongated portion of it sets axially. and now theoretic questions rush in upon him. is this new force a true repulsion, or is it merely a differential attraction? might not the apparent repulsion of diamagnetic bodies be really due to the greater attraction of the medium by which they are surrounded? he tries the rarefaction of air, but finds the effect insensible. he is averse to ascribing a capacity of attraction to space, or to any hypothetical medium supposed to fill space. he therefore inclines, but still with caution, to the opinion that the action of a magnet upon bismuth is a true and absolute repulsion, and not merely the result of differential attraction. and then he clearly states a theoretic view sufficient to account for the phenomena. 'theoretically,' he says, 'an explanation of the movements of the diamagnetic bodies, and all the dynamic phenomena consequent upon the action of magnets upon them, might be offered in the supposition that magnetic induction caused in them a contrary state to that which it produced in ordinary matter.' that is to say, while in ordinary magnetic influence the exciting pole excites adjacent to itself the contrary magnetism, in diamagnetic bodies the adjacent magnetism is the same as that of the exciting pole. this theory of reversed polarity, however, does not appear to have ever laid deep hold of faraday's mind; and his own experiments failed to give any evidence of its truth. he therefore subsequently abandoned it, and maintained the non-polarity of the diamagnetic force. he then entered a new, though related field of inquiry. having dealt with the metals and their compounds, and having classified all of them that came within the range of his observation under the two heads magnetic and diamagnetic, he began the investigation of the phenomena presented by crystals when subjected to magnetic power. this action of crystals had been in part theoretically predicted by poisson,[ ] and actually discovered by plucker, whose beautiful results, at the period which we have now reached, profoundly interested all scientific men. faraday had been frequently puzzled by the deportment of bismuth, a highly crystalline metal. sometimes elongated masses of the substance refused to set equatorially, sometimes they set persistently oblique, and sometimes even, like a magnetic body, from pole to pole. 'the effect,' he says, 'occurs at a single pole; and it is then striking to observe a long piece of a substance so diamagnetic as bismuth repelled, and yet at the same moment set round with force, axially, or end on, as a piece of magnetic substance would do.' the effect perplexed him; and in his efforts to release himself from this perplexity, no feature of this new manifestation of force escaped his attention. his experiments are described in a memoir communicated to the royal society on december , . i have worked long myself at magne-crystallic action, amid all the light of faraday's and plucker's researches. the papers now before me were objects of daily and nightly study with me eighteen or nineteen years ago; but even now, though their perusal is but the last of a series of repetitions, they astonish me. every circumstance connected with the subject; every shade of deportment; every variation in the energy of the action; almost every application which could possibly be made of magnetism to bring out in detail the character of this new force, is minutely described. the field is swept clean, and hardly anything experimental is left for the gleaner. the phenomena, he concludes, are altogether different from those of magnetism or diamagnetism: they would appear, in fact, to present to us 'a new force, or a new form of force, in the molecules of matter,' which, for convenience sake, he designates by a new word, as 'the magne-crystallic force.' he looks at the crystal acted upon by the magnet. from its mass he passes, in idea, to its atoms, and he asks himself whether the power which can thus seize upon the crystalline molecules, after they have been fixed in their proper positions by crystallizing force, may not, when they are free, be able to determine their arrangement? he, therefore, liberates the atoms by fusing the bismuth. he places the fused substance between the poles of an electro-magnet, powerfully excited; but he fails to detect any action. i think it cannot be doubted that an action is exerted here, that a true cause comes into play; but its magnitude is not such as sensibly to interfere with the force of crystallization, which, in comparison with the diamagnetic force, is enormous. 'perhaps,' adds faraday, 'if a longer time were allowed, and a permanent magnet used, a better result might be obtained. i had built many hopes upon the process.' this expression, and his writings abound in such, illustrates what has been already said regarding his experiments being suggested and guided by his theoretic conceptions. his mind was full of hopes and hypotheses, but he always brought them to an experimental test. the record of his planned and executed experiments would, i doubt not, show a high ratio of hopes disappointed to hopes fulfilled; but every case of fulfilment abolished all memory of defeat; disappointment was swallowed up in victory. after the description of the general character of this new force, faraday states with the emphasis here reproduced its mode of action: 'the law of action appears to be that the line or axis of magne-crystallic force (being the resultant of the action of all the molecules) tends to place itself parallel, or as a tangent, to the magnetic curve, or line of magnetic force, passing through the place where the crystal is situated.' the magne-crystallic force, moreover, appears to him 'to be clearly distinguished from the magnetic or diamagnetic forces, in that it causes neither approach nor recession, consisting not in attraction or repulsion, but in giving a certain determinate position to the mass under its influence.' and then he goes on 'very carefully to examine and prove the conclusion that there was no connection of the force with attractive or repulsive influences.' with the most refined ingenuity he shows that, under certain circumstances, the magne-crystallic force can cause the centre of gravity of a highly magnetic body to retreat from the poles, and the centre of gravity of a highly diamagnetic body to approach them. his experiments root his mind more and more firmly in the conclusion that 'neither attraction nor repulsion causes the set, or governs the final position' of the crystal in the magnetic field. that the force which does so is therefore 'distinct in its character and effects from the magnetic and diamagnetic forms of force. on the other hand,' he continues, 'it has a most manifest relation to the crystalline structure of bismuth and other bodies, and therefore to the power by which their molecules are able to build up the crystalline masses.' and here follows one of those expressions which characterize the conceptions of faraday in regard to force generally:--'it appears to me impossible to conceive of the results in any other way than by a mutual reaction of the magnetic force, and the force of the particles of the crystals upon each other.' he proves that the action of the force, though thus molecular, is an action at a distance; he shows that a bismuth crystal can cause a freely suspended magnetic needle to set parallel to its magne-crystallic axis. few living men are aware of the difficulty of obtaining results like this, or of the delicacy necessary to their attainment. 'but though it thus takes up the character of a force acting at a distance, still it is due to that power of the particles which makes them cohere in regular order and gives the mass its crystalline aggregation, which we call at other times the attraction of aggregation, and so often speak of as acting at insensible distances.' thus he broods over this new force, and looks at it from all possible points of inspection. experiment follows experiment, as thought follows thought. he will not relinquish the subject as long as a hope exists of throwing more light upon it. he knows full well the anomalous nature of the conclusion to which his experiments lead him. but experiment to him is final, and he will not shrink from the conclusion. 'this force,' he says, 'appears to me to be very strange and striking in its character. it is not polar, for there is no attraction or repulsion.' and then, as if startled by his own utterance, he asks--'what is the nature of the mechanical force which turns the crystal round, and makes it affect a magnet?'... 'i do not remember,' he continues 'heretofore such a case of force as the present one, where a body is brought into position only, without attraction or repulsion.' plucker, the celebrated geometer already mentioned, who pursued experimental physics for many years of his life with singular devotion and success, visited faraday in those days, and repeated before him his beautiful experiments on magneto-optic action. faraday repeated and verified plucker's observations, and concluded, what he at first seemed to doubt, that plucker's results and magne-crystallic action had the same origin. at the end of his papers, when he takes a last look along the line of research, and then turns his eyes to the future, utterances quite as much emotional as scientific escape from faraday. 'i cannot,' he says, at the end of his first paper on magne-crystallic action, 'conclude this series of researches without remarking how rapidly the knowledge of molecular forces grows upon us, and how strikingly every investigation tends to develop more and more their importance, and their extreme attraction as an object of study. a few years ago magnetism was to us an occult power, affecting only a few bodies, now it is found to influence all bodies, and to possess the most intimate relations with electricity, heat, chemical action, light, crystallization, and through it, with the forces concerned in cohesion; and we may, in the present state of things, well feel urged to continue in our labours, encouraged by the hope of bringing it into a bond of union with gravity itself.' supplementary remarks a brief space will, perhaps, be granted me here to state the further progress of an investigation which interested faraday so much. drawn by the fame of bunsen as a teacher, in the year i became a student in the university of marburg, in hesse cassel. bunsen's behaviour to me was that of a brother as well as that of a teacher, and it was also my happiness to make the acquaintance and gain the friendship of professor knoblauch, so highly distinguished by his researches on radiant heat. plucker's and faraday's investigations filled all minds at the time, and towards the end of , professor knoblauch and myself commenced a joint investigation of the entire question. long discipline was necessary to give us due mastery over it. employing a method proposed by dove, we examined the optical properties of our crystals ourselves; and these optical observations went hand in hand with our magnetic experiments. the number of these experiments was very great, but for a considerable time no fact of importance was added to those already published. at length, however, it was our fortune to meet with various crystals whose deportment could not be brought under the laws of magne-crystallic action enunciated by plucker. we also discovered instances which led us to suppose that the magne-crystallic force was by no means independent, as alleged, of the magnetism or diamagnetism of the mass of the crystal. indeed, the more we worked at the subject, the more clearly did it appear to us that the deportment of crystals in the magnetic field was due, not to a force previously unknown, but to the modification of the known forces of magnetism and diamagnetism by crystalline aggregation. an eminent example of magne-crystallic action adduced by plucker, and experimented on by faraday, was iceland spar. it is what in optics is called a negative crystal, and according to the law of plucker, the axis of such a crystal was always repelled by a magnet. but we showed that it was only necessary to substitute, in whole or in part, carbonate of iron for carbonate of lime, thus changing the magnetic but not the optical character of the crystal, to cause the axis to be attracted. that the deportment of magnetic crystals is exactly antithetical to that of diamagnetic crystals isomorphous with the magnetic ones, was proved to be a general law of action. in all cases, the line which in a diamagnetic crystal set equatorially, always set itself in an isomorphous magnetic crystal axially. by mechanical compression other bodies were also made to imitate the iceland spar. these and numerous other results bearing upon the question were published at the time in the 'philosophical magazine' and in 'poggendorff's annalen'; and the investigation of diamagnetism and magne-crystallic action was subsequently continued by me in the laboratory of professor magnus of berlin. in december, , after i had quitted germany, dr. bence jones went to the prussian capital to see the celebrated experiments of du bois reymond. influenced, i suppose, by what he there heard, he afterwards invited me to give a friday evening discourse at the royal institution. i consented, not without fear and trembling. for the royal institution was to me a kind of dragon's den, where tact and strength would be necessary to save me from destruction. on february , , the discourse was given, and it ended happily. i allude to these things, that i may mention that, though my aim and object in that lecture was to subvert the notions both of faraday and plucker, and to establish in opposition to their views what i regarded as the truth, it was very far from producing in faraday either enmity or anger. at the conclusion of the lecture, he quitted his accustomed seat, crossed the theatre to the corner into which i had shrunk, shook me by the hand, and brought me back to the table. once more, subsequently, and in connection with a related question, i ventured to differ from him still more emphatically. it was done out of trust in the greatness of his character; nor was the trust misplaced. he felt my public dissent from him; and it pained me afterwards to the quick to think that i had given him even momentary annoyance. it was, however, only momentary. his soul was above all littleness and proof to all egotism. he was the same to me afterwards that he had been before; the very chance expression which led me to conclude that he felt my dissent being one of kindness and affection. it required long subsequent effort to subdue the complications of magne-crystallic action, and to bring under the dominion of elementary principles the vast mass of facts which the experiments of faraday and plucker had brought to light. it was proved by reich, edmond becquerel, and myself, that the condition of diamagnetic bodies, in virtue of which they were repelled by the poles of a magnet, was excited in them by those poles; that the strength of this condition rose and fell with, and was proportional to, the strength of the acting magnet. it was not then any property possessed permanently by the bismuth, and which merely required the development of magnetism to act upon it, that caused the repulsion; for then the repulsion would have been simply proportional to the strength of the influencing magnet, whereas experiment proved it to augment as the square of the strength. the capacity to be repelled was therefore not inherent in the bismuth, but induced. so far an identity of action was established between magnetic and diamagnetic bodies. after this the deportment of magnetic bodies, 'normal' and 'abnormal'; crystalline, amorphous, and compressed, was compared with that of crystalline, amorphous, and compressed diamagnetic bodies; and by a series of experiments, executed in the laboratory of this institution, the most complete antithesis was established between magnetism and diamagnetism. this antithesis embraced the quality of polarity,--the theory of reversed polarity, first propounded by faraday, being proved to be true. the discussion of the question was very brisk. on the continent professor wilhelm weber was the ablest and most successful supporter of the doctrine of diamagnetic polarity; and it was with an apparatus, devised by him and constructed under his own superintendence, by leyser of leipzig, that the last demands of the opponents of diamagnetic polarity were satisfied. the establishment of this point was absolutely necessary to the explanation of magne-crystallic action. with that admirable instinct which always guided him, faraday had seen that it was possible, if not probable, that the diamagnetic force acts with different degrees of intensity in different directions, through the mass of a crystal. in his studies on electricity, he had sought an experimental reply to the question whether crystalline bodies had not different specific inductive capacities in different directions, but he failed to establish any difference of the kind. his first attempt to establish differences of diamagnetic action in different directions through bismuth, was also a failure; but he must have felt this to be a point of cardinal importance, for he returned to the subject in , and proved that bismuth was repelled with different degrees of force in different directions. it seemed as if the crystal were compounded of two diamagnetic bodies of different strengths, the substance being more strongly repelled across the magne-crystallic axis than along it. the same result was obtained independently, and extended to various other bodies, magnetic as well as diamagnetic, and also to compressed substances, a little subsequently by myself. the law of action in relation to this point is, that in diamagnetic crystals, the line along which the repulsion is a maximum, sets equatorially in the magnetic field; while in magnetic crystals the line along which the attraction is a maximum sets from pole to pole. faraday had said that the magne-crystallic force was neither attraction nor repulsion. thus far he was right. it was neither taken singly, but it was both. by the combination of the doctrine of diamagnetic polarity with these differential attractions and repulsions, and by paying due regard to the character of the magnetic field, every fact brought to light in the domain of magne-crystallic action received complete explanation. the most perplexing of those facts were shown to result from the action of mechanical couples, which the proved polarity both of magnetism and diamagnetism brought into play. indeed the thoroughness with which the experiments of faraday were thus explained, is the most striking possible demonstration of the marvellous precision with which they were executed. footnotes to chapter [ ] see heat as a mode of motion, ninth edition, p. . [ ] see sir wm. thomson on magne-crystallic action. phil. mag., . chapter . magnetism of flame and gases--atmospheric magnetism when an experimental result was obtained by faraday it was instantly enlarged by his imagination. i am acquainted with no mind whose power and suddenness of expansion at the touch of new physical truth could be ranked with his. sometimes i have compared the action of his experiments on his mind to that of highly combustible matter thrown into a furnace; every fresh entry of fact was accompanied by the immediate development of light and heat. the light, which was intellectual, enabled him to see far beyond the boundaries of the fact itself, and the heat, which was emotional, urged him to the conquest of this newly-revealed domain. but though the force of his imagination was enormous, he bridled it like a mighty rider, and never permitted his intellect to be overthrown. in virtue of the expansive power which his vivid imagination conferred upon him, he rose from the smallest beginnings to the grandest ends. having heard from zantedeschi that bancalari had established the magnetism of flame, he repeated the experiments and augmented the results. he passed from flames to gases, examining and revealing their magnetic and diamagnetic powers; and then he suddenly rose from his bubbles of oxygen and nitrogen to the atmospheric envelope of the earth itself, and its relations to the great question of terrestrial magnetism. the rapidity with which these ever-augmenting thoughts assumed the form of experiments is unparalleled. his power in this respect is often best illustrated by his minor investigations, and, perhaps, by none more strikingly than by his paper 'on the diamagnetic condition of flame and gases,' published as a letter to mr. richard taylor, in the 'philosophical magazine' for december, . after verifying, varying, and expanding the results of bancalari, he submitted to examination heated air-currents, produced by platinum spirals placed in the magnetic field, and raised to incandescence by electricity. he then examined the magnetic deportment of gases generally. almost all of these gases are invisible; but he must, nevertheless, track them in their unseen courses. he could not effect this by mingling smoke with his gases, for the action of his magnet upon the smoke would have troubled his conclusions. he, therefore, 'caught' his gases in tubes, carried them out of the magnetic field, and made them reveal themselves at a distance from the magnet. immersing one gas in another, he determined their differential action; results of the utmost beauty being thus arrived at. perhaps the most important are those obtained with atmospheric air and its two constituents. oxygen, in various media, was strongly attracted by the magnet; in coal-gas, for example, it was powerfully magnetic, whereas nitrogen was diamagnetic. some of the effects obtained with oxygen in coal-gas were strikingly beautiful. when the fumes of chloride of ammonium (a diamagnetic substance) were mingled with the oxygen, the cloud of chloride behaved in a most singular manner,--'the attraction of iron filings,' says faraday, 'to a magnetic pole is not more striking than the appearance presented by the oxygen under these circumstances.' on observing this deportment the question immediately occurs to him,--can we not separate the oxygen of the atmosphere from its nitrogen by magnetic analysis? it is the perpetual occurrence of such questions that marks the great experimenter. the attempt to analyze atmospheric air by magnetic force proved a failure, like the previous attempt to influence crystallization by the magnet. the enormous comparative power of the force of crystallization i have already assigned as a reason for the incompetence of the magnet to determine molecular arrangement; in the present instance the magnetic analysis is opposed by the force of diffusion, which is also very strong comparatively. the same remark applies to, and is illustrated by, another experiment subsequently executed by faraday. water is diamagnetic, sulphate of iron is strongly magnetic. he enclosed 'a dilute solution of sulphate of iron in a tube, and placed the lower end of the tube between the poles of a powerful horseshoe magnet for days together,' but he could produce 'no concentration of the solution in the part near the magnet.' here also the diffusibility of the salt was too powerful for the force brought against it. the experiment last referred to is recorded in a paper presented to the royal society on the nd august, , in which he pursues the investigation of the magnetism of gases. newton's observations on soap-bubbles were often referred to by faraday. his delight in a soap-bubble was like that of a boy, and he often introduced them into his lectures, causing them, when filled with air, to float on invisible seas of carbonic acid, and otherwise employing them as a means of illustration. he now finds them exceedingly useful in his experiments on the magnetic condition of gases. a bubble of air in a magnetic field occupied by air was unaffected, save through the feeble repulsion of its envelope. a bubble of nitrogen, on the contrary, was repelled from the magnetic axis with a force far surpassing that of a bubble of air. the deportment of oxygen in air 'was very impressive, the bubble being pulled inward or towards the axial line, sharply and suddenly, as if the oxygen were highly magnetic.' he next labours to establish the true magnetic zero, a problem not so easy as might at first sight be imagined. for the action of the magnet upon any gas, while surrounded by air or any other gas, can only be differential; and if the experiment were made in vacuo, the action of the envelope, in this case necessarily of a certain thickness, would trouble the result. while dealing with this subject, faraday makes some noteworthy observations regarding space. in reference to the torricellian vacuum, he says, 'perhaps it is hardly necessary for me to state that i find both iron and bismuth in such vacua perfectly obedient to the magnet. from such experiments, and also from general observations and knowledge, it seems manifest that the lines of magnetic force can traverse pure space, just as gravitating force does, and as statical electrical forces do, and therefore space has a magnetic relation of its own, and one that we shall probably find hereafter to be of the utmost importance in natural phenomena. but this character of space is not of the same kind as that which, in relation to matter, we endeavour to express by the terms magnetic and diamagnetic. to confuse these together would be to confound space with matter, and to trouble all the conceptions by which we endeavour to understand and work out a progressively clearer view of the mode of action, and the laws of natural forces. it would be as if in gravitation or electric forces, one were to confound the particles acting on each other with the space across which they are acting, and would, i think, shut the door to advancement. mere space cannot act as matter acts, even though the utmost latitude be allowed to the hypothesis of an ether; and admitting that hypothesis, it would be a large additional assumption to suppose that the lines of magnetic force are vibrations carried on by it, whilst as yet we have no proof that time is required for their propagation, or in what respect they may, in general character, assimilate to or differ from their respective lines of gravitating, luminiferous, or electric forces.' pure space he assumes to be the true magnetic zero, but he pushes his inquiries to ascertain whether among material substances there may not be some which resemble space. if you follow his experiments, you will soon emerge into the light of his results. a torsion-beam was suspended by a skein of cocoon silk; at one end of the beam was fixed a cross-piece / inch long. tubes of exceedingly thin glass, filled with various gases, and hermetically sealed, were suspended in pairs from the two ends of the cross-piece. the position of the rotating torsion-head was such that the two tubes were at opposite sides of, and equidistant from, the magnetic axis, that is to say from the line joining the two closely approximated polar points of an electro-magnet. his object was to compare the magnetic action of the gases in the two tubes. when one tube was filled with oxygen, and the other with nitrogen, on the supervention of the magnetic force, the oxygen was pulled towards the axis, the nitrogen being pushed out. by turning the torsion-head they could be restored to their primitive position of equidistance, where it is evident the action of the glass envelopes was annulled. the amount of torsion necessary to re-establish equidistance expressed the magnetic difference of the substances compared. and then he compared oxygen with oxygen at different pressures. one of his tubes contained the gas at the pressure of inches of mercury, another at a pressure of inches of mercury, a third at a pressure of inches, while a fourth was exhausted as far as a good air-pump renders exhaustion possible. 'when the first of these was compared with the other three, the effect was most striking.' it was drawn towards the axis when the magnet was excited, the tube containing the rarer gas being apparently driven away, and the greater the difference between the densities of the two gases, the greater was the energy of this action. and now observe his mode of reaching a material magnetic zero. when a bubble of nitrogen was exposed in air in the magnetic field, on the supervention of the power, the bubble retreated from the magnet. a less acute observer would have set nitrogen down as diamagnetic; but faraday knew that retreat, in a medium composed in part of oxygen, might be due to the attraction of the latter gas, instead of to the repulsion of the gas immersed in it. but if nitrogen be really diamagnetic, then a bubble or bulb filled with the dense gas will overcome one filled with the rarer gas. from the cross-piece of his torsion-balance he suspended his bulbs of nitrogen, at equal distances from the magnetic axis, and found that the rarefaction, or the condensation of the gas in either of the bulbs had not the slightest influence. when the magnetic force was developed, the bulbs remained in their first position, even when one was filled with nitrogen, and the other as far as possible exhausted. nitrogen, in fact, acted 'like space itself'; it was neither magnetic nor diamagnetic. he cannot conveniently compare the paramagnetic force of oxygen with iron, in consequence of the exceeding magnetic intensity of the latter substance; but he does compare it with the sulphate of iron, and finds that, bulk for bulk, oxygen is equally magnetic with a solution of this substance in water 'containing seventeen times the weight of the oxygen in crystallized proto-sulphate of iron, or . times its weight of metallic iron in that state of combination.' by its capability to deflect a fine glass fibre, he finds that the attraction of this bulb of oxygen, containing only . of a grain of the gas, at an average distance of more than an inch from the magnetic axis, is about equal to the gravitating force of the same amount of oxygen as expressed by its weight. these facts could not rest for an instant in the mind of faraday without receiving that expansion to which i have already referred. 'it is hardly necessary,' he writes, 'for me to say here that this oxygen cannot exist in the atmosphere exerting such a remarkable and high amount of magnetic force, without having a most important influence on the disposition of the magnetism of the earth, as a planet; especially if it be remembered that its magnetic condition is greatly altered by variations of its density and by variations of its temperature. i think i see here the real cause of many of the variations of that force, which have been, and are now so carefully watched on different parts of the surface of the globe. the daily variation, and the annual variation, both seem likely to come under it; also very many of the irregular continual variations, which the photographic process of record renders so beautifully manifest. if such expectations be confirmed, and the influence of the atmosphere be found able to produce results like these, then we shall probably find a new relation between the aurora borealis and the magnetism of the earth, namely, a relation established, more or less, through the air itself in connection with the space above it; and even magnetic relations and variations, which are not as yet suspected, may be suggested and rendered manifest and measurable, in the further development of what i will venture to call atmospheric magnetism. i may be over-sanguine in these expectations, but as yet i am sustained in them by the apparent reality, simplicity, and sufficiency of the cause assumed, as it at present appears to my mind. as soon as i have submitted these views to a close consideration, and the test of accordance with observation, and, where applicable, with experiments also, i will do myself the honour to bring them before the royal society.' two elaborate memoirs are then devoted to the subject of atmospheric magnetism; the first sent to the royal society on the th of october, and the second on the th of november, . in these memoirs he discusses the effects of heat and cold upon the magnetism of the air, and the action on the magnetic needle, which must result from thermal changes. by the convergence and divergence of the lines of terrestrial magnetic force, he shows how the distribution of magnetism, in the earth's atmosphere, is effected. he applies his results to the explanation of the annual and of the diurnal variation: he also considers irregular variations, including the action of magnetic storms. he discusses, at length, the observations at st. petersburg, greenwich, hobarton, st. helena, toronto, and the cape of good hope; believing that the facts, revealed by his experiments, furnish the key to the variations observed at all these places. in the year , i had the honour of an interview with humboldt, in berlin, and his parting words to me then were, 'tell faraday that i entirely agree with him, and that he has, in my opinion, completely explained the variation of the declination.' eminent men have since informed me that humboldt was hasty in expressing this opinion. in fact, faraday's memoirs on atmospheric magnetism lost much of their force--perhaps too much--through the important discovery of the relation of the variation of the declination to the number of the solar spots. but i agree with him and m. edmond becquerel, who worked independently at this subject, in thinking, that a body so magnetic as oxygen, swathing the earth, and subject to variations of temperature, diurnal and annual, must affect the manifestations of terrestrial magnetism.[ ] the air that stands upon a single square foot of the earth's surface is, according to faraday, equivalent in magnetic force to lbs. of crystallized protosulphate of iron. such a substance cannot be absolutely neutral as regards the deportment of the magnetic needle. but faraday's writings on this subject are so voluminous, and the theoretic points are so novel and intricate, that i shall postpone the complete analysis of these researches to a time when i can lay hold of them more completely than my other duties allow me to do now. footnote to chapter [ ] this persuasion has been greatly strengthened by the recent perusal of a paper by mr. baxendell. chapter . speculations: nature of matter: lines of force the scientific picture of faraday would not be complete without a reference to his speculative writings. on friday, january , , he opened the weekly evening-meetings of the royal institution by a discourse entitled 'a speculation touching electric conduction and the nature of matter.' in this discourse he not only attempts the overthrow of dalton's theory of atoms, but also the subversion of all ordinary scientific ideas regarding the nature and relations of matter and force. he objected to the use of the term atom:--'i have not yet found a mind,' he says, 'that did habitually separate it from its accompanying temptations; and there can be no doubt that the words definite proportions, equivalent, primes, &c., which did and do fully express all the facts of what is usually called the atomic theory in chemistry, were dismissed because they were not expressive enough, and did not say all that was in the mind of him who used the word atom in their stead.' a moment will be granted me to indicate my own view of faraday's position here. the word 'atom' was not used in the stead of definite proportions, equivalents, or primes. these terms represented facts that followed from, but were not equivalent to, the atomic theory. facts cannot satisfy the mind: and the law of definite combining proportions being once established, the question 'why should combination take place according to that law?' is inevitable. dalton answered this question by the enunciation of the atomic theory, the fundamental idea of which is, in my opinion, perfectly secure. the objection of faraday to dalton might be urged with the same substantial force against newton: it might be stated with regard to the planetary motions that the laws of kepler revealed the facts; that the introduction of the principle of gravitation was an addition to the facts. but this is the essence of all theory. the theory is the backward guess from fact to principle; the conjecture, or divination regarding something, which lies behind the facts, and from which they flow in necessary sequence. if dalton's theory, then, account for the definite proportions observed in the combinations of chemistry, its justification rests upon the same basis as that of the principle of gravitation. all that can in strictness be said in either case is that the facts occur as if the principle existed. the manner in which faraday himself habitually deals with his hypotheses is revealed in this lecture. he incessantly employed them to gain experimental ends, but he incessantly took them down, as an architect removes the scaffolding when the edifice is complete. 'i cannot but doubt,' he says, 'that he who as a mere philosopher has most power of penetrating the secrets of nature, and guessing by hypothesis at her mode of working, will also be most careful for his own safe progress and that of others, to distinguish the knowledge which consists of assumption, by which i mean theory and hypothesis, from that which is the knowledge of facts and laws.' faraday himself, in fact, was always 'guessing by hypothesis,' and making theoretic divination the stepping-stone to his experimental results. i have already more than once dwelt on the vividness with which he realised molecular conditions; we have a fine example of this strength and brightness of imagination in the present 'speculation.' he grapples with the notion that matter is made up of particles, not in absolute contact, but surrounded by interatomic space. 'space,' he observes, 'must be taken as the only continuous part of a body so constituted. space will permeate all masses of matter in every direction like a net, except that in place of meshes it will form cells, isolating each atom from its neighbours, itself only being continuous.' let us follow out this notion; consider, he argues, the case of a non-conductor of electricity, such for example as shell-lac, with its molecules, and intermolecular spaces running through the mass. in its case space must be an insulator; for if it were a conductor it would resemble 'a fine metallic web,' penetrating the lac in every direction. but the fact is that it resembles the wax of black sealing-wax, which surrounds and insulates the particles of conducting carbon, interspersed throughout its mass. in the case of shell-lac, therefore, space is an insulator. but now, take the case of a conducting metal. here we have, as before, the swathing of space round every atom. if space be an insulator there can be no transmission of electricity from atom to atom. but there is transmission; hence space is a conductor. thus he endeavours to hamper the atomic theory. 'the reasoning,' he says, 'ends in a subversion of that theory altogether; for if space be an insulator it cannot exist in conducting bodies, and if it be a conductor it cannot exist in insulating bodies. any ground of reasoning,' he adds, as if carried away by the ardour of argument, 'which tends to such conclusions as these must in itself be false.' he then tosses the atomic theory from horn to horn of his dilemmas. what do we know, he asks, of the atom apart from its force? you imagine a nucleus which may be called a, and surround it by forces which may be called m; 'to my mind the a or nucleus vanishes, and the substance consists in the powers of m. and indeed what notion can we form of the nucleus independent of its powers? what thought remains on which to hang the imagination of an a independent of the acknowledged forces?' like boscovich, he abolishes the atom, and puts a 'centre of force' in its place. with his usual courage and sincerity he pushes his view to its utmost consequences. 'this view of the constitution of matter,' he continues, 'would seem to involve necessarily the conclusion that matter fills all space, or at least all space to which gravitation extends; for gravitation is a property of matter dependent on a certain force, and it is this force which constitutes the matter. in that view matter is not merely mutually penetrable;[ ] but each atom extends, so to say, throughout the whole of the solar system, yet always retaining its own centre of force.' it is the operation of a mind filled with thoughts of this profound, strange, and subtle character that we have to take into account in dealing with faraday's later researches. a similar cast of thought pervades a letter addressed by faraday to mr. richard phillips, and published in the 'philosophical magazine' for may, . it is entitled 'thoughts on ray-vibrations,' and it contains one of the most singular speculations that ever emanated from a scientific mind. it must be remembered here, that though faraday lived amid such speculations he did not rate them highly, and that he was prepared at any moment to change them or let them go. they spurred him on, but they did not hamper him. his theoretic notions were fluent; and when minds less plastic than his own attempted to render those fluxional images rigid, he rebelled. he warns phillips moreover, that from first to last, 'he merely threw out as matter for speculation the vague impressions of his mind; for he gave nothing as the result of sufficient consideration, or as the settled conviction, or even probable conclusion at which he had arrived.' the gist of this communication is that gravitating force acts in lines across space, and that the vibrations of light and radiant heat consist in the tremors of these lines of force. 'this notion,' he says, 'as far as it is admitted, will dispense with the ether, which, in another view is supposed to be the medium in which these vibrations take place.' and he adds further on, that his view 'endeavours to dismiss the ether but not the vibrations.' the idea here set forth is the natural supplement of his previous notion, that it is gravitating force which constitutes matter, each atom extending, so to say, throughout the whole of the solar system. the letter to mr. phillips winds up with this beautiful conclusion:-- 'i think it likely that i have made many mistakes in the preceding pages, for even to myself my ideas on this point appear only as the shadow of a speculation, or as one of those impressions upon the mind which are allowable for a time as guides to thought and research. he who labours in experimental inquiries, knows how numerous these are, and how often their apparent fitness and beauty vanish before the progress and development of real natural truth.' let it then be remembered that faraday entertained notions regarding matter and force altogether distinct from the views generally held by scientific men. force seemed to him an entity dwelling along the line in which it is exerted. the lines along which gravity acts between the sun and earth seem figured in his mind as so many elastic strings; indeed he accepts the assumed instantaneity of gravity as the expression of the enormous elasticity of the 'lines of weight.' such views, fruitful in the case of magnetism, barren, as yet, in the case of gravity, explain his efforts to transform this latter force. when he goes into the open air and permits his helices to fall, to his mind's eye they are tearing through the lines of gravitating power, and hence his hope and conviction that an effect would and ought to be produced. it must ever be borne in mind that faraday's difficulty in dealing with these conceptions was at bottom the same as that of newton; that he is in fact trying to overleap this difficulty, and with it probably the limits prescribed to the intellect itself. the idea of lines of magnetic force was suggested to faraday by the linear arrangement of iron filings when scattered over a magnet. he speaks of and illustrates by sketches, the deflection, both convergent and divergent, of the lines of force, when they pass respectively through magnetic and diamagnetic bodies. these notions of concentration and divergence are also based on the direct observation of his filings. so long did he brood upon these lines; so habitually did he associate them with his experiments on induced currents, that the association became 'indissoluble,' and he could not think without them. 'i have been so accustomed,' he writes, 'to employ them, and especially in my last researches, that i may have unwittingly become prejudiced in their favour, and ceased to be a clear-sighted judge. still, i have always endeavoured to make experiment the test and controller of theory and opinion; but neither by that nor by close cross-examination in principle, have i been made aware of any error involved in their use.' in his later researches on magne-crystallic action, the idea of lines of force is extensively employed; it indeed led him to an experiment which lies at the root of the whole question. in his subsequent researches on atmospheric magnetism the idea receives still wider application, showing itself to be wonderfully flexible and convenient. indeed without this conception the attempt to seize upon the magnetic actions, possible or actual, of the atmosphere would be difficult in the extreme; but the notion of lines of force, and of their divergence and convergence, guides faraday without perplexity through all the intricacies of the question. after the completion of those researches, and in a paper forwarded to the royal society on october , , he devotes himself to the formal development and illustration of his favourite idea. the paper bears the title, 'on lines of magnetic force, their definite character, and their distribution within a magnet and through space.' a deep reflectiveness is the characteristic of this memoir. in his experiments, which are perfectly beautiful and profoundly suggestive, he takes but a secondary delight. his object is to illustrate the utility of his conception of lines of force. 'the study of these lines,' he says, 'has at different times been greatly influential in leading me to various results which i think prove their utility as well as fertility.' faraday for a long period used the lines of force merely as 'a representative idea.' he seemed for a time averse to going further in expression than the lines themselves, however much further he may have gone in idea. that he believed them to exist at all times round a magnet, and irrespective of the existence of magnetic matter, such as iron filings, external to the magnet, is certain. no doubt the space round every magnet presented itself to his imagination as traversed by loops of magnetic power; but he was chary in speaking of the physical substratum of those loops. indeed it may be doubted whether the physical theory of lines of force presented itself with any distinctness to his own mind. the possible complicity of the luminiferous ether in magnetic phenomena was certainly in his thoughts. 'how the magnetic force,' he writes, 'is transferred through bodies or through space we know not; whether the result is merely action at a distance, as in the case of gravity; or by some intermediate agency, as in the case of light, heat, the electric current, and (as i believe) static electric action. the idea of magnetic fluids, as applied by some, or of magnetic centres of action, does not include that of the latter kind of transmission, but the idea of lines of force does.' and he continues thus:--'i am more inclined to the notion that in the transmission of the [magnetic] force there is such an action [an intermediate agency] external to the magnet, than that the effects are merely attraction and repulsion at a distance. such an affection may be a function of the ether; for it is not at all unlikely that, if there be an ether, it should have other uses than simply the conveyance of radiations.' when he speaks of the magnet in certain cases, 'revolving amongst its own forces,' he appears to have some conception of this kind in view. a great part of the investigation completed in october, , was taken up with the motions of wires round the poles of a magnet and the converse. he carried an insulated wire along the axis of a bar magnet from its pole to its equator, where it issued from the magnet, and was bent up so as to connect its two ends. a complete circuit, no part of which was in contact with the magnet, was thus obtained. he found that when the magnet and the external wire were rotated together no current was produced; whereas, when either of them was rotated and the other left at rest currents were evolved. he then abandoned the axial wire, and allowed the magnet itself to take its place; the result was the same.[ ] it was the relative motion of the magnet and the loop that was effectual in producing a current. the lines of force have their roots in the magnet, and though they may expand into infinite space, they eventually return to the magnet. now these lines may be intersected close to the magnet or at a distance from it. faraday finds distance to be perfectly immaterial so long as the number of lines intersected is the same. for example, when the loop connecting the equator and the pole of his barmagnet performs one complete revolution round the magnet, it is manifest that all the lines of force issuing from the magnet are once intersected. now it matters not whether the loop be ten feet or ten inches in length, it matters not how it may be twisted and contorted, it matters not how near to the magnet or how distant from it the loop may be, one revolution always produces the same amount of current electricity, because in all these cases all the lines of force issuing from the magnet are once intersected and no more. from the external portion of the circuit he passes in idea to the internal, and follows the lines of force into the body of the magnet itself. his conclusion is that there exist lines of force within the magnet of the same nature as those without. what is more, they are exactly equal in amount to those without. they have a relation in direction to those without; and in fact are continuations of them.... 'every line of force, therefore, at whatever distance it may be taken from the magnet, must be considered as a closed circuit, passing in some part of its course through the magnet, and having an equal amount of force in every part of its course.' all the results here described were obtained with moving metals. 'but,' he continues with profound sagacity, 'mere motion would not generate a relation, which had not a foundation in the existence of some previous state; and therefore the quiescent metals must be in some relation to the active centre of force,' that is to the magnet. he here touches the core of the whole question, and when we can state the condition into which the conducting wire is thrown before it is moved, we shall then be in a position to understand the physical constitution of the electric current generated by its motion. in this inquiry faraday worked with steel magnets, the force of which varies with the distance from the magnet. he then sought a uniform field of magnetic force, and found it in space as affected by the magnetism of the earth. his next memoir, sent to the royal society, december , , is 'on the employment of the induced magnetoelectro current as a test and measure of magnetic forces.' he forms rectangles and rings, and by ingenious and simple devices collects the opposed currents which are developed in them by rotation across the terrestrial lines of magnetic force. he varies the shapes of his rectangles while preserving their areas constant, and finds that the constant area produces always the same amount of current per revolution. the current depends solely on the number of lines of force intersected, and when this number is kept constant the current remains constant too. thus the lines of magnetic force are continually before his eyes, by their aid he colligates his facts, and through the inspirations derived from them he vastly expands the boundaries of our experimental knowledge. the beauty and exactitude of the results of this investigation are extraordinary. i cannot help thinking while i dwell upon them, that this discovery of magneto-electricity is the greatest experimental result ever obtained by an investigator. it is the mont blanc of faraday's own achievements. he always worked at great elevations, but a higher than this he never subsequently attained. footnotes to chapter [ ] he compares the interpenetration of two atoms to the coalescence of two distinct waves, which though for a moment blended to a single mass, preserve their individuality, and afterwards separate. [ ] in this form the experiment is identical with one made twenty years earlier. see page . chapter . unity and convertibility of natural forces: theory of the electric current. the terms unity and convertibility, as applied to natural forces, are often employed in these investigations, many profound and beautiful thoughts respecting these subjects being expressed in faraday's memoirs. modern inquiry has, however, much augmented our knowledge of the relationship of natural forces, and it seems worth while to say a few words here, tending to clear up certain misconceptions which appear to exist among philosophic writers regarding this relationship. the whole stock of energy or working-power in the world consists of attractions, repulsions, and motions. if the attractions and repulsions are so circumstanced as to be able to produce motion, they are sources of working-power, but not otherwise. let us for the sake of simplicity confine our attention to the case of attraction. the attraction exerted between the earth and a body at a distance from the earth's surface is a source of working-power; because the body can be moved by the attraction, and in falling to the earth can perform work. when it rests upon the earth's surface it is not a source of power or energy, because it can fall no further. but though it has ceased to be a source of energy, the attraction of gravity still acts as a force, which holds the earth and weight together. the same remarks apply to attracting atoms and molecules. as long as distance separates them, they can move across it in obedience to the attraction, and the motion thus produced may, by proper appliances, be caused to perform mechanical work. when, for example, two atoms of hydrogen unite with one of oxygen, to form water the atoms are first drawn towards each other--they move, they clash, and then by virtue of their resiliency, they recoil and quiver. to this quivering motion we give the name of heat. now this quivering motion is merely the redistribution of the motion produced by the chemical affinity; and this is the only sense in which chemical affinity can be said to be converted into heat. we must not imagine the chemical attraction destroyed, or converted into anything else. for the atoms, when mutually clasped to form a molecule of water, are held together by the very attraction which first drew them towards each other. that which has really been expended is the pull exerted through the space by which the distance between the atoms has been diminished. if this be understood, it will be at once seen that gravity may in this sense be said to be convertible into heat; that it is in reality no more an outstanding and inconvertible agent, as it is sometimes stated to be, than chemical affinity. by the exertion of a certain pull, through a certain space, a body is caused to clash with a certain definite velocity against the earth. heat is thereby developed, and this is the only sense in which gravity can be said to be converted into heat. in no case is the force which produces the motion annihilated or changed into anything else. the mutual attraction of the earth and weight exists when they are in contact as when they were separate; but the ability of that attraction to employ itself in the production of motion does not exist. the transformation, in this case, is easily followed by the mind's eye. first, the weight as a whole is set in motion by the attraction of gravity. this motion of the mass is arrested by collision with the earth; being broken up into molecular tremors, to which we give the name of heat. and when we reverse the process, and employ those tremors of heat to raise a weight, as is done through the intermediation of an elastic fluid in the steam-engine, a certain definite portion of the molecular motion is destroyed in raising the weight. in this sense, and this sense only, can the heat be said to be converted into gravity, or more correctly, into potential energy of gravity. it is not that the destruction of the heat has created any new attraction, but simply that the old attraction has now a power conferred upon it, of exerting a certain definite pull in the interval between the starting-point of the falling weight and its collision with the earth. so also as regards magnetic attraction: when a sphere of iron placed at some distance from a magnet rushes towards the magnet, and has its motion stopped by collision, an effect mechanically the same as that produced by the attraction of gravity occurs. the magnetic attraction generates the motion of the mass, and the stoppage of that motion produces heat. in this sense, and in this sense only, is there a transformation of magnetic work into heat. and if by the mechanical action of heat, brought to bear by means of a suitable machine, the sphere be torn from the magnet and again placed at a distance, a power of exerting a pull through that distance, and producing a new motion of the sphere, is thereby conferred upon the magnet; in this sense, and in this sense only, is the heat converted into magnetic potential energy. when, therefore, writers on the conservation of energy speak of tensions being 'consumed' and 'generated,' they do not mean thereby that old attractions have been annihilated and new ones brought into existence, but that, in the one case, the power of the attraction to produce motion has been diminished by the shortening of the distance between the attracting bodies, and that in the other case the power of producing motion has been augmented by the increase of the distance. these remarks apply to all bodies, whether they be sensible masses or molecules. of the inner quality that enables matter to attract matter we know nothing; and the law of conservation makes no statement regarding that quality. it takes the facts of attraction as they stand, and affirms only the constancy of working-power. that power may exist in the form of motion; or it may exist in the form of force, with distance to act through. the former is dynamic energy, the latter is potential energy, the constancy of the sum of both being affirmed by the law of conservation. the convertibility of natural forces consists solely in transformations of dynamic into potential, and of potential into dynamic, energy, which are incessantly going on. in no other sense has the convertibility of force, at present, any scientific meaning. by the contraction of a muscle a man lifts a weight from the earth. but the muscle can contract only through the oxidation of its own tissue or of the blood passing through it. molecular motion is thus converted into mechanical motion. supposing the muscle to contract without raising the weight, oxidation would also occur, but the whole of the heat produced by this oxidation would be liberated in the muscle itself. not so when it performs external work; to do that work a certain definite portion of the heat of oxidation must be expended. it is so expended in pulling the weight away from the earth. if the weight be permitted to fall, the heat generated by its collision with the earth would exactly make up for that lacking in the muscle during the lifting of the weight. in the case here supposed, we have a conversion of molecular muscular action into potential energy of gravity; and a conversion of that potential energy into heat; the heat, however, appearing at a distance from its real origin in the muscle. the whole process consists of a transference of molecular motion from the muscle to the weight, and gravitating force is the mere go-between, by means of which the transference is effected. these considerations will help to clear our way to the conception of the transformations which occur when a wire is moved across the lines of force in a magnetic field. in this case it is commonly said we have a conversion of magnetism into electricity. but let us endeavour to understand what really occurs. for the sake of simplicity, and with a view to its translation into a different one subsequently, let us adopt for a moment the provisional conception of a mixed fluid in the wire, composed of positive and negative electricities in equal quantities, and therefore perfectly neutralizing each other when the wire is still. by the motion of the wire, say with the hand, towards the magnet, what the germans call a scheidungs-kraft--a separating force--is brought into play. this force tears the mixed fluids asunder, and drives them in two currents, the one positive and the other negative, in two opposite directions through the wire. the presence of these currents evokes a force of repulsion between the magnet and the wire; and to cause the one to approach the other, this repulsion must be overcome. the overcoming of this repulsion is, in fact, the work done in separating and impelling the two electricities. when the wire is moved away from the magnet, a scheidungs-kraft, or separating force, also comes into play; but now it is an attraction that has to be surmounted. in surmounting it, currents are developed in directions opposed to the former; positive takes the place of negative, and negative the place of positive; the overcoming of the attraction being the work done in separating and impelling the two electricities. the mechanical action occurring here is different from that occurring where a sphere of soft iron is withdrawn from a magnet, and again attracted. in this case muscular force is expended during the act of separation; but the attraction of the magnet effects the reunion. in the case of the moving wire also we overcome a resistance in separating it from the magnet, and thus far the action is mechanically the same as the separation of the sphere of iron. but after the wire has ceased moving, the attraction ceases; and so far from any action occurring similar to that which draws the iron sphere back to the magnet, we have to overcome a repulsion to bring them together. there is no potential energy conferred either by the removal or by the approach of the wire, and the only power really transformed or converted, in the experiment, is muscular power. nothing that could in strictness be called a conversion of magnetism into electricity occurs. the muscular oxidation that moves the wire fails to produce within the muscle its due amount of heat, a portion of that heat, equivalent to the resistance overcome, appearing in the moving wire instead. is this effect an attraction and a repulsion at a distance? if so, why should both cease when the wire ceases to move? in fact, the deportment of the wire resembles far more that of a body moving in a resisting medium than anything else; the resistance ceasing when the motion is suspended. let us imagine the case of a liquid so mobile that the hand may be passed through it to and fro, without encountering any sensible resistance. it resembles the motion of a conductor in the unexcited field of an electro-magnet. now, let us suppose a body placed in the liquid, or acting on it, which confers upon it the property of viscosity; the hand would no longer move freely. during its motion, but then only, resistance would be encountered and overcome. here we have rudely represented the case of the excited magnetic field, and the result in both cases would be substantially the same. in both cases heat would, in the end, be generated outside of the muscle, its amount being exactly equivalent to the resistance overcome. let us push the analogy a little further; suppose in the case of the fluid rendered viscous, as assumed a moment ago, the viscosity not to be so great as to prevent the formation of ripples when the hand is passed through the liquid. then the motion of the hand, before its final conversion into heat, would exist for a time as wave-motion, which, on subsiding, would generate its due equivalent of heat. this intermediate stage, in the case of our moving wire, is represented by the period during which the electric current is flowing through it; but that current, like the ripples of our liquid, soon subsides, being, like them, converted into heat. do these words shadow forth anything like the reality? such speculations cannot be injurious if they are enunciated without dogmatism. i do confess that ideas such as these here indicated exercise a strong fascination on my mind. is then the magnetic field really viscous, and if so, what substance exists in it and the wire to produce the viscosity? let us first look at the proved effects, and afterwards turn our thoughts back upon their cause. when the wire approaches the magnet, an action is evoked within it, which travels through it with a velocity comparable to that of light. one substance only in the universe has been hitherto proved competent to transmit power at this velocity; the luminiferous ether. not only its rapidity of progression, but its ability to produce the motion of light and heat, indicates that the electric current is also motion.[ ] further, there is a striking resemblance between the action of good and bad conductors as regards electricity, and the action of diathermanous and adiathermanous bodies as regards radiant heat. the good conductor is diathermanous to the electric current; it allows free transmission without the development of heat. the bad conductor is adiathermanous to the electric current, and hence the passage of the latter is accompanied by the development of heat. i am strongly inclined to hold the electric current, pure and simple, to be a motion of the ether alone; good conductors being so constituted that the motion may be propagated through their ether without sensible transfer to their atoms, while in the case of bad conductors this transfer is effected, the transferred motion appearing as heat.[ ] i do not know whether faraday would have subscribed to what is here written; probably his habitual caution would have prevented him from committing himself to anything so definite. but some such idea filled his mind and coloured his language through all the later years of his life. i dare not say that he has been always successful in the treatment of these theoretic notions. in his speculations he mixes together light and darkness in varying proportions, and carries us along with him through strong alternations of both. it is impossible to say how a certain amount of mathematical training would have affected his work. we cannot say what its influence would have been upon that force of inspiration that urged him on; whether it would have daunted him, and prevented him from driving his adits into places where no theory pointed to a lode. if so, then we may rejoice that this strong delver at the mine of natural knowledge was left free to wield his mattock in his own way. it must be admitted, that faraday's purely speculative writings often lack that precision which the mathematical habit of thought confers. still across them flash frequent gleams of prescient wisdom which will excite admiration throughout all time; while the facts, relations, principles, and laws which his experiments have established are sure to form the body of grand theories yet to come. footnotes to chapter [ ] mr. clerk maxwell has recently published an exceedingly important investigation connected with this question. even in the non-mathematical portions of the memoirs of mr. maxwell, the admirable spirit of his philosophy is sufficiently revealed. as regards the employment of scientific imagery, i hardly know his equal in power of conception and clearness of definition. [ ] one important difference, of course, exists between the effect of motion in the magnetic field, and motion in a resisting medium. in the former case the heat is generated in the moving conductor, in the latter it is in part generated in the medium. chapter . summary. when from an alpine height the eye of the climber ranges over the mountains, he finds that for the most part they resolve themselves into distinct groups, each consisting of a dominant mass surrounded by peaks of lesser elevation. the power which lifted the mightier eminences, in nearly all cases lifted others to an almost equal height. and so it is with the discoveries of faraday. as a general rule, the dominant result does not stand alone, but forms the culminating point of a vast and varied mass of inquiry. in this way, round about his great discovery of magneto-electric induction, other weighty labours group themselves. his investigations on the extra current; on the polar and other condition of diamagnetic bodies; on lines of magnetic force, their definite character and distribution; on the employment of the induced magneto-electric current as a measure and test of magnetic action; on the revulsive phenomena of the magnetic field, are all, notwithstanding the diversity of title, researches in the domain of magneto-electric induction. faraday's second group of researches and discoveries embrace the chemical phenomena of the current. the dominant result here is the great law of definite electro-chemical decomposition, around which are massed various researches on electro-chemical conduction and on electrolysis both with the machine and with the pile. to this group also belongs his analysis of the contact theory, his inquiries as to the source of voltaic electricity, and his final development of the chemical theory of the pile. his third great discovery is the magnetization of light, which i should liken to the weisshorn among mountains--high, beautiful, and alone. the dominant result of his fourth group of researches is the discovery of diamagnetism, announced in his memoir as the magnetic condition of all matter, round which are grouped his inquiries on the magnetism of flame and gases; on magne-crystallic action, and on atmospheric magnetism, in its relations to the annual and diurnal variation of the needle, the full significance of which is still to be shown. these are faraday's most massive discoveries, and upon them his fame must mainly rest. but even without them, sufficient would remain to secure for him a high and lasting scientific reputation. we should still have his researches on the liquefaction of gases; on frictional electricity; on the electricity of the gymnotus; on the source of power in the hydro-electric machine, the last two investigations being untouched in the foregoing memoir; on electro-magnetic rotations; on regelation; all his more purely chemical researches, including his discovery of benzol. besides these he published a multitude of minor papers, most of which, in some way or other, illustrate his genius. i have made no allusion to his power and sweetness as a lecturer. taking him for all in all, i think it will be conceded that michael faraday was the greatest experimental philosopher the world has ever seen; and i will add the opinion, that the progress of future research will tend, not to dim or to diminish, but to enhance and glorify the labours of this mighty investigator. chapter . illustrations of character. thus far i have confined myself to topics mainly interesting to the man of science, endeavouring, however, to treat them in a manner unrepellent to the general reader who might wish to obtain a notion of faraday as a worker. on others will fall the duty of presenting to the world a picture of the man. but i know you will permit me to add to the foregoing analysis a few personal reminiscences and remarks, tending to connect faraday with a wider world than that of science--namely, with the general human heart. one word in reference to his married life, in addition to what has been already said, may find a place here. as in the former case, faraday shall be his own spokesman. the following paragraph, though written in the third person, is from his hand:--'on june , , he married, an event which more than any other contributed to his earthly happiness and healthful state of mind. the union has continued for twenty-eight years and has in no wise changed, except in the depth and strength of its character.' faraday's immediate forefathers lived in a little place called clapham wood hall, in yorkshire. here dwelt robert faraday and elizabeth his wife, who had ten children, one of them, james faraday, born in , being father to the philosopher. a family tradition exists that the faradays came originally from ireland. faraday himself has more than once expressed to me his belief that his blood was in part celtic, but how much of it was so, or when the infusion took place, he was unable to say. he could imitate the irish brogue, and his wonderful vivacity may have been in part due to his extraction. but there were other qualities which we should hardly think of deriving from ireland. the most prominent of these was his sense of order, which ran like a luminous beam through all the transactions of his life. the most entangled and complicated matters fell into harmony in his hands. his mode of keeping accounts excited the admiration of the managing board of this institution. and his science was similarly ordered. in his experimental researches, he numbered every paragraph, and welded their various parts together by incessant reference. his private notes of the experimental researches, which are happily preserved, are similarly numbered: their last paragraph bears the figure , . his working qualities, moreover, showed the tenacity of the teuton. his nature was impulsive, but there was a force behind the impulse which did not permit it to retreat. if in his warm moments he formed a resolution, in his cool ones he made that resolution good. thus his fire was that of a solid combustible, not that of a gas, which blazes suddenly, and dies as suddenly away. and here i must claim your tolerance for the limits by which i am confined. no materials for a life of faraday are in my hands, and what i have now to say has arisen almost wholly out of our close personal relationship. letters of his, covering a period of sixteen years, are before me, each one of which contains some characteristic utterance;--strong, yet delicate in counsel, joyful in encouragement, and warm in affection. references which would be pleasant to such of them as still live are made to humboldt, biot, dumas, chevreul, magnus, and arago. accident brought these names prominently forward; but many others would be required to complete his list of continental friends. he prized the love and sympathy of men--prized it almost more than the renown which his science brought him. nearly a dozen years ago it fell to my lot to write a review of his 'experimental researches' for the 'philosophical magazine.' after he had read it, he took me by the hand, and said, 'tyndall, the sweetest reward of my work is the sympathy and good will which it has caused to flow in upon me from all quarters of the world.' among his letters i find little sparks of kindness, precious to no one but myself, but more precious to me than all. he would peep into the laboratory when he thought me weary, and take me upstairs with him to rest. and if i happened to be absent, he would leave a little note for me, couched in this or some other similar form:--'dear tyndall,--i was looking for you, because we were at tea--we have not yet done--will you come up?' i frequently shared his early dinner; almost always, in fact, while my lectures were going on. there was no trace of asceticism in his nature. he preferred the meat and wine of life to its locusts and wild honey. never once during an intimacy of fifteen years did he mention religion to me, save when i drew him on to the subject. he then spoke to me without hesitation or reluctance; not with any apparent desire to 'improve the occasion,' but to give me such information as i sought. he believed the human heart to be swayed by a power to which science or logic opened no approach, and, right or wrong, this faith, held in perfect tolerance of the faiths of others, strengthened and beautified his life. from the letters just referred to, i will select three for publication here. i choose the first, because it contains a passage revealing the feelings with which faraday regarded his vocation, and also because it contains an allusion which will give pleasure to a friend. 'royal institution. [ this is crossed out by faraday ] 'ventnor, isle of wight, june , . 'my dear tyndall,--you see by the top of this letter how much habit prevails over me; i have just read yours from thence, and yet i think myself there. however, i have left its science in very good keeping, and i am glad to learn that you are at experiment once more. but how is the health? not well, i fear. i wish you would get yourself strong first and work afterwards. as for the fruits, i am sure they will be good, for though i sometimes despond as regards myself, i do not as regards you. you are young, i am old.... but then our subjects are so glorious, that to work at them rejoices and encourages the feeblest; delights and enchants the strongest. 'i have not yet seen anything from magnus. thoughts of him always delight me. we shall look at his black sulphur together. i heard from schonbein the other day. he tells me that liebig is full of ozone, i.e., of allotropic oxygen. 'good-bye for the present. 'ever, my dear tyndall, 'yours truly, 'm. faraday.' the contemplation of nature, and his own relation to her, produced in faraday a kind of spiritual exaltation which makes itself manifest here. his religious feeling and his philosophy could not be kept apart; there was an habitual overflow of the one into the other. whether he or another was its exponent, he appeared to take equal delight in science. a good experiment would make him almost dance with delight. in november, , he wrote to me thus:--'i hope some day to take up the point respecting the magnetism of associated particles. in the meantime i rejoice at every addition to the facts and reasoning connected with the subject. when science is a republic, then it gains: and though i am no republican in other matters, i am in that.' all his letters illustrate this catholicity of feeling. ten years ago, when going down to brighton, he carried with him a little paper i had just completed, and afterwards wrote to me. his letter is a mere sample of the sympathy which he always showed to me and my work. 'brighton, december , . 'my dear tyndall,--i cannot resist the pleasure of saying how very much i have enjoyed your paper. every part has given me delight. it goes on from point to point beautifully. you will find many pencil marks, for i made them as i read. i let them stand, for though many of them receive their answer as the story proceeds, yet they show how the wording impresses a mind fresh to the subject, and perhaps here and there you may like to alter it slightly, if you wish the full idea, i.e., not an inaccurate one, to be suggested at first; and yet after all i believe it is not your exposition, but the natural jumping to a conclusion that affects or has affected my pencil. 'we return on friday, when i will return you the paper. 'ever truly yours, 'm. faraday.' the third letter will come in its proper place towards the end. while once conversing with faraday on science, in its relations to commerce and litigation, he said to me, that at a certain period of his career, he was forced definitely to ask himself, and finally to decide whether he should make wealth or science the pursuit of his life. he could not serve both masters, and he was therefore compelled to choose between them. after the discovery of magneto-electricity his fame was so noised abroad, that the commercial world would hardly have considered any remuneration too high for the aid of abilities like his. even before he became so famous, he had done a little 'professional business.' this was the phrase he applied to his purely commercial work. his friend, richard phillips, for example, had induced him to undertake a number of analyses, which produced, in the year , an addition to his income of more than a thousand pounds; and in a still greater addition. he had only to will it to raise in his professional business income to l. a year. indeed double this sum would be a wholly insufficient estimate of what he might, with ease, have realised annually during the last thirty years of his life. while restudying the experimental researches with reference to the present memoir, the conversation with faraday here alluded to came to my recollection, and i sought to ascertain the period when the question, 'wealth or science,' had presented itself with such emphasis to his mind. i fixed upon the year or , for it seemed beyond the range of human power to pursue science as he had done during the subsequent years, and to pursue commercial work at the same time. to test this conclusion i asked permission to see his accounts, and on my own responsibility, i will state the result. in , his professional business income, instead of rising to l., or more, fell from l. s. to l. s. from this it fell with slight oscillations to l. in , and to zero in . between and , it never, except in one instance, exceeded l.; being for the most part much under this. the exceptional year referred to was that in which he and sir charles lyell were engaged by government to write a report on the haswell colliery explosion, and then his business income rose to l. from the end of to the day of his death, faraday's annual professional business income was exactly zero. taking the duration of his life into account, this son of a blacksmith, and apprentice to a bookbinder, had to decide between a fortune of , l. on the one side, and his undowered science on the other. he chose the latter, and died a poor man. but his was the glory of holding aloft among the nations the scientific name of england for a period of forty years. the outward and visible signs of fame were also of less account to him than to most men. he had been loaded with scientific honours from all parts of the world. without, i imagine, a dissentient voice, he was regarded as the prince of the physical investigators of the present age. the highest scientific position in this country he had, however, never filled. when the late excellent and lamented lord wrottesley resigned the presidency of the royal society, a deputation from the council, consisting of his lordship, mr. grove, and mr. gassiot, waited upon faraday, to urge him to accept the president's chair. all that argument or friendly persuasion could do was done to induce him to yield to the wishes of the council, which was also the unanimous wish of scientific men. a knowledge of the quickness of his own nature had induced in faraday the habit of requiring an interval of reflection, before he decided upon any question of importance. in the present instance he followed his usual habit, and begged for a little time. on the following morning, i went up to his room and said on entering that i had come to him with some anxiety of mind. he demanded its cause, and i responded:--'lest you should have decided against the wishes of the deputation that waited on you yesterday.' 'you would not urge me to undertake this responsibility,' he said. 'i not only urge you,' was my reply, 'but i consider it your bounden duty to accept it.' he spoke of the labour that it would involve; urged that it was not in his nature to take things easy; and that if he became president, he would surely have to stir many new questions, and agitate for some changes. i said that in such cases he would find himself supported by the youth and strength of the royal society. this, however, did not seem to satisfy him. mrs. faraday came into the room, and he appealed to her. her decision was adverse, and i deprecated her decision. 'tyndall,' he said at length, 'i must remain plain michael faraday to the last; and let me now tell you, that if i accepted the honour which the royal society desires to confer upon me, i would not answer for the integrity of my intellect for a single year.' i urged him no more, and lord wrottesley had a most worthy successor in sir benjamin brodie. after the death of the duke of northumberland, our board of managers wished to see mr. faraday finish his career as president of the institution, which he had entered on weekly wages more than half a century before. but he would have nothing to do with the presidency. he wished for rest, and the reverent affection of his friends was to him infinitely more precious than all the honours of official life. the first requisite of the intellectual life of faraday was the independence of his mind; and though prompt to urge obedience where obedience was due, with every right assertion of manhood he intensely sympathized. even rashness on the side of honour found from him ready forgiveness, if not open applause. the wisdom of years, tempered by a character of this kind, rendered his counsel peculiarly precious to men sensitive like himself. i often sought that counsel, and, with your permission, will illustrate its character by one or two typical instances. in , i was appointed examiner under the council for military education. at that time, as indeed now, i entertained strong convictions as to the enormous utility of physical science to officers of artillery and engineers, and whenever opportunity offered, i expressed this conviction without reserve. i did not think the recognition, though considerable, accorded to physical science in those examinations at all proportionate to its importance; and this probably rendered me more jealous than i otherwise should have been of its claims. in trinity college, dublin, a school had been organized with reference to the woolwich examinations, and a large number of exceedingly well-instructed young gentlemen were sent over from dublin, to compete for appointments in the artillery and the engineers. the result of one examination was particularly satisfactory to me; indeed the marks obtained appeared so eloquent that i forbore saying a word about them. my colleagues, however, followed the usual custom of sending in brief reports with their returns of marks. after the results were published, a leading article appeared in 'the times,' in which the reports were largely quoted, praise being bestowed on all the candidates, except the excellent young fellows who had passed through my hands. a letter from trinity college drew my attention to this article, bitterly complaining that whereas the marks proved them to be the best of all, the science candidates were wholly ignored. i tried to set matters right by publishing, on my own responsibility, a letter in 'the times.' the act, i knew, could not bear justification from the war office point of view; and i expected and risked the displeasure of my superiors. the merited reprimand promptly came. 'highly as the secretary of state for war might value the expression of professor tyndall's opinion, he begged to say that an examiner, appointed by his royal highness the commander-in-chief, had no right to appear in the public papers as professor tyndall has done, without the sanction of the war office.' nothing could be more just than this reproof, but i did not like to rest under it. i wrote a reply, and previous to sending it took it up to faraday. we sat together before his fire, and he looked very earnest as he rubbed his hands and pondered. the following conversation then passed between us:-- f. you certainly have received a reprimand, tyndall; but the matter is over, and if you wish to accept the reproof, you will hear no more about it. t. but i do not wish to accept it. f. then you know what the consequence of sending that letter will be? t. i do. f. they will dismiss you. t. i know it. f. then send the letter! the letter was firm, but respectful; it acknowledged the justice of the censure, but expressed neither repentance nor regret. faraday, in his gracious way, slightly altered a sentence or two to make it more respectful still. it was duly sent, and on the following day i entered the institution with the conviction that my dismissal was there before me. weeks, however, passed. at length the well-known envelope appeared, and i broke the seal, not doubting the contents. they were very different from what i expected. 'the secretary of state for war has received professor tyndall's letter, and deems the explanation therein given perfectly satisfactory.' i have often wished for an opportunity of publicly acknowledging this liberal treatment, proving, as it did, that lord panmure could discern and make allowance for a good intention, though it involved an offence against routine. for many years subsequently it was my privilege to act under that excellent body, the council for military education. on another occasion of this kind, having encouraged me in a somewhat hardy resolution i had formed, faraday backed his encouragement by an illustration drawn from his own life. the subject will interest you, and it is so sure to be talked about in the world, that no avoidable harm can rise from its introduction here. in the year , sir robert peel wished to offer faraday a pension, but that great statesman quitted office before he was able to realise his wish. the minister who founded these pensions intended them, i believe, to be marks of honour which even proud men might accept without compromise of independence. when, however, the intimation first reached faraday in an unofficial way, he wrote a letter announcing his determination to decline the pension; and stating that he was quite competent to earn his livelihood himself. that letter still exists, but it was never sent, faraday's repugnance having been overruled by his friends. when lord melbourne came into office, he desired to see faraday; and probably in utter ignorance of the man--for unhappily for them and us, ministers of state in england are only too often ignorant of great englishmen--his lordship said something that must have deeply displeased his visitor. all the circumstances were once communicated to me, but i have forgotten the details. the term 'humbug,' i think, was incautiously employed by his lordship, and other expressions were used of a similar kind. faraday quitted the minister with his own resolves, and that evening he left his card and a short and decisive note at the residence of lord melbourne, stating that he had manifestly mistaken his lordship's intention of honouring science in his person, and declining to have anything whatever to do with the proposed pension. the good-humoured nobleman at first considered the matter a capital joke; but he was afterwards led to look at it more seriously. an excellent lady, who was a friend both to faraday and the minister, tried to arrange matters between them; but she found faraday very difficult to move from the position he had assumed. after many fruitless efforts, she at length begged of him to state what he would require of lord melbourne to induce him to change his mind. he replied, 'i should require from his lordship what i have no right or reason to expect that he would grant--a written apology for the words he permitted himself to use to me.' the required apology came, frank and full, creditable, i thought, alike to the prime minister and the philosopher. considering the enormous strain imposed on faraday's intellect, the boy-like buoyancy even of his later years was astonishing. he was often prostrate, but he had immense resiliency, which he brought into action by getting away from london whenever his health failed. i have already indicated the thoughts which filled his mind during the evening of his life. he brooded on magnetic media and lines of force; and the great object of the last investigation he ever undertook was the decision of the question whether magnetic force requires time for its propagation. how he proposed to attack this subject we may never know. but he has left some beautiful apparatus behind; delicate wheels and pinions, and associated mirrors, which were to have been employed in the investigation. the mere conception of such an inquiry is an illustration of his strength and hopefulness, and it is impossible to say to what results it might have led him. but the work was too heavy for his tired brain. it was long before he could bring himself to relinquish it and during this struggle he often suffered from fatigue of mind. it was at this period, and before he resigned himself to the repose which marked the last two years of his life, that he wrote to me the following letter--one of many priceless letters now before me--which reveals, more than anything another pen could express, the state of his mind at the time. i was sometimes censured in his presence for my doings in the alps, but his constant reply was, 'let him alone, he knows how to take care of himself.' in this letter, anxiety on this score reveals itself for the first time. 'hampton court, august , . 'my dear tyndall,--i do not know whether my letter will catch you, but i will risk it, though feeling very unfit to communicate with a man whose life is as vivid and active as yours; but the receipt of your kind letter makes me to know that, though i forget, i am not forgotten, and though i am not able to remember at the end of a line what was said at the beginning of it, the imperfect marks will convey to you some sense of what i long to say. we had heard of your illness through miss moore, and i was therefore very glad to learn that you are now quite well; do not run too many risks or make your happiness depend too much upon dangers, or the hunting of them. sometimes the very thinking of you, and what you may be about, wearies me with fears, and then the cogitations pause and change, but without giving me rest. i know that much of this depends upon my own worn-out nature, and i do not know why i write it, save that when i write to you i cannot help thinking it, and the thoughts stand in the way of other matter. * * * * * 'see what a strange desultory epistle i am writing to you, and yet i feel so weary that i long to leave my desk and go to the couch. 'my dear wife and jane desire their kindest remembrances: i hear them in the next room:... i forget--but not you, my dear tyndall, for i am 'ever yours, 'm. faraday.' this weariness subsided when he relinquished his work, and i have a cheerful letter from him, written in the autumn of . but towards the close of that year he had an attack of illness, from which he never completely rallied. he continued to attend the friday evening meetings, but the advance of infirmity was apparent to us all. complete rest became finally essential to him, and he ceased to appear among us. there was no pain in his decline to trouble the memory of those who loved him. slowly and peacefully he sank towards his final rest, and when it came, his death was a falling asleep. in the fulness of his honours and of his age he quitted us; the good fight fought, the work of duty--shall i not say of glory?--done. the 'jane' referred to in the foregoing letter is faraday's niece, miss jane barnard, who with an affection raised almost to religious devotion watched him and tended him to the end. i saw mr. faraday for the first time on my return from marburg in . i came to the royal institution, and sent up my card, with a copy of the paper which knoblauch and myself had just completed. he came down and conversed with me for half an hour. i could not fail to remark the wonderful play of intellect and kindly feeling exhibited by his countenance. when he was in good health the question of his age would never occur to you. in the light and laughter of his eyes you never thought of his grey hairs. he was then on the point of publishing one of his papers on magnecrystallic action, and he had time to refer in a flattering note to the memoir i placed in his hands. i returned to germany, worked there for nearly another year, and in june, , came back finally from berlin to england. then, for the first time, and on my way to the meeting of the british association, at ipswich, i met a man who has since made his mark upon the intellect of his time; who has long been, and who by the strong law of natural affinity must continue to be, a brother to me. we were both without definite outlook at the time, needing proper work, and only anxious to have it to perform. the chairs of natural history and of physics being advertised as vacant in the university of toronto, we applied for them, he for the one, i for the other; but, possibly guided by a prophetic instinct, the university authorities declined having anything to do with either of us. if i remember aright, we were equally unlucky elsewhere. one of faraday's earliest letters to me had reference to this toronto business, which he thought it unwise in me to neglect. but toronto had its own notions, and in , at the instance of dr. bence jones, and on the recommendation of faraday himself, a chair of physics at the royal institution was offered to me. i was tempted at the same time to go elsewhere, but a strong attraction drew me to his side. let me say that it was mainly his and other friendships, precious to me beyond all expression, that caused me to value my position here more highly than any other that could be offered to me in this land. nor is it for its honour, though surely that is great, but for the strong personal ties that bind me to it, that i now chiefly prize this place. you might not credit me were i to tell you how lightly i value the honour of being faraday's successor compared with the honour of having been faraday's friend. his friendship was energy and inspiration; his 'mantle' is a burden almost too heavy to be borne. sometimes during the last year of his life, by the permission or invitation of mrs. faraday, i went up to his rooms to see him. the deep radiance, which in his time of strength flashed with such extraordinary power from his countenance, had subsided to a calm and kindly light, by which my latest memory of him is warmed and illuminated. i knelt one day beside him on the carpet and placed my hand upon his knee; he stroked it affectionately, smiled, and murmured, in a low soft voice, the last words that i remember as having been spoken to me by michael faraday. it was my wish and aspiration to play the part of schiller to this goethe: and he was at times so strong and joyful--his body so active, and his intellect so clear--as to suggest to me the thought that he, like goethe, would see the younger man laid low. destiny ruled otherwise, and now he is but a memory to us all. surely no memory could be more beautiful. he was equally rich in mind and heart. the fairest traits of a character sketched by paul, found in him perfect illustration. for he was 'blameless, vigilant, sober, of good behaviour, apt to teach, not given to filthy lucre.' he had not a trace of worldly ambition; he declared his duty to his sovereign by going to the levee once a year, but beyond this he never sought contact with the great. the life of his spirit and of his intellect was so full, that the things which men most strive after were absolutely indifferent to him. 'give me health and a day,' says the brave emerson, 'and i will make the pomp of emperors ridiculous.' in an eminent degree faraday could say the same. what to him was the splendour of a palace compared with a thunderstorm upon brighton downs?--what among all the appliances of royalty to compare with the setting sun? i refer to a thunderstorm and a sunset, because these things excited a kind of ecstasy in his mind, and to a mind open to such ecstasy the pomps and pleasures of the world are usually of small account. nature, not education, rendered faraday strong and refined. a favourite experiment of his own was representative of himself. he loved to show that water in crystallizing excluded all foreign ingredients, however intimately they might be mixed with it. out of acids, alkalis, or saline solutions, the crystal came sweet and pure. by some such natural process in the formation of this man, beauty and nobleness coalesced, to the exclusion of everything vulgar and low. he did not learn his gentleness in the world, for he withdrew himself from its culture; and still this land of england contained no truer gentleman than he. not half his greatness was incorporate in his science, for science could not reveal the bravery and delicacy of his heart. but it is time that i should end these weak words, and lay my poor garland on the grave of this just and faithful knight of god. images generously made available by the internet archive/american libraries.) a study of splashes _by the same author._ * * * * * a first course of physical laboratory practice. containing experiments. _with illustrations. crown vo, s. d._ * * * * * dynamics of rotation. an elementary introduction to rigid dynamics. _crown vo, s. d._ * * * * * longmans, green, and co. london, new york, bombay, and calcutta. [illustration: permanent splashes left where a projectile has 	entered an armour-plate. [_see page ._] a study of splashes by a. m. worthington c.b., m.a., f.r.s. headmaster and professor of physics at the royal naval engineering college, devonport with illustrations from instantaneous photographs longmans, green, and co. paternoster row, london new york, bombay, and calcutta _all rights reserved_ dedicated to the natural history society of rugby school and its former president arthur sidgwick in remembrance of the encouragement given to the early observations made in boyhood by my old school-friend h. f. newall from which this study sprang. preface this publication is an attempt to present in a form acceptable to the general reader the outcome of an inquiry conducted by the aid of instantaneous photography, which was begun about fourteen years ago. the author, in , had occasion to lecture at the royal institution on the "splash of a drop," of which he had already made a somewhat prolonged study. that lecture, which was subsequently reprinted in the "romance of science" series by the society for promoting christian knowledge, dealt largely with the splash of a drop falling on a solid plate, with which the present volume is not concerned. at the close of the lecture were exhibited for the first time a few photographs of some of the phenomena now dealt with, which the author had just succeeded in taking with the help of his friend mr. r. s. cole. the success of the photographs and the additional information they afforded led to a long photographic investigation, which formed the subject of two papers[a] in the _transactions_ of the royal society. except for two magazine articles,[b] the results of this work have not been presented to the general public. moreover, in the illustrations printed by the royal society much of the beauty of the original photographs was lost in the reproduction, or was sacrificed in a selection of which the only object was the elucidation of points of technical scientific interest. if the present volume is so fortunate as to find many readers among the general public, as the author hopes it may, especially among the young whose eyes are still quick to observe, and whose minds are eager, it will be on account of admiration for the exquisite beauty of some of the forms assumed, of surprise at the revelation of so much where so little was expected, and because of the peculiar fascination that is always felt in following any gradually changing natural phenomenon, in which the sequence of events can, partly at any rate, be anticipated and understood. for the sake of serious students of physics who may be interested in unexpected phenomena of fluid motion, all references that seem necessary have been given in footnotes, and it may be mentioned that the later photographs of series i and those of series i-a and iii, have not been previously published, and afford new information on certain points. in taking these photographs the author has been much helped by his friends dr. g. b. bryan and mr. g. f. page. a. m. w. tavistock, _sept. , _. [added _march , _.] a slight delay in the publication of this book has afforded the opportunity of obtaining the new and quite unexpected information given in the supplementary chapter. contents page chapter i preliminary--methods of observation and apparatus chapter ii the splash of a drop--low fall chapter iii principles involved chapter iv the splash continued chapter v higher falls--bubble-building chapter vi below the surface chapter vii the two kinds of splashes of solid spheres chapter viii the transition from the smooth or "sheath" splash to the rough or "basket" splash chapter ix the explanation of the cause of difference between the two splashes chapter x conclusion chapter xi (supplementary) a new phenomenon that appears with an increase in the velocity of entry of a rough sphere footnotes: [a] "impact with a liquid surface," by worthington and cole. _phil. trans. roy. soc._, a , , and a , . [b] _pearson's magazine_, july and august, . a study of splashes chapter i preliminary--methods of observation and apparatus there will be but few of my readers who have not, in some heavy shower of rain, beguiled the tedium of enforced waiting by watching, perhaps half-unconsciously, the thousand little crystal fountains that start up from the surface of pool or river; noting now and then a surrounding coronet of lesser jets, or here and there a bubble that floats for a moment and then vanishes. it is to this apparently insignificant transaction, which always has been and always will be so familiar, and to others of a like nature, that i desire to call the attention of those who are interested in natural phenomena; hoping to share with them some of the delight that i have myself felt, in contemplating the exquisite forms that the camera has revealed, and in watching the progress of a multitude of events, compressed indeed within the limits of a few hundredths of a second, but none the less orderly and inevitable, and of which the sequence is in part easy to anticipate and understand, while in part it taxes the highest mathematical powers to elucidate. in these modern days of kinematographs and snapshot cameras it might seem an easy matter to follow, by the aid of photography, even a splashing drop. but in reality the task is not so simple, for the changes of form that take place in a splash are far too rapid to come within reach of any ordinary kinematograph, and even the quickest photographic shutter is also much too slow, so that it is necessary to have recourse to the far shorter exposure of a suitable electric spark. the originals of the photographs which illustrate this book were taken by means of a spark, whose duration was certainly less than three-millionths of a second, an interval of time which bears to a whole second about the same proportion as a day to a thousand years. in order to obtain the photographs, advantage was taken of the fact that whatever be the sequence of events in any particular splash, this sequence will be exactly repeated every time that a falling drop strikes the surface under exactly the same conditions, and the problem to be solved was, therefore, as follows:--to cause a drop of definite size to fall from a definite height in absolute darkness so as to strike the surface of the liquid into which it falls at a spot towards which is directed a photographic camera with uncovered lens, and armed with an exceptionally sensitive plate, and to illuminate the drop at the instant that it just touches the surface by a flash of such excessively short duration that no appreciable change of form can take place while the drop is illuminated. this gives us a photograph of the earliest stage. the plate must then be removed and a fresh one substituted; a second drop, of exactly the same size, must be let fall from exactly the same place, and photographed in just the same way, but the flash must now be so timed as to take place at a slightly later stage of the splash, say, one-thousandth of a second later. the photographic plate must be then again removed and a third substituted, on which a still later stage is to be depicted, and in this way the phenomenon can be followed step by step. by adopting this process, and not attempting to follow the same individual splash throughout, we avoid two great difficulties: ( ) the necessity of shifting our photographic plate or film through a distance equal to the breadth of the whole picture every five hundredth or thousandth of a second (if we wish to obtain pictures of stages so near together as this); and ( ) the difficulty of obtaining brilliant flashes of light of sufficiently short duration at these very short intervals. for these we substitute two other difficulties: ( ) that of delivering the drops exactly as required; and ( ) that of timing the flash on each occasion within one or two thousandths of a second, so as to pick out the exact stage we wish to photograph. i will now describe how these two problems have been solved. it is easy enough to arrange for the production of small drops of almost exactly equal size. they may be allowed to fall one by one at a steady rate from the end of a fine glass tube connected to a vessel in which the liquid is maintained at a constant level, as in fig. , or they may be squeezed out slowly as required by means of a syringe held in a clip as in fig. . any required number of these small drops can be caught, and allowed to run together if a larger drop is to be experimented with. [illustration: fig. ] [illustration: fig. ] if the liquid used is mercury, the drops may be caught in any little glass cup such as a deeply concave watch-glass; but other liquids, such as water or milk, would wet the glass and stick to it. if, however, the inside surface of the watch-glass be first carefully smoked in the flame of a candle, then even water or milk will roll over it without sticking, and the drop thus made up will retain a spheroidal form, and can be conveyed to the place of observation in the dark room, where it is transferred to the "dropping cup." this consists of a similar, deep, smoked watch-glass (w)--see plate i--supported on the end of a small horizontal lever, a light cylindrical rod of about the dimensions of an ordinary uncut lead pencil, pivoted about a horizontal axle near the end to which the watch-glass is attached. the other end is armed with a small light piece of iron (i) and is held in position by means of an electro-magnet (m), against the action of a spring. on cutting off the current from the electro-magnet the spring, acting as a catapult, tosses up the longer arm of the lever and thus removes the watch-glass from below the drop (d), which is left unsupported in mid-air, so that it falls from a definite fixed distance into a bowl of water placed below it, towards the surface of which the camera (c) is directed. this solves problem number one. of course, if we wish to observe the splash of a solid sphere, there is no need to smoke the surface of the watch-glass. indeed, the sphere may be more conveniently supported on a small ring. now for the production and timing of the flash. two large leyden jars (jj) are provided, and charged by an electrical machine on their inner coats, one positively and one negatively. stout wires lead from the outer coats to the dark room, and terminate in a spark-gap (s) between magnesium terminals close over the surface of the water in the bowl just mentioned. if the inner coats are now connected together, the positive and negative charges unite with a dazzling flash and a simultaneous discharge and flash takes place between the two outer coats across the spark-gap in the dark room. this latter is the illuminating spark; we have now to time it correctly. for this purpose it is arranged that the discharge shall be effected by means of a falling metal sphere (t) which i shall call the timing sphere, which passes between two terminals s and s connected one to the inside of one jar and one to the inside of the other. these terminals are just too far apart for a spark to leap across, till the timing sphere passes between them and thus shortens the gap; then the discharge takes place, with its accompanying flash in the dark room. the release of the timing sphere is effected by an arrangement of lever and spring controlled by an electro-magnet exactly similar to that which releases the drop in the dark room, and the two electro-magnets _are on the same electric circuit_, so that the drop and timing sphere _are released simultaneously_. but while the drop always falls the same distance, the height through which the timing sphere has to fall before producing discharge can be adjusted at will, and to great nicety, by moving its releasing-lever up or down a vertical support with a scale attached. if, for example, a particular stage of the splash is photographed when the timing sphere falls just four feet to the gap, then by raising its releasing-lever about two-fifths of an inch, the laws of falling bodies tell us that we shall postpone the flash by just one-thousandth of a second, and the next photograph will accordingly reveal a stage just so much later. [illustration: plate i arrangement of apparatus for photographing splashes. laboratory e is the electrical machine. j j are the leyden jars whose inner coats are connected to the sparking knobs s s. l is the lever for releasing the timing sphere t. c is the catapult. i is the light strip of iron held down by the electro-magnet m. dark room d is the drop resting on the smoked watch-glass w. m is the electro-magnet holding down the lever against the action of the catapult, by means of the thin strip of iron i. c is the camera directed towards the liquid l into which the drop will fall. s is the spark-gap between magnesium terminals connected to the outer coats of the leyden jars. r is the concave mirror.] it ought still to be mentioned that to make the utmost use of the illuminating power of the spark, it is necessary to place close behind it a little concave mirror (r), by means of which a compact beam of rays, which would otherwise have been wasted, is directed to the required spot. by this addition we imitate, in miniature, the search-light of a man-of-war. * * * * * as with all experimental devices, the precision attainable with this arrangement is limited by several circumstances. in the first place, the demagnetization of the iron cores of the electro-magnets, when the current is cut off, is not truly instantaneous, and the time required depends on the strength of the magnetizing current and on the temperature of the iron, which in turn will depend on the length of time for which the current has been running. this variation would be of no importance if the two magnets were exactly alike and the springs of exactly equal strength, conditions which can be nearly but not perfectly fulfilled. [illustration: plate ii photographs taken to test the accuracy of the "timing." ] a more important source of uncertainty arises from the fact that the time at which the spark takes place depends partly on the magnitude of the + and - charges which have been allowed to accumulate on the discharging knobs connected to the two leyden jars, for when these charges are larger, then the spark will be longer and will take place earlier and before the timing sphere has reached the mid-position. the charging has therefore to be carefully watched by means of the indications of a suitable electrometer, and the timing sphere must on each occasion be released when the charges have just reached the right value. but even this does not entirely suffice, for the passage of the spark depends also partly on the state of the surface of the knobs, which cannot be kept at any high degree of polish. still, when care is taken to keep the conditions as nearly as possible constant, neither of these sources of error is serious, and the reader can judge for himself of the accuracy of the timing from the photographs given on plate ii, in which a solid sphere was let fall in the dark room past a metre scale. the timing sphere was arranged, in the first four photographs, to illuminate it at the same stage in its fall, after a descent of thirty centimetres; if the timing had been perfect the sphere would appear on each occasion at the same mark on the scale. it will be observed that in the first, second, and fourth photographs the falling sphere is almost accurately bisected by the long line of the three-inch mark on the right-hand edge of the scale. the greatest difference of position being just about one millimetre (as read off the left-hand scale), which would correspond to an error of about / of a second. but the third photograph is earlier, showing the sphere · millimetres higher up, a distance which implies an error of just / of a second. a fifth photograph was then taken, with the timing arranged so as to illuminate the sphere one centimetre higher up, and it will be seen that if we compare this with no. , the error is again only one millimetre. thus nos. and agree very closely, but disagree with nos. , , and by about / of a second. the photographs themselves supply the reason. for there happens to be visible on each an (out-of-focus) image of the spark, and this image is very much the same in , and , but much larger and brighter in and , showing that the knobs were then more highly charged, which would account for the spark occurring a little too early. but when we are watching the splash made by the fall of a liquid drop, instead of a solid sphere, there is a new and more serious source of difficulty. for the drop as it lies on the smoked glass cup is not perfectly spherical, but is flattened by its own weight, as shown in fig. , and on the sudden removal of the supporting cup it oscillates between an oval form, elongated vertically, and a flattened form (see fig. ). these oscillations are unavoidable, and their extent will depend partly on the amount of adhesion between the smoked surface and the drop, and as this adhesion is never entirely absent and is variable, depending partly on the length of time that the drop has been lying in the cup, it follows that the drop will always receive a slight tug downwards at starting, which will be greater on some occasions than on others. on this account not only will the time taken to reach the water vary slightly, but the drop will strike it sometimes when elongated and sometimes when flattened, and the resulting splash will be affected by this circumstance. [illustration: fig. ] [illustration: fig. ] the four photographs on the next page were taken in succession in order to afford the reader an opportunity of judging for himself the sort of accuracy attainable when a liquid drop was concerned. the fall was centim., and the greatest discrepancy is · millimetres, corresponding to / of a second. thus even here the error does not amount to two-thousandths of a second. [illustration: photographs taken to test the timing of a falling drop. ] with higher falls the timing sphere is moving more quickly past the discharging knobs, and the error due to a longer or shorter spark is correspondingly less, so that it appears safe to say that the accuracy of the timing was such that, when all precautions were taken, any desired stage could be picked out within two-thousandths of a second. it is not however pretended that the precautions necessary for the most accurate timing were always taken, especially in the earlier series of photographs, for the main object of the experiments was to find out what happened, and only incidentally to ascertain exactly how long it took to happen, and there is no doubt that on some occasions, through the smoke-film being allowed to wear away, adhesions to the dropping cup occurred, with a corresponding disturbance of the timing, before the defect was noticed and remedied. [illustration: fig. photograph of the edge of a rapidly whirling disc.] it remains to mention, for the sake of those interested in photography, that notwithstanding the sensitiveness of the plates and the brilliance of the illuminating spark, its duration was so short that the negatives were always "under-exposed."[c] i have mentioned that the effective duration of the spark was less than three-millionths of a second. the evidence for this is the accompanying photograph (fig. ), taken of a cardboard disc when rotating at a rate of fifty-three turns per second; the disc was cm. in diameter, and had been roughly graduated round the edge with pen and ink. the photograph of the part that was in focus shows no perceptible blurring of the edge of the marks, and with a lens, a blurring of one-tenth of a millimetre would be easily detectable. since the edge was moving at a rate of · metres per second (about miles per hour), the time taken to traverse one-tenth of a millimetre would be rather less than three-millionths of a second. hence we may conclude that the illumination did not last so long as this. the weakness of the negatives was met by a prolonged development of about forty minutes in a saturated solution of eikonogen. this forbade the use of any artificial light, and all the photographic processes had to be conducted in absolute darkness. to avoid the tedium of long waiting in the dark room, a light-tight tray was constructed, in which several developing dishes could be placed, and the whole brought out into the daylight and suitably rocked. in this way ten or twelve photographs could be developed simultaneously. it may be worth while to mention here that the bright spark given by breaking the _primary_ circuit of an induction coil at the surface of mercury was found to be of much too long duration to be useful for the purposes of splash-photography. footnotes: [c] the plates which i have mostly used have been thomas's a ordinary. chapter ii the splash of a drop--low fall we will now turn to the photographic record itself. the first series shows the splash of a drop of water weighing · of a gram, and therefore · millimetres (or rather less than one-third of an inch) in diameter, falling cm. (about inches) into milk mixed with water. the object of adding milk to the water was to make it more visible. the addition of milk makes, as we shall see, a little but not much difference in the general character of the splash. the scale of the figures is three-quarters of the actual size. the number written against any figure gives, on the assumption that no unobserved error has crept in, the time in decimal parts of a second that has elapsed since the stage marked "_t = _," which is nearest to the first instant of contact. the reader will understand from what has been said that the error in any of these times may be as much as two-thousandths of a second, but is not likely to be more than that, when all precautions were taken. it will be observed that as the drop descends into the liquid the upper portion is at first not appreciably distorted, but that a little cup or crater of liquid is thrown up round it. as the drop descends further, this crater grows wider and higher and thicker in the wall, and jets are shot out from its edge or rim. these jets are visible even in the second figure. the black marks on the inside wall of the crater are due to the lamp-black carried down with the drop from the smoked surface of the supporting cup: though in one sense a disfigurement, they serve to show by their presence that the interior of the crater _is lined by the original liquid which formed the drop_, and thus afford useful information as to the nature of the flow. the crater rises with great rapidity up to fig. . in fig. the walls are beginning to grow thicker, while the next three figures show the crater subsiding and widening, till in fig. it lies as a mere ring of lobes on the surface, surrounding a central hollow. [illustration: series i milk into water ( cm.). scale / . t = · sec. · sec. · sec. · sec. · sec. · sec. · sec.] fig. shows the beginning of the rebound, in the rising of a central column. it will be seen that the lamp-black is now all swept to the middle, indicating that _the liquid of the original drop emerges at the head_ of the central column. full confirmation of this is obtained from fig. , which represents the emergent column obtained when the circumstances are all the same, except that we have a drop of milk falling into water instead of water falling into milk. it will be observed that the upper part only of the column is visible, precisely because it contains nearly all the milk of the drop, while the lower part, consisting chiefly of transparent water, remains invisible. [illustration: series i--(_continued_) · sec. · sec. · sec.] no. shows the column at its greatest height, and it should be noticed that figs. and show a tendency on the part of the head of this column to split off as a separate drop. [illustration: series i--(_continued_) · sec. · sec. · sec.] the column in subsiding forms a "cake" of liquid round the base. the edge of this circular cake (see figs. , , and ) is the first well-marked ripple spreading outwards in an ever-widening circle. [illustration: series i--(_continued_) · sec. · sec. · sec.] if fig. is reached without the top of the column having separated, then the splash follows the course shown in figs. -a to -a, in which it will be observed that the disappearance of the first column is very quickly followed by the rise of a secondary column very different in shape, which itself subsides again, but has not yet (in -a) formed, as it ultimately will, a second "cake" on the top of the first. thus the second ripple follows late after the first. [illustration: series i--(_continued_) alternative (_a_). -a · sec. -a · sec. -a · sec. -a · sec.] if, however, the summit of the primary column succeeds in breaking off (as in fig. -b), or even in very nearly breaking off, then the impact of this newly-formed drop forms a second slight crater on the top of the first cake, and we have the series ( -b to -b), in which it will be observed that the rim of the secondary crater spreads rapidly outwards, so that a second well-marked circular ripple in this case _quickly_ follows the first. the secondary column that is thrown up in fig. -b is very like that which emerged at a much earlier stage in the (_a_) series. the photographs of this (_b_) series show very beautifully the manner in which the advancing edge of the ripple degenerates into smaller ripples travelling with greater speed. [illustration: series i--(_continued_) alternative (_b_). -b · sec. -b · sec. -b · sec. -b · sec.] [illustration: series i--(_continued_) alternative (_b_). -b · sec. -b · sec. -b · sec.] it will be readily understood that if the splitting off of the head of the primary column happens to take place a little earlier, or on the other hand is nearly, but not quite, complete when it descends below the surface, then subsequent configurations will differ somewhat from either of the sub-series here shown. since any figure photographed might belong to either sequence, the disentanglement of the two series required careful consideration and long experimenting. the reappearance of the original drop at the head of the rebounding column, of which the explanation has been given in this chapter, is easily verified by naked-eye observation. let the reader when he next receives a cup of tea or coffee to which no milk has yet been added, make the simple experiment of dropping into it from a spoon, at the height of fifteen or sixteen inches above the surface, a single drop of milk. he will have no difficulty in recognizing that the column which emerges carries the white milk-drop at the top only slightly stained by the liquid into which it has fallen. in the same way naked-eye observation reveals the crater thrown up by the entry of a big rain-drop into a pool of water. in either case what we are able to glimpse is a "stationary" stage. the rebounding column reaches a maximum height, remains poised for an instant, and then descends. the same is true of the crater. it is the relatively long duration of the moment of poise that produces on the eye a clear impression where all else is blurred by rapid change. but there is frequently a curious illusion. we often seem to see the crater with the column standing erect in the middle of it. we know now that in reality the crater has vanished before the column appears. but the image of the crater has not time to fade before that of the column is superposed on it. those who are accustomed "to believe nothing that they hear and only half of what they see" may be glad to find at least the latter part of their maxim so completely justified. chapter iii principles involved the reader's attention has now been directed to various features which, with certain modifications, will be found in many of the splashes that we shall examine; but so far the language used has been simply descriptive and in no way explanatory. instead of going on to describe other splashes in the same way, and thus to accumulate a great mass of uncoördinated descriptive detail, it will be better to pause for a moment in order to become acquainted with certain principles connected with the behaviour of liquids, the application of which will go a long way towards explaining what we see going on in any splash. the first principle to be understood is that the surface layers of any liquid behave like a uniformly stretched skin or membrane, which is always endeavouring to contract and to diminish its area. if the surface is flat, like the surface of still liquid in a bowl, this surface-tension has only the effect of exerting a small inward pull on the walls of the bowl. but if the surface is curved, with a convexity outwards, then the surface layers, on account of their tension, press the interior liquid back, and thus tend to check the growth of any protuberance; while, on the other hand, if the surface is concave outwards, then the surface-tension tends to pull the interior liquid forward, and so to diminish the concavity. direct evidence of this surface-tension is easy to cite. we have it in any pendent drop, such as any of those shown in the accompanying figures. [illustration: water. turpentine. pendent drops (magnified - / times).] if we ask ourselves how it is that the liquid in the interior of one of these drops does not flow out, pressed as it is by the liquid above it, the answer is that everywhere the stretched skin presses it back. a soap-bubble too presses on the air in its interior, both the outside layers and the inside layers of the thin film being curved over the interior space. this is the reason that a soap-bubble blown on the bowl of a pipe will slowly collapse again if we remove the stem of the pipe from our mouth. the bubble drives the interior air back through the pipe. and it is easy to show that if two soap-bubbles be blown on the ends of two tubes which can be connected together by opening a tap between them, then the smaller will collapse and blow out the larger. the reason of this is that in the bubble of smaller radius the surface layers are more sharply curved, and therefore exert a greater pressure on the air within. thus if a strap be pulled at each end with a total tension t and bent over a solid cylinder of small radius, as in fig. , it is easy to see that the pressure on the surface of the part of the cylinder touched by the strap is less than if the strap be bent over an equal area on a cylinder of larger radius (fig. ). the tension of the surface layers of a liquid causes them to act on the liquid within, exactly as does the stretched strap on the solid in these figures. if at any place the liquid presents, as it generally does, not a cylindrical surface, but one with curvature in two directions, then the pressure corresponds to what would be produced by two straps crossing at right angles, laid one over the other, each with the curvature of the surface in its direction (fig. ). [illustration: fig. ] [illustration: fig. ] [illustration: fig. ] [illustration: fig. ] we can now understand why the drop that has been lying on the watch-glass should oscillate in its descent. the sharp curvature of the edge aa of the drop (see fig. ) tells us that the liquid there is pushed back by the pressure of the stretched surface layers, and when the supporting glass is removed the sides of the drop move inwards, driving the liquid into the lower part, the tendency being to make the drop spherical, and so to equalize the pressure of the surface at all points. but in the process the liquid overshoots the mark, and the drop becomes elongated vertically and flattened at the sides. this causes the curvature at top and bottom to be sharper than at the sides, and on this account the back-pressure of the ends soon checks the elongation and finally reverses the flow of liquid, and the drop flattens again. as an example of the way in which a _concavity_ of the surface is pulled out by the surface-tension may be cited the dimples made by the weight of an aquatic insect, where its feet rest on the surface without penetrating it. this same surface-tension checks the rise of the crater, and would cause it to subside again even without the action of gravity. thus the pressures of the sharply curved crater-edge on the liquid between the crater walls are indicated by the dotted arrows in fig. , and arise from the surface-tension indicated by the full arrows. during the early part of the splash the surface-tension is more important than gravity in checking the rise of the walls. for, as the numbers show, the crater of series i is already at about its maximum height in no. , i.e. about seven-thousandths of a second after first contact. in this time the fall due to gravity would be only about / of an inch. thus if gravity had not acted the crater would only have risen about / of an inch higher. the same reasoning applies to the rise of the central column, but here the curvature at the summit is much less sharp. the numbers show that the column reaches its maximum height in about / of a second after its start in no. , and in this time the fall due to gravity is about half an inch, so that gravity has reduced the height by this amount. [illustration: fig. ] the second principle which i will now mention enables us to explain the occurrence of the jets and rays at the edge of the crater and their splitting into drops. it was shown in by the blind belgian philosopher, plateau,[d] that a cylinder of liquid is not a figure of stable equilibrium if its length exceeds about - / times its diameter. thus a long cylindrical rod of liquid, such as fig. , if it could be obtained and left for a moment to itself, would at once topple into a row of sensibly equal, equidistant drops, the number of which is expressed by a very simple law, viz. that for every - / times the diameter there is a drop, or that the distance between the centres of the drops is equal to the circumference of the cylinder. [illustration: fig. ] the cause of this instability is the action of the same skin-tension that we have already spoken of. calculation shows, and plateau was able to confirm the calculation by experiment, that if through chance agitations lobes are formed at a nearer distance apart than - / times the radius, with hollows between as in the accompanying fig. , then the curvatures will be such as to make the skin-tension push the protuberances back and pull the hollows out. but if the protuberances occur at any greater distances apart than the length of the perimeter, then the sharper curvature of the narrower parts will drive the liquid there into the parts already wider, thus any such an initial accidental inequality of diameter will go on increasing, or the whole will topple into drops. [illustration: fig. ] at the last moment the drops are joined by narrow necks of liquid (fig. ), which themselves split up into secondary droplets (fig. ). [illustration: fig. ] [illustration: fig. ] what we have said of a straight liquid cylinder applies also to an annulus of liquid made by bending such a cylinder into a ring. this also will spontaneously segment or topple into drops according to the same law.[e] now the edge of the crater is practically such a ring, and it topples into a more or less regular set of protuberances, the liquid being driven from the parts between into the protuberances. now while the crater is rising the liquid is flowing up from below towards the rim, and the spontaneous segmentation of the rim means that channels of easier flow are created, whereby the liquid is driven into the protuberances, which thus become a series of jets. these are the jets or arms which we see at the edge of the crater. examination with a lens of some of the craters will show that the lines of easier flow leading to a jet are often marked by streaks of lamp-black in series i, or by streaks of milk in series ii. this explanation of the formation of the jets applies also to a similar phenomenon on a much larger scale, with which the reader will be already familiar. if he has ever watched on a still day, on a straight, slightly shelving sandy shore, the waves that have just impetus enough to curl over and break, he will have noticed that up to a certain moment the wave presents a long, smooth, horizontal cylindrical edge (see fig. -a) from which, at a given instant, are shot out an immense array of little jets which speedily break into foam, and at the same moment the back of the wave, hitherto smooth, is seen to be furrowed or combed (see fig. -b). the jets are due to the segmentation of the cylindrical rim according to plateau's law, and the ridges between the furrows mark the lines of easier flow determined by the position of the jets. [illustration: fig. -b fig. -a diagrams of a breaking wave.] the tendency of the central column of series i to separate into two parts is only another illustration of the same instability of a liquid cylinder. the column, however, is much thicker than the jets, and its surface is therefore less sharply curved, and consequently the inward pressure of the stretched curved surface is relatively slight and the segmentation proceeds only slowly. since this segmentation must originate in some accidental tremor, we see how it is that the summit of the column may succeed in separating off on some occasions and not on others. as a matter of fact, the height of fall for this particular splash was purposely selected, so that the column thrown up should _just_ not succeed in dividing in order that the formation of the subsequent ripples might not be disturbed by the falling in of the drops split off. but, as the reader will have perceived, the margin allowed was not quite sufficient. the two principles that i have now explained, viz. the principle of the skin-tension, and the principle of the instability and spontaneous segmentation of a liquid cylinder, jet, or annulus, will go far to explain much that we shall see in any splash, but it is well that the reader should realize how much has been left unexplained. why, for example, should the crater rise so suddenly and vertically immediately round the drop as it enters? why should the drop spread itself out as a lining over the inside of the crater, turning itself inside out, as it were, and making an inverted umbrella of itself? why when the crater subsides should it flow inwards rather than outwards, so as to throw up such a remarkable central column? these questions, which demand that we should trace the motion of every particle of the water back to the original impulse given by the impact of the drop, are much more difficult to answer, and can only be satisfactorily dealt with by a complicated mathematical analysis. something, however, in the way of a general explanation will be given in a later chapter. footnotes: [d] _statique expérimentale et théorique des liquides._ [e] see worthington on the "segmentation of a liquid annulus," _proc. roy. soc._, no. , . chapter iv the splash continued i have stated that the addition of the milk to the water made but little difference in the character of the resulting splash. it does, however, make certain differences in detail, as will be gathered from an examination of the next series i-a, which shows the effect of letting the water-drop fall from the same height into water instead of into milk. such a splash is difficult to photograph unless the illumination is from behind. as shown in this way, the early figures of the crater might be unintelligible to the reader had he not already studied the same crater lighted up from the side. sometimes, though the front of the crater is hardly visible directly, yet every lobe on it can be clearly traced in the inverted image seen by reflection. the most noticeable difference between the two splashes is perhaps the very much greater number of ripples seen with the splash in pure water. this is partly because, with the illumination behind, such ripples are more easily visible, but arises chiefly from the fact that ripples are not so readily propagated over the surface of milk on account both of its smaller surface-tension and its greater viscosity. the first appearance of outward-spreading ripples is in no. , just round the subsiding crater. [illustration: series i-a water into water ( cm. fall). scale / . t = · sec. · sec. · sec.] [illustration: series i-a--(_continued_) · sec. · sec. · sec. · sec.] since the origination of these ripples is an interesting phenomenon from a physical point of view, as throwing light on the dispersion of waves travelling with different velocities, special precautions were taken to secure the most favourable conditions, and in order to clean the surface after the arrival of each drop, which inevitably brings down a little adherent lamp-black, a continuous slow stream of fresh water was maintained which swept the contaminated surface-liquid away over the edge of the vessel. the effect of this precaution is seen by a comparison of the photographs no. and no. -a. in the first the surface was kept quite clean in the way described; in the second it had only been cleaned by skimming it with a fine wire-gauze dish. the beginning of the descent of the first central column seems to be marked by the appearance of a slight depression round its base, which has just not begun in no. -a, and has just begun in no. , and goes on increasing in figs. and . [illustration: series i-a--(_continued_) running water. scale reduced to / . · sec. · sec. · sec. -a · sec.] [illustration: series i-a--(_continued_) running water. scale / . · sec. · sec. · sec. · sec.] the same feature marks the beginning of the descent of the secondary central column, which is still rising in fig. , is just poised in fig. , and thence onwards shows a gradually increasing central depression. these last four figures carry us to a rather later stage than was reached in series i. it should be noticed that in this series the water-drop used was of smaller diameter than that of series i, weighing · grams as against · grams. by employing the smaller drop, we diminish irregularities due to oscillations of form set up on release, for the smaller drop is more spherical when lying on the dropping cup than the larger; a few photographs taken for comparison with the full-sized drop showed, however, extremely little difference in the splashes at this height of fall. [illustration: series i-a--(_continued_) still water. scale / . · sec. · sec. · sec.] [illustration: series i-a--(_continued_) still water. scale / . · sec. · sec. · sec.] chapter v higher falls--bubble-building it might well be expected that the effect of increasing the height of fall of our drop to cm. would be simply to emphasize the phenomena already observed, and to obtain a higher crater and a taller rebounding column. such an expectation would be mistaken. a new phenomenon makes its appearance. the crater does indeed rise to a greater height, but its mouth closes so as to form a bubble on the surface of the liquid. if the height be not too great the closing is either incomplete or at any rate only temporary, and the bubble reopens at the top to make way for the column which rises as before from the base, but is now much thicker and hardly so high as before. in the series ii, which is now given, the drop was of milk, · mm. in diameter, and fell cm. into water. photographs and --to which is added -a, though taken under slightly different conditions--show that the drop on entering punches a sheer-walled hole, for the fine line of light seen above the level of the top of the drop in figs. and -a marks the circular cliff-like edge of the as yet undisturbed liquid. up the vertical sides of this circular pit the liquid of the drop is streaming. this cliff is highest and perhaps clearest in fig. -a. the closing of the mouth of the crater, which is just beginning in fig. , is to be explained as follows. if the crater were a simple thin-walled cylinder of liquid, it would contract under the influence of the surface-tension just as does a soap-bubble, but not so fast, since the walls have only a horizontal curvature. if the wall is thinner above than below, then the upper part will contract faster than the lower, through there being less liquid to accelerate. now the supply of liquid is from below, and will thicken the lower part of the walls first, and thus account for the faster closing of the mouth. on the other hand, the uppermost edge of the crater is the place where the checking influence of the surface-tension on the upward flow is first felt, with the result that the edge of the rim is thickened by the influx from below, so that a more or less regular rope-like annulus is formed round the edge. now calculation shows that such an annulus, so long as its thickness is not more than · times the thickness of the wall below, will contract quicker than the wall, and this will tend to close the crater, somewhat as a bag would be closed by the contraction of an elastic cord round the mouth. this rope-like thickening of the edge is to be seen in figs. and , and especially in figs. and of series iii on page . [illustration: series ii milk into water ( cm. fall). · sec. -a · sec. · sec. · sec. · sec. · sec. · sec.] the photographs , , and (obtained after adding a little milk to the water in order to render it more visible) were at first very puzzling. what happens is that the bubble sometimes reopens very soon (or perhaps does not quite close) as in fig. , and makes way for the column which rises from the base exactly as in the previous series. this column may be dimly seen through the walls of the bubble in fig. , and no. shows the column alone, the bubble having opened early and receded with great velocity, a few drops round the base being all that is left of it. nos. -a and -b illustrate this reopening. in -a the milk-drop was allowed to fall again into quite pure water, and the photograph shows very beautifully the summit of the column, with the original milk-drop at the top, emerging through the reopening mouth of the bubble; and fig. -b shows the same at a very slightly later stage when the bubble has completely retreated. [illustration: series ii--(_continued_) · sec. · sec. · sec. -a · sec. -b · sec.] in fig. the bubble has been too firmly closed to reopen, and the summit has been struck by the column within. the next figure (no. ) shows how in such a case the emergent column becomes entangled in the liquid of the bubble when it bursts. under the influence, however, of the surface-tension, which pushes back the protuberances and pulls out the hollows, regularity of form is soon regained. thus fig. shows the emergent columns at a later stage after such an encounter, already much more symmetrical; and the subsequent photographs (for which a good deal of milk was added for the sake of greater visibility) show a column of uniformly sedate and respectable rotundity, betraying no traces of any youthful irregularities. [illustration: series ii--(_continued_) · sec. · sec. · sec. · sec. · sec.] [illustration: series ii--(_continued_) · sec. · sec. · sec. · sec.] series iii shows the effect of still further increasing the height of fall of the water-drop (to cm., or about ft. in.), and at the same time doubling its volume so that it now weighs · gram. the crater now closes in about / of a second, and forms a comparatively permanent bubble. the rope-like thickening of the edge, already alluded to, is well seen in figs. and . in its earlier stages the bubble is thick-walled, rough, and furrowed, but becomes smoother and thinner the longer it lasts, both because the liquid drains down the sides and because it becomes more uniformly distributed under the equalizing influence of the surface-tension. [illustration: series iii water-drop weighing · grams falling cm. ( - / feet) into milk mixed with water. scale / . t = · sec. · sec. · sec.] such a bubble may remain long closed, as in fig. , becoming every moment more delicate and exquisite, or it may open at an even earlier stage, as in fig. . there is a characteristic difference between the arms of a closing and of an opening bubble. it will be noticed that up to the moment of closing the arms slope outwards. the upper portions have been projected at an earlier stage when the mouth of the crater was wider open and the flow was either actually outwards or more nearly vertical; then as the mouth contracts the arms are left behind in the upper parts. [illustration: series iii--(_continued_) · sec. · sec. · sec. · sec.] in an opening bubble, on the other hand, the arms are at first vertical, and later have the very characteristic inward slope of the last figure, which is also well seen in fig. -a of the last series. here the edge of the opened bubble retreats outwards and downwards, leaving the arms behind. such is the origin of the bubbles raised by the big drops of a thunder shower on the surface of a pool. the building of each fairy dome is accomplished in less than two-hundredths of a second, and before one-tenth of a second has elapsed the whole construction may have vanished. one can almost regret that so beautiful a process should have been so long unwatched. to build these bubbles a large drop is essential. with a drop weighing only · of a gram, even though it fall from a height of cm., there is no bubble, and the splash is almost exactly that of series i-a. the exact time required for the closing of the bubble probably depends a good deal on the phase of oscillation of the drop at the moment of entry, and, as already observed, a big drop, which lies very flat in the dropping cup, is set vibrating more strongly on liberation than a small one. we shall see in chapter vii that the impact of a rough solid sphere, if falling from a sufficient height, produces a very exquisite bubble; in this case irregularities due to oscillation are absent, and the closing can be timed with greater precision. [illustration: series iii--(_continued_) · sec. · sec. · sec.] [illustration: fig. arrangement for taking photographs below the surface of the liquid.] chapter vi below the surface[f] our investigation has so far been limited to what we can see from above the surface of the liquid; nor perhaps would it occur to any one acquainted only with so much as we have yet examined that it might be worth while to look below the general level of the surface. the discovery, however, that when the splash is made by a solid sphere very remarkable phenomena, which will be described in the next chapter, take place below the surface, led at a much later date to a similar examination in the case of a liquid drop. a suitable arrangement of the apparatus in the dark room is shown in the accompanying diagram (fig. ). the water into which the drop is to fall is placed in a thin glass vessel ab, with parallel sides. (an inverted clock-shade makes a very convenient vessel.) the water fills the vessel to the brim, and is allowed to overflow it in a steady stream, thus presenting a surface which, being perpetually renewed, is maintained perfectly clean. close behind the vessel is a plate p of finely roughened glass, on which the light from the spark-gap f, in front of its concave reflector m, is thrown by means of the condenser lens l taken from an optical lantern. this provides a very uniformly illuminated background against which the splash is viewed by means of the camera c, whose optic axis is horizontal, either a little below the level of the liquid surface or at that level. by having it just at the level of the surface we secure simultaneous pictures of what is going on both above and below the surface. there is, to be sure, a narrow band or region of confusion stretching across the photographs in which the images obtained by reflection, both external and internal, overlap the direct images, and it should also be mentioned that the two pictures will not be quite in focus together, for the optical effect of the water, through which the part below the surface is viewed, is to bring the image forward. the photographs of series iv were obtained in this way from the splash of a drop of water weighing · grams falling cm. into water. (the same splash as that of series i-a.) the perfectly spherical form presented by the cavity below the surface is very remarkable. in the present case, this spherical cavity when at its deepest, as in fig. , would contain about fifty of the original drops, and in other cases--e.g. with a drop of / the volume, falling from cm.--the cavity would contain as many as of the original drops. in figs. , , and the depth of the cavity is nearly constant, but the diameter is steadily increasing. the spherical form, however, is still maintained. the last figure shows the central column just beginning to rise. [illustration: series iv the splash of series i-a viewed below the surface. t = · sec. · sec. · sec. · sec. · sec. · sec. · sec.] there can be no doubt that the liquid of the original drop is spread out in an excessively thin lining over the interior of this sphere. the reader has seen for himself part of the evidence in the streaks of milk that are carried up the inner walls of the crater when a milk-drop falls into water (series ii); in the streaks of lamp-black that are carried there when the drop is of milk, and it may here be mentioned that other photographs that cannot be reproduced here have enabled me to trace the gradual deformation of the drop into this thin layer and show that it passes through configurations like figs. , , and . [illustration: fig. ] [illustration: fig. ] [illustration: fig. ] it appears possible that the study of this remarkable spherical excavation may afford a clue that will lead to a solution of the very difficult hydro-dynamical questions involved, and the matter is still being investigated. footnotes: [f] the information conveyed in this chapter was first published in a communication to the mathematical and physical section of the british association at leicester in . chapter vii the two kinds of splashes of solid spheres in the present chapter will be described the splash that follows the entry of a _solid_ sphere falling vertically into a liquid from a small height, and i should like to persuade the reader, if possible before he begins to read, or at any rate afterwards, to make a very simple experiment. let a few child's marbles be taken--not glass "marbles," for these are seldom round enough or smooth enough, but what are sold in the toy-shops as "stone" marbles--and let one of these be well rubbed and polished with a dry handkerchief, and then dropped from a height of about cm., or, say, foot, into a deep bowl or basin of water, the bottom of which may be conveniently protected from breakage by a few folds of fine copper gauze. if the polishing has been good, and the surface of the sphere has not been dimmed by subsequent handling with hot or greasy fingers, it will be observed that the splash is singularly insignificant, the sphere slipping noiselessly into the liquid with very little disturbance of the surface. but if the same sphere be fished out of the water, and again let fall from the same height without being first dried, or, better still, if another marble be taken, which has been previously roughened with sand-paper, the resulting splash is totally different. there is now a noise of bubbles, which may be seen rising through the liquid, and a tall jet is seen to be tossed into the air. ( ) the splash of a rough sphere. to understand the cause of this really surprising difference we must turn to the photographic record, and we will take first the case of a rough sphere falling into water to which milk has been added for the sake of clearness in the photographs. the diameter of the sphere was · cm. (or / inch), and the height of fall cm., or just about inches. the sphere on each occasion was fished out, redried, and re-roughened with sand- or emery-paper. examination of the first photographs of series v shows that the liquid, instead of flowing over and wetting the surface of the sphere, is driven violently away, so that as far as can be seen from above the upper portion is, at first at any rate, unwetted by the liquid. the crater that is subsequently formed is very similar to that which was thrown by the liquid drop in series i, the main difference being that in the present-case the crater is thinner in the wall and more regular. this greater regularity is chiefly to be attributed to the fact that the solid sphere enters the liquid with a true spherical form, and is not distorted by the oscillations and tremors which disturb a falling drop. the gradual thickening of the wall and the corresponding reduction in the number of lobes as the subsidence proceeds is beautifully shown in figs. , , , and , the last-mentioned figure being hardly distinguishable from the corresponding fig. of series i, p. . this stage is in each case reached in about / of a second. [illustration: series v rough sphere. "basket splash." diameter of sphere, · centim. height of fall, centim. t = · sec. · sec. ] [illustration: series v rough sphere--(_continued_). · sec. · sec. · sec. · sec.] now from the depths of the crater there rises with surprising velocity the exquisite jet of fig. , which in obedience to the law of segmentation at once splits up in its upper portion into little drops, while at the same time it gathers volume from below, and rises ultimately as a tall, graceful column to a height which may be even greater than that from which the sphere fell. this is the emergent jet which one sees with the naked eye whenever a sufficiently rough sphere is dropped from a small height into water, but if we are to ascertain how this column originates, we must follow the sphere below the surface of the liquid. the arrangement already described on p. enables this to be done. we let the sphere fall into clear water contained in a narrow, flat-sided, inverted clock-shade and illuminate this from behind while the camera stands straight in front. [illustration: series v rough sphere--(_continued_). · sec. · sec. · sec. · sec.] in this manner were obtained the photographs of series vi, which require a little explanation. in the first figure we see the sphere just entering the liquid. the faint horizontal line shows the level of the surface. above this line we see the internally reflected image of the part that has already entered, while still higher in the figure may be discerned the summit of the sphere itself. the slight lateral displacement of the part below the surface is due to refraction consequent on the camera having been set with its optic axis not quite perpendicular to the face of the vessel. in the subsequent figures it will be observed that the sphere, as it descends, drags with it the surface of the liquid in the form of a gradually deepening pocket or bag, the upper part of the sphere being for a long time quite unwetted by the liquid. the sides of this pocket or bag of air not being quite smooth, give a somewhat distorted appearance to the sphere within. also, since the sides are sloping, their reflected image in the level surface slopes in the opposite direction and produces an angle where the two meet. this angle marks very clearly the level of the surface. above the surface-line in figs. to is seen the beaded lip of the crater which we have already viewed from above, but this is somewhat out of focus, for the camera had to be focused on the sphere as seen under water, and the effect of the water is to bring the sphere optically nearer. hence only the nearer part of the crater, i.e. the middle part of the front edge, is distinctly shown. [illustration: series vi the splash of a rough sphere as seen below the surface. diameter, · centim. height of fall, centim. t = · sec. · sec. · sec. · sec.] coming now to fig. , we perceive that the long cylindrical hollow has begun to divide. in this spontaneous division we have another illustration of the law of instability which regulated the sub-division of the jets and columns of earlier series. this law is the same whether the cylinder be of air surrounded by liquid or of liquid surrounded by air. hitherto we have only seen it operating in jets of liquid in air; now we have a jet of air in a liquid. the lower part of the long cylinder of air splits off into a bubble just behind the sphere, and follows in its wake to the bottom of the vessel, and is only detached and rises to the surface when the sphere strikes the bottom. many years ago, through the kindness of the curator of the brighton aquarium, i was enabled to watch this bubble of air following in the wake of the sphere to the bottom of the deepest tank. figs. , , and show the two parts gradually separating. [illustration: series vi--(_continued_) scale reduced to about / . · sec. · sec. · sec.] fig. shows specially well the ripples on the surface of the descending bubble. these undulations sometimes become so accentuated that the upper part of this descending bubble is detached, and then the curious phenomenon may be seen of this detached part still following the rest downwards through the liquid with an unsteady, lurching motion. meanwhile the upper half of the divided air-column is seen in fig. to resemble a deep basin which now rapidly fills up by the influx of liquid from all sides. it is from the confluence of this inflowing liquid into channels which necessarily narrow as the centre is approached that the great velocity with which the liquid spirts upwards is obtained. in fig. the jet is just discernible above the surface, and in fig. it is well-established. [illustration: series vi--(_continued_) · sec. · sec. · sec.] on increasing the height of fall of a rough sphere to cm., we obtain a higher crater which closes and forms a bubble, exactly as when we increased the height of fall of a liquid drop. the process as viewed from above the surface is shown in series vii. the first figure of this series shows very well how completely the liquid is driven away from the surface of the sphere the first moment of contact. the subsequent crater and bubble are of exquisite delicacy. this bubble, though it closes completely as in the last figure, is doomed to almost immediate destruction. for we see, on looking below the surface, that the proceedings there are of the same kind as in the case of the lower fall already described, and result in the formation of an upward-directed jet. [illustration: series vii rough sphere falling cm. scale / . t = · sec. · sec. · sec. · sec.] thus the first three figures of series viii show the last moments of a bubble which has burst, spontaneously, and so has made way for the jet of fig. . (these are taken from a splash into petroleum with · cm. fall.) but the last two figures, and (taken with a cm. fall), show how a bubble which might otherwise have been permanent, is stabbed by the rising jet and destroyed. with water and cm. fall the jet appears sometimes to rise quite unimpeded, and sometimes to be checked by the still closed bubble. before leaving the splash of a rough sphere, i desire to call the reader's attention to another point. such figures as , , and of series v, p. , show that the surface of the liquid beyond the walls of the crater is still flat and undisturbed; yet we now know from the corresponding figs. , , and of series vi, p. , that a large volume of liquid has been displaced, much larger than the quantity required to form the crater wall. the inference is that the level of the surface has been slightly raised even at a great distance from the place of the splash. figs. , , and of series vi themselves confirm the impression of the undisturbed flatness of the surface at even a small distance from the splash. ( ) the splash of a smooth sphere. the reader who has been sufficiently interested to make for himself the simple experiment suggested at the beginning of this chapter, will have already realized that the splash of a smooth sphere is totally different from that of a rough one. the photographs of series ix show that the difference is quite pronounced from the first instant of contact. in this series the sphere was of polished stone · cm. in diameter and fell cm. the scale of magnification is / . the second figure shows that the liquid, instead of being driven away from the surface as was the case with a rough sphere, now rises up in a thin, closely-fitting sheath which (see fig. ) completely envelops the sphere even before its summit has reached the water-level. figs. and show the comparatively insignificant column that remains to mark the spot where the sphere has entered. fig. was the result of a lucky accident, which left the sphere rough on the right-hand side, smooth on the left. nothing could show better than this photograph the essential difference between the two splashes. [illustration: series viii rough sphere. splashes viewed below the surface. the bursting of the bubble. · sec. from a splash into petroleum · cm. fall. · sec. · sec. from a splash into petroleum cm. fall. · sec. · sec.] the reader's attention is directed to the remarkably deep furrows which characterize the whole sheath in fig. and the left-hand (smooth splash) part in fig. . about these furrows we shall have something to say later. a better idea of the extreme thinness of the enveloping sheath is obtained when the illumination is from behind as in series x, in which the sphere was of highly polished serpentine stone · cm. (or just over inch) in diameter, the fall being cm. (or not quite inches). [illustration: series ix the "sheath" splash of a smooth sphere. t = · sec. · sec. · sec. · sec. ] examination of either series ix or series x shows that with the smooth sphere as with the rough the amount of water lifted above the surface in the immediate neighbourhood of the splash is much less than the whole volume displaced, so that we are again driven to the conclusion that the surface at even a considerable distance must be bodily lifted without its flatness being sensibly disturbed. this conclusion was confirmed by a direct experiment. the not very wide vessel of fig. a was taken and filled brimful with milk, and the lower edge of a card millimetre scale was placed just in contact with the liquid surface at one side. the reader should notice that the liquid is not quite up to the level of the spout on the right-hand side of this figure. then the sphere was dropped in and the photograph of fig. b was taken when the sphere was about two-thirds immersed. the rise at the edge of the scale is about millimetres, and there is an apparently equal rise at the spout, where, however, the surface appears quite flat. [illustration: fig. a] [illustration: fig. b] it seems probable, then, that whenever a stone is thrown into a lake the impulse accompanying its entry travels with the velocity of a compressional wave (i.e. with the velocity of sound) through the liquid, and is therefore almost instantly felt and produces a minute rise of level even in remote parts of the lake long before the arrival of any ripple or surface disturbance. [illustration: series x polished serpentine sphere falling cm. into water. · sec. · sec. · sec. · sec. · sec. · sec.] it may here be observed that whether the sphere be rough or smooth, its size makes little or no difference in the character of the splash, within a range of diameter from to millimetres--i.e. from about / inch to about - / inches. no doubt with a very large sphere, taking a long time to enter, the splash would be controlled more by gravity than by surface-tension, but so long as the sphere is within the limits mentioned this is not the case unless the height of fall be made very small indeed. chapter viii the transition from the smooth or "sheath" splash to the rough or "basket" splash the influence of velocity. if we gradually increase the velocity with which a well-polished sphere enters the liquid we find that there is a gradual transition from the silent "smooth" or "sheath" splash taking down no air and giving rise to only an insignificant column, to the noisy, "rough," "basket" splash taking down much air and throwing up a tall and conspicuous jet. thus in the fourth figure of series xi, in which the height of fall has been increased from to cm. (i.e. from inches to feet), the sphere being of polished ivory, we see that the enveloping sheath has in many places broken away from the surface before the summit has been covered. it is well known that a sphere moving through a liquid pushes away the liquid in front of it, which flowing round closes in at the back of the sphere. although the surface round the column of fig. is still very flat, the liquid just below it must be streaming inwards,[g] as is indicated by the radial striæ. to the meeting of these converging streams we must attribute the large access of liquid that is forced up into the column, whose subsequent toppling into drops is accompanied by the curious, characteristic, lop-sided curvature of the later figures. [illustration: series xi polished ivory sphere, · centim. in diameter, falling cm. into water mixed with milk. t = · sec. · sec. · sec. · sec. · sec.] series xii shows how even with a very highly polished metal sphere falling into water from the still greater height of cm. the characteristic sheath of the "smooth" splash is no longer so closely fitting even at an early stage, but is beginning to resemble the earlier stages of the basket-shaped crater of the "rough" splash; yet no air was taken down at this height. the transition was also watched by means of photographs taken below the water-line. it may be well here to guard the reader against a possible misconception. the curved outline of the liquid in these photographs does not represent the path followed by the particles. each particle must have travelled in a nearly straight line from the moment it left the surface of the sphere, and must still be moving upwards and outwards. gravity has not had time to produce any sensible displacement. this applies also to the curved outlines in many other early figures. influence of the condition of the surface. by very careful rubbing of such a polished, steel sphere, it was found possible to increase the height of fall to · cm. (well over feet) and yet to secure a perfectly "airless," "smooth" splash. but the equilibrium of the splash, if i may use the phrase, is, at this high velocity of entry ( cm. per sec., or about feet per sec.), very unstable, and was found to depend on minute differences in the condition of the surface. how minute this difference may be, which yet makes the whole difference in the character of the splash, may be gathered from the following extract from the original paper:-- "a polished steel sphere · cm. in diameter was found (by naked-eye observation) to give an airless splash when falling into water from a height of · cm.; at · cm., there was much air taken down. this observation at · cm. was repeated three times, observer c. doing the polishing. then observer w. polished, and the splash was first _nearly_ airless and then _quite_ airless. then, by persevering in the rubbing, the height of fall was gradually raised to · cm., and a perfectly airless splash was secured, and even at · cm. the record was 'very little air indeed.' "again, a polished marble sphere · cm. in diameter falling into water from a height of cm. was found to take down 'much air' when rubbed with a certain clean handkerchief a, and 'none at all, or only very little,' when rubbed with clean handkerchief b. this result was confirmed four times with b and five with a. these handkerchiefs were subsequently examined under the microscope, but were found to be extremely similar, and the cause of the difference remained for the time beyond conjecture. "on another occasion, of two similar nickel-plated steel spheres, each millimetres in diameter, and each treated in exactly the same way, falling cm. into paraffin oil, one would always take down much air and the other little or none, and again microscopic examination showed only a very slight difference in the surfaces." [illustration: series xii smooth sphere of polished serpentine falling centim. into water. scale / . t = · sec. · sec.] by wetting the surface of a smooth sphere we can always convert a smooth or "sheath" splash into a rough or "basket" splash. thus when the ivory sphere (which when dry and well-polished gave, with a fall of cm., the splash of series xi, p. ), was allowed to fall _wet_ into the liquid, all other circumstances remaining the same, the splash of series xiii, p. , was obtained, which is entirely different from the first. the wetting was effected by dipping the sphere into the bowl of milky water into which it was to fall, and then shaking off as much as possible of the adherent liquid, but in all cases the splash quickly became unsymmetrical, probably through the liquid, during the fall, drifting to one side of the sphere. influence of the nature of the liquid. the nature of the liquid employed has a great influence in determining whether at a given height the splash shall be "rough" or "smooth." thus with paraffin oil the maximum height that could be reached with an airless splash with highly polished nickel-plated spheres, well rubbed on a selvyt cloth, was found to be only · cm. (about inches), but, with water, a fall of cm. (over feet) could be reached. the paraffin oil used in these experiments had, at a temperature of °· centigrade, a specific gravity · and a surface-tension about · of that of water. since only a small increase of height was required with this liquid to make a smooth sphere give the same splash as a rough one, this liquid was found much more convenient than water in investigating the transition. when water is made more viscid by the gradual addition of glycerine,[h] the surface-tension and the specific gravity are but little altered though the viscosity is steadily and sensibly increased. an admixture of two parts of glycerine to fifty-one of water produced no perceptible change in the splashes observed. when the glycerine was increased to six volumes in fifty-one of water, though this made the viscosity half as great again, the change was noticeable but still slight, the chief difference being, with a smooth sphere, the greater salience of the ribs or flutings in some of the earlier stages of the glycerine splash, and the much greater reluctance of the subsequent jets to topple into droplets. this latter feature is well seen in the first figure on page , showing the entry of a smooth sphere of polished serpentine stone into this glycerine mixture from a height of cm. [illustration: series xiii splash of a smooth wet sphere. t = · sec. · sec. · sec.] with pure glycerine, which is much more viscous, the splash of the same polished serpentine sphere falling from cm. (about - / feet), is shown in series xiv. in the original photographs the radial furrows on the right-hand side of fig. are very pronounced, and even in fig. the fluting of the film is seen to be already well developed on the left-hand side; but these details have proved rather too delicate for reproduction in the plate. two photographs taken of stage had each of them an isolated jet, owing probably to the fact that when working with so sticky a liquid it was difficult to avoid contaminating the cloth on which the sphere was each time repolished after washing in water, with the result that the spheres behaved as if locally rough. the relatively great length and height of this jet brings out well the part played by viscosity, both in delaying segmentation into droplets and also in hindering the flow of the rest of the liquid sheath which has remained in contact with the sphere. with a rough sphere falling into pure glycerine from the same height of cm., except for an occasional jet that may escape as in fig. of series xv, the proceedings are uneventful, as a glance at the series will show. with the same height of fall into water we should have had an exquisite crater fringed with a multitude of fine jets, and ultimately closing to form a bubble. we thus see how little play is given to the action of the surface-tension in a very viscous liquid. [illustration: polished stone sphere falling centim. into water mixed with glycerine.] [illustration: series xiv polished stone sphere falling centim. into pure glycerine. scale / . ] [illustration: series xv rough sphere falling centim. into pure glycerine. scale / . t = · sec. · sec. · sec.] the influence of temperature. it was found that if a polished sphere was heated in boiling water, quickly rubbed dry, and let fall while still hot, a very marked difference was produced. with the sphere hot, the height of fall can be much increased before the splash becomes "rough." thus with paraffin oil, the height with a nickel-plated sphere rose from · cm. to · cm., and with water from cm. to cm. the remarkable influence of a flame held near the liquid, and traversed by the sphere in its fall. in our search for the explanation of the difference between the rough and the smooth splash, it occurred to us to let the smooth sphere drop through a flame held near the liquid, and the result was very remarkable. with paraffin oil (and the sphere hot) the airless height now rose from · cm. to · cm., and with water and a cold sphere, it rose from cm. to over cm., which was the greatest height that the laboratory would permit. either the luminous flame of a bat's-wing burner or the flame of a bunsen burner held nearly horizontal produces the effect, provided the flame is held near enough to the surface of the liquid, and it is a very striking experiment to let the polished sphere fall several times from a height which gives a large volume of bubbles rising with much noise to the surface, and then to let it fall through the flame, and to observe the complete change in the phenomenon. on a sphere already roughened the flame has no observable effect. the supposition of electrification tested and rejected. the behaviour with a flame led at first to the supposition that we had to deal with an electrical phenomenon, for a flame would certainly discharge completely any electrified sphere passing through it, and it appeared reasonable to suppose that the sphere might become electrified by friction with the air through which it fell. it required a long series of experiments, into the details of which i need not now ask my readers to enter, to prove that this tempting explanation was untenable, and that there was no reason to believe that electrification had anything to do with the matter. experiments in vacuo. it remained to examine what part was played by the air in the whole transaction. this could only be settled by removing the air and letting the spheres, whether rough or smooth, fall through a vacuum into the liquid, or rather through a space occupied only by the vapour of the liquid in use. instantaneous photographs obtained under these conditions showed that the presence of the air has no material influence on the early course of the splash, and that a sphere which gives a "smooth" splash in air will give a "smooth" splash in vacuo, while if the splash is "rough" in air, it will also be "rough" in vacuo. footnotes: [g] some useful information about the internal flow of the liquid was obtained by the device of letting the sphere descend between two slowly ascending streams of very minute bubbles liberated by electrolysis at two electrodes placed in the liquid. these streams, initially straight and vertical, were displaced and distorted as the sphere passed near them and afforded a measure of the displacement of the fluid at different points. for details see _phil. trans. roy. soc._, vol. , p. ( ). [h] glycerine was found to be a rather treacherous liquid, requiring special precautions for which the reader who desires details is referred to the original memoir. _phil. trans. roy. soc._, series a, . vol. , p. . chapter ix the explanation of the cause of difference between the two splashes i have some hope that, by the enumeration of the many surprising and puzzling facts mentioned in the last chapter, i may have succeeded in producing in the mind of my reader some sympathy with the state of perplexity of mr. cole and myself when, after four years of experimenting, we found ourselves still unable to answer the question, "why does the rough sphere make one kind of splash, the smooth sphere another kind?" by reflecting, however, on all the facts at our disposal, we were at last led to what seems to be an entirely satisfactory explanation, and one moreover which we were able to test by further experiment. this explanation may be stated as follows:-- when a sphere, either rough or smooth, first strikes the liquid, there is an impulsive pressure between the two, and the column of liquid lying vertically below the elementary area of first contact is compressed. for very rapid displacements the liquid on account of its viscosity behaves like a solid. in the case of a solid rod we know that the head would be somewhat flattened out by a similar blow, and a wave of compression would travel down it; to this flattening or broadening out of the head of the column corresponds the great outward radial velocity, tangential to the surface, initiated in the liquid, of which we have abundant evidence in many of the photographs. (see pp. , , and .) into this outward-flowing sheath the sphere descends, and since each successive zone of surface which enters is more nearly parallel to the direction of motion of the sphere, the displacement of liquid is most rapid at the lowest point, from the neighbourhood of which fresh liquid is supplied to flow along the surface. whether the rising sheath shall leave the surface of the sphere, or shall follow it, depends upon the efficiency of the adhesion to the sphere. if the sphere is smooth and clean, the molecular forces of cohesion will guide the nearest layers of the advancing edge of the sheath, and will thus cause the initial flow to be along the surface of the sphere. to pull any portion of the advancing liquid out of its rectilinear path the sphere must have rigidity. if the advancing liquid meets loosely attached particles, e.g. of dust, these will constitute places of departure from the surface of the sphere; the dust will be swept away by the momentum of the liquid which, being no longer in contact with the sphere, perseveres in its rectilinear motion. if the dust particles are few and far between, the cohesion of the neighbouring liquid will bring back the deserting parts, but if the places of departure are many, then the momentum of the deserters will prevail. thus at every instant there is a struggle between the momentum of the advancing edge of the sheath and the cohesion of the sphere; the greater the height of fall the greater will be the momentum of the rising liquid, and the less likely is the cohesion to prevail, and the presence or absence of dust particles may determine the issue of the struggle. roughness of the surface will be equally efficient in causing the liquid to leave the sphere. for the momentum will readily carry the liquid past the mouth of any cavities (see fig. ), into which it can only enter with a very sharp curvature of its path. it is to be observed that the surface-tension of the air-liquid surface of the sheath will act at all times in favour of the cohesion of the sphere, and even if the film has left the sphere the surface-tension will tend to make it close in again, but we should not be right in attributing much importance to this capillary pressure which, with finite curvatures, is a force of a lower order of magnitude than the cohesion, and, as the photographs now to be shown will clearly show, is incompetent to produce the effects observed. [illustration: fig. ] having arrived at this general explanation, we proceeded to test it. experiments on the influence of dust. in the first place, to test the influence of dust, the experiment was made of deliberately dusting the surface of the sphere. for this purpose a highly polished nickelled sphere was held in a pair of crucible tongs by an electrified person standing on an insulating stool, and by him presented to any dusty object that stood or could be brought within reach. particles of dust soon settled on the electrified sphere, which was then carefully placed on the dropping ring with the dusty side lowest. the liquid used was paraffin oil, and the height of fall was · cm., at which this sphere when not dusted gave always a quite airless splash. when dusted an enormous bubble of air was carried down on each occasion. although the sphere when laid on the dropping ring must have completely lost the electrical charge, yet it seemed worth while to go through the same electrifying process without dusting. the result showed that no change was produced. in order to see how far the influence of dust would go, the height of fall was now reduced, and it was found that with sphere ( ) a fall of · cm. gave a perfectly rough splash when the surface was visibly dimmed with fine dust, and with a second similar sphere a fall of · cm. availed. if the surface was only slightly dusty, then at these low heights the splash remained "smooth." it then occurred to us to try the effect of partial or local dusting, for we had already found by experimenting with a marked sphere that the method of dropping did not impart any appreciable rotation to the sphere, which reached the liquid in the attitude with which it started from the dropping ring. accordingly, after dusting the sphere in the manner already described, the dust was carefully rubbed away from all but certain parts whose position was recorded. the experiments were very successful, and the results are shown on page . the liquid used was water, and the sphere was of polished serpentine, · centim. in diameter, falling centim. in fig. of series xvi the sphere was dusted on the _right-hand side_, and a "sound of splash" was recorded. on the left side we see that there is no disturbance of the "smooth splash"; on the right is a "pocket" of air such as was obtained by accident in series ix, fig. (see p. ). the point of departure at which the liquid left the sphere is well marked, and a tangent from this point passes through the outermost conspicuous droplets that must have been projected from it. in fig. the sphere was dusted _at the top and on the right-hand side, but not much more than half-way down_, and the configuration corresponds entirely to the facts. here again a tangent from the well-marked drops on the right-hand side leads very nearly to the place of departure from the surface of the sphere. in fig. the sphere was dusted near the bottom only. the appearance on the left-hand side seems to show that the liquid has, after leaving the sphere, again been brought within reach. this recovery at an early stage is explained by reference to photographs of series vi (p. ) of the splash of a rough sphere, which show that even the rough sphere is soon wetted for some distance up the sides, by the gradual passage of the sphere into the divergently flowing cone of liquid which surrounds the lower part. when the liquid again touches a polished part the film will be again guided up it in the manner already explained. [illustration: series xvi spheres dusted at one side. ] we observe that in figs. and (as also in fig. of page ) the continuous film or shell of liquid no longer reaches the outermost droplets that once have been at its edge. it must evidently have been pulled in by its own surface-tension, which of course will cease to exercise any inward pull on a drop that has once separated. the influence of dust, thus incontestably proved, seems also to afford a satisfactory explanation of-- ( ) the effect of a flame. ( ) the effect of heating. ( ) the variable and uncertain effects of electrification. for, ( ), we may suppose that the flame burns off minute particles of dust; ( ), we know from aitken's experiments[i] that dust from the atmosphere will not settle on a surface hotter than the air; ( ) an electrified sphere descending through the air would attract dust to its surface unless it happened, as well might happen, that the air round about it, with its contained dust, had become itself similarly charged through the working of the electrical machine. in further confirmation of our view that the leading clue to the explanation of the motion is the struggle between the adhesion of the rigid sphere and the tangential momentum of the liquid, we may cite the following points:-- a _liquid_ sphere makes a "rough" splash, and the photographs obtained show that the lower part of the in-falling drop is swept away by the tangential flow, while the upper part is still undistorted. here we have cohesion but no rigidity. also we find that the "rough" splash is obtained by any process which gives a non-rigid surface to the sphere. thus the splash made by a marble freshly roughened by sand-papering, or by grinding between two files and let fall from the very small height of · cm., can be practically controlled by attending to the condition of the surface. if the surface is quite dry and still covered with the fine powder resulting from the process of roughening, the splash is "rough," and a great bubble of air is taken down. but if this coat of powder, which has neither cohesion nor shearing strength, be removed by rubbing, the splash (under this low velocity) is "smooth." again, a marble freshly sand-papered and covered with the resulting powder, if let fall from or cm., gives a rough splash. the same marble picked out of the liquid and very quickly dropped in again from the same height, will give again a rough splash. here the liquid film is thick and "shearable." but if the same sphere be allowed to drain or be lightly wiped, the splash will be smooth. we may conjecture that in this case enough fluid is left to fill up the interstices, but that the coat is not thick enough to shear easily. if, however, the sphere be thoroughly dried, the splash becomes "rough" again. this gives us the explanation of the facts already recorded in respect of the splash of a wet sphere. this splash was always irregular; the liquid drifted to one side where it would shear, while it disappeared from the other or became there too thin to shear, though sufficient to fill up crevices. explanation of the ribs or flutings in the splash of a smooth sphere. the fact thus established experimentally, that the surface of a smooth sphere must be rigid if the film is to envelop it closely, suggests also a satisfactory explanation of the flutings. for we know from other researches on the motion of liquids,[j] that a layer of liquid actually in contact with a solid can have no motion relative to the solid, but must move with it. thus in the film or sheath which rises over and envelops the sphere, the layer of liquid next to the solid must be moving downwards with it, while the outermost layers at least are moving upwards; thus there must be a strong viscous shear in the film impeding its rise. if by any fortuitous oscillation a radial rib arises, this will be a channel in which the liquid, being farther from the surface, will be less affected by the viscous drag; it will therefore be a channel of more rapid flow and diminished pressure, into which, therefore, the neighbouring liquid will be forced from either side. thus a rib once formed is in stable equilibrium, and will correspond to a jet at the edge of the rim. this explains the persistence of the ribs when once established, and we may attribute their regular distribution to the fact that they first originate in the spontaneous segmentation of the annular rim at the edge of the advancing sheath. this explanation quite accords with the appearance of such figures as fig. of page and figs. and of page , in which, firstly, we see that the flutings are absent from that part of the sheath which has left the sphere, and, secondly, we see how much higher in every case the continuous film has risen in that part which has left the sphere than in the part which has clung to it, and has been hindered by the viscous drag. especially is this the case in fig. , series xiv (p. ), where the liquid was pure glycerine. the effect of the viscous drag is, in fact, most marked in the most viscous liquid, and it is also in the viscous liquid that the ribs are most strongly marked. influence of the nature of the liquid explained. finally, in confirmation of our explanation, we have the fact that with a liquid of small density and surface-tension, such as paraffin oil, a much smaller velocity of impact with a highly polished sphere suffices to give a "rough" splash than with water, a liquid of greater density and surface-tension, the reason being without doubt that the tangential velocity given by the impact is greater with the lighter liquid, as, indeed, is proved to be the case by the greater height to which the surrounding sheath is thrown up. the surface-tension also being smaller, the less is the abatement of velocity on account of work done in extending the surface. footnotes: [i] see _nature_, vol. xxix., january , . [j] see whetham on "the alleged slipping at the boundary of a liquid in motion." _phil. trans. roy. soc._, vol. ( ). chapter x conclusion we have now reached the end of the story, as far at least as i am able to tell it. but there is certainly more to be found out. no one has yet examined what happens when a rough sphere enters a liquid with a very high velocity. that the motion set up must differ from that at a low velocity is apparent to any one who has thrown stones from a low bridge into deep water below. the stone that is thrown with a great velocity makes neither quite the same sound nor the same kind of splash as a slow-falling stone, and though in the light of our present knowledge we may conjecture the kind of difference to be expected, yet experience has taught me that the subject is so full of unexpected turns that it is better to wait for the photographic record than to speculate without it. it would be an immense convenience, as was suggested in the first chapter, if we could use a kinematograph and watch such a splash in broad daylight, without the troublesome necessity of providing darkness and an electric spark. but the difficulties of contriving an exposure of the whole lens short enough to prevent blurring, either from the motion of the object, or from that of the rapidly-shifting sensitive film, are very great, and any one who may be able to overcome them satisfactorily, will find a multitude of applications awaiting his invention. but even were the photographic record complete, what does it amount to? all that we have done has been merely to follow the rapid changes of form that take place in the bounding surface of the liquid. the interior particles of the liquid itself have remained invisible to us. but it is precisely the motion of these particles that the student of hydrodynamics desires to be able to trace. his study is so difficult that even in the apparently simple case of the gently-undulating surface of deep water, the reasoning necessary to discover the real path of any particle can at present only be followed by the highly-trained mathematician. in other and more complicated cases such as are exemplified by the sudden disturbances that we have studied, any definite information that can be obtained, even as to the motion of the surface, may afford a clue to the solution of important questions; and i have been encouraged to hope that the observations here recorded may serve as a useful basis of experimental fact in a confessedly difficult subject. to take a single illustration of a possible application in an unexpected quarter, i would invite the attention of the reader to the two photographs in the frontispiece, which exhibit the splash of a projectile on striking the steel armour-plate of a battleship. these are ordinary photographs taken after the plate had been used as a target. they represent the side on which the projectile has entered. in one picture the projectile is still seen embedded in the plate. no one looking at these photographs can fail to be struck with the close resemblance to some of the splashes that we have studied. there is the same _slight_ upheaval of the neighbouring surface, the same crater, with the same curled lip, leading to the inference that under the immense and suddenly applied pressure, the steel has behaved like a liquid. such flow of metals under great pressure is familiar enough to mechanical engineers, but what i desire to suggest is, that from a study of the motions set up in a liquid in an analogous case, it may be possible to deduce information about the distribution of internal stress, which may apply also to a solid, and may thus lead to improvements in the construction of a plate that is intended to resist penetration. chapter xi (supplementary) a new phenomenon that appears with an increase in the velocity of entry of a rough sphere a slight delay in the passage of this book through the press has enabled me to obtain some of the missing information referred to in the opening paragraph of the last chapter. if any reader who may have been persuaded to try for himself the simple experiment mentioned at the beginning of chapter vii, will extend his observations by increasing the height of fall of the roughened marble to or feet (say to centim.), he will find that while, as before, much air is still carried down, there is nevertheless, now, no rebounding jet projected high into the air, such as is invariably seen with the lower fall of feet ( centim.), and he will notice a curious "seething" appearance at the surface.[k] thinking that this appearance which the naked eye detects must be due to an entanglement of the rising jet with the bubble, which entanglement was likely to produce confused motions that could not be profitably studied, i had not till now been sufficiently curious to examine what really happened. but certain recent observations of the persistence with which the seething motion again and again recurred when a stone was dropped or thrown into a river, led me to suspect that something required investigation. i was, however, quite unprepared to find the remarkable change of procedure that is revealed by the following series of photographs (series xvii), in the taking of which i owe much to the kind and skilful assistance of dr. bryan. the earlier figures show the very rapid rise of the crater and its closing as a bubble much before the entrapped column of air divides. before the division takes place, the liquid now flowing in from all sides closes over the upper end of the long air-tube, separates it from the air outside, and _forms a downward jet which shoots down the middle of the air-tube in pursuit of the sphere_. the first formation of this jet is not easy to observe, because the view is obscured by much splashing and turbulent vortical motion resulting apparently from the collision of the streams that converge from all sides on the axis of the air-tube at its upper end. thus in fig. the jet is not yet well established, or at least not easily discerned; but in fig. the turbulence has cleared away from the upper part, and from this stage onwards the jet is well seen in all the figures, and it persists long after the segmentation of the air column has taken place. the reader must not suppose that this jet is a mere _falling_ of the water under the action of gravity, for the rapidity with which it advances is far greater than could be accounted for in this way; indeed, as the "times" show, the effect of gravity during the establishment of the jet is insignificant. [illustration: series xvii rough sphere falling cm. into water. scale / . · sec. · sec. · sec. · sec. · sec.] the segmentation of the air column appears to be independent of the jet; but some photographs, such as fig. , show the jet striking the side and breaking into the surrounding liquid with a great accompaniment of "air-dust." [illustration: series xvii--(_continued_) · sec. · sec.] n.b.--each of these figures is made up from two photographs; one of the upper and one of the lower portion taken from different splashes, but with the same "timing." the reader will observe that after division of the air-tube has taken place, say from fig. onwards, the water entering the jet at the top and coming out again at the bottom must circulate as in a vortex ring, part of the core of which is filled with the air surrounding the jet. it is also to be observed that after the establishment of the jet, there is a steady increase in the size of the heap above the surface; but it is not easy in any given photograph to say how much of this protuberance is air and how much is water. an examination of figs. , , and shows that the place of origin of the jet is gradually lifted above the level of the free surface. that the jet we now see should be directed downwards rather than upwards may, i think, be explained in a general way as follows:--the great initial momentum of the sphere causes it to continue in rapid motion after the bubble has closed, thus the sphere acts as a sort of piston, which by increasing the length of the air-tube diminishes the pressure in it and so sucks in the bubble, which is driven down by the greater atmospheric pressure above. the converging horizontal inflow near the mouth of the air-tube cannot, of course, produce the downward-directed jet without an equal and opposite generation of momentum upwards; but this is now expended, not in producing a similar upward jet, but in balancing the excess of atmospheric pressure. the reaction, in fact, to the projection of the jet downwards, is the force which holds up and slowly raises the roof of the long air-shaft. [illustration: series xvii--(_continued_) · sec. · sec. · sec. · sec.] when, as in the last figure of series vi, p. , we saw the upward-directed jet, then also there must have been an equal and opposite generation of downward momentum distributed in some way through the liquid below the basin, of which, however, there could be no visible sign. hence we see that the present downward jet is, in a sense, not a new phenomenon, but one which, having existed unnoticed before, is now rendered visible to us by reason of its being produced in air instead of in water. by means of a hole bored through the ceiling of the dark room, the fall was then increased to centim. (just over feet). the very beautiful earlier stages of the splash at this height are shown in series xviii. fig. shows very well the internal splashing at the top of the air-column which accompanies the initiation of the jet. some later photographs taken at this height (not yet quite presentable) show the jet passing right down the narrow neck of air-tube and probably striking the top of the sphere, the descent of which must thus be liable to a curious irregularity. a further increase of the height of fall to centim. ( - / feet) was found to produce but little change in the phenomena. [illustration: series xviii early stages of the splash of a rough sphere (diam. · centim.) falling centim. (about feet) into water. t = · sec. · sec. ] footnotes: [k] i can recommend any reader who is not afraid of being late for breakfast to keep a bag of marbles in his bath-room. printed by william brendon and son, ltd plymouth [transcriber's note: the following changes have been made to the original text. page : "the same in , and " changed to "the same in , and " page : "· of a gram" changed to " · of a gram" page : added full stop to image caption " · sec." sidelights on relativity by albert einstein contents ether and the theory of relativity an address delivered on may th, , in the university of leyden geometry and experience an expanded form of an address to the prussian academy of sciences in berlin on january th, . ether and the theory of relativity an address delivered on may th, , in the university of leyden how does it come about that alongside of the idea of ponderable matter, which is derived by abstraction from everyday life, the physicists set the idea of the existence of another kind of matter, the ether? the explanation is probably to be sought in those phenomena which have given rise to the theory of action at a distance, and in the properties of light which have led to the undulatory theory. let us devote a little while to the consideration of these two subjects. outside of physics we know nothing of action at a distance. when we try to connect cause and effect in the experiences which natural objects afford us, it seems at first as if there were no other mutual actions than those of immediate contact, e.g. the communication of motion by impact, push and pull, heating or inducing combustion by means of a flame, etc. it is true that even in everyday experience weight, which is in a sense action at a distance, plays a very important part. but since in daily experience the weight of bodies meets us as something constant, something not linked to any cause which is variable in time or place, we do not in everyday life speculate as to the cause of gravity, and therefore do not become conscious of its character as action at a distance. it was newton's theory of gravitation that first assigned a cause for gravity by interpreting it as action at a distance, proceeding from masses. newton's theory is probably the greatest stride ever made in the effort towards the causal nexus of natural phenomena. and yet this theory evoked a lively sense of discomfort among newton's contemporaries, because it seemed to be in conflict with the principle springing from the rest of experience, that there can be reciprocal action only through contact, and not through immediate action at a distance. it is only with reluctance that man's desire for knowledge endures a dualism of this kind. how was unity to be preserved in his comprehension of the forces of nature? either by trying to look upon contact forces as being themselves distant forces which admittedly are observable only at a very small distance--and this was the road which newton's followers, who were entirely under the spell of his doctrine, mostly preferred to take; or by assuming that the newtonian action at a distance is only _apparently_ immediate action at a distance, but in truth is conveyed by a medium permeating space, whether by movements or by elastic deformation of this medium. thus the endeavour toward a unified view of the nature of forces leads to the hypothesis of an ether. this hypothesis, to be sure, did not at first bring with it any advance in the theory of gravitation or in physics generally, so that it became customary to treat newton's law of force as an axiom not further reducible. but the ether hypothesis was bound always to play some part in physical science, even if at first only a latent part. when in the first half of the nineteenth century the far-reaching similarity was revealed which subsists between the properties of light and those of elastic waves in ponderable bodies, the ether hypothesis found fresh support. it appeared beyond question that light must be interpreted as a vibratory process in an elastic, inert medium filling up universal space. it also seemed to be a necessary consequence of the fact that light is capable of polarisation that this medium, the ether, must be of the nature of a solid body, because transverse waves are not possible in a fluid, but only in a solid. thus the physicists were bound to arrive at the theory of the "quasi-rigid" luminiferous ether, the parts of which can carry out no movements relatively to one another except the small movements of deformation which correspond to light-waves. this theory--also called the theory of the stationary luminiferous ether--moreover found a strong support in an experiment which is also of fundamental importance in the special theory of relativity, the experiment of fizeau, from which one was obliged to infer that the luminiferous ether does not take part in the movements of bodies. the phenomenon of aberration also favoured the theory of the quasi-rigid ether. the development of the theory of electricity along the path opened up by maxwell and lorentz gave the development of our ideas concerning the ether quite a peculiar and unexpected turn. for maxwell himself the ether indeed still had properties which were purely mechanical, although of a much more complicated kind than the mechanical properties of tangible solid bodies. but neither maxwell nor his followers succeeded in elaborating a mechanical model for the ether which might furnish a satisfactory mechanical interpretation of maxwell's laws of the electro-magnetic field. the laws were clear and simple, the mechanical interpretations clumsy and contradictory. almost imperceptibly the theoretical physicists adapted themselves to a situation which, from the standpoint of their mechanical programme, was very depressing. they were particularly influenced by the electro-dynamical investigations of heinrich hertz. for whereas they previously had required of a conclusive theory that it should content itself with the fundamental concepts which belong exclusively to mechanics (e.g. densities, velocities, deformations, stresses) they gradually accustomed themselves to admitting electric and magnetic force as fundamental concepts side by side with those of mechanics, without requiring a mechanical interpretation for them. thus the purely mechanical view of nature was gradually abandoned. but this change led to a fundamental dualism which in the long-run was insupportable. a way of escape was now sought in the reverse direction, by reducing the principles of mechanics to those of electricity, and this especially as confidence in the strict validity of the equations of newton's mechanics was shaken by the experiments with beta-rays and rapid kathode rays. this dualism still confronts us in unextenuated form in the theory of hertz, where matter appears not only as the bearer of velocities, kinetic energy, and mechanical pressures, but also as the bearer of electromagnetic fields. since such fields also occur _in vacuo_--i.e. in free ether--the ether also appears as bearer of electromagnetic fields. the ether appears indistinguishable in its functions from ordinary matter. within matter it takes part in the motion of matter and in empty space it has everywhere a velocity; so that the ether has a definitely assigned velocity throughout the whole of space. there is no fundamental difference between hertz's ether and ponderable matter (which in part subsists in the ether). the hertz theory suffered not only from the defect of ascribing to matter and ether, on the one hand mechanical states, and on the other hand electrical states, which do not stand in any conceivable relation to each other; it was also at variance with the result of fizeau's important experiment on the velocity of the propagation of light in moving fluids, and with other established experimental results. such was the state of things when h. a. lorentz entered upon the scene. he brought theory into harmony with experience by means of a wonderful simplification of theoretical principles. he achieved this, the most important advance in the theory of electricity since maxwell, by taking from ether its mechanical, and from matter its electromagnetic qualities. as in empty space, so too in the interior of material bodies, the ether, and not matter viewed atomistically, was exclusively the seat of electromagnetic fields. according to lorentz the elementary particles of matter alone are capable of carrying out movements; their electromagnetic activity is entirely confined to the carrying of electric charges. thus lorentz succeeded in reducing all electromagnetic happenings to maxwell's equations for free space. as to the mechanical nature of the lorentzian ether, it may be said of it, in a somewhat playful spirit, that immobility is the only mechanical property of which it has not been deprived by h. a. lorentz. it may be added that the whole change in the conception of the ether which the special theory of relativity brought about, consisted in taking away from the ether its last mechanical quality, namely, its immobility. how this is to be understood will forthwith be expounded. the space-time theory and the kinematics of the special theory of relativity were modelled on the maxwell-lorentz theory of the electromagnetic field. this theory therefore satisfies the conditions of the special theory of relativity, but when viewed from the latter it acquires a novel aspect. for if k be a system of co-ordinates relatively to which the lorentzian ether is at rest, the maxwell-lorentz equations are valid primarily with reference to k. but by the special theory of relativity the same equations without any change of meaning also hold in relation to any new system of co-ordinates k' which is moving in uniform translation relatively to k. now comes the anxious question:--why must i in the theory distinguish the k system above all k' systems, which are physically equivalent to it in all respects, by assuming that the ether is at rest relatively to the k system? for the theoretician such an asymmetry in the theoretical structure, with no corresponding asymmetry in the system of experience, is intolerable. if we assume the ether to be at rest relatively to k, but in motion relatively to k', the physical equivalence of k and k' seems to me from the logical standpoint, not indeed downright incorrect, but nevertheless inacceptable. the next position which it was possible to take up in face of this state of things appeared to be the following. the ether does not exist at all. the electromagnetic fields are not states of a medium, and are not bound down to any bearer, but they are independent realities which are not reducible to anything else, exactly like the atoms of ponderable matter. this conception suggests itself the more readily as, according to lorentz's theory, electromagnetic radiation, like ponderable matter, brings impulse and energy with it, and as, according to the special theory of relativity, both matter and radiation are but special forms of distributed energy, ponderable mass losing its isolation and appearing as a special form of energy. more careful reflection teaches us, however, that the special theory of relativity does not compel us to deny ether. we may assume the existence of an ether; only we must give up ascribing a definite state of motion to it, i.e. we must by abstraction take from it the last mechanical characteristic which lorentz had still left it. we shall see later that this point of view, the conceivability of which i shall at once endeavour to make more intelligible by a somewhat halting comparison, is justified by the results of the general theory of relativity. think of waves on the surface of water. here we can describe two entirely different things. either we may observe how the undulatory surface forming the boundary between water and air alters in the course of time; or else--with the help of small floats, for instance--we can observe how the position of the separate particles of water alters in the course of time. if the existence of such floats for tracking the motion of the particles of a fluid were a fundamental impossibility in physics--if, in fact, nothing else whatever were observable than the shape of the space occupied by the water as it varies in time, we should have no ground for the assumption that water consists of movable particles. but all the same we could characterise it as a medium. we have something like this in the electromagnetic field. for we may picture the field to ourselves as consisting of lines of force. if we wish to interpret these lines of force to ourselves as something material in the ordinary sense, we are tempted to interpret the dynamic processes as motions of these lines of force, such that each separate line of force is tracked through the course of time. it is well known, however, that this way of regarding the electromagnetic field leads to contradictions. generalising we must say this:--there may be supposed to be extended physical objects to which the idea of motion cannot be applied. they may not be thought of as consisting of particles which allow themselves to be separately tracked through time. in minkowski's idiom this is expressed as follows:--not every extended conformation in the four-dimensional world can be regarded as composed of world-threads. the special theory of relativity forbids us to assume the ether to consist of particles observable through time, but the hypothesis of ether in itself is not in conflict with the special theory of relativity. only we must be on our guard against ascribing a state of motion to the ether. certainly, from the standpoint of the special theory of relativity, the ether hypothesis appears at first to be an empty hypothesis. in the equations of the electromagnetic field there occur, in addition to the densities of the electric charge, _only_ the intensities of the field. the career of electromagnetic processes _in vacuo_ appears to be completely determined by these equations, uninfluenced by other physical quantities. the electromagnetic fields appear as ultimate, irreducible realities, and at first it seems superfluous to postulate a homogeneous, isotropic ether-medium, and to envisage electromagnetic fields as states of this medium. but on the other hand there is a weighty argument to be adduced in favour of the ether hypothesis. to deny the ether is ultimately to assume that empty space has no physical qualities whatever. the fundamental facts of mechanics do not harmonize with this view. for the mechanical behaviour of a corporeal system hovering freely in empty space depends not only on relative positions (distances) and relative velocities, but also on its state of rotation, which physically may be taken as a characteristic not appertaining to the system in itself. in order to be able to look upon the rotation of the system, at least formally, as something real, newton objectivises space. since he classes his absolute space together with real things, for him rotation relative to an absolute space is also something real. newton might no less well have called his absolute space "ether"; what is essential is merely that besides observable objects, another thing, which is not perceptible, must be looked upon as real, to enable acceleration or rotation to be looked upon as something real. it is true that mach tried to avoid having to accept as real something which is not observable by endeavouring to substitute in mechanics a mean acceleration with reference to the totality of the masses in the universe in place of an acceleration with reference to absolute space. but inertial resistance opposed to relative acceleration of distant masses presupposes action at a distance; and as the modern physicist does not believe that he may accept this action at a distance, he comes back once more, if he follows mach, to the ether, which has to serve as medium for the effects of inertia. but this conception of the ether to which we are led by mach's way of thinking differs essentially from the ether as conceived by newton, by fresnel, and by lorentz. mach's ether not only _conditions_ the behaviour of inert masses, but _is also conditioned_ in its state by them. mach's idea finds its full development in the ether of the general theory of relativity. according to this theory the metrical qualities of the continuum of space-time differ in the environment of different points of space-time, and are partly conditioned by the matter existing outside of the territory under consideration. this space-time variability of the reciprocal relations of the standards of space and time, or, perhaps, the recognition of the fact that "empty space" in its physical relation is neither homogeneous nor isotropic, compelling us to describe its state by ten functions (the gravitation potentials g_(mn)), has, i think, finally disposed of the view that space is physically empty. but therewith the conception of the ether has again acquired an intelligible content, although this content differs widely from that of the ether of the mechanical undulatory theory of light. the ether of the general theory of relativity is a medium which is itself devoid of _all_ mechanical and kinematical qualities, but helps to determine mechanical (and electromagnetic) events. what is fundamentally new in the ether of the general theory of relativity as opposed to the ether of lorentz consists in this, that the state of the former is at every place determined by connections with the matter and the state of the ether in neighbouring places, which are amenable to law in the form of differential equations; whereas the state of the lorentzian ether in the absence of electromagnetic fields is conditioned by nothing outside itself, and is everywhere the same. the ether of the general theory of relativity is transmuted conceptually into the ether of lorentz if we substitute constants for the functions of space which describe the former, disregarding the causes which condition its state. thus we may also say, i think, that the ether of the general theory of relativity is the outcome of the lorentzian ether, through relativation. as to the part which the new ether is to play in the physics of the future we are not yet clear. we know that it determines the metrical relations in the space-time continuum, e.g. the configurative possibilities of solid bodies as well as the gravitational fields; but we do not know whether it has an essential share in the structure of the electrical elementary particles constituting matter. nor do we know whether it is only in the proximity of ponderable masses that its structure differs essentially from that of the lorentzian ether; whether the geometry of spaces of cosmic extent is approximately euclidean. but we can assert by reason of the relativistic equations of gravitation that there must be a departure from euclidean relations, with spaces of cosmic order of magnitude, if there exists a positive mean density, no matter how small, of the matter in the universe. in this case the universe must of necessity be spatially unbounded and of finite magnitude, its magnitude being determined by the value of that mean density. if we consider the gravitational field and the electromagnetic field from the stand-point of the ether hypothesis, we find a remarkable difference between the two. there can be no space nor any part of space without gravitational potentials; for these confer upon space its metrical qualities, without which it cannot be imagined at all. the existence of the gravitational field is inseparably bound up with the existence of space. on the other hand a part of space may very well be imagined without an electromagnetic field; thus in contrast with the gravitational field, the electromagnetic field seems to be only secondarily linked to the ether, the formal nature of the electromagnetic field being as yet in no way determined by that of gravitational ether. from the present state of theory it looks as if the electromagnetic field, as opposed to the gravitational field, rests upon an entirely new formal _motif_, as though nature might just as well have endowed the gravitational ether with fields of quite another type, for example, with fields of a scalar potential, instead of fields of the electromagnetic type. since according to our present conceptions the elementary particles of matter are also, in their essence, nothing else than condensations of the electromagnetic field, our present view of the universe presents two realities which are completely separated from each other conceptually, although connected causally, namely, gravitational ether and electromagnetic field, or--as they might also be called--space and matter. of course it would be a great advance if we could succeed in comprehending the gravitational field and the electromagnetic field together as one unified conformation. then for the first time the epoch of theoretical physics founded by faraday and maxwell would reach a satisfactory conclusion. the contrast between ether and matter would fade away, and, through the general theory of relativity, the whole of physics would become a complete system of thought, like geometry, kinematics, and the theory of gravitation. an exceedingly ingenious attempt in this direction has been made by the mathematician h. weyl; but i do not believe that his theory will hold its ground in relation to reality. further, in contemplating the immediate future of theoretical physics we ought not unconditionally to reject the possibility that the facts comprised in the quantum theory may set bounds to the field theory beyond which it cannot pass. recapitulating, we may say that according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether. according to the general theory of relativity space without ether is unthinkable; for in such space there not only would be no propagation of light, but also no possibility of existence for standards of space and time (measuring-rods and clocks), nor therefore any space-time intervals in the physical sense. but this ether may not be thought of as endowed with the quality characteristic of ponderable media, as consisting of parts which may be tracked through time. the idea of motion may not be applied to it. geometry and experience an expanded form of an address to the prussian academy of sciences in berlin on january th, . one reason why mathematics enjoys special esteem, above all other sciences, is that its laws are absolutely certain and indisputable, while those of all other sciences are to some extent debatable and in constant danger of being overthrown by newly discovered facts. in spite of this, the investigator in another department of science would not need to envy the mathematician if the laws of mathematics referred to objects of our mere imagination, and not to objects of reality. for it cannot occasion surprise that different persons should arrive at the same logical conclusions when they have already agreed upon the fundamental laws (axioms), as well as the methods by which other laws are to be deduced therefrom. but there is another reason for the high repute of mathematics, in that it is mathematics which affords the exact natural sciences a certain measure of security, to which without mathematics they could not attain. at this point an enigma presents itself which in all ages has agitated inquiring minds. how can it be that mathematics, being after all a product of human thought which is independent of experience, is so admirably appropriate to the objects of reality? is human reason, then, without experience, merely by taking thought, able to fathom the properties of real things. in my opinion the answer to this question is, briefly, this:--as far as the laws of mathematics refer to reality, they are not certain; and as far as they are certain, they do not refer to reality. it seems to me that complete clearness as to this state of things first became common property through that new departure in mathematics which is known by the name of mathematical logic or "axiomatics." the progress achieved by axiomatics consists in its having neatly separated the logical-formal from its objective or intuitive content; according to axiomatics the logical-formal alone forms the subject-matter of mathematics, which is not concerned with the intuitive or other content associated with the logical-formal. let us for a moment consider from this point of view any axiom of geometry, for instance, the following:--through two points in space there always passes one and only one straight line. how is this axiom to be interpreted in the older sense and in the more modern sense? the older interpretation:--every one knows what a straight line is, and what a point is. whether this knowledge springs from an ability of the human mind or from experience, from some collaboration of the two or from some other source, is not for the mathematician to decide. he leaves the question to the philosopher. being based upon this knowledge, which precedes all mathematics, the axiom stated above is, like all other axioms, self-evident, that is, it is the expression of a part of this _a priori_ knowledge. the more modern interpretation:--geometry treats of entities which are denoted by the words straight line, point, etc. these entities do not take for granted any knowledge or intuition whatever, but they presuppose only the validity of the axioms, such as the one stated above, which are to be taken in a purely formal sense, i.e. as void of all content of intuition or experience. these axioms are free creations of the human mind. all other propositions of geometry are logical inferences from the axioms (which are to be taken in the nominalistic sense only). the matter of which geometry treats is first defined by the axioms. schlick in his book on epistemology has therefore characterised axioms very aptly as "implicit definitions." this view of axioms, advocated by modern axiomatics, purges mathematics of all extraneous elements, and thus dispels the mystic obscurity which formerly surrounded the principles of mathematics. but a presentation of its principles thus clarified makes it also evident that mathematics as such cannot predicate anything about perceptual objects or real objects. in axiomatic geometry the words "point," "straight line," etc., stand only for empty conceptual schemata. that which gives them substance is not relevant to mathematics. yet on the other hand it is certain that mathematics generally, and particularly geometry, owes its existence to the need which was felt of learning something about the relations of real things to one another. the very word geometry, which, of course, means earth-measuring, proves this. for earth-measuring has to do with the possibilities of the disposition of certain natural objects with respect to one another, namely, with parts of the earth, measuring-lines, measuring-wands, etc. it is clear that the system of concepts of axiomatic geometry alone cannot make any assertions as to the relations of real objects of this kind, which we will call practically-rigid bodies. to be able to make such assertions, geometry must be stripped of its merely logical-formal character by the co-ordination of real objects of experience with the empty conceptual frame-work of axiomatic geometry. to accomplish this, we need only add the proposition:--solid bodies are related, with respect to their possible dispositions, as are bodies in euclidean geometry of three dimensions. then the propositions of euclid contain affirmations as to the relations of practically-rigid bodies. geometry thus completed is evidently a natural science; we may in fact regard it as the most ancient branch of physics. its affirmations rest essentially on induction from experience, but not on logical inferences only. we will call this completed geometry "practical geometry," and shall distinguish it in what follows from "purely axiomatic geometry." the question whether the practical geometry of the universe is euclidean or not has a clear meaning, and its answer can only be furnished by experience. all linear measurement in physics is practical geometry in this sense, so too is geodetic and astronomical linear measurement, if we call to our help the law of experience that light is propagated in a straight line, and indeed in a straight line in the sense of practical geometry. i attach special importance to the view of geometry which i have just set forth, because without it i should have been unable to formulate the theory of relativity. without it the following reflection would have been impossible:--in a system of reference rotating relatively to an inert system, the laws of disposition of rigid bodies do not correspond to the rules of euclidean geometry on account of the lorentz contraction; thus if we admit non-inert systems we must abandon euclidean geometry. the decisive step in the transition to general co-variant equations would certainly not have been taken if the above interpretation had not served as a stepping-stone. if we deny the relation between the body of axiomatic euclidean geometry and the practically-rigid body of reality, we readily arrive at the following view, which was entertained by that acute and profound thinker, h. poincare:--euclidean geometry is distinguished above all other imaginable axiomatic geometries by its simplicity. now since axiomatic geometry by itself contains no assertions as to the reality which can be experienced, but can do so only in combination with physical laws, it should be possible and reasonable--whatever may be the nature of reality--to retain euclidean geometry. for if contradictions between theory and experience manifest themselves, we should rather decide to change physical laws than to change axiomatic euclidean geometry. if we deny the relation between the practically-rigid body and geometry, we shall indeed not easily free ourselves from the convention that euclidean geometry is to be retained as the simplest. why is the equivalence of the practically-rigid body and the body of geometry--which suggests itself so readily--denied by poincare and other investigators? simply because under closer inspection the real solid bodies in nature are not rigid, because their geometrical behaviour, that is, their possibilities of relative disposition, depend upon temperature, external forces, etc. thus the original, immediate relation between geometry and physical reality appears destroyed, and we feel impelled toward the following more general view, which characterizes poincare's standpoint. geometry (g) predicates nothing about the relations of real things, but only geometry together with the purport (p) of physical laws can do so. using symbols, we may say that only the sum of (g) + (p) is subject to the control of experience. thus (g) may be chosen arbitrarily, and also parts of (p); all these laws are conventions. all that is necessary to avoid contradictions is to choose the remainder of (p) so that (g) and the whole of (p) are together in accord with experience. envisaged in this way, axiomatic geometry and the part of natural law which has been given a conventional status appear as epistemologically equivalent. _sub specie aeterni_ poincare, in my opinion, is right. the idea of the measuring-rod and the idea of the clock co-ordinated with it in the theory of relativity do not find their exact correspondence in the real world. it is also clear that the solid body and the clock do not in the conceptual edifice of physics play the part of irreducible elements, but that of composite structures, which may not play any independent part in theoretical physics. but it is my conviction that in the present stage of development of theoretical physics these ideas must still be employed as independent ideas; for we are still far from possessing such certain knowledge of theoretical principles as to be able to give exact theoretical constructions of solid bodies and clocks. further, as to the objection that there are no really rigid bodies in nature, and that therefore the properties predicated of rigid bodies do not apply to physical reality,--this objection is by no means so radical as might appear from a hasty examination. for it is not a difficult task to determine the physical state of a measuring-rod so accurately that its behaviour relatively to other measuring-bodies shall be sufficiently free from ambiguity to allow it to be substituted for the "rigid" body. it is to measuring-bodies of this kind that statements as to rigid bodies must be referred. all practical geometry is based upon a principle which is accessible to experience, and which we will now try to realise. we will call that which is enclosed between two boundaries, marked upon a practically-rigid body, a tract. we imagine two practically-rigid bodies, each with a tract marked out on it. these two tracts are said to be "equal to one another" if the boundaries of the one tract can be brought to coincide permanently with the boundaries of the other. we now assume that: if two tracts are found to be equal once and anywhere, they are equal always and everywhere. not only the practical geometry of euclid, but also its nearest generalisation, the practical geometry of riemann, and therewith the general theory of relativity, rest upon this assumption. of the experimental reasons which warrant this assumption i will mention only one. the phenomenon of the propagation of light in empty space assigns a tract, namely, the appropriate path of light, to each interval of local time, and conversely. thence it follows that the above assumption for tracts must also hold good for intervals of clock-time in the theory of relativity. consequently it may be formulated as follows:--if two ideal clocks are going at the same rate at any time and at any place (being then in immediate proximity to each other), they will always go at the same rate, no matter where and when they are again compared with each other at one place.--if this law were not valid for real clocks, the proper frequencies for the separate atoms of the same chemical element would not be in such exact agreement as experience demonstrates. the existence of sharp spectral lines is a convincing experimental proof of the above-mentioned principle of practical geometry. this is the ultimate foundation in fact which enables us to speak with meaning of the mensuration, in riemann's sense of the word, of the four-dimensional continuum of space-time. the question whether the structure of this continuum is euclidean, or in accordance with riemann's general scheme, or otherwise, is, according to the view which is here being advocated, properly speaking a physical question which must be answered by experience, and not a question of a mere convention to be selected on practical grounds. riemann's geometry will be the right thing if the laws of disposition of practically-rigid bodies are transformable into those of the bodies of euclid's geometry with an exactitude which increases in proportion as the dimensions of the part of space-time under consideration are diminished. it is true that this proposed physical interpretation of geometry breaks down when applied immediately to spaces of sub-molecular order of magnitude. but nevertheless, even in questions as to the constitution of elementary particles, it retains part of its importance. for even when it is a question of describing the electrical elementary particles constituting matter, the attempt may still be made to ascribe physical importance to those ideas of fields which have been physically defined for the purpose of describing the geometrical behaviour of bodies which are large as compared with the molecule. success alone can decide as to the justification of such an attempt, which postulates physical reality for the fundamental principles of riemann's geometry outside of the domain of their physical definitions. it might possibly turn out that this extrapolation has no better warrant than the extrapolation of the idea of temperature to parts of a body of molecular order of magnitude. it appears less problematical to extend the ideas of practical geometry to spaces of cosmic order of magnitude. it might, of course, be objected that a construction composed of solid rods departs more and more from ideal rigidity in proportion as its spatial extent becomes greater. but it will hardly be possible, i think, to assign fundamental significance to this objection. therefore the question whether the universe is spatially finite or not seems to me decidedly a pregnant question in the sense of practical geometry. i do not even consider it impossible that this question will be answered before long by astronomy. let us call to mind what the general theory of relativity teaches in this respect. it offers two possibilities:-- . the universe is spatially infinite. this can be so only if the average spatial density of the matter in universal space, concentrated in the stars, vanishes, i.e. if the ratio of the total mass of the stars to the magnitude of the space through which they are scattered approximates indefinitely to the value zero when the spaces taken into consideration are constantly greater and greater. . the universe is spatially finite. this must be so, if there is a mean density of the ponderable matter in universal space differing from zero. the smaller that mean density, the greater is the volume of universal space. i must not fail to mention that a theoretical argument can be adduced in favour of the hypothesis of a finite universe. the general theory of relativity teaches that the inertia of a given body is greater as there are more ponderable masses in proximity to it; thus it seems very natural to reduce the total effect of inertia of a body to action and reaction between it and the other bodies in the universe, as indeed, ever since newton's time, gravity has been completely reduced to action and reaction between bodies. from the equations of the general theory of relativity it can be deduced that this total reduction of inertia to reciprocal action between masses--as required by e. mach, for example--is possible only if the universe is spatially finite. on many physicists and astronomers this argument makes no impression. experience alone can finally decide which of the two possibilities is realised in nature. how can experience furnish an answer? at first it might seem possible to determine the mean density of matter by observation of that part of the universe which is accessible to our perception. this hope is illusory. the distribution of the visible stars is extremely irregular, so that we on no account may venture to set down the mean density of star-matter in the universe as equal, let us say, to the mean density in the milky way. in any case, however great the space examined may be, we could not feel convinced that there were no more stars beyond that space. so it seems impossible to estimate the mean density. but there is another road, which seems to me more practicable, although it also presents great difficulties. for if we inquire into the deviations shown by the consequences of the general theory of relativity which are accessible to experience, when these are compared with the consequences of the newtonian theory, we first of all find a deviation which shows itself in close proximity to gravitating mass, and has been confirmed in the case of the planet mercury. but if the universe is spatially finite there is a second deviation from the newtonian theory, which, in the language of the newtonian theory, may be expressed thus:--the gravitational field is in its nature such as if it were produced, not only by the ponderable masses, but also by a mass-density of negative sign, distributed uniformly throughout space. since this factitious mass-density would have to be enormously small, it could make its presence felt only in gravitating systems of very great extent. assuming that we know, let us say, the statistical distribution of the stars in the milky way, as well as their masses, then by newton's law we can calculate the gravitational field and the mean velocities which the stars must have, so that the milky way should not collapse under the mutual attraction of its stars, but should maintain its actual extent. now if the actual velocities of the stars, which can, of course, be measured, were smaller than the calculated velocities, we should have a proof that the actual attractions at great distances are smaller than by newton's law. from such a deviation it could be proved indirectly that the universe is finite. it would even be possible to estimate its spatial magnitude. can we picture to ourselves a three-dimensional universe which is finite, yet unbounded? the usual answer to this question is "no," but that is not the right answer. the purpose of the following remarks is to show that the answer should be "yes." i want to show that without any extraordinary difficulty we can illustrate the theory of a finite universe by means of a mental image to which, with some practice, we shall soon grow accustomed. first of all, an observation of epistemological nature. a geometrical-physical theory as such is incapable of being directly pictured, being merely a system of concepts. but these concepts serve the purpose of bringing a multiplicity of real or imaginary sensory experiences into connection in the mind. to "visualise" a theory, or bring it home to one's mind, therefore means to give a representation to that abundance of experiences for which the theory supplies the schematic arrangement. in the present case we have to ask ourselves how we can represent that relation of solid bodies with respect to their reciprocal disposition (contact) which corresponds to the theory of a finite universe. there is really nothing new in what i have to say about this; but innumerable questions addressed to me prove that the requirements of those who thirst for knowledge of these matters have not yet been completely satisfied. so, will the initiated please pardon me, if part of what i shall bring forward has long been known? what do we wish to express when we say that our space is infinite? nothing more than that we might lay any number whatever of bodies of equal sizes side by side without ever filling space. suppose that we are provided with a great many wooden cubes all of the same size. in accordance with euclidean geometry we can place them above, beside, and behind one another so as to fill a part of space of any dimensions; but this construction would never be finished; we could go on adding more and more cubes without ever finding that there was no more room. that is what we wish to express when we say that space is infinite. it would be better to say that space is infinite in relation to practically-rigid bodies, assuming that the laws of disposition for these bodies are given by euclidean geometry. another example of an infinite continuum is the plane. on a plane surface we may lay squares of cardboard so that each side of any square has the side of another square adjacent to it. the construction is never finished; we can always go on laying squares--if their laws of disposition correspond to those of plane figures of euclidean geometry. the plane is therefore infinite in relation to the cardboard squares. accordingly we say that the plane is an infinite continuum of two dimensions, and space an infinite continuum of three dimensions. what is here meant by the number of dimensions, i think i may assume to be known. now we take an example of a two-dimensional continuum which is finite, but unbounded. we imagine the surface of a large globe and a quantity of small paper discs, all of the same size. we place one of the discs anywhere on the surface of the globe. if we move the disc about, anywhere we like, on the surface of the globe, we do not come upon a limit or boundary anywhere on the journey. therefore we say that the spherical surface of the globe is an unbounded continuum. moreover, the spherical surface is a finite continuum. for if we stick the paper discs on the globe, so that no disc overlaps another, the surface of the globe will finally become so full that there is no room for another disc. this simply means that the spherical surface of the globe is finite in relation to the paper discs. further, the spherical surface is a non-euclidean continuum of two dimensions, that is to say, the laws of disposition for the rigid figures lying in it do not agree with those of the euclidean plane. this can be shown in the following way. place a paper disc on the spherical surface, and around it in a circle place six more discs, each of which is to be surrounded in turn by six discs, and so on. if this construction is made on a plane surface, we have an uninterrupted disposition in which there are six discs touching every disc except those which lie on the outside. [figure : discs maximally packed on a plane] on the spherical surface the construction also seems to promise success at the outset, and the smaller the radius of the disc in proportion to that of the sphere, the more promising it seems. but as the construction progresses it becomes more and more patent that the disposition of the discs in the manner indicated, without interruption, is not possible, as it should be possible by euclidean geometry of the the plane surface. in this way creatures which cannot leave the spherical surface, and cannot even peep out from the spherical surface into three-dimensional space, might discover, merely by experimenting with discs, that their two-dimensional "space" is not euclidean, but spherical space. from the latest results of the theory of relativity it is probable that our three-dimensional space is also approximately spherical, that is, that the laws of disposition of rigid bodies in it are not given by euclidean geometry, but approximately by spherical geometry, if only we consider parts of space which are sufficiently great. now this is the place where the reader's imagination boggles. "nobody can imagine this thing," he cries indignantly. "it can be said, but cannot be thought. i can represent to myself a spherical surface well enough, but nothing analogous to it in three dimensions." [figure : a circle projected from a sphere onto a plane] we must try to surmount this barrier in the mind, and the patient reader will see that it is by no means a particularly difficult task. for this purpose we will first give our attention once more to the geometry of two-dimensional spherical surfaces. in the adjoining figure let _k_ be the spherical surface, touched at _s_ by a plane, _e_, which, for facility of presentation, is shown in the drawing as a bounded surface. let _l_ be a disc on the spherical surface. now let us imagine that at the point _n_ of the spherical surface, diametrically opposite to _s_, there is a luminous point, throwing a shadow _l'_ of the disc _l_ upon the plane _e_. every point on the sphere has its shadow on the plane. if the disc on the sphere _k_ is moved, its shadow _l'_ on the plane _e_ also moves. when the disc _l_ is at _s_, it almost exactly coincides with its shadow. if it moves on the spherical surface away from _s_ upwards, the disc shadow _l'_ on the plane also moves away from _s_ on the plane outwards, growing bigger and bigger. as the disc _l_ approaches the luminous point _n_, the shadow moves off to infinity, and becomes infinitely great. now we put the question, what are the laws of disposition of the disc-shadows _l'_ on the plane _e_? evidently they are exactly the same as the laws of disposition of the discs _l_ on the spherical surface. for to each original figure on _k_ there is a corresponding shadow figure on _e_. if two discs on _k_ are touching, their shadows on _e_ also touch. the shadow-geometry on the plane agrees with the the disc-geometry on the sphere. if we call the disc-shadows rigid figures, then spherical geometry holds good on the plane _e_ with respect to these rigid figures. moreover, the plane is finite with respect to the disc-shadows, since only a finite number of the shadows can find room on the plane. at this point somebody will say, "that is nonsense. the disc-shadows are _not_ rigid figures. we have only to move a two-foot rule about on the plane _e_ to convince ourselves that the shadows constantly increase in size as they move away from _s_ on the plane towards infinity." but what if the two-foot rule were to behave on the plane _e_ in the same way as the disc-shadows _l'_? it would then be impossible to show that the shadows increase in size as they move away from _s_; such an assertion would then no longer have any meaning whatever. in fact the only objective assertion that can be made about the disc-shadows is just this, that they are related in exactly the same way as are the rigid discs on the spherical surface in the sense of euclidean geometry. we must carefully bear in mind that our statement as to the growth of the disc-shadows, as they move away from _s_ towards infinity, has in itself no objective meaning, as long as we are unable to employ euclidean rigid bodies which can be moved about on the plane _e_ for the purpose of comparing the size of the disc-shadows. in respect of the laws of disposition of the shadows _l'_, the point _s_ has no special privileges on the plane any more than on the spherical surface. the representation given above of spherical geometry on the plane is important for us, because it readily allows itself to be transferred to the three-dimensional case. let us imagine a point _s_ of our space, and a great number of small spheres, _l'_, which can all be brought to coincide with one another. but these spheres are not to be rigid in the sense of euclidean geometry; their radius is to increase (in the sense of euclidean geometry) when they are moved away from _s_ towards infinity, and this increase is to take place in exact accordance with the same law as applies to the increase of the radii of the disc-shadows _l'_ on the plane. after having gained a vivid mental image of the geometrical behaviour of our _l'_ spheres, let us assume that in our space there are no rigid bodies at all in the sense of euclidean geometry, but only bodies having the behaviour of our _l'_ spheres. then we shall have a vivid representation of three-dimensional spherical space, or, rather of three-dimensional spherical geometry. here our spheres must be called "rigid" spheres. their increase in size as they depart from _s_ is not to be detected by measuring with measuring-rods, any more than in the case of the disc-shadows on _e_, because the standards of measurement behave in the same way as the spheres. space is homogeneous, that is to say, the same spherical configurations are possible in the environment of all points.* our space is finite, because, in consequence of the "growth" of the spheres, only a finite number of them can find room in space. * this is intelligible without calculation--but only for the two-dimensional case--if we revert once more to the case of the disc on the surface of the sphere. in this way, by using as stepping-stones the practice in thinking and visualisation which euclidean geometry gives us, we have acquired a mental picture of spherical geometry. we may without difficulty impart more depth and vigour to these ideas by carrying out special imaginary constructions. nor would it be difficult to represent the case of what is called elliptical geometry in an analogous manner. my only aim to-day has been to show that the human faculty of visualisation is by no means bound to capitulate to non-euclidean geometry. file was produced from images generously made available by the internet archive.) soap-bubbles and the forces which mould them. [illustration: experiment for showing by intermittent light the apparently stationary drops into which a fountain is broken up by the action of a musical sound. (_see_ page .)] soap-bubbles and the forces which mould them. _being a course of three lectures_ delivered in the theatre of the london institution on the afternoons of dec. , , jan. and , , before a juvenile audience. by c. v. boys, a.r.s.m., f.r.s., assistant professor of physics at the royal college of science, south kensington. published under the direction of the general literature committee. society for promoting christian knowledge, london: northumberland avenue, w.c.; , queen victoria street, e.c. brighton: , north street. new york: e. & j. b. young & co. . to g. f. rodwell, the first science-master appointed at marlborough college, _this book is dedicated_ by the author as a token of esteem and gratitude, and in the hope that it may excite in a few young people some small fraction of the interest and enthusiasm which his advent and his lectures awakened in the author, upon whom the light of science then shone for the first time. preface. i would ask those readers who have grown up, and who may be disposed to find fault with this book, on the ground that in so many points it is incomplete, or that much is so elementary or well known, to remember that the lectures were meant for juveniles, and for juveniles only. these latter i would urge to do their best to repeat the experiments described. they will find that in many cases no apparatus beyond a few pieces of glass or india-rubber pipe, or other simple things easily obtained are required. if they will take this trouble they will find themselves well repaid, and if instead of being discouraged by a few failures they will persevere with the best means at their disposal, they will soon find more to interest them in experiments in which they only succeed after a little trouble than in those which go all right at once. some are so simple that no help can be wanted, while some will probably be too difficult, even with assistance; but to encourage those who wish to see for themselves the experiments that i have described, i have given such hints at the end of the book as i thought would be most useful. i have freely made use of the published work of many distinguished men, among whom i may mention savart, plateau, clerk maxwell, sir william thomson, lord rayleigh, mr. chichester bell, and prof. rücker. the experiments have mostly been described by them, some have been taken from journals, and i have devised or arranged a few. i am also indebted to prof. rücker for the use of various pieces of apparatus which had been prepared for his lectures. soap-bubbles, and the forces which mould them. i do not suppose that there is any one in this room who has not occasionally blown a common soap-bubble, and while admiring the perfection of its form, and the marvellous brilliancy of its colours, wondered how it is that such a magnificent object can be so easily produced. i hope that none of you are yet tired of playing with bubbles, because, as i hope we shall see during the week, there is more in a common bubble than those who have only played with them generally imagine. the wonder and admiration so beautifully portrayed by millais in a picture, copies of which, thanks to modern advertising enterprise, some of you may possibly have seen, will, i hope, in no way fall away in consequence of these lectures; i think you will find that it will grow as your knowledge of the subject increases. you may be interested to hear that we are not the only juveniles who have played with bubbles. ages ago children did the same, and though no mention of this is made by any of the classical authors, we know that they did, because there is an etruscan vase in the louvre in paris of the greatest antiquity, on which children are represented blowing bubbles with a pipe. there is however, no means of telling now whose soap they used. it is possible that some of you may like to know why i have chosen soap-bubbles as my subject; if so, i am glad to tell you. though there are many subjects which might seem to a beginner to be more wonderful, more brilliant, or more exciting, there are few which so directly bear upon the things which we see every day. you cannot pour water from a jug or tea from a tea-pot; you cannot even do anything with a liquid of any kind, without setting in action the forces to which i am about to direct your attention. you cannot then fail to be frequently reminded of what you will hear and see in this room, and, what is perhaps most important of all, many of the things i am going to show you are so simple that you will be able without any apparatus to repeat for yourselves the experiments which i have prepared, and this you will find more interesting and instructive than merely listening to me and watching what i do. there is one more thing i should like to explain, and that is why i am going to show experiments at all. you will at once answer because it would be so dreadfully dull if i didn't. perhaps it would. but that is not the only reason. i would remind you then that when we want to find out anything that we do not know, there are two ways of proceeding. we may either ask somebody else who does know, or read what the most learned men have written about it, which is a very good plan if anybody happens to be able to answer our question; or else we may adopt the other plan, and by arranging an experiment, try for ourselves. an experiment is a question which we ask of nature, who is always ready to give a correct answer, provided we ask properly, that is, provided we arrange a proper experiment. an experiment is not a conjuring trick, something simply to make you wonder, nor is it simply shown because it is beautiful, or because it serves to relieve the monotony of a lecture; if any of the experiments i show are beautiful, or do serve to make these lectures a little less dull, so much the better; but their chief object is to enable you to see for yourselves what the true answers are to questions that i shall ask. [illustration: fig. .] now i shall begin by performing an experiment which you have all probably tried dozens of times. i have in my hand a common camel's-hair brush. if you want to make the hairs cling together and come to a point, you wet it, and then you say the hairs cling together because the brush is wet. now let us try the experiment; but as you cannot see this brush across the room, i hold it in front of the lantern, and you can see it enlarged upon the screen (fig. , left hand). now it is dry, and the hairs are separately visible. i am now dipping it in the water, as you can see, and on taking it out, the hairs, as we expected, cling together (fig. , right hand), because they are wet, as we are in the habit of saying. i shall now hold the brush in the water, but there it is evident that the hairs do not cling at all (fig. , middle), and yet they surely are wet now, being actually in the water. it would appear then that the reason which we always give is not exactly correct. this experiment, which requires nothing more than a brush and a glass of water, then shows that the hairs of a brush cling together not only because they are wet, but for some other reason as well which we do not yet know. it also shows that a very common belief as to opening our eyes under water is not founded on fact. it is very commonly said that if you dive into the water with your eyes shut you cannot see properly when you open them under water, because the water gums the eyelashes down over the eyes; and therefore you must dive in with your eyes open if you wish to see under water. now as a matter of fact this is not the case at all; it makes no difference whether your eyes are open or not when you dive in, you can open them and see just as well either way. in the case of the brush we have seen that water does not cause the hairs to cling together or to anything else when under the water, it is only when taken out that this is the case. this experiment, though it has not explained why the hairs cling together, has at any rate told us that the reason always given is not sufficient. i shall now try another experiment as simple as the last. i have a pipe from which water is very slowly issuing, but it does not fall away continuously; a drop forms which slowly grows until it has attained a certain definite size, and then it suddenly falls away. i want you to notice that every time this happens the drop is always exactly the same size and shape. now this cannot be mere chance; there must be some reason for the definite size, and shape. why does the water remain at all? it is heavy and is ready to fall, but it does not fall; it remains clinging until it is a certain size, and then it suddenly breaks away, as if whatever held it was not strong enough to carry a greater weight. mr. worthington has carefully drawn on a magnified scale the exact shape of a drop of water of different sizes, and these you now see upon the diagram on the wall (fig. ). these diagrams will probably suggest the idea that the water is hanging suspended in an elastic bag, and that the bag breaks or is torn away when there is too great a weight for it to carry. it is true there is no bag at all really, but yet the drops take a shape which suggests an elastic bag. to show you that this is no fancy, i have supported by a tripod a large ring of wood over which a thin sheet of india-rubber has been stretched, and now on allowing water to pour in from this pipe you will see the rubber slowly stretching under the increasing weight, and, what i especially want you to notice, it always assumes a form like those on the diagram. as the weight of water increases the bag stretches, and now that there is about a pailful of water in it, it is getting to a state which indicates that it cannot last much longer; it is like the water-drop just before it falls away, and now suddenly it changes its shape (fig. ), and it would immediately tear itself away if it were not for the fact that india-rubber does not stretch indefinitely; after a time it gets tight and will withstand a greater pull without giving way. you therefore see the great drop now permanently hanging which is almost exactly the same in shape as the water-drop at the point of rupture. i shall now let the water run out by means of a syphon, and then the drop slowly contracts again. now in this case we clearly have a heavy liquid in an elastic bag, whereas in the drop of water we have the same liquid but no bag that is visible. as the two drops behave in almost exactly the same way, we should naturally be led to expect that their form and movements are due to the same cause, and that the small water-drop has something holding it together like the india-rubber you now see. [illustration: fig. .] [illustration: fig. .] let us see how this fits the first experiment with the brush. that showed that the hairs do not cling together simply because they are wet; it is necessary also that the brush should be taken out of the water, or in other words it is necessary that the surface or the skin of the water should be present to bind the hairs together. if then we suppose that the surface of water is like an elastic skin, then both the experiments with the wet brush and with the water-drop will be explained. let us therefore try another experiment to see whether in other ways water behaves as if it had an elastic skin. i have here a plain wire frame fixed to a stem with a weight at the bottom, and a hollow glass globe fastened to it with sealing-wax. the globe is large enough to make the whole thing float in water with the frame up in the air. i can of course press it down so that the frame touches the water. to make the movement of the frame more evident there is fixed to it a paper flag. now if water behaves as if the surface were an elastic skin, then it should resist the upward passage of the frame which i am now holding below the surface. i let go, and instead of bobbing up as it would do if there were no such action, it remains tethered down by this skin of the water. if i disturb the water so as to let the frame out at one corner, then, as you see, it dances up immediately (fig. ). you can see that the skin of the water must have been fairly strong, because a weight of about one quarter of an ounce placed upon the frame is only just sufficient to make the whole thing sink. this apparatus which was originally described by van der mensbrugghe i shall make use of again in a few minutes. [illustration: fig. .] i can show you in a more striking way that there is this elastic layer or skin on pure clean water. i have a small sieve made of wire gauze sufficiently coarse to allow a common pin to be put through any of the holes. there are moreover about eleven thousand of these holes in the bottom of the sieve. now, as you know, clean wire is wetted by water, that is, if it is dipped in water it comes out wet; on the other hand, some materials, such as paraffin wax, of which paraffin candles are made, are not wetted or really touched by water, as you may see for yourselves if you will only dip a paraffin candle into water. i have melted a quantity of paraffin in a dish and dipped this gauze into the melted paraffin so as to coat the wire all over with it, but i have shaken it well while hot to knock the paraffin out of the holes. you can now see on the screen that the holes, all except one or two, are open, and that a common pin can be passed through readily enough. this then is the apparatus. now if water has an elastic skin which it requires force to stretch, it ought not to run through these holes very readily; it ought not to be able to get through at all unless forced, because at each hole the skin would have to be stretched to allow the water to get to the other side. this you understand is only true if the water does not wet or really touch the wire. now to prevent the water that i am going to pour in from striking the bottom with so much force as to drive it through, i have laid a small piece of paper in the sieve, and am pouring the water on to the paper, which breaks the fall (fig. ). i have now poured in about half a tumbler of water, and i might put in more. i take away the paper but not a drop runs through. if i give the sieve a jolt then the water is driven to the other side, and in a moment it has all escaped. perhaps this will remind you of one of the exploits of our old friend simple simon, "who went for water in a sieve, but soon it all ran through." but you see if you only manage the sieve properly, this is not quite so absurd as people generally suppose. [illustration: fig. .] if now i shake the water off the sieve, i can, for the same reason, set it to float on water, because its weight is not sufficient to stretch the skin of the water through all the holes. the water, therefore, remains on the other side, and it floats even though, as i have already said, there are eleven thousand holes in the bottom, any one of which is large enough to allow an ordinary pin to pass through. this experiment also illustrates how difficult it is to write real and perfect nonsense. you may remember one of the stories in lear's book of nonsense songs. "they went to sea in a sieve, they did, in a sieve they went to sea: in spite of all their friends could say, on a winter's morn, on a stormy day, in a sieve they went to sea. * * * "they sailed away in a sieve, they did, in a sieve they sailed so fast, with only a beautiful pea-green veil, tied with a riband by way of a sail, to a small tobacco-pipe mast;" and so on. you see that it is quite possible to go to sea in a sieve--that is, if the sieve is large enough and the water is not too rough--and that the above lines are now realized in every particular (fig. ). [illustration: fig. .] i may give one more example of the power of this elastic skin of water. if you wish to pour water from a tumbler into a narrow-necked bottle, you know how if you pour slowly it nearly all runs down the side of the glass and gets spilled about, whereas if you pour quickly there is no room for the great quantity of water to pass into the bottle all at once, and so it gets spilled again. but if you take a piece of stick or a glass rod, and hold it against the edge of the tumbler, then the water runs down the rod and into the bottle, and none is lost (fig. ); you may even hold the rod inclined to one side, as i am now doing, but the water runs down the wet rod because this elastic skin forms a kind of tube which prevents the water from escaping. this action is often made use of in the country to carry the water from the gutters under the roof into a water-butt below. a piece of stick does nearly as well as an iron pipe, and it does not cost anything like so much. [illustration: fig. .] i think then i have now done enough to show that on the surface of water there is a kind of elastic skin. i do not mean that there is anything that is not water on the surface, but that the water while there acts in a different way to what it does inside, and that it acts as if it were an elastic skin made of something like very thin india-rubber, only that it is perfectly and absolutely elastic, which india-rubber is not. you will now be in a position to understand how it is that in narrow tubes water does not find its own level, but behaves in an unexpected manner. i have placed in front of the lantern a dish of water coloured blue so that you may the more easily see it. i shall now dip into the water a very narrow glass pipe, and immediately the water rushes up and stands about half an inch above the general level. the tube inside is wet. the elastic skin of the water is therefore attached to the tube, and goes on pulling up the water until the weight of the water raised above the general level is equal to the force exerted by the skin. if i take a tube about twice as big, then this pulling action which is going on all round the tube will cause it to lift twice the weight of water, but this will not make the water rise twice as high, because the larger tube holds so much more water for a given length than the smaller tube. it will not even pull it up as high as it did in the case of the smaller tube, because if it were pulled up as high the weight of the water raised would in that case be four times as great, and not only twice as great, as you might at first think. it will therefore only raise the water in the larger tube to half the height, and now that the two tubes are side by side you see the water in the smaller tube standing twice as high as it does in the larger tube. in the same way, if i were to take a tube as fine as a hair the water would go up ever so much higher. it is for this reason that this is called capillarity, from the latin word _capillus_, a hair, because the action is so marked in a tube the size of a hair. [illustration: fig. .] supposing now you had a great number of tubes of all sizes, and placed them in a row with the smallest on one side and all the others in the order of their sizes, then it is evident that the water would rise highest in the smallest tube and less and less high in each tube in the row (fig. ), until when you came to a very large tube you would not be able to see that the water was raised at all. you can very easily obtain the same kind of effect by simply taking two square pieces of window glass and placing them face to face with a common match or small fragment of anything to keep them a small distance apart along one edge while they meet together along the opposite edge. an india-rubber ring stretched over them will hold them in this position. i now take this pair of plates and stand it in a dish of coloured water, and you at once see that the water creeps up to the top of the plates on the edge where they meet, and as the distance between the plates gradually increases, so the height to which the water rises gradually gets less, and the result is that the surface of the liquid forms a beautifully regular curve which is called by mathematicians a rectangular hyperbola (fig. ). i shall have presently to say more about this and some other curves, and so i shall not do more now than state that the hyperbola is formed because as the width between the plates gets greater the height gets less, or, what comes to the same thing, because the weight of liquid pulled up at any small part of the curve is always the same. [illustration: fig. .] if the plates or the tubes had been made of material not wetted by water, then the effect of the tension of the surface would be to drag the liquid away from the narrow spaces, and the more so as the spaces were narrower. as it is not easy to show this well with paraffined glass plates or tubes and water, i shall use another liquid which does not wet or touch clean glass, namely, quicksilver. as it is not possible to see through quicksilver, it will not do to put a narrow tube into this liquid to show that the level is lower in the tube than in the surrounding vessel, but the same result may be obtained by having a wide and a narrow tube joined together. then, as you see upon the screen, the quicksilver is lower in the narrow than in the wide tube, whereas in a similar apparatus the reverse is the case with water (fig. ). [illustration: fig. .] i want you now to consider what is happening when two flat plates partly immersed in water are held close together. we have seen that the water rises between them. those parts of these two plates, which have air between them and also air outside them (indicated by the letter _a_ in fig. ), are each of them pressed equally in opposite directions by the pressure of the air, and so these parts do not tend to approach or to recede from one another. these parts again which have water on each side of each of them (as indicated by the letter _c_) are equally pressed in opposite directions by the pressure of the water, and so these parts do not tend to approach or to recede from one another. but those parts of the plates (_b_) which have water between them and air outside would, you might think, be pushed apart by the water between them with a greater force than that which could be exerted by the air outside, and so you might be led to expect that on this account a pair of plates if free to move would separate at once. but such an idea though very natural is wrong, and for this reason. the water that is raised between the plates being above the general level must be under a less pressure, because, as every one knows, as you go down in water the pressure increases, and so as you go up the pressure must get less. the water then that is raised between the plates is under a less pressure than the air outside, and so on the whole the plates are pushed together. you can easily see that this is the case. i have two very light hollow glass beads such as are used to decorate a christmas tree. these will float in water if one end is stopped with sealing-wax. these are both wetted by water, and so the water between them is slightly raised, for they act in the same way as the two plates, but not so powerfully. however, you will have no difficulty in seeing that the moment i leave them alone they rush together with considerable force. now if you refer to the second figure in the diagram, which represents two plates which are neither of them wetted, i think you will see, without any explanation from me, that they should be pressed together, and this is made evident by experiment. two other beads which have been dipped in paraffin wax so that they are neither of them wetted by water float up to one another again when separated as though they attracted each other just as the clean glass beads did. [illustration: fig. .] if you again consider these two cases, you will see that a plate that is wetted tends to move towards the higher level of the liquid, whereas one that is not wetted tends to move towards the lower level, that is if the level of the liquid on the two sides is made different by capillary action. now suppose one plate wetted and the other not wetted, then, as the diagram imperfectly shows, the level of the liquid between the plates _where it meets_ the non-wetted plate is higher than that outside, while where it meets the wetted plate it is lower than that outside; so each plate tends to go away from the other, as you can see now that i have one paraffined and one clean ball floating in the same water. they appear to repel one another. you may also notice that the surface of the liquid near a wetted plate is curved, with the hollow of the curve upwards, while near a non-wetted plate the reverse is the case. that this curvature of the surface is of the first importance i can show you by a very simple experiment, which you can repeat at home as easily as the last that i have shown. i have a clean glass bead floating in water in a clean glass vessel, which is not quite full. the bead always goes to the side of the vessel. it is impossible to make it remain in the middle, it always gets to one side or the other directly. i shall now gradually add water until the level of the water is rather higher than that of the edge of the vessel. the surface is then rounded near the vessel, while it is hollow near the bead, and now the bead sails away towards the centre, and can by no possibility be made to stop near either side. with a paraffined bead the reverse is the case, as you would expect. instead of a paraffined bead you may use a common needle, which you will find will float on water in a tumbler, if placed upon it very gently. if the tumbler is not quite full the needle will always go away from the edge, but if rather over-filled it will work up to one side, and then possibly roll over the edge; any bubbles, on the other hand, which were adhering to the glass before will, the instant that the water is above the edge of the glass, shoot away from the edge in the most sudden and surprising manner. this sudden change can be most easily seen by nearly filling the glass with water, and then gradually dipping in and taking out a cork, which will cause the level to slowly change. so far i have given you no idea what force is exerted by this elastic skin of water. measurements made with narrow tubes, with drops, and in other ways, all show that it is almost exactly equal to the weight of three and a quarter grains to the inch. we have, moreover, not yet seen whether other liquids act in the same way, and if so whether in other cases the strength of the elastic skin is the same. you now see a second tube identical with that from which drops of water were formed, but in this case the liquid is alcohol. now that drops are forming, you see at once that while alcohol makes drops which have a definite size and shape when they fall away, the alcohol drops are not by any means so large as the drops of water which are falling by their side. two possible reasons might be given to explain this. either alcohol is a heavier liquid than water, which would account for the smaller drop if the skin in each liquid had the same strength, or else if alcohol is not heavier than water its skin must be weaker than the skin of water. as a matter of fact alcohol is a lighter liquid than water, and so still more must the skin of alcohol be weaker than that of water. [illustration: fig. .] we can easily put this to the test of experiment. in the game that is called the tug-of-war you know well enough which side is the strongest; it is the side which pulls the other over the line. let us then make alcohol and water play the same game. in order that you may see the water, it is coloured blue. it is lying as a shallow layer on the bottom of this white dish. at the present time the skin of the water is pulling equally in all directions, and so nothing happens; but if i pour a few drops of alcohol into the middle, then at the line which separates the alcohol from the water we have alcohol on one side pulling in, while we have water on the other side pulling out, and you see the result. the water is victorious; it rushes away in all directions, carrying a quantity of the alcohol away with it, and leaves the bottom of the dish dry (fig. ). [illustration: fig. .] this difference in the strength of the skin of alcohol and of water, or of water containing much or little alcohol, gives rise to a curious motion which you may see on the side of a wine-glass in which there is some fairly strong wine, such as port. the liquid is observed to climb up the sides of the glass, then to gather into drops, and to run down again, and this goes on for a long time. this is explained as follows:--the thin layer of wine on the side of the glass being exposed to the air, loses its alcohol by evaporation more quickly than the wine in the glass. it therefore becomes weaker in alcohol or stronger in water than that below, and for this reason it has a stronger skin. it therefore pulls up more wine from below, and this goes on until there is so much that drops form, and it runs back again into the glass, as you now see upon the screen (fig. ). there can be no doubt that this movement is referred to in proverbs xxiii. : "look not thou upon the wine when it is red, when it giveth his colour in the cup, when it moveth itself aright." if you remember that this movement only occurs with strong wine, and that it must have been known to every one at the time that these words were written, and used as a test of the strength of wine, because in those days every one drank wine, then you will agree that this explanation of the meaning of that verse is the right one. i would ask you also to consider whether it is not probable that other passages which do not now seem to convey to us any meaning whatever, may not in the same way have referred to the common knowledge and customs of the day, of which at the present time we happen to be ignorant. [illustration: fig. .] ether, in the same way, has a skin which is weaker than the skin of water. the very smallest quantity of ether on the surface of water will produce a perceptible effect. for instance, the wire frame which i left some time ago is still resting against the water-skin. the buoyancy of the glass bulb is trying to push it through, but the upward force is just not sufficient. i will however pour a few drops of ether into a glass, and simply pour the vapour upon the surface of the water (not a drop of _liquid_ is passing over), and almost immediately sufficient ether has condensed upon the water to reduce the strength of the skin to such an extent that the frame jumps up out of the water. there is a well-known case in which the difference between the strength of the skins of two liquids may be either a source of vexation or, if we know how to make use of it, an advantage. if you spill grease on your coat you can take it out very well with benzine. now if you apply benzine to the grease, and then apply fresh benzine to that already there, you have this result--there is then greasy benzine on the coat to which you apply fresh benzine. it so happens that greasy benzine has a stronger skin than pure benzine. the greasy benzine therefore plays at tug-of-war with pure benzine, and being stronger wins and runs away in all directions, and the more you apply benzine the more the greasy benzine runs away carrying the grease with it. but if you follow the directions on the bottle, and first make a ring of clean benzine round the grease-spot, and then apply benzine to the grease, you then have the greasy benzine running away from the pure benzine ring and heaping itself together in the middle, and escaping into the fresh rag that you apply, so that the grease is all of it removed. there is a difference again between hot and cold grease, as you may see, when you get home, if you watch a common candle burning. close to the flame the grease is hotter than it is near the outside. it has therefore a weaker skin, and so a perpetual circulation is kept up, and the grease runs out on the surface and back again below, carrying little specks of dust which make this movement visible, and making the candle burn regularly. you probably know how to take out grease-stains with a hot poker and blotting-paper. here again the same kind of action is going on. a piece of lighted camphor floating in water is another example of movement set up by differences in the strength of the skin of water owing to the action of the camphor. i will give only one more example. if you are painting in water-colours on greasy paper or certain shiny surfaces the paint will not lie smoothly on the paper, but runs together in the well-known way; a very little ox-gall, however, makes it lie perfectly, because ox-gall so reduces the strength of the skin of water that it will wet surfaces that pure water will not wet. this reduction of the surface tension you can see if i use the same wire frame a third time. the ether has now evaporated, and i can again make it rest against the surface of the water, but very soon after i touch the water with a brush containing ox-gall the frame jumps up as suddenly as before. it is quite unnecessary that i should any further insist upon the fact that the outside of a liquid acts as if it were a perfectly elastic skin stretched with a certain definite force. suppose now that you take a small quantity of water, say as much as would go into a nut-shell, and suddenly let it go, what will happen? of course it will fall down and be dashed against the ground. or again, suppose you take the same quantity of water and lay it carefully upon a cake of paraffin wax dusted over with lycopodium which it does not wet, what will happen? here again the weight of the drop--that which makes it fall if not held--will squeeze it against the paraffin and make it spread out into a flat cake. what would happen if the weight of the drop or the force pulling it downwards could be prevented from acting? in such a case the drop would only feel the effect of the elastic skin, which would try to pull it into such a form as to make the surface as small as possible. it would in fact rapidly become a perfectly round ball, because in no other way can so small a surface be obtained. if, instead of taking so much water, we were to take a drop about as large as a pin's head, then the weight which tends to squeeze it out or make it fall would be far less, while the skin would be just as strong, and would in reality have a greater moulding power, though why i cannot now explain. we should therefore expect that by taking a sufficiently small quantity of water the moulding power of the skin would ultimately be able almost entirely to counteract the weight of the drop, so that very small drops should appear like perfect little balls. if you have found any difficulty in following this argument, a very simple illustration will make it clear. many of you probably know how by folding paper to make this little thing which i hold in my hand (fig. ). it is called a cat-box, because of its power of dispelling cats when it is filled with water and well thrown. this one, large enough to hold about half a pint, is made out of a small piece of the _times_ newspaper. you may fill it with water and carry it about and throw it with your full power, and the strength of the paper skin is sufficient to hold it together until it hits anything, when of course it bursts and the water comes out. on the other hand, the large one made out of a whole sheet of the _times_ is barely able to withstand the weight of the water that it will hold. it is only just strong enough to allow of its being filled and carried, and then it may be dropped from a height, but you cannot throw it. in the same way the weaker skin of a liquid will not make a large quantity take the shape of a ball, but it will mould a minute drop so perfectly that you cannot tell by looking at it that it is not perfectly round every way. this is most easily seen with quicksilver. a large quantity rolls about like a flat cake, but the very small drops obtained by throwing some violently on the table and so breaking it up appear perfectly round. you can see the same difference in the beads of gold now upon the screen (fig. ). they are now solid, but they were melted and then allowed to cool without being disturbed. though the large bead is flattened by its weight, the small one appears perfectly round. finally, you may see the same thing with water if you dust a little lycopodium on the table. then water falling will roll itself up into perfect little balls. you may even see the same thing on a dusty day if you water the road with a water-pot. [illustration: fig. .] [illustration: fig. .] if it were not for the weight of liquids, that is the force with which they are pulled down towards the earth, large drops would be as perfectly round as small ones. this was first beautifully shown by plateau, the blind experimentalist, who placed one liquid inside another which is equally heavy, and with which it does not mix. alcohol is lighter than oil, while water is heavier, but a suitable mixture of alcohol and water is just as heavy as oil, and so oil does not either tend to rise or to fall when immersed in such a mixture. i have in front of the lantern a glass box containing alcohol and water, and by means of a tube i shall slowly allow oil to flow in. you see that as i remove the tube it becomes a perfect ball as large as a walnut. there are now two or three of these balls of oil all perfectly round. i want you to notice that when i hit them on one side the large balls recover their shape slowly, while the small ones become round again much more quickly. there is a very beautiful effect which can be produced with this apparatus, and though it is not necessary to refer to it, it is well worth while now that the apparatus is set up to show it to you. in the middle of the box there is an axle with a disc upon it to which i can make the oil adhere. now if i slowly turn the wire and disc the oil will turn also. as i gradually increase the speed the oil tends to fly away in all directions, but the elastic skin retains it. the result is that the ball becomes flattened at its poles like the earth itself. on increasing the speed, the tendency of the oil to get away is at last too much for the elastic skin, and a ring breaks away (fig. ), which almost immediately contracts again on to the rest of the ball as the speed falls. if i turn it sufficiently fast the ring breaks up into a series of balls which you now see. one cannot help being reminded of the heavenly bodies by this beautiful experiment of plateau's, for you see a central body and a series of balls of different sizes all travelling round in the same direction (fig. ); but the forces which are acting in the two cases are totally distinct, and what you see has nothing whatever to do with the sun and the planets. [illustration: fig. .] [illustration: fig. .] we have thus seen that a large ball of liquid can be moulded by the elasticity of its skin if the disturbing effect of its weight is neutralized, as in the last experiment. this disturbing effect is practically of no account in the case of a soap-bubble, because it is so thin that it hardly weighs anything. you all know, of course, that a soap-bubble is perfectly round, and now you know why; it is because the elastic film, trying to become as small as it can, must take the form which has the smallest surface for its content, and that form is the sphere. i want you to notice here, as with the oil, that a large bubble oscillates much more slowly than a small one when knocked out of shape with a bat covered with baize or wool. the chief result that i have endeavoured to make clear to-day is this. the outside of a liquid acts as if it were an elastic skin, which will, as far as it is able, so mould the liquid within it that it shall be as small as possible. generally the weight of liquids, especially when there is a large quantity, is too much for the feebly elastic skin, and its power may not be noticed. the disturbing effect of weight is got rid of by immersing one liquid in another which is equally heavy with which it does not mix, and it is hardly noticed when very small drops are examined, or when a bubble is blown, for in these cases the weight is almost nothing, while the elastic power of the skin is just as great as ever. lecture ii. i did not in the last lecture by any direct experiment show that a soap-film or bubble is really elastic, like a piece of stretched india-rubber. a soap-bubble consisting, as it does, of a thin layer of liquid, which must have of course both an inside and an outside surface or skin, must be elastic, and this is easily shown in many ways. perhaps the easiest way is to tie a thread across a ring rather loosely, and then to dip the ring into soap water. on taking it out there is a film stretched over the ring, in which the thread moves about quite freely, as you can see upon the screen. but if i break the film on one side, then immediately the thread is pulled by the film on the other side as far as it can go, and it is now tight (fig. ). you will also notice that it is part of a perfect circle, because that form makes the space on one side as great, and therefore on the other side, where the film is, as small, as possible. or again, in this second ring the thread is double for a short distance in the middle. if i break the film between the threads they are at once pulled apart, and are pulled into a perfect circle (fig. ), because that is the form which makes the space within it as great as possible, and therefore leaves the space outside it as small as possible. you will also notice, that though the circle will not allow itself to be pulled out of shape, yet it can move about in the ring quite freely, because such a movement does not make any difference to the space outside it. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] i have now blown a bubble upon a ring of wire. i shall hang a small ring upon it, and to show more clearly what is happening, i shall blow a little smoke into the bubble. now that i have broken the film inside the lower ring, you will see the smoke being driven out and the ring lifted up, both of which show the elastic nature of the film. or again, i have blown a bubble on the end of a wide pipe; on holding the open end of the pipe to a candle flame, the outrushing air blows out the flame at once, which shows that the soap-bubble is acting like an elastic bag (fig. ). you now see that, owing to the elastic skin of a soap-bubble, the air inside is under pressure and will get out if it can. which would you think would squeeze the air inside it most, a large or a small bubble? we will find out by trying, and then see if we can tell why. you now see two pipes each with a tap. these are joined together by a third pipe in which there is a third tap. i will first blow one bubble and shut it off with the tap (fig. ), and then the other, and shut it off with the tap . they are now nearly equal in size, but the air cannot yet pass from one to the other because the tap is turned off. now if the pressure in the largest one is greatest it will blow air into the other when i open this tap, until they are equal in size; if, on the other hand, the pressure in the small one is greatest, it will blow air into the large one, and will itself get smaller until it has quite disappeared. we will now try the experiment. you see immediately that i open the tap the small bubble shuts up and blows out the large one, thus showing that there is a greater pressure in a small than in a large bubble. the directions in which the air and the bubble move is indicated in the figure by arrows. i want you particularly to notice and remember this, because this is an experiment on which a great deal depends. to impress this upon your memory i shall show the same thing in another way. there is in front of the lantern a little tube shaped like a u half filled with water. one end of the u is joined to a pipe on which a bubble can be blown (fig. ). you will now be able to see how the pressure changes as the bubble increases in size, because the water will be displaced more when the pressure is more, and less when it is less. now that there is a very small bubble, the pressure as measured by the water is about one quarter of an inch on the scale. the bubble is growing and the pressure indicated by the water in the gauge is falling, until, when the bubble is double its former size, the pressure is only half what it was; and this is always true, the smaller the bubble the greater the pressure. as the film is always stretched with the same force, whatever size the bubble is, it is clear that the pressure inside can only depend upon the curvature of a bubble. in the case of lines, our ordinary language tells us, that the larger a circle is the less is its curvature; a piece of a small circle is said to be a quick or a sharp curve, while a piece of a great circle is only slightly curved; and if you take a piece of a very large circle indeed, then you cannot tell it from a straight line, and you say it is not curved at all. with a part of the surface of a ball it is just the same--the larger the ball the less it is curved; and if the ball is very large indeed, say miles across, you cannot tell a small piece of it from a true plane. level water is part of such a surface, and you know that still water in a basin appears perfectly flat, though in a very large lake or the sea you can see that it is curved. we have seen that in large bubbles the pressure is little and the curvature is little, while in small bubbles the pressure is great and the curvature is great. the pressure and the curvature rise and fall together. we have now learnt the lesson which the experiment of the two bubbles, one blown out by the other, teaches us. [illustration: fig. .] [illustration: fig. .] a ball or sphere is not the only form which you can give to a soap-bubble. if you take a bubble between two rings, you can pull it out until at last it has the shape of a round straight tube or cylinder as it is called. we have spoken of the curvature of a ball or sphere; now what is the curvature of a cylinder? looked at sideways, the edge of the wooden cylinder upon the table appears straight, _i. e._ not curved at all; but looked at from above it appears round, and is seen to have a definite curvature (fig. ). what then is the curvature of the surface of a cylinder? we have seen that the pressure in a bubble depends upon the curvature when they are spheres, and this is true whatever shape they have. if, then, we find what sized sphere will produce the same pressure upon the air inside that a cylinder does, then we shall know that the curvature of the cylinder is the same as that of the sphere which balances it. now at each end of a short tube i shall blow an ordinary bubble, but i shall pull the lower bubble by means of another tube into the cylindrical form, and finally blow in more or less air until the sides of the cylinder are perfectly straight. that is now done (fig. ), and the pressure in the two bubbles must be exactly the same, as there is a free passage of air between the two. on measuring them you see that the sphere is exactly double the cylinder in diameter. but this sphere has only half the curvature that a sphere half its diameter would have. therefore the cylinder, which we know has the same curvature that the large sphere has, because the two balance, has only half the curvature of a sphere of its own diameter, and the pressure in it is only half that in a sphere of its own diameter. [illustration: fig. .] i must now make one more step in explaining this question of curvature. now that the cylinder and sphere are balanced i shall blow in more air, making the sphere larger; what will happen to the cylinder? the cylinder is, as you see, very short; will it become blown out too, or what will happen? now that i am blowing in air you see the sphere enlarging, thus relieving the pressure; the cylinder develops a waist, it is no longer a cylinder, the sides are curved inwards. as i go on blowing and enlarging the sphere, they go on falling inwards, but not indefinitely. if i were to blow the upper bubble till it was of an enormous size the pressure would become extremely small. let us make the pressure nothing at all at once by simply breaking the upper bubble, thus allowing the air a free passage from the inside to the outside of what was the cylinder. let me repeat this experiment on a larger scale. i have two large glass rings, between which i can draw out a film of the same kind. not only is the outline of the soap-film curved inwards, but it is exactly the same as the smaller one in shape (fig. ). as there is now no pressure there ought to be no curvature, if what i have said is correct. but look at the soap-film. who would venture to say that that was not curved? and yet we had satisfied ourselves that the pressure and the curvature rose and fell together. we now seem to have come to an absurd conclusion. because the pressure is reduced to nothing we say the surface must have no curvature, and yet a glance is sufficient to show that the film is so far curved as to have a most elegant waist. now look at the plaster model on the table, which is a model of a mathematical figure which also has a waist. [illustration: fig. .] let us therefore examine this cast more in detail. i have a disc of card which has exactly the same diameter as the waist of the cast. i now hold this edgeways against the waist (fig. ), and though you can see that it does not fit the whole curve, it fits the part close to the waist perfectly. this then shows that this part of the cast would appear curved inwards if you looked at it sideways, to the same extent that it would appear curved outwards if you could see it from above. so considering the waist only, it is curved both towards the inside and also away from the inside according to the way you look at it, and to the same extent. the curvature inwards would make the pressure inside less, and the curvature outwards would make it more, and as they are equal they just balance, and there is no pressure at all. if we could in the same way examine the bubble with the waist, we should find that this was true not only at the waist but at every part of it. any curved surface like this which at every point is equally curved opposite ways, is called a surface of no curvature, and so what seemed an absurdity is now explained. now this surface, which is the only one of the kind symmetrical about an axis, except a flat surface, is called a catenoid, because it is like a chain, as you will see directly, and, as you know, _catena_ is the latin for a chain. i shall now hang a chain in a loop from a level stick, and throw a strong light upon it, so that you can see it well (fig. ). this is exactly the same shape as the side of a bubble drawn out between two rings, and open at the end to the air.[ ] [illustration: fig. .] [illustration: fig. .] [footnote : if the reader finds these geometrical relations too difficult to follow, he or she should skip the next pages, and go on again at "we have found...." p. .] let us now take two rings, and having placed a bubble between them, gradually alter the pressure. you can tell what the pressure is by looking at the part of the film which covers either ring, which i shall call the cap. this must be part of a sphere, and we know that the curvature of this and the pressure inside rise and fall together. i have now adjusted the bubble so that it is a nearly perfect sphere. if i blow in more air the caps become more curved, showing an increased pressure, and the sides bulge out even more than those of a sphere (fig. ). i have now brought the whole bubble back to the spherical form. a little increased pressure, as shown by the increased curvature of the cap, makes the sides bulge more; a little less pressure, as shown by the flattening of the caps, makes the sides bulge less. now the sides are straight, and the cap, as we have already seen, forms part of a sphere of twice the diameter of the cylinder. i am still further reducing the pressure until the caps are plane, that is, not curved at all. there is now no pressure inside, and therefore the sides have, as we have already seen, taken the form of a hanging chain; and now, finally, the pressure inside is less than that outside, as you can see by the caps being drawn inwards, and the sides have even a smaller waist than the catenoid. we have now seen seven curves as we gradually reduced the pressure, namely-- . outside the sphere. . the sphere. . between the sphere and the cylinder. . the cylinder. . between the cylinder and the catenoid. . the catenoid. . inside the catenoid. [illustration: fig. .] now i am not going to say much more about all these curves, but i must refer to the very curious properties that they possess. in the first place, they must all of them have the same curvature in every part as the portion of the sphere which forms the cap; in the second place, they must all be the curves of the least possible surface which can enclose the air and join the rings as well. and finally, since they pass insensibly from one to the other as the pressure gradually changes, though they are distinct curves there must be some curious and intimate relation between them. this though it is a little difficult, i shall explain. if i were to say that these curves are the roulettes of the conic sections i suppose i should alarm you, and at the same time explain nothing, so i shall not put it in that way; but instead, i shall show you a simple experiment which will throw some light upon the subject, which you can try for yourselves at home. [illustration: fig. .] i have here a common bedroom candlestick with a flat round base. hold the candlestick exactly upright near to a white wall, then you will see the shadow of the base on the wall below, and the outline of the shadow is a symmetrical curve, called a hyperbola. gradually tilt the candle away from the wall, you will then notice the sides of the shadow gradually branch away less and less, and when you have so far tilted the candle away from the wall that the flame is exactly above the edge of the base,--and you will know when this is the case, because then the falling grease will just fall on the edge of the candlestick and splash on to the carpet,--i have it so now,--the sides of the shadow near the floor will be almost parallel (fig. ), and the shape of the shadow will have become a curve, known as a parabola; and now when the candlestick is still more tilted, so that the grease misses the base altogether and falls in a gentle stream upon the carpet, you will see that the sides of the shadow have curled round and met on the wall, and you now have a curve like an oval, except that the two ends are alike, and this is called an ellipse. if you go on tilting the candlestick, then when the candle is just level, and the grease pouring away, the shadow will be almost a circle; it would be an exact circle if the flame did not flare up. now if you go on tilting the candle, until at last the candlestick is upside down, the curves already obtained will be reproduced in the reverse order, but above instead of below you. you may well ask what all this has to do with a soap-bubble. you will see in a moment. when you light a candle, the base of the candlestick throws the space behind it into darkness, and the form of this dark space, which is everywhere round like the base, and gets larger as you get further from the flame, is a cone, like the wooden model on the table. the shadow cast on the wall is of course the part of the wall which is within this cone. it is the same shape that you would find if you were to cut a cone through with a saw, and so these curves which i have shown you are called conic sections. you can see some of them already made in the wooden model on the table. if you look at the diagram on the wall (fig. ), you will see a complete cone at first upright (a), then being gradually tilted over into the positions that i have specified. the black line in the upper part of the diagram shows where the cone is cut through, and the shaded area below shows the true shape of these shadows, or pieces cut off, which are called sections. now in each of these sections there are either one or two points, each of which is called a focus, and these are indicated by conspicuous dots. in the case of the circle (d fig. ), this point is also the centre. now if this circle is made to roll like a wheel along the straight line drawn just below it, a pencil at the centre will rule the straight line which is dotted in the lower part of the figure; but if we were to make wheels of the shapes of any of the other sections, a pencil at the focus would certainly not draw a straight line. what shape it would draw is not at once evident. first consider any of the elliptic sections (c, e, or f) which you see on either side of the circle. if these were wheels, and were made to roll, the pencil as it moved along would also move up and down, and the line it would draw is shown dotted as before in the lower part of the figure. in the same way the other curves, if made to roll along a straight line, would cause pencils at their focal points to draw the other dotted lines. [illustration: fig. .] we are now almost able to see what the conic section has to do with a soap-bubble. when a soap-bubble was blown between two rings, and the pressure inside was varied, its outline went through a series of forms, some of which are represented by the dotted lines in the lower part of the figure, but in every case they could have been accurately drawn by a pencil at the focus of a suitable conic section made to roll on a straight line. i called one of the bubble forms, if you remember, by its name, catenoid; this is produced when there is no pressure. the dotted curve in the second figure b is this one; and to show that this catenary can be so drawn, i shall roll upon a straight edge a board made into the form of the corresponding section, which is called a parabola, and let the chalk at its focus draw its curve upon the black board. there is the curve, and it is as i said, exactly the curve that a chain makes when hung by its two ends. now that a chain is so hung you see that it exactly lies over the chalk line. all this is rather difficult to understand, but as these forms which a soap-bubble takes afford a beautiful example of the most important principle of continuity, i thought it would be a pity to pass it by. it may be put in this way. a series of bubbles may be blown between a pair of rings. if the pressures are different the curves must be different. in blowing them the pressures slowly and _continuously_ change, and so the curves cannot be altogether different in kind. though they may be different curves, they also must pass slowly and continuously one into the other. we find the bubble curves can be drawn by rolling wheels made in the shape of the conic sections on a straight line, and so the conic sections, though distinct curves, must pass slowly and continuously one into the other. this we saw was the case, because as the candle was slowly tilted the curves did as a fact slowly and insensibly change from one to the other. there was only one parabola, and that was formed when the side of the cone was parallel to the plane of section, that is when the falling grease just touched the edge of the candlestick; there is only one bubble with no pressure, the catenoid, and this is drawn by rolling the parabola. as the cone is gradually inclined more, so the sections become at first long ellipses, which gradually become more and more round until a circle is reached, after which they become more and more narrow until a line is reached. the corresponding bubble curves are produced by a gradually increasing pressure, and, as the diagram shows, these bubble curves are at first wavy (c), then they become straight when a cylinder is formed (d), then they become wavy again (e and f), and at last, when the cutting plane, _i. e._ the black line in the upper figure, passes through the vertex of the cone the waves become a series of semicircles, indicating the ordinary spherical soap-bubble. now if the cone is inclined ever so little more a new shape of section is seen (g), and this being rolled, draws a curious curve with a loop in it; but how this is so it would take too long to explain. it would also take too long to trace the further positions of the cone, and to trace the corresponding sections and bubble curves got by rolling them. careful inspection of the diagram may be sufficient to enable you to work out for yourselves what will happen in all cases. i should explain that the bubble surfaces are obtained by spinning the dotted lines about the straight line in the lower part of fig. as an axis. as you will soon find out if you try, you cannot make with a soap-bubble a great length of any of these curves at one time, but you may get pieces of any of them with no more apparatus than a few wire rings, a pipe, and a little soap and water. you can even see the whole of one of the loops of the dotted curve of the first figure (a), which is called a nodoid, not a complete ring, for that is unstable, but a part of such a ring. take a piece of wire or a match, and fasten one end to a piece of lead, so that it will stand upright in a dish of soap water, and project half an inch or so. hold with one hand a sheet of glass resting on the match in middle, and blow a bubble in the water against the match. as soon as it touches the glass plate, which should be wetted with the soap solution, it will become a cylinder, which will meet the glass plate in a true circle. now very slowly incline the plate. the bubble will at once work round to the lowest side, and try to pull itself away from the match stick, and in doing so it will develop a loop of the nodoid, which would be exactly true in form if the match or wire were slightly bent, so as to meet both the glass and the surface of the soap water at a right angle. i have described this in detail, because it is not generally known that a complete loop of the nodoid can be made with a soap-bubble. [illustration: fig. .] [illustration: fig. .] we have found that the pressure in a short cylinder gets less if it begins to develop a waist, and greater if it begins to bulge. let us therefore try and balance one with a bulge against another with a waist. immediately that i open the tap and let the air pass, the one with a bulge blows air round to the one with a waist and they both become straight. in fig. the direction of the movement of the air and of the sides of the bubble is indicated by arrows. let us next try the same experiment with a pair of rather longer cylinders, say about twice as long as they are wide. they are now ready, one with a bulge and one with a waist. directly i open the tap, and let the air pass from one to the other, the one with a waist blows out the other still more (fig. ), until at last it has shut itself up. it therefore behaves exactly in the opposite way that the short cylinder did. if you try pairs of cylinders of different lengths you will find that the change occurs when they are just over one and a half times as long as they are wide. now if you imagine one of these tubes joined on to the end of the other, you will see that a cylinder more than about three times as long as it is wide cannot last more than a moment; because if one end were to contract ever so little the pressure there would increase, and the narrow end would blow air into the wider end (fig. ), until the sides of the narrow end met one another. the exact length of the longest cylinder that is stable, is a little more than three diameters. the cylinder just becomes unstable when its length is equal to its circumference, and this is - / diameters almost exactly. [illustration: fig. .] i will gradually separate these rings, keeping up a supply of air, and you will see that when the tube gets nearly three times as long as it is wide it is getting very difficult to manage, and then suddenly it grows a waist nearer one end than the other, and breaks off forming a pair of separate and unequal bubbles. if now you have a cylinder of liquid of great length suddenly formed and left to itself, it clearly cannot retain that form. it must break up into a series of drops. unfortunately the changes go on so quickly in a falling stream of water that no one by merely looking at it could follow the movements of the separate drops, but i hope to be able to show to you in two or three ways exactly what is happening. you may remember that we were able to make a large drop of one liquid in another, because in this way the effect of the weight was neutralized, and as large drops oscillate or change their shape much more slowly than small, it is more easy to see what is happening. i have in this glass box water coloured blue on which is floating paraffin, made heavier by mixing with it a bad-smelling and dangerous liquid called bisulphide of carbon. [sidenote: _see diagram at the end of the book._ fig. .] the water is only a very little heavier than the mixture. if i now dip a pipe into the water and let it fill, i can then raise it and allow drops to slowly form. drops as large as a shilling are now forming, and when each one has reached its full size, a neck forms above it, which is drawn out by the falling drop into a little cylinder. you will notice that the liquid of the neck has gathered itself into a little drop which falls away just after the large drop. the action is now going on so slowly that you can follow it. fig. contains forty-three consecutive views of the growth and fall of the drop taken photographically at intervals of one-twentieth of a second. for the use to which this figure is to be put, see page . if i again fill the pipe with water, and this time draw it rapidly out of the liquid, i shall leave behind a cylinder which will break up into balls, as you can easily see (fig. ). i should like now to show you, as i have this apparatus in its place, that you can blow bubbles of water containing paraffin in the paraffin mixture, and you will see some which have other bubbles and drops of one or other liquid inside again. one of these compound bubble drops is now resting stationary on a heavier layer of liquid, so that you can see it all the better (fig. ). if i rapidly draw the pipe out of the box i shall leave a long cylindrical bubble of water containing paraffin, and this, as was the case with the water-cylinder, slowly breaks up into spherical bubbles. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] having now shown that a very large liquid cylinder breaks up regularly into drops, i shall next go the other extreme, and take as an example an excessively fine cylinder. you see a photograph of a spider on her geometrical web (fig. ). if i had time i should like to tell you how the spider goes to work to make this beautiful structure, and a great deal about these wonderful creatures, but i must do no more than show you that there are two kinds of web--those that point outwards, which are hard and smooth, and those that go round and round, which are very elastic, and which are covered with beads of a sticky liquid. now there are in a good web over a quarter of a million of these beads which catch the flies for the spider's dinner. a spider makes a whole web in an hour, and generally has to make a new one every day. she would not be able to go round and stick all these in place, even if she knew how, because she would not have time. instead of this she makes use of the way that a liquid cylinder breaks up into beads as follows. she spins a thread, and at the same time wets it with a sticky liquid, which of course is at first a cylinder. this cannot remain a cylinder, but breaks up into beads, as the photograph taken with a microscope from a real web beautifully shows (fig. ). you see the alternate large and small drops, and sometimes you even see extra small drops between these again. in order that you may see exactly how large these beads really are, i have placed alongside a scale of thousandths of an inch, which was photographed at the same time. to prove to you that this is what happens, i shall now show you a web that i have made myself by stroking a quartz fibre with a straw dipped in castor-oil. the same alternate large and small beads are again visible just as perfect as they were in the spider's web. in fact it is impossible to distinguish between one of my beaded webs and a spider's by looking at them. and there is this additional similarity--my webs are just as good as a spider's for catching flies. you might say that a large cylinder of water in oil, or a microscopic cylinder on a thread, is not the same as an ordinary jet of water, and that you would like to see if it behaves as i have described. the next photograph (fig. ), taken by the light of an instantaneous electric spark, and magnified three and a quarter times, shows a fine column of water falling from a jet. you will now see that it is at first a cylinder, that as it goes down necks and bulges begin to form, and at last beads separate, and you can see the little drops as well. the beads also vibrate, becoming alternately long and wide, and there can be no doubt that the sparkling portion of a jet, though it appears continuous, is really made up of beads which pass so rapidly before the eye that it is impossible to follow them. (i should explain that for a reason which will appear later, i made a loud note by whistling into a key at the time that this photograph was taken.) [illustration: fig. .] [illustration: fig. .] lord rayleigh has shown that in a stream of water one twenty-fifth of an inch in diameter, necks impressed upon the stream, even though imperceptible, develop a thousandfold in depth every fortieth of a second, and thus it is not difficult to understand that in such a stream the water is already broken through before it has fallen many inches. he has also shown that free water drops vibrate at a rate which may be found as follows. a drop two inches in diameter makes one complete vibration in one second. if the diameter is reduced to one quarter of its amount, the time of vibration will be reduced to one-eighth, or if the diameter is reduced to one-hundredth, the time will be reduced to one-thousandth, and so on. the same relation between the diameter and the time of breaking up applies also to cylinders. we can at once see how fast a bead of water the size of one of those in the spider's web would vibrate if pulled out of shape, and let go suddenly. if we take the diameter as being one eight-hundredth of an inch, and it is really even finer, then the bead would have a diameter of one sixteen-hundredth of a two-inch bead, which makes one vibration in one second. it will therefore vibrate sixty-four thousand times as fast, or sixty-four thousand times a second. water-drops the size of the little beads, with a diameter of rather less than one three-thousandth of an inch, would vibrate half a million times a second, under the sole influence of the feebly elastic skin of water! we thus see how powerful is the influence of the feebly elastic water-skin on drops of water that are sufficiently small. i shall now cause a small fountain to play, and shall allow the water as it falls to patter upon a sheet of paper. you can see both the fountain itself and its shadow upon the screen. you will notice that the water comes out of the nozzle as a smooth cylinder, that it presently begins to glitter, and that the separate drops scatter over a great space (fig. ). now why should the drops scatter? all the water comes out of the jet at the same rate and starts in the same direction, and yet after a short way the separate drops by no means follow the same paths. now instead of explaining this, and then showing experiments to test the truth of the explanation, i shall reverse the usual order, and show one or two experiments first, which i think you will all agree are so like magic, so wonderful are they and yet so simple, that if they had been performed a few hundred years ago, the rash person who showed them might have run a serious risk of being burnt alive. [illustration: fig. .] [illustration: fig. .] you now see the water of the jet scattering in all directions, and you hear it making a pattering sound on the paper on which it falls. i take out of my pocket a stick of sealing-wax and instantly all is changed, even though i am some way off and can touch nothing. the water ceases to scatter; it travels in one continuous line (fig. ), and falls upon the paper making a loud rattling noise which must remind you of the rain of a thunder-storm. i come a little nearer to the fountain and the water scatters again, but this time in quite a different way. the falling drops are much larger than they were before. directly i hide the sealing-wax the jet of water recovers its old appearance, and as soon as the sealing-wax is taken out it travels in a single line again. now instead of the sealing-wax i shall take a smoky flame easily made by dipping some cotton-wool on the end of a stick into benzine, and lighting it. as long as the flame is held away from the fountain it produces no effect, but the instant that i bring it near so that the water passes through the flame, the fountain ceases to scatter; it all runs in one line and falls in a dirty black stream upon the paper. ever so little oil fed into the jet from a tube as fine as a hair does exactly the same thing. [illustration: fig. .] i shall now set a tuning-fork sounding at the other side of the table. the fountain has not altered in appearance. i now touch the stand of the tuning-fork with a long stick which rests against the nozzle. again the water gathers itself together even more perfectly than before, and the paper upon which it falls is humming out a note which is the same as that produced by the tuning-fork. if i alter the rate at which the water flows you will see that the appearance is changed again, but it is never like a jet which is not acted upon by a musical sound. sometimes the fountain breaks up into two or three and sometimes many more distinct lines, as though it came out of as many tubes of different sizes and pointing in slightly different directions (fig. ). the effect of different notes could be very easily shown if any one were to sing to the piece of wood by which the jet is held. i can make noises of different pitches, which for this purpose are perhaps better than musical notes, and you can see that with every new noise the fountain puts on a different appearance. you may well wonder how these trifling influences--sealing-wax, the smoky flame, or the more or less musical noise--should produce this mysterious result, but the explanation is not so difficult as you might expect. i hope to make this clear when we meet again. lecture iii. at the conclusion of the last lecture i showed you some curious experiments with a fountain of water, which i have now to explain. consider what i have said about a liquid cylinder. if it is a little more than three times as long as it is wide, it cannot retain its form; if it is made very much more than three times as long, it will break up into a series of beads. now, if in any way a series of necks could be developed upon a cylinder which were less than three diameters apart, some of them would tend to heal up, because a piece of a cylinder less than three diameters long is stable. if they were about three diameters apart, the form being then unstable, the necks would get more pronounced in time, and would at last break through, so that beads would be formed. if necks were made at distances more than three diameters apart, then the cylinder would go on breaking up by the narrowing of these necks, and it would most easily break up into drops when the necks were just four and a half diameters apart. in other words, if a fountain were to issue from a nozzle held perfectly still, the water would most easily break into beads at the distance of four and a half diameters apart, but it would break up into a greater number closer together, or a smaller number further apart, if by slight disturbances of the jet very slight waists were impressed upon the issuing cylinder of water. when you make a fountain play from a jet which you hold as still as possible, there are still accidental tremors of all kinds, which impress upon the issuing cylinder slightly narrow and wide places at irregular distances, and so the cylinder breaks up irregularly into drops of different sizes and at different distances apart. now these drops, as they are in the act of separating from one another, and are drawing out the waist, as you have seen, are being pulled for the moment towards one another by the elasticity of the skin of the waist; and, as they are free in the air to move as they will, this will cause the hinder one to hurry on, and the more forward one to lag behind, so that unless they are all exactly alike both in size and distance apart they will many of them bounce together before long. you would expect when they hit one another afterwards that they would join, but i shall be able to show you in a moment that they do not; they act like two india-rubber balls, and bounce away again. now it is not difficult to see that if you have a series of drops of different sizes and at irregular distances bouncing against one another frequently, they will tend to separate and to fall, as we have seen, on all parts of the paper down below. what did the sealing-wax or the smoky flame do? and what can the musical sound do to stop this from happening? let me first take the sealing-wax. a piece of sealing-wax rubbed on your coat is electrified, and will attract light bits of paper up to it. the sealing-wax acts electrically on the different water-drops, causing them to attract one another, feebly, it is true, but with sufficient power where they meet to make them break through the air-film between them and join. to show that this is no fancy, i have now in front of the lantern two fountains of clean water coming from separate bottles, and you can see that they bounce apart perfectly (fig. ). to show that they do really bounce, i have coloured the water in the two bottles differently. the sealing-wax is now in my pocket; i shall retire to the other side of the room, and the instant it appears the jets of water coalesce (fig. ). this may be repeated as often as you like, and it never fails. these two bouncing jets are in fact one of the most delicate tests for the presence of electricity that exist. you are now able to understand the first experiment. the separate drops which bounced away from one another, and scattered in all directions, are unable to bounce when the sealing-wax is held up, because of its electrical action. they therefore unite, and the result is, that instead of a great number of little drops falling all over the paper, the stream pours in a single line, and great drops, such as you see in a thunder-storm, fall on the top of one another. there can be no doubt that it is for this reason that the drops of rain in a thunder-storm are so large. this experiment and its explanation are due to lord rayleigh. [illustration: fig. .] [illustration: fig. .] the smoky flame, as lately shown by mr. bidwell, does the same thing. the reason probably is that the dirt breaks through the air-film, just as dust in the air will make the two fountains join as they did when they were electrified. however, it is possible that oily matter condensed on the water may have something to do with the effect observed, because oil alone acts quite as well as a flame, but the action of oil in this case, as when it smooths a stormy sea, is not by any means so easily understood. when i held the sealing-wax closer, the drops coalesced in the same way; but they were then so much more electrified that they repelled one another as similarly electrified bodies are known to do, and so the electrical scattering was produced. you possibly already see why the tuning-fork made the drops follow in one line, but i shall explain. a musical note is, as is well known, caused by a rapid vibration; the more rapid the vibration the higher is the pitch of the note. for instance, i have a tooth-wheel which i can turn round very rapidly if i wish. now that it is turning slowly you can hear the separate teeth knocking against a card that i am holding in the other hand. i am now turning faster, and the card is giving out a note of a low pitch. as i make the wheel turn faster and faster, the pitch of the note gradually rises, and it would, if i could only turn fast enough, give so high a note that we should not be able to hear it. a tuning-fork vibrates at a certain definite rate, and therefore gives a definite note. the fork now sounding vibrates times in every second. the nozzle, therefore, is made to vibrate, but almost imperceptibly, times a second, and to impress upon the issuing cylinder of water imperceptible waists every second. now it just depends what size the jet is, and how fast the water is issuing, whether these waists are about four and a half diameters apart in the cylinder. if the jet is larger, the water must pass more quickly, or under a greater pressure, for this to be the case; if the jet is finer, a smaller speed will be sufficient. if it should happen that the waists so made are anywhere about four diameters apart, then even though they are so slightly developed that if you had an exact drawing of them, you would not be able to detect the slightest change of diameter, they will grow at a great speed, and therefore the water column will break up regularly, every drop will be like the one behind it, and like the one in front of it, and not all different, as is the case when the breaking of the water merely depends upon accidental tremors. if the drops then are all alike in every respect, of course they all follow the same path, and so appear to fall in a continuous stream. if the waists are about four and a half diameters apart, then the jet will break up most easily; but it will, as i have said, break up under the influence of a considerable range of notes, which cause the waists to be formed at other distances, provided they are more than three diameters apart. if two notes are sounded at the same time, then very often each will produce its own effect, and the result is the alternate formation of drops of different sizes, which then make the jet divide into two separate streams. in this way, three, four, or even many more distinct streams may be produced. [illustration: fig. .] i can now show you photographs of some of these musical fountains, taken by the instantaneous flash of an electric spark, and you can see the separate paths described by the drops of different sizes (fig. ). in one photograph there are eight distinct fountains all breaking from the same jet, but following quite distinct paths, each of which is clearly marked out by a perfectly regular series of drops. you can also in these photographs see drops actually in the act of bouncing against one another, and flattened when they meet, as if they were india-rubber balls. in the photograph now upon the screen the effect of this rebound, which occurs at the place marked with a cross, is to hurry on the upper and more forward drop, and to retard the other one, and so to make them travel with slightly different velocities and directions. it is for this reason that they afterwards follow distinct paths. the smaller drops had no doubt been acted on in a similar way, but the part of the fountain where this happened was just outside the photographic plate, and so there is no record of what occurred. the very little drops of which i have so often spoken are generally thrown out from the side of a fountain of water under the influence of a musical sound, after which they describe regular little curves of their own, quite distinct from the main stream. they, of course, can only get out sideways after one or two bouncings from the regular drops in front and behind. you can easily show that they are really formed below the place where they first appear, by taking a piece of electrified sealing-wax and holding it near the stream close to the nozzle and gradually raising it. when it comes opposite to the place where the little drops are really formed, it will act on them more powerfully than on the large drops, and immediately pull them out from a place where the moment before none seemed to exist. they will then circulate in perfect little orbits round the sealing-wax, just as the planets do round the sun; but in this case, being met by the resistance of the air, the orbits are spirals, and the little drops after many revolutions ultimately fall upon the wax, just as the planets would fall into the sun after many revolutions, if their motion through space were interfered with by friction of any kind. there is only one thing needed to make the demonstration of the behaviour of a musical jet complete, and that is, that you should yourselves see these drops in their different positions in an actual fountain of water. now if i were to produce a powerful electric spark, then it is true that some of you might for an instant catch sight of the drops, but i do not think that most would see anything at all. but if, instead of making merely one flash, i were to make another when each drop had just travelled to the position which the one in front of it occupied before, and then another when each drop had moved on one place again, and so on, then all the drops, at the moments that the flashes of light fell upon them, would occupy the same positions, and thus all these drops would appear fixed in the air, though of course they really are travelling fast enough. if, however, i do not quite succeed in keeping exact time with my flashes of light, then a curious appearance will be produced. suppose, for instance, that the flashes of light follow one another rather too quickly, then each drop will not have had quite time enough to get to its proper place at each flash, and thus at the second flash all the drops will be seen in positions which are just behind those which they occupied at the first flash, and in the same way at the third flash they will be seen still further behind their former places, and so on, and therefore they will appear to be moving slowly backwards; whereas if my flashes do not follow quite quickly enough, then the drops will, every time that there is a flash, have travelled just a little too far, and so they will all appear to be moving slowly forwards. now let us try the experiment. there is the electric lantern sending a powerful beam of light on to the screen. this i bring to a focus with a lens, and then let it pass through a small hole in a piece of card. the light then spreads out and falls upon the screen. the fountain of water is between the card and the screen, and so a shadow is cast which is conspicuous enough. now i place just behind the card a little electric motor, which will make a disc of card which has six holes near the edge spin round very fast. the holes come one after the other opposite the hole in the fixed card, and so at every turn six flashes of light are produced. when the card is turning about - / times a second, then the flashes will follow one another at the right rate. i have now started the motor, and after a moment or two i shall have obtained the right speed, and this i know by blowing through the holes, when a musical note will be produced, higher than the fork if the speed is too high, and lower than the fork if the speed is too low, and exactly the same as the fork if it is right. to make it still more evident when the speed is exactly right, i have placed the tuning-fork also between the light and the screen, so that you may see it illuminated, and its shadow upon the screen. i have not yet allowed the water to flow, but i want you to look at the fork. for a moment i have stopped the motor, so that the light may be steady, and you can see that the fork is in motion because its legs appear blurred at the ends, where of course the motion is most rapid. now the motor is started, and almost at once the fork appears quite different. it now looks like a piece of india-rubber, slowly opening and shutting, and now it appears quite still, but the noise it is making shows that it is not still by any means. the legs of the fork are vibrating, but the light only falls upon them at regular intervals, which correspond with their movement, and so, as i explained in the case of the water-drops, the fork appears perfectly still. now the speed is slightly altered, and, as i have explained, each new flash of light, coming just too soon or just too late, shows the fork in a position which is just before or just behind that made visible by the previous flash. you thus see the fork slowly going through its evolutions, though of course in reality the legs are moving backwards and forwards times a second. by looking at the fork or its shadow, you will therefore be able to tell whether the light is keeping exact time with the vibrations, and therefore with the water-drops. now the water is running, and you see all the separate drops apparently stationary, strung like pearls or beads of silver upon an invisible wire (_see_ frontispiece). if i make the card turn ever so little more slowly, then all the drops will appear to slowly march onwards, and what is so beautiful,--but i am afraid few will see this,--each little drop may be seen to gradually break off, pulling out a waist which becomes a little drop, and then when the main drop is free it slowly oscillates, becoming wide and long, or turning over and over, as it goes on its way. if it so happens that a double or multiple jet is being produced, then you can see the little drops moving up to one another, squeezing each other where they meet and bouncing away again. now the card is turning a little too fast and the drops appear to be moving backwards, so that it seems as if the water is coming up out of the tank on the floor, quietly going over my head, down into the nozzle, and so back to the water-supply of the place. of course this is not happening at all, as you know very well, and as you will see if i simply try and place my finger between two of these drops. the splashing of the water in all directions shows that it is not moving quite so quietly as it appears. there is one more thing that i would mention about this experiment. every time that the flashing light gains or loses one complete flash, upon the motion of the tuning-fork, it will appear to make one complete oscillation, and the water-drops will appear to move back or on one place. i must now come to one of the most beautiful applications of these musical jets to practical purposes which it is possible to imagine, and what i shall now show are a few out of a great number of the experiments of mr. chichester bell, cousin of mr. graham bell, the inventor of the telephone. to begin with i have a very small jet of water forced through the nozzle at a great pressure, as you can see if i point it towards the ceiling, as the water rises eight or ten feet. if i allow this stream of water to fall upon an india-rubber sheet, stretched over the end of a tube as big as my little finger, then the little sheet will be depressed by the water, and the more so if the stream is strong. now if i hold the jet close to the sheet the smooth column of liquid will press the sheet steadily, and it will remain quiet; but if i gradually take the jet further away from the sheet, then any waists that may have been formed in the liquid column, which grow as they travel, will make their existence perfectly evident. when a wide part of the column strikes the sheet it will be depressed rather more than usual, and when a narrow part follows, the depression will be less. in other words, any very slight vibration imparted to the jet will be magnified by the growth of waists, and the sheet of india-rubber will reproduce the vibration, but on a magnified scale. now if you remember that sound consists of vibrations, then you will understand that a jet is a machine for magnifying sound. to show that this is the case i am now directing the jet on to the sheet, and you can hear nothing; but i shall hold a piece of wood against the nozzle, and now, if on the whole the jet tends to break up at any one rate rather than at any other, or if the wood or the sheet of rubber will vibrate at any rate most easily, then the first few vibrations which correspond to this rate will be imparted to the wood, which will impress them upon the nozzle and so upon the cylinder of liquid, where they will become magnified; the result is that the jet immediately begins to sing of its own accord, giving out a loud note (fig. ). i will now remove the piece of wood. on placing against the nozzle an ordinary lever watch, the jolt which is imparted to the case at every tick, though it is so small that you cannot detect it, jolts the nozzle also, and thus causes a neck to form in the jet of water which will grow as it travels, and so produce a loud tick, audible in every part of this large room (fig. ). now i want you to notice how the vibration is magnified by the action i have described. i now hold the nozzle close to the rubber sheet, and you can hear nothing. as i gradually raise it a faint echo is produced, which gradually gets louder and louder, until at last it is more like a hammer striking an anvil than the tick of a watch. [illustration: fig. .] [illustration: fig. .] i shall now change this watch for another which, thanks to a friend, i am able to use. this watch is a repeater, that is, if you press upon a nob it will strike, first the hour, then the quarters, and then the minutes. i think the water-jet will enable you all to hear what time it is. listen! one, two, three, four;... ting-tang, ting-tang;... one, two, three, four, five, six. six minutes after half-past four. you notice that not only did you hear the number of strokes, but the jet faithfully reproduced the musical notes, so that you could distinguish one note from the others. i can in the same way make the jet play a tune by simply making the nozzle rest against a long stick, which is pressed upon a musical-box. the musical-box is carefully shut up in a double box of thick felt, and you can hardly hear anything; but the moment that the nozzle is made to rest against the stick and the water is directed upon the india-rubber sheet, the sound of the box is loudly heard, i hope, in every part of the room. it is usual to describe a fountain as playing, but it is now evident that a fountain can even play a tune. there is, however, one peculiarity which is perfectly evident. the jet breaks up at certain rates more easily than at others, or, in other words, it will respond to certain sounds in preference to others. you can hear that as the musical-box plays, certain notes are emphasized in a curious way, producing much the same effect that follows if you lay a penny upon the upper strings of a horizontal piano. [illustration: fig. .] now, on returning to our soap-bubbles, you may remember that i stated that the catenoid and the plane were the only figures of revolution which had no curvature, and which therefore produced no pressure. there are plenty of other surfaces which are apparently curved in all directions and yet have no curvature, and which therefore produce no pressure; but these are not figures of revolution, that is, they cannot be obtained by simply spinning a curved line about an axis. these may be produced in any quantity by making wire frames of various shapes and dipping them in soap and water. on taking them out a wonderful variety of surfaces of no curvature will be seen. one such surface is that known as the screw-surface. to produce this it is only necessary to take a piece of wire wound a few times in an open helix (commonly called spiral), and to bend the two ends so as to meet a second wire passing down the centre. the screw-surface developed by dipping this frame in soap-water is well worth seeing (fig. ). it is impossible to give any idea of the perfection of the form in a figure, but fortunately this is an experiment which any one can easily perform. [illustration: fig. .] then again, if a wire frame is made in the shape of the edges of any of the regular geometrical solids, very beautiful figures will be found upon them after they have been dipped in soap-water. in the case of the triangular prism these surfaces are all flat, and at the edges where these planes meet one another there are always three meeting each other at equal angles (fig. ). this, owing to the fact that the frame is three-sided, is not surprising. after looking at this three-sided frame with three films meeting down the central line, you might expect that with a four-sided or square frame there would be four films meeting each other in a line down the middle. but it is a curious thing that it does not matter how irregular the frame may be, or how complicated a mass of froth may be, there can never be more than three films meeting in an edge, or more than four edges, or six films, meeting in a point. moreover the films and edges can only meet one another at equal angles. if for a moment by any accident four films do meet in the same edge, or if the angles are not exactly equal, then the form, whatever it may be, is unstable; it cannot last, but the films slide over one another and never rest until they have settled down into a position in which the conditions of stability are fulfilled. this may be illustrated by a very simple experiment which you can easily try at home, and which you can now see projected upon the screen. there are two pieces of window-glass about half an inch apart, which form the sides of a sort of box into which some soap and water have been poured. on blowing through a pipe which is immersed in the water, a great number of bubbles are formed between the plates. if the bubbles are all large enough to reach across from one plate to the other, you will at once see that there are nowhere more than three films meeting one another, and where they meet the angles are all equal. the curvature of the bubbles makes it difficult to see at first that the angles really are all alike, but if you only look at a very short piece close to where they meet, and so avoid being bewildered by the curvature, you will see that what i have said is true. you will also see, if you are quick, that when the bubbles are blown, sometimes four for a moment do meet, but that then the films at once slide over one another and settle down into their only possible position of rest (fig. ). the air inside a bubble is generally under pressure, which is produced by its elasticity and curvature. if the bubble would let the air pass through it from one side to the other of course it would soon shut up, as it did when a ring was hung upon one, and the film within the ring was broken. but there are no holes in a bubble, and so you would expect that a gas like air could not pass through to the other side. nevertheless it is a fact that gases can slowly get through to the other side, and in the case of certain vapours the process is far more rapid than any one would think possible. [illustration: fig. .] [illustration: fig. .] ether produces a vapour which is very heavy, and which also burns very easily. this vapour can get to the other side of a bubble almost at once. i shall pour a little ether upon blotting-paper in this bell jar, and fill the jar with its heavy vapour. you can see that the jar is filled with something, not by looking at it, for it appears empty, but by looking at its shadow on the screen. now i tilt it gently to one side, and you see something pouring out of it, which is the vapour of ether. it is easy to show that this is heavy; it is only necessary to drop into the jar a bubble, and so soon as the bubble meets the heavy vapour it stops falling and remains floating upon the surface as a cork does upon water (fig. ). now let me test the bubble and see whether any of the vapour has passed to the inside. i pick it up out of the jar with a wire ring and carry it to a light, and at once there is a burst of flame. but this is not sufficient to show that the ether vapour has passed to the inside, because it might have condensed in sufficient quantity upon the bubble to make it inflammable. you remember that when i poured some of this vapour upon water in the first lecture, sufficient condensed to so weaken the water-skin that the frame of wire could get through to the other side. however, i can see whether this is the true explanation or not by blowing a bubble on a wide pipe, and holding it in the vapour for a moment. now on removing it you notice that the bubble hangs like a heavy drop; it has lost the perfect roundness that it had at first, and this looks as if the vapour had found its way in, but this is made certain by bringing a light to the mouth of the tube, when the vapour, forced out by the elasticity of the bubble, catches fire and burns with a flame five or six inches long (fig. ). you might also have noticed that when the bubble was removed, the vapour inside it began to pass out again and fell away in a heavy stream, but this you could only see by looking at the shadow upon the screen. [illustration: fig. ] you may have noticed when i made the drops of oil in the mixture of alcohol and water, that when they were brought together they did not at once unite; they pressed against one another and pushed each other away if allowed, just as the water-drops did in the fountain of which i showed you a photograph. you also may have noticed that the drops of water in the paraffin mixture bounced against one another, or if filled with the paraffin, formed bubbles in which often other small drops, both of water and paraffin, remained floating. in all these cases there was a thin film of something between the drops which they were unable to squeeze out, namely, water, paraffin, or air, as the case might be. will two soap-bubbles also when knocked together be unable to squeeze out the air between them? this you can try at home just as well as i can here, but i will perform the experiment at once. i have blown a pair of bubbles, and now when i hit them together they remain distinct and separate (fig. ). [illustration: fig. .] i shall next place a bubble on a ring, which it is just too large to get through. in my hand i hold a ring, on which i have a flat film, made by placing a bubble upon it and breaking it on one side. if i gently press the bubble with the flat film, i can push it through the ring to the other side (fig. ), and yet the two have not really touched one another at all. the bubble can be pushed backwards and forwards in this way many times. [illustration: fig. .] i have now blown a bubble and hung it below a ring. to this bubble i can hang another ring of thin wire, which pulls it a little out of shape. since the pressure inside is less than that corresponding to a complete sphere, and since it is greater than that outside, and this we can tell by looking at the caps, the curve is part of one of those represented by the dotted lines in c or e, fig. . however, without considering the curve any more, i shall push the end of the pipe inside, and blow another bubble there, and let it go. it falls gently until it rests upon the outer bubble; not at the bottom, because the heavy ring keeps that part out of reach, but along a circular line higher up (fig. ). i can now drain away the heavy drops of liquid from below the bubbles with a pipe, and leave them clean and smooth all over. i can now pull the lower ring down, squeezing the inner bubble into a shape like an egg (fig. ), or swing it round and round, and then with a little care peel away the ring from off the bubble, and leave them both perfectly round every way (fig. ). i can draw out the air from the outer bubble till you can hardly see between them, and then blow in, and the harder i blow, the more is it evident that the two bubbles are not touching at all; the inner one is now spinning round and round in the very centre of the large bubble, and finally, on breaking the outer one the inner floats away, none the worse for its very unusual treatment. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] there is a pretty variation of the last experiment, which, however, requires that a little green dye called fluorescine, or better, uranine, should be dissolved in a separate dish of the soap-water. then you can blow the outer bubble with clean soap-water, and the inner one with the coloured water. then if you look at the two bubbles by ordinary light, you will hardly notice any difference; but if you allow sunlight, or electric light from an arc lamp, to shine upon them, the inner one will appear a brilliant green, while the outer one will remain clear as before. they will not mix at all, showing that though the inner one is apparently resting against the outer one, there is in reality a thin cushion of air between. now you know that coal-gas is lighter than air, and so a soap-bubble blown with gas, when let go, floats up to the ceiling at once. i shall blow a bubble on a ring with coal-gas. it is soon evident that it is pulling upwards. i shall go on feeding it with gas, and i want you to notice the very beautiful shapes that it takes (fig. , but imagine the globe inside removed). these are all exactly the curves that a water-drop assumes when hanging from a pipe, except that they are the other way up. the strength of the skin is now barely able to withstand the pull, and now the bubble breaks away just as the drop of water did. [illustration: fig. .] i shall next place a bubble blown with air upon a ring, and blow inside it a bubble blown with a mixture of air and gas. it of course floats up and rests against the top of the outer bubble (fig. ). now i shall let a little gas into the outer one, until the surrounding gas is about as heavy as the inner bubble. it now no longer rests against the top, but floats about in the centre of the large bubble (fig. ), just as the drop of oil did in the mixture of alcohol and water. you can see that the inner bubble is really lighter than air, because if i break the outer one, the inner one rises rapidly to the ceiling. [illustration: fig. .] instead of blowing the first bubble on a heavy fixed ring, i shall now blow one on a light ring, made of very thin wire. this bubble contains only air. if i blow inside this a bubble with coal-gas, then the gas-bubble will try and rise, and will press against the top of the outer one with such force as to make it carry up the wire ring and a yard of cotton, and some paper to which the cotton is tied (fig. ); and all this time, though it is the inner one only which tends to rise, the two bubbles are not really touching one another at all. [illustration: fig. .] [illustration: fig. .] i have now blown an air-bubble on the fixed ring, and pushed up inside it a wire with a ring on the end. i shall now blow another air-bubble on this inner ring. the next bubble that i shall blow is one containing gas, and this is inside the other two, and when let go it rests against the top of the second bubble. i next make the second bubble a little lighter by blowing a little gas into it, and then make the outer one larger with air. i can now peel off the inner ring and take it away, leaving the two inner bubbles free, inside the outer one (fig. ). and now the multiple reflections of the brilliant colours of the different bubbles from one to the other, set off by the beautiful forms which the bubbles themselves assume, give to the whole a degree of symmetry and splendour which you may go far to see equalled in any other way. i have only to blow a fourth bubble in _real_ contact with the outer bubble and the ring, to enable it to peel off and float away with the other two inside. [illustration: fig. .] we have seen that bubbles and drops behave in very much the same way. let us see if electricity will produce the same effect that it did on drops. you remember that a piece of electrified sealing-wax prevented a fountain of water from scattering, because where two drops met, instead of bouncing, they joined together. now there are on these two rings bubbles which are just resting against one another, but not really touching (fig. ). the instant that i take out the sealing-wax you see they join together and become one (fig. ). two soap-bubbles, therefore, enable us to detect electricity, even when present in minute quantity, just as two water fountains did. [illustration: fig. .] [illustration: fig. .] we can use a pair of bubbles to prove the truth of one of the well-known actions of electricity. inside an electrical conductor it is impossible to feel any influence of electricity outside, however much there may be, or however near you go to the surface. let us, therefore, take the two bubbles shown in fig. , and bring an electrified stick of sealing-wax near. the outer bubble is a conductor; there is, therefore, no electrical action inside, and this you can see because, though the sealing-wax is so near the bubble that it pulls it all to one side, and though the inner one is so close to the outer one that you cannot see between them, yet the two bubbles remain separate. had there been the slightest electrical influence inside, even to a depth of a hundred-thousandth of an inch, the two bubbles would have instantly come together. [illustration: fig. .] there is one more experiment which i must show, and this will be the last; it is a combination of the last two, and it beautifully shows the difference between an inside and an outside bubble. i have now a plain bubble resting against the side of the pair that i have just been using. the instant that i take out the sealing-wax the two outer bubbles join, while the inner one unharmed and the heavy ring slide down to the bottom of the now single outer bubble (fig. ). and now that our time has drawn to a close i must ask you whether that admiration and wonder which we all feel when we play with soap-bubbles has been destroyed by these lectures; or whether now that you know more about them it is not increased. i hope you will all agree with me that the actions upon which such common and every-day phenomena as drops and bubbles depend, actions which have occupied the attention of the greatest philosophers from the time of newton to the present day, are not so trivial as to be unworthy of the attention of ordinary people like ourselves. practical hints. i hope that the following practical hints may be found useful by those who wish themselves to successfully perform the experiments already described. _drop with india-rubber surface._ a sheet of thin india-rubber, about the thickness of that used in air-balls, as it appears _before_ they have been blown out, must be stretched over a ring of wood or metal eighteen inches in diameter, and securely wired round the edge. the wire will hold the india-rubber better if the edge is grooved. this does not succeed if tried on a smaller scale. this experiment was shown by sir w. thomson at the royal institution. _jumping frame._ this is easily made by taking a light glass globe about two inches in diameter, such, for instance, as a silvered ball used to ornament a christmas-tree or the bulb of a pipette, which is what i used. pass through the open necks of the bulb a piece of wire about one-twentieth of an inch in diameter, and fix it permanently and water-tight upon the wire by working into the necks melted sealing-wax. an inch or two above the globe, fasten a flat frame of thin wire by soldering, or if this is too difficult, by tying and sealing-wax. a lump of lead must then be fastened or hung on to the lower end, and gradually scraped away until the wire frame will just be unable to force its way through the surface of the water. none of the dimensions or materials mentioned are of importance. _paraffined sieve._ obtain a piece of copper wire gauze with about twenty wires to the inch, and cut out from it a round piece about eight inches in diameter. lay it on a round block, of such a size that it projects about one inch all round. then gently go round and round with the hands pressing the edge down and keeping it flat above, until the sides are evenly turned down all round. this is quite easy, because the wires can allow of the kind of distortion necessary. then wind round the turned-up edge a few turns of thick wire to make the sides stiff. this ought to be soldered in position, but probably careful wiring will be good enough. melt some paraffin wax or one or two paraffin candles of the best quality in a clean flat dish, not over the fire, which would be dangerous, but on a hot plate. when melted and clear like water, dip the sieve in, and when all is hot quickly take it out and knock it once or twice on the table to shake the paraffin out of the holes. leave upside down until cold, and then be careful not to scratch or rub off the paraffin. this had best be done in a place where a mess is of no consequence. there is no difficulty in filling it or in setting it to float upon water. _narrow tubes and capillarity._ get some quill-glass tube from a chemist, that is, tube about the size of a pen. if it is more than, say, a foot long, cut off a piece by first making a firm scratch in one place with a three-cornered file, when it will break at the place easily. to make very narrow tube from this, hold it near the ends in the two hands very lightly, so that the middle part is high up in the brightest part of an ordinary bright and flat gas flame. keep it turning until at last it becomes so soft that it is difficult to hold it straight. it can then be bent into any shape, but if it is wanted to be drawn out it must be held still longer until the black smoke upon it begins to crack and peel up. then quickly take it out of the flame, and pull the two ends apart, when a long narrow tube will be formed between. this can be made finer or coarser by regulating the heat and the manner in which it is pulled out. no directions will tell any one so much as a very little practice. for drawing out tubes the flame of a bunsen burner or of a blow-pipe is more convenient; but for bending tubes nothing is so good as the flat gas flame. do not clean off smoke till the tubes are cold, and do not hurry their cooling by wetting or blowing upon them. in the country where gas is not to be had, the flame of a large spirit-lamp can be made to do, but it is not so good as a gas-flame. the narrower these tubes are, the higher will clean water be observed to rise in them. to colour the water, paints from a colour-box must not be used. they are not liquid, and will clog the very fine tubes. some dye that will quite dissolve (as sugar does) must be used. an aniline dye, called soluble blue, does very well. a little vinegar added may make the colour last better. _capillarity between plates._ two plates of flat glass, say three to five inches square, are required. provided they are quite clean and well wetted there is no difficulty. a little soap and hot water will probably be sufficient to clean them. _tears of wine._ these are best seen at dessert in a glass about half filled with port. a mixture of from two to three parts of water, and one part of spirits of wine containing a very little rosaniline (a red aniline dye), to give it a nice colour, may be used, if port is not available. a piece of the dye about as large as a mustard-seed will be enough for a large wine-glass. the sides of the glass should be wetted with the wine. _cat-boxes._ every school-boy knows how to make these. they are not the boxes made by cutting slits in paper. they are simply made by folding, and are then blown out like the "frog," which is also made of folded paper. _liquid beads._ instead of melting gold, water rolled on to a table thickly dusted with lycopodium, or other fine dust, or quicksilver rolled or thrown upon a smooth table, will show the difference in the shape of large and small beads perfectly. a magnifying-glass will make the difference more evident. in using quicksilver, be careful that none of it falls on gold or silver coins, or jewellery, or plate, or on the ornamental gilding on book-covers. it will do serious damage. _plateau's experiment._ to perform this with very great perfection requires much care and trouble. it is easy to succeed up to a certain point. pour into a clean bottle about a table-spoonful of salad-oil, and pour upon it a mixture of nine parts by volume spirits of wine (not methylated spirits), and seven parts of water. shake up and leave for a day if necessary, when it will be found that the oil has settled together by itself. fill a tumbler with the same mixture of spirit and water, and then with a fine glass pipe, dipping about half-way down, slowly introduce a very little water. this will make the liquid below a little heavier. dip into the oil a pipe and take out a little by closing the upper end with the finger, and carefully drop this into the tumbler. if it goes to the bottom, a little more water is required in the lower half of the tumbler. if by chance it will not sink at all, a little more spirit is wanted in the upper half. at last the oil will just float in the middle of the mixture. more can then be added, taking care to prevent it from touching the sides. if the liquid below is ever so little heavier, and the liquid above ever so little lighter than oil, the drop of oil perhaps as large as a halfpenny will be almost perfectly round. it will not appear round if seen through the glass, because the glass magnifies it sideways, but not up and down, as may be seen by holding a coin in the liquid just above it. to see the drop in its true shape the vessel must either be a globe, or one side must be made of flat glass. spinning the oil so as to throw off a ring is not material, but if the reader can contrive to fix a disc about the size of a threepenny-piece upon a straight wire, and spin it round without shaking it, then he will see the ring break off, and either return if the rotation is quickly stopped, or else break up into three or four perfect little balls. the disc should be wetted with oil before being dipped into the mixture of spirit and water. _a good mixture for soap-bubbles._ common yellow soap is far better than most of the fancy soaps, which generally contain a little soap and a lot of rubbish. castille soap is very good, and this may be obtained from any chemist. bubbles blown with soap and water alone do not last long enough for many of the experiments described, though they may sometimes be made to succeed. plateau added glycerine, which greatly improves the lasting quality. the glycerine should be pure; common glycerine is not good, but price's answers perfectly. the water should be pure distilled water, but if this is not available, clean rain-water will do. do not choose the first that runs from a roof after a spell of dry weather, but wait till it has rained for some time, the water that then runs off is very good, especially if the roof is blue slate or glass. if fresh rain-water is not to be had, the softest water should be employed that can be obtained. instead of castille soap, plateau found that a pure soap prepared from olive-oil is still better. this is called oleate of soda. it should be obtained freshly prepared from a manufacturing chemist. old, dry stuff that has been kept a long time is not so good. i have always used a modification of plateau's formula, which professors reinold and rücker found to answer so well. they used less glycerine than plateau. it is best made as follows. fill a clean stoppered bottle three-quarters full of water. add one-fortieth part of its weight of oleate of soda, which will probably float on the water. leave it for a day, when the oleate of soda will be dissolved. nearly fill up the bottle with price's glycerine and shake well, or pour it into another clean bottle and back again several times. leave the bottle, stoppered of course, for about a week in a dark place. then with a syphon, that is, a bent glass tube which will reach to the bottom inside and still further outside, draw off the clear liquid from the scum which will have collected at the top. add one or two drops of strong liquid ammonia to every pint of the liquid. then carefully keep it in a stoppered bottle in a dark place. do not get out this stock bottle every time a bubble is to be blown, but have a small working bottle. never put any back into the stock. in making the liquid _do not warm or filter it_. either will spoil it. never leave the stoppers out of the bottles or allow the liquid to be exposed to the air more than is necessary. this liquid is still perfectly good after two years' keeping. i have given these directions very fully, not because i feel sure that all the details are essential, but because it exactly describes the way i happen to make it, and because i have never found any other solution so good. castille soap, price's glycerine, and rain-water will almost certainly answer every purpose, and the same proportions will probably be found to work well. _rings for bubbles._ these may be made of any kind of wire. i have used tinned iron about one-twentieth of an inch in diameter. the joint should be smoothly soldered without lumps. if soldering is a difficulty, then use the thinnest wire that is stiff enough to support the bubbles steadily, and make the joint by twisting the end of the wire round two or three times. rings two inches in diameter are convenient. i have seen that dipping the rings in melted paraffin is recommended, but i have not found any advantage from this. the nicest material for the light rings is thin aluminium wire, about as thick as a fine pin (no. to , b. w. g.), and as this cannot be soldered, the ends must be twisted. if this is not to be had, very fine wire, nearly as fine as a hair (no. , b. w. g.), of copper or of any other metal, will answer. the rings should be wetted with the soap mixture before a bubble is placed upon them, and must always be well washed and dried when done with. _threads in ring._ there is no difficulty in showing these experiments. the ring with the thread may be dipped in the soap solution, or stroked across with the edge of a piece of paper or india-rubber sheet that has been dipped in the liquid, so as to form a film on both sides of the thread. a needle that has also been wetted with the soap may be used to show that the threads are loose. the same needle held for a moment in a candle-flame supplies a convenient means of breaking the film. _blow out candle with soap-bubble._ for this, the bubble should be blown on the end of a short wide pipe, spread out at one end to give a better hold for the bubble. the tin funnel supplied with an ordinary gazogene answers perfectly. this should be washed before it is used again for filling the gazogene. _bubbles balanced against one another._ these experiments are most conveniently made on a small scale. pieces of thin brass tube, three-eighths or half an inch in diameter, are suitable. it is best to have pieces of apparatus, specially prepared with taps, for easily and quickly stopping the air from leaving either bubble, and for putting the two bubbles into communication when required. it should not be difficult to contrive to perform the experiments, using india-rubber connecting tubes, pinched with spring clips to take the place of taps. there is one little detail which just makes the difference between success and failure. this is to supply a mouth-piece for blowing the bubble, made of glass tube, which has been drawn out so fine that these little bubbles cannot be blown out suddenly by accident. it is very difficult, otherwise, to adjust the quantity of air in such small bubbles with any accuracy. in balancing a spherical against a cylindrical bubble, the short piece of tube, into which the air is supplied, must be made so that it can be easily moved to or from a fixed piece of the same size closed at the other end. then the two ends of the short tube must have a film spread over them with a piece of paper, or india-rubber, but there must be _no_ film stretched across the end of the fixed tube. the two tubes must at first be near together, until the spherical bubble has been formed. they may then be separated gradually more and more, and air blown in so as to keep the sides of the cylinder straight, until the cylinder is sufficiently long to be nearly unstable. it will then far more evidently show, by its change of form, than it would if it were short, when the pressure due to the spherical bubble exactly balances that due to a cylindrical one. if the shadow of the bubbles, or an image formed by a lens on a screen, is then measured, it will be found that the sphere has a diameter which is very accurately double that of the cylinder. _thaumatrope for showing the formation and oscillations of drops._ the experiment showing the formation of water-drops can be very perfectly imitated, and the movements actually made visible, without any necessity for using liquids at all, by simply converting fig. (at end of book) into the old-fashioned instrument called a thaumatrope. what will then be seen is a true representation, because the forms in the figure are copies of a series of photographs taken from the moving drops at the rate of forty-three photographs in two seconds.[ ] [footnote : for particulars see _philosophical magazine_, september .] obtain a piece of good card-board as large as the figure, and having brushed it all over on one side with thin paste, lay the figure upon it, and press it down evenly. place it upon a table, and cover it with a few thicknesses of blotting-paper, and lay over all a flat piece of board large enough to cover it. weights sufficient to keep it all flat may be added. this must be left all night at least, until the card is quite dry, or else it will curl up and be useless. now with a sharp chisel or knife, but a chisel if possible, cut out the forty-three slits near the edge, accurately following the outline indicated in black and white, and keeping the slits as narrow as possible. then cut a hole in the middle, so as to fit the projecting part of a sewing-machine cotton-reel, and fasten the cotton-reel on the side away from the figure with glue or small nails. it must be fixed exactly in the middle. the edge should of course be cut down to the outside of the black rim. now having found a pencil or other rod on which the cotton-reel will freely turn, use this as an axle, and holding the disc up in front of a looking-glass, and in a good light, slowly and steadily make it turn round. the image of the disc seen through the slit in the looking-glass will then perfectly represent every feature of the growing and falling drop. as the drop grows it will gradually become too heavy to be supported, a waist will then begin to form which will rapidly get narrower, until the drop at last breaks away. it will be seen to continue its fall until it has disappeared in the liquid below, but it has not mixed with this, and so it will presently appear again, having bounced out of the liquid. as it falls it will be seen to vibrate as the result of the sudden release from the one-sided pull. the neck which was drawn out will meanwhile have gathered itself in the form of a little drop, which will then be violently hit by the oscillations of the remaining pendant drop above, and driven down. the pendant drop will be seen to vibrate and grow at the same time, until it again breaks away as before, and so the phenomena are repeated. in order to perfectly reproduce the experiment, the axle should be firmly held upon a stand, and the speed should not exceed one turn in two seconds. the effect is still more real if a screen is placed between the disc and the mirror, which will only allow one of the drops to be seen. _water-drops in paraffin and bisulphide of carbon._ all that was said in describing the plateau experiment applies here. perfectly spherical and large drops of water can be formed in a mixture so made that the lower parts are very little heavier, and the upper parts very little lighter, than water. the addition of bisulphide of carbon makes the mixture heavier. this liquid--bisulphide of carbon--is very dangerous, and has a most dreadful smell, so that it had better not be brought into the house. the form of a hanging drop, and the way in which it breaks off, can be seen if water is used in paraffin alone, but it is much more evident if a little bisulphide of carbon is mixed with the paraffin, so that water will sink slowly in the mixture. pieces of glass tube, open at both ends from half an inch to one inch in diameter, show the action best. having poured some water coloured blue into a glass vessel, and covered it to a depth of several inches with paraffin, or the paraffin mixture, dip the pipe down into the water, having first closed the upper end with the thumb or the palm of the hand. on then removing the hand, the water will rush up inside the tube. again close the upper end as before, and raise the tube until the lower end is well above the water, though still immersed in the paraffin. then allow air to enter the pipe very slowly by just rolling the thumb the least bit to one side. the water will escape slowly and form a large growing drop, the size of which, before it breaks away, will depend on the density of the mixture and the size of the tube. to form a water cylinder in the paraffin the tube must be filled with water as before, but the upper end must now be left open. then when all is quiet the tube is to be rather rapidly withdrawn in the direction of its own length, when the water which was within it will be left behind in form of a cylinder, surrounded by the paraffin. it will then break up into spheres so slowly, in the case of a large tube, that the operation can be watched. the depth of paraffin should be quite ten times the diameter of the tube. to make bubbles of water in the paraffin, the tube must be dipped down into the water with the upper end open all the time, so that the tube is mostly filled with paraffin. it must then be closed for a moment above and raised till the end is completely out of the water. then if air is allowed to enter slowly, and the tube is gently raised, bubbles of water filled with paraffin will be formed which can be made to separate from the pipe, like soap-bubbles from a "churchwarden," by a suitable sudden movement. if a number of water-drops are floating in the paraffin in the pipe, and this can be easily arranged, then the bubbles made will contain possibly a number of other drops, or even other bubbles. a very little bisulphide of carbon poured carefully down a pipe will form a heavy layer above the water, on which these compound bubbles will remain floating. cylindrical bubbles of water in paraffin may be made by dipping the pipe down into the water and withdrawing it quickly without ever closing the top at all. these break up into spherical bubbles in the same way that the cylinder of liquid broke up into spheres of liquid. _beaded spider-webs._ these are found in the spiral part of the webs of all the geometrical spiders. the beautiful geometrical webs may be found out of doors in abundance in the autumn, or in green-houses at almost any time of the year. to mount these webs so that the beads may be seen, take a small flat ring of any material, or a piece of card-board with a hole cut out with a gun-wad cutter, or otherwise. smear the face of the ring, or the card, with a very little strong gum. choose a freshly-made web, and then pass the ring, or the card, across the web so that some of the spiral web (not the central part of the web) remains stretched across the hole. this must be done without touching or damaging the pieces that are stretched across, except at their ends. the beads are too small to be seen with the naked eye. a strong magnifying-glass, or a low power microscope, will show the beads and their marvellous regularity. the beads on the webs of very young spiders are not so regular as those on spiders that are fully grown. those beautiful beads, easily visible to the naked eye, on spider lines in the early morning of an autumn day, are not made by the spider, but are simply dew. they very perfectly show the spherical form of small water-drops. _photographs of water-jets._ these are easily taken by the method described by mr. chichester bell. the flash of light is produced by a short spark from a few leyden-jars. the fountain, or jet, should be five or six feet away from the spark, and the photographic plate should be held as close to the stream of water as is possible without touching. the shadow is then so definite that the photograph, when taken, may be examined with a powerful lens, and will still appear sharp. any rapid dry plate will do. the room, of course, must be quite dark when the plate is placed in position, and the spark then made. the regular breaking up of the jet may be effected by sound produced in almost any way. the straight jet, of which fig. is a representation, magnified about three and a quarter times, was regularly broken up by simply whistling to it with a key. the fountains were broken up regularly by fastening the nozzle to one end of a long piece of wood clamped at the end to the stand of a tuning-fork, which was kept sounding by electrical means. an ordinary tuning-fork, made to rest when sounding against the wooden support of the nozzle, will answer quite as well, but is not quite so convenient. the jet will break up best to certain notes, but it may be tuned to a great extent by altering the size of the orifice or the pressure of the water, or both. _fountain and sealing-wax._ it is almost impossible to fail over this very striking yet simple experiment. a fountain of almost any size, at any rate between one-fiftieth and a quarter of an inch in the smooth part, and up to eight feet high, will cease to scatter when the sealing-wax is rubbed with flannel and held a few feet away. a suitable size of fountain is one about four feet high, coming from an orifice anywhere near one-sixteenth of an inch in diameter. the nozzle should be inclined so that the water falls slightly on one side. the sealing-wax may be electrified by being rubbed on the coat-sleeve, or on a piece of fur or flannel which is _dry_. it will then make little pieces of paper or cork dance, but it will still act on the fountain when it has ceased to produce any visible effect on pieces of paper, or even on a delicate gold-leaf electroscope. _bouncing water-jets._ this beautiful experiment of lord rayleigh's requires a little management to make it work in a satisfactory manner. take a piece of quill-glass tube and draw it out to a very slight extent (see a former note), so as to make a neck about one-eighth of an inch in diameter at the narrowest part. break the tube just at this place, after first nicking it there with a file. connect each of these tubes by means of an india-rubber pipe, or otherwise, with a supply of water in a bottle, and pinch the tubes with a screw-clip until two equal jets of water are formed. so hold the nozzles that these meet in their smooth portions at every small angle. they will then for a short time bounce away from one another without mixing. if the air is very dusty, if the water is not clean, or if air-bubbles are carried along in the pipes, the two jets will at once join together. in the arrangement that i used in the lantern, the two nozzles were nearly horizontal, one was about half an inch above the other, and they were very slightly converging. they were fastened in their position by melting upon them a little sealing-wax. india-rubber pipes connected them with two bottles about six inches above them, and screw-clips were used to regulate the supply. one of the bottles was made to stand on three pieces of sealing-wax to electrically insulate it, and the corresponding nozzle was only held by its sealing-wax fastening. the water in the bottles had been filtered, and one was coloured blue. if these precautions are taken, the jets will remain distinct quite long enough, but are instantly caused to recombine by a piece of electrified sealing-wax six or eight feet away. they may be separated again by touching the water issuing near one nozzle with the finger, which deflects it; on quietly removing the finger the jet takes up its old position and bounces off the other as before. they can thus be separated and made to combine ten or a dozen times in a minute. _fountain and intermittent light._ this can be successfully shown to a large number of people at once only by using an electric arc, but there is no occasion to produce this light if not more than one person at a time wishes to see the evolution of the drops. it is then merely necessary to make the fountain play in front of a bright background such as the sky, to break it up with a tuning-fork or other musical sound as described, and then to look at it through a card disc equally divided near the edge into spaces about two or three inches wide, with a hole about one-eighth of an inch in diameter between each pair of spaces. a disc of card five inches in diameter, with six equidistant holes half an inch from the edge, answers well. the disc must be made to spin by any means very regularly at such a speed that the tuning-fork, or stretched string if this be used, when looked at through the holes, appears quiet, or nearly quiet, when made to vibrate. the separate drops will then be seen, and everything described in the preceding pages, and a great deal more, will be evident. this is one of the most fascinating experiments, and it is well worth while to make an effort to succeed. the little motor that i used is one of cuttriss and co.'s p. . motors, which are very convenient for experiments of this kind. it was driven by four grove's cells. these make it rotate too fast, but the speed can be reduced by moving the brushes slightly towards the position used for reversing the motor, until the speed is almost exactly right. it is best to arrange that it goes only just too fast, then the speed can be perfectly regulated by a very light pressure of the finger on the end of the axle. _mr. chichester bell's singing water-jet._ for these experiments a very fine hole about one seventy-fifth of an inch in diameter is most suitable. to obtain this, mr. bell holds the end of a quill-glass tube in a blow-pipe flame, and constantly turns it round and round until the end is almost entirely closed up. he then suddenly and forcibly blows into the pipe. out of several nozzles made in this way, some are sure to do well. lord rayleigh makes nozzles generally by cementing to the end of a glass (or metal) pipe a piece of thin sheet metal in which a hole of the required size has been made. the water pressure should be produced by a head of about fifteen feet. the water must be quite free from dust and from air-bubbles. this may be effected by making it pass through a piece of tube stuffed full of flannel, or cotton-wool, or something of the kind to act as a filter. there should be a yard or so of good black india-rubber tube, about one-eighth of an inch in diameter inside, between the filter and the nozzle. it is best not to take the water direct from the water-main, but from a cistern about fifteen feet above the nozzle. if no cistern is available, a pail of water taken up-stairs, with a pipe coming down, is an excellent substitute, and this has the further advantage that the head of water can be easily changed so as to arrive at the best result. the rest of the apparatus is very simple. it is merely necessary to stretch and tie over the end of a tube about half an inch in diameter a piece of thin india-rubber sheet, cut from an air-ball that has not been blown out. the tube, which may be of metal or of glass, may either be fastened to a heavy foot, in which case a side tube must be joined to it, as in fig. , or it may be open at both ends and be held in a clamp. it is well to put a cone of card-board on the open end (fig. ), if the sound is to be heard by many at a time. if the experimenter alone wishes to hear as well as possible when faint sounds are produced, he should carry a piece of smooth india-rubber tube about half an inch in diameter from the open end to his ear. this, however, would nearly deafen him with such loud noises as the tick of a watch. _bubbles and ether._ experiments with ether must be performed with great care, because, like the bisulphide of carbon, it is dangerously inflammable. the bottle of ether must never be brought near a light. if a large quantity is spilled, the heavy vapour is apt to run along the floor and ignite at a fire, even on the other side of a room. any vessel may be filled with the vapour of ether by merely pouring the liquid upon a piece of blotting-paper reaching up to the level of the edge. very little is required, say half a wine-glassful, for a basin that would hold a gallon or more. in a draughty place the vapour will be lost in a short time. bubbles can be set to float upon the vapour without any difficulty. they may be removed in five or ten seconds by means of one of the small light rings with a handle, provided that the ring is wetted with the soap solution and has _no_ film stretched across it. if taken to a light at a safe distance the bubble will immediately burst into a blaze. if a neighbouring light is not close down to the table, but well up above the jar on a stand, it may be near with but little risk. to show the burning vapour, the same wide tube that was used to blow out the candle will answer well. the pear shape of the bubble, owing to its increased weight after being held in the vapour for ten or fifteen seconds, is evident enough on its removal, but the falling stream of heavy vapour, which comes out again afterwards, can only be shown if its shadow is cast upon a screen by means of a bright light. _experiment with internal bubbles._ for these experiments, next to a good solution, the pipe is of the greatest importance. a "churchwarden" is no use. a glass pipe / inch in diameter at the mouth is best. if this is merely a tube bent near the end through a right angle, moisture condensed in the tube will in time run down and destroy the bubble occasionally, which is very annoying in a difficult experiment. i have made for myself the pipe of which fig. is a full size representation, and i do not think that it is possible to improve upon this. those who are not glass-blowers will be able, with the help of cork, to make a pipe with a trap as shown in fig. , which is as good, except in appearance and handiness. in knocking bubbles together to show that they do not touch, care must be taken to avoid letting either bubble meet any projection in the other, such as the wire ring, or a heavy drop of liquid. either will instantly destroy the two bubbles. there is also a limit to the violence which may be used, which experience will soon indicate. in pushing a bubble through a ring smaller than itself, by means of a flat film on another ring, it is important that the bubble should not be too large; but a larger bubble can be pushed through than would be expected. it is not so easy to push it up as down because of the heavy drop of liquid, which it is difficult to completely drain away. [illustration: fig. . length of stem inches] [illustration: fig. . length of stem inches] to blow one bubble inside another, the first, as large as an average orange, should be blown on the lower side of a horizontal ring. a light wire ring should then be hung on to this bubble to slightly pull it out of shape. for this purpose thin aluminium rings are hardly heavy enough, and so either a heavier metal should be used, or a small weight should be fastened to the handle of the ring. the ring should be so heavy that the sides of the bubble make an angle of thirty or forty degrees with the vertical, where they meet the ring as indicated in fig. . the wetted end of the pipe is now to be inserted through the top of the bubble, until it has penetrated a clear half inch or so. a new bubble can now be blown any size almost that may be desired. to remove the pipe a slow motion will be fatal, because it will raise the inner bubble until it and the outer one both meet the pipe at the same place. this will bring them into true contact. on the other hand, a violent jerk will almost certainly produce too great a disturbance. a rather rapid motion, or a slight jerk, is all that is required. it is advisable before passing the pipe up through the lower ring, so as to touch the inner bubble, and so drain away the heavy drop, to steady this with the other hand. the superfluous liquid can then be drained from both bubbles simultaneously. care must be taken after this that the inner bubble is not allowed to come against either wire ring, nor must the pipe be passed through the side where the two bubbles are very close together. to peel off the lower ring it should be pulled down a very little way and then inclined to one side. the peeling will then start more readily, but as soon as it has begun the ring should be raised so as not to make the peeling too rapid, otherwise the final jerk, when it leaves the lower ring, will be too much for the bubbles to withstand. bubbles coloured with fluorescine, or uranine, do not show their brilliant fluorescence unless sunlight or electric light is concentrated upon them with a lens or mirror. the quantity of dye required is so small that it may be difficult to take little enough. as much as can be picked up on the last eighth of an inch of a pointed pen-knife will be, roughly speaking, enough for a wine-glassful of the soap solution. if the quantity is increased beyond something like the proportion stated, the fluorescence becomes less and very soon disappears. the best quantity can be found in a few minutes by trial. to blow bubbles containing either coal-gas or air, or a mixture of the two, the most convenient plan is to have a small t-shaped glass tube which can be joined by one arm of the t to the blow-pipe by means of a short piece of india-rubber tube, and be connected by its vertical limb with a sufficient length of india-rubber pipe, one-eighth of an inch in diameter inside, to reach to the floor, after which it may be connected by any kind of pipe with the gas supply. the gas can be stopped either by pinching the india-rubber tube with the left hand, if that is at liberty, or by treading on it if both hands are occupied. meanwhile air can be blown in by the other arm of the t, and the end closed by the tongue when gas alone is required. this end of the tube should be slightly spread out when hot by rapidly pushing into it the _cold_ tang of a file, and twisting it at the same time, so that it may be lightly held by the teeth without fear of slipping. if a light t-piece or so great a length of small india-rubber tube cannot be obtained, then the mouth must be removed from the pipe and the india-rubber tube slipped in when air is to be changed for gas. this makes the manipulation more difficult, but all the experiments, except the one with three bubbles, can be so carried out. the pipe must in every case be made to enter the highest point of a bubble in order to start an internal one. if it is pushed horizontally through the side, the inner bubble is sure to break. if the inner bubble is being blown with gas, it will soon tend to rise. the pipe must then be turned over in such a manner that the inner bubble does not creep along it, and so meet the outer one where penetrated by the pipe. a few trials will show what is meant. the inner bubble may then be allowed to rest against the top of the outer one while being enlarged. when it is desired after withdrawing the pipe to blow more air or gas into either the inner or the outer bubble, it is not safe after inserting the pipe again to begin to blow at once; the film which is now stretched across the mouth of the pipe will probably become a third bubble, and this, under the circumstances, is almost certain to cause a failure. an instantaneous withdrawal of the air destroys this film by drawing it into the pipe. air or gas may then be blown without danger. if the same experiment is performed upon a light ring with cotton and paper attached, the left hand will be occupied in holding this ring, and then the gas must be controlled by the foot, or by a friend. the light ring should be quite two inches in diameter. if, when the inner bubble has begun to carry away the ring, &c., the paper is caught hold of, it is possible, by a judicious pull, to cause the two bubbles to leave the ring and so escape into the air one inside the other. for this purpose the smallest ring that will carry the paper should be used. with larger rings the same effect may be produced by inclining the ring, and so allowing the outer bubble to peel off, or by placing the mouth of the pipe against the ring and blowing a third bubble in real contact with the ring and the outer bubble. this will assist the peeling process. to blow three bubbles, one inside the other two, is more difficult. the following plan i have found to be fairly certain. first blow above the ring a bubble the size of a large orange. then take a small ring about an inch in diameter, with a straight wire coming down from one side to act as a handle, and after wetting it with the solution, pass it carefully up through the fixed ring so that the small ring is held well inside the bubble. now pass the pipe, freshly dipped in the solution, into the outer or no. bubble until it is quite close to the small ring, and begin to blow the no. bubble. this must be started with the pipe almost in contact with the inner ring, as the film on this ring would destroy a bubble that had attained any size. withdraw the pipe, dip it into the liquid, and insert it into the inner bubble, taking care to keep these two bubbles from meeting anywhere. now blow a large gas-bubble, which may rest against the top of no. while it is growing. no. may now rest against the top of no. without danger. remove pipe from no. by gently lowering it, and let some gas into no. to make it lighter, and at the same time diminish the pressure between nos. and . presently the small ring can be peeled off no. and removed altogether. but if there is a difficulty in accomplishing this, withdraw the pipe from no. and blow air into no. to enlarge it, which will make the process easier. then remove the pipe from no. . the three bubbles are now resting one inside the other. by blowing a fourth bubble, as described above, against the fixed ring, no. bubble will peel off, and the three will float away. no. can, while peeling, be transferred to a light wire ring from which paper, &c. are suspended. this description sounds complicated, but after a little practice the process can be carried out almost with certainty in far less time than it takes to describe it; in fact, so quickly can it be done, and so simple does it appear, that no one would suppose that so many details had to be attended to. _bubbles and electricity._ these experiments are on the whole the most difficult to perform successfully. the following details should be sufficient to prevent failure. two rings are formed at the end of a pair of wires about six inches long in the straight part. about one inch at the opposite end from the ring is turned down at a right angle. these turned-down ends rest in two holes drilled vertically in a non-conductor such as ebonite, about two or three inches apart. then if all is right the two rings are horizontal and at the same level, and they may be moved towards or away from one another. separate them a few inches, and blow a bubble above or below each, making them nearly the same size. then bring the two rings nearer together until the bubbles just, and only just, rest against one another. though they may be hammered together without joining, they will not remain long resting in this position, as the convex surfaces can readily squeeze out the air. the ebonite should not be perfectly warm and dry, for it is then sure to be electrified, and this will give trouble. it must not be wet, because then it will conduct, and the sealing-wax will produce no result. if it has been used as the support for the rings for some of the previous experiments, it will have been sufficiently splashed by the bursting of bubbles to be in the best condition. it must, however, be well wiped occasionally. a stick of sealing-wax should be held in readiness under the arm, in a fold or two of _dry_ flannel or fur. if the wax is very strongly electrified, it is apt to be far too powerful, and to cause the bubbles, when it is presented to them, to destroy each other. a feeble electrification is sufficient; then the instant it is exposed the bubbles coalesce. the wax may be brought so near one bubble in which another one is resting, that it pulls them to one side, but the inner one is screened from electrical action by the outer one. it is important not to bring the wax very near, as in that case the bubble will be pulled so far as to touch it, and so be broken. the wetting of the wax will make further electrification very uncertain. in showing the difference between an inner and an outer bubble, the same remarks with regard to undue pressure, electrification, or loss of time apply. i have generally found that it is advisable in this experiment not to drain the drops from both the bubbles, as their weight seems to steady them; the external bubble may be drained, and if it is not too large, the process of electrically joining the outer bubbles, without injury to the inner one, may be repeated many times. i once caused eight or nine single bubbles to unite with the outer one of a pair in succession before it became too unwieldy for more accessions to be possible. * * * * * it would be going outside my subject to say anything about the management of lanterns. i may, however, state that while the experiments with the small bubbles are best projected with a lens upon the screen, the larger bubbles described in the last lecture can only be projected by their shadows. for this purpose the condensing lens is removed, and the bare light alone made use of. an electric arc is far preferable to a lime-light, both because the shadows are sharper, and because the colours are so much more brilliant. no oil lamp would answer, even if the light were sufficient in quantity, because the flame would be far too large to cast a sharp shadow. in these hints, which have in themselves required a rather formidable chapter, i have given all the details, so far as i am able, which a considerable experience has shown to be necessary for the successful performance or the experiments in public. the hints will i hope materially assist those who are not in the habit of carrying out experiments, but who may wish to perform them for their own satisfaction. though people who are not experimentalists may consider that the hints are overburdened with detail, it is probable that in repeating the experiments they will find here and there, in spite of all my care to provide against unforeseen difficulties, that more detail would have been desirable. though it is unusual to conclude such a book as this with the fullest directions for carrying out the experiments described, i believe that the innovation in the present instance is good, more especially because many of the experiments require none of the elaborate apparatus which so often is necessary. the end. _richard clay & sons, limited, london & bungay._ [illustration: fig. thaumatrope for showing the formation and oscillation of drops.] [transcriber's notes all apparent printer's errors retained. variation in punctuation are as in the original, but missing full stops at end of paragraphs have been supplied. there are inconsistencies in the use of italics, spacing of words and use of full stop after 'axiome', abbreviations etc. all are retained to match text. there is a great variation in spelling including multiple spellings of the same word, all spelling has been retained to match text. there are several instances of obviously missing letters or inverted n & u. these have been changed or obvious letters replaced, with the changes surrounded by {}. all instances are detailed at the end of the text. it should also be noted that in the original text there is a missing line at the end of page in original text. there are a number of instances in the original text where 'that' is immediately followed by a second 'that' in the sentence. these could be potential printer's errors or, since several of them make sense, part of the author's style. they have been left in the text as they appear in the original text. the original text has many sidenotes, some are true sidenotes, introductions to paragraphs etc, some acting as footnotes with some marked in original text with *. these have been dealt with in three ways with the footnotes placed after their relevant paragraph and sidenotes place before their relevant paragraph. ) footnotes marked with capital letter. these were sidenotes in original text marked with * in the original text and thus acting like footnotes. ) footnotes marked with number. these were sidenotes in original text that were unmarked but acting like normal footnotes. the anchor in the text was placed at the most suitable relevant place in comparison with the placement of the sidenote text in the margin, but still should be considered only an approximate placement. ) sidenotes placed at start of the relevant paragraph. some sidenotes were considered not to be relevant as footnotes, introductions to paragraphs etc, and were left as sidenotes before their relevant paragraph.] * * * * * a discourse _presented_ to the most serene don cosimo ii. great duke _of_ tuscany, concerning the _natation_ of bodies vpon, and _submersion_ in, the water. by galileus galilei: philosopher and mathematician unto his most serene highnesse. englished from the second edition of the italian, compared with the manuscript copies, and reduced into propositions: by _thomas salusbury_, esq; _london_: printed by william leybourn: _m dc lxiii._ * * * * * [decoration] a discovrse presented to the most serene don cosimo ii. great duke of _tuscany_: concerning _the natation of bodies upon, or submersion_ _in, the water._ considering (most serene prince) that the publishing this present treatise, of so different an argument from that which many expect, and which according to the intentions i proposed in my [a] astronomicall _adviso_, i should before this time have put forth, might peradventure make some thinke, either that i had wholly relinquished my farther imployment about the new celestiall observations, or that, at least, i handled them very remissely; i have judged fit to render an account, aswell of my deferring that, as of my writing, and publishing this treatise. [a] his nuncio siderio. as to the first, the last discoveries of _saturn_ to be tricorporeall, and of the mutations of figure in _venus_, like to those that are seen in the moon, together with the consequents depending thereupon, have not so much occasioned the demur, as the investigation of the times of the conversions of each of the four medicean planets about _jupiter_, which i lighted upon in _april_ the year past, , at my being in _rome_; where, in the end, i assertained my selfe, that the first and neerest to _jupiter_, moved about _gr._ & _m._ of its sphere in an houre, makeing its whole revolution in one naturall day, and hours, and almost an halfe. the second moves in its orbe _gr._ _min._ or very neer, in an hour, and its compleat conversion is consummate in dayes, hours, and one third, or thereabouts. the third passeth in an hour, _gr._ _min._ little more or less of its circle, and measures it all in dayes, hours, or very neer. the fourth, and more remote than the rest, goes in one houre, _gr_ _min._ and almost an halfe of its sphere, and finisheth it all in dayes, and very neer hours. but because the excessive velocity of their returns or restitutions, requires a most scrupulous precisenesse to calculate their places, in times past and future, especially if the time be for many moneths or years; i am therefore forced, with other observations, and more exact than the former, and in times more remote from one another, to correct the tables of such motions, and limit them even to the shortest moment: for such exactnesse my first observations suffice not; not only in regard of the short intervals of time, but because i had not as then found out a way to measure the distances between the said planets by any instrument: i observed such intervals with simple relation to the diameter of the body of _jupiter_; taken, as we have said, by the eye, the which, though they admit not errors of above a minute, yet they suffice not for the determination of the exact greatness of the spheres of those stars. but now that i have hit upon a way of taking such measures without failing, scarce in a very few seconds, i will continue the observation to the very occultation of _jupiter_, which shall serve to bring us to the perfect knowledge of the motions, and magnitudes of the orbes of the said planets, together also with some other consequences thence arising. i adde to these things the observation of some obscure spots[ ], which are discovered in the solar body, which changing, position in that, propounds to our consideration a great argument either that the sun revolves in it selfe, or that perhaps other starrs, in like manner as _venus_ and _mercury_, revolve about it, invisible in other times, by reason of their small digressions, lesse than that of _mercury_, and only visible when they interpose between the sun and our eye, or else hint the truth of both this and that; the certainty of which things ought not to be contemned, nor omitted. [ ] the authors observations of the solar spots _continuall observation hath at last assured me that these spots are matters contiguous to the body of the sun, there continually produced in great number, and afterwards dissolved, some in a shorter, some in a longer time, and to be by the conversion or revolution of the sun in it selfe, which in a lunar moneth, or thereabouts, finisheth its period, caried about in a circle, an accident great of it selfe, and greater for its consequences._ as to the other particular in the next place [b] many causes have moved me to write the present tract, the subject whereof, is the dispute which i held some dayes since, with some learned men of this city, about which, as your highnesse knows, have followed many discourses: the principall of which causes hath been the intimation of your highnesse, having commended to me writing, as a singular means to make true known from false, reall from apparent reasons, farr better than by disputing vocally, where the one or the other, or very often both the disputants, through too greate heate, or exalting of the voyce, either are not understood, or else being transported by ostentation of not yeilding to one another, farr from the first proposition, with the novelty, of the various proposals, confound both themselves and their auditors. [b] the occasion inducing the author to write this treatise. moreover, it seemed to me convenient to informe your highnesse of all the sequell, concerning the controversie of which i treat, as it hath been advertised often already by others: and because the doctrine which i follow, in the discussion of the point in hand, is different from that of _aristotle_; and interferes with his principles, i have considered that against the authority of that most famous man, which amongst many makes all suspected that comes not from the schooles of the peripateticks, its farr better to give ones reasons by the pen than by word of mouth, and therfore i resolved to write the present discourse: in which yet i hope to demonstrate that it was not out of capritiousnesse, or for that i had not read or understood _aristotle_, that i sometimes swerve from his opinion, but because severall reasons perswade me to it, and the same _aristotle_ hath tought me to fix my judgment on that which is grounded upon reason, and not on the bare authority of the master[ ]; and it is most certaine according to the sentence of _alcinoos_, that philosophating should be free. nor is the resolution of our question in my judgment without some benefit to the universall[ ], forasmuch as treating whether the figure of solids operates, or not, in their going, or not going to the bottome in water, in occurrences of building bridges or other fabricks on the water, which happen commonly in affairs of grand import, it may be of great availe to know the truth. [ ] _aristotle_ prefers reason to the authority ofan author. [ ] the benefit of this argument. i say therfore, that being the last summer in company with certain learned men, it was said in the argumentation; that condensation was the propriety of cold[ ], and there was alledged for instance, the example of ice: now i at that time said, that, in my judgment, the ice should be rather water rarified than condensed[ ], and my reason was, because condensation begets diminution of mass, and augmentation of gravity, and rarifaction causeth greater lightness, and augmentarion of masse: and water in freezing, encreaseth in masse, and the ice made thereby is lighter than the water on which it swimmeth. [ ] condensation the propriety of cold, according to the peripateticks. [ ] ice rather water rarified, than condensed, and why: _what i say, is manifest, because, the medium subtracting from the whole gravity of sollids the weight of such another masse of the said medium; as_ archimedes _proves in his_ [c] first booke de insidentibus humido; _when ever the masse of the said solid encreaseth by distraction, the more shall the_ medium _detract from its entire gravity; and lesse, when by compression it shall be condensed and reduced to a lesse masse._ [c] in lib: . of natation of bodies prop. . [sidenote: figure operates not in the natation of sollids.] it was answered me, tha{t} that proceeded not from the greater levity, but from the figure, large and flat, which not being able to penetrate the resistance of the water, is the cause that it submergeth not. i replied, that any piece of ice, of whatsoever figure, swims upon the water, a manifest signe, that its being never so flat and broad, hath not any part in its floating: and added, that it was a manifest proofe hereof to see a piece of ice of very broad figure being thrust to the botome of the water, suddenly return to flote atoppe, which had it been more grave, and had its swimming proceeded from its forme, unable to penetrate the resistance of the _medium_, that would be altogether impossible; i concluded therefore, that the figure was in sort a cause of the natation or submersion of bodies, but the greater or lesse gravity in respect of the water: and therefore all bodyes heavier than it of what figure soever they be, indifferently go to the bottome, and the lighter, though of any figure, float indifferently on the top: and i suppose that those which hold otherwise, were induced to that beliefe, by seeing how that diversity of formes or figures, greatly altereth the velosity, and tardity of motion; so that bodies of figure broad and thin, descend far more leasurely into the water, than those of a more compacted figure, though both made of the same matter: by which some might be induced to believe that the dilatation of the figure might reduce it to such amplenesse that it should not only retard but wholly impede and take away the motion, which i hold to be false. upon this conclusion, in many dayes discourse, was spoken much, and many things, and divers experiments produced, of which your highnesse heard, and saw some, and in this discourse shall have all that which hath been produced against my assertion, and what hath been suggested to my thoughts on this matter, and for confirmation of my conclusion: which if it shall suffice to remove that (as i esteem hitherto false) opinion, i shall thinke i have not unprofitably spent my paynes and time. and although that come not to passe, yet ought i to promise another benefit to my selfe, namely, of attaining the knowledge of the truth, by hearing my fallacyes confuted, and true demonstrations produced by those of the contrary opinion. and to proceed with the greatest plainness and perspicuity that i can possible, it is, i conceive, necessary, first of all to declare what is the true, intrinsecall, and totall cause, of the ascending of some sollid bodyes in the water, and therein floating; or on the contrary, of their sinking and so much the rather in asmuch as i cannot satisfie myselfe in that which _aristotle_ hath left written on this subject. [sidenote: the cause of the natation & submersion of solids in the water.] i say then the cause why some sollid bodyes descend to the bottom of water, is the excesse of their gravity, above the gravity of the water; and on the contrary, the excess of the waters gravity above the gravity of those, is the cause that others do not descend, rather that they rise from the bottom, and ascend to the surface. this was subtilly demonstrated by _archimedes_ in his book of the natation of bodies: conferred afterwards by a very grave author, but, if i erre not invisibly, as below for defence of him, i shall endeavour to prove. i, with a different method, and by other meanes, will endeavour to demonstrate the same, reducing the causes of such effects to more intrinsecall and immediate principles, in which also are discovered the causes of some admirable and almost incredible accidents, as that would be, that a very little quantity of water, should be able, with its small weight, to raise and sustain a solid body, an hundred or a thousand times heavier than it. and because demonstrative order so requires, i shall define certain termes, and afterwards explain some propositions, of which, as of things true and obvious, i may make use of to my present purpose. definition i. _i then call equally grave_ in specie, _those matters of which equall masses weigh equally._ as if for example, two balls, one of wax, and the other of some wood of equall masse, were also equall in weight, we say, that such wood, and the wax are _in specie_ equally grave. definition ii. _but equally grave in absolute gravity, we call two sollids, weighing equally, though of mass they be unequall._ as for example, a mass of lead, and another of wood, that weigh each ten pounds, i call equall in absolute gravity, though the mass of the wood be much greater then that of the lead. _and, consequently, less grave_ in specie. definition iii. _i call a matter more grave_ in specie _than another, of which a mass, equall to a mass of the other, shall weigh more._ and so i say, that lead is more grave _in specie_ than tinn, because if you take of them two equall masses, that of the lead weigheth more. definition iv. _but i call that body more grave absolutely than this, if that weigh more than this, without any respect had to the masses._ and thus a great piece of wood is said to weigh more than a little lump of lead, though the lead be _in specie_ more heavy than the wood. and the same is to be understood of the less grave _in specie_, and the less grave absolutely. these termes defined, i take from the mechanicks two principles: the first is, that axiome. i. _weights absolutely equall, moved with equall velocity, are of equall force and moment in their operations._ _definition v._ moment, amongst mechanicians, signifieth that vertue, that force, or that efficacy, with which the mover moves, and the moveable resists. _which vertue dependes not only on the simple gravity, but on the velocity of the motion, and on the diverse inclinations of the spaces along which the motion is made: for a descending weight makes a greater impetus in a space much declining, than in one less declining; and in summe, what ever is the occasion of such vertue, it ever retaines the name of moment; nor in my judgement, is this sence new in our idiome, for, if i mistake not, i think we often say; this is a weighty businesse, but the other is of small moment: and we consider lighter matters and let pass those of moment; a metaphor, i suppose, taken from the mechanicks._ as for example, two weights equall in absolute gravity, being put into a ballance of equall arms, they stand in _equilibrium_, neither one going down, nor the other up: because the equality of the distances of both, from the centre on which the ballance is supported, and about which it moves, causeth that those weights, the said ballance moving, shall in the same time move equall spaces, that is, shall move with equall velocity, so that there is no reason for which this weight should descend more than that, or that more than this; and therefore they make an _equilibrium_, and their moments continue of semblable and equall vertue. the second principle is; that axiome ii. _the moment and force of the gravity, is encreased by the velocity of the motion._ so that weights absolutely equall, but conjoyned with velocity unequall, are of force, moment and vertue unequall: and the more potent, the more swift, according to the proportion of the velocity of the one, to the velocity of the other. of this we have a very pertinent example in the balance or stiliard of unequall arms, at which weights absolutely equall being suspended, they do not weigh down, and gravitate equally, but that which is at a greater distance from the centre, about which the beam moves, descends, raising the other, and the motion of this which ascends is slow, and the other swift: and such is the force and vertue, which from the velocity of the mover, is conferred on the moveable, which receives it, that it can exquisitely compensate, as much more weight added to the other slower moveable: so that if of the arms of the balance, one were ten times as long as the other, whereupon in the beames moving about the centre, the end of that would go ten times as far as the end of this, a weight suspended at the greater distance, may sustain and poyse another ten times more grave absolutely than it: and that because the stiliard moving, the lesser weight shall move ten times faster than the bigger. it ought alwayes therefore to be understood, that motions are according to the same inclinations, namely, that if one of the moveables move perpendicularly to the horizon, then the other makes its motion by the like perpendicular; and if the motion of one were to be made horizontally; that then the other is made along the same horizontall plain: and in summe, alwayes both in like inclinations. this proportion between the gravity and velocity is found in all mechanicall instruments: and is considered by _aristotle_, as a principle in his _mechanicall questions_; whereupon we also may take it for a true assumption, that axiome iii. _weights absolutely unequall, do alternately counterpoyse and become of equall moments, as oft as their gravities, with contrary proportion, answer to the velocity of their motions._ that is to say, that by how much the one is less grave than the other, by so much is it in a constitution of moving more swiftly than that. having prefatically explicated these things, we may begin to enquire, what bodyes those are which totally submerge in water, and go to the bottom, and which those that by constraint float on the top, so that being thrust by violence under water, they return to swim, with one part of their mass visible above the surface of the water: and this we will do by considering the respective operation of the said solids, and of water: which operation followes the submersion and sinking; and this it is[ ], that in the submersion that the solid maketh, being depressed downwards by its proper gravity, it comes to drive away the water from the place where it successively subenters, and the water repulsed riseth and ascends above its first levell, to which ascent on the other side it, as being a grave body of its own nature, resists: and because the descending solid more and more immerging, greater and greater quantity of water ascends, till the whole sollid be submerged; its necessary to compare the moments of the resistance of the water to ascension, with the moments of the pressive gravity of the solid: and if the moments of the resistance of the water, shall equalize the moments of the solid, before its totall immersion[ ]; in this case doubtless there shall be made an _equilibrium_, nor shall the body sink any farther. but if the moment of the solid, shall alwayes exceed the moments wherewith the repulsed water successively makes resistance[ ], that solid shall not only wholly submerge under water, but shall descend to the bottom. but if, lastly, in the instant of totall submersion, the equality shall be made between the moments of the prement solid, and the resisting water[ ]; then shall rest, ensue, and the said solid shall be able to rest indifferently, in whatsoever part of the water. by this time is manifest the necessity of comparing the gravity of the water, and of the solid[ ]; and this comparison might at first sight seem sufficient to conclude and determine which are the solids that float a-top, and which those that sink to the bottom in the water, asserting that those shall float which are lesse grave _in specie_ than the water, and those submerge, which are _in specie_ more grave. for it seems in appearance, that the sollid in sinking continually, raiseth so much water in mass, as answers to the parts of its own bulk submerged: whereupon it is impossible, that a solid less grave _in specie_, than water, should wholly sink, as being unable to raise a weight greater than its own, and such would a mass of water equall to its own mass be. and likewise it seems necessary, that the graver solids do go to the bottom, as being of a force more than sufficient for the raising a masse of water, equall to its own, though inferiour in weight. nevertheless the business succeeds otherwise: and though the conclusions are true, yet are the causes thus assigned deficient, nor is it true, that the solid in submerging, raiseth and repulseth masses of water, equall to the parts of it self submerged; but the water repulsed, is alwayes less than the parts of the solid submerged[ ]: and so much the more by how much the vessell in which the water is contained is narrower: in such manner that it hinders not, but that a solid may submerge all under water, without raising so much water in mass, as would equall the tenth or twentieth part of its own bulk: like as on the contrary, a very small quantity of water, may raise a very great solid mass[ ], though such solid should weigh absolutely a hundred times as much, or more, than the said water, if so be that the matter of that same solid be _in specie_ less grave than the water. and thus a great beam, as suppose of a weight, may be raised and born afloat by water, which weighs not : and this happens when the moment of the water is compensated by the velocity of its motion. [ ] how the submersion of solids in the water, is effected. [ ] what solids shall float on the water. [ ] what solids shall sinke to the botome. [ ] what solids shall rest in all places of the water. [ ] the gravitie of the water and solid must be compared in all problems, of natation of bodies. [ ] the water repelled is ever less than the parts of the sollid submerged. [ ] _a_ small quantity of water, may float a very great solid mass. but because such things, propounded thus in abstract, are somewhat difficult to be comprehended, it would be good to demonstrate them by particular examples; and for facility of demonstration, we will suppose the vessels in which we are to put the water, and place the solids, to be inviron'd and included with sides erected perpendicular to the plane of the horizon, and the solid that is to be put into such vessell to be either a streight cylinder, or else an upright prisme. _the which proposed and declared, i proceed to demonstrate the truth of what hath been hinted, forming the ensuing theoreme._ _theoreme i._ [sidenote: the proportion of the water raised to the solid submerged.] the mass of the water which ascends in the submerging of a solid, prisme or cylinder, or that abaseth in taking it out, is less than the mass of the said solid, so depressed or advanced: and hath to it the same proportion, that the surface of the water circumfusing the solid, hath to the same circumfused surface, together with the base of the solid. _let the vessell be a b c d, and in it the water raised up to the levell e f g, before the solid prisme h i k be therein immerged; but after that it is depressed under water, let the water be raised as high as the levell l m, the solid h i k shall then be all under water, and the mass of the elevated water shall be l g, which is less than the masse of the solid depressed, namely of h i k, being equall to the only part e i k, which is contained under the first levell e f g. which is manifest, because if the solid h i k be taken out, the water i g shall return into the place occupied by the mass e i k, where it was continuate before the submersion of the prisme. and the mass l g being equall to the mass e k: adde thereto the mass e n, and it shall be the whole mass e m, composed of the parts of the prisme e n, and of the water n f, equall to the whole solid h i k: and, therefore, the mass l g shall have the same proportion to e m, as to the mass h i k: but the mass l g hath the same proportion to the mass e m, as the surface l m hath to the surface m h: therefore it is manifest, that the mass of water repulsed l g, is in proportion to the mass of the solid submerged h i k; as the surface l m, namely, that of the water ambient about the sollid, to the whole surface h m, compounded of the said ambient water, and the base of the prisme h n. but if we suppose the first levell of the water the according to the surface h m, and the prisme allready submerged h i k; and after to be taken out and raised to e a o, and the water to be faln from the first levell h l m as low as e f g; it is manifest, that the prisme e a o being the same with h i k, its superiour part h o, shall be equall to the inferiour e i k: and remove the common part e n, and, consequently, the mass of the water l g is equall to the mass h o; and, therefore, less than the solid, which is without the water, namely, the whole prisme e a o, to which likewise, the said mass of water abated l g, hath the same proportion, that the surface of the waters circumfused l m hath to the same circumfused surface, together with the base of the prisme a o: which hath the same demonstration with the former case above._ [illustration] _and from hence is inferred, that the mass of the water, that riseth in the immersion of the solid, or that ebbeth in elevating it, is not equall to all the mass of the solid, which is submerged or elevated, but to that part only, which in the immersion is under the first levell of the water, and in the elevation remaines above the first levell: which is that which was to be demonstrated. we will now pursue the things that remain._ and first we will demonstrate that, theoreme ii. [sidenote: the proportion of the water abated, to the solid raised.] _when in one of the above said vessels, of what ever breadth, whether wide or narrow, there is placed such a prisme or cylinder, inviron'd with water, if we elevate that solid perpendicularly, the water circumfused shall abate, and the abatement of the water, shall have the same proportion to the elevation of the prisme, as one of the bases of the prisme, hath to the surface of the water circumfused._ [illustration] imagine in the vessell, as is aforesaid, the prisme a c d b to be placed, and in the rest of the space the water to be diffused as far as the levell e a: and raising the solid, let it be transferred to g m, and let the water be abased from e a to n o: i say, that the descent of the water, measured by the line a o, hath the same proportion to the rise of the prisme, measured by the line g a, as the base of the solid g h hath to the surface of the water n o. the which is manifest: because the mass of the solid g a b h, raised above the first levell e a b, is equall to the mass of water that is abased e n o a. therefore, e n o a and g a b h are two equall prismes; for of equall prismes, the bases answer contrarily to their heights: therefore, as the altitude a o is to the altitude a g, so is the superficies or base g h to the surface of the water n o. if therefore, for example, a pillar were erected in a waste pond full of water, or else in a well, capable of little more then the mass of the said pillar, in elevating the said pillar, and taking it out of the water, according as it riseth, the water that invirons it will gradually abate, and the abasement of the water at the instant of lifting out the pillar, shall have the same proportion, that the thickness of the pillar hath to the excess of the breadth of the said pond or well, above the thickness of the said pillar: so that if the breadth of the well were an eighth part larger than the thickness of the pillar, and the breadth of the pond twenty five times as great as the said thickness, in the pillars ascending one foot, the water in the well shall descend seven foot, and that in the pond only / of a foot. [sidenote: why a solid less grave _in specie_ than water, stayeth not under water, in very small depths:] this demonstrated, it will not be difficult to show the true cause, how it comes to pass, that, theoreme iii. _a prisme or regular cylinder, of a substance specifically less grave than water, if it should be totally submerged in water, stayes not underneath, but riseth, though the water circumfused be very little, and in absolute gravity, never so much inferiour to the gravity of the said prisme._ let then the prisme a e f b, be put into the vessell c d f b, the same being less grave _in specie_ than the water: and let the water infused rise to the height of the prisme: i say, that the prisme left at liberty, it shall rise, being born up by the water circumfused c d e a. for the water c e being specifically more grave than the solid a f, the absolute weight of the water c e, shall have greater proportion to the absolute weight of the prisme a f, than the mass c e hath to the mass a f (in regard the mass hath the same proportion to the mass, that the weight absolute hath to the weight absolute, in case the masses are of the same gravity _in specie_.) but the mass c e is to the mass a f, as the surface of the water a c, is to the superficies, or base of the prisme a b; which is the same proportion as the ascent of the prisme when it riseth, hath to the descent of the water circumfused c e. [illustration] therefore, the absolute gravity of the water c e, hath greater proportion to the absolute gravity of the prisme a f; than the ascent of the prisme a f, hath to the descent of the said water c e. the moment, therefore, compounded of the absolute gravity of the water c e, and of the velocity of its descent, whilst it forceably repulseth and raiseth the solid a f, is greater than the moment compounded of the absolute gravity of the prisme a f, and of the tardity of its ascent, with which moment it contrasts and resists the repulse and violence done it by the moment of the water: therefore, the prisme shall be raised. [sidenote: the proportion according to which the submersion & natation of solids is made.] it followes, now, that we proceed forward to demonstrate more particularly, how much such solids shall be inferiour in gravity to the water elevated; namely, what part of them shall rest submerged, and what shall be visible above the surface of the water: but first it is necessary to demonstrate the subsequent lemma. lemma i. [sidenote: the absolute gravity of solids, are in a proportion compounded of their specifick gravities, and of their masses.] _the absolute gravities of solids, have a proportion compounded of the proportions of their specificall gravities, and of their masses._ [illustration] let a and b be two solids. i say, that the absolute gravity of a, hath to the absolute gravity of b, a proportion compounded of the proportions of the specificall gravity of a, to the specificall gravity of b, and of the mass a to the mass b. let the line d have the same proportion to e, that the specifick gravity of a, hath to the specifick gravity of b; and let e be to f, as the mass a to the mass b: it is manifest, that the proportion of d to f, is compounded of the proportions d and e; and e and f. it is requisite, therefore, to demonstrate, that as d is to f, so the absolute gravity of a, is to the absolute gravity of b. take the solid c, equall in mass to the solid a, and of the same gravity _in specie_ with the solid b. because, therefore, a and c are equall in mass, the absolute gravity of a, shall have to the absolute gravity of c, the same proportion, as the specificall gravity of a, hath to the specificall gravity of c, or of b, which is the same _in specie_; that is, as d is to e. and, because, c and b are of the same gravity _in specie_, it shall be, that as the absolute weight of c, is to the absolute weight of b, so the mass c, or the mass a, is to the mass b; that is, as the line e to the line f. as therefore, the absolute gravity of a, is to the absolute gravity of c, so is the line d to the line e: and, as the absolute gravity of c, is to the absolute gravity of b, so is the line e to the line f: therefore, by equality of proportion, the absolute gravity of a, is to the absolute gravity of b, as the line d to the line f: which was to be demonstrated. i proceed now to demonstrate, how that, theoreme iv. [sidenote: the proportion of water requisite to make a solid swim:] _if a solid, cylinder, or prisme, lesse grave specifically than the water, being put into a vessel, as above, of whatsoever greatnesse, and the water, be afterwards infused, the solid shall rest in the bottom, unraised, till the water arrive to that part of the altitude, of the said prisme, to which its whole altitude hath the same proportion, that the specificall gravity of the water, hath to the specificall gravity of the said solid: but infusing more water, the solid shall ascend._ [illustration] let the vessell be m l g n of any bigness, and let there be placed in it the solid prisme d f g e, less grave _in specie_ than the water; and look what proportion the specificall gravity of the water, hath to that of the prisme, such let the altitude d f, have to the altitude f b. i say, that infusing water to the altitude f b, the solid d g shall not float, but shall stand in _equilibrium_, so, that that every little quantity of water, that is infused, shall raise it. let the water, therefore, be infused to the levell a b c; and; because the specifick gravity of the solid d g, is to the specifick gravity of the water, as the altitude b f is to the altitude f d; that is, as the mass b g to the mass g d; as the proportion of the mass b g is to the mass g d, as the proportion of the mass g d is to the mass a f, they compose the proportion of the mass b g to the mass a f. therefore, the mass b g is to the mass a f, in a proportion compounded of the proportions of the specifick gravity of the solid g d, to the specifick gravity of the water, and of the mass g d to the mass a f: but the same proportions of the specifick gravity of g d, to the specifick gravity of the water, and of the mass g d to the mass a f, do also by the precedent _lemma_, compound the proportion of the absolute gravity of the solid d g, to the absolute gravity of the mass of the water a f: therefore, as the mass b g is to the mass a f, so is the absolute gravity of the solid d g, to the absolute gravity of the mass of the water a f. but as the mass b g is to the mass a f; so is the base of the prisme d e, to the surface of the water a b; and so is the descent of the water a b, to the elevation of the prisme d g; therefore, the descent of the water is to the elevation of the prisme, as the absolute gravity of the prisme, is to the absolute gravity of the water: therefore, the moment resulting from the absolute gravity of the water a f, and the velocity of the motion of declination, with which moment it forceth the prisme d g, to rise and ascend, is equall to the moment that results from the absolute gravity of the prisme d g, and from the velocity of the motion, wherewith being raised, it would ascend: with which moment it resists its being raised: because, therefore, such moments are equall, there shall be an _equilibrium_ between the water and the solid. and, it is manifest, that putting a little more water unto the other a f, it will increase the gravity and moment, whereupon the prisme d g, shall be overcome, and elevated till that the only part b f remaines submerged. which is that that was to be demonstrated. corollary i. [sidenote: _h_ow far solids less grave _in specie_ than water, do submerge.] _by what hath been demonstrated, it is manifest, that solids less grave_ in specie _than the water, submerge only so far, that as much water in mass, as is the part of the solid submerged, doth weigh absolutely as much as the whole solid._ for, it being supposed, that the specificall gravity of the water, is to the specificall gravity of the prisme d g, as the altitude d f, is to the altitude f b; that is, as the solid d g is to the solid b g; we might easily demonstrate, that as much water in mass as is equall to the solid b g, doth weigh absolutely as much as the whole solid d g; for, by the _lemma_ foregoing, the absolute gravity of a mass of water, equall to the mass b g, hath to the absolute gravity of the prisme d g, a proportion compounded of the proportions, of the mass b g to the mass g d, and of the specifick gravit{y} of the water, to the specifick gravity of the prisme: but the gravity _in specie_ of the water, to the gravity _in specie_ of the prisme, is supposed to be as the mass g d to the mass g b. therefore, the absolute gravity of a mass of water, equall to the mass b g, is to the absolute gravity of the solid d g, in a proportion compounded of the proportions, of the mass b g to the mass g d, and of the mass d g to the mass g b; which is a proportion of equalitie. the absolute gravity, therefore, of a mass of water equall to the part of the mass of the prisme b g, is equall to the absolute gravity of the whole solid d g. corollary ii. [sidenote: _a_ rule to equilibrate solids in the water.] _it followes, moreover, that a solid less grave than the water, being put into a vessell of any imaginable greatness, and water being circumfused about it to such a height, that as much water in mass, as is the part of the solid submerged, do weigh absolutely as much as the whole solid; it shall by that water be justly sustained, be the circumfused water in quantity greater or lesser._ [illustration] for, if the cylinder or prisme m, less grave than the water, _v. gra._ in subsequiteriall proportion, shall be put into the capacious vessell a b c d, and the water raised about it, to three quarters of its height, namely, to its levell a d: it shall be sustained and exactly poysed in _equilibrium_. the same will happen; if the vessell e n s f were very small, so, that between the vessell and the solid m, there were but a very narrow space, and only capable of so much water, as the hundredth part of the mass m, by which it should be likewise raised and erected, as before it had been elevated to three fourths of the height of the solid: which to many at the first sight, may seem a notable paradox, and beget a conceit, that the demonstration of these effects, were sophisticall and fallacious: but, for those who so repute it, the experiment is a means that may fully satisfie them. but he that shall but comprehend of what importance velocity of motion is, and how it exactly compensates the defect and want of gravity, will cease to wonder, in considering that at the elevation of the solid m, the great mass of water a b c d abateth very little, but the little mass of water e n s f decreaseth very much, and in an instant, as the solid m before did rise, howbeit for a very short space: whereupon the moment, compounded of the small absolute gravity of the water e n s f, and of its great velocity in ebbing, equalizeth the force and and moment, that results from the composition of the immense gravity of the water a b c d, with its great slownesse of ebbing; since that in the elevation of the sollid m, the abasement of the lesser water e s, is performed just so much more swiftly than the great mass of water a c, as this is more in mass than that which we thus demonstrate. [sidenote: _t_he proportion according to which water riseth and falls in different vessels at the immersion and elevation of solids.] in the rising of the solid m, its elevation hath the same proportion to the circumfused water e n s f, that the surface of the said water, hath to the superficies or base of the said solid m; which base hath the same proportion to the surface of the water a d, that the abasement or ebbing of the water a c, hath to the rise or elevation of the said solid m. therefore, by perturbation of proportion, in the ascent of the said solid m, the abasement of the water a b c d, to the abasement of the water e n s f, hath the same proportion, that the surface of the water e f, hath to the surface of the water a d; that is, that the whole mass of the water e n s f, hath to the whole mass a b c d, being equally high: it is manifest, therefore, that in the expulsion and elevation of the solid m, the water e n s f shall exceed in velocity of _m_otion the water a b c d, asmuch as it on the other side is exceeded by that in quantity: whereupon their moments in such operations, are mutually equall. [illustration] _and, for ampler confirmation, and clearer explication of this, let us consider the present figure, (which if i be not deceived, may serve to detect the errors of some practick mechanitians who upon a false foundation some times attempt impossible enterprizes,) in which, unto the large vessell e i d f, the narrow funnell or pipe i c a b is continued, and suppose water infused into them, unto the levell l g h, which water shall rest in this position, not without admiration in some, who cannot conceive how it can be, that the heavie charge of the great mass of water g d, pressing downwards, should not elevate and repulse the little quantity of the other, contained in the funnell or pipe c l, by which the descent of it is resisted and hindered: but such wonder shall cease, if we begin to suppose the water g d to be abased only to q d, and shall afterwards consider, what the water c l hath done, which to give place to the other, which is descended from the levell g h, to the levell q o, shall of necessity have ascended in the same time, from the levell l unto a b. and the ascent l b, shall be so much greater than the descent g q, by how much the breadth of the vessell g d, is greater than that of the funnell i c; which, in summe, is as much as the water g d, is more than the water l c: but in regard that the moment of the velocity of the motion, in one moveable, compensates that of the gravity of another what wonder is it, if the swift ascent of the lesser water c l, shall resist the slow descent of the greater g d?_ the same, therefore, happens in this operation, as in rhe stilliard, in which a weight of two pounds counterpoyseth an other of , asoften as that shall move in the same time, a space times greater than this: which falleth out when one arme of the beam is an hundred times as long as the other. let the erroneous opinion of those therefore cease, who hold that a ship is better, and easier born up in a great abundance of water, then in a lesser quantity[ ], (_this was believed by_ aristotle _in his problems, sect. , probl. ._) it being on the contrary true, that its possible, that a ship may as well float in ten tun of water, as in an ocean. [ ] a ship flotes as well in ten tun of water as in an ocean. [sidenote: a solid specifiaclly graver than the water, cannot be born up by any quantity of it.] but following our matter, i say, that by what hath been hitherto demonstrated, we may understand how, that corollary iii. _one of the above named solids, when more grave_ in specie _than the water, can never be sustained, by any whatever quantity of it._ for having seen how that the moment wherewith such a solid, as grave _in specie_ as the water, contrasts with the moment of any mass of water whatsoever, is able to retain it, even to its totall submersion, without its ever ascending; it remaineth, manifest, that the water is far less able to raise it up, when it exceeds the same _in specie_: so, that though you infuse water till its totall submersion, it shall still stay at the bottome, and with such gravity, and resistance to elevation, as is the excess of its absolute gravity, above the absolute gravity of a mass equall to it, made of water, or of a matter _in specie_ equally grave with the water: and, though you should moreover adde never so much water above the levell of that which equalizeth the altitude of the solid, it shall not, for all that, encrease the pression, or gravitation, of the parts circumfused about the said solid, by which greater pression, it might come to be repulsed; because, the resistance is not made, but only by those parts of the water, which at the motion of the said solid do also move, and these are those only, which are comprehended by the two superficies equidistant to the horizon, and their parallels, that comprehend the altitude of the solid immerged in the water. i conceive, i have by this time sufficiently declared and opened the way to the contemplation of the true, intrinsecall and proper causes of diverse motions, and of the rest of many solid bodies in diverse _mediums_, and particularly in the water, shewing how all in effect, depend on the mutuall excesses of the gravity of the moveables and of the _mediums_: and, that which did highly import, removing the objection, which peradventure would have begotten much doubting, and scruple in some, about the verity of my conclusion, namely, how that notwithstanding, that the excess of the gravity of the water, above the gravity of the solid, demitted into it, be the cause of its floating and rising from the bottom to the surface, yet a quantity of water, that weighs not ten pounds, can raise a solid that weighs above pounds: in that we have demonstrated, that it sufficeth, that such difference be found between the specificall gravities of the _mediums_ and moveables, let the particular and absolute gravities be what they will: insomuch, that a solid, provided that it be specifically less grave than the water, although its absolute weight were pounds, yet may it be born up and elevated by ten pounds of water, and less: and on the contrary, another solid, so that it be specifically more grave than the water, though in absolute gravity it were not above a pound, yet all the water in the sea, cannot raise it from the bottom, or float it. this sufficeth me, for my present occasion, to have, by the above declared examples, discovered and demonstrated, without extending such matters farther, and, as i might have done, into a long treatise: yea, but that there was a necessity of resolving the above proposed doubt, i should have contented my self with that only, which is demonstrated by _archimedes_, in his first _book de insidentibus humido_[ ]: where in generall termes he infers and confirms the same conclusions, namely, that solids (_a_) less grave than water, swim or float upon it, the (_b_) more grave go to the bottom, and the (_c_) equally grave rest indifferently in all places, yea, though they should be wholly under water. [ ] _of natation_ (a) _lib. , prop. ._ (b) _id. lib. . prop. ._ (c) _id. lib. . prop. ._ [sidenote: the authors defence of _archimedes_ his doctrine, against the oppositions of _buonamico_.] but, because that this doctrine of archimedes, perused, transcribed and examined by _signor francesco buonamico_, in his _fifth book of motion, chap. _, and afterwards by him confuted, might by the authority of so renowned, and famous a philosopher, be rendered dubious, and suspected of falsity; i have judged it necessary to defend it, if i am able so to do, and to clear _archimedes_, from those censures, with which he appeareth to be charged. _buonamico_ rejecteth the doctrine of _archimedes_, first[ ], as not consentaneous with the opinion of _aristotle_, adding, that it was a strange thing to him, that the water should exceed the earth in gravity[ ], seeing on the contrary, that the gravity of water, increaseth, by means of the participation of earth. and he subjoyns presently after[ ], that he was not satisfied with the reasons of _archimedes_, as not being able with that doctrine, to assign the cause whence it comes, that a boat and a vessell, which otherwise, floats above the water, doth sink to the bottom, if once it be filled with water; that by reason of the equality of gravity, between the water within it, and the other water without, it should stay a top; but yet, nevertheless, we see it to go to the bottom. [ ] his first objection against the doctrine of _archimedes_. [ ] his second objection. [ ] his third objection. he farther addes[ ], that _aristotle_ had clearly confuted the ancients, who said, that light bodies moved upwards[ ], driven by the impulse of the more grave ambient: which if it were so, it should seem of necessity to follow, that all naturall bodies are by nature heavy, and none light: for that the same would befall the fire and air, if put in the bottom of the water. and, howbeit, _aristotle_ grants a pulsion in the elements, by which the earth is reduced into a sphericall figure, yet nevertheless, in his judgement; it is not such that it can remove grave bodies from their naturall places, but rather, that it send them toward the centre, to which (as he somewhat obscurely continues to say,) the water principally moves, if it in the interim meet not with something that resists it, and, by its gravity, thrusts it out of its place: in which case, if it cannot directly, yet at least as well as it can, it tends to the centre: but it happens, that light bodies by such impulsion, do all ascend upward: but this properly they have by nature, as also, that other of swimming. he concludes, lastly[ ], that he concurs with _archimedes_ in his conclusions; but not in the causes, which he would referre to the facile and difficult separation of the _medium_, and to the predominance of the elements, so that when the moveable superates the power of the _medium_; as for example, lead doth the continuity of water, it shall move thorow it, else not. [ ] his fourth objection. [ ] the _a_ncients denyed _a_bsolute levity. [ ] the causes of natation & submersion, according to the peripateticks. this is all that i have been able to collect, as produced against _archimedes_ by _signor buonamico_: who hath not well observed the principles and suppositions of _archimedes_; which yet must be false, if the doctrine be false, which depends upon them; but is contented to alledge therein some inconveniences, and some repugnances to the doctrine and opinion of _aristotle_. in answer to which objections, i say, first[ ], that the being of _archimedes_ doctrine, simply different from the doctrine of _aristotle_, ought not to move any to suspect it, there being no cause, why the authority of this should be preferred to the authority of the other: but, because, where the decrees of nature are indifferently exposed to the intellectuall eyes of each, the authority of the one and the other, loseth all a{u}thenticalness of perswasion, the absolute power residing in reason; therefore i pass to that which he alledgeth in the second place[ ], as an absurd consequent of the doctrine of _archimedes_, namely, that water should be more grave than earth. but i really find not, that ever _archimedes_ said such a thing, or that it can be rationally deduced from his conclusions: and if that were manifest unto me, i verily believe, i should renounce his doctrine, as most erroneous. perhaps this deduction of _buonamico_, is founded upon that which he citeth of the vessel, which swims as long as its voyd of water, but once full it sinks to the bottom, and understanding it of a vessel of earth, he infers against _archimedes_ thus: thou sayst that the solids which swim, are less grave than water: this vessell swimmeth: therefore, this vessell is lesse grave than water. if this be the illation. i easily answer, granting that this vessell is lesse grave than water, and denying the other consequence, namely, that earth is less grave than water. the vessel that swims occupieth in the water, not only a place equall to the mass of the earth, of which it is formed; but equall to the earth and to the air together, contained in its concavity. and, if such a mass compounded of earth and air, shall be less grave than such another quantity of water, it shall swim, and shall accord with the doctrine of _archimedes_; but if, again, removing the air, the vessell shall be filled with water, so that the solid put in the water, be nothing but earth, nor occupieth other place, than that which is only possest by earth, it shall then go to the bottom, by reason that the earth is heavier than the water: and this corresponds well with the meaning of _archimedes_. see the same effect illustrated, with such another experiment, in pressing a viall glass to the bottom of the water, when it is full of air, it will meet with great resistance, because it is not the glass alone, that is pressed under water, but together with the glass a great mass of air, and such, that if you should take as much water, as the mass of the glass, and of the air contained in it, you would have a weight much greater than that of the viall, and of its air: and, therefore, it will not submerge without great violence: but if we demit only the glass into the water, which shall be when you shall fill the glass with water, then shall the glass descend to the bottom; as superiour in gravity to the water. [ ] the authors answer to the first objection. [ ] the authors answer to the second objection. returning, therefore, to our first purpose; i say, that earth is more grave than water, and that therefore, a solid of earth goeth to the bottom of it; but one may possibly make a composition of earth and air, which shall be less grave than a like mass of water; and this shall swim: and yet both this and the other experiment shall very well accord with the doctrine of _archimedes_. but because that in my judgment it hath nothing of difficulty in it, i will not positively affirme that _signor buonamico_, would by such a discourse object unto _archimedes_ the absurdity of inferring by his doctrine, that earth was less grave than water, though i know not how to conceive what other accident he could have induced thence. perhaps such a probleme (in my judgement false) was read by _signor buonamico_ in some other author, by whom peradventure it was attributed as a singular propertie, of some particular water, and so comes now to be used with a double errour in confutation of _archimedes_, since he saith no such thing, nor by him that did say it was it meant of the common element of water. [sidenote: the authors answer to the third objection.] the third difficulty in the doctrine of _archimedes_ was, that he could not render a reason whence it arose, that a piece of wood, and a vessell of wood, which otherwise floats, goeth to the bottom, if filled with water. _signor buonamico_ hath supposed that a vessell of wood, and of wood that by nature swims, as before is said, goes to the bottom, if it be filled with water; of which he in the following chapter, which is the of the fifth book copiously discourseth: but i (speaking alwayes without diminution of his singular learning) dare in defence of _archimedes_ deny this experiment, being certain that a piece of wood which by its nature sinks not in water, shall not sinke though it be turned and converted into the forme of any vessell whatsoever, and then filled with water: and he that would readily see the experiment in some other tractable matter, and that is easily reduced into several figures, may take pure wax, and making it first into a ball or other solid figure, let him adde to it so much lead as shall just carry it to the bottome, so that being a graine less it could not be able to sinke it, and making it afterwards into the forme of a dish, and filling it with water, he shall finde that without the said lead it shall not sinke, and that with the lead it shall descend with much slowness: & in short he shall satisfie himself, that the water included makes no alteration. i say not all this while, but that its possible of wood to make barkes, which being filled with water, sinke; but that proceeds not through its gravity, encreased by the water, but rather from the nailes and other iron workes, so that it no longer hath a body less grave than water, but one mixt of iron and wood, more grave than a like masse of water. therefore let _signor buonamico_ desist from desiring a reason of an effect, that is not in nature: yea if the sinking of the woodden vessell when its full of water, may call in question the doctrine of _archimedes_, which he would not have you to follow, is on the contrary consonant and agreeable to the doctrine of the peripateticks, since it aptly assignes a reason why such a vessell must, when its full of water, descend to the bottom; converting the argument the other way, we may with safety say that the doctrine of _archimedes_ is true, since it aptly agreeth with true experiments, and question the other, whose deductions are fastened upon erroneouss conclusions. as for the other point hinted in this same instance, where it seemes that _benonamico_ understands the same not only of a piece of wood, shaped in the forme of a vessell, but also of massie wood, which filled, _scilicet_, as i believe, he would say, soaked and steeped in water, goes finally to the bottom that happens in some porose woods, which, while their porosity is replenished with air, or other matter less grave than water, are masses specificially less grave than the said water, like as is that viall of glass whilest it is full of air: but when, such light matter departing, there succeedeth water into the same porosities and cavities, there results a compound of water and glass more grave than a like mass of water: but the excess of its gravity consists in the matter of the glass, and not in the water, which cannot be graver than it self: so that which remaines of the wood, the air of its cavities departing, if it shall be more grave _in specie_ than water, fil but its porosities with water, and you shall have a compost of water and of wood more grave than water, but not by vertue of the water received into and imbibed by the porosities, but of that matter of the wood which remains when the air is departed: and being such it shall, according to the doctrine of _archimedes_, goe to the bottom, like as before, according to the same doctrine it did swim. [sidenote: the authors answer to the fourth objection.] as to that finally which presents itself in the fourth place, namely, that the _ancients_ have been heretofore confuted by _aristotle_, who denying positive and absolute levity, and truely esteeming all bodies to be grave, said, that that which moved upward was driven by the circumambient air, and therefore that also the doctrine of _archimedes_, as an adherent to such an opinion was convicted and confuted: i answer first, that _signor buonamico_ in my judgement hath imposed upon _archimedes_, and deduced from his words more than ever he intended by them, or may from his propositions be collected, in regard that _archimedes_ neither denies, nor admitteth positive levity, nor doth he so much as mention it: so that much less ought _buonamico_ to inferre, that he hath denyed that it might be the cause and principle of the ascension of fire, and other light bodies[ ]: having but only demonstrated, that solid bodies more grave than water descend in it, according to the excess of their gravity above the gravity of that, he demonstrates likewise, how the less grave ascend in the same water[ ], accordng to its excess of gravity, above the gravity of them. so that the most that can be gathered from the demonstration of _archimedes_ is, that like as the excess of the gravity of the moveable above the gravity of the water, is the cause that it descends therein, so the excess of the gravity of the water above that of the moveable, is a sufficient cause why it descends not, but rather betakes it self to swim: not enquiring whether of moving upwards there is, or is not any other cause contrary to gravity: nor doth _archimedes_ discourse less properly than if one should say: if the south winde shall assault the barke with greater _impetus_ than is the violence with which the streame of the river carries it towards the south, the motion of it shall be towards the north: but if the _impetus_ of the water shall overcome that of the winde, its motion shall be towards the south. the discourse is excellent and would be unworthily contradicted by such as should oppose it, saying: thou mis-alledgest as cause of the motion of the bark towards the south, the _impetus_ of the stream of the water above that of the south winde; mis-alledgest i say, for it is the force of the north winde opposite to the south, that is able to drive the bark towards the south. such an objection would be superfluous, because he which alledgeth for cause of the motion the stream of the water, denies not but that the winde opposite to the south may do the same, but only affirmeth that the force of the water prevailing over the south wind, the bark shall move towards the south: and saith no more than is true. and just thus when _archimedes_ saith, that the gravity of the water prevailing over that by which the moveable descends to the bottom, such moveable shall be raised from the bottom to the surface alledgeth a very true cause of such an accident, nor doth he affirm or deny that there is, or is not, a vertue contrary to gravity, called by some levity, that hath also a power of moving some matters upwards. let therefore the weapons of _signor buonamico_ be directed against _plato_[ ], and other _ancients_, who totally denying _levity_, and taking all bodies to be grave, say that the motion upwards is made, not from an intrinsecal principle of the moveable, but only by the impulse of the _medium_; and let _archimedes_ and his doctrine escape him, since he hath given him no cause of quarelling with him. but if this apologie, produced in defence of _archimedes_, should seem to some insufficient to free him from the objections and arguments, produced by _aristotle_ against _plato_, and the other _ancients_, as if they did also fight against _archimedes_, alledging the impulse of the water as the cause of the swimming of some bodies less grave than it[ ], i would not question, but that i should be able to maintaine the doctrine of _plato_ and those others to be most true, who absolutely deny levity, and affirm no other intrinsecal principle of motion to be in elementary bodies save only that towards the centre of the earth[ ], nor no other cause of moving upwards, speaking of that which hath the resemblance of natural motion, but only the repulse of the _medium_, fluid, and exceeding the gravity of the moveable[ ]: and as to the reasons of _aristotle_ on the contrary, i believe that i could be able fully to answer them, and i would assay to do it, if it were absolutely necessary to the present matter, or were it not too long a digression for this short treatise. i will only say, that if there were in some of our ellementary bodies an intrinsecall principle and naturall inclination to shun the centre of the earth, and to move towards the concave of the moon, such bodies, without doubt, would more swiftly ascend through those _mediums_ that least oppose the velocity of the moveable, and these are the more tenuous and subtle; as is, for example, the air in comparison of the water, we daily proving that we can with farre more expeditious velocity move a hand or a board to and again in one than in the other[ ]: nevertheless, we never could finde any body, that did not ascend much more swiftly in the water than in the air. yea of bodies which we see continually to ascend in the water, there is none that having arrived to the confines of the air, do not wholly lose their motion[ ]; even the air it self, which rising with great celerity through the water, being once come to its region it loseth all [ ] of natation, lib. . prop. . [ ] of natation, lib. . prop. . [ ] _plato_ denyeth positive levity. [ ] the authors defence of the doctrine of _plato_ and the _ancients_, who absolutely deny levity: [ ] according to _plato_ there is no principle of the motion, of descent in naturall bodies, save that to the centre. [ ] no cause of the motion of ascent, save the impulse of the _medium_, exceeding the moveable in gravitie. [ ] bodies ascend much swifter in the water, than in the air. [ ] all bodies ascending through water, lose their motion, comming to the confines of the air. [sidenote: the lighter bodies ascend more swiftly through water.] and, howbeit, experience shewes, that the bodies, successively less grave, do most expeditiously ascend in water, it cannot be doubted, but that the ignean exhalations do ascend more swiftly through the water, than doth the air: which air is seen by experience to ascend more swiftly through the water, than the fiery exhalations through the air[ ]: therefore, we must of necessity conclude, that the said exhalations do much more expeditiously ascend through the water, than through the air; and that, consequently, they are moved by the impulse of the ambient _medium_, and not by an intrinsick principle that is in them, of avoiding the centre of the earth; to which other grave bodies tend. [ ] fiery exhalations ascend thorow the water more swiftly than doth the air; & the air ascends more swiftly thorow the water, than fire thorow the air. [sidenote: _t_he authors confutation of the peripateticks causes of natation & submersion.] to that which for a finall conclusion, _signor buonamico_ produceth of going about to reduce the descending or not descending, to the easie and uneasie division of the _medium_, and to the predominancy of the elements: i answer, as to the first part, that that cannot in any manner be admitted as a cause, being that in none of the fluid _mediums_, as the air, the water, and other liquids, there is any resistance against division[ ], but all by every the least force, are divided and penetrated, as i will anon demonstrate: so, that of such resistance of division there can be no act, since it self is not in being. as to the other part, i say, that the predominancy of the elements in moveables[ ], is to be considered, as far as to the excesse or defect of gravity, in relation to the _medium_: for in that action, the elements operate not, but only, so far as they are grave or light: therefore, to say that the wood of the firre sinks not, because air predominateth in it, is no more than to say, because it is less grave than the water. yea, even the immediate cause, is its being less grave than the water[ ]: and it being under the predominancy of the air, is the cause of its less gravity: therefore, he that alledgeth the predominancy of the element for a cause, brings the cause of the cause, and not the neerest and immediate cause. now, who knows not that the true cause is the immediate, and not the mediate[ ]? moreover, he that alledgeth gravity, brings a cause most perspicuous to sence[ ]: the cause we may very easily assertain our selves; whether ebony, for example, and firre, be more or less grave than water: but whether earth or air predominates in them, who shall make that manifest? certainly, no experiment can better do it than to observe whether they swim or sink. so, that he who knows, not whether such a solid swims, unless when he knows that air predominates in it, knows not whether it swim, unless he sees it swim, for then he knows that it swims, when he knows that it is air that predominates, but knows not that air hath the predominance, unless he sees it swim: therefore, he knows not if it swims, till such time as he hath seen it swim. [ ] water & other fluids void of resistance against division. [ ] _t_he predominancy of elements in moveables to be considered only in relation to their excess or defect of gravity in reference to the _medium_. [ ] _t_he immediate cause of natation is that the moveable is less grave than the water. [ ] _t_he peripateticks alledge for the reason of natation the cause of the cause. [ ] gravity a cause most perspicuous to sence. let us not then despise those hints, though very dark, which reason, after some contemplation, offereth to our intelligence, and lets be content to be taught by _archimedes_, that then any body shall submerge in water[ ], when it shall be specifically more grave than it, and that if it shall be less grave[ ], it shall of necessity swim, and that it will rest indifferently in any place under water, if its gravity be perfectly like to that of the water. [ ] lib . of natation prop. [ ] id. lib. . prop. . these things explained and proved[ ], i come to consider that which offers it self, touching what the diversity of figure given unto the said moveable hath to do with these motions and rests; and proceed to affirme, that, [ ] id. lib . prop. . theoreme v. [sidenote: diversity of figure no cause of its absolute natation or submersion.] _the diversity of figures given to this or that solid, cannot any way be a cause of its absolute sinking or swimming._ so that if a solid being formed, for example, into a sphericall figure, doth sink or swim in the water, i say, that being formed into any other figure, the same figure in the same water, shall sink or swim: nor can such its motion by the expansion or by other mutation of figure, be impeded or taken away. [sidenote: the expansion of figure, retards the velocity of the ascent or descent of the moveable in the water; but doth not deprive it of all motion.] the expansion of the figure may indeed retard its velocity, aswell of ascent as descent, and more and more according as the said figure is reduced to a greater breadth and thinness: but that it may be reduced to such a form as that that same matter be wholly hindred from moving in the same water, that i hold to be impossible. in this i have met with great contradictors, who producing some experiments, and in perticular a thin board of ebony, and a ball of the same wood, and shewing how the ball in water descended to the bottom, and the board being put lightly upon the water submerged not, but rested; have held, and with the authority of _aristotle_, confirmed themselves in their opinions, that the cause of that rest was the breadth of the figure, u{n}able by its small weight to pierce and penetrate the resistance of the waters crassitude, which resistance is readily overcome by the other sphericall figure. this is the principal point in the present question, in which i perswade my self to be on the right side. therefore, beginning to investigate with the examination of exquisite experiments that really the figure doth not a jot alter the descent or ascent of the same solids, and having already demonstrated that the greater or less gravity of the solid in relation to the gravity of the _medium_ is the cause of descent or ascent: when ever we would make proof of that, which about this effect the diversity of figure worketh, its necessary to make the experiment with matter wherein variety of gravities hath no place. for making use of matters which may be different in their specifical gravities, and meeting with varieties of effects of ascending and descending, we shall alwayes be left unsatisfied whether that diversity derive it self really from the sole figure, or else from the divers gravity also. we may remedy this by takeing one only matter, that is tractable and easily reduceable into every sort of figure. moreover, it will be an excellent expedient to take a kinde of matter, exactly alike in gravity unto the water: for that matter, as far as pertaines to the gravity, is indifferent either to ascend or descend; so that we may presently observe any the least difference that derives it self from the diversity of figure. [sidenote: an experiment in wax, that proveth figure to have no operation in natation & submersion.] now to do this, wax is most apt, which, besides its incapacity of receiveing any sensible alteration from its imbibing of water, is ductile or pliant, and the same piece is easily reduceable into all figures: and being _in specie_ a very inconsiderable matter inferiour in gravity to the water, by mixing therewith a little of the fileings of lead it is reduced to a gravity exactly equall to that of the water. this matter prepared, and, for example, a ball being made thereof as bigge as an orange or biger, and that made so grave as to sink to the bottom, but so lightly, that takeing thence one only grain of lead, it returns to the top, and being added, it submergeth to the bottom, let the same wax afterwards be made into a very broad and thin flake or cake; and then, returning to make the same experiment, you shall see that it being put to the bottom, it shall, with the grain of lead rest below, and that grain deducted, it shall ascend to the very surface, and added again it shall dive to the bottom. and this same effect shall happen alwaies in all sort of figures, as wel regular as irregular: nor shall you ever finde any that will swim without the removall of the grain of lead, or sinke to the bottom unless it be added: and, in short, about the going or not going to the bottom, you shall discover no diversity, although, indeed, you shall about the quick and slow descent: for the more expatiated and distended figures move more slowly aswel in the diveing to the bottom as in the rising to the top; and the other more contracted and compact figures, more speedily. now i know not what may be expected from the diversity of figures, if the most contrary to one another operate not so much as doth a very small grain of lead, added or removed. me thinkes i hear some of the adversaries to raise a doubt upon my produced experiment[ ]. and first that they offer to my consideration, that the figure, as a figure simply, and disjunct from the matter workes not any effect, but requires to be conjoyned with the matter; and, furthermore, not with every matter, but with those only, wherewith it may be able to execute the desired operation. like as we see it verified by experience, that the acute and sharp angle is more apt to cut, than the obtuse; yet alwaies provided, that both the one and the other, be joyned with a matter apt to cut, as for example, with steel. therefore, a knife with a fine and sharp edge, cuts bread or wood with much ease, which it will not do, if the edge be blunt and thick: but he that will instead of steel, take wax, and mould it into a knife, undoubtedly shall never know the effects of sharp and blunt edges: because neither of them will cut, the wax being unable by reason of its flexibility, to overcome the hardness of the wood and bread. and, therefore, applying the like discourse to our purpose, they say, that the difference of figure will shew different effects, touching natation and submersion, but not conjoyned with any kind of matter, but only with those matters which, by their gravity, are apt to resist the velocity of the water, whence he that would elect for the matter, cork or other light wood, unable, through its levity, to superate the crassitude of the water, and of that matter should forme solids of divers figures, would in vain seek to find out what operation figure hath in natation or submersion; because all would swim, and that not through any property of this or that figure, but through the debility of the matter, wanting so much gravity, as is requisite to superate and overcome the density and crassitude of the water. [ ] an objection against the experiment in water. its needfull, therefore, if wee would see the effect wrought by the diversity of figure, first to make choice of a matter of its nature apt to penetrate the crassitude of the water. and, for this effect[ ], they have made choice of such a matter, as fit, that being readily reduced into sphericall figure, goes to the bottom; and it is ebony, of which they afterwards making a small board or splinter, as thin as a lath, have illustrated how that this, put upon the surface of the water, rests there without descending to the bottom: and making, on the otherside, of the same wood a ball, no less than a hazell nut, they shew, that this swims not, but descendes. from which experiment, they think they may frankly conclude, that the breadth of the figure in the flat lath or board, is the cause of its not descending to the bottom, for as much as a ball of the same matter, not different from the board in any thing but in figure, submergeth in the same water to the bottom. the discourse and the experiment hath really so much of probability and likelyhood of truth in it, that it would be no wonder, if many perswaded by a certain cursory observation, should yield credit to it; nevertheless, i think i am able to discover, how that it is not free from falacy. [ ] an experiment in ebany, brought to disprove the experiment in wax. beginning, therefore, to examine one by one, all the particulars that have been produced, i say, that figures, as simple figures, not only operate not in naturall things, but neither are they ever seperated from the corporeall substance[ ]: nor have i ever alledged them stript of sensible matter, like as also i freely admit, that in our endeavouring to examine the diversity of accidents, dependant upon the variety of figures, it is necessary to apply them to matters, which obstruct not the various operations of those various figures: and i admit and grant, that i should do very ill; if i would experiment the influence of acutenesse of edge with a knife of wax, applying it to cut an oak, because there is no acuteness in wax able to cut that very hard wood. but yet such an experiment of this knife, would not be besides the purpose, to cut curded milk, or other very yielding matter: yea, in such like matters, the wax is more commodious than steel; for finding the diversity depending upon angles, more or less acute, for that milk is indifferently cut with a raisor, and with a knife, that hath a blunt edge. it needs, therefore, that regard be had, not only to the hardness, solidity or gravity of bodies, which under divers figures, are to divide and penetrate some matters, but it forceth also, that regard be had, on the other side, to the resistance of the matters, to be divided and penetrated. but since i have in making the experiment concerning our contest; chosen a matter which penetrates the resistance of the water; and in all figures descendes to the bottome, the adversaries can charge me with no defect; yea, i have propounded so much a more excellent method than they, in as much as i have removed all other causes, of descending or not descending to the bottom, and retained the only sole and pure variety of figures, demonstrating that the same figures all descende with the only alteration of a grain in weight: which grain being removed, they return to float and swim; it is not true, therefore, (resuming the example by them introduced) that i have gon{e} about to experiment the efficacy of acuteness, in cutting with matters unable to cut, but with matters proportioned to our occasion, since they are subjected to no other variety, then that alone which depends on the figure more or less acute. [ ] figure is unseperable from corporeall substance. [sidenote: the answer to the objection against the experiment of the wax.] but let us proceed a little farther, and observe, how that indeed the consideration, which, they say, ought to be had about the election of the matter, to the end, that it may be proportionate for the making of our experiment, is needlessly introduced, declaring by the example of cutting, that like as acuteness is inefficient to cut, unless when it is in a matter hard and apt to superate the resistance of the wood or other matter, which we intend to cut; so the aptitude of descending or not descending in water, ought and can only be known in those matters, that are able to overcome the renitence, and superate the crassitude of the water. unto which, i say, that to make distinction and election, more of this than of that matter, on which to impress the figures for cutting or penetrating this or that body, as the solidity or obdurateness of the said bodies shall be greater or less, is very necessary: but withall i subjoyn, that such distinction, election and caution would be superfluous and unprofitable, if the body to be cut or penetrated, should have no resistance, or should not at all withstand the cutting or penitration: and if the knife were to be used in cutting a mist or smoak, one of paper would be equally serviceable with one of _damascus_ steel: and so by reason the water hath not any resistance against the penitration of any solid body, all choice of matter is superfluous and needless, and the election which i said above to have been well made of a matter reciprocall in gravity to water, was not because it was necessary, for the overcoming of the crassitude of the water, but its gravity, with which only it resists the sinking of solid bodies: and for what concerneth the resistance of the crassitude, if we narrowly consider it, we shall find that all solid bodies, as well those that sink, as those that swim, are indifferently accomodated and apt to bring us to the knowledge of the truth in question. nor will i be frighted out of the belief of these conclusions, by the experiments which may be produced against me, of many severall woods, corks, galls, and, moreover, of subtle slates and plates of all sorts of stone and mettall, apt by means of their naturall gravity, to move towards the centre of the earth, the which, nevertheless, being impotent, either through the figure (as the adversaries thinke) or through levity, to break and penetrate the continuity of the parts of the water, and to distract its union, do continue to swimm without submerging in the least: nor on the other side, shall the authority of _aristotle_ move me, who in more than one place, affirmeth the contrary to this, which experience shews me. [sidenote: no solid of such levity, nor of such figure, but that it doth penetrate the crassitude of the water.] [sidenote: bodies of all figures, laid upon the water, do penetrate its crassitude, and in what proportion.] i return, therefore, to assert, that there is not any solid of such levity, nor of such figure, that being put upon the water, doth not divide and penetrate its crassitude: yea if any with a more perspicatious eye, shall return to observe more exactly the thin boards of wood, he shall see them to be with part of their thickness under water, and not only with their inferiour superficies, to kisse the superiour of the water, as they of necessity must have believed, who have said, that such boards submerge not, as not being able to divide the tenacity of the parts of the water: and, moreover, he shall see, that subtle shivers of ebony, stone or metall, when they float, have not only broak the continuity of the water, but are with all their thickness, under the surface of it; and more and more, according as the matters are more grave: so that a thin plate of lead, shall be lower than the surface of the circumfused water, by at least twelve times the thickness of the plate, and gold shall dive below the levell of the water, almost twenty times the thickness of the plate, as i shall anon declare. but let us proceed to evince, that the water yields and suffers it self to be penetrated by every the lightest body; and therewithall demonstrate, how, even by matters that submerge not, we may come to know that figure operates nothing about the going or not going to the bottom, seeing that the water suffers it self to be penetrated equally by every figure. [sidenote: the experiment of a cone, demitted with its base, and after with its point downwards.] make a cone, or a piramis of cypress, of firre, or of other wood of like gravity, or of pure wax, and let its height be somewhat great, namely a handfull, or more, and put it into the water with the base downwards: first, you shall see that it will penetrate the water, nor shall it be at all impeded by the largeness of the base, nor yet shall it sink all under water, but the part towards the point shall lye above it: by which shall be manifest, first, that that solid forbeares not to sink out of an inability to divide the continuity of the water, having already divided it with its broad part, that in the opinion of the adversaries is the less apt to make the division. the piramid being thus fixed, note what part of it shall be submerged, and revert it afterwards with the point downwards, and you shall see that it shall not dive into the water more than before, but if you observe how far it shall sink, every person expert in geometry, may measure, that those parts that remain out of the water, both in the one and in the other experiment are equall to an hair: whence he may manifestly conclude, that the acute figure which seemed most apt to part and penetrate the water, doth not part or penetrate it more than the large and spacious. and he that would have a more easie experiment, let him take two cylinders of the same matter, one long and small, and the other short, but very broad, and let him put them in the water, not distended, but erect and endways: he shall see, if he diligently measure the parts of the one and of the other, that in each of them the part submerged, retains exactly the same proportion to that out of the water, and that no greater part is submerged of that long and small one, than of the other more spacious and broad: howbeit, this rests upon a very large, and that upon a very little superficies of water: therefore the diversity of figure, occasioneth neither facility, nor difficulty, in parting and penetrating the continuity of the water, and, consequently, cannot be the cause of the natation or submersion. he may likewise discover the non operating of variety of figures, in arising from the bottom of the water, towards the surface, by taking wax, and tempering it with a competent quantity of the filings of lead, so that it may become a considerable matter graver than the water: then let him make it into a ball, and thrust it unto the bottom of the water; and fasten to it as much cork, or other light matter, as just serveth to raise it, and draw it towards the surface: for afterwards changing the same wax into a thin cake, or into any other figure, that same cork shall raise it in the same manner to a hair. this silenceth not my antagonists, but they say, that all the discourse hitherto made by me little importeth to them, and that it serves their turn, that they have demonstrated in one only particular, and in what matter, and under what figure pleaseth them, namely, in a board and in a ball of ebony, that this put in the water, descends to the bottom, and that stays atop to swim: and the matter being the same, and the two bodies differing in nothing but in figure, they affirm, that they have with all perspicuity demonstrated and sensibly manifested what they undertook; and lastly, that they have obtained their intent. nevertheless, i believe, and thinke, i can demonstrate, that that same experiment proveth nothing against my conclusion. [sidenote: in experiments of natation, the solid is to be put into, not upon the water.] and first, it is false, that the ball descends, and the board not: for the board shall also descend, if you do to both the figures, as the words of our question requireth; that is, if you put them both into the water. [sidenote: the question of natation stated.] _the words were these. that the antagonists having an opinion, that the figure would alter the solid bodies, in relation to the descending or not descending, ascending or not ascending in the same_ medium, _as_ v. gr. _in the same water, in such sort, that, for example, a solid that being of a sphericall figure, shall descend to the bottom, being reduced into some other figure, shall not descend: i holding the contrary, do affirm, that a corporeall solid body, which reduced into a sphericall figure, or any other, shall go to the bottom, shall do the like under whatsoever other figure, {&}c._ [sidenote: place defined according to aristotle.] but to be in the water, implies to be placed in the water, and by _aristotles_ own definition of place, to be placed, importeth to be invironed by the superficies of the ambient body, therefore, then shall the two figures be in the water, when the superficies of the water, shall imbrace and inviron them: but when the adversaries shew the board of ebony not descending to the bottom, they put it not into the water, but upon the water, where being by a certain impediment (as by and by we will shew) retained, it is invironed, part by water, and part by air, which thing is contrary to our agreement, that was, that the bodies should be in the water, and not part in water, and part in air. _the which is again made manifest, by the questions being put as well about the things which go to the bottom, as those which arise from the bottom to swimme, and who sees not that things placed in the bottom, must have water about them._ [sidenote: the confutation of the experiment in the ebany.] it is now to be noted, that the board of ebany and the ball, put into the water, both sink, but the ball more swiftly, and the board more slowly; and slower and slower, according as it shall be more broad and thin, and of this tardity the breadth of the figure is the true cause: but these broad boards that slowly descend, are the same, that being put lightly upon the water, do swimm: therefore, if that were true which the adversaries affirm, the same numerical figure, would in the same numericall water, cause one while rest, and another while tardity of motion, which is impossible: for every perticular figure which descends to the bottom[ ], hath of necessity its own determinate tardity and slowness, proper and naturall unto it, according to which it moveth, so that every other tardity, greater or lesser is improper to its nature: if, therefore, a board, as suppose of a foot square, descendeth naturally with six degrees of tardity, it is impossible, that it should descend with ten or twenty, unless some new impediment do arrest it. much less can it, by reason of the same figure rest, and wholly cease to move; but it is necessary, that when ever it resteth, there do some greater impediment intervene than the breadth of the figure. therefore, it must be somewhat else, and not the figure, that stayeth the board of ebany above water, of which figure the only effect is the retardment of the motion, according to which it descendeth more slowly than the ball. let it be confessed, therefore, rationally discoursing, that the true and sole cause of the ebanys going to the bottom, is the excess of its gravity above the gravity of the water: and the cause of the greater or less tardity, the breadth of this figure, or the contractedness of that: but of its rest, it can by no means be allowed, that the quallity of the figure, is the cause thereof: aswell, because, making the tardity greater, according as the figure more dilateth, there cannot be so immense a dilatation, to which there may not be found a correspondent immence tardity without redusing it to nullity of motion; as, because the figures produced by the antagonists for effecters of rest, are the self same that do also go to the bottom. [ ] every perticular figure hath its own peculiar tardity. i will not omit another reason, founded also upon experience, and if i deceive not my self, manifestly concluding, how that the introduction of the breadth or amplitude of figure, and the resistance of the water against penetration, have nothing to do in the effect of descending, or ascending, or resting in the water. [d]take a piece of wood or other matter, of which a ball ascends from the bottom of the water to the surface, more slowly than a ball of ebony of the same bignesse, so that it is manifest, that the ball of ebony more readily divideth the water in descending, than the other in ascending; as for example, let the wood be walnut-tree. then take a board of walnut-tree, like and equall to that of ebony of the antagonists, which swims; and if it be true, that this floats above water, by reason of the figure, unable through its breadth, to pierce the crassitude of the same, the other of wallnut-tree, without all question, being thrust unto the bottom, will stay there, as less apt, through the same impediment of figure, to divide the said resistance of the water. but if we shall find, and by experience see, that not only the thin board, but every other figure of the same wallnut-tree will return to float, as undoubtedly we shall, then i must desier my opposers to forbear to attribute the floating of the ebony, unto the figure of the board, in regard that the resistance of the water is the same, as well to the ascent, as to the descent, and the force of the wallnut-trees ascension, is lesse than the ebonys force in going to the bottom. [d] the figure & resistance of the medium against division, have nothing to do with the effect of natation or submersion, by an experiment in wallnut tree. [sidenote: an experiment in gold, to prove the non-operating of figure in natation and submersion.] nay, i will say more, that if we shall consider gold in comparison of water, we shall find, that it exceeds it in gravity almost twenty times, so that the force and impetus, wherewith a ball of gold goes to the bottom, is very great. on the contrary, there want not matters, as virgins wax, and some woods, which are not above a fiftieth part less grave than water, whereupon their ascension therein is very slow, and a thousand times weaker than the _impetus_ of the golds descent: yet notwithstanding, a plate of gold swims without descending to the bottom, and, on the contrary, we cannot make a cake of wax, or thin board of wood, which put in the bottom of the water, shall rest there without ascending. now if the figure can obstruct the penetration, and impede the descent of gold, that hath so great an _impetus_, how can it choose but suffice to resist the same penetration of the other matter in ascending, when as it hath scarce a thousandth part of the _impetus_ that the gold hath in descending? its therefore, necessary, that that which suspends the thin plate of gold, or board of ebony, upon the water, be some thing that is wanting to the other cakes and boards of matters less grave than the water; since that being put to the bottom, and left at liberty, they rise up to the surface, without any obstruction: but they want not for flatness and breadth of figure: therefore, the spaciousnesse of the figure, is not that which makes the gold and ebony to swim. and, because, that the excess of their gravity above the gravity of the water, is questionless the cause of the sinking of the flat piece of ebony, and the thin plate of gold, when they go to the bottom, therefore, of necessity, when they float, the cause of their staying above water, proceeds from levity, which in that case, by some accident, peradventure not hitherto observed, cometh to meet with the said board, rendering it no longer as it was before, whilst it did sink more ponderous than the water, but less. now, let us return to take the thin plate of gold, or of silver, or the thin board of ebony, and let us lay it lightly upon the water, so that it stay there without sinking, and diligently observe its effect. and first, see how false the assertion of _aristotle_, and our oponents is, to wit, that it stayeth above water, through its unability to pierce and penetrate the resistance of the waters crassitude: for it will manifestly appear, not only that the said plates have penetrated the water, but also that they are a considerable matter lower than the surface of the same, the which continueth eminent, and maketh as it were a rampert on all sides, round about the said plates, the profundity of which they stay swimming: and, according as the said plates shall be more grave than the water, two, four, ten or twenty times, it is necessary, that their superficies do stay below the universall surface of the water, so much more, than the thickness of those plates, as we shal more distinctly shew anon. in the mean space, for the more easie understanding of what i say, observe with me a little the present scheme: in which let us suppose the surface of the water to be distended, according to the lines f l d b, upon which if one shall put a board of matter specifically more grave than water, but so lightly that it submerge not, it shall not rest any thing above, but shall enter with its whole thickness into the water: and, moreover, shall sink also, as we see by the board a i, o i, whose breadth is wholly sunk into the water, the little ramperts of water l a and d o incompassing it, whose superficies is notably higher than the superficies of the board. see now whether it be true, that the said board goes not to the bottom, as being of figure unapt to penetrate the crassitude of the water. [illustration] [sidenote: why solids having penitrated the water, do not proceed to a totall submersion.] but, if it hath already penetrated, and overcome the continuity of the water, & is of its own nature more grave than the said water, why doth it not proceed in its sinking, but stop and suspend its self within that little dimple or cavitie, which with its ponderosity it hath made in the water? i answer; because that in submerging it self, so far as till its superficies come to the levell with that of the water, it loseth a part of its gravity, and loseth the rest of it as it submergeth & descends beneath the surface of the water, which maketh ramperts and banks round about it, and it sustaines this loss by means of its drawing after it, and carrying along with it, the air that is above it, and by contact adherent to it, which air succeeds to fill the cavity that is invironed by the ramperts of water; so that that which in this case descends and is placed in the water, is not only the board of ebony or plate of iron, but a composition of ebony and air, from which resulteth a solid no longer superiour in gravity to the water, as was the simple ebony, or the simple gold. and, if we exactly consider, what, and how great the solid is, that in this experiment enters into the water, and contrasts with the gravity of the same, it will be found to be all that which we find to be beneath the surface of the water, the which is an aggregate and compound of a board of ebony, and of almost the like quantity of air, or a mass compounded of a plate of lead, and ten or twelve times as much air. but, gentlemen, you that are my antagonists in our question, we require the identity of matter, and the alteration only of the figure; therefore, you must remove that air, which being conjoyned with the board, makes it become another body less grave than the water, and put only the ebony into the water, and you shall certainly see the board descend to the bottom; and, if that do not happen, you have got the day. and to seperate the air from the ebony[ ], there needs no more but only to bath the superficies of the said board with the same water: for the water being thus interposed between the board and the air, the other circumfused water shall run together without any impediment, and shall receive into it the sole and bare ebony, as it was to do. [ ] how to seperate the air from solids in demitting them into the water. but, me thinks i hear some of the adversaries cunningly opposing this, and telling me, that they will not yield, by any means, that their board be wetted, because the weight added thereto by the water, by making it heavier than it was before, draws it to the bottom, and that the addition of new weight is contrary to our agreement, which was, that the matter be the same. to this, i answer, first; that treating of the operation of figure in bodies put into the water, none can suppose them to be put into the water without being wet; nor do i desire more to be done to the board, then i will give you leave to do to the ball. moreover, it is untrue, that the board sinks by vertue of the new weight added to it by the water, in the single and slight bathing of it: for i will put ten or twenty drops of water upon the same board, whilst it is sustained upon the water; which drops, because not conjoyned with the other water circumfused, shall not so encrease the weight of it, as to make it sink: but if the board being taken out, and all the water wiped off that was added thereto, i should bath all its superficies with one only very small drop, and put it again upon the water, without doubt it shall sink, the other water running to cover it, not being retained by the superiour air; which air by the interposition of the thin vail of water, that takes away its contiguity unto the ebony, shall without renitence be seperated, nor doth it in the least oppose the succession of the other water: but rather, to speak better, it shall descend freely; because it shall be all invironed and covered with water, as soon as its superiour superficies, before vailed with water, doth arrive to the levell of the universall surface of the said water. to say, in the next place, that water can encrease the weight of things that are demitted into it, is most false; for water hath no gravity in water[ ], since it descends not: yea, if we would well consider what any immense mass of water doth put upon a grave body; that is placed in it, we shall find experimentally, that it, on the contrary, will rather in a great part deminish the weight of it[ ], and that we may be able to lift an huge stone from the bottom of the water, which the water being removed, we are not able to stir. nor let them tell me by way of reply, that although the superposed water augment not the gravity of things that are in it, yet it increaseth the ponderosity of those that swim, and are part in the water and part in the air, as is seen, for example, in a brass ketle[ ], which whilst it is empty of water, and replenished only with air shall swim, but pouring of water therein, it shall become so grave, that it shall sink to the bottom, and that by reason of the new weight added thereto. to this i will return answer, as above, that the gravity of the water, contained in the vessel is not that which sinks it to the bottom, but the proper gravity of the brass, superiour to the specificall gravity of the water: for if the vessel were less grave than water, the ocean would not suffice to submerge it[ ]. and, give me leave to repeat it again, as the fundamentall and principall point in this case, that the air contained in this vessel before the infusion of the water, was that which kept it a-float[ ], since that there was made of it, and of the brass, a composition less grave than an equall quantity of water: and the place that the vessel occupyeth in the water whilst it floats, is not equall to the brass alone, but to the brass and to the air together, which filleth that part of the vessel that is below the levell of the water: moreover, when the water is infused, the air is removed, and there is a composition made of brass and of water, more grave _in specie_ than the simple water, but not by vertue of the water infused, as having greater specifick gravity than the other water, but through the proper gravity of the brass, and through the alienation of the air. now, as he that should say that brass, that by its nature goes to the bottom, being formed into the figure of a ketle[ ], acquireth from that figure a vertue of lying in the water without sinking, would say that which is false; because that brass fashioned into any whatever figure, goeth always to the bottom, provided, that that which is put into the water be simple brass; and it is not the figure of the vessel that makes the brass to float, but it is because that that is not purely brass which is put into the water, but an aggregate of brass and of air: so is it neither more nor less false, that a thin plate of brass or of ebony, swims by vertue of its dilated & broad figure: for the truth is, that it bares up without submerging, because that that which is put in the water, is not pure brass or simple ebony, but an aggregate of brass and air, or of ebony and air. and, this is not contrary unto my conclusion, the which, (having many a time seen vessels of mettall, and thin pieces of diverse grave matters float, by vertue of the air conjoyned with them) did affirm, that figure was not the cause of the natation or submersion of such solids as were placed in the water. nay more, i cannot omit, but must tell my antagonists, that this new conceit of denying that the superficies of the board should be bathed, may beget in a third person an opinion of a poverty of arguments of defence on their part, since that such bathing was never insisted upon by them in the beginning of our dispute, and was not questioned in the least, being that the originall of the discourse arose upon the swiming of flakes of ice, wherein it would be simplicity to require that their superficies might bedry: besides, that whether these pieces of ice be wet or dry they alwayes swim, and as the adversaries say, by reason of the figure. [ ] water hath no gravity in water. [ ] water deminisheth the gravity of solids immerged therein. [ ] the experiment of a brass ketle swiming when empty, & sinking when full, alledged to prove that water gravitates in water, answered. [ ] an ocean sufficeth not to sink a vessel specifically less grave than water. [ ] air, the cause of the natation of empty vessels of matters graver _in specie_ than the water. [ ] neither figure, nor the breadth of figure, is the cause of natation. some peradventure, by way of defence, may say, that wetting the board of ebony, and that in the superiour superficies, it would, though of it self unable to pierce and penetrate the water, be born downwards, if not by the weight of the additionall water, at least by that desire and propension that the superiour parts of the water have to re-unite and rejoyn themselves: by the motion of which parts, the said board cometh in a certain manner, to be depressed downwards. [sidenote: the bathed solid descends not out of any affectation of union in the upper parts of the water.] this weak refuge will be removed, if we do but consider, that the repugnancy of the inferiour parts of the water, is as great against dis-union, as the inclination of its superiour parts is to union: nor can the uper unite themselves without depressing the board, nor can it descend without disuniting the parts of the nether water: so that it doth follow, by necessary consequence, that for those respects, it shall not descend. moreover, the same that may be said of the upper parts of the water, may with equall reason be said of the nether, namely, that desiring to unite, they shall force the said board upwards. happily, some of these gentlemen that dissent from me, will wonder, that i affirm, that the contiguous superiour air is able to sustain that plate of brass or of silver, that stayeth above water; as if i would in a certain sence allow the air[ ], a kind of magnetick vertue of sustaining the grave bodies, with which it is contiguous. to satisfie all i may, to all doubts, i have been considering how by some other sensible experiment i might demonstrate, how truly that little contiguous and superiour air sustaines those solids, which being by nature apt to descend to the bottom, being placed lightly on the water submerge not, unless they be first thorowly bathed; and have found, that one of these bodies having descended to the bottom, by conveighing to it (without touching it in the least) a little air, which conjoyneth with the top of the same, it becometh sufficient, not only, as before to sustain it, but also to raise it, and to carry it back to the top, where it stays and abideth in the same manner, till such time, as the assistance of the conjoyned air is taken away. and to this effect, i have taken a ball of wax, and made it with a little lead, so grave, that it leasurely descends to the bottom, making with all its superficies very smooth and pollite: and this being put gently into the water, almost wholly submergeth, there remaining vissible only a little of the very top[ ], the which so long as it is conjoyned with the air, shall retain the ball a-top, but the contiguity of the air taken away by wetting it, it shall descend to the bottom and there remain. now to make it by vertue of the air, that before sustained it to return again to the top, and stay there, thrust into the water a glass reversed with the mouth downwards, the which shall carry with it the air it contains, and move this towards the ball, abasing it till such time that you see, by the transparency of the glass, that the contained air do arrive to the summity of the _b_all[ ]: then gently withdraw the glass upwards, and you shall see the _b_all to rise, and afterwards stay on the top of the water[ ], if you carefully part the glass and the water without overmuch commoving and disturbing it. there is, therefore, a certain affinity between the air and other bodies, which holds them unied, so, that they seperate not without a kind of violence. the same likewise is seen in the water[ ]; for if we shall wholly submerge some body in it, so that it be thorowly bathed, in the drawing of it afterwards gently out again, we shall see the water follow it, and rise notably above its surface, before it seperates from it. solid bodies, also[ ], if they be equall and alike in superficies, so, that they make an exact contact without the interposition of the least air, that may part them in the seperation and yield untill that the ambient _medium_ succeeds to replenish the place, do hold very firmly conjoyned, and are not to be seperated without great force but, because, the air, water, and other liquids, very expeditiously shape themselves to contact with any solid _b_odies, so that their superficies do exquisitely adopt themselves to that of the solids, without any thing remaining between them, therefore, the effect of this conjunction and adherence is more manifestly and frequently observed in them, than in hard and inflexible bodies, whose superficies do very rarely conjoyn with exactness of contact[ ]. this is therefore that magnetick vertue, which with firm connection conjoyneth all bodies, that do touch without the interposition of flexible fluids; and, who knows, but that that a contact, when it is very exact, may be a sufficient cause of the union and continuity of the parts of a naturall _b_ody? [ ] _a_ magnetisme in the _a_ir, by which it bears up those solids in the water, that are contiguous with it. [ ] the effect of the airs contiguity in the natation of solids. [ ] the force of contact. [ ] _a_n affectation of conjunction betwixt solids and the air contiguous to them. [ ] the like affectation of conjunction betwixt solids & the water. [ ] also the like affectation and conjunction betwixt solids themselves. [ ] contact may be the cause of the continuity of naturall bodies. now, pursuing my purpose, i say; that it needs not, that we have recourse to the tenacity, that the parts of the water have amongst themselves, by which they resist and oppose division, distraction, and seperation, because there is no such coherence and resistance of division for if there were, it would be no less in the internall parts than in those nearer the superiour or externall surface, so that the same board, finding alwayes the same resistance and renitence, would no less stop in the middle of the water than about the surface, which is false. moreover, what resistance can we place in the continuity of the water, if we see that it is impossible to find any body of whatsoever matter, figure or magnitude, which being put into the water, shall be obstructed and impeded by the tenacity of the parts of the water to one another, so, but that it is moved upwards or downwards, according as the cause of their motion transports it? and, what greater proof of it can we desier, than that which we daily see in muddy waters, which being put into vessels to be drunk, and being, after some hours setling[ ], still, as we say, thick in the end, after four or six dayes they are wholly setled, and become pure and clear? nor can their resistance of penetration stay those impalpable and insensible atomes of sand, which by reason of their exceeding small force, spend six dayes in descending the space of half a yard. [ ] the settlement of _m_uddy water, proveth that that element hath no aversion to division. _nor let them say, that the seeing of such small bodies, consume six dayes in descending so little a way, is a sufficient argument of the waters resistance of division; because that is no resisting of division, but a retarding of motion; and it would be simplicity to say, that a thing opposeth division[ ], and that in the same instant, it permits it self to be divided: nor doth the retardation of motion at all favour the adversaries cause, for that they are to instance in a thing that wholly prohibiteth motion, and procureth rest; it is necessary, therefore, to find out bodies that stay in the water, if one would shew its repugnancy to division, and not such as move in it, howbeit but slowly._ [ ] water cannot oppose division, and at the same time permit it self to be divided. what then is this crassitude of the water, with which it resisteth division? what, i beseech you, should it be, if we (as we have said above) with all diligence attempting the reduction of a matter into so like a gravity with the water, that forming it into a dilated plate it rests suspended as we have said, between the two waters, it be impossible to effect it, though we bring them to such an equiponderance, that as much lead as the fourth part of a grain of musterd-seed, added to the same expanded plate, that in air [_i. e. out of the water_] shall weigh four or six pounds, sinketh it to the bottom, and being substracted, it ascends to the surface of the water? i cannot see, (if what i say be true, as it is most certain) what minute vertue and force we can possibly find or imagine, to which the resistance of the water against division and penetration is not inferiour; whereupon, we must of necessity conclude that it is nothing: because, if it were of any sensible power, some large plate might be found or compounded of a matter alike in gravity to the water, which not only would stay between the two waters; but, moreover, should not be able to descend or ascend without notable force. we may likewise collect the same from an other experiment[ ], shewing that the water gives way also in the same manner to transversall division; for if in a setled and standing water we should place any great mass that goeth not to the bottom, drawing it with a single womans hair, we might carry it from place to place without any opposition, and this whatever figure it hath, though that it possess a great space of water, as for instance, a great beam would do moved side-ways. perhaps some might oppose me and say, that if the resistance of water against division, as i affirm, were nothing; ships should not need such a force of oars and sayles for the moving of them from place to place in a tranquile sea, or standing lake. to him that should make such an objection, i would reply[ ], that the water contrasteth not against, nor simply resisteth division, but a sudden division, and with so much greater renitence, by how much greater the velocity is: and the cause of this resistance depends not on crassitude, or any other thing that absolutely opposeth division, but because that the parts of the water divided, in giving way to that solid that is moved in it, are themselves also necessitated locally to move, some to the one side, and some to the other, and some downwards: and this must no less be done by the waves before the ship, or other body swimming through the water, than by the posteriour and subsequent; because, the ship proceeding forwards, to make it self a way to receive its bulk, it is requisite, that with the prow it repulse the adjacent parts of the water, as well on one hand as on the other, and that it move them as much transversly, as is the half of the breadth of the hull: and the like removall must those waves make, that succeeding the poump do run from the remoter parts of the ship towards those of the middle, successively to replenish the places, which the ship in advancing forwards, goeth, leaving vacant. now, because, all motitions are made in time[ ], and the longer in greater time: and it being moreover true, that those bodies that in a certain time are moved by a certain power such a certain space, shall not be moved the same space, and in a shorter time, unless by a greater power: therefore, the broader ships move slower than the narrower, being put on by an equall force: and the same vessel requires so much greater force of wind, or oars, the faster it is to move. [ ] an hair will draw a great mass thorow the water; which proveth, that it hath no resistance against transversall division. [ ] how ships are moved in the water. [ ] bodies moved a certain space in a certain time, by a certain power, cannot be moved the same space and in a shorter time, but by a greater power. _but yet for all this, any great mass swimming in a standing lake, may be moved by any petit force; only it is true, that a lesser force more slowly moves it: but if the waters resistance of division, were in any manner sensible, it would follow, that the said mass, should, notwithstanding the percussion of some sensible force, continue immoveable, which is not so[ ]. yea, i will say farther, that should we retire our selves into the more internall contemplation of the nature of water and other fluids, perhaps we should discover the constitution of their parts to be such, that they not only do not oppose division, but that they have not any thing in them to be divided: so that the resistance that is observed in moving through the water[ ], is like to that which we meet with in passing through a great throng of people, wherein we find impediment, and not by any difficulty in the division, for that none of those persons are divided whereof the croud is composed, but only in moving of those persons side-ways which were before divided and disjoyned: and thus we find resistance in thrusting a stick into an heap of sand, not because any part of the sand is to be cut in pieces, but only to be moved and raised[ ]. two manners of penetration, therefore, offer themselves to us, one in bodies, whose parts were continuall, and here division seemeth necessary, the other in the aggregates of parts not continuall, but contiguous only[ ], and here there is no necessity of dividing but of moving only. now, i am not well resolved, whether water and other fluids may be esteemed to be of parts continuall or contiguous only[ ]; yet i find my self indeed inclined to think that they are rather contiguous (if there be in nature no other manner of aggregating, than by the union, or by the touching of the extreams:) and i am induced thereto by the great difference that i see between the conjunction of the parts of an hard or solid body[ ], and the conjunction of the same parts when the same body shall be made liquid and fluid: for if, for example, i take a mass of silver or other solid and hard mettall, i shall in dividing it into two parts, find not only the resistance that is found in the moving of it only[ ], but an other incomparably greater, dependent on that vertue, whatever it be, which holds the parts united: and so if we would divide again those two parts into other two, and successively into others and others, we should still find a like resistance, but ever less by how much smaller the parts to be divided shall be; but if, lastly, employing most subtile and acute instruments, such as are the most tenuous parts of the fire, we shall resolve it (perhaps) into its last and least particles, there shall not be left in them any longer either resistance of division, or so much as a capacity of being farther divided, especially by instruments more grosse than the acuities of fire: and what knife or rasor put into well melted silver can we finde, that will divide a thing which surpasseth the separating power of fire? certainly none: because either the whole shall be reduced to the most minute and ultimate divisions, or if there remain parts capable still of other subdidivisions, they cannot receive them, but only from acuter divisors than fire; but a stick or rod of iron, moved in the melted metall, is not such a one. of a like constitution and consistence, i account the parts of water[ ], and other liquids to be, namely, incapable of division by reason of their ienuity; or if not absolutely indivisible, yet at least not to be divided by a board, or other solid body, palpable unto the hand, the sector being alwayes required to be more sharp than the solid to be cut. solid bodies, therefore, do only move, and not divide the water[ ], when put into it; whose parts being before divided to the extreamest minuity, and therefore capable of being moved, either many of them at once, or few, or very few, they soon give place to every small corpuscle, that descends in the same: for that, it being little and light, descending in the air, and arriving to the surface of the water, it meets with particles of water more small, and of less resistance against motion and extrusion, than is its own prement and extrusive force; whereupon it submergeth, and moveth such a portion of them, as is proportionate to its power. there is not, therefore, any resistance in water against division, nay, there is not in it any divisible parts. i adde; moreover, that in case yet there should be any small resistance found (which is absolutely false)[ ] haply in attempting with an hair to move a very great natant machine, or in essaying by the addition of one small grain of lead to sink, or by removall of it to raise a very broad plate of matter, equall in gravity with water, (which likewise will not happen, in case we proceed with dexterity) we may observe that that resistance is a very different thing from that which the adversaries produce for the cause of the natation of the plate of lead or board of ebony, for that one may make a board of ebony, which being put upon the water swimmeth, and cannot be submerged, no not by the addition of an hundred grains of lead put upon the same, and afterwards being bathed, not only sinks, though the said lead be taken away, but though moreover a quantity of cork, or of some other light body fastened to it, sufficeth not to hinder it from sinking unto the bottome: so that you see, that although it were granted that there is a certain small resistance of division found in the substance of the water, yet this hath nothing to do with that cause which supports the board above the water, with a resistance an hundred times greater than that which men can find in the parts of the water: nor let them tell me, that only the surface of the water hath such resistance[ ], and not the internall parts, or that such resistance is found greatest in the beginning of the submersion, as it also seems that in the beginning, motion meets with greater opposition, than in the continuance of it; because, first, i will permit, that the water be stirred, and that the superiour parts be mingled with the middle[ ], and inferiour parts, or that those above be wholly removed, and those in the middle only made use off, and yet you shall see the effect for all that, to be still the same: moreover, that hair which draws a beam through the water, is likewise to divide the upperparts, and is also to begin the motion, and yet it begins it, and yet it divides it: and finally, let the board of ebony be put in the midway, betwixt the bottome and the top of the water, and let it there for awhile be suspended and setled, and afterwards let it be left at liberty, and it will instantly begin its motion, and will continue it unto the bottome. nay, more, the board so soon as it is dimitted upon the water, hath not only begun to move and divide it, but is for a good space dimerged into it._ [ ] the parts of liquids, so farre from resisting division, that they contain not any thing that may be divided. [ ] the resistance a solid findeth in moving through the water, like to that we meet with in passing through a throng of people: [ ] or in thrusting a stick into an heap of sand. [ ] two kinds of penetration, one in bodies continuall, the other in bodies only contiguous. [ ] water consists not of continuall, but only of contiguous parts. [ ] _se{e} what satisfaction he hath given, as to this point, in lib. de motu. dial. ._ [ ] great difference betwixt the conjunction of the parts of a body when solid, and when fluid. [ ] water consists of parts that admit of no farther division. [ ] solids dimitted into the water, do onely move, and not divide it. [ ] if there were any resistance of division in water, it must needs be small, in that it is overcome by an hair, a grain of lead, or a slight bathing of the solid. [ ] the uper parts of the water, do no more resist division than the middle or lower parts. [ ] waters resistance of division, not greater in the beginning of the submersion. let us receive it, therefore, for a true and undoubted conclusion, that the water hath not any renitence against simple division, and that it is not possible to find any solid body, be it of what figure it will, which being put into the water, its motion upwards or downwards, according as it exceedeth, or shall be exceeded by the water in gravity (although such excesse and difference be insensible) shall be prohibited, and taken away, by the crassitude of the said water. when, therefore, we see the board of ebony, or of other matter, more grave than the water, to stay in the confines of the water and air, without submerging, we must have recourse to some other originall, for the investing the cause of that effect, than to the breadth of the figure, unable to overcome the renitence with which the water opposeth division, since there is no resistance; and from that which is not in being, we can expect no action. it remains most true, therefore, as we have said before, that this so succeds, for that that which in such manner put upon the water, not the same body with that which is put _into_ the water: because this which is put _into_ the water, is the pure board of ebony, which for that it is more grave than the water, sinketh, and that which is put _upon_ the water, is a composition of ebony, and of so much air, that both together are specifically less grave than the water, and therefore they do not descend. i will farther confirm this which i say. gentlemen, my antagonists, we are agreed, that the excess or defect of the gravity of the solid, unto the gravity of the water, is the true and proper cause of natation or submersion. [sidenote: great caution to be had in experimenting the operation of figure in natation.] now, if you will shew that besides the former cause, there is another which is so powerfull, that it can hinder and remove the submersion of those very solids, that by their gravity sink, and if you will say, that this is the breadth or ampleness of figure, you are oblieged, when ever you would shew such an experiment, first to make the circumstances certain, that that solid which you put into the water, be not less grave _in specie_ than it, for if you should not do so, any one might with reason say, that not the figure, but the levity was the cause of that natation. but i say, that when you shall dimit a board of ebony into the water, you do not put therein a solid more grave _in specie_ than the water, but one lighter, for besides the ebony, there is in the water a mass of air, united with the ebony, and such, and so light, that of both there results a composition less grave than the water: see, therefore, that you remove the air, and put the ebony alone into the water, for so you shall immerge a solid more grave then the water, and if this shall not go to the bottom, you have well philosophized and i ill. now, since we have found the true cause of the natation of those bodies, which otherwise, as being graver than the water, would descend to the bottom, i think, that for the perfect and distinct knowledge of this business, it would be good to proceed in a way of discovering demonstratively those particular accidents that do attend these effects, and, probl. i. [sidenote: to finde the proportion figures ought to have to the waters gravity, that by help of the contiguous air, they may swim.] _to finde what proportion severall figures of different matters ought to have, unto the gravity of the water, that so they may be able by vertue of the contiguous air to stay afloat._ [illustration] let, therefore, for better illustration, d f n e be a vessell, wherein the water is contained, and suppose a plate or board, whose thickness is comprehended between the lines i c and o s, and let it be of matter exceeding the water in gravity, so that being put upon the water, it dimergeth and abaseth below the levell of the said water, leaving the little banks a i and b c, which are at the greatest height they can be, so that if the plate i s should but descend any little space farther, the little banks or ramparts would no longer consist, but expulsing the air a i c b, they would diffuse themselves over the superficies i c, and would submerge the plate. the height a i b c is therefore the greatest profundity that the little banks of water admit of. now i say, that from this, and from the proportion in gravity, that the matter of the plate hath to the water, we may easily finde of what thickness, at most, we may make the said plates, to the end, they may be able to bear up above water: for if the matter of the plate or board i s were, for example, as heavy again as the water, a board of that matter shall be, at the most of a thickness equall to the greatest height of the banks, that is, as thick as a i is high: which we will thus demonstrate. let the solid i s be double in gravity to the water, and let it be a regular prisme, or cylinder, to wit, that hath its two flat superficies, superiour and inferiour, alike and equall, and at right angles with the other laterall superficies, and let its thickness i o be equall to the greatest altitude of the banks of water: i say, that if it be put upon the water, it will not submerge: for the altitude a i being equall to the altitude i o, the mass of the air a b c i shall be equall to the mass of the solid c i o s: and the whole mass a o s b double to the mass i s; and since the mass of the air a c, neither encreaseth nor diminisheth the gravity of the mass i s, and the solid i s was supposed double in gravity to the water; therefore as much water as the mass submerged a o s b, compounded of the air a i c b, and of the solid i o s c, weighs just as much as the same submerged mass a o s b: but when such a mass of water, as is the submerged part of the solid, weighs as much as the said solid, it descends not farther, but resteth, as by (_a_) _archimedes_[ ], and above by us, hath been demonstrated: therefore, i s shall descend no farther, but shall rest. and if the solid i s shall be sesquialter in gravity to the water, it shall float, as long as its thickness be not above twice as much as the greatest altitude of the ramparts of water, that is, of a i. for i s being sesquialter in gravity to the water, and the altitude o i, being double to i a, the solid submerged a o s b, shall be also sesquialter in mass to the solid i s. and because the air a c, neither increaseth nor diminisheth the ponderosity of the solid i s: therefore, as much water in quantity as the submerged mass a o s b, weighs as much as the said mass submerged: and, therefore, that mass shall rest. and briefly in generall. [ ] of natation lib. . prop. . [illustration] theoreme. vi. [sidenote: the proportion of the greatest thickness of solids, beyond which encreased they sink.] _when ever the excess of the gravity of the solid above the gravity of the water, shall have the same proportion to the gravity of the water, that the altitude of the rampart, hath to the thickness of the solid, that solid shall not sink, but being never so little thicker it shall._ [illustration] let the solid i s be superior in gravity to the water, and of such thickness, that the altitude of the rampart a i, be in proportion to the thickness of the solid i o, as the excess of the gravity of the said solid i s, above the gravity of a mass of water equall to the mass i s, is to the gravity of the mass of water equall to the mass i s. i say, that the solid i s shall not sinke, but being never so little thicker it shall go to the bottom: for being that as a i is to i o, so is the excess of the gravity of the solid i s, above the gravity of a mass of water equall to the mass i s, to the gravity of the said mass of water: therefore, compounding, as a o is to o i, so shall the gravity of the solid i s, be to the gravity of a mass of water equall to the mass i s: and, converting, as i o is to o a, so shall the gravity of a mass of water equall to the mass i s, be to the gravity of the solid i s: but as i o is to o a, so is a mass of water i s, to a mass of water equall to the mass a b s o: and so is the gravity of a mass of water i s, to the gravity of a mass of water a s: therefore as the gravity of a mass of water, equall to the mass i s, is to the gravity of the solid i s, so is the same gravity of a mass of water i s, to the gravity of a mass of water a s: therefore the gravity of the solid i s, is equall to the gravity of a mass of water equall to the mass a s: but the gravity of the solid i s, is the same with the gravity of the solid a s, compounded of the solid i s, and of the air a b c i. therefore the whole compounded solid a o s b, weighs as much as the water that would be comprised in the place of the said compound a o s b: and, therefore, it shall make an _equilibrium_ and rest, and that same solid i o s c shall sinke no farther. but if its thickness i o should be increased, it would be necessary also to encrease the altitude of the rampart a i, to maintain the due proportion: but by what hath been supposed, the altitude of the rampart a i, is the greatest that the nature of the water and air do admit, without the waters repulsing the air adherent to the superficies of the solid i c, and possessing the space a i c b: therefore, a solid of greater thickness than i o, and of the same matter with the solid i s, shall not rest without submerging, but shall descend to the bottome: which was to be demonstrated. in consequence of this that hath been demonstrated, sundry and various conclusions may be gathered, by which the truth of my principall proposition comes to be more and more confirmed, and the imperfection of all former argumentations touching the present question cometh to be discovered. _and first we gather from the things demonstrated, that,_ theoreme vii. [sidenote: the heaviest bodies may swimme.] _all matters, how heavy soever, even to gold it self, the heaviest of all bodies, known by us, may float upon the water._ because its gravity being considered to be almost twenty times greater than that of the water, and, moreover, the greatest altitude that the rampart of water can be extended to, without breaking the contiguity of the air, adherent to the surface of the solid, that is put upon the water being predetermined, if we should make a plate of gold so thin, that it exceeds not the nineteenth part of the altitude of the said rampart, this put lightly upon the water shall rest, without going to the bottom: and if ebony shall chance to be in sesquiseptimall proportion more grave than the water, the greatest thickness that can be allowed to a board of ebony, so that it may be able to stay above water without sinking, would be seaven times more than the height of the rampart tinn, _v. gr._ eight times more grave than water, shall swimm as oft as the thickness of its plate, exceeds not the th part of the altitude of the rampart. [sidenote: _he elsewhere cites this as a proposition, therefore i make it of that number._] and here i will not omit to note, as a second corrollary dependent upon the things demonstrated, that, theoreme viii. [sidenote: natation and submersion, collected from the thickness, excluding the length and breadth of plates.] _the expansion of figure not only is not the cause of the natation of those grave bodies, which otherwise do submerge, but also the determining what be those boards of ebony, or plates of iron or gold that will swimme, depends not on it, rather that same determination is to be collected from the only thickness of those figures of ebony or gold, wholly excluding the consideration of length and breadth, as having no wayes any share in this effect._ it hath already been manifested, that the only cause of the natation of the said plates, is the reduction of them to be less grave than the water, by means of the connexion of that air, which descendeth together with them, and possesseth place in the water; which place so occupyed, if before the circumfused water diffuseth it self to fill it, it be capable of as much water, as shall weigh equall with the plate, the plate shall remain suspended, and sinke no farther. now let us see on which of these three dimensions of the solid depends the terminating, what and how much the mass of that ought to be, that so the assistance of the air contiguous unto it, may suffice to render it specifically less grave than the water, whereupon it may rest without submersion. it shall undoubtedly be found, that the length and breadth have not any thing to do in the said determination, but only the height, or if you will the thickness: for, if we take a plate or board, as for example, of ebony, whose altitude hath unto the greatest possible altitude of the rampart, the proportion above declared, for which cause it swims indeed, but yet not if we never so little increase its thickness; i say, that retaining its thickness, and encreasing its superficies to twice, four times, or ten times its bigness, or dminishing it by dividing it into four, or six, or twenty, or a hundred parts, it shall still in the same manner continue to float: but encreasing its thickness only a hairs breadth, it will alwaies submerge, although we should multiply the superficies a hundred and a hundred times. now forasmuch as that this is a cause, which being added, we adde also the effect, and being removed, it is removed; and by augmenting or lessening the length or breadth in any manner, the effect of going, or not going to the bottom, is not added or removed: i conclude, that the greatness and smalness of the superficies hath no influence upon the natation or submersion. and that the proportion of the altitude of the ramparts of water, to the altitude of the solid, being constituted in the manner aforesaid, the greatness or smalness of the superficies, makes not any variation, is manifest from that which hath been above demonstrated, and from this, that, _the prisms and cylinders which have the same base, are in proportion to one another as their heights._ whence cylinders or prismes[ ], namely, the board, be they great or little, so that they be all of equall thickness, have the same proportion to their conterminall air, which hath for base the said superficies of the board, and for height the ramparts of water; so that alwayes of that air, and of the board, solids, are compounded, that in gravity equall a mass of water equall to the mass of the solids, compounded of air, and of the board: whereupon all the said solids do in the same manner continue afloat. we will conclude in the third place, that, [ ] prismes and cylinders having the same base, are to one another as their heights. theoreme. ix. [sidenote: all figures of all matters, float by hep of the rampart replenished with air, and some but only touch the water.] _all sorts of figures of whatsoever matter, albeit more grave than the water, do by benefit of the said rampart, not only float, but some figures, though of the gravest matter, do stay wholly above water, wetting only the inferiour surface that toucheth the water._ and these shall be all figures, which from the inferiour base upwards, grow lesser and lesser; the which we shall exemplifie for this time in piramides or cones, of which figures the passions are common. we will demonstrate therefore, that, _it is possible to form a piramide, of any whatsoever matter preposed, which being put with its base upon the water, rests not only without submerging, but without wetting it more then its base._ for the explication of which it is requisite, that we first demonstrate the subsequent lemma, namely, that, lemma ii. [sidenote: solids whose masses are in contrary proportion to their specifick gravities are equall in absolute gravity.] _solids whose masses answer in proportion contrarily to their specificall gravities, are equall in absolute gravities._ [illustration] let a c and b be two solids, and let the mass a c be to the mass b, as the specificall gravity of the solid b, is to the specificall gravity of the solid a c: i say, the solids a c and b are equall in absolute weight, that is, equally grave. for if the mass a c be equall to the mass b, then, by the assumption, the specificall gravity of b, shall be equall to the specificall gravity of a c, and being equall in mass, and of the same specificall gravity they shall absolutely weigh one as much as another. but if their masses shall be unequall, let the mass a c be greater, and in it take the part c, equall to the mass b. and, because the masses b and c are equall; the absolute weight of b, shall have the same proportion to the absolute weight of c, that the specificall gravity of b, hath to the specificall gravity of c; or of c a, which is the same _in specie_: but look what proportion the specificall gravity of b, hath to the specificall gravity of c a, the like proportion, by the assumption, hath the mass c a, to the mass b, that is, to the mass c: therefore, the absolute weight of b, to the absolute weight of c, is as the mass a c to the mass c: but as the mass a c, is to the mass c, so is the absolute weight of a c, to the absolute weight of c: therefore the absolute weight of b, hath the same proportion to the absolute weight of c, that the absolute weight of a c, hath to the absolute weight of c: therefore, the two solids a c and b are equall in absolute gravity: which was to be demonstrated. having demonstrated this, i say, theoreme x. [sidenote: there may be cones and piramides of any matter, which demitted into the water, rest only their bases.] _that it is possible of any assigned matter, to form a piramide or cone upon any base, which being put upon the water shall not submerge, nor wet any more than its base._ [illustration] let the greatest possible altitude of the rampart be the line d b, and the diameter of the base of the cone to be made of any matter assigned b c, at right angles to d b: and as the specificall gravity of the matter of the piramide or cone to be made, is to the specificall gravity of the water, so let the altitude of the rampart d b, be to the third part of the piramide or cone a b c, described upon the base, whose diameter is b c: i say, that the said cone a b c, and any other cone, lower then the same, shall rest upon the surface of the water b c without sinking. draw d f parallel to b c, and suppose the prisme or cylinder e c, which shall be tripple to the cone a b c. and, because the cylinder d c hath the same proportion to the cylinder c e, that the altitude d b, hath to the altitude b e: but the cylinder c e, is to the cone a b c, as the altitude e b is to the third part of the altitude of the cone: therefore, by equality of proportion, the cylinder d c is to the cone a b c, as d b is to the third part of the altitude b e: but as d b is to the third part of b e, so is the specificall gravity of the cone a b c, to the specificall gravity of the water: therefore, as the mass of the solid d c, is to the mass of the cone a _b_ c, so is the specificall gravity of the said cone, to the specificall gravity of the water: therefore, by the precedent lemma, the cone a b c weighs in absolute gravity, as much as a mass of water equall to the mass d c: but the water which by the imposition of the cone a b c, is driven out of its place, is as much as would precisely lie in the place d c, and is equall in weight to the cone that displaceth it: therefore, there shall be an _equilibrium_, and the cone shall rest without farther submerging. and its manifest, corolary i. [sidenote: amongst cones of the same base, those of least altitude shall sink the least.] _that making upon the same basis, a cone of a less altitude, it shall be also less grave, and shall so much the more rest without submersion._ corolary ii. [sidenote: there may be cones and piramides of any matter, which demitted with the point downwards do float atop.] _it is manifest, also, that one may make cones and piramids of any matter whatsoever, more grave than the water, which being put into the water, with the apix or point downwards, rest without submersion._ because if we reassume what hath been above demonstrated, of prisms and cylinders, and that on bases equall to those of the said cylinders, we make cones of the same matter, and three times as high as the cylinders, they shall rest afloat, for that in mass and gravity they shall be equall to those cylinders, and by having their bases equall to those of the cylinders, they shall leave equall masses of air included within the ramparts. this, which for example sake hath been demonstrated, in prisms, cylinders, cones and piramids, might be proved in all other solid figures, but it would require a whole volume (such is the multitude and variety of their symptoms and accidents) to comprehend the particuler demonstration of them all, and of their severall segments: but i will to avoid prolixity in the present discourse, content my self, that by what i have declared every one of ordinary capacity may comprehend, that there is not any matter so grave, no not gold it self, of which one may not form all sorts of figures, which by vertue of the superiour air adherent to them, and not by the waters resistance of penetration, do remain afloat, so that they sink not. nay, farther, i will shew, for removing that error, that, theoreme xi. [sidenote: a piramide or cone, demitted with the point downwards shal swim, with its base downward shall sink.] _a piramide or cone put into the water, with the point downward shall swimme, and the same put with the base downwards shall sinke, and it shall be impossible to make it float._ now the quite contrary would happen, if the difficulty of penetrating the water, were that which had hindred the descent, for that the said cone is far apter to pierce and penetrate with its sharp point, than with its broad and spacious base. and, to demonstrate this, let the cone be _a b c_, twice as grave as the water, and let its height be tripple to the height of the rampart _d a e c_: i say, first, that being put lightly into the water with the point downwards, it shall not descend to the bottom: for the aeriall cylinder contained betwixt the ramparts _d a c e_, is equall in mass to the cone _a b c_; so that the whole mass of the solid compounded of the air _d a c e_, and of the cone _a b c_, shall be double to the cone _a c b_: and, because the cone _a b c_ is supposed to be of matter double in gravity to the water, therefore as much water as the whole masse _d a b c e_, placed beneath the levell of the water, weighs as much as the cone _a b c_: and, therefore, there shall be an _equilibrium_, and the cone _a b c_ shall descend no lower. now, i say farther, that the same cone placed with the base downwards, shall sink to the bottom, without any possibility of returning again, by any means to swimme. [illustration] let, therefore, the cone be _a b d_, double in gravity to the water, and let its height be tripple the height of the rampart of water l b: it is already manifest, that it shall not stay wholly out of the water, because the cylinder being comprehended betwixt the ramparts _l b d p_, equall to the cone _a b d_, and the matter of the cone, beig double in gravity to the water, it is evident that the weight of the said cone shall be double to the weight of the mass of water equall to the cylinder _l b d p_: therefore it shall not rest in this state, but shall descend. [illustration] corolary i. [sidenote: much less shall the said cone swim, if one immerge a part thereof.] _i say farther; that much lesse shall the said cone stay afloat, if one immerge a part thereof._ which you may see, comparing with the water as well the part that shall immerge as the other above water. let us therefore of the cone a b d, submergeth part n t o s, and advance the point n s f above water. the altitude of the cone f n s, shall either be more than half the whole altitude of the cone f t o, or it shall not be more: if it shall be more than half, the cone f n s shall be more than half of the cylinder e n s c: for the altitude of the cone f n s, shall be more than sesquialter of the altitude of the cylinder e n s c: and, because the matter of the cone is supposed to be double in specificall gravity to the water, the water which would be contained within the rampart e n s c, would be less grave absolutely than the cone f n s; so that the whole cone f n s cannot be sustained by the rampart: but the part immerged n t o s, by being double in specificall gravity to the water, shall tend to the bottom: therefore, the whole _c_one f t o, as well in respect of the part submerged, as the part above water shall descend to the bottom. but if the altitude of the point f n s, shall be half the altitude of the whole cone f t o, the same altitude of the said cone f n s shall be sesquialter to the altitude e n: and, therefore, e n s c shall be double to the cone f n s; and as much water in mass as the _c_ylinder e n s c, would weigh as much as the part of the _c_one f n s. but, because the other immerged part n t o s, is double in gravity to the water, a mass of water equall to that compounded of the _c_ylinder e n s c, and of the solid n t o s, shall weigh less than the _c_one f t o, by as much as the weight of a mass of water equall to the solid n t o s: therefore, the _c_one sha{l}l also descend. again, because the solid n t o s, is septuple to the cone f n s, to which the _c_ylinder e s is double, the proportion of the solid n t o s, shall be to the _c_ylinder e n s c, as seaven to two: therefore, the whole solid compounded of the _c_ylinder e n s c, and of the solid n t o s, is much less than double the solid n t o s: therefore, the single solid n t o s, is much graver than a mass of water equall to the mass, compounded of the _c_ylinder e n s c, and of n t o s. corolary ii. [sidenote: part of the cones towards the cuspis removed, it shall still sink.] _from whence it followeth, that though one should remove and take away the part of the cone f n s, the sole remainder n t o s would go to the bottom._ corolary iii. [sidenote: the more the cone is immerged, the more impossible is its floating.] _and if we should more depress the cone f t o, it would be so much the more impossible that it should sustain it self afloat, the part submerged n t o s still encreasing, and the mass of air contained in the rampart diminishing, which ever grows less, the more the cone submergeth._ that cone, therefore, that with its base upwards, and its _cuspis_ downwards doth swimme, being dimitted with its base downward must of necessity sinke. they have argued farre from the truth, therefore, who have ascribed the cause of natation to waters resistance of division, as to a passive principle, and to the breadth of the figure, with which the division is to be made, as the efficient. i come in the fourth place, to collect and conclude the reason of that which i have proposed to the adversaries, namely, theoreme xii. [sidenote: solids of any figure & greatnesse, that naturally sink, may by help of the air in the rampart swimme.] _that it is possible to fo{r}m solid bodies, of what figure and greatness soever, that of their own nature goe to the bottome; but by the help of the air contained in the rampart, rest without submerging._ [illustration] the truth of this proposition is sufficiently manifest in all those solid figures, that determine in their uppermost part in a plane superficies: for making such figures of some matter specifically as grave as the water, putting them into the water, so that the whole mass be covered, it is manifest, that they shall rest in all places, provided, that such a matter equall in weight to the water, may be exactly adjusted: and they shall by consequence, rest or lie even with the levell of the water, without making any rampart. if, therefore, in respect of the matter, such figures are apt to rest without submerging, though deprived of the help of the rampart, it is manifest, that they may admit so much encrease of gravity, (without encreasing their masses) as is the weight of as much water as would be contained within the rampart, that is made about their upper plane surface: by the help of which being sustained, they shall rest afloat, but being bathed, they shall descend, having been made graver than the water. in figures, therefore, that determine above in a plane, we may cleerly comprehend, that the rampart added or removed, may prohibit or permit the descent: but in those figures that go lessening upwards towards the top, some persons may, and that not without much seeming reason, doubt whether the same may be done, and especially by those which terminate in a very acute point, such as are your cones and small piramids. touching these, therefore, as more dubious than the rest, i will endeavour to demonstrate, that they also lie under the same accident of going, or not going to the bottom, be they of any whatever bigness. let therefore the cone be a b d, made of a matter specifically as grave as the water; it is manifest that being put all under water, it shall rest in all places (alwayes provided, that it shall weigh exactly as much as the water, which is almost impossible to effect) and that any small weight being added to it, it shall sink to the bottom: but if it shall descend downwards gently, i say, that it shall make the rampart e s t o, and that there shall stay out of the water the point a s t, tripple in height to the rampart e s: which is manifest, for the matter of the cone weighing equally with the water, the part submerged _s b d t_, becomes indifferent to move downwards or upwards; and the cone _a s t_, being equall in mass to the water that would be contained in the concave of the rampart _e s t o_, shall be also equall unto it in gravity: and, therefore, there shall be a perfect _equilibrium_, and, consequently, a rest. now here ariseth a doubt, whether the cone _a b d_ may be made heavier, in such sort, that when it is put wholly under water, it goes to the bottom, but yet not in such sort, as to take from the rampart the vertue of sustaining it that it sink not, and, the reason of the doubt is this: that although at such time as the cone _a b d_ is specifically as grave as the water, the rampart _e s t o_ sustaines it, not only when the point _a s t_ is tripple in height to the altitude of the rampart _e s_, but also when a lesser part is above water; [for although in the descent of the cone the point _a s t_ by little and little diminisheth, and so likewise the rampart _e s t o_, yet the point diminisheth in greater proportion than the rampart, in that it diminisheth according to all the three dimensions, but the rampart according to two only, the altitude still remaining the same; or, if you will, because the cone _s {a} t_ goes diminishing, according to the proportion of the cubes of the lines that do successively become the diameters of the bases of emergent cones, and the ramparts diminish according to the proportion of the squares of the same lines; whereupon the proportions of the points are alwayes sesquialter of the proportions of the cylinders, contained within the rampart; so that if, for example, the height of the emergent point were double, or equall to the height of the rampart, in these cases, the cylinder contained within the rampart, would be much greater than the said point, because it would be either sesquialter or tripple, by reason of which it would perhaps serve over and above to sustain the whole cone, since the part submerged would no longer weigh any thing;] yet, nevertheless, when any gravity is added to the whole mass of the cone, so that also the part submerged is not without some excesse of gravity above the gravity of the water, it is not manifest, whether the cylinder contained within the rampart, in the descent that the cone shall make, can be reduced to such a proportion unto the emergent point, and to such an excesse of mass above the mass of it, as to compensate the excesse of the cones specificall gravity above the gravity of the water: and the scruple ariseth, because that howbeit in the descent made by the cone, the emergent point _a s t_ diminisheth, whereby there is also a diminution of the excess of the cones gravity above the gravity of the water, yet the case stands so, that the rampart doth also contract it self, and the cylinder contained in it doth deminish. nevertheless it shall be demonstrated, how that the cone _a b d_ being of any supposed bignesse, and made at the first of a matter exactly equall in gravity to the water, if there may be affixed to it some weight, by means of which i{t} may descend to the bottom, when submerged under water, it may also by vertue of the rampart stay above without sinking. [illustration] [illustration] let, therefore, the cone _a b d_ be of any supposed greatnesse, and alike in specificall gravity to the water. it is manifest, that being put lightly into the water, it shall rest without descending; and it shall advance above water, the point _a s t_, tripple in height to the height of the rampart _e s_: now, suppose the cone _a b d_ more depressed, so that it advance above water, only the point _a i r_, higher by half than the point _a s t_, with the rampart about it _c i r n_. and, because, the cone _a b d_ is to the cone _a i r_, as the cube of the line _s t_ is to the cube of the line _i r_, but the cylinder _e s t o_, is to the cylinder _c i r n_, as the square of _s t_ to the square of _i r_, the cone _a s t_ shall be octuple to the cone _a i r_, and the cylinder _e s t o_, quadruple to the cylinder _c i r n_: but the cone _a s t_, is equall to the cylinder _e s t o_: therefore, the cylinder _c i r n_, shall be double to the cone _a i r_: and the water which might be contained in the rampart _c i r n_, would be double in mass and in weight to the cone _a i r_, and, therefore, would be able to sustain the double of the weight of the cone _a i r_: therefore, if to the whole cone _a b d_, there be added as much weight as the gravity of the cone _a i r_, that is to say, the eighth part of the weight of the cone _a s t_, it also shall be sustained by the rampart _c i r n_, but without that it shall go to the bottome: the cone _a b d_, being, by the addition of the eighth part of the weight of the cone _a s t_, made specifically more grave than the water. but if the altitude of the cone _a i r_, were two thirds of the altitude of the cone _a s t_, the cone _a s t_ would be to the cone _a i r_, as twenty seven to eight; and the cylinder _e s t o_, to the cylinder _c i r n_, as nine to four, that is, as twenty seven to twelve; and, therefore, the cylinder _c i r n_, to the cone _a i r_, as twelve to eight; and the excess of the cylinder _c i r n_, above the cone _a i r_, to the cone _a s t_, as four to twenty seven: therefore if to the cone _a b d_ be added so much weight as is the four twenty sevenths of the weight of the cone _a s t_, which is a little more then its seventh part, it also shall continue to swimme, and the height of the emergent point shall be double to the height of the rampart. this that hath been demonstrated in cones, exactly holds in piramides, although the one or the other should be very sharp in their point or cuspis[ ]: from whence we conclude, that the same accident shall so much the more easily happen in all other figures, by how much the less sharp the tops shall be, in which they determine, being assisted by more spacious ramparts. [ ] natatio{n} easiest effected in figures broad toward the top. theoreme xiii. [sidenote: all figures sink or swim, upon bathing or not bathing of their tops.] _all figures, therefore, of whatever greatnesse, may go, and not go, to the bottom, according as their sumities or tops shall be bathed or not bathed._ and this accident being common to all sorts of figures, without exception of so much as one. figure hath, therefore, no part in the production of this effect, of sometimes sinking, and sometimes again not sinking, but only the being sometimes conjoyned to, and sometimes seperated from, the supereminent air: which cause, in fine, who so shall rightly, and, as we say, with both his eyes, consider this business, will find that it is reduced to, yea, that it really is the same with, the true, naturall and primary cause of natation or submersion; to wit, the excess or deficiency of the gravity of the water, in relation to the gravity of that solid magnitude, that is demitted into the water. for like as a plate of lead, as thick as the back of a knife, which being put into the water by it self alone goes to the bottom, if upon it you fasten a piece of cork four fingers thick, doth continue afloat, for that now the solid that is demitted in the water, is not, as before, more grave than the water, but less, so the board of ebony, of its own nature more grave than water; and, therefore, descending to the bottom, when it is demitted by it self alone into the water, if it shall be put upon the water, conjoyned with an expanded vail of air, that together with the ebony doth descend, and that it be such, as that it doth make with it a compound less grave than so much water in mass, as equalleth the mass already submerged and depressed beneath the levell of the waters surface, it shall not descend any farther, but shall rest, for no other than the universall and most common cause, which is that solid magnitudes, less grave _in specie_ than the water, go not to the bottom. so that if one should take a plate of lead, as for example, a finger thick, and an handfull broad every way, and should attempt to make it swimme, with putting it lightly on the water, he would lose his labour, because that if it should be depressed an hairs breadth beyond the possible altitude of the ramparts of water, it would dive and sink; but if whilst it is going downwards, one should make certain banks or ramparts about it, that should hinder the defusion of the water upon the said plate, the which banks should rise so high, as that they might be able to contain as much water, as should weigh equally with the said plate, it would, witho{u}t all question, descend no lower, but would rest, as being sustained by vertue of the air contained within the aforesaid ramparts: and, in short, there would be a vessell by this means formed with the bottom of lead. but if the thinness of the lead shall be such, that a very small height of rampart would suffice to contain so much air, as might keep it afloat, it shall also rest without the artificiall banks or ramparts, but yet not without the air, because the air by it self makes banks sufficient for a small height, to resist the superfusion of the water: so that that which in this case swimmes, is as it were a vessell filled with air, by vertue of which it continueth afloat. i will, in the last place, with an other experime{n}t, attempt to remove all difficulties, if so be there should yet be any doubt left in any one, touching the opperation of this [e]continuity of the air, with the thin plate which swims, and afterwards put an end to this part of my discourse. [e] or rather contiguity, i suppose my self to be questioning with some of my oponents. whether figure have any influence upon the encrease or diminution of the resistance in any weight against its being raised in the air[ ]; and i suppose, that i am to maintain the affirmative, asserting that a mass of lead, reduced to the figure of a ball, shall be raised with less force, then if the same had been made into a thinne and broad plate, because that it in this spacious figure, hath a great quantity of air to penetrate, and in that other, more compacted and contracted very little: and to demonstrate the truth of such my opinion, i will hang in a small thred first the ball or bullet, and put that into the water, tying the thred that upholds it to one end of the ballance that i hold in the air, and to the other end i by degrees adde so much weight, till that at last it brings up the ball of lead out of the water: to do which, suppose a gravity of thirty ounces sufficeth; i afterwards reduce the said lead into a flat and thinne plate, the which i likewise put into the water, suspended by three threds, which hold it parallel to the surface of the water, and putting in the same manner, weights to the other end, till such time as the plate comes to be raised and drawn out of the water: i finde that thirty six ounces will not suffice to seperate it from the water, and raise it thorow the air: and arguing from this experiment, i affirm, that i have fully demonstrated the truth of my proposition. here my oponents desires me to look down, shewing me a thing which i had not before observed, to wit, that in the ascent of the plate out of the water, it draws after it another plate (_if i may so call it_) of water, which before it divides and parts from the inferiour surface of the plate of lead, is raised above the levell of the other water, more than the thickness of the back of a knife: then he goeth to repeat the experiment with the ball, and makes me see, that it is but a very small quantity of water, which cleaves to its compacted and contracted figure: and then he subjoynes, that its no wonder, if in seperating the thinne and broad plate from the water, we meet with much greater resistance, than in seperating the ball, since together with the plate, we are to raise a great quantity of water, which occurreth not in the ball: he telleth me moreover, how that our question is, whether the resistance of elevation be greater in a dilated plate of lead, than in a ball, and not whether more resisteth a plate of lead with a great quantity of water, or a ball with a very little water: he sheweth me in the close, that the putting the plate and the ball first into the water, to make proofe thereby of their resistance in the air, is besides our case, which treats of elivating in the air, and of things placed in the air, and not of the resistance that is made in the confines of the air and water, and by things which are part in air and part in water: and lastly, they make me feel with my hand, that when the thinne plate is in the air, and free from the weight of the water, it is raised with the very same force that raiseth the ball. seeing, and understanding these things, i know not what to do, unless to grant my self convinced, and to thank such a friend, for having made me to see that which i never till then observed: and, being advertised by this same accident, to tell my adversaries, that our question is, whether a board and a ball of ebony, equally go to the bottom in water, and not a ball of ebony and a board of ebony, joyned with another flat body of air: and, farthermore, that we speak of sinking, and not sinking to the bottom, in water, and not of that which happeneth in the confines of the water and air to bodies that be part in the air, and part in the water; nor much less do we treat of the greater or lesser force requisite in seperating this or that body from the air; not omitting to tell them, in the last place, that the air doth resist, and gravitate downwards in the water, just so much as the water (if i may so speak) gravitates and resists upwards in the air, and that the same force is required to sinke a bladder under water, that is full of air, as to raise it in the air, being full of water, removing the consideration of the weight of that filme or skinne, and considering the water and the air only. and it is likewise true, that the same force is required to sink a cup or such like vessell under water, whilst it is full of air, as to raise it above the superficies of the water, keeping it with the mouth downwards; whilst it is full of water, which is constrained in the same manner to follow the cup which contains it, and to rise above the other water into the region of the air, as the air is forced to follow the same vessell under the surface of the water, till that in this c{a}se the water, surmounting the brimme of the cup, breaks in, driving thence the air, and in that case, the said brimme coming out of the water, and arriving to the confines of the air, the water falls down, and the air sub-enters to fill the cavity of the cup: upon which ensues, that he no less transgresses the articles of the _convention_, who produceth a plate conjoyned with much air, to see if it descend to the bottom in water, then he that makes proof of the resistance against elevation in air with a plate of lead, joyned with a like quantity of water. [ ] an experiment of the operation of figures, in encreasing or lessening of the airs resistance of division. [sidenote: _aristotles_ opinion touching the operation of figure examined.] i have said all that i could at present think of, to maintain the assertion i have undertook. it remains, that i examine that which _aristotle_ hath writ of this matter towards the end of his book de cælo[ ]; wherein i shall note two things: the one that it being true as hath been demonstrated, that figure hath nothing to do about the moving or not moving it self upwards or downwards, its seemes that _aristotle_ at his first falling upon this speculation, was of the same opinion, as in my opinion may be collected from the examination of his words. 'tis true, indeed, that in essaying afterwards to render a reason of such effect, as not having in my conceit hit upon the right, (which in the second place i will examine) it seems that he is brought to admit the largenesse of figure, to be interessed in this operation. as to the first particuler, hear the precise words of _aristotle_. [ ] _aristot. de cælo_ lib. . cap . _figures are not the causes of moving simply upwards or downwards, but of moving more slowly or swiftly[ ][ ], and by what means this comes to pass, it is not difficult to see._ [ ] _aristotle_ makes not figure the cause of motion absolutely, but of swift or slow motion, [ ] lib. . cap. : text. . here first i note, that the terms being four, which fall under the present consideration, namely, motion, rest, slowly and swiftly: and _aristotle_ naming figures as causes of tardity and velocity, excluding them from being the cause of absolute and simple motion, it seems necessary, that he exclude them on the other side, from being the cause of rest, so that his meaning is this. figures are not the causes of moving or not moving absolutely, but of moving quickly or slowly: and, here, if any should say the mind of _aristotle_ is to exclude figures from being causes of motion, but yet not from being causes of rest, so that the sence would be to remove from figures, there being the causes of moving simply, but yet not there being causes of rest, i would demand, whether we ought with _aristotle_ to understand, that all figures universally, are, in some manner, the causes of rest in those bodies, which otherwise would move, or else some particular figures only, as for example, broad and thinne figures: if all indifferently, then every body shall rest: because every body hath some figure, which is false; but if some particular figures only may be in some manner a cause of rest, as, for example, the broad, then the others would be in some manner the causes of motion: for if from seeing some bodies of a contracted figure move, which after dilated into plates rest, may be inferred, that the amplitude of figure hath a part in the cause of that rest; so from seeing such like figures rest, which afterwards contracted move, it may with the same reason be affirmed, that the united and contracted figure, hath a part in causing motion, as the remover of that which impeded it: the which again is directly opposite to what _aristotle_ saith, namely, that figures are not the causes of motion. besides, if _aristotle_ had admitted and not excluded figures from being causes of not moving in some bodies, which moulded into another figure would move, he would have impertinently propounded in a dubitative manner, in the words immediately following, whence it is, that the large and thinne plates of lead or iron, rest upon the water, since the cause was apparent, namely, the amplitude of figure. let us conclude, therefore, that the meaning of _aristotle_ in this place is to affirm, that figures are not the causes of absolutely moving or not moving, but only of moving swiftly or slowly: which we ought the rather to believe, in regard it is indeed a most true conceipt and opinion. now the mind of _aristotle_ being such, and appearing by consequence, rather contrary at the first sight, then favourable to the assertion of the oponents, it is necessary, that their interpretation be not exactly the same with that, but such, as being in part understood by some of them, and in part by others, was set down: and it may easily be indeed so, being an interpretation consonent to the sence of the more famous interpretors, which is, that the adverbe _simply_ or _absolutely_, put in the text, ought not to be joyned to the verbe to _move_, but with the noun _causes_: so that the purport of _aristotles_ words, is to affirm, that figures are not the causes absolutely of moving or not moving, but yet are causes _secundum quid_, _viz._ in some sort; by which means, they are called auxiliary and concomitant causes: and this proposition is received and asserted as true by _signor buonamico lib. . cap. ._ where he thus writes. _there are other causes concomitant, by which some things float, and others sink, among which the figures of bodies hath the first place_, &c. concerning this proposition, i meet with many doubts and difficulties, for which me thinks the words of _aristotle_ are not capable of such a construction and sence, and the difficulties are these. first in the order and disposure of the words of _aristotle_, the particle _simpliciter_, or if you will _absoluté_, is conjoyned with the verb _to move_, and seperated from the noun _causes_, the which is a great presumption in my favour, seeing that the writing and the text saith, figures are not the cause of moving simply upwards or downwards, but of quicker or slower motion: and, saith not, figures are not simply the causes of moving upwards or downwards, and when the words of a text receive, transposed, a sence different from that which they sound, taken in the order wherein the author disposeth them, it is not convenient to inverte them. and who will affirm that _aristotle_ desiring to write a proposition, would dispose the words in such sort, that they should import a different, nay, a contrary sence? contrary, i say, because understood as they are written; they say, that figures are not the causes of motion, but inverted, they say, that figures are the causes of motion, &c. moreover, if the intent of _aristotle_ had been to say, that figures are not simply the causes of moving upwards or downwards, but only causes _secundum quid_, he would not have adjoyned those words, _but they are causes of the more swift or slow motion_; yea, the subjoining this would have been not only superfluous but false, for that the whole tenour of the proposition would import thus much. figures are not the absolute causes of moving upwards or downwards, but are the absolute cause of the swift or slow motion; which is not true: because the primary causes of greater or lesser velocity, are by _aristotle_ in the th of his _physicks_, _text. ._ attributed to the greater or lesser gravity of moveables, compared among themselves, and to the greater or lesser resistance of the _medium's_, depending on their greater or less crassitude: and these are inserted by _aristotle_ as the primary causes; and these two only are in that place nominated: and figure comes afterwards to be considered, _text. ._ rather as an instrumentall cause of the force of the gravity, the which divides either with the figure, or with the _impetus_; and, indeed, figure by it self without the force of gravity or levity, would opperate nothing. i adde, that if _aristotle_ had an opinion that figure had been in some sort the cause of moving or not moving, the inquisition which he makes immediately in a doubtfull manner, whence it comes, that a plate of lead flotes, would have been impertinent; for if but just before he had said, that figure was in a certain sort the cause of moving or not moving, he needed not to call in question, by what cause the plate of lead swims, and then ascribing the cause to its figure; and framing a discourse in this manner. figure is a cause _secundum quid_ of not sinking: but, now, if it be doubted, for what cause a thin plate of lead goes not to the bottom; it shall be answered, that that proceeds from its figure: a discourse which would be indecent in a child, much more in _aristotle_; for where is the occasion of doubting? and who sees not, that if _aristotle_ had held, that figure was in some sort a cause of natation, he would without the least hesitation have writ; that figure is in a certain sort the cause of natation, and therefore the plate of lead in respect of its large and expatiated figure swims; but if we take the proposition of _aristotle_ as i say, and as it is written, and as indeed it is true, the ensuing words come in very oppositely, as well in the introduction of swift and slow, as in the question, which very pertinently offers it self, and would say thus much. figures are not the cause of moving or not moving simply upwards or downwards, but of moving more quickly or slowly: but if it be so, the cause is doubtfull, whence it proceeds, that a plate of lead or of iron broad and thin doth swim, &c. and the occasion of the doubt is obvious, because it seems at the first glance, that the figure is the cause of this natation, since the same lead, or a less quantity, but in another figure, goes to the bottom, and we have already affirmed, that the figure hath no share in this effect. lastly, if the intent of _aristotle_ in this place had been to say, that figures, although not absolutely, are at least in some measure the cause of moving or not moving: i would have it considered, that he names no less the motion upwards, than the other downwards: and because in exemplifying it afterwards, he produceth no other experiments than of a plate of lead, and board of ebony, matters that of their own nature go to the bottom, but by vertue (as our adversaries say) of their figure, rest afloat; it is fit that they should produce some other experiment of those matters, which by their nature swims, but retained by their figure rest at the bottom. but since this is impossible to be done, we conclude, that _aristotle_ in this place, hath not attributed any action to the figure of simply moving or not moving. but though he hath exquisitely philosophiz'd, in investigating the solution of the doubts he proposeth, yet will i not undertake to maintain, rather various difficulties, that present themselves unto me, give me occasion of suspecting that he hath not entirely displaid unto us, the true cause of the present conclusion: which difficulties i will propound one by one, ready to change opinion, whenever i am shewed, that the truth is different from what i say; to the confession whereof i am much more inclinable than to contradiction. [sidenote: _aristotle_ erred in affirming a needle dimitted long wayes to sink.] _aristotle_ having propounded the question, whence it proceeds, that broad plates of iron or lead, float or swim; he addeth (as it were strengthening the occasion of doubting) forasmuch as other things, less, and less grave, be they round or long, as for instance a needle go to the bottom. now i here doubt, or rather am certain that a needle put lightly upon the water, rests afloat, no less than the thin plates of iron or lead. i cannot believe, albeit it hath been told me, that some to defend _aristotle_ should say, that he intends a needle demitted not longwayes but endwayes, and with the point downwards; nevertheless, not to leave them so much as this, though very weak refuge, and which in my judgement _aristotle_ himself would refuse, i say it ought to be understood, that the needle must be demitted, according to the dimension named by _aristotle_, which is the length: because, if any other dimension than that which is named, might or ought to be taken, i would say, that even the plates of iron and lead, sink to the bottom, if they be put into the water edgewayes and not flatwayes. but because _aristotle_ saith, broad figures go not to the bottom, it is to be understood, being demitted broadwayes: and, therefore, when he saith, long figures as a needle, albeit light, rest not afloat, it ought to be understood of them when demitted longwayes. _moreover, to say that_ aristotle _is to be understood of the needle demitted with the point downwards, is to father upon him a great impertinency; for in this place he saith, that little particles of lead or iron, if they be round or long as a needle, do sink to the bottome; so that by his opinion, a particle or small grain of iron cannot swim: and if he thus believed, what a great folly would it be to subjoyn, that neither would a needle demitted endwayes swim? and what other is such a needle, but many such like graines accumulated one upon another? it was too unworthy of such a man to say, that one single grain of iron could not swim, and that neither can it swim, though you put a hundred more upon it._ lastly, either _aristotle_ believed, that a needle demitted longwayes upon the water, would swim, or he believed that it would not swim: if he believed it would not swim, he might well speak as indeed he did; but if he believed and knew that it would float, why, together with the dubious problem of the natation of broad figures, though of ponderous matter, hath he not also introduced the question; whence it proceeds, that even long and slender figures, howbeit of iron or lead do swim? and the rather, for that the occasion of doubting seems greater in long and narrow figures, than in broad and thin, as from _aristotles_ not having doubted of it, is manifested. no lesser an inconvenience would they fasten upon _aristotle_, who in his defence should say, that he means a needle pretty thick, and not a small one; for take it for granted to be intended of a small one; and it shall suffice to reply, that he believed that it would swim; and i will again charge him with having avoided a more wonderfull and intricate probleme, and introduced the more facile and less wonderfull. we say freely therefore, that _aristotle_ did hold, that only the broad figure did swim, but the long and slender, such as a needle, not. the which nevertheless is false, as it is also false in round bodies: because, as from what hath been predemonstrated, may be gathered, little balls of lead and iron, do in like manner swim. [sidenote: _aristotle_ affirmeth some bodies volatile for their minuity, text. .] he proposeth likewise another conclusion, which likewise seems different from the truth, and it is, that some things, by reason of their littleness fly in the air, as the small dust of the earth, and the thin leaves of beaten gold: but in my opinion, experience shews us, that that happens not only in the air, but also in the water, in which do descend, even those particles or atomes of earth, that disturbe it, whose minuity is such, that they are not deservable, save only when they are many hundreds together. therefore, the dust of the earth, and beaten gold, do not any way sustain themselves in the air, but descend downwards, and only fly to and again in the same, when strong windes raise them, or other agitations of the air commove them: and this also happens in the commotion of the water, which raiseth its sand from the bottom, and makes it muddy. but _aristotle_ cannot mean this impediment of the commotion, of which he makes no mention, nor names other than the lightness of such minutiæ or atomes, and the resistance of the crassitudes of the water and air, by which we see, that he speakes of a calme, and not disturbed and agitated air: but in that case, neither gold nor earth, be they never so small, are sustained, but speedily descend. [sidenote: _democritus_ placed the cause of natation in certain fiery atomes.] he passeth next to confute _democritus_[ ], which, by his testimony would have it, that some fiery atomes, which continually ascend through the water, do spring upwards, and sustain those grave bodies, which are very broad, and that the narrow descend to the bottom, for that but a small quantity of those atomes, encounter and resist them. [ ] _aristot. de cælo_ lib. . cap. . text. . i say, _aristotle_ confutes this position[ ], saying, that that should much more occurre in the air, as the same _democritus_ instances against himself, but after he had moved the objection, he slightly resolves it, with saying, that those corpuscles which ascend in the air, make not their _impetus_ conjunctly. here i will not say, that the reason alledged by _democritus_ is true[ ], but i will only say, it seems in my judgement, that it is not wholly confuted by _aristotle_, whilst he saith, that were it true, that the calid ascending atomes, should sustain bodies grave, but very broad, it would much more be done in the air, than in water, for that haply in the opinion of _aristotle_, the said calid atomes ascend with much greater force and velocity through the air, than through the water. and if this be so, as i verily believe it is, the objection of _aristotle_ in my judgement seems to give occasion of suspecting, that he may possibly be deceived in more than one particular: first, because those calid atomes, (whether they be fiery corpuscles, or whether they be exhalations, or in short, whatever other matter they be, that ascends upwards through the air) cannot be believed to mount faster through air, than through water: but rather on the contrary, they peradventure move more impetuously through the water, than through the air, as hath been in part demonstrated above. and here i cannot finde the reason, why _aristotle_ seeing, that the descending motion of the same moveable, is more swift in air, than in water, hath not advertised us, that from the contrary motion, the contrary should necessarily follow; to wit, that it is more swift in the water, than in the air: for since that the moveable which descendeth, moves swifter through the air, than through the water, if we should suppose its gravity gradually to diminish, it would first become such, that descending swiftly through the air, it would descend but slowly through the water: and then again, it might be such, that descending in the air, it should ascend in the water: and being made yet less grave, it shall ascend swiftly through the water, and yet descend likewise through the air: and in short, before it can begin to ascend, though but slowly through the air, it shall ascend swiftly through the water: how then is it true, that ascending moveables move swifter through the air, than through the water? [ ] _democritus_ confuted by _aristotle_, text . [ ] _aristotles_ confutation of _democritus_ refuted by the author. that which hath made _aristotle_ believe, the motion of ascent to be swifter in air, than in water, was first, the having referred the causes of slow and quick, as well in the motion of ascent, as of descent, only to the diversity of the figures of the moveable, and to the more or less resistance of the greater or lesser crassitude, or rarity of the _medium_; not regarding the comparison of the excesses of the gravities of the moveables, and of the _mediums_: the which notwithstanding, is the most principal point in this affair: for if the augmentation and diminution of the tardity or velocity, should have only respect to the density or rarity of the _medium_, every body that descends in air, would descend in water: because whatever difference is found between the crassitude of the water, and that of the air, may well be found between the velocity of the same moveable in the air, and some other velocity: and this should be its proper velocity in the water, which is absolutely false. the other occasion is, that he did believe, that like as there is a positive and intrinsecall quality, whereby elementary bodies have a propension of moving towards the centre of the earth, so there is another likewise intrinsecall[ ], whereby some of those bodies have an _impetus_ of flying the centre, and moving upwards: by vertue of which intrinsecall principle, called by him levity, the moveables which have that same motion more easily penetrate the more subtle _medium_, than the more dense: but such a proposition appears likewise uncertain, as i have above hinted in part, and as with reasons and experiments, i could demonstrate, did not the present argument importune me, or could i dispatch it in few words. [ ] lib. . cap. . the objection therefore of _aristotle_ against _democritus_, whilst he saith, that if the fiery ascending atomes should sustain bodies grave, but of a distended figure, it would be more observable in the air than in the water, because such corpuscles move swifter in that, than in this, is not good; yea the contrary would evene, for that they ascend more slowly through the air: and, besides their moving slowly, they ascend, not united together, as in the water, but discontinue, and, as we say, scatter: and, therefore, as _democritus_ well replyes, resolving the instance they make not their push or _impetus_ conjunctly. _aristotle_, in the second place, deceives himself, whilst he will have the said grave bodies to be more easily sustained by the said fiery ascending atomes in the air than in the water: not observing, that the said bodies are much more grave in that, than in this, and that such a body weighs ten pounds in the air, which will not in the water weigh / an ounce; how can it then be more easily sustained in the air, than in the water? [sidenote: _democritus_ confuted by the authour.] let us conclude, therefore, that _democritus_ hath in this particular better philosophated than _aristotle_. but yet will not i affirm, that _democritus_ hath reason'd rightly, but i rather say, that there is a manifest experiment that overthrows his reason, and this it is, that if it were true, that calid ascending atomes should uphold a body, that if they did not hinder, would go to the bottom, it would follow, that we may find a matter very little superiour in gravity to the water, the which being reduced into a ball, or other contracted figure, should go to the bottom, as encountring but few fiery atomes; and which being distended afterwards into a dilated and thin plate, should come to be thrust upwards by the impulsion of a great multitude of those corpuscles, and at last carried to the very surface of the water: which wee see not to happen; experience shewing us, that a body _v. gra._ of a sphericall figure, which very hardly, and with very great leasure goeth to the bottom, will rest there, and will also descend thither, being reduced into whatsoever other distended figure. we must needs say then, either that in the water, there are no such ascending fiery atoms, or if that such there be, that they are not able to raise and lift up any plate of a matter, that without them would go to the bottom: of which two positions, i esteem the second to be true, understanding it of water, constituted in its naturall coldness. but if we take a vessel of glass, or brass, or any other hard matter, full of cold water, within which is put a solid of a flat or concave figure, but that in gravity exceeds the water so little, that it goes slowly to the bottom; i say, that putting some burning coals under the said vessel, as soon as the new fiery atomes shall have penetrated the substance of the vessel, they shall without doubt, ascend through that of the water, and thrusting against the foresaid solid, they shall drive it to the superficies, and there detain it, as long as the incursions of the said corpuscles shall last, which ceasing after the removall of the fire, the solid being abandoned by its supporters, shall return to the bottom. but _democritus_ notes, that this cause only takes place when we treat of raising and sustaining of plates of matters, but very little heavier than the water, or extreamly thin: but in matters very grave, and of some thickness, as plates of lead or other mettal, that same effect wholly ceaseth: in testimony of which, let's observe that such plates, being raised by the fiery atomes, ascend through all the depth of the water, and stop at the confines of the air, still staying under water: but the plates of the opponents stay not, but only when they have their upper superficies dry, nor is there any means to be used, that when they are within the water, they may not sink to the bottom. the cause, therefore, of the supernatation of the things of which _democritus_ speaks is one, and that of the supernatation of the things of which we speak is another. but, returning to _aristotle_[ ], methinks that he hath more weakly confuted _democritus_, than _democritus_ himself hath done: for _aristotle_ having propounded the objection which he maketh against him, and opposed him with saying, that if the calid ascendent corpuscles were those that raised the thin plate, much more then would such a solid be raised and born upwards through the air, it sheweth that the desire in _aristotle_ to detect _democritus_, was predominate over the exquisiteness of solid philosophizing: which desire of his he hath discovered in other occasions, and that we may not digress too far from this place, in the text precedent to this chapter which we have in hand[ ]; where he attempts to confute the same _democritus_ for that he, not contenting himself with names only, had essayed more particularly to declare what things gravity and levity were; that is, the causes of descending and ascending, (and had introduced repletion and vacuity) ascribing this to fire, by which it moves upwards, and that to the earth, by which it descends; afterwards attributing to the air more of fire, and to the water more of earth. but _aristotle_ desiring a positive cause, even of ascending motion, and not as _plato_, or these others, a simple negation, or privation, such as vacuity would be in reference to repletion[ ], argueth against _democritus_ and saith: if it be true, as you suppose, then there shall be a great mass of water, which shall have more of fire, than a small mass of air, and a great mass of air, which shall have more of earth than a little mass of water, whereby it would ensue, that a great mass of air, should come more swiftly downwards, than a little quantity of water: but that is never in any case soever: therefore _democritus_ discourseth erroneously. [ ] _aristotle_ shews his desire of finding _democritus_ in an error, to exceed that of discovering truth. [ ] cap. . text . [ ] id. ibid. but in my opinion, the doctrine of _democritus_ is not by this allegation overthrown, but if i erre not, the manner of _aristotle_ deduction either concludes not, or if it do conclude any thing, it may with equall force be restored against himself. _democritus_ will grant to _aristotle_, that there may be a great mass of air taken, which contains more earth, than a small quantity of water, but yet will deny, that such a mass of air, shall go faster downwards than a little water, and that for many reasons. first, because if the greater quantity of earth, contained in the great mass of air, ought to cause a greater velocity than a less quantity of earth, contained in a little quantity of water, it would be necessary, first, that it were true, that a greater mass of pure earth, should move more swiftly than a less: but this is false, though _aristotle_ in many places affirms it to be true: because not the greater absolute, but the greater specificall gravity, is the cause of greater velocity[ ]: nor doth a ball of wood, weighing ten pounds, descend more swiftly than one weighing ten ounces, and that is of the same matter: but indeed a bullet of lead of four ounces, descendeth more swiftly than a ball of wood of twenty pounds: because the lead is more _grave in specie_ than the wood. therefore, its not necessary, that a great mass of air, by reason of the much earth contained in it, do descend more swiftly than a little mass of water[ ], but on the contrary, any whatsoever mass of water, shall move more swiftly than any other of air, by reason the participation of the terrene parts _in specie_ is greater in the water, than in the air. let us note, in the second place, how that in multiplying the mass of the air, we not only multiply that which is therein of terrene, but its fire also: whence the cause of ascending, no less encreaseth, by vertue of the fire, than that of descending on the account of its multiplied earth. it was requisite in increasing the greatness of the air, to multiply that which it hath of terrene only, leaving its fire in its first state, for then the terrene parts of the augmented air, overcoming the terrene parts of the small quantity of water, it might with more probability have been pretended, that the great quantity of air, ought to descend with a greater _impetus_, than the little quantity of water. [ ] the greater specificall, not the greater absolute gravity, is the cause of velocity. [ ] any mass of water shal move more swiftly, than any of air, and why. therefore, the fallacy lyes more in the discourse of _aristotle_, than in that of _democritus_, who with severall other reasons might oppose _aristotle_, and alledge; if it be true, that the extreame elements be one simply grave, and the other simply light, and that the mean elements participate of the one, and of the other nature; but the air more of levity, and the water more of gravity, then there shall be a great mass of air, whose gravity shall exceed the gravity of a little quantity of water, and therefore such a mass of air shall descend more swiftly than that little water: but that is never seen to occurr: therefore its not true, that the mean elements do participate of the one, and the other quality. this argument is fallacious, no less than the other against _democritus_. lastly, _aristotle_ having said, that if the position of _democritus_ were true, it would follow, that a great mass of air should move more swiftly than a small mass of water, and afterwards subjoyned, that that is never seen in any case: methinks others may become desirous to know of him in what place this should evene, which he deduceth against _democritus_, and what experiment teacheth us, that it never falls out so. to suppose to see it in the element of water, or in that of the air is vain, because neither doth water through water, nor air through air move, nor would they ever by any whatever participation others assign them, of earth or of fire: the earth, in that it is not a body fluid, and yielding to the mobility of other bodies, is a most improper place and _medium_ for such an experiment: _vacuum_, according to the same _aristotle_ himself, there is none, and were there, nothing would move in it: there remains the region of fire, but being so far distant from us, what experiment can assure us, or hath assertained _aristotle_ in such sort, that he should as of a thing most obvious to sence, affirm what he produceth in confutation of _democritus_, to wit, that a great mass of air, is moved no swifter than a little one of water? but i will dwell no longer upon this matter, whereon i have spoke sufficiently: but leaving _democritus_, i return to the text of _aristotle_, wherein he goes about to render the true reason, how it comes to pass, that the thin plates of iron or lead do swim on the water; and, moreover, that gold it self being beaten into thin leaves, not only swims in water, but flyeth too and again in the air. he supposeth that of continualls[ ], some are easily divisible, others not: and that of the easily divisible, some are more so, and some less: and these he affirms we should esteem the causes. he addes that that is easily divisible, which is well terminated, and the more the more divisible, and that the air is more so, than the water, and the water than the earth. and, lastly, supposeth that in each kind, the lesse quantity is easlyer divided and broken than the greater. [ ] _de cælo_ l. . c. . t. . here i note, that the conclusions of _aristotle_ in generall are all true, but methinks, that he applyeth them to particulars, in which they have no place, as indeed they have in others, as for example, wax is more easily divisible than lead, and lead than silver, inasmuch as wax receives all the terms more easlier than lead, and lead than silver. its true, moreover, that a little quantity of silver is easlier divided than a great mass: and all these propositions are true, because true it is, that in silver, lead and wax, there is simply a resistance against division, and where there is the absolute, there is also the respective. but if as well in water as in air, there be no renitence against simple division, how can we say, that the water is easlier divided than the air? we know not how to extricate our selves from the equivocation: whereupon i return to answer, that resistance of absolute division is one thing, and resistance of division made with such and such velocity is another. but to produce rest, and to abate the motion, the resistance of absolute division is necessary; and the resistance of speedy division, causeth not rest, but slowness of motion. but that as well in the air, as in water, there is no resistance of simple division, is manifest, for that there is not found any solid body which divides not the air, and also the water: and that beaten gold, or small dust, are not able to superate the resistance of the air, is contrary to that which experience shews us, for we see gold and dust to go waving to and again in the air, and at last to descend downwards, and to do the same in the water, if it be put therein, and separated from the air. and, because, as i say, neither the water, nor the air do resist simple division, it cannot be said, that the water resists more than the air. nor let any object unto me, the example of most light bodies, as a feather, or a little of the pith of elder, or water-reed that divides the air and not the water, and from this infer, that the air is easlier divisible than the water; for i say unto them, that if they do well observe, they shall see the same body likewise divide the continuity of the water[ ], and submerge in part, and in such a part, as that so much water in mass would weigh as much as the whole solid. and if they shal yet persist in their doubt, that such a solid sinks not through inability to divide the water, i will return them this reply, that if they put it under water, and then let it go, they shall see it divide the water, and presently ascend with no less celerity, than that with which it divided the air in descending: so that to say that this solid ascends in the air, but that coming to the water, it ceaseth its motion, and therefore the water is more difficult to be divided, concludes nothing: for i, on the contrary, will propose them a piece of wood, or of wax, which riseth from the bottom of the water, and easily divides its resistance, which afterwards being arrived at the air, stayeth there, and hardly toucheth it; whence i may aswell say, that the water is more easier divided than the air. [ ] _archimed. de insident. humi_ lib. . prop. . i will not on this occasion forbear to give warning of another fallacy of these persons, who attribute the reason of sinking or swimming to the greater or lesse resistance of the crassitude of the water against division, making use of the example of an egg, which in sweet water goeth to the bottom, but in salt water swims; and alledging for the cause thereof, the faint resistance of fresh water against division, and the strong resistance of salt water. but if i mistake not, from the same experiment, we may aswell deduce the quite contrary; namely, that the fresh water is more dense, and the salt more tenuous and subtle, since an egg from the bottom of salt water speedily ascends to the top, and divides its resistance, which it cannot do in the fresh, in whose bottom it stays, being unable to rise upwards. into such like perplexities, do false principles lead men: but he that rightly philosophating, shall acknowledge the excesses of the gravities of the moveables and of the mediums, to be the causes of those effects, will say, that the egg sinks to the bottom in fresh water, for that it is more grave than it, and swimeth in the salt, for that its less grave than it: and shall without any absurdity, very solidly establish his conclusions. therefore the reason totally ceaseth, that _aristotle_ subjoyns in the text saying[ ]; the things, therefore, which have great breadth remain above, because they comprehend much, and that which is greater, is not easily divided. such discoursing ceaseth, i say, because its not true, that there is in water or in air any resistance of division; besides that the plate of lead when it stays, hath already divided and penetrated the crassitude of the water, and profounded it self ten or twelve times more than its own thickness: besides that such resistance of division, were it supposed to be in the water, could not rationally be affirmed to be more in its superiour parts than in the middle, and lower: but if there were any difference, the inferiour should be the more dense, so that the plate would be no less unable to penetrate the lower, than the superiour parts of the water; nevertheless we see that no sooner do we wet the superiour superficies of the board or thin piece of wood, but it precipitatly, and without any retension, descends to the bottom. [ ] text . i believe not after all this, that any (thinking perhaps thereby to defend _aristotle_) will say, that it being true, that the much water resists more than the little, the said board being put lower descendeth, because there remaineth a less mass of water to be divided by it: because if after the having seen the same board swim in four inches of water, and also after that in the same to sink, he shall try the same experiment upon a profundity of ten or twenty fathom water, he shall see the very self same effect. and here i will take occasion to remember, for the removall of an error that is too common; that that ship or other whatsoever body, that on the depth of an hundred or a thousand fathom, swims with submerging only six fathom of its own height, [_or in the sea dialect, that draws six fathom water_] shall swim in the same manner in water, that hath but six fathom and half an inch of depth[ ]. nor do i on the other side, think that it can be said, that the superiour parts of the water are the more dense, although a most grave authour hath esteemed the superiour water in the sea to be so, grounding his opinion upon its being more salt, than that at the bottom: but i doubt the experiment, whether hitherto in taking the water from the bottom, the observatour did not light upon some spring of fresh water there spouting up: but we plainly see on the contrary, the fresh waters of rivers to dilate themselves for some miles beyond their place of meeting with the salt water of the sea, without descending in it, or mixing with it, unless by the intervention of some commotion or turbulency of the windes. [ ] a ship that in fathome water draweth fathome, shall float in fathome and / an inch of depth. but returning to _aristotle_, i say, that the breadth of figure hath nothing to do in this business more or less, because the said plate of lead, or other matter, cut into long slices, swim neither more nor less[ ]; and the same shall the slices do, being cut anew into little pieces, because its not the breadth but the thickness that operates in this business. i say farther, that in case it were really true, that the renitence to division were the proper cause of swimming[ ], the figures more narrow and short, would much better swim than the more spacious and broad, so that augmenting the breadth of the figure, the facility of supernatation will be deminished, and decreasing, that this will encrease. [ ] thickness not breadth of figure to be respected in natation. [ ] were renitence the cause of natation, breadth of figure would hinder the swiming of bodies. and for declaration of what i say, consider that when a thin plate of lead descends, dividing the water, the division and discontinuation is made between the parts of the water, invironing the perimeter or circumference of the said plate, and according to the bigness greater or lesser of that circuit, it hath to divide a greater or lesser quantity of water, so that if the circuit, suppose of a board, be ten feet in sinking it flatways, it is to make the seperation and division, and to so speak, an incission upon ten feet of water; and likewise a lesser board that is four feet in perimeter, must make an incession of four feet. this granted, he that hath any knowledge in geometry, will comprehend, not only that a board sawed in many long thin pieces, will much better float than when it was entire, but that all figures, the more short and narrow they be, shall so much the better swim. let the board a b c d be, for example, eight palmes long, and five broad, its circuit shall be twenty six palmes; and so many must the incession be, which it shall make in the water to descend therein: but if we do saw ir, as suppose into eight little pieces, according to the lines e f, g h, {&}c. making seven segments, we must adde to the twenty six palmes of the circuit of the whole board, seventy others; whereupon the eight little pieces so cut and seperated, have to cut ninty six palmes of water. and, if moreover, we cut each of the said pieces into five parts, reducing them into squares, to the circuit of ninty six palmes, with four cuts of eight palmes apiece; we shall adde also sixty four palmes, whereupon the said squares to descend in the water, must divide one hundred and sixty palmes of water, but the resistance is much greater than that of twenty six; therefore to the lesser superficies, we shall reduce them, so much the more easily will they float: and the same will happen in all other figures, whose superficies are simular amongst themselves, but different in bigness: because the said superficies, being either deminished or encreased, always diminish or encrease their perimeters in subduple proportion; to wit, the resistance that they find in penetrating the water; therefore the little pieces gradually swim, with more and more facility as their breadth is lessened. [illustration] _this is manifest; for keeping still the same height of the solid, with the same proportion as the base encreaseth or deminisheth, doth the said solid also encrease or diminish; whereupon the solid more diminishing than the circuit, the cause of submersion more diminisheth than the cause of natation: and on the contrary, the solid more encreasing than the circuit, the cause of submersion encreaseth more, that of natation less._ and this may all be deduced out of the doctrine of _aristotle_ against his own doctrine. lastly, to that which we read in the latter part of the text[ ], that is to say, that we must compare the gravity of the moveable with the resistance of the medium against division, because if the force of the gravity exceed the resistance of the _medium_, the moveable will descend, if not it will float. i need not make any other answer, but that which hath been already delivered; namely, that its not the resistance of absolute division, (which neither is in water nor air) but the gravity of the _medium_ that must be compared with the gravity of the moveables; and if that of the _medium_ be greater, the moveable shall not descend, nor so much as make a totall submersion, but a partiall only; because in the place which it would occupy in the water, there must not remain a body that weighs less than a like quantity of water: but if the moveable be more grave, it shall descend to the bottom, and possess a place where it is more conformable for it to remain, than another body that is less grave. and this is the only true proper and absolute cause of natation and submersion, so that nothing else hath part therein: and the board of the adversaries swimmeth, when it is conjoyned with as much air, as, together with it, doth form a body less grave than so much water as would fill the place that the said compound occupyes in the water; but when they shall demit the simple ebony into the water, according to the tenour of our question, it shall alwayes go to the bottom, though it were as thin as a paper. [ ] lib. . c. . text . [decoration] finis. [decoration] * * * * * [detailed transcriber's notes the text has been made to match the original text as much as possible including variation in spelling, punctuation, italics etc. the following, details apparent printer's errors as well as changes or additions to aid readability of text. page , missing full stop after abbreviation gr. ' gr min.'. page , sidenote, missing space between words 'the authority ofan author.'. page , printer's error, augmentarion for augmentation 'and augmentarion of masse'. page , missing letter t 'tha{t} that proceeded not'. page , printer's error or inconsistent punctuation, 'my paynes and time. and although'. page , inconsistent punctuation, full stop after axiome where as there are none after those following 'axiome. i.'. page , missing full stop added to end of paragraph 'or else an upright prisme.'. page , printer's error, missing letter c in illustration, 'the prisme a c d b to be placed'. page , printer's error or archaic lettering, final y looking like a in original text'and of the specifick gravit{y}'. page , printer's error, letter n for t in text to refer to illustration, 'if the vessell e n s f'. page , printer's error, duplicate word in text 'equalizeth the force and and moment,'. page , printer's error, rhe for the 'as in rhe stilliard,'. page , missing space between words 'asoften as that'. page , sidenote, printer's error, specifiaclly for specifically 'a solid specifiaclly graver'. page , potential printer's error, properly for property, 'but this properly they have'. page , printer's error, n for u 'loseth all a{u}thenticalness'. page , printer's error or variation in spelling, benonamico for buonamico 'it seemes that benonamico'. page , printer's error, missing i 'accordng to its excess'. page , missing line at the end of page in original text 'its region it loseth all'. page , missing letter n 'u{n}able by its small weight'. page , missing letter e 'that i have gon{e} about'. page , unclear symbol in original text 'other figure, {&}c.'. page , potential printer's error, comma in unusual position 'whatever figure, goeth always'. page , missing space between words 'superficies might bedry:'. page , missing letter t, unied for united 'which holds them unied'. page , printer's error, motitions for motions 'all motitions are made'. page , sidenote, possible missing letter e, 'se{e} what satisfaction'. page , printer's error, subdidivisions for subdivisions, 'other subdidivisions,'. page , missing letter i, dminishing for diminishing 'or dminishing it by dividing'. page , sidenote, printer's error, missing letter l, hep for help 'float by hep of'. page , printer's error, missing letter n, beig for being 'beig double in gravity'. page , printer's error, missing letter l, 'sha{l}l also descend.'. page , printer's error, missing letter r, 'to fo{r}m solid bodies'. page , printer's error, missing letter a, 'cone s {a} t'. page , printer's error, missing letter t, 'of which i{t} may descend'. page , sidenote, printer's error, inverted n, 'natatio{n} easiest effected'. page , missing letter u, 'witho{u}t all question,'. page , printer's error, inverted n, 'with an other experime{n}t'. page , potential printer's error, sidenote ends with comma, 'or rather contiguity,'. page , missing letter a, 'that in this c{a}se the water,'. page , printer's error, ir for it, 'but if we do saw ir,'. page , unclear symbol in original text '{&}c. making seven segments'. ] reading the weather [illustration: shower behind valley forge _courtesy of richard f. warren_] reading the weather by t. morris longstreth illustrated with photographs by richard f. warren outing handbooks number new york outing publishing company mcmxv copyright, , by outing publishing company all rights reserved. dedicated with love, to my grandmother mary gibson haldeman herself responsible for so much sunshine. contents chapter page forecast i i our well-ordered atmosphere ii the clear day iii the storm cycle iv sky signs for campers the clouds the winds temperatures rain and snow dew and frost the thunderstorm exposed the tornado the hurricane the cloudburst the halo v the barometer vi the seasons vii the weather bureau viii a chapter of explosions condensations signs of fair weather signs of coming storm signs of clearing when will it rain? signs of temperature change some unsolved weather problems what the weather flags mean our four world's records,--and others illustrations shower behind valley forge _frontispiece_ page cirrus deepening to cirro-stratus cirro stratus with cirro-cumulus beneath cirro-cumulus to alto-stratus alto-stratus cumulus stratus nimbus forecast science is certainly coming into her own nowadays,--and into everybody else's. every activity of man and most of nature's have felt her quickening hand. her eye is upon the rest. drinking is going out because the drinker is inefficient. the fly is going out because he carries germs. and for everything that goes out something else comes in that makes people healthier and more comfortable, and, perhaps, wiser. one strange thing about this flood-tide of science is that it overwhelms the old, buttressed superstitions the easiest of all, once it really sets about it. for instance, nothing could have been better fortified for centuries than the fact that night air is injurious and should be shut out of house. then, science turned its eye upon night air, found it a little cooler, a trifle moister, and somewhat cleaner than day air with the result that we all invite it indoors, now, and even go out to meet it. once interested in the air, science soon began to take up that commonplace but baffling phase of it called the weather. now, of all matters under the sun the weather was the deepest intrenched in superstition and hearsay. from the era of noah it had been made the subject of more remarks unrelieved by common sense than any other. it was at once the commonest topic for conversation and the rarest for thought. considering the opportunities for study of the weather this conclusion, we must admit, is more surprising than complimentary to the human race. but it is so. the fact that science had to face was this: that the weather had been and remained a tremendous, dimly-recognized factor in our level of living. so talk about it all must. and science set about finding some easy fundamental truths to talk instead of the hereditary gossip about old-fashioned winters or the usual meaningless conversational coin. two groups of men had always known a good deal about the weather from experience: the sailor had to know it to save his life, and the farmer had to cultivate a weather eye along with his early peas. but the ordinary business man (and wife), the town-dweller, and even the suburbanite knew so few of the proven facts that the weather from day to day, from hour to hour, was a continual puzzle to them. the rain not only fell upon the just and unjust but it fell unquestioned, or misunderstood. at last science established some sort of a weather bureau in , in our country, and after this had triumphed over great handicaps, the government set it upon its present footing in . an intelligent interest in the weather was in likelihood of being aroused by maps, pamphlets, frost and flood warnings that saved dollars and lives. then suddenly, or almost suddenly, a new force was felt in every community. it was the call of outdoors. the new land of woods and lakes was explored. men learned that living by bread alone (without air) made a very stuffy existence. hence the man in town opened all his windows at night, the suburban majority planned to build sleeping porches, the youngsters begged to go to camp, their fathers went hunting and fishing in increasing numbers, and, most important of all, the fathers' wives began to accompany them into the woods. thus, living has been turned inside out,--the very state of things that old scientist plato recommended some thirty thousand moons ago. and among the manifestations of nature the weather is holding its place, important and even fascinating. for the person who most depends on umbrellas and the subway in the city needs to watch the sky most carefully in the woods. that old academic question as to whether it be wise or foolish to come in out of the wet was never settled by the wilderness veteran. the veteran's wife settles it very quickly. she considers the cloud. when the commuter goes camping he rightly likes his comforts. a wet skin is not one of these. therefore he studies the feel of the wind. and so it comes about that the person who talks about storm centers and areas of high pressure and cumulus clouds is no longer regarded as slightly unhinged. men are eager to learn the laws of the snowstorm and the cold wave; for, with the knowledge that snow is not poison and cold not necessarily discomfort, january has been opened up for enjoyments that july could never give. bookwriting and camping are both explained by the same fact,--a certain fondness for the thing. i wanted to see the commoner weather pinned down to facts. the following chapters resulted. they constitute a sort of overhead baedeker, it being their pleasure to show up the sureties of the sun and rain and to star the weather signs that can be relied upon. for, after all, even the elements, although unruled, are law-abiding. reading the weather chapter i our well-ordered atmosphere if there is anything that has been overlooked more than another it is our atmosphere. but it absolutely cannot be avoided--in books on the weather. it deserves a chapter, anyway, because if it were not for the atmosphere this earth of ours would be a wizened and sterile lump. it would float uselessly about in the general cosmos like the moon. to be sure the earth does not loom very large in the eye of the sun. it receives a positively trifling fraction of the total output of sunheat. so negligible is this amount that it would not be worth our mentioning if we did not owe our existence to it. it is thanks to the atmosphere, however, that the earth attains this (borrowed) importance. it is thanks to this thin layer of gases that we are protected from that fraction of sunheat which, however trifling when compared with the whole, would otherwise be sufficient to fry us all in a second. without this gas wrapping we would all freeze (if still unfried) immediately after sunset. the atmosphere keeps us in a sort of thermos globe, unmindful of the burning power of the great star, and of the uncalculated cold of outer space. yet, limitless as it seems to us and inexhaustible, our invaluable atmosphere is a small thing after all. half of its total bulk is compressed into the first three and a half miles upward. only one sixty-fourth of it lies above the twenty-one mile limit. compared with the thickness of the earth this makes a very thin envelope. light as air, we say, forgetting that this stuff that looks so thin and inconsequential weighs fifteen pounds to the square inch. we walk around carrying our fourteen tons gaily enough. the only reason that we don't grumble is because the gases press evenly in all directions permeating our tissues and thereby supporting this crushing burden. a layer of water thirty-four feet thick weighs just about as much as this air-pack under which we feel so buoyant. but if these gases get in motion we feel their pressure. we say the wind is strong to-day. as it blows along the surface of the earth this wind is mostly nitrogen, oxygen, moisture, and dust. the nitrogen occupies nearly eight-tenths of a given bulk of air, the oxygen two-tenths, and the moisture anything up to one-twentieth. five other gases are present in small quantities. the dust and the water vapor occupy space independently of the rest. as one goes up mountains the water vapor increases for a couple of thousand feet and then decreases to the seven mile limit after which it has almost completely vanished. the lightest gases have been detected as high up as two hundred miles and scientists think that hydrogen, the lightest of all, may escape altogether from the restraint of gravity. one strange fact about all of these gases is that they do not form a separate chemical combination, although they are thoroughly mixed. at first glance the extreme readiness of the atmosphere to carry dust and bacteria does not seem a point in its favor. in reality it is. most bacteria are really allies of the human race. they benefit us by producing fermentations and disintegrations of soils that prepare them for plant food. it is a pity that the few disease breeding types of bacteria should have given the family a bad name. without bacteria the sheltering atmosphere would have nothing but desert rock to protect. further, rain is accounted for only by the dust. of course this sounds very near the world's record in absurdities. but it is a half truth at least, for moisture cannot condense on nothing. every drop of rain, every globule of mist must have a nucleus. consequently each wind that blows, each volcano that erupts is laying up dust for a rainy day. apparently the atmosphere is empty. actually it is full enough of dust-nuclei to outfit a fullgrown fog if the dewpoint should be favorable. if there were no dust in the air all shadows would be intensest black, the sunlight blinding. but the dust particles fulfill their greatest mission as heat collectors,--they and the particles of water vapor which have embraced them. it is in reality owing to these water globules and not to the atmosphere that supports them that we are enabled to live in such comfortable temperatures. for the air strata above seven miles where the tides of oxygen and nitrogen have rid themselves of water and dust absorb very little of the solar radiation. the heat is grabbed by the lowest layer of air as it goes by. the air snatches it both going and coming. the little particles get about half of it on the way down and when it is radiated back very little escapes them. so it comes about that the heavy moist air near the earth is the warmest of all. it would, of course, get very warm if, as it collected its heat, it didn't have a tendency to rise. as it rises, moreover, it must fight gravity, that arch enemy of all rising things. and as it fights it loses energy, which is heat. so high altitudes and low temperatures are found together for these two reasons. but after the limit of moisture content has been reached the temperature gets no lower according to reliable investigations. instead a monotony of ° below zero eternally prevails-- ° is called the absolute zero of space. the vertical heating arrangements of the atmosphere appear somewhat irregular. but horizontally it is in a much worse way. the surface of the globe is three quarters water and one quarter land and irregularly arranged at that. the shiny water surfaces reflect a good deal of the heat which they receive, they use up the heat in evaporation and what they do absorb penetrates far. the land surfaces, on the contrary, absorb most of the heat received, but it does not penetrate to any depth. as a consequence of these differences land warms up about four times as quickly as water and cools off about four times as fast. therefore the temperature of air over continents is liable to much more rapid and extreme changes than the air over the oceans. the disparity of temperature is also rendered much greater because of differing areas of cloud and clear skies, because of interfering mountain masses, because of the change from day to night, or the constant progress of the seasons. at first blush it seems remarkable that the atmosphere should not be hopelessly unsettled in its habits, that there should belong to it any hint of system. as a matter of fact, in the main its courses are as well-ordered as the sun's. cause and not caprice are at the bottom of the wind's listings. its one desire is rest. [illustration: cirrus deepening to cirro-stratus _courtesy of richard f. warren_ cirrus clouds first appear as feathery lines converging toward one or two points on the horizon, often merging into bands of darker clouds, arranged horizontally. a sky like this appears when there is little wind. if the wind shifts to an easterly direction by way of north there will likely be snow within hours; if it works around by way of the southwest and south hours will probably pass before rain. if the mares' tails, as here, are absent and yet the stratified clouds are present there is little likelihood of a storm. cirrus clouds precede every disturbance of magnitude. sometimes they are hidden by a lower cloud layer.] but rest it rarely succeeds in finding. forever warming, rising, cooling, falling, it rushes about to regain its equilibrium. with so many opposing forces at work the calm day is the real marvel, our weeks of indian summer the ranking miracle of our climate. the very evolution of the myriad patches of air quilted over the earth with their different opportunities to become heated, to cool their heels, precludes stability in our so called temperate zone. but over great stretches of the earth's surface conditions are continuous enough to discipline the atmosphere into strict routine. conjure the globe before your eyes and you will find the scheme of atmospheric circulation something like this: a broad band of heated air perpetually rises from the sweltering equatorial belt of lands and seas. the supply never ceases, the warming process goes on night and day, and to a great height the light warm incense mounts. then, cooling, from this altitude it begins to run down hill toward the poles. this is happening all the way around the globe. so naturally the common centers, the poles, cannot accommodate all this downrush of air. therefore as it approaches the goal it falls into a majestic file about the center, very much as water does in running out of a hole in the center of a circular basin. the nearer north, the cooler this vast maelstrom grows and the nearer has it sunk to the earth. it descends circuitously and, by the force arising from the earth's rotation, is sheered to the right in the northern hemisphere, to the left in the southern. watching the water circle out of the basin you will notice the outside whirl is in no hurry to get to the center. this corresponds to the easterly trades of commerce, geography, and fiction. the direction of the upper currents flowing back to the poles is from southwest to northeast; but in our middle zones this becomes almost from west to east, is constant and is known to the profession as the prevailing westerlies. look up some day when wisps of clouds are floating very high. you will notice that their port is in the east, mattering not what wind may be blowing where you are. they are above the petty disturbances of the shallow surface winds. they follow a gulf stream of immeasurable grandeur. onward, always onward, they sail, emblems of a great serenity. beneath this vast drift of air, which increases in velocity as it nears the pole, is an undertow from two to three miles thick. it is the movements of this undertow that affect our lives. these movements are influenced by all the changes of temperature and by the configurations of land. they take the form of whirls. these whirls may be small eddies, local in effect, or vast cyclones with diameters of fifteen hundred miles. small or large they roll along under the westerlies, translated by friction, and invariably moving for most of their course in an easterly direction, like their tractor above. they circle across the united states every few days. their courses do not vary a great deal, and yet enough to make each one a matter for conjecture. and all the conjecturing centers upon the condition of the atmosphere,--the changing atmosphere which is yet so dependable. the weather we are used to, the daily weather that catches us unprepared, and yet that does not mistreat us all the time is the product of these little whirls, which are so remotely connected with the grander atmospheric movements of our planet. remembering this, we can at last come back to earth and set about our real business which is to see why certain kinds of weather come at such uncertain times and how to tell when they will arrive. chapter ii the clear day we owe our fair weather to that department of atmospheric activity called anticyclone by the weatherman. the anticyclone is an accumulation of air which has become colder than the air surrounding it. this accumulation oftener than not has an area near the center where the air is coldest. about this coldest area the air currents revolve in the direction of a clock's hands. and since this cold air is contracted and denser than its warmer environment it has a perpetual tendency to whirl outward from the center into this warmer environment. one comes to think, therefore, of the anticyclone as a huge pyramid of cold air moving slowly across the country from west to east and all the while melting down on all sides, like a plate of ice-cream, into the surrounding territory. it is such an immense accumulation that often while its head is reared over montana the first shivers of its approach are beginning to be felt in texas and pennsylvania. it does not extend equally far, however, to the north and west of its head, which is really sometimes where its tail ought to be. that is, a long slope of increasing pressure and cold will sweep in a gentle gradient from pennsylvania to montana and will then decrease by a very steep gradient to the pacific coast. the anticyclone draws its power from the inexhaustible supplies of cold air from the upper levels. this air is very dry and accounts for the almost invariably clear skies of the anticyclone. in winter when the intensity of all the atmospheric activities is greatly increased, the anticyclone develops into the cold wave. the rapidly rising pressure rears its head and rushes along upon the heels of a storm like a vast tidal wave at sixty miles an hour, tumbling the mercury thirty, forty, fifty degrees. these cold waves first appear in the northwest. they cannot well originate over either ocean and a high-pressure area building up over the southern half of the country will not attain the sufficient degree of frigidity to earn the title, for even cold waves have been standardized by the government. but although nearly all the cold waves choose montana or the dakotas as a base, they have at least two definite lines of action. those which are born amid the mountains or on the great plains of montana have a curious habit of bombarding the texas coast before starting on their eastward march. it is not unusual for us to read of zero weather in the panhandle and freezing on the gulf while the mercury may still be standing as high as fifty in new york city. it is this rapid onslaught from montana to texas that produces those notorious blizzards of that section called northers, during which the cattle used to be frozen on the hoof. the record time for a drive of this extent is about twelve hours and the normal about twenty-four which gives scant time for the weather bureau to warn the vast interests of the impending assault. when the cold wave, after following this path, does swing toward the atlantic coast, as most of them do, it has lost interest and usually produces only seasonably cold weather along the appalachians. those cold waves that recruit their strength in canada and enter the united states through minnesota or, rarely, this side of the lakes move along the border and supply intensely cold weather for a night or two to new england and the middle atlantic states. cold waves almost always follow a storm. the storm, being an area of low pressure makes a fit receptacle for the surplus of the high pressure, and since the whole business of the weather is to seek peace and pursue it, the greater the discrepancies the more violent the pursuit. consequently we have the spectacle of a ridge of cold dry air following and trying to level up a fleeing hollow of warm moist air--but rarely succeeding. this principle of action and reaction is almost the sole principle of the weather and is nowhere more clearly demonstrated than in the winter's succession of storm and cold wave. in summer the anticyclones are not only actually but relatively more moderate than in winter. but their influence is still the same,--clear skies, cooler nights, dry, westerly winds. during the year the anticyclone furnishes us with about sixty per cent. of our weather. the cyclone is responsible for the remaining forty per cent. the weather depends on the cyclone for its variety and upon the anticyclone for its reputation. so it is well to be able to recognize an anticyclone when one appears. the first and most reliable symptom of the approach of an anticyclone is the west wind. this sign is valid the country over, and is one of the very few signs that hold true for most of the north temperate zone. in summer over our country the west wind comes from the southwest, to be irish, and in winter from the northwest. but for nearly all of our forty-eight states for nearly all of the year the westerly winds are those that bring us fair days and nights. and it is these crisp, clear days and cloudless, brilliant nights which we have in mind when we boast to english friends of our american weather. the west wind is so popular because it has a slight downward flowing tendency. it also blows from land to sea over all america except the narrow pacific coast. these downward, outward directions allow it to gather only enough moisture to keep it from becoming seriously dry. its upper sources supply it with ozone. its density gives it weight and by its superior weight it prevails. it dries roads faster than a brace of suns could do it. it is tonic. and curiously enough, although the anticyclone loads half a ton excess weight upon us we like it. the greater the burden the more we feel like leaping and shouting. our good cheer seems to be ground out of us, like street pianos. the reverse holds, too. for when the anticyclone moves off us and the cyclone hovers over us, removing half a ton of pressure, instead of feeling relieved we feel depressed, out of spirit. the animals share this reaction with us. in fact barnyards antedated barometers as forecasters, because all the domestic creatures, with pigs in particular, evidenced the disagreeable leniency of the low pressure areas upon their persons. "grumphie smells the weather an' grumphie smells the wun' he kens when clouds will gather an' smoor the blinkin' sun." the only trouble about this rather extravagant tribute to the pig, versatile though he is, is that he can tell only a very few hours ahead about the coming changes and it takes so much more skill to judge what his actions mean than to read the face of the sky that the science of meteorology finally comes to supplant barnyardology. the coming of the anticyclone is foretold by the shifting of the wind from any quarter to the west. the course that the center of the anticyclone is keeping may be watched by the same agency. since the circulation from the cone of cold air follows the hour hands of a clock it follows that if the center is moving north of you the wind, blowing outward from the center, will work from west to northwest and from northwest to north and slightly east of north. if the wind has shifted into the west on a wednesday, it will likely be cold by wednesday night and colder on thursday. by friday morning the wind will be coming from the north, likely, with the lowest temperature of all. by saturday the cold will moderate, the wind will tire and gradually die to a calm or become weakly variable. the four day supremacy of the anticyclone will be over. but, mind you, there are a dozen variations of this routine. i am only suggesting a usual one. if after blowing two or three days from the west the wind shifts to the southwest and south, you may know that the central cold area is passing south of you and that its intensity will not be great. while these anticyclones that float down and to the right of their normal path linger longer, they are never so severely cold, nor, alas, so uniformly clear as the others. it is a profound law of anticyclones and even more particularly of cyclones, that if they deviate to the right they weaken, if they are pushed by an obstacle to the left they increase greatly in intensity. occasionally the central portion of an anticyclone passes over your locality. then the wind will fall. the frost will be keen and the cold will be notably dry and invigorating. in summer although the sunlight may be powerfully bright and the heat great, yet the air will have a buoyant effect, the body a resilience. and the nights will cool swiftly. soon after the center passes from the locality a wind will spring up from the east with rapidly rising temperature and increased humidity. the coldest part of the anticyclone is not, as one would suppose, at the center, but in advance of it; and its authority, like a schoolmaster's, is rapidly dissipated after its back is turned upon a place. the intensity of an anticyclone is measured by its wind velocity and by the degree of cold obtaining under its influence. but the greatest cold occurs rarely in conjunction with the greatest velocity of the wind. the calms that occur at sunrise enable radiation to take an extra spurt which pushes the mercury lower by a degree or so than happens when the wind is blowing. but, windy or calm, the period about sunrise is normally the coldest of the day, even extending in midwinter for as much as half an hour after sunrise, so slow are the feeble rays at restoring the balance of loss and gain of heat. the greatest falls occur at the advent of the cold wave, no matter whether it arrives at ten in the morning or at midnight. if the temperature starts to decline gradually during the day, a further and decided fall may be expected at nightfall if the sky is clear. and if the temperature rises gradually during the night the normal processes are being displaced and a change from fair to foul is a surety. in summer the hottest time of day is not at noon, any more than the coldest part of the winter day was at midnight, for the reason that the sun can pour in its heat faster than the earth can radiate it, and the hour for the maximum temperature is pushed as far along toward evening as four or five or even six o'clock. the average anticyclone continues its influence for clearness for about four days. some, however, hurry the whole thing through in two. others are interrupted by a more vigorous cyclone and are put to rout. others are held up by an inherent weakness and are forced to mark time over one locality until strengthened or dissipated. and a few great ones hold sway over the country for a week. these choose the north-center of the country in which to locate. there they pile up the cold air until its very weight causes it to move majestically on. its skirts sweep the gulf coast where they are a bit bedraggled by invading cyclones. it gives the new englanders a fortnight of nipping, brisk days and the mercury in minnesota and the dakotas does not emerge above zero. once, in montana, one of these refrigerating systems established the record of sixty-three degrees below zero. but in siberia where the immense extent of the land surface collaborates with a prolonged night, an anticyclone built up an area of superior chilliness that left a world's record of ninety-one below. in summer a succession of these highs causes the frequent droughts of weeks which harass the west and new england. the air becomes so dry that it parches and then shrivels the green leaves. any little cyclones that, under ordinary conditions, would suck in moist air from the gulf and relieve the situation with a rain are dried out and frustrated by the unclouded sun. it requires a cyclone of great depth to overthrow the supremacy of these summer anticyclones. while the anticyclone furnishes fair weather the sky is not necessarily or even usually free from clouds under its influence. in summer the evaporation during the long days overloads the air for the time being. normally about eleven in the morning little balls and patches of white clouds dot the blue. these increase in number and size until about three in the afternoon when they will have grown little black bellies and fluffy white tops. by five they will have dwindled and by eight entirely vanished. these heaped clouds, known as cumulus, are a guarantee of a normal atmosphere and continued fair weather. they mean that currents of warm, moist air have risen until they have struck a level so cool as to cause them to condense part of their moisture. this condensation sinks until it enters a warmer stratum and the cloud is dissipated. the total movement is a reasonable exchange that preserves the equilibrium of the air, very much as a person bends one way and then another to maintain his balance. in winter there is not such an opportunity offered and the few clouds that form because of the daily variation in temperature are flatter and are called stratus clouds. sometimes these stratus clouds may cover the sky at midday, but in thin platings and not leadenly. in winter as in summer they tend to disappear toward evening. they are often accompanied by an unpleasant wind, but rarely by the snow flurry which is the "april shower" of the winter months. but when the snow flurry does come there is no better sign for the woodsman of coming cold; it never fails. the morning will have begun brilliantly, but soon great summery puffs of cloud form and increase and darken on their under sides. their tops are vague and wear a veil. it is the snow. the reason is simple. the coming anticyclone strikes the upper air before it hits the earth's surface. the sudden cold causes rapid condensation. hence the flurries. but the anticyclone is an agent of dryness, hence their short duration. sometimes the veil of snow does not reach the earth. sometimes it blots out everything in a spirited squall. but it never lasts long, except in the northwest states. and it is invariably followed by a period of colder weather. in summer local evaporation may be so long-continued or so vigorous that the cumulus clouds cannot hold all their moisture content when cooled. a shower is the result, usually a trifling one and mostly without thunder. the great thunderstorms are always in connection with the passing of a cyclone. the small heat thunderstorms are only the indulgences of a spell of fair weather. these tiny showers are daily and sometimes hourly accompaniments of clear weather in the mountains. the air warms rapidly in the valleys and is speedily cooled on rushing up a mountain side and a threat and a sprinkle are the result. when a performance of this sort is going on nobody need fear unpleasant weather of long duration. another pledge of a clear day that does not appear too credible on the face of it is the morning fog in summer. in winter it is a different matter. in august and september particularly the rapidly lengthening nights allow so much heat to evaporate that the surplus moisture in the air is condensed to the depth of several hundred feet. by ten o'clock the sun has eaten into this lowest stratum, heated it and yet begins to decline in power before the balance swings the other way, so that a cloudless day often follows a fog in those months. about three mornings of fog, however, are enough to discourage the sun and a rain follows. of course this is because the anticyclone with its special properties has been losing power. when these conditions of clear nights with no wind follow the first two or three windy days of the anticyclone, particularly in autumn and spring, frost results. in winter the chances that a fog will be dissipated are rather slim. but if it shows a tendency to rise all may yet be well. [illustration: cirro-stratus with cirro-cumulus beneath _courtesy of richard f. warren_ the fine-spun lines of the cirrus proper drag this veil of whitish cloud over the sky. the sun sometimes is surrounded by a colored halo due to the refraction of the light by the ice crystals. but more often it vanishes behind the veil. the mottled clouds below the veil show that a rather rapid condensation of the moisture in the air is taking place. this sky is distinctly threatening, although the direction and force of the wind will more accurately foretell the severity of the coming storm. with this sky expect rain or snow within hours.] an excellent sign of clear weather is this fact of the morning mist rising from ravines in the mountains. and even if you haven't any mountain ravines at command the altitude of clouds can be observed. it is safer to have them lessen in number rather than increase, scatter rather than combine. the higher the clouds the finer the weather. and if the sky through the rifts is a clear untarnished blue the prospects of settled weather are much better than with fewer clouds and a milky blue sky beyond. after the direction of the wind and the shapes of the clouds the colors of the sky are a great help in the reading of the morrow's promise. and the best time to read this promise is in the morning or evening when the half lights emphasize the coloring. soon after the close observation of cloud colors has commenced the amazing discovery is made that the same color at sunrise means exactly the reverse of its meaning at sunset. "sky red in the morning a sailor's sure warning, sky red at night a sailor's delight." christ seized upon this phenomenon to throw confusion into the pharisees and sadducees when they asked that he would show them a sign from heaven. as matthew reports it:--"he answered and said unto them, when it is evening ye say, it will be fair weather for the sky is red. and in the morning it will be foul weather to-day for the sky is red and lowering. o ye hypocrites, ye can discern the face of the sky; but ye cannot discern the signs of the times." the reasons for this contradictory evidence of color are not nearly so obvious as the fact itself. taking the scientist's word for it need not stretch one's credulity overmuch if he can be followed step by step. he says that sunlight is white light, and white is the sublime combination of every color. if no atmosphere existed about us the light would all come through, leaving the sky black. the atmosphere, however, which is full of dust and water particles, breaks up these rays, these white sheaves of light, into their various colors. the longest vibrations, which are the red, and the shortest, which are the violet, get by and the rest are turned back, mixing up into the color which we call our blue sky. if the dust and water particles grow so large and numerous as to divert more of the short rays than usual we get a redder glow than usual. this is most noticeable when the sun and clouds are near the horizon for the air through which they appear is nearer the earth and consequently dirtier. if these water globules mass together so as to reflect all the rays alike the result is a whitish appearance. that is why a fog bank, composed of tiny droplets, each reflecting with all its might, can make the sky a dull and uniform gray. as evening approaches the temperature of the normal day lowers. as the temperature lowers it is the tendency of the moisture in the air to condense about the little dust particles in the air. and as these particles increase in size their tendency is to reflect more and more of the waning rays of light. therefore if the sky is gray in the evening it means that the atmosphere already contains a good deal of condensed moisture. if the cooling should go on through the night, as it normally would, condensation would continue with rain as the likely result. if, on the other hand, after the evening's cooling has progressed and yet the colors near the horizon are prevailingly red it means that there is so little moisture in the atmosphere that the further increase due to the night's condensation will not be sufficient to cause rain. hence the natural delight of the sailor. a gray morning sky implies an atmosphere full of water precisely as an evening gray does. the difference lies in the ensuing process. by morning the temperature has reached its lowest point and if this has not been sufficient to cause cooling to the rainpoint the chance for rain will be continually lessened by the growing heat of the rising sun. the gray, therefore, is the normal indication of a clear cool night which has permitted radiation and therefore condensation to this degree. it is for this reason that we have the heavy fogs of august and september followed by cloudless days. a red morning sky shows, like the red evening sky, that condensation has not taken place to any extent. but this is abnormal for a clear night causes condensation. the red therefore means that a layer of heavy moist air above the surface levels has prevented the normal radiation. hence when the day's evaporation adds more moisture to that already at the higher levels the total humidity is likely to increase beyond the dewpoint with the resultant rain. these two color auguries are among the most reliable of all the weather signs. unfortunately the sunrises are scarcely ever on hand to be examined except by milkmen. but a careful scrutiny of the sunset will make one proficient in shades. in summer when the sun burns round and clear-cut and red on the rim of the horizon the air contains much dust and smoke, the accompaniment of dry weather. and as dry weather has a way of perpetuating itself such a sun makes dry and continued weather a safe prophecy. in winter the same red and flaming sun setting brilliant as new minted gold is a sure indication of clear and cold weather. in all seasons the light tints of the evening sky mean the atmosphere at its best. a golden sunset, a light breeze from the west, a glowing horizon as the sun goes down, slow fading colors all constitute a hundred to one bet for continued fair weather. the sunset colors that are surely followed by storm will be discussed in the next chapter. the sky is too little regarded. architects that do not consider the sky are behind in their calling. maxfield parrish has made himself famous by allying himself with its seas of color. the hunter can read it and learn whether he may sleep dry without his tent. only we who shut ourselves within rooms and behind newspapers forget that there is a sky--until it falls and we are taken to a sanitarium. from the night itself much may be discovered about the continuance of fair weather. a sky well sown with stars is a good sign. if only a few stars are visible the clear spell is about over. stars twinkle because of abrupt variation in the temperature of the air strata. if the wind is from the west cold and clear will result no matter how much may twinkle twinkle little star. but if he twinkle with the wind from the south or east the cloud will soon fly. that is the way with these weather signs. one sign does not make a prophecy. it is the combination that has strength and reliability. furthermore the eye must be trained by many comparisons. of all the conditions that make night forecasting easy the later evenings of the moon are the best. the moon furnishes just the proper amount of illumination to betray the air conditions. if she swims clear and triumphant well and good. if she rides bright while dark bellying clouds sweep over her in summer, inconsequential showers may follow. but if she disappears by faint degrees behind a thin but close knit curtain of cloud the clear weather is being definitely concluded. a great many changes in the weather take place after three in the morning. most campers are accustomed to waking anyway once or twice to replenish the fire, and a glance at the stars will show the sleepiest what changes are occurring in the eternal panorama. a man may have gone to bed in security to get up in a snowstorm, whereas a survey of the skies at three would have noted the coming change. the habit of waking in the dead of night,--which isn't really so dead after all,--is not an unpleasant one. its compensations are set forth in a beautiful and vivid chapter of stewart edward white's "the forest." every camper knows them, and this added mastery that a knowledge of the skies gives him lends a sense of power, which lasts until the unexpected happens. for the unexpected happens to the best regulated of all forecasters, the government. equipped with every instrument and with an army whose business is nothing else than to hunt down storms and warn the public, the weather bureau is still surprised fifteen times out of a hundred by unforeseeable changes in atmospheric pressure. it is scarcely likely then that amateurs without flawless barometers and without reports of the current weather in three hundred places could hope to foretell with complete accuracy. but there is a place for the amateur, aside from his own personal gratification and profit. the weather bureau within the limits of the present appropriation cannot expect to predict for every village and borough. that the amateur may do and with as great accuracy for the few hours immediately in advance. the weather bureau may predict with this large percentage of accuracy-- %--for forty-eight hours in advance because its scope is country wide. it may even forecast in a general way for seven days and still maintain a considerable advantage over almanac guesswork. but the man who is relying upon local signs is limited to ten or at most twelve hours. of course he may guess beyond that but it is only a guess. the work that the bureau does and that he may do within his limits is not guesswork. meteorology is an exact science, and forecasting is an art. both may be studied now in classes under professors with degrees in the same way that any other science and art may be studied. the old sort of weather wisdom which was a startling compound of wisdom, superstition, and inanity has passed away, or is passing away as rational weather talk spreads. these limits of the layman--ten hours with no instruments--are further defined by his locality. in mountainous country changes come more quickly than in level localities, in winter than in summer, so that one's prophetic time-limit is shortened. while the best indications of the clear day are the great fundamental ones, there are many little signs that bolster up one's confidence in one's own predictions. the lessened humidity coincident with clear weather is responsible often for many little household prognostics. salt is dry. the windows (of your summer cottage) do not stick. the children are less restless. smoke ascends, or if the wind is blowing is not flattened to the ground. flies are merely insects, for the time being, and not the devil. swallows and the other birds that eat insects fly high because that is where the insects are. spiders do not hesitate to make their webs on the lawn. they welcome dew but distrust rain. cow and sheep feed quietly, rarely calling to one another as they do before a storm. in short the general aspect of these is normal and therefore remains unnoticed. but all these household prognostics may be advertising the most placid weather while only twelve hours away and coming at sixty miles an hour may be the severest storm of the season. the weather bureau with its maps and barometers follows its every movement. the man in the woods whose comfort in summer and whose life in winter may depend upon his preparedness for the approaching storm does well to read its warnings and know its laws. chapter iii the storm cycle doubtless those who hope for a hereafter of unmitigated ease and song, desire, on this earth, one long, sweet anticyclone. but theirs, in most of the united states, is disappointment. with an irregularity that seems perversely regular at times our fair weather is interrupted by a storm which in turn gives way to some more fair weather or another storm,--there is no telling which very long in advance. and that is why american weather ranks high among our speculative interests. to emphasize this irregularity a seemingly regular succession of events may be noticed. it will cloud up, let's say, on a sunday, rain on monday and tuesday, clear on wednesday, staying clear until sunday when it will cloud up for the repeat. during this past season it rained on a dozen washdays in succession. the newspapers grew jocular about it. and very often one notices two or three rainy sundays in a row. by actual observation this year we enjoyed fifteen clear thursdays in succession in a normal spring. the weather gets into a rut. and if the anticyclones and cyclones were all of the same intensity it is conceivable that the rainy sundays might go on until the day of rest was changed by statute. but the intensities of the whirls differ. before long an anticyclone feebler than ordinary is overtaken by a cyclone and annihilated. or one stronger than the average may dominate the situation for several days. or the great body of cold air in winter over the interior of canada may send a succession of moderate antis across our country making a barrier of dry cold air through which the lurking cyclone can not push. mostly, however, three days of anticyclonic influence and three days of cyclonic influence with one day in between for rest, the transition period, make up a normal week of it. let the american farmer thank his stars (and clouds) for that. for no other regions of the earth are so consistently watered and sunned all the year round as the great expanse of the north american continent. the cyclone is that activity of the atmosphere which prevents us from suffering from an eternal drought. the cyclone is an accumulation of air which has become warmer than the air about it. this area of air usually has a central portion that is warmest of all. since warmth expands, this air grows lighter and rises. nature, steadfast in her grudge against a vacuum, causes the surrounding air to rush it. since these contending currents cannot all occupy the central area at once they fall into a vast ascending spiral that spins faster and faster as it approaches the center. imagine an inverted whirlpool. it is a replica on a much smaller scale of the great polar influx, except that the latter has a descending motion. the cyclone thus is tails to the anticyclone's heads, the reverse of the coin. where the anti's air was cool and dry the cyclone's is warm and moist. the anti had a downward tendency and a motion, in our hemisphere, flowing outward from the apex in generous curves in the direction of clock hands. the cyclone has an upward tendency, flowing inward to the core contrariwise to clock hands. from these two great actions and reactions come all the varieties of our weather. to understand the procession of the cyclones and anticyclones across our plateaus, our mountains, our plains, and our eastern highland is to know why, and often when, it will be clear or not. to mentally visualize the splendid sweep of the elements on their transcontinental run is to glimpse grandeur in the order of things which will go far to offset the petty annoyances of fog or sleet. ignorance may be bliss, but knowledge is preparedness. the anticyclone suggests a pyramid of cold, dry air. the cyclone suggests a shallow circular tank in leisurely whirl. but all comparisons are misleading and a caution is needed right here. for a storm is not a watering cart driven across our united skies by jupiter tonans pluvius. it is not a receptacle from which rain drips until the supply is exhausted. a cyclone is a much more delicate operation than that. it is a process. it can renew itself and become a driving rain storm after it had all the appearance of being a sucked orange for a thousand miles. suppose that our cyclone, this organization of warm, moist air with its curving winds, enters the state of washington on a wednesday, from the north pacific. as early as the monday afternoon before the wind throughout all that section of the country would have shifted out of the west and have started to blow in some easterly direction,--northeast in british columbia and southeast in lower idaho. but since these winds are blowing from the interior they are dry, and consequently rain does not fall much before the storm center is near, that is on the wednesday. if the storm center passes north of tacoma the winds, shifting by south and southwest, bring in the ocean moisture and heavy rain commences which continues until the rising barometer and westerly winds indicate the approach of another anticyclone. so much for western washington. as the cyclone passes eastward it mounts the cascades and its temperature is lowered, its moisture is squeezed out, and it stalks over montana, the mere ghost of its former self, as far as energy and rainfall are concerned. to be sure it preserves its essential characteristics of relative warmth, and inwhirling winds. but let it continue. as its influence begins to be felt over wisconsin and the lake region the moister air is sucked into the whirl and rain, evaporated from superior, falls on minnesota. the east winds are the humid ones now, the west ones the dry. eastward the center moves, over indiana, ohio, new york, the rainfall steadily increasing as the ocean reservoirs are tapped. the first time you tell a new englander that his easterly storms come from the west you are in danger, unless he be a child, for it is to the children that one may safely appeal. indeed it is the increasing number of children who are learning these fundamental weather facts in the public schools that the weather bureau relies upon for a more intelligent support in the next generation. they teach their parents. these latter find it difficult to believe, however, that the storms which hurl the fishing fleets upon the coast in a blinding northeaster have not originated far out at sea, but have come across the continent. for the safe handling of boats knowledge of the rotary motion of storms is necessary that one may be able to tell by the direction of the winds and the way they are shifting where lies the center of the storm and its greatest intensity. in tacoma when the wind shifted by way of southeast, south, and southwest that was proof that the storm center was passing north of the city. likewise if in new york the winds shift by way of northeast, north, and northwest the storm center is passing south of that city. as it drifts out to sea it is gradually dissipated by the changing influences on the north atlantic. very few of our storms ever reach europe, although some have been traced to siberia. the government has put its sleuths on the track of every storm that has crossed the united states in the last thirty years. these weather detectives with a thousand eyes have made diagrams of their actions, mapped their courses, computed their speeds, and if we don't know where all our discarded storms go to, we at least know where most of them came from and how they acted when with us. about a hundred and ten areas of low-pressure affect the country during the normal year. of these all but seven, speaking in averages, come from the west so that the boston mechanic who will not believe that the nor'easter comes via the mississippi valley is right about / of the time. but even that small fraction is no exception to the general law, because those seven storms are not born in newfoundland but in our east gulf states. they come up the coast, and the wind blows from the northeast and north into their centers while they are still on the carolina coast. the great hurricanes which are cradled in the tropics and march westward under the influence of the trades are genuine exceptions to the general westward rule, although they always eventually turn toward the east. they will be given the prominence they demand later, since the eastbound schedule must not be sidetracked now. [illustration: cirro-cumulus to alto-stratus _courtesy of richard f. warren_ the wispy edges of the cloud at the brightest part are cirrus, the fleecy cloud at the extreme top is a thin alto-cumulus, and the dark base of the sky is stratus. but this stratus is too high for that classification and so they call it alto-stratus. this sky shows that the temperatures are moderate, a cold sky being much better packed, and a warm one fluffier. the fact that a veil of cirrus has not preceded the heavier clouds argues that the coming storm will not be of much consequence. this sort of cloud bank arising after a period of cold weather is the best possible prediction of a thaw. slight rain might follow within a few hours.] three cyclones a year form over the lower ohio river basin. on account of their origin over land instead of over water they rarely acquire much energy. once in a decade such depressions deepen rapidly. it was one of these ohio river storms that increased greatly in energy while moving from west virginia to the jersey coast that gave philadelphia her christmas blizzard, a surprise to her citizens and to the weather bureau, for most of the snow fell with the mercury above freezing. the flare-back which gave taft his big inaugural snowstorm is another example of the way a depression may deepen on approaching the coast. until the upper atmosphere is as well understood and watched as the lower, or until instruments are perfected whereby the weather conditions can be made self-announcing such surprises are absolutely unavoidable. under conditions that warrant any suspicion of sudden developments the bureau at washington is careful to order extra observations in the areas likely to be affected, but no surface observations can quite suffice. fifteen storms a year originate over the west gulf states, or, drifting in from the pacific over arizona and new mexico begin to acquire energy in texas. twelve are set up over the colorado mountains. these usually dip down into texas before starting their drive toward the northeast. after both these sets of storms get under way they strike resolutely for the same locality,--the st. lawrence valley. the conformation of the st. lawrence region provides an irresistible attraction for american storms. occasionally a very strong anticyclone holds that territory and pushes the cyclone off the coast at hatteras or even makes them drift across the country to florida. but such occasions are exceptional. give the ordinary cyclone its head, and, ten to one, you will find it on the way to the st. lawrence. the inhabitants will confirm this statement, i am sure. they do not feel discriminated against in the matter of weather. they get nearly everything that is going. since they have to accommodate from seventy to eighty cyclones in fifty-two weeks they have very little time to brood over any one variety of weather. with the optimism of that section of the country they say, "if you don't like our weather, wait a minute." ten storms a year originate over the rocky mountain plateau, north of colorado. about twenty cross over from the canadian provinces of alberta and british columbia. and all our other storms, about forty each year, enter our country from the north pacific by way of washington and oregon. many of these drift across the northern tier of states without any great display of energy, at least before they reach the lake region. but the majority loop down somewhat into the middle west as far south as kansas, and then make their turn toward the inevitable st. lawrence. they usually require four days to make the trip from coast to coast by this route, as also by the more direct northern route, because on that they travel more at leisure. but the storms from texas, whose energy is greatest because of greater heat and moisture, occasionally speed from oklahoma to new york in thirty-six hours. in summer all speeds are reduced. this is because the disparities in temperature are less. in winter where greater extremes of temperature are brought into conjunction the processes of the storm are all more violent. and it is a bit disheartening to know that a storm is aggravated to even greater endeavors by its own exertions. its energy provides the conditions to stimulate greater energy, and, like a fire, it increases as it goes. if it did not run out of the zone which nourished it and proceed into another zone where conditions were distinctly discouraging the limits of the storm would be much extended, and vast territories would be devastated by the self-propelling combination of wind and water. to the generality of us the word storm means rain. to the scientist it means wind. in reality the cyclone is rare that crosses our country without causing rain somewhere along its track. the curiosity of the weather bureau to find out the paths of the storm centers is abundantly justified because it is along these paths that the heaviest rainfall and the severest winds occur. but whether or not there is precipitation on the path of the cyclone it is rated as a storm if there is a lowering of pressure and consequent wind-shift. the storm centers are not always well-defined, and quite often the circulation of the wind about them is not complete. such cyclones never amount to much, although there is always the possibility of their closing in and developing a complete circulation with the attendant increase of energy. the incomplete cyclones over the desert and plateau regions are lame affairs, lacking interest and advancing timidly if at all. but once let them drift into a locality where they can be supplied with moist air, they pick up energy, keep a definite course, and advance with increasing speed. very often the center will split up, the circulation perfecting itself around both centers of depression. one of these will likely be over minnesota and the other over texas and the organization will steam-roller the states to the east in the manner of a gigantic dumb-bell. this formation is more likely to have been caused by the two centers appearing simultaneously than by a split in an original center. the weather reports call this fashion of storm a trough of low pressure. the southern center is the one that develops the more energy on its turn to the northeast. if the two centers should unite on reaching the northeast a very heavy precipitation is the invariable result. all cyclones have much greater length than breadth. they frequently stretch from unknown latitudes in canada into unrecorded distances into the gulf, while on the other hand it is a very large storm that rains simultaneously upon the mississippi and the atlantic. behind a cyclone of pronounced energy a second whirl, called a secondary depression, often develops, in which case the period of wet weather is prolonged. also, more rarely, an offshoot forms ahead of the main depression. a sluggish, sulky cyclone either in winter or summer provides more opportunity to humanity for self-discipline than almost any other feature of our national environment. in winter when the depression slows up it settles down upon one community in the guise of fog, and stays by the locality until an anticyclone blows in and noses it out. fog is aggravation, but a hot wave is suffering and the hot wave is caused by a depression weak in character but generous in dimensions getting held up on the northern half of our country. by its nature it attracts the air from all sides, and being in the north, the direction of the wind over most of the country would be southerly. air from the west and north has a downward tendency, but south and east winds are surface currents. consequently these winds, blowing over leagues of heated soil, become dry and parching. if the depression lingers long the entire country to the east, south, and west soon suffers from superheated air. at last the very intensity of the heat defeats itself and the reaction to cooler is effected dramatically through a thunderstorm. the well-developed cyclone in winter causes what we all know as a three days' rain, although continuous precipitation rarely lasts over ten hours. the rest of the time is occupied by general cloudiness with occasional sprinkles and a final downpour as the wind shifts to the west and the anticyclone nears. in summer the depressions, being shallower, rarely cause continuous cloudiness for three days, although their influence often lasts as long as that in the guise of a series of thunderstorms. the line of storms extends several hundred miles, bombarding all the towns from albany to richmond. these thunderstorms sometimes achieve in an hour or two even greater results than their winter relatives can accomplish in three days in the matter of rainfall, wind velocity, and general destructiveness. our wettest months are july and august and not december and january. the freedom of the wind has been the subject of much poetic and prosaic license. as a matter of fact the wind is the veriest slave of all the elements. it is harried about from cyclone to anticyclone, wound up in tornadoes, directed hither and thither by changing temperatures. it blows, not where it listeth, but where it has to. and circuitously at that. for once the path of duty is not straight. that is another fact that the boston mechanic would have been slow to accept,--that the wind blows in curves. a little consideration, however, of the fact that the wind is perpetually unwinding in great curves from the anticyclone and winding up on the cyclone will show that nowhere can it be blowing in a perfectly straight line. thus it becomes the surest indication that a cyclone is to the west of one if the wind blows from an easterly point. the storm is bound to move toward the east, therefore the rapidity with which the clouds move and thicken will signify when the area of precipitation will reach the observer. the cycle of the storm is normally this: after a cloudless and windless night a light air springs up from a little north of east. at the same time strands of thin wavy clouds appear, very high up. they may be seen to be moving from the southwest or northwest. their velocity is great. their name is cirrus, and they are called mares' tails by the sailors. they are followed by several hours of clear skies, usually; but if the storm is smaller and close at hand there is no clear interval. before the larger storms these cirrus clouds are sent up as storm signals twenty-four and even forty-eight hours in advance. the day that intervenes is very clear, the air feels softer, the temperature is higher. in midafternoon more cirrus appears, and as condensation follows the quick cooling the silky lines increase in number. beneath them a thicker formation, known as cirro-stratus, forms a dense bank in the west and southwest. the sun sets in a gray obscurity. if there is a moon it fades by degrees behind the veil of alto-stratus, and the halo which first was seen wide enough to enclose several stars narrows until it chokes the moon in its ever-thickening cocoon of vapor. there is no value whatever in the old superstition that the number of stars within the halo foretells the number of days that it will rain or snow. the same halo that encloses three stars at eight o'clock may have narrowed down to one by midnight, or none at all, so that the prophetic circle is bound in the very nature of its increase to contradict itself. the presence of a halo is a pretty sure sign of some precipitation within twenty-four or thirty hours. it fails about thirteen times in a hundred. if the halo is observed around the sun it is an even surer sign, failing only seven times the hundred. during the time of cloud-increase the wind will probably lull before a snow, so that the hour or so before precipitation begins is one of intense brooding calm. or if there is no calm the wind, now easterly, will be very gentle. soon after the precipitation begins the wind will begin to freshen and will continue to increase in velocity until the center of the storm is close to the locality. this will require about eight hours for the average storm. as storms vary an average is a very misleading thing and the best way to judge of the length and severity of the storm is by watching the wind. if it increases gradually the storm will be of long duration. if the wind rises fitfully and swiftly it will not likely be long but may be severe. if the wind reaches any considerable velocity before the rain or snow begins the storm is sure to be short and severe. the color and formation of the clouds will tell when the precipitation is about to begin. in summer, no matter how striking and black are the shapes and shadows of the clouds, rain will not fall until a gray patch, a uniform veil called nimbus is seen. in the little showers of april this patch of unicolored cloud is there, as well as behind the great arch of the onrushing thunderstorm. in winter raindrops are smaller and the tendency of the clouds is to appear a dull, uniform gray at all times. but the careful observer can detect a difference between the nature of the clouds several hours before precipitation and their color immediately before. when snow is about to fall no seams are visible. an impenetrable film obscures all the joints. from such a sky as this snow is sure to fall. but if seams are visible, if parts of the skyscape are darker than others, then, no matter whether the temperature on the ground is below freezing a rain storm will ensue. very often these winter rains begin in snow or sleet, but the clouds register the moment when the change from snow to rain is to be made. the presence of swift-flying low clouds from the east is a certain sign that the change to a temperature above freezing has been effected in the upper strata of the atmosphere. this variety of cloud is called scud, and accompanies rain and wind rather than foretelling it long in advance. if the storm is approaching from the southwest the precipitation begins near the coast about twelve hours after the cirrus clouds commence to thicken and about twenty-four after they were first seen. in some localities as much as thirty-six and even forty-eight hours are sometimes required for the east wind to bring the humidity to the dew-point. just a little observation will enable one to gauge the ordinary length of time required to bring things to the rain-pitch in one's own country. of course no two storms in succession make the trip under the same auspices and with the same speed. the sign of the universe should be a pendulum. one period of cyclone, anticyclone, cyclone will traverse the country rapidly. then there will be a halt all along the line, and the next series,--anticyclone, cyclone, anticyclone, will take three days longer to make the crossing. otherwise our weather would have a deadening regularity. on an average our storms cross the country at the rate of about six hundred miles a day. this is the average. some delay, linger, and wait for days over one locality. others do a thousand miles in the twenty-four hours. they thicken up enough to cause rain from two hundred to six hundred miles in advance of their centers. it stops raining not long after the actual center has passed. but for picnic purposes the storm is far from being over. for even though continuous raining has stopped the low pressure still induces a degenerate sort of precipitation called showers, or oftener mist for another twelve hours (usually in winter). then as the cooling influence of the anticyclone approaches the rain recommences. this time it is not for long, however, and is followed by permanent clearing, the wind shifting into the west. sometimes the change to blue sky is abrupt. but if the subsequent anticyclone is not very well defined, cloudy conditions may linger for a couple of days. such clouds are usually much broken and show white at the edges and never cause more than a chilly feeling. this attempt to outline the customary cycle of the storm,--clear sky, cirrus cloud, wind-shift to the east, the denser cirro-stratus, the pavement-like stratus, the woolly nimbus, the first continuous hours of rain, the misty interval, the windshift to the west, the final shower, and breaking cloud, the all-blue sky--this storm-schedule is always subject to change. but the fundamentals are there in disguise every time. they only have to be looked for and there is some satisfaction in penetrating the disguise. when a storm comes up the atlantic coast, as happens a few times a winter, the process is shortened, because the effects of the larger easterly quadrants are felt only at sea. the most prominent recent illustration of this type of storm was the severe snowstorm that swept the coast states from carolina to maine the saturday before easter, . its calendar read as follows: friday, p. m., cirrus clouds thickening into cirro-stratus. midnight, stars faintly visible, wind from northeast, miles an hour. sunrise, stratus clouds, wind rising in gusts at philadelphia to miles; a. m., rapid consolidation of clouds with snow shortly after, although the temperature at the surface of the earth was as high as seven degrees above the freezing point. this rapidly dropped to freezing. flakes were irregular in size. until one o'clock in the afternoon the snow thickened with gusts of wind up to forty miles. snowfall for five hours was inches, an unprecedented fall for this locality. then the storm waned for five hours more, inches more of snow falling. precipitation practically ceased at p. m. by sunrise on sunday the skies were free of clouds and the wind blew gently from the northwest. occasionally a high pressure area out at sea and beyond the ken of the weather bureau causes one of these coast storms to curve inward to the surprise of everybody. occasionally, too, the transcontinental storms are driven north or south of their accustomed paths. while the divergence may be slight, it causes a margin of variance from the accuracy of the bureau's report. then arises a second storm,--one of indignation--from all the people on one side of the strip who carried umbrellas to no purpose, and from the others,--who didn't. this pushing aside of the cyclone is caused by pressure variation that only hourly reports from many localities could detect. vast hidden influences shift the weights ever so little and the meteorological express is wrecked. but this happens, at most, fifteen times in a hundred, and remembering the unseen agencies to be coped with people are refraining more and more from the tart criticisms of former times, not in charity but in justice, although there is small tendency yet to forward eulogies to the bureau in recognition of the eighty-five times it is right. chapter iv sky signs for campers the weather-wise, even more so than poets, are born. but that only goes to say that weather-wisdom can be fathered. for poetry and canoeing and the art of making fires, once the desire for these things is born, may be aided infinitely by observation and practice. nobody can teach a man the smell of the wind. but the chap who feels nature beating under his heart can, by taking thought, add anything to his stature. so it is with those who are called weather-wise. an unconscious desire, a little conscious knowledge, a good deal of experimentation with the cycle of days, and you have a weatherman. these chapters aim to put the little conscious knowledge into the hands of the people with the unconscious desire, so that when they take their week in the woods for the first time (and their month for the second time) they may enjoy the shifting scenery of the sky-ocean and, incidentally, a dry skin. for i take it that everybody will soon be camping. maine and the adirondacks have become a family barracks. it is hudson bay for bachelors. and over this expanse of woods and children the weather problem ranks with the domestic one. for naturally if a soaking would endanger his vacation the husband must not permit a rain,--unexpectedly. in all seriousness, it is of avail to know the skies if one is going into the wilds just as it is of avail to know what severed arteries demand, what woods burn well, and what mushrooms can be eaten, even though one can get along without knowing these things until perchance the artery is severed or the arched squall catches one far from shore. at the very least, one grain of weather wisdom prevents a mush of discomfort. and if, fellow-camper, the following observations gathered on a thousand thoughtless walks do not tally (for the northeastern states) with yours, write me, so that in the end we may finally contrive together a completer handbook of our weather. the clouds clouds are signposts on the highway of the winds. every phase of the weather, except stark clearness, is commented upon by a cloud of some sort. when danger is close they thicken. when it passes they disappear. the aviators of the future will be cloud-wary. he who flies must read or never fly again. the cirrus cloud is always the first to appear in the series that leads up to the storm. it looks like the tail of pegasus and for it the old forecasters in their forecastles made a special proverb. "mackerel scales and mares' tails make lofty ships carry low sails." these white plumes and scrolls which are in reality glistening ice-breath, fly at the height of five, six, seven, and even eight miles. and as a sign of coming storm they are about as infallible as anything may be in this erratic world. they were born in the cradle of a storm. the storm center was breathing warmed air upward to great heights, and although the disc of the storm itself was only two or three miles deep, its nucleus, crater-like, shot warm columns twice as far. with just enough moisture content to make a showing against the blue these streamers flowed to the eastward. at those dizzy heights the prevailing westerlies are in full force, blowing from eighty to two hundred miles an hour night after night and day after day. these westerlies caught the storm exhalations, the streamers, and hurled them eastward at greater speed than the main body of the storm. and that is the reason that we see these cirrus clouds always eight, mostly twelve, often twenty-four and sometimes forty-eight hours before the storm is due. just a few strands of cirrus have little significance. they may be condensation from a local disturbance, or a back fling from a past storm. but if the procession of the cirri has some continuity and broadens to the western horizon it is a sign about eight times in ten that a cyclone is approaching. occasionally the storm center is too far to the south or north to cause rains at your locality, but the cirri bank up on the horizon and their lacework covers the sky. if they appear to be moving toward the region of greatest cloudiness it is not a sign of precipitation. this condition is most apparent at philadelphia when the storm center over alabama or mississippi floats out to sea by way of florida without having the energy to turn north. then the cirrus is seen thickly on our southern horizon. looking closely one sees that the cirri are moving from the northwest, and are being drawn into the storm area instead of proceeding in advance of it. careful watching will sometimes enable one to tell whether the tails are increasing or decreasing in size. if they dissolve it means that the cyclone from which they were projected is losing strength because of new conditions. cloudiness may follow but no precipitation of consequence. the plumy tails are expressive: pointing upward they mean that the upward currents are strong and rain will follow; pointing downward they mean that the cold dry upper currents have the greater weight and clear weather is likely. in summer the cirrus cloud formations are not such certain advance agents of rain because all depressions are weaker and less able to confront a well-intrenched drought. as the proverb goes, "all signs of rain fail in dry weather," and there is some truth in it. the fine wavy cirrus clouds often increase in number, develop in texture until the blue sky has become veiled with a muslin-like layer of mist. this is the cirro-stratus, and is a development of the cirrus, but it does not fly so high. its significance is of greater humidity and is the first real confirmation of the earlier promise of the cirri. another form that the cirro stratus may assume is the mackerel sky,--clouds with the light and shade of the scales of a fish. if this formation is well-defined and following cirrus it is a fairly accurate storm indicator. it is not quite infallible, however, as the same forms may be assumed when the process is from wet to dry. the old proverb, "mackerel sky, soon wet or soon dry," expresses this uncertainty. if dry is to follow the scales will appreciably lessen in size and perhaps disappear. if the cirro-stratus or scaly clouds are followed by a conspicuous lowering it is only a question of a few hours until precipitation begins. the cirro-stratus at a lower level is called alto-stratus and this becomes heavy enough to obscure the sun. the cloud process from stratus on is slow or rapid, depending upon the energy of the coming storm and the rate of its approach. in most cases the clouds darken, solidify, and become a uniform gray, no shadows thrown, no joints. soon after the leaden hues are thus seamless the first snowflake falls. if it doesn't it is a sign that the process of condensation is halting: the storm will not be severe. sometimes there is no precipitation after all this preparation, but under these circumstances the wind has not ventured much east of north. from the time that the snow starts the clouds have chance to tell little. only by a process of relative lightening or darkening can the progress of the storm be followed and the wind, and not the clouds at all, is the factor to be watched; for occasionally the sun may shine through the tenuous snowclouds without presaging any genuine clearing so long as the wind is in the east. but in summer the clouds become even more eloquent than the wind. the rain-cloud, called the nimbus, becomes different from the dull winter spectacle. in summer air becomes heated much more quickly and the warm currents pour up into the cold altitudes where they condense into the marvelous mont blancs (or ice-cream cones) of a summer afternoon. these piled masses of vapor are cumulus clouds, and if they don't overdo the matter are a sign of fair weather. they should appear as little cottony puffs about ten or eleven in the morning, increase slowly in size, rear their dazzling heads and then start to melt about four in the afternoon. but perhaps the upward rush of warm, moist air has been so great in the morning that the afternoon cooling cannot dispose of it all without spilling. then occurs a little shower,--the april sort. often in our mountainous districts it showers every day for this reason. the great thunderstorms come for greater reasons: they are yoked to a low pressure area and represent the summer's brother to the winter's three-day storm. cumulus clouds are called fair weather clouds until their bellies swell and blacken and they begin to form a combination in restraint of sunlight. even then it will not rain so much out of the blackness as out of the grayness behind it, and if there is no grayness chances are that you will escape a wetting. one can almost always measure the amount of rain that is imminent by the density of the curtain being let down from the rear of the cloud. if you can see the other clouds through it or the landscape the shower will be slight. if a gray curtain obscures everything behind it you had better pull your canoe out of the water and hide under it if time is less valuable than a dry skin. such showers may be successive but rarely continuous. rain clouds have been observed within yards of the ground. very often it can be seen to rain from lofty clouds and the fringe of moisture apparently fail to reach the earth, because the condensation was licked up and totally absorbed on entering a stratum of warmer air. the reverse of this occurs on rare occasions;--condensation takes place so rapidly that a cloud does not have time to form, and rain comes from an apparently clear sky. this phenomenon has been witnessed oftenest in dry regions and never for very long or in great amounts, although a half hour of this sort of disembodied storm is on record. if the cumulus clouds of the summer's afternoon do not decrease in size as evening approaches showers may be looked for during the night. and if the morning sky is full of these puffy little clouds the day's evaporation on adding to them will probably cause rain. a trained eye will distinguish between a stale and fresh appearance in cloud formation, the light, newly made, fresh clouds, like fresh bread, contain more moisture. if the clouds have much white about them they need not be feared as rain-bearers. clouds are much higher in summer than in winter and the raindrops of warm air are larger than those of cool. if cumulus clouds heap up to leeward, that is, to the north, or northwest on a south or southwest wind a heavy storm is sure to follow. this is notably so as regards the series of showers in connection with the passage of a low-pressure area. the wind will bear heavy showers from the south (in summer) for a whole morning and half the afternoon with intervals of brilliant sky and burning sun. or perhaps the south wind will not produce showers, but all the time along the northwest horizon a bank of cloud grows blacker and approaches the zenith, flying in the face of the wind or tacking like a squadron against it. about the time that the lightning becomes noticeable and the thunder is heard the wind drops suddenly, veers into the west, and the face of things darkens with the onrush of the tempest. although no rain may have fallen while the wind was in the southern quarter yet that constituted the first half of the storm and the onslaught of rain and thunder the second. while the storm area moved from the west to the east the circulation of air about the center was vividly demonstrated by the south wind blowing into the depression, whose center was epitomized by the moment of calm before the charge of the plumed thunderheads from the northwest. most camping is done either in hilly or mountainous country where the movement of clouds is swifter and more changeable than over flat lands. there is one sign of great reliability: if the mountains put on their nightcaps the weather is changing for the wetter, and if clouds rise on the slopes of the hills or up ravines, or increase their height noticeably over the mountain-tops, the weather is changing for the dryer. in the mountains where abrupt cliffs toss the winds with all their moisture to heights that cool clouds form and condense rapidly and the weather changes quickly. but even in the mountains the big changes give plenty of warning. often clouds may be noticed moving in two or even three directions on different levels at once. the upper stratum will probably be cirrus from the west. cumulus or stratus may be floating up from the south. a light drift of vapor called scud may fly on the surface easterly wind. such a confused condition of wind circulation betokens an unsettled system of air pressures and as frequent collisions of the air bodies at varying temperatures are inevitable rains, probably heavy, will follow. on clear days one will be surprised to see isolated clouds, usually the torn, thin sort, drifting across the sky from the east. a change will follow soon. in winter black, hard clouds betoken a bleak wind. clear winter days several times a season show a brilliant blue sky filling with great cumulus clouds of dark blue, blurred at the top and gray at the base. they will sprinkle snow in smart, short flurries, and are ushering in a period of clear and much colder weather. a sky full of white clouds and much light is a cheerful sign of continuing fair weather. the softer the sky the milder the weather and the more gentle the wind. it is the dark gloomy blues that bring the wind. but do not mistake the woolly softness of the rolling clouds before a thunderstorm. a sudden and often violent gust follows. tumbling clouds in any event should make one wary of venturing on water. summer drownings would not be so numerous if the portent of the squall were heeded. to this data might be added many singular cloud formations that are not observed often. the funnel shaped cloud of the tornado, the green shades of the hurricane cloud, the green sky of cold weather showing out between layers of steel blue, coppery tints that show before heavy storms sometimes, variations of color at sunset each of which has a meaning which practice in deciphering will make clear. but enough has been given to show sky-searchers how many are the tips of coming weather that may be read from a conglomeration of fog particles. nobody with eyes should be caught unawares by day. the look of the sunset shadows forth much of the coming night. and throughout all this truth holds: the greater the coming storm the longer and clearer are the warnings given to the watchful. the winds the wind is the ring-master of the clouds. it whistles and they obey. therefore to be windwise is to be weatherwise, almost. one can get a hold on the wind by learning to gauge its strength. look at the trees or the smoke from your city chimneys and guess how fast it blows at eight o'clock in the morning, or eight at night. the weather report the next day will tell you how nearly you were right. beginning is easy; anybody can guess a calm. when the leaves are just moving lazily the weather bureau calls it a light or gentle breeze, moving from to miles an hour. a fresh breeze, from to miles will stir the twigs at first and finally swing the branches about. from to miles, a brisk wind, will cause white caps on the lakes, tossing the tops of the trees, but breaking only small twigs. increasing from to miles it becomes a high wind that breaks branches on trees, wrecks signs in the towns, causes high waves at sea and roars like the ocean in heavy squalls through the woods. from to miles an hour makes a gale. sailing craft are now in danger. the pressure at miles an hour is pounds to the square foot, having risen from three-quarters of an ounce at miles. this pressure becomes pounds per foot when the wind reaches a velocity of miles. at trees are uprooted, chimneys may go, it is difficult to walk against, the noise becomes very great but rather inspires than frightens. as the gale increases from to (which velocity the bureau rather weakly calls a storm wind), danger rapidly increases. trees are prostrated, the uproar becomes terrifying, walking without aid is impossible, the great ocean liners are in danger, the sea becomes a whitened surface of driving spume that heaps up into piles of water thirty or more feet high, windows are blown in and frame houses cannot stand much greater velocities. anything from miles an hour up is well called a hurricane. everything goes at . at galveston the machine that registered the wind velocity blew away at . they have better instruments now, and in many places velocities of over a hundred miles an hour have been recorded. as high as miles was registered on the top of mt. washington, and in a single gust at montreal. the great hurricane winds are most felt at a few of the exposed places on our coasts. cape mendocino, on the pacific, has miles an hour to its credit in a january hurricane. but enough destruction is done at miles. fields are stripped of their crops, or leveled; houses are demolished unless they are specially built, like the new york sky-scrapers, to withstand much higher velocities. in the small whirling storms called tornadoes the wind is estimated to reach a velocity of to miles, and nothing but the cyclone cellar will shelter one from the fury of the elements when they are really unleashed. the higher one goes the greater the velocity of the wind. on the top of mt. washington miles is rather common for hours at a time and is recorded now and then. that is only feet above boston. if such a force struck boston for a minute it would be blown _en masse_ into the bay. velocities on land are less than those at sea, because of the resulting friction from obstacles. velocities in summer are lower (thunder gusts excepted) than in winter. since the wind is caused by differences in atmospheric pressure, and that in turn by disparities in temperature, winter holds the palm for greater velocities because the wide whirl of a cyclone over the great plains may cause to mix air from texas with a temperature of degrees with air from montana of degrees below zero, while the summer temperatures in both states might easily be degrees. throughout most of our land certain winds have always the same bearing upon the weather and this correspondence is roughly the same over most of the country. west winds, for instance, are an almost universal guarantee of clear weather. the pacific coast and western florida are the exceptions. northwest winds bring clear skies and cool weather everywhere. in winter in the north plateau section heavy snows arrive in advance of the severe cold waves that come on these northwest gales. north winds are the cold bearing ones. clear skies prevail under their influence. northeast winds are cold, raw snow-bearing winds in winter and spring and bring chilly rains in midsummer. east winds are the surest rainbringers of all for the eastern two-thirds of the country, and are soon followed by rain with a shift of wind over the other third. their temperatures are more moderate than those of the northeast storms. the greatest falls of rain occur, however, with the southeast winds, whose moisture content is greater than that of the others because they are warmer and blow off water except in rocky mountain districts. south winds are warm and contain much moisture, which falls in showers rather than in continuous rains. the southwest winds of winter precede a thaw and are much damper than west winds. in summer over much of our country they are hot, parching winds that injure vegetation. the average velocity of the wind from these different quarters is variable in different parts of the country, the severest being on the southeast and northwest quadrants. the highest winds are always where the steepest gradients are; that is, where the barometric pressure decreases or increases the fastest. the steepest gradients are usually on the northeast and northwest sides of the storm center, with the exception of the atlantic coast where the southeast winds are often highest. the average for the northeast quadrant is miles, for s. e. , for s. w. , and for the n. w. miles an hour. but averages can deceive. as a matter of fact single instances of great wind velocities occur from each point of the compass. the greatest velocity ever recorded at philadelphia occurred in october, , when the wind blew seventy-five miles an hour from the southeast. but the record velocities for eight of the other months were registered in the northwest quadrant. [illustration: alto-stratus _courtesy of richard f. warren_ not so high as cirro-stratus, and yet partaking of the same skeiny texture. this would be a normal sky in winter about six hours after the veil of cirrus had begun to throw its haze about the sun. no other cloud formations appear, however, and so the area of precipitation is still pretty far away. in summer such a sky is less common. if the disturbance is to amount to anything the cirro-cumulus will soon form. if the wind is from a westerly quarter the blanket of cloud is doubtless a drift from some distant storm, which will not affect this locality. the wind is always blowing toward a storm and away from clear weather.] the period of time when the barometer is beginning to rise after having been very low is that when the strongest winds blow. some sections of our country have special kinds of wind that are peculiarly their own, notably colorado, wyoming, and montana where the chinook reigns. this phenomenon belongs only to the cold season and only to the coldest days of it. it is a warm wind that begins to blow without much warning from the southern quarter. it is caused by a body of cold air suddenly falling from a great height. as it falls its descent heats it and it causes a rise in the temperature of the surrounding locality that greatly exceeds any rise from other causes. the increase in temperature will be as much as forty degrees in fifteen minutes. this sudden dry heat is a great snow-eater. if it were not for the chinook the snow-blanket would stay so much longer on the cattle ranges that they would be useless as such. in northeastern sections of our country and canada the warm winds blowing in from the ocean at the approach of a cyclone do away with the snow rapidly but with nothing like the speed of the chinook. another phenomenon of the air that is of tremendous benefit to man is the sea-breeze. during the intense heat of a hot wave the wind may shift to the east in boston and in fifteen minutes coats are comfortable. such a shift may bring relief to a strip of land two hundred miles wide along our entire eastern seaboard. the sea-breeze is explained by the fact that the land cools more quickly than the sea and also warms more easily. during the whole forenoon of a summer's day the sun has been pouring upon land and sea, but the land-air has become much hotter than the air over the sea. it rises and the sea-air rushes landward. by midnight the land has cooled off even more than the sea and the heavier air now presses out to sea again. on every normal day this balancing process takes place. if it doesn't conditions are abnormal and chances are that mischief is brewing. this ebb and flow of warmer and cooler air is, on a small scale, exactly what is happening on a vastly larger field of operations between cyclone and anticyclone. and it is the dominance of the anticyclone with its prolonged rush of air from the northwest that interrupts the sea breeze for two or three days in winter, as the cyclone prevents the night land breeze from taking place when it is central off the eastern coast. the exchange of air between mountain side and valley is similar to the land-and-sea breeze. the rarer air on the mountain side heats faster by day and cools faster by night than the denser air in the valley. therefore during the day it rises and the valley air rushes up to take its place; during the night it cools and sinks into the valley. this is a great help when one is shut up in a secluded valley for several days and cannot get a good view of the skies. the atmosphere is acting properly and will remain settled so long as the air blows up your ravine for most of the day, and turns about sundown and blows out and down the ravine like a flood of refreshing water. of course many valleys are so large as to be affected, not by these local causes, but by the larger movements of the anticyclones when the sure-clear west wind may blow up the valley for three days at a time. but, nevertheless, for most mountainous places the logic holds and you may expect rain if the wind does not blow coolly down the ravine at night. of course watch your clouds for confirmation. in times of calm prepare for storm. an eminent meteorologist has frowned upon me for saying that. it is not the whole truth, i admit, but there is a certain kind of calm which happens often enough to justify the remark. it happens this way. a severe storm has passed. the customary anticyclone with its brisk northwest winds has arrived and is blowing with all the vigor necessary to induce one to believe that the clear weather is to continue for the usual length of time; that is, three or four days. but suddenly in the early afternoon, just when it should be blowing its hardest, the wind drops, lulls, shows a tendency to change its direction. there is only one explanation. another cyclone has developed off in the west. it has knocked the anticyclone on the flank, taken the teeth out of the gale. the wind shows this before clouds can. the absence of wind when there ought to be a lot shows it before even the first cirrus swims overhead. the chance is that when the flow of anticyclonic air has been thus rudely cut off and stillness follows, it will be storming by morning. it is best to keep an eye on these abnormal, precipitous calms. in times of peace prepare for rain. but the eminent meteorologist was eminently right when he said that the statement was misleading unless explained. for there are many kinds of calms that do not portend coming storms. nearly every day, winter and summer, but particularly in summer, the wind drops to a calm at sunset. that is a time of adjustment. after sunset when the accounts are all in the wind springs up with as much force as it had in the afternoon and continues until dawn. at sunrise, however, there is another truce. if this truce is neglected either at sunrise or at sunset it is a sign that either a cyclone on an anticyclone is very much in the ascendency. these truces are most often observed at the seashore when you are out sailing and the smell of supper fills your nostrils but is not sufficient to fill your sails. these calms are normal and the best sign of a fair day on the morrow, provided the other signs agree. during the great transition period from summer to winter comes that autumnal truce, indian summer, which is the chief claim to fame of american weather. for day after day a brooding haze sleeps in the air, sometimes for weeks there is no wind of any strength. winter advances insidiously in the fall but retreats in commotion, and the cooling off process permits of these still days while they are uncommon in the spring. the wind checks off more mileage in march than in any other month. while the regular day's end calm and the calm of the year's exhaustion mean continued fair weather, there is one calm that everybody knows, which is the most dramatic moment in the whole repertory of the weather: the foreboding, ten-count wait before the knockout blow of the thunderstorm. but when that calm comes every one is already sitting tight so that it is not much account as a warning. they say that the intense stillness before the hurricane strikes is uncanny. whether inshore or afloat the wind is to be watched if you would know what weather is to be. it is only another of nature's paradoxes that the most unstable element should be the most reliable guide of all on the uncertain trail of the next day's weather. temperatures considering that the temperature of the sun is , degrees fahrenheit and the temperature of space is absolute zero, degrees below ours, we do very well on earth to be as comfortable as we are. and we owe it all to the atmosphere which keeps the sun from concentrating upon us. our place in the sun is so very small that we intercept only one-half of one billionth of the heat which it is giving off night and day. but that is sufficient to do a lot of damage if it could get at us. but even the paltry range of temperatures so far recorded on our planet,--from degrees above zero one day in california, to degrees below zero one night in siberia,--is by no means a fair statement of the extremes we are called upon to bear. only twice a decade in our country does the mercury vary as much as sixty degrees in twenty-four hours, and there are vast areas where the daily change amounts to only a few degrees. the changes that do come so suddenly to us, particularly in winter and that are known as cold waves, are in reality beneficial. to them we americans may owe our energy, our vivacity, our changeability of mood. the refrigerated, revivified air sweeping down from the north is tonic. it is heavy, and issuing from antiseptic altitudes, drives the humid, germ-nursing air from our city streets. if we had arranged a process of refreshment like this at vast expense we should have been intensely proud of it. as it is we are intensely annoyed at it and occasionally a few people are frozen to death. the weather bureau warnings and the coal clubs are reducing the loss in property and lives. if you are sleeping out it is of great importance to know when the mercury is going to take one of these swoops, for sleeping cold means little real rest because one's muscles are tense, and the next day's packing needs all the relaxation one can get. two generalizations govern pretty much every change of temperature: the mercury will rise before a storm and it will fall after one, winter and summer, but much more conspicuously in winter. there are two reasons for this. our cyclones usually cross our country over such a northern track that over most of the country the air drawn into them comes from the southern quarters and is therefore warmer than the air previously flowing from the anticyclone. also the process of precipitation causes heat. this is true to such an extent on the coast of ireland where it rains most of the time that a scientist has computed that the inhabitants get from one-third to one-half as much heat from the rainfall as they do directly from the sun. thus a normal storm is doubly sure to warm up the environment. in summer the reverse is partially true, for very often the rain does not begin until the actual center of depression has passed and the west winds have begun to exercise their cooling influence. so that in summer we have a sultry, sunny day as the first half of the storm area and then a cooling shower. also after two or three days of warm weather in spring and autumn we have a rainstorm of the winter type which lowers the temperature instead of raising it. this is because the heat produced by the storm is less than that of the sun's rays intercepted by the clouds. the clear skies of the preceding anticyclone had permitted the land to warm up very fast under the midsummer sun, and the clouds of the cyclone, by cutting off the supply, had made a relative chill. in winter the sunrays are so much feebler because of their slant and radiation proceeds so rapidly under the dry air of the anticyclone that a much greater degree of cold is produced than when the cyclonic clouds prevent the radiation. therefore the rainy area is the warmest of all. even in summer the winds from the southeast, south, and southwest are warmer than those from the opposite quarters, not only because they blow from a quarter naturally warmer on account of the sun, but because they are surface winds and have absorbed some of the heat from the soil. being denser, they absorb it more readily and hold it longer. the change, then, from the period of fair weather to that of storm brings an increase of temperature. but the rate of increase varies. the faster the storm is approaching the faster the temperature will rise; and the route of the storm's center makes all the difference as to the amount of the rise. if the wind shifts by way of the north and holds in the northeast until precipitation begins the rise in temperature will be very slight. the great snowstorms of the northern half of the country occur under just such a circumstance. if the wind shifts by way of the north but gets around to the east or even southeast before the precipitation starts the rise in temperature will be more pronounced, as much as thirty degrees sometimes in a few hours, and the winter storm that started in as snow soon changes to sleet and rain. if the wind shifts by way of the south and then into the southeast the rise will be vigorous and the storm will likely be a comparatively warm rain. if the wind shifts only so far as the south the rise will be highest of all and blue sky will often appear between the showers, showing that the air is heated to a considerable height. the progress of the temperature changes from the maximum of the cyclonic area to the minimum of the anticyclone is also dependent upon the wind. if the storm center is passing south and the wind begins to pull into the northeast and north the temperature will fall steadily and slowly. the rain or snow often cease gradually by the time the wind has reached the north, but the temperature continues to fall slowly until it reaches very low levels in mid-winter. if the storm center is passing north of you the wind which has brought most of the rain while it was in the southeast with comparatively high temperatures swings into the southwest, the temperature falls somewhat. there is usually a final downpour and a rapid shift of the wind into the west or northwest, but almost never directly into the north. the temperature falls several degrees in a few minutes, quite unlike the gradual decline of the northeast-by-north shift, and clear skies come at once with rapidly diminishing temperatures. in the vicinity of philadelphia a fall of twenty-five degrees would be most unusual on the northeast shift,--such storms reaching degrees and falling to , while with the other shift a fall from degrees to would not be unusual. of course any one set of figures given could only show the tendency and not the rule or limits. after the manner of the wind-shift the intensity of the storm is a good gauge of the temperature change to be expected by the camper. as a rule the greater the intensity of the storm the greater will be the degree of cold that follows it. the storms that have a complete wind circulation about them are always more severe than those with incomplete circulation and are invariably followed up by some reduction in temperature. if the decrease is not proportionately great and the subsequent wind has only a moderate clearing quality look out for another cyclone. in such a case the temperature is the best witness of the contemplated change. for instance, after a summer thunderstorm a decided coolness is _de rigeur_. if this does not occur it means nearly every time that there is another thunderstorm in process of construction. there may be not a cloud in the sky, there may be no wind (although there should be) so that the course of the thermometer is the only means of telling what is to be the next event. anybody can take a thermometer with him although a barometer--the most accurate forecaster of all--may be thought too much expense and bother. at some future date the weather bureau will be able to predict the temperature of seasons in advance. this, together with the amount of rain scheduled to fall, will be an invaluable aid to everybody and to the farmers most of all. at present mild seasons that have severe storms without the appropriate degree of cold after them cannot be entirely explained, let alone being prediscovered. they all hinge upon the more or less permanent areas of high and low air pressure over the oceans and international meteorological service has not progressed far enough to support many ocean stations as yet. sometimes clear weather may intensify, growing brighter, stiller, colder. this is because the pressure is increasing. cold seasons are distinguished usually by a succession of anticyclones. there is no way of telling how long a certain spell of cold weather is to last, but i have noticed that the same characteristics rarely predominate for longer than a month at a time. in other words, if december has been warm and rainy, january will likely be cold and dry. of course, that is precisely the unscientific sort of generalization which the bureau very rightly frowns upon, but which one may nurse privately until science has provided a substitute as she already has in so many instances. with a little practice it is an easy matter to estimate the temperature to within a very few degrees. try guessing for a few mornings and then look at the thermometer. you will hit within three degrees every time after a week of this. allowance must be made for the amount of moisture in the air and for the force of the wind. damp air feels colder by several degrees than crisp, dry air, and a breeze increases the difference still more. air in motion is not necessarily colder than calm air. as a matter of fact the lowest temperatures of all are recorded about sunrise after a still, clear night. the amount of radiation accomplished during the last hours of the night is amazing, and the downward impetus of the thermometer is often carried on for an hour or more after the sun has appeared above the horizon. a self-recording thermometer is an amusing toy which will show this and becomes a valuable instrument if one raises fruit. in winter three o'clock of an afternoon sees the highest temperature usually, and in summer this maximum occurs as late as half-past five, due to the fact that the sun can pour in its heat faster than the earth can radiate it off. for the half hour before and after sunset, particularly in winter, the loss of heat is relatively greatest; then the pace slackens till three or four in the morning, when the plunge of the mercury is accelerated until the rays of the rising sun counteract the radiation. if the mercury does not rise appreciably on a clear winter's day it is a sign that a cold wave is stealing in, due, doubtless, to a gradual increase in pressure without its customary bluster. very often snow flurries predict its approach, but this may be so gradual that only the restriction of the daily thermal rise may indicate it. by the next morning the temperature will likely be twenty degrees colder. if the mercury does not fall on a clear winter's night it is a sign that a layer of moist air not far above the surface of the earth is checking the normal night radiation. unsettled weather is almost sure to follow unless this wet blanket is itself dissipated and the mercury takes its customary tumble before morning. if the temperature falls while the sky is still covered with clouds clearing, possibly after a little precipitation, will soon follow. hot waves approach insidiously. a night will not cool off as it properly should, the sun will rise coppery, and while the day is yet young everybody begins to realize that all is not exactly right. but the heat increases usually for several days, not only by reason of steadily lowering pressure, but also by accumulation. finally when a climax is reached it departs abruptly on the toe of a thunderstorm. a cold wave reverses the process. it arrives abruptly on the heels of a departing cyclone and, after losing power, steals away without any commotion whatever. its rate of progress is in close relation to the cyclone ahead of it. our mountains play a great part in our weather. they are a right arm of providence to our agricultural communities. due to their north and south trend a cold wave of any severity reaches the pacific coast only once a generation. just once has snow been observed to fall at san diego and it is so rare south of san francisco that many people never have seen a flake. east of the mountains the belt of desert makes natural crops impossible for a thousand miles, but if they crossed the continent all the territory north of them would have such a cold climate that none of the present enormous crops of canada and our northern states could possibly be grown. it is also due to the wide insweep of winds from the gulf that the plains states are so well watered. [illustration: cumulus _courtesy of richard f. warren_ the tops of cumulus are irregular, looking like wool-packs; the bases are flat. the true cumulus shows a sharp outline all the way round. its shape is in constant change due to the strong winds it is encountering. it is caused by the swift uprush of warm air on a sunny day. this cloud is a sign of fair weather, because the base is not large, compact, or dark enough to threaten rain and its comrades are also disjointed. if the cumulus grow darker toward the horizon and increase toward evening a squall is likely.] in lesser fashion the appalachians protect the atlantic seaboard. they withstand the impact of the cold waves to a great extent, although they are not high enough to divert the flow of cold air entirely toward the south and it is not desirable that they should. as things are the cold strikes alabama before it hits new jersey, and is often more severe there. comparative cold is often registered by the green color of the sky. a fiery red continues the prevailing heat. the day that is ushered in by a fog, in summer, will likely be warm, providing the fog lifts by ten o'clock. the temperature of a night with even a thin covering of clouds will be a good deal higher than if the sky is clear. in the british isles the whole difference between freezing and no freezing lies with the fairness of the heavens. everywhere frost will not form while the sky is covered, although the temperature may be below the freezing point. in summer radiation on a still clear night may be so rapid that frost may follow a temperature of fifty degrees at nightfall. the temperature at the surface of the earth may easily deceive, as a colder or warmer stratum of air may overlie that immediately next to the ground. i have seen water particles fall when the temperature was as low as degrees above zero, showing that the stratum of cold air was very thin. our sleet storms in which immense damage is done to trees and telegraph wires occurs from just such a situation,--a cold, shallow layer of air close to the earth, with the warm moisture-bearing air flowing over it. the reverse of this situation is not uncommon--the sight of a snowstorm proceeding merrily along with the ground temperature at or even degrees. coming warmth may be noticed by the increase in size of snow flakes, with finally hail and rain. coming cold is foreshadowed by hail mixed with the rain and lastly snow flakes which have a tendency to decrease in size. colors of the clouds predict temperature changes, but it takes much practice to distinguish the cold, hard grays from the soft, warm ones. a warm sky is always less uniform in color than a cold one. the colors of winter sunsets are, as a rule, much brighter than those of summer skies. the stars seem brighter on a night that is to be cold. if they twinkle it is because of rushing air currents, and if the wind is from the northwest the result may be a subsequent lowering of temperatures. the whole question of whether it will be colder and how much is vital to the camper and if the signs of change are taken along with the look of the clouds and the direction of the wind he need never be wrong as to the direction the mercury is going, and will soon be able to guess the distance pretty fairly. rain and snow east of the mississippi river rain falls with the utmost impartiality upon every locality. thirty to fifty inches are delivered at intervals of three or four days throughout the year. and if there is a slight irregularity in delivery one can be sure that from to of the days will be rainy. occasionally there is a more or less serious hold up of supplies, but this rarely happens in the spring of the year and never happens to all sections at once. and if there is a desire to make amends for the drought, we have what we call a flood and blame it on the weather instead of on our precipitous denudation of the watersheds. west of the mississippi particular people have to go to particular places for their rain. if they like a lot of it they must go to the coast districts of washington or oregon where they can have it almost every day. it rains a good deal at eastport, maine,--about inches a year; that is, nearly an inch a week,--but at neal bay, washington, at about the same latitude, in one year it rained inches, and it never stops short of inches any year. on the other hand, if the washington people are tired of it they need only escape to arizona where it rains about two inches a year, and they can live in an enterprising hotel down there whose manager believes that it pays to advertise the sun. he guarantees to provide free board on every day that the sun doesn't shine. in the plateau section enough snow falls every year to store up enough water for irrigation purposes, and the little rain that falls arrives in just the right season to do the most good, the spring. in california what the farmers lose in amount they make up in the regularity of its arrival. north of the ohio river most of the precipitation from november to april is snow. about inches of it falls on the average over this tremendous territory. and it is more useful than rain,--the handy blanket that makes lumber-hauling easy, that keeps the ground from freezing to arctic depths, that fertilizes the soil, and that acts as a great reservoir, holding over the meat and drink of the vegetable kingdom till the thirsty time arrives. in upper michigan and maine the average depth becomes inches. averages are very misleading when snowfall is being considered, some winters producing very scanty amounts and others heaping it on to the depth of inches once at north volney, new york. south of the ohio the depth varies from substantial amounts in some winters to almost nothing in others. snow has been observed, however, in every part of our country except the extreme southern tip of florida. once and only once on the records a great three-day snowstorm visited all of southern california, extending to the mexican border and to the coast. the strip of country between the parallels of new york city and richmond comprises the section wherein each winter storm is one large guess as to whether the precipitation is to be snow or rain. a compromise is usually affected in this way. before the clouding up began the mercury may have stood at ten degrees below zero. as soon as the wind acquired an easterly slant the temperature increased. as it neared the freezing point the snow would begin, first in flakes of medium size which would enlarge until after a particularly heavy fall of a few minutes they would at once almost cease. hail soon would succeed, the mercury still rising, and often the hail would have turned to rain before the freezing point of the air of the immediate surface of the earth had been reached, turning the snow already on the ground to slush and making a holiday for germs. one can always tell when this change to warmer is about to occur because the clouds which have been part and parcel with the obscuring snow suddenly show, not lighter but darker. the sudden increase in size of the flakes is another infallible symptom of increasing warmth in the atmosphere for each large flake is a compound of many smaller ones. when the temperature is low the flakes are very small, being grains and spicules in the severe blizzards of the west and falling as snow-dust in the arctic. in the heavy storms of the guessing-belt the flakes are not necessarily small. i have noticed (in the latitude of philadelphia) that our largest storms begin very leisurely indeed with small and regular-sized flakes. a quarter of an inch may not fall in the first hour. as the center nears the snow comes ever faster and larger, but not large, flakes are mixed with the original-sized flakes. snow dust is apparent. at the height of the storm flakes of all sizes except the very large are falling, denoting great activity in the strata of air within the storm influence. in the ordinary storm an accumulation at the rate of an inch an hour denotes a storm of considerable intensity. the snow will likely keep on falling as long as the flakes are irregular in size. if they grow large and few or very small a cessation is likely, even though the wind is still blowing from an easterly quarter. the amount of snow likely to fall can be gauged not only by the process of flake-change but by the rate at which the wind rises. a storm's intensity is measured by the amount of wind. a storm can be a storm without a drop of rain or flake of snow if only there be enough wind. and as long as the wind in a snowstorm keeps rising the storm is likely to go on, probably increasing in volume of precipitation. if the wind shows a tendency to edge around to the southeast there is danger of the snow turning to rain; if the wind veers slowly to the northeast the temperature will fall slowly and the rate of precipitation will likely increase for a while. in such instances the snow does not continue to fall after the wind has swung west of north. often clearing takes place with the wind still in the north or even a point east of north. contrary to superstition snow may begin to fall at any hour of the day or night. but certain hours seem more propitious than others, owing no doubt to the tendency of cooling air to condense. three o'clock of an afternoon and eight o'clock in the morning are favorite times, the one being the hour of a winter afternoon when cooling is begun, the other the hour when the coldest time is reached and condensation likely if at all. of course, one remembers storms beginning at nine, ten, eleven, and every other hour. storms that begin in the morning seldom reach much activity before three o'clock in the afternoon, while those that begin then quickly increase in intensity as evening draws near and the sun's warmth is withdrawn from the upper air-strata. more snow falls at night than in the daytime, also. snow is more delicate than rain and perhaps more responsive than rain to the subtle changes of the atmosphere. possibly there is no ground on the bureau records for these ideas, possibly storms have a tendency to start from the gulf on their northeastward journey and so reach philadelphia oftener at one time than another. i would like my notions confirmed that snowstorms increase at nightfall, and that they prefer to start operations at sunrise and about sunset. for the camper the snowstorm need have no terrors. it gives a long warning of its approach. it comes mostly without destructive winds. its upholstery protects and warms the walls of one's tent. it adds beauty to the leafless woods, interest to the trailer, and a hundred amusements among the hills. but the value of snowy weather is not only measured by its beauties and commercial uses. there is another way: make it read character for you. watch the reactions toward the first snowfall of half a dozen kinds of people. it will show you what they are; give you a very fair measure of their youth. our atmosphere contains a lot of moisture that never gets precipitated. you can prove this on any warm day by noticing the way the atmosphere acts toward a glass of ice-water. when the air of the room is much warmer than the surface of the glass it surrenders its moisture willy nilly. sometimes this condensation is enough to cause a miniature rainstorm that trickles down the outside of the tumbler. if a small cold surface can wring so much water out of a little air it is small wonder that we get an inch or so of rain from vast currents of air at unequal temperatures. try to visualize the process. a stream of vapor has been warmed and is ascending. a mile up and it has cooled not only by the reason of altitude but also by the process itself. about each little dust-particle in the surrounding area vapor forms--vapor cannot form without something to form on, there being always enough dust from deserts and volcanoes to go round. if the cooling proceeds the tiny globules enlarge and as they increase in weight they settle and fall. falling, they unite with others. if the air-strata are very warm and thick the drops may grow to a very considerable size. we see these in the middle of our great winter rains when the insweep of southern winds with all their warmth and moisture is very extensive. also the first few drops that come from the thick, hot lips of the thundercloud are usually immense. the best way to measure the size of a raindrop is to have it fall in a box of dry sand. it rolls up the sand and measurements can be easily and accurately made. but the most interesting way is to let the first drops of the thunderstorm fall upon a sheet of blotting paper. if the same sort of blotting paper is used the measurements will be of just as much importance for comparison. circles as big as teacups are formed sometimes. heavy drops in winter mean a heavy fall, because they denote high temperatures which are uncommon and are bound to be followed by considerable condensation as the cooling proceeds back to normal temperatures. small drops in summer mean either cooler weather, or sudden condensation. small drops in winter are a sign of very thin moisture-bearing strata, or low temperatures, indicating that the rain will be light, protracted, and liable to change to snow. hail is frozen rain. winter hail is small and harmless and rarely falls to any depth because the exact temperatures that bring forth the hail rarely continue for very long at a time. hail in winter is merely the stepping stone to either rain or snow. but in summer hail is a serious matter. it shows that there is a violent disturbance of the atmosphere in progress. vertical air currents, probably abetted by electricity,--the authorities are not sure--often carry the stones up several times. they take on layer after layer, coalesce, and sometimes fall the size of eggs, apples, or any other fruit, barring melons. the usual summer hail does not exceed the size of a robin's egg. even a projectile of that size, however, falling for a half mile or more has a tremendous destructive power. greenhouses suffer, birds are killed, cattle stunned, and loss of life has been known to follow. in august in in new hampshire hailstones fell to the weight of ounces, diameter inches, circumference inches. in pittsburgh stones weighing a full pound have crashed down, and in europe where many destructive storms have occurred there are official records of even greater phenomena. the lightning accompanying these hailstones is usually very severe. a flake or ball of snow forms the nucleus of a hailstone. if a thundercloud looks particularly black or if it can be seen in commotion think of hail and seek shelter. it is pretty difficult to predict exactly when hail is going to fall in summer. it is a possibility with every large storm, but a probability with only a very few during the summer. it accompanies tornadoes. in winter hail falls before a rainstorm, even when the ground temperature precludes the possibility of snow; some lingering stratum of cold air has ensnared the drops on their way down. snow is not frozen rain. it has an origin of its own. it is born in a temperature consistently below freezing and on the condensation of the invisible moisture becomes visible as a tiny crystal. these infinitesimal crystals unite and form larger, hexagonal shapes, elongated or starry. they are wafted along, sinking, all slightly differing one from another, although forming a few types. these types have been photographed and catalogued and very often the altitude from which the snow is coming may be learned from their shape and design. but this branch of science is young yet and confusing and the outdoor man has surer signs of the vicissitudes of the storm, in the general size of the flakes, the power and direction of the wind, the clouds and temperature. the possibilities of flake-study as a means of forecasting are many and of value as is anything that tends to unveil the secrets of the greater heights. snowflakes are so light that after the storm processes are over and the sun has come out the residue may still float lazily to the ground. the wild disorder of the snow flurry will only last a few minutes and never leave much snow on the ground. snowstorms that come on the wings of the west wind may be severe, but they will be short. they are unusual in the east, but sometimes the heaviest snows of the western states come on the sudden cooling that follows the shift to west. snowstorms arriving on a high wind last only a few hours. snowstorms that are long in gathering and increase to considerable intensity continue a long while. those that follow a sudden clouding up are of no importance. the snowstorms that leave on a high wind from the west or northwest are followed by a cold wave. those that continue after the storm wind has died away are succeeded by calm, clear, and usually warmer weather. in northern districts a snowstorm may be looked for after a period of cold weather. in middle districts if the cold has been severe the reaction to warmer may bring rain instead. in such cases generalities are of no use, and the possibilities must be determined by the man on the spot. the best conditions for snow through the middle districts are occasioned by an area of low-pressure with its attendant precipitation crossing the southern half of the country while the northern half is under the influence of an area of high-pressure with its attendant frigidity. the cold air flows into the southern storm with the result that the middle districts get the northern quadrants of the storm which are the usual snow-bearing ones instead of the southern rain-bearing quadrants that they would have got if the center of the storm had pursued its usual course up the ohio and down the st. lawrence. if the storm has two centers, one over texas and the other over montana, as is so frequently the case in winter, the subsequent high pressure will come too late to affect the temperature of the zone of precipitation and the latter will likely be rain in the middle districts. sometimes the cyclones cross the country on the canadian border and enough warm air is sucked over the line to give the inhabitants of montreal a thaw and rain. this happens to them only once or twice a winter. and even more rarely a cyclone over the gulf with an anticyclone above it will give the gulf states a taste of winter, but rarely more than a few flakes. it really all depends on the influx of air, its rate and direction. it rains in alaska and snows in georgia on the same day merely because at one place the air is coming off the pacific, and at the other it is flowing from the center of a refrigerated continent. and the progress of these storms is one of nature's greatest poems if you take a minute to think of them sweeping on in majesty, the one thing that man cannot control. even the snow which is the citizens' curse as well as the farmers' blessing becomes epic when it beleaguers an empire for half a year. dew and frost the very process that made the tumbler of ice-water sweat on the hot day causes dew. and the formation of frost is analogous to that of snow. frost is not frozen dew, but the formation of moisture crystals at the temperature of ° or below. frost or dew form only on still, cloudless nights. even if no clouds are visible, neither will form if a stratum of humid air has prevented radiation. hence either dew or frost is a fairly good sign of clear weather. three white frosts on successive mornings are followed by a rain. this saying holds water not because there is any virtue in frost to cause rain, but because a storm is normally due once a week. the frosts did not form when the anticyclonic winds were blowing and usually not more than three mornings elapse between the time that the anticyclone has lost its influence and the time for the next cyclone to appear. frost indicates a considerable amount of moisture in the atmosphere, also, which tends to increase as the cyclone approaches. the heaviest dews come in late summer and the heaviest frosts in mid-autumn because the change in temperature is greatest then and there is a greater chance that there will be a calm at sunrise. the greatest frost damage occurs in the spring because the tenderer crops are growing then. summer frosts used to occur in the northern parts of minnesota and along the southern boundaries of the inland canadian provinces before the forests were cleared off. the march of civilization has actually pushed back the frost line some distance. frost may occur when the amount of humidity in the air is low and the barometer rising at any temperature under degrees at nightfall, the clear skies permitting radiation enough under those circumstances to produce the necessary cooling. an evening temperature of degrees with the clear skies and faint west breeze will almost surely produce a frost, provided the wind drops. in such circumstances the only hope for the farmer is that there is enough humidity in the air to cause a fog before the frost-point is reached. a temperature touching degrees would not bring frost, however, if the sky was at all overcast. frost is difficult to predict because a night shift in the wind, cloudiness that forms after midnight, or even a wind arising before the coolest period at dawn will prevent its formation. on the other hand, clouds may disperse, the wind may fall or radiation may be so rapid before sunrise as to cause a killing frost unawares. the farmer who lives in areas disputed by winter and spring may never be quite sure, but precautions should be taken on the still, clear, dry nights with the thermometer at fifty or below. fruit-growers resort to fires or to coverings to protect their crops. the fires are particularly worth while, not so much for their heat which at best cannot be expected to warm up the great outdoors much, but for the smoke which prevents radiation. a line of smudges such as campers use to ward off the mosquito would spread a pall of smoke over an orchard efficaciously. a snowstorm, the soft fluffy sort that falls in april or may, can do much less damage to vegetation than a severe frost. temperatures are much lower on the ground than even six feet above the grass. naturally these temperatures are those that really influence most vegetation and in england temperatures on the grass are given in the weather report with the ordinary observations, being as much as six or eight degrees lower on clear nights. in some of the hot, dry countries, such as arabia and egypt, most of the moisture that they receive falls in the form of dew. falls, of course, is a loose expression as the dew forms and does not fall, being different from the minute particles of fog. the fog particles in suspension in the air are estimated to be as small as - th of an inch. when they grow to - th of an inch in diameter they commence to fall. fogs are chiefly caused by the soil being warmer than the air above it; the vapor on rising condenses and becomes visible. in the spring and fall currents of air blow over rivers at different temperatures and the result is a fog. one does not have a fog in the desert. there are places in the ocean with cold and warm currents with the air above them correspondingly different where fog is of almost constant occurrence. the gulf stream off the grand banks of newfoundland has a temperature of degrees, while the water on the banks is degrees so that fogless days are rare along the line of meeting. frost is known in every part of our country, many localities in the plateau section being exposed to it every month of the year. the thin air and cloudless skies of the altitudes make radiation very easy and the daily variation of temperature is much wider than along the humid coasts. those who have never looked into frost conditions throughout our country will be surprised to read the warnings of the weather bureau. from the station at pensacola, florida (frost-proof florida!), comes this statement: "vegetables are subject to damage by frost during all seasons of the year." pittsburgh, pennsylvania, "frost is likely to damage fruit or other crops in may and september." phoenix, arizona, "frost is likely to do damage in december, february, and march." baker city, oregon, "fruit and other crops are most liable to damage by frost in april, may, june, september, and october." kalispell, montana, "frost damage for fruit, may th to july th; for grain, june th to august st." montgomery, alabama, "during march, april, and may fruit and early vegetables are subject to damage by frost." the thunderstorm exposed probably nothing in the world causes more terror than a flash of lightning. in an able-bodied thunderstorm playing about a city there are several dozen flashes, and every one of them brings trepidation, fright, or positive terror to thousands of human beings,--oftenest women, sometimes men, and occasionally children. yet probably there is no alarm in the world so ill-founded. thunderstorms play pretty generally over our three million square miles with their hundred million population. yet lightning picks out of this crowd only three hundred people a year who are foolish enough to be killed. that is, only three persons in each million to be sacrificed to the most astounding and beautiful display in the world, a mere handful compared to the mounds of motor car victims or to the , deaths a year attributable to railroads and the perils of track-walking. the trouble about the thunderstorm is that it does not lull one into the sense of insecure repose. it is too obviously after one. if the thunder were toned down a bit and the lightning a trifle duller the alliance might claim its thousands, like the inconspicuous housefly, and never meet an objection. but until the thunderstorm foregoes its bravado it will continue to bully the ladies into hysterics. of course, there is always the sporting chance that you are one of the three in your particular million to perish. but you can lessen the chance. you must not seek refuge under a tree. you should not take doubtful shelter in a barn. and you had best not sit in a draft by an open window if there is a tree just outside it. by these three avenues most of the thoughtless three hundred (a year) invite their end. trees that are tall and otherwise exposed are struck oftenest. the electricity in the cloud and the electricity in the earth are always endeavoring to combine. when this tendency becomes so strong that the resistance of the intervening air is counteracted the electric discharge between thundercloud and earth takes place. this happens most frequently from some pointed thing as a steeple, a tree if they are good conductors. men and animals are sometimes charged with the electricity opposite to that of the cloud. when the lightning is discharged, even at a distance, the bodies revert rapidly from the electric to the natural state. this return shock or concussion occasionally proves fatal. that is the reason that trees are such poor protectors from the storm's fury. better a wet skin in the middle of a field than precarious dryness under an oak or cherry or tall pine or almost any other tree. if it should hail hard enough to stove in your head take to a beech or a small spruce. barns are struck so often because the body of warm, dry air in them favors the passage of electricity. those who hide in barns are sometimes cremated. after a severe thunderstorm in the poconos i have seen as many as three barns on fire at once. open windows, porches, and exposure generally are safe, but not safest. the cellar, that old stamping ground, is where instinct takes a few. any closed room on the side of a house away from trees is good enough. but the risk of annihilation is so very small that one is repaid for taking it by the spectacle. a great thunderstorm surpasses anything in nature in the matter of architecture, coloring, directness, and surprise,--which, with selection, comprise the essentials of art. imagine the crowds that would pay to wonder at the sight if a thunderstorm could be staged, say, at the hippodrome! some hot morning, if you have time to watch, you may see a thunderstorm born in the mountains. the warm, moist air flows up the mountainside and the essential start is made. cooling, this air first shows as a fluffy cloud that soon grows harder in appearance and becomes tufted at the top. its little belly swells and grows blacker. it hovers over the valley. others add to it. suddenly a sort of adolescent thunder is heard. the tension has become too great. a definite consolidation is visible, a fringe lowers, and a few drops of rain may reach you. the incipient storm moves off, and having started a whirl within itself, increases, like a rumor, as it goes. before it has moved beyond your horizon it may have become a large patch of dark blue with billowy white crests on the top, and underneath hangs a curtain of rain. chances are that it will not go far before encountering conditions that dispel it, but it may cover half a dozen counties before nightfall. as a rule these little heat thunderstorms do not amount to a great deal. they are originated by local conditions and leave things pretty much as they found them. but when a cyclone is passing in summer a series of thunderstorms or heavy showers with some thunder frequently take place instead of the all day winter rain. these thunderstorms mount up against the wind. their clouds are black. the word black is an indulgence of the human weatherman meaning, of course, any dark color,--a black sky would terrify the most hardened of meteorologists. the cyclone winds come from the south or southeast just as they do in winter, but this quarter may not bring the heaviest rainfall in summer. there may be showers or even clear skies, but the day will be humid and hot. a haze of cirro-stratus cloud will gradually overspread the sky from the west, darkening into a blue from the original whitish or gray. lightning does not appear from the cirrus, but after the sky has grown pretty dark a ridge or tumbled cloud will be seen low on the western horizon. meanwhile the wind will have died down. the lightning, at first only a faint glimmer, will have become more frequent and noticeable. if it is striking at a distance of fifteen miles the thunder will not be heard. as soon as the storm center, where the heaviest rain and the electrical display are taking place, gets within the fifteen-mile radius thunder will be heard to growl, and the tumbled cumulus clouds which may have lain along the horizon for hours will begin to approach. the storm will be upon you in ten minutes likely after the arc of foreboding blue and white cottony cloud has begun its charge across the sky. light quickly fades from the heavens. the wind drops entirely. streaks of lightning burn downward. behind the arc stretches a curtain of uniform blue or gray. if the gray is lighter in places the rainfall will not be heavy. if the curtain is a uniform blue a heavy rain is sure. if the bow of clouds can be seen to tumble or is continuous and approaches fast the wind is certain to be severe,--may be from to miles an hour for the first few minutes. sometimes a cloud of dust advancing before it demonstrates its force. this moment immediately before the storm breaks is the dramatic moment of the entire cyclone. as in a tragedy, the interest has built up to this supreme occasion, this knife thrust, from which interest recedes until clear skies show that the play is over. from to hours is the usual time required in winter. in summer the cyclone takes even longer to pass a given point, but the period of rainfall, in which the winter storm's amount is often surpassed, may not last fifteen minutes. first the blow, then a crash of thunder, and the rain in big drops, which lessen rapidly in size as the whole world seems involved in the vast forces of the storm center. most of the precipitation occurs in the first fifteen minutes, sometimes in the first five. a hearty storm will deliver an inch in short order. although the rain continues often for an hour and sometimes in the storms that are attached to a well-defined cyclonic system there will be two or three robust thunderstorms in succession, yet the first downpour is usually the torrential one and the others die away until the conditions that caused the outbreak have passed off. with the severer storms hail falls. the general condition of the air after a thunderstorm is cooler, dryer, and more invigorating than before. ozone has been liberated, dust has been washed from the air and vegetation. the surest sign of a continuation of unsettled weather is the failure of the atmosphere to cool off. if the air remains sultry and heavy and depressing another shower is due. in such circumstances the wind will not have begun to blow with any great promise from the west. a close, sultry morning is the best indication of a thunder-gust. the large piles of cumulus clouds are called thunderheads for the very reason that they almost always precede a thunderstorm. the heaviest electrical disturbances have cirrus clouds a few hours in advance of them very much as their winter relatives. a thunderstorm that does not cause the barometer to fall considerably will not amount to a great deal. at night the different kinds of lightning furnish a running commentary to the storm. on calm evenings the sky will be cloudless, with perhaps the exception of a low rim on the northern horizon. yet flashes of lightning, of course without thunder, may be seen illuminating that entire quadrant of the sky. this is called heat lightning and is popularly supposed to be the result of the heat only. as a matter of fact it is caused by a normal thunderstorm that is operating below the horizon. reflections from this storm are shown on the rim of clouds, or if no clouds are visible, on the bowl of the sky. if you see lightning be sure that there is a storm somewhere. if this disembodied sort of lightning continues to flash from the western sky it is quite possible that the storm will reach you. if it shows on the northwest or north of you the chances are that the storm will be carried around. if the wind is from the southwest and the lightning appears there only the progress of the clouds will show whether the storm is pursuing the normal track from the west and around you or whether it is edging up toward you. one cannot be very well surprised by a thunderstorm of any energy in camp as the lightning shows as much as two hours before the storm breaks and the thunder gives fifteen minutes' notice on most occasions. the sort of lightning that spends itself illuminating the clouds in serpents and willowy branches confines itself to the altitudes and is very beautiful and harmless. it is accompanied by thunder that sounds hollow, that rumbles over the sky, and usually does not end with the crash and thud of the more vigorous variety. such lightning and such thunder are more often connected with the sort of storm that comes up very swiftly on a western wind. it gives shorter warning than any other sort of thunderstorm and is not connected with the cyclonic area. i have known such a storm to manifest itself low in the west, approach, and break within twenty minutes. much wind results and not much rain, although the temperature falls. lightning with storms of this impromptu kind rarely does any damage. but if the storm rises slowly against the wind, requiring an hour or two or three to approach and break, the lightning will grow almost continuously, some of the flashes being broad streamers cleaving the western sky. it is this sort of lightning that does the damage. the thunder, instead of rolling like an empty barrel, hits into a series of concussions. if the lightning strikes an object nearby the crash is rather appalling. there are several freak sorts of lightning such as the ball form, which are rare. the approach of the center of disturbance may be gauged by the length of time that elapses between flash and crash. in reality the thunder occurs immediately after the discharge of electricity, but sound travels so slowly, compared to light, that a minute may intervene between stroke and clap. you may count the seconds, noticing the regular decrease, signifying the nearing of the crisis. soon a flash in front and a simultaneous peal will show you that you are in the thick of things. the next bolt or two may hit very close and you can appreciate what it means to be on the firing line. then the next river of fire with its detonation streams behind you and you are saved. in a severe thunderstorm there are several centers, several nuclei that shed destruction like great batteries and their progress over and beyond you has its thrills. you may find the exact number of feet away that the bolt hit by multiplying the number of seconds elapsing between the lightning and thunder by . but an easier way is to allow a mile for every five seconds on the watch. one or two seconds, and you are pretty near the center of the fray. lightning compresses the air, leaving a partial vacuum. the other air rushing in to fill this partial vacuum forms the wave motion that produces the noise. that is the whole why of thunder. the reason thunder rolls is that the lightning is a series of discharges each of which gives rise to a particular detonation. if lightning were but one discharge, the thunder would be but one stupefying crash. reflections from the clouds and from layers of air of different densities and from the ground are agencies that prolong the sound. our atmosphere is never lacking in electricity. this electricity is always positive in clear weather and sometimes negative in cloudy. science concludes, then, that negative electricity invariably indicates rain, hail, or snow within a radius of forty miles. moist air is a good conductor. our powerful motors can now produce a spark of electricity several feet long. but some of the flashes that shoot across the sky in a big storm extend over five miles. the duration of the flash varies from - th of a second to a second. the reason that lightning does not always pass imperially along a straight line is that some air, either moister or warmer than the air around it, offers less resistance. the lightning takes this line of least resistance along the pathway of warmer or less dense air. altitudes of thunderclouds vary. they may hover above the earth at feet. they may be a mile high. they have been observed on peaks of mountains three miles high. many other electrical phenomena are observed in the mountains. the study of these will undoubtedly benefit meteorology and perhaps go far to explain the unsolved problems of the service. one kind of thunderstorm that is rather rare is that which arrives in winter with the passage of an energetic cyclone. often when the wind, having been in the southeast for most of the storm, is passing around and reaches the south or southwest the rainfall culminates in a deluge and thunder is heard. one or two such storms are a winter's complement. they usually terminate the rainfall for that particular cyclone. i have never heard of damage caused by these winter electrical storms, and they occur only in exceptionally well-developed areas of low pressure. lightning has many times been observed during heavy snow storms. i have never heard any thunder with it. the discharge must have been very faint. [illustration: stratus _courtesy of richard f. warren_ stratus is merely lifted fog in a horizontal form, the lowest of all, and the simplest as regards structure. it means neither rain nor snow and the apparent clearness of the blue above it would indicate clear weather to come. but through the break in the stratus near the horizon shows a cloud of firmer texture, which is less reassuring. stratus over the land in winter takes the appearance of long bolsters of gray through which a pale blue sky shines. such clouds may blanket the sky for days without causing a drop of rain. if they show a tendency to glaze over expect snow or rain, but not in large quantities.] the fascination that a thunderstorm has for many people is explained partially by the fact that one sees the whole process from beginning to end. the officials of the weather bureau have this privilege as regards cyclones. it is their business and pleasure to watch the setting up of these vast storms, to follow them on their journey. it is small wonder then that they find the spectacle fascinating. the tornado the birds, the flowers, and the tornadoes are all busiest in spring. and the tornadoes probably make the largest impression. a tornado is merely a whirl of air, caused, as are all the other whirls, by a striking difference in temperature in adjacent areas. a tornado is a local and restricted example of the same thing that a cyclone is. but a tornado rarely crosses more than a single state; a cyclone strides continents. a tornado lasts, in one place, about a minute; a cyclone affects the weather for three days. a tornado never survives the night; a cyclone plods on for a week. and yet if you are betting on destruction put your money on the tornado. what it lacks in the realms of space and time it makes up in intensity. its sting is fatal. tornadoes occur chiefly in the spring because the temperature changes are greatest then and it is from these that the tornado sucks its nourishment. over the plains, for example, a limited area is abnormally heated by a local cause. abnormal cold comes in contact with the abnormal heat. the great difference in pressure results in a spiral as it did in the cyclone, only in a very small spiral, and once begun its energy is self-aggravating. the whole thing moves off toward the northeast attended by the black cloud of its condensation. from the black cloud a funnel like an elephant's trunk sways back and forth, now touching the ground and now escaping it. the black cloud has been in the southwest for some time probably before it has commenced to move. the day has been very oppressive. the sun rose rather coppery, in all likelihood. as the black cloud with the swaying funnel nears a roaring is heard. darkness falls. the roar increases.... instantly it is over. now that you've been through a tornado you know how it feels,--almost. after the funnel passes hail falls, lightning flashes through the lessening murk. heavy rain succeeds, and if you're alive you go out and rescue the perishing. the wind velocity in the path of a tornado is enormous,--anything up to miles an hour,--but no instruments have been devised to withstand the strain. varying pressures are responsible for the destruction. as the funnel passes over a house where the normal air pressure is about , pounds to the square foot it removes , pounds for an instant. naturally the outside walls cannot withstand this enormous inside out pressure and the house explodes like a projectile. only under such conditions could the vagaries of matter,--straws piercing logs and chickens bereft of every feather--be perhaps not explained but pardoned. stories of any degree of incredibility crop up after each tornado, often with accompanying photographs as proof. people are plastered with mud, pianos are deposited in neighboring lots, babies are hung up unhurt by their clothes in tree-tops, and often one person is killed and another nearby escapes unhurt, bible-fashion. tornadoes may form almost anywhere, but they are never found on the immediate pacific coast. they are most common in the mississippi valley, are rather common in the gulf states, and have occurred throughout most of the east at one time or another. since there is no way of stopping them the next best thing is to know the conditions that make for their formation. if the weather bureau predicts a cold wave for sections of the country where the weather is already abnormally warm the line of meeting will probably produce a tornado somewhere. the officials, however, advise you not to worry until you see the intensely black cloud in the southwest trailing its funnel. see where this funnel is tending and run the other way. all tornadoes progress from the southwest to the northeast. bad as they are, this makes them far less terrifying than if they whipped back and forth over a town or chased you around the pasture. if you happen to be in the house, take to the cellar, the southwest corner of it. if you can't escape lie face down to the ground. the only tornado that i have ever witnessed was an undeveloped one in england, and a bit lethargic compared to those of the prairie states. but even this blew an entire train off the track. it had all the other appurtenances of a tornado, the hail, the twisted trees, the narrow southwest to northeast path. the fact that the houses had only corners of their roofs blown off showed that as a tornado it was distinctly second-grade and without power to explode. england, shortly after, was raided by three water-spouts. these phenomena are caused by precisely the same conditions as are the tornadoes. they form over the sea, and the funnel is composed of water. they take considerable bodies of water up into the skies and torrential rains result over adjacent districts. if i remember correctly, two of the english water-spouts broke against the cliffs and the other, moving inland in modified form, gave gloucester a nine-inch rain. ships have been known to fire cannon at these spouts. if one hit a boat directly damage might be caused, but they have little of the destructive force of the tornado. as our country builds up the destruction from this most powerful of all phenomena is likely to increase. bureau warnings over phones may result in the saving of some lives; cellars will undoubtedly be built in the principal zones. but the problem is an interesting one, for unlike the waterspout, cannon cannot be employed to shatter an emptiness that stalks the more malignantly the emptier it is. the hurricane the tropical hurricane is undoubtedly nature's mightiest exhibit. the hurricane is the cyclone par excellence. it does not differ from our ordinary weekly cyclone in the essentials of wind rotation or pressures or rainfall; but it does differ in place of birth, in its course, and chiefly in its intensity. the genuine hurricane is a west indian production. it is generally cradled in those islands south and east of jamaica and cuba. it is nursed by the trade-winds. the first notice of its birth is an alteration in these winds, which are among the most regular observances on our planet. an extensive formation of cirrus clouds spreads over the sky and the barometer, which has been stationary for some days, edges off and begins a long and gradual fall. great rollers are noticed for a day or two before the winds rise. a hurricane moves slowly. this tropical organization is superior in depth to our shallow, disc-like, continental cyclone which is one and rarely over two miles thick. the hurricane rears its head three, four, and even five miles high. instead, too, of dissipating its force over thousands of miles at once it is only a few hundred miles in diameter. its center moves methodically along at the not very impressive speed of fifteen miles an hour, while our cyclones hurry along at thirty. but the hurricane is thorough. the wind about its center reaches a velocity of miles an hour. this velocity has never yet been attained on the surface of the earth by our trans-continental cyclone. our cyclone always has an eastward trend; the hurricane has a parabolic course. it begins by moving west on the trades, drifting and dealing destruction to the banana and sugar plantations of jamaica. it enters the gulf of mexico, and since it is then pretty much out of the influence of the trades it curves to the right and begins to act like any other storm by heading directly for the st. lawrence. if it passes out through the florida straits it never reaches the st. lawrence but speeds up the coast and out to sea, usually at hatteras to follow the shipping routes across the north atlantic. but if it has become so involved in the gulf of mexico that it cannot escape to sea again, it comes up through the gulf states and on toward new england. fortunately as it goes inland its intensity diminishes because it has not so much energy-giving moisture to draw from. also its sphere of action widens, its embrace is less mighty, its characteristics more those of an ordinary continental cyclone. it manages, however, to deliver gales of miles an hour along the coastal plain, increasing to at the exposed places such as hatteras and block island. the intensest hours of a hurricane are those when its course is changing from westward to eastward. enormous rainfalls accompany these storms, amounting to six inches in some instances. since one inch of rain amounts to tons per acre, and , tons to a square mile one can imagine the great amount of evaporation that has taken place to so saturate the air as to drench vast territories to such an extent. while scarcely a year goes by without one of these west indian hurricanes distinguishing itself on our shores the one that visited galveston in eclipsed all. it chose to turn in the vicinity of the city. the gale increased to over miles an hour and the wind gauge then blew away. the waters of the bay were heaped up and three thousand lives were lost in the flood and wreck of flying houses. this peculiar storm did not turn northeast at once but ascended the mississippi, turning at the lakes and proceeding down the st. lawrence after having spent a week in our country. the listless doldrums have sent us of these storms in the last generation. june has seen , july , august , september , and october . sea-yarners have seized upon the hurricane to energize many a flagging chapter, and particularly have they emphasized the eye of the storm. the eye is that vortex where contending winds neutralize each other into a calm, where the sun shines out through the scud, where the waves, relieved of the great pressure, leap upward in wild disorder. then the center passes and the wind flings itself upon the unlucky bark from the opposite quarter. its first onslaught is always represented as being the fiercest of the whole storm and gradually lessening as the center drives farther away. this is true in the same way that the first attack of the thunderstorm is usually the fiercest, both being when the pressure begins to rise. this savage change to the northwest is naturally the hardest of all for the ships to bear as they must steady at once against the severest blast instead of gradually bracing for its culmination. in no department of meteorology has fiction adhered so closely to the facts as in the sea-rover accounts of the hurricane. but in real life there is very little excuse for the vessel to be caught anywhere near the disastrous center of the storm. indeed, for generations sea-captains have known how to escape the deadly eye. by watching the barometer and noticing in which direction the wind is working round they can tell the course to a nicety and estimate its speed. then the wise ones run the other way for even the _olympics_ and _imperators_ of the sea are cowed by the might of the west indian. the typhoons of the west pacific are similar manifestations. the hurricane moves off from its birthplace so slowly that our weather bureau has an opportunity to size it up, to chart its probable course, and to warn shipping interests. the ship-owners, as a class, appreciate the service of the bureau and obey its warnings. vessels with cargoes of a total value of $ , , were known to have been detained in port on the atlantic coast by the bureau's warnings of a single hurricane. now that a much vaster commerce will steam through these dangerous waters toward the panama canal the warnings will assume an even greater importance. the best description of a hurricane that it has been my fortune to read is in a story entitled "chita," one of the remarkable fictions of lafcadio hearn. as truthfully as a scientist and with great beauty of style he has pictured the long days of burning sun, the foreboding calm, the thickening haze, the ominous increasing swell of the ocean, a breathless night with the lightning glowing from between piling towers of cloud, the startling suddenness of the wind's attack, its fury, the hissing rain, the shrill crescendo of the gale. cloudburst it is the american tendency to exaggerate. we call every snowstorm a blizzard, every breeze a gale, every shower a cloudburst. in our generous vocabulary it never rains but it pours. consequently if we, in the east, ever had a real blizzard or a real cloudburst we should be at a considerable loss to find words for an unprofane description. i do not know how they manage out west where these things occur. a genuine cloudburst must be an amazing spectacle. it is caused by a furious updraft of wind keeping a rainstorm in suspense until so much water has accumulated that it has to let go all at once and the accumulation descends like a wet blanket. this phenomenon is staged in the mountains; most often in the rockies where melting snow and desert-hot ravines provide the necessary extremes of temperature. wind blowing up a mountain-side can maintain considerable force,--so much that a man cannot possibly walk against it. black thunder clouds brew on the peaks. suddenly the collapse, and the person who tells the story afterward finds himself struggling in a torrent that a minute before had been a dry gulch. the moral of the story seems to be that if you are camping in the mountains and there is a strong upstream wind blowing and the clouds darken about the hill-tops and the thunder mumbles then don't make your bed in the creek-bottom lands. the high water marks of former freshets, but not of cloudbursts, show on the side of the stream. even in the less impulsive east a couple of inches of rain make a surprising rise in a little creek. the halo the halo is a luminous circle around the moon or the sun. it is caused by the refraction of light passing through moisture, which at the usual height is in the form of ice-crystals. the halo when complete consists of two large circles whose diameters are constant, and degrees. then there are often other arches in contact. at each point of contact occurs a parhelion which is a mock sun of brilliant colors and called a sun-dog. since the sun-dog is brighter than the other parts of the halo it sometimes appears when the rest of the halo cannot be seen. sun-dogs hunt in pairs or fours. if the halo is colored the red is on the inside. when the colors are caused by diffraction instead of refraction, the red is on the outside of the prismatic ring and the halo is called a corona. having now satisfied the demands of science all that can be forgotten except that the halo around either sun or moon means excess moisture in the atmosphere. the wide halos are seen in the high cirrus clouds , , hours in advance of a cyclone. at first the ring is very wide and faint with several stars in it. if the storm is advancing rapidly the halo brightens and narrows and the stars fade. this is proof to show that the proverb stating that the number of stars inside the ring is a forecast of the number of days of storm is sheer nonsense. for presently the ring closes and the stars disappear which would show according to the proverb that the storm had changed its mind and would cut down the number of days from several to none. the moon grows paler. the light that it casts upon the earth is eerie at this stage. within a few hours the cocoon of mist is completely woven about the moon. the circle has closed. snow or rain begins within a few hours after the moon has entirely disappeared. if it does not so begin it shows that the process of increasing humidity is a very slow one and the storm center is probably passing far to one side of the observer. also if the snow begins before the light of the moon is entirely suppressed the disturbance is a shallow one and the storm will be light. when the halo is actually a corona (red outside) the approach of the storm can be gauged by the rapidity with which the circle grows smaller. for a decrease in diameter denotes that the size of the moisture drops is increasing and therefore the storm is approaching. as a matter of fact the corona will have disappeared long before the time for rain. still it is useful to know that if the corona increases in size the conditions are clearing. with the halo the reverse holds. for when the clouds are very high the halo looks small, and high clouds imply swifter winds and a greater distance from the storm center. the zuñi indians who have an eye for the picturesque as well as for the truth state the chief fact about haloes happily: "when the sun is in his house it will rain soon." another saying of theirs anent cumulus clouds holds for our country as well as for theirs: "when the clouds rise in terraces of white, soon will the country of the corn-priests be pierced with the arrows of rain." there are many little observations which the man who has kept the corner of his eye open may profit by and yet which are rather difficult to express in type. who could describe an egg for instance whose springtide of youth was far behind and yet was not quite ready for the discard! in nature it is the fleeting moment of transition, the half-tones of the border that are so hard to catch, so difficult to portray, and yet so very important not to miss if one is to become sure. there follow some of the baldest and most communicable half-facts about the weather that should be used oftener to bolster up some opinion gleaned from more positive sources than to mould one in their own strength. moisture in the atmosphere helps sight to a certain extent. for when the air is full of moisture its temperature tends to become equalized, obliterating irregularities which would otherwise reflect the vibrations producing sight and sound. so if one hears better or sees better on a certain day it augurs a moister atmosphere,--an auxiliary sign if there is a view that you are fond of looking at many times a day. in the city, alas, clearer vision on one day than another means merely that less coal is being used. but in camp there is very often a perceptible difference in one's seeing ability even on days that could all be classed as clear. another thing that the haunter of the woods may notice is that his smelling capacity is increased before a storm. the increase of humidity which precedes a rain buoys up odors and depresses smoke. even in dry weather if you will stroll by a marsh you will notice how rank the vegetation smells and how the smells float in layers in the air strata of different humidity. one's sense of smell is a very slender thread on which to hang a storm, however. fires burn more briskly in dry air than in moist, but to tell the difference (if you can't feel it) you must be very sure that your wood is as dry on one day as on another. before a rain many plants close their flowers or shift their leaves. the dandelion, pimpernel, red clover, silver maple are good examples of this, but they would not be of much use in the north woods. the closing, too, takes place only a few hours before rain and is merely confirmation of the signals rendered more adequately by clouds and winds. bugs and flies are particularly annoying before a storm and it is surprising that the spider should not take advantage of this to get a meal. but spiders are cautious and they never spin a web on the grass, at least on the day that brings a storm. the insects do not fly so high on these weather-breeding days and consequently the birds that feed on them fly lower. the chimney swifts are a particularly good guide to the different altitudes at which insects fly. the stars are on a par with bugs as weather guides, although there are many proverbs that grant them much. one circumstance should not be neglected, however, and that is that wind mixes air and when air is well mixed atmospheric inequalities are less disturbing to vision. hence when one can see the stars and the moon well wind currents are oftenest the cause. even if it is not blowing on earth these wind currents may yet be blowing above to reach the earth later. in this way cold waves arrive. there is an old proverb about this condition, applying it to the moon, "sharp horns do threaten windy weather." but the stars are of second rate importance because they are so soon obscured. if you can't see them it is cloudy, but you do not know what kind of cloud it is. if only the brightest show, a veil of cirrus is arriving. a dark sky with only a few dim stars is an omen of storms. if the stars twinkle it is because the varying currents of the upper air are in juxtaposition. if they twinkle while the northwest wind is on it is a sign of colder weather,--not because they are twinkling but because of the northwest wind. in the days when almanacs were the sole guides to the weather a man with a sense of humor, butler by name, got out one and dedicated it to "torpid liver and inflammatory rheumatism, the most insistent weather prophets known to suffering mortals." rheumatism is following the almanac to the scrap heap, and it would be harder for a camper to guess what a torpid liver was like than to forecast the weather, yet for the majority of "suffering mortals" there is still much truth in the amiable observation of mr. butler, "as old sinners have old points o' the compass in their bones and joints." chapter v the barometer whatever the foregoing chapters may imply as to the whole world going camping the fact is that the woods are still, unfortunately, for the few. the woodsman must yield gracefully to the suburbanite,--in numbers. but the weather is for everybody. to be sure the sunrise that talks so confidentially to the hunter of the coming day does not exist for the commuter. but the coming day does, even though the things it means are essentially different. to the hunter with his seasoned clothes and well-earned health a rain is only of concern in so much as it affects the business of the day; personally it is of small moment. but to the commuter what does the weather mean? dollars and cents, of course. his business goes on, but to his person one unexpected shower = the cost of pressing a suit; one thorough soaking = one doctor's bill. for you cannot expect the man to throw off a chill who can quiet his conscience on the matter of daily exercise by watering the geraniums and reading the newspaper. weather wisdom is necessary for the hunter; for the commuter it pays. the hunter had to rely on local weather signs. the commuter can go him one better by investing $ (how finance will creep in!) in a little aneroid barometer. the local weather signs were good for twelve hours at the longest. the barometer is a faithful instrument that adds another twelve hours to a man's knowledge. half a day, or even a day before any local sign of changing wind or growing cloud appears the barometer is on the job. it will register in philadelphia the news of a disturbance approaching the mississippi. so sensitive is it that it is the slave to every wave of the great air ocean. the barometer gauges for the eye the amount of atmosphere that is piled above one. if the amount is normal and at sea-level the instrument will measure . inches. this air pressure is equivalent to a column of water feet high. as this would make unwieldy prognosticators the scientists use mercury instead, which requires a column less than three feet long. and for general purposes this is supplanted by the handy little aneroid (which means "without fluid"). this is so fixed that the pressure of the air influences the upper surface of a vacuum chamber, balanced perfectly between this pressure and a main spring. this action is transmitted to an index hand moving across the dial marked into fractions of inches after the manner of the recognized standard, the mercurial barometer. when the warm moist light air of a cyclone invades a locality the pressure is partially removed, the vacuum chamber is not pressed so hard and the dial hand or the mercury subsides. when the cold, dry, heavy air of the anticyclone lumbers in more pressure is applied and the mercury, or the dial hand, climbs. so a falling barometer means a storm, a rising one fair weather. that is a generality that glitters. if that were all there was to it weather officials would have a sinecure. but each cyclone varies in size, intensity, and rate of progress. some do not advance for days. therefore there has grown up a pretty large body of information as each storm has had to be watched and the barometric movements recorded. the most important variations follow: remembering that . inches is sea-level normal, if the barometer is steady at . or . the weather will remain fair as long as the steadiness continues, and on the turn, if the fall proceeds slowly with the wind from a westerly direction fair to partly cloudy weather with slowly rising temperature will follow for two days. if the barometer rises rapidly from . the fall will be equally rapid and rain or snow may be expected within a couple of days. since the depressions of the atmosphere tend to a certain regularity about the center of the storm it follows that the reactions will follow the actions in similar manner,--a long rise portending a long fall and a variable glass meaning unsettled conditions. the barometer does not rise with wind from an easterly direction unless a shift is imminent. in winter the air is so much colder over the land than over the sea that the air brought in by an easterly wind is soon condensed. consequently with winds from the south or southeast, even if the barometer is . or . and falling slowly rain usually arrives (and rain of course is meant to include snow whenever the mercury is below the freezing point) within hours. if the fall is rapid there may be precipitation within hours, and the wind will rapidly increase and the temperature rise. if the wind is from the east or northeast and the barometer . or above and falling slowly it means rain within hours in winter. in summer if the wind is light rain may not fall for a day or so. if the fall is rapid in winter rain with increasing winds will often set in when the barometer begins its fall and the wind gets to a point a little east of north. if the barometer is . or below and falling slowly with northeast to southeast winds the storm will continue to hours. if the barometer falls rapidly the wind will be high with rain and the change to rising barometer with clearing and colder will probably come within to hours. if the barometer is below . but rising slowly the clear weather will last several days. if the barometer is . or below and falling rapidly with winds south of east a severe storm is at hand to be followed within hours by clearing and colder. under the same conditions but with northeast winds there will occur heavy snow followed by a cold wave. if these promises do not always bear fruit it is because they will have been interrupted by an unseen shifting of the atmospheric weights. but the barometer will record them. a rapid rise may be checked in ascent and the instrument may fluctuate like a stock-ticker. its tale is of very unsettled weather conditions and consequently no particular brand of weather will last for very long at a time. a sudden rise of the barometer may bring its gale of wind as well as a sudden fall. but the tendency will be toward clearing and much colder. a fall of the barometer on a west wind is not common. it means rain. a rise on a south wind means fair. a low barometer and a cold south wind mean a change to west with squalls for a while. on the other hand, a high barometer with warmer weather means a shift of the wind to southerly quarters and an imminent fall. if the barometer rises fast and the temperature does, too, look for another storm. this is often noticed in summer. there is a slight daily oscillation of the mercury, which, if other things are steady, registers highest at a. m. and p. m. and lowest at a. m. and p. m. if this data confuses bear in mind the simple ordinary progress of the barometer in the usual storm: first, it will stand steady for a day or so at any point between . and . . then the glass will begin (for most storms) to fall gradually. as the center nears the fall hastens. after the lowest point has been reached a slight rise will be followed by another slight fall and then the final long rise will commence. the rain begins and ceases at different stages for different storms, depending upon the wind's velocity and direction. for every feet of altitude the height of the mercury is about one inch less. do not complain that your barometer is inaccurate if you are living up in the mountains and your readings are not the same as the weather reports which are reduced to sea level. all the figures given in this chapter are for sea level and if your house is feet above you must move the copper hand of your aneroid . inches from the pressure hand. if the pressure hand would read . the adjustable copper hand would read . which is the sea level reading. one good thing to remember is that a barometer falls lower for high winds than for heavy rain. a fall of two- or three-tenths of an inch in four hours brings a gale. in the ordinary gale the wind blows hardest when the barometer begins its rise from a very low point. in summer a suddenly falling barometer foretells a thunderstorm, and if the corresponding rise does not at once take place the unsettled conditions will continue with probably another thunderstorm. if you see the thunderstorm first, that is, if the barometer is not affected by the approaching black cloud you may be sure that the storm will amount to nothing. the man in the fields or along the shore has many natural barometers in animal life. but these natural barometers only corroborate; they do not foretell, at least very long before. some are useful at times and among these the birds are foremost. the observant zuñis have incorporated this in one of their pretty proverbs, "when chimney swallows circle and call they speak of rain." as a matter of fact the swallows are circling most of the time after insects. if they are flying high it is because the bugs are flying high and that is because there is no danger of rain. as the rain nears the air gets moister, the bugs and the birds fly lower. whether they do this because their instinct is to avoid a wetting or because the lighter atmosphere of a cyclone makes flying more difficult, particularly at altitudes, i do not know. for weather purposes it is enough to watch their comparative levels. wild geese are excellent signs, i am told, but it would be a dry country that waits for a sight of them for its rain. bees localize before a storm and will not swarm. flies crowd upon the screens of houses when humidity is high, possibly because the appetizing odors from within are buoyed afar by the heavy air. cuckoos seek the higher ground in fair weather and disappear into bottom lands before a rain. although they are called rain-crows they are heard in all weathers. smoke is as good an evidence of barometric pressure as anything except the instrument itself. on clear, still days it will mount; on humid days without wind it will cling to the hill. there is that difference. but it takes skill and many comparisons to gauge its angles in the wind. it becomes a test in observation and finally rewards one by becoming an excellent sign not only of air texture but of the direction of its currents. no reference to barometers would be complete without mentioning spiders. they show a most delicate apprehension of changing conditions. if the day is to be fine and without wind they will run out long threads and be rather active. if the rain is nearing they strengthen their webs, shorten the filaments and sit dully in the center. fresh webs on the lawn insure a clear day. but for the commuter, whose time is money, there is little leisure to consider the spider. as a natural result of the variation in altitude affecting the barometer the words which are printed on the face become entirely useless. in some places it would be impossible for the needle to point higher than "very stormy." even at sea level a sudden fall to "fair" would cause a rain, much to the indignation of the person who thought that he had purchased a self-registering weather prophet. disregard the words but watch the needle and you will never be surprised at what the weather is doing next. chapter vi the seasons too great emphasis cannot be laid upon the futility, at present, of trying to forecast the weather for more than a very few days in advance. long range efforts are not made by the bureau because with its present limited knowledge of the factors that control seasons and with the present limited facilities for collecting data the process of looking into next month has not been perfected, and the attempt to investigate next winter's weather proves scientifically impossible. as usual, fakers step in where science fears to tread. with goose-bones (not their own) and hickory nuts they prophesy with all their might. and if their prophecies come true, as sometimes they must, there is wide rejoicing in the newspapers and the cause of science is set back by just so much. but science cannot be thwarted in the end and every year new discoveries are made, new speculations proved true or forever false, and some time, doubtless, the weather will be predicted from year to year with the same % accuracy with which the hour forecast is now made. experimenting is worth the little that it costs, too, for to know when the summer is to be dry or wet, hot or cold will be a boon to everybody and to the farmer most of all. one conclusion has already been reached by officials in the weather bureau and scientists generally. it has been decided by long search through creditable records, painstaking comparisons of averages coupled with the most accurate investigations for half a century, that, on the basis of ten years, our seasons do not change. that is, counting the decade as a unit, our weather keeps to the same level of efficiency through the centuries. this statement comes always as a blow. it always provokes argument and citations of grandmother's blizzards. there is a great and universal hesitation in believing that our weather is as good to-day as it used to be. the good old times when there was a general debauch of snow and you could skate all winter on anything but the atlantic ocean certainly appear no more. as a matter of fact there has been a change, but it has been in our memories. in grandmother's youth the trains,--if they had trains then,--doubtless were stalled by a big snow for then they did not have rotary plows. in father's day they may have had an unbroken winter of sleighing. we couldn't now; sleighs are extinct. but in our time, in fact every year, some record is being broken and the records go back a respectable length of time. for example in philadelphia the most accurate records made by standard instruments have been kept for years. during this time the highest wind velocity was recorded in ( miles an hour). the greatest rainfall in hours occurred in ( . inches). the lowest temperature was registered in ( degrees below zero); the highest in ( degrees). the greatest number of thunderstorms for any one year took place in when we had . as late as the heaviest snowfall ever recorded at this station, amounting to inches, occurred. and just a few weeks ago (april rd, ) it snowed inches in half as many hours. all these items do not indicate a climate decreasing in virility very swiftly. but there is more evidence yet that philadelphia is experiencing the same varieties of weather in about the same proportions. diaries of observant men running back to show that almost any kind of memory could be founded on fact, that the same violent changes in temperature, the same deep snows and unseasonable seasons that we endure to-day were noticed then. to quote: "the whole winter of was intensely cold. the delaware was closed from the st of december to the th of march. the ice was from two to three feet thick." we despaired of ever living up to this until three years ago when the same thing happened and sleighs crossed the river a little above the city. and despite the new ice-boats! "the winter of was very mild, particularly the month of february when trees were in blossom." "on the st of december, , the delaware was frozen completely over in one night, and the weather continued cold until the th of march with snow about two and a half feet deep." "the winter of was very mild. the first snow was as late as the th of march." and so it goes. was mild; "one of the coldest since the settlement of the country"; was intensely cold, mild, very mild after the th of january, long, stormy and severely cold. the upshot of it all is that february violets and april snows were just as well known to general washington as they are to us. [illustration: nimbus _courtesy of richard f. warren_ nimbus is any cloud from which rain is falling, and the important thing to know is how to judge from the formless thing how much longer it is to rain. the wind is the surest guide. in this picture the nimbus cloud is only that at the end of the cape. all the rest is torn stratus and cumulus, which needs to condense a little further before it becomes nimbus. this will likely happen because the cloud at the left is very dark. the broken appearance denotes some wind. rain does not fall from a mottled sky nor yet a streaky one; the nimbus is uniform in appearance. in summer a break in the nimbus will show a veil of cirro-stratus above. just nimbus by itself will not support much of a storm. in winter if the nimbus is particularly seamless snow is about to fall.] but though all facts point to the fact that the climate does not change in a decade or a generation or a dozen generations, there is some comfort for those who are not satisfied in knowing that it doesn't stay the same forever. during the carboniferous times the poles were as warm as the tropics and when the ice age came on it was very chilly everywhere. if one might only live an eon or two he might then well complain of the changing climate. climate, however, is one thing, weather another. the climate is the sum total of the weather. climate is as enduring as our constitution, the weather is as changeable as our city governments. no matter how proud a scientist may be of the lasting qualities of the climate, he has to admit that our weather, taken day by day or even year by year, is versatile in the extreme. and the question he has set himself to solve is how to explain the variations of the seasonable weather. he wants to find out why all winters are not alike, and why no two successive springs are the same. then he will be on firm ground at last and able to make scientific forecasts for the ensuing year. the obvious thing was to find out as accurately as possible what had happened and science's keenest eye was focused on records in the hope of discovering fixed periods of warmth or wetness, cycles of cold and drought. so far no cycles have been discovered that are beyond dispute. nothing has been found that cannot be contradicted successfully. this is discouraging. one of the most frequent starting places for investigators is the spots on the sun. they found that periods of three, eight, eleven, and thirty-five years should bear some resemblance; was eagerly looked forward to. they wanted it to correspond with the remarkably cool summer of . when it started off in july with a temperature of degrees, the highest ever recorded in philadelphia, they concluded that the sunspots were fooling them. a connection between sunspots and weather has not been established, therefore, although they are now known to affect the electrical condition of the earth's atmosphere. longer periods of observation will permit comparisons that may yet define concurrent cycles of sunspots and weather. a definite weather cycle has not yet been discovered, but one step in the way has been cleared up. we now are pretty sure of one cause for unusual single seasons of heat and cold. there exist in winter great bodies of cold, dry air heaped up over canada and siberia, which are formed by the greater rapidity of radiation over land surfaces than over water. these mounds of cold air build up during december, january, and february and form great so-called permanent areas of high barometer. it is on the skirts of the canadian high that the smaller highs form which sweep over our country, giving us our cold waves. also in winter permanent lows form over the north pacific and north atlantic where warm currents afford continuous supplies of warm moist air. from the great aleutian (pacific) low spring most of the cyclones which swing down below the border of the canadian high, make their turn somewhere in the mississippi valley, and then head for the icelandic low. it can be seen that if the canadian high is a little stronger than usual and spreads a little farther south, then the northern half of our country will come more directly under its influence and we will experience an unusually severe winter. as the storms are pushed south and as the cold air pours into the northern quadrants the snow line is pushed south too. hence all abnormally snowy winters are caused by a strengthening of the permanent canadian high which may be central anywhere north of our dakota or montana borders. conversely, if this high is weaker than usual the cyclones can cross the country on a line farther north, there will be less snow, and the cold waves that follow will be less severe or even non-existent. in summer the reverse occurs. great oceanic highs are built up over the south atlantic and south pacific and a permanent low occupies the center of our continent. the character of the season is determined by the strength and position of these areas. the eastern states are affected especially by the slow movements of the south atlantic low. the puzzle is why should these areas change their power and position, and if they must change why don't they do it regularly? the puzzle will undoubtedly be solved. these great centers of action will be plotted against and observed from every vantage point by a thousand observers. a fascinating field for scientific speculation opens. at present our government exchanges daily observations with stations in siberia, canada, and the west indies. the great storm-breeder, the aleutian low, is watched from alaskan shores. in the atlantic the bureau needs stationary ships to record the growth and decline of the high over the azores. knowledge of the wind circulation from this would inform us whether our storms were to be shunted farther north and pushed somewhat inland. a storm which is pushed to the left of its normal track increases tremendously in intensity. whereas a cyclone that limps slackly to the right of its normal line loses intensity at once. it misses coil. in this respect storms seem to resemble rattlesnakes. the energy of the azores high influences the number and destructiveness of the west indian hurricanes: the larger the area is the closer do the hurricanes hug our shores and the more destruction do they accomplish. the very sureness that the general average of the seasons is to be the same enables us to guess pretty accurately for individual purposes as to the kind of season coming next. a guess, let me add, is not a forecast. it is a gamble and disapproved of by the bureau, but until they supply us with a basis for judgment we will have to go on guessing, for human curiosity is as near to perpetual motion as the weather is to the lacking fourth dimension. one of these guesses is that if the winter has been a warm one the summer will be cool, for the very good reason that the yearly average does depart so slightly from the fixture. unfortunately one hot summer does not mean that the following summer will be cool. certain sequences of the seasons have been observed often enough to have been gathered into proverbs. everybody agrees that "a late spring never deceives." "a year of snow, fruit will grow." "a green winter makes a full churchyard." of the many hundreds of proverbs relating to the seasons a few are sage, some outworn, and many sheer nonsense. nearly all refer to the obvious fact that one kind of season is followed by another rather unlike it, not much telling what. and there, unsatisfactorily enough, they leave one. but much is to be hoped for from the scientific explorations now in progress. and until they are heard from few of us will realize how many seasonable seasons we really enjoy. chapter vii the weather bureau at the cost of a cent and a half a year apiece we americans are supplied with detailed information in advance about the weather. and the information is correct for more than four-fifths of the time. if stock brokers never missed oftener, what reputations would accrue! cheapness, accuracy, and a certain modesty are the three qualities that distinguish the out-givings of the bureau from the old-fashioned predictions of the weather which used to appear in almanacs. almanacs have probably kept appearing ever since the art of printing first allowed unscrupulous persons to juggle with words. they cost fifty cents and their predictions were based on nothing but the strength of their author's imagination. of course, it was impossible for him to guess wrong more than half the time so that when he announced in january that july would be hot with thunderstorms he was often right. this gave him prestige, but aided his clients little. the weather bureau was in about the same position in regard to the quack predictions of the almanacs as was the honest doctor of the last decade who could only prescribe good food and fresh air and moderate exercise for the patient who much preferred the expensive allurements of the medicinal cure-all as advertised. in humility the bureau said that as things stood it could not forecast with accuracy for more than hours, and its honesty brought it into disregard. but, although the weather bureau,--like the christian church and other things that have had to combat superstition at every step--has grown slowly it has grown surely and its work is being recognized more widely and relied upon more understandingly every month. it was an american scientist who discovered the rotary motion of cyclones and their progressive character, but due to the conservative nature of our government three other nations had established weather services before we had. in the war department was authorized to start a system of observations that would permit of a rough sort of forecasting. the forecasts proved of so much value to shippers and sailors that the work was handed over to the department of agriculture and enlarged ( ). to-day every part of our country contributes to the knowledge of existing weather conditions. at a. m. observations are made at hundreds of stations and wired to the central office at washington. the chief there, knowing these conditions, is enabled to locate a storm, to gauge its rate of speed, to learn its course, and to measure its intensity. he can dictate storm warnings and be sure that within an hour every sailing master will have a copy. he can detect a cold wave at its entrance into our territory and know that within an hour every shipper, every truckster (who has signified that he wishes to be informed) will have the facts that will save him money. at p. m. the same stations telegraph the changed conditions, and if any very violent disturbance is in progress an observation is made at noon. besides the washington distributing station there are others from which warnings are sent by telegraph, telephone, or mail. there are , addresses on the mailing list and , , telephone subscribers can get them within an hour. the newspapers reach many millions. and all this at a cost of - / cents a year. if we, in a fit of generosity, should pay cents, or even - / the government would be enabled to work out many of the larger problems awaiting only a larger appropriation to be attacked. the people's investment of $ , , a year is a good investment. in one year the service saves a great many hundred per cent. a few known savings are worth giving; $ , , worth of protection was made possible from one exceptionally severe cold wave; the california citrus growers estimated that one warning saved $ , , worth of fruit; $ , , of shipping (and cargoes) was known to have been detained in port just on account of one hurricane warning, and there are many warnings of gales every year. uncalculated savings have been effected among the growers of tobacco, sugar, cranberries, truck. the railway and transportation companies save, through use of the forecasts, in shipments of bananas, oysters, fish, and eggs. farmers, manufacturers, raisin driers, photographers, insurance companies, and about a hundred and fifty other occupations increase their profits by a systematic study of the forecasts. the people who live along the rivers often owe their lives and frequently much of their property to telephone warnings of approaching floods. the flood stages in all the principal rivers and streams have been calculated and losses are reduced by per cent. by accurate predictions as to when the crest of the flood may be expected and how high it will reach. a hundred uses of river forecasts, even when flood stages are not expected are given in the booklet, "the weather bureau" which you can have from washington for the asking, like many another of their publications. yet, with all the good it does, the man on the street still regards the bureau as an uninteresting, undependable exhibit in the upper corner of the newspaper,--if he regards it at all. it is his child, however, who is instructing him. for his child is being taught in the public school all about it and he takes his teaching home and becomes the teacher. the child is father of the (old) man in lots of instances. the most impressive thing about the whole output of the bureau to the child is its map. the bureau issues a map every day which is posted in post-offices and railroad stations and in schools, too, if they ask for it. and every day this map shows in all its gripping details the way our storms are sidling across the continent or rushing up our coasts. it prints the word low where the stormy area of low barometer is. about the low run continuous black lines numbered . , . , . , etc., which show where in the country the pressures are the same. as the numbers run up to . , . , . they begin to circle about the word high which denotes where the pressure is highest. little circles will be observed on the map. some are clear, indicating clear weather; others are half clear, half black, indicating partly cloudy conditions; others are all black, showing clouds; others have r. or s. inside them, telling where it is raining. the numbers under the circles show how much it has rained or snowed and the numbers under the other numbers are the velocities of the winds. the arrows through the circles fly with the wind. a little zig-zag locates each thunderstorm and the shaded portions show over what portions of the country it has rained during the last hours. as an intelligent puzzle picture the map is unequaled and no wonder the child likes it. with this map you can tell at a glance what the weather is doing to your uncle in tacoma and to your cousin in missouri. with two successive maps you can find out about how fast the storms are traveling, in what direction, and how low the temperatures are under their influence, and so estimate for yourself the weather for the next three days. besides the invaluable daily weather map the bureau issues many other maps that present the phenomena of the week, the month, and the season in graphic form. masters of vessels are now coöperating with the government to provide observations at sea, and both on our northwest and southeast coasts such information is very valuable. in the west several hundreds of stations are maintained in the mountains for the purpose of obtaining the depth and content of the great snowfalls there. estimates can then be given out as to the amount of water to be available for irrigating purposes. in addition to the stations of the first class there are coöperative stations at which observations are made and mailed to centers for distribution. special local data help to establish the relations between climate and forestry, agriculture, water resources, and allied subjects. many bulletins are compiled by experts in their respective lines and these are for free distribution. a study of forest cover is being made in colorado and the effects of denudation on the flow of streams will soon be scientifically established. as soon as practicable the bureau hopes to extend its period of forecasting. weekly forecasts have been tried in a general way with success, but long-range forecasting depends upon so many relationships of the air that present knowledge and facilities do not warrant its adoption. chapter viii a chapter of explosions in the good old times when a man was born, spent his life, and died in the same village the weather proverb was fashioned. generations had watched the clouds gather under certain circumstances and scatter under certain others and they naturally drew conclusions. these conclusions crystallized until they resembled nuggets of golden weather wisdom. some were even used as charms. and all contained a deal of truth so long as they were only meant to refer to the country in which they had originated. but nowadays when the very idea of remaining in the same place for very long at a time is obnoxious the weather proverb suffers. it suffers chiefly by transportation. the weather in county cork is so very different from the weather that makes chicago famous that the same weather lore does not fit. yet it is often applied. the old truths, treasured in picturesque phrase and jingle, were brought over the ocean unchanged and made to do duty,--a case of new wine in old bottles again, for a gentle old irish proverb splits up the back when it tries to accommodate itself to a week of our reckless but magnificent weather. fairy stories are jewels to be cherished. and it is a careless and unimaginative race that perpetuates no legends. even old saws are quaint and should be preserved: "see a pin and pick it up, all the day you'll have good luck." let that sort of thing go on because it adds richness to our conversation. but if a thousand men, after having picked up their morning pins, sat around waiting for the ensuing luck the progress of scientific business management would be halted. and precisely that way is the knowledge of ordinary weather facts halted,--a full-grown superstition sits in the path. instead of relying upon their eyes the majority of people rely upon a bit of doggerel. for example, millions of people firmly believe that the ground-hog is a key to the weather. they say that if the ground-hog does not see his shadow on the nd of february that winter is over! this is the sort of thing that obscures the findings of science not to mention common-sense. few of these people have ever seen a ground-hog. few of the rest have ever studied its habits. the ant, the mouse, the fly, the rat, and the mosquito have far more influence upon our lives than the ground-hog has and the most ambitious animal cannot expect to influence atmospheric pressure, which is responsible for our weather. yet as often as the nd of february comes around the hopes of many are either dashed or raised according to the actions of this creature. as a matter of fact, whether february nd is clear or cloudy can have no influence on the rest of the winter. almost all the other proverbs have a basis of reason. but this puts its believers in the wrong either way. if they say that it is the actions of the animal that they rely upon they depend upon a characteristic thoroughly and surely disproved. no animal, although it may sense a change in the weather a few hours in advance, is able to feel it for three days ahead to say nothing of six weeks. if these people say, on the other hand, that a cloudy february nd means an immediate and complete let up of winter, or that a clear february nd means a certain continuance of cold weather for six weeks, they have only to trouble themselves to look at the files of the nearest weather bureau for the last forty years. they will find no connection. the trouble is that they will not look, but keep on repeating the bit of nonsense and believing in it, although the strength of their convictions probably does not reduce their coal-bills. the same people are fond of saying that the first three days of december show what the winter will be like. that is, if the st is fair so will december be; if the nd is cold so will january be; and if it snows on the rd, so will it snow in february. if all three should be clear and warm certainly a remarkable winter would follow! no rain, no snow, no cold! you see how absurd this superstition is. "a dry moon lies on its back!" after the ground-hog the moon is supposed to have the most influence on our seasons. the government and many scientists connected with no governments have made careful, exhaustive and conclusive investigations. no relation between the moon and our weather has been discovered except as she causes our tides and they affect atmospheric pressure in an infinitesimal degree. we would still have just as much and just as variable weather if there were no moon. the weather changes with the changing moon, and it does not change as the moon changes, and the chances are about even that the times of change will coincide. so there is, therefore, absolutely no foundation for the dozens of proverbs that yoke the changes of the moon with the changes of our weather. neither in science nor in observation has any sequence been deduced. so the moon may lie on its back or on its side or stand on its head and the weather will remain dry if no low pressure areas cross the country, and it can lie on its back for days and the country be drowned out if they do. there are enough pretty things to say about the moon, anyway, and will be more all the time for, to commit a paraphrase: science is stranger than superstition. "it will rain for forty days straight if it rains on st. swithin's day," which, i might as well say for the benefit of those who don't know their saints, falls on july th every year. it would be interesting to know how many people in a hundred really believe this, or really believe all the other things that are attributed to the saints,--quite a few, probably. luckily for st. swithin july and august are wet months, with often several days of showers or thunderstorms in succession. but never once in philadelphia has it rained for forty days, one right after another, although half the july ths have been rained on. this proverb is one of those that had better never been transplanted from its native ireland where rain for days would excite scarcely a curse. "long and loud singing of robins denotes rain." it does not. oftener than not it denotes the time of day. just watch the robins and listen to them and see what they do before a storm, during it, after it, and then you will see how little the songs of birds can be depended upon to supplant the barometer. "if march comes in like a lion it will go out like a lamb," and the other way round. i have seen march come in like a lion and go out like a lion, come in like a lion and go out like a lamb, come in like a lamb and go out like a lion, and come in like a lamb and go out like a noah's ark. but i never have seen march do anything dependable. it is quite impossible to tell how march is going out on march th, and absolutely impossible to tell on march st. but there is this much observation expressed in the proverb, that march is so changeable that, if it comes in cold, windy, unsettled, there is not so much chance for such weather still to be going on at the end of the month, and still less in england where the proverb came from. this is a harmless proverb unless it should lead people to actually count upon a pleasant spring just because march had an unpleasant inception. misfortunes rarely come singly, even on the weather calendar. "when squirrels are scarce in autumn the winter will be severe." aside from the scientific truth that the animals cannot know in advance about the seasons there is little evidence on either side to base a contention. nobody has made a squirrel census; nobody, probably, has found out whether they increase in numbers for six years and then die off in great quantities as do the rabbits in the north country on the seventh; nobody has connected their apparent numbers year after year with the actual severities of the winters. and so nobody has a right to promulgate the report (except as a bit of nonsense like april fool) that the ensuing winter is going to be a record breaker because the squirrels have disappeared. it would be far truer to say that "when squirrels are scarce in autumn the hunters have been busy," and let it go at that. there are a lot of proverbs in this connection about goose bones and hickory nuts and wild geese, which sound plausible but are never proved. if the birds have all the sense credited to them it is strange that some allow themselves to be caught by an early snowstorm in the fall and decimated. also it is not uncommon for early migrations in the spring to arrive in the north to be slain by the thousand by a belated blizzard. it is granted that animals and birds, having a far greater sensitiveness than man, occasionally sense a catastrophe some hours before it is evidenced by any visual signs, but seasonal wisdom has not been proved in any one instance and disproved in many. none of the proverbs relating to the animals and birds are to be depended upon. they deceive, much to the regret of all the meteorologists who would welcome any genuine clue to nature of the coming season. any farmer would be only too glad to keep a menagerie of squirrels and wild geese and toads if only he might be assured by them of the coming seasonal conditions. the proverbs given indicate the range, possibly, but certainly not the full absurdity of the old weather sayings. there are many other proverbs that contain at least a half truth. "enough blue sky to make a dutchman's breeches indicates clearing," is one that is true if the wind has changed to the west. if the wind still blows from an easterly quarter blue sky for a dutchman's whole wardrobe would not insure clear weather. all sayings must be tested many times before they are believed implicitly. "there is always a thaw in january," is about as true a generalization as can be made about things for which generalizations are never strictly in place. even in canada the severity of the winter is often broken by a spell of warmer weather with a rain, perhaps, in the dead of winter. in the united states a winter without some break in each of the months would be a most unusual occurrence. so that it is quite reasonable to expect the "january thaw" any time from christmas until the middle of february. "a late spring never deceives," unless it is so very late, like the phenomenal spring of , that the jump is made, perforce, into summer. that is a cruel deception. what is meant of course is that if the freezing weather continues consistently, well past the average, the likelihood of frost-damage to fruit is slight. there is nothing much worse than for the blossoms to be forced by a period of warm weather early, for there is only a slim chance that it will continue past the danger limit. it is surprising how late frost may occur,--the last date for killing frost in pennsylvania is about may th on the _average_, which makes it possible till june. "the first robins indicate the approach of spring." but certainly not its arrival. "if the moon rises clear expect fair weather." right; because if it is summer even the eastern horizon would show the humidity necessary enough to cause a thunderstorm, and in winter the cirrus clouds give several hours' warning. but, again, the wind is the chief factor to be considered. proverbs, representing variations of the truth, could be given about every manifestation of the skies as well as about things that were never manifest except in the imagination, for every country has contributed to the volume of weather-lore. but, unfortunately, neither age nor amount of repetition are as good as the truth and they should be discarded if they are false. the way to discard is not to repeat. the man who desires weather-wisdom should seek it with his eyes. his comparison will be that which he sees with that which he has seen, and he will soon form all the weather axioms he needs for himself. the local bureau or the bureau at washington will answer all his inquiries, cheerfully, promptly, and free of charge. of course there are things that the bureau wants to know itself. it is very curious about the higher strata of air. small balloons have carried very light instruments to an altitude of fifteen miles and brought considerable knowledge to earth, but each bit makes more knowledge imperative. the cry of "last frontier" hurts the adventurous, the exploring, the woods-loving as no other cry has power to hurt. with the poles gone and alaska in harness we are inclined to think that it is all over. we resign ourselves to our trammelling globe,--as the gold-fish do,--forgetting. but there is plenty of interest left. the birds must be brought back. forests must be made and patrolled, and the air-ocean is still unknown. that, at any rate, has remained unspoiled by man. the seas have been charted and the mountains have been disemboweled, but the atmosphere is unconquered. more must be known. squadrons of aëroplanes cannot ride out the gale until their pilots know all about the gale. until that time there need be no cry of last frontier, for until that time the weather will continue to be our overlord, whose dominions are flaunted before the watcher on the porch and the runner on the trail. condensations look for continued fair weather when: a gentle wind blows from the west, northwest, or a little south of west. the sun sets in a cloudless sky. the sunset is composed of light tints, inclining to red or yellow. the sunset is followed by a glowing and slow-fading western sky. the sun sets like a ball of fire (warmer). the sun rises out of a gray sky. the clouds are noticeably high for the season. the clouds rise on the mountains. the clouds have frequent breaks showing blue sky between. the puffy cumulus clouds show a lot of white. the cumulus clouds decrease toward nightfall. the winter sky is mottled with a northwest wind. the summer morning fog breaks before ten o'clock. the dawn is low. the blue sky has a tendency to show green near the northern horizon (colder). the sun breaks through a departing thunderstorm and makes a rainbow. snow-flurries drift down a north wind (colder). cirrus clouds, or others, dissolve, or cirrus have tails down. spiders spin on the grass. there is a moderate dew or frost. the temperature is normal or colder than normal, other signs being right. the sky is sown with stars. the moon rises clear. the wind blows down mountain ravines after nightfall. the salt is dry, smoke ascends, birds fly high, and animals act normally. the barometer rises slowly, or is steady at or above . . no change need be feared as the anticyclone nears, or for three days after clear conditions are established so long as the wind remains brisk from some westerly quarter. the direction of the wind, the kind of cloud, and the temperature changes are the factors to watch if you have no barometer. * * * * * look for a change toward storms when: the west wind suddenly drops. the west wind shifts to south or northeast. the cirrus clouds appear in well-organized lines. the cirrus clouds merge into cirro-stratus. the sky looks like fish scales, so-called mackerel sky. light scud drifts across the sky from east to west. the summer cumulus clouds increase in size as the afternoon proceeds. walls grow damp, flies are more of a burden than usual, swallows fly low. smoke falls to the ground. there have been three white frosts. a halo appears around either the moon or sun. when sun-dogs appear about the sun, denoting ice-particles in the air. the summer morning is sultry and the wind variable. the temperature is much above the normal. few stars are visible and those are indistinct. the clouds gather about the mountain tops, or drop down the mountain-sides. the wind continues to blow up ravines after nightfall. the sunset is a dull gray, or the sun sets into a livid cloudbank. the sunrise is a fiery red, and the dawn is high. the sun gradually is smothered in fine-textured clouds and the wind shifts. the temperature does not fall at night. the signs most to be heeded are the shift of wind to a point east of north or south, the gradual filming of the sky with cirrus and cirro-stratus, and the increase of temperature. of course, the barometer is the best indicator of all. * * * * * look for a change toward clearing when: the wind shifts from the easterly quarter into the west. the temperature falls rapidly. the clouds rise, or break, or lighten perceptibly in color. patches of blue sky appear through the rifts in the clouds, wind north. raindrops grow smaller after the windshift. snowflakes drive less busily, float lazily down, or thin out conspicuously. seams appear in the clouds, snow will cease and rain probably. the thunder and lightning occur only in the eastern quarter. permanent clearing will not be effected until the change of the wind to the points on the western half of the compass show that the cyclone has definitely passed to the north or south or over the locality. in winter the cloud covering may move off slowly, but there will be little precipitation after the wind has reached north or west. the bank of cirro-stratus gets thinner and the moon or the sun gradually shines through. in summer clearing is much more abrupt, as is the clouding up. the ability to sense accurately the moment when the weights are shifted and the change to clearing commences takes some observation to acquire, but the advantage is worth it. * * * * * rain (or snow) will fall: within five minutes after the arch of the thundercloud is seen to move toward one. within five minutes when the curtain of falling drops obscures the landscape to the west of one. within a few minutes after the bottoms of cumulus clouds turn from black to gray, letting down visible trailing showers. within a short while after the winter sky has become uniform in color. within an hour after the pavement-like, but scarcely discernible, thundercloud consolidates along the west, if the wind is from the southwest. if the wind is from the southeast this cloud may take four hours to rise. from two to eight hours after the sun or moon has vanished behind the cirro-stratus. from eight to forty-eight hours after the first cirrus is seen, depending upon the distance from the sea and the time of year. every little while from southwest showers in the passing of a summer low. for about eight to twelve hours continuously in a winter storm, and intermittently until the wind swings west. for a very short while from a thunder cloud rising on a west wind. for an hour or more from a thundercloud that rises on a southwest or southeast wind. * * * * * the temperature will fall when: a thunderstorm breaks, continuing low if the wind blows from the west after clearing. nightfall approaches and the sky is free from clouds. the mercury remains at the same level during the sunny hours. a cyclone is departing and the anticyclone moving in. the wind swings north of east in a storm,--the fall will be gradual. the wind swings west of south in a storm,--the fall will be sudden. a snowstorm begins, for a short time only. a cloudy day clears at sunset. snow flurries are seen. the sky shows green and the clouds look hard. * * * * * the temperature will rise when: a thunderstorm is brewing, or a day or two before a winter cyclone. after a thunderstorm if another is to follow. the morning is free from clouds and if it is not the first day of a cold wave. the wind dips south of west or south of northeast, the former shift bringing the more sudden rise. the sun sets as a ball of fire, at which one can easily look. a snowstorm gets under way, unless the wind is swinging toward the north. a page of problems one satisfying thing about meteorology is that there is a constantly widening field for conquest. among the questions that await solution are: what are the relative densities of clouds? what is the original atmospheric electricity, its distribution and laws? what are the causes and nature of precipitation? will aërial ascents on all sides of an atmospheric disturbance discover the mechanism of storms? what relations are there of solar radiation to our atmosphere? what influence do lunar tides bear to our weather? on what does the permanence of the summer lows over the rockies depend? these questions are only samples. many certainties can be attained by merely complete observations over a longer period of time, others by new systems of observations that await a more generous appropriation. even the upper air investigations on mt. weather, va., have had to be curtailed. the bureau's record has proved it efficient, of enormous benefit to the country, and deserving of the encouragement instead of the depreciation of every citizen. what the weather flags mean in every city the bureau causes flags to be flown from some prominent place so that a glance may show shippers and everybody who may be concerned at the shortest possible notice just what the approaching weather conditions are. a plain white flag means fair weather. a black triangle stands for temperature and is always exhibited with some other flag. its relative position, either above or below indicates higher or lower temperature. therefore white flag with the black below means fair and colder. the white flag with the black above means fair and warmer. a white flag with a black square in the center means a cold wave. a blue flag means either rain or snow. the blue with the black above would mean rain or snow and warmer. the blue with the black below would mean rain or snow and colder. a blue and white flag means a local shower. the same meanings are attached to the black triangle in connection with the blue and white. a red triangle indicates a dangerous local storm, is called the information flag meaning that shippers should apply to the bureau for news of the direction in which the storm is travelling. a red square with a black center means severe winds. . southwesterly with a white triangle below. . northwesterly with a white triangle above. . northeasterly with a red triangle above. . southeasterly with a red triangle below. our four world's records,--and others maximum temperature united states, at greenland ranch, cal., july, . world, at greenland ranch, cal. minimum temperature united states, - at miles city, mont., january, . world, - at verkhojansk, siberia. absolute zero of space - degrees fahrenheit. maximum annual precipitation united states, . inches at glenora, oreg., in . world, . inches, cherrapunji, india, . maximum monthly precipitation united states, . inches at helen mine, cal., january, . world, inches, cherrapunji, india, july, . maximum hour precipitation united states, inches at alexandria, la. minimum annual precipitation united states, none at bagdad, cal., in . (only . inches fell at bagdad during period to , inclusive.) maximum annual snowfall united states, inches at tamarack, cal., . maximum monthly snowfall united states, inches at tamarack, cal., january, . maximum wind velocity united states, miles per hour at mt. washington, on jan. , . (much higher velocities have undoubtedly occurred in tornadoes, etc., but have not been susceptible of instrumental measurement.) the end _outing publishing company--new york_ outing handbooks _the textbooks for outdoor work and play_ ¶ each book deals with a separate subject and deals with it thoroughly. if you want to know anything about airedales an outing handbook gives you all you want. if it's apple growing, another outing handbook meets your need. the fisherman, the camper, the poultry-raiser, the automobilist, the horseman, all varieties of out-door enthusiasts, will find separate volumes for their separate interests. there is no waste space. ¶ the series is based on the plan of one subject to a book and each book complete. the authors are experts. each book has been specially prepared for this series and all are published in uniform style, flexible cloth binding. ¶ two hundred titles are projected. the series covers all phases of outdoor life, from bee-keeping to big-game shooting. among the books now ready or in preparation are those described on the following pages. price seventy cents per vol. net, postage c. extra the numbers make ordering easy. . exercise and health, by dr. woods hutchinson. dr. hutchinson takes the common-sense view that the greatest problem in exercise for most of us is to get enough of the right kind. the greatest error in exercise is not to take enough, and the greatest danger in athletics is in giving them up. he writes in a direct matter-of-fact manner with an avoidance of medical terms, and a strong emphasis on the rational, all-round manner of living that is best calculated to bring a man to a ripe old age with little illness or consciousness of bodily weakness. [illustration] . camp cookery, by horace kephart. "the less a man carries in his pack the more he must carry in his head," says mr. kephart. this book tells what a man should carry in both pack and head. every step is traced--the selection of provisions and utensils, with the kind and quantity of each, the preparation of game, the building of fires, the cooking of every conceivable kind of food that the camp outfit or woods, fields or streams may provide--even to the making of desserts. every recipe is the result of hard practice and long experience. [illustration] . backwoods surgery and medicine, by charles s. moody, m. d. a handy book for the prudent lover of the woods who doesn't expect to be ill but believes in being on the safe side. common-sense methods for the treatment of the ordinary wounds and accidents are described--setting a broken limb, reducing a dislocation, caring for burns, cuts, etc. practical remedies for camp diseases are recommended, as well as the ordinary indications of the most probable ailments. includes a list of the necessary medical and surgical supplies. . apple growing, by m. c. burritt. the various problems confronting the apple grower, from the preparation of the soil and the planting of the trees to the marketing of the fruit, are discussed in detail by the author. chapter headings are:--the outlook for the growing of apples--planning for the orchard--planting and growing the orchard--pruning the trees--cultivation and cover cropping--manuring and fertilizing--insects and diseases affecting the apple--the principles and practice of spraying--harvesting and storing--markets and marketing--some hints on renovating old orchards--the cost of growing apples. . the airedale, by williams haynes. the book opens with a short chapter on the origin and development of the airedale, as a distinctive breed. the author then takes up the problems of type as bearing on the selection of the dog, breeding, training and use. the book is designed for the non-professional dog fancier, who wishes common sense advice which does not involve elaborate preparations or expenditure. chapters are included on the care of the dog in the kennel and simple remedies for ordinary diseases. . the automobile.--its selection, care and use, by robert sloss. this is a plain, practical discussion of the things that every man needs to know if he is to buy the right car and get the most out of it. the various details of operation and care are given in simple, intelligent terms. from it the car owner can easily learn the mechanism of his motor and the art of locating motor trouble, as well as how to use his car for the greatest pleasure. a chapter is included on building garages. . fishing kits and equipment, by samuel g. camp. a complete guide to the angler buying a new outfit. every detail of the fishing kit of the freshwater angler is described, from rodtip to creel, and clothing. special emphasis is laid on outfitting for fly fishing, but full instruction is also given to the man who wants to catch pickerel, pike, muskellunge, lake-trout, bass and other freshwater game fishes. prices are quoted for all articles recommended and the approved method of selecting and testing the various rods, lines, leaders, etc., is described. [illustration] . the fine art of fishing, by samuel g. camp. combine the pleasure of catching fish with the gratification of following the sport in the most approved manner. the suggestions offered are helpful to beginner and expert anglers. the range of fish and fishing conditions covered is wide and includes such subjects as "casting fine and far off," "strip-casting for bass," "fishing for mountain trout" and "autumn fishing for lake trout." the book is pervaded with a spirit of love for the streamside and the out-doors generally which the genuine angler will appreciate. a companion book to "fishing kits and equipment." the advice on outfitting so capably given in that book is supplemented in this later work by equally valuable information on how to use the equipment. . the horse--its breeding, care and use, by david buffum. mr. buffum takes up the common, every-day problems of the ordinary horse-users, such as feeding, shoeing, simple home remedies, breaking and the cure for various equine vices. an important chapter is that tracing the influx of arabian blood into the english and american horses and its value and limitations. chapters are included on draft-horses, carriage horses, and the development of the two-minute trotter. it is distinctly a sensible book for the sensible man who wishes to know how he can improve his horses and his horsemanship at the same time. . the motor boat--its selection, care and use, by h. w. slauson. the intending purchaser is advised as to the type of motor boat best suited to his particular needs and how to keep it in running condition after purchased. the chapter headings are: kinds and uses of motor boats--when the motor balks--speeding of the motor boat--getting more power from a new motor--how to install a marine power plant--accessories--covers, canopies and tops--camping and cruising--the boathouse. . outdoor signalling, by elbert wells. mr. wells has perfected a method of signalling by means of wig-wag, light, smoke, or whistle which is as simple as it is effective. the fundamental principle can be learned in ten minutes and its application is far easier than that of any other code now in use. it permits also the use of cipher and can be adapted to almost any imaginable conditions of weather, light, or topography. . tracks and tracking, by josef brunner. after twenty years of patient study and practical experience, mr. brunner can, from his intimate knowledge, speak with authority on this subject. "tracks and tracking" shows how to follow intelligently even the most intricate animal or bird tracks. it teaches how to interpret tracks of wild game and decipher the many tell-tale signs of the chase that would otherwise pass unnoticed. it proves how it is possible to tell from the footprints the name, sex, speed, direction, whether and how wounded, and many other things about wild animals and birds. all material has been gathered first hand; the drawings and half-tones from photographs form an important part of the work. [illustration] . wing and trap-shooting, by charles askins. contains a full discussion of the various methods, such as snap-shooting, swing and half-swing, discusses the flight of birds with reference to the gunner's problem of lead and range and makes special application of the various points to the different birds commonly shot in this country. a chapter is included on trap shooting and the book closes with a forceful and common-sense presentation of the etiquette of the field. . profitable breeds of poultry, by arthur s. wheeler. mr. wheeler discusses from personal experience the best-known general purpose breeds. advice is given from the standpoint of the man who desires results in eggs and stock rather than in specimens for exhibition. in addition to a careful analysis of stock--good and bad--and some conclusions regarding housing and management, the author writes in detail regarding plymouth rocks, wyandottes, orpingtons, rhode island reds, mediterraneans and the cornish. . rifles and rifle shooting, by charles askins. a practical manual describing various makes and mechanisms, in addition to discussing in detail the range and limitations in the use of the rifle. treats on the every style and make of rifle as well as their use. every type of rifle is discussed so that the book is complete in every detail. . sporting firearms, by horace kephart. this book is the result of painstaking tests and experiments. practically nothing is taken for granted. part i deals with the rifle, and part ii with the shotgun. the man seeking guidance in the selection and use of small firearms, as well as the advanced student of the subject, will receive an unusual amount of assistance from this work. the chapter headings are: rifles and ammunition--the flight of bullets--killing power--rifle mechanism and materials--rifle sights--triggers and stocks--care of rifle--shot patterns and penetration--gauges and weights--mechanism and build of shotguns. . the yachtsman's handbook, by herbert l. stone. the author and compiler of this work is the editor of "yachting." he treats in simple language of the many problems confronting the amateur sailor and motor boatman. handling ground tackle, handling lines, taking soundings, the use of the lead line, care and use of sails, yachting etiquette, are all given careful attention. some light is thrown upon the operation of the gasoline motor, and suggestions are made for the avoidance of engine troubles. . scottish and irish terriers, by williams haynes. this is a companion book to "the airedale," and deals with the history and development of both breeds. for the owner of the dog, valuable information is given as to the use of the terriers, their treatment in health, their treatment when sick, the principles of dog breeding, and dog shows and rules. . navigation for the amateur, by capt. e. t. morton. a short treatise on the simpler methods of finding position at sea by the observation of the sun's altitude and the use of the sextant and chronometer. it is arranged especially for yachtsmen and amateurs who wish to know the simpler formulae for the necessary navigation involved in taking a boat anywhere off shore. illustrated with drawings. chapter headings: fundamental terms--time--the sumner line--the day's work, equal altitude, and ex-meridian sights--hints on taking observations. . outdoor photography, by julian a. dimock. a solution of all the problems in camera work out-of-doors. the various subjects dealt with are: the camera--lens and plates--light and exposure--development--prints and printing--composition--landscapes--figure work--speed photography--the leaping tarpon--sea pictures--in the good old winter time--wild life. . packing and portaging, by dillon wallace. mr. wallace has brought together in one volume all the valuable information on the different ways of making and carrying the different kinds of packs. the ground covered ranges from man-packing to horse-packing, from the use of the tump line to throwing the diamond hitch. . the bull terrier, by williams haynes. this is a companion book to "the airedale" and "scottish and irish terriers" by the same author. its greatest usefulness is as a guide to the dog owner who wishes to be his own kennel manager. a full account of the development of the breed is given with a description of best types and standards. recommendations for the care of the dog in health or sickness are included. the chapter heads cover such matters as:--the bull terrier's history--training the bull terrier--the terrier in health--kenneling--diseases. [illustration] . the fox terrier, by williams haynes. as in his other books on the terrier, mr. haynes takes up the origin and history of the breed, its types and standards, and the more exclusive representatives down to the present time. training the fox terrier--his care and kenneling in sickness and health--and the various uses to which he can be put--are among the phases handled. . suburban gardens, by grace tabor. illustrated with diagrams. the author regards the house and grounds as a complete unit and shows how the best results may be obtained by carrying the reader in detail through the various phases of designing the garden, with the levels and contours necessary, laying out the walks and paths, planning and placing the arbors, summer houses, seats, etc., and selecting and placing trees, shrubs, vines and flowers. ideal plans for plots of various sizes are appended, as well as suggestions for correcting mistakes that have been made through "starting wrong." [illustration] . fishing with floating flies, by samuel g. camp. this is an art that is comparatively new in this country although english anglers have used the dry fly for generations. mr. camp has given the matter special study and is one of the few american anglers who really understands the matter from the selection of the outfit to the landing of the fish. his book takes up the process in that order, namely--how to outfit for dry fly fishing--how, where, and when to cast--the selection and use of floating flies--dry fly fishing for brook, brown and rainbow trout--hooking, playing and landing--practical hints on dry fly fishing. . the gasoline motor, by harold whiting slauson. deals with the practical problems of motor operation. the standpoint is that of the man who wishes to know how and why gasoline generates power and something about the various types. describes in detail the different parts of motors and the faults to which they are liable. also gives full directions as to repair and upkeep. various chapters deal with types of motors--valves--bearings--ignition--carburetors--lubrication--fuel--two cycle motors. . ice boating, by h. l. stone. illustrated with diagrams. here have been brought together all the available information on the organization and history of ice-boating, the building of the various types of ice yachts, from the small footer to the -foot racer, together with detailed plans and specifications. full information is also given to meet the needs of those who wish to be able to build and sail their own boats but are handicapped by the lack of proper knowledge as to just the points described in this volume. . modern golf, by harold h. hilton. mr. hilton is the only man who has ever held the amateur championship of great britain and the united states in the same year. in addition to this, he has, for years, been recognized as one of the most intelligent, steady players of the game in england. this book is a product of his advanced thought and experience and gives the reader sound advice, not so much on the mere swinging of the clubs as in the actual playing of the game, with all the factors that enter into it. he discusses the use of wooden clubs, the choice of clubs, the art of approaching, tournament play as a distinct thing in itself, and kindred subjects. . intensive farming, by l. c. corbett. a discussion of the meaning, method and value of intensive methods in agriculture. this book is designed for the convenience of practical farmers who find themselves under the necessity of making a living out of high-priced land. . practical dog breeding, by williams haynes. this is a companion volume to practical dog keeping, described below. it goes at length into the fundamental questions of breeding, such as selection of types on both sides, the perpetuation of desirable, and the elimination of undesirable, qualities, the value of prepotency in building up a desired breed, etc. the arguments are illustrated with instances of what has been accomplished, both good and bad, in the case of well-known breeds. . practical dog keeping, by williams haynes. mr. haynes is well known to the readers of the outing handbooks as the author of books on the terriers. his new book is somewhat more ambitious in that it carries him into the general field of selection of breeds, the buying and selling of dogs, the care of dogs in kennels, handling in bench shows and field trials, and at considerable length into such subjects as food and feeding, exercise and grooming, disease, etc. [illustration] . the vegetable garden, by r. l. watts. this book is designed for the small grower with a limited plot of ground. the reader is told what types of vegetables to select, the manner of planting and cultivation, and the returns that may be expected. . amateur rodmaking, by perry d. frazer. illustrated. a practical manual for all those who want to make their own rod and fittings. it contains a review of fishing rod history, a discussion of materials, a list of the tools needed, description of the method to be followed in making all kinds of rods, including fly-casting, bait-fishing, salmon, etc., with full instructions for winding, varnishing, etc. . pistol and revolver shooting, by a. l. a. himmelwright. a new and revised edition of a work that has already achieved prominence as an accepted authority on the use of the hand gun. full instructions are given in the use of both revolver and target pistol, including shooting position, grip, position of arm, etc. the book is thoroughly illustrated with diagrams and photographs and includes the rules of the united states revolver association and a list of the records made both here and abroad. . pigeon raising, by alice macleod. this is a book for both fancier and market breeder. full descriptions are given of the construction of houses, the care of the birds, preparation for market, and shipment. descriptions of the various breeds with their markings and characteristics are given. illustrated with photographs and diagrams. . fishing tackle, by perry d. frazer. illustrated. the subtitle is descriptive. "hints for beginners in the selection, care, and use of rods, reels, lines, etc." it tells all the fisherman needs to know about making and overhauling his tackle during the closed season and gives full instructions for tournament casting and fly-casting. chapters are included on cases and holders for the care of tackle when not in use. . automobile operation, by a. l. brennan, jr. illustrated. tells the plain truth about the little things that every motorist wants to know about his own car. do you want to cure ignition troubles? overhaul and adjust your carbureter? keep your transmission in order? get the maximum wear out of your tires? do any other of the hundred and one things that are necessary for the greatest use and enjoyment of your car? then you will find this book useful. . the fox hound, by roger d. williams. author of "horse and hound". illustrated. the author is the foremost authority on fox hunting and foxhounds in america. for years he has kept the foxhound studbook, and is the final source of information on all disputed points relating to this breed. his book discusses types, methods of training, kenneling, diseases and all the other practical points relating to the use and care of the hound. an appendix is added containing the rules and regulations of hound field trials. . salt water game fishing, by charles f. holder. mr. holder covers the whole field of his subject devoting a chapter each to such fish as the tuna, the tarpon, amberjack, the sail fish, the yellow-tail, the king fish, the barracuda, the sea bass and the small game fishes of florida, porto rico, the pacific coast, hawaii, and the philippines. the habits and habitats of the fish are described, together with the methods and tackle for taking them. the book concludes with an account of the development and rules of the american sea angling clubs. illustrated. . winter camping, by warwick s. carpenter. a book that meets the increasing interest in outdoor life in the cold weather. mr. carpenter discusses such subjects as shelter equipment, clothing, food, snowshoeing, skiing, and winter hunting, wild life in winter woods, care of frost bite, etc. it is based on much actual experience in winter camping and is fully illustrated with working photographs. . woodcraft for women, by mrs. kathrene gedney pinkerton. the author has spent several years in the canadian woods and is thoroughly familiar with the subject from both the masculine and feminine point of view. she gives sound tips on clothing, camping outfit, food supplies, and methods, by which the woman may adjust herself to the outdoor environment. . small boat building, by h. w. patterson. illustrated with diagrams and plans. a working manual for the man who wants to be his own designer and builder. detail descriptions and drawings are given showing the various stages in the building, and chapters are included on proper materials and details. . *reading the weather, by t. morris longstreth. the author gives in detail the various recognized signs for different kinds of weather based primarily on the material worked out by the government weather bureau, gives rules by which the character and duration of storms may be estimated, and gives instructions for sensible use of the barometer. he also gives useful information as to various weather averages for different parts of the country, at different times of the year, and furnishes sound advice for the camper, sportsman, and others who wish to know what they may expect in the weather line. . boxing, by d. c. hutchison. practical instruction for men who wish to learn the first steps in the manly art. mr. hutchison writes from long personal experience as an amateur boxer and as a trainer of other amateurs. his instructions are accompanied with full diagrams showing the approved blows and guards. he also gives full directions for training for condition without danger of going stale from overtraining. it is essentially a book for the amateur who boxes for sport and exercise. . tennis tactics, by raymond d. little. out of his store of experience as a successful tennis player, mr. little has written this practical guide for those who wish to know how real tennis is played. he tells the reader when and how to take the net, discusses the relative merits of the back-court and volleying game and how their proper balance may be achieved; analyzes and appraises the twist service, shows the fundamental necessities of successful doubles play. . *how to play tennis, by james burns. this book gives simple, direct instruction from the professional standpoint on the fundamentals of the game. it tells the reader how to hold his racket, how to swing it for the various strokes, how to stand and how to cover the court. these points are illustrated with photographs and diagrams. the author also illustrates the course of the ball in the progress of play and points out the positions of greatest safety and greatest danger. . taxidermy, by leon l. pray. illustrated with diagrams. being a practical taxidermist, the author at once goes into the question of selection of tools and materials for the various stages of skinning, stuffing and mounting. the subjects whose handling is described are, for the most part, the every-day ones, such as ordinary birds, small mammals, etc., although adequate instructions are included for mounting big game specimens, as well as the preliminary care of skins in hot climates. full diagrams accompany the text. . the canoe--its selection, care and use, by robert e. pinkerton. illustrated with photographs. with proper use the canoe is one of the safest crafts that floats. mr. pinkerton tells how that state of safety may be obtained. he gives full instructions for the selection of the right canoe for each particular purpose or set of conditions. then he tells how it should be used in order to secure the maximum of safety, comfort and usefulness. his own lesson was learned among the indians of canada, where paddling is a high art, and the use of the canoe almost as much a matter of course as the wearing of moccasins. . horse packing, by charles j. post. illustrated with diagrams. this is a complete description of the hitches, knots, and apparatus used in making and carrying loads of various kinds on horseback. its basis is the methods followed in the west and in the american army. the diagrams are full and detailed, giving the various hitches and knots at each of the important stages so that even the novice can follow and use them. it is the only book ever published on this subject of which this could be said. full description is given of the ideal pack animal, as well as a catalogue of the diseases and injuries to which such animals are subject. . *learning to skate, by j. f. verne. the general problem of the art of skating is taken up from the standpoint of the man or woman who puts on skates for the first time. fundamental rules are laid down for learning the simpler strokes, carrying the reader on through to speed and fancy skating. advice is included on the proper skates and clothing. . *touring afoot, by dr. c. p. fordyce. illustrated. this book is designed to meet the growing interest in walking trips and covers the whole field of outfit and method for trips of varying length. various standard camping devices are described and outfits are prescribed for all conditions. it is based on the assumption that the reader will want to carry on his own back everything that he requires for the trip. . *the marine motor, by lieut. frank w. sterling, u. s. n. illustrated with diagrams. this book is the product of a wide experience on the engineering staff of the united states navy. it gives careful descriptions of the various parts of the marine motor, their relation to the whole and their method of operation; it also describes the commoner troubles and suggests remedies. the principal types of engines are described in detail with diagrams. the object is primarily to give the novice a good working knowledge of his engine, its operation and care. the storm. an essay. title: an elegy on the author of the true-born-english-man. with an essay on the late storm. by the author of the hymn to the pillory. the storm. an essay. i'm told, _for we have news among the dead_, heaven lately spoke, but few knew what it said; the voice, in loudest tempests spoke, and storms, which nature's strong foundation shook. _i felt it hither_, and i'd have you know i heard the voice, and knew the language too. think it not strange i heard it here, no place is so remote, but when _he speaks_, they hear. besides, tho' i am dead in fame, i never told you where i am. tho' i have lost poetick breath, i'm not in perfect state of death: from whence this _popish consequence_ i draw, _i'm in the_ limbus _of the law_. let me be where i will i heard the storm, from every blast _it eccho'd thus, reform_; i felt the mighty shock, and saw the night, when guilt look'd pale, and own'd the fright; and every time the raging element shook _london_'s lofty towers, at every rent the falling timbers gave, _they cry'd, repent_. i saw, when all the stormy crew, newly commission'd from on high, newly instructed what to do, in lowring, cloudy, troops drew nigh: they hover'd o'er the guilty land, as if they had been backward to obey; as if they wondred at the sad command, and pity'd those they shou'd destroy. but heaven, that long had gentler methods tried, and saw those gentler methods all defied, had now resolv'd to be obey'd. the queen, an emblem of the _soft, still, voice_, had told the nation how to make their choice; told them the only way to happiness was by the blessed door of peace. but the unhappy genius of the land, deaf to the blessing, as to the command, scorn the high caution, and contemn the news, and all the blessed thoughts of peace refuse. since storms are then the nation's choice, _be storms their portion, said the heavenly voice_: he said, and i could hear no more, so soon th' obedient troops began to roar: so soon the blackning clouds drew near, and fill'd with loudest storms the trembling air: i thought i felt the world's foundation shake, and lookt when all the wondrous frame would break. i trembl'd as the winds grew high, and so did many a braver man than i: for he whose valour scorns his sence, has chang'd his courage into impudence. man may to man his valour show, and 'tis his vertue to do so. but if he's of his maker not afraid, he's not courageous then, but mad. soon as i heard the horrid blast, and understood how long 'twould last, view'd all the fury of the element, consider'd well by whom 'twas sent, and _unto whom_ for punishment: it brought my hero to my mind, _william_, the glorious, great, and good, and kind. short epithets to his just memory; the first he was to all the world, _the last to me_. the mighty genius to my thought appear'd, just in the same concern he us'd to show, when private tempests us'd to blow, storms which the monarch more than death or battel fear'd. when party fury shook his throne, and made their mighty malice known, _i've heard the sighing monarch say_, the publick peace so near him lay, it took the pleasure of his crown away. it fill'd with cares his royal breast; often he has those cares prophetickly exprest, that when he should the reins let go, heaven would some token of its anger show, to let the thankless nation see how they despis'd their own felicity. this robb'd the hero of his rest, disturb'd the calm of his serener breast. when to the queen the scepter he resign'd, with a resolv'd and steady mind, tho' he rejoic'd to lay the trifle down, he pity'd her to whom he left the crown: foreseeing long and vig'rous wars, foreseeing endless, private, party jarrs, would always interrupt her rest, and fill with anxious cares her royal breast. for storms of court ambition rage as high almost as tempests in the sky. could i my hasty doom retrieve, and once more in the land of poets live, i'd now the men of flags and fortune greet, and write an elegy upon the fleet. first, those that on the shore were idly found, _whom other fate protects_, while better men were drown'd, they may thank god for being knaves on shore, but sure the q---- will never trust them more. they who rid out the storm, and liv'd, but saw not whence it was deriv'd, sensless of danger, or the mighty hand, that could to cease, as well as blow, command, let such unthinking creatures have a care, for some worse end prepare. let them look out for some such day, when what the sea would not, _the gallows may_. those that in former dangers shunn'd the fight, but met their ends in this disast'rous night, have left this caution, tho' too late, that all events are known to fate. cowards avoid no danger when they run, and courage scapes the death it would not shun; 'tis nonsence from our fate to fly, all men must once have heart enough to die. those sons of plunder are below my pen, because they are below the names of men; who from the shores presenting to their eyes the fatal _goodwin_, where the wreck of _navies_ lyes, a thousand dying saylors talking to the skies. from the sad shores they saw the wretches walk, by signals of distress they talk; there with one tide of life they're vext, for all were sure to die the next. the barbarous shores with men and boats abound, the men more barbarous than the shores are found; off to the shatter'd ships they go, and for the floating purchase row. they spare no hazard, or no pain, but 'tis to save the goods, and not the men. within the sinking supplaints reach appear, as if they'd mock their dying fear. then for some trifle all their hopes supplant, with cruelty would make a _turk_ relent. if i had any _satyr_ left to write, cou'd i with suited spleen indite, my verse should blast that fatal town, and drowned saylors widows pull it down; no footsteps of it should appear, and ships no more cast anchor there. the barbarous hated name of _deal_ shou'd die, or be a term of infamy; and till that's done, the town will stand a just reproach to all the land. the ships come next to be my theme, the men's the loss, i'm not concern'd for them; for had they perish'd e'er they went, where to no purpose they were sent, the ships might ha' been built again, and we had sav'd the money and the men. there the mighty wrecks appear, _hic jacent_, useless things of war. graves of men, and tools of state, there you lye too soon, there you lye too late. but o ye mighty ships of war! what in winter did you there? wild _november_ should our ships restore to _chatham_, _portsmouth_, and the _nore_, so it was always heretofore, for heaven it self is not unkind, if winter storms he'll sometimes send, since 'tis suppos'd the men of war are all laid up, and left secure. nor did our navy feell alone, the dreadful desolation; it shook the _walls of flesh_ as well as stone, and ruffl'd all the nation. the universal fright made guilty _h----_ expect his fatal night; his harden'd soul began to doubt, and storms grew high within, as they grew high without. flaming meteors fill'd the air, but _asgil_ miss'd his _fiery chariot_ there; recall'd his black blaspheming breath, and trembling paid his homage unto death. _terror appear'd in every face_, even _vile blackbourn_ felt some shocks of grace; began to feel the hated truth appear, began to fear, after _he had burlesqu'd a god_ so long, he should at last be in the wrong. some power he plainly saw, (and seeing, felt a strange unusual awe;) some secret hand he plainly found, was bringing some strange thing to pass, and he that neither god nor devil own'd, must needs be at a loss to guess. fain he would not ha' guest the worst, but guilt will always be with terror curst. hell shook, for devils dread almighty power, at every shock they fear'd the fatal hour, the adamantine pillars mov'd, and satan's _pandemonium_ trembl'd too; the tottering _seraphs_ wildly rov'd, doubtful what the almighty meant to do; for in the darkest of the black abode, _there's not a devil but believes a god_. old _lucifer_ has sometimes try'd _to have himself be deify'd_; but devils nor men the being of god deny'd, till men of late found out new ways to sin, and turn'd the devil out to let the atheist in. but when the mighty element began, and storms the weighty truth explain, almighty power upon the whirlwind rode, and every blast proclaim'd aloud _there is, there is, there is_, a god. plague, famine, pestilence, and war, are in their causes seen, the true originals appear before the effects begin: but storms and tempests are above our rules, here our philosophers are fools. the _stagyrite_ himself could never show, from whence, nor how they blow. tis all sublime, 'tis all a mystery, they see no manner how, nor reason why; _all sovereign being_ is the amazing theme, 'tis all resolv'd to power supreme; from this first cause our tempest came, and let the atheists spight of sense blaspheme, they can no room for banter find, till they produce another father for the wind. _satyr_, thy sense of sovereign being declare, he made the mighty prince o'th' air, and devils recognize him by their fear. ancient as time, and elder than the light, ere the first day, or antecedent night, ere matter into settl'd form became, and long before existence had a name; before th' expance of indigested space, while the vast _no-where_ fill'd the room of place. liv'd _the first cause_ the first great _where_ and _why_, existing _to and from_ eternity, of his great self, and _of necessity_. _this i call god_, that one great word of fear, at whose great sound, when from his mighty breath 'tis eccho'd round, nature pays homage with a trembling bow, and conscious men would faintly disallow; the secret trepidation racks the soul, and while he says, no god, replies, thou fool. _but call it what we will_, _first being it had_, does space and substance fill. eternal self-existing power enjoy'd, and whatsoe'er is so, _that same is god_. if then it should fall out, as who can tell, but that there is a heaven and hell, mankind had best consider well for fear 't should be too late when their mistakes appear; such may in vain reform, unless they do't before another storm. they tell us _scotland_ scap'd the blast; no nation else have been without a taste: all _europe_ sure have felt the mighty shock, 't has been a universal stroke. but heaven has other ways to plague the _scots_, as poverty and plots. her majesty confirms it, what she said, i plainly heard it, tho' i'm dead. the dangerous sound has rais'd me from my sleep, i can no longer silence keep, here _satyr_'s thy deliverance, a plot in _scotland_, hatch'd in _france_, and liberty the old pretence. prelatick power with popish join, the queens just government to undermine; this is enough to wake the dead, the call's too loud, it never shall be said the lazy _satyr_ slept too long, when all the nations danger claim'd his song. rise _satyr_ from thy sleep of legal death, and reassume satyrick breath; what tho' to seven years sleep thou art confin'd, thou well may'st wake with such a wind. such blasts as these can seldom blow, but they're both form'd above and heard below. then wake and warn us now the storms are past, lest heaven return with a severer blast. wake and inform mankind of storms that still remain behind. if from this grave thou lift thy head, they'll surely mind one risen from the dead. tho' _moses_ and the prophets can't prevail, a speaking _satyr_ cannot fail. tell 'em while secret discontents appear, there'll ne'er be _peace and union_ here. they that for trifles so contend, have something farther in their end; but let those hasty people know, the storms above reprove the storms below, and 'tis too often known, the storms below do storms above forerun; they say this was a high-church storm, sent out the nation to reform; but th' emblem left the moral in the lurch, for't blew the steeple down upon the church. from whence we now inform the people, the danger of the church is from the steeple. and we've had many a bitter stroke, from pinacle and weather-cock; from whence the learned do relate, that to secure the church and state, the time will come when all the town to save the church, will pull the steeple down. two tempests are blown over, now prepare for storms of treason and intestine war. the high-church fury to the north extends, in haste to ruin all their friends. occasional conforming led the way, and now occasional rebellion comes in play, to let the wond'ring nation know, that high-church honesty's an empty show, a phantasm of delusive air, that as occasion serves can disappear, and loyalty's a sensless phrase, an empty nothing which our interest sways, and as that suffers this decays. who dare the dangerous secret tell, _that church-men can rebel_. faction we thought was by the whigs engross'd, and _forty one_ was banter'd till the jest was lost. _bothwel_ and _pentland-hills_ were fam'd, and _gilly cranky_ hardly nam'd. if living poets dare not speak, _we that are dead_ must silence break; and boldly let them know the time's at hand. when ecclesiastick tempests shake the land. prelatick treason from the crown divides, and now rebellion changes sides. their volumes with their loyalty may swell, but in their turns too they rebel; can plot, contrive, assassinate, and spight of passive laws disturb the state. let fair pretences fill the mouths of men, no fair pretence shall blind my pen; they that _in such a reign as this_ rebel must needs be in confederacy with hell. oppressions, tyranny and pride, may give some reason to divide; but where the laws with open justice rule, he that rebels _must be both knave and fool_. may heaven the growing mischief soon prevent, and traytors meet reward in punishment. _finis._ contributions from the museum of history and technology: paper the natural philosophy of william gilbert and his predecessors _w. james king_ by w. james king the natural philosophy of william gilbert and his predecessors until several decades ago, the physical sciences were considered to have had their origins in the th century--mechanics beginning with men like galileo galilei and magnetism with men like the elizabethan physician and scientist william gilbert. historians of science, however, have traced many of the th century's concepts of mechanics back into the middle ages. here, gilbert's explanation of the loadstone and its powers is compared with explanations to be found in the middle ages and earlier. from this comparison it appears that gilbert can best be understood by considering him not so much a herald of the new science as a modifier of the old. the author: w. james king is curator of electricity, museum of history and technology, in the smithsonian institution's united states national museum. the year saw the publication by an english physician, william gilbert, of a book on the loadstone. entitled _de magnete_,[ ] it has traditionally been credited with laying a foundation for the modern science of electricity and magnetism. the following essay is an attempt to examine the basis for such a tradition by determining what gilbert's original contributions to these sciences were, and to make explicit the sense in which he may be considered as being dependent upon earlier work. in this manner a more accurate estimate of his position in the history of science may be made. [ ] william gilbert, _de magnete, magneticisque corporibus et de magno magnete tellure; physiologia nova, plurimis & argumentis, & experimentis, demonstrata_, london, , pp., with an introduction by edward wright. all references to gilbert in this article, unless otherwise noted, are to the american translation by p. fleury mottelay, pp., published in new york in , and are designated by the letter m. however, the latin text of the edition has been quoted wherever i have disagreed with the mottelay translation. a good source of information on gilbert is dr. duane h. d. roller's doctoral thesis, written under the direction of dr. i. b. cohen of harvard university. dr. roller, at present curator of the de golyer collection at the university of oklahoma, informed me that an expanded version of his dissertation will shortly appear in book form. unfortunately his researches were not known to me until after this article was completed. one criterion as to the book's significance in the history of science can be applied almost immediately. a number of historians have pointed to the introduction of numbers and geometry as marking a watershed between the modern and the medieval understanding of nature. thus a. koyré considers the archimedeanization of space as one of the necessary features of the development of modern astronomy and physics.[ ] a. n. whitehead and e. cassirer have turned to measurement and the quantification of force as marking this transition.[ ] however, the obvious absence[ ] of such techniques in _de magnete_ makes it difficult to consider gilbert as a founder of modern electricity and magnetism in this sense. [ ] alexandre koyré, _Études galiléennes_, paris, . [ ] alfred n. whitehead, _science and the modern world_, new york, , ch. ; ernst cassirer, _das erkenntnisproblem_, ed. , berlin, , vol. , pp. - , - . [ ] however, see m: pp. , , , . [illustration: figure .--william gilbert's book on the loadstone, title page of the first edition, from a copy in the library of congress. (_photo courtesy of the library of congress._)] there is another sense in which it is possible to contend that gilbert's treatise introduced modern studies in these fields. he has frequently been credited with the introduction of the inductive method based upon stubborn facts, in contrast to the methods and content of medieval aristotelianism.[ ] no science can be based upon faulty observations and certainly much of _de magnete_ was devoted to the destruction of the fantastic tales and occult sympathies of the romans, the medieval writers, and the renaissance. however, let us also remember that gilbert added few novel empirical facts of a fundamental nature to previous observations on the loadstone. gilbert's experimental work was in large part an expansion of petrus peregrinus' _de magnete_ of ,[ ] and a development of works like robert norman's _the new attractive_,[ ] in which the author discussed how one could show experimentally the declination and inclination of a magnetized needle, and like william borough's _discourse on the variation of the compass or magnetized needle_,[ ] in which the author suggested the use of magnetic declination and inclination for navigational purposes but felt too little was known about it. that other sea-going nations had been considering using the properties of the magnetic compass to solve their problems of navigation in the same manner can be seen from simon stevin's _de havenvinding_.[ ] [ ] for example, william whewell, _history of the inductive sciences_, ed. , new york, , vol. , pp. and ; charles singer, _a short history of science to the nineteenth century_, oxford, , pp. and ; and a. r. hall, _the scientific revolution_, boston, , p. . [ ] _petri peregrini maricurtenis, de magnete, seu rota perpetui motus, libellus_, a reprint of the angsburg edition in j. g. g. hellmann, _rara magnetica_, berlin, , not paginated. a number of editions of peregrinus, work, both ascribed to him and plagiarized from him, appeared in the th century (see heinz balmer, _beiträge zur geschichte der erkenntnis des erdmagnetismus_, aarau, , pp. - ). [ ] hellmann, _ibid._, robert norman, _the newe attractive, containyng a short discourse of the magnes or lodestone, and amongest other his vertues, of a newe discovered secret and subtill propertie, concernyng the declinyng of the needle, touched therewith under the plaine of the horizon. now first founde out by robert norman hydrographer_. london, . the possibility is present that norman's work was a direct stimulus to gilbert, for wright's introduction to _de magnete_ stated that gilbert started his study of magnetism the year following the publication of norman's book. [ ] hellman, _ibid._, william borough, _a discourse of the variation of the compasse, or magneticall needle. wherein is mathematically shewed, the manner of the observation, effects, and application thereof, made by w. b. and is to be annexed to the newe attractive of r. n._ london, . [ ] hellman, _ibid._, simon stevin, _de havenvinding_, leyden, . it is interesting to note that wright translated stevin's work into english. instead of new experimental information, gilbert's major contribution to natural philosophy was that revealed in the title of his book--a new philosophy of nature, or physiology, as he called it, after the early greeks. gilbert's attempt to organize the mass of empirical information and speculation that came from scholars and artisans, from chart and instrument makers, made him "the father of the magnetic philosophy."[ ] [ ] as edward wright was to call him in his introduction. gilbert's _de magnete_ was not the first attempt to determine the nature of the loadstone and to explain how it could influence other loadstones or iron. it is typical of greek philosophy that one of the first references we have to the loadstone is not to its properties but to the problem of how to explain these properties. aristotle[ ] preserved the solution of the first of the ionian physiologists: "thales too ... seems to suppose that the soul is in a sense the cause of movement, since he says that a stone has a soul because it causes movement to iron." plato turned to a similar animistic explanation in his dialogue, _ion_.[ ] such an animistic solution pervaded many of the later explanations. [ ] aristotle, _on the soul_, translated by w. s. hett, loeb classical library, london, , a (see also a : "some think that the soul pervades the whole universe, whence perhaps came thales' view that everything is full of gods"). [ ] plato, _ion_, translated by w. r. m. lamb, loeb classical library, london, , (see also ). that a mechanical explanation is also possible was shown by plato in his _timaeus_.[ ] he argued that since a vacuum does not exist, there must be a plenum throughout all space. motion of this plenum can carry objects along with it, and one could in this manner explain attractions like that due to amber and the loadstone. [ ] plato, _timaeus_, translated by r. g. bury, loeb classical library, london, , . it is difficult to determine which explanation plato preferred, for in both cases the speaker may be only a foil for plato's opinion rather than an expression of these opinions. another mechanical explanation was based upon a postulated tendency of atoms to move into a vacuum rather than upon the latter's non-existence. lucretius restated this epicurean explanation in his _de rerum natura_.[ ] atoms from the loadstone push away the air and tend to cause a vacuum to form outside the loadstone. the structure of iron is such that it, unlike other materials, can be pushed into this empty space by the thronging atoms of air beyond it. [ ] lucretius, _de rerum natura_, translated by w. h. d. rouse, loeb classical library, london, , bk. vi, lines - . galen[ ] returned to a quasi-animistic solution in his denial of epicurus' argument, which he stated somewhat differently from lucretius. one can infer that galen held that all things have, to a greater or lesser degree, a sympathetic faculty of attracting its specific, or proper, quality to itself.[ ] the loadstone is only an inanimate example of what one finds in nutritive organs in organic beings. [ ] galen, _on the natural faculties_, translated by a. s. brock, loeb classical library, london, , bk. and bk. . a view similar to this appeared in plato, _timaeus_, (see footnote ). [ ] this same concept was to reappear in the middle ages as the _inclinatio ad simile_. one of the few writers whose explanations of the loadstone gilbert mentioned with approval is st. thomas aquinas. although the medieval scholastic philosophy of st. thomas seems foreign to our way of thinking, it formed a background to many of gilbert's concepts, as well as to those of his predecessors, and it will assist our discussion to consider briefly thomist philosophy and to make its terminology explicit at this point.[ ] [ ] the background for much of the following was derived from annaliese maier, _an der grenze von scholastik und naturwissenchaft_, ed , rome, . in scholastic philosophy, all beings and substances are a coalescence of inchoate matter and enacting form. form is that which gives being to matter and which is responsible for the "virtus" or power to cause change, since matter in itself is inert. moreover, forms can be grasped intellectually, whence the nature of a being or a substance can be known. any explanation of phenomena has to be based upon these innate natures, for only if the nature of a substance is known can its properties be understood. inanimate natures are determined by observation, abstraction, and induction, or by classification.[ ] [ ] st. thomas' epistemology for the natural inanimate world was based upon aristotle's dictum: that which is in the mind was in the senses first. the nature of a substance is causally prior to its properties; while the definition of the nature is logically prior to these properties. thus, what we call the theory of a substance is expressed in its definition, and its properties can be deduced from this definition. the world of st. thomas is not a static one, but one of the aristotelian motions of quantity (change of size), of quality (alteration), and of place (locomotion). another kind of change is that of substance, called generation and corruption, but this is a mutation, occurring instantly, rather than a motion, that requires time. in mutation the essential nature is replaced by a new substantial form. all these changes are motivated by a causal hierarchy that extends from the first cause, the "dator formarum," or creator, to separate intellectual substances that may be angels or demons, to the celestial bodies that are the "generantia" of the substantial forms of the elements and finally to the four prime qualities (dry and wet, hot and cold) of the substantial forms. accidental forms are motivated by the substantial forms through the instrumentality of the four prime qualities, which can only act by material contact. the only causal agents in this hierarchy that are learned through the senses are the tangible qualities. usually the prime qualities are not observed directly, but only other qualities compounded of them. one of the problems of scholastic philosophy was the incorporation, into this system of efficient agents, of other qualities, such as the qualities of gravity and levity that are responsible for upward and downward motion. besides the causal hierarchy of forms, the natural world of st. thomas existed in a substantial and spatial hierarchy. all substances whether an element or a mixture of elements have a place in this hierarchy by virtue of their nature. if the material were removed from its proper place, it would tend to return. in this manner is obtained the natural downward motion of earth and the natural upward motion of fire. local motion can also be caused by the "virtus coeli" generating a new form, or through the qualitative change of alteration. since each element and mixture has its own natural place in the hierarchy of material substances, and this place is determined by its nature, changes of nature due to a change of the form can produce local motion. if before change the substance is in its natural place, it need not be afterwards, and if not, would then tend to move to its new natural place. it will be noted that the scholastic explanation of inanimate motion involved the action and passion of an active external mover and a passive capacity to be moved. whence the definition of motion that descartes[ ] was later to deride, "motus est actus entis in potentia prout quod in potentia." [ ] rené descartes, _oeuvres_, charles adam and paul tannery, paris, - , vol. , p. (letter to mersenne, oct., ), and vol. (le monde), p. . the original definition can be found in aristotle, _physics_, translated by p. h. wickstead and f. m. cornford, loeb classical library, london, , a . aquinas rephrases the definition as "_motus est actus existentis in potentia secundum quod huius modi._" see st. thomas aquinas, _opera omnia_, antwerp, , vol. , _physicorum aristotelis expositio_, lib. , lect. , cap. a, p. . we have seen above that the "motor essentialis" for terrestial change is the "virtus coeli." thus the enacting source of all motion and change is the heavens and the heavenly powers, while the earth and its inhabitants becomes the focus or passive recipient of these actions. in this manner the scholastic restated in philosophical terms the drama of an earth-centered universe. although change or motion is normally effected through the above mentioned causal hierarchy, it is not always necessary that actualization pass from the first cause down through each step of the hierarchy to terminate in the qualities of the individual being. some of the steps could be by-passed: for instance man's body is under the direct influence of the celestial bodies, his intellect under that of the angels and his will under god.[ ] another example of effects not produced through the tangible prime qualities is that of the tide-producing influence of the moon on the waters of the ocean or the powers of the loadstone over iron. such causal relations, where some members of the normal causal chain have been circumvented, are called occult.[ ] [ ] st. thomas aquinas, _op. cit._ (footnote ), vol. , _summa contra gentiles_, lib. , cap. (quo modo dicitur aliquis bene fortunatus et quo modo adjuvatur homo ex superioribus causis), p. . [ ] st. thomas aquinas, op. cit. (footnote ), vol. _opuscula, de operationibus occultis naturae ad queindam militem ultramontem_, pp. - . while st. thomas referred to the loadstone in a number of places as something whose nature and occult properties are well known, it was always as an example or as a tangential reference. one does not find a systematic treatment of the loadstone in st. thomas, but there are enough references to provide a fairly explicit statement of what he considered to be the nature of the magnet. in one of his earliest writings, st. thomas argued that the magnet attracts iron because this is a necessary consequence of its nature.[ ] respondeo dicendum, quod omnibus rebus naturaliter insunt quaedam principia, quibus non solum operationes proprias efficere possunt, sed quibus etiam eas convenientes fini suo reddant, sive sint actiones quae consequantur rem aliquam ex natura sui generis, sive consequantur ex natura speciei, ut magneti competit ferri deorsum ex natura sui generis, et attrahere ferrum ex natura speciei. sicut autem in rebus agentibus ex necessitate naturae sunt principia actionum ipsae formae, a quibus operationes proprie prodeunt convenientes fini.... due to its generic form, the loadstone is subject to natural motion of place of up and down. however, the "virtus" of its specific form enabled it to produce another kind of motion--it could draw iron to itself. normally the "virtus" of a substance is limited to those contact effects that could be produced by the form operating through the active qualities of one substance, on the relatively passive qualities of another. st. thomas asserted the loadstone to be one of these minerals, the occult powers of whose form goes beyond those of the prime qualities.[ ] forma enim elementi non habet aliquam operationem nisi quae fit per qualitates activas et passivas, quae sunt dispositiones materiae corporalis. forma autem corporis mineralis habet aliquam operationem excedentem qualitates activas et passivas, quae consequitur speciem ex influentia corporis coelestis, ut quod magnes attrahit ferrum, et quod saphirus curat apostema. that this occult power of the loadstone is a result of the direct influence of the "virtus coeli" was expounded at greater length in his treatise on the soul.[ ] quod quidem ex propriis formarum operationibus perpendi potest. formae enim elementorum, quae sint infimae et materiae propinquissime, non habent aliquam operationem excedentem qualitates activas et passivas, ut rarum et densum, et aliae huiusmodi, qui videntur esse materiae dispositiones. super has autem sunt formae mistorum quae praeter praedictas operationes, habent aliquam operationem consequentem speciem, quam fortiuntur ex corporibus coelestibus; sicut quod magnes attrahit ferrum non propter calorem aut frigiis, aut aliquid huiusmodi; sed ex quadam participatione virtutis coelestis. super has autem formas sint iterum animae plantarum, quae habent similitudinem non solum ad ipsa corpora coelestia, sed ad motores corporum coelestium, inquantum sunt principia cuiusdam motus, quibusdam seipsa moventibus. super has autem ulterius sunt animae brutorum, quae similitudinem iam habent ad substantiam moventem coelestia corpora, non solum in operatione qua movent corpora, sed etiam in hoc quod in seipsis cognoscitivae sunt, licet brutorum cognitio sit materialium tantum et materialiter.... st. thomas placed the form of the magnet and its powers in the hierarchy of forms intermediate between the forms of the inanimate world and the forms of the organic world with its hierarchy of plant, animal and rational souls. the form of the loadstone is then superior to that of iron, which can only act through its active and passive qualities, but inferior to the plant soul, that has the powers of growth from the "virtus coeli." this is similar to galen's comparison of the magnet's powers to that of the nutritive powers of organic bodies. in his commentary on aristotle's _physics_, st. thomas explained how iron is moved to the magnet. it is moved by some quality imparted to the iron by the magnet.[ ] illud ergo trahere dicitur, quod movet alterum ad seipsum. movere autem aliquid secundum locum ad seipsum contingit tripliciter. uno modo sicut finis movet; unde et finis dicitur trahere, secundum illud poetate: "trahit sua quemque voluptas": et hoc modo potest dici quod locus trahit id, quod naturaliter movetur ad locum. alio modo potest dici aliquid trahere, quia movet illud ad seipsum alterando aliqualiter, ex qua alteratione contingit quod alteratum moveatur secundum locum: et hoc modo magnes dicitur trahere ferrum. sicut enim generans movet gravia et levia, inquantum dat eis formarum per quam moventur ad locum, ita et magnes dat aliquam qualitatem ferro, per quam movetur ad ipsum. et quod hoc sit verum patet ex tribus. primo quidem quia magnes non trahit ferrum ex quacumque distantia, sed ex propinquo; si autem ferrum moveretur ad magnetem solum sicut ad finem, sicut grave ad suum locum, ex qualibet distantia tenderet ad ipsum. secundo, quia, si magnes aliis perungatur, ferrum attrahere non potest; quasi aliis vim alterativam ipsius impedientibus, aut etiam in contrarium alterantibus. tertio, quia ad hoc quod magnes attrahat ferrum, oportet prius ferrum liniri cum magnete, maxime si magnes sit parvus; quasi ex magnete aliquam virtutem ferrum accipiat ut ad eum moveatur. sic igitur magnes attrahit ferrum non solum sicut finis, sed etiam sicut movens et alterans. tertio modo dicitur aliquid attrahere, quia movet ad seipsum motu locali tantum. et sic definitur hic tractio, prout unum corpus trahit alteram, ita quod trahens simul moveatur cum eo quod trahitur. as the "generans" of terrestrial change moves what is light and heavy to another place by implanting a new form in a substance, so the magnet moves the iron by impressing upon it the quality by which it is moved. by virtue of the new quality, the iron is not in its natural place and moves accordingly. st. thomas proved that the loadstone acts as a secondary "generans" in three ways: ( ) the loadstone produces an effect not from any distance but only from a nearby position (showing that this motion is due to more than place alone), ( ) rubbing the loadstone with garlic acts as if it impedes or alters the "virtus magnetis," and ( ) the iron must be properly aligned with respect to the loadstone in order to be moved, especially if the loadstone is small. thus the iron is moved by the magnet not only to a place, but also by changing and altering it: one has not only the change of locomotion but that of alteration. moreover the source of this alteration in the iron is not the heavens but the loadstone. accordingly the loadstone could cause change in another substance because it could influence the nature of the other substance. [ ] st. thomas aquinas, _op. cit._ (footnote ), vol , _scriptum in quartum librum sententiarum magistri petri lombardi_, lib. , disq. (de diversis coniugii legibus), art. (utrum habere plures uxores sit contra legem naturae), p. . the same statement occurs in one of his most mature works, _op. cit._ vol. , _summa theologica_, pars (supplementum), quaestio (de pluralitate uxorum in quinque articulos divisa), art. (utrum habere plures uxores sit contra legem naturae), p. . [ ] st. thomas aquinas, _op. cit._ (footnote ), vol. , _quaestio unica: de spiritualibus creaturis_, art. (utrum substantia spiritualis possit uniri corpori), p. . see also vol. , _summa contra gentiles_, lib. , cap. (quomodo dicitur aliquis bene fortunatus, et quomodo adjuvatur homo ex superioribus causis), p. ; and vol. , _opuscula, de operationibus occultis naturae ad queindam militem ultramontem_, pp. - . [ ] st. thomas aquinas, _op. cit._ (footnote ), vol. , _quaestio unica: de anima_, art. (utrum anima humana possit esse forma et hoc aliquid), p. . see also vol. , _quaestio: de veritate_, quaestio (de providentia), art. (utrum humani actus a divina providentia gubernentur mediis corporibus coelestibus), p. . [ ] st. thomas aquinas, _op. cit._ (footnote ), vol. , _physicorum aristotelis expositio_, lib. , lect. , cap. g (probatur in motu locali quod movens et motum oportet esse simul), p. (quoted in gilbert, m: p. ). about the time that st. thomas was writing his letter _de operationibus occultis naturae_ to a certain knight, petrus peregrinus was writing from a military camp a letter in which he showed how certain relatively new effects could be produced by the loadstone. he was more interested in what he could do with the magnet than in explaining these effects. however, he discussed it at sufficient length for one to find that his explanation of magnetic phenomena was basically similar to that of his contemporary, st. thomas. peregrinus based his discussion of the loadstone upon its nature and analyzed magnetic phenomena in terms of the change of alteration. in magnetic attraction, the nature of the iron is altered by having a new quality impressed upon it,[ ] and the loadstone is the agent that makes the iron the same species as the stone.[ ] ... oportet enim quod illud quod iam conversum est ex duobus in unum, sit in eadem specie cum agente; quod non esset, si natura istud impossible eligeret. this impressed similarity to the agent, peregrinus realized, is not a pole of the same polarity but one opposite to that of the inducing pole. to produce this effect, the virtue of the stronger agent dominates the weaker patient and impresses the virtue of the stronger on the weaker so that they are made similar.[ ] ... in cuius attractione, lapis fortioris virtutis agens est; debilioris vero patiens. a further instance of alteration occurs in the reversal of polarity of magnetized iron when one brings two similar poles together. again, the stronger agent dominates the weaker patient and the iron is left with a similarity to the last agent.[ ] ... causa huis est impressio ultimi agentis, confundentis et alterantis virtutem primi. in this assimilation of the agent to the patient, another effect is produced: the agent not only desires to assimilate the patient to itself, but to unite with it to become one and the same. speaking of the motion to come together, he says:[ ] huius autem rei causam per hanc viam fieri existimo: agens enim intendit suum patiens non solum sibi assimilare, sed unire, ut ex agente et patiente fiat unum, per numerum. et hoc potes experiri in isto lapide mirabili in hunc modum.... agens ergo, ut vides experimento, intendit suum paciens sibi unire; hoc autem fit ratione similitudinis inter ea. oportet ergo ... virtute attractionis, fiat una linea, ex agente et patiente, secundum hunc ordinem ... the nature of the magnet, as an active cause, tends to enact, and since it acts in the best manner in which it is able, it acts so as to preserve the similarities of opposite poles.[ ] natura autem, que tendet ad esse, agit meliori modo quo potest, eligit primum ordinem actionis, in quo melius salvatur idemptitas, quam in secundo ... thus unlike poles tend to come together when a dissected magnet is reassembled. like st. thomas, peregrinus argued that the magnet receives its powers from the heavens. but he further specified this by declaring that different virtues from the different parts of the heavens flow into their counterpart in the loadstone--from the poles of the heavens the virtue flows into the poles of the magnet,[ ] praeterea cum ferrum, vel lapis, vertatur tarn ad partem meridionalem quam ad partem septemtrionalem ... existima cogimur, non solum a partem septemtrionali, verum etiam a meridionali virtutem influi in polos lapidis, magis quam a locis minere ... omnes autem orbes meridiani in polis mundi concurrent; quare, a polis mundi, poli magnetis virtutem recipiunt. et ex hoc apparet manifeste quod non ad stellam nauticam movetur, cum ibi non concurrant orbes meridiani, sed in polis; stella enim nautica, extra orbem meridianum cuiuslibet regionis semper invenitur, nisi bis, in completa firmanenti revolutione. ex hiis ergo manifestum est quod a partibus celi, partes magnetis virtutem recipiunt. and similarly for the other parts of the heavens and the other parts of the loadstone.[ ] ceteras autem partes lapidis merito estimare potes, influentiam a reliquis celi partibus retinere, ut non sic solum polos lapidis a polis mundi, sed totum lapidem a toto celo, recipere influentiam et virtutem, estimes. physical proof for such influences was adduced by peregrinus from the motions of the loadstone. that the poles of the loadstone receive their virtue from the poles of the heavens follows experimentally from north-south alignment of a loadstone. that not only the poles but the entire loadstone receives power from corresponding portions of the heavens follows from the fact that a spherical loadstone, when "properly balanced," would follow the motion of the heavens.[ ] quod tibi tali modo consulo experire: ... et si tunc lapis moveatur secundum celi motum, gaudeas te esse assecutum secretum mirabile; si vero non, imperitie tue, potiusquam nature, defectus imputetur. in hoc autem situ, seu modo positionis, virtutes lapidis huius estimo conservari proprie, et in reliquis sitibus celi virtutem eius obsecari, seu ebetari, potiusquam conservari puto. per hoc autem instrumentum excusaberis ab omni horologio; nam per ipsum scire poteris ascensus in quacumque hora volueris, et omnes alias celi dispositiones, quas querunt astrologi. as the heavens move eternally, so the spherical loadstone must be a "perpetuum mobile". another of the scholars whose explanation of the loadstone gilbert noted with approval was cardinal nicholas of cusa.[ ] the latter's references to it were not as direct as those of st. thomas, but he did use it as an image several times to provide a microcosmic example of the relation of god to his creation. from this one can infer that he explained the preternatural motion of the magnet and the iron by impressed qualities, the heavens being the agent for the loadstone, and the loadstone, the agent for iron. [ ] hellmann, _op. cit._ (footnote ), peregrinus, pt. , ch. . the magnet attracts the iron "secundum naturalem appetitum lapidis ... sine resistentia." there is no natural resistence to this motion since it is no longer contrary to the nature of the iron. the nature of the iron has changed. [ ] _ibid._, pt. , ch. . [ ] _ibid._, pt. , ch. . [ ] _ibid._, pt. , ch. . [ ] _ibid._, pt. , ch. . [ ] _ibid._, pt. , ch. . see also footnote . [ ] _ibid._, pt. , ch. . see also ch. . [ ] _ibid._, pt. , ch. . see also ch. . [ ] _ibid._, pt. , ch. . [ ] however, he may not always have approved of him. see m: ; "overinquisitive theologians, too, seek to light up god's mysteries and things beyond man's understanding by means of the loadstone and amber." in the _idiota de sapientia_ the cardinal used the image of the magnet and the iron to provide a concrete instance of his "coincidentia oppositorum," to illustrate how eternal wisdom, in the neoplatonic sense, could, at the same time, be principle or cause of being, its complement and also its goal.[ ] si igitur in omni desiderio vitae intellectualis attenderes, a quo est intellectus, per quod movetur et ad quod, in te comperires dulcedinem sapientiae aeternae illam esse, quae tibi facit desiderium tuum ita dulce et delectabile, ut in inerrabili affectu feraris ad eius comprehensionem tanquam ad immortalitatem vitae tue, quasi ad ferrum et magnetem attendas. habet enim ferrum in magnete quoddam sui effluxus principium; et dum magnes per sui praesentiam excitat ferrum grave et ponderosum, ferrum mirabili desiderio fertur etiam supra motum naturae, quo secundum gravitatem deorsum tendere debet, et sursum movetur se in suo principio uniendo. nisi enim in ferro esset quaedam praegustatio naturalis ipsius magnetis, non moveretur plus ad magnetem quam ad alium lapidem; et nisi in lapide esset major inclinatio ad ferrum quam cuprum, non esset illa attractio. habet igitur spiritus noster intellectualis ab aeterna sapientia principium sic intellectualiter essendi, quod esse est conformius sapientae quam aliud non intellectuale. hinc irraditio seu immissio in sanctam animam est motus desideriosus in excitatione. by virtue of the principle that flows from the magnet to the iron--which principle is potentially in the iron, for the iron already has a foretaste for it--the excited iron could transcend its gravid nature and be preternaturally moved to unite with its principle. reciprocally, the loadstone has a greater attraction to the iron than to other things. just as the power of attraction comes from the loadstone, so the deity is the source of our life. just as the principle implanted in the magnet moves the iron against its heavy nature, so the deity raises us above our brutish nature so that we may fulfill our life. as the iron moves to the loadstone, so we move to the deity as to the goal and end of our life. in _de pace fidei_, cusa[ ] again used the iron and magnet as an example of motion contrary to and transcending nature. he explained this supernatural motion as being due to the similarity between the nature of the iron and the magnet, and this in turn is analogous to the similarity between human spiritual nature and divine spiritual nature. as the iron can move upward to the loadstone because both have similar natures, so man can transcend his own nature and move towards god when his potential similitude to god is realized. another image used by cusa was the comparison of christ to the magnetic needle that takes its power from the heavens and shows man his way.[ ] [ ] nicholas of cusa (nicolaus cusaneus), _nicolaus von cues, texte seiner philosophischen schriften_, ed. a. petzelt, stuttgart, , bk. , _idiota de sapientia_, p. (quoted in gilbert, m: ). it is interesting that cusa held that the loadstone has an inclination to iron, as well as the converse! [ ] cusa, _cusa schriften_, vol. , _de pace fidei_, translated by l. mohler, leipzig, , ch. , p. . [ ] cusa, _exercitationes_, ch. , and , quoted in, f. a. scharpff, _des cardinals und bischofs nicolaus von cusa wichtigste schriften in deutscher uebersetzung_, freiburg, , p. . see also martin billinger, _das philosophische in den excitationen des nicolaus von cues_, heidelberg, , and _cusa schriften_ (see footnote ), vol. , p. , note . gilbert (m: p. ) called the compass "the finger of god." the elizabethan englishman robert norman also turned to the deity to explain the wonderful effects of the loadstone.[ ] now therefore ... divers have whetted their wits, yea, and dulled them, as i have mine, and yet in the end have been constrained to fly to the cornerstone: i mean god: who ... hath given virtue and power to this stone ... to show one certain point, by his own nature and appetite ... and by the same vertue, the needle is turned upon his own center, i mean the center of his circular and invisible vertue ... and surely i am of opinion, that if this would be found in a sphericall form, extending round about the stone in great compass, and the dead body stone in the middle therof: whose center is the center of his aforesaid vertue. and this i have partly proved, and made visible to be seen in the same manner, and god sparing me life, i will herein make further experience. again, one can infer that the heavens impart a guiding principle to the iron which acts under the influence of this superior cause. one of the points made in st. thomas' argument on motion due to the loadstone was that there is a limit to the "virtus" of the loadstone, but he did not specify the nature of it. norman refined the thomist concept of a bound by making it spherical in form, foreshadowing gilbert's "orbis virtutis." gilbert's philosophy of nature does not move far from scholastic philosophy, except away from it in logical consistency. as the concern of aristotle and of st. thomas was to understand being and change by determining the nature of things, so gilbert sought to write a logos of the physis, or nature, of the loadstone--a physiology.[ ] this physiology was not formally arranged into definitions obtained by induction from experience, but nevertheless there was the same search for the quiddity of the loadstone. once one knew this nature then all the properties of the loadstone could be understood. [ ] hellmann, _op. cit._ (footnote ), norman, bk. , ch. . [ ] m: p. . gilbert described the nature of the loadstone in the terms of being that were current with his scholarly contemporaries. this was the same ontology that scholasticism had taught for centuries--the doctrine of form and matter that we have already found in st. thomas and nicholas of cusa. thus we find richard hooker[ ] remarking that form gives being and that "form in other creatures is a thing proportionable unto the soul in living creatures." francis bacon,[ ] in speaking of the relations between causes and the kinds of philosophy, said: "physics is the science that deals with efficient and material causes while metaphysics deals with formal and final causes." john donne[ ] expressed the problem of scholastic philosophy succinctly: this twilight of two yeares, not past or next, some embleme is of me, ... ... of stuffe and forme perplext, whose _what_ and _where_, in disputation is ... as we shall see, gilbert continued in the same tradition, but his interpretation of form and formal cause was much more anthropomorphic than that of his predecessors. gilbert began his _de magnete_ by expounding the natural history of that portion of the earth with which we are familiar.[ ] having declared the origin and nature of the loadstone, we hold it needful first to give the history of iron also ... before we come to the explication of difficulties connected with the loadstone ... we shall better understand what iron is when we shall have developed ... what are the causes and the matter of metals ... his treatment of the origin of minerals and rocks agreed in the main with that of aristotle,[ ] but he departed somewhat from the peripatetic doctrine of the four elements of fire, air, water, and earth.[ ] instead, he replaced them by a pair of elements.[ ] (if the rejection of the four aristotelian elements were clearer, one might consider this a part of his rejection of the geocentric universe but he did not define his position sufficiently.)[ ] [ ] richard hooker. _of the laws of ecclesiastical polity_, bk. , ch. , sect. (_works_, oxford, clarendon press, , vol. , p. ) [ ] francis bacon, _de augmentis scientiarum_, bk. , ch. , in _works_, ed. j. spedding, r. l. ellis, and d. d. heath, boston, n.d. ( ?), vol. , p. . [ ] _the poems of john donne_, ed. h. j. c. grierson, london, oxford university press, , p. ("to the countesse of bedford, on new yeares day"). [ ] m: pp. , . [ ] m: pp. , . aristotle, _works_, ed. w. d. ross, oxford, -- , vol. , _de generatione et corruptione_, translated by h. h. joachim, , vol. , _meteorologica_, translated by e. w. webster, . [ ] m: pp. , , , , , . dr. h. guerlac has kindly brought to my attention the similarity between the explanation given in gilbert and that given in the _meteorologica_, bk. , ch. . p. . [ ] m: p. . [ ] a statement of the relation between aristotle's four elements and place can be found in maier, _op. cit._ (footnote ), pp. - . according to gilbert the primary source of matter is the interior of the earth, where exhalations and "spiritus" arise from the bowels of the earth and condense in the earth's veins.[ ] if the condensations, or humors, are homogeneous, they constitute the "materia prima" of metals.[ ] from this "materia prima," various metals may be produced,[ ] according to the particular humor and the specificating nature of the place of condensation.[ ] the purest condensation is iron: "in iron is earth in its true and genuine nature."[ ] in other metals, we have instead of earth, "condensed and fixed salts, which are efflorescences of the earth."[ ] if the condensed exhalation is mixed in the vein with foreign earths already present, it forms ores that must be smelted to free the original metal from dross by fire.[ ] if these exhalations should happen to pass into the open air, instead of being condensed in the earth, they may return to the earth in a (meteoric) shower of iron.[ ] [ ] m: pp. , , , , . [ ] m: pp. , , , ; see, however, pp. - : "iron ore, therefore, as also manufactured iron, is a metal slightly different from the homogenic telluric body because of the metallic humor it has imbibed ..." [ ] m: pp. , , , , , . [ ] m: pp. , , , . [ ] m: pp. , , , ; on p. he says that iron is "more truly the child of the earth than any other metal"; it is the hardest because of "the strong concretion of the more earthy substance." [ ] m: pp. , , , . [ ] m: pp. , . [ ] m: pp. , . gilbert was indeed writing a new physiology, both in the ancient sense of the word and the modern. the process of the formation of metals had many biological overtones, for it was a kind of metallic epigenesis.[ ] "within the globe are hidden the principles of metals and stones, as at the earth's surface are hidden the principles of herbs and plants."[ ] in all cases, the "spiritus" acts as semen and blood that inform and feed the proper womb in the generation of animals.[ ] "the brother uterine of iron,"[ ] the loadstone, is formed in this manner. as the embryo of a certain species is the result of the specificating nature of the womb in which the generic seed has been placed, so the kind of metal is the result of a certain humor condensing in a particular vein in the body of the earth. [ ] gilbert's terminology strongly suggests that he was familiar with alchemical literature, as well as that of medical chemistry. he has been credited as being highly skilled in chemistry. see sir walter langdon-brown, "william gilbert: his place in the medical world," _nature_, vol. , pp. - , . [ ] _ibid._, p. . [ ] m: pp. , , , . see also galen, _op. cit._ (footnote ) bk. , ch. . [ ] m: pp. , . gilbert developed this biological analogy further by ascribing to metals a process of decay after reaching maturity. once these solid materials have been formed, they will degenerate unless protected, forming earths of various kinds as a result.[ ] the "rind of the earth"[ ] is produced by this process of growth and decay. if these earths are soaked with humors, transparent materials are formed.[ ] [ ] m: pp. , , , , , , . [ ] m: p. . [ ] m: p. . as we shall see below, the ultimate cause of this internal and superficial life is the motion of the earth, which animation is the expression of the magnetic soul of this sphere.[ ] as the life of animals results from the constant working of the heart and arteries,[ ] so the daily motion of the earth results in a constant generation of mineral life within the earth. in contrast to aristotle's[ ] making the motion of the heavens the cause of continuous change, gilbert made that of the earth the remote cause.[ ] however, unlike the constant cyclical transmutation of substances in aristotle, there is only generation and decay. [ ] m: pp. , , . [ ] m: p. . a somewhat different opinion, although not necessarily inconsistent is expressed on p. , where he says the surface is due to the action of the atmosphere, the waters, and the radiations and other influences of heavenly bodies. [ ] aristotle, _op. cit._ (footnote ), _de generatione et corruptione_, bk. , ch. . [ ] m: pp. , , . gilbert made a number of successive generalizations in order to arrive at the induction that the form of the loadstone is a microcosmic "anima" of that of the earth.[ ] after comparing the properties of the loadstone and of iron, his first step in this induction was that the two materials, found everywhere,[ ] are consanguineous:[ ] "these two associated bodies possess the true, strict form of one species, though because of the outwardly different aspect and the inequality of the selfsame innate potency, they have hitherto been held to be different ..." good iron and good loadstone are more similar than a good and a poor loadstone, or a good and a poor iron ore.[ ] moreover, they have the same potency,[ ] for the innate potency of one can be passed to the other:[ ] "the stronger invigorates the weaker, not as if it imparted of its own substances or parted with aught of its own strength, nor as if it injected into the other any physical substance; but rather the dormant power of the one is awakened by the other's without expenditure." in addition, the potency can be passed only to the other.[ ] finally they both have the same history: we see both the finest magnet and iron ore visited as it were by the same ills and diseases, acting in the same way and with the same indications, preserved by the same remedies and protective measures, and so retaining their properties ... they are both impaired by the action of acrid liquids as though by poison[ ] ... each is saved from impairment by being kept in the scrapings of the other. [so] ... form, essence and appearance are one.[ ] any difference between the loadstone proper and the iron proper is due to a difference in the actual power of the magnetic virtue:[ ] "weak loadstones are those disfigured with dross metallic humors and with foreign earth admixtures, [hence one may conclude] they are further removed from the mother earth and are more degenerate." [ ] m: pp. xlvii, , . [ ] m: pp. , , , , . [ ] m: pp. , , . [ ] m: pp. , . [ ] m: p. . [ ] m: pp. , . [ ] m: p. . [ ] m: p. . [ ] m: p. . [ ] m: pp. , , , , , , . gilbert's second induction was that they are "true and intimate parts of the globe,"[ ] that is, that they are piece of the "materia prima" of all we see about us. for they "seem to contain within themselves the potency of the earth's core and of its inmost viscera."[ ] whence, in gilbert's philosophy, the earthy matter of the elements was not passive or inert[ ] as it was in aristotle's, but already had the magnetic powers of loadstone. being endowed with properties, it was, in peripatetic terms, a simple body. [ ] m: p. . [ ] m: pp. , . [ ] m: p. . gilbert is confusing aristotelian matter and an element. he includes cold and dry, with formless and inert! see also maier, _op. cit._ (footnote ). if these pieces of earth proper, before decay, are loadstones, then one may pass to the next induction that the earth itself is a loadstone.[ ] conversely, a terrella has all the properties of the earth:[ ] "every separate fragment of the earth exhibits in indubitable experiments the whole impetus of magnetic matter; in its various movements it follows the terrestial globe and the common principle of motion."[ ] [ ] m: p. ; bk. , ch. . [ ] m: pp. , - , - , - , - . [ ] m: p. . see also pp. and . it is not clear, at this point, whether he believed a "properly balanced" terrella would be a _perpetuum mobile_. the next induction that gilbert made was that as the magnet possesses verticity and turns towards the poles, so the loadstone-earth possesses a verticity and turns on an axis fixed in direction.[ ] he could now discuss the motions of a loadstone in general, in terms of its nature, just as an aristotelian discussed the motion of the elements in terms of their nature. [ ] m: pp. , - , , , - , , , - gilbert implied (m: p. ), that a terrella does not rotate as peregrinus said, due to resistance (m: p. ), or due to the mutual nature of coition (m: p. ); or even to the rotation of the earth (m: p. ). however (m: p. ), he also mentioned that a terrella would revolve by itself! but before reaching this point in his argument, gilbert digressed to classify the different kinds of attractions and motions which the elements produce. in particular, he distinguished electric attraction from magnetic coition, and pointed out the main features of electrical attraction. since the resultant motions were different, the essential natures of electric and magnetic substances had to differ. gilbert introduced his treatment of motion by discussing the attraction of amber. all sufficiently light solids[ ] and even liquids,[ ] but not flame or air[ ] are attracted by rubbed amber. heat from friction,[ ] but not from alien sources like the sun[ ] or the flame,[ ] produce this "affection." by the use of a detector modeled after the magnetic needle, which we would call an electroscope but which he called a "versorium,"[ ] gilbert was able to extend the list of substances that attract like amber.[ ] these gilbert called "electricae."[ ] [ ] m: pp. , , , . [ ] m: pp. , , . [ ] m: pp. , . [ ] m: pp. , . [ ] m: pp. , , . [ ] m: pp. , , , . [ ] m: p. . [ ] m: pp. - , . [ ] m: p. . the definition gilbert gave of an electric in the glossary at the beginning of his treatise was not an experimental one: "electricae, quae attrahunt eadem ratione ut electrum." possibly as a result of testing experimentally statements like that of st. thomas, on the effect of garlic on a loadstone, gilbert discovered that the interposition of even the slightest material (except a fluid like olive oil) would screen the attraction of electrics.[ ] hence the attraction is due to a material cause, and, since it is invisible, it is due to an effluvium.[ ] it must be much rarer than air,[ ] for if its density were that of air or greater, it would repel rather than attract.[ ] [ ] m: pp. , , . [ ] m: pp. , . [ ] m: p. . [ ] m: pp. , , . the source of the effluvia could be inferred from the properties of the electrics. many but not all of the electrics are transparent, but all are firm and can be polished.[ ] since they retain the appearance and properties of a fluid in a firm solid mass,[ ] gilbert concluded that they derived their growth mostly from humors or were concretions of humors.[ ] by friction, these humors are released and produce electrical attraction.[ ] [ ] m: pp. , , . [ ] m: p. . [ ] m: pp. , . see also aristotle, _op. cit._ (footnote ), _meteorologica_, bk. . [ ] m: p. . this humoric source of the effluvia was substantiated by gilbert in a number of ways. electrics lose their power of electrical attraction upon being heated, and this is because the humor has been driven off.[ ] bodies that are about equally constituted of earth and humor, or that are mostly earth, have been degraded and do not show electrical attraction.[ ] bodies like pearls and metals, since they are shiny and so must be made of humors, must also emit an effluvium upon being rubbed, but it is a thick and vaporous one without any attractive powers.[ ] damp weather and moist air can weaken or even prevent electrical attraction, for it impedes the efflux of the humor at the source and accordingly diminishes the attraction.[ ] charged bodies retain their powers longer in the sun than in the shade, for in the shade the effluvia are condensed more, and so obscure emission.[ ] [ ] m: pp. , . [ ] m: p. . [ ] m: p. . see also p. . [ ] m: pp. , - , . (see particularly the heated amber experiment described on p. ). [ ] m: p. . all these examples seemed to justify the hypothesis that the nature of electrics is such that material effluvia are emitted when electrics are rubbed, and that the effluvia are rarer than air. gilbert realized that as yet he had not explained electrical attraction, only that the pull can be screened. the pull must be explained by contact forces,[ ] as aristotle[ ] and aquinas[ ] had argued. accordingly, he declared, the effluvia, or "spiritus,"[ ] emitted take "hold of the bodies with which they unite, enfold them, as it were, in their arms, and bring them into union with the electrics."[ ] [ ] m: p. . [ ] aristotle, _physics_, translated by p. h. wicksteed and f. m. cornford, loeb classical library, london, , bk. , ch. , b . [ ] st. thomas aquinas, _op. cit._ (footnote ), vol. , _physicorum aristotelis expositio_, lib. , lect. (in moventibus et motis non potest procedi in infinitum, sed oportet devenire ad aliquid primum movens immobile), cap. d, p. . [ ] m: p. . [ ] m: p. . it can be seen how this uniting action is effected if objects floating on water are considered, for solids can be drawn to solids through the medium of a fluid.[ ] a wet body touching another wet body not only attracts it, but moves it if the other body is small,[ ] while wet bodies on the surface of the water attract other wet bodies. a wet object on the surface of the water seeks union with another wet object when the surface of the water rises between both: at once, "like drops of water, or bubbles on water, they come together."[ ] on the other hand, "a dry body does not move toward a wet, nor a wet to a dry, but rather they seem to go away from one another."[ ] moreover, a dry body does not move to the dry rim of the vessel while a wet one runs to a wet rim.[ ] [ ] m: p. . [ ] m: pp. , . [ ] m: p. . [ ] m: p. . [ ] m: p. . by means of the properties of such a fluid, gilbert could explain the unordered coming-together that he called coacervation.[ ] different bodies have different effluvia, and so one has coacervation of different materials. thus, in gilbert's philosophy air was the earth's effluvium and was responsible for the unordered motion of objects towards the earth.[ ] [ ] m: p. . [ ] m: p. (see also p. ). although gilbert does not make it explicit, this would solve the medieval problem of gravitation without resorting to a ptolemaic universe. in addition, since coacervation is electric, and electric forces can be screened, it should have been possible to reduce the downward motion of a body by screening! the analogy between electric attraction and fluids is a most concrete one, yet lying beneath this image is a hypothesis that is difficult to fix into a mechanical system based upon contact forces. this is the assumption that under the proper conditions bodies tend to move together in order to participate in a more complete unity.[ ] the steps in electrical attraction were described as occurring on two different levels of abstraction: first one has physical contact through an effluvium or "spiritus" that connects the two objects physically. then, as a result of this contact, the objects somehow sense[ ] that a more intimate harmony is possible, and move accordingly. gilbert called the motion that followed contact, attraction. however, this motion did not connote what we would call a force:[ ] it did not correspond directly to a push or pull, but it followed from what one might term the apprehension of the possibility of a more complete participation in a formal unity. the physical unity due to the "spiritus" was the prelude to a formal organic unity, so that _humor_ is "rerum omnium unitore." gilbert's position can be best seen in the following:[ ] spiritus igitur egrediens ex corpora, quod ab humore aut succo aqueo concreverat, corpus attrahendum attingit, attactum attrahenti unitur; corpus peculiari effluviorum radio continguum, unum effecit ex duobus: unita confluunt in conjunctissimam convenientiam, quae attractio vulgo dicitur. quae unitas iuxta pythagorae opinionem rerum omnium principium est, per cuius participationem unaquaeque res una dicitur. quoniam enim nullo actio a materia potest nisi per contactum, electrica haec non videntur tangere, sed ut necesse erat demittitur aliquid ab uno ad aliud, quod proxime tangat, et eius incitationis principium sit. corpora omnia uniuntur & quasi ferruminantur quodammodo humore ... electrica vero effi via peculiaria, quae humoris fusi subtilissima sunt materia, corpuscula allectant. aër (commune effluvium telluris) & partes disjunctis unit, & tellus mediante aëre ad se revocat corpora; aliter quae in superioribus locis essent corpora, terram non ita avide appelerent. electrica effluvia ab aëre multum differunt, & u aër telluris effluvium est, ita electrica suahabent effluvia & propria; peculiaribus effluviis suus cuique; est singularis ad unitatem ductus, motus ad principium, fontem, & corpus effluvia emittens. a similar hypothesis will reappear in his explanation of magnetic attraction. [ ] m: pp. , : "this unity is, according to pythagoras, the principle, through participation, in which a thing is said to be one" (see footnotes and ). [ ] "sense" is probably too strong a term, and yet the change following contact is difficult to describe in gilbert's phraseology without some such subjective term. see gilbert's argument on the soul and organs of a loadstone, m: pp. - . [ ] m: pp. , . [ ] gilbert, _de magnete_, london, , bk. , ch. , pp. - . following the tradition of the medieval schoolmen gilbert started his examination of the nature of the loadstone by pointing out the different kinds of motion due to a magnet. the five kinds (other than up and down) are:[ ] ( ) coitio (vulgo attractio, dicta) ad unitatem magneticam incitatio. ( ) directio in polos telluris, et telluris in mundi destinatos terminos verticitas et consistentia. ( ) variatio, a meridiano deflexio, quem motum nos depravatum dicimus. ( ) declinatio, infra horizontem poli magnetici descensus. ( ) motus circularis, seu revolutio. of the five he initially listed, three are not basic ones. variation and declination he later explained as due to irregularities of the surface of the earth, while direction or verticity is the ordering motion that precedes coition.[ ] this leaves only coition and revolution as the basic motions. how these followed from "the congregant nature of the loadstone can be seen when the effusion of forms has been considered." coition (he did not take up revolution at this point) differed from that due to other attractions. there are two and only two kinds of bodies that can attract: electric and magnetic.[ ] gilbert refined his position further by arguing that one does not even have magnetic attraction[ ] but instead the mutual motion to union that he called coition.[ ] in electric attraction, one has an action-passion relation of cause and effect with an external agent and a passive recipient; while in magnetic coition, both bodies act and are acted upon, and both move together.[ ] instead of an agent and a patient in coition,[ ] one has "conactus." coition, as the latin origin of the term denoted, is always a concerted action. [ ] this can be seen from the motions of two loadstones floating on water.[ ] the mutual motion in coition was one of the reasons for gilbert's rejection of the perpetual motion machine of peregrinus.[ ] [ ] _ibid._, ch. , pp. - . [ ] m: pp. , . [ ] m: pp. , , , , . [ ] m: p. . [ ] m: pp. , , , , . it need hardly be pointed out that coitus is not an impersonal term. [ ] m: p. . [ ] m: p. . [ ] m: pp. , , , , , , . [ ] m: pp. , . [ ] m: pp. , . see also footnote . magnetic coition, unlike electric attraction, cannot be screened.[ ] hence it cannot be corporeal for it travels freely through bodies[ ] and especially magnetic bodies;[ ] one can understand the action of the armature on this basis.[ ] since coition cannot be prevented by shielding, it must have an immaterial cause.[ ] [ ] m: pp. , , , , , , , , . this is, of course, contrary to modern experience. [ ] m: pp. , , , , , , , . [ ] m: pp. , , , , . [ ] m: pp. - . [ ] m: p. . yet, unless one has the occult action-at-a-distance, change must be caused by contact forces. gilbert resolved the paradox of combining contact forces with forces that cannot be shielded, by passing to a higher level of abstraction for the explanation of magnetic phenomena: he saw the contact as that of a form with matter. although gilbert remarked that the cause of magnetic phenomena did not fall within any of the categories of the formal causes of the aristotelians, he did not renounce for this reason the medieval tradition. actually there are many similarities between gilbert's explanation of the loadstone's powers and that of st. thomas. magnetic coition is not due to any of the generic or specific forms of the aristotelian elements, nor is it due to the primary qualities of any of their elements, nor is it due to the celestial "generans" of terrestrial change.[ ] relictis aliorum opinionibus de magnetis attractione; nunc coitionis illius rationem, et motus illius commoventem naturam docebimus. cum vero duo sint corporum genera, quae manifestis sensibus nostris motionibus corpora allicere videntur, electrica et magnetica; electrica naturalibus ab humore effluviis; magnetica formalibus efficientiis, seu potius primariis vigoribus, incitationes faciunt. forma ilia singularis est, et peculiaris, non peripateticorum causa formalis, et specifica in mixtis, est secunda forma, non generantium corporum propagatrix; sed primorum et praeciporum globorum forma; et partium eorum homogenearum, non corruptarum, propria entitas et existentia, quam nos primariam, et radicalem, et astream appellare possumus formam; non formam primam aristotelis; sed singularem illam, quae globum suum proprium tuetur et disponit. talis in singulis globis, sole, lunas et astris, est una; in terra etiam una, quae vera est ilia potentia magnetica, quam nos primarium vigorem appellamus. quare magnetica natura est telluris propria, eiusque omnibus verioribus partibus, primaria et stupenda ratione, insita; haec nec a caelo toto derivatur procreaturve, per sympathiam, per influentiam, aut occultiores qualitates; nec peculiari aliquo astro: est enim suus in tellure magneticus vigor, sicut in sole et luna suae formae; frustulumque; lunae, lunatice ad eius terminos, et formam componit se; solarque; ad solem, sicut magnes ad tellurem, et ad alterum magnetem, secundum naturam sese inclinando et alliciendo. differendum igitur de tellure quae magnetica, et magnes; tum etiam de partibus eius verioribus, quae magneticae sunt; et quomodo ex coitione difficiuntur. instead, he declared it to be due to a form that is natural and proper to that element that he made the primary component of the earth.[ ] to understand his argument, let us briefly recall the peripatetic theory of the elements. in this philosophy of nature each element or simple body is a combination of a pair of the four primary qualities that informs inchoate matter. these qualities are the instruments of the elemental forms and determine the properties of the element. thus the element fire is a compound of the qualities hot and dry, and the substantial form of fire acts through these qualities. similarly for the other elements, earth, water, and air: their forms determine a proper place for each element, and a motion to that place natural to each element.[ ] [ ] m: p. , and gilbert, _de magnete_, london, , bk. ch. , p. . [ ] m: p. . [ ] m: pp. , . gilbert had previously declared that the primary substance of the earth is an element. since it is an element, it has a motion natural to it, and this motion is magnetic coition. as an aristotelian considered the substantial form of the element, fire, to act through the qualities of hot and dry, and to cause an upward motion; so gilbert argued that the substantial form of his element, pure loadstone, acts through the magnetic qualities and causes magnetic coition. this motion is due to its primary form, and is natural to the element earth.[ ] it is instilled in all proper and undegenerate parts of the earth,[ ] but in no other element.[ ] [ ] m: pp. , , , , , , , . for rotation, see footnote . [ ] m: pp. , . that each part is informed with the properties of the whole is an argument favoring an animistic explanation of the nature of this form. [ ] m: p. . to the medieval philosopher, the "generantia" of the occult powers of the loadstone are the heavenly bodies. gilbert, however, endowed the earth with these heavenly powers which were placed in the earth in the beginning[ ] and caused all magnetic materials to conform with it both physically and formally.[ ] such magnetic powers are the property of all parts of the earth;[ ] they give the earth its rotating motion[ ] and hold the earth together in spite of this motion.[ ] [ ] m: pp. , . [ ] m: pp. , , , . [ ] m: pp. , , . [ ] m: pp. , , , , , , , - , - . [ ] m: pp. , ; see also electric attraction, p. . indeed, each of the main stellar bodies, sun, moon, stars, and earth, has such a form or principle unique to itself that causes its parts not only to conform with itself but to revolve.[ ] thus, if one removes a piece of the moon from this body, it will tend to align itself with the moon and then to return to its proper place; and a fragment of the sun would similarly tend to return after proper orientation.[ ] moreover, there is a farther-ranging, though weaker, mutual action of the heavenly bodies so that one has a causal hierarchy of these specific conforming powers. the form of the sun is superior to that of the inferior globes and is responsible for the order and regularity of planetary orbits.[ ] in like manner, the moon is responsible for the tides of the ocean.[ ] [ ] m: pp. , - . [ ] m: pp. , . [ ] m: pp. , , , , , , . [ ] m: pp. , , . by virtue of the causal hierarchy of forms, the loadstone acquires its magnetic powers from the earth.[ ] as the earth has its natural parts, so has the stone.[ ] although the geometrical center of a terrella is the center of the magnetic forces,[ ] objects do not tend to move to the center but to its poles,[ ] where the magnetic energy is most conspicuous.[ ] however, in a sense, the energy is everywhere equal: the virtue is spread throughout the entire mass of the loadstone,[ ] and all the parts direct the forces to the poles.[ ] the poles become the "thrones" of the magnetic powers.[ ] on the other hand, the directive force is stronger where coition is weaker and accordingly, verticity is most prominent at the equator.[ ] [ ] m: pp. - , , . this is not quite the same argument as that the powers of the loadstone are identical with those of the earth. see footnote . [ ] m: pp. , . [ ] m: p. . [ ] m: pp. , . [ ] m: pp. , , . [ ] m: pp. , , , , , , . [ ] m: pp. , , , , , , . [ ] m: p. . [ ] m: pp. , , . the strength of a loadstone depends upon its shape and mass. a bar magnet has greater powers than a spherical one because it tends to concentrate the magnetic powers more in the ends.[ ] for a given purity and shape, the heavier the loadstone, the greater its strength.[ ] a loadstone has a maximum degree of magnetic force that cannot be increased.[ ] however, weaker ones can be strengthened by stronger ones.[ ] similarly, the shape and weight of the iron determine the magnetic force in coition.[ ] [ ] m: pp. , , - . [ ] m: pp. , , , , , , . [ ] m: p. . [ ] m: p. . [ ] m: p. . the formal forces of a loadstone emanate in all directions from it,[ ] but there is a bound to it that gilbert called the "orbis virtutis."[ ] the shape of this "orbis virtutis" is determined by the shape of the stone.[ ] this insensible effusion is analogous to the spreading of light that reveals its presence only by opaque bodies.[ ] similarly, the magnetic forms are effused from the stone,[ ] and can only reveal their presence by coition with another loadstone or by "awakening" magnetic bodies within the "orbis virtutis."[ ] unmagnetized iron that comes within the "orbis virtutis" is altered, and the magnetic virtue renews a form that is already potentially in the iron.[ ] the formal energy is drawn not only from the stone but from the iron.[ ] this is not generation, or alteration in the sense of a new impressed quality, but alteration in the sense of the entelechy or the activation of a form potentially present.[ ] those bodies magnetized by coming within the "orbis virtutis" have in turn an efflux of their own.[ ] iron can also receive verticity directly from the earth without the intervention of an ordinary loadstone.[ ] such verticity can be expelled and annulled by the presence of another loadstone.[ ] [ ] m: pp. , , , , , , , . [ ] gilbert defined the _orbis virtutis_ in the glossary at the beginning of his treatise as, "... totum illud spatium, per quod quaevis magnetis virtus extenditur." this is the core of the difference between electric and magnetic forces. the substantial form of an electric could not be "effused," but was "imprisoned" in matter (as the neoplatonic soul in the human body); while the primary form of a magnet did not require a material carrier and its effusion was similar to the propagation of a species in light. [ ] m: pp. , , . [ ] m: pp. , . [ ] m: pp. - . see also p. , where it is stated that the sun and earth could awaken souls. [ ] m: pp. , , , , , , , . this awakening of the iron within the "orbis virtutis" is comparable (pp. , ) to the birth of a child under the influence of the stars. [ ] m: pp. , , , , , . see also footnote . [ ] m: p. . [ ] m: pp. , , . [ ] m: pp. , . [ ] m: pp. , , - . [ ] m: p. . although one does not normally find iron to be magnetized, a loadstone always has some magnetism. that two bodies such as iron and loadstone should have different properties is the result of the loss of a form by the iron, but this form is still potentially present in the iron. the iron that has been obtained from an ore has been deformed,[ ] for it has been placed "outside its nature" by the fire.[ ] the nature has not been removed, since, once the iron has cooled, the confused form can be reformed by a loadstone. [ ] the latter "awakens" the proper form of iron.[ ] after smelting, the magnetized iron may manifest stronger powers than a loadstone of equal weight, but this is because the primary matter of the earth is purer in the iron than in the loadstone.[ ] if fire does not deform a loadstone too much, it can be remagnetized,[ ] but a burnt loadstone cannot be reformed.[ ] corruption from external causes may also deform a loadstone or iron so that it can not be magnetized.[ ] bodies mixed with the degenerate substance of the earth or with aqueous humor spoilt by contamination with earth, do not show either electric attraction or magnetic coition.[ ] [ ] m: pp. , , . [ ] m: p. . [ ] m: pp. , , . [ ] m: pp. , , , , . [ ] m: pp. , . [ ] m: pp. , . [ ] m: p. . [ ] m: pp. , , , , . [ ] m: p. . in a manner suggestive of peregrinus, gilbert wrote that, "magnetic bodies seek formal unity."[ ] thus a dissected loadstone not only tends to come back together, as in the unordered coacervation of electric attraction, but to restore the organization it had before dissection.[ ] accordingly, opposite poles appear on the interfaces of the sections, not "from an opposition" but from "a concordance and a conformance."[ ] this ensures that when the parts are joined together again, they have the same orientation as before. gilbert compared this power of restoring the original loadstone with that of a plant's vital power under the process of cutting and grafting; the plant can be revived only when the parts are in a certain order.[ ] [ ] m: p. . [ ] m: pp. - . see also footnote . [ ] m: pp. , . [ ] m: pp. - . a hypothesis similar to that used to explain electric attraction lay beneath the explanation of magnetic coition: that bodies brought into contact will move together. in electric attraction, the contact is material and due to the "spiritus" from the electric body; in magnetic coition, it is formal and depends on the action of a primary form that spreads from a magnetized body to its limit of effusion, the "orbis virtutis." if iron is inside the "orbis virtutis," the two bodies "enter into alliance and are one and the same"[ ] for within it "they have absolute continuity, and are joined by reason of their accordance, albeit the bodies themselves be separated."[ ] gilbert's treatment of coition can be analyzed into the same two steps as can electric attraction. first occurs a contact, which in this case is not physical but formal, and from this initial formal contact follows movement to a more complete unity. both the contact and the movement to unity are described on the same level of abstraction, instead of on two different levels as in electric attraction. again one does not find any clear-cut concept of force as a push or pull,[ ] but instead, a motion to a formal unity, this time a cooperative motion. the parts of a magnetic body are in greater harmony when they are assembled in a certain pattern and so they move accordingly. [ ] m. p. . [ ] m: p. . [ ] see, however, m: pp. , . as to the nature of the primary form itself, gilbert agreed with thales that it is like a soul,[ ] "for the power of self-movement seems to betoken a soul."[ ] with galen and st. thomas he placed the form of the loadstone superior to that of inanimate matter.[ ] in a sense, gilbert even made it superior to organic matter, for it is incapable of error.[ ] like the soul, the primary form cannot be fragmented; when a loadstone is divided, one does not separate the poles but each part acquires its own poles and an equator. [ ] m: pp. , . [ ] m: p. . [ ] m: p. . [ ] m: pp. - . like the soul, fire does not destroy it.[ ] like the soul of astral bodies, and of the earth itself, it produces complex but regular motions; the motion of two loadstones on water offers such an example.[ ] like the soul of a newborn child, whose nature depends on the configuration of the heavens, the properties in the newly awakened iron depend upon its position in the "orbis virtutis."[ ] whence gilbert declared: ... the earth's magnetic force and the animate form of the globes, that are without senses, but without error ... exert an unending action, quick, definite, constant, directive, motive, imperant, harmonious through the whole mass of matter; thereby are the generation and the ultimate decay of all things on the superficies propagated.[ ] the bodies of the globes ... to the end that they might be in themselves, and in their nature endure, had need of souls to be conjoined to them, for else there were neither life, nor prime act, nor movement, nor unition, nor order, nor coherence, nor _conactus_, nor _sympathia_, nor any generation nor alteration of seasons, and no propagation; but all were in confusion....[ ] wherefore, not with reason, thales ... declares the loadstone to be animate, a part of the animate mother earth and her beloved offspring.[ ] gilbert ended book of his treatise on the magnet with a persuasive plea for his magnetic philosophy of the cosmos, yet his conceptual scheme was not too successful an induction in the eyes of his contemporaries. in particular the man from whom the royal society took the inspiration for their motto, "nullius in verba," did not value his magnetic philosophy very highly. whether francis bacon was alluding to gilbert when he expounded his parable of the spider and the ant[ ] is not explicit, but he certainly had him in mind when he wrote of the idols of the cave and the idols of the theater.[ ] [ ] m: p. . [ ] m: p. . [ ] m: p. . [ ] m: p. . [ ] m: pp. , . [ ] m: p. . [ ] francis bacon, _op. cit._ (footnote ), vol. , _novum organum_, bk. , ch. , p. . [ ] _ibid._, ch. and ch. (pp. and ). few of the subsequent experimenters and writers on magnetism turned to gilbert's work to explain the effects they discussed. although both his countrymen sir thomas browne[ ] and robert boyle[ ] described a number of the experiments already described by gilbert and even used phrases similar to his in describing them, they tended to ignore gilbert and his explanation of them. instead, both turned to an explanation based upon magnetic effluvia or corpuscles. the only direct continuation of gilbert's _de magnete_ was the _philosophia magnetica_ of nicolaus cabeus.[ ] the latter sought to bring gilbert's explanation of magnetism more directly into the fold of medieval substantial forms. [ ] sir thomas browne, _pseudodoxia epidemica_, ed. , london, , bk. , ch. , , . [ ] robert boyle, _experiments and notes about the mechanical production of magnetism_, london, . [ ] nicolaus cabeaus, _philosophia magnetica_, ferarra, . however, gilbert's efforts towards a magnetic philosophy did find approval in two of the men that made the seventeenth century scientific revolution. while galileo galilei[ ] was critical of gilbert's arguments as being unnecessarily loose, he nevertheless saw in them some support for the copernican world-system. johannes kepler[ ] found in gilbert's explanation of the loadstone-earth a possible physical framework for his own investigations on planetary motions. [ ] galileo galilei, _dialogue on the great world systems_, in the translation of t. salusbury, edited and corrected by g. de santillana, university of chicago press, , pp. - . [ ] cassirer, _op. cit._ (footnote ), vol. , p. - . yet galileo and kepler had moved beyond gilbert's world of intellectual experience. they were no longer concerned with determining the nature of material things in order to explain their qualities. instead, they had passed into the realm of the mathematical relations of kinematics: quantitative law had replaced qualitative experience of cause and effect. gilbert had some intimations of the former, but he was primarily concerned with explaining magnetism in terms of substance and attribute. he had to ascertain the nature of the loadstone and of the earth in order to explain their properties and their motions. he even went further and explained the nature of the form of the loadstone. his method of determining the nature of a substance was a rather primitive one--it was not by a process of induction and deduction, nor by synthesis and analysis, nor by "resolutio" and "compositio," but by the use of analogies. he compared the natural history of metals and rocks with that of plants, and gave the two former the same kind of principle as the last. he determined the nature of the entity behind electric attraction by finding that such attractions could be screened, and hence it had to be corporeal. after comparing this "corporeal" attraction with that of the surface forces of a fluid, he concluded that the entity was a subtle fluid. he determined the nature of the entity behind magnetic coition by (incorrectly) finding that it cannot be screened, and hence the cause had to be a formal one. since both stars and the loadstone can carry out regular motions, and stars had souls, the form of the loadstone had to be a soul. the method of analogy was used again in his comparison of the properties of a magnetized needle placed over a terrella with the properties of a compass placed over the earth, whence he concluded the earth to be a giant loadstone. since the earth resembled the other celestial globes, it had to have, the circular inertia of these globes.[ ] as for his magnetic experiments to show physically that the earth moved, and his unbridled speculations on the "animae" of the celestial globes, one is inclined to agree with bacon's estimate of his magnetic philosophy. one might consider gilbert's book as a renaissance recasting of aristotle's _de caelo_ with the earth in the role of a heavenly body. so it might well be, for gilbert was still concerned with distinguishing the nature of the heavenly body, earth, that caused the coitional and revolving motions, from those natures for which up and down, and coacervation were the natural motions. because the natural motions were different, the natures had to be different, and these different natures led to a universe and a concept of space neither of which were aristotelian. one no longer had a central reference point for absolute space; there was no "motor essentialis" focused upon the earth but one had only the mutual motion of the heavenly bodies. the natural distinction between heaven and earth was gone, for the earth was no longer an inert recipient but a source of wonder, and so the stage was set for the universe of giordano bruno.[ ] the aristotelian philosophy of nature was used to justify a new cosmology, but there was no break with the past such as one finds in galileo and kepler. instead he followed the chimera of the world organism, as paracelsus had, and of the world soul, as bruno had. consequently gilbert's physiology did not enter into the main stream of science. [ ] because the earth has the same nature as a celestial globe, its revolution and circular inertia require no more explanation than those of any other heavenly body. [ ] one wonders if bruno might not have been another of the stimuli for gilbert. the latter's interest in magnetism began shortly before bruno visited england and lectured on his interpretation of the copernican theory. yet this is not to deny gilbert's services to natural philosophy. although not all of his experimental distinction between electric and magnetic forces has been retained, still, some of it has. his "orbis virtutis" was to become a field of force, and his class of electrics, insulators of electricity. his practice of arming a loadstone was to be of considerable importance in the period before the invention of the electromagnet. his limited recognition of the mutual nature of forces and their quantitative basis in mass was ultimately to appear in newton's second and third laws of motion. in spite of the weaknesses of the method of analogy, gilbert's experimental model of the terrella to interpret the earth's magnetism was as much a contribution to scientific method as to the theory of magnetism. consequently, in spite of an explanation of electricity and magnetism that one would be amused to find in a textbook today, we can still read his _de magnete_ with interest and profit. but more important than his scientific speculations, is the insight he can give us into a renaissance philosophy of nature and its relation to medieval thought. one does not find in _de magnete_ a prototype of modern physical science in the same sense one can in the writings of galileo and kepler. instead one finds here a full-fledged example of an earlier kind of science, and this is gilbert's main value to the historian today. produced from scanned images of public domain material from the google print project and from the internet archive: american libraries.) the philosophy of the weather. and a guide to its changes. by t. b. butler. new york: d. appleton & company, nos. & broadway. . entered according to act of congress, in the year , by t. b. butler, in the clerks office of the district court of the district of connecticut. electrotyped by thomas b. smith, & beekman street. printed by j. f. trow, broadway. introduction. the atmospheric conditions and phenomena which constitute "the weather" are of surpassing interest. now, we rejoice in the genial air and warm rains of spring, which clothe the earth with verdure; in the alternating heat and showers of summer, which insure the bountiful harvest; in the milder, ripening sunshine of autumn; or the mantle of snow and the invigorating air of a moderate winter's-day. now, again, we suffer from drenching rains and, devastating floods, or excessive and debilitating heat and parching drought, or sudden and unseasonable frost, or extreme cold. and now, death and destruction come upon us or our property, at any season, in the gale, the hurricane, or the tornado; or a succession of sudden or peculiar changes blight our expected crops, and plant in our systems the seeds of epidemic disease and death. these, and other normal conditions, and varied changes, and violent extremes, potent for good or evil, are continually alternating above and around us. they affect our health and personal comfort, and, through those with whom we are connected, our social and domestic enjoyments. they influence our business prosperity directly, or indirectly, through our near or remote dependence upon others. they limit our pleasures and amusements--they control the realities of to-day, and the anticipations of to-morrow. none can prudently disregard them; few can withhold from them a constant attention. scientific men, and others, devote to them daily hours of careful observation and registration. devout christians regard them as the special agencies of an over-ruling providence. the prudent, fear their sudden, or silent and mysterious changes; the timid, their awful manifestations of power; and they are, to each and all of us, ever present objects of unfailing interest. this _interest_ finds constant expression in our intercourse with each other. a recent english writer has said: "the germ of meteorology is, as it were, innate in the mind of every englishman--the weather is his first thought after every salutation." in the qualified sense in which this was probably intended, it is, doubtless, equally true of us. indeed, it is often not only a "first thought" _after_ a salutation, but a part of the salutation itself--an offspring of the same friendly feeling, or a part of the same habit, which dictates the salutation--an expression of sympathy in a subject of common and absorbing interest--a sorrowing or rejoicing with those who sorrow or rejoice in the frowns and smiles of an ever-changing, ever-influential atmosphere. if consistent with our purpose, it would be exceedingly interesting to trace the varied forms of expression in use among different classes and callings, and see how indicative they are of character and employment. the sailor deals mainly with the winds of the hour, and to him all the other phases of the weather are comparatively indifferent. he speaks of airs, and breezes, and squalls, and gales, and hurricanes; or of such appearances of the sky as prognosticate them. the citizens, whose lives are a succession of _days_, deal in such adjectives as characterize the weather of _the day_, according to their class, or temperament, or business; and it is pleasant, or fine, or _very_ pleasant or fine; beautiful, delightful, splendid, or glorious; or unpleasant, rainy, stormy, dismal, dreadful or horrible. the farmer deals with the weather of considerable periods; with forward or backward _seasons_, with "cold snaps" or "hot spells," and "wet spells" or "dry spells." and there are many intermediate varieties. the acute observer will find much in them to instruct and amuse him, and will probably be surprised to find how much they have to do with his "first impressions" of others. but i have a more important object in view. i propose to deal with "the philosophy _of the weather_"--to examine the nature and operation of the arrangements from which the phenomena result; to strip the subject, if possible, of some of the complication and mystery in which traditionary axioms and false theories continue to envelop it; to endeavor to grasp _its principles_, and unfold them in a plain, concise, and systematic manner, to the comprehension of "_the many_," who are equal partners with the scientific in its practical, if not in its philosophic interest; and to deduce a few general rules by which its changes may be understood, and, ultimately, to a considerable extent, foreseen. this is not an easy, perhaps not a prudent undertaking. nor is my position exactly that of a volunteer. a few words seem necessary, therefore, by way of apology and explanation. in the fall of , in the evening of a fair autumnal day, i started for hartford, in the express train. just above meriden, an acquaintance sitting beside me, who had been felicitating himself on the prospect of fine weather for a journey to the north, called my attention to several small patches of scud--clouds he called them--to the eastward of us, between us and the full clear moon, which seemed to be enlarging and traveling south--and asked what they meant. "ah!" said i, "they are scud, forming over the central and northern portions of connecticut, induced and attracted by the influence of a storm which is passing from the westward to the eastward, over the northern parts of new england, and are traveling toward it in a southerly surface wind, which we have run into. they seem to go south, because we are running north faster than they. you see them at the eastward because they are forming successively as the storm and its influence passes in that direction, and are most readily seen in the range of the moon; but when we reach hartford you will see them in every direction, more numerous and dense, running north to underlie that storm." i had seen such appearances too many times to be deceived. it was so. when we arrived at hartford they were visible in all directions, running to the northward at the rate of twenty-five miles an hour. in the space of forty minutes we had passed from a clear, calm atmosphere (and which still remained so), into a cloudy, damp air, and brisk wind blowing in the same direction we were traveling, and toward a heavy storm. my friend passed on, and met the southern edge of the rain at deerfield, and had a most unpleasant journey during the forenoon of the next day. taking the cars soon afterwards, in the afternoon, for the south, i found him on his return. "shall i have fair weather now till i get home?" said he. "there are no indications of a storm here, or at present," i replied, "but we may observe them elsewhere, and at nightfall." he kept a sharp look-out, and, as we neared new haven, discovered faint lines of cirrus cloud low down in the west, extending in parallel bars, contracting into threads, up from the western horizon, in an e. n. e. direction toward the zenith. "now, what is that?" said he. "the eastern outlying edge of a n. e. storm, approaching from the w. s. w. it is now raining from to miles to the westward of the eastern extremity of those bars of cirrus-condensation; perhaps more, perhaps less; and under those bars of condensation the wind is attracted, and is blowing from the n. e. toward the body of the storm, and where the condensation is sufficiently dense to drop rain. that dense portion will reach here, and it will rain from twelve to fifteen hours hence. as we pass along the shore, and run under that out-lying advance cirrus-condensation, we shall see that the vessels in the sound have the wind from the n. e., freshening, but we shall continue to have this light and scarcely-perceptible air from the northward for a time--_the n. e. wind always setting in toward an approaching storm, out on the sound, much sooner than upon the land_." as we approached the storm, and the storm us, the evidence of denser condensation at the west, and of wind from the east, blowing toward it, became more apparent. the fore and aft vessels were running "up sound" with "sheet out and boom off," before a fresh n. e. breeze, and my friend was astonished. "i must understand this," said he; "how is it?" "all very simple. the page of nature spread out above us is intelligible to him who will attentively study it. the laws which produce the impressions and changes upon that page, are few and comprehensible. although there is great variety, even upon the limited portion which is bounded by our horizon, there is also substantial uniformity; and, although the changes are always extensive, often covering an area of one thousand miles or more, and our vision can not extend in any direction more than from thirty to fifty, yet those changes are always, to a considerable extent, intelligible, and may often be foreseen." "has meteorology made such progress?" "by no means. it has, indeed, been raised to the dignity of a science, and professorships endowed for its advancement. some books have been written, and many theories broached in relation to it; and innumerable observations of the states of the barometer and thermometer, of the clouds, and the quantity of fallen rain, and the direction and force of the wind--made and recorded simultaneously in different countries--have been published and compared; and a great many important facts established, and tables of '_means_' constructed, and just inferences drawn, yet the _few and simple arrangements_ upon which all the phenomena depend, and _their philosophy_, have not yet been clearly elicited or understood." "have not the 'american association for the advancement of science' arrived at some definite and sound conclusion upon the subject?" "no; it has been with them, for many years, an interesting subject for papers and debate. some very valuable articles, upon particular topics, or branches of the subject, have been read and published. but the _cyclonologists_, as they term themselves, and who seem to think the great question is, '_are storms whirlwinds?_' appear with new editions and phases of their favorite views as regularly as the annual meeting recurs; and, though they have not convinced, they seem to have silenced their opponents. the only conclusion, however, judging from their debates, to which the association appear to have come with any considerable unanimity, is, that they are yet without sufficient _authentic observations_ and well-established facts, to authorize the adoption of the huttonian, daltonian, gyratory, or aspiratory, or any of the other numerous theories which abound. and they are right. the subject is mystified by these theories and speculations of the study, founded on barometrical and thermometrical records, and the direction and force of the surface winds. "the qualities of heat were among the earlier discoveries of science, and all the phenomena of the weather were forthwith attributed to its influence. hastily-formed and erroneous views of its power, and the manner of its action in particular localities, and under particular circumstances, have retained the credence accorded to them when first announced, although subsequent discoveries have shown their fallacy; some new theory of _modification_ having been invented to reconcile the discrepancies as soon as they appeared. perhaps it is not too much to say (however it may seem to one not thoroughly acquainted with the subject, who does not know that the _primary_ and secondary modifying hypotheses found in kämtz, may be counted by hundreds) that there is not remaining in any other science, and possibly in all others, an equal amount of false and absurd theory, and of forced and unnatural grouping of admitted facts to sustain it, as in meteorology as at present taught and received. astronomy, as a science, is almost perfected--the nature, and size, and orbits, of the distant worlds around us are known--while constant changes and alternating atmospheric conditions, which all occur _within less than six miles of us_, affecting all our important interests, and obvious to our senses, although much talked off, and made the objects of many theories, are but little understood." "how, then, did you acquire the information you seem to possess?" "by studying '_the countenance of the sky_,' for in no other way has such information ever been, or can it ever be, acquired. by a long-continued, daily, and sometimes hourly observation of the clouds and currents of the atmosphere, in connection with such reports of the then state of the weather elsewhere, as have fallen under my notice, and the effect of its changes upon the animal creation--for very much can be learned from them. yonder flock of black ducks that sit on that inshore rock, above the tide--the wildest and most suspicious of all their tribe--although the air is calm about them, know well that a storm is at hand. they probably both see and feel it. as twilight approaches they will fly away inland, forty or fifty miles perhaps, and settle among the lilies or grass which surround some fresh-water pond, certain of remaining while the storm lasts, and for one day at least, out of danger, and undisturbed. many a time, in my boyhood, have i heard, in the stillness of evening, the whistling of their wings, as they swept up the connecticut valley, to seek, on the borders of the coves, and in the creeks of the meadows, a concealed and safe feeding-place during a coming storm. and many a time in the autumn, after they had all passed down for the season, when the indications of an approaching storm were clearly visible at nightfall, have i waited for them to return, on the eastern margin of a bend in the cove, on the eastern side of a creek, to shoot them, though invisible, by shooting across the head of the wake, which they made upon the water in alighting, and from which the few remaining rays of twilight that came from the western sky were reflected. "but i am far from being singular in this. that page is more extensively read than is generally supposed. many plain, unassuming men--farmers, shipmasters, and others within the circle of my acquaintance--know more, practically, of the weather than the most learned closet-theorist, or the most indefatigable recorder of its changes. every one, by studying the page of nature above him, as he would the page of any other science, and testing, by observation, the numerous theories invented to account for the varied phenomena, may learn much, very much, that will be useful and interesting to him, and which he can never learn from books, or instruments, or theories alone." "well," said my friend, "i am too far advanced in life, as are many others, to commence such observations, and you must publish." i demurred, and he insisted. "it is difficult to spare the time; and i can not neglect my profession," i urged. "where there is a will there is a way," he replied. "it is difficult to make one's self understood without many illustrations." "very well, they are easily obtained." "but they cost money, and it is said 'science will not pay its way' like fiction and humbug." "that," said he, "is a libel--such science will. every one is interested in the weather--all talk about it--and thousands would carefully observe it, if they could be correctly guided in their observations." "i may get into unpleasant controversy." "suppose you do; you can yield your position if wrong, and maintain it if right, and _magna est veritas_." "but i may be mistaken in some of the views to which it will be necessary to advert, if i attempt to systematize the subject." "be it so--your mistakes may lead others to the discovery of the truth. besides, the weather is _common property_, and every one has a right to theorize about it, or to talk about it, as they please--even to call a stormy day a pleasant one, or make any other mistaken remark concerning it; and every other person is entitled to a like latitude of reply. and further," said he, with some emphasis, "no important observation, in relation to a subject of such interest, should be lost; and, if you have observed one new fact, or drawn one new and just inference from those which have been observed by others; and especially if, from observation and reading, you can deduce from the phenomena an intelligible, _observable, general system_, it is not only your right, but duty, to make it known. such a knowledge of the true system is greatly desired by every considerate man." to my friend's last argument i was compelled to yield. i could make no reply consistent with the great principles of fraternity, which i shall ever recognize. the promise was given. my friend went on his way, and i went to the daguerreotypist to procure a copy of the then appearance of the sky, as the first step toward its fulfillment. the fulfillment of that promise, reader, you will find in the following work. it was commenced as an article for a magazine, but it has grown on my hands to a volume. justice could not well be done to the subject in less space. it has been written during occasional and distant intervals of relaxation from professional avocations, or during convalescence from sickness, and it is, for these reasons, somewhat imperfect in style and arrangement. but i have no time to rewrite. there is much in it which will be old to those who read journals of science, but new to those who do not. there is more which will be new to all classes of readers, and may, perhaps, be deemed heretical and revolutionary by conservative meteorologists; yet i feel assured that the work is a step in the right direction--that it contains a substantially accurate exposition of the philosophy of the weather, and valuable suggestions for the practical observer. i have inserted my name in the title-page, contrary to my original intention, and at the suggestion of others; for i have no scientific reputation which will aid the publisher to sell a copy. nor do i desire to acquire such reputation. it can never form any part of my "capital in life." nor has it influenced me at all in preparing the work. i have aimed to fulfill a promise, too hastily given, perhaps--to put on record the observations i have made, and the inferences i have drawn from those of others--to induce and assist further observations, and, if possible, of a _general_ and _connected character_--and to impress those who may read what i have written with the belief, that _they will derive a degree of pleasure from a daily familiarity with, and intelligent understanding of, the "countenance of the sky," not exceeded by that which any other science can afford them_. i have examined, with entire freedom and fearlessness (but i trust in a manner which will not be deemed censurable or in bad taste) the theories and supposed erroneous views of others, for, in my judgment, the advancement of the science requires it. says sir george harvey, in his able article on meteorology, written for the encyclopædia metropolitana: "it is humiliating to those who have been most occupied in cultivating the science of meteorology, to see an agriculturist or a waterman, who has neither instruments nor theory, foretell the future changes of the weather many days before they happen, with a precision which the philosopher, aided by all the resources of science, would be unable to attain." the admissions contained in this paragraph, in relation to the comparative uselessness of instruments and theories, and the value of practical observation, are both in a good measure true. and the time has come, or should speedily come, when "_pride of opinion_," and "_esprit du corps_," among theorists and philosophers, should neither be indulged in, nor respected; and when their theories should be freely discussed, and rigidly tested by the observations of practical men. such measure, therefore, as i have meted, i invite in return. let whatever i have advanced, that is new, or adopted that is old, be _as_ rigidly tested, and _as_ freely discussed. let the errors, if there be any--and doubtless there are--be detected and exposed. let the truth be sought by all; and meteorology, as a practical science, advance to that full measure of perfection and usefulness, of which it is unquestionably susceptible. table of contents. page chapter i. heat and moisture are indispensable to the fertillity of the earth--arrangements exist for their diffusion and distribution, and all the phenomena of the weather result from their operation--heat furnished or produced mainly by the direct action of the sun's rays--manner in which it is diffused over the earth--other causes operate besides the sun's rays--the earth intensely heated in its interior--heat derived from the great oceanic currents, and the aerial currents which flow from the tropics to the poles, and from magnetism and electricity--water distributed by an atmospheric machinery as extensive as the globe--evidences of this--its distribution over the continents of north america--explanation of it--source from whence our supply of water is derived, and from which our rivers return chapter ii. our rivers return in the form of clouds, and in storms and showers--definition and character of storms--differences in the character of the clouds which constitute them--nomenclature of howard--its imperfections--new order of description--low fog--high fog--storm fog--storm scud--n. w. scud--cumulus-- stratus--cirrus--compounds of the two latter--recapitulation in tabular form chapter iii. our rivers do not return from the north atlantic--all storms and showers move from the westward to the eastward--seeming clouds seen moving from the eastward to the westward are scud--they are incidents of the storm, and not a necessary part of it--the storm clouds are above them, moving to the eastward--occasions when this may be seen--admitted facts prove it--investigations prove it--may be known from analogy--from the fact that there is an aerial current pursuing the same course in which the storms originate--character of this current--its influence upon our country--importance of a knowledge of its origin, cause, and the reciprocal action between it and the earth--to this end necessary to go down "to the chambers of the south" chapter iv. the trade wind region--its extent and arrangements--its belt of daily rains and movable character--the trade winds--the extra tropical belt of rains--connection between them and their annual movements--the counter-trades--their origin and situation--one of them constitutes our aerial current--it originates in the south atlantic as a surface-trade--anomalies of the trade wind region--dry seasons--humboldt's description of them--exist where the surface trades are situated--the rainless countries-- concentrated counter-trade--monsoons--received theory in relation to them a fallacy--cause of the great central phenomena-- calorific theory a fallacy--land not hotter under the belt of rains, nor sea materially so--theory should be abandoned chapter v. the agent, magnetism--its character and currents--oxygen magnetic--precipitation at the belt of rains occasioned by depolarization--storms originate in this central belt, and move toward the poles chapter vi. course and functions of the counter-trade--ours come from the south atlantic--reason why it can not come from the pacific-- mistake of mr. redfield and lieutenant maury in regard to it-- all our storms originate in it--proofs of this--state of the weather, whether hot or cold affected by it--proofs of this--all our surface winds are incidents of it, and due to its conditions and attractions--proofs of this--character of the different winds--anomalies of mr. blodgett accounted for--received theory in regard to sea and land breezes a mistaken one--proofs of this--peculiar character of the n. w. wind--identity with the winter mexican northers--character of the west india hurricanes-- of the thunder-gust--of the tornado--sundry particulars in relation to the latter--due to currents of electricity-- proportions of winds in different localities--examination of the work of professor coffin upon that subject--examination of lieutenant maury's theory of the monsoons chapter vii. height of the counter-trade in different latitudes--cause of the calms of cancer--influence of mountains upon the counter-trade-- reports of herndon and gibbon--focus of precipitation in the extra-tropical belt north of its southern line--evidences of this--the elevation of the counter-trade above the earth varies in the same latitude with the variations in the phenomena of the weather--temperature of the counter-trade--rain dust, its origin and indications--volcanic ashes--how far they indicate its course of progression--question whether there is an eastern progression of the body of the atmosphere above the machinery of distribution chapter viii. important to understand the precise character of the reciprocal action between the earth and the counter-trade--connection between the width and movements of the belt of inter-tropical rains and the volume of the trades--its peculiarities over africa, the atlantic, and south america--the magnetic equator-- character of the storms which originate in the inter-tropical belt indicate local magnetic action--supposed influence of volcanic action--gulf stream changes its position--this the result of magnetic action--alternating contrasts of heat and cold, and rain and drought--dr. webster's history of the weather--spots upon the sun--their character and influence--cold or warm periods during the same decade, and during different decades--connection between the spots and magnetic disturbances and variations--influence of the moon upon the weather--no decisive inference to be drawn from these facts, and a more critical examination necessary chapter ix. examination of existing theories--calorific theory the prevailing one--lateral overflow of professor dove--absurdity of his views in relation to them--his theory of hurricanes--its absurdity--a new theory by mr. dobson--three theories advanced by meteorologists of this country--professor espy's theory--mr. bassnett's theory--mr. redfield's theory--extended examination of the latter--his theory in relation to the fall of the barometer contradictory in its character--philosophy of the barometric change--no aid to be derived from these theories chapter x. further inquiry in relation to the reciprocal action between the earth and the counter-trade--terrestrial magnetism, and what we know of it--its elements, and their variations--their connection with the variations of atmospheric condition--magnetism acts through its connection with electricity--character of the latter and its variations--their connection with atmospheric conditions-- electricity as well as magnetism in excess over this country-- effects of it upon our climate--closer consideration of the atmospheric phenomena--their diurnal changes and connections compared with those of magnetism and electricity--grouping of all the diurnal variations--particular and separate examination of them--classification of storms--examination in detail of the several classes and the primary influence of the earth or counter-trade in relation to each chapter xi. prognostics the philosophy of the weather. chapter i. heat and moisture are indispensable to the fertility of the earth. without suitable arrangements for their diffusion and distribution, and within the limits of certain minima and maxima, it would not have been habitable, or the design of its creator perfected. these arrangements therefore exist, and "while the earth remaineth seed time and harvest shall not cease." few and simple in their character, though necessarily somewhat complicated and irregular in their operation, the ultimate result is always attained. a beautiful system of compensations supplies the losses of every apparent irregularity in one section or crop, by the abundance of others. from the operation of these few, simple, connected, and intelligible arrangements for the diffusion of heat and the distribution of moisture over the earth, result all the phenomena which constitute the weather; and by studying them, and their operation, we may acquire an accurate knowledge of its "_philosophy_." the necessary heat is furnished, or produced, mainly by the direct action of the sun's rays; and the most obvious feature in the arrangements for its diffusion is that by which the sun is made to shine successively and alternately upon different portions of the earth. nothing animate or organic could endure his burning rays, if they shone continuously or vertically upon one point, or could exist without their occasional presence. hence the provision for a diurnal rotation, to prevent the exposure of any portion of the globe to the action of those rays for twenty-four consecutive hours, except for a limited period, and at a considerable angle, in the polar regions. but the earth is spheroidal, and a diurnal revolution would still leave that portion which lies under the equator too much, and the other too little, exposed to the action of the sun. this is obviated by an annual revolution of the earth around the sun, and an obliquity of its axis, by reason of which the northern and southern portions are alternately and, as far as the tropics vertically, exposed to the sun; and it is made to travel (so to speak) from tropic to tropic, producing summer and winter, and other important phenomena. this obliquity and consequent change of exposure are in degree precisely what the wants of the earth would seem to require. if it was greater, the sun would travel further north and south, but the alternate winters would be longer and more severe. if it was less, the end would not be as perfectly attained. the direct action of the sun's rays upon the earth, particularly those portions which lie north and south of the tropics, is not the only source from which the supply of heat is derived. although there is a general increase of heat in spring and summer when the sun travels north, and of cold when he travels south in winter, yet there are frequent irregularities attending both. very sudden and great changes occur in each of them. frost sometimes, cool weather often, occurs in midsummer, and considerable heat and tornadoes in midwinter. and ordinarily the maxima and minima of each month and, indeed, of each week are widely apart. even in the polar regions, in midwinter, _where the sun does not shine at all_, the same moderating changes with which we are conversant occur in degree. an extract or two from the register found in dr. kane's narrative of the "grinnell expedition" will illustrate this. january , (latitude about °, longitude about °). date. wind. force. ther. bar. sky and weather. jan. calm - . . blue sky, m. " w. gent breeze - . . blue sky, detached clouds, m. " w. by n. gent breeze - . . blue sky, m., clouded over. " w. by s. light breeze - . . clouded over, m., snow. " w. gent breeze - . . blue sky, detached clouds, m. " w.s.w. light air - . . blue sky, m. " w.n.w. light air - . . blue sky. " nw. by w. light air - . . clouded over, m. " nw. by w. gent breeze - . . clouded over, snow. feb. w. light breeze - . . cloudy, blue sky, m. " w. light air - . . blue sky, detached clouds, m. these extracts are instructive. it will be seen that on the d of january, when the sun had been absent some weeks, it was calm, the thermometer stood at ° below zero (the - or minus mark before the figures indicates that), and the barometer at . , with blue sky, somewhat misty or hazy--(the letter "m." standing for misty or hazy)--a state of the air which existed most of the time when it did not snow or rain, and therefore is of no importance in this connection. the next day the thermometer began to rise, and the barometer to fall. on the th it clouded over, and the thermometer rose rapidly, and on the th it had risen more than °, and snow fell. on the th it cleared off, the thermometer fell rapidly, and the barometer rose. on the th the thermometer had fallen to ° below zero, and the barometer had risen to . . another instance, in all respects similar, occurred the latter part of the month. we shall see hereafter that these changes are precisely like those which occur with us, and every where. that, as in the polar regions, and whether the sun be present or absent, or obscured by clouds, and by night as well as by day, the changes from warm to cold and from cold to warm are sudden and great, and that the latter are connected with the fall of rain and snow--that every where in winter it _moderates to storm_. many other instructive instances, especially in relation to the great difference in the seasons in our own country, and upon the same parallels elsewhere, might be cited if it were necessary. but they will more appropriately appear in the sequel. [illustration: fig. . in the above cut the isothermal lines are centigrade. the zero of the centigrade thermometer is the freezing point of water, or ° of fahrenheit. the boiling point of water is ° centigrade, or ° fahrenheit. a degree of centigrade is equal to one degree and four-fifths, fahrenheit. the ° line of the cut, therefore, is ° of fahrenheit--the line of ° above is ° fahrenheit--the line of ° below is ° fahrenheit, and so on. the reader, who is not familiar with the difference in the scale of the thermometer, is desired to remember this; for we shall make occasional extracts in which the temperature is given in the centigrade scale.] the cause of those irregularities, especially in the same seasons of different years, and when very great, is often sought and supposed to be found in the presence or absence of spots on the sun, ice floes and bergs in the atlantic, etc., etc. but neither the spots, nor ice, nor other local causes produce them. the cause will be found in the character of the arrangements we are considering, and the irregular action of the power which controls them. nor is the temperature of the northern hemisphere, north of the tropics, equal in the same latitudes. very great diversities exist in the "annual mean" as well as the "mean" of the different seasons. accurate observations at many points have enabled men of science to demonstrate this by drawing isothermal lines (_i. e._, lines of equal average annual heat) from point to point around the earth, which show at a glance these differences. the annexed cut is a polar projection of the isothermal lines of the northern hemisphere, as far down as the tropic, copied from kaemtz's meteorology. the dotted lines show the parallels of latitude, the dark lines the isothermal lines, or lines of equal annual average temperature. the reader is desired to observe how rarely they correspond with the parallels of latitude, and how they fall below in a few instances, and in others with great uniformity rise almost to the pole. take, for example, the isothermal line of or zero--that is, the line where the mean or _average_ height of the thermometer _for the year_ is at zero. at behring's straits this line is a little below the arctic circle, or the parallel of . north latitude. passing east over north america, it descends into canada, almost to lake superior, and to about the th parallel: that is to say, it is on an average during the year as cold on our continent at the th parallel as it is near behring's straits at the th parallel. passing east, the line of zero rises again over the atlantic ocean until, in the meridian of spitzbergen, it reaches, within the arctic circle, up almost to the th parallel. so, too, the isothermal of ° below zero, which is below the th parallel in siberia, rises in the north sea, above behring's straits, to the parallel of °, descending on the continent in north america to the th parallel, and rising again almost to the pole at spitzbergen, to descend again in siberia, while the isothermals of ° and ° below zero, which in north america are but just above the latitude of ° and ° respectively, ascend abruptly _surrounding the magnetic pole_, and _falling short of the geographical one_. let this projection of the lines of equal temperature, and particularly the situation of the magnetic poles, be studied well, for we shall recur to it hereafter in illustration of many important portions of our subject. it is apparent from these facts, and were it necessary might be rendered still more so by referring to others, that other causes operate in the distribution of heat over the earth besides the direct action of the sun's rays upon it. doubtless very considerable allowance is to be made for the difference of seasons, and difference during the same season upon the land and upon the ocean; in mountainous countries and level ones. but making every allowance for them, the fact that other causes have a _controlling_ influence in producing the deviations still remains most obvious. neither the difference of temperature between the land and the ocean, or land surfaces of unequal elevations, will account for the elevation of the isothermal lines on different portions of the ocean, or their extension around the magnetic poles. returning to a consideration of the arrangements for the diffusion of heat, we observe: first, that the earth itself is intensely heated in its interior. this is inferred, and justly, from the fact that the thermometer is found to rise about one degree for every fifty-five feet of descent--whether in boring artesian wells, exploring caves, or sinking shafts in mines. it is demonstrated, also, by the existence of hot springs and the action of volcanoes. heat is supposed to be conducted from the center toward the surface every where, but with difficulty and slowly. it is also supposed to be conducted from the tropical regions toward the poles. such is the opinion of humboldt. (cosmos, vol. i. p. .) probably it reaches the surface and exerts an influence, also, upon the weather through the ocean, and by heating it in its greatest depths. little attention has been paid, so far as i am informed, to the question how far the ocean is thus heated in _tropical latitudes_. doubtless a portion of the warmth of the ocean there is derived from that source, and it has its influence in changing the temperature of the deep-seated cold polar currents of, the great oceans. perhaps it may yet be found that the icebergs are detached by it in the polar seas--the observations of dr. kane point to such a result. (grinnell expedition, p. , and also chap. .) little need be said of the inconsiderable quantities of heat supposed to be derived by radiation from the stars, the planets, and from space. if any such are derived they are too inconsiderable to be of importance in this inquiry. heat is also carried, and in quantities which exert very considerable influence upon the weather, from the tropics to the poles by the great oceanic currents which flow unceasingly from one to the other. the most important of these with which we are acquainted is the gulf stream of the atlantic. gathering in the south atlantic, and passing north through the caribbean sea and the gulf of mexico, it issues out through the bahama channel, and flows north along the eastern coast of the united states, but some distance from it, to newfoundland, and from thence continuing to the north-east and spreading out over the surface of the ocean--a portion of it mingling with the waters of the north atlantic in passing--it flows up on the western coast of europe, around the faroe islands, and spitzbergen, to the polar sea; passing around greenland, and perhaps through its fiords, it descends again through the sounds and channels of the arctic regions into baffin's bay, and through davis's straits, burdened with the icebergs and floes of the polar waters, to return again to the south atlantic. for reasons which will appear in the sequel, it has comparatively little influence upon the weather of the united states. western europe, however, greenland, the islands which lie in its course, and the polar seas, are most materially influenced. although not the only cause, it has very much to do with the remarkable elevation of the isothermal lines over the northern atlantic, and upon western europe, as seen upon the map. a like oceanic current exists in the pacific ocean, the influence of which may also be traced upon the map by the elevation of the isothermal lines at the northern extremity of that ocean, and upon the north-west coast of north america. a vast amount of heat is transported from the tropical to the temperate and frozen regions of the earth by these great oceanic currents. another supply is derived from aerial currents which flow from the tropics toward the poles. these currents exist every where over the entire surface of the earth, but in more concentrated volumes along the great "lines of no variation," and greater magnetic intensity, on the western side of the great oceans, over the eastern portions of the two continents of north america and asia. not, as meteorological writers suppose, in the upper portions of the atmosphere, having risen in the trade-wind region and run off at the top toward the poles by force of gravity, but near, and sometimes in contact with the earth. the influence of these aerial currents upon the temperature of the atmosphere, and in producing the phenomena we are to consider, is exceedingly important. we shall have occasion to examine them with great care and minuteness under another head, for upon them, more than any other portion of the arrangements, depend not only the diffusion of heat, but also the distribution of moisture. still another supply of heat, during the sudden changes, at least, is produced by the action of terrestrial magnetism and electricity. very great progress has been made within a short period, in the investigation of the nature of these agents. the identity, or at least intimate association or connection of heat, light, electricity, and magnetism, always suspected, has been in various ways, and by a variety of experiments demonstrated. the influence of magnetism if distinct from gravitation, is second only to that; and its agency in producing the phenomena we are considering is primary and controlling. we will only, in this connection, ask the reader to note the situation of the north magnetic poles (for there are two of them); the manner in which the isothermal lines _surround_ them; the fact that they are _poles of cold_, _i. e._, that it is colder there than even to the north of them. we shall recur to this part of the subject again. such, briefly considered, are the principal arrangements by which heat is diffused over the earth. equally marked by infinite wisdom, and equally interesting and important, are the arrangements by which moisture is distributed. doubtless the general belief is that this is a simple process; that water evaporates and rises till it meets a colder stratum of atmosphere, and then condenses and falls again; or that, according to the huttonian theory, currents of air of different temperatures mingle and equalize their heat, and the aggregate mass when equalized in temperature is cooler, and therefore is unable to hold as much moisture in solution as the most heated portion had, and the excess falls in rain. but the process is by no means so simple, nor is heat the sole or most powerful agent concerned in it. currents of air do not mingle, but stratify. evaporation from the surface of any given portion of the earth outside of the tropics does not alone supply that portion with rain. _vast and wonderful, coextensive with the globe itself, and perfectly connected, is the machinery by which that supply is furnished even to the most inconsiderable portion of its surface._ take your map of north america and note, in this respect, its peculiarities. it extends from the isthmus of darien to the arctic regions, and from the th to the th meridian of west longitude from greenwich, and has upon its surface a type of every climate in the world. for the purpose of simplifying and illustrating the matter in hand, let us divide it into five sections. let the first section embrace central america and southern mexico, south of °; the second, northern mexico and southern new mexico, california, etc., between the parallels of ° and °; the third, northern california, utah, southern oregon, and western new mexico, north of the parallel of °; the fourth, the entire continent north of °; and the fifth, the eastern united states, east of the meridian of °. these divisions are not intended to be entirely accurate in their separation, but substantially so for the purpose of illustrating the differences which exist in each. the accompanying diagram shows approximately, by dotted lines, the divisions. [illustration: fig. .] now let us see in what a diverse manner, and to what a different extent, they are severally supplied with moisture. central america and southern mexico lie within the tropics--their rains are tropical rains. the season is divided into wet and dry, as are the seasons of all tropical countries which are not rainless. during the rainy season it rains a portion of nearly every day, and during the dry season the sky is clear, the air is pure, and rain seldom falls. all around the earth within the tropics, over the land and over the sea, there is a belt of almost daily rains, varying in width, north and south, in different sections, but averaging about five hundred miles. this belt of daily rains is formed at and by the meeting of n. e. and s. e. trades, and travels north and south with them, as they do with the sun, _encircling the globe_. by this narrow belt a portion of the earth's surface, an average of some ° of latitude, is supplied with moisture. wherever it is situated at any given period, the tropical rainy season exists; and when it is absent in its northern or southern transit, the dry season prevails. southern mexico is within the range of this moving belt, and in its course to the northward with the sun, in our summer from may to october, it arrives over, and covers that country with a rainy season. when the sun returns to the south, taking with it the trades and this belt of tropical rains, that portion of mexico is without rain, and dry, and so continues until the rainy belt returns in the following year. while the belt is over southern mexico it is nearly all _precipitation_, and there is little _evaporation_; while that belt is _absent_ it is all _evaporation_, with little or no _rain_. surely this is not consistent with the prevailing belief of simple evaporation, ascent to a colder stratum, commingling, and condensation, and rain. southern mexico at least is not supplied by mere evaporation from its surface, and must therefore form an exception to that belief, and to the huttonian theory. but we shall recur again to the peculiarity of distribution within the tropics. turn now for a brief space to northern mexico, southern new mexico, and southern california. in northern mexico, southern new mexico, utah, and california, between the parallels of ° and °, and particularly west of the mountain ranges, we find an almost rainless region, sterile and worthless, resembling that which is found upon nearly the same parallels of north latitude in northern africa, egypt, arabia, beloochistan, afghanistan, and north-western india; and in corresponding latitudes south of the equator, in peru, a portion of southern africa, and the northern and middle portions of new holland. why northern mexico and the other countries named are thus sterile and comparatively rainless, we shall see hereafter, when we examine critically the machinery of distribution as it operates within the tropics. it is the fact that it is thus sterile and rainless to which we desire to call attention in this place. mr. bartlett thus describes it: "on leaving the head waters of the concho, nature assumes a new aspect. here shrubs and trees disappear, except the thorny chaparral of the deserts; the water-courses all cease, nor does any stream intervene until the rio grande is reached, three hundred and fifty miles distant, except the muddy pecos, which, rising in the rocky mountains, near santa fé, crosses the great desert plain west of the llano estacado, or staked plain. "from the rio grande to the waters of the pacific, pursuing a westerly course along the d parallel, near el paso del norte, there is no stream of a higher grade than a small creek. i know of none but the san pedro and the santa cruz--the latter but a rivulet, losing itself in the sands near the gila--the other but a diminutive stream, scarcely reaching that river. at the head-waters of the concho, therefore, begins that great desert region, which, with no interruption save a limited valley or bottom-land along the rio grande, and lesser ones near the small courses mentioned, extends over a district embracing sixteen degrees of longitude, or about a thousand miles, and is wholly unfit for agriculture. it is a desolate, barren waste, which can never be rendered useful for man or beast, save for a public highway."--_bartlett's personal narrative_, vol. i. p. . turning now to central and upper california, and utah, and southern oregon, we find still another peculiarity. like southern mexico, they have a rainy and dry season, but at a different period, and for a different reason. the dry season of california, etc., is the summer of the northern hemisphere, and her rainy season the winter. _california_ is, therefore, _dry_ when southern _mexico_ is _wet_, and _vice versâ_. the belt of rains which supplies california with moisture during her rainy seasons is the belt of _extra-tropical_ rains, which extends from the northern limit of the north-east trades to the poles, encircling the earth. the southern edge of this extra-tropical belt is _carried up_ on the western coast of america, and in that portion of the continent in _summer_, when the sun and trades, and the inter-tropical rainy belt travel to the north, and uncover california, etc., leaving them without rain for a period of about six months. [illustration: fig. . in summer.] as the sun, with the trades, travels south, the southern edge of the belt of extra-tropical rain follows, and covers california, etc., again extending gradually from the north to the south, and thus their wet season returns. the annexed diagrams by the shading will show the situation of the rainy belts which cover mexico, utah, new mexico, and california in summer and winter, and that the belts of rains are entirely distinct and different in character. [illustration: fig. . in winter.] here again in this section of the continent, as in mexico, evaporation is going on for six months of the year, and were it not for the return of the belt of rains from the north, in the fall, would go on for the entire year without precipitation; and for the other six months precipitation is vastly in excess. nor can this be reconciled with, or explained by, the huttonian or any other received theory of rain. here again it is obvious that evaporation alone, however great or long continued, will not furnish the evaporating section with rain. the northern portion of the continent lies beneath the zone of extra-tropical rains, and north of the northern limit of the n. e. trades--is never uncovered from it, and has no distinct rainy or dry season, although more rain falls at certain periods, and in certain localities, than at others. the climate of that part of oregon which lies upon the pacific, and the character of its rains, resemble those of north-western europe, and will be further explained hereafter. coming to the portion of the continent which we occupy, the th section, we find it different still--a most favored region. portions of it--eastern texas, for instance--are upon the same parallels of latitude as the rainless regions of northern mexico, etc. eastern texas, however, is not rainless. other portions are upon the same parallels as california, etc., yet have no distinct rainy and dry season. we repeat, this section is a most favored region--without a parallel upon any portion of the earth's surface, except, in degree, in china and some other portions of eastern asia. it is not only without a distinct rainy and dry season, but it is watered by an average, annually, of more than forty inches of rain, while europe, although bounded on three sides by seas and oceans, and apparently much more favorably situated, receives annually an average of only about twenty-five--if we except norway, and one or two other places, where the fall is excessive. the distribution of this supply of moisture over the united states is, in other respects, wonderful. iowa, in the interior of the continent, far away from the great oceans, on the east or west, or the gulf of mexico on the south, receives fifty inches; some ten or fifteen inches more than fall upon the slope east of the alleghanies, and contiguous to the great atlantic (from which all our storms are, erroneously, supposed to be derived), and the average over the entire great interior valley is about forty-five inches, falling at all seasons of the year. observe, then, by way of recapitulation: southern mexico has a rainy season furnished by the belt of _inter_-tropical rains, which _travels up over it from the south_ in summer. california has a rainy season, which is furnished by the _extra_-tropical belt of rains, which travels _down from the north_, and covers it in winter. northern mexico and the adjoining regions west of the th meridian are between the limits of the two, and neither travels far enough to reach them, except for brief and uncertain periods; they are comparatively rainless; while the eastern portion of the continent, _in all latitudes_, unlike the others, is without a distinctly marked dry season, or a rainless region, and with the exception of occasional droughts, is abundantly supplied with rain at all seasons of the year. and now, what is the explanation of all this? what produces the extra-tropical belt of regular rains surrounding the earth, north of the parallel of ° north, in some places, and ° in others, extending to the pole, with its southern edge traveling up ten or more degrees in summer, leaving large portions of the earth subject to a dry season; and back again in the winter to give them a rainy one? what produces the narrow belt of inter-tropical rains, encircling the earth; traveling up and down every year over an average of ° of latitude, supplying every portion of it alternately with rain? and what connects the two together over the eastern portion of north america, so as to leave no distinctly marked wet and dry season, and no rainless and sterile portion there? are all these the result of simple evaporation, ascent to a colder region, condensation, and descent again? demonstrably not. of the forty inches which fall annually upon the middle and eastern portions of the united states, an average probably of one-half or twenty inches, runs off by the rivers to the ocean, or is carried away eastward by the westerly and north-westerly evaporating winds. the same is true, in degree, of the rain which falls upon the other portions. evaporation, therefore, could not keep up the supply. from whence, then, does it come? this twenty inches, thus lost by the rivers and winds, and with such wonderful regularity every year. "all the rivers run into the sea, yet the sea is not full. _note the place whence the rivers come, hither they return again._" but how is it that they thus return with such wonderful regularity, in a narrow traveling belt of daily rains within the tropics, and a movable belt of irregular rains without the tropics, extending to the poles, leaving a space on each side of the equator encircling the earth in like manner (except at two points, _viz._, eastern asia and eastern north america), from which they do not go, and to which they do not return, and which is almost entirely unfurnished with rain? and all this without any relation, whatever, to the contiguity of the oceans? obviously this is not the work of mere evaporation, or of the accidental or irregular commingling of winds with different dew points, or quantities of moisture in solution, or accidental, irregular changes of barometric pressure. _it is one vast, wonderful, connected, and regular system--co-extensive with the globe--necessary to the return of moisture from the oceans upon the most inconsiderable portion of it, and to the condensation of the local moisture of evaporation; and by it the waters are returned from the oceans as regularly and bountifully upon the far interior of the great continents in the same latitudes, as upon the "isles which rest in their bosoms."_ chapter ii. before proceeding to an examination of this connected atmospheric machinery, and an investigation of the particular ocean from which our rivers return, it may be well to look at the form in which they appear to return, that we may have a clear understanding of terms. they seem to return in the form of clouds, and in storms and showers, although, in truth, they return in regular, uniform, ordinarily invisible currents, and the storms and showers are but condensations in, and discharges from portions of those currents, aided by the local moisture of evaporation. the term _storms_, seems to be used by european meteorologists to denote what we term thunder showers or gusts, and tornados; while what we call storms are denominated by them regular rains. as the terms are extensively in use in this country, we must adhere to the meaning attached to them _here_ rather than _there_. storms with us, then, are regular rains of from six to forty-eight or more hours' continuance: generally without lightning, or thunder, or gusts, and usually with wind of more or less force, from some easterly point. they are called north-east storms, or south-east storms, according to the point from which the surface winds blow. practically we shall find that this distinction is of some importance, for the north-east storms are the longest, lasting generally twenty-four hours, or more, while the south-east ones seldom, if ever, continue as long. these storms extend over a considerable surface, rarely less than one hundred miles in one direction or another, and sometimes fifteen hundred, or more. distinct showers cover but a small surface, sometimes not more than forty to one hundred rods, as in the tornado, and rarely more than ten miles. belts of showers, each new one forming a little more to the south, often, in summer, pass across the country, following each other in succession; and these belts may be of considerable width, say thirty to one hundred and fifty miles. the clouds which constitute the storms and showers differ in appearance and character, as well in the active as in the forming state. clouds are of distinct characters, alike, substantially, every where under like circumstances; and a distinct nomenclature has been applied to them by dr. howard, of london. he notes three kinds of primary clouds: _viz._, cirrus, stratus, and cumulus; and inasmuch as the boundary line between them is not very distinct, certain compounds of the three, _viz._: cirro-stratus, cirro-cumulus, and cumulo-stratus. this nomenclature is every where received, and portions of it are of great practical importance. the three principal descriptions of cloud, _viz._: the cirrus, the stratus, and the cumulus, we have very much as they have in europe, and doubtless as they exist every where outside of the tropics. the nimbus, another cloud described by him, is not distinct from the cumulus or stratus. an isolated, limited thunder-shower in a clear sky, presents the appearance of a nimbus, as shown in the cuts, but the basis of it is a cumulus, and it differs from an ordinary fair-weather cumulus merely in the dark and fringe-like appearance of the rain as it is falling from its lower surface, and sometimes in the existence of a stratus above and in connection with it. a similar form is often assumed by the peculiar clouds of the n. w. winds in march or november, when they assume the form of _squalls_, and drop flurries of snow. the nimbus, therefore, is not a distinct cloud, but an appearance which the cumulus, stratus, or cirro-stratus has in a stormy or showery state, and does not deserve a distinct name. it is but a cumulus, or a stratus, or cirro-stratus dissolving in snow or rain. it is important that this term should be abandoned. it tends to confuse and prevent a clear understanding of the difference in the character of the clouds, and in relation to which precision is both difficult and desirable. the figures on pages and , show the different kinds of clouds as designated by howard. they are copied from the engravings in the sixth edition of maury's "sailing directions." [illustration: fig. .] [illustration: fig. .] figure . the cirrus is indicated by bird. the cirro-cumulus by " the cirro-stratus by " the cumulo-stratus by " figure . the cirrus by " the cumulus by " the stratus by " the nimbus by " how far these representations correspond with the actual appearance of the different compound forms in england, i can not say. but although they convey a _general_ idea, _they are not sufficiently accurate for practical illustration or observation here_. indeed howard himself has omitted from his last edition his plate of the clouds, assigning as a reason, "that the real student will acquire his knowledge in a more solid manner by the observation of nature, without the aid of drawings, and that the _more superficial are liable to be led into error by them_." the collection of forms in the cuts _does not contain some very important ones_, and contains some which are not distinct forms; but they may aid us somewhat in this inquiry, and, therefore, i have copied them. it is well, also, for the reader to have the generally received description before him. but for the purpose of _practical_ illustration hereafter, and greater precision, i shall follow a somewhat different order in describing them, and introduce two forms of _scud_ quite as important, practically, as any other. first, then, commencing at the earth, we have what may be properly termed _fog_, or low fog. this forms, in still clear weather, in the valleys, and over the surface of the rivers and other bodies of water, during the night, and most frequently the latter part of it, and is at its acmé at sunrise, or soon after, limiting vision horizontally and perpendicularly, and dissolving away during the forenoon. it is rarely more than from two to four hundred feet in height at its upper surface, and often much less, and is composed of vesicular condensed vapor, sometimes sufficiently dense to fall in mist, and is doubtless in composition substantially what the clouds are in the other strata of the atmosphere, as observed by us, or passed through by aeronauts. i have never seen it carried up to any considerable height into the other strata by any of the supposed ascending currents, to form permanent clouds, and shall have occasion to allude to the fact in another connection. it disappears usually before mid-day, and has, when thus formed, no connection with any clouds which furnish rain. to this dr. howard originally gave the name of stratus, and so it is represented upon the cut; but the latter term may be with greater propriety applied to the smooth uniform cloud in the superior strata from which the rain or snow is known to fall, and i shall retain and so apply it. the next in order, ascending, is high fog. this is usually from one to two thousand feet in height at its lower surface. it forms, like low fog, during the night and in still weather; and is rarely, if ever, connected with clouds which furnish rain. it breaks away and disappears between ten and twelve in the forenoon, usually passing off to the eastward. this fog is most commonly seen in summer and autumn, particularly the latter, and unless distinguished from cloud will deceive the weather-watcher. it is readily distinguishable. although often very dense, obscuring the light of the sun as perfectly as the clouds of a north-east storm, it differs from them. it forms in still clear weather, is present only in the morning, is perfectly uniform, and, before its dissolution commences, without breaks, or light and shade, or apparent motion, and unaccompanied by scud or surface wind. the storm clouds are never entirely uniform, or without spots of light and shade, by which their nature can be discerned, and rarely, when as dense as high fog, without scud running under them and surface winds. there is another fog still, connected with rain storms, but it does not often precede them; occurring at all seasons, but most commonly in connection with the warm s. e. thaws and rains of winter and spring; and which usually comes on _after_ the rain has commenced and continued for awhile, and the easterly wind has abated; occupying probably the entire space from the earth to the inferior surface of the rain clouds or stratus. practically this does not require any further notice. it is an _incident_ of the storm. when formed it remains while the storm clouds remain, and passes off with them. it is sometimes exceedingly dense in february and march, when it accompanies a thaw, and if there is a considerable depth of snow, it has the credit of aiding essentially in its dissolution. mingled with the smoke of london, it produced there the memorable _dark day_ of the th of february, , and at various other times has produced others of like character. (see howard's climate of london, vol. iii. pp. , , .) these fogs have been so dense there that every kind of locomotion was dangerous, even _with lanterns, at mid-day_. the next in order, ascending, are the storm scud, which float in the north-east or easterly, south-east or southerly wind, before and during storms. these, as the reader will hereafter see, are, _practically_, very important forms of cloud condensation--although they have found no place in any practical or scientific description given of the clouds, and are not upon the cuts. they are patches of foggy seeming clouds of all sizes, more or less connected together by thin portions of similar condensation, often passing to the westward, south-westward, north-westward, or northward with great rapidity. their average height is about half a mile, but they often run much lower. they are usually of an "ashy gray" color. the annexed cut shows one phase of them, from among many taken by daguerreotype. the arrows pointing to the west show the scud distinguished from the smooth partially formed stratus above. this view was taken a few hours prior to the setting in of a heavy s. e. rain storm. it is a northerly view. [illustration: fig. .] at about the same height, but in a _different state of the atmosphere_, float the peculiar fair-weather clouds of the n. w. wind. they usually form in a clear sky, and pass with considerable rapidity to the s. e. sometimes they are quite large, approaching the cumulus in form, and white, with dark under surfaces, and at others, in the month of november particularly, are entirely dark, and assume the character of squalls and drop flurries of snow; and then resemble the nimbus of howard. they assume at different times and in different seasons, different shapes like those of the scud, the cumulus, or the stratus. [illustration: fig. .] they form and float in the peculiar n. w. current which is usually a fair-weather wind, and are never connected with storms. in mild weather they are usually white, and in cold weather sometimes very black, and at all times differ _in color_ from the ashy gray scud of the storm. this variety is not represented upon the general cuts. the annexed diagram shows one phase of them, but they are readily observable at all seasons of the year, when the n. w. wind is prevailing; differing in appearance according to the season. let these, as well as the storm scud, be carefully observed and studied by the reader, and let no opportunity to familiarize himself with their appearance be lost. a brief glance at each recurrence of easterly or north-westerly wind will suffice. [illustration: summer cumuli.] the _cumuli_ appear in isolated clouds of every size, or in vast clouds composed of aggregated masses, as the peculiar cloud of the thunder shower. they form as low down as the scud or fair-weather cloud of the n. w. wind, which, for convenience, i will call n. w. _scud_; and often in violent showers, and particularly in hail storms, extend up as far as the density of the atmosphere will permit them to form. professor espy thinks he has measured their tops at an altitude of ten miles. others have estimated their height, when most largely developed, at twelve miles; but it is very doubtful whether the atmosphere can contain the moisture necessary to form so dense a cloud at that elevation. it is their immense height, however, whether it be six, or eight, or ten miles, together with the sudden and violent electric action, condensing suddenly all the moisture contained in the atmosphere within the space occupied by the cloud, which produces such sudden and heavy falls of rain or hail. as the rain drops or hail, when formed at such an elevation, in falling through the partially condensed vapor of the cloud must necessarily enlarge by accretion from the particles with which they come in contact, and probably also by attraction, their size when they reach the earth, though frequently very considerable, is not a matter of astonishment. the cumulus is represented in the general plate with sufficient accuracy to show its peculiar character. in summer, when the air is calm, the weather warm, and no storm is approaching, there is always, in the day time, a tendency to the formation of cumuli. this tendency exhibits itself about ten o'clock in the forenoon, and they gradually form and enlarge until about two in the afternoon; and after that, if they do not continue to enlarge and form showers, they melt away and disappear before nightfall. sometimes in july and august the atmosphere will be studded with them at mid-day, floating about three-quarters of a mile from the earth (in a level country), gently and slowly away to the eastward. at times it may seem as if they must coalesce and form showers, yet they frequently do not, but gradually melt away, as before stated. the cumulus is the principal cloud of the tropics, and is not often seen with us except in summer, or when our weather is tropical in character. the engraving on the preceding page, shows a phase of these fair-weather summer cumuli. the last in order occupying (with their compounds) the higher portions of the atmosphere, are the cirrus and stratus. the cirrus is often the skeleton of the other, and precedes it in formation. these are the proper clouds of the storm, in our sense of the term. while, however, the cirrus remains a cirrus, it furnishes no rain. when it extends and expands, and its threads widen and coalesce into cirro-stratus and stratus, or it induces a layer of stratus below it, the rain forms. the following is dr. howard's description of cirrus: "parallel, flexuous or diverging fibers, extensible by increase in any or in all directions. clouds in this modification appear to have the least density, the greatest elevation, and the greatest variety of extent and direction. they are the earliest appearance after serene weather. they are first indicated by a few threads penciled, as it were, on the sky. these increase in length, and new ones are in the mean time added to them. often the first-formed threads serve as stems to support numerous branches, which in their turn, give rise to others." the illustrations in the general cut are imperfect, and do not represent the delicate fibers of the cloud, for it is a difficult cloud to daguerreotype or engrave, but the representation is sufficiently accurate to give the reader a general idea of the different varieties, and enable him to discover them readily by observation. they are the most elevated forms, always of a light color, and often illuminated about sunset by the rays of the sun shining upon their inferior surface; the sun, however, often illuminates, in like manner, the dense forms of cirro-stratus, and the latter, from their greater density, are susceptible of a brighter and more vivid illumination. the stratus is a smooth, uniform cloud--the true rain cloud of the storm; often forming without much cirrus above, or connected with it. it may be seen in its partially formed state in the bank in the west, at nightfall, or in the circle around the moon in the night. when it becomes sufficiently condensed, rain always falls from it, but in moderation. if there be large masses of scud running beneath it for its drops to fall through (especially as is sometimes the case, in two or more currents), the rain may be very heavy. but more of this hereafter. [illustration: fig. .] the annexed cut shows the forming stratus, light and thin, passing to the east, as indicated by the short arrows just before a storm, while the scud beneath is running to the west. it was copied from a daguerreotype view, facing northwardly. intermediate between the fibrous, tufted, cirrus, and the smooth uniform stratus, there is a variety of forms partaking more or less of the character of one or the other, and termed _cirro-stratus_. no single correct representation of cirro-stratus as a distinct cloud, can be given--but several varieties will be hereafter alluded to, under the head of prognostics. several modifications are represented with tolerable accuracy upon the cuts. the cirro-cumulus is a collection in patches of very small distinct heaps of white clouds; they are called fleecy clouds, from their resemblance to a collection of fleeces of wool, and are imperfectly represented on the general cut. they do not appear often, and are usually _fair-weather clouds_. this form has none of the characteristics of the cumulus, and does not appear in the same stratum. it was probably called cumulus because its small masses are distinct, as are those of the ordinary cumulus. it occurs in the same stratum as cirro-stratus, and properly belongs to that modification. i retain the name inasmuch as the cloud is of some practical importance. the cumulo-stratus is seldom seen in our climate, as it is represented in the cut. stratus condensation _above_, and in connection with cumulus condensation, is not uncommon, but that precise form is rare. this, too, is practically of no consequence, and i shall take no further notice of it. recapitulating, i give (in a tabular form) the three principal strata and their modifications, located with sufficient accuracy for illustration. the clouds which are found in an upper or lower portion of a stratum are so represented by the location of their names; those which appear at all heights in the stratum, with the names across. the elevation is the average one--although there is no limit to the cirrus above, except the absence of sufficient moisture. it was seen by guy lussac, and has been by other aeronauts, at an elevation of five miles, or more, when too delicate to be visible below. +------------------------------------------------+ | | miles. |cirrus. | | cirro-cumulus. | | cirro-stratus. | | | primary | { cumulus extending up | stratum. | { in violent showers. | | | | | |stratus. | |------------------------------------------------| | | - / miles. scud & |n. w. scud. { cumulus storm scud. | cumulus |fair-weather { ordinarily and | stratum. | { its base always. | | | |------------------------------------------------| | | / mile. fog |high fog. storm fog. | stratum. | | | low fog at the surface of the earth. | | | +------------------------------------------------+ with the assistance of this table of elevations, and a careful observation, the reader can soon become familiar with the forms of clouds and their relative situations. chapter iii. having thus taken a brief view of the different clouds, let us return to the inquiry, from what ocean, and by what machinery, _our_ "rivers return." not wholly or mainly from the north atlantic, although it lies adjacent to us, and they often _seem_ to do so; for, first, all storms, showers, and clouds, which furnish, _independently_, any appreciable quantity of rain to the united states, and even adjacent to the atlantic, or indeed to the atlantic itself, come from a westerly point, and pass to the eastward. _this is a general, uniform, and invariable law, although there is in different places, and in the same place at different times, some variation in their direction; ranging in storms from w. by s. to s. s. w., and in showers between s. w. and n. w., to the opposite easterly points of the compass; the most general direction, east of the alleghanies, being from w. s. w. to e. n. e._ but do we not see, you inquire--at least those of us who live east of the alleghanies--that when it rains, the wind is from the eastward; and that the _clouds_ follow the wind from the east to the west? you do indeed, generally, in all considerable storms, observe that the wind blows from some easterly point, and that _seeming_ clouds are blown by it to the westward; but what you see, and call clouds, are not the clouds which furnish the rain. far above the seeming clouds you notice, directly over your head when it rains or snows, are the rain or snow clouds, dense and dark, passing to the eastward, how strong soever the wind may blow from the quarter to which they tend, or any other quarter, between you and them. what you see below them are _scud_. so the sailors call them, and so i have termed them. it is a "dictionary name," and a good one, expressive of a distinction between them and _clouds_. they are thin, and the sun shines through them, although with some difficulty, when the rain clouds above are absent or broken. _this east wind and the scud are not the storm, or essential parts of it._ storms occasionally exist, particularly in april, without either. they are but _incidents_, _useful_, but not _necessary incidents_, as all surface winds are. if you could see a section of the storm, you would see the rain cloud above, moving to the east, and the scud beneath running to the west, as indicated by the arrows in the cut on page . opportunities frequently occur when these appearances may be seen. storms are sometimes very long, a thousand miles, perhaps, from w. s. w. to e. n. e., and not more than one to three hundred miles wide from s. e. to n. w., and their sides, particularly the northern ones, regular, and without extensive partial condensation. then the storm cloud above, moving to the eastward, and the scud running under to the westward, may be seen as in the cut. so they may be seen before, at the commencement, and at the conclusion of easterly storms, in a majority of cases, and the reader is desired to notice them particularly as opportunities occur. the term _running_, too, is a very expressive one, used by sailors as applicable to _scud_. for while the forming or formed storm clouds may be moving moderately along, at the rate of twelve to fifteen or twenty miles an hour, from about w. s. w. to e. n. e., the scud may be running under them in a different direction--opposite, or diagonal, or both--at the rate of twenty, fifty, sixty, and, in hurricanes, even ninety miles an hour. you have doubtless seen these scud running from n. e. to s. w., and without dropping any moisture, a day or sometimes two days, before the storm coming from the s. w. reached the place where you were; and then, sometimes the storm cloud slipped by to the southward, and the expected storm at that point proved "a dry northeaster." sometimes the condensation, although sufficiently dense to influence and attract the surface atmosphere, and create an easterly wind and scud, does not become sufficiently dense to drop rain, and then, too, we have a dry northeaster, which may melt away or increase to a storm after it has passed over us. _i have never seen, except, perhaps, in a single instance, one of these masses of scud, however dense, which had not a rain (stratus) cloud above it, drop moisture enough to make the eaves run._ so you see it may be true, and if you will examine carefully, you may satisfy yourself that it is true, that the storms all move from a westerly point to the eastward, notwithstanding the wind under them is blowing, and the scud under them are running to the westward. there are many other methods by which the reader may determine this matter himself. he may catch an opportunity for a view, when there is a break in the stratus cloud above, and the sun or moon, no longer obscured by the _storm cloud_, shines through the scud beneath. then he may see they are moving in different directions. _the upper cloud, if there be any of it left, always to the eastward._ again, we may see the storm approach from the westward, as it often does, before the wind commences to blow, and the scud to run from the eastward; particularly snow storms in winter, and the gentle showers and storms of spring. again, thunder storms, we know, come from the westward, and apparently against an east wind. it is sometimes said they approach from the east, but it is a mistake. during thirty years attentive observation in different localities, i have never seen an instance. they sometimes _form_ over us, or just east of us, or one may form at the east and another at the west, and as they _spread out in forming_, one may seem to be coming from the east, or there may be an easterly current, with dense flocculent scud at the under surface of the shower cloud running westward, but they finally pass off to the eastward, and never to the westward. it is possible that a _patch of scud_ may become sufficiently _dense_ and _electrified_ to make a _shower_, but i have never observed one. such an _apparent_ instance may be found recorded in "sillman's journal," vol. xxxix. page . i have seen the scud assume a distinct cumulus form, but never to become sufficiently dense to make a thunder shower. thunder and lightning sometimes attend portions of regular storms in spring and autumn, but the thunder is always heard first in the west, and last in the east. again, there are admitted facts with which you are conversant, which prove this proposition. when it has been raining all day, and just at night the storm has nearly all passed over to the eastward, and the sun shines under the western edge of it, and "_sets clear_," as it is termed--you say that "_it will be clear the next day_." why? because the storm will not pass to the westward, covering the sun and continuing, how strong soever the wind may be from the east; and because it is passing, and will continue to pass off to the eastward, leaving the sky clear. _the easterly wind will stop as soon as the storm clouds have passed, and it will fall calm, or the wind will "come out" from the westward._ so, too, when the clouds are dark in the west in the morning, and the sun rises clear, but "_goes into a cloud_," as it is expressed, you say that it will rain. and if the clouds are dense this generally proves true; because there is a storm or shower approaching from the west, and passing over to the east, the western edge of whose advance condensation has met the sun in his coming, and obscured him from your vision. when, too, it has been storming, and lights up in the n. w. you say it will clear off; the n. w. wind will blow all the clouds away. it is, indeed, generally true that when it so lights up it is about to clear off; although it sometimes shuts down again, in consequence of the approach of another storm from the westward, following closely behind the one which is passing off. it is a great mistake, however, to suppose the n. w. wind blows away the clouds. watch the smooth stratus rain cloud at its lower edge, where the clear sky is seen, and you will see that it is moving on steadily to the n. e., in obedience to the laws of its current, and will do so, even when its retreating edge has passed up to the zenith, and down to the s. e. the storm uncovers us from the n. w. by the contraction of its width, _or_ because it has a _southern lateral extension_ and _dissolution_, and not by being blown away by the n. w. wind; although that wind, by its peculiar fair-weather clouds, may be, perhaps, observed beneath, ready to follow its retreating edge. again, when it has been clear all day, and the sun sets in a bank of cloud, you say--"_it will rain to-morrow, the sun did not set clear_," and unless that bank is a thunder cloud, merely, which will pass over or by you, with or without rain, before morning, it is generally true that it will. the bank will prove the eastern edge of an approaching storm. from these generally admitted and understood facts, you may know that storms pass from the west to the east. this proposition is also proved by all the investigations of storms, which have taken place since the settlement of this country. storms of great severity attract particular attention, and are said to "back up" against the wind, because they are observed to commence storming first at the westward, although the wind is from the eastward. doubtless you recollect many such instances recorded in the newspapers. no season occurs without such notices. many storms have been investigated by mr. redfield, for the purpose of sustaining his theory. many others by professor espy, to sustain his. one by professor loomis, with great research and ability--and some by others, accounts of all which have been published; and every one yet investigated, north of the parallel of °, has been shown to pass from a westerly to an easterly point. so, too, we may know it from analogy. the laws of nature are uniform. there is a great end to be accomplished, _viz._: the distribution of forty inches of water, at regular intervals, over a large extent of country. the rivers are to return, and the clouds are to drop fatness, and seed time and harvest are not to cease. it is to be done and is done, by means of storms and showers, and pursuant to general laws, as immutable as the result. most of these storms and showers, it has been found, and may be observed, move from the westward to the eastward. then we may know, from analogy, that they do so in obedience to a general, uniform law; and so i might say with confidence, if our inquiry stopped here, it will ever be found by those who may hereafter examine them. but, d. there is a current in the atmosphere, all over the continent north of the n. e. trades, but in great volume over the united states, east of the meridian of ° w. from greenwich--varying in different seasons, and upon different parallels, and flowing near the earth, when no surface wind interposes between them. in the vicinity of new york, the usual course of this current is from about w. s. w. to e. n. e. in the western and south-western portion of the united states, it is, doubtless, more southerly--varying somewhat according to the season--and in other sections varies in obedience to the general law of its origin, and progress. i have observed its course in many places, between the parallels of ° and ° n. _this current comes from the south atlantic ocean._ it is our portion of the aerial current, which flows every where from the tropics toward the poles, to which i have already alluded in connection with the distribution of heat. _it brings to us the twenty inches of rain which we lose by the rivers, and by the westerly winds, which carry off a portion of the local moisture of evaporation, and its action precipitates the remaining portion of that moisture. it spreads out over the face of our country, with considerable, but not entire uniformity. all our great storms originate in it, and all our showers originate in or are induced and controlled by it._ _from the varied action, inherent or induced, of this current, most of our meteorological phenomena, whether of wet or dry, or cold or warm weather, result_; and a thorough knowledge of its origin, cause, and the reciprocal action between it and the earth, is essential to a knowledge of the "_philosophy of the weather_." let us then go down to the "chambers of the south," to the inter-tropical regions, of which we have said something in connection with a notice of southern mexico, and see where, and how this great aerial current originates. chapter iv. between the parallels of ° north latitude, and ° south latitude--changing its location within this limit at different seasons of the year--encircling the earth, and covering about one-half of its area--we find the trade-wind region. in this region are the simple and uniform arrangements, which extend every where, and produce all the atmospheric phenomena. in the center of it we find that movable belt of continual or daily rains, and comparative calms, particularly _near its center_, about four hundred and fifty miles in width upon the atlantic, and over africa, and the eastern portions of the pacific, and something more over south america and the west indies, the western portion of the pacific and the indian ocean, to which we have already alluded. this belt of rains and calms follows the trades and sun, in their transit north and south, from one tropic to the other--its width and extension depending upon the volume of trade-winds existing on the sides of it. its southern edge, when the sun is at the southern solstice, extends to ° south in the atlantic, to ° south in the indian ocean, and still further, probably, over south america: on this point i do not pretend to be accurate, for accuracy is not essential. when the sun is at the northern solstice the southern edge is carried up as far as ° north, over the atlantic, and still further over the northern portions of south america, the west indies, and mexico. it travels, therefore, from south to north, over from twenty to forty degrees of latitude. the presence of this belt of rains over any given portion of the inter-tropics, gives that portion its rainy season, and its absence, as it moves to the north, or the south, gives the portion from which it has moved, its dry season. it passes in its transit twice each year over some portions of the country, bogota, for instance, and two corresponding rainy and dry seasons result. its presence, and character, and movements, are as fixed and regular, over from twenty-five to forty degrees of the earth's surface, _and all around it_, as the presence and movements of the sun over the same area. at the northern edge of this movable belt of rain, and extending in some places, particularly in the pacific ocean, north about °, or about one thousand four hundred miles, and in other places a less distance, the n. e. trade winds prevail, blowing toward and into it from n. n. e., n. e., and e. n. e., averaging about n. e. at the south line of this belt of rains, extending south from twenty-five to thirty degrees, or from sixteen hundred to two thousand miles, the s. e. trades blow toward and into it, from the s. e., s. s. e., or e. s. e., averaging about s. e. of course the northern limit of the n. e. trades travels north and south with the belt of rain, toward which it blows; and so the southern limit of the s. e. trades travel in like manner with the rainy belt, or rather, to speak with entire accuracy, the belt of rain moves with the trades, and the trades follow the verticality of the sun. the following diagrams exhibit approximately, and with sufficient accuracy for illustration, the situations of the rainy belt and the trades, when at their northern and southern limit, as well as the manner in which it must give certain localities two rainy seasons each year, in its transit north and south. at the northern and southern limits of the trade-winds, and extending from them to the poles, are found the variable winds and irregular extra-tropical rains, all over the earth, which are shown by the shading on the maps. this line of extra-tropical rains descends to the south, following the retreating trades as they descend in our winter, and recedes north before the trades when they return in spring and summer, so that at the outer limit of the trades respectively, toward the poles, the line of extra-tropical rains will be found, receding or following that limit, as the trades pass up and down with the sun. from the north pole to the northern limit of the n. e. trade-winds, wherever found, whether at ° north latitude, as in some places in summer when the sun is at the tropic of cancer; or whether at ° to ° north latitude, as in our winter, when the sun is at the tropic of capricorn; the extra-tropical rains prevail. a state of things precisely similar exists between the south pole and the southern limit of the s. e. trades. between this northern limit of the n. e. trades and the northern line of the inter-tropical belt of rains, wherever situated (with two exceptions, to which we have alluded and shall allude again), there is, for the time being, a dry season; and a like dry season between the southern line of the belt of rains and the southern limit of the s. e. trades. we have, therefore, extending around the earth, a belt of daily tropical rains, near the center,--two belts of drought which are mainly trade-wind surfaces, one on each side of the central rainy belt,--extending to the outward limits of the trades and the line of extra-tropical rains; and these rainy and dry belts, moving up and down after the sun, a distance of from twenty to forty degrees of latitude, each year. [illustration: fig. . in summer.] [illustration: fig. . in winter.] such are the _main_ phenomena, _at the surface_, in the trade-wind region. ascending a step higher in the atmosphere, we find, above the surface-trades, a counter-trade, running, not in the opposite direction, but at right angles, or nearly so. the counter-trade which issues from the northern side of the rainy belt, running to the n. w. or w. n. w., and the counter trade which issues from the southern side, running to the s. w. or w. s. w., varying, as the trades do in direction in different localities. these counter-trades are continuations of the surface trades, which, ascending in their course, have threaded their way through the opposite trade in the rainy belt, and are continuing on at the same angle, and in the same direction at which they blew upon the surface, and in obedience to the same law. this is apparent from several considerations. st. they issue at the same angle, and over the top of the surface trades. in the west indies and elsewhere, this has been ascertained and proved by the course of the storms, and the rotation of their surface winds, and observation. d. we can not suppose the n. e. trade to be reflected, and turn back over itself at a right angle. that would be impossible, even if there were a wall of solid material there for it to blow against. air is a peculiar fluid, and it stratifies with astonishing ease. he who supposes that a current of air put in motion can be turned aside by another current, or by the atmosphere at rest, or can be made to mingle, is mistaken. it will stratify, and force itself onward through the adjacent and opposing atmosphere, and in a right line. i have observed some remarkable instances of this character. d. the cause which operates to produce the surface trades, still operates upon the current to carry it over into the other hemisphere; a counter-trade, as we shall see. it is impossible, therefore, to believe that the surface-trades as they arrive at the belt of rains and calms, turn at a right angle, or at any angle, and return: and impossible to doubt that they pass through each other in this belt, and out at the opposite side, as upper currents, at the same angle at which they entered. of course the n. e. trade of the atlantic becomes the n. e. counter-trade of south america, carrying their storms in a s. w. direction, and the s. e. trade of the atlantic the s. e. counter-trade of the west indies, carrying all their storms in a n. w. direction; and what is true of them is true of the trade winds _every where, all over the globe, over the land and over the sea_. doubtless here some one will say, our upper current is a s. w. current. true, the s. e. trade which enters the belt of rains, and issues out on the north, a s. e. upper current or counter-trade, keeps that course until it arrives at the northern limit of the surface trade, when, in _obedience to another law_, which we shall notice, it gradually _decends near the surface, curves to the eastward_, and becomes _the s. w. current which passes over us_. and so we have the s. e. trade-wind of the south atlantic, with its moisture, warmth, electricity, and polarity, over, and perhaps sometimes around us, dropping the electric rain which makes glad our fields; giving us, when not prevented by other conditions, the balmy air of spring, the indian summer of autumn, and the mild mitigating changes of winter; and thus, _our rivers, which run into the sea, return to us again_. but let us go back to the trade-wind region--the region of regularity and uniformity--and examine somewhat more attentively its features, that we may more fully understand the character of this counter-trade. here are ° at least of the ° of the earth's surface, and at its largest diameter, covered in the course of the year, and of their travels, by the trade-winds at the surface, the counter-trades above, and the belt of rains and comparative calms, formed by the action of the opposite trades, as they thread their way through each other, to assume the relation of counter-trades. truly the magnitude, simplicity, and regularity of this machinery are most wonderful. there are, however, some _apparent_ anomalies which deserve attention. here are most distinctly marked the _rainy_ and _dry seasons_, existing side by side. here are the _rainless portions_ of the earth, already but briefly alluded to; here the _monsoons_, and another peculiarity, _viz._: the _gathering of the counter-trades_ upon the western sides of the two great oceans, into two _aerial currents of greater volume_, _analogous_ somewhat to the two _gulf streams_ of those oceans. let us examine these anomalies. the rainy and dry seasons depend, as we have seen, upon the transit north and south of the rainy belt, or belt of comparative calms. wherever this belt may happen on any given day to be situated, each side of it the trades prevail, it is dry, the earth is parched, and vegetation withers. these changes are graphically described by humboldt in his "views of nature," as they occur on the northern portions of south america, as follows: "when, beneath the vertical rays of the bright and cloudless sun of the tropics, the parched sward crumbles into dust, then the indurated soil cracks and bursts, as if rent asunder by some mighty earthquake. the hot and dusty earth forms a cloudy vail, which shrouds the heavens from view, and increases the stifling oppression of the atmosphere; while the east wind (_i. e._ trade-wind), when it blows over the long heated soil, instead of cooling, adds to the burning glow. "gradually, too, the pools of water, which had been protected from evaporation by the now seared foliage of the fan-palm, disappear. as in the icy north animals become torpid from cold, so here the crocodile and the boa-constrictor lie wrapped in unbroken sleep, deeply buried in the dried soil. every where the drought announces death, yet every where the thirsty wanderer is deluded by the phantom of a moving, undulating, watery surface, created by the deceptive play of the reflected rays of light (the mirage). a narrow stratum separates the ground from the distant palm-trees, which seem to hover aloft, owing to the contact of currents of air having different degrees of heat, and therefore of density. shrouded in dark clouds of dust, and tortured by hunger and burning thirst, oxen and horses scour the plain, the one belowing dismally, the other with outstretched necks snuffing the wind, in the endeavor to detect, by the moisture in the air, the vicinity of some pool of water not yet wholly evaporated. "even if the burning heat of day be succeeded by the cool freshness of the night, here always of equal length, the wearied ox and horse enjoy no repose. huge bats now attack the animals during sleep, and vampyre-like suck their blood; or, fastening on their backs, raise festering wounds, in which mosquitos, hippobosces, and a host of other stinging insects, burrow and nestle. such is the miserable existence of these poor animals, when the heat of the sun has absorbed the waters from the surface of the earth. "when, after a long drought, the genial season of rain arrives, the scene suddenly changes. the deep azure of the hitherto cloudless sky assumes a lighter hue. scarcely can the dark space in the constellation of the southern cross be distinguished at night. the mild phosphorescence of the magellanic clouds fades away. even the vertical stars of the constellations aquila and ophiuchus, shine with a flickering and less planetary light. like some distant mountain, a single cloud is seen rising perpendicularly on the southern horizon. misty vapors collect and gradually overspread the heavens, while distant thunder proclaims the approach of the vivifying rain. scarcely is the surface of the earth moistened, before the teeming steppe becomes covered with killingiæ, with the many-panicled paspalum, and a variety of grasses. excited by the power of light, the herbaceous mimosa unfolds its dormant, drooping leaves, hailing, as it were, the rising sun in chorus with the matin song of the birds, and the opening flowers of aquatics. horses and oxen, buoyant with life and enjoyment, roam over and crop the plains. the luxuriant grass hides the beautiful and spotted jaguar, who, lurking in safe concealment, and carefully measuring the extent of the leap, darts, like the asiatic tiger, with a cat-like bound on his passing prey." such is humboldt's description of the dry season on the orinoco, and the return of the belt of rains from the south. again, within this trade-wind region are the _rainless countries_. these are portions of the earth which the equatorial rainy belt does not ascend far enough north in summer to cover, nor does the southern edge of the extra-tropical regular rains descend, in winter, far enough south to cover them, and where, of course, rain seldom, if ever, falls. such are the central parts of the desert of sahara, egypt, arabia, portions of affghanistan, beloochistan, and the western parts of hindoostan, to the north of the inter-tropical belt, and a similar state of things exists south of the equator in parts of south america, africa, and new holland, although upon a comparatively small surface. again, another anomaly is the gathering of the trade winds into greater volumes, on the westerly side of the great oceans, and the consequent carrying of the equatorial rainy belt up to the region of extra-tropical rains, on the eastern side of the great continents of asia and north america, and the peculiar liability of these aerial gulfs to hurricanes and typhoons. such an aerial gulf gathers over the caribbean sea, and the west indies. passing across the gulf of mexico, it enters over texas, and louisiana, and the other southern states; its western edge passing north in autumn and winter, on the eastern side of the highlands of western texas, new mexico, and the great desert; curving, as all counter-trades do, to the eastward as soon as it passes the limit of the n. e. trades, and spreading out over our favored country, leaving the evidence of its pathway in the greater quantities of rain, which fall annually upon its surface. this gathering deprives a portion of the atlantic, north of the tropics, of its share of the counter trade, and there, as every where, where the volume of counter-trade is small, storms and gales are infrequent, and of less force, and comparative calms prevail. that portion of the atlantic has long been known as "the horse latitudes," a name given to it by our yankee sailors, because, there, in former times, the old-fashioned, low-decked, flat-bottomed, horse-carrying craft of new england, bound for the west indies, often floundered about in the calms and baffling winds, until their animals perished for want of water, and were thrown overboard. lieutenant maury, in his most praiseworthy and exceedingly useful investigation of "the winds and currents of the ocean," has defined the situation of these calms and baffling winds at different seasons--for they move up and down, of course, with the motion of the whole machinery--and enabled navigators to avoid them, by running _east_ before they attempt to make _southing_; and very materially shortened the voyages to the equator. a like gathering, in volume, of the s. e. trade, on the western side of the pacific, enters over asia, and covers china and malaysia, extending, in its western course, nearly as far as the western edge of hindoostan. in this concentrated volume of counter-trade, and owing to its concentrated action, form and float the typhoons of the china sea, and of the bay of bengal; and to this anomalous aerial gulf stream, the s. e. portions of asia, from the western desert of hindoostan, to the eastern portion of china, north of the rainy belt, owe their great supply of moisture and fertility, and their peculiar climate. the western line of this volume of counter-trade is marked by the eastern portion of the rainless region of beloochistan, and the north-western deserts of india, as the western edge of our concentrated volume of counter-trade, is marked by the arid plains of northern mexico, western texas, and new mexico. on the south of the equatorial rainy belt, there is no corresponding aerial gulf of equal volume, as there is no corresponding gulf stream of equal magnitude. on the western side of the indian ocean we find a gathering of the n. e. trades from the bay of bengal and the indian ocean, in which form and travel the hurricanes which prevail--traveling to the southward and westward--about the isle of france or mauritius; and the lagullus oceanic current, which runs down to the s. w. toward the cape of good hope. but the extension of south america to the eastward, under, or just south of the n. e. trades, does not permit the formation of such a concentrated volume on the western side of the atlantic, nor is the strength or regularity of the n. e. trades, on that ocean, equal to those of the s. e. nor is the magnetic intensity on the eastern and middle portions of the pacific, sufficient to produce such a concentration, in large volume, there. the trades over that ocean, therefore, curve without concentration, except a partial one, over the western groups of polynesia, which the asiatic line of magnetic intensity approaches and where hurricanes are sometimes found, until we arrive near the eastern line of magnetic intensity, on the eastern side of asia. we shall, hereafter, have occasion to follow the anomalous concentrated volumes of the s. e. counter-trade, of the northern tropic, on the western side of the great oceans, in explanation of some of the phenomena which we find north of the trade-wind region. suffice it here to add, that if it were not for the concentration of these counter-trades, on the western side of the great oceans, the rainless region between the parallels of ° and ° would encircle the earth; and china and the eastern united states would have a distinctly marked rainy and dry season, as have california, the barbary states, syria, persia, and other countries which lie north of the rainless region, within the summer range of the n. e. trades, but also within the winter descending range of the belt of extra-tropical rains. another anomaly which we find in the trade-wind region, is the monsoon. there are several of them, but they are found, in the greatest strength and regularity, in the indian ocean. another, defined by the investigations of maury, is found on the west coast of africa, extending out over the atlantic. another prevails on the western coast of south and central america. the etesian winds of the mediterranean are but the n. e. trades, whose northern limit is carried up in summer, by the transit of the connected machinery, to the north, over that sea. the n. e. and s. e. monsoons, so called, of the indian ocean, are but the regular trades, blowing when the belt of rains is absent, as they do all over the globe. the n. w. monsoon, south of the equator, in the vicinity of new holland; the s. w. monsoon which blows from the arabian sea, in upon hindoostan; the s. w. monsoon of the atlantic, south of the cape de verde islands; and the variable west monsoon winds of the west coast of southern and central america, and southern mexico (known under several different names, but chiefly by that of tapayaguas), are all that deserve attention as such. at first sight they appear to be anomalies, but the facts declare their character with perfect certainty. first, they are not continuous, like the trades, but _prevailing_ winds, and are _storm winds_; _they always blow toward a region_, _or portion of the ocean_, _covered at the time by clouds and falling weather_. second, they do not blow upon, or toward, heated surfaces of land or water--_i. e._, toward the dry and parched surfaces, where the dry season prevails, or from adjoining cold waters on to warm surfaces, but toward the land or water _situated under the rainy belt_. they are therefore incident storm winds, (as our easterly winds are incident storm winds) of the rain clouds of the tropics. they blow in upon the land, under the belt of rains, while that belt with its daily cloud, and inducing electric action, is over it, and follow that belt in its transit north and south. they blow from the warm south polar current of the atlantic, which flows n. w. from the coast of africa, toward the inshore north polar current, which is there flowing south, but under the belt of rains. in the indian ocean they blow from the center of that ocean, and the arabian sea, toward the belt which hangs over hindoostan, from the s. w.; and when the rainy belt travels south they still blow toward, and under it, from the indian ocean, but of course from the n. w. the heated character of the waters of the indian ocean and arabian sea, which receive no polar currents, but heated waters from the persian gulf, and from rivers which flow into the bay of bengal over the heated plains of a tropical country, explain this. so, too, the monsoon of the atlantic ocean, does not blow north of the cape de verde islands,--where the heated surface of sahara, burning with the rays of a vertical sun, has a temperature sometimes ranging from one hundred and forty to one hundred and sixty degrees--but remains under the rainy belt, drawn from the heated waters which flow up from the south atlantic, and travels north as the rainy belt travels north in summer, and south to the gulf of guinea, as that travels south in winter. the same is true of the pacific monsoon, the tapayaguas, the least marked of all, which blows in during the rainy season upon the west coast of southern mexico, and of southern and central america. they are all incident rain or storm winds, blowing in upon the land, or on to a colder surface of different polarity, _during the rainy season_; and if it were possible to catch one of our north-easters, in its passage over our country to the eastward, and anchor it to the alleghanies, "paying out" so to have it reach in part over the atlantic, and keep it there in operation six months, we should have a continual easterly wind under it; a _monsoon_ more strongly marked than the monsoons of the indian, or atlantic oceans. _the received theory in relation to them is a fallacy._ recapitulating, then, all the phenomena, we have,--_surface-trades_, blowing toward the center, passing through each other, and continuing on as upper or counter-trades; a _belt of rains_, with calms near the center, formed by the trades where they meet and pass through each other, which travels with them north and south following the sun; _two belts of drought_, following the belt of rains and the trades, and followed by the _extra_-tropical line of rains, as it travels with the trades and the rainy belt, leaving a part of the earth which the equatorial rainy belt does not travel far enough north, nor the extra-tropical line of rains far enough south to cover, and which is consequently a _rainless region_; _the monsoons_, which are but incidents of the rainy belt, and the _gathered volumes_ of counter-trade, on the west of the two great oceans, which usurp the place of the n. e. trades, carrying the rainy belt up to the region of extra-tropical rain, and preventing the rainless region from encircling the earth. upon _what cause_ do these great central phenomena, so vast, so regular, so wonderful, depend? what is the _motive power_ of this connected atmospheric machinery, whose action and influence extend over the entire globe? "_heat, heat_," say the text books, the professors, the votaries of meteorology. "all these phenomena are owing to the heat of the sun. it heats the ocean and the earth--the air is thereby heated and rises, the cold air rushes in from below, then the ascended current rolls off each way at the top toward the pole, acquiring a westerly motion from the rotation of the earth, slipping away from under it, and a different, _viz._: an easterly motion, after reaching the latitude of °, from the _same rotation_; and all the winds and disturbances of the atmosphere are produced in the same way. they are produced by the action of heated surfaces upon the adjacent atmosphere." this is the great theory of meteorologists, by which they attempt to account for the various atmospherical disturbances, of both tropical and extra-tropical regions. the whole theory is a fallacy--it will not stand the test of a careful examination. the bases of the theory, which are assumed to be facts, are not so. the agent has not the power claimed for it. a heated surface, alone, never caused any considerable ascending current, or if it did, never produced a mile of wind. i repeat it, the theory and all incidental ones--the thousand explanatory and modifying theories, and hypotheses--_the whole system_--is without foundation in fact, and will not bear a critical examination. let us see if this language is stronger than the facts will warrant. the theory assumes that both the land and water, under this central belt, where the air is supposed to be rising are _materially hotter_ than the land and ocean are on _either side of it_. now, how much hotter are the air and the land under the belt of rains and calms, upon hindoostan, or africa, or south america, where the former is supposed to be acquiring heat and expansion so rapidly, and to be ascending, than under, and in the dry belts on either side? none; it is cooler by the thermometer--_much cooler_. the central belt of rains in midsummer over africa, extends up as far as ° north latitude, and perhaps further. north of this line over the whole surface of the desert, the barbary states, a part of the mediterranean, and some portion of italy, the dry season extends, and from the entire surface the n. e. trade blow into the central belt.[ ] over the desert they all pass. now this desert is a sea of sand, under a vertical sun, intensely heated, blistering the skin with which it comes in contact, and often acquiring a temperature of ° to ° of fahrenheit. under the central belt of rains neither the earth nor air exceed the temperature of °. and yet the hot air of the desert does not ascend, but blows into this cooler central belt; and when it is felt as it blows off the western coast by the mariner, or even in guinea, when the belt of rains has gone south in winter, as it often is as the _harmattan_, it is suffocating and intolerable. there, then, not only is it untrue, that the land and the air over it under the rainy belt are hotter, but it is true that intensely heated air blows horizontally from the desert of sahara. nay, as it will appear in the sequel, this hottest of all surfaces not only can not have a vortex, but it can not induce a monsoon, and scarcely a sea breeze. the same is true in a great degree of the surface, and the air over it, on either side of the supposed vortex of the rainy belt upon south america. see the description of humboldt, already given, where the thermometer stood as high as ° of fahrenheit in the shade, while the n. e. winds, the regular trades, were blowing over the land. and it is equally true of arabia, and indeed of every portion of the earth. there is not a spot upon the globe where the land and the air are cooler _by the side_ of the central belt of rains, than _under it_. _and the opposite is true every where upon the land._ how much hotter is the ocean and air under this supposed vortex? but little hotter than they are on the side where the sun is not vertical, _and none on the other_. let us be a little more particular. the temperature of the atlantic under the belt of rains in our winter, and on the south of the belt at the latitude of ° south, and down to ° or more south, is °. the air may range a degree, or possibly two, higher than the water at either point. on the north this difference is from nothing at the meeting of the trades and belt of rains, to about ° at their northern limit. this is too _trifling_ to be worth one moment's consideration. it is less, far less than the difference between the water and air of the gulf stream which runs along our coast, and the adjoining waters and air over them. while on the south side of the belt of rains the _difference is actually against the theory_--and the same state of things is reversed in summer, when the sun is vertical at the north. from the log of an intelligent shipmaster, found in the wind and current charts of lieutenant maury, i abridge the following, which will illustrate this. captain young in february, found the n. e. trades at about ° north latitude, with the water at ° and air at °, trade-wind n. e. at ° ' the water was ° the air ° wind n. e. feb. d. ° ' " - / ° " ° " n. e. " d. ° ' " ° " ° " n. e. " th. no obs. " - / ° " ° " n. e., e. s. e. rain. " th. ° ' " ° " ° " e. s. e. rain. " th. no obs. " ° " ° " s. e. to e. s. e. hazy, rain & sqs. " th. ° ' " ° " ° " calm, with rain. " th. no obs. " ° " ° " calm rain. march st. ° ' " ° " ° " e. s. e. sqs. rain. " d. ° ' s. l. " ° " ° " s. e. sqs. rain. " d. ° ' " ° " ° " s. e. & s. s. e. weather settled. " th. ° ' " ° " ° " s. s. e. & s. e. fair weather. " th. ° ' " ° " ° " s. e. fair wthr. " th. ° ' " ° " ° " s. e. & e. s. e. fair weather. here the air was seven degrees colder at the extreme limit of the n. e. trades than in the _center_ of the belt of rains, as it is, usually, in mid-winter, but not in summer. on the other hand, _after he left the region of calms and rains_, where the water and air stood with almost entire uniformity at °, on the d of march, and for three days thereafter, during which he was in the s. e. trades with fair weather, the water was the same as under the supposed vortex, _viz._, °, _and the air rose to ° and °_! _this is demonstration._ i also take from a letter of lieutenant walsh to lieutenant maury, relative to the cruise of the "taney" the following, showing the warmth of the gulf stream compared with the adjacent ocean. "we first crossed the gulf stream on the st of october; we struck it in latitude ° ', longitude ° ' as indicated by the temperature of the water, which was as follows: a.m. water at surface ° " " " ° " " " ° " " " ° ° was the highest temperature found in crossing at this time. re-crossing it in may, in latitude ° ', longitude ° ', he found the water as follows: a.m. water at surface ° ' " " " ° " " " ° ' " " " ° ' m. " " ° ' ° being the highest temperature found." the average difference between the temperature of the water of the gulf stream and the adjoining ocean, at the line of division, is about ten degrees, increasing to more than twenty on approaching the coast, and within one hundred miles--a far greater difference than is ever found on the winter side of the inter-tropical rainy belt. it is not only not so, then, that the surface of the ocean is materially warmer under the belt of rains than the adjoining surface under the trades, especially on the summer side, but if it were so, the trades would not be created thereby, any more than upon the gulf stream. and the opposite is true of the land where the line of calms, and rains, and drought meet, all around the globe. the fact assumed is therefore untrue. the hottest surfaces, even at the rainless portion, where there is no vortex, no storm, and no wind but the continual uniform n. e. horizontal trade-wind, _never_ created, by reason of the heat alone, a mile of wind, a storm or shower. but, again, the belt of calms, where the air is supposed to rise and create a suction which draws the trades on either side a distance of from one thousand to two thousand miles, an average of three thousand miles in all, at least, is not itself, on an average, over five hundred miles in breadth from north to south. what a wonder of meteorology is here! with a breadth of five hundred miles, the rising of the atmosphere is supposed to be so rapid and of such immense volume that it draws the surface atmosphere, one thousand to fifteen hundred miles on one side and two thousand on the other, with a uniform steady velocity of twenty miles per hour. is this vast suction found by the unlucky mariner who may be drawn within the vortex? _not at all._ he finds no rapid suction there, but _horizontal currents_, not steady, indeed, like the trades, and sometimes calms _at the center_, but still the _currents are there_, and, _except near the center, there as squalls, showers, and baffling winds and as monsoons_. again, is there at the mouth of this vortex, or as you approach it, an increased rapidity in the trade corresponding to the magnitude of its influence? does the trade become a hurricane as it approaches the spot where it is to supply the place of that which has suddenly "expanded by heat, and been forced to rise, boil over, and run off at the top in turn?" not at all. it blows gently, even up to the very line of the rainy belt, and becomes squally and baffling, falls gradually calm near the center, or changes to a monsoon. but, again, the belt of rains is so far from being a belt of calms strictly, that its monsoons in the indian, atlantic, and pacific oceans, at times, extend hundreds of miles out over the ocean. that of the atlantic, triangular, with its base resting on africa, according to lieutenant maury, extends sometimes almost to the coast of south america, a distance of one thousand miles, and thus under the supposed ascending vortex. where is the great uprising suction during the prevalence of this extensive surface horizontal monsoon beneath it? manifestly it does not exist. nay, that monsoon is blowing from the warm current which sets up from the cape of good hope toward the caribbean sea, and over the cold north polar current, which runs down between the continent and the cape de verdes. equally untrue is the presumption that the air rises over heated portions of the earth elsewhere, and by reason of such heating. _perpendicular currents of the atmosphere are rarely seen, never extensive, or attaining any considerable altitude._ i have watched for them thirty years. i have seen currents of air ascend, with their moisture condensing as they ascended, and unite with the under surface of a highly electrified cloud--the advance condensation of a thunder shower--but that cloud was moving horizontally at a distance of from one to two thousand feet above the surface of the earth, and did not rise. i have seen patches of scud rising from the surface during the intervals of a showery and highly electrified storm, toward, and uniting with, the clouds above, when very low, as i have seen them approach and unite horizontally; and doubtless there is a tendency upwards of the wind, created and attracted by the summer shower, as may be seen in the ascending dust before the rain, but i have never been able to detect an ascending current, except as induced and attracted by a cloud above moving horizontally, in the hottest day or dryest time. none of the clouds of our climate, even when the earth is heated and parched by a two months' unbroken drought, can be detected rising above the strata in which they form. i have watched the cumuli at such periods when they filled the air, and can assert that they never rise. the atmosphere moves, invariably, in horizontal strata, and the whole theory of ascending currents is fallacious. but let us look still further at the tropical currents. the true harmattan of north-western africa (for the term is sometimes misapplied), hot and blistering, generated upon the sand of the desert--why does it blow from sahara horizontally, on or over cooler surfaces, following the belt of rains as a n. e. trade? why does it not ascend? the sirocco of north sahara, the kamsin or chamsin of eastern sahara, and the simoon of arabia, which blow hot and suffocating from those deserts--why do they blow _from_ heated surfaces and _horizontally over_ cooler ones? why do they not ascend? arabia is surrounded on three sides by seas and gulfs, from which evaporation is rapid. her interior deserts are extensive and intensely hot--why are they rainless? why do they not have a _vortex_, a _monsoon_, or even a _shower_? because there is no such law or action as this theory supposes. those winds blow horizontally in obedience to other laws, and under the control of other and more powerful agents. but further still, what heating and ascending process is it that makes the variable winds north of the tropics? that brings in the warm air and fog of the gulf stream upon our _snow-clad coast_, in mid-winter, to increase the january thaw? nay, what heating process is it that disturbs the calms of the polar regions with fresh breezes and gales, sometimes of the force of , when the _sun does not shine_, the thermometer is from ° to ° below zero, the _earth and sea one frozen surface_, and the hardy explorer dressed in furs, barely lives in his cabin covered by an embankment of snow, and heated by a stove? gentlemen, meteorologists, it will not do. the theory is unsound; the assumed facts do not exist. the whole universe has not an agent, organic or inorganic, which can play such absurd and inconsistent pranks in the face of its creator, as your various and complicated theories assign to caloric. away with the theory and all its incidental and complicated and mystified hypotheses, they rest like a pall upon the science;--away with the whole system, and let us seek some agent whose _power_ and _adaptation_ correspond with the _extent_, and _simplicity_, and _magnificence_ of the phenomena, and, in some degree, with the _power_ and _wisdom of their author_. chapter v. one, and the principal end attained by the power of the agent, is the gathering of a volume of atmosphere from, or near, the _surface_ of the land and sea, so as to ensure its possession of all the moisture of evaporation which rises from the locality, and the highest degree of temperature, and from a space ranging from one to two thousand miles in width, in one hemisphere, and to carry it over into the other. not over the top, or upon the top, of the whole mass of atmosphere situated in the opposite hemisphere--_out of reach of all influences from the earth_--but through it, and curving gradually down near to, and within influential distance of the surface of the earth, soon after it passes the outward limit of its fellow trade; and to continue the current onward, leaving portions of it and its heat and moisture on the way, but taking a considerable volume up and around the magnetic poles--it being impossible for the entire volume to be thus carried around the poles in consequence of the diminished circumference of the earth. to this end it is obvious it must possess _polarity_. another end to be attained is to combine the moisture of evaporation with the air, so that the cold atmosphere through which, or the earth over which it passes, may not be _continually condensing its moisture_, and thereby _enveloping the earth in a perpetual mist_; but so that it may part with it at _intervals_, making _cloudy_ and _clear days_; and part with it in _portions_, so that a _regular_ and _necessary supply_ may be furnished to the _entire hemisphere_, even up to the geographical poles. is there such an agent? there is, precisely and perfectly adapted to the ends to be attained, ever there and ever active, and that agent is _magnetism_. [illustration: fig. .] the earth is a magnet. it has its magnetic poles, and they are distinct from its geographical ones; and there are two in each hemisphere. they are situated from ° to ° distant from the geographical poles; and ours is not far from longitude ° w. from greenwich, and ° north latitude. navigators have gone north and north-west of it, and found its situation by the declination of the needle. from these poles, lines of magnetic intensity extend to the opposite and corresponding pole of the other hemisphere, and upon or near those lines the needle points north without variation; and toward these lines of no variation the needle every where, on either side declines. the foregoing diagram shows the situation of our magnetic pole and line of no variation, the dip of the needle by the arrows, and the magnetic equator. recent discoveries have shown that the magnetic force is exerted in lines and currents; that such currents, as physical lines of force, surround magnets, and currents of electricity. doubtless such lines of force exist around the earth and the magnetic poles. there are also _longitudinal_ lines of force existing and active, between the poles, and extending from one side of the center to the other, occupying nearly one third of the magnet. if you take a large needle thoroughly magnetized, place it upon paper and drop filings of iron upon it, they will become arranged about it in circular and perpendicular, and also in _longitudinal lines_, conforming to the currents. [illustration: fig. .] this experiment is illustrated in all our books on natural philosophy. the foregoing diagram, copied from olmstead's philosophy, does not show as accurately as faraday's projection of the lines upon a globe-magnet the comparative distance from the poles of the needle, at which the longitudinal currents commence and terminate, and _where the filings will not adhere_ to any considerable extent. the lines shown upon the needle should bear the same proportion to its length as the trade-winds bear to that of the earth, measured from pole to pole, and if the needle had a globular form they would so appear. these lines are made by currents arising from one side of the magnetic equator, and passing over to the other. doubtless, just such currents rise, and pass over upon the earth. magnetic and electric currents carry the air with them. this is well settled by experiment. _oxygen_, too, is _magnetic_, and capable both of receiving and retaining polarity and of combining with, or attracting and retaining vapor, and of course the moisture of evaporation. here then we have a power existing, capable of producing the result--precisely, and with evident wisdom adapted to its production--ever present and active; and no other known agent can. is it not then the agent? let us look a little further. this result is affected by the action of the sun: the trades with the central belts of rains travel north and south after it; so does the sun affect the magnetic currents every where, even the magnetic needle is daily affected by its action, as it increases the intensity of the terrestrial magnetic currents, and hence its well established diurnal oscillations. again, along the eastern lines of the continents which skirt the great oceans on the west, run the northerly and southerly lines of no variation, and of greatest magnetic intensity. here are the trade currents gathered into a volume, which curve and carry unusual fertility to south-eastern asia, and north america, and in those great aerial gulf streams we find the _intense_ electric action which produces the typhoons of the former, and the hurricanes of the latter. it may still be said that these conditions and phenomena of the trade-wind region, are not produced by magnetism or magneto-electricity, _but the objector can point to no other adequate power_. that it must be heat, electricity, or magnetism, must be admitted. there is no other power known. heat demonstrably can not produce them. magnetism or electricity therefore must, and they are doubtless states or phases of the same power, producing in their different states or phases the different results. and even heat--atmospheric temperature, is often, if not always the result of their action. in the present state of science, it is enough for me that the _magnetic longitudinal currents are there_; that they are _lines of force_ and _adequate_; that _oxygen is magnetic_, and therefore the atmosphere must be affected by them--that so far as we can reason from analogy, they ought to produce the effect upon the atmosphere which we find produced, and until further light is thrown upon the subject i shall presume that they do. every step we take hereafter in this investigation will confirm the presumption. there is one peculiarity to be more particularly noticed before we leave the trade-wind region, and we are now prepared to notice it. the belt of rains, formed by the currents of the two trades, threading their way through each other--how are they produced? why should the place where the currents thus pass through each other be a place of almost daily precipitation? there is, in fact, no ascension, except that which the currents have in their line of ascent to attain the elevation which the magnetic law of the current requires. the trades have passed over an evaporating surface and are charged with moisture. this moisture they hold in magneto-electric combination. _evaporation_ does not depend upon _temperature_. ice and snow evaporate at all temperatures (howard, vol. , p. ). so the cold n. w. wind, full of positive electricity, will lap up, as it were, the pools from the earth, with astonishing quickness; and when this electricity is deranging the action of the machinery and material of the manufacturer, he allays it by a supply of moisture, with which the electricity can combine. nor does the air lose its moisture when below the freezing point. in all parts of the atmosphere, as at the surface of the earth in winter, moisture is held in large quantities in the coldest and severest weather; and it is not till it moderates, and a perceptible _electric_ change takes place, that it is precipitated as rain or snow. doubtless there is an exposure of considerable surfaces, of opposite currents, charged with opposite polarity, and a constant depolarization where their surfaces meet. may there not be a consequent dissolution of the electro-magnetic combination between the air and moisture, or the excitation of that electric action which attends or produces like rains every where? and hence the constant precipitation. this is rendered probable, by the fact that precipitation, at the meeting of the trades, takes place in level countries in the day-time, between a. m. and sunset, in showers, with thunder and lightning, as with us in summer, although among the mountains the rain sometimes falls in the night also. the precipitation in the heat of the day is obviously induced by the action of the sun, although it is by no means certain that the friction of the opposing surfaces does not assist in the operation. i am well aware that the lines of magnetic force curve upward and carry the trades with them, and that, therefore, precipitation by condensation from the mere cold of the upper stratum of the atmosphere is possible. but, there are three reasons why i do not believe such to be the fact. st. precipitation takes place in the day time mainly, and in sudden, isolated, heavy showers and not in steady continuous rain. nor is there condensation or continual mist at other hours of the day. d. they occur at a time of day when the sun is affecting the magnetic currents most powerfully, _viz._, between ten o'clock a. m. and sunset, and mainly at the time of greatest heat. d. the counter-trades _do not precipitate_ after they leave the rainy belt, although at a great elevation, until they reach the outward limits of the trades; and they _do precipitate again_, although they gradually descend _nearer the earth_, as soon as they become subject to the action of the currents of an opposite magnetism. their precipitation is partial too, even then, and they carry a portion of their moisture through an atmosphere of the coldest temperature up to the geographical poles. a similar result attends the action of the sun in the extra-tropical regions. cumuli commence forming in the counter-trade, or at the line between that and the surface current, at the same time of day that the diurnal motion of the magnetic needle commences, or the rain clouds form in the tropics; they continue to enlarge here as there, till about the same hour of the day that the _needle_ obtains its maximum diurnal variations; and when the influence of the sun upon the needle ceases, and it returns to its original status, the cumuli disappear. hail storms too, it is said, always, or generally occur in the day time. in like manner the sea-breezes and other fair-weather surface winds, rise in the forenoon with the influence of the sun upon the magnetic currents and the needle, and die away at nightfall when the influence ceases. there are other electro-magnetic, or to speak more correctly, magneto-electric, effects of the sun's action equally illustrative, which tend to show that the precipitation at the passing of the trades, is the result of their action upon each other, aided by the sun, to which we shall allude when we come to speak of the causes and character of the surface winds of the extra-tropical regions. as, however, this takes place only, or mainly, where the threading surfaces meet, it is but partial, and the body of the respective polarized currents pursue their way unaffected, toward the opposite magnetic pole--and there for the present we leave them. storms sometimes originate in these currents, when concentrated, as in the west indies, the china sea, the bay of bengal, and indian ocean, while passing through the rainy belt, and move with the current to the north-west if issuing on the north side of it, and to the south-west if issuing on the south side of it, until they respectively get beyond the extreme limits of the trades, and then they curve to the eastward, imbedded in and following their current. the peculiar extension of the land to the east on the northern portions of south america, prevents the gathering of an aerial gulf similar to the one which we have described to the north-west, entering upon our division of the continent over the gulf of mexico. it is otherwise in the indian ocean, and there the storms are found issuing from the rainy belt on the southern side, sweeping over the mauritius and other islands of that ocean, and _often simultaneously_ with storms issuing on the north over the bay of bengal. colonel reid mentions instances and gives a diagram.[ ] these storms in milder forms issue from the rain belt at other points, and may issue any where, but will always be found most extensive and most violent, that is to say, as hurricanes and typhoons, in the concentrated volumes of counter-trade on the western side of the great oceans, within a few hundred miles of the lines of magnetic intensity and no variation, and when they form in the rainy belt they are highly electric. most frequently, however, as we shall see, they form in these currents after they have issued from the rainy belt, and after they have passed the extreme limits of the trades and become subject to the circular and perpendicular magnetic currents which exist north and south of the longitudinal ones, and which when seen upon the magnetic needle, attract the filings and cause them to adhere--although but slight attraction or adhesion takes place where the longitudinal currents exist. such, then, are the atmospheric arrangements and phenomena of the trade-wind region, and the cause that produces them; such is the character and cause of the enlarged volume of counter-trade, which spreads out and blows over our country as permanently as the s. e. trades blow on the south atlantic and south america, returning to us the rivers which had run from us to the sea. chapter vi. coming back now, to a consideration of the course and functions of the counter-trade after it leaves the northern limit of the surface-trades, we find it curves to the eastward and gradually assumes about an e. n. e. course, and becomes a w. s. w. current where it crosses the line of no variation, and continues on until it passes off over the atlantic; and this course and curve is analogous to what may be found true of the counter-trades every where. it is best illustrated by the course of all the storms (in the american sense of the word, as distinguished from thunder showers and other brief rains), which have been traced north or south of the limits of the trades. it was found by mr. redfield in most of the storms investigated by him, which originated within, or north of the tropics. doubtless it was the actual course of the others, and that the investigation was imperfect. all the great autumnal, winter, or spring storms which have traversed the whole or any considerable portion of the territory of the united states, east of new mexico, which have been investigated by professors espy, loomis, redfield, or others, have been found to follow this course. a storm which passed over madeira, appears from the investigations of colonel reid to have followed the same law of curvature. and so, doubtless, did another which he has described as passing over the levant. the storms which supply the winter rains of california and utah, reach them by this law of curvature and progress, after the northern limits of the trades have descended to the south with the sun, so that the counter-trades of the pacific may descend to the surface and curve in upon them. but the absence of a concentration of the counter-trade, and its deficient action because of its passage over mountain ranges, and their location so near the northern limit of the trades that their storms can not expand and become extensive, as well as their weaker magnetic intensity, prevent their storms from becoming violent, and their supply of rain is not large and much of it falls in showers. the same is true of the barbary states, of syria, and persia, and of southern europe; and indeed of all the countries of the globe which lie between the winter and summer extreme limits of the surface-trades, and without the limits of the two concentrated counter-trades. enough appears in the writings of the meteorologists of europe to show, that their long continued rains, which are analogous to our storms and are _preceded by the formation of the true cirrus of the counter-trade_, follow the same great law of curvature and progress; although the presence of the gulf stream with its mass of south polar waters on the western side of the british islands, denmark, and norway supplies them with showers, and fogs, and cumuli from the west and north-west, and makes the mean of the surface winds of their storms somewhat variant from ours. a like law reversed prevails in the southern hemisphere. the storms of new holland and the indian ocean, south of the limits of the trade, curve to the eastward and travel about south-east, their _south-west_ being a _clearing off wind_ as our _north-west_ is, and _precisely similar in all its other characteristics_, where the relation of magnetic intensity is the same. the storms of the pacific on the s. w. coast of south america, in like manner travel to the s. e., flooding the western slopes of the mountain ranges with rain, and aggravated by the intensity of the magnetic currents at the extremity of the continent in a high latitude, meet the mariner in the face as he emerges from under the lee of the land and attempts to pass the horn. it will ultimately be shown that the precipitation which takes place, as the storms and counter-trades pass north and east in the northern hemisphere and south and east in the southern hemisphere, is owing less to cold than increased magnetic intensity. and all this is the result of one great uniform law, existing every where, varying in its phenomena only in consequence of the difference in volume, and magneto-electric intensity of the portions of the counter-trade, as of the surface-trade at different places, and the different magnetic intensity of the local perpendicular and circular currents of the earth over which they pass, at different periods and at different points. mr. redfield and lieutenant maury have assumed that our s. w. current comes from the pacific ocean. aside from the adverse evidence which the investigations of the former in relation to the course of the west indian storms, and their curving over the continent, furnish to the contrary, and that which has herein before been stated in relation to the law of curvature, it is obvious they are mistaken, for another and conclusive reason. in order to reach us from the pacific in a direction from s. w. to n. e., it must pass the table lands and mountain ranges of mexico and new mexico, and it would supply them bountifully, even if it did not thereby leave us comparatively rainless and sterile. every where currents passing from the ocean _over mountain ranges_ part with a large share of their moisture. thus the counter-trade which curves over the andes and over peru, is deprived of its moisture and leaves the western coast rainless. so in degree of the counter-trade which curves over the himalaya and kuenlon mountains, and from there passes over the desert of cobi, to the north and east--it is deprived by those elevated ranges of its moisture. so the mountains on the south-western coast of south america are drenched with rain, while patagonia, which lies on the east of them is comparatively dry. and so of every other country similarly situated. now the mountain ranges and table lands of mexico are not thus supplied with moisture. for the space of four months in southern and less in northern mexico, and in summer, and while the belt of the tropics is extended up over them, they have rain and in daily showers which _travel up from the south_, indicating the course of the counter-trade. (see bartlett's personal narrative, vol. ii. p. .) at other seasons, and while we are bountifully supplied, they are dry. in short, there are no two portions of the earth that differ more widely in regard to their supply of moisture, and all their climatic characteristics and relations. it is therefore, according to all analogy, impossible that our counter-trade should come from the south pacific across the continent and below °, and in this also those gentlemen are mistaken. messrs. espy and redfield recognizing the existence of "a prevailing" s. w. current, but considering the surface-winds beneath it as the principal actors in producing the atmospherical conditions and changes, have attributed no office to that current, except that of giving direction and progression to our storms. this is their great mistake. it plays no such unimportant part in the philosophy of the weather, as we have already incidentally seen, and will proceed still further to consider. _all our storms originate in it._ this we may know from analogy. _where there is no counter-trade, outside of the equatorial belt of rains, and within influential distance of the earth, there are neither storms nor rain._ so, when, as we have seen, the concentration of the volume of northern counter-trade in the west indies, gathered by the hauling of the s. e. trades more from the east, as they approach the central belt, diminishing the volume of the counter-trade over the north atlantic, the calms and drought of the horse-latitudes are found. and when the counter-trade is small in volume and weak in intensity, by reason of the fact that the surface-trades from the opposite hemisphere which constitute it, formed upon land where evaporation was small, as upon southern africa and new holland, or formed where the magnetic intensity was weak, or passed over mountain ranges in their course, the annual supply of rain, the ranges of the barometer, and the alternations of atmospheres conditions are remarkably less. we have already seen where the rainless portions of the earth are, and why they are so; because those lying north of the northern limit of the equatorial rainy belt were yet too far south to be covered by the line of extra-tropical rains; or in other words, too far south to be uncovered by the surface n. e. trades and the longitudinal magnetic currents, and to be covered by the counter-trades in contact, or nearly so with the earth, and influenced by the perpendicular north polar magnetic currents. thus we have seen that the rains of southern mexico were summer rains, due to the northern extension of the equatorial rainy belt; those of california were winter rains, due to the southern extension of the extra-tropical rains following the n. e. surface trades. we have also briefly alluded to the fact that either side of the equatorial rainy belt, evaporation is going on for months under a vertical sun, without precipitation--unless it be from an occasional brief storm of great intensity which originates in that belt at the line of it, and passing on in the counter-trade, reverses, for the time being, by its concentrated and powerful action, like a magnetic body introduced into the field of another magnet, the surface-trades. mere evaporation then, does not produce the storm, or shower, or rain, where most active in the dry torrid zone. it may be said that those dry portions are, for the time being (as the rainless portions of the earth are continually), within the operation of the surface-trades, and that therefore the evaporated moisture is carried away by them toward the equatorial rainy belt. precisely so; but why carried away? why should it not condense, occasionally, at least, and drop the rain as it passes along, if a great supply of moisture from excessive evaporation could furnish rain. perhaps it may still be said it is going from a cold to a warm section. this is not true, as we have shown. but, it may be said that the rainless regions at any rate receive no moisture, and therefore can not supply any by evaporation. this would not meet the case, as it would still be true that when the rainy belt has left a given spot, the dry weather sets in with excessive evaporation, and the north-east trades in summer, blowing from the countries lying north of the rainless regions, and which have been supplied during the interval by the extra-tropical rains, and are loaded with evaporation, are passing over the rainless regions on their way to enter the central belt. so blow the n. e. trades from the mediterranean, and the barbary states _over the desert of sahara_ and into the rainy belt south of it; but drop no moisture on their way, because exposed to no magnetic currents of an opposite polarity. but it is not true that all the rainless regions are without evaporation. egypt is an exception. the annual freshets of the nile saturate its central valley, and vast reservoirs of water are saved from it and let out over its surface, and it all evaporates, but produces no rain. and so are large quantities turned aside and scattered over the bottom lands of northern mexico, and other countries, during the dry season, and their evaporation furnishes no rain. hygrometers and dew points are of no consequence there--nor are they of any, on either side of the rainy belt, where six perpendicular feet of moisture is evaporated in six months. again we have alluded to a strip of coast on the pacific west of the mountain ranges of south america, lying partly in peru, partly in bolivia, and partly in northern chili, which, although long and narrow, washed by the broad pacific ocean, is without rain. south america has no other _wholly_ rainless region, so far as is known. a part of this region would lie between the equatorial belt of rain, and the southern extra-tropical one, and never be covered by either; but the volume of n. e. trades from the atlantic, although from the make of the land not concentrated to so great an extent as the volume of s. e. trade on the north, and therefore not so liable to hurricanes and other violent storms, is yet sufficiently so to carry the southern line of the equinoctial rainy belt down in winter to the summer line of extra-tropical rains, and give a supply of rain to all the continent--leaving no strictly rainless region south of the equatorial rainy belt and east of the andes. those mountains, however, present a barrier to its south-western progress which it doubtless passes to some extent, but deprived of its moisture, and unable to supply the rainless coast region of peru, bolivia, and northern chili. there is, therefore, a portion of this rainless line of coast which is within the region of extra-tropical rains, over which a portion of the n. e. trades of the atlantic, as a counter-trade, should or do, curve, and where there should therefore be extra-tropical rains. it is washed by the pacific, an evaporating surface, and westerly and south-west breezes are drawn in from that ocean over it. why then is it rainless? the only reason which can be assigned why rain does not fall there is that the high mountain ranges of the andes intercept and perhaps in part divert the counter-trade, and deprive that portion of it which passes them, of its moisture, by that reciprocal action of opposite polarities which takes place whenever and wherever the trade approaches so near the earth; and it curves over the narrow line of coast with the feeble condensation, and imperfect forms, and varied coloring which mark so peculiarly the rainless clouds of that region. (see stewart's journal of a voyage to the sandwich islands, page .) again, it is estimated, and on reliable data, that twelve perpendicular feet of water are annually evaporated from the surface of the red sea, between nubia on one side, and arabia on the other; yet they are both rainless countries, except so far as the inter-tropical belt of rains extends up on to a small portion of them. the moisture of evaporation, floated up from a surface covered by the surface-trade is invariably so combined as to remain uncondensed till it has passed south into the equatorial rainy belt, and over to the opposite hemisphere, and been exposed to the currents of an opposite magnetism. again, the n. e. trades extended up in summer over the mediterranean sea, an evaporating surface, blow over the barbary states in june and july, but furnish no rain. and so of the s. e. or n. e. trades which blow over brazil and other countries in the absence north or south of the tropical belt of rains. it is obvious from these facts--and more like them might be cited--that mere evaporation, however copious or long continued, does not make the storm or shower in the locality where it takes place, and _without the existence and influential agency_ of a counter-trade; and that _reciprocal action_, whatever it may be, that takes place _between it and the earth_. again, our own experience is conclusive of this. we have no surface-trade north of °, and yet a long drought and great evaporation may follow a wet spring. belts of droughts and frequent rains occur every year in different portions of the country side by side, and _the dividing line follows the course of the counter-trade_, and is sometimes distinctly marked for weeks. when a change occurs in the counter-trade, whether from causes existing there or the influence of terrestrial magnetism (in relation to which we shall inquire hereafter), showers form or storms come on: until it does they will not. efforts at condensation will occasionally appear, but they will be feeble and ineffectual, and occasion a repetition of the axiom that "all signs fail in a drought." and we may know it from direct observation. the first indications of a storm, and of most if not all showers, are observable in the counter-trade. these indications, so far as they are visible, are of course to be looked for in the west; although the direction and character of the surface-winds are often indicative of these changes when not visible at the west as we shall see. the indications are those of condensation, and vary very much in different seasons of the year. it is not my purpose in this place to examine them particularly. they will be alluded to hereafter under the head of prognostics. suffice it now to say, then, that whether it be the long threads or lines of cirrus which occur in the trade in the winter after a period of severe cold, following the interposition of a large volume of n. w. cold air and the elevation of the counter-trade; or the forms of cirrus which occur at other times and other seasons; or whether it be the ordinary bank at night-fall, or the evening condensation which makes the "circle" around the moon, or the morning cirro-stratus haze which gradually thickens, passes over and obscures the sun, all which may be followed by the easterly scud and winds: they are alike condensation in the trade, the advance or forming condensation of a storm or showers. the state of the weather, whether hot or cold, is extensively affected by this trade current. as we have already suggested, the mere presence of the sun in its summer solstice, or its absence in winter, is not an adequate cause of all the sudden and various changes to which we are subject. the state of the counter-trade, which is always over, or _within influential distance of us_, and sometimes probably in contact with us--the nature of the surface-winds which it is at any given time creating and attracting around us, and the electric condition of the surface-atmosphere _induced_ by it, or by the immediate action of the earth's magnetism, produce those sudden changes which mark our climate. when no intervening surface-winds elevate it above us, and there is no storm or other condensation within influential distance, it induces the gentle balmy s. w. wind of spring--the cooling s. w. wind of summer--the peculiar indian summer air of autumn, or the comparatively moderate, although cold, open weather of winter. if there be a partial tendency to condensation in it, the cumuli form under the magnetic influence excited by the sunbeams from ten to three o'clock in the day, and float gently away to the eastward, disappearing before night-fall. if the disposition to condensation is stronger, whether inherent or induced by an increased local activity of terrestrial magnetism, these cumuli will increase toward night-fall, or earlier, and terminate me showers; and if it is in a highly electrical state, the still oppressive sultriness which precedes the tornado, and that devastating scourge may appear. if this disposition to condensation becomes extensive, cirri form and run into cirro-stratus, or they extend, coalesce, and form stratus; the surface-wind will be attracted under them, the thermometer fall in summer or rise in winter, and a storm begin. intense action and sudden cold may exist in and under this counter-trade over the southern portion of the country, while all is calm, warm, and balmy at the north. heavy snow storms sometimes pass at the south when there are none at the north, and a corresponding state of the weather follows. if a large body of snow fall at the north, the winter is cold, regular, and "old fashioned;" if little snow falls at the north and more at the south, the winter at the north is open and broken. i have known the ice make several inches thick at baltimore and washington, when none could be obtained for the ice-houses on the connecticut shore of long island sound. in short, although heat and cold are mainly dependent upon the altitude of the sun, aided by the other arrangements we have alluded to, yet the counter-trade, and the reciprocal action which takes place between it and the earth, are most powerful agents, mitigating the rigors of winter, bringing about the changes from cold to warm weather which the sun is too far south to produce. and on the other hand, by this reciprocal action, producing the electrical phenomena, the gusts, the tornadoes, the hail storms, and the cool seasons of summer, and the period of intense cold in winter. _all our surface-winds, except the light, peculiar w. s. w. wind which is felt where the counter-trade is in contact with the earth, and which is a part of it, and perhaps the genuine n. w. wind which is very peculiar, are incidents of the trade, and are due to its conditions and attractions._ we have already said this was true of the easterly wind and scud of a storm--it is alike true of all. the storm winds east of the alleghanies are usually, though not always, from the eastward. they are sometimes from the southward, as they doubtless are still more frequently in the interior of the continent. there is occasionally a southerly afternoon wind, followed by short rains in spring and fall, or a succession of showers in summer, which is rather a precedent wind than a storm wind; blowing toward and under an advance portion of the storm at the north, and hauling to the eastward when the rain sets in, or to the westward when the showers reach us. when there are no storms, or showers, or inducing electric action in the counter-trade, within influential distance to disturb the surface atmosphere, it is calm. if a storm approaches, or forms within inducing distance, the surface atmosphere is _affected_ and _attracted toward the storm_, from one or more points, and "blows," as we say, toward and under it. it commences blowing first nearest the storm, and extends as the storm travels, or becomes more intense and extends its inducing influence. i have repeatedly noticed this in traveling on steamboats and railroads running _toward_ or _from_, and in several instances _through_ a storm, and telegraphic notices and other investigations prove it. the point from which the surface atmosphere is attracted and blows, depends very much upon the position of the storm in relation to bodies of water and the point of observation, and its shape; and the force with which it may blow will depend much upon its intensity. let us take an instance or two by way of illustration of all these points; and as i have given instances of summer in the introduction, we will take those of winter. it is january of an "old fashioned winter;" the snow is about three feet deep in canada, about one foot in southern new york, and a few inches in philadelphia, and so extends west to the alleghanies at least. for several days the sky has been clear, the thermometer rising in the day-time, in the vicinity of new york to about ° fahrenheit, falling at night to about °, with light airs from the n. w. during the middle and latter part of the day; the counter-trade and the barometer both running high; cold but pleasant, steady, winter weather. there is a warm south-east rain and thaw coming, as one or more such almost invariably occur in january. how coming? the sun is far south, and shines aslant, but through a pure and windless atmosphere; he has tried for several days to melt the snow from the roof; a few icicles are pendant from the eaves; but the body of the snow is still there. how can a thaw come? not from the sun, surely. no, indeed, not from the action of the sun directly, upon our country, nor from the atlantic or the gulf stream which is off our coast. but a portion of the current of counter-trade is coming, heated by his rays and the warm water in the south atlantic, in an intense magneto-electric state, capable of inducing an electro-thermal change in the surface atmosphere which it approaches, and of being reciprocally acted upon by the north polar terrestrial magnetism. it is now over northern texas and western louisiana, it will be here day after tomorrow. the day passes as the day previous had passed; the sleigh-bells jingle merrily in the evening; the moon shines clear all night; the storm is coming steadily on, but its influence has not reached us, and the morning and midday are like those which preceded it. as nightfall approaches, however, the thermometer does not fall as rapidly as on the day previous; the sun shines dimly and through lines of whitish cirrus cloud extending from the horizon at the west, appearing darker as the sun descends and shines more _horizontally_ through them--perhaps mainly in the n. w.--and which extend up and over toward the e. n. e. the air next the earth begins to feel raw; it is changing, not from warm to cold, but _electrically_ from positive to negative; and dampening, from a tendency to condensation by induction, as we shall see--the same condensation which in warm weather may be seen on flagging stones, and walls, and vessels containing cold water. the advance cirrus condensation of the storm is over us and affecting us; the earth too is affecting the adjacent atmosphere by action extended from beneath the storm. still there is no wind, although sounds seem to be heard a little more distinctly from the east, and so ends the day. evening comes, and the moon wades in a smooth bank of cirro-stratus haze, with a very large circle around her; the cirrus bands of haze have coalesced and formed a thin stratus. the storm is coming steadily on, its condensation is seen to be thicker as it approaches, it is now raining from one hundred to one hundred and fifty miles to the west, but we do not know it. that it is about to storm all believe, for all are conscious of a change. the candle if extinguished will not relight as readily, if at all, on being blown; there is a crackling almost too faint for snow in the fire; the sun did not set clear; the old rheumatic joints complain, and the venerable corns ache. morning comes, and the storm is on. the wind is blowing from the s. e., the scud are running rapidly from the same quarter to the n. w., the thermometer continues rising, and it rains. the storm has reached us and the thaw has commenced. gradually, as the densest portion of the storm cloud reaches us, it darkens; the scud are nearer the earth, and run with more rapidity; the rain falls more heavily and continuously, and by the middle of the day a thick fog has enveloped the earth; the wind is dying away, and the trade itself, with its southern tendency to fog, has settled near us; the barometer has fallen, the thermometer is up to fifty degrees, the water is running down the hills, the snow is saturated with water and is disappearing under the influence of the fog, the rain, and the warm air. evening comes; the south-east wind and the rain have ceased; the rain clouds have passed off to the eastward; the fog has followed on and disappeared; there is a light trade air from the s. w.; the moon shines out, and a few patches of stratus, broken up into fragments and melting away, are following on in the trade: the storm is past. hark! to the tones of boreas as he bursts forth from the n. w., and rushing, whistling, howling, dashes on between the trade and the earth, following the storm. now the barometer rises rapidly, the thermometer falls, and in an incredibly short time all is congealed, and cold and wintery as before. the cold n. w. wind has again interposed between the trade and the earth; the trade is elevated a mile or more above it and is entirely free from its influence and from condensation; the deep blue of a sky "as pure as the spirit that made it" is over us, and steady winter reigns again. it is obvious that there was nothing in the action of the sun upon our snow-clad country, to induce the thaw or the storm. it began, continued, approached, and passed off to the n. e. in the counter-trade. the s. e. wind which existed every where within its influence: in the interior states, missouri, illinois, indiana, ohio, michigan, and in canada, as well as upon the atlantic coast, commencing in the former earlier than upon the last, was the result of its induction and attraction. of the n. w. wind that followed we shall speak hereafter. if any one doubts whether this be a true sketch let him examine the investigation of a storm published by professor loomis, or observe for himself hereafter. if, however, the storm of professor loomis is referred to, it should be remembered that his notes show the occurrence of a slight distinct snow storm at the n. w. stations one day in advance of the principal storm. the latter appears first as rain at fort towson, on the nineteenth, moving north and curving to the east--its center passing near st. louis, and south of quebec, and the whole storm enlarging as it advanced. take another instance. since the thaw it has not been quite as cold as before; but the rain-soaked snow is hard and solid, the ground, where the snow was blown or worn off, icy and slippery--the thermometer falls during the night to about °, and rises to about °; the sun makes no impression upon the snow; the firmament is of the deepest blue, the borealis at night vivid. "o, for a storm of some kind, to mitigate the still severe cold;" for the thaw has made us more sensitive, and storm winds do blow warm in their season. but patience, it will come. another day, or two, perhaps, pass: the sun rises as usual, the thermometer has the same range still. "long cold snap," we exclaim; "how long will it last?" a change is coming, but this time it will snow. about an hour or two after sunrise the cirrus threads are discoverable again in the west, but now they are most numerous in the s. w. as the day passes on they thicken and advance toward the e. n. e., the sun begins to be obscured, the thermometer rises, and it slowly "_moderates_." there is a snow storm approaching from the s. w. but the thermometer rises slowly; it must get up to ° or ° before it can snow much. i have known in one instance, at norwalk, a considerable fall of snow, although much mingled with hail, when the thermometer stood at ° above zero, and one, a moderate fall, some two inches, with it at °, but these were exceptions. the snow range of the thermometer on the parallel of ° north latitude, and south of it, is from ° to ° above °; when colder or warmer it may snow to whiten the ground, or perhaps barely cover it, but usually rains or hails. we have seen that in the polar regions, according to dr. kane, it is about zero, but the rise of the thermometer there, previous to the snow, was about the same as here, _i. e._, from ° to °. this fact is instructive. since the foregoing was written, and on the th of february, , a snow-storm of considerable length set in, with the thermometer at °, and continued more than twenty-four hours, the thermometer gradually rising. the snow was very fine, like that described by arctic voyagers as falling in extreme cold weather. as the dense and darker portions of the storm approach, and although the sun is obscured, and the ground frozen, it continues to moderate, and at evening, when the thermometer is up to °, and the dense portion of the storm has reached us, gently and in calmness the snow begins to fall. perhaps a light air following the storm, or the presence of the trade near the earth, at first inclines the snow-flakes to the eastward. this is frequently so at the commencement of snow storms. ere long, however, the wind rises from the n. e., and the snow is driven against the windows, rounded and hardened by the attrition of its flakes upon each other, in their descent through the eddying and opposite currents. the next day we rise to witness a heavy fall of snow, perhaps, and a continued driving n. e. storm, in full blast; the snow whirling and settling in drifts under the lee of every fence or building. can it be, you ask, that this driving wind is but an _incident_ of the storm? the result of _attraction_, while the storm clouds are sailing quietly and undisturbed on in the counter-trade above, directly over the gale which is blowing below? it is even so. nor has it "backed up," as it is termed by those who have ascertained that it has commenced snowing first, and cleared off first, at a point west of them. you saw, or might have seen, the cirro-stratus cloud passing to the e. n. e. in the afternoon, and until the snow-flakes filled the air, and the clouds became invisible. you may still see that the wind will die away before the storm breaks, and "come out" gently from the s. w., unless it should back into the northward and westward, and in either event you may see the last of the storm clouds, as you did see, or might have seen the first of them, pass to the eastward. toward night the wind dies away, and the storm passes off abruptly, or the sky becomes clear in the n. w. now you may see the smooth stratus storm cloud, continuous, or breaking up into fragments and passing off to the east, even at the edge which borders the clear sky in the west or north-west, to be followed that evening or the next day, by the north-west wind and its peculiar fair-weather scud. i have given these as instances illustrating the manner in which rain and snow storms originate the surface easterly winds in winter. but it must not be supposed that they commence with precisely the same appearances in every case in winter; much less in summer. there is very great diversity in this respect, in different seasons, and in different storms during the same season. a great many different and accurate descriptions might be given, if time and space would permit, which all would recognize as truthful. very frequently in summer, and sometimes in winter, the wind will set in from the eastward, and blow fresh toward a storm, before the condensation in the trade, which forms the eastern and approaching edge of the storm, has assumed the form of a distinct cloud. not unfrequently, when it is calm next the surface, a narrow stratum of easterly wind, a half a mile or a mile above the earth, may be seen with a continuous fog, condensing, but not in considerable patches like the usual scud, running with great rapidity toward the storm. such a stream of fog blew with great rapidity for thirty-six hours toward the storm which inundated virginia and pennsylvania, in , and carried away the potomac bridge at washington. such a stream of fog was visible the evening before the great flood of , which inundated connecticut, and curried away so many railroad and other bridges. i have also seen such a stream of fog running at about the same height, when it was calm at the surface, from the s. w. toward a violent storm which formed over central new england--and from the north toward a heavy storm passing south of us. such strata form, as far as i have been able to discover, the _middle current_ of storms which are accompanied with very heavy falls of rain. these double currents are much more common than is supposed. east of the alleghanies, short and heavy rain storms, which commence north-east, hauling to the south and lighting up about mid-day _after a very rainy forenoon_, frequently have a s. e. or s. s. e. middle current of this character, which involves the whole surface atmosphere when the storm has nearly passed, and the n. e. wind dies away, and the wind seems to haul to the s. s. e. and s.; so that it is rather the prevalence of a _different_ and _coexisting current_, than a hauling of the _same wind_, which marks the period of lighting up in the south. sometimes the easterly wind will set in and blow a day or two before the border of the storm reaches us. sometimes the storm is passing, or will pass, in its lateral southern extension, south of us, and the condensation in the trade extends over us sufficiently dense to induce an easterly current beneath it, but not dense enough to drop rain, and then we have a dry north-easter. i can not, within the limits i have prescribed, allude to all the peculiarities attending the induction and attraction of an easterly wind, by the storm in the counter-trade. they are readily noticeable by the attentive and discriminating observer, and their existence and cause is all with which i have to do at present. winds from the north, or any point from n. n. e. to n. n. w., are comparatively infrequent in the united states, east of the alleghanies--though it is otherwise in the vicinity of the great lakes. sometimes the wind "backs," as sailors term it, during a n. e. storm, from the n. e. through the n. n. e., n., and n. n. w. to n. w. when this takes place, it is toward the close of the storm. occasionally, though very rarely, it continues to storm after the wind has passed the point of n. n. e., and until it gets n. w. i have known a few instances in the course of thirty years, and but a few. they are exceptions--rare exceptions. when the wind thus backs from the n. e. to the n. w. through the n., you may be very certain that the body of the storm, or at least the point of greatest intensity and greatest attraction, is at the time passing to the southward of you. this is most commonly the course of the wind when the storm extends far south and lasts several days, and does not extend north far, or if so, with much intensity, beyond the point of observation. the change of the wind is explained by the situation of the focus of intensity and attraction, to the south of the observer, and its passage by on that side. probably in locations further north and (as i think i have observed) south of the lakes, it may be more frequent than upon the parallel of ° east of the alleghanies (which is as far north as i have observed), inasmuch as the further north the locality, the more likely storms and other disturbances in the counter-trade will be to pass to the southward of it. between the n. e. and s. e. the wind may blow from any point, before and during storms, and in a clear day in the morning, as a light variable breeze, or, after mid-day, toward approaching showers. i have known it blow all day during a storm from due east; to change back and forth between south-east and north-east, and to blow for hours from any intermediate point--as different portions of the storm were of different intensity, and exerted a more or less powerful inducing influence; and doubtless this often takes place at sea. it depends upon the situation of the focus of attraction of the storm, its shape relative to the particular locality, and with reference to the atmosphere east of it, and peculiar local magnetic action; or, as is sometimes the case in low latitudes, is owing to the fact that the storm is made up of many imperfectly connected showers, which have different force, and induce changeable and baffling winds. the inducing and attracting influence of the approaching storm is exerted sooner, and with most force, upon the surface atmosphere, over bodies of water like the ocean and the lakes. thus, the wind will set from the eastward toward an approaching storm out upon long island sound, for hours before it is felt upon either shore; and when all is calm in the evening on land, and often before the moon forms a halo or circle in the milky condensation of the approaching storm, or any sign of condensation is visible, the breaking of the waves upon the shores may be heard. doubtless this may be observed on the shores of the atlantic at other points. this power of attracting the surface atmosphere from bodies of water like the ocean and the great lakes, will account for two apparent anomalies, mentioned by mr. blodget in a valuable and instructive article read to the scientific convention, in , regarding the annual fall of rain over the united states. first--the influence of mountains in extracting the water from the atmospheric currents which pass over them, is well known and readily explainable. mr. blodget, however, found that the source of our rains, whatever it might be, when it reached the alleghanies, was so far exhausted of its moisture that those mountains extracted less from it than fell to the westward, by some five to ten inches annually; and that the fall of rain upon them was less than upon the atlantic slope eastward of them, to the ocean. this does not accord with observation elsewhere, but is easily explained. as the storm approaches the ocean, it attracts in under it the surface atmosphere of the ocean, loaded with vapor, condensing in the form of fog and scud, as it becomes subject to the increasing influence of the storm. although the scud and fog would not of itself make rain, it aids materially in increasing the quantity of that which falls through it. the drops, by attraction and contact, enlarge themselves as they pass through, in the same manner as a drop of water will do in running down a pane of glass which is covered with moisture. the small drop which starts from the upper portion of a fifteen-inch pane, will sometimes more than double its size before it reaches the bottom. _it is by this power of attracting the surface atmosphere, which contains the moisture of evaporation, under it, and inducing condensation in it, that the moisture of evaporation which rarely rises very far in the atmosphere is made to fall again during storms and showers._ this attraction of a moist atmosphere from the ocean accounts for the excess of rain on the east of the alleghanies, compared with its fall upon them. so the great valley of the mississippi is comparatively level, and less of its water runs off than of that which falls upon the alleghanies. there is, therefore, more moisture of evaporation in the atmosphere of the former to be thus precipitated and add to the annual supply of rain upon that valley, and it exceeds that which falls upon the alleghanies. those mountains, too, are elevated but about , feet above the table-lands at their base, and exert little influence on the counter-trade. if they, were , or , feet high, a different state of things would exist. second--mr. blodget found the quantity of rain which fell in iowa, and to the south and west of the lake region, to be greater than fell over the lake region itself. this is doubtless in part owing to the same cause. the counter-trade, in a stormy state, attracts the surface atmosphere from the lake region, with its evaporated moisture, before it arrives over it, and therefore more rain falls s. w. of the lake region than upon it. this power of attracting the surface wind of the ocean in under it, produces the heavy gales which affect our coast, and which are rarely felt west of the alleghanies to any considerable degree; and a storm coming from the w. s. w., extending a thousand miles or more from s. s. e. to n. n. w., may have the wind set in violently at s. e. on the _southern coast first_, and at later periods, successively, at points further north, and thus induce the belief that the storm traveled from south to north. mr. redfield finding that some of the gales which he investigated, particularly that of september d, , did not extend far inland, and commenced at later periods regularly, at more northern points, concluded that the gale traveled along the line of the coast to the northward. in this, and in relation to the storm of (and perhaps some others), he has been deceived. my recollections of that storm are accurate and distinct. but i shall recur to this again when i come to speak of his theory. toward storms, or belts of showers which would be storms if it were not summer and the tropical tendency to showers active in the trade, which pass mainly to the north of us, or commence north and pass over us, condensing south while progressing east, the wind may commence blowing before the body of the storm reaches us, from any point between south by west and south east, particularly in the summer season and in the afternoon. when the rain in a storm of this character sets in, in the night, it will sometimes haul into the s. e., if the focus of attraction be situated north of us, and so remain until just before the storm is to break. there are, however, a class of southerly summer winds which deserve more particular notice. for two or three months in the year--say from the middle of june to the th of august--storms on the eastern part of the continent, except in wet seasons, are rare, and most of our rain is derived from showers. during these periods belts of drought are frequent, sometimes in one locality, and sometimes in another, extending with considerable regularity from w. s. w. to e. n. e. in the course of the counter-trade, while rain falls in frequent and almost daily showers to the northward or southward of them. if the daily rains are at the north, over the belt of drought, s. s. w. and s. w. by s. winds blow, sometimes with cumuli or scud, during the middle of the day and afternoon, to underlie the showery counter-trade on the north of the line of drought. thus, sometimes nearly every day for several days, the evaporated moisture of the dry belt will be carried over to increase the store of those who have a sufficient supply without. during the latter part of the afternoon the clouds in the west may look very much like a gathering shower, but the attractions of the counter-trade fifty or one or two hundred miles to the north, will absorb them all, and at nightfall the wind will haul to the s. w. on a line with the counter-trade, and die away. if there be a drought on any given line of latitude, and frequent showers or heavy rains at the south of it, although there may not be a like surface-wind, with cumuli and fog, blowing from the north toward it, yet a general, gentle set of the atmosphere, from the n. n. w., or n. w., or other northerly point, toward the belt of rains, some distance above the earth, will often be observable, with a barometer continually depressed, and perhaps a cool atmosphere. during set fair weather, when the attracting belt of rains is far north, on the north shore of long island sound, the wind, like a sea breeze, will set in gently from about s. s. e. or s. by e. in the forenoon, blowing a gentle breeze through the day, and hauling to w. s. w. on a line with the trade at nightfall, and dying away. during a drought i have known this to happen for seventeen successive days. it is obvious to an attentive observer that this is the result of the influence of the sun in exciting the magnetic influence of the earth, and producing a state of the trade not unlike that which induces the formation of cumuli, and which attracts the surface atmosphere from the sound in over the land: for the _tendency to cumulus condensation precedes the breeze_, and the breeze is often wanting in the hottest days where no such tendency to the formation of cumuli exists. the same is true of sea breezes elsewhere. they do not blow in upon some of the hottest surfaces. where they do exist, they do not always blow, but are wanting during the hottest days; and careful observers have identified their appearance with the formation of cumuli, or other condensation, upon the hills inland. they are not, therefore, the result of ascending currents of heated air. the received theory regarding sea and land breezes is a mistaken one in another respect. there is no such thing as a land wind corresponding in force to, and the opposite of, the sea breeze--occasioned by the comparative warmth of the ocean. these breezes blow mainly within the trade-wind region. of course they are either beneath the belt of rains or the adjoining trades. they are said to be, and doubtless are, most active and strongly marked on lines of coast, particularly the malabar coast, and where the trade-winds are drawing usually from them. in the day-time, when the action of the sun increases the action of the magnetic currents upon the land, or there are _elevations inland_ which approach the counter-trade, and especially if it is elevated near the coast, as the malabar coast is by the ghauts, the attraction of this atmosphere over it _reverses the trade_, or inclines it in upon the land, and it blows in obliquely or perpendicularly, according to the relative trending of the coast and the direction of the surface-trade. thus, where islands are situated within the range of the trades, the latter will be _reversed_ during the day on the _leeward_ side, but continue to blow as land winds during the night. so they are sometimes deflected in upon the land on the sides, during the day, and in like manner return to their course in the night. so, too, the north-east trades of northern africa, are occasionally (though feebly where the coast is flat) deflected during the day-time, and blow in as n. w. winds. upon the southern coast of africa the s. e. trade is deflected, and blows in as a s. w. wind. upon the south-western coast of north america, the n. e. trades are deflected in like manner, and so are the s. e. trades upon the western coast of south america. where the coast mountain ranges are very elevated, as upon the western coast of the american continent, this attracting influence and consequent deflection extends to a considerable distance seaward, and hence the westerly winds of california, etc. it must be understood that we are now speaking of the winds which blow within the range and during the existence of the trade-winds or the presence of the dry belt--for the trades are not always perceptible on the land. captain fitzroy thus describes the sea breezes of the western coast of peru, at ° south latitude. "the tops of the hills on the coast of peru are frequently covered with heavy clouds. the prevailing winds are from s. s. e. to s. w., seldom stronger than a fresh breeze, and often very slight. _sometimes during the summer, for three or four successive days, there is not a breath of wind, the sky is beautifully clear, with a nearly vertical sun._ on the days that a sea breeze sets in, it generally commences about ten in the morning, then light and variable, but gradually increasing till one or two in the afternoon. from that time a steady breeze prevails till near sunset, when it begins to die away, and soon after the sun is down there is a calm. about eight or nine in the evening _light winds_ come off the land, and continue till sun-rise, when it again becomes calm until the sea breeze sets in as before." to illustrate this further, i take the following letter from professor espy's philosophy of storms: clinton hotel, n. y., dec. , . to professor espy, dear sir,--understanding you are desirous of collecting curious meteorological facts, i take the liberty of communicating to you what i saw in the month of december, , at the island of owhyhee. i lay at that island in the cavrico bay,[ ] in which captain cook was killed, three weeks, and every day during that time, very soon after the sea breeze set in, say about nine o'clock, a cloud began to form round the lofty conical mountain in that island, in the form of a ring, as the wooden horizon surrounds the terrestrial artificial globe, and it soon began to rain in torrents, and continued through the day. in the evening the sea breeze died away and the rain ceased, and the cloud soon disappeared, and it remained entirely clear till after the sea breeze set in next morning. the land breeze prevailed during the night, and was so cool as to render fires pleasant to the natives, which i observed they constantly kindled in the evening. i was particularly struck with the phenomena of the cloud surrounding the mountain, when none was ever seen in any other part of the sky, and none then till after the sea breeze set in, in the morning, which it did with wonderful regularity. the mountain stood in bold relief, and its top could always be seen from where the ship lay, above the cloud, even when it was the densest and blackest, with the lightning flashing and the thunder rolling, as it did every day. i passed up through the cloud once, and i know, therefore, how violently it rains, especially at the lower side of the cloud. this rain never extends beyond the base of the mountain;[ ] and all round the horizon there is eternally a cloudless sky. the dews, however, are very heavy, and there seems to be no suffering for want of rain. that this state of things continues all the year, i have no doubt, from what an american, by name sears, who had spent four years there, told me; he had seen no change in regard to the rain. caleb williams. providence, r. i. similar citations might be made to show that the sea breeze is induced by the same cause which forms the clouds over the land--that it is frequently wanting for three or four days under a vertical sun, and that the land breeze blows gently and not with corresponding force where there is no surface trade, or where it is deflected, not reversed. a succession of showers passing across the country to the north, within one hundred to one hundred and fifty miles, almost always produces a southerly wind to the southward of them. there is more that is peculiar about these belts of showers. although they consist of large highly-electrified cumuli, there is a strong tendency to cirro-stratus condensation in the lower part of the trade over them; and it is that condensation rather than the cumuli, which attracts the surface atmosphere from the south. they would be storms, if the atmosphere had not a summer-tropical tendency to showers. there is, too, a tendency in these belts to extend to the south, and it is generally, as far as i have observed, the extension southerly of those belts, by the formation of new showers which terminate the "hot spells" or "heated terms" of mid-summer. the very oppressive and fatal one of the summer of , was, in character, a type of all--although exceeding them in severity. the first three or four days were calm, hot, and smoky--an appearance which attends all similar periods more or less, refracting the red ray of the light, and giving the sun a peculiar dry-weather, red appearance. (this smoky haze is usually atmospheric, and occasionally seen even in march, although not unfrequently fires in the woods fill the air with actual smoke, and very much increase it, and when this is so, the odor of the smoke is often perceptible.) then we began to have a fresh south-west by south breeze in the day-time, hauling to the south-west, and dying away at nightfall. the next day, the tendency to condensation and consequent belt of showers having extended further south and approached nearer to us, the s. s. w. wind blew _fresher_ toward it, and _did not die away at nightfall_. during the evening the reflection of the lightning playing upon the tops of the thunder clouds, just visible at the north (heat-lightning, it is termed, because supposed to be unaccompanied by thunder, but in reality lightning reflected from clouds at too great a distance for the thunder to be heard), and the continuance of the southerly wind after nightfall, gave sure evidence of the coming showers the next day, and an end of the excessive heat for that time. so ended both of those long-to-be-remembered "heated terms" of . the same is probably true of the interior of the country every where. lieutenant maury, in the course of his investigations, and in order to ascertain the direction of the winds in the mississippi valley during rain, addressed a number of gentlemen, and received their replies, which are published with his wind and current charts. several answered, among other things, that, "whenever the lightning appears to linger at the north at eventide, rain almost invariably follows speedily; not so in the south." thus it frequently is with us. if, during a hot, dry time, of a few days continuance, the lightning so lingers in the evening, and the wind continues to blow _fresh_ from the southward _after nightfall_, showers will generally follow within forty-eight hours, most commonly the next day, and a cool n. n. w. or n. w. wind with a favorable change ensue. such, at least, has been the result of my observation for many years. indeed this seems to be the general law in summer in the mississippi valley, where the easterly winds are not so common as with us. to illustrate this further, i copy from a recent work by t. bassnett, entitled the "mechanical theory of storms," two short extracts, showing the manner in which belts of showers extend southerly, while progressing north-eastwardly, at ottawa. the first occurred in august, ; the last, december, . the first was a belt of showers; the latter would have been in august, but the lateness of the season changed its character somewhat, though not entirely, to a more regular rain, especially toward the close. "august th.--very fine and clear all day: wind from s. w.; a light breeze; p.m. frequent flashes of lightning in the northern sky; p.m., a _low bank of dense clouds in north_, fringed with cirri, visible during the flash of the lightning; p.m., same continues. " th.--very fine and clear morning; wind s. w. moderate; noon, clouds accumulating in the northern half of the sky; _wind fresher_, _s. w._; p.m., a clap of thunder over head, and black cumuli in west, north, and east; p.m., much thunder and scattered showers; six miles west rained very heavily; p.m., the heavy clouds passing over to the south; p.m., clear again in north. " th.--clear all day; wind the same (s. w.); a hazy bank visible all along on _southern horizon_. "december st, .--wind n. e., fine weather. " d.--thick, hazy morning, wind east, much lighter in s. e. than in n. w.; a.m., a clear arch in s. e. getting more to south; noon, _very black in w. n. w._; above, a broken layer of cirro-cumulus, the sun visible sometimes through the waves; wind around to s. e., and fresher; getting thicker all day; p.m., _wind south, strong_; thunder, lightning, and heavy rain all night, with strong squalls from south. " d.--wind s. w., moderate, drizzly day; p.m., wind west, and getting clearer." it is obvious that the showers at the north passed east on the evening of the th of august; that new showers, taking the same course, originated in the north, but more southerly next day, with s. w. wind, and that they passed east, and others formed successively further south, which passed over the place of observation late in the afternoon, and that others formed south and passed east during the night and next day, visible in a bank on the southern horizon. later or earlier in the spring and autumn, these brisk afternoon southerly winds continuing after nightfall, indicate moderate rains from a rainy belt extending in a similar manner, without the cumuli and thunder which attend those of mid-summer. i shall recur to this class of showers and storms when we come to their classification. light surface winds from south-west to west are not often storm-winds, and are usually those which the trade near the earth draws after it. sometimes the trade seems to draw the surface wind from the s. w. and w. s. w. with considerable rapidity, and some scud a little distance above the earth. when this is so, it will be found that a storm has passed to the north of us, or a belt of rains is passing north, which may or may not have sufficient southern extension to reach us. when there have been heavy storms at the south in the spring, especially if of snow, the s. w. wind which the trade draws after it, and which comes from the snowy or chilled surface, is exceedingly "raw"--that is, damp and chilly, although not thermometrically very cold. probably every one has noticed these "_raw_" s. w. winds of spring. usually, when storms and showers, which have not a southern lateral extension, pass off, the trade is very near the earth, and a light s. w. wind or calm follows for a longer or shorter period. not unfrequently, however, our n. e. storms terminate with a s. w. wind, shifting suddenly, perhaps, just at the close of the storm, during what is sometimes called a "clearing-off-shower," or, more frequently, dying gradually away as a n. e. wind, and coming out gently from the s. w., following the retreating cloud of the storm. in such cases it is said to "clear off warm." with us the wind rarely blows from the west, except while slowly hauling from some southerly point to the n. w. it is probably otherwise east of the lakes and in some other localities to the north-west. occasionally, and most frequently in march, a w. to w. n. w. wind follows storms, and blows with considerable severity, with large irregular, squally masses of scud, and sometimes a gale. such was the character of the dry gale which crossed the country, particularly northern new york, in march, , doing great damage. these westerly winds are always accompanied by a continued depression of the barometer, and peculiar, foggy, scuddy, condensation, and should be distinguished with care from the regular and peculiar n. w. wind, as they may be, by the continued depression of the barometer, and the character of the scud. they are doubtless magnetic storms. the remaining surface wind, the n. w., the genuine boreas of our climate, the invariable fair-weather wind, is one of great interest. it is unique and peculiar. it is not the left-hand wind of a rotary gale, and has no immediate connection with the storm. i have known it blow moderately, fifteen successive days in winter; rising about ten a.m., and dying away at nightfall. occasionally, but very rarely indeed, a light wind exists from the n. w. during a storm, owing probably to a focus of intensity in relation to some surface the storm covers, like the focus which exhibits itself as a clearing-off shower near the close of a storm; but the real fair-weather boreas is a different affair altogether. let us observe with care its peculiarities; they are instructive. st. it rarely blows with any considerable force beneath the trade while there are storm clouds, or any considerable condensation in it. it does not interfere with that reciprocal action which takes place between the trade and the earth, during approaching or existing storms. i have frequently seen it with its peculiar scud clouds in the n. w., waiting for the storm condensation of the trade to pass by, that full of positive electricity it might commence its sports; rushing and eddying along the surface, licking up the warm, south polar, electric rain, which stood in pools upon the ground, or rose in steamy vapor from the surface, and with its cool breath dry up the muddy roads as no degree of heat can dry them. the annexed figure ( ) shows the appearance of the northern edge of a stratus storm cloud, passing off e. n. e. at the close of the storm, which was "_clearing off from the north-west_." it is from a daguerreotype view, looking w. n. w., taken at eight o'clock in the morning, in the fall of the year. near the horizon maybe seen the n. w. scud, forming in the n. w. wind, which is about to follow the retreating edge of the storm cloud. figure is from a daguerreotype view, taken at eleven o'clock the same day, when the storm cloud had passed off and its edge remained visible only south of the zenith, and the north-east scud had risen up and covered the northern half of the sky, and the wind was blowing a gale from that quarter. [illustration: fig. .] another view was taken about two p.m. of the same day, when the scud had a very dark, gloomy appearance--as _dark_ and _gloomy_ as those of a mexican norther--too dark to represent by a cut. not unfrequently in a moist summer season, after a day of showers or rain, which have had an extending formation or lateral extension from north to south, it will commence blowing in the morning, and encourage the hay-maker with the hope of fine weather. but often before noon, the milky stratus condensation above with cumuli below, will appear in the trade; the n. w. wind die away and variable airs from the east or south appear, to be followed toward night by an enlargement of the cumuli and showers. it rarely, if ever, blows fresh till the storm condensation of the trade has passed; or continues to blow after that condensation reappears. when it commences blowing after a storm, and the northern edge of the storm is not over us, we may frequently see the latter low down in the s. e. passing eastward. [illustration: fig. . north view.] d. its scud are peculiar. every one, probably, has noticed them. they are distinct, more or less disconnected, irregular, with every form between those of the easterly scud, cumulus, and stratus, according to the season. if large, with _dark under surfaces_; forming _rapidly_ and as _rapidly dissolving_; rarely dropping any rain, sometimes dropping a flurry of snow, in november or march, oftener than at any other period; sailing away to the s. e., and casting a traveling shadow as they pass on over the surface of the earth. their electricity, particularly when white, is probably always positive, as that of all whitish clouds is supposed to be. d. _it is emphatically a surface wind._ the incident storm winds, the n. e. and s. e., frequently _commence blowing_ under the storm, toward its point of greatest intensity, _up near the line of cirro-stratus condensation_, evidenced by the running scud; or blow there with most rapidity, and so continue for hours before the whole surface atmosphere from thence to the earth becomes involved in the movement; and sometimes without being felt below at all. not so with the n. w. wind; it _begins at the surface_ and blows there with more rapidity than above; it seems to be attracted by the earth; it interposes between the earth and the trade, wedging the trade up and occupying its place. it blows under at all seasons of the year, but most readily and strongly from a surface of snow whose electricity is always positive. hence it blows most strongly and _continuously_ when snow has fallen at the north, and prevails during winter very much in proportion to the extent and continuance of the covering of snow which invests the earth in that direction. it follows after storms, and particularly warm rains, during the autumn, winter, and spring months, which have a lateral southern extension. whether it is increased by the snow from the surface from which it blows, or is caused by the same magnetic action which causes the great fall of snow, is a question we shall consider hereafter. th. it does not connect or mingle with the trade current in any way, or change or divert the course of that current; but interposes between it and the earth, elevating the trade in proportion to its own volume, above the influences of the earth (when the trade becomes free from condensation, and singularly, clear); and raising _proportionately_ the barometer. an experienced observer can frequently estimate, with considerable accuracy, the rise of the barometer, by measuring with his eye, (when the clouds will enable him to do so,) the depth of this interposed n. w. current. the barometer rarely rises after a storm, for twenty-four or forty-eight hours if the wind continues at any point from s. w. to w. n. w., but always rapidly as soon as the genuine n. w. current with any considerable depth interposes and elevates the trade. it will be obvious to every one, i think, certainly, if they will hereafter study the subject and observe for themselves, that the n. w. wind does not blow away the storm; and that it follows after it, blowing over the surface which is uncovered by the storm; rarely, if ever, with any force when the body of the storm passed south of us; and that it is a purely surface wind, seemingly attracted by the peculiar magneto-electric state in which the surface of the earth is left, compared with a snow-clad surface to the north, by a recent storm, or that peculiar state of the trade which is left by the action of the storm. it seems to follow that magnetic wave which, passing from north to south, acts in its course upon the counter-trade, producing the storm, or belt of showers, and giving them their southern lateral extension, and will well repay future telegraphic investigation. its electricity is intensely positive--that of the earth by the action of the storm as intensely negative. th. this n. w. wind occurs in all parts of the northern hemisphere, so far as we have data to determine, and its corresponding wind from the s. w. occurs in the southern hemisphere. it is identical with a class of the northers of the gulf of mexico, as a brief analysis of the character of the latter will show. st. the fall and winter _norther_ is a dry wind without rain or falling weather--so is our n. w. wind. d. it is preceded by a falling barometer; s. e. scud and rain at the point where it blows, or to the eastward of it. so is ours when it blows a gale in the fall and spring months, which bear the nearest resemblance in climatic character to the periods when the northers blow. with this distinction, however, that our precedent rains either pass over us or to the southward, the direction of storms being e. n. e.; their precedent storms passing over or to the eastward of them as they move more to the northward. d. it is often preceded by a copious dew; so is ours--such dews often following light fall rains in our climate, and preceding n. w. wind. th. the most peculiar characteristic, however, is that the barometer rises rapidly and invariably while the norther prevails, and very much in proportion to its violence. the same is true of our genuine n. w. wind, and is not true _of any other wind_ on this continent which i have observed or read of. th. while they are thus alike in these respects, they are unlike in no respect. mr. redfield has traced them in _supposed_ connection with storms which continue from that vicinity across the united states to the e. n. e., and endeavored to connect them with those storms, as the left-hand winds of a rotary gale. obviously, i think, they are identical with our n. w. winds which also _follow_, indeed, but _are distinct from the storms_. there are a class of northers in the gulf of mexico--the "nortes del muero colorado"--sometimes occurring in the summer months, beginning at n. e., veering about and settling at n. n. w., and as they decline hauling round by the west to the southward. these winds correspond precisely with the hurricane winds of the west indies, and are doubtless the incident winds of a storm traveling thence to the n. n. w. precisely as our n. e. or e. n. e gales are incident storm winds to the n. e. storms of our latitude. in this connection we will look at the peculiarities of a west india hurricane. "it is not a little remarkable," says mr. espy, speaking of the storms and hurricanes of the west indies, "that all these storms, and _all others which have been traced to the west indies_, traveled n. w. almost at right angles to the direction of the trade-wind in those latitudes, but very nearly, if not exactly, in the direction of an upper current of the air known to exist there toward the n. w." substantially the same facts have been repeated by mr. redfield, and demonstrated by his able investigations, both there and in the eastern pacific, and are confirmed by the observations of edwards, lawson, and others, while residents there. it is a matter of surprise that gentlemen like messrs. redfield and espy, who have certainly displayed great ability in the investigations of meteorological phenomena, should fail to recognize a more intimate relation between this upper current and the storms they were investigating, and to detect the general laws which govern both. the storms and hurricanes of the west indies are comparatively of small diameter, and have little advance condensation. when they pass on to the south-western portion of north america and curve to the n. e., as they frequently do, they enlarge in front and at the sides, and their advance condensation, which is not dense enough to drop rain, extends in some cases from one to three hundred miles; and the storm itself, by the time it reaches the alleghanies, may extend one thousand to fifteen hundred miles, and perhaps in certain magnetic states of the surface, and occasionally, may cover the entire portion of the continent, from north to south. such, probably, was very nearly the extension of the storm investigated by professor loomis. in the west indies, however, at the commencement, they vary from twenty to one hundred miles, or possibly more, in width. first, they are preceded by a hot, sultry and oppressive atmosphere--_as are electric storms every where_--a peculiar electric state of the earth and adjacent air. second, the black clouds and lightning which indicate the approaching hurricane are seen to the s., s. e., and e. s. e., according to the season of the year, as we see them at the westward. during the rainy season, and when the storm, as is usual at that period, is small, and the s. e. trade blows more eastwardly, the wind at the windward islands, possibly, may set in at the north, and back round by the east as it progresses. so colonel reid thinks it sometimes does, at barbadoes. but when the belt of rains is south, and the hurricane comes from the south-east, and is larger and more violent in its action, and the north-east winds prevail, the first effect is an increase of these trades. soon, however, the wind hauls to the north and north-west, in opposition to its course, bearing the same relation to it that our east and north-east winds bear to storms in the united states; and the wind hauls around during the passage of the storm to the west, south-west, and south-east, and at the latter point it clears off. mr. edwards in his history of jamaica says--and as a resident, his authority should be decisive as to this island--"_that all hurricanes begin from the north_, veer back to the w. n. w., w., and s. s. w., and when they get around to the s. e. the foul weather breaks up." doubtless the same is true of the class of northers of which we are speaking on the gulf of mexico. _but with this class the barometer does not rise during the gale, and in proportion to its length and violence._ with the other class of n. w. winds--the northers of winter--it does. the following description of two winter northers, copied from colonel reid's valuable work, will illustrate what has been said. _precisely such changes from s. e. rains to n. w. winds, with blue sky and detached dark clouds--fair-weather n. w. scud--occur every autumn in october and november_, and the falling of the thermometer and rising of the barometer, after rain, and a change of the wind, are perfectly characteristic. ------------------------------------------------------------------------ . | wind. |force.|weather.| bar.|ther.| ------------------------------------------------------------------------ jan. .| | | | | | a.m. . |s. s. w. | |b. c. | . | |off tampico. noon. |south. | |b. c. r.| . | | {lat. ° ' n., p.m. . |south. | |b. c. r.| . | | {long. ° ' w. jan. .| | | | | | a.m. . |s. easterly.| |b. c. | . | | {between and | | | | | | {a.m., wind was | | | | | | {variable. noon. |n. by w. | |c. q. w.| . | |norther commenced at | | | | | | a. m. p.m. . |n. n. w. | |c. | . | |lat. ° ' n., | | | | | | long. ° ' w. feb. . | | | | | | a.m. . |n. n. w. | |c. g. | . | |lat. ° ' n., | | | | | | long. ° ' w. noon. |westerly. | |c. | . | | p.m. . |calm. | |c. | . | | ------------------------------------------------------------------------ feb. .| | | | | | a.m. . |s. e. | |b. c. r.| . | |at sacraficios. noon. |s. w. | |b. c. | . | |norther comc'd at . | | | | | | p.m. p.m. . |n. w. by n. | |c. q. u.| . | | feb. .| | | | | | a.m. . |n. w. by n. | |c. q. u.| . | | {gale moderated and | | | | | | {again freshened | | | | | | {about a.m. noon. |n. w. by n. | |c. g. q.| . | | p.m. . |n. w. | |c. g. | . | | feb. .| | | | | | a.m. . |n. w. | |q. | . | | p.m. . | n. n. w. | | c. g. | . | | ------------------------------------------------------------------------ b. indicates blue sky--c. detached clouds--r. rain--v. visibility of objects--q. squalls--w. wet dew--u. ugly threatening appearance--g. gloomy weather. the exact counterpart of the first norther may be observed with us every fall. on the th january, with a rising thermometer and falling barometer, there was rain at midday. the night following was moist--the next day, about ten a.m., the wind came out n. w., with squalls and gloomy weather, a falling thermometer, and rising barometer. the norther of feb. th differed from the other only in regard to the time of the day when it commenced; the order of events was the same. the rain fell in the night--it cleared off early in the day, and the norther followed in the afternoon. this also is frequently the case with us, as every one may observe. this brief notice of the surface winds of our climate would be incomplete without a description of those of the thunder-gust and tornado. the former is exceedingly simple. the showers, which are accompanied with much wind, form suddenly in hot weather, and have a considerable advance condensation (frequently with obvious lateral internal action), extending eastwardly from the line of smooth cloud from which the rain is falling, or rather where the falling rain obscures the inequalities of the cloud. _the gust is never felt until the advancing condensation has passed over us_, when it takes the place of the gentle easterly breeze which previously set toward the shower. _the gust ceases as soon as the cloud has passed._ it is obviously the result of the inducing and attracting influence of the cloud upon the atmosphere near the surface of the earth as it passes over it. let the reader watch attentively this advance condensation, from its eastern edge to the line of smooth cloud and falling rain, and he will understand at a glance this internal action of gust-clouds. the whole phenomena are simple and intelligible. a cloud approaching from a westerly point, dark and irregular from its eastern edge to the line of falling rain, where it appears smooth and of a light color; wind from the east blowing gently toward it, till the condensation is over us; then the gust following the cloud; then the rain, and in a few minutes the cloud, and wind, and rain have passed on to the east, and "sunshine" returns. the tornado, as it is termed when it occurs upon land, "spout," if on the water, is sometimes of a different character, and as it undoubtedly had great influence in inducing the gyrating theory of mr. redfield, and the aspiratory theory of mr. espy, and has been cited by both in support of their respective theories, it deserves a more particular notice. there are several marked peculiarities attending it which determine its character. st. it occurs during a _peculiarly sultry and electric_ state of the trade and surface atmosphere, and at a time when thunder showers are prevailing in and around the locality, and at every period of the year when such a state of the atmosphere exists. one recently occurred in brandon, ohio, in midwinter. d. there is always a cloud above, but very near the earth, between which and the earth the tornado forms and rages. it is usually described as a black cloud, ranging about feet or less above the earth, often with a whitish shaped cone projecting from it, and forming a connection with the earth; at intervals rising and breaking the connection, and again descending and renewing it with devastating energy. its width at the surface varies from forty to one hundred and eighty rods--the most usual width being from sixty to ninety rods. sometimes when still wider, they have more the character of thunder-gusts, and are brightly luminous. d. two motions are usually visible, one ascending one near the earth and in the middle, and a gyratory one around the other. the latter is rarely felt, or its effects observed, near the earth. occasionally, and at intervals, objects are thrown obliquely backward by it. th. it is composed, at the surface of the earth, of _two lateral currents_, a northerly and southerly one, varying in direction, but normally at right angles in most cases, although not always, with its course of progression, extending from the extreme limits of its track to the axis; which currents are most distinctly defined toward the center, and upward. these currents prostrate trees, or elevate and remove every thing in their way which is detached and movable. there does not seem to be any current in advance of these lateral ones tending toward the tornado, save in rare and excepted cases, and then owing to the make of the ground or the irregular action of the currents; nor any following, except that made by the curving of the lateral currents toward the center of the spout as it moves on, and perhaps a tendency of the air to follow and supply the place of that which has been carried upward and forward, like that of water following the stern of a vessel. the south current is always the strongest, and often a little in advance of the other, and covers the greatest area. the proportion of the two currents to each other is much the same that the s. e. trades bear to the n. e. this excess in volume and strength of the southerly current will explain the irregularities in most cases, and the fact that objects are so often _taken up and carried from the south to the north side_, and so rarely from the north and carried south of the axis. these irregularities are such as attend all violent forces, and something can be found which will favor almost any theory; but the two lateral currents appear always to be the principal actors, except, perhaps, when it widens out and assumes more the character of a straightforward gust. see a collection by professor loomis, american journal of science, vol. xliii. p. . the following diagram is a section of the new haven tornado, from professor olmstead's map accompanying his article in the "american journal of science and art," vol. . p. . the manner in which the main currents flow is shown by their early and unresisted effect in a cornfield, as represented by the dotted lines. the direction in which the fragments of buildings were carried by the greater power of the southerly currents is shown also. and so is this irregular action, where a part of the southerly current broke through the northerly one, and prostrated two or three trees backward on the north side of the axis. [illustration: fig. .] th. this cloud, and its spout, move generally with the course of the counter-trade in the locality--_i. e._, from some point between s. w. and w., to the eastward, but occasionally a little south of east, deflected by the magnetic wave beneath the belt of showers. th. several exceedingly instructive particulars have been observed and recorded. _a_. _no wind is felt outside of the track_, as those assert who have stood very near it, and its effects show. _b_. the track is often as distinctly marked, where it passed through a wood, as if the grubbers had been there with their axes to open a path for a rail-road. the branches of the trees, projecting within its limits, are found twisted and broken off, or stripped of their leaves, while not a leaf is disturbed at the distance of a foot or two on the opposite side of the tree, and outside of the track. _c_. as the spout passes over water, the latter seems to _boil up_ and _rise to meet it_, and _flow up_ its trunk in a _continued stream_. _d_. as it passes over the land, and over buildings, fences, and other movable things, they appear to _shoot up_, instantaneously, as it were, into the air, and into fragments. if buildings are not destroyed or removed, the doors may be burst open _on the leeward side_, and gable ends _snatched out_, and roofs taken off on the _same side_, while that portion of the building which is to the windward remains unaffected. _e_. articles of clothing, and other light articles, have been carried out of buildings through open doors, or chimneys, or holes made in the roofs, and to a great distance, without _any opening_ being made for the air to _blow_ in. _f_. if there be a discharge of electricity up the spout from the earth, like that of lightning, the intense action ceases for a time or entirely. _g_. vegetation in the track is often scorched and killed, and so of the leaves on one side of a tree, which is within the track, while those on the other side, and without the track remain unaffected. (espy's philosophy of storms, , cited from peltier.) _h_. the active agent whatever it is, has been known to _seize hold of a chain attached to a plow_ and _draw the plow about, turning the stiff sod for a considerable distance_. (see loomis on the tornado at stow, ohio, american journal of science, vol. xxxiii. p. .) _i_. in passing over ponds, the spout has taken up all the water and fish, and scattered them in every direction, and to a great distance. _j_. the barometer falls very little during the passage of the spout. (see the natchez hurricane of , espy page .) not more than it _frequently_ does during gentle showers. _k_. persons have been taken up, carried some distance, and if not projected against some object in the way, or some object against them, have usually been _set down gently and uninjured_. _l_. buildings which stood upon posts, with a free passage for the air under them, although in the path of the tornado, escaped undisturbed. (olmstead's account of the new haven tornado, american journal of science, vol. xxxvii. p .) _m_. a chisel taken from a chest of tools, and stuck fast in the wall of the house. (ibid.) _n_. fowls have had all their feathers stripped from them in an instant and run about naked but uninjured.[ ] _o_. articles of furniture, etc., have been found torn in pieces by antagonistic forces. _p_. frames taken from looking-glasses without breaking the glass. nails drawn from the roofs of houses without disturbing the tiles. _q_. hinges taken from doors--_mud taken from the bed of a stream_ (the water being first removed), and let down on a house covering it completely--a farmer taken up from his wagon and carried thirty rods, his horses carried an equal distance in another direction, _the harness stripped from them_, and the wagon carried off also, _one wheel not found at all_. (american journal of science, vol. xxxvii. p. .) pieces of timber, boards, and clapboard, driven into the side of a hill, _as no force of powder could drive them, etc., etc._ now to my mind, these circumstances indicate clearly, that it is not wind, _i. e._, mere currents of air, which produces the effect, but that a _continuous current_ or _stream of electricity_ from the earth to the cloud exists, and carries with it from near the earth, such articles as are movable: that this stream collects from the _northerly_ and _southerly_ side upon the _magnetic meridian_, in _two currents_ with _polarity_, which meet in their passage up at the center; curving toward the center in the posterior part as the spout moves on, when acting in a normal manner, and making the "_law of curvature_" observed: that no conceivable movement of the air alone in such limited spaces could produce such effects; or if so, that no agent but electricity could so move the air: that the air in a building could not shoot the roof upward, and into fragments; much less could the air in a cellar by any conceivable force, be made to elevate _or shoot up_ the entire house, and its inmates, and contents--effects so totally unlike what takes place in gales, hurricanes, and typhoons: that elastic free air never did nor could take hold of the plow chain, and plow up the ground; or scorch and kill the vegetation; or twist the _limbs_ from one side of a tree, while the most delicate leaves on the other, and within two or three feet, remained unaffected and undisturbed; or pick the chickens: that even if the expansion of the air could produce these effects--if a sudden vacuum were produced--_nothing but currents of electricity could produce the sudden vacuum_, by removing the air above. it is well settled that atmospheric electricity can and does flow in currents with light, by experiments in relation to the brush discharge, etc. that it may do so without light or disruptive discharge, and in a stream, or as it is termed, by convection, with the force and effect seen in the tornado, is perfectly consistent with what we know of it--and it is, i think clearly evinced that such is the character of the phenomena, by the fact that a sudden powerful _disruptive_ discharge, _with light, up the spout_, produces an instantaneous partial or total suspension of its action; to be renewed as the cloud passes over _another_ and more highly charged _portion_ of the _earth's surface_. peltier gives instances where the spout has been entirely and instantaneously destroyed by such a sudden and powerful discharge of electricity; marking the spot where it was so destroyed by a large hole in the earth, from which the discharge issued. and in fact these tornados are often steadily luminous, and so much so, when they occur in the night, as to enable persons to read without difficulty. the lateral inward and upward currents, are accompanied, after they meet and unite, or seem to unite, by gyratory or circular ones. how are they produced? this question can only be answered by analogy. no permanent impressions are left by the circular currents, except to a limited extent, and in occasional instances; and observation of them has been, and must necessarily be limited and uncertain. i have witnessed one or two on a moderate scale; but owing to the suddenness of their passage, and the confusion of the objects taken up, it was difficult to determine what the circular currents were. when the southerly current is much the strongest, it appears sometimes to cross the axis, and curve round the northerly one. perhaps this may be all the curving that really takes place, except at the posterior part of the axis, for evidence of a curving on the south of the axis is rarely, if ever seen. assuming, however, that the main currents unite and form one from the earth to the cloud, _induced_ circular currents would be in perfect keeping with the known laws of electricity. such currents, and with magnetic properties, are always induced by powerful currents of voltaic electricity passing through wires. and doubtless _in all cases_ powerful currents of electricity _induce attendant circular currents_. this may account for the external gyration of the spout. or it may be that the two lateral currents of air which attend the currents of electricity, do not unite; having opposite polarity, but pass by and around each other, in connection with the circular magnetic currents. future observation and perhaps experimental research will determine this. but it may not be accomplished by the present generation; for the belief that tornados are mere whirlwinds, produced by the action of the sun in heating the land, is adhered to, notwithstanding they cross the intense magnetic area of ohio in mid-winter, and seems to be ineradicable. the proportions of different winds vary in different localities. for the benefit of those who are curious, i copy a table from an able compilation by professor coffin, published by the smithsonian institute, showing the proportion of the winds at new haven (the station nearest to me). it will be noticed that during the year the n. w. winds blow the greatest number of days; the s. w. next; the n. e. and s. e. less than either, and about equal. it may be observed that the two latter bear about the same proportion to the whole, that our number of cloudy and stormy days, averaging about ninety, bear to the whole number of days in the year. +------------------------------------------------+ |course.| . | . | . | . | total. | |------------------------------------------------| | n. | | | | | | | n. e. | | | | | | | e. | | | | | | | s. e. | | | | | | | s. | | | | | | | s. w. | | | | | | | w. | | | | | | | n. w. | | | | | | +------------------------------------------------+ this work of mr. coffin has been brought to my notice since the foregoing pages were written. the facts embodied in it will be found to comport with what i have observed and stated. in relation to the proportionate number of days in the year during which the wind blows from the different points of the compass at the several stations it is very full and able. but it has cardinal defects. it does not show the _main currents_ of the atmosphere. it treats the surface-winds, which are incidental, as principals. the direction of the main currents is indeed shown frequently by the mean course of the surface winds, but not uniformly or intelligibly. nor does it distinguish between the fair weather and storm winds; nor always between the trade winds during their northern transit, and the variable winds north of the trade-wind region. hence, the deductions derived from it disclose no general system, and sustain no theory, although many very important facts appear. some of these, professor coffin found it difficult to reconcile with received theories, or satisfactorily explain. for instance, he found the prevailing winds of the united states, in louisiana and texas, s. and s. e.; in western arkansas, and missouri, southerly, and in iowa and wisconsin, s. w., forming a curve, and evidently connected together. thus, alluding to the winds west of the mississippi, and between the parallels of ° and °, he says: "on the american continent, west of the mississippi, there appears to be more diversity in the mean direction of the wind, yet here it is westerly at sixteen stations out of twenty, from which observations have been obtained. the most peculiar feature in this region, is the _line_ of southerly winds on the western borders of arkansas and missouri. it seems to form a connecting link between the winds of this zone and the south-easterly ones that we find south of it; and, in some degree, to favor an idea that has been advanced, that there is a vast eddy, extending from the western shore of the gulf of mexico, to the eastern shore of the atlantic; that the easterly trade-winds of the atlantic ocean, when they strike the american continent, veer northwardly, and then n. e., and thus recross the atlantic, and follow down the coast of portugal and africa, till they complete the circuit." this mean prevalence of the curving winds indicates the course of the western portion of the concentrated counter-trade, of which we have so fully spoken, and to which that portion owes its rains and fertility. doubtless the curve would have been traced somewhat further west, if observations had been obtained from more westerly stations. the idea of an eddy, to which professor coffin alludes, is of course unsound; that of a counter-trade, most fully confirmed; the curve corresponding with that of the regular rains and fertility as they are known to exist. professor coffin is a believer in the generally-received theory of rarefaction, as the cause of all winds. his work is published by the smithsonian institution, and the theory is, so far forth, nationalized. but he found it very difficult to reconcile all the facts he obtained, with the theory, and, possessing a truth-loving mind, he frankly admits it. alluding to the prevalence of n. e. winds off the coast of africa in the summer months, as shown by certain numbered wind-roses, he says: "nos. , , , and , have caused me much perplexity. the arrows for the warmer months evidently indicate a point of rarefaction situated to the _south_ or _south-west_, and yet all the observations from which they were computed were taken within a few hundred miles of the african coast and desert of sahara; a region, the annual range of whose temperature must be exceedingly great. the only way in which i can account for a fact so astonishing, is, by supposing the deflecting forces at these numbers to be secondary to the influence which we see so strongly marked in nos. , , and . let us, then, first devote our attention to these." (we have not space for the map of professor coffin, nor is it necessary to insert it. the numbers , , , and , refer to respective portions of the atlantic, west of africa, north of the cape de verdes, of ° of latitude each, where the n. e. trades are drawing off from the coast. the nos. , , and refer to like portions _below_ the cape de verde, where the s. w. monsoons are found under the rainy belt; and the explanation of the distinguished author is an attempt to account for the blowing of the trades _from_ sahara, by supposing them connected with the monsoons further south, which seem to blow toward it.) "the intense heat of the great desert rarefies the air exceedingly from june to october, inclusive, and hence the arrows of unparalleled length (plate xii.)," (showing the monsoon winds below the cape de verdes,) "pointing toward it during those months, the longest being longer than that which represents the most uniform of the trade-winds, in the ratio of to . the influence of this rarefaction is sufficient to curve the powerful current of the trade-winds in the manner exhibited on plate vii. nos. and , and to produce the not less remarkable change in no. , holding the current back and retarding it, so that its progressive motion in the _three_ months of july, august, and september united, hardly exceeds that during any _one_ of the colder months of the year. but while this is so, the trades on the western side of the atlantic are pursuing nearly their regular track, being but slightly affected by these influences. as a consequence, the latter must leave, as it were, a partial vacuum behind them, which is filled by air flowing in from the north-east and south-east. this will account for the seeming anomaly of having a somewhat strong deflecting force directed toward mid-ocean, in the hottest part of the year, as in the numbers above referred to. _and yet it may be very naturally asked, why does not the air from these parts supply the great desert directly, instead of taking a circuitous route to supply the region that supplies it? a question which, i confess, it seems difficult to answer._" (the italicization in the foregoing extract is mine). here the worthy professor finds a fact inconsistent with the theory of rarefaction--viz.: that the winds blow off shore, and toward mid-ocean, opposite sahara, and he is "perplexed and astonished." the theory, however, must be maintained, and one of those modifying hypotheses which have made meteorology such a complicated piece of patch-work, must be invented; some "deflecting forces" found. there is the great desert, bordering upon the ocean, north of the cape de verde islands, for a distance of six hundred miles, widening as it extends inland, whose temperature, as he says, "_must be exceedingly great_;" and doubtless is so, and yet the air, instead of blowing in upon it in a hurricane, is actually drawing off from it, and blowing towards the s. w., where the water and air do not rise above °. well may he be "perplexed and astonished." turning south, however, to the distance of five hundred miles or more, he finds the s. w. monsoon winds, which in those months blow under the belt of rains, toward the land, in the direction of, but at a great distance from, sahara. it is an easy matter to suppose that they reach the great desert and supply its vortex of rarefaction, inasmuch as they blow in a direction toward it, and distance is no impediment to supposition. then it is necessary to _suppose_ that the s. e. and n. e. trades, at the south-west, draw so strongly to the westward as to create a partial vacuum to the s. w. of sahara, which is filled by the winds which draw off shore, and then we have the supply brought from the distance of five hundred miles or more, by an ascending vortex, which creates a vacuum, and the air near the vortex taken away in _another_ direction by a _partial_ vacuum; and so an ascending _vortex_, which creates a vacuum is supplied from a distance, and a _partial vacuum_ at a distance is supplied by the air near the perfect vacuum. such an idea of a supply by a circuitous route, and secondary influence, is not very philosophical, to say the least, and professor coffin feels it; and to the question, why is it so? which, he says, may very naturally be asked, he confesses there is no answer. and there would be none, even if his suppositions were based upon facts. but other questions might be asked equally difficult to be answered, viz.: st. is there any rarefaction which can draw the trades to the west, and in that particular locality, in opposition to the supposed vortex of sahara, by creating a _partial vacuum_? d. are they in fact so drawn? d. do the s. w. winds, south of the cape de verdes, and _under the rainy belt_, which in the summer months extend up to these islands, _reach the desert at all_? these are pertinent questions, _and every one of them must be answered in the negative_. the hypothesis is without foundation, and professor's coffin's perplexity and astonishment must remain, until he abandons the theory of rarefaction entirely. the winds which so perplex him are nothing but the regular n. e. trades, made to originate on the coast and continent of africa, in summer, by the northern transit of the whole machinery. they not only draw off from the desert coast, but they _blow over the desert itself_ on to the ocean, and into the rainy belt upon the land, as we have already seen, and the supposed vortex of rarefaction does not exist. that the monsoons do not reach the desert is demonstrated by the tables of professor coffin, and to set it at rest we will make the necessary extracts. commencing with the region from the equator to ° n., and from ° to ° w. longitude, we have the observed winds in proportion, as follows, for july and august--the south-east trades prevailing, inasmuch as the belt of rains is at this season situated further north. latitude ° to °, longitude from greenwich ° to °. +-------------------------------------------------------+ | course. | july. | august. | course. | july. | august. | |-------------------------------------------------------| | north. | | | s. s. w.| | | | n. n. e.| | | s. w. | | | | n. e. | | | w. s. w.| | | | e. n. e.| | | west. | | | | east. | | | w. n. w.| | | | e. s. e.| | | n. w. | | | | s. e. | | | n. n. w.| | | | s. s. e.| | | calm. | | | | south. | | |---------------------------| | | | | total | | , | +-------------------------------------------------------+ here, it is evident that the s. e. trades are the prevailing winds, but their course is variable. ascending to the region between ° and ° north latitude, and ° to ° west longitude, the northern part of which at this season is covered by the rainy belt; we find the monsoon, the s., s. s. w., and s. w. winds, the prevailing ones in august, although the winds are variable, as usual under the rainy belt. +-------------------------------------------------------+ | course. | july. | august. | course. | july. | august. | |-------------------------------------------------------| | north. | | | s. s. w.| | | | n. n. e.| | | s. w. | | | | n. e. | | | w. s. w.| | | | e. n. e.| | | west. | | | | east. | | | w. n. w.| | | | e. s. e.| | | n. w. | | | | s. e. | | | n. n. w.| | | | s. s. e.| | | calm. | | | | south. | | |---------|-------|---------| | | | | total | , | , | +-------------------------------------------------------+ ascending to the region of ° to ° north latitude, and ° to ° west longitude, we find the winds exceedingly variable, and the monsoons diminished remarkably. if professor coffin's theory was correct, they should increase as they approach the desert; but they in fact, diminish, and the n. e. trades are found at the north portion. +-------------------------------------------------------+ | course. | july. | august. | course. | july. | august. | |-------------------------------------------------------| | north. | | | s. s. w.| | | | n. n. e.| | | s. w. | | | | n. e. | | | w. s. w.| | | | e. n. e.| | | west. | | | | east. | | | w. n. w.| | | | e. s. e.| | | n. w. | | | | s. e. | | | n. n. w.| | | | s. s. e.| | | calm. | | | | south. | | |---------|-------|---------| | | | | total | | | +-------------------------------------------------------+ ascending to the region between ° and ° north latitude, and ° to ° west longitude, we get north of the belt of rains _and lose the monsoons entirely although still below the desert_; and find the regular n. e. trades, with less variable winds than are found in almost any other part of the ocean. +-------------------------------------------------------+ | course. | july. | august. | course. | july. | august. | |-------------------------------------------------------| | north. | | | s. s. w.| | | | n. n. e.| | | s. w. | | | | n. e. | | | w. s. w.| | | | e. n. e.| | | west. | | | | east. | | | w. n. w.| | | | e. s. e.| | | n. w. | | | | s. e. | | | n. n. w.| | | | s. s. e.| | | calm | | | | south. | | |---------|-------|---------| | | | | total, | | | +-------------------------------------------------------+ ascending still further to the region between ° and ° north latitude, and ° and ° west longitude, which borders, in part, on the s. w. corner of the desert, and we have not, during the month of august, a single wind between s. s. e. and w. n. w., which blows in upon the land; and _only twelve instances out of three hundred and ninety-four in this hottest month in the year, and on the southern portion of the desert, when the wind blows on shore from any quarter_. this is demonstration. the monsoon winds are confined to the rainy belt; they do not reach the desert, nor does the desert attract the winds from the ocean, or reverse, hold back, or disturb the trades. +-------------------------------------------------------+ | course. | july. | august. | course. | july. | august. | |-------------------------------------------------------| | north. | | | s. s. w.| | | | n. n. e.| | | s. w. | | | | n. e. | | | w. s. w.| | | | e. n. e.| | | west. | | | | east. | | | w. n. w.| | | | e. s. e.| | | n. w. | | | | s. e. | | | n. n. w.| | | | s. s. e.| | | calm. | | | | south. | | |---------|-------|---------| | | | | total, | | | +-------------------------------------------------------+ ascending once more, to the region between the degrees of and , north latitude, and and , west longitude, we find it bounded east entirely on the center of the desert. now here, certainly, there must be evidence of the truth of the rarefaction theory, if any where on the face of the earth. yet here, in july and august, we find the trades as regular as any where, and not more variable winds than are found in the trades toward their northern limits every where, and in august, only forty out of four hundred and twenty-nine winds, blowing directly or indirectly on shore. +-------------------------------------------------------+ | course. | july. | august. | course. | july. | august. | |-------------------------------------------------------| | north. | | | s. s. w.| | | | n. n. e.| | | s. w. | | | | n. e. | | | w. s. w.| | | | e. n. e.| | | west. | | | | east. | | | w. n. w.| | | | e. s. e.| | | n. w. | | | | s. e. | | | n. n. w.| | | | s. s. e.| | | calm. | | | | south. | | |---------|-------|---------| | | | | total, | | | +-------------------------------------------------------+ it would seem to be impossible for any man to believe in the theory of rarefaction, after an examination of these tables. professor coffin discovers other anomalies, for which he finds it difficult to account. among these are the northerly tendency, in the afternoon, of the winds in ohio, south of lake erie; the winds of south-western asia, which, he says, "are so irregular as to defy all attempts to reduce them to system;" particularizing the n. w. at jerusalem, the westerly at bagdad, the n. e. at constantinople, the northerly at trebizond, etc., etc. jerusalem has the mediterranean at the n. w., bagdad has it at the west, constantinople has the black sea at the n. e., trebizond n. n. w. and n. e., and the counter-trade, as it passes over them, draws its storm-surface wind or sea-breeze, from the quarter where evaporation is greatest, and the atmosphere is most susceptible of electrical inductive influence. precisely as it draws from the ocean and the eastward, east of the alleghanies, from the lake region, west of the lakes, and from the northward, south of the lakes, and from the westward, east of them. this law of attraction will explain, too, the mean prevalence of easterly winds north of the parallel of °, at the stations named in his work. great bear lake, great slave lake, and fort enterprise, lie east of the rocky mountain range which interposes between them and the pacific, and have hudson's bay and other large bodies of water on the east and north. hence, easterly winds prevail at these places. at norway house, on nelson's river, near the north end of lake winnipeg, a large body of water, which stretches off to the south, we find the south wind the prevalent one, especially in december, when the northern and north-eastern waters are frozen up, and the n. e. largely present at all seasons of the year. at new hernhut, in winter, when davis' straits are covered with floes, the prevailing wind is east, drawn from the warm, open sea east of greenland, where the gulf stream is evaporating. but in june and july, when evaporation is going on over davis' straits and baffin's bay, the prevailing winds are west and south, and the east winds fall off. other stations are equally instructive, but i must forbear. in relation, however, to the easterly zone of wind, of which professor coffin speaks, it should be added that the counter-trade, south of the magnetic pole, in high latitudes, pursues an easterly course, is near the earth, and attracts an opposite wind as it does on the east and north of the pole, in localities where the surface atmosphere is not peculiarly susceptible to its influence, and, therefore, the _winds are mainly opposite to its course_. thus, at melville island, they are almost all westerly and north-westerly, for there the remnant of the counter-trade is passing west around the magnetic pole. these westerly and north-westerly winds are very light, and like the gentle easterly breeze which sets toward the cumulus clouds and summer showers. since most of this work was written, i have procured, and read with great pleasure, lieutenant maury's "geography of the sea." it is a work of great interest, and should be in the hands of every one. the extent of ground covered, however, made it necessary for lieutenant maury to introduce much matter not derived from his own investigations. in doing this, he has taken received opinions, and has thereby introduced much heresy. the view he adopts in relation to the monsoons, although the popular one with philosophers, is of that character. he says (page ): "monsoons are, for the most part, formed of trade-winds. when a trade-wind is turned back, or diverted, by over-heated districts, from its regular course at stated seasons of the year, it is regarded as a monsoon. thus, the african monsoons of the atlantic, the monsoons of the gulf of mexico, and the central american monsoons of the pacific, are, for the most part, formed of the north-east trade-winds, which are turned back to restore the equilibrium which the over-heated plains of africa, utah, texas, and new mexico have disturbed. when the monsoons prevail for five months at a time--for it takes about a month for them to change and become settled--then both they and the trade-winds, of which they are formed, are called monsoons." again (§ - ): "the agents which produce monsoons reside on the land. these winds are caused by the rarefaction of the air over large districts of country situated on the polar edge, or near the polar edge, of the trade-winds. thus, the monsoons of the indian ocean are caused by the intense heat which the rays of a cloudless sun produce, during the summer time, upon the desert of cobi and the burning plains of central asia. when the sun is north of the equator, the force of his rays, beating down upon these wide and thirsty plains, is such as to cause the vast superincumbent body of air to expand and ascend. there is, consequently, a rush of air, especially from toward the equator, to restore the equilibrium; and, in this case, the force which tends to draw the north-east trade-winds back becomes greater than the force which is acting to propel them forward. consequently, they obey the stronger power, turn back, and become the famous south-west monsoons of the indian ocean, which blow from may to september inclusive. "of course, the vast plains of asia are not brought up to monsoon heat _per saltum_, or in a day. they require time both to be heated up to this point and to be cooled down again. hence, there is a conflict for a few weeks about the change of the monsoon, when neither the trade wind nor the monsoon force has fairly lost or gained the ascendency. this debatable period amounts to about a month at each change. so that the monsoons of the indian ocean prevail really for about five months each way, viz.: from may to september, from the south-west, in obedience to the influence of the over-heated plains, and from november to march inclusive from the north-east, in obedience to the trade-wind force." what the "trade-wind force" is, lieutenant maury tells us in another paragraph, viz.: "calorific action of the sun and diurnal rotation of the earth"--the received calorific theory. i have already shown, i think, conclusively, that there is no expansion and ascent in the supposed region of calms, which induces, or can induce, the trades; and that, in point of fact, the air on the land is cooler under the belt of rains. but as lieutenant maury, whose reputation is national, adopts the theory, i shall be pardoned for copying the following table, showing the difference of temperature at two cities of india, before, after, and while the belt of inter-tropical rains is over them. it will be seen that the temperature is actually less when the belt is there, viz., in july and august, than in april and may. _this should be conclusive upon that point._ +----------------------------------------------------+ | | anjarakandy. | calcutta. | | months. |--------------------|-------------------| | | rain. | temp. | rain. | temp. | |-----------|----------|---------|---------|---------| | | m. m. | | m. m.| | | january, | , | °, | , | °, | | february, | , | °, | , | °, | | march, | , | °, | , | °, | | april, | , | °, | , | °, | | may, | , | °, | , | °, | | june, | , | °, | , | °, | | july, | , | °, | , | ,° | | august, | , | °, | , | °, | | september,| , | °, | , | °, | | october, | , | °, | , | °, | | november, | , | °, | , | °, | | december, | , | °, | , | °, | |-----------|----------|---------|---------|---------| | year, | , | °, | , | °, | +----------------------------------------------------+ anjarakandy is on the malabar coast, between ° and ° north latitude. calcutta in an angle of the bay of bengal, at ° ' north latitude. the former is in and near the focus of the monsoons, and has a temperature in july (when inches of rain fall), about as low as in december. in the foregoing table from kaemptz, the rain is in millimetres, about twenty-five of which make an inch, and the temperature is centigrade, which may be raised to fahrenheit by adding four fifths of the quantity and also °--thus, if the height of the centigrade thermometer be °, add to this four fifths of °, which is °, and also °, the result is °. twenty-five centigrade is therefore equal to seventy-seven fahrenheit. lieutenant maury is not, and should not be a theorist. he occupies the position, in some sort, of a national _investigator_, and, of course, of national _instructor_. opinions which emanate from him, or which are endorsed by him, should be accurate. sooner or later that which he has adopted in relation to the monsoons, and some others, must be abandoned. in addition to what has already been said, i wish to call his, and the reader's attention, to several other facts and considerations in relation to the monsoons, and particularly those of india. st. the deserts of cobi and bucharia, which constitute the "burning plains" of _central_ asia, north-east of the indian ocean, lie between ° and ° of north latitude, and under the zone of extra-tropical rains. they are not wholly rainless. they partake of that saline character which affects so much of asia and the western part of this continent. south of them, running nearly east and west, are the lofty ranges of the himmalaya and kuenlun mountains, and the table lands of thibet. to their saline character, in part, but mainly to the interposition of these mountain ranges, depriving the counter-trade of moisture, they owe their comparative sterility. _if bountifully supplied with rains, this salt would doubtless ere this have been washed to the ocean, as it has been from other countries, once as salt as they._ but they have some rain, and more or less vegetation, and are not intensely hot. they lie too far north, and are too elevated. their temperature is not materially different from that of the western, and comparatively desert portions of our own country, and they are utterly incapable of creating a monsoon at the indian ocean, and especially from the long line of malabar coast, where the south-west monsoons are found in most strength. the sterile portions of utah, new mexico, and texas are alike incapable of such effect upon the atmosphere of central america and mexico. these monsoons commence in may, and prevail until october, and the temperature of the air where they blow ranges with considerable regularity between ° at night, and ° at mid-day, on the malabar coast, and a trifle lower in central america. at fort fillmore, el paso, new mexico, in latitude ° , the mean temperature for may is ° june " °, ' july " °, ' august " °, ' september " °, ' ------- and for the whole period, °, ' at santa fé, new mexico, the mean for may is °, ' june " °, ' july " °, ' august " °, ' september " °, ' ------- and for the whole period, °, ' mean of the two united, °, ' the mean of western texas is about ° higher than at fort fillmore, and of utah not materially different; and the mean of _central_ asia between ° and ° does not materially vary from them. now, it is perfectly evident that during may and september the temperature of central asia is far below that of the indian ocean and india, and never materially exceeds it. central asia is hot, "burning," if you please, compared with more elevated, fertile, or better watered territory _in the same latitude_, and so it has been characterized; but not so, compared with the indian ocean, or india, where the sun is vertical. during the greater part of the time, therefore, that the monsoons are in full blast, utah, texas, and new mexico, and cobi, and the burning plains of asia, are from ° to ° colder than the temperature of the place where the monsoons are blowing. would not such a fact be perfectly conclusive in any other science except theory-swathed meteorology? d. the theory assumes that the heated air has an ascensive force, which causes it to rise and create a vacuum, and this vacuum, by its suction, draws in the adjoining air, which immediately ascends. the adjoining air, drawn away from its locality, leaves a vacuum, and that is filled by another rush from the s. w., and so on, till the indian ocean is reached, and the monsoons are accounted for. now, look at the difficulties: the highest temperature that can be assumed for the air over cobi, at any time, without disregarding facts and analogy, is °. what is the ascensive power of an area of atmosphere of °? for this we have no problem or formula, although problems and formulas abound in the science. professor espy relied on heated air only to give the storm a _start_. his main reliance was on the latent heat supposed to be given out during condensation, for his ascensive storm power. but over these "burning plains" there is, according to the theory, no storm or cloud, or condensation on which that supposed reliance for expansion can be placed. what, then, is the ascension force of air at °? _we ought to know, for we sometimes have it as high, or within two or three degrees as high, in all the eastern and middle states._ the monsoons blow at from twenty to twenty-five miles an hour, and sometimes more. is that the ascensive force of air at °? at miles an hour it would be , feet; at miles, , feet; and at miles, feet per minute. does any man believe that either current exists? why, then, do we not have our hats taken off, or light objects carried up, or have a monsoon, or, at least, have the clouds running up, when we have such elevated temperatures. _nothing of the kind occurs with us._ our hottest days are comparatively still days; and i have seen the cumulus sailing gently to the east, horizontally, when the air was at °. why should we be exempt? is not our air the same and our heat the same? again, suppose we grant that the ascensive force is equal to or even miles an hour, will not the adjoining air hold back somewhat to avoid leaving behind an entire vacuum? or, will it all voluntarily rush in, and leave a new complete vacuum? and, if so, why the preference of vacuums by the air, and _when, where, and why_, should the _successive vacuums stop_? nay, would not gravity fill the second vacuum from _above_, rather than from the south-west side? and will not the air incline to rush in, to some or all these successive vacuums, from some other side than south-west? or, have these deserts the power of selecting the quarter from which their vacuum shall be filled, and of delegating it to succeeding vacuums? would it not incline to rush in from the east and west where there are no elevations, rather than from the s. w. and over the kuenlun mountains, the intervening ridges and valleys of thibet, the lofty himmalayas, the extent of india, and the ghaut mountains, from three to four thousand feet high, on its eastern coast? would it not, at least, _leak in a little_, and lessen the force with which the vacuums would draw from the far-off indian ocean, so that the monsoon could not blow with equal force? or, if cobi and its fellow deserts _must_ and _can_ draw from an _ocean_, why not from the head of the arabian sea, or bay of bengal, or the china sea, which are nearer, or from the japan sea, which is still nearer, or the yellow sea, which is close by? why draw only from under the central belt of rains? nay, what shall be done with professor dove? in a recent article, republished in the american journal of science and art, for january, , he says: "a greatly diminished atmospheric pressure taking place in summer over the _whole continent_ of asia must produce an influx from all surrounding parts; and thus we have west winds in europe, north winds in the icy sea, east winds on the east coast of asia, and south winds in india. _the monsoon itself becomes, as we see, in this point of view, only a secondary phenomena._" this looks very like _antagonism_. who shall we believe? again, suppose you get one atmosphere from the whole area, raised up by the supposed ascensive force, and at the rate of twenty-five, twenty, or even ten miles an hour, and a new volume drawn in from the south-west, and _over the mountains_: will it not take a _little time_ for _that_ to _heat up_? does it heat so fast as to _keep up the ascensive force_ without intermission, at twenty-five, or twenty, or ten miles the hour? what says mr. ericsson to this? can he not arrange with a moderate lens, to move his engine with the rays of the summer sun? nay, lieutenant maury says they can not heat up "_per saltum_, or in a day." but according to a reasonable calculation, they must heat up the air from °, or less, to °, at the rate of , feet per minute. heating , feet in depth, in the proportion of ° per minute, night and day, for five months, is "_per saltum_" in a minute, and , "_saltums_" per day! and further still, the indian ocean, from which the monsoons are drawn to cobi and central asia to the n. e., is during those months covered by the belt of calms and rains, as heretofore stated; and the s. e. trades blowing into it are attributed to the suction created by the ascent of heated air _there_. so, then, the monsoons are blowing away from under the rainy belt, from to miles, to cobi and the burning plains of asia, while the ascensive force of that belt is such as to draw the s. e. trades toward the very spot, a distance of , or , miles, at miles an hour! what must the ascensive force over cobi, etc., be, if, as a "stronger power," it can overcome an ascensive force over the indian ocean sufficient to draw the s. e. trades , miles, at miles an hour; and, in addition to the force necessary to resist this central suction, not only stop or hold back the n. e. trade, but reverse it and draw it back, at miles an hour, as a monsoon? must it not be, at least, double that of the belt of calms, or the "great region of expansion," as professor dove calls it? now, i am irresistibly tempted to ask whether a meteorological theory can be too absurd for credence, and whether it would not be as well to endow the deserts with ribs and lungs, and a proboscis long enough to reach the indian ocean, and the necessary power of inspiration and expiration? such a theory would avoid all difficulties, conflict with no more analogies, and, in my judgment, be as much entitled to credit as the one to which meteorologists adhere. d. north of the malabar coast, in the north-west of india, lies an extensive desert. west of that is beloochistan, with its rainless deserts. further west are the rainless deserts of arabia, and these three, including the persian deserts further north, cover _as much surface_ as the deserts of cobi and bucharia--have the sun vertical in part, and nearly so over the entire surface--_are more intensely hot_, and lie within _one third of the distance_ which intervenes between that desert and the indian ocean off the malabar coast, with _an open sea and_ no _mountains between_. now, look at it. the north-west desert of india, and the rainless deserts of beloochistan and arabia _reverse no trade_ and _have no monsoon_, although the arabian sea heads right up among them. they do not attract one from the indian ocean off the malabar coast, although not more than one third of the distance off, and without such mountains and table lands intervening as separate that coast from cobi. it is said by lieutenant maury that the monsoons, "_obey the stronger force_." but which is the stronger force? cobi, not _wholly_ rainless, lying north of °, under the zone of extra-tropical rains, with india and the ghauts, the himmalaya mountains, the table lands of thibet, and the kuenlun mountains between? or the deserts of india, beloochistan, and arabia, _wholly rainless_, and _intensely hot, near by_, and in _open view_. there can be but one answer to this question. nothing in the way of desert barrenness, or elevated temperature, unless it be those of sahara, can exceed the deserts about the head of the arabian sea and persian gulf. certainly those of cobi can not compare with them; yet the trades blow steadily over them, although more northerly there, as every where, near their northern limits, especially on land. says hopkins, in his atmospheric changes: "if any one part of the broad expanse of the continent of asia could be heated so as to draw air from the arabian sea and the indian ocean during the summer, it would be that part which lies between hindoostan and the lake of aral, including the region between the valley of the oxus and persia, and the land of this part, unlike hindoostan, is not screened from the sun by thick vapors. but what says burnes respecting the winds of this part? why, that about the latter end of june, though the thermometer was at ° in the day, 'in this country a steady wind generally blows from the north.' and on the d of august, after having passed the oxus--'the heat of the sand rose to °, and that of the atmosphere exceeded °, but the wind blew steadily, nor do i believe that it would be possible to traverse this tract in summer if it ceased to blow. the steady manner in which it comes from one direction is remarkable in this inland country.' again--'the air itself was not disturbed but by the usual north wind that blows steadily in this desert.' and he has many other similar passages." here there is a vast tract of country south of ° which has a temperature often of °, and does not reverse the trade and create a monsoon. how utterly unphilosophical, then, to attribute the monsoons to cobi because they "obey the stronger force!" or to attribute them to it at all. th. the monsoons can not be _traced from_ the malabar coast _to cobi_. they do not exist on the south-west of cobi and near it, where they should in greatest force, and there is no connection, in fact, shown between them. they do not often extend more than twenty-five miles inland, or to the east of the ghauts. there are no corresponding intervening monsoons crossing india to the mountains--none over the mountains and table lands--none under the northern lee of the mountains--nor, in short, on the whole track, nor any s. w. winds except such as naturally belong to the action of the curving counter-trade. finally, the investigations of commodore wilkes on mauna loa, a mountain upon hawaii, more than , feet high, and the observations of professor wise and other aeronauts are sufficient to put this whole matter of heated lands and ascent of the atmosphere as the cause of winds, at rest. commodore wilkes was encamped for about _twenty days_ on pendulum peak, in december and january . although not up to the elevation of the counter-trade in that latitude, he was above the local clouds which form over the island during the day, where the sea breezes blow in with as great strength as any where. indeed, he was on the top of the "lofty conical mountain" to which caleb williams alludes in the letter to professor espy i have quoted, and above the spot where professor espy assumed that the clouds were rising with such force as to induce the strong sea breezes of that island. during this time there were two snow-storms on mauna loa, and they had the wind from the s. w. during the storm, as might be expected, looking at the situation of the mountain on the western side of the island. these storms moved to the n. w., and were observed at the other islands in that direction as rain. the local clouds lay over the island every day, as they do over active volcanic islands which are very elevated, although it was the dry season. _nothing like an ascent of the clouds or of the currents of air from the ocean was observed._ on the contrary, the clouds formed before the sea breezes set in, and the latter blew from the different sides of the island in under the clouds, and outward again, probably on the opposite side. the whole interior of the island is elevated, and its temperature low; and _there was no elevation of temperature on the high portions of the island over which the clouds formed, and toward which the winds blew, which could create an upward current_. "during our stay on the summit, we took much pleasure and interest in watching the various movements of the clouds; this day in particular, they attracted our attention; the whole island beneath us was covered with a dense white mass, in the center of which was the cloud of the volcano rising like an immense dome. all was motionless until the hour arrived when the sea-breeze set in from the different sides of the island; a motion was then seen in the clouds, at the opposite extremities, both of which seemed apparently moving toward the same center, in undulations, until they became quite compact, and so contracted in space as to enable us to see a well defined horizon; at the same time there was a wind from the mountain, at right angles, that was affecting the mass, and drawing it asunder in the opposite direction. the play of these masses was at times in circular orbits, as they became influenced alternately by the different forces, until the whole was passing to and from the center in every direction, assuming every variety of form, shape and motion. "on other days clouds would approach us from the s. w. when we had a strong n. e. trade-wind blowing, coming up with cumulus front, reaching the height of about eight thousand feet, spreading horizontally, and then dissipating. at times they would be seen lying over the island in large horizontal sheets as white as the purest snow, with a sky above of the deepest azure blue that fancy can depict. i saw nothing in it approaching to blackness at any time." (exploring expedition, vol iv. p. ). here, in the last paragraph, we have the whole truth disclosed. the n. e. trade was blowing on mauna loa, , feet above the sea, and the sea-breeze blew in on the _leeward side_, its moisture condensing over the volcanic island, but without rising _up the mountain_, or _through the surface-trade_, or _above , feet_. so, too, the celebrated aeronaut, mr. wise, in the course of more than a hundred ascensions, some during high wind, and others during rain storms, never met with an ascending current, except in a single instance, in the body of a hail-cloud, and then there were descending currents also, the usual intestine motion of hail-cloud with its opposite polarities. i copy a description of his passage through the clouds of a rain-storm, and his floating a long period above them; and there was no ascending current which disturbed their horizontal repose or progression. the double layer is not uncommon--condensation taking place at the connection of the upper and lower portions of the trades, with the surrounding atmosphere; or in the trade, and by _induction in the surface atmosphere_ at the same time. such instances are frequently visible, and if his ascensions had been undertaken at other times in stormy weather he would have seen more of them. "before i passed the limits of the borough, a parachute, containing an animal, was dropped, which descended fast and steady, and, just as it reached the earth, my ærial ship entered a dense black body of clouds. ten minutes were consumed in penetrating this dismal ocean of rainy vapor, occasionally meeting with great chasms, ravines, and defiles, of different shades of light and darkness. when i emerged from this ocean of clouds, a new and wonderfully magnificent scene greeted my eyes. a faint sunshine shed its warmth and luster over the surface of this vast cloud sea. the balloon rose more rapidly after it got above it. viewing it from an elevation above the surface, i discovered it to present the same shape of the earth beneath, developing mountains and valleys, corresponding to those on the earth's surface. the profile of the cloud-surface was more depressed than that on the earth, and, in the distance of the cloud-valley a magnificent sight presented itself. pyramids and castles, rocks and reefs, icebergs and ships, towers and domes--every thing belonging to the grand and magnificent could be seen in this distant harbor; the half-obscured sun shedding his mellow light upon it, gave it a rich and dazzling luster. they were really "castles in the air," formed of the clouds. casting my eyes upward, i was astonished in beholding another cloud-stratum, far above the lower one; it was what is commonly termed a "mackerel sky," the sun faintly shining through it. the balloon seemed to be stationary; the clouds above and below appeared to be quiescent; the air castles, in the distance, stood to their places; silence reigned supreme; it was solemnly sublime. solitary and alone in a mansion of the skies, my very soul swelled with emotion; i had no companion to pour out my feelings to. great god, what a scene of grandeur! such were my thoughts; a reverence for the works of nature, an admiration indescribable. the solemn grandeur--the very stillness that surrounded me--seemed to make a sound of praise. "this was a scene such that i never beheld one before or after exactly like it. two perfect layers of clouds, one not a mile above the earth; the other, about a mile higher; and, between the two, a clear atmosphere, in the midst of which the balloon stood quietly in space. it was, indeed, a strange sight--a meteorological fact, which we cannot possibly see or make ourselves acquainted with, without soaring above the surface of the earth." (history and practice of aeronautics, p. ). this is graphic. perhaps in relation to the conformity of the upper surface of the inferior layer of clouds, to the irregularities of the earth's surface, he was misled during the enthusiasm of the moment. he is certainly mistaken as to the possibility of observing these double layers from the earth; i have seen them in hundreds of instances. but in relation to the _quiescence_ of the clouds for an hour, and _the entire absence of ascending currents_, he could not be mistaken. and now, in the absence of all direct proof to sustain the hypothesis, that the heating of the land produces ascending currents, and thereby the winds, and especially the monsoons, and in view of all the adverse evidence, i put it to lieutenant maury, and every sincere searcher after meteorological truth, whether the theory should not be abandoned. chapter vii. the counter-trade of the northern hemisphere ranges at different heights in different latitudes, in the same latitude at different seasons, and also upon different days of the same season; and, like the line of perpetual snow, has its greatest elevation in the tropics, descending gradually to the surface of the ocean at the poles. at the northern limit of the n. e. trades, it does not, ordinarily, approach the earth sufficiently near for decided reciprocal action. hence, at that point, storms do not often originate; the winds are lighter and more variable, and calms are more frequent than at any point, except at the meeting and elevation of the trades, or in the polar regions. doubtless this state of things is increased by the feebler action of north polar magnetism, and the irregular action of the longitudinal magnetic currents, evinced by the irregular, and often, feeble action of the trades, near their extreme limits. they are not unfrequently wholly wanting, near the northern limit, for several days in succession, and calms and baffling winds are found in their place--another effect of the irregular action of terrestrial magnetism, consequent upon the ever-changing transit of central activity from south to north, and from north to south. upon the islands, however, and continents, which have elevated mountain peaks and ridges, especially if of volcanic origin and activity, which approach more nearly the path of the counter-trade, a different state of things exists. there, showers and gusts are frequent. thus, upon the sandwich island, kauai, the most northern one, which is within the region of the n. e. trade during ten months of the year, and upon its volcanic peaks and elevated table-lands, and north-easterly from them, over the district of waioli, rain falls in abundance during the year, while the coastlines upon other portions of the island can not be cultivated without irrigation. (see wilkes' exploring expedition, vol. iv. pp. and ; and american journal of science and art, for may, ). a like state of things, in degree, may be found upon the canaries, and the more elevated of the west india islands. the cape de verdes are an exception, and the christian world are quite often called upon for contributions of provisions, to save the inhabitants of these islands from starvation. they lie at the northern limit of the equatorial belt, and for a period of two months only (july and august), are supplied with rain. if, from any cause, the belt does not move as far north as usual during any season, unbroken drought and famine are sure to overtake them. the islands contain some elevated peaks, and are of volcanic origin, but not of present volcanic activity, and the counter-trades as they issue from the equatorial belt at their highest elevation, are too far above them for reciprocal, influential action. if the islands could be placed ° further north, we should hear no more of drought or famine from them, and their quantity of rain and fertility would be not only more permanent, but much increased. superadded to this, is the fact, that at that point the belt of rains precipitates feebly because the s. e. trade originates upon the southern part of the continent of africa, and the n. e. mainly, upon the desert and the barbary states--and both are sparingly supplied with moisture. the same state of things is strikingly obvious upon continents wherever the mountains are sufficiently elevated, even within the trade-wind region. thus, in south america, the andean ranges are of great elevation, and spurs and table-lands extend from them a considerable distance to the eastward. there, the s. e. and n. e. trades of the atlantic meet in very considerable volumes, and not only is the equatorial belt much wider than upon the atlantic and pacific, but the counter-trades are met upon the elevated peaks and mountain-ranges, and showers and storms on their eastern slopes and summits are frequent during the dry season--down even to the extra-tropical belt. i have already said that it was probable that the great elevation of the andes diverted and turned south a portion of the n. e. counter-trade which would otherwise pass over the western coast of peru. the report of lieutenant herndon, which has come to my notice since that was written, states facts which strongly corroborate that opinion. it seems that the trades and counter-trades actually _bank up_, in their passage to the westward, against those mountains, and the true elevation of their eastern slopes can not be barometrically ascertained. (see report of the exploration of the amazon, p. ). lieutenant herndon says: "i was surprised to find the temperature of boiling water at egas to be but ° ', the same within ' of a degree that it was at a point one day's journey below tingo maria, which village is several hundred miles above the last rapids of the huallaga river; at santa cruz, two days above the mouth of the huallaga, it was ° '; at nauta, three hundred and five miles below this, it was ° '; at pebas, one hundred and seventy miles below nauta, ° '. i was so much surprised at these results that i had put the apparatus away, thinking that its indications were valueless; but i was still more surprised, upon making the experiment at egas, to find that the temperature of the boiling water had fallen ° below what it was at santa cruz, thus giving to egas an altitude of fifteen hundred feet above that village, which is situated more than a thousand miles up stream of it. i continued my observations from egas downward, and found a regular increase in the temperature of the boiling water until our arrival at pará, where it was ° '. "from an after-investigation, i am led to believe that the cause of this phenomenon arises from the fact that the trade-winds are dammed up by the andes, and that the atmosphere in those parts is, from this cause, compressed, and, consequently, heavier than it is further from the mountains, though over a less elevated portion of the earth. the discovery of this fact has led me to place little reliance in the indications of the barometer for elevation, at the eastern foot of the andes. it is reasonable, however, to suppose that this cause would no longer operate at egas, nearly one thousand miles below the mouth of the huallaga." the report of lieutenant gibbon, is also exceedingly instructive. separating from lieutenant herndon at tarma, upon the andes, he pursued a southern course, along the eastern slopes of the chain from ° ' south, almost to ° south, at ohuro, making a journey of about ° ' of latitude. a considerable portion of this journey was over eastern and less elevated portions of the andes; but little below, however, the line of perpetual snow. here, during the dry season, he met with frequent showers and fogs from the eastward, but left them as he descended into the plains upon the table-land. there he found the dry season more distinctly marked; but occasional irregularities were found upon the table-lands, as every where upon corresponding elevations. the s. e. trades, however, were there obvious, during the dry season, notwithstanding the irregularities. the rainy season, from december to may, he spent at cochabamba, and at its close he traveled north down the madeira and its tributaries, to the amazon. although scarcely consistent with my prescribed limits, i can not forbear making a few extracts. thus, when on the mountains, east of huanvelica, in the n. e. counter-trade, he says: "our course is to the eastward. the snow-capped mountains are in sight to the west. temperature of a spring °; air °. lightning flashes all around us; as the wind whirls from _north-east_ to south-west, rain and snow-flakes become hail, half the size of peas. thunder roars and echoes through the mountains; the mules hang their heads, and travel slowly; the thinly-clad aboriginal walks shivering as he drives the train ahead; the dark cumulus cloud seems to wrap itself around us." again, at the bombam post-house, in the focus of change from cirrus to cumulus, and stratus, and storm: "the winds are very gentle, and curl the cirrus or hairy clouds in most graceful shapes about the hoary-headed andes, in rich and delicate clusters; when the peak is concealed, all but the blue tinge below the snow, we see a natural bridal vail. an _easterly wind_ lifts and turns them to dark, cumulus clouds, settled on the frosty crown, like an old man's winter cap; the physiognomical expression is that of anger. the change is accompanied by thunder, and seems to command all around to clothe themselves for storms. the cold rain comes down in _fine drops_ upon us; the day grows darker, and the _clouds press close upon the earth_." during an excursion east of cuzco-- "turning from the river, we ascend a steep ridge of mountains--the eastern range at last. a heavy mist _wafts upward as the winds drive it against the side of the andes_, so that our view is shortened to a few hundred yards. we hope the curtain will rise that we may view the productions of the tropical valley below; but the mist thickens, and the day gets dark with heavy, heaped-up black clouds; a rain-storm follows. the grasses are thrifty, and the top of the ridge covered with a thick sod. by barometer, we stand eleven thousand one hundred feet above the level of the sea." in may following, having spent the rainy season in cochabamba, he travels north-- "our route from tarma to oruro was south. we traveled ahead of the sun. in december, when we arrived in cochabamba, the sun had just passed us. as soon as he did so, the rains descended heavily on this side of the ridge; it was impossible to proceed. the roads were flooded, the ravines impassable, and the arrieros put off their journey until the dry season had commenced. after the sun passed the zenith of cochabamba, and had fairly moved the rain belt after him toward the north, then we came out from under shelter, and are now walking behind the rain belt in dry weather, while the inhabitants are actively employed in tending their crops." so on the north of the equatorial belt, along the whole line of the andes, up to the northern boundary of the desert valley of the gila, rain falls on the high mountain-ranges, owing to the contiguity of the counter-trade and the diversion of showers to the north, along their eastern sides. during the survey of the boundary line between mexico and california, etc., by the commission under mr. bartlett, it became necessary to find some spot where water and grass were abundant, for the head quarters of the commission. this was found, and _could only be found_, upon the mimbres mountains, at an old abandoned spanish copper mine, , or , feet above the level of the sea, surrounded with peaks of still greater height. these elevated ranges were within influential distance of the counter-trade, and here snow fell in the winter, from the extra-tropical belt, and rain, in showers, in summer, at the period of the most northerly extension of the tropical belt; when fifteen miles off, in the valley, it was unbroken drought. mr. bartlett thus describes it in his personal narrative: "we reached this district on the d of may. vegetation was then forward, though there had been no rain. but it must be remembered that during the winter there is snow, and hence a good deal of moisture in the earth when the spring opens. the months of may and june were moderately warm. on the third of july the first rain fell. it then came in torrents, accompanied by hail, and lasted three or four hours. many of our adobe houses were deluged with water, and the mountain-sides exhibited cataracts in every direction. the arroyo, which passes through the village, and which furnishes barely water enough for our party and the animals, became so much swollen as to render it difficult to cross; and, by the time it had received the numerous mountain torrents, which fall into it within a mile from our camp, it became impassable for wagons, or even mules. the dry gullies became rapid streams, five or six feet deep, and sometimes fifty feet or more across. on this day, a party, in coming to the copper mines, from the plain below, _where there had been no rain_, found themselves suddenly in a region overflowing with water, so that their progress was arrested, and they were obliged to wait until the flood had subsided. after this we had occasional showers, during the months of july and august." the location of this mountain station is near the thirty-third degree of north latitude, while the northern limit of the equatorial belt, nowhere, except upon the mountain ranges and table-lands of mexico, extends above °. there, for the reason we have been considering, it does extend further north during july and august, in occasional showers, and in the vicinity of mount picacho, mr. bartlett met one of its mountain thunder-storms on the th of july, on his return south through mexico, in latitude °, in the following year. (personal narrative, vol. ii. p. ). these showers originated in strata of counter-trade, which had followed up along the eastern side of the mountains and not from strata which had crossed them and curved to the eastward, as is shown by the course of progression of the showers. let us look, in this connection, at a fact or two of great interest, though not directly connected with the point in hand. the southern limit of the extra-tropical belt in winter, on the pacific coast of north america, is in the vicinity of san diego, at about °. in summer, that limit is carried up above astoria, which is in latitude ° '--about °--yet new mexico receives little if any rain in winter in the vicinity of albuquerque, but does receive a limited supply of about seven inches in summer and autumn, five and a half inches of which falls in june, july, and august. albuquerque is in latitude ° ', below the southern summer limit of the extra-tropical belt, and north of the northern limit of the equatorial belt. this anomaly is explained by the extension west over northern new mexico, of the extreme western edge of our concentrated counter-trade, by reason of its issuing further west from the equatorial belt in its northern extension in the summer months. this western edge, in curving to the east, north-east of new mexico, covers the north-western states, iowa, minnesota, wisconsin, etc., and furnishes them that great excess of summer precipitation which is a peculiarity of their climate; and its absence further east in winter, and the very great elevation of the rocky mountains and other ranges over which their ordinary counter-trade of that season curves, account for the absence of much precipitation and snow there, or over the valley of the rio grande in new mexico, in winter. we may now see, too, why the western coast and the pacific region of the continent, below °, are so deficient in moisture. the s. e. trades, which arise from the western portion of the south atlantic and the continent of south america, which, if it were not for the andes chain, in their natural course, after passing the equatorial belt, would continue on to the north-west until they passed the limits of the n. e. trades, and curve in upon the western portion of our continent below °, and supply it bountifully with rain, are, in part, perhaps, diverted along the eastern side of those mountains to swell the volume of our counter-trade, and in part pass them, almost exhausted of their supply of moisture by their contiguous reciprocal action. hence, too, the deficiency of precipitation at the base of the andes, on the western side, and the peculiar and irregular character of the winds under the western lee of the andean range. baffling airs and bands of calms prevail on this portion of the pacific, except where the mountains fall off, and then there is a westerly or south-westerly monsoon under the equatorial belt. says lieutenant maury in his charts, sixth edition, p. : "the passage, under canvass, from panama to california, as at present made, is the most tedious, uncertain, and vexatious that is known to navigators. "my investigations have been carried far enough to show that at certain seasons of the year a vessel bound from panama to california, must cross at least three, at some seasons four, such meetings of winds or bands of calms, before she can enter the region of the n. e. trades. hence the tedious passage." such will ever be the state of things on this continent and upon the eastern pacific, so long as the s. e. counter-trades are compelled to pass over the mountain chain of south and central america. again, if we examine carefully the belt or zone of extra-tropical rains, we shall find that the focus of greatest precipitation is considerably north of its southern limit, and that, other things being equal, this focus travels north in summer, and gives to higher latitudes their needed summer rains. this is very apparent upon the north-western portion of our continent, as the following table will show: +----------------------------------------------------------- | | lat. |jan.|feb.|mar.|apr.|may.|june.| | |-------|----|----|----|----|----|-----| |san diego, cal. | ° '| . | . | . | . | . | . | |san francisco. | ° '| . | . | . | . | . | . | |cant., far w., cal.| ° '| . | . | . | . | . | . | |astoria, oregon. | ° '| . | . | . | . | . | . | |puget's s'd, ore. | ° '| . | . | . | . | . | . | |sitka, russ. am. | ° '| . | . | . | . | . | . | +----------------------------------------------------------- ---------------------------------------+ |july.|aug.|sept.|oct.| nov.|dec.|year.| |-----|----|-----|----|-----|----|-----| | . | . | . | . | . | . | . | | . | . | . | . | . | . | . | | . | . | . | . | . | . | . | | . | . | . | . | . | . | . | | . | . | . | . | . | . | . | | . | . | . | . | . | . | . | ---------------------------------------+ the figures are for inches and tenths of an inch of rain. thus, it will be seen that in january, when the southern line is at san diego, at the south line of california, the focus of precipitation is over oregon; and that in august and september when the southern line is carried up and over oregon, the focus has traveled north to sitka, and that it is always at least ° north of the southern line of the belt upon that coast. the increased quantities of rain which fall at the focus of precipitation there, from oregon up, are doubtless much enhanced by the equatorial oceanic current which flows over opposite that part of the continent. a like effect, precisely, is produced in europe. the quantity of rain which falls at bergen, in norway, being - / inches per year, more than three times the average for that continent. the difference shown in the foregoing table, between astoria and puget's sound, is owing to the fact that the latter lies in the interior and within the coast range of mountains, while astoria is situated at the mouth of the columbia river, with an open view of the ocean. a like comparative increase of precipitation in northern latitudes, in summer, is found every where varying according to the local influences which operate in the particular case. thus, ------------------------------------------------------------------------ there falls in |winter.|spring.|summer.|aut'mn.| year. ---------------------------------|-------|-------|-------|-------|------ burlington, vt., lat. ° ' | . | . | . | . | . albany, n. y., lat. ° ' | . | . | . | . | . minnesota, iowa, lat. ° ' | . | . | . | . | . st. peters'g, russ., lat. ° '| . | . | . | . | . pekin, china, lat. ° | . | . | . | . | . ------------------------------------------------------------------------ pekin lies in the northern part of china, and would have a much larger fall of rain from a concentrated counter-trade, but for the numerous mountain-ranges which intersect its path in winter, but over which it passes at a greater elevation during the summer--a peculiarity from which the eastern section of this country is most remarkably and happily free. thus, it is obvious that the focus of precipitation in the zone of extra tropical rains, is some ° to ° north of its southern line, and travels with the whole machinery in its annual transit north and south. it is a question of some difficulty, perhaps, whether this focus is increased by the increase of magnetic action at this point, for both the line of descent of the counter-trade, and the focus of magnetic action, are carried up in a like manner, and for a like cause, and, in all probability, both concur in the result. there is exceeding wisdom in this provision for the gradual subsidence of the counter-trade, and gradual increase of magnetic intensity, and consequent gradual precipitation. on the european continent, and over western asia, there are ° of latitude to be supplied with moisture by this polar belt of rains. if the focus of precipitation was at its southern border, the counter-trade would be deprived of its moisture at that point, and little would reach the more northern portions of the globe which are to be supplied by it. but the movement of the whole machinery carries up the southern line from the south boundaries of the barbary states on to the mediterranean and portions of southern europe, and the focus of precipitation and of near approach of the counter-trade to the earth, being situated far north of the southern line, is carried up correspondingly, while the combination of the moisture with the atmosphere by south polar magnetism and electricity, and the gradual descent of the counter-trade, enable it to resist, to some extent, the influence of north polar magnetism and cold, and thus retain portions of its moisture for distribution in the polar regions. _the elevation of the counter-trade above the earth varies in the same latitude with the variations in the phenomena of the weather._ an attentive observation of the clouds of our climate will soon satisfy any one of this, after he has become familiar with them, so as to distinguish with certainty the clouds of the trade. its range, in this country, is from , feet, or less, to , feet above the earth, and its depth with us probably, from , to , feet. gay-lussac, in his scientific experimental balloon ascension, the first of _that character_ ever made, except an imperfect one just previous, by himself and biot, found it at about , feet over paris, and about , feet in depth. it is detected by the thermometer when much elevated. the atmosphere grows cool as it is ascended on mountains, or by balloons. the rate of cooling is ordinarily about ° of fahrenheit for every feet. if it were not for the equatorial current, this progressive decrease of temperature would doubtless be perfectly uniform. of gay-lussac's ascension, on this point it was said: "at forty minutes after o'clock, on the morning of the th september, , the scientific voyager ascended, as before, from the garden of the repository of models. the barometer then stood at . english inches, the thermometer at ° fahrenheit, and the hygrometer at - / °. the sky was unclouded, but misty. "during the whole of this gradual ascent, he noticed, at short intervals, the state of the barometer, the thermometer, and the hygrometer. of these observations, amounting in all to twenty-one, he has given a tabular view. we regret, however, that he has neglected to mark the times at which they were made, since the results appear to have been very materially modified by the progress of the day. it would likewise have been desirable to have compared them with a register, noted every half hour, at the observatory. from the surface of the earth to the height of , feet, the temperature of the atmosphere decreased regularly, from ° to ° ' by fahrenheit's scale; _but afterward it increased again, and reached to ° ' at the altitude of , feet_; evidently owing to the influence of the warm currents of air which, as the day advanced, rose continually from the heated ground. from that point the temperature diminished, with only slight deviations from a perfect regularity. at the height of , feet the thermometer subsided to ° ', on the verge of congelation; but it sunk to ° ' at the enormous altitude of , feet above paris, or , feet above the level of the sea, the utmost limit of the balloon's ascent." the high range of the barometer indicated a very considerable elevation of the trade at the time gay-lussac made his ascension. i am not aware that it has since been found at so great an elevation, in so high a latitude, though it is undoubtedly elevated by the interposition of a large volume of n. w. air, upon some occasions, to nearly the same altitude with us. in the extract in relation to the ascension of gay-lussac, we have another of the thousand hastily-adopted and absurd hypotheses connected with the caloric theory. it is obviously and utterly _impossible_ that in addition to the ordinary accumulation of heat at the surface of the earth "_as the day advanced_"--that is, _during the forenoon_, warm currents should ascend, unobserved by gay-lussac during an ascent of , feet--not _affecting in the least_ so large an intervening body of the atmosphere or his thermometer, and in such immense volumes as to increase the warmth of a stratum of , feet in depth, an average of ° of fahrenheit, and to the extent of ° at the center. very few balloon ascensions have been made with a view to scientific and accurate observation. but other aeronauts have met the counter-trade at different altitudes, and in both clear and stormy weather. recently, in , four ascensions were made in england, under the direction of the kew observatory committee, of the british association. i copy from the august number of the "london, edinburg, and dublin magazine," for , the following condensed amount of the result: "the ascents took place on august th, august th, october st, and november th, , from the vauxhall gardens, with mr. c. green's large balloon. "the principal results of the observations may be briefly stated as follows: "each of the four series of observations shows that the progress of the temperature is not regular at all heights, but that at a certain height (_varying on different days_) the regular diminution becomes arrested, and for the space of about , feet the temperature remains constant, or even increases by a small amount. it afterward resumes its downward course, continuing, for the most part, to diminish regularly throughout the remainder of the height observed. there is thus, in the curves representing the progression of temperature with height, an appearance of _dislocation_, always in the same direction, but varying in amount from ° to °. "in the first two series, viz.: august th and th, this peculiar interruption of the progress of temperature is strikingly coincident with a _large_ and _rapid fall_ in the temperature of the _dew-point_. the same is exhibited in a less marked manner on november th. on october st a dense cloud existed at a height of about , feet; the temperature decreased uniformly from the earth up to the _lower_ surface of the cloud. when a slight rise commenced, the rise continuing through the cloud, and to about feet above its upper surface, when the regular descending progression was resumed. at a short distance above the cloud, the dew-point fell considerably, but the rate of diminution of temperature does not appear to have been affected in this instance in the same manner as in the other series; the phenomenon so strikingly shown in the other three cases being perhaps modified by the existence of moisture in a _condensed_ or vesicular form. "it would appear, on the whole, that about the principal plane of condensation heat is developed in the atmosphere, which has the effect of raising the temperature of the higher air above what it would have been had the rate of decrease continued uniformly from the earth upward." these gentlemen do not adopt the absurd explanation of the french philosophers; they account for the phenomenon by supposing heat to be _developed_ at that particular part of the atmosphere; but they are equally wide of the mark. they found the excess of heat there to the extent of ° to °, and on days when there was no condensation, or other assignable cause for its _development_. the temperature of the counter-trade partakes, doubtless, of the temperature of the adjoining strata at its upper and lower portion, and has never been found much, if any, higher than ° at the center. nor could it be expected. the trade, in its upward curving course, within the tropics, attains a considerable altitude where the atmosphere is comparatively cold, and necessarily loses a portion of its heat there, and during its northern flow. probably its central summer range, in the latitude of paris, is not far from °, and with us °. the contrast between the trade and the surrounding atmosphere, in winter, is much more striking, and this has been observed particularly upon the brocken of the alps, and in the polar regions. "in all seasons the temperature is higher on the brocken, on a serene, than on a cloudy day, and, in the month of january, _the serene days were warmer than at berlin_." (kämtz's meteorology, by walker, p. .--note.) as the portion of the counter-trade, which does not become depolarized--in diminished volume--progresses toward the polar regions, it settles nearer the earth, and within the arctic circle is found but little way above it. thus, in december, , parry, at winter island, in latitude ° ', flew a kite, with a thermometer attached, to the height of feet, and found that the temperature, instead of falling - / °, the usual ratio of decrease, rose / of a degree. the same thing was observed at spitzbergen, in latitude ° ' north, and at bosekop, latitude ° ', by a scientific commission, and by means of kites, confined balloons, and the ascent of elevations. "in winter the temperature goes on increasing with the height, up to a certain limit, which is variable, according to the different atmospheric circumstances, the influence of which is not yet very exactly known. the hour of the day appears to be indifferent, since there exists no thermometric diurnal variation in the strata of the surface. the mean of thirty-six experiments, made with kites, or with captive balloons, at bosekop, latitude ° ' north, has given a mean rate of increase of ° ' for the first hundred meters.[ ] beyond this limit, and even beyond the first or meters, the temperature again becomes decreasing, at first very slowly, but afterward the decrease is accelerated. the observations that have been made on the flanks, or on the summits, of mountains, during the same expeditions, entirely confirm these results. the cooling influence of a soil, that radiates its own heat for several weeks, without receiving any thing on the part of the sun, in compensation of its losses, the influence of _counter-currents from above_, coming from the west and the south-west, with a high temperature, account for this anomaly, which, in winter, represents the normal state of the most northern parts of the european continent." (walker's kämtz, p. .--note.) mr. walker is the only author, so far as i know, who has suspected the true cause of the phenomenon, viz.: "currents from above coming from the west and south-west, with a high temperature;" but the caloric theory "sticks like a burr," and he adheres also to the idea that a snow-clad surface, in the absence of the sun, can aid, by radiation, in warming the atmosphere for a distance of several hundred yards above it, increasing the warmth as the distance from the earth increases! this contrast between the counter-trade and the adjacent atmosphere, in winter, in latitudes as low as that of the brocken, is probably heightened by the increased warmth of the former, at that season. the s. e. trades then form under a vertical sun, and the difference of temperature can not be less than from ° to °. not unfrequently in winter and spring the rain will fall with a temperature of ° to °, when the atmosphere near the earth is ° or ° or more, below those points; and it is frozen to every object upon which it falls. the trade stratum, from which it descends, is not warmed by "radiation" or by ascending currents from a snow-clad surface, and during a cloudy day; nor by a "development of heat" at that particular altitude, but it has brought its heat from the south atlantic, and imparts it to the rain which forms within it. there is every reason to believe that the counter-trade flows north in a regular descending plane, not materially differing from that of the line of perpetual snow. the descent of the latter is well ascertained to be from about , feet at the equator, to _the surface_ at the poles. the plane of the counter-trade is probably much the same, varying over different localities, from the varied action between it and the earth which we are considering; and probably both correspond with the increase of magnetic intensity. lieutenant maury, in an able and original article upon the circulation of the atmosphere, conceives the bands of comparative calms at the northern limits of the trades, which he appropriately terms the "_calms of cancer_," to be nodes in the circulation of the atmosphere, and that the upper or counter-trade here decends and becomes a surface wind from the s. w., as the n. e. trade is a surface wind; and that an upper current from the poles approaches and descends at the same node, to make the n. e. trade. but it is evident he adopted that conclusion too hastily, as he obviously did the conclusion that the calms of the horse latitudes were a type of all. we have seen that the latter are increased by a diversion of the counter-trade, and that they are avoided by making easting. so it may be observed that our upper current is a s. w. current, and no northerly upper current is visible, or exists over the country, however it may be in western europe and the north pacific, on the west of the magnetic poles, where cold, dry northerly and north-easterly winds are found. the origin and progress of storms withal demonstrates that no such node can exist. two points have been made in relation to the course of the counter-trade in the tropics, and are relied upon to show its progress there to the n. e., which deserve consideration. in the first place, it is well known that "rain dust" falls in considerable quantities on the western coast of africa, particularly about the cape de verde islands, and also upon the mediterranean and south-western europe, where it is termed "sirocco dust." "this dust," says lieutenant maury, "when subjected to microscopic examination, is found to consist of infusoria and organisms, whose _habitat_ (place of abode) is not africa, but south america, and in the s. e. trade-wind region of south america. professor ehrenberg has examined specimens of sea dust, from the cape de verdes and the regions thereabout, from malta, genoa, lyons, and the tyrol, and he has found such a similarity among them as would not have been more striking had these specimens been all taken from the same pile. "south american forms he recognizes in all of them; indeed, they are the prevailing form in every specimen he has examined. "it may, i think, be now regarded as an established fact, that there is a perpetual upper current of air from south america to north africa, and that the volume of air in these upper currents, which flows to the northward, is nearly equal to the volume which flows to the southward with the n. e. trade-winds, there can be no doubt," etc. now, it is doubtless true that this dust is transported in a counter-trade, and that such dust is found in south america, and is taken up there by sand-spouts, like those of the ocean in form and action. both humboldt and gibbon have graphically described them. yet i do not think the point well taken. south-eastward of the cape de verdes, where the surface-trades--which, becoming counter-trades, pass over these islands, and, recurving, pass over the mediterranean and south-western europe--should originate, there is a vast extent of unexplored continent in the same latitude as the portion of south america where the dust is found; and the same dry seasons, and the same spouts, in all probability, exist in both. until it be shown that such forms have no "_habitat_" in central and southern and unexplored africa, upon the same latitudes as in south america, it may fairly be presumed that the dust is taken up there. indeed, the _curve_ upon which this dust is found to fall, in the greatest quantities, is very remarkable, and corresponds remarkably with the _law of curvature_ of the counter-trade we have considered, and with the progress of a storm upon that coast, and over the mediterranean, investigated by colonel reid. (see reid, on storms and variable winds, p. .) this _curve clearly indicates the origin of the dust in south africa_. the second point is, that ashes from the volcanos of mexico and central america have fallen to the north-east of the place where they were ejected. mr. redfield has grouped these instances of volcanic eruption usually cited, and i copy from him: "we learn from humboldt, that in the great eruption of jorullo, a volcano of southern mexico, which is , feet above the sea, in latitude ° ', longitude ° ', the roofs of the houses in queretaro, more than miles north, ° east from the volcano, were covered with the volcanic dust. in january, , an eruption took place in the volcano of cosiguina, on the pacific coast of central america, in latitude ° north, and having an elevation of , feet, the ashes from which fell on the island of jamaica, distant miles north, ° east from the volcano. the elevated currents by which volcanic ashes are thus transported are seldom or never of a transient or fortuitous character; and these results, therefore, afford us one of the best indications of their general course. thus, the progress of the higher portion of the trade-wind was marked by the eruption of tuxtla, latitude ° ', longitude °, which covered the houses in vera cruz with ashes, at the distance of miles north, ° west, and also at peroté, miles north, ° west. the ashes from the volcano, at st. vincent, which fell at barbadoes, and east of that island, in , mark the course of a current from the westward, which appears there at times, in the region of clouds, and may, perhaps, be connected with the permanent winds on the pacific coast of mexico." as to one of the instances cited in the foregoing paragraph, that of tuxtla, it may be laid out of the case--the direction conforming substantially to the assumed course of the counter-trade at that point. st. vincent lies w. n. w., or nearly so, of barbadoes, and a n. w. or westerly surface-wind, prior to, and during storms, is common in the west indies as the n. e. is here--both alike, blowing in opposition to the progressive course of the storm. there is nothing strange or peculiar, therefore, respecting that instance, or the existence of variable and especially s. w. currents, between the trades, with occasional partial condensation. the falling of the ashes from cosiguina, upon jamaica, has long and often been cited, as proof that in the west indies the prevailing upper currents run from the s. w. but it has been ascertained that, _during the same eruption, ashes fell miles to the westward, on the deck of the conway_, a vessel then upon the pacific ocean. that case, therefore, does not prove the absence of the s. e. counter-trade at the time, but only the presence of another, and a different current above or below it--and it may have been either, and transient. so of the jorullo instance. investigation would probably have shown that ashes fell to the n. w., and that they were carried n. e. by a transient s. w. wind produced by the existence of a storm to the eastward, or one of those states of partial condensation of the counter-trade which often produce currents at greater distances without a storm. not one of these cases disproves the existence of a s. e. counter-trade, and the invariable n. w. progression of the storms of those latitudes demonstrates it. occasional anomalous currents, depending upon storm action at considerable distance, are found in our atmosphere, and doubtless are there also. thus, although the n. w. wind is almost invariably a surface wind, i have, in a few instances, seen a n. w. set at a considerable elevation, converging toward a peculiarly stormy state of atmosphere far south of us, about the period of the spring equinox. and so in one or two instances i think i have seen light cirro-stratus clouds _above_ the counter-trade, when it ran very low, setting from the n. e., although the usual and almost invariable location of the n. e. wind is below the counter-trade and the stratus clouds of the storm. aeronauts, too, have found these secondary currents beneath a serene and cloudless sky. indeed, the s. e. counter-trade doubtless often induces a thin secondary current of s. w. wind between itself and the surface-trade, in the same manner that similar currents are induced with us, and every where. a question arises here of considerable interest, which, i confess, i can not answer to my own satisfaction. it is, whether there be, or not, _an eastern progression of the body of the atmosphere above the machinery of distribution_. i have thought there was, and that in set fair weather i had seen a peculiar kind of cirro-cumulus cloud, in patches, the small cumuli very distinct and rounded, moving due east, which indicated such a current. but i am not satisfied, from my own observation, that it is so, nor is it easy to determine the question. the moisture of evaporation rarely, if ever, ascends to any considerable elevation, and the upper strata must be very dry. hence, condensation, if it takes place, is thin, and perhaps often undiscernable. investigations upon mountains prove little, for the winds of the inferior strata rush up their sides and over them. it is an open question, and future observation may solve it. the prevailing opinion seems to be that there is. if the theory of oersted, in relation to the circular currents of a magnet, be true, there should be such a progression produced by opposite secondary currents, unless, indeed, it be also true that those currents are inoperative at so great a distance, or their influence barely suffices to retain the attenuated atmosphere in its place. perhaps the investigations of ampère conflict with it. but it is worth while, i think, for philosophers to inquire whether the transverse position of the needle upon the wire is not the effect of the central _longitudinal_ currents, conforming to the circular currents of the wire, and whether it is not owing to the production of the same currents in a globe by the circular currents of ampère, that the globe is magnetized, and the needles made to dip. chapter viii. it is exceedingly desirable, in a practical point of view, to understand the precise character of the reciprocal action which takes place between the earth and the counter-trade, and produces the varied phenomena which mark our climate. we have seen that the same laws, other things being equal, operate every where, and that analogies may be sought in the character of those phenomena elsewhere, under the same, or different, modifying circumstances. looking, therefore, at the magneto-electric movable machinery as a whole, and its influence upon the atmospheric circulation and conditions, we find many facts which point to a primary action in the counter-trade, and others that point as significantly to a primary local-inducing-action in the earth. let us briefly review those to which we have alluded, and advert to some others, and see what solution of the question they will justify: the belt of inter-tropical rains appears to be, in width, and amount of precipitation, and annual travel north and south, proportionate to the volume of trades which blow into it, the quantity of moisture they contain, and the elevation of the surface over which they meet. south america is the most thoroughly-watered country within the tropics, except, perhaps, portions of hindoostan, burmah, siam, etc., on south-eastern asia. the contrast between both, and africa, as far as explored, and as shown by its rivers, is most obvious. the amazon, alone, delivers more water to the ocean than all the rivers of africa. of the width of the belt of rains over africa, in the interior, we know little. its northern extension is less, by from ° to °, than the same belt over south america, the west indies, and mexico. probably its southern is also. upon south america, the southern edge is carried down to cochabamba, in latitude °, and probably to °, to the northern edge of the coast-desert of peru, while it is rarely, if ever, found over the atlantic below °, a difference of ° to °. over south america, too, the quantity of water which falls is also vastly in excess of that which falls upon the atlantic. the main cause of these differences is obvious. the n. e. counter-trades which blow over africa, originate on a surface which is rainless, as eastern sahara, egypt, arabia, etc., or subject to a dry season by the northern ascent of the southern line of the extra-tropical belt, as the barbary states, syria, persia, etc., and their supply of moisture is necessarily scanty. on the south, the s. e. trades originate, in part, upon the eastern portion of southern africa, and, in part, upon the indian ocean, and from the latter source, and a portion of the mediterranean, doubtless most of the water which falls upon central africa, is derived. the n. e. and s. e. trades which blow into the inter-tropical belt upon the eastern portion of the atlantic, originate upon similar surfaces, and with like effect. thus, the s. e. trades, in summer, are from the southern portion of africa, and the n. e., in part, from the mediterranean; and, in winter, the n. e. from the deserts, senegambia, nigritia, etc., and the s. e., owing to the narrowing of the african continent, mainly from the south atlantic and indian oceans. going west, the belt widens, and its range increases until the andes are reached; but under their lee, on the western side, a totally different state of things is found, and the belt of the coast becomes broken and irregular, as we have seen in the citation from maury. the width, extension, and excessive precipitation of the belt, over south america, follow the same law. the south atlantic widens out by the trending of the coast to the s. w., and furnishes a large area for the unobstructed formation and evaporative action of the s. e. trades. so the trending of the coast to the n. w., from ° south to the northward, opens a large area for a like formation and action of the n. e. trades. no correspondingly favorable circumstances exist any where, except, perhaps, around hindoostan, and there the fall of rain is very excessive in some places, as on the kassaya hills, to the extent of inches per annum. in addition to this, the magnetic line of no variation, and of greater intensity, which runs from our magnetic pole, obliquely, s. s. e., to its opposite and corresponding pole in the southern hemisphere, enters the atlantic on the coast of north carolina, and traverses it, and the eastern portion of south america, through the whole trade-wind region. the table-lands, and slopes, and high mountain peaks, meet the trades successively, as they go west, and the latter wrench from them, to an unusual extent, their moisture; depressing the line of perpetual snow, by an increase of quantity on the eastern sides, several thousand feet, as it is for a like cause depressed on the southern side of the himmalayas. on the eastern slopes and tops of the andes, as we have seen, and owing to their elevation, falls the moisture which, according to the working of the machinery, and the law of curvature, should bless the coast line of peru and northern chili, the eastern pacific, northern mexico, california, utah, and new mexico; and, while the andes stand, the curse of comparative aridity must rest upon them all. southern chili, and western patagonia are supplied by the n. e. trades, which originate in the west indies, the gulf of mexico, and the caribbean sea, and the pacific, off central america, in the neighborhood of the bay of panama. but there, again, the same effect of elevation is seen. the mountain slopes of southern chili and patagonia are abundantly supplied, and their mountain ranges are drenched with rain, while eastern patagonia and southern buenos ayres, under their lee, are comparatively dry. so the s. e. trades, which originate off the western coast of south america, curve in upon, and aided by the oceanic currents, supply, abundantly, the n. w. coast of this continent, north of california; and there, too, the coast, and its elevated ranges, receive, as we have seen, a very large proportionate supply of their moisture. substantially, the same state of things, as far as circumstances permit, is reproduced upon malaysia, hindoostan, etc., and the interposition of arid new holland upon the evaporating trade-surface may be distinctly traced upon south-western asia. deserts abound there; the caspian sea receives the drainage of a very large surface, without an outlet; their southern line of extra-tropical rains is carried up very far in summer, and their dry season is intensely hot. (see an article in the american journal of science, for july, , by azariah smith.) another fact in this connection is worthy of a moment's consideration. the magnetic equator, as sought by the dipping needle, is not coincident with the geographical one. humboldt found it, on the andes, at ° ' south, and it has been found still lower in the atlantic. over africa it rises above the geographical equator, and descends again on the indian ocean. about midway the pacific, it becomes coincident with the equator of the earth again. (see diagram, on page .) perhaps it is not known, with certainty, why this is so. the south pole may be situated nearer the geographical pole than the north one--but this is not believed to be so, nor could it make the difference. the greatest southern depression of the magnetic equator is found where the lines of greatest intensity, and of no variation, are found; and at the more intense of these lines exists the greatest depression. from this, i think, it may be inferred that the needle is affected by the greater magnetic intensity of the northern hemisphere, to which it may yet appear the obliquity of the earth's axis is owing. however this may be, or whatever the cause, no marked effect is produced upon the trades. the s. e. trades, by reason of the greater extent of ocean-surface on which they originate, are every where the most extensive, regular, and forcible. the south polar waters, from which they rise, are every where trenching upon, and overriding, the north polar ones; and thus, by a most beneficent provision, the greater portion of the habitable surface is placed in the northern hemisphere, and the principal portion of the southern is left open to an extensive, active evaporative action, which supplies the northern habitable surface with a large excess of the needed moisture. the condensation, and consequent precipitation, which takes place at the passing of the trades, as we have already said, over the ocean and lowlands, takes place mainly in the day-time. upon the table-lands and mountain-ranges, it often continues during the evening and night. the morning, and early part of the day, however, in tropical countries, are generally fair at all elevations. storms also originate in the equatorial belt, and issuing forth in great volume and with great intensity of action, find their way up even within the arctic circle. those which pass over this continent, or the northern atlantic, generally originate in the west indies, some of them over the caribbean sea, some over the islands, and some over the open ocean to the east of them; and, nearly all the most violent, during the months of august, september, and october. it would seem most probable that the primary action in such cases was in the trades themselves, but it is by no means certain that such is the case. this is the class of storms of which mr. redfield has industriously investigated some twenty or more; mr. espy some, and lieutenant porter two. their course, when very violent, is often more directly north than that of storms, however violent, which originate north of the calms of cancer, owing, perhaps, to their greater paramagnetic character. this course i have myself observed, in several instances, about the period of the autumnal equinox--never, however, more southerly than from s. w. to n. e., on the parallel of °, except in three, and, perhaps, four, instances, when it has been s. w. by s. to n. e. by n. i know of no class of storms in relation to which the evidence of primary action in the counter-trade is stronger than in those of the class which originate on the ocean east of the windward islands. but it is not satisfactory as to them. doubtless the conflict of polarities between the passing trades is sufficient to produce the showers and rains which are ordinarily found over the ocean and lowlands, in the equatorial belt; but it is doubtful whether it is sufficient to produce such extensive, long-continued, and violent action, as that which characterizes the hurricane autumnal gales. they occur, too, at the time when the whole machinery of distribution has reversed its course, and is rapidly pursuing its journey south. it is a period of great magnetic disturbance, over both land and sea; of more active gales and local-increased precipitation. at the magnetic observatory of toronto, canada west, these disturbances are carefully and systematically observed, and their maxima, or periods of greatest disturbance occur in april and september. (see silliman's journal, new series, vol. xvii. p. .) the tendency to volcanic action is not as great at the autumnal, as at the vernal equinox, for the reason that most of the volcanic action of the western hemisphere develops itself now upon south rather than north america. but both exist, and are active, and what are improperly termed equinoctial storms, and gales, and rains, are proverbial during, or just subsequent to, both periods with us--as they are when the same change, called the breaking up of the monsoons, takes place in the line of magnetic intensity, over southern and eastern asia. a volume might be filled with extracts, showing, at least, most remarkable coincidences between violent volcanic action and great atmospheric disturbance. perhaps the increased fall of rain at and after the equinoxes, in the northern hemisphere, and in certain localities subject to volcanic activity, is as strikingly illustrated by the register, kept by mr. johnson, on the volcanic island of kauai, one of the hawaiian group, already alluded to, as in any other case, although it is by no means a singular one. the greatest fall of rain, in any month except april and october, was eight inches. in april, the fall was fourteen inches, in october, eighteen inches. neither the equatorial, nor extra-tropical belt, were over the island during those months; but they were the n. e. trades, and the result was owing solely to the interposition of high volcanic mountains, _in a state of disturbance_, into, or near, the strata of the counter-trade. mr. dobson, in stating a theory to which we shall hereafter advert, advances the following proposition: " . _cyclones (hurricanes) begin in the immediate neighborhood of active volcanoes._ the mauritius cyclones begin near java; the west indian, near the volcanic series of the caribbean islands; those of the bay of bengal, near the volcanic islands on its eastern shores; the typhoons of the china sea, near the philippine islands, etc." the peculiar stormy state of the atmosphere, over the gulf stream, to which i have alluded, certainly affords no evidence of primary atmospheric action. it is a body of south polar water, pursuing its way under the guidance of magnetism--maintaining its polarity--arched somewhat like the roof of a house, by the outward pressure of a cold north polar current which it has met to the east of the banks of newfoundland, and forced to take an in-shore course to the southward, and the bodies of water which the rivers discharge, and a conflict with the north polar surface-winds which sweep over it, and fogs, and thunder, and rain, are a matter of course. dr. kane met a portion of this singular current in baffin's bay, north of °, which had preserved its characteristics and a considerable proportionate excess of heat, although it probably had been around greenland, or found its way to the west, toward the magnetic pole, through some of its northern fiords or straits. (grinnel expedition, p. .) the investigations of lieutenant maury show, that when the gulf stream turns to the eastward, crossing the lines of declination at right angles, as the counter-trades also seem to do in the same latitude, it is _carried up, in summer, several degrees to the north_, and descends again in winter--thus demonstrating its connection with the shifting magnetic machinery which controls alike the ocean, the atmosphere, and the temperature of the earth.[ ] there are other irregularities which deserve to be noticed, in this connection, although the analogical evidence they afford is far from being decisive. i have already said that it was within my own observation, that alternating lines of heat and cold, as well as rain and drought, existed frequently, without regard to latitude, following, to some extent, the course of the counter-trade. such lines have been observed by others. thus, mr. espy, after describing a snow-storm, which was followed by a very cold n. w. wind, of several days' continuance, says: "this cold air covered the whole country, from michigan to the eastern coast of the united states, till the beginning of the great storm of the th january; and, what is worthy of particular notice is, that _the temperature began to increase first in the north and north-west_. on the morning of the th, in the north-western parts of pennsylvania, and northern parts of new york, the _thermometer_ had already _risen in some places °_, and, in others, _above °_. while in the s. e. corner of pennsylvania, and in the s. e. corner of new york it had not _begun to rise_. the _wind_ also began to change from the _north-west_ to _south_ and _south-east_, _first_ in the north-west parts of pennsylvania and new york, some time before it commenced in the south-east of those states; and, during the whole of the th, the thermometer, in the north of new york, continued to rise, though the wind was blowing from the southward, where the thermometer was many degrees lower." thus, too, mr. redfield (american journal of science, november, , p. ): "on the contrary, in times of the greatest depression of the thermometer, in numerous instances, the cold period has been found to have first taken effect in, or near, the tropical latitudes, and the gulf of mexico, and has thence been propagated toward the eastern portions of the united states, in a manner corresponding to the observed progression of storms." this was because the cold n. w. wind which _followed_ storms began to follow them as the storms curved and passed to the n. e. they occur in europe also. says kämtz: "such contrasts are not uncommon in europe, and, in this respect, the alps form a remarkable limit; for they separate the climates of the north of europe from the mediterranean climates, where the distribution of rain is not the same as in the center of europe. hence the differences between the climates of the north and south of france. _if the winter is mild in the north_, the newspapers are filled with the lamentations of the _italians_ and _provençals_ at the _severity of the cold_." these facts seem to indicate a primary action in the counter-trade. probably in connection with one class of storms they do, and with another do not. i shall endeavor to show the distinction when i come to the classification of storms. the difference of seasons in this country, and over the entire northern hemisphere, is often very great. in a remarkable work of a remarkable man--"a brief history of epidemic and pestilential diseases," by noah webster, published in , vols.--a history of the weather for about two centuries-- to inclusive, is given generally, and then in a tabular form. those who think that every considerable extreme which occurs exceeds any thing before known, will do well to consult that work. droughts are described, where "there was not a drop of rain for three or four months, and cattle were fed upon the leaves of the trees." winters, so intensely cold that the thermometer fell to ° below zero, at brandywine; or so mild that there was little frost, and people upon connecticut river plowed their fields, and the _peach trees blossomed in pennsylvania in february_. these extremes generally existed in europe and america at the same time, but occasionally they were opposite and alternate. says mr. webster, in summing up the facts (vol. ii. p. ): "it is to be observed that in some cases a severe winter extends to both hemispheres, sometimes to one only, and in a few cases to a part of a hemisphere only. thus in - , - , - , - , - , - , the severity extended to both hemispheres. in - , - , and in other instances, the severe winter in europe preceded, by one year, a similar winter in america. in a few instances, severe frost takes place in one hemisphere during a series of mild winters in the others; but this is less common. in general, the severity happens in both hemispheres at once, or in two winters, in immediate succession; and, as far as this evidence has yet appeared, this severity is closely attendant on volcanic discharges, with very few exceptions." it will be seen that dr. webster (ll.d. and not m.d., and therefore the remarkable character of the work) attributes great influence to earthquakes and volcanic action. probably he is correct in this. the present active volcanic action of the western hemisphere is nearly all within the trade-wind region, from mexico to peru inclusive. the west india islands are of volcanic origin, and the influence of volcanic action is not confined to a concussion of the earth, or the eruption of mud and lava. its connection with magnetic action, and disturbance, is unquestionable. but whether they operate to increase or diminish the trades, and the extent to which they induce violent electric action and storms within and without the tropics, is a question which further observation must determine. the ripples of the ocean, compared by lieutenant banvard to that of a "boiling cauldron, or such as is formed by water being forced from under the gate of a mill-pond," are met with in the vicinity of volcanic islands, where hurricanes and water-spouts originate, and have been observed to precede storms, and be connected with a falling barometer. but whether they are volcanic or magneto-electric, it is difficult to determine. dr. webster remarks, as the result of observation, during the th century, that earthquakes had a n. w. and s. e. progression in the united states, and especially in new england. in a recent article, professor dana has examined, with great ability, the general and remarkable trending of coast lines, groups of islands, and ranges of mountains, from n. e. to s. w. and from n. w. to s. e. (american journal of science, may, .) the line of magnetic intensity, which connects our magnetic pole with its opposite, is now upon this continent nearly a n. w. and s. e. line, and the pole is fast traveling to the west. it may, and probably will yet, be established, that there is an intimate connection between the cause of volcanic action within the earth, to which the upheaval of the n. w. and s. e., and n. e. and s. w. ranges were due, and of magnetic action without, and between both, and the cause of _the s. e. extension_ of our summer storms and belts of showers and barometric _waves_, and the _peculiar n. w. wind_. our limits do not permit us to pursue the subject. much influence upon the weather has been attributed to the spots upon the sun. these spots are supposed to be breaks or openings in the luminous atmosphere or photosphere of the sun, through which its dark nucleus body is seen. counselor schwabe, of dessau, has made them his study since , and has arrived at some singular results. they seem to be numerous--in groups--and to appear periodically with minima and maxima of ten years. as the result of his observations, from to , he gives us the following table and remarks: +-----------------------------------------------+ | year. | groups. | days showing | days of | | | | no spots. | observation. | |-------|---------|--------------|--------------| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | +-----------------------------------------------+ "i observed large spots, visible to the naked eye, in almost all the years not characterized by the minimum; the largest appeared in , , , , , , , , . i regard all spots, whose diameter exceeds ", as large, and it is only when of such a size that they begin to be visible to even the keenest unaided sight. "the spots are, undoubtedly, closely connected with the formation of faculæ, for i have often observed faculæ, or narben, formed at the same points from whence the spots had disappeared, while new solar spots were also developed within the faculæ. every spot is surrounded by a more or less bright, luminous cloud. i do not think that the spots exert any influence on the annual temperature. i register the height of the barometer and thermometer three times in the course of each day, but the annual mean numbers deduced from their observations have not hitherto indicated any appreciable connection between the temperature and the number of the spots. nor, indeed, would any importance be due to the apparent indication of such a connection in individual cases, unless the results were found to correspond with others derived from many different parts of the earth. if the solar spots exert any slight influence on our atmosphere, my tables would, perhaps, rather tend to show that the years which exhibit _a larger number of spots_ had a _smaller number of fine days_ than those exhibiting few spots." these observations _seem_ to show that the spots exert no influence upon the weather, and to be satisfactory. but, perhaps, they are not entirely so. no effect would, of course, be expected from day to day, and perhaps the annual mean may not be seriously disturbed, and yet the spots may seriously affect the seasons. popular tradition has fixed upon certain periods, of , , and years, for the return of winters of unusual severity; and the tables of mr. webster, and other facts, show that it is not wholly without foundation. if we, and those we have cited, are not mistaken in most of the views expressed, the natural effect of a partial interception or failure of the sun's rays, by or from the existence of the spots, would be to decrease the exciting power of the solar rays upon terrestrial magnetism, and, as a consequence, the volume of the trades and their amount of moisture. this would increase the _mean_ heat of the summer in the temperate zone--for the _less_ the volume of trade, the less precipitation and variable wind, and succeeding polar waves of cooler air, and the greater mean heat. on the other hand, the same cause, and the feebler heating power of the sun's rays, would make the winters more severe, both from an absence of a portion of heat, derived directly from the sun's rays, and a less mitigating influence, from the action of the trade, by reason of its decreased volume. so, too, the absence of spots, and a more powerful influence from the solar rays, may gradually carry the machinery further north in summer, and further south in winter, and thus make the _seasons extreme_ without seriously disturbing the mean of the year. and both these may occur in a more marked degree over our intense magnetic area than in europe. i am satisfied that they do so occur. that the partial failure of the sun's rays limits the transit of the machinery, and the volume of the trades during the latter half of the decade, and extends the transit and increases the volume during the first half, producing an occasional severe summer drought and severe winter, in the warmest portion of the decade. and that the variations correspond with the difference in the character and number of the spots in different decades, and hence the longer and shorter periods. turning to the tables of dr. webster, we find that a general tendency to extreme seasons does seem to exist from the th to the th year of every decade, and especially of every alternate decade. the periods of - , , and , - , - , - , - , are those in which the tendency was seen most decided. these tables are very general. the thermometer was not perfected till about , and did not get into general use before . there were very few meteorological registers kept, or accessible to dr. webster. hence he was obliged to resort to such other sources of information as were open to him, and such statements as he found are not always entirely reliable. the oldest inhabitant is apt to express himself very strongly respecting present extremes, and fail somewhat in his recollection of those which have past. still his tables afford general and obvious evidence of the regularity of those periodic conditions. +---------------------------------------------------------+ |a. d.| summer. | winter. | |-----|-------------------------|-------------------------| | | hot and dry | .... | | | hot and dry | .... | | | .... | .... | | | dry europe | .... | | | .... | .... | | | hot, dry europe | .... | | | very hot | .... | | | .... | very severe | | | .... | .... | | | .... | .... | | | .... | cold europe | | | wet england | .... | | | wet england | mild | | | dry and hot | .... | | | dry | .... | | | very dry | severe | | | .... | severe | | | hot and wet | .... | | | .... | cold america | | | dry europe | .... | | | .... | .... | | | cold, wet | .... | | | .... | cold | | | wet england | .... | | | wet england | .... | | | .... | .... | | | dry, hot amer. | .... | | | hot amer. | severe europe | | | .... | .... | | | .... | very cold eng. | | | .... | .... | | | .... | severe amer. | | | dry eng. | .... | | | .... | .... | | | wet | .... | | | wet | .... | | | .... | very severe am. | | | .... | .... | | | wet england | very severe eng. | | | .... | very severe am. | | | .... | .... | | | .... | severe syria | | | hot | .... | | | .... | .... | | | .... | .... | | | .... | .... | | | hot and dry | severe | | | dry | .... | | | very dry | .... | | | very hot | very severe | | | wet england | severe amer. | | | very hot amer. | .... | | | .... | severe | | | .... | mild amer. | | | .... | severe europe | | | .... | severe syria | | | .... | .... | | | hot | .... | | | .... | severe | | | .... | .... | | | very dry amer. | .... | | | very dry amer. | severe | | | .... | .... | | | hot europe | .... | | | hot europe | severe europe | | | hot and dry eur. | very severe | | | .... | cold | | | hot | .... | | | hot | .... | | | wet england | .... | | | wet am. & eng. | cold europe | | | hot america | am., great snow | | | .... | .... | | | .... | severe europe | | | .... | .... | | | hot | severe europe | | | .... | .... | | | hot | mild | | | hot eng. | very severe | | | .... | .... | | | .... | .... | | | dry amer. | .... | | | hot | very severe | | | hot | .... | | | dry europe | cold | | | cool | cold | | | cool | .... | | | rainy amer. | cold | | | cool spring, hot summer | severe eur., mild amer. | | | .... | .... | | | very hot am. | cold | | | .... | .... | | | hot, dry am. | mild amer. | | | .... | severe europe | | | amer., hot, rainy | .... | | | autumn very dry am. | cold amer. | | | cool am. | severe amer. | | | very hot } | { long & severe | | | very dry am. } | { amer. & eur. | +---------------------------------------------------------+ still more definite evidence is found in the meteorological tables of dr. holyoke and dr. hildreth, and an account, by dr. hildreth, of the seasons when the ohio river was closed or obstructed by ice, found in silliman's journal, new series, vol. xiii. p. . thus, we have, from the tables of dr. holyoke, the following annual means, from to , inclusive. i have arranged them in periods of five years. it will be seen that there are three peculiarities observable. first, a marked difference between the first and second periods of the decade, corresponding, generally, with the presence or absence of the spots. second, a difference in the mean of the decades which may well be supposed to correspond with the difference in the number or size of the spots since a like difference is observable in number and size, and the time when they reached their maxima and minima, in the table of schwabe. and, third, there are occasional single cold years during the warm period, and these correspond with what the tables of dr. webster show for both the sixteenth and seventeenth centuries. in relation to this, it should be remembered that volcanic action is a frequent and powerful disturber of the regular action of terrestrial magnetism, and that the extremes, for that reason, are frequently meridional or local and alternating; and to that cause very great extremes, and marked exceptions, may be due, notwithstanding the spots upon the sun may exert an influence in producing hot summers and cold winters toward the close of each decade. thus, to select an instance to illustrate this and explain an anomaly: the coldest season during the whole period, embraced in the following tables, is that of . this occurs during the decrease of spots, and the warm half of the decade. turning to the table of volcanic action, and of earthquakes, found in the report of the british association for , we find that year was remarkable for earthquakes in the united states and south america. in december, , earthquakes commenced in the valley of the mississippi, ohio, and arkansas, felt also at places in tennessee, kentucky, missouri, indiana, virginia, north and south carolina, georgia, and florida, though not so severely east of the alleghanies, _which continued until _. about the same time they commenced in caraccas, and, in march, , became severe over the greater portion of the northern section of south america, and in the atlantic. no such general and continued succession of earthquakes occurred during the other periods embraced in the tables, and the mean of the following five years was very low, embracing the memorable cold summer of . +---------------------------------------------------------------+ | cold period. | warm period. | cold period. | warm period. | |---------------|---------------|---------------|---------------| | °. | °. | °. | °. | | °. | °. | °. | °. | | °. | °. | °. | °. | | °. | °. | °. | °. | | °. | °. | °. | °. | |---------------|---------------|---------------|---------------| |mean of | | | | |period °. |mean °. |mean °. |mean °. | |---------------|---------------|---------------|---------------| |---------------|---------------|---------------|---------------| | °. | °. | °. | °. | | °. | °. | °. | °. | | °. | °. | °. | °. | | °. | °. | °. | °. | | °. | °. | °. | °. | |---------------|---------------|---------------|---------------| |mean °. |mean °. |mean °. |mean °. | +---------------------------------------------------------------+ the tables of dr. hildreth, from to , inclusive, furnish, generally, evidence of a like character. there are, however, an anomaly or two which will be observed. from to , the mean is high during the period when spots were at a maximum. but that maximum embraced a much less number of spots than the two succeeding ones. a contrast appears in the tables of dr. hildreth, during the early period, for dr. holyoke's register, for , puts it _below the mean_, but dr. hildreth's one of the _highest of the half century_. in commenced a period when the spots were much more numerous, and from to , inclusive, the seasons were correspondingly below the mean. from that period to a gradual and slightly irregular rise took place, excepting the year , when another cold year intervened. the table of earthquakes, published by the british association, closes with , and i have not access to any others. the occurrence of such cold years, in the warm period, at intervals during the two centuries previous, and in , and onward, and evidently owing to increased volcanic action beneath the western portion of the northern hemisphere, justifies the belief that the low temperature of was owing to the same cause. the following are the means from the tables of dr. hildreth: +----------------------------------------------------------------+ | °. | °. | °. | °. | °. | | °. | °. | °. | °. | °. | | °. | °. | °. | °. | °. | | °. | °. | °. | °. | °. | | °. | °. | °. | °. | °. | |------------|------------|------------|------------|------------| |mean °. |mean °. |mean °. |mean °. |mean °. | +----------------------------------------------------------------+ the observations of dr. holyoke were made at salem, massachusetts; those of dr. hildreth at marietta, ohio. the following, in relation to the freezing of the ohio river, is evidence of a different kind, but shows the same general correspondence, and particularly _the mildness of the winters when there were few spots_, and their severity from to , inclusive, when the spots were most numerous: .--river open all winter--some floating ice. .--river closed th january. .--floating ice--closed d january--opened th february. .--closed in december, which was a very cold month--opened january , and remained open all winter. .--open all winter. .--open all winter. .--closed january --opened the last of the month--cold. .--closed th january--opened th february. .--closed from th december to th february. cold year. .--closed from th january to th march. cold year. .--closed from th december to th january. .--closed th december--opened th january. .--closed d january--opened th do. .--open all winter. .--closed th november--opened th december--open all the rest of the winter. .--open all winter. .--open all winter. .--closed th december--opened again a few days--closed again on the th. it is not stated how long it remained closed. .--open all winter. .--much floating ice, but not closed--heavy rains and floods. .--floating ice in january, but not closed. .--floating ice, but not closed. .--open all winter--a little ice. (december in the above table, means december previous). this is more reliable as to the winter season than the tables of annual means--although the evidence they afford, making due allowance for the exceptions, is very striking. i shall return to this part of the subject again. but there is other evidence of the influence of these spots. their connection with the irregular magnetic disturbance of the earth has been distinctly traced. colonel sabine, president of the british association, in his opening address, september, , after reviewing the recent discoveries in magnetism, says:-- "it is not a little remarkable that this periodical magnetic variation is found to be identical in period, and in epochs of maxima and minima, with the periodical variation in the frequency and magnitude of the _solar spots_, which m. schwabe has established by twenty-six years of unremitting labor. from a cosmical connection of this nature, supposing it to be finally established, it would follow that the decennial period, which we measure by our magnetic instrument, is, in fact, a solar period, manifested to us, also, by the alternately increasing and decreasing frequency and magnitude of observations on the surface of the solar disc. may we not have in these phenomena the indication of a cycle, or period of _secular change in the magnetism of the sun_, affecting visibly his gaseous atmosphere or photosphere, and sensibly modifying the magnetic influence which he exercises on the surface of our earth?"--american journal of science, new series, vol. xiv. p. . i think it may fairly be inferred, that although these spots do not occasion the "cold spells" and "hot spells," and other transient peculiarities, they do materially affect the _mean_ temperature of the year, and exert an obvious influence when at their maxima; and there is a tendency to an increase of the heat and dryness of summer, and the severity of winter, at the periods named, in our excessive climate, and a well-established connection between the spots and magnetic disturbances and variations. popular opinion has ever attributed to the moon a controlling effect upon the changes of the weather. if it be dry, a storm is expected _when the moon changes_; or if it be wet, dry weather. such popular opinions are usually entitled to respect, and founded in truth. but every attempt to verify _this opinion_, by careful observation and registration, has failed. weather-tables and lunar phases, compared for nearly one hundred years, show four hundred and ninety-one new or full moons attended by a change of the weather, and five hundred and nine without. the celebrated olbers, after _fifty years of careful observation_ and comparison, decided against it. so did the more celebrated arago, at a more recent date--summing up the result of his observations by saying--"whatever the progress of the sciences, never will observers, who are trustworthy and careful of their reputation, venture to foretell the state of the weather." still, the moon may influence the weather, though she may not effect changes at her syzygies or quadratures, and this subject should not be too summarily dismissed. that the moon can not effect changes at the periods named seems philosophically obvious. she changes, for the _whole earth_, within the period of twenty-four hours; yet, how varied the state of things on different portions of its surface. the equatorial belts of trades, and drought, and rains, cover from fifty to sixty degrees of its surface, and know nothing of lunar disturbance. the extra-tropical belt of rains and variable weather moves up in its season, uncovering °, or more, of latitude, and admitting the trades and a six months' drought over it, as in california, regardless of the moon. under the zone of extra-tropical rains, even upon the eastern part of the continent of north america, "dry spells" and "wet spells" exist side by side; the focus of precipitation is now in one parallel, and now in another--_storms_ exist _here_ and _fair weather there_, on the same continent at the same time; and as the moon's rays in her northing pass round the northern hemisphere during the twenty-four hours, they, doubtless, pass from ten to thirty or more storms, of all characters and intensities, moving in opposition to her orbit--and as many larger intervening areas of fair weather, not one of which are indebted to her for their existence, or "take thought of her coming." the storm, which originates in the tropics, pursues its curving way now n. w., then n. e., and again north, to the arctic circle, and, perhaps, around the magnetic pole, over gulf, and continent, and ocean, _occupying one third the time of a lunation, and two changes, perhaps, in its progress_, without any perceptible or conceivable influence from her. yet every inhabitant of mother-earth, influenced by _coincidences remembered_, and uninfluenced by _exceptions forgotten_, looks up within his limited horizon, and devoutly expects from the agency of some phase of the moon, a change for the special benefit of his _dot_ upon the earth's surface. upon how many of these countless dots is the moon at a particular phase, or relative distance from the sun, to change fair weather to foul, or foul to fair? upon none. the storms keep on their way;--the wet spells, and the dry spells, the cold and the hot spells alternate in their time, and though the moon turns toward them in passing, her dark face, her half face, or her full orb (the gifts of the sun, which confer no power), they do not heed her. they are originated, and are continued, by a more potent agent. they are the work of an atmospheric mechanism, as _ceaseless_ in its operation as _time_, as _regular_ as the _seasons_, _as extensive as the globe_. indeed, it seems as if it was expressly designed by the creator that the moon should not interfere materially with this atmospheric machinery. she is the nearest orb; her influence would be controlling and continuous; would follow her monthly path from south to north, and with changes too violent, and intervals too long; and would interfere with the regular fundamental operation in the trade-wind region, where she is _vertical_. aside from the attraction of gravitation, therefore, she seems to have been so created as to be incapable of exerting any influence. she is without an atmosphere; the rays which she reflects are polarized, and without chemical or magnetic power; and, if it be true that melloni has recently detected heat in them, by the use of a lens three feet in diameter, which could not previously be effected, its quantity is exceedingly small, and incapable of influence. doubtless, the attraction of her mass is felt upon the earth, as the tides attest; and upon the atmosphere as well as the ocean. but the atmosphere is comparatively _attenuated_, and exceedingly so at its upper surface. her attraction, therefore, although felt, is not influential. she seemed, to dr. howard, to produce in her northing and southing, a lateral tide which the barometer disclosed, but owing to the attenuated character of the atmosphere, neither the sun nor moon create an easterly and westerly tide, that is observable, except with the most delicate instruments. sabine is believed to have detected such a tide by the barometer, at st. helena, of one four thousandth of an inch. but even this _infinitesimal influence_ may prove an error upon further investigation. there is a diurnal variation of the barometer, but it is not the result of her attraction, for it is not later each day as are the tides, exists in the deepest mines as well as upon the surface, and is demonstrably connected with the _group_ of _diurnal_ changes produced by the action of the sun-light and heat upon the earth's magnetism. can the lateral tide, if there be one, affect the weather? for in the present state of science it seems entirely certain that the moon can exert an influence in no other way. if the received idea of many, perhaps most, meteorologists, on which all wheel barometers are constructed, that a _high barometer_ necessarily produces _fair weather_, and a _low one foul_, were true, she certainly might do so. but that idea can not be sustained, and there is no known certain influence exerted by the moon upon the weather, in relation to which we have any reliable practical data. humboldt appears to have adopted the impression of sir w. herschell, that the moon aids in the dispersion of the clouds. (cosmos, vol. iv. p. .) but the tendency to such dispersion is always rapid during the latter part of the day and evening, when there is no storm approaching, and the full moon renders their dissolution visible, and attracts attention to them. the greenwich observations, also, carefully examined by professor loomis, fail to confirm the impression of herschell and humboldt, and those eminent philosophers are doubtless in this mistaken. from this general and somewhat desultory view of the general facts, which bear analogically upon the question, no decisive inference can be drawn in relation to the seat of the primary influence which produces the atmospheric changes. the preponderance is in favor of the magnetic, or magneto-electric, action of the earth. we must come back to our own country and grapple with the question at home. chapter ix. before proceeding to do this, however, it may be well to look at some theories which have been advanced, and to a greater or less extent adopted, and at their bearing upon the question. the calorific theory is at present the prevailing one in europe and in this country. meteorologists there and here refer all atmospheric conditions and phenomena to the influence of heat. the principal applications of that theory have been considered. but within the last few years the elasticity and tension of the aqueous vapor of the atmosphere have received much attention, as exerting an auxiliary or modifying influence. professor dove, of berlin, who ranks perhaps as the most distinguished meteorologist of that continent, attributes barometric variations to _lateral overflows_, and, in the upper regions, resulting from the elevation of the atmosphere by expansion; and in this view meteorologists of europe seem generally to acquiesce. in an article sent to colonel sabine, and recently republished in the american journal of science, january, , in thus attempting to account for the annual variation of barometric pressure, which occurs in europe and asia, and, indeed, over the entire hemisphere. he says: "from the combined action or the variations of aqueous vapor, and of the dry air, we derive immediately the periodical variations of the whole atmospheric pressure. as the dry air and the aqueous vapor mixed with it, press in common on the barometer, so that the up-borne column of mercury consists of two parts, one borne by the dry air, the other by the aqueous vapor, we may well understand that as with increasing temperature the air expands, and by reason of its augmented volume rises higher, and _its upper portion overflows laterally_," etc. and in another place he says: "from the magnitude of the variations in the northern hemisphere, and the extent of the region over which it prevails, we must infer that _at the time of diminished pressure a lateral overflow probably takes place_," etc. doubtless, the mean pressure of the atmosphere, in summer, in the northern hemisphere, is less than in winter, in some localities, and greater in others, and it differs in different countries of equal temperature. and this is all very intelligible. the mean of the pressure for the month is made up by _averaging_ all the _elevations_ and _depressions_. during a month, showing a very low mean, the barometer may, at times, attain its _highest altitude_, if the depressions below the mean are great or more frequent. the barometer is depressed during storms, and ranges high during _set fair_ weather. ordinarily, therefore, the more stormy the season the more diminished the mean pressure; and it is a mistake to look to an overflow to account for the fact. the changes in the location of the atmospheric machinery, and consequent change in the amount and severity of falling weather, and the periodic frequency and character of storms, and consequent _periodic_ depressions and elevations of the barometer, explain the annual mean variations, as they do the other phenomena. but it is perfectly consistent with the calorific theory to attempt to account for these differences by another of those ever-necessary modifications, viz.: the different tension and elasticity of aqueous vapor in different countries of equal temperature; and then to _suppose_ an expansion of the whole body of the atmosphere and a lateral overflow from the place where the air is expanded, on to some other, where it is not; and thus _suppose_ all necessary currents in the upper regions, setting hither and yon, by the force of gravity alone. and apparently he who is best at supposition becomes the most distinguished meteorologist. perhaps i have already said all that i ought to be pardoned for saying, in relation to the utter absurdity of attributing all meteorological phenomena to the agency of heat; but when i find such views as those which that article contains, emanating from so distinguished a man, sanctioned by the president of the british association, and copied into the leading journal of science in this country, i can not forbear a further and a somewhat critical examination of them. there is more error of supposition and less truth in it, than in any other article regarding the science, of equal length, which has fallen under my notice. what is the height of this expansion? the moisture of evaporation ascends, ordinarily, but a few thousand feet. the atmosphere grows regularly cooler, from the earth to the trade, and _the increased warmth that is felt at the surface extends but little way_. currents of warm air do not ascend. the strata maintain, substantially, their relative positions; and this is a most beneficent provision. in northern latitudes of the temperate zone, all the warmth derived from a few hours' sunshine is needed at the surface; and, deplorable, indeed, would be our condition, if the atmosphere, as fast as warmed by the rays of the sun, were to hasten up, and the frigid strata descend in its place. the earth would not be habitable. all the warm air on its surface would be rising as soon as it became warmed, and the cold air above be descending, and enveloping us with the chilling strata which are ever floating within two or three miles above us. no. infinite wisdom has ordered it otherwise. the laws of magnetism and of static-electric induction and attraction keep the strata in their places, and preserve to us the warmth which the solar rays afford or produce. the inhabitant of the valley, in a high northern latitude, in summer, can plant, and sow, and reap, at the base of the mountain whose summit penetrates the stratum of continual congelation, and up its sides, almost to the line of perpetual snow; and, as he looks upon the fruits of his labor, and up to the snow-clad peak that towers above him, can thank his maker for placing a warm equatorial current, a perpetual barrier, between the fertility and warmth which surround him, and the cold destructive strata above; and thank him for not creating such a state of things, as certain meteorologists insist we shall believe he has created. again, where are the _upper regions_, from which the lateral overflow takes place? the atmosphere is differently estimated, at from thirty to forty-five miles, or more, in height. whatever its height may be, it is exceedingly attenuated in its "upper regions." gay-lussac marked the barometer at - / inches at the height of , feet. two thirds of the atmospheric density, then, is within five miles of the earth. air, too, is _compressible_. allowing for the latter and the attenuation, how many miles in vertical depth, of its "_upper regions_," must move from one portion to another, to depress the barometer two inches--its range sometimes in twenty-four hours--or even half an inch? let the computation be made, and see how startling the proposition, how utterly impossible that the theory can be true. the distinguished professor, in the paper referred to, introduces his theory of the formation of hurricanes, and we quote-- "if we suppose the upper portions of the air ascending over asia and africa to flow off laterally, and if this takes place suddenly, it will check the course of the upper or counter-current above the trade-wind, and force it to break into the lower current. "an east wind coming into a s. w. current must necessarily occasion a rotatory movement, turning in the opposite direction to the hands of a watch. a rotatory storm, moving from s. e. to n. w., in the lower current or trade, would, in this view, be the result of the encounter of two masses of air, impelled toward each other at many places in succession, the further cause of the rotation (originating primarily in this manner) being that described by me in detail in a memoir 'on the law of storms,' translated in the 'scientific memoirs,' vol. iii. art. . thus, it happens that the west india hurricanes, and the chinese typhoons occur near the lateral confines on either side of the great region of atmospheric expansion, the typhoons being probably occasioned by the direct pressure of the air from the region of the trade-winds over the pacific, into the more expanded air of the monsoon region, and being distinct from the storms appropriately called by the portuguese 'temporales,' which accompany the out-burst of the monsoon when the direction of the wind is reversed." the analogy between this, and a theory of mr. redfield's, will be noticed further on. but i remark, in passing, that there is not a fact or inference in this paragraph which will bear examination. . there is no such regular s. w. wind over the surface trade, as he supposes. doubtless, there are, occasionally, secondary s. w. currents between the counter-trade and the surface one, with partial condensation, for much of both becomes depolarized by their reciprocal action and precipitation, and these induced s. w. currents are sometimes so strong as to usurp the place of the surface-trade, and become very violent in the latter part of hurricanes; but such is not the usual course of the upper currents of the west indies, as the progress of storms there, and observation, prove. . there can not be any _periods_ of extensive and _sudden_ expansion over africa. if there is any place on the earth which has a more uniformly progressive temperature, either way, and is more free from _sudden_ extremes, or which is more arid and destitute of aqueous vapor, and sudden aqueous expansions, than another, it is africa. no such occasional sudden expansions are there possible. . winds do not, and can not, "_encounter_." they stratify upon each other. they are produced by the action of opposite electricity, and are _connected together_ in their origin and action. the atmosphere is never free from the regular and irregular currents, however invisible for the want of condensation. aeronauts find them in the most serene days. they exist without encounter or tendency to rotation, every where, and at all times; even over the head of the distinguished professor, whether he sleeps or is awake. we can all see them when there is condensation, and it is rarely the case that there is not some degree of it in some of them. . that "great region of expansion" is a chimera. it does not exist. it is a region of _lower temperature_, and of _condensation_, instead of _expansion_ of _aqueous vapor_. the trade does not rise in it, or the s. w. wind overflow from it. see the table cited page . . the hurricanes do not originate _in the surface trades_, as he supposes. they originate in the belt of rains, the supposed "region of expansion," and issue out of it; or in the counter-trade, where volcanic elevations rise far into or above the surface trade. . this hypothesis can not be sustained upon his own principles. the distance between africa and the west india islands, where most of the hurricanes originate, is from , to , miles. these gales are small when they commence, not ordinarily over one or two hundred miles in diameter, and often less. there are trades all the way over from africa, and s. w. winds also, if they exist, as he supposes, in the west indies. how can it happen that this lateral overflow should pass _without effect_, over , miles of s. w. wind and trade, and concentrating the overflow of a continent over one small and chosen spot of the west indies, _pitch down_ there, and there only, and crowd the s. w. wind into the trade below? this is too much for sensible men to believe. what does professor dove mean by the term _impulsion_, as applied to the winds? how are they _impelled_? it is the fundamental idea of his calorific theory, that they are _drawn_ by the _suction_ caused by a _vacuum_, and the vacuum created by expansion and overflow above, in obedience to the law of gravity; that the s. e. trade is drawn to the great region of expansion, and the s. w. runs from it as an overflow. but if the s. w. is driven down into the plane and place of the surface-trades, how does it continue to be impelled, and why is it not then subject to the suction of the vacuum which draws the trade? does that vacuum _select its air_, and so attract the trade, in preference to the depressed portion of the s. w. current, that the former runs around the latter to get to the vacuum, and the latter around the former to get away from it? and does the trade, when it has got around the s. w. current, instead of going to the vacuum, continue to gyrate, and the s. w. current, instead of pursuing its regular course, gyrate also about the trade, and both move off together, regardless of the vacuum of the great region of expansion, in a new direction to the n. w., in an independent, self-sustaining, cyclonic movement, increasing in power and extent, involving extended and increasing condensation, producing the most violent electrical phenomena, and thus continuing up, even to the arctic circle? yes, says professor dove. no, say all fact, all analogy, and his own principles. . his theory relative to the typhoons is unintelligible. if they originate near the lateral confines of the great region of atmospheric expansion, they originate in the region of the trade-winds, for the two are identical. how the direct pressure of the air from the trade-wind over the pacific, in the more expanded air of the monsoon region, can occasion a typhoon upon any principles, passes my comprehension. if, as lieutenant maury supposes, the monsoons are reversed trades, then the trade-wind and monsoon region are identical. if the monsoons are found in the belt of rains, then, the trades, upon professor dove's principles, pass into the monsoon region by attraction or suction, without pressure. either way the theory is undeserving of consideration. a new theory has recently been started by mr. thomas dobson, and, although it is (like all other efforts to get the _upper strata down_ to produce condensation, or those below _up_, that they may be condensed), without foundation, his collection of facts is brief and interesting. i copy his article from the london, edinburgh, and dublin phil. mag., for december, . it adds to the collection of facts in relation to the connection between volcanic action and storms for the seventeenth century, made by dr. webster: the following appear to be the main facts which are available as a basis for a theory which shall comprehend all the meteors in question: st. the eruption of a submarine volcano has produced water-spouts. "during these bursts the most vivid flashes of lightning continually issued from the densest part of the volcano, and the volumes of smoke rolled off in large masses of fleecy clouds, gradually expanding themselves before the wind in a direction nearly horizontal, and drawing up _a quantity of water-spouts_."--(captain tilland's description of the upheaval of sabrina island in june, , phil. trans.) with this significant fact may be compared the following analogous ones: "in the aleutian archipelago a new island was formed in . it was first observed _after a storm_, at a point in the sea from which a column of smoke had been seen to rise."--(lyell, principles of geology.) "among the aleutian islands a new volcanic island appeared in the midst of _a storm_, attended with flames and smoke. after the sea was calm, a boat was sent from unalaska with twenty russian hunters, who landed on this island on june st, ."--(journal of science, vol. vii.) "on july th, , a submarine eruption broke out between the mainland of orkney and the island of strousa. amid thunder and lightning, a very dense jet black cloud was seen to rise from the sea, at a distance of five or six miles, which _traveled toward the north-east_. on passing over strousa, the wind from a slight air became _a hurricane_, and a thick, well-defined belt of large hailstones was left on the island. the barometer fell two inches."--(transactions royal society, edinburg, vol. ix.) d. hurricanes, whirlwinds, and hailstones accompany the paroxysms of volcanos. " . a great volcanic eruption at lancerote island, and _a storm_, which was equally new and terrifying to the inhabitants, as they had never known one in the country before."--(lyell, principles of geology, vol. ii.) " . in the philippine islands a terrible volcanic eruption destroyed the town of taal and several villages. darkness, hurricanes, thunder, lightning, and earthquakes, alternated in frightful succession."--(edinburgh philosophical journal.) "in , , , and , during eruptions of etna, caravans in the deserts of africa perished by violent whirlwinds. in , while vesuvius was in eruption, a whirlwind destroyed a caravan."--(rev. w. b. clarke in tasw. journal.) " , java. a tremendous eruption of tombow mountain. between nine and ten p.m., ashes began to fall, and soon after _a violent whirlwind_ took up into the air the largest trees, men, horses, cattle, etc."--(raffles' history of java.) " , dec. vesuvius in eruption. in the evening _a hail storm_, accompanied with red sand."--(journal of science, vol. v.) " , banda. a frightful volcanic eruption, and in the evening an earthquake and a violent hurricane."--(annales de chimie.) " , oct. eruption of vesuvius. toward its close the volcanic thunder-storm produced an exceedingly violent and abundant fall of rain."--(humboldt, aspects of nature.) " , jan. etna in eruption. violent hurricanes at genoa, in the bay of biscay, and in great britain. " , feb. destructive earthquakes in the west indies, a volcanic eruption at guadaloupe, followed by hurricanes in the atlantic." " , june . volcano of white island, new zealand, in eruption. heavy squalls of wind and hail; it blew as hard as in a typhoon."--(commodore hayes, r.n., in naut. mag., .) " , march . volcanic eruption and earthquake in java; and on the st of march, and d of april, violent hurricanes."--(java courant.) " , aug. . a frightful eruption of the long dormant volcano of the pelée mountain, martinique. aug. . hurricane at st. thomas, etc.; earthquake at jamaica, etc. " , april . earthquake at hawaii, and on the th a great volcanic eruption. on the th _a gale of unusual violence_ lasted thirty-six hours, and did great damage."--(the polynesian, april , .) d. in volcanic regions, earthquakes and hurricanes often occur almost simultaneously, but in no certain order, and without any volcanic eruption being observed. in , , , and , earthquakes and hurricanes occurred together at jamaica; in at carthagena; in at barbadoes; in at charleston; in at tobago; in and at antigua; in , an awful storm at montreal, rain of a dark inky color, and a slight earthquake. people conjectured that a volcano had broken out. in the great martinique hurricane, a _waterspout_ burst on mount pelée and overwhelmed the place. same night, an earthquake. , oct. . manilla.--twenty four hours' rain and two heavy earthquakes. p.m., a severe hurricane. " , sept. . manilla--an earthquake destroyed a great part of the city; many vessels wrecked by a great hurricane in the adjacent seas, between the th and th of september."--(singapore times.) " , oct. calcutta.--furious hurricane and violent earthquake; , lives lost." " , may . bombay.--hurricane and earthquakes; , lives lost."--(madras lit. tran., .) " . ongole, india, and in , at ceylon, a hurricane and earthquake shocks."--(piddington.) " . cyprus.--an earthquake and a frightful hurricane."--(hecker.) " . bagdad.--an earthquake and _a storm_--an event quite unprecedented. " , dec. zante.--great earthquake and hurricane, with manifestations of a submarine eruption."--(edinburg phil. journal.) " , dec. navigator's islands.--hurricane and earthquakes."--(williams' missionary enterprise.) " , oct., nov. new zealand.--succession of earthquake shocks, and several tempests. " , oct. at valparaiso, a destructive tempest and severe earthquakes."--(nautical magazine, .) when an earthquake of excessive intensity occurs, as at lisbon, in , the volcanic craters, which act as the safety-valves of the regions in which they are placed, are supposed to be sealed up; and it is a remarkable and highly-suggestive fact, that _no hurricane follows such an earthquake_. the number of instances of the concurrence of ordinary earthquakes and hurricanes might easily be increased, but the preceding suffice to show the _generality_ of their coincidence, both as _to time_ and place. th. the breaking of water-spouts on mountains sometimes accompanies hurricanes. in , during the great martinique hurricane, before cited. " , nov. at teneriffe, enormous and most destructive water-spouts fell on the culminating tops of the mountains, and a furious cyclone raged around the island. the same occurred in and in ."--(espy and grey's western australia.) " . moray.--floods and earthquakes, preceded by water-spouts and a tremendous storm."--(sir t. d. lander.) " , june. hurricanes, accompanied by water-spouts and fall of avalanches, in the white mountains."--(silliman's american journal, vol. xv.) th. the fall of an avalanche sometimes produces a hurricane. " , dec. a part ( , , cubic feet) of the glacier fell from the weisshorn ( , feet). at the instant, when the snow and ice struck the inferior mass of the glacier, the pastor of the village of randa, the sacristan, and some other persons, _observed a light_. a frightful hurricane immediately succeeded."--(edinburg philosophical journal, .) th. water-spouts occur frequently near active volcanos. this is well known with regard to the west indies and the mediterranean. the following notices refer to the malay archipelago and the sandwich islands: "water-spouts are often seen in the seas and straits adjacent to singapore. in oct., , i saw _six_ in action, attached to one cloud. in august, , one passed over the harbor and town of singapore, dismasting one ship, sinking another, and carrying off the corner of the roof of a house, in its passage landward."--(journal of indian archipelago.) " . an immense water-spout broke over the harbor of honolulu. a few years before, one broke on the north side of the island (oahu), washed away a number of houses, and drowned several inhabitants."--(jarves' history of sandwich islands.) th. cyclones begin in the immediate neighborhood of active volcanos. the mauritius cyclones begin near java; the west indian, near the volcanic series of the caribbean islands; those of the bay of bengal, near the volcanic islands, on its eastern shores; the typhoons of the china sea, near the philippine islands, etc. th. within the tropics, cyclones move toward the west; and, in middle latitudes, cyclones and water-spouts move toward the n. e., in the northern hemisphere, and toward the s. e. in the southern hemisphere. th. in the northern hemisphere, cyclones rotate in a horizontal plane, in the order n. w., s. e.; and in the southern hemisphere, in the order n. e., s. w. by applying the principles of electro-dynamics to the electricity of the atmosphere, i shall endeavor to connect and explain the preceding well-defined facts. the continuous observations of quetelet, on the electricity of the atmosphere, from to (literary journal, february, ), show that it is always positive, and increases as the temperature diminishes. it therefore increases rapidly with the height above the earth's surface. we may, consequently, regard the upper and colder regions of the atmosphere as an immense reservoir of electric fluid enveloping the earth, which is insulated by the intermediate spherical shell formed by the lower and denser atmosphere. now, whenever a vertical column of this atmosphere is suddenly displaced, the surrounding aqueous vapor will be immediately condensed and aggregated, and the cold rarefied air and moisture will form a vertical conductor for the descent of the electrical fluid. this descent will take place down a spiral, gyrating in the order n. w., s. e., in the northern hemisphere, since the electric current is under the same influence as that of the south pole of a magnet; and in the order n. e., s. w., in the southern hemisphere. the air exterior to the conducting cylinder will partake of the violent revolving motion, and a tornado or cyclone will be produced. upon the foregoing facts i shall comment in another place. three theories have been advanced by meteorologists of this country, two of which profess to explain all the phenomena of the weather. professor espy attributed the production of storms and rain to an ascending column of air, rarefied by heat, and the rarefaction increased by the latent heat of vapor given out during condensation, and an inward tendency of the air, from all directions, toward the ascending vortex, constituting the prevailing winds. thus, professor espy conceived, and to some extent proved, that the wind blew inward, from all sides, toward the center of a storm, either as a circle, or having a long central line, and he conceived that it ascended in the middle, and spread out above; and that clouds, rain, hail, and snow, were formed by condensation consequent upon the expansion and cooling of the atmosphere, as it attained an increased elevation. _this ascent_ was not, in fact, _proved_ by professor espy, _has not been found by others_, and _is not discoverable, according to my observations_. the theory was ingenious, founded on the theory of dalton, that the vapor was maintained in the atmosphere by reason of a large quantity of latent heat, which was given out when condensation took place. this theory is also unsound. no such elevation of temperature is found in clouds or fogs when they form near the earth, however dense. thus the two principal elements of professor espy's theory are found to be untrue, and the theory untenable. but it was sustained with great ability and research, and the distinguished theorist deserves much for the discovery and record of important facts in relation to the weather. aside from its theoretical views, his book contains a great mass of valuable information, and will well repay the cost of purchase and perusal. another theory, by mr. bassnett, is of recent date, founded on the influence of the moon, and the supposed creation of vortices in the ether above, whose influence extends to the earth, producing storms and other phenomena. no one can peruse his book without conceding to him great ability and scientific attainment; and if his theory was true, the periods of fair and foul weather could be calculated with great mathematical certainty. but it contains inherent and insuperable objections. i will only add that all herein before contained is in direct opposition to it. mr. w. c. redfield, of new york, as early as , first advanced in this country the theory of gyration in storms, and investigated their lines of progress on our coast and continent. his theory is limited in its character, and does not profess, except indirectly, to explain all, or indeed any, of the other phenomena of the weather. as far as it goes, however, it is generally received in this country and europe, and has been adopted by reed, piddington, and others, who have written on the law of storms. the position of mr. redfield is honorable to himself and his country. science and navigation are much indebted to him for his industry in the collection of facts. nevertheless, his theory is not in accordance with my observation, and i deem it unsound. although expressed disbelief of the theory has been characterized as an "attack" upon its author, i propose, with that _respect_ which is due to him, but with that _freedom_ and _independence_ which a search for _truth_ warrants, to examine it with some particularity. it is a part of the subject, and i can not avoid it. when the theory was first announced, i adopted it as probably true; and being then engaged in a different profession, which took me much into the open air by night and day, i watched with renewed care the clouds and currents for evidence to confirm it. i discovered none; on the contrary, i found much, very much, absolutely and utterly inconsistent with its truth. the substance only of these observations will be adduced. mr. redfield admits that the progression of our storms in the vicinity of new york, is from some point between s. s. w. and w. s. w., to some point between n. n. e. and e. n. e. according to my observation, except perhaps in occasional autumnal gales, they are not often, if ever, from s. of s. w., and the great majority of them, including, i believe, all n. e. storms, are between s. w. and w. s. w. now, the card of mr. redfield, moving over any place from any point between s. w. and w. s. w., calls for a s. e. wind at its axis, an e. wind at its north front, and a s. wind at its south front, and does not call _for a n. e. wind on its front at all, except at the north extreme_, where it could _not continue for any considerable period_. [illustration: fig. .] in relation to this, i observe, st. _about one-half of our n. e. storms, including some of the most severe ones, not only set in n. e., but continue in that quarter without veering at all, during the entire period that the storm cloud is over us_; usually for twenty-four hours; not unfrequently for forty-eight hours, sometimes for seventy-two or more hours. this every one can observe for himself, and it can not, of course, be reconciled with his theory. d. n. e. storms, whether they set in from that quarter in the commencement, or veer to it afterward, when they do "change" round, more frequently veer by the s. to the s. w. in clearing off, than back through the n. into the n. w. the former, in accordance with his theory, they can not do, as the reader can see by passing the left side of the card over his place of residence on the map from s. w. to n. e. d. n. e. storms often pass off without hauling by s. or backing by n., and with or without a clearing off shower, the _wind shifting and coming out suddenly at s. w._ this they could not do in accordance with his theory, as slipping the card will show. th. from june to february it is _exceedingly uncommon_ for a n. e. storm to back into the n. w. they do so more frequently from february to may, especially about the time of the vernal equinox and after; and then, because the focus of precipitation and storm intensity of the extra tropical zone of rains is s. of ° east of the alleghanies. his theory requires them to back by n. into n. w. _in all cases, when they set in n. e._ th. when they do back from the n. e. into the n. w., it rarely indeed continues to storm after the wind leaves the point of n. e. by n., and generally, if it does continue stormy, _the wind is light_, and not a gale, how violent soever the gale from the eastward may have been. usually, by the time the wind gets n. w., it has cleared off. this, mr. redfield, as we shall see, evades by embracing the n. w. fair wind as a part of the same gale. according to my observation, therefore, a _very large proportion_ of the _n. e. storms_, and they are a majority of the most violent ones of our climate east of the alleghanies, do not _commence, continue_, or _veer_ in accordance with his theory, but the _reverse_; and so long as this is so, i can not receive his theory as true. th. s. e. storms do not always, or indeed often, conform to the requirements of his card. when they set in violently at s. e., and continue so for hours without veering, the axis of the storm should be over us, and the wind should change _suddenly_ to n. w. this did not occur in the storm of sept. , , nor does it often, if ever, occur in the summer or early gales of the autumnal months. in the later storms of autumn, and as often in those which are very gentle as any, and in the winter months when s. e. gales are rare, it does sometimes so change after the storm cloud has passed. but in the winter months, as in the storm investigated by professor loomis, the storms are frequently long from s. e. to n. w., and the s. e. wind blows nearly in coincidence with its long axis, for a thousand or fifteen hundred miles, till the barometric minimum is passed, and the inducing and attracting force of this part of the storm cloud is spent, and then the n. w. wind follows; sometimes blowing in under the storm cloud, turning the rain to snow; but oftener following the storm within a few hours, or the next day. the storm of professor loomis, when over texas, was not probably more than four or five hundred miles in length. as it curved more, and passed north and east, it extended laterally, its center traveling with most rapidity, and when it reached the eastern coast was about fifteen hundred miles long, and not more than six hundred broad. along the eastern part of that storm, except when by its more rapid progress the front projected much further eastward over new england than its previously existing line, the s. e. winds blew. when it bulged out, so to speak, by reason of the increased progress of the center, the wind veered to the n. e. the center of the storm passed near st. louis and south of quebec, as the _fall of rain_, the _bulging_ of the _rapidly-moving center_, and the _line of subsequent cold_, attest. it is utterly impossible for any unbiased mind to look at the description of that storm, and attribute to it a rotary character. with all the data before him, mr. redfield himself has not attempted it directly.[ ] the september storm of was more violent in character than any which have since occurred. my recollection of it is as distinct as if it occurred yesterday. peculiar circumstances, not important in this connection, fixed my attention upon the weather during that day and night. there were cirro-stratus clouds passing all day, from about s. w. to n. e., thickening toward night with fresh s. s. w. wind and flocculent scud, such as i have since seen at the setting-in of s. e. autumnal gales. in the evening the wind (in the immediate neighborhood of hartford, ct.), veered to s. e., the cloud floated low, it became very dark, and the wind blew a most violent gale. the trees were falling about the house where i then resided, the windows were burst in, and i was up and observant. when the cloud passed off to the east, it was suddenly light, and almost calm. the western edge of the storm cloud was as perpendicular as a steep mountain side, and was enormously elevated, and very black. i have sometimes seen the western side of a summer thunder cloud, which had drawn a violent gust along beneath it, as elevated and perpendicular, but never a storm cloud. no cloud of that _depth_, or _intensity_ as exhibited by its peculiar blackness, ever floated or will float so near the earth, without inducing a devastating current beneath. after it had passed the ridges east of the connecticut valley, its top could be seen for a long and unusual period over the elevated ranges. now that storm was but an _intense portion_ of an extensive stratus-rain cloud. such portions frequently exist, and mr. redfield admits the fact. another like portion, in the same storm, passed over norfolk, virginia, and the adjacent section, where the wind was n. e., and veered round by n. w. to s. w. baltimore, and some vessels at sea, were between the two intense portions of the storm, and were not affected by either. its northern limit was bounded by a line, drawn from some point not far north of trenton, new jersey, north-eastward, and north of worcester, massachusetts. i was about forty miles south of its northern limit, and north of its center. during that day, and the next, there was wind from s. w. to s. e., inclusive, including the gale, and _from no other quarter_. it did not at any time veer to the w. or n. w. after the passage of the storm-cloud, the wind was very light. when this intense portion of the storm passed over the valley of the connecticut, its longest axis was from s. s. e. to n. n. w., and the _wind was s. e. the whole length of it_. in its passage from the longitude of trenton to boston, there was n. w. wind at one point, and but one, and that was in the iron region, at the n. w. corner of connecticut, at the northern limit of the intense cloud, and owing, doubtless, to some local cause. the direction of the wind in that storm was in accordance with what is generally true of our storms. the wind on the front of the storm depends upon its shape. if the storm is long in proportion to its width (and no other _violent_ autumnal or winter storm has been investigated, to my knowledge), the wind blows axially, or obliquely, on its front. thus, if long from s. e. to n. w., the wind on its front will blow from the s. e. so, if the storm is long from s. w. to n. e., and has a south-eastern lateral extension, with an easterly progression, the wind will blow axially in the center, and obliquely at the edges. instances might be multiplied, but i refer to one of recent date and striking character. all of us remember the drought of . it ended in drenching rain on the th of september. this rain fell from a belt, half showery and half stormy in character, which had a s. e. lateral extension. the evening of the previous day there was some lightning visible at the north, and the usual s. s. w. afternoon wind _continued fresh after nightfall_. the next day we had a brisk wind from the same quarter, and, after noon, the clouds appeared to pile up in the far north, seeming very elevated. they continued to do so, extending southerly during the afternoon, _with a high wind from s. s. w._, the cumulus clouds moving e. n. e. at p.m., gentlemen who left new york at p.m., reported that a dispatch had been received from albany, dated p.m., stating that it was raining very heavily there. about p.m., the belt reached us, and it rained heavily from that time till morning. not far from p.m., and during the heaviest rain, the wind shifted from the s. s. w. to n. e., and blew fresh and cold from that quarter during the night, and till the belt had passed south, and then from n. e. by n., cool, with heavy scud, during the forenoon, veering gradually to the n. n. e., and dying away. after the rain ceased, the northern edge of the belt was distinctly visible in the s. and s. e., its stratus-cloud moving e. n. e., and its scud to the westward. the front of that storm did not pass over us. it was long and narrow. the wind blew somewhat obliquely inward, along its southern border, to the eastward, and, in like manner, to the westward, on its northern border, but from the n. e. axially along its central portions. in the last instance, the wind changed from s. w. to n. e. this, too, is impossible, according to mr. redfield's theory. similar instances, in summer, and early autumn, are not uncommon. but i shall recur to this in connection with the different _classes_ of storms. again, the manner in which these s. e. winds co-exist with the n. e., and become the prevailing wind, toward the close of the storm, is instructive, and inconsistent with the theory of mr. redfield. in the west indies, the first effect of the storm is to increase the n. e. trade; the wind then becomes baffling, but settles in the n. w. or n. n. w., _in direct opposition to the admitted progress of the storm_. at this point, or at s. w., it blows with most force. sometimes it veers gradually, and sometimes falls calm, and comes out from the s. w., blowing violently. it ends by veering to the s. e., following gently the course of the storm. thus, mr. edwards, in the third volume of his history of jamaica, as herein before cited, "_all hurricanes begin from the north, veer back to w. n. w., w., and s. s. w., and when they get round to s. e. the foul weather breaks up_." a short, sudden gale, resembling those of our summer thunder-showers, is sometimes met with from the s. e.; but the violent hurricanes of any considerable continuance are, in almost every case, as just stated. now, there is, in our latitudes, an obvious law on the subject, and it is this:--if the storm is not disproportionately long, northerly and southerly, there is a general tendency to induce and attract a surface current, in opposition to the course of the storm on its front, and especially its north front. at the same time, there is a tendency to induce a lateral current on its side, particularly the southerly side, and sometimes its south front: that the latter current is, in the first part of the storm, above the former; in the middle and latter part, it becomes the prevailing current at the surface, and the wind changes accordingly, with or without a calm--that this lateral change sometimes takes place on either side, but usually occurs on the side where the water is warmest, or there is, for other and local reasons, a _greater susceptibility in the atmosphere to inductive and attractive influence_. thus, our n. e. storms very frequently have a southerly current also, drawn from the ocean, south of us, which forms the middle current, and, in the middle and latter part of it, becomes the prevailing one. _i have seen more than a hundred such instances, clearly and distinctly marked._ since i have been writing this chapter, january th, , such an instance has occurred. on sunday, the th, the cirro-stratus were all day passing from the s. w. to n. e., and gradually thickening with light air from the e. n. e., in the afternoon. during the evening the wind set in _violently_ from the n. e., with a deluging rain. during the night, and after a brief calm, it changed suddenly to the southward, and blew in like manner. this morning the storm was gone, and with it, six inches of hard, frozen icy snow; the trade was clear, with the exception of here and there a broken, melting piece of stratus, but scud were still running from the southward, and the wind has been from the south, veering to s. w., all day, with sunshine. as i have before remarked, this middle current is always present, in this locality, in stratus storms, when there is a heavy fall of rain or snow, although, when the latter happens, the middle current is sometimes from the northward; if it be from the southward, it turns the snow first into very large flakes, and then to rain in our part of the storm. doubtless, the same thing occurs every where. in the west indies, and especially over the leeward islands, the middle current is most commonly from the stream of warm water which runs off to the westward into the caribbean sea; as the s. w. moonsoon is from the same current below the cape de verdes. the s. w. winds, which come from those south polar waters, in the west indies, appear to be the most violent. but it may be on either or both sides. the hurricane cloud of the west indies moves confessedly n. w. in most instances, and undoubtedly it does in all. there is an immutable law that requires it. the seeming exceptions are not such; they are but instances imperfectly investigated. now, a circular storm moving n. w. can set in n. w. only on the left front, and _can not change to s. w. on that side of the axis_. nor can the wind blow at the axis from n. w. at all. it should be n. e. in first half, and s. w. in last half. strange as it may seem, the axis of a west india hurricane in conformity with mr. redfield's theory, and a n. w. progression, has never been found, with perhaps a single exception, in any one of which i have seen a description. on the west coast of europe, the gale is commonly from the atlantic, either following under the storm from the s. w., or blowing in diagonally from the w. or n. w.; the n. e. wind of western europe being a cold, dry wind, which there is reason to believe has been around the siberian pole and is returning, as the cold northerly winds of the north pacific have around the north american magnetic pole. "if the n. e. winds always prevailed," says kämtz, speaking of berlin, "even at a considerable height it would never rain." this was based on an observation of showers, and not fully reliable. but the dry and cool character of the n. e. wind of western europe is unquestionable. the s. e. wind is also a storm wind, but owing to the character of the surface from which it is attracted, it is not as violent as the westerly winds are. such, too, is the general course and character of the side wind in the southern hemisphere. there gales are less frequent, the magnetic intensity is less, the counter-trades are less; it is not in "the order of providence" that as much rain shall fall there. nevertheless, gales occur, although rarely, if ever, with equal violence. about new holland, where storms are pursuing a s. e. course, they have the wind n. e., corresponding to our s. e., veering from thence, _by the north_, to the westward, clearing off from s. w., with a rising barometer, as ours do from n. w. in the bay of bengal, the indian ocean, and the arabian sea, there is more irregularity. but the law of progress and lateral winds can be distinctly traced as _present_ and prevailing, notwithstanding the irregularities. our limits do not permit an analysis. in the celebrated case of the charles heddle, there was much evidence to show that she was driven across the front of the storm by one lateral wind, and back by another. (diagram of colonel reid, p. .) the waters of the indian ocean are hot and confined. storms there are often composed of detached masses, move slower--sometimes not more than three or four miles an hour--and they curve over the ocean, where it is hotter than in any similar latitude. yet, notwithstanding all peculiarities and irregularities, the law we have been considering is probably the _prevailing_ law there. no man knows better the existence of these different currents than mr. redfield. doubtless it has escaped his attention that the upper of two, after the passage of a considerable proportion of the storm, becomes the lower, and causes a seeming change of the same wind. in a series of elaborate articles, substantially reviewing the whole subject, published in the american journal of science, for , he says: "in nearly all great storms which are accompanied with rain, there appear two distinct classes of clouds, one of which, comprising the storm scuds in the active portion of the gale, has already been noticed. above this is an extended stratum of stratus cloud, which is found moving with the general or local current of the lower atmosphere which overlies the storm. it covers not only the area of rain, but often extends greatly beyond this limit, over a part of the dry portion of the storm, partly in a broken or detached state. this stratus cloud is often concealed from view by the nimbus, and scud clouds in the rainy portion of the storm, but by careful observations, may be sufficiently noticed to determine the general uniformity of its specific course, and, approximately, its general elevation. "the more usual course of this extended cloud stratum, in the united states, is from some point in the horizon between s. s. w. and w. s. w. its course and velocity do not appear influenced in any perceptible degree by the activity or direction of the storm-wind which prevails beneath it. on the posterior or dry side of the gale, it often disappears before the arrival of the newly condensed cumuli and cumulo-stratus which not unfrequently float in the colder winds, on this side of the gale." "the general height of the great stratus cloud which covers a storm, in those parts of the united states which are near the atlantic, can not differ greatly from one mile; and perhaps is oftener below than above this elevation. this estimate, which is founded on much observation and comparison, appears to comprise, at the least, the limit or thickness of the proper storm-wind, which constitutes the revolving gale. "it is not supposed, however, that this disk-like stratum of revolving wind is of equal height or thickness throughout its extent, nor that it always reaches near to the main canopy of stratus cloud. it is probably higher in the more central portions of the gale than near its borders, in the low latitudes, than in the higher, and may thin out entirely at the extremes, except in those directions where it coincides with an ordinary current. moreover, in large portions of its area, there may be, and often is, more than one storm-wind overlying another, and severally pertaining to contiguous storms. in the present case, we see, from the observations of professor snell and mr. herrick, at amherst, massachusetts, and at hamden, maine ( and b.), that the true storm wind, at those places, was super-imposed on another wind; and various facts and observations may be adduced to show that brisk winds, of great horizontal extent, are often limited, vertically to a very thin sheet or stratum." much of the foregoing is graphically described, and unquestionably true. but it may well be asked how he, or others, distinguish which of two or more currents (for there are frequently three, and sometimes four visible), are the true currents of the storm, and which interlopers from another storm? is the true one always the upper one, and why? if the upper one, why is the interloper at the surface noted and quoted to prove what a storm is? how does he know what proportions of the winds he has recorded to show the revolving motion of gales, were the true storm winds of the particular storm? or, that every one of them was not an interloping wind on which the true storm wind was superimposed? these inquiries are pertinent, for obviously, unless some rule for distinguishing between the currents is given, and there be evidence of direct observation to show that the surface wind, whose direction is noted, is the true wind of the storm, and that the _latter_ is not _superimposed_, no reliance can be placed upon logs, or newspaper accounts, or registers. there is another element besides direction, viz.: superimposition, a determination of which _is_ essential to _truth_. it will be difficult for mr. redfield to say that a determination of that element has been made, with certainty, in a single storm he has investigated; and in relation to the convergence of storms, and blending, and superimposition of their winds, i think he is mistaken. mr. redfield is right in saying (american journal of science, vol. ii., new series, p. ) that "too much reliance may be placed upon mere observations of the surface winds in meteorological inquiries," and yet _they_ only have thus far been regarded, and he has proved gyration in no other way. i have frequently, with a vane in sight, asked intelligent men how the wind was, and been amused and instructed by their inability to state it correctly. mr. redfield, in his inquiries, often found two reports of the weather at the _same time_, from the _same place_, materially different; and i have known, from my own observation, newspapers and meteorological registers to be several points out of the way; and this, because the vanes are influenced by local elevations, and change several points, and very often; because few know the exact points of the compass in their own localities, and because entire accuracy has not been deemed essential. for these reasons, newspaper and telegraphic reports are not always reliable; and therefore, and because, also, storm-winds are easterly and fair winds westerly, and the former veer from east around to west, on one or both sides in many cases, there are few storms which can not be represented as whirlwinds, by a proper _selection_ of _reports_, a corresponding _location_ of the _center_, and an _extension_ of the lines of supposed gyration, so as to include the _preceding_ winds, the actual winds of the storm, and the _lateral_, and _succeeding_ fair weather ones. but, again, mr. redfield is right in saying there is, in such cases, "an extended stratum of stratus cloud," and it is always present. but why does he say this _covers the storm_? is it distinct from it, and if so, what is it doing there? what power placed it there, and for what purpose? has this extended stratum of cloud, which forms the canopy of a vast chamber--five hundred to one thousand miles in diameter, and less than two miles in vertical depth, while the earth forms the floor--any agency in producing the whirl that is supposed to be going on within it, and if so, what? has the earth any agency, and if so, what? if neither the ceiling nor floor of the chamber have any agency in producing it, what does? are we to consider the _storm-scud_ as possessing the power, and as waltzing around the aerial chamber, carrying the air with them in a hurricane-dance of devastation? _what, in short, is the power, and how is it exerted?_ to these questions, mr. redfield's essays furnish no comprehensive answer. there is an intimation that the cause of storms will be, at some future day, developed. one attempt, and but one, has thus far been made, and that i quote entire: "we have seen that the two cuba storms, as well as the mexican northers, have appeared to come from the contiguous border of the pacific ocean. "now, are there any peculiarities in the winds and aerial currents of those regions, which may serve to induce or support a leftwise rotation in extensive portions of the lower atmosphere, while moving on, or near the earth's surface? i apprehend there are such peculiarities, which have an extensive, constant, and powerful influence. first, we find on the eastern portion of the pacific, from upper california to near the bay of panama, an almost constant prevalence of north-westerly winds at the earth's surface. next, we have an equally constant wind from the southern and south-western quarter, which, having swept the western coast of south america, _extends across the equator to the vicinity of panama_, thus meeting, and commonly over-sliding the above-mentioned westerly winds, and tending to a deflection or rotation of the same, from right to left. as this influence may thus become extended to the caribbean or honduras sea, we have, next, the upper or s. e. trade of this sea, which is here frequently a surface-wind, and must tend to aid and quicken the gyrative movement, ascribed to the two previous winds; and lastly we have the n. e. or lower trade, from the tropic, which, coinciding with the northern front of the gyration, serves still further to promote the revolving movement which may thus result from the partial coalescence of these great winds of central america, and the contiguous seas. "thus, while a great storm is, in part, on the pacific ocean, its n. e. wind may be felt in great force on that side of the continent, through the great gorges or depressions near the bays of papagayo or tehuantepec, as noticed by humboldt, captain basil hall, and others, the elevations which there separate the two seas being but inconsiderable; and, when the gyration is once perfected, the whole mass will gradually assume the movement of the predominant current, which is generally the higher one, and will move off with it, integrally, as we see in the cases of the vortices, which are successively found in particular portions of a stream, where subject to disturbing influences." the analogy between this and the theory of professor dove, cited above, and prior, in point of time, is obvious. they are substantially alike in principle, with different locations. they differ also in this, professor dove appears to think something more than over-sliding necessary, and assigns the duty of crowding the upper current down in to the lower, to make an _encounter_, to a lateral overflow from africa. mr. redfield seems to think there may be a tendency to deflection when they "over-slide" each other. they are both closet hypotheses, the poetry of meteorology, with something more than poetical license as to facts. in the first place, _no such concurring winds exist in the same locality at the same time_. when the inter-tropical belt of rains is over central america and southern mexico, a s. w. monsoon blows in under it, but it usurps the place of all other surface winds; and, when the belt is absent, that portion of the eastern pacific is most remarkably calm, or is covered by the n. e. trades. secondly, the _trade-winds every where pursue their appointed course without "tendency to deflection" by the meeting, or "over-sliding," or "breaking in," or "encounter,"_ of other winds. the great laws of circulation do not admit of any such _confusion_. and, lastly, _no storm ever came over the eastern united states from that quarter_. the unchangeable laws of atmospheric circulation forbid it. recent observations also have shown that the storms on the west coast of central america, and the eastern pacific, pursue a n. w. course, precisely as in the west indies, and every where over the surface-trades of the northern hemisphere. indeed _mr. redfield himself has recently investigated several of them, and admits their course to be north-westerly_. (see american journal of science, new series, vol. xviii. p. .) but, suppose the co-existence of the winds and the course of the storms admitted as claimed, let us seek for clearer views. what do these gentlemen mean? do they intend to have us believe the air has inherent moving power, and that the "tendency" of which they speak is an attribute of the winds, and that when they thus meet, and "come into each other," "encounter," or "over-slide," and become acquainted, they wheel into a waltz, and move off northward, "integrally," with unceasing circular movement, even until they arrive at the arctic circle? or is it a mere mechanical effect of meeting, "coming into each other," or "over-sliding?" if the latter, why a tendency to rotation from right to left? the trade-winds, at least, are _continuous, unbroken sheets_, and not disconnected portions which meet and blow past each other, and there is no warrant for placing them _side and side_, and attributing to them any such mechanical effect, and as little respecting the other winds. outside of the fanciful hypothesis, there are no facts to show such a tendency one way rather than the other; and, in accordance with the known facts regarding stratification of the currents of air, no such "tendency" can exist. but what _power_ impels the winds, which thus meet at these points? if they be impelled, is it consistent with the action of this power that the _winds_ it has _created_ and _controls_, should thus assume an _opposite "tendency,"_ and whirl away to the north-eastward, regardless of the power that originated and controls them? what must this "_tendency_" be, which thus _occasionally_ not only diverts the winds from the _usually regular course_ given them by their originating power, but increases their action, from gentle, ordinary winds, to hurricanes? nay, which gives them a new, resistless gyratory and electric energy, increasing as the new, independent, supposed cyclonic organization moves off, "_integrally_," away from "the home of its many fathers," on a devastating journey towards the north pole? and, further, if all this were true as to the west indies and central america, what is to be said of the billions of other storms, originating on a thousand other portions of the earth's surface, and how are they to be accounted for, inasmuch as such other "meetings," "coming into each other," and "over-sliding," and "tendency to deflection," is not assumed to exist? these questions cannot be satisfactorily answered. the distinguished theorists are mistaken. the stratus-cloud does not over-lie or cover the storm. it is the storm. the winds beneath, whether surface or superimposed, are but its incidents, due to its static induction and attraction. their _direction_ depends on the shape of the storm cloud, and its course of progression, and the susceptibility of the surface atmosphere in this direction or that, to its inductive and attractive influence. their _force_ to its depth, its contiguity to the earth, and the intensity of its action; and the scud, are but patches of condensation, occasioned by the same inductive action which affects and attracts the surface current in which they form. another objection to mr. redfield's theory of gyration is based upon the fact that in order to constitute his _storm_, to get the _gyration_, he has to include, at least, an equal amount, generally a great deal more, of _fair weather_. the n. w. wind, the "posterior, or dry side of the gale," as he calls it (in the foregoing extract), is a _fair weather wind_. it is _necessary_, however, to complete the supposed _circle_, and it is _pressed into the service_. the practical answer given to the question, "_what are storms?_" is, they are cyclones, part storm, so called, and _part fair weather_; that is, the stratus-cloud, the scud, the easterly wind, and rain or snow of day before yesterday, were the _wet side_, or front part of the storm, and the sunshine, clear sky, and n. w. wind of yesterday, to-day, and, perhaps, to-morrow, are the posterior or dry side. when a storm clears off from the n. w. it is not _over_, it is, perhaps, _just begun_; and, inasmuch as it storms again, very soon after the wind changes back from the n. w. to the southward, in winter, our weather then is pretty much all _storms_. the statement of this claim seems so absurd that it may appear like injustice to make it. but gyration can not be made out without it, and it is evident in the extract quoted above; in the claim that the winter northers of the mexican gulf are parts of passing storms; and clearly and unequivocally advanced as a distinct proposition, as follows: " . the body of the gale usually comprises an area of rain or foul weather, together with another, and, perhaps equal, or greater, area of fair or bright weather." (am. jour. of science, vol. xlii. p. .) now, in the first place, we must distinguish between a storm and fair weather, before we can tell what the former is, and it is difficult to assent to a theory which explains what a s. e. storm of _twelve hours'_ continuance is, by including _two or three days of succeeding n. w. fair weather wind_, as a part of it. there is no proportionate relation as to _time_, nor any relation as to _qualities_, or the attending conditions of the atmosphere, nor any conceivable _connection_, except the hypothetical one of _gyration_, between the two winds. and, in the second place, it is true, and mr. redfield is well aware of the fact, that winds often blow for many days from the n. e., s. w., or n. w., without any preceding or succeeding winds to which they have any discoverable relation. if, therefore, truth would justify mr. redfield in including the fair weather wind, a difficulty would remain which his theory does not cover or explain. no american, except mr. redfield, has been able to discover satisfactory evidence of the gyration of storms, by actual careful observation, or a careful unbiased collation of the observation of others. professor coffin is reported to have read to the scientific association, at their buffalo meeting, a paper, confirmatory, in part, but i have not been able to see it. the tracks of tornados have been searched as with candles. when they have been narrow, from forty to eighty rods, their action has been substantially similar, and, although, as we have herein before stated, some irregularities have been found which were consistent with gyration--for irregularities attend the violent action of all forces, and particularly the motion of electricity through the atmosphere, as every one who has seen the zig-zag course of a flash of lightning knows--yet the evidence of two lateral inward currents, or lines of force, has predominated over all others. in all cases, where the path is narrow, those lateral currents are the actors; they constitute the tornado; their _irregularities_ of action produce the exceptions; but the exceptions are neither numerous nor uniform, and do not prove either the theory of mr. espy or that of mr. redfield. the action is not that of moving air, merely, but of a power exceeding in force that of powder, which nothing but electricity or magnetism can exert. as the path widens, the wind becomes more like the straight-line gust which follows beneath the ordinary severe thunder-showers. his theory finds no substantial confirmation or support in the path of the tornado. several storms were investigated by professor espy, some of them the same which mr. redfield had attempted to show were of a rotary character; one or two by the franklin institute of philadelphia; one by professor loomis, already alluded to; and recently, two by lieutenant porter, from logs returned to the national observatory. none of these investigations confirm the theory of mr. redfield. indeed, mr. redfield himself has found it necessary to resort to suppositions of _modifying causes_ to explain the evident inconsistencies. it is assumed that the axis, or center, oscillates, and describes a series of circles; and thus, one class of difficulties is avoided. again, it is assumed that simultaneous storms converge and blend upon the same field, and another class of difficulties are surmounted. and, again, inasmuch as it is notorious that violent gales are rarely if ever felt with equal violence around the area of a circle, but from one or two points only, it is assumed, that the storm winds ascend, superimpose, and descend again, when they return to the place of their first violent action, etc. the _simple truth_ requires no such resort to _modifying hypothesis_. still, another objection is, that the changes in the barometer, which occur before, during, and after storms, do not sustain the claims of mr. redfield or the requirements of his theory. the barometer sometimes rises before storms. it generally commences falling about the time, or soon after the storm sets in, continues to fall during its progress, and rises again, sooner or later, afterward. this is the general rule. on this subject mr. redfield's claim is this: "effect of the gale's rotation on the barometer.--the extraordinary fall of the mercury in the barometer, which takes place in gales or tempests, has attracted attention since the earliest use of this instrument by meteorologists. but i am not aware that the principal cause of this depression had ever been pointed out, previously to my first publication in this journal, in april, , when i took the occasion to notice this result as being obviously due to the _centrifugal force_ of the revolving motion found in the body of the storm. "since that period, inquiries have been continued by meteorologists in regard to the periodical and other fluctuations of the barometer, and the relations of these fluctuations to temperature and aqueous vapor. but these incidental causes of variation, in the atmospheric pressure, prove to be of minor influence, and we are left to the sufficient and only satisfactory solution of this marked phenomenon which is found in the centrifugal force of rotation." the average pressure of the atmosphere, at the surface of the ocean, or in the interior of the country, allowing for elevation, is about equal to the weight of a column of quicksilver, thirty inches in height; hence the barometer is said to stand at about thirty inches at the level of the sea. this is sufficiently accurate for the northern hemisphere, north of the n. e. trades; but the average is somewhat lower in the trades and in the southern hemisphere. thus, the average of sixteen months, during which the grinnell expedition was absent, was . / . from a large number of logs examined by lieutenant maury, the mean elevation in the n. e. trades of the atlantic was . / ; the s. e. trades of the atlantic, . / ; off cape horn, . / ; s. e. trades of the pacific, . / ; n. e. trades of the pacific, . / . the height of the barometer off cape horn is not a fair index of the general elevation of the southern hemisphere, inasmuch as it stands lower there than at the coast of patagonia and chili, or at most, if not all, other stations in that hemisphere. as the barometer is constantly oscillating up and down (irrespective of its diurnal oscillation), it has no known fair weather standard. the point of inches is taken only as it is a mean. i have known it to commence storming when the barometer was at . , and not to fall before it cleared off, below . . and i have known it to be below for several days consecutively, with fair weather. in our climate there is no reliable fair weather standard for the barometer. it falls below without storming; it rises far above, and storms without falling below. no reliance can be placed upon its elevation, except by comparison; but of that hereafter. the general rule, nevertheless, is, that it falls more or less during storms, whatever its height, and rises sooner or later, more or less, after they clear off. the difference between its highest and lowest points is called its range. the greatest range observed, and recorded, is about inches--from about to --but this range is rare. the range, in the trade-wind region, is comparatively small; in this country it is greater than in europe; and, generally, the range will be found greatest where the volume of counter-trade, and magnetic intensity, and the corresponding amount of precipitation, and extremes of heat and cold are greatest. one of the greatest ranges during one storm, or two successive portions of a storm, in this country, which i have seen recorded, occurred at boston, in february, . it was as follows--counting the hours as , and from midnight: feb. .. h.. . . " .. h.. . fall of . in hours. " .. h.. . rise of . in hours. " .. h.. . stationary hours. " .. h.. . fall of . in hours. " .. h.. . rise of . in hours. amount of oscillation, . in days, hours. these ranges were owing to the alternation of s. e. storms, and n. w. winds. taking the first range as a basis, and allowing the height of the atmosphere to be , feet for the first inch, we have nearly , feet displaced during one day, if we look for the displacement near the earth, or some or miles, if we soar aloft in the upper regions to look for the _lateral overflow_ of professor dove, and about the same quantity restored the next. this brings us to the inquiry, how was it done? it is perfectly idle to talk about _difference_ of _temperature_ or _tension_ of _vapor_, the _ascent_ of warm air, or _descent_ of cold in a case like this; or to say that they were occasioned by a lateral overflow of some thirty miles of its upper portion, first this way and then that, in such a brief space of time. the change is equal to nearly / of the weight of the whole atmosphere, and the cause, whatever it was, existed within two or three miles of the earth. mr. redfield's explanation i give in his own words, at length: "one of the most important deductions which may be drawn from the facts and explications which are now submitted, is an explanation of the causes which produce the fall of the barometer on the approach of a storm. this effect we ascribe to the centrifugal tendency or action which pertains to all revolving or rotary movements, and which must operate with great energy and effect upon so extensive a mass of atmosphere as that which constitutes a storm. let a cylindrical vessel, of any considerable magnitude, be partially filled with water, and let the rotative motion be communicated to the fluid, by passing a rod repeatedly through its mass, in a circular course. in conducting this experiment, we shall find that the surface of the fluid immediately becomes depressed by the centrifugal action, except on its exterior portions, where, owing merely to the resistance which is opposed by the sides of the vessel, it will rise above its natural level, the fluid exhibiting the character of a miniature vortex or whirlpool. let this experiment be carefully repeated, by passing the propelling rod around the exterior of the fluid mass, in continued contact with the sides of the vessel, thus producing the whole rotative impulse, by an external force, analagous to that which we suppose to influence the gyration of storms and hurricanes, and we shall still find a corresponding result, beautifully modified, however, by the quiescent properties of the fluid; for, instead of the deep and rapid vortex before exhibited, we shall have a concave depression of the surface, of great regularity: and, by the aid of a few suspended particles, may discover the increased degree of rotation, which becomes gradually imparted to the more central portions of the revolving fluid. the last-mentioned result obviates the objection, which, at the first view, might, perhaps, be considered as opposed to our main conclusion, grounded on the supposed equability of rotation, in both the interior and exterior portions of the revolving body, like that which pertains to a wheel, or other solid. it is most obvious, however, that all fluid masses are, in their gyrations, subject to a different law, as is exemplified in the foregoing experiment; and this difference, or departure from the law of solids, is doubtless greater in aëriform fluids than in those of a denser character. "the whole experiment serves to demonstrate that such an active gyration as we have ascribed to storms, and have proved, as we deem, to appertain to some, at least, of the more violent class; must necessarily expand and spread out, _by its centrifugal action, the stratum of atmosphere subject to its influence, and which must, consequently, become flattened or depressed by this lateral movement, particularly toward the vortex or center of the storm_; lessening thereby the weight of the incumbent fluid, and producing a consequent fall of the mercury in the barometrical tube. this effect must increase, till the gravity of the circumjacent atmosphere, superadded to that of the storm itself, shall, by its counteracting effect, have produced an equilibrium in the two forces. should there be no overlaying current in the higher regions, moving in a direction different from that which contains the storm, the rotative effect may, perhaps, be extended into the region of perpetual congelation, till the medium becomes too rare to receive its influence. but whatever may be the limit of this gyration, its effect must be to _depress_ the _cold stratum_ of the upper atmosphere, particularly toward the more central portions of the storm; and, by thus bringing it in contact with the humid stratum of the surface, to produce a permanent and continuous stratum of clouds, together with a copious supply of rain, or a deposition of congelated vapor, according to the state of the temperature prevailing in the lower region." the italics in the foregoing extract are mine; and, in relation to it, i observe: st. there is no cylindrical vessel around storms, and _air will not thus resist air_. confessedly, such resistance is necessary. let any one watch his cigar smoke, and see how readily it moves on, with little momentum. let any one try the experiment of creating a whirl in the _open air_, or in a room, or box of paper, or other material, which can be suddenly removed, with air colored by smoke. i am exceedingly mistaken if he does not find the presence of a "cylindrical vessel," absolutely essential to prevent the instantaneous tangential escape of the air. d. turn back to page and look at the fall of the barometer in the polar regions (recorded in the extract from dr. kane), with _scarcely any wind_, and _as little variation_ in its _direction_, and see how utterly mr. redfield's theory fails to account for the phenomena. d. if i understand mr. redfield correctly, he has abandoned the claim as originally made, that the wind moves in circles, expanding, and _spreading out_ by a "_lateral movement_," and now asserts that it blows spirally inward, and elevates the air in the center. i quote: "vortical inclination of the storm wind.--by this is meant some degree of involution from a true circular course. in the new england storm above referred to, this convergence of the surface-winds appeared equal to an average of about ° from a circle. in the present case, such indication seems more or less apparent in the arrows on the storm figures of the several charts, where the concentrical circle afford us means for a just comparison of the general course of wind which is approximately shown by the several observations. "perhaps we may estimate the average of the vorticose convergence, as observed in the entire storm for three successive days, at from ° to °--out of the ° which would be requisite for a congeries of _centripetal_ or center-blowing winds. this rough estimate of the degree of involution is founded only on a bird's-eye view of the plotted observations. but, however estimated, this involution seems to afford a measure of the air and vapor which finds its way to a _higher elevation_ by means of the vortical movement in the body of the storm." if the elevation of the air at the borders of the storm, and depression in the middle, resulted from the outward tendency and "lateral movement" of the revolving air, and from the _centrifugal force_, as in the experiment with the water in a cylindrical vessel, as stated in the first paragraph quoted, an _involution_ of from ° to ° from the action of a _centripetal force_, must carry the air _inward_, and the _barometer should stand highest in the middle of the storm_. the change is fatal to his theory. the two are diametrically opposite in character and effect. in one, the superior strata would be brought down in the center by the _lateral pressure outward_; in the other, they would be elevated by the _involution_, which "affords a measure of the air and vapor which finds its way to a higher elevation," etc. it is perfectly obvious mr. redfield has refuted his own hypothesis. in doing this, he is met by the other difficulty alluded to, which he does not attempt to explain. this gathering of the air inward, spirally, by a centripetal force, if it took place, not only would not depress, but _must elevate the barometer in the center, above that of the adjoining atmosphere_. when he first attributed the depression of the barometer to a lateral movement and centrifugal force, he supposed the superior strata descended into the depression, and their frigidity occasioned the condensation, and cloud, and rain. how he now proposes to account for the formation of cloud and rain during storms, while the warm air of the inferior stratum finds its way to a higher elevation in the center of the storm, he does not inform us, and we must wait his time. "i have," he says, "long held the proper inquiry to be, _what are storms_? and not, _how are storms produced_? as has been well expressed by another. it is only when the former of these inquiries has been solved that we can enter advantageously upon the latter." the former does not seem to be yet solved, or the solution of the latter commenced. mr. redfield tells us (page , and onward), that there is an extended stratum of stratus-cloud, which overlies the storm, and that it does not differ greatly from one mile in height. we are not told how the air, which finds its way to a higher elevation during several days continuance of such a storm, _gets through the stratum_. if he is right it _must_ do so, and it would not answer to _suppose_ a very small opening or gentle current through it, to carry off all the air which works inward in a hurricane, during several days continuance. but he does not seem to recognize either the necessity or existence of any _vent_ at all; nor is there any; and this fact is open to the observation of every school-boy in the country; and it is equally open to his observation that _when and where the barometer is most depressed, the stratus storm-cloud is nearest the earth_. colonel reid has much to say about the "_storm's eye_," or "treacherous center" of a storm. a careful analysis of the instances where the "storm's eye" is noticed will show that the term is applied, in the northern hemisphere, to that lighting up in the w. or n. w., which is the commencement of the clearing-off process, and attended with a shift of wind to the fair-weather quarter: _i. e._, to w. or n. w. just such an "eye" as is seen when the last of the storm cloud has passed so far to the east as to admit the rays of the sun under the western or north-western edge of it. the same kind of "storm's eye" is described in the southern hemisphere, except that the wind shifts to s. w. instead of n. w., that being the clearing-off wind there. no instance of a "_storm's eye_" in the center of the extended stratum of stratus-cloud, which overlies the storm, can be found recorded, to my knowledge; and it is obvious that colonel reid adopts the view of mr. redfield, that the westerly and n. w. _fair weather_ winds are a part of the storm. so long as these gentlemen hold to that opinion they will never solve the question, "_what are storms?_" or reach the other, "_how are storms produced?_" notwithstanding, mr. redfield asserts, or adopts the assertion, that the inquiry should be, "what are storms?" not "how are storms produced?" that inquiry should be a _rational_ one, and should not violate all analogy, or call for an explanation which science can not _rationally_ furnish. mr. redfield does not seem to have formed any just conception of the _immeasurable power_ of a hurricane, _five hundred miles in diameter_; or of the nature of that _rod_ which the _almighty must insert in it, to whirl it with such violent and long-continued force_; nor any just conception of the tendency of the whirling mass, in the absence of his "cylindrical vessel," to fly off, tangentially, into the surrounding air; or of the nature or power of the centripetal force necessary to hold the gyratory mass in its current, and gather it in involute spirals toward a center. nor has any other man who has witnessed, or read of mountain-tossed waves; of the largest ships blown down and engulfed; of towns submerged, and vessels carried far inland, and left in cultivated fields, by the subsidence of the sea; of sturdy forests and strongly-built edifices prostrated; or listened to the howling of the tempest, and felt his own house rock beneath him, been able to conceive of any known form of calorific or mechanical, or other power, acting from a comparatively small center, which could hold such an immense irresistable mass of whirling air in a circle, and _gather it_ in toward the center in gradually contracting spirals. i confess that, to my mind, it seems little less than a mockery of our intelligence for mr. redfield, or professor dove, or any other man, how distinguished soever he may be, to tell us that all this is the result of a "tendency to left-wise rotation" of ordinary winds, "coming into each other," or "over-sliding," or "meeting," or "encountering," on this "front," or that, down in central america, or in the west indies, or the monsoon region; or to talk of "lateral overflows" from mere gravity; of the ascent of warm air, or the descent of cold strata; of the _resistance of adjacent passive air_, or other mere _atmospheric resistances_ in connection with such _awful manifestations of power_. their explanations of these phenomena are not rational, nor can they be believed by any rational man, who will bestow upon them half an hour of _comprehensive, unbiased reflection_. waiving many minor points of great force, for this notice of mr. redfield's theory is already too much extended for my limits, i am constrained to take issue with him on the fact, and to assert, unhesitatingly, that in a _majority of instances no such barometric curve exists_. doubtless the depression beneath the storm is found, and exterior lateral elevations may also be had by _extending the line into the usual fair weather elevation on each side_, as mr. redfield is obliged to do, to get his supposed circle of winds at all. doubtless, too, the seamen sailing out of a storm, on either _side_, and approaching fair weather, will have a rising barometer. but from _front to rear, on the line of progression_, in tropical storms, the curve does not exist on shore, in this latitude, oftener than in two, or possibly three, cases in ten; and then only upon a single state of facts--that is, when there is an interposition of n. w. wind; and this, at some seasons, rarely occurs. an elevation usually occurs before the storm, on its front, if it present an extensive easterly front, as one of these classes does, and a _depression is left_ after it has passed off, unless a considerable body of n. w. wind interposes, as heretofore stated. but when there is not such interposition of n. w. wind (for w., w. n. w., or even n. w. by w. will not suffice), there is not an immediate rise of the barometer corresponding in rapidity and extent with the fall, and frequently none during the first twenty-four hours of bright, fair weather. let the reader, if he has access to a barometer, note this fact, for it is obvious and conclusive. finally, there are other atmospheric conditions to which the barometric changes are obviously due: st. the counter-trade is of a different _volume_, at different times, over the same locality, and hence a difference in the normal elevations of the barometer. d. it is at a different _elevation_, at different times, over the same locality. it was so found by the investigations of the kew observatory committee referred to; has been so found by other aeronauts, and may readily be seen by a careful, practiced observer. it is highest, with a high barometer, in serene weather, when a storm is not at hand; and can sometimes be plainly seen to ascend when a considerable volume of n. w. wind is blowing in beneath, and elevating, simultaneously, the trade and the barometer. opportunities occur every year, when the northern edge of the dissolving stratus-cloud is attenuated, and the storm is clearing off in the n. w., with wind from that quarter, and a rising barometer, when its gradual elevation may be observed to correspond with the _volume_ of that wind. d. during storms, with a low barometer, the _trade_ and the _clouds run low_. this, too, is clearly observable, especially when the stratus-cloud passes off abruptly, very soon after the rain ceases. in such cases the barometer will remain depressed for a considerable time, unless another storm supervenes speedily, or the wind sets in from the n. w. th. the _trade, in a stormy state, moves faster_ than when in a normal condition. this is observable during the partial breaks which frequently occur in storms, and at other times. it is also inferable from the more rapid progress of the more intense center, and other intense portions of storms, and the consequent greater depression of the barometer, under such centers or intense portions. (see the storm of professor loomis.) it is obvious, also, from the greater rapidity of progress attending the more intense and violent storms which all investigations discloses. these simple facts explain all the phenomena: st. the trade stratum is a continuous unbroken sheet, and its descent must displace a portion of the surface atmosphere. a portion of it is impelled forward, aiding in the precedent elevation of the barometer, and a portion is attracted backward, into the space from which a like portion had been previously attracted by the passing storm cloud, forming the easterly wind. d. the increased progress of the stormy portion of the counter-trade occasions an accumulation in front of the storm, and an elevation of the barometer, and tends also to increase the _depression_ under the spot from which it moves. the latter is, to some extent, counteracted by the thin sheets of surface wind which are drawn in under the stratus from the sides. that which is drawn from the front in successive portions, fills the space from which like portions had been drawn to the westward, and left behind in a passive state by the passing storm. thus, the surface atmosphere of new england may pass under the entire width of a storm, as a gale; moving now in puffs with great violence, as it passes beneath irregular and intense portions of the cloud, and now moderately; and be left, in a passive state, in kentucky, occupying the space from which the atmosphere had been previously drawn by the same storm, _in like manner_, on to northern texas. d. the nearer the stratus-cloud to the earth, the greater the displacement of surface atmosphere, the lower the barometer, and, ordinarily, the more violent the wind. first, because the same intensity, which, by attraction, brings the trade near the earth, acts with greater force upon the surface atmosphere; and, secondly, the storm winds, which are often most rapid beneath the clouds and above the earth, are likely to be felt with more violence at its surface, where the stratus cloud runs low, especially at sea. i desire to commend all these facts, in relation to the theory of mr. redfield, to the careful attention and observation of those who, although believers in the theory, are not wedded to it; and who have a sincere desire to understand the phenomena which are continually, and thus far, _mysteriously_, occurring within two or three miles of us, while our knowledge of the distant worlds around us--the science of astronomy--seems almost perfect. i will return to a further and a careful consideration of the nature of the reciprocal action between the earth and the counter-trade, and the facts bearing upon the question, in another chapter. it is obvious that received theories can not aid us materially in the inquiry. chapter x. we are yet ignorant of the true nature of magnetism. we trace its lines, as in the diagrams, upon and around the magnet; but we can only do this with soft iron, or other substance, in which magnetic action may be induced. we know that these lines are currents, or lines of force, for that force produces sensible effects, and we measure it by the movements of the needle. we know that these lines may be _deflected_ by other magnetic bodies, and concentrated upon them. we know that the earth, and the smallest magnets, exhibit properties in common. the poles of the magnet are some distance from its extreme ends--so are those of the earth. the intensity increases, from the center, or near it, to the poles of the magnet, as shown by its attraction; and the same increase of magnetic intensity, from the magnetic equator to the magnetic poles, or near them, is traced upon the earth. we know that there are two lines, or rather _areas_, of greater intensity upon the globe. one extending from the american magnetic pole, south-eastwardly, to a corresponding pole in the southern hemisphere; and another, the asiatic, extending from the siberian pole to a corresponding southern one, in like manner. we know that, from those lines or areas, the intensity, east and west, on the same parallel of latitude, decreases each way, to about midway between them. thus, calling the intensity where humboldt found the magnetic equator over south america, in ° ' south latitude, , or unity--the least intensity known is, . , found at the magnetic equator, over the south atlantic, and at its most southern depression; and it increases to . in the west indies, and to . upon one or more points of the north american continent, south of the magnetic pole, and about the meridian of °. that it is . , at warren, ohio, in latitude ° ', and longitude ° ', and decreases to . at new haven, connecticut, in latitude ° '. that it is but . at paris, nearly one third less than on the same latitude in some portions of this continent. that the line of equal intensity, or "_iso-dynamic_" line, of - / , is a closed curve of an oval shape, extending somewhat below °, in the longitude of cincinnati, and reaches off nearly to bhering's straits, on the west; rising in a similar manner, though not so abruptly, on the east; including the great northern lakes and a considerable part of hudson's bay. while the iso-dynamic lines of - / , and - / , are smaller ovals, included within the former. such, at least, is the present belief from such investigations as have been made. (see an article by professor loomis, american journal of science, new series, vol. iv. p. .) our subject demands a still closer examination of the elements of magnetism and its associated electricities, and their influence upon climate and the atmosphere with a view to the solution of the questions in hand, and we will pursue the inquiry in the present chapter. waiving, for the present, any further notice of the fact that the counter-trades are concentrated over, and contiguous to, this area of intensity, for the purpose of examining the magnetic phenomena independently, and intending to return to a consideration of their connection with it, we observe:--that it is now well settled that the iso-geothermal lines, or lines of equal terrestrial heat, are coincident, or nearly so, with the lines of equal magnetic intensity. the points where the magnetic intensity is at a minimum, on the magnetic meridian, are the warmest points of that meridian, and those where it is most intense, the coldest. the magnetic elements of a place may be computed from its thermal ones. the laws producing or governing the distribution of one, have an intimate physical relation with those producing or governing the other. professor norton ably sums up a discussion of the subject (in the american journal of science for september, ), omitting the theoretic propositions, as follows: " . all the magnetic elements of any place on the earth may be deduced from the thermal elements of the same; and all the great features of the distribution of the earth's magnetism may be theoretically derived from certain prominent features in the distribution of its heat. " . of the magnetic elements, the horizontal intensity is nearly proportional to the mean temperature, as measured by fahrenheit's thermometer; the vertical intensity is nearly proportional to the difference between the mean temperatures, at two points situated at equal distances north and south of the place, in a direction perpendicular to the iso-geothermal line; and, in general, the direction of the needle is nearly at right angles to the iso-geothermal line, while the precise course of the inflected line to which it is perpendicular may be deduced from brewster's formula for the temperature, by differentiating and putting the differential equal to zero. " . as a consequence, the laws of the terrestrial distribution of the physical principles of magnetism and heat must be the same, or nearly the same; and these principles themselves must have, toward one another, the most intimate physical relations." the magnetic elements, of which professor norton speaks, are the declination, dip, and horizontal and vertical forces or intensities. i have said, that toward the areas of greatest magnetic intensity, the needle every where declines. so as intensity increases, from the magnetic equator toward the poles, the needle, when so suspended as to permit of the motion, _dips_, inclines downward, and the dip is greatest, on the same parallel, where intensity is greatest. to my mind, the magnetic elements are very intelligible. they are all attributable to attraction, and attraction is greatest where intensity is greatest. there is nothing in the earth or atmosphere to make the needle point northerly rather than in any other direction, except magnetic intensity. thus, the greater intensity of magnetism near the northern and southern points of the globe, attracts the corresponding ends of the needle in those directions. and, as magnetism increases in quantity or intensity, and the poles are approached, the attraction increases, and the needle dips more and more, till the focus of intensity and attraction is reached, and then it becomes perpendicular. so magnetism is unequally diffused, meridionally, in or over the earth, and there are two equidistant areas where its quantity or intensity is greatest. these exert a lateral attraction upon the needle; it yields to this attraction, and hence its declination. if it is carried on to one area of intensity, and to the center of it, it will point to the northern focus of intensity or magnetic pole; and, if carried a trifle further west, it will yield to an eastern attraction, and point directly north. if carried still further west, its declination _east_ will increase. thus its normal direction is to the pole, on the central focus of intensity, and when it points directly north it is west of the central line of intensity. and thus, it seems to me, all the magnetic elements may be resolved into the one element of attraction by excess of intensity or activity. this impression is strengthened by the fact that the needle moves to the east in the morning, when the solar rays increase magnetic activity in that direction, and west again, as their influence increases there. now, these elements--the declination and horizontal and vertical forces--all these periodical, regular, and irregular variations of magnetic activity, are intimately connected with the variations of atmospheric condition: first, they show an increase of activity during certain hours of the day, corresponding to, and obviously connected with, the diurnal atmospheric changes. second, they show an increase of activity during the northern transit of the atmospheric machinery--an _annual_ variation. third, they show an increase in that activity during the latter portion of each decennial period, conforming to the occurrence of solar spots. and, fourth, _irregular variations_ of activity, corresponding with the _irregular changes_ of atmospheric condition. we will examine these results, and in doing so, take those of the element of declination--one answering for all. the magnetic needle moves to the west in summer, from about a.m. till about p.m., and the extent of its progress, during that period, constitutes the magnitude of its daily variation. it is found that this variation differs in different months, and that it is normally greatest in the summer months, and least in the winter, in the ratio of about two to one. it is further found, that in different years the maximum activity occurs in different months, and that the years differ also, and there is a distinctly marked decennial period, corresponding most remarkably with the decennial maxima of recurring solar spots, as observed by schwabe. dr. lamont, of munich, gives us the following table of magnitude of declination there, for the ten years preceding , which clearly exhibits this fact, and also the greater intensity during the northern transit of the atmospheric machinery. he says: "the magnitude of the variations of declination have a period of ten years. for five years there is a uniform increase, and during the following five years a uniform decrease in the variations. with us the magnetic declination is a minimum at about eight o'clock in the morning, and is greatest at two o'clock in the afternoon. subtracting the declination at eight o'clock from that at two o'clock, we obtain _the magnitude of the diurnal motion_. from the hourly observations, conducted in this observatory since the month of august, , we ascertain the following to be the magnitude of the diurnal motion for each month separately." +-------------------------------------------------------------------+ | | jan. | feb. | march.| april.| may. | june. | july. | aug. | +-------------------------------------------------------------------| | | . | . | . | . | . | . | . | . | | | . | . | . | . | . | . | . | . | | | . | . | . | . | . | . | . | . | | | . | . | . | . | . | . | . | . | | | . | . | . | . | . | . | . | . | | | . | . | . | . | . | . | . | . | | | . | . | . | . | . | . | . | . | | | . | . | . | . | . | . | . | . | | | . | . | . | . | . | . | . | . | | | . | . | . | . | . | . | . | . | +-------------------------------------------------------------------+ +----------------------------------------------------+ | sept. | oct. | nov. | dec. | autmn | spring| year.| | | | | |& wint.| & sum.| | |----------------------------------------------------| | . | . | . | . | . | . | . | | . | . | . | . | . | . | . | | . | . | . | . | . | . | . | | . | . | . | . | . | . | . | | . | . | . | . | . | . | . | | . | . | . | . | . | . | . | | . | . | . | . | . | . | . | | . | . | . | . | . | . | . | | . | . | . | . | . | . | . | | . | . | . | . | . | . | . | +----------------------------------------------------+ the philadelphia and toronto observations disclose the same state of facts. dr. lamont, also, in his article, gives us the following table of the magnitude of the variations derived from observations at gottingen: +--------------------+ | year.|mean of year.| |--------------------| | | . | | | . | | | . | | | . | | | . | | | . | | | . | +--------------------+ a comparison of these tables, and particularly the latter, with schwabe's table of spots, is interesting. there is obviously a greater mean variation when the spots are most numerous. comparing the two with the tables of hildreth, in relation to the temperature, from to , there is, to say the least, a most remarkable coincidence. and there are others equally remarkable. there are also irregularities of action disclosed by all, in different months of the different years, and of the same year, which are obviously connected with the difference of the seasons; and there are constantly occurring irregularities and disturbances which correspond with the, as constantly occurring, irregular atmospheric phenomena. a wide field is here opened for investigation and research. i have not time or opportunity to pursue it. enough appears, so far as i have examined, to confirm the belief that magnetism is actively concerned in the production of the varied changes, as well as the normal conditions of the weather. in what manner does it act? an answer to this requires an extension of the inquiry. the lines of magnetic force are every instant passing upward from the earth, _around_ and _through_ us. their connection with heat is unquestionable. they are intimately associated, also, with another equally obvious and intensely active agent--electricity. we speak of this as an independent, imponderable, elementary body, but how little we yet know of it. it is every where, in every thing, easily excited into action, and then traceable to a certain, but limited extent. it is set in motion, and becomes obvious to us, by the chemical action of the acids and metals of a galvanic apparatus. we separate it from the atmosphere by friction and excitation, upon non-conductors, as in the electric machine; by the cleavage of crystals and other exciting operations. we obtain it from magnets, by the magneto-electric machine, and from the lines of magnetic force which are ever passing into the atmosphere from the earth, by intersecting them with a movable iron wire, properly insulated. _from the current of magnetism which has passed through us from the earth, electricity may thus be separated and collected over our heads._ we set it in motion, and obtain it _by heating_ different metals in connection, or the same metal unequally; and from certain animals--like the torpedo and the gymnotus--whose organization is such as to enable them to evolve it. in all these cases, and they constitute an epitome of the principal methods by which we obtain it in a distinct form, it is made to flow in currents. when thus obtained, and imprisoned in non-conductors, it may be discharged, and with somewhat different effect, as it is discharged in a mass, disruptively, as it is called, as from the clouds in lightning, or permitted to flow convectively, in currents, along the wires of a galvanic apparatus, or in heated air, as from the earth to a cloud in the tornado. it is, moreover, capable of division into positive and negative, and when concentrated or disturbed in one body, it tends to create a similar disturbance or division in a contiguous mass. to this action of electricity, the term static induction is applied. thus, a positively electrified body _induces_ a division of the electricity in a contiguous body, if both are insulated or surrounded by a non-conducting medium; the negative electricity of the contiguous body being attracted by, and tending to pass to, the positive of the adjoining body, and the positive being repelled to the opposite side. that, in its turn, if sufficiently powerful, tends to disturb the electricity of its neighbor, and attract away its negative electricity; or, if the body which contains it is free to move, to attract that. thus, by the conflicting action of a positive atmosphere, and a negative earth, and perhaps counter-trade, influenced by magnetism and the solar rays, the currents and winds of the atmosphere are produced, the atmosphere moving with exceeding ease and rapidity. electricity, excited into currents, or obtained and discharged in either of the methods enumerated, is identical in character, and produces certain well-known effects: st. physiological.--shocking and convulsing the animal system; producing a peculiar sensation on the tongue, and a flash before the eyes, and in sufficient quantity destroying life. d. magnetic.--_deflecting the needle_, and, by a suitable arrangement of wire into helices, _conferring magnetic power_, or constituting magnets. d. luminous.--producing light--by a spark, as it does in natural phenomena--by the glow, the brush discharge, the ball of flame, the flash, or the chain of lightning, and probably the aurora. th. evolving heat.--melting metallic substances by concentration, with a great intensity of heat--as the wire of the galvanic apparatus, and as is sometimes seen in the effects of lightning in fusing metals on persons stricken; and setting combustibles on fire. th. attraction and repulsion.--attraction, when the currents flow parallel with each other, or are of opposite natures, and repelling when of like character. th. induction.--inducing attendant circular or other secondary currents, such as may be seen in the atmosphere during its most violent displays of active energy. th. capable of being dissipated by heated air, or carried off by moisture, although isolated by dry air, of ordinary temperature, which is a bad conductor. now, although magnetism can not be collected, imprisoned, or discharged, like electricity, or collected at all, but by its adherence to some substance capable of magnetization, it is obvious there is an intimate association, at least, between it and electricity. _they are never found alone._ all _electricity_ will _magnetize_. all _magnetism_ will evolve electricity. all _currents_ of _electricity_ have _encircling currents_ of _magnetism_, and all deflect the magnetic needle. all magnetic currents give out to intersecting wires, _currents of electricity_, and all magnets _induce_ them. electricity, therefore, whether identical in substance with magnetism, but differing in form, or whether merely associated with it, as is variously believed, should be present with magnetism in greater quantity or intensity where magnetism is most intense, and active, and whenever present, should be active and influential. and so we find, from observation, the fact to be. no inconsiderable effort has been made by the advocates of the caloric and mechanical theories, to ignore the agency of electricity and of magnetism, in the production of the varied meteorological phenomena. but it will not do. the phenomena, grouped and analyzed, disclose a potential-controlling, magneto-electric agency, and meteorology will advance rapidly to perfection, as a simple, intelligible, and practical science, _as soon as that agency is admitted_. electricity is always perceptibly present in storms and showers within the tropics. most of the rain, from the tropical belt, falls from "thunder showers." so hurricanes and typhoons, and all tropical storms, are confessedly, and in proportion to their intensity, "_highly electric_." this excess of quantity or activity of electricity, exists in connection with the movable atmospheric machinery. when it moves up north in summer, and arrives at its highest point of northern transit, _storms_ are very _uncommon_, and the tropical forms of cloud and showers, with thunder and lightning, prevail. this is most obvious, if not most influential, where the magnetic intensity is greatest. violent showers, and gusts, and tornadoes, are more frequent in this country than in europe; and over the area of greatest intensity, as in ohio, than at a distance on the extreme eastern or western coast. and the same is true over the intense magnetic area of asia. electricity, too, like magnetism, has its diurnal, and doubtless its annual and decennial variations, and also its irregular ones, and they are most obviously and intimately connected. magnetism and electricity together, constitute the aurora. its culmination is in the magnetic meridian--it affects the telegraph wires--is connected with the irregular disturbances which affect the magnetic needle, and does not exist in the limits of the trades, although occasionally seen from thence, when it passes south, and near them. the aurora sometimes extends south in waves, as do the magneto-electric, atmospheric, periodical changes of cold and heat, and storm, and sunshine. _the aurora is connected with the formation of cloud_, and with a smoky atmosphere, similar to that with which we are familiar in summer and autumn. thus humboldt (cosmos, vol. i. pp. , ). "this connection of the polar light with the most delicate cirrus clouds, deserves special attention, because it shows that the electro-magnetic evolution of light is a part of a meteorological process. terrestrial magnetism here manifests its influence on the atmosphere, and on the condensation of aqueous vapor. the fleecy clouds seen in iceland, by thienemann, and which he considered to be the northern light, have been seen in recent times by franklin and richardson, near the american north pole, and by admiral wrangel on the siberian coast of the polar sea. all remarked 'that the aurora flashed forth in the most vivid beams when masses of cirrus-strata were hovering in the upper regions of the air, and when these were so thin that their presence could only be recognized by the formation of a halo round the moon.' these clouds sometimes range themselves, even by day, in a similar manner to the beams of the aurora, and then disturb the course of the magnetic needle in the same manner as the latter. on the morning after every distinct nocturnal aurora, the same superimposed strata of clouds have still been observed that had previously been luminous. the apparently converging polar zones (streaks of clouds in the direction of the magnetic meridian), which constantly occupied my attention during my journeys on the elevated plateaux of mexico, and in northern asia, belong, probably, to the same group of diurnal phenomena." mr. william stevenson gives us (in the london, edinburgh, and dublin philosophical magazine for july, ) an interesting article on the connection between aurora and clouds. his observations on this most important branch of the subject trace a connection between the aurora and the formation of cloud, and open up, as he says, "a most interesting field for observation which promises to lead to very important results." such observations point with great significance, to the primary influence of the magneto-electricity of the earth. to the difference in the magnetic intensity of the eastern portion of this continent, compared with europe and our western coast, very much of the difference of climate, so far as temperature is involved, may be attributed. we have seen in what manner the iso-thermal lines surround these areas of intensity. so the most excessive climate--that is, the climate where the greatest extremes alternate, other things being equal, is upon or near the line or area of greatest magnetic intensity. i say other things being equal, because large bodies of water modify climates by equalizing the seasons--making the summers cooler and the winters warmer than the mean of the parallel. thus, our great interior lakes modify the climate in relation to temperature in their vicinity. their summers are cooler and their winters warmer; but westward of them the same line of equal summer temperature, or iso-thermal line, rises with considerable abruptness, and the winter, or iso-cheimal line of equal temperature, falls in a similar manner. thus, the range of the thermometer, from the highest elevation to the lowest depression, for the year, is very great, while in the tropics the range is comparatively small. from observations made at the military posts of the united states, dr. forrey deduced summer and winter lines of equal temperature, starting from the vicinity of boston and running west, which showed most remarkably the rise of the summer lines as intensity increased, and the fall of the winter lines in like manner. the influence of the lakes was also most obvious. the elevation of the earth increases, going west, to about feet at the surface of the lakes, and to nearly , feet at the eastern base of the rocky mountains; and, although temperature does not decrease to as great a degree when the elevation above the level of the sea is _gradual_, yet some allowance should doubtless be made for that elevation on this line. when that allowance is made, the ascent of the summer line, to the north, over the area of greatest intensity, is strikingly apparent. dr. forrey also instituted a comparison between fort snelling, where the climate is as excessive, and the range of the thermometer as great, as in any portion of the continent in the same latitude, with key west, and i copy his diagram. it is very instructive, showing the gradual mean rise of the temperature, from january to december, inclusive, while the cross lines show the _extremes of each month_. perhaps the most interesting part of it, is the illustration of the monthly extremes, and the contrast between them, in the excessive climate of fort snelling, and the tropical one of key west. each is a type of the climate in which it is situated. the annual range and monthly extremes are small in tropical countries, and large in extra-tropical ones. the extreme range, or greatest elevation of heat, contrary to what is generally supposed, is greater at fort snelling than at key west. but the climate of the latter is modified by the adjoining ocean. i copy, also, a table (p. ), showing the range of the thermometer for the year, and the maxima and minima, during each month, at several other places in this country, and at london and rome, for the purpose of showing the extent of the ranges compared with those places; and also, that these great changes in each month occur very uniformly all over the country, and may always be expected, and with considerable regularity. they are incident to our climate. i wish i could engrave the foregoing diagram, and the following table, upon the mind of every man, woman, and child in the country; and under it, in ever-visible letters, these words of precaution: conform to the peculiarities of your climate, and clothe yourselves, at all times, in accordance with the alternations of the weather. if heeded, they would save thousands, every year, from premature death. [illustration: fig. .] the effect of this difference of magnetic intensity upon the climate of europe is marked. there, the excessive summer heat, which our greater magnetic intensity and larger volume of counter trade give us, is unknown. hence, while we can grow indian corn (which requires the excessive summer heat) over all the eastern states, up to °, and in some localities east of the lakes to ° ', and to ° west of them, to the base of the rocky mountains, and notwithstanding the increase of elevation, they can not grow it except over a limited area, and with limited success. nor can they, or the inhabitants of any other country except china, grow profitably the kind of cotton which is so successfully grown in the southern states of the union. nor can china do so to a considerable extent, because of the mountainous character of the surface. to a level and remarkably watered country, greater magnetic and electric intensity, and a greater volume of counter-trade, we are, and ever shall remain, indebted, for an almost exclusive monopoly in the growth of two of the most important staple productions of the earth. on the other hand, although the same magnetic intensity, and its winter excess of positive electricity and cold, make our winters extreme, there are but few of the productions of temperate latitudes which we can not grow successfully, and they are comparatively unimportant. a fort vancouver, oregon territory b fort brady, outlet of lake sup. c hancock barracks, houlton, me. d fort armstrong, rock island, ill. e west point, new york f washington, d. c. g jefferson barracks, near st. louis h fort king, interior of east florid. i environs of london k rome, italy a b c d e f h i j k lat. ° ° ° ° ° ° ° ° ° ° ' ' ' ' ' ' ' ' ' ' annual range. jan. min. - - - - max. feb. min. - - - max. mar. min. - - max. apr. min. max. may. min. max. june. min. max. july. min. max. aug. min. max. sept. min. max. oct. min. max. nov. min. max. dec. min. - - max. this excess of magnetic intensity and electricity not only gives a peculiar character to our vegetation, but also to our race, our animals, and every thing. he who supposes that the restless activity and energy of the people of the united states is the result of habit, or education, or any fortuitous circumstances alone, is mistaken. let him watch the contrast in his own feelings during those occasional languid, damp, and sultry, although not thermometrically, hot days--which so much resemble the summer weather of england--with those days of bright, bracing, n. w. and s. w. air, so much more frequent here, and he will appreciate the difference. that term "bracing," so much in use, will express the effect of this peculiar weather. it "girds up the loins," both of body and mind. men and animals can work with more ease, even in our peculiar extremes of heat, than they can in england, and fatten with less. a similar difference in degree is found between our climate and that of the pacific portion of our country. something is due to the difference in the volume and moisture of the counter-trades, and something to the contiguity of the pacific ocean; but to the difference in magneto-electric intensity, the contrast is mainly due. corn and cotton will be grown, to some extent, in the valleys west of the meridian of °, but never as successfully as east of it. the aurora is periodical, like all the other atmospheric phenomena, but its periodicity is not accurately ascertained. it is believed to have occurred much oftener during the second quarter of this century, than during the first. it is known, however, to occur most frequently in the spring and fall; and during those periods when the active and rapid transit of the atmospheric machinery produces the greatest degree of magnetic disturbance. this identifies it with terrestrial magnetism. dalton gives us the following table of observations, arranged according to the months when they were seen. jan. feb. mar. apr. may. june. july. aug. sept. oct. nov. dec. ( ) ( ) ( ) ( ) ( ) contains those observed by him at kendall; ( ) are taken from another list; ( ) is marian's list of those observed before ; and ( ), those seen in the state of new york in and . mr. stevenson's table of those observed by him at dunse, from to , inclusive, is as follows: jan. feb. mar. apr. may. june. july. aug. sept. oct. nov. dec. observations in this country correspond substantially with the foregoing. they are, however, seen here in the summer months more frequently than in europe. see an article by mr. herrick (american journal of science, vol. . p. ). in this, also, they conform to our greater magnetic intensity and more excessive climate. the auroras appear to follow the polar belts of condensation and precipitation. dalton considers them indications of fair weather. they are often most brilliant just after a storm has passed, but their continuance is no indication that another will not follow within the usual period. the condensation with which the aurora is connected, is not, in my judgment, often in the counter-trade, or below it, but above, where feeble condensation has been seen by aeronauts when invisible at the surface of the earth. neither the height of this condensation, not that of the aurora, have been satisfactorily ascertained. the aurora of april th, , was a favorable one for observation. it was carefully and attentively watched by professor olmsted, mr. herrick, dr. ellsworth, and others, and they are intelligent and skillful observers.[ ] but the nature of the aurora forbids reliance on parallax, or measurements founded on the time when, any portion of the bow or arch rises in range of a particular star. the bow or arch moves southwardly, but the same rays or currents do not. the wave of magnetic _activity_ moves south, and each successive current, as it is reached by the _impulse_, becomes luminous. hence the observers, when distant, do not see, at the same time, or at different times, the same rays. the phenomenon is unquestionably magneto-electric. electricity becomes luminous in a vacuum, and de la rive, by combining the electric currents with those of magnetism, produced all the peculiarities of the aurora. the magnetic currents, passing from the earth, have associated electric ones in connection, and these, in the upper attenuated atmosphere, become luminous. whether, as de la rive supposes, by combining with the positive electricity existing there, or because the associated electric currents are _then_ in excess, not being intercepted by atmospheric vapor and returned to the earth in rain, we can not know, nor is it very important we should. having thus taken a general view of the nature of magnetism and its associated electricities, and their connection with the general and obvious peculiarities of climate, let us approach more nearly the varied atmospheric phenomena, resulting from variations of pressure, temperature, condensation, and wind, and give them a closer consideration. they all have regularity and periodicity--they all occur in degree, and in connection with magnetism and electricity, during the twenty-four hours of every serene and normal summer's day. grouped together, in comparison with the changes in the activity and force of the magnetic elements, their connection is clearly discernible. the day may be said, with truth, to commence, in some portion of the summer, at a.m. the atmospheric does at all seasons. at that hour the barometer is at its morning minimum. it has, as we have said, a perceptible diurnal variation of two maxima and two minima. its periods of depression are at a.m., and p.m., and of elevation at a.m., and p.m. the difference between the elevation and depression is considerable within the tropics, where humboldt tells us the hour of the day can be known by the height of the barometer, and it decreases toward the poles. at a.m. it is then at one of its minima, and rises till o'clock. at, or about the same period, and sometimes when the barometer is falling, and previous thereto, there is a tendency to fog in localities subject to that condensation. this tendency is sometimes observed at the other barometric minimum, late in the afternoon or early in the evening, but less frequently. the tendency to fog condensation is greatest in this country about the morning minimum. it seems to be owing to the influence of the earth; it is confined to the surface atmosphere, and is apparently produced by the inductive agency of the negative electricity of the earth. it disappears, whether it be high or low fog, about the time when the barometer attains its morning maximum, or about a.m. at about that period, when there has been fog, or earlier, when there has not, and sometimes as early as a.m., there is a tendency to trade condensation--cirrus in mid-winter, and a cumulus in mid-summer, and, during the intermediate time, a tendency to cirro-stratus, partaking more or less of the character of one or the other, according to the season. temperature, in summer, commences its diurnal elevation about a.m., also, and rises till about p.m. from that time it falls with very little variation till o'clock the next morning. it has but one maximum and one minimum in the twenty-four hours. as the morning barometric maximum approaches, and the heat increases the magnetic activity, condensation in the trade appears, or induced condensation in the upper portion of the surface atmosphere, that portion near the earth is affected and attracted--and the "wind rises," according to the locality, the season, and the activity of the condensation. the tendency to blow increases with the tendency to trade and cumulus condensation, and continues till toward night, when it gradually dies away, unless there be a storm approaching. as the heat increases, and stimulates magnetism into activity, the magnetic needle commences moving to the west, its regular diurnal variation, and continues to do so until about p.m., when it commences returning to the east, and so continues to return until p.m., when it moves west again until a.m., and from thence to the east, till a.m. similar variations also take place in the horizontal force, as evinced by the action of the magnetometer needle, and in the vertical force, as shown by the oscillations. so that it is evident that there are two maxima, and two minima of magnetic activity every day, shown by all the methods by which we measure magnetic action and force--more than double at the acme of northern summer transit over that of winter, and proceeding _pari passu_, with the other daily phenomena--evincing the same irregular action which the other phenomena evince. still another phenomenon, which has a daily change, is electric tension, or the increase or decrease in the tension of the positive or true atmospheric electricity. [illustration: fig. .] the following table shows the mean two hourly tensions for three years, at kew, viz.: hours p.m. a.m. a.m. a.m. a.m. a.m. number of observations , , tension . . . . . . hours a.m. p.m. p.m. p.m. p.m. p.m. number of observations , tension . . . . . from this it will be seen that the tension of electricity is at a minimum at a.m., also, that it rises till , falls till p.m., but not as rapidly, rises till , falls again till a.m., or the close of the meteorological day--having two maxima and minima, as have most of the phenomena thus far considered. in order to see what the connections between these ever-present, daily phenomena are, and their connection with other phenomena, and that we may understand their normal conditions, i will trace them approximately in a diagram (figure .) the foregoing diagram of the daily phenomena of a summer's day, when no disturbing causes are in operation, no storm existing within influential distance, and no unusual intensity or irregular action of any of the forces present, affords a basis for considering the various phenomena of the weather in all its changes and conditions. it is obvious that the other phenomena do not all depend upon temperature merely, if indeed any of them do. temperature has but one maximum and minimum, and that is exceedingly regular, and does not correspond with any other. the barometer has two; electric tension, two; magnetic activity, two; condensation, two--one the formation of cloud, and the other the formation of fog and dew; wind, one--resembling temperature in that respect, but embracing a much less period. fog forms at one barometric minimum, and cloud at another. fog forms at one period of the magnetic variation, cloud at another. the formation of cloud corresponds with the greatest intensity of magnetic action, and its associate electricities. but the oscillations of the barometer do not correspond with either. and thus, then, we connect them: cause. | effect. | effect. | | increase of magnetic|decrease of pressure. |increase of primary or magneto-electric | |condensation. activity, as shown |of positive electric | by declination and |tension. |of wind. increase of | | horizontal and |of surface condensation,|of electrical disturbance vertical force. |_i. e._, fog and dew. |and phenomena in the | |trade and its vicinity. this connection is equally obvious if the order is reversed--thus; cause. | effect. | effect. | | decrease of magnetic|increase of pressure. |disappearance of primary or magneto-electric | |condensation. activity. |of tension of | |atmospheric electricity.|of wind, and | | |of surface condensation,|of electric disturbance |_i. e._, fog and dew. |in the trade and its | |vicinity. if we examine still more particularly the different phenomena, we shall find the same relative action of the forces carried into all the atmospheric conditions, however violent. . the barometer falls when horizontal magnetic force, and a tendency to cloud and wind, increase; and rises when they decrease. this corresponds with the character of the irregular barometric oscillation. barometric depressions accompany clouds and winds, and are in proportion to them, and are all greatest where magnetic force is greatest. the barometer also rises as the magnetic energy decreases. do the magnetic currents, passing upward with increased force, lift, elevate the atmosphere? how, then, are we to explain the increased range of the oscillations, as the center of atmospheric machinery is reached, where magnetism has least intensity, and the perpendicular currents are less, and attraction is less? attraction is greatest where intensity is greatest, and there the barometer stands highest, and the diurnal range is least. is it then the attraction of magnetism which produces the barometric oscillations? if so, how then can we explain the diurnal fall while magnetism is most active? perhaps we have not yet arrived at such a knowledge of the nature of magnetism as is necessary to a correct answer of those questions. faraday has taught us that the lines of magnetic force are close curves, passing into the atmosphere, and over to the opposite hemisphere, and returning through the earth, out on the opposite side in like manner, and back again, passing twice through the earth and twice through the atmosphere. all we know of this is what the iron filings indicate, and we do not know how much reliance to place upon the indications they give. but if faraday is right, the sun will, twice each day, intersect and stimulate into increased activity the same closed magnetic curve--once when it is coming out of the earth, during our day, when its influence will be the most active, and once when it is returning on the opposite side of the earth; and a second, but feebler magnetic and electric maximum, may be occasioned by its action on the opposite and returning closed curve of the same current. however this may be, it is exceedingly difficult to conceive, of any adequate influence exerted by the tension of vapor. so the mid-day barometric minimum may be caused by the attraction of the earth, in a state of increased magnetic activity and intensity, upon the counter-trade, and its consequent approach or settling toward the earth. observation, as i have already said, pointedly indicates such a state of things. so the increased magnetic activity, with or by its associate electricity, acts upon the electricity of the counter-trade, condensation takes place, the electricity is disturbed in the surface-atmosphere, by induction, and its tension is changed. opposite electrical conditions are induced in the surface strata, and attraction takes place. the air moves easily, and thus the attractions originate the winds. secondary currents are induced, as in all other cases of electric activity, and winds, in _different strata_ and directions, occur, with or without cumulus, or scud condensation, according to their activity, and the proportion of moisture of evaporation they may contain. i am well aware that the various received theories of meteorology attribute condensation to the action of cold, mingling of colder strata, etc. but i think that view will have to be abandoned. it assumes that moisture is evaporated and held in the atmosphere by latent heat, which is given out during condensation, and actually warms the surrounding atmosphere. thus, the kew committee undertook to explain the development of greater heat, at the elevation where they, in fact, found the counter-trade. but how unphilosophical to suppose a portion of the air or vapor contained in it, can give out to another adjoining portion _more heat than is necessary to produce an equilibrium_. this can, indeed, be done by experiment--_but the experiment is made with currents of electricity_. how unphilosophical, too, to talk of latent heat in connection with evaporation, _at the lowest temperature known_. meteorologists must revise their opinions on the subject of condensation. this latent heat has never been actually met with; on the contrary, the most sudden and complete condensations of the vapor of the atmosphere are attended by as sudden and extraordinary productions of cold, and consequent hail, and the connection between condensation and electricity is shown by too many facts to permit the old theory to stand. _fog never forms with the thermometer below °._ it is mainly a _summer condensation_, especially high fog. it has been attributed to the cooling effect of an atmosphere colder than the earth, but it often occurs when the earth is the coldest, and when the vapor, as it rises, is colder than the air, and could not give out heat to a warmer medium. (see american journal of science, vol. xliv. p. .) again, it is not mere condensation, but a formation of globules or vesicles, hollow, and the air expanded in them, by means of which they float like a soap bubble which contains the warm air of the breath. is not every vesicle a model shower, positively electrified on the outside, negatively in the center, or the reverse, according to the strata, with the air expanded in the middle by the excess of heat which negative electricity detains? look at them, as they attach themselves to the slender nap of the cloth you wear, when passing through them, and see how many of them it would require to form a large drop of rain. the clouds are of a similar vesicular character, and rain does not fall till the vesicles unite to form drops. sudden and extreme cold is indeed produced in the hail-storm, when, above, below, and around it, the temperature is unaffected. testu, wise, and other aeronauts, have so found it, and the hail tells us it is so. but it is idle to say it results from radiation. all the phenomena of the sudden, violent hail-storms are electric in an extraordinary degree. the electricity is disturbed and separated--the associated heat continues with the negative, and leaves the positive portion of the cloud, and a corresponding reduction of temperature results. so masson found in his eudiometrical analytical experiments the _negative_ wire would heat to fusion, while the positive was cold. (see london, edinburgh, and dublin journal of science for december, .) this disturbed electricity is diffused over the vesicles. listen to the thousand _crackling_ sounds which initiate the clap of thunder, and may be heard when the lightning strikes near you; produced by the gathering of the lightning from as many points of the cloud where it was diffused, to unite in one current and produce the "clap" or "peal"--and to the "pouring" of the rain, which follows the union of the vesicles, after the excess of repelling electricity is discharged. no _change_ of temperature is observed when fogs form, except the ordinary change between night and day; and it seems perfectly obvious, in looking at all the phenomena, that fogs form at a temperature of ° or °, in consequence of the electric influence of the earth upon the adjoining surface-atmosphere; and, when formed, they withstand the most intense action of a summer sun, till the time of day arrives for the barometric and electric tension to fall, condensation to take place in the counter-trade above, and wind to be induced. who that has noticed the almost blistering force of the solar rays, as they break through a section of high fog, about a.m., can forget them. fogs form near the earth, during the night, when the atmosphere above is loaded with moisture many degrees colder, and yet remains free from condensation. on the other hand, during the heat of the day, and of the hottest days, the heavy rains condense above--nay, they frequently fall at a temperature of ° to °, in the tropics, and of ° to ° in mid-winter here. thus far, an adherence to the opinion that condensation was simply a cooling process; the driving out of its latent heat, not merely to another body to make an equilibrium, but "_getting rid of it_" by positive active radiation, or in some other way, so as to cool off and condense, has involved the formation and classification of clouds in obscurity. hopkins (atmospheric changes, p. ) laments this, but fettered by a false and imperfect theory, in relation to the tension of vapor, he falls into a similar error. now, there are, as we have seen, peculiar, distinctly-marked varieties of cloud, connected with peculiar and distinctly-marked conditions of the atmosphere, _irrespective of temperature_. none of the theories advanced, account, or profess to account for the differences in either. no modification of the calorific theory will account for them. they differ in shape, in color, in tendency to precipitation, in line of progress, and in electrical character. the explanation of this is found in the fact, that they form in distinct and different strata, partake of the positive electric character of the one, or the negative of the other; or are secondary, induced by the action of a primary condensation in a different stratum. there is not any mingling of the different strata, as has been supposed; and many other facts than those to which we have alluded, show that the formation of cloud is a magneto-electric process. the observations of reid show that every violent shower cloud has the electricities disturbed, and portions of it are positive, and others negative. howard gives us the following _résumé_ of reid's observations: "from an attentive examination of reid's observations i have been able to deduce the following general results: " . _the positive electricity, common to fair weather, often yields to a negative state before rain._ " . _in general, the rain that first falls, after a depression of the barometer, is_ negative. " . _above forty cases of rain, in one hundred, give negative_ electricity; although the state of the atmosphere is positive, before and afterward. " . _positive rain, in a positive atmosphere, occurs more rarely_: perhaps fifteen times in one hundred. " . _snow and hail, unmixed with rain, are positive, almost without exception._ " . _nearly forty cases of rain, in one hundred, affected the apparatus with both kinds_ of electricity; sometimes with an interval, in which no rain fell; and so, that a positive shower was succeeded by a negative; and, _vice versâ_; at others, the two kinds alternately took place during the same shower; and, it should seem, _with a space of non-electric rain between them_." howard attributes, with great apparent probability, the successive differences in the electrical character of the rain, to the passage of different portions of the cloud, having different polarity, over the place of observation. so _positive hail_, and _negative rain_ fall in _parallel bands_ from the same cloud. many such instances are on record. it should be remembered that he is describing the phenomena in the showery climate of england. but the most decisive, perhaps, as well as practically important evidence of the influence of magnetism, or magneto-electricity, in meteorological phenomena, is derived from the action of storms. my observation has been limited, for my life has been, and must be, a practical one. but, subject to future, and i hope speedy corroboration, or correction, by extensive systematic observation, i think i may venture to divide all storms into four kinds: . those which come to us from the tropics, and constitute the class investigated by mr. redfield. that these are of a magneto-electric character is evident. they originate near the line of magnetic intensity, over, or in the vicinity of, the volcanic islands of the tropics; are largely accompanied by electrical phenomena; extend laterally as they progress north; induce and create a change of temperature in advance of them, and do not abate until they pass off over the atlantic to the e. or n. e., and perhaps not until they reach the arctic circle. their extensive and continued action is not owing to any mere _mechanical agency_ of the adjoining passive air, or other supposed currents, originated, no man can tell how, but they concentrate upon themselves the local magnetic currents as they pass over and intersect them, and, by their inductive action upon the surface-atmosphere, in different directions, attract it under them, and within their more active influence. here the action of the magnetic currents is probably the primary cause, but the power of the storm to concentrate upon itself the new magnetic currents which it intersects as it enters each new, successive field, enables them to maintain and extend their action. the following diagram illustrates the course and gradual enlargement of a mid-autumn tropical storm, which induces a s. e. wind in front, and occasions a thaw. [illustration: fig. .] . another class originate at the n. w., and extend gradually south easterly on the magnetic meridian. these are most frequent in summer, forming belts of showers, but occur, i believe, at all seasons of the year. they seem to be produced by magnetic waves passing south, and are followed in autumn and winter, and sometimes in summer, by the peculiar n. w. wind and scud, and a term of cooler weather. thus, it is believed that many, perhaps all of the alternating terms of heat and cold, are dependent on magnetic waves passing over the country in a similar manner, with a greater or less belt of condensation between them, and depending on peculiar magnetic action traveling in the same way. the s. e. extension of showers and storms, and the cooler changes of temperature which immediately follow them; with light n. w. wind in mid-summer, and with it fresher at earlier and later periods, in the form of northers blowing violently, according to the season, are intimately connected, and indicate such waves. the indication is strengthened also by the frequent progress of auroras in like manner, occurring usually after the belt of condensation has passed, and frequently following it. the clouds and currents of the atmosphere, so far as i have been able to discover, show no permanent current from the pole to the atmospheric equator, compensating for the counter-trade; and that compensation is furnished by the periodical but frequent atmospheric waves, connected with the periodical changes of storm, and cloud, and sunshine, which gradually extend from north to south, in or near the magnetic meridian. perhaps such compensating currents are found west of the magnetic poles, as we have suggested, and make the n. e. and northerly dry winds of western europe and the pacific; but, in the present state of our knowledge, it is impossible to say that they are. if it be so, the compensation they furnish must be small; for the volume of counter-trade which is not depolarized before it reaches the arctic circle, and which passes round the magnetic pole, must be very small. a majority of our periodical changes, during the northern transit, and i believe at all seasons, are of this character; and, i have reason to believe, from observation, in one or two cases, that where belts of rains and showers begin, over _any locality_ in the united states, they may assume this character. i have been in saratoga when an easterly storm commenced _south of that place_; the condensation and mackerel sky being visible at the south, and no cloud formation or rain occurring there at the time, and have traced it afterward as a belt which had a lateral extension south-eastward. leaving that place immediately after a belt had passed south, i have overtaken it by railroad, and run into it again before arriving at new york; and witnessed its subsequent extension south-eastwardly, out over the atlantic. i have witnessed the approach of such a belt in the spring, at sandusky, upon lake erie, and its passage over to the s. e., followed by the n. w. wind, as mr. bassnett describes them at ottawa, and run under the attenuated edge of the same belt, on the same day, on the way to pittsburg, leaving the n. w. wind behind, but finding it present again with clear sky on the following morning. i have seen hundreds of them approach from the north, and pass to s. e., out over the atlantic; followed by the n. w. wind in spring and autumn. this class of storms pass off toward, and doubtless over the track, of our european steamers and packets. i know this, for i witness it nearly every month in the year. it is not a matter of speculation, but of actual, long-continued observation. probably, as one approaches the gulf stream, and when over it, its induced winds may be more violent. it is time our navigators understood this; and that all the gales of the north atlantic, certainly, are not rotary; and do not approach from the s. w. in the same manner as the class investigated by mr. redfield do. where a fresh southerly or south-westerly wind is followed by any considerable cirro-stratus or stratus-condensation, it is usually of this character. the following diagram exhibits the peculiarities of this class of storms. it is intended to represent the same storm or belt of showers, on _two successive_ days, and, of course, its usual rate of southerly extension: [illustration: fig. .] this class of storms, or belts of showers, present the following succession of phenomena in summer: . still warm weather, one or more days. . fresh southerly wind, one or more days; if more than one, dying away at the s. w., at night-fall, but continuing into the evening of the day before the belt of condensation arrives. . belt of condensation, with or without rain or showers, with the easterly wind blowing axially, if the condensation is heavy and the belt wide; westerly if the condensation is feeble or the belt narrow--the clouds moving about e. n. e. . cooler air, light n. w. in summer, heavy n. w. in autumn, winter, and spring. and, the next period-- . still warm weather or light airs. . southerly wind, fresh. . belt of condensation. . cool northerly wind. and so on, successively, unless broken in upon by some other class. sometimes these periods are exceedingly regular, at other times the other classes prevail. i have much reason to believe that this is the _normal, periodic_ provision for condensation of our portion of the northern hemisphere, and probably of every other where rain falls regularly in the summer season, and that the other classes are exceptions, as the hurricanes are exceptions to the normal condition of the weather every where. perhaps in some seasons, during the northern transit, the exceptions may equal the rule, but i do not now remember such a season. in other years nearly all the storms are of this character. thus, dr. hildreth (in silliman's journal for ), speaking of the year , in a note to his register of that year, says: "there have been, this year, an unusual number of winds from n. or n. w. nearly every rain the past summer has been followed with winds from the northward, when, in many previous summers, the wind continued to the southward after rain." the immediate occurrence of northerly wind after the passage of the belt of condensation, is a peculiar feature of this class of storms. as this also will be new, and is of great practical interest, i shall be pardoned for referring to other evidence. bermuda is in latitude ° north. in the summer season they are within the range of the calms of cancer, as lieutenant maury terms them, and not subject to storms. from november to may, inclusive, they have successions of revolving wind. colonel reid gave them much attention, and studied them barometrically: that is, he studied the changes of the wind during the successive periodic depressions. he found them revolving like ours, and hence inferred the truth of the gyratory theory in relation to all winds. but it is perfectly evident the same polar belts which pass over us reach them during the southern transit. the precedent southerly wind, the _central condensation_, the appearance of lightning, and the rotation of the wind by both the east and west, but most frequently by west, are the same. in his chapter on observations at the bermudas, he gives us many examples. probably the existence of the gulf stream to the west and north has a modifying influence upon them, and their action becomes less intense in that latitude, but they are very similar. i copy a record of the weather, for a month, which may be found on pages , , and , and a portion of his remarks: "the month of december, , presents a continual succession of revolving winds passing over the bermudas, with scarcely an irregularity, as regards the fall and rise of the barometer accompanying the veering of the wind. one, however, occurred on the th and th. the s. w. wind abated, and changed to w. n. w., with the barometer still falling. but in the column of remarks it is noted that there was lightning seen in the n. and n. w., from p.m., during the night. this irregularity may, therefore, have been occasioned by a gale passing over the banks of newfoundland, influencing the direction of the wind at bermuda. "revolving winds. +-----------------------------------------------------------------+ | date. | hour. |direction of| wind's | weather. | bar.|ther.| | | | wind. | force. | | | | |--------|---------|------------|--------|-----------|------|-----| | . | | | | | | | |nov. |midnight.| s. s. e. | |b. c. | · | | |dec. | noon. | s. s. w. | |b. c. | · | | | | " | s. w. | |g. m. q. | · | | | | " | s. s. w. | |g. c. | · | " | | | " | s. w. | |g. m. r. | · | | | | " | w. n. w. | |p. q. | · | " | | | " | n. w. | |p. q. |* · | " | | | " | n. n. w. | |b. c. | · | | | " |midnight.| n. n. w. | |b. c. | · | | | | noon. | w. n. w. | |b. c. | · | | | | " | s. s. w. | |p. q. | · | | | | " | s. w. | |b. c. | · | " | | | " | w. n. w. | |b. c. m. |* · | | | | " | s. s. w. | " |b. v. | · | | | | " | n. n. by w.| " |b. v. | · | | | | " | n. n. w. | |b. c. v. | · | | | " |midnight.| n. w. | |b. c. p. | · | | | | noon. | s. w. by s.| |g. m. r. | · | | | " | p.m. | s. s. w. | |m. q. r. | · | | | " | " | s. s. w. | " |g. m. q. r.| · | " | | " | " | w. s. w. | " |q. w. |* · | " | | " | " | n. w. | |b. c. q. | · | " | | " | " | n. n. w. | " |b. c. | · | " | | | noon. | n. w. | |b. c. m. | · | | | | " | n. w. by n.| " |p. q. | · | | | | " | n. w. | |c. q. | · | " | | | " | n. w. by n.| |m. q. r. |* · | | | | " | n. n. w. | " |p. q. c. | · | | | | " | n. w. by n.| |c. q. | · | | | " |midnight.| s. w. | |b. c. | · | | | | dawn. | ---- | | | | | | " | noon. | s. s. w. | |g. m. | · | | | " | p.m. | s. | |g. m. | · | " | | " | " | s. s. e. | " |g. m. r. | · | " | | " | " | s. s. e. | " |w. r. | · | " | | " | " | s. e. | " |m. w. r. | · | " | | | noon. | s. w. | |b. c. m. |* · | | | | " | w. n. w. | " |b. m. | · | | | | " | w. n. w. | |b. c. | · | | | | " | n. | |c. | · | | | | " | s. e. | |c. q. r. | · | | | | " | s. w. | |c. q. | · | | | " |midnight.| s. s. w. | " |b. c. | · | | | | noon. | s. w. | |c. b. |* · | | | | " | w. n. w. | |b. c. q. | · | | | | " | n. w. | |b. c. | · | | +-----------------------------------------------------------------+ "_remark printed in the register._ "the changes of the wind during the december gales have been nearly the same in all: _i. e._, commencing with a southerly wind at first, the wind has veered by the west, toward the north-west, sometimes ending as far round as n. n. w." these extracts show the passage of several successive belts, each with the phenomena in regular order. the first commences with blue sky and detached clouds, barometer up, thermometer down to °, and nearly calm, on the th of november. dec. (at noon). wind freshens from s. s. w.; thermometer rises; barometer still up. dec. . barometer has fallen; thermometer up; wind increasing from s. w., with gloomy, squally appearance. dec. . wind s. s. w.; barometer slowly falling; thermometer slightly. dec. . wind fresh; s. w.; condensation and rain has reached them, and it carries barometer and thermometer down. dec. . wind shifting by the west, and squally. dec. . winds gets n. w.; blows fresh; barometer at its minimum, probably at the time of the change of wind, although the register does not show the precise time. dec. . wind n. n. w.; blue sky and detached clouds (n. w. scud), cleared off; barometer elevated by the n. w. wind, from . to . . midnight: blue sky; detached clouds (n. w. scud probably); barometer up to . ; thermometer fallen, from the cooler character of the northerly wind. dec. . wind having lulled as a northerly wind has got round to s. w. again; thermometer up; barometer falling, and another belt approaching, and so on. the first and last part of december show each two regular occurrences of substantially the same phenomena. the middle is somewhat more irregular. there were five distinctly-marked periods, and one squally, long-continued period, with a slight tendency to condensation, and a slight fall of barometer and rain on the th (n. w. squall probably), but not sufficient to reverse the wind to the south. in colonel reid's opinion there were five revolving gales which passed over bermuda during the month. in my opinion, there were five perfect polar waves of condensation, and one imperfect one, with as many successive southerly winds preceding the condensation, with or without rain in the center, followed by as many cold n. w. or n. n. w. winds, with squalls, in the rear, about five days apart. (see the * in the barometric column.) _we are at issue._ let the question be determined by _actual observation_, and not by _speculation_. it is of fundamental and exceeding importance to the science. now, let us take a month in summer, from the observations of mr. bassnett, at ottawa. here the climate differs somewhat from that east of the alleghanies; the magnetic intensity is greater, and the action more violent and irregular. that part of the country, it should be remembered, has a greater fall of rain in summer, for reasons we have stated, and those periodic revolutions are more frequent. "a brief abstract from a journal of the weather for one sidereal period of the moon, in . "_june_ st. fine clear morning (s. fresh): noon very warm °; p.m., plumous _cirri in south_; ends clear. " d. hazy morning (s. very fresh) arch of cirrus in west; p.m., black in w. n. w.; p.m., overcast and rainy; p.m., a heavy gust from south; . p.m., blowing furiously (s. by w.); p.m., tremendous squall, uprooting trees and scattering chimneys; p.m., more moderate (w.). " d. clearing up (n. w.); a.m., quite clear; a.m., bands of mottled cirri pointing n. e. and s. w., ends cold (w. n. w.); the cirri seem to rotate from left to right, or with the sun. " th. fine clear, cool day, begins and ends (n. w.). " th. clear morning (n. w. light); p.m. (e.), calm; tufts of tangled cirri in north, intermixed with radiating streaks, all passing eastward; ends clear. " th. hazy morning (s. e.), cloudy; noon, a heavy, windy-looking bank in north (s. fresh), with dense cirrus fringe above, on its upper edge; clear in s. " th. clear, warm (w.); bank in north; noon bank covered all the northern sky, and fresh breeze; p.m., a few flashes to the northward. " th. uniform dense cirro-stratus (s. fresh); noon showers all round; p.m., a heavy squall of wind, with thunder and rain (s. w. to n. w.); p.m., a line of heavy cumuli in south; . p.m., a very bright and high cumulus in s. w., protruding through a layer of dark stratus; . p.m., the cloud bearing e. by s., with three rays of electric light. " th. a stationary stratus over all (s. w. light); clear at night, but distant lightning in s. " th. stratus clouds (n. e. almost calm); a.m., raining gently; p.m., stratus passing off to s.; p.m., clear, pleasant. "_july_ st. fine and clear; a.m., cirrus in sheets, curls, wisps, and gauzy wreaths, with patches beneath of darker shade, all nearly motionless; close and warm (n. e.); a long, low bank of haze in s., with one large cumulus in s. w., but very distant. " d. at a.m., overcast generally, with hazy clouds and fog of prismatic shades, chiefly greenish-yellow; a.m. (s. s. e. freshening), thick in w.; a.m. (s. fresh), much cirrus, thick and gloomy; a.m., a clap of thunder, and clouds hurrying to n.; a reddish haze all around; at noon the margin of a line of yellowish-red cumuli just visible above a gloomy-looking bank of haze in n. n. w. (s. very fresh); warm, °; more cumuli in n. w.; the whole line of cumuli n. are separated from the clouds south by a clearer space. these clouds are borne rapidly past the zenith, but never get into the clear space--they seem to melt or to be turned off n. e. the cumuli in n. and n. w., slowly spreading e. and s.; p.m., the bank hidden by small cumuli; p.m., very thick in north, magnificent cumuli visible sometimes through the breaks, and beyond them a dark, watery back-ground (s. strong); . p.m., wind round to n. w. in a severe squall; p.m., heavy rain, with thunder, etc.--all this time there is a bright sky in the south visible through the rain ° high; p.m., clearing (s. w. mod.). " d. very fine and clear (n. w.); noon, a line of large cumuli in n., and dark lines of stratus below, the cumuli moving eastward; p.m., their altitude ° '. velocity, ° per minute; p.m., much lightning in the bank north. " th. a.m., a line of small cumulo-stratus, extending east and west, with a clear horizon north and south ° high. this band seems to have been thrown off by the central yesterday, as it moves slowly south, preserving its parallelism, although the clouds composing it move eastward. fine and cool all day (n. w. mod.)--lightning in n. " th. cloudy (n. almost calm), thick in e., clear in w.; same all day. " th. fine and clear (e. light); small cumuli at noon; clear night. " th. warm (s. e. light); cirrus bank n. w.; noon (s.) thickening in n.; p.m., hazy but fine; p.m., lightning in n.; p.m., the lightning shows a heavy line of cumuli along the northern horizon; calm and very dark, and incessant lightning in n. " th. last night after midnight commencing raining, slowly and steadily, but leaving a line of lighter sky south; much lightning all night, but little thunder. " th. a.m., very low scud ( feet high) driving south, still calm below (n. light); a.m., clearing a little; a bank north, with cirrus spreading south; same all day; p.m., wind freshening (n. stormy); heavy cumuli visible in s.; . p.m., quite clear, but a dense watery haze obscuring the stars; p.m., again overcast; much lightning in s. and n. w. " th. last night ( a.m. of th) squall from n. w. very black; a.m., still raining and blowing hard, the sky a perfect blaze, but very few flashes reach the ground; a.m., raining hard; a.m. (n. w. strong); a constant roll of thunder; noon (n. e.); p.m. (n.); p.m., clearing; p.m., a line of heavy cumuli in s., but clear in n. w., n., and n. e. " th. a.m., overcast, and much lightning in south (n. mod.); a.m., clear except in south; p.m. (e.); p.m., lightning south; p.m., auroral rays long, but faint, converging to a point between epsilon virginis and denebola, in west; low down in west, thick with haze; on the north the rays converged to a point still lower; lightning still visible in south. this is an aurora in the west. " th. fine, clear morning (n. e.); same all day; no lightning visible to-night, but a bank of clouds low down in south, ° high, and streaks of dark stratus below the upper margin. " th. fine and clear (n. e.); noon, a well-defined arch in s. w., rising slowly; the bank yellowish, with prismatic shades of greenish-yellow on its borders. this is the o. a. at p.m., the bank spreading to the northward. at p.m., thick bank of haze in north, with bright auroral margin; one heavy pyramid of light passed through cassiopeia, traveling _westward_ - / ° per minute. this moves to the other side of the pole, but not more inclined toward it than is due to prospective, if the shaft is very long; . p.m., saw a mass of light more diffuse due east, reaching to _markab_, then on the prime vertical. it appears evident this is seen in profile, as it inclines downward at an angle of ° or ° from the perpendicular. it does not seem very distant. p.m., the aurora still bright, but the brightest part is now west of the pole, before it was east. " th. a.m., clear, east and north; bank of cirrus in n. w., _i. e._, from n. n. e. to w. by s.; irregular branches of cirrus clouds, reaching almost to south-eastern horizon; wind changed (s. e. fresh); a.m., the sky a perfect picture; heavy regular shafts of dense cirrus radiating all around, and diverging from a thick nucleus in north-west, the spaces between being of clear, blue sky. the shafts are rotating from north to south, the nucleus advancing eastward. "at noon (same day), getting thicker (s. e. very fresh); p.m., moon on meridian, a prismatic gloom in south, and very thick stratus of all shades; p.m., very gloomy; wind stronger (s. e.); p.m., very black in south, and overcast generally. " th. last night, above p.m., commenced raining; a.m., rained steadily; a.m., same weather; . a.m., a line of low storm-cloud, or scud, showing very sharp and white on the dark back-ground all along the southern sky. this line continues until noon, about ° at the highest, showing the northern boundary of the storm to the southward; p.m., same bank visible, although in rapid motion eastward; same time clear overhead, with cirrus fringe pointing north from the bank; much lightning in south (w. fresh); so ends. " th. last night a black squall from n. w. passed south without rain; at a.m., clear above but, very black in south (calm below all the time); a.m., the bank in south again throwing off rays of cirri in a well-defined arch, whose vortex is south; these pass east, but continue to form and preserve their linear direction to the north; no lightning in south to-night. " th. clear all day, without a stain, and calm. " th. fine and clear (n. e. light); p.m., calm. " th. fair and cloudy (n. e. light); p.m., calm. " th. fine and clear (n. fresh); i. v. visible in s. w. " th. a.m., bank in n. w., with beautiful cirrus radiations; a.m., getting thick, with dense plates of cream-colored cirrus visible through the breaks; gloomy looking all day (n. e. light)." the letters in a parenthesis signify the direction of the wind. during this month there were three distinctly marked periods of belts of showers, preceded by "fresh" or "strong" south wind, and followed by the n. w. there was a period when a belt of less intense stratus, without much wind, occurred ( th, th, and th of june). this was followed by a distinct belt of showers and _fresh_ s. wind, on the d of july, and by the n. w. wind and clear weather, on the d. during the rest of july it was more irregular, with the exception of the th, th, and th, when another belt and revolution occurred. now, these periods, when distinctly marked, exhibit the same succession of phenomena--viz., elevation of temperature, fresh southerly wind, belt of condensation, cumulus or stratus with cirrus running east, but extending south, followed by n. w. wind, and clear, cold air. can any one believe they were successive rotary gales? i wish, in this connection, to make a suggestion to lieutenant maury and others. the descriptions of m. bassnett, although not perfect, are very intelligible. he describes things as they were, and as they should be described. he distinguishes the clouds, and the scud, and other appearances. but colonel reid's descriptions are unmeaning and unintelligible. g. m.--gloomy, misty! gloomy from what? fog, or stratus, or a stratum of scud, or what? we can not know. again, c. the table tells us this stands for detached clouds. but of what kind? cumulus, broken stratus, patches of cirro-cumulus or cirro-stratus, or scud? all these, and indeed every kind of cloud or fog formation, except low fog, may exist in detached portions. these abbreviations will not answer; they do not describe the weather. the clouds must be studied and described. there is no difficulty in doing it. sailors will learn them very soon after their teachers have; and those who teach them should see to it that the logs contain terms of description which convey the meaning which may, and ought to be, conveyed. the use of these indefinite terms can not be continued without culpability. again, the observations of seamen off our coast are in accordance with the progress of this class of storms on land, and prove that they continue s. e. over the atlantic, abating in action as they approach the tropics. there is abundant evidence of this in the work of colonel reid, and the charts of lieutenant maury, but i can not devote further space to them. the third class form in the counter-trade, over some portion of the country, from excessive volume or action of the counter-trade, or local magnetic activity, without coming from the tropics or being connected with a regular polar wave of magnetic disturbance. the following diagram exhibits their form, progress, and accompanying induced winds. [illustration: fig. .] the gentle rains of spring, particularly april, and the moderate and frequent snow-storms of winter, are often of this character; and so are the heavy rains, which commence at the morning barometric minimum, rain heavily through the forenoon, and light up near mid-day in the south, followed by gentle, warm, s. w. winds. this class are more frequent in some years than others--probably the early years of the decade, while polar storms are, during the later ones. it is this class which have _violent_ easterly winds _in front_, and on the _south side_, with two or more currents, and which mr. redfield has also supposed to be cyclones. the fourth class are isolated showers, occurring over particular localities, or belts of drought and showers alternating; sometimes a general disposition to cloudy and showery weather for a longer or shorter interval over the whole country; at others, limited to particular localities in the course of the trade. such a period occurred during the wheat harvest of . this class i attribute to a general increased magnetic action, but it may be induced by an increased volume, or greater south polar magnetic intensity of the counter-trade, exciting and concentrating the regular currents of the field, and increasing their activity and energy. these also often work off south gradually, and are followed by a cold n. w. air for a day or two; showing a tendency, in the excited magnetism, to pass as a wave toward the tropics. the following diagram will give some idea of this class: [illustration: fig. .] there are sometimes very obvious local tendencies to precipitation over portions adjoining an area affected with drought, as there are other magnetic irregularities over particular areas. all these classes of storms are variant in intensity. sometimes the general or local cloud-formation is weak, and does not produce precipitation at all; so of that which extends southerly. probably the tropical storm are always sufficiently dense and active to precipitate. their action is often violent over particular localities, and hence the more frequent occurrence of the tornado over the more intense area of ohio, and other portions of the west. all violent local storms are doubtless owing to local magneto-electric activity. chapter xi. the reader who has attentively perused and considered the facts stated, and the principles deduced, in the preceding pages, and is ready to make a practical application of them by careful observation, will have little difficulty in understanding the varied atmospheric conditions; and will soon be able to form a correct judgment of the immediate future of the weather, so far as his limited horizon will permit. but there are other facts and considerations, not specifically alluded to, which will materially aid him in his observations; and there is a degree of philosophical truth in the proverbs and signs, which ancient popular observation accumulated, and poetry and tradition have preserved, that meteorologists have been slow to discover or admit, but which will be obvious upon examination, and commend them to his attention. the classical reader is doubtless familiar with that part of the first georgic of virgil, which contains a description of the signs indicative of atmospheric changes. much of it is beautifully poetic, and, if read in the light of a correct philosophy, is equally truthful. i copy from a creditable translation, found in the first volume of howard's "climate of london": "all that the genial year successive brings, showers, and the reign of heat, and freezing gales, appointed signs foreshow; the sire of all decreed what signs the southern blast should bring, decreed the omens of the varying moon: that hinds, observant of the approaching storm, might tend their herds more near the sheltering stall." prognostics.--_ st. of wind._ "when storms are brooding--in the leeward gulf dash the swell'd waves; the mighty mountains pour a harsh, dull murmur; far along the beach rolls the deep rushing roar; the whispering grove betrays the gathering elemental strife. scarce will the billows spare the curved keel; for swift from open sea the cormorants sweep, with clamorous croak; the ocean-dwelling coot sports on the sand; the hern her marshy haunts deserting, soars the lofty clouds above; and oft, when gales impend, the gliding star nightly descends athwart the spangled gloom, and leaves its fire-wake glowing white behind. light chaff and leaflets flitting fill the air, and sportive feathers circle on the lake." _ d. of rain._ "but when grim boreas thunders; when the east and black-winged west, roll out the sonorous peal, the teeming dikes o'erflow the wide champaign, and seamen furl their dripping sails. the shower, forsooth, ne'er took the traveler unawares! the soaring cranes descried it in the vale, and shunn'd its coming; heifers gazed aloft, with nostrils wide, drinking the fragrant gale; skimm'd the sagacious swallow round the lake, and croaking frogs renew'd their old complaint. oft, too, the ant, from secret chambers, bears her eggs--a cherished treasure--o'er the sand, along the narrow track her steps have worn. high vaults the thirsty bow; in wide array the clamorous rooks from every pasture rise with serried wings. the varied sea-fowl tribes, and those that in cäyster's meadows seek, amid the marshy pools, their skulking prey, fling the cool plenteous shower upon their wings, crouch to the coming wave, sail on its crest, and idly wash their purity of plume. the audacious crow, with loud voice, hails the rain a lonesome wanderer on the thirsty sand. maidens that nightly toil the tangled fleece, divine the coming tempest; in the lamp crackles the oil; the gathering wick grows dim." _ d. of fair weather._ "nor less, by sure prognostics, mayest thou learn (when rain prevails), in prospect to behold warm suns, and cloudless heavens, around thee smile. brightly the stars shine forth; cynthia no more glimmers obnoxious to her brother's rays; nor fleecy clouds float lightly through the sky. the chosen birds of thetis, halcyons, now spread not their pinions on the sun-bright shore; nor swine the bands unloose, and toss the straw. the clouds, descending, settle on the plain; while owls forget to chant their evening song, but watch the sunset from the topmost ridge. the merlin swims the liquid sky, sublime, while for the purple lock the lark atones: where she, with light wing, cleaves the yielding air, her shrieking fell pursuer follows fierce-- the dreaded merlin; where the merlin soars, _her_ fugitive swift pinion cleaves the air. and now, from throat compressed, the rook emits, treble or fourfold, his clear, piercing cry; while oft amid their high and leafy roosts, bursts the responsive note from all the clan, thrill'd with unwonted rapture--oh! 'tis sweet, when bright'ning hours allow, to seek again their tiny offspring, and their dulcet homes. yet deem i not, that heaven on them bestows foresight, or mind above their lowly fate; but rather when the changeful climate veers, obsequious to the humor of the sky; when the damp south condenses what was rare, the dense relaxing--or the stringent north rolls back the genial showers, and rules in turn, the varying impulse fluctuates in their breast: hence the full concert in the sprightly mead-- the bounding flock--the rook's exulting cry." _ th. the moon's aspects, etc._ "mark with attentive eye, the rapid sun-- the varying moon that rolls its monthly round; so shalt thou count, not vainly, on the morn; so the bland aspect of the tranquil night will ne'er beguile thee with insidious calm. when luna first her scatter'd fires recalls, if with blunt horns she holds the dusky air, seamen and swains predict th' abundant shower. if rosy blushes tinge her maiden cheek, wind will arise: the golden phoebe still glows with the wind. if (mark the ominous hour!) the clear fourth night her lucid disk define, that day, and all that thence successive spring, e'en to the finished month, are calm and dry; and grateful mariners redeem their vows to glaucus, inöus, or the nereid nymph." _ th. the sun's aspects, etc._ "the sun, too, rising, and at that still hour, when sinks his tranquil beauty in the main, will give thee tokens; certain tokens all, both those that morning brings, and balmy eve. when cloudy storms deform the rising orb, or streaks of vapor in the midst bisect, beware of showers, for then the blasting south (foe to the groves, to harvests, and the flock), urges, with turbid pressure, from above. but when, beneath the dawn, red-fingered rays through the dense band of clouds diverging, break, when springs aurora, pale, from saffron couch, ill does the leaf defend the mellowing grape; leaps on the noisy roof the plenteous hail, fearfully crackling. nor forget to note, when sol departs, his mighty day-task done, how varied hues oft wander on his brow; azure betokens rain: the fiery tint is eurus's herald; if the ruddy blaze be dimm'd with spots, then all will wildly rage with squalls and driving showers: on that fell night, none shall persuade me on the deep to urge my perilous course, or quit the sheltering pier. but if, when day returns, or when retires, bright is the orb, then fear no coming rain: clear northern airs will fan the quiv'ring grove. lastly, the sun will teach th' observant eye what vesper's hour shall bring; what clearing wind shall waft the clouds slow floating--what the south broods in his humid breast. who dare belie the constant sun?" i copy also the following from howard: "dr. jenner's signs of rain--an excuse for not accepting the invitation of a friend to make a _country_ excursion. "the hollow winds begin to blow, the clouds look black, the glass is low, the soot falls down, the spaniels sleep, and spiders from their cobwebs creep. last night the sun went pale to bed, the moon in halos hid her head, the boding shepherd heaves a sigh, for see! a rainbow spans the sky. the walls are damp, the ditches smell; closed is the pink-eyed pimpernel. hark! how the chairs and tables crack; old betty's joints are on the rack. loud quack the ducks, the peacocks cry; the distant hills are looking nigh. how restless are the snorting swine!-- the busy flies disturb the kine. low o'er the grass the swallow wings; the cricket, too, how loud it sings! puss, on the hearth, with velvet paws, sits smoothing o'er her whisker'd jaws. through the clear stream the fishes rise and nimbly catch the incautious flies; the sheep were seen, at early light, cropping the meads with eager bite. though _june_, the air is cold and chill; the mellow blackbird's voice is still; the glow-worms, numerous and bright, illumed the dewy dell last night; at dusk the squalid toad was seen, hopping, crawling, o'er the green. the frog has lost his yellow vest, and in a dingy suit is dress'd. the leech, disturbed, is newly risen quite to the summit of his prison. the whirling wind the dust obey and in the rapid eddy plays. my dog, so altered in his taste, quits mutton-bones, on grass to feast; and see yon rooks, how odd their flight! they imitate the gliding kite: or seem precipitate to fall, as if they felt the piercing ball. 'twill surely rain; i see, with sorrow, our jaunt must be put off to-morrow." howard attributes the foregoing to jenner; but hone, in his "every-day book," attributes it to darwin, and gives it, with several couplets, not found in that attributed to jenner. these i add from hone, as follows: "her corns with shooting pains torment her-- and to her bed untimely send her." that couplet is included by hone with what is said of aunt betty. "the smoke from chimneys right ascends, then spreading back to earth it bends. the wind unsteady veers around; or, settling in the south is found." those are as philosophically accurate and valuable as any. "the tender colts on back do lie; nor heed the traveler passing by. in fiery red the sun doth rise, then wades through clouds to mount the skies." the first of those couplets is untrue. it is doubtless alluded to as one of the acts of the animal creation, indicating sleepiness and inaction, which precede storms; but colts do not lie on the back. the other couplet is both true and important. this collection entire, whether written by darwin or jenner, contains most of the signs which have been preserved, and which are of much practical importance in our climate. it is unquestionably true that "appointed signs foreshow the weather," to a great extent, every where, but with more certainty in the climate in which virgil wrote than in our variable and excessive one. "showers" and "freezing gales" we can, perhaps, as well understand; but the "_reign of heat_," by which he probably meant the dry period, when the southern edge of the extra-tropical belt of rains is carried up to the north of them, we do not experience. something like it we did indeed have, during the excessive northern transit, in the summer of ; but it was an exception, not the rule. some of the most important of those signs from virgil and jenner i propose to allude to in detail; but it is necessary to look; in the first place, to the character of the season and the month. we have seen that the years differ during different periods of the same decade. that they incline to be hot and irregular during the early part of it, and cool, regular, and productive during the latter portion--subject, however, to occasional exceptions. the latter half of the third decade of this century ( to , inclusive) was comparatively warm; and, in the latitude of °, was very unhealthy, and so continued during the early part of the next, over the hemisphere, embracing the _cholera seasons_. the spots upon the sun were much less numerous than usual, during the latter half of the third decade. thus the spots from to , inclusive, were to " " to " " and the size of those from to exceeded those of the other years. the attentive observer will very soon be satisfied that the seasons have a character; and those of every year differ in a greater or less degree from those of other years in the same decade, and those of one decade not unfrequently from those of some other. _periodicity_ is stamped upon all of them, and upon all resulting consequences. like seasons come round, and, like productiveness or unproductiveness, healthy or epidemic diatheses, attend them. we have seen that, in relation to mean temperature, there are such periodical diversities, but they are more strongly marked in the character of storms, and other successions of phenomena. "_all signs fail in a drouth_," for then all attempts at condensation are partial, imperfect, and ineffectual. "_it rains very easy_," it is said, at other times, and so it seems to do, and with comparatively little condensation. in the one case, no great reliance can be placed upon indications which are entirely reliable in the other. so "_all our storms clear off cold_," or, "_all our storms clear off warm_," are equally common expressions--as the _prevailing classes_ of storms give a _character_ to the _seasons_. it "_rains every sunday now_," is sometimes said, and is often peculiarly true--the storm waves having just then a weekly or semi-weekly period, and one falls upon sunday for several successive weeks; and when it is so, _that_ coincidence is sure to be noticed and commented upon, and the other perhaps disregarded. if the seasons depended upon the northward and southward journey of the sun alone, entire regularity might be expected--for we have no reason to believe that magnetism and electricity contain, within themselves, inherently, any tendency to irregularity, or periodicity; and, the sun being constant in his _periods_, would be constant in his _influence_. but he is inconstant and variable in his influence, and it is apparently traceable to the existence of spots; but i am not quite sure that it is occasioned by the _observable_ spots alone. grant that the intensity and power of his rays differ on the same day, in different years, and that difference may be attributable in part to causes which our telescopes can not discover. but the differences in the seasons do not depend on the variability of the sun's influence alone. this appears from the frequent meridional and latitudinal diversities and contrasts, to which allusion has been made. the sun can not be supposed to exert a _less_ influence on a middle, than a more northern latitude; nor on one series of meridians, than another. there must, therefore, be another local and powerful disturbing cause, varying the magnetic and electric activity and influence upon the trades, as well in their incipiency as in their circuits, and thus controlling the atmospheric conditions locally and in _the opposite hemispheres_. that other disturbing cause is _volcanic action_. we can conceive of none other, and we can detect and trace the influence of that to a considerable extent. unfortunately we know, and can practically know, comparatively little of it. it has been busy with the earth since the creation, and will continue to be so till, possibly, by a collision, it shall burst into asteroids--its molten interior flowing out in seeming combustion--each fragment retaining its magnetic polarities entire, and continuing on in an independent orbit in the heavens, an asteroid, or meteorite. while, therefore, the agency of magnetism in itself may be regular, and the transit of the sun is regular, and "seed-time and harvest shall not cease," yet the sun is not regular in his influence, and the magnetic agency is disturbed by another and irregular power. and, although we can trace the influence of both upon the seasons, we can not measure that influence, and from it reliably foretell the weather. the discoveries of swabe, and future ones, relative to solar irregularities, will assist us, but, till we understand better, and to some extent anticipate, the changes of volcanic action, we shall not be able to understand or foresee all the differences in the seasons. that time may come; for progress is yet to be read in the front of meteorology, and simultaneous practical observations made and interchanged at every important point on the globe. nevertheless, the seasons have a character--often a regular one--one class of storms prevailing over all others--one series of phenomena occurring to the exclusion of others--and we must regard it if we would arrive at intelligent estimates of their future condition. the most difficult part to understand are the meridional contrasts. last year we had one of the worst drouths which has occurred since the settlement of the country. but while all the eastern portion of the united states was dry, new mexico was unusually wet; and the north-western states, on the same curving line of the counter-trade, were not affected by the drouth. extract from a letter written by governor merriweather, to mr. bennett, in answer to a circular, published in the "new york herald," and dated "santa fe, new mexico, oct. th, . "more rain has fallen during the last six months, within this territory, than ever was known to have fallen in the same length of time, in this usually dry climate. generally, little or no crops have been produced without irrigation; but this season some good crops have been produced without any artificial watering." we have seen that there was an apparent connection between the remarkable volcanic action, exerted beneath the western continents during the second decade of this century, and the remarkable coldness of that decade. and it is easy to see that the comparative absence of volcanic action from immediately beneath the old world, and its presence in great excess beneath the new, may disturb the regular action of terrestrial magnetism above it in the earth's-crust here, and affect seasons, diatheses, and health unfavorably; while from its absence they may be favorably affected there. i have some general views in relation to this, but they are necessarily speculative, for the data are few, and i reserve them. i am, however, induced to believe that the transit of the atmospheric machinery is greater over some portions of the northern hemisphere, in some seasons, than others. the most natural explanation of the unusual contrast between the drouth of the eastern states, and the wet of the territories, during the last summer, is, that the concentrated counter-trade was carried west, by some irregular magnetic action in the south atlantic or west indies. but there was much evidence that the northern extension of the atmospheric machinery was greater than usual. the transit began _early_--it was evidently _rapid_; the rains of may fell in april, and the spring was wet; _summer set in earlier_--all the appearances then were unusually tropical--the polar belts of condensation descended upon us, but they were feeble, as they doubtless become, when they reach the tropics, and did not precipitate; the summer continued full twenty days later--no rain falling till about the th of september. the season throughout was excessive, but otherwise regular. spring came earlier; summer commenced earlier and continued longer; autumn held off later, and cold weather, when it came, was uniform and severe. this season the transit has seemed to be less than for several years.[ ] the spring was backward; the summer cool, but exceedingly regular; the autumn thus far without extremes, and the whole year healthy and productive. it is the normal period of the decade, between the irregular heat of the first part, and the irregular cold of the last; and it has been normal in character, and conformed beautifully to its location. if the transit of was further north than the mean, as it seemed to be over this country, that of itself would convey the showers which follow up in the western portion of the concentrated trade, on the east of the mountains of mexico, and cause them to precipitate further north, over new mexico, and thus, rather than from a diverted trade, they may have derived their unusual supply of moisture during the summer of . on this subject i can but conjecture, and leave to future observation a discovery of the truth. enough appears, however, to show the importance of taking the location of the year in the decade, and even the character of the decade itself, into the account. but whatever the remote cause of the difference in the seasons, the character of the seasons is directly influenced by the character of storms, or periodic changes. sometimes the tropical storms are most numerous; at others the polar waves; and at others the irregular local storms, or general tendency to showers. the seasons when the polar waves are most prevalent, are the most regular, healthy, and productive. those where the tropical tendency is greatest, are irregular; and so are those where the other classes predominate. these differences in the character of the storms, are but the varying forms in which magnetic action develops itself. i have said that there was a decided tendency to cirrus without cumulus, in mid-winter, and cumulus without cirro-stratus or stratus, in midsummer, and during the intermediate time an intermediate tendency. but there is a difference between spring and autumn. dry westerly (not n. w.) gales prevail in march, and n. e. storms in april and may, but violent s. e. gales are not as common. on the other hand, the dry westerly gales of march are comparatively unknown in autumn, and the violent, tropical, south-easters are then common. snow-storms occur during the northern transit, not unfrequently in april and may; but they do not occur so near the acme of the northern transit on its return; nor until it approaches very near its southern limit. the quiet, warm, and genial air of april, is reproduced in the indian summer of autumn, but they present widely different appearances. those, and many other peculiarities of the seasons, deserve the attentive consideration of every one who would become familiar with the weather and its prognostics. these irregularities in the character of the seasons have doubtless always existed, and always been the objects of popular observation. there are some very old proverbs which show this. i copy a few of the many, which may be found in foster's collection. mr. graham hutchison does not seem to think any of those ancient proverbs worthy of notice. but he misjudges. they are the result of popular observation, and many of them accord with the true philosophy of the weather. _irregular_ seasons are unhealthy, and unreliable for productiveness. when the southern transit was late, or limited, and the autumn ran into winter, our ancestors feared the consequences in both particulars, and expressed their fears, and hopes also, in proverbs. thus, "a green winter makes a fat churchyard." there is very great truth in this proverb. again, "if the grass grows green in janiveer, it will grow the worse for it all the year." this is emphatically true, for the season which commences irregularly will be likely to continue to be irregular in other respects. another of the same tenor: "if janiveer calends be summerly gay, it will be winterly weather till calends of may." janiveer is an alteration of the french name for january, and the proverb is very old. so march should be normally dry and windy. this, too, they understood, and hence the strong proverb: "a bushel of march _dust_ is worth a king's ransom." and another: "march hack ham, come in like a lion, go out like a lamb." so april and may should be cool and moist. it is their normal condition in regular, healthy, and productive seasons. the grass and grain require such conditions; and the spring rains are needed to supply the excessive summer evaporation. this, too, they well understood. and hence the proverbs: "a cold april the barn will fill." "a cool may, and a windy, makes a full barn and a findy." and-- "april and may are the keys of the year." this was not very favorable, to be sure, for corn; but their consolation was found, as we find it, in the truth of another proverb: "look at your corn in may, and you'll come sorrowing away; look again in june, and you'll come singing in another tune." this difference in the character of the seasons occasioned the adoption of a great variety of "almanac days;" and they are still very much regarded. candlemas-day ( d of february) was one of them. says hone, in his "every-day book": "bishop hall, in a sermon, on candlemas-day, remarks, that 'it has been (i say not how true) an old note, that hath been wont to be set on this day, that if it be clear and sunshiny, it portends hard weather to come; if cloudy and lowering, a mild and gentle season ensuing.'" to the same effect is one of ray's proverbs: "the hind had as lief see his wife on her bier, as that candlemas-day should be pleasant and clear." st. paul's day, or the th of january, was another great "almanac day," and so the verse: "if saint paul's day be fair and clear, it does betide a happy year; but if it chance to snow or rain, then will be dear all kinds of grain. if clouds or mists do dark the sky, great store of birds and beasts shall die; and if the winds do fly aloft, then war shall vex the kingdom oft." st. swithin's day was another of these "almanac days." gay said truly, "let no such vulgar tales debase thy mind; nor paul, nor swithin, rule the clouds or wind." yet "_almanac days_" are still in vogue to a considerable extent--such as the _three first days_ of the year, old style--the first three of the season--the last of the season--different days of the month--of the lunation, etc., etc. and some still look to the breastbone of a goose, in the fall, to judge, by its whiteness, whether there is to be much snow during the winter, etc. these _almanac days should all be abandoned_; they have no foundation in philosophy or truth. there is one proverb, however, in relation to candlemas-day, which the "oldest inhabitant" will remember, and which it may be well to retain. it has a practical application for the farmer, and in relation to the length of the winter: "just half of your wood and half of your hay should be remaining on candlemas-day." the months, too, have a character which must be remembered and regarded. _january_ is the coldest month of the year, in most localities. the atmospheric machinery reaches its extreme southern transit, for the season, during the month--usually about the middle. it remains stationary a while--usually till after the th of february. one or more thaws, resulting from tropical storms, occur during the month, in normal winters, but they are of brief duration. boreas follows close upon the retreating storm with his icy breath. there is a remarkable uniformity in the progress of the depression of temperature, to the extreme attained in this month, over the entire hemisphere. it differs in degree according to latitude and magnetic intensity; but it progresses to that degree, whatever it may be, with as great uniformity in a southern as northern latitude. the table, copied from dr. forrey, discloses the fact, and so does the following one, taken from mr. blodget's valuable paper, published in the patent office report for : table showing the mean temperature for each month at several places, viz.: +---------------------------------------------------------------------+ | | lat. | jan. | feb. |march.|april.| may. |june. | |-------------------|-------|------|------|------|------|------|------| |quebec, canada e. | ° '| . | . | . | . | . | . | |new york, n. y. | ° '| . | . | . | . | . | . | |albany, n. y. | ° '| . | . | . | . | . | . | |rochester, n. y. | ° '| . | . | . | . | . | . | |baltimore, md. | ° '| . | . | . | . | . | . | |savannah, ga. | ° '| . | . | . | . | . | . | |key west, fla. | ° '| . | . | . | . | . | . | |mobile, ala. | ° '| . | . | . | . | . | . | |new orleans, la. | ° '| . | . | . | . | . | . | |marietta, ohio | ° '| . | . | . | . | . | . | |san antonio, tex. | ° '| . | . | . | . | . | . | |san francisco, cal.| ° '| . | . | . | . | . | . | +---------------------------------------------------------------------+ +-----------------------------------------+ |july. | aug. | sept.| oct. | nov. | dec. | |------|------|------|------|------|------| | . | . | . | . | . | . | | . | . | . | . | . | . | | . | . | . | . | . | . | | . | . | . | . | . | . | | . | . | . | . | . | . | | . | . | . | . | . | . | | . | . | . | . | . | . | | . | . | . | . | . | . | | . | . | . | . | . | . | | . | . | . | . | . | . | | . | . | . | . | . | . | | . | . | . | . | . | . | +-----------------------------------------+ snows during this month are much heavier, and more frequent, in some localities than others. the reasons why this is so have been stated. the mountainous portions of the country receive the heaviest falls. they affect condensation somewhat, and according to their elevation. they intercept the flakes before they melt, and retain them longer without change. the thaws, or tropical storms, also sometimes have a current of cold air, with snow setting under them on their northern and north-western border. such was the case with that investigated by professor loomis. january is without other marked peculiarities. it shows, of course, those extremes of temperature found, to a greater or less degree, in all the months, and differs, as the others differ, in different seasons. normally, in temperate latitudes, it is a healthy month. the digestive organs have recovered from that tendency to bilious diseases which characterizes the summer extreme northern transit, and the tendency to diseases of the respiratory organs, which characterizes the southern extreme and the commencement of its return, is not often developed till february. february, in its normal condition until after the th, and about the middle, is much like january. often the first ten days of february are the coldest of the season. the average of the month is a trifle higher, in most localities, as the tables show. this results from the increasing warmth of the latter part of the month. there are localities, however, where the entire month is as cold as january. such (as will appear from blodget's table) are albany and rochester, in the state of new york, and new orleans, in louisiana. at most places the difference is slight, either way. south of the latitude of ° heavy snows are more likely to occur in the last half of january and first half of february than earlier. about the middle of the month we may expect thaws of more permanence in normal seasons. they are followed, as in january, by n. w. wind and cold weather, but it is not usually as severe. many years since, an observing old man said to me, "_winter's back breaks about the middle of february_." and i have observed that there is usually a yielding of the extreme weather about that period. here, again, it is interesting and instructive to look at the tables, and see how regularly and uniformly the temperature rises in all latitudes, at the same time; as early and as rapidly at quebec as at new orleans or san antonio; and subsequently rises with greatest rapidity where the descent was greatest. the elevation of temperature does not progress northwardly, a wave of heat accompanying the sun, but is a magneto-electric change, commencing about the same time over the whole country, and indeed over the hemisphere. march is a peculiar month--the month of what is termed, and aptly termed, "unsettled weather." it, may "come in like a lion," or be variable at the outset. the northern transit is fairly started, and is progressing rapidly, and there is great magnetic irritability. a reference to the table of dr. lamont will show that the declination has increased with great rapidity. normally, the early part is like the latter part of february, and the latter part approaches the milder but still changeable weather of april. its distinguishing feature is violent westerly wind. not the regular n. w. only--although that is prevalent--but a peculiar westerly wind, ranging from w. by n. to n. w. by w., often blowing with hurricane violence. this wind was alluded to on page . with the change and active transit to the north, in february and in march, comes the tendency to diseases of the respiratory organs--pneumonias and lung fevers--and this is the most dangerous period of the year for aged people. april is a milder and more agreeable month. during some period of it, in normal seasons, and at other times in march, there is a warm, quiet, genial, "lamb-"like _spell_, exceedingly favorable for oat seeding. when it comes, advantage should be taken of it, for long heavy n. e. storms are liable to occur, and frequently with snow. on the latitude of ° heavy snow-storms are not uncommon in april. within the last fifteen years two such have occurred after the th of the month. april, as we have seen, should be cool and moist. if dry, the early crops are endangered by a spring drouth; if very wet, there is danger of an extreme northern transit, and an early summer drouth. it is emphatically true that "april and may are the keys of the year." its distinguishing peculiar feature is the gentle, _warm_, _trade_ rains--"_april showers_"--which, in the absence of great magnetic irritability, that current drops upon us. there is great _mean_ magnetic activity, but it is not so _irregularly excessive_ as in march. may, in our climate, should be, and normally is, a wet month, and a cool one, considering the altitude of the sun. the atmospheric machinery which the sun moves is, however, ordinarily about six weeks behind it--the latter reaching the tropic the th of june, and the former its farthest northern extension about six weeks later. hence it is not a cause for alarm if may be wet and cool. the great staples, wheat, grass, and oats, are benefited; and corn, according to the proverb, will not be seriously retarded. the movable belt of excessive magneto-electric action, with its tropical electric rains, so exciting to vegetation, and its periods or terms of excessive heat, is on its way north, and sure to arrive in season, and remain long enough to mature the corn. there have been but two seasons in this century when corn did not mature in the latitude of °. one during the cold decade, and the cold part of it, between and ; and the other, during the cold half of the fourth decade, between and . the distinguishing feature, if there be one, of may, is its long, and, for the season, cool storms. these have, in different localities, different names. in pastoral sections we hear of the "_sheep storms_"--those which effect the sheep severely when newly shorn--killing them or reducing them in flesh by their coldness and severity. in relation to this too early shearing, there is an old english proverb, in "forster's collection," viz.: "shear your sheep in may, and you will shear them all away." so there are others called "_quaker storms_," which occur about the time when that estimable sect hold their yearly meeting. and there are other names given in different localities to these long spring storms. but they are all _mere coincidences_--equinoctial and all. notwithstanding the storms, however, the temperature rises at a mean. the declination is often as great as in mid-summer. the earth is growing warmer by the increase of magneto-electric action, whatever the state of the atmosphere. the yellow, sickly blade of corn is extending its roots and preparing to "_jump_" when the atmosphere becomes hot, as it is sure to do, when the machinery attains a sufficient altitude, how backward soever it may seem to be. the farmer need not mourn over its backwardness, unless the season is a very extraordinary one, like those of and . the storms ensure his hay, wheat, and oat crops; the warming earth is at work with the roots of his corn, and is filling with water, and preparing for the hot and rapidly-evaporating suns of mid-summer. the earth would grow warmer if every day was cloudy. by the middle of june the atmospheric machinery approaches its northern acme, the summer sets in, and not unfrequently, as extremely hot days occur during the latter part of the month, as at any period of the summer. but the heat is not so continuous, or great, at a mean. from the middle of june to the latter part of august is summer in our climate, and during that period from one to three or four terms of extreme heat occur, continuing from one to five or six days, and possibly more, terminating finally in a belt of showers overlaid with more or less cirro-stratus condensation in the trade, and controlled by the s. e. polar wave of magnetism, and followed by a cool but gentle northerly wind. during these "heated terms," a general showery disposition sometimes, though rarely, appears, with isolated showers, which bring no mitigation of the heat. not until a southern extension of them appears, followed by a n. w. air, does the term change, so far as i have observed. by the th of august, in the latitude of °, an evident change of transit is observable, by one who watches closely, although the range of the thermometer in the day-time may not disclose it. a greater tendency to cirrus-formation is visible. the nights grow cooler in proportion to the days. the swallows are departing, or have departed; the blackbirds, too, and the boblinks, with their winter jackets on, _their plumage all changed to the same colors_, are flocking for the same purpose, and hurrying away. the pigeons begin to appear in flocks from the north, and the first of the blue-winged teal and black duck are seen straggling down the rivers. at this season, and nearly coincident with the change, the peculiar annual catarrhs return. these are colds (so called) which at some period of the person's life were taken about or soon after the period of change, and have returned every year, at, or near the same period. they soon become _habitual_, and no care or precaution will prevent them. i know one gentleman who has had this annual cold in august for twenty-seven years, with entire regularity; and another who has had it nineteen years; and many others for shorter periods. i never knew one which had recurred for two or three years that could be afterward prevented, or broken up. _very instructive are these annual catarrhs_ to those who think health worth preserving, and in relation to the change of transit. _the change is felt over the entire hemisphere._ between the th of august and the th of september hurricanes originate in the tropics and pursue their curving and recurving way up over us; or long "north-easters" commence in the interior and pass off to e. n. e. on to the atlantic, followed now in a more marked degree by the peculiar n. w. wind, so common over the entire continent in autumn and winter. by the th of september the pigeons may be seen in flocks in the morning, and just prior to the setting in of a brisk n. w. wind, hurrying away southward with a sagacity that we scarcely appreciate, to avoid the anticipated rigors of winter, and to be followed soon by all the migratory feathered tribes that remain. the nights grow cooler, although the sun shines hot in the day-time, and woe to the person, unless with an iron constitution, who disregards the change, and exposes himself or herself without additional protection, to its influence. nature has taken care of those who depend upon her, or upon instinct, for protection. the feathers of birds and water-fowl are full; the hair and the fur are grown. beasts and birds have been preparing for the change, and are ready when it begins. they know that the earth is changing. the shifting machinery is fast carrying south that excess of negative electricity which has so much to do with giving it its summer heat. they feel its absence, even during the day, and the contrast between that and the positively electrified northern atmosphere, which now follows every retreating wave of condensation. the musk-rat builds, of long grass and weeds, his floating nest in the pond, that he may have a place to retire to, when the rain fills it up and drives him from his burrow in its banks. but man, with all his intellect, is too heedless of the change. additional clothing is now as necessary to him as to animals, but it is burdensome to him in the day time, and therefore he will not wear it, how much soever it would add to his comfort and safety during the night. he stands with his thin summer soles upon the changed ground, or sits in a current, or in the night air, less protected than the animals, and dysentery or fever sends him to his long home. he has _intelligence_, but he lacks _instinct_. he has time for the changes of dress which fashion may require, but none for those which atmospherical changes demand. _fashion_ has attention in _advance_; _death_ none till _at the door_. now the southern line of the extra-tropical belt of rains descends upon those who, living between the areas of magnetic intensity, have a dry season; and the focus of precipitation in that belt descends every where. "_winter no come till swamps full_," the indians told our fathers, and there is truth in the remark; although like other general truths respecting the weather, it is not always so in our climate. rains fall during the autumnal months, as during the spring months, and while the transit of the machinery is active and the evaporation is less. and the magnetic comparative rest, and the seed time and equable "spell" of april is reproduced in the indian summer of autumn. the machinery gradually and irresistibly descends, and with an excess of polar positive electricity, comes snow; boreas controls, and winter sets in, reaching its maximum of cold in january again. remembering, then, the differences in the normal conditions of the seasons and months, and the different characters that the winds, and storms, and clouds, and other phenomena bear in them respectively, let us now look at the signs of foul or fair weather not herein before fully stated, upon which practical reliance may be placed. in the first place, we must look to the forming condensation. there are many days when the atmosphere is without visible clouds, but few when it is entirely without condensation. such days are seen during the dry season in the trade-wind region; and with us, in mid-summer drouths, which partake of this tropical character; and when, at any season, but particularly in winter, the n. w. wind in large volume has elevated the trade very high. condensation is not necessarily in form of visible cloud. it may be of that smoky character which sometimes attends mid-summer drouths, giving the sun a blood-red appearance; or it may be like that change from deep azure to a "lighter hue," obscuring the vision, which humboldt describes as preceding the arrival of the inter-tropical belt of rains. gay-lussac, and other aeronauts, have seen a thin cloud stratum at the height of , to , feet, not visible at the earth, although some degree of mistiness and obscurity were observed. at that elevation the clouds are thin, and always white and positive. some degree of turbidness is frequent; it may occur, as we have stated, with n. w. wind, but, if it does, the wind soon changes round to the southward. this turbidness or mistiness, where it exists, and indicates rain, does not disappear toward night, as it should do if but the daily cloudiness which results from ordinary diurnal magnetic activity, but becomes more obvious at nightfall; and, when hardly visible at mid-day, or during the afternoon, may then be observed, obscuring in a degree, the sun's rays; and, later in the evening, forming a circle round the moon. thus jenner-- "last night the sun went _pale to_ bed, the moon in _halos_ hid her head." and so, too, virgil-- "the sun, too, rising, and at that still hour, when sinks his tranquil beauty in the main, will give thee tokens; certain tokens all, both those that morning brings, and balmy eve. * * * * * when sol departs, his mighty day-task done, how varied hues oft wander on his brow. * * * * * if the ruddy blaze be _dimm'd_ with _spots_, then all will wildly rage with squalls and driving showers: on that fell night none shall persuade me on the deep to urge my perilous course, or quit the sheltering pier. but if, when day returns, or when retires, _bright_ is the orb, then fear no coming rain: clear northern airs will fan the quiv'ring grove. lastly, the sun will teach th' observant eye what vesper's hour shall bring; what clearing wind shall waft the clouds slow floating--what the south broods in his humid breast. who dare belie the constant sun?" more frequently this kind of condensation is sufficiently dense at night-fall to take shape, and show a bank when the sun shines horizontally through a mass of it. i am now speaking of _storm_ condensation, or that which indicates the approach of a storm. thunder clouds at nightfall, dark, dense, and isolated, are, of course, to be distinguished. those, every one understands to indicate a shower, and immediate succeeding fair weather. the halos do not, in cases of incipient storm condensation, always appear. the moon may not be present: though, in her absence, i have seen them in the light of the primary planets; or she may be in the eastern portion of the heavens. when this is so, and the condensation forms slowly, there may be less appearance of it, after the sun disappears, than before, although a storm is approaching, and sure to be on by the middle of next day, and perhaps with great violence. when the failure of the light no longer reveals the denser condensation in the west, the stars may shine, as did the sun, dimly but visibly, through the partial and invisible condensation; and one who did not notice the bank in the west, at nightfall and before dark, may be deceived by the seeming clearness of the evening. thus virgil-- "mark, with attentive eye, the rapid sun-- the varying moon that rolls its monthly round; so shalt thou count, not vainly, on the morn; _so the bland aspect of the tranquil night will ne'er beguile thee with insidious calm_." all early condensation and indications derived from it, must be looked for in the west. from that quarter all storms come. these indications at nightfall are of a varied character. they may consist of primary condensation in the trade, or of secondary condensation, scud running north toward a storm, the condensation of which has not yet visibly reached us, but which will extend south and pass over us. it may be a heavy bank, or consist of narrow cirrus bands. cirro-stratus cloud banks, in the s. w., in the fall and winter, of a foggy and uniform character, are indicative of snow. the body of the storm will pass south of us, and a portion over us, the wind be north of east, and the snow will not be likely to turn to rain before it reaches the earth, by reason of a southern middle current. banks in the n. w. indicate rain at all seasons. the storm is north of us, working southerly, and such storms rain on the southern border--in winter even--because they have the wind on that border from south of east. it may, indeed, snow, but if so, probably in large flakes, soon turning to rain. there are other appearances at nightfall which deserve consideration. a red sun, with smoky air, is indicative of continued dry weather, a frequent appearance in dry terms, lasting three or four days, at least, from the commencement. so is a red appearance of the sky, when there are no clouds, indicative of a fair day following. on this subject we have an allusion to the weather, by our saviour while on earth, which, like all such allusions found in the bible, is of remarkable philosophical accuracy. it is found in matthew, chapter xvi., verses and : "he answered and said unto them, when it is evening ye say, it will be fair weather, for the sky is red. and in the morning, it will be foul weather to-day, for the sky is red and lowering. o, ye hypocrites, ye _can discern_ the face of the sky," etc. another allusion to the weather, though not applicable to this point, i will refer to in passing. it is found in luke, chapter xii., verses and : "and he said also to the people, when ye see a cloud rise out of the west straightway ye say, there cometh a shower; and so it is. and when ye see the south wind blow, ye say, there will be heat; and it cometh to pass." this is all very true, and might have been cited to show the universality of the phenomena. but to return. we have an old english proverb alluding to the same phenomena, of great value and truth, viz.: "an evening red and a morning gray are sure signs of a fair day; be the evening gray and the morning red, put on your hat or you'll wet your head." the sky is red if there be no condensation at the west to obscure the rays of the sun; if there be, it is gray, or there is a bank or cloud, and it is obscured. so if there be no condensation over, or to the east of us, in the morning, to reflect the rays of the sun, the sky is gray; if there be such condensation, the sun is reflected from it, and the sky is red. such morning condensation is indicative of foul weather. it is, as we have said, the eastern edge of an approaching storm, on, or under which, the sun shines and illumines it. thus, at night, it shines through a portion at the west, which is situate between the sun and us, making the sky gray: but shines on, or under, a portion in the morning, east of us, but not far enough east to obscure the horizon, and the rays of the rising sun are reflected from it. in either case the red or gray appearance results from the relative situation of the sun and the eastern edge of an approaching storm. the following couplet of darwin is an apt description of the morning appearance: "in fiery red the sun doth rise, then wades through clouds to mount the skies." the sun is often reflected in vivid colors, from the under surface of clouds, at sunset. this is an indication of fair weather. it is evident the sun shines through a _clear atmosphere beyond the cloud_, or his rays would not reach and illume the lower surface of the cirro-stratus with such distinctness. he "_sets clear_," as is said; the clouds are passing off, and there are none beyond. it is this appearance, in different forms, when there happen to be patches of broken, melting cirro-stratus above the horizon, which makes the beautiful sunsets that attract attention. so the sun is reflected, in beautiful colors sometimes, from the cumulus clouds which have passed over to the east. the most beautiful and variegated i have ever seen, were reflected from that imperfect cumulus condensation which takes place occasionally during long drouths--doubtless resembling that which is seen over peru, hereinbefore alluded to, as described by stewart. it is not, then, the presence of cloud condensation at the west, at nightfall, which alone indicates foul weather; but such condensation, whatever its form, as evinces that it is not the _dissolving_ cloud of the day, but the eastern, approaching portion of a _still denser portion beyond, through, or under which, the sun can not shine clearly, but which wholly or partially obscures it_. _remembering this philosophy of the matter_, the observer will soon be able to detect the various forms of condensation which originate or exhibit themselves at nightfall, and whether they indicate an approaching storm or not, without a more explicit specification of them. it is an important hour for observation; "let not the sun go down" without attention. when the condensation is obvious, but thin, at nightfall, it may not, as i have said, be discernible in the evening. but there are methods by which the incipient storm condensation may be detected. the number of the stars visible, and the _distinctness_ with which they may be seen, indicate the absence or presence of condensation and its density. virgil, alluding to the indications of fair weather, says: "_brightly_ the stars shine forth; cynthia no more _glimmers_ obnoxious to her brother's rays; nor fleecy clouds float lightly through the sky." the brightness of the stars and the clear appearance of the moon show the absence of condensation and the _dissolution_ of the fleecy clouds at the close of the day is, as we have seen, always a fair-weather indication. there is much true philosophy in the allusions of virgil to the moon. thus-- "when luna first her scatter'd fires recalls, if with _blunt horns_ she holds the _dusky_ air, seamen and swains predict th' abundant shower." the horns, or angles of the moon will, of course, appear distinct and sharp or indistinct and blunt, in proportion to the amount of condensation in the atmosphere which impedes the passage of the light. for the same reason, when the moon is new, her entire disk is visible when the atmosphere is very clear, by reason, as is supposed, of light reflected from the earth to the moon and back to us. this double reflection can only take place when the atmosphere is very clear. hence, virgil alludes to it, and correctly, as an indication of continued fair weather: "if (mark the ominous hour!) the clear fourth night her lucid disk define, that day, and all that thence successive spring, e'en to the finished month, are calm and dry." probably virgil alluded to a month of the summer trade-wind drouth which reaches up on southern italy. but that appearance of the moon is occasionally seen here, and the indication is, in degree, philosophically true. it is somewhat more difficult to determine what will be the result of the condensation seen at the west in the morning, and which is not so far east, or of such a character, as to reflect the rays of the sun; for, although always suspicious, it is sometimes of a foggy character, and disappears between eight and nine o'clock. if it increases in density after ten o'clock, or is of a dense cirro-stratus character, rain may generally be expected. if of a decided _cirro-cumulus_ character, it is certain to disappear. cirro-cumulus is seen in small patches, with small, distinct, and rounded masses, in summer, in the morning, and sometime, during the day, after high fog has disappeared, and at other times, and is always, when of that _distinct_ character, a fair weather indication. i have seen it thus when the wind was blowing from the n. e., and the scud running toward a storm passing near, but to the south of us, when those who relied upon the existence of the wind and scud as evidences that we were to have the desired rain, were deceived. thus, the couplet from an old almanac: "if _woolly fleeces_ strew the heavenly way, be sure no rain disturb the summer day." when this morning condensation is not high fog, and is dense and passing east with a wavy appearance, it is very certain to rain. jenner says: "the boding shepherd heaves a sigh, _for see, a rainbow spans the sky_." an old almanac had the following verse: "a rainbow in the morning is the shepherd's warning; a rainbow at night is the shepherd's delight." so the proverb was originally made; but as our ancestors were not shepherds, and had a horror of ocean storms, it was commonly quoted, in this country, in the following form: "a rainbow in the morning, the sailors take warning," etc. rainbows are not reflected from _clouds_, but falling rain, and a morning rainbow at the west is, of course, evidence that it is _actually raining there_, and will, in all probability, pass over us. "thunder in the morning, rain before night," is a common saying, and a true one. there is a belt of showers, or showery period approaching, of unusual intensity--for thunder showers in the morning are rare. the afternoon is their most common period, and they are very apt to appear then, when the morning is showery. of the different forms of cirrus and cirro-stratus, which appear during the day, and indicate approaching storms, or of cumulus indicative of showers, it is difficult to give an intelligible description without very many illustrations. i have many daguerreotype views, taken at different seasons of the year, and at a time when different forms of cirrus and cirro-stratus condensation, indicative of storms, exhibited themselves. they differ, as i have said, and it must be remembered, very much at _different seasons_ of the year, and in _different years_, and their delicate shades are taken with difficulty by the artist, and reproduced with difficulty, and only at considerable expense, by the engraver; and i have omitted them. the time will come when a knowledge of their language will be sought for and read--when the "countenance of the sky" will be an object of intelligent interest to all whose business may be affected by the weather, or who love to learn of nature. but it is not yet. this is the age of theory and speculation. the time of actual, practical, connected observation and prognostication, which may justify expensive illustration, is yet to arrive. the reader will find in the general plates representations of several kinds of cirri. they are delicate, always white, more or less fibrous, and form in the upper part of the trade or the adjoining atmosphere above it. their character and elevation should be studied, and the observer should be careful to distinguish which is the most elevated. not unfrequently it may seem, to a hasty observer, that the cirrus is below the cirro-stratus or forming stratus. but the genuine cirrus never is. it forms near, and above, the point of congelation, and is often composed of crystals of ice or snow. if they fall, they melt and evaporate, when there is no storm, before reaching the earth. aeronauts have met with them and their crystals when there was no fall of moisture at the surface of the earth; and the angles of reflection exhibited by halos and other optical phenomena which form in them, enable us to detect their crystallization and the form of it. they are produced by electric changes which condense the vapor, and the coldness of the air at that elevation freezes it at the _instant of its condensation_. congelation is crystallization, and all crystallization is electric, or magneto-electric. the snow-flakes differ in form and size according to the suddenness of the condensation, the amount of moisture condensed, the polarity of the strata through which they pass, and their consequent attraction and adhesion to each other. the connection of electricity with these formations of cirri has frequently been admitted, and it is perfectly obvious that the long fibrous bands, shooting from horizon to horizon, could not be formed by commingling of currents any more than the perfectly isolated, distinct, enlarging-outward cumulus hail-storm, could be so formed. cirri form at the line of meeting, between the trade and the upper atmosphere, and in one or the other, or both, very much according to the season, and the suddenness with which storms are produced. these often _induce_ a layer of cirro-stratus or stratus at the lower line of the counter-trade, and in the surface-atmosphere, which precipitates; and this operation is clearly discernible, and very frequently, before gentle rains. condensation in the whole body of the trade is usually in the form of turbidness or mistiness, a bank or incipient stratus, without cirri. it seems matter of astonishment that water should float so far condensed, in strata where the air is so much lighter, without being precipitated. but electric attraction and repulsion between the different strata and the vesicles, explain it. in mid-winter, the cirrus forms are prevalent and most distinct. after severe cold weather, when a storm approaches, the cirri form in long, narrow threads, parallel to each other, extending from about w. s. w. to e. n. e., gradually thickening and forming, or inducing, cirro-stratus and stratus, and dropping snow. this form is called the _linear_-cirrus. the tufted, and other fibrous forms, are seen in patches also, in great distinctness, during these mid-winter days, when the wind gets around to the southward, and the weather is pleasant. such days are called "_weather-breeders_," and their _offspring_ the patches of cirrus, which are to extend and compose, or induce the storm, and indeed are an advance part of it, are then never absent. a clear, moderate day, in a normal winter, with wind from any southern point, however light, between the st of january and the middle of february, without these patches of cirrus, is very uncommon. watch and see whether they tend to cirro-stratus, or whether the wind gets around to the n. w. at nightfall, and they disappear. if the former, a storm may be expected; if the latter, fair weather. thus there are three peculiarities attending the forming cirrus of mid-winter ( st of january to th of february): long, fibrous, parallel bands in the morning (linear cirrus), gradually coalescing as the day advances, after severe cold; the comoid, curled, or tufted cirrus, in curling bunches, called "_mares'-tails_," and the _transverse_, when the fibers are in bands or threads, which are not parallel, but cross each other at angles, more or less acute. the two former varieties are represented on figure , page , indicated by one bird, but the last form is a very prevalent one in our atmosphere. various names have been given to different forms of _cirro-stratus_. those represented in figure , page , are the "_cymoid_" on the right, the "_mottled_" on the left, below the cirro-cumulus; and the "_linear_" below that. the form known as the "_mackerel sky_" is not represented there. it consists of regular forms, resembling the _waves_ on the surface of the water when the wind blows a gentle breeze. but the _wavy_ form, and of all sizes, is very frequently assumed by cirro-stratus, which is rapidly condensing, and turning to stratus. in the "mackerel sky," strictly so called, the waves are small, parallel, nearly distinct and equi-distant, and resembling the appearance of a school of mackerel, swimming in the same direction, one above another. all _wavy_ forms of cirro-stratus indicate a disposition to increased condensation and rain. when the waves are very large and dense, and cross obliquely, or unite at one end, rain is very certain to fall soon, if the line of progress of the condensation is over the observer, and the clouds are seen in the western or n. w. quarter of the sky. but there are few forms which are not occasionally seen when no rain or snow falls. the intensity of the electric action which produces them may not be sufficient to effect precipitation, or they may be the attendant, attenuated _lateral_ condensation, which frequently "thins out" a considerable distance from the dense, precipitating portions of the storm. if that denser portion is north of us, the probabilities of rain are greater, for there is always a probability that the storm may be of the character which is extended south, by a polar wave. the observer must watch the formation of cirri, and the different forms of cirro-stratus and stratus, and become familiar with their appearance. it is not a difficult task. with the aid of a few general directions he will soon be familiar with them: . get a correct idea of the different characters of the primary clouds. the true fibrous _cirrus_--the different forms of _cirro-stratus_--the smooth, uniform _stratus_--the _cirro-cumulus_, which is nothing but a cirro-stratus, separated into _distinct masses_ by the repulsion of static electricity--and the _cumulus_, too distinct ever to be mistaken. there is no difficulty, except with the varied forms of cirro-stratus. it is useless to attempt to give, or the observer to rely on, names for these numerous forms, without as numerous illustrations. those in use are rarely applied correctly. i have never met with ten persons who applied even the term "mackerel sky" to the same precise form of cirro-stratus. in relation to all of them it is to be observed that polar belts of condensation, and local appearances of considerable extent, are often too feeble in action to precipitate, even when the mackerel form is present; and all may be the lateral attendants of passing storms. therefore, . satisfy yourself whether the cirrus or cirro-stratus increases in density and tends to the formation, or induction, of stratus; and whether it is isolated, or an extension of the condensation of a storm, and if the latter, _where that storm is_. the time will come when an intelligent use of the telegraph will do this for you. . look also to the character of the wind, if there be any. on this subject i have perhaps said all that is necessary in the preceding pages. next to condensation, the direction and character of the wind is the most valuable prognostic. indeed it often tells us that a storm is approaching, and the quarter from which it will come, and its character, before the condensation is visible. . see if there is any _secondary_ condensation or scud. these are sometimes seen running toward a storm, when there are not distinct clouds visible in the western horizon, at nightfall, or in the evening, as in the instance stated in the introduction, and sometimes from the north-east, as in cases heretofore so often stated. but the easterly scud do not often form in winter, until after the cirrus has passed into the form of cirro-stratus, or has induced the latter forms in the inferior portion of the trade, or the surface atmosphere. the inductive effect of the primary condensation, therefore, is not always, and especially in winter, sufficient to create the easterly current and scud, and it is often the case that the easterly wind is not felt, or the scud seen, in snow-storms, until the snow has begun to fall, and the first snow will fall with a s. w. air, as i have heretofore stated. but when the condensation has so far advanced toward stratus that the easterly wind and scud are obvious, there is little or no doubt that rain or snow will fall speedily. the occasional occurrence of easterly wind and scud, without rain, however--dry north-easters, as i have termed them--in connection with storms passing south of us, or condensation too feeble to precipitate, should be remembered. the long, dry, north-easterly winds of spring have been attributed to the icebergs, but they are overlaid by feeble stratus or cirro-stratus condensation, or are the result of attraction, by a more southern precipitation. the observer must be careful to distinguish between the various forms of n. w. scud and cirro-stratus, which they sometimes resemble. this he may do _from the direction in which they move_. cirro-stratus always moves from some point between s. s. w. and w. s. w. to some point between n. n. e. and e. n. e. the various forms of n. w. scud move to the s. e. the march, foggy scud, from between w. and n. w., rarely have any cirro-stratus above them, but rather a peculiar turbid condensation. the character of the primary condensation, the direction and force of the wind, and the direction of the secondary condensation or scud, must be the main reliance of the observer. but i must reiterate that they all differ in different kinds of storms, in different seasons of the same year, and the same seasons of different years; and the observer must be careful to make due allowance for those differences. there are, however, divers other secondary signs, which, although not alone to be relied upon, will aid the observer, if carefully studied, when the character of the clouds, and the pressure of easterly or southerly wind and scud, are not decisive. of these, a large class are electrical. the smoke descends the adjoining chimney-flues, or outside of the chimney, toward the ground. thus, darwin, as quoted by hone: "the smoke from chimneys right ascends, then, _spreading_, back to earth it bends." smoke is electrified _positively_, by the act of combustion; the earth and the adjacent atmosphere, when storms are gathering or approaching, is _negative_. hence the smoke spreads, and is attracted downward by an opposite electricity. on the other hand, it is interesting to see, at other times, and when the difference in temperature is not material, but the whole atmosphere is positive, with what rapidity and compactness the smoke will ascend in a _straight and elevated column_ from the chimney, repelled by a similar electricity. i am aware it is generally supposed the smoke descends because the _air is lighter_. but it is a mistake. i have seen it descend when the barometer was at °. , or . above the mean. there is, too, a draught downward in chimneys, in such cases when there is no smoke or fire in any of its flues. thus jenner says: "the soot falls down;" whether he meant by this that there was an actual fall of soot other than what is occasioned by the rain falling in through the chimney top, and disturbing the soot, as sometimes happens, i do not know. it occurs rarely, and is of very little practical importance. but every housewife knows that chimneys, which have been used in winter, and are full of soot, _smell_ before storms. the odor results from a downward draught and the dampness of the air. so the smoke from one flue will descend another, into some unused room, on such occasions. another class of these electrical signs are felt by those who are suffering from chronic diseases, which have affected the nerves and made them sensitive. thus jenner: "old betty's joints are on the rack." and hone adds: "her corns with shooting pains torment her, and to her bed untimely send her." but old betty's rheumatism or corns are not alone in this. those whose bones have been broken feel it. all invalids feel it. and, indeed, all observing healthy persons may, and do, although all are not distinctly conscious of it. it is common for such to say, i feel sleepy, or i feel dull, or, it _feels_ like snow, or _feels_ like rain, and thus from their own feelings to be able to predict, not only falling weather, but its _character_, whether snow or rain, at a time when either may occur consistently with appearances. this change is a change from the positive electricity which is so congenial to the active--"bracing" is the usual term--to negative and damp--for this change is accompanied by condensation, as i believe all changes from positive to negative are. certain it is, if the atmosphere is highly charged with negative electricity, condensation takes place; if with positive, evaporation. perhaps it is a change of the associated electricity which accompanies magnetism, and not of the free atmospheric electricity alone. hence another phenomenon alluded to by jenner: "the walls are damp, the ditches smell." there are localities where this dampness is very obvious. the celebrated william cobbett, many years since, when a farmer on long island, observed and published the fact that the stones grew damp before a storm. i know of flagging stones that usually grow damp two or three hours before rain, especially in spring and fall, and every step taken upon them is made visible by a corresponding increase of condensation. the reverse of this takes place just before the close of storms. flagging stones, and walls under cover, will frequently become dry before the rain ceases. the negative electricity becomes less as the positive prevails, although the clouds above are still dropping rain. in the comparatively moist, showery climate of england, these changes from positive to negative alternate rapidly between successive showers; but observations of electric phenomena, or of clouds, in that climate, are not, without qualification, safe guides for us. so "the ditches smell," particularly in the evening before a rain, when the immediate surface-atmosphere is charged with negative electricity, and the _condensing moisture_ prevents the diffusion of the odors. for the same reason the candle will not relight, and there is crackling in the ashes or lamp. thus, again, virgil: "maidens that nightly toil the tangled fleece divine the coming tempest; in the lamp _crackles_ the oil, the gathering wick grows dim." virgil did not live in our cold climate, and knew nothing of the crackling in the fire, or in the ashes or coals which remain after the wood is consumed. the lamp exhibits it on a smaller scale, and perhaps he had noticed it when in company with the maidens. but it is sometimes noticeable even in the lamp or candle with us. a small particle of moisture will produce it, in a marked degree, at any time. in winter, when the air is highly positive and cold, the candle can be blown out, and by another puff of the breath relighted, with ease. but when the electricity before a storm becomes negative, and partial condensation takes place, this can not be done. this partial condensation before storms and showers shows itself upon vessels containing cold-water, in summer. it seems to be the received opinion, that the condensation is evidence of a greater _quantity_ of moisture in the atmosphere. but this, too, is a mistake, and hence the little reliance to be placed on hygrometers. this partial condensation is sometimes visible. when the sun shines clearly, at the east or west, through a _small opening_ in the clouds, the condensing vapor is shown by the streaks of sunlight, just as the fine particles of dust are seen in a dark room, when a few rays of sunlight are admitted through a small aperture. this phenomenon is often observed, and it is said of it--"it's a going to rain; _the sun is drawing water_." virgil alludes to this as seen in the east in the morning, thus: "but when beneath the dawn _red-fingered rays_ through the dense band of clouds _diverging_ break, * * * * * ill does the leaf defend the mellowing grape; leaps on the noisy roof the plenteous hail, fearfully crackling." it is well ascertained that storm-clouds of great intensity have polarity in the different portions, and that in the less intense magneto-electrical climate of england isolated showers are often of this character--the polarity existing in rings. showers are doubtless thus found with us. mr. wise got into one of them; see his description (theory and practice of aeronautics page ). i have, in another place, alluded to the upward attraction of the dust beneath the advance condensation of a shower. jenner alludes to it in the following lines: "the whirling winds the _dust_ obeys, and in the rapid eddy plays." so virgil: "light chaff and leaflets, _flitting, fill the air_, and sportive feathers circle on the lake." all these are electrical. in england, where the action of such isolated clouds is less intense, the different electricities in different portions of the cloud, whose opposite and changing action produce all the phenomena, the condensation, the cold and congelation, the currents, etc., have been accurately ascertained. we can not get into the situation occupied by mr. wise. but every man may observe these _intestine motions_ occasionally, in the advance condensation of an isolated thunder-shower, in front of, but near the smooth line of falling rain. they are more lateral than upward or downward, and are often exceedingly rapid in movement. i have said that hail has often been found to fall from particular and well-defined portions of a cloud, and rain from the other portions, the hail being positive, and rain negative. an instance of very striking character may be found in espy's philosophy of storms (introduction, page xx.) doubtless in all cases thunder-showers, which are isolated and distinct, have opposite electricity in different portions, to whose active agency all the phenomena are owing. and the return of electricity to the earth in the rain explains the greater fertilizing effect of the latter compared with all artificial watering. he was a true philosopher who attempted to stimulate vegetation by electricity. sounds may sometimes aid the observer in doubtful cases in foretelling the weather. the roar of the surf, or breaking of the waves on the shore, when great bodies of water are disturbed by a precedent storm-wind, often heard before the wind is perceived on the land, i have already alluded to. and thus virgil: "when storms are brooding--in the _leeward gulf_ dash the swelled waves; the mighty mountains pour a harsh, dull murmur; far along the beach rolls the deep rushing roar." the moaning or whistling of the wind all have noticed. it is not uncommon to hear the expression, "the wind sounds like rain." jenner says: "the _hollow_ winds begin to blow." and virgil: "the _whispering_ grove betrays the gathering elemental strife." this whispering is the motion of the leaves; and they are often stirred by a peculiar motion which is not that of wind. sometimes every leaf upon a tree may be seen _vibrating_ with an _upward and downward_ motion, when there is not wind enough to stir a twig. this interesting phenomenon is electrical. trees, and all vegetables, confessedly discharge electricity, and such discharges move the leaves, when very active. with us, sounds can be heard more distinctly from the east or south, before storms, according to the character of the coming wind. howard mentions an instance when he heard carriages five miles off. steamboat paddles, rail-road cars, and other sounds, are often heard a great distance. the distance at which the now common steam-whistle is heard, and the direction, is not an unimportant auxiliary indication of the weather. howard attributes these peculiar phenomena to the "_sounding board_," made by the _stratum of cloud_; but sounds may be heard from the north-west, when there is no condensation, and the wind is from that quarter, and also from the east when it is not cloudy; and in a level country the village bells often tell the direction of the current of air just over our heads when we do not feel it at the surface. the wind is undoubtedly moving in a rapid, and perhaps invisible current, not far above us. if from the east or south, it betokens rain; if from the western quarter, fair weather. the conduct of the different animals furnish a considerable portion of the signs alluded to by virgil and jenner, and are never unimportant auxiliary evidence of the approaching changes, whether from dry to wet, or wet to dry. the observer will find, in the conduct of our birds and animals, especially those which are not domestic, ample evidence of the truth of the descriptions of virgil. he denies the animals and birds foresight, but he does not seem to have observed that the swallow leaves for the south as soon as the _autumnal_ change begins to be felt, and in august; nor the evident sagacity of other _migratory_ birds. they do not act from the "_varying impulse_" produced by an actual state of things, but a knowledge or apprehension of those which are to come. this is nothing more or less than foresight. so foresight tends to prudence and skill, and they exercise both, and with reference to the future. the goldfinch does not build her nest in the hole of the tree, or in the crotch of the limb; but _hangs it_ with _exquisite skill_ on the slender _waving, outward branch_, where no animal, or larger bird, or any depredator, can be sustained. she is not more timid than others; why does she invariably thus build? what makes her "_impulses_" differ from those of other birds, and always in the _same manner_? jenner, too, has grouped, in admirably descriptive language, many of the peculiarities exhibited by animals and birds before approaching storms, some of which exhibit foresight, and others not. perhaps the rooster, who keeps ceaseless watch over his harem, is the most reliable weather-watcher we have. in my earlier days, when it was the practice to keep valuable birds of the kind much longer than it now is, and they had opportunity to become _experienced_, it was interesting to observe how closely they watched the weather. i well remember a venerable chanticleer, who, perched on the tree among his hens, would always foretell the coming storm of the morrow, by sounding forth _in the evening_, and _often_, his defiant note. such note in the evening was invariable evidence of foul weather. and during the night, their earlier and more frequent crowing is often indicative of it. it is, however, in the earlier part of the day, in doubtful cases, that no inconsiderable reliance may be placed on their sagacity. often, when a storm is gathering in the forenoon, they will announce it by an almost incessant crowing. the habits of an _experienced_, old-fashioned bird, of this kind, will well repay attention; but i can not answer for the shanghai and other _fancy breeds_. jenner says: "the leech disturbed, is newly risen quite to the summit of his prison." few have had, or will have, opportunities to observe this, but it is strikingly true. it is difficult to conceive how mere condensation, from an increase of vapor in the atmosphere, should be foreseen by the leech in his watery prison. it is obvious, i think, there is an electric change which reaches him, as it does the whole animal creation, the once broken bones, and the joints of aunt betty. thus much of the philosophy of signs. _the barometer_ is a useful instrument, in connection with observations of the other phenomena. it is especially useful to the sailor, as its indications relative to the winds are much the most certain. but it is not, _alone_, to be relied upon. this is well settled, although the reasons for it have not been understood. why it should rise sometimes before storms, in opposition to the general rule--or fall at others without rain--or rise occasionally during the heaviest gales, has been a mystery, and impaired the confidence in its accuracy and usefulness even of the class of philosophers of whom sir george harvey spoke, in the sentence quoted in the introduction. but, as i have already intimated, it is all very intelligible. i have said that the barometer has no fair weather standard--the mean of inches at the level of the sea being an _average_ of the _fair weather_ elevations and the _foul weather_ depressions. its fair weather position, it would seem, must be above the mean, therefore, and as much above as its foul weather depressions are below. but this is not precisely true. its extreme fair weather range is inches, and it rarely reaches that; while its lowest storm range is down to , and is the most often reached of the two. my barometer stands about feet above ordinary high-water mark. it is not a "wheel," but an open, "scale" barometer, and a perfectly good one. its most reliable fair weather standard is about - / inches. it is its _most common summer, set fair position_, but that position is often at other and different elevations, at other periods of the year, during fair weather. the reader must observe for his own locality, and satisfy himself what the most common set fair position for the barometer is, at the different periods of the year, where he resides. when he has ascertained this, he may apply the following principles to illustrate its exceptional action, and in judging of the future of the weather: st. _as to its rise before storms._--supposing it to have been stationary, at or about a set fair position, _for the period_, and for one or two or more days, a very _gradual_ and _moderate_ rise is an indication of continued fair weather; and a _sudden_ and _considerable rise_ is indicative of a storm. if the sudden and considerable rise occurs in the latter part of spring, summer, or early autumn, it indicates a storm of the _first_ or _third classes_ described in chapter x., if in winter, a storm of the _first class_ only. if the elevation is _very_ sudden and considerable, the storm will probably be _severe_. the philosophy of this, according to my present apprehension of it, is, that these storms present an _extended easterly front_--_settle very near the earth_--and _have a rapid progress_--thus accumulating the atmosphere somewhat, in advance of them. d. _as to its fall before storms without previous rise._--this is always very regular before the second class of storms, or polar belts of showers and storms. it is very fairly exemplified in the table from reid, on page . the barometer, so far as i have opportunity to observe, does not rise from a stationary position on the approach of this class of storms. at the commencement of heated, summer, dry terms, my barometer has most frequently ranged at about . , and gradually, but slowly, fallen below inches before the belt of showers arrived, and the term closed. the fourth rule of dalton (meteorology, page ) indicates a similar law in england. it is as follows: "in summer, after a long continuance of fair weather, with the barometer high, it generally falls gradually, and for one, two, or more days, before there is much appearance of rain. if the fall be sudden and great for the season, it will probably be followed by thunder." d. _it falls frequently and considerably without rain._--this is owing to the fact that _all_ regular, periodic efforts at condensation do not result in rain. the second, third, and fourth classes of storms described, may not (as we have said) _be sufficiently active to precipitate_, although the _series of phenomena_ (including the fall of the barometer) may be, in other respects, perfect. such an instance may be found in reid's table, on page , and on the th of the month. but the fall in such cases is not as great, unless the wind be violent. th. _it rises during considerable gales._--but these are of the kind so often alluded to--viz., the n. w., in the northern hemisphere, and the s. w., in the southern; and the _philosophy_ of it has been explained, and is observable. with these explanations, the reader will be able to understand, and practically apply, the barometric changes, in connection with the other phenomena, in forming an opinion of the weather. _the thermometer_ is also an auxiliary. it _rises_, during the winter half of the year, in the _advance portion of the storm_, and falls when it passes off again; and the reverse is true, as we have seen, when its range is very high in summer. it is, therefore, to some extent, a useful auxiliary, although of minor importance. _the hygrometer_ is of less importance still. it is not in general use as a practical guide to the changes of the weather, and does not deserve to be. a question, which has been much mooted, deserves a passing notice in this connection--viz., whether our climate has gradually become ameliorated and milder on the eastern part of our continent, since its settlement. i have not space left for its discussion. humboldt (aspects of nature, page ) is of opinion that there has been no material change. he says: "the statements so frequently advanced, although unsupported by measurements, that since the first european settlements in new england, pennsylvania, and virginia, the destruction of many forests on both sides of the alleghanys, has rendered the climate more equable--making the winters milder and the summers cooler--are now generally discredited. no series of thermometric observations worthy of confidence extend further back, in the united states, than seventy-eight years. we find, from the philadelphia observations, that from to , the mean annual heat has hardly risen °. fahrenheit--an increase that may fairly be ascribed to the extension of the town, its greater population, and to the numerous steam-engines. this annual increase of temperature may also be owing to accident, for in the same period i find that there was an increase of the mean winter temperature of ° fahrenheit; but, with this exception, the seasons had all become somewhat warmer. thirty-three years' observation, at salem, in massachusetts, show scarcely any difference, the mean of each one oscillating within ° of fahrenheit, about the mean of the whole number; and the winters of salem, instead of having been rendered more mild, as conjectured, from the eradication of the forests, have become colder, by ° fahrenheit, during the last thirty-three years." the facts hereinbefore stated show that there is nothing like a _regular_ amelioration; that the seasons differ during the same decade, and different decades. the cold decade, from to , has not been reproduced. but it may be, and we know not how soon. since that period there has certainly been a change--for even the cold period from to did not equal that from to , nor indeed those of to or to . but as these variations, so far as we are enabled to judge, depend upon the varying influence of the sun's rays, and of volcanic action, it is impossible to say that equally cold periods will not return, during the latter half of this century. if the influence of the sun was constant, and volcanic action regular, two causes would tend to modify the seasons: st. the exposure of the surface to a more effective action of the solar rays, by a removal of the forests, and by drainage. that such action would be more effective upon a surface thus uncovered and drained, can not be doubted. d. _the movement of the area of magnetic intensity, and the magnetic pole, to the west._--there is such a movement, and its progress can be measured by the increase of declination on the east of it, and its decrease on the west. and the effect of it on climate is unquestionable. in all probability it has had an influence upon ours; and a removal of that area and pole still further west-- ° or °--would change the location of the concentrated trade, and the gulf stream, and restore to greenland the fertility she once had, and which the faroe islands now enjoy. and, on the other hand, its removal as far east of its present position would again depopulate greenland, and render it again inaccessible. but i can not pursue this subject. finally, assistance may be derived from the occasional, although imperfect, accounts of the state of the weather elsewhere, which the newspapers afford. i have been much indebted to the associated press of new york for intelligence contained in their telegraphic reports. occasionally they have been very full and instructive. on this point, however, there is less of reality in the present than of hope in the future. the time must come when the collection and dissemination of meteorological truth, will be deemed an object of national importance, and national duty. population is increasing, by immigration and propagation, in a rapidly progressive ratio. there has been great danger that it would outrun agricultural production. a short crop this year would have been disastrous to our prosperity--and the danger was imminent. every description of business, and every financial circle, felt that fever of anxiety it was so well calculated to induce. the importance of extended agricultural production, and the dependence of all classes upon its success, are now in a greater measure appreciated; and none can fail to see the value of a correct understanding of the weather to the agriculturist, how short-sighted soever they may be, in relation to its direct influence upon their own prosperity and happiness. our country is, physically, a most favored one. the facts disclosed or alluded to in this volume show that it is without a parallel on the face of the globe; and our facilities for meteorological observation, and the ascertainment and practical application of meteorological truth, are equally pre-eminent. the great extent and unbroken surface of the eastern portion of the continent; its excessive supply of magnetism and atmospheric currents, and the consequent marked character of the phenomena; the existence and prospective increase of telegraph lines over most of its surface; the homogeneous and energetic character of a population united, upon so large a surface, under one government; the freedom of that government from debt, and the excess of its revenue; the possession of a national observatory, with a competent philosopher at its head; and a national institution, liberally endowed, and adapted to the collection and diffusion of practical and scientific intelligence, give us an opportunity and a capacity for connected observation and investigation, and an ability to profit by it, that no other nation can boast. we have, too, a just national pride. our exploring ships have penetrated and made discoveries in both hemispheres, and our travelers have visited successfully every clime; and thus our national interests, and obligations, and pride, demand an organization, practical and permanent, in relation to this subject, and the time will come when we shall have it. when that time comes--when the present _limited horizon_ of each of us is _practically extended over the entire country_--and when the actual state of the weather over every part of it is known, at the same time, to the inhabitants of every other, and every where _read in the light of a correct philosophy_, prognostication will be comparatively simple and certain; and a progress will have been made, productive of an amount of pecuniary, intellectual, and social benefit to the people, which can not be overestimated. may it come before the shadows of the night of death have gathered around us, that we may have a more perfect view of that atmospheric machinery which distinguishes our planet from others, and is, with such infinite wisdom, adapted to make it a fit habitation for man! the end. appendix. since this work was completed i have received a very valuable publication, entitled, the "army meteorological register." it is a compilation of the observations made by the officers of the medical department of the army, at the military posts of the united states, from to inclusive, prepared under the supervision of the surgeon-general, and published by direction of the secretary of war. to this, there is appended a report or general review of the prominent features of american climatology, so far as the basis afforded by the published observation of the army medical bureau would warrant positive deduction, by mr. lorin blodget, a distinguished meteorologist, accompanied by temperature and rain charts, for each of the four seasons;--exhibiting the various local differences and peculiarities relative to temperature and precipitation in each. these local differences and peculiarities and contrasts are deduced and delineated by mr. blodget with much ability. he was fettered, however, by the prevailing calorific theories, and the unfortunate practice of grouping the phenomena into means for the seasons, spring, summer, autumn, and winter, which grouping is arbitrary, and comparatively uninstructive. hence, he failed to discover what the tables and summaries most clearly disclose--the principles and system unfolded in the foregoing work. but the summaries of this register contain observations made at posts in western and southwestern texas, in kansas and nebraska, and in new mexico and california, where there has been a dearth of such observations hitherto, and enable me to demonstrate, more conclusively, and i think so that none can fail to understand it, the truth of the philosophy i have endeavored to exhibit. to do this, i will take a _year_,--divide it into two seasons, the periods of northern and southern transit, the only natural and correct division--and note the phenomena in each, as each progresses. and i will take the year , because that is the last year for which the record of observation is complete; because it had marked peculiarities which are remembered; and because i have alluded to those peculiarities, and those allusions should be confirmed or disproved by the record. unless i mistake exceedingly, the confirmation will be found signal and convincing. i have assumed, pp. , , that the transits were greater in some seasons than others; that the drought of was owing to an extreme northern transit, or to an extension west of the concentrated counter-trade, or both, leaving us less supplied with moisture than usual. in point of fact, it appears from these observations that it resulted from _both_ causes, operating _connectedly_; and the annals of science rarely furnish a more striking instance of analogical inference proved true by subsequent investigation. commencing then with the commencement of the northern transit about the st of february, we are enabled to trace the then location of our concentrated trade, and its subsequent progress to the north till august, and its influence upon temperature and precipitation. and we can also trace the situation during the same period, of the intervening drought, and the inter-tropical belt of rains, and the extension of the latter north over florida and the cotton-planting states. on the st of february, , our counter-trade was somewhat more concentrated on its extreme winter curve, over the southern states, than usual. its line of excess reached up from fort brooke, on the peninsula of florida, to the northwest, a little east of pensacola on the gulf, cutting mount vernon arsenal north of pensacola, and extending thence north-westwardly on to eastern louisiana, and curving thence and passing n. e. or e. n. e., to the atlantic, about the waters of the chesapeake bay. it thinned out to the west over new orleans and baton rouge, supplying them moderately, but did not extend to the forts of texas on the west, nor the posts in the indian territory at the n. w. it was east of fort towson, which is the south-eastern one. it did not reach st. louis on the north, nor extend north of the ohio river, as will appear from the tables hereinafter given. the following cut shows substantially its situation on the st of february. [illustration] now, during the month of january, we find the following state of things. _under_ this concentrated trade, the temperature was above the mean, even if forts monroe and mchenry on the atlantic are included; but mr. blodget discredits their returns, and some others which do not conform to general results. on the west and north of its curving line, both precipitation and temperature were below the mean. under the counter trade, we have the following stations, with their actual and mean temperature. i have inserted the temperature for several subsequent months, to show a depression in april. table i. ------------------------------------------------------------------------- | lat. | lon. | jan. | feb. | mar. | april.| may. | ------------------------------------------------------------------------| fort moultrie | . | . | . | . | . | . | . | mean of yrs.| | | . | . | . | . | . | fort pierce | . | . | . | . | . | . | . | mean of yrs. | | | . | . | . | . | . | fort meade | . | . | . | . | . | . | . | mean of yrs. | | | . | . | . | . | . | fort brooke | . | . | . | . | . | . | . | mean of yrs.| | | . | . | . | . | . | fort myers | . | . | . | . | . | . | . | mean of yrs. | | | . | . | . | . | . | key west | . | . | . | . | . | . | . | mean of yrs.| | | . | . | . | . | . | fort barrancas | . | . | . | . | . | . | . | mean of yrs.| | | . | . | . | . | . | mt. vernon ars'l| . | . | . | . | . | . | . | mean of yrs.| | | . | . | . | . | . | baton rouge | . | . | . | . | . | . | . | mean of yrs.| | | . | . | . | . | . | ------------------------------------------------------------------------- ------------- june. | july. ------------- . | . . | . . | . . | . . | . . | . . | . . | . . | . . | . . | . . | . . | . . | . . | . . | . . | . . | . ------------- it will be seen that the temperature was above the mean in january at every post except baton rouge, and there it was at the mean. we shall see hereafter that baton rouge was near its western line. under this trade during this month, and at the same posts, the fall of rain was as follows, compared with the mean:-- table ii. -------------------------------------------------------------- | january. | febr'y. | march. | -------------------------------------------------------------- | . | mean.| . | mean.| .| mean.| -------------------------------------------------------------- key west. | . | . | . | . | . | . | fort myers. | . | . | . | . | . | . | " brooke. | . | . | . | . | . | . | " mead. | . | . | . | . | . | . | " pierce. | . | . | . | . | . | . | " barrancas. | . | . | . | . | . | . | mt. vernon ars'l | . | . | . | . | . | . | baton rouge. | . | . | . | . | . | . | fort moultrie. | . | . | . | . | . | . | -------------------------------------------------------------- ------------------------------------------- | april. | may. | june.| july. | .| mean.| . | mean.| | ------------------------------------------- | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . ------------------------------------------- it will be observed that in february the counter-trade and extra-tropical belt had moved up from key west, and a drought, which sometimes intervenes between the concentrated counter-trade and the inter-tropical belt, appeared there in february and march. in april, the inter-tropical belt appeared at that point, and went on increasing till september. as the counter-trade commenced moving north in february, an increased precipitation above the mean commenced at all the more southern stations under the concentrated-trade--an earnest of that irregularity which followed, and marked the season as the most excessive of the century. in march, the intervening drought appeared at the other posts on the peninsula, and also at fort moultrie, followed _much more closely than usual_, by the inter-tropical belt of rains. in april, the drought appeared at fort barrancas and mount vernon arsenal (the wave of precipitation having moved to the west), and slightly in comparison at baton rouge. if now we look at the condition of things, _west_ and _north_ of the curving line of concentrated trade, from fort brown, at the mouth of the rio grande, in south-western texas, through that state, the indian territory, arkansas, missouri, kentucky, and northern pennsylvania, to the atlantic, we find the thermometer every where in january below the mean. the following table will show this, and the precipitation for that month and february:-- table iii. ------------------------------------------------------------------------ | january. | february. | march. | |-----------------------------------------------| | . | mean. | . | mean. | . | mean. | -----------------------------------------------------------------------| _western texas._ | | | | | | | fort brown | . | . | . | . | . | . | " ewell | . | . | . | . | . | . | " inge | . | . | . | . | . | . | | | | | | | | _indian territory._ | | | | | | | fort towson. | . | . | . | . | . | . | forts gibson, washita, | | | | | | | and arbuckle, in much| | | | | | | the same proportions.| | | | | | | | | | | | | | _arkansas._ | | | | | | | fort smith. | . | . | . | . | . | . | | | | | | | | _missouri._ | | | | | | | st. louis arsenal. | . | . | . | . | . | . | | | | | | | | _kentucky._ | | | | | | | newport barracks. | . | . | . | . | . | . | | | | | | | | _pennsylvania._ | | | | | | | allegheny arsenal. | . | . | . | . | . | . | | | | | | | | _delaware._ | | | | | | | fort delaware | . | . | . | . | . | . | | | | | | | | _new york harbor._ | | | | | | | fort columbus. | . | . | . | . | . | . | ------------------------------------------------------------------------ -------------------------------------- | rain in january. | rain in february. -------------------------------------- | . | . | . | . | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . -------------------------------------- we find, also, from this and table first, that every where, except at fort brown, and upon the atlantic coast, the temperature had risen above the mean in february. the situation of the belt which supplied the western coast in winter, and its excess of precipitation, are also represented upon the cut. the intervening area was not without counter-trade and precipitation--the latter, of course, greatest over the area of intensity--but they were _comparatively_ less, as the tables will show. the following cut and table show the situation of the concentrated counter-trade in march. [illustration] table iv. ------------------------------------------------------------------------ | jan.|febr.| mar.| apr.| may.|june.|july. ------------------------------------------------------------------------ fort barrancas, pensacola bay| . | . | . | . | . | . | . mean. | . | . | . | . | . | . | . baton rouge, louisiana | . | . | . | . | . | . | . mean. | . | . | . | . | . | . | . fort towson, indian territory| . | . | . | . |recr'd stops here. mean. | . | . | . | . | | | fort gibson, indian territory| . | . | . | . | . | . | . mean. | . | . | . | . | . | . | . fort smith, arkansas | . | . | . | . | . | . | . mean. | . | . | . | . | . | . | . st. louis arsenal | . | . | . | . | . | . | . mean. | . | . | . | . | . | . | . newport barracks, kentucky | . | . | . | . | | | (no mean given.) | | | | | | | ------------------------------------------------------------------------ we see from this table that its focus had extended west in florida over fort barrancas, and over baton rouge in louisiana; n. w. to forts towson and gibson in the indian territory, and smith in arkansas; north to st. louis arsenal at st. louis, and to newport barracks in kentucky; but it was spread over a larger surface east of the mountains. its greatest progress for the month, was a west and north-west progress. in april, we find it had progressed rapidly west and north-west, and its position is shown by the following cut and table. [illustration] table v. ------------------------------------------------------------------------ | jan.|febr.| mar.| apr.| may.|june.|july. ------------------------------------------------------------------------ fort riley, kansas. | . | . | . | . | . | . | . fort leavenworth, kansas. | . | . | . | . | . | . | . mean | . | . | . | . | . | . | . alleghany arsenal, pittsburgh | . | . | . | . | . | . | . mean | . | . | . | . | . | . | . fort columbus, new york harbor| . | . | . | . | . | . | . mean | . | . | . | . | . | . | . fort independence, boston | . | . | . | . | . | . | west point. | . | . | . | . | . | . | mean | . | . | . | . | . | . | ---------------------------------------------------------------------- we see, too, that both east and west of the mountains, its focus of precipitation was one month in advance of the mean. at all the stations where the greatest fall was in march, it should have been in april, and the fall at those points was greatly in excess of the usual quantity. and the same was true of stations reached in april. the concentrated trade, instead of spreading out, and precipitating over the whole south-eastern portion of the continent (its normal condition), was gathered into a wave of greater volume, resulting in greater precipitation, and was rapidly hastening its curve to the west over texas, and to the north-west over the indian territory, and northward on its usual curve to the north and east of them. the observations for april disclose another singular and instructive condition. the temperature, that had every where been above the mean in march, fell below it in april under the concentrated trade. and snow fell on three days in some localities, and four in others. along the ohio river, it fell to the depth of to inches on the th, and east of the mountains to a greater depth on the th, one day later. it fell to the depth of inches at marietta on the th also. dr. hilldreth, american journal of science for march, , says:-- "it is a singular fact that the deepest snow, inches, fell on the th of april, and at the head waters about pittsburg over a foot. also, on the th of the month, at marietta, inches, a very rare occurrence." this depression of the temperature was quite general, but the fall of snow was local. the latter was north of a line drawn from fort laramie, at the base of the rocky mountains, in an e. s. e. direction--north of forts kearney and leavenworth, and of st. louis, but south of newport barracks in kentucky, and from thence to the atlantic. snow fell at every station north of this line, at no station south of it. the depression of temperature, however, was experienced over the continent, east of the rocky mountains, under, and south of, the belt of precipitation. now what occasioned this general depression of temperature, and local fall of snow? it will not do to say, as perhaps some calorific theorist may be inclined to say, because the concentrated trade had been carried up where it was cold, a month too soon; or that the sun had heated the land in advance of it, and drawn it up. for, st, it might be asked how, if it was warm enough to draw it up, could it be cold enough to make it snow; or, d, how happened it to start, when, as we have seen, it was warmer than the mean under it, and colder than the mean to the north and west of it, when it commenced its journey? but again, it snowed at posts north of the line, while the thermometer remained above the mean; and the thermometer fell below the mean down to fort brown in south-western texas, and at key west in the southern part of florida; and what is more remarkable still, at key west, fort barrancas, and every other south-eastern station, except forts brooke and moultrie, it not only fell below the _mean_ of the month, but _below the actual temperature of march_. (see table i.) at forts brooke and moultrie it did not rise above that temperature. west of the rocky mountains the depression was not felt; nor at stations north, or north-west of the belt of precipitation. it is obvious, the calorific theory can furnish no rational explanation of this matter; for the reason that, whatever the cause, it operated not only under, but south, and far south of the belt of precipitation. it could not have been spots upon the sun, or other general cause, for then it would have operated in new mexico and california, and at the north-western stations. it operated most intensely in florida and the south-eastern states, which approach most nearly the volcanic areas of south america and the west indies. i believe it to have been occasioned by volcanic action affecting the local magnetism of our intense area; but it is a most important development, and should be thoroughly investigated. we may find in it the key to the mysterious, but unquestionable, influence of volcanic upon magnetic action; and i hope the distinguished surgeon-general will cause the records of that month to be published "in extenso." in may and june, the trade became more concentrated, a perfectly developed belt from the rio grande to the lakes and british possessions, and doubtless to the atlantic, with every where a central focus of excessive precipitation, gathering to itself in one vast wave the current that should have been spread out over the whole country; and leaving every where on its eastern and southern borders, down to the northern edge of the inter-tropical belt of rains--(which extended up to lines drawn from baton rouge to charleston)--a _perfectly well developed_ and _defined drought_. that drought will long be remembered. the following cuts show, approximately, the location of the belt of precipitation and drought for those months, and the table which follows will show their correctness. the tables also show that this wave was occasionally a double, or divided one--evinced by an intervening _partial_ precipitation. tables iv., v., and vi., also show the commencement of the drought at the several stations, as the wave moved to the west and north. [illustration: may.] [illustration] table vi. jan. febr. mar. apr. may. june. july. aug. sept. fort brown . . . . . . . . . mean . . . . . . . . . ringgold barracks . . . . . . . . . mean . . . . . . . . . fort merrill . . . . . . . . . mean . . . . . . . . . fort duncan . . . . . . . . . mean . . . . . . . . . fort inge . . . . . . . . . mean . . . . . . . . . fort mckavet . . . . . . . . . " belknap . . . . . . . . . " massachusetts, northern new mexico . . . . . fort kearney . . . . . . . . . mean . . . . . . . . . fort laramie . . . . . . . . . mean . . . . . . . . . fort ridgley . . . . . . . . . " snelling . . . . . . . . . mean . . . . . . . . . fort ripley . . . . . . . . . mean . . . . . . . . . fort mackinac . . . . . . . . . mean . . . . . . . . . fort brady . . . . . . . . . mean . . . . . . . . . fort niagara . . . . . . . . . mean . . . . . . . . . but the belt of trade continued its progress to the west and north, and during the months of july and august the drought extended in both directions, reaching, in august, from mississippi, alabama, georgia, and south carolina, to the lakes, and from the rocky mountains to the atlantic. its position is shown by the following cut, and the position of the belt of precipitation by the following table. [illustration] table vii. _situation of the focus of precipitation in july and august._ ----------------------------------------------------------- | june.| july.| aug. | sept.| oct. ----------------------------------------------------------- _new mexico._ | | | | | | | | | | fort thorne | . | . | . | . | . albuquerque | . | . | . | . | . santa fe | . | . | . | . | . fort defiance | . | . | . | . | . " yuma | . | . | . | . | . san diego | . | . | . | . | . fort snelling, minnesota | . | . | . | . | . " brady | . | . | . | . | . " mackinac | . | . | . | . | . ----------------------------------------------------------- i have not space for all the comment which this exposition is calculated to induce. the reader will not only find in it an explanation of the extraordinary character of the summer of , but will see from the _means_, that it was but an _excessive development_ of an annual phenomenon,--the progress of a concentrated counter-trade. it is not necessary to follow with particularity the return transit. it required no great degree of sagacity to predict, at the time, that the drought would continue in the vicinity of new york till about the th of september. the return of the belt to that latitude, was not to be expected before that time, and the drought continued, in fact, until the th of september. its return progress was slow, and it was every where behind time. the autumn was warm, and so, indeed, were december and january, west of the area of magnetic intensity, although upon, and east of it, there was a depression in december. the retreating but lingering edge of counter-trade, with its excess of snow for the season, caught the iron horse, with its train and passengers, upon the prairies of the west, and laid its embargoing hands upon them. few, if any, can have forgotten the thrilling accounts which reached us from that section, of the sufferings endured by those who were thus embargoed for days and nights, far from the comfortable habitations of their fellow men. but the return transit, though slow, was extreme, and february and march were exceedingly cold for the season. the transit to the north, again, did not commence as early as usual, and the spring was backward, and the summer cool. both were without irregularity, and the season was productive. the following table exhibits the temperature on a line of posts, running north and south at the west, during the winter months of , and will illustrate what has been said. table viii. --------------------------------------------------- . |january |february.| march.| april. --------------------------------------------------- key west | . | . | . | . mean | . | . | . | . fort snelling | . | . | . | . mean | . | . | . | . fort kearney | . | . | . | . mean | . | . | . | . fort laramie | . | . | . | . mean | . | . | . | . fort arbuckle | . | . | . | . mean | . | . | . | . fort belknap | . | . | . | . mean | . | . | . | . fort chadbourne | . | . | . | . mean | . | . | . | . fort mckavitt | . | . | . | . mean | . | . | . | . fort merrill | . | . | . | . mean | . | . | . | . fort brown | . | . | . | . mean | . | . | . | . fort inge | . | . | . | . mean | . | . | . | . --------------------------------------------------- the return transit to the south for this winter, - , has been an extreme one. it is too early yet (feb. th) to write its history, but the extreme southern transit is as obvious as the unusual severity of the cold. the rains which usually fall upon the southern states are precipitated further south upon the west indies, and threaten a deterioration of their sugar crop. the snow, and cold winds, and ice, of the middle latitudes, are felt even in florida. our sheet of counter-trade has been exceedingly thin, and the barometer has ranged, in fair weather, much below the mean. occasional, and for a part of the time, _weekly_ periods of an increase of its volume, with a corresponding elevation of the barometer, and a consequent moderation of the intense cold, and a storm, have occurred. but those periods have been few and brief. no regular thaw has yet occurred. from the th of december to this date, at norwalk, there have been but two periods when the wind has blown from the south-west with sufficient force to stir the limbs of the trees. there has been no wind from south of that point, or east of north-east; and even our storm-winds, with one exception, have been north of north-east--owing to the situation of the focus of precipitation far to the south of us--and there is reason to fear that a cold summer like those of and may follow. if this extreme transit is owing to defect in the influence of the sun, from spots, or other causes, such will probably be the result. if from volcanic action at the south, the influence of that action may cease, and a rapid return transit, and an ordinary season, may follow. believing in the laws of periodicity in relation to the weather and disease, i planted an early kind of corn (the dutton), in , and had a crop when few around me succeeded. we must watch this return transit, with hope, indeed, but not without fear, and be wise in time. there is a mass of other evidence in these summaries which shows the truth of what i have written. there is not a deduction of mr. blodget which it will not explain. the ascent of the summer lines of temperature to the west is explained by the diminution of magnetic intensity. their descent in winter by the location and attractions of the concentrated trade. the excess of precipitation in alabama and mississippi by the succession of summer and winter belts. that of the interior of the atlantic slope in summer, by the showers which fall upon the elevations; and of the coast, by the easterly storms and their attraction of the surface atmosphere of the ocean, at other seasons. but i cannot further particularize. even the influence of the spots is clearly demonstrated by the observations at _interior stations_, which were unaffected by contiguous oceans or elevations. at forts washita, gibson, scott, smith, and others, the years and were below the mean. all that evidence, and those deductions, however, i must pass by for want of space, and take leave of the subject. footnotes: [ ] see the diagram for summer at page . [ ] law of storms, p. . [ ] kearakakua bay (called cavrico above), is on the s. w. side of the island, and the trade was reversed during the day by the cloud condensation inland. [ ] lieutenant wilkes spent twenty days upon the top of this or an adjoining mountain, and his observations there will be alluded to in another connection. [ ] all attempts to produce this result by the sudden exhaustion of air about the chickens in receivers, or shooting them from cannons, have failed, and no patent for a chicken-picker has been applied for. [ ] a meter is yard, and . of a yard. [ ] see his map, accompanying the geography of the sea. [ ] see am. jour. of science, new series, vol. . p. . [ ] their estimate was to miles. [ ] since the text was in type, and, as might have been anticipated, we have intelligence confirmatory of this, from the cape de verde islands. the inter-tropical belt of rains has not moved as far north as the northern islands--they have had no rain--and the people are in a starving condition. transcriber's notes: passages in italics are indicated by _italics_. punctuation has been corrected without note. the following misprints have been corrected: "appearnces" corrected to "appearances" (page ) "faroday's" corrected to "faraday's" (page ) "gentleman" corrected to "gentlemen" (page ) "two" corrected to "too" (page ) "surise" corrected to "sunrise" (page ) "acion" corrected to "action" (page ) "stanta corrected to "santa" (page ) "augugst" corrected to "august" (page ) "baloon's" corrected to "balloon's" (page ) "mannner" corrected to "manner" (page ) " " corrected to " " (page ) "sevententh" corrected to "seventeenth" (page ) "maner" corrected to "manner" (page ) "particulary" corrected to "particularly" (page ) "are are" corrected to "are" (page ) "iso-theral" corrected to "iso-thermal" (page ) "the the" corrected to "the" (page ) "phenonema" corrected to "phenomena" (page ) "calorifice" corrected to "calorific" (page ) other than the corrections listed above, inconsistencies in spelling and hyphenation have been retained from the original. tables throughout this text version have been adjusted for readability. transcriber's notes: text enclosed by underscores is in italics (_italics_), and text enclosed by equal signs is in bold (=bold=). this book contains macmillan & co.'s september book catalogue, which follows the index for the main book. additional transcriber's notes are at the end. michael faraday. [illustration] [illustration: _michael faraday_ _from a photograph by john watkins. parliament street._] michael faraday. by j. h. gladstone, ph.d., f.r.s. _third edition, with portrait._ london: macmillan and co. . [_the right of translation and reproduction is reserved._] london: r. clay, sons, and taylor, printers, bread street hill. preface. shortly after the death of michael faraday, professor auguste de la rive, and others of his friends, gave to the world their impressions of his life, his character, and his work; professor tyndall drew his portrait as a man of science; and after a while dr. bence jones published his biography in two octavo volumes, with copious extracts from his journals and correspondence. in a review of this "life and letters" i happened to mention my thought of giving to the public some day my own reminiscences of the great philosopher; several friends urged me to do so, not in the pages of a magazine, but in the form of a little book designed for those of his fellow-countrymen who venerate his noble character without being able to follow his scientific researches. i accepted the task. professor tyndall and dr. bence jones, with messrs. longman, the publishers, kindly permitted me to make free use of their materials; but i am indebted to the corporation of the trinity house, and to many friends, for a good deal of additional information; and in compiling my book i have preferred, where practicable, to illustrate the character of faraday by documents or incidents hitherto unpublished, or contained in those sketches of the philosopher which are less generally known. it is due to myself to say that i had pretty well sketched out the second part of this book before i read m. dumas' "eloge historique." the close similarity of my analysis of professor faraday's character with that of the illustrious french chemist may perhaps be accepted as an additional warrant for the correctness of our independent estimates. preface to second edition. the very favourable reception which my book has met with, both from the press and the public, seems to call for my grateful acknowledgment on the issue of a second edition. in revising the former, i have added some further particulars about faraday, especially in regard to "his method of working;" and an engraving from a photograph by watkins, which best recalls to my recollection the features and the usual expression of the genial philosopher. contents. sect. page i.--the story of his life ii.--study of his character iii.--fruits of his experience iv.--his method of working v.--the value of his discoveries supplementary portraits appendix:--list of honorary fellowships, etc. index michael faraday. section i. the story of his life. at the beginning of this century, in the neighbourhood of manchester square, london, there was an inquisitive boy running about, playing at marbles, and minding his baby-sister. he lived in jacob's well mews, close by, and was learning the three r's at a common day-school. few passers-by would have noticed him, and none certainly would have imagined that this boy, as he grew up, was to achieve the truest success in life, and to die honoured by the great, the wise, and the good. yet so it was; and to tell the story of his life, to trace the sources of this success, and to depict some of the noble results of his work, are the objects of this biographical sketch. it was not at jacob's well mews, but in newington butts, that the boy had been born, on september , , and his parents, james and margaret faraday, had given this, their third child, the unusual name of michael. the father was a journeyman blacksmith, a skilful workman who, in spite of poverty and feeble health, strove to bring up his children in habits of industry and the love of god. of course young michael must soon do something for his living. there happened to be a bookseller's shop in blandford street, a few doors from the entrance to the mews, kept by a mr. riebau, an intelligent man, who is said to have had a leaning to astrology; and there he went as errand boy when thirteen years old. many a weary walk he had, carrying round newspapers to his master's customers; but he did his work faithfully; and so, after a twelvemonth, the bookseller was willing to take him as an apprentice, and that without a premium. now, a boy in a bookseller's shop can look at the inside as well as the outside of the books he handles, and young faraday took advantage of his position, and fed on such intellectual food as watts's "improvement of the mind," mrs. marcet's "conversations on chemistry," and the article on "electricity" in the _encyclopædia britannica_, besides such lighter dishes as miss burney's "evelina;" nor can we doubt that when he was binding lyons' "experiments on electricity," and boyle's "notes about the producibleness of chymicall principles," he looked beyond the covers.[ ] and his thirst for knowledge did not stop with reading: he must see whether mrs. marcet's statements were correct, and so, to quote his own words, "i made such simple experiments in chemistry as could be defrayed in their expense by a few pence per week, and also constructed an electrical machine, first with a glass phial, and afterwards with a real cylinder, as well as other electrical apparatus of a corresponding kind." he kept too a note-book called "the philosophical miscellany," intended, he tells us, "to promote both amusement and instruction, and also to corroborate or invalidate those theories which are continually starting into the world of science;" and miscellaneous indeed were the scraps he gathered from the magazines of the time. one day, early in , walking somewhere in the neighbourhood of fleet street, he saw in a shop-window a bill announcing that lectures on natural philosophy were delivered by mr. tatum, at , dorset street, at eight in the evening, price of admission one shilling. he wanted to hear these lectures. his master's permission was obtained, but where was the money to come from? the needful shillings were given him by his elder brother, robert, who earned them as a blacksmith; and so michael faraday made his first acquaintance with scientific lectures. and not with lectures only, for tatum's house was frequented by other earnest students, and lifelong friendships were formed. among these students was benjamin abbott, a young quaker, who had received a good education, and had then a situation in a city house as confidential clerk. with him faraday chatted on philosophy or anything else, and happily for us he chatted on paper, in letters of that fulness and length which the penny post and the telegraph have well-nigh driven out of existence; and happily for us, too, abbott kept those letters, and dr. bence jones has published them. they are wonderful letters for a poor bookseller's apprentice; they bear the stamp of an innate gentleman and philosopher. long afterwards, when benjamin abbott was an old man, he used to tell how faraday made his first experiments in the kitchen of his house, and delivered his first lecture from the end of that kitchen table. the electrical machine made by him in those early days came into the possession of sir james south, and now forms one of the treasures of the royal institution. as the eager student drank in the lectures of tatum, he took notes, and he afterwards wrote them out carefully in a clear hand, numbering and describing the different experiments that he saw performed, and making wonderfully neat drawings of the apparatus, in good perspective. these notes he bound in four volumes, adding to each a copious index, and prefixing to the first this dedication to his master:-- "to mr. g. riebau. "sir, "when first i evinced a predilection for the sciences, but more particularly for that one denominated electricity, you kindly interested yourself in the progress i made in the knowledge of facts relating to the different theories in existence, readily permitting me to examine those books in your possession that were in any way related to the subjects then occupying my attention. to you, therefore, is to be attributed the rise and existence of that small portion of knowledge relating to the science which i possess, and accordingly to you are due my acknowledgments. "unused to the arts of flattery, i can only express my obligations in a plain but sincere way. permit me, therefore, sir, to return thanks in this manner for the many favours i have received at your hands and by your means and believe me, "your grateful and obedient servant, "m. faraday." now there happened to be lodging at mr. riebau's a notable foreigner of the name of masquerier. he was a distinguished artist, who had painted napoleon's portrait, and had passed through the stirring events of the first french revolution, not without serious personal danger, and was now finding a refuge and a home in london. he was struck with the intelligence of the apprentice, whose duty it was to do various offices for him; and he lent the young man his books, and taught him how to make the drawings in perspective which have already been alluded to. but the lectures in dorset street were not the only ones that michael faraday attended; and as the royal institution is the central scene of all his subsequent history, we must pay a mental visit to that building. turning from the busy stream of piccadilly into the quiet of albemarle street, we see, in a line with the other houses, a large grecian façade with fourteen lofty pilasters. between these are folding doors, which are pushed open from time to time by grave-looking gentlemen, many of them white-headed; but often of an afternoon, and always on friday evening during the season, the quiet street is thronged with carriages and pedestrians, ladies and gentlemen, who flock through these folding doors. entering with them, we find ourselves in a vestibule, with a large stone staircase in front, and rooms opening on the right and left. the walls of these rooms are lined with myriads of books, and the tables are covered with scientific and other periodicals of the day, and there are cabinets of philosophical apparatus and a small museum. going up the broad staircase and turning to the right, we pass through an ante-room to the lecture theatre. there stands the large table, horseshoe-shaped, with the necessary appliances for experiments, and behind it a furnace and arrangements for black-board and diagrams; while round the table as a centre range semicircular seats, rising tier above tier, and surmounted by a semicircular gallery, the whole capable of seating persons. on the basement is a new chemical laboratory, fitted up with modern appliances, and beyond it the old laboratory, with its furnaces and sand-bath, its working tables and well-stored shelves, flanked by cellars that look like dark lumber-rooms. a narrow private staircase leads up to the suite of apartments in which resides the director of the house. such is the royal institution of great britain, incorporated by royal charter in the year , "for the diffusing knowledge and facilitating the general introduction of useful mechanical inventions and improvements, and for teaching, by courses of philosophical lectures and experiments, the application of science to the common purposes of life;"--with the motto, "illustrans commoda vitæ." fifty or sixty years ago the building was essentially what it is now, except the façade and entrance, and that the laboratory, which was considered a model of perfection, was even darker than at present, and in the place of the modern chemical room there was a small theatre. the side room, too, was fitted up for actual work, though even at mid-day it had to be artificially lighted; and beyond this there was, and still is, a place called the froggery, from a certain old tradition of frogs having been kept there. the first intention of the founders to exhibit useful inventions had not been found very practicable, but the place was already famous with the memories of rumford and young; and at that time the genius of sir humphry davy was entrancing the intellectual world with brilliant discoveries, and drawing fashionable audiences to albemarle street to listen to his eloquent expositions. among the customers of the bookseller in blandford street was a mr. dance, who, being a member of the royal institution, took young faraday to hear the last four public lectures of davy. the eager student sat in the gallery, just over the clock, and took copious notes of the professor's explanations of radiant matter, chlorine, simple inflammables, and metals, while he watched the experiments that were performed. afterwards he wrote the lectures fairly out in a quarto volume, that is still preserved--first the theoretical portions, then the experiments with drawings, and finally an index. "the desire to be engaged in scientific occupation, even though of the lowest kind, induced me," he says, "whilst an apprentice, to write, in my ignorance of the world and simplicity of my mind, to sir joseph banks, then president of the royal society. naturally enough, 'no answer' was the reply left with the porter." on the th of october his apprenticeship expired, and on the next day he became a journeyman bookbinder under a disagreeable master--who, like his friend the artist, was a french _émigré_. no wonder he sighed still more for congenial occupation. towards the end of that same october sir humphry davy was working on a new liquid which was violently explosive, now known as chloride of nitrogen,--and he met with an accident that seriously injured his eye, and produced an attack of inflammation. of course, for a while he could not write, and, possibly through the introduction of m. masquerier,[ ] the young bookseller was employed as his amanuensis. this, however, faraday himself tells us lasted only "some days;" and in writing years afterwards to dr. paris, he says, "my desire to escape from trade, which i thought vicious and selfish, and to enter into the service of science, which i imagined made its pursuers amiable and liberal, induced me at last to take the bold and simple step of writing to sir h. davy, expressing my wishes, and a hope that, if an opportunity came in his way, he would favour my views; at the same time i sent the notes i had taken of his lectures." davy, it seems, called with the letter on one of his friends--at that time honorary inspector of the models and apparatus--and said, "pepys, what am i to do? here is a letter from a young man named faraday; he has been attending my lectures, and wants me to give him employment at the royal institution--_what can i do?_" "do?" replied pepys; "put him to wash bottles: if he is good for anything, he will do it directly; if he refuses, he is good for nothing." "no, no," replied davy, "we must try him with something better than that." so davy wrote a kind reply, and had an interview with the young man upon the subject; in which, however, he advised him to stick to his business, telling him that "science was a harsh mistress, and, in a pecuniary point of view, but poorly rewarding those who devoted themselves to her service." he promised him the work of the institution, and his own besides. but shortly afterwards the laboratory assistant was discharged for misconduct, and so it happened that one night the inhabitants of quiet weymouth street were startled by the unusual apparition of a grand carriage with a footman, which drew up before the house where faraday lived, when the servant left a note from sir humphry davy. the next morning there was an interview, which resulted in the young aspirant for scientific work being engaged to help the famous philosopher. his engagement dates from march , , and he was to get _s._ per week, and a room in the house. the duties had been previously laid down by the managers:--"to attend and assist the lecturers and professors in preparing for, and during lectures. where any instruments or apparatus may be required, to attend to their careful removal from the model room and laboratory to the lecture-room, and to clean and replace them after being used; reporting to the managers such accidents as shall require repair, a constant diary being kept by him for that purpose. that in one day in each week he be employed in keeping clean the models in the repository, and that all the instruments in the glass cases be cleaned and dusted at least once within a month." the young assistant did not confine himself to the mere discharge of these somewhat menial duties. he put in order the mineralogical collection; and from the first we find him occupying a higher position than the minute quoted above would indicate. in the course of a few days he was extracting sugar from beet-root; but all his laboratory proceedings were not so pleasant or so innocent as that, for he had to make one of the worst smelling of all chemical compounds, bisulphide of carbon; and as davy continued to work on the explosive chloride of nitrogen, his assistant's career stood some chance of being suddenly cut short at its commencement. indeed it seems that before the middle of april he had run the gauntlet of four separate explosions. knowing that the liquid would go off on the slightest provocation, the experimenters wore masks of glass, but this did not save them from injury. in one case faraday was holding a small tube containing a few grains of it between his finger and thumb, and brought a piece of warm cement near it, when he was suddenly stunned, and on returning to consciousness found himself standing with his hand in the same position, but torn by the shattered tube, and the glass of his mask even cut by the projected fragments. nor was it easy to say when the compound could be relied on, for it seemed very capricious; for instance, one day it rose quietly in vapour in a tube exhausted by the air-pump, but the next day, when subjected to the same treatment, it exploded with a fearful noise, and sir humphry was cut about the chin, and was struck with violence on the forehead. this seems to have put an end to the experiments. nevertheless, in spite of disagreeables and dangers, the embryo philosopher worked on with a joyful heart, beguiling himself occasionally with a song, and in the evening playing tunes on his flute. the change in michael faraday's employment naturally made him more earnest still in the pursuit of knowledge. he was admitted as a member of the "city philosophical society," a fraternity of thirty or forty men in the middle or lower ranks of life, who met every wednesday evening for mutual instruction; and here is a contemporary picture of him at one of its debates:-- "but hark! a voice arises near the chair! its liquid sounds glide smoothly through the air; the listening muse with rapture bends to view the place of speaking, and the speaker too. neat was the youth in dress, in person plain; his eye read thus, _philosopher in grain_; of understanding clear, reflection deep; expert to apprehend, and strong to keep. his watchful mind no subject can elude, nor specious arts of sophists e'er delude; his powers, unshackled, range from pole to pole; his mind from error free, from guilt his soul. warmth in his heart, good humour in his face, a friend to mirth, but foe to vile grimace; a temper candid, manners unassuming, always correct, yet always unpresuming. such was the youth, the chief of all the band; his name well known, sir humphry's right hand. with manly ease towards the chair he bends, with watts's logic at his finger-ends." another way in which he strove to educate himself is thus described in his own words:--"during this spring magrath and i established the mutual improvement plan, and met at my rooms up in the attics of the royal institution, or at wood street at his warehouse. it consisted, perhaps, of half-a-dozen persons, chiefly from the city philosophical society, who met of an evening to read together, and to criticise, correct, and improve each other's pronunciation and construction of language. the discipline was very sturdy, the remarks very plain and open, and the results most valuable. this continued for several years." seven months after his appointment there began a new passage in faraday's life, which gave a fresh impulse to his mental activity, and largely extended his knowledge of men and things. sir humphry davy, wishing to travel on the continent, and having received a special pass from the emperor napoleon, offered to take him as his amanuensis: he accepted the proposal, and for a year and a half they wandered about france, italy, and switzerland, and then they returned rapidly by the tyrol, germany, and holland. from letters written when abroad we can catch some of the impressions made on his mind by these novel scenes. "i have not forgot," he writes to abbott, "and never shall forget, the ideas that were forced on my mind in the first days. to me, who had lived all my days of remembrance in london, a city surrounded by a flat green country, a hill was a mountain, and a stone a rock; for though i had abstract ideas of the things, and could say rock and mountain, and would talk of them, yet i had no perfect ideas. conceive then the astonishment, the pleasure, and the information which entered my mind in the varied county of devonshire, where the foundations of the earth were first exposed to my view, and where i first saw granite, limestone, &c., in those places and in those forms where the ever-working and all-wonderful hand of nature had placed them. mr. ben., it is impossible you can conceive my feelings, and it is as impossible for me to describe them. the sea then presented a new source of information and interest; and on approaching the shores of france, with what eagerness, and how often, were my eyes directed to the south! when arrived there, i thought myself in an uncivilized country; for never before nor since have i seen such wretched beings as at morlaix." his impression of the people was not improved by the fact of their having arrested the travellers on landing, and having detained them for five days until they had sent to paris for verification of their papers. again, to her towards whom his heart was wont to turn from distant lands with no small longing: "i have said nothing as yet to you, dear mother, about our past journey, which has been as pleasant and agreeable (a few things excepted, in reality nothing) as it was possible to be. sir h. davy's high name at paris gave us free admission into all parts of the french dominions, and our passports were granted with the utmost readiness. we first went to paris, and stopped there two months; afterwards we passed, in a southerly direction, through france to montpellier, on the borders of the mediterranean. from thence we went to nice, stopping a day or two at aix on our way; and from nice we crossed the alps to turin, in piedmont. from turin we proceeded to genoa, which place we left afterwards in an open boat, and proceeded by sea towards lerici. this place we reached after a very disagreeable passage, and not without apprehensions of being overset by the way. as there was nothing there very enticing, we continued our route to florence; and, after a stay of three weeks or a month, left that fine city, and in four days arrived here at rome. being now in the midst of things curious and interesting, something arises every day which calls for attention and observations. the relics of ancient roman magnificence, the grandeur of the churches, and their richness also--the difference of habits and customs, each in turn engages the mind, and keeps it continually employed. florence, too, was not destitute of its attractions for me, and in the academy del cimento and the museum attached to it is contained an inexhaustible fund of entertainment and improvement; indeed, during the whole journey, new and instructive things have been continually presented to me. tell b. i have crossed the alps and the apennines; i have been at the jardin des plantes; at the museum arranged by buffon; at the louvre, among the _chefs d'oeuvre_ of sculpture and the masterpieces of painting; at the luxembourg palace, amongst rubens' works; that i have seen a glowworm!!! waterspouts, torpedo, the museum at the academy del cimento, as well as st. peter's, and some of the antiquities here, and a vast variety of things far too numerous to enumerate." but he kept a lengthy journal, and as we turn over the pages--for the best part of it is printed by bence jones--we meet vivid sketches of the provokingly slow custom-house officers, the postilion in jack-boots, and the thin pigs of morlaix--pictures of paris, too, when every frenchman was to him an unintelligible enemy; when the apollo belvidere, the venus de medici, and the dying gladiator were at the louvre, and when the first napoleon visited the senate in full state. "he was sitting in one corner of his carriage, covered and almost hidden from sight by an enormous robe of ermine, and his face overshadowed by a tremendous plume of feathers that descended from a velvet hat." we watch sir humphry as ampère and others bring to him the first specimens of iodine, and he makes experiments with his travelling apparatus on the dark lustrous crystals and their violet vapour; we seem, too, to be present with the great english chemist and his scholar as they burn diamonds at florence by means of the grand duke's gigantic lens, and prove that the invisible result is carbonic acid; or as they study the springs of inflammable gas at pietra mala, and the molten minerals of vesuvius. the whole, too, is interspersed with bits of fun, and this culminates at the roman carnival, where he evidently thoroughly enjoyed the follies of the corso, the pelting with sugar-plums, and the masked balls, to the last of which he went in a nightgown and nightcap, with a lady who knew all his acquaintances; and between the two they puzzled their friends mightily. this year and a half may be considered as the time of faraday's education; it was the period of his life that best corresponds with the collegiate course of other men who have attained high distinction in the world of thought. but his university was europe; his professors the master whom he served, and those illustrious men to whom the renown of davy introduced the travellers. it made him personally known, also, to foreign _savants_, at a time when there was little intercourse between great britain and the continent; and thus he was associated with the french academy of sciences while still young, his works found a welcome all over europe, and some of the best representatives of foreign science became his most intimate friends. in may , his engagement at the royal institution was renewed, with a somewhat higher position and increased salary, which was again raised in the following year to _l._ per annum. the handwriting in the laboratory note-book changes in september , from the large running letters of brande to the small neat characters of faraday, his first entry having reference to an analysis of "dutch turf ash," and then soon occur investigations into the nature of substances bearing what must have been to him the mysterious names of paligenetic tincture, and _baphe eugenes chruson_. it is to be hoped that the constituents of this golden dye agreed together better than the greek words of its name. we can imagine the young philosopher taking a deeper interest in the researches on flame which his master was then carrying out, and in the gradual perfection of the safety-lamp that was to bid defiance to the explosive gases of the mine; this at least is certain, that davy, in the preface to his celebrated paper on the subject, expresses himself "indebted to mr. michael faraday for much able assistance," and that the youthful investigator carefully preserved the manuscript given him to copy. part of his duty, in fact, was to copy such papers; and as sir humphry had a habit of destroying them, he begged leave to keep the originals, and in that way collected two large volumes of precious manuscripts. but there came a change. hitherto he had been absorbing; now he was to emit. the knowledge which had been a source of delight to himself must now overflow as a blessing to others: and this in two ways. his first lecture was given at the city philosophical society on january , , and in the same year his first paper was published in the _quarterly journal of science_. the lecture was on the general properties of matter; the paper was an analysis of some native caustic lime from tuscany. neither was important in itself, but each resembled those little streams which travellers are taken to look at because they are the sources of mighty rivers, for faraday became the prince of experimental lecturers, and his long series of published researches have won for him the highest niche in the temple of science. when he began to investigate for himself, it could not have been easy to separate his own work from that which he was expected to do for his master. hence no small danger of misunderstandings and jealousies; and some of these ugly attendants on rising fame did actually throw their black shadows over the intercourse between the older and the younger man of genius. in these earlier years, however, all appears to have been bright; and the following letter, written from rome in october , will give a good idea of the assistant's miscellaneous duties, and of the pleasant feelings of davy towards him. it may be added that in another letter he is requested to send some dozens of "flies with pale bodies" to florence, for sir humphry loved fly-fishing as well as philosophy. "to mr. faraday. "i received the note you were so good as to address to me at venice; and by a letter from mr. hatchett i find that you have found the parallax of mr. west's sirius, and that, as i expected, he is mistaken. "if when you write to me you will give the per cents. and _long annuities_, it will be enough. "i will thank you to put the enclosed letters into the post, except those for messrs. morland and messrs. drummond, which perhaps you will be good enough to deliver. "mr. hatchett's letter contained praises of you which were very gratifying to me; and pray believe me there is no one more interested in your success and welfare than your sincere well-wisher and friend, "h. davy. "rome." it must not be supposed, however, that he had any astronomical duties, for the parallax he had found was not that of the dog-star, but of a reputed new metal, sirium, which was resolved in faraday's hands into iron, nickel, and sulphur. but the impostor was not to be put down so easily, for he turned up again under the _alias_ of vestium; but again he was unable to escape the vigilant eye of the young detective, for one known substance after another was removed from it; and then, says faraday, "my vestium entirely disappeared." his occupations during this period were multifarious enough. we must picture him to ourselves as a young-looking man of about thirty years of age, well made, and neat in his dress, his cheerfulness of disposition often breaking out in a short crispy laugh, but thoughtful enough when something important is to be done. he has to prepare the apparatus for brande's lectures, and when the hour has arrived he stands on the right of the professor, and helps him to produce the strange transformations of the chemical art. and conjurers, indeed, the two appear in the eyes of the youth on the left, who waits upon them, then the "laboratory assistant," now the well-known author, mr. william bollaert, from whom i have learnt many details of this period. when not engaged with the lectures, faraday is manufacturing rare chemicals, or performing commercial analyses, or giving scientific evidence on trials. one of these was a famous one, arising from the imperial insurance company resisting the claim of severn and king, sugar-bakers; and in it appeared all the chemists of the day, like knights in the lists, on opposite sides, ready to break a lance with each other. all his spare time faraday was occupied with original work. chlorine had a fascination for him, though the yellow choking gas would get out into the room, and he investigated its combinations with carbon, squeezed it into a liquid, and applied it successfully as a disinfectant when fatal fever broke out in the millbank penitentiary. iodine too, another of davy's elements, was made to join itself to carbon and hydrogen; and naphthaline was tormented with strong mineral acids. long, too, he tried to harden steel and prevent its rusting, by alloying it with small quantities of platinum and the rarer metals; the boy blew the bellows till the crucibles melted, but a few ordinary razors seem to have been the best results. far more successful was he in repeating and extending some experiments of ampère on the mutual action of magnets and electric currents; and when, after months of work and many ingenious contrivances, the wire began to move round the magnet, and the magnet round the wire, he himself danced about the revolving metals, his face beaming with joy--a joy not unmixed with thankful pride--as he exclaimed, "there they go! there they go! we have succeeded at last." after this discovery he thought himself entitled to a treat, and proposed to his attendant a visit to the theatre. "which shall it be?" "oh, let it be astley's, to see the horses." so to astley's they went; but at the pit entrance there was a crush; a big fellow pressed roughly upon the lad, and faraday, who could stand no injustice, ordered him to behave himself, and showed fight in defence of his young companion. the rising philosopher indulged, too, in other recreations. he had a wonderful velocipede, a progenitor of the modern bicycle, which often took him of an early morning to hampstead hill. there was also his flute; and a small party for the practice of vocal music once a week at a friend's house. he sang bass correctly, both as to time and tune. and though the city philosophical society was no more, the ardent group of students of nature who used to meet there were not wholly dispersed. they seem to have carried on their system of mutual improvement, and to have read the current scientific journals at mr. nicol's house till he married, and then alternately at those of mr. r. h. solly, mr. ainger, and mr. hennel, of apothecaries' hall, who came to a tragical end through an explosion of fulminating silver. several of them, including mr. cornelius varley, joined the society of arts, which at that time had committees of various sciences, and was very democratic in its management; and, finding that by pulling together they had great influence, they constituted themselves a "caucus," adopting the american word, and meeting in private. magrath was looked upon as a "chair-maker," and faraday in subsequent years held the office of chairman of the committee of chemistry, and occasionally he presided at the large meetings of the society. during this time ( ) the athenæum club was started, not in the present grecian palace in pall mall, but in a private house in waterloo place. its members were the aristocracy of science, literature, and art, and they made faraday their honorary secretary; but after a year he transferred the office to his friend magrath, who held it for a long period. among the various sects into which christendom is divided, few are less known than the sandemanians. about a century and a half ago, when there was little light in the presbyterian church of scotland, a pious minister of the name of john glas began to preach that the church should be governed only by the teaching of christ and his apostles, that its connection with the state was an error, and that we ought to believe and to practise no more and no less than what we find from the new testament that the primitive church believed and practised. these principles, which sound very familiar in these days, procured for their asserter much obloquy and a deposition by the church courts, in consequence of which several separate congregations were formed in different parts of great britain, especially by robert sandeman, the son-in-law of mr. glas, and from him they received their common appellation. in early days they taught a simpler view of faith than was generally held at that time; it was with them a simple assent of the understanding, but produced by the spirit of god, and its virtue depended not on anything mystical in the operation itself, but on the grandeur and beauty of the things believed. now, however, there is little to distinguish them in doctrine from other adherents of the puritan theology, though they certainly concede a greater deference to their elders, and attach more importance to the lord's supper than is usual among the puritan churches. their form of worship, too, resembles that of the presbyterians; but they hold that each congregation should have a plurality of elders, pastors, or bishops, who are unpaid men; that on every "first day of the week" they are bound to assemble, not only for prayers and preaching, but also for "breaking of bread," and putting together their weekly offerings; that the love-feast and kiss of charity should continue to be practised; that "blood and things strangled" are still forbidden as food; and that a disciple of christ should not charge interest on loans except in the case of purely business transactions, or lay up wealth for the unknown future, but rather consider all he possesses as at the service of his poorer brethren, and be ready to perform to them such offices of kindness as in the early church were expressed by washing one another's feet. but what gives the remarkable character to the adherents of this sect is their perfect isolation from all christian fellowship outside their own community, and from all external religious influence. they have never made missionary efforts to win men from the world, and have long ceased to draw to themselves members from other churches; so they have rarely the advantage of fresh blood, or fresh views of the meaning of scripture. they commonly intermarry, and are expected to "bear one another's burthens;" so the church has acquired somewhat of the additional character of a large intertwined family and of a mutual benefit society. this rigid separation from the world, extending now through three or four generations, has produced a remarkable elevation of moral tone and refinement of manner; and it is said that no one unacquainted with the inner circle can conceive of the brotherly affection that reigns there, or the extent to which hospitality and material help is given without any ostentation, and received without any loss of self-respect. the body is rendered still more seclusive by demanding, not merely unity of spirit among its members, but unanimity of opinion in every church transaction. in order to secure this, any dissentient who persists in his opinion after repeated argument is rejected; the same is also the consequence of neglect of church duties, as well as of any grave moral offence: and in such a community excommunication is a serious social ban, and though a penitent may be received back once, he can never return a second time. it was in the midst of this little community that faraday received his earliest religious impressions, and among them he found his ecclesiastical home till the day of his entrance into the church above. among the elders of the sandemanian church in london was mr. barnard, a silversmith, of paternoster row. the young philosopher became a visitor at his house, and though he had previously written,-- "what is't that comes in false deceitful guise, making dull fools of those that 'fore were wise? 'tis love." --he altered his opinion in the presence of the citizen's third daughter, sarah, and wrote to her what was certainly not the letter of a fool:-- "you know me as well or better than i do myself. you know my former prejudices and my present thoughts--you know my weaknesses, my vanity, my whole mind; you have converted me from one erroneous way, let me hope you will attempt to correct what others are wrong.... again and again i attempt to say what i feel, but i cannot. let me, however, claim not to be the selfish being that wishes to bend your affections for his own sake only. in whatever way i can best minister to your happiness, either by assiduity or by absence, it shall be done. do not injure me by withdrawing your friendship, or punish me for aiming to be more than a friend by making me less; and if you cannot grant me more, leave me what i possess,--but hear me." the lady hesitated, and went to margate. there he followed her, and they proceeded together to dover and shakspeare's cliff, and he returned to london full of happiness and hope. he loved her with all the ardour of his nature, and in due course, on june , , they were married. the bridegroom desired that there should be no bustle or noise at the wedding, and that the day should not be specially distinguished; but he calls it himself "an event which more than any other contributed to his happiness and healthful state of mind." as years rolled on the affection between husband and wife became only deeper and deeper; his bearing towards her proved it, and his letters frequently testify to it. doubtless at any time between their marriage and his final illness he might have written to her as he did from birmingham, at the time of the british association:--"after all, there is no pleasure like the tranquil pleasures of home, and here--even here--the moment i leave the table, i wish i were with you in quiet. oh! what happiness is ours! my runs into the world in this way only serve to make me esteem that happiness the more." he took his bride home to albemarle street, and there they spent their wedded life; but until mr. barnard's death it was their custom to go every saturday to the house of the worthy silversmith, and spend sunday with him, returning home usually in the evening of that day. his own father died while he was at riebau's, but his mother, a grand-looking woman, lived long afterwards, supported by her son, whom she occasionally visited at the institution, and of whose growing reputation she was not a little proud. with a mind calmed and strengthened by this beautiful domestic life, he continued with greater and greater enthusiasm to ask questions of nature, and to interpret her replies to his fellow-men. just before his marriage he had been appointed at the royal institution superintendent of the house and laboratory, and in february , after a change in the management of the institution, he was placed as director in a position of greater responsibility and influence. one of his first acts in this capacity was to invite the members to a scientific evening in the laboratory; this took place three or four times in , and in the following years these gatherings were held every week from feb. to june ; and though the labour devolved very much upon faraday, other philosophers sometimes brought forward discoveries or useful inventions. thus commenced those friday evening meetings which have done so much to popularize the high achievements of science. faraday's note-books are still preserved, containing the minutes of the committee-meetings every thursday afternoon, the duke of somerset chairman, and he secretary; also the record of the friday evenings themselves, who lectured, and on what subject, and what was exhibited in the library, till june , when other arrangements were probably made. the year was otherwise fruitful in lectures: in the spring, a course of twelve on chemical manipulation at the london institution; after easter, his first course at albemarle street, six lectures on chemical philosophy (he had helped professor brande in );[ ] and at christmas, his desire to convey knowledge, and his love to children, found expression in a course of six lectures to the boys and girls home for their holidays. these were a great success; indeed, he himself says they "were just what they ought to have been, both in matter and manner,--but it would not answer to give an extended course in the same spirit." he continued these juvenile lectures during nineteen years. the notes for courses of lectures were written in school copy-books, and sometimes he appends a general remark about the course, not always so favourable as the one given above. thus he writes, "the eight lectures on the operations of the laboratory, april , were not to my mind." of the course of twelve in the spring of , he says he "found matter enough in the notes for at least seventeen." up to faraday was bringing the forces of nature in subjection to man on a salary of only _l._ per annum, with house, coals, and candles, as the funds of the institution would not at that time afford more; but among the sedate _habitúes_ of the place was a tall, jovial gentleman, who lounged to the lectures in his old-fashioned blue coat and brass buttons, grey smalls, and white stockings, who was a munificent friend in need. this was john fuller, a member of parliament. he founded a professorship of chemistry with an endowment that brings in nearly _l._ a year, and gave the first appointment to faraday for life. when the institution became richer, his income was increased; and when, on account of the infirmities of age, he could no longer investigate, lecture, or keep accounts, the managers insisted on his still retaining in name his official connection with the place, with his salary and his residence there. nor indeed could they well have acted otherwise; for though the royal institution afforded in the first instance a congenial soil for the budding powers of faraday, his growth soon became its strength; and eventually the blooming of his genius, and the fruit it bore, were the ornament and glory of the institution. it will be asked, was this _l._ or _l._ per annum the sole income of faraday? no; in early days he did commercial analyses, and other professional work, which paid far better than pure science. in his gains from this source amounted to , _l._, and in to considerably more; they might easily have been increased, but at that time he made one of his most remarkable discoveries--the evolution of electricity from magnetism,[ ]--and there seemed to lie open before him the solution of the problem how to make one force exhibit at will the phenomena of magnetism or of common or voltaic electricity. and then he had to face another problem--his own mental force might be turned either to the acquisition of a fortune, or to the following up of those great discoveries; it would not do both: which should he relinquish? the choice was deliberately made: nature revealed to him more and more of her secrets, but his professional gains sank in to _l._ _s._, and during no subsequent year did they amount even to that. still his work was not entirely confined to his favourite studies. in a letter to lord auckland, long afterwards, he says:--"i have given up, for the last ten years or more, all professional occupation, and voluntarily resigned a large income that i might pursue in some degree my own objects of research. but in doing this i have always, as a good subject, held myself ready to assist the government if still in my power, _not for pay_; for, except in one instance (and then only for the sake of the person joined with me), i refused to take it. i have the honour and pleasure of applications, and that very recently, from the admiralty, the ordnance, the home office, the woods and forests, and other departments, all of which i have replied to, and will reply to as long as strength is left me." he had declined the professorship of chemistry at the london university--now university college,--but in he accepted a lectureship at the royal academy, woolwich, and held it for about twenty years. in he became scientific adviser to the trinity house, and his letter to the deputy master also shows his feelings in reference to such employment:--"you have left the title and the sum in pencil. these i look at mainly as regards the character of the appointment; you will believe me to be sincere in this, when you remember my indifference to your proposition as a matter of interest, though _not as a matter of kindness_. in consequence of the goodwill and confidence of all around me, i can at any moment convert my time into money, but i do not require more of the latter than is sufficient for necessary purposes. the sum, therefore, of _l._ is quite enough in itself, but not if it is to be the indicator of the character of the appointment; but i think you do not view it so, and that you and i understand each other in that respect; and your letter confirms me in that opinion. the position which i presume you would wish me to hold is analogous to that of a standing counsel." for nearly thirty years faraday continued to report on all scientific suggestions and inventions connected with lighthouses or buoys, not for personal gain or renown, but for the public good. his position was never above that of a "standing counsel." in his own words: "i do not know the exact relation of the board of trade and the trinity house to each other; i am simply an adviser upon philosophical questions, and am put into action only when called upon." in regard to the lectureship at woolwich, mr. abel, his successor, writes thus:--"faraday appears to have enjoyed his weekly trips to woolwich, which he continued for so many years, as a source of relaxation. he was in the habit of going to woolwich in the afternoon or evening preceding his lecture at the military academy, then preparing at once for his experiments, and afterwards generally taking a country ramble. the lecture was delivered early the following morning. no man was so respected, admired, and beloved as a teacher at the military academy in former days as faraday. many are the little incidents which have been communicated to me by his pupils illustrative of his charms as a lecturer, and of his kindly feelings for the youths to whom he endeavoured to impart a taste for, if not a knowledge of, science. but for some not ill-meant, though scarcely judicious, proposal to dictate modifications in his course of instruction, faraday would probably have continued for some years longer to lecture at woolwich. in may , soon after i had been appointed his successor, faraday wrote to me requesting the return of some tubes of condensed gases which he left at the academy. this letter ends thus:--'i hope you feel yourself happy and comfortable in your arrangements at the academy, and have cause to be pleased with the change. i was ever very kindly received there, and that portion of regret which one must ever feel in concluding a long engagement would be in some degree lessened with me by hearing that you had reason to be satisfied with your duties and their acceptance.--ever very truly yours, m. faraday.'" for year after year the life of faraday afforded no adventure and little variety, only an ever-growing skill in his favourite pursuit, higher and higher success, and ever-widening fame. but simple as were his mind and his habits, no one picture can present him as the complete man; we must try to make sketches from various points of view, and leave it to the reader's imagination to combine them. let us watch him on an ordinary day. after eight hours' sleep, he rises in time to breakfast at eight o'clock, goes round the institution to see that all is in order, and descends into the laboratory, puts on a large white apron, the stains and holes in which tell of previous service, and is busy among his pieces of apparatus. the faithful anderson, an old soldier, who always did exactly what he was told, and nothing more,[ ] is waiting upon him; and as thought flashes after thought through his eager--perhaps impatient--brain, he twists his wires into new shapes, and re-arranges his magnets and batteries. then some conclusion is arrived at which lights up his face with a gleam of satisfaction, but the next minute a doubt comes across that expressive brow--may the results not be due to something else yet imperfectly conceived?--and a new experiment must be devised to answer that. in the meantime perhaps one of his little nieces has been left in his charge. she sits as quiet as a mouse with her needlework; but now and then he gives her a nod, or a kind word, and throwing a little piece of potassium on to a basin of water for her amusement, he shows her the metal bursting into purple flame, floating about in fiery eddies, and the crack of the fused globule of potash at the end. presently there is handed to him the card of some foreign _savant_, who makes his pilgrimage to the famous institution and its presiding genius; he puts down his last result on a slate, comes upstairs, and, disregarding the interruption, chats with his visitor with all cordiality and openness. then to work again till dinner-time, at half-past two. in the afternoon he retires to his study with its plain furniture and the india-rubber tree in the window, and writes a letter full of affection to some friend, after which he goes off to the council meeting of one of the learned bodies. then back again to the laboratory, but as evening approaches he goes upstairs to his wife and niece, and then there is a game at bagatelle or acting charades; and afterwards he will read aloud from shakspeare or macaulay till it is time for supper and the simple family worship which now is not liable to the interruptions that generally prevent it in the morning. and so the day closes. or if it be a fine summer evening, he takes a stroll with his wife and the little girl to the zoological gardens, and looks at all the new arrivals, but especially the monkeys, laughing at their tricks till the tears run down his cheeks. but should it be a friday evening, faraday's place is in the library and theatre of the institution, to see that all is right and ready, to say an encouraging word to the lecturer, and to welcome his friends as they arrive; then taking his seat on the front bench near the right hand of the speaker, he listens with an animated countenance to his story,[ ] sometimes bending forwards, and scarcely capable of keeping his fingers off the apparatus--not at all able if anything seems to be going wrong; when the discourse is over, a warm shake of the hand, with "thank you for a pleasant hour," and "good night" to those around him, and upstairs with his wife and some particularly congenial friends to supper. on the dining-table is abundance of good fare and good wine, and around it flows a pleasant stream of lively and intellectual conversation. but suppose it is his own night to lecture. the subject has been carefully considered, an outline of his discourse has been written on a sheet of foolscap, with all the experiments marked and numbered, and during the morning everything has been arranged on the table in such order that his memory is assisted by it; the audience now pours in, and soon occupies all the seats, so that late comers must be content with sitting on the stairs or standing in the gangways, or at the back of the gallery. faraday enters, and placing himself in the centre of the horse-shoe table, perfect master of himself, his apparatus, and his audience, commences a discourse which few that are present will ever forget. here is a picture by lady pollock:--"it was an irresistible eloquence, which compelled attention and insisted upon sympathy. it waked the young from their visions, and the old from their dreams. there was a gleaming in his eyes which no painter could copy, and which no poet could describe. their radiance seemed to send a strange light into the very heart of his congregation; and when he spoke, it was felt that the stir of his voice and the fervour of his words could belong only to the owner of those kindling eyes. his thought was rapid, and made itself a way in new phrases--if it found none ready made--as the mountaineer cuts steps in the most hazardous ascent with his own axe. his enthusiasm sometimes carried him to the point of ecstasy when he expatiated on the beauties of nature, and when he lifted the veil from her deep mysteries. his body then took motion from his mind; his hair streamed out from his head; his hands were full of nervous action; his light, lithe body seemed to quiver with its eager life. his audience took fire with him, and every face was flushed. whatever might be the after-thought or the after-pursuit, each hearer for the time shared his zeal and his delight."[ ] is it possible that he can be happier when lecturing to the juveniles? the front rows are filled with the young people; behind them are ranged older friends and many of his brother philosophers, and there is old sir james south, who is quite deaf, poor man, but has come, as he says, because he likes to see the happy faces of the children. how perfect is the attention! faraday, with a beaming countenance, begins with something about a candle or a kettle that most boys and girls know, then rises to what they had never thought of before, but which now is as clear as possible to their understandings. and with what delight does he watch the performances of nature in his experiments! one could fancy that he had never seen the experiments before, and that he was about to clap his hands with boyish glee at the unexpected result! then with serious face the lecturer makes some incidental remark that goes far beyond natural philosophy, and is a lesson for life. some will remember one of these occasions which forms the subject of a painting by mr. blaikley. within the circle of the table stands the lecturer, and waiting behind is the trusty anderson, while the chair is occupied by the prince consort, and beside him are the young prince of wales and his brother, the present duke of edinburgh; while the rev. john barlow and dr. bence jones sit on the left of the princes; sir james south stands against the door, and murchison, de la rue, mrs. faraday, and others may be recognized among the eager audience. let us now suppose that it is a sunday on which we are watching this prince among the aristocracy of intellect, and we will assume it to be during one of the periods of his eldership, namely between and , or after . the first period came to a close through his separation both from his office and from the church itself. the reason of this is unknown except to the parties immediately concerned, but it will be readily understood how easily differences may arise in such a community as that of the sandemanians between an original and conscientious mind and his brethren in the faith. he, however, continued to worship among his friends, and was after a while restored to the rights of membership, and eventually to the office of elder. in the morning he and his family group find their way down to the plain little meeting-house in paul's alley, red-cross street, since pulled down to make room for the metropolitan railway. the day's proceedings commence with a prayer meeting, during which the worshippers gradually drop in and go to their accustomed seats, faraday taking his place on the platform devoted to the elders: then the more public service begins; one of a metrical but not rhyming version of the psalms is sung to a quaint old tune; the lord's prayer and another psalm follow; he rises and reads in a slow, reverent manner the words of one of the evangelists, with a most profound and intelligent appreciation of their meaning; or he offers an extempore prayer, expressing perfect trust and submission to god's will, with deep humility and confession of sin. it may be his turn to preach. on two sides of a card he has previously sketched out his sermon with the illustrative texts, but the congregation does not see the card, only a little bible in his hand, the pages of which he turns quickly over, as, fresh from an earnest heart, there flows a discourse full of devout thought, clothed largely in the language of scripture. after a loud simultaneous "amen" has closed the service, the church members withdraw to their common meal, the feast of charity; and in the afternoon there is another service, ending by invariable custom with the lord's supper. the family group do not reach home till half-past ; then there is a quiet evening, part of which is spent by faraday at his desk, and they retire to rest at an early hour. again on wednesday evening he is among the little flock. the service is somewhat freer, for not the officers of the church only, but the ordinary members are encouraged to express whatever thoughts occur to them, so as to edify one another. at these times, faraday, especially when he was not an elder, very often had some word of exhortation, and the warmth of his temperament would make itself felt, for he was known in the small community as an experimental rather than a doctrinal preacher. the notes of his more formal discourses which i have had the opportunity of seeing, indicate, as might be expected from the tenets of his church, a large acquaintance with the words of scripture, but no knowledge of modern exegesis. they appear to have impressed different hearers in different ways. one who heard him frequently and was strongly attached to him, says that his sermons were too parenthetical and rapid in their delivery, with little variety or attractiveness; but another scientific friend, who heard him occasionally, writes: "they struck me as resembling a mosaic work of texts. at first you could hardly understand their juxtaposition and relationship, but as the well-chosen pieces were filled in, by degrees their congruity and fitness became developed, and at last an amazing sense of the power and beauty of the whole filled one's thoughts at the close of the discourse." his first sermon as an elder was on christ's character and example as shown in matthew xi. - : "learn of me; for i am meek and lowly in heart." among the latest of his sermons was one that he preached at dundee about four years before his death. he began by telling his audience that his memory was failing, and he feared he could not quote scripture with perfect accuracy; and then, as said one of the elders who had been present, "his face shone like the face of an angel," as he poured forth the words of loving exhortation. when a mind is stretched in the same direction week-day and sunday, the tension is apt to become too great. with faraday the first symptom was loss of memory. then his devoted wife had to hurry him off to the country for rest of brain. once he had to give up work almost entirely for a twelvemonth. during this time he travelled in switzerland, and extracts from his diary are given by bence jones. his niece, mrs. deacon, gives us her recollections of a month spent at walmer:--"how i rejoiced to be allowed to go there with him! we went on the outside of the coach, in his favourite seat behind the driver. when we reached shooter's hill, he was full of fun about falstaff and the men in buckram, and not a sight nor a sound of interest escaped his quick eye and ear. at walmer we had a cottage in a field, and my uncle was delighted because a window looked directly into a blackbird's nest built in a cherry-tree. he would go many times in a day to watch the parent birds feeding their young. i remember, too, how much he was interested in the young lambs, after they were sheared at our door, vainly trying to find their own mothers. the ewes, not knowing their shorn lambs, did not make the customary signal. in those days i was eager to see the sun rise, and my uncle desired me always to call him when i was awake. so, as soon as the glow brightened over pegwell bay, i stole downstairs and tapped at his door, and he would rise, and a great treat it was to watch the glorious sight with him. how delightful, too, to be his companion at sunset! once i remember well how we watched the fading light from a hill clothed with wild flowers, and how, as twilight stole on, the sounds of bells from upper deal broke upon our ears, and how he watched till all was grey. at such times he would be well pleased if we could repeat a few lines descriptive of his feelings." and then she tells us about their examining the flowers in the fields by the aid of "galpin's botany," and how with a candle he showed her a spectre on the white mist outside the window; of reading lessons that ended in laughter, and of sea-anemones and hermit crabs, with the merriment caused by their odd movements as they dragged about the unwieldy shells they tenanted. "but of all things i used to like to hear him read 'childe harold;' and never shall i forget the way in which he read the description of the storm on lake leman. he took great pleasure in byron, and coleridge's 'hymn to mont blanc' delighted him. when anything touched his feelings as he read--and it happened not unfrequently--he would show it not only in his voice, but by tears in his eyes also." a few days at brighton refreshed him for his work. he was in the habit of running down there before his juvenile lectures at christmas, and at easter he frequently sought the same sea-breezes. but it was not always that faraday could run away from london when the mental tension became excessive. a shorter relaxation was procured by his taking up a novel such as "ivanhoe," or "jane eyre," or "monte christo." he liked the stirring ones best, "a story with a thread to it." or he would go with his wife to see kean act, or hear jenny lind sing, or perhaps to witness the performance of some "wizard of the north." now and then he would pay a visit to some scene of early days. one of his near relatives tells me: "it is said that mr. faraday once went to the shop where his father had formerly been employed as a blacksmith, and asked to be allowed to look over the place. when he got to a part of the premises at which there was an opening into the lower workshop, he stopped and said: 'i very nearly lost my life there once. i was playing in the upper room at pitching halfpence into a pint pot close by this hole, and having succeeded at a certain distance, i stepped back to try my fortune further off, forgetting the aperture, and down i fell; and if it had not been that my father was working over an anvil fixed just below, i should have fallen on it, broken my back, and probably killed myself. as it was, my father's back just saved mine.'" business, as well as pleasure, sometimes took him away from home. he often joined the british association, returning usually on saturday, that he might be among his own people on the lord's day. during the meeting he would generally accept the hospitality of some friend; and it was one of these occasions that gave rise to the following _jeu d'esprit_:-- "'that p will change to f in the british tongue is true (quoth professor phillips), though the instances are few;' an entry in my journal then i ventured thus to parody, 'i this day dined with fillips, where i hobbed and nobbed with pharaday.' "t. t. "oxford, _june , _." at the liverpool meeting, in , he was president of the chemical section, and on two other occasions he was selected to deliver the evening lecture, but though repeatedly pressed to undertake the presidency of the whole body, he could not be prevailed upon to accept the office. my first personal intercourse with him, of any extent, was at the ipswich meeting in . i watched him with all the interest of an admiring disciple, and there is deeply engraven on my memory the vivacity of his conversation, the eagerness with which he entered into some mathematico-chemical speculations of dumas, and the playfulness with which, when we were dining together, he cut boomerangs out of card, and shot them across the table at his friends. professional engagements also took him not unfrequently into the country. some of these will be described in the later sections, that treat of his mode of working and its valuable results. to comprehend a man's life it is necessary to know not merely what he does, but also what he purposely leaves undone. there is a limit to the work that can be got out of a human body or a human brain, and he is a wise man who wastes no energy on pursuits for which he is not fitted; and he is still wiser who, from among the things that he can do well, chooses and resolutely follows the best. faraday took no part in any of the political or social movements of his time. to politics indeed he seems to have been really indifferent. it was during the intensely interesting period of - that he was on the continent with davy, but he alludes to the taking of paris by the allied troops simply because of its bearing on the movements of the travellers, and on march , , he made his remarkable entry in his journal: "i heard for news that bonaparte was again at liberty. being no politician, i did not trouble myself much about it, though i suppose it will have a strong influence on the affairs of europe." in later days he seems to have awaked to sufficient interest to read the debates, and to show a conservative tendency; he became a special constable in , and was disposed generally to support "the powers that be,"--though that involved some perplexity at a change of government. it is more singular that a man of his benevolent spirit should never have taken a prominent part in any philanthropic movement. in some cases his religious views may have presented an obstacle, but this reason can hardly apply to many of those social movements in which the influence of his name, and his occasional presence and advice, would have been highly valuable. during the latter half of his life, he, as a rule, avoided serving on committees even for scientific objects, and was reluctant to hold office in the learned societies with which he was connected. i believe, however, that this arose not so much from want of interest, as from a conviction that he was ill-suited by natural temperament for joining in discussions on subjects that roused the passions of men, or for calmly weighing the different courses of action, and deciding which was the most judicious. it is remarkable how little even of his scientific work was done in conjunction with others. neither did he spend time in rural occupations, or in literary or artistic pursuits. beasts and birds and flowers he looked at, but it was for recreation, not for study. music he was fond of, and occasionally he visited the opera, but he did not allow sweet sounds to charm him away from his work. he stuck closely to his fireside, his laboratory, his lecture table, and his church. he lived where he worked, so that he had only to go downstairs to put to the test of experiment any fresh thought that flitted across his brain. he almost invariably declined dinner-parties, except at lady davy's, and at mr. and mrs. masquerier's at brighton, towards whom he felt under an obligation on account of former kindnesses. if he went to a _soirée_, he usually stayed but a short time; and even when away from home he generally refused private hospitality. thus he was able to give almost undivided attention to the chief pursuit of his life. his residence in so accessible a part of london did, however, expose him to the constant invasion of callers, and his own good nature often rendered fruitless the efforts that were considerately made to restrict these within reasonable limits. of course he suffered from the curious and the inconsiderate of the human species; and then there were those pertinacious bores, the dabblers in science. "one morning a young man called on him, and with an air of great importance confided to him the result of some original researches (so he deemed them) in electrical philosophy. 'and pray,' asked the professor, taking down a volume of rees' cyclopædia, 'did you consult this or any elementary work to learn whether your discovery had been anticipated?' the young man replied in the negative. 'then why do you come to waste my time about well-known facts, that were published forty years ago?' 'sir,' said the visitor, 'i thought i had better bring the matter to head-quarters immediately.' 'all very well for you, but not so well for head-quarters,' replied the professor, sharply, and set him down to read the article." "a grave, elderly gentleman once waited upon him to submit to his notice 'a new law of physics.' the visitor requested that a jug of water and a tumbler might be brought, and then producing a cork, 'you will be pleased to observe,' said he, 'how persistently this cork clings to the side of the glass when the vessel is half filled.' 'just so,' replied the professor. 'but now,' resumed this great discoverer, 'mark what happens when i fill the tumbler to the brim. there! you see the cork flies to the centre--positively repelled by the sides!' 'precisely so,' replied the amused electrician, with the air of a man who felt perfectly at home with the phenomenon, and indeed regarded it quite as an old friend. the visitor was evidently disconcerted. 'pray how long have you known this?' he ventured to ask faraday. 'oh, ever since i was a boy,' was the rejoinder. crestfallen--his discovery demolished in a moment--the poor gentleman was retiring with many apologies, when the professor, sincerely concerned at his disappointment, comforted him by suggesting that possibly he might some day alight upon something really new."[ ] but there were other visitors who were right welcome to a portion of his time. one day it might be a young man, whom a few kind words and a little attention on the part of the great philosopher would send forward on the journey of life with new energy and hopes. another day it might be some intellectual chieftain, who could meet the prince of experimenters on equal terms. but these are hardly to be regarded as interruptions;--rather as a part of his chosen work. here is one instance in the words of mr. robert mallet. "... i was, in the years that followed, never in london without paying him a visit, and on one of those times i ventured to ask him (if not too much engaged) to let me see where he and davy had worked together. with the most simple graciousness he brought me through the whole of the royal institution, albemarle street. brande's furnaces, davy's battery, the place in the laboratory where he told me he had first observed the liquefaction of chlorine, are all vividly before me--but nothing so clear or vivid as our conversation over a specimen of green (crown) glass, partially devitrified in floating opaque white spheres of radiating crystals: he touched luminously on the obscure relation of the vitreous and crystalloid states, and on the probable nature of the nuclei of the white spheres. my next visit to faraday that i recollect was not long after my paper 'on the dynamics of earthquakes' had appeared in the transactions of the royal irish academy. he almost at once referred to it in terms of praise that seemed to me so far beyond my due, that even now i recall the very humble way i felt, as the thought of faraday's own transcendent merits rushed across my mind. i ventured to ask him, had the paper engaged his attention sufficiently that i might ask him--did he consider my explanation of the before supposed _vorticose_ shock sufficient? to my amazement he at once recited _nearly word for word_ the paragraph in which i took some pains to put my views into a demonstrative shape, and ended with, 'it is as plain and certain as a proposition of euclid!' and yet the subject was one pretty wide away from his own objects of study." often, too, if some interesting fact was exhibited to him, he would send to his brother _savants_ some such note as this:-- "royal institution, _ th may, _. "my dear wheatstone, "dr. dubois-raymond will be making his experiments _here_ next thursday, the th, from and after o'clock. i wish to let you know, that you may if you like join the select few. "ever truly yours, "m. faraday." it was indeed his wont to share with others the delight of a new discovery. thus sir henry holland tells me that he used frequently to run to his house in brook street with some piece of scientific news. one of these visits was after reading bunsen and kirchhoff's paper on spectrum analysis; and he did not stop short with merely telling the tale of the special rays of light shot forth by each metallic vapour, as the following letter will show. it is addressed to the present baroness burdett coutts. "royal institution, _friday, th may_. "dear miss coutts, "to-morrow, at o'clock, immediately after max müller's lecture, i shall show sir henry holland an apparatus which has arrived from munich to manifest the phenomena of light which have recently been made known to us by bunsen and kirchhoff. mr. barlow will be here, and he suggests that you would like to know of the occasion. if you are inclined to see how philosophers work and live, and so are inclined to climb our narrow stairs (for i must show the experiments in my room), we shall be most happy to see you. the experiments will not be beautiful except to the intelligent. "ever your faithful servant, "m. faraday." sometimes, too, the exhibition of a scientific fact would take him away from home. thus, when her majesty and the prince consort once paid a private visit to the polytechnic, mr. pepper arranged a surprise for the royal party, by getting faraday in a quiet room to explain the ruhmkorff's coil--the latest development of his own inductive currents. this he did with his usual vivacity and enthusiasm, and the interview is said to have gratified the philosopher as well as the queen. he could not, however, escape the inroads made upon his time by correspondence. people would write and ask him questions. once a solitary prisoner wrote to tell him, "it is indeed in studying the great discoveries which science is indebted to you for, that i render my captivity less sad, and make time flow with rapidity,"--and then he proceeds to ask, "_what is the most simple_ combination to give to a voltaic battery, in order to produce a spark capable of setting fire to powder under water, or under ground? up to the present i have only seen employed to that purpose piles of thirty to forty pairs constructed on dr. wollaston's principles. they are very large and inconvenient for field service. could not the same effect be produced by two spiral pairs only? and if so, what can be their smallest dimension?" and who was the prisoner who thus speculated on the applications of science to war? it was no other than prince louis napoleon, then immured in the fortress of ham, and now the ex-emperor of the french. at another time he wrote asking for his advice in the manufacture of an alloy which should be about as soft as lead, but not so fusible,--a question which also had evident bearing upon the art of war; and offering at the same time to pay the cost of any experiments that might be necessary. often, too, the correspondents of faraday thought that they were doing him a kindness. he says somewhere: "the number of suggestions, hints for discovery, and propositions of various kinds, offered to me very freely and with perfect goodwill and simplicity on the part of the proposers, for my exclusive investigation and final honour, is remarkably great, and it is no less remarkable that but for one exception--that of mr. jenkin--they have all been worthless.... i have, i think, universally found that the man whose mind was by nature or self-education fitted to make good and worthy suggestions, was also the man both able and willing to work them out." both the askers of questions and the givers of advice expected answers--and the answers came. most of faraday's letters, indeed, are of a purely business character: sometimes they are very laconic, as the note in which he announced to dr. paris one of his principal discoveries:-- "dear sir, "the _oil_ you noticed yesterday turns out to be liquid chlorine. "yours faithfully, "m. faraday." but in other letters, as may be expected, there is found the enthusiasm of his ardent nature, or the glow of his genial spirit. an instance or two may suffice. "royal institution, _ th march, _. "dear sir, "i have received and at once looked at your paper. many thanks for so good a contribution to the beloved science. what glorious steps electricity has taken in the days within our remembrance, and what hopes are held out for the future! the great difficulty is to remove the mists which dim the dawn of a subject, and i cannot but consider your paper as doing very much that way for a most important part of natural knowledge. "i am, my dear sir, "most truly yours, "m. faraday. "j. p. joule, esq." "royal institution, _ th oct., _. "my dear miss moore, "the summer is going away, and i never (but for one day) had any hopes of profiting by your kind offer of the roof of your house in clarges street. what a feeble summer it has been as regards sunlight! i have made a good many preliminary experiments at home, but they do not encourage me in the direction towards which i was looking. all is misty and dull, both the physical and the mental prospect. but i have ever found that the experimental philosopher has great need of patience, that he may not be downcast by interposing obstacles, and perseverance, that he may either overcome them, or open out a new path to the bourn he desires to reach. so perhaps next summer i may think of your housetop again. many thanks for your kind letter and all your kindnesses uswards. my wife had your note yesterday, and i enjoyed the violets, which for a time i appropriated. "with kindest remembrances and thoughts to all with you and her at hastings, "i am, my dear friend, "very faithfully yours, "m. faraday." the following is written to mr. frank barnard, then an art student in paris:-- "royal institution, _ th nov., _. "my dear nephew, "though i am not a letter-writer and shall not profess to send you any news, yet i intend to waste your time with one sheet of paper: first to thank you for your letter to me, and then to thank you for what i hear of your letters to others. you were very kind to take the trouble of executing my commissions, when i know your heart was bent upon the entrance to your studies. your account of m. arago was most interesting to me, though i should have been glad if in the matter of health you could have made it better. he has a wonderful mind and spirit. and so you are hard at work, and somewhat embarrassed by your position: but no man can do just as he likes, and in many things he has to give way, and may do so honourably, provided he preserve his self-respect. never, my dear frank, lose that, whatever may be the alternative. let no one tempt you to it; for nothing can be expedient that is not right; and though some of your companions may tease you at first, they will respect you for your consistency in the end; and if they pretend not to do so, it is of no consequence. however, i trust the hardest part of your probation is over, for the earliest is usually the hardest; and that you know how to take all things quietly. happily for you, there is nothing in your pursuit which need embarrass you in paris. i think you never cared for home politics, so that those of another country are not likely to occupy your attention, and a stranger can be but a very poor judge of a new people and their requisites. "i think all your family are pretty well, but i know you will hear all the news from your appointed correspondent jane, and, as i said, i am unable to chronicle anything. still, i am always very glad to hear how you are going on, and have a sight of all that i may see of the correspondence. "ever, my dear frank, "your affectionate uncle, "m. faraday." his scientific researches were very numerous. the royal society catalogue gives under the name of faraday a list of papers, published in various scientific magazines or learned transactions. many of these communications are doubtless short, but a short philosophical paper often represents a large amount of brain work; a score of them are the substance of his friday evening discourses; while others are lengthy treatises, the records of long and careful investigations; and the list includes the thirty series of his "experimental researches in electricity." these extended over a period of twenty-seven years, and were afterwards reprinted from the "philosophical transactions," and form three goodly volumes, with , numbered paragraphs--one of the most marvellous monuments of intellectual work, one of the rarest treasure-houses of newly-discovered knowledge, with which the world has ever been enriched. faraday never published but one book in the common acceptation of the term--it was on "chemical manipulation,"--but there appeared another large volume of reprinted papers: and three of his courses of lectures were also published as separate small books, though not by himself. it is very tempting to linger among these papers; but this is not intended as a scientific biography, and those readers who wish to make themselves better acquainted with his work will find an admirable summary of it in professor tyndall's "faraday as a discoverer." in sections iv. and v., however, i have endeavoured to give an idea of his manner of working, and of the practical benefits that have flowed to mankind from some of his discoveries. as these papers appeared his fame grew wider and wider. when a comparatively young man he was naturally desirous of appending the mystic letters "f.r.s." to his name, and he was balloted into the royal society in january , not without strong opposition from his master, sir humphry davy, then president. he paid the fees, and never sought another distinction of the kind. but they were showered down upon him. the philosophical society of cambridge had already acknowledged his merits, and the learned academies of paris and florence had enrolled him amongst their corresponding members. heidelberg and st. petersburg, philadelphia and boston, copenhagen, berlin, and palermo, quickly followed: and as the fame of his researches spread, very many other learned societies in europe and america, as well as at home, brought to him the tribute of their honorary membership.[ ] he thrice received the degree of doctor, oxford making him a d.c.l., prague a ph.d., and cambridge an ll.d., besides which he was instituted a chevalier of the prussian order of merit, a commander of the legion of honour, and a knight commander of the order of st. maurice and st. lazarus. among the medals which he received were each of those at the disposal of the royal society--indeed the copley medal was given him twice--and the grande médaille d'honneur at the time of the french exhibition. altogether it appears he was decorated with ninety-five titles and marks of merit,[ ] including the blue ribbon of science, for in he was chosen one of the eight foreign associates of the french academy. though he had never passed through a university career, he was made a member of the senate of the university of london, which he regarded as one of his chief honours; and he showed his appreciation of the importance of the office by a diligent attendance to its duties. as the recognized prince of investigators, it is no wonder that on the resignation of lord wrottesley, an attempt was made to induce him to become president of the royal society. a deputation waited upon him and urged the unanimous wish of the council and of scientific men. faraday begged for time to consider. tyndall gives us an insight into the reasons that led him to decline. he tells us: "on the following morning i went up to his room, and said, on entering, that i had come to him with some anxiety of mind. he demanded its cause, and i responded, 'lest you should have decided against the wishes of the deputation that waited on you yesterday.' 'you would not urge me to undertake this responsibility,' he said. 'i not only urge you,' was my reply, 'but i consider it your bounden duty to accept it.' he spoke of the labour that it would involve; urged that it was not in his nature to take things easy; and that if he became president, he would surely have to stir many new questions, and agitate for some changes. i said that in such cases he would find himself supported by the youth and strength of the royal society. this, however, did not seem to satisfy him. mrs. faraday came into the room, and he appealed to her. her decision was adverse, and i deprecated her decision. 'tyndall,' he said at length, 'i must remain plain michael faraday to the last; and let me now tell you, that if i accepted the honour which the royal society desires you to confer upon me, i would not answer for the integrity of my intellect for a single year.'" in sir robert peel desired to confer pensions as honourable distinctions on faraday and some other eminent men. lord melbourne, who succeeded him as prime minister, in making the offer at a private interview, gave utterance to some hasty expressions that appeared to the man of science to reflect on the honour of his profession, and led to his declining the money. the king, william iv., was struck with the unusual nature of the proceeding, and kept repeating the story of faraday's refusal; and about a month afterwards the premier, dining with dr. (now sir henry) holland, begged him to convey a letter to the professor and to press on him the acceptance of the pension. the letter was couched in such honourable and conciliatory terms, that faraday's personal objection could no longer apply, and he expressed his willingness to receive this mark of national approval. a version of the matter that found its way into the public prints caused fresh annoyance, and nearly produced a final refusal, but through the kind offices of friends who had interested themselves throughout in the matter, a friendly feeling was again arrived at, and the pension of £ a year was granted and accepted. in the queen offered him a house at hampton court. it was a pretty little place, situated in the well-known green in front of the palace; and in that quiet retreat faraday spent a large portion of his remaining years. in october he wrote a letter to the managers of the royal institution, resigning part of his duties, in which he reviewed his connection with them. "i entered the royal institution in march , nearly forty-nine years ago, and, with the exception of a comparatively short period during which i was abroad on the continent with sir h. davy, have been with you ever since. during that time i have been most happy in your kindness, and in the fostering care which the royal institution has bestowed upon me. thank god, first, for all his gifts. i have next to thank you and your predecessors for the unswerving encouragement and support which you have given me during that period. my life has been a happy one, and all i desired. during its progress i have tried to make a fitting return for it to the royal institution, and through it to science. but the progress of years (now amounting in number to three-score and ten) having brought forth first the period of development, and then that of maturity, have ultimately produced for me that of gentle decay. this has taken place in such a manner as to make the evening of life a blessing; for whilst increasing physical weakness occurs, a full share of health free from pain is granted with it; and whilst memory and certain other faculties of the mind diminish, my good spirits and cheerfulness do not diminish with them." when he could no longer discharge effectually his duties at the trinity house, the corporation quietly made their arrangements for transferring them, and, with the concurrence of the board of trade, determined that his salary of _l._ per annum should continue as long as he lived. sir frederick arrow called upon him at albemarle street, and explained how the matter stood, but he found it hard to persuade the professor that there was no injustice in his continuing to receive the money; then, taking hold of sir frederick by one hand and dr. tyndall by the other, faraday, with swimming eyes, passed over his office to his successor. gradually but surely the end approached. the loss of memory was followed by other symptoms of declining power. the fastenings of his earthly tabernacle were removed one by one, and he looked forward to "the house not made with hands, eternal in the heavens." this was no new anticipation. calling on the friend who had long directed with him the affairs of the institution, but who was then half paralysed, he had said, "barlow, you and i are waiting; that is what we have to do now; and we must try to do it patiently." he had written to his niece, mrs. deacon: "i cannot think that death has to the christian anything in it that should make it rare, or other than a constant, thought; out of the view of death comes the view of the life beyond the grave, as out of the view of sin (the true and the real view which the holy spirit alone can give to a man) comes the glorious hope.... my worldly faculties are slipping away day by day. happy is it for all of us that the true good lies not in them. as they ebb, may they leave us as little children trusting in the father of mercies, and accepting his unspeakable gift." and when the dark shadow was creeping over him, he wrote to the comte de paris: "i bow before him who is lord of all, and hope to be kept waiting patiently for his time and mode of releasing me according to his divine word, and the great and precious promises whereby his people are made partakers of the divine nature." his niece, miss jane barnard, who tended him with most devoted care, thus wrote from hampton court on the th june:--"the kind feelings shown on every side towards my dear uncle, and the ready offers of help, are most soothing. i am thankful to say that we are going on very quietly; he keeps his bed and sleeps much, and we think that the paralysis gains on him, but between whiles he speaks most pleasant words, showing his comfort and trust in the finished work of our lord. the other day he repeated some verses of the th psalm, and yesterday a great part of the rd. we can only trust that it may be given us to say truly, 'thy will be done;' indeed, the belief that all things work together for good to them that believe, is an anchor of hope, sure and steadfast, to the soul. we are surrounded by most kind and affectionate friends, and it is indeed touching to see what warm feelings my dear uncle has raised on all sides." when his faculties were fading fast, he would sit long at the western window, watching the glories of the sunset; and one day when his wife drew his attention to a beautiful rainbow that then spanned the sky, he looked beyond the falling shower and the many-coloured arch, and observed, "he hath set his testimony in the heavens." on august , , quietly, almost imperceptibly, came the release. there was a philosopher less on earth, and a saint more in heaven. the funeral, at his own request, was of the simplest character. his remains were conveyed to highgate cemetery by his relations, and deposited in the grave, according to the practice of his church, in perfect silence. few of his scientific friends were in london that bright summer-time, but professor graham and one or two others came out from the shrubbery, and joining the group of family mourners, took their last look at the coffin. but when this sun had set below our earthly horizon, there seemed to spring up in the minds of men a great desire to catch some of the rays of the fading brightness and reflect them to posterity. a "faraday memorial" was soon talked of, and the work is now in the sculptor's hands; the chemical society has founded a "faraday lectureship;" one of the new streets in paris has been called "rue faraday;" biographical sketches have appeared in many of the british and continental journals; successive books have told the story of his life and work; and in a thousand hearts there is embalmed the memory of this christian gentleman and philosopher. footnotes: [ ] these books, with others bound by faraday, are preserved in a special cabinet at the royal institution, together with more valuable documents,--the laboratory notes of davy and those of faraday, his notes of tatum's and davy's lectures, copies of his published papers with annotations and indices, notes for lectures and friday evening discourses, account books, and various memoranda, together with letters from wollaston, young, herschel, whewell, mitscherlich, and many others of his fellow-workers in science. these were the gift of his widow, in accordance with his own desire. [ ] this idea was suggested by some remarks of faraday to the baroness burdett coutts. [ ] sir roderick murchison used to tell how he was attending brande's lectures, when one day, the professor being absent, his assistant took his place, and lectured with so much ease that he won the complete approval of the audience. this, he said, was faraday's first lecture at the royal institution. [ ] the laboratory note-book shows that at this very time he was making a long series of commercial analyses of saltpetre for mr. brande. [ ] the following anecdote has been sent me on the authority of mr. benjamin abbott:--"sergeant anderson was engaged to attend to the furnaces in mr. faraday's researches on optical glass in , and was chosen simply because of the habits of strict obedience his military training had given him. his duty was to keep the furnaces always at the same heat, and the water in the ashpit always at the same level. in the evening he was released, but one night faraday forgot to tell anderson he could go home, and early next morning he found his faithful servant still stoking the glowing furnace, as he had been doing all night long." a more probable and better authenticated version of this story is that after nightfall anderson went upstairs to faraday, who was already in bed, to inquire if he was to remain still on duty. [ ] one evening, when the rev. a. j. d'orsey was lecturing "on the study of the english language," he mentioned as a common vulgarism that of using "don't" in the third person singular, as "he don't pay his debts." faraday exclaimed aloud, "that's very wrong." [ ] the _st. paul's magazine_, june . [ ] _british quarterly review_, april . [ ] see appendix. [ ] no wonder the celebrated electrician p. riess, of berlin, once addressed a long letter to him as "professor michael faraday, member of all academies of science, london." section ii. study of his character. in the previous section we have traced the leading events of a life which was quietly and uniformly successful. we have watched the passage of the errand-boy into the philosopher, and we have seen how at first he begged for the meanest place in a scientific workshop, and at last declined the highest honour which british science was capable of granting. his success did not lie in the amassing of money--he deliberately turned aside from the path of proffered wealth; nor did it lie in the attainment of social position and titles--he did not care for the weight of these. but if success consists in a life full of agreeable occupation, with the knowledge that its labours are adding to the happiness and wealth of the world, leading on to an old age full of honour, and the prospect of a blissful immortality,--then the highest success crowned the life of faraday. how did he obtain it? not by inheritance, and not by the force of circumstances. the wealth or the reputation of fathers is often an invaluable starting-point for sons: a liberal education and the contact of superior minds in early youth is often a mighty help to the young aspirant: the favour of powerful friends will often place on a vantage-ground the struggler in the battle of life. but faraday had none of these. accidental circumstances sometimes push a man forward, or give him a special advantage over his fellows; but faraday had to make his circumstances, and to seize the small favours that fortune sometimes threw in his way. the secret of his success lay in the qualities of his mind. it is only fair, however, to remark that he started with no disadvantages. there was no stain in the family history: he had no dead weight to carry, of a disgraced name, or of bad health, or deficient faculties, or hereditary tendencies to vice. it must be acknowledged, too, that he was endowed with a naturally clear understanding and an unusual power of looking below the surface of things. the first element of success that we meet with in his biography is the faithfulness with which he did his work. this led the bookseller to take his poor errand-boy as an apprentice; and this enabled his father to write, when he was : "michael is bookbinder and stationer, and is very active at learning his business. he has been most part of four years of his time out of seven. he has a very good master and mistress, and likes his place well. he had a hard time for some while at first going; but, as the old saying goes, he has rather got the head above water, as there is two other boys under him." this faithful industry marked also his relations with davy and brande, and the whole of his subsequent life; and at last, when he found that he could no longer discharge his duties, it made him repeatedly press his resignation on the managers of the royal institution, and beg to be relieved of his eldership in the church. his love of study, and hunger after knowledge, led him to the particular career which he pursued, and that power of imagination, which reveals itself in his early letters, grew and grew, till it gave him such a familiarity with the unseen forces of nature as has never been vouchsafed to any other mortal. as a source of success there stands out also his enthusiasm. a new fact seemed to charge him with an energy that gleamed from his eyes and quivered through his limbs, and, as by induction, charged for the time those in his presence with the same vigour of interest. plücker, of bonn, was showing him one day in the laboratory at albemarle street his experiments on the action of a magnet on the electric discharge in vacuum tubes. faraday danced round them; and as he saw the moving arches of light, he cried, "oh! to live in it!" mr. james heywood once met him in the thick of a tremendous storm at eastbourne, rubbing his hands with delight because he had been fortunate enough to see the lightning strike the church tower, and displace a pinnacle. this enthusiasm led him to throw all his heart into his work. nor was the energy spasmodic, or wasted on unworthy objects; for, in the words of bence jones, his was "a lifelong lasting strife to seek and say that which he thought was true, and to do that which he thought was kind." indeed, his perseverance in a noble strife was another of the grand elements in his success. his tenacity of purpose showed itself equally in little and in great things. arranging some apparatus one day with a philosophical instrument maker, he let fall on the floor a small piece of glass: he made several ineffectual attempts to pick it up. "never mind," said his companion, "it is not worth the trouble." "well, but, murray, i don't like to be beaten by something that i have once tried to do." the same principle is apparent in that long series of electrical researches, where for a quarter of a century he marched steadily along that path of discovery into which he had been lured by the genius of davy. and so, whatever course was set before him, he ran with patience towards the goal, not diverted by the thousand objects of interest which he passed by, nor stopping to pick up the golden apples that were flung before his feet. this tremendous faculty of work was relieved by a wonderful playfulness. this rarely appears in his writings, but was very frequent in his social intercourse. it was a simple-hearted joyousness, the effervescence of a spirit at peace with god and man. it not seldom, however, assumed the form of good-natured banter or a practical joke. indications of this playfulness have already been given, and i have tried to put upon paper some instances that occur to my own recollection, but the fun depended so much upon his manner, that it loses its aroma when separated from himself. however, i will try one story. i was spending a night at an hotel at ramsgate when on lighthouse business. early in the morning there came a knock at the bed-room door, but, as i happened to be performing my ablutions, i cried, "who's there?" "guess." i went over the names of my brother commissioners, but heard only "no, no," till, not thinking of any other friend likely to hunt me up in that place, i left off guessing; and on opening the door i saw faraday enjoying with a laugh my inability to recognize his voice through a deal board. a student of the late professor daniell tells me that he remembers faraday often coming into the lecture-room at king's college just when the professor had finished and was explaining matters more fully to any of his pupils who chose to come down to the table. one day the subject discoursed on and illustrated had been sulphuretted hydrogen, and a little of the gas had escaped into the room, as it perversely will do. when faraday entered he put on a look of astonishment, as though he had never smelt such a thing before, and in a comical manner said, "ah! a savoury lecture, daniell!" on another occasion there was a little ammonia left in a jar over mercury. he pressed daniell to tell him what it was, and when the professor had put his head down to see more clearly, he whiffed some of the pungent gas into his face. occasionally this humour was turned to good account, as when, one friday evening before the lecture, he told the audience that he had been requested by the managers to mention two cases of infringement of rule. the first related to the red cord which marks off the members' seats. "the second case i take to be a hypothetical one, namely, that of a gentleman wearing his hat in the drawing-room." this produced a laugh, which the professor joined in, bowed, and retired. this faithful discharge of duty, this almost intuitive insight into natural phenomena, and this persevering enthusiasm in the pursuit of truth, might alone have secured a great position in the scientific world, but they alone could never have won for him that large inheritance of respect and love. his contemporaries might have gazed upon him with an interest and admiration akin to that with which he watched a thunderstorm; but who feels his affections drawn out towards a mere intellectual jupiter? we must look deeper into his character to understand this. there is a law well recognized in the science of light and heat, that a body can absorb only the same sort of rays which it is capable of emitting. just so is it in the moral world. the respect and love of his generation were given to faraday because his own nature was full of love and respect for others. each of these qualities--his respect for and love to others, or, more generally, his reverence and kindliness--deserves careful examination. throughout his life, michael faraday appeared as though standing in a reverential attitude towards nature, man, and god. towards nature, for he regarded the universe as a vast congeries of facts which would not bend to human theories. speaking of his own early life, he says: "i was a very lively imaginative person, and could believe in the 'arabian nights' as easily as in the 'encyclopædia;' but facts were important to me, and saved me. i could trust a fact, and always cross-examined an assertion." he was indeed a true disciple of that philosophy which says, "man, who is the servant and interpreter of nature, can act and understand no farther than he has, either in operation or in contemplation, observed of the method and order of nature."[ ] and verily nature admitted her servant into her secret chambers, and showed him marvels to interpret to his fellow-men more wonderful and beautiful than the phantasmagoria of eastern romance. his reverence towards man showed itself in the respect he uniformly paid to others and to himself. thoroughly genuine and simple-hearted himself, he was wont to credit his fellow-men with high motives and good reasons. this was rather uncomfortable when one was conscious of no such merit, and i at least have felt ashamed in his presence of the poor commonplace grounds of my words and actions. to be in his company was in fact a moral tonic. as he had learned the difficult art of honouring all men, he was not likely to run after those whom the world counted great. "we must get garibaldi to come some friday evening," said a member of the institution during the visit of the italian hero to london. "well, if garibaldi thinks he can learn anything from us, we shall be happy to see him," was faraday's reply. this nobility of regard not only preserved him from envying the success of other explorers in the same field, but led him heartily to rejoice with them in their discoveries. dumas gives us a picture of foucault showing faraday some of his admirable experiments, and of the two men looking at one another with eyes moistened, but full of bright expression, as they stood hand in hand, silently thankful--the one for the pleasure he had experienced, the other for the honour that had been done him. he also tells how, on another occasion, he breakfasted at albemarle street, and during the meal mr. faraday made some eulogistic remarks upon davy, which were coldly received by his guest. after breakfast, he was taken downstairs to the ante-room of the lecture theatre, when faraday, walking up to the portrait of his old master, exclaimed, "wasn't he a great man!" then turning round to the window next the entrance door, he added, "it was there that he spoke to me for the first time." the frenchman bowed. they descended the stairs again to the laboratory. faraday pulled out an old note-book, and turning over its pages showed where davy had entered the means by which the first globule of potassium was produced, and had drawn a line round the description, with the words, "capital experiment." the french chemist owned himself vanquished, and tells the tale in honour of him who remembered the greatness and forgot the littlenesses of his teacher. and the respect he showed to others he required to be shown to himself. it is difficult to imagine anyone taking liberties with him, and it was only in early life that there were small-minded creatures who would treat him not according to what he was, but according to the position from which he had risen. his servants and workpeople were always attentive to the smallest expression of his wish. still, he did not "go through his life with his elbows out." he once wrote to matteucci: "i see that that moves you which would move me most, viz. the imputation of a want of good faith; and i cordially sympathize with anyone who is so charged unjustly. such cases have seemed to me almost the only ones for which it is worth while entering into controversy. i have felt myself not unfrequently misunderstood, often misrepresented, sometimes passed by, as in the cases of specific inductive capacity, magneto-electric currents, definite electrolytic action, &c. &c.: but it is only in the cases where moral turpitude has been implied, that i have felt called upon to enter on the subject in reply." yet, where he felt that his honour was impugned, none could be more sensitive or more resolute. this desire to clear himself, combined with his delicate regard for the feelings of others, struck me forcibly in the following incident. at mr. barlow's, one friday evening after the discourse, two or three other chemists and myself were commenting unfavourably on a public act of faraday, when suddenly he appeared beside us. i did not hesitate to tell him my opinion. he gave me a short answer, and joined others of the company. a few days afterwards he found me in the laboratory preparing for a lecture, and, without referring directly to what i had said, he gave me a full history of the transaction in such a way as to show that he could not have acted otherwise, and at the same time to render any apology on my part unnecessary. intimately connected with his respect for man as well as reverence for truth, was the flash of his indignation against any injustice, and his hot anger against any whom he discovered to be pretenders. when, for instance, he had convinced himself that the reputed facts of table-turning and spiritualism were false, his severe denunciation of the whole thing followed as a matter of course. thus, too, a story is told of his once taking the side of the injured in a street quarrel by the pump in savile row. one evening also at my house, a young man who has since acquired a scientific renown was showing specimens of some new compounds he had made. a well-known chemist contemptuously objected that, after all, they were mere products of the laboratory: but faraday came to the help of the young experimenter, and contended that they were chemical substances worthy of attention, just as much as though they occurred in nature. his reverence for god was shown not merely by that homage which every religious man must pay to his creator and redeemer, but by the enfolding of the words of scripture and similar expressions in such a robe of sacredness, that he rarely allowed them to pass his lips or flow from his pen, unless he was convinced of the full sympathy of the person with whom he was holding intercourse. this characteristic reverence was united to an equally characteristic kindliness. this word does not exactly express the quality intended; but unselfishness is negative, goodness is too general, love is commonly used with special applications; kindness, friendship, geniality, and benevolence are only single aspects of the quality. let the reader add these terms all together, and the resultant will be about what is meant.[ ] faraday's love to children was one way in which this kindliness was shown. having no children of his own, he surrounded himself usually with his nieces: we have already had a glimpse of him heartily entering into their play, and we are told how a word or two from uncle would clear away all the trouble from a difficult lesson, that a long sum in arithmetic became a delight when he undertook to explain it, and that when the little girl was naughty and rebellious, he could gently win her round, telling her how he used to feel himself when he was young, and advising her to submit to the reproof she was fighting against. nor were his own relatives the only sharers of his kindness. one friend cherishes among his earliest recollections, that of faraday making for him a fly-cage and a paper purse, which had a real bright half-crown in it. when the present mr. baden powell was a little fellow of thirteen, he used to give short lectures on chemistry in his father's house, and the philosopher of albemarle street liked to join the family audience, and would listen and applaud the experiments heartily. when one day my wife and i called on him with our children, he set them playing at hide-and-seek in the lecture theatre, and afterwards amused them upstairs with tuning-forks and resounding glasses. at a _soirée_ at mr. justice grove's, he wanted to see the younger children of the family; so the eldest daughter brought down the little ones in their nightgowns to the foot of the stairs, and faraday expressed his gratification with "ah! that's the best thing you have done to-night." and when his faculties had nearly faded, it is remembered how the stroking of his hand by mr. vincent's little daughter quickened him again to bright and loving interest. it would be easy to multiply illustrations of this kindliness in various relations of life. here is one of his own telling, where certainly the effect produced was not owing to any knowledge of how princely an intellect underlay the loving spirit. it is from a journal of his tour in wales:-- "_tuesday, july th._--after dinner i set off on a ramble to melincourt, a waterfall on the north side of the valley, and about six miles from our inn. here i got a little damsel for my guide who could not speak a word of english. we, however, talked together all the way to the fall, though neither knew what the other said. i was delighted with her burst of pleasure as, on turning a corner, she first showed me the waterfall. whilst i was admiring the scene, my little welsh damsel was busy running about, even under the stream, gathering strawberries. on returning from the fall i gave her a shilling that i might enjoy her pleasure: she curtsied, and i perceived her delight. she again ran before me back to the village, but wished to step aside every now and then to pull strawberries. every bramble she carefully moved out of the way, and ventured her bare feet to try stony paths, that she might find the safest for mine. i observed her as she ran before me, when she met a village companion, open her hand to show her prize, but without any stoppage, word, or other motion. when we returned to the village i bade her good-night, and she bade me farewell, both by her actions and, i have no doubt, her language too." in a letter which mr. abel, the director of the chemical department of the war establishment, has sent me, occur the following remarks:-- "early in i was appointed, partly through the kind recommendation of faraday, to instruct the senior cadets and a class of artillery officers in the arsenal, in practical chemistry. on the occasion of my first attendance at woolwich, when, having just reached manhood, i was about to deliver my first lecture as a recognized teacher, i was naturally nervous, and was therefore dismayed when on entering the class-room i perceived faraday, who, having come to woolwich, as usual, to prepare for his next morning's lecture at the military academy, had been prompted by his kindly feelings to lend me the support of his presence upon my first appearance among his old pupils. in a moment faraday put me completely at my ease; he greeted me heartily, saying, 'well, abel, i have come to see whether i can assist you;' and suiting action to word, he bustled about, persisting in helping me in the arrangement of my lecture-tables,--and at the close of my demonstration he followed me from pupil to pupil, aiding each in his first attempt at manipulation, and evidently enjoying most heartily the self-imposed duty of assistant to his young _protégé_." another scientific friend, mr. w. f. barrett, writes:--"my first interview with mr. faraday ten years ago left an impression upon me i can never forget. young student as i then was, thinking chiefly of present work and little of future prospects, and till then unknown to mr. faraday, judge of my feelings when, taking my hand in both of his, he said, 'i congratulate you upon choosing to be a _philosopher_: it is an arduous life, but a noble and a glorious one. work hard, and work carefully, and you will have success.' the sweet yet serious way he said this made the earnestness of work become a very vivid reality, and led me to doubt whether i had not dared to undertake too lofty a pursuit. after this mr. faraday never forgot to remember me in a number of thoughtful and delicate ways. he would ask me upstairs to his room to describe or show him the results of any little investigation i might have made: taking the greatest interest in it all, his pleasure would seem to equal and thus heighten mine, and then he would add words of kind suggestion and encouragement. in the same kindly spirit he has invited me to his house at hampton court, or would ask me to join him at supper after the friday evening's lecture. his kindness is further shown by his giving me a volume of his researches on chemistry and physics, writing therein, 'from his friend michael faraday.' those who live alone in london, unknown and uncared-for by any around them, can best appreciate these marks of attention which mr. faraday invariably showed, and not only to myself, but equally to my fellow-assistant in the chemical laboratory." the following instance among many that might be quoted will illustrate his readiness to take trouble on behalf of others. when dr. noad was writing his "manual of electricity," a doubt crossed his mind as to whether sir snow harris's unit jar gave a true measure of the quantity of electricity thrown into a leyden jar: he asked faraday, and his doubt was confirmed. shortly afterwards he received a letter beginning thus:-- "my dear sir, "whilst looking over my papers on induction, i was reminded of our talk about harris's unit jar, and recollected that i had given you a result just the _reverse_ of my old conclusions, and, as i believe, of the truth. i think the jar _is a true measure_, so long as the circumstances of position, &c., are not altered; for its discharge and the quantity of electricity thus passed on depends on the constant relation of the balls connected with the inner and outer surface coating to each other, and is independent of their joint relation to the machine, battery, &c.... perhaps i have not made my view clear, but next time we meet, remind me of the matter. "ever truly yours, "m. faraday." and just a week afterwards dr. noad received a second letter, surmounted by a neat drawing, and describing at great length experiments that the professor had since made in order to place the matter beyond doubt. and it was not merely for friends and brother _savants_ that he would take trouble. old volumes of the _mechanics' magazine_ bear testimony to the way in which he was asked questions by people in all parts of the kingdom, and that he was accustomed to give painstaking answers to such letters. "do to others as you would wish them to do to you," was a precept often on his lips. but i have heard that he was sometimes charged with transgressing it himself, inasmuch as he took an amount of trouble for other people which he would have been greatly distressed if they had taken for him. his charities were very numerous,--not to beggars; for them he had the mendicity society's tickets,--but to those whose need he knew. the porter of the royal institution has shown me, among his treasured memorials, a large number of forms for post-office orders, for sums varying from _s._ to _l._, which faraday was in the habit of sending in that way to different recipients of his thoughtful bounty. two or three instances have come to my knowledge of his having given more considerable sums of money--say _l._--to persons who he thought would be benefited by them. in some instances the gift was called a loan, but he lent "not expecting again," and entered into the spirit of the injunction, "when thou doest alms, let not thy left hand know what thy right hand doeth." this principle was in fact stated in one of his letters to a friend: "as a case of distress i shall be very happy to help you as far as my means allow me in such cases; but then i never let my name go to such acts, and very rarely even the initials of my name." his contributions to the general funds of his church were kept equally secret. from all these circumstances, therefore, it is impossible to gauge the amount of his charitable gifts; but when it is remembered that for many years his income from different sources must have been , _l._ or , _l._, that he and mrs. faraday lived in a simple manner--comfortably, it is true, but not luxuriously--and that his whole income was disposed of in some way, there can be little doubt that his gifts amounted to several hundred pounds per annum. but it was not in monetary gifts alone that his kindness to the distressed was shown. time was spent as freely as money; and an engrossing scientific research would not be allowed to stand in the way of his succouring the sorrowful. many persons have told me of his self-denying deeds on behalf of those who were ill, and of his encouraging words. he had indeed a heart ever ready to sympathize. thus meeting once in the neighbourhood of hampton court an old friend who had retired there invalided and was being drawn about in a bath chair, he is said to have burst into tears. when eight years ago my wife and my only son were taken away together, and i lay ill of the same fatal disease, he called at my house, and in spite of remonstrances found his way into the infected chamber. he would have taken me by the hand if i had allowed him; and then he sat a while by my bedside, consoling me with his sympathy and cheering me with the christian hope. it is no wonder that this kindliness took the hearts of men captive; and this quality was, like mercy, "twice blessed; it blesseth him that gives, and him that takes." the feeling awakened in the minds of others by this kindliness was indeed a source of the purest pleasure to himself; trifling proofs of interest or love could easily move his thankfulness; and he richly enjoyed the appreciation of his scientific labours. this would often break forth in words. thus in the middle of a letter to a. de la rive, principally on scientific matters, he writes:-- "do you remember one hot day, i cannot tell how many years ago, when i was hot and thirsty in geneva, and you took me to your house in the town and gave me a glass of water and raspberry vinegar? that glass of drink is refreshing to me still." again: "tyndall, the sweetest reward of my work is the sympathy and good-will which it has caused to flow in upon me from all quarters of the world." but to estimate rightly this amiability of character, it must be distinctly remembered that it was not that superabundance of good-nature which renders some men incapable of holding their own, or rebuking what they know to be wrong. in proof of this his letters to the spiritualists might be quoted; but the following have not hitherto seen the light. they are addressed to two different parties whose inventions came officially before him. "you write 'private' on the outside of your official communication, and 'confidential' within. i will take care to respect these instructions as far as falls within my duty; but i can have nothing private or confidential _as regards the trinity house_, which is my chief. whatever opinion i send to them i must accompany with the papers you send me. if therefore you wish anything held back from them, send me another official answer, and i will return you the one i have, marked 'confidential.' our correspondence is indeed likely to become a little irregular, because your papers have not come to me through the trinity house. you will feel that i cannot communicate any opinion i may form to you: i am bound to the trinity house, to whom i must communicate in confidence. i have no objection to your knowing my conclusions; but the _trinity house_ is the fit judge of the use it may make of them, or the degree of confidence they may think they deserve, or the parties to whom they may choose to communicate them." by a foot-note it appears that the _private and confidential_ communication was returned to the writer, by desire, four days afterwards. "sir, "i have received your note and read your pamphlet. there is nothing in either which makes it at all desirable to me to see your apparatus, for i have not time to spare to look at a matter two or three times over. in referring to ----, i suppose you refer also to his application to the trinity house. in that case i shall hear from him _through the trinity house_. he has, however, certain inquiries (which i have no doubt have gone to him long ago through the trinity house) to answer before i shall think it necessary to take any further steps in the matter. with these, however, i suppose you have nothing to do. "are you aware that many years ago our institution was lighted up for months, if not for years together, by oil-gas (or, as you call it, olefiant gas), compressed into cylinders to the extent of thirty atmospheres, and brought to us from a distance? i have no idea that the patent referred to at the bottom of page could stand for an hour in a court of law. i think, too, you are wrong in misapplying the word _olefiant_. it already belongs to a particular gas, and cannot, without confusion, be used as you use it. "i am sir, "your obedient servant, "m. faraday." "sir, "thanks for your letter. at the close of it you ask me _privately_ and confidingly for the encouragement my opinion might give you if _this power_ gas-light is fit for lighthouses. i am unable to assent to your request, as my position at the trinity house requires that i should be able to take up any subject, applications, or documents they may bring before me in a perfectly unbiassed condition of mind. "i am, sir, "yours very truly, "m. faraday." the kindliness which shed its genial radiance on every worthy object around, glowed most warmly on the domestic hearth. little expressions in his writings often reveal it, as when we read in his swiss journal about interlaken: "clout-nail making goes on here rather considerably, and is a very neat and pretty operation to observe. i love a smith's shop, and anything relating to smithery. my father was a smith." when he was sitting to noble for his bust, it happened one day that the sculptor, in giving the finishing touches to the marble, made a clattering with his chisels: noticing that his sitter appeared _distrait_, he said that he feared the jingling of the tools had annoyed him, and that he was weary. "no, my dear mr. noble," said faraday, putting his hand on his shoulder, "but the noise reminded me of my father's anvil, and took me back to my boyhood." this deep affection peeps out constantly in his letters to different members of his family, "bound up together," as he wrote to his sister-in-law, "in the one hope, and in faith and love which is in jesus christ." but it was towards his wife that his love glowed most intensely. yet how can we properly speak of this sacred relationship, especially as the mourning widow is still amongst us? it may suffice to catch the glimpse that is reflected in the following extract from a letter he wrote to mrs. andrew crosse on the death of her husband:-- "_july , ._ "... believe that i sympathize with you most deeply, for i enjoy in my life-partner those things which you speak of as making you feel your loss so heavily. "it is the kindly domestic affections, the worthiness, the mutual aid in sorrow, the mutual joy in happiness that has existed, which makes the rupture of such a tie as yours so heavy to bear; and yet you would not wish it otherwise, for the remembrance of those things brings solace with the grief. i speak, thinking what my own trouble would be if i lost my partner; and i try to comfort you in the only way in which i think i could be comforted. "m. faraday." there was, as tyndall has observed, a mixture of chivalry with this affection. in his book of diplomas he made the following remarkable entry:-- "_ th january, _. "amongst these records and events, i here insert the date of one which, as a source of honour and happiness, far exceeds all the rest. we were _married_ on june , . "m. faraday." on the character of faraday, these two qualities of reverence and kindliness have appeared to me singularly influential. among the ways in which they manifested themselves was that beautiful combination of firmness and gentleness which has been frequently remarked: intimately associated with them also were his simplicity and truthfulness. these points must have made themselves evident already, but they deserve further illustration. in his early days, "one sabbath morning his swift and sober steps were carrying him along the holborn pavement towards his meeting-house, when some small missile struck him smartly on the hat. he would have thought it an accident and passed on, when a second and a third rap caused him to turn and look just in time to perceive a face hastily withdrawn from a window in the upper story of a closed linendraper's establishment. roused by the affront, he marched up to the door and rapped. the servant opening it said there was no one at home, but faraday declared he knew better, and desired to be shown upstairs. opposition still being made, he pushed on, made his way up through the house, opened the door of an upper room, discovering a party of young drapers' assistants, who at once professed they knew nothing of the motive of this sudden visit. but the hunter had now run his game to earth: he taxed them sharply with their annoyance of wayfarers on the sabbath, and said that unless an apology were made at once, they should hear from their employer of something much to their disadvantage. an apology was made forthwith."[ ] long, long after this event, dr. and mrs. faraday, with dr. tyndall, were returning one evening from mr. gassiot's, on clapham common: a dense fog came on, and they did not know where they were. the two gentlemen got out of their vehicle, and walked to a house and knocked. a man appeared, first at a window and afterwards at the door, very angry indeed at the disturbance, and demanded to know their business. faraday, in his calm, irresistible manner, explained the situation and their object in knocking. the man instantly changed his tone, looked foolish, and muttered something about being in a fright lest his house of business was on fire. as to simplicity of character: when, in the course of writing this book, i have spoken to his acquaintances about faraday, the most frequent comment has been in such words as, "oh! he was a beautiful character, and so simple-minded." i have tried to ascertain the cause of this simple-mindedness, and i believe it was the consciousness that he was meaning to do right himself, and the belief that others whom he addressed meant to do right too, and so he could just let them see everything that was passing through his mind. and while he knew no reason for concealment, there was no trace of self-conceit about him, nor any pretence at being what he was not. to illustrate this quality is not so easy; the indications of it, like his humour, were generally too delicate to be transferred to paper; but perhaps the following letter will do as well as anything else, for there are few philosophers who could have written so naturally about the pleasures of a pantomime and then about his highest hopes:-- "royal institution, london, w. "_ st january, ._ "my dear miss coutts, "you are very kind to think of our pleasure and send us entrance to your box for to-morrow night. we thank you very sincerely, and i mean to enjoy it, for i still have a sympathy with children and all their thoughts and pleasure. permit me to wish you very sincerely a happy year; and also to mrs. brown. with some of us our greatest happiness will be content mingled with patience; but there is much happiness in that and the expected end. "ever your obliged servant, "m. faraday."[ ] as to truthfulness: he was not only truthful in the common acceptation of the word, but he did not allow, either in himself or others, hasty conclusions, random assertions, or slippery logic. "at such times he had a way of repeating the suspicious statement very slowly and distinctly, with an air of wondering scrutiny as if it had astonished him. his irony was then irresistible, and always produced a modification of the objectionable phrase." one friday evening there was exhibited an improved davy lamp, with an eulogistic description. faraday added the words, "the opinion of the inventor." "an acquaintance rather given to inflict tedious narratives on his friends was descanting to faraday on the iniquity of some coachman who had set him down the previous night in the middle of a dark and miry road,--'in fact,' said the irksome drawler, 'in a perfect morass; and there i was, as you may imagine, half the night, plunging and struggling to get out of this dreadful morass.' 'more ass you!' rapped out the philosopher at the top of his scale of laughter." this was a rare instance, for it was only when much provoked that he would perpetrate a pun, or depart from the kind courtesy of his habitual talk. that he was quite ready to give up a statement or view when it was proved by others to be incorrect, is shown by the preface to the volumes in which are reprinted his "experimental researches." "in giving advice," says his niece miss reid, "he always went back to first principles, to the true right and wrong of questions, never allowing deviations from the simple straightforward path of duty to be justified by custom or precedent; and he judged himself strictly by the same rule which he laid down for others." these beauties of character were not marred by serious defects or opposing faults. "he could not be too closely approached. there were no shabby places or ugly comers in his mind." yet he was very far from being one of those passionless men who resemble a cold statue rather than throbbing flesh and blood. he was no "model of all the virtues," dreadfully uninteresting, and discouraging to those who feel such calm perfection out of their reach. his inner life was a battle, with its wounds as well as its victory. proud by nature, and quick-tempered, he must have found the curb often necessary; but notwithstanding the rapidity of his actions and thoughts, he knew how to keep a tight rein on that fiery spirit. i have listened attentively to every remark in disparagement of faraday's character, but the only serious ones have appeared to me to arise from a misunderstanding of the man, a misunderstanding the more easy because his standard of right and wrong, and of his own duty, often differed from the notions current around him. still, it may be true that his extreme sensitiveness led him sometimes to do scant justice to those who, he imagined, were treading too closely in his own footsteps; as, for instance, when nobili brought out some beautiful experiments on magnetism, just after the short notice of his own discoveries in which faraday had sent to m. hachette, and which was communicated to the académie des sciences. it is true also that, with his great caution and his repugnance to moral evil, he was more disposed to turn away in disgust from an erring companion than to endeavour to reclaim him. it has also been imputed to him as a fault that he founded no school, and took no young man by the hand as davy had taken him. that this was rather his misfortune than his fault, would appear from words he once wrote to miss moore: "i have often endeavoured to discover a genius, but have not been very successful, though many cases seemed promising at first." the world would doubtless have been the gainer if he had stamped his own image on the minds of a group of disciples: but a man cannot do everything; and had faraday been more of a teacher, he would perhaps have been less of an investigator. of course faraday was subject, like other men, to errors of judgment, and it was impossible, even if desirable, always to avoid giving offence. thus he was constantly pestered for his autograph; and instead of throwing the applications into his waste-paper basket, he had a formal circular lithographed excusing himself from complying. this offended more than one recipient; and he was roughly made aware of it by once having the circular returned from st. louis with a scurrilous comment, and the postage from america not prepaid. he never again used the printed form, miss barnard undertaking to answer all such requests. it has been previously remarked that faraday took little part in social movements, and went little into society, but it must not be supposed that he was by any means unsocial. it seems probable that his freedom in this matter was somewhat hampered by the principles in which he had been brought up: it is certain that he was restrained by the desire to give all the time and energy he could to scientific research. yet pleasant stories are told of his occasional appearances at social gatherings. thus he liked to attend the royal academy dinners, and in earlier days he enjoyed the artistic and musical _conversaziones_ at hullmandel's, where stanfield turner and landseer met garcia and malibran; and sometimes he joined this pleasant company at supper and charades, at others in their excursions up the river in an eight-oared cutter. captain close has described to me how, when the french lighthouse authorities put up the screw-pile light on the sands near calais, they invited the trinity house officers and faraday to inspect it. a dinner was arranged for them after the inspection, and m. reynaud proposed the health of the _étranger célèbre_. a young engineer took exception to faraday being called a stranger--since he had been at st. cyr he had known the great englishman well by his works. the professor replied to the compliment in the language of his hosts, with a few of his happy and kindly remarks. a gentleman high in the diplomatic service, who was present, remarked that faraday had said many things which were not french, but not a word which ought not to be so. more unrestricted was faraday's sympathy with nature. he felt the poetry of the changing seasons, but there were two aspects of nature that especially seemed to claim communion with his spirit: he delighted in a thunderstorm, and he experienced a pleasurable sadness as the orange sunset faded into the evening twilight. there are other minds to which both these sensations are familiar, but they seem to have been felt with great intensity by him. no doubt his electrical knowledge added much to his interest in the grand discharges from the thunder-clouds, but it will hardly account for his standing long at a window watching the vivid flashes, a stranger to fear, with his mind full of lofty thoughts, or perhaps of high communings. sometimes, too, if the storm was at a little distance, he would summon a cab, and, in spite of the pelting rain, drive to the scene of awful beauty. on a clear starry night captain close quoted to him the words of lorenzo in the "merchant of venice:"-- ... "look, how the floor of heaven is thick inlaid with patines of bright gold; there's not the smallest orb, which thou behold'st, but in his motion like an angel sings, still quiring to the young-eyed cherubins; such harmony is in immortal souls; but, whilst this muddy vesture of decay doth grossly close us in, we cannot hear it." faraday, who happened not to be familiar with the passage, made his friend repeat it over and over again as he drank in the whole meaning of the poetry, for there is a true sense in which no other mortal had ever opened his ears so fully to the harmony of the universe. * * * * * from the plains of mental mediocrity there occasionally rise the mountains of genius, and from the dead level of selfish respectability there stand out now and then the peaks of moral greatness. neither kind of excellence is so common as we could wish it, and it is a rare coincidence when, as in socrates, the two meet in the same individual. in faraday we have a modern instance. there are persons now living who watched this man of strong will and intense feelings raising himself from the lower ranks of society, yet without losing his balance; rather growing in simplicity, disinterestedness, and humility, as princes became his correspondents and all the learned bodies of the world vied with each other to do him homage; still finding his greatest happiness at home, though reigning in the affections of all his fellows,--loving every honest man, however divergent in opinion, and loved most by those who knew him best. this is the phenomenon. by what theory is it to be accounted for? the secret did not lie in the nature of his pursuits. this cannot be better shown than in the following incident furnished me by mrs. crosse:--"one morning, a few months after we were married, my husband took me to the royal institution to call on mr. and mrs. faraday. i had not seen the laboratory there, and the philosopher very kindly took us over the institution, explaining for my information many objects of interest. his great vivacity and cheeriness of manner surprised me in a man who devoted his life to such abstruse studies, but i have since learnt to know that the highest philosophical nature is often, indeed generally, united with an almost childlike simplicity. "after viewing the ample appliances for experimental research, and feeling impressed by the scientific atmosphere of the place, i turned and said, 'mr. faraday, you must be very happy in your position and with your pursuits, which elevate you entirely out of the meaner aspects and lower aims of common life.' "he shook his head, and with that wonderful mobility of countenance which was characteristic, his expression of joyousness changed to one of profound sadness, he replied: 'when i quitted business, and took to science as a career, i thought i had left behind me all the petty meannesses and small jealousies which hinder man in his moral progress; but i found myself raised into another sphere, only to find poor human nature just the same everywhere--subject to the same weaknesses and the same self-seeking, however exalted the intellect.' "these were his words as well as i can recollect; and, looking at that good and great man, i thought i had never seen a countenance which so impressed me with the characteristic of perfect unworldliness. we know how his life proved that this rare qualification was indeed his." "childlike simplicity:" "unworldliness." where was the tree rooted that bore such beautiful blossoms? faraday had learnt in the school of christ to become "a little child," and he loved not the world because the love of the father was in him. we have a charming glimpse of this in an extract which professor tyndall has given from an old paper in which he wrote his impressions after one of his earliest dinners with the philosopher:--"at two o'clock he came down for me. he, his niece, and myself formed the party. 'i never give dinners,' he said; 'i don't know how to give dinners; and i never dine out. but i should not like my friends to attribute this to a wrong cause. i act thus for the sake of securing time for work, and not through religious motives as some imagine.' he said grace. i am almost ashamed to call his prayer a 'saying' of grace. in the language of scripture, it might be described as the petition of a son into whose heart god had sent the spirit of his son, and who with absolute trust asked a blessing from his father. we dined on roast beef, yorkshire pudding, and potatoes, drank sherry, talked of research and its requirements, and of his habit of keeping himself free from the distractions of society. he was bright and joyful--boylike, in fact, though he is now sixty-two. his work excites admiration, but contact with him warms and elevates the heart. here, surely, is a strong man. i love strength, but let me not forget the example of its union with modesty, tenderness, and sweetness, in the character of faraday." but his religion deserves a closer attention. when an errand-boy, we find him hurrying the delivery of his newspapers on a sunday morning so as to get home in time to make himself neat to go with his parents to chapel: his letters when abroad indicate the same disposition; yet he did not make any formal profession of his faith till a month after his marriage, when nearly thirty years of age. of his spiritual history up to that period little is known, but there seem to be good grounds for believing that he did not accept the religion of his fathers without a conscientious inquiry into its truth. it would be difficult to conceive of his acting otherwise. but after he joined the sandemanian church, his questionings were probably confined to matters of practical duty; and to those who knew him best nothing could appear stronger than his conviction of the reality of the things he believed. in order to understand the life and character of faraday, it is necessary to bear in mind not merely that he was a christian, but that he was a sandemanian. from his earliest years that religious system stamped its impress deeply on his mind, it surrounded the blacksmith's son with an atmosphere of unusual purity and refinement, it developed the unselfishness of his nature, and in his after career it fenced his life from the worldliness around, as well as from much that is esteemed as good by other christian bodies. to this small self-contained sect he clung with warm attachment; he was precluded from christian communion or work outside their circle, but his sympathies at least burst all narrow bounds. thus the abbé moigno tells us that at faraday's request he one day introduced him to cardinal wiseman. the interview was very cordial, and his eminence did not hesitate frankly and good-naturedly to ask faraday if, in his deepest conviction, he believed all the church of christ, holy, catholic, and apostolical, was shut up in the little sect in which he bore rule. "oh no!" was the reply; "but i do believe from the bottom of my soul that christ is with us." there were other points, too, in his character which reflected the colouring of the religious school to which he belonged. thus, while humility is inseparable from a christian life, there is a special phase of that virtue bred of those doctrines which teach that all our righteousness must be the unmerited gift of another: these doctrines are strongly insisted upon in the sandemanian church, and this humility was acquired in an intense degree by its minister. again, while all christians deplore the terrible amount of folly and sin in the world, most recognize also a large amount of good, and believe in progressive improvement; but small communities are apt to take gloomy views, and so did faraday, notwithstanding his personal happiness, and his firm conviction that "there is one above who worketh in all things, and who governs even in the midst of that misrule to which the tendencies and powers of men are so easily perverted." in writing to professor schönbein and a few other kindred spirits he would turn naturally enough from scientific to religious thoughts, and back again to natural philosophy, but he generally kept these two departments of his mental activity strangely distinct; yet of course it was well known that the professor at albemarle street was one of that long line of scientific men, beginning with the _savants_ of the east, who have brought to the redeemer the gold, frankincense, and myrrh of their adoration. but the peculiar features of faraday's spiritual life are matters of minor importance: the genuineness of his religious character is acknowledged by all. we have admired his faithfulness, his amiability of disposition, and his love of justice and truth; how far these qualities were natural gifts, like his clearness of intellect, we cannot precisely tell; but that he exercised constant self-control without becoming hard, ascended the pathway of fame without ever losing his balance, and shed around himself a peculiar halo of love and joyousness, must be attributed in no small degree to a heart at peace with god, and to the consciousness of a higher life. footnotes: [ ] bacon's "novum organum," i. . [ ] bence jones has used the greek agapê; and it was just this ideal of christian love which faraday set before himself. [ ] for this anecdote, and some others in inverted commas, i am indebted to mr. frank barnard. [ ] in another letter that lady burdett coutts has kindly sent me, faraday says: "we had your box once before, i remember, for a pantomime, which is always interesting to me because of the immense concentration of means which it requires." in a third he makes admiring comments on fechter. section iii. fruits of his experience. those who loved faraday would treasure every word that he wrote, and to them the life and letters which bence jones has given to the world will be inestimable; but from the multitude who knew him only at a distance, we can expect no enthusiasm of admiration. yet all will readily believe that through the writings of such a genius there must be scattered nuggets of intellectual gold, even when he is not treating directly of scientific subjects. some of these relate to questions of permanent interest, and such nuggets it is my aim to separate and lay before the reader. when quite a young man he drew the following ideal portrait:--"the philosopher should be a man willing to listen to every suggestion, but determined to judge for himself. he should not be biassed by appearances, have no favourite hypothesis, be of no school, and in doctrine have no master. he should not be a respecter of persons, but of things. truth should be his primary object. if to these qualities be added industry, he may indeed hope to walk within the veil of the temple of nature." this ideal he must steadily have kept before him, and not unfrequently in after days he gave utterance to similar thoughts. here are two instances, the first from a lecture thirty years afterwards, the second from a private letter:--"we may be _sure_ of facts, but our interpretation of facts we should doubt. he is the wisest philosopher who holds his theory with some doubt; who is able to proportion his judgment and confidence to the value of the evidence set before him, taking a fact for a fact, and a supposition for a supposition; as much as possible keeping his mind free from all source of prejudice, or, where he cannot do this (as in the case of a theory), remembering that such a source is there." the letter is to mr. frederick field, and relates to a paper on the existence of silver in the water of the ocean. "royal institution, _ st october, _. "my dear sir, "your paper looks so well, that though i am of course unable to become security for the facts, i have still thought it my duty to send it to the royal society. whether it will appear there or not i cannot say,--no one can say even for his own papers; but for my part, i think that as facts are the foundation of science, however they may be interpreted, so they are most valuable, and often more so than the interpretations founded upon them. i hope your further researches will confirm those you have obtained: but i would not be too hasty with them,--rather wait a while, and make them quite secure. "i am, sir, your obliged servant, "m. faraday." how pleasant it would have been to peep into his mind and watch the process by which he was transformed into the very image of his ideal philosopher! he has partially told us the secret in two remarkable lectures, one of which was delivered before the city philosophical society when he was only twenty-seven years of age, while the other formed part of a series on education at albemarle street. copious extracts from the first are given by dr. bence jones; the second was published at the time. in the early lecture, which is "on the forms of matter," he points out the advantages and dangers of systematizing, and winds up his remarks with-- "nothing is more difficult and requires more care than philosophical deduction, nor is there anything more adverse to its accuracy than fixidity of opinion. the man who is certain he is right is almost sure to be wrong, and he has the additional misfortune of inevitably remaining so. all our theories are fixed upon uncertain data, and all of them want alteration and support. ever since the world began opinion has changed with the progress of things; and it is something more than absurd to suppose that we have a sure claim to perfection, or that we are in possession of the highest stretch of intellect which has or can result from human thought. why our successors should not displace us in our opinions, as well as in our persons, it is difficult to say; it ever has been so, and from analogy would be supposed to continue so; and yet, with all this practical evidence of the fallibility of our opinions, all, and none more than philosophers, are ready to assert the real truth of their opinions." in his discourse entitled "observations on mental education," like a skilful physician he first determines what is the great intellectual disease from which the community suffers--"deficiency of judgment,"--and then he lays down rules by which each man may attempt his own cure. for this self-education, "it is necessary that a man examine himself, and that not carelessly.... a first result of this habit of mind will be an internal conviction of _ignorance in many things respecting which his neighbours are taught_, and that his opinions and conclusions on such matters ought to be advanced with reservation. a mind so disciplined will be _open to correction upon good grounds in all things_, even in those it is best acquainted with; and should familiarize itself with the idea of such being the case.... it is right that we should stand by and act on our principles, but not right to hold them in obstinate blindness, or retain them when proved to be erroneous." and then he gives cases from his own mental history:--"i remember the time when i believed a spark was produced between voltaic metals as they approached to contact (and the reasons why it might be possible yet remain); but others doubted the fact and denied the proofs, and on re-examination i found reason to admit their corrections were well founded. years ago i believed that electrolites could conduct electricity by a conduction proper; that has also been denied by many through long time: though i believed myself right, yet circumstances have induced me to pay that respect to criticism as to re-investigate the subject, and i have the pleasure of thinking that nature confirms my original conclusions. so, though evidence may appear to preponderate extremely in favour of a certain decision, it is wise and proper to hear a counter-statement. you can have no idea how often, and how much, under such an impression, i have desired that the marvellous descriptions which have reached me might prove, in some points, correct; and how frequently i have submitted myself to hot fires, to friction with magnets, to the passes of hands, &c., lest i should be shutting out discovery;--encouraging the strong desire that something might be true, and that i might aid in the development of a new force of nature." he turns then to another evil, and its cure: "the _tendency to deceive ourselves_ regarding all we wish for, and the necessity of _resistance to these desires_; ... the force of the temptation which urges us to seek for such evidence and appearances as are in favour of our desires, and to disregard those which oppose them, is wonderfully great. in this respect we are all, more or less, active promoters of error." he winds up his remarks upon this subject with the italicized sentence: "i will simply express my strong belief that that point of self-education which consists in teaching the mind to resist its desires and inclinations until they are proved to be right, is the most important of all, not only in things of natural philosophy, but in every department of daily life." he turns then to the necessity of a "habit of forming clear and precise ideas," and of expressing them in "clear and definite language:"--"when the different data required are in our possession, and we have succeeded in forming a clear idea of each, the mind should be instructed to _balance them_ one against another, and not suffered carelessly to hasten to a conclusion." "as a result of this wholesome mental condition, we should be able to form a _proportionate judgment_;" that is, one proportionate to the evidence, ranging through all degrees of probability--while he adds: "frequently the exercise of the judgment ought to end in _absolute reservation_." "the education which i advocate," says faraday, "will require _patience_ and _labour of thought_ in every exercise tending to improve the judgment. it matters not on what subject a person's mind is occupied; he should engage in it with the conviction that it will require mental labour." "because the education is internal, it is not the less needful; nor is it more the duty of a man that he should cause his child to be taught, than that he should teach himself. indolence may tempt him to neglect the self-examination and experience which form his school, and weariness may induce the evasion of the necessary practices; but surely a thought of the prize should suffice to stimulate him to the requisite exertion; and to those who reflect upon the many hours and days devoted by a lover of sweet sounds to gain a moderate facility upon a mere mechanical instrument, it ought to bring a correcting blush of shame if they feel convicted of neglecting the beautiful living instrument wherein play all the powers of the mind." at the commencement of this discourse the lecturer felt called upon to limit the range of his remarks:--"high as man is placed above the creatures around him, there is a higher and far more exalted position within his view; and the ways are infinite in which he occupies his thoughts about the fears, or hopes, or expectations of a future life. i believe that the truth of that future cannot be brought to his knowledge by any exertion of his mental powers, however exalted they may be; that it is made known to him by other teaching than his own, and is received through simple belief of the testimony given. let no one suppose for a moment that the self-education i am about to commend in respect of the things of this life extends to any considerations of the hope set before us, as if man by reasoning could find out god. it would be improper here to enter upon this subject further than to claim an absolute distinction between religious and ordinary belief. i shall be reproached with the weakness of refusing to apply those mental operations which i think good in respect of high things to the very highest. i am content to bear the reproach. yet, even in earthly matters, i believe that 'the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and godhead;' and i have never seen anything incompatible between those things of man which can be known by the spirit of man which is within him, and those higher things concerning his future which he cannot know by that spirit." there is of course a certain truth in this passage; spiritual discernment is a real thing possessed by some, and not by others; yet is there this absolute distinction between religious and ordinary belief? surely there is the same opportunity and the same necessity for careful judgment, and for resistance to prejudice or preference, when we are weighing the credentials of anything that may come before us purporting to be a revelation from above; surely too, if we have satisfied ourselves that we possess such a revelation, we must seek for the same clearness of ideas, and must exercise the same patience and labour of thought, if we would understand it aright. that mental discipline which fits us to interpret the works of god cannot but be akin to the intellectual training required for interpreting his word. since faraday thought and wrote, the question of public education has taken a far deeper hold on the feelings and the hopes of the nation, and it is not merely the extent of the instruction, but its nature also, that is discussed. it is held to be no longer right that the minds of our youth should be fed almost exclusively on the dry husks of classic or mediæval knowledge, while the rich banquet of modern discovery remains untasted. yet it is hard for natural science to gain an honoured place in our venerable scholastic institutions. faraday, however, had long formed his conclusions on this subject. in one of his friday evening discourses he says: "the development of the applications of physical science in modern times has become so large and so essential to the well-being of man, that it may justly be used as illustrating the true character of pure science as a department of knowledge, and the claims it may have for consideration by governments, universities, and all bodies to whom is confided the fostering care and direction of learning. as a branch of learning, men are beginning to recognize the right of science to its own particular place; for, though flowing in channels utterly different in their course and end from those of literature, it conduces not less, as a means of instruction, to the discipline of the mind, whilst it ministers, more or less, to the wants, comforts, and proper pleasure, both mental and bodily, of every individual of every class in life. until of late years, the education for, and recognition of it by the bodies which may be considered as governing the general course of all education, have been chiefly directed to it only as it could serve professional services, viz. those which are remunerated by society; but now the fitness of university degrees in science is under consideration, and many are taking a high view of it, as distinguished from literature, and think that it may well be studied for its own sake, _i.e._ as a proper exercise of the human intelligence, able to bring into action and development all the powers of the mind. as a branch of learning, it has (without reference to its applications) become as extensive and varied as literature; and it has this privilege, that it must ever go on increasing." on the subject of scientific education faraday was examined by the public schools commission, november th, , and his sentiments of course appear in their report. he said to them: "that the natural knowledge which has been given to the world in such abundance during the last fifty years should remain untouched, and that no sufficient attempt should be made to convey it to the young mind growing up and obtaining its first views of those things, is to me a matter so strange that i find it difficult to understand. though i think i see the opposition breaking away, it is yet a very hard one to overcome. that it ought to be overcome i have not the least doubt in the world." lord clarendon asked him: "you think it is now knocking at the door, and there is a prospect of the door being opened?" "yes," answered faraday, "and it will make its way, or we shall stay behind other nations in our mode of education." he had been led to the conviction that the exclusive attention to literary studies created a tendency to regard other things as nonsense, or belonging only to the artisan, and so the mind is positively injured for the reception of real knowledge. he says: "it is the highly educated man that we find coming to us again and again, and asking the most simple question in chemistry or mechanics; and when we speak of such things as the conservation of force, the permanency of matter, and the unchangeability of the laws of nature, they are far from comprehending them, though they have relation to us in every action of our lives. many of these instructed persons are as far from having the power of judging of these things as if their minds had never been trained." he gives his own opinion as to the precise course to be pursued with great diffidence; but it is evident that he would begin the education in natural science at a pretty early age, and in all cases carry it up to a certain point. one-fifth of a boy's time might be devoted to this purpose at present, though in less than half a century he thinks science will deserve and obtain a far larger share. supposing a boy of eleven years of age and of ordinary intelligence at one of our public schools: "i would teach him," he says, "all those things that come before classics in the programme of the london university,--mechanics, hydrostatics, hydraulics, pneumatics, acoustics, and optics. they are very simple and easily understood when they are looked at with attention by both man and boy. with a candle, a lamp, and a lens or two, an intelligent instructor might teach optics in a very short time; and so with chemistry. i should desire all these." much would depend on the competency and earnestness of the teacher. "good lectures might do a great deal. they would at all events remove the absolute ignorance which sometimes now appears, but would give a very poor knowledge of natural things." perhaps these opinions of one whose lips are now silent will yet have their weight in the discussion of this question both in our highest seats of learning and in those educational parliaments which have been lately called into existence in almost every town and district of our country. from the somewhat disparaging remarks about lectures quoted above, it must not be supposed that this prince of lecturers depreciated his office. "lectures," he said, "depend entirely for their value upon the manner in which they are given. it is not the matter, it is not the subject, so much as the man; but if he is not competent, and does not feel that there is a need of competency, to convey his ideas gently and quietly and simply to the young mind, he simply throws up obstacles, and will be found using words which they will not comprehend." these were the words of his later days, but fortunately he felt "the need of competency" before his own habits were formed, and in four letters to abbott we find wonderfully sagacious observations on the matter, which it would be well for any young lecturer to study. he describes the proper arrangement of a lecture-room, dwelling on the necessity of good ventilation; and then, having considered the fittest subjects for popular lectures, he turns to the character of the audience, and shows how that must be studied; for some expect to be entertained by the manner of the lecturer as well as his subject, while others care for something which will instruct. he dwells on the superiority of the eye over the ear as a channel of knowledge, and lays down some rules for this kind of instruction, which he of all men subsequently carried out to perfection. "apparatus is an essential part of every lecture in which it can be introduced.... diagrams and tables, too, are necessary, or at least add in an eminent degree to the illustration and perfection of a lecture. when an experimental lecture is to be delivered, and apparatus is to be exhibited, some kind of order should be observed in the arrangement of them on the lecture table. every particular part illustrative of the lecture should be in view; no one thing should hide another from the audience, nor should anything stand in the way of or obstruct the lecturer. they should be so placed, too, as to produce a kind of uniformity in appearance. no one part should appear naked and another crowded, unless some particular reason exists and makes it necessary to be so. at the same time the whole should be so arranged as to keep one operation from interfering with another." a good delivery comes in for its share of praise; "for though to all true philosophers science and nature will have charms innumerable in every dress, yet i am sorry to say that the generality of mankind cannot accompany us one short hour unless the path is strewed with flowers." then, "a lecturer should appear easy and collected, undaunted and unconcerned, his thoughts about him, and his mind clear and free for the contemplation and description of his subject. his action should not be hasty and violent, but slow, easy, and natural, consisting principally in changes of the posture of the body, in order to avoid the air of stiffness or sameness that would otherwise be unavoidable. his whole behaviour should evince respect for his audience, and he should in no case forget that he is in their presence." he allows a lecturer to prepare his discourse in writing, but not to read it before the audience, and points out how necessary it is "to raise their interest at the commencement of the lecture, and by a series of imperceptible gradations, unnoticed by the company, keep it alive as long as the subject demands it." this of course forbids breaks in the argument, or digressions foreign to the main purpose, and limits the length of the lecture to a period during which the listeners can pay unwearied attention. he castigates those speakers who descend so low as "to angle for claps," or who throw out hints for commendation, and shows that apologies should be made as seldom as possible. experiments should be to the point, clear, and easily understood: "they should rather approach to simplicity, and explain the established principles of the subject, than be elaborate and apply to minute phenomena only.... 'tis well, too, when the lecturer has the ready wit and the presence of mind to turn any casual circumstance to an illustration of his subject." but experiments should be explained by a satisfactory theory; or if the scientific world is divided in opinion, both sides of the question ought to be stated with the strongest arguments for each, that justice may be done and honour satisfied. often in later days was his experience in lecturing made use of for the benefit of others. "if," he once remarked to a young lecturer, "i said to my audience, 'this stone will fall to the ground if i open my hand,' i should open my hand and let it fall. take nothing for granted as known; inform the eye at the same time as you address the ear." i remember him once giving me hints on the laying of the lecture table at the institution, and telling me that where possible he was accustomed to arrange the apparatus in such a way as to suggest the order of the experiments. an incident told me by dr. carpenter will illustrate some of the foregoing points. the first time he heard faraday lecture at the royal institution, the professor was explaining the researches of melloni on radiant heat. during the discourse he touched on the refraction and polarization of heat; and to explain refraction he showed the simple experiment of fixing some coloured wafers at the bottom of a basin, and then pouring in water so as to make them apparently rise. dr. carpenter, who had come up from bristol with grand ideas of the lectures at albemarle street, wondered greatly at the introduction of so commonplace an experiment. of course there were many other illustrations, and beautiful ones too. he went down, however, after the lecture, to the table, and among the crowd chatting there was an old gentleman who remarked, "i think the best experiment to-night was that of the wafers in the basin." when a young lecturer, faraday took lessons in elocution from mr. smart, and was at great pains to cure himself of any defect of pronunciation or manner; for this purpose he would get a friendly critic to form part of his audience. on the fly-leaves of many of the notes of his lectures are written the reminders--"stand up"--"don't talk quick." indeed, in early days it was so much a matter of anxiety to him that everything in his lectures should be as perfect as possible, that he not only was accustomed to go over everything again and again in his mind, but the difficulty of satisfying himself used to trouble his dreams. i was told this, if i am not mistaken, by himself; and it goes far to explain how his discourses possessed such a fascination. some of his feelings in regard to lecturing may be learnt from the following particulars, for which i am indebted to mr. charles tomlinson. they relate to a course of lectures he delivered in on statical electricity. the first lecture began thus:--"time moves on, and brings changes to ourselves as well as to science. i feel that i must soon resign into the hands of my successors the position which i now occupy at this table. indeed, i have long felt how much rather i would sit among you and be instructed than stand here and attempt to instruct. i have always felt my position in this institution as a very strange one. coming after such a man as davy, and associated with such a man as brande, and having had to make a position for myself, i have always felt myself here in a strange position. you will wonder why i make these remarks. it is not from any affectation of modesty that i do so, but i feel that loss of memory may soon incapacitate me altogether for my duties. without, however, troubling you more about myself, let us proceed to the subject before us, and fall back upon the beginnings of the wonderful science of electricity. i shall have to trouble you with very little of theory. the facts are so wonderful that i shall not attempt to explain them." at the second lecture, "faraday advanced to the table at three o'clock, and began to apologize for an obstruction of voice, which indeed was painfully evident. he said that, 'in an engagement where the contracting parties were one and many, the one ought not on any slight ground to break his part of the engagement with the many, and therefore, if the audience would excuse his imperfect utterance, he would endeavour--' murmurs arose: 'put off the lecture.' faraday begged to be allowed to go on. a medical man then rose and said he had given it as his opinion that it would be dangerous to dr. faraday to proceed. faraday again urged his wish to proceed--said it was giving so much trouble to the ladies, who had sent away their carriages, and perhaps put off other engagements. on this the whole audience rose as by a single impulse, and a number of persons surrounded faraday, who now yielded to the general desire to spare him the pain and inconvenience of lecturing." a fortnight elapsed before he could again make his appearance, but he continued his course later than usual, in order not to deprive his audience of any of the eight lectures he had undertaken to give them. prince albert came to one of these extra lectures. faraday's opinion as to the honours due to scientific men from society or from government, may be gathered from the following extract from a letter written me by his private friend mr. blaikley:--"on one occasion, when making some remark in reference to a movement on behalf of science, i inadvertently spoke of the proper honour due to science. he at once remarked, 'i am not one who considers that science can be honoured.' i at once saw the point. his views of the grandeur of truth, when once apprehended, raised it far beyond any honour that man could give it; but man might honour himself by respecting and acknowledging it." professor george wilson, of edinburgh, has thus described his first visit to the philosopher: "faraday was very kind, showed me his whole laboratory with labours going on, and talked frankly and kindly; but to the usual question of something to do, gave the usual round o answer, and treated me to a just, but not very cheering animadversion on the government of this country, which, unlike that of every other civilized country, will give no help to scientific inquiry, and will afford no aid or means of study for young chemists." "take care of your money," was his advice to mr. joule, then another young aspirant to scientific honours, but who has since rendered the highest service to science, without leaning on any hopes of government help or public support. but the impressions given in conversation may not be always correct. happily there exist his written opinions on this subject. in a letter addressed to professor andrews of belfast, and dated nd february, , there occurs this passage:--"as to the particular point of your letter about which you honour me by asking my advice, i have no advice to give; but i have a strong feeling in the matter, and will tell you what i should do. i have always felt that there is something degrading in offering rewards for intellectual exertion, and that societies or academies, or even kings and emperors, should mingle in the matter does not remove the degradation, for the feeling which is hurt is a point above their condition, and belongs to the respect which a man owes to himself. with this feeling, i have never since i was a boy aimed at any such prize; or even if, as in your case, they came near me, have allowed them to move me from my course; and i have always contended that such rewards will never move the men who are most worthy of reward. still, i think rewards and honours _good_ if properly distributed, but they should be given for what a man has done, and not offered for what he is to do, or else talent must be considered as a thing marketable and to be bought and sold, and then down falls that high tone of mind which is the best excitement to a man of power, and will make him do more than any commonplace reward. when a man is rewarded for his deserts, he honours those who grant the reward, and they give it not as a moving impulse to him, but to all those who by the reward are led to look to that man for an example." eleven years afterwards faraday expressed similar views, but more fully, in a letter to the late lord wrottesley as chairman of the parliamentary committee of the british association:-- "royal institution, _march th, _. "my lord, "i feel unfit to give a deliberate opinion on the course it might be advisable for the government to pursue if it were anxious to improve the position of science and its cultivators in our country. my course of life, and the circumstances which make it a happy one for me, are not those of persons who conform to the usages and habits of society. through the kindness of all, from my sovereign downwards, i have that which supplies all my need; and in respect of honours, i have, as a scientific man, received from foreign countries and sovereigns, those which, belonging to very limited and select classes, surpass in my opinion anything that it is in the power of my own to bestow. "i cannot say that i have not valued such distinctions; on the contrary, i esteem them very highly, but i do not think i have ever worked for or sought after them. even were such to be now created here, the time is past when these would possess any attraction for me; and you will see therefore how unfit i am, upon the strength of any personal motive or feeling, to judge of what might be influential upon the minds of others. nevertheless, i will make one or two remarks which have often occurred to my mind. "without thinking of the effect it might have upon distinguished men of science, or upon the minds of those who, stimulated to exertion, might become distinguished, i do think that a government should _for its own sake_ honour the men who do honour and service to the country. i refer now to honours only, not to beneficial rewards; of such honours i think there are none. knighthoods and baronetcies are sometimes conferred with such intentions, but i think them utterly unfit for that purpose. instead of conferring distinction, they confound the man who is one of twenty, or perhaps fifty, with hundreds of others. they depress rather than exalt him, for they tend to lower the especial distinction of mind to the commonplaces of society. an intelligent country ought to recognize the scientific men among its people as a class. if honours are conferred upon eminence in any class, as that of the law or the army, they should be in this also. the aristocracy of the class should have other distinctions than those of lowly and high-born, rich and poor, yet they should be such as to be worthy of those whom the sovereign and the country should delight to honour, and, being rendered very desirable and even enviable in the eyes of the aristocracy by birth, should be unattainable except to that of science. thus much i think the government and the country ought to do, for their own sake and the good of science, more than for the sake of the men who might be thought worthy of such distinction. the latter have attained to their fit place, whether the community at large recognize it or not. "but besides that, and as a matter of reward and encouragement to those who have not yet risen to great distinction, i think the government should, in the very many cases which come before it having a relation to scientific knowledge, employ men who pursue science, provided they are also men of business. this is perhaps now done to some extent, but to nothing like the degree which is practicable with advantage to all parties. the right means cannot have occurred to a government which has not yet learned to approach and distinguish the class as a whole. * * * "i have the honour to be, my lord, "your very faithful servant, "m. faraday." sometimes people's views on these matters change when the despised distinction is actually offered, but it was not so with him; for once, when indirectly sounded as to whether a knighthood would be acceptable, he declined the honour, preferring to "remain plain michael faraday to the last." in this day, when so many allow their names to be used for offices of which they never intended to discharge the duties, the following letter may convey an appropriate lesson:-- "royal institution, _oct. th, _. "my dear percy, "i cannot be on the committee; i avoid everything of that kind, that i may keep my stupid mind a little clear. as to being on a committee and not working, that is worse still. * * * "ever yours and mrs. percy's, "m. faraday." it is well known that he waged implacable war with the spiritualists. eighteen years ago tables took to spinning mysteriously under the fingers of ladies and gentlemen who sat or stood around the animated furniture; much was said about a new force, much too about strange revelations from another sphere, but faraday made a simple apparatus which convinced him and most others that the tables moved through the unconscious pressure of the hands that touched them. the account of this will be found in the _athenæum_ of july , . three weeks afterwards he wrote to his friend schönbein: "i have not been at work except in turning the tables upon the table-turners, nor should i have done that, but that so many inquiries poured in upon me, that i thought it better to stop the inpouring flood by letting all know at once what my views and thoughts were. what a weak, credulous, incredulous, unbelieving, superstitious, bold, frightened,--what a ridiculous world ours is, as far as concerns the mind of man! how full of inconsistencies, contradictions, and absurdities it is!" but the believers in these occult phenomena, some of them holding high positions about the court, would not let him alone; and there are many indications of the annoyance and irritation they caused him. he declined to meet the professors of the mysterious art, and the following letter will serve to show the way in which he regarded them:-- "royal institution, _nov. , _. "sir, "i beg to thank you for your papers, but have wasted more thought and time on so-called spiritual manifestation than it has deserved. unless the spirits are utterly contemptible, _they_ will find means to draw my attention. "how is it that your name is not signed to the testimony that you give? are you doubtful even whilst you publish? i've no evidence that any natural or unnatural power is concerned in the phenomena that requires investigation or deserves it. if i could consult the spirits, or move them to make themselves honestly manifest, i would do it. but i cannot, and am weary of them. "i am, sir, your obedient servant, "m. faraday." there was once a strange statement put forth to the effect that faraday said electricity was life.[ ] he himself denied it indignantly; but as most falsehoods are perversions of some truth, this one probably originated in his experiments on the gymnotus. he felt an intense interest in those marine animals that give shocks, and sought "to identify the living power which they possess, with that which man can call into action from inert matter, and by him named electricity."[ ] the most powerful of these is the gymnotus, or electrical eel, and a live specimen of this creature, forty inches long, was secured by the adelaide gallery--a predecessor of the polytechnic--in the summer of . four days after its arrival the poor creature lost an eye; for two months it could not be coaxed to eat either meat or fish, worms or frogs; but at last one day it killed and devoured four small fishes, and afterwards swallowed about a fish per diem. it was accustomed to swim round and round the tank, till a live fish was dropped in, when in some cases bending round its victim, it would discharge a shock that made the fish float on its back stunned and ready to be sucked into the jaws of its assailant. faraday examined this eel and the water around it, both with his hands and with special collectors of electricity, and satisfied himself not merely of the shock, which was easy enough, but of its power to deflect a galvanometer, to make a magnet, to effect chemical decomposition, and to give a spark. his account of the experiments terminates with some speculations on the connection of this animal electricity with nervous power; but there the matter rested. his own views were thus expressed to his friend dumas:--"as living creatures produce heat, and a heat certainly identical with that of our hearths, why should they not produce electricity also, and an electricity in like manner identical with that of our machines? but if the heat produced during life, and necessary to life, is not life after all, why should electricity itself be life? like heat, like chemical action, electricity is an implement of life, and nothing more." whether the belief that electricity is life would be inconsistent with the christian faith or not, it is clear that when an infidel preacher asserts that faraday held such an opinion, his assertion will influence few who are not already disposed to materialism. far more damaging is it to the cause of religion when her ministers repeat the assumption of the infidel that those who study the truths of nature are particularly prone to disbelieve. yet such statements have been made, even with reference to faraday. i have it on the best authority that one of the leading clergymen of the day, preaching on a special occasion from peter's words, "the elements shall melt with fervent heat, the earth also and the works that are therein shall be burned up," spoke in antagonism to scientific men, alluding to faraday by name, and to his computation of the tremendous electrical forces that would be produced by sundering the elements of one drop of water. "they shall be confuted by their own element--fire," added the preacher, careless of the conclusion which his audience might legitimately draw from such a two-edged argument. the accuser of the men of science was much astonished when told after his sermon, by a brother clergyman, that faraday and other eminent physicists of the day were believers in a divine revelation. it may be doubted whether faraday ever tried to form a definite idea of the relation in which the physical forces stand to the supreme intelligence, as newton did, or his own friend sir john herschel; nor did he consider it part of his duty as a lecturer to look beyond the natural laws he was describing. his practice in this respect has been well described by the rev. professor pritchard:[ ]--"this great and good man never obtruded the strength of his faith upon those whom he publicly addressed; upon principle he was habitually reticent on such topics, because he believed they were ill suited for the ordinary assemblages of men. yet on more than one occasion when he had been discoursing on some of the magnificent pre-arrangements of divine providence so lavishly scattered in nature, i have seen him struggle to repress the emotion which was visibly striving for utterance; and then, at the last, with one single far-reaching word, he would just hint at his meaning rather than express it. on such occasions he only who had ears to hear, could hear." in his more familiar lectures to the cadets at woolwich, however, he more than hinted at such elevated thoughts. in conversation, too, faraday has been known to express his wonder that anyone should fail to recognize the constant traces of design; and in his writings there sometimes occur such passages as the following:--"when i consider the multitude of associated forces which are diffused through nature--when i think of that calm and tranquil balancing of their energies which enables elements most powerful in themselves, most destructive to the world's creatures and economy, to dwell associated together and be made subservient to the wants of creation, i rise from the contemplation more than ever impressed with the wisdom, the beneficence, and grandeur beyond our language to express, of the great disposer of all!" faraday's journals abound with descriptions of "nature and human nature." he had evidently a keen eye for the beauties of scenery, and occasionally the objects around him suggested higher thoughts. here are two instances taken from his notes of a swiss tour in :-- "_monday, th._--very fine day; walk with dear sarah on the lake side to oberhofen, through the beautiful vineyards; very busy were the women and men in trimming the vines, stripping off leaves and tendrils from the fruit-bearing branches. the churchyard was beautiful, and the simplicity of the little remembrance-posts set upon the graves very pleasant. one who had been too poor to put up an engraved brass plate, or even a painted board, had written with ink on paper the birth and death of the being whose remains were below, and this had been fastened to a board, and mounted on the top of a stick at the head of the grave, the paper being protected by a little edge and roof. such was the simple remembrance, but nature had added her pathos, for under the shelter by the writing a caterpillar had fastened itself, and passed into its deathlike state of chrysalis, and, having ultimately assumed its final state, it had winged its way from the spot, and had left the corpse-like relics behind. how old and how beautiful is this figure of the resurrection! surely it can never appear before our eyes without touching the thoughts." "_august th, brienz lake._--george and i crossed the lake in a boat to the giessbach--he to draw, and i to saunter.... this most beautiful fall consists of a fine river, which passes by successive steps down a very deep precipice into the lake. in some of these steps there is a clear leap of water of feet or more, in others most beautiful combinations of leap, cataract, and rapid, the finest rocks occurring at the sides and bed of the torrent. in one part a bridge passes over it. in another a cavern and a path occur under it. to-day every fall was foaming from the abundance of water, and the current of wind brought down by it was in some parts almost too strong to stand against. the sun shone brightly, and the rainbows seen from various points were very beautiful. one at the bottom of a fine but furious fall was very pleasant. there it remained motionless, whilst the gusts and clouds of spray swept furiously across its place, and were dashed against the rock. it looked like a spirit strong in faith and stedfast in the midst of the storm of passions sweeping across it; and though it might fade and revive, still it held on to the rock as in hope and giving hope; and the very drops which in the whirlwind of their fury seemed as if they would carry all away, were made to revive it and give it greater beauty. "how often are the things we fear and esteem as troubles made to become blessings to those who are led to receive them with humility and patience." * * * * * in concluding this section it may be well to string together a few gems from faraday's lectures or correspondence, though they are greatly damaged by being torn away from their original setting:-- "after all, though your science is much to me, we are not friends for science sake only, but for something better in a man, something more important in his nature, affection, kindness, good feeling, moral worth; and so, in remembrance of these, i now write to place myself in your presence, and in thought shake hands, tongues, and hearts together." this was addressed to schönbein. "i should be glad to think that high mental powers insured something like a high moral sense, but have often been grieved to see the contrary: as also, on the other hand, my spirit has been cheered by observing in some lowly and uninstructed creature such a healthful and honourable and dignified mind as made one in love with human nature. when that which is good mentally and morally meet in one being, that that being is more fitted to work out and manifest the glory of god in the creation, i fully admit." "let me, as an old man who ought by this time to have profited by experience, say that when i was younger i found i often misinterpreted the intentions of people, and found they did not mean what at the time i supposed they meant; and further, that as a general rule, it was better to be a little dull of apprehension when phrases seemed to imply pique, and quick in perception when, on the contrary, they seemed to imply kindly feeling. the real truth never fails ultimately to appear; and opposing parties, if wrong, are sooner convinced when replied to forbearingly, than when overwhelmed." "man is an improving animal. unlike the animated world around him, which remains in the same constant state, he is continually varying; and it is one of the noblest prerogatives of his nature, that in the highest of earthly distinctions he has the power of raising and exalting himself continually. the transitory state of man has been held up to him as a memento of his weakness: to man _degraded_ it may be so with justice; to man as he ought to be it is no reproach; and in knowledge, that man only is to be contemned and despised who is _not_ in a state of transition." "it is not the duty or place of a philosopher to dictate belief, and all hypothesis is more or less matter of belief; he has but to give his facts and his conclusions, and so much of the logic which connects the former with the latter as he may think necessary, and then to commit the whole to the scientific world for present, and, as he may sometimes without presumption believe, for future judgment." footnotes: [ ] i myself once heard this advanced by an infidel lecturer on paddington green. [ ] "electrical researches," series xv. [ ] "analogies in the progress of nature and grace," p. . section iv. his method of working. it is on record that when a young aspirant asked faraday the secret of his success as a scientific investigator, he replied, "the secret is comprised in three words--work, finish, publish." each of these words, we may be sure, is full of meaning, and will guide us in a useful inquiry. already in the "story of his life" we have caught some glimpses of the philosopher at work in his laboratory; but before looking at him more closely let us learn from a foreigner with what feelings to enter a place that is hallowed by so many memories sacred in the history of science. professor schönbein, of basle, who visited england in , says: "during my stay in london, i once worked with faraday for a whole day long in the laboratory of the royal institution, and i cannot forbear to say that this was one of the most enjoyable days that i ever spent in the british capital. we commenced our day's work with breakfast; and when that was over, i was supplied with one of the laboratory dresses of my friend, which, when i was presented in it to the ladies, gave occasion to no little amusement, as the dimensions of faraday are different from those of my precious body. "to work with a man like faraday was in itself a great pleasure; but this pleasure was not a little heightened in doing so in a place where such grand secrets of nature had been unfolded, the most brilliant discoveries of the century had been made, and entirely new branches of knowledge had been brought forth. for the empty intellect circumstances of this nature are indeed of little special value; but they stand in quite another relation to our power of imagination and inner nature. "i do not deny that my surroundings produced in me a very peculiar feeling; and whilst i trod the floor upon which davy had once walked--whilst i availed myself of some instrument which this great discoverer had himself handled--whilst i stood working at the very table at which the ever-memorable man sought to solve the most difficult problems of science, at which faraday enticed the first sparks out of the magnet, and discovered the most beautiful laws of the chemical action of current electricity, i felt myself inwardly elevated, and believed that i myself experienced something of the inbreathing of the scientific spirit which formerly ruled there with such creative power, and which still works on."[ ] the habit of faraday was to think out carefully beforehand the subject on which he was working, and to plan his mode of attack. then, if he saw that some new piece of apparatus was needed, he would describe it fully to the instrument maker with a drawing, and it rarely happened that there was any need of alteration in executing the order. if, however, the means of experiment existed already, he would give anderson a written list of the things he would require, at least a day before--for anderson was not to be hurried. when all was ready, he would descend into the laboratory, give a quick glance round to see that all was right, take his apron from the drawer, and rub his hands together as he looked at the preparations made for his work. there must be no tool on the table but such as he required. as he began, his face would be exceedingly grave, and during the progress of an experiment all must be perfectly quiet; but if it was proceeding according to his wish, he would commence to hum a tune, and sometimes to rock himself sideways, balancing alternately on either foot. then, too, he would often talk to his assistant about the result he was expecting. he would put away each tool in its own place as soon as done with, or at any rate when the day's work was over, and he would not unnecessarily take a thing away from its place: thus, if he wanted a perforated cork, he would go to the drawer which contained the corks and cork-borers, make there what he wanted, replace the borers, and shut the drawer. no bottle was allowed to remain without its stopper; no open glass might stand for a night without a paper cover; no rubbish was to be left on the floor; bad smells were to be avoided if possible; and machinery in motion was not permitted to grate. in working, also, he was very careful not to employ more force than was wanted to produce the effect. when his experiments were finished and put away, he would leave the laboratory, and think further about them upstairs. this orderliness and this economy of means he not only practised himself, but he expected them also to be followed by any who worked with him; and it is from conversation with these that i have been enabled to give this sketch of his manner of working.[ ] this exactness was also apparent in the accounts he kept with the royal institution and trinity house, in which he entered every little item of expenditure with the greatest minuteness of detail. it was through this lifelong series of experiments that faraday won his knowledge and mastered the forces of nature. the rare ingenuity of his mind was ably seconded by his manipulative skill, while the quickness of his perceptions was equalled by the calm rapidity of his movements. he had indeed a passion for experimenting. this peeps out in the preface to the second edition of his "chemical manipulation," where he writes, "being intended especially as a book of instruction, no attempts were made to render it pleasing, otherwise than by rendering it effectual; for i concluded that, if the work taught clearly what it was intended to inculcate, the high interest always belonging to a well-made or successful experiment would be abundantly sufficient to give it all the requisite charms, and more than enough to make it valuable in the eyes of those for whom it was designed." he could scarcely pass a gold leaf electrometer without causing the leaves to diverge by a sudden flick from his silk handkerchief. i recollect, too, his meeting me at the entrance to the lecture theatre at jermyn street, when lyon playfair was to give the first, or one of the first lectures ever delivered in the building. "let us go up here," said he, leading me far away from the central table. i asked him why he chose such an out-of-the-way place. "oh," he replied, "we shall be able here to find out what are the acoustic qualities of the room." the simplicity of the means with which he made his experiments was often astonishing, and was indeed one of the manifestations of his genius. a good instance is thus narrated by sir frederick arrow. "when the electric light was first exhibited permanently at dungeness, on th june, , a committee of the elder brethren, of which i was one, accompanied faraday to observe it. we dined, i think, at dover, and embarked in the yacht from there, and were out for some hours watching it, to faraday's great delight--(a very fine night)--and especially we did so from the varne lightship, about equidistant between it and the french light of grisnez, using all our best glasses and photometers to ascertain the relative value of the lights: and this brings me to my story. before we left dover, faraday, with his usual bright smile, in great glee showed me a little common paper box, and said, 'i must take care of this; it's my special photometer,'--and then, opening it, produced a lady's ordinary black shawl-pin,--jet, or imitation perhaps,--and then holding it a little way off the candle, showed me the image very distinct; and then, putting it a little further off, placed another candle near it, and the relative distance was shown by the size of the image. he lent me this afterwards when we were at the varne lightship, and it acted admirably; and ever since i have used one as a very convenient mode of observing, and i never do so but i think of that night and dear good faraday, and his genial happy way of showing how even common things may be made useful." after this faraday modified his glass-bead photometer, and he might be seen comparing the relative intensity of two lights by watching their luminous images on a bead of black glass, which he had threaded on a string, and was twirling round so as to resolve the brilliant points into circles of fainter light; or he fixed the black glass balls on pieces of cork, and, attaching them to a little wheel, set them spinning for the same purpose. some of these beads are preserved by the trinity house, with other treasures of a like kind, including a flat piece of solder of an irregular oval form, turned up at one side so as to form a thumb-rest, and which served the philosopher as a candlestick to support the wax-light that he used as a standard. the museum of the royal institution contains a most instructive collection of his experimental apparatus, including the common electrical machine which he made while still an apprentice at riebau's, and the ring of soft iron, with its twisted coils of wire isolated by calico and tied with common string, by means of which he first obtained electrical effects from a magnet. in lecturing to the young he delighted to show how easily apparatus might be extemporized. thus, in order to construct an electrical machine he once inverted a four-legged stool to serve for the stand, and took a white glass bottle for the cylinder. a cork was fitted into the mouth of this bottle, and a bung was fastened with sealing-wax to the other end: into the cork was inserted a handle for rotating the bottle, and in the centre of the bung was a wooden pivot on which it turned; while with some stout wire he made crutches on two of the legs of the stool for the axles of this glass cylinder to work upon. the silk rubber he held in his hand. a japanned tin tea-canister resting on a glass tumbler formed the conductor, and the collector was the head of a toasting fork. with this apparently rough apparatus he exhibited all the rudimentary experiments in electricity to a large audience. wishing to carry home in good condition a flower that had been given him, he rolled a piece of writing-paper round a cork, tied it tightly with string, and filled the little tube with water. he had thus a perfectly efficient bouquet-holder. a lady, calling on his wife, happened to mention that a needle had been once broken into her foot, and she did not know whether it had been all extracted or not. "oh!" said faraday, "i will soon tell you that,"--and taking a finely suspended magnetic needle, he held it close to her foot, and it dipped to the concealed iron. on this subject schönbein has also some good remarks. "the laboratory of the institution is indeed efficiently arranged, though anything but large and elaborately furnished. and yet something extraordinary has happened in this room for the extension of the limits of knowledge; and already more has been done in it than in many other institutions where the greatest luxury in the supply of apparatus prevails, and where there is the greatest command of money. but when men work with the creative genius of a davy, and the intuitive spirit of investigation and the wealth of ideas of a faraday, important and great things must come to pass, even though the appliances at command should be of so limited a character. for the experimental investigator of nature, it is especially desirable that, according to the kind of his researches, he should have at command such and such appliances, that he should possess a 'philosophical apparatus,' a laboratory, &c.; but for the purpose of producing something important, of greatly widening the sphere of knowledge, it in no way follows that a superfluity of such things is necessary to him.... he who understands how to put appropriate questions to nature, generally knows how to extract the answers by simple means; and he who wants this capacity will, i fear, obtain no profitable result, even though all conceivable tools and apparatus may be ready to his hand." nor did faraday require elaborate apparatus to illustrate his meaning. steaming up the thames one july day in a penny boat, he was struck with the offensiveness of the water. he tore some white cards into pieces, wetted them so as to make them sink easily, and dropped them into the river at each pier they came to. their sudden disappearance from sight, though the sun was shining brightly, was proof enough of the impurity of the stream; and he wrote a letter to the _times_ describing his observations, and calling public attention to the dangerous state of the river.[ ] at a meeting of the british association he wished to explain the manner in which certain crystallized bodies place themselves between the poles of an electro-magnet: two or three raw potatoes furnished the material out of which he cut admirable models of the crystals. wishing to show the electrical nature of gun-cotton, he has been known to lay his watch upon the table, balance on it a slender piece of wood, and, charging a morsel of the gun-cotton by drawing it along his coat sleeve, cause the wood to revolve towards the electric fibres. "an artist was once maintaining that in natural appearances and in pictures, up and down, and high and low, were fixed indubitable realities; but faraday told him that they were merely conventional acceptations, based on standards often arbitrary. the disputant could not be convinced that ideas which he had hitherto never doubted had such shifting foundations. 'well,' said faraday, 'hold a walking-stick between your chin and your great toe; look along it and say which is the upper end.' the experiment was tried, and the artist found his idea of perspective at complete variance with his sense of reality; either end of the stick might be called 'upper,'--pictorially it was one, physically it was the other." faraday's manner of experimenting may be further illustrated by the recollections of other friends who have had the opportunity of watching him at work. mr. james young, who was in the laboratory of university college in , thus writes:--"about that time professor graham had got from paris thilorier's apparatus for producing liquid and solid carbonic acid; hearing of this, mr. faraday came to graham's laboratory, and, as one might expect, showed great interest in this apparatus, and asked graham for the loan of it for a friday evening lecture at the royal institution, which of course graham readily granted, and faraday asked me to come down to the institution and give him the benefit of my experience in charging and working the apparatus; so i spent a long evening at the royal institution laboratory. there was no one present but faraday, anderson, and myself. the principal thing we did was to charge the apparatus and work with the solid carbonic acid, mr. faraday working with great activity; his motions were wonderfully rapid; and if he had to cross the laboratory for anything, he did not walk at an ordinary step, but ran for it, and when he wanted anything he spoke quickly. faraday had a theory at that time that all metals would become magnetic if their temperature were low enough; and he tried that evening some experiments with cobalt and manganese, which he cooled in a mixture of carbonic acid and ether, but the results were negative." among the deep mines of the durham coal-field is one called the haswell colliery. one saturday afternoon, while the men were at work in it as usual, a terrible explosion occurred: it proceeded from the fire-damp that collects in the vaulted space that is formed in old workings, when the supporting pillars of coal are removed and the roof falls in: the suffocating gases rushed along the narrow passages, and overwhelmed ninety-five poor fellows with destruction. of course there was an inquiry, and the government sent down to the spot as their commissioners professor faraday and sir charles lyell. the two gentlemen attended at the coroner's inquest, where they took part in the examination of the witnesses; they inspected the shattered safety-lamps; they descended into the mine, spending the best part of a day in the damaged and therefore dangerous galleries where the catastrophe had occurred, and they did not leave without showing in a practical form their sympathy with the sufferers. when down in the pit, an inspector showed them the way in which the workmen estimated the rapidity of the ventilation draught, by throwing a pinch of gunpowder through the flame of a candle, and timing the movement of the little puff of smoke. faraday, not admiring the free and easy way in which they handled their powder, asked where they kept their store of it, and learnt that it was in a large black bag which had been assigned to him as the most comfortable seat they could offer. we may imagine the liveliness with which he sprang to his feet, and expostulated with them on their culpable carelessness. my own opportunities of observing faraday at work were nearly confined to a series of experiments, which are the better worth describing here as they have escaped the notice of previous biographers. the royal commission appointed to inquire into our whole system of lights, buoys, and beacons, perceived a great defect that rendered many of our finest shore or harbour lights comparatively ineffective. the great central lamp in a lighthouse is surrounded by a complicated arrangement of lenses and prisms, with the object of gathering up as many of the rays as possible and sending them over the surface of the sea towards the horizon. now, it is evident that if this apparatus be adjusted so as to send the beam two or three degrees upwards, the light will be lost to the shipping and wasted on the clouds; and if two or three degrees downwards, it will only illuminate the water in the neighbourhood: in either case the beautiful and expensive apparatus would be worse than useless. it is evident also that if the eye be placed just above the wick of the lamp, it will see through any particular piece of glass that very portion of the landscape which will be illuminated by a ray starting from the same spot; or the photographic image formed in the place of the flame by any one of the lenses will tell us the direction in which that lens will throw the luminous rays. this simple principle was applied by the commissioners for testing the adjustment of the apparatus in the different lights, and it was found that few were rightly placed, or rather that no method of adjustment was in use better than the mason's plumbline. the royal commissioners therefore in drew the attention of all the lighthouse authorities to this fact, and asked the elder brethren of the trinity house, with faraday and other parties, to meet them at the lights recently erected at the north foreland and whitby. i, as the scientific member of the commission, had drawn out in detail the course of rays from different parts of the flame, through different parts of the apparatus, and i was struck with the readiness with which faraday, who had never before considered the matter,[ ] took up the idea, and recognized its importance and its practical application. with his characteristic ingenuity, too, he devised a little piece of apparatus for the more exact observation of the matter inside the lighthouse. he took to mr. ladd, the optical instrument maker, a drawing, very neatly executed, with written directions, and a cork cut into proper shape with two lucifer matches stuck through it, to serve as a further explanation of his meaning: and from this the "focimeter," as he called it, was made. the position of the glass panels at whitby was corrected by means of this little instrument, and there were many journeys down to chance's glassworks near birmingham, where, declining the hospitality of the proprietor in order to be absolutely independent, he put up at a small hotel while he made his experiments, and jotted down his observations on the cards he habitually carried in his pocket. at length we were invited down to see the result. faraday explained carefully all that had been done, and at the risk of sea-sickness (no trifling matter in his case) accompanied us out to sea to observe the effect from various directions and at various distances. the experience acquired at whitby was applied elsewhere, and in may the trinity house appointed a visiting committee, "to examine all dioptric light establishments, with the view of remedying any inaccuracies of arrangement that may be found to exist." faraday had instructed and practised captain nisbet and some others of the elder brethren in the use of the focimeter, and now wrote a careful letter of suggestions on the question of adjustment between the lamp and the lenses and prisms; so thoughtfully did he work for the benefit of those who "go down to the sea in ships, that do business in great waters." as to the mental process that devised, directed, and interpreted his experiments, it must be borne in mind that faraday was no mathematician; his power of appreciating an _à priori_ reason often appeared comparatively weak. "it has been stated on good authority that faraday boasted on a certain occasion of having only once in the course of his life performed a mathematical calculation: that once was when he turned the handle of babbage's calculating machine."[ ] though there was more pleasantry than truth in this professed innocence of numbers, probably no one acquainted with his electrical researches will doubt that, had he possessed more mathematical ability, he would have been saved much trouble, and would sometimes have expressed his conclusions with greater ease and precision. yet, as sir william thomson has remarked with reference to certain magnetic phenomena, "faraday, without mathematics, divined the result of the mathematical investigation; and, what has proved of infinite value to the mathematicians themselves, he has given them an articulate language in which to express their results. indeed, the whole language of the magnetic field and 'lines of force' is faraday's. it must be said for the mathematicians that they greedily accepted it, and have ever since been most zealous in using it to the best advantage." the peculiarity of his mind was indeed well known to himself. in a letter to dr. becker he says: "i was never able to make a fact my own without seeing it; and the descriptions of the best works altogether failed to convey to my mind such a knowledge of things as to allow myself to form a judgment upon them. it was so with _new_ things. if grove, or wheatstone, or gassiot, or any other told me a new fact, and wanted my opinion either of its value, or the cause, or the evidence it could give on any subject, i never could say anything until i had seen the fact. for the same reason i never could work, as some professors do most extensively, by students or pupils. all the work had to be my own." thus we are told what took place "when dr. tyndall brought mr. faraday into the laboratory to look at his new discovery of calorescence. as faraday saw for the first time a piece of cold, black platinum raised to a dazzling brightness when held in the focus of dark rays, a point undistinguishable from the air around, he looked on attentively, putting on his spectacles to observe more carefully, then ascertained the conditions of the experiment, and repeated it for himself; and now quite satisfied, he turned with emotion to dr. tyndall, and almost hugged him with pleasure."[ ] the following story by mr. robert mallet also serves as an illustration:--"it must be now eighteen years ago when i paid him a visit and brought some slips of flexible and _tough_ muntz's yellow metal, to show him the instantaneous change to complete brittleness with rigidity produced by dipping into pernitrate of mercury solution. he got the solution, and i _showed_ him the facts; he obviously did not doubt what he saw _me_ do before and close to him: but a sort of experimental instinct seemed to require he should try it himself. so he took one of the slips, bent it forwards and backwards, dipped it, and broke it up into short bits between his own fingers. he had not before spoken. _then_ he said, 'yes, it _is_ pliable, and it _does_ become instantly brittle.' and after a few moments' pause he added, 'well, now have you any more facts of the sort?' and seemed a little disappointed when i said, 'no; none that are new.' it has often since occurred to me how his mind needed absolute satisfaction that he had grasped a _fact_, and then instantly rushed to colligate it with another if possible." but as the professor watched these new facts, new thoughts would shape themselves in his mind, and this would lead to fresh experiments in order to test their truth. the answers so obtained would lead to further questions. thus his work often consisted in the defeat of one hypothesis after another, till the true conditions of the phenomena came forth and claimed the assent of the experimenter and ultimately of the scientific world. a. de la rive has some acute observations on this subject. he explains how faraday did not place himself before his apparatus, setting it to work, without a preconceived idea. neither did he take up known phenomena, as some scientific men do, and determine their numerical data, or study with great precision the laws which regulate them. "a third method, very different from the preceding, is that which, quitting the beaten track, leads, as if by inspiration, to those great discoveries which open new horizons to science. this method, in order to be fertile, requires one condition--a condition, it is true, which is but rarely met with--namely, genius. now, this condition existed in faraday. endowed, as he himself perceived, with much imagination, he dared to advance where many others would have recoiled: his sagacity, joined to an exquisite scientific tact, by furnishing him with a presentiment of the possible, prevented him from wandering into the fantastic; while, always wishing only for facts, and accepting theories only with difficulty, he was nevertheless more or less directed by preconceived ideas, which, whether true or false, led him into new roads, where most frequently he found what he sought, and sometimes also what he did not seek, but where he constantly met with some important discovery. "such a method, if indeed it can be called one, although barren and even dangerous with mediocre minds, produced great things in faraday's hands; thanks, as we have said, to his genius, but thanks also to that love of truth which characterized him, and which preserved him from the temptation so often experienced by every discoverer, of seeing what he wishes to see, and not seeing what he dreads." this love of truth deserves a moment's pause. it was one of the most beautiful and most essential of his characteristics; it taught him to be extremely cautious in receiving the statements of others or in drawing his own conclusions,[ ] and it led him, if his scepticism was overcome, to adopt at once the new view, and to maintain it, if need be, against the world. "the thing i am proudest of, pearsall, is that i have never been found to be wrong," he could say in the early part of his scientific history without fear of contradiction. after his death a. de la rive wrote, "i do not think that faraday has once been caught in a mistake; so precise and conscientious was his mode of experimenting and observing." this is not absolutely true; but the extreme rarity of his mistakes, notwithstanding the immense amount of his published researches, is one of those marvels which can be appreciated only by those who are in the habit of describing what they have seen in the mist land that lies beyond the boundaries of previous knowledge. into this unknown region his mental vision was ever stretched. "i well remember one day," writes mr. barrett, a former assistant at the royal institution, "when mr. faraday was by my side, i happened to be steadying, by means of a magnet, the motion of a magnetic needle under a glass shade. mr. faraday suddenly looked most impressively and earnestly as he said, 'how wonderful and mysterious is that power you have there! the more i think over it the less i seem to know:'--and yet he who said this knew more of it than any living man." it is easy to imagine with what wonder he would stand before the apples or leaves or pieces of meat that swung round into a transverse position between the poles of his gigantic magnet, or the sand that danced and eddied into regular figures on plates of glass touched by the fiddle-bow, or gold so finely divided that it appeared purple and when diffused in water took a twelvemonth to settle. it is easy, too, to imagine how he would long to gain a clear idea of what was taking place behind the phenomena. but it is far from easy to grasp the conceptions of his brain: language is a clumsy vehicle for such thoughts. he strove to get rid of such figurative terms as "currents" and "poles;" in discussing the mode of propagation of light and radiant heat he endeavoured "to dismiss the ether, but not the vibrations;" and in conceiving of atoms, he says: "as to the little solid particles ... i cannot form any idea of them apart from the forces, so i neither admit nor deny them. they do not afford me the least help in my endeavour to form an idea of a particle of matter. on the contrary, they greatly embarrass me." yet he could not himself escape from the tyranny of words or the deceitfulness of metaphors, and it is hard for his readers to comprehend what was his precise idea of those centres of forces that occupy no space, or of those lines of force which he beheld with his mental eye, curving alike round his magnetic needle, and that mightiest of all magnets--the earth. as he was jealous of his own fame, and had learnt by experience that discoveries could be stolen, he talked little about them till they were ready for the public; indeed, he has been known to twit a brother electrician for telling his discoveries before printing them, adding with a knowing laugh, "i never do that." he was obliged, however, to explain his results to professor whewell, or some other learned friend, if he wished to christen some new idea with a greek name. one of whewell's letters on such an occasion, dated trinity college, cambridge, october , , begins thus:-- "my dear sir, "i am always glad to hear of the progress of your researches, and never the less so because they require the fabrication of a new word or two. such a coinage has always taken place at the great epochs of discovery; like the medals that are struck at the beginning of a new reign, or rather like the change of currency produced by the accession of a new sovereign; for their value and influence consists in their coming into common circulation." * * * * * during the whole time of an investigation faraday had kept ample notes, and when all was completed he had little to do but to copy these notes, condensing or re-arranging some parts, and omitting what was useless. the paper then usually consisted of a series of numbered paragraphs, containing first a statement of the subject of inquiry, then a series of experiments giving negative results, and afterwards the positive discoveries. in this form it was sent to the royal society or some other learned body. yet this often involved considerable labour, as the following words written to miss moore in from a summer retreat in upper norwood will show:--"i write and write and write, until nearly three papers for the royal society are nearly completed, and i hope that two of them will be good if they do justify my hopes, for i have to criticise them again and again before i let them loose. you shall hear of them at some of the next friday evenings." this criticism did not cease with their publication, for he endeavoured always to improve on his previous work. thus, in he bound his papers together in one volume, and the introduction on the fly-leaf shows the object with which it was done:-- "papers of mine, published in octavo, in the _quarterly journal of science_, and elsewhere, since the time that sir h. davy encouraged me to write the analysis of caustic lime. "some, i think (at this date), are good, others moderate, and some bad. but i have put _all_ into the volume, because of the utility they have been of to me--and none more than the bad--in pointing out to me in future, or rather after times, the faults it became me to watch and to avoid. "as i never looked over one of my papers a year after it was written, without believing, both in philosophy and manner, it could have been much better done, i still hope the collection may be of great use to me. "m. faraday. "_august , _." * * * * * this section may be summed up in the words of dumas when he gave the first "faraday lecture" of the chemical society:--"faraday is the type of the most fortunate and the most accomplished of the learned men of our age. his hand in the execution of his conceptions kept pace with his mind in designing them; he never wanted boldness when he undertook an experiment, never lacked resources to ensure success, and was full of discretion in interpreting results. his hardihood, which never halted when once he had undertaken a task, and his wariness, which felt its way carefully in adopting a received conclusion, will ever serve as models for the experimentalist." footnotes: [ ] "mittheilungen aus dem reisetagebuche eines deutschen naturforschers," p. . [ ] since the publication of the first edition i have been struck with how precisely his practice corresponded with his precept in the introduction to his book on "chemical manipulation:"--"when an experiment has been devised, its general nature and principles arranged in the mind, and the causes to be brought into action, with the effect to be expected, properly considered, then it has to be performed. the ultimate objects of an experiment, and also the particular contrivance or mode by which the results are to be produced, being mental, there remains the mere performance of it, which may properly enough be expressed by the term _manipulation_. "notwithstanding this subordinate character of manipulation, it is yet of high importance in an experimental science, and particularly in chemistry. the person who could devise only, without knowing how to perform, would not be able to extend his knowledge far, or make it useful; and where every doubt or question that arises in the mind is best answered by the result of an experiment, whatever enables the philosopher to perform the experiment in the simplest, quickest, and most direct manner, cannot but be esteemed by him as of the utmost value." [ ] _punch's_ cartoon next week represented professor faraday holding his nose, and presenting his card to father thames, who rises out of the unsavoury ooze. [ ] since writing the above i have come across a letter written by faraday in answer to one by captain welier as far back as th sept. , in which he pointed out the mal-adjustment of the dioptric apparatus at orfordness. in july of the following year he made lengthy suggestions to the trinity house, in which he proposed using a flat white circle or square, half an inch across, on a piece of black paper or card, as a "focal object." this was to be looked at from outside, in order to test the regularity of the glass apparatus. he also suggested observations on the divergence by looking at this white circle at a distance of twenty feet at most. another plan he proposed was that of lighting the lamp and putting up a white screen outside. these methods of examining he carried out very shortly afterwards at blackwall, on french and english refractors, but it seems never to have occurred to him to place his eye in the focus, or in any other manner to observe the course of the rays from inside the apparatus. [ ] dr. scoffern, _belgravia_, october . [ ] mr. barrett, _nature_, sept. , . [ ] a good instance of his caution in drawing conclusions is contained in one of his letters to me:-- "royal institution of great britain, "_ july, _. "my dear gladstone, "although i have frequently observed lights from the sea, the only thing i have learnt in relation to their _relative brilliancy_ is that the average of a very great number of observations would be required for the attainment of a moderate approximation to truth. one has to be some miles off at sea, or else the observation is not made in the chief ray, and then one does not know the state of the atmosphere about a given lighthouse. strong lights like that of cape grisnez have been invisible when they should have been strong; feeble lights by comparison have risen up in force when one might have expected them to be relatively weak; and after inquiry has not shown a state of the air at the lighthouse explaining such differences. it is probable that the cause of difference often exists at sea. "besides these difficulties there is that other great one of not seeing the two lights to be compared in the field of view at the same time and same distance. if the eye has to turn ° from one to the other, i have no confidence in the comparison; and if both be in the field of sight at once, still unexpected and unexplained causes of difference occur. the two lights at the south foreland are beautifully situated for comparison, and yet sometimes the upper did not equal the lower when it ought to have surpassed it. this i referred at the time to an upper stratum of haze; but on shore they knew nothing of the kind, nor had any such or other reason to expect particular effects. "ever truly yours, "m. faraday." as an instance of his unwillingness to commit himself to an opinion unless he was sure about it, may be cited a letter he wrote to sir g. b. airy, the astronomer royal, who asked for his advice in regard to the material of which the national standard of length should be made:--"i do not see any reason why a pure metal should be particularly free from internal change of its particles, and on the whole should rather incline to the hard alloy than to soft copper, and yet i hardly know why. i suppose the labour would be too great to lay down the standard on different metals and substances; and yet the comparison of them might be very important hereafter, for twenty years seem to _do_ or _tell_ a great deal in relation to standard measures." bronze was finally chosen. section v. the value of his discoveries. science is pursued by different men from different motives. "to some she is the goddess great; to some the milch-cow of the field; their business is to calculate the butter she will yield." now, faraday had been warned by davy before he entered his service that science was a mistress who paid badly; and in we have seen him deliberately make his calculation, give up the butter, and worship the goddess. for the same reason also he declined most of the positions of honour which he was invited to fill, believing that they would encroach too much on his time, though he willingly accepted the honorary degrees and scientific distinctions that were showered upon him.[ ] and among those who follow science lovingly, there are two very distinct bands: there are the philosophers, the discoverers, men who persistently ask questions of nature; and there are the practical men, who apply her answers to the various purposes of human life. many noble names are inscribed in either bead-roll, but few are able to take rank in both services: indeed, the question of practical utility would terribly cramp the investigator, while the enjoyment of patient research in unexplored regions of knowledge is usually too ethereal for those who seek their pleasures in useful inventions. the mental configuration is different in the two cases; each may claim and receive his due award of honour. faraday was pre-eminently a discoverer; he liked the name of "philosopher." his favourite paths of study seem to wander far enough from the common abodes of human thought or the requirements of ordinary life. he became familiar, as no other man ever was, with the varied forces of magnetism and electricity, heat and light, gravitation and galvanism, chemical affinity and mechanical motion; but he did not seek to "harness the lightnings," or to chain those giants and to make them grind like samson in the prison-house. his way of treating them reminds us rather of the old fable of proteus, who would transform himself into a whirlwind or a dragon, a flame of fire or a rushing stream, in order to elude his pursuer; but if the wary inquirer could catch him asleep in his cave, he might be constrained to utter all his secret knowledge: for the favourite thought of faraday seems to have been that these various forces were the changing forms of a proteus, and his great desire seems to have been to learn the secret of their origin and their transformations. thus he loved to break down the walls of separation between different classes of phenomena, and his eye doubtless sparkled with delight when he saw what had always been looked upon as permanent gases liquefy like common vapours under the constraint of pressure and cold--when the wires that coiled round his magnets gave signs of an electric wave, or coruscated with sparks--when the electricities derived from the friction machine and from the voltaic pile yielded him the same series of phenomena--when he recognized the cumulative proof that the quantity of electricity in a galvanic battery is exactly proportional to the chemical action--when his electro-static theory seemed to break down the barrier between the conductors and insulators, and many other barriers beside--when he sent a ray of polarized light through a piece of heavy glass between the poles of an electro-magnet, and on making contact saw that the plane of polarization was rotated, or, as he said, the light was magnetized--and when he watched pieces of bismuth, or crystals of iceland spar, or bubbles of oxygen, ranging themselves in a definite position in the magnetic field. "i delight in hearing of exact numbers, and the determinations of the equivalents of force when different forms of force are compared one with another," he wrote to joule in ; and no wonder, for these quantitative comparisons have proved many of his speculations to be true, and have made them the creed of the scientific world. when he began to investigate the different sciences, they might be compared to so many different countries with impassable frontiers, different languages and laws, and various weights and measures; but when he ceased they resembled rather a brotherhood of states, linked together by a community of interests and of speech, and a federal code; and in bringing about this unification no one had so great a share as himself. he loved to speculate, too, on matter and force, on the nature of atoms and of imponderable agents. "it is these things," says the great german physicist professor helmholz, "that faraday, in his mature works, ever seeks to purify more and more from everything that is theoretical, and is not the direct and simple expression of the fact. for instance, he contended against the action of forces at a distance, and the adoption of two electrical and two magnetic fluids, as well as all hypotheses contrary to the law of the conservation of force, which he early foresaw, though he misunderstood it in its scientific expression. and it is just in this direction that he exercised the most unmistakeable influence first of all on the english physicists."[ ] while, however, faraday was pre-eminently an experimental philosopher, he was far from being indifferent to the useful applications of science. his own connection with the practical side of the question was threefold: he undertook some laborious investigations of this nature himself; he was frequently called upon, especially by the trinity house, to give his opinions on the inventions of others; and he was fond of bringing useful inventions before the members of the royal institution in his friday evening discourses. the first of these, on february , , was on india-rubber, and was illustrated by an abundance of specimens both in the raw and manufactured states. he traced the history of the substance, from the crude uncoagulated sap to the sheet rubber and waterproof fabrics which mr. hancock and mr. macintosh had recently succeeded in preparing. in this way also he continued to throw the magic of his genius around morden's machinery for manufacturing bramah's locks, ericsson's caloric engine, brunel's block machinery at portsmouth, petitjean's process for silvering mirrors, the prevention of dry-rot in timber, de la rue's envelope machinery, artificial rubies, bonelli's electric silk loom, barry's mode of ventilating the house of lords, and many kindred subjects. it may not be amiss to describe the last of his friday evenings, in which he brought before the public mr. c. w. siemens' regenerative gas furnace. the following letter to the inventor will tell the first steps:-- "royal institution, _march , _. "my dear sir, "i have just returned from birmingham--and there saw at chance's works the application of your furnaces to glass-making. i was very much struck with the whole matter. "as our managers want me to end the f. evenings here after easter, i have looked about for a thought, for i have none in myself. i think i should like to speak of the effects i saw at chance's, if you do not object. if you assent, can you help me with any drawings or models, or illustrations either in the way of thoughts or experiments? do not say much about it out of doors as yet, for my mind is not settled in what way (if you assent) i shall present the subject. "ever truly yours, "m. faraday. "c. w. siemens, esq." of course the permission was gladly given, and mr. siemens met him at birmingham, and for two days conducted him about works for flint and crown glass, or for enamel, as well as about ironworks, in which his principle was adopted, wondering at the professor's simplicity of character as well as at his ready power of grasping the whole idea. then came the friday evening, th june, , in which he explained the great saving of heat effected, and pictured the world of flame into which he had gazed in some of those furnaces. but his powers of lecturing were enfeebled, and during the course of the hour he burnt his notes by accident, and at the conclusion he very pathetically bade his audience farewell, telling them that he felt he had been before them too long, and that the experience of that evening showed he was now useless as their public servant, but he would still endeavour to do what he could privately for the institution. the usual abstract of the lecture appeared, but not from his unaided pen. inventors, and promoters of useful inventions, frequently benefited by the advice of faraday, or by his generous help. a remarkable instance of this was told me by cyrus field. near the commencement of his great enterprise, when he wished to unite the old and the new worlds by the telegraphic cable, he sought the advice of the great electrician, and faraday told him that he doubted the possibility of getting a message across the atlantic. mr. field saw that this fatal objection must be settled at once, and begged faraday to make the necessary experiments, offering to pay him properly for his services. the philosopher, however, declined all remuneration, but worked away at the question, and presently reported to mr. field:--"it can be done, but you will not get an instantaneous message." "how long will it take?" was the next inquiry. "oh, perhaps a second." "well, that's quick enough for me," was the conclusion of the american; and the enterprise was proceeded with. as to the electric telegraph itself, faraday does not appear among those who claim its parentage, but he was constantly associated with those who do; his criticisms led ritchie to develop more fully his early conception, and he was constantly engaged with batteries and wires and magnets, while the telegraph was being perfected by others, and especially by his friend wheatstone, whose name will always be associated with what is perhaps the most wonderful invention of modern times. as to faraday's own work in applied science, his attempts to improve the manufacture of steel, and afterwards of glass for optical purposes, were among the least satisfactory of his researches. he was more successful in the matter of ventilation of lamp-burners. the windows of lighthouses were frequently found streaming with water that arose from the combustion of the oil, and in winter this was often converted into thick ice. he devised a plan by which this water was effectually carried away, and the room was also made more healthy for the keepers. at the athenæum club serious complaints were made that the brilliantly lighted drawing-room became excessively hot, and that headaches were very common, while the bindings of the books were greatly injured by the sulphuric acid that arose from the burnt coal-gas. faraday cured this by an arrangement of glass cylinders over the ordinary lamp chimneys, and descending tubes which carried off the whole products of combustion without their ever mixing with the air of the room. this principle could of course be applied to brackets or chandeliers elsewhere, but the professor made over any pecuniary benefit that might accrue from it to his brother, who was a lamp manufacturer, and had aided him in the invention. the achievements of faraday are certainly not to be tested by a money standard, nor by their immediate adaptation to the necessities or conveniences of life. "practical men" might be disposed to think slightly of the grand discoveries of the philosopher. their ideas of "utility" will probably be different. one man may take his wheat corn and convert it into loaves of bread, while his neighbour appears to lose his labour by throwing the precious grain into the earth: but which is after all most productive? the loaves will at once feed the hungry, but the sower's toil will be crowned in process of time by waving harvests. yet some of faraday's most recondite inquiries did bear practical fruit even during his own lifetime. in proof of this i will take one of his chemical and two of his electrical discoveries. long ago there was a portable gas company, which made oil-gas and condensed it into a liquid. this liquid faraday examined in , and he found the most important constituent of it to be a light volatile oil, which he called bicarburet of hydrogen. the gas company, i presume, came to an end; but what of the volatile liquid? obtained from coal-tar, and renamed benzine or benzol, it is now prepared on a large scale, and used as a solvent in some of our industrial arts. but other chemists have worked upon it, and torturing it with nitric acid, they have produced nitrobenzol--a gift to the confectioner and the perfumer. and by attacking this with reducing agents there was called into existence the wondrous base aniline,--wondrous indeed when we consider the transformations it underwent in the hands of hofmann, and the light it was made to throw on the internal structure of organic compounds. faraday used sometimes to pay a visit to the royal college of chemistry, and revel in watching these marvellous reactions. but aniline was of use to others besides the theoretical chemist. tortured by fresh appliances, this base gave highly-coloured bodies which it was found possible to fix on cotton as well as woollen and silken fabrics, and thence sprang up a large and novel branch of industry, while our eyes were delighted with the rich hues of mauve and magenta, the bleu de paris, and various other "aniline dyes." everyone who is at all acquainted with the habits of electricity knows that the most impassable of obstacles is the air, while iron bolts and bars only help it in its flight: yet, if an electrified body be brought near another body, with this invisible barrier between them, the electrical state of the second body is disturbed. faraday thought much over this question of "induction," as it is called, and found himself greatly puzzled to comprehend how a body should act where it is not. at length he satisfied himself by experiment that the interposed obstacle is itself affected by the electricity, and acquires an electro-polar state by which it modifies electric action in its neighbourhood. the amount varies with the nature of the substance, and faraday estimated it for such dielectrics as sulphur, shellac, or spermaceti, compared with air. he termed this new property of matter "specific inductive capacity," and figured in his own mind the play of the molecules as they propagated and for a while retained the force. now, these very recondite observations were opposed to the philosophy of the day, and they were not received by some of the leading electricians, especially of the continent, while those who first tried to extend his experiments blundered over the matter. however, the present professor sir william thomson, then a student at cambridge, showed that while faraday's views were rigorously deducible from coulomb's theory, this discovery was a great advance in the philosophy of the subject. when submarine telegraph wires had to be manufactured, thomson took "specific inductive capacity" into account in determining the dimensions of the cable: for we have there all the necessary conditions--the copper wire is charged with electricity, the covering of gutta-percha is a "dielectric," and the water outside is ready to have an opposite electric condition induced in it. the result is that, as faraday himself predicted, the message is somewhat retarded; and of course it becomes a thing of importance so to arrange matters that this retardation may be as small as possible, and the signals may follow one another speedily. now this must depend not only on the thickness of the covering, but also on the nature of the substance employed, and it was likely enough that gutta-percha was not the best possible substance. in fact, when professor fleeming jenkin came to try the inductive capacity of gutta-percha by means of the red sea cable, he found it to be almost double that of shellac, which was the highest that faraday had determined, and attempts have been made since to obtain some substance which should have less of this objectionable quality and be as well adapted otherwise for coating a wire. there is hooper's material, the great merit of which is its low specific inductive capacity, so that it permits of the sending of four signals while gutta-percha will only allow three to pass along; and mr. willoughby smith has made an improved kind of gutta-percha with reduced capacity. of course no opinion is expressed here on the value of these inventions, as many other circumstances must be taken into account, such as their durability and their power of insulation,--that is, preventing the leakage of the galvanic charge; but at least they show that one of the most abstruse discoveries of faraday has penetrated already into our patent offices and manufactories. two students in the physical laboratory at glasgow have lately determined with great care the inductive capacity of paraffin, and there can be little doubt that the speculations of the philosopher as to the condition of a dielectric will result in rendering it still more easy than at present to send words of information or of friendly greeting to our cousins across the atlantic or the indian ocean. the history of the magneto-electric light affords another remarkable instance of the way in which one of faraday's most recondite discoveries bore fruit in his own lifetime; and it is the more interesting as it fell to his own lot to assist in bringing the fruit to maturity. "brighton, _november , _. "dear phillips, "for once in my life i am able to sit down and write to you without feeling that my time is so little that my letter must of necessity be a short one; and accordingly i have taken an extra large sheet of paper, intending to fill it with news. "but how are you getting on? are you comfortable? and how does mrs. phillips do; and the girls? bad correspondent as i am, i think you owe me a letter; and as in the course of half an hour you will be doubly in my debt, pray write us, and let us know all about you. mrs. faraday wishes me not to forget to put her kind remembrances to you and mrs. phillips in my letter.... "we are here to refresh. i have been working and writing a paper that always knocks me up in health; but now i feel well again, and able to pursue my subject; and now i will tell you what it is about. the title will be, i think, 'experimental researches in electricity:'--i. on the induction of electric currents; ii. on the evolution of electricity from magnetism; iii. on a new electrical condition of matter; iv. on arago's magnetic phenomena. there is a bill of fare for you; and, what is more, i hope it will not disappoint you. now, the pith of all this i must give you very briefly; the demonstrations you shall have in the paper when printed...." so wrote faraday to his intimate friend richard phillips, on november th, , and the letter goes on to describe the great harvest of results which he had gathered since the th of august, when he first obtained evidence of an electric current from a magnet. a few days afterwards he was at work again on these curious relations of magnetism and electricity in his laboratory, and at the round pond in kensington gardens, and with father thames at waterloo bridge. on the th of february he entered in his note-book: "this evening, at woolwich, experimented with magnet, and for the first time got the magnetic spark myself. connected ends of a helix into two general ends, and then crossed the wires in such a way that a blow at _a b_ would open them a little. then bringing _a b_ against the poles of a magnet, the ends were disjoined, and bright sparks resulted." next day he repeated this experiment at home with mr. daniell's magnet, and then invited some of his best friends to come and see the tiny speck of light.[ ] but what was the use of this little spark between the shaken wires? "what is the use of an infant?" asked franklin once, when some such question was proposed to him. faraday said that the experimentalist's answer was, "endeavour to make it useful." but he passed to other researches in the same field. "i have rather been desirous," he says, "of discovering new facts and new relations dependent on magneto-electric induction, than of exalting the force of those already obtained; being assured that the latter would find their full development hereafter." and in this assurance he was not mistaken. electro-magnetism has been taken advantage of on a large scale by the metallurgist and the telegrapher; and even the photographer and sugar-refiner have attempted to make it their servant; but it is its application as a source of light that is most interesting to us in connection with its discoverer. many "electric lights" were invented by "practical men," the power being generally derived from a galvanic battery; and it was discovered that by making the terminals of the wires of charcoal, the brilliancy of the spark could be enormously increased. some of these inventions were proposed for lighthouses, and so came officially under the notice of faraday as scientific adviser to the trinity house. thus he was engaged in and with the beautiful electric light of dr. watson, which he examined most carefully, evidently hoping it might be of service, and at length he wrote an elaborate report pointing out its advantages, but at the same time the difficulties in the way of its practical adoption. the trinity corporation passed a special vote of thanks for his report, and hesitated to proceed further in the matter. but faraday's own spark was destined to be more successful. in some large magneto-electric machines were set up in paris for producing combustible gas by the decomposition of water. the scheme failed, but a mr. f. h. holmes suggested that these expensive toys might be turned to account for the production of light. "my propositions," he told the royal commissioners of lighthouses, "were entirely ridiculed, and the consequence was, that instead of saying that i thought i could do it, i promised to do it by a certain day. on that day, with one of duboscq's regulators or lamps, i produced the magneto-electric light for the first time; but as the machines were ill-constructed for the purpose, and as i had considerable difficulty to make even a temporary adjustment to produce a fitting current, the light could only be exhibited for a few minutes at a time." he turned his attention to the reconstruction of the machines, and after carrying on his experiments in belgium, he applied to the trinity board in february . here was the tiny spark, which faraday had produced just twenty-five years before, exalted into a magnificent star, and for faraday it was reserved to decide whether this star should shed its brilliance from the cliffs of albion. a good piece of optical apparatus, intended for the bishop rock in the scillies, happened to be at the experimental station at blackwall, and with this comparative experiments were made. we can imagine something of the interest with which faraday watched the light from woolwich, and asked questions of the inventor about all the details of its working and expense; and we can picture the alternations of hope and caution as he wrote in his report, "the light is so intense, so abundant, so concentrated and focal, so free from under-shadows (caused in the common lamp by the burner), so free from flickering, that one cannot but desire it should succeed. but," he adds, "it would require _very careful_ and progressive introduction--men with peculiar knowledge and skill to attend it; and the means of instantly substituting one lamp for another in case of accident. the common lamp is so simple, both in principle and practice, that its liability to failure is very small. there is no doubt that the magneto-electric lamp involves a great number of circumstances tending to make its application more refined and delicate; but i would fain hope that none of these will prove a barrier to its introduction. nevertheless, it must pass into practice only through the ordeal of a full, searching, and prolonged trial." this trial was made in the upper of the two light towers at the south foreland; but it was not till the th december, , that the experiment was commenced. faraday made observations on it for the first two days, but it did not act well, and was discontinued till march , , when it again shot forth its powerful rays across the channel. it was soon inspected by faraday inside and outside, by land and by sea. his notes terminate in this way:--"went to the hills round, about a mile off, or perhaps more, so as to see both upper and lower light at once. the effect was very fine. the lower light does not come near the upper in its power, and, as to colour, looks red whilst the upper is white. the visible rays proceed from both horizontally, but those from the low light are not half so long as those from the electric light. the radiation from the upper light was beautifully horizontal, going out right and left with intenseness like a horizontal flood of light, with blackness above and blackness below, yet the sky was clear and the stars shining brightly. it seemed as if the lanthorn[ ] only were above the earth, so dark was the path immediately below the lanthorn, yet the whole tower was visible from the place. as to the shadows of the uprights, one could walk into one and across, and see the diminution of the light, and could easily see when the edge of the shadow was passed. they varied in width according to the distance from the lanthorn. with upright bars their effect is considerable at a distance, as seen last night; but inclining these bars would help in the distance, though not so much as with a light having considerable upright dimension, as is the case with an oil-lamp. "the shadows on a white card are very clear on the edge--a watch very distinct and legible. on lowering the head near certain valleys, the feeble shadow of the distant grass and leaves was evident. the light was beautifully steady and bright, with no signs of variation--the appearance was such as to give confidence to the mind--no doubt about its continuance. "as a light it is unexceptionable--as a magneto-electric light wonderful--and seems to have all the adjustments of quality and more than can be applied to a voltaic electric light or a ruhmkorff coil." the royal commissioners and others saw with gratification this beautiful light, and arrangements were made for getting systematic observations of it by the keepers of all the lighthouses within view, the masters of the light-vessels that guard the goodwin sands, and the crews of pilot cutters; after which faraday wrote a very favourable report, saying, among other things: "i beg to state that in my opinion professor holmes has practically established the fitness and sufficiency of the magneto-electric light for lighthouse purposes, so far as its nature and management are concerned. the light produced is powerful beyond any other that i have yet seen so applied, and in principle may be accumulated to any degree; its regularity in the lanthorn is great, its management easy, and its care there may be confided to attentive keepers of the ordinary degree of intellect and knowledge."[ ] the elder brethren then wished a further trial of six months, during which time the light was to be entirely under their own control. it was therefore again kindled on august , and the experiment happened soon to be exposed to a severe test, as one of the light-keepers, who had been accustomed to the arrangement of the lamps in the lantern, was suddenly removed, and another took his place without any previous instruction. this man thought the light sufficiently strong if he allowed the carbon points to touch, as the lamp then required no attendance whatever, and he could leave it in that way for hours together. on being remonstrated with, he said, "it is quite good enough." notwithstanding such difficulties as these, the experiment was considered satisfactory, but it was discontinued at the south foreland, for the cliffs there are marked by a double light, and the electric spark was so much brighter than the oil flames in the other house, that there was no small danger of its being seen alone in thick weather, and thus fatally misleading some unfortunate vessel. after this faraday made further observations, estimates of the expense, and experiments on the divergence of the beam, while mr. holmes worked away at northfleet perfecting his apparatus, and the authorities debated whether it was to be exhibited again at the start, which is a revolving light, or at dungeness, which is fixed. the scientific adviser was in favour of the start, but after an interview with mr. milner gibson, then president of the board of trade, dungeness was determined on; a beautiful small combination of lenses and prisms was made expressly for it by messrs. chance, and at last, after two years' delay, the light again shone on our southern coast. it may be well to describe the apparatus. there are permanent magnets, weighing about lbs. each, ranged on the periphery of two large wheels. a steam-engine of about three-horse power causes a series of soft iron cores, surrounded by coils of wire, to rotate past the magnets. this calls the power into action, and the small streams of electricity are all collected together, and by what is called a "commutator" the alternative positive and negative currents are brought into one direction. the whole power is then conveyed by a thick wire from the engine-house to the lighthouse tower, and up into the centre of the glass apparatus. there it passes between two charcoal points, and produces an intensely brilliant continuous spark. at sunset the machine is started, making about revolutions per minute; and the attendant has only to draw two bolts in the lamp, when the power thus spun in the engine-room bursts into light of full intensity. the "lamp" regulates itself, so as to keep the points always at a proper distance apart, and continues to burn, needing little or no attention for three hours and a half, when, the charcoals being consumed, the lamp must be changed, but this is done without extinguishing the light. again there were inspections, and reports from pilots and other observers, and faraday propounded lists of questions to the engineer about bolts and screws and donkey-engines, while he estimated that at the varne light-ship, about equidistant from cape grisnez and dungeness, the maximum effect of the revolving french light was equalled by the constant gleam from the english tower. but delays again ensued till intelligent keepers could be found and properly instructed; but on the th june, , faraday's own light, the baby grown into a giant, shone permanently on the coast of britain. france, too, was alert. berlioz's machine, which was displayed at the international exhibition in london, and which was also examined by faraday, was approved by the french government, and was soon illuminating the double lighthouse near havre. these magneto-electric lights on either side of the channel have stood the test of years; and during the last two years there has shone another still more beautiful one at souter point, near tynemouth; while the narrow strait between england and france is now guarded by these "sentinels of peaceful progress," for the revolving light at grisnez has been lately illuminated on this principle, and on the st of january, , the two lights of the south foreland flashed forth with the electric flame.[ ] in describing thus the valuable applications of faraday's discoveries of benzol, of specific inductive capacity, and of magneto-electricity, it is not intended to exalt these above other discoveries which as yet have paid no tribute to the material wants of man. the good fruit borne by other researches may not be sufficiently mature, but it doubtless contains the seeds of many useful inventions. yet, after all, we must not measure the worth of faraday's discoveries by any standard of practical utility in the present or in the future. his chief merit is that he enlarged so much the boundaries of our knowledge of the physical forces, opened up so many new realms of thought, and won so many heights which have become the starting-points for other explorers. footnotes: [ ] de la rive points this out in his brief notice of faraday immediately on receiving the news of his death:--"je n'ai parlé que du savant, je tiens aussi à dire un mot de l'homme. alliant à une modestie vraie, parcequ'elle provenait de l'élévation de son âme, une droiture à toute épreuve et une candeur admirable, faraday n'aimait la science que pour elle-même. aussi jouissait-il des succès des autres au moins autant que des siens propres; et quant à lui, s'il a accepté, avec une sincère satisfaction, les honneurs scientifiques qui lui out été prodigués à si juste titre, il a constamment refusé toutes les autres distinctions et les récompenses qu'on eût voulu lui décerner. il s'est contenté toute sa vie de la position relativement modeste qu'il occupait à l'institution royale de londres; avoir son laboratoire et strictement de quoi vivre, c'est tout ce qu'il lui fallait.--presinge, le août, .--a. de la rive." [ ] preface to "faraday und seine entdeckungen." [ ] i am indebted to sir charles wheatstone for the following impromptu by herbert mayo:-- "around the magnet faraday was sure that volta's lightnings play: but how to draw them from the wire? he drew a lesson from the heart: 'tis when we meet, 'tis when we part, breaks forth the electric fire." [ ] the room with glass sides, from which the light is exhibited at the top of a lighthouse, is called by this name. [ ] one night there was a beautiful aurora. mr. holmes remarked that his poor electric light could not compare with that for beauty; but faraday rejoined, "don't abuse your light. the aurora is very beautiful, and so is a wild horse, but you have tamed it and made it valuable." [ ] the illuminating apparatus at dungeness is one of what is termed the sixth order, millimetres (about inches) in diameter. mr. chance constructed one for souter point of the third order, one metre (nearly inches) in diameter, with special arrangements for giving artificial divergence to the beam in a vertical direction, in order to obviate the danger arising from the luminous point not being always precisely in the same spot. it has also additional contrivances for utilizing the back light. similar arrangements were made for the south foreland lights, which are also of the third order; and every portion of the machinery and apparatus is in duplicate in case of accident, and the double force can be employed in times of fog. supplementary portraits. it has been said that there is no photograph or painting of faraday which is a satisfactory likeness; not because good portraits have never been published, but because they cannot give the varied and ever-shifting expression of his features. similarly, i fear that the mental portraiture which i have attempted will fail to satisfy his intimate acquaintance. yet, as one who never saw him in the flesh may gain a good idea of his personal appearance by comparing several pictures, so the reader may learn more of his intellectual and moral features by combining the several estimates which have been made by different minds. earlier biographies have been already referred to, but my sketch may well be supplemented by an anonymous poem that appeared immediately after his death, and by the words of two of the most distinguished foreign philosophers--messrs. de la rive and dumas. "statesmen and soldiers, authors, artists,--still the topmost leaves fall off our english oak: some in green summer's prime, some in the chill of autumn-tide, some by late winter's stroke. "another leaf has dropped on that sere heap-- one that hung highest; earliest to invite the golden kiss of morn, and last to keep the fire of eve--but still turned to the light. "no soldier's, statesman's, poet's, painter's name was this, thro' which is drawn death's last black line; but one of rarer, if not loftier fame-- a priest of truth, who lived within her shrine. "a priest of truth: his office to expound earth's mysteries to all who willed to hear-- who in the book of science sought and found, with love, that knew all reverence, but no fear. "a priest, who prayed as well as ministered: who grasped the faith he preached; and held it fast: knowing the light he followed never stirred, howe'er might drive the clouds thro' which it past. "and if truth's priest, servant of science too, whose work was wrought for love and not for gain: not one of those who serve but to ensue their private profit: lordship to attain "over their lord, and bind him in green withes, for grinding at the mill 'neath rod and cord; of the large grist that they may take their tithes-- so some serve science that call science lord. "one rule his life was fashioned to fulfil: that he who tends truth's shrine, and does the hest of science, with a humble, faithful will, the god of truth and knowledge serveth best. "and from his humbleness what heights he won! by slow march of induction, pace on pace, scaling the peaks that seemed to strike the sun, whence few can look, unblinded, in his face. "until he reached the stand which they that win a bird's-eye glance o'er nature's realm may throw; whence the mind's ken by larger sweeps takes in what seems confusion, looked at from below. "till out of seeming chaos order grows, in ever-widening orbs of law restrained, and the creation's mighty music flows in perfect harmony, serene, sustained; "and from varieties of force and power, a larger unity, and larger still, broadens to view, till in some breathless hour all force is known, grasped in a central will, "thunder and light revealed as one same strength-- modes of the force that works at nature's heart-- and through the universe's veinèd length bids, wave on wave, mysterious pulses dart. "that cosmic heart-beat it was his to list, to trace those pulses in their ebb and flow towards the fountain-head, where they subsist in form as yet not given e'en _him_ to know. "yet, living face to face with these great laws, great truths, great myst'ries, all who saw him near knew him for child-like, simple, free from flaws of temper, full of love that casts out fear: "untired in charity, of cheer serene; not caring world's wealth or good word to earn; childhood's or manhood's ear content to win; and still as glad to teach as meek to learn. "such lives are precious: not so much for all of wider insight won where they have striven, as for the still small voice with which they call along the beamy way from earth to heaven." _punch_, september , . the estimate of m. a. de la rive is from a letter he addressed to faraday himself:-- "i am grieved to hear that your brain is weary; this has sometimes happened on former occasions, in consequence of your numerous and persevering labours, and you will bear in mind that a little rest is necessary to restore you. you possess that which best contributes to peace of mind and serenity of spirit--a full and perfect faith, a pure and tranquil conscience, filling your heart with the glorious hopes which the gospel imparts. you have also the advantage of having always led a smooth and well-regulated life, free from ambition, and therefore exempt from all the anxieties and drawbacks which are inseparable from it. honour has sought you in spite of yourself; you have known, without despising it, how to value it at its true worth. you have known how to gain the high esteem, and at the same time the affection, of all those acquainted with you. "moreover, thanks to the goodness of god, you have not suffered any of those family misfortunes which crush one's life. you should, therefore, watch the approach of old age without fear and without bitterness, having the comforting feeling that the wonders which you have been able to decipher in the book of nature must contribute to the greater reverence and adoration of their supreme author. "such, my dear friend, is the impression that your beautiful life always leaves upon me; and when i compare it with our troubled and ill-fulfilled life-course, with all that accumulation of drawbacks and griefs by which mine in particular has been attended, i put you down as very happy, especially as you are worthy of your good fortune. this leads me to reflect on the miserable state of those who are without that religious faith which you possess in so great a degree." in m. dumas' eloge at the académie des sciences, occur the following sentences:-- "i do not know whether there is a _savant_ who would not feel happy in leaving behind him such works as those with which faraday has gladdened his contemporaries, and which he has left as a legacy to posterity: but i am certain that all those who have known him would wish to approach that moral perfection which he attained to without effort. in him it appeared to be a natural grace, which made him a professor full of ardour for the diffusion of truth, an indefatigable worker, full of enthusiasm and sprightliness in his laboratory, the best and most amiable of men in the bosom of his family, and the most enlightened preacher amongst the humble flock whose faith he followed. "the simplicity of his heart, his candour, his ardent love of the truth, his fellow-interest in all the successes, and ingenuous admiration of all the discoveries of others, his natural modesty in regard to what he himself discovered, his noble soul--independent and bold,--all these combined gave an incomparable charm to the features of the illustrious physicist. "i have never known a man more worthy of being loved, of being admired, of being mourned. "fidelity to his religious faith, and the constant observance of the moral law, constitute the ruling characteristics of his life. doubtless his firm belief in that justice on high which weighs all our merits, in that sovereign goodness which weighs all our sufferings, did not inspire faraday with his great discoveries, but it gave him the straightforwardness, the self-respect, the self-control, and the spirit of justice, which enabled him to combat evil fortune with boldness, and to accept prosperity without being puffed up.... "there was nothing dramatic in the life of faraday. it should be presented under that simplicity of aspect which is the grandeur of it. there is, however, more than one useful lesson to be learnt from the proper study of this illustrious man, whose youth endured poverty with dignity, whose mature age bore honours with moderation, and whose last years have just passed gently away surrounded by marks of respect and tender affection." appendix. list of learned societies to which michael faraday belonged. anno . corresponding member of the academy of sciences, paris. corresponding member of the accademia dei georgofili, florence. honorary member of the cambridge philosophical society. honorary member of the british institution. . fellow of the royal society. honorary member of the cambrian society, swansea. fellow of the geological society. . member of the royal institution. corresponding member of the society of medical chemists, paris. . honorary member of the westminster medical society. . correspondent of the société philomathique, paris. . fellow of the natural society of science, heidelberg. . honorary member of the society of arts, scotland. . honorary member of the imperial academy of sciences, st. petersburg. . honorary member of the college of pharmacy, philadelphia. honorary member of the chemical and physical society, paris. fellow of the american academy of arts and sciences, boston. member of the royal society of science, copenhagen. . corresponding member of the royal academy of sciences, berlin. honorary member of the hull philosophical society. . foreign corresponding member of the academy of sciences and literature, palermo. . corresponding member of the royal academy of medicine, paris. honorary member of the royal society, edinburgh. honorary member of the institution of british architects. honorary member of the physical society, frankfort. honorary fellow of the medico-chirurgical society, london. . senator of the university of london. honorary member of the society of pharmacy, lisbon. honorary member of the sussex royal institution. foreign member of the society of sciences, modena. foreign member of the natural history society, basle. . honorary member of the literary and scientific institution, liverpool. . honorary member of the institution of civil engineers. foreign member of the royal academy of sciences, stockholm. . member of the american philosophical society, philadelphia. honorary member of the hunterian medical society, edinburgh. . foreign associate of the royal academy of sciences, berlin. . honorary member of the literary and philosophical society, manchester. honorary member of the useful knowledge society, aix-la-chapelle. . foreign associate of the academy of sciences, paris. honorary member of the sheffield scientific society. . corresponding member of the national institute, washington. corresponding member of the société d'encouragement, paris. . honorary member of the society of sciences, vaud. . member of the academy of sciences, bologna. foreign associate of the royal academy of sciences of belgium. fellow of the royal bavarian academy of sciences, munich. correspondent of the academy of natural sciences, philadelphia. . foreign honorary member of the imperial academy of sciences, vienna. . honorary member, first class, of the institut royal des pays bas. foreign correspondent of the institute, madrid. . corresponding associate of the accademia pontificia, rome. foreign associate of the academy of sciences, haarlem. . member of the royal academy of sciences, the hague. corresponding member of the batavian society of experimental philosophy, rotterdam. fellow of the royal society of sciences, upsala. . foreign associate of the royal academy of sciences, turin. honorary member of the royal society of arts and sciences, mauritius. . corresponding associate of the royal academy of sciences, naples. . honorary member of the imperial society of naturalists, moscow. corresponding associate of the imperial institute of sciences of lombardy. . corresponding member of the netherlands' society of sciences, batavia. member of the imperial royal institute, padua. . member of the institute of breslau. corresponding associate of the institute of sciences, venice. member of the imperial academy, breslau. . corresponding member of the hungarian academy of sciences, pesth. . foreign associate of the academy of sciences, pesth. honorary member of the philosophical society, glasgow. . honorary member of the medical society, edinburgh. . foreign associate of the imperial academy of medicine, paris. . foreign associate of the royal academy of sciences, naples. index. a. abbott, benjamin, . abel, f. a., reminiscences by, , . anderson, sergeant, . apparatus, simplicity of, - . arrow, sir frederick, anecdote by, . astley's theatre, adventure at, . athenæum club, , . atoms, or centres of force? . autograph persecution, . b. barlow, rev. john, ; incident at his house, . barnard, f., anecdotes by, , , . barnard, miss jane, . barrett, w. f., reminiscences by, , , . blacksmith's shop, , . blaikley's painting, . bollaert, william, . bores, . british association, . c. carpenter, dr., anecdote by, . character of faraday, . charitable gifts, . "chemical manipulation," ; quotations from, , . chemical society, . children and faraday, , , . churchyard at oberhofen, . city philosophical society, , . close, captain, anecdotes by, , . colliery explosion at haswell, . committees, , . continent, visits to the, , . correspondence, . crosse, mrs. a., visit of, . d. daniell, professor, . davy, sir humphry, , , , , ; his safety-lamp, . de la rive, a., ; sketches by, , , . deacon, mrs., recollections by, . discoveries, value of, , . domestic affection, . dumas, sketches by, , , . e. education, views on, , - . electrical machines, primitive, , . enthusiasm, . experiment, love of, , . explosions, . f. faithfulness, . faraday, michael, his birth, ; apprenticed to a bookseller, , ; begins to experiment, , ; attends tatum's lectures, ; davy's, ; becomes journeyman bookbinder, ; engaged by davy, , ; his attempts at self-improvement, , , ; travels on the continent, ; gives his first lecture, ; writes his first paper, ; assists professor brande, ; his amusements, , , ; marries, ; gives courses of lectures, ; appointed fullerian professor, ; his income, , ; accepts lectureship at woolwich, ; becomes scientific adviser to trinity house, ; his usual day's work, ; his friday evenings, ; his juvenile lectures, ; his sunday engagements, ; his wednesday meetings, ; his visits to the country, ; his correspondence, ; his publications, ; his honours, , ; declines presidentship of royal society, ; refuses and accepts pension, ; resigns his appointments, , ; his last illness, ; his death, . faraday's father, , , , , . " mother, , . field, cyrus, . firmness with gentleness, . force, a proteus, . foucault, visit to, . friday evenings at the royal institution, , , . fuller, john, . funeral, . g. giessbach falls, . government and science, . graham, professor, , . gymnotus, . h. hampton court, house at, . helmholz, professor, quoted, . holland, sir henry, , . holmes, f. h., , . home life, , , , . honours, scientific, , ; views on, - . humility, . humour, . i. imagination, . indignation against wrong, . infidelity, accusation of, . inner conflicts, . j. jermyn street, incident at, . jones, dr. h. bence, quoted, ; his "life and letters of faraday," , , . journals, , . juvenile lectures at royal institution, , . k. kindliness, - , , . l. laboratory work, , . lectures at royal institution, , , . lecturing, views on, - . letters from faraday to abbott, b., ; abel, f. a., ; airy, sir g. b. (astronomer royal), ; andrews, prof., ; auckland, lord, ; barnard, f., ; barnard, miss sarah, ; becker, dr., ; coutts, lady burdett, , ; crosse, mrs. andrew, ; deacon, mrs., ; faraday, mrs. (his mother), ; faraday, mrs. (his wife), , ; field, f., ; gladstone, j. h., ; inventors, ; joule, j. p., , ; managers of royal institution, ; matteucci, ; moore, miss, , ; noad, dr., ; paris, comte de, ; paris, dr., , ; percy, dr., ; phillips, r., ; riebau, g., ; schönbein, , ; siemens, c. w., ; spiritualist, ; wheatstone, sir charles, ; wrottesley, lord, . letters to faraday, from bonaparte, louis napoleon, ; davy, sir humphry, ; de la rive, a., ; whewell, dr., . lighthouses, adjustment of apparatus in, ; illuminated by electricity, - . love of study, . love to children, . m. magnetism, wonder at, . magneto-electric light, - . magrath, mr., , . mallet, robert, reminiscences by, . . masquerier, m., , . mathematics, want of, . mayo, herbert, impromptu by, . melbourne, lord, . mental and moral greatness conjoined, , . mental education, views on, . music, , . n. napoleon iii., . natural theology, views on, . noad, dr., . noble, mr. (the sculptor), . note-books, , , , . o. orderliness, . p. peel, sir robert, . philosopher portrayed, , . philosophers and practical men, , . photometer, special, playfulness, , . poetry of nature, , . politics, indifference to, . pollock, lady, description of friday evening discourse, . potato models, . practical applications of science, - . preaching, style of, . prince consort, . pritchard, rev. c., quoted, . progress, necessity of, , . publications, scientific, , . public schools commission, evidence before, . _punch_, verses in, . q. queen victoria, , . r. reid, miss, reminiscences by, . religious belief, views on, . religious character, - . researches, early, ; on electricity and magnetism, , , , , ; electrical eel, ; telegraphy, ; ventilation, ; benzol, . respect paid to others, . reverence, , , . roman carnival, . royal commission on lights, . royal institution, , , , , ; faraday laboratory assistant at, , ; superintendent of house at, ; fullerian professorship, ; relics at, , . royal society, fellowship, ; presidentship declined, ; communications to, , . s. sandemanians, , ; faraday's eldership among, . schönbein, prof., quoted, , science a branch of education, . sciences linked together, . self-respect, , . sensitiveness, , . sermons, faraday's, , . simple-hearted joyousness, , . simplicity of character, , . sirium _alias_ vestium, . social character, , . society of arts, . spectrum analysis, . spiritualists, opinion of, . submarine cables, . swiss tour, . t. table-turning explained, . tenacity of purpose, . thames impure, . thomson, sir william, , . thunderstorms enjoyed, , . tomlinson, c., reminiscence by, . trinity house, , , , , , - . truthfulness, , . tyndall, professor, reminiscences by, , , ; his "faraday as a discoverer," . u. unworldliness, . v. velocipede riding, . visitors, attention to, , . visits to the sick, . w. walmer, visit to, . welsh damsel at waterfall, . william iv., . wiseman, cardinal, visit of, . woolwich academy, , , . working, method of, , . y. young, james reminiscence by, . the end. london: r. clay, sons, and taylor, printers, bread street hill. bedford street, covent garden, london. _september ._ _macmillan & co.'s catalogue of works in belles lettres, including poetry, fiction, etc._ =allingham.=--laurence bloomfield in ireland; or, the new landlord. by william allingham. new and cheaper issue, with a preface. fcap. vo. cloth. _s._ _d._ "_it is vital with the national character.... it has something of pope's point and goldsmith's simplicity, touched to a more modern issue._"--athenÆum. =an ancient city, and other poems.=--by a native of surrey. extra fcap. vo. _s._ =archer.=--christina north. by e. m. archer. two vols. crown vo. _s._ "_the work of a clever, cultivated person, wielding a practised pen. the characters are drawn with force and precision, the dialogue is easy: the whole book displays powers of pathos and humour, and a shrewd knowledge of men and things._"--spectator. =arnold.=--the complete poetical works. vol. i. narrative and elegiac poems. vol. ii. dramatic and lyric poems. by matthew arnold. extra fcap. vo. price _s._ each. _the two volumes comprehend the first and second series of the poems, and the new poems._ "_thyrsis is a poem of perfect delight, exquisite in grave tenderness of reminiscence, rich in breadth of western light, breathing full the spirit of gray and ancient oxford._"--saturday review. =atkinson.=--an art tour to the northern capitals of europe. by j. beavington atkinson. vo. _s._ "_we can highly recommend it; not only for the valuable information it gives on the special subjects to which it is dedicated, but also for the interesting episodes of travel which are interwoven with, and lighten, the weightier matters of judicious and varied criticism on art and artists in northern capitals._"--art journal. =baker.=--cast up by the sea; or, the adventures of ned grey. by sir samuel baker, m.a., f.r.g.s. with illustrations by huard. fifth edition. crown vo. cloth gilt. _s._ _d._ "_an admirable tale of adventure, of marvellous incidents, wild exploits, and terrible dénouements._"--daily news. 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"_the story is rendered with consummate beauty._"--literary churchman. =buist.=--birds, their cages and their keep: being a practical manual of bird-keeping and bird-rearing. by k. a. buist. with coloured frontispiece and other illustrations. crown vo. _s._ =burnand.=--my time, and what i've done with it. by f. c. burnand. crown vo. _s._ =cabinet pictures.=--oblong folio, price _s._ contents:--_"childe harold's pilgrimage" and "the fighting téméraire," by j. m. w. turner; "crossing the bridge," by sir w. a. callcott; "the cornfield," by john constable; and "a landscape," by birket foster._ _the daily news says of them,_ "_they are very beautifully executed, and might be framed and hung up on the wall, as creditable substitutes for the originals._" cabinet pictures. a second series. _containing:--"the baths of caligula" and "the golden bough," by j. m. w. turner; "the little brigand," by t. uwins; "the lake of lucerne," by percival skelton; "evening rest," by e. m. wimperis._ oblong folio. _s._ =carroll.=--works by "lewis carroll:"-- alice's adventures in wonderland. with forty-two illustrations by tenniel. th thousand. crown vo. cloth. _s._ a german translation of the same. with tenniel's illustrations. crown vo. gilt. _s._ a french translation of the same. with tenniel's illustrations. crown vo. gilt. _s._ an italian translation of the same. by t. p. rossette. with tenniel's illustrations. crown vo. _s._ "_beyond question supreme among modern books for children._"--spectator. "_one of the choicest and most charming books ever composed for a child's reading._"--pall mall gazette. "_a very pretty and highly original book, sure to delight the little world of wondering minds, and which may well please those who have unfortunately passed the years of wondering._"--times. through the looking-glass, and what alice found there. with fifty illustrations by tenniel. crown vo. gilt. _s._ th thousand. "_quite as rich in humorous whims of fantasy, quite as laughable in its queer incidents, as loveable for its pleasant spirit and graceful manner, as the wondrous tale of alice's former adventures._"--illustrated london news. "_if this had been given to the world first it would have enjoyed a success at least equal to 'alice in wonderland.'_"--standard. =children's (the) garland,= from the best poets. selected and arranged by coventry patmore. new edition. with illustrations by j. lawson. crown vo. cloth extra. _s._ =christmas carol (a).= printed in colours from original designs by mr. and mrs. trevor crispin, with illuminated borders from mss. of the th and th centuries. imp. to. cloth inlaid, gilt edges, £ _s._ also a cheaper edition, _s._ "_a most exquisitely got up volume. legend, carol, and text are preciously enshrined in its emblazoned pages, and the illuminated borders are far and away the best example of their art we have seen this christmas. the pictures and borders are harmonious in their colouring, the dyes are brilliant without being raw, and the volume is a trophy of colour-printing. the binding by burn is in the very best taste._"--times. =church (a. j.)=--horÆ tennysonianÆ, sive eclogæ e tennysono latine redditæ. cura a. j. church, a.m. extra fcap. vo. _s._ "_of mr. church's ode we may speak in almost unqualified praise, and the same may be said of the contributions generally._"--pall mall gazette. =clough (arthur hugh).=--the poems and prose remains of arthur hugh clough. with a selection from his letters and a memoir. edited by his wife. with portrait. two vols. crown vo. _s._ "_taken as a whole," the spectator says, "these volumes cannot fail to be a lasting monument of one of the most original men of our age._" "_full of charming letters from rome," says the morning star, "from greece, from america, from oxford, and from rugby._" the poems of arthur hugh clough, sometime fellow of oriel college, oxford. fourth edition. fcap. vo. _s._ "_from the higher mind of cultivated, all-questioning, but still conservative england, in this our puzzled generation, we do not know of any utterance in literature so characteristic as the poems of arthur hugh clough._"--fraser's magazine. =clunes.=--the story of pauline: an autobiography. by g. c. clunes. crown vo. _s._ "_both for vivid delineation of character and fluent lucidity of style, 'the story of pauline' is in the first rank of modern fiction._"--globe. "_told with delightful vivacity, thorough appreciation of life, and a complete knowledge of character._"--manchester examiner. =collects of the church of england.= with a beautifully coloured floral design to each collect, and illuminated cover. crown vo. _s._ also kept in various styles of morocco. "_this is beyond question," the art journal says, "the most beautiful book of the season._" _the guardian thinks it_ "_a successful attempt to associate in a natural and unforced manner the flowers of our fields and gardens with the course of the christian year._" =cox.=--recollections of oxford. by g. v. cox, m.a., late esquire bedel and coroner in the university of oxford. second and cheaper edition. crown vo. _s._ _the times says that it_ "_will pleasantly recall in many a country parsonage the memory of youthful days._" =culmshire folk.=--by ignotus. three vols. crown vo. _s._ _d._ "_its sparkling pleasantness, its drollery, its shrewdness, the charming little bits of character which frequently come in, its easy liveliness, and a certain chattiness which, while it is never vulgar, brings the writer very near, and makes one feel as if the story were being told in lazy confidence in an hour of idleness by a man who, while thoroughly good-natured, is strongly humorous, and has an ever-present perception of the absurdities of people and things._"--spectator. =dante.=--dante's comedy, the hell. translated by w. m. rossetti. fcap. vo. cloth. _s._ "_the aim of this translation of dante may be summed up in one word--literality. to follow dante sentence for sentence, line for line, word for word--neither more nor less, has been my strenuous endeavour._"--author's preface. =days of old;= stories from old english history. by the author of "ruth and her friends." new edition. mo. cloth, extra. _s._ _d._ "_full of truthful and charming historic pictures, is everywhere vital with moral and religious principles, and is written with a brightness of description, and with a dramatic force in the representation of character, that have made, and will always make, it one of the greatest favourites with reading boys._"--nonconformist. =deane.=--marjory. by milly deane. third edition. with frontispiece and vignette. crown vo. _s._ _d._ _the times of september th says it is_ "_a very touching story, full of promise for the after career of the authoress. it is so tenderly drawn, and so full of life and grace, that any attempt to analyse or describe it falls sadly short of the original. we will venture to say that few readers of any natural feeling or sensibility will take up 'marjory' without reading it through at a sitting, and we hope we shall see more stories by the same hand._" _the morning post calls it_ "_a deliciously fresh and charming little love story._" =de vere.=--the infant bridal, and other poems. by aubrey de vere. fcap. vo. _s._ _d._ "_mr. de vere has taken his place among the poets of the day. pure and tender feeling, and that polished restraint of style which is called classical, are the charms of the volume._"--spectator. =doyle (sir f. h.)=--lectures on poetry, delivered before the university of oxford in . by sir francis hastings doyle, professor of poetry in the university of oxford. crown vo. _s._ _d._ "_full of thoughtful discrimination and fine insight: the lecture on 'provincial poetry' seems to us singularly true, eloquent, and instructive._"--spectator. =estelle russell.=--by the author of "the private life of galileo." new edition. crown vo. _s._ _full of bright pictures of french life. the english family, whose fortunes form the main drift of the story, reside mostly in france, but there are also many english characters and scenes of great interest. it is certainly the work of a fresh, vigorous, and most interesting writer, with a dash of sarcastic humour which is refreshing and not too bitter._ "_we can send our readers to it with confidence._"--spectator. =evans.=--brother fabian's manuscript, and other poems. by sebastian evans. fcap. vo. cloth. _s._ "_in this volume we have full assurance that he has 'the vision and the faculty divine.'... clever and full of kindly humour._"--globe. =evans.=--the curse of immortality. by a. eubule evans. crown vo. _s._ "_never, probably, has the legend of the wandering jew been more ably and poetically handled. the author writes as a true poet, and with the skill of a true artist. the plot of this remarkable drama is not only well contrived, but worked out with a degree of simplicity and truthful vigour altogether unusual in modern poetry. in fact, since the date of byron's 'cain,' we can scarcely recall any verse at once so terse, so powerful, and so masterly._"--standard. =fairy book.=--the best popular fairy stories. selected and rendered anew by the author of "john halifax, gentleman." with coloured illustrations and ornamental borders by j. e. rogers, author of "ridicula rediviva." crown vo. cloth, extra gilt. _s._ (golden treasury edition. mo. _s._ _d._) "_a delightful selection, in a delightful external form,_"--spectator. "_a book which will prove delightful to children all the year round._"--pall mall gazette. =fletcher.=--thoughts from a girl's life. by lucy fletcher. second edition. fcap. vo. _s._ _d._ "_the poems are all graceful; they are marked throughout by an accent of reality; the thoughts and emotions are genuine._"--athenÆum. =garnett.=--idylls and epigrams. chiefly from the greek anthology. by richard garnett. fcap. vo. _s._ _d._ "_a charming little book. for english readers, mr. garnett's translations will open a new world of thought._"--westminster review. =gilmore.=--storm warriors; or, life-boat work on the goodwin sands. by the rev. john gilmore, m.a., rector of holy trinity, ramsgate, author of "the ramsgate life-boat," in _macmillan's magazine_. crown vo. _s._ "_the stories, which are said to be literally exact, are more thrilling than anything in fiction. mr. gilmore has done a good work as well as written a good book._"--daily news. =gladstone.=--juventus mundi. the gods and men of the heroic age. by the right hon. w. e. gladstone, m.p. crown vo. cloth extra. with map. _s._ _d._ second edition. "_to read these brilliant details," says the athenÆum,_ "_is like standing on the olympian threshold and gazing at the ineffable brightness within._" _according to the westminster review,_ "_it would be difficult to point out a book that contains so much fulness of knowledge along with so much freshness of perception and clearness of presentation._" =gray.=--the poetical works of david gray. new and enlarged edition. edited by henry glassford bell, late sheriff of lanarkshire. crown vo. _s._ =guesses at truth.=--by two brothers. with vignette title and frontispiece. new edition, with memoir. fcap. vo. _s._ also see golden treasury series. =halifax.=--after long years. by m. c. halifax. crown vo. _s._ _d._ "_a story of very unusual merit. the entire story is well conceived, well written, and well carried out; and the reader will look forward with pleasure to meeting this clever author again._"--daily news. "_this is a very pretty, simple love story.... the author possesses a very graceful, womanly pen, and tells the story with a rare tender simplicity which well befits it._"--standard. =hamerton.=--a painter's camp. second edition, revised. extra fcap. vo. _s._ book i. _in england_; book ii. _in scotland_; book iii. _in france._ "_these pages, written with infinite spirit and humour, bring into close rooms, back upon tired heads, the breezy airs of lancashire moors and highland lochs, with a freshness which no recent novelist has succeeded in preserving._"--nonconformist. =heaton.=--happy spring time. illustrated by oscar pletsch. with rhymes for mothers and children. by mrs. charles heaton. crown vo. cloth extra, gilt edges. _s._ _d._ "_the pictures in this book are capital._"--athenÆum. =hervey.=--duke ernest, a tragedy; and other poems. fcap. vo. _s._ "_conceived in pure taste and true historic feeling, and presented with much dramatic force.... thoroughly original._"--british quarterly. =higginson.=--malbone: an oldport romance. by t. w. higginson. fcap. vo. _s._ _d._ _the daily news says:_ "_who likes a quiet story, full of mature thought, of clear, humorous surprises, of artistic studious design? 'malbone' is a rare work, possessing these characteristics, and replete, too, with honest literary effort._" =hillside rhymes.=--extra fcap. vo. _s._ =home.=--blanche lisle, and other poems. by cecil home. fcap. vo. _s._ _d._ =hood (tom).=--the pleasant tale of puss and robin and their friends, kitty and bob. told in pictures by l. frÖlich, and in rhymes by tom hood. crown vo. gilt. _s._ _d._ "_the volume is prettily got up, and is sure to be a favourite in the nursery._"--scotsman. "_herr frölich has outdone himself in his pictures of this dramatic chase._"--morning post. =keary (a.)=--works by miss a. keary:-- janet's home. new edition. globe vo. _s._ _d._ "_never did a more charming family appear upon the canvas; and most skilfully and felicitously have their characters been portrayed. each individual of the fireside is a finished portrait, distinct and lifelike.... the future before her as a novelist is that of becoming the miss austin of her generation._"--sun. clemency franklyn. new edition. globe vo. _s._ _d._ "_full of wisdom and goodness, simple, truthful, and artistic.... it is capital as a story; better still in its pure tone and wholesome influence._"--globe. oldbury. three vols. crown vo. _s._ _d._ "_this is a very powerfully written story._"--globe. "_this is a really excellent novel._"--illustrated london news. "_the sketches of society in oldbury are excellent. the pictures of child life are full of truth._"--westminster review. =keary (a. and e.)=--works by a. and e. keary:-- the little wanderlin, and other fairy tales. mo. _s._ _d._ "_the tales are fanciful and well written, and they are sure to win favour amongst little readers._"--athenÆum. the heroes of asgard. tales from scandinavian mythology. new and revised edition, illustrated by huard. extra fcap. vo. _s._ _d._ "_told in a light and amusing style, which, in its drollery and quaintness, reminds us of our old favourite grimm._"--times. =kingsley.=--works by the rev. charles kingsley, m.a., rector of eversley, and canon of westminster:-- "westward ho!" or, the voyages and adventures of sir amyas leigh. ninth edition. crown vo. _s._ _fraser's magazine calls it_ "_almost the best historical novel of the day._" two years ago. fifth edition. crown vo. _s._ "_mr. kingsley has provided us all along with such pleasant diversions--such rich and brightly tinted glimpses of natural history, such suggestive remarks on mankind, society, and all sorts of topics, that amidst the pleasure of the way, the circuit to be made will be by most forgotten._"--guardian. hypatia; or, new foes with an old face. seventh edition. crown vo. _s._ hereward the wake--last of the english. second edition. crown vo. _s._ yeast: a problem. sixth edition. crown vo. _s._ alton locke. new edition. with a new preface. crown vo. _s._ _d._ _the author shows, to quote the spectator,_ "_what it is that constitutes the true christian, god-fearing, man-living gentleman._" the water babies. a fairy tale for a land baby. new edition, with additional illustrations by sir noel paton, r.s.a., and p. skelton. crown vo. cloth, extra gilt. _s._ "_in fun, in humour, and in innocent imagination, as a child's book we do not know its equal._"--london review. "_mr. kingsley must have the credit of revealing to us a new order of life.... there is in the 'water babies' an abundance of wit, fun, good humour, geniality, élan, go._"--times. the heroes; or, greek fairy tales for my children. with coloured illustrations. new edition. mo. _s._ _d._ "_we do not think these heroic stories have ever been more attractively told.... there is a deep under-current of religious feeling traceable throughout its pages which is sure to influence young readers powerfully._"--london review. "_one of the children's books that will surely become a classic._"--nonconformist. phaethon; or, loose thoughts for loose thinkers. third edition. crown vo. _s._ "_the dialogue of 'phaethon' has striking beauties, and its suggestions may meet half-way many a latent doubt, and, like a light breeze, lift from the soul clouds that are gathering heavily, and threatening to settle down in misty gloom on the summer of many a fair and promising young life._"--spectator. poems; including the saint's tragedy, andromeda, songs, ballads, etc. complete collected edition. extra fcap. vo. _s._ _the spectator calls "andromeda"_ "_the finest piece of english hexameter verse that has ever been written. it is a volume which many readers will be glad to possess._" prose idylls. new and old. second edition. crown vo. _s._ contents:--_a charm of birds; chalk-stream studies; the fens; my winter-garden; from ocean to sea; north devon._ "_altogether a delightful book.... it exhibits the author's best traits, and cannot fail to infect the reader with a love of nature and of out-door life and its enjoyments. it is well calculated to bring a gleam of summer with its pleasant associations, into the bleak winter-time; while a better companion for a summer ramble could hardly be found._"--british quarterly review. =kingsley (h.)=--works by henry kingsley:-- tales of old travel. re-narrated. with eight full-page illustrations by huard. fourth edition. crown vo. cloth, extra gilt. _s._ "_we know no better book for those who want knowledge or seek to refresh it. as for the 'sensational,' most novels are tame compared with these narratives._"--athenÆum. "_exactly the book to interest and to do good to intelligent and high-spirited boys._"--literary churchman. the lost child. with eight illustrations by frÖlich. crown to. cloth gilt. _s._ _d._ "_a pathetic story, and told so as to give children an interest in australian ways and scenery._"--globe. "_very charmingly and very touchingly told._"--saturday review. oakshott castle. vols. crown vo. _s._ _d._ "_no one who takes up 'oakshott castle' will willingly put it down until the last page is turned.... it may fairly be considered a capital story, full of go, and abounding in word pictures of storms and wrecks._"--observer. =knatchbull-hugessen.=--works by e. h. knatchbull-hugessen, m.p.:-- _mr. knatchbull-hugessen has won for himself a reputation as a teller of fairy-tales._ "_his powers," says the times, "are of a very high order; light and brilliant narrative flows from his pen, and is fed by an invention as graceful as it is inexhaustible._" "_children reading his stories," the scotsman says, "or hearing them read, will have their minds refreshed and invigorated as much as their bodies would be by abundance of fresh air and exercise._" stories for my children. with illustrations. fourth edition. crown vo. _s._ "_the stories are charming, and full of life and fun._"--standard. "_the author has an imagination as fanciful as grimm himself, while some of his stories are superior to anything that hans christian andersen has written._"--nonconformist. crackers for christmas. more stories. with illustrations by jellicoe and elwes. fourth edition. crown vo. _s._ "_a fascinating little volume, which will make him friends in every household in which there are children._"--daily news. moonshine: fairy tales. with illustrations by w. brunton. sixth edition. crown vo. cloth gilt. _s._ "_a volume of fairy tales, written not only for ungrown children, but for bigger, and if you are nearly worn out, or sick, or sorry, you will find it good reading._"--graphic. "_the most charming volume of fairy tales which we have ever read.... we cannot quit this very pleasant book without a word of praise to its illustrator. mr. brunton from first to last has done admirably._"--times. tales at tea-time. fairy stories. with seven illustrations by w. brunton. fifth edition. crown vo. cloth gilt. _s._ "_capitally illustrated by w. brunton.... in frolic and fancy they are quite equal to his other books. the author knows how to write fairy stories as they should be written. the whole book is full of the most delightful drolleries._"--times. queer folk. fairy stories. illustrated by s. e. waller. fourth edition. crown vo. cloth gilt. _s._ "_decidedly the author's happiest effort.... one of the best story books of the year._"--hour. =knatchbull-hugessen (louisa).=--the history of prince perrypets. a fairy tale. by louisa knatchbull-hugessen. with eight illustrations by weigand. new edition. crown to. cloth gilt. _s._ _d._ "_a grand and exciting fairy tale._"--morning post. "_a delicious piece of fairy nonsense._"--illustrated london news. =knox.=--songs of consolation. by isa craig knox. extra fcap. vo. cloth extra, gilt edges. _s._ _d._ "_the verses are truly sweet; there is in them not only much genuine poetic quality, but an ardent, flowing devotedness, and a peculiar skill in propounding theological tenets in the most graceful way, which any divine might envy._"--scotsman. =latham.=--sertum shaksperianum, subnexis aliquot aliunde excerptis floribus. latine reddidit rev. h. latham, m.a. extra fcap. vo. _s._ =lemon.=--the legends of number nip. by mark lemon. with illustrations by c. keene. new edition. extra fcap. vo. _s._ _d._ =life and times of conrad the squirrel.= a story for children. by the author of "wandering willie," "effie's friends," &c. with a frontispiece by r. farren. second edition. crown vo. _s._ _d._ "_having commenced on the first page, we were compelled to go on to the conclusion, and this we predict will be the case with every one who opens the book._"--pall mall gazette. =little estella,= and other fairy tales for the young. mo. cloth extra. _s._ _d._ "_this is a fine story, and we thank heaven for not being too wise to enjoy it._"--daily news. =lowell.=--works by j. russell lowell:-- among my books. six essays. dryden--witchcraft--shakespeare once more--new england two centuries ago--lessing--rousseau and the sentimentalists. crown vo. _s._ _d._ "_we may safely say the volume is one of which our chief complaint must be that there is not more of it. there are good sense and lively feeling forcibly and tersely expressed in every page of his writing._"--pall mall gazette. complete poetical works of james russell lowell. with portrait, engraved by jeens. mo. cloth extra. _s._ _d._ "_all readers who are able to recognise and appreciate genuine verse will give a glad welcome to this beautiful little volume._"--pall mall gazette. =lyttelton.=--works by lord lyttelton:-- the "comus" of milton, rendered into greek verse. extra fcap. vo. _s._ the "samson agonistes" of milton, rendered into greek verse. extra fcap. vo. _s._ _d._ "_classical in spirit, full of force, and true to the original._"--guardian. =maclaren.=--the fairy family. a series of ballads and metrical tales illustrating the fairy mythology of europe. by archibald maclaren. with frontispiece, illustrated title, and vignette. crown vo. gilt. _s._ "_a successful attempt to translate into the vernacular some of the fairy mythology of europe. the verses are very good. there is no shirking difficulties of rhyme, and the ballad metre which is oftenest employed has a great deal of the kind of 'go' which we find so seldom outside the pages of scott. the book is of permanent value._"--guardian. =macmillan's magazine.=--published monthly. price _s._ volumes i. to xxix. are now ready. _s._ _d._ each. =macquoid.=--patty. by katharine s. macquoid. third and cheaper edition. crown vo. _s._ "_a book to be read._"--standard. "_a powerful and fascinating story._"--daily telegraph. _the globe considers it_ "_well-written, amusing, and interesting, and has the merit of being out of the ordinary run of novels._" =maguire.=--young prince marigold, and other fairy stories. by the late john francis maguire, m.p. illustrated by s. e. waller. globe vo. gilt. _s._ _d._ "_the author has evidently studied the ways and tastes of children and got at the secret of amusing them; and has succeeded in what is not so easy a task as it may seem--in producing a really good children's book._"--daily telegraph. =marlitt (e.)=--the countess gisela. translated from the german of e. marlitt. crown vo. _s._ _d._ "_a very beautiful story of german country life._"--literary churchman. =masson (professor).=--works by david masson, m.a., professor of rhetoric and english literature in the university of edinburgh. british novelists and their styles. being a critical sketch of the history of british prose fiction. crown vo. _s._ _d._ wordsworth, shelley, keats, and other essays. crown vo. _s._ chatterton: a story of the year . crown vo. _s._ the three devils: luther's, milton's, and goethe's; and other essays. crown vo. _s._ =mazini.=--in the golden shell; a story of palermo. by linda mazini. with illustrations. globe vo. cloth gilt. _s._ _d._ "_as beautiful and bright and fresh as the scenes to which it wafts us over the blue mediterranean, and as pure and innocent, but piquant and sprightly as the little girl who plays the part of its heroine, is this admirable little book._"--illustrated london news. =merivale.=--keats' hyperion, rendered into latin verse. by c. merivale, b.d. second edition. extra fcap. vo. _s._ _d._ =milner.=--the lily of lumley. by edith milner. crown vo. _s._ _d._ "_the novel is a good one and decidedly worth the reading._"--examiner. "_a pretty, brightly-written story._"--literary churchman. "_a tale possessing the deepest interest._"--court journal. =milton's poetical works.=--edited with text collated from the best authorities, with introduction and notes by david masson. three vols. vo. with three portraits engraved by c. h. jeens and radcliffe. (uniform with the cambridge shakespeare.) =mistral (f.)=--mirelle, a pastoral epic of provence. translated by h. crichton. extra fcap. vo. _s._ "_it would be hard to overpraise the sweetness and pleasing freshness of this charming epic._"--athenÆum. "_a good translation of a poem that deserves to be known by all students of literature and friends of old-world simplicity in story-telling._"--nonconformist. =mitford (a. b.)=--tales of old japan. by a. b. mitford, second secretary to the british legation in japan. with illustrations drawn and cut on wood by japanese artists. new and cheaper edition. crown vo. _s._ "_they will always be interesting as memorials of a most exceptional society; while, regarded simply as tales, they are sparkling, sensational, and dramatic, and the originality of their ideas and the quaintness of their language give them a most captivating piquancy. the illustrations are extremely interesting, and for the curious in such matters have a special and particular value._"--pall mall gazette. =mr. pisistratus brown, m.p.=, in the highlands. new edition, with illustrations. crown vo. _s._ _d._ "_the book is calculated to recall pleasant memories of holidays well spent, and scenes not easily to be forgotten. to those who have never been in the western highlands, or sailed along the frith of clyde and on the western coast, it will seem almost like a fairy story. there is a charm in the volume which makes it anything but easy for a reader who has opened it to put it down until the last page has been read._"--scotsman. =mrs. jerningham's journal.= a poem purporting to be the journal of a newly-married lady. second edition. fcap. vo. _s._ _d._ "_it is nearly a perfect gem. we have had nothing so good for a long time, and those who neglect to read it are neglecting one of the jewels of contemporary history._"--edinburgh daily review. "_one quality in the piece, sufficient of itself to claim a moment's attention, is that it is unique--original, indeed, is not too strong a word--in the manner of its conception and execution._"--pall mall gazette. =mudie.=--stray leaves. by c. e. mudie. new edition. extra fcap. vo. _s._ _d._ contents:--"his and mine"--"night and day"--"one of many," &c. _this little volume consists of a number of poems, mostly of a genuinely devotional character._ "_they are for the most part so exquisitely sweet and delicate as to be quite a marvel of composition. they are worthy of being laid up in the recesses of the heart, and recalled to memory from time to time._"--illustrated london news. =murray.=--the ballads and songs of scotland, in view of their influence on the character of the people. by j. clark murray, ll.d., professor of mental and moral philosophy in mcgill college, montreal. crown vo. _s._ "_independently of the lucidity of the style in which the whole book is written, the selection of the examples alone would recommend it to favour, while the geniality of the criticism upon those examples cannot fail to make them highly appreciated and valued._"--morning post. =myers (ernest).=--the puritans. by ernest myers. extra fcap. vo. cloth. _s._ _d._ "_it is not too much to call it a really grand poem, stately and dignified, and showing not only a high poetic mind, but also great power over poetic expression._"--literary churchman. =myers (f. w. h.)=--poems. by f. w. h. myers. containing "st. paul," "st. john," and others. extra fcap. vo. _s._ _d._ "_it is rare to find a writer who combines to such an extent the faculty of communicating feelings with the faculty of euphonious expression._"--spectator. "_'st. paul' stands without a rival as the noblest religious poem which has been written in an age which beyond any other has been prolific in this class of poetry. the sublimest conceptions are expressed in language which, for richness, taste, and purity, we have never seen excelled._"--john bull. =nichol.=--hannibal, a historical drama. by john nichol, b.a. oxon., regius professor of english language and literature in the university of glasgow. extra fcap. vo. _s._ _d._ "_the poem combines in no ordinary degree firmness and workmanship. after the lapse of many centuries, an english poet is found paying to the great carthagenian the worthiest poetical tribute which has as yet, to our knowledge, been afforded to his noble and stainless name._"--saturday review. =nine years old.=--by the author of "st. olave's," "when i was a little girl," &c. illustrated by frÖlich. third edition. extra fcap. vo. cloth gilt. _s._ _d._ _it is believed that this story, by the favourably known author of "st. olave's," will be found both highly interesting and instructive to the young. the volume contains eight graphic illustrations by mr. l. frölich. the examiner says:_ "_whether the readers are nine years old, or twice, or seven times as old, they must enjoy this pretty volume._" =noel.=--beatrice, and other poems. by the hon. roden noel. fcap. vo. _s._ "_it is impossible to read the poem through without being powerfully moved. there are passages in it which for intensity and tenderness, clear and vivid vision, spontaneous and delicate sympathy, may be compared with the best efforts of our best living writers._"--spectator. =norton.=--works by the hon. mrs. norton:-- the lady of la garaye. with vignette and frontispiece. new edition. fcap. vo. _s._ _d._ "_full of thought well expressed, and may be classed among her best efforts._"--times. old sir douglas. cheap edition. globe vo. _s._ _d._ "_this varied and lively novel--this clever novel so full of character, and of fine incidental remark._"--scotsman. "_one of the pleasantest and healthiest stories of modern fiction._"--globe. =oliphant.=--works by mrs. oliphant:-- agnes hopetoun's schools and holidays. new edition with illustrations. royal mo. gilt leaves. _s._ _d._ "_there are few books of late years more fitted to touch the heart, purify the feeling, and quicken and sustain right principles._"--nonconformist. "_a more gracefully written story it is impossible to desire._"--daily news. a son of the soil. new edition. globe vo. _s._ _d._ "_it is a very different work from the ordinary run of novels. the whole life of a man is portrayed in it, worked out with subtlety and insight._"--athenÆum. =our year.= a child's book, in prose and verse. by the author of "john halifax, gentleman." illustrated by clarence dobell. royal mo. _s._ _d._ "_it is just the book we could wish to see in the hands of every child._"--english churchman. =olrig grange.= edited by hermann kunst, philol. professor. extra fcap. vo. _s._ _d._ "_a masterly and original power of impression, pouring itself forth in clear, sweet, strong rhythm.... it is a fine poem, full of life, of music and of clear vision._"--north british daily mail. =oxford spectator, the.=--reprinted. extra fcap. vo. _s._ _d._ "_there is,_" _the saturday review says,_ "_all the old fun, the old sense of social ease and brightness and freedom, the old medley of work and indolence, of jest and earnest, that made oxford life so picturesque._" =palgrave.=--works by francis turner palgrave, m.a., late fellow of exeter college, oxford:-- the five days' entertainments at wentworth grange. a book for children. with illustrations by arthur hughes, and engraved title-page by jeens. small to. cloth extra. _s._ "_if you want a really good book for both sexes and all ages, buy this, as handsome a volume of tales as you'll find in all the market._"--athenÆum. "_exquisite both in form and substance._"--guardian. lyrical poems. extra fcap. vo. _s._ "_a volume of pure quiet verse, sparkling with tender melodies, and alive with thoughts of genuine poetry.... turn where we will throughout the volume, we find traces of beauty, tenderness, and truth; true poet's work, touched and refined by the master-hand of a real artist, who shows his genius even in trifles._"--standard. original hymns. third edition, enlarged, mo. _s._ _d._ "_so choice, so perfect, and so refined, so tender in feeling, and so scholarly in expression, that we look with special interest to everything that he gives us._"--literary churchman. golden treasury of the best songs and lyrics. edited by f. t. palgrave. see golden treasury series. shakespeare's sonnets and songs. edited by f. t. palgrave. gem edition. with vignette title by jeens. _s._ _d._ "_for minute elegance no volume could possibly excel the 'gem edition.'_"--scotsman. =parables.=--twelve parables of our lord. illustrated in colours from sketches taken in the east by mceniry with frontispiece from a picture by john jellicoe, and illuminated texts and borders. royal to. in ornamental binding. _s._ _the times calls it_ "_one of the most beautiful of modern pictorial works;_" _while the graphic says_ "_nothing in this style, so good, has ever before been published."_ =patmore.=--the children's garland, from the best poets. selected and arranged by coventry patmore. new edition. with illustrations by j. lawson. crown vo. gilt. _s._ golden treasury edition. mo. _s._ _d._ "_the charming illustrations added to many of the poems will add greatly to their value in the eyes of children._"--daily news. =pember.=--the tragedy of lesbos. a dramatic poem. by e. h. pember. fcap. vo. _s._ _d._ _founded upon the story of sappho._ "_he tells his story with dramatic force, and in language that often rises almost to grandeur."_--athenÆum. =poole.=--pictures of cottage life in the west of england. by margaret e. poole. new and cheaper edition. with frontispiece by r. farren. crown vo. _s._ _d._ "_charming stories of peasant life, written in something of george eliot's style.... her stories could not be other than they are, as literal as truth, as romantic as fiction, full of pathetic touches and strokes of genuine humour.... all the stories are studies of actual life, executed with no mean art._"--times. =population of an old pear tree.= from the french of e. van bruyssel. edited by the author of "the heir of redclyffe." with illustrations by becker. cheaper edition. crown vo. gilt. _s._ _d._ "_this is not a regular book of natural history, but a description of all the living creatures that came and went in a summer's day beneath an old pear tree, observed by eyes that had for the nonce become microscopic, recorded by a pen that finds dramas in everything, and illustrated by a dainty pencil.... we can hardly fancy anyone with a moderate turn for the curiosities of insect life, or for delicate french esprit, not being taken by these clever sketches._"--guardian. "_a whimsical and charming little book._"--athenÆum. =prince florestan of monaco, the fall of.= by himself. new edition, with illustration and map. vo. cloth. extra gilt edges, _s._ a french translation, _s._ also an edition for the people. crown vo. _s._ "_those who have read only the extracts given, will not need to be told how amusing and happily touched it is. those who read it for other purposes than amusement can hardly miss the sober and sound political lessons with which its light pages abound, and which are as much needed in england as by the nation to whom the author directly addresses his moral._"--pall mall gazette. "_this little book is very clever, wild with animal spirits, but showing plenty of good sense, amid all the heedless nonsense which fills so many of its pages._"--daily news. "_in an age little remarkable for powers of political satire, the sparkle of the pages gives them every claim to welcome._"--standard. =rankine.=--songs and fables. by w. j. mcquorn rankine, late professor of civil engineering and mechanics at glasgow. with illustrations. crown vo. _s._ "_a lively volume of verses, full of a fine manly spirit, much humour and geniality. the illustrations are admirably conceived, and executed with fidelity and talent._"--morning post. =realmah.=--by the author of "friends in council." crown vo. _s._ =rhoades.=--poems. by james rhoades. fcap. vo. _s._ _d._ =richardson.=--the iliad of the east. a selection of legends drawn from valmiki's sanskrit poem, "the ramayana." by frederika richardson. crown vo. _s._ _d._ "_it is impossible to read it without recognizing the value and interest of the eastern epic. it is as fascinating as a fairy tale, this romantic poem of india._"--globe. "_a charming volume, which at once enmeshes the reader in its snares._"--athenÆum. =roby.=--story of a household, and other poems. by mary k. roby. fcap. vo. _s._ =rogers.=--works by j. e. rogers:-- ridicula rediviva. old nursery rhymes. illustrated in colours, with ornamental cover. crown to. _s._ _d._ "_the most splendid, and at the same time the most really meritorious of the books specially intended for children, that we have seen._"--spectator. "_these large bright pictures will attract children to really good and honest artistic work, and that ought not to be an indifferent consideration with parents who propose to educate their children._"--pall mall gazette. mores ridiculi. old nursery rhymes. illustrated in colours, with ornamental cover. crown to. _s._ _d._ "_these world-old rhymes have never had and need never wish for a better pictorial setting than mr. rogers has given them._"--times. "_nothing could be quainter or more absurdly comical than most of the pictures, which are all carefully executed and beautifully coloured._"--globe. =rossetti.=--goblin market, and other poems. by christina rossetti. with two designs by d. g. rossetti. second edition. fcap. vo. _s._ "_she handles her little marvel with that rare poetic discrimination which neither exhausts it of its simple wonders by pushing symbolism too far, nor keeps those wonders in the merely fabulous and capricious stage. in fact, she has produced a true children's poem, which is far more delightful to the mature than to children, though it would be delightful to all._"--spectator. =runaway (the).= a story for the young. by the author of "mrs. jerningham's journal." with illustrations by j. lawson. globe vo. gilt. _s._ _d._ "_this is one of the best, if not indeed the very best, of all the stories that has come before us this christmas. the heroines are both charming, and, unlike heroines, they are as full of fun as of charms. it is an admirable book to read aloud to the young folk when they are all gathered round the fire, and nurses and other apparitions are still far away._"--saturday review. =ruth and her friends.= a story for girls. with a frontispiece. fourth edition. mo. cloth extra. _s._ _d._ "_we wish all the school girls and home-taught girls in the land had the opportunity of reading it._"--nonconformist. =scouring of the white horse; or, the long= vacation ramble of a london clerk. illustrated by doyle. imp. mo. cheaper issue. _s._ _d._ "_a glorious tale of summer joy._"--freeman. "_there is a genial hearty life about the book._"--john bull. "_the execution is excellent.... like 'tom brown's school days,'the 'white horse' gives the reader a feeling of gratitude and personal esteem towards the author._"--saturday review. =shairp (principal).=--kilmahoe, a highland pastoral, with other poems. by john campbell shairp, principal of the united college, st. andrews. fcap. vo. _s._ "_kilmahoe is a highland pastoral, redolent of the warm soft air of the western lochs and moors, sketched out with remarkable grace and picturesqueness._"--saturday review. =shakespeare.=--the works of william shakespeare. cambridge edition. edited by w. george clark, m.a. and w. aldis wright, m.a. nine vols. vo. cloth. _l._ _s._ _d._ _the guardian calls it an_ "_excellent, and, to the student, almost indispensable edition;_" _and the examiner calls it_ "_an unrivalled edition._" =shakespeare's tempest.= edited with glossarial and explanatory notes, by the rev. j. m. jephson. new edition. mo. _s._ =slip (a) in the fens.=--illustrated by the author. crown vo. _s._ "_an artistic little volume, for every page is a picture._"--times. "_it will be read with pleasure, and with a pleasure that is altogether innocent._"--saturday review. =smith.=--poems. by catherine barnard smith. fcap. vo. _s._ "_wealthy in feeling, meaning, finish, and grace; not without passion, which is suppressed, but the keener for that._"--athenÆum. =smith (rev. walter).=--hymns of christ and the christian life. by the rev. walter c. smith, m.a. fcap. vo. _s._ "_these are among the sweetest sacred poems we have read for a long time. with no profuse imagery, expressing a range of feeling and expression by no means uncommon, they are true and elevated, and their pathos is profound and simple._"--nonconformist. =spring songs.= by a west highlander. with a vignette illustration by gourlay steele. fcap. vo. _s._ _d._ "_without a trace of affectation or sentimentalism, these utterances are perfectly simple and natural, profoundly human and profoundly true._"--daily news. =stanley.=--true to life.--a simple story. by mary stanley. crown vo. _s._ _d._ "_for many a long day we have not met with a more simple, healthy, and unpretending story._"--standard. =stephen (c. e.)=--the service of the poor; being an inquiry into the reasons for and against the establishment of religious sisterhoods for charitable purposes. by caroline emilia stephen. crown vo. _s._ _d._ "_it touches incidentally and with much wisdom and tenderness on so many of the relations of women, particularly of single women, with society, that it may be read with advantage by many who have never thought of entering a sisterhood._"--spectator. =stephens (j. b.)=--convict once. a poem. by j. brunton stephens. extra fcap. vo. _s._ _d._ "_it is as far more interesting than ninety-nine novels out of a hundred, as it is superior to them in power, worth, and beauty. we should most strongly advise everybody to read 'convict once.'_"--westminster review. =streets and lanes of a city:= being the reminiscences of amy dutton. with a preface by the bishop of salisbury. second and cheaper edition. globe vo. _s._ _d._ "_one of the most really striking books that has ever come before us._"--literary churchman. =thring.=--school songs. a collection of songs for schools. with the music arranged for four voices. edited by the rev. e. thring and h. riccius. folio. _s._ _d._ _the collection includes the "agnus dei," tennyson's "light brigade," macaulay's "ivry," etc. among other pieces._ =tom brown's school days.=--by an old boy. golden treasury edition, _s._ _d._ people's edition, _s._ with seven illustrations by a. hughes and sydney hall. crown vo. _s._ "_the most famous boy's book in the language._"--daily news. =tom brown at oxford.=--new edition. with illustrations. crown vo. _s._ "_in no other work that we can call to mind are the finer qualities of the english gentleman more happily portrayed._"--daily news. "_a book of great power and truth._"--national review. =trench.=--works by r. chenevix trench, d.d., archbishop of dublin. (for other works by this author, see theological, historical, and philosophical catalogues.) poems. collected and arranged anew. fcap. vo. _s._ _d._ elegiac poems. third edition. fcap. vo. _s._ _d._ calderon's life's a dream: the great theatre of the world. with an essay on his life and genius. fcap. vo. _s._ _d._ household book of english poetry. selected and arranged, with notes, by archbishop trench. second edition. extra fcap. vo. _s._ _d._ "_the archbishop has conferred in this delightful volume an important gift on the whole english-speaking population of the world._"--pall mall gazette. sacred latin poetry, chiefly lyrical. selected and arranged for use. by archbishop trench. third edition, corrected and improved. fcap. vo. _s._ justin martyr, and other poems. fifth edition. fcap. vo. _s._ =trollope (anthony).=--sir harry hotspur of humblethwaite. by anthony trollope, author of "framley parsonage," etc. cheap edition. globe vo. _s._ _d._ _the athenÆum remarks:_ "_no reader who begins to read this book is likely to lay it down until the last page is turned. this brilliant novel appears to us decidedly more successful than any other of mr. trollope's shorter stories._" =turner.=--works by the rev. charles tennyson turner:-- sonnets. dedicated to his brother, the poet laureate. fcap. vo. _s._ _d._ small tableaux. fcap. vo. _s._ _d._ =under the limes.=--by the author of "christina north." second edition. crown vo. _s._ "_the readers of 'christina north' are not likely to have forgotten that bright, fresh, picturesque story, nor will they be slow to welcome so pleasant a companion to it as this. it abounds in happy touches of description, of pathos, and insight into the life and passion of true love._"--standard. "_one of the prettiest and best told stories which it has been our good fortune to read for a long time._"--pall mall gazette. =vittoria colonna.=--life and poems. by mrs. henry roscoe. crown vo. _s._ "_it is written with good taste, with quick and intelligent sympathy, occasionally with a real freshness and charm of style._"--pall mall gazette. =waller.=--six weeks in the saddle: a painter's journal in iceland. by s. e. waller. illustrated by the author. crown vo. _s._ "_an exceedingly pleasant and naturally written little book.... mr. waller has a clever pencil, and the text is well illustrated with his own sketches._"--times. =wandering willie.= by the author of "effie's friends," and "john hatherton." third edition. crown vo. _s._ "_this is an idyll of rare truth and beauty.... the story is simple and touching, the style of extraordinary delicacy, precision, and picturesqueness.... a charming gift-book for young ladies not yet promoted to novels, and will amply repay those of their elders who may give an hour to its perusal._"--daily news. =webster.=--works by augusta webster:-- "_if mrs. webster only remains true to herself, she will assuredly take a higher rank as a poet than any woman has yet done._"--westminster review. dramatic studies. extra fcap. vo. _s._ "_a volume as strongly marked by perfect taste as by poetic power._"--nonconformist. a woman sold, and other poems. crown vo. _s._ _d._ "_mrs. webster has shown us that she is able to draw admirably from the life; that she can observe with subtlety, and render her observations with delicacy; that she can impersonate complex conceptions and venture into which few living writers can follow her._"--guardian. portraits. second edition. extra fcap. vo. _s._ _d._ "_mrs. webster's poems exhibit simplicity and tenderness ... her taste is perfect.... this simplicity is combined with a subtlety of thought, feeling, and observation which demand that attention which only real lovers of poetry are apt to bestow._"--westminster review. prometheus bound of Æschylus. literally translated into english verse. extra fcap. vo. _s._ _d._ "_closeness and simplicity combined with literary skill._"--athenÆum. "_mrs. webster's 'dramatic studies' and 'translation of prometheus' have won for her an honourable place among our female poets. she writes with remarkable vigour and dramatic realization, and bids fair to be the most successful claimant of mrs. browning's mantle._"--british quarterly review. medea of euripides. literally translated into english verse. extra fcap. vo. _s._ _d._ "_mrs. webster's translation surpasses our utmost expectations. it is a photograph of the original without any of that harshness which so often accompanies a photograph._"--westminster review. the auspicious day. a dramatic poem. extra fcap. vo. _s._ "_the 'auspicious day' shows a marked advance, not only in art, but, in what is of far more importance, in breadth of thought and intellectual grasp._"--westminster review. "_this drama is a manifestation of high dramatic power on the part of the gifted writer, and entitled to our warmest admiration, as a worthy piece of work._"--standard. yu-pe-ya's lute. a chinese tale in english verse. extra fcap. vo. _s._ _d._ "_a very charming tale, charmingly told in dainty verse, with occasional lyrics of tender beauty._"--standard. "_we close the book with the renewed conviction that in mrs. webster we have a profound and original poet. the book is marked not by mere sweetness of melody--rare as that gift is--but by the infinitely rarer gifts of dramatic power, of passion, and sympathetic insight._"--westminster review. =when i was a little girl.= stories for children. by the author of "st. olave's." fourth edition. extra fcap. vo. _s._ _d._ with eight illustrations by l. frÖlich. "_at the head, and a long way ahead, of all books for girls, we place 'when i was a little girl.'_"--times. "_it is one of the choicest morsels of child-biography which we have met with._"--nonconformist. =white.=--rhymes by walter white. vo. _s._ _d._ =whittier.=--john greenleaf whittier's poetical works. complete edition, with portrait engraved by c. h. jeens. mo. _s._ _d._ "_mr. whittier has all the smooth melody and the pathos of the author of 'hiawatha,' with a greater nicety of description and a quainter fancy._"--graphic. =wolf.=--the life and habits of wild animals. twenty illustrations by joseph wolf, engraved by j. w. and e. whymper. with descriptive letter-press, by d. g. elliot, f.l.s. super royal to, cloth extra, gilt edges. _s._ _this is the last series of drawings which will be made by mr. wolf, either upon wood or stone. the pall mall gazette says:_ "_the fierce, untameable side of brute nature has never received a more robust and vigorous interpretation, and the various incidents in which particular character is shown are set forth with rare dramatic power. for excellence that will endure, we incline to place this very near the top of the list of christmas books._" _and the art journal observes,_ "_rarely, if ever, have we seen animal life more forcibly and beautifully depicted than in this really splendid volume._" also, an edition in royal folio, handsomely bound in morocco elegant, proofs before letters, each proof signed by the engravers. price _l._ _s._ =wollaston.=--lyra devoniensis. by t. v. wollaston, m.a. fcap. vo. _s._ _d._ "_it is the work of a man of refined taste, of deep religious sentiment, a true artist, and a good christian._"--church times. =woolner.=--my beautiful lady. by thomas woolner. with a vignette by arthur hughes. third edition. fcap. vo. _s._ "_no man can read this poem without being struck by the fitness and finish of the workmanship, so to speak, as well as by the chastened and unpretending loftiness of thought which pervades the whole._"--globe. =words from the poets.= selected by the editor of "rays of sunlight." with a vignette and frontispiece. mo. limp., _s._ "_the selection aims at popularity, and deserves it._"--guardian. =yonge (c. m.)=--works by charlotte m. yonge. the heir of redclyffe. twentieth edition. with illustrations. crown vo. _s._ heartsease. thirteenth edition. with illustrations. crown vo. _s._ the daisy chain. twelfth edition. with illustrations. crown vo. _s._ the trial: more links of the daisy chain. twelfth edition. with illustrations. crown vo. _s._ dynevor terrace. sixth edition. crown vo. _s._ hopes and fears. fourth edition. crown vo. _s._ the young stepmother. fifth edition. crown vo. _s._ clever woman of the family. third edition. crown vo. _s._ the dove in the eagle's nest. fourth edition. crown vo. _s._ "_we think the authoress of 'the heir of redclyffe' has surpassed her previous efforts in this illuminated chronicle of the olden time._"--british quarterly. the caged lion. illustrated. third edition. crown vo. _s._ "_prettily and tenderly written, and will with young people especially be a great favourite._"--daily news. "_everybody should read this._"--literary churchman. the chaplet of pearls; or, the white and black ribaumont. crown vo. _s._ new edition. "_miss yonge has brought a lofty aim as well as high art to the construction of a story which may claim a place among the best efforts in historical romance._"--morning post. "_the plot, in truth, is of the very first order of merit._"--spectator. "_we have seldom read a more charming story._"--guardian. the prince and the page. a tale of the last crusade. illustrated. mo. _s._ _d._ "_a tale which, we are sure, will give pleasure to many others besides the young people for whom it is specially intended.... this extremely prettily-told story does not require the guarantee afforded by the name of the author of 'the heir of redclyffe' on the title-page to ensure its becoming a universal favourite._"--dublin evening mail. the lances of lynwood. new edition, with coloured illustrations. mo. _s._ _d._ "_the illustrations are very spirited and rich in colour, and the story can hardly fail to charm the youthful reader._"--manchester examiner. the little duke: richard the fearless. new edition. illustrated. mo. _s._ _d._ a storehouse of stories. first and second series. globe vo. _s._ _d._ each. contents of first series:--history of philip quarll--goody twoshoes--the governess--jemima placid--the perambulations of a mouse--the village school--the little queen--history of little jack. "_miss yonge has done great service to the infantry of this generation by putting these eleven stories of sage simplicity within their reach._"--british quarterly review. contents of second series:--family stories--elements of morality--a puzzle for a curious girl--blossoms of morality. a book of golden deeds of all times and all countries. gathered and narrated anew. new edition, with twenty illustrations by frÖlich. crown vo. cloth gilt. _s._ (see also golden treasury series). cheap edition. _s._ "_we have seen no prettier gift-book for a long time, and none which, both for its cheapness and the spirit in which it has been compiled, is more deserving of praise._"--athenÆum. little lucy's wonderful globe. pictured by frÖlich, and narrated by charlotte m. yonge. second edition. crown to. cloth gilt. _s._ "_'lucy's wonderful globe' is capital, and will give its youthful readers more idea of foreign countries and customs than any number of books of geography or travel._"--graphic. cameos from english history. from rollo to edward ii. extra fcap. vo. _s._ second edition, enlarged. _s._ a second series. the wars in france. extra fcap. vo. _s._ "_instead of dry details,_" _says the nonconformist,_ "_we have living pictures, faithful, vivid, and striking._" p's and q's; or, the question of putting upon. with illustrations by c. o. murray. second edition. globe vo. cloth gilt. _s._ _d._ "_one of her most successful little pieces ... just what a narrative should be, each incident simply and naturally related, no preaching or moralizing, and yet the moral coming out most powerfully, and the whole story not too long, or with the least appearance of being spun out._"--literary churchman. the pillars of the house; or, under wode, under rode. second edition. four vols. crown vo. _s._ "_a domestic story of english professional life, which for sweetness of tone and absorbing interest from first to last has never been rivalled._"--standard. "_miss yonge has certainly added to her already high reputation by this charming book, which, although in four volumes, is not a single page too long, but keeps the reader's attention fixed to the end. indeed we are only sorry there is not another volume to come, and part with the underwood family with sincere regret._"--court circular. lady hester; or, ursula's narrative. second edition. crown vo. _s._ "_we shall not anticipate the interest by epitomizing the plot, but we shall only say that readers will find in it all the gracefulness, right feeling, and delicate perception which they have been long accustomed to look for in miss yonge's writings._"--guardian. macmillan's golden treasury series. uniformly printed in mo., with vignette titles by sir noel paton, t. woolner, w. holman hunt, j. e. millais, arthur hughes, &c. engraved on steel by jeens. bound in extra cloth, _s._ _d._ each volume. also kept in morocco and calf bindings. 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"_it includes specimens of all the great masters in the art of poetry, selected with the matured judgment of a man concentrated on obtaining insight into the feelings and tastes of childhood, and desirous to awaken its finest impulses, to cultivate its keenest sensibilities._"--morning post. =the book of praise.= from the best english hymn writers. selected and arranged by lord selbourne. _a new and enlarged edition._ "_all previous compilations of this kind must undeniably for the present give place to the book of praise.... the selection has been made throughout with sound judgment and critical taste. the pains involved in this compilation must have been immense, embracing, as it does, every writer of note in this special province of english literature, and ranging over the most widely divergent tracks of religious thought._"--saturday review. =the fairy book;= the best popular fairy stories. selected and rendered anew by the author of "john halifax, gentleman." "_a delightful selection, in a delightful external form; full of the physical splendour and vast opulence of proper fairy tales._"--spectator. =the ballad book.= a selection of the choicest british ballads. edited by william allingham. "_his taste as a judge of old poetry will be found, by all acquainted with the various readings of old english ballads, true enough to justify his undertaking so critical a task._"--saturday review. =the jest book.= the choicest anecdotes and sayings. selected and arranged by mark lemon. "_the fullest and best jest book that has yet appeared._"--saturday review. =bacon's essays and colours of good and evil.= with notes and glossarial index. by w. aldis wright, m.a. "_the beautiful little edition of bacon's essays, now before us, does credit to the taste and scholarship of mr. aldis wright.... it puts the reader in possession of all the essential literary facts and chronology necessary for reading the essays in connection with bacon's life and times._"--spectator. =the pilgrim's progress= from this world to that which is to come. by john bunyan. "_a beautiful and scholarly reprint._"--spectator. =the sunday book of poetry for the young.= selected and arranged by c. f. alexander. "_a well-selected volume of sacred poetry._"--spectator. =a book of golden deeds= of all times and all countries. gathered and narrated anew. by the author of "the heir of redclyffe." "_... to the young, for whom it is especially intended, as a most interesting collection of thrilling tales well told; and to their elders, as a useful handbook of reference, and a pleasant one to take up when their wish is to while away a weary half-hour. we have seen no prettier gift-book for a long time._"--athenÆum. =the poetical works of robert burns.= edited, with biographical memoir, notes, and glossary, by alexander smith. two vols. "_beyond all question this is the most beautiful edition of burns yet out._"--edinburgh daily review. =the adventures of robinson crusoe.= edited from the original edition by j. w. clark, m.a. fellow of trinity college, cambridge. "_mutilated and modified editions of this english classic are so much the rule, that a cheap and pretty copy of it, rigidly exact to the original, will be a prize to many book-buyers._"--examiner. =the republic of plato.= translated into english, with notes by j. ll. davies, m.a. and d. j. vaughan, m.a. "_a dainty and cheap little edition._"--examiner. =the song book.= words and tunes from the best poets and musicians. selected and arranged by john hullah, professor of vocal music in king's college, london. "_a choice collection of the sterling songs of england, scotland, and ireland, with the music of each prefixed to the words. how much true wholesome pleasure such a book can diffuse, and will diffuse, we trust through many thousand families._"--examiner. =la lyre française.= selected and arranged, with notes, by gustave massÓn, french master in harrow school. _a selection of the best french songs and lyrical pieces._ =tom brown's school days.= by an old boy. "_a perfect gem of a book. the best and most healthy book about boys for boys that ever was written._"--illustrated times. =a book of worthies.= gathered from the old histories and written anew by the author of "the heir of redclyffe." with vignette. "_an admirable addition to an admirable series._"--westminster review. =a book of golden thoughts.= by henry attwell, knight of the order of the oak crown. "_mr. attwell has produced a book of rare value.... happily it is small enough to be carried about in the pocket, and of such a companion it would be difficult to weary._"--pall mall gazette. =guesses at truth.= by two brothers. new edition. =the cavalier and his lady.= selections from the works of the first duke and duchess of newcastle. with an introductory essay by edward jenkins, author of "ginx's baby," &c. mo. _s._ _d._ "_a charming little volume._"--standard. =theologia germanica.=--which setteth forth many fair lineaments of divine truth, and saith very lofty and lovely things touching a perfect life. edited by dr. pfeiffer, from the only complete manuscript yet known. translated from the german, by susanna winkworth. with a preface by the rev. charles kingsley, and a letter to the translator by the chevalier bunsen, d.d. =milton's poetical works.=--edited, with notes, &c., by professor masson. two vols. mo. _s._ =scottish song.= a selection of the choicest lyrics of scotland. compiled and arranged, with brief notes, by mary carlyle aitkin. mo. _s._ _d._ "_miss aitken's exquisite collection of scottish song is so alluring, and suggests so many topics, that we find it difficult to lay it down. the book is one that should find a place in every library, we had almost said in every pocket, and the summer tourist who wishes to carry with him into the country a volume of genuine poetry, will find it difficult to select one containing within so small a compass so much of rarest value._"--spectator. macmillan's globe library. _beautifully printed on toned paper and bound in cloth extra, gilt edges, price s. d. each; in cloth plain, s. d. also kept in a variety of calf and morocco bindings at moderate prices._ books, wordsworth says, are "the spirit breathed by dead men to their kind;" and the aim of the publishers of the globe library has been to make it possible for the universal kin of english-speaking men to hold communion with the loftiest "spirits of the mighty dead;" to put within the reach of all classes _complete_ and _accurate_ editions, carefully and clearly printed upon the best paper, in a convenient form, at a moderate price, of the works of the master-minds of english literature, and occasionally of foreign literature in an attractive english dress. the editors, by their scholarship and special study of their authors, are competent to afford every assistance to readers of all kinds: this assistance is rendered by original biographies, glossaries of unusual or obsolete words, and critical and explanatory notes. the publishers hope, therefore, that these globe editions may prove worthy of acceptance by all classes wherever the english language is spoken, and by their universal circulation justify their distinctive epithet; while at the same time they spread and nourish a common sympathy with nature's most "finely touched" spirits, and thus help a little to "make the whole world kin." _the saturday review says:_ "_the globe editions are admirable for their scholarly editing, their typographical excellence, their compendious form, and their cheapness._" _the british quarterly review says:_ "_in compendiousness, elegance, and scholarliness, the globe editions of messrs. macmillan surpass any popular series of our classics hitherto given to the public. as near an approach to miniature perfection as has ever been made._" =shakespeare's complete works.= edited by w. g. clark, m.a., and w. aldis wright, m.a., of trinity college, cambridge, editors of the "cambridge shakespeare." with glossary. pp. , . _the athenÆum says this edition is_ "_a marvel of beauty, cheapness, and compactness.... for the busy man, above all for the working student, this is the best of all existing shakespeares._" _and the pall mall gazette observes:_ "_to have produced the complete works of the world's greatest poet in such a form, and at a price within the reach of every one, is of itself almost sufficient to give the publishers a claim to be considered public benefactors._" =spenser's complete works.= edited from the original editions and manuscripts, by r. morris, with a memoir by j. w. hales, m.a. with glossary. pp. lv., . "_worthy--and higher praise it needs not--of the beautiful 'globe series.' the work is edited with all the care so noble a poet deserves._"--daily news. =sir walter scott's poetical works.= edited with a biographical and critical memoir by francis turner palgrave, and copious notes. pp. xliii., . "_we can almost sympathise with a middle-aged grumbler, who, after reading mr. palgrave's memoir and introduction, should exclaim--'why was there not such an edition of scott when i was a schoolboy?'_"--guardian. =complete works of robert burns.=--the poems, songs, and letters, edited from the best printed and manuscript authorities, with glossarial index, notes, and a biographical memoir by alexander smith. pp. lxii., . "_admirable in all respects._"--spectator. "_the cheapest, the most perfect, and the most interesting edition which has ever been published._"--bell's messenger. =robinson crusoe.= edited after the original editions, with a biographical introduction by henry kingsley. pp. xxxi., . "_a most excellent and in every way desirable edition._"--court circular. "_macmillan's 'globe' robinson crusoe is a book to have and to keep._"--morning star. =goldsmith's miscellaneous works.= edited, with biographical introduction, by professor masson. pp. lx., . "_such an admirable compendium of the facts of goldsmith's life, and so careful and minute a delineation of the mixed traits of his peculiar character as to be a very model of a literary biography in little._"--scotsman. =pope's poetical works.= edited, with notes and introductory memoir, by adolphus william ward, m.a., fellow of st. peter's college, cambridge, and professor of history in owens college, manchester. pp. lii., . _the literary churchman remarks:_ "_the editor's own notes and introductory memoir are excellent, the memoir alone would be cheap and well worth buying at the price of the whole volume._" =dryden's poetical works.= edited, with a memoir, revised text, and notes, by w. d. christie, m.a., of trinity college, cambridge. pp. lxxxvii., . "_an admirable edition, the result of great research and of a careful revision of the text. the memoir prefixed contains, within less than ninety pages, as much sound criticism and as comprehensive a biography as the student of dryden need desire._"--pall mall gazette. =cowper's poetical works.= edited, with notes and biographical introduction, by william benham, vicar of addington and professor of modern history in queen's college, london. pp. lxxiii., . "_mr. benham's edition of cowper is one of permanent value. the biographical introduction is excellent, full of information, singularly neat and readable and modest--indeed too modest in its comments. the notes are concise and accurate, and the editor has been able to discover and introduce some hitherto unprinted matter. altogether the book is a very excellent one._"--saturday review. =morte d'arthur.=--sir thomas malory's book of king arthur and of his noble knights of the round table. the original edition of caxton, revised for modern use. with an introduction by sir edward strachey, bart. pp. xxxvii., . "_it is with perfect confidence that we recommend this edition of the old romance to every class of readers._"--pall mall gazette. =the works of virgil.= rendered into english prose, with introductions, notes, running analysis, and an index. by james lonsdale, m.a., late fellow and tutor of balliol college, oxford, and classical professor in king's college, london; and samuel lee, m.a., latin lecturer at university college, london. pp. . "_a more complete edition of virgil in english it is scarcely possible to conceive than the scholarly work before us._"--globe. =the works of horace.= rendered into english prose, with introductions, running analysis, notes, and index. by john lonsdale, m.a., and samuel lee, m.a. _the standard says,_ "_to classical and non-classical readers it will be invaluable as a faithful interpretation of the mind and meaning of the poet, enriched as it is with notes and dissertations of the highest value in the way of criticism, illustration, and explanation._" london: r. clay, sons, and taylor, printers. * * * * * transcriber's notes: footnotes have been moved to the end of each chapter and renumbered consecutively through the document. [oe] changed to oe: p. (d'oeuvre) punctuation has been made consistent. variations in spelling and hyphenation were retained as they appear in the original publication, except as noted below. changes have been made as follows: page , footnote : greek transliterated to english (greek agapê;) page : "mallett" changed to "mallet" (mallet, robert, reminiscences) catalog p. : "book's" changed to "book" (wish 'sybil's book' a wide) catalog p. : "j. w. m." changed to "j. m. w." (by j. m. w. turner; "the) catalog p. : "gree" changed to "greek" (from the greek anthology.) catalog p. : "is" changed to "it" (it puts the reader in possession) transcriber's note: text enclosed by underscores is in italics (_italics_). in view of the difficulty of reliably distinguishing th-century variant spellings from typographical errors, the text has been reproduced entirely as printed. * * * * * experiments and observations on electricity, made at _philadelphia_ in _america_, by mr. benjamin franklin, and communicated in several letters to mr. p. collinson, of _london_, f. r. s. * * * * * * _london_: printed and sold by e. cave, at _st. john's gate_. . (_price s. d._) the preface. _it may be necessary to acquaint the reader, that the following observations and experiments were not drawn up with a view to their being made publick, but were communicated at different times, and most of them in letters wrote on various topicks, as matters only of private amusement._ _but some persons to whom they were read, and who had themselves been conversant in electrical disquisitions, were of opinion, they contain'd so many curious and interesting particulars relative to this affair, that it would be doing a kind of injustice to the publick, to confine them solely to the limits of a private acquaintance._ _the editor was therefore prevailed upon to commit such extracts of letters, and other detach'd pieces as were in his hands to the press, without waiting for the ingenious author's permission so to do; and this was done with the less hesitation, as it was apprehended the author's engagements in other affairs, would scarce afford him leisure to give the publick his reflections and experiments on the subject, finish'd with that care and precision, of which the treatise before us shews he is alike studious and capable. he was only apprized of the step that had been thus taken, while the first sheets were in the press, and time enough for him to transmit some farther remarks, together with a few corrections and additions, which are placed at the end, and may be consulted in the perusal._ _the experiments which our author relates are most of them peculiar to himself; they are conducted with judgment, and the inferences from them plain and conclusive; though sometimes proposed under the terms of suppositions and conjectures._ _and indeed the scene he opens, strikes us with a pleasing astonishment, whilst he conducts us by a train of facts and judicious reflections, to a probable cause of those phænomena, which are at once the most awful, and, hitherto, accounted for with the least verisimilitude._ _he exhibits to our consideration, an invisible, subtile matter, disseminated through all nature in various proportions, equally unobserved, and, whilst all those bodies to which it peculiarly adheres are alike charged with it, inoffensive._ _he shews, however, that if an unequal distribution is by any means brought about; if there is a coacervation in one part of space, a less proportion, vacuity, or want, in another; by the near approach of a body capable of conducting the coacervated part to the emptier space, it becomes perhaps the most formidable and irresistible agent in the universe. animals are in an instant struck breathless, bodies almost impervious by any force yet known, are perforated, and metals fused by it, in a moment._ _from the similar effects of lightening and electricity our author has been led to make some propable conjectures on the cause of the former; and at the same time, to propose some rational experiments in order to secure ourselves, and those things on which its force is often directed, from its pernicious effects; a circumstance of no small importance to the publick, and therefore worthy of the utmost attention._ _it has, indeed, been of late the fashion to ascribe every grand or unusual operation of nature, such as lightening and earthquakes, to electricity; not, as one would imagine, from the manner of reasoning on these occasions, that the authors of these schemes have, discovered any connection betwixt the cause and effect, or saw in what manner they were related; but, as it would seem, merely because they were unacquainted with any other agent, of which it could not positively be said the connection was impossible._ _but of these, and many other interesting circumstances, the reader will be more satisfactorily informed in the following letters, to which he is therefore referred by_ _the_ editor. [illustration] letter i. from mr benj. franklin, in _philadelphia_. to mr peter collinson, f.r.s. _london_. _july , _. _sir_, the necessary trouble of copying long letters, which perhaps when they come to your hands may contain nothing new, or worth your reading (so quick is the progress made with you in electricity) half discourages me from writing any more on that subject. yet i cannot forbear adding a few observations on m. _muschenbroek_'s wonderful bottle. . the non-electric contain'd in the bottle differs when electrised from a non-electric electrised out of the bottle, in this: that the electrical fire of the latter is accumulated _on its surface_, and forms an electrical atmosphere round it of considerable extent: but the electrical fire is crouded _into the substance_ of the former, the glass confining it. . at the same time that the wire and top of the bottle, &c. is electrised _positively_ or _plus_, the bottom of the bottle is electrised _negatively_ or _minus_, in exact proportion: _i. e._ whatever quantity of electrical fire is thrown in at top, an equal quantity goes out of the bottom. to understand this, suppose the common quantity of electricity in each part of the bottle, before the operation begins, is equal to ; and at every stroke of the tube, suppose a quantity equal to is thrown in; then, after the first stroke, the quantity contain'd in the wire and upper part of the bottle will be , in the bottom . after the second, the upper part will have , the lower , and so on 'till after strokes, the upper part will have a quantity of electrical fire equal to , the lower part none: and then the operation ends: for no more can be thrown into the upper part, when no more can be driven out of the lower part. if you attempt to throw more in, it is spued back thro' the wire, or flies out in loud cracks thro' the sides of the bottle. . the equilibrium cannot be restored in the bottle by _inward_ communication or contact of the parts; but it must be done by a communication formed _without_ the bottle, between the top and bottom, by some non-electric, touching both at the same time; in which case it is restored with a violence and quickness inexpressible: or, touching each alternately, in which case the equilibrium is restored by degrees. . as no more electrical fire can be thrown into the top of the bottle, when all is driven out of the bottom, so in a bottle not yet electrised, none can be thrown into the top, when none _can_ get out at the bottom; which happens either when the bottom is too thick, or when the bottle is placed on an electric _per se_. again, when the bottle is electrised, but little of the electrical fire can be _drawn out_ from the top, by touching the wire, unless an equal quantity can at the same time _get in_ at the bottom. thus, place an electrised bottle on clean glass or dry wax, and you will not, by touching the wire, get out the fire from the top. place it on a non-electric, and touch the wire, you will get it out in a short time; but soonest when you form a direct communication as above. so wonderfully are these two states of electricity, the _plus_ and _minus_, combined and balanced in this miraculous bottle! situated and related to each other in a manner that i can by no means comprehend! if it were possible that a bottle should in one part contain a quantity of air strongly comprest, and in another part a perfect vacuum, we know the equilibrium would be instantly restored _within_. but here we have a bottle containing at the same time a _plenum_ of electrical fire, and a _vacuum_ of the same fire; and yet the equilibrium cannot be restored between them but by a communication _without_! though the _plenum_ presses violently to expand, and the hungry vacuum seems to attract as violently in order to be filled. . the shock to the nerves (or convulsion rather) is occasion'd by the sudden passing of the fire through the body in its way from the top to the bottom of the bottle. the fire takes the shortest course, as mr _watson_ justly observes: but it does not appear, from experiment, that, in order for a person to be shocked, a communication with the floor is necessary; for he that holds the bottle with one hand, and touches the wire with the other, will be shock'd as much, though his shoes be dry, or even standing on wax, as otherwise. and on the touch of the wire (or of the gun-barrel, which is the same thing) the fire does not proceed from the touching finger to the wire, as is supposed, but from the wire to the finger, and passes through the body to the other hand, and so into the bottom of the bottle. experiments _confirming the above_. experiment i. place an electrised phial on wax; a small cork-ball suspended by a dry silk-thread held in your hand, and brought near to the wire, will first be attracted, and then repelled: when in this state of repellency, sink your hand, that the ball may be brought towards the bottom of the bottle; it will there be instantly and strongly attracted, 'till it has parted with its fire. if the bottle had an electrical atmosphere, as well as the wire, an electrified cork would be repelled from one as well as from the other. experiment ii. fig. . from a bent wire (_a_) sticking in the table, let a small linen thread (_b_) hang down within half an inch of the electrised phial (_c_). touch the wire of the phial repeatedly with your finger, and at every touch you will see the thread instantly attracted by the bottle. (this is best done by a vinegar cruet, or some such belly'd bottle). as soon as you draw any fire out from the upper part by touching the wire, the lower part of the bottle draws an equal quantity in by the thread. experiment iii. fig. . fix a wire in the lead, with which the bottom of the bottle is armed, (_d_) so as that bending upwards, its ring-end may be level with the top or ring-end of the wire in the cork (_e_), and at three or four inches distance. then electricise the bottle, and place it on wax. if a cork suspended by a silk thread (_f_) hang between these two wires, it will play incessantly from one to the other, 'till the bottle is no longer electrised; that is, it fetches and carries fire from the top to the bottom of the bottle, 'till the equilibrium is restored. experiment iv. fig. . place an electricised phial on wax; take a wire (_g_) in form of a c, the ends at such a distance when bent, as that the upper may touch the wire of the bottle, when the lower touches the bottom: stick the outer part on a stick of sealing wax (_h_) which will serve as a handle. then apply the lower end to the bottom of the bottle, and gradually bring the upper-end near the wire in the cork. the consequence is, spark follows spark till the equilibrium is restored. touch the top first, and on approaching the bottom with the other end, you have a constant stream of fire, from the wire entering the bottle. touch the top and bottom together, and the equilibrium will soon be restored, but silently and imperceptibly; the crooked wire forming the communication. experiment v. fig. . let a ring of thin lead or paper surround a bottle (_i_), even at some distance from or above the bottom. from that ring let a wire proceed up, 'till it touch the wire of the cork (_k_). a bottle so fixt cannot by any means be electrised: the equilibrium is never destroyed: for while the communication between the upper and lower parts of the bottle is continued by the outside wire, the fire only circulates: what is driven out at bottom, is constantly supply'd from the top. hence a bottle cannot be electrised that is foul or moist on the outside. experiment vi. place a man on a cake of wax, and present him the wire of the electrified phial to touch, you standing on the floor, and holding it in your hand. as often as he touches it, he will be electrified _plus_; and any one standing on the floor may draw a spark from him. the fire in this experiment passes out of the wire into him; and at the same time out of your hand into the bottom of the bottle. experiment vii. give him the electrified phial to hold; and do you touch the wire; as often you touch it he will be electrified _minus_, and may draw a spark from any one standing on the floor. the fire now passes from the wire to you, and from him into the bottom of the bottle. experiment viii. lay two books on two glasses, back towards back, two or three inches distant. set the electrified phial on one, and then touch the wire; that book will be electrified _minus_; the electrical fire being drawn out of it by the bottom of the bottle. take off the bottle, and holding it in your hand, touch the other with the wire; that book will be electrised _plus_; the fire passing into it from the wire, and the bottle at the same time supply'd from your hand. a suspended small cork-ball will play between these books 'till the equilibrium is restored. experiment ix. when a body is electrised _plus_ it will repel an electrified feather or small cork-ball. when _minus_ (or when in the common state) it will attract them, but stronger when _minus_ than when in the common state, the difference being greater. experiment x. tho', as in exper. vi. a man standing on wax may be electrised a number of times, by repeatedly touching the wire of an electrised bottle (held in the hand of one standing on the floor) he receiving the fire from the wire each time: yet holding it in his own hand, and touching the wire, tho' he draws a strong spark, and is violently shock'd, no electricity remains in him; the fire only passing thro' him from the upper to the lower part of the bottle. observe, before the shock, to let some one on the floor touch him to restore the equilibrium in his body; for in taking hold of the bottom of the bottle, he sometimes becomes a little electrised _minus_, which will continue after the shock; as would also any _plus_ electricity, which he might have given him before the shock. for, restoring the equilibrium in the bottle does not at all affect the electricity in the man thro' whom the fire passes; that electricity is neither increased nor diminish'd. experiment xi. the passing of the electrical fire from the upper to the lower part of the bottle, to restore the equilibrium is render'd strongly visible by the following pretty experiment. take a book whose cover is filletted with gold; bend a wire of eight or ten inches long in the form of (_m_) fig. , slip it on the end of the cover of the book over the gold line, so as that the shoulder of it may press upon one end of the gold line, the ring up, but leaning towards the other end of the book. lay the book on a glass or wax; and on the other end of the gold lines, set the bottle electrised: then bend the springing wire, by pressing it with a stick of wax till its ring approaches the ring of the bottle wire; instantly there is a strong spark and stroke, and the whole line of gold, which completes the communication between the top and bottom of the bottle, will appear a vivid flame, like the sharpest lightning. the closer the contact between the shoulder of the wire, and the gold at one end of the line, and between the bottom of the bottle and the gold at the other end, the better the experiment succeeds. the room should be darkened. if you would have the whole filletting round the cover appear in fire at once, let the bottle and wire touch the gold in the diagonally opposite corners. _i am_, &c. b. franklin. letter ii. from mr benj. franklin, in _philadelphia_. to mr peter collinson, f.r.s. _london_. _sept. , ._ _sir_, in my last i informed you that, in pursuing our electrical enquiries, we had observed some particular phænomena, which we looked upon to be new, and of which i promised to give you some account, tho' i apprehended they might possibly not be new to you, as so many hands are daily employ'd in electrical experiments on your side the water, some or other of which would probably hit on the same observations. the first is the wonderful effect of pointed bodies, both in _drawing off_ and _throwing off_ the electrical fire. for example: place an iron shot of three or four inches diameter, on the mouth of a clean dry glass bottle. by a fine silken thread from the cieling, right over the mouth of the bottle, suspend a small cork-ball, about the bigness of a marble; the thread of such a length, as that the cork-ball may rest against the side of the shot. electrify the shot, and the ball will be repelled to the distance of four or five inches, more or less, according to the quantity of electricity.--when in this state, if you present to the shot the point of a long slender sharp bodkin, at six or eight inches distance, the repellency is instantly destroy'd, and the cork flies to the shot. a blunt body must be brought within an inch, and draw a spark, to produce the same effect. to prove that the electrical fire is _drawn off_ by the point, if you take the blade of the bodkin out of the wooden handle, and fix it in a stick of sealing wax, and then present it at the distance aforesaid, or if you bring it very near, no such effect follows; but sliding one finger along the wax till you touch the blade, and the ball flies to the shot immediately.--if you present the point in the dark, you will see, sometimes at a foot distance, and more, a light gather upon it like that of a fire-fly or glow-worm; the less sharp the point, the nearer you must bring it to observe the light; and at whatever distance you see the light, you may draw off the electrical fire, and destroy the repellency.--if a cork-ball so suspended be repelled by the tube, and a point be presented quick to it, tho' at a considerable distance, 'tis surprizing to see how suddenly it flies back to the tube. points of wood will do as well as those of iron, provided the wood is not dry; for perfectly dry wood will no more conduct electricity than sealing wax. to shew that points will _throw off_ as well as _draw off_ the electrical fire; lay a long sharp needle upon the shot, and you cannot electrise the shot, so as to make it repel the cork-ball.--or fix a needle to the end of a suspended gun-barrel, or iron rod, so as to point beyond it like a little bayonet; and while it remains there, the gun-barrel, or rod, cannot by applying the tube to the other end be electrised so as to give a spark, the fire continually running out silently at the point. in the dark you may see it make the same appearance as it does in the case beforementioned. the repellency between the cork-ball and the shot is likewise destroy'd; . by sifting fine sand on it; this does it gradually. . by breathing on it. . by making a smoke about it from burning wood.[ ] . by candle light, even tho' the candle is at a foot distance: these do it suddenly.--the light of a bright coal from a wood fire; and the light of red-hot iron do it likewise; but not at so great a distance. smoke from dry rosin dropt on hot iron, does not destroy the repellency; but is attracted by both shot and cork-ball, forming proportionable atmospheres round them, making them look beautifully, somewhat like some of the figures in _burnet_'s or _whiston_'s theory of the earth. _n. b._ this experiment should be made in a closet where the air is very still. the light of the sun thrown strongly on both cork and shot by a looking-glass for a long time together, does not impair the repellency in the least. this difference between fire-light and sun-light, is another thing that seems new and extraordinary to us. we had for some time been of opinion, that the electrical fire was not created by friction, but collected, being really an element diffus'd among, and attracted by other matter, particularly by water and metals. we had even discovered and demonstrated its afflux to the electrical sphere, as well as its efflux, by means of little light windmill wheels made of stiff paper vanes, fixed obliquely and turning freely on fine wire axes. also by little wheels of the same matter, but formed like water wheels. of the disposition and application of which wheels, and the various phænomena resulting, i could, if i had time, fill you a sheet. the impossibility of electrising one's self (tho' standing on wax) by rubbing the tube and drawing the fire from it; and the manner of doing it by passing the tube near a person or thing standing on the floor, &c. had also occurred to us some months before mr _watson_'s ingenious _sequel_ came to hand, and these were some of the new things i intended to have communicated to you.--but now i need only mention some particulars not hinted in that piece, with our reasonings thereupon; though perhaps the latter might well enough be spared. . a person standing on wax, and rubbing the tube, and another person on wax drawing the fire; they will both of them, (provided they do not stand so as to touch one another) appear to be electrised, to a person standing on the floor; that is, he will perceive a spark on approaching each of them with his knuckle. . but if the persons on wax touch one another during the exciting of the tube, neither of them will appear to be electrised. . if they touch one another after exciting the tube, and drawing the fire as aforesaid, there will be a stronger spark between them, than was between either of them and the person on the floor. . after such strong spark, neither of them discover any electricity. these appearances we attempt to account for thus. we suppose as aforesaid, that electrical fire is a common element, of which every one of the three persons abovementioned has his equal share, before any operation is begun with the tube. _a_, who stands on wax and rubs the tube collects the electrical fire from himself into the glass; and his communication with the common stock being cut off by the wax, his body is not again immediately supply'd. _b_, (who stands on wax likewise) passing his knuckle along near the tube, receives the fire which was collected by the glass from _a_; and his communication with the common stock being likewise cut off, he retains the additional quantity received.--to _c_, standing on the floor, both appear to be electrised: for he having only the middle quantity of electrical fire, receives a spark upon approaching _b_, who has an over quantity; but gives one to _a_, who has an under quantity. if _a_ and _b_ approach to touch each other, the spark is stronger, because the difference between them is greater; after such touch there is no spark between either of them and _c_, because the electrical fire in all is reduced to the original equality. if they touch while electrising, the equality is never destroy'd, the fire only circulating. hence have arisen some new terms among us: we say, _b_, (and bodies like circumstanced) is electrised _positively_; _a_, _negatively_. or rather, _b_ is electrised _plus_; _a_, _minus_. and we daily in our experiments electrise bodies _plus_ or _minus_ as we think proper.--to electrise _plus_ or _minus_, no more needs to be known than this, that the parts of the tube or sphere that are rubbed, do, in the instant of the friction attract the electrical fire, and therefore take it from the thing rubbing: the same parts immediately, as the friction upon them ceases, are disposed to give the fire they have received, to any body that has less. thus you may circulate it, as mr _watson_ has shewn; you may also accumulate or substract it upon or from any body, as you connect that body with the rubber or with the receiver, the communication with the common stock being cut off. we think that ingenious gentleman was deceived, when he imagined (in his _sequel_) that the electrical fire came down the wire from the cieling to the gun-barrel, thence to the sphere, and so electrised the machine and the man turning the wheel, _&c._ we suppose it was _driven off_, and not brought on thro' that wire; and that the machine and man, _&c._ were electrised _minus_; _i. e._ had less electrical fire in them than things in common. as the vessel is just upon sailing, i cannot give you so large an account of american electricity as i intended: i shall only mention a few particulars more.--we find granulated lead better to fill the phial with, than water, being easily warmed, and keeping warm and dry in damp air.--we fire spirits with the wire of the phial.--we light candles, just blown out, by drawing a spark among the smoke between the wire and snuffers.--we represent lightning, by passing the wire in the dark over a china plate that has gilt flowers, or applying it to gilt frames of looking-glasses, _&c._--we electrise a person twenty or more times running, with a touch of the finger on the wire, thus: he stands on wax. give him the electrised bottle in his hand. touch the wire with your finger, and then touch his hand or face; there are sparks every time.--we encrease the force of the electrical kiss vastly, thus: let _a_ and _b_ stand on wax; give one of them the electrised phial in hand; let the other take hold of the wire; there will be a small spark; but when their lips approach, they will be struck and shock'd. the same if another gentleman and lady, _c_ and _d_, standing also on wax, and joining hands with _a_ and _b_, salute, or shake hands.--we suspend by fine silk thread a counterfeit spider, made of a small piece of burnt cork, with legs of linnen thread, and a grain or two of lead stuck in him to give him more weight. upon the table, over which he hangs, we stick a wire upright as high as the phial and wire, two or three inches from the spider; then we animate him by setting the electrified phial at the same distance on the other side of him; he will immediately fly to the wire of the phial, bend his legs in touching it, then spring off, and fly to the wire in the table; thence again to the wire of the phial, playing with his legs against both in a very entertaining manner, appearing perfectly alive to persons unacquainted. he will continue this motion an hour or more in dry weather.--we electrify, upon wax in the dark, a book that has a double line of gold round upon the covers, and then apply a knuckle to the gilding; the fire appears every where upon the gold like a flash of lightning: not upon the leather, nor, if you touch the leather instead of the gold. we rub our tubes with buckskin, and observe always to keep the same side to the tube, and never to sully the tube by handling; thus they work readily and easily, without the least fatigue; especially if kept in tight pastboard cases, lined with flannel, and fitting closeto the tube.[ ]--this i mention because the _european_ papers, on electricity, frequently speak of rubbing the tube, as a fatiguing exercise. our spheres are fixed on iron axes, which pass through them. at one end of the axis there is a small handle, with which we turn the sphere like a common grindstone. this we find very commodious, as the machine takes up but little room, is portable, and may be enclosed in a tight box, when not in use. 'tis true, the sphere does not turn so swift, as when the great wheel is used: but swiftness we think of little importance, since a few turns will charge the phial, _&c._ sufficiently. _i am_, &c. b. franklin. [illustration] letter iii. from mr benj. franklin, in _philadelphia_. to mr peter collinson, f.r.s. _london_. _farther_ experiments _and_ observations _in_ electricity. _ ._ _sir_, § . there will be the same explosion and shock, if the electrified phial is held in one hand by the hook, and the coating touch'd with the other, as when held by the coating, and touch'd at the hook. . to take the charg'd phial safely by the hook, and not at the same time diminish its force, it must first be set down on an electric _per se_. . the phial will be electrified as strongly, if held by the hook, and the coating apply'd to the globe or tube; as when held by the coating, and the hook apply'd. . but the _direction_ of the electrical fire being different in the charging, will also be different in the explosion. the bottle charged thro' the hook, will be discharged thro' the hook; the bottle charged thro' the coating, will be discharged thro' the coating, and not other ways: for the fire must come out the same way it went in. . to prove this; take two bottles that were equally charged thro' the hooks, one in each hand; bring their hooks near each other, and no spark or shock will follow; because each hook is disposed to give fire, and neither to receive it. set one of the bottles down on glass, take it up by the hook, and apply its coating to the hook of the other; then there will be an explosion and shock, and both bottles will be discharged. . vary the experiment, by charging two phials equally, one thro' the hook, the other thro' the coating: hold that by the coating which was charged thro' the hook; and that by the hook which was charg'd thro' the coating: apply the hook of the first to the coating of the other, and there will be no shock or spark. set that down on glass which you held by the hook, take it up by the coating, and bring the two hooks together: a spark and shock will follow, and both phials be discharged. in this experiment the bottles are totally discharged, or the equilibrium within them restored. the _abounding_ of fire in one of the hooks (or rather in the internal surface of one bottle) being exactly equal to the _wanting_ of the other: and therefore, as each bottle has in itself the _abounding_ as well as the _wanting_, the wanting and abounding must be equal in each bottle. see §. , , , . but if a man holds in his hands two bottles, one fully electrify'd, the other not at all, and brings their hooks together, he has but half a shock, and the bottles will both remain half electrified, the one being half discharged, and the other half charged. . place two phials equally charged on a table at five or six inches distance. let a cork-ball, suspended by a silk thread, hang between them. if the phials were both charged through their hooks, the cork, when it has been attracted and repell'd by the one, will not be attracted, but equally repelled by the other. but if the phials were charged, the one through the hook, and the other[ ] through the coating, the ball, when it is repelled from one hook, will be as strongly attracted by the other, and play vigorously between them, 'till both phials are nearly discharged. . when we use the terms of _charging_ and _discharging_ the phial, 'tis in compliance with custom, and for want of others more suitable. since we are of opinion, that there is really no more electrical fire in the phial after what is called its _charging_, than before, nor less after its _discharging_; excepting only the small spark that might be given to, and taken from, the non-electric matter, if separated from the bottle, which spark may not be equal to a five hundredth part of what is called the explosion. for if, on the explosion, the electrical fire came out of the bottle by one part, and did not enter in again by another; then, if a man standing on wax, and holding the bottle in one hand, takes the spark by touching the wire hook with the other, the bottle being thereby _discharged_, the man would be _charged_; or whatever fire was lost by one, would be found in the other, since there is no way for its escape: but the contrary is true. . besides the phial will not suffer what is called a _charging_, unless as much fire can go out of it one way, as is thrown in by another. a phial cannot be charged standing on wax or glass, or hanging on the prime conductor, unless a communication be form'd between its coating and the floor. . but suspend two or more phials on the prime conductor, one hanging to the tail of the other; and a wire from the last to the floor, an equal number of turns of the wheel shall charge them all equally, and every one as much as one alone would have been. what is driven out at the tail of the first, serving to charge the second; what is driven out of the second charging the third; and so on. by this means a great number of bottles might be charged with the same labour, and equally high, with one alone, were it not that every bottle receives new fire, and loses its old with some reluctance, or rather gives some small resistance to the charging, which in a number of bottles becomes more equal to the charging power, and so repels the fire back again on the globe, sooner than a single bottle would do. . when a bottle is charged in the common way, its _inside_ and _outside_ surfaces stand ready, the one to give fire by the hook, the other to receive it by the coating; the one is full, and ready to throw out, the other empty and extremely hungry; yet as the first will not _give out_, unless the other can at the same instant _receive in_; so neither will the latter receive in, unless the first can at the same instant give out. when both can be done at once, 'tis done with inconceivable quickness and violence. . so a strait spring (tho' the comparison does not agree in every particular) when forcibly bent, must, to restore itself, contract that side which in the bending was extended, and extend that which was contracted; if either of these two operations be hindered, the other cannot be done. but the spring is not said to be _charg'd_ with elasticity when bent, and discharg'd when unbent; its quantity of elasticity is always the same. . glass, in like manner, has, within its substance, always the same quantity of electrical fire, and that a very great quantity in proportion to the mass of glass, as shall be shewn hereafter. . this quantity, proportioned to the glass, it strongly and obstinately retains, and will have neither more nor less, though it will suffer a change to be made in its parts and situation; _i. e._ we may take away part of it from one of the sides, provided we throw an equal quantity into the other. . yet when the situation of the electrical fire is thus altered in the glass; when some has been taken from one side, and some added to the other, it will not be at rest or in its natural state, till 'tis restored to its original equality.--and this restitution cannot be made through the substance of the glass, but must be done by a non-electric communication formed without, from surface to surface. . thus, the whole force of the bottle, and power of giving a shock, is in the glass itself; the non-electrics in contact with the two surfaces, serving only to _give_ and _receive_ to and from the several parts of the glass; that is, to give on one side, and take away from the other. . this was discovered here in the following manner. purposing to analyse the electrified bottle, in order to find wherein its strength lay, we placed it on glass, and drew out the cork and wire which for that purpose had been loosely put in. then taking the bottle in one hand, and bringing a finger of the other near its mouth, a strong spark came from the water, and the shock was as violent as if the wire had remained in it, which shewed that the force did not lie in the wire. then to find if it resided in the water, being crouded into and condensed in it, as connfi'd by the glass, which had been our former opinion, we electrify'd the bottle again, and placing it on glass, drew out the wire and cork as before; then taking up the bottle we decanted all its water into an empty bottle, which likewise stood on glass; and taking up that other bottle, we expected if the force resided in the water, to find a shock from it; but there was none. we judged then, that it must either be lost in decanting, or remain in the first bottle. the latter we found to be true: for that bottle on trial gave the shock, though filled up as it stood with fresh unelectrified water from a tea-pot.--to find then, whether glass had this property merely as glass, or whether the form contributed any thing to it; we took a pane of sash-glass, and laying it on the stand, placed a plate of lead on its upper surface; then electrify'd that plate, and bringing a finger to it, there was a spark and shock. we then took two plates of lead of equal dimensions, but less than the glass by two inches every way, and electrified the glass between them, by electrifying the uppermost lead; then separated the glass from the lead, in doing which, what little fire might be in the lead was taken out and the glass being touched in the electrified parts with a finger, afforded only very small pricking sparks, but a great number of them might be taken from different places. then dexterously placing it again between the leaden plates, and compleating a circle between the two surfaces, a violent shock ensued.--which demonstrated the power to reside in glass as glass, and that the non-electrics in contact served only, like the armature of a loadstone, to unite the force of the several parts, and bring them at once to any point desired: it being a property of a non-electric, that the whole body instantly receives or gives what electrical fire is given to or taken from any one of its parts. . upon this, we made what we call'd an _electrical-battery_, consisting of eleven panes of large sash-glass, arm'd with thin leaden plates pasted on each side, placed vertically, and supported at two inches distance on silk cords, with thick hooks of leaden wire, one from each side, standing upright, distant from each other, and convenient communications of wire and chain, from the giving side of one pane, to the receiving side of the other; that so the whole might be charged together, and with the same labour as one single pane; and another contrivance to bring the giving sides, after charging, in contact with one long wire, and the receivers with another, which two long wires would give the force of all the plates of glass at once through the body of any animal forming the circle with them. the plates may also be discharged separately, or any number together that is required. but this machine is not much used, as not perfectly answering our intention with regard to the ease of charging, for the reason given § . we made also of large glass panes, magical pictures, and self-moving animated wheels, presently to be described. . i perceive by the ingenious mr _watson_'s last book, lately received, that dr _bevis_ had used, before we had, panes of glass to give a shock; though, till that book came to hand, i thought to have communicated it to you as a novelty. the excuse for mentioning it here, is, that we tried the experiment differently, drew different consequences from it, (for mr _watson_ still seems to think the fire _accumulated on the non-electric_ that is in contact with the glass, page ) and, as far as we hitherto know, have carried it farther. . the magical picture is made thus. having a large metzotinto with a frame and glass, suppose of the king, (god preserve him) take out the print, and cut a pannel out of it, near two inches distant from the frame all round. if the cut is through the picture 'tis not the worse. with thin paste or gum-water, fix the border that is cut off on the inside of the glass, pressing it smooth and close; then fill up the vacancy by gilding the glass well with leaf gold or brass. gild likewise the inner edge of the back of the frame all round except the top part, and form a communication between that gilding and the gilding behind the glass: then put in the board, and that side is finished. turn up the glass, and gild the fore side exactly over the back gilding, and when it is dry, cover it by pasting on the pannel of the picture that had been cut out, observing to bring the corresponding parts of the border and picture together, by which the picture will appear of a piece as at first, only part is behind the glass, and part before.--hold the picture horizontally by the top, and place a little moveable gilt crown on the king's-head. if now the picture be moderately electrified, and another person take hold of the frame with one hand, so that his fingers touch its inside gilding, and with the other hand endeavour to take off the crown, he will receive a terrible blow, and fail in the attempt. if the picture were highly charged, the consequence might perhaps be as fatal as that of high-treason; for when the spark is taken through a quire of paper laid on the picture, by means of a wire communication, it makes a fair hole through every sheet, that is, through leaves, (though a quire of paper is thought good armour against the push of a sword or even against a pistol bullet) and the crack is exceeding loud. the operator, who holds the picture by the upper-end, where the inside of the frame is not gilt, to prevent its falling, feels nothing of the shock, and may touch the face of the picture without danger, which he pretends is a test of his loyalty.--if a ring of persons take the shock among them, the experiment is called, _the conspirators_. . on the principle, in § , that hooks of bottles, differently charged, will attract and repel differently, is made, an electrical wheel, that turns with considerable strength. a small upright shaft of wood passes at right angles through a thin round board, of about twelve inches diameter, and turns on a sharp point of iron fixed in the lower end, while a strong wire in the upper-end passing thro' a small hole in a thin brass plate, keeps the shaft truly vertical. about thirty _radii_ of equal length, made of sash glass cut in narrow strips, issue horizontally from the circumference of the board, the ends most distant from the center being about four inches apart. on the end of every one, a brass thimble is fixed. if now the wire of a bottle electrified in the common way, be brought near the circumference of this wheel, it will attract the nearest thimble, and so put the wheel in motion; that thimble, in passing by, receives a spark, and thereby being electrified is repelled and so driven forwards; while a second being attracted, approaches the wire, receives a spark, and is driven after the first, and so on till the wheel has gone once round, when the thimbles before electrified approaching the wire, instead of being attracted as they were at first, are repelled, and the motion presently ceases.--but if another bottle which had been charged through the coating be placed near the same wheel, its wire will attract the thimble repelled by the first, and thereby double the force that carries the wheel round; and not only taking out the fire that had been communicated to the thimbles by the first bottle, but even robbing them of their natural quantity, instead of being repelled when they come again towards the first bottle, they are more strongly attracted, so that the wheel mends its pace, till it goes with great rapidity twelve or fifteen rounds in a minute, and with such strength, as that the weight of one hundred _spanish_ dollars with which we once loaded it, did not seem in the least to retard its motion.--this is called an electrical jack; and if a large fowl were spitted on the upright shaft, it would be carried round before a fire with a motion fit for roasting. . but this wheel, like those driven by wind, water, or weights, moves by a foreign force, to wit, that of the bottles. the self-moving wheel, though constructed on the same principles, appears more surprising. 'tis made of a thin round plate of window-glass, seventeen inches diameter, well gilt on both sides, all but two inches next the edge. two small hemispheres of wood are then fixed with cement to the middle of the upper and under sides, centrally opposite, and in each of them a thick strong wire eight or ten inches long, which together make the axis of the wheel. it turns horizontally on a point at the lower end of its axis, which rests on a bit of brass cemented within a glass salt-celler. the upper end of its axis passes thro' a hole in a thin brass plate cemented to a long strong piece of glass, which keeps it six or eight inches distant from any non-electric, and has a small ball of wax or metal on its top to keep in the fire. in a circle on the table which supports the wheel, are fixed twelve small pillars of glass, at about four inches distance, with a thimble on the top of each. on the edge of the wheel is a small leaden bullet communicating by a wire with the gilding of the _upper_ surface of the wheel; and about six inches from it is another bullet communicating in like manner with the _under_ surface. when the wheel is to be charged by the upper surface, a communication must be made from the under surface to the table. when it is well charg'd it begins to move; the bullet nearest to a pillar moves towards the thimble on that pillar, and passing by electrifies it and then pushes itself from it; the succeeding bullet, which communicates with the other surface of the glass, more strongly attracts that thimble on account of its being before electrified by the other bullet; and thus the wheel encreases its motion till it comes to such a height as that the resistance of the air regulates it. it will go half an hour, and make one minute with another twenty turns in a minute, which is six hundred turns in the whole; the bullet of the upper surface giving in each turn twelve sparks, to the thimbles, which make seven thousand two hundred sparks; and the bullet of the under surface receiving as many from the thimbles; those bullets moving in the time near two thousand five hundred feet.--the thimbles are well fixed, and in so exact a circle, that the bullets may pass within a very small distance of each of them.--if instead of two bullets you put eight, four communicating with the upper surface, and four with the under surface, placed alternately; which eight, at about six inches distance, compleats the circumference, the force and swiftness will be greatly increased, the wheel making fifty turns in a minute; but then it will not continue moving so long.--these wheels may be applied, perhaps, to the ringing of chimes, and moving of light-made orreries. . a small wire bent circularly with a loop at each end; let one end rest against the under surface of the wheel, and bring the other end near the upper surface, it will give a terrible crack, and the force will be discharged. . every spark in that manner drawn from the surface of the wheel, makes a round hole in the gilding, tearing off a part of it in coming out; which shews that the fire is not accumulated on the gilding, but is in the glass itself. . the gilding being varnish'd over with turpentine varnish, the varnish tho' dry and hard, is burnt by the spark drawn thro' it, and gives a strong smell and visible smoke. and when the spark is drawn through paper, all round the hole made by it, the paper will be blacked by the smoke, which sometimes penetrates several of the leaves. part of the gilding torn off, is also found forcibly driven into the hole made in the paper by the stroke. . 'tis amazing to observe in how small a portion of glass a great electrical force may lie. a thin glass bubble, about an inch diameter, weighing only six grains, being half-filled with water, partly gilt on the outside, and furnish'd with a wire hook, gives, when electrified, as great a shock as a man can well bear. as the glass is thickest near the orifice, i suppose the lower half, which being gilt was electrified, and gave the shock, did not exceed two grains; for it appeared, when broke, much thinner than the upper half.--if one of these thin bottles be electrified by the coating, and the spark taken out thro' the gilding, it will break the glass inwards at the same time that it breaks the gilding outwards. . and allowing (for the reasons before given, § , , ,) that there is no more electrical fire in a bottle after charging, than before, how great must be the quantity in this small portion of glass! it seems as if it were of its very substance and essence. perhaps if that due quantity of electrical fire so obstinately retained by glass, could be separated from it, it would no longer be glass; it might lose its transparency, or its brittleness, or its elasticity.--experiments may possibly be invented hereafter, to discover this. . we are surprized at the account given in mr _watson_'s book, of a shock communicated through a great space of dry ground, and suspect there must be some metaline quality in the gravel of that ground; having found that simple dry earth, rammed in a glass tube, open at both ends, and a wire hook inserted in the earth at each end, the earth and wires making part of a circle, would not conduct the least perceptible shock, and indeed when one wire was electrify'd, the other hardly showed any signs of its being in connection with it.--even a thoroughly wet pack-thread sometimes fails of conducting a shock, tho' it otherwise conducts electricity very well. a dry cake of ice, or an icicle held between two in a circle, likewise prevents the shock; which one would not expect, as water conducts it so perfectly well.--gilding on a new book, though at first it conducts the shock extremely well, yet fails after ten or a dozen experiments, though it appears otherwise in all respects the same, which we cannot account for. . there is one experiment more which surprizes us, and is not hitherto satisfactorily accounted for; it is this. place an iron shot on a glass stand, and let a ball of damp cork, suspended by a silk thread, hang in contact with the shot. take a bottle in each hand, one that is electrify'd through the hook, the other through the coating: apply the giving wire to the shot, which will electrify it _positively_, and the cork shall be repelled: then apply the requiring wire, which will take out the spark given by the other; when the cork will return to the shot: apply the same again, and take out another spark, so will the shot be electrify'd _negatively_; and the cork in that case shall be repelled equally as before. then apply the giving wire to the shot, and give the spark it wanted, so will the cork return: give it another, which will be an addition to its natural quantity, so will the cork be repelled again: and so may the experiment be repeated as long as there is any charge in the bottles. which shews that bodies having less than the common quantity of electricity, repel each other, as well as those that have more. chagrined a little that we have hitherto been able to produce nothing in this way of use to mankind; and the hot weather coming on, when electrical experiments are not so agreeable, 'tis proposed to put an end to them for this season, somewhat humorously, in a party of pleasure, on the banks of _skuylkill_.[ ] spirits, at the same time, are to be fired by a spark sent from side to side through the river, without any other conductor than the water; an experiment which we some time since performed, to the amazement of many. a turkey is to be killed for our dinner by the _electrical shock_, and roasted by the _electrical jack_, before a fire kindled by the _electrified bottle_; when the healths of all the famous electricians in _england_, _holland_, _france_, and _germany_, are to be drank in [ ]_electrified bumpers_, under the discharge of guns from the _electrical battery_. _april , ._ [illustration] letter iv. containing observations _and_ suppositions, _towards forming a new_ hypothesis, _for explaining the several_ phænomena _of_ thunder-gusts.[ ] _sir_, §. . non-electric bodies, that have electric fire thrown into them, will retain it 'till other non-electrics, that have less, approach; and then 'tis communicated by a snap, and becomes equally divided. . electrical fire loves water, is strongly attracted by it, and they can subsist together. . air is an electric _per se_, and when dry will not conduct the electrical fire; it will neither receive it, nor give it to other bodies; otherwise no body surrounded by air could be electrified positively and negatively: for should it be attempted positively, the air would immediately take away the overplus; or negatively, the air would supply what was wanting. . water being electrified, the vapours arising from it will be equally electrified; and floating in the air, in the form of clouds, or otherwise, will retain that quantity of electrical fire, till they meet with other clouds or bodies not so much electrified, and then will communicate as beforementioned. . every particle of matter electrified is repelled by every other particle equally electrified. thus the stream of a fountain, naturally dense and continual, when electrified, will separate and spread in the form of a brush, every drop endeavouring to recede from every other drop. but on taking out the electrical fire, they close again. . water being strongly electrified (as well as when heated by common fire) rises in vapours more copiously; the attraction of cohesion among its particles being greatly weakened, by the opposite power of repulsion introduced with the electrical fire; and when any particle is by any means disengaged, 'tis immediately repelled, and so flies into the air. . particles happening to be situated as _a_ and _b_, are more easily disengaged than _c_ and _d_, as each is held by contact with three only, whereas _c_ and _d_ are each in contact with nine. when the surface of water has the least motion, particles are continually pushed into the situation represented by fig. . . friction between a non-electric and an electric _per se_, will produce electrical fire; not by _creating_, but _collecting_ it: for it is equally diffused in our walls, floors, earth, and the whole mass of common matter. thus the whirling glass globe, during its friction against the cushion, draws fire from the cushion, the cushion is supplied from the frame of the machine, that from the floor on which it stands. cut off the communication by thick glass or wax placed under the cushion, and no fire can be _produced_, because it cannot be _collected_. . the ocean is a compound of water, a non-electric, and salt an electric _per se_. . when there is a friction among the parts near its surface, the electrical fire is collected from the parts below. it is then plainly visible in the night; it appears at the stern and in the wake of every sailing vessel; every dash of an oar shows it, and every surff and spray: in storms the whole sea seems on fire.--the detach'd particles of water then repelled from the electrified surface, continually carry off the fire as it is collected; they rise, and form clouds, and those clouds are highly electrified, and retain the fire 'till they have an opportunity of communicating it. . the particles of water rising in vapours, attach themselves to particles of air. . the particles of air are said to be hard, round, separate and distant from each other; every particle strongly repelling every other particle, whereby they recede from each other, as far as common gravity will permit. . the space between any three particles equally repelling each other, will be an equilateral triangle. . in air compressed, these triangles are smaller; in rarified air they are larger. . common fire joined with air, increases the repulsion, enlarges the triangles, and thereby makes the air specifically lighter. such air among denser air, will rise. . common fire, as well as electrical fire gives repulsion to the particles of water, and destroys their attraction of cohesion; hence common fire, as well as electrical fire, assists in raising vapours. . particles of water, having no fire in them, mutually attract each other. three particles of water then being attached to the three particles of a triangle of air, would by their mutual attraction operating against the air's repulsion, shorten the sides and lessen the triangle, whereby that portion of air being made denser, would sink to the earth with its water, and not rise to contribute to the formation of a cloud. . but if every particle of water attaching itself to air, brings with it a particle of common fire, the repulsion of the air being assisted and strengthened by the fire, more than obstructed by the mutual attraction of the particles of water, the triangle dilates, and that portion of air becoming rarer and specifically lighter rises. . if the particles of water bring electrical fire when they attach themselves to air, the repulsion between the particles of water electrified, joins with the natural repulsion of the air, to force its particles to a greater distance, whereby the triangles are dilated, and the air rises, carrying up with it the water. . if the particles of water bring with them portions of _both sorts_ of fire, the repulsions of the particles of air is still more strengthened and increased, and the triangles farther enlarged. . one particle of air may be surrounded by twelve particles of water of equal size with itself, all in contact with it; and by more added to those. . particles of air thus loaded would be drawn nearer together by the mutual attraction of the particles of water, did not the fire, common or electrical, assist their repulsion. . if air thus loaded be compressed by adverse winds, or by being driven against mountains, &c. or condensed by taking away the fire that assisted it in expanding; the triangles contract, the air with its water will descend as a dew; or, if the water surrounding one particle of air comes in contact with the water surrounding another, they coalesce and form a drop, and we have rain. . the sun supplies (or seems to supply) common fire to all vapours, whether raised from earth or sea. . those vapours which have both common and electrical fire in them, are better supported, than those which have only common fire in them. for when vapours rise into the coldest region above the earth, the cold will not diminish the electrical fire, if it doth the common. . hence clouds formed by vapours raised from fresh waters within land, from growing vegetables, moist earth, &c. more speedily and easily deposite their water, having but little electrical fire to repel and keep the particles separate. so that the greatest part of the water raised from the land is let fall on the land again; and winds blowing from the land to the sea are dry; there being little use for rain on the sea, and to rob the land of its moisture, in order to rain on the sea, would not appear reasonable. . but clouds formed by vapours raised from the sea, having both fires, and particularly a great quantity of the electrical, support their water strongly, raise it high, and being moved by winds may bring it over the middle of the broadest continent from the middle of the widest ocean. . how these ocean clouds, so strongly supporting their water, are made to deposite it on the land where 'tis wanted, is next to be considered. . if they are driven by winds against mountains, those mountains being less electrified attract them, and on contact take away their electrical fire (and being cold, the common fire also;) hence the particles close towards the mountains and towards each other. if the air was not much loaded, it only falls in dews on the mountain tops and sides, forms springs, and descends to the vales in rivulets, which united make larger streams and rivers. if much loaded, the electrical fire is at once taken from the whole cloud; and, in leaving it, flashes brightly and cracks loudly; the particles instantly coalescing for want of that fire, and falling in a heavy shower. . when a ridge of mountains thus dams the clouds, and draws the electrical fire from the cloud first approaching it; that which next follows, when it comes near the first cloud, now deprived of its fire, flashes into it, and begins to deposite its own water; the first cloud again flashing into the mountains; the third approaching cloud, and all the succeeding ones, acting in the same manner as far back as they extend, which may be over many hundred miles of country. . hence the continual storms of rain, thunder, and lightning on the east-side of the _andes_, which running north and south, and being vastly high, intercept all the clouds brought against them from the _atlantic_ ocean by the trade winds, and oblige them to deposite their waters, by which the vast rivers _amazons_, _la plata_, and _oroonoko_ are formed, which return the water into the same sea, after having fertilized a country of very great extent. . if a country be plain, having no mountains to intercept the electrified clouds, yet is it not without means to make them deposite their water. for if an electrified cloud coming from the sea, meets in the air a cloud raised from the land, and therefore not electrified; the first will flash its fire into the latter, and thereby both clouds shall be made suddenly to deposite water. . the electrified particles of the first cloud close when they lose their fire; the particles of the other cloud close in receiving it: in both, they have thereby an opportunity of coalescing into drops.--the concussion or jerk given to the air, contributes also to shake down the water, not only from those two clouds but from others near them. hence the sudden fall of rain immediately after flashes of lightning. . to shew this by an easy experiment. take two round pieces of pasteboard two inches diameter; from the center and circumference of each of them suspend by fine silk threads eighteen inches long, seven small balls of wood, or seven peas equal in bigness; so will the balls appending to each pasteboard, form equal equilateral triangles, one ball being in the center, and six at equal distances from that, and from each other; and thus they represent particles of air. dip both sets in water, and some cohering to each ball they will represent air loaded. dexterously electrify one set, and its balls will repel each other to a greater distance, enlarging the triangles. could the water supported by the seven balls come into contact, it would form a drop or drops so heavy as to break the cohesion it had with the balls, and so fall.--let the two sets then represent two clouds, the one a sea cloud electrified, the other a land cloud. bring them within the sphere of attraction, and they will draw towards each other, and you will see the separated balls close thus; the first electrified ball that comes near an unelectrified ball by attraction joins it, and gives it fire; instantly they separate, and each flies to another ball of its own party, one to give, the other to receive fire; and so it proceeds through both sets, but so quick as to be in a manner instantaneous. in the collision they shake off and drop their water, which represents rain. . thus when sea and land clouds would pass at too great a distance for the flash, they are attracted towards each other till within that distance; for the sphere of electrical attraction is far beyond the distance of flashing. . when a great number of clouds from the sea meet a number of clouds raised from the land, the electrical flashes appear to strike in different parts; and as the clouds are jostled and mixed by the winds, or brought near by the electrical attraction, they continue to give and receive flash after flash, till the electrical fire is equally diffused. . when the gun-barrel (in electrical experiments) has but little electrical fire in it, you must approach it very near with your knuckle, before you can draw a spark. give it more fire, and it will give a spark at a greater distance. two gun-barrels united, and as highly electrified, will give a spark at a still greater distance. but if two gun-barrels electrified will strike at two inches distance, and make a loud snap, to what a great distance may , acres of electrified cloud strike and give its fire, and how loud must be that crack! . it is a common thing to see clouds at different heights passing different ways, which shews different currents of air, one under the other. as the air between the tropics is rarified by the sun, it rises, the denser northern and southern air pressing into its place. the air so rarified and forced up, passes northward and southward, and must descend in the polar regions, if it has no opportunity before, that the circulation may be carried on. . as currents of air, with the clouds therein, pass different ways, 'tis easy to conceive how the clouds, passing over each other, may attract each other, and so come near enough for the electrical stroke. and also how electrical clouds may be carried within land very far from the sea, before they have an opportunity to strike. . when the air, with its vapours raised from the ocean between the tropics, comes to descend in the polar regions, and to be in contact with the vapours arising there, the electrical fire they brought begins to be communicated, and is seen in clear nights, being first visible where 'tis first in motion, that is, where the contact begins, or in the most northern part; from thence the streams of light seem to shoot southerly, even up to the zenith of northern countries. but tho' the light seems to shoot from the north southerly, the progress of the fire is really from the south northerly, its motion beginning in the north being the reason that 'tis there first seen. for the electrical fire is never visible but when in motion, and leaping from body to body, or from particle to particle thro' the air. when it passes thro' dense bodies 'tis unseen. when a wire makes part of the circle, in the explosion of the electrical phial, the fire, though in great quantity, passes in the wire invisibly: but in passing along a chain, it becomes visible as it leaps from link to link. in passing along leaf-gilding 'tis visible: for the leaf-gold is full of pores; hold a leaf to the light and it appears like a net; and the fire is seen in its leaping over the vacancies.--and as when a long canal filled with still water is opened at one end, in order to be discharged, the motion of the water begins first near the opened end, and proceeds towards the close end, tho' the water itself moves from the close towards the opened end: so the electrical fire discharged into the polar regions, perhaps from a thousand leagues length of vaporiz'd air, appears first where 'tis first in motion, _i. e._ in the most northern part, and the appearance proceeds southward, tho' the fire really moves northward. this is supposed to account for the _aurora borealis_. . when there is great heat on the land, in a particular region (the sun having shone on it perhaps several days, while the surrounding countries have been screen'd by clouds) the lower air is rarified and rises, the cooler denser air above descends; the clouds in that air meet from all sides, and join over the heated place; and if some are electrified, others not, lightning and thunder succeed, and showers fall. hence thunder-gusts after heats, and cool air after gusts; the water and the clouds that bring it, coming from a higher and therefore a cooler region. . an electrical spark, drawn from an irregular body at some distance is scarce ever strait, but shows crooked and waving in the air. so do the flashes of lightning; the clouds being very irregular bodies. . as electrified clouds pass over a country, high hills and high trees, lofty towers, spires, masts of ships, chimneys, _&c._ as so many prominencies and points, draw the electrical fire, and the whole cloud discharges there. . dangerous, therefore, is it to take shelter under a tree during a thunder-gust. it has been fatal to many, both men and beasts. . it is safer to be in the open field for another reason. when the clothes are wet, if a flash in its way to the ground should strike your head, it will run in the water over the surface of your body; whereas, if your clothes were dry, it would go thro' the body. hence a wet rat cannot be killed by the exploding electrical bottle, when a dry rat may. . common fire is in all bodies, more or less, as well as electrical fire. perhaps they may be different modifications of the same element; or they may be different elements. the latter is by some suspected. . if they are different things, yet they may and do subsist together in the same body. . when electrical fire strikes thro' a body, it acts upon the common fire contained in it, and puts that fire in motion; and if there be a sufficient quantity of each kind of fire, the body will be inflamed. . when the quantity of common fire in the body is small, the quantity of the electrical fire (or the electrical stroke) should be greater: if the quantity of common fire be great, less electrical fire suffices to produce the effect. . thus spirits must be heated before we can fire them by the electrical spark. if they are much heated a small spark will do; if not, the spark must be greater. . till lately we could only fire warm vapours; but now we can burn hard dry rosin. and when we can procure greater electrical sparks, we may be able to fire not only unwarm'd spirits, as lightning does, but even wood, by giving sufficient agitation to the common fire contained in it, as friction we know will do. . sulphureous and inflammable vapours arising from the earth, are easily kindled by lightning. besides what arise from the earth, such vapours are sent out by stacks of moist hay, corn, or other vegetables, which heat and reek. wood rotting in old trees or buildings does the same. such are therefore easily and often fired. . metals are often melted by lightning, tho' perhaps not from heat in the lightning, nor altogether from agitated fire in the metals.--for as whatever body can insinuate itself between the particles of metal, and overcome the attraction by which they cohere (as sundry menstrua can) will make the solid become a fluid, as well as fire, yet without heating it: so the electrical fire, or lightning, creating a violent repulsion between the particles of the metal it passes thro', the metal is fused. . if you would, by a violent fire, melt off the end of a nail, which is half driven into a door, the heat given the whole nail before a part would melt, must burn the board it sticks in. and the melted part would burn the floor it dropp'd on. but if a sword can be melted in the scabbard, and money in a man's pocket, by lightning, without burning either, it must be a cold fusion. . lightning rends some bodies. the electrical spark will strike a hole thro' a quire of strong paper. . if the source of lightning, assigned in this paper, be the true one, there should be little thunder heard at sea far from land. and accordingly some old sea-captains, of whom enquiry has been made, do affirm, that the fact agrees perfectly with the hypothesis; for that, in crossing the great ocean, they seldom meet with thunder till they come into soundings; and that the islands far from the continent have very little of it. and a curious observer, who lived years at _bermudas_, says, there was less thunder there in that whole time than he has sometimes heard in a month at _carolina_. additional papers. to mr. peter collinson, f.r.s. _london_. philadelphia, _july , _ _sir_, as you first put us on electrical experiments, by sending to our library company a tube, with directions how to use it; and as our honourable proprietary enabled us to carry those experiments to a greater height, by his generous present of a compleat electrical apparatus; 'tis fit that both should know from time to time what progress we make. it was in this view i wrote and sent you my former papers on this subject, desiring, that as i had not the honour of a direct correspondence with that bountiful benefactor to our library, they might be communicated to him through your hands. in the same view i write, and send you this additional paper. if it happens to bring you nothing new (which may well be, considering the number of ingenious men in _europe_, continually engaged in the same researches) at least it will show, that the instruments, put into our hands, are not neglected; and, that if no valuable discoveries are made by us, whatever the cause may be, it is not want of industry and application. _i am, sir, your much obliged humble servant_, b. franklin. opinions and conjectures, _concerning the properties and effects of the electrical matter, arising from experiments and observations, made in_ philadelphia, . § . the electrical matter consists of particles extreamly subtile, since it can permeate common matter, even the densest metals, with such ease and freedom, as not to receive any perceptible resistance. . if any one should doubt, whether the electrical matter passes thro' the substance of bodies, or only over and along their surfaces, a shock from an electrified large glass jar, taken thro' his own body, will probably convince him. . electrical matter differs from common matter in this, that the parts of the latter mutually attract, those of the former mutually repel, each other. hence the appearing divergency in a stream of electrified effluvia. . but tho' the particles of electrical matter do repel each other, they are strongly attracted by all other matter.[ ] . from these three things, the extreme subtilty of the electrical matter, the mutual repulsion of its parts, and the strong attraction between them and other matter, arise this effect, that when a quantity of electrical matter, is applied to a mass of common matter, of any bigness or length within our observation (which has not already got its quantity) it is immediately and equally diffused through the whole. . thus common matter is a kind of spunge to the electrical fluid. and as a spunge would receive no water, if the parts of water were not smaller than the pores of the spunge; and even then but slowly, if there were not a mutual attraction between those parts and the parts of the spunge; and would still imbibe it faster, if the mutual attraction among the parts of the water did not impede, some force being required to separate them; and fastest, if, instead of attraction, there were a mutual repulsion among those parts, which would act in conjunction with the attraction of the spunge. so is the case between the electrical and common matter. . but in common matter there is (generally) as much of the electrical, as it will contain within its substance. if more is added, it lies without upon the surface, and forms what we call an electrical atmosphere: and then the body is said to be electrified. . 'tis supposed, that all kinds of common matter do not attract and retain the electrical, with equal strength and force; for reasons to be given hereafter. and that those called electrics _per se_, as glass, &c. attract and retain it strongest, and contain the greatest quantity. . we know that the electrical fluid is _in_ common matter, because we can pump it _out_ by the globe or tube. we know that common matter has near as much as it can contain, because, when we add a little more to any protion of it, the additional quantity does not enter, but forms an electrical atmosphere. and we know that common matter has not (generally) more than it can contain, otherwise all loose portions of it would repel each other, as they constantly do when they have electric atmospheres. . the beneficial uses of this electrical fluid in the creation, we are not yet well acquainted with, though doubtless such there are, and those very considerable; but we may see some pernicious consequences, that would attend a much greater proportion of it. for had this globe we live on as much of it in proportion, as we can give to a globe of iron, wood, or the like, the particles of dust and other light matters that get loose from it, would, by virtue of their separate electrical atmospheres, not only repel each other, but be repelled from the earth, and not easily be brought to unite with it again; whence our air would continually be more and more clogged with foreign matter, and grow unfit for respiration. this affords another occasion of adoring that wisdom which has made all things by weight and measure! . if a piece of common matter be supposed intirely free from electrical matter, and a single particle of the latter be brought nigh, 'twill be attracted and enter the body, and take place in the center, or where the attraction is every way equal. if more particles enter, they take their places where the balance is equal between the attraction of the common matter and their own mutual repulsion. 'tis supposed they form triangles, whose sides shorten as their number increases; 'till the common matter has drawn in so many, that its whole power of compressing those triangles by attraction, is equal to their whole power of expanding themselves by repulsion; and then will such piece of matter receive no more. . when part of this natural proportion of electrical fluid, is taken out of a piece of common matter, the triangles formed by the remainder, are supposed to widen by the mutual repulsion of the parts, until they occupy the whole piece. . when the quantity of electrical fluid taken from a piece of common matter is restored again, it enters, the expanded triangles being again compressed till there is room for the whole. . to explain this: take two apples, or two balls of wood or other matter, each having its own natural quantity of the electrical fluid. suspend them by silk lines from the ceiling. apply the wire of a well-charged vial, held in your hand, to one of them (a) fig. . and it will receive from the wire a quantity of the electrical fluid; but will not imbibe it, being already full. the fluid therefore will flow round its surface, and form an electrical atmosphere. bring a into contact with b, and half the electrical fluid is communicated, so that each has now an electrical atmosphere, and therefore they repel each other. take away these atmospheres by touching the balls, and leave them in their natural state: then, having fixed a stick of sealing wax to the middle of the vial to hold it by, apply the wire to a, at the same time the coating touches b. thus will a quantity of the electrical fluid be drawn out of b, and thrown on a. so that a will have a redundance of this fluid, which forms an atmosphere round it, and b an exactly equal deficiency. now bring these balls again into contact, and the electrical atmosphere will not be divided between a and b, into two smaller atmospheres as before; for b will drink up the whole atmosphere of a, and both will be found again in their natural state. . the form of the electrical atmosphere is that of the body it surrounds. this shape may be rendered visible in a still air, by raising a smoke from dry rosin, dropt into a hot tea-spoon under the electrised body, which will be attracted and spread itself equaly on all sides, covering and concealing the body. and this form it takes, because it is attracted by all parts of the surface of the body, tho' it cannot enter the substance already replete. without this attraction it would not remain round the body, but dissipate in the air. . the atmosphere of electrical particles surrounding an electrified sphere, is not more disposed to leave it or more easily drawn off from any one part of the sphere than from another, because it is equally attracted by every part. but that is not the case with bodies of any other figure. from a cube it is more easily drawn at the corners than at the plane sides, and so from the angles of a body of any other form, and still most easily from the angle that is most acute. thus if a body shaped as a, b, c, d, e, in fig. , be electrified, or have an electrical atmosphere communicated to it, and we consider every side as a base on which the particles rest and by which they are attracted, one may see, by imagining a line from a to f, and another from e to g, that the portion of the atmosphere included in f, a, e, g, has the line a, e, for its basis. so the portion of atmosphere included in h, a, b, i, has the line a, b, for its basis. and likewise the portion included in k, b, c, l, has b, c, to rest on; and so on the other side of the figure. now if you would draw off this atmosphere with any blunt smooth body, and approach the middle of the side a, b, you must come very near before the force of your attracter exceeds the force or power with which that side holds its atmosphere. but there is a small portion between i, b, k, that has less of the surface to rest on, and to be attracted by, than the neighbouring portions, while at the same time there is a mutual repulsion between its particles and the particles of those portions, therefore here you can get it with more ease or at a greater distance. between f, a, h, there is a larger portion that has yet a less surface to rest on and to attract it; here therefore you can get it away still more easily. but easiest of all between l, c, m, where the quantity is largest, and the surface to attract and keep it back the least. when you have drawn away one of these angular portions of the fluid, another succeeds in its place, from the nature of fluidity and the mutual repulsion beforementioned; and so the atmosphere continues flowing off at such angle, like a stream, till no more is remaining. the extremities of the portions of atmosphere over these angular parts are likewise at a greater distance from the electrified body, as may be seen by the inspection of the above figure; the point of the atmosphere of the angle c, being much farther from c, than any other part of the atmosphere over the lines c, b, or b, a: and besides the distance arising from the nature of the figure, where the attraction is less, the particles will naturally expand to a greater distance by their mutual repulsion. on these accounts we suppose electrified bodies discharge their atmospheres upon unelectrified bodies more easily and at a greater distance from their angles and points than from their smooth sides.--those points will also discharge into the air, when the body has too great an electrical atmosphere, without bringing any non-electric near, to receive what is thrown off: for the air, though an electric _per se_, yet has always more or less water and other non-electric matters mixed with it; and these attract and receive what is so discharged. . but points have a property, by which they _draw on_ as well as _throw off_ the electrical fluid, at greater distances than blunt bodies can. that is, as the pointed part of an electrified body will discharge the atmosphere of that body, or communicate it farthest to another body, so the point of an unelectrified body, will draw off the electrical atmosphere from an electrified body, farther than a blunter part of the same unelectrified body will do. thus a pin held by the head, and the point presented to an electrified body, will draw off its atmosphere at a foot distance; where if the head were presented instead of the point, no such effect would follow. to understand this, we may consider, that if a person standing on the floor would draw off the electrical atmosphere from an electrified body, an iron crow and a blunt knitting kneedle held alternately in his hand and presented for that purpose, do not draw with different forces in proportion to their different masses. for the man, and what he holds in his hand, be it large or small, are connected with the common mass of unelectrified matter; and the force with which he draws is the same in both cases, it consisting in the different proportion of electricity in the electrified body and that common mass. but the force with which the electrified body retains its atmosphere by attracting it, is proportioned to the surface over which the particles are placed; i.e. four square inches of that surface retain their atmosphere with four times the force that one square inch retains its atmosphere. and as in plucking the hairs from the horse's tail, a degree of strength insufficient to pull away a handful at once, could yet easily strip it hair by hair; so a blunt body presented cannot draw off a number of particles at once, but a pointed one, with no greater force, takes them away easily, particle by particle. . these explanations of the power and operation of points, when they first occurr'd to me, and while they first floated in my mind, appeared perfectly satisfactory; but now i have wrote them, and consider'd them more closely in black and white, i must own i have some doubts about them: yet as i have at present nothing better to offer in their stead, i do not cross them out: for even a bad solution read, and its faults discover'd, has often given rise to a good one in the mind of an ingenious reader. . nor is it of much importance to us, to know the manner in which nature executes her laws; 'tis enough if we know the laws themselves. 'tis of real use to know, that china left in the air unsupported will fall and break; but _how_ it comes to fall, and _why_ it breaks, are matters of speculation. 'tis a pleasure indeed to know them, but we can preserve our china without it. . thus in the present case, to know this power of points, may possibly be of some use to mankind, though we should never be able to explain it. the following experiments, as well as those in my first paper, show this power. i have a large prime conductor made of several thin sheets of fuller's pasteboard form'd into a tube, near feet long and a foot diameter. it is cover'd with _dutch_ emboss'd paper, almost totally gilt. this large metallic surface supports a much greater electrical atmosphere than a rod of iron of times the weight would do. it is suspended by silk lines, and when charg'd will strike at near two inches distance, a pretty hard stroke so as to make one's knuckle ach. let a person standing on the floor present the point of a needle at or more inches distance from it, and while the needle is so presented, the conductor cannot be charged, the point drawing off the fire as fast as it is thrown on by the electrical globe. let it be charged, and then present the point at the same distance, and it will suddenly be discharged. in the dark you may see a light on the point, when the experiment is made. and if the person holding the point stands upon wax, he will be electrified by receiving the fire at that distance. attempt to draw off the electricity with a blunt body, as a bolt of iron round at the end and smooth (a silversmith's iron punch, inch-thick, is what i use) and you must bring it within the distance of three inches before you can do it, and then it is done with a stroke and crack. as the pasteboard tube hangs loose on silk lines, when you approach it with the punch iron, it likewise will move towards the punch, being attracted while it is charged; but if at the same instant a point be presented as before, it retires again, for the point discharges it. take a pair of large brass scales, of two or more feet beam, the cords of the scales being silk. suspend the beam by a packthread from the cieling, so that the bottom of the scales may be about a foot from the floor: the scales will move round in a circle by the untwisting of the packthread. set the iron punch on the end upon the floor, in such a place as that the scales may pass over it in making their circle: then electrify one scale by applying the wire of a charged phial to it. as they move round, you see that scale draw nigher to the floor, and dip more when it comes over the punch; and if that be placed at a proper distance, the scale will snap and discharge its fire into it. but if a needle be stuck on the end of the punch, its point upwards, the scale, instead of drawing nigh to the punch and snapping, discharges its fire silently through the point, and rises higher from the punch. nay, even if the needle be placed upon the floor near the punch, its point upwards, the end of the punch, tho' so much higher than the needle, will not attract the scale and receive its fire, for the needle will get it and convey it away, before it comes nigh enough for the punch to act. and this is constantly observable in these experiments, that the greater quantity of electricity on the pasteboard tube, the farther it strikes or discharges its fire, and the point likewise will draw it off at a still greater distance. now if the fire of electricity and that of lightening be the same, as i have endeavour'd to show at large in a former paper, this pasteboard tube and these scales may represent electrified clouds. if a tube of only feet long will strike and discharge its fire on the punch at two or three inches distance, an electrified cloud of perhaps , acres, may strike and discharge on the earth at a proportionably greater distance. the horizontal motion of the scales over the floor, may represent the motion of the clouds over the earth; and the erect iron punch, a hill or high building; and then we see how electrified clouds passing over hills or high buildings at too great a height to strike, may be attracted lower till within their striking distance. and lastly, if a needle fix'd on the punch with its point upright, or even on the floor below the punch, will draw the fire from the scale silently at a much greater than the striking distance, and so prevent its descending towards the punch; or if in its course it would have come nigh enough to strike, yet being first deprived of its fire it cannot, and the punch is thereby secured from the stroke. i say, if these things are so, may not the knowledge of this power of points be of use to mankind, in preserving houses, churches, ships, &c. from the stroke of lightning, by directing us to fix on the highest parts of those edifices, upright rods of iron made sharp as a needle, and gilt to prevent rusting, and from the foot of those rods a wire down the outside of the building into the ground, or down round one of the shrouds of a ship, and down her side till it reaches the water? would not these pointed rods probably draw the electrical fire silently out of a cloud before it came nigh enough to strike, and thereby secure us from that most sudden and terrible mischief? . to determine the question, whether the clouds that contain lightning are electrified or not, i would propose an experiment to be try'd where it may be done conveniently. on the top of some high tower or steeple, place a kind of sentry-box, (as in fig. .) big enough to contain a man and an electrical stand. from the middle of the stand, let an iron rod rise and pass bending out of the door, and then upright or feet, pointed very sharp at the end. if the electrical stand be kept clean and dry, a man standing on it when such clouds are passing low, might be electrified and afford sparks, the rod drawing fire to him from a cloud. if any danger to the man should be apprehended (though i think there would be none) let him stand on the floor of his box, and now and then bring near to the rod, the loop of a wire that has one end fastened to the leads, he holding it by a wax handle; so the sparks, if the rod is electrified, will strike from the rod to the wire, and not affect him. . before i leave this subject of lightning, i may mention some other similarities between the effects of that, and these of electricity. lightning has often been known to strike people blind. a pigeon that we struck dead to appearance by the electrical shock, recovering life, droop'd about the yard several days, eat nothing though crumbs were thrown to it, but declined and died. we did not think of its being deprived of sight; but afterwards a pullet struck dead in like manner, being recovered by repeatedly blowing into its lungs, when set down on the floor, ran headlong against the wall, and on examination appeared perfectly blind. hence we concluded that the pigeon also had been absolutely blinded by the shock. the biggest animal we have yet killed or try'd to kill with the electrical stroke, was a well-grown pullet. . reading in the ingenious dr. _hales_'s account of the thunder storm at _stretham_, the effect of the lightning in stripping off all the paint that had covered a gilt moulding of a pannel of wainscot, without hurting the rest of the paint, i had a mind to lay a coat of paint over the filleting of gold on the cover of a book, and try the effect of a strong electrical flash sent through that gold from a charged sheet of glass. but having no paint at hand, i pasted a narrow strip of paper over it; and when dry, sent the flash through the gilding; by which the paper was torn off from end to end, with such force, that it was broke in several places, and in others brought away part of the grain of the turky-leather in which it was bound; and convinced me, that had it been painted, the paint would have been stript off in the same manner with that on the wainscot at _stretham_. . lightning melts metals, and i hinted in my paper on that subject, that i suspected it to be a cold fusion; i do not mean a fusion by force of cold, but a fusion without heat. we have also melted gold, silver, and copper, in small quantities, by the electrical flash. the manner is this: take leaf gold, leaf silver, or leaf gilt copper, commonly called leaf brass or _dutch_ gold: cut off from the leaf long narrow strips the breadth of a straw. place one of these strips between two strips of smooth glass that are about the width of your finger. if one strip of gold, the length of the leaf, be not long enough for the glass, add another to the end of it, so that you may have a little part hanging out loose at each end of the glass. bind the pieces of glass together from end to end with strong silk thread; then place it so as to be part of an electrical circle, (the ends of gold hanging out being of use to join with the other parts of the circle) and send the flash through it, from a large electrified jar or sheet of glass. then if your strips of glass remain whole, you will see that the gold is missing in several places, and instead of it a metallic stain on both the glasses; the stains on the upper and under glass exactly similar in the minutest stroke, as may be seen by holding them to the light; the metal appeared to have been not only melted, but even vitrified, or otherwise so driven into the pores of the glass, as to be protected by it from the action of the strongest _aqua fortis_ and _ag: regia_. i send you enclosed two little pieces of glass with these metallic stains upon them, which cannot be removed without taking part of the glass with them. sometimes the stain spreads a little wider than the breadth of the leaf, and looks brighter at the edge, as by inspecting closely you may observe in these. sometimes the glass breaks to pieces: once the upper glass broke into a thousand pieces, looking like coarse salt. these pieces i send you, were stain'd with _dutch_ gold. true gold makes a darker stain, somewhat reddish; silver, a greenish stain. we once took two pieces of thick looking-glass, as broad as a gunter's scale, and inches long; and placing leaf gold between them, put them betwixt two smoothly plain'd pieces of wood, and fix'd them tight in a book-binder's small press; yet though they were so closely confined, the force of the electrical shock shivered the glass into many pieces. the gold was melted and stain'd into the glass as usual. the circumstances of the breaking of the glass differ much in making the experiment, and sometimes it does not break at all: but this is constant, that the stains in the upper and under pieces are exact counterparts of each other. and though i have taken up the pieces of glass between my fingers immediately after this melting, i never could perceive the least warmth in them. . in one of my former papers, i mention'd, that gilding on a book, though at first it communicated the shock perfectly well, yet fail'd after a few experiments, which we could not account for. we have since found, that one strong shock breaks the continuity of the gold in the filleting, and makes it look rather like dust of gold, abundance of its parts being broken and driven off; and it will seldom conduct above one strong shock. perhaps this may be the reason; when there is not a perfect continuity in the circle, the fire must leap over the vacancies; there is a certain distance which it is able to leap over according to its strength; if a number of small vacancies, though each be very minute, taken together exceed that distance, it cannot leap over them, and so the shock is prevented. . from the before mentioned law of electricity, that points, as they are more or less acute, draw on and throw off the electrical fluid with more or less power, and at greater or less distances, and in larger or smaller quantities in the same time, we may see how to account for the situation of the leaf of gold suspended between two plates, the upper one continually electrified, the under one in a person's hand standing on the floor. when the upper plate is electrified, the leaf is attracted and raised towards it, and would fly to that plate were it not for its own points. the corner that happens to be uppermost when the leaf is rising, being a sharp point, from the extream thinness of the gold, draws and receives at a distance a sufficient quantity of the electrical fluid to give itself an electrical atmosphere, by which its progress to the upper plate is stopt, and it begins to be repelled from that plate, and would be driven back to the under plate, but that its lowest corner is likewise a point, and throws off or discharges the overplus of the leaf's atmosphere, as fast as the upper corner draws it on. were these two points perfectly equal in acuteness, the leaf would take place exactly in the middle space, for its weight is a trifle, compared to the power acting on it: but it is generally nearest the unelectrified plate, because, when the leaf is offered to the electrified plate at a distance, the sharpest point is commonly first affected and raised towards it; so that point, from its greater acuteness, receiving the fluid faster than its opposite can discharge it at equal distances, it retires from the electrified plate, and draws nearer to the unelectrified plate, till it comes to a distance where the discharge can be exactly equal to the receipt, the latter being lessened, and the former encreased; and there it remains as long as the globe continues to supply fresh electrical matter. this will appear plain, when the difference of acuteness in the corners is made very great. cut a piece of _dutch_ gold (which is fittest for these experiments on account of its greater strength) into the form of fig. the upper corner a right angle, the two next obtuse angles, and the lowest a very acute one; and bring this on your plate under the electrified plate, in such a manner as that the right-angled part may be first raised (which is done by covering the acute part with the hollow of your hand) and you will see this leaf take place much nearer to the upper than to the under plate; because, without being nearer, it cannot receive so fast at its right-angled point, as it can discharge at its acute one. turn this leaf with the acute part uppermost, and then it takes place nearest the unelectrified plate, because otherwise it receives faster at its acute point than it can discharge at its right-angled one. thus the difference of distance is always proportioned to the difference of acuteness. take care in cutting your leaf to leave no little ragged particles on the edges, which sometimes form points where you would not have them. you may make this figure so acute below and blunt above, as to need no under plate, it discharging fast enough into the air. when it is made narrower, as the figure between the pricked lines, we call it the _golden fish_, from its manner of acting. for if you take it by the tail, and hold it at a foot or greater horizontal distance from the prime conductor, it will, when let go, fly to it with a brisk but wavering motion, like that of an eel through the water; it will then take place under the prime conductor, at perhaps a quarter or half an inch distance, and keep a continual shaking of its tail like a fish, so that it seems animated. turn its tail towards the prime conductor, and then it flies to your finger, and seems to nibble it. and if you hold a plate under it at six or eight inches distance, and cease turning the globe, when the electrical atmosphere of the conductor grows small, it will descend to the plate and swim back again several times with the same fish-like motion, greatly to the entertainment of spectators. by a little practice in blunting or sharpening the heads or tails of these figures, you may make them take place as desired, nearer, or farther from the electrified plate. . it is said in section , of this paper, that all kinds of common matter are supposed not to attract the electrical fluid with equal strength; and that those called electrics _per se_, as glass, &c. attract and retain it strongest, and contain the greatest quantity. this latter position may seem a paradox to some, being contrary to the hitherto received opinion; and therefore i shall now endeavour to explain it. . in order to this, let it first be considered, _that we cannot, by any means we are yet acquainted with, force the electrical fluid thro' glass_. i know it is commonly thought that it easily pervades glass, and the experiment of a feather suspended by a thread in a bottle hermetically sealed, yet moved by bringing a nibbed tube near the outside of the bottle, is alledged to prove it. but, if the electrical fluid so easily pervades glass, how does the vial become _charged_ (as we term it) when we hold it in our hands? would not the fire thrown in by the wire pass through to our hands, and so escape into the floor? would not the bottle in that case be left just as we found it, uncharged, as we know a metal bottle so attempted to be charged would be? indeed, if there be the least crack, the minutest solution of continuity in the glass, though it remains so tight that nothing else we know of will pass, yet the extremely subtile electrical fluid flies through such a crack with the greatest freedom, and such a bottle we know can never be charged: what then makes the difference between such a bottle and one that is sound, but this, that the fluid can pass through the one, and not through the other?[ ] . it is true there is an experiment that at first sight would be apt to satisfy a slight observer, that the fire thrown into the bottle by the wire, does really pass thro' the glass. it is this: place the bottle on a glass stand, under the prime conductor; suspend a bullet by a chain from the prime conductor, till it comes within a quarter of an inch right over the wire of the bottle; place your knuckle on the glass stand, at just the same distance from the coating of the bottle, as the bullet is from its wire. now let the globe be turned, and you see a spark strike from the bullet to the wire of the bottle, and the same instant you see and feel an exactly equal spark striking from the coating on your knuckle, and so on spark for spark. this looks as if the whole received by the bottle was again discharged from it. and yet the bottle by this means is charged![ ] and therefore the fire that thus leaves the bottle, though the same in quantity, cannot be the very same fire that entered at the wire; for if it were, the bottle would remain uncharged. . if the fire that so leaves the bottle be not the same that is thrown in through the wire, it must be fire that subsisted in the bottle, (that is, in the glass of the bottle) before the operation began. . if so, there must be a great quantity in glass, because a great quantity is thus discharged even from very thin glass. . that this electrical fluid or fire is strongly attracted by glass, we know from the quickness and violence with which it is resumed by the part that had been deprived of it, when there is an opportunity. and by this, that we cannot from a mass of glass draw a quantity of electrical fire, or electrify the whole mass _minus_, as we can a mass of metal. we cannot lessen or increase its whole quantity, for the quantity it has it holds; and it has as much as it can hold. its pores are filled with it as full as the mutual repellency of the particles will admit; and what is already in, refuses, or strongly repels, any additional quantity. nor have we any way of moving the electrical fluid in glass, but one; that is, by covering part of the two surfaces of thin glass with non-electrics, and then throwing an additional quantity of this fluid on one surface, which spreading in the non-electric, and being bound by it to that surface, acts by its repelling force on the particles of the electrical fluid contained in the other surface, and drives them out of the glass into the non-electric on that side, from whence they are discharged, and then those added on the charged side can enter. but when this is done, there is no more in the glass, nor less than before, just as much having left it on one side as it received on the other. [illustration] . i feel a want of terms here, and doubt much whether i shall be able to make this part intelligible. by the word _surface_, in this case, i do not mean mere length and breadth without thickness; but when i speak of the upper or under surface of a piece of glass, the outer or inner surface of the vial, i mean length, breadth, and half the thickness, and beg the favour of being so understood. now, i suppose, that glass in its first principles, and in the furnace, has no more of this electrical fluid than other common matter: that when it is blown, as it cools, and the particles of common fire leave it, its pores become a vacuum: that the component parts of glass are extremely small and fine, i guess from its never showing a rough face when it breaks, but always a polish; and from the smallness of its particles i suppose the pores between them must be exceeding small, which is the reason that aqua-fortis, nor any other menstruum we have, can enter to separate them and dissolve the substance; nor is any fluid we know of, fine enough to enter, except common fire, and the electrical fluid. now the departing fire leaving a vacuum, as aforesaid, between these pores, which air nor water are fine enough to enter and fill, the electrical fluid (which is every where ready in what we call the non-electrics, and in the non-electric mixtures that are in the air,) is attracted in: yet does not become fixed with the substance of the glass, but subsists there as water in a porous stone, retained only by the attraction of the fixed parts, itself still loose and a fluid. but i suppose farther, that in the cooling of the glass, its texture becomes closest in the middle, and forms a kind of partition, in which the pores are so narrow, that the particles of the electrical fluid, which enter both surfaces at the same time, cannot go through, or pass and repass from one surface to the other, and so mix together; yet, though the particles of electrical fluid, imbibed by each surface, cannot themselves pass through to those of the other, their repellency can, and by this means they act on one another. the particles of the electrical fluid have a mutual repellency, but by the power of attraction in the glass they are condensed or forced nearer to each other. when the glass has received and, by its attraction, forced closer together so much of this electrified fluid, as that the power of attracting and condensing in the one, is equal to the power of expansion in the other, it can imbibe no more, and that remains its constant whole quantity; but each surface would receive more, if the repellency of what is in the opposite surface did not resist its entrance. the quantities of this fluid in each surface being equal, their repelling action on each other is equal; and therefore those of one surface cannot drive out those of the other: but, if a greater quantity is forced into one surface than the glass would naturally draw in; this increases the repelling power on that side, and overpowering the attraction on the other, drives out part of the fluid that had been imbibed by that surface, if there be any non-electric ready to receive it: such there is in all cases where glass is electrified to give a shock. the surface that has been thus emptied by having its electrical fluid driven out, resumes again an equal quantity with violence, as soon as the glass has an opportunity to discharge that over-quantity more than it could retain by attraction in its other surface, by the additional repellency of which the vacuum had been occasioned. for experiments favouring (if i may not say confirming) this hypothesis, i must, to avoid repetition, beg leave to refer you back to what is said of the electrical phial in my former papers. . let us now see how it will account for several other appearances.--glass, a body extremely elastic (and perhaps its elasticity may be owing in some degree to the subsisting of so great a quantity of this repelling fluid in its pores) must, when rubbed, have its rubbed surface somewhat stretched, or its solid parts drawn a little farther asunder, so that the vacancies in which the electrical fluid resides, become larger, affording room for more of that fluid, which is immediately attracted into it from the cushion or hand rubbing, they being supply'd from the common stock. but the instant the parts of the glass so open'd and fill'd have pass'd the friction, they close again, and force the additional quantity out upon the surface, where it must rest till that part comes round to the cushion again, unless some non electric (as the prime conductor) first presents to receive it.[ ] but if the inside of the globe be lined with a non-electric, the additional repellency of the electrical fluid, thus collected by friction on the rubb'd part of the globe's outer surface, drives an equal quantity out of the inner surface into that non-electric lining, which receiving it, and carrying it away from the rubb'd part into the common mass, through the axis of the globe and frame of the machine, the new collected electrical fluid can enter and remain in the outer surface, and none of it (or a very little) will be received by the prime conductor. as this charg'd part of the globe comes round to the cushion again, the outer surface delivers its overplus fire into the cushion, the opposite inner surface receiving at the same time an equal quantity from the floor. every electrician knows that a globe wet within will afford little or no fire, but the reason has not before been attempted to be given, that i know of. . so if a tube lined with a [ ]non-electric, be rubb'd, little or no fire is obtained from it. what is collected from the hand in the downward rubbing stroke, entering the pores of the glass, and driving an equal quantity out of the inner surface into the non-electric lining: and the hand in passing up to take a second stroke, takes out again what had been thrown into the outer surface, and then the inner surface receives back again what it had given to the non-electric lining. thus the particles of electrical fluid belonging to the inside surface go in and out of their pores every stroke given to the tube. put a wire into the tube, the inward end in contact with the non-electric lining, so it will represent the _leyden_ bottle. let a second person touch the wire while you rub, and the fire driven out of the inward surface when you give the stroke, will pass through him into the common mass, and return through him when the inner surface resumes its quantity, and therefore this new kind of _leyden_ bottle cannot so be charged. but thus it may: after every stroke, before you pass your hand up to make another, let the second person apply his finger to the wire, take the spark, and then withdraw his finger; and so on till he has drawn a number of sparks; thus will the inner surface be exhausted, and the outer surface charged; then wrap a sheet of gilt paper close round the outer surface, and grasping it in your hand you may receive a shock by applying the finger of the other hand to the wire: for now the vacant pores in the inner surface resume their quantity, and the overcharg'd pores in the outer surface discharge that overplus; the equilibrium being restored through your body, which could not be restored through the glass.[ ] if the tube be exhausted of air, a non electric lining in contact with the wire is not necessary; for _in vacuo_, the electrical fire will fly freely from the inner surface, without a non-electric conductor: but air resists its motion; for being itself an electric _per se_, it does not attract it, having already its quantity. so the air never draws off an electric atmosphere from any body, but in proportion to the non-electrics mix'd with it: it rather keeps such an atmosphere confin'd, which from the mutual repulsion of its particles, tends to dissipation, and would immediately dissipate _in vacuo_.--and thus the experiment of the feather inclosed in a glass vessel hermetically sealed, but moving on the approach of the rubbed tube, is explained: when an additional quantity of the electrical fluid is applied to the side of the vessel by the atmosphere of the tube, a quantity is repelled and driven out of the inner surface of that side into the vessel, and there affects the feather, returning again into its pores, when the tube with its atmosphere is withdrawn; not that the particles of that atmosphere did themselves pass through the glass to the feather.----and every other appearance i have yet seen, in which glass and electricity are concern'd, are, i think, explain'd with equal ease by the same hypothesis. yet, perhaps, it may not be a true one, and i shall be obliged to him that affords me a better. . thus i take the difference between non electrics and glass, an electric _per se_, to consist in these two particulars. st, that a non-electric easily suffers a change in the quantity of the electrical fluid it contains. you may lessen its whole quantity by drawing out a part, which the whole body will again resume; but of glass you can only lessen the quantity contain'd in one of its surfaces; and not that, but by supplying an equal quantity at the same time to the other surface; so that the whole glass may always have the same quantity in the two surfaces, their two different quantities being added together. and this can only be done in glass that is thin; beyond a certain thickness we have yet no power that can make this change. and, dly, that the electrical fire freely removes from place to place, in and through the substance of a non-electric, but not so through the substance of glass. if you offer a quantity to one end of a long rod of metal, it receives it, and when it enters, every particle that was before in the rod, pushes its neighbour quite to the further end, where the overplus is discharg'd; and this instantaneously where the rod is part of the circle in the experiment of the shock. but glass, from the smallness of its pores, or stronger attraction of what it contains, refuses to admit so free a motion; a glass rod will not conduct a shock, nor will the thinnest glass suffer any particle entring one of its surfaces to pass thro' to the other. . hence we see the impossibility of success, in the experiments propos'd, to draw out the effluvial virtues of a non-electric, as cinnamon for instance, and mixing them with the electrical fluid, to convey them with that into the body, by including it in the globe, and then applying friction, etc. for though the effluvia of cinnamon, and the electrical fluid should mix within the globe, they would never come out together through the pores of the glass, and so go to the prime conductor; for the electrical fluid itself cannot come through; and the prime conductor is always supply'd from the cushion, and that from the floor. and besides, when the globe is filled with cinnamon, or other non-electric, no electrical fluid can be obtain'd from its outer surface, for the reason before-mentioned. i have try'd another way, which i thought more likely to obtain a mixture of the electrical and other effluvia together, if such a mixture had been possible. i placed a glass plate under my cushion, to cut off the communication between the cushion and floor; then brought a small chain from the cushion into a glass of oil of turpentine, and carried another chain from the oil of turpentine to the floor, taking care that the chain from the cushion to the glass touch'd no part of the frame of the machine. another chain was fix'd to the prime conductor, and held in the hand of a person to be electrised. the ends of the two chains in the glass were near an inch distant from each other, the oil of turpentine between. now the globe being turn'd, could draw no fire from the floor through the machine, the communication that way being cut off by the thick glass plate under the cushion: it must then draw it through the chains whose ends were dipt in the oil of turpentine. and as the oil of turpentine being an electric _per se_, would not conduct what came up from the floor, was obliged to jump from the end of one chain, to the end of the other, through the substance of that oil, which we could see in large sparks; and so it had a fair opportunity of seizing some of the finest particles of the oil in its passage, and carrying them off with it: but no such effect followed, nor could i perceive the least difference in the smell of the electrical effluvia thus collected, from what it has when collected otherwise; nor does it otherwise affect the body of a person electrised. i likewise put into a phial, instead of water, a strong purgative liquid, and then charged the phial, and took repeated shocks from it, in which case every particle of the electrical fluid must, before it went through my body, have first gone through the liquid when the phial is charging, and returned through it when discharging, yet no other effect followed than if it had been charged with water. i have also smelt the electrical fire when drawn through gold, silver, copper, lead, iron, wood, and the human body, and could perceive no difference; the odour is always the same where the spark does not burn what it strikes; and therefore i imagine it does not take that smell from any quality of the bodies it passes through. and indeed, as that smell so readily leaves the electrical matter, and adheres to the knuckle receiving the sparks, and to other things; i suspect that it never was connected with it, but arises instantaneously from something in the air acted upon by it. for if it was fine enough to come with the electrical fluid through the body of one person, why should it stop on the skin of another? but i shall never have done, if i tell you all my conjectures, thoughts, and imaginations, on the nature and operations of this electrical fluid, and relate the variety of little experiments we have try'd. i have already made this paper too long, for which i must crave pardon, not having now time to make it shorter. i shall only add, that as it has been observed here that spirits will fire by the electrical spark in the summer time, without heating them, when _fahrenheit_'s thermometer is above ; so, when colder, if the operator puts a small flat bottle of spirits in his bosom, or a close pocket, with the spoon, some little time before he uses them, the heat of his body will communicate warmth more than sufficient for the purpose. additional experiment, _proving that the_ leyden bottle _has no more electrical fire in it when charged, than before; nor less when discharged: that in discharging, the fire does not issue from the wire and the coating at the same time, as some have thought, but that the coating always receives what is discharged by the wire, or an equal quantity; the outer surface being always in a negative state of electricity, when the inner surface is in a positive state_. place a thick plate of glass under the rubbing cushion, to cut off the communication of electrical fire from the floor to the cushion; then, if there be no fine points or hairy threads sticking out from the cushion, or from the parts of the machine opposite to the cushion, (of which you must be careful) you can get but a few sparks from the prime conductor, which are all the cushion will part with. hang a phial then on the prime conductor, and it will not charge, tho' you hold it by the coating.----but form a communication by a chain from the coating to the cushion, and the phial will charge. for the globe then draws the electrical fire out of the outside surface of the phial, and forces it, through the prime conductor and wire of the phial, into the inside surface. thus the bottle is charged with its own fire, no other being to be had while the glass plate is under the cushion. hang two cork balls by flaxen threads to the prime conductor; then touch the coating of the bottle, and they will be electrified and recede from each other. for just as much fire as you give the coating, so much is discharged through the wire upon the prime conductor, whence the cork balls receive an electrical atmosphere. but take a wire bent in the form of a c, with a stick of wax fixed to the outside of the curve, to hold it by; and apply one end of this wire to the coating, and the other at the same time to the prime conductor, the phial will be discharged; and if the balls are not electrified before the discharge, neither will they appear to be so after the discharge, for they will not repel each other. now if the fire discharged from the inside surface of the bottle through its wire, remained on the prime conductor, the balls would be electrified and recede from each other. if the phial really exploded at both ends, and discharged fire from both coating and wire, the balls would be _more_ electrified and recede _farther_: for none of the fire can escape, the wax handle preventing. but if the fire, with which the inside surface is surcharged, be so much precisely as is wanted by the outside surface, it will pass round through the wire fixed to the wax handle, restore the equilibrium in the glass, and make no alteration in the state of the prime conductor. accordingly we find, that if the prime conductor be electrified, and the cork balls in a state of repellency before the bottle is charged, they continue so afterwards. if not, they are not electrified by that discharge. corrections _and_ additions _to the_ preceding papers. page , sect. . we since find, that the fire in the bottle is not contained in the non-electric, but _in the glass_. all that is after said of the _top_ and _bottom_ of the bottle, is true of the _inside_ and _outside_ surfaces, and should have been so expressed. _see sect._ , p. . page , line . the equilibrium will soon be restored _but silently_, etc. this must have been a mistake. when the bottle is full charged, the crooked wire cannot well be brought to touch the top and bottom so quick, but that there will be a loud spark; unless the points be sharp, without loops. ibid. line ult. _outside_: add, such moisture continuing up to the cork or wire. page , line . _by candle-light_ etc. from some observations since made, i am inclined to think, that it is not the light, but the smoke or non-electric effluvia from the candle, coal, and red-hot iron, that carry off the electrical fire, being first attracted and then repelled. page , line . _windmil wheels_, &c. we afterwards discovered, that the afflux or efflux of the electrical fire, was not the cause of the motions of those wheels, but various circumstances of attraction and repulsion. page , line . _let_ a _and_ b _stand on wax_, &c. we soon found that it was only necessary for one of them to stand on wax. page . in the title r. _on_. page , line . r. contact, line . confined. page , line . for _stand_ r. _hand_. page , line . _the consequence might perhaps be fatal_, &c. we have found it fatal to small animals, but 'tis not strong enough to kill large ones. the biggest we have killed is a hen. page , line . _ringing of chimes_, &c. this is since done. page , line . _fails after ten or twelve experiments._ this was by a small bottle. and since found to fail after with a large glass. page , sect. , . _spirits must be heated before we can fire them_, &c. we have since fired spirits without heating, when the weather is warm. _finis._ books printed and sold by edward cave, at st. _john's gate_. i. geography reform'd: or, a new system of general geography according to an accurate analysis of the science, augmented with several necessary branches omitted by former authors. in four parts. . of the nature and principles of geography; 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on the use of the trepan; of wounds in the brain, exfoliation of the cranium, cases of pregnant women, faulty anus in new born children, abscesses in the fundament, stones encysted in the bladder, obstructions to the ejaculation of the semen, an inverted eyelid, extraneous bodies retained in the oesophagus, discharged through abscesses; of bronchotomy, gastrotomy, native hare-lips; of the cæsarean operation; a new method of extracting the stone from the bladder, on a cancer of the breast, an elastic truss for hernias, remarkable hernias of the stomach, and through the foramen ovale. of a pulmonary abscess, &c. translated from the original, dedicated to the french king. in two volumes, octavo. price s. iii. a treatise of comets, containing, . an explication of all the various appearances of the late comet, both in its own trajectory and the firmament of fixt stars, to its setting in the sun beams: illustrated with a plan of the earth's and comet's orbits. . the history of comets from the earliest account of those kinds of planets to the present time; wherein the sentiments of the ancient and modern philosophers are occasionally displayed. with remarks on the intentional end of comets, and the nature and design of saturn's ring. the distance, velocity, size, solidity, and other properties of those bodies considered; and the wonderful phænomena of their tails and atmospheres accounted for. illustrated also by a copper-plate. by g. smith. price s. iv. the natural history of mount vesuvius, with the explanation of the various phenomena that usually attend the eruptions of this celebrated volcano. translated from the original italian, composed by the royal academy of sciences at naples, by order of the king of the two sicilies. price s. stitch'd, or s. d. bound. footnotes. [ ] we suppose every particle of sand, moisture, or smoke, being first attracted and then repelled, carries off with it a portion of the electrical fire; but that the same still subsists in those particles, till they communicate it to something else; and that it is never really destroyed.--so when water is thrown on common fire, we do not imagine the element is thereby destroyed or annihilated, but only dispersed, each particle of water carrying off in vapour its portion of the fire, which it had attracted and attached to itself. [ ] our tubes are made here of green glass, or inches long, as big as can be grasped. electricity is so much in vogue, that above one hundred of them have been sold within these four months past. [ ] to charge a bottle commodiously through the coating, place it on a glass stand; form a communication from the prime conductor to the coating, and another from the hook to the wall or floor. when it is charged, remove the latter communication before you take hold of the bottle, otherwise great part of the fire will escape by it. [ ] the river that washes one side of _philadelphia_, as the _delaware_ does the other; both are ornamented with the summer habitations, of the citizens, and the agreeable mansions of the principal people of this colony. [ ] an electrified bumper, is a small thin glass tumbler, near filled with wine, and electrified as the bottle. this when brought to the lips gives a shock, if the party be close shaved, and does not breathe on the liquor. [ ] thunder-gusts are sudden storms of thunder and lightning, which are frequently of short duration, but sometimes produce mischievous effects. [ ] see the ingenious essays on electricity in the transactions, by mr ellicot. [ ] see the first sixteen sections of my former paper, called _farther experiments_, &c. [ ] see § of _farther experiments_, &c. [ ] in the dark the electrical fluid may be seen on the cushion in two semi-circles or half-moons, one on the fore part, the other on the back part of the cushion, just where the globe and cushion separate. in the fore crescent the fire is passing out of the cushion into the glass; in the other it is leaving the glass, and returning into the back part of the cushion. when the prime conductor is apply'd to take it off the glass, the back crescent disappears. [ ] gilt paper, with the gilt face next the glass, does well. [ ] see farther experiments, § . _by the same author._ =fun with magnetism.= a book and complete outfit for _sixty-one experiments_. =fun with electricity.= a book and complete outfit for _sixty experiments_. =fun with puzzles.= a book and complete outfit for _four hundred puzzles_. =fun with soap-bubbles.= a book and complete outfit for _fancy bubbles and films_. =hustle-ball.= an american game. played by means of magic wands and polished balls of steel. =jingo.= the great war game, including =jingo junior=. =how two boys made their own electrical apparatus.= a book containing complete directions for making all kinds of simple apparatus for the study of elementary electricity. =the study of elementary electricity and magnetism by experiment.= this book is designed as a text-book for amateurs, students, and others who wish to take up a systematic course of simple experiments at home or in school. _in preparation._ =things a boy should know about electricity.= this book explains, in simple, straightforward language, many things about electricity; things in which the american boy is intensely interested; things he wants to know; things he should know. _ask your toy dealer, stationer, or bookseller for our books, games, puzzles, educational amusements, etc._ thomas m. st. john, west st st., new york. the study of elementary electricity and magnetism by experiment containing two hundred experiments performed with simple, home-made apparatus by thomas m. st. john, met. e. author of "fun with magnetism," "fun with electricity," "how two boys made their own electrical apparatus," etc. [illustration: logo] new york thomas m. st. john west st street copyright, , by thomas m. st. john. to the student. this book is designed as a text-book for amateurs, students, and others who wish to take up a systematic course of elementary electrical experiments at home or in school. the student is advised to begin at the beginning, to perform the experiments in the order given, and to understand each step before proceeding. certain principles and explanations necessarily precede the practical and perhaps more interesting applications of those principles. in selecting the apparatus for the experiments in this book, the author has kept constantly in mind the fact that the average student will not buy the expensive pieces usually described in text-books. the two hundred experiments given can be performed with simple, inexpensive apparatus; in fact, the student should make at least a part of his own apparatus. for the benefit of those who wish to make their own apparatus, the author has given, throughout the work, explanations that will aid in the construction of certain pieces especially adapted to these experiments. for those who have the author's "how two boys made their own electrical apparatus," constant references have been made to it as the "apparatus book," as this contains full details for making almost all kinds of simple apparatus needed in "the study of elementary electricity and magnetism by experiment." thomas m. st. john. _new york, april, ._ the study of elementary electricity and magnetism by experiment part i--magnetism part ii--static electricity part iii--current electricity the study of elementary electricity and magnetism by experiment. table of contents. part i.--magnetism. page. chapter i. =iron and steel= introduction.--kinds of iron and steel.--exp. , to study steel.--discussion.--exp. , to find whether a piece of hard steel can be made softer.--annealing.--exp. , to find whether a piece of annealed steel can be hardened.--hardening; tempering.--exp. , to test the hardening properties of soft iron.--discussion. chapter ii. =magnets= kinds of magnets.--exp. , to study the horseshoe magnet.--poles; equator.--exp. , to ascertain the nature of substances attracted by a magnet.--magnetic bodies; diamagnetic bodies.--practical uses of magnets.--exp. , to study the action of magnetism through various substances.--magnetic transparency; magnetic screens.--exp. , to find whether a magnet can give magnetism to a piece of steel.--discussion; bar magnets.--exp. , to make small magnets.--exp. , to find whether a freely-swinging bar magnet tends to point in any particular direction.--north-seeking poles; south-seeking poles; pointing power.--the magnetic needle; the compass.--exp. , to study the action of magnets upon each other.--exp. , to study the action of a magnet upon soft iron.--laws of attraction and repulsion.--exp. , to learn how to produce a desired pole at a given end of a piece of steel.--rule for poles.--our compass.--review; magnetic problems.--exp. , to find whether the poles of a magnet can be reversed.--discussion; reversal of poles.--exp. , to find whether we can make a magnet with two n poles.--exp. , to study the bar magnet with two n poles.--discussion; consequent poles.--exp. , to study consequent poles. exp. , to study the theory of magnetism.--theory of magnetism; magnetic saturation.--exp. , to find whether soft iron will permanently retain magnetism.--retentivity or coercive force; residual magnetism.--exp. , to test the retentivity of soft steel.--discussion.--exp. , to test the retentivity of hard steel.--exp. , to test the effect of heat upon a magnet.--discussion.--exp. , to test the effect of breaking a magnet.--discussion. chapter iii. =induced magnetism= exp. , to find whether we can magnetize a piece of iron without touching it with a magnet.--temporary magnetism; induced magnetism.--exp. , to find whether a piece of steel can be permanently magnetized by induction.--exp. , to study the inductive action of a magnet upon a piece of soft iron.--polarization; pole pieces.--exps. - , to study pole pieces. chapter iv. =the magnetic field= exp. , to study the space around the magnet, in which pieces of iron become temporary magnets by induction.--discussion; the magnetic field.--exp. , to study the magnetic field of a bar magnet.--magnetic figures; lines of magnetic force.--exps. - , to study the magnetic fields of various combinations of bar magnets.--exps. - , to study the lifting power of combinations of bar magnets.--discussion; compound magnets.--exps. - , to study the magnetic field of the horseshoe magnet.--discussion; resistance to lines of force.--exp. , to show that lines of force are on all sides of a magnet.--discussion.--exp. , to study a horseshoe magnet with movable poles.--discussion; advantages of horseshoe magnets. chapter v. =terrestrial magnetism= the magnetism of the earth.--declination.--exp. , to study the lines of force above and below a bar magnet placed horizontally.--the dip or inclination of the magnetic needle.--exp. , to study the dip or inclination of the magnetic needle due to the action of the earth.--discussion; balancing magnetic needles.--exps. - , to study the inductive influence of the earth.--discussion.--natural magnets.--exp. , to test the effect of twisting a wire held north and south in the earth's magnetic field.--exp. , to test for magnetism in bars of iron, tools, etc. part ii.--static electricity. chapter vi. =electrification= some varieties of electricity.--exp. - , to study electrification by friction.--discussion.--electrified and neutral bodies.--force; resistance; work; potential energy; electrification.--heat and electrification.--exps. - , to study electrical attraction.--discussion.--exp. , to study mutual attractions.--mutual attractions.--exps. - , to study electrical repulsions.--the carbon electroscope.--discussion of experiments , , .--exp. , to study the electrification of glass.--questions.--exp. , to compare the electrification produced by ebonite and flannel with that produced by glass and silk.--discussion.--laws. chapter vii. =insulators and conductors= exps. - , to study insulators.--conductors.--exp. , to study conduction.--discussion.--exp. , to study conduction.--telegraph line using static electricity.--discussion.--relation between conductors and insulators.--electrics and non-electrics.--exp. , to study the effect of moisture upon an insulator.--discussion.--exp. , to test the effects of moisture upon bodies to be electrified. chapter viii. =charging and discharging conductors= the electrophorus.--exp. , to learn how to use the electrophorus.--exp. , to study "charging by conduction."--exp. , to study potential; electromotive force.--pressure; potential; electromotive force; current, spark.--theories about electrifications.--exp. , to study some methods of discharging an electrified body.--disruptive, conductive and convective discharges.--exp. , to study intermittent or step-by-step discharges.--discussion.--exp. , to ascertain the location of the charge upon an electrified conductor.--discussion.--hollow and solid conductors.--exp. , to study the effect of points upon a charged conductor.--electric density.--electric wind. chapter ix. =induced electrification= electric fields; lines of force.--exp. , to study electric induction.--electric polarization; theory of induction.--exp. , to learn how to charge a body by induction.--free and bound electrifications.--exp. , to show that a neutral body is polarized before it is attracted by a charged one.--polarization precedes attraction.--exp. , to find whether electric induction will act through an insulator.--dielectrics.--exp. , to find whether a polarized conductor can act inductively upon another conductor.--successive induction.--inductive capacity.--exp. , to study the action of the electrophorus.--discussion.--details of action.--exp. , to see, hear, and feel the results of inductive influence and polarization.--discussion. chapter x. =condensation of electrification= exp. , to find whether a large surface will hold more electrification than a small one.--electrical capacity.--exp. , to find whether the capacity of a given conductor can be increased without increasing its size.--condensation; condensers.--the leyden jar.--fulminating panes.--induction coil condensers.--submarine cables.--exp. , to study the condensation of electrification.--discussion.--exp. , to study the action of the condenser.--discussion.--exp. , to study the effect of electrical discharges upon the human body.--shocks; dischargers.--exps. - , to show the strong attraction between opposite electrifications in the condenser.--discussion.--exp. , to show how the condenser may be slowly discharged.--the electric chime.--exp. , to ascertain the location of a charge in a condenser.--discussion.--exp. , to find whether any electrification remains in the condenser after it has once been discharged.--residual charge.--exp. , to study successive condensation; the chime cascade.--discussion. chapter xi. =electroscopes= electroscopes.--our leaf electroscope.--exp. , to study the leaf electroscope; charging by conduction.--discussion.--exp. , to learn some uses of the electroscope.--discussion.--the proof-plane. chapter xii. =miscellaneous experiments= exp. , to show that friction always produces two kinds of electrification.--discussion.--exp. , to show "successive sparks."--exp. , to show to the eye the strong attraction between a charged and a neutral body.--exp. , to feel the strong attraction between a charged and a neutral body.--exp. , the human body a frictional electric machine.--static electric machines. chapter xiii. =atmospheric electricity= atmospheric electricity.--lightning.--thunder.--lightning rods.--causes of atmospheric electricity.--st. elmo's fire.--aurora borealis. part iii.--current electricity. chapter xiv. construction and use of apparatus exp. , to study the effect of the electric current upon the magnetic needle.--electrical connections.--current detectors.--exp. , to study the construction and use of a simple "key."--exp. , to study the construction and use of a simple "current reverser."--exp. , to study the simple current detector.--exp. , to study the construction and use of the simple galvanoscope.--discussion; true readings.--exp. , to study the construction and use of a simple astatic needle.--astatic needles.--exp. , to study the construction and use of a simple astatic galvanoscope.--astatic galvanoscopes. chapter xv. galvanic cells and batteries exp. , to study the effect of dilute sulphuric acid upon carbon and various metals.--to amalgamate.--dilute sulphuric acid.--discussion.--exp. , to study the effect of dilute sulphuric acid upon various combinations of metals.--discussion.--exp. , to study the construction of a simple voltaic or galvanic cell.--the electric current.--source of the electrification.--the electric circuit; open and closed circuits.--plates or elements.--direction of current.--poles or electrodes.--chemical action in the simple galvanic cell.--action in cell using impure zinc; action using pure zinc.--exp. , to see what is meant by "local currents" in the cell.--local action; local currents.--reasons for amalgamating zinc plates.--exp. , to study the "single-fluid" galvanic cell.--the simple cell.--polarization of cells.--effects of polarization.--remedies for polarization; depolarizers.--exp. , to study the "two-fluid" galvanic cell.--setting up the two-fluid cell.--care of two-fluid cell.--copper sulphate solution.--chemical action in the two-fluid cell.--various galvanic cells; open and closed circuit cells.--the leclanché cell--dry cells.--the bichromate of potash cell.--the daniell cell.--the gravity cell. chapter xvi. the electric circuit exp. , to see what is meant by "divided circuits" and "shunts."--divided circuits; shunts.--exp. , to see what is meant by "short circuits." chapter xvii. electromotive force electromotive force.--unit of e. m. f.; the volt.--exp. , to see whether the e. m. f. of a cell depends upon the materials used in its construction.--discussion.--electromotive series.--exp. , to see whether the e. m. f. of a cell depends upon its size.--discussion. chapter xviii. electrical resistance resistance.--exp. , to study the general effect of "resistance" upon a current.--external resistance; internal resistance; unit of resistance; the ohm.--resistance coils; resistance boxes.--simple resistance coil.--exp. , to test the power of various substances to conduct galvanic electricity.--conductors and nonconductors.--exp. , to find the effect of sulphuric acid upon the conductivity of water.--internal resistance.--exp. , to find what effect the length of a wire has upon its electrical resistance.--discussion.--exp. , to find what effect the size (area of cross-section) of a wire has upon its electrical resistance.--discussion.--exp. , to compare the resistance of a divided circuit with the resistance of one of its branches. discussion.--exp. , to study the effect of decreasing the resistance in one branch of a divided circuit.--current in divided circuits. chapter xix. measurement of resistance exp. , to study the construction and use of a simple wheatstone's bridge.--the simple bridge.--equipotential points.--example.--exp. , to measure the resistance of a wire by means of wheatstone's bridge; the "bridge method."--allowances for connections.--exps. - , to measure the resistances of various wires, coils, etc., by the "bridge method."--table.--exp. , to study the effect of heat upon the resistance of metals.--effect of heat upon resistance.--exp. , to measure the resistance of a wire by the "method of substitution."--simple rheostat.--exp. , to measure the e. m. f. of a cell by comparison with the two-fluid cell.--exp. , to measure the internal resistance of a cell by the "method of opposition." chapter xx. current strength strength of current.--unit of current strength; the ampere.--measurement of current strength.--the tangent galvanometer.--the ammeter.--the voltameter.--unit of quantity; the coulomb.--electrical horse-power; the watt.--ohm's law.--internal resistance and current strength.--exp. , having a cell with large plates, to find how the strength of the current is affected by changes in the position of the plates, the external resistance being small.--exp. , same as exp. , but with small plates.--exp. , to find whether the changes in current strength, due to changes in internal resistance, are as great when the external resistance is large, as they are when the external resistance is small.--discussion, with examples.--arrangement of cells and current strength.--cells in series.--cells abreast.--exp. , to find the best way to join two similar cells when the external resistance is small.--exp. , to find the best way to join two similar cells when the external resistance is large.--best arrangement of cells. chapter xxi. chemical effects of the electric current chemical action and electricity.--electrolysis.--exp. , to study the electrolysis of water.--composition of water.--electromotive force of polarization.--exp. , to coat iron with copper.--exp. , to study the electrolysis of a solution of copper sulphate.--electroplating.--exp. , to study the chemistry of electroplating.--discussion.--electrotyping.--voltameters.--exp. , to study the construction and action of a simple "storage" cell.--secondary or storage cells. chapter xxii. electromagnetism electromagnetism.--exp. , to study the lines of force about a straight wire carrying a current.--ampere's rule.--lines of force about parallel wires.--exp. , to study the lines of force about a coil of wire like that upon the galvanoscope.--exp. , to study the magnetic field about a small coil of wire.--coils.--polarity of coils.--exp. , to test the attracting and "sucking" power of a magnetized coil or helix.--exp. , to find whether a piece of steel can be permanently magnetized by an electric current.--exp. , to study the effect of a piece of iron placed inside of a magnetized coil of wire. chapter xxiii. electromagnets electromagnets.--cores of electromagnets.--exps. - , to study straight electromagnets; lifting power; residual magnetism of core; magnetic tick; magnetic figures; magnetic field.--horseshoe electromagnets.--use of yoke.--experimental magnets.--method of joining coils.--exps. - , to study horseshoe electromagnets; to test the poles; to study the inductive action of one core upon the other; magnetic figures; permanent magnetic figures; lifting power; residual magnetism when magnetic circuit is closed.--closed magnetic circuits. chapter xxiv. thermoelectricity exp. , to find whether electricity can be produced by heat.--home-made thermopile.--thermoelectricity.--peltier effect.--thermopiles. chapter xxv. induced currents electromagnetic induction.--exp. , to find whether a current can be generated with a bar magnet and a hollow coil of wire.--discussion.--induced currents and work.--exp. , to find whether a current can be generated with a bar magnet and a coil of wire having an iron core.--exp. , to find whether a current can be generated with a horseshoe magnet and a coil of wire having an iron core.--induced currents and lines of force.--exp. , to find whether a current can be generated with an electromagnet and a hollow coil of wire.--exp. , to find whether a current can be generated with an electromagnet and a coil of wire having an iron core.--discussion of exps. - .--exp. , to study the effect of starting or stopping a current near a coil of wire or other closed circuit.--exp. , to study the effect of starting or stopping a current in a coil placed inside of another coil.--discussion of exps. - .--direction of induced current.--laws of induction.--primary and secondary currents.--exp. , to see what is meant by alternating currents.--direct and alternating currents.--self-induction; extra currents. chapter xxvi. the production of motion by currents currents and motion.--exp. , motion produced with a hollow coil and a piece of iron.--exp. , motion with hollow coil and bar magnet.--exp. , motion with electromagnet and piece of iron.--exp. , motion with electromagnet and bar magnet.--exp. , motion with electromagnet and horseshoe magnet.--exp. , motion with two electromagnets.--discussion of exps. - .--exp. , rotary motion with a hollow coil of wire and a permanent magnet.--exp. , rotary motion with an electromagnet and a permanent magnet.--discussion of exps. - . chapter xxvii. applications of electricity things electricity can do.--exp. , to study the action of a simple telegraph sounder.--discussion.--telegraph line; connections.--operation of line.--exp. , to study the action of the "relay" on telegraph lines.--the relay.--exp. , to study the action of a two-pole telegraph instrument.--exp. , to study the action of a simple "single needle telegraph instrument."--exp. , to study the action of a simple automatic contact breaker, or current interrupter.--automatic current interrupters.--exp. , to study the action of a simple electric bell, or a "buzzer."--electric bells and buzzers.--exp. , to study the action of a simple telegraph "recorder."--exp. , to study the action of a simple "annunciator."--discussion.--exp. , to study the shocking effects of the "extra current." induction coils.--action of induction coils.--transformers.--the dynamo.--the electric motor.--exp. , to study the action of the telephone.--the telephone.--the bell, or magneto-transmitter.--the receiver.--the carbon transmitter.--induction coils in telephone work.--electric lighting and heating.--arc lamps.--the incandescent lamp. chapter xxviii. wire tables apparatus list index magnetism a few dont's for young students. don't fail to make at least a part of your own apparatus; there is a great deal of satisfaction and pleasure in home-made apparatus. don't experiment in all parts of the house, if working at home. fit up a small room for your den, and carry the key. don't begin an experiment before you really know what you are trying to do. read the directions carefully, then begin. don't rush through an experiment to see what happens at the end of it. see what happens at each step, and notice every little thing that seems unusual. don't try to do all parts of an experiment at the same time. understand one part, then proceed. don't fail to ask yourself questions, and form an opinion about the results of an experiment before you read what the author has to say about it. don't fail to keep a note-book. keep all the data and arithmetical work for future reference. don't leave the apparatus around after you have finished the day's work. part i.--magnetism. chapter i. iron and steel. _= . introduction.=_ we should know something about iron and steel at the start, because we are to use them in nearly every experiment. the success with some of the experiments will depend largely upon the quality of the iron and steel used. when we buy a piece of iron from the blacksmith, we get more than iron for our money. hidden in this iron are other substances (carbon, phosphorus, silicon, etc.), which are called "impurities" by the chemist. if all the impurities were taken out of the iron, however, we should have nothing but a powder left; this the chemist would call "chemically pure iron," but it would be of no value whatever to the blacksmith or mechanic. the impurities in iron and steel are just what are needed to hold the particles of iron together, and to make them valuable. by regulating the amount of carbon, phosphorus, etc., manufacturers can make different grades and qualities of iron or steel. when carbon is united with the _pure_ iron, we get what is commonly called iron. _= . kinds of iron and steel.=_ _cast iron_ is the most impure form of iron. stoves, large kettles, flatirons, etc., are made of cast iron. _wrought iron_ is the purest form of commercial iron. it usually comes in bars or rods. blacksmiths hammer these into shapes to use on wagons, machinery, etc. _steel_ contains more carbon than wrought iron, and less than cast iron. _soft steel_ is very much like wrought iron in appearance, and it is used like wrought iron. _hard steel_ has more carbon in it than soft steel. tools, needles, etc., are made of this. =experiment . to study steel.= _apparatus._ a steel sewing-needle (no. ).[a] [footnote a: _=note. each piece of apparatus used in the following experiments has a number. see "apparatus list" at the back of this book for details. the numbers given under "apparatus," in each experiment, refer to this list.=_] = . directions.= (a) bend a sewing-needle until it breaks. is the steel brittle? (b) if you have a file, test the hardness of the needle. _= . discussion.=_ "needle steel" is usually of good quality. it will be very useful in many experiments. do you know how to make the needle softer? =experiment . to find whether a piece of hard steel can be made softer.= [illustration: fig. .] _apparatus._ fig. . a needle; a cork, ck (no. ); lighted candle (no. ). the bottom of the candle should be warmed and stuck to a pasteboard base. = . directions.= (a) stick the point of the needle into ck, fig. , then hold the needle in the flame until it is red-hot. (the upper part of the flame is the hottest.) (b) allow the needle to cool in the air. (c) test the brittleness of the steel by bending it. test its hardness with a file (exp. ). _= . annealing.=_ this process of softening steel by first heating it and then allowing it to cool slowly, is called _annealing_. all pieces of iron and steel are, of course, hard; but you have learned that some pieces are much harder than others. =experiment . to find whether a piece of annealed steel can be hardened.= _apparatus._ the needle just annealed and bent; cork, etc., of exp. ; a glass of cold water. = . directions.= (a) heat the bent portion of the needle in the candle flame (exp. ) until it is red-hot, then immediately plunge the needle into the water. (b) test its brittleness and hardness, as in exp. . _= . hardening; tempering.=_ good steel is a very valuable material; the same piece may be made hard or soft at will. by sudden cooling, the steel becomes very hard. this process is called _hardening_, but it makes the steel too brittle for many purposes. by _tempering_ is meant the "letting down" of the steel from the very hard state to any desired degree of hardness. this may be done by suddenly cooling the steel when at the right temperature, it not being hot enough to produce extreme hardness. (the approximate temperature of hot steel can be told by the colors which form on a clean surface. these are due to oxides which form as the steel gradually rises in temperature.) =experiment . to test the hardening properties of soft iron.= _apparatus._ a piece of soft iron wire about in. ( . cm.) long (no. ); the candle, water, etc., of exp. . = . directions.= (a) test the wire by bending and filing. (b) heat the wire in the candle flame as you did the needle (fig. ), then cool it suddenly with the water. study the results. _= . discussion.=_ soft iron contains much less carbon than steel. the hardening quality which steel has is due to the proper amount of carbon in it. if you have performed the experiments so far, you will be much more able to understand later ones, and you will see why we are obliged to use soft iron for some parts of electrical apparatus, and hard steel for other parts. chapter ii. magnets. _= . kinds of magnets.=_ among the varieties of magnets which we shall discuss, are the natural, artificial, temporary, permanent, bar, horseshoe, compound, and electro-magnet. [illustration: fig. .] _the horseshoe magnet_, h m (fig. ), is the most popular form of small magnets. the red paint has nothing to do with the magnetism. the piece, a, is called its _armature_, and is made of soft iron, while the magnet itself should be made of the best steel, properly hardened. the armature should always be in place when the magnet is not in use, and care should be taken to thoroughly clean the ends of the magnet before replacing the armature. the horseshoe magnet is _artificial_, and it is called a _permanent_ magnet, because it retains its strength for a long time, if properly cared for. =experiment . to study the horseshoe magnet.= _apparatus._ fig. . the horseshoe magnet, h m (no. ). = . directions.= (a) remove the armature, a, from the magnet, then move a about upon h m to see ( ) if the curved part of h m has any attraction for a, and ( ) to see if there is any attraction for a at points between the curve and the extreme ends of h m. _= . poles; equator.=_ the ends of a magnet are called its _poles_. the end marked with a line, or an n, should be the _north_ pole. the unmarked end is the _south_ pole. n and s are abbreviations for north and south. the central part, at which there _seems_ to be no magnetism, is called the _neutral point_ or _equator_. =experiment . to ascertain the nature of substances attracted by a magnet.= _apparatus._ the horseshoe magnet, h m (fig. ); silver, copper, and nickel coins; iron filings (no. ), nails, tacks, pins, needles; pieces of brass, lead, copper, tin, etc. (ordinary tin is really sheet iron covered with tin.) use the various battery plates for the different metals. = . directions.= (a) try the effect of h m upon the above substances, and upon any other substances thought of. _= . magnetic bodies; diamagnetic bodies.=_ substances which are attracted by a magnet are said to be _magnetic_. a piece of soft iron wire is magnetic, although not a magnet. very strong magnets show that nickel, oxygen, and a few other substances not containing iron, are also magnetic. some elements are actually repelled by a powerful magnet; these are called _diamagnetic_ bodies. it is thought that all bodies are more or less affected by a magnet. _= . practical uses of magnets.=_ many practical uses are made of magnets, such as the automatic picking out of small pieces of iron from grain before it is ground into flour, and the separation of iron from other metals, etc. the most important uses of magnets are in the compass and in connection with the electric current, as in machines like dynamos and motors. (see experiments with electro-magnets.) =experiment . to study the action of magnetism through various substances.= _apparatus._ horseshoe magnet, h m; a sheet of stiff paper; pieces of sheet glass, iron, zinc, copper, lead, thin wood, etc.; sewing-needle. (a tin box may be used for the iron, and battery plates for the other metals.) = . directions.= (a) place the needle upon the paper and move h m about immediately under it. (b) in place of the paper, try wood, glass, etc. (c) invent an experiment to show that magnetism will act through your hand. (d) invent an experiment to show that magnetism will act through water. _= . magnetic transparency; magnetic screens.=_ substances, like paper, are said to be _transparent_ to magnetism. iron does not allow magnetism to pass through it as readily as paper and glass; in fact, thick iron may act as a _magnetic screen_. =experiment . to find whether a magnet can give magnetism to a piece of steel.= = . note.= you have seen that the horseshoe magnet can lift nails, iron filings, etc.; you have used this lifting power to show that the magnet was really a magnet, and not merely an ordinary piece of iron painted red. can we give some of its magnetism to another piece of steel? can we pass the magnetism along from one piece of steel to another? _apparatus._ the horseshoe magnet, h m; two sewing-needles that have never been near a magnet; iron filings. = . directions.= (a) test the needles for magnetism with the iron filings, and be sure that they are not magnetized. (b) remove the armature, a, from h m, then touch the point of one of the needles to one pole of h m. (c) lay h m aside, and test the point of the needle for magnetism. (d) if you find that the needle is magnetized, rub its point upon the point of the other needle; then test the point of the second needle for magnetism. _= . discussion; bar magnets.=_ a piece of good steel will attract iron after merely touching a magnet. to thoroughly magnetize it, however, a mere touch is not sufficient. there are several ways of making magnets, depending upon the size, shape, and strength desired. for these experiments, the student needs only a good horseshoe magnet, or, better still, the electro-magnets described later; with these any number of small magnets may be made. straight magnets are called _bar magnets_. =experiment . to make small magnets.= _apparatus._ fig. . the horseshoe magnet, h m; sewing-needles; iron filings. (see apparatus book, pg. , for various kinds of steel suitable for small magnets.) = . directions.= (a) hold h m (fig. ) in the left hand, its poles being uppermost. grasp the point of the needle with the right hand, and place its point upon the n or marked pole of h m. (b) pull the needle along in the direction of its length (see the arrow), continuing the motion until its head is at least an inch from the pole. (c) raise the needle at least an inch above h m, lower it to its former position (fig. ), and repeat the operation or times. do not slide the needle back and forth upon the pole, and be careful not to let it accidentally touch the s pole of h m. (d) test the needle for magnetism with iron filings, and save it for the next experiment. [illustration: fig. .] [illustration: fig. .] =experiment . to find whether a freely-swinging bar magnet tends to point in any particular direction.= _apparatus._ fig. . a magnetized sewing-needle (exp. ); the flat cork, ck (no. ); a dish of water. (you can use a tumbler, but a larger dish is better.) = . note.= an oily sewing-needle may be floated without the cork by carefully lowering it to the surface of the water. all magnets, pieces of iron and steel, knives, etc., should be removed from the table when trying such experiments. why? = . directions.= (a) place the little bar magnet (the needle) upon the floating cork, turn it in various positions, and note the result. _= . north-seeking poles; south-seeking poles; pointing power.=_ it should be noted that the _point_ swings to the north, provided the needle is magnetized as directed in exp. . this is called the north, or north-seeking pole. the n-seeking pole is sometimes called the marked pole. for convenience, we shall hereafter speak of the n-seeking pole as the n pole, and of the s-seeking pole as the s pole. we shall hereafter speak of the tendency which a bar magnet has to point n and s, as its _pointing power_. an unmagnetized needle has no pointing power. _= . the magnetic needle; the compass.=_ a small bar magnet, supported upon a pivot, or suspended so that it may freely turn, is called a _magnetic needle_. when balanced upon a pivot having under it a graduated circle marked n, e, s, w, etc., it is called a _compass_. these have been used for centuries. (see apparatus book for home-made magnetic needles.) =experiment . to study the action of magnets upon each other.= _apparatus._ two magnetized sewing-needles (magnetized as in exp. ); the cork, etc., of exp. . = . directions.= (a) float each little bar magnet (needles) separately to locate the n poles. (b) leave one magnet upon the cork, and with the hand bring the n pole of the other magnet immediately over the n pole of the floating one. note the result. (c) try the effect of two s poles upon each other. (d) what is the result when a n pole of one is brought near a s pole of the other? =experiment . to study the action of a magnet upon soft iron.= _apparatus._ a magnetized sewing-needle; cork, etc., of exp. ; a piece of soft iron wire, in. long; iron filings. = . directions.= (a) test the wire for magnetism with filings. be sure that it is not magnetized. if it shows any magnetism, twist it thoroughly before using. (exp. .) (b) float the magnetized needle (exp. ), then bring the end of the wire near one pole of the needle and then near the other pole. (c) place the wire upon the cork, hold the needle in the hand and experiment. _= . laws of attraction and repulsion.=_ from experiments and are derived these laws: (_= =_) _=like poles repel each other=_; (_= =_) _=unlike poles attract each other=_; (_= =_) _=either pole attracts and is attracted by unmagnetized iron or steel.=_ the attraction between a magnet and a piece of iron or steel is mutual. attraction, alone, simply indicates that at least one of the bodies is magnetized; repulsion proves that both are magnetized. =experiment . to learn how to produce a desired pole at a given end of a piece of steel.= _apparatus._ same as in exp. . = . directions.= (a) magnetize a sewing-needle (exp. ) by rubbing it upon the n pole of h m from _point to head_. float it and locate its n pole. (b) take another needle that has not been magnetized, and rub it on the same pole (n) from _head to point_. locate its n pole. (c) magnetize another needle by rubbing it from _point to head_ upon the s pole of h m; locate its n pole. can you now determine, beforehand, how the poles of the needle magnet will be arranged? _= . rule for poles.=_ the end of a piece of steel which last touches a n pole of a magnet, for example, becomes a s pole. _= . our compass=_ (no. ). while the floating magnetic needle described in exp. , and shown in fig. , does very well, it will be found more convenient to use a compass whenever poles of pieces of steel are to be tested. fig. shows merely the cover of the box which serves as a base for the magnetic needle furnished. we shall hereafter speak of this apparatus as _our compass_, o c. (see apparatus book, chap. vii, for various forms of home-made magnetic needles and compasses.) = . review; magnetic problems.= to be sure that you understand and remember what was learned in exp. , do these problems: . using the s pole of the horseshoe magnet, magnetize a needle so that its head will become a n pole. test with floating cork, as in exp. . . using the n pole of the horseshoe magnet, magnetize a needle so that its head shall be a s pole. test. . magnetize two needles, one on the n and one on the s pole of the horseshoe magnet, in such a way that the two points will repel each other. test. if the student cannot do these little problems at once, and test the results satisfactorily to himself, he should study the previous experiments again before proceeding. [illustration: fig. .] [illustration: fig. .] =experiment . to find whether the poles of a magnet can be reversed.= _apparatus._ fig. . the horseshoe magnet, h m; a thin wire nail, w n, in. ( cm.) long; a piece of stiff paper, cut as shown, to hold w n; thread with which to suspend the paper; compass, o c (no. ). = . directions.= (a) magnetize w n so that its point shall be a s pole. test with o c to make sure that you are right. (b) swing w n in the paper (fig. ), then _slowly_ bring the s pole of h m near its point. note result. (c) _quickly_ bring the s pole of h m near the point. is w n still repelled? has its s pole been reversed? _= . discussion; reversal of poles.=_ the poles of weak magnets may be easily reversed. this often occurs when the apparatus is mixed together. it is always best, before beginning an experiment, to remagnetize the pieces of steel which have already served as magnets. the same may be shown by magnetizing a needle, rubbing it first in one direction, and then in another upon the magnet, testing, in each case, the poles produced. =experiment . to find whether we can make a magnet with two n poles.= _apparatus._ the horseshoe magnet, h m; an unmagnetized sewing-needle; compass, o c (no. ). = . note.= you have already learned that the polarity of a weak magnet can be changed (exp. ). can you think of any method by which _two n poles_ can be made in one piece of steel? = . directions.= (a) place the needle upon h m, as in fig. . (b) keeping the part, c, in contact with the n pole of h m, and using the n pole of h m as a pivot, turn the needle end for end so that its head will be in contact with the s pole of h m. (c) pull the needle straight from h m, being careful not to slide it in either direction. (d) test the polarity of the ends with o c (fig. ), and save it for the next experiment. [illustration: fig. .] [illustration: fig. .] =experiment . to study the bar magnet with two n poles.= _apparatus._ the strange magnet just made (exp. ); iron filings; compass, o c (no. ). = . directions.= (a) sprinkle filings over the whole length of the needle and then raise it (fig. ). (b) break the needle at its center, and test, with o c, the two new ends produced at that point. remember that repulsion is the test for polarity. _= . discussion; consequent poles.=_ iron filings cling to a magnet where poles are located. in this case, two small magnets were made in one piece of steel; they had a common s pole at the center. the pointing power (§ ) of such a magnet is very slight; would it have _any_ pointing power if we could make the end poles of equal strength? intermediate poles, like those in the needle just discussed, are called _consequent poles_. practical uses are made of consequent poles in the construction of motors and dynamos. =experiment . to study consequent poles.= _apparatus._ an unmagnetized sewing-needle; horseshoe magnet, h m (no. ); iron filings (no. ); compass (no. ). = . directions.= (a) let _w_, _x_, _y_, and _z_ stand for four places along the body of the needle, _w_ being at its point and _z_ at its head. (b) touch _w_ with the n pole of h m, _x_ with the s pole, _y_ with the n pole, and _z_ with the s pole. do not slide h m along on the needle, just _touch_ the needle as directed. (c) cover the needle with filings, then lift it. =experiment . to study the theory of magnetism.= _apparatus._ a thin bar magnet, b m (no. ); iron filings; a sheet of paper. fig. shows simply the edge of b m and the paper. b m should be magnetized as directed in exp. . [illustration: fig. .] = . directions.= (a) sprinkle some iron filings upon a sheet of paper. (b) bring one pole of b m in contact with the filings (fig. ), and lightly sweep it through them several times, always in the same direction. are the filings _simply_ pushed about? (c) do the same with a stick, and compare the result with that produced with b m. _= . theory of magnetism; magnetic saturation.=_ this bringing into line the particles of iron indicates that each particle became a magnet. this experiment should aid in understanding what is thought to take place when steel is magnetized. the pile of filings represents the body to be magnetized, and each little filing stands for a particle of that body. a bar of steel is composed of extremely small particles, called _molecules_. they are very close together and do not move from place to place as easily as the pieces of filings. a magnet, however, when properly rubbed upon the steel, seems to have power to make the molecules point in the same direction. this produces an effect upon the whole bar. each molecule of the steel is supposed to be a magnet. when these little magnets pull together, the whole bar becomes a strong magnet. when a magnet is jarred, and the little magnetized molecules are mixed again, they pull in all sorts of directions upon each other. this lessens the attraction for outside bodies. steel is said to be _saturated_, when it contains as much magnetism as possible. a piece of steel becomes slightly longer when magnetized. it is thought, by many, that there is a current of electricity around each molecule, making a little magnet of it. (see electro-magnets.) =experiment . to find whether soft iron will permanently retain magnetism.= _apparatus._ a piece of soft iron wire, or in. ( . to cm.) long (no. ); the horseshoe magnet, h m; iron filings; flat cork, f c (no. ), and the dish of water used in exp. (fig. ). = . directions.= (a) magnetize the wire (exp. ). notice that the wire clings strongly to h m. (b) test the lifting power of the little wire magnet by seeing about how many iron filings its poles will raise. (c) test the pointing power (§ ) of the wire by floating it on f c (fig. ). (d) holding one end of the wire in the hand, thoroughly jar it by striking the other end several times against a hard surface. (e) test the lifting and pointing powers, as in b and c. _= . retentivity or coercive force; residual magnetism.=_ soft iron loses _most_ of its magnetism when simply removed beyond the action of a magnet. we say that it does not retain magnetism; that is, it has very little _retentivity or coercive force_. this is an important fact, the action of many electric machines and instruments depending upon it. a slight amount of magnetism remains, however, in the softest iron, after removing it from a magnet. this is called _residual magnetism_. a piece of iron may show poles, when tested with the compass, although it may have almost no pointing power. =experiment . to test the retentivity of soft steel.= _apparatus._ a wire nail, w n (no. ); horseshoe magnet, h m; iron filings; flat cork, f c; the dish of water (exp. , fig. ). = . directions.= (a) with h m magnetize the nail; this is made of soft steel. (b) test the lifting and pointing powers of w n (exp. ). (c) strike w n several times with a hammer to jar it. (d) again test its lifting and pointing powers. _= . discussion.=_ soft steel has a greater retentivity than soft iron. it contains less carbon than cast or tool steel, and is called mild steel or machinery steel. you do not want soft steel for permanent magnets. =experiment . to test the retentivity of hard steel.= _apparatus._ a hard steel sewing-needle (no. ); other articles used in exp. . = . directions.= (a) magnetize the needle with h m. (b) test its lifting and pointing powers (exp. ). (c) hammer the needle and test again as in (b). =experiment . to test the effect of heat upon a magnet.= _apparatus._ a magnetized sewing-needle; the candle, cork, etc., of exp. . (see fig. .) = . directions.= (a) test the needle for magnetism. (b) stick the needle into the cork (fig. ), and heat it until it is red-hot. (c) test the needle again for magnetism. (d) see if you can again magnetize the needle. _= . discussion.=_ heating a body is supposed to thoroughly stir up its molecules. jarring or twisting a magnet tends to weaken it. (see exp. .) the molecules of steel do not move about or change their relative positions as readily as those of soft iron. when the molecules of hard steel are once arranged, by magnetizing them, for example, they strongly resist any outside influences which tend to mix them up again. a magnet does not attract a piece of red-hot iron. the particles of the hot iron are supposed to vibrate too rapidly to be brought into line; that is, the iron cannot become polarized by induction. (see exp. .) =experiment . to test the effect of breaking a magnet.= _apparatus._ a magnetized sewing-needle; iron filings; compass, o c (no. ). [illustration: fig. .] = . directions.= (a) break the little bar magnet (needle), and test the two new ends produced for magnetism, with the iron filings. (fig. ). (b) touch the two new poles together to see whether they are like or unlike. (c) test the nature of the poles with o c (fig. ) (d) break one of the halves and test its parts. _= . discussion.=_ the above results agree with the theory that each molecule is a magnet (exp. ). no matter into how many pieces a magnet is broken, each part becomes a magnet. (fig. ). this shows that those molecules near the equator of the magnet really have magnetism. their energy, however, is all used upon the adjoining molecules; hence no external bodies are attracted at that point. chapter iii. induced magnetism. [illustration: fig. .] =experiment . to find whether we can magnetize a piece of iron without touching it with a magnet.= _apparatus._ horseshoe magnet, h m; iron filings, i f (fig. ). = . directions.= (a) hold the armature of the magnet in a vertical position (fig. ), its lower end being directly in a little pile of iron filings. (b) bring the n pole of h m near the upper end of a, but do not let them touch each other. (c) keeping a and the pole of h m the same distance apart, lift them. do any filings cling to a? (d) without moving or jarring a, take h m away from it and note result upon the filings. _= . temporary magnetism; induced magnetism.=_ the armature, a, was induced to become a magnet without even touching h m. its magnetism was _temporary_, however, as the filings dropped as soon as the _inductive action_ of h m was removed. a small amount of residual magnetism ( ) remained in a. soft iron is exceedingly valuable, because it has very little retentivity ( ), and because it can be easily _magnetized by induction_. the armature was made of soft iron. it had _induced magnetism_. it was a _temporary magnet_. =experiment . to find whether a piece of steel can be permanently magnetized by induction.= _apparatus._ an unmagnetized sewing-needle; horseshoe magnet, h m; iron filings; sheet of stiff paper. = . directions.= (a) test the needle for magnetism. (b) place the unmagnetized needle upon the paper, then move h m about immediately under it, so that the needle will be attracted. (c) test the needle again for permanent magnetism. [illustration: fig. .] =experiment . to study the inductive action of a magnet upon a piece of soft iron.= _apparatus._ horseshoe magnet, h m; iron filings, i f; a piece of soft iron wire about an inch long, i w (fig. ), placed upon the n pole of h m; compass, o c (no. ), (§ ). = directions.= (a) test the lower end of i w for magnetism with i f. (b) leaving i w upon the n pole of h m, test the pole at the lower end of i w with o c, to determine whether it is n or s. (c) jar i w (exp. ), then place it upon the s pole of h m, and again test the polarity of the lower end. _= . polarization; pole pieces.=_ the wire, i w (fig. ), was acted upon by induction (exp. ) and behaved like a magnet. poles were produced in it, so we say that the wire was _polarized_. pieces of iron, placed upon the poles of a magnet, are called _pole pieces_. it should be noted that the lower end of the wire has a pole _like_ the pole of h m, to which it is attached. =experiments - . to study pole pieces.= _apparatus for experiments - ._ horseshoe magnet, h m; soft iron wires; iron filings, i f. = . directions.= (a) suspend two wires, each about an inch long (fig. ) from one pole of h m. do their lower ends attract or repel each other? [illustration: fig. .] [illustration: fig. .] =experiment .= = . directions.= (a) place the two wires just used so that one shall cling to the n pole of h m, and the other to the s pole of h m (fig. ). (b) bring the lower ends of the wires near each other. do they attract or repel each other? =experiment .= = . directions.= (a) bend a -inch iron wire, as in fig. , and place it upon the poles of h m. (b) see if its central part, marked x, will strongly attract filings. [illustration: fig. .] [illustration: fig. .] =experiment .= = . directions.= (a) bend the wire just used a little more, and place its ends upon _one_ pole of h m (fig. ). (b) see if the iron filings and small wires will cling to its central part. chapter iv. the magnetic field. =experiment . to study the space around a magnet, in which pieces of iron become temporary magnets by induction.= _apparatus._ a bar magnet, b m (no. ); a compass (no. ); a sheet of stiff paper about ft. ( cm.) square, with a center line, c l, drawn parallel to one of its sides (fig. - / ), and with another line, e w, drawn perpendicular to c l. (see apparatus book, chap. vi., for various ways of making home-made permanent magnets.) = . directions.= (a) lay the paper upon the table, and place the compass over the center of the line, c l, previously drawn. (b) place the eye directly over the compass-needle, then turn the paper until the line is n and s; that is, until the line is parallel to the length of the needle. pin the paper to the table to hold its center line n and s. (c) place b m upon the paper, as shown (fig. - / ), its n pole to the north, and its center at the cross line, e w. [illustration: fig. - / .] (d) slowly move the compass entirely around and near b m, and note the various positions taken by the needle. note especially the way in which its n pole points. this is to get a general idea of the action of the needle. (e) place the compass in the position marked , which is on e w, about in. from the line, c l. press the wooden support down firmly upon the paper to show, by the dent made in the paper by the pin-head, the exact place on the paper that is under the center of the compass-needle. before removing the compass from this position, look down upon it again, and make a dot on the paper with a pencil directly under each end of the needle. remove the compass, and draw a line through the dent and the two dots just made. this will show a plan of the exact position of the needle. (f) repeat this for the various points marked , , in. from c l, always marking on the plan the position of the n pole of the needle. do the same with the other points marked on fig. - / by dots, and study the resulting diagram. _= . discussion; the magnetic field.=_ the compass-needle was decidedly affected all around b m (fig. ), showing that induction can take place in a considerable space around a magnet; this space is called the _magnetic field_ of the magnet. let us consider _one_ position taken by the compass-needle in the field of b m (fig. ), as, for example, the one in which the needle has been made black. the s pole of the black needle is attracted by the n pole of b m, and is repelled by the s pole of b m. the n pole of the compass-needle is attracted by the s pole of b m, and is repelled by b m's n pole. the position which it takes, therefore, is due to the action of these forces, together with its tendency to point n and s. [illustration: fig. .] every magnet has a certain magnetic field, with its lines of force passing through the surrounding air in certain definite positions. as soon, however, as a piece of iron or another magnet is brought within the field, the original position of the lines of force is changed. this has to be considered in the construction of electrical machinery. =experiment . to study the magnetic field of a bar magnet.= _apparatus._ a sheet of stiff paper; iron filings, i f; bar magnet, b m (no. ); a sifter for the filings (no. ); (see apparatus book, § , , , for home-made sifters.) = . directions.= (a) place b m upon the table, and lay the paper over it. (b) with the sifter sprinkle some filings upon the paper directly over b m, then tap the paper gently, to assist the particles to take final positions. study the results. _= . magnetic figures; lines of magnetic force.=_ the filings clearly indicated the extent and nature of the magnetic field of b m. you should notice how the filings radiate from the poles, and how they form curves from one pole to the other. they make upon the paper a _magnetic figure_. each particle of the filings becomes a little magnet, by induction (exp. ), and takes a position which depends upon attractions and repulsions, as discussed in exp. . magnetism seems to reach out in lines from the poles of a magnet. the position and direction of some of the lines are shown by the lines of filings. they are very distinct near the poles, and are considered, for convenience, to start from the n pole of a magnet, where they separate. they then pass through the air on all sides of the magnet, and finally enter it again at the s pole. these lines are called _lines of force_ or _lines of magnetic induction_. the poles must not be considered mere points at the ends of a magnet. as shown by magnetic figures, the lines of magnetic induction flow from a considerable portion of the magnet's ends. =experiments - . to study the magnetic fields of various combinations of bar magnets.= _apparatus for exps. - ._ two bar magnets, b m (nos. , ); an iron ring, i r (no. ); iron filings, i f; a sheet of stiff paper; the sifter (no. ). = . note.= the student will find it very helpful to make the magnetic figures of the combinations given. thoroughly magnetize the bar magnets upon an electro-magnet, or upon a strong horseshoe magnet, and mark their n poles in some way. the n poles may be marked by sticking a small piece of paper to them. = . directions.= (a) arrange the two magnets, b m, as in fig. , with their unlike poles about an inch apart. (the dotted circle indicates the iron ring to be used in the _next_ experiment. about a quarter, only, of the magnets are shown.) (b) place the paper over the magnets, and sift filings upon it immediately over the unlike poles. note particularly the lines of filings between n and s. (c) make a sketch of the result. (see experiments with electromagnets, and the illustrations of magnetic figures with them.) =experiment .= = . directions.= (a) leaving the opposite poles an inch apart, as in exp. , place the iron ring, i r (no. ), between them (fig. , dotted circles). (b) place the paper over it all, and sprinkle filings upon it to get the magnetic figure. (c) make a sketch of the resulting figure, and compare it with the figure made in exp. . why do the lines of force appear indistinct in the center of the ring and around it? (see § .) [illustration: fig. .] [illustration: fig. .] =experiment .= = . directions.= (a) arrange the two bar magnets, as in exp. , but with their two n poles an inch apart. (b) make the magnetic figure of the combination. do the lines of force flow from one n pole directly to the n pole of the other? do the particles of filings reaching out from one b m attract or repel those from the other b m? =experiment .= = . directions.= (a) place the two bar magnets side by side, so that their unlike poles shall be arranged as in fig. . (b) make the magnetic figure. =experiment .= = . directions.= (a) turn one b m end for end, so that their like poles shall be near each other, but otherwise arranged as in fig. . (b) make and study the magnetic figure. =experiments - . to study the lifting power of combinations of bar magnets.= _apparatus for exps. - ._ two bar magnets, b m (no. , ), of about equal strength; iron filings, i f. = . directions.= (a) find out about how many filings you can lift with the n pole of one magnet. (b) place the two magnets together (fig. ), their _like_ poles being in contact; then see whether the two n poles will lift more or less filings than one pole. [illustration: fig. .] =experiment .= = . directions.= (a) remove all filings from the two magnets just used, and hold them tightly together (fig. ), with their _unlike_ poles in contact. (b) compare the amount of filings you can lift at one end of this combination with that lifted in exp. (a) and (b). _= . discussion; compound magnets.=_ many lines of force pass into the air from two like poles. such a combination is called a _compound magnet_. a piece of thin steel can be magnetized more strongly in proportion to its weight than a thick piece, because the magnetism does not seem to penetrate beyond a certain distance into the steel. thin steel may be magnetized practically through and through. a thick magnet has but a crust of magnetized molecules; in fact, a thick magnet may be greatly weakened by eating the outside crust away with acid. by riveting several thin bar or horseshoe magnets together, thick permanent magnets of considerable strength are made. _= .=_ lines of force, in passing from the n to the s pole of a magnet, meet a resistance in the air, which does not carry or conduct them as easily as iron or steel. in the arrangement of exp. the lines of force are not obliged to push their way through the air, as each magnet serves as a return conductor for the lines of force of the other. either magnet may be considered an armature for the other. to show in another way that few lines of force pass into the air, the student may lay the above combination upon the table and make a magnetic figure. (see apparatus book, p. , for method of making home-made compound magnets.) in the case where a ring was placed between the poles of two bar magnets (exp. ), the lines of force from the n pole jumped across the first air-space. they then disappeared in the body of the ring, until they were obliged to jump across the second air-space, to get to the s pole. the weakness of the field in the central space was clearly shown by the filings. there were no stray lines of force passing through the air, because it was easier for them to go through the iron ring. this will be discussed again under "dynamos and motors." (see also § .) =experiments - . to study the magnetic field of the horseshoe magnet.= _apparatus for exps. - ._ horseshoe magnet, h m; iron filings, i f; sheet of stiff paper. = . directions.= (a) place h m, with its armature removed, flat upon the table, and cover it with the paper; then make the magnetic figure. (exp. .) (b) compare the number of well-defined curves at the poles with the number at the equator. =experiment .= = . directions.= (a) make the magnetic figure of h m with its armature in place. (b) is the attraction for outside bodies increased or decreased by placing the armature on h m? =experiment .= = . directions.= (a) lay h m flat upon the table, and place one or two matches between its poles and the armature; cover with paper as before, and make the magnetic figure. do lines of force still pass through the armature? _= . discussion; resistance to lines of force.=_ it is evident, from the last experiments, that lines of force will pass through iron whenever possible, on their way from the n to the s pole of a magnet. when the armature of a horseshoe magnet is in place, most of the lines of magnetic induction crowd together and pass through it rather than push their way through the air. air is not a good conductor of lines of force; and the magnet has to do work to overcome the resistance of the air, when the armature is removed, in order to complete the magnetic circuit. this work causes a magnet to become gradually weaker. the soft iron armature is an excellent conductor of lines of force; it completes the magnetic circuit so perfectly that very little work is left for the magnet to do. =experiment . to show that lines of force are on all sides of a magnet.= _apparatus._ our compass, o c (no. ); horseshoe magnet, h m; glass tumbler, g t; sheet of stiff paper; iron filings, i f. arrange as in fig. . h m may be supported in a vertical position by placing paper, or a handkerchief, under it. the poles should just touch the stiff paper placed over the tumbler. [illustration: fig. .] = . directions.= (a) sprinkle iron filings upon the paper, and study the resulting magnetic figure. (b) place o c upon the paper in different positions. does the magnetic needle always come to rest about parallel to the lines of filings? _= . discussion.=_ the student should keep in mind the fact that the filings in the magnetic figure show the approximate extent and form of the magnetic field simply in one plane. if the paper were held in some other position near the magnet (in a tilted position, for example,) the lines of filings would not be the same as those produced in exp. - . the lines of force come out of every side of the n pole. when a magnetic needle is placed in any magnetic field, its n pole points in the direction in which the lines of force are passing; that is, it points towards the s pole of the magnet producing the field. =experiment . to study a horseshoe magnet with movable poles.= _apparatus._ a narrow strip of spring steel, s s (no. ); iron filings, i f. = . directions.= (a) magnetize the spring steel, s s. (b) bend s s until its poles are about / in. apart, then using it as a horseshoe magnet, and keeping its poles the same distance apart, see about how many filings you can lift. (c) clean the poles of s s, press them tightly together, then again test its lifting power with filings. [illustration: fig. .] _= . discussion; advantages of horseshoe magnets.=_ when the opposite poles of the flexible magnet are pressed together, the lines of force do not have to pass through the air; there is very little attraction for outside bodies. the same effect is produced with the armature (exp. ). a horseshoe magnet has a strong attraction for its armature, because it has a _double power to induce and to attract_. suppose the n pole of a bar magnet, b m (fig. ), be placed near one end of a piece of iron, as, for example, the armature, a. a will become a temporary magnet by induction (exp. ). the s pole of a, polarized by induction, will be attracted by b m, while its n pole will be repelled by b m; so, you see, that a bar magnet does not pull to advantage. chapter v. terrestrial magnetism. _= . the magnetism of the earth.=_ the student must have guessed, before this, that the earth acts like a magnet. it causes the magnetic needle to take a certain position at every place upon its surface, and this position depends upon the earth's attractions and repulsions for it. the earth has lines of force which flow from its n magnetic pole, and these lines, before they can get to the earth's s magnetic pole, must spread out through the air on all sides of the earth. as the magnetic needle points to the earth's n magnetic pole (which is more than , miles from its _real_ n pole), it is evident that the compass-needle does not show the _true_ north for all places upon the earth's surface. in fact, the n pole of the needle may point e, w, or even s. this effect would be seen by carrying a compass around the earth's n magnetic pole. [illustration: fig. .] _= . declination.=_ for convenience, we shall represent the true n and s, at the place where you are experimenting, by the full line, n s, in fig. . the dotted line shows the direction taken by the compass-needle. the angle, a, between them, is called the _angle of variation_ or the _declination_. this angle is not the same for all places; and, in fact, it changes slowly at any given place; so it becomes necessary to construct _magnetic maps_ for the use of mariners and others. =experiment . to study the lines of force above and below a bar magnet placed horizontally.= _apparatus._ a bar magnet, b m (no. ); compass, o c (no. ). = . directions.= (a) lay b m upon the table and place o c upon its center. note the position of the compass-needle. (b) slide o c along from one end of b m to the other, and study the effect upon its needle. do lines of force curve _over_ b m as well as around its sides, as shown in exp. ? (c) place o c upon the table. hold b m horizontally above o c, and move o c back and forth under b m. does the needle remain horizontal, or does it show that lines of force pass _under_ b m on their way from its n to its s pole? [illustration: fig. .] _= . the dip or inclination of the magnetic needle.=_ the needle is said to dip when it takes positions like those in fig. . compass-needles should be horizontal, when properly balanced, and entirely free from all effects other than those of the earth. the excessive dip shown (fig. ) is due, of course, to the efforts of the magnetic needle to place itself in the direction in which the lines of force of b m pass. =experiment . to study the dip or inclination of the magnetic needle, due to the action of the earth.= _apparatus._ fig. . our compass, o c (no. ); horseshoe magnet, h m (no. ); piece of paper. = . directions.= (a) place o c upon the table, and mark upon a piece of paper the height of the n pole of its needle above the table. (fig. .) the paper should be held in a vertical position, and near the pole. [illustration: fig. .] (b) with h m reverse the poles of the compass-needle (exp. ), so that its former n pole shall become a s pole. (c) place the needle upon its pivot again, and mark upon the paper, as before, the height of its new n pole above the table. does the needle remain horizontal? (d) remagnetize the needle, and reverse its poles so that it will again balance. [illustration: fig. .] _= . discussion; balancing magnetic needles.=_ if a piece of unmagnetized steel be balanced and then magnetized, it will no longer remain horizontal; it will dip. try this. compass-needles are balanced after they are magnetized. can you now see why the needle did not remain horizontal after its poles were changed? a piece of steel first balanced and then magnetized, has to have its s pole slightly weighted, as suggested by the line at s (fig. x), to make it horizontal. the magnetic needle does not tend to dip at the earth's equator, because the lines of force of the earth are nearly horizontal at the equator. as we pass toward the north or south on the earth, the lines of force slant more and more as they come from or enter the earth's magnetic poles. what position would the needle take if we should hold it directly over the earth's n magnetic pole? fig. shows what the needle does when held near the poles of a bar magnet. =experiments - . to study the inductive influence of the earth.= _apparatus for exps. - ._ compass, o c, (no. ); an iron stove poker, or other rod of iron; a hammer. (the iron and hammer are not furnished.) = . note.= you have seen (exp. ), that iron becomes magnetized by induction when placed near a magnet. as the earth acts like a huge magnet, having poles, lines of force, etc., will it magnetize pieces of iron which are in the air or upon its surface? = . directions.= (a) test the poker for poles with o c, remembering that _repulsion_ is necessary to prove that it is polarized. if the poker has very weak poles, proceed; but if it shows some strength, hold it in an east and west direction, and hit it several sharp blows on the end with the hammer. test for polarity again. (b) with one hand hold the poker in the n and s line, give it a dip toward the north, and strike it several times with the hammer to thoroughly stir up its molecules. (c) test again for poles with o c, and note especially whether the lower end (of the poker) became a n or a s pole. =experiment .= = . directions.= (a) turn the poker end for end (see exp. ); repeat the striking, and test again the pole produced at the lower and north end of it. (b) now hold the poker horizontally in the east and west line, and pound it. (c) test for poles. has this strengthened or weakened the poker magnet? _= . discussion.=_ dipping the poker places it nearly in the same direction as that taken by the earth's lines of force. the magnetic influence of the earth acts to advantage upon the poker, by induction, only when the poker is properly held. it no doubt occurs to the student that the end of a magnetic needle which points to the north is really opposite in nature to the north magnetic pole of the earth. the n pole of a needle, then, must be in reality a s pole to be attracted by the earth's n pole. it has been agreed, for convenience, to call the n-seeking pole of a magnet its n pole. _= . natural magnets.=_ nearly all pieces of iron become more or less magnetized by the inductive action of the earth's magnetism. your poker was slightly magnetized at the start, perhaps, from standing in a dipping position. induction takes place along lines of force. in northern latitudes the earth's lines of force have a dip to the north. you should now see why the greatest effect was produced upon the poker when it, also, was made to dip. parts of machinery, steel frames of bridges and buildings, tools in the shop, and even certain iron ores, become polarized by this inductive action. these might all be called natural magnets. magnetic iron ore, called lodestone, is referred to, however, when speaking of _natural magnets_. lodestone was used thousands of years ago to indicate n and s, and it was discovered, later, that it could impart its power to pieces of steel when the two were rubbed together. =experiment . to test the effect of twisting a wire held north and south in the earth's magnetic field.= _apparatus._ compass, o c (no. ); a piece of soft iron wire, in. ( cm.) long (no. ). bend up about an inch of the wire at each end so that it may be firmly held when twisting it. =note.= you have seen that we can _pound_ magnetism into or out of a piece of iron at will. can we _twist_ it into a wire and out again without the use of magnets? = . directions.= (a) test the wire for poles with o c. (b) hold the wire in a n and s direction, dipping it at the same time, as directed in exp. for the poker, and twist it back and forth. (c) test again for poles with o c. as the poles of the wire may be very weak, bring them _slowly_ toward the compass-needle (see exp. ), and note the _first_ motions produced upon the needle. (d) hold the wire horizontally east and west, twist and test again. has its magnetism become weaker or stronger than before? =experiment . to test for magnetism in bars of iron, tools, etc.= _apparatus._ steel drills; files; chisels; bars or rods of iron that have been standing in an upright position; stove-lid lifters; stove pokers, etc., etc.; a compass. = . directions.= (a) with the compass test the ends of the above for magnetism, and note which ends are s. notes. static electricity part ii.--static electricity chapter vi. electrification. _= . some varieties of electricity.=_ _static electricity_ does not seem to "flow in currents" as readily as some other varieties; its tendency is to stand still, hence the name, static. the simplest way to produce it is by friction. _thermo electricity_ is produced by changes in temperature. when certain combinations of metals become hotter or colder, a current is produced. _voltaic_ or _galvanic electricity_ is produced by chemical action. batteries give this variety. _induced electricity_ is produced by other currents, and by combinations of magnets and moving coils of wire, as in the dynamo. this is, by far, the most important variety of electricity, and the dynamo is the most important producer of it. each of the above varieties of electricity will be studied experimentally with simple apparatus. =experiments - .= to study electrification by friction. _apparatus._ ebonite sheet, e s (no. ); flannel cloth, f c (no. ). see what is said in preface about static electricity. = . directions.= (a) examine e s. note that its surface is not smooth, like that of ordinary hard-rubber combs. can you think of any reason for this? (b) hold its flat surface near your face, then near the back of your hand. do you feel anything unusual? (c) lay e s upon a flat board, or uncovered wooden table, and slide it about. can you easily pick it up? (d) place e s flat upon the table again; keep it from sliding about with your left hand, and rub it _vigorously_ for a _minute_ with f c. does e s act exactly as it did before in (b) and (c)? (e) repeat the experiment in a dark room. (f) thoroughly electrify e s, and see if it will cling to the wall strongly enough to support its own weight. _= . discussion; electrified and neutral bodies.=_ the ebonite sheet became _electrified_ or _charged_; and as the _electrification_ was produced by friction, we may say that the action of the ebonite indicated the presence of _frictional electricity_. no one can tell _just_ why the ebonite acted so queerly, but we can learn a great deal by experimenting. bodies which are not charged are said to be _neutral_. the table, chairs, earth, etc., are neutral. we may consider that a neutral body has been _discharged_. _= . force; resistance; work; potential energy; electrification.=_ it takes _force_ to raise water into a tank placed on the roof. in raising the water, _work_ has to be done, and _we_ have to do the work; but when we once have the water in the tank we have accomplished something. the water has _potential energy_; that is, on account of its high _position_, we can make it do some work by simply turning a stop-cock so that the water can run out and turn a water-wheel, for example. it takes _force_ to vigorously rub a piece of ebonite with a flannel cloth, for _resistance_ has to be _overcome_; that is, _work_ has to be done. several things are accomplished by this work; heat is produced, for we can _feel_ that the ebonite gets warm; we can _hear_ sounds and _see_ sparks. the simple muscular exertion on our part has been changed to heat, light, and sound. the most wonderful part of it all, however, is that we have electrified or charged the ebonite. _we_ did the work at first, and now the ebonite has the power to do something, as you will soon see. _electrification_ is, then, a sort of potential energy. _= . heat and electrification.=_ we say that heat passes to or from a body to make it hot or cold. heat _produces_ the sensation of warmth, but heat isn't warmth. we can force a cold body to become hot; in other words, we can get it into a hot condition in various ways, such as rubbing it, hammering it, or by placing it near or in contact with another hot body. electrification is, also, a condition or state into which we can force a body; but electrification isn't electricity. we know whether a body is hot or cold by its effects upon us, upon thermometers, and upon other bodies. we can tell, also, whether a body is electrified or not by the way it acts, and, in certain cases, by the sound, heat, and light which accompany the electrification. do not get the idea that an electrified body is covered with a layer of electricity just as a board is covered with a layer of paint. [illustration: fig. .] =experiment .= = . directions.= repeat exp. , but in place of the ebonite, use hot tissue-paper, hot brown paper, hot newspaper, or a hot silk handkerchief. rub your hand vigorously over them. do these become charged? =experiments - . to study electrical attractions.= _apparatus._ the ebonite sheet, e s (no. ); flannel cloth, f c (no. ); small pieces of dry tissue-paper, t p (no. ); thread (no. ). = . directions.= (a) thoroughly electrify e s as before, then lift and hold it in the air. (fig. .) (b) see what the paper and thread will do when held loosely near e s. _= . discussion.=_ exp. shows that _an electrified body attracts neutral ones_. this much was known about electricity over , years ago. they didn't have ebonite then, but some of the educated men of greece knew that amber would attract light bodies after being rubbed. the greek word for amber is _elektron_, and from this has been made the word _electricity_. =experiment .= = . directions.= charge a sheet of hot paper by friction; lift it, by its opposite ends, and lower it over small pieces of tissue-paper placed on the table. what happens to the little pieces? =experiment . to study mutual attractions.= _apparatus._ the support and its attachments (see § ); support wire, s w (no. ); silk thread, s t (no. ), or a rubber band, r b (no. ); ebonite rod, e r (no. ); flannel cloth, f c (no. ); wire swing, w s (no. ). tie one end of s t to w s, fig. ; tie the other end of s t to s w; adjust w s by bending, if necessary, so that it will securely hold e r. it will be found convenient to use a rubber band instead of s t; if you do, let w s straddle one end of r b (fig. ), and hang the other end of r b upon s w. = . the support= consists of a support base (s b, fig. ), a support rod (s r, fig. ), and a support wire (s w, fig. ). there is a small hole in one end of s r to receive the wire, s w, and a large hole in the other end to take the short ebonite which holds the insulating table (fig. ). a little paper should be wound around the lower end of s r, so that it will stand solidly in the spool which forms a part of the base. = . directions.= (a) electrify e r with f c, and place e r in the swing, w s (fig. ). [illustration: fig. .] (b) bring your finger near one side of the rubbed end of e r, then near the unrubbed end, and compare the results. = . mutual attractions.= _a neutral body_, like the hand, for example, _attracts electrified ones_. from exp. , , , it is seen that the attraction between a neutral and an electrified body is mutual; each attracts the other. =experiment . to study electrical repulsions.= _apparatus._ same as for exp. ; ebonite sheet, e s (no. ). = . directions.= (a) charge e r, and place it in w s, fig. . (b) charge e s, and bring it slowly near one side of the charged end of e r. =experiment . to study electrical repulsions.= _apparatus._ a sheet of tissue-paper, t p (no. ); shears or a knife. cut t p, as in fig. . each leg should be about / in. wide and or in. long. = . directions.= (a) heat the paper, then place it flat upon the table and electrify it by rubbing it with your hand. you must rub away from the uncut part, or you will break the legs. (b) raise t p, holding it by the uncut part. note the action of legs, and make a sketch of them. [illustration: fig. .] [illustration: fig. .] =experiment . to study electrical repulsions.= _apparatus._ ebonite rod, e r (no. ); a carbon electroscope, c e, fig. (see § ); the support complete (see § ); small piece of damp tissue-paper. _= . the carbon electroscope.=_ light an ordinary match, and let it burn until it is charred through and through. the black substance remaining is _carbon_. this is very light; it has, also, another important property which you will soon understand. tie a small piece of the carbon to one end of a dry _silk_ thread, and fasten the other end of the thread to the support wire, s w, which is fastened to the support (fig. ). we shall call this piece of apparatus the _carbon e-lec-tro-scope_. (see electroscopes, chapter xviii., apparatus book.) = . directions.= (a) electrify e r, then hold it near the carbon of the electroscope. (b) bring the charged rod near little pieces of _damp_ tissue-paper. _= . discussion of experiments , , .=_ in the two pieces of ebonite were made of the same material, and both were rubbed with flannel. they must have been similarly electrified. in , different parts of the same piece of paper were similarly electrified. in , the little piece of carbon took some of the electrification from the charged rod, just as it would take molasses from your finger should your sticky finger touch it. the electrification on the carbon must have been of the same kind as that on the rod. the carbon was _charged by contact_. we learn, then, that _two bodies repel each other when they have the same kind of electrification_. do two charged bodies _always_ repel each other? is it possible that there are different kinds of electrifications? =experiment . to study the electrification of glass.= _apparatus._ the sheet of glass, g (no. ), heated (a hot bottle or lamp chimney will do); a piece of silk large enough to rub g. (a silk handkerchief is just the thing, but in case you have no silk, use the flannel cloth, f c, no. .) = . directions.= (a) vigorously rub the hot glass with the silk (or flannel), also heated. (b) test g for electrification by means of little pieces of tissue-paper and the carbon electroscope, exp. . _= . questions.=_ will two pieces of electrified glass repel each other? arrange an experiment to show whether you are right or not. is the charge on the glass exactly like that on the ebonite? do you know how to find out? =experiment . to compare the electrification produced by ebonite and flannel with that produced by glass and silk.= _apparatus._ the support (see § ); wire swing, w s (no. ); ebonite rod, etc., of exp. (fig. ); the glass, g, and silk of exp. . = . directions.= (a) electrify e r, and place it in w s, fig. . (b) bring the uncharged glass near e r, noting the action of e r. (c) heat and electrify g; bring it near e r, and carefully note whether the attraction between them is stronger or weaker than before, or whether they repel each other. _= . discussion.=_ we know that the glass was electrified, because it lifted tissue-paper; hence, its charge was not of the same kind as that on the ebonite. had the electrifications been exactly alike, we should have had a repulsion (exps. , , ). the exact difference between these two kinds of electrifications is not known. it has been agreed, for convenience, to call that produced by glass and silk a _positive_ electrification. with ebonite and flannel a _negative_ electrification is produced. the sign + is generally written for the word positive, and - for negative. these signs indicate _kind_, and not more or less, as in arithmetic. _= . laws.=_ we have learned from the experiments these facts, which are called _laws_: ( ) charges of the same kind repel each other; ( ) charges of unlike kinds attract each other; ( ) either kind of a charge attracts, and is attracted by a neutral body. chapter vii. insulators and conductors. =experiment . to study insulators.= _apparatus._ ebonite rod, e r (no. ); flannel cloth, f c (no. ); tissue-paper, t p (no. ). = . directions.= (a) holding one end of e r in the hand, charge the other end by rubbing it with f c. (b) with bits of the t p test each end of e r for a charge, and compare the results. =experiment . to study insulators.= _apparatus._ the ebonite sheet, e s (no. ); flannel cloth, f c (no. ). = . directions.= (a) thoroughly electrify e s (exp. , d), then lift and hold it in the air, as in fig. . (b) by moving your rounded knuckle about near the surface of e s, see if you can get more than one spark from it. =experiment . to study insulators.= _apparatus._ a hard-rubber comb (not furnished); flannel cloth, f c (no. ); dull pointed nail (no. ). = . directions.= (a) electrify the comb with f c. (b) move the nail along near the teeth of the comb, and listen carefully. _= . discussion of experiments , , ; insulators.=_ in the electrification remained at one end of the rod. in and the sparks showed that all parts of the ebonite were not discharged at the same time. a substance, like ebonite, which will not allow electrification to pass from one part of it to another, is called an _insulator_. silk and glass are also insulators. do you now see why a silk thread was used to make the carbon electroscope? why do they fasten telegraph wires to glass insulators? _= . conductors.=_ it has already been stated that water in an elevated tank has potential energy. we can allow the water to flow through a conducting pipe to another tank a little lower than the first, and it will still retain much of the potential energy, but not all. can we conduct from one place to another this peculiar state of things, this queer form of potential energy which we call electrification? it is clear, from the last experiments, that in order to do it we need something besides ebonite, which really acts like a closed stop-cock to the flow of electrification. to keep electrification in one place we need an insulator; to get it from one place to another we need a _conductor_. insulators are as important as conductors. you saw that sparks went to the finger from the ebonite, so we call the finger a conductor. you have learned that attractions and repulsions show the presence of electrification. can we have our charged body in one place and get attractions or repulsions at some other place? [illustration: fig. .] =experiment . to study conduction.= _apparatus._ fig. ; the support (see § ); a bent hairpin, h p (no. ); ebonite sheet, e s; flannel cloth, f c; tin disk, b f b (no. ), which is the bottom of the flat-box, f b; the insulating table, i t (see § ). = . the insulating table= consists of a tin box (exactly like that used for the electrophorus cover), and an ebonite rod about - / in. long. see § for full details about fitting the rod into the box, etc. the lower end of the short rod fits into the large hole in one end of the support rod, s r. arrange as in fig. . b f b should swing about very easily. = . directions.= (a) charge e s, then rub it upon i t, as shown, noting the action of b f b. _= . discussion.=_ ebonite being an insulator (§ ), we say that i t, h p and b f b were _insulated_. you can see that the electrification must have passed through i t and h p to get to the disk, b f b. h p was the _conductor_, allowing the disk, also, to become charged. the wood, s r, is a conductor, and, as it was not insulated from the earth, s r was neutral. account for the attraction. (see § .) [illustration: fig. .] =experiment . to study conduction.= _apparatus._ a copper wire, c w (no. ); insulating rubber band, r b (no. , fig. ); wire swing, w s (no. ); the other half of the flat box, t f b (no. ); apparatus of exp. . = . telegraph line.= to have our telegraph line using frictional electricity complete, we must have: ( ) some way of generating or making the electricity; ( ) some means of getting it or its effects to the other end of the line; ( ) some way of showing that it has been taken there. the charged e s will be the source of the electrification. new york will represent the end at which we _send_ the message, so at n. y. we must have a _sending instrument_. see fig. , which explains itself. r b or a silk thread must be used to _insulate_ the sender. around one leg of w s is twisted one bare end of the _conductor_, c w. boston will represent the end of the line at which the message is received, and there we need a _receiving instrument_. this is similar to the apparatus described in exp. , fig. . in addition to this, tie the middle of a moist cotton thread that is in. long, to b c (fig. ), and let its two free ends lie over the top and reach down against the bottom of the tin; that is, on the left-hand side. fig. will give you an idea in regard to the looks of the thread; at first, however, it should be close to the bottom of the tin. twist the other bare end of the copper wire around b c. when the line is properly constructed and ready for use, both instruments and c w are entirely insulated. do not let any part of c w touch the table or your clothing. = . directions.= (a) touch the insulated sending instrument with the charged ebonite sheet, and watch for any motion in the receiving instrument. =note.= better results will be obtained by using the charged electrophorus cover as the source of electrification, instead of e s. (exp. .) _= . discussion.=_ the action here was like that in the previous experiment, the difference being that a longer _conductor_ was used. electrification is always looking for some place to get to the earth, just as water will run from a roof to the ground. you will understand more about it a little later. in our apparatus just described, the only way that the earth could be reached was through the wooden rod s r. do not get the idea that real messages are sent in any such way, or that electricity flows through a wire as water flows through a pipe. _= . relation between conductors and insulators.=_ the above terms are merely relative. static electricity is easily conducted by dry wood, while galvanic electricity is practically insulated by it. a substance may be an insulator for currents of low potential, while at the same time it will conduct high potential currents. (see potential § .) _= . electrics and non-electrics.=_ bodies like glass, sealing-wax, amber, etc., were called electrics by the first students of electricity, because it was upon these substances that they could easily produce electrification. they called iron and other metals non-electrics, because they could detect no electrification after rubbing them. can you explain why they did not detect any electrification on metals? can you devise an experiment to prove that metals may be charged? do you see any relation between a non-electric and a conductor? =experiment . to study the effect of moisture upon an insulator.= _apparatus._ same as for exp. , with the exception of the copper wire; this is to be replaced by a dry silk thread about feet ( cm.) long (no. ). = . directions.= (a) see if a charge can be sent through the thread, in the same manner as it was through the copper. is dry silk a conductor? (b) thoroughly wet the thread, being careful not to wet the rubber band insulator (fig. ); see if wet silk is a conductor. _= . discussion.=_ dry silk is an insulator, while wet silk is a good conductor of _static_ electricity. it is the water, however, which really does the conducting. even small amounts of moisture on glass, or other insulators, will allow the charge to escape. glass collects much moisture from the air. do you now see why it is necessary, to get good results, to have the paper, glass, etc., hot before electrifying them? =experiment . to test the effects of moisture upon bodies to be electrified.= _apparatus._ two pieces of newspaper, each about in. ( cm.) square. = . directions.= (a) heat one piece to make it thoroughly dry, and leave the other cold. (b) stroke each, say times, with your hand, pressing them upon the table; then place them upon the wall at the same time, being careful not to let them touch your clothing. see which will cling to the wall the longer. chapter viii. charging and discharging conductors. _= . the electrophorus.=_ while the ebonite sheet alone, or a good hard-rubber comb, may be used for many experiments in frictional electricity, the sparks produced are small, and the ebonite has to be electrified as often as it is discharged. to obtain real good sparks, and to avoid this continual rubbing, the student should be provided with an _e-lec-troph'-o-rus_. this is, really, a simple, cheap, and efficient frictional electric machine. an electrophorus consists of insulators and conductor--that is, of parts: ( ) insulating handle, ( ) cover, and ( ) a plate or base of insulating material. [illustration: fig. .] = . our electrophorus= is shown in fig. . for the insulating _handle_ use the ebonite rod, e r (no. ); for the _plate_, use the ebonite sheet, e s (no. ). the _electrophorus cover_, e c (no. ), furnished, is a tin box with a fancy top. a hole has been punched in the center of its top, and into the hole has been riveted a short tube, so that the handle, e r, can be firmly held. the hole has been made a little larger than e r for convenience. to make e r fit tightly in the hole, so that you can lift e c, wrap a small piece of paper around the end of e r before pushing it into the hole. you can easily find out how much paper to use to make a good fit. with a knife cut away all loose points of paper that stick out of the hole around e r; this is _important_. the top and bottom of e c should be pressed firmly together. first learn how to use the electrophorus. with the large amount of electrification produced we can then find out how it works. =experiment . to learn how to use the electrophorus.= _apparatus._ shown in figs. , . _do not fail to read_ § . = . directions.= (a) place e s upon a _flat_, uncovered, wooden table, and rub it _vigorously_ for a _minute_ with the _warm_ flannel, f c, to thoroughly charge it. do not let e s slide about, and do not lift it from the table. (b) with the right hand grasp e r at its extreme end, and place e c upon e s. (c) touch e c for an instant with a finger of your left hand (fig. ). (d) remove your finger entirely from e c, then lift e c by its insulating handle, e r, at the same time holding e s down to the table, if it tries to follow e c. [illustration: fig. .] [illustration: fig. .] (e) bring your left hand near e c (fig. ). you should get a good spark from e c. (f) it is not necessary to immediately rub e s again. you have discharged e c by taking a spark from it. to _recharge_ it, simply place it upon e s again; let it remain there while you count ; touch it as before, and then lift by e r. = . extra notes.= you may repeat the above operation many times. as soon as the sparks begin to get small, electrify e s again. the charge on e c is +, although that on e s is -. you will understand, later, why this is so. =if you do not get a good spark= from the electrophorus, read the directions again. the ebonite must be well electrified; the cover must be lifted by the _end_ of its handle; you must _touch_ the cover and _withdraw your finger_ from it _before_ lifting. you must allow the cover to remain upon the ebonite or seconds each time. the board, or table, upon which e s rests, must be _flat_, and not warped, so that e c will fit down perfectly upon e s. =experiment . to study "charging by conduction."= _apparatus._ fig. . to one end of a _silk_ thread, s t, is tied a little bent clamp, b c (no. ); the other end of s t is tied to the support wire, s w (no. ); the bottom of the flat box, b f b (no. ), is supported by b c, and thus _insulated_ from the table and earth; the electrophorus (exp. ) is also necessary. = . directions.= (a) charge e c (exp. ), and bring it near b f b (fig. ). note the spark. (b) repeat (a) twice, noting the relative sizes of the sparks. does b f b continue to be attracted by e c? (c) bring your knuckle slowly towards the charged disk, b f b. [illustration: fig. .] [illustration: fig. .] =experiment . to study potential; electro-motive force.= _apparatus._ the insulating table, i t, fig. . (for details see exp. ; the electrophorus exp. ). = . directions.= (a) pass a spark from the thoroughly charged e c (exp. ) to i t. (b) recharge e c, and see how many times i t will take good sparks from it, and note the relative sizes of the sparks. (c) as soon as i t refuses to take more sparks from e c, touch e c to see if it is completely discharged. (d) touch i t. _= . pressure; potential; electro-motive force.=_ water runs down hill. it always tries to run from a high place to a lower one. electrification acts very much like water in this respect. we say that water has a _pressure_, or a _head_ of so many feet. in speaking of a charge, we say that it has a _potential, or an electro-motive force_. water may have a high or low pressure, and a charge may have a high or low potential. the greater the pressure of water, the harder it tries to break away and get somewhere; the greater the potential of a charge, the farther it will jump to your hand. _= a. current; spark.=_ electrification will easily pass from a place of high potential to one of low potential through a conductor, and when it _passes_ we say we have an _electric current_, or a _current of electricity_. water has no desire to flow on a dead level, and the electric current does not care to flow between two places of equal potential. the potential of the earth and of all neutral bodies is zero; that is, they have no charge, no potential; so it is very easy for a charge to escape into the earth. dry air is a pretty good insulator, but when the attraction between a charged and a neutral body gets great enough, the spark rips right through the air. benjamin franklin proved by experiment that lightning is caused by the electrification in the clouds and air. (see atmospheric electricity.) = . theories about electrifications.= _the "one-fluid" theory_ suggests that neutral bodies have a certain amount of electrification, and that they have a certain potential called zero potential. if the potential of a body becomes greater than that of the earth, the body is said to be positively electrified; if the potential of the body is less than that of the earth, it is said to be negatively electrified. if we fill a bottle with sea water, we have a great deal of water when we compare it with the bottle, but a very little water when we compare it with the sea. the earth is so large that small amounts of electrification taken from it or added to it do not affect its potential to any extent. = .= _the "two-fluid" theory_ suggests that there are two absolutely different kinds of electrification, one called positive (+), and the other negative (-). when these two are equal in quantity, the body is said to be neutral. if the body contains more + than -, the body is said to be charged positively. it is evident then, if the two-fluid theory be accepted, that no matter how strongly a body is charged positively there must be in it _some_ negative electrification; that is, we may charge a neutral body + by adding + electrification to it, or by taking - electrification from it. there must always be, then, some + and - electrifications in a body. these theories do not require much consideration by the student of elementary electricity. the best thing he can do is to learn what electricity can do, and how it can be used. [illustration: fig. .] =experiment . to study some methods of discharging an electrified body.= _apparatus._ the electrophorus (exp. ); an ordinary pin (fig. ). = . note.= you have seen sparks pass from e c to your rounded knuckle, and to other conductors. in all of these cases the discharge was _sudden_, one spark doing the work. can we _slowly_ discharge e c, or discharge it without sounds? = . directions.= (a) thoroughly charge e c, and test it with your knuckle to be sure that it is working properly. (b) charge e c again; hold the pin in your left hand (fig. ), and _slowly_ bring its _head_ toward e c; listen for sparks. (c) recharge e c, and bring the _point_ of the pin slowly toward it. touch e c to see whether it has been discharged or not. _= . disruptive, conductive, and convective discharges.=_ sudden discharges, accompanied by bright sparks, are said to be _disruptive_. when the electrification is continuously carried away by a conductor, there is a _conductive_ discharge. there is a _convective_ discharge when the electrification escapes from points into the air. (see § .) the nature of the discharge depends upon the potential of the charge, upon the nature of the charged conductor, and upon the nature of the surrounding air and objects. convective discharges are often _silent_, as in exp. (c). in this case, electrification passed from the earth through the pin-point to the cover to neutralize it. (see induced electricity.) [illustration: fig. .] =experiment . to study intermittent or step-by-step discharges.= _apparatus._ electrophorus (exp. ); carbon electroscope (§ ), (exp. ). = . directions.= (a) charge e c, then hold your hand on one side of the carbon (fig. ), and hold e c upon the opposite side. what should the carbon do? _= . discussion.=_ the carbon and e c were insulated, while the hand was "grounded"--that is, it was connected with the earth. carbon is a good conductor; it may be quickly charged and discharged. =experiment . to ascertain the location of the charge upon an electrified conductor.= _apparatus._ the electrophorus (exp. ); the insulating table, i t (exp. ); the tin box, t b (no. ), fig. ; a piece of moist cotton thread, c t, or in. long, bent double, and hung over the edge of the open box, t b. one-half of c t should be inside of t b, which, in turn, should stand on i t. [illustration: fig. .] = . directions.= (a) charge e c; pass a spark to t b, and note the action of both parts of c t. _= . hollow and solid conductors.=_ the moist thread, being a conductor, became charged as well as the box. the electrification seemed to be entirely on the outside of t b. a hollow conductor will hold as large a charge as a solid one having the same amount of surface. this refers to charges of static electricity, not to currents. an electric current passes through the whole substance of a conductor. =experiment . to study the effect of points upon a charged conductor.= _apparatus._ the electrophorus (fig. ); a pin, bent slightly to keep it from rolling. = . directions.= (a) charge e c; test its charge with your knuckle. be sure that you get a good spark. (b) charge e c again, and hold it by its insulating handle, e r, long enough to count before discharging it with your knuckle. be sure that it holds its charge during this time. (c) while e c is upon e s (fig. ), lay the bent pin upon e c, so that its point will project into the air. the point should stick out about / in. from the edge of e c. (d) touch e c; raise it by e r; count as before; then test with your knuckle to see if e c is still charged. _= . electric density; electric wind.=_ a charge resides upon the outside of a conductor (exp. ), and it continually tries to escape. it seems to pile up at points and corners, and we say that it is denser at such places than at well-rounded parts of a charged conductor. all points and sharp places should be removed from a conductor, if it is desired to keep a charge for any length of time. electrification may escape from a point so rapidly that currents are produced in the surrounding air. as the particles of air become charged, they repel each other. the movement of the air particles may be so great that a lighted candle will be affected when placed near the point. this current of air is called _electric wind_. electrification easily passes from points, and the electrophorus may be easily and silently discharged by holding a pointed pin near it (exp. , c). thorns, leaves with sharp edges, etc., have a great effect upon atmospheric electricity. they allow a silent escape of electrification from the earth to neutralize that in the clouds which is opposite in nature. (see atmospheric electricity.) chapter ix. induced electrification. _= . electric field; lines of force.=_ in our study of magnetism you learned that a magnet can act through the air, and induce a piece of iron to become a magnet. you saw how the iron filings arranged themselves around the magnet, showing that the lines of force reached out from the poles in a very peculiar manner. there is an _electric field_ all around a charged conductor, just as there is a magnetic field about a magnet. the lines of force in the electric field pass from the positively charged body to the negatively charged one, or to some neutral one, which, you will soon see, is practically the same thing. when the positively charged electrophorus cover is held above the negatively charged ebonite sheet, a very strong electric field exists between them. = . note.= you have seen that we can _charge_ an insulated conductor by _touching_ it with the charged cover, or by allowing a spark to pass to the conductor. what effect, if any, has a charged body upon an insulated conductor _before_ they touch each other, and before any spark passes to the conductor? [illustration: fig. .] =experiment . to study electric induction.= _apparatus._ fig. . the insulating table, i t (for details see exp. ); tin box, t b (no. , fig. ); moist cotton thread, c t; the electrophorus (exp. ); tie c t around one end of the closed t b, and leave the ends of c t long enough to hang down over the end. place a match on each side of t b to keep it from rolling. = . directions.= _part ._--(a) pass a spark from the charged e c to t b, and note the action of the thread, which will be our electroscope. remove e c. (b) touch the charged t b with the finger, watching c t. _part ._ (c) bring the re-charged e c near the neutral t b, and parallel to its end surface; but keep them at least an inch apart, so that a spark cannot pass. watch c t. (d) withdraw e c, and try to explain the action of c t. _= . electric polarization; theory of induction.=_ this experiment should remind the student of exp. , in magnetism, in which a piece of soft iron was magnetized by the inductive action of a magnet. the soft iron was in a magnetic field; it became polarized. is it possible that the box, t b, was polarized, being in the electric field of e c? we know, by the action of c t (fig. ), that the top end of t b was charged while e c was in place. the charge was not conducted. you know, from previous experiments, that + and - electrifications rush together whenever possible. why can we not suppose that a neutral body, like the box at the start, contains an equal amount of both kinds, and that these different electrifications have already rushed together? if you imagine a small army of positive soldiers struggling, "man to man," with the same number of equally strong negative soldiers, you can readily see that one-half of them can hold the other half from running away. a body remains neutral, then, according to this idea, as long as it has an equal quantity of the two opposite kinds of electrification. (see theories, § , .) as soon as the positively charged e c was brought near t b, it destroyed the neutrality of t b, by pulling at its - electrification, and by pushing back its + electrification to the top end and into c t. we say that the charged e c produced a separation of the combined electrifications of t b by _induction_, and not by contact. as soon as the inductive action of e c was removed, t b became neutral again. [illustration: figs. - .] = . note.= figs. and may aid the student. in fig. , t b is supposed to be neutral. the "double sign" means that the + and - electrifications are united; and, as there are an equal number of both kinds, none are left free to tell the tale. fig. shows what happens when the + e c is near. what would happen if we could cut into t b at the middle with an insulated knife while it is polarized by e c? =experiment . to learn how to charge a body by induction.= _apparatus._ fig. , same as in exp. . = . directions.= (a) bring the charged e c within an inch of the bottom of t b, and as soon as c t is repelled, showing that t b is polarized (exp. ), touch t b with your finger; then remove your finger while you still hold e c in place. (b) withdraw e c and its inductive action. explain the motions of c t during the experiment. is it still repelled by t b after e c is removed? _= . free and bound electrifications.=_ as explained in exp. , and as shown in fig. , t b became polarized. the - electrification was drawn towards e c; it was held or _bound_ there as long as e c was near. the + was actually repelled by e c, and it was _free_ to escape through your arm as soon as t b was touched, leaving the top end of t b neutral. as soon as e c was removed, the - electrification, no longer held by e c, spread all over t b and on to c t. t b was _charged by induction_. it was charged negatively by driving out + electrification. =experiment . to show that a neutral body is polarized before it is attracted by a charged one.= _apparatus._ the electrophorus (exp. ); dry tissue-paper, t p. cut out pieces of t p, each about / inch square. = . directions.= (a) place the bits of dry t p upon a board or table, and convince yourself that they are attracted equally by the charged e c. (b) slightly moisten one piece of t p only. see if one is attracted by e c more readily than the other. _= . polarization precedes attraction.=_ dry tissue-paper is not a good conductor; you have seen (exp. ) that it can be electrified, which indicates that it is at least a partial insulator. insulators are not easily polarized. (why?) even if the pieces of t p were polarized, the opposite electrifications were so near each other that the attraction of e c for the - was nearly overcome by the repulsion for the +; the result being that t p was not strongly attracted by e c until the + had a chance to escape. the moist tissue-paper allowed its + to escape more quickly than the dry piece. a conductor is attracted by a charged body more strongly than an insulator, because the latter is not easily polarized. a neutral body, then, is really no longer neutral when it is in the electric field. _polarization precedes attraction._ =experiment . to find whether electric induction will act through an insulator.= _apparatus._ small bits of carbon (exp. ); bits of moist tissue-paper, t p; one-half of the flat box, t f b (no. ); sheet of glass, g (no. ); electrophorus (exp. ). place the carbon and t p into t f b (fig. ), and cover with the glass. = . directions.= (a) charge the electrophorus cover, e c (exp. ), move it about a little above the glass, and see if the carbon, etc., are attracted. _= . dielectrics.=_ the carbon must have been polarized and attracted _through_ the glass. you saw, exp. , that the lines of magnetic force could penetrate and act through paper, glass, etc.; it is now evident that the electric field is not easily fenced in, even by an insulator. substances, like the glass, which allow this inductive influence to act through them, are called _dielectrics_. [illustration: fig. .] [illustration: fig. .] =experiment . to find whether a polarized conductor can act inductively upon another conductor.= _apparatus._ fig. . insulating table, i t (for details see exp. ); ebonite sheet, e s (no. ); flat box complete f b (nos. , ); sheet of glass, g (no. ); small piece of slightly moist tissue-paper, t p; charged electrophorus cover, e c. arrange as shown. = . directions.= (a) hold e c, charged, near and under i t, then bring your finger, f, near t p. explain the action of t p. _= . successive induction.=_ the inductive influence of e c first polarized i t; this acted through the dielectric, e s, and polarized f b, which, in turn, polarized t p through the second dielectric, g. this induction after induction is called _successive induction_. _= . inductive capacity.=_ dielectrics are insulators. two substances may be equally good insulators, that is, they may equally well resist the _spread_ of electrification _over_ their surfaces, or the _flow_ of the electric current _through_ them, while one may be, nevertheless, a better _dielectric_ than the other. the better the dielectric, the easier it is for the electric field to polarize a conductor placed beyond the dielectric. a good dielectric is said to have a high _inductive power or capacity_. glass is about times as good a dielectric as dry air; and as the latter (under certain conditions) is taken as the standard, or as unity, we may say that the _specific inductive capacity_ of glass is about . =experiment . to study the action of the electrophorus.= _apparatus._ the electrophorus (exp. ); small bits of moist tissue-paper, t p. = . directions.= (a) thoroughly electrify e s, fig. , and place e c upon it by its handle, e r. (b) touch e c, as directed in exp. , and listen for a small spark which should pass from e c to your finger. (c) again, place a little piece of t p upon e c before lowering it upon e s. do not touch e c, but bring your finger near t p. what does t p do? now, touch e c and see, when you bring your finger near it, if t p acts as it did before. (d) again, place several pieces of t p upon e c (e s being thoroughly charged); touch e c, then lift it by its handle. note action of t p, which should be slightly moist. _= . discussion.=_ the electrification upon the ebonite is negative (exp. ). although e s and e c (fig. ) seem quite smooth, there are many little hills, valleys, and air-spaces between them, which keep them from touching each other perfectly. the ebonite has the electric field at the start, and it really acts across these minute air-spaces _by induction_ (exp. ), and polarizes e c. the air-spaces form the dielectric (exp. ). the - electrification of e c being repelled by the - of e s, it is driven to the top of e c, while the + is drawn to the bottom. this + is kept from rushing to the - of e s by the air dielectric, and because e s is a non-conductor. by touching e c the free - escapes to the earth, leaving e c _positively_ charged when it is lifted. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] = . details of action.= the different steps in the action of the electrophorus are shown graphically in figs. to . fig. shows e s negatively charged. e c is neutral at first, fig. ; that is, it is supposed to contain both + and -, as shown by the "double sign" (§ ). fig. shows that e c has been polarized by the inductive action of e s. the repelled - escapes to the finger (this escaping is what gave the small spark to the finger and charged the t p in the last experiment), leaving the top uncharged, while the + is _bound_ (fig. ). as soon as e c is lifted (fig. ) the + spreads all over e c, which is then charged. the +, upon going to the top, charged the pieces of t p (exp. , d), causing them to be repelled. the charge of - upon e s has not been removed, so the operation may be repeated many times before e s must be again electrified. the - electrification on the ebonite acts inductively through e s, drawing up + electrification from the earth. to make this action easier a "sole," or metal conductor, is often placed under the ebonite. =experiment . to see, hear, and feel the results of inductive influence and polarization.= _apparatus._ ebonite sheet, e s (no. ); insulating table, i t; flannel cloth, f c. = . directions.= (a) thoroughly charge e s with f c. with the right hand bring e s near and parallel to the top surface of i t, but do not let them touch each other. (b) remove e s, then touch i t to see if it is charged. (c) repeat (a), and while you hold e s about / inch from i t, their flat surfaces being parallel, touch i t. watch for any sparks, and note any peculiar actions of e s. (d) remove your finger from i t, then withdraw e s; finally touch i t with your knuckle. _= . discussion.=_ this apparatus is really the electrophorus upside down. it shows very clearly ( ) the escape of the - electrification from i t, by the spark; ( ) that the attraction between i t and e s is much greater than before, when this - is removed; and ( ) it shows the different steps of the inducing and charging process, as described in exp. , and as shown in figs. and . chapter x. condensation of electrification. =experiment . to find whether a large surface will hold more electrification than a small one.= _apparatus._ the insulating table (for details, see exp. ); a large tin basin or pan (not furnished); the electrophorus (exp. ). = . directions.= (a) test the electrophorus and be sure that it is working properly. (b) as in exp. , see how many good sparks i t will take from e c (which should be recharged at each trial) before the potential of i t is raised so that it equals the potential of e c. (c) carefully set the basin or pan upon i t, then count the number of good sparks you can pass to it from e c (recharged at each trial). compare the number of sparks necessary to raise the potential of the large surface until it equals that of e c, with the number found in part (b). _= . electrical capacity.=_ it takes more heat to raise the temperature of a gallon of ice-water to the boiling point, than it takes for a quart of ice-water. you have just seen that a large insulated surface will take more sparks from a charged body than a small one, before its potential is raised to that of the small one, and to that of the charging body. we say that a large surface has a greater _capacity_ than a small one, the shape and other conditions being the same. =experiment . to find whether the capacity of a given conductor can be increased without increasing its size.= _apparatus._ fig. . insulating table. i t (exp. ); the extra ebonite sheet, e s (no. ); the complete flat box, f b (no. , ); the charged electrophorus cover, e c (exp. ). arrange, as shown, i t being insulated from the earth by e s. f b should rest upon a wooden table or other large conductor. = . directions.= (a) see how many good sparks i t will take from e c. re-charge e c at each count, and note the relative sizes of the sparks. (b) discharge i t by touching it with your knuckle. [illustration: fig. .] _= . condensation; condensers.=_ as i t easily held more sparks than it would take before (exp. ), we say that its _capacity_ has been increased. its potential didn't increase, because that could not get greater than the potential of e c, the charging body. to describe this state of affairs, we say that the electrification was denser than before, and that it was _condensed_. the _capacity of i t was greatly increased by the presence of another conductor, f b, insulated from i t, but "grounded_." such a combination, conductors, with a dielectric between them, is called a condenser. a condenser can hold much more electrification at a certain potential than an equal amount of surface can hold when not properly arranged. we might call a condenser a storage battery for static electricity. the capacity of a condenser depends, among other things, upon the area of the conducting surfaces, and upon the thickness and nature of the dielectric. among the various forms of condensers may be mentioned the leyden jar, and the fulminating pane. = . the leyden jar= consists of a wide-mouthed glass jar, with tin-foil pasted upon the inside and outside to within or inches of the top. the inner coat or conductor is connected to a knob or ball at the top by means of a chain. to charge the jar, the outer coat is connected with the earth by holding it in the hand, or by resting it upon a table while the electrification is passed to the knob. a _leyden battery_ consists of or more connected jars, the object being to increase the area of the surface. the jar is discharged by touching one end of a _discharger_ (§ ) to the outer coat, and swinging its other end over to the knob, when a bright spark will pass between the knob and discharger. (see exp. .) = . fulminating panes=, or franklin's plates, are practically the same as a leyden jar. the tin-foil, however, is pasted upon the opposite sides of a pane of glass, a margin of about an inch being left all around. one side of the pane is charged, and takes the place of the inside coat of the jar. the other side is grounded. the pane is discharged by connecting the two sheets of foil. = . induction coil condensers= consist of sheets of tin-foil separated by sheets of paraffined paper, which act as the dielectric. (see induction coils.) = . submarine cables=, with the surrounding water, act like condensers, the result being that the condensing effect slows up the electric current and retards the signals. these make a condenser of enormous capacity. the wires inside form one conductor, and the water the other, while the insulation around the wires forms the dielectric. =experiment . to study the condensation of electrification.= _apparatus._ same as in last experiment, but arrange so that f b and i t shall be near each other at one side; that is, so that the edge of e s shall be even with the edges of the two tins. = . directions.= (a) pass good sparks to i t from the charged e c until something happens. watch the side where i t and f b are near each other. _= . discussion.=_ we may say that the electrification was condensed, in this experiment, until the charge became so great that the _condenser_ suddenly discharged itself. condensers may be made in many ways, but they all consist of conductors, with a dielectric between them. one conductor is insulated, and receives the charge; the other conductor is grounded. [illustration: fig. .] =experiment . to study the action of the condenser.= _apparatus._ fig. . the insulating table, i t; ebonite sheet, e s (no. ); flat box, f b, complete (nos. , ); the electrophorus (exp. ). note that this is really the same apparatus as that just used; both conductors of this condenser, however, are insulated and reversed in position. = . directions.= (a) see that your electrophorus works properly, then find out how many good sparks you can pass from e c to f b, recharging e c each time. note the relative sizes of the sparks, and compare the result with the number taken by the condenser in the last experiment. (b) when f b seems to be fully charged, touch i t with your knuckle. (from your study of induction what should be the result?) (c) now see if f b will again take good sparks from the charged e c. pass sparks to f b until it seems fully charged. (d) again touch i t, then repeat (a) and (b) several times, until a bright spark passes from f b over the edge of e s to i t. _= . discussion.=_ the action of the condenser, as clearly shown, depends upon induction. you should now be able to explain and show by diagram the different steps. e c was positively charged (exp. ). this also charged f b positively by contact. f b acted inductively through the dielectric, e s, drawing up _some_ of the - in i t, and repelling _some_ of the +. as i t was insulated, this free + electrification could not escape. before we touched i t, its + and - electrifications, although partially separated, were struggling against this inductive action; and, on account of their strong attraction for each other, our efforts to charge the condenser were retarded. upon touching i t the free + escaped to the earth. (this was the cause of the spark.) this left _some_ - electrification bound on the underside of e s, and some + bound on the upperside of e s. the capacity of f b was increased by this process, as the + already put into it was very much occupied by the attractions of the induced - just under e s. as more + was given to f b, more - was drawn up under e s and more + was pushed out of i t. this action went on until the two conductors were strongly and oppositely charged. this action goes on continuously when the lower conductor is grounded. the spark between the tins was due to the rushing together of the + and - electrifications; it showed that there was a _momentary current of electricity_. =experiment . to study the effect of electrical discharges upon the human body.= _apparatus._ the condenser (fig. ), with e s centrally placed so that the apparatus cannot discharge itself; the hairpin discharger, h p d (no. ); the electrophorus. = . directions.= (a) charge the condenser (exp. ) with good sparks from e c, then touch i t (fig. ). (b) recharge the condenser with sparks, then touch f b. discharge it by again touching i t as in (a). (c) recharge with sparks; then place your thumb against f b, and quickly swing the first finger of the same hand over to i t, and get a slight shock. (d) recharge with as many sparks as you think you can stand. (e) instead of using your hand to discharge the condenser, try the bent hairpin. keeping one end against f b, swing the other end over near i t. _= . shocks; dischargers.=_ the two conductors being oppositely charged in the condenser (exp. ), it is only necessary to place some conductor between them to allow the charges to rush together. any conductor so used is called a _discharger_. the hand carried the whole current which caused the _shock_. when i t was touched first, the current was obliged to pass through your body, through the floor, and up the table-legs into f b. always touch the "grounded" conductor first with the discharger, so that you will get a good spark and _not_ a shock. =experiments - . to show the strong attraction between the opposite electrifications in the condenser.= _apparatus._ flat box, f b (nos. , ); sheet of glass, g (no. ); electrophorus (exp. ). the two parts of f b are used for the conductors of the condenser (fig. ) for the sake of lightness. the bottoms should be next to the glass, which is used for the dielectric on account of its stiffness. the lower tin should rest upon the table. the glass should be perfectly clean and dry (hot). = . directions.= (a) charge the condenser with or good sparks from e c. (b) lift the condenser by one corner of g (fig. ), being careful not to discharge it. explain why the lower conductor follows the glass. [illustration: fig. .] =experiment .= = . directions.= (a) charge and lift the condenser as just explained (exp. ). fig. . (b) with your right hand touch the upper tin alone, then the lower tin alone. (c) touch both tins at the same time, and note the action of the lower one. _= . discussion.=_ this clearly shows how strongly the two electrifications are _bound_ in the condenser. each refuses to escape to the earth, but they instantly rush together at the first opportunity. the dielectric may be shattered in a very heavily-charged condenser by this strong attraction. [illustration: fig. .] =experiment . to show how the condenser maybe slowly discharged.= _apparatus._ fig. . the condenser (exp. ); the carbon electroscope with support (exp. ); the electrophorus (exp. ). = . directions.= (a) charge the condenser by means of the electrophorus; then hang the carbon so that it can swing between the upper conductor and e c placed as shown. _= . the electric chime.=_ the charging and discharging of the carbon being rapid, it acts like a _chime_ as it taps against the tins. =experiment . to ascertain the location of the charge in the condenser.= _apparatus._ the condenser, consisting of flat box, f b (nos. , ); ebonite sheet, e s (no. ); insulating table, i t (exp. ); (when charging, arrange as in fig. .); the electrophorus; hairpin discharger, h p d (no. ). = . directions.= (a) charge the condenser with or good sparks from e c. (b) lift i t away from e s by its insulating handle, and set it upon the table. (it may be necessary to hold e s down.) (c) lift e s directly up and away from f b. (lift by corners; do not scrape e s along on f b; do not allow e s to touch your clothing.) (d) replace e s and then i t by its handle quickly, making the condenser complete again. (e) with h p d see if the condenser still holds a charge. touch f b first (exp. ). _= . discussion.=_ as the _conductors_ were completely discharged, being left for a few moments upon the table, it is evident that the opposite electrifications must reside in and upon the _dielectric_. the conductors allow an even and _rapid_ discharge from all parts of the dielectric at the same time. the dielectric is considerably strained when a condenser is heavily charged. this strain, caused by the attraction of the opposite electrifications, may be great enough to break or puncture the dielectric. =experiment . to find whether any electrification remains in the condenser after it has once been discharged.= _apparatus._ the condenser (fig. ); the electrophorus (exp. ); hairpin discharger, h p d. = . directions.= (a) thoroughly charge the condenser. (b) discharge it with h p d, being sure to touch f b first, and to touch i t for an instant while h p d is against f b. (c) after a few moments use h p d again, and see if you get a slight spark. _= . residual charge.=_ the two electrifications on the opposite sides of the dielectric have such an attraction for each other, when the condenser is charged, that they seem to penetrate, or soak into, the dielectric. these do not completely soak out again at the discharge. the small amount left is called a _residual charge_. [illustration: fig. .] =experiment . to study successive condensation; the chime cascade.= _apparatus._ fig. . this really consists of two condensers, joined by a wire. the upper condenser consists of t f b (no. ), e s (no. ), and the insulating table, i t. (see exp. .) the lower condenser consists of the cover of the tin box, c t b (no. ), the sheet of glass g (no. ), and b f b (no. ). the tin box, t b (no. ), is placed under this to raise it, simply. a wire or hairpin, h p, is hung upon the edge of t f b, its lower end being inside of c t b and not quite touching it. this acts like a pendulum, which is to swing to c t b at the proper time. the source of electrification is e c. =note.= you have learned that in charging the condenser with the positively charged e c, + electrification is driven from f b into the earth. can we use this to charge a second condenser? = . directions.= (a) pass or good sparks from e c to the under side of i t (fig. ), noting the action of h p. (b) hold e c in the hand, and, with its insulating handle, poke h p away from the condensers. do not discharge them. (c) with h p d test the lower condenser for a charge, touching t b first. (d) with h p d touch t f b first (why?), and discharge the upper condenser. _= . discussion.=_ a long row of condensers may be charged in this way. there is no advantage in it, as the electrification is merely divided between them. how can two condensers be joined to get the advantages of a large surface? chapter xi. electroscopes. _= . electroscopes=_ are instruments to show the presence, relative amount, or kind of electrification on a body. (see apparatus book, chap. xviii, for home-made electroscopes.) the _carbon electroscope_ has been described (exp. ). the _pith-ball electroscope_ is made by using pith from elder, corn-stalk, or milk-weed, in place of the carbon. the _gold-leaf electroscope_ is a very delicate instrument. the gold-leaf is supported, as suggested in fig. , at the lower end of a wire conductor which sticks through and hangs from the cork of a glass jar or flask. to the top end of the wire is soldered a ball or disk. the glass jar insulates the gold-leaf, and keeps it dry and free from dust. [illustration: fig. .] = . our leaf electroscope= (fig. ) is made with aluminum-leaf. gold-leaf is too delicate for unskilful handling, and aluminum will do for all ordinary experiments. to cut it into any desired shape, place it between two sheets of paper, then cut through paper and all. = . construction.= bend one leg of a hairpin, h p, as in fig. , and slide it onto i t. hang a wire, w, or another hair pin straightened, then bent, from the horizontal leg of h p. this is to support the "leaves," l, which are made from a strip of aluminum-leaf about in. long and / in. wide. moisten the under side of the horizontal part of w with paste or mucilage; press it upon the middle of the strip laid flat upon the table, and then lift w. the leaves should cling to w. each leaf should be, then, in. long. they should hang close together when not in use. a large chimney, or fruit-jar, may be used to surround the leaves, and to keep currents of air from them. the leaves should not touch the side of the jar when spread. =experiment . to study the leaf electroscope; charging by conduction.= _apparatus._ the leaf electroscope (fig. , § , ); ebonite rod, e r (no. ); flannel cloth, f c (no. ). = . directions.= (a) thoroughly charge e r, then scrape it along upon i t, noting the action of the leaves, l. (b) see if the leaves will remain spread for some time. (c) touch i t to discharge it, and note the action of l. _= . discussion.=_ no explanation should be necessary for this. are the leaves charged alike? as they were charged by contact, is the electrification on them + or -? =experiment . to charge the leaf electroscope by induction.= _apparatus._ our electroscope (fig. , § ); ebonite sheet, e s (no. ); flannel cloth, f c (no. ). = . directions.= (a) charge e s with f c, then hold e s above i t (fig. ), their surfaces being kept parallel and about or inches apart. watch the leaves. (b) withdraw e s. do the leaves remain spread? (c) repeat (a), and before removing e s, touch i t. (d) remove your finger from i t, then withdraw e s. do the leaves now remain spread? _= . discussion.=_ the permanent divergence of l was due to a charge given by induction. (exp. .) as e s was -, what was the kind of a charge in l? did any electrification go to the electroscope from e s? in (c) what became of the charge in l? explain why the leaves again diverged in (d). the electroscope was charged with + electrification by taking - out of it. =experiment . to learn some uses of the electroscope.= _apparatus._ our electroscope (fig. , § ); ebonite rod, e r (no. ); ebonite sheet, e s (no. ); glass, g (no. ); flannel cloth, f c (no. ). = . directions.= (a) with the charged e r charge the electroscope negatively by conduction (exp. ). note the amount of permanent divergence of the leaves. (b) electrify the glass, which will be +, (or use the + e c), and _slowly_ lower it over i t, noting the effect upon l. raise and lower g or e c several times. does g, which has an opposite charge to the electroscope, make l diverge more or less? (c) discharge the electroscope and recharge as in (a). (d) slowly lower the charged e s over i t. (e) slowly lower the palm of your hand over i t. =note.= if the + g is brought too near the -ly charged electroscope, l will first collapse and then instantly diverge again with a + charge by contact. the _first_ motions should be observed. _= . discussion.=_ as a neutral body causes a slight _collapse_ of the leaves, as well as a body charged positively (when the charge in the leaves is -), an increase of divergence is really the only sure test to tell how a body is charged. the - leaves collapse when a + body is brought near i t, because the - in them is drawn up towards the body. the leaves diverge more when a - body is brought near, because the - in i t is repelled into them. _= . the proof-plane.=_ since charges of static electricity reside upon the outside of conductors, it is an easy matter to take samples of the electrification. this may be done with a little instrument called a carrier, or proof-plane. it consists of a small conductor with an insulating handle. a ring or coin may be used for the conductor, and a silk thread for the handle. by touching the carrier to any charged body, it, also, becomes charged; and the nature of the charge may be determined by the use of a previously charged leaf electroscope (exp. ). a delicate gold-leaf electroscope would be ruined by coming in contact with a heavily charged body. the carrier allows a small sample to be tested. chapter xii. miscellaneous experiments. [illustration: fig. .] =experiment . to show that friction always produces two kinds of electrifications.= _apparatus._ fig. . the carbon electroscope (exp. ); flannel cloth, f c, doubled twice to make thicknesses (see fig. ); ebonite sheet, e s (no. ); ebonite rod, e r (no. ); charged electrophorus cover, e c. = . directions.= (a) vigorously rub e s with f c (folded as in fig. ). see if you can discover any attraction between them. (b) rub e s again, but do not lift f c from it with the hand alone. slip e r under the top fold in f c (fig. ), and lift f c straight up from e s. do not let f c touch the table or your hand. (c) see if f c is charged, using or different tests. (d) charge the electroscope with f c until the carbon is strongly repelled. (e) bring the positively charged e c slowly near the carbon, and note the result. (f) slowly bring the negatively charged e s near the carbon that has been charged by contact with f c. _= . discussion.=_ this experiment showed that while the ebonite was negatively charged, the flannel was positively charged. one kind of electrification is never produced without the other. it can also be shown that the two kinds are equal in amount. =experiment . to show "successive sparks."= _apparatus._ fig. . the electrophorus (exp. ); the extra ebonite sheet, e s (no. ); three coins (marked a, b, c, in fig. ). the coins should nearly touch each other, and rest upon e s. a part, only, of the electrophorus cover is shown. = . directions.= (a) thoroughly charge the electrophorus cover. (b) place your finger upon the coin marked a, to "ground" it, then quickly touch the coin c with the charged cover, at the same time watching for sparks between the coins. if you cannot see the sparks, darken the room a little, and look at the center coin, b, while doing the experiment. [illustration: fig. .] [illustration: fig. .] =experiment . to show to the eye the strong attraction between a charged and a neutral body.= _apparatus._ the flat box, f b (nos. , ); the electrophorus (see exp. ). = . directions.= (a) stand f b upon its edge upon a level table, then bring the charged electrophorus cover near it. (b) instead of the above, use light hoops made of paper, eggshells, feathers, sawdust, etc. =experiment . to feel the strong attraction between a charged and a neutral body.= _apparatus._ fig. . the flat box f b (nos. , ); the electrophorus (exp. ). = . directions.= (a) hold f b in the left hand, as shown, then _slowly_ bring near it the charged cover, at the same time looking between them so that you can keep them the same distance apart all the way round. (b) bring them near enough to allow a spark to pass from e c to f b. =experiment . the human body a frictional electric machine.= _apparatus._ yourself; a carpet; a room with dry air, easily had on a cold winter's day. = . directions.= (a) scrape your feet along upon the carpet, then quickly touch your finger to some conductor, as, for example, a friend's nose. (b) it is possible to light the gas by the above process. have a friend turn on the gas just before you bring your finger to the jet, and be sure that the spark from your finger passes through the gas on its way to the conductor, the jet. (c) bring your finger quickly near a small piece of tissue paper after you have scraped your feet along to charge your body. _= . static electric machines=_ are used to produce large quantities of static electricity. in the early forms, the electrification was produced by friction. modern machines depend upon the principle of induction. the electrophorus (exp. ) is really a very simple form of induction machine. the potential of these machines is very great, as the spark may jump many inches. thousands of galvanic cells would be needed to make a spark an inch long. when the spark passes through the air it meets with an extremely high resistance, as air practically insulates ordinary electricity. this high resistance in the circuit reduces the strength of the current. while the potential is very high, the strength of the current is very low. (see ohm's law.) chapter xiii. atmospheric electricity. _= . atmospheric electricity.=_ the air is generally electrified, even in clear weather. its charge is usually +. clouds are sometimes +, and sometimes -. the cause of atmospheric electricity is not thoroughly understood. it is thought, by some, to be due to the friction of the particles of vapor upon each other. it is also thought that the evaporation of sea water, and the friction of winds, produce it. _= . lightning.=_ benjamin franklin, in , proved by his famous kite experiment that atmospheric and frictional electricities were of the same nature. by means of a kite, the string being wet by the rain, he succeeded, during a thunder-storm, in drawing sparks, charging condensers, etc. lightning may be produced by the passage of electricity between clouds, or between the cloud and the earth, which, with the intervening air, have the effect of a condenser. if one cloud is charged, it acts inductively upon another, producing in it the opposite kind of electrification. when the attraction between the two electrifications becomes great enough, it overcomes the resistance of the air, and lightning is produced. the flash is practically instantaneous. the direction taken seems to depend upon the conditions of the surrounding air. it has generally a zigzag motion, and is then called _chain lightning_. the air in the path of the electricity becomes intensely heated; it is this effect, and not the electricity which we see. in hot weather flashes are often seen which light up whole clouds, no thunder being heard. this is called _heat lightning_, and is generally considered to be due to distant discharges, the light of which is reflected by the clouds. the _potential_ of the lightning spark is beyond all calculation. the spark jumps through miles of air, which is, really, an insulator. this spark represents billions and billions of volts. _= . thunder=_ is caused by the violent disturbances produced in the atmosphere by lightning. the nature of the sound depends, among other things, upon the distance of the observer from the discharge, and upon the length and shape of the path taken. clouds, hills, and other objects produce echoes, which also modify the original sound. it takes nearly five seconds for the sound to travel one mile. _= . lightning-rods=_, when well constructed, often prevent violent discharges of atmospheric electricity. they have pointed prongs at the top, which allow the negative of the earth (which is being attracted by the cloud when it is positively charged) to pass quietly into the air above; this neutralizes the cloud. in case of a discharge, the rods aid in conducting the electricity to the earth. _= . causes of atmospheric electricity.=_ there are several theories about this. some think that it is due to the rotation of the earth, different parts being acted upon differently by the heat of the sun. some claim that the evaporation of the water in the ocean produces it, while others say that the electrification is produced in the clouds by the friction of their particles upon each other. the matter has not been settled. _= . st. elmo's fire=_ electrification from the earth is often drawn up through the masts of ships to neutralize that in the clouds (see § ), and, as it escapes from the points of the masts, light is produced. this may be clearly shown by repeating exp. in the dark; the head of the pin (fig. ) will represent the end of a mast, and the charged electrophorus cover will be the charged cloud. _= . aurora borealis=_, also called northern lights, are luminous effects often seen in the north. they often occur at the same time with magnetic storms, at which times telegraph and telephone work may be disturbed. the exact cause of this light is not known, but it is thought by many to be due to disturbances in the earth's magnetism, caused by the action of the sun. current electricity. part iii.--current electricity. chapter xiv. construction and use of apparatus. =note.=--before taking up the study of cells and the electric current, let us perform a few experiments in order to understand the construction and use of some of the apparatus needed for such study. a dry cell will be used as the source of the electricity for these first experiments, because it is convenient. you will understand its action later. use this cell only as directed; improper use of it might spoil it. [illustration: fig. .] =experiment . to study the effect of the electric current upon the magnetic needle.= _apparatus._ a compass (no. ); a dry cell, d c (no. ); wires with spring connectors attached (§ ) for making connections. fig. shows a plan or top view of the arrangement. any other form of cell will do in place of d c. = . electrical connections.= one must constantly join wires, connect wires with apparatus, or connect one piece of apparatus to another, to make the proper electrical connections. a very simple method of connections has been used in all the apparatus described in these experiments. a little arrangement which we shall call a spring connector, s c, (fig. ), gives us a means of quickly making connections; that is, it does away with expensive binding-posts. it is made of brass, nickel plated, and may be used anywhere without affecting the magnetic needle. six or eight wires, about no. gauge, each about - / ft. long, should be prepared with a connector at each end. you may use wire furnished (no. ). scrape the insulation from the ends of the wires for about - / inches, then twist the bare ends around the connectors as shown in fig. . the wire should pass around tightly at least or times and then be twisted a little, as shown, to help tighten it. do not put it on so poorly or in such quantity that the part, b, will spread. = .= fig. shows how the connector should be slipped upon a thin piece of metal, m, like that on the galvanoscope, for example. the wire, w, from the apparatus itself is permanently fastened under the head of the screw, s, while the wire from any other apparatus is one of those kept on hand as above mentioned and connected with s c. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] = .= fig. shows how several wires may be quickly joined, electrically, by slipping the connectors at their ends upon a thin metal plate, m p, which may be a piece of tin, zinc, copper, etc. m p should not be too thick. in case the connectors become too much spread to pinch the plate, squeeze the part, a, a little more together. = .= fig. shows the method of connecting with the special form of binding-post used, for example, upon the resistance coil, r c. the end, c, of s c, is pressed down into the tube, t, until you feel, by moving it, that it springs firmly against the sides of the tube. in case you wish s c to fit very tightly in t, one of the legs may be slightly bent outwards. [illustration: fig. .] = .= the connector may be used in still another way; that is, by pushing the part, a, into the hole of an ordinary binding-post, (fig. ), and using it just the same as a thick wire. = . directions.= (a) stand the compass and d c near each other (fig. ). attach one end of an insulated copper wire, c w, to the binding-post, c, which is on the carbon plate of the cell. do _not_ join the other end to the other binding-post, zn, of the zinc plate. (b) with the left hand hold c w above and near the compass-needle, and in the n and s line, so that it will extend over the entire length of the needle. (c) take the free end of c w in the right hand and touch binding-post, zn, for an instant only, watching the needle. repeat. _= . current detectors.=_ we know that a magnet can act, by induction, through the air upon a piece of iron or upon another magnet. the deflection of the needle in this experiment shows that there must be a magnetic field around a wire carrying a current. this fact is of the greatest possible importance. the simple magnetic needle, when used as above, becomes a detector of electricity. [illustration: fig. .] [illustration: fig. .] =experiment . to study the construction and use of a simple "key."= _apparatus._ a key, k (no. ) (§ ); a dry cell, d c, (no. ); a compass, o c (no. ). _arrange_ as shown in fig. , which is a top view or plan. connect the pieces of apparatus with wires and spring connectors (§ ). binding-post, c, is joined to i (in) of the key; o (out) of key is joined to binding-post, zn, the wire, c w, passing directly over and near o c, which is to be used as a detector. the current cannot pass until the lever, l, is pressed. a metal plate, m p, is used to connect two short wires (§ ) in case c w is not long enough. [illustration: fig. .] = . a key= is merely a piece of apparatus by which the circuit can be conveniently and rapidly opened and closed at the will of the operator; that is, by it the electricity can be quickly turned on or off. fig. shows a simple form of key. to the base, b, are fastened two metal pieces or straps, the upper one, l, being the lever or key proper. the front end of l is raised above o, so that the two do not touch each other unless l is firmly pressed down. a screw, s, keeps l from springing too far above o. for convenience we shall suppose that the wire leading to the key joins it at i (in); the wire _from_ the key is joined to o (out), by means of connectors (§ ). the key may be put into any circuit by first cutting a wire and then joining the ends to i and o. spring connectors make the best connections with this form of key. (for home-made keys see apparatus book.) = . directions.= (a) the magnetic needle being directly under the wire, press l down for an _instant only_ and note the action of the needle. (b) press l again, hold it down for seconds, not over that, and watch the needle. _=discussion.=_ the key allows us to easily regulate the length of time during which the current passes. this experiment shows, also, that the magnetic field about the wire disappears as soon as the current ceases to pass. [illustration: fig. .] =experiment . to study the construction and use of a simple "current reverser."= _apparatus._ a dry cell, d c (no. ); a compass, o c (no. ); a current reverser, c r (no. ) (see § ); an insulated copper wire, c w, or feet long, with spring connectors joined to its ends (§ ). _arrange_ as in fig. . the wire, c w, leading from x should be held by the left hand so that it will be just above (or below) and parallel to the magnetic needle. the current cannot pass through c w until one of the straps or levers on c r is pressed. (see apparatus book for home-made reversers.) = . the current reverser.= (no. .) to the wooden base (fig. ) are fastened four metal straps, each turned up at the end so that spring connectors (§ ) can be slipped on to make electric connections with other pieces of apparatus. suppose that at c and z connections are made with the carbon and zinc of the cell, by means of wires and spring connectors (§ ). the current comes from the cell to c. as the two straps, and , press firmly up against strap , and do not touch , it is evident that no current can pass from to or to until they are pressed down upon . two wires are joined by spring connectors to and at their turned up ends, x and y, and these wires lead to any desired instrument. [illustration: fig. .] = . directions.= (a) press down lever (fig. ), for an instant only, at the same time noting carefully in which direction the n pole of the needle is deflected. (b) after allowing lever to spring up again, and after the needle comes to rest, press down lever for an instant, watching the needle. is the n pole of the needle deflected in the same direction as it was in (a)? _= . discussion.=_ the reverser gives us a quick and easy means of reversing the current which is to pass through any desired instrument, first in one direction and then in the opposite direction. suppose (fig. ) that the current enters c r at c, as it does when c is joined to the carbon of the cell; the current can go no farther until one lever is lowered. if lever (fig. ) be now pressed down, as in part (a), the current will pass along , which does not now touch , out through x to a coil of wire or any instrument, and back to the reverser by the wire joined to y. it will then pass from onto , to z, and back to the cell; that is, the current enters c w at x. when lever is pressed, the current still entering c r at c, the electricity will pass onto and out at y, and back through x, and z to the cell. the current, then, can be made to pass out of x or y at will by pressing the proper lever. this experiment also teaches something about currents, but these will be discussed later. [illustration: fig. .] =experiment . to study the simple current detector.= _apparatus._ the compass (no. ); dry cell, d c (no. ); current reverser, c r (no. ); copper wire, c w, a few feet long, with spring connectors on its ends. (see apparatus book, chapter xiii, for home-made detectors.) = . directions.= (a) join the ends of the wire to x and y of the reverser, c r, as in the last experiment. coil up c w so that you can hold the coil with your left hand, as shown in fig. , the magnetic needle being inside of it and parallel to it. (b) press lever of the reverser for an instant only. is the needle deflected more or less than it was when the wire simply passed over or under it once? (c) reverse the current through c w by pressing lever , and note the result. (d) get clearly in mind which way the n pole of the needle is deflected when the current enters c w at x, also when it enters at y. _= . discussion.=_ the current passed _over_ the needle in one direction, and _under_ it in the opposite direction; that is, the part of the wire above _helps_ that below. each turn of the wire increases the strength of the magnetic field about the coil, and helps to deflect the needle. in this way, by increasing the number of turns, detectors may be made that will show the presence of very weak currents. the magnetic fields about wires and coils will be studied in a later chapter. [illustration: fig. .] =experiment . to study the construction and use of the simple galvanoscope.= _apparatus._ the galvanoscope, g v, complete (no. ), described in § - ; dry cell, d c (no. ); current reverser, c r (no. ) (§ ); wires, with spring connectors, to join the different pieces of apparatus (§ ). (see apparatus book, chapter xiii, for home-made galvanoscopes.) = . the galvanoscope= (fig. ) is more than a mere detector of electricity. with it we shall be able to study, more fully, cells, currents, etc., etc. we must first understand its construction. = . the coil-support=, c s, is fastened to the cross-piece, c p, on which are the binding-posts or coil-ends, l, m and r (left, middle, right). the legs, g l, are screwed to c p in such a way that c p is held a little above the table: this allows c s to be tipped to the front or rear to adjust it vertically. on account of the peculiar arrangement of the legs, the galvanoscope can be made to stand firmly, even upon uneven surfaces. the screws holding g l should not be put in far enough to tear the threads in the wood, c p. = . the galvanoscope coils=, g c, are two in number, both being fastened to the coil-support, c s. the first coil has five turns of wire, its ends being fastened to l and m; the other coil has _ten_ turns, with ends at m and r. the current can, at will, be made to pass through , or turns of wire by making the proper connections. suppose that we have two wires direct from a cell, or from the current reverser, with spring connectors on them so that we can slip them onto l, m or r, which stand for left, middle or right. when the wires are joined to l and m the current can pass through but turns; when joined to m and r it will go through turns; and when to l and r it will pass through the entire turns. when the current enters the galvanoscope at l and passes out at m or r, it will pass through the turns of wire from left to right, at the top; that is, it will pass in a "clockwise" direction. = . the compass-needle=, furnished with o c (no. ), will do also for this galvanoscope. it should be placed upon the pin-point after fixing on the pointers (§ ). the length of the needle should be parallel to the plane of the coil when no current passes; that is, the coil and coil-support should be in the n and s line. the needle can be centered in regard to right and left, and in regard to up and down, by properly adjusting the position of the pin-point support, p p s; this is held firmly to c s by two spring-connectors. by removing s c, the support, p p s, may be raised or lowered. = . to place the coil= in the n and s line, simply swing the galvanoscope bodily around, at the same time looking down upon the needle, until the length of the needle becomes parallel to the coil-support. when once carefully adjusted n and s, a line may be drawn upon the table as a guide for its position in future experiments. the coil should stand in a vertical plane, and this straight up and down position can be easily adjusted. to place the coil in the e and w line, turn it until the pointers are at the ° ( degree) marks,--the ° (zero degree) marks remaining, of course, as described above. = . the degree-card, g d c= (fig. ) has a dot at its center, to show where to make a pin-hole for the pin that supports the compass-needle. with this you can tell how many degrees the needle is deflected when the current passes. this card, g d c, should be pressed down over the pin-point. the zero points of g d c should be n and s, also, when the coil is in that position; that is, they should be in the plane of the coil. the pointers on the needle (§ ) will then be at o, when the needle is at rest, no current passing through the coils. (see apparatus book § for home-made degree-card.) g d c may be held permanently in position after it is adjusted, by sticking a short pin through it into p p s. do not let this pin interfere, however, with the swinging of the needle. [illustration: fig. .] = . pointers= (fig. ) should be fastened to the needle, in order to make the readings of degrees accurate. fasten to the compass-needle a piece of no. insulated copper wire, as shown. it may be cut to the proper length after it is wound around the needle. see that the wire does not touch the pin when needle is in place; balance needle by cutting a little from the heavier end of wire with shears; bend the ends of wire so that they are at opposite sides of the degree-card, both pointing at o, for example. the needle must swing freely, be nicely balanced, and the wire must not touch pin or degree-card. [illustration: fig. .] = . directions.= (a) arrange as in fig. , the coil being n and s (§ ). join the ends of the wires, and , with the -turn coil of g v as shown. wire, , is connected to l (fig. ). press lever of c r (fig. ) for an instant, watching the compass-needle and noting how many degrees it swings the first time. get thoroughly in mind the direction in which the n pole of the needle is deflected when the current passes around g c in a "clockwise" direction. there must be no magnets, iron, or pieces of steel within feet of a g. (b) press lever, , for an instant, watching the needle. the current will now pass in an "anti-clockwise" direction. is the needle deflected about the same number of degrees as in (a)? (c) change the ends of the wires, and , to the -turn coil (§ ) and repeat (a) and (b). (d) change and to l and r (fig. ), thus allowing the current to pass around turns; then repeat (a) and (b). _= . discussion; true readings.=_ is not possible to get the magnetic needle, m, exactly in the center of g c; the pointers will not exactly be in the axis of m; the coils will not be exactly n and s: hence, if you pass a certain current through the coil and the pointer reads degrees, you will find, if you reverse the current, that the pointer may read degrees on the other side of the zero mark. to get the _true reading_, average the two, in this case the average being degrees. the galvanoscope gives us an instrument with which we can study, more fully, cells, currents, etc. = . note of caution.= it has already been stated that the compass-needle should be in the center of the coil (§ ), and that the coil should be in the n and s line (§ ). in addition to the above, see that there are no magnets near g v, when using it; tap g v occasionally to be sure that the needle swings freely, hold the eye directly over the pointers when reading degrees; the pointers should be at zero when no current passes through g v; be sure that the electrical connections are good. there are several sources of error in taking readings, and in all the quantitative experiments given. the author takes it for granted that such errors will be looked out for by the teacher. [illustration: fig. .] =experiment . to study the construction and use of a simple astatic needle.= _apparatus._ two unmagnetized sewing-needles (no. ); horseshoe magnet, h m (no. ); piece of stiff paper doubled and cut as in fig. ; a pin-point on which to support the paper. the pin may be stuck through a cork, or that of o c (no. ) may be used. = . directions.= (a) draw each needle across the n pole of h m five times from point to head (exp. ). this should make them of nearly equal strength, both points being n poles. (b) stick the needles through the paper as shown, the n poles being at the same end of the paper. balance the paper upon the pin-point. has this combination a strong or weak pointing-power? (c) turn one of the needles end for end. again test the pointing-power. _= . discussion; astatic needles.=_ by arranging the needles so that their poles oppose each other, the pointing-power becomes almost nothing. this sort of a needle is needed in some experiments in electricity. their magnetic fields are still retained. the combination is called an _astatic needle_; it is used to detect very feeble currents. the more nearly equal the magnets are in strength, the better. they are usually arranged with one above the other (fig. ). [illustration: fig. .] [illustration: fig. .] =experiment . to study the construction and use of a simple astatic galvanoscope.= _apparatus._ an astatic galvanoscope, a g (no. ) (§ - ); dry cell, d c (no. ); current reverser, c r (no. ) (§ ); wires for connections (§ ). _arrange_ as shown in fig. , which is a top view. the wires from c r are connected to the binding-posts of a g at the back, the spring connectors being slipped into them (§ ). = . construction of the astatic galvanoscope.= when not to be used for a long time, or for shipping, the legs, a (fig. ) may be removed, and the whole packed inside of the box, b. the _coil_, c, has a resistance of about ohms, and is fastened to the coil-support, c s. the ends of the coil are permanently fastened to the binding-posts, l and r (left and right). the ends are so arranged that when the current enters at l it will pass around the coil in a clockwise direction. = . the astatic needle= (exp. ) is supported by a small thread, t, which is tied to the thread-wire, t w. this t w springs into an eyelet, e, which, in turn, rests in a hole made in the end of b. e should turn easily in the hole, but it should not wabble. fig. shows a sectional view of the coil and needle. the wire, w, should be bent, as shown, so that the magnets can be as near the center-line of c as possible. fig. shows a front view of the needle. as a matter of convenience it will be best to arrange the poles of the needles, as shown, to agree with the descriptions of the experiments. to keep the needle from being affected by air currents, the glass plate (no. ) may be placed in front of the box, b. stand it upon the legs, a, and tie a string around it, and b, to hold it in place. [illustration: fig. .] [illustration: fig. .] = . adjusting the needle.= as t is tied to t w, the needle may be swung completely around by turning t w. this should be done until the length of the needle is parallel to the turns of c. the up and down position of the needle should be fixed as nearly as possible when fastening t to t w, the exact place being finally fixed by raising or lowering t w through e. the spring in t w should hold it firmly in e after adjustment. the wire, w, joining the needle-magnets should not touch the coil. it may be made to just swing free from c by tilting the box forward or backward a little. the construction of the legs, etc., makes it possible to tilt the box, and to make it stand firmly upon an irregular surface. [illustration: fig. .] = . directions.= (a) see that there are no magnets within feet of a g. test the astatic needle, after you have it properly suspended, to convince yourself that it does not try to swing around in a n and s line. in case the needle-magnets have been in contact with other magnets, or are not equally magnetized, remagnetize them as directed in exp. . they must remain in any position given them by turning t w. finally, bring them parallel to the turns of the coil. (see § .) arrange as in fig. . (b) press lever of c r (§ ) for an instant only. this allows the current to enter a g at l. repeat several times until you thoroughly fix in your mind the direction in which the right-hand end of the needle is deflected. does the needle jump suddenly when the current passes? (c) press lever for an instant only. study the result. _= . astatic galvanoscopes.=_ it is evident that in the ordinary current detector (exp. ), the pointing power of the needle has to be overcome by the magnetic field about the coil, before the needle can be forced from its n and s line. very weak currents will not visibly move the needle in ordinary detectors. to make a sensitive instrument we must have strong fields about both the needle and coil, and we must, at the same time, decrease the pointing power of the needle. both of these things are accomplished by using an _astatic needle_ in connection with a coil containing considerable wire. the uses of the astatic galvanoscope will be studied more fully in later experiments. chapter xv. galvanic cells and batteries. =experiment . to study the effect of dilute sulphuric acid upon carbon and various metals.= _apparatus._ a piece each of copper and iron wire in., ( cm.) long; two narrow strips of sheet zinc (no. ), one being amalgamated (§ ); a carbon rod (no. ); a tumbler (no. ), partly filled with dilute sulphuric acid, (§ ); mercury (no. ). = . to amalgamate= one of the above zinc strips is to coat it with mercury. remove all jewelry from the hands before proceeding. wash the zinc with water, and with a cloth remove all dirt from its surface. amalgamated zinc is very brittle, so lay it flat upon a piece of board or upon a plate, after dipping it for an instant in the dilute acid. place a small drop or two of mercury upon the strip, and rub the mercury about upon both sides of the zinc with a cloth made wet with the dilute acid. mercury clings strongly to zinc or tin, so you may use a narrow piece of either as a spoon to carry a small drop from the supply to the zinc. tap it upon the zinc to dislodge the drops. do not get on too much mercury, just enough to coat it, or, at least, that part of it that will be under the acid. be careful not to break the thin zinc when amalgamating it, as it gets very brittle. it should look bright. (see apparatus book § , .) _note._ should any mercury get upon copper plates it may be removed by heating them in a flame. = . dilute sulphuric acid=, for these experiments, should be made by mixing part, by volume, of concentrated acid, with parts of water. do not let the acid get upon clothes or carpets. do not add water to acid. mix by _slowly_ adding the acid to the water in a glass or earthen dish, stirring at the same time. mix over a sink or out of doors. (for fuller details see app. book; § , , , , .) to save time, make at least a quart of the mixture, bottle, and label it for use. = . directions.= (a) bend over one end of each of the wires and metal strips, and hang them upon the edge of the tumbler (fig. ), so that their lower ends shall be in the acid. do not let them touch each other. stand the carbon rod in the acid. if there is no visible action upon any of the above substances, add a few drops of concentrated acid to the tumbler. note carefully what takes place in the tumbler, and state which of the substances are dissolved, which simply made brighter, and which not acted upon at all. _= . discussion.=_ the bubbles of gas that arise from the zinc when it dissolves are hydrogen, and they indicate that a vigorous chemical action is going on in the tumbler, and that the zinc is being eaten away. [illustration: fig. .] [illustration: fig. .] =experiment . to study the effect of dilute sulphuric acid upon various combinations of metals.= _apparatus._ the same as in the last experiment. a small piece of amalgamated zinc, however, will be better than the whole strip. = . directions.= (a) twist one end of the clean copper wire around the small piece of amalgamated zinc (fig. ). hold one end of the wire in the hand and dip the combination into the acid. what takes place? watch the surface of the copper, remembering that each, alone, was not acted upon by the acid (exp. ). (b) use the clean iron wire in place of the copper wire, and repeat (a). watch the surface of the iron. (c) with a string or thread tie a small piece of well amalgamated zinc to the carbon rod (fig. ), then dip the combination into the acid. watch the surface of the carbon. _= . discussion.=_ while amalgamated zinc is not rapidly dissolved by dilute sulphuric acid, a vigorous action of some kind takes place when it is in contact with another metal or with carbon in the acid. the bubbles of hydrogen that are liberated do not seem to come from the zinc; they appear to grow, in the fluid, directly at the surface of the copper, iron, or other metal used with the zinc. this shows that something besides the mere dissolving of a metal takes place. can we arrange our apparatus so that we can get some useful results from this action? =experiment . to study the construction of a simple voltaic or galvanic cell.= _apparatus._ a narrow strip of zinc (no. ), amalgamated as directed in § . (an amalgamated zinc rod (no. ) may be used in place of the strip); a narrow strip of sheet copper (no. ); the tumbler of dilute acid of exp. ; a flexible copper wire about feet long, with spring connectors (no. ) attached to its ends. (see electrical connections, § .) [illustration: fig. .] = . directions.= (a) holding the amalgamated zinc strip in one hand and the copper strip in the other (fig. ), dip them into the acid, but do not let them touch each other. note any chemical action. (b) touch the copper and zinc together, _below_ the surface of the acid. watch the copper. (c) separate the lower ends of the strips, then touch them together _above_ the acid. anything still happen to the copper? (d) slip one spring connector with the attached wire upon the zinc strip, then stand the strips in the tumbler, so that they can not touch each other. now touch the copper strip with the free end of the wire, at the same time watching the copper. (e) raise the wire from cu, touch it to cu again, and repeat several times until you are sure that something takes place every time the wire touches cu. _= . the electric current.=_ something must happen in or through the wire, and it can only happen when the two metals are joined in some way. this peculiar, invisible action in the wire is called the _electric current_, and such an arrangement is called a _galvanic cell_. _= . source of the electrification.=_ when two different metals are placed in acid they are electrified unequally by chemical action. each has a higher potential than the acid, but their potentials are not the same. this electrification tends to pass from the place of higher to the place of lower potential, and the conducting wire allows this transfer to take place. as the difference of potential is kept up by the continued chemical action, the current is continuous, and not simply instantaneous, as in discharges of frictional electricity. as heat is produced by the burning of coal, so electrification is produced by the chemical burning of zinc. chemical energy is the source of electrification in the galvanic cell, just as muscular energy was the source of the electrification in the experiments with frictional electricity. _= . the electric circuit; open and closed circuits.=_ the simple galvanic cell just used, together with the wire which joined the metal strips, is called an _electric circuit_. if the current should pass through a telegraph instrument, for example, on its way from one strip to the other, the telegraph instrument would also be in the circuit. when the wire is cut or removed from one metal strip, the circuit is said to be _open_--that is, we have an _open circuit_. when the current passes, the circuit is _closed_. we also say _make_ and _break_ the circuit, and that the circuit has been _broken_. _= . plates or elements.=_ the copper and zinc strips are called the _plates or elements_ of the cell. the zinc, zn, fig. , is dissolved by the acid, and is called the positive plate (+ plate). the copper, cu, is the negative plate (- plate). the copper is also called the _cathode_, and the zinc the _anode_. _= . direction of current.=_ it has been agreed, for convenience, that _in_ the cell the current passes from the zinc through the liquid to the copper, where the hydrogen bubbles are deposited. it then passes through the wire, or other conductor furnished, back to the zinc, through the liquid to cu again, and so on around and around thousands of times per second. the current really starts at the surface of the zinc, where the chemical action is. when carbon and zinc are used, the action and direction of the current are the same, carbon being the - plate. [illustration: fig. .] _= . poles or electrodes.=_ if the wire were cut, the electricity coming from the + plate would be stopped at the end of the wire marked +, fig. , after passing through the acid and up cu. this end of the wire is called the + _pole or electrode_ (positive). the end of the wire joined to zn is called the - _pole_ or _electrode_; that is, the + electrode is the end of the wire attached to the - plate. the tops of cu and zn may be considered electrodes. the top of cu is the + _pole_ of the cell, while cu is the - _plate_. = . chemical action in the simple galvanic cell.= the chemical formula of sulphuric acid is h_{ }so_{ } (read h, , s, o, ). this means that it is a compound of hydrogen (h), sulphur (s), and oxygen (o). the h_{ }so_{ } stands for _molecule_ of acid, and the small figures show that the molecule is made up of _atoms_ of hydrogen (h_{ }), one of sulphur (s), and of oxygen (o_{ }). the atoms are held together by _chemical attraction_ or _affinity_. there is a stronger chemical affinity between zinc (zn) and so_{ } than between h_{ } and so_{ }; so, as soon as the zn gets a chance, as it does in the cell, it drives out the h_{ }, and it takes its place in the molecule. this chemical _reaction_ may be shown by the following chemical _equation_: zn + h_{ }so_ = znso_ + h_ . zinc and sulphuric acid produce zinc sulphate and hydrogen. the zinc sulphate produced weakens the effect of the acid; in fact, the acid has to be renewed occasionally, as it is changed to the sulphate which remains in solution. = . action in cell using impure zinc.= the above action takes place in the cell when impure zinc is used, even when no current passes, heat being produced by the reaction instead of useful electricity. (see local currents.) =action in cell using pure zinc.= when pure zinc (or impure zinc that has been properly amalgamated) is used in the cell, it dissolves, or is eaten away, only when the current passes. it should be noted that the bubbles of hydrogen do not even then appear at the zinc; they are not seen throughout the body of the liquid. there seems to be an unseen transfer of hydrogen from the zinc, through the liquid, to the copper (or other - plate used), and it appears there in the shape of bubbles. the larger the quantity of pure zinc dissolved, the stronger the current, and the larger the amount of hydrogen produced. as the zinc dissolves it parts with its latent energy, and this energy forces the electric current through the circuit. while the hydrogen of the decomposed acid makes its way towards the - plate, the so_ part of it travels towards the zn plate, where the znso_ is formed. (see § .) =experiment . to see what is meant by "local currents" in the cell.= _apparatus._ tumbler of dilute sulphuric acid. (§ ); strip of unamalgamated zinc; crystal of copper sulphate (blue vitriol) (no. ); a galvanized iron nail (no. ), this being iron covered with zinc. = . directions.= (a) hold the nail in the acid for a few seconds, and note result. (b) rub the copper sulphate upon the zinc simply in one spot, then place the zinc in the acid, noting the result at the spot. _= . local action; local currents.=_ ordinary commercial zinc contains such impurities as carbon, iron, lead, etc., in small quantities. it was seen, exp. , that when different metals were in contact with the zinc, the zinc was rapidly dissolved by the acid. the impurities in the zinc act like the copper plate in the simple cell, thus producing _local currents_ in the zinc, which rapidly destroy it without doing any good. these currents pass from zinc to impurities, and back to the zinc, without going out into the main wire. this local action takes place even when the main circuit is open. _= . reasons for amalgamating zinc plates.=_ pure zinc is not affected by dilute sulphuric acid, but it is too expensive to use in cells; so amalgamated zinc is used instead, because it is cheaper, and acts the same as pure zinc. the impurities are removed from the surface of the zinc, as the zinc alone is dissolved by the mercury. there is, then, a liquid layer of pure zinc with mercury upon the surface of the amalgamated plate. this is not acted upon by the acid when the circuit is open. a stronger and more regular current is produced with amalgamated zinc than with the impure unamalgamated zinc. =experiment . to study the "single-fluid" galvanic cell.= _apparatus._ the galvanoscope g v (no. ), (see § , etc.); the simple cell arranged as described in § , the zinc being amalgamated. = . the simple cell= should be arranged so that the plates will be held firmly in position. the zinc, zn (no. ), and copper, cu (no. ), should be fastened to the wooden cross-piece, w c p (no. ), as shown in fig. . care should be taken not to use longer screws than those provided for (no. ). if the screws touch each other they will short circuit the cell. partly fill the tumbler (no. ) with dilute sulphuric acid (§ ), join wires with connectors to the plates. the free ends of the wires are then ready to join to apparatus. the ends of wires _may_ be fastened under the screw-heads instead of using connectors on the plates. do not put the plates into the acid until you read the "directions." = . directions.= (a) arrange as in fig. . place the coil of g v, n and s (§ ). _before_ putting the plates in the acid join them to the -turn coil of g v (§ ). the compass-needle should point to zero. see that the needle swings freely. (b) place the plates in the acid, and _quickly_ bring the needle to rest with the aid of the hand, so that you can take the reading at once before the hydrogen bubbles entirely cover the copper plate. watch the action of the needle for a few minutes. make a note of the reading, in degrees, at the beginning of the experiment and at the end of five minutes. (see note.) [illustration: fig. .] [illustration: fig. .] =note.= if no change takes place in the position of the needle, the change beginning inside of seconds after the plates are let down into the acid, withdraw the plates, then clean and thoroughly dry the copper to remove all traces of hydrogen. this may be done by heating the copper over a gas flame. let the copper remain in the air minutes, then try again. in taking the first reading you must work quickly. catch the needle during its _first_ swing. if you allow it to swing back and forth until it comes to rest, your first reading will not be what it should be. (c) after the needle has remained in one position, without change, for or minutes, take hold of the wooden cross-piece, move the plates back and forth in the acid to dislodge the hydrogen bubbles, and note carefully the action of the needle. does the current seem stronger when the plates are moved? can you get the needle back to the first reading? (d) remove the plates from the acid, dry and clean the copper, let them stand in the air for minutes, then take another quick reading and compare it with the first. _= . polarization of cells.=_ the acid gets a little weaker, of course, as it is decomposed by the zinc (§ ), but the chief cause of the weakening of the current is hydrogen, which forms a filmy coating upon the copper plate. this coating even seems to soak into the copper, and it takes some time for it to be thoroughly removed. the zinc plate is kept comparatively free from hydrogen by amalgamation. _= . effects of polarization.=_ the hydrogen bubbles weaken the current in at least two ways. in the _first_ place, hydrogen is not a conductor of electricity; so it holds the current back, as any other resistance would. _secondly_, acid acts upon hydrogen as it would upon another metal. when the copper plate becomes well covered with hydrogen, the acid cannot touch it; so we really have a _hydrogen plate_ in the cell. hydrogen acts very much like zinc in the acid; we say that it is more electro-positive than copper. the result is, then, that a new current starts up, and as this is towards the zinc, in the acid, it partially destroys or neutralizes the main zinc-to-copper current. practical use is made of the principles of polarization (see secondary batteries). _= . remedies for polarization; depolarizers.=_ any scheme by which the hydrogen may be destroyed and kept from the inactive, or negative plate, will prevent polarization. _mechanical_ means have been employed to brush away the hydrogen by keeping up a constant circulation of the liquid. _chemical action_ is another means by which the hydrogen may be side-tracked before it gets to the - plate in single-fluid cells. substances like nitric acid and bichromate of potash, called _depolarizers_, contain large quantities of oxygen, and, during the chemical changes that take place, this oxygen unites with the hydrogen. these substances are used in zinc-carbon cells. (see § , etc. for various forms of cells.) there is another form of cell, the _two-fluid_ type, in which _electro-chemical_ means are employed, and in which a metal is deposited upon the - plate instead of hydrogen. the - plate is usually copper, copper being deposited upon it. =experiment . to study the "two-fluid" galvanic cell.= _apparatus._ the glass tumbler, g t, (no. ); porous cup, p c, (no. ); the strip of zinc (no. ), well amalgamated (§ ), or the amalgamated zinc rod (no. ); piece of sheet copper (no. ), bent so that it will surround p c; copper wires, c w, with connectors; a saturated solution of copper sulphate, commonly called blue vitriol or blue stone (see § ); dilute sulphuric acid (see § ). with the above, set up the two-fluid cell (see § ). the galvanoscope, g v, complete, is also needed, and if quantitative work is to be done, a pair of scales weighing to . gram is necessary. (see app. book, chapter i, for home-made two-fluid cells.) [illustration: fig. .] = . setting up the two-fluid cell.= fig. . stand the amalgamated zinc rod, zn, in p c, then place p c in the tumbler, g t; put in the copper plate as shown. pour diluted acid (§ ) into p c until it stands about - / in. deep; then at once pour the copper solution (§ ) into g t, on the outside of p c, until it stands at the same height as the acid _in_ p c. as soon as the liquids have soaked into p c the cell will be ready for use; but it is better to connect it with the galvanoscope at once and note the increase of current during the first few minutes while the liquids soak through p c. a crystal of copper sulphate should be put outside of p c to keep the solution full strength. this is a form of the well-known daniell cell. fig. shows a form of home-made two-fluid cell as described in apparatus book. if you have the one furnished, use the rod instead of sheet zinc, and use connectors on the plates. = . care of two-fluid cell.= this experimental cell should be taken apart when not in use. it should not be left in open circuit, even for half an hour. even after the plates are removed, copper may be deposited upon p c in case there are any metallic impurities on it. remove the plates and p c, and thoroughly wash them. the copper solution should be put into a bottle to prevent evaporation; the dilute acid may be thrown away to be replaced by fresh acid for the next experiment. = . copper sulphate solution= is made by adding the blue crystals to water until no more will dissolve--that is, the solution should be "saturated," extra crystals always being left in the stock bottle. an ounce of the crystals to half a tumbler of water will be about right, but a pint or so should be made at a time and be kept bottled to save time. = . directions.= (a) in case you have access to a pair of scales that weigh to within . gram, carefully weigh both copper and zinc before proceeding. they should be washed and dried with a cloth. see that there are no drops of mercury upon the zinc that may fall off during the experiment. (b) replace the plates in the cell, and connect them with the -turn coil of g v, placed n and s. allow the circuit to remain closed for half an hour, and record the position of the needle every minutes. (c) again wash, dry with a cloth without rubbing, and weigh both the zinc and copper. compare the new weights with those found in (a). _= . chemical action in the two-fluid cell.=_ in this form of cell the hydrogen is not allowed to collect upon the copper plate. the action inside of p c is like that explained in § , hydrogen being set free. as soon as this hydrogen (h_{ }) comes in contact with the copper sulphate (cuso_{ }), and it begins to do this in the walls of p c, a new chemical reaction takes place. hydrogen has a stronger attraction for so_{ } than cu has, so it unites with the so_{ }, forming h_{ }so_{ } (sulphuric acid), and this at the same time throws out the cu bodily. this cu is then free, instead of hydrogen, to be deposited upon the copper plate. the current is constant, as there is no polarization. _= . various galvanic cells; open and closed circuit cells.=_ there is no one form of cell that is best for all kinds of work. if momentary currents only are wanted, such as for bells, telephones, etc., in which the cell has plenty of time to rest, _open circuit_ cells are used. these cells polarize, however, when the circuit has to be closed for any length of time. this form of cell is always ready for use, and may not need attention for months at a time. the most common forms of the open circuit cells are the _leclanché_ (§ ) and _dry_ cell (§ ). open circuit cells polarize quickly, because the depolarizer (§ ) is slow in destroying the hydrogen. when a strong current is needed for a considerable time, such as for telegraph lines, motors, etc., a _closed circuit_ cell is necessary. the depolarization must be rapid and constant. the _bichromate_ (§ ) and the _daniell_ cell (§ ) are very common forms of closed circuit cells. (see, also, storage cells.) = . the leclanché cell= is an open circuit cell in which carbon and zinc are the plates. the carbon is surrounded with dioxide of manganese, a depolarizer; the two are either packed together in a porous cup, or the latter is compressed into blocks, which are fastened to the carbon. the porous cup stands in a jar containing a solution of sal ammoniac (ammonium chloride), which acts as the exciting fluid, and in which stands a zinc rod. the zinc is not acted upon when the circuit is open. the hydrogen given off by the decomposition of the ammonium chloride is destroyed by the oxygen contained in the manganese dioxide. the e. m. f. is nearly . volts. = . dry cells= are for open circuit work. sheet zinc forms at the same time the active plate and the outside cylinder or case. a carbon plate acts as the inactive or - plate. the exciting fluid is kept from spilling by its being absorbed by one of the various substances used for that purpose. = . the bichromate of potash cell= is a very common one for laboratory use. it gives a strong current, and although a single fluid cell, it does not readily polarize. zinc and carbon plates are used. in the sulphuric acid, which is the exciting fluid, is dissolved bichromate of potash. this cell is used for running small motors, induction coils, etc. it has, with fresh solutions, an e. m. f. of about volts. (see apparatus book, chapter i., for home-made batteries.) = . the daniell cell=, of which the two-fluid cell used in exp. is a form, is noted for its constant current. the e. m. f. is a little over volt, and it should be kept working through a resistance when not in regular use; it should not be left in open circuit. the porous cup keeps the two fluids from mixing, but it does not stop the current. = . the gravity cell= is a form of the above. as one of the fluids is heavier than the other, no porous cup is needed. gravity, together with the action of the current, tends to keep the fluids separated. a copper plate is placed in the bottom of the jar, and upon this is put the copper sulphate solution. the zinc plate is supported by the top of the jar and rests in a solution of zinc sulphate, which is lighter than the blue solution below. an insulated wire extends from the copper through the liquids. this cell is used for telegraph and similar work. (see apparatus book for home-made gravity cell, its regulation, etc.) chapter xvi. the electric circuit. =experiment . to see what is meant by "divided circuits" or "shunts."= _apparatus._ the galvanoscope, g v (no. ); astatic galvanoscope, a g (no. ); two-fluid cell, -f c (see § ); wires with connectors; small thin pieces of tin or other metal, m p, for rapidly making connections (§ ). _arrange_ as in fig. . the wires, and , from -f c, lead to the metal plates m p-a and m p-b, for convenience. the wires, and , from g v, are also connected with these plates. the wires, and (dotted lines), lead from a g, to be used as directed in part (b) of the experiment. see that g v is properly placed. see that a g is adjusted. [illustration: fig. .] = . directions.= (a) without a g in place, take the reading of g v. the current now passes from cu through , m p-b, , g v, , m p-a, to zn. (b) connect wires and to the plates, as shown by the dotted lines. again take reading of g v, and compare it with the first reading. does some of the current pass through a g? _= . divided circuits; shunts.=_ the current divides at m p-b into two parts; one part may be called a _shunt_ of the other. the circuit is said to be _divided_; it has two branches. if the two ends of a wire be fastened to another as in fig. , the circuit is also divided. when two or more conductors lead side by side from one point to another, they are called _parallel_ circuits; that is, the conductors are joined in parallel. as strong currents would injure delicate galvanometers, a small part only of the current may be allowed to pass through the galvanometer by using a shunt. fig. shows such an arrangement, in which most of the current passes through the shunt, s. there are many practical uses of shunts. [illustration: fig. .] =experiment . to see what is meant by "short circuits."= _apparatus._ about the same as in exp. , fig. . the astatic galvanoscope is not needed; in place of it provide a short piece of metal, such as a battery-plate, or even a jack-knife. _arrange_ as in fig. , but without a g. = . directions.= (a) with the current passing as described in exp. (a), take the reading of g v. (b) lay the ends of the metal, or other thick conductor, upon m p-a and m p-b. compare the new reading of g v with that in part (a). (c) remove the conductor used to short circuit g v, take the reading in degrees, then touch m p-a to m p-b; watch g v. _= . short circuits=_ are very apt to occur unless care is taken. do not allow uninsulated wires to touch each other. as shown by the above experiment, practically the whole of the current may be side-tracked by a _shunt of low resistance_. a galvanic cell is short-circuited by connecting the plates directly by a wire or other conductor. chapter xvii. electromotive force. _= . electromotive force.=_ it has been stated that a galvanic cell has the _power_ to charge one of its plates positively and the other negatively; this power is called _electromotive force_, and, for short, e. m. f. is written. the e. m. f. of a cell depends upon the kinds of plates used and their condition, the chemicals used in the exciting fluids, etc. the greater the e. m. f. of a cell the greater its power to force the current through wires, etc. the e. m. f. of a cell does not depend upon the size of its plates, as will be seen by later experiments. _= . unit of e. m. f.; the volt.=_ a certain amount of e. m. f. has been taken as the standard, and, in honor of volta, it has been called the volt. the e. m. f. of the two-fluid cell used in exp. is not far from volt. if a certain cell has the power to keep up twice the difference of potential between its terminals that the daniell cell has, we say that it has an e. m. f. of about volts. _=voltmeters=_ are instruments to measure e. m. f. =experiment . to see if the e. m. f. of a cell depends upon the materials used in its construction.= _apparatus._ tumbler two-thirds full of dilute sulphuric acid ( ); strips of zinc, zn (no. ); copper strips, cu (no. ); iron strip, i (no. ); lead strip, l (no. ); carbon rod (no. ); the galvanoscope, g v (no. ); wires with connectors (§ ), so that the plates can be changed quickly; the wooden cross-piece, w c p (no. ). _arrange_ as in fig. . the metal strips are all of the same size; they may be held with the hand firmly against w c p, in order to have them the same distance apart in each trial. they should be lowered to the bottom of the tumbler in each case, in order to have them acted upon by the same amount of acid. place g v properly. [illustration: fig. .] = . directions.= (a) with zn and cu connected to g v as shown (fig. ), take the reading in degrees, and note in which direction (east or west) the n end of the needle is deflected. tabulate results, as shown in fig. , filling in each column of your table made out on paper. (b) in like manner try the following combinations in the order given, in each case connecting the first-mentioned plate with the left-hand binding-post, l, of g v. for (b) use zinc-iron. (c) use zinc-lead; (d) iron-copper; (e) iron-lead; (f) lead-copper; (g) copper-carbon. [illustration: +------+---------------+------------+-------------+------------+ | part | plates. | liquid. | deflection. | current in | | | | | | cell from | +------+-------+-------+------------+-------------+------------+ | (a) | zinc. |copper.| dil. sulp. | ° west. | cu to zn | | | | | acid. | | | | (b) | | | | | | | | | | | | | | (c) | | | | | | fig. .] = . note.= some of the combinations produce but slight currents. in case g v is not delicate enough to show clearly which way the current passes, use the astatic galvanoscope in its place for such combinations. _= . discussion.=_ exp. clearly showed that different combinations of metals in the acid have different powers of pushing electricity through the galvanoscope. although some of the pairs of metals furnished so weak a current that it was necessary to use the astatic galvanoscope to study the current, all produced _some_ current, and from the results can be formed an electromotive series (§ ). the strength of acid, condition of plates, etc., affect the e. m. f. of a cell. = . electromotive series.= all metals are not acted upon to the same degree by dilute acid. from the results of exp. it is seen, part (b), that iron is electronegative to zinc; that is, the current in the cell flows from zinc to iron. part (d) showed that iron is electropositive to copper, as the current flowed from iron to copper in the cell. it is possible to arrange the metals in a series, one below the other, in such a way that any one will be electronegative to those above it and electropositive to those below it; that is, the list should have the most electropositive metal at the top, and the one least acted upon by the acid at the bottom. make such a list from your results. the farther the metals used are apart in the _list_, the greater will be the e. m. f. of the cell. good carbon is acted upon the least of all, so zinc and carbon are better than zinc and copper. =experiment . to see whether the electromotive force of a cell depends upon its size.= _apparatus._ galvanoscope; two glass tumblers; dilute acid; two wooden cross-pieces; two copper and two zinc strips, the same size as those used for exp. . (see § ). these materials will form two simple cells like fig. . have about in. of acid in one tumbler, and but in. in the other. the plates of one cell will then be about - / in. in acid, and those of the other cell only / in. in acid. this gives us the same effect as a large and a small cell. = . directions.= (a) join the large and small cells with g v so that their currents will oppose each other. to do this, join the two zinc plates by means of a wire and connectors. with two other wires connect the two copper plates with the galvanoscope binding-posts, and watch for any indication of current. does one cell oppose the other? _= . discussion.=_ the e. m. f. of a cell, then, does not depend upon the size of its plates. the small piece of zinc--that is, the one in but little acid--had the same potential as the large piece; they must have had, as they were joined. the large cell will give a stronger current, under certain conditions, than the small one; but this depends upon other things than e. m. f. (see experiments under current strength.) a zinc-copper cell, like the one just used (exp. ), has the same _voltage_ as one of the same kind would have, even though it were made as large as a barrel. chapter xviii. electrical resistance. _= . resistance.=_ it is harder for a horse to draw a wagon through deep sand than over a smooth pavement. we may say that the sand holds the horse back--that is, it offers a resistance. the electric current does not pass through all sorts of substances with the same ease, and when it succeeds in pushing its way through a circuit of considerable resistance, we cannot expect it to arrive at the end of its journey without being weaker than when it started. do we expect this of a man or horse? we shall soon see that there is a definite relation between resistance and the strength of the current at the end of its journey. =experiment . to study the general effect of "resistance" upon a current.= _apparatus._ galvanoscope, g v (no. ); resistance coil, r c (no. ) (§ ); two-fluid cell, -f c (§ ); wires with connectors (§ ). _arrange_ as in fig. . the current passes as shown by the arrow, and the circuit may be opened and closed at the metal plate, m p, or by using a key in its place. properly place g v. = . directions.= (a) take the reading of g v in degrees, the current passing through the entire length of r c. (see § .) (b) change the end of wire from binding-post r to m, on r c, so that the current will pass through one-half only of r c. note the reading of g v. (c) remove r c entirely and connect wires and by means of a metal plate. compare the readings of (a), (b) and (c). what do they show? [illustration: fig. .] _= . external resistance; internal resistance.=_ when we consider a circuit like that shown in fig. we see that it is composed of two parts, and that we have two kinds of resistances. the wires, instruments, etc., make up what is called the _external resistance_ of the circuit; that is, the part that is external to the cell. the liquids in the cell offer a resistance to the current; this is called _internal resistance_. (see § .) the strength of the current depends upon the relation between these two resistances, as will be seen by future experiments, as well as upon the e. m. f. of the cell. as liquids are not as good conductors as metals, the internal resistance of cells may be quite high. _= . unit of resistance; the ohm.=_ whenever anything is to be measured, a standard, or unit, is necessary. the unit of resistance is called the ohm, in honor of ohm, who made careful investigations upon this subject. a column of mercury having a length of a little over feet has been taken as a unit. (the column taken is . cm. with a weight of . grams; it has a cross-section of about sq. mm., at a temperature of °c.) mercury is a liquid, and has no "grain" to affect the resistance. for the use of students, ft. in. of no. copper wire, or ft. in. of no. copper wire will make a fairly good ohm. we might, of course, take any other length as _our_ standard; the above, however, will give results that are approximately correct. (see wire tables at the end of this book.) _= . resistance coils; resistance boxes.=_ coils of wire, having carefully-measured resistances, are called _resistance coils_. the wire for any coil is doubled at the center before it is wound into coils or upon spools (fig. ) to avoid the magnetic effect. the ends of the coils are attached to binding-posts, or to brass blocks, in regular instruments, so that one or more coils can be used at a time; that is, so that they may be handled in a manner similar to that in which the different coils on the galvanoscope are used. if we have coils of , , , and ohms resistance, we shall be able to use any number of ohms from to by making the proper connections. (see apparatus book, chapter xvii, for home-made resistance coils.) for protection and convenience, coils are usually placed in a box, the whole being called a _resistance box_. the ends of the coils are joined to brass blocks, placed near each other on the top of the box, and between which may be pressed plugs when it is desired to short circuit the coils. by removing a plug, the coil, whose ends are joined to the blocks touching it, is brought into the circuit. [illustration: fig. .] [illustration: fig. .] = . simple resistance coil.= fig. shows a simple form of coil, r c (no. ). the total resistance is ohms, l (left) and r (right) being binding-posts to which the ends of the coil, c, are joined. m (middle) connects with the middle of the wire, at which point the wire is doubled. the coil is fastened to a stiff pasteboard base, b. =connections.= when ohms resistance are wanted, let the current enter at l and leave at r (or the reverse). when ohm is wanted, let the current leave or enter at m, the other wire being joined to l or to r. connections should be made with spring connectors. see § . =experiment . to test the power of various substances to conduct galvanic electricity.= _apparatus._ galvanoscope, g v (no. ); dry cell, d c, or two-fluid cell, -f c; pieces of different metals; wood, dry and damp; tumbler of pure water; rubber; ebonite; silk; glass, etc., etc. _arrange_ as in fig. , leaving out r c, and instead of having m p between wires and , use their free ends to press firmly upon the ends of the substance to be tested; that is, the body under test should take the place of m p in the fig. g v will show a deflection, of course, when the particular thing under test is a conductor. = . directions.= (a) make tests with the above substances, and with any others at hand, and note which are conductors and which are not. _= . conductors and nonconductors.=_ it is evident, from the experiments, that bodies which conduct static electricity do not necessarily conduct galvanic electricity. the greater the e. m. f. of a current, the greater its power to overcome resistance. some bodies, like dry wood, that readily conduct the high potential static electricity, make fairly good insulators for the low potential galvanic currents. for convenience, substances may be divided into _good conductors_, _partial conductors_, and _insulators_, or nonconductors. _=good conductors.=_ metals, charcoal, graphite, acids, etc. _=partial conductors.=_ dry wood, paper, cotton, etc. _=insulators.=_ oils, porcelain, silk, resin, shellac, ebonite, paraffine, glass, dry air. [illustration: fig. .] =experiment . to find the effect of sulphuric acid upon the conductivity of water.= _apparatus._ galvanoscope, g v; cell; -f c; connecting wires; saucer or tumbler, s; a little sulphuric acid. _arrange_ as in fig. . = . directions.= (a) put a little pure water in s, and see if enough current can pass through it to deflect the needle of g v. the ends of the wires, and , should be gradually moved toward each other, the needle being watched. (b) put or drops of concentrated acid into the water; stir it, then repeat the test. what effect has the acid? _= . internal resistance.=_ as found in exp. , pure water is not a good conductor of galvanic electricity. the acid in the simple cell, and in other single-fluid cells, acts upon the zinc and at the same time makes it possible for the current to pass, as it reduces the internal resistance. as seen later, this resistance in cells is greatly diminished by bringing the plates near each other, and by increasing the surface of the plates that are in contact with the acid. the larger the plates the less the internal resistance, other things remaining the same. the internal resistance of a _battery_ can be changed by connecting the cells differently. (see chap. on arrangement of cells.) [illustration: fig. .] =experiment . to find what effect the length of a wire has upon its electrical resistance.= _apparatus._ a no. german-silver wire, g-s w, a little over two meters long, un-insulated (no. ); the two-fluid cell, -f c (exp. ); galvanoscope, g v (no. ); plate binding-posts, x, y and z (no. - - ); copper washers (no. ). _arrange_ as in fig. , so that the current will flow, at first, as shown by the arrow. the metal plates, m p and m p , are used so that the connections may be changed without disturbing g v. the binding-posts may be fastened directly to the top of the table; but it will be more convenient to permanently fix them to a board, b, as shown, so that the same arrangement can be used for future experiments. the binding-posts, x and y, should be about / in. apart, just far enough so that their edges do not touch each other. the binding-post, z, should be fastened to b with its inside end meter( centimeters, cm.) from the ends of x and y. marks should be made upon b, centimeters apart, as indicated by the cross lines. this distance may be taken from the scale on the rule (no. ). fasten one end of the no. wire, g-s w, to x. to do this twist its end around the screw, s, between x and the copper washer, then turn the screw in with a screw-driver until it firmly holds x to the board. pass the wire around the screw in z, and bring its free end to the other binding-post, y, to be fastened (fig. ). two meters of wire then form a path for the current from x to y. have the board wide enough so that another set of binding-posts can be put by the side of y. it will be best to permanently leave the no. wire upon the board, and to fasten the no. wire (next experiment) to another set of binding-posts, placed in the same manner as those in fig. . make holes in the wood with an awl before forcing in the screws. = . note.= this experiment is usually done with a reverser in the circuit, first taking readings with the current passing in one direction, and then in the opposite direction. considerable time will be saved by taking all the readings for one direction of the current at a time, simply using different lengths of german-silver wire, and allowing the current to flow constantly during each part. this obviates all danger of poor contacts in the reverser, etc.; it saves the trouble of handling the reverser, and much of the time needed for the needle to come to rest. [illustration: +--------------------+----+----+----+---+---+---+---+---+----+---+ |length of circ., cm.| | | | | | | | | | o | +--------------------+----+----+----+---+---+---+---+---+----+---+ |deflection; west | ° | ° | °| | | | | | °| °| +--------------------+----+----+----+---+---+---+---+---+----+---+ |deflection; east | ° | ° | °| | | | | | °| °| +--------------------+----+----+----+---+---+---+---+---+----+---+ | average | . | . | | | | | | | . | | +--------------------+----+----+----+---+---+---+---+---+----+---+ fig. .] = . directions.= (a) with the circuit arranged as in fig. , and with g v properly placed, take the reading of g v, the current passing through cm. of no. g-s w. record your results in a diagram made like fig. . the row of figures across the top shows the length of the circuit. the table is started with results from one experiment. your results will probably be different from these. (b) get the deflection with the current passing through cm. of wire. to do this press a piece of copper (o, fig. ) upon the wire at the mark cm. from z, another thin piece of metal, u, having been slipped under the wire. this will allow the current to pass across from one wire to the other. record the deflection in the col. marked . (c) record the deflections for the lengths, cm., , , , , and ; then repeat (a) to be sure that the cell has been working uniformly. this deflection should agree with that in (a). (d) change the direction of the current through g v; to do this, change wire, , from m p to m p , and wire to m p . this must be done without disturbing g v. (e) repeat (a), (b), and (c), and record the deflections for the different lengths. (f) get the average deflections. (g) take, for future use, the deflection produced without g-s w being in the circuit. swing the end of wire, , that is joined to y, around to m p . the current will then pass simply through g v. record deflection in col. marked o. =note.= it is best to do the next experiment at once with the same cell, so that the results of the two experiments can be compared. in case this is impossible, get your cell to produce the same deflection when you use it again, as shown in col. o, fig. . you can regulate the deflection of the needle of g v by varying the strength and quantity of the acid in p c. _= . discussion.=_ the resistance of a wire evidently depends (exp. ) upon its length. the _exact_ relation between resistance and length cannot be seen from these results, however, which are used in the next experiment. it will be shown later that in a wire, other things remaining the same, the resistance varies directly as its length. =experiment . to find what effect the size (area of cross-section) of a wire has upon its electrical resistance.= _apparatus._ same as in last experiment, with one change, however. replace the no. g-silver wire with a no. g-silver wire (no. ), or, what is better still, fasten it to another set of binding-posts on the board and leave the no. for future use. the two should be stretched side by side for constant use. = . directions.= (a) see that your cell is in the same condition as for exp. ; that is, it should produce the same deflection of the needle of g v as before, when the two, only, are in the circuit. (see exp. , g.) the deflection may be changed by changing the strength and quantity of the dilute acid and copper solution. (b) find the average deflection of the needle with the meters of no. g-s wire in the circuit, arranged as in fig. . (c) compare this average deflection with the results obtained in exp. , in order to find what length of the no. wire has the same resistance as meters of no. wire. to find how many times greater one length is than another, we divide the larger length by the smaller; hence, to find the relation between the two lengths of wire that gave the same deflection,--lengths of equal resistance,--we divide the centimeters (the length of the no. ) by the length of no. found as directed. (d) from the wire tables it will be found that the area of cross-section of no. wire is about . times that of no. wire. how does this quotient, or ratio, compare with that found in part (c)? what is the relation between the area of cross-section of a wire and its resistance? (see § , also exp. .) _= . discussion.=_ if we find that a certain wire, x, which is feet long, has the same resistance as a shorter one, y, feet long, we see ( divided by ) that the ratio of their lengths is . . this means that the longer one, x, is . times as good a _conductor_ as y; or, in other words, that the _resistance_ of y is . times that of x. it is easier for water to flow through a large pipe than it is through a small one. the same general principle is true of electricity. a large wire offers less resistance to the current than a small one of the same material. if one wire is twice the size of another of equal length, it will be twice as good a conductor as the other; that is, it will have one-half the resistance of the smaller, provided they are of the same material. (see laws.) =experiment . to compare the resistance of a divided circuit with the resistance of one of its branches.= _apparatus._ same as in last experiment. arrange as in fig. . = . directions.= (a) note the deflection of the needle when the current passes through meter of g-s wire, as shown. this will be considered as one branch of the divided circuit. (b) still allowing the current to pass as in part (a), press a piece of copper firmly across the binding-posts x and y, to electrically connect them, and note the reading of the needle. in this case the current divides at z through the two branches. what is learned from the results of (a) and (b)? (c) see if you can show the same results with apparatus arranged as in fig. . [illustration: fig. .] [illustration: fig. .] _= . discussion.=_ two wires placed side by side as in (b), exp. , really form a conductor having twice the size (area of cross section) of one of the branches. the more paths a current has in going from one place to another, the less the resistance. (see exp. .) the wires are said to be in "parallel" or in "multiple arc." [illustration: fig. .] [illustration: fig. .] =experiment . to study the effect of decreasing the resistance in one branch of a divided circuit.= _apparatus._ galvanoscope, g v (no. ); resistance coil, r c (no. ); two-fluid cell, -f c (§ ), or a dry cell; connecting wires; metal plates, m p. _arrange_ as in fig. , so that the current divides into two branches at m p . the branches unite at m p . = . directions.= (a) take the reading of g v with ohms resistance in the lower branch; that is, with the whole of r c in circuit. (b) take the reading of g v with one ohm in circuit; that is, with the end of wire, , connected to m instead of to r. (c) cut out r c from the lower branch by replacing it with a metal plate, thus joining wires and . compare the results from (a), (b), and (c), and explain. _= . current in divided circuits.=_ let us consider a circuit like that shown in fig. . if the points, c and z, were at the same potential, no current would pass from c to z. as the current does pass, z must be at a lower potential than c; there is a _fall of potential_ from c to z. if the branch, a b, has the same resistance as r x, the same amount of current will pass through each. exp. has shown that when the branches have unequal resistances, most of the current passes through the one of small resistance. if r x has a greater resistance than a b, most of the current will pass through a b. chapter xix. measurement of resistance. [illustration: fig. .] =experiment . to study the construction and use of a simple "wheatstone's bridge."= _apparatus._ fig. . a wheatstone's bridge, w b (no. ), (§ ); astatic galvanoscope, a g (no. ); dry cell, d c (no. ); key, k (no. ); wires with spring connectors, two of which, r and x, are equal in length; metal plate, m p, for connecting wires. _arrange_ as in fig. . the carbon of d c is joined to k, and this to the point, c, of the bridge. the zinc of d c connects with the point z on w b. the a g is placed between the branches for clearness. wire is joined to the left-hand binding-post of a g, and wire joins m p with the right-hand one. when the end of wire does not touch g-s w, it is evident that as soon as k is pressed, the current divides at c on its way to z, where the branches unite again. k is used so that d c will not be polarized by steady use. [illustration: fig. .] = . the simple wheatstone's bridge= (fig. ) consists of a wooden base, w, at the ends of which are fastened two aluminum conductors, and . at one side of w is fastened another conductor, . in fig. are side views of the conductors. these are used merely for convenience in making connections, and take the place of the metal plates used in previous experiments. a german-silver wire, g-s w, is stretched between and , and under this is a scale, s, divided into small parts, these being tenths of the larger divisions. the ends of g-s w are held between eyelets, as shown at e, fig. . [illustration: fig. .] =reading the scale.= the value of part a can be read directly from the scale, using the lower row of figures. the point marked p, for example, would be read . (three and seven-tenths large divisions); b would be . , found by subtracting . from . the sum of a and b must always equal . the . may also be read directly by using the upper row of figures for the whole numbers, counting the tenths to the left. try to divide the smallest divisions into halves, at least; that is, if a = . , b = . . take the readings carefully. = . directions.= (a) touch the free end of wire, , to the point, c, which has a higher potential than m p. press down k for an instant only. some current should pass through a g, as a shunt. should it pass from c to m p or the reverse? note in which direction the right-hand end of the astatic needle is deflected. (b) swing the end of around and touch it to the point, z, which has a lower potential than m p. press k for an instant, watch the needle, and compare with the results in (a). (c) move the free end of along on g-s w, touching k at intervals, until a point is found at which the needle of a g is not deflected. how does the potential of this point compare with that of m p? _= . discussion; equipotential points.=_ since one end of the g-s w has a higher, and its other end has a lower potential than m p, there must be, somewhere on it, a point at which the potential is the same as at m p. this place is quickly found by sliding the free end of wire, , along, pressing k occasionally, until a g shows that no current tends to pass through it in either direction, when the current passes from c to z through the two branches of the divided circuit. this point and m p are called _equipotential points_. if the resistance of the part, x, be increased, it should be evident that the part of the bridge-wire, b, should be also increased to find a point having the same potential as m p; that is, the end of should be moved towards c. we have, in the bridge-wire, a simple means of varying the resistance of its parts, a and b. = . use of wheatstone's bridge.= it will be found, upon trial, if we put a resistance of ohms in place of r, fig. , and ohms in place of x, that the free end of wire will have to be at the center of the bridge-wire in order to get a "balance"; that is, to find the place where a g is not affected. no matter what the resistance of r and x are, provided they are equal, this will be true. the value of both a and b, on the scale, will be whole spaces, no tenths. from this we see that a: b:: r: x, which reads a _is to_ b _as_ r _is to_ x; this means that a × x = b × r. supplying the values of the letters, we have × = × . if we did not know the value of x, that is, if we were measuring the resistance of a coil of wire, using a -ohm coil as the standard, or r, we could find the value of x, knowing the other parts of the proportion. × x = × , which means that times the value of x is ; hence the value of x is ÷ = ohms. suppose that we have r = ohms, which is the standard resistance coil (no. ), and are trying to find the resistance of a coil, x. we slide the end of wire, , along on the bridge-wire until the correct place is found. (see exp. , , for details.) take the values of a and b (§ ), supply them in the equation given, and work out the value of x. = . example.= r = ohms; a = . ; b = . ; to find the value of x in ohms. a: b:: r: x, which means that a × x = b × r, or . × x = . × . x must equal, then ( . × ) ÷ . = . ohms. =note.= in practice it is most convenient to make connections as shown in fig. when measuring resistances (exp. ). the arrangement given in fig. is simply for explanation. it will be seen that the smaller a is, compared with b, the larger the unknown resistance compared with your standard. [illustration: fig. .] =experiment . to measure the resistance of a wire by means of wheatstone's bridge; the "bridge method."= _apparatus._ same as in exp. ; the two-ohm resistance coil, r c (no. ); a coil of wire, x, as, for example, the -turn coil on the galvanoscope, g v (no. ). _arrange_ as in fig. . you will observe that the central conductor of the bridge ( , fig. ) takes the place of m p in previous explanations. we still have the same kind of a divided circuit as explained in exp. , a g being connected with points of equal potential. it will be found convenient to have d c at the right, and a g facing you at the left, the key being in front. (see exp. in regard to adjusting a g.) notice that you have a standard resistance ( ohms) in place of r, fig. , and an unknown resistance (galvanoscope coil) in place of x. (see § .) = . directions.= (a) touch the free end of wire, , to the left-hand side of the bridge-wire, press the key for an instant, only, and note the direction taken by the right-hand end of the needle. move the end of wire, , to the right-hand side of the bridge-wire, touch key, watching needle. does the needle move more or less than before? in the same or opposite direction? if the deflections are opposite, the point that has the same potential as binding-post, , must be _between_ the two points touched. (b) be sure that all connections are good. find the point on g-s w, at which there is no deflection, as directed in exp. (c). note the readings on the scale, as explained in § . (c) make the proper calculation, § , , and find the resistance of the coil of g v, the resistances of the wires joining r c and g v to the bridge being neglected. (d) make proper allowances for the resistances of the wires just mentioned (see § ), and compare them with the results found in part (c). = . allowances for connections.= it should be remembered that the wires joining r c and g v to the bridge also have some resistance. such connections, in regular instruments, are made by heavy copper straps or by thick, short wires, so that their resistances can be neglected. in case you use the ordinary no. copper wire, as directed, the resistances of the pieces can be measured by means of the bridge, or you can calculate their resistances from the wire tables. the resistances should be allowed for. it is evident that your standard resistance is ohms _plus_ the resistance of the connecting wires, and that the resistance of the coil, x, is found by deducting the resistance of its connecting wires from that found from the proportion previously used. _example._ we see from the table that the resistance of about ft. in. of no. , b and s copper wire is ohm. this equals in. if in. have a r (resistance) of ohm, in. will have a r of one- th of an ohm; that is divided by , which equals a little over . ohm. for every inch of no. wire used, then, for connections, we may allow . ohm. this will be near enough for our purposes. suppose that each connection is in. long, the regular wires with connectors being used. the r of the in. joined to r c will then be times . = . ohm. our standard r must then be considered as . ohm. if we substitute this in the example, as stated in § , we have . × x = . × . . x must equal ( . × . )/ . = . ohm, which includes the unknown resistance and in. of connections, the r of which is . ohm; . - . = . , the resistance of x alone. compare this with the answer to example, § . make allowances according to length of connectors used. _=note.=_--carefully keep all the results of these experiments in a note book for future reference. be sure that connections are good. =experiments - . to measure the resistances of various wires, coils, etc., by the "bridge method."= _apparatus._ the coils of wire, etc., as stated in the "directions" of each experiment. the details of each piece of apparatus may be found by referring, from the numbers given, to the "apparatus list," and to descriptions in the paragraphs mentioned. also all the apparatus of exp. . =note.= make proper allowances for connections (§ ) in all experiments in measuring resistances. =experiment .= = . directions.= (a) as explained in exp. , measure the resistance of the -turn coil of g v, allowing for connections (§ ). read the bridge-scale carefully. (b) use one-half of the -ohm coil as standard and repeat. =experiment .= = . directions.= (a) measure the resistance of the -turn coil of g v (see exp. , etc.), using ohms as standard. (b) use ohm as standard, repeat, and compare results. (c) add the resistances of the and -turn coils, and compare the sum with the resistance of the -turn coil, as found in exp. , d. the difference should be but a few hundredths of an ohm. =experiment .= = . directions.= (a) measure the resistance of the coil of no. copper wire (no. ). this coil is used for later experiments. spring connectors are fastened to the ends of this coil, allowing it to be directly connected to the conductor on the bridge, so no allowance should be made for its connecting wires. (see exp. for details.) mark the resistance upon the coil for future use. (see note.) =note.= the student will be surprised, perhaps, to find that different results are obtained for the resistance of a given wire in case he uses different standard resistances in the various tests; that is, he will probably get a different result in exp. (a) from the result of exp. (b). the difference here, however, may not be large. the best results are obtained by making the standard resistance as nearly equal as possible to the resistance to be measured, so that a balance can be found when the end of wire (fig. ) is near the center of the bridge-wire. if r, fig. , is much larger or smaller than x, the point desired on g-s w will be near one of its ends, and large errors thereby produced. the approximate resistance of x can be found by trial, then more or less resistance can be used for r to suit. the student should make several coils as explained in apparatus book, chapter xvii. the resistance of the different coils furnished should be measured and marked. these can be used to vary the value of r. =experiment .= = . directions.= (a) measure the resistance of the coil of no. copper wire (no. ). (see exp. for details and the note, exp. .) =experiment .= = . directions.= (a) measure the combined resistance of the two coils used in exps. and , when they are joined in "series"; that is, when one end of one coil is joined to one end of the other by means of a metal plate, the free ends being connected to the bridge (exp. ). the current has to travel through the entire length of both coils. (b) compare this result with the sum of their separate resistances found in exps. and . (see exp. , note.) =experiment .= = . directions.= (a) measure the resistance of the two coils (exp. ) when they are joined "in parallel." (see § .) they may be joined in parallel by connecting them both to the bridge at the same time, one end of each being slipped onto (fig. ), the other end of each being joined to . in this way the current has two paths, side-by-side, to get from to . (see exp. , note.) (b) compare this resistance with that of exp. . =experiment .= = . directions.= (a) measure the resistance of meter of no. german-silver wire. use the wire as arranged on a board, exp. (figs. and ), making the connections with the bridge from binding-posts, x and z. (see exp. , note.) the wires connecting the bridge with the ends of the g-s wire will each have to be about ft. long. in making deductions (§ ) figure according to the length used. (b) divide the total resistance by to get the resistance of cm. of the wire, and carefully mark off the board into cm. this will give parts between x and z. =experiment .= = . directions.= (a) using the no. g-s wire on the board, as arranged for exp. , measure the resistance of the meters in series, the connections being made with the bridge from x and y, fig. . (b) compare the result with that of exp. . what is the relation between the length of a wire and its resistance? see summary of laws. (see exp. , note.) =experiment .= = . directions.= (a) measure the resistance of the above two meters of no. g-s wire when joined in parallel. (§ .) the binding-posts, x and y, can be joined by a short wire with connectors on its ends, or by clamping a thin strip across by means of spring connectors. use the -ohm coil as the standard, and make proper allowances. (§ .) (b) from the results of exps. and what can be said about the resistances of parallel circuits as compared with the resistances of the separate branches? =experiment .= = . directions.= (a) arrange the meters of no. g-s wire on the table or board, again (exp. , fig. ). (b) measure the resistance of one meter. find the value of x approximately, and use a resistance for r that will suit. (see exp. , note.) (c) divide the result by to get the resistance of cm. of the wire. (d) compare the resistance of one meter of no. g-s wire, found in exp. , with the resistance of meter of no. g-s wire. what is the relation, then, between the size (area of cross-section) of a wire and its resistance? (see the results of exp. , and § , also summary of laws.) =experiment .= = . directions.= (a) measure the resistance of meters of no. copper wire, arranged on a board as in fig. . (see exp. , note.) get the resistance of meter. (b) compare the conductivities of copper and german silver by studying the results of exps. and . which has the greater resistance? to find out how many times greater one resistance is than the other, divide the larger by the smaller. =experiment . to study the effect of heat upon the resistance of metals.= _apparatus._ same as for exp. ; the coil of no. wire (no. ); a lamp or other source of heat. arrange as in fig. . = . directions.= (a) measure the resistance of the coil as before, exp. . the result should nearly agree with that of exp. , provided connections, etc., are the same. (b) remove the coil from the bridge, hold it about a foot above a lamp or stove, to warm it thoroughly, but do not heat it enough to injure the covering. it will take a minute or so to warm it so that the heat will get to the inside also. (c) replace the coil, measure its resistance, and compare the result with its resistance when cold. does heat increase or decrease the resistance of a copper wire? _= . effect of heat upon resistance.=_ although there was but the fraction of an ohm difference in the resistances of the hot and cold coil, it was evident that changes of temperature affect the conducting power of copper. this is true of all metals; but german silver and other alloys are much less affected than pure metals, so they are used in making standard resistance coils. the resistance of liquids that can be decomposed by the electric current decreases as the temperature rises. carbon acts like the liquids, while the resistance of metals _increases_ as their temperature rises. =experiment . to measure the resistance of a wire by the method of "substitution."= _apparatus._ the coil of no. wire (no. ), the resistance of which has been measured, but which will be considered an unknown resistance, x; g v, -f c, m p, connecting wires, etc., previously used; rheostat (§ ). arrange as in fig. first, then as in fig. . _= . simple rheostat.=_ the no. and no. g-s wires stretched upon the board (fig. ), make a convenient form of rheostat. the resistance per cm. being known from the results of exp. and , the resistance for any number of cm. is easily found. the -cm. divisions should be divided into centimeters. these spaces may be marked off from the rule (no. ). [illustration: fig. .] = . directions.= (a) be sure that -f c gives a constant current, shown by the uniform deflection at g v, when arranged as in fig. . do not use a cell that quickly polarizes. the coil, x, forms a part of the circuit; it is joined to wires, and , by means of metal plates, so that it may be quickly removed without disturbing either g v or -f c. carefully read the deflection at g v. (b) remove x from the circuit, and join the free end of wire, , to binding-post, x, and the free end of wire, , to a small piece of sheet copper, which can be firmly pressed upon the g-s wire to make a contact. move this along on the g-s wire until the deflection produced equals that of part (a), remembering that the longer the g-s wire in the circuit the less the deflection. make two or three trials, as one or two cm. difference in length make but a little difference in the deflection. note the number of cm. of g-s wire used, the resistance of which must equal that of the coil, x. (c) find the resistance of x by multiplying the length just found by the resistance of each cm., and compare the result with the value found by using the bridge method directly. [illustration: fig. .] =experiment . to measure the e. m. f. of a cell by comparison with the two-fluid cell.= _apparatus._ rheostat (§ ); the two-fluid cell, -f c (exp. ), the e. m. f. of which may be taken as volt; dry cell, d c; galvanoscope, g v. arrange first as in fig. . = . directions.= (a) be sure that -f c gives a constant current. take the reading of g v without the rheostat in the circuit; that is, with wires, and , joined directly. the deflection should be or degrees at least, and be constant. (b) attach a small piece of copper to the end of , and firmly rub it along upon the g-s wire, thus introducing resistance into the circuit, until the deflection is, say, ° ( or degrees will do). note the length of g-s wire used and call it (b). (c) gradually add more resistance by moving the end of along until the deflection is °, degrees less than before. (if the original was ° make the new °). call the number of cm. of wire used (c). (d) replace -f c with the dry cell d c. add resistance, as before, until g v indicates a deflection of °, being careful not to keep the circuit closed long enough to partially polarize d c. make or trials, allowing d c to rest a few minutes between each. call the number of cm. of g-s wire used (d). (e) again add more resistance, as in (c), until the deflection is reduced to °. call the length used (e). = . calculation.= it is known that resistances that are able to reduce the strength of the currents equally are proportional to the electromotive forces; that is, the electromotive forces of the two cells are to each other as the two resistances necessary to produce equal changes in the deflections, which, of course, indicate equal changes in the strength of the currents. since the resistances used in the two cases are directly proportional to the lengths used, we have: length (c-b): length (e-d):: e. m. f. of -f c: e. m. f. of d c. substitute the values found and find the e. m. f. of d c. =experiment . to measure the internal resistance of a cell by the "method of opposition."= _apparatus._ all the apparatus of exp. . two simple cells (§ ), the plates of which should be of the same size, the same distance apart, and immersed in acid to the same extent in both. the acid in both should be of the same strength. = . directions.= (a) connect the two cells in opposition, so that no current will be generated by them, and so that the two can be treated as a dead resistance. do this by joining the two zinc plates by a wire with connectors, and use wires to connect the copper plates to the bridge like any other unknown resistance. (b) measure the resistance of the two by the regular bridge method, allowing for wires used for connections. one-half of the resistance found will give the internal resistance of one cell. (see note.) =note.=--the standard resistance will have to be arranged to suit each particular case to make the calculations even approximately correct. (see exp. , note.) the standard resistance may be increased by adding the various coils and rheostat wires, their values being known. _= . summary of laws of resistance.=_ . _the resistance of a wire is directly proportional to its length, provided its cross-section, material, etc., are uniform._ =example.= if . ft. of no. copper wire has a resistance of ohm, . ft. will have a resistance of ohms, because . is twice . ; . ft. will have a resistance of . ohms, as ( . ÷ . = . ) it is . times . . . _the resistance of a wire is inversely proportional to its area of cross-section._ the areas of cross-section of round wires vary as the squares of their diameters; so _the resistance of a wire is also inversely proportional to the square of its diameter, other things being equal_. =example.= a no. wire has a diameter of about . inch, while the diameter of a no. wire is about . in.; that is, the no. has _twice_ the diam. that the no. has. the area of cross-section of the no. , however, is four times that of the no. , so its resistance is but / that of the no. , the lengths, etc., being the same. (see wire tables.) . _the resistance of a wire depends upon its material, as well as upon its length, size, etc._ . _the resistance of a wire depends upon its temperature._ (see elementary electrical examples.) chapter xx. current strength. _= . strength of current.=_ the water in a certain tank may be under great pressure, but if it is obliged to pass through long tubes before it can turn a water-wheel, for example, it is evident that the work done will depend not only upon the pressure in the tank, but upon the resistance to be overcome before the water gets to the wheel. the work that the water can do depends upon its _rate of flow_, and may be used to measure the _strength_ of the current. the strength of a current of electricity is measured also by the _work_ that it can do, and it depends upon its _rate of flow_ at the point measured. the strength may be determined from its magnetic, heating, or chemical effects. _= . unit of current strength; the ampere.=_ a current having the strength of ampere, when passed through a solution of silver nitrate under proper conditions, will deposit . gramme of silver in _one second_; if passed through a solution of copper sulphate, copper plates being used for the electrodes, in the solution, . gramme of copper will be deposited in _one second_. (see chemical effects of the current.) the thousandth of an ampere is called the milliampere. the strength of a current is proportional to the amount of chemical work that it can do per second. (see § .) _= . measurement of current strength.=_ the _galvanoscope_ previously described simply shows the presence of a current, or whether one current is larger or smaller than another. when the degree-card is used to get the relative deflections, the instrument may be called a _galvanometer_. _=the tangent galvanometer=_ is made on the same general idea as our galvanoscope, the diameter of the coil being twenty times, or more, the length of the needle. in these the strengths of the two currents compared are proportional to the tangents of the angles of deflection produced. (see elementary electrical examples.) there are several varieties of galvanometers, each designed for its special work. they are often calibrated or standardized so that the amperes of current passing through them can be read off directly from the scale. _= . the ammeter=_ is really a galvanometer from which may be read directly the strength of a current. the coil has a low resistance so that it will not greatly reduce the strength of the current to be tested. _=the voltameter=_ measures the strength of a current by chemical means. _= . unit of quantity; the coulomb.=_ a current having a strength of ampere will do more chemical work by flowing one hour than it can do in second. in speaking of the _quantity_ of electricity we introduce the element of _time_. the unit of quantity is called the _coulomb_, just as a cubic foot of water may be taken as a unit of quantity for water. a coulomb is the quantity of electricity given, in one second, by a current having a strength of ampere. coulombs are found by multiplying amperes by seconds; thus, a current of amperes will give coulombs in seconds. _= . electrical horse-power; the watt.=_ the electric current has power to do work, and we speak of the horse-power of an electric motor in the same way as for a steam-engine. a current with the strength of ampere and an e. m. f. of volt has a unit of power called the watt. watts make an electrical horse-power. watts = amperes × volts. watts ÷ = the number of horse-power. (see transformers, also elementary electrical examples.) _= . ohm's law.=_ it was first shown by ohm that the strength of a current is equal to its e. m. f. divided by the resistance in the circuit; that is, strength of current (amperes) = e. m. f. (volts). / resistance (ohms). if we let c stand for the strength in amperes, e for the e. m. f. in volts, and r for the resistance in ohms, we have the short formula, easily remembered, c = e/r _= . an ampere=_ would be produced by a current of volt pushing its way through a resistance of ohm. knowing any two of the three, c, e, or, r, the other may be found. the resistance, r, it must be remembered, is the total resistance in the circuit, and is composed of the total internal and external resistances. (see elementary electrical examples.) _= . internal resistance and current strength.=_ it is evident that the internal resistance of a cell varies with the position and size of the plates. we shall now study the effects of these changes upon the strength of the current. [illustration: fig. .] =experiment . having a cell with large plates, to find how the strength of the current is affected by changes in the position of the plates, the external resistance being small.= _apparatus._ galvanoscope, g v; materials for simple cell (exp. ); connecting wires. arrange as in figure , omitting the wooden cross-piece. = . directions.= (a) connect the wires with the -turn coil of g v, which has but little resistance. have the tumbler nearly full of dilute acid to get the effect of large plates; that is, the current has a large liquid conductor to pass through in the cell, and the _internal_ resistance will be small. g v should be properly placed n and s. (b) place the copper and zinc plates as far apart as possible in the acid, and press them against the bottom of the tumbler. note the reading of g v. it is not necessary to take readings with reversed current. (c) still pressing them against the bottom of the glass, to keep the same amount of surface under acid, slowly bring them near each other and watch the needle. (d) hold the plates about an inch apart, and against the bottom, and note the reading of g v. slowly raise the plates, keeping them the same distance apart until they are out of the acid. watch the action of the needle. make a note of your readings in degrees and write your conclusions. does a change in internal resistance affect the strength of the current? =experiment . having a cell with small plates to find how the strength of the current is affected by changes in the position of the plates, the external resistance being small.= _apparatus._ same as in exp. , the acid, however, being but in. deep in the tumbler; that is, we have the effect of a cell with small plates, each being about in. by / in. = . directions.= (a) repeat (b) and (c) of exp. , recording the reading of g v in each case. (b) compare the results with those of exp. , remembering that the _internal_ resistance is larger than before. is the current as strong with small plates as with large plates when the external resistance is small? when the external resistance is small (the -turn coil, for example), should the cell have a high or low internal resistance to produce the greatest effect upon the needle? [illustration: fig. .] =experiment . to find whether the changes in current strength, due to changes in internal resistance, are as great when the external resistance is large, as they are when the external resistance is small.= _apparatus._ same as for exp. , , also the rheostat containing the two meters of g-s wire (exp. ). = . directions.= (a) arrange as in fig. , the external resistance being meters of no. g-s wire in series with g v. the -f c in the fig. is replaced, however, by the simple cell as in exp. . (b) find the effect upon the strength of the current of moving the plates about when but in. of acid is in the tumbler. (c) nearly fill the tumbler with acid and repeat (b), taking readings with plates near each other and as far apart as possible. lift them nearly out of the acid and take the reading. (d) still increase the external resistance of the circuit by adding coils of wire or the meter of no. g-s wire and repeat. is the strength of the current greatly affected by _slight_ changes in the internal resistance when the external resistance is large? _= . discussion.=_ we shall study, by means of figures, how changes in internal resistance affect the strength of the current. let r stand for the total external resistance of a circuit, and r for the total internal resistance of the cell or cells; ohm's law, then, will be expressed by c = e / (r + r) =example.= let us take a circuit (a) when the external resistance, r, is small, and (b) when r is large compared with r, e being taken as volt in both cases. (a) let r = , and r = ; substituting these values in the formula above, we have: c = / ( + ) = / = . + ampere. now let the internal resistance, r, be slightly increased from to ohms; the value of c then becomes / ampere, as r + r = . the change in c, then, is the difference between / and / ; and this expressed in decimals becomes . - . = . ampere. (b) let r = ohms, and r = ohms as in (a). substituting these values we have, c = / ( + ) = / = . ampere. increasing r from to , as before, etc., we find that c = divided by = . ampere. the above shows clearly (a) that the value of c is changed considerably by changes in r when r is _small_, and (b) that changes in r produce very slight changes in c when r is _large_. review your results of exps. - . (see elementary electrical examples.) _= . arrangement of cells and current strength.=_ we have seen that internal resistance affects current strength. in joining cells, then, attention must be given to the internal resistance as well as to the e. m. f. of the combination. _= . cells in series.=_ it has been shown by careful experiments that the e. m. f. of two cells joined in series (fig. ) is equal to the sum of the e. m. f. of each. ten cells, joined in series, have ten times the e. m. f. of one cell, provided they have the same e. m. f. as the zn of one is joined to the cu of the other, the current is obliged to pass through one solution after the other; that is, the internal resistance of the two in series is equal to the sum of their internal resistances. ten cells, joined in series, have ten times the internal resistance of one cell, provided they have equal internal resistances. [illustration: fig. .] [illustration: fig. .] _= . cells abreast.=_ when the positive plates are joined together and the negative plates are also joined together (fig. ), the cells are said to be _abreast_, _in parallel_, or in _multiple arc_. it has been shown that two cells of equal strength, joined abreast, have the same e. m. f. as one cell. the two cu plates, being joined, must have the same potential; all the zn plates have the same potential, so the difference of potential at the terminals of the combination is the same as that at the terminals of a single cell. in two cells abreast (fig. ) the current has two liquid paths, side by side, to get from cu to zn; this makes the internal resistance one-half that of one cell, provided their internal resistances are equal. ten cells, of equal internal resistance, when joined abreast, have one-tenth the internal resistance of one cell. =experiment . to find the best way to join two similar cells when the external resistance is small.= _apparatus._ two simple cells using dilute sulphuric acid, with copper and zinc elements, as in exp. ; galvanoscope, g v; connecting wires, etc. have the zincs well amalgamated. remove them from the acid as soon as readings are taken. = . directions.= (a) partly fill the tumblers with the acid. join the cells in series (fig. ), then connect wire (fig. ) with the left-hand binding-post of g v, and wire with the middle one, thus putting the -turn coil into the circuit. take the reading of g v. (b) join the cells in multiple arc (fig. ), connecting them as in (a) with g v. write down the reading, and compare it with that found in (a). (c) take the reading with but cell joined to g v. =experiment . to find the best way to join two similar cells when the external resistance is large.= _apparatus._ same as for exp. , also the rheostat containing metres of no. or g-s wire. arrange the g-s wire in series with the -turn coil of g v, as shown in fig. , two simple cells being used, however, instead of -f c as shown. = . directions.= (a) take the reading of g v when the two cells are in series (exp. ), the external resistance being the -turn coil and g-s wire. (b) join the cells in parallel and take the reading, using the same external resistance as in (a). (c) increase the external resistance by adding coils of wire or metres of no. g-s wire and repeat (a) and (b). what does the experiment show? (d) take the reading with cell and large external resistance. _= . best arrangement of cells.=_ it will be seen by experiments that with a given number of cells the strongest current is produced when they are arranged so that the internal resistance of the battery nearly equals the external resistance of the circuit. when the external resistance is small, the internal resistance may be kept down by joining the cells in parallel; and, although the e. m. f. is also kept small, the value of c will be larger than it would be with a larger internal resistance and a larger e. m. f. when the external resistance is large, the internal resistance can be made large by joining the cells in series. the advantage comes, however, from having a large value of e. a large resistance can not hold back a current of large e. m. f. by joining the cells in series the value of e is made large, and the value of c becomes large even though there is an increased internal resistance. (see elementary electrical examples.) chapter xxi. chemical effects of the electric current. _= . chemical action and electricity.=_ we have learned that the electric current is produced, in the cell, by chemical action. there is a definite relation between the chemical action and the current produced. we are now to study the changing of electrical energy back, again, to chemical energy. [illustration: fig. .] _= . electrolysis=_ is the name given to the process of decomposing chemical compounds by passing the electric current through them. the compound decomposed is the _electrolyte_. fig. shows a tumbler of liquid (electrolyte) through which the current is to pass in the direction of the arrow. two carbon plates, a and c, are in the liquid, and are joined to the source of electricity. the current enters at a (_anode_) and leaves at c (_cathode_). [illustration: fig. .] =experiment . to study the electrolysis of water.= _apparatus._ the two simple cells (§ ) joined in series (§ ), although two daniell or two dry cells will be better. a tumbler of water containing a few drops of sulphuric acid to make the water a conductor. two pieces of sheet copper will serve as the electrodes. the galvanoscope may also be put into the circuit as in fig. . = . directions.= (a) allow the current to pass, and note ( ) whether gas is set free at both electrodes, a and c, and ( ) at which the quantity of gas is the greater. if very little gas is produced use more cells. (b) remove a and c from the liquid, to remove the gas, then watch the action of the needle of g v as the water is again decomposed. _= . composition of water.=_ the two gases liberated in exp. were hydrogen (h) and oxygen (o). the chemical formula for water is h_{ }o, which means that it is composed of two parts, by volume, of h and one part of o. with proper apparatus these gases may be collected, tested, and the amounts measured. _= . electromotive force of polarization.=_ we know that h and o have a strong chemical attraction, or affinity, for each other. in order, then, for the current to decompose water, this attraction between the gases must be overcome; and as soon as the current ceases, these gases try to rush together again to form water. this sets up an electromotive force of almost . volts; in fact, a current is produced if the h and o be allowed to form water again (see storage cells). to decompose water the current must have an e. m. f. of over . volts to overcome this e. m. f. of polarization. it was seen in the study of simple cells that the current became rapidly weaker as hydrogen was deposited upon the copper plate, on account of this opposing electromotive force. in decomposing other compounds, the anode is made of the metal which is to be deposited at the cathode. if copper is to be deposited from a solution of copper sulphate the anode should be a copper plate; this keeps the solution at same strength, and avoids the opposing e. m. f. of polarization; that is, a very weak current will do the work (see exp. ), because the electrodes are of the same metal. =experiment . to coat iron with copper.= _apparatus._ iron nail, solution of copper sulphate (§ ). = . directions.= (a) clean the nail with sandpaper, then hold it in the copper solution for a few seconds. machinists often cover iron or steel tools with a thin coating of copper in this way. [illustration: fig. .] =experiment . to study the electrolysis of a solution of copper sulphate.= _apparatus._ galvanoscope, g v; two-fluid cell, -f c; a tumbler, t, containing about an inch of copper sulphate solution (§ ); a wooden cross-piece to which is fastened a copper strip; carbon rod, c; wire is held to c by a rubber band. _arrange_ as in fig. , so that cu will be the _anode_ (§ ), the current passing as shown by arrow. a dry cell may be used for short experiments instead of the -f c. = . directions.= (a) the carbon being clean, allow the current to pass, c and cu being kept about / in. apart. watch the surface of c, and note the beautiful color of the deposited copper. save the coated rod for the next experiment. has the cu plate been acted upon? _= . electroplating=_ is the name given to the process of coating substances with metal with the aid of the electric current. the copper sulphate, cuso_{ }, is broken up into cu and so_{ } by the current. the cu goes to the cathode, and the so_{ } attacks the anode, gradually dissolving it if it be copper; that is, the _metal_ part of cuso_{ } is carried in the direction of the current. most metals are coated with copper before they are silver or gold plated. a solution of silver is used for silver plating, silver being used as the anode. =experiment . to study the chemistry of electroplating.= _apparatus._ same as in last experiment, but use two carbon rods for the electrodes. arrange as in fig. , with the cu replaced by another carbon. two simple cells (§ ) are also needed. = . directions.= (a) allow the current to pass as before. is copper still deposited? does anything occur now at the surface of the anode? is the copper deposited as rapidly as before? (b) try the effect of the two simple cells joined in series, instead of the two-fluid cell. (c) after a fair coating of copper has been deposited upon the carbon cathode, reverse the direction of the current through the copper solution; that is, use the coated rod for the anode. allow the current to pass until a change takes place in the anode. _= . discussion.=_ ions are the names given to the parts into which an electrolyte is decomposed by the electric current. in the case of cuso_{ }, the ions are cu and so_{ }, which is called an acid radical. this so_{ } can not dissolve carbon or platinum, so these are used when water is to be electrolyzed. where copper is used as the anode for copper plating, the so_{ } attacks it, forming cuso_{ } again, and this keeps the solution strong. if carbon were used instead, the so_{ } would take h_{ } from the water around the anode and h_{ }so_{ } (sulphuric acid) would be formed, the oxygen of the water being set free at the anode. the amount of cu dissolved from the copper anode equals nearly the amount deposited upon the cathode. exp. shows that the metal is carried in the direction of the current. as hydrogen is produced at the cathode it is chemically considered a metal. _= . electrotyping=_ consists in making a copy in metal, of a woodcut, page of type, etc. a mould or impression of the type is first made in wax, or other suitable material (the pages of this book, for example, as set up by the printer). these moulds are, of course, the reverse of the type. they are coated with graphite to make them conduct electricity, and hung as the cathode, in a bath of copper sulphate. after a thin coat of copper has been deposited by an electric current, the wax is removed and the thin copper backed with soft metal. the metal surface next to the wax will be just like the type, only made of copper. these plates or _electrotypes_ can be printed from, the original type being used to set up another page. (see "things a boy should know about electricity.") _= . voltameters=_ are cells used to measure the strength of an electric current. in the _water voltameter_ the hydrogen and oxygen produced are measured. the h acts like a metal and goes to the cathode, two parts of h being formed to one of o. _copper voltameter._ this cell measures the amount of copper deposited in a given time by a current. the copper cathode is weighed before and after the current flows. the weight of cu deposited is then divided by the number of seconds during which the current passed, and this result, in turn, by . , which will give the average strength of the current in amperes. (see § .) other forms of voltameters are also used. in all voltameters the quantity of metal deposited is proportional to the time that the current flows, and to its strength. [illustration: fig. .] =experiment . to study the construction and action of a simple "storage" cell.= _apparatus._ two lead plates, l p, (nos. , ) fastened to a wooden cross-piece (§ ). the spring-connectors should not be forced upon the thick lead. fasten one end of the wire under the screw-head. a tumbler two-thirds full of dilute sulphuric acid (§ ); the astatic galvanoscope, a g; wires to form connections; the two simple cells joined in series. _arrange_ as in fig. . one l p is joined to binding-post, l, of a g by the wire marked ; wire connects the other l p to the copper cu. wire joins the zinc to any thin metal plate, m p, which is used for convenience, so that the spring connectors can be quickly slipped on or off. wire joins m p with binding-post r of a g. = . directions.= (a) get clearly in mind the direction in which the right-hand end of the astatic needle is deflected when the current passes, remembering that it passes into a g at l and leaves at r. allow the current to flow for or minutes through the circuit, at the same time watching the needle to see whether the strength of the current remains constant. (b) remove the connector from cu, swing it over into the position of the dotted line (fig. ), slip the connector upon m p and watch the needle. this cuts the cells out of the circuit; but, if you desire, also remove wire from m p. does the storage cell, s c, produce any current? does it pass through a g in the same direction as that which came directly from the two cells? (c) try the dry cell in place of the two simple cells. try other cells in series if you have them. _= . secondary or storage cells=_ must be charged by a current before they can give out a current. _electricity_ is not really stored. chemical changes are produced in the storage cell by the charging current, as in the voltameter or electroplating bath; and it is, then, potential chemical energy that is stored. when the new compounds are allowed to go back to their original condition by joining the electrodes of the charged cell a current is produced. in other words, an electric current produces chemical changes in the cell by electrolysis, and these new compounds have an e. m. f. of polarization because they are constantly willing and anxious to get back to their old state. the plates are lead and are usually coated with compounds of lead. hydrogen and oxygen are given out at the electrodes. the current from a dynamo is used to charge secondary batteries. (see "things a boy should know about electricity.") chapter xxii. electromagnetism. _= . electromagnetism=_ is the name given to magnetism that is developed by electricity. you have already seen that if a magnetic needle be placed in the magnetic field of a _magnet_, its n pole will point in the direction in which the lines of force pass on their way from the n to the s pole of the magnet. you have also seen that in the galvanoscope, etc., a coil of wire acts like a magnet when a current passes through it. can we not, then, use the needle to study the lines of force about wires and coils? [illustration: fig. .] [illustration: fig. .] =experiment . to study the lines of magnetic force about a straight wire carrying a current.= _apparatus._ the compass, o c; key, k; dry cell, d c. arrange as in fig. . = . directions.= (a) arrange the wire so that the current will flow through it from n to s over the compass-needle as soon as the circuit is closed (fig. , a). press k for an instant only, and note the direction in which the n pole is deflected. repeat two or three times until you get clearly in mind the direction taken by the needle. sketch the result in your note-book, and compare with fig. , a. the arrow shows the direction of the current. (b) let the current pass for an instant from n to s and _under_ the needle, as shown in fig. , b. sketch result. (c) let the current pass for an instant from s to n _above_ the needle (fig. , c). sketch result. (d) let it pass from s to n _under_ the needle (fig. , d). sketch result. (e) let it pass through the wire from east to west (fig. , f) above the needle, then under it, and note result. compare the results with those indicated in fig. . [illustration: fig. .] [illustration: fig. .] _= . lines of force about a wire.=_ when a current passes through a wire, the needle, over or under it, tends to take a position at right angles to the wire. this shows that the lines of force pass _around_ the wire and not in the direction of its length. the needle does not swing entirely perpendicular to the wire; that is, to the e and w line, because the earth is at the same time pulling its n pole towards the n. if the needle had no pointing power, and at the same time retained its magnetic field, it would point exactly at right angles to the wire as soon as the current passed. if you look along the wire, fig. , from the point, c, towards the positions, a and b, you will see (a) that _under_ the wire the lines of force pass to the left, and that _above_ the wire (b) they pass towards the right. this is because the n pole points in the directions mentioned. (see fig. .) looking along the wire from z towards position, d and c, you will see just the opposite to the above, as the current comes _towards_ you. _rule._--when the current goes from you, the lines of force pass around the wire in a clockwise direction, and when the current comes toward you they pass around it in an anti-clockwise direction. _= . ampere's rule=_ may be used to remember what has been learned in exp. . _if you imagine yourself swimming in the wire with the current, always facing the needle, the n-seeking pole of the needle will always be deflected towards your left hand._ when the needle is above the wire you must imagine that you swim upon your back, in order to _face_ the needle. _another rule._--hold the right hand with the thumb extended and with the fingers pointing in the direction of the current, the palm being towards the needle and on the opposite side of the wire from the needle. the n-seeking pole will then be deflected in the direction in which the thumb points. _= .=_ if a wire carrying a strong current be dipped in iron filings, the magnetic field about the wire acts by induction upon the particles of filings, making magnets of them. these cling to each other simply because they are little magnets. _= . lines of force about parallel wires.=_ when a current passes in the same direction in two parallel wires the lines of force pass around the wires in the same direction in both, and the magnetic fields attract each other. when the currents flow in opposite directions the magnetic fields repel each other. =experiment . to study the lines of force about a coil of wire like that upon the galvanoscope.= _apparatus._ galvanoscope, g v; dry cell; key; compass. arrange as in fig. , using g v instead of the compass shown. the coil of g v should be placed in the e and w line. the current can pass only when the key is pressed. connect the wires with g v, so that the current will pass through the -turn coil from w to e on top of the coil; that is, so that the current will have a "clockwise" motion. fig. represents a front view of the coil. [illustration: fig. .] [illustration: fig. .] = . directions.= (a) hold the compass in the various places marked with a dot (fig. ) and note the directions taken by its n pole. make a circle similar to the one shown to represent the coil, and sketch upon it the way in which the lines of force pass around it according to your observations. (b) make a diagram like fig. , which represents a cross-section of the coil through the center. imagine that you have removed the top half of the coil and that you are looking down upon the ends of the wire of the lower half. draw curved arrows about the coil at w and e to show which way the lines of force are passing. compare your results with those in fig. , remembering that at e, fig. , the current is going away from you. (c) move o c back and forth on the center-line that runs n and s through the coil, and note the positions of the compass-needle. does the coil seem to have poles? (d) reverse the current through the coil and repeat your observations. =experiment . to study the magnetic field about a small coil of wire.= _apparatus._ a coil of wire (no. ), described in § ; current reverser, c r (no. ); dry cell; connecting wires, etc. = . coils= of wire for some of the following experiments should be wound upon wooden spools that have been turned down thin, so that the wire will be as near the central hole as possible. they should be wound with a winder. (see apparatus book, chapter x.) for convenience we shall call the starting end of the coil, that is, the end that comes from the wire that is near the center, the _inside end_, i e. the end of the last layer of the coil we shall call the _outside end_, o e. these letters should be noted in the diagrams. see apparatus list for details of the special coils used in these experiments. [illustration: fig. .] = . directions.= (a) arrange as in fig. , so that the axis of the coil will lie in the e and w line. place o c about in. from the e end of the coil. press one lever of c r so that the current will pass around the coil for an instant in a clockwise direction; that is, so that it will enter the coil at o e. note the action of the needle. if the needle is not affected move it nearer the coil and press the lever again. get clearly in mind the connections, the direction in which the n end of the needle is deflected, etc. is the e end of the coil a n or a s pole? (b) reverse the current through the coil. what effect has it upon the polarity of the e end of the coil? (c) place o c at the west end of the coil and repeat (a) and (b). (d) place o c in various positions about the coil and note the action of the needle when the current passes. does this coil act like a magnet, having poles, magnetic field, etc.? [illustration: fig. .] _= . polarity of coils.=_ it is evident from exps. and that a coiled conductor has poles, magnetic field, etc., when a current passes, and that it strongly resembles a magnet, even though no iron enters into its construction. we may say that the coil becomes magnetized by the electric current. fig. shows a right handed coil or helix of wire, the current passing as shown by the small arrows. the left-hand end is a s pole because the current passes around it in a clockwise direction. when you face the right-hand end of the coil the current is seen to pass around it in an anti-clockwise direction; this produces a n pole. as the n pole of the magnetic needle is attracted toward the s pole of the coil, it is clear that the lines of force pass through the inside of the coil as shown by the large arrows. they then curve through the air and return to the s pole as with magnets. =experiment . to test the attracting and "sucking" power of a magnetized coil or helix.= _apparatus._ the coil, battery, etc., used in exp. , fig. ; a sewing-needle. [illustration: fig. .] = . directions.= (a) arrange the coil, etc., as described in exp. . the coil need not lie in the e and w line, however, and a key may be used instead of the current reverser. (b) magnetize the needle so that its point will be a n pole. (c) tie a thread about the center of the magnetized needle, hold the thread in the hand so that the s pole of the needle will swing freely at the hole at the right-hand end of the coil (fig. ). if the current passes as directed, the right-hand end of the coil will be a n pole. what happens to the needle when the key is pressed for an instant. (d) change the needle to the left end of the coil and repeat. (e) try a nail, pen, iron, etc., instead of the needle. =experiment . to find whether a piece of steel can be permanently magnetized by an electric current.= _apparatus._ same as for last experiment; an unmagnetized sewing-needle; the compass. = . directions.= (a) be sure that the needle is not magnetized. it should attract both ends of the compass-needle. how can any magnetism in the needle be removed? (b) place the needle inside of the coil with its _point_ to the east; that is, with its point at the n pole of the coil, and its head at the s pole. close the circuit for an instant. test the needle again for poles. is the point a n or a s pole? (c) turn the needle end for end in the coil, and see whether its polarity can be reversed. (d) experiment with iron wire, nails, steel pens, spring steel, etc. [illustration: fig. .] =experiment . to study the effect of a piece of iron placed inside of a magnetized coil of wire.= _apparatus._ same as in exp. ; a short rod or iron _core_, i c, of soft iron (no. ) that will fit inside of the coil. this combination is called an electromagnet. = . directions.= (a) arrange first as for exp. , fig. , with the coil in the e and w line, no core being used, and place o c about in. from the right-hand end of the coil. (b) press the lever for an instant to see whether the field of the coil is strong enough to move the compass-needle at that distance. move o c a little nearer or farther from the coil until the needle _just_ moves, when the circuit is closed. (c) place i c inside of the coil (fig. ), and repeat (b) to see whether the magnetic field of the coil is stronger or weaker than before. (d) study the location of the poles. can they be reversed? chapter xxiii. electromagnets. _= . electromagnets=_ are important to the student of electricity. they form the principal part of nearly every electrical instrument. you have seen that a wire has a magnetic field about it the instant a current passes through it. a coil, or helix of wire, has a stronger field than a straight wire carrying the same current, because each turn, or convolution, adds its field to the fields of the other turns. by having a _core_ of soft iron instead of air, wood, or other non-magnetic material, the strength of the magnet is greatly increased. the central core may be permanently fixed in the coil, or it may be removable. (see apparatus book, chapter ix, for home-made electromagnets.) _= . cores of electromagnets.=_ a strong magnet has more lines of force passing from its n pole through the air to its s pole than a weak magnet. by increasing the number of lines of force we increase the strength of a magnet. it has been seen, in experiments with permanent magnets, that lines of force do not pass as readily through air as through soft iron, and that lines of force will go out of their way to pass through iron. it was learned in exp. that inside of a helix (fig. ) the lines of force pass from the s to the n pole; they then spread out through the air and pass back on all sides of the coil to its s pole, as in the case of permanent magnets. the air around and inside of a helix offers a great resistance to the lines of force, and tends to weaken the magnetic field. when part of the circuit consists of an iron core, which is a splendid conductor of lines of force, the magnetic field is greatly increased in strength. =experiments - . to study straight electromagnets.= _apparatus._ a good dry cell or other source of a fairly strong current; coil with soft iron core; key; wires with connectors, etc.; small nails; iron-filings; compass; large wire nail; tin box (no. ) to act as a base for the electromagnets. =experiment . lifting power.= = . directions.= (a) join the cell, key, and coil, as explained in exp. , so that the current will pass only when the key is pressed. place the core inside of the coil (fig. ). two good cells in series can be used to advantage. (b) hold the coil in a vertical position near small nails, iron filings, tin boxes, etc.; then press the key and raise coil; carry the clinging iron to another place, break the circuit at the key, and explain the result. why do nails cling more strongly to the core than filings after the circuit is broken? =experiment . residual magnetism of core.= = . directions.= (a) after the current has passed through the coil with the core in place, remove the core and test it for magnetism with the compass. will the small end of the core attract both poles of the compass-needle, or is it slightly magnetized? (b) if there is any residual magnetism, strike the core with a hammer and test again. (c) use a soft steel wire nail for the core, and repeat (a) and (b). why does soft iron make a better core than steel for electromagnets? which should be the more easily magnetized? =experiment . magnetic tick.= = . directions.= (a) join the electromagnet with the cell and key as before (exp. ). hold one end of the core firmly against the top of a tin box which should stand upon the table and which should act as a sounding-board. the flat boxes used in the experiments on static electricity are good for this, or use the tin box, no. , for a base. rapidly open and close the circuit by means of the key and listen for any clicks made by the core. (b) listen for this sound in telegraph sounders, electric bells, etc., if you have them. the armature should be held, of course, so that slight sounds can be heard. _= . discussion.=_ a bar of iron becomes slightly longer when it is magnetized, the particles of iron being made to point in the same direction. as soon as the current ceases to flow through the coil the particles of the soft core nearly all resume their mixed positions. the click heard is supposed to be due to the changes in the molecules of iron. the core becomes gradually warmer when it is rapidly magnetized and demagnetized by a strong current. [illustration: fig. .] =experiment . magnetic figures.= = . directions.= (a) arrange as in fig. . the key should be used in case a dry cell acts as the source of the current. two good cells joined in series can be used to advantage. lay the coil flat upon the table and place on it a piece of stiff, smooth paper, or a sheet of glass. (b) sprinkle a few iron filings upon the glass, which may be held in place by books. gently tap the glass with a pencil while you close the circuit at the key. do the filings arrange themselves as in the case of permanent magnets? make a sketch of the field, remembering that you have both n and s poles, and compare it with previous results. [illustration: fig. .] =experiment . magnetic figures.= = . directions.= (a) arrange as in fig. , but stand the coil on end, using the base as directed in § , to hold it firmly in position. join the ends, o e and i e, to the key as before. fig. shows a top view of the coil and base. (b) with books, etc., fix a piece of stiff, smooth paper, or glass just over the top of the core, and proceed as in exp. to study the field. see § for making permanent pictures of magnetic fields. =experiment . magnetic field.= = . directions.= (a) use same arrangement as for exp. , except filings and glass, which are replaced by the compass. (b) hold the compass about in. from the top pole of the electromagnet, close the circuit for a second or two and note action of needle. is the top n or s, when the current enters the coil at o e? compare result with § . (c) move the compass quickly about the pole, the circuit being closed, and note action of needle. compare result with directions taken by particles of iron filings in exp. . (d) reverse the direction of the current through the coil and test the nature of the pole at the top. [illustration: fig. .] [illustration: fig. .] _= . horseshoe electromagnets.=_ fig. shows a simple form of electromagnet with two coils which have a bent piece of iron as a core for both. the coils have to be wound on by hand in this form. as this is troublesome, the coils are generally wound on two separate cores which are joined by a _yoke_ (§ ), which takes the place of the curved part in fig. . the separate coils can be quickly made with a "winder" and joined to suit. (see apparatus book, chapter ix, for home-made electromagnets.) fig. shows a top view of a home-made experimental horseshoe electromagnet. the coils are joined by an iron strap, called the _yoke_, which is screwed to a wooden base. a strip of iron placed above the magnets to be attracted by them, when the current passes, is called the _armature_. (see telegraph sounders.) _= . use of yoke.=_ it has been explained (§ ) why horseshoe magnets are, in general, better than straight ones. the same is true of electromagnets; there are two poles to attract, and two to induce. the lines of force pass through the yoke on their way from one core to the other, and this reduces the resistance to them. the strength of the horseshoe magnet would be greatly reduced if the lines of force were obliged to pass through two air spaces instead of one; in fact, if there were no yoke we should have simply two straight magnets. the yoke should be made of soft iron. [illustration: fig. .] [illustration: fig. .] = . experimental magnets= are quickly joined to a tin base (no. ), which has holes punched in, through which screws can be put to hold the cores in place. fig. shows plan of tin. fig. shows how removable cores are fastened to the base, the coils being on the spools, and fig. shows how home-made coils on bolts can be used. the coils on bolts should be wound as directed in apparatus book, chapter x. the tin base also serves as the yoke. _removable cores._ fig. . these are of soft iron (no. , ). in one end of each is a hole for the screws, s. part of the tin has been cut away in the fig. the copper washer, c w, should be used. (see § .) connectors are fastened to the ends of the coils (§ - ). _bolt cores._ fig. . after winding on the coils, as directed in apparatus book, remove the nut and put on an extra washer, e w, so that the ends of the coils will not be pressed against the tin, but come out between the two washers. push the screw-end of the bolt through holes (about in. apart) punched in the tin, then put on the nut, as shown. do not force the nut on too far,--just far enough to hold the cores in place. the ends of the wires are not shown in figs. , . connectors are fastened to them (§ ). [illustration: fig. .] = . method of joining coils.= to produce the best results the poles of the horseshoe electromagnet should be unlike. as the coils are wound alike, their ends must be joined in such a manner that the current will pass around them in opposite directions; that is, if the current enters one coil at the outside end, o e, it must enter the other coil at the inside end, i e. fig. shows a plan of the connections, spring connectors being fastened to the coil-ends, to allow rapid and easy changes in the arrangement. l, m, and r are pieces of metal fastened to a strip of wood (no. ), used to make connections from cells or other apparatus. they are turned up at each end as in fig. , . care should be taken not to get short circuits by allowing two wires to touch the tin base. by changing the ends of the coils upon l, m, and r (left, middle, and right), and by changing the direction in which the current enters the "combination connecting plates" (no. ), it is evident that the nature of the poles can be regulated to suit. =experiments - . to study horseshoe electromagnets.= _apparatus._ coils of wire with cores and yoke like those explained in this chapter. coils fastened to tin base or yoke with wires leading from them to the combination connecting plates (no. , fig. ), are very handy. cells; iron filings; compass; iron strip (no. ). =experiment . to test the poles.= = . directions.= (a) arrange as in fig. , but use the experimental magnets and combination connections (fig. ) in place of the single coil shown in fig. . join o of the key with l, and zn of the cell with r of fig. . when the key is pressed the current will enter the magnets from l and leave at r. (b) with the compass test the polarity of the cores as in exp. , b, c. make a sketch of the arrangement, and note which pole is n and which s. (c) see which way the current must pass around each coil, by the way it is wound, and compare the results of (b) with exp. , fig. . =experiment . to test the poles.= = . directions.= (a) arrange as in exp. , but reverse the direction of the current through the coils. do this by joining o of the key (fig. ) with r of fig. , and zn of the cell with l. (b) repeat (b) and (c) of exp. and study results. [illustration: fig. .] =experiment . to test the poles.= = . directions.= (a) arrange all connections as in exp. , then reverse the positions of o e and i e of coil a; that is, join o e to m, and i e to l, fig. . this will make unlike ends come together at m; in other words, when the current enters at l and leaves at r it will pass around both coils in the same direction. (b) study the nature of the poles, as in exps. , , and note results. _note._--fig. shows simply the two cores of a horseshoe electromagnet with arrows to indicate in which direction the current is passing in each coil to produce n and s poles. =experiment . to study the inductive action of one core upon the other.= = . directions.= (a) arrange as for exp. , but join the wire from zn of the cell to m (fig. ). in this way coil b will be cut out of the circuit. place the coils in the e and w line. (b) find about how far the residual magnetism of the core of b can act upon the compass-needle, holding the compass on the side away from coil a, no current passing. (c) press the key for an instant, and note whether the magnetism of coil b has been made stronger or weaker. explain the action of core a on core b. =experiment . magnetic figures.= = . directions.= (a) arrange as in exp. . with books, etc., fix a piece of smooth, stiff paper, or a sheet of glass, just above the poles of the electromagnets. (b) sprinkle iron filings upon the glass, and gently tap it while the circuit is closed at the key for a few seconds. make a sketch of the magnetic figure produced. do the lines of force from the opposite poles attract or repel each other? see § for making permanent figures. (see "things a boy should know about electricity" for drawings of magnetic figures.) _note._--if possible, use two or three good cells in series for making magnetic figures, as a fairly strong field is best. =experiment . magnetic figures.= = . directions.= (a) arrange apparatus as for exp. , and make the magnetic figure for this combination, as directed in exp. . sketch and study the results. =experiment . magnetic figures.= = . directions.= (a) arrange the apparatus and connections as in exp. , and make the magnetic figure of this combination as directed in exp. . in this case the poles are alike. sketch and study the results. =experiment . magnetic figures.= = . directions.= (a) arrange apparatus and connections as in exp. , and make the magnetic figure of the combination as directed in exp. . compare the figure produced with that of exp. . in this case the current passes through but one coil. = . permanent magnetic figures= can be made in several ways for future study and comparison. (a) _paraffine paper figures._ make paraffine paper as directed in apparatus book, page . for this purpose smooth, stiff, _white_ paper is best, so that the filings will show plainly, and but a thin coating of paraffine should be given. place the magnets upon the table, lay over them a piece of unparaffined paper, and fix the paraffine paper directly over this. this is necessary, as the coated paper sticks when heated. for electromagnets it will be necessary to support the edges of the paper with books, etc. sprinkle on the filings and tap the paper to make them arrange themselves while the circuit is closed. after the lines of force show plainly, the current need not be used again, provided the paper be kept perfectly still. pass the flame of a bunsen burner over the paper to melt the coating. this will, no doubt, make the two pieces of paper stick together, and permanently fix the particles of filings in place. do not heat the paper too much--just enough to melt the paraffine. if you have no gas, hold a fire-shovel, containing hot coals, over the paper. as soon as the paraffine cools, the figures will stand considerable handling. _blue print figures_ are very pretty, and last indefinitely. get some blue-print paper at a photographer's, who will give you directions about "developing" it with water. keep this in the dark, and take out but one sheet at a time for experiments. to make the figures, take your apparatus near a window where bright sunlight comes in. pull down the curtain so that you have but a dim light when you make the magnetic figure, as directed before. after the lines of force show plainly, raise the curtain, and let the bright sunlight shine on it for or minutes, or until the surface of the paper has a rich, bronze color. the paper cannot be acted upon by the light under the particles of filings. quickly shake the filings from the paper, and wash it in changes of water to "develop" it, then pin the paper up to dry. =experiment . lifting power.= = . directions.= (a) arrange the apparatus as in exp. . hold an iron strip (no. ), a screw-driver, or other iron bar directly over and near the poles of the experimental electromagnet. close the circuit at the key, then lift the magnets by the "armature," as the iron strip may be called, the circuit being kept closed for a few seconds. if your cell is good there should be no trouble in lifting the magnets by the armature. open the circuit, and see whether the magnets drop. (b) hold the magnets upside down directly over nails, tin boxes, iron filings, or other pieces of iron. close the circuit, move the attracted iron to another place on the table, and open the circuit. can this principle be used for practical purposes? _note._--some experiments illustrating practical uses of electromagnets will be given in a future chapter. =experiment . residual magnetism when magnetic circuit is closed.= = . directions.= (a) arrange as in exp. . you have already seen that each core retains some magnetism after the circuit is closed. place the iron strip firmly across the poles, close the circuit for an instant, open the circuit, then see whether the armature still clings to the cores with some strength. the armature should fit well upon the cores for this experiment. (b) again press the armature upon the cores, no current being used; then lift it as in (a). compare the attraction with that found in (a). _= . closed magnetic circuits.=_ it was seen in the study of the permanent horseshoe magnet, that the armature clung strongly to the magnet. the armature closed the magnetic circuit, the lines of force having almost no resistance. in the case of electromagnets the magnetic circuit becomes closed when the armature touches both poles at the same time. the armature clings strongly to the poles even after the current ceases to flow. as soon as the magnetic circuit is broken, however, but little residual magnetism remains. the armatures of electromagnets are usually arranged so that they can not quite touch the cores, to avoid this sticking. chapter xxiv. thermoelectricity. [illustration: fig. .] =experiment . to find whether electricity can be produced by heat.= _apparatus._ the home-made thermopile described in § ; astatic galvanoscope; connecting wires; candle or alcohol lamp. = . home-made thermopile.= (fig. .) for this you need hairpins, copper wire, a piece of wood about in. long and in. square on the ends, pieces of tin, and some small nails. straighten the hairpins and scrape the coating off with sandpaper or a file. scrape the insulation from pieces of copper wire, each about in. long. twist the ends of the copper wire about the ends of the hairpins (fig. ), and then fasten the hairpins to the block. they may be held firmly by small nails which should be driven partly into the block and bent over. the hairpins at the right-hand side of the fig. are shown to be near but not touching each other. this allows all to be heated at the same time. the tin binding-posts may be nailed or screwed to the block, and if the bare copper wires and be placed under x and y before they are screwed down they will be electrically connected. the ends of and may be held under the screw-heads. the block may be supported upon other blocks to raise it to the proper height, which will depend upon the length of the candle. [illustration: fig. .] a thermopile in the form of a circle with several pairs of metals, can easily be made by fastening the hairpins to a piece of cardboard (fig. ) with a hole at the center. this may be supported by blocks, the heat being applied under the center. = . directions.= (a) arrange the apparatus as in fig. . see that the astatic needle is properly adjusted, no magnets being near it. (b) heat the joints as shown, and watch the needle. can a current be produced by heat? (c) remove the connector on wire from y to m, thus cutting one pair out of the circuit. heat the joints again and compare the strength of the current with that produced in (b). (d) see whether much current is produced by one pair. from results obtained do you see any relation between the strength of the current and the number of pairs? _= . thermoelectricity=_ is produced by heating the junction between two metals. different pairs of metals produce different results. antimony and bismuth are often used. if the end of a strip of bismuth be soldered to the end of a similar strip of antimony, and the free ends be connected to a galvanometer of low resistance, the presence of a current will be shown when the point of contact becomes hotter than the rest of the circuit. the current will flow from the bismuth to antimony across the joint. by cooling the junction below the temperature of the rest of the circuit a current will be produced in the opposite direction. thermoelectric currents have a low potential. the energy of the current is kept up by the heat absorbed. _= . peltier effect.=_ the action noted in § can be reversed; that is, if a current from a battery be sent through the metals, the parts at the junction become slightly warmer or cooler than before, depending upon the direction of the current. this is known as the _peltier effect_, the heat not being due to the resistance to the current. _= . thermopiles.=_ as the e. m. f. of the current produced by a single pair of metals is small, several pairs are usually joined in series in such a way that the different currents help each other and flow in the same direction. such combinations, usually made of antimony and bismuth, are called thermoelectric piles, or simply thermopiles. they are useful in detecting very small differences in temperature. the heat of a match, or the cold of a piece of ice, will produce a current even at some distance, the thermopile being connected with a sensitive short-coil astatic galvanometer. (see "things a boy should know about electricity.") chapter xxv. induced currents. _= . electromagnetic induction.=_ you have seen, by experiments, that a magnet has the power to induce another piece of iron or steel to become a magnet. you have also seen, in the study of static electricity, that an electrified body has the power to act through space upon another conductor. a body may be polarized and charged with static electricity by induction. several questions now come up. can a _current_ of electricity in a conductor induce a _current_ in another conductor not in any way connected with the first? can current electricity produce effects through space? is there an electromagnetic induction? it has been seen that a current-carrying wire has a magnetic field, and that magnetic fields can act through space. it is evident, then, that a conductor will be surrounded and cut by lines of force when it is placed in a magnetic field, or near a wire or coil through which a current passes. let us study this by experiments. =experiments - . to study induced currents.= _apparatus._ the two coils of wire (nos. , ); two short, soft iron cores (nos. , ); long iron core (no. ); bar magnet (no. ); astatic galvanoscope (no. ); dry cell (no. ); key (no. ); horseshoe magnet; connecting wires with spring connectors (no. ) on the ends (§ - ); coil of wire (no. ) wound on an iron core; compass. [illustration: fig. .] =experiment . to find whether a current can be generated with a bar magnet and a hollowed coil of wire.= = . directions.= (a) arrange as in fig. . the coil (no. ) of fine wire is joined to a g (no. ) as shown. small pieces of tin or copper, and , are used to make connections between the coil ends and wires, and , which are attached to the galvanoscope. it is best to use the wires, and , so that the coil will be feet at least, from a g; otherwise the needle of a g might be affected by the magnet, m (no. ). (b) get clearly in mind in which direction the right-hand end of the needle is deflected when a current enters a g at l, the left-hand binding-post. if you have forgotten the results of previous experiments, use the cell for an instant, touching the wire from the carbon to l and that from the zinc to r. if any currents come from the coil, later, you should be able to tell in which direction they flow, the coil and a g forming a closed circuit. (c) hold the magnet, m, as shown, and quickly push it into the coil until it has the place of a core, at the same time watching the needle. if a current is produced, in which direction does it flow from the coil? does the needle remain deflected? is the current constant or temporary? (d) after the magnet, m, has been placed in the coil, as in (c), and the needle has come to rest, quickly pull m from the coil, watching the needle. if a current is produced, does it pass from the coil in the same direction as before, in (c)? (e) turn m end for end, repeat (c) and (d), and study the results. are lines of force made to cut the turns of the coil? (f) repeat (c) and (d), moving m slowly. _= . discussion.=_ an induced current, produced as in the above experiment, is a momentary one. no current passes when the magnet and coil are still; at least one of them has to be in motion. when the magnet is inserted, the induced current is said to be an _inverse_ one, as it passes in a direction opposite to that which would be necessary to give the magnet its poles, it being considered a core magnetized by the current. a _direct_ current is produced when the magnet is withdrawn from the coil. rapid movements produce stronger currents than slow ones. (see § .) _= . induced currents and work.=_ it takes force to move a magnet through the center of a coil, and it is this work that is the source of the induced current. when the coil is pushed on to the magnet, or when it is moved through a magnetic field, force is also required. we have, in this simple experiment, the key to the action of the dynamo and other important electrical machines. these will be discussed later. =experiment . to find whether a current can be generated with a bar magnet and a coil of wire having an iron core.= = . directions.= (a) arrange as in exp. , fig. , and, in addition, place an iron core (no. ) inside of the coil (no. ). (b) hold the bar magnet (no. ) as in fig. , and quickly lower it until it touches the core, at the same time watching the needle. study results, direction of current, etc., as before. (c) suddenly withdraw m from the core. is the current produced in the same direction as that from (b)? (d) turn m end for end and repeat (b) and (c). (e) repeat (c) and (d), moving magnet slowly. how does the strength of the current compare with that of exp. ? are lines of force made to cut the turns of the coil? [illustration: fig. .] =experiment . to find whether a current can be generated with a horseshoe magnet and a coil of wire having an iron core.= = . directions.= (a) arrange the apparatus as in exp. , but use the horseshoe magnet, h m, instead of the bar magnet. fig. shows the coil (no. ) with one pole of h m held over the core. (b) study the effect of quickly lowering and raising first one pole and then the other over the core, as with the bar magnet. get clearly in mind the direction in which the induced current flows in each case. _= . induced currents and lines of force.=_ in the experiments just given, it should be remembered that the permanent magnets are sending out thousands of lines of force from their n poles, and receiving them again at their s poles. as the magnet is pushed into the coil (exp. ), the lines of force not only cut through the turns of the coil, but the number of lines of force that cut the coil at any instant varies rapidly as the magnet is moved. motion is necessary, with this arrangement, to make a change in the number of cutting lines of force. the current passes only while the magnet moves; and the direction of the current at any moment depends upon whether the number of lines of force is increasing or decreasing at that moment. (see § , .) [illustration: fig. .] =experiment . to find whether a current can be generated with an electromagnet and a hollow coil of wire.= = . directions.= (a) the hollow coil (no. ) should be joined to the astatic galvanoscope, as shown in fig. . instead of the bar magnet in fig. , an electromagnet is to be used, and this should be joined in series with a cell and key, as shown in fig. . the current from the cell will pass only when k is pressed. (b) note from the winding which way the current must pass around the coil when the circuit is closed at k, and determine whether the lower end of the long iron core, l i c (no. ) should be n or s. with the compass test the poles of the core to be sure you are right. (c) quickly lower the end of l i c into the hollow coil (h, fig. ), the circuit being kept closed long enough to allow the needle to partially come to rest again. withdraw l i c before you open the circuit. explain action of needle. (d) reverse the direction of the current through the electromagnet, by changing the connections, and repeat (c). does any induced current pass through a g when the core is held still in the coil h, even though a current passes through coil e? [illustration: fig. .] =experiment . to find whether a current can be generated with an electromagnet and a coil of wire having an iron core.= = . directions.= (a) fig. shows simply the arrangement of coils. coil h (no. ) with core, is joined to the galvanoscope as in fig. . coil e, with short core, should be joined to key and cell as shown in fig. . (b) keeping in mind the polarity of the lower end of core e, quickly lower it to the core of h, the circuit being kept closed for a few seconds. does the needle remain deflected after the motion ceases? (c) quickly raise e, the circuit being still closed, then open the circuit. compare the directions taken by the induced currents in (b) and (c). _= . discussion of exps. , .=_ this motion in straight lines is not suitable for producing currents strong enough for commercial purposes. in order to produce currents of considerable strength, the coils of wire have to be pushed past magnets with great speed. special machines (see dynamos) are constructed in which the coils are wound so that they can be given a rapid _rotary motion_ as they fly past strong electromagnets. in this way the coil can keep on passing the same magnets, in the same direction, as long as force is applied to the shaft that carries them. [illustration: fig. .] =experiment . to study the effect of starting or stopping a current near a coil of wire or other closed circuit.= = . directions.= (a) arrange as in fig. . place the two coils, h and e, on the same core, l i c. connect e with the key and cell as before (fig. ). connect h with the astatic galvanoscope, a g, as in fig. . keep the coils or feet from a g, so that the needle will not be affected by them. (b) close the circuit at the key, watching the needle, then as soon as the needle regains its former position, open the circuit again. compare the direction of the induced current in h with that of the current in e, ( ) when the main circuit is closed, and ( ) when it is opened. is any current induced in h by a steady current in e? (see transformers.) [illustration: fig. .] =experiment . to study the effect of starting or stopping a current in a coil placed inside of another coil.= = . directions.= (a) arrange as in fig. . join coil h with the astatic galvanoscope, a g. place the small coil p (no. ) with core, inside of h, and connect the ends of p with the key and cell, as shown. (b) close the circuit at k; watch the needle, and as soon as it regains its position, open the circuit again. compare the direction of the induced current in h with that of the inducing current in p, ( ) when the inducing circuit is closed, and ( ) when it is broken. (see induction coils.) _= . discussion of exps. , .=_ when a current suddenly begins to flow through a coil, the effect upon a neighboring coil is the same as that produced by suddenly bringing a magnet near it; and when the current stops, the opposite effect is produced. we may consider that when the inducing circuit is closed, the lines of force shoot out through the turns of the outside coil. upon opening the circuit the lines of force cease to exist; that is, we may imagine them drawn in again. [illustration: fig. .] _= . direction of induced current.=_ fig. shows the magnet on its way into the coil; the number of lines of force is increasing in the coil, and the induced current passes in an anti-clockwise direction when looking down into the coil along the lines of force. this produces an _indirect_ current. if a current from a cell were passed through the coil in the direction of this indirect current, the lower end of a bar of iron would become a s pole. (see § .) _= . laws of induction.=_ ( ) an increase in the number of lines of force that pass through a closed circuit produces an indirect induced current; while a decrease produces a direct one. (see § .) ( ) the e. m. f. of the induced current is equal to the rate of increase or decrease in the number of lines of force that pass through the circuit. ( ) a constant current produces no induced current, provided there is no motion. ( ) closing a circuit produces an indirect current. ( ) opening a circuit produces a direct current. ( ) _lenz's law._ induced currents have a direction that tends to stop the motion that produces them. _= . primary and secondary currents.=_ in the preceding experiments in induction, it must be kept in mind that the current from the cell did not pass through the galvanoscope. there were two entirely separate circuits, in no way connected. the _primary_ current comes from the cell, while the _secondary_ current is an induced one. [illustration: fig. .] =experiment . to see what is meant by alternating currents.= = . directions.= (a) arrange as in fig. . connect coil h with a g, as before. place one pole of h m against the end of the core i c, hold h with one hand, and with the other quickly push the other pole of h m onto the core. this should produce a momentary current through a g, first in one direction, and then in the other. let the needle come to rest. (b) move h m back and forth upon the end of i c, changing its polarity rapidly. a minute's practice will enable you to slide the core from one pole of h m to the other and back again rapidly-- complete vibrations per second being about right. the needle should be parallel to the coil of a g, and if properly done, the needle will be made to vibrate back and forth slightly at each change in the polarity of i c. _= . direct and alternating currents.=_ a current that flows steadily in one direction is said to be a _direct_ current. a cell gives a direct current when the circuit is closed. when the current passes in one direction for an instant, and then reverses immediately and flows in the opposite direction, it is said to _alternate_. the induced current which flowed through the galvanoscope in exp. was an alternating one. currents of this class have great practical uses. _= . self-induction; extra currents.=_ it has been shown that a magnetized coil can act through space and induce a current in a neighboring coil. the lines of force which reach out from an electromagnet will generate a current in any conductor which happens to be in the field, or which is moved across the lines. it is evident, then, since the lines of force from each turn of a coil cut all the other turns of the same coil, that each turn acts as a conductor placed in the field of every other turn. the instant a current begins to flow through a coil, there is an inverse current of self-induction started in the coil, which opposes the current in the cell. when the circuit is broken, this _extra current_, as it is also called, is a direct one and adds its strength to that of the current from the cell; as this takes place at the instant the circuit is broken, a bright spark is seen at the key, and this shows that the e. m. f. of this extra current is high. practical uses are made of it. chapter xxvi. the production of motion by currents. _= . currents and motion.=_ we have seen, in the experiments on induced currents, that a current of electricity can be generated by properly moving magnets near coils of wire. (see dynamo-electric machines.) can we reverse this process? can motion be produced by the electric current? =experiments - . to study the production of motion by means of the electric current.= _apparatus._ the support, including base, rod, and support wire, s w (fig. .) coils of wire (no. , ); iron cores for coils; cell; key; connecting wires; compass; current reverser; bar magnet; horseshoe magnet. [illustration: fig. .] =experiment . motion produced with a hollow coil and a piece of iron.= = . directions.= (a) arrange as in fig. . coil h (no. ) is to be used as a pendulum, and can be supported by fastening a string to it, the upper end of which should be tied to s w. connect the ends of h with k and d c. there will be a slight magnetic field about h as soon as the circuit is closed. (b) hold i c near the end of the coil. close the circuit for an instant. is there any motion produced in h? while the motion will be slight, there should be enough to be noticed if the cell is strong. (c) swing the suspended coil back and forth like a pendulum for a minute, until you get in mind the rapidity of its vibrations. stop it, then repeat (b), closing and opening the circuit at regular intervals, so that the little impulses given by the attraction for i c will gradually cause h to vibrate. the wires leading from h should not drag upon the table. =experiment . motion with hollow coil and bar magnet.= = . directions.= (a) substitute the bar magnet m (no. ) for the iron of exp. (fig. ). get clearly in mind the polarity of the coil from the way the current flows through it, then test it with the compass to find whether you are right. (b) hold the n pole of m near the left-hand end of the coil, close the circuit for an instant and study results. (c) reverse the magnet and repeat (b). compare the results with those of exp. . try to make the coil vibrate. =experiment . motion with electromagnet and piece of iron.= = . directions.= (a) arrange as described in exp. , fig. . place a short core inside of the coil and repeat. (see § for directions.) why is the motion produced much larger than that given by a hollow coil? (b) the coil can gradually be made to swing through quite a little space by closing and opening the circuit regularly (§ , c). could any use be made of such a motion, if it were on a large scale? could it be made to run a machine? [illustration: fig. .] =experiment . motion with electromagnet and bar magnet.= = . directions.= (a) arrange as in fig. , the coil being suspended and connected as in exp. (fig. ). (b) study the effect of closing the circuit when the n pole of m is held near the core of h. reverse m, and repeat. [illustration: fig. .] =experiment . motion with electromagnet and horseshoe magnet.= = . directions.= (a) arrange as in fig. . the ends of h (no. ) are joined to x and y of the current reverser c r (no. ). it is evident, then, that the direction of the current through h can be easily and rapidly reversed by c r. (see exp. .) either pole of the horseshoe magnet h m will attract i c when it is not magnetized. (b) place the end of i c near the n pole of h m so that it will be attracted to it. you have learned that like poles repel each other, so press the lever of c r that will produce a n pole at the left-hand end of i c. the core i c should be repelled by the n pole of h m and be instantly attracted by its s pole. (c) rapidly reverse the current and make i c jump back and forth from one pole to the other. the results of this experiment should be remembered, as they will aid in understanding motors. a core / in. in diameter can be placed in between the poles and be made to vibrate rapidly as the current is reversed. [illustration: fig. .] =experiment . motion with two electromagnets.= = . directions.= (a) arrange as in fig. . join the two coils, h and e, in parallel. connect their two outside ends o e to a metal plate a, and their inside ends i e to b. join wires and to k, d c, a and b, as shown. when the circuit is closed at k, the current will pass along wire and divide at a, entering e and h at the same time by wires and and returning through and to b, and thence to d c. (b) close the circuit for an instant with wires arranged as in fig. . do the electromagnets attract or repel each other? study out the direction in which the current passes around the coils, and see whether they _should_ attract or repel. (c) change wire to b, and wire to a. the polarity of h, only, will be changed when this circuit is closed. press the key for an instant and study the results. _= . discussion of exps. - .=_ from the results it is evident that motion can be produced with the aid of the electric current in many different ways. it can be produced at the ends of wires which simply reach across the room, or which reach miles from the source of the current. to get practical results for commercial purposes we require a proper source of current, proper conductors, and proper apparatus to convert the motions into useful work. the motions given to the parts of the apparatus in the previous experiments are not suitable for commercial purposes, as they are in straight lines. a rotary motion is needed to do good work; and when this is applied to a shaft, belts can be used to run all sorts of machinery. (see electric motors.) [illustration: fig. .] =experiment . rotary motion with a hollow coil of wire and a permanent magnet.= = . directions.= (a) arrange as in fig. . a key can be used instead of the reverser. the coil of the galvanoscope, g v, has a magnetic field about it when the circuit is closed. the needle has a permanent field. (b) close the circuit for an instant, let the needle swing back past the zero mark, close the circuit again, etc., until the added impulses give the needle a complete turn. (c) keep the needle turning on its axis by opening and closing the circuit at the proper time. with a little practice you can make it turn rapidly. (d) reverse the motion of the needle. (see § .) [illustration: fig. .] =experiment . rotary motion with an electromagnet and a permanent magnet.= = . directions.= (a) arrange as in fig. . place the compass a short distance from the end of the core of the coil h (no. ). close the circuit, and as soon as the needle gets part way around open it again, closing it at the proper time to give the needle a new impulse. the speed can be regulated, somewhat, by changing its distance from the core. a key may be used in place of a reverser. (b) reverse the direction of rotation. _= . discussion of exps. - .=_ we have, in these experiments, the key to the action of electric motors. by properly opening and closing the circuit, the rotary motion can be kept up as long as current is supplied. if a small pulley were attached to the top of the compass-needle in exp. , a tiny belt could be attached, and we should have a machine that could do, perhaps, a fly-power of work. (see electric motors.) chapter xxvii. applications of electricity. _= . things electricity can do.=_ among the almost countless things that electricity can do are the following: it signals without wires. it drills rock, coal, and teeth. it cures diseases and kills criminals. it protects, heats, and ventilates houses. it photographs the bones of the human body. it rings church bells and plays church organs. it lights streets, cars, boats, mines, houses, etc. it pumps water, cooks food, and fans you while eating. it runs all sorts of machinery, elevators, cars, boats, and wagons. it sends messages with the telegraph, telephone, and search-light. it cuts cloth, irons clothes, washes dishes, blackens boots, welds metals, prints books, etc., etc. as this book deals almost exclusively with experiments, to be performed with simple, home-made apparatus, space cannot be given for a discussion of the many instruments and machines which make electricity a practical every-day thing. (see "things a boy should know about electricity.") the principles upon which a few important instruments depend, however, will be given. [illustration: fig. .] =experiment . to study the action of a simple "telegraph sounder."= = . directions.= (a) arrange as in fig. . the electromagnet is supported upon its base, as directed in § . coil h, k, and d c are joined in series. the iron strip, i, can be held by the left hand, while k is worked with the right. (b) press the key, closing the circuit for different lengths of time, and note that the _armature_, i, responds exactly to the motions at k. _= . discussion.=_ the downward click makes a distinct sound, and in regular instruments the armature is allowed to make an upward click, also. the time between the two clicks can be short or long to represent _dots_ or _dashes_, which, together with _spaces_, represent letters. (for telegraph alphabet, and complete directions for making and connecting a home-made telegraph line, see apparatus book.) [illustration: fig. .] _= . telegraph line; connections.=_ fig. shows complete connections for a home-made telegraph line. the capital letters are used for the right side, r, and small letters for the left side, l. gravity cells, b and b, are used. the _sounders_ s and s, and the _keys_, k and k, are shown by a top view, or plan. the broad black lines of s and s represent the armatures, which are directly over the electromagnets. the keys have switches, e and e. the two stations, r and l, may be near each other or in different houses. the _return wire_, r w, passes from the copper of b to the zinc of b. this is important, as the cells must help each other; that is, they are in series. the _line wire_, l w, passes from one station to the other, and the return may be through a wire, r w, or through the earth; but for short lines a return wire is best. _= . operation of line.=_ suppose r (right) and l (left) have a line. fig. shows that r's switch, e, is open, while e is closed. the entire circuit, then, is broken at but one point. as soon as r presses his key, the circuit is closed, and the current from both cells rushes around from b through k, s, l w, s, k, b, r w and back to b. this makes the armatures of s and s come down with a click at the same time. (see exp. .) as soon as the key is raised, the armatures raise, making the up-click. (see § .) as soon as r has finished, he closes his switch, e. l then opens e and answers r. both e and e are closed when the line is not in use, so that either can open his switch at any time and call up the other. closed circuit cells are used for such lines. on large lines the current from a dynamo is used. [illustration: fig. .] =experiment . to study the action and use of the "relay" on telegraph lines.= = . directions.= (a) arrange as in fig. . place k and d c at one end of the table to represent the sending station. at the other end of the table place e, which is the electromagnet of the relay, and h, the electromagnet of the sounder. connect the ends of e with k and d c, l w being the line wire, and r w the return. in practice, the return is through the earth. the relay armature, r a, should vibrate towards e every time k is pressed. c is a piece of copper against which r a presses each time it is attracted by e, and this closes what is called the local circuit. connect the poles of another battery, l b, with c and h, and the other end of coil h with r a. the sounder armature, s a, should be arranged as in exp. . small springs are shown on the two armatures, and these keep them away from the cores when the circuits are open. (b) fasten the parts to a board, and study the connections and action of this home-made outfit. = . the relay= replaces the sounder in the line wire circuit, and its coils are usually wound with many turns of fine wire, so that a feeble current will move its nicely adjusted armature. owing to the large resistance of long telegraph lines, the current is weak when it reaches a distant station, and not strong enough to work an ordinary sounder. the current passes back from the relay to the sending station through the earth. the relay armature acts as an automatic key to open and close the local circuit, which includes also a battery and sounder. the line current does not enter the sounder. (see "things a boy should know about electricity.") [illustration: fig. .] =experiment . to study the action of a two-pole telegraph instrument.= = . directions.= (a) arrange as in fig. . connect the two coils to the connecting plates, as described in § . join a strip of copper cu with wire leading from d c, and join the zinc of d c to m. the ends of wires and should be near cu but they must not touch it. if cu be slightly curved so that its ends are raised above the table, the ends of wires and may be put directly under the ends of cu; each half of cu can then be used as a key. two armatures, a and b, should be held as shown. d c can be placed at one side, of course, its terminals being joined to m and cu. (b) press first one end and then the other of cu, so that the current will pass through h or e at will. (c) paste pieces of paper to the armatures, the left one being marked with a dot, and the other with a dash. the one who sends the message can make dots or dashes at the instrument by pressing the proper key. this form of instrument can be easily made by boys, and the messages are more easily read by the eye than by the ear, as in regular sounders. [illustration: fig. .] =experiment . to study the action of a simple "single needle telegraph instrument."= = . directions.= (a) arrange as in fig. . stick a pin on each side of the n pole of the galvanoscope-needle through the degree-card, so that the needle can make but part of a turn when the circuit is closed. (b) touch one lever of the reverser c r, then the other, to see whether connections are right. the needle should be forced against one pin and then against the other. if motions to the left represent _dots_, and those to the right _dashes_, combinations of dots and dashes can be used for letters as in the "sounder" (exp. ). (c) arrange the apparatus shown in fig. so that messages can be sent. [illustration: fig. .] =experiment . to study the action of a simple automatic "contact breaker," or "current interrupter."= = . directions.= (a) arrange as in fig. . slip a spring connector attached to wire upon the iron strip i, a short distance from its end. hold the left-hand end of i firmly in one hand, and with the other hold the connector on wire just above that on . the right-hand end of i should be just above the core of h. (b) allow the current to pass through the circuit by touching the two connectors together gently. does the armature make one click, as in the telegraph sounder, or does it vibrate rapidly? (c) try the connectors in various positions on i. _= . automatic current interrupters=_ are used on bells, buzzers, induction coils, etc. the principle upon which they work is shown in the above experiment (fig. ). the current, as it comes from the carbon of d c, is obliged to stop when it reaches i, unless the two connectors touch. as soon as the current passes, i is pulled down and away from the upper connector, and this breaks the circuit. i, being held firmly in the hand, immediately springs back to its former position, closing the circuit. the rapidity of the vibrations depends somewhat upon the position of the connectors upon i. in regular instruments, a platinum point is used where the circuit is broken; this stands the constant sparking at that point. [illustration: fig. .] =experiment . to study the action of a simple "electric bell," or a "buzzer."= = . directions.= (a) fig. shows the circuit explained in exp. , with a key or push-button put in, so that the circuit can be closed at a distance from the vibrating armature. (b) have a friend work the key while you hold i and wires and as directed in exp. . the circuit must not be broken at two places, of course, so begin by holding the two connectors together. the armature should vibrate rapidly each time k is pressed. _= . electric bells and buzzers=_ are very nearly alike in construction; in fact, you will have a buzzer by removing the bell from an ordinary electric bell. buzzers are used in places where the loud sound of a bell would be objectionable. by placing a bell near the end of the vibrating armature (fig. ), so that the bell would be struck by it at each vibration, we should have an electric bell. by making the wires and long, the bell or buzzer can be worked at a distance. (see apparatus book, chapter xv, for home-made bells and buzzers.) [illustration: fig. .] =experiment . to study the action of a simple telegraph "recorder."= = . directions.= (a) cut from a tin box, or can, a piece of tin about in. long and - / in. wide. bend this double to make two thicknesses. this will serve as an armature i (fig. ). nail to one end of i a small spool, s, and into this put a short length of lead-pencil, p, which may be held firmly in s by wrapping a little paper around it. connect the ends of coil h to a key and cell as in fig. . (b) hold or fasten i in place, and have a friend make dots and dashes at the key, while you draw a piece of paper past the end of p. a little adjusting will be necessary to get the pencil to write only while the circuit is closed. in regular machines all the parts are automatic. [illustration: fig. .] =experiment . to study the action of a simple "annunciator."= = . directions.= (a) arrange as in fig. . fasten the two electromagnets, h and e, to a board or a piece of stiff cardboard. they may be held in place by passing strings over them and through the board, tying on the other side. the ends of coils h and e should be joined to pieces of tin, a, b, c, by means of connectors. k and k are keys or push-buttons, which in real instruments are in different rooms. two steel pens may be swung on pins a short distance from the ends of the cores, so that their lower ends will be attracted to the cores the instant the current passes through them. the residual magnetism should hold them against the cores until removed. hairpins, nails, or needles can be used instead of pens. (b) press first one k and then the other to see whether your connections are correct. _= . annunciators.=_ there are many forms of annunciators in use to indicate, in a hotel for example, a certain room when a bell rings at the office. if a bell be included in the circuit between d c and a in fig. , it will ring each time a key is pushed. this will call attention to the fact that some one has rung, and the annunciator will show the location of the special call. large instruments are made with hundreds of electromagnets, each one answering to a special room. the instrument should be set, of course, after each call. a nail or screw wound with insulated wire can be used for the electromagnets of a home-made annunciator. =experiment . to study the shocking effects of the "extra current."= = . directions.= (a) use the two electromagnets joined to the connecting plates (fig. ), to generate a self-induced or extra current. connect r of fig. with the zinc of a dry cell, and between l and the carbon of the cell place a key; in other words, join the electromagnets, cell, and key in series. two good cells in series can be used to advantage. (b) wet the ends of two fingers of the left hand, press one upon l and the other on r, thus making a shunt with your hand. with the right hand work the key rapidly. if the current is strong enough you should feel a slight shock in the fingers each time the circuit is broken. the extra current (§ ) causes the shock as it shoots through the fingers. (c) if you have electric bells or telegraph sounders use them for this experiment. _= . induction coils=_ are instruments for producing induced currents of high e. m. f. the apparatus shown in fig. forms a simple induction coil. the _primary_ coil is made of coarser wire and has less turns of wire than the _secondary_ coil. the current in the primary circuit is usually interrupted by an _automatic interrupter_ (exp. ), thus producing an alternating current in the secondary coil, the voltage of which depends upon the relative number of turns in the two coils. induction coils are used in telephone work, for medical purposes, for x-ray work, etc., etc. (for home-made induction coils see apparatus book, chapter xi.) [illustration: fig. .] = . action of induction coils.= fig. shows a top view of one of the home-made induction coils described, in full, in the apparatus book. wires and are the ends of the primary coil, while wires and are the terminals of the secondary coil. the battery wires should be joined to binding-posts w and x, and the handles to y and z. fig. shows the details of the automatic interrupter which is placed in the primary circuit. [illustration: fig. .] if the current enters at w, it will pass through the primary coil and out at x, after going through , r, f, s i, b, e and c. the instant the current passes, the bolt becomes magnetized; this attracts a, which pulls b away from the end of s i, thus automatically opening the circuit. b at once springs back to its former position against s i, as a is no longer attracted; the circuit being closed, the operation is rapidly repeated. (for commercial forms and uses of induction coils see "things a boy should know about electricity.") _= . transformers=_, like induction coils, are instruments for changing the e. m. f. and strength of currents. there is very little loss of energy in well-made transformers. they consist of two coils of wire on the same core; in fact, an induction coil may be considered a transformer. if the secondary coil has times as many turns of wire as the primary, a current with an e. m. f. of volts can be taken from the secondary coil, when the e. m. f. of the current passing through the primary is volt; but the _strength_ (amperes) of the secondary current will be but one-hundredth that of the primary current. by using the coil of fine wire as the primary, the e. m. f. of the current that comes from the other coil will be but one-hundredth that in the fine coil. it will have times its strength, however. continuous currents from cells or dynamos must be interrupted, as in induction coils, to be transformed from one e. m. f. to another. transformers are now largely used in lighting and power circuits, etc. (see "things a boy should know about electricity.") _= . the dynamo.=_ we saw in the exps. of chapter xxv. that currents of electricity can be generated in a coil of wire (closed circuit) by rapidly moving it through the field of a magnet. as shown by the experiments, this can be accomplished in many ways. the dynamo is a machine for doing this on a large scale, the coils being given a rotary motion in a very strong magnetic field; and as the number of lines of force that cut the coil is constantly changing, there is a current in the coil as long as power is applied, and this current is led from the machine by proper devices. _the dynamo is a machine for converting mechanical energy into an electric current, through electromagnetic induction._ if a loop of wire (fig. ) be so arranged on bearings at its ends that it can be made to revolve, a current will flow through it in one direction during one-half of the revolution, and in the opposite direction during the other half, it being insulated from all external conductors. such a current inside of the machine would be of no value; it must be led out to external conductors. some sort of sliding contact is necessary to connect a revolving conductor with a stationary one. [illustration: fig. .] [illustration: fig. .] fig. shows the ends of a coil joined to two rings, x, y, which are insulated from each other, and which rotate with the coil. two stationary pieces of carbon, a, b, called _brushes_, press against the rings, and to these are joined wires which complete the circuit, and which lead out where the current can do work. the arrows show the direction of the current during one-half of a revolution. the rings form a _collector_, and this arrangement gives an alternating current. [illustration: fig. .] in fig. the ends of the coil are joined to the two halves of a cylinder. these halves, x and y, are insulated from each other and from the axis. the current flows from x onto the brush a, through some external circuit where it does work, and thence back through brush b onto y. by the time that y gets around to a the direction of the current in the loop has reversed, so that it passes towards y; but it still enters the outside circuit through a because y is then in contact with a. this device is called a _commutator_, and it allows a constant or direct current to leave the machine. in regular machines there are many loops of wire and several segments to the commutator. the rotating coils are wound upon an iron core, so that the lines of force, in passing from one pole to the other, will meet with as little resistance as possible. the coils, core, and commutator, taken together, are called the _armature_. the magnets which furnish the field are called the _field-magnets_. these are electromagnets, the current from the dynamo, or a part of it, being used to excite them. there are many forms of dynamos, and many ways of winding the armature and field-magnets, but space will not permit a discussion of them here. (see "things a boy should know about electricity.") _= . the electric motor.=_ experiments have shown that motion can be produced by the electric current in many ways. the galvanoscope may be considered a tiny motor. _an electric motor is a machine for transforming electric energy into mechanical power._ while the electric motor is similar in construction to the dynamo, it is opposite to it in action. motors receive current and produce motion. the motion is a rotary one, the power being applied to other machines by means of belts or gears. [illustration: fig. .] =experiment . to study the action of the telephone.= = . directions.= (a) join the ends of coil h (fig. ) to the astatic galvanoscope. move magnet m back and forth in front of the soft iron core, while h is held in position. watch the needle. imagine that vibrations in the air caused by the voice are strong enough to give m a slight motion to and fro, and you can see how a current would be sent through the galvanoscope by speaking against m. _= . the telephone=_ is an instrument for reproducing sounds at a distance, and electricity is the agent by which this is generally accomplished. the part spoken to is called the _transmitter_, and the part which gives the sound out again is called the _receiver_. sound itself does not pass over the line. although the same apparatus may be used for both transmitter and receiver, they are generally different in construction. [illustration: fig. .] _= . the bell or magneto-transmitter=_ generates its own current, and is, strictly speaking, a dynamo that is run by the voice. you have seen, by experiments, that a current can be generated in a coil of wire by moving a magnet back and forth in front of its soft iron core. in the telephone this process is reversed, soft iron in the shape of a thin disc (d, fig. ) being made to vibrate by the voice immediately in front of a coil having a permanent magnet, m, for a core. the soft iron diaphragm is fixed near, but it does not touch the magnet. the coil consists of many turns of fine insulated wire. the current generated is an alternating one and exceedingly feeble; in fact, it can not be detected by a galvanoscope. _= . the receiver=_ has the same construction as the bell transmitter, and receives the currents from the line. as the diaphragm is always attracted by the magnet, it is under a constant strain. this strain is increased when a current passes through the coil in a direction that adds strength to the magnet, and decreased when the current weakens the magnet. when the current through the coil is always in the same direction, but varies in strength, the diaphragm will vibrate on account of the varying pull upon it. [illustration: fig. .] when the current through the coil is an alternating one, the same result is obtained, as the magnet gets weaker and stronger many times per minute. fig. shows two bell instruments joined, either being used as the transmitter and the other as the receiver. _= . the carbon transmitter=_ does not in itself generate a current like the magneto-transmitter; it merely produces changes in the strength of a current that flows through it, and that comes from some outside source. in fig. , x and y are two carbon buttons, x being attached to the diaphragm, d. button y presses gently against x. when d is caused to vibrate by the voice, x is made to press more or less against y, and this allows more or less current to pass through the circuit, in which also is the receiver, r. this direct undulating current changes the pull upon the diaphragm of r, causing it to vibrate and reproduce the original sounds spoken into the transmitter. [illustration: fig. .] _= . induction coils in telephone work.=_ as the resistance of telephone lines is large, a current with a fairly high e. m. f. is desired. while the current from one or two cells is sufficient to work the transmitter, it is not strong enough to force its way over a long line. to get around this difficulty an induction coil is used to transform the battery current, that flows through the transmitter and primary coil, into a current with a high e. m. f. that can go into the main line and force its way to a distant receiver. the battery current in the primary coil is undulating, but always in the same direction, the magnetic field around the core getting weaker and stronger. this causes an alternating current in the secondary coil and main line. [illustration: fig. .] fig. shows the two coils, p, s, of the induction coil. the primary, p, is joined in series with a cell and transmitter. the secondary coil, s, is joined to the receiver. one end of s can be grounded, the current completing the circuit through the earth and into the receiver through another wire entering the earth. there are many forms of transmitters. (see "things a boy should know about electricity.") _= . electric lighting and heating.=_ whenever resistance is offered to the electric current, heat is produced. by proper appliances, the heat of resistance can be applied just where it is needed, and many commercial processes depend upon electricity for their success. dynamos are used to generate currents for lighting and heating purposes. there are two great systems of lighting, the one by _arc_ lamps and the other by _incandescent_ lamps. (see "things a boy should know about electricity.") _= . arc lamps=_ produce a light when a current passes from one carbon rod to the other across an air-space. as the current starts through the lamp, the ends of the carbons touch, and the imperfect contact causes resistance enough to heat the ends red-hot. they are then automatically separated, and the current passes from one to the other, causing the "arc." the resistance of the air-space is reduced by the intensely heated vapor and flying particles of carbon. _= . the incandescent lamp=_ consists of a glass bulb, in which is a vacuum, and the light is caused by the passage of a current through a thin fibre of vegetable carbon, enclosed in the vacuum. the fibre would burn instantly if allowed to come in contact with the air. the fibres have a high resistance, and are easily heated to incandescence. chapter xxviii. wire tables. _copper wire tables_ are very convenient, and a necessity when working electrical examples. the tables here given are taken from a dealer's catalogue, and will be found sufficiently accurate for ordinary work. _explanation of tables._ in the _first_ column are given the sizes of wires by numbers. the b & s or american gauge is used. in the table below is given a comparison between the b & s and the birmingham gauges. the _second column_ gives the diameters of wires. the diameter of no. wire is thousandths of an inch; the diameter of no. wire is a little over thousandths or hundredths of an inch. the _third column_ contains what is called circular mils, a mil being a thousandth of an inch. the figures in this column are obtained by squaring those in the second; thus, for no. wire, × = . this column is useful when working examples where the squares of the diameters are wanted. the rest of the table explains itself. the table at the bottom gives a comparison between the fractional and decimal parts of an inch. space can not be given here for a series of examples showing the many uses of this table. (see "elementary electrical examples.") copper wire tables. (based on the b. a. unit.) =====+=======+=========+=======+===================================+ gauge| diam- |sectional|capac- | ohms | | eter. | area | ity. | | -----+-------+---------+-------+-----------+----------+------------+ b.&s.| in | in |in amp-| per | per | per | no. | ths|circular | eres. | , | mile. | pound. | | | mils. | | feet. | | | -----+-------+---------+-------+-----------+----------+------------+ |. | . | . | . | . | . | |. | . | . | . | . | . | |. | . | . | . | . | . | |. | . | . | . | . | . | |. | . | . | . | . | . | |. | . | . | . | . | . | |. | . | . | . | . | . | |. | . | . | . | . | . | |. | . | . | . | . | . | |. | . | . | . | . | . | |. | . | . | . | . | . | |. | . | . | . | . | . | |. | . | . | . | . | . | |. | . | . | . | . | . | |. | . | . | . | . | . | |. | . | . | . | . | . | |. | . | . | . | . | . | |. | . | . | . | . | . | |. | . | . | . | . | . | |. | . | . | . | . | . | |. | . | . | . | . | . | |. | . | . | . | . | . | |. | . | .... | . | . | . | |. | . | .... | . | . | . | |. | . | .... | . | . | . | |. | . | .... | . | . | . | |. | . | .... | . | . | . | |. | . | .... | . | . | . | |. | . | .... | . | . | . | |. | . | .... | . | . | . | |. | . | .... | . | . | . | |. | . | .... | . | . | . | |. | . | .... | . | . | . | |. | . | .... | . | . | . | |. | . | .... | . | . | . | |. | . | .... | . | . | . | |. | . | .... | . | . | . | |. | . | .... | . | . | . | |. | . | .... | . | . | . | |. | . | .... | . | . | . | |. | . | .... | . | . | . | |. | . | .... | . | . | . | |. | . | .... | . | . | . | |. | . | .... | | . | . | -----+-------+---------+-------+-----------+----------+------------+ =====+=======================+======================== gauge| feet. | pounds. | | -----+-----------+-----------+-----------+------------ b.&s.| per | per | per | per no. | pound. | ohm. | , feet.| ohm. -----+-----------+-----------+-----------+------------ | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . | . -----+-----------+-----------+-----------+------------ comparative table of the fractional and decimal parts of an inch. +-----------------+ | / = . | | / = . | | / = . | | / = . | | / = . | | / = . | | / = . | | / = . | | / = . | | / = . | | / = . | | / = . | | / = . | | / = . | | / = . | | / = . | | / = . | | / = . | | / = . | | / = . | | / = . | | / = . | | / = . | | / = . | | / = . | | / = . | | / = . | | / = . | | / = . | | / = . | | / = . | | / = . | +-----------------+ comparative table of b. and s. and b. w. gauges in decimal parts of an inch. +------------+--------------+-------------+ |birmingham | american | no. of | |wire gauge. | (b. and s.) | wire gauge. | | | wire gauge. | | +------------+--------------+-------------+ | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | . | | | . | | | | . | | | | . | | | | . | | +------------+--------------+-------------+ list of apparatus for the study of elementary electricity and magnetism by experiment. the = = pieces of apparatus in the following list are referred to, by number, in the experiments contained in "the study of elementary electricity and magnetism by experiment." this list is furnished to give those who wish to make their own apparatus an idea of the approximate size, etc., of the various articles used. the author is preparing a price catalogue of the articles included in this list, and of odds and ends needed in the construction of simple, home-made apparatus. =no. .= a package of steel sewing-needles. to be suitable for experiments in magnetism, these should be of good, hard steel, and not too thick. =no. .= a flat cork, about in. in diameter and / in. thick. =no. .= a candle for annealing steel. =no. - .= one dozen assorted annealed iron wires, from in. to in. in length. the iron should be very soft. =no. .= one english horseshoe magnet, - / in. long, best quality. =no. .= a small box of iron filings from soft iron. =no. .= a compass (fig. ). the needle swings very freely; it is enclosed in a wooden pill box, the cover of which forms the support. =no. , .= two soft steel wire nails, in. long. =no. , .= two pieces of spring steel, about in. long and / in. wide, to be magnetized by the student and used as bar magnets. =no. .= an iron ring, or washer, about / in. in diameter. =no. .= a sifter for iron filings. this consists of a pasteboard pill box: prick holes through the bottom with a pin. =no. .= a thin, flexible piece of spring steel, about in. long and / in. wide. =no. , .= two ebonite sheets (e s, fig. ), each in. square. these are made with a special surface. they are very much better than the ordinary smooth ebonite. =no. .= one ebonite rod (e r, fig. ), - / in. long, with special surface. =no. .= one ebonite rod, - / in. long, with special surface, used to support the insulating table, no. (i t, fig. ). =no. .= one piece of flannel cloth, in. square. =no. .= six sheets of tissue-paper, each in. square. =no. .= a few feet of white cotton thread. =no. .= a few feet of black silk thread. =no. .= one support base (s b, fig. ). this is of thin wood, about - / in. by - / in., to one end of which is fastened a spool for holding the support rod (no. ). =no. .= one support rod (s r, fig. ), in. long and / in. in diameter. this rod has a hole in each end. the small hole is for holding the support wire (no. ); the large hole is for the ebonite rod (no. ). =no. .= one support wire (s w, fig. ). =no. .= one wire swing (w s, fig. ). =no. .= one sheet of glass, in. square. =no. .= one bent hairpin (h p, fig. ). =no. .= bottom of flat box (b f b, fig. ), - / in. in diameter. =no. .= top of flat box (t f b, fig. ). =no. .= one electrophorus cover (e c, fig. ), - / in. in diameter. this has rounded edges, and a small tube is riveted into the top of it to hold the insulating handle, e r. =no. .= one insulating table (i t, fig. ). this is made the same as no. , and is supported by no. . =no. .= one insulated copper wire, - / feet long. =no. .= one rubber band (r b, fig. ). =no. .= six bent wire clamps (b c, fig. ). =no. .= one tin box conductor (t b, fig. ). this cylindrical conductor is about the size of an ordinary baking powder box. =no. .= one hairpin discharger for the condenser. =no. .= two sheets of aluminum-leaf for the leaf electroscope (fig. ) and other experiments. =no. .= one bent wire (h p, fig. ) used in connection with the leaf electroscope. =no. .= a dry cell, ordinary size about in. high and - / in. in diameter. =no. .= enough mercury to amalgamate battery zincs. a wooden pill box containing about half a thimbleful will do. =no. .= a coil containing feet of no. insulated copper wire for connections. =no. .= one dozen spring connectors (fig. ) for making connections. these are made of brass, nickel plated, and do not affect the compass-needle. =no. .= a telegraph key (fig. ) without switch. the metal straps are made of aluminum; they are / in. wide, and are fastened to a neat wooden base. =no. .= three metal plates, each about in. by / in., on which spring connectors (no. ) are to be pushed in order to join two wires. =no. .= a current reverser (fig. ). the straps are made of aluminum and are fastened to a neat wooden base. =no. .= a galvanoscope (fig. ) including a degree-card (no. ). the cardboard coil-support, c s, is × - / in., and the hole in it is - / in. in diameter. the coil is - / in. in diameter, made of no. insulated copper wire. =no. .= an astatic galvanoscope (fig. ). the whole may be taken apart and mailed in the containing box, b, which is - / × - / × in. the coil is made of no. wire, and has a resistance of about ohms. spring connectors are used to join a wire to the apparatus by pushing the connectors into the tubular binding-posts, l and r. =no. - .= four strips of sheet zinc, in. by / in., not amalgamated. =no. .= a carbon rod, in. long (fig. ). =no. , .= two glass tumblers (fig. ). =no. , .= two strips of sheet copper, in. by / in. (fig. ). =no. .= one galvanized iron nail. =no. , .= two wooden cross-pieces (fig. ). =no. .= one dozen brass screws, / in. long, size no. , with round heads. =no. .= a porous cup (p c, fig. ) that will stand inside of the tumblers (no. ). =no. .= a zinc rod, about / in. in diameter, like those used in leclanché cells. =no. .= a sheet copper plate for the two-fluid cell (c, fig. ). this is in. wide; it nearly surrounds the porous cup, and is supported upon the edge of the tumbler by a narrow strip, a, with which connections are made by spring connectors (no. ). =no. .= one strip of sheet iron, in. by / in. =no. , .= two strips of sheet lead, in. by / in. =no. .= a resistance coil (fig. ). the coil is made of no. insulated copper wire; it has a resistance of ohms (nearly) and is fastened to a cardboard base. it is so arranged that either one or two ohms can be used at will. =no. .= a wheatstone's bridge (fig. ), including a scale (no. ). the aluminum straps, , , , are fastened to a neat wooden base, in. long by in. wide. a no. german-silver wire is used for the bridge. =no. .= a piece of no. uncovered german-silver wire, . meters long, used for resistance (fig. ). =no. .= a piece of no. uncovered german-silver wire, . meters long. =no. - .= three plate binding-posts, consisting of bent straps of sheet aluminum (x, y, z, fig. ). =no. .= two ounces of copper sulphate, commonly called bluestone. the crystals may be kept in a large wooden pill box. =no. .= one dozen copper washers. =no. .= one combination rule, ft. long, marked with english and metric systems. =no. .= a hollow coil of no. insulated copper wire (fig. ). the spool, on which the wire is wound, has a hole for a five-sixteenths inch core. it is turned down thin, so that the wire is near the core. the coil is about - / in. long and in. in diameter. spring connectors are joined to the ends of the coil. =no. .= a hollow coil of no. insulated copper wire, similar to no. , with spring connectors attached to its ends. =no. .= carbon rod for electroplating. =no. , .= two soft iron cores, with screws (i c, fig. ). these cores are / in. in diameter, and have a threaded hole in one end for fastening them to no. . =no. .= a tin box with three holes punched in its top (fig. ). this serves as a base, as well as a yoke, for the two electromagnets, a, b, shown in plan. =no. .= combination connecting plates (fig. ). three aluminum straps are fastened to a wooden base. they are turned up at their ends so that spring connectors can be easily pushed upon them. =no. .= one long iron core (l i c, fig. ). this is of soft iron, / in. in diameter, and long enough to pass through both coils (no. , ). =no. .= bar magnet, about in. long and / in. in diameter. =no. .= coil of insulated wire wound on a soft iron core, to act as a primary coil for induction experiments. this coil fits inside of the hollow coils (nos. , ). =no. .= a printed degree-card for the galvanoscope (no. ). this is printed on stiff cardboard, about in. in diameter. =no. .= a printed scale for the wheatstone's bridge (no. ). this is printed on stiff paper. the scale is in. long, and is divided into large divisions, each of which is subdivided into parts, thus making parts in all. index. numbers refer to paragraphs. see table of contents for the various experiments. abreast, arrangement of cells, . accumulators. (see storage cells.) action, local, . air, as insulator, , _a_. alternating currents, . amalgamating, , . amber, . ammeter, . ampere, the, , . ampere's rule, . annealing, . annunciators, . anode, , . applications of electricity, chap. xxvii. arc lamp, . arrangement, of cells, to . armature, the, , , . astatic needles, , , ; galvanoscope, , . atmospheric electricity, chap. xiii., ; causes of, . attraction, mutual, ; and repulsion, laws of magnetic, ; electric, laws of, . aurora borealis, . batteries, chap. xv.; storage, . bell, electric, . bell telephone, . bichromate cell, . bound electrification, , . breaking a magnet, . bridge, wheatstone's, to . brushes, . buzzers, electric, . cable, submarine, as condenser, . capacity, inductive, ; electrical, , . carbon, transmitter, ; electroscope, . cathode, . cell, galvanic, chap. xv.; arrangement of, , , , ; chemical action in, , ; direction of current in, ; local action in, ; open and closed circuit, ; polarization of, ; poles of, ; simple, ; secondary, ; single-fluid, ; two-fluid, , ; various galvanic, to . charge, in condenser, ; residual of condensers, . charging conductors, chap. viii. chemical action, . chemical effects of current, chap. xxi. circuit, electric, ; divided, ; short, . coercive force, , . coils, ; induction, , ; method of joining, ; polarity of, ; resistance, ; simple resistance, . commutator, . compass, ; our, ; needle, , . compound magnets, . condensation of electrification, chap. x., . condensers, ; action of, , ; induction coil, ; submarine cables, . conductive discharge, , . conductors, , , ; hollow and solid, ; and insulators, relation between, ; and non-conductors, . connections, electrical, to . contact breaker, exp. ; § . convective discharge, . copper sulphate solution, . cores, of electromagnets, . coulomb, the, . current electricity, part iii. current, , _a_, ; detectors, , ; direction of in cell, ; direct and alternating, ; extra, ; interrupters, ; primary and secondary, ; measurement of, ; reverser, , ; strength of, chap. xx., , , to ; unit of, . daniell cell, . declination, . depolarizers, , . detectors, current, , . diamagnetic bodies, . dielectric, , , . dielectrics, . dip, . direct currents, , . dischargers, . discharges, kinds of, . divided circuits, , . dry cells, . dynamo, , . earth's magnetism, , , . electric, bells, ; chime, ; circuit, , chap. xvi.; current, , _a_, ; density, ; field, ; horse-power, ; lighting, ; machines, static, ; motor, ; polarization, ; resistance, chap. xviii.; wind, . electricity, static, part ii.; current, part iii.; applications of, chap. xxvii., ; kinds of, ; derivation of name, ; atmospheric, chap. xiii., . electrification, chap. vi., , , , ; and heat, ; condensation of, chap. x.; escape of, ; free and bound, ; induced, chap. ix.; kinds of, ; of earth, ; source of in cells, ; theories about, ; two kinds of, , . electrics and non-electrics, . electrified bodies, , . electrodes, . electromagnetism, chap. xxii., . electromagnets, chap. xxiii., ; cores of, ; horseshoe, . electromotive force, , chap. xvii., , , ; measurement of, exp. ; of polarization, , ; series, ; unit of, . electrophorus, ; action of, , ; our, . electrolysis, . electrolyte, . electroplating, , . electroscope, action of, , ; carbon, ; pith-ball, ; our leaf, , . electroscopes, chap. xi. electrotyping, . equator of magnet, . equipotential points, . external resistance, , . extra current, ; exp. . field, electric, ; magnetic, chap. iv., , ; magnets, . figures, magnetic, ; permanent, . force, ; lines of magnetic, , , , ; lines of electric, ; lines of about a wire, . franklin, benjamin, . free electrification, . frictional electricity, part ii. fulminating panes, . galvanic cells, chap. xv., ; chemical action in, ; various kinds of, to . galvanoscope, to . glass, as insulator, . gold-leaf electroscope, , . gravity cell, . hardening steel, , . heat, effect on resistance, ; effect on magnet, . horse-power, electric, . horseshoe magnet, ; advantages of, ; electromagnets, . hydrogen, , , , , , . inclination of needle, . induced currents, chap. xxv.; and work, ; and lines of force, , , ; direction of, . induced magnetism, chap. iii. induction coils, ; action of, ; condensers of, ; with telephone, . induction, electromagnetic, ; laws of, ; static, theory of, ; successive, . inductive capacity, . insulators, chap. vii., . internal resistance, , , , , . iron and steel, chap. i. iron, hardening properties of, exp. ; impurities of, ; kinds of, ; soft, . jar, leyden, . key, , . lamp, arc, ; incandescent, . laws, of electrification, ; of induction, ; of magnetism, ; of resistance, . leclanché cell, . leyden jar, . lighting, , , . lightning, , _a_; ; rods, . lines of force, about a wire, , ; electric, ; and induced currents, , ; magnetic, , , , , ; resistance to, , . local action, . local currents, . lodestone, . magnetic, bodies, ; circuits, closed, ; field, , ; figures, permanent, ; figures, , exp. , , exps. to ; force, lines of, , , ; induction of the earth, ; needle, ; needles, balancing of, ; needle, dip of, ; problems, ; saturation, ; screens, ; tick, exp. ; transparency, . magnetism, part i.; induced, chap. iii., ; residual, , ; temporary, ; terrestrial, chap. v.; theory of, ; of earth, ; laws of, . magnets, bar, ; compound, ; effect of breaking, ; equator of, , ; experimental, ; kinds of, ; natural, ; poles of, , ; practical uses of, . mercury, . motion, production of, chap. xxvi., , , . motors, electric, . mutual attractions, . natural magnets, . needle, astatic, , , ; magnetic, , . negative electrification, . neutral bodies, . non-conductors, . non-electrics, . north-seeking poles, . ohm, the, . ohm's law, . one-fluid theory, . open and closed circuits, ; cells, . oxygen, , , . peltier effect, . pith-ball electroscope, . plates or elements, . polarization of cells, ; effects of, ; electric, , ; electromotive force of, ; magnetic, ; remedies for, . poles, , , , ; consequent, ; of coils, ; of electrodes, ; reversal of, ; rule for, . pole pieces, . positive electrification, . potential, , ; energy, . primary current, . proof-plane, . quantity, unit of, . recorder, exp. . relay, telegraph, . repulsion, laws of electrostatic, ; laws of magnetic, . residual, charge in condenser, ; magnetism, ; magnetism of core, exp. . resistance, coils, ; effect of heat on, ; electrical, chap. xviii., , , ; external and internal, , , ; internal, , ; laws of, ; to lines of force, ; measurement of, chap. xix.; unit of, . retentivity, , . reverser, current, , . rheostat, simple, . saturation, magnetic, . secondary cells, ; current, . self-induction, . series arrangement of cells, . shocks, . short circuits, . shunts, . silk, as insulator, . single-fluid cell, . single needle telegraph, exp. . sounder, telegraph, . spark, , _a_. static electricity, part ii. static electric machines, . steel, chap. i.; kinds of, ; magnetism of, , , . st. elmo's fire, . storage cells, . successive, induction, ; condensation, . sulphuric acid, , , . tangent galvanometer, . telegraph, line, , ; relay, ; single needle instrument, exp. ; sounder, ; static, . telephone, the, ; bell, ; carbon transmitter, ; with induction coils, ; receiver, . tempering steel, . temporary magnetism, . terrestrial magnetism, chap. v. thermoelectricity, chap. xxiv., . thermopile, ; home-made, . thunder, . transformers, . transmitters, , . two-fluid cell, , ; care of, ; chemical action in, . two-fluid theory of electrification, . unit of, current strength, ; e. m. f., ; of power, ; quantity, ; resistance, . variation, angle of, . varieties of electricity, . volt, the, . voltameters, , , . water, composition of, . watt, the, . wheatstone's bridge, to . wind, electric, . wire tables, chap. xxviii. yoke, use of, . zero, potential, , _a_. zinc, chemical action with, ; with commercial, . zinc plates, reasons for amalgamating, . notes. notes. electrical books electrical apparatus games puzzles educational amusements [illustration] thomas m. st. john, met. e. a word to parents about games and educational amusements. systematic play is as important as systematic work. the best games and home amusements are as valuable to a child as school-studies; in fact, they bring out and stimulate qualities in a child, which no school-study can. fascinating home amusements are as necessary as school-books. boys and girls like to be busy. their amusements should be entered into as heartily, chosen as carefully, and purchased as willingly as school-books. =games.=--jingo and hustle-ball are good games. they are interesting and full of action. they arouse a child's common-sense. they cultivate an ability to think rapidly, judge correctly, and decide quickly. they educate the eye and hand at the same time. they are very simple, and may be played at once. =educational amusements.=--there is a peculiar fascination about electricity and magnetism, which makes these subjects appeal to every boy and girl. there is nothing better than science studies, to teach children to observe and to see what they look at; besides, it is _fun_ to experiment. "fun with electricity" and "fun with magnetism" are educational amusements. they contain fascinating experiments and are systematically arranged. juvenile work in electricity. _from the electrical engineer, may , ._ the position that young america is now taking in the electric and magnetic field is very clearly shown at the electrical show now being held at madison square garden, by an exhibit of simple experimental apparatus made by young boys from the browning school, of this city. the models shown cover every variety of apparatus that is dear to the heart of a boy, and yet, along the whole line from push-buttons to motors, one is struck by the extreme simplicity of design and the ingenious uses made of old tin tomato cans, cracker boxes, bolts, screws, wire, and the wood that a boy can get from a soap box. the apparatus in this exhibit was made by boys , and years of age, from designs made by mr. thomas m. st. john, of the browning school. it clearly shows that good, practical apparatus can be made from cheap materials by an average boy. the whole exhibit is wired and in working order, and it attracts the attention of a large number of parents and boys who hover around to see, in operation, the telegraph instruments, buzzers, shocking coils, current detectors, motors, etc. mr. st. john deserves the thanks of every boy who wants to build his own electrical apparatus for amusement or for experimental purposes, as he has made the designs extremely simple, and has kept constantly in mind the fact that the average boy has but a limited supply of pocket money, and an equally limited supply of tools. how two boys made their own electrical apparatus. =contents:= _chapter_ i. cells and batteries.--ii. battery fluids and solutions.--iii. miscellaneous apparatus and methods of construction.--iv. switches and cut-outs.--v. binding-posts and connectors.--vi. permanent magnets.--vii. magnetic needles and compasses.--viii. yokes and armatures.--ix. electro-magnets.--x. wire-winding apparatus.--xi. induction coils and their attachments.--xii. contact breakers and current interrupters.--xiii. current detectors and galvanometers.--xiv. telegraph keys and sounders.--xv. electric bells and buzzers.--xvi. commutators and current reversers.--xvii. resistance coils.--xviii. apparatus for static electricity.--xix. electric motors.--xx. odds and ends.--xxi. tools and materials. "the author of this book is a teacher and writer of great ingenuity, and we imagine that the effect of such a book as this falling into juvenile hands must be highly stimulating and beneficial. it is full of explicit details and instructions in regard to a great variety of apparatus, and the materials required are all within the compass of very modest pocket-money. moreover, it is systematic and entirely without rhetorical frills, so that the student can go right along without being diverted from good helpful work that will lead him to build useful apparatus and make him understand what he is about. the drawings are plain and excellent. we heartily commend the book."--_electrical engineer._ "those who visited the electrical exhibition last may cannot have failed to notice on the south gallery a very interesting exhibit, consisting, as it did, of electrical apparatus made by boys. the various devices there shown, comprising electro-magnets, telegraph keys and sounders, resistance coils, etc., were turned out by boys following the instructions given in the book with the above title, which is unquestionably one of the most practical little works yet written that treat of similar subjects, for with but a limited amount of mechanical knowledge, and by closely following the instructions given, almost any electrical device may be made at very small expense. that such a book fills a long-felt want may be inferred from the number of inquiries we are constantly receiving from persons desiring to make their own induction coils and other apparatus."--_electricity._ "at the electrical show in new york last may one of the most interesting exhibits was that of simple electrical apparatus made by the boys in one of the private schools in the city. this apparatus, made by boys of thirteen to fifteen years of age, was from designs by the author of this clever little book, and it was remarkable to see what an ingenious use had been made of old tin tomato-cans, cracker-boxes, bolts, screws, wire, and wood. with these simple materials telegraph instruments, coils, buzzers, current detectors, motors, switches, armatures, and an almost endless variety of apparatus were made. in his book mr. st. john has given directions in simple language for making and using these devices, and has illustrated these directions with admirable diagrams and cuts. the little volume is unique, and will prove exceedingly helpful to those of our young readers who are fortunate enough to possess themselves of a copy. for schools where a course of elementary science is taught, no better text-book in the first-steps in electricity is obtainable."--_the great round world._ exhibit of experimental electrical apparatus at the electrical show, madison square garden, new york. while only pieces of simple apparatus were shown in this exhibit, it gave visitors something of an idea of what young boys can do if given proper designs. [illustration: "how two boys made their own electrical apparatus" gives proper designs--designs for over things.] just published. how two boys made their own electrical apparatus. containing complete directions for making all kinds of simple electrical apparatus for the study of elementary electricity. by professor thomas m. st. john, new york city. the book measures × - / in., and is beautifully bound in cloth. it contains pages and illustrations. complete directions are given for making different pieces of apparatus for the practical use of students, teachers, and others who wish to experiment. price, post-paid, $ . . the shocking coils, telegraph instruments, batteries, electromagnets, motors, etc., etc., are so simple in construction that any boy of average ability can make them; in fact, the illustrations have been made directly from apparatus constructed by young boys. the author has been working along this line for several years, and he has been able, _with the help of boys_, to devise a complete line of simple electrical apparatus. _the apparatus is simple because the designs and methods of construction have been worked out practically in the school-room, absolutely no machine-work being required._ _the apparatus is practical because it has been designed for real use in the experimental study of elementary electricity._ _the apparatus is cheap because most of the parts can be made of old tin cans and cracker boxes, bolts, screws, wires and wood._ address, thomas m. st. john, west st street, new york. fun with magnetism. book and complete outfit for sixty-one experiments in magnetism.... [illustration] children like to do experiments; and in this way, better than in any other, _a practical knowledge of the elements of magnetism_ may be obtained. these experiments, although arranged to amuse boys and girls, have been found to be very _useful in the class-room_ to supplement the ordinary exercises given in text-books of science. to secure the best _possible quality of apparatus_, the horseshoe magnets were made at sheffield, england, especially for these sets. they are new and strong. other parts of the apparatus have also been selected and made with great care, to adapt them particularly to these experiments.--_from the author's preface._ =contents.=--experiments with horseshoe magnet.--experiments with magnetized needles.--experiments with needles, corks, wires, nails, etc.--experiments with bar magnets.--experiments with floating magnets.--miscellaneous experiments.--miscellaneous illustrations showing what very small children can do with the apparatus.--diagrams showing how magnetized needles may be used by little children to make hundreds of pretty designs upon paper. =amusing experiments.=--something for nervous people to try.--the jersey mosquito.--the stampede.--the runaway.--the dog-fight.--the whirligig.--the naval battle.--a string of fish.--a magnetic gun.--a top upsidedown.--a magnetic windmill.--a compass upsidedown.--the magnetic acrobat.--the busy ant-hill.--the magnetic bridge.--the merry-go-round.--the tight-rope walker.--a magnetic motor using attractions and repulsions. _the book and complete outfit will be sent, post-paid, upon receipt of cents, by_ thomas m. st. john, w. st st., new york. a few off-hand statements that have been made about "fun with magnetism" and "fun with electricity" in letters of inquiry to the author. (these statements were absolutely unsolicited.) "my little boy has your 'fun with magnetism' and enjoys it so much that if the 'fun with electricity' is ready i would like to have it for him. please let me know," etc. "i have had much fun with 'fun with magnetism.'" "my boy has 'fun with magnetism' and has enjoyed it very much and would like the other. will you," etc. "please let me know when 'fun with electricity' is upon the market, for if it is as good as this, i shall certainly want it." "i have just received 'fun with magnetism' and am delighted with it. please send me sets," etc. "i have 'fun with magnetism' and 'fun with electricity' and have enjoyed them very much. please send," etc. "i am much pleased with 'fun with electricity' and would like to have," etc. "'fun with electricity' is fine and i have had lots of fun with it. please send," etc. "having experimented with both of your apparatus 'fun with magnetism' and 'fun with electricity,' and having found them both amusing and instructive, i wish to ask," etc. "i have purchased your outfits 'fun with electricity' and 'fun with magnetism,' and though they are designed for amusement, i find them a great help in my studies. will you please," etc. "i have one of your outfits of 'fun with electricity,' and i enjoy it very much, some of the experiments being very astonishing. will you please," etc. "i have enjoyed 'fun with magnetism' and 'fun with electricity' very much." "my little boy has your book 'fun with electricity,' which has given him much amusement. he would like to have," etc. "i am very much pleased with both outfits. i am very much in favor of such things for boys; it keeps them occupied with something that is both amusing and instructive. send me," etc. fun with electricity. book and complete outfit for sixty experiments in electricity.... [illustration] enough of the principles of electricity are brought out to make the book instructive as well as amusing. the experiments are systematically arranged, and make a fascinating science course. no chemicals, no danger. the book is conversational and not at all "schooly," harry and ned being two boys who perform the experiments and talk over the results as they go along. "the book reads like a story."--"an appropriate present for a boy or girl."--"intelligent parents will appreciate 'fun with electricity.'"--"very complete, because it contains both book and apparatus."--"there is no end to the fun which a boy or girl can have with this fascinating amusement." =there is fun in these experiments.=--chain lightning.--an electric whirligig.--the baby thunderstorm.--a race with electricity.--an electric frog pond.--an electric ding-dong.--the magic finger.--daddy long-legs.--jumping sally.--an electric kite.--very shocking.--condensed lightning.--an electric fly-trap.--the merry pendulum.--an electric ferry-boat.--a funny piece of paper.--a joke on the family cat.--electricity plays leap-frog.--lightning goes over a bridge.--electricity carries a lantern.--and _= others=_. the _=outfit=_ contains different articles. the _=book of instruction=_ measures × - / inches, and has illustrations, pages, good paper and clear type. _the book and complete outfit will be sent, by mail or express, charges prepaid, upon receipt of cents, by_ thomas m. st. john, w. st st., new york. fun with puzzles. book, key, and complete outfit for four hundred puzzles.... the book measures × - / inches. it is well printed, nicely bound, and contains chapters, pages, and illustrations. the key is illustrated. it is bound with the book, and contains the solution of every puzzle. the complete outfit is placed in a neat box with the book. it consists of numbers, counters, figures, pictures, etc., for doing the puzzles. =contents=: _chapter_ ( ) secret writing. ( ) magic triangles, squares, rectangles, hexagons, crosses, circles, etc. ( ) dropped letter and dropped word puzzles. ( ) mixed proverbs, prose and rhyme. ( ) word diamonds, squares, triangles, and rhomboids. ( ) numerical enigmas. ( ) jumbled writing and magic proverbs. ( ) dissected puzzles. ( ) hidden and concealed words. ( ) divided cakes, pies, gardens, farms, etc. ( ) bicycle and boat puzzles. ( ) various word and letter puzzles. ( ) puzzles with counters. ( ) combination puzzles. ( ) mazes and labyrinths. "fun with puzzles" is a book that every boy and girl should have. it is amusing, instructive,--educational. it is just the thing to wake up boys and girls and make them think. they like it, because it is real fun. this sort of educational play should be given in every school-room and in every home. "fun with puzzles" will puzzle your friends, as well as yourself; it contains some real brain-splitters. over new and original puzzles are given, besides many that are hundreds of years old. =secret writing.= among the many things that "f. w. p." contains, is the key to _secret writing_. it shows you a very simple way to write letters to your friends, and it is simply impossible for others to read what you have written, unless they know the secret. this, alone is a valuable thing for any boy or girl who wants to have some fun. _the book, key, and complete outfit will be sent, postpaid, upon receipt of cents, by_ thomas m. st. john, west st st., new york city. fun with soap-bubbles. book and complete outfit for fancy bubbles and films.... [illustration] =the outfit= contains everything necessary for thousands of beautiful bubbles and films. all highly colored articles have been carefully avoided, as cheap paints and dyes are positively dangerous in children's mouths. the outfit contains the following articles: one book of instructions, called "fun with soap-bubbles," metal base for bubble stand, wooden rod for bubble stand, large wire rings for bubble stand, small wire ring, straws, package of prepared soap, bubble pipe, water-proof bubble horn. the complete outfit is placed in a neat box with the book. (extra horns, soap, etc., furnished at slight cost.) =contents of book.=--twenty-one illustrations.--introduction.--the colors of soap-bubbles.--the outfit.--soap mixture.--useful hints.--bubbles blown with pipes.--bubbles blown with straws.--bubbles blown with the horn.--floating bubbles.--baby bubbles.--smoke bubbles.--bombshell bubbles.--dancing bubbles.--bubble games.--supported bubbles.--bubble cluster.--suspended bubbles.--bubble lamp chimney.--bubble lenses.--bubble basket.--bubble bellows.--to draw a bubble through a ring.--bubble acorn.--bubble bottle.--a bubble within a bubble.--another way.--bubble shade.--bubble hammock.--wrestling bubbles.--a smoking bubble.--soap films.--the tennis racket film.--fish-net film.--pan-shaped film.--bow and arrow film.--bubble dome.--double bubble dome.--pyramid bubbles.--turtle-back bubbles.--soap-bubbles and frictional electricity. "there is nothing more beautiful than the airy-fairy soap-bubble with its everchanging colors." _=the best possible amusement for old and young.=_ _the book and complete outfit will be sent, post-paid, upon receipt of cents, by_ thomas m. st. john, west st st., new york city. things a boy should know about electricity. (in preparation.) this book explains, in simple, straightforward language, many things about electricity; things in which the american boy is intensely interested; things he wants to know; things he should know. it is free from technical language and rhetorical frills, but it tells how things work, and why they work. it is brimful of illustrations--the best that can be had--illustrations that are taken directly from apparatus and machinery, and that show what they are intended to show. this book does not contain experiments, or tell how to make apparatus; our other books do that. after explaining the simple principles of electricity, it shows how these principles are used and combined to make electricity do every-day work. the following are _some of the things electricity can do:_ it signals without wires. it drills rock, coal, and teeth. it cures diseases and kills criminals. it protects, heats, and ventilates houses. it photographs the bones of the human body. it rings church bells and plays church organs. it lights streets, cars, boats, mines, houses, etc. it pumps water, cooks food, and fans you while eating. it runs all sorts of machinery, elevators, cars, boats, and wagons. it sends messages with the telegraph, telephone, telautograph, and search-light. it cuts cloth, irons clothes, washes dishes, blackens boots, welds metals, prints books, etc., etc. _everyone should know about electricity._ =things a boy should know about electricity= will interest _you_. we shall be glad to send you complete information as soon as it is ready. send us your address now. dewey flag poles =are little models of real flag poles....= [illustration] they are appropriate for any occasion, and suitable for any kind of decoration. they should stand on tables, mantels, pianos, etc.; in fact, there is no better ornament for general use. 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[illustration] [illustration] '_a hustle from the word go._' _=this exciting game=_ will be sent, _=charges prepaid=_, by mail or express, upon receipt of cents. address thomas m. st. john, west st street, new york city. jingo the great war game social exciting interesting simple. a thorough war game:--infantry against infantry, cavalry against cavalry, etc. jingo is really a great war contest between england and america. upon the game-board are beautiful war scenes, each lithographed in colors. american and english flags, coats of arms, cannon, torpedoes, etc., aid in making this game artistic, handsome and attractive. the following companies, ships, etc., are shown:--_american_, th u. s. infantry,-- th u. s. cavalry,-- d u. s. light artillery,--u. s. mortar battery,--u. s. monitor "miantonomoh,"--u. s. ram "katahdin,"--u. s. battleship "indiana,"--u. s. torpedo boat "cushing,"--u. s dynamite cruiser "vesuvius." _english_, th east lancashire,-- st royal dragoons,--royal horse artillery,--royal artillery,--h. m. s. "thunderer,"--h. m. s. "seagull,"--h. m. s. "nile,"--h. m. s. "australia,"--h. m. s. "dart." the game board is over inches square when opened. jingo is made and finished in a manner which makes it the most beautiful, artistic, and practical game ever published. "just what every boy likes." "a good idea well carried out." "the game-board is a work of art." "any child can play jingo at once." "it is the handsomest game on the market." jingo junior is the greatest game ever invented for little folks. it is played upon the jingo board with the extra ammunition furnished. these two great games make a most complete and beautiful outfit for home amusement. jingo and jingo junior. two fascinating and entirely different games, played with one outfit, and complete in one box. [illustration] _this handsome outfit_ for playing the _two great war games_ will be sent _charges prepaid_ upon receipt of _=$ . =_. address thomas m. st. john, west st street, new york city. transcriber's notes in the text version, italic text is denoted by _underscores_ and bold text by =equal signs=. obvious punctuation errors have been repaired. the book contains some inconsistent hyphenation which has been left as printed. p. xi. (toc) "constructiou" changed to "construction" p. xiv. (toc) "the prodution of motion" changed to "the production of motion" p. . para. . "thick permament magnets" changed to "thick permanent magnets" p. . para . "wabble" may be a typo for wobble but has been left on the off chance that this could be what was intended. p. . fig . the final column has been scored through but appears to read "cu to zn" p. . para. , . german-silver wire, g-s w used here but previously g s w used. p. . para . "circuit in closed" changed to "circuit is closed" p. . para . "core inside of the c l" changed to "core inside of the coil" after checking original scans. p. - . it appears that a word has been omitted across the page break. "the copper washer, c w, be used." has been changed to "the copper washer, c w, should be used.". (alternative words are possible!) p. . no. . - in. changed to / in. p. . no. , . - in. changed to / in. p. . no. . and no. . - in. changed to / in. p. . entry for coulomb moved from end of "c" to above current. experiments and observations tending to illustrate the nature and properties of electricity. in one letter to _martin folkes_, esq; president, and two to the _royal society_. by _william watson_, f. r. s. _london_: printed for c. davis, printer to the _royal society_, against _gray's-inn, holborn_. mdccxlvi. [price one shilling.] the preface. _the following sheets were not intended to be made publick, but as part of the philosophical transactions. as those works are printed in the order of time they are read; these observations, communicated to the_ royal society _at different meetings, would, upon that account, have been publish'd separate in different numbers of those transactions. to satisfy therefore the impatience of several learned and very valuable friends, to whose importunities i have neither will, nor inclination to deny any thing in my power to grant, i caused a few copies to be printed, that the whole might be seen together, and then broke up the press. this has excited the curiosity of the publick, and raised a demand for these experiments much beyond what i had reason to expect. i therefore found it necessary to send them to the press a second time, lest some of those over-officious gentlemen, who are always ready on these occasions, should do it for me; so that whoever has an inclination, may now be made acquainted, by what means the several surprizing phænomena of electricity have been brought about._ _i chose to lay these papers before the publick in the same dress wherein they appeared before the very honourable and learned body, to whom, as the various effects of electricity presented themselves, they were regularly communicated, and from whom they met with a very favourable reception. many members of the_ royal society, _as well as several other persons of great rank and quality, have been repeated witnesses of the facts which are here laid before the world; particularly the present worthy president_, martin folkes, esq; _whose extensive abilities and great knowledge in every branch of useful literature are exceeded only by his candour and zeal in promoting science. the advice and assistance of this gentleman, whose friendship i shall always esteem as one of the greatest happinesses of my life, has been of great moment in the prosecution of these discoveries. i therefore take this publick manner of testifying my sincerest obligations as well to him, as to sir_ hans sloane, bart. _who, although retired from business, is nevertheless attentive to whatever tends to the advancement of philosophy. upon a report made to him of these experiments and observations, he, as surviving executor of sir_ godfrey copley, _was pleased to appoint me last year to receive the annual prize-medal of gold, given by the_ royal society _in consequence of sir_ godfrey's _benefaction. the honour of being so particularly taken notice of by gentlemen of such distinguished merit, as it cannot but give me the highest pleasure, so shall it ever continue to raise in me sentiments of the truest gratitude, and most profound respect._ _if it should be asked, to what useful purposes the effects of electricity can be applied, it may be answered, that we are not as yet so far advanced in these discoveries as to render them conducive to the service of mankind. perfection in any branch of philosophy is to be attained but by slow gradations. it is our duty to be still going forward; the rest we must leave to the direction of that providence, which we know assuredly, has created nothing in vain. but i make no scruple to assert, that notwithstanding the great advances, which have been made in this part of natural philosophy within these few years, many and great properties remain still undiscover'd. future philosophers (some perhaps even of the present age) may deduce from electrical experiments, uses extremely beneficial to society in general._ _no present advantage accrued to those persons, or to that age, which first discover'd the properties of the magnet. many hundreds of years intervened, before they were applied to the great uses of navigation. had these remain'd a secret till now, what other methods could have been substituted in their place, by which we could securely traverse the vast ocean? all the advantages we receive from distant commerce, we must still have been strangers to, but for this fortunate application of the magnetical power. and even the discoveries thus far had been very imperfect, without the knowledge of the variation of the compass. but the present age, and even this nation, boasts of a gentleman[ ], who seems to be entrusted with the magnetical powers themselves. he makes artificial magnets, increases in a few minutes the powers of real ones to a surprizing degree, changes at pleasure their poles, and makes that newly acquired polarity, permanent. the world, i hope, will not long be deprived of the manner, by which these extraordinary changes are produced, which as yet this gentleman thinks proper to conceal. as electricity has some properties in common with magnetism, as will be shewn in the course of these observations; some new lights probably may be thrown upon both. but to return; admitting even, that no substantial advantages could arise from the inquiries before us, (which, however, we can by no means grant, upon our considering the effects we already perceive of its operations upon human bodies) whatever tends to enlarge the conceptions of the mind, and to implant in us still more lofty ideas of the almighty author of nature, deserves certainly, independent of other considerations, our highest regard._ _these experiments were all made with glass tubes of about two foot long; the bore about an inch in diameter. but a scrupulous exactness in these proportions is no ways necessary. the thinner and lighter these tubes are, the sooner they are excited; though they, 'tis true, don't retain their power so long as those, which are more thick and substantial. but where you intend to communicate the electrical power, as fast as you excite it, i should prefer a light tube; though it ought never to be less than / of an inch thick, because of the danger of breaking it by the friction._ _the tube, before it is rubbed, should be always made dry and warm, which may be done by laying it before the fire. but i cannot omit hereupon making one further remark_; viz. _that glass tubes, exactly of the same dimensions, made at the same time, and with the same materials, vary considerably with regard to their fitness for electrical purposes. clear and dry air with some degree of cold is most eligible, though i have succeeded in the greatest fogs, but with more difficulty._ to _martin folkes_, esq; p. r. s. _sir_, the society having heard from some of their correspondents in _germany_, that what they call a vegetable quintessence had been fired by electricity, i take this opportunity to acquaint you, that on _friday_ evening last i succeeded, after having been disappointed in many attempts, in setting spirits of wine on fire by that power. the preceding part of the week had been remarkably warm, and the air very dry, than which nothing is more necessary towards the success of electrical trials; to these i may add, that the wind was then easterly and inclining to freeze. i that evening used a glass sphere as well as a tube; but i always find myself capable of sending forth much more fire from the tube than from the sphere, probably from not being sufficiently used to the last. i had before observ'd, that although[ ] non-electric bodies made electrical, lose almost all that electricity by coming either within or near the contact of _non-electrics_ not made electrical; it happens otherwise with regard to _electrics per se_, when excited by rubbing, patting, _&c_; because from the rubbed tube i can sometimes procure five or six flashes from different parts, as though the tube of two foot long, instead of being one continued cylinder, consisted of five or six separate segments of cylinders, each of which gave out its electricity at a different explosion. the knowledge of this theorem is of the utmost consequence towards the success of electrical experiments; inasmuch as you must endeavour by all possible means to collect the whole of this fire at the same time. professor _hollman_ seems to have endeavour'd at this and succeeded, by having a tin tube, in one end of which he put a great many threads, whose extremities touch'd the sphere when in motion, and each thread collected a quantity of electrical fire, the whole of which center'd in the tin tube, and went off at the other extremity. another thing to be observ'd, is to endeavour to make the flashes follow each other so fast, as that a second may be visible before the first is extinguish'd. when you transmit the electrical fire along a sword or other instrument, whose point is sharp, it often appears as a number of disseminated sparks, like wet gunpowder or _wild-fire_; but if the instrument has no point, you generally perceive a pure bright flame, like what is vulgarly call'd the _blue-ball_, which gives the appearance of stars to fired rockets. the following is the method i made use of and was happy enough to succeed in. i suspended a poker in silk lines; at the handle of which i hung several little bundles of white thread, the extremities of which were about a foot at right angles from the poker. among these threads, which were all attracted by the rubbed tube, i excited the greatest electrical fire i was capable, whilst an assistant near the end of the poker held in his hand a spoon, in which were the warm spirits. thus the thread communicated the electricity to the poker, and the spirit was fired at the other end. it must be observ'd in this experiment, that the spoon with the spirit must not touch the poker; if it does, the electricity without any flashing is communicated to the spoon, and to the assistant in whose hand it is held, and so is lost in the floor. by these means, i fired several times not only the ætherial liquor or phlogiston of _frobenius_ and rectified spirit of wine, but even common proof spirit. these experiments, as i before observ'd, were made last _friday_ night, the air being perfectly dry. _sunday_ proved wet and _monday_ somewhat warm, so that the air was full of vapour; wind south-west and cloudy. under these disadvantages, on _monday_ night i attempted again my experiments; they succeeded, but with infinitely more labour than the preceeding, because of the unfitness of the evening for such trials. _your candour_ will not permit you to think my minuteness trivial, with regard to the circumstances of the weather, who know, how many things must concur to make these experiments succeed. i shall wait with impatience for a proper opportunity to have these experiments repeated in your presence, and am, with the utmost respect, _sir, your most obedient,_ _humble servant_, w. watson. _aldersgate-street, march . ._ to the royal society. gentlemen, i lately acquainted you, that i had been able to fire spirit of wine, _phlogiston_ of _frobenius_, and common proof spirit, by the power of electricity. since which (till yesterday) we have had but one very dry fine day; _viz._ _monday, april _. wind e. n. e.; when about four o'clock in the afternoon, i got my _apparatus_ ready, and fired the spirit of wine four times from the poker as before, three times from the finger of a person electrified, standing upon a cake of wax, and once from the finger of a second person standing upon wax, communicating with the first by means of a walking cane held between their arms extended. the horizontal distance in this case between the glass tube and the spirit was at least ten feet. you all know, that there is the repulsive power of electricity, as well as the attractive; inasmuch as you are able, when a feather or such-like light substance is replete with electricity, to drive it about a room, which way you please. this repulsive power continues, until either the tube loses its excited force, or the feather attracts the moisture from the air, or comes near to some non-electric substance; if so, the feather is attracted by, and its electricity lost in, whatever non-electric it comes near. in electrified bodies, you see a perpetual endeavour to get rid of their electricity. this induced me to make the following experiment. i placed a man upon a cake of wax, who held in one of his hands a spoon with the warm spirits, and in the other a poker with the thread. i rubbed the tube amongst the thread, and electrified him as before. i then ordered a person not electrified to bring his finger near the middle of the spoon; upon which, the flash from the spoon and spirit was violent enough to fire the spirit. this experiment i then repeated three times. in this method, the person by whose finger the spirit of wine is fired, feels the stroke much more violent, than when the electrical fire goes from him to the spoon. this method for the sake of distinction, we will call the repulsive power of electricity. the late dr. _desaguliers_ has observed in his excellent dissertation concerning electricity, "that there is a sort of capriciousness attending these experiments, or something unaccountable in their phænomena, not to be reduced to any rule. for sometimes an experiment, which has been made several times successively, will all at once fail." now i imagine that the greatest part, if not the whole of this matter, depends upon the moisture or dryness of the air, a sudden though slight alteration in which, perhaps not sufficient to be obvious to our faculties, may be perceived by the very subtle fire of electricity. for _ st_, i conceive, that the air itself (as has been observed by dr. _desaguliers_) is an _electric per se_ and of the vitreous kind; therefore it repels the electricity arising from the glass tube, and disposes it to electrify whatever non-electrical bodies receive the effluvia from the tube. _ dly_, that water is a _non-electric_, and of consequence a conductor of electricity; this is exemplified by a jett of water being attracted by the tube, _from either electric_'s _per se_ conducting electricity, and _non-electric_'s more readily when wetted; but what is more to my present purpose, is, that if you only blow through a dry glass tube, the moisture from your breath will cause that tube to be a conductor of electricity. these being premised; in proportion as the air is replete with watery vapours, the electricity arising from the tube, instead of being conducted as proposed, is, by means of these vapours, communicated to the circum-ambient atmosphere and dissipated as fast as excited. this theory has been confirmed to me by divers experiments, but by none more remarkably than on the evening of the day i made those before-mention'd; when the vapours, which in the afternoon by the sun's heat, and a brisk gale were dissipated, and the air perfectly dry, descended again in great plenty upon the absence of both, and the evening was very damp. for between seven and eight o'clock, i attempted again the same experiments in the same manner, without being able to make any of them succeed; though all those mention'd in this paper with others of less note, were made in half an hour's time. i am the more particular in this, being willing to save the labour of those, who are desirous of making these kind of trials; for although some of the lesser experiments may succeed almost at any time, yet i never could find that the more remarkable ones would succeed but in dry weather. _i am, gentlemen,_ _your most obedient,_ _humble servant,_ w. watson. _london, april . ._ [illustration] to the royal society. gentlemen, in some papers i lately did myself the honour to lay before you, i acquainted you of some experiments in electricity; particularly i took notice of having been able to fire spirit of wine by what i call'd the repulsive power thereof; which i have not heard had been thought of by any of those _german_ gentlemen, to whom the world is obliged for many surprizing discoveries in this part of natural philosophy. how far strictly speaking the spirit in this operation may be said to be fired by the repulsive power of electricity, or how far that power, which repels light substances when fully impregnated with electricity, fires the spirit, may probably be the subject of a future inquiry; but as i am unwilling to introduce more terms into any demonstration than what are absolutely necessary for the more ready conception thereof, and as inflammable substances may be fired by electricity two different ways, let the following definitions at present suffice of each of these methods. but first give me leave to premise, that no inflammable substances will take fire, when brought into or near the contact of _electrics per se_ excited to electricity. this effect must be produced by non-electrical substances impregnated with electricity received from the exciting _electrics per se_. but to return, _ st_, i suppose that inflammable substances are fired by the attractive power of electricity, when this effect arises from their being brought near excited non-electrics. _ dly_, that inflammable substances are fired by the repulsive power of electricity; when it happens, that the inflammable substances, being first electrified themselves, are fired by being brought near non-electrics not excited. this matter will be better illustrated by an example. suppose that either a man standing upon a cake of wax, or a sword suspended in silk lines are electrified, and the spirit, being brought near them, is fired, this is said to be perform'd by the attractive power of electricity. but if the man electrified as before holds a spoon in his hand containing the spirit, or the same spoon and spirit are placed upon the sword, and a person not electrified applies his finger near the spoon, and the spirit is fired from the flame arising from the spoon and spirit upon such application; this i call being fired by the repulsive power. of the two mention'd kinds i generally find the repulsive power strongest. since my last communication, the spirit has been fired both by the attractive and repulsive power through four persons standing upon electrical cakes; each communicating with the other either by the means of a walking cane, a sword, or any other non-electric substance. it has likewise been fired from the handle of a sword held in the hand of a third person. i have not only fired _frobenius_'s phlogiston, rectified-spirit and common proof-spirit, but also sal volatile oleosum, spirit of lavender, dulcified spirit of nitre, peony water, _daffy_'s elixir, _helvetius_'s stiptic, and some other mixtures where the spirit has been very considerably diluted; likewise distilled vegetable oils, such as that of turpentine, lemon, orange peels and juniper, and even those of them, which are specifically heavier than water, as oil of sassafras; also resinous substances, such as balsam capivi and turpentine; all which send forth, when warmed, an inflammable vapour. but expressed vegetable oils, as those of olives, linseed, and almonds, as well as tallow, all whose vapours are uninflammable, i have not been able yet to fire; but these indeed will not fire on the application of lighted paper. besides, if these last would fire with lighted paper, unless their vapours were inflammable, i can scarce conceive they would fire by electricity; because in firing spirits, _&c._ i always perceive that the electricity snaps before it comes in contact with their surfaces, and therefore only fires their inflammable vapours. as an excited non-electric emits almost all its fire, if once touch'd by a non-electric not excited, i was desirous of being satisfy'd, whether or no the fire emitted would not be greater or less in proportion to the volume of the electrified body. in order to this i procur'd an iron bar about five feet long and near pounds in weight; this i electrified lying on cakes of wax and rosin, but observed the flashes arising therefrom not more violent than those from a common poker. in making this experiment, being willing to try the repulsive force, it once happen'd that whilst the bar was at one end electrifying, a spoon lay upon the other, and upon an assistant's pouring some warm spirit into the spoon, the electrical flash from the spoon snapped and fired the first drop of the spirit, which unexpectedly fired not only the whole jett as it was pouring, but kindled likewise the whole quantity in the pot, in which i usually have it warm'd. i find, in firing inflammable substances from the finger of a man standing upon wax, that _cæteris paribus_ the success is more constant, if the man instead of holding the thread (the use of which i communicated in a former paper) in his hand, the thread is suspended at the end of an iron rod held in one hand, and he touches the spirit with one of the fingers of the other. if a man, standing upon the electrical cake with a dish or deep plate of water in one hand, and the iron rod with the thread in the other, is made electrical; and a person not electrified touches any part either of the plate or water, the flashes of fire come out plentifully, and wherever you bring your finger very near, the water rises up in a little cone, from the point of which the fire is produced, and your finger, though not in actual contact, is made wet. the same experiment succeeds through three or more people. in firing inflammable substances, the person who holds the spoon in his hand to receive the electrical flashes, when the finger of the electrified person is brought near thereto, not only feels a tingling in his hand, but even a slight pain up to his elbow. this is most perceptible in dry weather, when the electricity is very powerful. there is a considerable difficulty in firing _electrics per se_, such as turpentine, and balsam capivi, by the repulsive power of electricity; because in this case these substances will not permit the electricity to pass through them; therefore when you would have this experiment succeed, the finger of the person, who is to fire them, is to be applied as near to the edge as possible of these substances when warm'd in a spoon, that the flashes from the spoon (for these substances will emit none) may snap, where they are spread the thinnest, and then fire their effluvia. this experiment, as well as several others, serves to confute that opinion, which has prevail'd with many, that the electricity floats only upon the surfaces of bodies. if an electrical cake is dipp'd in water, it is thereby made a conductor of electricity, the water hanging about it transmitting the electrical effluvia in such a manner, that a person standing thereon can by no means be electrified enough to attract the leaf gold at the smallest distance; though the person standing upon the same cake when dry, attracted a piece of fine thread hanging at the distance of two feet from his finger. we must here observe that the cake being of an unctuous substance, the water will no where lie uniformly thereon, but adhere in separate moleculæ; so that in this instance the electricity jumps from one particle of water to another, till the whole is dissipated. from the appearance of the threads amongst which i rub the tube, i can frequently judge, though the spirit may be many feet distant from them, whether or no it will fire; because when the persons standing upon the wax are made electrical enough to fire the spirit, the threads repel each other at their lower parts, where they are not confin'd, to a considerable distance, and this distance is in proportion as the threads are made electrical. if two persons stand upon electrical cakes at about a yard's distance from each other, one of which persons, for the sake of distinction, we will call a, the other b: if a when electrified touches b, a loses almost all his electricity at that touch only, which is receiv'd by b and stopp'd by the electrical cake; if a is immediately electrified again to the same degree as before and touches b, the snapping is less upon the touch; and this snapping, upon electrifying a, grows less and less, till b being impregnated with electricity, though receiv'd at intervals, the snapping will no longer be sensible. that glass will repel and not conduct the electricity of glass, has been mention'd by others, who have treated of this subject; but the experiments to determine this matter must be conducted with a great deal of caution; for unless the glass tube, intended to conduct the electricity, be as warm as the external air, it will seem to prove the contrary, unless in very dry places and seasons. thus, i sometimes have brought a cold, though dry, glass tube near three feet long into a room, where there has been a number of people; when upon placing the tube upon silk lines, and laying some leaf silver upon a card at one end and rubbing another glass tube at the other, the silver has, contrary to expectation, been thrown off as readily as from an iron rod. at first i was surpriz'd at this appearance, but then conjectur'd, that it must arise from the coldness of the glass, condensing the floating vapour of the room; in order then to obviate this, i warm'd the tube sufficiently, and this effect was no longer produced, but the silver lay perfectly still. if a number of pieces of finely spun glass cut to about an inch in length, little bits of fine wire of the same length of what metal you please, and small cork balls, are either put all together, or each by themselves, into a dry pewter plate, or upon a piece of polish'd metal, they make in the following manner a very odd and surprizing appearance. let a man, standing upon electrical cakes, hold this plate in his hand with the bits of glass, wire, _&c._ detached from each other, as much as conveniently may be; when he is electrified, let him cause a person standing upon the ground to bring another plate, his hand, or any other non-electric, exactly over the plate containing these bodies. when his hand, _&c._ is about eight inches over them, let him bring it down gently: as it comes near, in proportion to the strength of the electricity, he will observe the bits of glass first raise themselves upright; and then, if he brings his hand nearer, dart directly up and stick to it without snapping. the bits of wire will fly up likewise, and as they come near the hand, snap aloud; you feel a smart stroke, and see the fire arising from them to the hand at every stroke; each of these, as soon as they have discharged their fire, falls down again upon the plate. the cork balls also fly up, and strike your hand, but fall again directly. you have a constant succession of these appearances as long as you continue to electrify the man, in whose hand the plate is held; but if you touch any part either of the man or plate, the pieces of glass, which before were upon their ends, immediately fall down. some few years ago, sir _james lowther_ brought some bladders fill'd with inflammable air, collected from his coal-mines, to the royal society. this air flam'd upon a lighted candle being brought near it. this inflammability has occasion'd many terrible accidents. mr. _maud_, a worthy member of this society, made at that time by art, and shew'd the society, air exactly of the same quality. i was desirous of knowing if this air would be kindled by electrical flashes. i accordingly made such air by putting an ounce of filings of iron, an ounce of oil of vitriol and four ounces of water into a florence flask; upon which an ebullition ensued, and the air, which arose from these materials, not only fill'd three bladders, but also, upon the application of the finger of an electrified person, took flame and burnt near the top and out of the neck of the flask a considerable time. when the flame is almost out, shake the flask and the flame revives. you must with your finger dipped in water, moisten the mouth of the flask as fast as it is dried by the heat within, or the electricity will not fire it: because the flask being an electric _per se_ will not snap at the application of the finger, without the glass being first made non-electric by wetting. it has sometimes happen'd, if the finger has been applied before the inflammable air has found a ready exit from the mouth of the flask, that the flash has fill'd the flask, and gone off with an explosion equal to the firing of a large pistol, and sometimes indeed it has burst the flask. the same effect is produced from the spirit of sea salt, as from oil of vitriol; but as the acid of sea salt is much lighter than that of vitriol, there is no necessity to add the water in this experiment. those who are not much acquainted with chemical philosophy, may think it very extraordinary, that from a mixture of cold substances, which both conjunctly and separately are uninflammable, this very inflammable vapour should be produced. in order to solve this, it may not be improper to premise, that iron is compounded of a metallic as well as a sulphurous part. this sulphur is so fix'd, that, after heating the iron red hot, and even melting it ever so often, the sulphur will not be disengaged therefrom: but upon the mixture of the vitriolic acid, and by the heat and ebullition which are almost instantly produced, the metallic part is dissolved, and the sulphur, which before was intimately connected therewith, being disengaged, becomes volatile. this heat and ebullition continues 'till the vitriolic acid is perfectly saturated with the metallic part of the iron, and the vapour once fired continues to flame, until this saturation being effected, no more of the sulphur flies off. i have heretofore mentioned, how considerably perfectly dry air conduces to the success of these experiments; but we have been lately informed by an extract of a letter, that _abbé nolet_ was of opinion, that they would succeed in wet weather, provided the tubes were made of glass, tinged blue with zaffer. i have procured tubes of this sort, but, after giving them many candid trials, i cannot think them equal to their recommendation. i first tried one of them in a smart shower of rain after a dry day, when the drops were large, and the spirit fired three times in about four minutes; the same effect succeeded, under the same circumstances, from the white one; but after three or four hours raining, when the air was perfectly wet, i never could make it succeed. and to illustrate this matter further, i have been able when the weather has been very dry, with once rubbing my hand down this blue tube, and applying it to the end of an iron rod six feet long, to throw off several pieces of leaf-silver lying upon a card at the other end of this rod, whereas i never have been able to throw it off by any means in very wet weather. besides, i am of opinion, that after the electrical fire is gone from the tube, the tube has no share in the conducting of it; my sentiments on that head i laid before you in a former paper: for if the silk lines are wetted, they diffuse all the electricity, and the same effects happen when the air is wet, be your glass of what colour it will. it may not be improper here to observe, that zaffer, which is used by the glass-makers and enamellers, is made of cobalt or mundick calcin'd after the subliming the flowers. this being reduced to a very fine powder, and mixt with twice or thrice its own weight of finely powdered flints, is moisten'd with water and put up in barrels, in which it soon runs into a hard mass and is call'd zaffer. a dry sponge hanging by a pack-thread at the end of an electrified sword, or from the hand of an electrified man, gives no signs of being made electrical; if it is well soak'd in water, wherever it is touched, you both see and feel the electrical sparks. not only so, but if it is so full of water, that it falls from the sponge, those drops in a dark room, receiv'd upon your hand, not only flash and snap, but you perceive a pricking pain. if you hold your hand, or any non-electrical substances, very near, the water which had ceased dropping when the sponge was not electrified, drops again upon its being electrified, and the drops fall in proportion to the receiv'd electricity, as though the sponge were gently squeez'd between your fingers. i was desirous to know if i was able to electrify a drop of cold water, dropping from the sponge, enough to fire the spirit; but after many unsuccessful trials, i was forced to desist; because the cold water dropping from the sponge not only cool'd the spirit too much, but also render'd it too weak; likewise, every drop carried with it great part of the electricity from the sponge. i then consider'd, in what manner, i could give a tenacity to the water, sufficient to make the drops hang a considerable time, and this i brought about by making a mucilage of the seeds of fleawort. a wet sponge then, squeez'd hard and fill'd with this cold mucilage, was held in the hand of an electrified man, when the drops forced out by the electricity, assisted by the tenacity of the liquor, hung some inches from the sponge, and by a drop of this i fir'd not only the spirit of wine, but likewise the inflammable air before mentioned, both with and without the explosion. what an extraordinary effect is this! that a drop of cold water (for the seeds contribute nothing but add consistence to the water) should be the medium of fire and flame. camphor is a vegetable resin, and of consequence an electric _per se_. this substance, notwithstanding its great inflammability, will not take fire from the finger of a man or any other body electrified, though made very warm and the vapours arise therefrom in great abundance. because, neither electric's _per se_ excited, or electrified bodies, exert their force by snapping upon electric's _per se_, though not excited. if you break camphor small and warm it in a spoon, it is not melted by heat like other resins; but if that heat were continued it would all prove volatile. to camphor thus warm'd, the finger of an electrified man, a sword or such-like, will in snapping exert its force upon the spoon, and the circum-ambient vapour of the camphor will be fired thereby, and light up the whole quantity exposed. the same experiment succeeds by the repulsive power of electricity. a poker thoroughly ignited put into spirit of wine, or into the distilled oil of vegetables, produces no flame in either; it indeed occasions the vapours to arise from the oil in great abundance. but if you electrify this heated poker, the electrical flashes presently kindle flame in either. the experiment is the same with camphor. these experiments, as well as the following, sufficiently evince, that the electrical fire is truly flame, and that extreamly subtil. i have made several trials in order to fire gunpowder alone, which i tried both warm and cold, whole and powder'd, but never could make it succeed; and this arises in part from its vapours not being inflammable, and in part from its not being capable of being fir'd by flame, unless the sulphur in the composition is nearly in the state of accension. this we see by putting gunpowder into a spoon with rectified spirit, which, when lighted, will not fire the powder, 'till by the heat of the spoon from the burning spirit, the sulphur is almost melted. likewise, if you hold gunpowder ground very fine in a spoon over a lighted candle, or any other flame, as soon as the spoon is hot enough to melt the sulphur, you see a blue flame, and instantly the powder flashes off. the same effects are observ'd in the _pulvis fulminans_, compos'd of nitre, sulphur, and fixed alkaline salt. besides, when the gunpowder is very dry and ground very fine, it (as you please to make the experiment) is either attracted, or repell'd; so that in the first case, the end of your finger when electrified, shall be cover'd over with the powder, though held at some distance; and in the other, if you electrify the powder, it will fly off at the approach of any non-electrified substance, and sometimes even without it. but i can at pleasure fire gunpowder, and even discharge a musket, by the power of electricity, when the gunpowder has been ground with a little camphor or with a few drops of some inflammable chemical oil. this oil somewhat moistens the powder, and prevents its flying away; the gunpowder then being warm'd in a spoon, the electrical flashes fire the inflammable vapour, which fires the gunpowder: but the time between the vapour firing the powder is so short, that frequently they appear as the same and not successive operations, wherein the gunpowder itself seems fired by the electricity; and indeed the first time this experiment succeeded, the flash was so sudden and unexpected, that the hand of my assistant, who touch'd the spoon with his finger, was considerably scorch'd. so that there seems a fourth ingredient necessary to make gunpowder readily take fire by flame, and that such a one, as will heighten the inflammability of the sulphur. in common cases the lighted match or the little portion of red hot glass, which falls among the powder, and is the result of the collision from the flint and steel, fires the charcoal and sulphur, and these the nitre. but if to these three ingredients you add a fourth, _viz._ a vegetable chemical oil, and gently warm this mixture, the oil by the warmth mixes intimately with the sulphur, lowers its consistence, and makes it readily take fire by flame. in these operations, notwithstanding i always made use of the finest scented oils of orange peel, lemons, and such like, yet upon the least warming the mixture, the rank smell of balsam (_i. e._ the ready solution) of sulphur was very obvious. read before the r. s. _oct. . _. _a continuation of the above._ read, _feb. . _. as water is a non-electric, and of consequence a conductor of electricity, i had reason to believe that ice was endowed with the same properties. upon making the experiment i found my conjectures not without foundation; for upon electrifying a piece of ice, wherever the ice was touched by a non-electric, it flashed and snapped. a piece of ice also held in the hand of an electrified man, as in the beforementioned processes, fired warm spirit, chemical vegetable oils, camphor, and gunpowder prepared as before. but here great care must be taken, that by the warmth of the hand, or of the air in the room, the ice does not melt; if so, every drop of water therefrom considerably diminishes the received electricity. in order to obviate this, i caused my assistant, while he was electrifying, to be continually wiping the ice dry upon a napkin hung to the buttons of his coat, and this being electrified as well as the ice, prevented any loss of the force of the electricity. the experiment will succeed likewise, if, instead of the ice, you electrify the spirit, _&c._ and bring the ice not electrified near them. i must observe, that ice is not so ready a conductor of electricity as water; so that i very frequently have been disappointed in endeavouring with it to fire inflammable substances, when it has been readily done by a sword or the finger of a man. in the first paper[ ] i had the honour to lay before you upon this subject, i took notice of my having observed two different appearances of the fire from electrified substances; _viz_. those large bright flashes, which may be procured from any part of electrified bodies, by bringing a non-electric unexcited near them, and with which we have fired all the inflammable substances mentioned in the course of these observations; and those, like the firing of wet gunpowder, which are only perceptible at the points or edges of excited non-electrics. these last also appear different in colour and form according to the substances from which they proceed: for from polished bodies, as the point of a sword, a silver probe, the points of scissors, and the edges of the steel-bar made magnetical by the ingenious dr. _knight_, the electrical fire appears like a pencil of rays, agreeing in colour with the fire from _boyle_'s phosphorus; but from unpolished bodies, as the end of a poker, a rusty nail or such-like, the rays are much more red. the difference of colour here, i am of opinion, is owing rather to the different reflection of the electrical fire from the surface of the body from which it is emitted, than to any difference in the fire itself. these pencils of rays issue successively as long as the bodies, from which they proceed, are exciting; but they are longer and more brilliant, if you bring any non-electric not excited near them, though it must not be close enough to make them snap. if you hold your hand at about two or three inches distance from these points, you not only feel successive blasts of wind from them, but hear also a crackling noise. where there are several points, you observe at the same time several pencils of rays. * * * * * it appears from experiments, that besides the several properties, that electricity is possess'd of peculiar to itself, it has some in common with magnetism and light. proposition i. in common with magnetism, electricity counteracts, and in light substances overcomes the force of gravity. like that extraordinary power likewise, it exerts its force _in vacuo_ as powerfully as in open air, and this force is extended to a considerable distance through various substances of different textures and densities. corollary. gravity is the general endeavour and tendency of bodies towards the center of the earth; this is overcome by the magnet with regard to iron, and by electricity with regard to light substances both in its attraction and repulsion; but i have never been able to discern that vortical motion, by which this effect was said to be brought about by the late dr. _desaguliers_ and others, having no other conception of its manner of acting than as rays from a center, which indeed is confirmed by several experiments. one of which, very easy to be tried, is, that if a single downy seed of cotton grass is dropped from a man's hand, and in its fall comes within the attraction of the rubbed tube; the down of this seed, which before seemed to stick together, separates, and forms rays round the center of the seed: or if you fasten many of these seeds with mucilage of gum arabic, round a bit of stick, the down of them when electrified, which otherwise hangs from the stick, is raised up, and forms a circular appearance round the stick. as these light bodies are directed in their motions, only by the force impressed upon them, and as their appearance is constantly _radiatim_, such appearance by no means squares with our idea of a vortex. some have imagined a polarity also, when they have observed one end of an excited glass tube repel light substances, and the other attract them. but this deception, arising from the whole length of the tube not being excited, but only such part of it as has been rubbed; so that as much of the tube as is held in the hand, remains in an unexcited state, and permits light substances to lie still thereon, though forcibly repell'd at the other end. this attractive power of electricity acts not only upon non-electrics, as leaf-gold, silver, thread, and such like, but also upon originally electrics, as silk, dry feathers, little pieces of glass and resin; it attracts all bodies, that are not of the same standard of electricity, (if i may be allowed the expression) as the excited body from which it proceeds. i have found no body however dense, whose pores are not pervious to electricity by a proper management, not even gold itself. proposition ii. in common with light, electricity pervades glass, but suffers no refraction therefrom; i having from the most exact observations found its direction to be in right lines, and that through glasses of different forms, included one within the other, and large spaces left between each glass. corollary. this rectilineal direction is observable only as far as the electricity can penetrate through unexcited originally-electrics, and those perfectly dry; nor is it at all material, whether these substances are transparent, as glass; semidiaphanous, as porcelain or thin cakes of white wax; or quite opake, as thick woollen cloth, as well as woven silk of various colours; it is only necessary that they be originally-electrics. but the case is widely different with regard to non-electrics; wherein the direction, given to the electricity by the excited originally-electric, is alter'd as soon as it touches the surface of a non-electric, and is propagated with a degree of swiftness scarcely to be measured in all possible directions to impregnate the whole non-electric mass in contact with it, or nearly so, however different in itself, and which must of necessity be terminated by an originally electric, before the electricity exerts the least attraction, and then this power is observed first at that part of the non-electric the most remote from the originally-electric. thus for example, by an excited tube held over it, leaf gold will be attracted through glass, cloth, _&c._ held horizontally in the hand of a man standing upon the floor, and this attraction is exerted to a considerable distance. on the contrary, the rubbed tube will not attract leaf gold or other light bodies, however near, through silver, tin, the thinnest board, paper, or any other non-electric, held in the manner before-mentioned. but if you rub the paper over with wax melted, and by that means introduce the originally-electric therein, you observe the electricity acts in right lines, and attracts powerfully. and here i must beg leave to remind you, not only of the former corollary, but of some of the former experiments also; by which it appears, that although, to make a non-electric exert any power, we must excite the whole mass thereof, yet we can excite what part, and what only, of an originally-electric we please. thus we observe, that leaf-gold, and the seed of cotton-grass, (which grows upon boggs and is a very proper subject for these inquiries) are attracted under a glass jar made warm[ ], and turned bottom upwards, upon which are placed books and several other non-electrics; and that the motions of the light bodies underneath correspond with the motions of the glass tube held over them, the electricity seeming instantaneously to pass through the books and the glass. but this does not happen, till the electricity has fully impregnated the non-electrics, which lie upon the glass, which received electricity is stopped by the glass, and then these non-electrics dart their power directly through the upper part of the glass after the manner of originally-electrics. but if the thinnest non-electric, even the finest paper, as i before mentioned, is held in the hand of a man at the smallest distance over the leaf-gold, and the electricity is not stopped, not the least power will be exerted, and the gold will lie still. i must here remark likewise, that this law of electricity is so constant and regular, that i have not found one deviation from it; so that even the quicksilver, spread thin as it usually is at the back of a plate of a looking-glass, will prevent the passing through of the electrical attraction, unless stopped by an originally-electric. this penetration of the electrical power through originally-electrics is much greater than has hitherto been imagined, and has caused the want of success to great numbers of experiments. i have been at no small pains to determine, how far this power can penetrate through a dry originally-electric; and have found by repeated trials, that either in a cake of wax alone, or of wax and resin mixed, when the electricity is very powerful, it has passed, i say, in straight lines through these cakes of the thickness of inches and / ; but i never could make it act through one of inches / , for in this it was perfectly stopped. so that the cakes commonly made use of to stop the electricity, by being too thin, suffer a considerable quantity of the electrical power to pervade them, and be lost in the floor. i make no doubt, if the electrical power could be more increased, it would penetrate much further through these originally-electric bodies. proposition iii. electricity, in common with light likewise, when its forces are collected and a proper direction given thereto upon a proper object, produces fire and flame. corollary. the fire of electricity (as i have before observed) is extremely delicate, and sets on fire, as far as i have yet experienced, only inflammable vapours. nor is this flame at all heightned by being superinduced upon an iron rod, red hot with coarser culinary fire, as in a preceeding experiment; nor diminished by being directed upon cold water. however i was desirous of knowing, if this flame would be effected by a still greater degree of cold; and in order to determine this, i made an artificial cold; by which the mercury, in a very nice thermometer adjusted to _fahrenheit_'s scale, was depressed in about minutes from degrees above the freezing point to degrees below it, that is, the mercury fell degrees. from this cold mixture, when electrified, the flashes were as powerful and the stroke as smart as from the red hot iron. i could have made the cold more intense, but the above was sufficient for my purpose. this experiment seems to indicate, that the fire of electricity is affected neither by the presence or absence of other fire. for as red hot iron, by sir _isaac newton_'s scale of heat, is fixed at degrees, and as the ratio between sir _isaac_'s degrees and _fahrenheit_'s is as to , it necessarily follows, that the difference of heat between the hot iron and the cold mixture is degrees; and nevertheless this vast difference makes no alteration in the appearance of the electrical flame. we find likewise, that as the fire, arising from the refraction of the rays of light by a _lens_, and brought to a _focus_, is observed first at some small distance from their surfaces, to set on fire combustible substances; the same effect, as i have before observ'd, is produced in like manner by electrical flame. i may perhaps be thought too minute in some of the before-mentioned particulars; but in inquiries abstruse as these are, where we have so little _a priori_ to direct us, the greatest attention must be had to every circumstance, if we are truly desirous of investigating the laws of this surprizing power. for, as has been said upon another occasion by my ever honoured friend martin folkes, esq; our most worthy president, "that electricity seems to furnish an inexhaustible fund for inquiry; and sure phænomena so various and so wonderful can arise only from causes very general and extensive, and such as must have been designed by the almighty author of nature for the production of very great effects, and such as are of great moment to the system of the universe." if these observations receive the countenance of this learned society, i shall think myself sufficiently recompensed, and am, _gentlemen, with the highest esteem,_ _your most obedient_ _humble servant_, w. watson. _finis._ ----- footnote : dr. _gowin knight_, f. r. s. footnote : i call _electrics per se_ or originally-_electrics_, those bodies, in which an attractive power towards light substances is easily excited by friction; such as glass, amber, sulphur, sealing-wax, and most dry parts of animals, as silk, hair, and such like. i call _non-electrics_ or conductors of electricity, those bodies, in which the above property is not at all or very slightly perceptible; such as wood, animals living or dead, metals and vegetable substances. see _gray_, _du fay_, _desaguliers_, _wheler_, in the philosophical transactions. footnote : page . footnote : i have constantly observed, that the electrical attraction through glass is much more powerful, when the glass is made warm, than when cold. this effect may proceed from a two-fold cause: first, warm glass does not condense the water from the air, which makes the glass, as has been before[ ] demonstrated, a conductor of electricity: secondly; as heat enlarges the dimensions of all known bodies, and consequently causes their constituent parts to recede from each other, the electrical effluvia, passing in straight lines, find probably a more ready passage through their pores. footnote : page . ------------------------------------------------------------------------ transcriber's note: all footnotes moved to the end of the text. page , 'contract' changed to 'contact,' "the contact of non-electrics..." page , 'power' changed to 'poker,' "i suspended a poker in silk lines;" page , second 'it' struck, "if it does..." page , 'o'clock' rejoined. page , period added to 'e. n. e.' page , 'erectified' changed to 'electrified,' "and a person not electrified..." page , 'it' changed to 'itself,' "not even gold itself." generously made available by the internet archive.) shilling scientific series [illustration: dr. aitken's dust-counter. r is the test-receiver; p the air-pump; m the measuring apparatus; l the illuminating arrangements; g the gasometer; a the pipe through which the tested air is drawn.] meteorology; or, weather explained. by j. g. m'pherson, ph.d., f.r.s.e., graduate with first-class honours, and for nine years extension lecturer on meteorology and mathematical examiner in the university of st. andrews; author of "tales of science," etc. london: t. c. & e. c. jack, henrietta street, w.c. and edinburgh. . the shilling scientific series _the following vols. are now ready or in the press_:-- balloons, airships, and flying machines. by gertrude bacon. motors and motoring. by professor harry spooner. radium. by dr. hampson. telegraphy with and without wires. by w. j. white. electric lighting. by s. f. walker, r.n., m.i.e.e. local government. by percy ashley, m.a. _others in preparation_ printed by ballantyne, hanson & co. at the ballantyne press contents chap. page i. introduction ii. the formation of dew iii. true and false dew iv. hoar-frost v. fog vi. the numbering of the dust vii. dust and atmospheric phenomena viii. a fog-counter ix. formation of clouds x. decay of clouds xi. it always rains xii. haze xiii. hazing effects of atmospheric dust xiv. thunder clears the air xv. disease germs in the air xvi. a change of air xvii. the old moon in the new moon's arms xviii. an autumn afterglow xix. a winter foreglow xx. the rainbow xxi. the aurora borealis xxii. the blue sky xxiii. a sanitary detective xxiv. fog and smoke xxv. electrical deposition of smoke xxvi. radiation from snow xxvii. mountain giants xxviii. the wind xxix. cyclones and anti-cyclones xxx. rain phenomena xxxi. the meteorology of ben nevis xxxii. the weather and influenza xxxiii. climate xxxiv. the "challenger" weather reports xxxv. weather-forecasting index prefatory note i am very much indebted to dr. john aitken, f.r.s., for his great kindness in carefully revising the proof sheets, and giving me most valuable suggestions. this is a sufficient guarantee that accuracy has not been sacrificed to popular explanation. j. g. m'p. ruthven manse, _june , _. meteorology chapter i introduction though by familiarity made commonplace, the "weather" is one of the most important topics of conversation, and has constant bearings upon the work and prospects of business-men and men of pleasure. the state of the weather is the password when people meet on the country road: we could not do without the humble talisman. "a fine day" comes spontaneously to the lips, whatever be the state of the atmosphere, unless it is peculiarly and strikingly repulsive; then "a bitter day" would take the place of the expression. yet i have heard "_terrible_ guid wither" as often as "_terrible_ bad day" among country people. scarcely a friendly letter is penned without a reference to the weather, as to what has been, is, or may be. it is a new stimulus to a lagging conversation at any dinner-table. all are so dependent on the weather, especially those getting up in years or of delicate health. i remember, when at strathpeffer, the great health-resort in the north of scotland, in , an anxious invalid at "the pump" asking a weather-beaten, rheumatic old gamekeeper what sort of a day it was to be, considering that it had been wet for some time. the keeper crippled to the barometer outside the doorway, and returned with the matter-of-fact answer: "she's faurer doon ta tay nur she wass up yestreen." the barometer had evidently fallen during the night. "and what are we to expect?" sadly inquired the invalid. "it'll pe aither ferry wat, or mohr rain"--a poor consolation! most men who are bent on business or pleasure, and all dwellers in the country who have the instruments, make a first call at the barometer in the lobby, or the aneroid in the breakfast-parlour, to "see what she says." a good rise of the black needle (that is, to the right) above the yellow needle is a source of rejoicing, as it will likely be clear, dry, and hard weather. a slight fall (that is, to the left) causes anxiety as to coming rain, and a big depression forebodes much rain or a violent storm of wind. in either case of "fall," the shutters come over the eyes of the observer. next, even before breakfast, a move is made to the self-registering thermometer (set the night before) on a stone, a couple of feet above the grass. a good reading, above the freezing-point in winter and much above it in summer, indicates the absence of killing rimes, that are generally followed by rain. a very low register accounts for the feeling of cold during the night, though the fires were not out; and predicts precarious weather. ordinarily careful observers--as i, who have been in one place for more than thirty years--can, with the morning indications of these two instruments, come pretty sure of their prognostics of the day's weather. of course, the morning newspaper is carefully scanned as to the weather-forecasts from the london meteorological office--direction of wind; warm, mild, or cold; rain or fair, and so on--and in general these indications are wonderfully accurate for twenty-four hours; though the "three days'" prognostics seem to stretch a point. we are hardly up to that yet. the lower animals are very sensitive as to the state of approaching extremes of weather. "thae sea beass," referring to sea-gulls over the inland leas during ploughing, are ordinary indicators of stormy weather. wind is sure to follow violent wheelings of crows. "beware of rain" when the sheep are restive, rubbing themselves on tree stumps. but all are familiar with jenner's prognostics of rain. science has come to the aid of ordinary weather-lore during the last twenty years, by leaps and bounds. time-honoured notions and revered fictions, around which the hallowed associations of our early training fondly and firmly cling, must now yield to the exact handling of modern science; and with reluctance we have to part with them. yet there is in all a fascination to account for certain ordinary phenomena. "the man in the street," as well as the strong reading man, wishes to know the "why" and the "how" of weather-forecasting. they are anxious to have weather-phenomena explained in a plain, interesting, but accurate way. the freshness of the marvellous results has an irresistible charm for the open mind, keen for useful information. the discoveries often seem so simple that one wonders why they were not made before. until about twenty years ago, meteorology was comparatively far back as a science; and in one important branch of it, no one has done more to put weather-lore on a scientific basis than dr. john aitken, f.r.s., who has very kindly given me his full permission to popularise what i like of his numerous and very valuable scientific papers in the _transactions of the royal society of edinburgh_. this i have done my best to carry out in the following pages. "the way of putting it" is my only claim. many scientific men are decoyed on in the search for truth with a spell unknown to others: the anticipation of the results sometimes amounts to a passion. many wrong tracks do they take, yet they start afresh, just as the detective has to take several courses before he hits upon the correct scent. when they succeed, they experience a pleasure which is indescribable; to them fame is more than a mere "fancied life in others' breath." dr. aitken's continued experiments, often of rare ingenuity and brilliancy, show that no truth is altogether barren; and even that which looks at first sight the very simplest and most trivial may turn out fruitful in precious results. small things must not be overlooked, for great discoveries are sometimes at a man's very door. dr. aitken has shown us this in many of his discoveries which have revolutionised a branch of meteorology. prudence, patience, observing power, and perseverance in scientific research will do much to bring about unexpected results, and not more so in any science than in accounting for weather-lore on a rational basis, which it is in the power of all my readers to further. "the old order changeth, giving place to new." with kaleidoscopic variety nature's face changes to the touch of the anxious and reverent observer. and some of these curious weather-views will be disclosed in these pages, so as, in a brief but readable way, to explain the weather, and lay a safe basis for probable forecastings, which will be of great benefit to the man of business as well as the man of pleasure. "felix, qui potuit rerum cognoscere causas." --virgil. chapter ii the formation of dew the writer of the book of job gravely asked the important question, "who hath begotten the drops of dew?" we repeat the question in another form, "whence comes the real dew? does it fall from the heavens above, or does it rise from the earth beneath?" until about the beginning of the seventeenth century, scientific men held the opinion of ordinary observers that dew fell from the atmosphere. but there was then a reaction from this theory, for nardius defined it as an exhalation from the earth. of course, it was well known that dew was formed by the precipitation of the vapour of the air upon a colder body. you can see that any day for yourself by bringing a glass of very cold water into a warm room; the outer surface of the glass is dimmed at once by the moisture from the air. m. picket was puzzled when he saw that a thermometer, suspended five feet above the ground, marked a lower temperature on clear nights than one suspended at the height of seventy-five feet; because it was always supposed that the cold of evening descended from above. again he was puzzled when he observed that a buried thermometer read higher than one on the surface of the ground. until recently the greatest authority on dew was dr. wells, who carefully converged all the rays of scientific light upon the subject. he came to the conclusion that dew was condensed out of the air. but the discovery of the true theory was left to dr. john aitken, f.r.s., a distinguished observer and a practical physicist, of whom scotland has reason to be proud. about twenty years ago he made the discovery, and it is now accepted by all scientific men on the continent as well as in great britain. what first caused him to doubt dr. wells' theory, so universally accepted, that dew is formed of vapour existing at the time in the air, and to suppose that dew is mostly formed of vapour rising from the ground, was the result of some observations made in summer on the temperature of the soil at a small depth under the surface, and of the air over it, after sunset and at night. he was struck with the unvarying fact that the ground, a little below the surface, was warmer than the air over it. by placing a thermometer among stems below the surface, he found that it registered ° fahr. higher than one on the surface. so long, then, as the surface of the ground is above the dew-point (_i.e._ the temperature when dew begins to be formed), vapour must rise from the ground; this moist air will mingle with the air which it enters, and its moisture will be condensed and form dew, whenever it comes in contact with a surface cooled below the dew-point. you can verify this by simple experiments. take a thin, shallow, metal tray, painted black, and place it over the ground after sunset. on dewy nights the _inside_ of the tray is dewed, and the grass inside is wetter than that outside. on some nights there is no dew outside the tray, and on all nights the deposit on the inner is heavier than that on the outside. if wool is used in the experiments, we are reminded of one of the forms of the dewing of gideon's fleece--the fleece was bedewed when all outside was dry. you therefore naturally and rightly come to the conclusion that far more vapour rises out of the ground during the night than condenses as dew on the grass, and that this vapour from the ground is trapped by the tray. much of the rising vapour is generally carried away by the passing wind, however gentle; hence we have it condensed as dew on the roofs of houses, and other places, where you would think that it had fallen from above. the vapour rising under the tray is not diluted by the mixture with the drier air which is occasioned by the passing wind; therefore, though only cooled to the same extent as the air outside, it yields a heavier deposit of dew. if you place the tray on bare ground, you will find on a dewy night that the inside of the tray is quite wet. on a dewy night you will observe that the under part of the gravel of the road is dripping wet when the top is dry. you will find, too, that around pieces of iron and old implements in the field, there is a very marked increase of grass, owing to the deposit of moisture on these articles--moisture which has been condensed by the cold metal from the vapour-charged air, which has risen from the ground on dewy nights. but all doubt upon this important matter is removed by a most successful experiment with a fine balance, which weighs to a quarter of a grain. if vapour rises from the ground for any length of time during dewy nights, the soil which gives off the vapour must lose weight. to test this, cut from the lawn a piece of turf six inches square and a quarter of an inch thick. place this in a shallow pan, and carefully note the weight of both turf and pan with the sensitive balance. to prevent loss by evaporation, the weighing should be done in an open shed. then place the pan and turf at sunset in the open cut. five hours afterwards remove and weigh them, and it will be found that the turf has lost a part of its weight. the vapour which rose from the ground during the formation of the dew accounts for the difference of weight. this weighing-test will also succeed on bare ground. when dealing with hoar-frost, which is just frozen dew, we shall find visible evidence of the rising of dew from the ground. you know the beautiful song, "annie laurie," which begins with-- "maxwelton's braes are bonnie, where early fa's the dew"-- well, you can no longer say that the dew "falls," for it rises from the ground. the song, however, will be sung as sweetly as ever; for the spirit of true poetry defies the cold letter of science. chapter iii true and false dew ever since men could observe and think, they have admired the diamond globules sparkling in the rising sun. these "dew-drops" were considered to be shed from the bosom of the morn into the blooming flowers and rich grass-leaves. ballantine's beautiful song of providential care tells us that "ilka blade o' grass keps it's ain drap o' dew." but, alas! we have to bid "good-bye" to the appellation "dew-drop." what was popularly and poetically called dew _is not dew at all_. then what is it? on what we have been accustomed to call a "dewy" night, after the brilliant summer sun has set, and the stars begin to peep out of the almost cloudless sky, let us take a look at the produce of our vegetable garden. on the broccoli are found glistening drops; but on the peas, growing next them, we find nothing. a closer examination shows us that the moisture on the plants is not arranged as would be expected from the ordinary laws of radiation and condensation. there is no generally filmy appearance over the leaves; the moisture is collected in little drops placed at short distances apart, along the edges of the leaves all round. now place a lighted lantern below one of the blades of the broccoli, and a revelation will be made. the brilliant diamond-drops that fringe the edge of the blade are all placed at the points where the nearly colourless veins of the blade come to the outer edge. the drops are not dew at all, but the exudation of the healthy plant, which has been conveyed up these veins by strong root-pressure. the fact is that the root acts as a kind of force-pump, and keeps up a constant pressure inside the tissues of the plant. one of the simplest experiments suggested by dr. aitken is to lift a single grass-plant, with a clod of moist earth attached to it, and place it on a plate with an inverted tumbler over it. in about an hour, drops will begin to exude, and the tip of nearly every blade will be found to be studded with a diamond-like drop. next substitute water-pressure. remove a blade of broccoli and connect it by means of an india-rubber tube with a head of water of about forty inches. place a glass receiver over it, so as to check evaporation, and leave it for an hour. the plant will be found to have excreted water freely, some parts of the leaves being quite wet, while drops are collected at the places where they appeared at night. if the water pressed into the leaf is coloured with aniline blue, the drops when they first appear are colourless; but before they grow to any size, the blue appears, showing that little water was held in the veins. the whole leaf soon gets coloured of a fine deep blue-green, like that seen when vegetation is rank; this shows that the injected liquid has penetrated through the whole leaf. again, the surfaces of the leaves of these drop-exuding plants never seem to be wetted by the water. it is because of the rejection of water by the leaf-surface that the exuded moisture from the veins remains as a drop. these observations and experiments establish the fact that the drops which first make their appearance on grass on dewy nights are not dew-drops at all, but the exuded watery juices of the plants. if now we look at dead leaves we shall find a difference of formation of the moisture on a dewy night: the moisture is spread equally over, where equally exposed. the moisture exuded by the healthy grass is always found at a _point_ situated near the tip of the blade, forming a drop of some size; but the true dew collects later on _evenly_ all over the blade. the false dew forms a large glistening diamond-drop, whereas the true dew coats the blade with a fine pearly lustre. brilliant globules are produced by the vital action of the plant, especially beautiful when the deep-red setting sun makes them glisten, all a-tremble, with gold light; while an infinite number of minute but shining opal-like particles of moisture bedecks the blade-surfaces, in the form of the gentle dew-- "like that which kept the heart of eden green before the useful trouble of the rain." chapter iv hoar-frost all in this country are familiar with the beauty of hoar-frost. the children are delighted with the funny figures on the glass of the bedroom window on a cold winter morning. frost is a wonderful artist; during the night he has been dipping his brush into something like diluted schist, and laying it gracefully on the smooth panes. and, as you walk over the meadows, you observe the thin white films of ice on the green pasture; and the clear, slender blades seem like crystal spears, or the "lashes of light that trim the stars." you all know what hoar-frost is, though most in the country give it the expressive name of "rime." but you are not all aware of how it is formed. hoar-frost is just frozen dew. in a learned paper, written in , professor wilson of glasgow made this significant remark: "this is a subject which, besides its entire novelty, seems, upon other accounts, to have a claim to some attention." he observed, in that exceptionally cold winter, that, when sheets of paper and plates of metal were laid out, all began to attract hoar-frost as soon as they had time to cool down to the temperature of the air. he was struck with the fact that, while the thermometer indicated degrees of frost a few feet above the ground and degrees of frost at the surface of the snow, there were only degrees of frost at a point inches below the surface of the snow. if he had only thought of placing the thermometer on the grass, under the snow, he would have found it to register the freezing-point only. and had he inserted the instrument below the ground, he would have found it registering a still higher temperature. that fact would have suggested to him the formation of hoar-frost; that the water-vapour from the warm soil was trapped by a cold stratum of air and frozen when in the form of dew. one of the most interesting experiments, without apparatus, which you can make is in connection with the formation of hoar-frost, when there is no snow on the ground, in very cold weather. if it has been a bright, clear, sunny day in january, the effect can be better observed. look over the garden, grass, and walks on the morning after the intense cold of the night; big plane-tree leaves may be found scattered over the place. you see little or no hoar-frost on the _upper_ surface of the leaves. but turn up the surface next the earth, or the road, or the grass, and what do you see? you have only to handle the leaf in this way to be brightly astonished. a thick white coating of hoar-frost, as thick as a layer of snow, is on the _under_ surface. if a number of leaves have been overlapping each other, there will be no coating of hoar-frost under the top leaves; but when you reach the lowest layer, next the bare ground, you will find the hoar-frost on the under surface of the leaves. now that is positive proof that the hoar-frost has not fallen from the air, but has risen from the earth. the sun's heat on the previous day warmed the earth. this heat the earth retained till evening. as the air chilled, the water-vapour from the warmer earth rose from its surface, and was arrested by the cold surface of the leaves. so cold was that surface that it froze the water-vapour when rising from the earth, and formed hoar-frost in very large quantities. when this happens later on in the season, one may be almost sure of having rain in the forenoon. as hoar-frost is just frozen dew, i can even more surely convince you of the formation of hoar-frost as rising from the ground by observations made by me at my manse in strathmore, in june . i mention this particularly because then was the most favourable testing-time that has _ever_ occurred during meteorological observations. june th was the warmest june day (with one exception) for twenty years. the thermometer reached ° fahr. in the shade. next day was the coldest june day (with one exception) for twenty years, when the thermometer was as low as ° in the shade. but during the night my thermometer on the grass registered °--the freezing point. on the evening of the sultry day i examined the soil at o'clock. it was damp, and the grass round it was filmy moist. the leaves of the trees were crackling dry, and all above was void of moisture. the air became gradually chilly; and as gradually the moisture rose in height on the shrubs and lower branches of small trees. the moon shone bright, and the stars showed their clear, chilly eyes. the soil soon became quite wet, the low grass was dripping with moisture, and the longer grass was becoming dewed. this gave the best natural evidence of the rising of the dew that i ever witnessed. but everything was favourable for the observation--the cold air incumbent on the rising, warm, moist vapour from the soil fixing the dew-point, when the projecting blades seized the moisture greedily and formed dew. had the temperature been a little below the freezing-point, hoar-frost would have been beautifully formed. chapter v fog to many nothing is more troublesome than a dense fog in a large town. it paralyses traffic, it is dangerous to pedestrians, it encourages theft, it chokes the asthmatic, and chills the weak-lunged. in the country it is disagreeable enough; but never so intensely raw and dense as in the city. on the sea, too, the fog is disagreeable and fraught with danger. the fog-horn is heard, in its deep, sombre note, from the lighthouse tower, when the strong artificial light is almost useless. but a peculiar sense of stagnation possesses the dweller of the large town, when enveloped in a dense fog. sometimes during the day, through a thinner portion, the sun will be dimly seen in copper hue, like the moon under an eclipse. the smoke-impregnated mass assumes a peculiar "pea-soup" colour. now, what is this fog? how is it formed? it has been ascertained that fogs are dependent upon _dust_ for their formation. without dust there could be no fogs, there would be only dew on the grass and road. instead of the dust-impregnated air that irritates the housekeeper, there would be the constant dripping of moisture on the walls, which would annoy her more. ocular demonstration can testify to this. if two closed glass receivers be placed beside each other, the one containing ordinary air, and the other filtered air (_i.e._ air deprived of its dust by being driven through cotton wool), and if jets of steam be successively introduced into these, a strange effect is noticed. in the vessel containing common air the steam will be seen rising in a dense cloud; then a beautiful white foggy cloud will be formed, so dense that it cannot be seen through. but in the vessel containing the filtered air, the steam is not seen at all; there is not the slightest appearance of cloudiness. in the one case, where there was the ordinary atmospheric dust, fog at once appeared; in the other case, where there was no dust in suspension, the air remained clear and destitute of fog. invisible dust, then, is necessary in the air for the formation of fogs. the reason of this is that a free-surface must exist for the condensation of the vapour-particles. the fine particles of dust in the air act as free-surfaces, on which the fog is formed. where there is abundance of dust in the air and little water-vapour present, there is an over-proportion of dust-particles; and the fog-particles are, in consequence, closely packed, but light in form and small in size, and take the lighter appearance of fog. accordingly, if the dust is increased in the air, there is a proportionate increase of fog. every fog-particle, then, has embosomed in it an invisible dust-particle. but whence comes the dust? from many sources. it is organic and inorganic. so very fine is the inorganic dust in the atmosphere that, if the two-thousandth part of a grain of fine iron be heated, and the dust be driven off and carried into a glass receiver of filtered air, the introduction of a jet of steam into that receiver would at once occasion an appreciable cloudiness. this is why fogs are so prevalent in large towns. next the minute brine-particles, driven into the air as fog forms above the ocean surface, are the burnt sulphur-particles emanating from the chimneys in towns. the brilliant flame, as well as the smoky flame, is a fog-producer. if gas is burnt in filtered air, intense fog is produced when water-vapour is introduced. products of combustion from a clear fire and from a smoky one produce equal fogging. the fogs that densely fill our large towns are generally less bearable than those that veil the hills and overhang the rivers. it is the sulphur, however, from the consumed coals, which is the active producer of the fogs of a large town. the burnt sulphur condenses in the air to very fine particles, and the quantity of burnt sulphur is enormous. no less than seven and a half millions of tons of coals are consumed in london. now, the average amount of sulphur in english coal is one and a quarter per cent. that would give no less than , tons of sulphur burned every year in london fires. now, if we reckon that on an average twice the quantity of coals is consumed there on a winter day that is consumed on a summer day, no less than tons of the products of combustion (in extremely fine particles) are driven into the superincumbent air of london every winter day. this is an enormous quantity, quite sufficient to account for the density of the fogs in that city. chapter vi the numbering of the dust if the shutters be all but closed in a room, when the sun is shining in, myriads of floating particles can be seen glistening in the stream of light. their number seems inexhaustible. according to milton, the follies of life are-- "thick and numberless, as the gay motes that people the sunbeams." can these, then, be counted? yes, dr. aitken has numbered the dust of the air. i shall never forget my rapt astonishment the day i first numbered the dust in the lecture-room of the royal society of edinburgh, with his instrument and under his direction. this wonderfully ingenious instrument was devised on this principle, that every fog-particle has entombed in it an invisible dust-particle. a definite small quantity of common air is diluted with a fixed large quantity of dustless air (_i.e._ air that has been filtered through cotton-wool). the mixture is allowed to be saturated with water-vapour. then the few particles of dust seize the moisture, become visible in fine drops, fall on a divided plate, and are there counted by means of a magnifying glass. that is the secret! i shall now give you a general idea of the apparatus. into a common glass flask of carafe shape, and flat-bottomed, of cubic inches capacity, are passed two small tubes, at the end of one of which is attached a small square silver table, inch in length. a little water having been inserted, the flask is inverted, and the table is placed exactly inch from the inverted bottom, so that the contents of air right above the table are cubic inch. this observing table is divided into equal squares, and is highly polished, with the burnishing all in one direction, so that during the observations it appears dark, when the fine mist-particles glisten opal-like with the reflected light in order that they may be more easily counted. the tube to which the silver table is attached is connected with two stop-cocks, one of which can admit a small measured portion of the air to be examined. the other tube in the flask is connected with an air-pump of cubic inches capacity. over the flask is placed a covering, coloured black in the inside. in the top of this cover is inserted a powerful magnifying glass, through which the particles on the silver table can be easily counted. a little to the side of this magnifier is an opening in the cover, through which light is concentrated on the table. to perform the experiment, the air in the flask is exhausted by the air-pump. the flask is then filled with filtered air. one-tenth of a cubic inch of the air to be examined is then introduced into the flask, and mixed with the cubic inches of dustless air. after one stroke of the air-pump, this mixed air is made to occupy an additional space of cubic inches; and this rarefying of the air so chills it that condensation of the water-vapour takes place on the dust-particles. the observer, looking through the magnifying-glass upon the silver table, sees the mist-particles fall like an opal shower on the table. he counts the number on a single square in two or three places, striking an average in his mind. suppose the average number upon a single square were five, then on the whole table there would be ; and these particles of dust are those which floated invisibly in the cubic inch of mixed air right above the table. but, as there are cubic inches of mixed air in the flask and barrel, the number of dust-particles in the whole is , . that is, there are , dust-particles in the same quantity of common air (one-tenth of a cubic inch) which was introduced for examination. in other words, a cubic inch of the air contained , dust-particles--nearly a quarter of a million. the day i used the instrument we counted , , of dust-particles in a cubic inch of the air outside of the room, due to the quantity of smoke from the passing trains. dr. aitken has counted in cubic inch of air immediately above a bunsen flame the fabulous number of , , of dust-particles. a small instrument has been constructed which can bring about results sufficiently accurate for ordinary purposes. it is so constructed that, when the different parts are unscrewed, they fit into a case - / inches by - / by - / deep--about the size of an ordinary cigar-case. after knowing this, we are apt to wonder why our lungs do not get clogged up with the enormous number of dust-particles. in ordinary breathing, cubic inches of air pass in and out at every breath, and adults breathe about fifteen times every minute. but the warm lung-surface repels the colder dust-particles, and the continuous evaporation of moisture from the surface of the air-tubes prevents the dust from alighting or clinging to the surface at all. chapter vii dust and atmospheric phenomena dr. aitken has devoted a vast amount of attention to the enumeration of dust-particles in the air, on the continent as well as in scotland, to determine the effects of their variation in number. on his first visit to hyères, in , he counted with the instrument , dust-particles in a cubic inch of the air: whereas in the following year he counted , . he observed, however, that where there was least dust, the air was very clear; whereas with the maximum of dust, there was a very thick haze. at mentone, the corresponding number was , , when the wind was blowing from the mountains; but increased to , , when the wind was blowing from the populous town. on his first visit to the rigi kulm, in switzerland, the air was remarkably clear and brilliant, and the corresponding number never exceeded , ; but, on his second visit, he counted no less than , . this was accounted for by a thick haze, which rendered the lower alps scarcely visible. the upper limit of the haze was well defined; and though the sky was cloudless, the sun looked like a harvest moon, and required no eagle's eye to keep fixed on it. next day there was a violent thunder-storm. at p.m. the storm commenced, and , dust-particles to the cubic inch of air were registered; but in the middle of the storm he counted only , . there was a heavy fall of hail at this time, and he accounts for the diminution of dust-particles by the down-rush of purer upper air, which displaced the contaminated lower air. at the lake of lucerne there was an exceptional diminution of the number in the course of an hour, viz. from , to , in a cubic inch. on looking about, he found that the direction of the wind had changed, bringing down the purer upper air to the place of observation. the bending downwards of the trees by the strong wind showed that it was coming from the upper air. returning to scotland, he continued his observations at ben nevis and at kingairloch, opposite appin, mr. rankin using the instrument at the top of the mountain. these observations showed in general that on the mountain southerly, south-easterly, and easterly winds were more impregnated with dust-particles, sometimes containing , per cubic inch. northerly winds brought pure air. the observations at sea-level showed a certain parallelism to those on the summit of the mountain. with a north-westerly wind the dust-particles reached the low number of per cubic inch, the lowest recorded at any low-level station. the general deductions which he made from his numerous observations during these two years are that ( ) air coming from inhabited districts is always impure; ( ) dust is carried by the wind to enormous distances; ( ) dust rises to the tops of mountains during the day; ( ) with much dust there is much haze; ( ) high humidity causes great thickness of the atmosphere, if accompanied by a great amount of dust, whereas there is no evidence that humidity alone has any effect in producing thickness; ( ) and there is generally a high amount of dust with high temperature, and a low amount of dust with low temperature. chapter viii a fog-counter next to the enumeration of the dust-particles in the atmosphere is the marvellous accuracy of counting the number of particles in a fog. the same ingenious inventor has constructed a fog-counter for the purpose; and the number of fog-particles in a cubic inch can be ascertained. this instrument consists of a glass micrometer divided into squares of a known size, and a strong microscope for observing the drops on the stage. the space between the micrometer and the microscope is open, so that the air passes freely over the stage; and the drops that fall on its surface are easily seen. these drops are very small; many of them when spread on the glass are no more than the five-hundredth of an inch in diameter. in observing these drops, the attention requires to be confined to a limited area of the stage, as many of the drops rapidly evaporate, some almost as soon as they touch the glass, whilst the large ones remain a few seconds. in one set of dr. aitken's observations, in february , the fog was so thick that objects beyond a hundred yards were quite invisible. the number of drops falling per second varied greatly from time to time. the greatest number was drops per square inch in one second. the high number never lasted for long, and in the intervals the number fell as low as , or to one-tenth. if we knew the size of these drops, we might be able to calculate the velocity of their fall, and from that obtain the number in a cubic inch. an ingenious addition is put to the instrument in order to ascertain this directly. it is constructed so as to ascertain the number of particles that fall from a known height. under a low-power microscope, and concentric with it, is mounted a tube inches long and - / inch in diameter, with a bottom and a cover, which are fixed to an axis parallel with the axis of the tube, so that, by turning a handle, these can be slid sideways, closing or opening the tube at both ends when required. in the top is a small opening, corresponding to the lens of the microscope, and in the centre of the bottom is placed the observing-stage illumined by a spot-mirror. the handle is turned, and the ends are open to admit the foggy air. the handle is quickly reversed, and the ends are closed, enabling the observer to count on the stage all the fog-particles in the two inches of air over it. the number of dust-particles in the air which become centres of condensation depends on the rate at which the condensation is taking place. the most recent observations show that quick condensation causes a large number of particles to become active, whereas slow condensation causes a small number. after the condensation has ceased, a process of differentiation takes place, the larger particles robbing the smaller ones of their moisture, owing to the vapour-pressure at the surface of the drops of large curvature being less than at the surface of drops of smaller curvature. by this process the particles in a cloud are reduced in number; the remaining ones, becoming larger, fall quicker. the cloud thus becomes thinner for a time. a strong wind, suddenly arising, will cause the cloud-particles to be rapidly formed: these will be very numerous, but very small--so small that they are just visible with great care under a strong magnifying lens used in the instrument. but in slowly formed clouds the particles are larger, and therefore more easily visible to the naked eye. though the particles in a fog are slightly finer, the number is about the same as in a cloud--that is, generally. as clouds vary in density, the number of particles varies. sometimes in a cloud one cannot see farther than yards; whereas in a few minutes it clears up a little, so that we can see yards. of course, the denser the cloud the greater the number of water-particles falling on the calculating-stage of the instrument. very heavy falls of cloud-particles seldom last more than a few seconds, the average being about on the square inch per second, the maximum reaching to . this is about four times the number counted in a fog. yet the particles are so very small that they evaporate instantly when they reach a slight increase of temperature. chapter ix formation of clouds in our ordinary atmosphere there can be no clouds without dust. a dust-particle is the nucleus that at a certain humidity becomes the centre of condensation of the water-vapour so as to form a cloud-particle; and a collection of these forms a cloud. this condensation of vapour round a number of dust-particles in visible form gives rise to a vast variety of cloud-shapes. there are two distinct ways in which the formation of clouds generally takes place. either a layer of air is cooled in a body below the dew-point; or a mass of warm and moist air rises into a mass which is cold and dry. the first forms a cloud, called, from being a layer, _stratus_; the second forms a cloud, called, from its heap appearance, _cumulus_. the first is widely extended and horizontal, averaging feet in height; the second is convex or conical, like the head of a sheaf, increasing upward from a level base, averaging from feet to feet in height. there are endless combinations of these two; but at the height of , feet, where the cloud-particles are frozen, the structure of the cloud is finer, like "mares' tails," receiving the name _cirrus_. when the cirrus and cumulus are combined, in well-defined roundish masses, what is familiarly described as a "mackerel sky" is beautifully presented. the dark mass of cloud, called _nimbus_, is the threatening rain-cloud, about feet in height. at the international meteorological conference at munich, in , twelve varieties of clouds were classified, but those named above are the principal. in a beautiful sunset one can sometimes notice two or three distances of clouds, the sun shedding its gold light on the full front of one set, and only fringing with vivid light the nearer range. although no man has wrought so hard as dr. aitken to establish the principle that clouds are mainly due to the existence of dust-particles which attract moisture on certain conditions, yet even twenty years ago he said that it was probable that sunshine might cause the formation of nuclei and allow cloudy condensation to take place where there was no dust. under certain conditions the sun gives rise to a great increase in the number of nuclei. accordingly, he has carefully tested a few of the ordinary constituents and impurities in our atmosphere to see if sunshine acted on them in such a way as to make them probable formers of cloud-particles. he tested various gases, with more or less success. he found that ordinary air, after being deprived of its dust-particles and exposed to sunshine, does not show any cloudy condensation on expansion; but, when certain gases are in the dustless air, a very different result is obtained. he first used ammonia, putting one drop into six cubic inches of water in a flask, and sunning this for one minute; the result was a considerable quantity of condensation, even with such a weak solution. when the flask was exposed for five minutes, the condensation by the action of the sunshine was made more dense. hydrogen peroxide was tested in the same way, and it was found to be a powerful generator of nuclei. curious is it that sulphurous acid is puzzling to the experimentalist for cloud formation. it gives rise to condensation in the dark; but sunshine very conclusively increases the condensation. chlorine causes condensation to take place without supersaturation; sulphuretted hydrogen (which one always associates with the smell of rotten eggs) gives dense condensation after being exposed to sunshine. though the most of these nuclei, due to the action of sunshine in the gases, remain active for cloudy condensation for a comparatively short space of time--fifteen minutes to half-an-hour--yet the experiments show that it is possible for the cloudy condensation to take place in certain circumstances in the absence of dust. this seems paradoxical in the light of the former beautiful experiments; but, in ordinary circumstances, dust is needed for the formation of clouds. however, supposing there is any part of the upper air free from dust, it is now found possible, when any of these gases experimented on be present, for the sun to convert them into nuclei of condensation, and permit of clouds being formed in dustless air, miles above the surface of the earth. in the lower atmosphere there are always plenty of dust-particles to form cloudy condensation, whether the sun shines or not. these are produced by the waste from the millions of meteors that daily fall into the air. but in the higher atmosphere, clouds can be formed by the action of the sun's rays on certain gases. this is a great boon to us on the earth; for it assures us of clouds being ever existing to defend us from the sun's extra-powerful rays, even when our atmosphere is fairly clear. this is surely of some meteorological importance. chapter x decay of clouds from the earliest ages clouds have attracted the attention of observers. varied are their forms and colours, yet in our atmosphere there is one law in their formation. cloud-particles are formed by the condensation of water-vapour on the dust-particles invisibly floating in the atmosphere, up to thousands--and even millions--in the cubic inch of air. but observers have not directed their attention so much to the decay of clouds--in fact, the subject is quite new. and yet how suggestive is the subject! the process of decay in clouds takes place in various ways. a careful observer may witness the gradual wasting away and dilution into thin air of even great stretches of cloud, when circumstances are favourable. in may my attention was particularly drawn to this at my manse in strathmore. in the middle of that exceptionally sultry month, i was arrested by a remarkable transformation scene. it was the hottest may for seventy-two years, and the driest for twenty-five years. the whole parched earth was thirsting for rain. all the morning my eyes were turned to the clouds in the hope that the much-desired shower should fall. till ten o'clock the sun was not seen, and there was no blue in the sky. nor was there any haze or fog. but suddenly the sun shone through a thinner portion of the enveloping clouds, and, to the north, the sky began to open. as if by some magic spell there was, in a quarter of an hour, more blue to be seen than clouds. at the same time, near the horizon, a haze was forming, gradually becoming denser as time wore on. in an hour the whole clouds were gone, and the glorious orb of day dispelled the moisture to its thin-air form. this was a pointed and rapid illustration of the decay from cloud-form to haze, and then to the pure vapoury sky. it was an instance of the _reverse_ process. as the sun cleared through, the temperature in the cloud-land rose and evaporation took place on the surface of the cloud-particles, until by an untraceable, but still a gradual process through fog, the haze was formed. even then the heat was too great for a definite haze, and the water-vapour returned to the air, leaving the dust-particles in invisible suspension. but clouds decay in another way. this i will illustrate in the next chapter on "it always rains." what strikes a close observer is the difference of structure in clouds which are in the process of formation and those which are in the process of decay. in the former the water-particles are much smaller and far more numerous than in the latter. while the particles in clouds in decay are large enough to be seen with the unaided eye, when they fall on a properly lighted measuring table, they are so small in clouds in rapid formation that the particles cannot be seen without the aid of a strong magnifying glass. observers have assumed that the whole explanation of the fantastic shapes taken by clouds is founded on the process of formation; but dr. aitken has pointed out that ripple-marked clouds, for instance, have been clouds of decay. when what is called a cirro-stratus cloud--mackerel-like against the blue sky--is carefully observed in fine weather, it will be found that it frequently changes the ripple-marked cirrus in the process of decay to vanishing. where the cloud is thin enough to be broken through by the clear air that is drawn in between the eddies, the ripple markings get nearer and nearer the centre, as the cloud decays. and, at last, when nearly dissolved, these markings are extended quite across the cloud. whether, then, we consider the cases of clouds gradually melting away back into their original state of blue water-vapour, or the constant fine raining from clouds and re-formation by evaporation, or the transformation of such clouds as the cirro-stratus into the ripple-marked cirrus, we are forced to the conclusion that in clouds there is not always development, but sometimes degeneration; not always formation, but sometimes decay. chapter xi it always rains all are familiar with the answer given by the native of skye to the irate tourist on that island, who, for the sixth day drenched, asked the question: "does it always rain here?" "na!" answered the workman, without at all understanding the joke; "feiles it snaas" (sometimes it snows). yet, strange to say, the tourist's question has been answered in the affirmative in every place where a cloud is overhead, visible or invisible. whenever a cloud is formed, it begins to rain; and the drops shower down in immense numbers, though most minute in size--"the playful fancies of the mighty sky." no doubt it is only in certain circumstances that these drops are attracted together so as to form large drops, which fall to the earth in genial showers to refresh the thirsty soil, or in a terrible deluge to cause great destruction. but when the temperature and pressure are not suitable for the formation of what we commonly know as the rain, the fine drops fall into the air under the cloud, where they immediately evaporate from their dust free-surfaces, if the air is dry and warm. this is, in other words, the decay of clouds. it is a curious fact that objects in a fog may not be wetted, when the number of water-particles is great. it seems that these water-particles all evaporate so quickly that even one's hand or face is not sensible of being wetted. the particles are minutely small; and they may evaporate even before reaching the warm skin, by reason of the heated air over the skin. there is a peculiarly warm sensation in the centre of a cumulus cloud, especially when it is not dense. a glow of heat seems to radiate from all points. yet the face and hands are quite dry, and exposed objects are not wetted; but it is really _always raining_. that is a curious discovery. it is radiant heat that is the cause of the remarkable result. the rays of the sun, which strike the upper part of the cloud, not only heat that surface but also penetrate the cloud and fall on the surface of bodies within, generating heat there. these heated surfaces again radiate heat into the air attached to them. this warm air receives the fine raindrops in the cloud, and dissolves the moisture from the dust-particles before the moisture can reach the surfaces exposed. that a vast amount of radiant heat rushes through a cloud is clearly shown by exposing a thermometer with black bulb _in vacuo_. on some occasions, a thermometer would indicate from ° to ° above the temperature of the air, thus proving the surface to be quite dry. these observations have been corroborated on mount pilatus, near lucerne-- feet higher and more isolated than the rigi. the summit was quite enveloped in cloud, and, though one might naturally conclude that the air was dense with moisture, yet the wooden seats, walls, and all exposed surfaces were quite dry. strange to say, however, the thermometers hung up got wet rapidly, and the pins driven into the wooden post to support them rapidly became moist. a thermometer lying on a wooden seat stood at °, while one hung up read only °. this difference was caused by radiant heat. it is well known that, when bodies are exposed to radiant heat, they are heated in proportion to their size; the smaller, then, may be moist, when the larger are dry by radiation. the effect of the sun's penetrating heat through the cloud is to heat exposed objects above the temperature of the air; and if the objects are of any size they are considerably heated, and retain their heat more, while at the same time around them is a layer of warm air which is quite sufficient to force the water-vapour to leave the dust-particles in the fine rain. hence seats, walls, posts, &c., are quite dry, though they are in the middle of a cloud. they are large enough to throw off the moisture by the retained heat, or by the large amount of surrounding heat; whereas, small bodies, which are not heated to the same degree and cannot therefore retain their heat so easily, have not heat-power sufficient to withstand the moisture, and they become wetted. hence, by the radiant heat, the large exposed objects are dry in the cloud; whereas small objects are damp, and, in some cases, dripping with wet. the fact is, then, that whenever a cloud overhangs, _rain is falling_, though it may not reach the earth on account of the dryness of the stratum of air below the cloud, and the heat of the air over the earth. so that on a summer day, with the gold-fringed, fleecy clouds sailing overhead, it is really raining; but the drops, being very small, evaporate long before reaching the earth. as ariel sings at the end of "the tempest" of shakespeare, "the rain, it raineth every day." it rains, but much of the melting of the clouds is reproduced by a wonderful circularity--the moisture evaporating, seizing other dust-particles, forming cloud-particles, falling again, and so on _ad infinitum_, during the existing circumstances. chapter xii haze what is haze? the dictionary says, "a fog." well, haze is _not_ a fog. in a fog, the dust-particles in the air have been fully clothed with water-vapour; in a haze, the process of condensation has been arrested. cloudy condensation is changed to haze by the reduction of its humidity. dr. aitken invented a simple apparatus for testing the condensing power of dust, and observing if water-vapour condensed on the deposited dust in unsaturated air. the dust from the air has first to be collected. this is done by placing a glass plate vertically, and in close contact with one of the panes of glass in the window, by means of a little india-rubber solution. the plate being thus rendered colder than the air in the room, the dust is deposited on it. construct a rectangular box, with a square bottom, - / inches a side and / inch deep, and open at the top. cover the top edge of the box with a thickness of india-rubber. place the dusty plate--a square glass mirror, inches a side--on the top of the india-rubber, and hold it down by spring catches, so as to make the box water-tight. the box has been provided with two pipes, one for taking in water and the other for taking away the overflow, with the bulb of a thermometer in the centre. clean the dust carefully off one half of the mirror, so that one half of the glass covering the box is clean and the other half dusty. pour cold water through the pipe into the box, so as to lower the temperature of the mirror, and carefully observe when condensation begins on the clean part and on the dusty part, taking a note of the difference of temperature. the condensation of the water-vapour will appear on the dust-particles before coming down to the natural dew-point temperature of the clean glass. and the difference between the two temperatures indicates the temperature above the dew-point at which the dust has condensed the water-vapour. magnesia dust has small affinity for water-vapour; accordingly, it condenses at almost exactly the same temperature as the glass. but gunpowder has great condensing power. all have noticed that the smoke from exploded gunpowder is far more dense in damp than in dry weather. in the experiment it will be found that the dust from gunpowder smoke begins to show signs of condensing the vapour at a temperature of ° fahr. above the dew-point. in the case of sodium dust, the vapour is condensed from the air at a temperature of ° above the dew-point. dust collected in a smoking-room shows a decidedly greater condensing power than that from the outer air. we can now understand why the glass in picture frames and other places sometimes appears damp when the air is not saturated. when in winter the windows are not often cleaned, a damp deposit may be frequently seen on the glass. any one can try the experiment. clean one half of a dusty pane of glass in cold weather, and the clean part will remain undewed and clear, while the dusty part is damp to the eye and greasy to the touch. these observations indicate that moisture is deposited on the dust-particles from air, which is not saturated, and that the condensation takes place while the air is comparatively dry, _before_ the temperature is lowered to the dew-point. there is, then, no definite demarcation between what seems to us clear air and thick haze. the clearest air has some haze, and, as the humidity increases, the thickness of the air increases. in all haze the temperature is above the dew-point. the dust-particles have only condensed a very small amount of the moisture so as to form haze, before the fuller condensation takes place at the dew-point. at the italian lakes, on many occasions when the air is damp and still, every stage of condensation may be observed in close proximity, not separated by a hard and fast line, but when no one could determine where the clear air ended and the cloud began. sometimes in the sky overhead a gradual change can be observed from perfect clearness to thick air, and then the cloud. a thick haze may be occasioned by an increased number of dust-particles with little moisture, or of a diminished number of dust-particles with much moisture, above the point of saturation. the haze is cleared by this temperature rising, so as to allow the moisture to evaporate from the dust-particles. whenever the air is dry and hazy, much dust is found in it; as the dust decreases the haze also decreases. for example, dr. aitken, at kingairloch, in one of the clearest districts of argyleshire, on a clear july afternoon, counted dust-particles in a cubic inch of the air; whereas, two days before, in thick haze, he counted no fewer than , in the cubic inch. at dumfries the number counted on a very hazy day in october increased twenty-fold over the number counted the day before, when it was clear. all know that thick haze is usual in very sultry weather. the wavy, will-o'-the-wisp ripples near the horizon indicate its presence very plainly. during the intense heat there is generally much dust in the atmosphere; this dust, by the high temperature, attracts moisture from the apparently dry air, though above the saturation point. in all circumstances, then, the haze can be accounted for by the condensing power of the dust-particles in the atmosphere, at a higher temperature than that required for the formation of fogs, or mists, or rain. chapter xiii hazing effects of atmospheric dust the transparency of the atmosphere is very much destroyed by the impurities communicated to it while passing over the inhabited areas of the country. dr. aitken devoted eighteen months to compare the amount of dusty impurities in different masses of air, or of different airs brought in by winds from different directions. he took falkirk for his centre of observations. this town lies a little to the north of a line drawn between edinburgh and glasgow, and is nearly midway between them. if we draw a line due west from it, and another due north, we find that, in the north-west quadrant so enclosed, the population of that part of scotland is extremely thin, the country over that area being chiefly mountainous. in all other directions, the conditions are quite different. in the north-east quadrant are the fairly well-populated areas of aberdeenshire, forfarshire, and the thickly populated county of fife. in the south-east quadrant are situated edinburgh and the well-populated districts of the south-east of scotland. and in the south-west quadrant are glasgow and the large manufacturing towns which surround it. the winds from three of these quadrants bring air polluted in its passage over populated areas, whereas the winds from the north-west come comparatively pure. the general plan of estimating the amount of haze is to note the most distant hill that can be seen through the haze. the distance in miles of the farthest away hill visible is then called "the limit of visibility" of the air at the time. for the observations made at falkirk, only three hills are available, one about four miles distant, the ochils about fifteen miles distant, and ben ledi about twenty-five miles distant--all in the north-west quadrant. when the air is thick, only the near hill can be seen; then the ochils become visible as the air clears; and at last ben ledi is seen, when the haze becomes still less. after ben ledi is visible, it then becomes necessary to estimate the amount of haze on it, in order to get the limit of visibility of the air at the time. thus, if ben ledi be half-hazed, then the limit of visibility will be fifty miles. in this way all the estimates of haze have been reduced to one scale for comparison. as the result of all the observations it was found that, as the dryness of the air increases, the limit of visibility also increases. a very marked difference in the transparency of the air was found with winds from the different directions. in the north-west quadrant the winds made the air very clear, whereas winds from all other directions made the air very much hazed. the winds in the other three areas are nearly ten times more hazed than those from the north-west quadrant. that is very striking. the conclusion come to is that the air from densely inhabited districts is so polluted that it is fully nine times more hazed than the air that comes from the thinly inhabited districts; in other words, the atmosphere at falkirk is about ten times thicker when the wind is east or south than it would be if there were no fires and no inhabitants. it is interesting to notice that the limit varies considerably for the same wind at the same humidity. this is what might have been expected, because from the observations made by the dust-counter the number of particles varied greatly in winds from the same directions, but at different times. this depends upon the rise and fall of the wind, changes in the state of trade, season of the year, and other causes. during a strike, the dearth of coal will make a considerable diminution in the number of dust-particles in the air of large towns. with a north wind, the extreme limits of visibility are to miles; and with a north-west wind, from to miles. an east wind has as limits to miles, and a south-east wind to miles. one interesting fact to be noticed, after wading through these tables, is this--that, as a general result, the transparency of the air increases about · times for any increase in dryness from ° to ° of wet-bulb depression. that is, the clearness of the air is inversely proportional to the relative humidity; or, put another way, if the air is four times drier it is about four times clearer. chapter xiv thunder clears the air the phrase "thunder clears the air" is familiar to all. it contains a very vital truth, yet even scientific men did not know its full meaning until just the other day. it came by experience to people who had been for ages observing the weather; and it is one of the most pointed of the "weather-lore" expressions. folks got to know, by a sort of rule-of-thumb, truths which scientifically they were unable to learn. and this is one. perhaps, therefore, we should respect a little more what is called "folk-lore," or ordinary people's sayings. experience has taught men many wonderful things. in olden times they were keener natural observers. they had few books, but they had plenty of time. they studied the habits of animals and moods of nature, and they came wonderfully near to reaching the full truth, though they could not give a reason for it. the awe-inspiring in nature has especially riveted the attention of man. and no appearance in nature joins more powerfully the elements of grandeur and awe than a heavy thunder-storm. when, suddenly, from the breast of a dark thunder-cloud a brilliant flash of light darts zigzag to the earth, followed by a loud crackling noise which softens in the distance into weaker volumes of sound, terror seizes the birds of the air and the cattle in the field. the man who is born to rule the storm rejoices in the powerful display; but kings have trembled at the sight. byron thus pictures a storm in the alps:-- "far along from peak to peak, the rattling crags among leaps the live thunder! not from one lone cloud, but every mountain now hath found a tongue, and jura answers, through her misty shroud, back to the joyous alps, who call to her aloud!" franklin found that lightning is just a kind of electricity. no one can tell how it is produced; yet a flash has been photographed. when the flash is from one cloud to another there is sheet-lightning, which is beautiful but not dangerous; but, when the electricity passes from a cloud to the earth in a forked form, it is very dangerous indeed. the flash is instantaneous, but the sound of the thunder takes some time to travel. roughly speaking, the sound takes five seconds or six beats of the pulse to the mile. all are now taught at school that it is the oxygen in the air which is necessary to keep us in life. if mice are put into a glass jar of pure oxygen gas, they will at once dance with exhilarating joy. it occurred to sir benjamin richardson, some time ago, that it would be interesting to continue some experiments with animals and oxygen. he put a number of mice into a jar of pure oxygen for a time; they breathed in the gas, and breathed out water-vapour and carbonic acid. after the mice had continued this for some time, he removed them by an arrangement. by chemical means he removed the water-vapour and carbonic acid from the mixed air in the vessel. when a blown-out taper was inserted, it at once burst into flame, showing that the remaining gas was oxygen. again, the mice were put into this vessel to breathe away. but, strange to say, the animals soon became drowsy; the smartness of the oxygen was gone. at last they died; there was nothing in the gas to keep them in life; yet, by the ordinary chemical tests, it was still oxygen. it had repeatedly passed through the lungs of the mice, and during this passage there had been an action in the air-cells which absorbed the life-giving element of the gas. it is oxygen, so far as chemistry is concerned, but it has no life-giving power. it has been _devitalised_. but the startling discovery still remains. sir benjamin had previously fitted up the vessel with two short wires, opposite each other in the sides--part outside and part inside. two wires are fastened to the outside knobs. these wires are attached to an electric machine, and a flash of electricity is made to pass between the inner points of the vessel. the wires are again removed; nothing strange is seen in the vessel. but, when living mice are put into the vessel, they dance as joyfully as if pure oxygen were in it. the oxygen in which the first mice died has now been quite refreshed by the electricity. the bad air has been cleared and made life-supporting by the electric discharge. it has been again vitalised. now, to apply this: before a thunder-storm, everything has been so still for days that the oxygen in the air has been to some extent robbed of its life-sustaining power. the air feels "close," a feeling of drowsiness comes over all. but, after the air has been pierced by several flashes of lightning, the life-force in the air is restored. your spirits revive; you feel restored; your breathing is far freer; your drowsiness is gone. then there is a burst of heavenly music from the exhilarated birds. thus a thunder-storm "clears the air." after the passage of lightning through the air ozone is produced--the gas that is produced after a flash of electricity. it is a kind of oxygen, with fine exciting effects on the body. if, then, the life-sustaining power of oxygen depends on a trace of ozone, and this is being made by lightning's work, how pleased should we be at the occasional thunder-storm! chapter xv disease-germs in the air the gay motes that dance in the sunbeams are not all harmless. all are annoying to the tidy housekeeper; but some are dangerous. there are living particles that float in the air as the messengers of disease and death. some, falling on fresh wounds, find there a suitable feeding-place; and, if not destroyed, generate the deadly influence. others are drawn in with the breath; and, unless the lungs can withstand them, they seize hold and spread some sickness or disease. from stagnant pools, common sewers, and filthy rooms, disease-germs are constantly contaminating the air. yet these can be counted. the simplest method is that of professor frankland. it depends on this principle: a certain quantity of air is drawn through some cotton-wool; this wool seizes the organisms as the air passes through; these organisms are afterwards allowed to feed upon a suitable nutritive medium until they reach maturity; they are then counted easily. about an inch from each end of a glass tube ( inches long and inch bore), the glass is pressed in during the process of blowing. some cotton-wool is squeezed in to form a plug at the farther constricted part of the glass. the important plug is now inserted at the same open end, but is not allowed to go beyond the constricted part at its end. a piece of long lead tubing is attached to the former end by an india-rubber tube. the other end of the lead tubing is connected with an exhausting syringe. sixty strokes of the cubic inches syringe will draw cubic inches of the air to be examined through the plugs, the first retaining the organisms. the impregnated plug is then put into a flask containing in solution some gelatine-peptone. the flask is made to revolve horizontally until an almost perfectly even film of gelatine and the organisms from the broken-up plug cover its inner surface. the flask is allowed to remain for an hour in a cool place, and is then placed under a bell-jar, at a temperature of ° fahr. there it remains, to allow the germs to incubate, for four or five days. the surface of the flask having been previously divided into equal parts by ink lines, the counting is now commenced. if the average be taken for each segment, the number of the whole is easily ascertained. a simple arithmetical calculation then determines the number of organisms in a cubic foot, since the number is known for the cubic inches. that is the process for determining the number of living organisms in a fixed quantity of air. no less than thirty colonies of organisms were counted in a cubic foot of air taken from the golden gallery of st. paul's cathedral, london, and from the air of the churchyard. an ordinary man would breathe there thirty-six micro-organisms every minute. electricity has a powerful effect in destroying these organisms. ozone is generated in the air by lightning, and it is detrimental to them. in fine ozoned highland air scarcely a disease-germ can be detected. open sea air contains about one germ in two cubic feet. it has been found that in paris the average in summer is about per cubic foot of air, but in bedrooms the number is double. during the twenty-four hours of the day the number of germs is highest about a.m., and lowest about mid-day. raindrops carry the germs to the ground. hence the advantage of a thunder plout in a sanitary way. a cubic foot of rain has been found to contain organic dust-germs, besides , , , of inorganic dust-particles. in a dirty town the rain will bring down in a year, upon a square foot of surface, no less than , , of bacteria, many of them being disease-bearing and death-bearing. no wonder, then, that scientific men are using every endeavour to protect the human frame, as well as the frame of the lower animals, from the baneful inroads of these floating nuclei of disease and death. chapter xvi a change of air for weakness of body and fatigue of mind a very common and essentially serviceable recommendation is "a change of air." of course, the change of scene from coast to country, or from town to hillside, may help much the depressed in body or mind; and this is very commendable. but, strange to say, there is a healing virtue in breathing different air. at first one is apt to think that air is the same all over, as he thinks water is--especially outside smoky towns; but both have varied qualities in different parts. you have only to be assured that in a cubic inch of bedroom air in the denser parts of a large town there are about , , of dust-particles, and in the open air of a heathery mountain-side there are only some hundreds, to see that there is something after all on the face of it in the "old wives' saw." not that the dust-particles are all injurious; for most of them are inorganic, and many of the organic particles are quite wholesome; yet there is a change wrought, often very marked, in going from one place to another for different air. even in the country, especially in summer-time, one distinctly notices the great difference in the air of lowland and highland localities. the ten miles change from strathmore to glenisla shows a marked difference in the air. below, it is close, weakening, enervating; above, it is exhilarating, invigorating, and strong. so people must have a change--at least those who can afford it--for health must be seen to first of all, if one has means to do so. oh! the blessing of good health! how many who enjoy it never think of the misery of its loss! in fact, health is the soul that animates all enjoyments of life; for without it those would soon be tasteless. a man starves at the best-spread table, and is poor in the midst of the greatest treasures without health. in these days half of our diseases come from the neglect of the body in the overwork of the brain. the wear and tear of labour and intellect go on without pause or self-pity. men may live as long as their forefathers, but they suffer more from a thousand artificial anxieties and cares. the men of old fatigued only the muscles, we exhaust the finer strength of the nerves. even more so now, then, do we require a change of air to soothe our overwrought nervous system. and when that magic power, concealed from mortal view, works such wonders on the health, surely it is one's duty to save up and have it, when it is within one's means. for is not health the greatest of all possessions? what a rich colour clothes the countenance of the young after a month's outing in the hill country! how fine and pure has the blood become! all stagnant humours, accumulated in winter town life, have been dispelled by the ozone-brightening charm. the weary looking office or shop man is now transfigured into a sprightly youth once more, ready with strongly recuperated power for another winter's labours. the pale wife, who has been stifled for months in close-aired rooms, has now a healthy flush on her becoming countenance that speaks of gladly restored health. and all this has been brought about by a "change of air"! for a thorough change to a town man, he should make for the highlands. there he is never tired of walking, for the air which he breathes is full of ozone. this revivifying element in the air is produced by the lightning-bursts from hill to hill. there is in the highlands a continual rush of electricity, whether seen or not. hence the air is very pure, free from organic germs, intensely exhilarating and buoyant. sportsmen are livingly aware of the recuperative power of the highland air. perhaps these city men do not benefit so much by the easy walking exercise on the grouse moors as in breathing the splendidly delight-inspiring air. what a change one feels there in a very few hours! "a change of air" is an old wives' adage. but much of the weather-lore of our forefathers was based on real scientific principles only now coming to light. nature is ever true, but it requires patience to unravel her secrets. we therefore advocate an occasional "change of air" to improve the health-- "the chiefest good, bestow'd by heaven, but seldom understood." chapter xvii the old moon in the new moon's arms after the sun's broad beams have tired the sight, the moon with more sober light charms us to descry her beauty, as she shines sublimely in her virgin modesty. there is always a most fascinating freshness in the first sight of the new moon. the superstition of centuries adds to this charm. why boys and girls like to turn over a coin in their pocket at this sight one cannot tell: yet it is done. no young lady likes to look at the new moon through a pane of glass. and farmers are always confident of a change of weather with a new moon: at least in bad weather they earnestly hope for it. but, banishing all superstition, we welcome the pale silver sickle in the heavens, once more appearing from the bosom of the azure. and no language can equal these beautiful words of the youthful shelley:-- "like the young moon, when on the sunlit limits of the night her white shell trembles amid crimson air, and while the sleeping tempest gathers might, doth, as the herald of his coming, bear the ghost of its dead mother, whose dim form bends in dark ether from her infant's chair." that is a more charming way of putting the ordinary expression, "the old moon in the new moon's arms." we are regularly accustomed to the moonshine, but only occasionally is the _earthshine_ on the moon so regulated that the shadowed part is visible. this is not seen at the appearance of every new moon. it depends upon the positions of the sun and moon, the state of the atmosphere, and the absence of heavy clouds. i never in my life saw the phenomenon so marvellously beautiful as on may th, , at my manse in strathmore. i took particular note of it, as some exceedingly curious things were connected with it. at nine o'clock in the evening, the new moon issued from some clouds in the western heavens, the sun having set, about an hour before. the crescent was thin and silvery, and the outline of the shadowed part was just visible. the sky near the horizon was clear and greenish-hued. as the night advanced the moon descended, and at ten o'clock she was approaching a purple stratum of clouds that stretched over the hills, while the position of the sun was only known a little to the east, by the back-thrown light upon the dim sky. through the moisture-laden air the sun's rays, reflected by the moon, threw a golden stream from the crescent moon, for the silvery shell became more golden-hued. the horns of the moon now seemed to project, and the shadowed part became more distinct, though the circle appeared smaller. by means of a field-glass i noticed that this was extra lighted, with points here and there quite golden-tinged. the darker spots showed the deep caverns; the brighter points brought into relief the mountain peaks. why was the surface brighter than usual? i cannot go into detail about the phases of the moon; but, in a word, i may say that, while the sun can illuminate the side of the moon turned towards it, it is unable to throw any light on the shadow, seeing that there is no atmosphere around the moon to refract the light. if we, in imagination, looked from the moon upon the earth, we should see the same phases as are now noticed in the moon; and when it is just before new moon on the earth, the earth will appear fully illuminated from the moon. we would also observe (from the moon) that the brightness of the illuminated part of the earth would vary from time to time, according to the changes in the earth's atmosphere. more light would be reflected to the moon from the clouds in our atmosphere than from the bare earth or cloudless sea, since clouds reflect more light than either land or sea. accordingly, we arrive at this curious fact--that the extra brightness of the _dark_ body of the moon is mainly determined by the amount of _cloud in our atmosphere_. accordingly, i concluded that there must be clouds to the west, though i could not see them, which reflected rays of light and faintly illuminated the shadowed part of the moon. it had become much colder, and i concluded that during the night the cloud-particles, if driven near by the wind, would condense into rain. and, assuredly, next morning i was gratified to find that rain had fallen in large quantities, substantiating the theory. there is much pleasure in verifying such an interesting problem. the dark body of the moon being more than usually visible is one of our well-known and oldest indications of coming bad weather. and at once came to my memory the lines of sir patrick spens, as he foreboded rain for his crossing the north sea:-- "i saw the new moon late yestreen wi' the auld moon in her arm; and if we gang to sea, master, i fear we'll come to harm." this lunar indication, then, has a sound physical basis, showing that near the observer there are vast areas of clouds, which are reflecting light upon the moon at the time, before they condense into rain by the chilling of the air. according to the old greek poet, aratus: "if the new moon is ruddy, and you can trace the shadow of the complete circle, a storm is approaching." chapter xviii an autumn afterglow a brilliant afterglow is welcomed for its surpassing beauty and a precursor of fine fixed weather. a glorious sunset has always had a charm for the lover of nature's beauties. the zenith spreads its canopy of sapphire, and not a breath creeps through the rosy air. a magnificent array of clouds of numberless shapes come smartly into view. some, far off, are voyaging their sun-bright paths in silvery folds; others float in golden groups. some masses are embroidered with burning crimson; others are like "islands all lovely in an emerald sea." over the glowing sky are splendid colourings. the flood of rosy light looks as if a great conflagration were below the horizon. i remember witnessing an especially brilliant sunset last autumn on the high-road between kirriemuir and blairgowrie. the setting sun shone upon the back of certain long trailing clouds which were much nearer me than a range behind. the fringes of the front range were brilliantly golden, while the face of those behind was sparklingly bright. then the sun disappeared over the western hills, and his place was full of spokes of living light. looking eastward, i observed on the horizon the base of the northern line of a beautiful rainbow--"the shepherd's delight" for fine weather. soon in the west the light faded; but there came out of the east a lovely flush, and the general sky was presently flamboyant with afterglow. the front set of clouds was darker except on the edges, the red being on the clouds behind; and the horizon in the east was particularly rich with dark red hues. gradually the eastern glow rose and reddened all the clouds, but the front clouds were still grey. the effect was very fine in contrast. the fleecy clouds overhead became transparently light red, as they stretched over to reach the silver-streaked west. the new moon was just appearing upright against a slightly less bright opening in the sky, betokening the firm hardness of autumn. soon the colouring melted away, and the peaceful reign of the later twilight possessed the land. now why that brilliancy of the east, when the west was colourless? most of all you note the immense variety and wealth of reds. these are due to dust in the atmosphere. we are the more convinced of this by the very remarkable and beautiful sunsets which occurred after the tremendous eruption at krakatoa, in the straits of sunda, thirty years ago. there was then ejected an enormous quantity of fine dust, which spread over the whole world's atmosphere. so long as that vast amount of dust remained in the air did the sunsets and afterglows display an exceptional wealth of colouring. all observers were struck with the vividly brilliant red colours in all shades and tints. the minute particles of dust in the atmosphere arrest the sun's rays and scatter them in all directions; they are so small, however, that they cannot reflect and scatter all; their power is limited to the scattering of the rays at the blue end of the spectrum, while the red rays pass on unarrested. the display of the colours of the blue end are found in numberless shades, from the full deep blue in the zenith to the greenish-blue near the horizon. if there were no fine dust-particles in the upper strata, the sunset effect would be whiter; if there were no large dust-particles, there would be no colouring at all. if there were no dust-particles in the air at all, the light would simply pass through into space without revealing itself, and the moment the sun disappeared there would be total darkness. the very existence of our twilight depends on the dust in the air; and its length depends on the amount and extension upwards of the dust-particles. but how have the particles been increased in size in the east? because, as the sun was sinking, but before its rays failed to illumine the heavens, the temperature of the air began to fall. this cooling made the dust-particles seize the water-vapour to form haze-particles of a larger size. the particles in the east first lose the sun's heat, and first become cool; and the rays of light are then best sifted, producing a more distinct and darker red. as the sun dipped lower, the particles overhead became a turn larger, and thereby better reflected the red rays. accordingly, the roseate bands in the east spread over to the zenith, and passed over to the west, producing in a few minutes a universal transformation glow. to produce the full effect often witnessed, there must be, besides the ordinary dust-particles, small crystals floating in the air, which increase the reflection from their surfaces and enhance the glow effects. in autumn, after sunset, the water-covered dust-particles become frozen and the red light streams with rare brilliancy, causing all reddish and coloured objects to glow with a rare brightness. then the air glows with a strange light as of the northern dawn. from all this it is clear that, though the colouring of sunset is produced by the direct rays of the sun, the afterglow is produced by reflection, or, rather, radiation from the illuminated particles near the horizon. the effect in autumn is a stream of red light, of varied tones, and rare brilliancy in all quarters, unseen during the warmer summer. we have to witness the sunsets at ballachulish to be assured that waller paton really imitated nature in the characteristic bronze tints of his richly painted landscapes. chapter xix a winter foreglow little attention has been paid to foreglows compared with afterglows, either with regard to their natural beauty or their weather forecasting. but either the ordinary red-cloud surroundings at sunrise, or the western foreglow at rarer intervals, betokens to the weather-prophet wet and gloomy weather. the farmer and the sailor do not like the sight, they depend so much on favourable weather conditions. of course, sunrise to the æsthetic observer has always its charms. the powerful king of day rejoices "as a bridegroom coming out of his chamber" as he steps upon the earth over the dewy mountain tops, bathing all in light, and spreading gladness and deep joy before him. the lessening cloud, the kindling azure, and the mountain's brow illumined with golden streaks, mark his approach; he is encompassed with bright beams, as he throws his unutterable love upon the clouds, "the beauteous robes of heaven." aslant the dew-bright earth and coloured air he looks in boundless majesty abroad, touching the green leaves all a-tremble with gold light. but glorious, and educating, and inspiring as is the sunrise in itself in many cases, there is occasionally something very remarkable that is connected with it. rare is it, but how charming when witnessed, though till very recently it was all but unexplained. this is the _foreglow_. it is in no respect so splendid as the afterglow succeeding sunset; but, because of its comparative rarity, its beauty is enhanced. i remember a foreglow most vividly which was seen at my manse, in strathmore, in january . my bedroom window looked due west; i slept with the blind up. on that morning i was struck, just after the darkness was fading away, with a slight colouring all along the western horizon. the skeleton branches of the trees stood out strongly against it. the colouring gradually increased, and the roseate hue stretched higher. the old well-known faces that i used to conjure up out of the thin blended boughs became more life-like, as the cheeks flushed. there was rare warmth on a winter morning to cheer a half-despairing soul, tired out with the long hours of oil reading, and pierced to the heart by the never-ceasing rimes; yet i could not understand it. i went to the room opposite to watch the sunrise, for i had observed in the diary that the appearance of the sun would not be for a few minutes. there were streaks of light in the east above the horizon, but no colour was visible. that hectic flush--slight, yet well marked--which was deepening in the western heavens, had no counterpart in the east, except the colourless light which marked the wintry sun's near approach. as soon as the sun's rays shot up into the eastern clouds, and his orb appeared above the horizon, the western sky paled, the colour left it, as if ashamed of its assumed glory. a foreglow like that i have very rarely seen, and its existence was a puzzle to me till i studied dr. aitken's explanation of the afterglows after sunset. i had never come across any description of a foreglow; and, of course, across no explanation of the curious phenomenon. the western heavens were coloured with fairly bright roseate hues, while the eastern horizon was only silvery bright before the sun rose; whereas, after the sun appeared and coloured the eastern hills and clouds, the western sky resumed its leaden grey and colourless appearance. why was that? what is the explanation? i have not space enough to repeat the explanation given already in the last chapter of the glorious phenomenon of the afterglow. but the explanation is similar. before sunrise, the rays of the sun are reflected by dust-particles in the zenith to the western clouds. the colouring is intensified by the frozen water-vapour on these particles in the west. one thing i carefully noted. ere mid-day, snow began to fall, and for some days a severe snow-storm kept us indoors. then, at any rate, the foreglow betokened a coming storm. it was, like a rainbow in a summer morning, a decided warning of the approaching wet weather. chapter xx the rainbow the poet wordsworth rapturously exclaimed-- "my heart leaps up when i behold a rainbow in the sky." and old and young have always been enchanted with the beautiful phenomenon. how glorious is the parti-coloured girdle which, on an april morning or september evening, is cast o'er mountain, tower, and town, or even mirrored in the ocean's depths! no colours are so vividly bright as when this triumphal arch bespans a dark nimbus: then it unfolds them in due prismatic proportion, "running from the red to where the violet fades into the sky." a plain description of the formation of the rainbow is not very easily given, but a short sketch may be useful. beautiful as is the ethereal bow, "born of the shower and colour'd by the sun," yet the marvellous effect is more exquisitely intensified in its gorgeous display when the hand of science points out the path in which the sun's rays, from above the western horizon, fall on the watery cloud, indicating fine weather--"the shepherd's delight." one law of reflection is that, when a ray of light falls on a plane or spherical surface, it goes off at the same angle to the surface as it fell. one law of refraction is that, when a ray of light passes through one medium and enters a denser medium (as from air to water), it is bent back a little. by refraction you see the sun's rays long after the sun has set; when the sun is just below the horizon, an observer, on the surface of the earth, will see it raised by an amount which is generally equal to its apparent diameter. the rays of different colours are bent back (when passing through the water) at different rates, some slightly, others more, from the red to the violet end. the rainbow, then, is produced by refraction and reflection of the several coloured rays of sunlight in the drops of water which make up falling rain. the sun is behind the observer, and its rays fall in a parallel direction upon the drops of rain before him. in each drop the light is dispersively refracted, and then reflected from the farther face of the drop; it travels back through the drop, and comes out with dispersing colours. according to the height of the sun, or the slope of its rays, a higher or lower rainbow will be formed. and, strange, no two people can see the very same bow; in fact the rainbow, as seen by the one eye, is not formed by the same water-drops as the rainbow seen by the other eye. when the primary bow is seen in most vivid colours on a dark cloud, a second arch, larger and fainter, is often seen. but the order of the colours is quite reversed. at a greater elevation, the sun's ray enters the lower side of a drop of rain-water, is refracted, reflected _twice_, and then refracted again before being sent out to the observer's eye. that is why the colours are reversed. _a one-coloured rainbow_ is a curious and rare phenomenon. it is a strange paradox, for the very idea of a rainbow brings up the seven colours--red, orange, yellow, green, blue, indigo, and violet. yet dr. aitken tells us of a rainbow with one colour which he observed on christmas day, in . he was taking his walk on the high ground south of falkirk. in the east he observed a strange pillar-like cloud, lit up with the light of the setting sun. then the red pillar extended, curved over, and formed a perfect arch across the north-eastern sky. when fully developed, this rainbow was the most extraordinary one which he had ever seen. there was no colour in it but red. it consisted simply of a red arch, and even the red had a sameness about it. outside the rainbow there was part of a secondary bow. the ochil hills were north of his point of observation. these hills were covered with snow, and the setting sun was glowing with rosy light. never had he seen such a depth of colour as was on them on this occasion. it was a deep, furnacy red. the sun's light was shorn of all the rays of short-wave length on its passage through the atmosphere, and only the red rays reached the earth. the reason why the ochils glowed with so deep a red was owing to their being overhung by a dense curtain of clouds, which screened off the light of the sky. the illumination was thus principally that of the direct softer light of the sun. chapter xxi the aurora borealis he must be a very careless observer who has not been struck with the appearance of the streamers which occasionally light up the northern heavens, and which farmers consider to be indicators of strong wind or broken weather. the time was when the phenomenon was considered to be supernatural and portentous, as the chroniclers of spectral battles, when "fierce, fiery warriors fought upon the clouds, in ranks and squadrons, and right form of war." and even in the rural districts of britain, the blood-coloured aurora, of october th, , was considered to be the reflection of an enormous prussian bonfire, fed by the beleaguered french capital. in joyful spirit, the shetlanders call the beautiful natural phenomenon, "merry dancers." burns associated their evanescence with the transitoriness of sensuous gratification:--"they flit ere you can point their place." and tennyson spoke of his cousin's face lit up with the colour and light of love, "as i have seen the rosy red flushing in the northern night." yet this phenomenon is to a great extent under the control of cosmical laws. one of the most difficult problems of our day has been to disentangle the irregular webwork of auroræ, and bring them under a law of periodicity, which depends upon the fluctuations of the sun's photosphere and the variations on the earth's magnetism, and which have such an important influence upon the fluctuations of the weather. the name "aurora borealis" was given to it by gassendi in . afterwards, the old almanacs described it as the "great amazing light in the north." in the lowlands of scotland, the name it long went by, of "lord derwentwater's lights," was given because it suddenly appeared on the night before the execution of the rebel lord. in ceylon auroræ were called "buddha lights." the first symptom of an aurora borealis is commonly a low arch of pale, greenish-yellow light, placed at right angles to the magnetic meridian. sometimes rays cover the whole sky, frequently showing tremulous motion from end to end; and sometimes they appear to hang from the sky like the fringes of a mantle. they are among the most capricious of natural phenomena, so full of individualities and vagaries. to the glitter of rapid movement they add the charm of vivid colouring. it is strongly asserted that auroræ are preceded by the same general phenomena as thunder-storms. this was borne out by piazzi smith (late astronomer-royal for scotland), who observed that their monthly frequency varies inversely with that of thunder-storms--both being safety-valves for the discharge of surplus electricity. careful observers have, moreover, noticed a remarkable coincidence between the display of auroræ and the maxima of the sun's spots and of the earth's magnetic disturbances. some have supposed that the light of the aurora is caused by clouds of meteoric dust, composed of iron, which is ignited by friction with the atmosphere. but there is this difficulty in the way, shooting stars are more frequent in the morning, while the reverse is the case with the aurora. the highest authorities have concluded, pretty uniformly, that auroræ are electric discharges through highly rarefied air, taking place in a magnetic field, and under the sway of the earth's magnetic induction. they are not inappropriately called "polar lightnings," for when electricity misses the one channel it must traverse the other. the natives of the arctic regions of north america pretend to foretell wind by the rapidity of the motions of the streamers. when they spread over the whole sky, in a uniform sheet of light, fine weather ensues. fitzroy believed that auroræ in northern latitudes indicated and accompanied stormy weather at a distance. the same idea is still current among many farmers and fishermen in scotland. is there any audible accompaniment to the brilliant spectacle? the natives of some parts, with subtle hearing-power, speak of the "whizzing" sound which is often heard during auroral displays. burns tells of their "hissing, eerie din," as echoes of the far-off songs of the valkyries. perhaps the most striking incident which corroborates this opinion occurred during the franco-prussian war. rolier, a practised aëronaut, left paris in a balloon, on his mission of city defence, and fourteen hours afterwards landed in norway. he had reached the height of two and a half miles. when descending, he passed through a peculiar cloud of sulphurous odour, which emitted flashed light and a slight scratching or rustling noise. on landing, he witnessed a splendid aurora borealis. he must, therefore, have passed through a cloud in which an electrical discharge of an auroral nature was proceeding, accompanied with an audible sound. there is, moreover, no improbability of such sounds being occasionally heard, since a somewhat similar phenomenon accompanies the brush discharge of the electric machinery, to which the aurora bears considerable resemblance. though no fixed conclusions are yet established about the causes of the brilliant auroral display, yet, as the results of laborious observations, we are assured that the stabler centre of our solar system holds in its powerful sway the several planets at their respective distances, supplying them all with their seasonable light and heat, vibrating sympathetic chords in all, and even controlling under certain--though to us still unknown--laws the electric streamers that flit, apparently lawlessly, in the distant earth's atmosphere. chapter xxii the blue sky if we look at the sky overhead, when cloudless in the sunshine, we wonder what gives the air such a deep-blue colour. we are not looking, as children seem to do, into vacancy, away into the far unknown. and even, if that were the case, would not the space be quite colourless? what, then, produces the blueness? some of the very fine dust-particles, even when clothed with an exceedingly thin coating of water-vapour, are carried very high; and, looking through a vast accumulation of these, we find the effect of a deep-blue colour. why so? because these particles are so small that they can only reflect the rays of the blue end of the spectrum; and the higher we ascend, the smaller are the particles and the deeper is the blue. but it is also because water, even in its very finest and purest form, is blue in colour. for long this was disputed. even sir robert christison concluded, after years of experimenting on highland streams, that water was colourless. of course, he admitted that the water in the indian and pacific oceans has frequent patches of red, brown, or white colour, from the myriads of animalcules suspended in the water. ehrenberg found that it was vegetable matter which gave to the red sea its characteristic name. but these, and similar waters, are not pure. it is to dr. aitken that the final discovery of the real colour of water is due. when on a visit to several towns on the shores of the mediterranean, he set about making some very interesting experiments, which the reader will follow with pleasure. it is a well-known fact that colour transmitted through different bodies differs considerably from colour reflected by them. in his first experiment he took a long empty metal tube, open at one end, and closed at the other end by a clear-glass plate. this was let down vertically into the water, near to a fixed object, which appeared of most beautiful deep and delicate blue at a depth of feet. scientific men know that, if the colour of water is due to the light reflected by extremely small particles of matter suspended in the water, then the object looked at through it would have been illuminated with yellow (the complementary colour of blue). a blackened tube was then filled with water (which had a clear-glass plate fixed to the bottom), and white, red, yellow, and purple objects were sunk in the water, and these colours were found to change in the same way as if they were looked at through a piece of pale-blue glass. the white object appeared blue, the red darkened very rapidly as it sank, and soon lost its colour; at the depth of seven feet a very brilliant red was so darkened as to appear dark brick-red. the yellow object changed to green, and the purple to dark blue. but, still further to satisfy himself that water is really blue in itself, even without any particles suspended in it, he tested the colour of _distilled_ water. he filled a darkened tube with this water (clear-glass plates being at the ends of the tube), and looked through it at a white surface. the effect was the same as before, the colour was blue, almost exactly of the same hue as a solution of prussian blue. this is corroborated by the fact that, the purer the water is in nature, the bluer is the tint when a large quantity is looked through. some highland lochs have crystal waters of the most extraordinary blue. of course, some cling to the old idea that this is accounted for by the reflected blue of the clear heavens above. no doubt, if the sky be deep blue, then this blue light, when reflected by the surface of the water, will enrich and deepen the hue. but the water itself is _really_ blue. at the same time, the dust-particles suspended in the water have a great effect in making the water appear more beautiful, brilliant, and varied in its colouring; because little or no light is reflected by the interior of a mass of water itself. if a dark metal vessel be filled with a weak solution of prussian blue, the liquid will appear quite dark and void of colour. but throw in some fine white powder, and the liquid will at once become of a brilliant blue colour. this accounts for the change of depth and brilliancy of colour in the several shores of the mediterranean. when, then, you look at the face of a deep-blue lake on a summer evening--the heavens all aglow with the unrivalled display of colour from the zenith, stretching in lighter hues of glory to the horizon--though to you the calm water appears like a lake of molten metal glowing with sky-reflected light, so powerful and brilliant as entirely to overpower the light which is internally reflected, yet blue is the normal colour of the water: _blueness is its inherent hue_. looking upwards, we observe three distinct kinds of blue in the sky from the horizon to the zenith. all painters in water-colours know that. newton thought that the colour of the sky was produced in the same way as the colours in thin plates, the order of succession of the colours gradually increasing in intensity. chapter xxiii a sanitary detective the impure state of the air in the rooms of a house can now be determined by means of colour alone. dr. aitken has invented a very simple instrument for that purpose; and this ought to be of great service to sanitary officers. it is called the koniscope--or dust-detective. the instrument consists of an air-pump and a metal tube with glass ends. near one end of the test-tube is a passage by which it communicates with the air-pump, and near the other end is attached a stop-cock for admitting the air to be tested. it is not nearly so accurate as the dust-counter; but it is cheaper, more easily wrought, and more handy for quick work. all the grades of blue, from what is scarcely visible to deep, dark blue, may be attached alongside the tube on pieces of coloured glass; and opposite these colours are the numbers of dust-particles in the cubic inch of the similar air, as determined by the dust-counter. while the number of particles was counted by means of the dust-counter, the depth of blue given by the koniscope was noted; and the piece of glass of that exact depth of blue attached. a metal tube was fitted up vertically in the room, in such a way that it could be raised to any desired height into the impure air near the ceiling, so that supplies of air of different degrees of impurity might be obtained. to produce the impurity, the gas was lit and kept burning during the experiments. the air was drawn down through the pipe by means of the air-pump of the koniscope, and it passed through the measuring apparatus of the dust-counter on its way to the koniscope. it may be remarked that, by a stroke of the air-pump, the air within the test-tube is rarefied and the dust-particles seize the moisture in the super-saturated air to form fog-particles; through this fog the colour is observed, and the shade of colour determines the number of dust-particles in the air. these colours are named "just visible," "very pale blue," "pale blue," "fine blue," "deep blue," and "very deep blue." when making a sanitary inspection, the pure air should be examined first, and the colour corresponding to that should be considered as the normal health colour. any increase from the depth would indicate that the air was being gradually contaminated; and the amount of increase in the depth of colour would indicate the amount of increase of pollution. as an illustration of what this instrument can detect, a room of by by feet was selected. the air was examined before the gas was lighted, and the colour in the test-tube was very faint, indicating a clear atmosphere. in all parts of the room this was found the same. a small tube was attached to the test-tube, open at the other end, for taking air from different parts of the room. three jets of gas were then lit in the centre of the room, and observations at once begun with the koniscope. within thirty-five seconds of striking the match to light the gas, the products of combustion had extended near the ceiling to the end of the room; this was indicated by the colour in the koniscope suddenly becoming a deep blue. in four minutes the deep-blue-producing air was got at a distance of two feet from the ceiling. in ten minutes there was strong evidence of the pollution all through the room. in half-an-hour the impurity at nine feet from the floor was very great, the colour being an intensely deep blue. the wide range of the indications of the instrument, from pure clearness to nearly black blue, makes the estimate of the impurity very easily taken with it; and, as there are few parts to get out of order, it is hoped it may come into general use for sanitary work. chapter xxiv fog and smoke just two hundred and forty years ago, mr. john evelyn, f.r.s., a well-known writer on meteorology, sent a curious tract to king charles ii., which was ordered to be printed by his majesty. it was entitled "fumifugium," and dealt with the great smoke nuisance in london. i find from the thesis that he had a very hazy idea of the connection between fog and smoke; and no wonder, for it is only lately that the connection has been fully explained. we know that without dust-particles there can be no fog, and that smoke supplies a vast amount of such particles. therefore, in certain states of the atmosphere, the more smoke the more fog. in mr. evelyn's day the fog, which he called "presumptuous smoake," was at times so dense that men could hardly discern each other for the "clowd." his majesty's only sister had complained of the damage done to her lungs by the contamination, and mr. evelyn was disgusted at the apathy of the people to do anything to remedy the nuisance. he deplored that that glorious and ancient city of london should wrap her stately head in "clowds of smoake, so full of stink and darknesse." he was of opinion that a method of charring coal so as to divest it of its smoke, while leaving it serviceable for many purposes, should be made the object of a very strict inquiry. and he was right. for it is now known that fog in a town is intensified by much smoke. in a city like london or glasgow, where a great river, fed by warm streams of water from gigantic works, passes through its centre, fogs can never be entirely obliterated, for the dust-particles in the air (often four millions and upwards in the cubic inch) will seize with terrible avidity the warm vapour rising from the river. that is the main reason why fogs cannot there be put down. smoke is being consumed to a great extent; yet we find particles of sulphur remaining, which seize the warm vapour and form fogs dense enough to check all traffic. the worst form of city fogs seems to be produced when the air, after first flowing slowly in one direction, then turns on its tracks and flows back over the city, bringing with it a black pall, the accumulated products of previous days, to which gets added the smoke and other impurities produced at the time. what irritated mr. evelyn was that, outside of london, the air was clear when passengers could not walk in safety within the city. so vexed was he about the contamination, that he made it the occasion of all the "cathars, phthisicks, coughs, and consumption in the city." he appealed to common sense to testify that those who repair to london soon take some serious illness. "i know a man," he said, "who came up to london and took a great cold, which he could never afterwards claw off again." mr. evelyn proposed that, by an act of parliament, the nuisance be removed; enjoining that all breweries, dye-works, soap and salt boilers, lime-burners, and the like, be removed five or six miles distant from london below the river thames. that would have materially helped his cause. but there is more difficulty in the purification than he anticipated. yet there was pluck in the old man pointing out the killing contamination and suggesting a possible remedy. he had the fond idea that thereby a certain charm, "or innocent magick," would make a transformation scene like arabia, which is therefore "styl'd the happy, attracting all with its gums and precious spices." in purer air fogs would be less dense, breathing would be easier, business would be livelier, life would be happier. few, i suppose, have laid their hands on this curious latin thesis, or its quaint translation, directing the king's attention to the fogs that were ruining london. since that time the city has increased, from little more than a village, to be the dwelling-place of six millions of human beings, yet too little improvement has been made in the removal of this fog nuisance. king edward's drive through london would be even more dangerous on a muggy, frosty day than was charles ii.'s, when science was little known. chapter xxv electrical deposition of smoke a good deal of scientific work is being done in the way of clearing away fog and smoke; and this, through time, may have some practical results in removing a great source of annoyance, illness, and danger in large towns. sir oliver lodge and dr. aitken have been throwing light upon the deposition of smoke in the air by means of electricity. if an electric discharge be passed through a jar containing the smoke from burnt magnesium wire, tobacco, brown paper, and other substances, the dust will be deposited so as to make the air clear. brush discharge, or anything that electrifies the air itself, is the most expeditious. if water be forced upwards through a vertical tube (with a nozzle one-twentieth of an inch in diameter), it will fall to the ground in a fine rain; but, if a piece of rubbed (electrified) sealing-wax be held a yard distant from the place where the jet breaks into drops, they at once fall in large spots as in a thunder-shower. if paper be put on the ground during the experiment, the sound of pattering will be observed to be quite different. if a kite be flown into a cloud, and made to give off electricity for some time, that cloud will begin to condense into rain. experiments with lord kelvin's recorder show that variations in the electrical state of the atmosphere precede a change of weather. then, with a very large voltaic battery, a tremendous quantity of electricity could be poured into the atmosphere, and its electrical condition could be certainly disturbed. if this could be made practically available, how useful it would be to farmers when the crops were suffering from excessive drought! it might be more powerfully available than the imagined condensation of a cloud into rain by the reverberation caused by the firing of a range of cannon. but what is the practical benefit of this information? if electricity deposits smoke, it might be made available in many ways. the fumes from chemical works might be condensed; and the air in large cities, otherwise polluted, might be purified and rendered innocuous. the smoke of chimneys in manufacturing works might be prevented from entering the atmosphere at all. in flour-mills and coal-mines the fine dust is dangerously explosive. in lead, copper, and arsenic works, it is both poisonous and valuable. lead smelters labour under this difficulty of condensing the fume which escapes along with the smoke from red-lead smelting furnaces; and it was considered that an electrical process of condensation might be made serviceable for the purpose. at bagillt, the method used for collecting or condensing the lead fume is a large flue two miles long; much is retained in this flue, but still a visible cloud of white-lead fume continually escapes from the top of the chimney. there is a difficulty in the way of depositing fumes in the flue by means of a sufficient discharge of electricity, viz. the violent draught which is liable to exist there, and which would mechanically blow away any deposited dust. but dr. aitken suggests that regenerators might be used along with the electricity. the warm fumes might be taken to a cold depositor, where (by the ordinary law of cold surfaces attracting warm dust-particles) the impurities would be removed, and, when purified, the air would again be taken through a hot regenerator before being sent up the chimney. by a succession of these chambers, with the assistance of electric currents, the air, impregnated with the most deleterious particles, or valuable dust, could be rendered innocuous. the sewage of our towns must be cleaned of its deleterious parts before being run into the streams which give drink to the lower animals, because an act of parliament enforces the process. why, then, ought we not to have similar compulsion for making the smoke from chemical and other noxious works quite harmless before being thrown into the air which contains the oxygen necessary for the life of human beings? there seems to be a good field before electricians to catch the smoke on the wing and deposit its dust on a large scale. this seems to be a matter beyond our reach at present, except in the scientist's laboratory; but certainly it is a "consummation devoutly to be wished." chapter xxvi radiation from snow one night a most interesting paper by dr. aitken, on "radiation from snow," was read by professor tait to the fellows of the royal society of edinburgh. i remember that dr. alex. buchan--the greatest meteorologist living--spoke afterwards in the very highest terms of the subject-matter of the paper. this was corroborated by lord kelvin, lord maclaren, and professor chrystal. dr. aitken had been testing the radiating powers of different substances. snow in the shade on a bright day at noon is ° fahr. colder than the air that floats upon it, whereas a black surface at the same is only ° colder. this difference diminishes as the sun gets lower; and at night both radiate almost equally well. i select, among the careful and numerous observations, the notes on january , ; for i took note of the cold of that day in my diary. it was the coldest day of the whole of that winter. the barometer was · inches, and the thermometer °--that is, ° of frost. according to dr. buchan, that january had only two equal in average cold for fifty years. on january , at a.m., when the air was at ° and the sky clear, a black surface registered ° and the upper layer of snow °, showing a difference of ° when both surfaces were colder than the superincumbent air. it is curious to note that, on february of the same year, at the same hour, when the sky was overcast, the air was at °, the black surface registered °, and the snow °, showing again the difference of °; but, in this case, both surfaces were warmer than the air. in both cases the radiation at night was equal. this small absorbing power of snow for heat reflected and radiated from the sky during the day must have a most important effect on the temperature of the air. the temperature of lands when covered with snow must be much lower than when free from it. and, when once a country has become covered with snow, there will be a tendency towards glacial conditions. but, besides being a bad absorber of heat from the sky, snow is also a very poor conductor of heat. on that very cold night (january ), when there was a depth of - / inches of snow on the ground, and the night clear, with strong radiation, the temperature of the surface of the snow was ° fahr., and a minimum thermometer on the snow showed that it had been down to zero some time before. a thermometer, plunged into the snow down to the grass, gave the most remarkable register of °. through the depth of - / inches of snow there was a difference of temperature of °. this was confirmed by removing the snow, and finding that the grass was unfrozen. as the ground was frozen when the snow fell, it would appear that the earth's heat slowly thawed it under the protection of the snow. the protection afforded by the bad-conducting power of snow is of great importance in the economy of nature. how vegetation would suffer, were it exposed to a low temperature, unprotected by the snow-mantle! so that, though the continued snow cools the air for animals that can look after their own heating, it keeps warm the soil; and vegetation prospers under the genial covering. the fine rich look of the young wheat-blades, after a continued snow has melted, must strike the most careless observer. instead of the half-blackened tips and semi-sickly blades, which we see in a field of young wheat after a considerable course of dry frost without snow, we have a rich, healthy green which shows the vital energy at work in the plants. or even in the town gardens, after a continued snow has been melted away by a soft, western breeze, we are struck with the white, peeping buds of the snowdrop and the finely springing grass in the sward. yet the snow-covering gives durability to cold weather. this has been demonstrated by dr. woeikof, the distinguished russian meteorologist. on this account the spring months of russia and siberia are intensely cold. the plants, then, which in winter are unable by locomotion to keep themselves in health, are protected by the snow-mantle which chills the air for animals that can keep themselves in heat by exercise. what a grand compensating power is here! chapter xxvii mountain giants some mysterious physical phenomena can be clearly explained by the aid of science. the mountain giants that at times haunt the lonely valleys, and strike with fear the superstitious dwellers there, are only the enlarged shadows of living human beings cast upon a dense mist. the two most startling of these "eerie" phenomena are the spectres of adam's peak and the brocken. the phenomena sometimes to be observed at adam's peak, in ceylon, are very remarkable. many travellers have given vivid accounts of these. on one occasion the hon. ralph abercromby, in his praiseworthy enthusiasm for meteorological research, went there with two scientific friends to witness the strange appearance. the conical peak, a mile and a half high, overlooks a gorge west of it. when, then, the north-east monsoon blows the morning mist up the valley, light wreaths of condensed vapour pass to the right of the peak, and catch the shadows at sunrise. this party reached the summit early one morning in february. the foreglow began to brighten the under-surface of the stratus-cloud with orange, and patches of white mist filled the hollows. soon the sun peeped through a chink in the clouds, and they saw the pointed shadow of the peak lying on the misty land. then a prismatic circle, with the red inside, formed round the shadow. the meteorologist waved his arms about, and immediately he found giant shadowy arms moving in the centre of the rainbow. soon they saw a brighter and sharper shadow of the peak, encircled by a double bow, and their own spectral arms more clearly visible. the shadow, the double bow, and the giant forms, combined to make this phenomenon the most marked in the whole world. the question has been frequently asked: why are such aërial effects not more widely observed? there are not many mountains of this height and of a conical shape; and still fewer can there be where a steady wind, for months together, blows up a valley so as to project the rising morning mist at a suitable height and distance on the western side, to catch the shadow of the peak at sunrise. the most famous place in europe for witnessing the awe-inspiring phenomenon is the brocken, in germany-- feet in height. the only great disappointment there is that the conditions rarely combine at sunrise or sunset to have "the spectre" successful. in july , my daughter and i were spending some weeks at harzburg, and, of course, we had to visit the brocken and take stock of the world-known phenomenon. at mid-day, the air at the flat summit was cold, clear, and hard. the boulders are of enormous size; and near the "noah's ark" hotel and observatory many are piled up in a mass, on which the observers stand at the appointed time for having their shadows projected on the misty air in the valleys. at five o'clock in the afternoon the sky was brilliantly clear on the summit of the brocken; but the wind was rising from the sun's direction, and the mist was filling up the wide-spread eastern valley. we stood on the "spectre" boulders, and our shadows were thrown on the grass, just as at home. however, they fell upon large patches of white heather, which there is very plentiful. at six o'clock the sun was still shining beautifully, and we anxiously waited for the time when it would be low enough to raise our shadows to the misty wall. an hour afterwards, a hundred visitors were out, and many of us were on the "spectre" stones. there was great excitement in anticipation of the weird appearances, which had attracted us from such a distance. but, almost at the moment of success, the sun descended behind a belt of purple cloud, and all we saw was part of a rainbow on the misty hollow. for the sun never appeared again. this was intensely saddening, seeing that, but for that stratum of cloud above the horizon, the phenomenon would have been graphically displayed. the cold became suddenly intense, and we had to sleep with a freezing mist enveloping the hotel. in vain did we wait for the wakening call, to tell us of sunrise; for the sun could not pierce the mist, and we had to return home disappointed. sometimes the rainbow colours assume the shapes of crosses instead of circles. occasionally a bright halo will be seen above the shadow-head of the observer, concentric rainbows enclosing all. in some recorded cases the grand effect must have been simply glorious. scientific observation has done much to dispel the superstition which has clung so tenaciously to the highland mind. the lonely grandeur of the weird mountain giants has been clearly explained as perfectly natural, yet the awe-striking feeling cannot be entirely driven off. chapter xxviii the wind once was the remark pointedly made: "the wind bloweth where it listeth." and that is nearly true still. the leading winds are under the calculation of the meteorologist, but the others will not be bound by laws. yet there are instruments for measuring the velocity and force of the wind, after it is on; but "whence it comes" is a different matter. a gentle air moves at the rate of miles an hour; a hurricane from to miles, pressing with lbs. on the square foot exposed to its fury. some of the gusts of the tay bridge storm, in , had a velocity of miles an hour, with a pressure of to lbs. to the square foot. before steamers supplanted so many sailing vessels, seamen required to be always on the alert as to the direction and strength of the wind, and the likelihood of any sudden change; and they chronicled twelve different strengths from "faint air" to a "storm." in general, the wind may be considered to be the result of a change of pressure and temperature in the atmosphere at the same level. the air of a warmer region, being lighter, ascends, and gives place to a current of wind from a colder region. these two currents--the higher and the lower--will continue to blow until there is equilibrium. the trade winds are regular and constant. these were much followed in the days of old. a vast amount of air in the tropics gets heated and ascends, being lighter, and travels to the colder north. a strong current rushes in from the north to take its place. but the earth rotates round its axis from west to east, and the combined motions make two slant wind directions, which are called the "trade winds," because they were so important in trade navigation. among the periodical winds are the "land and sea breezes." during the day, the land on the sea coast is warmer than the sea; accordingly, the air over the land becomes heated and ascends, the fine cool breeze from the sea taking its place. towards evening there is the equilibrium of temperature which produces a temporary calm. soon the earth chills, and the sea is counterbalancingly warm--as sea-water is steadier as to temperature than is land--the air over the sea becomes warmer, and ascends, the current from the land rushing in to take its place. hence during the night the wind is reversed, until in the morning again the equilibrium is restored and there is a calm, so far as these are concerned. these are therefore called the "land and sea breezes." of course, it is within the tropics that these breezes are most marked. by the assistance of other winds, a hurricane will there occasionally destroy towns and bring about much damage and loss of life; but better that hundreds should perish by a hurricane than thousands by the pestilence which, but for the storm, would have done its dire work. in countries where the differences of pressure are more marked than the differences of temperature, in the surrounding regions the strength of the wind thereby occasioned is far stronger than the land and sea breezes. the variable winds are more conflicting. these depend on purely local causes for a time, such as "the nature of the ground, covered with vegetation or bare; the physical configuration of the surface, level or mountainous; the vicinity of the sea or lakes, and the passage of storms." among these winds are the simoom and sirocco. the _east_ winds, which one does not care about in the british islands during the spring time, are occasioned by the powerful northern current which rushes south from the northern regions in europe. dr. buchan points out a very common mistake among even intelligent observers who shudder at the hard east winds. it is generally held that these winds are damp. they are unhealthy, but they are dry. it is quite true that many easterly winds are peculiarly moist; all that precede storms are so far damp and rainy; and it is owing to this circumstance that, on the east coast of scotland, the east winds are searching and carry most of the annual rainfall there. but all of these moist easterly winds, however, soon turn to some westerly point. the real east wind, so much feared by invalids, does not turn to the west; it is exceeding dry. curious is it that brain diseases, as well as consumption, reach their height in britain while east winds prevail. once in edinburgh, during the early spring, i had rheumatic fever, and during my convalescence my medical adviser, dr. menzies, would not let me have a short drive until the wind changed to the west. the first thing i anxiously watched in the morning was the flag on the castle; and for nearly two months it always waved from the east. how heart-depressing! creatures are we in the hands of nature's messengers. we so much depend upon the weather for our happiness. joyful are we when the honey-laden zephyr waves the long grass in june, or when "the gentle wind, a sweet and passionate wooer, kisses the blushing leaf." compared with this, how terrible is shakespeare's allusion to the appalling aspects of the storm:-- "i have seen tempests, when the scolding winds have rived the knotty oaks; and i have seen the ambitious ocean swell, and rage and foam, to be exalted with the threat'ning clouds; but never till to-night, never till now, did i go through a tempest dropping fire." chapter xxix cyclones and anti-cyclones the criticism of the weather in the meteorological column of our daily newspapers invariably speaks of "cyclones." it is, therefore, advisable to give as plain an explanation of these as possible. cyclones are "storm-winds." their nature has to be carefully studied by meteorologists, who are industriously at work to ascertain some scientific basis for the atmospheric movements. what is the cause of the spiral movement in storm-winds? in their centre the depression of the barometer is lowest, because the atmosphere there is lightest. as the walls of the spiral are approached, the barometer rises. dr. aitken has ingeniously hit upon an experiment to illustrate a spiral in air. all that is necessary is a good fire, a free-going chimney, and a wet cloth. the cloth is hung up in front of the fire, and pretty near it, so that steam rises readily from its surface; and, when there are no air-currents in the room, the steam will rise vertically, keeping close to the cloth. but if the room has a window in the wall, at right angles to the fireplace, so as to cause the air coming from it to make a cross-current past the fire, then a cyclone will be formed, and the vapour from the cloth will be seen circling round. when the cyclone is well formed, all the vapour is collected into the centre of the cyclone, and forms a white pillar extending from the cloth to the chimney. this experiment shows that no cyclone can form without some tangential motion in the air entering the area of low-pressure. now to illustrate the spiral approach. fill with water a cylindrical glass vessel, say inches in diameter and inches deep. have an orifice with a plug a little from the centre of the bottom. remove the plug, the water runs out, passing round the vessel in a vortex form. but, as the passage between the orifice (or centre of the cyclone) and the temporary division is narrower than in any other place, the water has to pass this part much more quickly than at any other place. and this curious result is observed: the top of the cyclone no longer remains over the orifice, but _travels_ in the direction of the water which is moving most speedily. similar to this is the cyclone in the atmosphere; its centre also moves in the direction of the quickest flowing wind that enters it. dr. aitken is of opinion that, in forecasting storms, too little attention has been paid to the _anti-cyclones_. they do more than simply follow and fill up the depression made by the cyclones. they initiate and keep up their own circulation, and collect the materials with which the cyclones produce their effect. neither could work efficiently without the other. suppose a large area on the earth over which the air is still in bright sunshine. after a time, when the air gets heated and charged with vapour, columns of air would begin to ascend in a disorderly fashion. but suppose an anti-cyclone is blowing at one side of this area. when the upper air descends to the earth, it spreads outwards in all directions; but the earth's rotation interferes and changes the radial into a spiral motion. the anti-cyclonic winds will prevent the formation of local cyclones, and drive all the moist, hot air to its circumference, just above the earth. the anti-cyclone forces its air tangentially into the cyclone, and gives it its direction and velocity of rotation, also the direction and rate of travel of the centre of depression. the earth's rotation is the original source of the rotatory movements, but both intensify the initial motion. accordingly, the cyclone must travel in the direction of the strongest winds blowing into it, just as the vortex in the vessel with the eccentric orifice travelled in the direction of the quickest moving water. this is verified by a study of the synoptic charts of the meteorological office. the sun's heat has always been looked upon as the main source of the energy of our winds, but some account must also be taken of the effects of cold. it is well known that the mean pressure over continental areas is high during winter and low during summer. as the sun's rays during summer give rise to the cyclonic conditions, so the cooling of the earth during winter gives rise to anti-cyclonic conditions. it is found during the winter months in several parts of the continent that as the temperature falls the pressure rises, producing anti-cyclones over the cold area; whereas, when the temperature begins to rise, the pressure falls, and cyclones are attracted to the warming area. small natural cyclones are often seen on dusty roads, the whirling column having a core of dusty air, and the centre of the vortex travelling along the road, tossing up the dust in a very disagreeable way to pedestrians. sometimes such a cyclone will toss up dry leaves to a height of four or five feet. they are very common; but it is only when dust, leaves, or other light material is present that they are visible to the eye. chapter xxx rain phenomena the soft rain on a genial evening, or the heavy thunder-showers on a broiling day, are too well known to be written about. sometimes rain is earnestly wished for, at other times it is dreaded, according to the season, seed-time or harvest. some years, like , are very deficient in rainfall, when the corn is stunted and everything is being burnt up; other years, like , there is an over-supply, causing great damage to agriculture. the year will long be remembered for its continuous rainfall; it is the record year; no year comes near it for the total rainfall all over the kingdom. rain is caused by anything that lowers the temperature of the air below the dew-point, but especially by winds. when a wind has blown over a considerable area of ocean on to the land, there is a likelihood of rain. when this wind is carried on to higher latitudes, or colder parts, there is a certainty of rain. of course, in the latter case the rain will fall heavier on the wind side than on the lee side. for short periods, the heaviest falls or "plouts" of rain are during thunder-storms. when the raindrops fall through a broad, cold stratum of air, they are frozen into hail, the particles of which sometimes reach a large size, like stones. of course, water-spouts now and again are of terrible violence. one of the heaviest rainfalls yet recorded in great britain was about - / inches in forty minutes at lednathie, forfarshire, in . now inch deep of rain means tons on an imperial acre; so the amount of water falling on a field during that short time is simply startling. the heaviest fall for one day was at ben nevis observatory, being fully - / inches in . in other parts of the world this is far exceeded. in one day at brownsville, texas, nearly inches fell in . on the khasi hills, india, inches on each of five successive days were registered. at gibraltar, inches were recorded in twenty-six hours. the heaviest rainfalls of the globe are occasioned by the winds that have swept over the most extensive ocean-areas in the tropics. on the summer winds the rainfall of india mainly depends; when this fails, there is most distressing drought. reservoirs are being erected to meet emergencies. from dr. buchan's statistics it is found that the annual rainfall at mahabaleshwar is inches; at sandoway ; and at cherra-pungi inches, the largest known rainfall anywhere on the globe. over a large part of the highlands of scotland more than inches fall annually, while in some of the best agricultural districts there it does not exceed inches. of all meteorological phenomena, rainfall is the most variable and uncertain. symons gives as tentative results from twenty years' observations in london--( ) in winter, the nights are wetter than the days; ( ) in spring and autumn, there is not much difference; ( ) in summer, nearly half as much again by day as by night. the wearisomeness of statistics may be here relieved by a short consideration of the _splash_ of a drop of rain. watching the drop-splashes on a rainy day in the outskirts of the city, when unable to get out, i brought to my recollection the marvellous series of experiments made by professor a. m. worthington in connection with these phenomena. of course, i could not see to proper advantage the formation of the splashes, as the heavy raindrops fell into these tiny lakes on the quiet road. there is not the effect of the huge thunder-drops in a stream or pool. the building up of the bubbles is not here perfect, for the domes fail to close, nor are the emergent columns visible to the naked eye. it is a pity; for r. l. stevenson once wrote of them in his delightful "inland voyage," when he canoed in the belgian canals, as thrown up by the rain into "an infinity of little crystal fountains." beautiful is this effect if one is under shelter, every dome seeming quite different in contour and individuality from all the rest. but terrible is it when out fishing on loch earn, even with the good-humoured old admiral, when the heavy thunder-drops splash up the crystal water, and one gets soaked to the skin, sportsman-like despising an umbrella. there is, however, a scientific interest about the splash of a drop. the phenomenon can be best seen indoors by letting a drop of ink fall upon the surface of pure water in a tumbler, which stands on white paper. it is an exquisitely regulated phenomenon, which very ideally illustrates some of the fundamental properties of fluids. when a drop of milk is let fall upon water coloured with aniline dye, the centre column of the splash is nearly cylindrical, and breaks up into drops before or during its subsequent descent into the liquid. as it disappears below the surface, the outward and downward flow causes a hollow to be again formed, up the sides of which a ring of milk is carried; while the remainder descends to be torn a second time into a beautiful vortex ring. this shell or dome is a characteristic of all splashes made by large drops falling from a considerable height, and is extremely pretty. sometimes the dome closes permanently over the imprisoned air, and forms a large bubble floating upon the water. the most successful experiments, however, have been carried through by means of instantaneous photography, with the aid of a leyden-jar spark, whose duration was less than the ten-millionth of a second. but the simple experiments, without the use of the apparatus, will while away a few hours on a rainy afternoon, when condemned to the penance of keeping within doors. chapter xxxi the meteorology of ben nevis several large and very important volumes of the royal society of edinburgh are devoted to statistics connected with the meteorology of ben nevis. most of the abstracts have been arranged by dr. buchan; while messrs. buchanan, omond, and rankine have taken a fair share of the work. this observatory, as mr. buchanan remarks, is unique, for it is established in the clouds; and the observations made in it furnish a record of the meteorology of the clouds. it is feet above the level of the sea; and as there is a corresponding observatory at fort william, at the base of the mountain, it is peculiarly well fitted for important observations and weather forecasting. the mountain, too, is on the west sea-coast of scotland, exposed immediately to the winds from the atlantic, catching them at first hand. it is lamentable to think that, when the importance of the observations made at the two observatories was becoming world known, funds could not be got to carry them on. ben nevis is the highest mountain in the british islands, best fitted for meteorological observations; yet these have been stopped for want of money. dr. buchan's valuable papers were published before any one dreamed of the stoppage of the work, which had such an important bearing on men engaged in business or taken up with open-air sport. from these i shall sift out a few facts that even "mute, inglorious" meteorologists may be interested in knowing. for a considerable time the importance of the study of the changes of the weather has come gradually to be recognised, and an additional impetus was given to the prosecution of this branch of meteorology when it was seen that the subject had intimate relations to the practical question of weather forecasts, including storm warnings. weather maps, showing the state of the weather over an extensive part of the surface of the globe, began to be constructed; but these were only indicators from places at the level of the sea. the singular advantages of a high-level observatory occurred to mr. milne home in ; and ben nevis was considered to be in every respect the most suitable in this country. the meteorological council of the royal society of london offered in , unsolicited, £ annually to the scottish meteorological society, to aid in the support of an observatory, the only stipulation being that the council be supplied with copies of the observations. from june to october, in , mr. wragge made daily observations at the top of the ben; and simultaneous observations were made, by mrs. wragge, at fort william. a second series, on a much more extended scale, was made in the following summer. funds were secured to build an observatory; and, in november , the regular work commenced, consisting of hourly observations by night as well as by day. until a short time ago, these were carried on uninterruptedly. telegraphic communications of each day's observations were sent to the morning newspapers; and now we are disappointed at not seeing them for comparison. the whole of the observations of temperature and humidity were of necessity eye-observations. for self-registering thermometers were comparatively useless when the wind was sometimes blowing at the rate of miles an hour. saturation was so complete in the atmosphere that everything exposed to it was dripping wet. every object exposed to the outside frosts of winter soon became thickly incrusted with ice. snowdrifts blocked up exposed instruments. accordingly, the observers had to use their own eyes, often at great risks. the instruments in the ben nevis observatory, and in the observing station at fort william, were of the best description. both stations were in positions where the effects of solar and terrestrial radiation were minimised. no other pair of meteorological stations anywhere in the world are so favourably situated as these two stations, for supplying the necessary observations for investigating the vertical changes of the atmosphere. it is to be earnestly hoped, therefore, that funds will be secured to resume the valuable work. the rate of the decrease of temperature with height there is ° fahr. for every feet of ascent, on the mean of the year. the rate is most rapid in april and may, when it is ° for each feet; and least rapid in november and december, when it is ° for feet. this rate agrees closely with the results of the most carefully conducted balloon ascents. the departures from the normal differences of temperature, but more especially the inversions of temperature, and the extraordinarily rapid rates of diminution with height, are intimately connected with the cyclones and anti-cyclones of north-western europe; and form data, as valuable as they are unique, in forecasting storms. the most striking feature of the climate of ben nevis is the repeated occurrence of excessive droughts. for instance, in the summer and early autumn of , low humidities and dew-points frequently occurred. corresponding notes were observed at sea-level. during nights when temperature falls through the effects of terrestrial radiation, those parts of the country suffer most from frosts over which very dry states of the air pass or rest; whereas, those districts, over which a more humid atmosphere hangs, will escape. on the night of august of that year, the potato crop on speyside was totally destroyed by the frost; whereas at dalnaspidal, in the district immediately adjoining, potatoes were scarcely--if at all--blackened. the mean annual pressure at ben nevis was · inches, and at fort william · , the difference being - / inches for the feet. for the whole year, the difference between the mean coldest hour, a.m., and the warmest hour, p.m., is °. for the five months, from october to february, the mean daily range of temperature varied only from o· to · . this is the time of the year when storms are most frequent; and this small range in the diurnal march of the temperature is an important feature in the climatology of ben nevis; for it presents, in nearly their simple form, the great changes of temperature accompanying storms and other weather changes, which it is so essential to know in forecasting weather. the daily maximum velocity of the wind occurs during the night, the daily differences being greatest in summer and least in winter. a blazing sun in the summer daily pours its rays on the atmosphere, and a thick envelope of cloud has apparently but little influence on the effect of the sun's rays. thunder-storms are essentially autumn and winter phenomena, being rare in summer. according to mr. buchanan, the weather on ben nevis is characterised by great prevalence of fog or mist. in continuously clear weather it practically never rains on the mountain at all. in continuously foggy weather, on the other hand, the average daily rainfall is inch. there is a large and continuous excess of pressure in clear weather over that of foggy weather. the mean temperature of the year is - / degrees higher in clear than in foggy weather. in june the excess is degrees. the nocturnal heating in the winter is very clearly observed. this has been noticed before in balloons as well as on mountains. the fog and mist in winter are much denser than in summer. whether wet or dry, the fog which characterises the climate of the mountain is nothing but _cloud_ under another name. chapter xxxii the weather and influenza some remarkable facts have been deduced by the late dr. l. gillespie, medical registrar, from the records of the royal infirmary of edinburgh. he considered that it might lead to interesting results if the admissions into the medical wards were contrasted with the varying states of the atmosphere. the repeated attacks of influenza made him pay particular attention to the influence of the weather on that disease. the meteorological facts taken comprise the weekly type of weather, _i.e._ cyclonic or anti-cyclonic, the extremes of temperature for the district for each week, and the mean weekly rainfall for the same district. more use is made of the extremes than of the mean, for rapid changes of temperature have a greater influence on disease than the actual mean. the period which he took up comprises the seven years - . there was a yearly average of admissions of ; so that he had a good field for observation. six distinct epidemics of influenza, varying in intensity, occurred during that period; yet there had been only twenty-three attacks between and . accordingly, these six epidemics must have had a great influence on the incidence of disease in the same period, knowing the vigorous action of the poison on the respiratory, the circulatory, and the nervous systems. the epidemics of influenza recorded in this country have usually occurred during the winter months. the first epidemic, which began on the th of december and continued for nine weeks, was preceded by six weeks of cyclonic weather, which was not, however, accompanied by a heavy rainfall. throughout the course of the disease, the type continued to be almost exclusively cyclonic, with a heavy rainfall, a high temperature, and a great deficiency of sunshine. the four weeks immediately following were also chiefly cyclonic, but with a smaller rainfall. the summer epidemic of followed a fine winter and spring, during which anti-cyclonic conditions were largely prevalent. but the epidemic was immediately preceded by wet weather and a low barometer. it took place in dry weather, and was followed by wet, cyclonic weather in turn. the great winter epidemic of followed an extremely wet and broken autumn. simultaneously with the establishment of an anti-cyclone, with east wind, practically no rain, and a lowering temperature, the influenza commenced. great extremes in the temperature followed, the advent of warmer weather and more equable days witnessing the disappearance of the disease. the fourth epidemic was preceded by a wet period, ushered in by dry weather, accompanied by great heat; and its close occurred in slightly wetter weather, but under anti-cyclonic conditions. the fifth outbreak began after a short anti-cyclone had become established over our islands, continued during a long spell of cyclonic weather with a considerable rainfall, but was drowned out by heavy rains. the last appearance of the modern plague, of which dr. gillespie's paper treats, commenced after cold and wet weather, continued in very cold but drier weather, and subsided in warmth with a moderate rainfall. the conditions of these six epidemics were very variable in some respects, and regular in others. the most constant condition was the decreased rainfall at the time, when the disease was becoming epidemic. anti-cyclonic weather prevailed at the time. according to dr. gillespie, the tables seem to suggest that a type of weather, which is liable to cause catarrhs and other affections of the respiratory tract, precedes the attacks of influenza; but that the occurrence of influenza in _epidemic form_ does not appear to take place until another and drier type has been established. as the weather changes, the affected patients increase with a rush. he is of opinion that the supposed rapid spread of influenza on the establishment of anti-cyclonic conditions may be explained in this way. the air in the cyclonic vortex, drawn chiefly from the atmosphere over the ocean, is moist, and contains none of the contagion; the air of the anti-cyclone, derived from the higher strata, and thus from distant cyclones, descending, blows gently over the land to the nearest cyclone, and, being drier, is more able to carry suspended particles with it. he considers that temperature has nothing to do with the problem, except in so far as the different types of weather may modify it. the infirmary records point to the occurrence of similar phenomena, recorded on previous occasions. accordingly, if such meteorological conditions are not indispensable to the spread of influenza in epidemic form, they at least afford favourable facilities for it. chapter xxxiii climate one is not far up in years, in scotland at any rate, without practically realising what climate means. he may not be able to put it in words, but easterly haars, chilling rimes, drizzling mists, dagging fogs, and soddening rains speak eloquently to him of the meaning of climate. climate is an expression for the conditions of a district with regard to temperature, and its influence on the health of animals and plants. the sun is the great source of heat, and when its rays are nearly perpendicular--as at the tropics--the heat is greater on the earth than when the slanted rays are gradually cooled in their passage. as one passes to a higher level, he feels the air colder, until he reaches the fluctuating snow-line that marks perpetual snow. the temperature of the atmosphere also depends upon the radiation from the earth. heat is quite differently radiated from a long stretch of sand, a dense forest, and a wide breadth of water. strange is it that a newly ploughed field absorbs and radiates more heat than an open lea. the equable temperature of the sea-water has an influence on coast towns. the gulf stream, from the gulf of mexico, heats the ocean on to the west coast of britain, and mellows the climate there. the rainfall of a district has a telling effect on the climate. boggy land produces a deleterious climate, if not malaria. over the world, generally, the prevailing winds are grand regulators of the climate in the distinctive districts. a wooded valley--like the greatest in britain, strathmore--has a health-invigorating power: what a calamity it is, then, that so many extensive woods, destroyed by the awful hurricane twelve years ago, are not replanted! some people can stand with impunity any climate; their "leather lungs" cannot be touched by extremes of temperature; but ordinary mortals are mere puppets in the hands of the goddess climate. hence health-resorts are munificently got up, and splendidly patronised by people of means. the poor, fortunately, have been successful in the struggle for existence, by innate hardiness, otherwise they would have had a bad chance without ready cash for purchasing health. it may look ludicrous at first sight, but it seems none the less true, that the variation of the spots on the sun have something to do with climate, even to the produce of the fields. on close examination, with a proper instrument, the disc of the sun is found to be here and there studded with dark spots. these vary in size and position day after day. they always make their first appearance on the same side of the sun, they travel across it in about fourteen days, and then they disappear on the other side. there is a great difference in the number of spots visible from time to time; indeed, there is what is called the minimum period, when none are seen for weeks together, and a maximum period, when more are seen than at any other time. the interval between two maximum periods of sun-spots is about eleven years. this is a very important fact, which has wonderful coincidences in the varied economy of nature. kirchhoff has shown, by means of the spectroscope, that the temperature of a sun-spot must be lower than that of the remainder of the solar surface. as we must get less heat from the sun when it is covered with spots than when there are none, it may be considered a variable star, with a period of eleven years. balfour stewart and lockyer have shown that this period is in some way connected with the action of the planets on the photosphere. as we have already mentioned, the variations of the magnetic needle have a period of the same length, its greatest variations occurring when there are most sun-spots. auroræ, and the currents of electricity which traverse the earth's surface, follow the same law. this remarkable coincidence set men a-thinking. can the varying condition of the sun exert any influences upon terrestrial affairs? is it connected with the variation of rainfall, the temperature and pressure of the atmosphere, and the frequency of storms? has the regular periodicity of eleven years in the sun-spots no effect upon climate and agricultural produce? mr. f. chambers, of bombay, has taken great trouble to strike, as far as possible, a connection between the recurring eleven years of sun-spots and the variation of grain prices. he arranged the years from to in nine groups of eleven years; and, from an examination of his tables, we find that there is a decided tendency for high prices to recur at more or less regular intervals of about eleven years, and a similar tendency for low prices. an occasional slight difference can be accounted for by some abnormal cause, as war or famine. amid all the apparently irregular fluctuations of the yearly prices, there is in every one of the ten provinces of india a periodical rise and fall of prices once every eleven years, corresponding to the regular variation which takes place in the number of sun-spots during the same period. if it were possible to obtain statistics to show the actual out-turn of the crops each year, the eleven yearly variations calculated therefrom might reasonably correspond with the sun-spot variations even more closely than do the price variations. this is a remarkable coincidence, if nothing more. what if it were yet possible to predict the variations of prices in the coming sun-spot cycle? such a power would be of immense service. by its aid it could be predicted that, as the present period of low prices has followed the last maximum of sun-spots, which was in the year , it will not last much longer, but that prices must gradually keep rising for the next five years. could science really predict this, it would be studied by many and blessed by more. yet the strange coincidence of a century's observations renders the conclusions not only possible, but to some extent probable. chapter xxxiv the "challenger" weather reports the _challenger_ expedition, commenced by sir wyville thomson, and after his death continued by sir john murray, with an able staff of assistants for the several departments, was one of the splendid exceptions to the ordinary british government stinginess in the furtherance of science. the results of the expedition were printed in a great number of very handsome volumes at the expense of the government. and the valuable deductions from the _challenger's_ weather reports by dr. alex. buchan, in his "atmospheric circulation," have thrown considerable light upon oceanic weather phenomena. for some of his matured opinions on these, i am here much indebted to him. humboldt, in , published a treatise on "isothermal lines," which initiated a fresh line for the study of atmospheric phenomena. an isotherm is an imaginary line on the earth's surface, passing through places having a corresponding temperature either throughout the year or at any particular period. an isobar is an imaginary line on the earth's surface, connecting places at which the mean height of the barometer at sea-level is the same. to isobars, as well as to isotherms, dr. buchan has devoted considerable attention. in , he published an important series of charts containing these, with arrows for prevailing winds over the earth for the months of the year. in this way what are called synoptic charts were established. in the _challenger_ report are shown the various movements of the atmosphere, with their corresponding causes. it is thus observed that the prevailing winds are produced by the inequality of the mass of air at different places. the air flows from a region of higher to a region of lower pressure, _i.e._ from where there is an excessive mass of air to fill up some deficiency. and this is the great principle on which the science of meteorology rests, not only as to winds, but as to weather changes. of the sun's rays which reach the earth, those that fall on the land are absorbed by the surface layer of about feet in thickness. but those that fall on the surface of the ocean penetrate, as shown by the observations of the _challenger_ expedition, to a depth of about feet. hence, in deep waters the temperature of the surface is only partially heated by the direct rays of the sun. in mid-ocean the temperature of the surface scarcely differs ° fahr. during the whole day, while the daily variation of the surface layer of land is sometimes °. the temperature of the air over the ocean is about three times greater than that of the surface of the open sea over which it lies; but, near land, this increases to five times. the elastic force of vapour is seen in its simplest form on the open sea, as disclosed by these reports. it is lowest at a.m. and highest at p.m. the relative humidity is just the reverse. when the temperature is highest, the saturation of the air is lowest, and _vice versâ_. so on land when the air, by radiation of heat from the earth, is cooled below the dew-point, dew is produced, and, at the freezing-point, hoar-frost. the _challenger_ reports, too, show that the force of the winds on the open sea is subject to no distinct and uniform daily variation, but that on nearing land the force of the wind gives a curve as distinctly marked as the ordinary curve of temperature. that force is lowest from to a.m., and highest from to p.m. each of the five great oceans gives the same result. at ben nevis, on the other hand, these forces are just reversed in strength. it is also shown by the _challenger_ observations that on the open sea the greatest number of thunder-storms occur from p.m. to a.m. and, from this, dr. buchan concludes that over the ocean terrestrial radiation is more powerful than solar radiation in causing those vertical disturbances in the equilibrium of the atmosphere which give rise to the thunder-storm. chapter xxxv weather-forecasting to foretell with any degree of certainty the state of the weather for twenty-four hours is of immense advantage to business men, tourists, fishermen, and many others. the weather is everybody's business. and the probabilities of accurate forecasts are so improving that all are more or less giving attention to the morning meteorological reports. weather-forecasting depends on the principle from vast experience that, if one event happens, a second is likely to follow. according to the extent and accuracy of the data, will be the strength of the probability of correct forecasts. and the great end of popular meteorology is to demonstrate this. we have given some explanations of the weather in some respects unique; and a careful consideration of these explanations will the more convince the reader of the importance of the subject. no doubt the changes of the weather are extremely complex, at times baffling; and the wonder is that forecasts come so near the truth. for instance, the year almost defied the ordinary rules of weather, for it broke the record for rainfall. and, last year, so repulsive and unseasonable was the spring, that there seemed to be a virtual "withdrawal" of the season. i wrote on it as "the recession of spring." speak about borrowing days! we had the equinoctial gales of march about the middle of april. on very few days had we "clear shining to cheer us after rain," for the bitter cold dried up any genial moisture. an old farmer remarked that "we're gaun ower faur north." no one could account for the backwardness of the season. unless for the cheering songs of the grove-charmers, one would have forgotten the time of the year. in march of this year, at strathmore, the barometer fell from · inches (the highest for years) to · in five days without unfavourable weather following. it again rose to · , then fell to · , followed by a rise to · without any peculiar change. but in two days it fell to · (the lowest for years), followed by a deluge of rain and a perfect hurricane for several hours, while the temperature was fortunately mild. it was only evident at the end that this universal storm had been "brewing" some days before. all are familiar with the ordinary prognostics of good and bad weather. a "broch" round the moon, in her troubled heaven, indicates a storm of rain or wind. when the dark crimson sun in the evening throws a brilliant bronzed light on the gables and dead leaves, we are sure that there is an intense radiation from the earth to form dew, or even hoar-frost. according to the meteorological folk-lore, the weather of the summer season is indicated by the foliation of the oak and ash trees. if the oak comes first into leaf, the summer will be hot and dry, if the ash has the precedence it will be wet and cold. looking over the observations of the budding of these two trees for half a century, i find that the weather-lore adage has been pretty correct. the ash was out before the oak a full month in the years , ' , ' , ' , ' , ' , ' , ' , ' , ' , ' , and ' ; and the summer and autumn in these years were unfavourable. again, the oak was out before the ash several weeks in the years , ' , ' , ' , ' , ' , ' , ' , ' , ' , ' , ' , ' , ' , ' , ' , ' , and ' ; the summers during these years were dry and warm, and the harvests were abundant. one can never think of this weather prognostic from nature without recalling the swallow song of tennyson's "princess":-- "why lingereth she to clothe her heart with love, delaying, as the tender ash delays to clothe herself, when all the woods are green?" on a muggy morning a sudden clearness in the south "drowns the ploughman." and yet enough blue in the sky "tae mak' a pair o' breeks" cheers one with the assurance of coming dry and sunny weather. the low flying of the swallows betokens rain, as well as any unseasonable dancing of midges in the evening. sore corns on the feet, and rheumatism in the joints, are direful precursors. the leaves are all a-tremble before the approach of thunder. but throughout this volume i have given many illustrations. but one of the largest and most important practical problems of meteorology is to ascertain the course which storms follow, and the causes by which that course is determined, so that a forecast may thereby be made, not only of the certain approach of a storm, but the particular direction and force of the storm. the method of conducting this large inquiry most effectively was devised by the french astronomer, le verrier--the great aspirant, with our own couch adams, for the discovery of the planet neptune. he began to carry this out in by the daily publication of weather data, followed by a synchronous weather map, which showed graphically for the morning of the day of publication the atmospheric pressure and the direction and force of the wind, together with tables of temperature, rainfall, cloud, and sea disturbances from a large number of places in all parts of europe. it is from similar maps that forecasts of storms are still framed, and suitable warnings issued; and a mass of information is being collected by telegraph from sixty stations in the british islands, &c., of the state of the weather at eight o'clock every morning, and analysed and arranged at the meteorological office in london for the evening's forecasts over the different districts of the country. a juster knowledge is being now acquired of those great atmospheric movements, and other changes, which form the groundwork of weather-forecasting. the meteorological office, westminster (entirely distinct from the royal meteorological society), is administered by a council (chairman, sir r. strachey; scottish member, dr. buchan), selected by the royal society. it employs a staff of over forty. the chief departments relate to: ( ) ocean meteorology, including the collection, tabulation, and discussion of meteorological data from british ships, the preparation of ocean weather charts, and the issue of meteorological instruments to the royal navy and mercantile marine; ( ) weather telegraphy, including the reception of telegrams thrice a day from selected stations for the preparation of the daily reports and weather forecasts. representatives of newspapers, &c., receive copies of the a.m. forecast based on the a.m. observations; and also of the . p.m. forecasts based on the observations received earlier in the day. in summer and autumn harvest forecasts are issued by telegraph to individuals who will defray the cost. the office also collects climatological data from a number of voluntary and some subsidised stations. the "first order" stations include valentia, falmouth, kew, and aberdeen. these have self-recording instruments of high precision, giving a continuous record of the meteorological elements. a government commission which sat last year, under the rt. hon. sir herbert maxwell, bart., have issued a report, recommending a number of changes in the management and constitution of the meteorological office; and considerable modifications are not unlikely to take place in the near future. in his evidence before that commission, the chairman of the council acknowledged that the great function of meteorologists is the collection of facts; but the interpretation of those collected facts, in a scientific manner, is still in a very immature condition. dr. buchan, in his evidence, confessed that forecasting by the council is purely "by rule of thumb." it is not possible to lay down hard and fast rules for forecasting. with regard to the storm-warning telegrams, as a rule, the earliest trustworthy indication of the approach of a dangerous storm to the coasts of the british isles precedes the storm by only a few hours. delays are therefore very serious. it is admitted by the best british meteorologists that the observations of the united states are better conducted, although the best instruments in the world are set and registered at kew, in england. the work of weather forecasts and storm warnings is carried on with the highest degree of promptitude and efficiency at the washington central office. this is because the work of predictions has been hitherto the chief work of the office: the entire time of the observers, on whose telegraphic reports the forecasts are based, is controlled by the united states weather bureau; and the right of precedence in the use of wires is maintained. professor brückner, of berne, has devoted a lifetime to the comparatively new treatment of climatic oscillations, based upon observations made at points on the earth's surface, distributed as follows: europe, ; asia, ; n. america, ; cen. and s. america, ; australia, ; africa, . one of his conclusions is that an average time of about thirty-five years is found to intervene between one period of excess or deficiency of warmth and the next, accompanied by the opposite relative condition of moisture. all are familiar with the hoisting of cone-warning as indication of a coming storm. this work is exceedingly important, especially for those connected with the sea by business or pleasure. on the known approach of a cyclone of dangerous intensity, special messages are sent from the london meteorological office, warning the coasts likely to be affected. when the cone is hoisted with its apex downwards, it means that strong south or south-west winds are to be looked for. when the cone is hoisted with its apex upwards, it indicates that strong winds from the north or north-east are expected. of course they are merely useful precautions; but they are universally attended to by people on the sea-coast. though one may have reasonable doubts about the use that can be made of weather forecasts for three days, such as are now regularly issued, on account of the finical, coy, spasmodic interludes on short notice, yet there is a wonderful certainty in the daily prognostics of the direction and strength of the wind, the temperature of the air, and the likelihood of rainy or fair weather, dependent on the broad uniformity of nature. this is very serviceable for people who have now to live at high pressure in business, in the enthralling days of keen competition. and it is a great boon to those who are in search of health by travelling, or who, in innocent pleasure, desire to live as much as possible in the open air. very little credit is given to the "gas" of the isolated "weather prophet"; but those who have confidence in the usual weather forecasts from the meteorological office are satisfied in their belief; and those who, in self-confidence, ignore all weather prognostics, are still weak enough to read them and act up to them. * * * * * in practical meteorology, in the scientific explanation of popular weather-lore, and in the study of atmospheric phenomena, which so powerfully influence us, for gladness or discomfort, we may, as with other branches of science, even all our days, cheerfully go on in "the noiseless tenor of our way," "nourishing a youth sublime, with the fairy tales of science and the long results of time." index abercromby, spectre on adam's peak, adam's peak, spectre, afterglow described, ; dust-particles to form, air, change of, ; clearness and dryness, ; devitalised, ; disease-germs in, ; thunder-clouds, aitken, dr., afterglows, ; anti-cyclones, ; colour of water, ; condensing power of dust, ; decay of clouds, ; dew-formation, ; dust and atmospheric phenomena, ; electrical deposition of smoke, ; false dew, ; fog-counter, ; foreglows, ; formation of clouds, ; haze, ; hazing effects of atmospheric dust, ; kingairloch experiments, ; one-coloured rainbow, ; radiation from snow, ; regenerators, ; sanitary detective, ammonia and cloud formation, annie laurie, anti-cyclones, forecasting by, ; formation, ; cause of influenza, aratus, forecasting by moon, ariel's song, aurora borealis, ; forebodings, - ; name by gassendi, ; other names, ; safety valve of electricity, ; sun's spots, ; sun control, ; symptoms, bagillt, condensing lead fumes, ballachulish, sunsets, ballantine's song, barometer, indications, ben nevis, dust-particles, ; instruments, ; meteorology, ; observations, ; rainfall, ; regret at stoppage of observatory, blairgowrie, personal description of afterglow, blue sky, ; cause of, , borrowing days, brocken, spectre, ; personal description, ; noah's ark, brückner, climatic oscillations, buchan, dr., aitken's radiation from snow, ; ben nevis, papers on, ; _challenger_ reports, ; cold of , ; east winds, ; isobars, ; rainfall statistics, ; on forecasting, buchanan, ben nevis observatory, ; great prevalence of fog, buddha's lights, of ceylon, burns, allusions to aurora, , byron, storm in alps, _challenger_ expedition, ; temperature, ; thunder-storms, ; winds, chambers on sun-spots and grain prices, change of air, ; strathmore to glenisla, charles ii., fog and smoke, chlorine and cloud formation, christison and colour of water, chrystal on aitken's radiation from snow, cirro-stratus cloud, mackerel-like, climate, _challenger_ notes, ; cone-warnings, ; gulf stream, ; oscillations, ; rainfall, ; sun-spots on, ; wooded country on, clouds, decay of, ; distances of, ; dry, ; even without dust, ; formation of, ; height of, ; numbering of cloud-particles, ; sunshine on cloud formation, ; varieties of, cone-warnings, continental winds, cyclones, ; formation of, , ; small natural, decay of clouds, ; in thin rain, ; process, ; ripple markings, dew, evidence of rising, ; experiments, , ; false dew, ; formation of, disease-germs in air, ; causes, ; deposited by rain, diseases, and east wind, ; personal notes, dumfries, dust in air at, dust, condensing power, ; from meteors, ; generally necessary for cloud formation, ; hazing effects, ; numbering, ; instruments for numbering, ; produces afterglows, ; produces foreglows, ; quantity in bunsen flame, ; at ben nevis, ; hyères, mentone, rigi kulm, ; lucerne, kingairloch, ; when not necessary, dust enumeration, deductions on, earn, loch, splash of drop at, earthshine, ehrenberg, on colour of water, evelyn, fumifugium, ; remedy for smoke, falkirk, dr. aitken's experiments on haze, false dew, fitzroy on aurora as a foreboder, fog, counter, ; dry, ; formation, ; more in towns, ; and smoke, folk-lore, foreglow, described, ; how produced, fort william observatory, frankland, disease-germs, franklin, lightning, gassendi, named aurora, gillespie, dr., on weather and influenza, glasgow, fog, glass, appearing damp, glenisla, ozoned air, grain crops and sun-spots, ; chambers' tables, great amazing light in the north, gulf stream, effects on climate, gunpowder, great condensing power, haze, what is, ; how produced, ; in clearest air, ; stages of condensation, ; in sultry weather, ; dryness of air and visibility, health improved by change of air, highland air, few disease-germs, hoar-frost, frozen dew, ; on under surfaces, humboldt, isotherms, hydrogen peroxide and cloud formation, hyères, dust-particles, indian ocean, colour, influenza, weather and, ; six distinct epidemics, ; spread of anti-cyclonic conditions, isobars by buchan, isotherms by humboldt, italian lakes, stages of condensation, job, on dew formation, kelvin recorder, ; aitken's radiation from snow, kew, instruments set, kingairloch, dust-particles, , kirchhoff, lower temperature of sun-spot, krakatoa, eruption of, dust-particles, le verrier and weathercharts, lockyer, and sun-spots, lightning, electricity, ; photographed, ; sheet and forked, ; ozone, lodge, electrical deposition of smoke, london, coals consumed, ; sulphur and fog, ; fog in reign of charles ii., ; meteorological office, , lord derwentwater's lights, lower animals, sensitiveness, lucerne, dust-particles, maclaren, aitken's radiation from snow, magnesia, small affinity for water-vapour, man in the street, mediterranean, brilliant colour, mentone, dust-particles, merry dancers of shetland, meteors, producing dust, meteorological council, london, ; office, ; cone-warnings, ; regular forecasts, milne home on ben nevis, milton, dust numberless, moon, old, in new moon's arms, ; weather indications, , mountain giants, ; adam's peak, ; brocken, munich, international meteorological conference, murray, _challenger_ expedition, nardius, dew exhalation, newton, colour of sky, nimbus, cloud, oak and ash, on climate, ochils, one-coloured rainbow, pacific, colour, paris, aurora, ; disease-germs, paton, waller, bronze tints in sunsets, piazzi smith, aurora, picket, dew-formation, pilatus, fine rain, polar lightnings, radiant heat, producing fine rain, radiation from snow, rain, ; heavy rainfalls, rainbow, ; forecasts, , ; formation, ; one-coloured, rains, it always, ; radiant heat in process, ; ariel's song, rankin, dust-particles, ben nevis, richardson, devitalised air, rigi kulm, dust-particles, rolier, aurora, st. paul's, london, disease-germs in air, sanitary detective, shakespeare, tempest, shelley, old moon in new moon's arms, simoom and sirocco, skye, rainy, smoke, electrical deposition of, ; regenerators, smoking-room, condensing power, snow, bad conducting, ; radiation from, sodium dust, condensing power, spens, forebodings of moon, splash of a drop, experiments, stevenson, r. l., splash of drop, stewart, sun-spots, strachey on forecasts, strathmore, observations on hoar-frost, ; on decay of clouds, ; to glenisla, change of air, ; observations on old moon in new moon's arms, ; afterglow described, ; foreglow, ; cold of , ; healthy by woods, ; observations on barometer, strathpeffer, sulphur as a fog-former, sulphuretted hydrogen and cloud-formation, sunshine on cloud-formation, sun's spots, and aurora, , ; and grain crops, symons, rainfall, synoptic charts, tait, on aitken's radiation from snow, tay bridge, fall of, tennyson, aurora, ; dew, ; oak and ash, thermometer, indications, thomson, wyville, _challenger_ expedition, thunder-storm described, valkyries, aurora, visibility, limit of, washington, meteorological office, water, pressure to show plant exudation, ; colour of, ; experiments on distilled, ; dust-particles vary colour, weather and influenza, weather-forecasting, ; advantages, ; principle, ; examples, ; old moon in new moon's arms, ; by moon, ; oak and ash, ; cone-warnings, ; three days', weather-lore, , weather talisman, ; call on barometer and thermometer, ; exceptional years, wells, dr., on dew, wilson, prof., on hoar-frost, wind, ; rates, ; trade, ; land and sea, woeikof, durability of cold, wordsworth, rainbow, worthington, splash of drop, wragge, observations at ben nevis, printed by ballantyne, hanson & co. edinburgh & london file was produced from images generously made available by the posner memorial collection (http://posner.library.cmu.edu/posner/)) vvilliam gilbert of colchester, physician of london. on the magnet, magnetick bodies also, and on the great magnet the earth; a new physiology, _demonstrated by many arguments_ & experiments. [illustration] london * * * * * imprinted at the chiswick press anno mcm. * * * * * [illustration] * * * * * {ij} [illustration] preface to the candid reader, studious of the magnetick philosophy. clearer proofs, in the discovery of secrets, and in the investigation of the hidden causes of things, being afforded by trustworthy experiments and by demonstrated arguments, than by the probable guesses and opinions of the ordinary professors of philosophy: so, therefore, that the noble substance of that great magnet, our common mother (the earth), hitherto quite unknown, and the conspicuous and exalted powers of this our globe, may be the better understood, we have proposed to begin with the common magnetick, stony, and iron material, and with magnetical bodies, and with the nearer parts of the earth which we can reach with our hands and perceive with our senses; then to proceed with demonstrable magnetick experiments; and so penetrate, for the first time, into the innermost parts of the earth. for after we had, in order finally to learn the true substance of the globe, seen and thoroughly examined many of those things which have been obtained from mountain heights or ocean depths, or from the profoundest caverns and from hidden mines: we applied much prolonged labour on investigating the magnetical forces; so wonderful indeed are they, compared with the forces of all other minerals, surpassing even the virtues of all other bodies about us. nor have we found this our labour idle or unfruitful; since daily during our experimenting, new and unexpected properties came to light; and our philosophy hath grown so much from the things diligently observed, that we have attempted to expound the interior parts of the terrene globe, and its native substance, upon magnetick principles; and to reveal to men the earth (our common mother), and to point it out as if with the finger, by real demonstrations and by experiments manifestly apparent to the senses. and as geometry ascends from sundry very small and very easy principles to the greatest and most difficult; by which the wit of man climbs above the firmament: so our magnetical doctrine and science first sets forth in convenient order the things which are less obscure; from these there come to light others that are more remarkable; and at length in due order there are opened the concealed and most secret things of the globe of the earth, and the causes are made known of those things which, either through the ignorance of the ancients or the neglect of moderns, have remained unrecognized and overlooked. but why should i, in so vast an ocean of books by which the minds of studious men are troubled and fatigued, through which very foolish productions the world and unreasoning men are intoxicated, and puffed up, rave and create literary broils, and while professing to be philosophers, physicians, mathematicians and astrologers, neglect and despise men of learning: why should i, i say, add aught further to this so-perturbed republick of letters, and expose this noble philosophy, which seems new and incredible by reason of so many things hitherto unrevealed, to be damned and torn to pieces by the maledictions of those who are either already sworn to the opinions of other men, or are foolish corruptors of good arts, learned idiots, grammatists, sophists, wranglers, and perverse little folk? but to you alone, true philosophizers, honest men, who seek knowledge not from books only but from things themselves, have i addressed these magnetical principles in this new sort of philosophizing. but if any see not fit to assent to these self-same opinions and paradoxes, let them nevertheless mark the great array of experiments and discoveries (by which notably every philosophy flourisheth), which have been wrought out and demonstrated by us with many pains and vigils and expenses. in these rejoice, and employ them to better uses, if ye shall be able. i know how arduous it is to give freshness to old things, lustre to the antiquated, light to the dark, grace to the despised, credibility to the doubtful; so much the more by far is it difficult to win and establish some authority for things new and unheard-of, in the face of all the opinions of all men. nor for that do we care, since philosophizing, as we deemed, is for the few. to our own discoveries and experiments we have affixed asterisks, larger and smaller, according to the importance and subtlety of the matter. whoso desireth to make trial of the same experiments, let him handle the substances, not negligently and carelessly, but prudently, deftly, and in the proper way; nor let him (when a thing doth not succeed) ignorantly denounce our discoveries: for nothing hath been set down in these books which hath not been explored and many times performed and repeated amongst us. many things in our reasonings and hypotheses will, perchance, at first sight, seem rather hard, when they are foreign to the {iij} commonly received opinion; yet i doubt not but that hereafter they will yet obtain authority from the demonstrations themselves. wherefore in magnetical science, they who have made most progress, trust most in and profit most by the hypotheses; nor will anything readily become certain to any one in a magnetical philosophy in which all or at least most points are not ascertained. this nature-knowledge is almost entirely new and unheard-of, save what few matters a very few writers have handed down concerning certain common magnetical powers. wherefore we but seldom quote antient greek authors in our support, because neither by using greek arguments nor greek words can the truth be demonstrated or elucidated either more precisely or more significantly. for our doctrine magnetical is at variance with most of their principles and dogmas. nor have we brought to this work any pretence of eloquence or adornments of words; but this only have we done, that things difficult and unknown might be so handled by us, in such a form of speech, and in such words as are needed to be clearly understood: sometimes therefore we use new and unusual words, not that by means of foolish veils of vocabularies we should cover over the facts with shades and mists (as alchemists are wont to do) but that hidden things which have no name, never having been hitherto perceived, may be plainly and correctly enunciated. after describing our magnetical experiments and our information of the homogenick parts of the earth, we proceed to the general nature of the whole globe; wherein it is permitted us to philosophize freely and with the same liberty which the egyptians, greeks, and latins formerly used in publishing their dogmas: whereof very many errors have been handed down in turn to later authors: and in which smatterers still persist, and wander as though in perpetual darkness. to those early forefathers of philosophy, aristotle, theophrastus, ptolemy, hippocrates, and galen, let due honour be ever paid: for by them wisdom hath been diffused to posterity; but our age hath detected and brought to light very many facts which they, were they now alive, would gladly have accepted. wherefore we also have not hesitated to expound in demonstrable hypotheses those things which we have discovered by long experience. farewell. * * * * * to the most eminent and learned man dr. william gilbert, _a distinguished doctor of medicine amongst the_ londoners, and father of magnetick philosophy, an encomiastic preface of edward wright on the subject of these books _magnetical_. _should there by chance be any one, most eminent sir, who reckons as of small account these magnetical books and labours of yours, and thinks these studies of yours of too little moment, and by no means worthy enough of the attention of an eminent man devoted to the weightier study of medicine: truly he must deservedly be judged to be in no common degree void of understanding. for that the use of the magnet is very important and wholly admirable is better known for the most part to men of even the lowest class than to need from me at this time any long address or commendation. nor truly in my judgment could you have chosen any topick either more noble or more useful to the human race, upon which to exercise the strength of your philosophic intellect; since indeed it has been brought about by the divine agency of this stone, that continents of such vast circuit, such an infinite number of lands, islands, peoples, and tribes, which have remained unknown for so many ages, have now only a short time ago, almost within our own memory, been quite easily discovered and quite frequently explored, and that the circuit of the whole terrestrial globe also has been more than once circumnavigated by our own countrymen, drake and cavendish; a fact which i wish to mention to the lasting memory of these men. for by the pointing of the iron touched by a loadstone, the points of south, north, east, and west, and the other quarters of the world are made known to navigators even under an overcast sky and in the darkest night; so that thus they always very easily understand to which point of the world they ought to direct their ship's course; which before the discovery of this wonderful virtue of the magnetick [greek: boreodeixis] was clearly impossible. hence in old times (as is established in histories), an incredible anxiety and immense danger was continually threatening sailors; for at the coming on of a tempest and the obscuring of the view of sun and stars, they were left entirely in ignorance whither they were making; nor could they find out this by any reasoning or skill. with what joy then may we suppose them to have been filled, to what feelings of delight must all shipmasters have given utterance, when that index magnetical first offered itself to them as a most sure guide, and as it were a mercury, for their journey? but neither was this sufficient for this magnetical mercury; to indicate, namely, the right way, and to point, as it were, a finger in the direction toward which the course must be {iiij} directed; it began also long ago to show distinctly the distance of the place toward which it points. for since the index magnetical does not always in every place look toward the same point of the north, but deviates from it often, either toward the east or toward the west, yet always has the same deviation in the same place, whatever the place is, and steadily preserves it; it has come about that from that deviation, which they call variation, carefully noticed and observed in any maritime places, the same places could afterwards also be found by navigators from the drawing near and approach to the same variation as that of these same places, taken in conjunction with the observation of the latitude. thus the portuguese in their voyages to the east indies had the most certain indications of their approach to the cape of good hope; as appears from the narrations of hugo van lynschoten and of the very learned richard hakluyt, our countryman. hence also the experienced skippers of our own country, not a few of them, in making the voyage from the gulf of mexico to the islands of the azores, recognized that they had come as near as possible to these same islands; although from their sea-charts they seemed to be about six hundred british miles from them. and so, by the help of this magnetick index, it would seem as though that geographical problem of finding the longitude, which for so many centuries has exercised the intellects of the most learned mathematicians, were going to be in some way satisfied; because if the variation for any maritime place whatever were known, the same place could very readily be found afterward, as often as was required, from the same variation, the latitude of the same place being not unknown._ _it seems, however, that there has been some inconvenience and hindrance connected with the observation of this variation; because it cannot be observed excepting when the sun or the stars are shining. accordingly this magnetick mercury of the sea goes on still further to bless all shipmasters, being much to be preferred to neptune himself, and to all the sea-gods and goddesses; not only does it show the direction in a dark night and in thick weather, but it also seems to exhibit the most certain indications of the latitude. for an iron index, suspended on its axis (like a pair of scales), with the most delicate workmanship so as to balance in æquilibrio, and then touched and excited by a loadstone, dips to some fixed and definite point beneath the horizon (in our latitude in london, for example, to about the seventy-second degree), at which it at length comes to rest. but under the æquator itself, from that admirable agreement and congruency which, in almost all and singular magnetical experiments, exists between the earth itself and a terrella (that is, a globular loadstone), it seems exceedingly likely (to say the very least), and indeed more than probable, that the same index (again stroked with a loadstone) will remain in æquilibrio in an horizontal position. whence it is evident that this also is very probable, that in an exceedingly small progress from the south toward the north (or contrariwise) there will be at least a sufficiently perceptible change in that declination; so that from that declination in any place being once carefully observed along with the latitude, the same place and the same latitude may be very easily recognized afterward, even in the darkest night and in the thickest mist by a declination instrument. wherefore to bring our oration at length back to you, most eminent and learned dr. gilbert (whom i gladly recognize as my teacher in this magnetick philosophy), if these books of yours on the magnet had contained nothing else, excepting only this finding of latitude from magnetick declination, by you now first brought to light, our shipmasters, britains, french, belgians, and danes, trying to enter the british channel or the straits of gibraltar from the atlantick ocean in dark weather, would still most deservedly judge them to be valued at no small sum of gold. but that discovery of yours about the whole globe of the earth being magnetical, although perchance it will seem to many "most paradoxical," producing even a feeling of astonishment, has yet been so firmly defended by you at all points and confirmed by so many experiments so apposite and appropriate to the matter in hand, in bk. , chap. ; bk. , chap. and ; and in almost the whole of the fifth book, that no room is left for doubt or contradiction. i come therefore to the cause of the magnetick variation, which hitherto has distracted the minds of all the learned; for which no mortal has ever adduced a more probable reason than that which has now been set forth by you for the first time in these books of yours on the magnet. the [greek: orthoboreodeixis] of the index magnetical in the middle of the ocean, and in the middle of continents (or at least in the middle of their stronger and more lofty parts), its inclining near the shore toward those same parts, even by sea and by land, agreeing with the experiments bk. , chap. , on an actual terrella (made after the likeness of the terrestrial globe, uneven, and rising up in certain parts, either weak or wanting in firmness, or imperfect in some other way),--this inclination having been proved, very certainly demonstrates the probability that that variation is nought else than a certain deviation of the magnetick needle toward those parts of the earth that are more vigorous and more prominent. whence the reason is readily established of that irregularity which is often perceived in the magnetick variations, arising from the inæquality and irregularity of those eminences and of the terrestrial forces. nor of a surety have i any doubt, that all those even who have either imagined or admitted points attractive or points respective in the sky or the earth, and those who have imagined magnetick mountains, or rocks, or poles, will immediately begin to waver as soon as they have perused these books of yours on the magnet, and willingly will march with your opinion. finally, as to the views which you discuss in regard to the circular motion of the earth and of the terrestrial poles, although to some perhaps they will seem most supposititious, yet i do not see why they should not gain some favour, even among the very men who do not recognize a sphærical motion of the earth; since not even they can easily clear themselves from many difficulties, which necessarily follow from the daily motion of the {v} whole sky. for in the first place it is against reason that that should be effected by many causes, which can be effected by fewer; and it is against reason that the whole sky and all the sphæres (if there be any) of the stars, both of the planets and the fixed stars, should be turned round for the sake of a daily motion which can be explained by the mere daily rotation of the earth. then whether will it seem more probable, that the æquator of the terrestrial globe in a single second (that is, in about the time in which any one walking quickly will be able to advance only a single pace) can accomplish a quarter of a british mile (of which sixty equal one degree of a great circle on the earth), or that the æquator of the _primum mobile_ in the same time should traverse five thousand miles with celerity ineffable; and in the twinkling of an eye should fly through about five hundred british miles, swifter than the wings of lightning, if indeed they maintain the truth who especially assail the motion of the earth). finally, will it be more likely to allow some motion to this very tiny terrestrial globe; or to build up with mad endeavour above the eighth of the fixed sphæres those three huge sphæres, the ninth (i mean), the tenth, and the eleventh, marked by not a single star, especially since it is plain from these books on the magnet, from a comparison of the earth and the terrella, that a circular motion is not so alien to the nature of the earth as is commonly supposed. nor do those things which are adduced from the sacred scriptures seem to be specially adverse to the doctrine of the mobility of the earth; nor does it seem to have been the intention of moses or of the prophets to promulgate any mathematical or physical niceties, but to adapt themselves to the understanding of the common people and their manner of speech, just as nurses are accustomed to adapt themselves to infants, and not to go into every unnecessary detail. thus in gen. i. v. , and psal. , the moon is called a great light, because it appears so to us, though it it is agreed nevertheless by those skilled in astronomy that many of the stars, both of the fixed and wandering stars, are much greater. therefore neither do i think that any solid conclusion can be drawn against the earth's mobility from psal. , v. ; although god is said to have laid the foundations of the earth that it should not be removed for ever; for the earth will be able to remain evermore in its own and self-same place, so as not to be moved by any wandering motion, nor carried away from its seat (wherein it was first placed by the divine artificer). we, therefore, with devout mind acknowledging and adoring the inscrutable wisdom of the triune divinity (having more diligently investigated and observed his admirable work in the magnetical motions), induced by philosophical experiments and reasonings not a few, do deem it to be probable enough that the earth, though resting on its centre as on an immovable base and foundation, nevertheless is borne around circularly._ _but passing over these matters (concerning which i believe no one has ever demonstrated anything with greater certainty), without any doubt those matters which you have discussed concerning the causes of the variation and of the magnetick dip below the horizon, not to mention many other matters, which it would take too long to speak of here, will gain very great favour amongst all intelligent men, and especially (to speak after the manner of the chemists) amongst the sons of the magnetick doctrine. nor indeed do i doubt that when you have published these books of yours on the magnet, you will excite all the diligent and industrious shipmasters to take no less care in observing the magnetick declination beneath the horizon than the variation. since (if not certain) it is at least probable, that the latitude itself, or rather the effect of the latitude, can be found (even in very dark weather) much more accurately from that declination alone, than can either the longitude or the effect of the longitude from the variation, though the sun itself is shining brightly or all the stars are visible, with the most skilful employment likewise of all the most exact instruments. nor is there any doubt but that those most learned men, peter plancius (not more deeply versed in geography than in observations magnetical), and simon stevinus, the most distinguished mathematician, will rejoice in no moderate degree, when they first see these magnetical books of yours, and observe their _[greek: limeneuretikê]_, or _haven-finding art_, enlarged and enriched by so great and unexpected an addition; and without doubt they will urge all their own shipmasters (as far as they can) to observe also everywhere the magnetick declination below the horizon no less than the variation. may your magnetical philosophy, therefore, most learned dr. gilbert, come forth into the light under the best auspices, after being kept back not till the ninth year only (as horace prescribes), but already unto almost a second nine, a philosophy rescued at last by so many toils, studyings, watchings, with so much ingenuity and at no moderate expense maintained continuously through so many years, out of darkness and dense mist of the idle and feeble philosophizers, by means of endless experiments skilfully applied to it; yet without neglecting anything which has been handed down in the writings of any of the ancients or of the moderns, all which you did diligently peruse and perpend. do not fear the boldness or the prejudice of any supercilious and base philosophaster, who by either enviously calumniating or stealthily arrogating to himself the investigations of others seeks to snatch a most empty glory. verily_ envy detracts from great homer's genius; _but_ whoever thou art, zoilus, thou hast thy name from him. _may your new physiology of the magnet, i say (kept back for so many years), come forth now at length into the view of all, and your philosophy, never to be enough admired, concerning the great magnet (that is, the earth); for, believe me_ (if there is any truth in the forebodings of seers), _these books of yours on the magnet will avail more for perpetuating the memory of your name than the monument of any great magnate placed upon your tomb._ {vj} * * * * * _interpretation of certain words.[ ]_ terrella, a globular loadstone. verticity, polar vigour, not [greek: peridinêsis] but [greek: peridineisios dunamis]: not a vertex or [greek: polos] but a turning tendency. electricks, things which attract in the same manner as amber. excited magnetick, that which has acquired powers from the loadstone. magnetick versorium, a piece of iron upon a pin, excited by a loadstone. non-magnetick versorium, a versorium of any metal, serving for electrical experiments. capped loadstone, which is furnished with an iron cap, or snout. meridionally, that is, along the projection of the meridian. paralleletically, that is, along the projection of a parallel. cusp, tip of a versorium excited by the loadstone. cross, sometimes used of the end that has not been touched and excited by a loadstone, though in many instruments both ends are excited by the appropriate termini of the stone. cork, that is, bark of the cork-oak. radius of the orbe of the loadstone, is a straight line drawn from the summit of the orbe of the loadstone, by the shortest way, to the surface of the body, which, continued, will pass through the centre of the loadstone. orbe of virtue, is all that space through which the virtue of any loadstone extends. orbe of coition, is all that space through which the smallest magnetick is moved by the loadstone. proof, for a demonstration shown by means of a body. magnetick coition: since in magnetick bodies, motion does not occur by an attractive faculty, but by a concourse or concordance of both, not as if there were an [greek: helktikê dunamis] of one only, but a [greek: sundromê] of both; there is always a coition of the vigour: and even of the body if its mass should not obstruct. declinatorium, a piece of iron capable of turning about an axis, excited by a loadstone, in a declination instrument. * * * * * index of chapters. _book ._ chap. . ancient and modern writings on the loadstone, with certain matters of mention only, various opinions, & vanities. chap. . magnet stone, of what kind it is, and its discovery. chap. . the loadstone has parts distinct in their natural power, & poles conspicuous for their property. chap. . which pole of the stone is the boreal: and how it is distinguished from the austral. chap. . loadstone seems to attract loadstone when in natural position: but repels it when in a contrary one, and brings it back to order. chap. . loadstone attracts the ore of iron, as well as iron proper, smelted & wrought. chap. . what iron is, and of what substance, and its uses. chap. . in what countries and districts iron originates. chap. . iron ore attracts iron ore. chap. . iron ore has poles, and acquires them, and settles itself toward the poles of the universe. chap. . wrought iron, not excited by a loadstone, draws iron. chap. . a long piece of iron (even though not excited by a loadstone) settles itself toward north & south. chap. . wrought iron has in itself certain parts boreal & austral: a magnetick vigour, verticity, and determinate vertices or poles. chap. . concerning other powers of loadstone, & its medicinal properties. chap. . the medicinal virtue of iron. chap. . that loadstone & iron ore are the same, but iron an extract from both, as other metals are from their own ores; & that all magnetick virtues, though weaker, exist in the ore itself & in smelted iron. chap. . that the globe of the earth is magnetick, & a magnet; & how in our hands the magnet stone has all the primary forces of the earth, while the earth by the same powers remains constant in a fixed direction in the universe. _book ._ chap. . on magnetick motions. chap. . on the magnetick coition, and first on the attraction of amber, or more truly, on the attaching of bodies to amber. chap. . opinions of others on magnetick coition, which they call attraction. chap. . on magnetick force & form, what it is; and on the cause of the coition. chap. . how the power dwells in the loadstone. chap. . how magnetick pieces of iron and smaller loadstones conform themselves to a terrella & to the earth itself, and by them are disposed. chap. . on the potency of the magnetick virtue, and on its nature capable of spreading out into an orbe. chap. . on the geography of the earth, and of the terrella. chap. . on the Æquinoctial circle of the earth and of a terrella. chap. . magnetick meridians of the earth. chap. . parallels. {vij} chap. . the magnetick horizon. chap. . on the axis and magnetick poles. chap. . why at the pole itself the coition is stronger than in the other parts intermediate between the æquator and the pole; and on the proportion of forces of the coition in various parts of the earth and of the terrella. chap. . the magnetick virtue which is conceived in iron is more apparent in an iron rod than in a piece of iron that is round, square, or of other figure. chap. . showing that movements take place by the magnetical vigour though solid bodies lie between; and on the interposition of iron plates. chap. . on the iron cap of a loadstone, with which it is armed at the pole (for the sake of the virtue), and on the efficacy of the same. chap. . an armed loadstone does not indue an excited piece of iron with greater vigour than an unarmed. chap. . union with an armed loadstone is stronger; hence greater weights are raised; but the coition is not stronger, but generally weaker. chap. . an armed loadstone raises an armed loadstone, which also attracts a third; which likewise happens, though the virtue in the first be somewhat small. chap. . if paper or any other medium be interposed, an armed loadstone raises no more than an unarmed one. chap. . that an armed loadstone draws iron no more than an unarmed one: and that an armed one is more strongly united to iron is shown by means of an armed loadstone and a polished cylinder of iron. chap. . the magnetick force causes motion toward unity, and binds firmly together bodies which are united. chap. . a piece of iron placed within the orbe of a loadstone hangs suspended in the air, if on account of some impediment it cannot approach it. chap. . exaltation of the power of the magnet. chap. . why there should appear to be a greater love between iron & loadstone, than between loadstone & loadstone, or between iron & iron, when close to the loadstone, within its orbe of virtue. chap. . the centre of the magnetick virtues in the earth is the centre of the earth; and in a terrella is the centre of the stone. chap. . a loadstone attracts magneticks not only to a fixed point or pole, but to every part of a terrella save the æquinoctial zone. chap. . on variety of strength due to quantity or mass. chap. . the shape and mass of the iron are of most importance in cases of coition. chap. . on long and round stones. chap. . certain problems and magnetick experiments about the coition, and separation, and regular motion of bodies magnetical. chap. . on the varying ratio of strength, and of the motion of coition, within the orbe of virtue. chap. . why a loadstone should be stronger in its poles in a different ratio; as well in the northern regions as in the southern. chap. . on a perpetual motion machine, mentioned by authors, by means of the attraction of a loadstone. chap. . how a more robust loadstone may be recognized. chap. . use of a loadstone as it affects iron. chap. . on cases of attraction in other bodies. chap. . on bodies which mutually repel one another. _book ._ chap. . on direction. chap. . the directive or versorial virtue (which we call verticity): what it is, how it exists in the loadstone; and in what way it is acquired when innate. chap. . how iron acquires verticity through a loadstone, and how that verticity is lost and changed. chap. . why iron touched by a loadstone acquires an opposite verticity, and why iron touched by the true northern side of a stone turns to the north of the earth, by the true southern side to the south; and does not turn to the south when rubbed by the northern point of the stone, and when by the southern to the north, as all who have written on the loadstone have falsely supposed. chap. . on the touching of pieces of iron of divers shapes. chap. . what seems an opposing motion in magneticks is a proper motion toward unity. chap. . a determined verticity and a disponent faculty are what arrange magneticks, not a force, attracting them or pulling them together, nor merely a strongish coition or unition. chap. . of discords between pieces of iron upon the same pole of a loadstone, and how they can agree and stand joined together. chap. . figures illustrating direction and showing varieties of rotations. chap. . on mutation of verticity and of magnetick properties, or on alteration in the power excited by a loadstone. chap. . on the rubbing of a piece of iron on a loadstone in places midway between the poles, and upon the æquinoctial of a terrella. chap. . in what way verticity exists in any iron that has been smelted though not excited by a loadstone. chap. . why no other body, excepting a magnetick, is imbued with verticity by being rubbed on a loadstone, and why no body is able to instil and excite that virtue, unless it be a magnetick. chap. . the placing of a loadstone above or below a magnetick body suspended in æquilibrio changes neither the power nor the verticity of the magnetick body. chap. . the poles, Æquator, centre in an entire loadstone remain and continue steady; by diminution and separation of some part they vary and acquire other positions. chap. . if the southern portion of a stone be lessened, something is also taken away from the power of the northern portion. chap. . on the use and excellence of versoria: and how iron versoria used as pointers in sun-dials, and the fine needles of the mariners' compass, are to be rubbed, that they may acquire stronger verticity. {viij} _book ._ chap. . on variation. chap. . that the variation is caused by the inæquality of the projecting parts of the earth. chap. . the variation in any one place is constant. chap. . the arc of variation is not changed equally in proportion to the distance of places. chap. . an island in ocean does not change the variation, as neither do mines of loadstone. chap. . the variation and direction arise from the disponent power of the earth, and from the natural magnetick tendency to rotation, not from attraction, or from coition, or from other occult cause. chap. . why the variation from that lateral cause is not greater than has hitherto been observed, having been rarely seen to reach two points of the mariners' compass, except near the pole. chap. . on the construction of the common mariners' compass, and on the diversity of the compasses of different nations. chap. . whether the terrestrial longitude can be found from the variation. chap. . why in various places near the pole the variations are much more ample than in a lower latitude. chap. . cardan's error when he seeks the distance of the centre of the earth from the centre of the cosmos by the motion of the stone of hercules; in his book , _on proportions_. chap. . on the finding of the amount of variation: how great is the arc of the horizon from its arctick to its antarctick intersection of the meridian, to the point respective of the magnetick needle. chap. . the observations of variation by seamen vary, for the most part, and are uncertain: partly from error and inexperience, and the imperfections of the instruments: and partly from the sea being seldom so calm that the shadows or lights can remain quite steady on the instruments. chap. . on the variation under the æquinoctial line, and near it. chap. . the variation of the magnetick needle in the great Æthiopick and american sea, beyond the æquator. chap. . on the variation in nova zembla. chap. . variation in the pacifick ocean. chap. . on the variation in the mediterranean sea. chap. . the variation in the interior of large continents. chap. . variation in the eastern ocean. chap. . how the deviation of the versorium is augmented and diminished by reason of the distance of places. _book ._ chap. . on declination. chap. . diagram of declinations of the magnetick needle, when excited, in the various positions of the sphere, and horizons of the earth, in which there is no variation of the declination. chap. . an indicatory instrument, showing by the virtue of a stone the degrees of declination from the horizon of each several latitude. chap. . concerning the length of a versorium convenient for declination on a terrella. chap. . that declination does not arise from the attraction of the loadstone, but from a disposing and rotating influence. chap. . on the proportion of declination to latitude, and the cause of it. chap. . explanation of the diagram of the rotation of a magnetick needle. chap. . diagram of the rotation of a magnetick needle, indicating magnetical declination in all latitudes, and from the rotation and declination, the latitude itself. chap. . demonstration of direction, or of variation from the true direction, at the same time with declination, by means of only a single motion in water, due to the disposing and rotating virtue. chap. . on the variation of the declination. chap. . on the essential magnetick activity sphærically effused. chap. . magnetick force is animate, or imitates life; and in many things surpasses human life, while this is bound up in the organick body. _book ._ chap. . on the globe of the earth, the great magnet. chap. . the magnetick axis of the earth persists invariable. chap. . on the magnetick diurnal revolution of the earth's globe, as a probable assertion against the time-honoured opinion of a primum mobile. chap. . that the earth moves circularly. chap. . arguments of those denying the earth's motion, and their confutation. chap. . on the cause of the definite time of an entire rotation of the earth. chap. . on the primary magnetick nature of the earth, whereby its poles are parted from the poles of the ecliptick. chap. . on the præcession of the Æquinoxes, from the magnetick motion of the poles of the earth, in the arctick & antarctick circle of the zodiack. chap. . on the anomaly of the præcession of the Æquinoxes, & of the obliquity of the zodiack. [illustration] * * * * * { } [illustration] william gilbert on the loadstone, bk. i. _chap. i._ ancient and modern writings on the loadstone, with certain matters of mention only, _various opinions, & vanities_. at an early period, while philosophy lay as yet rude and uncultivated in the mists of error and ignorance, few were the virtues and properties of things that were known and clearly perceived: there was a bristling forest of plants and herbs, things metallick were hidden, and the knowledge of stones was unheeded. but no sooner had the talents and toils of many brought to light certain commodities necessary for the use and safety of men, and handed them on to others (while at the same time reason and experience had added a larger hope), than a thorough examination began to be made of forests and fields, hills and heights; of seas too, and the depths of the waters, of the bowels of the earth's body; and all things began to be looked into. and at length by good luck the magnet-stone was discovered in iron lodes, probably by smelters of iron or diggers of metals. this, on being handled by metal folk, quickly displayed that powerful and strong attraction for iron, a virtue not latent and obscure, but easily proved by all, and highly praised and commended. and in after time when it had emerged, as it were out of darkness and deep dungeons, and had become dignified of men on account of its strong and amazing attraction for iron, many philosophers as well as physicians of ancient days discoursed of it, in short celebrated, as it were, its memory only; as for instance plato in the _io_[ ], aristotle in the _de anima_[ ], in book i. only, theophrastus the lesbian, dioscorides, c. plinius secundus, and julius solinus[ ]. as handed down by them the loadstone merely attracted iron, the rest of its virtues were all undiscovered. but that the story of the { } loadstone might not appear too bare and too brief, to this singular and sole known quality there were added certain figments and falsehoods, which in the earliest times, no less than nowadays, used to be put forth by raw smatterers and copyists to be swallowed of men. as for instance, that if a loadstone be anointed with garlick, or if a diamond be near, it does not attract iron[ ]. tales of this sort occur in pliny, and in ptolemy's _quadripartitum_; and the errors have been sedulously propagated, and have gained ground (like ill weeds that grow apace) coming down even to our own day, through the writings of a host of men, who, to fill put their volumes to a proper bulk, write and copy out pages upon pages on this, that, and the other subject, of which they knew almost nothing for certain of their own experience. such fables of the loadstone even georgius agricola himself, most distinguished in letters, relying on the writings of others, has embodied as actual history in his books _de natura fossilium_. galen noted its medicinal power in the ninth book of his _de simplicium medicamentorum facultatibus_, and its natural property of attracting iron in the first book of _de naturalibus facultatibus_; but he failed to recognize the cause, as dioscorides before him, nor made further inquiry. but his commentator matthiolus repeats the story of the garlick and the diamond, and moreover introduces mahomet's shrine vaulted with loadstones[ ], and writes that, by the exhibition of this (with the iron coffin hanging in the air) as a divine miracle, the public were imposed upon. but this is known by travellers to be false. yet pliny relates that chinocrates the architect had commenced to roof over the temple of arsinoe at alexandria with magnet-stone[ ], that her statue of iron placed therein might appear to hang in space. his own death, however, intervened, and also that of ptolemy, who had ordered it to be made in honour of his sister. very little was written by the ancients as to the causes of attraction of iron; by lucretius and others there are some short notices; others only make slight and meagre mention of the attraction of iron: all of these are censured by cardan for being so careless and negligent in a matter of such importance and in so wide a field of philosophizing; and for not supplying an ampler notion of it and a more perfect philosophy: and yet, beyond certain received opinions and ideas borrowed from others and ill-founded conjectures, he has not himself any more than they delivered to posterity in all his bulky works any contribution to the subject worthy of a philosopher. of modern writers some set forth its virtue in medicine only, as [ ]antonius musa brasavolus, baptista montanus, amatus lusitanus, as before them oribasius in his thirteenth chapter _de facultate metallicorum_, aetius amidenus, avicenna, serapio mauritanus, hali abbas, santes de ardoynis, petrus apponensis, marcellus[ ], arnaldus. bare mention is made of certain points relating to the loadstone in very few words by marbodeus callus, albertus, { } matthæus silvaticus, hermolaus barbarus, camillus leonhardus, cornelius agrippa, fallopius, johannes langius, cardinal cusan, hannibal rosetius calaber; by all of whom the subject is treated very negligently, while they merely repeat other people's fictions and ravings. matthiolus compares the alluring powers of the loadstone which pass through iron materials, with the mischief of the torpedo, whose venom passes through bodies and spreads imperceptibly; guilielmus pateanus in his _ratio purgantium medicamentorum_ discusses the loadstone briefly and learnedly. thomas erastus[ ], knowing little of magnetical nature, finds in the loadstone weak arguments against paracelsus; georgius agricola, like encelius[ ] and other metallurgists, merely states the facts; alexander aphrodiseus in his _problemata_ considers the question of the loadstone inexplicable; lucretius carus, the poet of the epicurean school, considers that an attraction is brought about in this way: that as from all things there is an efflux of very minute bodies, so from the iron atoms flow into the space emptied by the elements of the loadstone, between the iron and the loadstone, and that as soon as they have begun to stream towards the loadstone, the iron follows, its corpuscles being entangled. to much the same effect johannes costæus adduces a passage from plutarch; thomas aquinas[ ], writing briefly on the loadstone in chapter vii. of his _physica_, touches not amiss on its nature, and with his divine and clear intellect would have published much more, had he been conversant with magnetick experiments. plato thinks the virtue divine. but when three or four hundred years afterwards, the magnetick movement to north and south was discovered or again recognized by men, many learned men attempted, each according to the bent of his own mind, either by wonder and praise, or by some sort of reasonings, to throw light upon a virtue so notable, and so needful for the use of mankind. of more modern authors a great number have striven to show what is the cause of this direction and movement to north and south, and to understand this great miracle of nature, and to disclose it to others: but they have lost both their oil and their pains; for, not being practised in the subjects of nature, and being misled by certain false physical systems, they adopted as theirs, from books only, without magnetical experiments, certain inferences based on vain opinions, and many things that are not, dreaming old wives' tales. marsilius ficinus ruminates over the ancient opinions, and in order to show the reason of the direction seeks the cause in the heavenly constellation of the bear, supposing the virtue of the bear to prevail in the stone and to be transferred to the iron. paracelsus asserted that there are stars, endowed with the power of the loadstone, which attract to themselves iron. levinus lemnius describes and praises the compass[ ], and infers its antiquity on certain grounds; he does not divulge the hidden miracle which he propounds. in the kingdom { } of naples the amalfians were the first (so it is said) to construct the mariners' compass: and as flavius blondus says the amalfians[ ] boast, not without reason, that they were taught by a certain citizen, johannes goia, in the year thirteen hundred after the birth of christ. that town is situated in the kingdom of naples not far from salerno, near the promontory of minerva; and charles v. bestowed that principality on andrea doria, that great admiral, on account of his signal naval services. indeed it is plain that no invention of man's device has ever done more for mankind than the compass: some notwithstanding consider that it was discovered by others previously and used in navigation, judging from ancient writings and certain arguments and conjectures. the knowledge of the little mariners' compass seems to have been brought into italy by paolo, the venetian[ ], who learned the art of the compass in the chinas about the year mcclx.; yet i do not wish the amalfians to be deprived of an honour so great as that of having first made the construction common in the mediterranean sea. goropius[ ] attributes the discovery to the cimbri or teutons, forsooth because the names of the thirty-two winds inscribed on the compass are pronounced in the german tongue by all ship-masters, whether they be french, british, or spaniards; but the italians describe them in their own vernacular. some think that solomon, king of judæa, was acquaint with the use of the mariners' compass, and made it known to his ship-masters in the long voyages when they brought back such a power of gold from the west indies: whence also, from the hebrew word _parvaim_[ ], arias montanus maintains that the gold-abounding regions of peru are named but it is more likely to have come from the coast of lower Æthiopia, from the region of cephala, as others relate. yet that account seems to be less true, inasmuch as the phoenicians, on the frontier of judæa, who were most skilled in navigation in former ages (a people whose talents, work, and counsel solomon made use of in constructing ships and in the actual expeditions, as well as in other operations), were ignorant of magnetick aid, the art of the mariners' compass: for had it been in use amongst them, without doubt the greeks and also italians and all barbarians would have understood a thing so necessary and made famous by common use; nor could matters of much repute, very easily known, and so highly requisite ever have perished in oblivion; but either the learning would have been handed down to posterity, or some memorial of it would be extant in writing. sebastian cabot was the first to discover that the magnetick iron varied[ ]. gonzalus oviedus[ ] is the first to write, as he does in the _historia_, that in the south of the azores it does not vary. fernelius in his book _de abditis rerum causis_ says that in the loadstone there is a hidden and abstruse cause, elsewhere calling it celestial; and he brings forth nothing but the unknown by means of what is still more unknown. { } for clumsy, and meagre, and pointless is his inquiry into hidden causes. the ingenious fracastorio, a distinguished philosopher, in seeking the reason for the direction of the loadstone, feigns hyperborean magnetick mountains attracting magnetical things of iron: this view, which has found acceptance in part by others, is followed by many authors and finds a place not in their writings only, but in geographical tables, marine charts, and maps of the globe: dreaming, as they do, of magnetick poles and huge rocks, different from the poles of the earth. more than two hundred years earlier than fracastorio there exists a little work, fairly learned for the time, going under the name of one peter peregrinus[ ], which some consider to have originated from the views of roger bacon, the englishman of oxford: in which book causes for magnetick direction are sought from the poles of the heaven and from the heaven itself. from this peter peregrinus, johannes taisnier of hainault[ ] extracted materials for a little book, and published it as new. cardan talks much of the rising of the star in the tail of the greater bear, and has attributed to its rising the cause of the variation: supposing that the variation is always the same, from the rising of the star. but the difference of the variation according to the change of position, and the changes which occur in many places, and are even irregular in southern regions, preclude the influence of one particular star at its northern rising. the college of coimbra[ ] seeks the cause in some part of the heaven near the pole: scaliger in section cxxxi. of his _exercitationes_ on cardan suggests a heavenly cause unknown to himself, and terrestrial loadstones nowhere yet discovered. a cause not due to those sideritic mountains named above, but to that power which fashioned them, namely that portion of the heaven which overhangs that northern point. this view is garnished with a wealth of words by that erudite man, and crowned with many marginal subtilities; but with reasonings not so subtile. martin cortes[ ] considers that there is a place of attraction beyond the poles, which he judges to be the moving heavens. one bessardus[ ], a frenchman, with no less folly notes the pole of the zodiack. jacobus severtius[ ], of paris, while quoting a few points, fashions new errors as to loadstones of different parts of the earth being different in direction: and also as to there being eastern and western parts of the loadstone. robert norman[ ], an englishman, fixes a point and region respective, not attractive; to which the magnetical iron is collimated, but is not itself attracted. franciscus maurolycus[ ] treats of a few problems on the loadstone, taking the trite views of others, and avers that the variation is due to a certain magnetical island mentioned by olaus magnus[ ]. josephus acosta[ ], though quite ignorant about the loadstone, nevertheless pours forth vapid talk upon the loadstone. livio sanuto[ ] in his italian _geographia_, discusses at length the question whether the prime magnetick { } meridian and the magnetick poles are in the heavens or in the earth; also about an instrument for finding the longitude: but through not understanding magnetical nature, he raises nothing but errors and mists in that so important notion. fortunius affaytatus[ ] philosophizes foolishly enough on the attraction of iron, and its turning to the poles. most recently, baptista porta[ ], no ordinary philosopher, in his _magia naturalis_, has made the seventh book a custodian and distributor of the marvels of the loadstone; but little did he know or ever see of magnetick motions; and some things that he noted of the powers which it manifested, either learned by him from the reverend maestro paolo, the venetian[ ], or evolved from his own vigils, were not so well discovered or observed; but abound in utterly false experiments, as will be clear in due place: still i deem him worthy of high praise for having attempted so great a subject (as he has done with sufficient success and no mean result in many other instances), and for having given occasion for further research. all these philosophizers of a previous age, philosophizing about attraction from a few vague and untrustworthy experiments, drawing their arguments from the hidden causes of things; and then, seeking for the causes of magnetick directions in a quarter of the heavens, in the poles, the stars, constellations, or in mountains, or rocks, space, atoms, attractive or respective points beyond the heavens, and other such unproven paradoxes, are whole horizons wrong, and wander about blindly. and as yet we have not set ourselves to overthrow by argument those errors and impotent reasonings of theirs, nor many other fables told about the loadstone, nor the superstitions of impostors and fabulists: for instance, franciscus rueus'[ ] doubt whether the loadstone were not an imposture of evil spirits: or that, placed underneath the head of an unconscious woman while asleep, it drives her away from the bed if an adulteress: or that the loadstone is of use to thieves by its fume and sheen, being a stone born, as it were, to aid theft: or that it opens bars and locks, as serapio[ ] crazily writes: or that iron held up by a loadstone, when placed in the scales, added nothing to the weight of the loadstone, as though the gravity of the iron were absorbed by the force of the stone: or that, as serapio and the moors relate, in india there exist certain rocks of the sea abounding in loadstone, which draw out all the nails of the ships which are driven toward them, and so stop their sailing; which fable olaus magnus[ ] does not omit, saying that there are mountains in the north of such great powers of attraction, that ships are built with wooden pegs, lest the iron nails should be drawn from the timber as they passed by amongst the magnetick crags. nor this: that a white loadstone may be procured as a love potion: or as hali abbas[ ] thoughtlessly reports, that if held in the hand it will cure gout and spasms: or that it makes one acceptable and in favour with princes, or eloquent, as pictorio[ ] has { } sung; or as albertus magnus[ ] teaches, that there are two kinds of loadstones, one which points to the north, the other to the south: or that iron is directed toward the northern stars by an influence imparted by the polar stars, even as plants follow the sun, as heliotrope does: or that there is a magnet-stone situated under the tail of the greater bear, as lucas gauricus the astrologer stated: he would even assign the loadstone, like the sardonyx and onyx, to the planet saturn, yet at the same time he assigns it with the adamant, jasper, and ruby, to mars; so that it is ruled by two planets. the loadstone moreover is said by him to pertain to the sign virgo; and he covers many such shameful pieces of folly with a veil of mathematical erudition. such as that an image of a bear is engraved on a loadstone when the moon faces towards the north, so that when hung by an iron wire it may conciliate the influence of the celestial bear, as gaudentius merula[ ] relates: or that the loadstone drew iron and directed it to the north, because it is superior in rank to iron, at the bear, as ficinus writes, and merula repeats: or that by day it has a certain power of attracting iron, but by night the power is feeble, or rather null: or that when weak and dulled the virtue is renewed by goats' blood, as ruellius[ ] writes: or that goats' blood sets a loadstone free from the venom of a diamond, so that the lost power is revived when bathed in goats' blood by reason of the discord between that blood and the diamond: or that it removed sorcery from women, and put to flight demons, as arnaldus de villanova dreams: or that it has the power to reconcile husbands to their wives, or to recall brides to their husbands, as marbodeus gallus[ ], chorus-leader of vanities, teaches: or that in a loadstone pickled in the salt of a sucking fish[ ] there is power to pick up gold which has fallen into the deepest wells, according to the narratives of cælius calcagninus. with such idle tales and trumpery do plebeian philosophers delight themselves and satiate readers greedy for hidden things, and unlearned devourers of absurdities: but after the magnetick nature shall have been disclosed by the discourse that is to follow, and perfected by our labours and experiments, then will the hidden and abstruse causes of so great an effect stand out, sure, proven, displayed and demonstrated; and at the same time all darkness will disappear, and all error will be torn up by the roots and will lie unheeded; and the foundations of a grand magnetick philosophy which have been laid will appear anew, so that high intellects may be no further mocked by idle opinions. some learned men there are who in the course of long voyages have observed the differences of magnetick variation: the most scholarly thomas hariot[ ], robert hues, edward wright, abraham kendall, all englishmen; others there are who have invented and produced magnetical instruments, and ready methods of observation, indispensable for sailors and to those travelling afar: { } as william borough[ ] in his little book on the _variation of the compass_ or magneticall needle, william barlowe[ ] in his _supply_, robert norman in his _newe attractive_. and this is that robert norman[ ] (a skilful seaman and ingenious artificer) who first discovered the declination of the magnetick needle. many others i omit wittingly; modern frenchmen, germans, and spaniards, who in books written for the most part in their native tongues either misuse the placets of others, and send them forth furbished with new titles and phrases as tricky traders do old wares with meretricious ornaments; or offer something not worthy of mention even: and these lay hands on some work filched from other authors and solicit some one as their patron, or go hunting after renown for themselves among the inexperienced and the young; who in all branches of learning are seen to hand on errors and occasionally add something false of their own. * * * * * chap. ii. magnet stone, of what kind it is, and its _discovery._ loadstone, the stone which is commonly called the magnet, derives its name either from the discoverer (though he was not pliny's fabulous herdsman[ ], quoted from nicander, the nails of whose shoes and the tip of whose staff stuck fast in a magnetick field while he pastured his flocks), or from the region of magnesia in macedonia, rich in loadstones: or else from the city magnesia in ionia in afia minor, near the river mæander. hence lucretius says, _the magnet's name the observing grecians drew_ _from the magnetick region where it grew._ it is called heraclean from the city heraclea, or from the invincible hercules, on account of the great strength and domination and power which there is in iron of subduing all things: it is also called _siderite_, as being of iron; being not unknown to the most ancient writers, to the greeks, hippocrates, and others, as also (i believe) to jewish and egyptian writers; for in the oldest mines of iron, the most famous in asia, the loadstone was often dug out with its uterine brother, iron. and if the tales be true which are told of the people of the chinas, they were not unacquainted in primitive times with magnetical experiments, for even amongst { } them the finest magnets of all are still found. the egyptians, as manetho relates, gave it the name os ori: calling the power which governs the turning of the sun orus, as the greeks call it apollo. but later by euripides, as narrated by plato, it was designated under the name of magnet. by plato in the _io_, nicander of colophon, theophrastus, dioscorides, pliny, solinus, ptolemy, galen, and other investigators of nature it was recognized and commended; such, however, is the variety of magnets and their points of unlikeness in hardness, softness, heaviness, lightness, density, firmness, and friability of substance: so great and manifold are the differences in colour and other qualities, that they have not handed down any adequate account of it, which therefore was laid aside or left imperfect by reason of the unfavourable character of the time; for in those times varieties of specimens and foreign products never before seen were not brought from such distant regions by traders and mariners as they have been lately, and now that all over the globe all kinds of merchandise, stones, woods, spices, herbs, metals, and ore in abundance are greedily sought after: neither was metallurgy so generally cultivated in a former age. there is a difference in vigour; as whether it is male or female: for it was thus that the ancients used often to distinguish many individuals of the same species. pliny quotes from sotacus five kinds; those from Æthiopia, macedonia, boeotia, the troad, and asia, which were especially known to the ancients: but we have posited as many kinds of loadstones as there are in the whole of nature regions of different kinds of soil. for in all climates, in every province, on every soil, the loadstone is either found, or else lies unknown on account of its rather deep site and inaccesible position; or by reason of its weaker and less obvious strength it is not recognized by us while we see and handle it. to the ancients the differences were those of colour[ ], how they are red and black in magnesia and macedonia, in boeotia red rather than black, in the troad black, without strength: while in magnesia in asia they are white, not attracting iron, and resemble pumice-stone. a strong loadstone of the kind celebrated so often nowadays in experiments presents the appearance of unpolished iron, and is mostly found in iron mines: it is even wont to be discovered in an unbroken lode by itself: loadstones of this sort are brought from east india, china, and bengal, of the colour of iron, or of a dark blood or liver colour; and these are the finest, and are sometimes of great size, as though broken off a great rock, and of considerable weight; sometimes single stones, as it were, and entire: some of these, though of only one pound weight, can lift on high four ounces of iron or a half-pound or even a whole pound. red ones are found in arabia, as broad as a tile, not equal in weight to those brought from china, but strong and good: they are a little darker in the island of elba in the tuscan sea, and together with { } these also grow white ones, like some in spain in the mines of caravaca: but these are of lesser power. black ones also are found, of lower strength, such as those of the iron mines in norway and in sea-coast places near the strait of denmark. amongst the blue-black or dusky blue also some are strong and highly commended. other loadstones are of a leaden colour, fissile and not-fissile, capable of being split like slates in layers. i have also some like gray marble of an ashen colour, and some speckled like gray marble, and these take the finest polish. in germany there are some perforated like honeycombs, lighter than any others, and yet strong. those are metallick which smelt into the best iron; others are not easily smelted, but are burned up. there are loadstones that are very heavy, as also others very light; some are very powerful in catching up pieces of iron, while others are weaker and of less capacity, others so feeble and barren that they with difficulty attract ever so tiny a piece of iron and cannot repel an opposite magnetick. others are firm and tough, and do not readily yield to the artificer. others are friable. again, there are some dense and hard as emery, or loose-textured and soft as pumice; porous or solid; entire and uniform, or varied and corroded; now like iron for hardness, yea, sometimes harder than iron to cut or to file; others are as soft as clay. not all magnets can be properly called stones; some rather represent rocks; while others exist rather as metallick lodes; others as clods and lumps of earth. thus varied and unlike each other, they are all endowed, some more, some less, with the peculiar virtue. for they vary according to the nature of the soil, the different admixture of clods and humours, having respect to the nature of the region and to their subsidence in this last-formed crust of the earth, resulting from the confluence of many causes, and the perpetual alternations of growth and decline, and the mutations of bodies. nor is this stone of such potency rare; and there is no region wherein it is not to be found in some sort. but if men were to search for it more diligently and at greater outlay, or were able, where difficulties are present, to mine it, it would come to hand everywhere, as we shall hereafter prove. in many countries have been found and opened mines of efficacious loadstones unknown to the ancient writers, as for instance in germany, where none of them has ever asserted that loadstones were mined. yet since the time when, within the memory of our fathers, metallurgy began to flourish there, loadstones strong and efficacious in power have been dug out in numerous places; as in the black forest beyond helceburg; in mount misena not far from schwartzenberg[ ]; a fairly strong kind between schneeberg and annaberg in joachimsthal, as was noticed by cordus: also near the village of pela in franconia. in bohemia it occurs in iron mines in the lessa district and other places, as georgious agricola and several other men learned in metallurgy { } witness. in like manner in other countries in our time it is brought to light; for as the stone remarkable for its virtues is now famous throughout the whole world, so also everywhere every land produces it, and it is, so to speak, indigenous in all lands. in east india, in china, in bengal near the river indus it is common, and in certain maritime rocks: in persia, arabia, and the islands of the red sea; in many places in Æthiopia, as was formerly zimiri, of which pliny makes mention. in asia minor around alexandria and the troad; in macedonia, boeotia, in italy, the island of elba, barbary; in spain still in many mines as aforetime. in england quite lately a huge power of it was discovered in a mine belonging to adrian gilbert, gentleman[ ]; also in devonshire and the forest of dean; in ireland, too, norway, denmark, sweden, lapland, livonia, prussia, poland, hungary. for although the terrestrial globe, owing to the varied humours and natures of the soil arising from the continual succession of growth and decay, is in the lapse of time efflorescing through all its ambit deeper into its surface, and is girt about with a varied and perishable covering, as it were with a veil; yet out of her womb ariseth in many places an offspring nigher to the more perfect body and makes its way to the light of day. but the weak and less vigorous loadstones, enfeebled by the flow of humours, are visible in every region, in every strath. it is easy to discover a vast quantity of them everywhere without penetrating mountains or great depths, or encountering the difficulties and hardships of miners; as we shall prove in the sequel. and these we shall take pains so to prepare by an easy operation that their languid and dormant virtue shall be made manifest. it is called by the greeks[ ] [greek: heraklios], as by theophrastus, and [greek: magnêtis]; and [greek: magnês], as by euripides, as quoted by plato in the _io_: by orpheus[ ] too [greek: magnêosa], and [greek: sideritês] as though of iron: by the latins _magnes_, _herculeus_; by the french _aimant_[ ], corruptly from _adamant_; by the spaniards _piedramant_: by the italians _calamita_[ ]; by the english loadstone and adamant stone[ ], by the germans _magness_[ ] and _siegelstein_: among english, french, and spaniards it has its common name from adamant; perhaps because they were at one time misled by the name _sideritis_ being common to both: the magnet is called [greek: sideritês] from its virtue of attracting iron: the adamant is called [greek: sideritês] from the brilliancy of polished iron. aristotle designates it merely by the name of _the stone_:[ ] [greek: eoike de kai thalês ex hôn apomnêmoneuousi, kinêtikon ti tên psuchên hupolabein, eiper ton lithon ephê psuchên echein, hoti ton sidêron kinei]: _de anima_, lib. i. the name of magnet is also applied to another stone differing from siderite, having the appearance of silver; it is like amianth in its nature; and since it consists of laminæ (like specular stone)[ ], it differs in form: in german _katzensilber_ and _talke_[ ]. * * * * * { } chap. iii. the loadstone has parts distinct in their natural _power, & poles conspicuous for their property._ the stone itself manifests many qualities which, though known afore this, yet, not having been well investigated, are to be briefly indicated in the first place so that students may understand the powers of loadstone and iron, and not be troubled at the outset through ignorance of reasonings and proofs. in the heaven astronomers assign a pair of poles for each moving sphere: so also do we find in the terrestrial globe natural poles preeminent in virtue, being the points that remain constant in their position in respect to the diurnal rotation, one tending to the bears and the seven stars; the other to the opposite quarter of the heaven. in like manner the loadstone has its poles, by nature northern and southern, being definite and determined points set in the stone, the primary boundaries of motions and effects, the limits and governors of the many actions and virtues. however, it must be understood that the strength of the stone does not emanate from a mathematical point, but from the parts themselves, and that while all those parts in the whole belong to the whole, the nearer they are to the poles of the stone the stronger are the forces they acquire and shed into other bodies: these poles are observant of the earth's poles, move toward them, and wait upon them. magnetick poles can be found in every magnet, in the powerful and mighty (which antiquity used to call the masculine) as well as in the weak, feeble and feminine; whether its figure is due to art or to chance, whether long, flat, square, three-cornered, polished; whether rough, broken, or unpolished; always the loadstone contains and shows its poles. * but since the spherical form, which is also the most perfect, agrees best with the earth, being a globe, and is most suitable for use and experiment, we accordingly wish our principal demonstrations by the stone to be made with a globe-shaped magnet as being more perfect and adapted for the purpose. take, then, a powerful loadstone, solid, of a just size, uniform, hard, without flaw[ ]; make of it a globe upon the turning tool used for rounding crystals and some other stones, or with other tools as the material and firmness of the stone requires, for sometimes it is difficult to be worked. the stone thus perpared is a true, homogeneous offspring of the earth and of the same shape with it: artificially possessed of the orbicular form which nature granted from the beginning to the common mother earth: and it is a physical corpuscle imbued with many virtues, by { } means of which many abstruse and neglected truths in philosophy buried in piteous darkness may more readily become known to men. this round stone is called by us a [greek: mikrogê] or _terrella_[ ]. to find, then, the poles conformable to the earth's, take the round stone in hand, and place upon the stone a needle or wire of iron: the ends of the iron move upon their own centre and suddenly stand still. mark the stone with ochre or with chalk where the wire lies and sticks: move the middle or centre of the wire to another place, and so on to a third and a fourth, always marking on the stone along the length of the iron where it remains at rest: those lines show the meridian circles, or the circles like meridians on the stone, or terrella, all of which meet as will be manifest at the poles of the stone. by the circles thus continued the poles are made out, the boreal as well as the southern, and in the middle space betwixt these a great circle may be drawn for an æquator, just as astronomers describe them in the heavens and on their own globes, or as geographers do on the terrestrial globe: for that line so drawn on this our terrella is of various uses in our demonstrations and experiments magnetical. poles are also found in a round stone by a versorium, a piece of iron touched with a loadstone, and placed upon a needle or point firmly fixed on a foot so as to turn freely about in the following way:[ ] [illustration] on the stone a b the versorium is placed in such a way that the versorium may remain in equilibrium: you will mark with chalk the course of the iron when at rest: move the instrument to another spot, and again make note of the direction and aspect: do the same thing in several places, and from the concurrence of the lines of direction you will find one pole at the point a, the other at b. a versorium placed near the stone also indicates the true pole; when at right angles it eagerly beholds the stone and seeks the pole itself directly, and is turned in a straight line through the axis to the { } centre of the stone. for instance, the versorium d faces toward a and f, the pole and centre, whereas e does not exactly respect * either the pole a or the centre f[ ]. a bit of rather fine iron wire, of the length of a barley-corn, is placed on the stone, and is moved over the regions and surface of the stone, until it rises to the perpendicular[ ]: for it stands erect at the actual pole, whether boreal or austral; the further from the pole, the more it inclines from the vertical. the poles thus found you shall mark with a sharp file or gimlet. * * * * * chap. iiii. which pole of the stone is the boreal: & how it is _distinguished from the austral_. one pole of the earth turns toward the constellation of the cynosure, and constantly regards a fixed point in the heaven (except so far as it changes by the fixed stars being shifted in longitude, which motion we recognize as existing in the earth, as we shall hereafter prove): while the other pole turns to the opposite face of heaven, unknown to the ancients, now visible on long voyages, and adorned with multitudinous stars: in the same way the loadstone has the property and power of directing itself north and south (the earth herself consenting and contributing force thereto) according to the conformation of nature, which arranges the movements of the stone towards its native situation. which thing is proved thus: place a magnetick stone (after finding the poles) in a round wooden vessel, a bowl or dish, at the same time place it together with the vessel (like a sailor in a skiff) upon water in some large vessel or cistern, so that it may be able to float freely in the middle, nor touch the edge of it, and where the air is not disturbed by winds, which would thwart the natural movement of the stone. hereupon the stone placed as it were in a ship, in the middle of the surface of the still and unruffled water, will at once put itself in motion along with the vessel that carries it, and revolve circularly, until its austral pole points to the north, and its boreal pole to the south. for it reverts from the contrary position to the poles: and although by the first too-vehement impulse it over-passes the poles; yet after returning again and again, it rests at length at the poles, or at the meridian (unless because of local reasons it is diverted some little from those points, or from the meridional line, by some sort of variation[ ], the cause of which we will hereafter state). however often you move it away from its place, so often by virtue of nature's noble dower does it seek again those sure and { } determined goals; and this is so, not only if the poles have been disposed in the vessel evenly with the plane of the horizon, but also in the case of one pole, whether austral or boreal, being raised in the vessel ten, or twenty, or thirty, or fifty or eighty degrees, above * the plane of the horizon, or lowered beneath it: still you shall see the boreal part of the stone seek the south, and the austral part seek the north; so much so that if the pole of the stone shall be only one degree distant from the zenith and highest point of the heaven, in the case of a spherical stone, the whole stone revolves until the pole occupies its own site; though not in the absolutely direct line, it will yet tend toward those parts, and come to rest in the meridian of the directive action. with a like impulse too it is borne if the austral pole have been raised toward the upper quarters, the same as if the boreal had been exalted above the horizon. but it is always to be noted that, though there are various kinds of unlikeness in the stones, and one loadstone may far surpass another in virtue and efficiency; yet all hold to the same limits, and are borne toward the same points. further it is to be remembered * that all who before our time wrote of the poles of the stone, and all the craftsmen and navigators, have been very greatly in error in considering the part of the stone which tended to the north as the north pole of the stone, and that which verged toward the south, the south pole, which we shall hereafter prove to be false. so badly hitherto hath the whole magnetick philosophy been cultivated, even as to its foundation principles. * * * * * chap. v. loadstone seems to attract loadstone when in natural position: but repels it when in a contrary one, and brings _it back to order_. first of all we must declare, in familiar language, what are the apparent and common virtues of the stone; afterward numerous subtilities, hitherto abstruse and unknown, hidden in obscurity, are to be laid open, and the causes of all these (by the unlocking of nature's secrets) made evident, in their place, by fitting terms and devices. it is trite and commonplace that loadstone draws iron; in the same way too does loadstone attract loadstone. place the stone which you have seen to have poles clearly distinguished, and marked austral and boreal, in its vessel so as to float; and let the poles be rightly arranged with respect to the plane of the horizon, or, at any rate not much raised or awry: hold in your hand another stone the poles of which are also known; in { } such a way that its austral pole may be toward the boreal pole of the one that is swimming, and near it, sideways: for the floating stone forthwith follows the other stone (provided it be within its force and dominion) and does not leave off nor forsake it until it adhæres; unless by withdrawing your hand, you cautiously avoid contact. in like manner if you set the boreal pole of the one you hold in your hand opposite the austral pole of the swimming stone, they rush together and follow each other in turn. for contrary poles allure contrary. if, however, you apply in the same way the northern to the northern, and the austral to the austral pole, the one stone puts the other to flight, and it turns aside as though a pilot were pulling at the helm and it makes sail in the opposite ward as one that ploughs the sea, and neither stands anywhere, nor halts, if the other is in pursuit. for stone disposeth stone; the one turns the other around, reduces it to range, and brings it back to harmony with itself. when, however, they come together and are conjoined according to the order of nature, they cohære firmly mutually. for instance, if you were to set the boreal pole of that stone which is in your hand before the tropic of capricorn of a round floating loadstone (for it will be well to mark out on the round stone, that is the terrella, the mathematical circles as we do on a globe itself), or before any point between the æquator and the austral pole; at once the swimming stone revolves, and so arranges itself that its austral pole touches the other's boreal pole, and forms a close union with it. in the same way, again, at the other side of the æquator, with the opposite poles, you may produce similar results; and thus by this art and subtilty we exhibit attraction, repulsion, and circular motion for attaining a position of agreement and for declining hostile encounters. moreover 'tis in one and the same stone that we are thus able to demonstrate all these things and also how the same part of one stone may on division become either boreal or austral. let a d be an oblong stone, in which a is the northern, d the southern pole; cut this into two equal parts, then set part a in its vessel on the water[ ], so as to float. [illustration] { } and you will then see[ ] that a the northern point will turn to the south, as before; in like manner also the point d will move to the north, in the divided stone, as in the whole one. whereas, of the parts b and c, which were before continuous, and are now divided, the one is southern b, the other northern c. b draws c, desirous to be united, and to be brought back into its pristine continuity: for these which are now two stones were formed out of one: and for this cause c of the one turning itself to b of the other, they mutually attract each other, and when freed from obstacles and relieved of their own weight, as upon the surface of water, they run together and are conjoined. but if you direct the part or point a to c in the other stone, the one repels or turns away from the other: for so were nature perverted, and the form of the stone perturbed, a form that strictly keeps the laws which it imposed upon bodies: hence, when all is not rightly ordered according to nature, comes the flight of one from the other's perverse position and from the discord, for nature does not allow of an unjust and inequitable peace, or compromise: but wages war and exerts force to make bodies acquiesce well and justly. rightly arranged, therefore, these mutually attract each other; that is, both stones, the stronger as well as the weaker, run together, and with their whole forces tend to unity, a fact that is evident in all magnets, not in the Æthiopian only, as pliny supposed. the Æthiopian magnets if they be powerful, like those brought from china, because all strong ones show the effect more quickly and more plainly, attract more strongly in the parts nearest the pole, and turn about until pole looks directly at pole. the pole of a stone more persistently attracts and more rapidly seizes the corresponding part (which they term the adverse part) of another stone; for instance, north pulls south; just so it also summons iron with more vehemence, and the iron cleaves to it more firmly whether it have been previously excited by the magnet, or is untouched. for thus, not without reason hath it been ordained by nature, that the parts nearer to the pole should more firmly attract: but that at the pole itself should be the seat, the throne, as it were, of a consummate and splendid virtue, to which magnetical bodies on being brought are more vehemently attracted, and from which they are with utmost difficulty dislodged. so the poles are the parts which more particularly spurn and thrust away things strange and alien perversely set beside them. * * * * * { } chap. vi. loadstone attracts the ore of iron, as well as iron _proper, smelted and wrought_. principal and manifest among the virtues of the * magnet, so much and so anciently commended, is the attraction of iron; for plato states that the magnet, so named by euripides, allures iron, and that it not only draws iron rings but also indues the rings with power to do the same as the stone; to wit, draw other rings, so that sometimes a long chain of iron objects, nails or rings is formed, some hanging from others. the best iron (like that which is called _acies_ from its use, or _chalybs_ from the country of the chalybes) is best and strongly drawn by a powerful loadstone; whereas the less good sort, which is impure, rusty, and not thoroughly purged from dross, and not wrought in second furnaces, is more feebly drawn; and yet more weakly when covered and defiled with thick, greasy, and sluggish humours. it also draws ores of iron, those that are rich and of iron colour; the poorer and not so productive ores it does not attract, except they be prepared with some art. a loadstone loses some attractive virtue, and, as it were, pines away with age, if exposed too long to the open air instead of being laid in a case with filings or scales of iron. whence it should be buried in such materials; for there is nothing that plainly resists this exhaustless virtue which does not destroy the form of the body, or corrode it; not even if a thousand adamants were conjoined. nor do i consider that there is any such thing as the theamedes[ ], or that it has a power opposite to that of the loadstone. although pliny, that eminent man and prince of compilers (for it is what others had seen and discovered, not always or mainly his own observations, that he has handed down to posterity) has copied from others the fable now made familiar by repetition: that in india there are two mountains near the river indus; the nature of one being to hold fast all that is iron, for it consists of loadstone; the other's nature being to repel it, for it consists of the theamedes. thus if one had iron nails in one's boots, one could not tear away one's foot on the one mountain, nor stand still on the other. albertus magnus writes that a loadstone had been found in his day which with one part drew to itself iron, and repelled it with its other end; but albertus observed the facts badly; for every loadstone attracts with one end iron that has been touched with a loadstone, and drives it away with the other; and draws iron that been touched with a loadstone more powerfully than iron that has not been so touched. * * * * * { } chap. vii. what iron is, and of what substance, _and its uses._ for that now we have declared the origin and nature of the loadstone, we think it necessary first to add a history of iron and to indicate the hitherto unknown forces of iron, before this our discourse goes on to the explanation of magnetick difficulties and demonstrations, and to deal with the coitions and harmonies of loadstone with iron. iron is by all reckoned in the class of metals, and is a metal livid in colour, very hard, glows red-hot before it melts, being most difficult of fusion, is beaten out under the hammer, and is very resonant. chemists say that if a bed of fixed earthy sulphur be combined with fixed earthy quicksilver, and the two together are neither pure white but of a livid whiteness, if the sulphur prevail, iron is formed. for these stern masters of metals who by many inventions twisting them about, pound, calcine, dissolve, sublime, and precipitate, decide that this metal, both on account of the earthy sulphur and of the earthy mercury, is more truly a son of the earth than any other; they do not even think gold or silver, lead, tin, or copper itself so earthy; for that reason it is not smelted except in the hottest furnaces, with bellows; and when thus fused, on having again grown hard it is not melted again without heavy labour; but its slag with the utmost difficulty. it is the hardest of metals, subduing and breaking all things, by reason of the strong concretion of the more earthy matter. wherefore we shall better understand what iron is, when we shall declare what are the causes and substance of metals, in a different way from those who before our time have considered them. aristotle takes the material of the metals to be vapour. the chemists in chorus pronounce their actual elements to be sulphur and quicksilver. gilgil mauritanus gives it as ashes moistened with water. georgius agricola makes it out to be water and earth mixed; nor, to be sure, is there any difference between his opinion and the position taken by mauritanus. but ours is that metals arise and effloresce at the summits of the earth's globe, being distinguished each by its own form, like some of the other substances dug out of it, and all bodies around us. the earth's globe does not consist of ashes or inert dust. nor is fresh water an element, but a more simple consistency of evaporated fluids of the earth. unctuous bodies, fresh water devoid of properties, quicksilver and sulphur, none of these are principia of metals: these latter, { } things are the results of a different nature, they are neither constant nor antecedent in the course of the generation of metals. the earth emits various humours, not begotten of water nor of dry earth, nor from mixtures of these, but from the substance of the earth itself: these humours are not distinguished by contrary qualities or substance, nor is the earth a simple substance, as the peripateticks dream. the humours proceed from vapours sublimated from great depths; all waters are extracts and, as it were, exudations from the earth. rightly then in some measure does aristotle make out the matter of metals to be that exhalation which in continuance thickens in the lodes of certain soils: for the vapours are condensed in places which are less hot than the spot whence they issued, and by help of the nature of the soils and mountains, as in a womb, they are at fitting seasons congealed and changed into metals: but it is not they alone which form ores, but they flow into and enter a more solid material, and so form metals. so when this concreted matter has settled down in more temperate beds, it begins to take shape in those tepid places, just as seed in the warm womb, or as the embryo acquires growth: sometimes the vapour conjoins with suitable matter alone: hence some metals are occasionally though rarely dug up native, and come into existence perfect without smelting: but other vapours which are mixed with alien soils require smelting in the way that the ores of all metals are treated, which are rid of all their dross by the force of fires, and being fused flow out metallick, and are separated from earthy impurities but not from the true substance of the earth. but in so far as that it becomes gold, or silver, or copper, or any other of the existing metals, this does not happen from the quantity or proportion of material, nor from any forces of matter, as the chemists fondly imagine; but when the beds and region concur fitly with the material, the metals assume forms from the universal nature by which they are perfected; in the same manner as all the other minerals, plants, and animals whatever: otherwise the species of metals would be vague and undefined, which are even now turned up in such scanty numbers that scarce ten kinds are known. why, however, nature has been so stingy as regards the number of metals, or why there should be as many as are known to man, it is not easy to explain; though the simple-minded and raving astrologers refer the metals each to its own planet. but there is no agreement of the metals with the planets, nor of the planets with the metals, either in numbers or in properties. for what connexion is there of iron with mars? unless it be that from the former numerous instruments, particularly swords and engines of war, are fashioned. what has copper to do with venus? or how does tin, or how does spelter correspond with jupiter? they should rather be dedicated to venus. but this is old wives' talk. vapour is then a remote cause in the generation of the metals; the fluid condensed from { } vapours is a more proximate one, like the blood and semen in the generation of animals. but those vapours and juices from vapours pass for the most part into bodies and change them into marcasites and are carried into lodes (for we have numerous cases of wood so transmuted), the fitting matrices of bodies, where they are formed as metals. they enter most often into the truer and more homogeneal substance of the globe, and in the process of time a vein of iron results; loadstone is also produced, which is nought else than a noble kind of iron ore: and for this reason, and on account of its substance being singular, alien from all other metals, nature very rarely, if ever, mixes with iron any other metal, while the other metals are very often minutely mixed, and are produced together. now when that vapour or those juices happen to meet, in fitting matrices, with efflorescences deformed from the earth's homogenic substance, and with divers precipitates (the forms working thereto), the remainder of the metals are generated (a specifick nature affecting the properties in that place). for the hidden primordial elements of metals and stones lie concealed in the earth, as those of herbs and plants do in its outer crust. for the soil dug out of a deep well, where would seem to be no suspicion of a conception of seed, when placed on a very high tower, produces, by the incubation of sun and sky, green herbage and unbidden weeds; and those of the kind which grow spontaneously in that region, for each region produces its own herbs and plants, also its own metals. _[ ]here corn exults, and there the grape is glad,_ _here trees and grass unbidden verdure add._ _so mark how tmolus yields his saffrone store,_ _but ivory is the gift of indian shore;_ _with incense soft the softer shebans deal;_ _the stark chalybeans' element is steel:_ _with acrid castor reek the pontic wares,_ _epirus wins the palm of elian mares._ but what the chemists (as geber, and others) call fixed earthy sulphur in iron is nothing else than the homogenic earth-substance concreted by its own humour, amalgamated with a double fluid: a metallick humour is inserted along with a small quantity of the substance of the earth not devoid of humour. wherefore the common saying that in gold there is pure earth, but in iron mostly impure, is wrong; as though there were indeed such a thing as natural earth, and that the globe itself were (by some unknown process of refining) depurate. in iron, especially in the best iron, there is earth in its own nature true and genuine; in the other metals there is not so much earth as that in place of earth and precipitates there are consolidated and (so to speak) fixed salts, which are efflorescences of the globe, and which differ also greatly { } in firmness and consistency: in the mines their force rises up along with a twofold humour from the exhalations, they solidify in the underground spaces into metallic veins: so too they are also connate by virtue of their place and of the surrounding bodies, in natural matrices, and take on their specific forms. of the various constitutions of loadstones and their diverse substances, colours, and virtues, mention has been made before: but, now having stated the cause and origin of metals, we have to examine ferruginous matter not as it is in the smelted metal, but as that from which the metal is refined. quasi-pure iron is found of its proper colour and in its own lodes; still, not as it will presently be, nor as adapted for its various uses. it is sometimes dug up covered with white silex or with other stones. it is often the same in river sand, as in noricum. a nearly pure ore of iron is now often dug up in ireland, which the smiths, without the labours of furnaces, hammer out in the smithy into iron implements. in france iron is very commonly smelted out of a liver-coloured stone, in which are glittering scales; the same kind[ ] without the scales is found in england, which also they use for craftsmen's ruddle[ ]. in sussex in england[ ] is a rich dusky ore and also one of a pale ashen hue, both of which on being dried for a time, or kept in moderate fires, presently acquire a liver-colour; here also is found a dusky ore square-shaped with a black rind of greater hardness. an ore having the appearance of liver is often variously intermingled with other stones: as also with the perfect loadstone which yields the best of iron. there is also a rusty ore of iron, one of a leaden hue tending to black, one quite black, or black mixed with true cobalt: there is another sort mixed either with pyrites, or with sterile plumbago. one kind is also like jet, another like bloodstone. the emery used by armourers, and by glaziers for glass-cutting, called amongst the english emerelstone, by the germans smeargel, is ferruginous; albeit iron is extracted from it with difficulty, yet it attracts the versorium. it is now and then found in deep iron and silver diggings. thomas erastus says he had heard from a certain learned man of iron ores, of the colour of iron, but quite soft and fatty, which can be smoothed with the fingers like butter, out of which excellent iron can be smelted: somewhat the same we have seen found in england, having the aspect of spanish soap. besides the numberless kinds of stony ores, iron is extracted from clay, from clayey earth, from ochre, from a rusty matter deposited from chalybeate waters; in england iron is copiously extracted in furnaces often from sandy and clayey stones which appear to contain iron not more than sand, marl, or any other clay soils contain it. thus in aristotle's book _de mirabilibus auscultationibus_[ ], "there is said" (he states) "to be a peculiar formation of chalybean and misenian iron, for instance the sort collected from river gravel; some say { } that after being simply washed it is smelted in the furnace; others declare that it and the sediment which subsides after several washings are cast in and purified together by the fire; with the addition of the stone pyrimachus which is found there in abundance." thus do numerous sorts of things contain in their various substances notably and abundantly this element of iron and earth. however, there are many stones, and very common ones, found in every soil, also earths, and various and mixed materials, which do not hold rich substances, but yet have their own iron elements, and yield them to skilfully-made fires, yet which are left aside by metallick men because they are less profitable; while other soils give some show of a ferruginous nature, yet (being very barren) are hardly ever smelted down into iron; and being neglected are not generally known. manufactured irons differ very greatly amongst themselves. for one kind is tenacious in its nature, and this is the best; one is of medium quality: another is brittle, and this is the worst. sometimes the iron, by reason of the excellency of the ore, is wrought into steel, as to-day in noricum. from the finest iron, too, well wrought and purged from all dross, or by being plunged in water after heating, there issues what the greeks call [greek: stomôma]; the latins _acies;_ others _aciarium,_ such as was at times called syrian, parthian, noric, comese, spanish; elsewhere it is named from the water in which it is so often plunged, as at como in italy[ ], bambola and tarazona in spain. _acies_ fetches a much larger price than mere iron. and owing to its superiority it better accords with the loadstone, from which more powerful quality it is often smelted, and it acquires the virtues from it more quickly, retains them longer at their full, and in the best condition for magnetical experiments. after iron has been smelted in the first furnaces, it is afterward wrought by various arts in large worksteads or mills, the metal acquiring consistency when hammered with ponderous blows, and throwing off the dross. after the first smelting it is rather brittle and by no means perfect. wherefore with us (english) when the larger military guns are cast, they purify the metal from dross more fully, so that they may be stronger to withstand the force of the firing; and they do this by making it pass again (in a fluid state) through a chink, by which process it sheds its recremental matter. smiths render iron sheets tougher with certain liquids, and by blows of the hammer, and from them make shields and breastplates that defy the blows of battle-axes. iron becomes harder through skill and proper tempering, but also by skill turns out in a softer condition and as pliable as lead. it is made hard by the action of certain waters into which while glowing it is plunged, as at bambola and tarazona in spain: it grows soft again, either by the effect of fire alone, when without hammering and without water, it is left to cool by itself; or by that of grease into which it is plunged; or { } (that it may the better serve for various trades) it is tempered variously by being skilfully besmeared. baptista porta expounds this art in book of his _magia naturalis_. thus this ferric and telluric nature is included and taken up in various bodies of stones, ores, and earths; so too it differs in aspect, in form, and in efficiency. art smelts it by various processes, improves it, and turns it, above all material substances, to the service of man in trades and appliances without end. one kind of iron is adapted for breastplates, another serves as a defence against shot, another protects against swords and curved blades (commonly called scimitars), another is used for making swords, another for horseshoes. from iron are made nails, hinges, bolts, saws, keys, grids, doors, folding-doors, spades, rods, pitchforks, hooks, barbs, tridents, pots, tripods, anvils, hammers, wedges, chains, hand-cuffs, fetters, hoes, mattocks, sickles, baskets, shovels, harrows, planes, rakes, ploughshares, forks, pans, dishes, ladles, spoons, spits, knives, daggers, swords, axes, darts, javelins, lances, spears, anchors, and much ship's gear. besides these, balls, darts, pikes, breastplates, helmets, cuirasses, horseshoes, greaves, wire, strings of musical instruments, chairs, portcullises, bows, catapults, and (pests of human kind) cannon, muskets, and cannon-balls, with endless instruments unknown to the latins: which things i have rehearsed in order that it may be understood how great is the use of iron, which surpasses a hundred times that of all the other metals; and is day by day being wrought by metal-workers whose stithies are found in almost every village. for this is the foremost of metals, subserving many and the greatest needs of man, and abounds in the earth above all other metals, and is predominant. wherefore those chemists are fools[ ] who think that nature's will is to perfect all metals into gold; she might as well be making ready to change all stones to diamonds, since diamond surpasses all in splendour and hardness, because gold excels in splendour, gravity, and density, being invincible against all deterioration. iron as dug up is therefore, like iron that has been smelted, a metal, differing a little indeed from the primary homogenic terrestrial body, owing to the metallick humour it has imbibed; yet not so alien as that it will not, after the manner of refined matter, admit largely of the magnetick forces, and may be associated with that prepotent form belonging to the earth, and yield to it a due submission. * * * * * { } chap. viii. in what countries and districts iron _originates._ plenty of iron mines exist everywhere, both those of old time recorded in early ages by the most ancient writers, and the new and modern ones. the earliest and most important seem to me to be those of asia. for in those countries which abound naturally in iron, governments and the arts flourished exceedingly, and things needful for the use of man were discovered and sought after. it is recorded to have been found about andria, in the region of the chalybes near the river thermodon in pontus; in the mountains of palestine which face arabia; in carmania: in africa there was a mine of iron in the isle of meroe; in europe in the hills of britain, as strabo writes; in hither spain, in cantabria. among the petrocorii and cubi biturges[ ] (peoples of gaul), there were worksteads in which iron used to be wrought. in greater germany near luna, as recorded by ptolemy; gothinian iron is mentioned by cornelius tacitus; noric iron is celebrated in the verses of poets; and cretan, and that of euboea; many other iron mines were passed over by these writers or unknown to them; and yet they were neither poor nor scanty, but most extensive. pliny[ ] says that hither spain and all the district from the pyrenees is ferruginous, and on the part of maritime cantabria washed by the ocean (says the same writer) there is (incredible to relate) a precipitously high mountain wholly composed of this material. the most ancient mines were of iron rather than of gold, silver, copper or lead; since mainly this was sought because of the demand; and also because in every district and soil they were easy to find, not so deep-lying, and less beset by difficulties. if, however, i were to enumerate modern iron workings, and those of this age and over europe only, i should have to write a large and bulky volume, and sheets of paper would run short quicker than the iron, and yet for one sheet they could furnish a thousand worksteads. for amongst minerals, no material is so ample; all metals, and all stones distinct from iron, are outdone by ferric and ferruginous matter. for you will not readily find any region, and scarcely any country district over the whole of europe (if you search at all deeply), that does not either produce a rich and abundant vein of iron or some soil containing or slightly charged with ferruginous stuff; and that this is { } true any expert in the arts of metals and chemistry will easily find. beside that which has ferruginous nature, and the metallick lode, there is another ferric substance which does not yield the metal in this way because its thin humour is burnt out by fierce fires, and it is changed into an iron slag like that which is separated from the metal in the first furnaces. and of this kind is all clay and argillaceous earth, such as that which apparently forms a large part of the whole of our island of britain: all of which, if subjected very vehemently to intense heat, exhibits a ferric and metallick body, or passes into ferric vitreous matter, as can be easily seen in buildings in bricks baked from clay, which, when placed next the fires in the open kilns (which our folk call _clamps_)[ ] and burned, present an iron vitrification, black at the other end. moreover all those earths as prepared are drawn by the magnet, and like iron are attracted by it. so perpetual and ample is the iron offspring of the terrestrial globe. georgius agricola says that almost all mountainous regions are full of its ores, while as we know a rich iron lode is frequently dug in the open country and plains over nearly the whole of england and ireland; in no other wise than as, says he, iron is dug out of the meadows at the town of saga in pits driven to a two-foot depth. nor are the west indies without their iron lodes, as writers tell us; but the spaniards, intent upon gold, neglect the toilsome work of iron-founding, and do not search for lodes and mines abounding in iron. it is probable that nature and the globe of the earth are not able to hide, and are evermore bringing to the light of day, a great mass of inborn matter, and are not invariably obstructed by the settling of mixtures and efflorescences at the earth's surface. it is not only in the common mother (the terrestrial globe) that iron is produced, but sometimes also in the air from the earth's exhalations, in the highest clouds. it rained iron in lucania, the year in which m. crassus was slain. the tale is told, too, that a mass of iron, like slag, fell from the air in the nethorian forest, near grina, and they narrate that the mass was many pounds in weight; so that it could neither be conveyed to that place, on account of its weight, nor be brought away by cart, the place being without roads. this happened before the civil war waged between the rival dukes in saxony. a similar story, too, comes to us from avicenna. it once rained iron in the torinese[ ], in various places (julius scaliger telling us that he had a piece of it in his house), about three years before that province was taken over by the king. in the year in the country bordering on the river abdua (as cardan writes[ ] in his book _de rerum varietate_) there fell from the sky stones, one weighing pounds, another or pounds, of a rusty iron colour and remarkably hard. these occurrences being rare are regarded as portents, like the showers of earth and stones mentioned in roman history. but that it ever rained other metals is not { } recorded; for it has never been known to rain from the sky gold, silver, lead, tin, or spelter[ ]. copper, however, has been at some time noticed to fall from the sky, and this is not very unlike iron; and in fact cloud-born iron of this sort, or copper, are seen to be imperfectly metallick, incapable of being cast in any way, or wrought with facility. for the earth hath of her store plenty of iron in her highlands, and the globe contains the ferric and magnetick element in rich abundance. the exhalations forcibly derived from such material may well become concreted in the upper air by the help of more powerful causes, and hence some monstrous progeny of iron be begotten. * * * * * chap. ix. iron ore attracts iron ore. from various substances iron (like all the rest of the * metals) is extracted: such substances being stones, earth, and similar concretions which miners call veins because it is in veins[ ], as it were, that they are generated. we have spoken above of the variety of these veins. if a properly coloured ore of iron and a rich one (as miners call it) is placed, as soon as mined, upon water in a bowl or any small vessel (as we have shown before in the case of a loadstone), it is attracted by a similar piece of ore brought near by hand, yet not so powerfully and quickly as one loadstone is drawn by another loadstone, but slowly and feebly. ores of iron that are stony, cindery, dusky, red, and several more of other colours, do not attract one another mutually, nor are they attracted by the loadstone itself, even by a strong one, no more than wood, or lead, silver, or gold. take those ores and burn, or rather roast them, in a moderate fire, so that they are not suddenly split up, or fly asunder, keeping up the fire ten or twelve hours, and gently increasing it, then let them grow cold, skill being shown in the direction in which they are placed: these ores thus prepared a loadstone will now draw, and they now show a mutual sympathy, and when skilfully arranged run together by their own forces. * * * * * { } chap. x. * iron ore has poles, and acquires them, and settles _itself toward the poles of the universe_. deplorable is man's ignorance in natural science, and modern philosophers, like those who dream in darkness, need to be aroused, and taught the uses of things and how to deal with them, and to be induced to leave the learning sought at leisure from books alone, and that is supported only by unrealities of arguments and by conjectures. for the knowledge of iron (than which nothing is in more common use), and that of many more substances around us, remains unlearned; iron, a rich ore of which, placed in a vessel upon water, by an innate property of its own directs itself, just like the loadstone, north and south, at which points it rests, and to which, if it be turned aside, it reverts by its own inherent vigour. but many ores, less perfect in their nature, which yet contain amid stone or earthy substances plenty of iron, have no such motion; but when prepared by skilful treatment in the fires, as shown in the foregoing chapter, they acquire a polar vigour (which we call verticity[ ]); and not only the iron ores in request by miners, but even earth merely charged with ferruginous matter, and many rocks, do in like manner tend and lean toward those portions of the heavens, or more truly of the earth, if they be skilfully placed, until they reach the desired location, in which they eagerly repose. * * * * * { } chap. xi. * wrought iron, not excited by a loadstone, _draws iron_. from the ore, which is converted, or separated, partly into metal, partly into slag, by the intense heat of fires, iron is smelted in the first furnaces in a space of eight, ten, or twelve hours, and the metal flows away from the dross and useless matter, forming a large and long mass, which being subjected to a sharp hammering is cut into parts, out of which when reheated in the second hearth of the forge, and again placed on the anvil, the smiths fashion quadrangular lumps, or more specially bars which are bought by merchants and blacksmiths, from which in smithies usually it is the custom to fashion the various implements. this iron we term _wrought_, and its attraction by the loadstone is manifest to all. but we, by more carefully trying everything[ ], have found out that iron merely, by itself alone, not excited by any loadstone, not charged by any alien forces, attracts other iron; though it does not so eagerly snatch and suddenly pluck at it as would a fairly strong loadstone; this you may know thus: a small piece of cork, the size of a hazel-nut, rounded, is traversed by an iron wire up to the middle of the wire: when set swimming on still water apply to one end of it, close (yet so as not to touch), the end of another iron wire; and wire draws wire, and one follows the other when slowly drawn back, and this goes on up to the proper boundaries. let a be the cork with the iron wire, b one end of it raised a little above the surface of the water, c the end of the second wire, showing the way in which b is drawn by c. you may prove it in another way in a larger body. let a long bright iron rod (such as is made for hangings and window curtains) be hung in balance by a slender silken cord: to one end of this as it rests in the air bring a small oblong mass of polished iron, with its proper { } end at the distance of half a digit. the balanced iron turns itself to the mass; do you with the same quickness draw back the mass in your hand in a circular path about the point of equilibrium of the suspension; the end of the balanced iron follows after it, and turns in an orbit. [illustration] * * * * * chap. xii. * a long piece of iron, even though not excited by a _loadstone, settles itself toward north and south._ every good and perfect piece of iron, if drawn out in length, points north and south, just as the loadstone or iron rubbed with a magnetical body does; a thing that our famous philosophers have little understood, who have sweated in vain to set forth the magnetick virtues and the causes of the friendship of iron for the stone. you may experiment with either large or small iron works, and either in air or in water. a straight piece of iron six feet long of the thickness of your finger is suspended (in the way described in the foregoing chapter) in exact æquipoise by a strong and slender silken cord. but the cord should be cross-woven of several silk filaments, not twisted simply in one way; and it should be in a small chamber with all doors and windows closed, that the wind may not enter, nor the air of the room be in any way disturbed; for which reason it is not expedient that the trial should be made on windy days, or while a storm is brewing. for thus it freely follows its bent, and slowly moves until at length, as it rests, it points with its ends north and south, just as iron touched with a loadstone does in shadow-clocks, and in compasses, and in the mariners' compass. you will be able, if curious enough, to balance all at the same time by fine threads a number of small rods, or iron wires, or long pins with which women knit stockings; you will see that all of them at the same time are in accord, unless there be some error in this delicate operation: for unless you prepare everything fitly and skilfully, the labour will be void. make trial of this thing in water also, which is done both more certainly and more easily. let an iron wire two or three digits long, more or less, be passed through a round cork, so that it may just float upon water; and as soon as you have committed it to the waves, it turns upon its own centre, and one end tends to the north, the other to the south; the causes { } of which you will afterwards find in the laws of the direction. this too you should understand, and hold firmly in memory, that * as a strong loadstone, and iron touched with the same, do not invariably point exactly to the true pole but to the point of the variation; so does a weaker loadstone, and so does the iron, which directs itself by its own forces only, not by those impressed by the stone; and so every ore of iron, and all bodies naturally endowed with something of the iron nature, and prepared, turn to the same point of the horizon, according to the place of the variation in that particular region (if there be any variation therein), and there abide and rest. * * * * * chap. xiii. * wrought iron has in itself certain parts boreal and austral: a magnetick vigour, verticity, and determinate _vertices, or poles_. iron settles itself toward the north and south; not with one and the same point toward this pole or that: for one end of the piece of ore itself and one extremity also of a wrought-iron wire have a sure and constant destination to the north, the other to the south, whether the iron hang in the air, or float on water, be the iron large rods or thinner wires. even if it be a little rod, or a wire ten or twenty or more ells in length; one end as a rule is boreal, the other austral. if you cut off part of that wire, and if the end of that divided part were boreal, the other end (which was joined to it) will be austral. thus if you divide it into several parts, before making an experiment on the surface of water, you can recognize the vertex[ ]. in all of them a boreal end draws an austral and repels a boreal, and contrariwise, according to the laws magnetical. yet herein wrought iron differs from the loadstone and from its own ore, inasmuch as in an iron ball of any size, such as those used for artillery or cannon, or bullets used for carbines or fowling-pieces, verticity is harder to acquire and is less apparent than in a piece of loadstone, or of ore itself, or than in a round loadstone. but in long and extended pieces of iron a power is at once discerned; the causes of which fact, and the methods by which it acquires its verticity and its poles without use of a loadstone, as well as the reasons for all the other obscure features of verticity, we shall set forth in describing the motion of direction. * * * * * { } chap. xiiii. concerning other powers of loadstone, and its _medicinal properties_. dioscorides prescribes loadstone to be given with sweetened water, three scruples' weight, to expel gross humours. galen writes that a like quantity of bloodstone avails. others relate that loadstone perturbs the mind and makes folk melancholick, and mostly kills. gartias ab horto[ ] thinks it not deleterious or injurious to health. the natives of east india tell us, he says, that loadstone taken in small doses preserves youth. on which account the aged king, zeilam, is said to have ordered the pans in which his victuals were cooked to be made of loadstone. the person (says he) to whom this order was given told me so himself. there are many varieties of loadstone produced by differences in the mingling of earths, metals, and juices; hence they are altogether unlike in their virtues and effects, due to propinquities of places and of agnate bodies, and arising from the pits themselves as it were from the matrices being soul. one loadstone is therefore able to purge the stomach, and another to check purging, to cause by its fumes a serious shock to the mind, to produce a gnawing at the vitals, or to bring on a grave relapse; in case of which ills they exhibit gold and emerald, using an abominable imposture for lucre. pure loadstone may, indeed, be not only harmless, but even able to correct an over-fluid and putrescent state of the bowels and bring them back to a better temperament; of this sort usually are the oriental magnets from china, and the denser ones from bengal, which are neither misliking nor unpleasant to the actual senses. plutarch and claudius ptolemy[ ], and all the copyists since their time, think that a loadstone smeared with garlick does not allure iron. hence some suspect that garlick is of avail against any deleterious power of the magnet: thus in philosophy many false and idle conjectures arise from fables and falsehoods. some physicians[ ] have that a loadstone has power to extract the iron of an arrow from the human body. but it is when whole that the loadstone draws, not when pulverized and formless, buried in plasters; for it does not attract by reason of its material, but is rather adapted for the healing of open wounds, by reason of exsiccation, closing up and drying the sore, an effect by which the arrow-heads would rather be retained in the wounds. thus vainly and preposterously do the sciolists { } look for remedies while ignorant of the true causes of things. the application of a loadstone for all sorts of headaches no more cures them (as some make out) than would an iron helmet or a steel cap. to give it in a draught to dropsical persons is an error of the ancients, or an impudent tale of the copyists, though one kind of ore may be found which, like many more minerals, purges the stomach; but this is due to some defect of that ore and not to any magnetick property. nicolaus puts a large quantity of loadstone into his divine plaster[ ], just as the augsburgers do into a black plaster[ ] for fresh wounds and stabs; the virtue of which dries them up without smart, so that it proves an efficacious medicament. in like manner also paracelsus to the same end mingles it in his plaster for stab wounds[ ]. * * * * * chap. xv. the medicinal virtue of iron.[ ] not foreign to our present purpose will it be to treat briefly also of the medicinal virtue of iron: for it is a prime remedial for some diseases of the human body, and by its virtues, both those that are natural and those acquired by suitable preparation, it works marvellous changes in the human body, so that we may the more surely recognize its nature through its medicinal virtue and through certain manifest experiments. so that even those tyros in medicine who abuse this most famous medicament may learn to prescribe it with better judgment for the healing of the sick, and not, as too often they use it, to their harm. the best iron, stomoma, or chalybs, acies, or aciarium, is reduced to a fine powder by a file; the powder is steeped in the sharpest vinegar, and dried in the sun, and again soused in vinegar, and dried; afterwards it is washed in spring water or other suitable water, and dried; then for the second time it is pulverized and reduced on porphyry, passed through a very fine sieve, and put back for use. it is given chiefly in cases of laxity and over-humidity of the liver, in enlargement of the spleen, after due evacuations; for which reason it restores young girls when pallid, sickly, and lacking colour, to health and beauty; since it is very siccative, and is astringent without harm. but some who in every internal malady always talk of obstruction { } of the liver and spleen, think it beneficial in those cases because it removes obstructions, mainly trusting to the opinions of certain arabians[ ]: wherefore they administer it to the dropsical and to those suffering from tumour of the liver or from chronic jaundice, and to persons troubled with hypochondrical melancholia or any stomachic disorder, or add it to electuaries, without doubt to the grievous injury of many of their patients. fallopius commends it prepared in his own way for tumours of the spleen, but is much mistaken; for loadstone is pre-eminently good for spleens relaxed with humour, and swollen; but it is so far from curing spleens thickened into a tumour that it mightily confirms the malady. for those drugs which are strong siccatives and absorb humour force the viscera when hardened into a tumour more completely into a quasi-stony body. there are some who roast iron in a closed oven with fierce firing, and burn it strongly, until it turns red, and they call this saffron of mars; which is a powerful siccative, and more quickly penetrates the intestines. moreover they order violent exercise, that the drug may enter the viscera while heated and so reach the place affected; wherefore also it is reduced to a very fine flour; otherwise it only sticks in the stomach and in the chyle and does not penetrate to the intestines. as a dry and earthy medicament, then, it is shown by the most certain experiments to be, after proper evacuations, a remedy for diseases arising from humour (when the viscera are charged and overflowing with watery rheum). prepared steel is a medicament proper for enlarged spleen. iron waters too are effectual in reducing the spleen, although as a rule iron is of a frigid and astringent efficiency, not a laxative; but it effects this neither by heat nor by cold, but from its own dryness when mixed with a penetrative fluid: it thus disperses the humour, thickens the villi, hardens the tissues, and contracts them when lax; while the inherent heat in the member thus strengthened, being increased in power, dissipates what is left. whereas if the liver be hardened and weakened by old age or a chronic obstruction, or the spleen be shrivelled and contracted to a schirrus, by which troubles the fleshy parts of the limbs grow flaccid, and water under the skin invades the body, in the case of these conditions the introduction of iron accelerates the fatal end, and considerably increases the malady. amongst recent writers there are some who in cases of drought of the liver prescribe, as a much lauded and famous remedy, the electuary of iron slag, described by rhazes[ ] in his ninth book _ad almansorem_, chap. , or prepared filings of steel; an evil and deadly advice: which if they do not some time understand from our philosophy, at least everyday experience, and the decline and death of their patients, will convince them, even the sluggish and lazy. whether iron be warm or cold is variously contended by { } many. by manardus, curtius, fallopius and others, many reasons are adduced on both sides; each settles it according to his own sentiment. some make it to be cold, saying that iron has the property of refrigerating, because aristotle in his _meteorologica_ would put iron in the class of things which grow concreted in cold by emission of the whole of their heat: galen, too, says that iron has its consistency from cold; also that it is an earthy and dense body. further that iron is astringent, also that chalybeate water quenches thirst: and they adduce the cooling effect of thermal iron waters. others, however, maintain that it is warm, because of hippocrates making out that waters are warm which burst forth from places where iron exists. galen says that in all metals there is considerable substance, or essence, of fire. paolo[ ] affirms that iron waters are warm. rhazes will have it that iron is warm and dry in the third degree. the arabians think that it opens the spleen and liver; wherefore also that iron is warm. montagnana recommends it in cold affections of the uterus and stomach. thus do the smatterers cross swords together, and puzzle inquiring minds by their vague conjectures, and wrangle for trifles as for goats' wool, when they philosophize, wrongly allowing and accepting properties: but these matters will appear more plainly by and by when we begin to discuss the causes of things; the clouds being dispersed that have so darkened all philosophy. filings, scales, and slag of iron are, as avicenna makes out, not wanting in deleterious power (haply when they are not well prepared or are taken in larger quantity than is fit), hence they cause violent pain in the bowels, roughness of the mouth and tongue, marasmus, and shrivelling of the limbs. but avicenna wrongly[ ] and old-womanishly makes out that the proper antidote to this iron poison is loadstone to the weight of a drachm taken as a draught in the juice of mercurialis or of beet; for loadstone is of a twofold nature, usually malefiant and pernicious, nor does it resist iron, since it attracts it; nor when drunk in a draught in the form of powder does it avail to attract or repel, but rather inflicts the same evils. * * * * * { } chap. xvi. that loadstone & iron ore are the same, but iron an _extract from both, as other metals are from their own_ ores; & that all magnetick virtues, though _weaker, exist in the ore itself & in smelted iron._ hitherto we have declared the nature & powers of the loadstone, & also the properties & essence of iron; it now remains to show their mutual affinities, & kinship, so to speak, & how very closely conjoined these substances are. at the highest part of the terrestrial globe, or at its perishable surface & rind, as it were, these two bodies usually originate & are produced in one and the same matrix, as twins in one mine. strong loadstones are dug up by themselves, weaker ones too have their own proper vein. both are found in iron mines. iron ore most often occurs alone, without strong loadstone (for the more perfect are rarely met with). strong loadstone is a stone resembling iron; out of it is usually smelted the finest iron, which the greeks call _stomoma_, the latins _acies_, the barbarians (not amiss) _aciare_, or _aciarium_. this same stone draws, repels, controls other loadstones, directs itself to the poles of the world, picks up smelted iron, and works many other wonders, some already set forth by us, but many more which we must demonstrate more fully. a weaker loadstone, however, will exhibit all these powers, but in a lesser degree; while iron ore, & also wrought iron (if they have been prepared) show their strength in all magnetick experiments not less than do feeble and weak * loadstones; & an inert piece of ore, & one possessed of no magnetick properties, & just thrown out[ ] of the pit, when roasted in the fire & prepared with due art (by the elimination of humours & foreign excretions) awakes, and becomes in power & potency a magnet, * occasionally a stone or iron ore is mined, which attracts forthwith without being prepared: for native iron of the right colour attracts and governs iron magnetically. one form then belongs to the one mineral, one species, one self-same essence. for to me there seems to be a greater difference, & unlikeness, between the strongest { } loadstone, & a weak one which scarce can attract a single chip of iron; between one that is stout, strong, metallick, & one that is soft, friable, clayey; amidst such variety of colour, substance, quality, & weight; than there is on the one hand between the best ore, rich in iron, or iron that is metallick from the beginning, and on the other the most excellent loadstone. usually, too, there are no marks to distinguish them, and even metallurgists cannot decide between them, because they agree together in all respects. moreover we see that the best loadstone and the ore of iron are both as it were distressed by the same maladies & diseases, both run to old age in the same way & exhibit the same marks of it, are preserved & keep their properties by the same remedies & safeguards; & yet again the one increases the potency of the other, & by artfully devised adjuncts marvellously intensifies, & exalts it. for both are impaired by the more acrid juices as by poisons, & the aqua fortis of the chemists inflicts on both the same wounds, and when exposed too long to harm from the atmosphere, they both alike pine away, so to speak, & grow old; each is preserved by being kept in the dust & scrapings of the other; & when a fit piece of steel or iron is adjoined above its pole, the loadstone's vigour is augmented through the firm union. the loadstone is laid up in iron filings, not that iron is its food; as though loadstone were alive and needed feeding, as cardan philosophizes[ ]; nor yet that so it is delivered from the inclemency of the weather (for which cause it as well as iron is laid up in bran by scaliger; mistakenly, however, for they are not preserved well in this way, and keep for years their own fixed forms): nor yet, since they remain perfect by the mutual action of their powders, do their extremities waste away, but are cherished & preserved, like by like. for just as in their own places, in the mines, bodies like to each other endure for many ages entire and uncorrupt, when surrounded by bodies of the same stuff, as the lesser interior parts in a great mass: so loadstone and ore of iron, when inclosed in a mound of the same material, do not exhale their native humour, do not waste away, but retain their soundness. a loadstone lasts longer in filings of smelted iron, & a piece of iron ore excellently also in dust of loadstone; as also smelted iron in filings of loadstone & even in those of iron. then both these allied bodies have a true & just form of one & the same species; a form which until this day was considered by all, owing to their outward unlikeness & the inequality of the potency that is the same innate in both, to be different & unlike in kind; the smatterers not understanding that the same powers, though differing in strength, exist in both alike. and in fact they both are true & intimate parts of the earth, & as such retain the prime natural properties of mutually attracting, of moving, & of disposing themselves toward the position of the world, { } and of the terrestrial globe; which properties they also impart to each other, and increase, confirm, receive, and retain each other's forces. the stronger fortifies the weaker, not as though aught were taken away from its own substance, or its proper vigour, nor because any corporeal substance is imparted, but the dormant virtue of the one is aroused by the other, without loss. for if with a single small stone you touch a thousand bits of iron for the use of mariners[ ], that loadstone attracts iron no less strongly than before; with the same stone weighing one pound, any one will be able to suspend in the air a thousand pounds of iron. for if any one were to fix high up on the walls so many iron nails of so great a weight, & were to apply to them the same number of nails touched, according to the art, by a loadstone, they would all be seen to hang in the air through the force of one small stone. so this is not solely the action, labour, or outlay of the loadstone; but the iron, which is in a sense an extract from loadstone, and a fusion of loadstone into metal, & conceives vigour from it, & by proximity strengthens the magnetick faculties, doth itself, from whatever lode it may have come, raise its own inborn forces through the presence & contact of the stone, even when solid bodies intervene. iron that has been touched, acts anew on another piece of iron by contact, & adapts it for magnetick movements, & this again a third. but if you rub with a loadstone any other metal, or wood, or bones, or glass, as they will not be moved toward any particular and determinate quarter of heaven, nor be attracted by any magnetick body, so they are able not to impart any magnetick property to other bodies or to iron itself by attrition, & by infection. loadstone differs from iron ore, as also from some weaker magnets, in that when molten in the furnace into a ferric & metallick fused mass, it does not so readily flow & dissolve into metal; but is sometimes burnt to ashes in large furnaces; a result which it is reasonable to suppose arises from its having some kind of sulphureous matter mixed with it, or from its own excellence & simpler nature, or from the likeness & common form which it has with the common mother, the great magnet. for earths, and iron stones, magnets abounding in metal, are the more imbued & marred with excrementitious metallick humours, and earthy corruptions of substance, as numbers of loadstones are weaker from the mine; hence they are a little further remote from the common mother, & are degenerate, & when smelted in the furnace undergo fusion more easily, & give out a more certain metallick product, & a metal that is softer, not a tough steel. the majority of loadstones (if not unfairly burnt[ ]) yield in the furnace a very excellent iron. but iron ore also agrees in all those primary qualities with loadstone; for both, being nearer and more closely akin to the earth above all bodies known to us, have in themselves { } a magnetick substance, & one that is more homogenic, true & cognate with the globe of the earth; less infested & spoiled by foreign blemish; less confused with the outgrowths of earth's surface, & less debased by corrupt products. and for this reason aristotle in the fourth book of his _meteora_ seems not unfairly to separate iron from all the rest of the metals. gold, he says, silver, copper, tin, lead, belong to water; but iron is of the earth. galen, in the fourth chapter of _de facultatibus simplicium medicamentorum_, says that iron is an earthy & dense body. accordingly a strong loadstone is on our showing especially of the earth: the next place is occupied by iron ore or weaker loadstone; so the loadstone is by nature and origin[ ]** of iron, and it and magnetick iron are both one in kind. iron ore yields iron in furnaces; loadstone also pours forth iron in the furnaces, but of a much more excellent sort, that which is called steel or blade-edge; and the better sort of iron ore is a weak loadstone, the best loadstone being a most excellent ore of iron, in which, as is to be shown by us, the primary properties are grand and conspicuous. weaker loadstone or iron ore is that in which these properties are more obscure, feeble, and are scarce perceptible to the senses. * * * * * chap. xvii. that the globe of the earth is magnetick, & a magnet; & _how in our hands the magnet stone has all the primary_ forces of the earth, while the earth by the _same powers remains constant in a fixed direction in the universe._ prior to bringing forward the causes of magnetical motions, & laying open the proofs of things hidden for so many ages, & our experiments (the true foundations of terrestrial philosophy), we have to establish & present to the view of the learned our new & unheard of doctrine about the earth; and this, when argued by us on the grounds of its probability, with subsequent { } experiments & proofs, will be as certainly assured as anything in philosophy ever has been considered & confirmed by clever arguments or mathematical proofs. the terrene mass, which together with the vasty ocean produces the sphærick figure & constitutes our globe, being of a firm & constant substance, is not easily changed, does not wander about, & fluctuate with uncertain motions, like the seas, & flowing waves: but holds all its volume of moisture in certain beds & bounds, & as it were in oft-met veins, that it may be the less diffused & dissipated at random. yet the solid magnitude of the earth prevails & reigns supreme in the nature of our globe. water, however, is attached to it, & as an appendage only, & a flux emanating from it; whose force from the beginning is conjoined with the earth through its smallest parts, and is innate in its substance. this moisture the earth as it grows hot throws off freely when it is of the greatest possible service in the generation of things. but the thews and dominant stuff of the globe is that terrene body which far exceeds in quantity all the volume of flowing streams and open waters (whatever vulgar philosophers may dream of the magnitudes and proportions of their elements), and which takes up most of the whole globe and almost fills it internally, and by itself almost suffices to endow it with sphærick shape. for the seas only fill certain not very deep or profound hollows, since they rarely go down to a depth of a mile and generally do not exceed a hundred or fathoms. for so it is ascertained by the observations of seamen when by the plumb-line and sinker its abysms are explored with the nautical sounder; which depths relatively to the dimensions of the globe, do not much deform its globular shape. small then appears to be that portion of the real earth that ever emerges to be seen by man, or is turned up; since we cannot penetrate deeper into its bowels, further than the wreckage of its outer efflorescence, either by reason of the waters which gush up in deep workings, as through veins, or for want of a wholesome air to support life in the miners, or on account of the vast cost that would be incurred in pumping out such huge workings[ ], and many other difficulties; so that to have gone down to a depth of four hundred, or (which is of rarest occurrence) of five hundred fathoms[ ] as in a few mines, appears to all a stupendous undertaking. but it is easy to understand how minute, how almost negligibly small a portion that fathoms is of the earth's diameter, which is , miles. it is then parts only of the earth's circumference and of its prominences that are perceived by us with our senses; and these in all regions appear to us to be either loamy, or clayey, or sandy, or full of various soils, or marls: or lots of stones or gravel meet us, or beds of salt, or a metallick lode, and metals in abundance. in the sea and in deep waters, however, either reefs, and huge boulders, or smaller stones, or sands, or mud { } are found by mariners as they sound the depths. nowhere does the aristotelian element of _earth_ come to light; and the peripateticks are the sport of their own vain dreams about elements. yet the lower bulk of the earth and the inward parts of the globe consist of such bodies; for they could not have existed, unless they had been related to and exposed to the air and water, and to the light and influences of the heavenly bodies, in like manner as they are generated, and pass into many dissimilar forms of things, and are changed by a perpetual law of succession. yet the interior parts imitate them, and betake themselves to their own source, on the principle of terrene matter, albeit they have lost the first qualities and the natural terrene form, and are borne towards the earth's centre, and cohære with the globe of the earth, from which they cannot be wrenched asunder except by force. but the loadstone and all magneticks, not the stone only, but every magnetick homogenic substance, would seem to contain the virtue of the earth's core and of its inmost bowels, and to hold within itself and to have conceived that which is the secret and inward principle of its substance; and it possesses the actions peculiar to the globe of attracting, directing, disposing, rotating, stationing itself in the universe, according to the rule of the whole, and it contains and regulates the dominant powers of the globe; which are the chief tokens and proofs of a certain distinguishing combination, and of a nature most thoroughly conjoint. for if among actual bodies one sees something move and breathe, and experience sensations, and be inclined and impelled by reason, will one not, knowing and seeing this, conclude that it is a man or something rather like a man, than that it is a stone or a stick? the loadstone far excels all other bodies known to us in virtues and properties pertaining to the common mother: but those properties have been far too little understood or realized by philosophers: for to its body bodies magnetical rush in from all sides and cleave to it, as we see them do in the case of the earth. it has poles, not mathematical points, but natural termini of force excelling in primary efficiency by the co-operation of the whole: and there are poles in like manner in the earth which our forefathers sought ever in the sky: it has an æquator, a natural dividing line between the two poles, just as the earth has: for of all lines drawn by the mathematicians on the terrestrial globe, the æquator is the natural boundary, and is not, as will hereafter appear, merely a mathematical circle. it, like the earth, acquires direction and stability toward north and south, as the earth does; also it has a circular motion toward the position of the earth, wherein it adjusts itself to its rule: it follows the ascensions and declinations of the earth's poles, and conforms exactly to the same, and by itself raises its own poles above the { } horizon naturally according to the law of the particular country and region, or sinks below it. the loadstone derives temporary properties, and acquires its verticity from the earth, and iron is affected by the verticity of the globe even as iron is by a loadstone: magneticks are conformable to and are regulated by the earth, and are subject to the earth in all their motions. all its movements harmonize with, and strictly wait upon, the geometry and form of the earth, as we shall afterwards prove by most conclusive experiments and diagrams; and the chief part of the visible earth is also magnetical, and has magnetick motions, although it be disfigured by corruptions and mutations without end. why then do we not recognize this the chief homogenic substance of the earth, likest of substances to its inner nature and closest allied to its very marrow? for none of the other mixed earths suitable for agriculture, no other metalliferous veins, nor stones, nor sand, nor other fragments of the earth which have come to our view possess such constant and peculiar powers. and yet we do not assume that the whole interior of this globe of ours is composed of stones or iron (although franciscus maurolycus, that learned man, deems the whole of the earth's interior to consist of solid stone). for not every loadstone that we have is a stone, it being sometimes like a clod, or like clay and iron either firmly compacted together out of various materials, or of a softer composition, or by heat reduced to the metallick state; and the magnetick substance by reason of its location and of its surroundings, and of the metallick matrix itself, is distinguished, at the surface of the terrene mass, by many qualities and adventitious natures, just as in clay it is marked by certain stones and iron lodes. but we maintain that the true earth is a solid substance, homogeneous with the globe, closely coherent, endowed with a primordial and (as in the other globes of the universe) with a prepotent form; in which position it persists with a fixed verticity, and revolves with a necessary motion and an inherent tendency to turn, and it is this constitution, when true and native, and not injured or disfigured by outward defects, that the loadstone possesses above all bodies apparent to us, as if it were a more truly homogenic part taken from the earth. accordingly native iron which _sui generis_ (as metallurgists term it), is formed when homogenic parts of the earth grow together into a metallick lode; loadstone being formed when they are changed into metallick stone, or a lode of the finest iron, or steel: so in other iron lodes the homogenic matter that goes together is somewhat more imperfect; just as many parts of the earth, even the high ground, is homogenic but so much more deformate. smelted iron is fused and smelted out of homogenic stuffs, and cleaves to the earth more tenaciously than the ores themselves. such then is our earth in its { } inward parts, possessed of a magnetick homogeneal nature, and upon such more perfect foundations as these rests the whole nature of things terrestrial, manifesting itself to us, in our more diligent scrutiny, everywhere in all magnetick minerals, and iron ores, in all clay, and in numerous earths and stones; while aristotle's simple element, that most empty terrestrial phantom of the peripateticks, a rude, inert, cold, dry, simple matter, the universal substratum, is dead, devoid of vigour, and has never presented itself to any one, not even in sleep, and would be of no potency in nature. our philosophers were only dreaming when they spoke of a kind of simple and inert matter. cardan does not consider the loadstone to be any kind of stone, "but a sort of perfected portion of some kind of earth that is absolute; a token of which is its abundance, there being no place where it is not found. and there is" (he says) "a power of iron in the wedded earth which is perfect in its own kind when it has received fertilizing force from the male, that is to say, the stone of hercules" (in his book _de proportionibus_). and later: "because" (he says) "in the previous proposition i have taught that iron is true earth." a strong loadstone shows itself to be of the inward earth, and upon innumerable tests claims to rank with the earth in the possession of a primary form, that by which earth herself abides in her own station and is directed in her courses. thus a weaker loadstone and every ore of iron, and nearly all clay, or clayey earth, and numerous other sorts (yet more, or less, owing to the different labefaction of fluids and slimes), keep their magnetick and genuine earth-properties open to view, falling short of the characteristic form, and deformate. for it is not iron alone (the smelted metal) that points to the poles, nor is it the loadstone alone that is attracted by another and made to revolve magnetically; but all iron ores, and other stones, as rhenish slates and the black ones from avignon (the french call them _ardoises_) which they use for tiles, and many more of other colours and substances, provided they have been prepared; as well as all clay, grit[ ], and some sorts of rocks, and, to speak more clearly, all the more solid earth that is everywhere apparent; given that that earth be not fouled with fatty and fluid corruptions; as mud, as mire, as accumulations of putrid matter; nor deformate by the imperfections of sundry admixtures; nor dripping with ooze, as marls; all are attracted by the loadstone, when simply prepared by fire, and freed from their refuse humour; and as by the loadstone so also by the earth herself they are drawn and controlled magnetically, in a way different from all other bodies; and by that inherent force settle themselves according to the orderly arrangement and fabric of the universe and of the earth, as will appear { } later. thus every part of the earth which is removed from it exhibits by sure experiments every impulse of the magnetick nature; by its various motions it observes the globe of the earth and the principle common to both. [illustration] * * * * * { } [illustration] book second. _chap. i._ on magnetick motions. divers things concerning opinions about the magnet-stone, and its variety, concerning its poles and its known faculties, concerning iron, concerning the properties of iron, concerning a magnetick substance common to both of these and to the earth itself, have been spoken briefly by us in the former book. there remain the magnetical motions, and their fuller philosophy, shown and demonstrated. these motions are incitements of homogeneal parts either among themselves or toward the primary conformation of the whole earth. aristotle admits only two simple motions of his elements, from the centre and toward the centre; of light ones upward, heavy ones downward; so that in the earth there exists one motion only of all its parts towards the centre of the world,--a rude and inert precipitation. but what of it is light, and how wrongly it is inferred by the peripateticks from the simple motion of the elements, and also what is its heavy part, we will discuss elsewhere. but now our inquiry must be into the causes of other motions, depending on its true form, which we have plainly seen in our magnetick bodies; and these we have seen to be present in the earth and in all its homogenic parts also. we have noticed that they harmonize with the earth, and are bound up with its forces. five movements[ ] or differences of motions are then observed by us: coition (commonly called attraction), the { } incitement to magnetick union; direction towards the poles of the earth, and the verticity and continuance of the earth towards the determinate poles of the world; variation, a deflexion from the meridian, which we call a perverted movement; declination, a descent of the magnetick pole below the horizon; and circular motion, or revolution. concerning all these we shall discuss separately, and how they all proceed from a nature tending to aggregation, either by verticity or by volubility. jofrancus offusius[ ] makes out different magnetick motions; a first toward a centre; a second toward a pole at seventy-seven degrees; a third toward iron; a fourth toward loadstone. the first is not always to a centre, but exists only at the poles in a straight course toward the centre, if the motion is magnetick; otherwise it is only motion of matter toward its own mass and toward the globe. the second toward a pole at seventy-seven degrees is no motion, but is direction with respect to the pole of the earth, or variation. the third and fourth are magnetick and are the same. so he truly recognizes no magnetick motion except the coition toward iron or loadstone, commonly called attraction. there is another motion in the whole earth, which does not exist towards the terrella or towards its parts; videlicet, a motion of aggregation, and that movement of matter, which is called by philosophers a right motion, of which elsewhere. * * * * * chap. ii. on the magnetick coition, and first on the attraction of amber, or more truly, on the _attaching of bodies to amber_. celebrated has the fame of the loadstone and of amber ever been in the memoirs of the learned. loadstone and also amber do some philosophers invoke when in explaining many secrets their senses become dim and reasoning cannot go further. inquisitive theologians also would throw light on the divine mysteries set beyond the range of human sense, by means of loadstone and amber; just as idle metaphysicians, when they are setting up and teaching useless phantasms, have recourse to the loadstone as if it were a delphick sword, an illustration always applicable to everything. but physicians even (with the authority of { } galen), desiring to confirm the belief in the attraction of purgative medicines by means of the likeness of substance and the familiarities of the juices--truly a vain and useless error--bring in the loadstone as witness as being a nature of great authority and of conspicuous efficacy and a remarkable body. so in very many cases there are some who, when they are pleading a cause and cannot give a reason for it, bring in loadstone and amber as though they were personified witnesses. but these men (apart from that common error) being ignorant that the causes of magnetical motions are widely different from the forces of amber, easily fall into error, and are themselves the more deceived by their own cogitations. for in other bodies a conspicuous force of attraction manifests itself otherwise than in loadstone; like as in amber, concerning which some things must first be said, that it may appear what is that attaching of bodies, and how it is different from and foreign to the magnetical actions; those mortals being still ignorant, who think that inclination to be an attraction, and compare it with the magnetick coitions. the greeks call it [greek: êlektron][ ] because it attracts straws to itself, when it is warmed by rubbing; then it is called [greek: harpax][ ]; and [greek: chrusophoron] from its golden colour. but the moors call it carabe[ ], because they are accustomed to offer the same in sacrifices and in the worship of the gods. for carab signifies to offer in arabic; so carabe, an offering: or seizing chaff, as scaliger quotes from abohalis, out of the arabic or persian language. some also call it amber, especially the indian and ethiopian amber, called in latin _succinum_, as if it were a juice[ ]. the sudavienses or sudini[ ] call it _geniter_, as though it were generated terrestrially. the errors of the ancients concerning its nature and origin having been exploded, it is certain that amber comes for the most part from the sea, and the rustics collect it on the coast after the more violent storms, with nets and other tackle; as among the sudini of prussia; and it is also found sometimes on the coast of our own britain. it seems, however, to be produced also in the soil and at spots of some depth, like other bitumens; to be washed out by the waves of the sea; and to become concreted more firmly from the nature and saltness of the sea-water. for it was at first a soft and viscous material; wherefore also it contains enclosed and entombed in pieces of it, shining in eternal sepulchres, flies, grubs, gnats, ants; which have all flown or crept or fallen into it when it first flowed forth in a liquid state[ ]. the ancients and also more recent writers recall (experience proving the same thing), that amber attracts straws and chaff[ ]. the same is also done by jet[ ], which is dug out of the earth in britain, in germany, and in very many lands, and is a rather hard concretion from black bitumen, and as it were a transformation into stone. there are many modern authors[ ] who have written and copied from others about amber and jet[ ] attracting chaff, and about other { } substances generally unknown; with whose labours the shops of booksellers are crammed. our own age has produced many books about hidden, abstruse, and occult causes and wonders, in all of which amber and jet are set forth as enticing chaff; but they treat the subject in words alone, without finding any reasons or proofs from experiments, their very statements obscuring the thing in a greater fog, forsooth in a cryptic, marvellous, abstruse, secret, occult, way. wherefore also such philosophy produces no fruit, because very many philosophers, making no investigation themselves, unsupported by any practical experience, idle and inert, make no progress by their records, and do not see what light they can bring to their theories; but their philosophy rests simply on the use of certain greek words, or uncommon ones; after the manner of our gossips and barbers nowadays, who make show of certain latin words to an ignorant populace as the insignia of their craft, and snatch at the popular favour. for it is not only amber and * jet (as they suppose) which entice small bodies[ ]; but diamond, sapphire, carbuncle, iris gem[ ], opal, amethyst, vincentina, and bristolla (an english gem or spar)[ ], beryl, and crystal[ ] do the same. similar powers of attraction are seen also to be possessed by glass (especially when clear and lucid), as also by false gems made of glass or crystal, by glass of antimony, and by many kinds of spars from the mines, and by belemnites. sulphur also attracts, and mastick, and hard sealing-wax[ ] compounded of lac tinctured of various colours. rather hard resin entices, as does orpiment[ ], but less strongly; with difficulty also and indistinctly under a suitable dry sky[ ], rock salt, muscovy stone, and rock alum. this one may see when the air is sharp and clear and rare in mid-winter, when the emanations from the earth hinder electricks less, and the electrick bodies become * more firmly indurated; about which hereafter. these substances draw everything, not straws and chaff only[ ], but all metals, woods, leaves, stones, earths, even water and oil, and everything which is subject to our senses, or is solid; although some write that amber does not attract anything but chaff and certain twigs; (wherefore alexander aphrodiseus falsely declares the question of amber to be inexplicable, because it attracts dry chaff only, and not basil leaves[ ]), but these are the utterly false and disgraceful tales of the writers. but in order that you may be able clearly to test how such attraction occurs[ ], and what those materials[ ] are which thus entice other bodies (for even if bodies incline towards some of these, yet on account of weakness they seem not to be raised by them, but are more easily turned), make yourself a versorium of any metal you like, three or four digits in length, resting rather lightly on its point of support after the manner of a magnetick needle, to one end of which bring up a piece of amber or a smooth { } [illustration] and polished gem which has been gently rubbed; for the versorium turns forthwith. many things are thereby seen to attract, both those which are formed by nature alone, and those which are by art prepared, fused, and mixed; nor is this so much a singular property of one or two things (as is commonly supposed), but the manifest nature of very many, both of simple substances, remaining merely in their own form, and of compositions, as of hard sealing-wax, & of certain other mixtures besides, made of unctuous stuffs. we must, however, investigate more fully whence that tendency arises, and what those forces be, concerning which a few men have brought forward very little, the crowd of philosophizers nothing at all. by galen three kinds of attractives in general were recognized in nature: a first class of those substances which attract by their elemental quality, namely, heat; the second is the class of those which attract by the succession of a vacuum; the third is the class of those which attract by a property of their whole substance, which are also quoted by avicenna and others. these classes, however, cannot in any way satisfy us; they neither embrace the causes of amber, jet, and diamond, and of other similar substances (which derive their forces on account of the same virtue); nor of the loadstone, and of all magnetick substances, which obtain their virtue by a very dissimilar and alien influence from them, derived from other sources. wherefore also it is fitting that we find other causes of the motions, or else we must wander (as in darkness), with these men, and in no way reach the goal. amber truly does* not allure by heat, since if warmed by fire and brought near straws, it does not attract them, whether it be tepid, or hot, or glowing, or even when forced into the flame. cardan (as also pictorio) reckons that this happens in no different way[ ] than with the cupping-glass, by the force of fire. yet the attracting force of the cupping-glass does not really come from the force of fire. but he had previously said that the dry substance wished to imbibe fatty humour, and therefore it was borne towards it. but these statements are at variance with one another, and also foreign to reason. for if amber had moved towards its food, or if other bodies had inclined towards amber as towards provender, there would have been a diminution of the one which was devoured, just as there would have been a growth of the other which was sated. then why should an attractive force of fire be looked for in amber? if the attraction existed from heat, why should not very many other bodies also attract, if warmed by fire, by the sun, or by friction? neither can the attraction be on account of the dissipating of the air, when it takes place in open air (yet lucretius the poet adduces this as the reason for magnetical motions). nor in the cupping-glass can heat or fire attract by feeding on air: in the cupping-glass air, having been exhausted into flame, { } when it condenses again and is forced into a narrow space, makes the skin and flesh rise in avoiding a vacuum. in the open air warm things cannot attract, not metals even or stones, if they should * be strongly incandescent by fire. for a rod of glowing iron, or a flame, or a candle, or a blazing torch, or a live coal, when they are brought near to straws, or to a versorium, do not attract; yet at the same time they manifestly call in the air in succession; because they consume it, as lamps do oil. but concerning heat, how it is reckoned by the crowd of philosophizers, in natural philosophy and in _materia medica_ to exert an attraction otherwise than nature allows, to which true attractions are falsely imputed, we will discuss more at length elsewhere, when we shall determine what are the properties of heat and cold. they are very general qualities or kinships of a substance, and yet are not to be assigned as true causes, and, if i may say so, those philosophizers utter some resounding words; but about the thing itself prove nothing in particular. nor does this attraction accredited to amber arise from any singular quality of the substance or kinship, since by more thorough research we find the same effect in very many other bodies; and all bodies, moreover, of whatever quality, are allured by all those bodies. similarity also is not the cause; because all things around us placed on this globe of the earth, similar and dissimilar, are allured by amber and bodies of this kind; and on that account no cogent analogy is to be drawn either from similarity or identity of substance. but neither do similars mutually attract one another, as stone stone, flesh flesh, nor aught else outside the class of magneticks and electricks. fracastorio would have it that "things which mutually attract one another are similars, as being of the same species, either in action or in right subjection. right subjection is that from which is emitted the emanation which attracts and which in mixtures often lies hidden on account of their lack of form, by reason of which they are often different in act from what they are in potency. hence it may be that hairs and twigs move towards amber and towards diamond, not because they are hairs, but because either there is shut up in them air or some other principle, which is attracted in the first place, and which bears some relation and analogy to that which attracts of itself; in which diamond and amber agree through a principle common to each." thus far fracastorio. who if he had observed by a large number of experiments that all bodies are drawn to electricks except those which are aglow and aflame, and highly rarefied, would never have given a thought to such things. it is easy for men of acute intellect, apart from experiments and practice, to slip and err. in greater error do they remain sunk who maintain these same substances to be not similar, but to be substances near akin; and hold that on that account a thing moves towards another, its like, by which it is brought to more perfection. but these are { } ill-considered views; for towards all electricks all things move[ ] except such as are aflame or are too highly rarefied, as air, which is the universal effluvium of this globe and of the world. vegetable substances draw moisture by which their shoots are rejoiced and grow; from analogy with that, however, hippocrates, in his _de natura hominis_, book i., wrongly concluded that the purging of morbid humour took place by the specifick force of the drug. concerning the action and potency of purgatives we shall speak elsewhere. wrongly also is attraction inferred in other effects; as in the case of a flagon full of water, when buried in a heap of wheat, although well stoppered, the moisture is drawn out; since this moisture is rather resolved into vapour by the emanation of the fermenting wheat, and the wheat imbibes the freed vapour. nor do elephants' tusks attract moisture, but drive it into vapour or absorb it. thus then very many things are said to attract, the reasons for whose energy must be sought from other causes. amber in a fairly large mass allures, if* it is polished; in a smaller mass or less pure it seems not to attract without friction. but very many electricks (as precious stones and some other substances) do not attract at all unless rubbed. on the other hand many gems, as well as other bodies, are polished, yet do* not allure, and by no amount of friction are they aroused; thus the emerald, agate, carnelian, pearls, jasper, chalcedony, alabaster, porphyry, coral, the marbles, touchstone, flint, bloodstone, emery[ ], do not acquire any power; nor do bones, or ivory, or the hardest woods, as ebony, nor do cedar, juniper, or cypress; nor do metals, silver, gold, brass, iron, nor any loadstone, though many of them are finely polished and shine. but on the other hand there are some other polished substances of which we have spoken before, toward which, when they have been rubbed, bodies incline. this we shall understand only when we have more closely looked into the prime origin of bodies. it is plain to all, and all admit, that the mass of the earth, or rather the structure and crust of the earth, consists of a twofold material, namely, of fluid and humid matter, and of material of more consistency and dry. from this twofold nature or the more simple compacting of one, various substances take their rise among us, which originate in greater proportion now from the earthy, now from the aqueous nature. those substances which have received their chief growth from moisture, whether aqueous or fatty, or have taken on their form by a simpler compacting from them, or have been compacted from these same materials in long ages, if they have a sufficiently firm hardness, if rubbed after they have been polished and when they remain bright with the friction--towards those substances everything, if presented to them in the air, turns, if its too heavy weight does not prevent it. for amber has been compacted of moisture, and jet also. lucid gems are made of water; just as crystal[ ], which has been concreted from clear water, not { } always by a very great cold, as some used to judge, and by very hard frost, but sometimes by a less severe one, the nature of the soil fashioning it, the humour or juices being shut up in definite cavities, in the way in which spars are produced in mines. so clear glass is fused out of sand, and from other substances, which have their origin in humid juices. but the dross of metals, as also metals, stones, rocks, woods, contain earth rather, or are mixed with a good deal of earth; * and therefore they do not attract. crystal, mica, glass, and all electricks do not attract if they are burnt or roasted; for their primordial supplies of moisture perish by heat, and are changed and exhaled. all things therefore which have sprung from a predominant moisture and are firmly concreted, and retain the appearance of spar and its resplendent nature in a firm and compact body, allure all bodies, whether humid or dry. those, however, which partake of the true earth-substance or are very little different from it, are seen to attract also, but from a far different reason, and (so to say) magnetically; concerning these we intend to speak afterwards. but those substances which are more mixed of water and earth, and are produced by the equal degradation of each element (in which the magnetick force of the earth is deformed and remains buried; while the watery humour, being fouled by joining with a more plentiful supply of earth, has not concreted in itself but is mingled with earthy matter), can in no way of themselves attract or move from its place anything which they do not touch. on this account metals, marbles, flints, woods, herbs, flesh, and very many other things can neither allure nor solicit any body either magnetically or electrically. (for it pleases us to call that an electrick force, which hath * its origin from the humour.) but substances consisting mostly of humour, and which are not very firmly compacted by nature (whereby do they neither bear rubbing, but either melt down and become soft, or are not levigable, such as pitch, the softer kinds of resin, camphor, galbanum, ammoniack[ ], storax, asafoetida, benzoin, asphaltum, especially in rather warm weather) towards them small bodies are not borne; for without rubbing most electricks do not * emit their peculiar and native exhalation and effluvium. the resin turpentine when liquid does not attract; for it cannot be rubbed; but if it has hardened into a mastick it does attract. but now at length we must understand why small bodies turn towards those substances which have drawn their origin from water; by what force and with what hands (so to speak) electricks seize upon kindred natures. in all bodies in the world two causes or principles have been laid down, from which the bodies themselves were produced, matter and form[ ]. electrical motions become strong from matter, but magnetick from form chiefly; and they differ widely from one another and turn out unlike, since the one is ennobled by numerous virtues and is prepotent; the other is ignoble and of less potency, and { } mostly restrained, as it were, within certain barriers; and therefore that force must at times be aroused by attrition or friction, until it is at a dull heat and gives off an effluvium and a polish is induced on the body. for spent air, either blown out of the mouth or given* off from moister air, chokes the virtue. if indeed either a sheet of paper or a piece of linen be interposed, there will be no movement. but a loadstone, without friction or heat, whether dry or suffused with moisture, as well in air as in water, invites magneticks, even with the most solid bodies interposed, even planks of wood or pretty thick slabs of stone or sheets of metal. a loadstone appeals to magneticks* only; towards electricks all things move. a loadstone[ ] raises great weights; so that if there is a loadstone weighing two ounces and strong, it attracts half an ounce or a whole ounce. an electrical substance only attracts very small weights; as, for instance, a piece of amber of three ounces weight, when rubbed, scarce raises a fourth part of a grain of barley. but this attraction of amber and of electrical substances must be further investigated; and since there is this particular affection of matter, it may be asked why is amber rubbed, and what affection is produced by the rubbing, and what causes arise which make it lay hold on everything? as a result of friction it grows slightly warm and becomes smooth; two results which must often occur together. a large polished fragment of amber or jet attracts indeed, even without friction, but less strongly; but if it be brought gently near a flame or a live coal, so that it equally becomes warm, it does not attract small bodies because* it is enveloped in a cloud from the body of the flaming substance, which emits a hot breath, and then impinges upon it vapour from a foreign body which for the most part is at variance with the nature of amber. moreover the spirit of the amber which is called forth is enfeebled by alien heat; wherefore it ought not to have heat excepting that produced by motion only and friction, and, as it were, its own, not sent into it by other bodies. for as the igneous heat emitted from any burning substance cannot be so used that electricks may acquire their force from it; so also heat from the solar rays does not fit an electrick by the loosening of its* right material, because it dissipates rather and consumes it (albeit a body which has been rubbed retains its virtue longer exposed to the rays of the sun than in the shade; because in the shade the effluvia are condensed to a greater degree and more quickly). then again the fervour from the light of the sun aroused by means of a* burning mirror confers no vigour on the heated amber[ ]; indeed it dissipates and corrupts all the electrick effluvia. again, burning* sulphur and hard wax, made from shell-lac, when aflame do not allure; for heat from friction resolves bodies into effluvia, which flame consumes away. for it is impossible for solid electricks to be resolved into their own true effluvia otherwise than by attrition, save { } in the case of certain substances which by reason of innate vigour emit effluvia constantly. they are rubbed with bodies which do not befoul their surface, and which produce a polish, as pretty stiff silk or a rough wool rag which is as little soiled as possible, or the dry palm. amber also is rubbed with amber, with diamond, and with glass, and numerous other substances. thus are electricks manipulated. these things being so, what is it which moves? is it the body itself, inclosed within its own circumference? or is it something imperceptible to us, which flows out from the substance into the ambient air? somewhat as plutarch opines, saying in his _quæstiones platonicæ_[ ]: that there is in amber something flammable or something having the nature of breath, and this by the attrition of the surface being emitted from its relaxed pores attracts bodies. and if it be an effusion does it seize upon the air whose motion the bodies follow, or upon the bodies themselves? but if amber allured the body itself, then what need were there of friction, if it is bare and smooth? nor does the force arise from the light which is reflected from a smooth and polished body; for a gem of vincent's rock[ ], diamond, and clear glass, attract when they are rough; but not so powerfully and quickly, because they are not so readily cleansed from extraneous moisture on the surface, and are not rubbed equally so as to be copiously resolved at that part. nor does the sun by its own beams of light and its rays, which are of capital importance in nature, attract bodies in this way; and yet the herd of philosophizers considers that humours are attracted by the sun, when it is only denser humours that are being turned into thinner, into spirit and air; and so by the motion of effusion they ascend into the upper regions, or the attenuated exhalations are raised up from the denser air. nor does it seem to take place from the effluvia attenuating the air, so that bodies impelled by the denser air penetrate towards the source of the rarefaction; in this case both hot and flaming bodies would also allure other bodies; but not even the lightest chaff, or any versorium moves towards a flame. if there is a flow and rush of air towards the body, how can a small diamond of the size of a pea[ ] summon towards itself so much air, that it seizes hold of a biggish long body placed in equilibrio (the air about one or other very small part of an end being attracted)? it ought also to have slopped or moved more slowly, before it came into contact with the body, especially if the piece of amber was rather broad and flat, from the accumulation of air on the surface of the amber and its flowing back again. if it is because the effluvia are thinner, and denser vapours come in return, as in breathing, then the body would rather have had a motion toward the electrick a little while after the beginning of the application; but when electricks which have been rubbed are applied quickly to * a versorium then especially at once they act on the versorium, and it is attracted more when near them. but if it is because the rarefied { } effluvia produce a rarefied medium, and on that account bodies are more prone to slip down from a denser to a more attenuated medium; they might have been carried from the side in this way or downwards, but not to bodies above them; or the attraction and apprehension of contiguous bodies would have been momentary only. but with a single friction jet and amber draw and attract bodies to them strongly and for a long time, sometimes for the twelfth part of an hour, especially in clear weather. but if the mass of amber be rather large, and the surface polished, it attracts without friction. flint is rubbed and emits by attrition an inflammable matter that turns into sparks and heat. therefore the denser effluvia of flint producing fire are very far different from electrical effluvia, which on account of their extreme attenuation do not take fire, nor are fit material for flame. those effluvia are not of the nature of breath, for when emitted they do not propel anything, but are exhaled without sensible resistance and touch bodies. they are highly attenuated humours much more subtile than the ambient air; and in order that they may occur, bodies are required produced from humour and concreted with a considerable degree of hardness. non-electrick bodies are not resolved into humid effluvia, and those effluvia mix with the common and general effluvia of the earth, and are not peculiar. also besides the attraction of bodies, they retain them longer. it is probable therefore that amber does exhale something peculiar to * itself, which allures bodies themselves, not the intermediate air. indeed it plainly does draw the body itself in the case of a spherical drop of water standing on a dry surface; for a piece of amber applied to it at a suitable distance pulls the nearest parts out of their position and draws it up into a cone; otherwise, if it were * drawn by means of the air rushing along, the whole drop would have moved. that it does not attract the air is thus demonstrated: take a very thin wax candle, which makes a very small and clear flame; bring up to this, within two digits or any convenient distance, a piece of amber or jet, a broad flat piece, well prepared * and skilfully rubbed, such a piece of amber as would attract bodies far and wide, yet it does not disturb the flame; which of necessity would have occurred, if the air was disturbed, for the flame would have followed the current of air. as far as the effluvia are sent out, so far it allures; but as a body approaches, its motion is accelerated, stronger forces drawing it; as also in the case of magneticks and in all natural motion; not by attenuating or by expelling the air, so that the body moves down into the place of the air which has gone out[ ]; for thus it would have allured only and would not have retained; since it would at first also have repelled approaching bodies just as it drives the air itself; but indeed a particle, be it ever so small, does not avoid the first application made very quickly after rubbing. an effluvium exhales from amber and is emitted by rubbing: pearls, carnelian, agate, jasper, chalcedony, coral, metals, { } and other substances of that kind, when they are rubbed, produce no effect. is there not also something which is exhaled from them by heat and attrition? most truly; but from grosser bodies more blended with the earthy nature, that which is exhaled is gross and spent; for even towards very many electricks, if they are rubbed * too hard, there is produced but a weak attraction of bodies, or none at all; the attraction is best when the rubbing has been gentle and very quick; for so the finest effluvia are evoked. the effluvia arise from the subtile diffusion of humour, not from excessive and turbulent violence; especially in the case of those substances which have been compacted from unctuous matter, which when the atmosphere is very thin, when the north winds, and amongst us (english) the east winds, are blowing, have a surer and firmer effect, but during south winds and in damp weather, only a weak one; so that those * substances which attract with difficulty in clear weather, in thick weather produce no motion at all; both because in grosser air lighter substances move with greater difficulty; and especially because the effluvia are stifled, and the surface of the body that has been rubbed is affected by the spent humour of the air, and the effluvia are stopped at their very starting. on that account in the case of amber, jet, and sulphur, because they do not so easily take up moist air on their surface and are much more plenteously set free, that force is not so quickly suppressed as in gems, crystal, glass, and substances of that kind which collect on their surface the moister breath which has grown heavy. but it may be asked why does amber allure water, when water placed on its surface removes its action? evidently because it is one thing to suppress it at its very start, and quite another to extinguish it when it has been * emitted. so also thin and very fine silk, in common language _sarcenet_, placed quickly on the amber, after it has been rubbed, * hinders the attraction of the body; but if it is interposed in the intervening space, it does not entirely obstruct it. moisture also from spent air, and any breath blown from the mouth, as well as water put on the amber, immediately extinguishes its force. but oil, which is light and pure, does not hinder it; for although amber * be rubbed with a warm finger dipped in oil, still it attracts. but * if that amber, after the rubbing, is moistened with _aqua vitæ_ or spirits of wine, it does not attract; for it is heavier than oil, denser, and when added to oil sinks beneath it. for oil is light and rare, and does not resist the most delicate effluvia. a breath therefore, proceeding from a body which had been compacted from humour or from a watery liquid, reaches the body to be attracted; the body that is reached is united with the attracting body, and the one body lying near the other within the peculiar radius of its effluvia makes one out of two; united, they come together into the closest accord, and this is commonly called attraction. this unity, according to { } the opinion of pythagoras, is the principle of all things, and through participation in it each several thing is said to be one. for since no action can take place by means of matter unless by contact, these electricks are not seen to touch, but, as was necessary, something is sent from the one to the other, something which may touch closely and be the beginning of that incitement. all bodies are united and, as it were, cemented together in some way by moisture; so that a wet body, when it touches another body, attracts it, if it is small. so wet bodies on the surface of water attract wet bodies. but the peculiar electrical effluvia, which are the most subtile material of diffuse humour, entice corpuscles. air (the common effluvium of the earth) not only unites the disjointed parts, but the earth calls bodies back to itself by means of the intervening air; otherwise bodies which are in higher places would not so eagerly make for the earth. electrical effluvia differ greatly from air; and as air is the effluvium of the earth, so electricks have their own effluvia and properties, each of them having by reason of its peculiar effluvia a singular tendency toward unity, a motion toward its origin and fount, and toward the body emitting the effluvia. but those substances which by attrition emit a gross or vapourous or aeriform effluvium produce no effect; for either such effluvia are alien to the humour (the uniter of all things), or being very like common air are blended with the air and intermingle with the air, wherefore they produce no effect in the air, and do not cause motions different from those so universal and common in nature. in like manner * bodies strive to be united and move on the surface of water, just [illustration] as the rod c, which is put a little way under water. it is plain that the rod e f, which floats on the water by reason of the cork h, and only has its wet end f above the surface of the water, is attracted by the rod c, if the rod c is wet a little above the surface of the water; they are suddenly united, just as a drop adjoining a drop is attracted. so a wet thing on the surface of water seeks union with a wet thing, since the surface of the water is raised on both; and they immediately flow together, just like drops or bubbles. but they are in much greater proximity than electricks, and are united by their clammy natures. if, however, the whole rod be dry * above the water, it no longer attracts, but drives away the stick e f. the same is seen in those bubbles also which are made on { } water. for we see one drive towards another, and the quicker the nearer they are. solids are impelled towards solids by the medium of liquid: for example, touch the end of a versorium with the end of a rod on which a drop of water is projecting; as soon as the versorium touches the top of the droplet, immediately it is joined * strongly by a swift motion to the body of the rod. so concreted humid things attract when a little resolved into air (the effluvia in the intermediate space tending to produce unity); for water has on wet bodies, or on bodies wet with abundant moisture on the top of water, the force of an effluvium. clear air is a convenient medium for an electrical effluvium excited from concreted humour. wet bodies projecting above the surface of water (if they are near) run together so that they may unite; for the surface of the water is raised around wet substances. but a dry thing is not impelled to a wet one, nor a wet to a dry, but seems to run away. for if all is dry above the water, the surface of the water close to it does not rise, but shuns it, the wave sinking around a dry thing. so neither does a wet thing move towards the dry rim of a vessel; but it seeks [illustration] a wet rim. a b is the surface of the water; c d two rods, which stand up wet above the water; it is manifest that the surface of the water is raised at c and d along with the rods; and therefore the rod c, by reason of the water standing up (which seeks its level and unity), moves with the water to d. on e, on the other hand, a wet rod, the water also rises; but on the dry rod f the surface is depressed; and as it drives to depress also the wave rising on e in its neighbourhood, the higher wave at e turns away from f[ ]; for it does not suffer itself to be depressed. all electrical attraction occurs through an intervening humour; so it is by reason of humour that all things mutually come together; fluids indeed and aqueous bodies on the surface of water, but concreted things, if they have been resolved into vapour, in air;--in air indeed, the effluvium of electricks being very rare, that it may the better permeate the medium and not impel it by its motion; for if that effluvium had been thick, as that of air, or of the winds, or of saltpetre burnt by fire, as the thick and foul effluvia given out with very great force, from other bodies, or air set free from humour by heat rushing out through a pipe (in the instrument of hero of alexandria, described in his { } book _spiritalia_), then the effluvium would drive everything away, not allure it. but those rarer effluvia take hold of bodies and embrace them as if with arms extended, with the electricks to which they are united; and they are drawn to the source, the effluvia increasing in strength with the proximity. but what is that effluvium from crystal, glass, and diamond, since these are bodies of considerable hardness and firmly concreted? in order that such an effluvium should be produced, there is no need of any marked or perceptible flux[ ] of the substance; nor is it necessary that the electrick should be abraded, or worn away, or deformed. some odoriferous substances are fragrant for many years, exhaling continually, yet are not quickly consumed. cypress wood as long as it is sound, and it lasts a very long time indeed, is redolent; as many learned men attest from experience. such an electrick only for a moment, when stimulated by friction, emits powers far more subtile and more fine beyond all odours; yet sometimes amber, jet, sulphur, when they are somewhat easily let free into vapour, also pour out at the same time an odour; and on this account they allure with the very gentlest rubbing, often even without rubbing; they also excite more strongly, and retain hold for a longer time, because they have stronger effluvia and last longer. but diamond, glass, rock-crystal, * and numerous others of the harder and firmly concreted gems first grow warm: therefore at first they are rubbed longer, and then they also attract strongly; nor are they otherwise set free into vapour. everything rushes towards electricks[ ] excepting flame, and flaming bodies, and the thinnest air. just as they do not draw flame, in like manner they do not affect a versorium, if on any side it is very near to a flame, either the flame of a lamp or of any burning matter. it is manifest indeed that the effluvia are destroyed by flame and igneous * heat; and therefore they attract neither flame nor bodies very near a flame. for electrical effluvia have the virtue of, and are analogous with, extenuated humour; but they will produce their effect, union and continuity, not by the external impulse of vapours, not by heat and attenuation of heated bodies, but by their humidity itself attenuated into its own peculiar effluvia. yet they entice * smoke sent out by an extinguished light; and the more that smoke is attenuated in seeking the upper regions, the less strongly is it turned aside; for things that are too rarefied are not drawn to them; and at length, when it has now almost vanished, it does not * incline towards them at all, which is easily seen against the light. when in fact the smoke has passed into air, it is not moved, as has been demonstrated before. for air itself, if somewhat thin, is not attracted in any way, unless on account of succeeding that which has vacated its place, as in furnaces and such-like, where the air is fed in by mechanical devices for drawing it in. therefore an effluvium resulting from a non-fouling friction, and one which { } is not changed by heat, but which is its own, causes union and coherency, a prehension and a congruence towards its source, if only the body to be attracted is not unfitted for motion, either by the surroundings of the bodies or by its own weight. to the bodies therefore of the electricks themselves small bodies are borne. the effluvia extend out their virtue--effluvia which are proper and peculiar to them, and _sui generis_, differing from common air, being produced from humour, excited by a calorifick motion from attrition and attenuation. and as if they were material rays[ ], they hold and take up chaff, straws, and twigs, until they become extinct or vanish away: and then they (the corpuscles) being loosed again, attracted by the earth itself, fall down to the earth. the difference between magneticks and electricks[ ] is that all magneticks run together with mutual forces; electricks only allure; that which is allured is not changed by an implanted force, but that which has moved up to * them voluntarily rests upon them by the law of matter. bodies are borne towards electricks in a straight line towards the centre of the electrick; a loadstone draws a loadstone directly at the poles only, in other parts obliquely and transversely, and in this way also they adhere and hang to one another. electrical motion is a motion of aggregation of matter; magnetical motion is one of disposition and conformation. the globe of the earth is aggregated and cohæres by itself electrically. the globe of the earth is directed and turned magnetically; at the same time also it both cohæres, and in order that it may be solid, is in its inmost parts cemented together. * * * * * chap. iii. opinions of others on magnetick coition, _which they call attraction_. discussion having now been made concerning electricks, the causes of magnetick coition must be set forth. we say coition, not attraction[ ]. the word attraction unfortunately crept into magnetick philosophy from the ignorance of the ancients; for there seems to be force applied where there is attraction and an imperious violence dominates. for, if ever there is talk about magnetick attraction, we understand thereby magnetick coition, or a primary running together. now in truth it will not be useless here first briefly to set forth the views given by others, both the ancient { } and the more modern writers. orpheus in his hymns[ ] narrates that iron is attracted by loadstone as the bride to the arms of her espoused. epicurus holds that iron is attracted by a loadstone just as straws by amber; "and," he adds, "the atoms and indivisible particles which are given off by the stone and by the iron fit one another in shape; so that they easily cling to one another; when therefore these solid particles of stone or of iron strike against one another, then they rebound into space, being brought against one another by the way, and they draw the iron along with them." but this cannot be the case in the least; since solid and very dense substances interposed, even squared blocks of marble, do not obstruct this power, though they can separate atoms from atoms; and the stone and the iron would be speedily dissipated into such profuse and perpetual streams of atoms. in the case of amber, since there is another different method of attracting, the epicurean atoms cannot fit one another in shape. thales, as aristotle writes, _de anima_, bk. i., deemed the loadstone to be endowed with a soul of some sort, because it had the power of moving and drawing iron towards it. anaxagoras also held the same view. in the _timæus_ of plato there is an idle fancy[ ] about the efficacy of the stone of hercules. for he says that "all flowings of water, likewise the fallings of thunderbolts, and the things which are held wonderful in the attraction of amber, and of the herculean stone, are such that in all these there is never any attraction; but since there is no vacuum, the particles drive one another mutually around, and when they are dispersed and congregated together, they all pass, each to its proper seat, but with changed places; and it is forsooth, on account of these intercomplicated affections that the effects seem to arouse the wonder in him who has rightly investigated them." galen does not know why plato should have seen fit to select the theory of circumpulsion rather than that of attraction (differing almost on this point alone from hippocrates), though indeed it does not agree in reality with either reason or experiment. nor indeed is either the air or anything else circumpelled; and the bodies themselves which are attracted are carried towards the attracting substance not confusedly, or in an orbe. lucretius, the poet of the epicurean sect, sang his opinion of it thus: [ ]_first, then, know,_ _ceaseless effluvia from the magnet flow,--_ _effluvia, whose superior powers expel_ _the air that lies between the stone and steel._ _a vacuum formed, the steely atoms fly_ _in a link'd train, and all the void supply;_ _while the whole ring to which the train is join'd_ _the influence owns, and follows close behind. &c._ { } such a reason plutarch also alleges in the _quæstiones platonicæ_: that that stone gives off heavy exhalations, whereby the adjacent air, being impelled along, condenses that which is in front of it; and that air, being driven round in an orbe and reverting to the place it had vacated, drags the iron forcibly along with it. the following explanation of the virtues of the loadstone and of amber is propounded by johannes costæus of lodi[ ]. for he would have it that "there is mutual work and mutual result, and therefore the motion is partly due to the attraction of the loadstone and partly to a spontaneous movement on the part of the iron: for as we say that vapours issuing from the loadstone hasten by their own nature to attract the iron, so also the air repelled by the vapours, whilst seeking a place for itself, is turned back, and when turned back, it impels the iron, lifts it up, as it were, and carries it along; the iron being of itself also excited somehow. so by being drawn out and by a spontaneous motion, and by striking against another substance, there is in some way produced a composite motion, which motion would nevertheless be rightly referred to attraction, because the terminus from which this motion invariably begins is the same terminus at which it ends, which is the characteristic proper of an attraction." there is certainly a mutual action, not an operation, nor does the loadstone attract in that way; nor is there any impulsion. but neither is there that origination of the motion by the vapours, and the turning of them back, which opinion of epicurus has so often been quoted by others. galen errs in his _de naturalibus facultatibus_, book i., chap. , when he expresses the view that whatever agents draw out either the venom of serpents or darts also exhibit the same power as the loadstone. now of what sort may be the attraction of such medicaments (if indeed it may be called attraction) we shall consider elsewhere. drugs against poisons or darts have no relation to, no similitude with, the action of magnetical bodies. the followers of galen (who hold that purgative medicaments attract because of similitude of substance) say that bodies are attracted on account of similitude, not identity, of substance; wherefore the loadstone draws iron, but iron does not draw iron. but we declare and prove that this happens in primary bodies, and in those bodies that are pretty closely related to them and especially like in kind one to another, on account of their identity; wherefore also loadstone draws loadstone and likewise iron iron; every really true earth draws earth; and iron fortified by a loadstone within the orbe of whose virtue it is placed draws iron more strongly than it does the loadstone. cardan asks why no other metal is attracted by any other stone; because (he replies) no metal is so cold as iron; as if indeed cold were the cause of the attraction, or as if iron were much colder than lead, which neither follows nor is deflected towards a loadstone. { } but that is a chilly story, and worse than an old woman's tale. so also is the notion that the loadstone is alive and that iron is its food. but how does the loadstone feed on the iron, when the filings in which it is kept are neither consumed nor become lighter? cornelius gemma, _cosmographia_, bk. x.[ ], holds that the loadstone draws iron to it by insensible rays, to which opinion he conjoins a story of a sucking fish and another about an antelope. guilielmus puteanus[ ] derives it, "not from any property of the whole substance unknown to any one and which cannot be demonstrated in any way (as galen, and after him almost all the physicians, have asserted), but from the essential nature of the thing itself, as if moving from the first by itself, and, as it were, by its own most powerful nature and from that innate temperament, as it were an instrument, which its substance, its effective nature uses in its operations, or a secondary cause and deprived of its intermediary"; so the loadstone attracts the iron not without a physical cause and for the sake of some good. but there is no such thing in other substances springing from some material form; unless it were primary, which he does not recognize. but certes good is shown to the loadstone by the stroke of the iron (as it were, association with a friend); yet it cannot either be discovered or conceived how that disposition may be the instrument of form. for what can temperament do in magnetical motions, which must be compared with the fixed, definite, constant motions of the stars, at great distances in case of the interposition of very dense and thick bodies? to baptista porta[ ] the loadstone seems a sort of mixture of stone and iron, in such a way that it is an iron stone or stony iron. "but i think" (he says) "the loadstone is a mixture of stone and iron, as an iron stone, or a stone of iron. yet do not think the stone is so changed into iron, as to lose its own nature, nor that the iron is so drowned in the stone, but it preserves itself; and whilst one labours to get the victory of the other, the attraction is made by the combat between them. in that body there is more of the stone than of iron; and therefore the iron, that it may not be subdued by the stone, desires the force and company of iron; that being not able to resist alone, it may be able by more help to defend itself.... the loadstone draws not stones, because it wants them not, for there is stone enough in the body of it; and if one loadstone draw another, it is not for the stone, but for the iron that is in it." as if in the loadstone the iron were a distinct body and not mixed up as the other metals in their ores! and that these, being so mixed up, should fight with one another, and should extend their quarrel, and that in consequence of the battle auxiliary forces should be called in, is indeed absurd. but iron itself, when excited by a loadstone, seizes iron no less strongly than the loadstone. therefore those fights, seditions, and conspiracies in the stone, as if it were nursing up perpetual quarrels, { } whence it might seek auxiliary forces, are the ravings of a babbling old woman, not the inventions of a distinguished mage. others have lit upon sympathy as the cause. there may be fellow-feeling, and yet the cause is not fellow-feeling; for no passion can rightly be said to be an efficient cause. others hold likeness of substance, many others insensible rays as the cause; men who also in very many cases often wretchedly misuse rays, which were first introduced in the natural sciences by the mathematicians. more eruditely does scaliger[ ] say that the iron moves toward the loadstone as if toward its parent, by whose secret principles it may be perfected, just as the earth toward its centre. the divine thomas[ ] does not differ much from him, when in the th book of his _physica_ he discusses the reasons of motions. "in another way," he says, "it may be said to attract a thing, because it moves it to itself by altering it in some way, from which alteration it happens that when altered it moves according to its position, and in this manner the loadstone is said to attract iron. for as the parent moves things whether heavy or light, in as far as it gives them a form, by means of which they are moved to their place; so also the loadstone gives a certain quality to the iron, in accordance with which it moves towards it." this by no means ill-conceived opinion this most learned man shortly afterwards endeavoured to confirm by things which had obtained little credence respecting the loadstone and the adverse forces of garlick. cardinal cusan[ ] also is not to be despised. "iron has," he says, "in the loadstone a certain principle of its own effluence; and whilst the loadstone by its own presence excites the heavy and ponderous iron, the iron is borne by a wonderful yearning, even above the motion of nature (by which in accordance with its weight it ought to tend downwards) and moves upwards, in uniting itself with its own principle. for if there were not in the iron a certain natural foretaste of the loadstone itself, it would not move to the loadstone any more than to any other stone; and unless there were in the stone a greater inclination for iron than for copper, there would not be that attraction." such are the opinions expressed about the loadstone attracting (or the general sense of each), all dubious and untrustworthy. but those causes of the magnetical motions, which in the schools of the philosophers are referred to the four elements and the prime qualities, we relinquish to the moths and the worms. * * * * * { } chap. iiii. on magnetick force & form, what it is; and on the _cause of the coition_. relinquishing the opinions of others on the attraction of loadstone, we shall now show the reason of that coition and the translatory nature of that motion. since there are really two kinds of bodies, which seem to allure bodies with motions manifest to our senses, electricks and magneticks, the electricks produce the tendency by natural effluvia from humour; the magneticks by agencies due to form, or rather by the prime forces. this form is unique, and particular, not the formal cause of the peripateticks, or the specifick in mixtures, or the secondary form; not the propagator of generating bodies, but the form of the primary and chief spheres and of those parts of them which are homogeneous and not corrupted, a special entity and existence, which we may call a primary and radical and astral form; not the primary form of aristotle, but that unique form, which preserves and disposes its own proper sphere. there is one such in each several globe, in the sun, the moon, and the stars; one also in the earth, which is that true magnetick potency which we call the primary vigour. wherefore there is a magnetick nature peculiar to the earth and implanted in all its truer parts in a primary and astonishing manner; this is neither derived nor produced from the whole heaven by sympathy or influence or more occult qualities, nor from any particular star; for there is in the earth a magnetick vigour of its own, just as in the sun and moon there are forms of their own, and a small portion of the moon settles itself in moon-manner toward its termini and form; and a piece of the sun to the sun, just as a loadstone to the earth and to a second loadstone by inclining itself and alluring in accordance with its nature. we must consider therefore about the earth what magnetical bodies are, and what is a magnet; then also about the truer parts of it, which are magnetical, and how they are affected as a result of the coition. a body which is attracted by an electrick is not changed by it, but remains unshaken and unchanged, as it was before, nor does it excel any the more in virtue. a loadstone draws magnetical substances, which eagerly acquire power from its strength, not in their extremities only, but in their inward parts and * their very marrow. for when a rod of iron is laid hold of, it is magnetically excited in the end by which it is laid hold of, and that { } force penetrates even to the other extremity, not through its surface only, but through the interior and all through the middle. electrical bodies have material and corporeal effluvia. is any such magnetical effluvium given off, whether corporeal or incorporeal? or is nothing at all given off that subsists? if it really has a body, that body must be thin and spiritual, since it is necessary that it should be able to enter into iron. or what sort of an exhalation is it that comes from lead, when quicksilver which is bright and fluid is bound together by the odour merely and vapour of the lead, and remains, as it were, a firm metal? but even gold, which is exceedingly solid and dense, is reduced to a powder by the thin vapour of lead. or, seeing that, as the quicksilver has entrance into gold, so the magnetical odour has entrance into the substance of the iron, how does it change it in its essential property, although no change is perceptible to our senses in the bodies themselves? for without ingression into the body, the body is not changed, as the chemists not incorrectly teach. but if indeed these things resulted from a material ingression, then if strong and dense and thick substances had been interposed between the bodies, or if magnetical substances had been inclosed in the centres of the most solid and the densest bodies, the iron particles would not have suffered anything from the loadstone. but none the less they strive to come together and are changed. therefore there is no such conception and origin of the magnetick powers; nor do the very minute portions of the stone exist, which have been wrongly imagined to exist by baptista porta, aggregated, as it were, into hairs, and arising from the rubbing of the stone which, sticking to the iron, constitute its strength. electrick effluvia are not only impeded by any dense matter, but also in like manner by flames, or if a small flame is near, they do not allure. but as iron is not hindered by any obstacle from receiving force or motion from a loadstone, so it will pass through the midst of flames to the body of the loadstone and adhære to the stone. let there be a flame or a candle near the stone; bring up a short piece of iron wire, and when it has come near, it will penetrate through the midst of the flames to the stone; * and a versorium turns towards the loadstone nor more slowly nor less eagerly through the midst of flames than through open air. so flames interposed do not hinder the coition. but if the iron itself became heated by a great heat, it is demonstrable that it would not be attracted. bring a strongly ignited rod of iron near a magnetized versorium; the versorium remains steady and does not turn towards * such iron; but it immediately turns towards it, so soon as it has lost somewhat of its heat. when a piece of iron has been touched by a loadstone, if it be placed in a hot fire until it is perfectly red hot * and remain in the fire some considerable time, it will lose that magnetick strength it had acquired. even a loadstone itself through a { } longish stay in the fire, loses the powers of attracting implanted and innate in it, and any other magnetick powers. and although certain veins of loadstone exhale when burnt a dark vapour of a black colour, or of a sulphurous foul odour, yet that vapour was not the soul, or the cause of its attraction of iron (as porta thinks), nor do all loadstones whilst they are being baked or burnt smell of or exhale sulphur. it is acquired as a sort of inborn defect from a rather impure mine or matrix. nor does anything analogous penetrate into the iron from that material corporeal cause, since the iron conceives the power of attracting and verticity from the loadstone, even if glass or gold or any other stone be interposed. then also cast iron acquires the power of attracting iron, and verticity, from the verticity of the earth, as we shall afterwards plainly demonstrate in _direction_. but fire destroys the magnetick virtues in a stone, not because it takes away any parts specially attractive, but because the consuming force of the flame mars by the demolition of the material the form of the whole; as in the human body the primary faculties of the soul are not burnt, but the charred body remains without faculties. the iron indeed may remain after the burning is completed and is not changed into ash or slag; nevertheless (as cardan not inaptly says) burnt iron is not iron, but something placed outside its nature until it is reduced. for just as by the rigour of the surrounding air[ ] water is changed from its nature into ice; so iron, glowing in fire, is destroyed by the violent heat, and has its nature confused and perturbed; wherefore also it is not attracted by a loadstone, and even loses that power of attracting in whatever way acquired, and acquires another verticity when, being, as it were, born again, it is impregnated by a loadstone or the earth, or when its form is revived, not having been dead but confused, concerning which many things are manifest in the change of verticity. wherefore fracastorio[ ] does not confirm his opinion, that the iron is not altered; "for if it were altered," he says, "by the form of the loadstone, the form of the iron would have been spoiled." this alteration is not generation, but the restitution and reformation of a confused form. there is not therefore anything corporeal which comes from the loadstone or which enters the iron, or which is sent back from the iron when it is stimulated; but loadstone disposes loadstone by its primary form; iron, however, which is closely related to it, loadstone at the same time recalls to its conformate strength, and settles it; on account of which it rushes to the loadstone and eagerly conforms itself to it (the forces of each in harmony bringing them together). the coition also is not vague or confused, not a violent inclination of body to body, no rash and mad congruency; no violence is here applied to the bodies; there are no strifes or discords; but there is that concord (without which the universe would go to pieces), that analogy, namely, of the { } perfect and homogeneous parts of the spheres of the universe to the whole, and a mutual concurrency of the principal forces in them, tending to soundness, continuity, position, direction, and to unity. wherefore in the case of such wonderful action and such a stupendous implanted vigour (diverse from other natures) the opinion of thales of miletus[ ] was not very absurd, nor was it downright madness, in the judgment of scaliger, for him to grant the loadstone a soul; for the loadstone is incited, directed, and orbitally moved by this force, which is all in all, and, as will be made clear afterwards, all in every part; and it seems to be very like a soul. for the power of moving itself seems to point to a soul; and the supernal bodies, which are also celestial, divine, as it were, are thought by some to be animated, because they move with admirable order. if two loadstones be set one over against the other, each in a boat, on the surface of water, they do not immediately run together, but first they turn towards one another, or the lesser conforms to the greater, by moving itself in a somewhat circular manner, and at length, when they are disposed according to their nature, they run together. in smelted iron which has not been excited by a magnet there is no need for such an apparatus; since it has no verticity, excepting what is adventitious and acquired, and that not stable and confirmed (as is the case with loadstone, even if the iron has been smelted from the best loadstone), on account of the confusion of the parts by fire when it flowed as a liquid; it suddenly acquires polarity and natural aptitude by the presence of the loadstone, by a powerful mutation, and by a conversion into a perfect magnet, and by an absolute metamorphosis; and it flies to the body of the magnet as if it were a real piece of loadstone. for a loadstone has no power, nor can a perfect loadstone do anything which iron when excited by loadstone cannot perform, even when it has not been touched but only placed in its vicinity. for when first it is within the orbe of virtue of the loadstone, though it may be some distance away, yet it is immediately changed, and has a renovated form, formerly indeed dormant and inert in body, now lively and strong, which will be clearly apparent in the demonstrations of _direction_. so the magnetick coition is a motion of the loadstone and of the iron, not an action of one[ ]; an [greek: entelecheia], of each, not [greek: ergon]; a [greek: sunentelecheia] or conjoint action, rather than a sympathy. there is properly no such thing as magnetick antipathy. for the flight and declination of the ends, or an entire turning about, is an action of each towards unity by the conjoint action and [greek: sunentelecheia] of both. it has therefore newly put on the form, and on account of this being roused, it then, in order that it may more surely acquire it, rushes headlong on the loadstone, not with curves and turnings, as a loadstone to a loadstone. for since in a loadstone both verticity and the power disponent have existed through many ages, or from the very beginnings, { } have been inborn and confirmed, and also the special form of the terrestrial globe cannot easily be changed by another loadstone, as iron is changed; it happens from the constant nature of each, that one has not the sudden power over another of changing its verticity, but that they can only mutually come to agreement with each other. again, iron which has been excited by a loadstone, * if that iron on account of obstacles should not be able to turn round immediately in accordance with its nature, as happens with a versorium, is laid hold of, when a loadstone approaches, on either side or at either end. because, just as it can implant, so it can suddenly change the polarity and turn about the formal energies to any part whatever. so variously can iron be transformed when its form is adventitious and has not yet been long resident in the metal. in the case of iron, on account of the fusion of the substance when magnetick ore or iron is smelted, the virtue of its primary form, distinct before, is now confused; but an entire loadstone placed near it again sets up its primal activity; its adjusted and arranged form joins its allied strength with the loadstone; and both mutually agree and are leagued together magnetically in all their motions towards unity, and whether joined by bodily contact or adjusted within the orbe, they are one and the same. for when iron is smelted out of its own ore, or steel (the more noble kind of iron) out of its ore, that is, out of loadstone, the material is loosed by the force of the fire, and flows away, and iron as well as steel flow out from their dross and are separated from it; and the dross is either spoiled by the force of the fire and rendered useless, or is a kind of dregs of a certain imperfection and of mixture in the prominent parts of the earth. the material therefore is a purified one, in which the metallick parts, which are now mixed up by the melting, since those special forces of its form are confused and uncertain, by the approach of a loadstone are called back to life, as if to a kind of disponent form and integrity. the material is thus awakened and moves together into unity, the bond of the universe and the essential for its conservation. on this account and by the purging of the material into a cleaner body, the loadstone gives to the iron a greater force of attracting than there is in itself. for if iron dust * or an iron nail be placed over a large loadstone, a piece of iron joined to it takes away the filings and nail from the loadstone and retains them so long as it is near the loadstone; wherefore iron attracts iron more than loadstone does, if it have been conformed by a loadstone and remains within the orbe of its communicated form. a piece of iron even, skilfully placed near the pole of a loadstone, lifts up more than the loadstone. therefore the material of its own ore is better, and by the force of fire steel and iron are re-purged; and they are again impregnated by the loadstone with its own forms; therefore they move towards it by a spontaneous { } approach as soon as they have entered within the orbe of the magnetick forces, because they were possessed by it before, connected and united with it in a perfect union; & they have immediately an absolute continuity within that orbe, & have been joined on account of their harmony, though their bodies may have been disjoined. for the iron is not taken possession of and allured by material effluvia, after the manner of electricks, but only by the immaterial action of its form or an incorporeal progression, which in a piece of iron as its subject acts and is conceived, as it were, in a continuous homogeneous body, and does not need more open ways. therefore (though the most solid substances be interposed) the iron is still moved and attracted, and by the presence of loadstone the iron moves and attracts the loadstone itself, and by mutual forces a concurrency is made towards unity, which is commonly called attraction of the iron. but those formal forces pass out and are united to one another by meeting together; a force also, when conceived in the iron, begins to flow out without delay. but julius scaliger, who by other examples contends that this theory is absurd, makes in his th exercise a great mistake. for the virtues of primary bodies are not to be compared with bodies formed from and mixed with them. he would now have been able (had he been still alive) to discern the nature of effused forms in the chapter on forms effused by spherical magneticks. but if iron is injured somewhat by rust, it is affected either only slightly or not at all by the stone. for the metal is spoiled when eaten away and deformed by external injuries or by lapse of time (just as has been said about the loadstone), and it loses its prime qualities which are conjoined to its form; or, being worn out by age, retains them in a languid and weak condition; indeed it cannot be properly re-formed, when it has been corrupted. but a powerful and fresh loadstone attracts sound and clean pieces of iron, and those pieces of iron (when they have conceived strength) have a powerful attraction for other iron wires and iron nails, not only one at a time, but even successively one behind another, three, four or five, end to end, sticking and hanging in order like a chain. the loadstone, however, would not attract the last one following in such a row, if there were no nails between. [illustration] a loadstone placed as at a draws a nail or a bar b; similarly behind b it draws c; and after c, d. but the nails b and c being removed, the loadstone a, if it remain at the same distance, does not raise the nail d into the air. this occurs for this reason: because in the case of a continuous row of nails the presence of the loadstone a, besides its own powers, raises the magnetick natures of the iron works b and c, and makes them, as it were, forces auxiliary to itself. but b and c, like a continuous magnetical body, extend as far as { } d the forces by which d is taken and conformed, though they are weaker than those which c receives from b. and those iron nails indeed from that contact only, and from the presence of the loadstone even without contact, acquire powers which they retain in their own bodies, as will be demonstrated most clearly in the passage _on direction_. for not only whilst the stone is present does the iron assume these powers, and take them, as it were, vicariously from the stone, as themistius lays down in his th book on physicks[ ]. the best iron, when it has been melted down (such is steel), is allured by a loadstone from a greater distance, is raised though of greater weight, is held more firmly, assumes stronger powers than the common and less expensive, because it is cast from a better ore or loadstone, imbued with better powers. but what is made from more impure ore turns out weaker and is moved more feebly. as to fracastorio's[ ] statement that he saw a piece of loadstone draw a loadstone by one of its faces, but not iron; by another face iron, but not loadstone; by another both; which he says is an indication that in one part there is more of the loadstone, in another more of the iron, in another both equally, whence arises that diversity of attraction; it is most incorrect and badly observed on the part of fracastorio, who did not know how to apply skilfully loadstone to loadstone. a loadstone draws iron and also a loadstone, if both are suitably arranged and free and unrestrained. that is removed more quickly from its position and place which is lighter; for the heavier bodies are in weight, the more they resist; but the lighter both moves itself to meet the heavier and is allured by the other. * * * * * chap. v. how the power dwells in the _loadstone_. that a loadstone attracts loadstone, iron and other magnetical bodies, has been shown above in the previous book, and also with what strength the magnetick coition is ordered; but now we must inquire how that vigour is disposed in a magnetick substance. and indeed an analogy must be inferred from a large loadstone. any magnetick substance joins itself with a loadstone strongly, if the loadstone itself is strong; but more weakly, when it is somewhat imperfect or has been weakened by some flaw. a loadstone does not draw iron equally well with every part; or a magnetick substance does not approach every part of a loadstone alike; because a loadstone has its points, that is its true poles, in which an exceptional virtue excels. parts nearer the pole are { } stronger, those far away more weak, and yet in all the power is in a certain way equal. the poles of a terrella are a, b; the æquinoctial is c, d. at a and b the alluring force seems greatest. [illustration] at c and d there is no force alluring magnetick ends to the body, for the forces tend toward both poles. but direction is powerful on the æquator. at c, d, the distances are equal from both poles; therefore iron which is at c, d, when it is allured in contrary ways, does not adhære with constancy; but it remains and is joined to the stone, if only it incline to the one or other side. at e there is a greater power of alluring than at f, because e is nearer the pole. this is not so because there is really greater virtue residing at the pole, but since all the parts are united in the whole, they direct their forces towards the pole. from the forces flowing from the plane of the æquinoctial towards the pole, the power increases. a fixed verticity exists at the pole, so long as the loadstone remains whole; if it is divided or broken, the verticity obtains other * positions in the parts into which it is divided. for the verticity always changes in consequence of any change in the mass, and for this cause, if the terrella be divided from a to b, so that there are two stones, the poles will not be a, b, in the divided parts, but f, g, and h, i. [illustration] { } although these stones now are in agreement with one another, so that f would not seek h, yet if a was previously the boreal pole[ ], f is now boreal, and h also boreal; for the verticity is not changed (as baptista porta incorrectly affirms in the fourth chapter of his seventh book); since, though f and h do not agree, so that the one would incline to the other, yet both turn to the same point of the horizon. if the hemisphere h i be divided into two quadrants, the one pole takes up its position in h, the other in i. the whole mass of the stone, as i have said, retains the site of its vertex constant; and any part of the stone, before it was cut out from the block[ ], might have been the pole or vertex. but concerning this more under _direction_. it is important now to comprehend and to keep firmly in mind that the vertices are strong on account of the force of the whole, so that (the command being, as it were, divided by the æquinoctial) all the forces on one side tend towards the north; but those of an opposite way towards the south, so long as the parts are united, as in the following demonstration. [illustration] for so, by an infinite number of curves from every point of the equator dividing the sphere into two equal parts, and from every point of the surface from the æquator towards the north, and from the æquator towards the southern pole, the whole force tends asunder toward the poles. so the verticity is from the æquinoctial { } circle towards the pole in each direction. such is the power reposed in the undivided stone. from a vigour is sent to b, from a, b, to c, from a, b, c, to d, and from them likewise to e. in like manner from g to h, and so forth, as long as the whole is united. but if a piece a b be cut out (although it is near the æquator), yet it will be as strong in its magnetical actions as c d or d e, if torn away from the whole in equal quantity. for no part excels in special worth in the whole mass except by what is owing to the other adjoining parts by which an absolute and perfect whole is attained. _diagram of magnetic vigour transmitted from the plane of the Æquator to the peripherery of the terella or of the earth_ [illustration] { } heq is a terrella, e a pole, m the centre, hmq the æquinoctial plane. from every point of the æquinoctial plane vigour extends to the periphery, but by various methods; for from a the formal force is transmitted towards c, f, n, e, and to every point from c up to e, the pole; but not towards b; so neither from g towards c. the power of alluring is not strengthened in the part fhg from that which is in gmfe, but fgh increases the force in the eminence fe. so no force rises from the internal parts, from the lines parallel to the axis above those parallels, but always inwards from the parallels to the pole. from every point of the plane of the equator force proceeds to the pole e, but the point f has its powers only from gh, and n from oh; but the pole e is strengthened from the whole plane hq. wherefore in it the mighty power excels (just as in a palace); but in the intermediate intervals (as in f) only so much force of alluring is exerted as the portion hg of the plane can contribute. * * * * * chap. vi. how magnetick pieces of iron and smaller _loadstones conform themselves to a terrella & to_ the earth itself, and by them are _disposed_. coition of those bodies which are divided, and do not naturally cohære, if they are free, occurs through another kind of motion. a terrella sends out in an orbe its powers in proportion to its vigour and quality. but when iron or any other magnetick of convenient magnitude comes within its orbe of virtue, it is allured; but the nearer it comes to the body, the more quickly it runs up to it. they move towards the magnet, not as * to a centre, nor towards its centre. for they only do this in the case of the poles themselves, when namely that which is being allured, and the pole of the loadstone, and its centre, are in the same straight line. but in the intervening spaces they tend obliquely, just as is evident in the following figure, in which it is shown how the influence is extended to the adjoining magneticks within the orbe; in the case of the poles straight out. { } [illustration] the nearer the parts are to the æquinoctial, the more obliquely are magneticals allured; but the parts nearer the poles appeal more directly, at the poles quite straight. the principle of the turning of all loadstones, of those which are round and those which are long, is the same, but in the case of the long ones the experiment is easier. for in whatever form they are the verticity exists, and there are poles; but on account of bad and unequal form, they are often hindered by certain evils. if the stone were long, the vertex is at the ends, not on the sides; it allures more strongly at the vertex. for the parts bring together stronger forces to the pole in right lines than oblique. so the stone and the earth conform their magnetick motions by their nature. * * * * * chap. vii. on the potency of the magnetick virtue, and on its nature capable of spreading out into an orbe. from about a magnetical body the virtue magnetical is poured out on every side around in an orbe; around a terrella; in the case of other shapes of stones, more confusedly and unevenly. but yet there exists in nature no orbe or permanent or essential virtue spread through the air, but a magnet { } only excites magneticks at a convenient distance from it. and as light comes in an instant (as the opticians teach), so much more[ ] quickly is the magnetick vigour present within the limits of its strength; and because its activity is much more subtile than light, and does not consent with a non-magnetick substance, it has no intercourse with air, water, or any non-magnetick; nor does it move a magnetick with any motion by forces rushing upon it, but being present in an instant, it invites friendly bodies. and as light strikes an object, so a loadstone strikes a magnetick body and excites it. and just as light does not remain in the air above vapours and effluvia, and is not reflected from those spaces, so neither is the magnetick ray held in air or water. the appearances of things are apprehended in an instant in mirrors and in the eye by means of light; so the magnetick virtue seizes upon magneticks. without the more intangible and shining bodies, the appearances of things are not seized or reflected; so without magnetical objects the magnetick power is not perceived, nor are the forces thus conceived sent back again to the magnetick substance. in this, however, the magnetick power excels light, in that it is not hindered by any opaque or solid substance, but proceeds freely, and extends its forces on every side. in a terrella and globe-shaped loadstone the magnetick power is extended outside the body in an orbe; in a longer one, however, not in an orbe, but it is extended in an ambit conformably to the shape of the stone. as in the somewhat long stone a, the vigour is extended to the ambient limit f c d, equidistant on every side from the stone a. [illustration] * * * * * { } chap. viii. on the geography of the earth, _and of the terrella_. desiring that what follows may be better understood, we must now say something also about magnetick circles and limits. astronomers, in order to understand and observe methodically the motion of the planets and the revolution of the heavens, and to describe with more accuracy the celestial attire of the fixed stars, settled upon certain circles and definite limits in the sky (which geographers also imitate), so that the varied face of the earth and the beauty of its districts might be delineated. but we, in a way differing from them, recognize those limits and circles, and have found very many fixed by nature, not merely conceived by the imagination, both in the earth and in our terrella. the earth they mark out[ ] chiefly by means of the æquator and the poles; and those limits indeed have been arranged and marked out by nature. the meridians also indicate straight paths from pole to pole through distinct points on the æquator; by which way the magnetick virtue directs its course and moves. but the tropics and arctic circles, as also the parallels, are not natural limits placed on the earth; but all parallel circles indicate a certain agreement of the lands situated in the same latitude, or diametrically opposite. all these the mathematicians use for convenience, painting them on globes and maps. in like manner also in a terrella all these are required; not, however, in order that its exterior appearance may be geographically delineated, since the loadstone may be perfect, even, and uniform on all sides. and there are no upper and lower parts in the earth, nor are there in a terrella; unless perchance some one considers those parts superior which are in the periphery, and those inferior which are situated more towards the centre. * * * * * { } chap. ix. on the Æquinoctial circle of the earth _and of a terrella_. as conceived by astronomers the æquinoctial circle is equidistant from both poles, cutting the world in the middle, measures the motions of their _primum mobile_ or tenth sphere, and is named the zone of the _primum mobile_. it is called æquinoctial, because when the sun stands in it (which must happen twice in the year) the days are equal to the nights. that circle is also spoken of as _æquidialis_, wherefore it is called by the greeks [greek: isêmerinos]. in like manner it is also properly called Æquator, because it divides the whole frame of the earth between the poles into equal parts. so also an æquator may be rightly assigned to a terrella, by which its power is naturally divided, and by the plane of which permeating through its centre, the whole globe is divided into equal parts both in quantity and strength (as if by a transverse septum) between verticities on both sides imbued with equal vigour. * * * * * chap. x. magnetick meridians of the earth. meridians have been thought out by the geographer, by means of which he might both distinguish the longitude and measure the latitude of each region. but the magnetick meridians are infinite, running in the same direction also, through fixed and opposite limits on the æquator, and through the poles themselves. on them also the magnetick latitude is measured, and declinations are reckoned from them; and the fixed direction in them tends to the poles, unless it varies from some defect and the magnetick is disturbed from the right way. what is commonly called a magnetick meridian is not really magnetick, nor is it really a meridian, but it is understood to pass through the termini of the variation on the horizon. the variation is a depraved deviation from a meridian, nor is it fixed and constant in various places on any meridian. * * * * * { } chap. xi. parallels. in parallel circles the same strength and equal power are perceived everywhere, when various magneticks are placed on one and the same parallel either on the earth or on a terrella. for they are distant from the poles by equal intervals and have equal tendencies of declination, and they are attracted and held, and they come together with like forces; just as those regions which are situated under the same parallel, even if they differ in longitude, yet we say possess the same quantity of daylight and a climate equally tempered. * * * * * chap. xii. the magnetick horizon. horizon is the name given to the great circle, separating the things which are seen from those which are not seen; so that a half part of the heaven always is open and easily seen by us, half is always hidden. this seems so to us on account of the great distance of the star-bearing orbe: yet the difference is as great as may arise from the ratio of the semi-diameter of the earth compared with the semi-diameter of the starry heaven, which difference is in fact not perceived by our senses. we maintain, however, that the magnetick horizon is a plane level throughout touching the earth or a terrella in the place of some one region, with which plane the semi-diameter, whether of the earth or of the terrella, produced to the place of the region, makes right angles on every side. such a plane is to be considered in the earth itself and also in the terrella, for magnetick proofs and demonstrations. for we consider the bodies themselves only, not the general appearances of the world. therefore not with the idea of outlook (which varies with the elevations of the lands), but taking it as a plane which makes equal angles with the perpendicular, we accept in magnetick demonstrations a sensible horizon or boundary, not that which is called by astronomers the rational horizon. * * * * * { } chap. xiii. on the axis and magnetick poles. let the line be called the axis which is drawn in the earth (as in a terrella) through the centre to the poles. they are called [greek: poloi] by the greeks from [greek: polein], to turn, and by the latins they are also called _cardines_ or _vertices_; because the world rotates and is perpetually carried around them. we are about to show, indeed, that the earth and a terrella are turned about them by a magnetick influence. one of them in the earth, which looks towards the cynosure, is called boreal and arctic; the other one, opposite to this, is called austral and antarctic. nor do these also exist on the earth or on a terrella for the sake of the turning merely; but they are also limits of direction and position, both as respects destined districts of the world, and also for correct turnings among themselves. * * * * * chap. xiiii. why at the pole itself the coition is stronger than in _the other parts intermediate between the æquator and the pole;_ and on the proportion of forces of the coition in _various parts of the earth and of the terrella_. observation has already been made that the highest power of alluring exists in the pole, and that it is weaker and more languid in the parts adjacent to the æquator. and as this is apparent in the declination, because that disponent and rotational virtue has an augmentation as one proceeds from the Æquator towards the poles: so also the coition of magneticks grows increasingly fresh by the same steps, and in the same proportion. for in the parts more remote from the poles the loadstone does not draw magneticks straight down towards its own viscera; but they tend obliquely and they allure obliquely. for as the smallest chords in a circle differ from the diameter, so much do the forces of attracting differ between themselves in different parts of the terrella. { } for since attraction is coition towards a body, but magneticks run together by their versatory tendency, it comes about that in the diameter drawn from pole to pole the body appeals directly, but in other places less directly. so the less the magnetick is turned toward the body, the less, and the more feebly, does it approach and adhære. [illustration] just as if a b were the poles and a bar of iron or a magnetick fragment c is allured at the part e; yet the end laid hold of does not tend towards the centre of the loadstone, but verges obliquely towards the pole; and a chord drawn from that end obliquely as the attracted body tends is short; therefore it has less vigour and likewise less inclination. but as a greater chord proceeds from a body at f, so its action is stronger; at g still longer; longest at a, the pole (for the diameter is the longest way) to which all the parts from all sides bring assistance, in which is constituted, as it were, the citadel and tribunal of the whole province, not from any worth of its own, but because a force resides in it contributed from all the other parts, just as all the soldiers bring help to their own commander. wherefore also a slightly longer stone attracts more than a spherical one, since the length from pole to pole is extended, even if the stones are both from the same mine and of the same weight and size. the way from pole to pole is longer in a longer stone, and the forces brought together from other parts are not so scattered as in a round magnet and terrella, and in a narrow one they agree more and are better united, and a united stronger force excels and is preeminent. a much weaker office, however, does a plane or oblong stone perform, when the length is extended according to the leading of the parallels, and the pole stops neither on the apex nor in the circle and orbe, but is spread over the flat. wherefore also it invites a friend wretchedly, and feebly retains him, so that it is esteemed as one of an abject and contemptible class, according to its less apt and less suitable figure. * * * * * { } chap. xv. * the magnetick virtue which is conceived in iron is more apparent in an iron rod than in a piece of iron that _is round, square, or of other figure_. duly was it said before that the longer magnet attracts the greater weight of iron[ ]; so also in a longish piece of iron which has been touched the magnetick force conceived is stronger when the poles exist at the ends. for the magnetick forces which are driven from the whole in every part into the poles are not scattered but united in the narrow ends. in square and other angular figures the influence is dissipated, and does not proceed in straight lines or in convenient arcs. suppose also an iron globe have the shape of the earth, yet for the same reasons it drags magnetick substances less; wherefore a small iron sphere, when excited, draws another piece of iron more sluggishly than an excited rod of equal weight. * * * * * chap. xvi. showing that movements take place by the magnetical vigour though solid bodies lie between; and on _the interposition of iron plates_. float a piece of iron wire on the surface of water by transfixing it through a suitable cork; or set a versatory piece of iron on a pin or in a seaman's compass (a magnet being brought near or moved about underneath), it is put into a state of motion; neither the water, nor the vessel, nor the compass-box offering resistance in any way. thick boards do not obstruct[ ], nor earthen vessels nor marble vases, nor the metals themselves; nothing is so solid as to carry away or impede the forces excepting an iron plate. everything which is interposed (even though it is very dense) does not carry away its influence or obstruct its path, or indeed in any way hinder, diminish, or retard it. but all the force is not suppressed by an iron plate, but it is in some measure diverted aside. for when the vigour passes into the middle of an iron plate within the orbe of the magnetick virtue or placed just { } opposite the pole of the stone, that virtue is scattered in very large measure towards its extremities; so that the edges of a small round * plate of suitable size allure iron wires on every side. this is also apparent in the case of a long iron wand, which, when it has been touched by a magnet in the middle, has a like verticity at either end. * [illustration] b is a loadstone, c d a long rod magnetized in the middle a; e being the boreal pole; c is an austral end or pole; in like manner also the end d is another austral pole. but observe here the exactness with which a versorium touched by a pole, when a round plate is interposed, turns towards the same pole in the same * way as before the interposition, only weaker; the plate not standing in the way, because the vigour is diverted through the edges of the small plate, and passes out of its straight course, but yet the plate retains in the middle the same verticity, when it is in the neighbourhood of that pole, and close to it; wherefore the versorium tends towards the plate, having been touched by the same pole. if a loadstone is rather weak, a versorium hardly turns when a plate is put in between; for the vigour of the rather weak loadstone, being diffused through the extremities, passes less through the * middle. but if the plate has been touched in this way by a pole in the middle and has been removed from the stone outside its orbe of virtue, then you will see the point of the same versorium tend in the contrary direction and desert the centre of the small plate, which formerly it desired; for outside the orbe of virtue it has an opposite verticity, in the vicinity the same; for in the vicinity it is, as it were, a part of the loadstone, and has the same pole. [illustration] a is an iron plate near the pole, b a versorium which tends with its point towards the centre of the small plate, which has been touched by the pole of the loadstone c. but if the same small plate be { } placed outside the orbe of magnetick virtue, the point will not turn towards its centre, but the cross e of the same versorium does. but an iron globe interposed (if it is not too large) attracts the * point of the iron on the other side of the stone. for the verticity of that side is the same as that of the adjoining pole of the stone. and this turning of the cusp (that is, of the end touched by that pole) as well as of the cross-end, at a greater distance, takes place with an iron globe interposed, which would not happen at all if * the space were empty, because the magnetick virtue is passed on and continued through magnetick bodies. [illustration] a is a terrella, b an iron globe; between the two bodies is f, a versorium whose point has been excited by the pole c. in the other figure a is a terrella, c its pole, b an iron globe; where the versorium tends towards c, the pole of the terrella, through the iron globe. so a versorium placed between a terrella and an iron globe vibrates more forcibly towards the pole of the terrella; because the loadstone sends an instantaneous verticity into the opposite globe. there is the same efficiency in the earth, produced from the same cause. for if a revolvable needle is shut up in a rather thick gold box (this metal indeed excels all others in density) or a glass or stone box, nevertheless that magnetick needle has its forces connected and united with the influences of the earth, and the iron will turn freely and readily (unhindered by its prison) to its desired points, north and south. * it even does this when shut up in iron caverns, if they are sufficiently spacious. whatever bodies are produced among us, or are artificially forged from things which are produced, consist of matter of the terrestrial globe; nor do those bodies hinder the prime forces of nature which are derived from their primary form, nor can they resist them except by contrary forms. but no forms of mixed bodies are inimical to the primary implanted earth-nature, although some often do not agree[ ] with one another. but in the case of all those substances which have a material cause for their inclining (as amber, { } jet, sulphur), their action is impeded by the interposition of a body (as paper, leaves, glass, or the like) when that way is impeded and obstructed, so that that which exhales[ ] cannot reach the corpuscle to be allured. terrestrial and magnetick coition and motion, when corporeal impediments are interposed, is demonstrated also by the efficiencies of other chief bodies due to their primary form. the moon (more than all the stars) agrees with internal parts of the earth on account of its nearness and similarity in form. the moon produces the movements of the waters and the tides of the sea; twice it fills up the shores and empties them whilst it moves from a certain definite point in the sky back to the same point in a daily revolution. this motion of the waters is incited and the seas rise and fall no less when the moon is below the horizon and in the lowest part of the heavens, than if it had been raised at a height above the horizon. so the whole mass of the earth interposed[ ] does not resist the action of the moon, when it is below the earth; but the seas bordering on our shores, in certain positions of the sky when it is below the horizon, are kept in motion, and likewise stirred by its power (though they are not struck by its rays nor illuminated by its light), rise, come up with great force, and recede. but about the reason of the tides anon[ ]; here let it suffice to have merely touched the threshold of the question. in like manner nothing on the earth can be hidden from the magnetick disposition of the earth or of the stone, and all magnetical bodies are reduced to order by the dominant form of the earth, and loadstone and iron show sympathy with a loadstone though solid bodies be interposed. * * * * * chap. xvii. on the iron cap of a loadstone, with which it is armed at the pole (for the sake of the _virtue) and on the efficacy of the same._ conceive a small round plate, concave in shape, of the breadth of a digit to be applied to the convex polar surface of a loadstone and skilfully attached; or a piece of iron shaped like an acorn, rising from the base into an obtuse cone, hollowed out a little and fitted to the surface of the stone, to be tied to the loadstone. let the iron be the best steel, smoothed, shining, and even. a loadstone with such an appliance, which before only bore four ounces of iron, will now raise twelve. but the greatest force of a combining or rather united nature is seen { } when two loadstones, armed with iron caps, are so joined by their concurrent (commonly called contrary) ends, that they mutually * attract and raise one another. in this way a weight of twenty ounces is raised, when either stone unarmed would only allure four ounces of iron. iron unites to an armed loadstone more firmly than to a loadstone; and on that account raises greater weights, because the pieces of iron stick more pertinaciously to one that is armed. for by the near presence of the magnet they are cemented together, and since the armature[ ] conceives a magnetick vigour from its presence and the other conjoined piece of iron is at the same time endued with vigour from the presence of the loadstone, they are firmly bound together. therefore by the mutual contact of strong pieces of iron, the cohesion is strong. which thing is also made clear and is exhibited by means of rods sticking together, bk. , chap [ ]; and also when the question of the concretion of iron dust into a united body was discussed. for this reason a piece of iron set near a loadstone draws away any suitable piece of iron from the loadstone, if only it touch the iron; otherwise it does not snatch it away, though in closest proximity. for magnetick pieces of iron within the orbe of virtue, or near a loadstone, do not rush together with a greater endeavour[ ] than the iron and the magnet; but joined they are united more strongly and, as it were, cemented together, though the substance remain the same with the same forces acting. * * * * * chap. xviii. an armed loadstone does not endow an excited piece of iron with greater vigour _than an unarmed_. suppose there are two pieces of iron, one of * which has been excited by an armed loadstone, the other by one unarmed; and let there be applied to one of them another piece of iron of a weight just proportional to its strength, it is manifest that the remaining one in like manner raises the same and no more. magnetick versoria also touched by an armed loadstone turn with the same velocity and constancy towards the poles of the earth as those magnetized by the same loadstone unarmed. * * * * * { } chap. xix. union with an armed loadstone is stronger; _hence greater weights are raised; but the_ coition is not stronger[ ], but _generally weaker_. an armed magnet raises a greater weight, as is manifest to all; but a piece of iron moves towards a stone at an equal, or rather greater, distance when it * is bare, without an iron cap. this must be tried with two pieces of iron of the same weight and figure at an equal distance, or with one and the same versorium, the test being made first with an armed, then with an unarmed loadstone, at equal distances. * * * * * [illustration] chap. xx. * an armed loadstone raises an armed loadstone, _which also attracts a third; which likewise_ happens, though the virtue in the first _be somewhat small_. magnets armed cohære firmly when duly joined, and accord into one; and though the first be rather weak, yet the second one adhæres to it not only by the strength of the first, but of the second, which mutually give helping hands; also to the second a third often adheres and in the case of robust stones, a fourth to the third. * * * * * { } chap. xxi. * if paper or any other medium be interposed, an armed loadstone raises no more than an _unarmed one_. observation has shown above that an armed loadstone does not attract at a greater distance than an unarmed one; yet raises iron in greater quantity, if it is joined to and made continuous with the iron. but if paper be placed between, that intimate cohæsion of the metal is hindered, nor are the metals cemented together at the same time by the operation of the magnet. * * * * * [illustration] chap. xxii. * that an armed loadstone draws iron no more than an _unarmed one: and that an armed one is more strongly united_ to iron is shown by means of an armed loadstone _and a polished cylinder of iron_. if a cylinder be lying on a level surface, of too great a weight for an unarmed loadstone to lift, and (a piece of paper being interposed) if the pole of an armed loadstone be joined to the middle of it; if the cylinder were drawn from there by the loadstone, it would follow rolling; but if no medium were interposed, the cylinder would be drawn along firmly united with the armed loadstone, and in no wise rolling. but if the same loadstone be unarmed, it will draw the cylinder rolling with the same speed as the armed loadstone with the paper between or when it was wrapped in paper. armed loadstones of diverse weights, of the same ore vigour * and form, cling and hang to pieces of iron of a convenient size and proportionate figure with an equal proportion of strength. the same is apparent in the case of unarmed stones. a suitable piece * of iron being applied to the lower part of a loadstone, which is * hanging from a magnetick body, excites its vigour, so that the loadstone hangs on more firmly. for a pendent loadstone clings { } more firmly to a magnetick body joined to it above with a hanging piece of iron added to it, than when lead or any other non-magnetick body is hung on. a loadstone, whether armed or unarmed, * joined by its proper pole to the pole of another loadstone, armed or unarmed, makes the loadstone raise a greater weight by the opposite end[ ]. a piece of iron also applied to the pole of a magnet produces the same result, namely, that the other pole will carry a greater weight of iron; just as a loadstone with a piece of iron superposed on it (as in this figure) holds up a piece of iron below, which it cannot hold, if the upper one be removed. * magneticks in conjunction make one magnetick. wherefore as the mass increases, the magnetick vigour is also augmented. an armed loadstone, as well as an unarmed * one, runs more readily to a larger piece of iron and combines more firmly with a larger piece than with a lesser one. * * * * * chap. xxiii. magnetick force causes motion towards unity, _and binds firmly together bodies which are united_. magnetick fragments cohære within their strength well and harmoniously together. pieces of iron in the presence of a loadstone (even if they are not * touching the loadstone) run together, seek one another anxiously and embrace one another, and when joined are as if they were cemented. iron * filings or the same reduced to powder inserted in paper tubes, placed upon a stone meridionally or merely brought rather close to it, coalesce into one body, and so many parts suddenly are concreted * and combine; and the whole company of corpuscles thus conspiring together affects another piece of iron and attracts it, as if it constituted one integral rod of iron; and above the stone it is directed toward the north and south. but when they are removed a long * { } way from the stone, the particles (as if loosed again) are separated and move apart singly. in this way also the foundations of the world are connected and joined and cemented together magnetically. so let ptolemy of alexandria, and his followers, and those philosophers of ours, be the less terrified if the earth do move round in a circle, nor threaten its dissolution. iron filings, after being heated for a long time, are attracted by a loadstone, yet not so strongly or from so great a distance as when not heated. a loadstone loses some of its virtue by too great a heat; for its humour is set free, whence its peculiar nature is marred. likewise also, if iron filings are well burnt in a reverberatory furnace and converted into saffron of mars, they are not attracted by a loadstone; but if they are heated, but not thoroughly burnt, they do stick to a magnet, but less strongly than the filings themselves not acted upon by fire. for the saffron has become totally deformate, but the heated metal acquires a defect from the fire, and the forces in the enfeebled body are less excited by a loadstone; and, the nature of the iron being now ruined, it is not attracted by a loadstone. * * * * * chap. xxiiii. a piece of iron placed within the orbe of a loadstone hangs suspended in the air, if on account _of some impediment it cannot approach it_. within the magnetick orbe a piece of iron moves towards the more powerful points of the stone, if it be not hindered by force or by the material of a body placed between them; either it falls down from above, or tends sideways or obliquely, or flies up above. but if the iron cannot reach the stone on account of some obstacle, it cleaves to it and remains there, but with a less firm and constant connection, since at greater intervals or distances the alliance is less amicable. fracastorio, in the eighth chapter of his _de sympathia_, says that a piece of iron is suspended in the air, so that it can be moved neither up nor down, if a loadstone be placed above which is able to draw the iron up just as much as the iron itself inclines downwards with equal force; for thus the iron would be supported in the air: which thing is absurd; because the force of a magnet is { } always the stronger the nearer it is. so that when a piece of iron is raised a very little from the earth by the force of the magnet, it needs must be drawn steadily on towards the magnet (if nothing else come in the way) and cleave to it. baptista porta suspends a piece of iron in the air[ ] (a magnet being fixed above), and, by no very subtile process, the iron is detained by a slender thread from its lower part, so that it cannot rise up to the stone. the iron is raised upright by the magnet, although the magnet does not * touch the iron, but because it is in its vicinity; but when the whole iron on account of its greater nearness is moved by that which erected it, immediately it hurries with a swift motion to the magnet and cleaves to it. for by approaching the iron is more and more excited, and the coition grows stronger. * * * * * chap. xxv. exaltation of the power of the magnet. one loadstone far surpasses another in power, since one draws iron of almost its own weight, another can hardly stir some shreds. whatever things, whether animals or plants, are endowed with life need some sort of nourishment, by which their strength not only persists but grows firmer and more vigorous. but iron is not, as it seemed to cardan and to alexander aphrodiseus, attracted by the loadstone in order that it may feed on shreds of it, nor does the loadstone take up vigour from iron filings as if by a repast on victuals. since porta had doubts on this and resolved to test it, he took a loadstone of ascertained weight, and buried it in iron filings of not unknown weight; and when he had left it there for many months, he found the stone of greater weight, the filings of less. but the difference was so slender that he was even then doubtful as to the truth. what was done by him does not convict the stone of voracity, nor does it show any nutrition; for minute portions of the filings are easily scattered in handling. so also a very fine dust is insensibly born on a loadstone in some very slight quantity, by which something might have been added to the weight of the loadstone but which is only a surface accretion and might even be wiped off with no great difficulty. some think that a weak and sluggish stone can bring itself back into better condition, and that a very powerful one also might present it with the highest powers. do they acquire strength like animals when { } they eat and are sated? is the medicine prepared by addition or subtraction? is there anything which can re-create this primary form or bestow it anew? and, certes, nothing can do this which is not magnetical. magneticks can restore a certain soundness to magneticks (when not incurable); some can even exalt them beyond their proper strength; but when a body is at the height of perfection in its own nature, it is not capable of being strengthened further. so that that imposture of paracelsus, who affirms that the force and virtue can be increased and transmuted tenfold, turns out to be the more infamous. the method of effecting this is as follows, viz., you make it semi-incandescent in a fire of charcoal (that is, you heat it very hot), so that it does not become red-hot, however, and immediately slake it, as much indeed as it can imbibe, in oil of saffron of mars, made from the best carynthian steel. "in this way you will be able so to strengthen a loadstone that it can draw a nail out of a wall and accomplish many other like wonderful things, which are not possible for a common loadstone." but a loadstone thus slaked in oil not only does not gain power, but suffers also a certain loss of its inborn strength. a loadstone is improved if polished and rubbed with steel. buried in filings of the best iron or of pure steel, not rusty, it preserves its strength. sometimes also a somewhat good and strong one gains [illustration] some strength when it is rubbed on the pole of another, on the opposite part, and receives virtue. in all these experiments it is an advantage to observe the pole of the earth, and to adjust according to magnetick laws the stone which we wish to strengthen; which we shall set forth below. a somewhat powerful and fairly large loadstone increases the strength of a loadstone as it does of iron. a loadstone being placed over the boreal pole of a loadstone, * { } the boreal pole becomes stronger, and an iron rod (like an arrow) sticks to the boreal pole a, but not at all to the pole b. the pole a also, when it is at the top in a right line with the axis of both loadstones joined in accordance with magnetick laws, raises the rod to the perpendicular, which it cannot do if the large loadstone be removed, on account of its own weaker strength. but as a small iron globe, when placed above the pole of a terrella, raises the rod to the * perpendicular, so, when placed at the side, the rod is not directed towards the centre of the globe, but is raised obliquely and cleaves anywhere, because the pole in a round piece of iron is always the point which is joined most closely to the pole of the terrella and is not constant as in a smaller terrella. the parts of the earth, as of all magneticks, are in agreement and take delight in their mutual proximity; if placed in the highest power, they do not harm their inferiors, nor slight them; there is a mutual love among them all, a perennial good feeling. the weaker loadstones are re-created by the more powerful, and the less powerful cause no harm to the stronger. but a powerful one attracts and turns a somewhat strong one more than it does an impotent one. because a strenuous one confers a stronger activity, and itself hastens, flies up to the other, and solicits it more keenly; therefore there is a more certain and a stronger co-action and cohærency. * * * * * chap. xxvi. why there should appear to be a greater love between _iron and loadstone, than between loadstone and loadstone, or_ between iron and iron, when close to the loadstone, _within its orbe of virtue._ magnet attracts magnet, not in every part and on every side with equal conditions, as iron, but at one and a fixed point; therefore the poles of both must be exactly disposed, otherwise they do not cleave together duly and strongly. but this disposition is not easy and expeditious; wherefore a loadstone seems not to conform to a loadstone, when nevertheless they agree very well together. a piece of iron by the sudden impression of a loadstone is not only allured by the stone, but is renewed, its forces being drawn forth; by which it follows and solicits the loadstone with no less impulse, and even leads another piece of iron captive. let there be a small iron spike above a loadstone clinging firmly to it; if you apply an unmagnetized rod of iron to the spike, not, however, { } so that it touches the stone, you will see the spike when it has touched the iron, leaving the loadstone, follow the rod, try to grasp it by leaning toward it, and (if it should touch it) cleave firmly to it: for a piece of iron, when united and joined to another piece of iron placed within the orbe of virtue of the loadstone, draws it more strongly than does the loadstone itself. the natural magnetick virtue, confused and dormant in the iron, is aroused by the loadstone, is linked to the loadstone, and rejoices with it in its primary form; then smelted iron becomes a perfect magnetick, as robust as the loadstone itself. for as the one imparts and stirs, so the other conceives, and being stirred remains in virtue, and pours back the forces also by its own activity. but since iron is more like iron than loadstone, and the virtue in both pieces of iron is exalted by the proximity of the loadstone, so in the loadstone itself, in case of equal strength, likeness of substance prevails, and iron gives itself up rather to iron, and they are united by their very similar homogenic powers. which thing happens not so much from a coition, as from a firmer unition; and a knob or snout of steel, fixed skilfully on the pole of the stone, raises greater weights of iron than the stone of itself could. when steel or iron is smelted from loadstone or iron ore, the slag and corrupt substances are separated from the better by the fusion of the material; whence (in very large measure) that iron contains the nature of the earth, purified from alien flaw and blemish, and more homogenic and perfect, though deformed by the fusion. and when that material indeed is provoked by a loadstone, it conceives the magnetick virtues, and within their orbe is raised in strength more than the weaker loadstone, which with us is often not free from some admixture of impurities. * * * * * chap. xxvii. * the centre of the magnetick virtues in the earth is the centre of the earth; and in a terrella _is the centre of the stone_. rays of magnetick virtue spread out in every direction in an orbe; the centre of this orbe is not at the pole (as baptista porta reckons, chap. ), but in the centre of the stone and of the terrella. so also the centre of the earth is the centre of the magnetick motions of the earth; though magneticks are not borne directly toward the centre by magnetical motion, except when they are attracted by the true pole. for since the formal { } power of the stone and of the earth does not promote anything but the unity and conformity of disjoined bodies, it comes about that everywhere at an equal distance from the centre or from the circumference, just as it seems to attract perpendicularly at one place, so at another it is able even to dispose and to turn, provided the stone is not uneven in virtue. for if at the distance c from the pole d the stone is able to allure a versorium, * at an equally long interval above the æquator at a that stone can also direct and turn the versorium. so the very centre and middle of the terrella is the centre of its virtue, and from this to the circumference of the orbe (at equal intervals on every side) its magnetick virtues are emitted. [illustration] * * * * * chap. xxviii. a loadstone attracts magneticks not only to a fixed point or pole, but to every part of a _terrella save the æquinoctial zone_. coitions are always more powerful when poles are near poles, since in them by the concordancy of the whole there exists a stronger force; wherefore the one embraces the other more strongly. places declining from the poles have attractive forces, but a little weaker and languid in the ratio of their distance; so that at length on the æquinoctial circle they are utterly enervated and evanescent. neither do even the poles attract as mathematical points; nor do magneticks come into conjunction by their own poles, only on the poles of a loadstone. but coition { } is made on every part of the periphery, both northern and southern, by virtue emanating from the whole body; magneticks nevertheless incline languidly towards magneticks in the parts bordering on the æquator, but quickly in places nearer the pole. wherefore not the poles, not the parts alone nearest to the pole allure and invite magneticks, but magneticks are disposed and turned round and combine with magneticks in proportion as the parts facing and adjoined unite their forces together, which are always of the same potency in the same parallel, unless they are distributed otherwise from causes of variation. * * * * * chap. xxix. on variety of strength due to quantity _or mass_. quite similar in potency are those stones which are of the same mine, and not corrupted by adjacent ores or veins. nevertheless that which excels in size shows greater powers, since it seizes greater weights and has a wider orbe of virtue. for a loadstone weighing one ounce does not lift a large nail as does one weighing a pound, nor does it rule so widely, nor extend its forces; and if from a loadstone of a pound weight a portion is taken away, something of its power will be seen to go also; for when a portion is abstracted the virtue is lessened. but if that part is properly applied and united to it, though it is not fastened * to nor grown into it, yet by the application it obtains its pristine power and its vigour returns. sometimes, however, when a part is taken away, the virtue turns out to be stronger on account of the * bad shape of the stone, namely, when the vigour is scattered through inconvenient angles. in various species the ratio is various, for one stone of a drachm weight draws more than another of twenty pounds. since in very many the influence is so effete that it can hardly be perceived, those weak stones are surpassed by prepared pieces of clay. but, it may be asked[ ], if a stone of the same species and goodness weighing a drachm would seize upon a drachm of iron, would a stone of an ounce weight seize on an ounce, a pound on a pound, and so on? and this is indeed true; for it both strains and remits its strength proportionately, so that if a loadstone, one drachm of which would attract one drachm of iron, were in equal proportion applied either to a suitably large obelisk or to an immense pyramid of iron, it would lift it directly in such { } proportion and would draw it towards itself with no greater effort of its nature or trouble than a loadstone of a drachm weight embraces a drachm. but in all such experiments as this let the vigour of the magnets be equal; let there be also a just proportion in all of the shapes of the stones, and let the shape of the iron to be attracted be the same, and the goodness of the metal, and let the position of the poles of the loadstones be most exact. this is also no less true in the case of an armed loadstone than of an unarmed one. for the sake of experiment, let there be given a loadstone of eight ounces weight, which when armed lifts twelve ounces of iron; if you cut off from that loadstone a certain portion, which when it has been * reduced to the shape of the former whole one is then only of two ounces, such a loadstone armed lifts a piece of iron applied to it of three ounces, in proportion to the mass. in this experiment also the piece of iron of three ounces ought to have the same shape as the former one of twelve ounces; if that rose up into a cone, it is necessary that this also in the ratio of its mass should be given a pyramidal shape proportioned to the former. * * * * * chap. xxx. the shape and mass of the iron are of most _importance in coition_. observation has shown above that the shape and mass of the loadstone have great influence in magnetick coitions; likewise also the shape and mass of the iron bodies give back more powerful and steady forces. oblong iron rods are both drawn more quickly to a loadstone and cleave to it with greater obstinacy than round or square pieces, for the same reasons which we have proven in the case of the loadstone. but, moreover, this is also worthy of observation, that a smaller piece of iron, to which is hung a weight of another material, so that it is altogether in weight equal to another large whole piece of iron of a right weight * (as regards the strength of the loadstone), is not lifted by the loadstone as the larger piece of iron would be. for a smaller piece of iron does not join with a loadstone so firmly, because it sends back less strength, and only that which is magnetick conceives strength; the foreign material hung on cannot acquire magnetick forces. * * * * * { } chap. xxxi. on long and round stones. pieces of iron join more firmly with a long stone than with a round one, provided that the pole of the stone is at the extremity and end of its length; because, forsooth, in the case of a long stone, a magnetick is directed at the end straight towards the body in which the virtue proceeds in straighter lines and through the longer diameter. but a somewhat long stone has but little power on the side, much less indeed than a round one. it is demonstrable[ ], indeed, that at a and b the coition is * stronger in a round stone than at c and d, at like distances from the pole. [illustration] * * * * * chap. xxxii. certain problems and magnetick experiments about the coition, and separation, and regular motion _of bodies magnetical_. equal loadstones come together with equal incitation. * also magnetick bodies of iron, if alike in all respects, * come together when excited with similar incitation. furthermore, bodies of iron not excited by a * loadstone, if they are alike and not weighed down by their bulk, move towards one another with equal motion. two loadstones, disposed on the surface of some water in { } suitable skiffs, if they are drawn up suitably within their orbes of virtue, incite one another mutually to an embrace. so a proportionate * piece of iron in one skiff hurries with the same speed towards the loadstone as the loadstone itself in its boat strives towards the iron. from their own positions, indeed, they are so borne together, that they are joined and come to rest at length in the middle of the space. two iron wires magnetically excited, floating in water by means of * suitable pieces of cork, strive to touch and mutually strike one another with their corresponding ends, and are conjoined. coition is firmer and swifter than repulsion and separation in * equal magnetick substances. that magnetick substances are more sluggishly repelled than they are attracted is manifest in all magnetical experiments in the case of stones floating on water in suitable skiffs; also in the case of iron wires or rods swimming (transfixed through corks) and well excited by a loadstone, and in the case of versoria. this comes about because, though there is one faculty of coition, another of conformation or disposition, repulsion and aversion is caused merely by something disposing; on the other hand, the coming together is by a mutual alluring to contact and a disposing, that is, by a double vigour. a disponent vigour is often only the precursor of coition, in order that the bodies may stand conveniently for one another before conjunction; wherefore also they are turned round to the corresponding ends, if they can [not][ ] reach them through the hindrances. [illustration] if a loadstone be divided through a meridian into two equal parts, the separate parts mutually repel one another, the poles being * placed directly opposite one another at a convenient and equal distance. they repel one another also with a greater velocity than when pole is put opposite pole incongruously. just as the part b of the loadstone, placed almost opposite the part a, repels it floating in its skiff, because d turns away from f, and e from c; but if b is exactly joined with a again, they agree and become one body { } magnetical; but in proximity they raise enmities. but if one part of the stone is turned round, so that c faces d and f faces e, then a pursues b within its orbe until they are united. the southern parts of the stone avoid the southern parts, and the northern parts the northern. nevertheless, if by force you move up the southern cusp of a piece of iron too near the southern part of the stone, the cusp is seized and both are linked together in friendly embraces: because it immediately reverses the implanted verticity of the iron, and it is changed by the presence of the more powerful stone, which is more constant in its forces than the iron. for they come together according to their nature, if by reversal and mutation true conformity is produced, and just coition, as also regular direction. loadstones of the same shape, size, and vigour, attract one another mutually with like efficacy, and in the opposite position repel one another mutually with a like vigour. iron rods not touched, though alike and equal, do yet often act * upon one another with different forces; because as the reasons of their acquired verticity, also of their stability and vigour, are different, so the more strongly they are excited, the more vigorously do they incite. pieces of iron excited by one and the same pole mutually repel * one another by those ends at which they were excited; then also the opposite ends to those in these iron pieces raise enmities one to another. in versoria whose cusps have been rubbed, but not their cross-ends, * the crosses mutually repel one another, but weakly and in proportion to their length. in like versoria the cusps, having been touched by the same * pole of the loadstone, attract the cross-ends with equal strength. in a somewhat long versorium the cross-end is attracted rather * weakly by the cusp of a shorter iron versorium; the cross of the shorter more strongly by the cusp of the longer, because the cross of the longer versorium has a weak verticity, but the cusp has a stronger. the cusp of a longer versorium drives away the cusp of a * shorter one more vehemently than the cusp of the shorter the cusp of the longer, if the one is free upon a pin, and the other is held in the hand; for though both were equally excited by the same loadstone, yet the longer one is stronger at its cusp on account of its greater mass. the southern end of an iron rod which is not excited attracts * the northern, and the northern the southern; moreover, also the southern parts repel the southern, and the northern the northern. if magnetick substances are divided or in any way broken in pieces, each part has a northern and a southern end. { } a versorium is moved as far off by a loadstone when an obstacle * is put in the way, as through air and an open medium. rods rubbed upon the pole of a stone strive after the same pole * and follow it. therefore baptista porta errs when he says, chapter [ ], "if you put that part to it from which it received its force, it will not endure it, but drives it from it, and draws to it the contrary and opposite part." the principles of turning round and inclining are the same in the case of loadstone to loadstone, of loadstone to iron, of iron also to iron. when magnetick substances which have been separated by force and dissected into parts flow together into a true union and are suitably connected, the body becomes one, and one united virtue, nor have they diverse ends. the separate parts assume two opposite poles, if the division has * not been made along a parallel: if the division has been made along a parallel, they are able to retain one pole in the same site as before. pieces of iron which have been rubbed and excited by a loadstone are more surely and swiftly seized by a loadstone at fitting ends than such as have not been rubbed. if a spike is set up on the pole of a loadstone, a spike or style * of iron placed on the upper end is strongly cemented to it, and draws away the erect spike from the terrella when motion is made. if to the lower end of the erect spike the end of another spike * is applied, it does not cohære with it, nor do they unite together. as a rod of iron draws away a piece of iron from a terrella, so is it also with a minute loadstone and a lesser terrella, though weaker in strength. [illustration] the piece of iron c comes into conjunction with the terrella a, and the vigour in it is magnetically exalted and excited, both in the adjoining end and in the other also which is turned away through { } its conjunction with the terrella. the end that is turned away also conceives vigour from the loadstone b; likewise the pole d of that loadstone is powerful on account of its suitable aspect and the nearness of the pole e of the terrella. several causes therefore concur why the piece of iron c should cleave to the terrella b, to which it is joined more firmly than to the terrella a; the vigour excited in the rod, the vigour also excited in the stone b, and the strength implanted in b concur; therefore d is more firmly cemented magnetically with c than e with c. but if you were to turn the vertex f round to the iron c, c would not adhære to f as formerly to d; for stones so arranged being within the orbe of virtue are placed contrary to natural order; wherefore f does not receive power from e. two loadstones or excited pieces of iron, duly cohæring, fly * asunder on the approach of another more powerful loadstone or magnetized piece of iron. because the new-comer repels the other with its opposing face, and dominates it, and ends the relationship of the two which were formerly joined. so the forces of the other are lessened and succumb; but if it conveniently could, being diverted of its association with the weaker, and rolling round, it would turn about to the stronger. wherefore also magnetick bodies suspended in the air fall when a loadstone is brought near them with an opposing face, not (as baptista porta teaches) because the faculty of both those which were joined before grows faint and torpid, for no face can be hostile to both the ends which cohære, but to one only; and when the stronger loadstone, coming fresh with opposing face, impels this further from it, it is put to flight by the friendly reception of the former. * * * * * chap. xxxiii. on the varying ratio of strength, and of the motion _of coition, within the orbe of virtue_. should a very large weight, which at a very small distance is drawn towards a loadstone, be divided into ever so many equal parts, and should the radius of the orbe of magnetick attraction be divided into the same number of parts, the like named parts of the weight will correspond to the intermediate parts of the radius. the orbe of virtue extends more widely than the orbe of motion of any magnetick; for the magnetick is affected at its extremity, even if it is not moved with local motion, which effect is produced { } by the loadstone being brought nearer. a small versorium also is turned when a good distance off, even if at the same distance it would not flow towards the loadstone, though free and disengaged from impediment. the swiftness of the motion of a magnetick body to a loadstone is dependent on either the power of the loadstone, on its mass, on its shape, on the medium, or on its distance within the magnetick orbe. a magnetick moves more quickly towards a more powerful * stone than towards a sluggish one in proportion to the strength, and [as appears] by a comparison of the loadstones together. a lesser mass of iron also is carried more quickly towards a loadstone, just as also one that is a little longer in shape. the swiftness of magnetick motion towards a loadstone is changed by reason of the medium; for bodies are moved more quickly in air than in water, and in clear air than in air that is thick and cloudy. by reason of the distance, the motion is quicker in the case of bodies near together than when they are far off. at the limits of the orbe of virtue of a terrella a magnetick is moved feebly and slowly. at very short distances close to the terrella the moving impetus is greatest. a loadstone which in the outmost part of its orbe of virtue * hardly moves a versorium when one foot removed from it, doth, if a long piece of iron is joined to it, attract and repel the versorium more strongly with its opposite poles when even three feet distant. the result is the same whether the loadstone is armed or unarmed. let the iron be a suitable piece of the thickness of the little finger. for the vigour of the loadstone excites verticity in the iron and proceeds in the iron and through the iron much further than it extends through the air. the vigour proceeds even through several pieces of iron (joined * to one another end to end), not so regularly, however, as through one continuous solid. dust of steel placed upon paper rises up when a loadstone is moved near above it in a sort of steely hairiness; but if the loadstone is placed below, such a hairiness is likewise raised. steel dust (when the pole of a loadstone is placed near) is cemented * into one body; but when it desires coition with the loadstone, the mass is split and it rises in conglomerated parts. but if there is a loadstone beneath the paper, the mass is split in the same way and many portions result, each of which consists of very many parts, and remains cemented together, as individual bodies. whilst the lower parts of these pursue greedily the pole of the loadstone placed directly beneath, even they also are raised up as magnetick wholes, just as a small iron wire of the length of a grain or two grains of barley is raised up, both when the loadstone is moved near both beneath and above. * * * * * { } chap. xxxiiii. why a loadstone should be stronger in its poles in a different ratio; as well in the northern _regions as in the southern_. the extraordinary magnetick virtue of the earth is * remarkably demonstrated by the subtility of the following magnetical experiment. let there be given a terrella of no contemptible power, or a long loadstone with equal cones as polar extremities; but in any other shape which is not exactly round error is easy, and the experiment difficult. in the northern regions, raise the true north pole of the terrella above the horizon straight toward the zenith; it is demonstrable that it raises up a larger iron spike on its north pole, than the south pole of the same terrella is able to raise, when turned in the same way toward the highest point of the sky. the same thing is shown by a small terrella placed in the same way above a larger. [illustration] let _a b_ be the earth or a somewhat large terrella, also _a b_ a smaller terrella. there is set up above the northern pole of the smaller terrella a spike larger than the pole _b_ of the smaller terrella can raise, if it is turned round to the higher parts. and the pole _a_ of the { } smaller terrella has its strength from the larger, declining from the zenith to the plane of the horizon or to the level. but now, if, * leaving the terrella disposed in the same way, you bring a piece of iron to the lower and southern pole, it will attract and retain a greater weight than the boreal pole could, if it were turned round to the lower parts. which thing is demonstrated thus: let a be the earth or a terrella; e the boreal pole or some place in some great latitude; b a rather large terrella above the earth or a smaller terrella on the top of a larger; d its southern pole. it is manifest that d (the southern pole) attracts a larger piece of iron, c, than f (the boreal pole) will be able to, if it is turned round downward to the position d, toward the earth or the terrella in the northern regions. [illustration] magneticks acquire strength through magneticks, if they are properly placed according to their nature, in near neighbourhood and within the orbe of virtue. wherefore when a terrella is placed on the earth or on a terrella, so that its southern pole is turned round toward the northern pole, its northern pole, however, turned away from the northern pole, the influence and strength of { } its poles are increased. and so the northern pole of a terrella in such a position lifts up a larger spike than the southern pole, if the southern pole is turned away. similarly the southern pole in a proper and natural arrangement, acquiring strength from the earth or from a larger terrella, attracts and retains larger rods of iron. in * the other part of the terrestrial globe toward the south, as also in the austral portion of a terrella, the reasoning is converse; for the southern pole of the terrella being turned away is more robust, as also the northern pole when turned round. the more a region on the earth is distant from the æquinoctial (as also in a larger terrella), the larger is the accession of strength perceived; near the æquator, indeed, the difference is small, but on the æquator itself null; at the poles finally it is greatest. * * * * * chap. xxxv. on a perpetual motion machine, mentioned by authors, by means of the attraction _of a loadstone_. cardan writes[ ] that out of iron and the herculean stone can be made a perpetual motion machine; not that he himself had ever seen one, but only conceived the idea from an account by antonius de fantis[ ], of treves. such a machine he describes, book , _de rerum varietate_. but they have been little practised in magnetick experiments who forge such things as that. for no magnetick attraction can be greater (by any skill or by any kind of instrument) than the retention. things which are joined and those which are approaching near are retained with a greater force than those which are enticed and set in motion, and are moved; and that coition is, as we have shown above, a motion of both, not an attraction of one. such a machine peter peregrinus feigned many centuries before or else depicted one which he had received from others, and one which was much better fitted for the purpose. johannes taysnier published it also, spoiled by wretched figures, and copied out the whole theory of it word for word. o that the gods would at length bring to a miserable end such fictitious, crazy, deformed labours, with which the minds of the studious are blinded! * * * * * { } chap. xxxvi. how a more robust loadstone may be _recognized_. very powerful loadstones sometimes lift into the air a weight of iron equal to their own; a weak one barely attracts a slender wire. those therefore are more robust which appeal to and retain larger bodies, if there is no defect in their form, or the pole of the stone is not suitably moved up. moreover, when placed in a boat a keener influence turns its own poles round more quickly to the poles of the earth or the limits of variation on the horizon. one which performs its function more feebly indicates a defect and an effete nature. there must always be a similar preparation, a similar figure, and a like size; for in such as are very dissimilar and unlike, the experiment is doubtful. the method of testing the strength is the same also with a versorium in a place somewhat remote from a loadstone; for the one which is able to turn the versorium round at the greater distance, that one conquers and is held the more potent. rightly also is the force of a loadstone weighed in a balance by b. porta; a piece of loadstone is placed in one scale-pan, in the other just as much weight of something else, so that the scale-pans hang level. soon a piece of iron lying on the table is adjusted so that it sticks to the loadstone placed in the scale, and they cling together most perfectly, according to their friendly points; into the other scale-pan sand is gradually thrown, and that until the scale in which the loadstone is placed is separated from the iron. thus by weighing the weight of sand, the magnetick force becomes known. similarly also it will be pleasing to try with another stone, in equilibrium, the weight of the sand being observed, and to find out the stronger by means of the weights of sand. such is the experiment of cardinal cusan in his _de staticis_[ ], from whom it would seem that b. porta learnt the experiment. the better loadstones turn themselves round more quickly toward the poles or points of variation; then they also lead along and turn round more quickly, according to the greater quantity and mass of wood, a boat and other stuff. in a declination instrument, the more powerful force of a loadstone is looked for and required. those therefore are more lively when they get through their work readily, and pass through and come back again with speed, and swiftly at length settle at their own point. languid and effete ones move more sluggishly[ ], settle more tardily, adhære more uncertainly, and are easily disturbed from their possession. * * * * * { } chap. xxxvii. use of a loadstone as it affects _iron._ by magnetick coition we test iron ore in a blacksmith's forge. it is burnt, broken in pieces, washed and dried, in which way it lays down its alien humours; in the bits collected from the washing is placed a loadstone, which attracts the iron dust to itself; this, being brushed off with feathers, is received in a crucible, and the loadstone is again placed in the bits collected from the washing, and the dust wiped off, as long as any remains which it will attract to itself. this is then heated in the crucible along with _sal nitri_[ ] until it is liquid, and from this a small mass of iron is cast. but if the loadstone draws the dust to itself quickly and readily, we conjecture that the iron ore is rich; if slowly, poor; if it seems altogether to reject it, there is very little iron in it or none at all. in like manner iron dust can be separated from another metal. many tricks there are also, when iron is secretly applied to lighter bodies, and, being attracted by the motion of a loadstone which is kept out of sight, causes movements which are amazing to those who do not know the cause. very many such indeed every ingenious mechanician will perform by sleight of hand, as if by incantations and jugglery[ ]. * * * * * chap. xxxviii. on cases of attraction in other bodies. very often the herd of philosophizers and plagiarists repeat from the records of others in natural philosophy opinions and errors about the attractions of various bodies; as that diamond attracts iron, and snatches it away from a magnet; that there are various kinds or magnets, some which attract gold, others silver, brass, lead; even some which attract flesh, water, fishes. the flame of sulphur is said to seek iron and stones; so white naphtha is said to attract fire. i have said above that { } inanimate natural bodies do not attract, and are not attracted by, others on the earth, excepting magnetically or electrically. wherefore it is not true that there are magnets which attract gold or other metals; because a magnetick substance draws nothing but magnetick substances. though fracastorio says that he has shown a magnet drawing silver; if this were true, it must have happened on account of iron skilfully mixed with that silver or concealed in it, or else because nature (as she does sometimes, but rarely) had mixed iron with the silver; iron indeed is rarely mixed with silver by nature; silver with iron very rarely or never. iron is mixed with silver by forgers of false coin or from the avarice of princes in the coining of money, as was the case with the denarius of antony[ ], provided that pliny is recording a true incident. so cardan (perhaps deceived by others) says that there is a certain kind of loadstone which draws silver; he adds a most foolish test of this: "if therefore" (he says) "a slender rod of silver be steeped in that in which a versatory needle has stood, it will turn toward silver (especially toward a large quantity) although it be buried; by this means anyone will be able easily to dig up concealed treasures." he adds that "it should be very good stone, such as he has not yet seen." nor indeed will either he or anyone else ever see such a stone or such an experiment. cardan brings forward an attraction of flesh, wrongly so named and very dissimilar from that of the loadstone; for his _magnes creagus_ or flesh-magnet, from the experiment that it sticks to the lips, must be hooted out from the assembly of loadstones, or by all means from the family of things attractive. lemnian earth, ruddle, and very many minerals do this, and yet they are fatuously said to attract. he will have it that there is another loadstone, as it were, a third species, into which, if a needle is driven and afterwards stuck into the body, it is not felt. but what has attraction to do with stupefaction, or stupor with a philosopher's intellect, when he is discoursing about attraction? there are many stones, both found in nature and made by art, which have the power of stupefying. sulphur flame is said by some to attract, because it consumes certain metals by its power of penetration. so white naphtha attracts flame, because it gives off and exhales an inflammable vapour, on which account it is kindled at some distance, just as the smoke of a recently extinguished candle takes fire again from another flame; for fire creeps to fire through an inflammable medium. why the sucking fish echineis or the remora should stay ships has been variously treated by philosophers, who are often accustomed to fit this fable (as many others) to their theories, before they find out whether the thing is so in nature. therefore, in order that they may support and agree with the fatuities of the ancients, they put forward even the most fatuous ratiocinations and ridiculous problems, cliffs that attract, where the { } sucking fish tarry, and the necessity of some vacuum, i know not what, or how produced. pliny and julius solinus make mention of a stone chatochitis[ ]. they say that it attracts flesh, and keeps hold of the hands, just as a loadstone does iron, and amber chaff. but that happens only from a stickiness and from glue contained in it, since it sticks more easily to the hands when they are warm. sagda or sagdo[ ], of the colour of a sard, is a precious stone mentioned by pliny, solinus, albertus, and evax[ ]; they describe its nature and relate, on the authority of others, that it specially attracts wood to itself. some even babble that woods cannot be wrenched away except they are cut off. some also narrate that a stone is found which grows pertinaciously into ships, in the same way as certain testacea on long voyages. but a stone does not draw because it sticks; and if it drew, it would certainly draw shreds electrically, encelius saw in the hands of a sailor such a stone of feeble virtue, which would hardly attract even the smallest twigs; and in truth, not of the colour of the sard. so diamond, carbuncle, crystal, and others do attract. i pass over other fabulous stones; pantarbe, about which philostratus writes that it draws other stones to itself; amphitane also, which attracts gold. pliny in his origin of glass will have it that a loadstone is an attractor of glass, as well as of iron. for in his method of preparing glass, when he has indicated its nature, he subjoins this about loadstone. "soon (such is the astute and resourceful craft) it was not content to have mixed natron; loadstone also began to be added, since it was thought to attract to itself the liquor of glass (as it does iron)." georgius agricola writes that to the material of glass (sand and natron) one part also of loadstone is added. "because that force is believed, in our times just as in former times, to attract the liquor of glass to itself, as it attracts iron to itself, purges it when drawn, and makes clear glass from green or muddy; but the fire afterwards burns up the loadstone." it is true indeed that some sort of _magnes_ (as the magnesia of the glass-makers imbued with no magnetick virtues) is sometimes put in and mixed with the material of the glass; not, however, because it attracts glass. but when a loadstone is burnt, it does not lay hold of iron at all, nor is iron when red-hot allured by any loadstone; and loadstone also is burnt up by more powerful fires and loses its attractive potency. nor is this a function of loadstone alone in the glass furnaces; but also of certain pyrites and of some easily combustible iron ores, which are the only ones used by our glass-makers, who make clear, bright glass. they are mixed with the sand, ashes, and natron (just as they are accustomed to make additions in the case of metallick ores whilst they are smelted), so that when the material slows down into glass, the green and muddy colour of the glass may be purged by the penetrating heat. for no other material becomes so hot, { } or bears the fire for such a convenient time, until the material of the glass is perfectly fluid, and is at the same time burnt up by that ardent fire. it happens, however, sometimes, that on account of the magnetick stone, the magnesia, or the ore, or the pyrites, the glass has a dusky colour, when they resist the fire too much and are not burnt up, or are put in in too great quantity. wherefore manufacturers are seeking for a stone suitable for them, and are observing also more diligently the proportion of the mixture. badly therefore did the unskilful philosophy of pliny impose upon georgius agricola and the more recent writers, so that they thought the loadstone was wanted by glass-makers on account of its magnetick strength and attraction. but scaliger in _de subtilitate ad cardanum_, in making diamond attract iron, when he is discussing magneticks, wanders far from the truth, unless it be that diamond attracts iron electrically, as it attracts wood, straws, and all other minute bodies when it is rubbed. fallopius reckons that quicksilver draws metals by reason of an occult property, just as a loadstone iron, amber chaff. but when quicksilver enters metals, it is wrongly called attraction. for metals imbibe quicksilver, just as clay water; nor do they do this unless they are touching, for quicksilver does not allure gold or lead to itself from afar, but they remain motionless in their places. * * * * * chap. xxxix. on bodies which mutually repel one another. writers who have discoursed on the forces of bodies which attract others have also spoken about the powers of bodies which repel, but especially those who have instituted classes for natural objects on the basis of sympathy and antipathy. wherefore it would seem necessary for us to speak also about the mutual strife of bodies, so that published errors should not creep further, and be received by all to the ruin of true philosophy. they say that, just as like things attract for the sake of preservation, so unlike and contrary things for the same purpose mutually repel and put one another to flight. this is evident in the reaction of many things, but it is most manifest in the case of plants and animals, which attract kindred and familiar things, and in like manner reject foreign and unsuitable things. but in other bodies there is not the same reason, so that when they are separated, they should come together by mutually { } attracting one another. animals take food (as everything which grows), and draw it into their interior; they absorb the nourishment by certain parts and instruments (through the action and operation of the _anima_). they enjoy by natural instinct only the things set in front of them and near them, not things placed afar off; and this without any alien force or motion. wherefore animals neither attract any bodies nor drive them away. water does not repel oil (as some think) because the oil floats on water; nor does water repel mud, because the mud, if mixed in water, settles down in time. this is a separation of unlike bodies or such as are not perfectly mixed as respects the material; the separated bodies nevertheless remain joined without any natural strife. wherefore a muddy sediment settles quietly on the bottom of vessels, and oil remains on the top of the water and is not sent further away. a drop of water remains intact on a dry surface, and is not expelled from the dry substance. wrongly therefore do those who discourse on these matters infer an antipathy (that is, the force of repelling by contrary passions); for there is no repelling force in them; and repulsion comes[ ] from action, not from passion. but their greek vocables please them too much. we, however, must inquire whether there is any body which drives anything else further off without material impetus, as a loadstone attracts. but a loadstone seems even to repel loadstone. for the pole of one loadstone repels the pole of another, which does not agree with it according to nature; by repelling, it turns it round in an orbit so that they may exactly agree according to their nature. but if a somewhat weak loadstone, floating freely on water, cannot readily be turned round on account of impediments, the whole loadstone is repelled and sent further away from the other. all electricks attract all things: they never repel or propel anything at all[ ]. as to what is related about certain plants (as about the cucumber, which turns aside when oil is applied to it), there is a material change from the vicinity, not a hidden antipathy. but when they show a candle flame put against a cold solid substance (as iron) turn away to the side, and allege antipathy as the cause, they say nothing. the reason of this they will see clearer than the day, when we discourse on what heat is[ ]. but fracastorio's opinion that a loadstone can be found, which would drive iron away, on account of some opposing principle lurking in the iron, is foolish. * * * * * { } [illustration] book third. _chap. i._ on direction. on referring to the earlier books it will be found shown that a loadstone has its poles, and that a piece of iron has also poles, and rotation, and a certain verticity; finally, that the loadstone and the iron direct their poles toward the poles of the earth. now, however, we must make clear the causes of these things and their admirable workings, pointed out indeed before, but not proven. all those who have written before us about these rotations have left us their opinions so briefly, so meagrely, and with such hesitating judgment that they seem hardly likely ever to persuade anyone, or even to be able to satisfy themselves; and all their petty reasons are rejected by the more prudent as useless, uncertain, and absurd, being supported by no proofs or arguments; whence also magnetick science, being all the more neglected and not understood, has been in exile. the true austral pole of a loadstone, not the boreal (as all before us used to think), * if the loadstone is placed in its boat on the surface of water, turns to the north; in the case of a piece of iron also, whether it has been excited by a loadstone or not, the southern end moves toward the north. an oblong piece of iron of three or four digits' length[ ], when skilfully rubbed with a loadstone, quickly turns north and south. wherefore mechanicians, taking a piece of iron prepared in this way, balance it on a pin in a box, and fit it up with the requisites of a sun-dial; or they prepare the versorium out of two curved pieces of iron with their ends touching one another, so that the motion may be more constant. in this way the mariners' versorium is arranged, which is an instrument beneficial, useful, and auspicious to sailors for indicating, like a good genius, safety and the right way. but it must be understood on the threshold of this argument (before we proceed further) that these pointings of the loadstone or of iron are not perpetually made { } toward the true poles of the world, do not always seek those fixed and definite points, or remain on the line of the true meridian; but usually diverge some distance to the east or to the west. sometimes also at certain places on land or sea they do indicate exactly the true poles. this discrepancy is called the _variation_ of the iron or of the loadstone; and since this is brought about by other causes, and is merely a certain disturbance and perversion of the true direction, we are directing our attention in this place to the true direction of the compass and of the magnetick iron (which would be equally toward the true poles and on the true meridian everywhere on the earth, unless other obstacles and an untoward pervertency hindered it). of its variation and the cause of the perversion we shall treat in the next book. those who wrote about the world and about natural philosophy a century ago, especially those remarkable elementary philosophers, and all those who trace their knowledge and training to them down to our own times, those men, i say, who represented the earth as always at rest and, as it were, a useless weight, placed in the centre of the universe at an equal distance from the sky on every side, and its nature to be simple, imbued only with the qualities of dryness and cold, sought diligently for the causes of all things and of all effects in the heavens, the stars, the planets, in fire, air, waters and substances of mixed natures. never indeed did they recognize that the terrestrial globe had, besides dryness and cold, some special, effective, and predominant properties, strengthening, directing, and moving the globe itself through its whole mass and its very deepest vitals; nor did they ever inquire whether there were any such. for this reason the crowd of philosophizers, in order to discover the reasons of the magnetical motions, called up causes lying remote and far away. and one man seems to me beyond all others worthy of censure, martin cortes, who, since there was no cause which could satisfy him in the whole of nature, dreamed that there was a point of magnetical attraction beyond the heavens, which attracted iron. peter peregrinus thinks that the direction arises from the poles of the sky. cardan thought that the turning of iron was caused by a star in the tail of the great bear; bessard, the frenchman, opines that a magnetick turns toward the pole of the zodiack. marsilius ficinus will have it that the loadstone follows its own arctick pole; but that iron follows the loadstone, straws amber; whilst this perhaps follows the antarctick pole--a most foolish dream. others have recourse to i know not what magnetick rocks and mountains. thus it is always customary with mortals, that they despise things near home, whilst foreign and distant things are dear and prized. but we study the earth itself and observe in it the cause of so great an effect. the earth, as the common mother, has these causes inclosed in her innermost parts; in accordance with her rule, { } position, condition, verticity, poles, æquator, horizons, meridians, centre, circumference, diameter, and the nature of the whole interior of her substance, must all magnetical motions be discussed. the earth has been ordered by the highest artificer and by nature in such a way that it should have parts dissimilar in position, bounds of the whole and complete body, ennobled by certain functions, by which it might itself remain in a definite direction. for just as a loadstone, when it is floated on water in a suitable vessel, or is hung by slender threads in the air, by its implanted verticity conforms its poles to the poles of the common mother in accordance with magnetick laws; so if the earth were to deviate from its natural direction and its true position in the universe, or if its poles were to be drawn aside (if this were possible) toward the sun-rising or the sun-setting or toward any other points whatsoever in the visible firmament, they would return again to the north and south by magnetical motion, and would settle at the same points at which they are now fixed. the reason why the terrestrial globe seems to remain more steadily with the one pole toward those parts and directed toward the cynosure, and why its pole diverges by degrees minutes, with a certain variation not sufficiently investigated as yet by astronomers, from the poles of the ecliptick, depends on its virtue magnetical. the causes of the precession of the æquinoxes and the progression of the fixed stars, and of the change, moreover, in the declinations of the sun and of the tropicks, must be sought from magnetick influences; so that neither that absurd motion of trepidation of thebit bencora[ ], which is at great variance with observations, nor the monstrous superstructures of other heavens, are any longer needed. a versatory iron turns to the position of the earth, and if disturbed ever so often returns always to the same points. for in the far regions of the north, in a latitude of or degrees (to which at the milder seasons of the year our sailors are accustomed to penetrate without injury from the cold); in the regions halfway between the poles; on the æquator in the torrid zone; and again in all the maritime places and lands of the south, in the highest latitude which has thus far been reached, always the iron magnetick finds its way, and points to the poles in the same manner (excepting for the difference of variation); on this side of the æquator (where we live), and on the other side to the south, less well known, but yet in some measure explored by sailors: and always the lily of the compass points toward the north. this we have had confirmed by the most eminent captains, and also by very many of the more intelligent sailors. these facts have been pointed out to me and confirmed by our most illustrious sea-god, francis drake, and by another circumnavigator of the globe, thomas candish; our terrella also indicates the same thing. this is demonstrated in the case of the { } [illustration] orbicular stone, whose poles are a and b; an iron wire cd, which is placed upon the stone, always points directly along the meridian toward the poles ab, whether the centre of the wire is on the central line or æquator of the stone, or on any other part situated between the æquator and the poles, as at h, g, f, e. so the cusp of a versorium on this side of the æquator points toward the north; * on the other side the cross is always directed toward the south; but the cusp or lily[ ] does not, as some one has thought, turn toward the south beyond the æquator. some inexperienced people indeed, who in distant parts beyond the æquator have seen the versorium sometimes become more sluggish and less prompt, thought that the distance from the arctick pole or from the magnetick rocks was the cause of this. but they are very much mistaken; for it is as powerful[ ], and adjusts itself as quickly to the meridian or to the point of variation in the southern as in the northern parts of the earth. yet sometimes the motion appears slower, namely, when the supporting pin by lapse of time and long voyaging has become somewhat blunt, or the magnetick iron parts have lost, by age or rust, some of their acquired vigour. this may also be shown experimentally by the versatory iron of a small sun-dial placed on a very short pin set perpendicular to the surface of the stone, for the iron when touched by a loadstone points toward the poles of the stone and leaves the poles of the earth; for the general and remoter cause is overcome by the particular and powerful cause which is so near at hand. magnetick bodies have of themselves an inclination toward the position of the earth and are influenced by a terrella. two equal stones of equal strength adjust themselves to a terrella in accordance with magnetick laws. the iron conceives vigour from the loadstone and is influenced by the magnetical motions. wherefore true direction is the motion of a magnetick body in regard to the verticity of the earth, the natures of both agreeing and working together toward a natural position and unity. for indeed we have found out at length, by many experiments and in many ways, that there is a disposing nature, moving them together by reason of their various positions by one form that is common { } to both, and that in all magnetick substances there is attraction and repulsion. for both the stone[ ] and the magnetick iron arrange themselves by inclination and declination, according to the common position of their nature and the earth. and the force of the earth by the virtue of the whole, by attracting toward the poles, and repelling, arranges all magneticks which are unfixed and loose. for in all cases all magneticks conform themselves to the globe of the earth in the same ways and by the same laws by which another loadstone or any magneticks do to a terrella.[ ] * * * * * chap. ii. the directive or versorial virtue (which we call verticity): what it is, how it exists in the loadstone; _and in what way it is acquired when innate._ directive force, which is also called by us verticity, is a virtue which spreads by an innate vigour from the æquator in both directions toward the poles. that power, inclining in both directions towards the termini, causes the motion of direction, and produces a constant and permanent position in nature, not only in the earth itself but also in all magneticks. loadstone is found either in veins of its own or in iron mines, when the homogeneous substance of the earth, either having or assuming a primary form, is changed or concreted into a stony substance, which besides the primary qualities of its nature has various dissimilitudes and differences in different quarries and mines, as if from different matrices, and very many secondary qualities and varieties in its substance. a loadstone which is dug out in this breaking up of the earth's surface and of protuberances upon it, whether formed complete in itself (as sometimes in china) or in a larger vein, is fashioned by the earth and follows the nature of the whole. all the interior parts of the earth mutually conspire together in combination and produce direction toward north and south. but those magnetical bodies which come together in the uppermost parts of the earth are not true united parts of the whole, but appendages and parts joined on, imitating the nature of the whole; wherefore when floating free on water, they dispose themselves just in the same way as they are placed in the terrestrial system of nature. we had a large loadstone of twenty pounds * weight, dug up and cut out of its vein, after we had first observed and marked its ends; then after it was dug out, we placed it in a boat on water, so that it could turn freely; then immediately the face which had looked toward the north in the quarry began to { } turn to the north on the waves and at length settled toward that point. for that face which looked toward the north in the quarry is the southern, and is attracted by the northern parts of the earth, [illustration] in the same way as pieces of iron which acquire their verticity from the earth. about this point we intend to speak afterwards[ ] under change of verticity. but there is a different rotation of the internal parts of the earth, which are perfectly united to the earth and which are not separated from the true substance of the earth by the interposition of bodies as are loadstones in the upper portion of the earth, which is maimed, corrupt, and variable. let a b be a piece of magnetick ore; between which and the uniform globe of the earth lie various soils or mixtures which separate the ore to a certain extent from the globe of the true earth. it is therefore influenced by the forces of the earth just in the same way as c d, a piece of iron, in the air. so the face b of some ore or of that piece of it is moved toward the boreal pole g, just as the extremity c of the iron, not a or d. but the condition of the piece e f is different, which piece is produced in one connected mass with the whole, and is not separated from it by any earthy mixture. for if the part e f were taken out and floated freely in a boat by itself, it is not e that would be directed toward the boreal pole, but f. so in those substances which acquire their verticity in the air, c is the southern part and is seen to be attracted by the boreal pole g. in the case of others which are found in the upper unstable portion of the earth, b is the south, and in like manner inclines toward the boreal pole. but if those pieces deep down which are produced along with the earth are dug up, they turn about on a different plan. for f turns toward the boreal parts of the earth, because * is the southern part; e toward the south, because it is the northern. so of a magnetick body, c d, placed close to the earth, the end c turns toward the boreal pole; of one that is adnate to it b a, b inclines to the north; of one that is innate in it, e f, e turns toward the southern pole; which is confirmed by the { } [illustration] following demonstration, and comes about of necessity according to all magnetick laws. let there be a terrella with poles a b; from its mass cut out a small part e f; if this be suspended by a fine thread above the hole or over some other place, e does not seek the pole a but the pole b, and f turns to a; very differently from a rod of iron c d; because c, touching some northern part of the terrella, being magnetically carried away makes a turn round to a, not to b. and yet here it should be observed, that if the pole a of * the terrella were moved toward the earth's south, the end e of the piece cut out by itself, if not brought too near to the stone, would also move of itself toward the south. but the end c of the piece of iron, placed beyond its orbe of virtue, will turn toward the north. the part e f of the terrella, whilst in the mass, produced the same direction as the whole; but when it is separated and suspended by a thread, e turns to b, and f to a. [illustration] { } so parts having the same verticity with the whole, when separated, are impelled in the contrary direction; for contrary parts solicit contrary parts. nor yet is this a true contrariety, but the highest concordancy, and the true and genuine conformation of bodies magnetical in the system of nature, if they shall have been divided and separated: for the parts thus divided should be raised some distance from the whole, as will be made clear afterwards. magnetick substances seek a unity as regards form; they do not so much respect their own mass. wherefore the part f e is not attracted into its former bed; but when once it is unsettled and at a distance, it is * solicited by the opposite pole. but if the small piece f e is placed back again in its bed or brought close to, without any substances intervening, it acquires its former combination, and, as a part of the whole once more united, accords with the whole and sticks readily in its former position; and e remains toward a, and f toward b, and they settle steadily in their mother's lap. the reasoning is the same when the stone is divided into equal parts through the poles. [illustration] a spherical stone is divided into two equal parts along the axis a b; * whether therefore the surface a b is in the one part facing upward (as in the former diagram) or lying on its face in both parts (as in * the latter), the end a tends toward b. but it must also be understood that the point a is not carried with a definite aim always toward the point b, because in consequence of the division the verticity proceeds to other points, as to f g, as appears in the fourteenth chapter of this book. and l m are now the axes in each, and a b is no longer the axis; for magnetick bodies, as soon as they are divided, become single magnetick wholes; and they have { } vertices in accordance with their mass, new poles arising at each end in consequence of the division. yet the axis and the poles always follow the leading of a meridian; because that force passes along the meridians of the stone from the æquator to the poles, by an everlasting rule, the inborn virtue of the substance agreeing thereto from the long and lasting position and the facing of a suitable substance toward the poles of the earth; by whose strength continued through many centuries it has been fashioned; toward fixed and determined parts of which it has remained since its origin firmly and constantly turned. * * * * * chap. iii. how iron acquires verticity through a loadstone, and how that verticity _is lost and changed_. friction between an oblong piece of iron and a loadstone imparts to the former magnetick virtues, which are not corporeal nor inherent and persistent in any body, as we showed in the discussion on coition. it is plain that the iron, when it has been rubbed hard with one end and applied to the stone for a pretty long time, receives no stony nature, acquires no weight; for if, before the iron is touched by the stone, you weigh * it in a small and very exact goldsmith's balance, you will see after the rubbing that it has exactly the same weight, neither diminished nor increased. but if you wipe the iron with cloths after it has been touched, or wash it in water, or scour it with sand or on a grindstone, still it in nowise lays aside its acquired strength. for the force is spread through the whole body and conceived in the inmost parts, and cannot in any way be washed or wiped away. let an experiment then be made in fire, that untamed tyrant of nature. take a piece of iron of the length of a palm and the thickness of a goosequill pen; let this iron be passed through a suitable round cork and placed on the surface of water, and observe the end which turns to the north; rub this particular end with the true southern end of a loadstone; the iron so rubbed turns toward the south. remove the cork, and place the end * which was excited in the fire until the iron is just red-hot; when it is cooled, it will retain the strength of the loadstone and the verticity, though it will not be so prompt, whether because the force of the fire had not yet continued long enough to overcome all its { } strength, or because the whole iron was not heated to redness, for the virtue is diffused through the whole. remove the cork a second time, and putting the whole iron in the fire, blow the fire with the bellows, so that it may be all aglow, and let it remain a little longer time red-hot; when cooled (so, however, that, whilst it is cooling, it does not rest in one position), place it again on the water with the cork, and you will see that it has lost the verticity * which it had acquired from the stone. from these experiments it is clear how difficult it is for the property of polarity implanted by the loadstone to be destroyed. but if a small loadstone had remained as long in the same fire, it would have lost its strength. iron, because it does not so easily perish, and is not so easily burnt up as very many loadstones, retains its strength more stably, and when it is lost can recover it again from a loadstone; but a loadstone when burnt does not revive. but now that iron, which has * been deprived of its magnetick form, moves in a different way from any other piece of iron, for it has lost its polar nature; and whereas before the touch of the loadstone it may have had a motion toward the north, and after contact toward the south; now it turns to no definite and particular point; but afterwards, very slowly and after * a long time, it begins to turn in a doubtful fashion toward the poles of the earth (having acquired some power from the earth). i have said that the cause of direction was twofold, one implanted in the stone and iron, but the other in the earth, implanted by the disponent virtue; and for that reason (the distinction of poles and the verticity in the iron having now been destroyed) a slow and weak directive power is acquired anew from the verticity of the earth. we may see, therefore, with what difficulty and only by the application of hot fires and by long ignition of the iron heated to softness, the imparted magnetick virtue is eradicated. when this ignition has overcome the acquired polarity, and it has been now completely subdued and not awakened again, that iron is left unsettled and utterly incapable of direction. but we must further inquire how iron remains affected by verticity. it is manifest that it strongly affects and changes the nature of the iron, because the presence of a loadstone attracts the iron to itself with an altogether wonderful readiness. nor is it only the part that is rubbed, but on account of the rubbing (on one end only) the whole iron is affected together, and gains by it a permanent though an unequal power. this is demonstrated as follows. rub an iron wire on the end so * that it is excited, and it will turn towards the north; afterward cut off some portion of it; you will see that it still turns toward the north (as before), but more feebly. for it must be understood that the loadstone excites a steady verticity in the whole iron (if the rod be not too long) more vigorous throughout the whole mass in a shorter bar, and as long as the iron remains touching the loadstone a little { } stronger. but when the iron is separated from contact with it, then it becomes much weaker, especially in the end that was not touched. just as a long rod, one end of which is placed in the fire and heated, grows exceedingly hot at that end, less so in the parts adjoining and in the middle, whilst at the other end it can be held in the hand, and that end is only warm; so the magnetical vigour diminishes from the excited end to the other end; but it is present there instantly, and does not enter after an interval of time nor successively, as the heat in the iron; for as soon as a piece of iron has been touched by a loadstone it is excited throughout its whole length. for the sake of experiment, let there be a rod of iron or * digits long, untouched by a loadstone; as soon as you touch one end only with a loadstone, the opposite end immediately, or in the twinkling of an eye, by the power that it has conceived, repels or attracts a versorium, if it be applied to it ever so quickly. * * * * * chap. iiii. why iron touched by a loadstone acquires an opposite _verticity, and why iron touched by the true northern side of a stone_ turns to the north of the earth, by the true southern side _to the south; and does not turn to the south when rubbed by the northern point of the stone, and when by the southern to the north, as all who have written on the loadstone have falsely supposed._ demonstration has already been given that the northern part of a loadstone does not attract the northern part of another stone, but the southern, and repels the northern part of another stone from its northern side when it is applied[ ] to it. that general magnet, the terrestrial globe, disposes iron touched by a loadstone in the same way, and likewise magnetick iron stirs this same iron by its implanted strength, and excites motion and controls it. for whether the comparison and experiment has been made between loadstone and loadstone, or loadstone and iron, or iron and iron, or the earth and loadstone, or the earth and iron conformed * by the earth or strengthened by the power of a loadstone, the strength and inclinations of each must mutually harmonize and accord in the same way. but the reason must be sought, why a piece of iron when touched by a loadstone acquires a disposition to motion toward the opposite pole of the earth, and not toward that { } pole of the earth to which that pole of that loadstone turned by which it was excited. it has been pointed out that iron and loadstone are of one primary nature; when the iron is joined to the loadstone, they become, as it were, one body, and not only is the end of the iron changed, but the remaining parts also are affected along with it. a, the north pole of a loadstone, is placed against the cusp of a piece of iron; the cusp of the iron has now become the southern part of the iron, [illustration] because it is touching the northern part of the stone; the cross-end of the iron has become the northern. for if that contiguous magnetick substance be separated from the pole of the terrella, or from the parts near the pole, the one end (or the end which, whilst the connection was kept up, was touching the northern part of the stone) is the southern, whilst the other is the northern. so also if a versorium excited by a loadstone be divided into ever so many parts (however small), those parts when separated will, it is clear, arrange themselves in the same disposition as that in which they were disposed before, when they were undivided. wherefore whilst the cusp remains over the northern pole a, it is not the southern end, but is, as it were, part of a whole; but when it is taken away from the stone, it is the southern end, because when rubbed it tended toward the northern parts of the stone, and the cross (the other end of the versorium) is the northern end. the loadstone and the iron make one body; b is the south pole of the whole; c (that is, the cross) is the northern end of the whole; divide the iron also at e, and e will be the southern end with respect to the cross; and e will likewise be the northern end in respect to b. a is the true northern pole of the stone and is attracted by the southern pole of the earth. the end of the iron which is touched by the true boreal part of the stone becomes the southern end, and turns to a, the north [pole] of the stone, if it be near; or if it be some distance from the stone it turns to the north [pole] of the earth. so always iron which is touched (if it is free and unrestrained) tends to the opposite part of the earth from that part to which the loadstone that touched it tends. nor does it * make any difference how it is rubbed, whether straight up or slanting in some way. for in any case the verticity flows into the iron, { } [illustration] provided it is touched by either end. wherefore all the cusps at b acquire the same verticity, after they are separated, but opposite to that pole of the stone; wherefore also they are united to the loadstone at the pole b; and all the crosses in the present figure have the opposite verticity to the pole e, and are moved and laid hold of by e when they are in a convenient position. it is exactly the same in the case of the long stone f h divided at g; f and h always move, both in the whole and in the divided stone, to opposite poles of the earth, and o and p mutually attract one another, the one of them being the northern, the other the southern. for, supposing h to have been the southern in the whole stone and f the northern, p will be the northern with respect to h in the divided stone, and o the southern with respect to f. so also f and h mutually incline to a connection, if they are turned a very little toward one another, and run together at length and join. but supposing the division of the stone to have been meridional (that is, according to the line of a meridian, not of any parallel circle), then they turn [illustration] round, and a attracts b, and the end b is attracted to a and attracts a, until, being turned round, they are connected and cemented together; because magnetick attraction is not made along the parallels, but meridionally. for this reason pieces of iron placed on a terrella whose poles are a b, near the æquator along parallels, * do not combine or stick together firmly: { } [illustration] but if applied to one another along a meridian they are immediately * joined firmly together, not only on and near the stone, but even at some distance within the force of the controlling orbe. thus they are joined and cemented together at e, but not at c in the other figure. for the opposite ends c and f meet and adhære together in the case of the iron just in the same way as a and b before in the case of the stone. but they are opposite ends, because the pieces of iron proceed from the opposite sides and poles of the terrella; and c in reference to the northern pole a is southern, and f is boreal in reference to the * southern pole b. in like manner also they are cemented together, if the rod c (being not too long[ ]) be moved further toward a, and f toward b, and they be joined together over the terrella, like a and b of the divided stone above. but now if the cusp a, * which has been touched by a loadstone, be the southern end, and you were to touch and rub with this the cusp of another iron needle b, which has not been touched, b will be northern, and will point to the south. but if you were to touch with the northern point b any other iron needle, still new, on its cusp, this again will be southern, and will turn to the north. the iron not only receives the necessary strength from the loadstone, if it be a good loadstone, but also imparts its acquired strength to another piece of iron, and the second to a third (always in strict accordance with magnetick laws). in all these demonstrations of ours it should always be borne in mind that the poles of a stone, as well as those of iron, whether touched or untouched, are always in fact and by nature opposite to the pole toward which they point and are so designated by us, as we have laid down above. for in them all it is always the northern * which tends to the south, either of the earth or of the stone, and the southern which tends to the north of the stone. northern parts are attracted by the southern of the earth; so in the boat they { } tend toward the south. a piece of iron touched by the northern parts of a loadstone becomes south at the one end and tends always (if it is near and within the orbe of the loadstone) to the north of the stone, and if it be free and left to itself at some distance from the stone, it tends to the northern part of the earth. the northern pole a of a loadstone turns to g, the south of the earth; a versorium touched at its cusp by the part a follows a, because it has become southern. but the versorium c, placed farther away from the loadstone, turns its cusp to f, the north of the earth, because * the cusp has become southern by contact with the boreal part of the stone. so the ends touched by the northern part of the stone are made southern, or are excited with a southern polarity, and tend toward the north of the earth; those touched by the southern pole are made northern, or are excited with a northern force, and turn to the south of the earth. [illustration] * * * * * chap. v. on the touching of pieces of iron _of divers shapes._ bars of iron, when touched by a loadstone, have one end north, the other south, and in the middle is the limit of verticity, like the æquinoctial circle on the globe of a terrella or on an iron globe. but when an iron ring is rubbed on one side on a * loadstone, then the one pole is on the place that was in contact, whilst the other is at the opposite point; and the magnetick power divides the ring into two parts by a natural distinction which, though not in shape, yet in power and effect is like an æquator. but if a thin straight rod be bent into a ring without any welding or union of the ends, and be touched in the middle by a loadstone, both ends will be of the same verticity. let a ring be taken which is whole and continuous, and which has been * touched by a loadstone at one place, and let it be divided afterward { } at the opposite point and straightened out, both ends will also be * of the same verticity, no otherwise than a thin rod touched in the middle or a ring not cohærent at the joint. * * * * * chap. vi. what seems an opposing motion in magneticks _is a proper motion toward unity_. [illustration] in things magnetical nature always tends to unity, not merely to confluence and agglomeration, but to harmony; in such a way that the rotational and disponent faculty should not be disturbed, as is variously shown in the following example. let c d be an entire body of some magnetick substance, in which c tends to b, the north of the earth, and d to the south, a. then[ ] divide it in the middle in its æquator, and it will be e that is tending toward a, and f tending toward b. for just as in the undivided body, so in the divided, nature aims at these bodies being united; the end e again joins with f harmoniously and * eagerly and they stick together, but e is never joined to d, nor f to c; for then c must be turned contrary to nature toward a, the south, or d toward b, the north, which is foreign to them and incongruous. separate the stone in the place where it is cut and turn d round to c; they harmonize and combine excellently. for d is tending to the south, as before, and c to the north; e and f, parts which were cognate in the ore, are now widely separated, for they do not move together on account of material affinity, but they take their motion and inclination from their form. so the ends, whether joined or divided, tend magnetically in the same way to the earth's poles in the first figure where there is one whole, or divided as in the second figure; and f e in the second figure is a perfect magnetick joined together into one body and c d, just as it was primarily produced in its ore, and f e in its boat, turn in { } this way to the poles of the earth and are conformed to them. * this harmony of the magnetick form is shown also in the forms of vegetables. let a b be a twig from a branch of osier or other * tree which sprouts easily. let a be the upper part, b the lower part toward the root; divide it at c d; i say that the end d, if grafted again to c by the primer's art, grows to it; just as also if b is grafted to a, they grow together and germinate. but d being grafted on a, or c on b, they are at variance, and never grow into one another, but one of them dies on account of the inverted and inharmonious arrangement, since the vegetative force, which moves in one way, is now impelled in opposite directions. [illustration] * * * * * chap. vii. a determined verticity and a disponent faculty are what arrange magneticks, not a force, attracting or pulling them _together, nor merely strongish coition or unition_. [illustration] { } in the neighbourhood of the æquinoctial a there is no coition of the ends of a piece of iron with the terrella; at the poles there is the strongest. the greater the distance from the æquinoctial, the stronger is the coition with the stone itself, and with any part of it, not with its pole alone. yet pieces of iron are not raised up on account of some peculiar attracting force or a stronger combined force, but on account of that common directing or conforming and rotating force; nor indeed is a spike in the part about b, even one that is very small and of no * weight[ ], raised up to the perpendicular by the strongest terrella, but cleaves to it obliquely. also just as a terrella attracts magnetick bodies variously with dissimilar forces, so also an iron snout placed on the stone obtains a different potency in proportion to the latitude, * just as a snout at l by its firmer connection resists a greater weight more stoutly than one at m, and at m than at n. but neither does the snout raise the spike to the perpendicular except at the poles, as is shown in the figure. a snout at l may hold and lift from the earth two ounces of iron in one piece; yet it is not strong enough to raise an iron wire of two grains weight to the perpendicular, which would happen if the verticity arose on account of a * stronger attraction, or rather coition or unition. * * * * * chap. viii. of discords between pieces of iron upon the same pole of a loadstone, and how they can agree and _stand joined together_. suppose two iron wires or a pair of needles stuck on the pole of a terrella; though they ought to stand perpendicularly, they mutually repel one another at the upper * end, and produce the appearance of a fork; and if one end be forcibly impelled toward the other, the other declines and bends away from association with it, as in the following figure. [illustration] { } a and b, iron spikes, adhære obliquely[ ] upon the pole on account of their nearness to one another; either alone would otherwise stand erect and perpendicular. for the extremities a b, being of the same verticity, mutually abhor and fly one another. for if c be the northern pole of the terrella, a and b are also northern ends; but the ends which are joined to and held at the pole c are both * southern. but if those spikes be a little longer (as, for example, of two digits length) and be joined by force, they adhære together and unite in a friendly style, and are not separated without force. for they are magnetically welded, and there are now no longer two distinct ends, but one end and one body; no less than a wire which is doubled and set up perpendicularly. but here is seen also another subtile point, that if those spikes were shorter, not as much as the * breadth of one digit, or even the length of a barleycorn, they are in no way willing to harmonize or to stand straight up at the same time, because naturally in shorter wires the verticity is stronger in the ends which are distant from the terrella and the magnetick discord more vehement than in long ones. wherefore they in no way admit of an intimate association and connection. [illustration] likewise if those lighter pieces of iron or iron wires be suspended, hanging, as a and b, from a very fine silk thread, not twisted * but braided, distant from the stone the length of a single barleycorn, then the opposing ends, a and b, being situated within the orbe of virtue above the pole, keep a little away from one another for the same reason; except when they are very near the pole of the stone c, the stone then attracting them more strongly toward one end. * * * * * { } chap. ix. figures illustrating direction and showing varieties _of rotations_. [illustration] passing from the probable cause of motion toward fixed points (according to magnetick laws and principles), it remains for us to indicate those motions. above a round loadstone (whose poles are a, b) let a versatory needle be placed whose cusp has been excited by the pole a; that cusp is certainly directed toward a, and is strongly attracted by a; because, having been touched by a, it is in true harmony with a, and combines with it; and yet it is called contrary, because when the versorium is separated from the stone, it is seen to be moved toward the opposite part of the earth to that toward which the pole a of the loadstone is moved. for if a be the northern pole of the terrella, the cusp is the southern end of the needle, of which the other end (namely, the cross) is pointed to b; so b is the southern pole of the loadstone, but the cross is the northern end of the versorium. so also the cusp is attracted by e, f, g, h, and by every * part of a meridian, from the æquator toward the pole, by the faculty disponent; and when the versorium is on the same parts of the meridian, the cusp is directed toward a. for it is not the point a that turns the versorium toward it, but the whole loadstone; as also the whole earth does, in the turning of loadstones to the earth. [illustration] _figures illustrating magnetick directions in a right sphere[ ] of stone, and in the right sphere of the earth, as well as the polar directions to the perpendicular of the poles._ all these cusps have been touched by the pole a; all the cusps are turned toward a, excepting that one which is repelled by b. { } [illustration] _figures illustrating horizontal directions above the body of a loadstone._ all the cusps that have been made southern by rubbing on the boreal pole, or some place round the northern pole a, turn toward the pole a, and turn away from the southern pole b, toward which all the crosses look. i call the direction horizontal, because it is arranged along the plane of the horizon; for nautical and * horological instruments are so constructed that the iron hangs or is supported in æquilibrium on the point of a sharp pin, which prevents the dipping of the versorium, about which we intend to speak later. and in this way it is of the greatest use to man, indicating and distinguishing all the points of the horizon and the winds. otherwise on every oblique sphere (whether of stone or the earth) versoria and all magnetick substances would have a dip by their own nature below the horizon; and at the poles the directions would be perpendicular, which appears in our discussion _on declination_. [illustration] _a round stone (or terrella) cut in two at the æquator;_ and all the cusps have been touched by the pole a. the points at the centre of the earth, and between the two parts of the terrella which has been cut in two through the plane of the æquator, { } are directed as in the present[ ] diagram. this would also happen in the same way if the division of the stone were through the plane of a tropick, and the mutual separation of the divided parts and the interval between them were the same as before, when the loadstone was divided through the plane of the æquator, and the parts separated. for the cusps are repelled by c, are attracted by d; and the versoria are parallel, the poles or the verticity in both ends mutually requiring it. [illustration] _half a terrella by itself and its directions, unlike the directions * of the two parts close to one another as shown in the figure above_. all the cusps have been touched by a; all the crosses below except the middle one tend toward the loadstone, not straight, but obliquely; because the pole is in the middle of the plane which before was the plane of the æquator. all cusps touched by places distant from the pole move toward the pole (exactly the same as if they had been rubbed upon the pole itself), not toward the place where they were rubbed, wherever that may have been in the undivided stone in some latitude between the pole and the æquator. and for this reason there are only two distinctions of regions, northern and southern, in the terrella, just { } as in the general terrestrial globe, and there is no eastern nor western place; nor are there any eastern or western regions, rightly speaking; but they are names used in respect of one another toward the eastern or western part of the sky. wherefore it does not appear that ptolemy did rightly in his _quadripartitum_, making eastern and western districts and provinces, with which he improperly connects the planets, whom the common crowd of philosophizers and the superstitious soothsayers follow. * * * * * chap. x. on mutation of verticity and of magnetick properties, or on alteration in the power _excited by a loadstone_. friction with a loadstone gives to a piece of iron a verticity strong enough; not, however, so stable that the iron may not by being rubbed on the opposite part (not only with a more powerful loadstone, but with the same) be changed and deprived of all its former verticity, and indued with a new and opposite one. take a piece of iron wire and rub each end of the wire equally with one and the same pole of a loadstone, and let it be passed through a suitable cork and place it on water. then truly one end of the wire will be directed toward that pole of the earth toward which that end of the stone will not turn. but which end of the iron wire will it be? that certainly which was rubbed last. rub the other end of this again with the same pole, and immediately * that end will turn itself in the opposite direction. again touch the former end of the iron wire only with the same pole of the loadstone as before; and that[ ] end, having gained the command, immediately changes to the contrary side. so you will be able to change the property of the iron frequently, and that end of the wire rules which has been touched the last. now then merely hold the boreal pole of the stone for some time near the boreal part of the wire which was last touched, so that it does not touch, but so that it is removed from it by one, two, or even three digits, if the stone have been pretty * strong; and again it will change its property and will turn round to the contrary side; which will also happen (albeit rather more feebly) even if the loadstone be removed to a distance of four digits. you will be able to do the same thing, moreover, with both the austral and the boreal part of the stone in all these experiments. verticity may likewise be acquired and changed when thin plates of gold, * silver, and glass are interposed between the stone and the end of the iron or iron wire, if the stone were rather strong, even if the { } intermediate lamina is not touched either by the iron or the stone. and these changes of verticity take place in smelted iron. indeed what the one pole of the stone implants and excites, the other disturbs and extinguishes, and confers a new force. for it does not require a stronger loadstone to take away the weaker and sluggish virtue and to implant the new one; nor is iron inebriated by the equal strength of loadstones, and made utterly uncertain and neutral, as baptista porta teaches; but by one and the same loadstone, or by loadstones endowed with equal power and might, its strength is, in accordance with magnetick rules, turned round and changed, excited, repaired, or disturbed. but a loadstone itself, by being rubbed on another, whether a larger or a more powerful stone, is not disturbed from its own property and verticity, nor does it turn round toward the opposite direction in its boat, or to the other pole opposite to that to which it inclines by its own nature and implanted verticity. for strength which is innate and has been implanted for a very long time abides more firmly, nor does it easily yield from its ancient holding; and that which has grown for a long time is not all of a sudden brought to nothing, without the destruction of the substance containing it. nevertheless in a long interval of time a change * does take place; in one year, that is to say, or two, or sometimes in a few months; doubtless when a weaker loadstone remains lying by a stronger one contrary to the order of nature, namely, with the northern pole of one loadstone adjoined to the northern pole of another, or the southern to the southern. for so the weaker strength gradually declines with the lapse of time. * * * * * chap. xi. on the rubbing of a piece of iron on a loadstone in places midway between the poles, and upon _the æquinoctial of a terrella_. select a piece of iron wire of three digits length, not touched by a loadstone (but it will be better if its acquired verticity be rather weak or have been damaged in some way); touch it and rub it on the æquator of a terrella, exactly on the æquinoctial line in the direction of its length, on the one end, or the ends only, or in all its parts; place the wire touched in this * way on water in a cork fitted for it; it will swim about doubtfully on the waves without any acquired verticity, and the verticity previously implanted will be disturbed. if, however, it float by chance toward the poles, it will be checked a little by the poles of the earth, and will at length by the influence of the earth be indued with verticity. * * * * * { } chap. xii. in what way verticity exists in any iron that has _been smelted though not excited by a lodestone_. [illustration] having thus far[ ] demonstrated natural and inborn causes and powers acquired by means of the stone, we will now examine the causes of magnetick virtues in smelted iron that has not been excited by a stone. loadstone and iron furnish and exhibit to us wonderful subtilities. it has been repeatedly shown above that iron not excited by a stone turns north and south; further that it has verticity, that is, special and peculiar polar distinctions, just as a loadstone, or iron which has been rubbed upon a loadstone. this indeed seemed to us at first wonderful and incredible; the metal of iron from the mine is smelted in the furnace; it runs out of the furnace, and hardens into a great mass; this mass is divided in great worksteads, and is drawn into iron bars, from which smiths again construct many instruments and necessary pieces of iron-work. thus the same mass is variously worked up and transformed into very many similitudes. what is it, then, which { } preserves its verticity, and whence is it derived? so take this first from the above[ ] smithy. let the blacksmith beat out upon his anvil a glowing mass of iron of two or three ounces weight into an iron spike of the length of a span of nine inches. let the smith be standing with his face to the north, his back to the south, so that * the hot iron on being struck has a motion of extension to the north; and let him so complete his work with one or two heatings of the iron (if that be required); let him always, however, whilst he is striking the iron, direct and beat out the same point of it toward the north, and let him lay down that end toward the north. let him in this way complete two, three, or more pieces of iron, nay, a hundred or four hundred; it is demonstrable that all those which are thus beaten out toward the north, and so placed whilst they are cooling, turn round on their centres; and floating pieces of iron (being transfixed, of course, through suitable corks) make a motion in the water, the determined end being toward the north. in the same way also pieces of iron acquire verticity from their direction whilst they are being beaten out and hammered or drawn out, * as iron wires are accustomed to do toward some point of the horizon between east and south or between south and west, or in the opposite direction. those, however, which are pointed or drawn out rather toward the eastern or western point, conceive * hardly any verticity or a very undecided one. that verticity is especially acquired by being beaten out. but a somewhat inferior iron ore, in which no magnetick powers are apparent, if put in a * fire (its position being observed to be toward the poles of the world or of the earth) and heated for eight or ten hours, then cooled away from the fire, in the same position towards the poles, acquires a verticity in accordance with the position of its heating and cooling. let a rod of cast iron be heated red-hot in a strong fire, in which it lies * meridionally (that is, along the path of a meridian circle), and let be removed from the fire and cooled, and let it return to its former temperature, remaining in the same position as before; then from this it will turn out that, if the same ends have been turned to the same poles of the earth, it will acquire verticity, and the end which looked toward the north on water with a cork before the heating, if it have been placed during the heating and cooling toward the fourth, now turns round to the south. but if perchance sometimes the rotation have been doubtful and somewhat feeble, let it be placed again in the fire, and when it is taken out at a red heat, let it be perfectly cooled toward the pole from which we desire the verticity, and the verticity will be acquired. let the same rod be heated * in the contrary position, and let it be placed so at a red heat it is cool; for it is from its position in cooling (by the operation of the verticity of the earth) that verticity is put into the iron, and it turns round to parts contrary to its former verticity. so { } the end which formerly looked toward the north now turns to the south. in accordance with these reasonings and in these ways the boreal pole of the earth gives to the end of a piece of iron turned toward it a southern verticity, and that end is attracted by that pole. * and here it must be observed that this happens to iron not only when it is cooled in the plane of the horizon, but also at any angle to it almost up to the perpendicular toward the centre of the earth. so the heated iron conceives vigour and verticity from the earth more quickly in the course of its return to its normal state, and in its recovery, as it were (in the course of which it is transformed), than by its mere position alone. this is effected better and more * perfectly in winter and in colder air, when the metal returns more certainly to its natural temperature, than in summer and in warm regions. let us see also what position alone and a direction toward the poles of the earth can effect by itself without fire and heat. iron rods which have been placed and fixed for a long time, twenty * or more years, from south to north (as they not infrequently are fixed in buildings and across windows), those rods, i say, by that long lapse of time acquire verticity and turn round, whether hanging in the air, or floating (being placed on cork), to the pole toward which they were pointing, and magnetically attract and repel a balanced iron magnetick; for the long continued position of the body toward the poles is of much avail. this fact (although conspicuous by manifest experiments) is confirmed by an incident related in an italian letter[ ] at the end of a book of maestro filippo costa, of mantua, _sopra le compositioni degli antidoti_ written in italian, which translated runs thus: "a druggist of mantua showed me a piece of iron entirely changed into a magnet, drawing another piece of iron in such a way that it could be compared with a loadstone. now this piece of iron, when it had for a long time held up a brick ornament on the top of the tower of the church of st. augustine at rimini, had been at length bent by the force of the winds, and remained so for a period of ten years. when the monks wished to bend it back to its former shape, and had handed it over to a blacksmith, a surgeon named maestro giulio caesare discovered that it was like a magnet and attracted iron." this was caused by the turning of its extremities toward the poles for so long a time. and so what has been laid down before about change of verticity should be borne in mind; how in fact the poles of iron spikes are altered, when a loadstone is placed against them only with its pole and points toward them, even at a rather long distance. clearly it is in the same way that that large magnet also (to wit, the earth itself) affects a piece of iron and changes its verticity. for, although the iron may not touch the pole of the earth, nor any magnetick part of the earth, yet verticity is acquired and changed; not because the poles of the earth and the point itself which is ° distant { } from our city of london, changes the verticity at a distance of so many miles; but because the whole magnetick earth, that which projects to a considerable height, and to which the iron is near, and that which is situated between us and the pole, and the vigour existing within the orbe of its magnetick virtue (the nature of the whole conspiring thereto), produces the verticity. for the magnetick effluence of the earth rules everywhere within the orbe of its virtue, and transforms bodies; but those things which are more similar to it, and specially connected with it by nature, it rules and controls; as loadstone and iron. wherefore in very many matters of business and actions it is clearly not superstitious and idle to observe the positions and conditions of lands, the points of the horizon and the places of the stars. for as when a babe is brought forth into the light from its mother's womb, and acquires respiration and certain animal activities, then the planets and celestial bodies[ ], according to their position in the universe, and according to that configuration which they have with regard to the horizon and the earth, instil peculiar and individual qualities into the newly born; so that piece of iron, whilst it is being formed and lengthened out, is affected by the common cause (to wit, the earth); whilst it is returning also from its heated condition to its former temperature, it is imbued with a special verticity in accord with its position. rather long pieces of iron sometimes have the same verticity * at each end; wherefore they have motions which are less certain and well ordered on account of their length and of the aforesaid processes, exactly as when an iron wire four feet long is rubbed at each end upon the same pole of a loadstone. * * * * * chap. xiii. why no other body, excepting a magnetick, is imbued _with verticity by being rubbed on a loadstone; and why no_ body is able to instil and excite that virtue, _unless it be a magnetick._ ligneous substances floating on water never by their own strength turn round toward the poles of the earth, save by chance. so wires of gold, silver, brass, tin, lead, or glass, pushed through corks and floating, have no sure direction; and for this reason they do not show poles or points of variation when rubbed with a loadstone. for those things which do not of themselves incline toward the poles and obey the earth are also not ruled by { } the touch of a loadstone; for the magnetick vigour has no entrance into their inward parts; neither is the magnetick form received by them, nor are their forms magnetically excited; nor, if it did enter, would it effect anything, because in those bodies (mixed up with various kinds of efflorescent humours and forms, corrupted from the original property of the earth) there are no primary qualities. but those prime qualities of iron are excited by the juxtaposition of a loadstone, just as brute animals or men, when they are awakened out of sleep, move and put forth their strength. here one must marvel at a demonstrable error of b. porta, who, while rightly opposing a very old falsehood about the diamond, in speaking of a power contrary to that of the loadstone, introduces another still worse opinion; that forsooth iron, when touched by a diamond, turns to the north. "if" (he says) "you rub a steel-needle on a diamond, and then put it in a boat, or thrust it through a reed, or hang it up by a thread, it will presently turn to the north, almost as well as if it had been touched with the loadstone; but something more faintly. and, what is worth noting, the contrary part will turn the iron to the south: and when i had tried this in many steel-needles, and put them all into the water, i found, that they all stood equi-distant, pointing to the north." this indeed would * be contrary to our magnetick rules. for this reason we made an experiment with seventy excellent diamonds, in the presence of many witnesses, on a large number of spikes and wires, with the most careful precautions, floating (thrust, of course, through their corks) on the surface of water; never, however, could we observe this. he was deceived by the verticity acquired from the earth (as stated above) in the spike or wire of iron itself, and the iron itself turned aside to its own definite pole; and he, being ignorant of this, thought it was done by the diamond. but let the investigators of natural phenomena take heed that they are not the more deceived by their own badly observed experiments, and disturb the commonwealth of letters with their errors and stupidities. diamond is sometimes designated by the name of _sideritis_, not because it is made of iron or because it draws iron, but on account of its lustre, resembling flashing steel; with such a lustre do the choicest pieces of diamond shine; hence by very many writers many qualities are imputed to diamond which really belong to siderite loadstone. * * * * * { } chap. xiiii. the placing of a loadstone above or below a magnetick body suspended in æquilibrium changes neither the power _nor the verticity of the magnetick body._ quietly to pass this over would be improper, because a recent error arising from a defective observation of baptista porta must be overthrown; on which he (by an unfortunate repetition) even writes three chapters, namely, the th, the st, and the nd. for if a loadstone or a piece of magnetick iron, hanging in æquilibrium or floating on water, is attracted and disposed toward certain definite points, when you bring above it a piece of iron or another loadstone, it will not, if you afterward put the same[ ] below it, turn round to the contrary parts; but the same ends of the iron or the loadstone will always be directed toward the same ends of the stone, even if the loadstone or the iron is suspended in any way in æquilibrium or is poised on a needle, so that it can turn round freely. he was deceived by the irregular shape of some stone, or because he did not arrange the experiment suitably. wherefore he is led astray by a vain opinion, and thinks he may infer that, just as a stone has an arctic and antarctic pole, so also it has a western and an eastern, and an upper and a lower pole. so from foolish ideas conceived and admitted arise other fallacies. * * * * * chap. xv. the poles, Æquator, centre in an entire loadstone _remain and continue steady; by diminution and_ separation of some part they vary and _acquire other positions._ [illustration] * suppose a b to be a terrella, whose centre is e, and whose diameter (as also its æquinoctial circle) is d f. if you cut off a portion (through the arctic circle, for example), g h, it is demonstrable that the pole which was at a now has a position at i. but the centre and the æquinoctial recede toward b { } merely so that they are always in the middle of the mass that is left between the plane of the arctick circle g i h and the antarctick pole b. therefore the segment of the terrella comprised between the plane of the former æquinoctial (that, of course, which was the æquator before cutting that part away) d e f and the newly acquired æquator m l n will always be equal to the half of that part which was cut off, g i h a. [illustration] * but if the portions have been taken away from the side c d, the poles and axis will not be in the line a b, but in e f, and the axis would be changed in the same proportion as the æquator in the former figure. for those positions of forces and virtues, or rather limits of the virtues, which are derived from the whole form, are moved forward by change of quantity and shape; since all these limits arise from the conspiring together of the whole and of all { } the parts united; and the verticity or the pole is not a virtue innate in one part, or in some definite limit, or fixed in the substance; but it is an inclination of the virtue to that part. and just as a terrella separated from the earth has no longer the earth's poles and æquator, but individual ones of its own; so also if it again be divided, those limits and distinctions of the qualities and virtues pass on to other parts. but if a loadstone be divided in any way, either along a parallel, or meridionally, so that by the change of shape either the poles or the æquator move to other positions, if the part cut off be merely applied in its natural position and joined to the whole, even without any agglutination or cementing together, the determining points of the virtues return again to their former sites, as if no part of the body had been cut off. when a body is entire, its form remains entire; but when the body is lessened, a new whole is made, and there arises a new entirety, determined for every loadstone, however small, even for magnetick gravel, and for the finest sand. * * * * * chap. xvi. if the southern portion of a stone be lessened, something is also taken away from the power _of the northern portion._ now although the southern end of a magnetick iron is attracted by a northern end, and repelled by a southern, yet the southern portion of a stone does not diminish, but increases the potency of the boreal part. wherefore if a stone be cut in two and divided through the arctick circle, or through the tropick of cancer or the æquator, the southern portion does not attract magnetick substances so strongly with its pole as before; because a new whole arises, and the æquator is removed from its old position and moves forward on account of that cutting of the stone. in the former condition, since the opposite portion of the stone increases the mass beyond the plane of the æquator, it strengthens also the verticity, and the potency, and the motion to unity. * * * * * { } chap. xvii. on the use and excellence of versoria: and how iron _versoria used as pointers in sun-dials, and the fine needles_ of the mariners' compass, are to be rubbed, that _they may acquire stronger verticity._ versoria prepared by the loadstone subserve so many actions in human life that it will not be out of place to record a better method of touching them and exciting them magnetically, and a suitable manner of operating. rich ores of iron and such as yield a greater proportion of metal are recognized by means of an iron needle suspended in æquilibrium and magnetically prepared; and magnetick stones, clays, and earths are distinguished, whether crude or prepared. an iron needle (the soul of the mariners' compass), the marvellous director in voyages and finger of god, one might almost say, indicates the course, and has pointed out the whole way around the earth (unknown for so many ages). the spaniards (as also the english) have frequently circumnavigated (by an immense circuit) the whole globe by aid of the mariners' compass. those who travel about through the world or who sit at home have sun-dials. a magnetick pointer follows and searches out the veins of ore in mines. by its aid mines are driven in taking cities; catapults and engines of war are aimed by night; it has been of service for the topography of places, for marking off the areas and position of buildings, and for excavating aqueducts for water under ground. on it depend instruments designed to investigate its own dip and variation. when iron is to be quickened by the stone, let it be clean and bright, disfigured by no rust or dirt, and of the best steel[ ]. let the stone itself be wiped dry, and let it not be damp with any moisture, but let it be filed gently with some smooth piece of iron. but the hitting of the stone with a hammer is of no advantage. by these means let their bare surfaces be joined, and let them be rubbed, so that they may come together more firmly; not so that the material substance of the stone being joined to the iron may cleave to it, but they are rubbed gently together with friction, and (useless parts being rubbed off) they are intimately united; whence a more notable [illustration] virtue arises in the iron that is excited. a is the best way of touching a versorium when the cusp touches the pole and faces it; b is a moderately good way, when, though facing it, it is a little way { } distant from the pole; also in like manner c is only moderately good on account of the cusp being turned away from the pole; d, which is farther distant, is hardly so good; f, which is prepared crosswise along a parallel, is bad; of no virtue and entirely irresponsive and feeble is the magnetick index l, which is rubbed along the æquator; oblique and not pointing towards the pole as g, and oblique, not pointing toward but turned away from the pole as h, are bad. these have been placed so that they might indicate the distinct forces of a round stone. but mechanicians very often have a stone tending more to a cone shape, and more powerful on account of that shape since the pole, on which they rub their wires, is at the apex of the projecting part. sometimes the stone has on the top and above its own pole an artificial acorn or snout made of steel for the sake of its power. iron needles are rubbed on the top of this; wherefore they turn toward the same pole as if they had been prepared on that part of the stone with the acorn removed. let the stone be large enough and strong; the needle, even if it be rather long, should be sufficiently thick, not very slender; with a moderate cusp, not too sharp, although the virtue is not in the cusp itself only, but in the whole piece of iron. a strong large stone is not unfit for rubbing all needles on, excepting that sometimes by its strength it occasions some dip and disturbance in the iron in the case of longer needles; so that one which, having been touched before, rested in equilibrium in the plane of the horizon, now when touched and excited dips at one end, as far as the upright pin on which it turns permits it. wherefore in the case of longer versoria, the end which is going to be the boreal, before it is rubbed, should be a little lighter, so that it may remain exactly in æquilibrio after it is touched. but a needle in this way prepared does its * { } work worse the farther it is beyond the æquinoctial circle. let the prepared needle be placed in its capsule, and let it not be touched by any other magneticks, nor remain in the near vicinity of them, lest by their opposing forces, whether powerful or sluggish, it should become uncertain and dull. if you also rub the other end of the needle on the other pole of the stone, the needle will perform its functions more steadily, especially if it be rather long. a piece of iron touched by a loadstone retains the magnetick virtue, excited in it even for ages[ ], firm and strong, if it is placed according to nature meridionally and not along a parallel, and is not injured by rust or any external injury from the surrounding medium. porta wrongly seeks for a proportion between the loadstone and the iron: because, he says, a little piece of iron will not be capable of holding much virtue; for it is consumed by the great force of the loadstone. a piece of iron receives its own virtue fully, even if it be only of the weight of one scruple, whilst the mass of the loadstone is a thousand pounds. it is also useless to make the needle rather flat at the end that is touched, so that it may be better and more perfectly magnetick, and that it may best receive and hold certain magnetick particles; since hardly any part will stick on a sharp point; because he thought that it was by the adhesion of parts of the loadstone (as it were, hairs) that the influence is imparted and conserved, though those particles are merely rubbed off by the rubbing of the iron over the softer stone, and the iron none the less points toward the north and south, if after it is touched it be scoured with sand or emery powder, or with any other material, even if by long rubbing of this kind the external parts of it are lessened and worn away. when a needle is being rubbed, one should always leave off at the end; otherwise, if it is rubbed on the loadstone from the point toward the middle, less verticity is excited in the iron, sometimes none at all, or very little. for where the last contact is, there is the pole and goal of verticity. in order that a stronger verticity may be produced in the iron by rubbing on the loadstone, one * ought in northern lands to turn the true northern pole of the loadstone toward the highest part of the sky; on this pole that end of the needle is going to be rubbed, which shall afterwards turn toward the north of the earth; whilst it will be an advantage for the other end of the needle to be rubbed on the southern pole of the terrella turned toward the earth, and this being so excited will incline toward the south. in southern regions beyond the æquator the plan is just the contrary. the reason of this dissimilarity is demonstrated, book ii., chap, xxxiv., in which it is shown (by a manifest combination of a terrella and the earth) why the poles of a loadstone, for different reasons, are one stronger than the other. if a needle be touched between the mutually accordant * poles of two loadstones, equal in power, shape, and mass, no strength { } [illustration] is acquired by the needle. a and b are two loadstones attracting one another, according to nature, at their dissimilar ends; c, the * point of a needle touched by both at once, is not excited (even if those loadstones be connected according to nature), if they are equal; but if they are not equal, virtue is acquired from the stronger. when a needle is being excited by a loadstone, begin in the middle, and draw the needle toward its end; at the end let the application be continued with a very gentle rubbing around the end for some time; that is to say, for one or two minutes; do not repeat the motion from the middle to the end (as is frequently done) for in this way the verticity is injured. some delay is desirable, for although the power is imparted instantly, and the iron excited, yet from the vicinity of the loadstone and a suitable delay, a more steady verticity arises, and one that is more firmly durable in the iron. although an armed stone raises a greater weight of iron than an unarmed one, yet a needle is not more strongly excited by an armed stone than by an unarmed one. let there be two iron wires of the same length, wrought from the same wire; let one be excited by an armed end, the other by an unarmed end; it is manifest that the same needles have a beginning of motion or a sensible inclination at equal distances from the same armed and unarmed loadstone; this is ascertained by measuring with a longish reed. but objects which are more powerfully excited move more quickly; those which are less powerfully excited, more feebly, and not unless brought rather close; the experiment is made on water with equal corks. * * * * * { } [illustration] book fourth. _chap. i._ on variation. direction has hitherto been spoken of as if in nature there were no variation; for in the preceding natural history we wished to omit and neglect this, inasmuch as in a terrestrial globe, perfect and in every sense complete, there would be none. since, however, in fact, the earth's magnetick direction, owing to some fault and slip, deviates from its right course and from the meridian, we must extract and demonstrate the obscure and hidden cause of that variance which has troubled and sore racked in vain the minds of many. those who before us have written on the magnetick movements have made no distinction between direction and variation, but consider the motion of magnetick iron to be uniform and simple. now true direction is the motion of the magnetick body to the true meridian and its continuance therein with its appropriate ends towards the poles. but it very often happens at sea and on land that the magnetick iron does not point to the true pole, and that not only a versorium and magnetick pieces of iron, and the needle of a compass, or a mariners' compass, but also a terrella in its boat, as well as * iron ore, iron stones, and magnetick earths, properly prepared, are drawn aside and deviate towards some point of the horizon very near to the meridian. for they with their poles frequently face termini away from the meridian. this variation { } (observed by means of instruments or a nautical variation compass) is therefore the arc of the horizon between the common point of intersecion of it with the true meridian, and the terminus of the deflecion on the horizon or projection of the deviating needle. that arc varies and differs with change of locality. to the terminus of the variation is commonly assigned a great circle, called the circle of variation, and also a magnetick meridian passing through the zenith and the point of variation on the horizon. in the northern regions of the earth this variation is either from the north toward the east or from the north toward the west: similarly in the southern regions it is from the south toward the east or toward the west. wherefore one should observe in the northern regions of the earth * that end of the versorium or compass which turns toward the north; but in the southern regions the other end looking to the south--which seamen and sciolists for the most part do not understand, for in both regions they observe only the boreal lily of the compass (that which faces north). we have before said that all the motions of the magnet and iron, all its turning, its inclination, and its settlement, proceed from bodies themselves magnetical and from their common mother the earth, which is the source, the propagatrix, and the origin of all these qualities and properties. accordingly the earth is the cause of this variation and inclination toward a different point of the horizon: but how and by what powers must be more fully investigated. and here we must at the outset reject that common opinion of recent writers concerning magnetick mountains, or any magnetick rock, or any phantasmal pole distant from the pole of the earth, by which the motion of the compass or versorium is controlled. this opinion, previously invented by others, fracastorio himself adopted and developed; but it is entirely at variance with experience. for in that case in different places at sea and on land the point of variation would change toward the east or west in proportion and geometrical symmetry, and the versorium would always respect the magnetick pole: but experience teaches that there is no such definite pole or fixed terminus on the earth to account for the variation. for the arcs of * variation are changed variously and erratically, not only on different meridians but on the same meridian; and when, according to this opinion of the moderns, the deviation should be more and more toward the east, then suddenly, with a small change of locality, the deviation is from the north toward the west as in the northern regions near nova zembla. moreover, in the southern regions, and at sea at a great distance from the æquator towards the antarctick pole, there are frequent and great variations, and not only in the northern regions, from the magnetick mountains. but the cogitations of others are still more vain and trifling, such as that of cortes about a moving influence beyond all the heavens; that of { } marsilius ficinus about a star in the bear; that of peter peregrinus about the pole of the world; that of cardan, who derives it from the rising of a star in the tail of the bear[ ]; of bessardus, the frenchman, from the pole of the zodiack; that of livio sanuto from some magnetick meridian; that of franciscus maurolycus from a magnetical island; that of scaliger from the heavens and mountains; that of robert norman, the englishman, from a point respective. leaving therefore these opinions, which are at variance with common experience or by no means proved, let us seek the true cause of the variation. the great magnet or terrestrial globe directs iron (as i have said) toward the north and south; and excited iron quickly settles itself toward those termini. since, however, the globe of the earth is defective and uneven on its surface and marred by its diverse composition, and since it has parts very high and convex (to the height of some miles), and those uniform neither in composition nor body, but opposite and dissimilar: it comes to pass that the whole of that force of the earth diverts magnetical bodies in its periphery toward the stronger and more prominent connected magnetick parts. hence on the outermost surface of the earth magnetical bodies are slightly perverted from the true meridian. moreover, since the surface of the globe is divided into high lands and deep seas, into great continental lands, into ocean and vastest seas, and since the force of all magnetical motions is derived from the constant and magnetick terrestrial nature which is more prevalent on the greater continent and not in the aquæous or fluid or unstable part; [ ]it follows that in certain parts there would be a magnetick inclination from the true pole east or west away from any meridian (whether passing through seas or islands) toward a great land or continent rising higher, that is, obviously toward a stronger and more elevated magnetick part of the terrestrial globe. for since the diameter of the earth is more than , german miles, those large lands can rise from the centre of the earth more than four miles above the depth of the ocean bottom, and yet the earth will retain the form of a globe although somewhat uneven at the top. wherefore a magnetical body is turned aside, so far as the true verticity, when disturbed, admits, and departs from its right (the whole earth moving it) toward a vast prominent mass of land as though toward what is stronger. but the variation does really take place, not so much because of the more prominent and imperfect terrestrial parts and continent lands as because of the inæquality of the magnetick globe, and because of the real earth, which stands out more under the continent lands than under the depths of the seas. we must see, therefore, how the _apodixis_ of this theory can be sustained by more definite observations. since throughout all the course from the coast of guinea to cape verde, the canary isles, and the border of the kingdom of morocco, and { } thence along the coasts of spain, france, england, belgium, germany, denmark, and norway, there lie on the right hand and toward the east a continent and extensive connected regions, and on the left extensive seas and a vast ocean lie open far and wide, it is consonant with the theory (as has been carefully observed by many) that magnetical bodies should turn slightly to the east from the true pole toward the stronger and more remarkable elevations of the earth. but it is far otherwise on the eastern shores of northern america; for from florida by virginia and norumbega to cape race and away to the north the versorium is turned toward the west. but in the middle spaces, so to speak, as in the more westerly azores, it looks toward the true pole. that any magnetick body turns itself similarly to the same regions of the earth is not, however, because of that meridian or because of the concordancy of the meridian with any magnetick pole, as the crowd of philosophizers reckon, for it is not so throughout the whole of that meridian. for on the same meridian * near brazil something very different occurs, as we will show further on. the variation (cæteris paribus) is always less near the æquator, greater in higher latitudes, with the limitation that it be not very near the pole itself. hence the variation is greater on the coast of * norway and belgium than on the coast of morocco or guinea: greater also near cape race than in the harbours of norumbega or of virginia. on the coast of guinea magnetick implements deviate by a third part of one rumbe to the east: in cape verde islands by a half: on the coast of morocco by two thirds: in england at the mouth of the thames by a whole rumbe: and at london by nearly eleven degrees and one third. for indeed the moving magnetick virtue is stronger in a higher latitude; and the larger regions extending toward the poles dominate the more, as is easily apparent anywhere on a terrella. for as in the case of true direction magnetick bodies tend toward the pole (namely, toward the stronger end, the whole earth causing the motion), so also do they incline a little toward the stronger and higher parts by the action of the whole along with the conjoint action of iron bodies. * * * * * { } chap. ii. that the variation is caused by the inæquality of the _projecting parts of the earth_. demonstration of this may manifestly be made [illustration] * by means of a terrella in the following way: let there be a round loadstone somewhat imperfect in some part, and impaired by decay (such an one we had with a certain part corroded to resemble the atlantick or great ocean): place upon it some fine iron wire of the length of two barleycorns, as in the following figure. a b, a terrella in certain parts somewhat imperfect and of unæqual virtue on the circumference. the versoria e, f, do not vary, but look directly to the pole a; for they are placed in the middle of the firm and sound part of the terrella and somewhat distant from the imperfect part: that part of the surface which is distinguished by dots and transverse lines is the weaker. the versorium o also does not vary (because it is placed in the middle of the imperfect part), but is directed toward the pole, { } just as near the western azores on the earth. the versoria h and l do vary, for they incline toward the sounder parts very near them. as this is manifest in a terrella whose surface is sensibly rather imperfect, so also is it in others whole and perfect, when often one part of the stone has stronger external parts, which nevertheless do not disclose themselves manifestly to the senses. in such a terrella the demonstration of the variation and the discovery of the stronger parts is on this wise. [illustration] * let a be the pole, b the place of the variation, c the stronger regions; then the horizontal versorium at b varies from the pole a toward c: so that both the variation is shown and the stronger places of the loadstone recognized. the stronger surface is also found by a fine iron wire of the length of two barleycorns: for since at the pole of the terrella it rears up perpendicularly, but in other places inclines toward the æquator, if in one and the same parallel circle it should be more erect in one place than in another; where the wire is raised more upright, there the part and surface of the terrella is stronger. also when the iron wire placed over the pole inclines more to one part than to another. [illustration] * { } let the experiment be made by means of a fine iron wire of three digits length placed over the pole a, so that its middle lies over the pole. then one end is turned away from b toward c, and is not willing to lie quietly toward b; but on a terrella which is perfect[ ] all round and even it rests on the pole directed toward any point of the æquator you please. otherwise, let there be two * [illustration] meridians meeting in the poles a b, let iron wires be reared just at the ends d and c of the equal arcs d a and c a; then the wire at d (the stronger region) will be more raised up than that at c, the weaker. and thus the sounder and stronger part of the loadstone is recognized, which otherwise would not be perceived by the touch. in a terrella which is perfect, and even, and similar in all its parts, there is, at equal distances from the pole, no variation[ ]. variation is shown by means of a terrella, a considerable part of which, forming a surface a little higher than the rest, does, although it be not decayed and broken, allure the versorium from the true * direction (the whole terrella co-operating). a terrella uneven in surface. [illustration] { } it is shown by a small spike placed over a terrella or by a small versorium; for they are turned by the terrella toward the mass that stands out and toward the large eminences. in the same way on the earth the verticity is perturbed by great continents, which are mostly elevated above the depths of the seas and make the versorium deviate sometimes from the right tracks (that is, from the true meridians). on a terrella it is thus demonstrated: the end of the versorium a is not directed straight to the pole p, if there be a large protuberance b on the terrella; so also the cusp c deviates from the pole because of the eminence f. in the middle between the two eminences the versorium g collimates to the true pole because, being at equal distances from the two eminences b and f, it turns aside to neither, but observes the true meridian, especially when the protuberances are of equal vigour. but the versorium n on the other side varies from the pole m toward the eminences h, and is not held back, stopped, or restrained by the small eminence o on the terrella (as it were, some island of land in the ocean). l, however, being unimpeded, is directed to the pole m. the variation is demonstrated in another way on a terrella, just as on the earth. let a be the pole of the earth, b the equator, c the parallel circle of latitude of degrees, d a great eminence spread out toward the pole, e another eminence spread out from the pole toward the æquator. it is manifest that in the middle of d the versorium f { } does not vary; while g is very greatly deflected: but h very little, because it is further removed from d. similarly also the versorium i placed directly toward e does not deviate from the pole: but l and m turn themselves away from the pole a toward the eminence e. [illustration] * * * * * chap. iii. the variation in any one place _is constant_. [illustration] vnless there should be a great dissolution of a continent and a subsidence of the land such as there was of the region atlantis of which plato and the ancients tell, the variation will continue perpetually immutable; the arc of the variation remains the same in the same place or region, whether it be at sea or on land, as in times past a magnetick body has declined toward the east or the west. the constancy of the variation and the pointing of the versorium to a definite point on the horizon in individual regions is demonstrated by a small versorium placed over a terrella the surface of which is uneven: for it always deviates from the meridian by an equal arc. it is also shown by the inclination of a versorium toward a second magnet; although in reality it is by the turning power of the whole, whether in the earth or in a terrella. place upon a plane a versorium whose cusp is directed toward the north a: place beside it a loadstone, b, at such a distance that the versorium may turn aside toward b to the point c, and not beyond. then move the needle of the versorium as often as you will (the box and the loadstone not being moved), and it will certainly always return to the point c. in the same manner, if you { } placed the stone so that it may be truly directed toward e, the cusp always reverts to e, and not to any other point of the compass. accordingly, from the position of the land and from the distinctive nature of the highest parts of the earth (certain terrene and more magnetick eminences of the regions prevailing), the variation indeed becomes definite in one and the same place, but diverse and unæqual from a change of place, since the true and polar direction originating in the whole terrestrial globe is diverted somewhat toward certain stronger eminences on the broken surface. * * * * * chap. iiii. the arc of variation is not changed equally _in proportion to the distance of places_. in the open sea, when a vessel is borne by a favourable wind along the same parallel, if the variation be changed by one degree in the course of one hundred miles, the next hundred miles do not therefore lessen it by another degree; for the magnetick [needle] varies erratically as respects position, form, and vigour of the land, and also because of the distance. as, for example, when a course from the scilly isles to newfoundland has proceeded so far that the compass is directed to the true pole, then, as the vessel proceeds, in the first part of the course the variation increases toward the north-west[ ], but rather indistinctly and with small difference: thence, after an equal distance, the arc is increased in a greater proportion until the vessel is not far from the continent: for then it varies most of all. but before it touches actual land or enters port, then at a certain distance the arc is again slightly diminished. but if the vessel in its course should decline greatly from that parallel either toward the south or the north, the magnetick [needle] will vary more or less, according to the position of the land and the latitude * of the region. for (cæteris paribus) the greater the latitude the greater the variation. * * * * * { } chap. v. an island in ocean does not change the variation[ ], as _neither do mines of loadstone_. islands, although they be more magnetick than the sea, yet do not change the magnetick directions or variations. for since direction is a motion derived from the power of the whole earth, not from the attraction of any hill but from the disposing and turning power of the whole; so variation (which is a perturbation of the direction) is an aberration of the real turning power arising from the great inequalities of the earth, in consequence of which it, of itself, slightly diverts movable magneticks toward those which are the largest and the more powerful. the cause now shown may suffice to explain that which some so wonder at about the island of elba (and although this is productive of loadstone, yet the versorium (or mariners' compass) makes no special inclination toward it whenever vessels approach it in the tyrrhenian sea); and the following causes are also to be considered, viz.: that the virtue of smaller magnetick bodies extends scarcely or not at all of itself beyond their own mines: for variation does not occur because of attraction, as they would have it who have imagined magnetick poles. besides, magnetick mines are only agnate to the true earth, not innate: hence the whole globe does not regard them, and magneticks are not borne to them, as is demonstrated by the diagram of eminences. * * * * * chap. vi. that variation and direction arise from the disponent _power of the earth, and from the natural magnetick tendency_ to rotation, not from attraction, or from coition, _or from other occult cause_. owing to the loadstone being supposed (amongst the crowd of philosophizers) to seize and drag, as it were, magnetick bodies; and since, in truth, sciolists have remarked no other forces than those so oft besung of attractive ones, they therefore deem every motion toward the north and south to be caused by some alluring and inviting quality. but the englishman, { } robert norman, first strove to show that it is not caused by attraction: wherefore, as if tending toward hidden principles, he imagined a _point respective_[ ], toward which the iron touched by a loadstone would ever turn, not a _point attractive_; but in this he erred greatly, although he effaced the former error about attraction. he, however, demonstrates his opinion in this way: [illustration] let there be a round vessel filled with water: in the middle of the surface of the water place a slender iron wire on a perfectly round cork, so that it may just float in æquilibrium on the water; let the wire be previously touched by a magnet, so that it may more readily show the point of variation, the point d as it were: and let it remain on the surface for some time. it is demonstrable that the wire together with the cork is not moved to the side d of the vessel: which it would do if an attraction came to the iron wire by d: and the cork would be moved out of its place. this assertion of the englishman, robert norman, is plausible and appears to do away with attraction because the iron remains on the water not moving about, as well in a direction toward the pole itself (if the direction be true) as in a variation or altered direction; and it is moved about its own centre without any transference to the edge of the vessel. but direction does not arise from attraction, but from the disposing and turning power which exists in the whole earth, not in the pole or in some other attracting part of the stone, or in any mass rising above the periphery of the true circle so that a [illustration] variation should occur because of the attraction of that mass. moreover, it is the directing power of the loadstone and iron and its natural power of turning around the centre which cause the motion of direction, and of conformation, in which is included also the motion of the dip. and the terrestrial pole does not attract as if the terrene force were implanted only in the pole, for the magnetick force exists in the whole, although it predominates and excels at the pole. wherefore that the cork should rest quiescent in the middle and that the iron excited by a loadstone should not be moved toward the side of the vessel are agreeable to and in conformity { } with the magnetick nature, as is demonstrated by a terrella: for an iron spike placed on the stone at c clings on at c, and is not pulled * further away by the pole a, or by the parts near the pole: hence it persists at d, and takes a direction toward the pole a; nevertheless it clings on at d and dips also at d in virtue of that turning power by which it conforms itself to the terrella: of which we will say more in the part _on declination_. * * * * * chap. vii. why the variation from that lateral cause is not _greater than has hitherto been observed, having been_ rarely seen to reach two points of the mariners' _compass, except near the pole_. the earth, by reason of lateral eminences of the stronger globe, diverts iron and loadstone by some degrees from the true pole, or true meridian. as, for example, with us english at london it varies eleven degrees and / : in some other places the variation is a little greater, but in no other region is the end of the iron ever moved aside very much more from the meridian. for as the iron is always directed by the true verticity of the earth, so the polar nature of the continent land (just as of the whole terrene globe) acts toward the poles: and even if that mass divert magnetick bodies from the meridian, yet the verticity of those lands (as also of the whole earth) controls and disposes them so that they do not turn toward the east by any greater arc. but it is not easy to determine by any general method how great the arc of variation is in all places, and how many degrees and minutes it subtends on the horizon, since it becomes greater or less { } from diverse causes. for both the strength of true verticity of the place and of the elevated regions, as well as their distances from the given place and from the poles of the world, must be considered and compared; which indeed cannot be done exactly: nevertheless by our method the variation becomes so known that no grave error will perturb the course at sea. if the positions of the lands were uniform and straight along meridians, and not defective and rugged, the variations near lands would be simple; such as appear in the following figure. [illustration] this is demonstrated by a long loadstone the poles of which are in the ends a b; let c d be the middle line and the æquinoctial, and let g h and e f (the lines) be for meridians on which versoria are disposed, the variations of which are greater at a greater distance from the æquator. but the inequalities of the maritime parts of the habitable earth, the enormous promontories, the very wide gulfs, the mountainous and more elevated regions, render the variations more unequal, or sudden, or more obscure; and, moreover, less certain and more inconstant in the higher latitude. * * * * * { } chap. viii. on the construction of the common mariners' compass[ ], and on the diversity of the compasses _of different nations_. in a round[ ] hollow wooden bowl, all the upper part of which is closed with glass, a versorium is placed upon a rather long pin which is fixed in the middle. the covering prevents the wind, and the motion of air from any external cause. through the glass everything within can be discerned. the versorium is circular, consisting of some light material (as card), to the under part of which the magnetick pieces of iron are attached. on the upper part spaces (which are commonly called _points_) are assigned to the same number of mathematical intervals in the horizon or winds which are distinguished by certain marks and by a lily indicating the north. the bowl is suspended in the plane of the horizon in æquilibrium in a brass ring which also is itself suspended transversely in another ring within a box sufficiently wide with a leaden weight attached; hence it conforms to the plane of the horizon even though the ship be tossed to and fro by the waves. the iron works are either a pair with their ends united, or else a single one of a nearly oval shape with projecting ends, which does its work more certainly and more quickly. this is to be fitted to the cardboard circle so that the centre of the circle may be in the middle of the magnetick iron. but inasmuch as variation arises horizontally from the point of the meridian which cuts the horizon at right angles, therefore on account of the variation the makers in different regions and cities mark out the mariners' compass in different ways, and also attach in different ways the magnetick needles to the cardboard circle on which are placed the divisions or points. hence there are commonly in europe different constructions and forms. first that of the states on the mediterranean sea, sicily, genoa, and the republick of venice. in all these the needles are attached under the rose or lily on the cardboard versorium, so that (where there is no variation) they are directed to the true north and south points. wherefore the north part marked with the lily always shows exactly the point of variation when the apex itself of the lily on the movable circle, together with the ends of the magnetick wires attached below, rests at the point of variation. yet another is that of dantzig, and throughout the baltic sea, and the belgian provinces; { } in which the iron works fixed below the circle diverge from the lily ¼ of a rumbe to the east. for navigation to russia the divergency is / . but the compasses which are made at seville, lisbon, rochelle, bordeaux, rouen, and throughout all england have an interval of ½ a rumbe. from those differences most serious errors have arisen in navigation, and in the marine science. for as soon as the bearings of maritime places (such as promontories, havens, islands) have been first found by the aid of the mariners' compass, and the times of sea-tide or high water determined from the position of the moon over this or that point (as they say) of the compass, it must be further inquired in what region or according to the custom of what region that compass was made by which the bearings of those places and the times of the sea-tides were first observed and discovered. for one who should use the british compass and should follow the directions of the marine charts of the mediterranean sea would necessarily wander very much out of the straight course. so also he that should use the italian compass in the british, german, or baltic sea, together with marine charts that are made use of in those parts, will often stray from the right way. these different constructions have been made on account of the dissimilar variations, so that they might avoid somewhat serious errors in those parts of the world. but pedro nuñez seeks the meridian by the mariners' compass, or versorium (which the spanish call the needle), without taking account of the variation: and he adduces many geometrical demonstrations which (because of his slight use and experience in matters magnetical) rest on utterly vicious foundations. in the same manner pedro de medina, since he did not admit variation, has disfigured his _arte de navegar_ with many errors. * * * * * chap. ix. whether the terrestrial longitude can be found from _the variation_. grateful would be this work to seamen, and would bring the greatest advance to geography. but b. porta in chap. of book is mocked by a vain hope and fruitless opinion. for when he supposes that the magnetick needle would follow order and proportion in moving along meridians, so that "the neerer it is to the east, the more it will decline from the meridian line, toward the east; and the neerer it comes to the west, the { } point of the needle will decline the more to the west" (which is totally untrue), he thinks that he has discovered a true index of longitude. but he is mistaken. nevertheless, admitting and assuming these things (as though they were perfectly true), he makes a large compass indicating degrees and minutes, by which these proportional changes of the versorium might be observed. but those very principles are false, and ill conceived, and very ill considered; for the versorium does not turn more to the east because a journey is made toward the east: and although the variation in the more westerly parts of europe and the adjoining ocean is to the east and beyond the azores is changed a little to the west, yet the variation is, in various ways, always uncertain, both on account of longitude and of latitude, and because of the approach toward extensive tracts of land, and also because of the form of the dominant terrestrial eminences; nor does it, as we have before demonstrated, follow the rule of any particular meridian. it is with the same vanity also that livio sanuto so greatly torments himself and his readers. as for the fact that the crowd of philosophizers and sailors suppose that the meridian passing through the azores marks the limits of variation, so that on the other and opposite side of that meridian a magnetick body necessarily respects the poles exactly, which is also the opinion of joannes baptista benedictus and of many other writers on navigation, it is by no means true. stevinus (on the authority of hugo grotius) in his _havenfinding art_ distinguishes the variation according to the meridians: "it may be seene in the table of variations, that in _coruo_ the magneticall needle pointeth due north: but after that, the more a man shal goe towards the east, so much the more also shall he see the needle varie towards the east [[greek: anatolizein]], till he come one mile to the eastward from _plimouth_, where the variation comming to the greatest is degr. min. from hence the northeasting [anatolismus] beginneth to decrease, til you come to _helmshude_ (which place is westward from the north cape of finmark) where againe the needle pointeth due north. now the longitude from _coruo_ to _helmshude_ is degr. which things being well weighed, it appeareth that the greatest variation [chalyboclysis] degr. minutes at _plimmouth_ (the longitude whereof is degr.) is in the midst betweene the places where the needle pointeth due north." but although this is in some part true in these places, yet it is by no means true that along the whole of the meridian of the island of corvo the versorium looks truly to the north; nor on the meridian of plymouth is the variation in other places deg. min.--nor again in other parts of the meridian of helmshuda does it point to the true pole. for on the meridian passing through plymouth in latitude degrees the north-easterly variation is greater: in latitude deg. much less; in latitude deg. very small indeed. on the meridian of corvo, although there is no variation near the { } island, yet in latitude degrees the variation is about ½ a rumbe to the north-west; in latitude deg. the versorium inclines ¼ of a rumbe toward the east. consequently the limits of variation are not conveniently determined by means of great circles and meridians, and much less are the ratios of the increment or decrement toward any part of the heavens properly investigated by them. wherefore the rules of the abatement or augmentation of northeasting or northwesting, or of increasing or decreasing the magnetick deviation, can by no means be discovered by such an artifice. the rules which follow later for variation in southern parts of the earth investigated by the same method are altogether vain and absurd. they were put forth by certain portuguese mariners, but they do not agree with the observations, and the observations themselves are admitted to be bad. but the method of haven-finding in long and distant voyages by carefully observed variation (such as was invented by stevinus, and mentioned by grotius) is of great moment, if only proper instruments are in readiness, by which the magnetick deviation can be ascertained with certainty at sea. * * * * * chap. x. why in various places near the pole the variations are much more ample than in a _lower latitude_. variations are often slight, and generally null, when the versorium is at or near the earth's æquator. in a higher latitude of , or deg. there are not seldom very wide variations. the cause of this is to be sought partly from the nature of the earth and partly from the disposition of the versorium. the earth turns magnetick bodies and at the æquator directs them strongly toward the pole: [ ]at the poles there is no direction, but only a strong coition through the congruent poles. direction is therefore weaker near the poles, because by reason of its own natural tendency to turn, the versorium dips very much, and is not strongly directed. but since the force of those elevated lands is more vigorous, for the virtue flows from the whole globe, and since also the causes of variation are nearer, therefore the versorium deflects the more from its true direction toward those eminences. it must also be known that the direction of the versorium on its pin along the plane of the horizon is much stronger at the æquator than anywhere else by reason of the disposition of the { } versorium; and this direction falls off with an increase of latitude. for on the æquator the versorium is, following its natural property, directed along the plane of the horizon; but in other places it is, contrary to its natural property, compelled into æquilibrium, and remains there, compelled by some external force: because it would, according to its natural property, dip below the horizon in proportion to the latitude, as we shall demonstrate in the book _on declination_. hence the direction falls off and at the pole is itself nothing: and for that reason a feebler direction is easily vanquished by the stronger causes of variation, and near the pole the versorium deflects the more from the meridian. it is demonstrated by means of a terrella: if an iron wire of two digits length be placed on its æquator, it will be strongly and rapidly directed toward the poles along the meridian, but more weakly so in the mid-intervals; while near the poles one may discern a precipitate variation. * * * * * chap. xi. cardan's error when he seeks the distance of the _centre of the earth from the centre of the cosmos by the_ motion of the stone of hercules; in his _book , on proportions_. one may very easily fall into mistakes and errors when one is searching into the hidden causes of things, in the absence of real experiments, and this is easily apparent from the crass error of cardan; who deems himself to have discovered the distances of the centres of the cosmos and of the earth through a variation of the magnetick iron of degrees. for he reckoned that everywhere on the earth the point of variation on the horizon is always distant nine degrees from the true north, toward the east: and from thence he forms, by a most foolish error, his demonstrative ratio of the separate centres. * * * * * { } chap. xii. on the finding of the amount of variation: how great _is the arc of the horizon from its arctick or antarctick_ intersection of the meridian, to the point _respective of the magnetick needle_. virtually the true meridian is the chief foundation of the whole matter: when that is accurately known, it will be easy by a mariners' compass (if its construction and the mode of attachment of the magnetick iron works are known) or by some other larger horizontal versorium to exhibit the arc of variation on the horizon. by means of a sufficiently large nautical variation compass (two equal altitudes of the sun being observed before and after midday), the variation becomes known from the shadow; the altitude of the sun is observed either by a staff or by a rather large quadrant. on land the variation is found in another way which is easier, and because of the larger size of the instrument, more accurate. let a thick squared board be made of some suitable wood, the surface of which is two feet in length and sixteen inches in width: describe upon it some semicircles as in the following figure, only more in number. in the centre let a brass style be reared perpendicularly: let there be also a movable pointer reaching from the centre to the outmost semicircle, and a magnetick versorium in a cavity covered over with glass: then let the board be exactly adjusted to the level of the horizon by the plane instrument with its perpendicular; and turn the lily of the instrument toward the north, so that the versorium may rest truly over the middle line of the cavity, which looks toward the point of variation on on the horizon. then at some convenient hour in the morning (eight or nine for instance) observe the apex of the shadow thrown by the style when it reaches the nearest semicircle and mark the place of the apex of this shadow with chalk or ink: then bring round the movable index to that mark, and observe the degree on the horizon numbered from the lily, which the index shows. in the afternoon see when the end of the shadow shall again reach the periphery of the same semicircle, and, bringing the index to the apex of the shadow, seek for the degree on the other side of the lily. from the difference of the degrees becomes known [illustration] { } the variation; the less being taken from the greater, half the remainder is the arc of variation. the variation is sought by many other instruments and methods in conjunction with a convenient mariners' compass; also by a globe, by numbers, and by the ratios of triangles and sines, when the latitude is known and one observation is made of the sun's altitude: but those ways and methods are of less use, for it is superfluous to try to find in winding and roundabout ways what can be more readily and as accurately found in a shorter one. for the whole art is in the proper use of the instruments by which the sun's place is expeditiously and quickly taken (since it does not remain stationary, but moves on): for either the hand trembles or the sight is dim, or the instrument makes an error. besides, to observe the altitude on both sides of the meridian is just as expeditious as to observe on one side only and at the same time to find the elevation of the pole. and he who can take one altitude by the instrument can also take another; but if the one altitude be uncertain, then all the labour with the globe, numbers, sines and triangles is lost; nevertheless those exercises of ingenious mathematicians are to be commended. it is easy for anyone, if he stand on land, to learn the variation by accurate observations and suitable instruments, especially in a nearly upright sphere; but on the sea, on account of the motion and the restlessness of the waters, exact experiments in degrees and minutes cannot be made: and with the usual instruments scarcely within the third or even the halt of a rumbe, especially in a higher latitude; hence so many false and bad records of the observations of navigators. we have, however, taken care for the finding of the deviation by a sufficiently convenient and ready instrument, by means of the rising of certain stars, by the rising or setting of the sun, and in northern regions by the pole star: for the variation is learned with greater certainty even by the skilful with an instrument which is at once simple and less sensitive to the waves of the sea. its construction is as follows. [ ]let an instrument be made of the form of a true and meridional mariners' compass of at least one foot in diameter (with a versorium which is either nude or provided with a cardboard circle): let the limb be divided into four quadrants, and each quadrant into degrees. the movable compass-box (as is usual in the nautical instrument) is to be balanced below by a heavy weight of sixteen pounds. on the margin of the suspended compass-box, where opposite quadrants begin, let a half-ring rising in an angular frame in the middle be raised (with the feet of the half-ring fixed on either side in holes in the margin) so that the top of the frame may be perpendicular to the plane of the compass; on its top let a rule sixteen digits in length be fastened at its middle on a joint like a balance beam, so that it may move, as it were, about a central axis. at the ends of the rule there are small plates with holes, { } [illustration] { } through which we can observe the sun or stars. the variation is best observed and expeditiously by this instrument at the equinoxes by the rising or setting sun. but even when the sun is in other parts of the zodiack, the deviation becomes known when we have the altitude of the pole: that being known, one can learn the amplitude on the horizon and the distance from the true east both of the sun and of the following fixed stars by means of a globe, or tables, or an instrument. then the variation readily becomes known by counting from the true east the degrees and minutes of the amplitude at rising. observe the preceding star of the three in the belt of orion as soon as it appears on the horizon; direct the instrument toward it and observe the versorium, for since the star has its rising in the true east about one degree toward the south, it can be seen how much the versorium is distant from the meridian, account being taken of that one degree. you will also be able to observe the arctick pole star when it is on the meridian, or at its greatest distance from the meridian of about three degrees (the pole star is distant deg. min. from the pole, according to the observations of tycho brahe), and by the instrument you will learn the variation (if the star be not on the meridian) by adding or subtracting, _secundum artem_, the proper reduction [_prostaphæresis_][ ] of the star's distance from the meridian. you will find when the pole star is on the meridian by knowing the sun's place and the hour of the night: for this a practised observer will easily perceive without great error by the visible inclination of the constellation: for we do not take notice of a few minutes, as do some who, when they toil to track the minutes of degrees at sea, are in error by a nearly whole rumbe. a practised observer will, in the rising of sun or stars, allow something for refraction, so that he may be able to use a more exact calculation. bright and conspicuous stars[ ] which are _not far distant from the equator which it will be useful to observe at their rising and setting: the amplitude at the horizon on rising being known from the altitude of the pole and from the declination of the stars, by means of a globe, or tables, or an instrument whence the variation is perceived by technical calculation._ { } _right ascension_ _declination_ |oculus tauri | ° ' | ° ' n | |sinister humerus orionis | ° ' | ° ' n | |dexter humerus orionis | ° ' | ° ' n | |præcedens in cingulo orionis | ° ' | ° ' s | |canis major | ° ' | ° ' s | |canis minor | ° ' | ° ' n | |lucida hydræ | ° ' | ° ' s | |caput geminorum australe | ° ' | ° ' n | |caput boreale | ° ' | ° ' n | |cor leonis | ° ' | ° ' n | |cauda leonis | ° ' | ° ' n | |spica virginis | ° ' | ° ' s | |arcturus | ° ' | ° ' n | |cor aquilæ | ° ' | ° ' n | _an instrument for finding the amplitude at rising on the horizon._ describe the circumference of a circle and let it be divided into quadrants by two diameters intersecting each other at right angles at its centre. one of these will represent the æquinoctial circle, the other the axis of the world. let each of these quadrants be divided (in the accustomed way) into degrees; on every fifth or tenth of which at each end of each diameter and on each side let marks (showing the numbers) be inscribed on the two limbs or margins made for that purpose outside the circumference. then from each degree straight lines are drawn parallel to the æquator. you will then prepare a rule or alhidade equal to the diameter of that circle and divided throughout into the same parts into which the diameter of the circle representing the axis of the world is divided. let there be left a small appendage attached to the middle of the rule, by which the middle of the fiducial line itself of the rule may be connected with the centre of the circle: but to every fifth or tenth part of that rule let numbers be attached proceeding from the centre toward each side. this circle represents the plane of the meridian; its centre the actual point of east or west, _i.e._, the common intersection of the horizon and æquator; all those lines æquidistant from the æquator denote the parallels of the sun and stars; the fiducial line of the rule or alhidade represents the horizon; and its parts signify the degrees of the horizon, beginning from the point of setting or of rising. { } [illustration] therefore if the fiducial line of the rule be applied to the given latitude of the place reckoned from either end of that diameter which represents the axis of the world; and if further the given declination of the sun or of some star from the æquator (less than the complement of the latitude of the place) be found on the limb of the instrument; then the intersection of the parallel drawn from that point of the declination with the horizon, or with the fiducial line of the rule or alhidade, will indicate for the given latitude of the place the amplitude at rising of the given star or the sun. * * * * * { } chap. xiii. the observations of variation by seamen vary, for the _most part, and are uncertain: partly from error and inexperience_, and the imperfections of the instruments; and partly _from the sea being seldom so calm that the shadows or lights can remain quite steady on the instruments_. after the variation of the compass had first been noticed, some more diligent navigators took pains to investigate in various ways the difference of aspect of the mariners' compass. yet, to the great detriment of the nautical art, this has not been done so exactly as it ought to have been. for either being somewhat ignorant they have not understood any accurate method or they have used bad and absurd instruments, or else they merely follow some conjecture arising from an ill-formed opinion as to some prime meridian or magnetick pole; whilst others again transcribe from others, and parade these observations as their own; and they who, very unskilful themselves, first of all committed their observations to writing are, as by the prerogative of time, held in esteem by others, and their posterity does not think it safe to differ from them. hence in long navigations, especially to the east indies, the records by the portuguese of the deviating compass are seen to be unskilful: for whoever reads their writings will easily understand that they are in error in very many things, and do not rightly understand the construction of the portuguese compass (the lily of which diverges by half a rumbe from the needles toward the west), nor its use in taking the variation. hence, while they show the variation of the compass in different places, it is uncertain whether they measure the deviation by a true meridional compass or by some other whose needles are displaced from the lily. the portuguese (as is patent in their writings) make use of the portuguese compass, whose magnetick needles are fixed aside from the lily by half of one rumbe toward the east. moreover on the sea the observation of the variation is a matter of great difficulty, on account of the motion of the ship and the uncertainty of the deviation, even with the more skilful observers, if they use the best made instruments hitherto known and used. hence there arise different opinions concerning the magnetick deviation: as, for instance, near the island of st. helena the portuguese rodriguez de { } lagos measures half a rumbe. the dutch in their nautical log fix it at a whole rumbe. kendall, the expert englishman, with a true meridional compass admits only a sixth part of a rumbe. a little to the east of cape agullias diego alfonso makes no variation, and shows by an astrolabe that the compass remains in the true meridian. rodriguez shows that the compass at cape agulhas has no variation if it is of portuguese construction, in which the needles are inclined half a rumbe to the east. and there is the same confusion, negligence, and vanity in very many other instances. * * * * * chap. xiiii. on the variation under the æquinoctial line, _and near it_. in the north the magnetick needle varies because of the boreal eminences of the continent; in the south because of the austral; at the æquator, if the regions on both sides were equal, there would be no variation. but because this rarely happens some variation is often observed under the æquator; and even at some distance from the æquator of three or degrees toward the north, there may be a variation arising from the south, if those very wide and influential southern continents be somewhat near on one side. * * * * * chap. xv. the variation of the magnetick needle in the great Æthiopick and american sea, beyond _the æquator_. discourse hath already been had of the mode and reason of the variation in the great atlantick ocean: but when one has advanced beyond the æquator off the east coast of brazil the magnetick needle turns aside toward the mainland, namely, with that end of it which points to the south; so that with that end of the versorium it deviates from the true meridian toward the west; which navigators observe at the other end and suppose a variation to occur toward the east. but throughout the whole way from the first promontory on the east of brazil, by { } cape st. augustine and thence to cape frio, and further still to the mouth of the strait of magellan, the variation is always from the south toward the west with that end of the versorium which tends toward the antarctick pole. for it is always with the accordant end that it turns toward a continent. the variation, however, occurs not only on the coast itself, but at some distance from land, such as a space of fifty or sixty german miles or even more. but when at length one has progressed far from land, then the arc begins to diminish: for the magnetick needle turns aside the less toward what is too far off, and is turned aside the less from what is present and at hand, since it enjoys what is present. in the island of st. helena (the longitude of which is less than is commonly marked on charts and globes) the versorium varies by one degree or nearly two. the portuguese and others taught by them, who navigate beyond the cape of good hope to the indies, set a course toward the islands of tristan d'acunha, in order that they may enjoy more favourable winds; in the former part of their course the change of variation is not great; but after they have approached the islands the variation increases; and close to the islands it is greater than anywhere else in the whole course. for the end of the versorium tending to the south (in which lies the greatest source of the variation) is caught and allured toward the south-west by the great promontory of the southern land. but when they proceed onward toward the cape of good hope the variation diminishes the more they approach it. but on the prime meridian in the latitude of degrees, the versorium tends to the south-east: and one who navigates near the coast from manicongo to the tropick, and a little beyond, will perceive that the versorium tends from the south to the east, although not much. at the promontory of agulhas it preserves slightly the variation which it showed near the islands of d'acunha, which nevertheless is very much diminished because of the greater remoteness from the cause of variation, and consequently there the southern end of the versorium does not yet face exactly to the pole. * * * * * chap. xvi. on the variation in nova zembla. variations in parts near the pole are greater (as has been shown before) and also have sudden changes, as in former years the dutch explorers observed not badly, even if those observations were not exact--which indeed is pardonable in them; for with the usual instruments it is with difficulty { } that the truth becomes known in such a high latitude (of about degrees). now, however, from the deviation of the compass the reason for there being an open course to the east by the arctick ocean appears manifest; for since the versorium has so ample a variation toward the north-west, it is demonstrable that a continent does not extend any great distance in the whole of that course toward the east. therefore with the greater hope can the sea be attempted and explored toward the east for a passage to the moluccas by the north-east than by the north-west. * * * * * chap. xvii. variation in the pacifick ocean. passing the strait of magellan the deviation on the shore of peru is toward the south-east, _i.e._, from the south toward the east. and a similar deflection would be continued along the whole coast of peru as far as the æquator. in a higher latitude up to deg. the variation is greater than near the æquator; and the deflection toward the south-east is in nearly the same proportion as was the deviation from the south toward the west on the eastern shore of south america. from the æquator toward the north there is little or no variation until one comes to new galicia; and thence along the whole shore as far as quivira the inclination is from the north toward the east. * * * * * chap. xviii. on the variation in the mediterranean sea. sicilian and italian sailors think that in the sicilian sea and toward the east up to the meridian of the peloponnesus (as franciscus maurolycus relates) the magnetick needle "græcizes," that is, turns from the pole toward what is called the greek wind or boreas; that on the shore of the peloponnesus it looks toward the true pole; but that when they have proceeded further east, then it "mistralizes," because it tends from the pole toward the mistral or north-west wind: which agrees with our rule for the variation. for as the mediterranean sea is extended toward the west from that meridian, so on the side { } toward the east the mediterranean sea lies open as far as palestine; as toward north and east lie open the whole archipelago and the neighbouring black sea. from the peloponnesus toward the north pole the meridian passes through the largest and most elevated regions of all europe; through achaia, macedonia, hungary, transylvania, lithuania, novogardia, corelia and biarmia. * * * * * chap. xix. the variation in the interior of large _continents_. most of the great seas have great variations; in some parts, however, they have none, but the true directions are toward the pole. on continents, also, the magnetick needle often deviates from the meridian, as on the edge of the land and near the borders; but it is generally accustomed to deviate by a somewhat small arc. in the middle, however, of great regions there are no variations. hence in the middle lands of upper europe, in the interior of asia, and in the heart of africa, of peru, and in the regions of north or mexican america, the versorium rests in the meridian. * * * * * chap. xx. variation in the eastern ocean. variation in the eastern ocean throughout the whole voyage to goa and the moluccas is observed by the portuguese; but they err greatly in many things, following, as they do, the first observers who note down variations in certain places with ill-adapted instruments, and by no means accurate observations, or by some conjectures. as, for instance, in brandöe island, they make the versorium deviate by degrees to the north-west. for in no region or place in the whole world, of not greater latitude, is there so great a deviation; and, in reality, there the deviation is slight. also when they make out that at mosambique the compass deviates by one rumbe to the north-west, it is false; even though they use (as they are accustomed to do) the portuguese compass: for beyond all doubt on the shore of { } mosambique the versorium inclines ¼ rumbe or even more to the south-west. very wrongly also beyond the æquator in the course to goa they make the little compass incline by ½ rumbe to the west: whereas they should rather have said that in the first part of the course the portuguese compass inclines by rumbe: but that the true meridional compass inclines by ½ rumbe only. in order that the amount of variation in the eastern ocean may be accurately settled in most places by our rules, there is needed a more exact and truer survey of the southern land, which spreads out from the south to the æquinoctial more than is commonly described on maps and globes. * * * * * chap. xxi. how the deviation of the versorium is augmented and _diminished by reason of the distance of places_. in the middle of great and continent lands there is no variation. nor, generally, in the middle of very great seas. on the margin of those lands and seas the variation is often ample, yet not so great as at a little further distance on the sea. as, for example, near cape st. augustine the compass varies; but at miles from land toward the east it varies more; and miles off it varies still more; and yet still more at a distance of miles. but from a distance of miles the diminutions of deviation are slower, when they are navigating toward the mainland, than at a distance of miles, and at a distance of miles than at : for the deviations change and are diminished rather more swiftly the more they approach and draw near land than when at a great distance off. as, for instance, navigating toward newfoundland the change of variation is more rapid (that is, it decreases a degree in a smaller arc of the course on the parallel) when they are not far from land than when they are a hundred miles distant: but when travelling on land toward the interiors of regions the changes are slower in the first parts of the journey than when they come more into the interior. the ratio of the arcs on a parallel circle, when a versorium is moved toward continents which extend to the pole, corresponds with the degrees of variation. let a be the pole; b the eminences of the dominant lands; at c there is no variation caused by b, for it is too far away; at d the variation is very great because the versorium is allured or turned by the whole earth toward the eminent { } land b; and moreover it is not hindered, or restrained or brought back to the pole by the verticity of the earth; but, tending of its own nature to the pole, it is nevertheless deflected from it by reason of the site, or position, and convenient distance of the dominant and high lands. [illustration] now from c toward d the variation increases; the versorium, however, does not deviate so rapidly in the first spaces as near d: for more miles are traversed on the parallel circle c d, near c, in order that the versorium may deviate by one degree from the pole a, than near d. so also in order that the variation may be diminished from d toward e more miles are required near d than near e. thus the deviations become equal in unequal courses, whether the variation be increasing or decreasing; and yet the variation decreases by lesser intervals than it increases. there intervene, however, many other causes which perturb this proportion. * * * * * { } [illustration] book fifth. _chap. i._ on declination. in due course we have now come to that notable experiment, and remarkable motion of magnetick bodies dipping below the horizon by their own rotatory nature; by the knowledge of which is revealed a unity, a concordancy, and a mutual agreement between the terrestrial globe and the loadstone (or the magnetick iron), which is wonderful in itself, and is made manifest by our teaching. this motion we have made known in many striking experiments, and have established its rules; and in the following pages we shall demonstrate the causes of it, in such a way that no sound, logical mind can ever rightly set at nought or disprove our chief magnetick principles. direction, as also variation, is demonstrated in a horizontal plane, when a balanced magnetick needle comes to rest at some definite point; but declination is seen to be the motion of a needle, starting from that point of the horizon, first balanced on its own axis, then excited by a loadstone, one end or pole of it tending toward the centre of the earth. and we have found that it takes place in proportion to the latitude of each region. but that motion arises in truth, not from any motion from the horizon toward the centre of the earth, but from the turning of the whole magnetick body toward the whole of the earth, as we shall show hereafter. nor does the iron dip from the horizontal in some oblique sphere, according to the number of degrees of elevation of the pole in the given region, or by an equal arc in the quadrant, as will appear hereafter. { } instrument of the declination [illustration] { } now how much it dips at every horizon may be ascertained in the first place by a contrivance, which, however, is not so easily made as is that in dials for measuring time, in which the needle turns to the points of the horizon, or in the mariners' compass. from a plank of wood let a smooth and circular instrument be prepared, at least six digits in diameter, and affix this to the side of a square pillar, which stands upright on a wooden base. divide the periphery of this instrument into quadrants: then each quadrant into degrees. at the centre of the instrument let there be placed a brass peg, at the centre of the end of which let there be a small hollow, well polished. to this wooden instrument let a brass circle or ring be fixed, about two digits in width, with a thin plate or flat rod of the same metal, representing the horizon, fixed across it, through the middle of the circle. in the middle of the horizontal rod let there be another hollow, which shall be exactly opposite the centre of the instrument, where the former hollow was made. afterward let a needle be fashioned out of steel, as versoria are accustomed to be made. divide this at right angles by a thin iron axis (like a cross) through the very middle and centre of the wire and the cross-piece. let this dipping-needle be hung (with the ends of the cross resting in the aforesaid holes) so that it can move freely and evenly on its axis in the most perfect æquilibrium, so accurately that it turns away from no one point or degree marked on the circumference more than from another, but that it can rest quite easily at any. let it be fixed upright to the front part of the pillar, whilst at the edge of the base is a small versorium to show direction. afterward touch the iron, suspended by this ingenious method, on both ends with the opposite ends of a loadstone, according to the scientifick method, but rather carefully, lest the needle be twisted in any way; for unless you prepare everything very skilfully and cleverly, you will secure no result. then let another brass ring be prepared, a little larger, so as to contain the former one; and let a glass or a very thin plate of mica be fitted to one side of it. when this is put over the former ring, the whole space within remains inclosed, and the versorium is not interfered with by dust or winds. dispose the instrument, thus completed, perpendicularly on its base, and with the small versorium horizontal, in such a way that, while standing perpendicularly, it may be directed toward the exact magnetical point respective. then the end of the needle which looks toward the north dips below the horizon in northern regions, whilst in southern regions the end of the needle which looks toward the south tends toward the centre of the earth, in a certain proportion (to be explained afterward) to the latitude of the district in question, from the æquator on either side. the needle, however, must be rubbed on { } a powerful loadstone; otherwise it does not dip to the true point, or else it goes past it, and does not always rest in it. a larger instrument may also be used, whose diameter may be or digits; but in such an instrument more care is needed to balance the versorium truly. care must be taken that the needle be of steel; also that it be straight; likewise that both ends of the cross-piece be sharp and fixed at right angles to the needle, and that the cross-piece pass through the centre of the needle. as in other magnetical motions there is an exact agreement between the earth and the stone, and a correspondence manifestly apparent to our senses by means of our experiments; so in this declination there is a clear and evident concordance of the terrestrial globe with the loadstone. of this motion, so important and so long unknown to all men, the following is the sure and true cause. a magnet-stone is moved and turned round until one of its poles being impelled toward the north comes to rest toward a definite point of the horizon. [ ]this pole, which settles toward the north (as appears from the preceding rules and demonstrations), is the southern, not the boreal; though all before us deemed it to be the boreal, on account of its turning to that point of the horizon. a wire or versorium touched on this pole of the stone turns to the south, and is made into a boreal pole, because it was touched by the southern terminal of the stone. so if the cusp of a versorium be excited in a similar manner, it will be directed toward the southern pole of the earth, and will adjust itself also to it; but the cross (the other end) will be southern, and will turn to the north of the earth (the earth itself being the cause of its motion); for so direction is produced from the disposition of the stone or of the excited iron, and from the verticity of the earth. but declination takes place when a magnetick is turned round toward the body of the earth, with its southern end toward the north, at some latitude away from the æquator. for this is certain and constant, that exactly under the coelestial æquator, or rather over the æquator of the terrestrial globe, there is no declination of a loadstone or of iron; but in whatever way the iron has been excited or rubbed, it settles in the declination instrument precisely along the plane of the horizon, if it were properly balanced before. now this occurs thus because, when the magnetick body is at an equal distance from either pole, it dips toward neither by its own versatory nature, but remains evenly directed to the level of the horizon, as if it were resting on a pin or floating free and unhindered on water. but when the magnetick substance is at some latitude away from the æquator, or when either pole of the earth is raised (i do not say raised above the visible horizon, as the commonly imagined pole of the revolving universe in the sky, but above the horizon or its centre, or its proper diameter, æquidistant from the plane of the visible horizon, which is the true elevation of the terrestrial pole), { } [illustration] then declination is apparent, and the iron inclines toward the body of the earth in its own meridian. let a b, for example, be the visible horizon of a place; c d the horizontal through the earth, dividing it into equal parts; e f the axis of the earth; g the position of the place. it is manifest that the boreal pole e is elevated above the point c by as much as g is distant from the æquator. wherefore, since at e the magnetick needle stands perpendicularly in its proper turning (as we have often shown before), so now at g there is a certain tendency to turn in proportion to the latitude (the magnetick dipping below the plane of the horizon), and the magnetick body intersects the horizon at unequal angles, and exhibits a declination below the horizon. for the same reason, if the declinatory needle be placed at g, its southern end, the one namely which is directed toward the north, dips below the plane of the visible horizon a b. and so there is the greatest difference between a right sphere[ ] and a polar or parallel sphere, in which the pole is at the very zenith. for in a right sphere the needle is parallel to the plane of the horizon; but when the coelestial pole is vertically overhead, or when the pole of the earth is itself the place of the region, then the needle is perpendicular to the horizon. this is shown by a round stone. let a small dipping-needle, of two digits length (rubbed with a magnet), be hung in the air like a balance, and let the stone be carefully placed under it; and first let the terrella be at right angles, as in a right sphere, and as in the first figure; for so the magnetick needle will remain in equilibrium. but in an oblique position of the terrella, as in an oblique sphere, and in the second figure, the needle dips obliquely at one end toward the near pole, but does not rest on the pole, nor is its dip ruled by the pole, but by the body and mass of the whole; for the { } dip in higher latitudes passes beyond the pole. but in the third position of the terrella the needle is perpendicular; because the pole of the stone is placed at the top, and the needle tending straight toward the body reaches to the pole. the cross in the preceding figures always turns toward the boreal pole of the terrella, having been touched by the boreal pole of the terrella; the cusp of the needle, having been touched by the southern pole of the stone, turns to the south. thus one may see on a terrella the level, oblique, and perpendicular positions of a magnetick needle. * [illustration] * * * * * chap. ii. diagram of declinations of the magnetick needle, when _excited, in the various portions of the sphere, and horizons_ of the earth, in which there is no variation _of the declination_. [illustration] { } as æquator let a b be taken, c the north pole, d the south, e g dipping-needles in the northern, h f in the southern part of the earth or of a terrella. in the diagram before us all the cusps have been touched by the true arctick pole of the terrella. here we have the level position of the magnetick needle on the æquator of the earth and the stone, at a and b, and its perpendicular position at c, d, the poles; whilst at the places midway between, at a distance of degrees, the crosses of the needle dip toward the south, but the cusps just as much toward the north. of which thing the reason will become clear from the demonstrations that follow. _* diagram of the rotation and declination of a terrella_ conforming to the globe of the earth, for a _latitude of degrees north._ [illustration] a is the boreal pole of the earth or of a rather large terrella, b the southern, c a smaller terrella, e the southern pole of the smaller terrella, dipping in the northern regions[ ]. the centre c is placed on the surface of the larger terrella, because the smaller terrella shows some variation on account of the length of the axis; inappreciable, however, on the earth. just as a magnetick needle dips in a regional latitude of degrees, so also the axis of a stone (of a spherical stone, of course) is depressed below the horizon, and its natural austral pole falls, and its boreal pole is raised on the { } south toward the zenith. in the same way also a circular disc of iron behaves, which has been carefully touched at opposite parts on its circumference; but the magnetical experiments are less clear on account of the feebler forces in round pieces of iron. _variety in the declinations of iron spikes at various latitudes of a terrella._ [illustration] the declination of a magnetick needle above a terrella is shown by means of several equal iron wires, of the length of a barleycorn, arranged along a meridian. the wires on the æquator are directed by the virtue of the stone toward the poles, and lie down upon its body along the plane of its horizon. the nearer they are brought to the poles, the more they are raised up by their versatory nature. at the poles themselves they point perpendicularly toward the very centre. but iron spikes, if they are of more than a due length, are not raised straight up except on a vigorous stone. * * * * * chap. iii. * an indicatory instrument, showing by the virtue of a _stone the degrees of declination from the horizon_ of each several latitude. { } [illustration] { } _description of the instrument, and its use._ take a terrella of the best strong loadstone, and homogeneous throughout, not weakened by decay or by a flaw in any parts; let it be of a fair size, so that its diameter is six or seven digits; and let it be made exactly spherical. having found its poles according to the method already shown, mark them with an iron tool; then mark also the æquinoctial circle. afterwards in a thick squared block of wood, one foot in size, make a hemispherical hollow, which shall hold half of the terrella, and such that exactly one half of the stone shall project above the face of the block. divide the limb close to this cavity (a circle having been drawn round it for a meridian) into quadrants, and each of these into degrees. let the terminus of the quadrants on the limb be near the centre of a quadrant described on the block, also divided into degrees. at that centre let a short, slender versorium (its other end being rather sharp and elongated like a pointer) be placed in æquilibrio on a suitable pin. it is manifest that when the poles of the stone are at the starting points of the quadrants, then the versorium lies straight, as if in æquilibrio, over the terrella. but if you move the terrella, so that the pole on the left hand rises, then the versorium rises on the meridian in proportion to the latitude, and turns itself as a magnetick body; and on the quadrant described on the flat surface of the wood, the degree of its turning or of the declination is shown by the versorium. the rim of the cavity represents a meridional circle, to which corresponds some meridian circle of the terrella, since the poles on both sides are within the circumference of the rim itself. these things clearly always happen on the same plan on the earth itself when there is no variation; but when there is variation, either in the direction or in the declination (a disturbance, as it were, in the true turning, on account of causes to be explained later), then there is some difference. let the quadrant be near the limb, or have its centre on the limb itself, and let the versorium be very short, so as not to touch the terrella, because with a versorium that is longer or more remote, there is some error; for it has a motion truly proportionate to the terrella only on the surface of the terrella. but if the quadrant, being far distant from the terrella, were moved within the orbe of virtue of the terrella toward the pole on some circle concentrick with the terrella, then the versorium would indicate the degrees of declination on the quadrant, in proportion to and symmetrically with that circle, not with the terrella. * * * * * { } chap. iiii. concerning the length of a versorium convenient _for declination on a terrella_. declination being investigated on the earth itself by means of a declination instrument, we may use either a short or a very long versorium, if only the magnetick virtue of the stone that touches it is able to permeate through the whole of its middle and through all its length. for the greatest length of a versorium has no moment or perceptible proportion to the earth's semi-diameter. on a terrella, however, or in a plane near a meridian of a terrella, a short versorium is desirable, of the length, say, of a barleycorn; for longer ones (because they reach further) dip and turn toward the body of the terrella suddenly and irregularly in the first degrees of declination. [illustration] for example, as soon as the long versorium is moved forward from the aequator a to c, it catches on the stone with its cusp (as if with a long extended wing), when the cusp reaches to the parts about b, which produce a greater rotation than at c. and the extremities of longer wires also and rods turn irregularly, just as iron wires and balls of iron and other orbicular loadstones are likewise turned about irregularly by a long non-orbicular loadstone. just so magneticks or iron bodies on the surface of a terrella ought not to have too long an axis, but a very short one; so that they may make a declination on the terrella truly and naturally proportionate to that on the earth. a long versorium also close to a terrella with difficulty stands steady in a horizontal direction on a right sphere, and, beginning to waver, it dips immediately to one side, especially the end that was touched, or (if both were touched) the one which felt the stone last. * * * * * { } chap. v. that declination does not arise from the attraction of the loadstone, but from a disposing and _rotating influence_. in the universe of nature that marvellous provision of its maker should be noticed, whereby the principal bodies are restrained within certain habitations and fenced in, as it were (nature controlling them). for this reason the stars, though they move and advance, are not thrown into confusion. magnetical rotations also arise from a disposing influence, whether in greater and dominating quantity, or in a smaller, and compliant quantity, even though it be very small. for the work is not accomplished by attraction, but by an incitation of each substance, by a motion of agreement toward fixed bounds, beyond which no advance is made. for if the versorium dipped by reason of an attractive force, then a terrella made from a very strong magnetick stone would cause the versorium to turn toward itself more than one made out of an average stone, and a piece of iron touched with a vigorous loadstone would dip more. this, however, never happens. moreover, an iron snout placed on a meridian in any latitude does not raise a spike more toward the perpendicular than the stone itself, alone and unarmed; although when thus equipped, it plucks up and raises many greater weights[ ]. but if a loadstone be sharper toward one pole, toward the other blunter, the sharp end or pole allures a magnetick needle more strongly, the blunt, thick end makes it rotate more strongly; but an orbicular stone * makes it rotate strongly and truly, in accordance with magnetick rules and its globular form. a long stone, on the other hand, extended from pole to pole, moves a versorium toward it irregularly; for in this case the pole of the versorium always looks down on the pole itself. similarly also, if the loadstone have been made in the shape of a circle, and its poles are on the circumference, whilst the body of it is plane, not globular, if the plane be brought near a versorium, the versorium does not move with the regular magnetick rotation, as on a terrella; but it turns looking always toward the pole of the loadstone, which has its seat on the circumference of the plane. moreover, if the stone caused the versorium to rotate by attracting it, then in the first degrees of latitude, it would attract the end of a short versorium toward the body itself of the terrella; yet it does not so attract it that they are brought into contact and unite; but the versorium rotates just so far as nature demands, as is clear from this example. { } [illustration] * for the cusp of a versorium placed in a low latitude does not touch the stone or unite with it, but only inclines toward it. moreover, when a magnetick body rotates in dipping, the pole of the versorium is not stayed or detained by the pole of the earth or terrella; but it rotates regularly, and does not stop at any point or bound, nor point straight to the pole toward which the centre of the versorium is advancing, unless on the pole itself, and once only between the pole and the æquator; but it dips as it advances, according as the change of position of its centre gives a reason for its inclination in accordance with rules magnetical. the declination of a magnetick needle in water also, as demonstrated in the following pages, is a fixed quantity[ ]; the magnetick needle does not descend to the bottom of the vessel, but remains steady in the middle, rotated on its centre according to its due amount of declination. this would not happen, if the earth or its poles by their attraction drew down the end of the magnetick needle, so that it dipped in this way. * * * * * chap. vi. on the proportion of declination to latitude[ ], and _the cause of it_. concerning the making of an instrument for finding declination, the causes and manner of declination, and the different degrees of rotation in different places, the inclination of the stone, and concerning an instrument indicating by the influence of a stone the degree of declination from any horizon we have already spoken. then we spoke about needles on the meridian of a stone, and their rotation shown for various latitudes by their rise toward the perpendicular. we must now, however, treat more fully of the causes of the degree of that inclination. whilst a loadstone and a magnetick iron wire are moved along a meridian from the aequator toward the pole, they rotate toward a round loadstone, as also toward the earth with a circular movement. on a right horizon (just as also on the æquinoctial of { } the stone) the axis of the iron, which is its centre line, is a line parallel to the axis of the earth. when that axis reaches the pole, which is the centre of the axis, it stands in the same straight line with the axis of the earth. the same end of the iron which at the æquator looks south turns to the north. for it is not a motion of centre to centre, but a natural turning of a magnetick body to a magnetick body, and of the axis of the body to the axis; it is not in consequence of the attraction of the pole itself that the iron points to the earth's polar point. under the æquator the magnetick needle remains in æquilibrio horizontally; but toward the pole on either side, in every latitude from the beginning of the first degree right up to the ninetieth, it dips. the magnetick needle does not, however, in proportion to any number of degrees or any arc of latitude fall below the horizon just that number of degrees or a similar arc, but a very different one: because this motion is not really a motion of declination, but is in [illustration] reality a motion of rotation, and it observes an arc of rotation according to the arc of latitude. therefore a magnetick body a, while it is advancing over the earth itself, or a little earth or terrella, from the æquinoctial g toward the pole b, rotates on its own centre, and halfway on the progress of its centre[ ] from the æquator to the pole b it is pointing toward the æquator at f, midway between the two poles. much more quickly, therefore, must the versorium rotate than its centre advances, in order that by rotating it may face straight toward the point f. wherefore the motion of this rotation is rapid in the first degrees from the æquator, namely, from a to l; but more tardy in the later degrees from l to b, when facing from the æquator at f to c. but if the declination were equal to the latitude (_i.e._, always just as many degrees from the horizon, as the centre of the versorium has receded from the æquator), then the magnetick needle would be following some potency and peculiar virtue of the centre, as if it { } were a point operating by itself. but it pays regard to the whole, both its mass, and its outer limits; the forces of both uniting, as well of the magnetick versorium as of the earth. * * * * * * chap. vii. explanation of the diagram of the rotation of _a magnetick needle_. [illustration] suppose a c d l to be the body of the earth or of a terrella, its centre m, Æquator a d, axis c l, a b the horizon, which changes according to the place. from the point f on a horizon distant from the æquator a by the length of c m, the semi-diameter of the earth or terrella, an arc is described to h as the limit of the quadrants of declination; for { } all the quadrants of declination serving the parts from a to c begin from that arc, and terminate at m, the centre of the earth. the semi-diameter of this arc is a chord drawn from the æquator a to the pole c; and a line produced along the horizon from a to b, equal to that chord, gives the beginning of the arc of the limits of arcs of rotation and revolution, which is continued as far as g. for just as a quadrant of a circle about the centre of the earth (whose beginning is on the horizon, at a distance from the æquator equal to the earth's semi-diameter) is the limit of all quadrants of declination drawn from each several horizon to the centre; so a circle about the centre from b, the beginning of the first arc of rotation, to g is the limit of the arcs of rotation. the arcs of rotation and revolution of the magnetick needle are intermediate between the arcs of rotation b l and g l. the centre of the arc is the region itself or place in which the observation is being made; the beginning of the arc is taken from the circle which is the limit of rotations, and it stops at the opposite pole; as, for example, from o to l, in a latitude of degrees. let any arc of rotation be divided into equal parts from the limit of the arcs of rotation toward the pole; for whatever is the degree of latitude of the place, the part of the arc of rotation which the magnetick pole on or near the terrella or the earth faces in its rotation is to be numbered similarly to this. the straight lines in the following larger diagram show this. the magnetick rotation at the middle point in a latitude of degrees is directed toward the æquator, in which case also that arc is a quadrant of a circle from the limit to the pole; but previous to this all the arcs of rotation are greater than a quadrant, whilst after it they are smaller; in the former the needle rotates more quickly, but in the succeeding positions gradually more slowly. for each several region there is a special arc of rotation, in which the limit to which the needle rotates is according to the number of degrees of latitude of the place in question; so that a straight line drawn from the place to the point on that arc marked with the number of degrees of latitude shows the magnetick direction, and indicates the degree of declination at the intersection of the quadrant of declination which serves the given place. take away the arc of the quadrant of declination drawn from the centre to the line of direction; that which is left is the arc of declination below the horizon. as, for example, in the rotation of the versorium n, whose line respective proceeds to d, from the quadrant of declination, s m, take away its arc r m; that which is left is the arc of declination: how much, that is, the needle dips in the latitude of degrees. * * * * * { } chap. viii. diagram of the rotation of a magnetick needle, _indicating magnetical declination in all latitudes, and_ from the rotation and declination, the _latitude itself_. in the more elaborate diagram a circle of rotations and a circle of declinations are adjusted to the body of the earth or terrella, with a first, a last, and a middle arc of rotation and declination. now from each fifth division of the arc which limits all the arcs of rotation (and which are understood[ ] as divided into equal parts) arcs are drawn to the pole, and from every fifth degree of the arc limiting the quadrants of declination, quadrants are drawn to the centre; and at the same time a spiral line is drawn, indicating (by the help of a movable quadrant) the declination in every latitude. straight lines showing the direction of the needle are drawn from those degrees which are marked on the meridian of the earth or a terrella to their proper arcs and the corresponding points on those arcs. to ascertain the elevation of the pole or the latitude of a place anywhere _in the world, by means of the following diagram, turned into a magnetick instrument, without the help of the coelestial bodies, sun, planets, or fixed stars, in fog and darkness_. [illustration] we may see how far from unproductive magnetick philosophy is, how agreeable, how helpful, how divine! sailors when tossed about on the waves with continuous cloudy weather, and unable by means of the coelestial luminaries to learn anything about the place or the region in which they are, with a very slight effort and with a small instrument are comforted, and learn the latitude of the place. with a declination instrument the degree of declination of the magnetick needle below the horizon is observed; that degree is noted on the inner arc of the quadrant, and the quadrant is turned round about the centre of the instrument until that degree on the quadrant touches the spiral line; then in the open space b at the centre of the quadrant the latitude of the region on { } the circumference of the globe is discerned by means of the fiducial line a b. let the diagram be fixed on a suitable flat board, and let the centre of the corner a of the quadrant be fastened to the centre of it, so that the quadrant may rotate on that centre. but it must be understood that there is also in certain places a variation in the declination on account of causes already mentioned (though not a large one), which it will be an assistance also to allow for on a likely estimate; and it will be especially helpful to observe this variation in various places, as it seems to present greater difficulty than the variation in direction; but it is easily learnt with a declination instrument, when it dips more or less than the line in the diagram. [illustration] _to observe magnetick declination at sea_. set upon our variation instrument a declination instrument; a wooden disc being placed between the round movable { } compass and the declination instrument: but first remove the versorium, lest the versorium should interfere with the dipping needle. in this way (though the sea be rough) the compass box will remain upright at the level of the horizon. the stand of the declination instrument must be directed by means of the small versorium at its base, which is set to the point respective of the variation, on the great circle of which (commonly called the magnetick meridian), the plane of the upright box is arranged; thus the declinatorium (by its versatory nature) indicates the degree of declination. in a declination instrument the magnetick needle, which _in a meridional position dips, if turned along a parallel hangs perpendicularly._ in a proper position a magnetick needle, while by its rotatory nature conformed to the earth, dips to some certain degree below the horizon on an oblique sphere. but when the plane of the instrument is moved out of the plane of the meridian, the magnetick needle (which tends toward the pole) no longer remains at the degree of its own declination, but inclines more toward the centre; for the force of direction is stronger than that of declination, and all power of declination is taken away, if the plane of the instrument is on a parallel. for then the magnetick needle, because it cannot maintain its due position on account of the axis being placed transversely, faces down perpendicularly to the earth; and it remains only on its own meridian, or on that which is commonly called the magnetick meridian. * * * * * chap. ix. demonstration of direction, or of variation from the _true direction, at the same time with declination, by_ means of only a single motion in water, due _to the disposing and rotating virtue._ [illustration] fix a slender iron wire of three digits length through * a round cork, so that the cork may support the iron in water. let this water be in a good-sized glass vase or bowl. pare the round cork little by little with a very sharp knife (so that it may remain round), until it will stay motionless one or two digits below the face of the water; and let the wire be evenly balanced. { } [ ]rub one end of the wire thus prepared on the boreal end of a loadstone and the other on the southern part of the stone (very skilfully, so that the cork may not be moved ever so little from its place) and again place it in the water; then the wire will dip with a circular motion on its own centre below the plane of the horizon, in proportion to the latitude of the region; and, even while dipping, will also show the point of variation (the true direction being perturbed). let the loadstone (that with which the iron is rubbed) be a strong one, such as is needed in all experiments on magnetick declination. when the iron, thus put into the water and prepared by means of the loadstone, has settled in the dip, the lower end remains at the point of variation on the arc of a great circle or magnetick meridian passing through the zenith or vertex, and the point of variation on the horizon, and the lowest point of the heavens, which they call the nadir. this fact is shown by placing a rather long magnetick versorium on one side a little way from the vase. this is a demonstration of a more absolute conformity of a magnetick body with the earth's body as regards unity; in it is made { } apparent, in a natural manner, the direction, with its variation, and the declination. but it must be understood that as it is a curious and difficult experiment, so it does not remain long in the middle of the water, but sinks at length to the bottom, when the cork has imbibed too much moisture. * * * * * chap. x. on the variation of the declination. direction has been spoken of previously, and also * variation, which is like a kind of dragging aside of the direction. now in declination such irregular motion is also noticed, when the needle dips beyond the proper point or when sometimes it does not reach its mark. there is therefore a variation of declination, being the arc of a magnetick meridian between the true and apparent declination. for as, on account of terrestrial elevations, magnetick bodies are drawn away from the true meridian, so also the needle dips (its rotation being increased a little) beyond its genuine position. for as variation is a deviation of the direction, so also, owing to the same cause, there is some error of declination, though often very slight. sometimes, also, when there is no variation of direction in the horizontal, there may nevertheless be variation of the declination; namely, either when more vigorous parts of the earth crop out exactly meridionally, _i.e._ under the very meridian; or when those parts are less powerful than nature in general requires; or when the virtue is too much intensified in one part, or weakened in another, just as one may observe in the vast ocean. and this discrepant nature and varying effect may be easily seen in certain parts of almost any round loadstone. inæquality of power is recognized in any part of a terrella by trial of the demonstration in chap. of this book. but the effect is clearly demonstrated by the instrument for showing declination in chap. of this book. * * * * * { } chap. xi. on the essential magnetick activity sphærically _effused._ discourse hath often been held concerning the * poles of the earth and of the stone, and concerning the æquinoctial zone; whilst lately we have been speaking about the declining of magneticks toward the earth and toward the terrella, and the causes of it. but while by various and complicated devices we have laboured long and hard to arrive at the cause of this declination, we have by good fortune found out a new and admirable (beyond the marvels of all virtues magnetical) science of the orbes themselves. for such is the power of magnetick globes, that it is diffused and extended into orbes outside the body itself, the form being carried beyond the limits of the corporeal substance; and a mind diligently versed in this study of nature will find the definite causes of the motions and revolutions. the same powers of a terrella exist also within the whole orbe of its power; and these orbes at any distance from the body of the terrella have in themselves, in proportion to their diameter and the magnitude of their circumference, their own limits of influences, or points wherein magnetick bodies rotate; but they do not look toward the same part of the terrella or the same point at any distance from the same (unless they be on the axis of the orbes and of the terrella); but they always tend to those points of their own orbes, which are distant by similar arcs from the common axis of the orbes. as, for example, in the following diagram, we show the body of a terrella, with its poles and æquator; and also a versorium on three other concentrick orbes around the terrella at some distance from it. in these orbes (as in all those which we may imagine without end) the magnetick body or versorium conforms to its own orbe in which it is located, and to its diameter and poles and æquator, not to those of the terrella; and it is by them and according to the magnitude of their orbes that the magnetick body is governed, rotated, and directed, in any arc of that orbe, both while the centre of the magnetick body stands still, and also while it moves along. and yet we do not mean that the magnetick forms and orbes exist in air or water or in any medium that is not magnetical; as if the air or the water were susceptible of them, or were induced by them; for the forms are only effused and really subsist when magnetick substances are there; whence a magnetick body is laid hold of within the forces and limits of the orbes; and within the orbes magneticks { } dispose magneticks and incite them, as if the orbes of virtue were solid and material loadstones. for the magnetick force does not pass through the whole medium or really exist as in a continuous body; so the orbes are magnetick, and yet not real orbes nor existent by themselves. _diagram of motions in magnetick orbes._ [illustration] a b is the axis of the terrella and of the orbes, c d the æquator. on all the orbes, as on the terrella, at the equator the versorium arranges itself along the plane of the horizon; on the axis it everywhere looks perpendicularly toward the centre; in the intermediate spaces e looks toward d; and g looks toward h, not toward f, as the versorium l does on the surface of the terrella. but as is the relation of l to f on the surface of the terella, so is that of g to h on its orbe and of e to d on its orbe; also all the rotations on { } the orbes toward the termini of the orbes are such as they are on the surface of the terrella, or toward the termini of its surface. but if in the more remote orbes this fails somewhat at times, it happens on account of the sluggishness of the stone, or on account of the feebler forces due to the too great distance of the orbes from the terrella. _demonstration._ set upon the instrumental diagram described farther back [chap. ] a plate or stiff circle of brass or tin, on which may be described the magnetick orbes, as in the diagram above; and in the middle let a hole be made according to the size of the terrella, so that the plate may lie evenly on the wood about the middle of the terrella on a meridional circle. then let a small versorium of the length of a barley-corn be placed on any orbe; upon which, when it is moved to various positions on the same circle, it will always pay regard to the dimensions of that orbe, not to those of the stone; as is shown in the diagram of the effused magnetick forms. while some assign occult and hidden virtues of substances, others a property of matter, as the causes of the wonderful magnetical effects; we have discovered the primary substantive form of globes, not from a conjectural shadow of the truth of reasons variously controverted; but we have laid hold of the true efficient cause, as from many other demonstrations, so also from this most certain diagram of magnetick forces effused by the form. though this (the form) has not been brought under any of our senses, and on that account is the less perceived by the intellect, it now appears manifest and conspicuous even to the eyes through this essential activity which proceeds from it as light from a lamp. and here it must be noted that a magnetick needle, moved on the top of the earth or of a terrella or of the effused orbes, makes two complete rotations in one circuit of its centre, like some epicycle about its orbit. * * * * * { } chap. xii. magnetick force is animate, or imitates life; and in many things surpasses human life, while this is bound _up in the organick body_. a loadstone is a wonderful thing in very many experiments, and like a living creature. and one of its remarkable virtues is that which the ancients considered to be a living soul in the sky, in the globes and in the stars, in the sun and in the moon. for they suspected that such various motions could not arise without a divine and animate nature, immense bodies turned about in fixed times, and wonderful powers infused into other bodies; whereby the whole universe flourishes in most beautiful variety, through this primary form of the globes themselves. the ancients, as thales, heraclitus, anaxagoras, archelaus, pythagoras, empedocles, parmenides, plato, and all the platonists, and not only the older greeks, but the egyptians and chaldæans, seek for some universal life in the universe, and affirm that the whole universe is endowed with life. aristotle affirms that not the whole universe is animate, but only the sky; but he maintains that its elements are inanimate; whilst the stars themselves are animate. we, however, find this life in globes only and in their homogenic parts; and though it is not the same in all globes (for it is much more eminent in the sun and in certain stars than in others of less nobility) yet in very many the lives of the globes agree in their powers. for each several homogenic part draws to its own globe in a similar manner, and has an inclination to the common direction of the whole in the universe; and the effused forms extend outward in all, and are carried out into an orbe, and have bounds of their own; hence the order and regularity of the motions and rotations of all the planets, and their courses, not wandering away, but fixed and determined. wherefore aristotle concedes life to the sphæres themselves and to the orbes of the heavens (which he feigns), because they are suitable and fitted for a circular motion and actions, and are carried along in fixed and definite courses. it is surely wonderful, why the globe of the earth alone with its emanations is condemned by him and his followers and cast into exile (as senseless and lifeless), and driven out of all the perfection of the excellent universe. it is treated as a small corpuscle in comparison with the whole, and in the numerous concourse of many thousands it is obscure, disregarded, and unhonoured. { } with it also they connect the kindred elements, in a like unhappiness, wretched and neglected. let this therefore be looked upon as a monstrosity in the aristotelian universe, in which everything is perfect, vigorous, animated; whilst the earth alone, an unhappy portion, is paltry, imperfect, dead, inanimate, and decadent. but on the other hand hermes, zoroaster, orpheus, recognize a universal life. we, however, consider that the whole universe is animated, and that all the globes, all the stars, and also the noble earth have been governed since the beginning by their own appointed souls and have the motives of self-conservation. nor are there wanting, either implanted in their homogenic nature or scattered through their homogenic substance, organs suitable for organic activity, although these are not fashioned of flesh and blood as animals, or composed of regular limbs, which are also hardly perceptible in certain plants and vegetables; since regular limbs are not necessary for all life. nor can any organs be discerned or imagined by us in any of the stars, the sun, or the planets, which are specially operative in the universe; yet they live and imbue with life the small particles in the prominences on the earth. if there be anything of which men can boast, it is in fact life, intelligence; for the other animals are ennobled by life; god also (by whose nod all things are ruled) is a living soul. who therefore will demand organs for the divine intelligences, which rise superior to every combination of organs and are not restrained by materialized organs? but in the several bodies of the stars the implanted force acts otherwise than in those divine existences which are supernaturally ordained; and in the stars, the sources of things, otherwise than in animals; in animals again otherwise than in plants. miserable were the condition of the stars, abject the lot of the earth, if that wonderful dignity of life be denied to them, which is conceded to worms, ants, moths, plants, and toadstools; for thus worms, moths, grubs would be bodies more honoured and perfect in nature; for without life no body is excellent, valuable, or distinguished. but since living bodies arise and receive life from the earth and the sun, and grass grows on the earth apart from any seeds thrown down (as when soil is dug up from deep down in the earth, and put on some very high place or on a very high tower, in a sunny spot, not so long after various grasses spring up unbidden) it is not likely that they can produce what is not in them; but they awaken life, and therefore they are living. therefore the bodies of the globes, as important parts of the universe, in order that they might be independent and that they might continue in that condition, had a need for souls to be united with them, without which there can be neither life, nor primary activity, nor motion, nor coalition, nor controlling power, nor harmony, nor endeavour, nor sympathy; and without which there would be no generation { } of anything, no alternations of the seasons, no propagation; but all things would be carried this way and that, and the whole universe would fall into wretchedest chaos, the earth in short would be vacant, dead, and useless. but it is only on the superficies of the globes that the concourse of living and animated beings is clearly perceived, in the great and pleasing variety of which the great master-workman is well pleased. but those souls which are restrained within a kind of barrier and in prison cells, as it were, do not emit immaterial effused forms outside the limits of their bodies; and bodies are not moved by them without labour and waste. they are brought and carried away by a breath; and when this has calmed down or been suppressed by some untoward influence, their bodies lie like the dregs of the universe and as the refuse of the globes. but the globes themselves remain and continue from year to year, move, and advance, and complete their courses, without waste or weariness. the human soul uses reason, sees many things, inquires about many more; but even the best instructed receives by his external senses (as through a lattice) light and the beginnings of knowledge. hence come so many errors and follies, by which our judgments and the actions of our lives are perverted; so that few or none order their actions rightly and justly. but the magnetick force of the earth and the formate life or living form of the globes, without perception, without error, without injury from ills and diseases, so present with us, has an implanted activity, vigorous through the whole material mass, fixed, constant, directive, executive, governing, consentient; by which the generation and death of all things are carried on upon the surface. for, without that motion, by which the daily revolution is performed, all earthly things around us would ever remain savage and neglected, and more than deserted and absolutely idle. but those motions in the sources of nature are not caused by thinking, by petty syllogisms, and theories, as human actions, which are wavering, imperfect, and undecided; but along with them reason, instruction, knowledge, discrimination have their origin, from which definite and determined actions arise, from the very foundations that have been laid and the very beginnings of the universe; which we, on account of the infirmity of our minds, cannot comprehend. wherefore thales, not without cause (as aristotle relates in his book _de anima_), held that the loadstone was animate, being a part and a choice offspring of its animate mother the earth. * * * * * { } [illustration] book sixth. _chap. i._ on the globe of the earth, the _great magnet_. hitherto our subject hath been the loadstone and things magnetical: how they conspire together, and are acted upon, how they conform themselves to the terrella and to the earth. now must we consider separately the globe itself of the earth. those experiments which have been proved by means of the terrella, how magnetick things conform themselves to the terrella, are all or at least the principal and most important of them, displayed by means of the earth's body: and to the earth things magnetical are in all respects associate. first, as in the terrella the æquator, meridians, parallels, axis, poles are natural boundaries, as numerous experiments make plain: so also in the earth these boundaries are natural, not mathematical only (as all before us used to suppose). these boundaries the same experiments display and establish in both cases alike, in the earth no less than in the terrella. just as on the periphery of a terrella a loadstone or a magnetick piece of iron is directed to its proper pole: so on the earth's surface are there turnings-about, peculiar, manifest, and constant on either side of the æquator. iron is indued with verticity by being extended toward a pole of the earth, just as toward a pole of the terrella: by its being placed down also, and cooling toward the earth's pole after the pristine verticity has { } been annulled by fire, it acquires new verticity, conformable to its position earthward. iron rods also, when placed some considerable time toward the poles, acquire verticity merely by regarding the earth; just as the same rods, if placed toward the pole of a loadstone, even without touching it, receive polar virtue. there is no magnetick body that in any way runs to the terrella which does not also wait upon the earth. as a loadstone is stronger at one end on one side or other[ ] of its æquator: so is the same property displayed by a small terrella upon the surface of a larger terrella. according to the variety and artistick skill in the rubbing of the magnetick iron upon the terrella, so do the magnetick things perform their function more efficiently or more feebly. in motions toward the earth's body, as toward the terrella a variation is displayed due to the unlikeness, inequality, and imperfection of its eminences: so every variation of the versorium or mariners' compass, everywhere by land or by sea, which thing has so sorely disturbed men's minds, is discerned and recognized as due to the same causes. the magnetick dip (which is the wonderful turning of magnetick things to the body of the terrella) in systematick course, is seen in clearer light to be the same thing upon the earth. and that single experiment, by a wonderful indication, as with a finger, proclaims the grand magnetick nature of the earth to be innate and diffused through all her inward parts. a magnetick vigour exists then in the earth just as in the terrella, which is a part of the earth, homogenic in nature with it, but rounded by art, so as to correspond with the earth's globous shape and in order that in the chief experiments it might accord with the globe of the earth. * * * * * chap. ii. the magnetick axis of the earth _persists invariable_. as in the very first beginnings of the moving world, the earth's magnetick axis passed through the midst of the earth: so now it tends through the centre to the same points of the superficies; the circle and plane of the æquinoctial line also persisting. for not without the vastest overthrow of the terrene mass can these natural boundaries be changed, as it is easy to gather from magnetick demonstrations. wherfore the opinion of dominicus maria of ferrara, a most talented man, who was the teacher of nicolas copernicus, must be cancelled; a view { } which, according to certain observations of his own, is as follows.[ ] "i," he says, "in former years while studying ptolemy's _geographia_ discovered that the elevations of the north pole placed by him in the several regions, fall short of what they are in our time by one degree and ten minutes: which divergence can by no means be ascribed to an error of the tables: for it is not credible that the whole series in the book is equally wrong in the figures of the tables: hence it is necessary to allow that the north pole has been tilted toward the vertical point. accordingly a lengthy observation has already begun to disclose to us things hidden from our forefathers; not indeed through any sloth of theirs, but because they lacked the prolonged observation of their predecessors: for before ptolemy very few places were observed with regard to the elevations of the pole, as he himself also bears witness at the beginning of his _cosmographia_: (for, says he) hipparchus alone hath handed down to us the latitudes of a few places, but a good many have noted those of distances; especially those which lie toward sunrise or sunset were received by some general tradition, not owing to any sloth on the part of authors themselves, but to the fact that there was as yet no practice of more exact mathematicks. 'tis accordingly no wonder, if our predecessors did not mark this very slow motion: for in one thousand and seventy years it shows itself to be displaced scarce one degree toward the apex of dwellers upon the earth. the strait of gibraltar shows this, where in ptolemy's time the north pole appears elevated degrees and a quarter from the horizon: whereas now it is and two-fifths. the like divergence is also shown at leucopetra in calabria, and at particular spots in italy, namely those which have not changed from ptolemy's time to our own. and so by reason of this movement, places now inhabited will some day become deserted, while those regions which are now parched at the torrid zone will, though long hence, be reduced to our temper of climate. thus, as in a course of three hundred and ninety five thousands of years, is that very slow movement completed." thus, according to these observations of dominicus maria, the north pole is at a higher elevation, and the latitudes of places are greater than formerly; whence he argues a change of latitudes. now, however, stadius, taking just the contrary view, proves by observations that the latitudes have decreased. for he says: "the latitude of rome in ptolemy's _geographia_ is degrees / : and that you may not suppose any error of reckoning to have crept in on the part of ptolemy, on the day of the Æquinox in the city of rome, the ninth part of the gnomon of the sun-dial is lacking in shadow, as pliny relates and vitruvius witnesseth in his ninth book." but the observation of moderns (according to erasmus rheinholdus) gives the same in our time as degrees with a sixth: so that you are in doubt as to half of one degree in { } the centre of the world, whether you show it to have decreased by the earth's obliquity of motion. one may see then how from inexact observations men rashly conceive new and contradictory opinions and imagine absurd motions of the mechanism of the earth. for since ptolemy only received certain latitudes from hipparchus, and did not in very many places make the observations himself; it is likely that he himself, knowing the position of the places, formed his estimate of the latitude of cities from probable conjecture only, and then placed it in the maps. thus one may see, in the case of our own britain, that the latitudes of cities are wrong by two or three degrees, as experience teaches. wherefore all the less should we from those mistakes infer a new motion, or let the noble magnetick nature of the earth be debased for an opinion so lightly conceived. moreover, those mistakes crept the more readily into geography, from the fact that the magnetick virtue was utterly unknown to those geographers. besides, observations of latitudes cannot be made sufficiently exactly, except by experts, using also finer instruments, and taking into account the refraction of the lights. * * * * * chap. iii. on the magnetick diurnal revolution of the earth's globe, as a probable assertion against the time-honoured _opinion of a primum mobile_. among the ancients heraclides of pontus and ecphantus, afterwards the pythagoreans, as nicetas of syracuse and aristarchus of samos, and some others (as it seems), used to think that the earth moves, and that the stars set by the interposition of the earth and rose by her retirement. in fact they set the earth moving and make her revolve around her axis from west to east, like a wheel turning on its axle. philolaus the pythagorean[ ] would have the earth to be one of the stars, and believed that it turned in an oblique circle around fire, just as the sun and moon have their own courses. he was a distinguished mathematician, and a most able investigator of nature. but after philosophy became a subject treated of by very many and was popularized, theories adapted to the vulgar intelligence or based on sophistical subtility occupied the minds of most men, and prevailed like a torrent, the multitude consenting. thereupon many valuable discoveries of the ancients were rejected, and were dismissed to perish in banishment; or at least by not being further cultivated and developed became obsolete. so that copernicus[ ] (among later discoverers, a man most deserving of literary honour) is the first who attempted to illustrate the [greek: phainomena] of { } moving bodies by new hypotheses: and these demonstrations of reasons others either follow or observe in order that they may more surely discover the phænomenal harmony of the movements; being men of the highest attainments in every kind of learning. thus supposed and imaginary orbs of ptolemy and others for finding the times and periods of the motions are not necessarily to be admitted to the physical inquiries of philosophers. it is then an ancient opinion and one that has come down from old times, but is now augmented by important considerations that the whole earth rotates with a daily revolution in the space of hours. well then, since we see the sun and moon and other planets and the glory of all the stars approach and retire within the space of one natural day, either the earth herself must needs be set in motion with a diurnal movement from west to east, or the whole heaven and the rest of nature from east to west. but, in the first place, it is not likely that the highest heaven and all those visible splendours of the fixed stars are impelled along that most rapid and useless course. besides, who is the master who has ever made out that the stars which we call fixed are in one and the same sphere, or has established by reasoning that there are any real and, as it were, adamantine sphæres? no one has ever proved this as a fact; nor is there a doubt but that just as the planets are at unequal distances from the earth, [ ]so are those vast and multitudinous lights separated from the earth by varying and very remote altitudes; they are not set in any sphærick frame or firmament (as is feigned), nor in any vaulted body: accordingly the intervals of some are from their unfathomable distance matter of opinion rather than of verification; others do much exceed them and are very far remote, and these being located in the heaven at varying distances, either in the thinnest æther or in that most subtile quintessence, or in the void: how are they to remain in their position during such a mighty swirl of the vast orbe of such uncertain substance. there have been observed by astronomers stars; besides these, numberless others are visible, some indeed faint to our senses, in the case of others our sense is dim and they are hardly perceived and only by exceptionally keen eyes, and there is no one gifted with excellent sight who does not when the moon is dark and the air at its rarest, discern numbers and numbers dim and wavering with minute lights on account of the great distance: hence it is credible both that these are many and that they are never all included in any range of vision. how immeasurable then must be the space which stretches to those remotest of fixed stars! how vast and immense the depth of that imaginary sphere! how far removed from the earth must the most widely separated stars be and at a distance transcending all sight, all skill and thought! how monstrous then such a motion { } would be! it is evident then that all the heavenly bodies set as if in destined places are there formed into sphæres, that they tend to their own centres, and that round them there is a confluence of all their parts. and if they have motion, that motion will rather be that of each round its own centre, as that of the earth is; or a forward movement of the centre in an orbit, as that of the moon: there would not be circular motion in the case of a too numerous and scattered flock. of these stars some situate near the Æquator would seem to be borne around at a very rapid rate, others nearer the pole to have a somewhat gentler motion, others, apparently motionless, to have a slight rotation. yet no differences in point of light, mass or colours are apparent to us: for they are as brilliant, clear, glittering and duskish toward the poles, as they are near the Æquator and the zodiack: those which remain set in those positions do not hang, and are neither fixed, nor bound to anything of the nature of a vault. all the more insane were the circumvolution of that fictitious _primum mobile_, which is higher, deeper, and still more immeasurable. moreover, this inconceivable _primum mobile_ ought to be material and of enormous depth, far surpassing all inferior nature in size: for nohow else could it conduct from east to west so many and such vast bodies of stars, and the universe even down to the earth: and it requires us to accept in the government of the stars a universal power and a despotism perpetual and intensely irksome. that _primum mobile_ bears no visible body, is nohow recognizable, is a fiction believed in by those people, accepted by the weak-minded folk, who wonder more at our terrestrial mass than at bodies so vast, so inconceivable, and so far separated from us. but there can be no movement of infinity and of an infinite body, and therefore no diurnal revolution of that vastest _primum mobile_. the moon being neighbour to the earth revolves in days; mercury and venus have their own moderately slow motions; mars finishes a period in two years, jupiter in twelve years, saturn in thirty. and those also who ascribe a motion to the fixed stars make out that it is completed in , years, according to ptolemy, in , years, according to copernicus' observations; so that the motion and the completion of the journey always become slower in the case of the greater circles. and would there then be a diurnal motion of that _primum mobile_ which is so great and beyond them all immense and profound? 'tis indeed a superstition and in the view of philosophy a fable now only to be believed by idiots, deserving more than ridicule from the learned: and yet in former ages, that motion, under the pressure of an importunate mob of philosophizers, was actually accepted as a basis of computations and of motions, by mathematicians. the motions of the bodies (namely planets) seem to take place eastward and following the order of the signs. { } the common run of mathematicians and philosophers also suppose that the fixed stars in the same manner advance with a very slow motion: and from ignorance of the truth they are forced to join to them a ninth sphære. whereas now this first and unthinkable _primum mobile_, a fiction not comprehended by any judgment, not evidenced by any visible constellation, but devised of imagination only and mathematical hypothesis, unfortunately accepted and believed by philosophers, extended into the heaven and beyond all the stars, must needs with a contrary impulse turn about from east to west, in opposition to the inclination of all the rest of the universe. whatsoever in nature is moved naturally, the same is set in motion both by its own forces and by the consentient compact of other bodies. such is the motion of parts to their whole, of all interdependent sphæres and stars in the universe: such is the circular impulse in the bodies of the planets, when they affect and incite one another's courses. but with regard to the _primum mobile_ and its contrary and exceeding rapid movement, what are the bodies which incite it or propel it? what is the nature that conspires with it? or what is that mad force beyond the _primum mobile_? since it is in bodies themselves that acting force resides, not in spaces or intervals. but he who thinks that those bodies are at leisure and keeping holiday, while all the virtue of the universe appertains to the very orbits and sphæres, is on this point not less mad than he who, in some one else's house, thinks that the walls and floors and roof rule the family rather than the wife and thoughtful paterfamilias. therefore not by the firmament are they borne along, or are moved, or have their position; much less are those confused crowds of stars whirled around by the _primum mobile_, nor are they torn away and huddled along by a contrary and extremely rapid movement. ptolemy of alexandria seems to be too timid and weak-minded in dreading the dissolution of this nether world, were the earth to be moved round in a circle. why does he not fear the ruin of the universe, dissolution, confusion, conflagration, and infinite disasters celestial and super-celestial, from a motion transcending all thoughts, dreams, fables, and poetic licences, insurmountable, ineffable, and inconceivable? wherefore we are carried along by a diurnal rotation of the earth (a motion for sure more congruous), and as a boat moves above the waters, so do we turn about with the earth, and yet seem to ourselves to be stationary, and at rest. great and incredible it seems to some philosophers, by reason of inveterate prejudice, that the earth's vast body should be swirled wholly round in the space of hours. but it would be more incredible that the moon should travel through her orbit, or complete an entire course in a space of hours; more so the sun or mars; still more jupiter and saturn; more than marvellous would be the velocity in the case of the { } fixed stars and the firmament; what in the world they would have to wonder at in the case of their ninth sphere, let them imagine as they like. but to feign a _primum mobile_ and to attribute to the thing thus feigned a motion to be completed in the space of hours, and not to allow this motion to the earth in the same interval of time, is absurd. for a great circle of the earth is to the ambit of the _primum mobile_ less than a furlong to the whole earth. if the diurnal rotation of the earth seem headlong, and not admissible in nature by reason of its rapidity, worse than insane will be the movement of the _primum mobile_ both for itself and the whole universe, agreeing as it does with no other motion in any proportion or likeness. it seems to ptolemy and the peripateticks that nature must be disordered, and the framework and structure of this globe of ours be dissolved, by reason of so swift a terrestrial revolution. the earth's diameter is german miles; the greatest elongation of the new moon is , the least is semi-diameters of the earth: the greatest altitude of the half moon is , the least : yet it is probable that its sphære is still larger and deeper. the sun in its greatest eccentricity has a distance of semi-diameters of the earth; mars, jupiter, saturn, being slower in motion, are so proportionately further remote from the earth. the distances of the firmament and of the fixed stars seem to the best mathematicians inconceivable. leaving out the ninth sphære, if the convexity of the _primum mobile_ be duly estimated in proportion to the rest of the sphæres, the vault of the _primum mobile_ must in one hour run through as much space as is comprised in great circles of the earth, for in the vault of the firmament it would complete more than ; but what iron solidity can be imagined so firm and tough as not to be disrupted and shattered to fragments by a fury so great and a velocity so ineffable. the chaldæans indeed would have it that the heaven consists of light. in light, however, there is no so-great firmness, neither is there in plotinus' fiery firmament, nor in the fluid or aqueous or supremely rare and transparent heaven of the divine moses, which does not cut off from our sight the lights of the stars. we must accordingly reject the so deep-set error about this so mad and furious a celestial velocity, and the forced retardation of the rest of the heavens. let theologians discard and wipe out with sponges those old women's tales of so rapid a spinning round of the heavens borrowed from certain inconsiderate philosophers. the sun is not propelled by the sphære of mars (if a sphære there be) and by his motion, nor mars by jupiter, nor jupiter by saturn. the sphære, too, of the fixed stars, seems well enough regulated except so far as motions which are in the earth are ascribed to the heavens, and bring about a certain change of phænomena. the superiors do not exercise a despotism over the inferiors; for the heaven of { } philosophers, as of theologians, must be gentle, happy, and tranquil, and not at all subject to changes: nor shall the force, fury, swiftness, and hurry of a _primum mobile_ have dominion over it. that fury descends through all the celestial sphæres, and celestial bodies, invades the elements of our philosophers, sweeps fire along, rolls along the air, or at least draws the chief part of it, conducts the universal æther, and turns about fiery impressions (as if it were a solid and firm body, when in fact it is a most refined essence, neither resisting nor drawing), leads captive the superior. o marvellous constancy of the terrestrial globe, the only one unconquered; and yet one that is holden fast, or stationary, in its place by no bonds, no heaviness, by no contiguity with a grosser or firmer body, by no weights. the substance of the terrestrial globe withstands and sets itself against universal nature. aristotle feigns for himself a system of philosophy founded on motions simple and compound, that the heavens revolve in a simple circle, its elements moving with a right motion, the parts of the earth seeking the earth in straight lines, falling on its surface at right angles, and tending together toward its centre, always, however, at rest therein; accordingly also the whole earth remains immovable in its place, united and compacted together by its own weight. that cohæsion of parts and aggregation of matter exist in the sun, in the moon, in the planets, in the fixed stars, in fine in all those round bodies whose parts cohære together and tend each to their own centres; otherwise the heaven would fall, and that sublime ordering would be lost: yet these coelestial bodies have a circular motion. whence the earth too may equally have her own motion: and this motion is not (as some deem it) unsuitable for the assembling or adverse to the generation of things. for since it is innate in the terrestrial globe, and natural to it; and since there is nothing external that can shock it, or hinder it by adverse motions, it goes round without any ill or danger, it advances without being forced, there is nothing that resists, nothing that by retiring gives way, but all is open. for while it revolves in a space void of bodies, or in the incorporeal æther, all the air, the exhalations of land and water, the clouds and pendent meteors, are impelled along with the globe circularly: that which is above the exhalations is void of bodies: the finest bodies and those which are least cohærent almost void are not impeded, are not dissolved, while passing through it. wherefore also the whole terrestrial globe, with all its adjuncts, moves bodily along, calmly, meeting no resistance. wherefore empty and superstitious is the fear that some weak minds have of a shock of bodies (like lucius lactantius, who, in the fashion of the unlettered rabble and of the most unreasonable men scoffs at an antipodes and at the sphærick ordering of the earth all round). so for these reasons, not only probable but manifest, does the diurnal rotation of the earth seem, { } since nature always acts through a few rather than through many; and it is more agreeable to reason that the earth's one small body should make a diurnal rotation, than that the whole universe should be whirled around. i pass over the reasons of the earth's remaining motions, for at present the only question is concerning its diurnal movement, according to which it moves round with respect to the sun, and creates a natural day (which we call a nycthemeron[ ]). and indeed nature may be thought to have granted a motion very suitable to the earth's shape, which (being sphærical) is revolved about the poles assigned it by nature much more easily and fittingly than that the whole universe, whose limit is unknown and unknowable, should be whirled round; and than there could be imagined an orbit of the _primum mobile_, a thing not accepted by the ancients, which aristotle even did not devise or accept as in any shape or form existing beyond the sphære of the fixed stars; which finally the sacred scriptures do not recognize any more than they do the revolution of the firmament. * * * * * chap. iiii. that the earth moves circularly. if then the philosophers of the common sort, with an unspeakable absurdity, imagine the whole heaven and the vast extent of the universe to rotate in a whirl, it yet remains that the earth performs a diurnal change. for in no third way can the apparent revolutions be explained. this day, then, which is called natural, is a revolution of some meridian of the earth from sun to sun. it revolves indeed in an entire course, from a fixed star round to that star again. those bodies which in nature are moved with a circular, æquable and constant motion, are furnished, in their parts, with various boundaries. but the earth is not a chaos nor disordered mass; but by reason of its astral virtue, it has boundaries which subserve the circular motion, poles not mathematical, an æquator not devised by imagination, meridians also and parallels; all of which we find permanent, certain and natural in the earth: which by numerous experiments the whole magnetick philosophy sets forth. for in the earth there are poles set in fixed bounds, and at them the verticity mounts up on either side from the plane of the earth's æquator, with forces which are mightier and præpotent from the common action of the whole; and with these poles the diurnal revolution is in agreement. but in no turnings-about of bodies, in none of the motions of the planets are there to be recognized, beheld, or assured to us by any reasoning any sensible or natural poles in the firmament, or in any _primum_ { } _mobile_; but those are the conception of an unsettled imagination. wherefore we, following an evident, sensible and tested cause, do know that the earth moves on its own poles, which are apparent to us by many magnetick demonstrations. for not only on the ground of its constancy, and its sure and permanent position, is the earth endowed with poles and verticity: for it might be directed toward other parts of the universe, toward east or west or some other region. by the wondrous wisdom then of the builder forces, primarily animate, have been implanted in the earth, that with determinate constancy the earth may take its direction, and the poles have been placed truly opposite[ ], that about them as the termini, as it were, of some axis, the motion of diurnal turning might be performed. but the constancy of the poles is regulated by the primary soul. wherefore, for the earth's good, the collimations of her verticities do not continually regard a definite point of the firmament and of the visible heaven. for changes of the æquinoxes take place from a certain deflection of the earth's axis; yet in regard to that deflection, the earth has a constancy of motion [illustration] derived from her own forces. the earth, that she may turn herself about in a diurnal revolution, leans on her poles. for since at a and b there is constant verticity, and the axis is straight; at c and d (the æquinoctial line) the parts are free, the whole forces on either side being spread out from the plane of the æquator toward the poles, in æther which is free from renitency, or else in a void; and a and b remaining constant, c revolves toward d both from innate conformity and aptitude, and for necessary good, and the avoidance of evil; but being chiefly moved forward by the diffusion of the solar orbes of virtues, and by their lights. and 'tis borne around, not upon a new and strange course, but (with the { } tendency common to the rest of the planets) it tends from west to east. for all planets have a like motion eastward according to the succession of the signs, whether mercury and venus revolve beneath the sun, or around the sun. that the earth is capable of and fitted for moving circularly its parts show, which when separated from the whole are not only borne along with the [illustration] straight movement taught by the peripateticks, but rotate also. a loadstone fixed in a wooden vessel is placed on water so as to swim freely, turn itself, and float about. if the pole b of the loadstone be set contrary to nature toward the south, f, the terrella is turned about its own centre with a circular motion in the plane of the horizon, toward the north, e, where it rests, not at c or d. so does a small stone if only of four ounces; it has the same motion also and just as quick, if it were a strong magnet of one hundred pounds. the largest magnetical mountain will possess the same turning-power also, if launched in a wide river or deep sea: and yet a magnetick body is much more hindered by water than the whole earth is by the æther. the whole earth would do the same, if the boreal pole were to be diverted from its true direction; for the boreal pole would run back with the circular motion of the whole around the centre toward the cynosure. but this motion by which the parts naturally settle themselves in their own { } resting-places is no other than circular. the whole earth regards the cynosure with her pole according to a steadfast law of her nature: and thus each true part of it seeks a like resting-place in the world, and is moved circularly toward that position. the natural movements of the whole and of the parts are alike: wherefore when the parts are moved in a circle, the whole also has the potency of [illustration] moving circularly. a sphærical loadstone placed in a vessel on water moves circularly around its centre (as is manifest) in the plane of the horizon, into conformity[ ] with the earth. so also it would move in any other great circle if it could be free; as in the declination instrument, a circular motion takes place in the meridian (if there were no variation), or, if there should be some variation, in a great circle drawn from the zenith through the point of variation on the horizon. and that circular motion of the magnet to its own just and natural position shows that the whole earth is fitted and adapted, and is sufficiently furnished with peculiar forces for diurnal circular motion. i omit what peter peregrinus[ ] constantly affirms, that a terrella suspended above its poles on a meridian moves circularly, making an entire revolution in hours: which, however, it has not happened to ourselves as yet to see; and we even doubt this motion on account of the weight of the stone itself, as well as because the whole earth, as she is moved of herself, so also is she propelled by other stars: and this does not happen in proportion (as it does in the terrella) { } in every part. the earth is moved by her own primary form and natural desire, for the conservation, perfection, and ordering of its parts, toward things more excellent: and this is more likely than that the fixed stars, those luminous globes, as well as the wanderers, and the most glorious and divine sun, which are in no way aided by the earth, or renewed, or urged by any virtue therein, should circulate aimlessly around the earth, and that the whole heavenly host should repeat around the earth courses never ending and of no profit whatever to the stars. the earth, then, which by some great necessity, even by a virtue innate, evident, and conspicuous, is turned circularly about the sun, revolves; and by this motion it rejoices in the solar virtues and influences, and is strengthened by its own sure verticity, that it should not rovingly revolve over every region of the heavens. the sun (the chief agent in nature) as he forwards the courses of the wanderers, so does he prompt this turning about of the earth by the diffusion of the virtues of his orbes, and of light. and if the earth were not made to spin with a diurnal revolution, the sun would ever hang over some determinate part with constant beams, and by long tarriance would scorch it, and pulverize it, and dissipate it, and the earth would sustain the deepest wounds; and nothing good would issue forth; it would not vegetate, it would not allow life to animals, and mankind would perish. in other parts, all things would verily be frightful and stark with extreme cold; whence all high places would be very rough, unfruitful, inaccessible, covered with a pall of perpetual shades and eternal night. since the earth herself would not choose to endure this so miserable and horrid appearance on both her faces, she, by her magnetick astral genius, revolves in an orbit, that by a perpetual change of light there may be a perpetual alternation of things, heat and cold, risings and settings, day and night, morn and eve, noon and midnight. thus the earth seeks and re-seeks the sun, turns away from him and pursues him, by her own wondrous magnetick virtue. besides, it is not only from the sun that evil would impend, if the earth were to stay still and be deprived of solar benefit; but from the moon also serious dangers would threaten. for we see how the ocean rises and swells beneath certain known positions of the moon: and if there were not through the daily rotation of earth a speedy transit of the moon, the flowing sea would be driven above its level into certain regions, and many shores would be overwhelmed with huge waves. in order then that earth may not perish in various ways, and be brought to confusion, she turns herself about by magnetick and primary virtue: and the like motions exist also in the rest of the wanderers, urged specially by the movement and light of other bodies. for the moon also turns herself about in a monthly course, to receive in succession the sun's beams in which she, like the earth, { } rejoices, and is refreshed: nor could she endure them for ever on one particular side without great harm and sure destruction. thus each one of the moving globes is for its own safety borne in an orbit either in some wider circle, or only by a rotation of its body, or by both together. but it is ridiculous for a man a philosopher to suppose that all the fixed stars and the planets and the still higher heavens revolve to no other purpose, save the advantage of the earth. it is the earth, then, that revolves, not the whole heaven, and this motion gives opportunity for the growth and decrease of things, and for the generating of things animate, and awakens internal heat for the bringing of them to birth. whence matter is quickened for receiving forms; and from the primary rotation of the earth natural bodies have their primary impetus and original activity. the motion then of the whole earth is primary, astral, circular, around its own poles, whose verticity arises on both sides from the plane of the æquator, and whose vigour is infused into opposite termini, in order that the earth may be moved by a sure rotation for its good, the sun also and the stars helping its motion. but the simple straight motion downwards of the peripateticks is a motion of weight, a motion of the aggregation of disjoined parts, in the ratio of their matter, along straight lines toward the body of the earth: which lines tend the shortest way toward the centre. the motions of disjoined magnetical parts of the earth, besides the motion of aggregation, are coition, revolution, and the direction of the parts to the whole, for harmony of form, and concordancy. * * * * * chap. v. arguments of those denying the earth's motion, and _their confutation._ now it will not be superfluous to weigh well the arguments of those who say the earth does not move; that we may be better able to satisfy the crowd of philosophizers who assert that this constancy and stability of the earth is confirmed by the most convincing arguments. aristotle does not allow that the earth moves circularly, on the ground that each several part of it would be affected by this particular motion; that whereas now all the separate parts of the earth are borne toward the middle in straight lines, that circular motion would be violent, and strange to nature, and not enduring. but it has been before proved that all actual portions of the earth move in a circle, and that all magnetick bodies (fitly disposed) are borne around in an orbe. they are borne, however, toward the centre of the earth in a { } straight line (if the way be open) by a motion of aggregation as though to their own origin: they move by various motions agreeably to the conformation of the whole: a terrella is moved circularly by its innate forces. "besides" (says he), "all things which are borne in an orbe, afterwards would seem to be abandoned by the first motion, and to be borne by several motions besides the first. the earth must also be borne on by two sorts of motion, whether it be situate around a mid-point, or in the middle site of the universe: and if this were so, there must needs be at one time an advance, at another time a retrogression of the fixed stars: this, however, does not seem to be the case, but they rise and set always the same in the same places." but it by no means follows that a double motion must be assigned to the earth. but if there be but one diurnal motion of the earth around its poles, who does not see that the stars must always in the same manner rise and set at the same points of the horizon, even although there be another motion about which we are not disputing: since the mutations in the smaller orbit cause no variation of aspect in the fixed stars owing to their great distance, unless the axis of the earth have varied its position, concerning which we raise a question when speaking of the cause of the præcession of the æquinoxes. in this argument are many flaws. for if the earth revolve, that we asserted must needs occur not by reason of the first sphære, but of its innate forces. but if it were set in motion by the first sphære, there would be no successions of days and nights, for it would continue its course along with the _primum mobile_. but that the earth is affected by a double movement at the time when it rotates around its own centre, because the rest of the stars move with a double motion, does not follow. besides, he does not well consider the argument, nor do his interpreters understand the same. [greek: toutou de sumbainontos, anankaion gignesthai parodous kai tropas tôn endedemenôn astrôn.] (arist. _de coelo_, ii. chap. .) that is, "if this be so, there must needs be changes, and retrogressions of the fixed stars." what some interpret as retrogressions or regressions, and changes of the fixed stars, others explain as diversions: which terms can in no way be understood of axial motion, unless he meant that the earth moved by the _primum mobile_ is borne and turned over other poles diverse even from those which correspond to the first sphære, which is altogether absurd. other later theorists suppose that the eastern ocean ought to be impelled so into western regions by that motion, that those parts of the earth which are dry and free from water would be daily flooded by the eastern ocean. but the ocean is not acted upon by that movement, since nothing opposes it; and even the whole atmosphere is carried round: and for that reason in the earth's course all the things in the air are not left behind by us nor do they seem to move toward the west: wherefore also the clouds { } are at rest in the air, unless the force of the winds drive them; and objects which are projected into the air fall again into their own place. but those foolish folk who think that towers, temples, and buildings must necessarily be shaken and overthrown by the earth's motion, may fear lest men at the antipodes should slip off into an opposite orbe, or that ships when sailing round the entire [ ]globe should (as soon as they have dipped under the plane of our horizon) fall into the opposite region of the sky. but those follies are old wives' gossip, and the rubbish of certain philosophizers, men who, when they essay to treat of the highest truths and the fabrick of the universe, and hazard anything, can scarce understand aught _ultra crepidam_. they would have the earth to be the centre of a circle; and therefore to rest motionless amid the rotation. but neither the stars nor the wandering globes move about the earth's centre: the high heaven also does not move circularly round the earth's centre; nor if the earth were in the centre, is it a centre itself, but a body around a centre. nor is it confident with reason that the heavenly bodies of the peripateticks should attend on a centre so decadent and perishable as that of the earth. they think that nature seeks rest for the generation of things, and for promoting their increase while growing; and that accordingly the whole earth is at rest. and yet all generation takes place from motion, without which the universal nature of things would become torpid. the motion of the sun, the motion of the moon, cause changes; the motion of the earth awakens the internal breath of the globe; animals themselves do not live without motion, and the ceaseless activity of the heart and arteries. for of no moment are the arguments for a simple straight motion toward the centre, that this is the only kind in the earth, and that in a simple body there is one motion only and that a simple one. for that straight motion is only a tendency toward their own origin, not of the parts of the earth only, but of those of the sun also, of the moon, and of the rest of the sphæres which also move in an orbit. joannes costæus, who raises doubts concerning the cause of the earth's motion, looking for it externally and internally, understands magnetick vigour to be internal, active, and disponent; also that the sun is an external promotive cause, and that the earth is not so vile and abject a body as it is generally considered. accordingly there is a diurnal movement on the part of the earth for its own sake and for its advantage. those who make out that that terrestrial motion (if such there be) takes place not only in longitude, but also in latitude, talk nonsense. for nature has set in the earth determinate poles, and definite unconfused revolutions. thus the moon revolves with respect to the sun in a monthly course; yet having her own definite poles, facing determinate parts of the heaven. to suppose that the air moves the earth would be { } ridiculous. for air is only exhalation, and is an enveloping effluvium from the earth itself; the winds also are only a rush of the exhalations in some part near the earth's surface; the height of its motion is slight, and in all regions there are various winds unlike and contrary. some writers, not finding in the matter of the earth the cause (for they say that they find nothing except solidity and consistency), deny it to be in its form; and they only admit as qualities of the earth cold and dryness, which are unable to move the earth. the stoicks attribute a soul to the earth, whence they pronounce (amid the laughter of the learned) the earth to be an animal. this magnetick form, whether vigour or soul, is astral. let the learned lament and bewail the fact that none of those old peripateticks, nor even those common philosophizers heretofore, nor joannes costæus, who mocks at such things, were able to apprehend this grand and important natural fact. but as to the notion that surface inequality of mountains and valleys would prevent the earth's diurnal revolution, there is nothing in it: for they do not mar the earth's roundness, being but slight excrescences compared with the whole earth; nor does the earth revolve alone without its emanations. beyond the emanations, there is no renitency. there is no more labour exerted in the earth's motion than in the march of the rest of the stars: nor is it excelled in dignity by some stars. to say that it is frivolous to suppose that the earth rather seeks a view of the sun, than the sun of the earth, is a mark of great obstinacy and unwisdom. of the theory of the rotation we have often spoken. if anyone seek the cause of the revolution, or of other tendency of the earth, from the sea surrounding it, or from the motion of the air, or from the earth's gravity, he would be no less silly as a theorist than those who stubbornly ground their opinions on the sentiments of the ancients. ptolemy's reasonings are of no weight; for when our true principles are laid down, the truth comes to light, and it is superfluous to refute them. let costæus recognize and philosophers see how unfruitful and vain a thing it becomes then to take one's stand on the principles and unproved opinions of certain ancients. some raise a doubt how it can be that, if the earth move round its own axis, a globe of iron or of lead dropped from the highest point of a tower falls exactly perpendicularly to a spot of the earth below itself. also how it is that cannon balls from a large culverin, fired with the same quantity and strength of powder, in the same direction and at a like elevation through the same air, would be cast at a like distance from a given spot both eastward and westward, supposing the earth to move eastward. but those who bring forward this kind of argument are being misled: not attending to the nature of primary globes, and the combination of parts with their globes, even though they be not adjoined by solid parts. whereas the motion of the earth in the diurnal revolution does not involve the separation of her more { } solid circumference from the surrounding bodies; but all her effluvia surround her, and in them heavy bodies projected in any way by force, move on uniformly along with the earth in general coherence. and this also takes place in all primary bodies, the sun, the moon, the earth, the parts betaking themselves to their first origins and sources, with which they connect themselves with the same appetence as terrene things, which we call heavy, with the earth. so lunar things tend to the moon, solar things to the sun, within the orbes of their own effluvia. the emanations hold together by continuity of substance, and heavy bodies are also united with the earth by their own gravity, and move on together in the general motion: especially when there is no renitency of bodies in the way. and for this cause, on account of the earth's diurnal revolution, bodies are neither set in motion, nor retarded; they do not overtake it, nor do they fall short behind it when violently projected toward east or west. [illustration] let e f g be the earth's globe, a its centre, l e the ascending effluvia: just as the orbe of the effluvia progresses with the earth, so also does the unmoved part of the circle at the straight line l e progress along with the general revolution. at l and e, a heavy body, m, falls perpendicularly toward e, taking the shortest way to the centre, nor is that right movement of weight, or of aggregation compounded with a circular movement, but is a simple right motion, never leaving the line l e. but when thrown with an equal force from e toward f, and from e toward g, it completes an equal distance on either side, even though the daily rotation of the earth is in process: just as twenty paces of a man mark an equal space whether toward east or west: so the earth's diurnal motion { } is by no means refuted by the illustrious tycho brahe, through arguments such as these. [illustration] the tendency toward its origin (which, in the case of the earth, is called by philosophers weight) causes no resistance to the diurnal revolution, nor does it direct the earth, nor does it retain the parts of the earth in place, for in regard to the earth's solidity they are imponderous, nor do they incline further, but are at rest in the mass. if there be a flaw in the mass, such as a deep cavity (say fathoms), a homogenic portion of the earth, or compacted terrestrial matter, descends through that space (whether filled with water or air) toward an origin more assured than air or water, seeking a solid globe. but the centre of the earth, as also the earth as a whole, is imponderous; the separated parts tend toward their own origin, but that tendency we call weight; the parts united are at rest; and even if they were ponderable, they would introduce no hindrance to the diurnal revolution. for if around the axis a b, there be a weight at c, it is balanced from e; if at f, from g; if at h, from i. so internally at l, they are balanced from m: the whole globe, then, having a natural axis, is balanced in æquilibrio, and is easily set in motion by the slighted cause, but especially because the earth in her own place is nowise heavy nor lacking in balance. therefore weight neither hinders the diurnal revolution, nor influences either the direction or continuance in position. wherefore it is manifest that no sufficiently strong reason has yet been found out by philosophers against the motion of the earth. * * * * * { } chap. vi. on the cause of the definite time, of an entire _rotation of the earth._ diurnal motion is due to causes which have now to be sought, arising from magnetick vigour and from the confederated bodies; that is to say, why the diurnal rotation of the earth is completed in the space of twenty-four hours. for no curious art, whether of clepsydras or of sand-clocks, or those contrivances of little toothed wheels which are set in motion by weights, or by the force of a bent steel band, can discover any degree of difference in the time. but as soon as the diurnal rotation has been gone through, it at once begins over again. but we would take as the day the absolute turning of a meridian of the earth, from sun to sun. this is somewhat greater than one whole revolution of it; in this way the yearly course is completed in and nearly ¼ turnings with respect to the sun. from this sure and regular motion of the earth, the number and time of days, hours, minutes, in solar tropical years is always certain and definite, except that there are some slight differences due to other causes. the earth therefore revolves not fortuitously, or by chance, or precipitately; but with a rather high intelligence, equably, and with a wondrous regularity, in no other way than all the rest of the movable stars, which have definite periods belonging to their motions. for the sun himself being the agent and incitor of the universe in motion, other wandering globes set within the range of his forces, when acted on and stirred, also regulate each its own proper courses by its own forces; and they are turned about in periods corresponding to the extent of their greater rotation, and the differences of their effused forces, and their intelligence for higher good. and for that cause saturn, having a wider orbit, is borne round it in a longer time, jupiter a shorter, and mars still less; while venus takes nine months, mercury days, on the hypotheses of copernicus; the moon going round the earth with respect to the sun in days, hours, minutes. we have asserted that the earth moves circularly about its centre, completing a day by an entire revolution with respect to the sun. the moon revolves in a monthly course around the earth, and, repeating a conjunction with the sun after a former synodic conjunction, constitutes the month or lunar day. the moon's mean concentrick orbit, according to numerous observations of copernicus and later astronomers, is found to be distant and about / diameters of the earth from the earth's centre. the moon's revolution with respect to the sun takes place in ½ days and minutes of time. we reckon the motion with respect to the sun, not the periodic motion, { } just as a day is one entire revolution of the earth with respect to the sun, not one periodick revolution; because the sun is the cause of lunar as of terrestrial motion: also, because (on the hypotheses of later observers) the synodical month is truly periodic, on account of the earth's motion in a great orbit. the proportion of diameters to circumferences is the same. and the concentrick orbit of the moon contains twice over and ½ great circles of the earth & a little more. the moon & the earth, then, agree together in a double proportion of motion; & the earth moves in the space of twenty-four hours, in its diurnal motion; because the moon has a motion proportional to the earth, but the earth a motion agreeing with the lunar motion in a nearly double proportion. there is some difference in details, because the distances of the stars in details have not been examined sufficiently exactly, nor are mathematicians as yet agreed about them. the earth therefore revolves in a space of hours, as the moon in her monthly course, by a magnetick confederation of both stars, the globes being forwarded in their movement by the sun, according to the proportion of their orbits, as aristotle allows, _de coelo_, bk. ii., chap. . "it happens" (he says) "that the motions are performed through a proportion existing between them severally, namely, at the same intervals in which some are swifter, others slower," but it is more agreeable to the relation between the moon and the earth, that that harmony of motion should be due to the fact that they are bodies rather near together, and very like each other in nature and substance, and that the moon has more evident effects upon the earth than the rest of the stars, the sun excepted; also because the moon alone of all the planets conducts her revolutions, directly (however diverse even), with reference to the earth's centre, and is especially akin to the earth, and bound to it as with chains. this, then, is the true symmetry and harmony between the motions of the earth and the moon; not that old oft-besung harmony of coelestial motions, which assumes that the nearer any sphære is to the _primum mobile_ and that fictitious and pretended rapidest prime motion, the less does it offer resistance thereto, and the slower it is borne by its own motion from west to east: but that the more remote it is, the greater is its velocity, and the more freely does it complete its own movement; and therefore that the moon (being at the greatest distance from the _primum mobile_) revolves the most swiftly. those vain tales have been conceded in order that the _primum mobile_ may be accepted, and be thought to have certain effects in retarding the motions of the lower heavens; as though the motion of the stars arose from retardation, and were not inherent and natural; and as though a furious force were perpetually driving the rest of the heaven (except only the _primum mobile_) with frenzied incitations. much more likely is it that the stars are borne around symmetrically by their own forces, with a certain mutual concert and harmony. * * * * * { } chap. vii. on the primary magnetick nature of the earth, whereby its poles are parted from the poles _of the ecliptick._ primarily having shown the manner and causes of the diurnal revolution of the earth, which is partly brought about from the vigour of the magnetick virtue, partly effected by the præ-eminence and light of the sun; there now follows an account of the distance of its poles from the poles of the ecliptick--a supremely necessary fact. for if the poles of the universe or of the earth remained fast at the poles of the zodiack, then the Æquator of the earth would lie exactly beneath the line of the ecliptick, and there would be no variation in the seasons of the year, no winter, no summer, nor spring, nor autumn: but one and the same invariable aspect of things would continue. the direction of the axis of the earth has receded therefore from the pole of the zodiack (for lasting good) just so far as is sufficient for the generation and variety of things. accordingly the declination of the tropicks and the inclination of the earth's pole remain perpetually in the twenty-fourth degree; though now only degrees minutes are counted; or, as others make out, minutes: but once it was degrees minutes, which are the extreme limits of the declinations hitherto observed. and that has been prudently ordained by nature, and is arranged by the primary excellence of the earth. for if those poles (of the earth and the ecliptick) were to be parted by a much greater distance, then when the sun approached the tropick, all things in the other deserted part of the globe, in some higher latitude, would be desolate and (by reason of the too prolonged absence of the sun) brought to destruction. as it is, however, all is so proportioned that the whole terrestrial globe has its own varying seasons in succession, and alternations of condition, appropriate and needful: either from the more direct and vertical radiation of light, or from its increased tarriance above the horizon. around these poles of the ecliptick the direction of the poles of the earth is borne: and by this motion the præcession of the æquinoxes is apparent to us. * * * * * { } chap. viii. on the præcession of the Æquinoxes, from the magnetick motion of the poles of the earth, in the arctick _and antarctick circle of the zodiack._ primitive mathematicians, since they did not pay attention to the inequælities of the years, made no distinction between the æquinoctial, or solstitial revolving year, and that which is taken from some one of the fixed stars. even the olympick years, which they used to reckon from the rising of the dogstar, they thought to be the same as those counted from the solstice. hipparchus of rhodes was the first to call attention to the fact that these differ from each other, and discovered that the year was longer when measured by the fixed stars than by the æquinox or solstice: whence he supposed that there was in the fixed stars also some motion in a common sequence; but very slow, and not at once perceptible. after him menelaus, a roman geometer, then ptolemy, and long afterward mahometes aractensis, and several more, in all their literary memoirs, perceived that the fixed stars and the whole firmament proceeded in an orderly sequence, regarding as they did the heaven, not the earth, and not understanding the magnetical inclinations. but we shall demomstrate that it proceeds rather from a certain rotatory motion of the earth's axis, than that that eighth sphære (so called) the firmament, or non-moving empyrean, revolves studded with innumerable globes and stars, whose distances from the earth have never been proved by anyone, nor can be proved (the whole universe gliding, as it were). and surely it should seem much more likely that the appearances in the heavens should be clearly accounted for by a certain inflection and inclination of the comparatively small body of the earth, than by the setting in motion of the whole system of the universe; especially if this motion is to be regarded as ordained solely for the earth's advantage: while for the fixed stars, or for the planets, it is of no use at all. for this motion the rising and settings of stars in every horizon, as well as their culminations at the height of the heavens, are shifted so much that the stars which once were vertical are now some degrees distant from the zenith. for nature has taken care, through the earth's soul or magnetick vigour, that, just as it was needful in tempering, receiving, and warding off the sun's rays and light, by suitable seasons, that the points toward which the earth's pole is directed should be degrees and more { } from the poles of the ecliptick[ ]: so now for moderating and for receiving the luminous rays of the fixed stars in due turn and succession, the earth's poles should revolve at the same distance from the ecliptick at the ecliptick's arctick circle; or rather that they should creep at a gentle pace, that the actions of the stars should not always remain at the same parallel circles, but should have a rather slow mutation. for the influences of the stars are not so forceful as that a swifter course should be desired. slowly, then, is the earth's axis inflected; and the stars' rays, falling upon the face of the earth, shift only in so long a time as a diameter of the arctick or polar circle is extended: whence the star at the extremity of the tail of the cynosure, which once was degrees minutes (namely, in the time of hipparchus) distant from the pole of the universe, or from that point which the pole of the earth used to face, is now only degrees and minutes distant from the same point; whence from its nearness it is called by the moderns _polaris._ some time it will be only ½ degree away from the pole: afterward it will begin to recede from the pole until it will be degrees distant; and this, according to the prutenical tables, will be in anno domini . thus _lucida lyræ_ (which to us southern britons now almost culminates) will some time approach to the pole of the world, to about the fifth degree. so all the stars shift their rays of light at the surface of the earth, through this wonderful magnetical inflection of the earth's axis. hence come new varieties of the seasons of the year, and lands become more fruitful or more barren; hence the characters and manners of nations are changed; kingdoms and laws are altered, in accordance with the virtue of the fixed stars as they culminate, and the strength thence received or lost in accordance with the singular and specifick nature of each; or on account of new configurations with the planets in other places of the zodiack; on account also of risings and settings, and of new concurrences at the meridian. the præcession of the æquinoxes arising from the aequable motion of the earth's pole in the arctick circle of the zodiack is here demonstrated. let a b c d be the ecliptick line; i e g the arctic circle of the zodiack. then if the earth's pole look to e, the æquinoxes are at d, c. let this be at the time of metho, when the horns of aries were in the æquinoctial colure. now if the earth's pole have advanced to i; then the æquinoxes will be at k, l; and the stars in the ecliptick c will seem to have progressed, in the order of the signs, along the whole arc k c: l will be moved on by the præcession, against the order of the signs, along the arc d l. but this would occur in the contrary order, if the point g were to face the poles of the earth, and the motion were from e to g: for then the æquinoxes would be m n, and the fixed stars would anticipate the same at c and d, counter to the order of the signs. [illustration] * * * * * { } chap. ix. on the anomaly of the præcession of the equinoxes, _and of the obliquity of the zodiack._ at one time the shifting of the æquinoxes is quicker, at another slower, being not always equal: because the poles of the earth travel unequally in the arctick and antarctick circle of the zodiack; and decline on both sides from the middle path: whence the obliquity of the zodiack to the Æquator seems to change. and as this has become known by means of long observations, so also has it been perceived, that the true æquinoctial points have been elongated from the mean æquinoctial points, on this side and on that, by minutes (when the prostaphæresis is greatest): but that the solstices either approach the equator unequally minutes nearer, or recede as far behind; so that the nearest approach is degrees minutes, and the greatest elongation degrees minutes. astronomers have given various explanations to account for this inequality of the præcession and also of the obliquity of the tropicks. thebit, with the view of { } laying down a rule for such considerable inequalities in the motion of the stars, explained that the eighth sphære does not move with a continuous motion from west to east; but is shaken with a certain motion of trepidation, by which the first points of aries and libra in the eighth heaven describe certain small circles with diameters equal to about nine degrees, around the first points of aries and libra in the ninth sphære. but since many things absurd and impossible as to motion follow from this motion of trepidation, that theory of motion is therefore long since obsolete. others therefore are compelled to attribute the motion to the eighth sphære, and to erect above it a ninth heaven also, yea, and to pile up yet a tenth and an eleventh: in the case of mathematicians, indeed, the fault may be condoned; for it is permissible for them, in the case of difficult motions, to lay down some rule and law of equality by any hypotheses. but by no means can such enormous and monstrous celestial structures be accepted by philosophers. and yet here one may see how hard to please are those who do not allow any motion to one very small body, the earth; and notwithstanding they drive and rotate the heavens, which are huge and immense above all conception and imagination: i declare that they feign the heavens to be three (the most monstrous of all things in nature) in order that some obscure motions forsooth[ ] may be accounted for. ptolemy, who compares with his own the observations of timocharis and hipparchus, one of whom flourished years, the other years before him, thought that there was this motion of the eighth sphære, and of the whole firmament; and proved by help of numerous phenomena that it took place over the poles of the zodiack, and, supposing its motion to be so far æquable, that the non-planetary stars in the space of years completed just one degree beneath the _primum mobile_. after him years albategnius discovered that one degree was completed in a space of years, so that a whole period would be , years. alphonsus made out that this motion was still slower, completing one degree and minutes only in years; and that thus the course of the fixed stars went on, though unequally. at length copernicus, by means of the observations of timocharis, aristarchus of samos, hipparchus, menelaus, ptolemy, mahometes aractensis, alphonsus, and of his own, detected the anomalies of the motion of the earth's axis: though i doubt not that other anomalies also will come to light some ages hence. so difficult is it to observe motion so slow, unless extending over a period of many centuries; on which account we still fail to understand the intent of nature, what she is driving after through such inequality of motion. let a be the pole of the ecliptick, b c the ecliptick, d the Æquator; when the pole of the earth near the arctick circle of the zodiack faces the point m, then there is an anomaly of the præcession of the æquinox at f; { } but when it faces n, there is an anomaly of the præcession at e. but when it faces i directly, then the maximum obliquity g is observed at the solstitial colure; but when it faces l, there is the minimum obliquity h at the solstitial colure. [illustration] _copernicus' contorted circlet in the arctick circle of the zodiack._ let f b g be the half of the arctick circle described round the pole of the zodiack: a b c the solstitial colure: a the pole of the zodiack; d e the anomaly of longitude minutes at either side on both ends: b c the anomaly of obliquity minutes: b the greater obliquity of degrees minutes: d the mean obliquity of degrees minutes: c the minimum obliquity of degrees minutes. { } [illustration] [illustration] { } the period of motion of the præcession of the æquinoxes is , Ægyptian years; the period of the obliquity of the zodiack is years, and a little more. the period of the anomaly of the præcession of the æquinoxes is years, and a little more. if the whole time of the motion ai were divided into eight equal parts: in the first eighth the pole is borne somewhat swiftly from a to b; in the second eighth, more slowly from b to c; in the third, with the same slowness from c to d; in the fourth, more swiftly again from d to e; in the fifth, with the same swiftness from e to f; again more slowly from f to g; and with the same slowness from g to h; in the last eighth, somewhat swiftly again from h to i. and this is the contorted circlet of copernicus, fused with the mean motion into the curved line which is the path of the true motion. and thus the pole attains the period of the anomaly of the præcession of the æquinoxes twice; and that of the declination or obliquity once only. it is thus that by later astronomers, but especially by copernicus (the restorer of astronomy)[ ], the anomalies of the motion of the earth's axis are described, so far as the observations of the ancients down to our own times admit; but there are still needed more and exact observations for anyone to establish aught certain about the anomaly of the motion of the præcessions, and at the same time that also of the obliquity of the zodiack. for ever since the time at which, by means of various observations, this anomaly was first observed, we have only arrived at half a period of the obliquity. so that all the more all these matters about the unequal motion both of the præcession and of the obliquity are uncertain and not well known: wherefore neither can we ourselves assign any natural causes for it, and establish it for certain. wherefore also do we to our reasonings and experiments magnetical here set an end and period.[ ] [illustration] * * * * * { } index. abano, pietro di (apponensis or apianus), . abbas, hali ('alí ibn al 'abb[=a]s, _al majúsi_, , . abohalis, . _see also_ avicenna. _aciarium_ or _acies_, also _aciare_, , , , . acosta, josephus, . adamant, . æquator, the magnetick, , . aetius amidenus, . affaytatus, fortunius, . agate, non electrick, , . agricola, georgius, , , , , , , . agrippa, h. cornelius, . _aimant_, . albategnius (muhammad ibn j[=a]bir, _al-batt[=a]ni_, . albertus magnus, , , , . alexander aphrodiseus, , , . alexandria, hero of, . alfonso, diego, . alfonsus the wise (alphonsus x.), . amalfians said to have first constructed the compass, . amatus lusitanus, . amber, , - , , , . amethyst, electrical properties of, . amianth, . amidenus, aetius, . amphitane, . anatolismus, or northeasting, . anaxagoras, , . andrea doria (admiral), . antonius de fantis, . antonius musa brasavolus, . antony, the denarius of, . apianus. _see_ abano. apponensis. _see_ abano. aquinas, thomas, , . aractensis, mahometes, , . archelaus, . ardoynis, santes de, . arias montanus, . aristarchus, , . aristotle: _de anima_, , , , . _de coelo_, , . _de mirabilibus auscultationibus_, . _meteorologica_, , . on material of the metals, , . on the element of earth, . on motions, , , . on primary form, . on the _primum mobile_, . on animate nature of planets, . armature, . armed loadstones, , , , . arnaldus de villa nova, , . arsinoe, temple of, . attraction, , , , , , , . avicenna (abu 'ali husain ibn 'abd allah, _ibn síná_; also called abohalis): writes on the magnet, . on falling masses of iron, . alleges loadstone an antidote to iron poison, . on the property of attraction, . augsburgers (augustani), the, prescribe loadstone in plaster, . axis, the magnetick, , , . azores, variation of compass at the, , , , . bacon, roger, . bambola, or bilbilis, . baptista montanus, . baptista porta. _see_ porta. barbarus, hermolaus, . barlowe, william (rev. archdeacon), his book, _the navigators supply_, . basil leaves alleged not to be attracted, . belemnites are electrical, . bencora (th[=a]bit ibn kurrah, _al harrani_; also called thebitius), , . benedictus, joannes baptista (giambattista benedetti), . beryl, electrick properties of, . bessardus (toussaincte de bessard), , , . blondus, flavius, the historian, . borough, william, his book on the _variation of the compass_, . borrholybicum (north-north-west), . brahe, tycho, , . brandoe, the island of, . brasavolus, antonius musa, . bristolla, or bristol gem, . burnt clay, magnetick properties of, , . { } cabot, sebastian, . cælius calcagninus, . cæsare, or cesare, giulio, . calaber, hannibal rosetius, , _calamita_ or _kalamita_, . calcagninus, cælius, . camillus leonhardus, . candish, or cavendish, thomas, *iij, . cap of iron for a loadstone, , , , . _carabe_, or _karabe_, . carbuncle, electrick properties of, , . cardan, hieronymo, . _de proportionibus_: on iron and earth, , , . on distance of centre of cosmos, . _de rerum varietate_: on fall of meteorick iron, . on attraction of amber, . on a perpetual motion engine, . _de subtilitate_: alleges magnet to feed on iron, , , . on magnet that draws silver, . on magnetick influence of star in tail of _ursa minor_, , , . carnelian, the, , . _catoblepas_, the antelope called, . cesare, giulio, . _chalybs_, , , . chatochitis, . chemists, the, , , , , , . china, , , , , , , . chinocrates, . circumpulsion, doctrine of, , . clamps (open kilns), . clay when burnt is magnetick, , , . clepsydra, . coimbra, college of, . coition (mutual attraction), , , , , , , , , , , , . definition of, *vj, . orbe of, *vj. colours of loadstones, , , . como, . compass, alleged invention of, by amalfians, . origin of the compass-card, , . the mariners' (_pyxis_), , , , , . the little (_pyxidula_), , . different forms of, italian, baltic, portuguese, english, , , , . conduction, magnetick, , , . consequent poles, , . copernican system, . copernicus, nicolas, , , , , , , . cordus, valerius, . cornelius agrippa, . cornelius gemma, . cornelius tacitus, . _corolla insorta_, or contorted circlet, , . cortes, martin, , , . corvo, island of, . costa, filippo (of mantua), . costæus, joannes, , , , . _creagus_, the, or flesh-magnet, . crystal, rock, , , , curtius, nicolaus, . cusan (michael khrypffs), cardinal de cusa, , , . cynosure, the, or pole-star, , , , , . dean, forest of, loadstone found in the, . decay of the magnetick virtue, , , , , . declination, the, or dip, . denarius of antony, . diamond, an electrick, , , , . alleged power to attract iron, , . alleged antipathy to magnet, , , , . experiments upon, . diego alfonso, . differences between electricks and magneticks, , , . dioscorides, , , , . dip, the, also called declination, , , - . dipping-needle, or declination instrument, , . direction, or directive force, , , , . dividing a loadstone, , , , , , , , , , . dominicus maria ferrariensis, , . doria, andrea (admiral), . drake, sir francis, *iij _bis_, . du puys (also called puteanus), , . earth, the, a great magnet, , , , , , , . _echeneis_ (the sucking fish), , , . ecphantus, . effluvia, electrical, , , , . magnetical, . electrical attraction, , , . electrick force, definition of, . electricks, *vj, - . _electrum_ ([greek: êlektron]), . emerald is non-electrick, . emery, , . empedocles, . encelius (or entzelt, christoph.), , . epicurius, , . erasmus rheinholdus, . erastus, thomas, , . errors in navigation, , . evax, king of arabia, . euripides, , , . { } fallopius, gabriellus, , , , . fantis, antonius de, . fernelius, joannes franciscus, . ficinus, marsilius (or marsiglio ficino), , , . filings of iron, , , , , , . filippo costa. _see_ costa. fire destroys magnetick properties, , , , . flame destroys electrification, . flame hinders not magnetick attractions, . flavius blondus. _see_ blondus. flies in amber, . form _versus_ matter, , . fra paolo, . fracastorio, hieronymo, , , , , , , , . franciscus maurolycus. _see_ maurolycus. franciscus rueus. _see_ rueus. gagates. _see_ jet. galen, , , , , , , , , , . gallus, marbodæus, , . garlick, its reputed antagonism to magnetism, , , . gartias ab horto, . gaudentius merula, . gauricus, lucas, . geber (j[=a]bir ibn háiyán, _al-tarsus[=i]_) . gemma, cornelius, . gems, electrick properties of, , . _geniter_, . georgius agricola. _see_ agricola. gilbert, adrian, . gilgil mauritanus, . gioia, or goia, of amalfi, . giulio cæsare, . glass, an electrick by friction, , , . use of loadstone in making, . goat's blood, . gonzalus oviedus, . goropius, henricus becanus, . grotius, hugo, , . haematite, , . hali abbas ('ali ibn al 'abbás, _al masúfí_), , . hannibal rosetius calaber, . hariot, thomas, . heat, effect of on loadstone, , , , , . helmshuda, . heraclea, the city of, . heraclean stone, or stone of hercules, , , , . heraclides, . heraclitus, . hermes, . hermolaus barbarus, . hero of alexandria, . hipparchus, , , , , . hippocrates, , , , . horizon, the magnetick, defined, . horto, gartias ab, . horus, the bone of, or _os ori_, . hot iron not magnetick, . hues, robert, . hugo grotius, , . inclination. _see_ dip. interposition of bodies, , , , , , . iris gem, the, . iron, its nature and occurrence, , , , . filings of, , , , , , . its various names and qualities, , , . its various uses, , , , , , . medical uses of, , . surpasses loadstone, , . verticity in, , , . iron ore is magnetick, , , , . has poles, . islands, magnetick influence of, , , . jacobus severtius, . jet, , , , , . joannes baptista porta. _see_ porta. joannes baptista montanus, . joannes costæus. _see_ costæus. joannes franciscus offusius, . joannes goia. _see_ gioia. joannes langius, . joannes taisner, or taisnier. _see_ taisnier. jofrancus offusius, . josephus acosta, . julius cæsar moderatus, . julius cæsar scaliger. _see_ scaliger. kendall, abraham, , . korrah, thebitius ben. _see_ bencora. lactantius, lucius, . lagos, rodriguez de, . langius, joannes, . _lapis magnetis_, . _lapis specularis_, muscovy stone, or mica, , , . latitude in relation to dip, , . leonardus (or leonhardus), camillus, . levinus lemnius, . { } lifting power of loadstones, , , . lily of the compass, , , , . liquids, electrical attraction of, . attraction on surface of, . livio sanuto, , , . loadstone armed and unarmed, , , . as medicine, . in plasters, . rock, the, , , , , . various names of, . colours of, , , . various sources of, , , . london, magnetick variation at, , . longitude, magnetick finding at, . long magnets, advantage of, , , , lucania, fall of meteorick stones in, . lucas gauricus, . lucretius, , , , , . lusitanus, amatus, . lynschoten, hugo van, *iiij. magnes, [greek: magnês], [greek: magnêtis], . magnesia, . magnetick axis of terrella, , . axis of earth, , , . horizon, . meridian, , . mountains or rocks, , , , , . islands, , , . motions, the five, . magnus, albertus. _see_ albertus. magnus, olaus, , . mahometes aractensis, , . mahomet's tomb, . manardus, joannes, . marbodæus gallus, , . marcellus empiricus, . marco polo (paulus venetus), . mariners' compass. _see_ compass. mars, saffron of (_crocus martis_), , . marsiglio ficino. _see_ ficinus. martin cortes, , , . matter and form, , . matthæus silvaticus, . matthiolus, petrus, , . mauritanus, gilgil, . mauritanus, serapio, , . maurolycus, franciscus, , , , . medicinal use of iron, . of loadstone, . medina, pedro de, . menelaus, , . meridian, magnetick, , , . merula, gaudentius, . meteorick stones, falls of, , . mica (or muscovy stone), , , . [greek: mikrogê]. _see_ terrella. moisture stops electrick action, , . montagnana, b., . montanus, arias, . montanus, joannes baptista, . moors, serapio and the, . mountains, magnetick, , , , , . movement of trepidation, . musa brasavolus, antonius, . muscovy stone, , , . _see also_ mica. myths of the magnet, , , , , , , , , , , , , , , , motions, the various magnetical, . names of amber, . names of the loadstone, . names given to the magnetick poles, , , , . nicander of colophon, , . nicetas, . nicolas copernicus, , , , , , , nicolaus myrepsus, or præpositas, . non-electrick bodies, , . nonius, petrus (pedro nuñez), . norman, robert, , , , , . supposes a point respective, , , , . his _newe attractive_, . discoverer of the dip, . norumbega, the city of, . nova zembla, , . offusius, jofrancus, . olaus magnus, , . opal becomes electrical, . orbe of virtue, , , , orbes of planets, , . oribasius, . orpheus, , , . oviedus, gonzalus (gonzalo fernandez de oviedo y valdès), . pantarbes, . paolo (paulus Æginæ), . paolo, rev. maestro (fra paolo sarpi), paolo the venetian (marco polo), . paracelsus (bombast von hohenheim). asserts the stars to attract iron, . his emplastrum of loadstone, . his method of strengthening loadstones, . parmenides, . pearls are not electrick, , . pedro de medina, . percussion excites verticity, . peregrinus, peter, his book, . on cause of magnetick direction, , , . on perpetual motion engine, . affirms a terrella to revolve daily, . { } peripateticks, the, , , , , , , , , , . perpetual motion machine, . peter peregrinus. _see_ peregrinus. peter plancius. _see_ plancius. petrus apponensis. _see_ abano, pietro di. petrus nonius. _see_ nonius or nuñez. philolaus, . philostratus, . pictorio, g., , . _piedramant_, . plancius, peter, *v _bis._ planets, influence of, , , . plasters, magnetick, , . plato, . in the _io_, discusses name and properties of the magnet, , , , . in the _timæus_, suggests the theory of circumpulsion, . his atlantis, . on life in the universe, . pliny (c. plinius secundus). on loadstone fables, , , , . his mistake about Æthiopian loadstones, . on the five kinds of loadstones, . on the alleged discovery of the loadstones, . on the alleged magnetick mountains, . on a locality where loadstone was found, . on the occurrence of iron in spain, . on the sagda and the catochites, . on the silver denarius of antony, . on the use of loadstone by glass-makers, . on the shadow of a gnomon of a sun-dial at rome, . plotinus, . plutarch, claudius. on the garlick fable, . says something flammable exists in amber, . his theory of circumpulsion, , . polarity. _see_ verticity. pole, the, elevation of, , . poles, magnetick, of a loadstone, , , , , . poles are not points, , , , . polo, marco, . porta, joannes baptista (giambattista della porta). his narration of marvels, . on various tempering of iron, . asserts loadstone a mixture of stone and iron, . on his assertion that loadstones have hairs, . asserts vapour to be cause of attraction, . his error as to change of verticity, . suspends iron upwards by a thread, . his error as to centre of the orbe of virtue, . his error as to the polarity which causes repulsion, . his error as to magnetick opposing forces, . experiment with a balance, . his error as to iron being intoxicated, . his error as to iron excited by a diamond, . his error as to the pointing of a magnet, . proportion between loadstone and iron, . his error as to variation and longitude, . præcession of the Æquinoxes, , . _primum mobile_, the, , , , , , , , . prostaphæresis, , . prutenical tables, the, . ptolemæus, claudius. on loadstone fables, , . on the occurrence of loadstone and of iron, , . on the dissolution of the earth, , , . alleged relation of regions with the planets, . on the elevation of the pole at different latitudes, , . on the _primum mobile_, and the diurnal movement of the stars, , , . on the anomalies of the earth's motion, . puteanus, gulielmus (du puys), , . pyrimachus (_i.e._, pyrites), . pythagoras, , . _pyxidula_, , . _pyxis_, , , , , . radius, the, of the earth's orbit, . rasis. _see_ rhazes. rays of magnetick virtue, . reinoldus, erasmus (or rheinholdus), . _remora_, the (or sucking fish), , , . resin becomes electrical by friction, , . respective points, , , , . reversal of polarity, , . revolution of the globe, , , , . repulsion, electrical, denied to exist, . rhazes (muhammad ibn zakar[=i]y[=a]), , . rings, on the verticity of, . rodriguez de lagos, . rosetius calaber, hannibal, . ruellius, joannes, . rueus, franciscus (de la rue), . saffron of mars, , , . sagda, or sagdo, the, . sanuto, livio, , , . sapphire, the, . scales of iron, . scaliger, julius cæsar. on cause of magnetick direction, , , . on a fall of meteorick iron, . on preservation of loadstones, . on amber, . on magnetick attraction, . admits the loadstone to have a soul, . on diamond attracting iron, . scoria or slag of iron, , . sealing wax is electrical, , . sebastian cabot. _see_ cabot. serapio, or serapio mauritania (yuhanná ibn sarapion), , . severtius, jacobus, . shielding, magnetick, by iron plate, , . { } _siderites_ ([greek: sideritês]) , , . _siegelstein_ . silk suspension for magnetick iron, , . silvaticus, matthæus, . silver, loadstone for, , . similars, doctrine of attraction of, , . simon stevinus, *v _bis_, , . slate, magnetick properties of, . smeargel (emery), . solinus, caius julius, , , . solomon the king, . sotacus, . stadius, . stars are at various distances, . steel, , , , , , , . stevinus, simon, *v _bis_, , . _stomoma_ ([greek: stomôma]) , , . strabo, . _succinum_. _see_ amber. sudini, or sudavienses, . sulphur, electrical by friction, , , , . [greek: sundromê], *vj. [greek: sunentelecheia], . sussex, iron ore in, . sympathy and antipathy, , , . tacitus, cornelius, . taisner, or taisnier, joannes, , . tariassiona or tarazona, . terrella. definition of, *vj, , . poles and axis of, , , , . divided into two parts, . magnetick vigour, diagram of, , . how small pieces of iron behave toward, , . orbe of virtue of, , , . "geography" of, . æquinoctial circle of, , . parallels of, , . magnetick horizon of, . proportion of the forces in, , . experiment with iron sphere, . small iron sphere and rod, , . centre of magnetick virtue in, . irregular terrella to exhibit variation, , . to illustrate the dip of the needle, , . analogy of, with the earth, , , , . testing loadstones, methods of, . thales of miletus, , , , , . _theamedes_, the, . thebitius, or thebit ben korrah, , . themistius, . theophrastus, , , . thomas aquinas, , . tides, the cause of, . tycho brahe, , . variation of the compass, , , , , - , , , . variation at the azores, , , , . versorium, magnetick, definition of, *vj. use of, , , . versorium, non-magnetick, use of, , , . verticity, , , - . acquired, , , , , , , , , , , , , . in iron plates touched by loadstone, . in iron sphere, . how, in iron, , , . in bracket in tower of st. augustine's church, rimini, . similar at ends of rod touched in middle, , . by percussion, . through interposed matter, . not in bodies other than magnetick, . æquator separates two kinds of, . possessed by the earth, as a "cause," . change of, through change of mass, . definition of, *vj. described, , , . destroyed by heat, , , . earth produces it in loadstone and iron, , , , . excited through greater distances in iron than in air, . exists in all shapes of loadstone, . helps the earth to keep its orbit, . inhærent in wrought iron, , . as a magnetick motion, . mutation of, , . magnitude of earth prevents variation of, , . none acquired by iron rubbed on æquator of terrella, . not affected by position of loadstone, . of one loadstone as affected by another, , . opposite, acquired by iron touched by loadstone, , , . parts having same repel, , . pole of, where last contact is, . strengthened in versoria, - . strength of, decreases at once in both poles, . villa nova, arnaldus de, , . vincentina, the, . vincent's rock, gem of, . weather affects electricks, , , , . weighing the magnetick force, . wright, edward, his prefatory address, *iij _bis_, . wrought iron is magnetick, , . youth preserved by loadstone, . zeilam, the king of, . zimiri, . zoroaster, . { } this treatise by william gilbert, of colchester, physician of london, on the magnet, was first publisht in the latin tongue in london in the year of our lord m.d.c.; this english translation, which was completed in the year m.c.m., is printed for the gilbert club, to the number of two hundred and fifty copies, by charles whittingham and company, at the chiswick press, tooks court, chancery lane, london. [illustration] * * * * * notes on the de magnete of dr. william gilbert [illustration] privately printed london mcmi "for out of olde feldes, as men seith, cometh al this newe corn fro yeer to yere; and out of olde bokes, in good feith, cometh al this newe science that men lere." --_chaucer._ "i finde that you have vsed in this your tr[=a]slation greate art, knowledge, and discretion. for walking as it were in golden fetters (as al translators doe) you notwithstanding so warilie follow your auctor, that where he trippeth you hold him vp, and where he goeth out of the way, you better direct his foote. you haue not only with the bee sucked out the best iuyce from so sweete a flower, but with the silke-worme as it were wouen out of your owne bowels, the finest silke; & that which is more, not rude & raw silke, but finely died with the fresh colour of your owne art, invention, and practise. if these adamantes draw you not to effect this which you haue so happilie begunne: then let these spurres driue you forward: viz. your owne promise, the expectation of your friends, the losse of some credit if you should steppe backe, the profit which your labours may yeeld to many, the earnest desire which you yourselfe haue to reviue this arte, and the vndoubted acceptation of your paines, if you performe the same."--(prefatory epistle of john case, d. of physicke, printed in r. haydocke's translation of _the artes of curious painting_, of lomatius, oxford, .) "this booke is not for every rude and unconnynge man to see, but for clerkys and very gentylmen that understand gentylness and scyence."--_caxton._ chiswick press: charles whittingham and co. tooks court, chancery lane, london. * * * * * {ij} [illustration] bibliography of _de magnete_. i. (the london folio of .) _fol. *j. title_ gvilielmi gil | berti colcestren | sis, medici londi- | nensis, | de magnete, magneti- | cisqve corporibvs, et de mag- | no magnete tellure; physiologia noua, | plurimis & argumentis, & expe- | rimentis demonstrata. | _printer's mark_ | londini | excudebat petrvs short anno | mdc. || _*j verso_ gilbert's coat of arms. || _*ij_ ad lectorem || _*iij verso_ ad gravissimvm doctissimvmqve ... || _*vj_ verborum quorundam interpretatio. || _*vj verso_ index capitum. || p. . gvilielmi gilberti | de magnete, lib. i. || p. . finis. | errata. without any colophon, printer's mark, or date at end. _folio. ll. of preliminary matter._ abcdefghiklmnopqrstv, _all ternions, making numbered leaves. one blank leaf at front and one at end. page at end of liber ii. blank. a folded woodcut plate inserted between p. and p. . woodcut initials, headlines and diagrams. all known copies except one have ink corrections in several pages, particularly pp. , , ._ ii. (the stettin quarto of .) _four preliminary unnumbered leaves, viz._ ( ) _bastard title_ gulielmi gilberti | tractatus | de magnete || _verso_ blank; ( ) _engraved title._ tractatvs | siue | physiologia nova | de magnete, | magneticisqve corpo- | ribvs et magno magnete | tellure sex libris comprehensus | ã | guilielmo gilberto colcestrensi, | medico londinensi | ... omnia nunc diligenter recognita & emen- | datius quam ante in lucem edita, aucta & figu- | ris illustrata operâ & studio | wolfgangi lochmans i.u.d. | & mathemati: | ad calcem libri adjunctus est index capi- | tum rerum et verborum locupletissimus | excvsvs sedini | typis gotzianis sumptibus | _ioh: hallervordij._ | anno mdc.xxviii || _verso_ blank; ( ) præfatio; ( ) amicorum acclamationes (verses) || _verso_ blank. _sig._ a ad lectorem candidum. _sig._ a _verso_ ad gravissimum doctissimum[=q] virum. _sig._ b verborum quorundam interpretatio. _verso_ blank, followed by twelve engraved plates numbered i. to xii. _sig._ b is numbered as p. , and begins gvilielmi gilberti | de magnete. | liber i. _sig._ c _begins as p. _; _sig._ d as p. ; and so forth. the collation therefore is: ll. unnumbered, abcdefghiklmnopqrstvxyzaabbccddeeffgghhiikkllmm, _all fours. pagination ends on_ p. , _which has sig._ h_ _in error for_ hh_ , _being the end of the text. verso of_ hh_ blank. index capitum _begins fol._ [hh_ ] _and_ with index verborum _continues to verso of_ mm_ . _last leaf_ [mm_ ] _contains errata, and instructions to binder to place plates: verso_ blank. _quarto. woodcut initials and diagrams. without any colophon, printer's mark, or date at end._ in some copies the engraved title differs, having the words _ioh: hallervordij._ replaced by the word _authoris_. {iij} iii. (the stettin quarto of .) _four preliminary unnumbered leaves_, viz., ( ) _title._ tractatus, sive physiologia nova | de | magnete, | magneticisq; corporibus & magno | magnate tellure, sex libris comprehensus, | a guilielmo gilberto colce- | strensi, medico londinensi. | ... omnia nunc diligenter recognita, & emendatius quam ante | in lucem edita, aucta & figuris illustrata, opera & studio d. | wolfgangi lochmans, i.u.d. | & mathematici. | ad calcem libri adiunctus est index capitum, rerum & verborum | locupletissimus, qui in priore æditione desiderabatur | sedini, | typis gotzianis. | anno m.dc. xxxii. || _verso_ blank; ( ) præfatio; ( ) amicorum acclamationes (verses) || _verso_ claudianus de magnete (verses); ( ) _ibid._ _sig._ a ad lectorem candidum. _sig._ a _verso_ ad gravissimum doctissimumq. virum. _sig._ b verborum quorundam interpretatio; _verso_ blank. _sig._ b is numbered as p. , and begins gvilielmi gilberti | de magnete. | liber i. _sig._ c begins as p. ; _sig._ d as p. ; and so forth. the collation therefore is: _ll._ unnumbered, a _to_ mm, _all fours_. pagination _ends on p. , which bears sig._ h _in error for_ hh . _verso of sig._ hh . errata. index capitum _begins_ hh , _and with_ index verborum _extends to verso_ of mm . _the last leaf_ [mm ] _bears the instructions to binder, with verso_ blank. _there is no colophon, printer's mark, or date at end. quarto. woodcut initials, and diagrams. twelve etched plates of various sizes inserted._ with the exception of the preliminary matter and the instructions to binder, the pagination is the same as in the edition of , the pages in the body of the work being reprinted word for word; though with exceptions. for example, p. in ed. is one line shorter than in ed. . the etched plates are entirely different. it has been thought from the pagination being alike that these two editions were really the same with different plates, titles, and preliminary matter. but they are really different. the spacing of the words, letters and lines is different throughout, and there are different misprints. the watermarks of the paper also differ. iv. (the berlin "facsimile" folio of .) this is a photozincograph reproduction of the london folio of . it lacks the ink emendations on pages , , , &c., found in the original, and is wanting also in some of the asterisks in the margins. v. (the american translation of .) frontispiece portrait || _p. i. title_ william gilbert | of colchester, | physician of london, | on the | loadstone and magnetic bodies, | and on | the great magnet the earth. | a new physiology, | demonstrated with many arguments and experiments. | a translation by | p. fleury mottelay, | ... | new york: | john wiley & sons, | east tenth street | . || _p. ii_ bears imprint of ferris bros. _printers_, pearl street, new york. || _p. iii._ reduced reproduction of title of edition || _verso_ the gilbert arms || _p. v._ translator's preface || _p. ix._ biographical memoir || _p. xxxi._ contents || _p. xxxvii._ address of edward wright || _p. xlvii._ author's preface. || _p. liii._ explanation of some terms. || pp. - text of the work. || p. reduced reproduction of title of edition. || p. _ditto_ of edition. || p. _ditto_ of gilbert's _de mundo nostro_ of . || pp. to general index. || pages _xxx_, _xlvi_, _lii_, and are blanks. there are no signatures. octavo. diagrams reduced from woodcuts of the folio of . some copies bear on title the imprint | london: | bernard quaritch, | piccadilly. || * * * * * { } [illustration] notes on the _de magnete_ of dr. william gilbert. during the work of revising and editing the english translation of _de magnete_, many points came up for discussion, requiring critical consideration, and the examination of the writings of contemporary or earlier authorities. discrepancies between the texts of the three known editions--the london folio of , and the two stettin quartos of and respectively--demanded investigation. passages relating to astrology, to pharmacy, to alchemy, to geography, and to navigation, required to be referred to persons acquainted with the early literature of those branches. phrases of non-classical latin, presenting some obscurity, needed explanation by scholars of mediæval writings. descriptions of magnetical experiments needed to be interpreted by persons whose knowledge of magnetism enabled them to infer the correct meaning to be assigned to the words in the text. in this wise a large amount of miscellaneous criticism has been brought to bear, and forms the basis for the following notes. to make them available to all students of gilbert, the references are given to page and line both of the latin folio of and of the english edition of . s. p. t. [ ] _the glossary:_ gilbert's glossary is practically an apology for the introduction into the latin language of certain new words, such as the nouns _terrella_, _versorium_, and _verticitas_, and the adjectival noun _magneticum_, which either did not exist in classical latin or had not the technical meaning which he now assigns to them. his _terrella_, or [greek: mikrogê], as he explains in detail on p. , is a little magnetic model of the earth, but in the glossary he simply defines it as _magnes globosus_. neither _terrella_ nor _versorium_ appears in any latin dictionary. no older writer had used either word, though peter peregrinus (_de magnete_, augsburg, ) had described experiments with globular loadstones, and pivotted magnetic needles suitable for use in a compass had been known for nearly three centuries. yet the pivotted needle was not denominated _versorium_. blondo (_de ventis_, venice, ) does not use the term. norman (_the newe attractiue_, london, ) speaks of the "needle or compasse," and of the "wyre." barlowe (_the navigators supply_, london, ) speaks of { } the "flie," or the "wier." the term _versorium_ (literally, the _turn-about_) is gilbert's own invention. it was at once adopted into the science, and appears in the treatises of cabeus, _philosophia magnetica_ (ferrara, ), and of kircher, _magnes sive de arte magnetica_ (coloniæ, ), and other writers of the seventeenth century. curiously enough, its adoption to denote the pivotted magnetic needle led to the growth of an erroneous suggestion that the mariners' compass was known to the ancients because of the occurrence in the writings of plautus of the term _versoriam_, or _vorsoriam_. this appears twice as the accusative case of a feminine noun _versoria_, or _vorsoria_, which was used to denote part of the gear of a ship used in tacking-about. forcellini defines _versoria_ as "funiculus quo extremus veli angulus religatur"; while _versoriam capere_ is equivalent to "reverti," or (metaphorically) "sententiam mutare." the two passages in plautus are: eut. si huc item properes, ut istuc properas, facias rectius, huc secundus ventus nunc est; cape modo vorsoriam; hic favonius serenu'st, istic auster imbricus: hic facit tranquillitatem, iste omnes fluctus conciet. (in _mercat._ act. v., sc. .) charm. stasime, fac te propere celerem recipe te ad dominum domum; . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . cape vorsoriam recipe te ad herum. (in _trinum._ act. iv., sc. .) the word _magneticum_ is also of gilbert's own coinage, as a noun; as an adjective it had been certainly used before, at least in its english form, _magneticall_, which appears on the title-page of william borough's _discourse of the variation of the compasse_ (london, ). gilbert does not use anywhere the noun _magnetismus_, _magnetism_. the first use of that noun occurs in william barlowe's _magneticall aduertisements_ ( ), in the _epistle dedicatorie_, wherein, when speaking of dr. gilbert, he says "vnto whom i communicated what i had obserued of my selfe, and what i had built vpon his foundation of the _magnetisme_ of the earth." gilbert speaks of the _virtus magnetica_, or _vis magnetica_; indeed, he has a rich vocabulary of terms, using, beside _virtus_ and _vis_, _vires_, _robur_, _potestas_, _potentia_, _efficientia_, and _vigor_ for that which we should now call _magnetism_ or _the magnetic forces_. nor does he use the verb _magnetisare_, or its participle, _magnetisatus_: he speaks of _ferrum tactum_, or of _ferrum excitatum a magnete_. in spite of certain obscurities which occur in places in his work, he certainly shows a nice appreciation of words and their use, and a knowledge of style. one finds occasionally direct quotations from, and overt references to, the classic authors, as in the references to plato and aristotle on page , and in the passage from the georgics of vergil on p. . but here and there one finds other traces of unmistakable scholarship, as in the reference to goat's wool on p. , or in the use, on p. , of the word _perplacet_, which occurs in the letter of cicero _ad atticum_, or in that of _commonstrabit_, occurring on p. , and found only in cicero, terence and plautus; whilst the phrase on p. , in which gilbert rallies the smatterers on having lost both their oil and their pains, has a delightfully classical echo. { } the term _orbis virtutis_, defined by gilbert in the glossary, and illustrated by the cuts on pages , , and , might be effectively translated by _sphere of influence_, or _orbit within which there is sensible attraction_. it has been preferred, however, to translate it literally as the _orbe of virtue_, or _orbe of magnetick virtue_. this choice has been determined by the desire to adopt such an english phrase as gilbert would himself have used had he been writing english. t. hood, writing in in his book _the vse of both the globes_, in using the word _orbe_, says that the word _globe_ signifies a solid body, while a _sphere_ is hollow, like two "dishes joyned by the brimme"; "the latines properly call _orbis_ an orbe"; "moreouer the word _sphaera_ signifieth that instrument made of brasen hoopes (wee call it commonly a ringed sphere) wherewith the astronomers deliuer unto the nouices of that science the vnderstanding of things which they imagine in the heauen." further, dr. marke ridley in his _treatise of magneticall bodies and motions_ ( ), has a chapter (xiiii) "of the distance and orbe of the magnets vertue," throughout which the term orbe is retained. sir thomas browne also writes of "the orb of their activities." the word _coitio_, used by gilbert for the mutual force between magnet and iron, has been retained in its english form, _coition_. gilbert evidently adopted this term after much thought. the newtonian conception of action and reaction being necessarily equal had not dawned upon the mediæval philosophers. the term _attraction_ had been used in a limited sense to connote an action in which a force was conceived of as being exerted on one side only. diogenes of apollonia, alexander aphrodiseus, democritus, and others, conceived the magnet to draw at the iron without the iron in any way contributing to that action. saint basil specially affirms that the magnet is not drawn by iron. on the other hand, albertus magnus had conceived the idea that the iron sought the magnet by a one-sided effort in which the magnet took no part. gilbert had the wit to discern that the action was mutual, and to mark the new conception he adopted the new term, and defined it as it stands in his glossary. it is "a concourse or concordancy of both," and to emphasize his meaning he adds, "not as if there were an [greek: helktikê dunamis] but a [greek: sundromê]" not a tractile power, but a running together. the adjective [greek: helktikê] is obviously related to the verb [greek: helkô], i draw: but its meaning puzzled the subsequent editors of the text, for in the two stettin editions of and , the phrase appears in the respective forms of [greek: helêtikê dunamis] and [greek: helkustikê dunamis]. in creech's english version of lucretius (edition of , p. a, in the footnote) is the commentary "galen, disputing against epicurus, uses the term [greek: helkein], which seems likewise too violent." it may be noted that the same verb occurs in the passage from the _io_ of plato quoted below. the term [greek: sundromê] applied by gilbert to explain his term _coitio_ is used by diodorus for the mutual onset of two hostile forces. a picturesque sentence from sir thomas browne's _pseudodoxia epidemica_ (london, , p. ) sets the matter succinctly forth. "if in two skiffs of cork, a loadstone and steel be placed within the orb of their activities, the one doth not move the other standing still, but both hoist sayle and steer unto each other; so that if the loadstone attract, the steel hath also its attraction; for in this action the alliency is reciprocall, which jointly felt, they mutually approach and run into each others arms." { } the page and line references given in these notes are in all cases first to the latin edition of , and secondly to the english edition of . [ ] page , line . page , line . _plato in ione._--the passage in the _io_ of plato is in chap. v. socrates addressing the poet io tells him that his facility in reciting homer is not really an art: [greek: theia de dunamis, hê se kinei hôsper en têi lithôi, hên euripidês men magnêtin ônomasen, hoi de polloi hêrakleian. kai gar autê hê lithos ou monon autous tous daktulious agei tous sidêrous, alla kai dunamin entithêsi tois daktuliois, ôst au dunasthai tautou touto poiein, hoper hê lithos, allous agein daktulious, hôst' enioth' hormathos makros panu sidêriôn kai daktuliôn ex allêlôn êrtêtai pasi de toutois ex ekeinês tês lithou hê dunamis anêrtêtai.] the idea is that as the loadstone in attracting an iron ring will make it into a magnet, which can in turn act magnetically on another ring, and this on yet another, so the inspiration of the muse is transferred to the poet, who in turn hands on the inspiration through the reciter to the listener. after further expanding the same idea of the transference of influence, socrates again mentions the magnet (chap. vii.): [greek: oisth' oun hoti outos estin ho theatês tôn daktuliôn ho eschatos, hôn egô elegon hupo tês hêrakleiôtidos lithou ap' allêlôn tên dunamin lambanein, ho de mesos su ho rhapsôdos kai hupokritês, ho de prôtos autos ho poiêtês? ho de theos dia pantôn toutôn helkei tên psuchên hopoi an boulêtai tôn anthrôpôn, k.t.l.] (edition didot of , vol. i., p. ; or stephanus, p. d). there is another reference in plato to the magnet, namely, in the _timæus_ (p. , vol. ii., edit. citat.). see the note to p. . the reference by euripides to the magnet occurs in the lost play of oeneus, in a fragment preserved by suidas. see _fragmenta euripidis_ (ed. didot, , p. , or nauck's edition, no. ). [greek: hôs euripidês en oinei; tas brotôn gnômas skopôn, hôste magnêtis lithos tên doxan helkei kai methistêsin palin.] [ ] page , line . page , line . the brief passage from aristotle's _de anima_ referring to thales is quoted by gilbert himself at the bottom of p. . [ ] page , line . page , line . the edition of inserts commas between theophrastus and lesbius, and between julius and solinus, as though these were four persons instead of two. [ ] page , line . page , line . _si allio magnes illitus fuerit, aut si adamas fuerit_. an excellent version of this myth is to be found in julius solinus, _polyhistor, de memorabilibus_, chap. lxiv., of which the english version of , by a. golding, runs thus: "the diamonde will not suffer the lodestone to drawe yron unto him: or if y^e lodestone haue alreadie drawne a peece of yron to it, the diamond snatcheth and pulleth away as hys bootye whatsoever the lodestone hath taken hold of." saint augustine repeats the diamond myth in his _de civitate dei_, lib. xxi. baptista porta says (p. of the english version of ): "it is a common opinion amongst sea-men, that onyons and garlick are at odds with the loadstone: and steers-men, and such as tend the mariners card are forbid to eat onyons or garlick, lest they make the index of the poles drunk. but when i tried all these things, found them to be false: for not onely breathing and belching upon the loadstone after eating of garlick, did not stop its vertues: but when it was all anoynted over with the juice of garlick, it did perform its office as well as if it had never been touched with it: and i could observe almost not the least difference, lest i should make void the endeavours of the ancients. { } and again, when i enquired of marines, whether it were so, that they were forbid to eat onyons and garlick for that reason; they said, they were old wives fables, and things ridiculous; and that sea-men would sooner lose their lives, then abstain from eating onyons and garlick." the fables respecting the antipathy of garlick and of the diamond to the operation of the magnet, although already discredited by ruellius and by porta, died hard. in spite of the exposure and denunciations of gilbert--compare p. --these tales were oft repeated during the succeeding century. in the appendix to sir hugh plat's _jewel house of art and nature_, in the edition of , by d. b. gent, it is stated there (p. ): "the loadstone which ... hath an admirable vertue not onely to draw iron to it self, but also to make any iron upon which it is rubbed to draw iron also, it is written notwithstanding, that being rubbed with the juyce of garlick, it loseth that vertue, and cannot then draw iron, as likewise if a diamond be layed close unto it." pliny wrote of the alleged antipathy between diamond and goat's blood. the passage as quoted from the english version of pliny's _natural historie of the world_, translated by philemon holland (london, , p. , chap, iv.), runs: "but i would gladly know whose invention this might be to soake the diamond in goats bloud, whose head devised it first, or rather by what chance was it found out and knowne? what conjecture should lead a man to make an experiment of such a singular and admirable secret, especially in a goat, the filthiest beast ... in the whole world? certes i must ascribe both this invention and all such like to the might and beneficence together of the divine powers: neither are we to argue and reason how and why nature hath done this or that? sufficient is it that her will was so, and thus she would have it." [ ] page , line . page , line . _machometis sacellum._ gilbert credits matthiolus (the well-known herbalist and commentator on dioscorides) with producing the fable as to mahomet's coffin being suspended in the air by a magnet. sir richard burton, in his famous pilgrimage to el medïnah in , effectually disposed of this myth. the reputed sarcophagus rests simply on bricks on the floor. but it had long been known that aerial suspension, even of the lightest iron object, in the air, without contact above or below, was impossible by any magnetic agency. in barlowe's _magneticall aduertisements_ (london, , p. ) is the following: "as for the turkes _mahomet_, hanging in the ayer with his yron chest it is a most grosse untruth, and utterly impossible it is for any thing to hange in the ayer by any _magneticall_ power, but that either it must touch the stone it selfe, or else some intermediate body, that hindreth it from comming to the stone (like as before i haue shewed) or else some stay below to keepe it from ascending, as some small wier that may scantly bee seene or perceived." [ ] page , line . page , line . _arsinoes templum._--the account in pliny of the magnetic suspension of the statue of arsinoe in the temple built by chinocrates is given as follows in the english version (london, ) of philemon holland (p. ): "and here i cannot chuse but acquaint you with the singular invention of that great architect and master deviser, of alexandria in Ægypt _dinocrates_, who began to make the arched roufe of the temple of _arsinoe_ all of magnet or this loadstone, to the end, that within that temple the statue of the said princesse made of yron, might seeme to hang in the aire by nothing. but prevented he was by death { } before hee could finish his worke, like as king _ptolomæe_ also, who ordained that temple to be built in the honour of the said _arsinoe_ his sister." there are a number of similar myths in ausonius, claudian, and cassiodorus, and in the writings of later ecclesiastical historians, such as rusinus and prosper aquitanus. the very meagre accounts they have left, and the scattered references to the reputed magical powers of the loadstone, suggest that there existed amongst the primitive religions of mankind a _magnet-worship_, of which these records are traces. [ ] page , line . page , line . _brasevolus_ [or _brasavola_].--the list of authorities here cited consists mostly of well-known mediæval writers on _materia medica_ or on minerals: the last on the list, _hannibal rosetius calaber_, has not been identified. the following are the references in the order named by gilbert: antonio musa brasavola. _examen omnium simplicium medicamentorum_, section (lugdun., ). joannes baptista montanus. _metaphrasis summaria eorum quæ ad medicamentorum doctrinà attinet_ (augustæ rheticæ, ). amatus lusitanus. _amati lusitani in dioscoridis anazarbei de materia medica libros quinque_ (venet., , p. ). oribasius. _oribasii sardiani ad eunapium libri quibus ... facultates simplicium ... continentur_ (venet., ). aetius amidenus. _aetii amideni librorum medicinalium ... libri octo nunc primum in lucem editi_ (greek text, aldine edition, venet., ). a latin edition appeared in basel, . see also his _tetrabiblos ex veteribus medicinæ_ (basil., ). avicenna (ibn sinâ). _canona medicinæ_ (venice, ), liber ii., cap. . serapio mauritanus (yuhanná ibn sarapion). in hoc volumine continentur ... _ioan. sarapionis arabis de simplicibus medicinis opus præclarum et ingens ..._ (edited by brunfels, argentorati, , p. ). hali abbas ('alí ibn al 'abb[=a]s). _liber totius medicinæ necessaria c[=o]tinens ... quem haly filius abbas edidit ... et a stephano ex arabica lingua reductus_ (lugd., , p. _verso_). santes de ardoniis (or ardoynis). _incipit liber de venenis quem magister santes de ardoynis ... edere cepit venetiis die octauo nou[=e]bris_, (venet., ). petrus apponensis (or petrus de abano). the loadstone is referred to in two works by this author. ( ) _conciliator differentiarum philosophorum: et precipue medicorum clarissimi viri petri de abano patauini feliciter incipit_ (venet., , p. , _verso_, quæstio li.). ( ) _tractatus de venenis_ (roma, , cap. xi.). marcellus (called marcellus empiricus). _de medicamentis_, in the volume _medici antiqui omnes_ (venet., , p. ). arnaldus (arnaldus de villa nova). _incipit tractatus de virtutibus herbarum_ (venet., ). see also _arnaldi villanovani opera omnia_ (basil., ). marbodeus gallus. _marbodei galli poetae vetustissimi de lapidibus pretiosis enchiridion_ (friburgi, [ ], p. ). albertus magnus. _de mineralibus et rebus metallicis_ (venet., , lib. ii., _de lapidibus preciosis_, p. ). there is a reference to the loadstone { } also in a work attributed falsely to albertus, but now ascribed to henricus de saxonia, _de virtutibus herbarum, de virtutibus lapidum_, etc. (rouen, , and subsequent editions). an english version, _the secrets of albertus magnus of the vertues of hearbs stones and certaine beasts_ was publisht in london in . matthæus silvaticus. _pandectæ medicinæ_ (lugduni, , cap. ). hermolaus barbarus. his work, _hermolai barbari patritii veneti et aqvileiensis patriarchæ corollarii libri quinque ..._ venet., , is an early herbal. on p. are to be found descriptions of _lapis gagatis_ and _lapis magnes_. the latter is mostly taken from pliny, and mentions the alleged theamedes, and the myth of the floating statue. camillus leonardus. _speculum lapidum_ (venet., , fol. xxxviii.). an english translation, _the mirror of stones_, appeared in london in . cornelius agrippa. _henrici cor. agrippæ ab nettesheym ... de occulta philosophia libri tres_ (antv., ). the english version _of the vanitie and uncertaintie of artes_ was publisht in london, , and again later. fallopius (gabriellus). _g. f. de simplicibus medicamentis purgantibus tractatus_ (venet., ). see also his _tractatus de compositione medicamentorum_ (venet., ). johannes langius. _epistolarum medicinalium volumen tripartitum_ (paris, , p. ). cardinalis cusanus (nicolas khrypffs, cardinal de cusa). _nicolai cusani de staticis experimentis dialogus_ (argentorati, ). the english edition, entitled _the idiot in four books_, is dated london, . [ ] page , line . page , line . _marcellus_.--"marcellus empiricus, médecin de théodose-le-grand, dit que l'aimant, appelé _antiphyson_, attire et repousse le fer." (klaproth, _sur l'invention de la boussole_, , p. .) the passage from marcellus runs: "magnetes lapis, qui antiphyson dicitur, qui ferrum trahit et abjicit, et magnetes lapis qui sanguinem emittit et ferrum ad se trahit, collo alligati aut circa caput dolori capitis medentur." (marcellus, _de medicamentis_: in the volume _medici antiqui omnes, qui latinis literis morborum genera persecuti sunt_. venet., , p. .) [ ] page , line . page , line . _thomas erastus_.--the work in question is _dispvtationvm de medicina nova philippi paracelsi, pars prima: in qua quæ de remediis svperstitiosis & magicis curationibus ille prodidit, præcipuè examinantur à thoma erasto in schola heydebergensi, professore_. (basiliæ, . parts and appeared the same year, and part in .) gilbert had no more love for paracelsus than for albertus magnus or others of the magic-mongers. indeed the few passages in paracelsus on the magnet are sorry stuff. they will mostly be found in the seventh volume of his collected works (_opera omnia_, frankfurt, ). a sample may be taken from the english work publisht in london, , with the title: _of the nature of things, nine books; written by philipp theophrastus of hohenheim, called paracelsvs_. "for any loadstone that mercury hath but touched, or which hath been smeered with mercuriall oyle, or only put into mercury will never draw iron more" (p. ). "the life of the loadstone is the spirit of iron; which may bee extracted, and taken away with spirit of wine" (p. ). [ ] page , line . page , line . _encelius_ (or _entzelt_, christoph) { } wrote a work publisht in at frankfurt, with the title _de re metallica, hoc est, de origine, varietate, et natura corporum metallicorum, lapidum, gemmarum, atque aliarum quæ ex fodinis eruuntur, rerum, ad medicine usum deservientium, libri iii_. this is written in a singular medley of latin and german. gilbert undoubtedly took from it many of his ideas about the properties of metals. see the note to p. on _plumbum album_. [ ] page , line . page , line . _thomas aquinas._--the reference is to his commentaries upon the _physica_ of aristotle. the passage will be found on p. _bis_ of the giunta edition (venet., ). the essential part is quoted by gilbert himself on p. . [ ] page , line . page , line . _pyxidem._--the word _pyxis_, which occurs here, and in the next sentence as _pyxidem nauticam_, is translated _compass_. eleven lines lower occurs the term _nautica pyxidula_. this latter word, literally the "little compass," certainly refers to the portable compass used at sea. compare several passages in book iv. where a contrasting use is made of these terms; for example, on pp. and . calcagninus, _de re nautica_, uses the term _pyxidecula_ for an instrument which he describes as "vitro intecta." on p. , line , gilbert uses the non-classical noun _compassus_, "boreale lilium compassi (quod boream respicit)," and again on p. , line . [ ] page , line . page , line . _melphitani._--the inhabitants of amalfi in the kingdom of naples. the claim of the discovery or invention of the mariners' compass in the year by one joannes goia, or gioia, also named as flavio goia, has been much disputed. in guthrie's _new system of modern geography_ (london, , p. ), in the chronology, is set down for the year : "the mariner's compass invented, or improved by givia, of naples. the flower de luce, the arms of the duke of anjou, then king of naples, was placed by him at the point of the needle, in compliment to that prince." in an elaborate treatise was printed at naples, by flaminius venanson with the title, _de l'invention de la boussole nautique_. venanson, who cites many authorities, endeavours to prove that if gioia did not discover magnetic polarity he at least invented the compass, that is to say, he pivotted the magnetic needle and placed it in a box, with a card affixed above it divided into sixteen parts bearing the names of the sixteen principal winds. he alleges in proof that the compass-card is emblazoned in the armorial bearings of the city of amalfi. this view was combatted in the famous letter of klaproth to humboldt publisht in paris in . he shows that the use of the magnetized needle was known in europe toward the end of the twelfth century; that the chinese knew of it and used it for finding the way on land still earlier; that there is no compass-card in the arms of the city of amalfi; but he concedes that gioia may have improved the compass in by adding the wind-rose card. the most recent contributions to the question are a pamphlet by signorelli, _sull' invenzione della bussola nautica, ragionamento di pietro napoli signorelli, segretario perpetuo della società pontaniana; letto nella seduta del settembre _; matteo camera's _memorie storico-diplomatiche dell' antica città e ducato di amalfi_ (salerno, ); and admiral luigi fincati's work _il magnete, la calamita, e la bussola_ (roma, ). an older mention of gioia is to be found in blundevile's _exercises_ ( rd edition, , pp. - ). see also crescentio _della nautica mediterranea_, (roma, , p. ), and azuni, _dissertazione sull' origine della bussola nautica_ (venezia, ). { } there appears to be a slip in gilbert's reference to andrea doria, as he has confounded the town of amalfi in principato citra with melfi in basilicata. one of the sources relied upon by historians for ascribing this origin of the compass is the _compendia dell' istoria del regno di napoli_, of collenuccio (venet., mdxci.), p. . "nè in questo tacerò amalfi, picciola terra, & capo della costa di picentia, alia quale tutti quelli, che'l mar caualcano, vfficiosamente eterno gratie debono referire, essendo prima in quella terra trovato l'vso, & l'artificio della calamita, & del bussolo, col quale i nauiganti, la stella tramontana infallibilmente mirando, direzzano il lor corso, si come è publica fama, & gli amalfitani si gloriano, nè senza ragione dalli piu si crede, essendo cosa certa, che gli antichi tale instromento non hebbero; nè essendo mai in tutto falso quello, che in molto tempo è da molti si diuolga." another account is to be found in the _historiarum sui temporis_, etc., of paulus jovius (florent., ), tom. ii., cap. , p. . "quum essem apud philippum superuenit ioachinus leuantius ligur a lotrechio missus, qui deposceret captiuos; sed ille negauit se daturum, quando eos ad ipsum andream auriam ammirantem deducendos esse iudicaret. vgonis uerò cadauer, ut illudentium barbarorum contumeliis eriperetur, ad amalphim urbem delatum est, in ædeque andreæ apostoli, tumultuariis exequiis tumulatum. in hac urbe citriorum & medicorum odoratis nemoribus æquè peramoena & celebri, magnetis usum nauigantibus hodie familiarem & necessarium, adinuentum suisse incolæ asserunt." flavius blondus, whom gilbert cites, gives the following reference, in which gioia's name is not mentioned, in the section upon campania felix of his italy (_blondi flavii forlinensis ... italia illustrata_, basiliæ, , p. ). "sed fama est qua amalphitanos audiuimus gloriari, magnetis usum, cuius adminiculo nauigantes ad arcton diriguntur, amalphi suisse inuentum, quicquid uero habeat in ea re ueritas, certû est id noctu nauigandi auxilium priscis omnino suisse incognitum." there is a further reference to the alleged amalphian in caelius calcagninus _de re nautica commentatio_. (_see thesaurus græcarum antiquitatum_, , vol. xi., p. .) on the other hand baptista porta, who wrote in naples in (_magia naturalis_) distinctly sets aside the claim as baseless. william barlowe, in _the navigators supply_ ( , p. a ), says: "who was the first inuentor of this instrument miraculous, and endued, as it were, with life, can hardly be found. the lame tale of one _flauius_ at _amelphis_, in the kingdome of _naples_, for to haue deuised it, is of very slender probabilitie. _pandulph collenutius_ writing the neapolitane historie telleth vs, that they of _amelphis_ say, it is a common opinion there, that it was first found out among them. but _polidore virgil_, who searched most diligently for the inuentors of things, could neuer heare of this opinion (yet himselfe being an italian) and as he confesseth in the later ende of his third booke _de inventoribus rerum_, could neuer vnderstand anything concerning the first inuention of this instrument." according to park benjamin (_intellectual rise in electricity_, p. ) the use of the pivotted compass arose and spread not from amalfi at the hands of italians in the fourteenth century, but from wisbuy, at the hands of the finns, in the middle of the twelfth century. { } hakewill (_an apologie or declaration of the power and providence of god_, london, , pp. - ) says: "but _blondus_, who is therein followed by _pancirollus_, both _italians_, will not haue _italy_ loose the praise thereof, telling vs that about yeares agoe it was found out at malphis or melphis, a citty in the kingdome of _naples_ in the _province_ of _campania_, now called _terra di lovorador_. but for the author of it, the one names him not, and the other assures vs, he is not knowne: yet _salmuth_ out of _ciezus & gomara_ confidently christens him with the name of _flavius_, and so doth _du bartas_ in those excellent verses of his touching this subject. "'w' are not to _ceres_ so much bound for bread, neither to _bacchus_ for his clusters red, as signior _flavio_ to thy witty tryall, for first inventing of the sea-mans dyall, th' vse of the needle turning in the same, divine device, o admirable frame!' "it may well be then that _flavius_ the _melvitan_ was the first inventor of guiding the ship by the turning of the needle to the _north_: but some _german_ afterwards added to the _compasse_ the points of the winde in his owne language, whence other nations haue since borrowed it." [ ] page , line . page , line . _paulum venetum_.--the reference is to marco polo. he returned in from his famous voyage to cathay. but the oft-repeated tale that he first introduced the knowledge of the compass into europe on his return is disposed of by several well-established facts. klaproth (_op. citat._, p. ) adduces a mention of its use in in the eastern mediterranean, recorded in a work written in by bailak of kibdjak. and the passages in the iceland chronicle, and in alexander of neckham are still earlier. [ ] page , line . page , line . _goropius_. see _hispanica ioannis goropii becani_ (plantin edition, antv., ), p. . this is a discussion of the etymologies of the names of the points of the compass: but is quite unauthoritative. [ ] page , line . page , line . _paruaim_.--respecting this reference, sir philip magnus has kindly furnisht the following note. a clue to the meaning of _parvaim_, which should be written in english letters with a _v_, not a _u_, will be found in _ chronicles_, iii. . in the verse quoted the author speaks of gold as the gold of parvaim, [hebrew: whazahab zhab parwayim], and [hebrew: prwym] parvaim is taken as a gold-producing region. it is regarded by some as the same as ophir. the word is supposed to be cognate with a sanskrit word _pûrva_ signifying "prior, anterior, oriental." there is nothing in the root indicating gold. a form similar to parvaim, and also a proper name, is sepharvaim, found in _ kings_, xix. , and in _isaiah_, xxxvii. , and supposed to be the name of a city in assyria. [ ] page , line . page , line . cabot's observation of the variation of the compass is narrated in the _geografia_ of livio sanuto (vinegia, , lib. i., fol. ). see also fournier's _hydrographie_, lib. xi., cap. . [ ] page , line . page , line . _gonzalus oviedus_.--the reference is to gonzalo fernandez de oviedo y valdès. _summario de la historia general y natural de las indias occidentales_, , p. , where the author speaks of the crossing of "la linea del diametro, donde las agujas hacen la { } diferencia del nordestear, ò noroestear, que es el parage de las islas de los açores." [ ] page , line . page , line . _petri cujusdam peregrini_.--this opusculum is the famous letter of peter peregrinus written in , of which some twenty manuscript copies exist in various libraries in oxford, rome, paris, etc., and of which the oldest printed edition is that of (augsburg). see also libri, _histoire des sciences mathématiques_ ( ); bertelli in boncompagni's _bull. d. bibliogr._ t. i. and t. iv. ( and ), and hellmann's _rara magnetica_ ( ). a summary of the contents of peregrinus's book will be found in park benjamin's _intellectual rise in electricity_ ( ), pp. - . [ ] page , line . page , line . _johannes taisner hannonius._--taisnier, or taysnier, of hainault, was a plagiarist who took most of the treatise of peregrinus and publisht it in his _opusculum... de natura magnetis_ (coloniæ, ), of which an english translation by richard eden was printed by r. jugge in . [ ] page , line . page , line . _collegium conimbricense_.--this is a reference to the commentaries on aristotle by the jesuits of coimbra. the work is _colegio de coimbra da companhia de jesu, cursus conimbricensis in octo libros physicorum_ (coloniæ, sumptibus lazari ratzneri, ). other editions: lugd. ; and colon., . the later edition of , in the british museum, has the title _commentariorum collegii conimbricensis in octo libros physicorum_. [ ] page , line . page , line . _martinus cortesius_.--his _arte de navegar_ (sevilla, ) went through various editions in spanish, italian, and english. eden's translation was publisht , and again in . [ ] page , line . page , line . _bessardus_.--toussaincte de bessard wrote a treatise, _dialogue de la longitude_ (rouen, ), which gives some useful notes of nautical practice, and of the french construction of the compass. speaking of the needle he says: "elle ne tire pas au pole du monde: ains regarde, au pole du zodiaque, comme il sera discoursu, cy apres" (p. ). on p. he speaks of "l'aiguille aymantine." on p. he refers to mercator's _carte générale_, and denies the existence of the alleged loadstone rock. on p. he gives the most naïve etymologies for the terms used: thus he assigns as the derivation of _sud_ the latin _sudor_, because the south is hot, and as that of _ouest_ that it comes from _ou_ and _est_. "come, qui diroit, ou est-il? à scauoir le soleil, qui estoit nagueres sur la terre." [ ] page , line . page , line . _jacobus severtius_.--jacques severt, whose work, _de orbis catoptrici sev mapparvm mvndi principiis descriptione ac usu libri tres_ (paris, ), would have probably lapsed into obscurity, but being just newly publisht was mentioned by gilbert for its follies. [ ] page , line . page , line . _robertus norman_.--author of the rare volume _the newe attractiue_, publisht in london, , and several times reprinted. this work contains an account of norman's discovery of the dip of the magnetic needle, and of his investigation of it by means of the dipping-needle, which he invented. he was a compassmaker of the port of london, and lived at limehouse. [ ] page , line . page , line . _franciscus maurolycus_.--the work to which the myth of the magnetic mountains is thus credited is, _d. francisci abbatis messanensis opuscula mathematica_, etc. (venet, mdlxxv, p. a). "sed cur sagitta, vel obelus à vero septentrione, quandoque ad dextram, { } quandoque ad sinistram declinat? an quia sagitta, sicut magnes (cuius est simia) non verum septentrionem, sed insulam quandam (quam olaus magnus gothus in sua geographia vocat insulam magnetum) semper ex natura inspicere cogitur?" [ ] page , line . page , line . _olaus magnus_.--the famous archbishop of upsala, who wrote the history of the northern nations (_historia de gentibus septentrionalibus_), of which the best edition, illustrated with many woodcuts, appeared in rome in . an english edition entitled _a compendious history of the goths, swedes, and vandals, and other northern nations_ was printed in london in ; but it is much abbreviated and has none of the quaint woodcuts. the reference on p. appears to be to the following passage on p. (ed. ). "demum in suppolaribus insulis magnetum montes reperiuntur, quorum fragmentis ligna fagina certo tempore applicata, in saxeam duritiem, et vim attractivam convertuntur," or the following on p. : "magnetes enim in extremo septentrionis veluti montes, unde nautica directio constat, reperiuntur: quorum etiam magnetum tam vehemens est operatio, ut certis lignis fagineis conjuncti, ea vertunt in sui duritiem, & naturam attractivam." on p. is a woodcut depicting the penalties inflicted by the naval laws upon any one who should maliciously tamper with the compass or the loadstone, "qui malitiosè nauticum gnomonem, aut compassum, & præcipuè portionem magnetis, unde omnium directio dependet, falsaverit." he was to be pinned to the mast by a dagger thrust through his hand. it will be noted that the ships carried both a compass, and a piece of loadstone wherewith to stroke the needle. there is in the basel edition of this work, , a note _ad lectorem_, on the margin of carta a, as follows: "insula milliarium in longitud. & latitud. polo arctico subjecta. "vltra quam directorium nauticum bossolo dic[~u] uires amittit: propterea quòd ilia insula plena est magnetum." this myth of the magnetic mountains, probably originating with nicander, appears, possibly from an independent source, in the east, in china, and in the tales of the arabian nights. ptolemy gives the following account in his _geographia_ (lib. vii., cap. ): [greek: pherontai de kai allai sunecheis deka nêsoi kaloumenai maniolai en ais phasi ta sidêrous echonta hêlous ploia katechesthai, mêpote tês hêrikleias lithou peri autas genomenês, kai dia touto epiourois naupêgeisthai.] some editions omit the name of the manioles from the passage. no two authorities agree as to the place of these alleged magnetic mountains. some place them in the red sea. fracastorio, _de sympathia et antipathia_, cap. (_opera omnia_, giunta edition, , p. ), gives the following reason for the variation of the compass: "nos igitur diligentius rem considerãtes dicimus causam, [~q] perpendiculum illud ad polum vertatur, esse montes ferri, & magnetis, qui sub polo sunt, vt negociatores affirmant, quorum species per incredibilem distantiam vsque ad maria nostra propagata ad perpendiculum vsq;, vbi est magnes, consuetam attractionem facit: propter distantiam autem quum debilis sit, non moueret quidem magnetem, nisi esset in perpendiculo: quare & si non trahit vsq; ac. principium, vnde effluxit, at mouet tam[~e], & propinquiorem facit, quo potest. quod si naues sorte vllæ propinquiores sint illis montibus, ferrum omne ear[~u] cuellitur, propter quod nauigijs incolæ vtuntur clauis ligneis astrictis." in the last chapter of his _de sympathia_, fracastorio returns to the subject { } in consequence of some doubts expressed by giambattista rhamnusio, seeing that the loadstones in the island of elba do not sensibly deflect the magnet. fracastorio replies thus (p. , _op. citat._): "primum igitur vtrum sub polo sint. magnetis mõtes, nec ne, sub ambiguo relinquamus, scimus enim esse, qui scribãt planas magis esse eas regiones, de quo paulus iouius e[~p]us nucerinus lucul[~e]tus historiar[~u] nostri t[~e]poris scriptor, circa eã sarmatiæ partem, quæ moscouia n[~u]c dicitur, diligent[~e] inquisitionem ab incolis fecit, qui ne eos etiã inueniri montes retulere, qui rhyphei ab antiquis dicti sunt: meminimus tam[~e] nos quasdam chartas vidisse earum, quas mundi mappas appellãt, in quibus sub polo montes notati erant (qui magnetis montes inscripti fuerant). siue igitur sint, siue non sint ij montes, nihil ad nos in præsentiarum attinet, quando per montes polo subiectos cathenam illam montium intelligimus, qui ad septentrionem spectant tanti, & tam vasti, ac ferri & magnetis feraces: qui, & si magis distant à nostro mari, [~q] iluæ insulæ montes, potentiores tamen sunt ad mouendum perpendiculum propter abundantiam & copiã ferri, & magnetis. fortasse autem, & qui in ilua est magnes, non multæ actionis est in ea minera: multi enim d[~u] in minera sunt, minus valent, [~q] extracti, [~q] spirituales species sua habeant impedimenta: signum autem parum valere in sua minera iluæ insulæ magnetem, [~q] tam propinquus quum sit nauigijs illac prætereuntibus, perpendiculum tamen non ad se cõuertit." aldrovandi in the _musæum metallicum_ (bonon., , p. ) gives another version of the fable: "nonnulli, animadversa hac magnetis natura, scripserunt naves, quibus in calecutanam regionem navigatur, clavis ferreis non figi, ob magneticorum frequentiam scopulorum, quoniam facilè dissolverentur. sed garzias in historia aromatum id fabulosum esse tradidit: quandoquidem plures naues calecutanæ regionis, & illius tractus, ferreis clauis iunctas obseruauit: immò addidit naues in insulis maldiuis ligneis quidem clauis copulari, non quia à magnete sibi metuant, sed quoniam ferri inopia laborant." according to aldrovandi (p. , _op. citat._) the magnetic mountains are stated by sir john mandeville to be in the region of pontus. lipenius in his _navigatio salomonis ophritica illustrata_ (witteb., ), which is a mine of curious learning, in discussing the magnetic mountains quotes the reply of socrates to the inquirer who asked him as to what went on in the infernal regions, saying that he had never been there nor had he ever met any one who had returned thence. the loadstone rock figures in several early charts. in nordenskiöld's _facsimile atlas_ (stockholm, ) is given a copy of the map of johan ruysch from an edition of ptolemy, publisht in rome in , which shows four islands within the ice-bound arctic regions. south of these islands and at the east of the coast of greenland is the inscription: _hic compassus navium non tenet, nec naves quæ ferrum tenent revertere valent._ to which (on p. ) nordenskiöld adds the comment: _sagan on magnetberg, som skulle draga till sig fartyg förande jern, är gamal._ and he recalls the reference of ptolemy to the magnetic rocks in the manioles. a second inscription is added to ruysch's map in the ornamental margin that borders the arctic islands. _legere est in libro de inventione fortunati sub polo arctico rupem esse excelsam ex lapide magnete miliarium germanorum ambitu._ this refers to a matter recorded in hakluyt's _principall navigations_ (lond., , p. ), namely: "a testimonie of the learned mathematician, maister john dee, { } touching the foresaid voyage of nicholas de linna. anno a frier of oxford, being a good astronomer, went in companie with others to the most northren islands of the world, and there leaving his company together, he travelled alone, and purposely described all the northern islands, with the indrawing seas: and the record thereof at his return he delivered to the king of england. the name of which booke is _inventio fortunata_ (_aliter fortunæ_) _qui liber incipit a gradu usq. ad polum_." the situation of the alleged loadstone rock is thus described by t. blundevile in his _exercises_ in the chapter entitled _a plaine and full description of peter plancius his vniuersall map, seruing both for sea and land, and by him lately put foorth in the yeare of our lord, _.... written in our mother tongue by m. blundeuill, anno domini . the passage is quoted from p. of the third edition ( ): "now betwixt the . and . degrees of north latitude he setteth downe two long ilands extending from the west towardes the east somewhat beyond the first meridian, and from the saide meridian more eastward he setteth downe other two long ilandes ... and hee saith further that right under the north pole there is a certaine blacke and most high rocke which hath in circuite thirtie and three leagues, which is nintie and nine miles, and that the long iland next to the pole on the west is the best and most healthfull of all the north parts. next to the foresaide ilandes more southward hee setteth downe the ilandes of crocklande and groynelande, making them to haue a farre longer and more slender shape then all other mappes doe.... moreouer at the east end of the last ilande somewhat to the southwarde, he placeth the pole of the lodestone which is called in latine magnes, euen as mercator doth in his mappe who supposing the first meridian to passe through saint marie or saint michael, which are two of the outermost ilandes of the azores eastwarde, placeth the pole of the stone in the seuentie fiue degree of latitude, but supposing the first meridian to passe through the ile coruo, which is the furthest ile of the azores westwarde, he placeth the pole of the lodestone in the seuentie seuen degree of latitude." further, in the chapter on _the arte of nauigation_ in the same work (p. , _ed. citat._), blundevile says: "but whereas mercator affirmeth that there should bee a mine or great rocke of adamant, wherunto all other lesser rockes or needles touched with the lodestone doe incline as to their chiefe fountaine, that opinion seemeth to mee verie straunge, for truely i rather beleeue with robert norman that the properties of the stone, as well in drawing steele, as in shewing the north pole, are secret vertues given of god to that stone for mans necessarie vse and behoofe, of which secrete vertues no man is able to shewe the true cause." the following is one of the inscriptions in the compartments of the great chart of mercator entitled _ad usum navigantium_, published in : "testatur franciscus diepanus peritissimus nauarchus volubiles libellas, magnetis virtute infectas recta mundi polum respicere in insulis c. viridis, solis, bonauista, et maio, cui proxime astipulantur qui in tercera, aut s. maria (insulæ sunt inter açores) id fieri dicunt, pauci in earundem occidentalissima corvi nomine id contingere opinantur. quia vero locorum longitudinis a communi magnetis et mundi meridiano iustis de causis initium sumere oportet, plurium testimonium sequutus primum meridianum per dictas c. viridis insulas protraxi, et quum alibi plus minusque a polo deuiante { } magnete polum aliquum peculiarem esse oporteat quo magnetes ex omni mundi parte despiciant, euum hoc quo assignaui loco existere adhibita declinatione magnetis ratisbonæ obseruata didici. supputaui autem eius poli situm etiam respectu insulæ corui, ut iuxta extremo primi meridiani positus extremi etiam termini, intra quos polum hunc inueniri necesse est, conspicui fierent, donec certius aliquod nauclerorum obseruatio attulerit." not all the map-makers were as frank as paulus merula, the author of a _cosmographia generalis_, printed by plantin in , at leyden. for in the description of his _tabula universalis_ (_op. citat._ lib. iii., cap. ) he says that he does not believe in the magnetic islands; but that he has put them into his chart lest unskilful folk should think that he had been so careless as to leave them out! in the well-known myth of ogier the dane, immortalized by william morris in the _earthly paradise_ (london, , vol. i., p. ), the loadstone rock is an island in the far north. but this story is not one of the scandinavian sagas, and belongs to the carlovingian cycle of heroic poems, of which the chief is the _chanson de roland_; and ogier le danois is really not a dane but an _ardennois_. in the middle-high german epic of kudrun, the adventures of the fleet of queen hilda when attracted by the loadstone mountain at givers, in the north sea, are narrated at some length. (see _kudrun, herausgegeben und erklärt von ernst martin_. halle, .) one stanza will serve as a sample: . ze givers vor dem berge | lac daz hilden her. swie guot ir anker wæren, | an daz vinster mer. magnêten die steine | heten si gezogen. ir guote segelboume | stuonden alle gebogen. which may be rendered: . at givers before the mountain | lay hilda's ships by. though good their anchors were, | upon the murky sea. magnets the stones were | had drawn them thither. their good sailing masts | stood all bent together. recent magnetic research has shown that while there are no magnetic mountains that would account for the declination of the compass in general, yet there are minor local variations that can only be accounted for by the presence of magnetic reefs or rocks. the reader is referred to the account of the magnetic survey of great britain in the _philosophical transactions_ ( ) by professors rücker and thorpe. the well-known rocky peak the riffelhorn above zermatt, in switzerland, produces distinct perturbations in the direction of the compass within half a mile of its base. such local perturbations are regularly used in sweden for tracing out the position of underground lodes of iron ore. see thalén, _sur la recherche des mines de fer à l'aide de mesures magnétiques_ (soc. royale des sciences d'upsal, ); or b. r. brough, _the use of the magnetic needle in exploring for iron ore_ (_scientific american_, suppl. no. , p. , aug. , ). quite recently dr. henry wilde, f.r.s., has endeavoured to elucidate the deviations of the compass as the result of the configurations of land and sea on the globe, by means of a model globe in which the ocean areas are covered with thin sheet iron. this apparatus dr. wilde calls a _magnetarium_. see _proc. roy. soc._, june, , jan., , and june, . { } an actual magnetic rock exists in scandinavia, the following account of it being given in the _electrical review_ of new york, may , : "the island of bornholm in the baltic, which consists of a mass of magnetic iron ore, is much feared by mariners. on being sighted they discontinue steering by compass, and go instead by lighthouses. between bornholm and the mainland there is also a dangerous bank of rock under water. it is said that the magnetic influence of this ore bank is so powerful that a balanced magnetic needle suspended freely in a boat over the bank will take a vertical position." [ ] page , line . page , line . _josephus costa._--this is unquestionably a misprint for _acosta_ (joseph de), the jesuit, whose work _historia natural y moral de las indias_ was publisht at seville in . an italian edition appeared at venice in . the english edition, translated by e. grimestone, _the naturall and morall historie of the east and west indies_, was publisht in london in and . there are in gilbert's book references to two writers of the name of costa or costæus, joannes costa of lodi, who edited galen and avicenna (see pp. and ), and filippo costa of mantua, who wrote on antidotes and medicaments (see p. ). the passage to which gilbert refers is in acosta's _historia_ (ed. , p. ). "deziame a mi vn piloto muy diestro portugues [~q] eran quatro puntos en todo el orbe, donde se afixaua el aguja con el norte, y contaualas por sus nombres, de que no me acuerdo bien. vno destos es el paraje de la isla del cueruo, en las terceras, o islas de açores, como es cosa y a muy sabida. passando di alli a mas altura, noruestea, que es dezir, [~q] declina al poniente ... que me digã la causa desta efecto?... porque vn poco de hierro de fregarse cõ la piedra iman ... "mejor es, como dize gregorio theologo, que a la fe se sujete la razon, pues aun en su casa no sabe bien entenderse...." [ ] page , line . page , line . _livius sanutus._--livio sanuto publisht at venice in a folio work, _geografia distinta in xii libri; ne' quali, oltre l'esplicatione di nostri luoghi di tolomeo, della bussola e dell' aguglia, si dichiarono le provincie ... dell' africa_. in this work all liber i. (pages - ) deals with observations of the compass, mentioning sebastian cabot, and other navigators. he gives a map of africa, showing the central lakes out of which flow the _zaires fluvius_ and the _zanberes fluvius_. [ ] page , line . page , line . _fortunius affaitatus._--the work of affaytatus, _physicæ ac astronomiæ considerationes_, was publisht in venice in . [ ] page , line . page , line . _baptista porta._--the reference is to his celebrated _magia naturalis_, the first edition of which came out in at naples. an english edition, _natural magick by john baptista porta, a neapolitaine_, was printed in london, . book seven of this volume treats "of the wonders of the load-stone." in the proem to this book porta says: "i knew at venice r. m. paulus, the venetian, that was busied in the same study: he was provincial of the order of servants, but now a most worthy advocate, from whom i not only confess, that i gained something, but i glory in it, because of all the men i ever saw, i never saw any man more learned, or more ingenious, having obtained the whole body of learning; and is not only the splendor and ornament of venice or italy, but of the whole world." the reference is to fra paolo sarpi, better known as the historian of the council of trent. sarpi was himself known to gilbert. { } his relations with gilbert are set forth in the memoir prefixt to the edition of his works, _opere di fra paolo sarpi, servita_ ... in helmstat, mdcclxi, p. . "fino a questi giorni continuava il sarpi a raccorre osservazioni sulla declinazione dell' ago calamitato; e poi ch' egli, atteso il variare di tal declinazione, assurdità alcuna non trovava riguardo al pensamento dell' inglese guglielmo gilberto, cioè, che l'interno del nostro globo fosse gran calamita...." here follows a quotation from a letter of sarpi to lescasserio: "... unde cuspidem trahi a tanta mole terrena, quæ supereminet non absurde putavit gullielmus gilbertus, et in eo meridiano respicere recta polum, cave putes observatorem errasse. est vir accuratissimus, et interfuit omnibus observationibus, quas plures olim fecimus, et aliquas in sui gratiam, et cum arcubus vertici cupreo innitentibus, et cum innatantibus aquæ, et cum brevibus, et cum longis, quibus modis omnibus et hierapoli usus suit." sarpi had correspondence with gilbert, bacon, grotius, and casaubon. he also wrote on magnetism and other topics _in materia di fisica_, but these writings have perisht. he appears to have been the first to recognize that fire destroyed the magnetic properties. (see _fra paolo sarpi, the greatest of the venetians_ by the rev. alexander robertson, london, ; see also the notice of sarpi in park benjamin's _intellectual rise in electricity_.) [ ] page , line . page , line .: _r. m. paulus venetus_. see preceding note. [ ] page , line . page , line .: _franciscus rueus_.--francois de la rue, author of _de gemmis aliquot_ ... (paris, ). amongst other fables narrated by rueus is that if a magnet is hung on a balance, when a piece of iron is attracted and adheres to the magnet, it adds nothing to the weight! [ ] page , line . page , line .: _serapio_.--this account of the magnetic mountains will be found in an early pharmacology printed in (argentorati, g. ulricher andlenus), with the title "in hoc volumine continetur insignium medicorum joan. serapionis arabis de simplicibus medicinis opus præclarum et ingens, averrois arabis de eisdem liber eximius, rasis filius zachariæ de eisdem opusculum perutile." it was edited by otho brunsels. achilles p. gasser, in his appendix to the augsburg edition of peregrinus, gives a reference to serapio mauritanus, parte , cap. , libri _de medicinis compositis._ [ ] page , line . page , line .: _olaus magnus_. see note to p. . [ ] page , line . page , line .: _hali abas_.--a reference is given in gasser's ( ) edition of peregrinus to haliabbas arabs, lib. , _practicæ_ cap. , _regalis dispositionis medicinæ_. the passage to which gilbert refers is found in the volume _liber totius medicinæ necessaria c[=o]tinens ... quem haly filius abbas ... edidit ... et a stephano ex arabica lingua reductus_. (lugd., , to.) liber primus. practice, cap xlv. _de speciebus lapidum_, § . "lapis magnetes filis e [=v]tute sadenego: & aiunt q[=m] si teneat^r in manu mitigat [=q] sunt in pedib^s ipis dolores ac spasm[=u]." mr. a. g. ellis identifies the noun _sadenegum_ as a latin corruption of the arabic name of hæmatite, _shâdanaj_. [ ] page , line . page , line .: _pictorius_.--his poem was publisht at basel, . see also note on marbodæus, p. , line , below. [ ] page , line . page , line .: _albertus magnus_.--albertus, the celebrated archbishop of ratisbon, is responsible for propagating sundry of the myths of the magnet; and gilbert never loses a chance of girding at him. { } the following examples are taken from the treatise _de mineralibus et rebus metallicis_ (liber ii. _de lapidibus preciosis_), venet., . p. . "et quod mirabile videtur multis his lapis [adamas] quando magneti supponitur ligat magnetem et non permittit ipsum ferrum trahere." p. . "vnctus aut[~e] lapis alleo non trahit, si superponitur ei adamas iterum non attrahit, ita quod paruus adamas magn[~u] ligat magnet[~e]. inventus aut[~e] est nostris t[~e]poribus magnes qui ab uno angulo traxit ferr[~u] et ab alio fugavit, et hunc aristot. ponit aliud genus esse magnetis. narrauit mihi quidam ex nostris sociis experim[~e]tator quod uidit federicum imperatorem habere magnetem qui non traxit ferrum, sed ferrum uiceuersa traxit lapidem." the first edition of this work _de mineralibus_ appears to have been publisht in venice as a folio in . [ ] page , line . page , line . _gaudentius merula_.--this obscure passage is from liber iiii., cap. xxi., _lapides_, of the work _memorabilium gaudentii merulæ..._ (lugd., ), where we find: "qui magneti vrsæ sculpserit imaginem, quãdo luna melius illuc aspiciat, & filo ferreo susp[~e]derit, compos fiet vrsæ cælestis virtutis: verùm cum saturni radiis vegetetur, satius fuerit eam imaginem non habere: scribunt enim platonici malos dæmones septentrionales esse" (p. ). "trahit autem magnes ferrum ad se, quod ferro sit ordine superior apud vrsum" (p. ). the almost equally obscure passage in the _de triplici vita_ of marsiglio ficino (basil., ) runs: "videmus in specula nautarum indice poli libratum acum affectum in extremitate magnete moueri ad vrsam, illuc uidelicet trahente magnete: quoniam & in lapide hoc præualet uirtus vrsæ, & hinc transfertur in ferrum, & ad vrsam trahit utrunq;. virtus autem eiusmodi tum ab initio infusa est, tum continue vrsæ radijs uegetatur, forsitan ita se habet succinum ad polum alterum & ad paleas. sed dic interea, cur magnes trahit ubiq; ferrum? non quia simile, alioquin & magnetem magnes traheret multo magis, ferrum[=q]; ferr[=u]: non quia superior in ordine corporum, imò superius est lapillo metallum ... ego autem quum hæc explorata hactenus habuissem admodum gratulabar, cogitabam[=q]; iuuenis adhuc magneti pro uiribus inscluperet (_sic_) coelestis vrsæ figuram, quando luna melius illuc aspiciat, & ferro t[=u]c filo collo suspendere. sperabam equidem ita demum uirtutis me sideris illius compotem fore," &c. (p. ). [ ] page , line . page , line . _ruellius_.--joannes ruellius wrote a herbal _de natura stirpium_, paris, , which contains a very full account of amber, and a notice of the magnet (p. ) and of the fable about garlic. but on p. of the same work he ridicules plutarch for recording this very matter. [ ] page , line . page , line . _marbodæus gallus_.--this rare little book is entitled _marbodei galli poetæ vetustissimi de lapidibus pretiosis enchiridion_. it was printed at paris in . the freiburg edition, also of , has the commentaries of pictorius. the poem is in latin hexameters. after a preface of twenty-one lines the virtues of stones are dealt with, the paragraph beginning with a statement that evax, king of the arabs, is said to have written to nero an account of the species, names and colours of stones, their place of origin and their potencies; and that this work formed the basis of the poem. the alleged magical powers of the magnet are recited in caput i., _adamas_. caput xliii., _magnes_, gives further myths. { } the commentary of pictorius gives references to earlier writers, pliny, dioscorides, bartholomæus anglicus, solinus, serapio, and to the book _de lapidibus_ erroneously ascribed to aristotle. the following is a specimen of the poem of marbodeus: _magnetes lapis est inuentus apud trogloditas,_ _qu[=e] lapid[=a] genetrix nihilominus india mittit._ _hic ferruginei cognoscitur esse coloris,_ _et ui naturæ uicinum tollere ferrum._ _ededon magus hoc primum ferè dic[=i]tur usus,_ _conscius in magica nihil esse potentius arte._ _post illum fertur famosa uenefica circe_ _hoc in præstigijs magicis specialiter usa._ this poem was reprinted ( ) in migne's _patrologia_. in johann beckmann issued an annotated variorum edition of marbodeus (_marbodi liber lapidvm sev de gemmis_..., göttingæ, ), in which there is a bibliography of the poem, the first edition of which appears to have been publisht in , at vienna, thirteen other editions being described. beckmann adds many illustrative notes, and a notice of the arabian evax, who is supposed to have written the treatise _de lapidibus_. not the least curious part is a french translation alleged to have been written in , of which chap. xix. on the magnet begins thus: magnete trovent trogodite, en inde e precieus est ditte. fer resemble e si le trait, altresi cum laimant fait. dendor lama mult durement. qi lusoit a enchantement. circe lus a dot mult chere, cele merveillose forciere, &c. [ ] page , line . page , line . _echeneidis._--the _echeneis_, or sucking-fish, reputed to have magical or magnetic powers, is mentioned by many writers. as an example, see fracastorio, _de sympathia et antipathia_, lib. i., cap. , _de echineide, quomodo firmare nauigia possit_ (giunta edition, venet., , p. ). for other references to the _echeneis_ see gaudentius merula (_op. citat._) p. . also dr. walter charleton, _physiologia epicuro gassendo-charltoniana_ (lond., ), p. . compare p. , line . [ ] page , line . page , line . _thomas hariotus_, etc.--the four englishmen named were learned men who had contributed to navigation by magnetic observations. harriot's account of his voyage to virginia is printed in hakluyt's _voyages_. robert hues (or hood) wrote a treatise _on globes_, the latin edition of which appeared in (dedicated to sir walter raleigh), and the english edition in . it was republisht by the hakluyt society, . edward wright, the mathematician and writer on navigation, also wrote the preface to gilbert's own book. abraham kendall, or abram kendal was "portulano," or sailing-master of sir robert dudley's ship the _bear_, and is mentioned in dudley's _arcano del mare_. on the return of dudley's expedition in , he joined drake's last expedition, which sailed that year, and died on the same day as drake himself, january, . (see _hakluyt_, ed. , iv., p. .) [ ] page , line . page , line . _guilielmus borough._--borough's book has the title: _a discours of the variation of the cumpas, or magneticall { } needle. wherein is mathematically shewed, the manner of the obseruation, effectes, and application thereof, made by w. b._ and is to be annexed to _the newe attractive_ of r. n., (london). [ ] page , line . page , line . _guilielmus barlo_.--archdeacon william barlowe (author, in , of the _magneticall aduertisements_) wrote in a little work called _the navigators supply_. it gives a description of the ordinary compass, and also one of a special form of meridian compass provided with sights for taking the bearings by the sun. [ ] page , line . page , line . _robertus normannus_. see note to p. . [ ] page , line . page , line . _illo fabuloso plinij bubulco_.--the following is pliny's account from philemon holland's english version of (p. ): "as for the name magnes that it hath, it tooke it (as _nicander_ saith) of the first inventor and deviser thereof, who found it (by his saying) upon the mountaine ida (for now it is to be had in all other countries, like as in spaine also;) and (by report) a neat-heard he was: who, as he kept his beasts upon the aforesaid mountaine, might perceive as he went up and downe, both the hob-nailes which were on his shoes, and also the yron picke or graine of his staffe, to sticke unto the said stone." [ ] page , line . page , line . _differentiæ priscis ex colore_.--pliny's account of the loadstones of different colours which came from different regions is mainly taken from sotacus. the white magnet, which was friable, like pumice, and which did not draw iron, was probably simply magnesia. the blue loadstones were the best. see p. of holland's translation of pliny, london, . st. isidore (_originum seu etymologiarum_, lib. xvi., cap. ) says: "omnis autem magnes tanta melior est, quanto [magis] cæruleus est." [ ] page , line . page , line . _suarcebergo ... snebergum & annæbergum_.--in the stettin editions of and these are spelled _swarcebergs ... schnebergum & annebergum_. the cordus given as authority for these localities is valerius cordus, the commentator on dioscorides. [ ] page , line . page , line . _adriani gilberti viri nobilis_.--"adrian gylbert of sandridge in the countie of devon, gentleman" is the description of the person to whom queen elizabeth granted a patent for the discovery of a north-west passage to china. see hakluyt's _voyages_, vol. iii., p. . [ ] page , line . page , line . _dicitur a græcis_ [greek: êraklios].--the discussion of the names of the magnet in different languages by gilbert in this place is far from complete. he gives little more than is to be found in pliny. for more complete discussions the reader is referred to buttmann, _bemerkungen über die benennungen einiger mineralien bei den alten, vorzüglich des magnetes und des basaltes_ (musæum der alterthumswissenschaft, bd. ii., pp. - , and - , ); g. fournier, _hydrographie_ (livre xi., chap. i, ); ulisse aldrovandi, _musæum metallicum_ (bononiæ, , lib. iv., cap. , p. ); klaproth, _lettre à m. le baron a. de humboldt, sur l'invention de la boussole_, paris, ; t. s. davies, _the history of magnetical discovery_ (thomson's _british annual_, , pp. - ); th. henri martin, _de l'aimant, de ses noms divers et de ses variétés suivant les anciens_ (mémoires présentés par divers savants a l'academie des inscriptions et belles-lettres, i^{re} série, t. vi., i^{re} partie, ); g. a. palm, _der magnet in alterthum_ (programm des k. württembergischen seminars maulbronn, stuttgart, { } ). of these works, those of klaproth and of martin are by far the most important. klaproth states that in modern greek, in addition to the name [greek: magnêtis], the magnet also has the names [greek: adamas] and [greek: kalamita]. the former of these, in various forms, _adamas_, _adamant_, _aimant_, _yman_, and _piedramon_, has gone into many languages. originally the word [greek: adamas] (the unconquered) was applied by the greeks to the hardest of the metals with which they were acquainted, that is to say, to hard-tempered iron or steel, and it was subsequently because of its root-signification also given by them to the diamond for the same reason; it was even given to the henbane because of the deadly properties of that plant. in the writings of the middle ages, in st. augustine, st. isidore, marbodeus, and even in pliny, we find some confusion between the two uses of _adamas_ to denote the loadstone as well as the diamond. certainly the word _adamas_, without ceasing to be applied to the diamond, also designated the loadstone. at the same time (says martin) the word _magnes_ was preserved, as pliny records, to designate a loadstone of lesser strength than the _adamas_. on the other hand, the word _diamas_, or _deamans_, had already in the thirteenth century been introduced into latin to signify the diamond as distinguisht from the magnet. _adamas_ was rendered _aymant_ in the romance version of the poem of marbodeus on stones (see beckmann's variorum edition of , p. ), and in this form it was for a time used to denote both the magnet and the diamond. then it gradually became restricted in use to the stone that attracts iron. some confusion has also arisen with respect to the hebrew name of the magnet. sir w. snow harris makes the following statement (_magnetism_, p. ): "in the talmud it [the loadstone] is termed _achzhàb'th_, the stone which attracts; and in their ancient prayers it has the european name _magn[=e]s_." on this point dr. a. löwy has furnisht the following notes. the loadstone is termed in one of the talmudical sections and in the midrash, _eben shoebeth_ (lapis attrahens). this would of course be written [hebrew: 'bn shw'bt]. omitting the [hebrew: w] which marks the participial construction, the words would stand thus: [hebrew: 'bn sh'bt] a person referring to buxtorf's _lexicon_ talmudicum would in the index look out for "lapis magnesius," or for "magnes." he would then, in the first instance, be referred to the two words already quoted. not knowing the value of the letters of the hebrew alphabet, he reads [hebrew: 'bn sh'bt] thus: [hebrew: 'kzsh'bt] achzhab'th. it is true that buxtorf has inserted in his _lexicon_ the vocable [hebrew: magniyseis], "corruptum ex gr. [greek: magnês, magnêtês, magnêtis], named after the asiatic city magnesia." he goes on to say, "inde achilles statius istum lapidem vocavit [greek: magnêsian lithon]. hinc [hebrew: 'bn hmgnjss chmshwk hbrzl]. lapis magnesius trahit ferrum." here he quotes from (sepher) ikkarem iv., cap. . kircher, in his _magnes, sive de arte magnetica_ (coloniæ, ), gives several other references to hebrew literature. others have supposed that the word [hebrew: chlmysh] _khallamish_, which signifies pebble, rock, or hard rock, to be used for the magnet. as to the other greek name, [greek: sidêritis], or [greek: lithos sidêritis] this was given not only to the loadstone but also to non-magnetic iron. in the _etymologicum magnum_ (under the word [greek: magnêtis]), and in photius (_quæst. amphiloch._, q. ), it is stated that the name _sideritis_ was given to the loadstone either because of its action on iron, or of its resemblance in aspect to iron, _or rather_, they say, _because the loadstone was originally found in the mines of this metal_. alexander of aphrodisias expressly says (_quætiones physicæ_, ii. ) that { } the loadstone appears to be nothing else than [greek: gê sidêritis], the earth which yields iron, or the earth of iron. [ ] page , line . page , line . _ab orpheo_.--the reference is to v. - of the [greek: lithika]. the passage, as given in abel's edition (berol., ), begins: [greek: tolma d' athanatous kai henêei meilissethai] [greek: magnêssêi, tên d' exoch' ephilato thousios arês,] [greek: houneken, hoppote ken pelasêi polioio sidêrou,] [greek: êute parthenikê terenochroa chersin helousa] [greek: êitheon sternôi prosptussetai himeroenti,] [greek: hôs hêg' harpazousa poti spheteron demas haiei] [greek: aps palin ouk ethelei methemen polemista sidêron.] [ ] page , line . page , line . _gallis aimant_.--the french word _aimant_, or _aymant_, is generally supposed to be derived from _adamas_. nevertheless klaproth (_op. citat._, p. ) suggests that the word _aimant_ is a mere literal translation into french of the chinese word _thsu chy_, which is the common name of the magnet, and which means _loving stone_, or _stone that loves_. all through the east the names of the magnet have mostly the same signification, for example, in sanskrit it is _thoumbaka_ (the kisser), in hindustani _tchambak_. [ ] page , line . page , line . _italis calamita_.--the name _calamita_, universal in italian for the magnet, is also used in roumanian, croatian, bosnian, and wendish. its supposed derivation from the hebrew _khallamîsh_ is repudiated by klaproth, who also points out that the use of [greek: kalamita] in greek is quite modern. he adds that the only reasonable explanation of the word _calamita_ is that given by father fournier (_op. citat._), who says: "ils (les marins français) la nomment aussi _calamite_, qui proprement en français signifie une _grenouille verte_, parce qu'avant qu'on ait trouvé l'invention de suspendre et de balancer sur un pivot l'aiguille aimantée, nos ancêtres l'enfermaient dans une fiole de verre demi-remplie d'eau, et la faisaient flotter, par le moyen de deux petits fétus, sur l'eau comme une grenouille." klaproth adds that he entirely agrees with the learned jesuit, but maintains that the word _calamite_, to designate the little green frog, called to-day _le graisset_, _la raine_, or _la rainette_, is essentially greek. for we read in pliny (_hist. nat._ lib. xxxii., ch. x.): "ea rana quam græci _calamiten_ vocant, quoniam inter arundines, fruticesque vivat, minima omnium est et viridissima." [ ] page , line . page , line . _anglis_ loadstone & adamant stone. the english term _loadstone_ is clearly connected with the anglo-saxon verb _loedan_, to lead, and with the icelandic _leider-stein_. there is no doubt that the spelling _lodestone_ would be etymologically more correct, since it means _stone that leads_ not _stone that carries a load_. the correct form is preserved in the word _lode-star_. the word _adamant_, from _adamas_, the mediæval word for both loadstone and diamond, also occurs in english for the loadstone, as witness shakespeare: "you draw me, you hard-hearted adamant but yet you draw not iron; for my heart is true as steel." _midsummer night's dream_, act ii, scene . [ ] page , line . page , { } line . _germanis magness_, & _siegelstein_. the stettin edition of reads _germanis_ magnetstein, _belgis_ seylsteen; while that of reads _germanis_ magnetstein, _belgis_ sylsteen. [ ] page , line . page , line . in this line the greek sentence is, in every known copy of the folio of , corrected in ink upon the text, [greek: thalês] being thus altered into [greek: thalês], and [greek: apomnemonuousi] into [greek: apomnemoneuousi]. four lines lower, brackets have been inserted around the words (lapidum specularium modo). these ink corrections must have been made at the printers', possibly by gilbert's own hand. they have been carried out as errata in the editions of and . the "facsimile" berlin reprint of has deleted them, however. other ink corrections on pp. , , , , , , and of the folio edition of are noted in due course. [ ] page , line . page , line . _lapis specularis_. this is the mediæval name for _mica_, but in elizabethan times known as talc or muscovy stone. cardan, _de rerum varietate_ (basil., , p. ), lib. xiiii., cap. lxxii., mentions the use of _lapis specularis_ for windows. [ ] page , line . page , line .: _germanis katzensilbar_ & _talke_.--in the editions of and this is corrected to _germanis_ katzensilber & talcke. goethe, in _wilhelm meister's travels_, calls mica "cat-gold." [ ] page , line . page , line . _integtum_ appears to be a misprint for _integrum_, which is the reading of editions and . [ ] page , line . page , line . [greek: mikrogê] _seu terrella_. although rounded loadstones had been used before gilbert's time (see peregrinus, p. of augsburg edition of , or baptista porta, p. , of english edition of ), gilbert's use of the spherical loadstone as a model of the globe of the earth is distinctive. the name _terrella_ remained in the language. in _pepys's diary_ we read how on october , , he "received a letter from mr. barlow with a terella." john evelyn, in his _diary_, july, , mentions a "pretty terella with the circles and showing the magnetic deviations." a terrella, ½ inches in diameter, was presented in by king charles i. to the royal society, and is still in its possession. it was examined in (see _phil. transactions_ for that year) by the society to see whether the positions of its poles had changed. in grew's _catalogue and description of the rarities belonging to the royal society and preserved at gresham college_ (london, , p. ) is mentioned a terrella contrived by sir christopher wren, with one half immersed in the centre of a plane horizontal table, so as to be like a globe with the poles in the horizon, having thirty-two magnet needles mounted in the margin of the table to show "the different respect of the _needle_ to the several _points_ of the _loadstone_." in sir john pettus's _fleta minor_, london, , in the _dictionary of metallick words_ at the end, under the word _loadstone_ occurs the following passage: "another piece of curiosity i saw in the hands of sir _william persal_ (since deceased also) _viz._, a _terrella_ or _load-stone_, of little more than _ inches diameter_, turned into a _globular form_, and all the _imaginery lines_ of our _terrestrial globe_, exactly drawn upon it: _viz._ the _artick _ and _antartick circles_, the _two tropicks_, the _two colures_, the _zodiack_ and _meridian_; and these _lines_, and the several _countryes_, artificially _painted_ on it, and all of them with their true _distances_, from the two _polar points_, and to find the truth of those _points_, he took two _little pieces_ of a _needle_, each of about _half_ { } _an inch in length_, and those he laid on the _meridian line_, and then with _brass compasses_, moved one of them towards the _artick_, which as it was moved, still raised it self at one end higher and higher, keeping the other end fixt to the _terrella_; and when it had compleated it journy to the very _artick points_, it stood upright upon that _point_; then he moved the other piece of _needle_ to the _antartick point_, which had its _elevations_ like the other, and when it came to the _point_, it fixt it self upon that _point_, and stood _upright_, and then taking the _terrella_ in my hand, i could perfectly see that the two _pieces_ of _needles_ stood so exactly one against the other, as if it had been one intire _long needle_ put through the _terrella_, which made me give credit to those who held, that there is an _astral influence_ that _darts_ it self through the _globe_ of _earth_ from _north_ to _south_ (and is as the _axel-tree_ to the _wheel_, and so called the _axis_ of the _world_) about which the _globe_ of the _earth_ is turned, by an _astral power_, so as what i thought _imaginary_, by this _demonstration_, i found _real_." [ ] page , line . page , line . the editions of and give a different woodcut from this: they show the terrella lined with meridians, equator, and parallels of latitude: and they give the compass needle, at the top, _pointing in the wrong direction_. [ ] page , line . page , line . the berlin "facsimile" reprint omits the asterisk here. [ ] page , line . page , line . _erectus_ altered in ink in the folio to _erecta_. but _erectus_ is preserved in editions and . in cap. iiii., on p. , both these stettin editions insert an additional cut representing the terrella a placed in a tub or vessel b floating on water. [ ] page , line . page , line . _variatione quad[=a]._ the whole of book iiii. is devoted to a discussion of the variation of the compass. [ ] page , line . page , line . _aquæ._--this curious use of the dative occurs also on p. , line . [ ] page , line . page , line . _videbis._--the reading _vibebis_ of the edition is an error. [ ] page , line . page , line . _theamedem._--for the myth about the alleged _theamedes_, or repelling magnet, see cardan, _de subtilitate_ (folio ed., , lib. vii., p. ). pliny's account, in the english version of (p. ), runs: "to conclude, there is another mountaine in the same Æthyopia, and not farre from the said zimiris, which breedeth the stone theamedes that will abide no yron, but rejecteth and driveth the same from it." martin cortes, in his _arte de nauegar_ (seville, ), wrote: "and true it is that tanxeades writeth, that in ethiope is found another kinde of this stone, that putteth yron from it" (eden's translation, london, ). [ ] page , line . page , line . _hic segetes, &c._--the english version of these lines from vergil's _georgics_, book i., is by the late mr. r. d. blackmore. [ ] page , line . page , line . _quale_, altered in ink in the folio text to _qualis_. the editions of and both read _qualis_. [ ] page , line . page , line . _rubrica fabrili_: in english _ruddle_ or _reddle_. see "sir" john hill, _a general natural history_, , p. . in the _de re metallica_ of entzelt (encelius), frankfurt, , p. , is a paragraph headed _de rubrica fabrili_, as follows: "rubrica fabrilis duplex { } est. à germanis añt utraque dicitur rottel, röttelstein, wie die zimmerleüt vnd steynmetzen brauchen. à græcis [greek: miltos tektonikê]. est enim alia nativa, alia factitia. natiua à germanis propriè dicitur berckrottel. haec apud nos est fossilis.... porro factitia est rubrica fabrilis, à germanis braunrottel, quæ fit ex ochra usta, ut theophrastus et dioscorides testantur." [ ] page , line . page , line . _in sussexia angliæ._--in camden's _britannia_ ( ) we read concerning the iron industry in the villages in sussex: "they are full of iron mines in sundry places, where, for the making and founding thereof, there be furnaces on every side; and a huge deal of wood is yearly burnt. the heavy forge-hammers, worked by water-power, stored in hammer-ponds, ceaselessly beating upon the iron, fill the neighbourhood round about, day and night, with continual noise." [ ] page , line . page , line . _in libro aristotelis de admirandis narrationibus._--the reference is to the work usually known as the _de mirabilibus auscultationibus_, cap. xlviii.: "fertur autem peculiarissima generatio esse ferri chalybici amisenique, ut quod ex sabulo quod a fluviis defertur, ut perhibent certe, conflatur. alii simpliciter lotum in fornace excoqui, alii vero, quod ex lotura subsedit, frequentius lotum comburi tradunt adjecto simul et pyrimacho dicto lapide, qui in ista regio plurimus reperiri fertur." (ed. didot, vol. ii., p. .) according to georgius agricola, the stone pyrimachus is simply iron pyrites. [ ] page , line . page , line . _vt in italia comi_, &c.--this is mostly taken from pliny. compare the following passage from philemon holland's translation ( ), p. : "but the most varietie of yron commeth by the meanes of the water, wherein the yron red-hot is eftsoones dipped and quenched for to be hardened. and verely, water only which in some place is better, in other worse, is that which hath ennobled many places for the excellent yron that commeth from them, as namely, bilbilis in spaine, and tarassio, comus also in italie; for none of these places have any yron mines of their owne, and yet there is no talke but of the yron and steele that commeth from thence." bilbilis is bambola, and tariassona the tarazona of modern spain. [ ] page , line . page , line . _quare vani sunt illi chemici._--gilbert had no faith in the alchemists. on pp. and he had poked fun at them for declaring the metals to be constituted of sulphur and quicksilver, and for pronouncing the fixed earth in iron to be sulphur. on p. he had denied their proposition that the differences between silver, gold, and copper could arise from proportions of their constituent materials; and he likewise denounced unsparingly the supposed relation between the seven metals and the seven planets. he now denounces the vain dreams of turning all metals into gold, and all stones into diamonds. later he rejects as absurd the magnetic curing of wounds. his detachment from the pseudo-science of his age was unique if not complete. [ ] page , line . page , line . _petro-coriis, & cabis biturgibus._--the petro-corii were a tribe in the neighbourhood of perigord; the cubi biturges another in that of bourges. [ ] page , line . page , line . pliny's account, as translated by p. holland (ed. , p. ), runs thus: "of all mines that be, the veine of this mettall is largest, and spreadeth it selfe into most lengths every way: as we may see in that part of biscay that coasteth along the sea, and upon which the ocean beateth: where there { } is a craggie mountaine very steep and high, which standeth all upon a mine or veine of yron. a wonderfull thing, and in manner incredible, howbeit, most true, according as i have shewed already in my cosmographie, as touching the circuit of the ocean." [ ] page , line . page , line . _quas clampas nostri vocant._--the name _clamp_ for the natural kiln formed by heaping up the bricks, with ventilating spaces and fuel within the heap, is still current. [ ] page , line . page , line . _pluebat in taurinis ferrum._--the occurrence is narrated by scaliger, _de subtilitate_, exercitat. cccxxiii.: "sed falsò lapidis pluviam creas tu ex pulvere hausto à nubibus, atque in lapidem condensato. at ferrum, quod pluit in taurinis, cuius frustum apud nos extat, qua ex fodina sustulit nubes? tribus circiter annis antè, quàm ab rege provincia illa recepta esset, pluit ferro multis in locis, sed raris" (p. , editio lutetiæ, ). "during the latter ages of the roman empire the _city_ of augusta taurinorum seems to have been commonly known (as was the case in many instances in transalpine gaul) by the name of the tribe to which it belonged, and is called simply taurini in the itineraries, as well as by other writers, hence its modern name of torino or turin" (smith's _dictionary of greek and roman geographies_, p. ). there exists a considerable literature respecting falls of meteors and of meteoric iron. livy, plutarch, and pliny all record examples. see also _remarks concerning stones said to have fallen from the clouds_, by edward king (london, ); chladni, _ueber den ursprung der von pallas gefundenen und anderer ihr ähnlicher eisenmassen_ (riga, ); _philosophical transactions_, vol. lxxviii., pp. and ; vol. lxxxv., p. ; vol. xcii., p. ; humboldt's _cosmos_, vol. i. (p. of london edition, ); c. rammelsberg, _die chemische natur der meteoriten_ (berlin, ); maskelyne, _some lecture-notes on meteorites_ printed in _nature_, vol. xii., pp. , , and , . maskelyne denominates as _siderites_ those meteorites which consist chiefly of iron. they usually contain from to per cent. of iron, often alloyed with nickel. this meteoric iron is sometimes so pure that it can at once be forged by the smith. an admirable summary of the whole subject is to be found in l. fletcher's _an introduction to the study of meteorites_, publisht by the british museum (nat. hist.), london, . [ ] page , line . page , line . _vt cardanus ... scribit._--the passage runs: "vidimus anno mdx cum cecidisset è coelo lapides circiter mcc in agrum fluvio abduæ conterminum, ex his unum cxx pondo, alium sexaginta delati fuerunt ad reges gallor[~u] satrapes, plurimi: colos ferrugineus, durities eximia, odor sulphureus" (cardan, _de rerum varietate_, lib. xiiii., cap. lxxii.; basil., , p. ). [ ] page , line . page , line . _aut stannum, aut plumbum album._ although most authorities agree in translating _plumbum album_ or _plumbum candidum_ as "tin" (which is unquestionably the meaning in such examples as pliny's _nat. hist._, xxxiv. , and iv. ; or strabo, iii. ), nevertheless it is certain that here _plumbum album_ is not given as a synonym of _stannum_ and therefore is not _tin_. that gilbert meant either spelter or pewter is pretty certain. he based his metallic terms mainly upon encelius (christoph entzelt) whose _de re metallica_ was published at frankfurt in . from this work are taken the following passages: { } p. . _de plumbo candido._ cap. xxxi. "veluti plumbum nigr[~u] uocatur à germanis blei simpliciter, od' schwartzblei: ita plumb[~u] candid[~u] ab his uocatur weissblei, od' ziñ. impropriè autem plumbum hoc nostrum candidum ziñ, stannum dicitur. et non sunt idem, ut hactenus voluerunt, stannum et plumbum candidum, unser ziñ. aliud est stannum, de quo mox agemus: et aliud plumbum candidum nostrum, unser ziñ, quod nigro plumbo quasi est quiddã purius et perfectius...." p. . _de stanno._ cap. xxxii. "in præcedenti capite indicauimus aliud esse stannum, aliud esse plumb[~u] candid[~u]. illa ergo definitio plumbi candidi, dess zinnes, etiã apud chimistas nõ de stanno, sed de plumbo candido (ut mihi uidetur) intelligenda est, cum dicunt: stannum (es soll heyssen plumbum candidum) est metallicum album, non purum, lividum...." p. . "sic uides stannum, secundum serapionem, metallicum esse quod reperitur in sua propria uena, ut forsitan apud nos bisemut[~u]: ecõtra nostr[~u] candid[~u] plumb[~u], est plinij candid[~u] plumb[~u], das zin, quod cõflatur ut plumbum nigrum, ex pyrite, galena, et lapillis nigris. deinde uides stannum plinio esse quiddã de plumbo nigro, nempe primum fluorem plumbi nigri, als wann man vnser bley ertz schmeltzet, das erst das do fleüsset, zwäre plinio stannum. et hoc docet plinius adulterari pl[~u]bo candido, mit vnserm zinn, vnd wann du ihm recht nachdenckest, daruon die kannen gemacht werden, das man halbwerck heist.... o ir losen vngelerten, vnckenbrenner. stannum proculdubio arabis metallum est preciosius nostro candido plumbo: sicuti apud nos bisemuthum quiddam plumbo preciosius." [ ] page , line . page , line . _venas ... venis._--it is impossible to give in english this play on words between veins of ore and veins of the animal body. [ ] page , line . page , line . _quem nos verticitatem dicimus._--see the notes on gilbert's glossary, _ante_. the word verticity remained in the language. on p. of joseph glanvill's _vanity of dogmatizing_ (lond., ) we read: "we believe the _verticity_ of the _needle_, without a certificate from the _dayes_ of _old_." [ ] page , line . page , line . _nos verò diligentiùs omnia experientes._--the method of carefully trying everything, instead of accepting statements on authority, is characteristic of gilbert's work. the large asterisks affixed to chapters ix. x. xi. xii. and xiii. of book i. indicate that gilbert considered them to announce important original magnetical discoveries. the electrical discoveries of book ii., chapter ii., are similarly distinguished. a rich crop of new magnetical experiments, marked with marginal asterisks, large and small, is to be found in book ii., from chapter xv. to chapter xxxiv.; while a third series of experimental magnetical discoveries extends throughout book iii. [ ] page , line . page , line . _verticem._--the context and the heading of the chapter appear to require _verticitatem_. all editions, however, read _verticem_. [ ] page , line . page , line . _gartias ab horto._--the passage from gartias ab horto runs as follows in the italian edition of , _dell' historia dei semplici aromati._... di don garzia dall' horto, medico portughese, ... venezia mdcxvi., p. . "nè meno è questa pietra velenosa, si come molti hanno tenuto; imperoche le genti di queste bande dicono che la calamita presa per bocca, però in poca { } quantità, conserva la gioventù. la onde si racconta, che il re di zeilan il vecchio' s'haveva fatto fare tutti i vasi, dove si cocevano le vivãde per lui, di calamita. et questo lo disse à me colui proprio, che fu à questo officio destinato." [ ] page , line . page , line . _plutarchus & c. ptolemæus._--the garlick myth has already been referred to in the note to p. . the originals are plutarch, _quæstiones platonicæ_, lib. vii., cap. , § ; c. ptolemæus, _opus quadripartitum,_ bk. i., cap. . the english translation of the latter, by whalley (london, ), p. , runs: "for if the _loadstone_ be _rubbed_ with _garlick_, the _iron will not be drawn by it_." [ ] page , line . page , line . _medici nonnulli._--this is apparently a reference to the followers of rhazes and paracelsus. the argument of gilbert as to the inefficacy of powdered loadstones is reproduced more fully by william barlowe in his _magneticall aduertisements_ ( , p. ), as follows: "it is the goodnesse of the _loadstone_ ioyned with a fit forme that will shew great force. for as a very good forme with base substance can doe but very litle, so the substance of the _loadstone_ bee it neuer so excellent, except it haue some conuenient forme, is not auaileable. for example, an excellent _loadstone_ of a pound waight and of a good fashion, being vsed artificially, may take vp foure pounds of iron; beate it into small pouder, and it shall bee of no force to take vp one ounce of iron; yea i am very well assured that halfe an ounce of a loadstone of good fashion, and of like vertue will take vp more then that pound will doe being beaten into powder. whence (to adde this by the way) it appeareth manifestly, that it is a great error of those physitions and surgeons, which to remedy ruptures, doe prescribe vnto their patients to take the pouder of a _loadstone_ inwardly, and the small filing of iron mingled in some plaister outwardly: supposing that herein the _magneticall_ drawing should doe great wonders." [ ] page , line . page , line . _nicolaus in emplastrum divinum._...--nicolaus myrepsus is also known as præpositas. in his _liber de compositione medicamentorum_ (ingoldstat, , to) are numerous recipes containing loadstone: for example, recipe no. , called "esdra magna," is a medicine given for inflammation of the stomach and for strangury, compounded of some forty materials including "litho demonis" and "lapis magnetis." the _emplastrum divinum_ does not, however, appear to contain loadstone. in the english tractate, _præpositas his practise, a worke ... for the better preservation of the health of man. wherein are ... approved medicines, receiptes and ointmentes. translated out of latin in to english by_ l. m. (london, , to), we read on p. , "an emplaister of d. n. [doctor nicolaus] which the pothecaries call divinum." this contains litharge, bdellium, and "green brasse," but no loadstone. luis de oviedo in his treatise _methodo de la coleccion y reposicion de las medicinas simples_, edited by gregorio gonçalez, boticario (madrid, ), gives (p. ) the following: "emplasto de la madre. _recibe_: nuezes moscadas, clauos, cinamono, artemisia, piedraimon. de cada uno dos onças.... entre otras differencias que ay de piedraiman se hallan dos. vna que por la parte que mira al septentrion, atrae el hierro, por lo quel se llama magnes ferrugineus. y otra que atrae la carne, a la qual llaman magnes creaginus." an "emplastrum sticticum" containing amber, mummy, loadstone, { } hæmatite, and twenty other ingredients, and declared to be "vulnerum ulcerumque telo inflictorum sticticum emplastrum præstantissimum," is described on p. of the _basilica chimica_ of oswaldus crollius (frankfurt, ). [ ] page , line . page , line . _augustani ... in emplastrum nigrum_....--amongst the physicians of the augsburg school the most celebrated were adolphus occo, ambrosio jung, and gereone seyler. this particular reference is to the _pharmacopoeia augustana_ ... _a collegio medico recognita_, published at augsburg, and which ran through many editions. the recipe for the "_emplastrum nigrum vulgo stichpflaster_" will be found on p. of the seventh edition ( - ). the recipe begins with oil of roses, colophony, wax, and includes some twenty-two ingredients, amongst them mummy, dried earthworms, and two ounces _lapidis magnetis præparati_. the recipe concludes: "fiat emplastrum secundùm artem. perquàm efficax ad recentia vulnera et puncturas, vndè denominationem habet." the volume is a handsome folio not unlike gilbert's own book, and bears at the end of the prefatory address _ad lectorem_ identically the same _cul de lampe_ as is found on p. of _de magnete_. the contradictions as to the alleged medicinal virtues of loadstone are well illustrated by galen, who in his _de facultatibus_ says that loadstone is like hæmatite, which is astringent, while in his _de simplici medicina_ he says it is purgative. [ ] page , line . page , line . _paracelsus in fodicationum emplastrum_.--paracelsus's recipe for a plaster against stab-wounds is to be found in _wundt vund leibartznei_ ... d. theoph. paracelsus (frankf., , pp. - ). [ ] page , line . page , line . _ferri vis medicinalis_.--this chapter on the medicinal virtues of iron is a summary of the views held down to that time. those curious to pursue the subject should consult waring's _bibliotheca therapeutica_ (london, ). nor should they miss the rare black-letter quarto by dr. nicholas monardus, of seville, _joyfull newes out of the new-found worlde_, translated by john frampton (london, ), in which are recited the opinions of galen, rhazes, avicenna, and others, on the medicinal properties of iron. in addition to the views of the arabic authors, against whom his arguments are directed, gilbert discusses those of joannes manardus, curtius, and fallopius. the treatise of manardus, _epistolarum medicinalium libri viginti_ (basil., ), is a _résumé_ of the works of galen and the arabic physicians, but gives little respecting iron. curtius (nicolaus) was the author of a book, _libellus de medicamentis præparatibus et purgantibus_ (giessæ cattorum, ). the works of fallopius are _de simplicibus medicamentis purgentibus tractatus_ (venet., , to), and _tractatus de compositione medicamentorum_ (venet., , to). [ ] page , line . page , line . _quorundã arabum opiniones_.--the arabian authorities referred to here or elsewhere by gilbert are: _albategnius_ (otherwise known as machometes aractensis), muhammad ibn j[=a]bir, _al-batt[=a]n[=i]_. _avicenna_ (otherwise abohali). abou-'ali al-'hoséin ben-'abd-allah ibn-sinâ, or, shortly, _ibn sîna._ _averroes._ muhammad ibn ahmed ibn-roschd, _abou al-walíd._ _geber._ ab[=u] m[=u]s[=a] j[=a]bir ibn haiy[=a]n, _al-tars[=u]si._ _hali abas._ 'alí ibn al-'abbás, _al majúsi_. { } _rhazes_, or _rasis_. muhammad ibn zakar[=i]y[=a]. _serapio._ yuhanná ibn sarapion. _thebit ben-kora_ (otherwise thabit ibn corrah). ab[=u] thabit ibn kurrah, _al harrani._ [ ] page , line .: page , line . _electuarium de scoria ferri descriptum à raze._--rhazes or rasis, whose arabic name was muhammad ibn zakar[=i]y[=a], wrote _de simplicibus, ad almansorem._ in chap. of this work he gives a recipe for a stomachic, which includes fennel, anise, origanum, black pepper, cinammon, ginger, and iron slag. in the splendid folio work of rhazes publisht at venice in , with the title _habes candide lector contin[~e]tem rasis_, libri ultimi, cap. , under the heading _de ferro,_ are set forth the virtues of iron slag: "virtus scorie est sicut virtus scorie [a]eris sed debilior in purgãdo: et erugo ferri est stiptica: et c[~u] superpositur retinet fluxus menstruor[~u].... ait paulus: aqua in qua extinguitur ferr[~u] calens.... dico: certificatus sum experientia [~q] valet contra emorryodas diabetem et fluxum menstruorum." [ ] page , line .: page , line . _paulus._--this is not fra paolo sarpi, nor marco polo, nor paulus jovius the historian, nor paulus nicolettus venetus, but paulus aeginæ. [ ] page , line .: page , line . _sed malè avicenna._--the advice of avicenna to administer a draught containing powdered loadstone, reads as follows in the giunta edition (venice, ): lib. ii., cap. , p. . "magnes quid est? est lapis qui attrahit ferrum, quum ergo aduritur, fit hæmatites, & virtus ejus est sicut virtus illius.... datur in potu [ad bibitionem limaturæ ferri, quum retinetur in ventre scoria ferri. ipse enim extrahit] ipsam, & associatur ei apud exitum. et dicitur, quando in potu sumuntur ex eo tres anulusat cum mellicrato, educit solutione humorem grossum malum." the passage is identical with that in the venetian edition of , in both of which the liquid prescribed is mellicratus--mead. gilbert says that the iron is to be given in juice of _mercurialis_. here he only follows matthiolus, who, in his _commentaries on dioscorides_, says (p. of the basil. edition of ): "sed (vt idem auicenna scribit) proprium hujusce ferrei pharmaci antidotum, est lapis magnes drachmæ pondere potus, ex mercurialis, vel betæ succo." serapio, in his _de simplicibus medicinis_ (brunfels' edition, argentorati, ), p. , refers to galen's prescription of iron scoriæ, and under the article _de lapide magnetis_, p. , quotes dioscorides as follows: "et uirtus huius lapidis est, ut quãdo dantur in potu duo onolosat ex eo c[~u] melicrato, laxat humores grossos." the original passage in dioscorides, _de materia medica,_ ch. (spengel's edition of ) runs: "[greek: tou de magnêtou lithou aristos estin ho ton sidêron eucherôs helkôn, kai tên chroan kuanizôn, puknos te kai ouk agan barus. dunamin de echei pachous agôgon didomenos meta melikratou triôbolou baros; enioi de touton kaiontes anti haimatitou pipraskousin.]." in the frankfurt edition of dioscorides, translated by ruellius ( ), the passage is: "magnes lapis optimus est, qui ferrum facile trahit, colore ad coeruleum uergente, densus, nec admodum gravis. datur cum aqua mulsa, trium obolorum pondere, ut crassos humores eliciat. sunt qui magnetem cremat[=u] pro hæmatite vendant...." in the _scholia_ of joannes lonicerus upon dioscorides _in dioscoridæ { } anazarbei de re medica libros a virgilio marcello versos, scholia nova, ioanne lonicero autore_ (marburgi, , p. ), occurs the following: "_de recremento ferri._ cap. xlix. "[greek: skôria sidêrou]. scoria vel recrementum ferri. quæ per ignem à ferro et cupro sordes separantur ac reijciuntur, et ab aliis metallis [greek: skôria] uocantur. omnis scoria, maxime uero ferri exiccat. acerrimo aceto macerauit galenus ferri scoriam, ac deinde excocto, pharmacum efficax confecit ad purulentas quæ multo tempore uexatæ erant, aures, admirando spectantium effectu. ardenti scoria uel recrementum [greek: helkusma], inquit galenus." see also the _enarrationes eruditissimæ_ of amatus lusitanus (venet., ), pp. and , upon iron and the loadstone. [ ] page , line . page , line . _eijcitur_ for _ejicitur_. [ ] page , line . page , line . _ut cardanus philosophatur._--cardan's nonsense about the magnet feeding on iron is to be found in _de subtilitate_, lib. vii. (basil., , p. ). [ ] page , line . page , line . _ferramenta ... in usum navigantium._--compare marke ridley's _a short treatise of magneticall bodies and motions_ (lond., ), p. _a _ in the _preface magneticall_, where he speaks of the "iron-workes" used in building ships. the phraseology of marke ridley throws much light on the latin terms used by gilbert. [ ] page , line . page , line . _vruntur;_ changed in ink to _vrantur_ in the folio of ; but _uruntur_ appears in the editions of and . [ ] page , line . page , line . _virumque;_ altered in ink to _virunque_ in all copies of the folio edition of . [ ] page , line . page , line . _ad tantos labores exantlandos._--pumping, as it was in mining before the invention of the steam engine, may best be realized by examining the woodcuts in the _de re metallica_ of georgius agricola (basil., froben, ). [ ] page , line . page , line . _quingentas orgyas._--gilbert probably had in his mind the works of the rorerbühel, in the district of kitzbühl, which in the sixteenth century had reached the depth of , feet. see humboldt's _cosmos_ (lond., , vol. i., p. ). [ ] page , line . page , line . _glis._--this word, here translated _grit_, does not appear to be classical latin; it may mean _ooze or slime_. [ ] page , line . page , line . _motus igitur ... quinque._ the five kinds of magnetic motions correspond in fact to the remaining sections of the book; as follows: _coitio_, book ii.; _directio_, book iii.; _variatio_, book iv.; _declinatio_, book v.; and _revolutio_, book vi. [ ] page , line . page , line . _jofrancus offusius._--the reference is to the treatise _de divina astrorum faculitate_ of johannes franciscus offusius (paris, ). [ ] page , line . page , line . _græci vocant_ [greek: êlektron], _quia ad se paleas trahit._ in this discussion of the names given to amber, gilbert apparently conceives [greek: êlektron] to be derived from the verb [greek: helkein]; which is manifestly a doubtful etymology. there has been much discussion amongst philologists as to the derivation of [greek: êlektron] or [greek: êlektron], and its possible connection with the word [greek: êlektôr]. this discussion has been somewhat obscured by the circumstance that the greek authors unquestionably used [greek: êlektron] (and the latins their word _electrum_) in two different significations, some of them using these words to mean amber, others to mean a shining { } metal, apparently of having qualities between those of gold and silver, and probably some sort of alloy. schweigger, _ueber das elektron der alten_ (greifswald, ), has argued that this metal was indeed no other than platinum: but his argument partakes too much of special pleading. those who desire to follow the question of the derivation of [greek: êlektron] may consult the following authorities: j. m. gessner, _de electro veterum_ (commentt. soc. reg. scientt. goetting., vol. iii., p. , ); delaunay, _mineralogie der alten_, part ii., p. ; buttmann, _mythologus_ (appendix i., _ueber das elektron_), vol. ii., p. , in which he adopts gilbert's derivation from [greek: helkein]; beckmann, _ursprung und bedeutung des bernsteinnamens elektron_ (braunsberg, ); th. henri martin, _du succin, de ses noms divers et de ses variétés suivant les anciens_ (mémoires de l'académie des inscriptions et belles-lettres, tome vi., ^{re} série, ^{re} partie, ); martinus scheins, _de electro veterum metallico_ (inaugural dissertation, berlin, ); f. a. paley, _gold worship in relation to sun worship_ (contemporary review, august, ). see also curtius, _grundzüge der griechischen etymologie_, pp. - . the net result of the disputations of scholars appears to be that [greek: êlektôr] (he who shines) is a masculine form to which there corresponds the neuter form [greek: êlektron] (that which shines). stephanus admits the accentuation used by gilbert, [greek: êlektron], to be justified from the _timæus_ of plato; see note to p. . [ ] page , line . page , line . [greek: harpax] dicitur, & [greek: chrusophoron].--with respect to the other names given to amber, m. th. henri martin has written (see previous note) so admirable an account of them that it is impossible to better it. it is therefore given here entire, as follows: "le succin a reçu chez les anciens des noms très-divers. sans parler du nom de [greek: lunkourion], lyncurium, qui peut-être ne lui appartient pas, comme nous le montrerons plus loin, il s'est nommé chez les grecs le plus souvent [greek: êlektron] au neutre,^ mais aussi [greek: êlektros] au masculin^ et même au féminin,^ [greek: chrusêlektros],^ [greek: chrusophoros]^ et peut-être, comme nous l'avons vu, [greek: chalkolithanon]; plus tard [greek: souchion]^ ou [greek: souchinos]^ , et [greek: êlektrianos lithos];^ plus tard encore [greek: berenikê], [greek: beronikê] ou [greek: bernikê];^ il s'est nommé [greek: harpax] chez les grecs établis en syrie;^{ } chez les latins _succinum_, _electrum_, et deux variétés, _chryselectrum_ et _sualiternicum_ { } ou _subalternicum_;^{ } chez les germains, _gless_;^{ } chez les scythes, _sacrium_;^{ } chez les egyptiens, _sacal_;^{ } chez les arabes, _karabé_^{ } ou _kahraba_;^{ } en persan, _káruba_.^{ } ce mot, qui appartient bien à la langue persane, y signifie _attirant la paille_, et par conséquent exprime l'attraction électrique, de même que le mot [greek: harpax] des grecs de syrie. en outre, le nom de _haur roumi_ (_peuplier romain_) était donné par les arabes, non-seulement à l'arbre dont ils croyaient que le succin était la gomme, mais au succin lui-même. _haur roumi_, transformé en _aurum_ par les traducteurs latins des auteurs arabes, et consondu mal à propos avec _ambar_ ou _ambrum_, nom arabe latinisé de l'ambre gris, a produit le nom moderne d'_ambre_, nom commun à l'_ambre jaune_ ou succin, qui est une résine fossile, et à l'_ambre gris_, concrétion odorante qui se forme dans les intestines des cachalots. on ne peut dire avec certitude si le nom de basse grécité [greek: bernikê] est la source ou le dérivé de _bern_, radical du nom allemand du succin (_bernstein_). quoi qu'il en soit, le mot [greek: bernikê] a produit _vernix_, nom d'une gomme dans la basse latinité, d'où nous avons fait _vernis_.^{ }" ^ voyez hérodote, iii., ; platon, _timée_, p. c; aristote, _météor._, iv., ; théophraste, _hist. des plantes_, ix., ( ), § ; _des pierres_, § et ; diodore de sic., v., ; strabon, iv., , n^o , p. (casaubon); dioscoride, _mat. méd._, i., ; plutarque, _questions de table_, ii., , § ; _questions platoniques_, vii., et ; lucien, _du succin et des cygnes_; le même, _de pastrologie_, § ; s. clément, _strom._ ii., p. (paris, , in-fol.); alexandre d'aphr., _quest. phys. et mor._, ii., ; olympiodore, _météor._, i., , fol. , t. i., p. (ideler) et l'abréviateur d'etienne de byzance au mot [greek: Êlektrides]. ^ voyez sophocle, _antigone_, v. , et dans eustathe, sur l'_iliade_, ii., ; elien, _nat. des animaux_, iv. ; quintus de smyrne, v., ; eustathe, sur la _périégèse_ de denys, p. (bernhardy), et sur l'_odyssée_, iv., ; et suidas au mot [greek: hualê]. ^ voyez alexandre, _problèmes_, sect. , prooem., p. (ideler); eustathe, sur l'_odyssée_, iv., , et tzetzès, _chiliade_ vi., . ^ voyez psellus, _des pierres_, p. (bernard et maussac). ^ voyez dioscoride, _mat. méd._, i., . ^ voyez s. clément, _strom._, ii., p. (paris, , in-fol.). il paraît distinguer l'un de l'autre [greek: to souchion] et [greek: to êlektron], probablement parce qu'il attribue à tort au métal [greek: êlektron] la propriété attractive du succin. ^ voyez le faux zoroastre, dans les _géoponiques_, xv., , § . ^ voyez le faux zoroastre, au même endroit. ^ voyez eustathe, sur l'_odyssée_, iv., ; tzetzès, _chil._ vi., ; nicolas myrepse, _antidotes_, ch. , et l'etymol. gud. au mot [greek: êlektron]. comparez saumaise, exert. plin., p. . ^{ } voyez pline, xxxvii., , s. , n^o . ^{ } voyez pline, xxxvii., , s. - , et tacite, _germanie_, ch. . la forme _sualiternicum_, dans pline (s. , n^o ), est donnée par le manuscrit de bamberg et par m. sillig (t. v., p. ), au lieu de la forme _subalternicum_ des éditions antérieures. ^{ } voyez tacite et pline, _ll. cc._ ^{ } voyez pline, xxxvii., , s. , n^o , comp. j. grimm, _gesch. der deutsch. sprache_, kap. x., p. (leipzig, , in- ). ^{ } pline, _l. c._ ^{ } voyez saumaise, _de homon. hyles iatricæ_, c. , p. ( , in-fol.). ^{ } voyez sprengel, sur dioscoride, t. ii., pp. - . ^{ } voyez m. de sacy, cité par buttmann, _mythologus_, t. ii., pp. - . ^{ } voyez saumaise, _ex. plin._, p. . il n'est pas probable que le mot [greek: bernikê] ou [greek: berenikê] nom du succin dans la grécité du moyen âge, soit lié étymologiquement avec le nom propre [greek: berenikê], qui vient de l'adjectif macédonien [greek: berenikos] pour [greek: pherenikos]. [ ] page , line . page , line . _mauri vero carabem appellant, quià solebant in sacrificijs, & deorum cultu ipsum libare. carab enim significat offerre arabicè; ita carabe, res oblata; aut rapiens paleas, vt scaliger ex abohali citat, ex linguâ arabicâ, vel persicâ._--the printed text, line , has "non rapiens paleas," but in all copies of the folio of , the "non" has been altered in ink into "aut," possibly by gilbert's own hand. nevertheless the editions of and both read "non." there appears to be no doubt that the origin of the word _carabe_, or _karabe_, as assigned by scaliger, is substantially correct. as shown in the preceding note, martin adopted this view. if any doubt should remain it will be removed by the following notes which are due to mr. a. houtum schindler (member of the institution of electrical engineers), of terahan. reference is made to the magnetic and electric properties of stones in three early persian lapidaries. there are three stones only mentioned, amber, loadstone, and garnet. the electric property of the diamond is not mentioned. the following extracts are from the _tansûk nâmah_, by nasîr ed dîn tûsi, a.d. . the two other treatises give the first extracts in the same words. "_kâhrubâ_, also _kahrabâ_ [amber], "is yellow and transparent, and has its name from the property, which it possesses, of attracting small, dry pieces of straw or grass, after it has been rubbed with cloth and become warm. [note. in persian, kâh = straw; rubâ = the robber, hence kâhrubâ = the straw-robber.] some consider it a mineral, and say that it is found in the mediterranean and caspian seas, floating on the surface, but this is not correct. the truth is that kâhrubâ { } is the gum of a tree, called jôz i rûmî [_i.e._, roman nut; walnut?], and that most of it is brought from rûm [here the eastern rome] and from the confines of sclavonia and russia. on account of its bright colour and transparency it is made into beads, rings, belt-buckles, &c. ... &c. * * * * * "the properties of attraction and repulsion are possessed by other substances than loadstone, for instance, by amber and bîjâdah,^ which attract straws, feathers, etc., and of many other bodies, it can be said that they possess the power of attraction. there is also a stone which attracts gold; it has a pure yellow colour. there is also a stone which attracts silver from distances of three or two yards. there are also the stone which attracts tin, very hard, and smelling like asafoetida, the stone attracting hair, the stone attracting meat, etc., but, latterly, no one has seen these stones: no proof, however, that they do not exist." avicenna (ibn sinâ) gives the following under the heading of _karabe_ (see _canona medicinæ_, giunta edition, venet., , lib. ii., cap. , p. ): "karabe quid est? gumma sicut sandaraca, tendens ad citrinitatem, & albedinem, & peruietatem, & quandoque declinat ad rubedinem, quæ attrahit paleas, & [fracturas] plantarum ad se, & propter hoc nominatur karabe, scilicet rapiens paleas, persicè.... karabe confert tremori cordis, quum bibitur ex eo medietas aurei cum aqua frigida, & prohibet sputum sanguinis valde.... retinet vomitum, & prohibet materias malas a stomacho, & cum mastiche confortat stomachum.... retinet fluxum sanguinis ex matrice, & ano, & fluxum ventris, & confert tenasmoni." scaliger in _de subtilitate_, _exercitatio_ ciii., § , the passage referred to by gilbert says: "succinum apud arabas uocatur, carabe: quod princeps aboali, rapiens paleas, interpretatur" (p. _bis_, editio lutetiæ, ). ^ _bîjâdah_ is classified by muhammad b. mansûr (a.d. ) and by ibn al mubârak (a.d. ) under "stones resembling ruby"; the tansûk nâmah describes it in a separate chapter. from the description it can be identified with the almandine garnet, and the method of cutting this stone _en cabochon_, with hollow back in order to display its colour better is specially mentioned. the tansûk nâmah only incidentally refers to the electric property of the _bîjâdah_ in the chapter on loadstone, but the other two treatises specially refer to it in their description of the stone. the one has: "_bîjâdah_ if rubbed until warm, attracts straws and other light bodies just as amber does"; the other: "_bîjâdah_, if rubbed on the hair of the head, or on the beard, attracts straws." surûri, the lexicographer, who compiled a dictionary in , considers the _bîjâdah_ "a red ruby which possesses the property of attraction." other dictionaries do not mention the attractive property, but some authors confound the stone with amber, calling it _kâbrubâ_, the straw-robber. the _bîjâdah_ is not rubellite (red tourmaline) for it is described in the lapidaries as common, whereas rubellite (from ceylon) has always been rare, and was unknown in persia in the thirteenth century. [ ] page , line . page , line . _succinum seu succum._--dioscorides regarded amber as the inspissated juice of the poplar tree. from the frankfurt edition of (_de medicinali materia, etc._) edited by ruellius, we have, liber i., p. : _populus._ cap. xciii. "... lachrymam populorum commemorant quæ in padum amnem defluat, durari, ac coire in succinum, quod electrum vocant, alii chrysophorum. id attritu jucundum odorem spirat, et aurum colore imitatur. tritum potumque stomachi ventrisque fluxiones sistit." to this ruellius adds the commentary: "succinum seu succina gutta à succo dicta, græcis [greek: êlektrom] [sic], esse { } lachryma populi albæ, vel etiam nigræ quibusdam videtur, ab ejusdem arboris resina. dioscoridi et galeno dicta differens et [greek: pterugophoros], id est paleas trahens, quoque vocatur, quantum ei quoque galenus tribuit li. , ca. . succinum scribit à quibusdam pinei generis arboribus, ut gummi à cerasis excidere autumno, et largum mitti ex germania septentrionali, et insulis maris germanici. quod hodie nobis est compertissimum: ad hæc liquata igni valentiore, quia à frigido intensiore concrevit. pineam aperte olet, calidum primo gradu, siccum secundo, stomachum roborat, vomitum, nauseam arcet. cordis palpitationi prodest. pravorem humorum generationem prohibet. "germani weiss und gelbaugstein et bre[=n]stein. "galli ambra vocant: vulgo in corollis precariis frequens." in the scholia of johann lonicer in his edition of dioscorides, we find, lib. i., cap. xcviii., _de nigra populo_: "[greek: aigeiros], populus nigra ... idem electrum vel succinum [greek: haigeirou] lachrymam esse adseverat [paulus], cui præter vires quæ ab dioscoride recensentur, tribuit etiam vim sistendi sanguinis, si tusum in potu sumatur. avicennæ charabe, ut colligitur ex joanne jacobo manlio, est electrum hoc dioscoridis, attestatur brunfelsius. lucianus planè nullum electrum apud eridanum seu padum inveniri tradit, quandoquidem ne populus quidem illa ab nautis ei demonstrari potuerit. plinius rusticas transpadanas ex electro monilia gestare adfirmat, quum à venetis primum agnoscere didicissent adversus nimirum vitia gutturis et tonsillarum. num sit purgamentum maris, vel lachryma populi, vel pinus, vel ex radiis occidentis solis nascatur, vel ex montibus sudinorum profluat, incertum etiam erasmus stella relinquit. sudinas tamen borussiorum opes esse constat." matthiolus (in _p. a. mattioli ... opera quæ extant omnia, hoc est commentarii in vi libros p. dioscoridis de materia medica_, frankfurt, , p. ) comments on the suggestion of galen that amber came from the _populus alba_, and also comments on the arabic, greek, and latin names of amber. the poplar-myth is commemorated by addison (in _italy_) in the lines: no interwoven reeds a garland made, to hide his brows within the vulgar shade; but poplar wreathes around his temples spread, and tears of amber trickled down his head. amber is, however, assuredly not derived from any poplar tree: it comes from a species of pine long ago extinct, called by göppert the _pinites succinifer_. gilbert does not go into the medicinal uses, real or fancied, that have been ascribed to amber in almost as great variety as to loadstone. pliny mentions some of these in his _natural historie_ (english version of , p. ): "he [callistratus] saith of this yellow amber, that if it be worne about the necke in a collar, it cureth feavers, and healeth the diseases of the mouth, throat, and jawes: reduced into pouder and tempered with honey and oile of roses, it is soveraigne for the infirmities of the eares. stamped together with the best atticke honey, it maketh a singular eyesalve for to help a dim sight: pulverized, and the pouder thereof taken simply alone, or else drunke in water with masticke, is soveraigne for the maladies of the stomacke." nicolaus myrepsus (recipe , _op. citat._) gives a prescription for { } dysentery and diabetes confiding chiefly of "electri vel succi nili (nili succum appellant arabes karabem)." [ ] page , line . page , line . _sudauienses seu sudini._--cardan in _de rerum varietate_, lib. iii., cap. xv. (editio basil., , p. ), says of amber: "colligitur in quadam penè insula sudinorum, qui nunc uoc[=a]tur brusci, in prussia, nunc borussia, juxta veneticum sinum, & sunt orientaliores ostiis vistulæ fluuii: ubi triginta pagi huic muneri destinati sunt," etc. he rejects the theory that it consists of hardened gum. there exists an enormous literature concerning amber and the prussian amber industry. amongst the earliest works (after theophrastus and pliny) are those of aurifaber (_bericht über agtstein oder börnstein_, königsberg, ); goebel (_de succino, libri duo, authore severino goebelio, medico doctore_, regiomont., ); and wigand (_vera historia de succino borussico_, jena, ). later on hartmann, p. j. (_succini prussici physica et civilis historia_, francofurti, ); and the splendid folio of nathaniel sendel (_historia succinorum corpora aliena involventium_, lipsiæ, ), with its wealth of plates illustrating amber specimens, with the various included fossil fauna and flora. georgius agricola (_de natura fossilium_, liber iv.), and aldrovandi (_musæeum metallicum_, pp. - ) must also be mentioned. bibliographies of the earlier literature are to be found in hartmann (_op. citat._), and in daniel gralath, _elektrische bibliothek_ (_versuche und abhandlungen der naturforschenden gesellschaft in danzig_, zweiter theil, pp. - , danzig and leipzig, ). see also karl müllenhoff, _deutsche altertumskunde_, vol. i., zweites buch, pp. - , zinn und bernsteinhandel (berlin, ), and humboldt's _cosmos_ (bohn's edition, london, , vol. ii., p. ). the ancient greek myth according to which amber was the tears of the heliades, shed on the banks of the river eridanus over phaethon, is not alluded to by gilbert. it is narrated in well-known passages in ovid and in hyginus. those interested in the modern handling of the myth should refer to müllenhoff (_op. citat._, pp. - , der bernsteinmythus), or to that delightful work _the tears of the heliades_, by w. arnold buffum (london, ). [ ] page , line . page , line . _quare & muscos ... in frustulis quibusdam comprehensos retinet._--the occurrence of flies in amber was well known to the ancients. pliny thus speaks of it, book xxxvii., chap. iii. (p. of p. holland's translation of ): "that it doth destill and drop at the first very clear and liquid, it is evident by this argument, for that a man may see diverse things within, to wit, pismires, gnats, and lizards, which no doubt were entangled and stucke within it when it was greene and fresh, and so remain enclosed within as it waxed harder." a locust embedded in amber is mentioned in the _musæum septalianum_ of terzagus (dertonæ, ). martial's epigram (_epigrammata_, liber vi., ) is well known: dum phaethontea formica vagatur in umbra implicuit tenuem succina gutta feram. see also hermann (daniel), _de rana et lacerta succino borussiaco insitis_ { } (cracov., ; a later edition, rigæ, ). the great work on _inclusa_ in amber is, however, that of nathaniel sendel. see the previous note. sir thomas browne must not be forgotten in this connexion. the _pseudodoxia_ (p. of the second edition, ) says: "lastly, we will not omit what bellabonus upon his own experiment writ from dantzich unto mellichius, as he hath left recorded in his chapter _de succino_, that the bodies of flies, pismires and the like, which are said oft times to be included in amber, are not reall but representative, as he discovered in severall pieces broke for that purpose. if so, the two famous epigrams hereof in martiall are but poeticall, the pismire of brassavolus imaginary, and cardans mousoleum for a flie, a meer phancy. but hereunto we know not how to assent, as having met with some whose reals made good their representments." see also pope's _epistle to dr. arbuthnot_, line . [ ] page , line . page , line . _commemorant antiqui quod succinum festucas et paleas attrahit._--pliny (book xxxvii., chap. ii., p. of the english edition of ) thus narrates the point: "hee [_niceas_] writeth also, that in aegypt it [amber] is engendered.... semblably in syria, the women (saith hee) make wherves of it for their spindles, where they use to call it harpax, because it will catch up leaves, straws, and fringes hanging to cloaths." p. . "to come to the properties that amber hath, if it bee well rubbed and chaufed betweene the fingers, the potentiall facultie that lieth within, is set on work, and brought into actual operation, whereby you shall see it to drawe chaffe strawes, drie leaves, yea, and thin rinds of the linden or tillet tree, after the same sort as loadstone draweth yron." [ ] page , line . page , line . _quod etiam facit gagates lapis._--the properties of jet were well known to the mediæval writers. _julius solinus_ writes in _de mirabilibus_, chapter xxxiv., _of britaine_ (english version of by a. golding): "moreover to the intent to passe the large aboundance of sundry mettals (whereof britaine hath many rich mynes on all sides), here is store of the stone called geate, and y^e best kind of it. if ye demaund y^e beautie of it, it is a black jewell: if the qualitie, it is of no weight: if the nature, it burneth in water, and goeth out in oyle; if the power, rubbe it till it be warme, and it holdeth such things as are laide to it; as amber doth. the realme is partlie inhabited of barbarous people, who even frõ theyr childhoode haue shapes of divers beastes cunninglye impressed and incorporate in theyr bodyes, so that beeing engraued as it were in theyr bowels, as the man groweth, so growe the marks painted vpon him...." pliny describes it as follows (p. , english edition of ): "the geat, which otherwise we call gagates, carrieth the name of a toune and river both in lycia, called gages: it is said also, that the sea casteth it up at a full tide or high water into the island leucola, where it is gathered within the space of twelve stadia, and no where else: blacke it is, plaine and even, of a hollow substance in manner of the pumish stone, not much differing from the nature of wood; light, brittle, and if it bee rubbed or bruised, of a strong flavour." (book xxxvi., chap. xviii.) in the commentary of joannes ruellius upon dioscorides, _pedanii dioscoridis anazarbei de medicinali materia libri sex, ioanne ruellio suessionensi interprete_ ... (frankfurt, , fol., liber quintus, cap. xcii.) is the following description: { } "in gagatarum lapidum genere, præferendus qui celeriter accenditur, et odorem bituminis reddit. niger est plerunque, et squalidus, crustosus, per quam levis. vis ei molliendi, et discutiendi. deprehendit sonticum morbum suffitus, recreatque uuluæ strangulationes. fugat serpentes nidore. podagricis medicaminibus, et a copis additur. in cilicia nasci solet, qua influens amnis in mare effunditur, proxime oppidum quod plagiopolis dicitur. vocatur autem et locus et amnis gagas, in cujus faucibus ii lapides inveniuntur. "gagates lapis colore atro, germanis schwartzer augstein, voce parum depravata, dicitur. odore dum uritur bituminis, siccat, glutinat, digerit admotus, in corollis precariis et salinis frequens." and in the _scholia_ upon dioscorides of joannes lonicer (marpurgi, , cap. xcvii., p. ) is the following: "_de gagate lapide._ ab natali solo, urbe nimirum gagae lyciae nomen habet. galenus se flumen isthuc et lapidem non invenisse, etiamsi naui parua totam lyciam perlustravit: ait, se autem in caua syria multos nigros lapides invenisse glebosos, qui igni impositi, exiguam flammam gignerent. meminit hujus nicander in theriacis nempe suffitum hujus abigere venenata." there is also a good account of _gagates_ (and of succinum) by langius, _epistola_ lxxv., p. , of the work _epistolarum medicinalium volumen tripartitum_ (francofurti, ). [ ] page , line . page , line . _multi sunt authores moderni._--the modern authors who raised gilbert's wrath by ignorantly copying out all the old tales about amber, jet, and loadstone, instead of investigating the facts, were, as he says at the beginning of the chapter, some theologians, and some physicians. he seems to have taken a special dislike to albertus magnus, to puteanus (du puys), and to levinus lemnius. [ ] page , line . page , line . _& gagate._--the editions of and both read _ex gagate_. [ ] page , line . page , line . _nam non solum succinum, & gagates (vt illi putant) allectant corpuscula._--the list of bodies known to become electrical by friction was not quite so restricted as would appear from this passage. five, if not six, other minerals had been named in addition to amber and jet. ( .) _lyncurium._ this stone, about which there has been more obscurity and confusion than about any other gem, is supposed by some writers to be the tourmaline, by others a jacinth, and by others a belemnite. the ancients supposed it to be produced from the urine of the lynx. the following is the account of theophrastus, _theophrastus's history of stones. with an english version_ ..., by "sir" john hill, london, , p. , ch. xlix.-l. "there is some workmanship required to bring the emerald to its lustre, for originally it is not so bright. it is, however, excellent in its virtues, as is also the _lapis lyncurius_, which is likewise used for engraving seals on, and is of a very solid texture, as stones are; it has also an attractive power, like that of amber, and is said to attract not only straws and small pieces of sticks, but even copper and iron, if they are beaten to thin pieces. this diocles affirms. the _lapis lyncurius_ is pellucid, and of a fire colour." see also w. watson in _philos. trans._, , l. i., p. , _observations concerning the lyncurium of the ancients_. ( .) _ruby._ ( .) _garnet._ the authority for both these is pliny, _nat. hist._, book xxxvii., chap. vii. (p. of english edition of ). { } "over and besides, i find other sorts of rubies different from those above-named;... which being chaufed in the sun, or otherwise set in a heat by rubbing with the fingers, will draw unto them chaffe, strawes, shreads, and leaves of paper. the common grenat also of carchedon or carthage, is said to doe as much, although it be inferiour in price to the former." ( .) _jasper._ affaytatus is the authority, in _fortunii affaitati physici atque theologi ... physicæ & astronomicæ c[=o]siderationes_ (venet., ), where, on p. , he speaks of the magnet turning to the pole, likening it to the turning of a "palea ab ambro vel iaspide et hujuscemodi lapillis lucidis." ( .) _lychnis._ pliny and st. isidore speak of a certain stone _lychnis_, of a scarlet or flame colour, which, when warmed by the sun or between the fingers, attracts straws or leaves of papyrus. pliny puts this stone amongst carbuncles, but it is much more probably _rubellite_, that is to say, red tourmaline. ( .) _diamond._ in spite of the confusion already noted, _à propos_ of _adamas_ (note to p. ), between loadstone and diamond, there seems to be one distinct record of an attractive effect having been observed with a rubbed diamond. this was recorded by fracastorio, _de sympathia et antipathia rerum_ (giunta edition, venice, mdlxxiiii, chap. v., p. _verso_), "cujus rei & illud esse signum potest, cum confricata quædã vt succinum, & adamas fortius furculos trahunt." and (on p. _recto_); "nam si per similitudine (vt supra diximus) fit hæc attractio, cur magnes non potius magnetem trahit, [~q] ferrum, & ferrum non potius ad ferrum movetur, quàm ad magnetem? quæ nam affinitas est pilorum, & furculorum cum electro, & adamante? præsertim [~q] si cum electro affines sunt, quomodo & cum adamante affinitatem habebunt, qui dissimilis electro est?" an incontestable case of the observation of the electrification of the diamond occurs in gartias ab horto. the first edition of his _historia dei semplici aromati_ was publisht at goa in india in . in chapter xlviii. on the diamond, occur these words (p. of the venetian edition of ): "questo si bene ho sperimentato io più volte, che due diamanti perfetti fregati insieme, si vniscono di modo insieme, che non di leggiero li potrai separare. et ho parimente veduto il diamante dopo di esser ben riscaldato, tirare à se le festuche, non men, che si faccia l'elettro." see also aldrovandi, _musæum metallicum_ (bonon., , p. ). levinus lemnius also mentions the diamond along with amber. see his _occulta naturæ miracula_ (english edition, london, , p. ). [ ] page , line . page , line . _iris gemma._--the name _iris_ was given, there can be little doubt, to clear six sided prisms of rock-crystal (quartz), which, when held in the sun's beams, cast a crude spectrum of the colours of the rainbow. the following is the account of it given in pliny, book xxxvii., chap. vii. (p. of the english version of ): "... there is a stone in name called iris: digged out of the ground it is in a certaine island of the red sea, distant from the city berenice three score miles. for the most part it resembleth crystall: which is the reason that some hath tearmed it the root of crystall. but the cause why they call it iris, is, that if the beames of the sunne strike upon it directly within house, it doth send from it against the walls that bee neare, the very resemblance both in forme and also in colour of a rainebow; and eftsoones it will chaunge the same in much varietie, to the great admiration of them that behold it. for certain it is knowne, that six angles it hath in manner of the crystall: but they say that some of them have their sides rugged, and the same { } unequally angled: which if they be laid abroad against the sunne in the open aire, do scatter the beames of the sunne, which light upon them too and fro: also that others doe yeeld a brightnes from themselves, and thereby illuminat all that is about them. as for the diverse colours which they cast forth, it never happeneth but in a darke or shaddowie place: whereby a man may know, that the varietie of colours is not in the stone iris, but commeth by the reverberation of the wals. but the best iris is that which representeth the greatest circles upon the wall, and those which bee likest unto rainebowes indeed." in the english translation of solinus's _de mirabilibus_ (_the excellent and pleasant worke of julius solinus containing the noble actions of humaine creatures, the secretes and providence of nature, the descriptions of countries ... tr. by a. golding, gent._, lond., ), chapter xv. on arabia has the following: "hee findeth likewise the iris in the red sea, sixe cornered as the crystall: which beeing touched with the sunnebeames, casteth out of him a bryght reflexion of the ayre like the raynebowe." iris is also mentioned by albertus magnus (_de mineralibus_, venet., , p. ), by marbodeus gallus (_de lapidibus_, par. , p. ), who describes it as "crystallo simulem sexangulam," by lomatius (_artes of curious paintinge_, haydocke's translation, lond., , p. ), who says, "... the sunne, which casting his beames vpon the _stone iris_, causeth the _raine-bowe_ to appeare therein ...," and by "sir" john hill (_a general natural history_, lond., , p. ). figures of the iris given by aldrovandi in the _musæum metallicum_ clearly depict crystals of quartz. [ ] page , line . page , line . _vincentina, & bristolla (anglica gemma siue fluor)_. this is doubtless the same substance as the _gemma vincentij rupis_ mentioned on p. , line (p. , line , of english version), and is nothing else than the so-called "bristol diamond," a variety of dark quartz crystallized in small brilliant crystals upon a basis of hæmatite. to the work by dr. thomas venner (lond., ), entitled _via recta_ or the _bathes of bathe_, there is added an appendix, _a censure concerning the water of saint vincents rocks neer bristol (urbs pulchra et emporium celebre)_, in which, at p. , occurs this passage: "this water of saint _vincents_ rock is of a very pure, cleare, crystalline substance, answering to those crystalline diamonds and transparent stones that are plentifully found in those clifts." in the _fossils arranged_ of "sir" john hill (lond., ), p. , is the following entry: "black crystal. small very hard heavy glossy. perfectly black, opake. bristol (grottos, glass)" referring to its use. the name _vincentina_ is not known as occurring in any mineralogical book. prof. h. a. miers, f.r.s., writes concerning the passage: "_anglica gemma sive fluor_ seems to be a synonym for _bristolla_, or possibly for _vincentina et bristolla_. both quartz and fluor are found at clifton. in that case vincentina and bristolla refer to these two minerals, and if so one would expect bristolla to be the bristol diamond, and vincentina to be the comparatively rare fluor spar from that locality." at the end of the edition of of sir hugh plat's _jewel house of art and nature_, is appended _a rare and excellent discourse of minerals, stones, gums, and rosins; with the vertues and use thereof_, by d. b. _gent_. here, p. , we read: "we have in england a stone or mineral called a bristol stone (because { } many are found thereabouts) which much resembles the adamant or diamond, which is brought out of arabia and cyprus; but as it is wanting of the same hardnesse, so falls it short of the like vertues." [ ] page , line . page , line . _crystallus._--rock-crystal. quartz. pliny's account of it (philemon holland's version of , p. ) in book xxxvii., chap, ii., is: "as touching crystall, it proceedeth of a contrarie cause, namely of cold; for a liquor it is congealed by extreame frost in manner of yce; and for proofe hereof, you shall find crystall in no place els but where the winter snow is frozen hard: so as we may boldly say, it is verie yce and nothing else, whereupon the greeks have give it the right name crystallos, _i._ yce.... thus much i dare my selfe avouch, that crystall groweth within certaine rockes upon the alps, and these so steepe and inaccessible, that for the most part they are constrained to hang by ropes that shall get it forth." [ ] page , line . page , line . _similes etiam attrahendi vires habere videntur vitrum ... sulphur, mastix, & cera dura sigillaris._ if, as shown above, the electric powers of diamond and ruby had already been observed, yet gilbert was the first beyond question to extend the list of _electrics_ beyond the class of precious stones, and his discovery that _glass_, _sulphur_, and _sealing-wax_ acted, when rubbed, like amber, was of capital importance. though he did not pursue the discovery into mechanical contrivances, he left the means of that extension to his followers. to otto von guericke we owe the application of sulphur to make the first electrical machine out of a revolving globe; to sir isaac newton the suggestion of glass as affording a more mechanical construction. electrical attraction by natural products other than amber after they have been rubbed must have been observed by the primitive races of mankind. indeed humboldt in his _cosmos_ (lond., , vol. i., p. ) records a striking instance: "i observed with astonishment, on the woody banks of the orinoco, in the sports of the natives, that the excitement of electricity by friction was known to these savage races, who occupy the very lowest place in the scale of humanity. children may be seen to rub the dry, flat and shining seeds or husks of a trailing plant (probably a _negretia_) until they are able to attract threads of cotton and pieces of bamboo cane." [ ] page , line . page , line . _arsenicum_.--this is _orpiment_. see the _dictionary of metallick words_ at the end of pettus's _fleta minor_. [ ] page , line . page , line . _in convenienti coelo sicco_.--the observation that only in a dry climate do rock-salt, mica, and rock-alum act as electrics is also of capital importance. compare page . [ ] page , line . page , line . _alliciunt hæc omnia non festucas modo & paleas._--gilbert himself marks the importance of this discovery by the large asterisk in the margin. the logical consequence was his invention of the first _electroscope_, the _versorium non magneticum_, made of any metal, figured on p. . [ ] page , line . page , line . _quod tantum siccas attrahat paleas, nec folia ocimi._--this silly tale that basil leaves were not attracted by amber arose in the _quæstiones convivales_ of plutarch. it is repeated by marbodeus and was quoted by levinus lemnius as true. gilbert denounced it as nonsense. cardan (_de subtilitate_, norimb., , p. ) had already contradicted the fable. "trahit enim," he says, "omnia levia, paleas, festucas, ramenta { } tenuia metallorum, & ocimi folia, perperam contradicente theophrasto." sir thomas browne specifically refuted it. "for if," he says, "the leaves thereof or dried stalks be stripped into small strawes, they arise unto amber, wax, and other electricks, no otherwise then those of wheat or rye." [ ] page , line . page , line . _sed vt poteris manifestè experiri...._ gilbert's experimental discoveries in electricity may be summarized as follows: . the generalization of the class of _electrics_. . the observation that damp weather hinders electrification. . the generalization that electrified bodies attract everything, including even metals, water, and oil. . the invention of the non-magnetic _versorium_ or electroscope. . the observation that merely warming amber does not electrify it. . the recognition of a definite class of _non-electrics_. . the observation that certain electrics do not attract if roasted or burnt. . that certain electrics when softened by heat lose their power. . that the electric effluvia are stopped by the interposition of a sheet of paper or a piece of linen, or by moist air blown from the mouth. . that glowing bodies, such as a live coal, brought near excited amber discharge its power. . that the heat of the sun, even when concentrated by a burning mirror, confers no vigour on the amber, but dissipates the effluvia. . that sulphur and shell-lac when aflame are not electric. . that polish is not essential for an electric. . that the electric attracts bodies themselves, not the intervening air. . that flame is not attracted. . that flame destroys the electrical effluvia. . that during south winds and in damp weather, glass and crystal, which collect moisture on their surface, are electrically more interfered with than amber, jet and sulphur, which do not so easily take up moisture on their surfaces. . that pure oil does not hinder production of electrification or exercise of attraction. . that smoke is electrically attracted, unless too rare. . that the attraction by an electric is in a straight line toward it. [ ] page , line . page , line . _quæ sunt illæ materiæ._--gilbert's list of electrics should be compared with those given subsequently by cabeus ( ), by sir thomas browne ( ), and by bacon. the last-named list occurs in his _physiological remains_, published posthumously in ; it contains nothing new. sir thomas browne's list is given in the following passage, which is interesting as using for the first time in the english language the noun _electricities_: "many stones also both precious and vulgar, although terse and smooth, have not this power attractive; as emeralds, pearle, jaspis, corneleans, agathe, heliotropes, marble, alablaster, touchstone, flint and bezoar. glasse attracts but weakely though cleere, some slick stones and thick glasses indifferently: arsenic but weakely, so likewise glasse of antimony, but crocus metallorum not at all. saltes generally but weakely, as sal gemma, alum, and also talke, nor very discoverably by any frication: but if gently warmed at the fire, and wiped with a dry cloth, they will better discover their electricities." _(pseudodoxia epidemica_, p. .) in the _philosophical transactions_, vol. xx., p. , is _a catalogue of electrical bodies_ by the late dr. rob. plot. it begins "non solum succinum," and ends "alumen rupeum," being identical with gilbert's list except that he calls "vincentina & bristolla" by the name "pseudoadamas bristoliensis." [ ] page , line . page , line . _non dissimili modo._--the _modus_ { } _operandi_ of the electrical attractions was a subject of much discussion; see cardan, _op. citat._ [ ] page , line . page , line . _appellunt._--this appears to be a misprint for _appelluntur_. [ ] page , line . page , line . _smyris._--emery. this substance is mentioned on p. as a magnetic body. [ ] page , line . page , line . _gemmæ ... vt crystallus, quæ ex limpidâ concreuit._ see the note to p. . [ ] page , line . page , line . _ammoniacum._--ammoniacum, or gutta ammoniaca, is described by dioscorides as being the juice of a ferula grown in africa, resembling galbanum, and used for incense. "_ammoniack_ is a kind of gum like frankincense; it grows in lybia, where _ammon's_ temple was." sir hugh plat's _jewel house of art and nature_ (ed. , p. ). [ ] page , line . page , line . _duæ propositæ sunt causæ ... materia & forma._--gilbert had imbibed the schoolmen's ideas as to the relations of matter and form. he had discovered and noted that in the magnetic attractions there was always a verticity, and that in the electrical attractions the rubbed electrical body had no verticity. to account for these differences he drew the inference that since (as he had satisfied himself) the magnetic actions were due to _form_, that is to say to something immaterial--to an "imponderable" as in the subsequent age it was called--the electrical actions must necessarily be due to _matter_. he therefore put forward his idea that a substance to be an electric must necessarily consist of a concreted humour which is partially resolved into an effluvium by attrition. his discoveries that electric actions would not pass through flame, whilst magnetic actions would, and that electric actions could be screened off by interposing the thinnest layer of fabric such as sarcenet, whilst magnetic actions would penetrate thick slabs of every material except iron only, doubtless confirmed him in attributing the electric forces to the presence of these effluvia. see also p. . there arose a fashion, which lasted over a century, for ascribing to "humours," or "fluids," or "effluvia," physical effects which could not otherwise be accounted for. boyle's tracts of the years and on "effluviums," their "determinate nature," their "strange subtilty," and their "great efficacy," are examples. [ ] page , line . page , line . _magnes vero...._--this passage from line to line states very clearly the differences to be observed between the magnetical and the electrical attractions. [ ] page , line . page , line . _succino calefacto._--ed. reads _succinum_ in error. [ ] page , line . page , line . _plutarchus ... in quæstionibus platonicis._--the following latin version of the paragraph in _quæstio sexta_ is taken from the bilingual edition publisht at venice in , p. _verso_, liber vii., cap. (or, _quæstio septima_ in ed. didot, p. ). "electrum uero quæ apposita sunt, nequaquàm trahit, quem admodum nec lapis ille, qui sideritis nuncupatur, nec quicqu[=a] à seipso ad ea quæ in propinquo sunt, extrinsecus assilit. verum lapis magnes effluxiones quasdam tum graves, tum etiam spiritales emittit, quibus aer continuatus & iunctus repellitur. is deinceps alium sibi proximum impellit, qui in orbem circum actus, atque ad inanem locum rediens, ui ferrum fecum rapit & trahit. at electrum uim quandam flammæ similem & spiritalem continet, quam quidem { } tritu summæ partis, quo aperiuntur meatus, foras eijcit. nam leuissima corpuscula & aridissima quæ propè sunt, sua tenuitate atque imbecillitate ad seipsum ducit & rapit, cum non sit adeo ualens, nec tantum habeat ponderis & momenti ad expellendam aeris copiam, ut maiora corpora more magnetis superare possit & uincere." [ ] page , line . page , line . _gemma vincentij rupis._--see the note to p. _supra_, where the name _vincentina_ occurs. [ ] page , line . page , line . _orobi._--the editions of and read _oribi_. [ ] page , line . page , line . _in euacuati._--the editions of and read _inevacuati_. [ ] page , line . page , line . _assurgentem vndam ... declinat ab f._--these words are wanting in the stettin editions. [ ] page , line . page , line . _fluore._--this word is conjectured to be a misprint for _fluxu_ but it stands in all editions. [ ] page , line . page , line . _ruunt ad electria._--this appears to be a slip for _electrica_, which is the reading of the editions of and . [ ] page , line . page , line . _tan[=q] materiales radij._--the suggestion here of material _rays_ as the _modus operandi_ of electric forces seems to foreshadow the notion of electric lines of force. [ ] page , line . page , line . _differentia inter magnetica & electrica._--though gilbert was the first systematically to explore the differences that exist between the magnetic attraction of iron and the electric attraction of all light substances, the point had not passed unheeded, for we find st. augustine, in the _de civitate dei_, liber xxi., cap. , raising the question why the loadstone which attracts iron should refuse to move straws. the many analogies between electric and magnetic phenomena had led many experimenters to speculate on the possibility of some connexion between electricity and magnetism. see, for example, tiberius cavallo, _a treatise on magnetism_, london, , p. . also the three volumes of j. h. van swinden, _receuil de mémoires sur l'analogie de electricité et du magnétisme_, la haye, . aepinus wrote a treatise on the subject, entitled _de similitudine vis electricæ et magneticæ_ (petropolis, ). this was, of course, long prior to the discovery, by oersted, in , of the real connexion between magnetism and the electric current. [ ] page , line . page , line . _coitionem dicimus, non attractionem._--see the remarks, at the outset of these notes, on gilbert's definitions of words. [ ] page , line . page , line . _orpheus in suis carminibus._--this passage is in the chapter [greek: lithika] of orpheus, verses to . see note to p. , line . [ ] page , line . page , line . _platonis in timæo opinio_.--the passage runs (edition didot, vol. ii., p. , or stephanus, p. , c.): [greek: kai dê kai ta tôn hudatôn panta rheumata eti de ta tôn keraunôn ptômata kai ta thaumazomena êlektrôn peri tês helxeôs kai tôn hêrakleiôn lithôn, pantôn toutôn holkê men ouk estin oudeni pote, to de kenon einai mêden periôthein te hauta tauta eis allêla, to te diakrinomena kai sunkrinomena pros tên hautôn diameibomena hedran hekasta ienai panta, toutois tois pathêmasi pros allêla sumplechtheisi tethaumatourgêmena tôi kata tropon zêtounti phanêsetai.] [ ] page , line . page , line . the english version of the lines of lucretius is from busby's translation. [ ] page , line . { } page , line . _iohannes costæus laudensis._--joannes costa, of lodi, edited galen and avicenna. he also wrote a _de universali stirpium natura_ (aug. taurin., ). [ ] page , line . page , line . _cornelius gemma . cosmocrit._--this refers to the work _de naturæ divinis characterismis ... libri ii. avctore d. corn. gemma_ (antv., , lib. i., cap. vii., p. ). "certè vt à magnete insensiles radij ferrum ad se attrahunt, ab echineide paruo pisciculo sistuntur plena nauigia, à catoblepa spiritu non homines solùm, sed & alta serpentum genera interimuntur, & saxa dehiscunt." see also kircher's _magneticum naturæ regnum_ (amsterodami, , p. ), sectio iv., cap. iii., de magnete navium, quæ remora seu echeneis dicitur. see the note to p. , line . [ ] page , line . page , line . _guilielmus puteanus._--puteanus (du puys) wrote a work _de medicamentorum quomodocunque purgantium facultatibus_, libri ii. (lugd., ), in which he talks vaguely about the substantial "form" of the magnet, and quotes aristotle and galen. [ ] page , line . page , line . _baptistæ portæ._--the passage in the translation is quoted from the english version of , pp. , . [ ] page , line . page , line . _eruditè magis scaliger._--gilbert pokes fun at scaliger, whose "erudite" guess (that the motion of iron to the magnet was that of the offspring toward the parent) is to be found in his book _de subtilitate, ad cardanum_, exercitatio cii. (lutetiæ, , p. _bis_). [ ] page , line . page , line . _diuus thomas._--on p. gilbert had already spoken of st. thomas aquinas as a man of intellect who would have added more about the magnet had he been more conversant with experiments. the passage here quoted is from the middle of liber vii. of his commentaries on the _de physica_ of aristotle, _expositio diui thome aquinatis doctoris angelici super octo libros physicorum aristotelis_, etc. (venice, giunta edition, , p. _verso_, col. ). [ ] page , line . page , line . _cardinalis etiam cusanus._--cardinal de cusa (nicolas khrypffs) wrote a set of dialogues on statics, _nicolai cusani de staticis experimentis dialogus_ ( ), of which an english version appeared in london in with the title, _the idiot in four books; the first and second of wisdom, the third of the minde, the fourth of statick experiments. by the famous and learned c. cusanus._ in the fourth book _of statick experiments, or experiments of the ballance_, occurs (p. ) the following: "_orat._ tell me, if thou hast any device whereby the vertues of stones may be weighed. "_id._ i thinke the vertue of the load-stone might be weighed, if putting some iron in one scale, and a load-stone in the other, untill the ballance were even, then taking away the load-stone, and some other thing of the same weight being put into the scale, the load-stone were holden over the iron, so that that scale wou'd begin to rise; by reason of the load-stones attraction of the iron, then take out some of the weight of the other scale, untill the scale wherein the iron is, doe sinke againe to the æquilibrium, or equality still holding the load-stone unmovable as it was; i beleeve that by weight of what was taken out of the contrary scale, one might come proportionably to the weight of the vertue or power of the load-stone. and in like manner, the vertue of a diamond, might be found hereby, because { } they say it hinders the load-stone from drawing of iron; and so other vertues of other stones, consideration, being alwayes had of the greatnesse of the bodyes, because in a greater body, there is a greater power and vertue." in the edition of baptista porta's _magiæ naturalis libri xx._, in lib. vii., cap. xviii., occurs the description of the use of the balance to which gilbert refers. [ ] page , line . page , line . _aëris rigore._--all editions read thus, but the sense seems to require _frigore_. [ ] page , line . page , line . _fracastorius._--see his _de sympathia_, lib. i., cap. (giunta edition, , p. ). [ ] page , line . page , line . _thaletis milesij._--see the note to p. , line . [ ] page , line . page , line . _ità coitio magnetica actus est magnetis, & ferri, non actio vnius._--see the introductory remarks to these notes. there is a passage in scaliger's _de subtilitate ad cardanum_ (exercitat. cii., cap. , p. _op. citat._) which may be compared with gilbert's for its use of greek terms: "nã cùm uita dicatur actus animæ, acceptus est abs te actus pro actione. sed actus ille est [greek: entelecheia], nõ autem [greek: ergon]. at magnetis attractio est [greek: ergon], non aut[~e] [greek: entelecheia]." to which gilbert retorts: "non actio unius, utriusque [greek: entelecheia]; non [greek: ergon], [greek: sunentelecheia] et conactus potius quam sympathia." he returns on p. to the attack on scaliger's metaphysical notions. there is a parallel passage in the _epitome naturalis scientiæ_ of daniel sennert (oxoniæ, ), in the chapter _de motu_. [ ] page , line . page , line . _vt in . physicorum themistius existimat._--see _omnia themistii opera_ (aldine edition, , p. ), book of his paraphrase on aristotle's _physica_. [ ] page , line . page , line . _quod verò fracastorius._--_op. citat._, lib. i., cap. , p. _verso_. [ ] page , line . page , line . _si a borealis._--the editions of and omit the twelve words next following. [ ] page , line . page , line . _ex minera._--_minera_ is not a recognized word, even in late latin. it occurs again, p. , line . [ ] page , line . page , line . _multo magis._--this is an _à fortiori_ argument. it is interesting to find gilbert comparing the velocity of propagation of magnetic forces in space with the velocity of light. the parallel is completed in line by the consideration that as the rays of light require to fall upon an object in order that they may become visible, so the magnetic forces require a magnetic object in order to render their presence sensible. [ ] page , line . page , line . _orbem terrarum distinguunt._--the editions of and here add a figure of a globe marked with meridians and parallels of latitude, but with an erroneous versorium pointing to the south. these editions also both read _existentiam_ for the word _existentium_ in line . [ ] page , line . page , line . _magnes longior maiora pondera ferri attollit._--gilbert discovered the advantage, for an equal mass of loadstone, of an elongated shape. it is now well known that the specific amount of magnetism retained by elongated forms exceeds that in a short piece of the same material subjected to equal magnetizing forces. [ ] page , line . page , line . _non obstant crassa tabulata._--gilbert has several times referred (_e.g._, on p. ) to the way in which magnetic forces penetrate solid bodies. the experimental investigation in this chapter { } is the more interesting because it shows that gilbert clearly perceived the shielding action of iron to be due to iron conducting aside or diverting the magnetic forces. [ ] page , line . page , line . _non conveniant._--the editions of and both read _et conveniant_. [ ] page , line . page , line . _illud quod exhalat._--literally, _that which exhales_, in the sense of that which escapes: but in modern english the verb exhale in the active voice is now not used of the substance that escapes, but is used of the thing which emits it. it must therefore be rendered _that which is exhaled_ (_i.e._, breathed out). [ ] page , line . page , line . _ita tota interposita moles terrestris._--gilbert's notion that the gravitational force of the moon in producing the tides acts _through_ the substance of the earth may seem curiously expressed. but the underlying contention is essentially true to-day. the force of gravity is not cut off or screened off by the interposition of other masses. a recent investigation by professor poynting, f.r.s., has shown that so far as all evidence goes all bodies, even the densest, are transparent with respect to gravitational forces. [ ] page , line . page , line . _sed de æstus ratione aliàs._--there is no further discussion of the tides in _de magnete_. but a short account is to be found in gilbert's posthumous work _de mundo nostro sublunari philosophia nova_ (amsterdam, elzevir, ), in lib. v., the part which in the manuscript was left in english, and was turned into latin by his brother. it comprises about fifteen quarto pages, from cap. x. to cap. xix. inclusive, beginning with a characteristic diatribe against taisnier, levinus lemnius, and scaliger. but in assigning causes he himself goes wide of the mark. proceeding by a process of elimination he first shows that the moon's light cannot be the cause that impels the tides. "luna," he says, "non radio, non lumine, maria impellit. quomodo igitur? sane corporum conspiratione, acque (ut similitudine rem exponam) magnetica attractione." this cryptic utterance he proceeds to explain by a diagram, and adds: "quare luna non tam attrahit mare, quàm humorem & spiritum subterraneum; nec plus resistit interposita terra, quàm mensa, aut quicquam aliud densum, aut crassum, magnetis viribus." [ ] page , line . page , line . _armatura._--here this means the cap or snout of iron with which the loadstone was armed. this is apparently the first use of the term in this sense. in the _dialogues of galileo_ (p. of salusbury's _mathematical collections_, dialogue iii.), sagredus and salviatus discuss the arming of the loadstone, and the increased lifting power conferred by adding an iron cap. salviatus mentions a loadstone in the florentine academy which, unarmed, weighed six ounces, lifting only two ounces, but which when armed took up ounces. whereupon galileo makes salviatus say: "i extreamly praise, admire, and envy this authour, for that a conceit so stupendious should come into his minde. ... i think him [_i.e._, gilbert] moreover worthy of extraordinary applause for the many new and true observations that he made, to the disgrace of so many fabulous authours, that write not only what they do not know, but whatever they hear spoken by the foolish vulgar, never seeking to assure themselves of the same by experience, perhaps, because they are unwilling to diminish the bulk of their books." [ ] page , line . page , line . the reference to _lib._ is { } a misprint for _lib._ . it is corrected in the edition of , but not in that of . [ ] page , line . page , line . _conactu._--the editions of and read _conatu_. [ ] page , line . page , line . _coitio verò non fortior._--this heading to chap. xix., taken with the seven lines that follow, and the contrast drawn between _unitio_ and _coitio_, throw much light on the fundamental sense attached by gilbert to the term _coitio_. it is here clearly used in the sense of _mutual tendency toward union_. note also the contrasted use in chap. xx. of the verbs _cohære_ and _adhære_. adhærence connotes a one-sided force (an impossibility in physics), cohærence a mutual force. [ ] page , line . page , line . _nempè vt alter polus maius pondus arripiat._--this acute observation is even now not as well known as it ought to be. only so recently as siemens patented the device of fastening a mass of iron to one end of an electromagnet in order to increase the power of the other end. the fact, so far as it relates to permanent magnets was known to servington savery. see _philos. transactions_, , p. . [ ] page , line . page , line . _suspendit in aëre ferrum baptista porta._--porta's experiment is thus described (_natural magick_, london, , p. ): "_petrus pellegrinus_ saith, he shewed in another work how that might be done: but that work is not to be found. why i think it extream hard, i shall say afterwards. but i say it may be done, because i have now done it, to hold it fast by an invisible band, to hang in the air; onely so, that it be bound with a small thread beneath, that it may not rise higher: and then striving to catch hold of the stone above, it will hang in the air, and tremble and wag itself." [ ] page , line . page , line . _sed quæri potest ..._--the question here raised by gilbert is whether the lifting-power of magnets of equal quality is proportional to their weight. if a stone weighing a drachm will lift a drachm, would a stone that weighs an ounce lift an ounce? gilbert erroneously answers that this is so, and that the lifting-power of a loadstone, whether armed or unarmed, is proportional to its mass. the true law of the tractive force or lifting-power of magnets was first given in by james hamilton (afterwards earl of abercorn) in a work entitled _calculations and tables relating to the attractive virtue of loadstones ... printed_ [at london?] _in the year_ . (see also a paper in the _philos. transactions_, - , vol. xxxvi., p. ). this work begins thus: "the principle upon which these tables are formed, is this: that if two _loadstones_ are perfectly homogeneous, that is, if their matter be of the same specifick gravity, and of the same virtue in all parts of one stone, as in the other; and that like parts of their surfaces are cap'd or arm'd with iron; then the weights they sustain will be as the squares of the cube roots of the weights of the _loadstones_; that is, as their surfaces." upon lifting-power see also d. bernoulli, _acta helvetica_, iii., p. , ; p. w. haecker, _zur theorie des magnetismus_, nürnberg, ; van der willigen, _arch. du musée teyler_, vol. iv., haarlem, ; s. p. thompson, _philos. magazine_, july, . in the book of james hamilton, p. , he mentions a small terrella weighing english grains, which would sustain no less than , grains, and was valued at £ s. ¾d. { } in the _musæum septalianum_ of terzagus (dertonæ, , p. ) is mentioned a loadstone weighing twelve ounces which would lift sixty pounds of iron. sir isaac newton had a loadstone weighing grains, which he wore in a ring. it would lift grains. thomson's _british annual_, , p. , gives the following reference: "in the _records of general science_, vol. iii., p. , there is an interesting description of a very powerful magnet which was sent from virginia in by the celebrated dr. franklin to professor anderson, of glasgow. it is now in the possession of mr. crichton. it weighs ½ grains, and is capable of supporting a load of grains, which is equivalent to times its own weight." [ ] page , line . page , line . _manifestum est._--in this, as in many other passages, gilbert uses this expression in the sense that _it is demonstrable_ rather than meaning that _it is obvious_: for the fact here described is one that is not at all self-evident, but one which would become plain when the experiment had been tried. for other instances of this use of _manifestum_ see pages , line ; , line ; , line . [ ] page , line . page , line . _si per impedim[=e]ta ... pervenire possunt._--all editions agree in this reading, but the sense undoubtedly requires _non possint_. compare p. , line . [ ] page , line . page , line . _capite_ .--this is a misprint for _capite_ , and is retained in the later editions. in the quotation from baptista porta, where the english version of is adhæred to, the words "& deturbat eam" have been omitted by the translator. [ ] page , line . page , line . _cardanus scribit._--the alleged perpetual motion machine is mentioned in _de rerum varietate_, _lib._ , cap. xlviii. (basil., , p. ). see also the note to p. . for peregrinus and for taisnier, see the note to p. , lines and . [ ] page , line . page , line . _antonij de fantis._--his work is: _tabula generalis scotice subtilitatis octo sectionibus vniuersam doctoris subtilis periti[=a] c[=o]plect[=e]s: ab excellentissimo doctore antonio de f[=a]tis taruisino edita ..._ lugd., . [ ] page , line . page , line . _cusani in staticis._--see the note to p. , line . [ ] page , line . page , line . _languidi ... tardiùs acquiescunt._--the editions of and omit these seven words. [ ] page , line . page , line . _halinitro._--either native carbonate of soda or native carbonate of potash might be meant, but not saltpetre. scaliger, in his _de subtilitate ad cardanum_ (lutet., , p. ), _exercitatio_ ciii., , under the title, _nitrum non est salpetræ_, says: "more tuo te, tuaque confundis. salpetræ inter salis fossilis ponis hîc. mox halinitrum inter salis, & nitri naturam, speciem obtinere." "_sal nitrum_ is salt which is boiled out of the earth, especially fat earth, as in stables, or any place of excrements." (_a chymicall dictionary explaining hard places and words met withall in the writings of paracelsus ..._, lond., .) [ ] page , line . page , line . _arte ioculatoriâ._--edition , _joculatoriâ_; edition , _jaculatoriâ_. [ ] page , line . page , line . _qualis fuit antonij denarius._--the elizabethan version of pliny (book xxxiii., ch. ix., p. ) runs thus: { } "to come now unto those that counterfeit money. _antonius_ whiles hee was one of the three usurping triumvirs, mixed yron with the romane silver denier. he tempered it also with the brasen coine, and so sent abroad false and counterfeit money." georgius agricola (_de natura fossilium_, p. ) says: "sed ea fraus capitalis est, non aliter ac eorum qui adulterinas monetas cudunt, argento miscentes multam plumbi candidi portionem, aut etiam ferri, qualis fuit antonii denarius, ut plinius memoriæ tradidit. nunc dicam de candido plumbo, nam majoris pretii est quàm aes. in quod plumbum album, inquit plinius, addita aeris tertia portione candidi adulteratur stannum." [ ] page , line . page , line . _meminerunt chatochitis lapis plinius, atque iulius solinus._--the passage in pliny (english version of , book xxxvii., ch. x., p. ) runs: "catochitis is a stone proper unto the island corsica: in bignesse it exceedeth ordinarie pretious stones: a wonderfull stone, if all be true that is reported thereof, and namely, that if a man lay his hand thereon, it will hold it fast in manner of a glewie gum." [ ] page , line . page , line . _sagda vel sagdo._--albertus magnus in _de mineralibus_ (venet., , p. ) says: "sarda quem alij dicunt sardo lapis est qui se habet ad tabulas ligni sicut magnes ad ferr[=u], et ideo adhæret ita fortiter tabulis nauium quòd euelli n[=o] possit, nisi abscindatur cum ipso ea pars tabulæ cui inhæserit, est aut[=e] in colore purissimus nitens." and pliny (_op. citat._, p. ): "sagda is a stone, which the chaldeans find sticking to ships, and they say it is greene as porrets or leekes." [ ] page , line . page , line . _euace._--evax, king of the arabs, is said to have written to nero a treatise on the names, colours, and properties of stones. see the note on marbodæus, p. , line . [ ] page , line . page , line . _repulsus sit._ the words read thus in all editions, but the sense requires _repulsa sint._ [ ] page , line . page , line . _electrica omnia alliciunt cuncta, nihil omninò fugant vnquam, aut propellunt._ this denial of electrical repulsion probably arose from the smallness of the pieces of electric material with which gilbert worked. he could hardly have failed to notice it had he used large pieces of amber or of sealing-wax. electrical repulsion was first observed by nicolas cabeus, _philosophia magnetica_, ferrara, ; but first systematically announced by otto von guericke in his treatise _experimenta nova (ut vocantur) magdeburgica, de vacuo spatio_ (amstel., ). [ ] page , line . page , line . _cùm de calore quid sit disputabimus._--the discussion of the nature of heat is to be found in gilbert's _de mundo nostro sublunari_ (amstel., ), lib. i., cap. xxvi., pp. - . [ ] page , line . page , line . _trium vel quatuor digitorum._--here as in all other places in gilbert, _digitus_ means a finger's breadth, so that three or four digits means a length of two or three inches, or from six to eight centimetres. [ ] page , line . page , line . _ille thebit bencoræ trepidationis motus._ "trepidation in the ancient astronomy denotes a motion which in the ptolemaic system was attributed to the firmament, in order to account for { } several changes and motions observed in the axis of the world, and for which they could not account on any other principle." (barlow's _mathematical dictionary_.) [ ] page , line . page , line . _cuspis is aut lilium._--gilbert uses _cuspis_ or _lilium_ always of the north-pointing end of the needle. sir thomas browne speaks of "the lilly or northern point"; but he differs from gilbert in saying "the _cuspis_ or southern point" (_pseudodoxia epidemica_, , p. ). only in one place (p. , line ) does gilbert speak of _cuspis meridionalis_. everywhere else the south-pointing end is called the _crux_. [ ] page , line . page , line . _nam æquè potens est._--later observation showed this view to be incorrect. the horizontal component of the earth's magnetic field is not equally strong all over the globe, and the sluggishness of the needle's return to its position of rest is not due to the supporting pin becoming blunt with wear. the value of the horizontal component is zero at the north magnetic pole, and increases toward the magnetic equator. it is greatest near singapore and in borneo, being there more than twice as great as it is at london. (see captain creak in _report of voyage of h.m.s. challenger, physics and chemistry_, vol. ii., part vi., .) [ ] page , line . page , line . _lapis._--both stettin editions read _lapidis._ [ ] page , lines - . page , lines - . the gist of the whole book is summarized in these lines. they furnish a cardinal example of that inductive reasoning which was practist by gilbert, and of which bacon subsequently posed as the apostle. compare pages and . [ ] page , line . page , line . _dicturi sumus_.--change of verticity is treated of in book iii., chap. x., pp. to . [ ] page , line . page , line . _appositam._--all editions give this word, though the sense requires _appositum._ [ ] page , line . page , line . _non nimis longum._--the editions of and read (wrongly) _minus_ instead of _nimis_. [ ] page , line . page , line . the word _hunc_ in the folio of is corrected in ink to _tunc_, and the stettin editions both read _tunc_. [ ] page , line . page , line . _minimus & nullius ponderis._--the editions of and both wrongly read _est_ for _&_. [ ] page , line . page , line . _nutat._--the editions of and both wrongly read _mutat_. [ ] page , line . page , line . _in rectâ sphærâ._--the meaning of the terms a _right_ or _direct sphere_, an _oblique sphere_ and a _parallel sphere_ are explained by moxon on pages to of his book _a tutor to astronomy and geography_ (lond., ): "a _direct sphere_ hath both the _poles_ of the _world_ in the horizon ... it is called a _direct sphere_, because all the _celestial_ bodies, as _sun_, _moon_, and _stars_, &c. by the _diurnal_ motion of the _primum mobile_, ascend directly above, and descend directly below the _horizon_. they that inhabit under the _equator_ have the _sphere_ thus posited." "an _oblique sphere_ hath the _axis_ of the _world_ neither _direct_ nor _parallel_ to the _horizon_, but lies aslope from it." "a _parallel sphere_ hath one _pole_ of the _world_ in the _zenith_, the other in the _nadir_, and the _equinoctial_ line in the _horizon_." [ ] page , line . page , line . _præsenti._--the editions of and read _sequenti_, to suit the altered position of the figure. [ ] page , line . { } page , line . _atque ille statim._--the stettin editions both wrongly read illi. [ ] page . there is a curious history to this picture of the blacksmith in his smithy striking the iron while it lies north and south, and so magnetizing it under the influence of the earth's magnetism. woodcuts containing human figures are comparatively rare in english art of the sixteenth century; a notable exception being foxe's _acts and monuments_ with its many crude cuts of martyrdoms. the artist who prepared this cut of the smith took the design from an illustrated book of fables by one cornelius kiliani or cornelius van kiel entitled _viridarium moralis philosophiæ, per fabulas animalibus brutis attributas traditæ, etc._ (coloniæ, ). this rare work, of which there is no copy in the british museum, is illustrated by some fine copper-plate etchings printed in the text. on p. of this work is an etching to illustrate the fable _ferrarii fabri et canis_, representing the smith smiting iron on the anvil, whilst his lazy dog sleeps beneath the bellows. the cut on p. of gilbert gives, as will be seen by a comparison of the pictures just the same general detail of forge and tools; but the position of the smith is reversed right for left, the dog is omitted, and the words _septrenio_ and _auster_ have been added. [illustration] in the stettin edition of the picture has again been turned into a copper-plate etching separately printed, is reversed back again left for right, while a compass-card is introduced in the corner to mark the north-south direction. in the stettin edition of the artist has gone back to kiliani's original { } plate, and has re-etched the design very carefully, but reversing it all right for left. as in the london version of , the dog is omitted, and the words _septentrio_ and _auster_ are added. some of the original details--for example, the vice and one pair of pincers--are left out, but other details, for instance, the cracks in the blocks that support the water-tub, and the dress of the blacksmith, are rendered with slavish fidelity. it is perhaps needless to remark that the twelve copper-plate etchings in the edition of , and the twelve completely different ones in that of , replace certain of the woodcuts of the folio of . for example, take the woodcut on p. of the edition, which represents a simple dipping-needle made by thrusting a versorium through a bit of cork and floating it, immersed, in a goblet of water. in the edition this appears, slightly reduced, as a small inserted copper-plate, with nothing added; but in the edition it is elaborated into a full-page plate (no. xi.) representing the interior, with shelves of books, of a library on the floor of which stands the goblet--apparently three feet high--with a globe and an armillary sphere; while beside the goblet, with his back to the spectator, is seated an aged man, reading, in a carved armchair. this figure and the view of the library are unquestionably copied--reversed--from a well-known plate in the work _le diverse & artificiose machine_ of agostino ramelli (paris, ). in the emblems of jacob cats (_alle de wercken_, amsterdam, , p. ) is given an engraved plate of a smith's forge, which is also copied--omitting the smith--from kiliani's _viridarium_. [ ] page , line .. page , line . _præcedenti._--this is so spelled in all editions, though the sense requires _præcedente_. [ ] page , line . page , line . _quod in epistolâ quâdam italicâ scribitur._--the tale told by filippo costa of mantua about the magnetism acquired by the iron rod on the tower of the church of st. augustine in rimini is historical. the church was dedicated to st. john, but in the custody of the augustinian monks. the following is the account of it given by aldrovandi, _musæum metallicum_ ( , p. ), on which page also two figures of it are given: "aliquando etiam ferrum suam mutat substantiam, dum in magnetem conuertitur, & hoc experientia constat, nam arimini supra turrim templi s. ioannis erat crux a baculo ferreo ponderis centum librarum sustentata, quod tractu temporis adeò naturam magnetis est adeptum, vt, illivs instar, ferrum traheret: hinc magna admiratione multi tenentur, qua ratione ferrum, quod est metallum in magnetem, qui est lapis transmutari possit; animaduertendum est id à maxima familiaritate & sympathia ferri, & magnetis dimanare cum aristoteles in habentibus symbolum facilem transitum semper admiserit. hoc in loco damus imaginem frusti ferri in magnetem transmutati, quod clarissimo viro vlyssi aldrouando iulius caesar moderatus diligens rerum naturalium inquisitor communicauit; erat hoc frustum ferri colore nigro, & ferrugineo, crusta exteriori quodammodo albicante." and further on p. . "preterea id manifestissimum est; quoniam arimini, in templo sancti ioannis, fuit crux ferrea, quæ tractu temporis in magnetem conuersa est, & ab vno latere ferrum trahebat, & ab altero respuebat." see also sir t. browne's _pseudodoxia epidemica_ (edition of , p. ), and boyle's tract, _experiments and notes about the mechanical production of magnetism_ (london, , p. ). { } another case is mentioned in dr. martin lister's _a journey to paris_ (lond., , p. ). "he [mr. butterfield] shewed us a loadstone sawed off that piece of the iron bar which held the stones together at the very top of the steeple of _chartres_. this was a thick crust of rust, part of which was turned into a strong loadstone, and had all the properties of a stone dug out of the mine. _mons. de la hire_ has printed a memoir of it; also mons. _de vallemont_ a treatise. the very outward rust had no magnetic virtue, but the inward had a strong one, as to take up a third part more than its weight unshod." gassendi and grimaldi have given other cases. other examples of iron acquiring strong permanent magnetism from the earth are not wanting. the following is from sir w. snow harris's _rudimentary magnetism_ (london, , p. ). "in the _memoirs of the academy of sciences_ for , we find an account of a large bell at marseilles having an axis of iron: this axis rested on stone blocks, and threw off from time to time great quantities of rust, which, mixing with the particles of stone and the oil used to facilitate the motion, became conglomerated into a hardened mass: this mass had all the properties of the native magnet. the bell is supposed to have been in the same position for years." [ ] page , line . page , line . _tunc planetæ & corpora coelestia._--gilbert's extraordinary detachment from all metaphysical and ultra-physical explanations of physical facts, and his continual appeal to the test of experimental evidence, enabled him to lift the science of the magnet out of the slough of the dark ages. this passage, however, reveals that he still gave credence to the _nativities_ of judicial astrology, and to the supposed influence of the planets on human destiny. [ ] page , line . page , line . _ijdem._--the editions of and erroneously read _iisdem_. [ ] page , line . page , line . _ex optimo aciario._--gilbert recommended that the compass-needle should be of the best steel. though the distinction between iron and steel was not at this time well established, there is no reason to doubt that by _aciarium_ was meant edge-steel as used for blades. barlowe, in his _magneticall advertisements_ (lond., ), p. , gives minute instructions for the fashioning of the compass-needle. he gives the preference to a pointed oval form, and describes how the steel must be hardened by heating to whiteness and quenching in water, so that it is "brickle in a manner as glass it selfe," and then be tempered by reheating it over a bar of red hot iron until it is let down to a blue tint. savery (_philos. trans._, ) appears to have been the first to make a systematic examination of the magnetic differences between hard steel and soft iron. instructions for touching the needle are given in the _arte de nauegar_ of pedro de medina (valladolid, , lib. vi., cap. ). [ ] page , line . page , line . _per multa sæcula._--compare porta's assertion (p. , english edition) "iron once rubbed will hold the vertue a hundred years." clearly not a matter within the actual experience of either porta or gilbert. [ ] page , line . page , line . _cardani ab ortu stellæ in cauda vrsæ._--what cardan said (_de subtilitate_, _edit. citat._, p. ) was: "ortum stellæ in cauda ursæ minoris, quæ quinque partibus orientalior est polo mundi, respicit." [ ] page , line . page , line . _sequitur quod versus terram magnam, siue continentem ... à vero polo inclinatio magnetica fiat._--gilbert { } goes on to point out how, at that date, all the way up the west european coast from morocco to norway, the compass is deflected eastward, or toward the elevated land. he argued that this was a universal law. in _purchas his pilgrimes_ (lond., ), in the narrative, in vol. iii., of bylot and baffin's voyage of , there is mentioned an island between whale-sound and smith's sound, where there had been observed a larger variation than in any other part of the world. purchas, in a marginal note, comments on this as follows: "variation of the compass ° to the west, which may make questionable d. gilbert's rule, tom. ., l. , c. , that where more earth is more attraction of the compass happeneth by variation towards it. now the known continents of asia, &c., must be unspeakably more than here there can be, & yet here is more variation then about jepan, brasil, or peru, &c." gilbert's view was in truth founded on an incomplete set of facts. at that time, as he tells us, the variation of the compass at london was - / degrees eastward. but he did not know of the secular change which would in about fifty-seven years reduce that variation to zero. still less did he imagine that there would then begin a westward variation which in the year should reach ° ', and which should then steadily diminish so that in the year it should stand at ° ' westward. for an early discussion of the changes of the variation see vol. i. of the _philosophical transactions_ (abridged), p. . still earlier is the classical volume of henry gellibrand, _a discovrse mathematical on the variation of the magneticall needle_ (lond., ). gilbert heads chapter iii. of book iiii. (p. ) with the assertion _variatio uniuscuiusque loci constans est_, declaring that to change it would require the upheaval of a continent. gellibrand combats this on p. of the work mentioned. he says: "thus hitherto (according to the tenents of all our _magneticall_ philosophers) we have supposed the variations of all particular places to continue one and the same. so that when a seaman shall happly returne to a place where formerly he found the same variation, he may hence conclude he is in the same former _longitude_. for it is the assertion of _mr. dr. gilberts_. _variatio vnicuiusq; loci constans est_, that is to say, the same place doth alwayes retaine the same variation. neither hath this assertion (for ought i ever heard) been questioned by any man. but most diligent magneticall observations have plainely offred violence to the same, and proved the contrary, namely that the variation is accompanied with a variation." in henry bond wrote in the _sea-mans kalendar_ that in the year the variation would be zero at london. compare bond's _longitude found_ (lond., , p. ). as to inconstancy of the variation in one place see further fournier's _hydrographie_ (paris, , liv. xi., ch. , p. ), and kircher, _magnes_ (colon. agripp., , p. ). [ ] page , line . page , line . _perfecto._--though this word is thus in all editions, it ought to stand _perfectâ_, as in line below. [ ] page , line . page , line . _varietas_, for _variatio_. [ ] page , line . page , line . _in borrholybicum._--this name for the north-west, or north-north-west, is rarely used. it is found on the chart or windrose of the names of the winds on pp. and of the _mécometrie de l'eyman_ of g. nautonier ( ). here the name _borrolybicus_ is given as a synonym for _nortouest galerne_, or [greek: olumpias], while the two winds on the points next on the western and northern sides respectively are called _upocorus_ and _upocircius_. { } in swan's _specvlvm mundi_ (camb., , p. ) is this explanation: "borrholybicus is the north-west wind." in kircher's _magnes_ (colon. agripp., , p. ) is a table of the names of the thirty-two winds in six languages, where _borrolybicus_ is given as the equivalent of _maestro_ or _north-west_. [ ] page , line . page , line . _insula in oceano variationem non mutat._--the conclusions derived from the magnetic explorations of the challenger expedition, - , are briefly these: that in islands north of the magnetic equator there is a tendency to produce a local perturbation, attracting the north-seeking end of the needle downwards, and horizontally towards the higher parts of the land; while south of the magnetic equator, the opposite effects are observed. (see _challenger reports, physics and chemistry_, vol. ii., part vi., _report on the magnetical results_ by staff-commander creak, f.r.s.) [ ] page , line . page , line . _quarè & respectiuum punctum ... excogitauit._--the passage referred to is in _the newe attractiue_ of robert norman (lond., ), chap. vi. "your reason towards the earth carrieth some probabilitie, but i prove that there be no _attractive_, or drawing propertie in neyther of these two partes, then is the _attractive_ poynt lost, and falsly called the poynt _attractive_, as shall be proved. but because there is a certayne point that the needle alwayes respecteth or sheweth, being voide and without any _attractive_ propertie: in my judgment this poynt ought rather to bee called the point respective ... this poynt _respective_, is a certayne poynt, which the touched needle doth alwayes _respect_ or shew ..." [ ] page , line . page , line . _de pyxidis nauticæ vsitatæ compositione._--gilbert's description of the usual construction of the mariner's compass should be compared with those given by levinus lemnius in _the secret miracles of nature_ (london, ); by lipenius in _navigatio salomonis ophiritica_ (witteb., , p. ); and with that given in barlowe's _navigators supply_ (london, ). see also robert dudley's _dell' arcano del mare_ (firenze, ). [ ] page deals with the construction; the process of magnetizing by the loadstone had already been discussed in pp. to . it is interesting to see that already the magnetized part attached below the compass-card was being specialized in form, being made either of two pieces bent to meet at their ends, or of a single oval piece with elongated ends. the marking of the compass-card is particularly described. it was divided into thirty-two points or "winds," precisely as the earlier "wind-rose" of the geographers, distinguisht by certain marks, and by a lily--or fleur-de-lys--indicating the north. stevin in the _havenfinding art_ (london, ), from which work the passage on p. is quoted, speaking on p. of "the instrument which we call the sea-directorie, some the nautical box, ... or the sea compasse," mentions the "floure de luce" marking the north. the legend which assigns the invention of the compass to one goia or gioja of amalfi in has been already discussed in the note to page . gilbert generously says that in spite of the adverse evidence he does not wish to deprive the amalfians of the honour of the construction adopted in the compasses used in the mediterranean. but baptista porta the neapolitan, who wrote forty years before gilbert, discredited the legend. "_flavius_ saith, an italian found it out first, whose name was _amalphus_, born in our { } campania. but he knew not the mariners card, but stuck the needle in a reed, or a piece of wood, cross over; and he put the needles into a vessel full of water that they might flote freely." (porta's _natural magick_, english translation, london, , p. .) see also lipenius (_op. citat._ p. ). the pivotting of the needle is expressly described in the famous _epistle_ on the magnet of peter peregrinus, which was written in . gasser's edition, _epistola petri peregrini ... de magnete_, was printed in augsburg in . in part ii., cap. , of this letter, a form of instrument is described for directing one's course to towns and islands, and any places in fact on land or sea. this instrument consists of a vessel like a turned box (or _pyxis_) of wood, brass, or any solid material, not deep, but sufficiently wide, provided with a cover of glass or crystal. in its middle is arranged a slender axis of brass or silver, pivotted at its two ends into the top and the bottom of the box. this axis is pierced orthogonally with two holes, through one of which is passed the steel needle, while through the other is fixed square across the needle another stylus of silver or brass. the glass cover was to be marked with two cross lines north-south and east-west; and each quadrant was to be divided into ninety degrees. this the earliest described pivotted compass was therefore of the cross-needle type, a form claimed as a new invention by barlowe in . the first suggestion of suspending a magnetic needle by a thread appears to be in the _speculum lapidum_ of camillus leonardus (venet., , fig. k ij, lines - ): "nã tacto ferro ex una [=p]te magnetis ex opposita eius [=p]te appropinquato fugat: ut ex[=p]i[~e]tia docet de acu appenso filo." the earliest known examples of the "wind-rose" are those in certain parchment charts preserved in the biblioteca marciana in venice. these go back to or , the best being ascribed to andrea bianco. they have the north indicated by a fleur-de-lys, a trident, a simple triangle, or a letter t; while the east is distinguisht by a cross. the west is marked with a p. (see fincati, _op. citat._). the eight marks in order, clock wise, run thus, [lily] (or t). g. [cross] (or l) s. o. a (or l). p. m. the letters correspond to the italian names of the principal winds: tramontano north. greco north-east. levante east. sirocco south-east. ostro south. africo or libeccio south-west. ponente west. maestro north-west. wind-roses marked with the names of the minor winds are found in nautonier's _mécometrie de l'eyman_ (vennes, - , pp. - ), and kircher's _magnes siue de arte magnetica_ (colon. agripp., , p. ). the description above given of the early venetian wind-roses _exactly_ describes the compass-card as depicted by pedro de medina in his _arte de nauegar_ (valladolid, , folio lxxx.), in the sixth book entitled "las aguias de navegar"; while in the _breve compendio de la sphera_ of martin cortes (sevilla, , cap. iii., _de la piedrayman_) a similar wind-rose, without the letters, is found. { } in the _de ventis et navigatione_ of michaele angelo blondo (venet., , p. ) is given a wind-rose, described as "pixis uel buxolus instrumentum et dux nauigantium," having twenty-six points inscribed with the names of the winds, there being six between north and east, and six between south and west, and only five in each of the other quadrants. in the middle is a smaller wind-rose exactly like the early italian ones just mentioned. in the _della guerra di rhodi_ of jacobo fontano (venet., , pages - ) is a chapter _dei venti, e della bvssola di nauicare di giovanni quintino_, giving a wind-rose, and a table of the names of the winds, the north being indicated by a pointer, at the cusp of which are seven stars, and the west by an image of the sun. the other cardinal points are marked with letters. barlowe, in _the navigators supply_ (lond., ), speaks thus: "the merueilous and diuine instrument, called the _sayling compasse_ (being one of the greatest wonders that this world hath) is a circle diuided commonly into . partes, tearmed by our seamen windes, _rumbes_, or points of compasse." it is a disputed point with whom the method of naming the winds originated. some ascribe it to charlemagne. michiel coignet (_instruction novvelle ... touchant l'art de naviguer_, anvers, , p. ) ascribes it to andronicus cyrrhestes. see varro, _de re rustica_, iii., , , and vitruvius, i., , . gilbert's complaint of the evil practice of setting the needles obliquely beneath the card, with the intention of allowing for the variation, is an echo of a similar complaint in norman's _newe attractiue_. in chapter x. of this work norman thus enumerates the different kinds of compasses: "of these common sayling compasses, i find heere (in _europa_) five sundry sortes or sets. the first is of _levant_, made in _scicile_, _genoüa_, and _venice_: and these are all (for the most parte) made meridionally, with the wyers directlye sette under the south, and north of the compasse: and therefore, duely shewing the poynt _respective_, in all places, as the bare needle. and by this compasse are the plats made, for the most part of all the _levants_ seas. "secondly, there are made in _danske_, in the sound of _denmarke_, and in _flanders_, that have the wyers set at quarters of a point to the eastwards of the north of the compasse, and also some at a whole point: and by these compasses they make both the plats and rutters for the sound. "thirdly, there hath beene made in this countrey particulary, for saint _nicholas_ and _ruscia_, compasses set at seconds of a point, and the first plats of that discoverie were made by this compasse. "fourthly the compasse made at _sevill_, _lisbone_, _rochell_, _bourdeaux_, _roan_, and heere in _england_, are moste commonly set at halfe a point: and by this compasse are the plats of the east and west _indies_ made for their pylotes, and also for our coastes neere hereby, as _france_, _spayne_, _portugall_, and _england_: and therefore best of these nations to bee used, because it is the most common sorte that is generally used in these coastes." bessard (_op. citat._, pages and ) gives cuts of compasses showing the needle displaced one rumbe to the east. gallucci, in his _ratio fabricandi horaria mobilia et permanentia cum magnetica acu_ (venet., ), describes the needle as inclined degrees from the south toward the south-west. the frontispiece of the work of pedro nuñez, _instrumenta artis navigandi_, basil., , depicts a compass with the lily set one point to the east. reibelt, _de physicis et pragmaticis magnetis mysteriis_ (herbipolis, ), depicts the compass with the needle set about degrees to the east of north. see also fournier, _hydrographie_ (paris ); de lanis, _magisterium natvræ et artis_ (brixiæ, ); milliet deschales, _cursus seu mundus { } mathematicus_ (lugd., ). both the latter works give pictures of the compass-cards as used in south europe, and in north europe, and of the various known shapes of needles. [ ] page , line . page , line . _directio igitur inualidior est propè polos._ here as in many passages _direction_ means _the force which directs_. a similar usage prevails with the nouns _variation_ and _declination_, meaning frequently the force causing variation or declination respectively. page , line . _perquirere._ the edition of reads _perquirero_, in error. [ ] page , line . page , line . _ad pyxidis nauticæ veræ & meridionalis formam ... fiat instrumentum._--an excellent form of portable meridian compass, provided with sights for taking astronomical observations, is described by barlowe (_the navigators supply_, london, ), and is depicted in an etched engraving. an identical engraving is repeated in dudley's _arcano del mare_ (firenze, ). gilbert's new instrument was considerably larger. [ ] page , line . page , line . _addendo vel detrahendo prostaphæresin._--"prosthaphæresis, conflata dictione, ex additione et subtractione speciebus logistices, nomen habet ab officio, quia vt in semicirculo altero ad æquabilem motum adijcitur, ita in altero subtrahitur, vt adparens motus ex æquabili taxetur: atque hinc fit, quòd quæ prosthaphæresis dicitur ptolemæo, ea vulgò æquatio vocetur." (stadius, _tabulæ bergenses_, colon. agripp., , p. .) [ ] page , line . page , line . _stellæ lucidæ._--according to dr. marke ridley (_magneticall animadversions_, london, , p. ), this chapter xii. of book iv., with the table of stars, was written by edward wright, the author of the prefatory epistle of _de magnete_. wright was lecturer on navigation to the east india company, and author of sundry treatises on navigation. [ ] page , line . page , line . _hic qui versus boream constitit ... meridionalis est, non borealis, quem antè nos omnes existimabant esse borealem._--earlier on, on pages and , gilbert had mentioned this point. his insistence caused barlowe (_magneticall aduertisements_, , p. ) to speak of the south-pointing end of the needle as the "true north," and thereby drew on himself the animadversions of marke ridley. [ ] page , line . page , line . _in rectâ sphærâ._--see note to p. . [ ] page , line . page , line . _declinans in borealibus._--dipping as it does in northern regions; that is, with the north-seeking or true-south pole downward. [ ] page , line . page , line . _multa maiora pondera._--many greater weights. all editions read _multa_, but the sense requires _multo_: "much greater weights." [ ] page , line . page , line . _constans est._--this must not be read "is constant," for it is constant only in any given latitude. [ ] page , line . page , line . _de proportione declinationis pro latitudinis ratione._--gilbert here announces, and proceeds in the next seven pages to develop, the proposition that to each latitude there corresponds a constant dip to a particular number of degrees. if this were accurately so, then a traveller by merely measuring the dip would be able to ascertain, by calculation, by reference to tables, or by aid of some geometrical appliance, { } the latitude of the place. in this hope gilbert fought to perfect the dipping-needle; and he also worked out, on pages and , an empirical theory, and a diagram. this theory was still further developed by him, and given to thomas blundevile (see the note to p. ). briggs of gresham college, on gilbert's suggestion, calculated a table of dip and latitude on this theory. it was found, however, that the observed facts deviated more or less widely from the theory. kircher (_magnes_, , p. ) gives a comparative table of the computed and observed values. further discovery showed the method to be impracticable, and gilbert's hope remained unfulfilled. [ ] page , line . page , line . _progressionis centri._--note gilbert's precision of phrase. [ ] page , line . page , line . _subintellig[=u]tur._--this is printed _subintelligitur_, and is altered in ink in all copies of the folio edition. the editions of and read _subintelliguntur_. similarly in line the word _ducit_ has had a small _r_ added in ink, making it read _ducitur_, as also the other editions. [ ] page . this figure of the experiment with the simple dipping needle suspended in water in a goblet is due to robert norman. in his _newe attractiue_ (london, , chap. vi.) he thus describes it: "then you shall take a deepe glasse, bowle, cuppe, or other vessell, and fill it with fayre water, setting it in some place where it may rest quiet, and out of the winde. this done, cut the corke circumspectly, by little and little, untill the wyre with the corke be so fitted, that it may remain under the superficies of the water two or three inches, both ends of the wyer lying levell with the superficies of the water, without ascending or descending, like to the beame of a payre of ballance beeing equalie poysed at both ends. "then take out of the same the wyer without mooving the corke, and touch it with the _stone_, the one end with the south of the _stone_, and the other end with the north, and then set it againe in the water, and you shall see it presentlie turne it selfe upon his owne center, shewing the aforesay'd _declining_ propertie, without descending to the bottome, as by reason it should, if there were any _attraction_ downewards, the lower part of the water being neerer that point, then the superficies thereof." [ ] page , line . page , line . _ex altera parte._--the sense seems to require _et altera parte_, but all editions read _ex_. [ ] page , line . page , line . the passage here quoted from dominicus maria ferrariensis, otherwise known as the astronomer novara, does not occur in any known writing of that famous man. it is, however, quoted as being by novara in at least three other writings of the same epoch. see the _tabulæ secvndorum mobilium coelestium_ of maginus (venet., , p. , line to p. , line ); the _eratosthenes batavvs_ of willebrord snell (lugd. batav., , pp. - ); and the _almagesti novi (pars posterior)_ of riccioli (bonon., , p. ). the original document appears to have perisht. see a notice by m. curtze in boncompagni's _bullettino di bibliografia_, t. iv., april, . [ ] page , line . page , line . _philolaus pythagoricus._ "philolaüs a le premier dit que la terre se meut en cercle; d'autres disent que c'est nicétas de syracuse." "les uns prétendent que le terre est immobile; mais philolaüs le pythagoricien dit qu'elle se meut circulairement autour du feu (central) et suivant un cercle oblique, comme le soleil et la lune."--(chaignet, _pythagore et la philosophie pythagoricienne_, paris, .) it appears that the first of these _dicta_ is taken from diogenes laërt., viii. ; and the second from plutarch, _placit. philos._, iii. . the latter { } passage may be compared with aristotle, _de coelo_, ii. , who, referring to the followers of pythagoras, says: "they say that the middle is fire, that the earth is a star, and that it is moved circularly about this centre; and that by this movement it produces day and night." [ ] page , line . page , line . _copernicus._--his work is _de revolutionibus orbium coelestium, libri vi._ (basil., ). [ ] page , line . page , line . _quæ ... in cælo varijs distantijs collocata sunt._--this remark appears to be gilbert's one contribution to the science of astronomy; the stars having previously been regarded as fixed in the eighth sphere all at the same distance from the central earth, around which it revolved. [ ] page , line . page , line . _quem nycthemeron vocamus._--the and editions read _nyctemoron_. [ ] page , line . page , line . _poli verè oppositi sint._--for _verè_, the and editions read _rectæ_. all editions read _sint_, though _sunt_ seems to make better sense. [ ] page , line . page , line . _ad telluris conformitatem._--the word _conformitas_ is unknown in classical latin. [ ] page , line . page , line . _omitto quod petrus peregrinus constanter affirmat, terrellam super polos suos in meridiano suspensam, moveri circulariter integrâ revolutione horis: quod tamen nobis adhuc videre non contingit; de quo motu etiam dubitamus._ this statement that a spherical loadstone pivotted freely with its axis parallel to the earth's axis will of itself revolve on its axis once a day under the control of the heavens, thus superseding clocks, is to be found at the end of chap. x. of peregrinus's _epistola de magnete_ (augsb., ). gilbert, who doubted this experiment because of the stone's own weight is taken to task by galileo, in the third of his dialogues, for his qualified admission. "i will speak of one particular, to which i could have wished that _gilbert_ had not lent an ear; i mean that of admitting, that in case a little sphere of loadstone might be exactly librated, it would revolve in it self; because there is no reason why it should do so" (p. of salusbury's _mathematical collections_, london, ). the jesuit fathers who followed gilbert, but rejected his copernican ideas, pounced upon this pseudo-experiment, as though by disproving it they had upset the copernican theory. [ ] page , line . page , line . this line is left out in the edition. in the edition it was also left out by the printer, and subsequently printed in in the margin, being page of that edition. [ ] page , line . page , line . _vt poli telluris respectus à polis._--if it may be permitted to read _respectu_ for _respectus_ the sense is improved, and the passage may then be translated thus: "that just as it was needful ... that the poles of the earth as to direction should be degrees and more from the poles of the ecliptick; so now, &c." [ ] page , line . page , line . _vt motus quidem obscuri saluarentur._--it has been conjectured that _quidem_ is here a misprint for _quidam_, but the adverb _quidem_ adds a satirical flavour to his argument against the folly of those who held the doctrine of the moving spheres. the verb _salvare_ does not occur in classical latin. [ ] page , line . page , line . _à copernico (astronomiæ instauratore)._--gilbert was the first in england to uphold the doctrines of { } copernicus as to the motion of the earth on its axis and its revolution around the sun. he considered that his magnetic observations brought new support to that theory, and his views are quoted with approbation by kepler, _epitome astronomiæ copernicanæ_ ... authore ioanne keplero ... (francofurti, ); and by galileo, _dialogus de systemate mundi_ (augustæ treboc., ), an english translation of which appeared in salusbury's _mathematical collections and translations_ (london, , pp. to ). for this the book _de magnete_ was considered by many as heretical. many of the copies existing in italy are found to be either mutilated or else branded with a cross. for example, the copy in the library of the collegio romano in rome has book vi. torn out. galileo states that the book of gilbert would possibly never have come into his hands "if a peripatetick philosopher, of great fame, as i believe to free his library from its contagion, had not given it me." in england barlowe, in his _magneticall aduertisements_ ( ), expressly repudiated gilbert's copernican notions, while praising his discoveries in magnetism. marke ridley, while upholding gilbert's views, in his _magneticall animadversions_ ( ) did not consider him "skilfull in copernicus." the jesuit writers, cabeus, kircher, fonseca, grandamicus, schott, leotaudus, millietus, and de lanis, one and all, who followed gilbert in their magnetic writings, repudiated the idea that the magnetism of the globe gave support to the heretical modern astronomy. the works referred to are: cabeus, _philosophia magnetica, in qua magnetis natura penitus explicatur ... auctore nicolao cabeo ferrarensi soc. jesv._ (ferrariæ, ). kircher, _magnes, siue de arte magnetica, libri tres, authore athanasio kirchero ... e soc. iesv._ (romæ, ). grandamicus, _nova demonstratio immobilitatis terræ petita ex virtute magnetica_ (flexiæ, ). this work is most beautifully illustrated with copper-plate etchings of cupids making experiments with terrellas. schott, gaspar, _thaumaturgus physicus_ (herbipolis, ). leotaudus, _r. p. vincentinii leotavdi delphinatis, societ. iesv., magnetologia; in qva exponitvr nova de magneticis philosophia_, (lvgdvni, ). millietus (milliet deschales), _cursus seu mundus mathematicus_ (lugd., ), _tomus primus, tractatus de magnete_. de lanis, _magisterium natvræ et artis. opus physico-mathematicvm p. francisci tertii de lanis, soc. jesv._ (brixiæ, ). [ ] page , line . page , line . _hic finem & periodum imponimus._ on february [ ] gilbert wrote to barlowe (see _magneticall aduertisements_, p. ): "i purpose to adioyne an appendix of six or eight sheets of paper to my booke after a while, i am in hand with it of some new inventions, and i would haue some of your experiments, in your name and inuention put into it, if you please, that you may be knowen for an augmenter of that arte." this he never did. perhaps his appointment (in february, ) as chief physician in personal attendance on the queen interfered with the project; or his death, of the plague, in , intervened before his intention had been carried into effect. but it is probable that the substance of the proposed additions is to be found in the chapter, publisht in gilbert's lifetime, in blundevile's _theoriques of the seuen planets_ (london, ), thus described in the title-page of the work: "there is also hereto added, { } the making, description, and vse, of two most ingenious and necessarie instruments for sea-men, to find out thereby the latitude of any place vpon the sea or land, in the darkest night that is, without the helpe of sunne, moone, or starre. first inuented by m. doctor gilbert, a most excellent philosopher, and one of the ordinarie physicians to her maiestie: and now here plainely set downe in our mother tongue by master blundeuile." of these two instruments the first consists of a mechanical device, with movable quadrants, to be cut out in cardboard, to be used in connection with the diagram of spiral lines which gilbert had given as a folding plate between pages and of _de magnete_. the intention was that the sea-man having found by experiment with a dipping-needle the amount of the dip at any place, should by applying this diagram and its moving quadrants, ascertain the latitude, according to the theory expounded in book v., chap. vii. the second instrument is a simplified portable dipping-needle, having the degrees engraved on the inner face of a cylindrical brass ring. blundevile adds a table, calculated by briggs, and "annexed to the former treatise by _edward wright_, at the motion of the right worshipful m. doctor _gilbert_." this gives the values of the dip for different latitudes, as calculated from gilbert's empirical theory. the other work, _de mundo nostro sublunari philosophia nova_, which gilbert left in manuscript at his death, does not contain any additional matter on the magnetical investigations. though it contains several direct references to the _de magnete_, and particularly to book vi. on the rotation of the earth, it is doubtful whether it was written after or before the publication of _de magnete_. on pages to of the posthumous edition (amsterdam, ) gilbert refers to peregrinus's alleged perpetually revolving sphere, and denies its possibility. the greater part of the work is an anti-aristotelian discussion on air, meteorology, astronomy, the winds, tides, and springs. [illustration] * * * * * { } index to authorities abano, pietro di, . acosta, josephus, . addison, joseph, . aepinus, . aetius amidenus, . affaytatus, , . agricola, georgius, , , , . agrippa, h. cornelius, . albategnius, . albertus magnus, , , , , , . aldrovandi, ulisse, , , . alexander aphrodiseus, , . amatus lusitanus, , . apponensis, petrus, . _see_ abano. aquinas, st. thomas, , . ardoynis, santes de, . aristotle, , , , . arnaldus de villa nova, . augustani, . augustine, st., , , . aurifaber, . averroes, . avicenna, , , , . azuni, . bacon, lord, , . barlow, peter, . barlowe, william, , , , , , , , , , , , . beckmann, johann, , , . bencora, or ben korrah (thebitius), . benjamin, park, , , . bernoulli, d., . bertelli, timoteo, . bessard, toussaincte de, , . bianco, andrea, . blackmore, r. d., . blondo, michaele angelo, , . blondus, flavius, . blundevile, thomas, , , , , . bond, henry, . borough, william, , . boyle, robert, , . brasavolus, antonius musa, , . briggs, henry, , . brough, r. b., . browne, sir thomas, , , , , . brunfels, otho, , . buffum, w. a., . burton, sir richard f., . buttmann, , . buxtorf, . cabeus, nicolas, , , , . cabot, sebastian, , . caesar (or cesare, giulio), . calaber, hannibal rosetius, . calcagninus, cælius, , . camden, william, . camera, matteo, . cardan, hieronymo, , , , , , , , , , . casaubon, . cats, jacob, . cavallo, tiberius, . chaignet, . charlemagne, . charles ii., . charleton, dr. w., . chladni, . coignet, michiel, . collenuccio, pandolfo, . conimbricenses, . cordus, valerius, . cortes, martin, , , . costa, filippo, , . costa, joseph. _see_ acosta. costaeus, joannes, of lodi, , . creak, captain, , . creech, t., . crescentius, . crollius, o., . curtius, . curtius, n., . curtze, m., . cusan (cardinal de cusa), , , . cyrrhestes, andronicus, . davies, t. s., . de la hire, . de lanis, , . delaunay, . diogenes laertius, . dioscorides, , , , , , . dominicus maria ferrariensis (novara), . drake, sir francis, . dudley, sir robert, , , . du puys (puteanus), , . { } encelius, _or_ entzelt, , , . erastus, thomas, . euripides, . evax, , . evelyn, john, . fallopius, gabriellus, , . fantis, antonius de, . ficino, marsiglio, . fincati, admiral, . fletcher, l., . fonseca, . fontano, jacopo, . forcellini, . fournier, g., , , , , . foxe, . fracastorio, hieronymo, , , , . galen, , , , . galileo, , . gallucci, . gartias ab horto, , , . gassendi, . gasser, achilles p., , . geber, . gellibrand, henry, . gemma, cornelius, . gessner, . gilbert, adrian, . gilbert, william, _de mundo nostro sublunari_, , , . gioia, _or_ goia, , . glanvill, joseph, . goebel, . goethe, . gonzalus oviedus, . goppert, . goropius, henricus becanus, . gralath, d., . grandamicus, . grew, n., . grimaldi, . grotius, hugo, . guericke, otto von, , . haecker, p. w., . hakewill, g., . hakluyt, , . hali abbas, , , . hamilton, james (earl of abercorn), . hariot, thomas, . harris, sir w. snow, , . hartmann, p. j., . hellmann, g., . hermann, d., . hermolaus barbarus, . hill, "sir" john, , , . hood, t., . hues, robert, . humboldt, , , , . hyginus, . isidore, st., , , . kendall, abraham, . kepler, . kiel, cornelius van (_or_ kiliani), . king, edward, . kircher, athanasius, , , , , , , . klaproth, , , , , , . kudrun, . langius, joannes, . lanis, f. de, , . leonardus, camillus, , . leotaudus, . levinus lemnius, , , , . libri, . linna, nicolas de, . lipenius, , . lister, martin, . livio sanuto, , . livy, . lonicer, joannes, , , . löwy, . lucretius, , . maginus, . magnus, sir philip, . manardus, joannes, . marbodeus, , , , , , . marcellus empiricus, , . marco polo, . martial, , . martin, th. henri, , , , . maskelyne, n. story, . matthæus silvaticus, . matthiolus, p., , , . maurolycus, franciscus, . medina, pedro de, , . mercator, , . merula, gaudentius, , . merula, p., . miers, h. a., . migne, . milliet deschales, , . monardus, nicolas, . montanus, joannes baptista, . morris, william, . moxon, joseph, . muellenhoff, k., . nautonier, g., , . neckham, alexander of, . newton, sir isaac, . nicander, . nicolaus myrepsius (_or_ præpositas), , . { } nonius petrus (_or_ nuñez), . nordenskjold, . norman, robert, , , , , , , . novara, dominicus maria, . offusius, joannes franciscus, . olaus magnus, , . oribasius, . orpheus, , . ovid, . oviedo, luis de, . oviedus, gonzalus, . paley, f. a., . palm, g. a., . paracelsus (bombast von hohenheim), , . paulus aeginæ, . paulus jovius, . paulus venetus (fra paolo sarpi), . pepys, samuel, . peregrinus, petrus, , , , , , . pettus, sir john, , . philolaus, . photius, . pictorio, g., . plancius, . plat, sir hugh, , , . plato, , . plautus, . pliny (caius plinius secundus), , , , , , , , , , , , , , , , . plot, rob., . plutarch, , , , . polo, marco, . porta, joannes baptista (giambattista della porta), , , , , , , , , . poynting, j. h., . præpositas (_or_ nicolas myrepsius), , . ptolemy, , . purchas, . puteanus, gulielmus, , . quintino, giovanni, . ramelli, agostino, . rammelsberg, . reibelt, . rhazes, , , . riccioli, . ridley, marke, , , , . robertson, rev. alexander, . rücker, arthur w., . ruellius, joannes, , , , . rueus, franciscus, . ruysch, johan, . salusbury, t., , . santes de ardoynis, . sanuto, livio, , . sarpi, fra paolo, , . savery, servington, , . scaliger, j. c., , , , , , . scheins, m., . schindler, a. houtum, . schott, g., . schweigger, j. c., . sendel, nathaniel, , . sennert, daniel, . serapio, , , . severt, jacques, . shakespeare, william, . siemens, . signorelli, . silvaticus, matthæus, . smith, dr. william, . snell, willebrord, . solinus, julius, , , . stadius, . stephanus, . stevinus, simon, . strabo, . swan, john, . swinden, j. h. van, . taisnier, joannes, , . terzagus, , . thalèn, . thales, . thebit ben korrah, , . themistius, . theophrastus, , , . thompson, silvanus p., . thomson, r. d., . thorpe, t. e., . vallemont, abbé de, . van swinden, . varro, . venanson, flaminius, . venner, dr. t., . vergil, , . virgil, polydore, . vitruvius, . waring, e. j., . watson, william, . wigand, johann, . wilde, henry, . willigen, van der, . wren, sir christopher, . wright, edward, , , . [illustration] chiswick press: charles whittingham and co. tooks court, chancery lane, london. william gilbert, and terrestrial magnetism in the time of queen elizabeth: a discourse by silvanus p. thompson, f.r.s. william gilbert and terrestrial magnetism in the time of queen elizabeth. william gilbert, the father of electrical science, was born in colchester in . educated at st. john's college, cambridge, where he took his degree as doctor of medicine in , he settled, after four years of foreign travel, in london in , and was admitted to the royal college of physicians, of which he became censor, treasurer, and, in , president. he was in february, , appointed personal physician to the queen, whom he attended in her last illness. he came of a well-known east anglian family, and held extensive landed estates in essex and suffolk. he survived the queen only eight months, dying november th, . gilbert's monumental work, the _de magnete_, published in , marks an era in magnetic science. for some four hundred years the employment of the magnetic needle in navigation had been known both in northern and southern europe. while it is possible that the primitive use of the loadstone may be ascribed to the baltic, it is certain that the employment of a pivotted needle, and the addition of a rose of the winds as a compass card both originated in the mediterranean. the pivotted needle is described in the epistle of peter peregrinus, written in ; while the earliest known compass-card marked with the initials of the names of the winds is that ascribed to jachobus giraldis, of , in the biblioteca marciana in venice. the manner of use in elizabethan times of the loadstone and of the compass may be gathered from olaus magnus (_historia de gentibus septentrionalibus_, ), from pedro de medina (_arte de nauegar_, ), martinus cortes (_breve compendio de la sphera_, ), blundevile (_exercises_, ), norman (_newe attractive_, ), borough (_a discours of the variation of the cumpas_, ), pedro nuñez (_instrumenta artis navigandi_, ), barlow (_the navigators supply_, ), nautonier (_mécometrie de l'eyman_, ), and stevin (_die havenvinding_, ). at the time when steering by the compass was introduced into navigation, the compass pointed in middle europe so nearly truly to the north that with the rough instrumental appliances at hand its deviation from the true north was seldom noticed, or if noticed ascribed to some error in the setting of the needle. later the compass-makers began to set their needles slightly askew beneath the card, according to the variation in the place of origin. norman ( ) states that those used in the levant, made in sicily, genoa, or venice, had the needles straight, while those used in denmark and flanders had them set at three-quarters of a point, or a whole point, to the eastward; while those made in spain, portugal, france, and england, had the needles set half a point to the east. those for russia were set at "three seconds of a point." gilbert denounced these devices as tending to obscure the true facts. gradually it became recognized, probably after the voyage of columbus, when the manifest change in the declination of the needle nearly caused mutiny of the sailors, that the direction of the needle differs at different places; and accordingly navigators began to collect data. the record of the voyage of columbus states that during his second voyage in he used for steering the observations made on the declination during his first voyage. the "secret" of sebastian cabot, which he declared when dying to be a divine revelation to him, can have been little else than the idea of using in navigation the local declinations of the compass. on the other hand, pedro de medina flatly denied the existence of the declination, adding that if the compass did not show the pole, the fault lay in the defective construction of the compass itself. columbus had found a point - / ° east of corvo, in the azores, where there was "no variation," and other navigators explored the "agonic" lines which crossed the atlantic and the indian ocean. according to humboldt, alonzo de santa cruz in constructed the first general variation chart. but along with this development of practical interest in the subject there grew up a crop of wild legends to account for the irregularities observed. the reason why the compass needle pointed north, and the reason why it did not point truly north, were alike proclaimed to be due to the stars, to the influence of spirits, or to the existence of loadstone mountains of uncertain locality and of fabulous power. the old traditions of the arabian nights, dressed in a newer setting, found themselves justified by the insertion in maps of loadstone rocks, the position of which changed at the fancy of the chartographer. ptolemy had located them in the manioles; olaus magnus declared them to be under the pole; garzias ab horto situated them in the region of calcutta. the map of johann ruysch, which adorned the edition of ptolemy, publisht at rome in , showed four magnetic islands in the arctic circle. martinus cortes placed the loadstone mountains in sarmatia. mercator in his great chart depicted two great rocks rising from the sea to the north of eastern siberia, one being drawn on the supposition that at st. michael the compass points due north, while the other is further north on the supposition that the compass points due north at corvo. the map of cornelius wytfliet, , shows the same phantom islands. blundevile, writing in of the now lost map of peter plancius, mentions that he sets down the pole of the loadstone somewhat to southward of the islands that lie east of groynelande. meantime another significant fact had been discovered in by robert norman, of limehouse, compass-maker, namely, the tendency of the magnetized needle to dip its northern end downwards. noticing this as a circumstance that occasioned him some trouble in the construction of his compasses, he thereupon devised a dipping-needle, and measured the dip, "which for this cyty of london i finde by exact obseruations to be about degrees mynutes." he attributed both the declination and the dip of the needle to the existence of a "poynt respective," which the needle respected or indicated, but toward which it was not attracted. the first authoritative treatise on the variation of the compass was the tract by william borough, comptroller to the navy, who in found an eastward declination of ° ' at limehouse. borough had himself travelled in northern regions and had found at vaigats a westerly declination of degrees, whereas by norman's theory of the respective point there should have been an easterly declination of ° '. the great navigators were continually bringing home fresh information. drake, lynschoten, cavendish, hariot all contributed; as did lesser men such as abraham kendall, sailing-master to sir robert dudley (the _soi-disant_ duke of northumberland), and afterward companion of drake in his last voyage. teachers of navigation such as simon stevin of bruges and edward wright, lecturer to the east india company, might record and tabulate: but a master-mind was wanting to forge some larger and consistent doctrine which should afford a grasp of the whole subject. such an one arose in dr. william gilbert. nurtured, as we have seen, in the cambridge which had so recently been the home of linacre and of kaye--the kaye who founded caius college--gilbert had, during his subsequent sojourn in italy, conversed with all the learned men of his time. he had experimented on the magnet with fra paolo sarpi: he had, there is reason to think, met giordano bruno: he was the friend and correspondent of giovanni francesco sagredo. being a man of means and a bachelor, he spent money freely upon books, maps, instruments, minerals, and magnets. for twenty years he experimented ceaselessly, and read, and wrote and speculated, and tested his speculations by new experiments. for eighteen years he kept beside him the manuscript of his treatise, which in the year saw the light under the title of _de magnete_, to which was added the sub-title: _magneticisque corporibus, et de magno magnete tellure, physiologia nova_. that which gilbert had in fact perceived, and which none before him had glimpsed even dimly, was that the globe of the earth itself acted as a great loadstone, and that the tendency of the needle to point in a polar direction was due to the globe acting as a whole. so he boldly put into his title-page the statement that his new philosophy was concerning the great magnet the earth: and in chapter after chapter he set himself to describe the experiments upon which he founded his famous induction. the phrase _terrestrial magnetism_ does not occur in any of the prior treatises, because the idea had not presented itself. gilbert piled proof upon proof, sometimes most cogently, as when he constructed loadstone globes, or _terrellas_ to serve as magnetic models of the earth; sometimes with indifferent logic, as when he pointed to the iron ore in the earth and reasoned that the magnet tended to conform to (_i.e._ turn itself toward) the homogenic substance of the body from which it had been dug. the local deviations of the compass he sought to account for by the irregularities of the earth's crust, and maintained that the compass tended always, at places off the coast of a continent, to be deflected somewhat toward that continent. his syllogism was based on the fact that at that date all the way up the atlantic seaboard of europe, from morocco to norway, the variation was eastward. he argued that this was a universal law. but even within one generation, as may be seen in _purchas his pilgrims_, in the narrative of the voyage of bylot and baffin, the generality of the law was questioned. gilbert reasoned on such knowledge as he had, and this did not include any notion of the secular changes in the declination. in his time, as he tells us, the variation of the compass at london was - / degrees. what he did not know was that this was a diminishing quantity which in fifty-seven years would be reduced to zero, to be succeeded by a westward declination that would last for nearly three hundred years. for the facts as known in the thirty years succeeding gilbert's death, see the remarkable and scarce volume of gellibrand: _a discourse mathematical on the variation of the magneticall needle_ ( ). gilbert's treatise is a skilful literary achievement in which there is no trace to reveal whether any part was written before the rest. it is divided systematically into six books. the sixth book only appears to suffer from some incompleteness. it relates not so much to the magnet as to the copernican theory of the universe, which doctrine gilbert had eagerly espoused, and which he was the first in england to proclaim. it is known from a letter to barlow, printed in , that he intended to add to it certain chapters descriptive of some of his instruments, but he had not completed these before his death. the first book treats of historic accounts of the loadstone, of its origin and properties, of iron ores in general, and of the fables and vain opinions which in the handling of paracelsus and of the schoolmen had grown up around the magnet. the second book is on the magnetic motions, and primarily on the attractions and repulsions between loadstones, between loadstone and iron, and between magnetic needles. in this book occurs the notable digression upon the subject of amber and the electric forces of amber and of other substances which when rubbed show, as he discovered, similar electrical powers. an analysis of this part, and a summary of gilbert's electrical discoveries will be found in the notes printed for the gilbert club to accompany the english translation ( ) of the _de magnete_. after this digression gilbert returns to the attractive properties of the loadstone, and to the way they are affected by giving it different shapes. in the course of this enquiry, he announces his discovery of the augmentation of the power of the loadstone by arming it with iron caps, an invention which caused galileo to say: "i extremely praise, admire, and envy this author for that a conception so stupendous should come into his mind. i think him moreover worthy of extraordinary applause for the many new and true observations that he has made." gilbert further pointed out that the loadstone is surrounded by a sort of atmosphere or "orbe of virtue" within which the magnetical effects can be observed. book , on the directive force of the magnet, is full of most instructive experiments, in which the terrella figures largely, relating to the question how one magnet influences another and tends to make it point toward it. all this was leading up to the theory of terrestrial magnetism; for we find him naming the parts of his loadstone globes with poles, equator and meridians. in this book he dilates on the observation that vertical iron rods, such as the finial on the church of st. john at rimini, spontaneously acquired magnetic properties. this he traced to the influence of the earth, and demonstrated the effect by magnetizing iron bars by simply hammering them on the anvil while they lay in a north and south position. book deals with the declination, or, as it was then called, the variation of the compass. he discusses its observation and measurement, the influence of islands, the results obtained by travellers to distant parts, nova zembla, the guinea coast, the canary isles, florida, virginia, cape race, and brazil. then he recounts his experiments with terrellas having uneven surfaces to represent the irregularities of the earth's crust. he points out errors arising from the fallacious practice of setting the needle obliquely under the card. he considers in separate chapters the variations in nova zembla, in the pacific, in the mediterranean, and in the eastern ocean. the fifth book is on the dip. gilbert seized with avidity on norman's discovery of this effect, and devised an improved form of dipping-needle. he experimented on the dip of compass-needles placed at different points over his terrella, and evolved a theory on the proportion which he conceived to exist between the latitude and the dip. arguing from all too imperfect data, he propounded the view that the dip was the same in any given latitude; and proposed that seamen should ascertain their latitudes by simply observing the dip. he was aware that local irregularities might occur, as they do in the declination; but was not deterred by this knowledge from propounding his theory with much circumstance and considerable geometrical skill. after the publication of his book he developed the theory still further and gave it to blundevile for publication. at gilbert's suggestion briggs of gresham college calculated out a table of dip and latitude. it was, however, soon found that the facts deviated more or less widely from the theory. further observations in other lands showed the method to be impracticable; and gilbert's hope to give to the mariner a magnetic measure of latitude remained unfulfilled. book closes with an eloquent passage in which gilbert affirmed the neo-platonic doctrine of the animate nature of the universe, and asserted that thales was right when he held (as aristotle relates in the _de anima_) that the loadstone was animate, being part of and indeed the choice offspring of its animate mother the earth. book , as already mentioned, is devoted to copernican ideas, and contains gilbert's one contribution to the science of astronomy, in his remark that the fixed stars (previously regarded as fixed in the eighth of the celestial spheres at one common distance from the central earth) were in reality set in the heavens at various distances from the earth. from this brief analysis it will be seen that gilbert's claims to eminence rest not upon any particular discovery or invention, but upon his having built up a whole experimental magnetic philosophy on a truly scientific basis, in place of the vague and wild speculations which had previously been accepted. by his magnificent generalization from the small scale models to the globe itself, supported from point to point by experimental researches, he created the science of terrestrial magnetism. if from the imperfection of the data at his disposal he fell into sundry errors of detail, he yet founded the method of philosophizing by which those errors were in due time corrected. and if for nothing else than his masterly vindication of scientific method, and his rescue of the subject of magnetism from the pedantry and charlatanry into which in the preceding ages it had lapsed, his memory must be held in high honour. alas that of the personality of so great a man so little should be known. a brief but characteristic biography of him is enshrined by old fuller in his _worthies_. the poet dryden, and the epigrammatist owen, celebrated him in still briefer verse. his portrait, which hung for nigh two hundred years in the schools gallery, at oxford, disappeared a century ago, leaving only a poor engraving to perpetuate his scholarly countenance. doubtless he is one of the four physicians depicted by the pencil of camden in his famous cartoon (now in the british museum), as walking in the funeral procession of queen elizabeth. of his handwriting not a vestige was known until about five years ago, when a signature was unearthed in the record office. subsequently four signatures were found in the books of st. john's college; and recently there has come to light a volume of aristotle bearing gilbert's own marginal notes. his will lies at somerset house, but it is only a copy. of his fine collection of minerals and loadstones, which with his maps, books, manuscripts, and correspondence with sarpi and sagredo and others, he bequeathed to the college of physicians, nothing remains: they perisht in the great fire of london. in a quiet corner of the city of colchester stands the quaint old house where he lived, and where, according to local tradition, he once received the queen. and hard by it is the church of holy trinity, in which a mural tablet records his virtues and marks his last resting place. but his true monument is the immortal treatise in which he laid the foundations of terrestrial magnetism and of the experimental science of electricity. to the names of the men who made great the age of queen elizabeth, who added lustre to the england over which she ruled, and made it famous in foreign discovery, in sea-craft, in literature, in poetry, and in drama, must be joined that of the man who equally added lustre in science, doctor william gilbert. this discourse on william gilbert and terrestrial magnetism in the time of queen elizabeth was delivered by silvanus p. thompson at the meeting of the royal geographical society on march twenty-third mdcccciii on the occasion of the tercentenary of the death of queen elizabeth, and is now printed by charles whittingham and company at the chiswick press. [illustration] transcriber's note: the original book used the long "s", which has been changed to the modern "s" here. other archaic spellings have not been changed. the illustrations are small decorations. generously made available by internet archive (https://archive.org) note: project gutenberg also has an html version of this file which includes the original illustrations. see -h.htm or -h.zip: (http://www.gutenberg.org/files/ / -h/ -h.htm) or (http://www.gutenberg.org/files/ / -h.zip) images of the original pages are available through internet archive. see https://archive.org/details/michaelfaradayma jerr transcriber's note: text enclosed by underscores is in italics (_italics_). [illustration: michael faraday.] michael faraday: man of science. by walter jerrold. "whose work was wrought for love, and not for gain." "one rule his life was fashioned to fulfil; that he who tends truth's shrine and does the hest of science, with a humble, faithful will, the god of truth and knowledge serveth best." fleming h. revell company, new york chicago toronto publishers of evangelical literature. [illustration] preface. "tyndall, i must remain plain michael faraday to the last." in these words, with which he replied to professor tyndall's urgent appeal to him to accept the presidency of the royal society, we have a key-note to the character of the illustrious yet modest scientist, the good and great man, whose life-story i have attempted to tell in the following pages. a life-story such as that of michael faraday is both easy and difficult to tell--it is easy in that he passed a simple and unadventurous life; it is difficult, partly, perhaps, for the same reason, and partly because the story of his life-work is a story of the wonderful advance made in natural science during the first half of the present century. any detailed account of that scientific work would be out of place in a biography such as the present, which aims at showing by the testimony of those who knew him and by an account of his relations with his fellow-men, how nobly unselfish, how simple, yet how grand and useful, was the long life of michael faraday. besides this, we are shown--how many an illustrious name in the bede-roll of our great men brings it to mind--that with an enthusiastic love for a particular study, and unflagging perseverance in pursuance of it, the most adverse circumstance of birth and fortune may be overcome, and he who has striven take rank among the great and good whose names adorn the annals of their country. such lives are useful, not alone for the work which is done, but for the example which they afford us, that we also--to use longfellow's well-known, yet beautifully true lines-- "may make our lives sublime, and departing, leave behind us footprints on the sands of time." "the true scientist," says mr. robert buchanan in a recent work, "should be patient like darwin and reverent like faraday." the latter, indeed, seems to me to have been a truly typical scientist. never have we seen an instance of a less selfish devotion to a man's chosen work. born the son of a journeyman blacksmith, brought up amidst the most unpromising surroundings, with but the scantiest schooling, we find michael faraday educating himself during his spare time, and gradually acquiring, by indomitable perseverance, that scientific knowledge for which he thirsted. we find him seeking employment, even in the humblest capacity, in a place that must have appeared to his youthful mind as the very home of science. once there, we find him advancing with marvellous rapidity not only in the acquirement of knowledge which had been gained by others, but, yet prouder position, we find him ever adding to that store of knowledge the discovery of new facts. the patience of the true scientist was assuredly his. we find him acknowledged by his great contemporaries not only as an equal but as a leader among them. we find him with wealth and high social position within his reach. all this do we find--and not this alone; for we find him at the same time unspoiled in the slightest degree by his success; caring not in the least for the wealth that might be his, and declining honours which most men would have considered as but the fair reward of work which they had done. we find him also the object of love and admiration, not of his family and intimate friends alone, but of all persons with whom he came into contact. we find him exploring all the hidden workings of nature--making known discovery after discovery in the same modest and enthusiastic manner; and despite all these inquiries into the secrets of nature, we find him retaining unshaken that firm faith with which he had started--that beautiful and unquestioning trust in "a far off divine event to which the whole creation moves." much of faraday's kindliness and good nature, his considerateness and his simple earnest faith could be revealed only by his letters and by the records of those who had known him personally--on this account i have found it necessary somewhat freely to make use of illustrative quotations. after studying his life, however, the kindliness, nay more, the true brotherhood of the man with all men is the feeling which most firmly clings to us; we do not alone remember the great electrician, experimentalist, and lecturer, but we have an ever-present idea of the sterling goodness of the man. "a purer, less selfish, more stainless existence, has rarely been witnessed. at last came the voice which the dying alone can hear, and the hand which the living may not see, beckoned him away; and then that noble intellect, awakening from its lethargy, like some sleeper roused from a heavy dream, rose up and passed through the gates of light into the better land, where, doubtless, it is now immersed in the study of grander mysteries than it ever attempted to explore on earth." in closing this preface i have much pleasure in recording my deep indebtedness to miss jane barnard, a niece of the great professor, and for some two and twenty years a member of his household, for several reminiscences of her uncle; and also for her kindness in allowing me to look through the many interesting manuscripts of faraday's which are in her possession. walter jerrold. [illustration: library, royal institution] contents. chapter i. page as child--newsboy and bookbinder chapter ii. the turning point chapter iii. "home thoughts from abroad" chapter iv. back at work chapter v. "science which i loved" chapter vi. as teacher and preacher chapter vii. overwork--the end chapter viii. as friend and lecturer chapter ix. notes on his work chapter x. about the royal institution [illustration] [illustration] michael faraday. chapter i. as child--newsboy and bookbinder. "a virtuous household, though exceeding poor! pure livers were they all, austere and grave, and fearing god; the very children taught stern self-respect, a reverence for god's word, and an habitual piety." wordsworth. among those of our great men who, born in humble circumstances and unfurnished with the benefits of early education, have yet secured for themselves honourable positions in the history of the world's progress, michael faraday holds a remarkable place. born the son of a journeyman blacksmith, michael yet gained for himself a conspicuous position among the very first scientists of his day, and at the time of his death was acknowledged as one of the leading philosophers--electricians--chemists--of this nineteenth century. our interest in a great man makes us always interested also in his family--we become anxious to know who and what he was apart from that which has made him great. who were his parents? from where did they come? what were they like? what did they do? and a number of similar questions are at once started as soon as we commence considering the lives of our "great and good." in the case of faraday we have only scanty information as to his family, but thus much we have gleaned:-- during the whole of last century there was living in or near the village of clapham, in yorkshire, a family of the name of faraday. between the years and the clapham parish register shows us that "richard ffaraday, stonemason, tiler, and separatist," recorded the births of ten children, and it is probable that he had in his large family yet another son, robert. whether, however, robert was his son or only his nephew is a matter of doubt, but it is known of him that he married elizabeth dean, the possessor of a small though comfortable house called clapham wood hall, and that he was the father of ten children, one of whom, james, was born in , and became the father of michael faraday. robert and elizabeth faraday's six sons were each of them brought up to some trade or craft, and were thus all of them fitted to go out into the world and fight the battle of life. one son became a grocer and (as his grandfather, "richard ffaraday," had been) tiler; one a farmer; one a shoemaker, and so on. the third son, james, to us the most interesting member of this large family, although he appears to have been of somewhat weak constitution and unfitted for so laborious a vocation, became a blacksmith, served his apprenticeship, and exercised his craft for some time in the neighbourhood of his birthplace. when he was five-and-twenty years old (in ), james married; his wife being margaret hastwell, the daughter of a farmer living near kirkby stephen, a place some few miles away from clapham, over the westmoreland border. for two years or thereabouts did the young blacksmith and his wife remain in the neighbourhood of clapham; but after that time had elapsed they determined to come up to london, and seek their fortunes in the great metropolis. to the young men and women of our rural places the very name of london has about it, even to-day, a ring as of genuine coin, that tempts them to leave in large numbers the homes of their childhood that they may plunge into the vortex of city life. a hundred years ago this strange attractive power of the metropolis was probably much greater, owing to the difficulty of reaching it and the vague stories that were told of its wealth. they who had "been to london" were looked upon in rural places as veritable travellers, and were to their "home-keeping" friends objects of greater curiosity than anyone who to-day returns from the farthest or wildest portion of the earth's surface. the old story of "the london streets being paved with gold"--the story that had buoyed up the spirits of the youthful whittington--seems yet in the last century to have gained some credence. whether they were induced to do so by promises of work, or merely attracted to london as a centre where work would probably be plentiful, we cannot say; but it is at any rate certain that the faradays removed from the yorkshire village to a london suburb some time before the autumn of . for it was on the nd of september in that year that there was born to them at newington butts their third child, michael, the future illustrious chemist and philosopher, upon the story of whose life we are now about to enter. of michael's early years we have but a very meagre account. when he was about five years old his family removed from newington butts, and went to live in jacob's well mews, charles street, manchester square, where they occupied rooms over a coach-house. james faraday found employment at this time in welbeck street, while his young son passed his time, as children so circumstanced generally do, in playing in the streets; in after years, indeed, that son, become a prominent man, would point out where in spanish place he used to play at marbles, and where in manchester square he had at a later time been proud of having to take care of his younger sister, margaret. it was from jacob's well mews, too, that michael went to school, and received such scant education as was to be his before it became necessary that he, as a youth of thirteen, should step into the ranks of the workers and begin the battle of life in earnest; such education as he received was of the "most ordinary description (to use his own words), consisting of little more than the rudiments of reading, writing, and arithmetic at a common day-school. my hours out of school were passed at home and in the streets." when faraday was a boy nine years of age, in the first year of the present century, there was a time of much distress, when the rate of wages was very low, and the price of food very high: corn, indeed, which is at the present time about forty shillings per quarter, cost then as much as £ for the same quantity. the distress, was felt very generally throughout the country, and the faraday family severely felt the hard times; michael, we are told, was allowed one loaf each week, and, it is added (poor michael!), that the loaf had to last him that time. [illustration: the house in jacob's well mews.] near by where the faraday family lived in jacob's well mews there was, at no. , blandford street, a worthy bookseller named riebau. in , when faraday was a boy of thirteen, he was employed as an errand boy by mr. riebau, "for one year on trial"--a trial that, as we shall shortly see, proved highly satisfactory. michael's duty as errand boy, when he commenced, was to carry round the newspapers which were lent out by his master. he would get up very early each sunday morning, and take the papers round, so that he might be able to call again for them while it was yet fairly early; frequently he would be told that he "must call again," as the paper was not done with. on such occasions he would beg to be allowed to have it at once, as the next place at which he had to call might be a mile off, and he would lose so much time going twice over his rounds that he would not be able to get home and make himself neat, so that he might go with his parents to their place of worship. mr. riebau's shop, it may be noted, has changed but little since the early part of this century, it is still a stationer's business, and on the front of the house is placed a plaque bearing the simple inscription "michael faraday, man of science," with the date of his apprenticeship there. this plaque has furnished the simple yet sufficient title for this volume. his father, it may here be noted, had joined the sandemanian church, or the followers of robert sandeman, who, with his father-in-law, the reverend john glas, had seceded from the scotch presbyterian church, and with him had started the sect which was named after sandeman, or, as they are still called in scotland, glasites. in joining the sandemanian church, james faraday was following the family tradition, for the large family of clapham faradays, to whom we have referred, were all members of the same body. michael's mother, although she had not formally become a member of the church, used regularly to attend as one of the congregation. michael, as we shall learn, joined the church later on, and continued a devout and sincere member of it up to the time of his death. for about a year did young faraday continue as mr. riebau's errand boy; for about a year, as professor tyndall puts it, "he slid along the london pavements, a bright-eyed errand boy, with a load of brown curls upon his head and a packet of newspapers under his arm." we learn from one of his nieces that in his later years he rarely saw a newsboy without making some kind remark about him; as he said on one such occasion, "i always feel a tenderness for those boys, because i once carried newspapers myself." he was reproached, he says, as a boy, with being a great questioner. "he that questioneth much," says lord bacon, "shall learn much;" but this truth is too often forgotten by their elders when children are "inquisitive," and, as in faraday's case, what is but the natural questioning of an awakening mind is put down to idle curiosity, and the child is told (as we may often hear) "not to ask so many questions." although faraday says he was thus "charged with being a great questioner," he could not recall what kind of questions he put; though he tells one story against himself which shows that all questioning, even that of a young philosopher, is not necessarily wise. he had called at a certain house to leave a newspaper, and whilst waiting for the door to be opened he put his head between the iron bars that separated the house from the next, and while in that position asked himself, somewhat strangely, which side of the railing he was on? no sooner had he started the question than the door behind him opened, he drew suddenly back, and, hitting himself so as to make his nose bleed, he forgot all about his question, which, without being answered, was yet it would seem somewhat definitely settled. when his year as errand boy expired, michael was apprenticed to mr. riebau to learn the trade of bookbinder and stationer. his indentures are dated october th, , and contain in one line an excellent testimonial to his character: "in consideration of his faithful service no premium is given." of the earlier part of his seven years' apprenticeship we know but little. his father wrote in to a brother at the old home at clapham, "michael is bookbinder and stationer, and is very active at learning his business. he has been most part of four years of his time out of seven. he has a very good master and mistress, and likes his place well. he had a hard time for some while at first going; but, as the old saying goes, he has rather got the head above water, as there are two boys under him." [illustration: "michael faraday, man of science, apprentice here."] in that he was placed within reach of many and good books, which should go a great way towards deciding his scientific and speculative bent of mind, a position such as that in mr. riebau's shop was as good a one as he could have had. not only were many scientific books, that had hitherto been unavailable, now placed ready to his hand, but he had in riebau a kind and considerate master; he was allowed, and it was a valuable privilege, to be out occasionally of an evening that he might attend the lectures on natural philosophy which a mr. tatum was delivering at that time at his house in dorset street, fleet street. michael saw bills announcing the lectures in shop windows, and became anxious to hear them, which he was enabled to do owing to the kindness of his master, mr. riebau, and the generosity of his elder brother robert, who at the time was following their father's business, and made michael a present on several occasions of the shilling which was charged for entrance to the lectures. towards the end of the year faraday's family removed from jacob's well mews, where their home had been for thirteen years, and went to live at , weymouth street, near portland place, and there, on october th of the following year, james faraday died. he had been out of health for some years, and seems indeed to have been quite physically unfitted for so laborious an occupation as that of blacksmith. in he had written to a brother at clapham, "i am sorry to say i have not had the pleasure of enjoying one day's health for a long time. although i am very seldom off work for a whole day together, yet i am under the necessity (through pain) of being from work part of almost every day." he then concludes his letter in that spirit of simple yet earnest devotion that appears to have been characteristic of the whole family: "but we, perhaps, ought to leave these matters to the overruling hand of him who has a sovereign right to do what seemeth good to him, both in the armies of heaven and amongst the inhabitants of the earth." michael's strong affection for his parents became, as he grew older, one of the most marked features of his character; his great love for his mother is shown in many ways, notably in every letter which he wrote to her. the following story illustrates, as do many others that are told of him, faraday's depth of feeling with regard to his family. after he had become recognised by the world as the great man that he was, and when sitting to noble for his bust, it happened that the sculptor, in giving the finishing touches to the marble, made a clattering with his chisels: noticing that his sitter appeared moved, he said he feared the jingling of the tools had distressed him, and that he was weary. "no, my dear mr. noble," said faraday, putting his hand upon his shoulder, "but the noise reminded me of my father's anvil, and took me back to my boyhood." gradually faraday's interest widened in those matters which later on were to entirely engross his attention. his apprenticeship at first gave him many opportunities of reading philosophical and scientific works. "i loved," he afterwards wrote, referring to this time, "to read the scientific books which were under my hands, and, amongst them, delighted in marcet's _conversations in chemistry_, and the electrical treatise in the _encyclopædia britannica_. i made," he adds, and the item is interesting as giving us a first glimpse at his experiments, "i made such simple experiments in chemistry as could be defrayed in their expense by a few pence per week, and also constructed an electrical machine, first with a glass phial, and afterwards with a real cylinder, as well as other electrical apparatus of a corresponding kind." watts' _on the mind_, was, he said, the first thing that made him really think; while his thoughts were directed towards science by an article on electricity, which he lighted upon in an encyclopædia entrusted to him to bind. such glimpses into the early reading--showing us how the bent of his genius is decided--are always interesting in the life of one who, as tennyson says, "has made by force his merit known." into faraday's early reading--or that part of his reading which bore upon the science with which his name is so intimately connected--we have indeed something more than a glimpse, for he compiled (during - ) a note book in which he wrote down the names of such books and articles connected with the sciences as interested him. this note book he called, "_the philosophical miscellany_: being a collection of notices, occurrences, events, etc., relating to the arts and sciences, collected from the public papers, reviews, magazines, and other miscellaneous works; intended to promote both amusement and instruction, and also to corroborate or invalidate those theories which are continually starting into the world of science." thus ambitiously did michael faraday, a youth of not yet twenty years, start upon his career as an investigator; thus early did he evince a desire to "corroborate or invalidate those theories which are continually starting into the world of science." among books and articles to which reference is made in the interesting _miscellany_, there are papers by dr. darwin,[ ] papers on a "description of a pyro-pneumatic apparatus," and "experiment on the ocular spectra of light and colours," frequent references to "lightning," "electric fish," and other electrical phenomena, showing his early leaning towards this particular branch of investigation. there is a reference to the short essay on the _formation of snow_, which forms the reading for december th, in that interesting, and at the present time neglected, work, sturm's _reflections on the works of god_. this book has perhaps been supplanted in a great measure by the many popular treatises on science and natural history which recent years have produced, but which, nevertheless, have not taken the place of the _reflections_, the simplicity and directness of which give to the volume a perennial charm such as but few books can maintain. other papers, such as that on "how to loosen glass stopples," included in the _miscellany_, show us faraday's interest in the science of everyday life, to which in his later years we owe those delightfully interesting lectures on "the chemical history of a candle," lectures to which fuller reference is made later on in this volume. one other reference in the _miscellany_ is at any rate worthy of passing note for obvious reasons, or for reasons which are obvious as soon as we learn how closely connected is the career of faraday with that of his great benefactor and predecessor in the field of research, sir humphry davy. the reference is from the _chemical observer_, to the effect that "mr. davy (he was knighted in ) has announced to the royal society a great discovery in chemistry--the fixed alkalies have been decomposed by the galvanic battery." from the lectures at mr. tatum's house our young philosopher gained something more than a knowledge of the subjects discussed--he gained several friends, intercourse and exchange of ideas with whom were to form no inconsiderable part of his education; that he might illustrate the lectures, too, he set to study perspective, being kindly assisted in his work by mr. masquarier, a french refugee artist who was lodging at the time at mr. riebau's, and whose kindness to him faraday never in after years forgot to acknowledge. about a dozen lectures at mr. tatum's were spread over rather more than eighteen months (february, --september, ). at them, faraday became acquainted with benjamin abbott, a confidential clerk in the city--an acquaintance that ripened into life-long friendship; here also he met huxtable, a medical student, to whom he addressed the earliest note of his which is extant. other kindred spirits with whom faraday entered into friendly relations at the dorset street lectures, were magrath, newton, nichol, and many more. there is a perverted and ridiculous story told of faraday's first hearing davy lecture, to the effect that "magrath happening, many years ago, to enter the shop of mr. riebau, observed one of the bucks of the paper bonnet zealously studying a book which he ought to have been binding. he approached; it was a volume of the old _britannica_, open at 'electricity.' he entered into talk with the journeyman, and was astonished to find in him a self-taught chemist, of no slender pretensions. he presented him with a set of tickets for davy's lectures at the royal institution; and daily thereafter might the nondescript be seen perched, pen in hand, and his eyes starting out of his head, just over the clock opposite the chair. at last the course terminated; but faraday's spirit had received a new impulse, which nothing but dire necessity could have restrained." this circumstantial yet exaggerated story, couched as it is in the worst of tastes, is yet quoted with approval in a recent work supposed of some authority. magrath, as we have seen, faraday had met earlier, and, as he tells us himself, the kindness of giving him tickets for davy's lectures was done him by mr. dance.[ ] the story quoted above says also that he might be seen _daily_, and that "at last" the course terminated. to show us how garbled is this account and in what it is true, we will turn to an account of this incident--this important incident--in his life, which faraday himself wrote out later at the request of a correspondent. "during my apprenticeship," he says, "i had the good fortune, through the kindness of mr. dance, who was a customer of my master's shop, and also a member of the royal institution, to hear four of the last lectures of sir h. davy in that locality. the dates of these lectures were february th, march th, april th and th, . of these i made notes, and then wrote out the lectures in a fuller form, interspersing them with such drawings as i could make. the desire to be engaged in scientific occupation, even though of the lowest kind, induced me, whilst an apprentice, to write, in my ignorance of the world and simplicity of my mind, to sir joseph banks, then president of the royal society. naturally enough, 'no answer' was the reply left with the porter." the four lectures which faraday heard during the spring of were, as we shall see in the next chapter, to mark an epoch in his life. at each of these lectures, we are told, the delighted youth listened to sir humphry davy, from a seat in the gallery immediately over the clock directly facing the illustrious lecturer;[ ] both speaker and listener being unaware of the close inter-connection there was destined to be between their two careers. but of this in the next chapter, for between faraday's hearing davy's lectures and his correspondence with that great man, there are one or two other interesting facts in connection with the life of our bookbinder's apprentice and would-be philosopher. in july of this year it was that michael commenced his long and interesting series of letters to benjamin abbott, letters that show us how keenly alive faraday was to all things connected with the work with which he was anxious to become more intimately connected, and at the same time how anxious he was to make up for his deficiencies of education. in all his letters we find a charm in the simple earnestness of the man, in his straightforward search for truth, in the unreserved openness which characterised him when corresponding with one whom he not only called a friend, but treated as such on all occasions. simplicity, in its best and highest meaning, was, if we can in one word sum up the character of a man, the chief feature of faraday in all his relations throughout life. through all his letters to his intimate friends, too, there runs a vein of unaffected pleasantry which shows us at once that he was no "mere scientist," no "dry-as-dust" philosopher, which is a character too often given by thoughtless and careless persons to men who earn their laurels in any special field of research. we find that the great chemist or philosopher is not only a great scientist, but that he is also, as faraday undoubtedly was, a man of a simple, earnest, reverent nature, a man whose married life was one series of years of love-making, who was a cheerful, pleasant friend and companion, and intense and earnest lover of children. perhaps i cannot better conclude this chapter than by giving a few passages from his early letters, passages that will fully bear out much of what is said in the preceding paragraph. it was in july, , three months before the articles of his apprenticeship ran out, that faraday began his letters to abbott; he was not as yet twenty-one years of age, his early education, as we have seen, had been chiefly the three r's, yet we find these letters eminently remarkable for their correctness and fluency, not less than for their kindness, courtesy, and candour. his first letter to abbott is, indeed, doubly interesting, for it gives us the earliest account we have of any of his experiments. after writing a good deal on what he considers to be the advantage of a correspondence, he continues: "i have lately made a few simple galvanic experiments, merely to illustrate to myself the first principles of the science.... i, sir, i my own self, cut out seven discs of the size of halfpennies each! i, sir, covered them with seven halfpence, and i interposed between seven, or rather six, pieces of paper soaked in a solution of muriate of soda!!! but laugh no longer, dear a.; rather wonder at the effects this trivial power produced. it was sufficient to produce the decomposition of sulphate of magnesia--an effect which extremely surprised me; for i did not, could not, have any idea that the agent was competent to the purpose." again, to the same friend, he writes: "what? affirm you have little to say, and yet a philosopher? what a contradiction! what a paradox! 'tis a circumstance i till now had no idea of, nor shall i at any time allow you to advance it as a plea for not writing. a philosopher cannot fail to abound in subjects, and a philosopher can scarcely fail to have a plentiful flow of words, ideas, opinions, etc., etc., when engaged on them; at least, i never had reason to suppose you deficient there. query by abbott: 'then pray, mike, why have you not answered my last before now since subjects are so plentiful?' 'tis neither more nor less, dear a., than a want of time. time, sir, is all i require, and for time will i cry out most heartily. oh that i could purchase at a cheap rate some of our modern gents' spare hours, nay, days; i think it would be a good bargain both for them and me. as for subjects, there is no want of them. i could converse with you, i will not say for ever, but for any finite length of time. philosophy would furnish us with matter; and even now, though i have said _nothing_, yet the best part of a page is covered." a little later he writes, acknowledging a letter from his friend, a letter which found him paper-hanging--"but what a change of thought it occasioned; what a concussion, confusion, conglomeration; what a revolution of ideas it produced--oh! 'twas too much; away went cloths, shears, paper, paste, and brush, all--all was too little, all was too light to keep my thoughts from soaring high, connected close with thine." this letter, after referring to his friend's electrical experiments, he finishes somewhat sadly, "you know i shall shortly enter on the life of a journeyman, and then i suppose time will be more scarce than it is even now." little did he dream how great a change in his prospects one short half year would make. [illustration] footnotes: [ ] erasmus darwin, author of _the botanic garden_, _loves of the plants_, etc., and grandfather of the more famous charles darwin. [ ] it may be noted here that there are several spurious stories told of faraday's first visit to the institution and his introduction to davy. the story as told here is as faraday himself told it to davy's biographer. [ ] it is interesting to note that sir humphry davy was only thirteen years the senior of michael faraday. [illustration] chapter ii. the turning point. "and nature, the old nurse, took the child upon her knee, saying: 'here is a story-book thy father has written for thee.' 'come, wander with me,' she said, 'into regions yet untrod; and read what is still unread in the manuscripts of god!'" longfellow. there is a story told of sir humphry davy, that, on being asked on a certain occasion to enumerate what he considered as his greatest discoveries, he named first one thing and then another,--now his wonderful safety-lamp, then some electrical discovery, finishing up with "but the greatest of all my discoveries was the discovery of michael faraday." in the autumn of , as we have seen, faraday was a bookbinder, whose apprenticeship was just at an end, and who was contemplating, as the only thing possible, the taking up of life as a journeyman at the craft at which for seven years he had been working; indeed, a journeyman bookbinder he became, for in october of that year he engaged himself to a mr. de la roche, who, though a quick-tempered, passionate man, seems to have really cared for faraday, so much so, indeed, that he said to him, "i have no child, and if you will stay with me you shall have all i have when i am gone." but michael was not thus to be tempted from the path which he desired to tread, as he wrote afterwards to davy's biographer, "my desire to escape from trade, which i thought vicious and selfish, and to enter into the service of science, which i imagined made its pursuers amiable and liberal, induced me at last to take the bold and simple step of writing to sir h. davy, expressing my wishes and a hope that if an opportunity came in his way he would favour my views; at the same time, i sent the notes i had taken of his lectures." shortly after sir humphry received faraday's application, speaking to a friend--the honorary inspector of the models and apparatus--he said, "pepys, what am i to do? here is a letter from a young man named faraday; he has been attending my lectures, and wants me to give him employment at the royal institution. _what can i do?_" "do?" was pepys' reply, "do? put him to wash bottles; if he is good for anything, he will do it directly; if he refuses, he is good for nothing." "no, no," said davy, "we must try him with something better than that." notwithstanding the fact that his similar application of some months before to sir joseph banks had met with no answer, faraday, in his desire to leave trade for science, had thus addressed another of the leading men of the day. davy's reply was "immediate, kind, and favourable." it was this-- "_december th, ._ "to mr. faraday, "sir,--i am far from displeased with the proof you have given me of your confidence, and which displays great zeal, power of memory, and attention. i am obliged to go out of town, and shall not be settled in town till the end of january; i will then see you at any time you wish. it would gratify me to be of any service to you; i wish it may be in my power. "i am, sir, your obedient humble servant, "h. davy." [illustration: sir humphry davy.] the young bookbinder's delight on receiving the great and kindly-natured man's note may easily be imagined, as also may his anxiety for davy's return. five weeks, however, are soon passed, and michael duly met sir humphry "by the window which is nearest to the corridor, in the ante-room to the theatre" at the royal institution. davy was much impressed by the sincerity and modesty of the applicant, but yet advised him to continue at his bookbinding, going so far, indeed, as to say that he would get the royal institution binding for him, and would recommend him to his friends.[ ] with this, for the present, faraday had to be content. he returned to his binding, delighted that he had met and conversed with the greatest chemist of his time, but still anxious for an opportunity to leave that trade to which, as he had said, he was so averse, and to become wholly the servant of that science to which he was so attached. the change in his vocation was to come far more rapidly than he could have anticipated. he was still living, at this time (early in ), at , weymouth street, and one night, not very long after his interview with davy, just as he was undressing to go to bed, there came a loud knock at the front door. michael went to the window to see if there was any evidence as to whom the unwonted visitor might be. a carriage was there, from which a footman had alighted and left a note for "mr. m. faraday." it proved to be from sir humphry, who had already an opportunity of serving the young enthusiast. the note requested michael to call on davy the next morning. this he did, and learned that an assistant in the laboratory of the royal institution was required at once, the former assistant having been dismissed the day before. michael instantly expressed his willingness to accept the position; he was to have twenty-five shillings a week salary, and two rooms at the top of the institution building. it was not long before arrangements were all completed. a meeting of the managers of the institution was held on march st; the following is entered in the minutes of that day's proceedings:--"sir humphry davy has the honour to inform the managers that he has found a person who is desirous to occupy the situation in the institution lately filled by william payne. his name is michael faraday. he is a youth of twenty-two years of age. as far as sir h. davy has been able to observe or ascertain, he appears well fitted for the situation. his habits seem good, his disposition active and cheerful, and his manner intelligent. he is willing to engage himself on the same terms as those given to mr. payne at the time of quitting the institution. _resolved_:--that michael faraday be engaged to fill the situation lately occupied by mr. payne on the same terms." the duties of the assistant were specified by the managers in the following manner, his work being something other than the washing of bottles, which pepys had recommended. it is a fact, also, that faraday, almost from the commencement of his engagement, was concerned in more important work than that herein particularised. he was "to attend and assist the lecturers and professors in preparing for, and during, lectures; when any instruments or apparatus may be required, to attend to their careful removal from the model room or laboratory to the lecture-room, and to clean and replace them after being used, reporting to the manager such accidents as shall require repair, a constant diary being kept by him for that purpose. that, in one day in each week, he be employed in keeping clean the models in the repository, and that all the instruments in the glass cases be cleaned and dusted at least once within a month." as has been said, faraday's work was almost from the first of a higher nature; he is reported to have set in order the mineralogical collection soon after his arrival. but a very short while elapsed between michael's appointment as assistant and his taking up the duties of his post, for, on the th of march, he writes to abbott, dating his letter from his new home, the two rooms at the top of the institution. his letter tells us that he was already concerned in the active duties of his post, as the following passages show: "it is now about nine o'clock, and the thought strikes me that the tongues are going both at tatum's and at the lecture in bedford street; but i fancy myself much better employed than i should have been at the lecture at either of those places. indeed, i have heard one lecture already to-day, and had a finger in it (i can't say a hand, for i did very little). it was by mr. powell on mechanics, or rather, on rotatory motion, and was a pretty good lecture, but not very fully attended. "as i know you will feel a pleasure in hearing in what i have been or shall be occupied, i will inform you that i have been employed to-day, in part, in extracting the sugar from a portion of beetroot, and also in making a compound of sulphur and carbon--a combination which has lately occupied in a considerable degree the attention of chemists." about a month after writing the letter of which the above forms a part, faraday again wrote to his friend abbott, giving him an account of some experiments, in which he had been assisting sir humphry davy, on "the detonating compound of chlorine and azote, and of four different and strong explosions of the substance, explosions from which neither he nor davy had altogether escaped unhurt." "of these," he says, "the most terrible was when i was holding between my thumb and finger a small tube containing - / grains of the compound. my face was within twelve inches of the tube; but i fortunately had on a glass mask. it exploded by the slight heat of a small piece of cement that touched the glass above half-an-inch from the substance, and on the outside. the explosion was so rapid as to blow my hand open, bear off a part of one nail, and has made my fingers so sore that i cannot yet use them easily. the pieces of the tube were projected with such force as to cut the glass face of the mask i had on." in the other three experiments also they each of them got more or less cut about by the explosion of the "terrible compound," as faraday calls it, davy, indeed, in the last one, getting somewhat seriously cut. he writes thus frequently to abbott during the summer of , giving him in the later letters some well thought-out ideas on lectures and lecturing, which we shall have occasion to glance at when we are considering faraday himself in the capacity of a lecturer,--one of the most popular and yet truly scientific lecturers of any time. in this year, his twenty-first, faraday joined the city philosophical society, which had been founded about five years earlier by mr. tatum, at whose house the meetings were held. the society consisted of some thirty or forty individuals, "perhaps all in the humble or moderate rank of life;" and certainly all of them anxious to improve themselves and add to their knowledge of scientific subjects. once a week the members gathered together for mutual instruction; each member opening the discussion in his turn by reading a paper of a literary or philosophical nature, any member failing to do so at his proper time being fined half-a-guinea. in addition, the members had what they modestly called a "class book," but probably very like what we should now call a manuscript magazine; in this each member wrote essays, and the work was passed round from one to another. michael, it will be seen, was not neglecting any opportunity of educating himself; as he had said in starting his correspondence with abbott, one of his objects was to improve himself in composition and to acquire a clear and simple method of expressing that which he had to say. yet another method had he of furthering his self-education. in the scanty notes which he wrote about his own life he says, "during this spring ( ) magrath and i established the mutual improvement plan, and met at my rooms up in the attics of the royal institution, or at wood street at his warehouse. it consisted, perhaps, of half-a-dozen persons, chiefly from the city philosophical society, who met of an evening to read together, and to criticise, correct, and improve each other's pronunciation and construction of language. the discipline was very sturdy, the remarks very plain and open, and the results most valuable. this continued for several years." it is a matter for wonder how faraday, with all these attempts to improve his language and method, and to avoid even the slightest peculiarity, managed yet to retain in all his work a remarkable simplicity and naturalness of style. on september , , faraday wrote to his uncle and aunt, giving them an account of himself because he had nothing else to say, and was asked by his mother to write the account:--"i was formerly a bookseller and binder, but am how turned philosopher, which happened thus: whilst an apprentice, i, for amusement, learnt a little of chemistry and other parts of philosophy, and felt an eager desire to proceed in that way further. after being a journeyman for six months, under a disagreeable master, i gave up my business, and, by the interest of sir humphry davy, filled the situation of chemical-assistant to the royal institution of great britain, in which office i now remain, and where i am constantly engaged in observing the works of nature and tracing the manner in which she directs the arrangement and order of the world. i have lately had proposals made to me by sir humphry davy to accompany him, in his travels through europe and into asia, as philosophical assistant. if i go at all, i expect it will be in october next, about the end, and my absence from home will perhaps be as long as three years. but, as yet, all is uncertain, i have to repeat that, even though i may go, my path will not pass near any of my relations, or permit me to see those whom i so much long to see." this continental trip with davy forms one of the chief episodes in faraday's life. he had, though two-and-twenty years of age, never before been further than a few miles out of london. the country through which he passed, the sea, and the mountains, all came to him as a revelation. the letters which he wrote home from abroad, and the journals which he kept, all express his wonder at the strange sights, and all breathe the kindliness of nature and affection for home and those at home which all his life long were strongly marked characteristics. his letters to his mother are especially pleasing. he was away for but little over eighteen months, yet an account of his travels merits a chapter to itself. the commencement of marked an epoch in his life, the close of the same year marked another. [illustration] footnote: [ ] some of the books which faraday bound for the royal institution are there now; kept carefully with other relics of the great chemist. see the chapter entitled "about the royal institution." [illustration] chapter iii. "home thoughts from abroad." "one rule his life was fashioned to fulfil: that he who tends truth's shrine, and does the hest of science, with a humble, faithful will, the god of truth and knowledge serveth best." "wednesday, october th, .--this morning formed an epoch in my life." thus commences the first entry in that journal, in which, all the while that he was away, faraday noted down particulars of what he saw and thought. and, indeed, the young traveller's remark is by no means an exaggeration, as we recognise when we consider that he had never been out of sight of the metropolis, that he was accompanying one of the leading chemists, and that he and davy, englishmen both, were allowed free passports through france, although this and that country were at the time at war with one another. the fact that davy was a scientist overshadowed the fact that he was an englishman in the eyes of the french authorities; as the former, he was permitted to travel anywhere, and to use libraries, museums, etc., at any time; as the latter, he would have been instantly taken prisoner. this was an early and pleasing recognition of the universality of science, of its more than political or national interest, nay, of its international importance. so minute are the descriptions of things seen, so clear and simple is the language employed, that faraday's journal is most delightful reading; while the letters written home and to his friends are no less pleasing; perhaps, indeed, they are more so as they are so eminently characteristic of the man. they are remarkable for the unaffected spirit of affection which breathes through them, and show us, as indeed was shown in all ways throughout his life, the keen sensitiveness of his feelings and the genuine earnestness with which he was at all times seeking for self-improvement. on reaching plymouth, faraday gives expression in his diary to the wonder which moved him at seeing the country for the first time. the journey, of course, had to be done by road, as it was long before the time of railways; but coach or carriage riding, during fine autumn weather, through some of the most delightful scenery of rural england, must at any time be preferable to, though less expeditious than, railway travelling; and that michael felt the full benefit of it is shown by the following passage from the journal:-- "_friday, october th._--reached plymouth this afternoon. i was more taken by the scenery to-day than by anything else i have ever seen. it came upon me unexpectedly, and caused a kind of revolution in my ideas respecting the nature of the earth's surface. that such a revolution was necessary is, i confess, not much to my credit; and yet i can assign to myself a very satisfactory reason, in the habit of ideas induced by an acquaintance with no other green surface than that within three miles of london. devonshire, however, presented scenery very different to this; the mountainous nature of the country continually put forward new forms and objects, and the landscape changed before the eye more rapidly than the organ could observe it. this day gave me some idea of the pleasures of travelling, and has raised my expectations of future enjoyment to a very high point." if the surface of the earth gave our amateur traveller cause for wonder, what must have been his feelings when he first went down to the sea-shore? or when, on the night of october th, he was on board in mid-channel, with the "immense waves," as he graphically puts it, "striding one after another at a considerable distance?" or when, again, to use his own words, the vessel "sank down into the valleys between the great waves, and we had nothing in view but the wall of waters around us." he carefully observed, on this occasion, remaining on deck all night for that purpose, the phosphorescence of the sea. the next day they reached the harbour of morlaix, on the french coast, where, after much examination of luggage, and searching in all possible and impossible places for contraband goods by the french customs' officers, they took up their lodging for a couple of nights. and on the nd, the carriage having been successfully put together (to ensure comfortable accommodation sir humphry had brought his own with him), the party commenced their tour, reaching paris, where a stay of three months' length was made, on the th. faraday's observant nature is made evident to us in every page of his journal, and the light, humorous style in which much of it--that part which admits of such treatment--is written, gives evidence of the abiding cheerfulness of his disposition. on the road to paris there was a temporary stoppage, owing to the breaking of one of the horses' traces. while the accident was being repaired by the postillion, faraday found, to his great delight, a glow-worm, the first that he had seen, and which gave him much food for reflection. so great an impression did the first sight of the luminous little grub make on him, that, writing to his mother six months afterwards, and enumerating some of the more important things he had seen in his travels, he says, "i have seen a glow-worm!" in paris davy stayed some three months, and faraday records the great disadvantage under which he laboured through not knowing the french language. despite this, however, he attended lectures with davy, and accompanied him on visits to the laboratories of the various french chemists of the day, among others to that of chevreul, who was even then (he was three years older than faraday) well known as one of the rising chemists of the day.[ ] it was well for davy, and his assistant too, perhaps, that the paris authorities did not read the entries which the young englishman "with a round chin, a brown beard, a large mouth, and a great nose,"[ ] made in his journal, for he records as follows a visit which he paid to the galerie napoléon:--"it is," he wrote, "both the glory and the disgrace of france. as being itself, and as containing specimens of those things which proclaim the power of man, and which point out the high degree of refinement to which he has risen, it is unsurpassed, unequalled, and must call forth the highest and most unqualified admiration; but when memory brings to mind the manner in which the works came here, and views them only as the gains of violence and rapine, she blushes for the people that even now glory in an act that made them a nation of thieves."[ ] although he thus discoursed in his journal about what he saw and thought, he did not by any means neglect his favourite science, and his journal during the stay in paris contains frequent reference to the experiments which sir humphry was carrying on with a new substance which had been discovered a short while before by a french chemist--m. courtois. this substance, now known as iodine, was the source of much interesting research. not only about the time of its discovery, but during the whole of the century it has afforded scope to chemists for much speculation and useful experiment. the race-prejudice, which early in the present century affected english opinion of all things french, is to be traced even in faraday, who, with all his fairness and open-mindedness, seems always congratulating himself on not belonging to the people among whom he finds himself. this insular spirit finds expression in such passages as the following, which he wrote after staying indoors all day with nothing better to do than to note the difference between the rooms in paris and those he was accustomed to in england. he sums his views up thus:--"french apartments are magnificent, english apartments are comfortable; french apartments are highly ornamented, english apartments are clean; french apartments are to be seen, english apartments enjoyed; and the style of each kind best suits the people of the respective countries." [illustration: torpedo fish.] from paris the small party--which consisted of sir humphry and lady davy and faraday, whose nominal position was that of "assistant in chemistry and experiments"--went south to montpelier, near the coast of the mediterranean and some seventy-five miles from marseilles. after about six weeks' stay they again started on their travels; and after a cold and adventurous journey across the alps, reached turin on february nd, at the close of the carnival. from turin they went to genoa--where faraday was much interested in several water-spouts which he saw in the bay--and then on to florence. various experiments were made by davy at each place, on iodine, on the electricity of the torpedo fish, etc.; while at each place faraday found some opportunity of helping to satisfy his craving for improvement. of the stay at florence the journal gives but little account other than of davy's experiment to find out of what a diamond is composed, and of the various attempts which were made with the assistance of the "duke's burning glass" to burn diamond. after noting these experiments, faraday concludes: "as yet it appears that the diamond is pure carbon." from rome, which was the next halting-place on their travels, michael wrote home to his mother a long letter, every line of which breathes a spirit of true affection. "i trust that you are well in health and spirits, and that all things have gone right since i left you.... mr. riebau and fifty other friends would be inquired after, could i but have an answer. you must consider this letter as a kind of general one, addressed to that knot of friends who are twined round my heart; and i trust that you will let them all know that, though distant, i do not forget them, and that it is not from want of regard that i do not write to each singly, but from want of convenience and propriety; indeed, it appears to me that there is more danger of my being forgot than of my forgetting. the first and last thing in my mind is england, home, and friends. it is the point to which my thoughts still ultimately tend, and the goal to which, looking over intermediate things, my eyes are still directed. but, on the contrary, in london you are all together, your circle being little or nothing diminished by my absence; the small void which was formed on my departure would soon be worn out, and, pleased and happy with one another, you will seldom think of me. such are sometimes my thoughts, but such do not rest with me; an innate feeling tells me that i shall not be forgot, and that i still possess the hearts and love of my mother, my brother, my sisters, and my friends.... whenever a vacant hour occurs i employ it by thinking on those at home. in short, when sick, when cold, when tired, the thoughts of those at home are a warm and refreshing balm to my heart. let those who think such thoughts useless, vain, and paltry, think so still; i envy them not their more refined and more estranged feelings: let them look about the world, unencumbered by such ties and heartstrings, and let them laugh at those who, guided more by nature, cherish such feelings. for me, i still will cherish them, in opposition to the dictates of modern refinement, as the first and greatest sweetness in the life of man." it is in his letters such as this that we get to understand faraday, and to appreciate how it was that his friends, members of his family, nay, even persons who casually met him, were always struck by the simplicity and lovableness of the man. altogether, michael got much pleasure, and a great deal of experience, both of life and of science, during his continental tour, although it was not a source of unmixed delight. his engagement was to accompany davy in the capacity of secretarial and scientific assistant, but some work certainly not included under that head fell to him owing to sir humphry's valet not accompanying the party at the last moment. had he been with davy alone this would have been of little matter, for davy was a kind and considerate man, and would have dispensed with a servant's attendance, and have recognised in faraday the scientific assistant only; but--unfortunately for michael--lady davy, as has been mentioned, accompanied her husband, and she was not so considerate; and, in consequence, faraday was treated at times almost as a servant. this, occasionally, was very trying to him; but michael was too much of a philosopher to give in because circumstances were not as he could wish, and he wrote to his friend abbott, that though he had to sacrifice much, "the glorious opportunity he enjoyed of improving in the knowledge of chemistry and the sciences continually determined him to finish the voyage with sir humphry davy." a decision of this nature is characteristic of faraday at all times: he rarely started any work without having carefully considered it; but, having started it, he was not one to take his hand from the plough before the furrow was completed. this quality is well illustrated in a story which is told of faraday when he had become a well-known chemist. he was arranging some apparatus with a scientific instrument maker, when a small piece of glass fell to the ground; faraday made several unsuccessful efforts to pick it up, when his companion said that it was not worth troubling over. "perhaps not," said faraday; "but i do not like to fail in accomplishing anything that i have attempted." the months of may and june were spent by the small party mostly in italy--first in rome, then naples, and afterwards travelling from place to place. at naples a stay of some days was made, and faraday's journal gives us an interesting account of two visits to mount vesuvius. on the second day the party, largely increased by other visitors, had a picnic on the mount. "cloths were laid on the smoking lava, and bread, chickens, turkey, cheese, wine and water, and eggs roasted on the mountain, brought forth, and a species of dinner taken at this place. torches were now lighted, and the whole had a singular appearance; and the surrounding lazzaroni assisted not a little in adding to the picturesque effect of the scene. after having eaten and drunk, old england was toasted, and 'god save the king!' and 'rule, britannia' sung; and two very entertaining russian songs by a gentleman, a native of that country, the music of which was peculiar and very touching." [illustration: naples and mount vesuvius.] from naples the journey is continued up north through all the magnificent scenery of italy; the journal giving us occasionally delightful word-pictures of the landscape, and recording the young traveller's observations on various natural phenomena. now, as we have seen, glowworms attract his attention, then waterspouts, and the magnificent spectacle presented by vesuvius; and again his attention is occupied with the beautiful fire-flies that appeared "in innumerable quantities; at a distance they covered the side of the mountain, and near us they passed over the fields, hovered on the edge or crossed the road, often attaching themselves to the harness, and emitting their bright and harmless flashes of light in a rapid and beautiful manner." in july our party found themselves settled in geneva, where some three months were passed very enjoyably in congenial society. davy was the guest of the elder de la rive, with whom he experimented in chemistry, and with whom, they both being ardent sportsmen, he went out fishing and shooting. "on these occasions," says professor tyndall, "faraday charged davy's gun, while de la rive charged his own. once the genevese philosopher found himself by the side of faraday, and in his frank and genial way entered into conversation with the young man. it was evident that a person possessing such a charm of manner and such high intelligence could be no mere servant. on inquiry de la rive was somewhat shocked to find that the _soi-disant domestique_ was really _preparateur_ in the laboratory of the royal institution; and he immediately proposed that faraday thenceforth should join the masters instead of the servants at their meals. to this davy, probably out of weak deference to his wife, objected; but an arrangement was come to that faraday thenceforward should have his food in his own room." for reasons such as these we can well understand that faraday's life during his continental journeying was not altogether as pleasant as he had anticipated it would be. in his letters his reserve on this matter is marvellous, for it is only twice, and in writing to his intimate friend, abbott, that he refers at all to his, at times, uncomfortable situation, and then it is to give point to what he has been saying in reply to his friend's complaint as to the sordid and unintellectual surroundings amid which he is compelled to live. in his journals also faraday's reticence with regard to those with whom he travelled is noticeable; he wrote impressions of what he saw, and of what he thought that was worth record, and this was done merely for his own future use and pleasure--he would never wish to recall any petty humiliations which circumstances compelled him to suffer, and they were very properly allowed to pass unrecorded. indeed, in the note quoted above, particulars of which were given professor tyndall by m. de la rive, we learn more of the discomforts of his post than faraday himself ever allowed to escape. it is indeed a great pity for his own good fame that davy should have allowed a "weak deference to his wife" to influence him in such a matter, as it was a great pity when a few years later he allowed a petty spirit of jealousy to make him oppose the election of faraday as a fellow of the royal society. from geneva many letters were written home to his mother and friends. this is characteristic: "here, dear mother, all goes well. i am in perfect health, and almost contented, except with my ignorance, which becomes more visible to me every day, though i endeavour as much as possible to remedy it." it is strange how different we find the faraday of the letters and the faraday of the journal. in the first case the cheerful kindliness, the affectionate, sympathetic side of the man's nature at once strikes us; while in the journal the clear and simple description, uncoloured by personal feeling or prejudice, is no less remarkable. the three months' stay at geneva at an end, the small party, bidding farewell to their hospitable and kindly host, de la rive, turned south again. in de la rive, michael, by his intelligence, his scientific enthusiasm, and his unassuming cheerful disposition had won a life-long friend. the route south may well be described briefly in michael's own words, from a letter to his mother written early in november at rome: "on leaving geneva we entered switzerland, and traversed that mountainous and extraordinary country with health and fine weather, and were much diverted with the curious dresses and customs of the country.... from switzerland we passed through the states of baden, on the lake of constance (they are very small), across an arm of the kingdom of wurtemburg, and into bavaria. in this route we had seen, though slightly, lausanne, vevey, zurich, schaffhausen and the falls of the rhine, in switzerland, and munich, and many other towns in germany. on leaving munich we proceeded to and across the tyrol, and got to padua, and from padua to venice. you will remember very well, i have no doubt, the picture which hung in the parlour over the fireplace, and which represented the rialto and the great canal of this town. the first i have had the pleasure of crossing several times, and the second i have partly traversed in a venetian gondola.... after seeing venice for three days we left it, and came towards italy, passing bologna and florence." before reaching florence the two philosophers went out of their way to inquire into a phenomenon at pietra mala which was much talked about. from certain tracts of ground in the neighbourhood sheets of flame of various sizes were said to burst out; the fire was said to burn anything combustible, although the ground where the flames were was not even heated; locally, it was said to be the remains of an ancient volcano. "though it was raining hard, yet that would not deter sir humphry from visiting those places; but, at the same time, it made us wish to be as quick as possible. sir humphry therefore went to the first place, and i went to the _acqua bollenti_, conducted by a man of the village, who carried some fire, some straw, and some water. i found the place in a cultivated field, not far from a mountain, apparently of limestone. it was simply a puddle perhaps formed by the present showers of rain. much gas rose from the earth, and passed through the water, which made it appear boiling, and had given rise to its name; but the water and the ground were quite cold. i made another puddle with the water we brought, near the one i found there, and i saw that the gas rose up through it also; and it appeared to be continually passing off from a surface of more than eighteen inches in diameter. the soil appeared deep, and close to the spot supported vegetation readily. the man inflamed some straw, and then laid it on the ground; immediately the gas inflamed, and the flame spread to some distance from the straw over the surface of the earth, waving about like the flame of weak spirits of wine; this flame burnt some moments. on putting a light to the bubbles which rose through the water they inflamed, and sometimes a flame ran quickly from them over the whole surface of the water. i filled a bottle with the gas, but i could not distinguish any smell in it. in pouring water into the bottle, and lighting the jet of gas that came out, a large clear flame was obtained. the whole of this flame was a very pale blue, like spirits of wine. it inflamed paper and matches readily, as might be expected; and when i held a dry bottle or knife over it, they appeared to become dim by condensing water: but this was uncertain, as the weather was so rainy. the water had no taste, and appeared pure rain water. i brought some of it and the gas away, and returned to the village." in the "almost deserted laboratory of the florence academy" experiments with the pietra mala gas convinced davy that it was "light hydrocarburet, pure." the second stay in rome extended over nearly four months, during which time the grand carnival took place. faraday had at this part of his tour a great deal of his time to himself, and earnestly devoted himself to continuing the study of the french and italian languages, on which indeed he had been working all the while he had been away from home. but he also continued his observations on men and manners, for during the carnival week he twice attended masked balls in a domino, besides being present at the horse-races on the corso, and at other of the events of the carnival. he was, however, anxious to be on his way home to england, and his letters occasionally show how sad he felt at not knowing how soon the return would be. it was, however, to be earlier than he anticipated. on january th, , he wrote to abbott: "now for news! we shall part in a few weeks (pray write quickly) for naples, and from thence proceed immediately to sicily. afterwards our road is doubtful; but this much i know, that application is made for passports to travel in the turkish empire, and to reside in constantinople; that it is sir humphry's intention to be among the greek islands in march, and at athens early in the spring.... adieu, dear friend. with you i have no ceremony. the warmest wishes that friendship can dictate are formed for you by m. faraday." thus had he written towards the end of january--within three months he was to be shaking hands with his friend at home! while on the road to naples, faraday heard of the escape of napoleon from elba on march th, and records it thus briefly in his journal: "tuesday, march th.--i heard for news that bonaparte was again at liberty. being no politician, i did not trouble myself about it, though i suppose it will have a strong influence on the affairs of europe." it is strange how quietly davy and his "assistant" passed through europe at a time when war was convulsing nearly the whole of it; quietly and apparently unconcernedly they went their way, seeing who and what was to be seen at the various stopping-places, prosecuting their researches in different branches of chemistry, and adding in many ways to their stores of knowledge, seemingly unaffected by "the time that tried men's souls." at naples faraday again ascended vesuvius, and on this occasion had the grand experience of seeing it in active eruption. he writes a full and graphic account in the journal, from which one passage, descriptive of the eruption itself, may well be quoted. this time faraday had ascended with a guide only, sir humphry having stayed part way up the mountain to see monte somma. "i saw a large shower of red-hot stones in the air," writes faraday, "and felt the strong workings of the mountain; but my care was now to get to the crater, and that was soon done. here the scene surpassed everything. before me was the crater, like a deep gulf, appearing bottomless from the smoke that rose from below. on the right hand this smoke ascended in enormous wreaths, rolling above us into all forms; on the left hand the crater was clear, except where the fire burst out from the side with violence, its product rising and increasing the volume of volatile matter already raised in the air. the ground was in continual motion, and the explosions were continual, but at times more powerful shocks and noises occurred; then might be seen rising high in the air numbers of red-hot stones and pieces of lava, which at times came so near as to threaten us with a blow. the appearance of the lava was at once sufficient to satisfy one of its pasty form. it rose in the air in lumps of various size, from / lb. to lb. or more. the form was irregular, but generally long, like splashes of thick mud; a piece would often split into two or more pieces in the air. they were red-hot, and, when they fell down, continued glowing for five, ten, or fifteen minutes.... i was there during one explosion of very great force, when the ground shook as with a strong earthquake, and the shower of lava and of stones ascended to a very great height, and at this moment the smoke increased much in quantity. the guide now said this place was not safe, from its exposed situation to the melted lava and to the smoke, and because it oftentimes happens that a portion of the edge of the crater is shaken down into the gulf below. we therefore retreated a little, and then sat down, listened, and looked." we have seen from the letter to abbott at the end of january that a somewhat lengthened tour had been planned out. on march the st faraday's journal says, "we left naples at five o'clock." from that time the return was rapid. at rome there was some delay owing to the lack of post-horses; the french troops under murat were advancing, and everybody was leaving the city; the pope had fled, and the cardinals were flying. after a delay of a couple of days carriage-horses were hired at a great expense, and the travellers proceeded on their homeward flight. at mantua delay again occurred, as the passports had to be "signed, re-signed, and countersigned." "at last," says faraday, "we saw the outside of the town, having, much against our will, remained two hours and a-half in it." faraday's last letter home is written from brussels on april th; it is to his mother, and is well worth reading: "my very dear mother,--it is with no small pleasure i write you my last letter from a foreign country, and i hope it will be with as much pleasure you will hear i am within three days of england. nay more, before you read this letter i hope to tread on british ground.... i am not acquainted with the reason of our sudden return; it is, however, sufficient for me that it has taken place. we left naples very hastily, perhaps because of the motions of the neapolitan troops, and perhaps for private reasons. we came rapidly to rome, we as rapidly left it. we ran up italy, we crossed the tyrol, we stepped over germany, we entered holland, and we are now at brussels, and talk of leaving it to-morrow for ostend; at ostend we embark, and at deal we land on a spot of earth which i will never leave again. you may be sure we shall not creep from deal to london, and i am sure i shall not creep to , weymouth street; and then--but it is of no use. i have a thousand times endeavoured to fancy a meeting with you and my relations and friends, and i am sure i have as often failed: the reality must be a pleasure not to be imagined or described.... you may be sure that my first moment will be in your company. if you have opportunities, tell some of my dearest friends, but do not tell everybody--that is, do not trouble yourself to do it.... my thoughts wander from one to another, my pen runs on by fits and starts; i do not know what to say, and yet cannot put an end to my letter. i would fain be talking to you, but must cease. adieu till i see you, dearest mother; and believe me ever your affectionate and dutiful son, "m. faraday." "'tis the shortest and (to me) the sweetest letter i ever wrote you." thus ended faraday's wanderings-- "but who may tell what forms he brought away, to stand reveal'd at mem'ry's spell, and glad some distant day!" he returned home better equipped for continuing the work of chemical research, for which he had so intense a liking, with his stores of knowledge vastly increased, and his energy and application not one whit abated. how he again took up the thread of his work must be told in another chapter. [illustration] footnotes: [ ] chevreul, many readers will remember, lived to celebrate his hundredth birthday in , and all his life continued experiments in his fascinating science. he died on the th of april, . [ ] so was faraday described in the passport issued to him at paris. [ ] the works of art which faraday refers to are the laocoön, the venus de medici, the dying gladiator, and other sculpture brought from rome to paris by napoleon after one of his italian campaigns. faraday must have been gratified at their return to rome the year after his words were written. [illustration] chapter iv. back at work. "a choice that from the passions of the world withdrew, and fixed me in a still retreat; sheltered, but not to social duties lost, secluded, but not buried; and with song cheering my days, and with industrious thought; with the ever-welcome company of books; with virtuous friendship's soul-sustaining aid, and with the blessings of domestic love." wordsworth. his friends and relations having had due attentions from him, faraday at once began to cast about for work. on going abroad with davy he had relinquished his position at the royal institution, though sir humphry had promised to befriend him on their return; this promise was, much to faraday's gratification, duly fulfilled. within a fortnight of his return michael found himself re-engaged at the institution in the capacity of "assistant in the laboratory and mineralogical collection, and superintendent of the apparatus," a high-sounding office that carried with it the none too substantial honorarium of thirty shillings a week, and, as before, rooms in the building. it was, however, a distinct rise, both in position and in wage, and faraday, we may be sure, was pleased to get back to his well-loved institution on such terms. a life spent in scientific research is, generally, an apparently uneventful one. faraday's life, far from being an exception to this rule, was rather an accentuation of it. the story of his life is indeed highly interesting; but its interest lies in it, not as a story of action and change, but as a life that may be said to have realised almost wholly the ideal which was set before it. from the very first moment when faraday gave expression to his hate for trade and his love for science, his whole life was a practical illustration of his feelings; as we shall find on following him through his great and honourable career, there were many occasions on which he refused not only titles and such like honours, but pecuniary benefits which might fairly be considered his dues--no, "his work was wrought for love and not for gain," as the line which i have placed on the title page of this little book so well expresses it. the tour on the continent, as has been noted, was the most striking episode in faraday's long life. from may th, (the date on which he rejoined the institution), onwards, his life was a time of steady intellectual growth, spent in chemical research, in the explaining of phenomena, and in what is by no means his least claim on our regard, the popularisation of scientific knowledge. we have seen in his early correspondence with benjamin abbott how, on his very earliest acquaintance with lecturing and lectures at the royal institution, he began to study the different styles of the various lecturers, to note their peculiarities, and in what lay the secret of their success; we have seen, too, how he was striving to improve himself in composition--in the clear and intelligent method of stating things. he was preparing himself betimes for what he felt to be part of his true vocation; how eminently successful--beyond his wildest imaginings--he was, will be seen as we follow his life-story year by year. [illustration: the davy safety lamp.] it was, as faraday frequently acknowledged, his good fortune to assist sir humphry davy in his experiments not only while abroad, but after their home-coming. one of the most important of all davy's discoveries was made in the year of their return. on august rd he acknowledged a letter which he had received from dr. gray, directing his attention to the awful destruction of human life by explosions in coal mines. on october st, davy announced to his correspondent that he had discovered a "safety lamp;" on november nd he read a paper on fire-damp before the royal society, and on december th submitted to dr. gray models of lamps and lanterns made on the principle of his discovery, "that fire-damp will not explode in tubes or feeders of a certain small diameter." in his experiments, in connection with this discovery, davy received considerable help from his laboratory assistant, who must have been much gratified by that passage in davy's paper on the "safety lamp," in which the great discoverer expressed himself as "indebted to mr. michael faraday for much able assistance." this was "mr. michael faraday's" first public recognition, and must have been very delightful to him, especially coming as it did from the man of all others for whom, in his scientific capacity, michael had the most profound admiration. davy gained, as is well known, much honour and no inconsiderable amount of money for his discovery. there are, however, circumstances in which the safety lamp is _not_ safe; faraday, and, it is to be presumed, davy himself, was aware of this. it is illustrative of faraday's stern regard for truth that, although he was at the time davy's own assistant, he did not, and would not, attest before a parliamentary committee to the universal safety of the davy lamp. early in we find faraday beginning to put into practice those ideas on lectures and lecturing which he had so carefully considered before. on the point of giving his first lecture, though, he seems to doubt himself, and in a letter to abbott occurs the following passage--"i intend making some experiments on that subject (lecturing) soon; i will defer it (his letter on lecturing) till after such experiments are made. in the meantime, as preparatory and introductory to such a course of experiments, i will ask your opinion of, and observations on, english composition--style, delivery, reading, oratory, grammar, pronunciation, perspicuity, and in general all the branches into which the _belles lettres_ divide themselves; and if by asking i procure, i shall congratulate myself on the acquisition of much useful knowledge and experience." the first lecture--on the "general properties of matter"--was duly and successfully delivered before the city philosophical society on january th. before trusting himself to go upon the platform, faraday carefully wrote out his lecture, word for word, as it was to be delivered; a plan which he followed in the case of each of the other six lectures which he gave before the same society during the year. these lectures i have had the pleasure of seeing, as they are neatly written out and bound by their author. they are, with many similar treasures, in the possession of miss jane barnard, a niece of faraday. this year was an important one in several ways; not only did faraday give his first lecture, but also his first printed paper appeared in the _quarterly journal of science_, which was edited by mr. brande, who succeeded sir humphry as chief of the royal institution. the paper was on an analysis of native caustic lime, which had been undertaken at davy's instigation. that his scientific friends and patrons were beginning to recognise something out of the common in their laboratory assistant is shown in various ways, notably by such a passage as the following, taken from faraday's note-book: "when mr. brande left london in august, he gave the _quarterly journal_ in charge to me; it has had very much of my time and care, and writing, through it, has been more abundant with me. it has, however, also been the means of giving me earlier information on some new objects of science." faraday's common-place book--a kind of continuation of his _philosophical miscellany_ of six years earlier--gives us a good deal of information as to his intellectual progress at this time; it shows us not only what scientific subjects were interesting him, but also how zealously he was continuing his study of composition and the mode of expressing what he had to say clearly and definitely. there are passages from the _spectator_, alongside of tests with arsenic, a description of a visit to a silk-ribbon dresser's, along with an account of zerah colburn, the american calculating boy. sir humphry davy wrote to faraday, saying, "mr. colburn, the father of the american boy who has such extraordinary powers of calculation, will explain to you the method his son uses in confidence. i wish to ascertain if it can be practically used." it has been remarked of faraday that his was a poet-nature expressing itself through science; and this estimate seems largely true, but the verses which he wrote in his common-place book, "on love," if they be his own composition, are extremely poor; there are other verses though which will merit quotation. they are written by mr. dryden, a fellow-member of the city philosophical society, and are entitled "quarterly night," october nd, , being descriptive of one of the periodical gatherings of the society. the following passage is of especial interest to us, as it shows how faraday impressed a young contemporary:-- "but hark! a voice arises near the chair! its liquid sounds glide smoothly through the air; the listening muse with rapture bends to view the place of speaking and the speaker too. neat was the youth in dress, in person plain; his eye read thus, _philosopher-in-grain_; of understanding clear, reflection deep; expert to apprehend, and strong to keep. his watchful mind no subject can elude, nor specious arts of sophist e'er delude; his powers, unshackled, range from pole to pole; his mind from error free, from guilt his soul. warmth in his heart, good humour in his face, a friend to mirth, but foe to vile grimace; a temper candid, manner unassuming, always correct, yet always unpresuming. such was the youth, the chief of all the band; his name well known, sir humphry's right hand. with manly ease towards the chair he bends, with watts' logic at his finger-ends, 'i rise (but shall not on the theme enlarge) to show my approbation of this charge: if proved it be, the censure should be passed or this offence be neither worst nor last. a precedent will stand from year to year, and _'tis the usual practice_ we shall hear. extreme severity 'tis right to shun, for who could stand were justice only done? and yet experience does most clearly show extreme indulgence oft engenders woe. in striving then to hit the golden mean-- to knowledge, prudence, wisdom, virtue seen-- let isaac then be censured, not in spite but merely to evince our love of right. truth, order, justice, cannot be preserved unless the laws which rule us are observed. i for the _principle_ alone contend, would lash the crime, but make the man my friend.'" faraday's progress during these first few years after his reinstatement at the royal institution was rapid: his lectures to the city philosophical society, his published papers, and his letters, all give evidence of it. in , as in the previous year, he delivered six lectures before the society. it is interesting to find that instead of being written out word for word, the lectures were now delivered from notes, showing how the young lecturer was becoming so assured of his own command of language as to make the earlier method no longer necessary. his common-place book for this year continues to show a wide range of reading and thoughtfulness. in the summer, when the lectures at the institution had ceased for the recess, faraday availed himself of an invitation from his friend huxtable, who was living at south moulton, and spent a month holiday-making in devonshire. his early impressions of that county, when he passed through it with davy on their way to the continent, must have made him especially delighted to visit it once more; more particularly as he had an opportunity on this occasion of making geological excursions and of studying "wavellite, hydrargellite, and such hard things." a letter which he wrote from barnstaple to his mother during this holiday is interesting, referring as it does to those country occupations amid which she, in her girlhood, passed her time:--"i have seen a great deal of country life since i left town, and am highly pleased with it, though i should by no means be contented to live away from town. i have been at sheep-shearings, merry-makings, junketings, etc., and was never more merry; and i must say of the country people (of devonshire, at least) that they are the most hospitable i could imagine. i have seen all your processes of threshing, winnowing, cheese and butter-making, and think i could now give _you_ some instruction, but all i have to say to you on these subjects shall be said verbally." each year of his life at this period faraday found himself becoming busier than the previous one. another five chemical lectures (on the metals, well known and little known) were given before the city philosophical society during , completing a course, extending over three years, of seventeen lectures on the chemical science, no mean accomplishment for a young man from twenty-three to twenty-six years of age. so much was his time now becoming occupied that we find a great falling-off in his letters this year, a falling-off not only in number, but also in length. the correspondence with abbott, commenced six years earlier, practically comes to an end in ; there was not, it is necessary to mention, the slightest abatement in the warmth of affection of the two friends; it was that, to a great extent, perhaps, the correspondence had done its work, and what is undoubtedly the more powerful reason, our young scientist was beginning to find his time so well occupied with his favourite work that he could not devote enough of it to the writing of long letters. abbott was yet, and always, sure of the heartiest hand-shake and the most unaffected welcome from one who to the end of his life was the staunchest of friends. on july st, , faraday read a highly interesting paper before the members of the city philosophical society, on "observations on the inertia of the mind," in which he drew, in an able manner, an analogy between a state of the mind and what in the physical world is known as the inertia of matter. it may be of interest to note a few passages from this lecture to illustrate the thoughtfulness and thoroughness of faraday's work at this time, and also to give an example of his early style as a lecturer. "unlike the animated world around him, which remains in the same constant state, man is continually varying, and it is one of the noblest prerogatives of his nature, that in the highest of earthly distinctions he has the power of raising and exalting himself continually. the transition state of man has been held up to him as a memento of his weakness; to man _degraded_ it may be so with justice; to man as he ought to be it is no reproach; and in knowledge that man only is to be contemned and despised who is _not_ in a state of transition.... "by advancement on the plain of life, i mean advancement in those things which distinguish men from beasts--sentient advancement. it is not he who has soared above his fellow-creatures in power, it is not he who can command most readily the pampering couch or the costly luxury; but it is he who has done most good to his fellows, he who has directed them in the doubtful moment, strengthened them in the weak moment, aided them in the moment of necessity, and enlightened them in their ignorance, that leads the ranks of mankind.... "there is a power in natural philosophy, of an influence universal, and yet withal so obscure, in its nature so unobtrusive, that for many ages no idea of it existed. it is called _inertia_. it tends to retain every body in its _present_ state, and seems like the spirit of constancy impressed upon matter. whatever is in motion is by it retained in motion, and whatever is at rest remains at rest under its sway. it opposes every _new_ influence, strengthens every _old_ one. is there nothing in the human mind which seems analogous to this power?... "inertia is an essential property of matter; is it a never-failing attendant on the mind? i hope it is; for as it seems to be in full force whenever the mind is passive, i trust it is also in power when she is actively engaged. was the idle mind ever yet pleased to be placed in activity? was the dolt ever willing to resign inanity for perception? or are they not always found contented to remain as if they were satisfied with their situation? they are like the shepherd magnus: although on a barren rock, their efforts to remove are irksome and unpleasant; and they seem chained to the spot by a power over which they have no control, of which they have no perception. again: in activity, what intellectual being would resign his employment? who would be content to forego the pleasures hourly crowding upon him? each new thought, perception, or judgment is a sufficient reward in itself for his past labours, and all the future is pure enjoyment. there is a labour in thought, but none who have once engaged in it would willingly resign it. intermissions i speak not of; 'tis the general habit and tenor of the mind that concerns us, and that which has once been made to taste the pleasures of its own voluntary exertions will not by a slight cause be made to forego them. "inertia, as it regards matter, is a term sufficiently well understood both in a state of rest and of motion. as it is not my intention to attempt a description of functions of the mind according to strict mathematical terms, i shall resign the exclusive use of the word at present, and adopt two others, which, according to the sense they have acquired from usage, will, i believe, supply its place with accuracy. apathy will represent the inertia of a passive mind; industry that of an active mind. "it is curious to consider how we qualify ideas essentially the same, according to the words made use of to represent them. i might talk of mental inertia for a long time without attaching either blame or praise to it--without the chance even of doing so; but mention apathy and industry, and the mind simultaneously censures the one and commends the other. yet the things are the same, both idleness and industry are habits, and habits result from inertia.... "inertia has a sway as absolute in natural philosophy over moving bodies as over those at rest. it therefore does not retard motion or change, but is as frequently active in continuing that state as in opposing it. now, is this the case with mental inertia?" these passages from faraday's early lectures serve to show us not only how he was attaining the art of expressing himself clearly, but how thoroughly he went into a subject on which he had once entered. it is not possible to follow in detail the work on which faraday was engaged. we have seen him learning assiduously, and essaying to teach in the friendly circle of the philosophical society. his work during the next few years continued on very similar lines to those which we have been regarding. year by year, about this time, his scientific writing increased--his work was increasing, his friends were increasing--he was beginning to be "somebody," though as yet but in a small world. he had commenced a correspondence with professor g. de la rive--the gentleman who at geneva had been so struck by him when he was acting as davy's travelling factotum--a correspondence which on the death of de la rive was continued with his son, professor auguste de la rive. in faraday married. before, however, we treat of this important step in his life, let us glance at the journal which he kept of a walking tour he took in wales during the summer of . this journal gives us further evidence of the genuine enjoyment which he found in scenery and nature in her wilder and more impressive aspects; it also gives further evidence of his simple yet direct way of describing things, of that true descriptive power of which his continental journal was often a good illustration. at five o'clock in the morning of july th, he mounted the top of the _regulator_ coach at the white horse cellar, piccadilly, and at ten o'clock of the same evening was set down at bristol. not at all a bad coach-ride for one day's journey. he afterwards visited cardiff, and went over the dowlais ironworks at merthyr; thence he and his companion wandered about at their own sweet will, unconfined by any artificial circumstances. they walked in that manner which adds so great a charm to a walking tour, never knowing one day whither they should bend their way on the next. the following is a delightful bit descriptive of a visit to the fall of scwd-yv-hên-rhyd, or glentaree, formed by the descent of the river hên-rhyd. "_monday, th._--proceeding onward into brecknockshire, we suddenly heard the roar of water where we least expected it, and came on the edge of a deep and woody dell. entering among the trees, we scrambled onwards after our guide, tumbling and slipping, and jumping, and swinging down the steep sides of the dingle, sometimes in the path of a running torrent, sometimes in the projecting fragments of slate, and sometimes where no path or way at all was visible. the thorns opposed our passage, the boughs dashed the drops in our faces, and stones frequently slipped from beneath our feet into the chasm below, in places where the view fell uninterrupted by the perpendicular sides of the precipices. by the time we had reached the bottom of the dingle, our boots were completely soaked, and so slippery that no reliance could be placed on steps taken in them. we managed, however, very well, and were amply rewarded by the beauty of the fall which now came in view. before us was a chasm enclosed by high perpendicular and water-worn rocks of slate, from the sides of which sprang a luxuriant vegetation of trees, bushes, and plants. in its bosom was a basin of water, into which fell from above a stream divided into minute drops from the resistance of its deep fall. here and there lay trunks of trees which had been brought down by the torrent--striking marks of its power--and the rugged bed of shingles and rocky masses further heightened the idea other objects were calculated to give of the force it possessed when swelled by rains. we stepped across the river on a few tottering and slippery stones placed in its bed, and passing beneath the overhanging masses ran round on projecting points, until between the sheet of water and the rock over which it descended; and there we remained some time admiring the scene. before us was the path of the torrent, after the fine leap which it made in this place; but the abundance of wood hid it ere it had proceeded many yards from the place where it fell. no path was discernible from hence, and we seemed to be enclosed on a spot from whence there was no exit, and where no cry for help could be heard because of the torrent-roar." yet another passage should be quoted from this journal; a passage descriptive of an ascent of cader idris during a thunder-storm. a thunder-storm was, all his life long, one of the most moving things to faraday. it seemed always to quicken him into new life. [illustration: cader idris.] "_sunday, july th._--ascent of cader idris. the thunder had gradually become more and more powerful, and now rain descended. the storm had commenced at the western extremity of the valley, and rising up cader idris traversed it in its length, and then passing over rapidly to the south-east, deluged the hills with rain. the waters descended in torrents from the very tops of the highest hills in places where they had never yet been observed, and a river which ran behind the house into the lake below rose momentarily, overflowing its banks, and extended many yards over the meadows. the storm then took another direction, passing over our heads to the spot in the west at which it had commenced, and having been very violent in its course, seemed there to be exhausted and to die away. the scene altogether was a very magnificent one--the lightning's vivid flash illuminated those parts which had been darkened by its humid habitation, and the thunder's roar seemed the agonies of the expiring clouds as they dissolved into rain; whilst the mountains in echoes mocked the sounds, and laughed at the fruitless efforts of the elements against them." the journeying was continued on to dolgelly and llangollen; and then back again to london, and to work on in his old indefatigable manner. sir humphry davy was in italy in - , investigating the questions with regard to the unrolling of papyri recovered from the ruins of herculaneum. in february, , he wrote to faraday saying, "i have sent a report on the state of the mss. to our government, with a plan for the undertaking of unrolling; one part of the plan is to employ a chemist for the purpose at naples; should they consent, i hope i shall have to make a proposition to you on the subject." at the end of the same year davy again wrote to his protégé in a similar strain, but nothing ever came of it. and delighted as faraday would doubtless have been to re-visit italy, he probably would not have undertaken the few months' work at naples if it had meant, as it would doubtless have done, his severing his connection with the royal institution. a much more important step was about to be taken by faraday. he had a friend, also a member and elder of the sandemanian church, by name barnard. mr. barnard was a silversmith who lived in paternoster row, and thither faraday often went, attracted by the charms of mr. barnard's third daughter, sarah. faraday was at this time twenty-nine years of age, the lady who was to exercise so great an influence over his life was but twenty, and what is more she did not favour his advances. at last, in july, , he wrote to her, and in a letter characterised by the depth no less than the warmth of his affection, begged at any rate to be heard. such letters, intended for the eyes of but one person, are, as a rule, and it is well they should be, too sacred to be freely reproduced for all the world to read. the letters have, however, before been printed, and it may assist us in forming a correct picture of michael faraday--of the earnest, affectionate nature which was his--to re-peruse a passage such as this:-- "again and again i attempt to say what i feel, but i cannot. let me, however, claim not to be that selfish being that wishes to bend your affections for his own sake only. in whatever way i can best minister to your happiness, either by assiduity or by absence, it shall be done. do not injure me by withdrawing your friendship, or punish me for aiming to be more than a friend by making me less; and if you cannot grant me more, leave me what i possess, but hear me." miss barnard showed the letter from which this passage is quoted to her father, whose reply was merely to the effect that "love made philosophers into fools." doubtful of her own decision on so momentous a question as this, involving the life-long happiness of two persons, miss barnard postponed making an immediate decision by accompanying a married sister to ramsgate. faraday made up his mind "to run all risks of a kind reception at ramsgate." he went there, and after a week of delightful holiday-making, returned to london on august the th, having won the consent of her for whom he had evinced so strong a passion. within twelve months (on june th, ), michael faraday and sarah barnard were married, and took up their residence in the royal institution. the union proved a perfect one, and a wedded life of nearly half-a-century's duration and of unclouded love was the result. from this time forward the kindliness, the affection, the love of home and of those persons forming "home," which had been earlier so marked in faraday's letters to his mother, become even yet more marked in the letters written to his wife any time between his marriage and his death. some of these we shall note as we come to treat of the period in which they were written. [illustration: _from a drawing by_] mrs. michael faraday. [_alexander blaikley_.] the year was an important one to faraday for other though less significant reasons: in it his first paper was read before the royal society, and he was also engaged with a mr. stodart, surgical instrument maker, in experimenting on alloys of steel with a view to improving its quality. for many years after, we are told, faraday used to present his friends with razors made of a particular alloy discovered at this time. the paper embodying the results of these experiments in alloys was duly published in the _quarterly journal of science_. a description of our hero (for hero he was--one of our true "heroes of peace"), written by a friend about the time of his marriage, is interesting as assisting us to realise what manner of man he really was in the flesh. "a young-looking man of about thirty years of age, well made, and neat in his dress, his cheerfulness of disposition often breaking out in a short crisp laugh, but thoughtful enough when something important is to be done." we find faraday now a young man of thirty, happily married, with a large circle of friends who are finding in him something of that genius which year by year henceforward was to manifest itself; we find him not only gaining the goodwill of these friends for his talents, but gaining their affectionate regard by his unselfishness and unremitting good nature. after their marriage in june, , mr. and mrs. faraday took up their residence in the institution, where they continued to live for close upon forty years. although fortune seemed thus to be smiling upon michael faraday it must not be supposed that his position at the royal institution was a highly remunerative one, his position was yet nominally that of laboratory assistant, and the return which he received for his services was a salary of one hundred pounds per annum, a suite of rooms, and coal and gas. one month after his marriage faraday made his confession of sin and profession of faith before the sandemanian church. it is characteristic of his whole attitude towards religion, and the great and serious regard which it demands from every _individual_, that when his wife asked him why he had not told her what he was about to do, he simply yet earnestly answered, "that is between me and my god." truth in all things was what he aimed at, and his whole life may be said to be a seeking after truth in the various branches of knowledge; to half know a thing was never sufficient for him, he could not rest there; he must test its truth, and either cast it away, having proved it worthless, or accept it with delight, having proved its truth. this is evidenced in all his life-work, in his social intercourse no less than in his scientific work, in his letters and journals no less than in his lectures and published papers. a circumstantial account has appeared in some of the newspapers of faraday's secession from the sandemanian church, and his penitent return to it. not having seen any reference to such a secession in the biographies of faraday i wrote to miss barnard, who in the following note, most emphatically denies the truth of the story:--"faraday never seceded from the church of which he became a member early in life ( ). it is true that for a few weeks in there was a cessation of his communion with this church, but the reasons for this were absolutely private, and had nothing to do with any conflict in his mind between his faith in the scriptures and his scientific work. the statement is altogether without foundation, and neither the scene described nor anything like it ever took place. "_ aug., ._ jane barnard." [illustration] chapter v. "science which i loved." "if i would strive to bring back times, and try the world's pure gold, and wise simplicity; if i would virtue set as she was young, and hear her speak with one, and her first tongue; if holiest friendship naked to the touch, i would restore and keep it ever such; i need no other arts but study thee, who prov'st all these were, and again may be." ben jonson. the year of faraday's marriage which, as we have seen in the previous chapter, was also important to him in other ways, was marked by one unpleasant incident which was talked about for some time afterward; but although faraday was then spoken of in no measured terms, it has been conclusively shown that far from any blame being attached to him, the facts of the case are much to his credit. to put the matter shortly it was this. dr. wollaston had an idea as to the possibility of electro-magnetic rotation; he expected, in other words, to be able to demonstrate that the "wire in the voltaic circuit would revolve on its own axis." he was at the institution one day in the early part of , and was making an experiment in the laboratory with sir humphry davy. faraday, who was not present during the experiment, came in in time to hear the conversation that followed. he afterwards made various experiments on this subject, and was invited by the editor of the _annals of philosophy_ to contribute an historical sketch of electro-magnetism. this sketch appeared in the _annals_ of the same year; faraday repeating nearly all of the experiments to which he referred. these experiments of faraday's led him to the discovery, early in september, of the "rotation of a wire in voltaic current round a magnet, and of a magnet round the wire." he could not make the wire and the magnet revolve on their own axis. "there was not the slightest indication that such was the case." before he published the paper descriptive of these "new electro-magnetical motions," faraday essayed to see dr. wollaston that he might get permission to refer to wollaston's experiments. the doctor was out of town, and the paper was published "by an error of judgment" without any reference to his opinions and intentions. directly afterwards faraday was extremely distressed at hearing rumours which "affected his honour and his honesty." he wrote at once, not only to stodart, but directly to dr. wollaston, whom he met, and after mutual explanation the matter dropped. faraday, however, continued his electro-magnetic experiments. it is one of these that is referred to by his brother-in-law, who was with him one christmas day when: "all at once he exclaimed, 'do you see, do you see, do you see, george?' as the small wire began to revolve. i shall never forget the enthusiasm expressed in his face, and the sparkling in his eyes." in the summer of faraday was at swansea for a fortnight with phillips, the editor of the _philosophical magazine_. before starting, however, he took his wife and her mother down to ramsgate, whither he addressed to his wife three letters, in which are evident the deep feelings which were his in regard to their relations. the first letter was written on his arrival in town after leaving ramsgate. after detailing what he has done, he breaks off, "and now, my dear girl, i must set business aside. i am tired of the dull detail of things, and want to talk of love to you; and surely there can be no circumstance under which i can have more right. the theme was a cheerful and delightful one before we were married, but it is doubly so now.... oh, my dear sarah, poets may strive to describe and artists to delineate the happiness which is felt by two hearts truly and mutually loving each other; but it is beyond their efforts, and beyond the thoughts and conceptions of anyone who has not felt it. i have felt it and do feel it, but neither i nor any other man can describe it; nor is it necessary. we are happy, and our god has blessed us with a thousand causes why we should be so. adieu for to-night." the letters from swansea give an account of his journey, and of his host's house, of work at the copper furnaces, and other places; of the many people there are at mr. vivian's (with whom he was staying), and of the late and long dinner, which he made up his mind to avoid if possible. he stayed out walking one evening, got back after dinner had commenced, and so stole up to his own room that he might write a long letter to his wife, in reply to one which he had received from her. "i could almost rejoice at my absence from you," he wrote, "if it were only that it has produced such an earnest and warm mark of affection from you as that letter. tears of joy and delight fell from my eyes on its perusal." early in the following year faraday was experimenting on chlorine, a subject that had attracted a great deal of davy's attention. at davy's suggestion he enclosed some of the gas in an hermetically sealed glass tube, that he might "work with it under pressure, and see what would happen by heat." what "happened" was that on several occasions the tube exploded, twice doing injury to faraday's eyes. on one of the occasions when faraday was at work, dr. paris happened to enter the laboratory, and seeing an oily liquid in the tube rallied him on his carelessness in using dirty vessels. when, afterwards, the end of the tube was filed off, there was an explosion, and the oily matter disappeared. early on the following day dr. paris received this note:-- "dear sir,--the _oil_ you noticed yesterday turns out to be liquid chlorine. "yours faithfully, m. faraday." he had in fact succeeded in converting chlorine gas into a liquid by means of its own pressure. this was an important discovery which led to numerous experiments with other gases, and with like results. on may st, , faraday's certificate as a candidate for fellowship in the royal society was read for the first time. such a distinction was no doubt a coveted one in faraday's eyes, and it must have been extremely painful to him when he found that sir humphry davy was opposed to his election. it is interesting to observe, however, that the very first signature on his certificate is that of dr. wollaston. such being the case it seems impossible that the old charge against faraday in regard to electro-magnetic rotation could have been revived, and yet so it was. wollaston himself had expressed perfect satisfaction, and the matter seemed definitely settled. much as this revival of an untrue charge must have distressed a man of faraday's uncompromising integrity, to find davy, of all men, opposing him must have been yet more distressing. that davy's opposition was active may be surmised from the following, which is told by faraday himself: "sir h. davy told me i must take down my certificate. i replied that i had not put it up; that i could not take it down, as it was put up by my proposers. he then said i must get my proposers to take it down. i answered that i knew they would not do so. then he said, i, as president, will take it down. i replied that i was sure sir h. davy would do what he thought was for the good of the royal society." this attitude of davy's naturally pained faraday exceedingly; many years afterwards some allusion by a friend to his early life led up to a mention of it; faraday rose abruptly from his seat, and took a rapid walk up and down the room, saying, "talk of something else, and never let me speak of this again. i wish to remember nothing but davy's kindness." there were tears in his eyes as he spoke, showing how deeply the man was moved. faraday also said that davy had walked for an hour about the courtyard of somerset house, arguing with one of his proposers that faraday should not be elected. we know none of the reasons for davy's opposition, and his attitude in this affair must ever remain a cloud on his fair fame; that he, a self-made man, who had risen to the first position among modern chemists, should oppose at this stage of his career a man somewhat similarly circumstanced, who was also moving upwards step by step to one of the highest positions among modern philosophers, as he loved always to be called, is indeed as strange as it is regrettable. the fact, sad as it is, has to be noted; but we will not dwell upon it; more gratifying is it to learn how, when the ballot was taken, michael faraday was almost unanimously elected a fellow of the royal society, there being but one black ball. this, in after years, he proudly mentioned, was the only one among his innumerable honours that he had sought for. scarce a year passed afterwards but some fresh distinction was conferred upon him. early in john wilson croker, with sir thomas lawrence, sir f. chantrey, and sir h. davy founded the athenæum club, which still flourishes. for a short while faraday acted as honorary secretary to the club; but his more congenial scientific labours could not be neglected, and he soon retired from the secretaryship, in which he was succeeded by his friend magrath, who continued to hold the post for many years. faraday's notes and papers contributed to the scientific journals and other periodicals were frequent, but it would profit little here to detail them. one discovery he made about this time is well worthy of mention as it has had an important effect on a particular industry--the discovery was that of benzol, benzine, or as faraday named it, "bicarburet of hydrogen." this is prepared now in large quantities, being employed in the manufacture of aniline colours. we have it on the authority of sir roderick murchison that faraday's first lecture at the royal institution was delivered in the following circumstances. brande, who had succeeded sir humphry davy as the professor of chemistry, was delivering a course of lectures; one day the lecturer, owing to illness or some other cause, was absent, but his assistant (faraday) took his place, and lectured with so much ease that he won the complete approval of his audience. in this connection too, it is interesting to note that it was towards the close of this same year that faraday began his experiments in magnetic electricity, the particular branch of research which was to occupy a great part of his later life, and in which he was destined to make some of his most brilliant discoveries. [illustration: the laboratory, royal institution.] it is pleasing to find that whatever may have been davy's object in opposing faraday's election into the royal society, he did not bear him any continued ill-will; this is shown us not only by davy's expressions of goodwill in his letters, but by such things as an entry in the minutes of a meeting of the managers of the royal institution in february, . from this entry we learn that sir humphry davy, "having stated that he considered the talents and services of mr. faraday, assistant in the laboratory, entitled to some mark of approbation from the managers, and these sentiments having met the cordial concurrence of the board; _resolved_:--that mr. faraday be appointed director of the laboratory, under the superintendence of the professor of chemistry." it was after receiving this appointment that faraday occasionally invited members of the institution to evening meetings in the laboratory, when he generally had something new and interesting to show them. in these meetings in the laboratory was the origin of those regular friday evening meetings in the theatre, which commenced in , which have had for many years a world-wide reputation, and which have drawn together, week after week and year after year, large numbers of persons interested in science and in its popular exposition. in , the year in which the first regular friday evening meetings took place, seventeen lectures were delivered, six of them being given by faraday himself, on such a variety of subjects as "caoutchouc," "lithography," "brunel's tunnel at rotherhithe," etc. his aim in inaugurating these "friday evenings" may be gathered from the scanty notes which he made for introducing one of the earliest of the lectures:--"evening opportunities--interesting, amusing; instructive also:--scientific research--abstract reasoning, but in a popular way--dignity;--facilitate our object of attracting the world, and making ourselves with science attractive to it." these notes, slight as they are, give us an idea of what faraday's objects were, and are at the same time interesting, as they may fairly be said to represent the aim of a large part of his lecturing work throughout his career, the aim that is, which always seemed to be his, to make the subject of which he was speaking amusing, interesting, and instructive. no other man had ever succeeded in attracting the world to science by making the science attractive to them. high as is faraday's position as a scientist and philosopher, he is also to be remembered with much gratitude as, in point of time as well as of ability, the first of all true popularisers of science. this may not at first sound a very high title to bestow, but yet it is far from an insignificant one, and one that must indeed have gratified faraday; much as he was pleased with the acknowledgment of himself as one of their peers by such men as davy, de la rive, and other scientists, the knowledge that he was interpreting the wonders of nature to a vast number of persons hitherto ignorant, or in a measure ignorant, of her marvellous ways, was yet more pleasing to him. we can fully understand his echoing the sentiment which the late james russell lowell, speaking of the poet, expresses in the following beautiful verses:-- "it may be glorious to write thoughts that shall glad the two or three high souls, like those far stars that come in sight once in a century;-- "but better far it is to speak one simple word, which now and then shall waken their free nature in the weak and friendless sons of men; "to write some earnest verse or line, which, seeking not the praise of art, shall make a clearer faith and manhood shine in the untutored heart. "he who doth this in verse or prose, may be forgotten in his day, but surely shall be crowned at last with those who live and speak for aye." in faraday's first book--_on chemical manipulation_--was produced. faraday had published a short while before an account of some discovery he had made with respect to the existence of fluid sulphurs; in this year he writes:--"i have just learned that signor bellani had observed the same fact in . m. bellani complains of the manner in which facts and theories which have been published by him are afterwards given by others as new discoveries; and though i find myself classed with gay-lussac, sir h. davy, daniell and bostock, in having thus erred, i shall not rest satisfied without making restitution, for m. bellani in this instance certainly deserves it at my hand." this is worthy of note as a slight illustration of the true integrity of faraday's character; much as he valued any original discovery he might make, he valued much more that absolute truth which made him render honour to any predecessor even at his own expense; this was done, too, always as a matter of course, without the slightest spirit of grudging. his behaviour on such occasions, which are indeed too trying to most persons, had perhaps a great deal to do with the feeling which he awakened in all who came in contact with him. never, perhaps, was there a more unselfish, as there was never a more universally beloved, man. "his friendship," as professor tyndall says, "was energy and inspiration." faraday was appointed member of a "committee for the improvement of glass for optical purposes;" one of the results of his investigation was that when delivering, in , the bakerian lecture at the royal institution, he took for his subject, "the manufacture of glass for optical purposes." for further investigation of this subject a special experimenting room and furnace had been built at the institution in , and a special assistant--sergeant anderson--engaged to assist faraday. one chief object of these experiments was to improve the glasses of telescopes. this desired result was, however, not attained, although some notable work was done; the glass then manufactured, for instance, became invaluable in some of faraday's later researches. in the glass-making investigation stopped, and in the year following the committee presented their report to the royal society which had appointed them. the recognition of faraday's importance in the world of science was now made more manifest each year; not only were honours done him by various english and continental societies, but in the managers of the royal institution "relieved him from his duty as chemical assistant at the lectures because of his occupation in research." in he was invited to attend the meetings of the managers. in he had been offered the professorship of chemistry in london university, but much as he felt the honour which was done him, faraday declined it, and from the noblest of motives, as will be seen in this passage from his letter to dr. lardner on the subject. "i think it a matter of duty and gratitude on my part to do what i can for the good of the royal institution in the present attempt to establish it firmly. the institution has been a source of knowledge and pleasure to me for the last fourteen years; and though it does not pay me in salary for what i _now_ strive to do for it, yet i possess the kind feelings and goodwill of its authorities and members, and all the privileges it can grant or i require; and, moreover, i remember the protection it has afforded me during the past years of my scientific life." in he was offered, and accepted, as it did not interfere with his royal institution work, a post as lecturer in chemistry at the royal academy, woolwich. in the same year died sir humphry davy--the great chemist to whom faraday owed so much, and to whom, as we have shown, he remained deeply attached to the last. davy had fought his way up as faraday had done, but, unlike faraday, had been in a measure spoiled by his success; he had very little self-control, and but little method and order, and was, perhaps, too anxious about his fame,--about how he would stand in the eyes of men. with faraday it was far different--he aimed at truth in his knowledge, and cared but little for what the world might consider as success. he was known to say, referring to his experiments under davy, "that the greatest of all his great advantages was that he had a model to teach him what to avoid." faraday and davy were, nevertheless, friends to the last, and the death of the latter at the comparatively early age of fifty-two must have been a great blow to the younger man. the year is an important one in the life of michael faraday, for it was then that he commenced his brilliant series of experiments in electro-magnetism. it is on his electrical research that his chief claim to be remembered as a scientist rests. he had earlier experimented in the same connection, but hitherto without attaining the results which he had anticipated. but from this time forward he devoted much energy to this branch of research, with such success that if we pick up any of the most recent works on electrical science we inevitably find an important position given in it to the name and discoveries of michael faraday. this is not the place to enter into a description of these experiments, though reference to them will of course be made later on in this biography in the chapter devoted to a consideration of faraday's discoveries. in the year faraday was appointed fullerian professor of chemistry in the royal institution for life, without the obligation of having to deliver lectures in connection with the professorship. in the year a boy living in a distant part of england wrote to professor faraday, saying that he was desirous of taking up a scientific career. doubtless remembering his own beginning, faraday sent "by return of post a kind and courteous reply," which that boy, grown to man's estate, and known as doctor j. scoffern, gratefully referred to in a graceful tribute which he wrote after faraday's death. it was during this early part of faraday's success that he once gave evidence in a judicial case, when the scientific testimony was so diverse that the judge, in summing up, levelled something very like a reproach at the scientific witnesses, saying, "science has not shone this day." faraday would never again appear as a witness in a court of law. this is, perhaps, the most fitting place in which we can refer to some slight account of faraday's home-life in the institution, which is given by his brother-in-law george (mrs. faraday's youngest brother) and miss reid (her niece). george barnard was much with the faradays in these earlier times. "all the years i was with harding i dined at the royal institution," he says. "after dinner we nearly always had our games just like boys--sometimes at ball, or with horse-chestnuts instead of marbles--faraday appearing to enjoy them as much as i did, and generally excelling us all. sometimes we rode round the theatre on a velocipede, which was then a new thing." it is said "that sometimes of an early summer morning the philosopher was to be seen going up hampstead hill on this velocipede." barnard tells, too, of faraday's unflagging good spirits and his faculty for entering with keen enjoyment into any fun that was going forward--pic-nics up the river, with rustic cookery, charades, or anything else the party seemed bent upon, faraday would join in with delight; how he used to attend hullmandel's conversaziones, where he met many of the leading singers and artists of the day--garcia and malibran, sir edwin landseer, clarkson stanfield, j. m. w. turner, and indeed most of the members of the royal academy. the last-named artist often applied to faraday for chemical information about his pigments; upon turner, and all artists who made similar requests, faraday would always impress the importance there was in their prosecuting experiments with regard to their colours themselves, giving them a hint to put some of their colour and colour-washes in a bright sunlight, covering up one half and leaving the other exposed, and then observing the effect of light and gases on the latter. mr. barnard says that during their various country trips faraday was in the habit of just "rambling about geologising or botanising." mrs. faraday's niece, miss reid, was peculiarly well fitted to give reminiscences of her uncle, as she was for nearly twenty years (from ) one of the family at the royal institution. when she first went there miss reid was only a little child; and when her aunt was going out she was taken down to faraday's laboratory, where, as she afterwards wrote, "i had, of course, to sit as still as a mouse, with my needlework; but my uncle would often stop and give me a kind word or a nod, or sometimes throw a bit of potassium into water to amuse me." "in all my childish troubles," miss reid continues, "he was my never-failing comforter, and seldom too busy, if i stole into his room, to spare me a few minutes; and when perhaps i was naughty and rebellious, how gently and kindly he would win me round, telling me what he used to feel himself when he was young, advising me to submit to the reproof i was fighting against. "i remember his saying that he found it a good and useful rule to listen to all corrections quietly, even if he did not see reason to agree with them. "if i had a difficult lesson, a word or two from him would clear away all my trouble; and many a long wearisome sum in arithmetic became quite a delight when he undertook to explain it." the same lady gives some admirable notes of a holiday the small family party spent at walmer, in kent. how they drove down on the outside of the coach, and how full of fun faraday was, when they reached shooter's hill, over falstaff and the men in buckram; "not a sight nor a sound of interest escaped his quick eye and ear." "at walmer we had a cottage in a field, and my uncle was delighted because a window looked directly into a blackbird's nest built in a cherry tree. he would go many times in a day to watch the parent birds feeding their young." sunrise and sunset were never-failing sources of delighted admiration to him; at such times he was the best of companions, and it has been described as a great treat to watch the glorious sight with him. "he carried galpin's _botany_ in his pocket, and used to make me examine any flower new to me as we rested in the fields. the first we got at walmer was the _echium vulgare_, and is always associated in my mind with his lesson. for when we met with it a second time he asked, 'what is the name of that flower?' 'viper's bugloss,' said i. 'no, no, i must have the latin name,' said he." on one occasion he called his wife and niece into his room to "see a spectre." it was about ten o'clock in the evening, a thick white mist had risen. he then placed a candle behind them as they stood by the window, and they saw two gigantic shadowy beings projected on the mist and imitating, of course, every movement they made. faraday had gone to walmer for rest and refreshment, and his niece says that she, the young one of the party, had to inveigle him away from his books and papers to which he would return, and tempt him out on some excursion to see or find something, on which occasion he was nothing loth. we see, indeed, at all times of his life how keen was the delight he took in the company of young people; how beautifully he could enter into the spirit which animated their play, as though he was still a child himself, and this valuable faculty was his up to the latest. of the walmer excursion his niece further says:--"one day he went far out among the rocks, and brought home a great many wonderful things to show me; for in those days i had never seen nor heard of hermit crabs and sea anemones. my uncle seemed to watch them with as much delight as i did; and how heartily he would laugh at some of the movements of the crabs! we went one night to look for glow-worms. we searched every bank and likely place near, but not one did we see. on coming home to our cottage he espied a tiny speck of light on one of the doorposts. it came from a small centipede; but though it was put carefully under a glass, it never showed its light again. "my uncle read aloud delightfully. sometimes he gave us one of shakespeare's plays or scott's novels. but of all things i used to like to hear him read _childe harold_; and never shall i forget the way in which he read the description of the storm on lake leman. he took great pleasure in byron, and coleridge's 'hymn to mont blanc' delighted him. when anything touched his feelings as he read--and it happened not infrequently--he would show it not only by tears in his voice, but by tears in his eyes also. nothing vexed him more than any kind of subterfuge or prevarication, or glossing over things." his niece mentioned on one occasion a professor who had been discovered abstracting some manuscript from a library. he instantly said, "what do you mean by abstracting? you should say stealing; use the right word, my dear." indecision of any kind faraday could not bear; not only should one decide, but quickly. indeed he thought that in trifling matters immediate decision was important; it was better to decide incorrectly than to remain hesitating. as soon as he left his study and laboratory faraday had the happy faculty of being able to throw aside his science, and would, on going into the sitting-room, "enter into all the nonsense that was going on as heartily as any one; and as we sat round the fire he would often play some childish game, at which he was usually the best performer; or he would take part in a charade, and i well recollect his being dressed up to act the villain, and very fierce he looked. another time i recollect him as the learned pig." [illustration: faraday's study.] as we learn such things as these about him we cease to wonder that faraday was the object of so much admiration and love from all persons, old and young, with whom he came in contact. his wonderful work as a scientist will, it is to be hoped, never be remembered as his only claim on our regard; for as one of the best and kindest and most helpful of men, whose singular modesty and gentleness of character endeared him to all, he certainly deserves to be kept ever in our recollection. we must not, as a friend wrote of him, allow the name of faraday "to be nothing but a peg on which to hang discoveries;" but must also recollect that his "time, thoughts, purse, everything was freely given to those who had need of them." [illustration] [illustration] chapter vi. as teacher and preacher. "'tis the man who with a man is an equal, be he king, or poorest of the beggar clan, or any other wondrous thing a man may be." keats. by the commencement of the year we find michael faraday, not yet forty-four years of age, generally acknowledged as one of their peers by the leading men of science, not only in england but also on the continent. we find him elected member of many of the most important scientific and philosophical societies of this and other countries; we find him honoured by the university of oxford with the degree of d.c.l.; and, as we shall shortly see, we find the government proposing to confer a pension on him in consideration of the services which he has already rendered to science. truly a wonderful change to be wrought in the life of a man who thirty years before was carrying round newspapers as a common errand boy. it is, however, always gratifying to note, and especially pleasing to remember, that however successful he might be, faraday was never spoiled by the honours that were done him; he was always the same kindly, helpful, simple man that he had been. those persons who had the great good fortune to visit him at the royal institution, either at the time of which we are treating or during his later life, never failed to find a cordial welcome; "a friendly chat in those quiet rooms was one of the greatest pleasures it was possible to enjoy. the frugal simplicity of the furniture was characteristic of faraday." the faradays lived quietly to themselves at the institution, though they often, after the friday evening lectures, went round to berkley street to tea to mr. richard barlow's house, that gentleman and his wife always being at home to their friends after the friday evenings. on such occasions as these gatherings, faraday, we learn, used to be the centre of much interest and delight; for he had, as may be gathered from what has already been said of his character, that happy disposition which placed him at once in sympathy with any person with whom he might be speaking; especially was this rare sympathy his with regard to children, with whom he seemed at once able to place himself on an equal footing; and this it was that made his lectures to young people not only so interesting but so widely popular as they were. this subject, however, deserves fuller consideration, and will be found treated in a later chapter. had he chosen to do so at this period of his career, faraday might have been in receipt of a pretty considerable income. in , indeed, he had undertaken several commercial analyses, and his income from this source alone came to as much as a thousand pounds. such work interfered with his research, and was therefore unhesitatingly given up, and two years afterwards his professional gains amounted to but little more than a hundred and fifty pounds for the year, and in after years they did not reach even that sum. early in faraday received an intimation from sir james south that had sir robert peel remained in office he had intended conferring a pension upon him. faraday wrote in reply, saying that he could not accept a pension. the matter after this remained in abeyance for a while. during the summer faraday spent a short holiday in switzerland, whence he wrote to his old friend magrath: "the weather has been most delightful, and everything in our favour, so that the scenery has been in the most beautiful condition. mont blanc, above all, is wonderful, and i could not but feel at it what i have often felt before, that painting is very far beneath poetry in cases of high expression; of which this is one. no artist should try to paint mont blanc; it is utterly out of his reach. he cannot convey an idea of it, and a formal mass or a commonplace model conveys more intelligence, even with respect to the sublimity of the mountain, than his highest efforts can do. in fact, he must be able to dip his brush in light and darkness before he can paint mont blanc. yet the moment one sees it lord byron's expressions come to mind, as they seem to apply. the poetry and the subject dignify each other." in the autumn of the same year, shortly after his return from the continent, the subject of a pension for faraday was re-opened. the independence and openness of his character came out in a remarkable manner in this matter. he was asked to wait upon lord melbourne, the prime minister, at the treasury, which he did on october th. however he may have spoken of faraday personally, lord melbourne spoke of literary and scientific men with but scant courtesy, and in effect seemed to consider the awarding them pensions as a piece of state humbug. we have seen how faraday resented a slur cast upon science in a court of law, and he was no less indignant on this occasion; he returned home and wrote a letter, the tone of which though dignified was very decided. this letter, in which he declined to accept or even further to consider the acceptance of a pension from the government, faraday intended to forward at once to lord melbourne. he finally, however, allowed somewhat wiser counsels to prevail; his father-in-law, while justly proud of michael's scientific attainments, was also a shrewd business-like man, and persuaded him to write a letter, which, although it was not one whit less dignified in its tone was less decided in its refusal of the proposed pension. after many fruitless efforts to make faraday change his decision, a lady, who was a friend both of the philosopher and of the prime minister, asked the former what he would require at the hand of lord melbourne to make him change his mind on the subject. "i should require," he replied, "from his lordship what i have no right or reason to expect that he would grant--a written apology for the words he permitted himself to use to me." to melbourne's credit, be it said, that as soon as he knew of this he apologised amply for, as he expressed it, the "too blunt and inconsiderate manner in which he had expressed himself." on december th of the same year the pension of three hundred pounds a year was awarded to michael faraday for his services to the cause of science. a pension, it may here be mentioned, half of which was continued to the professor's widow, and on her death to his niece, miss jane barnard. he was not yet forty-five, we must recollect, when he was thought to have fairly earned this reward. early in further honour was done to him by his being appointed scientific adviser to the trinity house; in accepting the position he wrote a characteristic letter, of which the following is a portion; it was addressed to captain pelly, deputy-master: "i consider your letter to me as a great compliment, and should view the appointment at the trinity house, which you propose, in the same light; but i may not accept even honours without due consideration. in the first place my time is of great value to me, and if the appointment you speak of involved anything like periodical routine attendances i do not think i could accept it. but if it meant that in consultation, in the examination of proposed plans and experiments, in trials, etc., made as my convenience would allow, and with an honest sense of a duty to be performed, then i think it would consist with my present engagements.... in consequence of the goodwill and confidence of all around me, i can at any moment convert my time into money, but i do not require more of the latter than is sufficient for necessary purposes. the sum, therefore, of £ is quite enough in itself, but not if it is to be the indicator of the character of the appointment. but i think you do not view it so, and that you and i understand each other in that respect; and your letter confirms me in that opinion. the position which i presume you would wish me to hold is analogous to that of a standing counsel. as to the title it might be what you pleased almost. chemical adviser is too narrow; for you would find me venturing into parts of the philosophy of light not chemical. scientific adviser (the title afterwards decided upon) you may think too broad--or in me too presumptuous--and so it would be, if by it was understood all science.... the thought occurs to me whether, after all, you want such a person as myself. this you must judge of; but i always entertain a fear of taking an office in which i may be of no use to those who engage me." this letter is, as i have said, characteristic of the writer; it is characteristic of his sensitiveness to any honour done to him, and of his unworldliness, of his conscientiousness in making sure that he will be able to perform anything that he may undertake, and of a half-diffidence with regard to himself as to whether he was able to do all that was anticipated of him. for nearly thirty years, with credit to himself and to the brethren of the trinity house, did michael faraday continue as their scientific adviser. frequently do we find him experimenting on lights and lighting--visiting the various lighthouses round the coast, trying the electric light for them, comparing the various lights, and reporting to the brethren--such work as this is, as has been said, to be frequently noted in looking over a record of the mass of work which during these years faraday was doing. it is pleasing to notice here that on her husband's death mrs. faraday presented such of his portfolios, of well-ordered and indexed manuscripts, as referred to this part of his work to the trinity house. so carefully were these notes made and kept that it is possible now to refer quite easily to any particular piece of work on which faraday was engaged during these thirty years. in this same year ( ) there appeared the _life of sir humphry davy_, by his brother, dr. john davy. in this work statements were made with relation to faraday and his patron which were not true; and painful indeed though it must have been to the former, he felt compelled to deny them. this he did in a long letter to his friend r. phillips, editor of the _philosophical magazine_, in which periodical the letter was published. "i regret," faraday wrote, "that dr. davy has made that necessary which i did not think before so; but i feel that i cannot, after his observation, indulge my earnest desire to be silent on the matter, without incurring the risk of being charged with something opposed to an _honest_ character. this i dare not risk; but in answering for myself i trust that it will be understood that i have been driven unwillingly into utterance." the subject must indeed have been a painful one; to have to assert his own right to be the discoverer of certain chemical results which were being credited to davy. in one or two cases, when he found that he had been preceded in the discovery of anything he was the first to acknowledge that all honour was due to his predecessor, and that strict regard for true honesty in all things very properly would not allow him to be silent now. he concludes his letter to phillips in these words, "believing that i have now said enough to preserve my own 'honest fame' from any injury it might have risked from the mistakes of dr. davy, i willingly bring this letter to a close, and trust that i shall never again have to address you on the subject." it was at about this time that another incident occurred which illustrates faraday's absolute integrity of character--integrity that would not wink at anything that was in the slightest degree not straightforward, even though it was against his own interest, which, indeed, rather shunned doing a good deed that might seem dictated by mere self-interest. his brother was working at the time as a gas-fitter, and there was a possibility of his getting the athenæum club work in connection with his trade. michael, writing on the subject, said, "few things would please me more than to help my brother in his business, or than to know that he had got the athenæum work; but i am exceedingly jealous of myself, lest i should endeavour to have that done for him as my brother which the committee might not like to do for him as a tradesman, and it is this which makes me very shy of saying a word about the matter." during these years faraday was getting through a vast quantity of work, his experimental researches in electricity were taking up a great part of his time, but other matters were not neglected. he was frequently lecturing before the royal institution or the royal society; while he wrote a large number of scientific papers for the various philosophical periodicals to which he contributed. besides presenting series after series of his brilliant _experimental researches_ to the royal society, he had also to attend to his lectures at woolwich, and his work for the trinity house. he must indeed, hard-working man that he was, have found his long day very fully occupied; and it is scarcely to be wondered at that in the strain began to tell upon him, and a period of rest became necessary. as years went on, such periods of rest were more frequent and yet more vitally important. in the british association[ ] held its annual meeting at liverpool, and faraday attended it, and was made, as he put it, "a most responsible person," president of the chemical section. from liverpool he wrote home to his wife, "to-day i think we made our section rather more interesting than was expected, and to-morrow i expect will be good also. in the afternoon daniell and i took a quiet walk; in the evening he dined with me here. we have been since to a grand conversazione at the town hall, and i have now returned to my room to talk with you as the pleasantest and happiest thing i can do. nothing rests me so much as communion with you. i feel it even now as i write; and i catch myself saying the words aloud as i write them, as if you were within hearing. dear girl, think of me till saturday evening. i find i can get home very well by that time; so you may expect me. "ever, my dear sarah, your affectionate husband, "m. faraday." this reference to getting home on saturday evening is especially interesting, for faraday always took his wife home to her father's house every saturday evening, that she might see her family; they all went to church together on the sunday. this was an unvarying rule of faraday's for very many years, as long, indeed, as it was possible. faraday's mother, after having lived to see "her michael" come to be one of the great men of his time, died at islington in march, . the loss must have been keenly felt by faraday, for between mother and son the tenderest affection had always subsisted. she was justly proud of the position which her boy had won for himself; and he ever retained that beautiful chivalrous kindness and deference to his mother that had characterised him all along. the passages from his earlier letters to his mother which have been given in previous pages are evidence of this, and his kindly consideration was ever the same. much as the death of his mother, and, a few years later, of his brother robert, affected him, faraday had in his beautiful clear-sighted faith in his religion a source of inextinguishable solace, and looking forward to a reunion hereafter could see a "beautiful and consoling influence in the midst of all these troubles." severe and long-continued mental work, as i have said, began to tell upon faraday in , and he was ordered by his doctor to take an absolute rest. he suffered from loss of memory and similar symptoms of an overworked brain. his wife, as her niece tells us, used to carry him off to brighton, or somewhere down into the country for a few days when he became dull and low-spirited, and the rest soon restored him. during such a sojourn at brighton, towards the close of , professor brande wrote to him, saying that the doctor said faraday was to remain thoroughly idle for a time; and he (brande) kindly offered at the same time to do anything he could to relieve faraday of any routine work. he had indeed read some of faraday's electricity lectures at the institution, although, as he terms it in his letter to faraday, "he began to fear the fate of phæton in the chariot of phoebus." as yet, however, faraday would not take a very long-continued rest, and he was before long back in albemarle street working, although less than he had been doing. in the year faraday's health made it necessary that his scientific labours should be reduced, and just about this time, although he was still adding to his series of _experimental researches_, he was husbanding his strength as far as possible. this year, too, deserves especial note, for it was now that he became an elder in the sandemanian church; he had before on some occasions exhorted those present at week-day meetings, but it now devolved upon him to deliver regular sermons. faraday, as we have already seen, on more than one occasion, was not a man to undertake anything without doing it to the best of his abilities; and if this was his character in matters of everyday concern, how much more so should we expect it to be, and not without reason, his character in so vital a question as that concerning his religion. or perhaps it would be more correct to say concerning the more obvious exercise of his religious faith, for the spirit of his religion coloured all that he did; it was indeed the moving force of his soul, and was not confined to any narrow circle. [illustration: interior of the old sandemanian meeting-house, paul's alley.] the flow and energy which characterised faraday as a lecturer were replaced in his sermons by a simplicity and earnestness that together are best described as true devoutness. his sermons were always extemporary, although the outlines of them were carefully prepared beforehand, a small card having noted down on either side of it the heads of the elder's discourse and reference to such passages in the bible as he wanted to quote. one of these cards is given that it may show with what slight notes the earnest and reverent preacher provided himself. a friend, describing him officiating at the chapel, which was situated in paul's alley, redcross street, city, but which has long since been pulled down, and the church transferred to barnsbury grove, n., said, "he read a long portion of one of the gospels slowly, reverently, and with such an intelligent and sympathising appreciation of the meaning, that i thought i had never before heard so excellent a reader." [illustration: +--------------------------------------------------------------------+ | - peter iii. , , . _a prophetic warning to christians._ | | | | +------------------ | | -| first, the power and grace and promises of the gospel. | | -| i. , by his power are given great and precious promises, | | -| , divine nature and brethren exhorted to give diligence, | | -| , whilst in this life up to v. . | | | | +------------------ | | -| _then_ cometh a warning of the state into which they _may_ | | -| fall, , , if they forget--as he stirs them up, , , , as | | -| escapers from the corruption, i. . | | | | +------------------ | | -| iii. . _wherefore, beloved, seeing ye look for such things_, | | -| their hope and expectation--it is to stir up their pure mind, | | -| iii., , by way of remembrance--hastening the day of the, | | -| v. , awful as that day will be, , , because of the | | -| deliverance from the plague of our own heart. | | | | | -| cor. iv. , , , look not at things seen--temporal. | | -| titus ii. , looking for the hope and glorious appearing. | | -| heb. x. , yet a little while, and he that shall come | | | will come. | | | +--------------------------------------------------------------------+ | | | -| the world make his forbearance a plea to forget him or | | -| deny him. | | | | | -| iii. , , perceiving him not in his works. his people see | | -| his mercy and long-suffering and look for his promise, , | | -| , and salvation, , and learn that he knoweth how to | | -| reserve, ii , , and preserve, hence | | | | +------------------ | | -| they are not to be slothful, prov. xxiv. . | | -| nor sleeping--matt xxv. . sleeping virgins | | -| nor doubting iii. . | | -| nor repining heb. xii. , , , lift up hands | | -| jas. v. , , be patient--husbandmen | | -| waiteth, but waiting, luke xii. , , , , | | -| peter , v. , refers to days of long-suffering. | | | | +------------------ | | -| wherefore, beloved, seeing ye know these things, beware, etc., | | -| danger of falling away in many parts, i. , ii. , , , | | -| great pride of the _formal_ adherers, ii. , . | | | | | -| but the assurance is at iii. --i. , . | | | +--------------------------------------------------------------------+ copy of one of the cards from which faraday preached.] another of his listeners said, "his object seemed to be to make the most use of the words of scripture, and to make as little of his own words as he could. hence a stranger was struck first by the number and rapidity of his references to texts in the old and new testaments, and secondly, by the devoutness of his manner." yet another friend, who had been privileged to hear faraday preach before his small flock, said of his sermons, "they struck me as resembling a mosaic work of texts. at first you could hardly understand their juxtaposition and relationship; but as the well-chosen pieces were filled in by degrees their congruity and fitness became developed, and at last an amazing sense of the power and beauty of the whole filled one's thoughts at the close of the discourse." this, his first period of eldership in the church, continued from until , when a slight misunderstanding having arisen between himself and the brethren, he for a time relinquished the office; occupying it again, however, later on in life. his earnest religious feeling was an abiding source of consolation to him in all his trials; it affected in no slight degree his life and life-work at all points, although, to his credit be it said, that it was rather the spirit of his religious feeling which was thus manifested, and it is not by any means to be understood that he was in even the slightest degree given to cant, such a thing being far from possible with him. his religion was a something too sacred and too immediately between himself and his god, as he said, for him to refer to it, except when circumstances especially called for it. then, in the earnest sympathetic words of comfort, which he addressed to those persons with whom he was intimate when they were in trouble, we may trace the true deep current of religion, which was so essentially a part of his nature. it is interesting to connect the name of our philosopher with a great institution such as the establishment of the penny post. in sir rowland hill tells us in his autobiography that he was sorely puzzled to find an ink that, having obliterated the postage stamps, should not be removable. "in my anxiety," he says, "i went so far as to trouble the greatest chemist of the age. kindly giving me the needful attention, though in an extremely depressed state of health, the result of excessive labour, a fact, of course, unknown to me when i made the application, mr. faraday approved of the course which i submitted to him: viz., that an aqueous ink should be used both for the stamp and for obliteration." referring to this same year we find an interesting entry in crabb robinson's diary. "_may th._--attended carlyle's second lecture.... it gave great satisfaction, for it had uncommon thoughts and was delivered with unusual animation.... in the evening heard a lecture by faraday. what a contrast to carlyle! a perfect experimentalist, with an intellect so clear! within his sphere, _un uomo compito_. how great would that man be who could be as wise on mind and its relations as faraday is on matter!" faraday's life as a scientific experimentalist and discoverer is divided into two periods by an interval of four years, during which he did but little, or, compared with his previous performances but little, work. such a time of rest was indeed rendered absolutely necessary by loss of memory and giddiness, which had troubled him occasionally before, and which now put a stop to his experiments. this period of partial rest commenced with a three months' trip in switzerland, where he was accompanied by his wife and her brother. dr. bence jones says, "in different ways he showed much of his character during this period of rest. the journal he kept of his swiss tour is an image of himself. it was written with excessive neatness, and it had the different mountain flowers which he gathered in his walks fixed in it as few but faraday himself could have fixed them. his letters are free from the slightest sign of mental disease. his only illness was overwork, and his only remedy was rest." a few passages from this swiss journal are all that can be given. the first stay of any length was made at thun, whence many walking excursions were undertaken, sometimes indeed faraday walking as much as forty-five miles in the one day, a sufficient proof that he was not at all bodily ill. the journal gives us many a word-picture of the scenery and of the people, with now and then quaint observations and humorous reflections; let the following passages speak for themselves:-- "_july th._--took a long walk to the valley called the simmenthal, which goes off from the valley of the lake.... the frogs were very beautiful, lively, vocal, and intelligent, and not at all fearful. the butterflies, too, became familiar friends with me, as i sat under the trees on the river's bank. it is wonderful how much intelligence all these animals show when they are treated kindly and quietly; when, in fact, they are treated as having their right and part in creation, instead of being frightened, oppressed, and destroyed. "_monday, th._--very fine day; walk with dear sarah on the lake side to oberhofen, through the beautiful vineyards; very busy were the women and men in trimming the vines, stripping off leaves and tendrils from fruit-bearing branches. the churchyard was beautiful, and the simplicity of the little remembrance posts set upon the graves very pleasant. one who had been too poor to put up an engraved brass plate, or even a painted board, had written with ink on paper the birth and death of the being whose remains were below, and this had been fastened to a board and mounted on the top of a stick at the head of the grave, the paper being protected by a little edge and roof. such was the simple remembrance; but nature had added her pathos, for under the shelter by the writing a caterpillar had fastened itself and passed into its death-like state of chrysalis; and having ultimately assumed its final state it had winged its way from the spot, and had left the corpse-like relics behind. how old and how beautiful is this figure of the resurrection! surely it can never appear before our eyes without touching the thoughts. "_tuesday, th._--more pleasant rambles: fine. now we shall think of a move, and really the changing character of the _table d'hôte_ and other things make me in love with the thoughts of home. dear england, dear home! dear friends! i long to be in and among them all; and where can i expect to be more happy, or better off in anything? dear home, dear friends, what is all this moving, and bustle, and whirl, and change worth compared to you? "_august nd._--interlaken.... the jungfrau has been occasionally remarkably fine: in the morning particularly, covered with tiers of clouds, whilst the snow between them was beautifully distinct; and in the evening showing a beautiful series of tints from the base to the summit, according to the proportion of light on the different parts. at one time the summit was beautifully bathed in golden light, whilst the middle part was quite blue, and the snow of its peculiar blue-green colour in the clefts.... clout-nail making goes on here rather considerably, and is a very neat and pretty operation to observe. i love a smith's shop, and anything relating to a smithy. my father was a smith." how beautiful is the following description of the waterfall at brienz lake: "the sun shone brightly, and the rainbows seen from various points were very beautiful. one at the bottom of a fine but furious fall was very pleasant. there it remained motionless, whilst the gusts and clouds of spray swept furiously across its place, and were dashed against the rock. it looked like a spirit strong in faith and steadfast in the storm of passions sweeping across it; and though it might fade and revive, still it held on to the rock as in hope, and giving hope, and the very drops which in the whirlwind of their fury seemed as if they would carry all away were made to revive it and give it greater beauty." at length, on september th, the small party reach london again, and faraday's journal ends thus:--"crossing the new london bridge street we saw m.'s pleasant face, and shook hands; and though we separated in a moment or two, still we feel and know we are where we ought to be--at home." faraday's allusion to his father in the extract above is very pleasing and interesting. we are told that he used to like to pay visits to the scenes of his boyhood and youth, and that he once went to the shop where his father had formerly been employed as a blacksmith, and asked to be allowed to look over the place. when he got to a part of the premises at which there was an opening into the lower workshop, he stopped and said, "i very nearly lost my life there once. i was playing in the upper room at pitching halfpence into a pint pot close by this hole, and having succeeded at a certain distance, i stepped back to try my fortune further off, forgetting the aperture, and down i fell; and if it had not been that my father was working over an anvil fixed just below, i should have fallen on it, broken my back, and probably killed myself. as it was, my father's back just saved mine." on his return from his swiss trip, faraday took up a great part of his work again, and was fully occupied with a few electrical experiments, lectures, and trinity house work. what has been termed his second great period of research did not commence until . he lectured frequently at the institution--so frequently indeed that we cannot refer to them here, but must leave them to the chapter on his lectures. indeed, merely to detail the work which faraday did would take up considerably more than the whole space of this little book. in faraday became one of the special commissioners appointed to investigate the haswell colliery explosion. in his brother robert met with a fatal accident, and faraday writes to his wife, who was staying at tunbridge wells:--"dear heart,--.... come home, dear. come and join in the sympathy and comfort needed by many.... my sister and her children have not forgotten the hope in which they were joined together with my dear robert, and i see its beautiful and consoling influence in the midst of all these troubles. i and you, though joined in the same trouble, have part in the same hope. come home, dearest. "your affectionate husband, "m. faraday." in he delivered his famous lectures on "the chemical history of a candle;" and in the following year he gave a series of six lectures on "some points of domestic chemical philosophy--a fire, a candle, a lamp, a chimney, a kettle, ashes." his work during these years is shown in his many published letters, in his correspondence which for years he maintained with many of the leading scientists, not only in england but abroad--with de la rive, liebig, humboldt, etc. his work, however, cannot be particularised, neither can the many honours that year after year were awarded to him. we find that he was a man nearly sixty years of age, in the front rank of the great chemists of his country, and acknowledged as such on every hand, and yet we find that he was still the same energetic and enthusiastic scientist, the same kindly and unselfish friend, the same honest and disinterested man that we have seen him all through. such, indeed, he continued until the very last, his character but "deepening"--as he said of his love for his wife--as the years passed by. his chivalrous deference to women of all ages and ranks was also a remarkable feature of his character, no less at this later part of his life, than when he was a younger man; his chivalry has, indeed, been often referred to, but it was, i learn from miss barnard, one of his most readily observed good qualities. [illustration] footnote: [ ] founded in , for the purpose of stimulating scientific inquiry. [illustration] chapter vii. overwork--the end. "have you found your life distasteful? my life did, and does, smack sweet. was your youth of pleasure wasteful? mine i saved, and hold complete. do your joys with age diminish? when mine fail me i'll complain. must in death your daylight finish? my sun sets to rise again." robert browning. the year of the great exhibition was a busy one for faraday; he was working in his old accustomed, unremitting manner at his magnetic, and electric, and general experiments, he was continuing to write those _experimental researches_ which he sent in to the royal society, and upon which rests so large a part of his reputation as a scientist. he had given up his professorship at woolwich academy the previous year. he was lecturing, however, a good deal, and not alone on his own account, for during the summer he delivered a lecture on ozone for his good friend professor schönbein. his health, however, was far from being as good as it had been, and he had to take frequent rests; so that, although he was working as earnestly and enthusiastically as ever, it was, so to speak, only intermittently. that the loss of memory from which he had before suffered was still afflicting him at times, is made evident by such passages from his letters as the following pathetic one from a letter to schönbein: "i have no doubt i answer your letters very badly; but, my dear friend, _do you remember_ that _i forget_, and that i can no more help it than a sieve can help the water running out of it. still you know me to be your old and obliged and affectionate friend, and all i can say is, the longer i know you the more i seem to cling to you. ever, my dear schönbein, yours affectionately." a pathetic interest attaches to the following reminiscence of faraday by his niece (miss jane barnard); she was reading to him an anecdote of the duke of marlborough's intimation to the king that as he felt that the time when his faculties would fade had arrived he did not wish again to attend any council meeting, and that if he should attend he desired that no heed should be given to anything he said. faraday after listening attentively to it, asked miss barnard to read that anecdote to him if at any time she felt that his judgment no longer controlled his wishes. so numerous were the honours which were showered upon michael faraday during the last forty years of his life, that to enumerate them would be as tedious as it would be profitless; suffice it to say that he was elected a member of all the chief scientific and philosophical bodies in europe. indeed it is said that a continental professor addressed a letter to him as "professor michael faraday, member of all the learned societies of europe." it is worthy of note, however, that he was elected a member of the senate of the university of london, and was asked to act as examiner for the same body, but declined. during the periods of rest which his failing health made necessary, faraday would go off to brighton or hastings with his wife, where he would spend a few days in quiet idleness. in february, , he was at brighton, where mr. masquerier, the french refugee who had in early life given him lessons in geometry, was living. in crabb robinson's diary the following entry, which is of much interest to us here, occurs against february : "(at masquerier's, brighton.) we had calls soon after breakfast. the one to be mentioned was that of faraday, one of the most remarkable men of the day, the very greatest of our discoverers in chemistry, a perfect lecturer in the unaffected simplicity and intelligent clearness in his statement; so that the learned are instructed and the ignorant charmed. his personal character is admirable. when he was young, poor, and altogether unknown, masquerier was kind to him; and now that he is a great man he does not forget his old friend." an interesting story is told by dr. scoffern of an incident that happened during this year; an incident that illustrates in a remarkable manner the unaltered good humour and geniality which belonged to faraday as much during his later as his earlier years. professor brande was lecturing at the time on a newly-discovered method of purifying sugar by sugar of lead; while they were in the laboratory scoffern accidentally let fall a retort of corrosive liquid. in an instant, he tells us, professor faraday "threw some soda upon the floor; then down on his hands and knees he went, slop cloth in hand, like any humble housemaid. laughing, i expressed my desire to photograph him then and there; he demurred to the pose, begged me to consult his dignity, and began laughing with a childish joyousness. hilariously boyish upon occasion he could be, and those who knew him best, knew he was never more at home, that he never seemed so pleased as when making an 'old boy' of himself, as he was wont to say, lecturing before a juvenile audience at christmas." faraday, as has been said earlier, attended some of the annual meetings of the british association; in this year of the meeting was held at ipswich, and on that occasion dr. j. h. gladstone says he first met faraday to have any intercourse with him. "i watched him," he writes, "with all the interest of an admiring disciple, and there is deeply engraven on my memory the vivacity of his conversation, the eagerness with which he entered into some mathematico-chemical speculations of dumas, and the playfulness with which, when we were dining together, he cut boomerangs out of card, and shot them across the table at his friends." yet another story of faraday's remarkable disinterestedness is given us by dr. scoffern, who, writing of the year , says that he had made an abstract of a course of lectures which faraday had delivered on the subject of the non-metallic elements; this abstract he wished to embody in a book which he was about to publish. the kindly old chemist at once gave his permission, and would not even listen to any proposal as to sharing the profits of the work. scoffern immediately suggested that he would be misunderstood by the publisher, who would not be able to comprehend such a piece of generosity on the great professor's part. "oh," said faraday, "we'll soon settle that by writing;" and he wrote out a formal letter of assignment. despite the fact that his time was always fully occupied, faraday found time to write many letters, not only the long friendly, yet scientific letters to such men as de la rive and schönbein, but letters of advice and sympathy to his nephews and nieces, and other friends. his advice was always given in so kindly a spirit that it could not be taken amiss, and his sympathy was tendered in that rare manner--sincere and unostentatious--which characterises this feeling in its highest manifestation. the following passage, from a letter to his nephew, frank barnard, who was just starting life, is an illustration of this: "and so you are hard at work, and somewhat embarrassed by your position; but no man can do just as he likes, and in many things he has to give way, and may do so honourably, provided he preserves his self-respect. never, my dear frank, lose that, whatever may be the alternative. let no one tempt you to it, for nothing can be expedient that is not right; and though some of your companions may tease you at first, they will respect you for your consistency in the end; and if they pretend not to do so it is of no consequence. however, i trust the hardest part of your probation is over, for the earliest is usually the hardest, and that you know how to take all things quietly." although i have made but little special reference to the work on which our great hero was engaged when treating of different periods of his life; it becomes necessary here to refer to the part which faraday took in exposing a popular delusion which was widely believed in at the time, and which yet has many supporters--the delusion as to table-turning. he wrote a long account fully exposing the error which so many people were willing to believe; and although his exposure convinced most persons who troubled themselves to follow him in his investigations, the popular mind refused to be disillusionised, and the turning of the tables was referred to electricity, magnetism, spirits, a new natural force, and other agencies. this occasion perhaps drew more emphatic utterance from faraday than any other; he had no patience with people who would not be enlightened, and his feeling is shown in a letter written in july, , to professor schönbein: "i have not been at work except in turning the tables upon the table-turners, nor should i have done that, but that so many inquiries poured in upon me, that i thought it better to stop the inpouring flood by letting all know at once what my views and thoughts were. what a weak, credulous, incredulous, unbelieving, superstitious, bold, frightened,--what a ridiculous world ours is, as far as concerns the mind of man. how full of inconsistencies, contradictions, and absurdities it is. i declare that, taking the average of many minds that have recently come before me (and apart from that spirit which god has placed in each), and accepting for a moment that average as a standard, i should far prefer the obedience, affections, and instincts of a dog before it. do not whisper this, however, to others. there is one above who worketh in all things, and who governs even in the midst of that misrule to which the tendencies and powers of men are so easily perverted." in his juvenile lectures, delivered at christmas of the same year, he again referred to this popular error, giving at the same time some sound advice to his young friends. "in conclusion, i must address a few words to the intending philosophers who form the juvenile part of my audience. study science with earnestness--search into nature--elicit the truth--reason on it, and reject all which will not stand the closest investigation. keep your imagination within bounds, taking heed lest it run away with your judgment. above all, let me warn you young ones of the danger of being led away by the superstitions which at this day of boasted progress are a disgrace to the age, and which afford astonishing proofs of the vast floods of ignorance overflowing and desolating the highest places. "educated man, misusing the glorious gift of reason which raises him above the brute, actually lowers himself below the creatures endowed only with instinct; inasmuch as he casts aside the natural sense which might guide him, and in his credulous folly pretends to discover and investigate phenomena which reason would not for a moment allow, and which, in fact, are utterly absurd. "let my young hearers mark and remember my words. i desire that they should dwell in their memory as a protest uttered in this institution against the progress of error. whatever be the encouragement it may receive elsewhere, may we, at any rate in this place, raise a bulwark which shall protect the boundaries of truth, and preserve them uninjured during the rapid encroachments of gross ignorance under the mask of scientific knowledge." faraday's high position in the world of science and his well-known thoroughness in investigating any subject in which he interested himself, made his utterances on the subject of spirit-rapping and table-turning convincing to a large number of people. he was, however, for many years occasionally pestered with questions about it, by persons who thought they could prove to him that he was wrong; perhaps in no matter did faraday so nearly lose his patience as over this; at no other time did he so nearly exhibit that volcano of fiery passion which, according to tyndall, underlay the sweetness and gentleness of disposition which were his ever-obvious qualities. he had, as tyndall well puts it, "through high self-discipline converted the fire into a central glow and motive power of life, instead of permitting it to waste itself in useless passion. 'he that is slow to anger,' saith the sage, 'is greater than the mighty, and he that ruleth his own spirit than he that taketh a city!' faraday was _not_ slow to anger, but he completely ruled his own spirit; and thus, though he took no cities, he captivated all hearts." miss barnard, from her long and intimate acquaintance with her uncle, quite endorses what professor tyndall says. she says that a most fiery passion was kept under by the most perfect master, and during all the years she knew him she could not recollect above two occasions when faraday, even for a moment, let his passion get the better of him. lightly as he looked upon honours such as are the ones usually appreciated by more worldly men, faraday was always well-pleased and more than gratified when recognised by leading men of science or literature. many as were the distinctions which had been and were still being heaped upon him, he would especially value such a one as was offered him in , when one who in a measure had been his pupil--henry mayhew--dedicated to him a volume on the _wonders of science_, illustrating the life and progress in scientific knowledge of young humphry davy. this dedication runs, "my dear sir, i inscribe your name on one of the fly-leaves of this little book, with the same devotion as youths are wont to carve upon the trunk of some forest tree the name of those whom they admire most in the world; and i do so for many reasons." and in concluding the dedication he shows us once more the helpfulness and goodness of faraday's nature: "and now, my dear sir, let me, in conclusion, thank you for your generous encouragement of my labours when i was engaged in inquiring into the condition of the 'london poor.' many know your wisdom, but none are better acquainted with your goodness than yours very truly, "henry mayhew." never, when a success beyond the wildest imaginings of his youth had crowned his devotion to science, did faraday forget the time of his early struggles, and the humble beginning which he had made. as we have before mentioned, he would frequently stop in the street to speak a kindly word of encouragement to young newspaper lads who were just starting in life in the way that he had done over half a century earlier. an incident such as that depicted in the illustration was, indeed, a not uncommon one, for, to refer again to the professor's own words, he could not but feel a tenderness for such boys, because he had once carried newspapers himself. [illustration: faraday and the newsboy.] in miss reid's recollections of her illustrious uncle, from which we have quoted in an earlier chapter, there was something said about the reading which interested the scientist in his hours of relaxation. this is always an interesting matter in connection with our great men; we are always glad to know what they read, and, if possible, why they read it. at a party about this time, faraday joined in a discussion which was being carried on on the subject of novel reading, and some one of those present took a few notes of such works as he mentioned as being specially interesting or entertaining to him. he liked novels, he said, with some stir and life in them, such as _paul ferrol_, _jane eyre_, too--although of this he characteristically said, "there's a touch of mesmerism and mystery at the end which would be better away." of scott's novels he was always a great admirer, liking particularly _ivanhoe_, _guy mannering_, and _waverley_; he also spoke admiringly of fanny burney's novel, _evelina_, a book that is hardly among the generally read novels of to-day. writing in to professor de la rive on the death of mrs. marcet,[ ] faraday mentions his early reading as follows: "do not suppose that i was a very deep thinker, or was marked as a precocious person, i could believe in the _arabian nights_ as easily as in the _encyclopædia_. but facts were important to me and saved me. i could trust a fact, and always cross-examined an assertion. so when i questioned mrs. marcet's book by such little experiments as i could find means to perform, and found it true to the facts as i could understand them, i felt that i had got hold of an anchor in chemical knowledge, and clung fast to it. thence my deep veneration for mrs. marcet." when, in , mr. cyrus field was in england preparing for the laying of the great telegraph cable across the atlantic ocean, he inquired of faraday as to what he thought of its practicability; the philosopher doubted the possibility of transmitting a message. field saw that an objection from so great an authority would prove well-nigh fatal, and that it must be removed at once; he therefore offered to pay faraday sufficiently for his services if he would undertake such experiments as were necessary. faraday declined the money, but undertook the experiments, and on their completion reported to field, "it can be done, but you will not get an instantaneous message." "how long will it take?" anxiously inquired the engineer. "oh, perhaps a second." "well, that's quick enough for me." the year is an interesting one in the life of michael faraday; for over forty years he had lived in the royal institution, and during that time had risen from being a journeyman bookbinder with a small circle of friends, to being the first of living philosophers, with a fame known all over the world, and with friends wherever his fame had penetrated. in this year, however, while still retaining his connection with the royal institution, he removed with his wife to a house at hampton court, which had been kindly placed at their disposal by her majesty the queen, at the instigation of the prince consort. faraday writes in april to prince albert's secretary acknowledging the extreme kindness of her majesty, but expressing himself as doubtful whether to accept or to decline. the house it appeared wanted some repairs which faraday felt doubtful about; he did not feel that he would be enabled to undertake them, but his mind was soon set at rest on this score, for in the summer of this year, writing to one of his nieces, he says, "the case is settled. the queen has desired me to dismiss all thoughts of the repairs, as the house is to be put into thorough repair both inside and out. the letter from sir c. phipps is most kind." in writing to sir c. phipps himself faraday said, "i find it difficult to write my thanks or express my sense of the gratitude i owe to her majesty; first, for the extreme kindness which is offered to me in the use of the house at hampton court, but far more for that condescension and consideration which, in respect of personal rest and health, was the moving cause of the offer. i feared that i might not be able properly to accept her majesty's most gracious favour. i would not bring myself to decline so honourable an offer, and yet i was constrained carefully to consider whether its acceptance was consistent with my own particular and peculiar circumstances. the enlargement of her majesty's favour has removed all difficulty. i accept with deep gratitude, and i hope that you will help me to express fitly to her majesty my thanks and feelings on this occasion." [illustration: faraday's house, hampton court green.] faraday's house, standing pleasantly on hampton court green, was, as will be seen from the illustration, a delightful creeper-embowered place, and with its open aspect and surrounding greenery, must have afforded a great and agreeable change to the tired philosopher and his wife. for some years after his removal faraday made frequent runs up to town to the institution, where he continued his research work and also delivered many lectures, notably, several courses of the now annual juvenile lectures. he was, however, not able to continue for long spells of work, but had to take occasional intervals of rest. he still made frequent reports in connection with trinity house, but refused to take up any further work. he declined even to prepare his juvenile lectures for publication, although other reasons than his own incapacity for sustained work here influenced him, as we see by the following letter:-- "royal institution, january , . "dear sir,--many thanks both to you and mr. bentley. mr. murray made me an unlimited offer like that of mr. bentley's many years ago, but for the reasons i am about to give you i had to refuse his kindness. he proposed to take them by shorthand, and so save me trouble, but i knew that would be a thorough failure; even if i cared to give time to the revision of the ms., still the lectures without the experiments and the vivacity of speaking would fall far behind those in the lecture room as to effect. and then i do not desire to give time to them, for money is no temptation to me. in fact, i have always loved science more than money; and because my occupation is almost entirely personal, i cannot afford to get rich. "again thanking you and mr. bentley, "i remain, very truly yours, "m. faraday." i have had to insist once or twice upon faraday's deeply religious nature; it comes out very clearly in some letters written about this time, when he was an old man--having very nearly attained to the threescore years and ten of man's life. in his work and in his conversation he never obtruded his religious convictions, but the innate religious feeling of the man coloured his every relation with his fellow men. in that we have but few direct writings of his on this subject, a grave interest attaches to the following letter to his niece: "i never heard of the saying that separation is the brother of death; i think that it does death an injustice, at least in the mind of the christian; separation simply implies no re-union; death has to the christian everything hoped for contained in the idea of re-union. i cannot think that death has to the christian anything in it that should make it a rare, or other than a constant thought; out of the view of death comes the view of the life beyond the grave, as out of the view of sin (that true and real view which the holy spirit alone can give to a man) comes the glorious hope; without the conviction of sin there is no ground of hope to the christian. as far as he is permitted for the trial of his faith to forget the conviction of sin, he forgets his hope, he forgets the need of him who became sin or a sin-offering for his people, and overcame death by dying. and though death be repugnant to the flesh, yet where the spirit is given, to die is gain. what a wonderful transition it is! for, as the apostle says, even whilst having the firstfruits of the spirit, the people of god groan within themselves, 'waiting for the adoption, to wit, the redemption of the body.' elsewhere he says, that whilst in the earthly house of this tabernacle we groan, earnestly desiring to be clothed upon with our house which is from heaven. "it is permitted to the christian to think of death; he is even represented as praying that god would teach him to number his days. words are given to him, 'o death, where is thy sting? o grave, where is thy victory?' and the answer is given him, 'thanks be to god, who giveth us the victory through our lord jesus christ.' and though the thought of death brings the thought of judgment, which is far above all the trouble that arises from the breaking of mere earthly ties, it also brings to the christian the thought of him who died, was judged, and who rose again for the justification of those who believe in him. though the fear of death be a great thought, the hope of eternal life is a far greater.... you see i chat now and then with you as if my thoughts were running openly before us on the paper, and so it is. my worldly faculties are slipping away day by day. happy is it for all of us that the true good lies not in them. as they ebb, may they leave us as little children, trusting in the father of mercies and accepting his unspeakable gift." in faraday became once more an elder in the sandemanian church, and retained that office for nearly four years, when he finally resigned it. the meeting of the british association was held in this year at oxford, and faraday was once more present, as he liked to be, at this scientific gathering. a friend, apropos of this visit, wrote the following _jeu d'esprit_, which is worth remembering-- "'that p will change to f in the british tongue is true (quoth professor phillips), though the instances are few.' an entry in my journal then i ventured thus to parody, 'i this day dined with fillips, where i hobbed and nobbed with pharaday.'" [illustration: faraday delivering his christmas juvenile lectures.] this same year is also notable as being the nineteenth, and last, in which faraday delivered the christmas juvenile lectures; for ten years in succession he had given them, the four lectures of this, his final course, being those well known and generally appreciated ones upon "the chemical history of a candle." an earlier course having been given some years before on the same subject. his failing health and memory made it necessary for him to discontinue much of his work, and in the following year his last experimental work was done, and (on june ) his last friday evening lecture delivered. a touching and pathetic interest attaches to the slight notes which he made for this, his last lecture. the notes are brief--but yet how much is there not expressed in them? "personal explanation--years of happiness here, but time of retirement; loss of memory and _physical endurance of the brain_. " . causes--_hesitation and uncertainty_ of the convictions which the speaker has to urge. " . _inability to draw_ upon the mind for the treasures of knowledge it has previously received. " . _dimness_, and forgetfulness of one's former _self-standard_ in respect of _right_, _dignity_, and _self-respect_. " . strong duty of _doing justice to others_, yet inability to do so. "_retire._" thus did the old man of seventy years touchingly bid farewell to work which he had been carrying on for the greater part of half a century--to that work which had received from him the untiring devotion of a life-time. in his memory, which had previously troubled him, became even less trustworthy; though his cheerfulness, faith, and innate optimism were never clouded for a moment, as is well-shown in a letter which he wrote to the wife of his old friend barlow. "i called at your house," faraday wrote, "and i rejoice to think that your absence is a sign of good health. _our love to you both._ i am enjoying the gradual decay of strength and life, for when i revive it is no great revival or desire to me, and that cheers me in the view of death near and round us." in his chief work was in connection with the trinity house, faraday continuing to report upon the value of the magneto-electric light for lighthouses, and visiting yet again, as he had frequently done for years past, dungeness and other stations for the purpose. despite his incapacity for sustained mental work owing to his failing memory, faraday continued fairly hale in body, and was yet active, for in february of he was at dungeness, and in the autumn of the same year he was in scotland for a fortnight, and wrote from glasgow to one who for over forty years had been his loving companion, a letter breathing an affection unaltered by the lapse of years, unless indeed it were, to use his own expression, that it had grown _deeper_. we have seen the letters which he wrote in the early years of his marriage; it is fitting that we should quote from this one to show how unchanged he was, despite the many years which had passed over him. "i long to see you, dearest," he wrote, "and to talk over things together, and call to mind all the kindness i have received. my head is full and my heart also, but my recollection rapidly fails, even as regards the friends that are in the room with me. you will have to resume your old function of being a pillow to my mind, and a rest, a happy-making wife.... dearest, i long to see and be with you. whether together or separate, your husband, very affectionate, "m. faraday." in he felt compelled to relinquish the active work in connection with the trinity house, without altogether retiring from his position, for after thirty years' work, during which he had been treated by the brethren with uniform kindness and consideration, he did not like to altogether sever his connection with friends with whom he had been so long and so harmoniously working. in accordance with faraday's wishes professor tyndall undertook this work for him. in the same year he felt it necessary to communicate with the managers of the royal institution, expressing his desire to be allowed, without severing his connection with it, to give up his active work for the institution. the last two years of his life were thus passed "waiting" as he once or twice expressed it. to an old friend of very many years' standing he had said, "barlow, you and i are waiting--that is what we have to do now; and we must try to do it patiently." and again, in reply to a friend who inquired as to how he was, he simply replied, "just waiting." thus gradually and quietly the end approached. one of his nieces writes of her annual visit in : "i spent june at hampton court. dear uncle kept up rather better than sometimes; but oh! there was always pain in seeing afresh how far the mind had faded away. still the sweet unselfish disposition was there, winning the love of all around him.... "i shall never look at the lightning flashes without recalling his delight in a beautiful storm. how he would stand at the window for hours, watching the effects and enjoying the scene; while we knew his mind was full of lofty thoughts; sometimes of the great creator, and sometimes of the laws by which he sees meet to govern the earth. "i shall also always connect the sight of the hues of a brilliant sunset with him, and especially he will be present to my mind while i watch the fading of the tints into the sombre grey of night. he loved to have us with him, as he stood or sauntered on some open spot, and spoke his thoughts, perhaps in the words of gray's elegy, which he retained in memory, clearly, long after many other things had faded quite away. then, as darkness stole on, his companions would gradually turn indoors, while he was well pleased to be left to solitary communings with his own thoughts." [illustration: faraday's tomb at highgate.] on the th of august, , he passed quietly away, dying in his chair in his study at hampton court. his niece, miss barnard, from whose recollections we have learned much in earlier chapters, had spent a good part of her life with her aunt and uncle, and had helped to nurse the latter during the last few months. "my occupation has gone," she pathetically wrote to dr. bence jones. on august th, the funeral took place, everything being conducted simply and quietly; it was, as faraday had himself expressed a desire that it should be, strictly private. a plain headstone in highgate cemetery, with the following simple inscription, marks the place where lies all that was mortal of one of england's noblest sons. "michael faraday, born, nd september, , died, th august, " if it were necessary to add anything to these simple words that mark his resting-place, there might be put, and it would apply to faraday as truly as to any man that ever lived, the well-known line-- "an honest man, the noblest work of god." [illustration] footnote: [ ] author of _conversations on chemistry_, a work which had had a considerable influence on faraday in his early youth. [illustration] chapter viii. as friend and lecturer. "i thought these men will carry hence promptings their former life above, and something of a finer reverence for beauty, truth, and love." lowell. at various periods of faraday's life his genial good-nature and kindliness have been brought home to us in different ways. from that early time when he used to take care of his little sister margaret in manchester square, up to the very latest course of juvenile lectures which he delivered at the royal institution in , he had always the same love for young people; and, as is usual with persons of such a disposition, he was ever a great favourite of the children, whether of those who used to hear his christmas lectures, or with those happier ones who met him more intimately. intimate seems perhaps a curious word to use with regard to the relations of young children and an old man; but yet it is the only word that really expresses what is meant; that really indicates that instant bond of sympathy that seems to connect children with men and women who possess the power of attracting and delighting them by becoming from the first as one of themselves. this quality, this delightful quality, belonged to michael faraday in an eminent degree. fortunate indeed were those children who listened to his christmas lectures to juveniles, and fortunate also were those older people who were present on the same occasions. many people who enjoyed the pleasure and privilege of hearing him at such a time recorded their impressions, and pleasant reading is the result. lady pollock, for instance, wrote: "when he lectured to children he was careful to be perfectly distinct, and never allowed his ideas to outrun their intelligence. he took great delight in talking to them, and easily won their confidence. the vivacity of his manner and of his countenance, his pleasant laugh, the frankness of his whole bearing, attracted them to him. they felt as if he belonged to them; and indeed he sometimes, in his joyous enthusiasm, appeared like an inspired child. he was not at all a man for evening parties; he was nothing of a ladies' man; but he was the true man for the juveniles, and would go to see a domestic charade when the boys acted in it, and suddenly appear behind the scene to offer a little help or suggest a new arrangement; and then, while he was in front, he would laugh and applaud so loudly, that his presence was the best encouragement which the young performers could have. or he would help the young people to wonder at the feats of a conjuror, or he would join in a round game, and romp quite noisily. but all was done with a natural impulse. there was no assumption of kindness, no air of condescension." another writer, who had the rare privilege of meeting the great man socially, said: "nothing indeed pleased him better than to be a boy again, and to mingle in the sports of the young, especially if they took a turn congenial at all to his own pursuits. he has been known to join a youthful party on a november evening to assist in a display of fireworks. there he might be seen running to and fro in a garden at night, with his pockets crammed with combustibles--now kindling lycopodium or burning potassium--then letting off blue fires, green fires, purple fires--sometimes dropping ignited crackers at the feet of the boys with an air of affected astonishment, or probably chasing the girls in order to streak their cheeks with phosphorus." as a lecturer to children faraday was indeed particularly successful and especially interesting. his first course of christmas juvenile lectures was given in --his last, on "the chemical history of a candle," was given in , and during that time he gave in all nineteen of these courses. the first consisted of six lectures on chemistry, and the first lecture of the course was illustrated with no fewer than eighty-six experiments, which were, it is scarcely necessary to say, carried out on the platform. in his note-book faraday made the following entry with regard to this course, "the six juvenile lectures were just what they ought to have been, both in matter and manner; but it would not answer to give an extended course in the same spirit." one secret of faraday's success as a lecturer, both to juveniles and others, was the carefulness with which he always tried to ascertain what was the best method. in early years he would have a friend (magrath, or some other) among the audience, who was to tell him afterwards of any peculiarities, either of manner or style, that wanted correcting; and miss reid tells us that in the early years of the juvenile lectures her uncle used to encourage her to tell him everything that struck her; and, when she did not fully understand him, where her difficulties lay. he would then enlarge upon those points in the next lecture, and thus he made a child's remark serve him in making things clear to children. he used also, at first, to have a card, on which was distinctly written the word _slow_, before him; and if he forgot it and became rapid, his assistant anderson had orders to place it before him. sometimes also when his lecture hour was nearly expired, he would arrange to have a card with the word _time_ on it, placed within his view. one of his early courses for children treated of that subject which always possessed a great fascination for him, and in which he did much of his greatest work--electricity. the slight notes which he made for the first lecture of this course are interesting. "an extraordinary power that i have to explain; not fear boldly entering into its consideration, because i think it ought to be understood by children--not minutely, but so as to think reasonably about it, and such effects as children can produce, or observe to take place in nature--simple instances of its power." this first lecture was illustrated by eighty experiments. we have dwelt upon his kindliness and sympathy with children; but, it may be noted, he had none of his own, though from almost the earliest year of his married life he always liked to have one or other of his nieces with him. his love for children is well shown in an anecdote which dr. gladstone tells of his later life. he was at a soirée at the house of mr. justice grove; the eldest daughter having heard him express himself disappointed at being too late to see any of the younger members of the family, brought down the little ones in their nightgowns to the foot of the stairs, when faraday showed how gratified he was by saying to her, "ah! that's the best thing you've done to-night." sometimes he would indulge in some slight practical jokes with the young people. for instance, a nephew visiting him in his study one morning, faraday said to him, "why, frank, what a tall boy you are growing; you can almost touch that brass ball--just try." nothing loth, the boy standing on tiptoe reached up and touched the ball with his fingers, when his playful uncle gave a turn to a wheel and the boy received a slight shock, and with it a first, and somewhat unexpected, lesson on the nature of electricity. [illustration: faraday lecturing before the prince consort, prince of wales and duke of edinburgh. _reduction of the picture painted_] [_in by alexander blaikley_.] the prince consort was a constant patron of the royal institution, and the young princes also were present at many of the juvenile lectures. after attending such a course h.r.h. the prince of wales, then a lad of fifteen, and his brother, h.r.h. prince alfred, duke of edinburgh, wrote from windsor castle the following letters to professor faraday:-- "dear sir,--i am anxious to thank you for the advantage i have derived from attending your most interesting lectures. their subject, i know very well, is of great importance, and i hope to follow the advice you gave us of pursuing it beyond the lecture-room; and i can assure you that i shall always cherish with great pleasure the recollection of having been assisted in my early studies in chemistry by so distinguished a man. "believe me, dear sir, yours truly, "albert edward." prince alfred's letter was as follows:-- "dear sir,--i write to thank you very much for the pleasure you have given me by your lectures, and i cannot help hoping they will not be the last i shall hear from you. their subject was very interesting, and your clear explanations made it doubly so. "believe me, dear sir, yours truly, "alfred." it is interesting to learn faraday's own views with regard to popular lectures, for never yet was there a truly scientific lecturer who was more truly popular; perhaps, indeed, there is no other single man who, without in any degree lowering his work for the purpose, succeeded so well in popularising scientific knowledge. these are his own words on the subject: "as to popular lectures (which at the same time are to be _respectable_ and _sound_) none are more difficult to find. lectures which _really teach_ will never be popular; lectures which are popular will never _really teach_." his own success as a lecturer was owing largely to the power he had of adapting himself to all minds, from the deepest thinkers to the liveliest youth; this power gave him a "wide range of influence, and his sympathy with the young among his listeners imparted more life and colour to his discourses than they might otherwise have possessed. he had the art of making philosophy charming, and this was due in no little measure to the fact that to grey-headed wisdom, he united wonderful juvenility of spirit." "he was," to quote once more from lady pollock's recollections of the illustrious lecturer, "completely master of the situation; he had his audience at his command, as he had himself and all his belongings; he had nothing to fret him, and he could give his eloquence full sway. it was an irresistible eloquence, which compelled attention and insisted upon sympathy. it waked the young from their visions and the old from their dreams. there was a gleaming in his eye which no painter could copy, and which no poet could describe.... his enthusiasm sometimes carried him to the point of ecstasy when he expatiated on the beauty of nature, and when he lifted the veil from her deep mysteries. his body then took motion from his mind; his hair streamed out from his head, his hands were full of nervous action, his light, lithe body seemed to quiver with its eager life. his audience took fire with him, and every face was flushed.... a pleasant vein of humour accompanied his ardent imagination, and occasionally, not too often, relieved the tension of thought imposed upon his pupils. he would play with his subject now and then, but very delicately; his sport was only just enough to enliven the effort of attention. he never suffered an experiment to allure him away from his theme. every touch of his hand was a true illustration of his argument." as he once remarked in giving advice to a young lecturer: "if i said to my audience, 'this stone will fall to the ground if i open my hand,' i should open my hand and let it fall. take nothing for granted as known; inform the eye at the same time as you address the ear." it is of interest here--after seeing how faraday as a lecturer impressed others--to note some of the remarks which he made on the subject of lectures, lecturers, and lecturing in his early correspondence with his friend, benjamin abbott. several long letters on this matter passed between the friends, and faraday (he was twenty-one at the time) not only speaks in a discriminating manner with regard to lectures, but he also treats with his native good sense of lecture rooms, apparatus, etc. all his remarks, he says in his earliest letter to abbott on the subject, are the result of his own personal observation. the most necessary quality for a lecturer, says the youthful faraday, is a good delivery; he then dwells upon the necessity of illustrating a lecture with experiments wherever possible. (how well he carried this rule into practice has been seen in an earlier part of this chapter, where we learned of some of his juvenile lectures being illustrated by upwards of eighty experiments.) "a lecturer," he goes on to say, "should appear easy and collected, undaunted and unconcerned,[ ] his thoughts about him, and his mind clear and free for the contemplation and description of his subject." he then says, and we instantly think of the "time" card he had placed before him later, "i disapprove of long lectures; one hour is long enough for anyone, nor should they be allowed to exceed that time." the last of this series of letters on lectures commences in a style of genial banter, which, as it illustrates the lighter side of faraday's character, merits quotation. "dear abbott," he writes, "as when on some secluded branch in forest far and wide sits perched an owl, who, full of self-conceit and self-created wisdom, explains, comments, condemns, ordains, and orders things not understood, yet full of his importance still holds forth to stocks and stones around--so sits and scribbles mike; so he declaims to walls, stones, tables, chairs, hats, books, pens, shoes, and all the things inert that be around him, and so he will to the end of the chapter." this playful mood comes out, also, in one or two anecdotes which are told of him, when his fame was established. for instance, an old lady friend being much troubled by some rancid butter, thought that she had hit upon a method of improving it, which she did by mixing with it a quantity of soda, she having a somewhat high opinion of the purifying virtues of that alkali, although, it is to be presumed, she little suspected the uses to which it was applied in the manufactures. by this addition of soda, she triumphantly claimed that her butter "was greatly improved." one evening, when professor faraday called upon her, the old lady produced a sample of her "improved" butter. a merry laugh rang out from the philosopher's lips as he exclaimed, "well done, mrs. w., you have improved your bad butter into very indifferent _soap_!" good-humoured and good-natured as faraday habitually was, he did not like to be worried unnecessarily over unimportant matters; and willing as he was to place even his invaluable time at the disposal of almost anyone who claimed his attention, he had no patience with persons who came to him thoughtlessly, as the following story shows: a young man called on him one morning, and with an air of great importance confided to him the result of some original researches in electrical philosophy. "and pray," asked the professor, taking down a volume of ree's _cyclopoedia_, "did you consult this, or any elementary work, to learn whether your discovery had been anticipated?" the young man replied in the negative. "then why do you come to waste my time about well-known facts that were published forty years ago?" "sir," said the visitor in self-excuse, and hoping to flatter the philosopher, "i thought i had better bring the matter to headquarters immediately." "all very well for you, but not so well for headquarters," replied the professor sharply, and he forthwith set his visitor to read the article in the _cyclopoedia_. yet another story is told of a grave old gentleman who once waited upon faraday that he might show to him "a new law of physics." the gentleman asked for a jug of water and a tumbler; they were brought, and he then produced a cork. "you will be pleased to observe," he then said, "how persistently this cork clings to the side of the glass when the vessel is half-filled." "just so," replied the professor. "but now," continued the discoverer of a new law of physics, "mark what happens when i fill the glass to the brim. there! you see the cork flies to the centre--positively repelled by the sides!" "precisely so," answered faraday in an amused tone, which showed that the "new law" was more familiar to him than to his visitor, who, somewhat abashed, said, "pray, how long have you known this?" "oh, ever since i was a boy," was the reply; but the innate kindliness of his nature must show itself even in such a case, for, seeing the old gentleman's disappointed look, he added that he was not to be grieved, he might possibly some day alight upon something really new. the last course of faraday's juvenile lectures--on "the chemical history of a candle"--has been referred to once or twice. these lectures are indeed of very great interest, not only in themselves as chemical illustrations, but as being part of professor faraday's best known works, and the only juvenile lectures of his which are obtainable in the form of a book. the way in which he introduced his subject will show us how simple, and yet how explicit he was in explaining to his young audience the phenomena which he brought before them. "i purpose," he said, to quote the beginning of the initial lecture of the series of six, "in return for the honour you do us by coming to see what are our proceedings here, to bring before you in the course of these lectures, the chemical history of a candle. i have taken this subject on a former occasion, and were it left to my own will, i should prefer to repeat it almost every year--so abundant is the interest that attaches itself to the subject, so wonderful are the varieties of outlet which it offers into the various departments of philosophy. there is not a law under which any part of this universe is governed which does not come into play, and is touched upon in these phenomena. there is no better, there is no more open door by which you can enter into the study of natural philosophy, than by considering the physical phenomena of a candle. i trust, therefore, i shall not disappoint you in choosing this for my subject rather than any newer topic, which could not be better, were it even so good. "and before proceeding, let me say this also--that though our subject be so great, and our intention that of treating it honestly, seriously, and philosophically, yet i mean to pass away from all those who are seniors amongst us. i claim the privilege of speaking to juveniles as a juvenile myself. i have done so on former occasions--and, if you please, i shall do so again. and though i stand here with the knowledge of having the words i utter given to the world, yet that shall not deter me from speaking in the same familiar way to those whom i esteem nearest to me on this occasion. "and now, my boys and girls, i must first tell you of what candles are made. some are great curiosities. i have here some bits of timber, branches of trees particularly famous for their burning. and here you see a piece of that very curious substance taken out of some of the bogs in ireland, called _candlewood_--a hard, strong, excellent wood, evidently fitted for good work as a resister of force, and yet withal burning so well that where it is found they make splinters of it, and torches, since it burns like a candle, and gives a very good light indeed. and in this wood we have one of the most beautiful illustrations of the general nature of a candle that i can possibly give. the fuel provided, the means of bringing that fuel to the place of chemical action, the regular and gradual supply of air to that place of action--heat and light--all produced by a little piece of wood of this kind, forming, in fact, a natural candle. "but we must speak of candles as they are in commerce. here are a couple of candles commonly called dips. they are made of lengths of cotton cut off, hung up by a loop, dipped into melted tallow, taken out again and cooled, then re-dipped until there is an accumulation of tallow round the cotton. in order that you may have an idea of the various characters of these candles, you see these which i hold in my hand--they are very small and very curious. they are, or were, the candles used by miners in coal mines. in olden times the miner had to find his own candles; and it was supposed that a small candle would not so soon set fire to the fire-damp in the coal mines as a larger one; and for that reason, as well as for economy's sake, he made candles of this sort--twenty, thirty, forty, or sixty to the pound. they have been replaced since then by the steel-mill, and then by the davy lamp, and other safety lamps of various kinds. i have here a candle that was taken out of the _royal george_, it is said, by colonel pasley. it has been sunk in the sea for many years, subject to the action of salt water. it shows you how well candles may be preserved; for though it is cracked about and broken a good deal, yet, when lighted, it goes on burning regularly, and the tallow resumes its natural condition as soon as it is fused." we have not space to quote further from these delightful lectures, which however, as i have said earlier, can be got in a little volume by themselves. these lectures were, as indeed were most of faraday's lectures, beautifully illustrated with a large number of experiments. on one occasion, when suffering much in health, faraday yet insisted upon taking his place at the lecture table at the royal institution; for an obstruction of voice, which was indeed too painfully apparent, he apologised, saying that "in an engagement where the contracting parties were one and many, the one ought not on any slight ground to break his part of the engagement with the many, and therefore, if the audience would excuse his imperfect utterance he would proceed." the audience murmured, and there were cries of "put off the lecture;" but faraday begged to be allowed to go on. a medical man rose and said it would in his opinion be dangerous for the professor to proceed. faraday still urged his desire to go on with the lecture; he could not give people all the trouble of coming there, having perhaps put off other engagements, for nothing. on this, as by a single impulse, the whole audience rose, and faraday yielded to the generally expressed desire to spare him the pain and inconvenience of lecturing. after a fortnight's rest he reappeared, and continued the broken course, carrying it on later that his audience should not lose any of the eight lectures which they had anticipated. it was on a reappearance such as this after illness that "as soon as his presence was recognised, the whole audience rose simultaneously, and burst into a spontaneous utterance of welcome, loud and long." footnote: [ ] it is interesting here to see what tyndall says, referring to faraday as a lecturer: "i doubt his unconcern, but his fearlessness was often manifested. it used to rise within him as a wave, which carried both him and his audience along with it. on rare occasions also, when he felt himself and his subject hopelessly unintelligible, he suddenly evoked a certain recklessness of thought; and without halting to extricate his bewildered followers, he would dash alone through the jungle into which he had unwittingly led them; thus saving them from ennui by the exhibition of a vigour which, for the time being, they could neither share nor comprehend." [illustration] chapter ix. notes on his work. "so that i draw the breath of finer air station is nought, nor footways laurel-strewn, nor rivals tightly belted for the race. god speed to them! my place is here or there; my pride is that among them i have place: and thus i keep this instrument in tune." george meredith. in treating of the life-story of michael faraday i have let particulars as to his various experiments and discoveries interfere as little as possible with the continuity of the narrative, and have thought it advisable to slightly refer to them in a special chapter. the value of his contributions to our fund of scientific knowledge is made manifest by the fact that whatever book on electricity and allied subjects we may take up now--works even bringing the science down to the very latest date--we always find the name and experiments of michael faraday quoted with great respect as a leader and an unquestioned authority. indeed, our debt to him for his electrical work is incalculable; we are now seeing the electric light carried day by day into more streets, lighting more public places, nay, even being used in illuminating private buildings. this light we owe, primarily, to michael faraday. writing nearly a quarter of a century ago professor tyndall answers this question as to "what is the use of it all?" thus explicitly and unhesitatingly--"as far as electricity has been applied for medical purposes, it has been almost exclusively faraday's electricity. you have noticed those lines of wire which cross the streets of london. it is faraday's currents that speed from place to place through these wires. approaching the point of dungeness the mariner sees an unusually brilliant light, and from the noble _phares_ of la hève the same light flashes across the sea. these are faraday's sparks exalted by suitable machinery to sunlike splendour. at the present moment the board of trade and the brethren of the trinity house, as well as the commissioners of northern lights, are contemplating the introduction of the magneto-electric light at numerous points upon our coasts; and future generations will be able to refer to those guiding stars in answer to the question, what has been the practical use of the labours of faraday? but i would again emphatically say that his work needs no such justification; and that if he had allowed his vision to be disturbed by considerations regarding the practical use of his discoveries, those discoveries would never have been made by him." in one of his very earliest lectures delivered before the city philosophical society on the subject of chlorine,[ ] faraday referred to the question too often and too thoughtlessly put on hearing of a new discovery. "before leaving this subject," he said, "i will point out the history of this substance, as an answer to those who are in the habit of saying to every new fact, 'what is its use?' benjamin franklin says to such, 'what is the use of an infant?' the answer of the experimentalist is, 'endeavour to make it useful.'" truly the infant electricity has already grown to goodly proportions. it is to his researches in connection with electrical science that we must look for the chief result of faraday's work. his later years were almost exclusively taken up in the investigation of this fascinating subject. the value of his contributions to the sum of knowledge on this new branch of science was testified to in a remarkable manner during the past summer, when the centenary of his birth was celebrated in a fitting manner at that institution which had been a "home" to him for so many years. it was indeed an unique incident in the history of modern science, when on the th of june, , many of the leading living scientists met in the theatre of the royal institution to hear a lecture by lord rayleigh on the life-work of "one of england's greatest worthies." "a quarter of a century has not elapsed," wrote a contemporary journal,[ ] "since his death, and yet we find the highest nobles of the land vieing with the most illustrious professors of our own and of foreign universities in testifying their admiration for this man of the people, who rose to be a leader of scientific men." "when the history of electricity comes to be written," continues the same authority, "a chapter of great extent and first importance must be given to the prolific life-work of faraday. he will be pointed to as the man who in the middle of the nineteenth century, waged an energetic and relentless warfare against the two fluid theories in electricity and magnetism, and who dealt its death-blow to the theory of action at a distance. and to show the powerful influence his master-mind exercised over contemporary science, the historian may merely refer to clerk-maxwell, sir william thomson, rayleigh, tyndall, and others, all admiring disciples and professed followers of the great michael faraday." the meeting that thus did honour to the memory of faraday, was probably the most fitting method of celebrating the anniversary of his birth that could have been devised; it was, we may feel sure, just such a celebration as faraday would have felt most proud of. wealth and social rank, as we have seen throughout his life, had no attraction for him; but he _did_ like to receive the appreciation of capable men, in whose appreciation he found the highest honour to which it was possible to attain. some of faraday's earliest experiments, as was incidentally mentioned in an earlier part of this little book, were in connection with chlorine, etc., and then on the making of glass for optical purposes; and it was not, indeed, until he had been at the institution for about eighteen years that he really entered with any degree of success into his electrical research. here it is of interest to note a remark which he once made in this connection to the effect that it requires twenty years of work to make a _man_ in physical science, the whole of the previous period being one of _infancy_. once, however, he had reached this scientific manhood his work was done with remarkable rapidity; he would, once on the track, so to speak, of a discovery, mature it in a space of time so short as to be nothing less than marvellous; and one after another of his "experimental researches" were carried out, completed, described, and the resultant paper submitted to the royal society with a rapidity, and at the same time with an accuracy which has never been equalled. he was asked once what was the secret of his success, and answered that the whole secret might be told in three words, they were these "work--finish--publish." perhaps the centre word is the one on which faraday would himself have lain most stress--he was always careful to finish everything before he announced it, which makes his almost unexceptional accuracy, considering the rapidity with which he worked, even more remarkable. it is, however, not inaccurate to say that the results which he definitely announced, were never found to be wrong; further developments have of course taken place, but the result of a research as announced by him was never found to be untrue, and has never had to be put aside. he had said, in the early part of his scientific career, "the thing that i am proudest of is that i have never been found to be wrong." and after the death of his friend, professor a. de la rive wrote, "i do not think that faraday has once been caught in a mistake; so precise and conscientious was his mode of experimenting and observing." dr. gladstone commenting upon this says, "the extreme rarity of his mistakes, notwithstanding the immense amount of his published researches, is one of those marvels which can be appreciated only by those who are in the habit of describing what they have seen in the mist-land that lies beyond the boundaries of previous knowledge." the proper treatment of faraday's discoveries could of course only be undertaken by one who was himself a scientist; the technicalities of the laboratory and the lecture-theatre would be somewhat out of place in a book such as this, which but aims at presenting in a popular form the facts in connection with the life of one of the greatest of england's scientists--one of the best of her sons. it may, however, here be pointed out that to those who would become acquainted with the details of faraday's scientific work, with particulars of his numerous experiments, a delightful introduction has been afforded by professor tyndall, who in his little work on _faraday as a discoverer_, has summarised much of the great man's work, and explains in a clear and delightful manner much about the experiments which were undertaken and the discoveries which were made by his illustrious predecessor and friend. it is of interest to notice here what discovery of faraday it is which tyndall selects as the greatest--it is the discovery of electro-magnetism, of which he says: "the beauty and exactitude of the results of this investigation are extraordinary. i cannot help thinking while i dwell upon them, that this discovery of magneto-electricity is the greatest experimental result ever obtained by an investigator. it is the mont blanc of faraday's own achievements. he always worked at great elevations, but a higher than this he never subsequently attained." the following impromptu lines with reference to faraday's great discovery of magneto-electricity were written by herbert mayo:-- "around the magnet faraday was sure that volta's lightnings play, but how to draw them from the wire? he drew a lesson from the heart.-- 'tis when we meet, 'tis when we part, breaks forth th' electric fire." of this same subject tyndall wrote shortly after faraday's death:--"seven and thirty years have passed since the discovery of magneto-electricity; but, if we except the _extra current_, until quite recently nothing of moment was added to the subject. faraday entertained the opinion that the discoverer of a great law or principle had a right to the 'spoils'--this was his term--arising from its illustration; and guided by the principle he had discovered, his wonderful mind, aided by his wonderful ten fingers, overran in a single autumn this vast domain, and hardly left behind him the shred of a fact to be gathered by his successors." this indeed is a quality which has been insisted upon by all who have as fellow scientists treated of the work which was done by michael faraday,--this quality, that is, of completion, of thoroughness in finishing that which he had commenced; he seemed to become aware almost as though by intuition of the full meaning of a discovery, and of its true bearing with regard to previous knowledge. great as was faraday's work in the service of science he not only did not aim at, but he frequently declined to accept what many men would have considered but just reward. he was a chemist, a scientist, a philosopher (to give him the name which he best liked, and of which he felt most proud), and did his work as such from the purest love for it, as i have tried to show in earlier chapters. he did not seek worldly position--he was above it; he did not seek for wealth, he had no use for it, as his wants were of the simplest; he did not seek for popular applause, for the suffrage of the multitude; but what he did all his life long most earnestly and most faithfully strive for was--truth. he ever aimed at fulfilling what the laureate has beautifully expressed in his dedication to _in memoriam_ wherein he says-- "let knowledge grow from more to more, and more of reverence in us dwell, that mind and soul according well may make one music as before, but vaster." he sought to make knowledge grow from more to more, but it is to be recollected that he never for one more moment swerved from his faithful adherence to his church. in all his research among physical phenomena he was never led to doubt, as some have done, the truth of that religion in which he always maintained a sincere and beautiful faith; his religion was, indeed, always a something far above his science, a something sacred and of moment to himself, as a single soul. we saw in his reply to his wife on his formally entering the sandemanian church shortly after his marriage, what was his attitude on this question. it was, as he had said, a matter between himself and his god; and thus we find in what he has written but very little about his religion, although one or two of his letters to relations, where he has been directly appealed to, breathe the sincere and earnest devotion of the man, and his true christian spirit. his whole life was, however, a practical expression of his religious faith; as is shown to us by what has been said or written by all who came in contact with him. the following tribute to his memory from monsieur dumas is yet one further proof of the universal feeling which his friendship inspired, "i do not know whether there is a _savant_ who would not feel happy in leaving behind him such works as those with which faraday has gladdened his contemporaries, and which he has left as a legacy to posterity; but i am certain that all those who have known him would wish to approach that moral perfection which he attained to without effort. in him it appeared to be a natural grace, which made him a professor, full of ardour for the diffusion of truth, an indefatigable worker, full of enthusiasm and sprightliness in his laboratory, the best and most amiable of men in the bosom of his family, and the most enlightened preacher among the humble flock whose faith he followed. "the simplicity of his heart, his candour, his ardent love of the truth, his fellow interest in all the successes, and ingenuous admiration of all the discoveries of others, his natural modesty in regard to what he himself discovered, his noble soul--independent and bold--all these combined gave an incomparable charm to the illustrious physicist. "i have never known a man more worthy of being loved,--of being admired,--of being mourned. fidelity to his religious faith, and the constant observance of the moral law, constitute the ruling characteristics of his life.... there is more than one useful lesson to be learnt from the proper study of this illustrious man, whose youth endured poverty with dignity, whose mature age bore honours with moderation, and whose last years passed gently away surrounded by marks of respect and tender affection." several stories are told that illustrate the constant habit of experimenting which seemed to be innate in faraday's mind, and also show how simple were the means which he often adopted to attain a required end. an example of the latter is given us by sir frederick arrow in describing a visit which he, as one of the committee of the elder brethren of the trinity house, paid, to observe the dungeness electric light, in june . the committee accompanied faraday, who had always been a most energetic worker in the cause of the trinity house. "we dined," says sir frederick arrow, "i think at dover, and embarked in the yacht from there, and were out for some hours watching it, to faraday's great delight--(a very fine night)--and especially we did so from the varne lightship about equi-distant between it and the french light of grisnez, using all our best glasses and photometers to ascertain the relative value of the lights; and this brings me to my story. before we left dover, faraday, with his usual bright smile, in great glee showed me a little common paper-box, and said, 'i must take care of this; it's my special photometer,'--and then, opening it, produced a lady's ordinary black shawl pin--jet, or imitation perhaps--and then, holding it a little way off the candle, showed me the image very distinct; and then putting it a little further off, placed another candle near it, and the relative distance was shown by the size of the image. he lent me this afterwards when we were at the varne lightship, and it acted admirably; ever since i have used one as a very convenient mode of observing, and i never do so but i think of that night and dear good faraday, and his genial happy way of showing how even common things may be made useful." such men as had occasion to work in the laboratory with faraday, were always struck by his lively enthusiasm, and the great activity with which he worked--"his motions were wonderfully rapid; and if he had to cross the laboratory for anything, he did not walk at an ordinary step but ran for it, and when he wanted anything he spoke quickly." in his methods of working he was most exact. having carefully planned out in his own mind work to be done, he would enter the laboratory, and with his table unencumbered with anything beyond such things as he was using, would set to work in a grave, silent manner. after a time, however, as the experiment proceeded and the result which he had anticipated began to manifest itself, he would begin humming a tune, and even speak to his attendant of the expected result. on finishing such experimental work for the day, everything had to be put carefully away, all bottles stoppered, open vessels covered over, all instruments and materials returned to their various drawers, all rubbish cleared from the floor, and the laboratory left ready for the professor to start work again. faraday would then go upstairs to his study, and think further on the subject on which he happened to be working. in his later years we are told that he invariably carried about with him convenient sized cards on which he could jot down at once--in the street, in the lecture room, at a friend's, indeed anywhere--such thoughts as should flash across his mind. a few words deserve to be said with regard to sergeant anderson, who for over thirty years acted as faraday's laboratory assistant at the royal institution. in , when faraday was working at experiments on the manufacture of glass for optical purposes, a special furnace was erected at the royal institution, and anderson was engaged to assist at it. after the glass experiments were over, however, anderson, who had demonstrated his usefulness, was retained, and he continued throughout the rest of faraday's life as his assistant, having won the good opinion not only of the professor but of all with whom he had anything to do. there is one good story told of anderson, who had been chosen for his post on account of the habits of strict obedience, which his military training had given him. his duty was to keep the furnaces always at the same heat, and the water in the ashpit always at the same level. in the evening he was released; but one night faraday forgot to tell anderson he could go, and early next morning he found his faithful servant still stoking the glowing furnace, as he had been doing all night long. footnotes: [ ] a non-metallic element first discovered in by scheele, and the subject of much research to succeeding chemists. [ ] _engineering_ for june th, . [illustration] chapter x. about the royal institution. "the heights by great men reached and kept were not attained by sudden flight, but they, while their companions slept, were toiling upward in the night. standing on what too long we bore, with shoulders bent and downcast eyes, we may discern--unseen before-- a path to higher destinies. nor deem the irrevocable past as wholly wasted, wholly vain, if, rising on its wrecks, at last to something nobler we attain." longfellow. the royal institution, which for so many years was "home" to michael faraday, must ever remain intimately associated with his name. it is not a hundred years since it was founded, yet its history is the history of sir humphry davy, michael faraday, and john tyndall--or perhaps it would be more correct to say that its history is in a large measure a history of experimental research during the century. before regarding the institution as it is especially connected with the life-story of michael faraday, it may be well to just glance at its origin. early in the year a party of noblemen and gentlemen met at the house of sir joseph banks for the purpose of forming themselves, at the suggestion of count rumford, into a "society for bettering the condition of the poor." count rumford and his friends were most anxious for the success of their undertaking; and having once made a start did not remain idle, but in january, , succeeded in having their society incorporated by royal charter. the society started perhaps on a somewhat narrower basis than that on which it now stands; its original object was that it should be "an institution for diffusing the knowledge, and facilitating the general introduction of useful mechanical inventions and improvements; and for teaching by courses of philosophical lectures and experiments, the application of science to the common purposes of life." in a guide to london published in the early part of the present century, no. , albemarle street is thus referred to: "here is also the society's house for the encouragement of improvements in arts and manufactures, or the royal institution. the front of this house is barricaded by double windows, to prevent the entrance of cold in winter and heat in summer. here is a room for experimental dinners, and a kitchen fitted up on the late count rumford's plan. adjoining this is a large workshop, in which a number of coppersmiths, braziers, etc., are employed, and over this a large room for the reception of such models of machinery as may be presented to the institution." it has been said that chemistry dates one of its chief epochs from the foundation of the royal institution laboratory. the large building in albemarle street cannot be mistaken, for there are along the front of it fourteen great fluted corinthian columns which give a striking appearance to the premises. these columns were built on to the face of the building in , at a cost of five hundred pounds, by mr. lewis vulliamy. [illustration: _from photo by_] royal institution, albemarle street. [_h. dixon & son_.] that the royal institution is, indeed, well worth visiting it must be quite unnecessary to say. even was there not much to be seen which is of itself interesting, the place would have an attraction as being the place where so much has been done for the advancement of science by faraday, his predecessors, davy, rumford, and brande, and by tyndall, and other successors. [illustration: _from photo by_] lecture-theatre, royal institution. [_h. dixon & son_.] on entering the building we find ourselves in a lofty hall; in front of us, at the head of a short flight of stone steps, is a large portrait of sir humphry davy, while to the right we see foley's fine and striking statue of faraday, which was placed there as being the most fitting memorial of the great man's connection with the institution. on going up the flight of steps to the right, we find ourselves in the well-appointed library, where we are shown under a glass case a beautiful little statuette of faraday, and also a large photograph portrait of the philosopher. we next visit the lecture-theatre, our eyes being immediately drawn to the "seat over the clock," where michael faraday as a boy first sat, and listened and marvelled at the wonders of chemistry unfolded before him by the great humphry davy. this theatre it may be noted is one of the best for its acoustic properties in london. well may we pause here--thinking of the great men who have lectured here, and of the great men who have come here to listen. it may be mentioned that the lectures are not strictly confined to scientific subjects, for it was here, in , that thomas campbell gave his course of lectures on poetry, and that another poet--thomas moore--was also invited to lecture. from the lecture-theatre we are taken downstairs to see the room where all the numerous instruments and materials are kept. here we are shown the primitive electrical machine, which faraday early constructed for himself, and many of the things which he used in his work; here, too, we have pointed out to us a large glass-case running along one side of the room, and divided into sections, each section containing the tools and appliances used by one or other of the great men of the institution, davy and brande and faraday himself. in several of the smaller rooms through which we are permitted to pass, we notice among the many portraits several of the subjects of this little work. and among other interesting things especially pointed out to us there is a locked glass-case "presented to the royal institution by michael and sarah faraday" (it was characteristic of faraday thus to put his wife in as one of the donors). this case contains several books which michael had himself bound in those days when, disliking trade, he was seeking to enter the service of science. there are, besides, several books of davy's and several manuscripts of his also, which his assistant had carefully kept. and not only is the building worthy a visit on account of the many interesting relics it contains of some of our greatest scientists, and on account of the memoirs of its many great men, but even to the unscientific there is much that is attractive in the friday evening lectures, which since that year , when faraday may be said to have inaugurated them, up to now, have been regularly carried on. no trouble is spared by the lecturers to make their matter understood, and innumerable experiments are presented on these occasions. the experiments, too, are such as often require a great expenditure of time and trouble in their preparation. as an instance of this i may mention an experiment which was made on the occasion of my latest attendance at a "friday evening." the lecturer was professor harold dixon; the subject of which he was treating was "the rate of explosion of gases." to show the rapidity with which an explosion of a certain gas travelled, the lecturer had fitted up a leaden piping all round the theatre; the ends of the piping rested upon either end of the table at which professor dixon was lecturing. the piping was filled with gas, and the professor applied a light at one end; a sharp explosion took place as the gas was fired, and was followed _almost instantaneously_ by an explosion at the other end of the pipe--the explosion having in that very short time travelled through a length of two hundred and twenty feet of piping! i quote this instance to show that no trouble is spared in preparing an illustrative experiment, although such experiment may be demonstrated in a minute or less. it may be appropriate, while considering the long connection of faraday with the scene of his many experimental triumphs, to refer more particularly to that unique meeting which took place last summer (june th, ) in celebration of the hundredth anniversary of the birth of faraday, and to which slight reference is made in the last chapter. the meeting, appropriately enough, took the form of a gathering in the theatre of the royal institution of many of the most able and distinguished chemists of the day; lord rayleigh delivering an address on the developments of faraday's discoveries. the chair was taken by his royal highness the prince of wales, who referred in his opening remarks to the time when he had sat in that theatre and listened to faraday himself. the letter from h.r.h., which is quoted on p. , was read, as was also the following letter which the prince wrote to mrs. faraday on the occasion of her husband's death. "wiesbaden, september , . "dear mrs. faraday,--although i have not the pleasure of knowing you, i cannot resist sending you a few lines to tell you how deeply grieved and distressed i am to hear of the death of your husband, professor faraday. having had the great pleasure of knowing him for some years, and having heard his interesting lectures when quite a boy, i can fully appreciate how great the loss must be, not only to you, but to the whole country at large, where his name was deeply venerated by all classes. his name will not only be remembered as a great and distinguished scientific man, but also as a good man, whose excellent and amiable qualities were so universally known. pardon my trespassing so soon on your great grief, and believe me, dear mrs. faraday, yours very sincerely, "albert edward." a very interesting yet pathetic letter was read from dr. tyndall, which, coming as it did from a man who had so well known and so thoroughly appreciated faraday, is of great interest to us. "as faraday recedes from me in time," wrote tyndall, "he becomes to me more and more beautiful. anything, therefore, calculated to do honour to his memory must command my entire sympathy. but the utmost liberty i can now allow myself is to be shifted from my bed to a couch, and wheeled to a position near the window, from which i can see the bloom of the gorse and the brown of the heather. thus, considerations affecting the body only present an insuperable barrier to my going to london on wednesday." not very far from albemarle street, is blandford street, where it will be remembered michael faraday began the battle of life as a newsboy. mr. riebau's shop (no. ) is yet standing, and is still a stationer's and bookseller's. over the shop front is now to be observed a plaque, on which are the simple words, "michael faraday, man of science, apprentice here, - ." professor tyndall tells us of a pleasing story of a visit which he paid with the ex-bookbinder to this scene of his early labours. "mr. faraday and myself quitted the institution one evening together, to pay a visit in baker street. he took my arm at the door, and pressing it to his side in his warm genial way, said, 'come, tyndall, i will now show you something that will interest you.' we reached blandford street; and after a little looking about, he paused before a stationer's shop, and then went in. on entering the shop, his usual animation seemed doubled; he looked rapidly at everything it contained. to the left on entering was a door, through which he looked down into a little room, with a window in front facing blandford street. drawing me toward him, he said eagerly, 'look there, tyndall, that was my working-place. i bound books in that little nook.' a respectable-looking woman stood behind the counter; his conversation with me was too low to be heard by her, and he now turned to the counter to buy some cards as an excuse for our being there. he asked the woman her name--her predecessor's name--his predecessor's name. 'that won't do,' he said, with good-humoured impatience; 'who was his predecessor?' 'mr. riebau,' she replied, and immediately added, as if suddenly recollecting herself, 'he, sir, was the master of sir charles faraday!' 'nonsense!' he responded, 'there is no such person!' great was her delight when i told her the name of her visitor; but she assured me that as soon as she saw him running about the shop, she felt--though she did not know why--that it must be _sir charles faraday_!" turning to our right on coming out of no. , blandford street, we shall notice on the opposite side of the way a small turning down under an archway. that turning is the beginning of jacob's well mews, where the faraday family lived, and of which an illustration has been given on an earlier page of this book. the place is interesting and worthy of a visit, as showing us that however poor and unpromising may be the surroundings of a man's childhood, he may yet win for himself an enduring name, as has michael faraday, not only in the annals of his own country, but in those of knowledge--whose annals are concerned not with one, but with all countries. a most interesting and pleasant trip, too, may be taken to hampton court green, where a visit can be paid to the house, the use of which her majesty the queen so kindly gave to the professor, and where he passed the greater part of the last ten years of his life. of the very many visitors to the famous palace and gardens of hampton court, there are, i fear, not a very large proportion who notice the charming little house facing the green, and not far from the entrance to the palace where the professor lived. "faraday house," however, appears much the same as it did when he whose name it now bears was living there. with its front all overgrown with ivy and virginian creeper, with its creeper-bowered archway from the gate to the front door, with its trees and shrubs all along the front, and with its view across the green to the trees in the palace grounds beyond, the old-fashioned house has a delightful aspect, and seems indeed an ideal spot to which a man of faraday's simple, unpretentious, yet nature-loving character, could retire after a long life of arduous and useful work. the following "in memoriam" poem, which appeared in the pages of _punch_ shortly after faraday's death, so beautifully sums up much of the man's life and character, that it may be fittingly quoted as a conclusion to this short account of the life of the illustrious philosopher, a life which must impress all who have studied it as one of the purest and most unselfish of which we have any record. "statesmen and soldiers, authors, artists,--still the topmost leaves fall off our english oak: some in green summer's prime, some in the chill of autumn-tide, some by late winter's stroke. another leaf has dropped on that sere heap-- one that hung highest; earliest to invite the golden kiss of morn, and last to keep the fire of eve--but still turned to the light. no soldier's, statesman's, poet's, painter's name was this, through which is drawn death's last black line; but one of rarer, if not loftier fame-- a priest of truth, who lived within her shrine. a priest of truth: his office to expound earth's mysteries to all who willed to hear-- who in the book of science sought and found, with love, that knew all reverence, but no fear. a priest who prayed as well as ministered: who grasped the faith he preached, and held it fast: knowing the light he followed never stirred, howe'er might drive the clouds through which it past. and if truth's priest, servant of science too, whose work was wrought for love and not for gain: not one of those who serve but to ensue their private profit: lordship to attain over their lord, and bind him in green withes, for grinding at the mill 'neath rod and cord; of the large grist that they may take their tithes-- so some serve science that call science lord. one rule his life was fashioned to fulfil: that he who tends truth's shrine, and does the hest of science, with a humble, faithful will, the god of truth and knowledge serveth best. and from his humbleness what heights he won! by slow march of induction, pace on pace, scaling the peaks that seem to strike the sun, whence few can look, unblinded, in his face. until he reached the stand which they that win a bird's-eye glance o'er nature's realm may throw; whence the mind's ken by larger sweeps takes in what seems confusion, looked at from below. till out of seeming chaos order grows, in ever-widening orbs of law restrained, and the creation's mighty music flows in perfect harmony, serene, sustained; and from varieties of force and power, a larger unity and larger still, broadens to view, till in some breathless hour all force is known, grasped in a central will, thunder and light revealed as one same strength-- modes of the force that works at nature's heart-- and through the universe's veinèd length bids, wave on wave, mysterious pulses dart. that cosmic heart-beat it was his to list, to trace those pulses in their ebb and flow towards the fountain-head, where they subsist in form as yet not given e'en _him_ to know. yet, living face to face with these great laws, great truths, great myst'ries, all who saw him near knew him for childlike, simple, free from flaws of temper, full of love that casts out fear: untired in charity, of cheer serene; not caring world's wealth or good word to earn; childhood's or manhood's ear content to win; and still as glad to teach as meek to learn. such lives are precious: not so much for all of wider insight won where they have striven, as for the still small voice with which they call along the beamy way from earth to heaven." the end. london: knight, printer, middle street, aldersgate, e.c. * * * * * transcriber's note: footnotes have been moved to the end of each chapter and renumbered consecutively through the document. illustrations have been moved to paragraph breaks near where they are discussed. [oe] changed to oe: p. (phoebus), p. (cyclopoedia in places) punctuation has been made consistent. variations in spelling and hyphenation were retained as they appear in the original publication. changes have been made as follows: page : "whe" changed to "who" (anyone who claimed his attention) page : "precedessor" changed to to "predecessor" (predecessor?' 'mr. riebau,) page : "virginian" changed to "virginian" (with ivy and virginian creeper) lamp.--xxiii. x-rays, and how the bones of the human body are photographed.--xxiv. the electric motor and how it does work.--xxv. electric cars, boats and automobiles.--xxvi. a word about central stations.--xxvii. miscellaneous uses of electricity. this book explains, in simple, straightforward language, many things about electricity; things in which the american boy is intensely interested; things he wants to know; things he should know. it is free from technical language and rhetorical frills, but it tells how things work, and why they work. it is brimful of illustrations--the best that can be had--illustrations that are taken directly from apparatus and machinery, and that show what they are intended to show. this book does not contain experiments, or tell how to make apparatus; our other books do that. after explaining the simple principles of electricity, it shows how these principles are used and combined to make electricity do every-day work. _everyone should know about electricity._ a very appropriate present third edition how two boys made their own electrical apparatus. containing complete directions for making all kinds of simple electrical apparatus for the study of elementary electricity. by professor thomas m. st. john, new york city. the book measures × ½ in., and is beautifully bound in cloth. it contains pages and illustrations. complete directions are given for making different pieces of apparatus for the practical use of students, teachers, and others who wish to experiment. price, post-paid, $ . . the shocking coils, telegraph instruments, batteries, electromagnets, motors, etc., etc., are so simple in construction that any boy of average ability can make them; in fact, the illustrations have been made directly from apparatus constructed by young boys. the author has been working along this line for several years, and he has been able, _with the help of boys_, to devise a complete line of simple electrical apparatus. =_the apparatus is simple because the designs and methods of construction have been worked out practically in the school-room, absolutely no machine-work being required._= =_the apparatus is practical because it has been designed for real use in the experimental study of elementary electricity._= =_the apparatus is cheap because most of the parts can be made of old tin cans and cracker boxes, bolts, screws, wires and wood._= =address, thomas m. st. john,= = west st street,= =new york.= how two boys made their own electrical apparatus. =contents:= _chapter_ i. cells and batteries.--ii. battery fluids and solutions.--iii. miscellaneous apparatus and methods of construction.--iv. switches and cut-outs.--v. binding-posts and connectors.--vi. permanent magnets,--vii. magnetic needles and compasses.--viii. yokes and armatures.--ix. electro-magnets.--x. wire-winding apparatus.--xi. induction coils and their attachments.--xii. contact breakers and current interrupters.--xiii. current detectors and galvanometers.--xiv. telegraph keys and sounders.--xv. electric bells and buzzers.--xvi. commutators and current reversers.--xvii. resistance coils.--xviii. apparatus for static electricity.--xix. electric motors.--xx. odds and ends.--xxi. tools and materials. "the author of this book is a teacher and wirier of great ingenuity, and we imagine that the effect of such a book as this falling into juvenile hands must be highly stimulating and beneficial. it is full of explicit details and instructions in regard to a great variety of apparatus, and the materials required are all within the compass of very modest pocket-money. moreover, it is systematic and entirely without rhetorical frills, so that the student can go right along without being diverted from good helpful work that will lead him to build useful apparatus and make him understand what he is about. the drawings are plain and excellent. we heartily commend the book."--_electrical engineer._ "those who visited the electrical exhibition last may cannot have failed to notice on the south gallery a very interesting exhibit, consisting, as it did, of electrical apparatus made by boys. the various devices there shown, comprising electro-magnets, telegraph keys and sounders, resistance coils, etc., were turned out by boys following the instructions given in the book with the above title, which is unquestionably one of the most practical little works yet written that treat of similar subjects, for with but a limited amount of mechanical knowledge, and by closely following the instructions given, almost any electrical device may be made at very small expense. that such a book fills a long-felt want may be inferred from the number of inquiries we are constantly receiving from persons desiring to make their own induction coils and other apparatus."--_electricity._ "at the electrical show in new york last may one of the most interesting exhibits was that of simple electrical apparatus made by the boys in one of the private schools in the city. this apparatus, made by boys of thirteen to fifteen years of age, was from designs by the author of this clever little book, and it was remarkable to see what an ingenious use had been made of old tin tomato-cans, cracker-boxes, bolts, screws, wire, and wood. with these simple materials telegraph instruments, coils, buzzers, current detectors, motors, switches, armatures, and an almost endless variety of apparatus were made, in this book mr. st. john has given directions in simple language for making and using these devices, and has illustrated these directions with admirable diagrams and cuts. the little volume is unique, and will prove exceedingly helpful to those of our young readers who are fortunate enough to possess themselves of a copy. for schools where a course of elementary science is taught, no better text-book in the first-steps in electricity is obtainable."--_the great round world._ exhibit of experimental electrical apparatus at the electrical show, madison square garden, new york. while only pieces of simple apparatus were shown in this exhibit, it gave visitors something of an idea of what young boys can do if given proper designs. [illustration: "how two boys made their own electrical apparatus" gives proper designs--designs for over things.] fun with photography book and complete outfit. [illustration] =photography= is now an educational amusement, and to many it is the most fascinating of all amusements. the magic of sunshine, the wonders of nature, and the beauties of art are tools in the hand of the amateur photographer. a great many things can be done with this outfit, and it will give an insight into this most popular pastime. =the outfit= contains everything necessary for making ordinary prints--together with other articles to be used in various ways. the following things are included: one illustrated book of instructions, called "fun with photography;" package of sensitized paper; printing frame, including glass, back, and spring; set of masks for printing frame; set of patterns for fancy shapes; book of negatives (patent pending) ready for use; sheets of blank negative paper; alphabet sheet; package of card mounts; package of folding mounts; package of "fixo." =contents of book:=--=chapter i. introduction.=--photography.--magic sunshine.--the outfit.--=ii. general instructions.=--the sensitized paper.--how the effects are produced.--negatives.--prints.--printing frames.--our printing frame.--putting negatives in printing frame.--printing.--developing.--fixing.--drying.--trimming.--fancy shapes.--mounting.--=iii. negatives and how to make them.=--the paper.--making transparent paper.--making the negatives.--printed negatives.--perforated negatives.--negatives made from magazine pictures.--ground glass negatives.--=iv. nature photography.=--aids to nature study.--ferns and leaves.--photographing leaves.--perforating leaves.--drying leaves, ferns, etc., for negatives.--flowers.--=v. miscellaneous photographs.=--magnetic photographs.--combination pictures.--initial pictures.--name plates.--christmas, easter and birthday cards. _the book and complete outfit will be sent, by mail or express, charges prepaid, upon receipt of cents, by_ =thomas m. st. john, w. st st., new york.= fun with magnetism. book and complete outfit for sixty-one experiments in magnetism... [illustration] children like to do experiments; and in this way, better than in any other, _a practical knowledge of the elements of magnetism_ may be obtained. these experiments, although arranged to _amuse_ boys and girls, have been found to be very _useful in the class-room_ to supplement the ordinary exercises given in text-books of science. to secure the _best possible quality of apparatus_, the horseshoe magnets were made at sheffield, england, especially for these sets. they are new and strong. other parts of the apparatus have also been selected and made with great care, to adapt them particularly to these experiments.--_from the author's preface._ =contents.=--experiments with horseshoe magnet.--experiments with magnetized needles.--experiments with needles, corks, wires, nails, etc.--experiments with bar magnets.--experiments with floating magnets.--miscellaneous experiments.--miscellaneous illustrations showing what very small children can do with the apparatus.--diagrams showing how magnetized needles may be used by little children to make hundreds of pretty designs upon paper. =amusing experiments.=--something for nervous people to try.--the jersey mosquito.--the stampede.--the runaway.--the dog-fight.--the whirligig.--the naval battle.--a string of fish.--a magnetic gun.--a top upsidedown.--a magnetic windmill.--a compass upsidedown.--the magnetic acrobat.--the busy ant-hill.--the magnetic bridge.--the merry-go-round.--the tight-rope walker.--a magnetic motor using attractions and repulsions. _the book and complete outfit will be sent, post-paid, upon receipt of cents, by_ =thomas m. st. john, w. st st., new york.= fun with shadows book and complete outfit for shadow pictures, pantomimes, entertainments, etc., etc. [illustration] =shadow making= has been a very popular amusement for several centuries. there is a great deal of _fun_ and instruction in it, and its long life is due to the fact that it has always been a source of keen delight to grown people as well as to children. in getting material together for this little book, the author has been greatly aided by english, french and american authors, some of whom are professional shadowists. it has been the author's special effort to get the subject and apparatus into a practical, cheap form for boys and girls. =the outfit= contains everything necessary for all ordinary shadow pictures, shadow entertainments, shadow plays, etc. the following articles are included: one book of instructions called "fun with shadows"; shadow screen; sheets of tracing paper; coil of wire for movable figures; cardboard frame for circular screen; cardboard house for stage scenery; jointed wire fish-pole and line; bent wire scenery holders; clamps for screen; wire figure support; wire for oar; spring wire table clamps; wire candlestick holder; cardboard plates containing the following printed figures that should be cut out with shears: character hats; boat; oar-blade; fish; candlestick; cardboard plate containing printed parts for making movable figures. =contents of book:= one hundred illustrations and diagrams, including ten full-page book plates, together with six full-page plates on cardboard. _chapter_ i. introduction.--ii. general instructions.--iii. hand shadows of animals.--iv. hand shadows of heads, character faces, etc.--v. moving shadow figures and how to make them.--vi. shadow pantomimes.--vii. miscellaneous shadows. _the book and complete outfit will be sent, =post-paid=, upon receipt of cents, by_ =thomas m. st. john, west st st., new york city.= fun with electricity. book and complete outfit for sixty experiments in electricity.... [illustration] enough of the principles of electricity are brought out to make the book instructive as well as amusing. the experiments are systematically arranged, and make a fascinating science course. no chemicals, no danger. the book is conversational and not at all "schooly," harry and ned being two boys who perform the experiments and talk over the results as they go along. "the book reads like a story."--"an appropriate present for a boy or girl."--"intelligent parents will appreciate 'fun with electricity.'"--"very complete, because it contains both book and apparatus."--"there is no end to the fun which a boy or girl can have with this fascinating amusement." =there is fun in these experiments.=--chain lightning.--an electric whirligig.--the baby thunderstorm.--a race with electricity.--an electric frog pond.--an electric ding-dong.--the magic finger.--daddy long-legs.--jumping sally.--an electric kite.--very shocking.--condensed lightning.--an electric fly-trap.--the merry pendulum.--an electric ferry-boat.--a funny piece of paper.--a joke on the family cat.--electricity plays leap-frog.--lightning goes over a bridge.--electricity carries a lantern.--and _= others=_. the =_outfit_= contains different articles. the =_book of instruction=_ measures x ½ inches, and has illustrations, pages, good paper and clear type. _the book, and complete outfit will be sent, by mail or express, charges prepaid, upon receipt of cents, by_ =thomas m. st. john, w. st st., new york.= fun with puzzles. book, key, and complete outfit for four hundred puzzles... the book measures × ½ inches. it is well printed, nicely bound, and contains chapters, pages, and illustrations. the key is illustrated. it is bound with the book, and contains the solution of every puzzle. the complete outfit is placed in a neat box with the book. it consists of numbers, counters, figures, pictures, etc., for doing the puzzles. =contents:= _chapter_ ( ) secret writing. ( ) magic triangles, squares, rectangles, hexagons, crosses, circles, etc. ( ) dropped letter and dropped word puzzles. ( ) mixed proverbs, prose and rhyme. ( ) word diamonds, squares, triangles, and rhomboids. ( ) numerical enigmas. ( ) jumbled writing and magic proverbs. ( ) dissected puzzles. ( ) hidden and concealed words. ( ) divided cakes, pies, gardens, farms, etc. ( ) bicycle and boat puzzles. ( ) various word and letter puzzles. ( ) puzzles with counters. ( ) combination puzzles. ( ) mazes and labyrinths. "fun with puzzles" is a book that every boy and girl should have. it is amusing, instructive,--educational. it is just the thing to wake up boys and girls and make them think. they like it, because it is real fun. this sort of educational play should be given in every school-room and in every home. "fun with puzzles" will puzzle your friends, as well as yourself; it contains some real brain-splitters. over new and original puzzles are given, besides many that are hundreds of years old. =secret writing.= among the many things that "f. w. p." contains, is the key to _secret writing_. it shows you a very simple way to write letters to your friends, and it is simply impossible for others to read what you have written, unless they know the secret. this, alone is a valuable thing for any boy or girl who wants to have some fun. _the book, key, and complete outfit will be sent, postpaid, upon receipt of cents, by_ =thomas m. st. john, west st st., new york city.= fun with soap-bubbles. book and complete outfit for fancy bubbles and films.... [illustration] =the outfit= contains everything necessary for thousands of beautiful bubbles and films. all highly colored articles have been carefully avoided, as cheap paints and dyes are positively dangerous in children's mouths. the outfit contains the following articles: one book of instructions, called "fun with soap-bubbles," metal base for bubble stand, wooden rod for bubble stand, large wire rings for bubble stand, small wire ring, straws, package of prepared soap, bubble pipe, water-proof bubble horn. the complete outfit is placed in a neat box with the book. (extra horns, soap, etc., furnished at slight cost.) =contents of book.=--twenty-one illustrations.--introduction.--the colors of soap-bubbles.--the outfit.--soap mixture.--useful hints.--bubbles blown with pipes.--bubbles blown with straws.--bubbles blown with the horn.--floating bubbles.--baby bubbles.--smoke bubbles.--bombshell bubbles.--dancing bubbles.--bubble games.--supported bubbles.--bubble cluster.--suspended bubbles.--bubble lamp chimney.--bubble lenses.--bubble basket.--bubble bellows.--to draw a bubble through a ring.--bubble acorn.--bubble bottle.--a bubble within a bubble.--another way.--bubble shade.--bubble hammock.--wrestling bubbles.--a smoking bubble.--soap films.--the tennis racket film.--fish-net film.--pan-shaped film.--bow and arrow film.--bubble dome.--double bubble dome.--pyramid bubbles.--turtle-back bubbles.--soap-bubbles and frictional electricity. "there is nothing more beautiful than the airy-fairy soap-bubble with its everchanging colors." _the best possible amusement for old and young._ _the book and complete outfit will be sent, =post-paid=, upon receipt of cents, by_ =thomas m. st. john, west st st., new york city.= the study of elementary electricity and magnetism by experiment. by thomas m. st. john, met. e. the book contains pages and illustrations; it measures × ½ in. and is bound in green cloth. price, post-paid, $ . . this book is designed as a text-book for amateurs, students, and others who wish to take up a systematic course of elementary electrical experiments at home or in school. full directions are given for....... _two hundred simple experiments._ the experiments are discussed by the author, after the student has been led to form his own opinion about the results obtained and the points learned. in selecting the apparatus for the experiments in this book, the author has kept constantly in mind the fact that the average student will not buy the expensive pieces usually described in text-books. the two hundred experiments given can be performed with simple apparatus; in fact, the student should make at least a part of his own apparatus, and for the benefit of those who wish to do this, the author has given, throughout the work, explanations that will aid in the construction of certain pieces especially adapted to these experiments. for those who have the author's "how two boys made their own electrical apparatus," constant references have been made to it as the "apparatus book," as this contains full details for making almost all kinds of simple apparatus needed in "the study of elementary electricity and magnetism by experiment." _if you wish to take up a systematic course of experiments--experiments that may be performed with simple, inexpensive apparatus,--this book will serve as a valuable guide._ condensed list of apparatus for "the study of elementary electricity and magnetism by experiment." _number_ . steel needles; package of twenty-five.-- . flat cork.-- . candle.-- - . annealed iron wires; assorted lengths.-- . horseshoe magnet; best quality; english.-- . iron filings.-- . parts for compass.-- , . wire nails; soft steel.-- , . spring steel; for bar magnets.-- . iron ring.-- . sifter; for iron filings.-- . spring steel; for flexible magnet.-- , . ebonite sheets; with special surface.-- . ebonite rod.-- . ebonite rod; short.-- . flannel cloth.-- . tissue paper.-- . cotton thread.-- . silk thread.-- . support base.-- . support rod.-- . support wire.-- . wire swing.-- . sheet of glass.-- . hairpin.-- . circular conductor.-- . circular conductor.-- . electrophorus cover.-- . insulating table.-- . insulated copper wire.-- . rubber band.-- . bent wire clamps.-- . cylindrical conductor.-- . discharger; for condenser.-- . aluminum-leaf.-- . wires. . dry cell.-- . mercury.-- . insulated copper wire; for connections.-- . spring connectors; two dozen.-- . parts for key.-- . metal connecting plates.-- . parts for current reverser.-- . parts for galvanoscope.-- . parts for astatic galvanoscope.-- - . zinc strips.-- . carbon rod.-- , . glass tumblers.-- , . copper strips.-- . galvanized iron nail.-- , . wooden cross-pieces.-- . brass screws; one dozen.-- . porous cup.-- . zinc rod.-- . copper plate.-- . iron strip.-- , . lead strips.-- . parts for resistance coil.-- . parts for wheatstone's bridge.-- . german-silver wire; size no. .-- . german-silver wire; no. .-- -- . plate binding-posts.-- . copper sulphate.-- . copper burs; one dozen.-- . combination rule.-- . coil of wire; on spool for electromagnet.-- . coil of wire; on spool for electromagnet.-- . carbon rod.-- , . soft iron cores with screws.-- . combined base and yoke.-- . combination connecting plates.-- . long iron core.-- . round bar magnet, × / in.-- . thin electromagnet.-- . degree-card; for galvanoscope.-- . scale for bridge.-- , . soft iron cores with heads.-- , . flat bar magnets; these are × ½ × ¼ in.; highly polished steel; poles marked.-- . compass. =_illustrated price catalogue upon application._= electrical apparatus for sale a complete electric and magnetic cabinet for students, schools and amateurs. six extraordinary offers =this cabinet of electrical experiments= contains three main parts: (_a_) apparatus; (_b_) text-book; (_c_) apparatus list. (_a_) =the apparatus= furnished consists of one hundred and five pieces. over three hundred separate articles are used in making up this set. most of it is ready for use when received. seven pieces, however, are not assembled; but the parts can be readily finished and put together. (sold, also, _all_ pieces assembled.) (_b_) =the text-book=--called "the study of elementary electricity and magnetism by experiment"--gives full directions for two hundred experiments. (see table of contents, etc.) price, post-paid, $ . . (_c_) =the apparatus list= is an illustrated book devoted entirely to this special set of apparatus. not given with first offer. _the apparatus is simple because the designs and methods of construction have been worked out with great care._ _the apparatus is practical because it has been designed for real use in "the study of elementary electricity and magnetism by experiment."_ _the apparatus is cheap because the various parts are so designed that they can be turned out in quantity by machinery._ = st offer:= pieces to $ . = d offer:= pieces to , with part (_c_) . = d offer:= pieces to , with part (_c_) . = th offer:= complete cabinet, parts (_a_), (_b_), (_c_) . = th offer:= apparatus only, all pieces assembled . = th offer:= complete cabinet, all pieces assembled . =_express charges must be paid by you. estimates given._= a "special catalogue," pertaining to the above, with complete price-list, will be mailed upon application. =thomas m. st. john, west st st., new york city= fun with telegraphy book and complete outfit. [illustration] =telegraphy= is of the greatest importance to all civilized nations, and upon it depend some of the world's most important enterprises. every boy and girl can make practical use of telegraphy in one way or another, and the time it takes to learn it will be well spent. =the outfit.=--mr. st. john has worked for a number of years to produce a telegraph outfit that would be simple, cheap, and practical for those who wish to make a study of telegraphy. after making and experimenting with nearly one hundred models, many of which were good, he has at last perfected an instrument so simple, original, and effective that it is now being made in large quantities. the sounders are so designed that they will work properly with any dry cell of ordinary strength, and this is a great advantage for practice lines. dry batteries are cheap and clean, and there are no dangers from acids. the outfit consists of the following articles, placed in a neat box: one book of instruction, called "fun with telegraphy"; one telegraph "key"; one telegraph "sounder"; insulated copper wires for connections. the "key" and "sounder" are mounted, with proper "binding-posts," upon a base of peculiar construction, which aids in giving a large volume of sound. =contents of book.=--telegraphy.--the outfit.--a complete telegraph line.--connections.--the telegraph key.--the sounder.--the battery.--a practice line.--a two-instrument line.--operation of line.--the morse telegraph alphabet.--aids to learning alphabet.--cautions.--office calls.--receiving messages.--remember.--extra parts. =about batteries.=--for those who cannot easily secure batteries, we will furnish small dry cells, post-paid, at cents each, in order to deliver the outfits complete to our customers. this price barely covers the total cost to us, postage alone being cents. _=fun with telegraphy, including book, key, sounder, and wire (no battery), post-paid, cents, by=_ =thomas m. st. john, ninth ave., new york= tool sets for students the following tool sets have been arranged especially for those who wish to make use of the designs contained in "how two boys made their own electrical apparatus," "real electric toy-making for boys," "electric instrument-making," etc. it is very poor economy to waste valuable time and energy in order to save the cost of a few extra tools. =note.=--save money by buying your tools in sets. we do not pay express or freight charges at the special prices below. =for $ . .=--one _steel punch_; round, knurled head.--one light _hammer_; polished, nickel-plated, varnished handle.--one _iron clamp_; japanned, ¼ in.--one _screw-driver_; tempered and polished blade, cherry stained hardwood handle, nickel ferrule.--one _wrench_; retinned skeleton frame, gilt adjusting wheel.--one _awl_; tempered steel point, turned and stained wood handle, with ferrule.--one _vise_; full malleable, nicely retinned, - / in. jaws, full malleable screw with spring.--one pair _steel pliers_; in. long, polished tool steel, unbreakable, best grooved jaw.--one pair of _shears_; carbonized steel blades, hardened edge, nickel-plated, heavy brass nut and bolt.--one _file_; triangular, good steel.--one _file handle_; good wood, brass ferrule.--one _foot rule_; varnished wood, has english and metric system.--one _soldering set_; contains soldering iron, solder, resin, sal ammoniac, and directions. one _center-punch_; finely tempered steel. =for $ . .=--all that is contained in the $ . set of tools, together with the following: one pair of _tinner's shears_; cut, ¾ in., cast iron, hardened, suitable for cutting thin metal.--one _hollow handle tool set_; very useful; polished handle holds tools, gimlet, brad-awls, chisel, etc.--one _try square_; -in. blue steel blade, marked in / s, strongly riveted.--one -lb. _hammer_; full size, polished head, wedged varnished hardwood handle.--one _hack saw_; steel frame, ½-in. polished steel blade, black enamel handle; very useful. =for $ . .=--two _steel punches_; different sizes, one solid round, knurled head, polished; the other, point and head brightly polished, full nickel, center part knurled.--one _light hammer_; polished and nickel plated, varnished handle.--one regular _machinist's hammer_; ball peen, solid cast steel, with varnished hardwood handle; a superior article.--two _iron clamps_; one opens ¼ in., the other in., japanned.--one _screw-driver_; tempered and polished blade, firmly set in cherry stained hardwood handle with nickel ferrule.--one _wrench_; retinned, skeleton frame, gilt adjusting wheel.--one _awl_; tempered steel blade, ground to point, firmly set in turned and stained handle with ferrule.--one _steel vise_; ¼-in., jaws, steel screw, bright polished jaws and handle; a good strong vise.--one pair of _steel pliers_; in. long, bright steel, flat nose, wire-cutters, practically unbreakable.--one pair of _shears_; carbonized steel blades, hardened edges, nickel plated, heavy brass nut and bolt.--one _file_; triangular and of good steel.--one _file handle_; good wood, with brass ferrule.--one _foot rule_; varnished wood, has both the english and metric systems.--one _soldering set_; contains soldering iron, solder, resin, sal ammoniac, and directions; a very handy article.--one _center-punch_; finely tempered steel.--one pair of _tinner's shears_; these are best grade, inlaid steel cutting edges, polished and tempered, japanned handles; thoroughly reliable.--one _hollow handle tool set_; very useful; the polished handle holds tools, gimlet, chisel, brad-awl, etc.--one _try square_; -in. blue steel blade, marked both sides in / s, strongly riveted with brass rivets.--one _hack saw_; steel frame, ½-in. polished steel blade, black enamel handle; very useful for sawing small pieces of wood. =for $ . = will be included everything in the $ . offer, and the following: one _glue-pot_; medium size, with brush and best wood glue; inside pot has hinge cover.--one _ratchet screw-driver_; great improvement over ordinary screw-drivers; well made and useful.--one _hand drill_; frame malleable iron; hollow screw top holding drills; bores from - to - -in. holes; solid gear teeth; -jawed nickel plated chuck; a superior tool, and almost a necessity. =give the boy a set of tools= =thomas m. st. john, ninth ave., new york= real electric toy-making for boys _by_ thomas m. st. john, met. e. this book contains pages and over one hundred original drawings, diagrams, and full-page plates. it measures x ½ in., and is bound in cloth. price, post-paid, $ . =contents:= _chapter_ i. toys operated by permanent magnets.--ii. toys operated by static electricity.--iii. making electromagnets for toys.--iv. electric batteries.--v. circuits and connections.--vi. toys operated by electromagnets. vii. making solenoids for toys.--viii. toys operated by solenoids.--ix. electric motors.--x. power, speed, and gearing.--xi. shafting and bearings.--xii. pulleys and winding-drums.--xiii. belts and cables.--xiv. toys operated by electric motors.--xv. miscellaneous electric toys.--xvi. tools.--xvii. materials.--xviii. various aids to construction. while planning this book, mr. st. john definitely decided that he would not fill it with descriptions of complicated, machine-made instruments and apparatus, under the name of "toy-making," for it is just as impossible for most boys to get the parts for such things as it is for them to do the required machine work even after they have the raw materials. great care has been taken in designing the toys which are described in this book, in order to make them so simple that any boy of average ability can construct them out of ordinary materials. the author can personally guarantee the designs, for there is no guesswork about them. every toy was made, changed, and experimented with until it was as simple as possible; the drawings were then made from the perfected models. as the result of the enormous amount of work and experimenting which were required to originate and perfect so many new models, the author feels that this book may be truly called "real electric toy-making for boys." =every boy should make electrical toys.= the electric shooting game> a most original and fascinating game patent applied for and copyrighted [illustration] _=shooting by electricity=_ =the electric shooting game= is an entirely new idea, and one that brings into use that most mysterious something--_electricity_. the game is so simple that small children can play it, and as there are no batteries, acids, or liquids of any kind, there is absolutely no danger. the electricity is of such a nature that it is perfectly harmless--but very active. the "_game-preserve_" is neat and attractive, being printed in colors, and the birds and animals are well worth hunting. each has a fixed value--and some of them must not be shot at all--so there is ample opportunity for a display of skill in bringing down those which count most. "_electric bullets_" are actually shot from the "_electric gun_" by electricity. this instructive game will furnish a vast amount of amusement to all. _=the "game-preserve,"--the "electric gun,"--the "shooting-box,"--the "electric bullets,"--in fact, the entire electrical outfit, together with complete illustrated directions, will be sent in a neat box, post-paid, upon receipt of cents, by=_ =thomas m. st. john, ninth ave., new york= * * * * * transcriber's note: obvious punctuation errors were corrected. page , "turnnd" changed to "turned" (be turned to ) page , word "a" added to text (in a glass jar) [illustration: first direct-connected electric generator unit of large capacity ever constructed up to the time it was made by thomas a. edison in june, . capacity, incandescent lamps of candle-power each] a-b-c of electricity by william h. meadowcroft harper & brothers publishers new york & london a-b-c of electricity copyright, , , by william h. meadowcroft copyright, , by harper & brothers printed in the united states of america published may, from the laboratory of thomas a. edison orange, n. j. mr. w. h. meadowcroft, _new york city_. _dear sir_: _i have read the ms. of your "a-b-c of electricity," and find that the statements you have made therein are correct. your treatment of the subject, and arrangement of the matter, have impressed me favorably. _yours truly_, _thos. a. edison_ contents chap. page introduction to new edition viii preface x i. ii. definitions iii. magnetism iv. the telegraph v. wireless telegraphy vi. the telephone vii. electric light viii. electric power ix. batteries x. conclusion introduction to new edition the favor with which this book has been received has brought about the preparation of this new edition. the present volume has been enlarged by the addition of certain new material and it has been entirely reset. some new illustrations have been made, and in its new dress the book, it is hoped, will be found to afford an even larger measure of usefulness. the principles of the science remain the same, but the author is glad of the opportunity to note certain developments in their application. w. h. m. edison laboratory, _april, _. preface while there is no lack of most excellent text-books for the study of those branches of electricity which are above the elementary stage, there is a decided need of text-books which shall explain, in simple language, to young people of, say, fourteen years and upward, a general outline of the science, as well as the ground-work of those electrical inventions which are to-day of such vast commercial importance. there is also a need for such a book among a large part of the adult population, for the reason that there have been great and radical changes in this science since the time they completed their studies, and they have not the time to follow up the subject in the advanced books. as instances of those changes just spoken of, the electric light, telephone, and storage batteries may be mentioned, which have been developed during the last ten or twelve years, with the result of adding very many features that were entirely new to electricians. with these ideas in view i have prepared this little volume. it is not intended, in the slightest degree, to be put forward as a scientific work, but it will probably give to many the information they desire without requiring too great a research into books which treat more extensively and deeply of this subject. w. h. m. a-b-c of electricity a-b-c of electricity i we now obtain so many of our comforts and conveniences by the use of electricity that all young people ought to learn something of this wonderful force, in order to understand some of the principles which are brought into practice. you all know that we have the telegraph, the telephone, the electric light, electric motors on street-cars, electric bells, etc., besides many other conveniences which the use of electricity gives us. every one knows that, by the laws of multiplication, twice two makes four, and that twice two can never make anything but four. well, these useful inventions have been made by applying the _laws of electricity_ in certain ways, just as well known, so as to enable us to send in a few moments a message to our absent friends at any distance, to speak with them at a great distance, to light our houses and streets with electric light, and to do many other useful things with quickness and ease. but you must remember that we do not know what electricity itself really is. we only know how to produce it by certain methods, and we also know what we can do with it when we have obtained it. in this little book we will try to explain the various ways by which electricity is obtained, and how it is applied to produce the useful results that we see around us. we will try and make this explanation such that it will encourage many of you to study this very important and interesting subject more deeply. in the advanced books on electricity there are many technical terms which are somewhat difficult to understand, but in this book it will only be necessary to use a few of the more simple ones, which it will be well for you to learn and understand before going further. ii definitions the three measurements most frequently used in electricity are the volt, the ampère, the ohm. we will explain these in their order. [illustration: fig. ] _the volt._--this term may be better understood by making a comparison with something you all know of. suppose we have a tank containing one hundred gallons of water, and we want to discharge it through a half-inch pipe at the bottom of the tank. suppose, further, that we wanted to make the water spout upward, and for this purpose the pipe was bent upward as in fig. . if you opened the tap the water would spout out and upward as in fig. . [illustration: fig. ] the cause of its spouting upward would be the _weight_ or _pressure_ of the water in the tank. this pressure is reckoned as so many _pounds_ to the square inch of water. now, if the tank were placed on the roof of the house and the pipe brought to the ground as shown in fig. , the water would spout up very much higher, because there would be _many more pounds_ of pressure on account of the height of the pipe. so, you see, the force or pressure of water is measured in pounds, and, therefore, a pound is the unit of pressure, or force, of water. now, in electricity the unit of pressure, or force, is called a volt. this word "volt" does not mean any weight, as the word "pound" weight does. you all know that if you have a pound of water you must have something to hold it, because it has weight, and, consequently, occupies some space. but _electricity itself has no weight_ and therefore cannot occupy any space. when we desire to carry water into a house or other building we do so by means of hollow pipes, which are usually made of iron. this is the way that water is brought into houses in cities and towns, so that it may be drawn and used in any part of a dwelling. now, the principal supply usually comes from a reservoir which is placed up on high ground so as to give the necessary pounds of pressure to force the water up to the upper part of the houses. if some arrangement of this kind were not made we could get no water in our bedrooms, because, as you know, water will not rise above its own level unless by force. the water cannot escape as long as there are no holes or leaks in the iron pipes, but if there should be the slightest crevice in them the water will run out. in electricity we find similar effects. the electricity is carried into houses by means of wires which are covered, or _insulated_, with various substances, such, for instance, as rubber. just as the iron of the pipes prevents the water from escaping, the insulation of the wire prevents the escape of the electricity. now, if we were to cause the pounds of pressure of water, in pipes of ordinary thickness, to be very greatly increased, the pipes could not stand the strain and would burst and the water escape. so it is with electricity. if there were too many volts of pressure the insulation would not be sufficient to hold it and the electricity would escape through the covering, or insulation, of the wire. it is a simple and easy matter to stop the flow of water from an ordinary faucet by placing your finger over the opening. as the water cannot then flow, your finger is what we will call a non-conductor and the water will be retained in the pipe. we have just the same effects in electricity. if we place some substance which is practically a non-conductor, or insulator, such as rubber, around an electric wire, or in the path of an electric current, the electricity, acted upon by the volts of pressure, cannot escape, because the insulation keeps it from doing so, just as the iron of the pipe keeps the water from escaping. thus, you see, the volt does not itself represent electricity, but only the pressure which forces it through the wire. there are other words and expressions in electricity which are sometimes used in connection with the word "volt." these words are "pressure" and "intensity." we might say, for instance, that a certain dynamo machine had an electromotive force of volts; or that the intensity of a cell of a battery was volts, etc. we might mention, as another analogy, the pressure of steam in a boiler, which is measured or calculated in pounds, just as the pressure of water is measured. so, we might say that pounds steam pressure used through the medium of a steam-engine to drive a dynamo could thus be changed to electricity at volts pressure. _the ampère._--now, in comparing the pounds pressure of water with the volts of pressure of electricity we used as an illustration a tank of water containing gallons, and we saw that this water had a downward force or pressure in pounds. let us now see what this pressure was acting upon. it was forcing the quantity of water to spout upward through the end of the pipe. now, as the quantity of water was gallons, it could not all be forced at once out of the end of the pipe. the pounds pressure of water acting on the gallons would force it out at a _certain rate_, which, let us say, would be one gallon per minute. this would be the _rate of the flow_ of water out of the tank. thus, you see, we find a second measurement to be considered in discharging the water-tank. the first was the force, or pounds of pressure, and the second the _rate_ at which the quantity of water was being discharged per minute by that pressure. this second measurement teaches us that a _certain quantity_ will pass out of the pipe in a _certain time_ if the pressure is steady, such quantity depending, of course, on the size or friction resistance of the pipe. in electricity the volts of pressure act so as to force the quantity of current to _flow through the wires at a certain rate_ per second, and the rate at which it flows is measured in ampères. for instance, let us suppose that an electric lamp required a pressure of volts and a current of one ampère to light it up, we should have to supply a current of electricity flowing at the rate of one ampère, acted upon by an electromotive force of volts. you will see, therefore, that while the volt does not represent any electricity, but only its pressure, the ampère represents the _rate of flow_ of the current itself. you should remember that there are several words sometimes used in connection with the word "ampère"--for instance, we might say that a lamp required a "current" of one ampère or that a dynamo would give a "quantity" of ampères. _the ohm._--you have learned that the _pressure_ would discharge the _quantity_ of water at a certain rate through the pipe. now, suppose we were to fix _two_ discharge-pipes to the tank, the water would run away very much quicker, would it not? if we try to find a reason for this, we shall see that a pipe can only, at a given pressure, admit so much water through it at a time. therefore, you see, this pipe would present a certain amount of _resistance_ to the passage of the total quantity of water, and would only allow a limited quantity at once to go through. but, if we were to attach two or more pipes to the tank, or one large pipe, we should make it easier for the water to flow, and, therefore, the total amount of resistance to the passage of the water would be very much less, and the tank would quickly be emptied. now, as you already know, water has substance and weight and therefore occupies some space, but electricity has neither substance nor weight, and therefore cannot occupy any space; consequently, to carry electricity from one place to another we do not need to use a pipe, which is hollow, but we use a solid wire. these solid wires have a certain amount of _resistance_ to the passage of the electricity, just as the water-pipe has to the water, and (as it is in the case of the water) the effect of the resistance to the passage of electricity is greater if you pass a larger quantity through than a smaller quantity. if you wanted to carry a quantity of electricity to a certain distance, and for that purpose used a wire, there would be a certain amount of resistance in that wire to the passage of the current through it; but if you used two or more wires of the same size, or one large wire, the resistance would be very much less and the current would flow more easily. suppose that, instead of emptying the water-tank from the roof through the pipe, we had just turned the tank over and let the water all pour out at once down to the ground. that would dispose of the water very quickly and by a short way, would it not? that is very easy to be seen, because there would be _no resistance_ to its passage to the ground. well, suppose we had an electric battery giving a certain quantity of current, say five ampères, and we should take a large wire that would offer no resistance to that quantity and put it from one side of the battery to the other, a large current would flow at once and tend to exhaust the battery. this is called a _short circuit_ because there is little or no resistance, and it provides the current with an easy path to escape. remember this, that _electricity always takes the easiest path_. it will take as many paths as are offered, but the largest quantity will always take the easiest. as the subject of resistance is one of the most important in electricity, we will give you one more example, because if you can obtain a good understanding of this principle it will help you to comprehend the whole subject more easily in your future studies. we started by comparison with a tank holding gallons of water, discharging through a half-inch pipe, and showed you that the pounds of pressure would force the quantity of gallons through the pipe. when the tap was first opened the water would spout up very high, but as the water in the tank became lower the pressure would be less, and, consequently, the water would not spout so high. so, if it were desired to keep the water spouting up to the height it started with, we should have to keep the tank full, so as to have the same pounds of pressure all the time. but, if we wanted the water to spout still higher we should have to use other means, such as a force-pump, to obtain a greater pressure. now, if we should use too many pounds pressure it would force the quantity of water more rapidly through the pipe and would cause the water to become heated because of the resistance of the pipe to the passage of that quantity acted upon by so great a pressure. this is just the same in electricity, except that the wire itself would become heated, some of the electricity being turned into heat and lost. if a wire were too small for the volts pressure and ampères of current of electricity the resistance of such wire would be overcome, and it would become red-hot and perhaps melt. electricians are therefore very careful to calculate the resistance of the wires they use before putting them up, especially when they are for electric lighting, in order to make allowances for the ampères of current to flow through them, so that but little of the electricity will be turned into heat and thus rendered useless for their purpose. the unit of resistance is called the _ohm_ (pronounced like "home" without the "h"). all wires have a certain resistance per foot, according to the nature of the metal used and the size of the wire--that is to say, the finer the wire the greater number of ohms resistance it has to the foot. water and electricity flow under very similar conditions--that is to say, each of them must have a channel, or conductor, and each of them requires pressure to force it onward. water, however, being a tangible substance, requires a hollow conductor; while electricity, being intangible, will flow through a solid conductor. the iron of the water-pipe and the insulation of the electric wire serve the same purpose--namely, that of serving to prevent escape by reason of the pressure exerted. there is another term which should be mentioned in connection with resistance, as they are closely related, and that is _opposition_. there is no general electrical term of this name, but, as it will be most easily understood from the meaning of the word itself, we have used it. let us give an example of what opposition would mean if applied to water. probably every one knows that a water-wheel is a wheel having large blades, or "paddles," around its circumference. when the water, in trying to force its passage, rushes against one of these paddles it meets with its opposition, but overcomes it by pushing the paddle away. this brings around more opposition in the shape of another paddle, which the water also pushes away. and so this goes on, the water overcoming this opposition and turning the wheel around, by which means we can get water to do useful work for us. you must remember, however, that it is only by putting opposition in the path of a pressure and quantity of water that we can get this work. the same principle holds good in electricity. we make electricity in different ways, and in order to obtain useful work we put in its path the instruments, lamps, or machines which offer the proper amount of resistance, or opposition, to its passage, and thus obtain from this wonderful agent the work we desire to have done. you have learned that three important measurements in electricity are as follows: the _volt_ is the practical unit of measurement of _pressure_; the _ampère_ is the practical unit of measurement of the _rate of flow_; and the _ohm_ is the practical unit of measurement of _resistance_. iii magnetism now we will try to explain to you something about magnets and magnetism. there are very few boys who have not seen and played with the ordinary magnets, shaped like a horseshoe, which are sold in all toy-stores as well as by those who sell electrical goods. well, you know that these magnets will attract and hold fast anything that is made of iron or steel, but they have no effect on brass, copper, zinc, gold, or silver, yet there is nothing that you can see which should cause any such effect. you will notice, then, that magnetism is like electricity; we cannot see it, but we can tell that it exists, because it produces certain effects. and here is another curious thing--magnetism produces electricity, and electricity produces magnetism. this seems to be a very convenient sort of a family affair, and it is owing to this close relation that we are able to obtain so many wonderful things by the use of electricity. we shall now show you how electricity produces magnetism, and, when we come to the subject of electric lighting we will explain how magnetism produces electricity. [illustration: fig. ] the easiest way to show how electricity makes magnetism is to find out how magnets are made. suppose we wanted to make a horseshoe magnet, just mentioned above; we would take a piece of _steel_ and wind around it some fine copper wire, commencing on one leg of the horseshoe and winding around until we came to the end of the other leg. then we should have two ends of wire left, as shown in the sketch. (fig. .) we connect these two ends with an electric battery, giving, say, two volts, and then the ampères of current of electricity will travel through the wire, and in doing so has such an influence on the steel that it is converted into a magnet, such as you have played with. the current is "broken"--that is to say, it is shut off several times in making a magnet of this kind, and then the wire is taken away from the battery and is unwound from the steel horseshoe, leaving it free from wire, just as you have seen it. this horseshoe is now a _permanent magnet_--that is, it will _always_ attract and hold pieces of iron and steel. now, if you were to do the same thing with a horseshoe made of soft iron instead of steel it would not be a magnet after you stopped the current of electricity from going through the wires, although the piece of _iron_ would be a stronger magnet while the electricity was going through the wire around it. the steel magnet is called a permanent magnet, and its ends, or "poles," are named north and south. there is usually a loose piece of steel or iron, called an "armature," put across the ends, which has the peculiar property of keeping the magnetism from becoming weaker, and thereby retaining the strength of the magnet. the strongest part of the magnet is at the poles, while, at the point marked + (which is called the neutral point) there is scarcely any magnetism. it will be well to remember the object of the _armature_ as we shall meet it again in describing dynamo machines. the magnets made of iron are called electromagnets because they exhibit magnetism only when the ampères of current of electricity are flowing around them. they also have two poles, north and south, as have permanent magnets. electromagnets are used in nearly all electrical instruments, not only because they are stronger than permanent magnets, but because they can be made to act instantly by passing a current of electricity through them at the most convenient moment, as you will see when we explain some of the electrical instruments which are used to produce certain effects. (fig. .) [illustration: fig. ] of course there are a great many different shapes in which magnets are made. the simplest is the _bar magnet_, which is simply a flat or round piece of iron or steel. suppose you made a magnet of a flat piece of steel and put on top of it a sheet of paper, and then threw on the paper some iron filings, you would see them arrange themselves as is shown in the following sketch. (fig. .) the filings would always arrange themselves in this shape, no matter how large or small the magnets were. and, if you were to cut it into two or half a dozen pieces, each piece would have the same effect. this shows you that each piece would itself become a magnet and would have its poles exactly as the large one had. [illustration: fig. ] now, we have another curious thing to tell you about magnets. if you present the north pole of a magnet to the south pole of another magnet, they will attract and hold fast to each other, but if you present a south pole to another south pole, or a north pole to a north pole, they will repel each other, and there will be no attraction. you can perform some interesting experiments by reason of this fact. we will give you one of them. take, say, a dozen needles and draw them several times in the same direction across the ends of a magnet so that they become magnetized. now stick each needle half-way through a piece of cork, and put the corks, with the needles sticking through them, into a bowl of water. then take a bar magnet and bring it gradually toward the middle of the bowl and you will see the corks advance or back away from the magnet. if the ends of the needles sticking up out of the water are south poles and the end of the magnet you present is a north pole, the needles will come to the center; but will go to the side of the bowl if you present the south pole. you can vary this pretty experiment by turning up the other ends of part of the needles. you will remember that when we explained what "resistance" meant, we told you that electricity would always take the easiest path, and while part of it will flow in a small wire, the largest portion will take an easier path if it can get to something larger that is a metallic substance. electricity will only flow easily through anything that is made of metal. you will also remember that you learned that when electricity took a short cut to get away from its proper path it was called a _short circuit_. all this must be taken into consideration when magnets are being made. in the first place, the wire we wind around steel or iron to make magnets must always be covered with an insulator of electricity. magnet wire is usually covered with cotton or silk. if it were left bare, each turn of the wire would touch the next turn, and so we should make such an easy path for the electricity that it would all go back to the battery by a short circuit, and then we would get no magnetic effect in the steel or iron. _the only way we can get electricity to do useful work for us is to put some resistance or opposition in its way._ so you see that if we make it travel through the wire around the iron or steel, there is just enough resistance or opposition in its way to give it work to get through the wire, and this work produces the peculiar effect of making the iron or steel magnetic. the covering on the wire, as you will remember, is called "insulation." iv the telegraph every one knows how very convenient the telegraph is, but there are not many who think how wonderful it is that we can send a message in a few seconds of time to a distant place, even though it were thousands of miles away. and yet, though the present system of telegraphing is a wonderful one, the method of sending a telegram is simple enough. the apparatus that is used in sending a telegram is as follows: the battery. the wire. the telegraph key. the sounder. the different kinds of electric batteries will be mentioned afterward, so we will not stop now to describe them, but simply state that a battery is used to produce the necessary electricity. as you all know what wire is, there is no necessity of describing it further. the telegraph key is shown in the sketch below. (fig. .) [illustration: fig. ] this instrument is usually made of brass, except that upon the handle there is the little knob which is of hard rubber. the handle, or lever, moves down when this knob is pressed, and a little spring beneath pushes it up again when let go. you will see a second smaller knob, the use of which we will explain later. the sounder is shown on the following page. (fig. .) the part consisting of the two black pillars is an electromagnet, and across the top of these pillars is a piece of iron called the "armature," which is held up by a spring. [illustration: fig. ] now let us see how the battery and wire are placed in connection with these instruments. you have seen that we usually have two wires for the electricity to travel in, one wire for it to leave the battery, and the other to return on. but you will easily see that if two wires had to be used in telegraphing it would be a very expensive matter, especially when they had to be carried thousands of miles. so, instead of using a second wire, we use the earth to carry back the electricity to the battery, because the earth is a better conductor even than wire. although a quantity of ground equal in size to the wire would offer thousands of times greater resistance than the wire, yet, owing to the great body of our earth, its total resistance is even less than any telegraph wire used. when two electric wires are run from a battery and connected together through some instrument, this is called a "circuit," because the electricity has a path in which it can travel back to the battery. this would be a "metallic" circuit; _but when one wire only_ is used, and the other side of the battery is connected with the earth, it is called a "ground" or "earth" circuit, because the electricity returns through the earth. [illustration: fig. ] if you look at this sketch (fig. ) you will see how the telegraph instruments are connected and will then be able to understand how a message can be sent. here we have two sets of telegraph apparatus, one of which, let us say, is in new york and the other in philadelphia. you will see that one wire from the battery is connected with the earth, and the other wire with the sounder. another wire goes from the sounder to one leg of the key so as to make the brass base of the key part of the circuit. the other leg of the key is "insulated" from the brass base by being separated therefrom with some substance which will not carry electricity, such, for instance, as hard rubber. we will suppose that there is already a wire strung up on poles between new york and philadelphia, and that the key, sounder, and battery in the latter city are connected in the same way as those in new york. now, to enable us to send a message from one city to the other we must connect the ends of the wires to the instruments in each city; so we connect one end to the insulated leg of the key in new york, and the other end to the insulated leg of the key in philadelphia. everything is now completed, and, as soon as we find out what is the use of that part of the key that has a little round, black handle, we shall be ready to start. this is called the "switch." if you will look once more at the picture of the key you will see under the long handle (or lever) a little point which the lever will touch when it is pressed down. now this little point is part of that insulated leg, and, therefore, this point is also insulated from the base. if a current of electricity were sent along the wire it could not get any farther than this point unless we put in some arrangement to complete the path, or circuit, for it to travel in. we therefore put in the switch. one end of the switch (which is made of brass with a rubber handle) is fastened on the base of the key, so that it may be moved to the right or left. the other end, when the switch is moved to the left (or "closed"), touches a piece of brass fastened to the little point we have mentioned, and so makes a free path for the electricity to go through the base of the key and through the wire to the sounder, and from there to the battery, and so back to the earth. this switch must be opened before the sounder near it will respond to its neighboring key. now we are ready to send a message. suppose we want to send a telegram from new york to philadelphia. the operator in new york opens his switch and presses down his key several times. the switch on the philadelphia key being closed, the electricity goes through to the sounder, and, this being made an electromagnet by the current passing through the wire, the iron armature is attracted by the magnetism and drawn down to the magnet with a snap. it will stay there as long as the new york operator keeps his lever pressed down, but, when he allows it to spring up, there is no current passing through the philadelphia sounder and there is no magnetism, consequently the armature springs up again with a click. as often as the operator presses down his key lever and lets it spring up again, the same action takes place in the sounder, and it makes that click, click, which you have heard if you have ever seen telegraph instruments in operation. let us continue, however, to send our message. the new york operator, having pressed down his key several times to signal the philadelphia operator, closes his switch to receive the answer from philadelphia. the operator in the latter city then opens his switch and presses down his key several times, which makes the new york sounder click, in the same way, to let the operator there know that he is ready to receive the message. he then closes his switch and receives the telegram which the new york operator sends after opening _his_ key. telegraphic messages are sent and received in this way and are read by the sound of the clicks. these sounds may be represented on paper by dots, dashes, and spaces. for instance, if you press down the key and let it spring back quickly, that would represent a dot. if you press down the key and hold it a little longer before letting it spring up again, it would represent a dash. a space would be represented by waiting a little while before pressing down the key again. we show you below the alphabet in these dots, dashes, and spaces, and these are the ones now used in sending all telegraphic messages. [illustration] thus, you see, if you were telegraphing the word "and" you would press down your key and let it return quickly, then press down again and return after a longer pause, which would give the letter a; then slowly and quickly, which would be n; then slowly and twice quickly, which would be d. any persevering boy can learn to operate a telegraph instrument by a little study and regular practice; and, as complete learner's sets can be purchased very cheaply, this affords a pleasant and useful recreation for boys. there are many cases where two boys living near each other have a set of telegraph instruments in their homes and run a wire from one house to the other, thus affording many hours of pleasant and profitable amusement. in giving the above explanation of telegraphing we have described only the simple and elementary form. in large telegraph lines, such as those of the western union, there are many more additional instruments used, which are very complicated and difficult to understand; such, for instance, as the quadruplex, by which four distinct messages can be sent over the same wire at the same time. we have, therefore, described only the simplest form in order to give the general idea of the working of the telegraph by electromagnetism, which is the principle of all telegraphing. when you study electricity more deeply you will find this subject and the many different instruments very interesting and wonderful. v wireless telegraphy if it has seemed extraordinary to you that only one wire should be necessary for sending a message by the electric telegraph, and that our earth can be used instead of a second wire, how much more wonderful it is to realize that in these days we can exchange telegraphic messages with different points without any connecting wires at all between them, even though the places be many hundred miles apart. thus, two ships on the ocean, entirely out of sight of each other, may intercommunicate, or may telegraph to or receive despatches from a far-distant shore; indeed, telegraphy without wires has been accomplished across the atlantic ocean. in the language of the day, this is called "wireless telegraphy," although it is more correct to think of it as aerial, or space, telegraphy. as you will naturally want to know how this is effected, we will try to explain the main principles in a simple manner. if you drop a stone into a quiet pond, you will see the water form into ring-like waves, or ripples, which travel on and on until they die away in the far distance. these waves are caused, as we have seen, by a disturbance of the body of water. probably you have already learned in school that all known space is said to be filled with a medium called "ether," and that this medium is so exceedingly thin that it penetrates, or permeates, everything, so that it exists in the densest bodies as well as in free space. for the sake of obtaining a clear idea of this theory we may imagine that the ether envelops and permeates every thing in the entire universe. hence we can easily realize that, although we cannot see or feel the ether, any disturbance of it will set it in wavelike motion. modern science accounts for light, radiant heat, and electrical phenomena by reason of wavelike disturbances, vibrations, or pulsations of this ether. thus, if you should strike a light, the ether would be disturbed, causing waves to form, which, like the waves in the water, would travel in every direction. when these waves reached the eyes of another person within seeing distance, that person's eyes would be so acted upon by the waves that he would see the light which you had made, and would see it instantly, for light waves travel about , miles per second. so, if you create an electrical disturbance, the same kind of an effect will be produced; that is to say, waves in the ether will be created, or propagated, and will travel on and on in every direction. now, if some form of electrical appliance can be made that will be of the right kind to respond to them (as the eye responds to light rays), these electric waves can be made practically useful for transmitting messages through space. this is just what has been done, and we will now give you a brief general description of one kind of apparatus used. for "sending," or "transmitting," as it is usually termed, there is used an induction-coil, having rather large brass balls on the secondary terminals; suitable batteries, a condenser, a morse telegraph key, and an "aerial," or wire which is carried away up into the air vertically, and is made fast to a pole or special tower. when these are connected properly, the closing of the circuit with the key will cause sparks to jump between the brass balls. this electrical discharge, or oscillation, is carried by the aerial into the upper air and causes intense pulsations in the ether, which set up waves as already mentioned. if the circuit is opened again the disturbance ceases. so, by alternately closing and opening the circuit, the morse characters can be imitated. but how can these signals be received by the man for whom they are intended, who may be a hundred miles or more away? he has a "receiving" set, consisting of a sensitive relay, batteries, resistance-coils, a morse register, an aerial, and a special device called a "coherer." this is the important part of the whole set, because it is sensitive to the electrical waves. it consists of a little glass tube about as large around as an ordinary lead-pencil, and perhaps two inches long. in the tube are two metallic plugs, each having a wire attached so that one wire projects from each end of the tube. the plugs are separated inside the tube by a very small space, and in this space are some metal filings. one wire from the coherer is connected to the aerial and the other to the ground. when there are no electrical ether waves to influence them, these filings, being loosely separated, are at rest and offer high resistance; but when the ether is disturbed by electrical vibrations and the waves arrive at the coherer (through the aerial), these filings are drawn together, or cohere. this lowers their resistance and they become a better conductor. now, the coherer wires are also connected through a battery to the relay, which in turn is connected through another battery to a morse register. therefore, when the filings become a conductor, the current flows through them and the circuit to the relay is closed. that attracts an armature which closes the circuit of the morse register and thus marks the electrical impulse on a strip of paper tape. in the mean time, a restoring device, called a "decoherer," operated also by the relay circuit, has tapped upon the coherer, thus shaking the filings loose again, so that they are ready to cohere again and register another impulse, or character. thus, by pressing the key at the transmitting end for long or short periods, to represent morse characters, long and short waves are propagated in the ether and are received and recorded at the receiving end through the coherer and other parts of the receiving set. in this way telegraphic messages are sent and received through space, between points separated by hundreds or thousands of miles. we have tried to describe to you the general principles underlying the art of wireless telegraphy as plainly as possible, using for illustration the simplest kind of apparatus employed for the practical sending and receiving of messages. at the present day there are several systems in actual practice, and with the growth of the art there have been many elaborations of apparatus that have come into use. for instance, the coherer is not as much used as formerly. in its place there are employed several kinds of "wave-detectors" as they are now termed, and in many of the systems the electrical pulsations are generated by a dynamo-machine instead of batteries. then, again, instead of the messages being recorded by a morse register at the receiving end, the operator receives them by means of a telephone receiver, through which he hears the morse characters and writes them down in words as he hears them. generally the aerial, or "antennæ," as it is sometimes named, consists of several wires, sometimes a large number, carried to a considerable height. there are a great many other details which might be written to explain all the complicated apparatus which is used in some of the systems, but it is not intended in this book to offer more than a general explanation of main principles. we must leave it to you to study the details elsewhere if you so desire after you have read these pages. vi the telephone you probably all know that the telephone is an electrical instrument by which one person may talk to another who is at a distance. not only can we talk to a person who is in a different part of the city, but such great improvements have been made in these instruments that we can talk through the telephone to a person in another city, even though it be hundreds of miles away. the main principle of the telephone is electromagnetism, as in the telegraph, but there are other important points in addition to those we mentioned in describing the latter. let us take first the induction-coil you will remember that an electromagnet is made by winding many turns of wire around a piece of iron and sending a current of electricity through this wire. now, suppose this current of electricity was being supplied by two cells of a battery. if you took in your hands the wires coming from these _two cells_, giving, say, four volts, you could not feel any shock; but if you were to take hold of the ends of _the wires_ on the _electromagnet_ and _separate_ them while this same current was going through, you would get a decided shock. this separation would "break" the circuit, and the reason you would get a shock is that, while the electricity is acting on the wire, the iron itself is magnetized, and on breaking the circuit reacts upon the wire, producing for a moment more volts of pressure in every turn of it. thus, you see, this weak pressure of electricity as it travels through the wire can yet produce, through its magnetism, strong momentary effects, but _you cannot feel it unless you break the circuit_. how the induction-coil is made the object of the induction-coil is to produce high intensity, or pressure, from a comparatively weak pressure and large current of electricity; so, if we add still more wire, the magnet has a larger number of turns to act upon and thus makes a very strong pressure, or large number of volts, but a lesser number of ampères. instead of taking one piece of iron, as we would for an ordinary electromagnet, we take a bundle of iron wires in making an induction-coil, as these give a stronger effect. around this bundle of wires we wrap many turns of insulated copper wire. this is called the _primary coil_, and the ends of this wire are to be attached to the battery. [illustration: fig. ] on top of, or over, this primary coil we wrap a great many turns of very fine wire, of which, as it is so fine, a great length can be used. this is called the _secondary coil_, and it is in this coil that the volts, or pressure, of electricity become strongest. above we show you a sketch of an induction-coil. (fig. .) at the left-hand side of the cut is a "circuit-breaker," which is simply a piece of iron (armature) on a spring placed opposite the iron core. this armature is made a part of the wire leading to the primary coil. when the current from the battery is sent through the wires, the core becomes magnetized and draws this armature away from a fixed contact point, thus breaking the circuit, but the spring pulls it back, again completing the circuit, and so it keeps going back and forth very rapidly with a br-r-r-ing sound. if you were now to take hold of the ends of the secondary coil you would get a continuous series of quick shocks which would feel like pins and needles running into you. perhaps most of you have taken hold of the handles of a medical battery and have had shocks therefrom. in so doing, you have simply had the current from the secondary of an induction-coil. the current may be made weaker by sliding a metallic cover over part of the iron core and so shutting off part of the magnetic effect. sparking coils while on this subject we may add that these coils will produce sparks from the two ends of the wire of the secondary coil. these sparks vary in length according to the amount of wire in the coil. small ones are made which give a spark a quarter of an inch in length, while others are made which will give sparks , , and inches in length. in the latter, however, there are many miles of wire in the secondary coil. the largest induction-coil known is one which was made for an english scientist. there are , turns, or miles, of wire in the secondary coil. with cells of grove battery this coil will give a spark inches in length. you may form some idea of the effect of this induction-coil when we state that if we desired to produce the same length of spark direct from batteries, without using an induction-coil, we should require the combined volts of pressure of , to , cells of battery. having explained to you briefly the induction-coil--how it is made and its action--we must ask you to bear these principles in mind, and presently we will tell you how it is used in the telephone. the next thing we shall try to explain will be the vibrating diaphragm did you ever take the end of a cane in your hand, raise it up over your head, and then bring it down suddenly and sharply, so that it nearly touched the ground, as though you were about to strike something? if not, try it now with a thin walking-cane or with a pine stick about three feet long and one-half inch thick, and you will find that there is a peculiar sound given out. it is not the stick that makes this sound, but it is owing to the fact that you have caused the air to vibrate, or tremble, and thus give out a sound. [illustration: fig. ] if you strike a tuning-fork sharply you will see the ends vibrate and a sound will be given. if you put your fingers on top of a silk hat and speak near it you will feel vibrations of your voice. every time you speak you cause vibrations of the air; and the louder and higher you speak the greater the number of vibrations. suppose you take a thin piece of wood in your hands (say, for instance, the lid of a cigar-box cut in the shape shown in the picture, fig. ) and hold it about two inches from your mouth and then speak. you will feel the wood tremble in your hand. this is because the vibrations of the air cause the wood to vibrate in the same manner. these vibrations are very minute and cannot be seen with the naked eye, but they actually take place, and could be measured with a delicately balanced instrument. [illustration: fig. ] now let us try another experiment in further illustration of this principle. we will take a tube about three inches long and one and one-half or two inches in diameter. this tube may be made of cardboard. now cut out a piece of thin cardboard which will just fit over one end of the tube. this piece we will call the "diaphragm." fasten the diaphragm by pasting it with two strips of thin paper to the tube. these strips of paper should be fastened only on the ends, and the middle of the paper allowed to be slack, as shown in the picture, so that the diaphragm may work backward and forward easily. take a small shot about the size seen in the sketch and tie it to a single thread of fine silk, then let it hang as shown in the sketch (fig. ), so that it will only just touch the diaphragm. now, if you speak into the open end of the tube the diaphragm will vibrate and the shot will be seen to move to and from it according to the strength of the vibrations. if we could by any means make a diaphragm in another tube reproduce these same vibrations, we should hear the same words respoken, if the tube were held to the ear. [illustration: fig. ] while the vibrations caused by the human voice are too minute to be seen, it may seem surprising that they can be made to produce power. this is done by an ingenious mechanism called a phonomotor, perfected by the great inventor thomas a. edison, of whom every one has probably heard. this mechanism, when spoken or sung at (or into) immediately responds by causing a wheel to revolve. no amount of blowing will start the wheel, but it can instantly be set in motion by the vibrations caused by sound. the phonomotor (which is shown in the engraving fig. ) has a diaphragm and mouthpiece. a spring, which is secured to the bedpiece, rests on a piece of rubber tubing placed against the diaphragm. this spring carries a pawl that acts on a ratchet or roughened wheel on the fly-wheel shaft. a sound made in the mouthpiece creates vibrations in the diaphragm; the vibrations of the diaphragm move the spring and pawl with the same impulses, and as the pawl thus moves back and forth on the ratchet-wheel it is made to revolve. the instrument, therefore, is of great value for measuring the mechanical force of sound waves, or vibrations, produced by the human voice. the transmitter that part of the telephone into which we speak is called the transmitter. this is usually a piece of hard rubber having a round mouthpiece cut through it. at the other side of this mouthpiece is placed a diaphragm made of a thin piece of metal, which is held m place by a light spring. behind this diaphragm, and very close to it, is placed a carbon button. between this carbon button and the diaphragm is a small piece of platinum, which is placed so as to touch both the button and diaphragm very lightly. this platinum contact piece is connected with one of the wires running to the primary of the induction-coil, and the spring attached to the carbon button is connected with the battery to which the other wire of the primary is connected. this is all shown in the sketch of a transmitter. (fig. .) [illustration: fig. ] a is the mouthpiece; b, the diaphragm; c, the carbon button; d, the wire at the end of which is the platinum contact; e, the battery; and f, the induction-coil; p, p are the wires to the primary, and s, s to the secondary wires. we will now say a few words about the receiver, and then describe the manner in which the telephone works. the receiver this is that part of the telephone which is held to the ear, and by which we can hear the words spoken into the transmitter of the telephone at the other end of the line. [illustration: fig. ] the receiver is made of hard rubber, and contains a permanent bar magnet, which is wound with wire so as to make it also an electromagnet when desired. in front of this magnet is placed loosely a diaphragm of thin sheet iron. this diaphragm is placed so as to be within the influence of the magnet, but just so that neither one can touch the other. fig. is a sketch of the receiver. a and b are the wires leading to the magnet, c, and d is the diaphragm. e and f are where the wires connect, one from the secondary of the induction-coil in the other telephone, and the other connected with the earth. the carbon button the little carbon button plays an important part in the telephone. you will see from the sketch of the transmitter that the current of electricity will flow through the carbon button to the contact point and through the wire to the primary of the induction-coil. now, carbon has a peculiarity, which is this, that if we press this carbon button, ever so slightly, against the platinum contact, there would be less resistance to the flow of the electricity through the wire to the primary, and the more we press it the less the resistance becomes. the consequence of this would be that more current would go to the primary, and the secondary would become correspondingly stronger. if the carbon button were left untouched, and nothing pressed against it, the flow of current through it would be perfectly even. having examined the inside of the transmitter and receiver, and understanding the effect of pressure on the carbon button, let us now see how the telephone works when we speak into the mouthpiece of the transmitter, the vibrations of the air cause the diaphragm to vibrate very rapidly, and, of course, every movement of the diaphragm presses _more or less_ against the carbon button, in consequence of which the currents passing through the primary of the induction-coil are constantly increased or diminished and thus produce similar effects, but magnified, in the secondary. the effect of this is that the magnet in the receiver of the other telephone is receiving a rapidly changing current, which, producing corresponding magnetic changes, makes the magnet alternately weaker or stronger. this influences, by magnetism, the iron diaphragm accordingly, and makes it reproduce the same vibrations that were caused by the speech at the transmitter of the sending telephone. thus, the same vibrations being _reproduced_, the original sounds are given out, and we can hear what the person at the sending telephone is saying. the action of the telephone illustrates well the wonderfully quick action of the electric current by the reproduction of these sound waves, or air vibrations, for they number many thousands in one minute's speech. vii electric light we have now arrived at a very interesting part of the study of electricity, as well as a more difficult part than we have yet told you of, but one which you can easily understand if you read carefully. you must all have seen electric lights, either in the streets or in some large buildings, for so many electric lights are now used that there are very few people who have not seen them. but perhaps some of you have only seen the large, dazzling lights that are used in the streets, and do not know that there is another kind of electric light which is in a globe about the size and shape of a large pear, and gives about the same light as a good gas-jet. these two kinds of electric lights have different names. the large, dazzling lights which you see in the streets are called "arc-lights," and the small, pear-shaped lamps, which give a soft, steady light, are called "incandescent lights." we will tell you later why these names are given to them. [illustration: fig. ] the incandescent lights are generally used in houses, stores, theaters, factories, steamboats, and other places where a number of small lights are more pleasant to the eyes. the arc-lights (fig. ) are used to light streets and large spaces where a great quantity of light is wanted. it would not be pleasant to have one of these dazzling arc-lamps in your parlor--although it would give a great deal of light--because your eyes would soon become tired. but two or three of the small incandescent lights (fig. ) would be very agreeable, because they would give you a nice, soft light to read or work by, and would not tire your eyes. so, you see, these two different kinds of lamps are very useful in their proper places. now, if you will read patiently and carefully, we will try and explain how both these lights are made. [illustration: fig. ] you have seen that the telegraph, telephone, electric bells, etc., are worked by batteries. electric lights, however, require such a large amount of current that it is too expensive to produce them in large quantities by batteries. a small number of lamps could be lighted by batteries, but if we were to attempt to use them to light or , lamps together, the expense would be so enormous as to make it entirely out of the question. there are many millions of incandescent lamps in use in the united states, but you will easily see that there could not be that number used if we had to depend on batteries to light them. you will understand this more thoroughly when you have finished reading this little book. well, you will ask, if we cannot use batteries, what is used to produce these electric lights? machines called "dynamo-electric machines," or "generators," which are driven by steam-engines or water-power, are used to produce the electricity which makes these lamps give us light. you will remember that in the chapter on magnetism we explained to you how electricity makes magnetism, and now we will explain how, in the dynamo, magnetism makes electricity. [illustration: fig. ] it has been found that the influence of a magnet is very strong at its poles, and that this influence is always in the same lines. this influence has been described as "lines of force," which you will see represented in the sketch above by the dotted lines (fig. ). of course, these lines of force are only imaginary and cannot be seen in any magnet, but they are always present. the meaning of this term "lines of force," then, is used to designate the strength of the magnet. many years ago the great scientist faraday made the discovery that, by passing a closed loop of wire through the magnetic lines of force existing between the poles of a magnet, the magnetism produced the peculiar effect of creating a current of electricity in the wire. if the closed loop of wire were passed down, say from u to d, the current flowed in the wire in one direction, and if it were passed upward, from d to u, the current flowed in the other direction. thus, you see, magnetism produces electricity in the closed loop of wire as it cuts through the magnetic lines of force. just why or how, nobody knows; we only know that electricity is produced in that way, and to-day we make practical use of this method of producing it by embodying this principle in dynamo-machines, as we will shortly explain. in carrying this discovery into practice in making dynamo-machines we use copper wire. if iron were used, there would be a current of electricity generated, but it would be much less in quantity, because iron wire has much greater resistance to the passage of electricity than the same size of copper wire. perhaps you can understand it more thoroughly if we state that when a closed loop of wire is passed up and down between the poles of a strong magnet there is a very perceptible opposition felt to the passage of the wire to and fro. this is due to the influence of the magnetism upon the current produced in the wire as it cuts through the lines of force, and, inasmuch as these lines of force are always present at the poles of a magnet, you will see that, no matter how many times you pass the loop of wire up and down, there will be created in it a current of electricity by its passage through the lines of force. [illustration: fig. ] suppose that, instead of using one single loop of copper wire, you wound upon a spool a long piece of wire like that in fig. , and that you turned this spool around rapidly between the poles of the magnet, you would thus be cutting the lines of force by the same wire a great many times, and every time one length of the wire cut through the lines of force some electricity would be generated in it, and this would continue as long as the spool was revolved. but, as each length would only be a part of the one piece of wire, you will easily see that there would be a great deal of electricity generated in the whole piece of wire. [illustration: fig. ] all we have to do, then, is to collect this electricity from the two ends of the wire, and use it. if we should attach two wires to the two ends of this wire on the spool, they would be broken off when it turned around, so we must use some other method. we fix on the end of the spool (which is called an "armature") two pieces of copper, so that they will not touch each other (as in fig. ), and fasten the ends of the wire to these pieces of copper. this is called a "commutator," and, as you see, is really the ends of the wire on the spool. now we get two thin, flat pieces of copper and fix them so that they will rest upon the copper bars of the commutator, but will not go round with it. these two flat pieces of copper are called the "brushes," and they will collect from the commutator the electricity which is gathered in the wire around the spool. as the brushes stand still, two wires can be fastened to them, and thus the ampères of current of electricity, acted upon by the volts pressure, can be carried away to be used in the lamps, for you must remember that as long as the spool turns around it gathers more electricity while there is any magnetism for the wire on the spool to pass through. the constant revolving of the spool creates so much electricity that it is driven out from the wire on the spool, through the commutator to the brushes, and there it finds a path to travel away from the pressure of the new electricity which is all the time being made. in this way we get a continuous current of electricity in the two wires leading from the commutator, and can use it to light electric lamps or for other useful purposes. in explaining this to you, so far, we have used as an illustration of the magnet one of the steel permanent magnets in order to make the explanation more simple, but now that you understand how the electricity is made, we must explain to you something about the magnets that are used in dynamo-machines. we can perhaps make this more clear by giving another example. suppose you had a dynamo which was lighting up of the incandescent lamps, each of ohms resistance and each requiring volts pressure. now each lamp would take just a certain quantity of electricity, say half an ampère; so, the lamps would require one hundred times that quantity. but, if you turned off of these lamps at once, the tendency would be for the pressure to rise above the volts required for the other , and they would be apt to burn out quicker. it is plainly to be seen, then, that we must have some means of regulating the magnetism so as to regulate the lines of force for the wire on the armature to cut through. we can do this with an electromagnet, but not with a permanent magnet, because _we cannot easily regulate the amount of magnetism which a permanent magnet will give_. there is another reason why we cannot use permanent magnets in a dynamo, and that is because _they cannot be made to give as much magnetism as an electromagnet will give_. thus you will see that there are very good reasons for using electromagnets in making dynamo-machines. let us see now how these electromagnets and dynamos are made, and then examine the methods which are followed to operate and use them. you must remember, to begin with, that in referring to wire used on magnets and armatures and for carrying the electricity away to the lamps, we always mean wire that is _covered_ or _insulated_. in electric lighting, insulated wire is _always_ used, except at the points where it is connected with, the dynamo, the lamps, a switch, or any point where we make what is called a "connection." as the shape of the magnets is different in the dynamos of various inventors, we will take for illustration the one that is nearest the shape of the horseshoe and the shape that is generally used in illustrating the principle of the dynamo. this is the form used by mr. edison, whom we have previously mentioned. this form is shown in fig. . now, although this magnet appears to be in one piece, it really consists of five parts screwed together so as to make, practically, one piece. the names of the parts are as follows: f, f are the "cores"; c the "yoke," which binds them together; and p, p the "pole pieces," where the magnetism is the strongest. these pole pieces are rounded out to receive the _armature_, which, as you will remember, is the part that turns around. [illustration: fig. ] the cores, f, f, are first wound with a certain amount of wire, which depends upon the use the dynamo is to be made for. thus, you will see, there will be on each core two loose ends of the wire that is wound around it--namely, the beginning of the wire and the end where we leave off winding, which on the two cores together will make four ends of wire. we will tell you presently what is done with them. after the cores are wound, they are screwed firmly to the yoke and to the pole pieces, so as to make, for all practical purposes, one whole piece pretty nearly the shape of a horseshoe magnet. [illustration: fig. ] now, to make the dynamo complete, we must put in the armature between the poles, which are rounded off, as you will see, to accommodate it. the armature is held up by two "bearings," which you will see in the sketch of the complete dynamo above. (fig. .) the armature in a practical dynamo-machine consists of a large spool made of thin sheets of iron firmly fastened together and having a steel shaft run through the center, upon which it revolves. this spool, or armature, is wound with a number of strands of copper wire. the commutator, instead of consisting of two bars, is made in many dynamos with as many bars as there are strands of wire, and the ends of these wires are fastened to the bars of the commutator so as to make, practically, one long piece of wire, just as we showed you in explaining how the electricity was produced. the brushes, resting upon the commutator, carry away the electricity from it into the wires with which they are connected. now we have our dynamo all put together and ready to start as soon as we properly connect these four loose ends of wire on the cores. if you will turn back to fig. you will see that two of the wires are marked i, and the other two o. the letter i means the inside wire, or where the winding began, and the letter o means the outside wire, or where we left off winding. now, if we fasten together (or "connect") the two ends of wires, i and o, near the top of the magnet, we make the two wires round the cores into one wire, which starts, say, at i near the poles, goes all around one core, crosses over and around the other core down to the other end of the wire to o, near the poles. so far we have called the iron a magnet, although it is not a magnet until electricity is put into it; so, when the dynamo is started for the first time, these two ends of wire, i and o, are connected to a battery or other source of current for the purpose of sending electricity through the wire on the cores. when the electricity goes into this wire the iron immediately becomes a magnet, and the lines of force are present at the poles. now, the armature is turned around rapidly by a steam-engine, and, as the wire on the armature cuts the lines of force with great rapidity and so frequently, there is quickly generated a large quantity of electricity, which passes out as fast as it is made through the commutator and the brushes to the lamp. and so long as the armature is revolved and the battery attached, the electricity will be made, or, as it is usually termed, "generated." as we stated above, a battery is used _the first time the dynamo is run_, and now we will explain why it is not needed afterward. although iron will not become a permanent magnet, like steel, it _does not lose all its magnetism_ after it has been once thoroughly charged. when the dynamo is stopped, after the first trial, and the battery is taken away, you will discover only traces of magnetism about the poles. they will not readily attract even a needle or iron filings; but there is, nevertheless, a very small amount of magnetism left in the iron. small as this magnetism is, however, it is enough to make very faint and weak lines of force at the poles of the magnet. after the battery is taken away, the ends of the wire on the cores, which were connected to the battery, are connected, instead, to the wires which carry away the electricity from the brushes to the lamps. thus, you will see, if any electricity goes from the dynamo to the lamps, part of it must also find its way through the wires which are around the cores. we will now start up the dynamo without having any battery attached and see what happens. the armature turns around and the wires upon it cut through those very faint lines of force which are always at the poles. this, as you know, makes some electricity; very little, to be sure, but it comes out through the brushes to the wires leading to the lamps, and there it finds the wires leading back to the cores. well, part of this weak current of electricity goes into these wires and travels back round the cores and so makes the magnetism stronger. the consequence of this is that the lines of force become stronger and, as the armature keeps turning around, the electricity naturally becomes stronger, and so there is more of it going through the wires back to the cores and increasing the strength of the magnet all the time, until the dynamo becomes strong enough to generate all the current it was intended to give for the lamps. of course, you understand that the stronger the magnet becomes, the greater will be the lines of force and the greater the amount of electricity made by the turning of the armature. now, there is naturally a limit to what can be done with any particular dynamo; so, while the electricity continues to strengthen the magnetism and the magnetism increases the electricity, this cannot go beyond what is called the "saturation" point of the magnet. saturation means that the iron is full of magnetism, and will hold that much but no more. you will learn more as to the saturation of magnets when you study electricity more deeply, and we therefore do not intend to enter into that subject in this book. we will only state, however, that the magnets of dynamos are not always charged up to their saturation point. the lamps so far you have learned how the current of electricity is produced, and now we will follow along the wires to find out how it makes the lamps give out both strong lights and the smaller, pleasant ones. suppose we take first the large, dazzling lights we see in the streets, which, as you know, are called arc-lights those who have seen the arc-lamps will readily recognize them from the picture in fig. . you will see that there are two sticks, or "pencils," of carbon. now you will remember that in the chapter on magnetism we told you that _in order to have electricity do work for us we must put some resistance or opposition in its way_. when we get light from an electric lamp it is because we make the electricity do some work in the lamp, and this work is in pushing its way through a resistance or opposition which is in the lamp. [illustration: fig. ] when we generate electricity in the dynamo and put two wires for it to travel in, the current goes away from the dynamo through one of the wires and will go back to the dynamo through the other one if it can possibly get a chance to get to this other one. now, the electricity which is constantly being made fills the wires and acts as a pressure to force the current through the wires back to the dynamo, and, if we put no resistance or opposition in the way, it would have a very easy path to travel in and would do no work at all. the wires leading to an electric lamp should have very little resistance, not sufficient to require any work from the current in passing through. so, if we bring the two carbons in an arc-lamp together they really form part of the wire, and do not interrupt the current in its travels, but, if we _separate the carbons_, we make a gap which the current must jump across if it wants to go on. as the volts, or pressure, is so great, the current must jump, and this _against the resistance or opposition_ in an arc-lamp is that which gives the current so much work to do. indeed, so hard is it for the current to jump across this gap that it breaks off from one carbon a shower of tiny particles as fine as the finest dust, and makes them white hot in passing to the other. this shower of fine carbon dust, together with the ends of the carbons, being white hot, of course makes a light, and this is the dazzling light which you see in the arc-lamp. of course, when the electricity has jumped over from one carbon to the other, it goes through it to the wire, and so passes on to the next lamp, where it has to jump again, and so on until it has gone through the last lamp, then it has an easy path to get back to the dynamo. now, we want you to understand more thoroughly how that much resistance or opposition will cause heat, so we will try to give you a simple example. most of you know that if you were holding a rope tightly in your hands and some one pulled it through them quickly and suddenly, it would get very hot and your hands would feel as though they were being burned. this is heat caused by your hands resisting or opposing the passage of the rope through them, and if you could hold on tightly enough and the rope was drawn through quickly enough, it would take fire. this fire would, therefore, cause heat and light. it is just this principle of resistance to the passage of the current which causes the light in an arc-lamp, as we have shown you. incandescent lamps you have just learned that the light in an arc-lamp is caused by the current forcing off from the carbon sticks tiny particles and heating them up until they give a brilliant light. so, you see, in an arc-light there is a wearing away of carbon by electricity, and therefore these sticks, or pencils, of carbon in time are all burned away. in practice the carbon pencils last about eight or ten hours, and then new ones must be put in. now, in the incandescent lamp there is also carbon used, but the light is not produced by the combustion or wasting away of the carbon, as we will show you. the picture below will show you the appearance of an incandescent lamp. (fig. .) [illustration: fig. ] you will see that this lamp consists of a pear-shaped globe, and inside is a long u-shaped strip of carbon no thicker than an ordinary thread. this is a strip of bamboo cane[ ] which has been carbonized to a thread of charcoal. it is joined to two wires which come through the glass. these two wires come down through the bottom of the globe, and one is fastened to a brass screw-ring, while the other wire is fastened to a brass button at the bottom of the lamp. these two (the ring and button) must, as you know, be separated from each other by something which will not carry electricity, or they would make a short circuit when the electricity was applied. we separate the ring and the button in various ways. now, if we took the ends of two wires which were charged with the proper amount of electricity and put one wire on the screw-ring and the other on the button, the lamp would light up, because there would be a complete path for the current to travel in. [illustration: fig. ] it will, however, be plain to you that it would be awkward to light the lamps in this way, so we use a "socket" into which the lamp is screwed. (fig. .) the wires from the dynamo carrying the electricity are connected in the socket, one wire with the screw thread into which the screw-ring fits, and the other with a button which the button on the lamp touches when the lamp is screwed into the socket. thus we have a connected path for the current to travel in, or, as it is termed, a _complete circuit_. you will notice that in the incandescent lamp the electricity does not need to jump, as it does in the arc-light, because we give it one continuous line to travel in. in order, however, to get the current to do work for us, we put some resistance in its path, which it must overcome in order to travel back to the dynamo. the resistance in an incandescent lamp is the u-shaped carbon strip (or, as it is called, "filament"). this charcoal filament has so much greater resistance than the wires that it opposes, or resists, the passage of the electricity through it; but the electricity _must_ go through, and, as it is strong enough to force its way, it overcomes this resistance and passes on through the carbon to the wire at the other end. you see it is a struggle between the carbon and the electricity, the current being determined to go on and the carbon trying to keep it back; and, in the end, the electricity, being the stronger, gets the best of it; but the struggle has been so hard that the carbon has been raised to a white heat, or incandescence, and so gives out a beautiful light, which continues as long as the current of electricity flows. you will remember that in the arc-light the carbons are slowly consumed and new ones must be put in. if the carbon in the incandescent light were consumed, it would not last many minutes, because it is only about the size of a horsehair. now, you will naturally inquire why this fine strip is not burned up when it is raised to so high a heat. well, we will tell you. you know that if you light a match and let it burn the wood will all be consumed. but did you ever light a match, put it into a small bottle, and put the cork in? if you never did, do so now as an experiment, and you will see that the match will keep lighted for an instant and then go out without consuming the wood. the reasons for this are very simple. in order to burn anything up entirely it is absolutely necessary to have the gas called oxygen present, and, as the air you live in contains a very large amount of oxygen, there is more than sufficient in your room to cause the wood of the match to be entirely consumed after it is lighted. but there is such a small quantity of oxygen in the bottle that it is not enough to keep the fire going in the match, and, consequently, it will not burn up the wood. the reason the filament in an incandescent lamp is not burned up is because there is _no oxygen_ inside the globe. after the carbon is put in its place all the oxygen is drawn out through a tube, and the glass is sealed up so that no more oxygen can get in. this is called obtaining a "vacuum," and vacuum means a space without air. there being no oxygen in the globe, it is impossible for the carbon to burn up; so the incandescent lamp will continue to give its light for a very long time, some of them lasting for thousands of hours. some day, however, from a great variety of obscure causes, the filament becomes weak in some particular spot and breaks, and the light ceases. when this happens, we unscrew the lamp and put another one in, and the light goes on as usual. now you have learned how the incandescent lamp is made to give light. we will add that it is a beautiful, soft, white light, almost without heat, it will not explode, throws off no poisonous fumes like gas or oil lamps, and has many other points of comfort and convenience which make it very desirable. electric-light wires before closing the subject of electric light you would perhaps like to know something about the way in which we place the wires leading to the lamps. [illustration: fig. ] if you remember what we told you about measurements in the beginning of this book, it will be easy to understand what follows: you know that if you have a very great pressure you can force a quantity through a small conductor. this is the principle upon which the arc-lamps are run. every arc-lamp takes about to volts and from to ampères to produce the light, and they are connected with the wires as shown in fig. . this is called running lamps in "series," and, as you will see from the sketch, the wire starts out from the dynamo and connects with one carbon of the first arc-lamp, and to the other carbon is connected another wire which goes on to the next lamp, and so on until the last lamp is reached, and then the wire goes back to the dynamo. this forms, practically, one continuous loop from one brush to the other of the dynamo. the current starts out, makes its way through the first lamp, goes on to the next, makes its way through that, and so on till it has jumped the last one; then it goes back to the dynamo. now, as each of these jumps requires a pressure of or volts, you will easily see that the total pressure, in volts, of the electricity must be as many times or volts as there are lamps to be lighted; so, if there were lamps in circuit, there would be , to , volts pressure, which, while it gives very fine lights, might cause instant death to any one touching the wires. suppose anything happened to the first lamp, which stopped the current from jumping through it. there would be no path for the current to travel farther, and, consequently, all the lights would go out. to get over this difficulty there is sometimes used what is called a "shunt," which only acts when the lamp will not light. this shunt carries the current round the lamp to the other wire, so that it may travel on and light up the other lamps. wires for incandescent lamps the wiring for incandescent lamps is carried out in an entirely different way, which you can see by comparing fig. a with fig. which shows the wiring for arc-lamps. [illustration: fig. a] this is called connecting in "multiple arc." you will notice that the two wires running out from the dynamo (which are called the main wires) do not form one continuous loop as in the arc-light system, but that a smaller wire is attached to one of the main wires and then connected with the screw-ring in the lamp-socket; then another wire is connected with the button in the socket and afterward to the other main wire. every lamp forms an independent path through which the current can travel back to the dynamo. now, if we turn one of these incandescent lamps out, we simply shut off one of these paths and the electricity travels through the other lamps, and, if we wish, we can turn out all the lamps but one and there will still be a way for the electricity to go back to the dynamo. in the arc-lamps we must have a very high number of volts pressure, because the electricity has only one path, and it all has to pass through the first and other lamps till it comes to the last one. in the incandescent light the electricity has as many paths as there are lamps, so we only need to keep _one_ certain _pressure_ in volts in the main wires all the time. this pressure is _even_ all the way through the main wires, and, therefore, it is ready to light a lamp the instant it is turned on, because, as you have seen, electricity will always get back to the dynamo if there is a possible chance, and the lamp opens a path. the volts pressure used to operate any number of incandescent lamps is altogether very much less than for a number of arc-lights. for example, in the edison system the pressure (sometimes called "electromotive force") is only about volts, which is very mild and not at all dangerous. this electromotive force would be _the same_ if there were _one lamp or ten thousand_ lighted. while this edison current would not hurt any one, you should remember that it is much the better plan not to touch _any_ electric-light wires until you have learned a great deal more on this subject. we may add that each of the standard incandescent lamps requires only about one-quarter of an ampère of current to make them give a light of candle-power, which is about the light given by a very good gas-jet, and while the electromotive force, or pressure, would only be about volts, whether there were one lamp or ten thousand lighted, there must be sufficient ampères in the wires to give each lamp its proper quantity. switches we have made mention several times of turning on or off one or more lights, and now, perhaps, you would like to know how this is done. suppose the electricity was traveling through wires to one or several lamps, it would light up those lamps as long as the wires provided a path to travel in, but if you were to cut out one of them, which is called "breaking the circuit," there would be no road for the electricity to follow, and, consequently, its course would be stopped short and the lamps would go out. you will remember that _electricity must have a complete circuit_ or it can do no work, and in electric lighting it is always a _metallic circuit_ that is used. now, the switch is simply a device which is used to break the circuit so that the current cannot pass on. the simplest form of switch is seen in the sketch. (fig. .) [illustration: fig. ] you will see that there is a wire cut in two, and to one piece is attached a metallic piece, a, which turns one way or the other, and when it is turned so as to touch the other part of the wire the circuit is closed and the electricity goes from the lower part of the wire through the metallic piece a to the other part of the wire, thus making a complete circuit or path for the electricity to travel in. if we turn the piece a away from the upper wire this breaks the circuit and cuts off the path, and, of course, the lamps would go out. this is the principle of the switch, and, although they are made in thousands of ways, switches all have the same object--namely, the closing and breaking of the circuit, whether it is for one or a hundred lamps. wire on dynamos in explaining to you the construction and working of dynamo-machines, we did not state anything about the amounts of wire used in winding the machine. it is not our intention to say exactly how much is used on any one dynamo, because that is among the things you will have to learn when you come to study the subject of electricity more deeply. we simply want to have you understand that upon the number of turns of wire on any one machine depends the effect that that amount of wire, carrying electricity, will have upon a certain weight of iron when the armature is revolved a certain number of turns per minute. a certain number of strands of wire on an armature will only do a certain amount of work at the most, so you will see that a small dynamo will not produce as much electricity as a larger one containing more iron and wire. for high pressure there must be more strands of wire cutting the lines of force more frequently than would be required for low pressure; and, to produce a great many ampères, the armature must be larger and the wire upon it thicker than it would need to be if only a small number of ampères were wanted. this of itself is a very deep and complicated subject, and many books have been written upon it alone. we shall, therefore, not attempt to go more deeply into it in this little book, but simply content ourselves with giving you the general idea, which will be sufficient until you make a thorough study of the subject. viii electric power one of the most convenient uses to which electricity is put is in producing motive power for driving all kinds of machines, from a sewing-machine to a railway train, and we will now try to explain how we can get this kind of work from electricity. to begin with, you all know that a piece of machinery is usually made to work by revolving a wheel which is part of the machine, either by means of a steam-engine or by water-power, or, as a sewing-machine, by foot-power. now, when we work a piece of machinery by electricity we do just the same thing by using, instead of the steam-engine or water or foot power, an electric-engine called an "electromotor," which operates in the same way--namely, by turning the wheel of the machine it is applied to. foot-power is hard work for the person who is applying the power, and, as you can easily see, one person can make only a very little power by use of the feet. steam and water power can be used for any large amount of work, but the work must be within a few hundred feet of the engine or the power cannot be used. if there were a factory using steam-power a block or two away from where you lived, and you had a lathe in your house which you would like to have run by the steam-power in the factory, it would be practically impossible to do this. now, if the factory were still farther away from your house, it would be still more impossible, and if it were a mile away it would be foolish to dream of taking steam-power from a place so far away. suppose, however, that this factory was lighted by electric lights, it would be a very easy matter to take some of the power over to your house. this could be done, even if the factory were miles away, by taking two wires from their electric-light wires and running them into your house to an electromotor connected with your lathe. this electromotor would then run your lathe just as well as if it were belted to a steam-engine. so, you see, power can be carried in the form of electricity through two wires over very great distances and made to do work at a long way from the engine which is turning the dynamo to make the electricity. thus, you may have brought into your house wires which will give lights and, at the same time, power to run a sewing-machine, a lathe, or any other piece of machinery. having learned so far that a dynamo will make a continuous current of electricity, and that two wires will carry this current to any place where it is wanted, let us now see what takes place in the electromotor to transform the electricity into power. an electromotor (which we will now call by its short name, motor) is simply a machine made like a dynamo. curious as it may seem to you, it is a fact that if you take two dynamo-machines exactly alike, and run one with the steam-engine so as to produce electricity, and then take the two main wires and attach them to the brushes of the other dynamo, the electricity will drive this other dynamo so as to produce a great deal of power which could be used for driving other machines. thus, the second dynamo would become a motor. in the chapter on dynamos we explained something about the way they were made and how the electricity was produced. the motor you will remember that the armature consists of a spool wound with wire. this spool is made of iron plates fastened together so as to form one solid piece. the armature of a motor may be made in the same way; in fact, the whole motor is practically a dynamo-machine. there is something more about magnetism which we will tell you of here, because you will more easily understand it in its relation to an electromotor. if we take an ordinary piece of iron and bring one end of it near to (but not touching) one pole of a magnet, this piece of iron will itself become a weaker magnet as long as it remains in this position. this is said to be magnetism by "induction." the end of the piece of iron nearest to the magnet will be of the opposite polarity. for instance, if the pole of the magnet were north, the end of the iron which was nearest to this north pole would be south, and, of course, the other end would be north. to make this more plain we show it in the following sketch. (fig. .) this would be the same whether the magnet were a permanent or an electromagnet. you will remember also that the north pole of one magnet will _attract the south pole_ of another magnet, but will _repel a north pole_. these are the principles made use of in an electromotor, and we will now try to show you how this is carried into practice. [illustration: steel permanent magnet iron fig. ] although a motor is made like a dynamo, we will show a different form of machine from the dynamo already illustrated, because it will help you to understand more easily. (fig. .) here we have an electromagnet with its poles, and an iron armature wound with wire, just as in the dynamo we have described, except that its form is different. [illustration: fig. ] a commutator and brushes are also used, but the electricity, instead of being taken away from the brushes, is taken _to_ them by the wires connected with them. two wires are also connected which take part of the electricity around the magnet, just as in the dynamo. now, when the volts pressure and ampères of electricity coming from a dynamo or battery are turned into the wires leading to the brushes of the motor, they go through the commutator into the armature and round the magnet, and so create the lines of force at the poles and magnetize the iron of the armature. let us see what the effect of this is. the poles of the magnet become north and south, and the four ends on the armature also become north and south, two of each. by referring to fig. again we shall see what takes place. the north pole of the magnet is doing two things: it is repelling, or forcing away, the upper north pole of the armature and at the same time drawing toward itself the lower south pole of the armature. in the mean time the south pole of the magnet is repelling the south pole of the armature and at the same time drawing toward itself the north pole of the armature. this, of course, makes the armature turn around, and the same poles are again presented to the magnet, when they are acted upon in the same manner, which makes the armature revolve again, and this action continues as long as electricity is brought through the wires to the brushes. thus, the armature turns around with great speed and strength, and will then drive a machine to which it is attached. the speed and strength of the motor are regulated by the amount of iron and wire upon it, and by the volts pressure and ampères of electricity supplied to the brushes. motors are made from a small size that will run a sewing-machine up to a size large enough to run a railway train, and are often operated through wires at a great distance from the place where the electricity is being made, sometimes miles away. they are also made in a great many different forms, but the principle is practically the same as we have just described to you. ix batteries so far we have only described one way of producing electricity--namely, by means of a dynamo-machine driven by steam or water power. the supply of electricity so obtained is regular and constant as long as the steam or water power is applied to the dynamo. there is another and very different way of producing electricity, and this is by means of a chemical process in what is called a battery. to obtain electricity from the dynamo we must spend money for the coal to make the steam which operates the steam-engine, or for the water which turns the water-wheel, as well as for an engineer in both cases. when we obtain electricity from a battery we must spend money for the chemicals and metals which are constantly consumed in the battery. primary batteries an electrical battery is a device in which one or more chemical substances act upon a metal and a carbon, or upon two different metals, producing thereby a current of electricity, which will continue as long as there is any action of the chemicals upon the metal and carbon, or upon the two metals. batteries for _producing_ electricity may be divided into two classes, called "open circuit" batteries and "closed circuit" batteries. open-circuit batteries are those which are used where the electricity is _not_ required constantly without intermission--for instance, in telephones, electric bells, burglar alarms, gas-lighting, annunciators, etc. closed-circuit batteries are those which are used where the effect produced must be continuous every moment, as, for instance, in electric lights and motors. the open-circuit battery is made in many different ways, so we only describe two of the principal ones. as we told you in an early part of this book, we do not know just what electricity is, nor why it is produced under the conditions existing in a battery. but we do know that by following certain processes and making certain chemical combinations we can make as much electricity and in such proportions as we want. the two metals, or the metal and carbon, in a battery are called the "elements," and to these are connected the wires which lead from the battery to the instruments to be worked by it. _the leclanché battery._--this form of open-circuit battery consists of a glass jar in which is placed the elements. one element consists of a rod of zinc, and the other element is carbon and powdered black oxide of manganese. these two (the carbon and black oxide of manganese) are placed in an earthenware vessel called a "porous cup." this is simply a small jar made of clay which is not glazed. thus, the liquid which is in the glass jar penetrates through the porous cup to the carbon and manganese which it contains, and so the chemicals affect both these and the zinc at once, for, in order to obtain electricity, you will remember that the chemical action must take place at the same time upon both the elements in the same vessel. (fig. .) the chemical substance used in this battery is sal-ammoniac, or salts of ammonia. a certain quantity of this salt is dissolved in water, and this solution is poured into the glass jar. when this is done the battery will generate electricity at once. [illustration: fig. ] it should be remembered that the proper term for the chemical mixture which acts upon the elements in any battery is "electrolyte." _the dry battery._--the cleanliness, convenience, high efficiency, and comparatively low internal resistance of the dry cell has brought it into great favor in the last few years. it is now extensively used in preference to the leclanché and other open-circuit batteries having liquid electrolyte for light work, such as bells, gas-lighting, burglar alarms, ignition on motor-boats, automobiles, etc. the dry cell is also used in great numbers for pocket flash-lamps, and in other ways where it would be impossible to employ batteries containing liquids. a dry cell consists of zinc, carbon, and the electrolyte, which is a mixture so made that it is in the form of a gelatinous or semi-solid mass, so that it will not run or slop over. a piece of sheet zinc is formed into a long tube, and a round, flat piece of zinc is soldered at one end, thus making a cup open at one end. this forms the cell itself, and at the same time becomes one of the elements. the other element is a piece of battery carbon which is long enough to project out of the top of the cell about half an inch or more. while the cell is being filled with the electrolyte the carbon is held up by a support so that it does not touch the zinc at the bottom of the cup. of course, the zinc cup and the carbon are provided with proper binding-posts or other attachments, so that conducting wires can be connected. the electrolyte is packed into the cup and around the carbon in such a way that the cup is entirely filled within about half an inch from the top, and then some melted tar or pitch is poured over the top of the electrolyte. this seals the cell and binds the contents solidly together. just before the sealing compound hardens, one or two holes are made in it so that the gases may escape. the composition of the electrolyte itself is not exactly alike in all dry cells, as the various manufacturers follow their own particular formulas. however, as you may be curious to know something about it, we would state that one formula embraces flour, water, plaster of paris, granulated carbon, zinc chloride, ammonium chloride, and manganese binoxide. you will remember that the leclanché and the dry batteries are purely open-circuit cells, and that they can be used to advantage for electric bells, annunciators, burglar alarms, gas ignition, etc., where _the current of electricity is not doing_ continuous work, but only for a few seconds at a time. consequently, the batteries have a little rest in between, if only for a few seconds. now, if we were to attempt to use open-circuit batteries for electric lights or motors, where the electricity must work constantly every second, the batteries would "polarize"--that is to say, they would only work a few minutes and then stop, because the chemicals used in them are of that kind that they will only allow the battery to do a little work at a time. the batteries we have been describing will do the ordinary work for which they are intended for sometimes a year without requiring any attention, but if we try to make them do work for which they were not intended, they would only last a few days. if we should want to operate electric lights or motors continuously from a battery we must, therefore, use closed-circuit batteries there is a great variety of ways in which closed-circuit batteries are made, but, as the main principles are very much alike, we will only describe two general kinds, those with and those without a porous cup.[ ] in the first place, we must state that closed-circuit batteries proper usually consist of a glass jar and two elements--carbon and zinc. sometimes a porous cup is used; for what reason you will soon learn. the chemicals that are used are usually different from those used in the open-circuit batteries and are much stronger. these chemicals are usually sulphuric acid and bichromate of potash (or chromic acid), which are mixed with water. we will now examine two of the types of closed-circuit batteries, taking first the one without the porous cup, of which the grenet is a good example. [illustration: fig. ] this battery, as you see, consists of a glass jar, in which are placed two plates of carbon and one of zinc. (fig. .) the latter is between the two carbon plates and is movable up and down, so that it may be drawn up out of the solution when it is not desired to use the battery. when the zinc is in the solution there is a steady and continuous current of electricity developed, which can be taken away by wires from the connections on top of the battery. if the zinc were left in the solution when the battery was not in use, the acid would act upon it almost as much as though the electricity were not being used, and thus the zinc would be eaten away and the acid would be neutralized, so that no more action could be had when we wanted more electricity. now, in the grenet battery we can light a lamp or run a motor for several hours continuously, but at the end of that time the solution would become black and it would do no more work. then we must throw out that solution and put in fresh, and the battery will do the same work again, and so on. if you should only want to light your lamp or run your motor for a few minutes, you could pull the zinc up from the solution and put it down again when you wanted the electricity once more. the carbon element in the battery is not consumed by the acid, although the zinc is. [illustration: fig. ] now you will see the use of the porous cup. we will take as an illustration of this type an ordinary battery in which a porous cup is used. (fig. .) here, you will see, the carbon is placed in the porous cup, while the zinc is outside in the glass jar. in the glass cell with the zinc is usually used water made slightly acid, and the strong solution of sulphuric acid and bichromate of potash (or chromic acid) is poured in the porous cup, where the carbon is placed. the strong solution penetrates the porous cup very slowly and gets to the zinc, when it immediately produces a current of electricity. but the acid does not get at the zinc so freely as it does in the battery without a porous cup, and, consequently, neither the acid nor the zinc is so rapidly used up. where porous cups are used, the batteries will give a continuous current for a very much longer time than without them, and will, sometimes, give many hours' work every day for several months without requiring any change of solution. _polarization._--there is one other reason why a longer working time can be had from a battery with a porous cup, and that is, in a battery without a porous cup the action of the acid upon the zinc is so rapid that the carbon plates become covered with gas, and, therefore, the proper action by the acid cannot take place upon them. thus, the battery ceases to work, and is said to be "polarized." when a porous cup is used, the action of the acid upon the zinc is slow enough to give off only a small amount of gas, and thus the acid has a chance to act upon the carbon plates and develop a steady current of electricity. the work done by batteries the pressure and quantity of electricity given off continuously by open and closed circuit batteries is very different. the pressure (or "electromotive force") of one cell of an ordinary open-circuit battery is only about one volt, and the current is usually very much less than one ampère, except in a dry cell, which may give more. in the closed-circuit batteries described, the electromotive force of each cell is about two volts, while the current varies from to perhaps ampères, according to the size of the zinc and carbon plates. it would not matter if you made one cell as big as a barrel, nor if you put in a _dozen carbons and zincs_, the _electromotive force would not exceed the volts mentioned for each type of battery_, but the _ampère capacity would be greater_ than in a smaller cell on account of the larger size of the carbon and zinc plates. _internal resistance._--there is one other point which affects the number of ampères which can be obtained from a closed-circuit battery, and that is whether there is a large or small internal resistance in the battery itself. this depends upon the solution which is used and the arrangement of the plates. if there is a high resistance in the battery itself (called "internal resistance"), the electricity must do work to overcome this resistance before it can get out of the battery to do useful work through the wires, and, consequently, the capacity in ampères is limited. if, on the other hand, there is very little resistance in the battery, the current has very little work to flow to the wires leading from the battery, and we can get a larger quantity, or greater number of ampères. thus, you will see that while the closed-circuit battery is the stronger, and will do all that the open-circuit battery will do, and even more, in a short time the latter, though weaker, will do about as much work for the same amount of zinc and carbon as the former, but takes a much longer time. batteries for electric light as we have explained to you, closed-circuit batteries are used for producing incandescent electric lights in small numbers, as well as for running motors. to operate incandescent lights, a number of batteries connected together are used. the number used depends upon the pressure which the lamps require to make them give the required light. we will now explain how the batteries are connected together for this purpose. [illustration: fig. ] suppose you wished to light an incandescent lamp of, say, three candle-power, which required six volts. we would take three closed-circuit batteries which would each give two volts, and connect by a piece of wire the zinc of the first to the carbon of the second, and the zinc of the second to the carbon of the third, as shown in the sketch. (fig. .) we would then attach a wire to the carbon of the first and one to the zinc of the third, and there would be six volts in these two wires, which would light up one six-volt lamp nicely. this is called connecting in series, or for intensity. now if each of these cells gave ten ampères alone, the three will only give ten ampères together when they are connected in series. if our lamp only required one ampère, you would naturally think that ten similar lamps put on the wires would give as good light as the one, but that is not so. although you might light up two lamps, the pressure would drop and the lights would become less brilliant if you put on the whole number. so, if we wished to put on the whole ten lights we would connect another battery and thus increase the pressure, which would probably make these ten lamps burn brightly. these rules hold good for connecting any number of batteries for lamps of any number of volts--that is to say, there should be calculated about two volts for each cell and an allowance made for drop in pressure. connecting in multiple there is another way of connecting batteries, and that is to obtain a larger number of ampères. this is called connecting in multiple arc, or for quantity. [illustration: fig. ] let us take again for an illustration the three cells giving each volts and ampères. this time we connect the carbon of the first to the carbon of the second, and the carbon of the second to that of the third; then we connect the zinc of the first to that of the second, and the zinc of the second to that of the third, as shown in the sketch. (fig. .) we then attach a wire to the zinc and one to the carbon in the third cell, and we then can obtain from these two wires _only volts_, but ampères. there are, again, many ways of connecting several of these sets together, but it is not intended in this book to go into these at length, for the reason that we only set out to give a simple explanation of the first principles of this subject. we shall therefore only give an illustration of one more method of connecting batteries which will be easy to understand. this is called multiple series the sketch we have last given shows three batteries connected in multiple. these we will call set no. . now, suppose we take three more batteries exactly similar and connect them together just in the same manner. let us call this set no. . now take the wire leading from the carbon of set no. and connect it with the wire leading from the zinc of set no. . then take a wire leading from the zinc of set no. , and a wire leading from the carbon of set no. , and connect them with the lamps or motors. these two sets being connected in multiple series, we shall get volts and ampères. this is called connecting in multiple series, and may be extended indefinitely with any number of batteries. we should add that one of the elements in a battery is called "positive," and the other "negative." the edison primary battery as this type of battery will work efficiently on _either_ open or closed circuit, we have thought best to describe it separately at this place, in order not to confuse your ideas while reading about batteries generally. the type of cell we will now describe was originated by an inventor named lalande, and was known by that name; but it has been greatly improved and rendered more efficient by edison, and is now manufactured and sold by him under the name of the edison primary battery. before describing the cell itself, let us consider the action that takes place in a battery of this kind. if certain metals are placed in a suitable solution, and are connected together, outside of the solution, by wires, vigorous chemical action will take place at the surfaces of the metals, and electrical energy will be produced. the plates must be of different metals, and the solution should be one that will dissolve neither of them except when an electric current is allowed to flow. one of the metals is usually zinc, which is gradually eaten away or dissolved by the solution while the battery is delivering electrical energy. it is the chemical combination of the zinc and the solution that produces this energy, which leaves the zinc in the form of an electric current, and passes through the solution to the other metal, out of the cell to the wire, and thence back by another wire to the zinc, where it is once more started on its circuit. at the surface of the other metal, which may be, and frequently is, copper, small bubbles of the gas called hydrogen are produced. this gas rises to the surface of the liquid and gradually passes off into the air. but its presence offers resistance to the passage of the current; so that generally there is associated with the copper a supply of the gas oxygen. oxygen and hydrogen are always very eager to mix with each other, and, therefore, when the hydrogen bubbles appear they are quickly taken up by the oxygen near by. the mixture of these two gases forms water, which becomes part of the solution. all of this happens so quickly that the hydrogen cannot be perceived so long as there is any oxygen left in the copper-oxide plate. [illustration: fig. ] in the edison primary battery (fig. ) the plates are zinc, known as the negative, and copper oxide (copper and oxygen), or the positive. these are suspended in a solution of caustic soda and water, the plates and solution being contained in jars of glass or porcelain. the plates are provided with suitable wires for connecting the cells with one another and with the lamps, motors, or other devices which they are to operate. there are usually two zinc plates and one copper-oxide plate, or multiples thereof. the quantity of current that may be withdrawn depends on the size and number of the plates, as well as upon their construction and arrangement. the voltage of these cells is low, being about . volt each; but this is more than compensated for by the fact that the internal resistance of the battery is so low that the voltage is not perceptibly affected even at continuous high-discharge rates, and that the voltage remains practically constant throughout the life of the cell. furthermore, when the battery is not in use there is practically no local action. consequently, the cells may remain on open circuit (that is, doing no work) for years and there will be no loss of energy. the cell will then operate with the same practical efficiency as if it were new. in some classes of work this battery remains in service from four to six years without attention. another peculiar advantage of this battery lies in the fact that the plates and the electrolyte are so well proportioned that they are all exhausted at the same time, and then new plates and solution can be put in the jar, restoring it to its original condition. these batteries are used in great numbers for railway signal work and for other purposes, such as fire and burglar alarm systems, various telephone functions, operation of electric self-winding and programme clock systems, small electric-motor work, for low candle-power electric lamps, gas-engine ignition, electro-plating, telegraph systems, chemical analysis, and other experimental work where batteries are required that will remain in use for long periods of time without requiring any attention or renewal. the remarks that have been made on previous pages about connecting up batteries in series, multiple, and multiple series apply also to these edison primary cells. fig. shows a battery of four of these cells connected in series. secondary, or storage, batteries the open and closed circuit batteries we have so far described are used to produce electricity by the action of the chemicals upon the elements contained in them. they are called primary batteries. [illustration: fig. ] the batteries which we will now tell you of are called secondary, or storage, batteries, and do not of themselves make any primary current, but simply act as reservoirs, so to speak, to hold the energy of the electric current which is led into them from a dynamo or primary battery. at the proper time and under proper conditions these secondary batteries will give back a large percentage of the energy of the electric current which has been stored in them. this class of battery has been called by these three names: "secondary battery," "accumulator," and "storage battery"; but as the latter name is used almost exclusively in this country, we shall use it in the following description. two types there are two distinct types of storage battery. one is called the "lead" or "acid" storage battery, and the other the "alkaline" or "nickel-iron" storage battery. each of them simply acts as a reservoir to hold the energy of the electric current which is led into it, and each of them, under proper conditions, will give back that energy. as the lead storage battery is the oldest in point of discovery and invention, we will describe it first. the lead storage battery a lead storage battery usually consists of a glass or hard-rubber jar containing lead plates and a solution consisting of water and sulphuric acid. a single unit is usually called a "cell." (fig. .) there are always at least two lead plates in a storage-battery cell of this kind, although there may be any number above that. for the sake of making a clearer explanation to you, we will take as an illustration a cell containing only two plates.[ ] [illustration: fig. ] we have, then, a glass or hard-rubber jar containing two lead plates and a solution consisting of water and sulphuric acid. these plates are called the "elements," and one is called the positive and the other the negative element. the solution is called the "electrolyte." the positive element is a sheet of lead upon which is spread a paste made of red-lead. the negative element is a similar sheet of lead upon which is spread a paste made of litharge. now, when these plates are thus prepared, they are put into the acid solution in the jar, and a wire attached to each plate is connected with the two wires from a dynamo or other source of electric current, just as a lamp would be connected. the electric current then goes into the storage-battery cell, entering by the positive plate and coming out by the negative. these plates and the paste upon them offer some resistance, or opposition, to the passage of the current, so the electricity must do some work to get from one to the other. the work it does in this case is to so act upon the paste that its chemical nature is changed. so, after the primary current has been passed from one plate to the other for some time, and after several "discharges," the storage battery may be disconnected, being now "formed." the paste on the lead plates is now found to have changed its chemical nature, the paste on the positive plate having been transformed into peroxide of lead, and that on the negative plate into spongy lead. on arriving at this condition, the paste on the plates is called "active material." this process of "formation" is absolutely essential before the lead storage battery is ready to be used for actual work. so, when the plates have been fully "formed," the storage battery may be again connected with a source of electric current which again enters by the positive plate and leaves by the negative. this current so acts on the active material that it combines with the acid solution and, through the energy of the charging current, forms other chemical compounds which may for convenience be called "sulphates." when the charging current has flowed through the battery long enough to produce these changes in the active material the battery is said to be "charged," and is ready for useful work. if the two wires attached to the plates are now connected with electric lamps, or a motor, or other device, the active material will develop energy in the effort to again change its nature. this energy takes the form of an electric current, which leaves the battery and passes through the conductors and operates the lamps, motors, or other devices in its passage. in this way the battery is said to be "discharged," and at the end of its discharge it can again be charged and discharged in a similar manner for a long time, until the active material is either used up or drops off the plates. so far as the actual details of construction are concerned, lead storage batteries are made in a great many different ways, but the materials are, in general, of the same nature as those we have mentioned above. the alkaline storage battery we shall now describe an entirely different type of storage battery, which contains neither lead nor acid. it is one of the many inventions of thomas a. edison. in the alkaline storage battery the gas called oxygen plays a very important part, and we will try to make it clear to you what this part is. you are well aware of the fact that if you leave your pocket-knife out in the air it will get rusty. the reason for this is that iron or steel quickly tends to combine with the oxygen of the air, and this combination of oxygen and iron is rust, otherwise called oxide of iron, or iron oxide. this iron oxide, or rust, is therefore the result of a chemical action between the iron and the oxygen. now as all chemical actions require the expenditure of energy, there has been developed either heat or electricity in the process. the oxygen may be taken away from the iron oxide, chemically; but here again would be another chemical action which would require energy to be once more expended. iron oxide may be made chemically in many different ways. it is frequently made in the form of a powder. therefore, we do not have to depend upon iron rust for a supply of this material. before going further we must consider another oxide--namely, nickel oxide. it is characteristic of nickel that when it is combined with oxygen to a certain degree so as to form the compound known as nickel oxide, it will receive still more oxygen. now, if under proper conditions we compel iron oxide to give up its oxygen to some other kind of chemical compound, such as nickel oxide, we must expend energy. but, on the other hand, if this nickel oxide gives back the oxygen to the iron--which it will do if opportunity is given--there is energy produced again in receiving the oxygen. in other words, the energy previously expended, or part of it, is now returned. this action and reaction are practically those that take place in the edison alkaline storage battery. for simplicity of illustration we will consider a cell containing only two plates, one positive and one negative. the negative plate is made up of a number of small, flat, perforated pockets containing iron oxide in the form of a fine powder. the positive plate is made up of small, perforated tubes containing nickel oxide mixed with very thin flakes of metallic nickel. (fig. illustrates these plates, the positive being in front.) [illustration: fig. ] these two elements, positive and negative, having wires or conductors attached, are placed in a nickeled-steel can containing the electrolyte, which consists of a potash solution. you will see that this differs from a lead storage battery, in which the electrolyte is sulphuric acid and water. if we were to put this acid solution into a metallic can (except one made of lead) the can would not last long, as the acid would quickly eat holes through it. now let us see what takes place in the edison alkaline storage battery. if an electric current from a dynamo or other source of electricity is caused to pass through the positive to the negative plate the oxygen present in the iron oxide passes to and remains with the nickel oxide. during all the time this is going on the battery is said to be "charging," and when all the oxygen has been removed from the iron oxide and is taken up by the nickel oxide, then the battery is said to be "charged," and the flow of current into the battery is stopped. a change has now taken place. the powder in the negative plate is no longer iron oxide, but has been reduced to metallic iron, because the oxygen has been removed. the powder in the positive plate is now raised to a higher or super oxide of nickel, because it has taken the oxygen that was in the iron. but the nickel oxide will readily give up its excess of oxygen, and the iron will receive it back freely if permitted. if the proper conditions are established, this transfer of oxygen will take place, but the iron cannot receive it without delivering energy. [illustration: fig. ] the proper conditions are established by providing a conducting circuit between the two elements, in which lamps, motors, or other electrical devices are placed. as soon as this circuit is provided, the opportunity is given to the iron to receive the oxygen. this it does, and in so doing develops electrical energy. this energy is in the form of electric current which is then delivered by the battery on what is called the "discharge," and this current may be used for lighting lamps or for operating motors or other electrical devices. the battery is said to be discharging as long as the iron is receiving oxygen from the nickel oxide. as soon as it becomes iron oxide once more, the giving out of energy ceases and the battery is said to be "discharged," and must again be charged to obtain further work from it. such a battery can be charged and discharged an indefinite number of times. this type of battery is very rugged, and its combinations are not self-destructive. it is very simple, as it provides chiefly for the movement of the oxygen back and forth; besides, it gives much more current for its weight than the lead type of storage battery. (fig. shows the plates of a standard edison cell removed from container.) connecting storage batteries on the discharge, one cell of a lead storage battery gives an average of about volts, and a cell of alkaline storage battery about . volts, no matter what its size or the number of plates may be. when there are more than two plates in one cell, all the positives in that cell are connected together by metallic strips or bands, and all negatives in the cell are connected together in a similar way. although we cannot obtain more than the above-named electromotive force from one cell of either type of storage battery, we can obtain a greater ampère capacity by using large plates instead of small ones, or by using a larger number of small size. the same effects are produced by connecting the cells in series, or multiple, or multiple series, as we showed you in regard to primary batteries; and the storage batteries may be charged as well as discharged when connected in any one of these ways. charging current the current which is used for charging must always be greater in pressure than that of the storage batteries which are being charged. if it is not, the storage batteries will be the stronger of the two and will overpower the charging current and so discharge themselves. x conclusion we will now bring this little volume to a close, having given you a brief outline of the simplest rudiments of that wonderful power of nature, electricity. we may compare this subject to a beautiful house the inside of which you would like to examine from top to bottom. we have opened the door for you; now walk in and examine everything. there may be a great many stairs to climb, but what you see and learn will repay for all the trouble. the end footnotes: [ ] the filaments in modern "mazda" lamps, as made at the edison lamp works, are strips of metallic tungsten. [ ] the batteries we will now describe are for closed-circuit work _only_, and they are never used for open-circuit work. but there is a type of battery made that is available for either open or closed circuit operation. this is the edison primary battery, which will be described later on. [ ] practically, there is always one more negative plate than positive plates in a _regular_ storage-battery cell. consequently, a standard cell always contains an odd number of plates. transcriber's note -plain print and punctuation errors fixed. grounds of natural philosophy divided into thirteen parts with an appendix containing five parts the second edition, much altered from the first, which went under the name of philosophical and physical opinions written by the thrice noble, illustrious, and excellent princess, the duchess of newcastle london, printed by a. maxwell, in the year . to all the universities in europe. most learned societies, all books, without exception, being undoubtedly under your iurisdiction, it is very strange that some authors of good note, are not asham'd to repine at it; and the more forward they are in judging others, the less liberty they will allow to be judg'd themselves. but, if there was not a necessity, yet i would make it my choice, to submit, willingly, to your censures, these _grounds of natural philosophy_, in hopes that you will not condemn them, because they want _art_, if they be found fraught with sense and reason. you are the _starrs of the first magnitude_, whose influence governs the _world of learning_; and it is my confidence, that you will be propitious to the birth of this beloved child of my brain, whom i take the boldness to recommend to your patronage; and as, if you vouchsafe to look on it favourably, i shall be extreamly obliged to your goodness, for its everlasting life: so, if you resolve to frown upon it, i beg the favour, that it be not buried in the hard and rocky grave of your displeasure; but be suffer'd, by your gentle silence, to lye still in the soft and easie bed of oblivion, which is incomparably the less punishment of the two. it is so commonly the error of indulgent parents, to spoil their children out of fondness, that i may be forgiven for spoiling this, in never putting it to suck at the breast of some learned nurse, whom i might have got from among your students, to have assisted me; but would, obstinately, suckle it my self, and bring it up alone, without the help of any scholar: which having caused in the first edition, (which was published under the name of _philosophical and physical opinions_) many imperfections; i have endeavoured in this second, by many alterations and additions, (which have forc'd me to give it another name) to correct them; whereby, i fear, my faults are rather _changed_ and _encreased_, than _amended_. if you expect fair proportions in the parts, and a beautiful symmetry in the whole, having never been taught at all, and having read but little; i acknowledg my self too illiterate to afford it, and too impatient to labour much for method. but, if you will be contented with _pure wit_, and the effects of _meer contemplation_; i hope, that somewhat of that kind may be found in this book, and in my other _philosophical, poetical_, and _oratorical works_: all which i leave, and this especially, to your kind protection, and am, your most humble servant, and admirer, margaret newcastle. a table of the contents. the first part. i. of matter ii. of motion iii. of the degrees of matter iv. of _vacuum_ v. the difference of the two self-moving parts of matter vi. of dividing and uniting of parts vii. of life and knowledg viii. of nature's knowledg, and perception ix. of perception in general x. of double perception xi. whether the triumphant parts can be perceived distinctly from each other xii. whether nature can know her self, or have an absolute power of her self, or have an exact figure xiii. nature cannot judg herself xiv. nature poyses or balances her actions xv. whether there be degrees of corporal strength xvi. of effects and cause xvii. of influence xviii. of fortune and chance xix. of time and eternity the second part. i. of creatures ii. of knowledg and perception of different kinds and sorts of creatures iii. of perception of parts, and united perception iv. whether the rational and sensitive parts, have a perception of each other v. of thoughts, and the whole mind of a creature vi. whether the mind of one creature, can perceive the mind of another creature vii. of perception, and conception viii. of human supposition ix. of information between several creatures x. the reason of several kinds and sorts of creatures xi. of the several properties of several kinds and sorts of creatures the third part. chap. . to . of productions in general viii. productions must partake of some parts of their producers ix. of resemblances of several off-springs, or producers x. of the several appearances of the exterior parts of one creature the fourth part. i. of animal productions, and of the difference between productions and transformations ii. of different figurative motions in man's production iii. of the quickning of a child, or any other sort of animal creatures iv. of the birth of a child v. of mischances, or miscarriages of breeding-creatures vi. of the encrease of growth and strength of mankind, or such like creatures vii. of the several properties of the several exterior shapes of several sorts of animals viii. of the dividing and uniting parts of a particular creature the fifth part. i. of man ii. of the variety of man's natural motions iii. of man's shape and speech iv. of the several figurative parts of human creatures v. of the several perceptions amongst the several parts of man vi. of divided and composed perceptions vii. of the ignorances of the several perceptive organs viii. of the particular and general perceptions of the exterior parts of human creatures ix. of the exterior sensitive organs of human creatures x. of the rational parts of the human organs xi. of the difference between the human conception, and perception xii. of the several varieties of actions of human creatures xiii. of the manner of information between the rational and sensitive parts xiv. of irregularities and regularities of the restoring-parts of human creatures xv. of the agreeing and disagreeing of the sensitive and rational parts of human creatures xvi. of the power of the rational; or rather, of the indulgency of the sensitive xvii. of human appetites and passions xviii. of the rational actions of the head and heart of human creatures xix. of passions and imaginations xx. that associations, divisions, and alterations, cause several effects xxi. of the differences between self-love, and passionate love the sixth part. i. of the motions of some parts of the mind, and of forrein objects ii. of the motions of some parts of the mind iii. of the motions of human passions and appetites; as also, of the motions of the rational and sensitive parts, towards forrein objects iv. of the repetitions of the sensitive and rational actions v. of the passionate love, and sympathetical endeavours, amongst the associate parts of a human creature vi. of acquaintance vii. of the effects of forrein objects of the sensitive body; and of the rational mind of a human creature viii. of the advantage and disadvantage of the encounters of several creatures ix. that all human creatures have the like kind and sorts of properties x. of the singularity of the sensitive, and of the rational corporeal motions xi. of the knowledg between the sensitive organs of a human creature xii. of human perception, or defects of a human creature xiii. of natural fools the seventh part. i. of the sensitive actions of sleeping and waking ii. of sleeping iii. of human dreams iv. of the actions of dreams v. whether the interior parts of a human creature, do sleep vi. whether all the creatures in nature, have sleeping and waking-actions vii. of human death viii. of the heat of human life, and the cold of human death ix. of the last act of human life, ibid. x. whether a human creature hath knowledg in death, or not xi. whether a creature may be new formed after a general dissolution xii. of foreknowledg the eighth part. i. of the irregularity of nature's parts ii. of the human parts of a human creature iii. of human humors iv. of blood, ibid. v. of the radical humors, or parts vi. of expelling malignant disorders in a human creature vii. of human digestions and evacuations viii. of diseases in general ix. of the fundamental diseases the ninth part. i. of sickness ii. of pain iii. of dizziness iv. of the brain seeming to turn round in the head v. of weakness vi. of swooning, ibid. vii. of numb and dead palsies, or gangren's viii. of madness ix. the sensitive and rational parts may be distinctly mad x. the parts of the head are not only subject to madness; but also, the other parts of the body xi. the rational and sensitive parts of a human creature, are apt to disturb each other xii. of diseases produced by conceit the tenth part. i. of fevers ii. of the plague iii. of the small-pox and measles iv. of the intermission of fevers, or agues v. of consumptions vi. of dropsies, ibid. vii. of sweating viii. of coughs ix. of gangren's x. of cancers and fistula's xi. of the gout, ib. xii. of the stone xii. of apoplexies and lethargies xiii. of epilepsies xiv. of convulsions and cramps xv. of cholicks, ibid. xvi. of shaking-palsies xvii. of the muther, spleen, and scurvy xviii. of food or digestions, ibid. xix. of surfeits xx. of natural evacuations and purgings xxi. of purging-drugs xxii. of the various humors of drugs xxiii. of cordials xxiv. of the different actions of the several sensitive parts of a human creature. xxv. of the antipathy of some human creatures, to some forrein objects xxvi. of the effects of forrein objects, on the human mind, ib. xxvii. of contemplation xxviii. of injecting the blood of one animal, into the veins of another animal the eleventh part. i. of the different knowledges in different kinds and sorts of creatures ii. of the variety of self-actions in particular creatures iii. of the variety of corporeal motions of one and the same sort and kind of motion iv. of the variety of particular creatures, ibid. v. of dividing, and rejoyning, or altering exterior figurative motions vi. of different figurative motions in particular creatures vii. of the alterations of exterior and innate figurative motions of several sorts of creatures viii. of local motion ix. of several manners or ways of advantages or disadvantages x. of the actions of some sorts of creatures, over others xi. of glassie-bodies xii. of metamorphoses, or transformations of animals and vegetables, xiii. of the life and death of several creatures xiv. of circles xv. human creatures cannot so probably treat of other sorts of creatures, as of their own the twelfth part. i. of the equality of elements ii. of several tempers iii. of the change and rechange; and of dividing of the parts of the elements iv. of the innate figurative motions of earth v. of the figurative motions of air, ibid. vi. of the innate figurative motions of fire vii. of the productions of elemental fire viii. of flame ix. of the two sorts of fire most different, ibid. x. of dead or dull fires xi. of the occasional actions of fire xii. fire hath not the property to change and rechange xiii. of the innate figurative motions of water xiv. the nature or property of water xv. of the alteration of the exterior figurative motion of water xvi. of oyl of vitriol, ibid. xvii. of mineral and sulphurous waters xviii. the cause of the ebbing and flowing of the sea xix. of overflows xx. of the figure of ice and snow xxi. of the change and rechange of water xxii. of water quenching fire, and fire evaporating water xxiii. of inflamable liquors xxiv. of thunder xxv. of vapour, smoak, wind and clouds xxvi. of wind xxvii. of light xxviii. of darkness xxix. of colours xxx. of the exterior motions of the planets xxxi. of the sun, and planets, and seasons xxxii. of air corrupting dead bodies. the thirteenth part. i. of the innate figurative motions of metal ii. of the melting of metals iii. of burning, melting, boyling, and evaporating iv. of stone v. of the loadstone vi. of bodies apt to ascend, or descend vii. why heavy bodies descend more forcibly than leight bodies ascend, viii. of several sorts of densities and rarities, gravities, and levities ix. of vegetables x. of the production of vegetables xi. of replanting vegetables appendix. the first part. i. whether there can be a substance that is not a body ii. of an immaterial iii. whether an immaterial be perceivable iv. of the difference between god and nature v. all the parts of nature, worship god, ibid. vi. whether god's decrees are limited vii. of god's decrees concerning the particular parts of nature viii. of the ten commandments ix. of several religions x. of rules and prescriptions xi. sins and punishments are material xii. of human conscience the second part. i. whether it is possible there could be worlds consisting only of the rational parts, and others only of the sensitive parts ii. of irregular and regular worlds iii. whether there be egress and regress between the parts of several worlds iv. whether the parts of one and the same society, could (after their dissolution, meet and unite v. whether, if a creature being dissolved, if it could unite again, would be the same vi. of the resurrection of human-kind vii. of the dissolution of a world viii. of a new heaven, and a new earth ix. whether there shall be a material heaven and hell, ibid. x. concerning the joys or torments of the blessed and cursed, after they are in heaven or hell the third part. the preamble. i. of the happy and miserable worlds ii. whether there be such kinds and sorts of creatures in the happy and blessed world, as in this world iii. of the births and deaths of the heavenly world, ibid. iv. whether those creatures could be named blessed, that are subject to dye v. of the productions of the creatures of the regular world vi. whether the creatures in the blessed world, do feed and evacuate vii. of the animals, and of the food of the humans of the happy world viii. whether it is not irregular for one creature to feed on another ix. of the continuance of life in the regular world x. of the excellency and happiness of the creatures of the regular world xi. of human creatures in the regular world xii. of the happiness of human creatures in the material world, ibid. the fourth part. i. of the irregular world ii. of the productions and dissolutions of the creatures of the irregular world iii. of animals, and of humans in the irregular world iv. of objects and perceptions v. the description of the globe of the irregular world, ibid. vi. of the elemental air, and light of the irregular world vii. of storms and tempests in the irregular world viii. of the several seasons; or rather, of the several tempers in the irregular world, ibid. ix. the conclusion of the irregular and unhappy, or cursed world the fifth part. fifteen sections concerning restoring-beds, or wombs the conclusion grounds of natural philosophy. the first part. chap. i. of matter. matter is that we name body; which matter cannot be less, or more, than body: yet some learned persons are of opinion, that there are substances that are not material bodies. but how they can prove any sort of substance to be no body, i cannot tell: neither can any of nature's parts express it, because a corporeal part cannot have an incorporeal perception. but as for matter, there may be degrees, as, _more pure_, or _less pure_; but there cannot be any substances in nature, that are between body, and no body: also, matter cannot be figureless, neither can matter be without parts. likewise, there cannot be matter without place, nor place without matter; so that matter, figure, or place, is but one thing: for, it is as impossible for one body to have two places, as for one place to have two bodies; neither can there be place, without body. chap. ii. of motion. though matter might be without motion, yet motion cannot be without matter; for it is impossible (in my opinion) that there should be an immaterial motion in nature: and if motion is corporeal, then matter, figure, place, and motion, is but one thing, _viz_. a corporeal figurative motion. as for a first motion, i cannot conceive how it can be, or what that first motion should be: for, an immaterial cannot have a material motion; or, so strong a motion, as to set all the material parts in nature, or this world, a-moving; but (in my opinion) every particular part moves by its own motion: if so, then all the actions in nature are self-corporeal, figurative motions. but this is to be noted, that as there is but one matter, so there is but one motion; and as there are several parts of matter, so there are several changes of motion: for, as matter, of what degree soever it is, or can be, is but matter; so motion, although it make infinite changes, can be but motion. chap. iii. of the degrees of matter. though matter can be neither more nor less than matter; yet there may be degrees of matter, as _more pure_, or _less pure_; and yet the purest parts are as much material, in relation to the nature of matter, as the grossest: neither can there be more than two sorts of matter, namely, that sort which is self-moving, and that which is not self-moving. also, there can be but two sorts of the self-moving parts; as, that sort that moves intirely without burdens, and that sort that moves with the burdens of those parts that are not self-moving: so that there can be but these three sorts; those parts that are not moving, those that move free, and those that move with those parts that are not moving of themselves: which degrees are (in my opinion) the rational parts, the sensitive parts, and the inanimate parts; which three sorts of parts are so join'd, that they are but as one body; for, it is impossible that those three sorts of parts should subsist single, by reason nature is but one united material body. chap. iv. of vacuum. in my opinion, there cannot possibly be any _vacuum_: for, though nature, as being material, is divisible and compoundable; and, having self-motion, is in perpetual action: yet nature cannot divide or compose _from_ her self, although she may move, divide, and compose in her self: but, were it possible nature's parts could wander and stray in, and out of _vacuum_, there would be a confusion; for, where unity is not, order cannot be: wherefore, by the order and method of nature's corporeal actions, we may perceive, there is no _vacuum_: for, what needs a _vacuum_, when as body and place is but one thing; and as the body alters, so doth the place? chap. v. the difference of the two self-moving parts of matter. the self-moving parts of nature seem to be of two sorts, or degrees; one being purer, and so more agil and free than the other; which (in my opinion) are the rational parts of nature. the other sort is not so pure; and are the architectonical parts, which are the labouring parts, bearing the grosser materials about them, which are the inanimate parts; and this sort (in my opinion) are the sensitive parts of nature; which form, build, or compose themselves with the inanimate parts, into all kinds and sorts of creatures, as animals, vegetables, minerals, elements, or what creatures soever there are in nature: whereas the rational are so pure, that they cannot be so strong labourers, as to move with burdens of inanimate parts, but move freely without burdens: for, though the rational and sensitive, with the inanimate, move together as one body; yet the rational and sensitive, do not move as one part, as the sensitive doth with the inanimate. but, pray mistake me not, when i say, the inanimate parts are grosser; as if i meant, they were like some densed creature; for, those are but effects, and not causes: but, i mean gross, dull, heavy parts, as, that they are not self-moving; nor do i mean by purity, rarity; but agility: for, rare or dense parts, are effects, and not causes: and therefore, if any should ask, whether the rational and sensitive parts were rare, or dense; i answer, they may be rare or dense, according as they contract, or dilate their parts; for there is no such thing as a single part in nature: for matter, or body, cannot be so divided, but that it will remain matter, which is divisible. chap. vi. of dividing and uniting of parts. though every self-moving part, or corporeal motion, have free-will to move after what manner they please; yet, by reason there can be no single parts, several parts unite in one action, and so there must be united actions: for, though every particular part may divide from particular parts; yet those that divide from some, are necessitated to join with other parts, at the same point of time of division; and at that very same time, is their uniting or joining: so that division, and composition or joining, is as one and the same act. also, every altered action, is an altered figurative place, by reason matter, figure, motion, and place, is but one thing; and, by reason nature is a perpetual motion, she must of necessity cause infinite varieties. chap. vii. of life and knowledg. all the parts of nature have life and knowledg; but, all the parts have not active life, and a perceptive knowledg, but onely the rational and sensitive: and this is to be noted, that the variousness, or variety of actions, causes varieties of lives and knowledges: for, as the self-moving parts alter, or vary their actions; so they alter and vary their lives and knowledges; but there cannot be an infinite particular knowledg, nor an infinite particular life; because matter is divisible and compoundable. chap. viii. of nature's knowledg and perception. if nature were not self-knowing, self-living, and also perceptive, she would run into confusion: for, there could be neither order, nor method, in ignorant motion; neither would there be distinct kinds or sorts of creatures, nor such exact and methodical varieties as there are: for, it is impossible to make orderly and methodical distinctions, or distinct orders, by chances: wherefore, nature being so exact (as she is) must needs be self-knowing and perceptive: and though all her parts, even the inanimate parts, are self-knowing, and self-living; yet, onely her self-moving parts have an active life, and a perceptive knowledg. chap. ix. of perception in general. _perception_ is a sort of knowledg, that hath reference to objects; that is, some parts to know other parts: but yet objects are not the cause of perception; for the cause of perception is self-motion. but some would say, _if there were no object, there could be no perception_. i answer: it is true; for, that cannot be perceived, that is not: but yet, corporeal motions cannot be without parts, and so not without perception. but, put an impossible case, as, that there could be a single corporeal motion, and no more in nature; that corporeal motion may make several changes, somewhat like _conceptions_, although not _perceptions_: but, nature being corporeal, is composed of parts, and therefore there cannot be a want of objects. but there are infinite several manners and ways of perception; which proves, that the objects are not the cause: for, every several kind and sort of creatures, have several kinds and sorts of perception, according to the nature and property of such a kind or sort of composition, as makes such a kind or sort of creature; as i shall treat of, more fully, in the following parts of this book. chap. x. of double perception. there is a _double perception_ in nature, the rational perception, and the sensitive: the rational perception is more subtil and penetrating than the sensitive; also, it is more generally perceptive than the sensitive; also, it is a more agil perception than the sensitive: all which is occasioned not onely through the _purity_ of the rational parts, but through the _liberty_ of the rational parts; whereas the sensitive being incumbred with the inanimate parts, is obstructed and retarded. yet all perceptions, both sensitive and rational, are in parts; but, by reason the rational is freer, (being not a painful labourer) can more easily make an united perception, than the sensitive; which is the reason the rational parts can make a whole perception of a whole object: whereas the sensitive makes but perceptions in part, of one and the same object. chap. xi. whether the triumphant parts can be perceived distinctly from each other. some may make this question,_ whether the three sorts of parts, the rational, sensitive, and inanimate, may be singly perceived?_ i answer, not unless there were single parts in nature; but, though they cannot be singly perceived, yet they singly perceive; because, every part hath its own motion, and so its own perception. and though those parts, that have not self-motion, have not perception; yet, being joined, as one body, to the sensitive, they may by the sensitive motion, have some different sorts of self-knowledg, caused by the different actions of the sensitive parts; but that is not _perception_. but, as i said, the _triumphant parts_ cannot be perceived distinctly asunder, though their actions may be different: for, the joining, or intermixing of parts, hinders not the several actions; as for example, a man is composed of several parts, or, (as the learned term them) _corporeal motions_; yet, not any of those different parts, or corporeal motions, are a hindrance to each other: the same between the _sensitive_ and _rational parts_. chap. xii. whether nature can know her self, or have an absolute power of her self, or have an exact figure. i was of an opinion, that nature, because infinite, could not know her self; because infinite hath no limit. also, that nature could not have an absolute power over her own parts, because she had infinite parts; and, that the infiniteness did hinder the absoluteness: but since i have consider'd, that the infinite parts must of necessity be self-knowing; and that those infinite self-knowing parts are united in one infinite body, by which nature must have both an united knowledg, and an united power. also, i questioned, whether nature could have an exact figure, (but, mistake me not; for i do not mean the figure of matter, but a composed figure of parts) because nature was composed of infinite variety of figurative parts: but considering, that those infinite varieties of infinite figurative parts, were united into one body; i did conclude, that she must needs have an exact figure, though she be infinite: as for example, this world is composed of numerous and several figurative parts, and yet the world hath an exact form and frame, the same which it would have if it were infinite. but, as for self-knowledg, and power, certainly god hath given them to nature, though her power be limited: for, she cannot move beyond her nature; nor hath she power to make her self any otherwise than what she is, since she cannot create, or annihilate any part, or particle: nor can she make any of her parts, immaterial; or any immaterial, corporeal: nor can she give to one part, the nature (_viz_. the knowledg, life, motion, or perception) of another part; which is the reason one creature cannot have the properties, or faculties of another; they may have the like, but not the same. chap. xiii. nature cannot judg her self. although nature knows her self, and hath a free power of her self; (i mean, a natural knowledg and power) yet, nature cannot be an upright, and just judg of her self, and so not of any of her parts; because every particular part is a part of her self. besides, as she is self-moving, she is self-changeing, and so she is alterable: wherefore, nothing can be a perfect, and a just judg, but something that is individable, and unalterable, which is the infinite god, who is unmoving, immutable, and so unalterable; who is the judg of the infinite corporeal actions of his servant nature. and this is the reason that all nature's parts appeal to god, as being the only judg. chap. xiv. nature poyses, or balances her actions. although nature be infinite, yet all her actions seem to be _poysed_, or _balanced_, by opposition; as for example, as nature hath dividing, so composing actions: also, as nature hath regular, so irregular actions; as nature hath dilating, so contracting actions: in short, we may perceive amongst the creatures, or parts of this world, slow, swift, thick, thin, heavy, leight, rare, dense, little, big, low, high, broad, narrow, light, dark, hot, cold, productions, dissolutions, peace, warr, mirth, sadness, and that we name _life_, and _death_; and infinite the like; as also, infinite varieties in every several kind and sort of actions: but, the infinite varieties are made by the self-moving parts of nature, which are the corporeal figurative motions of nature. chap. xv. whether there be degrees of corporeal strength. as i have declared, there are (in my opinion) two sorts of self-moving parts; the one sensitive, the other rational. the rational parts of my mind, moving in the manner of conception, or inspection, did occasion some disputes, or arguments, amongst those parts of my mind. the arguments were these: _whether there were degrees of strength, as there was of purity, between their own sort, as, the rational and the sensitive?_ the major part of the argument was,_ that self-motion could be but self-motion: for, not any part of nature could move beyond its power of self-motion_. but the minor part argued, _that the self-motion of the rational, might be_ _stronger than the self-motion of the sensitive._ but the major part was of the opinion, _that there could be no degrees of the power of nature, or the nature of nature: for matter, which was nature, could be but self-moving, or not self-moving; or partly self-moving, or not self-moving._ but the minor argued, _that it was not against the nature of matter to have degrees of corporeal strength, as well as degrees of purity: for, though there could not be degrees of purity amongst the parts of the same sort, as amongst the parts of the rational, or amongst the parts of the sensitive; yet, if there were degrees of the rational and sensitive parts, there might be degrees of strength._ the major part said, _that if there were degrees of strength, it would make a confusion, by reason there would be no agreement; for, the strongest would be tyrants to the weakest, in so much as they would never suffer those parts to act methodically or regularly._ but the minor part said, _that they had observed, that there was degrees of strength amongst the sensitive parts._ the major part argued,_ that they had not degrees of strength by nature; but, that the greater number of parts were stronger than a less number of parts. also, there were some sorts of actions, that had advantage of other sorts. also, some sorts of compositions are stronger than other; not through the degrees of innate strength, nor through the number of parts; but, through the manner and form of their compositions, or productions._ thus my thoughts argued; but, after many debates and disputes, at last my rational parts agreed, that, if there were degrees of strength, it could not be between the parts of the same degree, or sort; but, between the rational and sensitive; and if so, the sensitive was stronger, being _less pure_; and the rational was more agil, being _more pure_. chap. xvi. of effects, and cause. to treat of infinite effects, produced from an an infinite cause, is an endless work, and impossible to be performed, or effected; only this may be said, that the effects, though infinite, are so united to the material cause, as that not any single effect can be, nor no effect can be annihilated; by reason all effects are in the power of the cause. but this is to be noted, that some effects producing other effects, are, in some sort or manner, a cause. chap. xvii. of influence. an _influence_ is this; when as the corporeal figurative motions, in different kinds, and sorts of creatures, or in one and the same sorts, or kinds, move sympathetically: and though there be antipathetical motions, as well as sympathetical; yet, all the infinite parts of matter, are agreeable in their nature, as being all material, and self-moving; and by reason there is no _vacuum_, there must of necessity be an influence amongst all the parts of nature. chap. xviii. of fortune and chance. _fortune_, is only various corporeal motions of several creatures, design'd to one creature, or more creatures; either to _that_ creature, or _those_ creatures advantage, or disadvantage: if advantage, man names it _good fortune_; if disadvantage, man names it _ill fortune_. as for _chance_, it is the visible effects of some hidden cause; and _fortune_, a sufficient cause to produce such effects: for, the conjunction of sufficient causes, doth produce such or such effects; which effects could not be produced, if any of those causes were wanting: so that, _chances_ are but the effects of fortune. chap. xix. of time and eternity. _time_ is not a thing by it self; nor is _time_ immaterial: for, _time_ is only the variations of corporeal motions; but eternity depends not on motion, but of a being without beginning, or ending. the second part. chap. i. of creatures. all creatures are composed-figures, by the consent of associating parts; by which association, they joyn into such, or such a figured creature: and though every corporeal motion, or self-moving part, hath its own motion; yet, by their association, they all agree in proper actions, as actions proper to their compositions: and, if every particular part, hath not a perception of all the parts of their association; yet, every part knows its own work. chap. ii. of knowledg and perception of different kinds and sorts of creatures. there is not any creature in nature, that is not composed of self-moving parts, (_viz._ both of rational and sensitive) as also of the inanimate parts, which are self-knowing: so that all creatures, being composed of these sorts of parts, must have a sensitive, and rational knowledg and perception, as animals, vegetables, minerals, elements, or what else there is in nature: but several kinds, and several sorts in these kinds of creatures, being composed after different manners, and ways, must needs have different lives, knowledges, and perceptions: and not only every several kind, and sort, have such differences; but, every particular creature, through the variations of their self-moving parts, have varieties of lives, knowledges, perceptions, conceptions, and the like; and not only so, but every particular part of one and the same creature, have varieties of knowledges, and perceptions, because they have varieties of actions. but, (as i have declared) there is not any different kind of creature, that can have the like life, knowledg, and perception; not only because they have different productions, and different forms; but, different natures, as being of different kinds. chap. iii. of perception of parts, and united perception. all the self-moving parts are perceptive; and, all perception is in parts, and is dividable, and compoundable, as being material; also, alterable, as being self-moving: wherefore, no creature that is composed, or consists of many several sorts of corporeal figurative motions, but must have many sorts of perception; which is the reason that one creature, as man, cannot perceive another man any otherwise but in parts: for, the rational, and sensitive; nay, all the parts of one and the same creature, perceive their adjoining parts, as they perceive foreign parts; only, by their close conjunction and near relation, they unite in one and the same actions. i do not say, they always agree: for, when they move irregularly, they disagree: and some of those united parts, will move after one manner, and some after another; but, when they move regularly, then they move to one and the same design, or one and the same united action. so, although a creature is composed of several sorts of corporeal motions; yet, these several sorts, being properly united in one creature, move all agreeably to the property and nature of the whole creature; that is, the particular parts move according to the property of the whole creature; because the particular parts, by conjunction, make the whole: so that, the several parts make one whole; by which, a whole creature hath both a general knowledg, and a knowledg of parts; whereas, the perceptions of foreign objects, are but in the parts: and this is the reason why one creature perceives not the whole of another creature, but only some parts. yet this is to be noted, that not any part hath another part's nature, or motion, nor therefore, their knowledg, or perception; but, by agreement, and unity of parts, there is composed perceptions. chap. iv. whether the rational and sensitive parts have a perception of each other. some may ask the question, _whether the rational and sensitive, have perception of each other?_ i answer: in my opinion, they have. for, though the rational and sensitive parts, be of two sorts; yet, both sorts have self-motion; so that they are but as one, as, that they are both corporeal motions; and, had not the sensitive parts incumbrances, they would be, in a degree, as agil, and as free as the rational. but, though each sort hath perception of each other, and some may have the like; yet they have not the same: for, not any part can have another's perception, or knowledg; but, by reason the rational and sensitive, are both corporeal motions, there is a strong sympathy between those sorts, in one conjunction, or creature. indeed, the rational parts are the designing parts; and the sensitive, the labouring parts; and the inanimate are as the material parts: not but all the three sorts are material parts; but the inanimate, being not self-moving, are the burdensome parts. chap. v. of thoughts, and the whole mind of a creature. as for thoughts, though they are several corporeal motions, or self-moving parts; yet, being united, by conjunction in one creature, into one whole mind, cannot be perceived by some parts of another creature, nor by the same sort of creature, as by another man. but some may ask, _whether the whole mind of one creature, as the whole mind of one man, may not perceive the whole mind of another man_? i answer, that if the mind was not joyn'd and mix'd with the sensitive and inanimate parts, and had not interior, as well as exterior parts, the whole mind of one man, might perceive the whole mind of another man; but, that being not possible, one whole mind cannot perceive another whole mind: by which observation we may perceive, there are no _platonick lovers_ in nature. but some may ask, whether the sensitive parts can perceive the rational, in one and the same creature? i answer, they do; for if they did not, it were impossible for the sensitive parts to execute the rational designs; so that, what the mind designs, the sensitive body doth put in execution, as far as they have power: but if, through irregularities, the body be sick, and weak, or hath some infirmities, they cannot execute the designs of the mind. chap. vi. whether the mind of one creature, can perceive the mind of another creature. some may ask the reason, _why one creature, as man, cannot perceive the thoughts of another man, as well as he perceives his exterior sensitive parts?_ i answer, that the rational parts of one man, perceive as much of the rational parts of another man, as the sensitive parts of that man doth of the sensitive parts of the other man; that is, as much as is presented to his perception: for, all creatures, and every part and particle, have those three sorts of matter; and therefore, every part of a creature is perceiving, and perceived. but, by reason all creatures are composed of parts, (_viz._ both of the rational and sensitive) all perceptions are in parts, as well the rational, as the sensitive perception: yet, neither the rational, nor the sensitive, can perceive all the interior parts or corporeal motions, unless they were presented to their perception: neither can one part know the knowledg and perception of another part: but, what parts of one creature are subject to the perception of another creature, those are perceived. chap. vii. of perception, and conception. although the exterior parts of one creature, can but perceive the exterior parts of another creature; yet, the rational can make conceptions of the interior parts, but not perception: for, neither the sense, nor reason, can perceive what is not present, but by rote, as after the manner of conceptions, or remembrances, as i shall in my following chapters declare: so that, the exterior rational parts, that are with the exterior sensitive parts of an object, are as much perceived, the one, as the other: but, those exterior parts of an object, not moving in particular parties, as in the whole creature, is the cause that some parts of one creature, cannot perceive the whole composition or frame of another creature: that is, some of the rational parts of one creature, cannot perceive the whole mind of another creature. the like of the sensitive parts. chap. viii. of human suppositions. although nature hath an infinite knowledg and perception; yet, being a body, and therefore divisible and compoundable; and having, also, self-motion, to divide and compound her infinite parts, after infinite several manners; is the reason that her finite parts, or particular creatures, cannot have a general or infinite knowledg, being limited, by being finite, to finite perceptions, or perceptive knowledg; which is the cause of _suppositions_, or imaginations, concerning forrein objects: as for example, a man can but perceive the exterior parts of another man, or any other creature, that is subject to human perception; yet, his rational parts may suppose, or presuppose, what another man thinks, or what he will act: and for other creatures, a man may suppose or imagine what the innate nature of such a vegetable, or mineral, or element is; and may imagine or suppose the moon to be another world, and that all the fixed starrs are sunns; which suppositions, man names _conjectures_. chap. ix. of information between several creatures. no question but there is _information_ between all creatures: but, several sorts of creatures, having several sorts of informations, it is impossible for any particular sort to know, or have perceptions of the infinite, or numberless informations, between the infinite and numberless parts, or creatures of nature: nay, there are so many several informations amongst one sort (as of mankind) that it is impossible for one man to perceive them all; no, nor can one man generally perceive the particular informations that are between the particular parts of his sensitive body; or between the particular informations of his rational body; or between the particular rational and sensitive parts: much less can man perceive, or know the several informations of other creatures. chap. x. the reason of several kinds and sorts of creatures. some may ask, _why there are such sorts of creatures, as we perceive there are, and not other sorts?_ i answer, that, 'tis probable, we do not perceive all the several kinds and sorts of creatures in nature: in truth, it is impossible (if nature be infinite) for a finite to perceive the infinite varieties of nature._ also they may ask, why the planets are of a spherical shape, and human creatures are of an upright shape, and beasts of a bending and stooping shape? also, why birds are made to flye, and not beasts? and for what cause, or design, have animals such and such sorts of shapes and properties? and vegetables such and such sorts of shapes and properties? and so of minerals and elements?_ i answer; that several sorts, kinds, and differences of particulars, causes order, by reason it causes distinctions: for, if all creatures were alike, it would cause a confusion. chap. xi. of the several properties of several kinds and sorts of creatures. as i have said, there are several kinds, and several sorts, and several particular creatures of several kinds and sorts; whereof there are some creatures of a mixt kind, and some of a mixt sort, and some of a mixture of some particulars. also, there are some kind of creatures, and sorts of creatures; as also particulars of a dense nature, others of a rate nature; some of a leight nature, some of a heavy nature; some of a bright nature, some of a dark nature; some of an ascending nature, some of a descending nature; some of a hard nature, some of a soft nature; some of a loose nature, and some of a fixt nature; some of an agil nature, and some of a slow nature; some of a consistent nature, and some of a dissolving nature: all which is according to the frame and form of their society, or composition. the third part. chap. i. of productions in general. the self-moving parts, or corporeal motions, are the producers of all composed figures, such as we name _creatures_: for, though all matter hath figure, by being matter; for it were non-sense to say, _figureless matter_; since the most pure parts of matter, have figure, as well as the grossest; the rarest, as well as the densed: but, such composed figures which we name _creatures_, are produced by particular associations of self-moving parts, into particular kinds, and sorts; and particular creatures in every kind, or sort. the particular kinds, that are subject to human perceptions, are those we name animals, vegetables, minerals, and elements; of which kinds, there are numerous sorts; and of every sort, infinite particulars: and though there be infinite varieties in nature, made by the corporeal motions, or self-moving parts, which might cause a confusion: yet, considering nature is intire in her self, as being only material, and as being but one united body; also, poysing all her actions by opposites; 'tis impossible to be any ways in extreams, or to have a confusion. chap. ii. of productions in general. the sensitive self-moving parts, or corporeal motions, are the labouring parts of all productions, or fabricks of all creatures; but yet, those corporeal motions, are parts of the creature they produce: for, production is only a society of particular parts, that joyn into particular figures, or creatures: but, as parts produce figures, by association; so they dissolve those figures by division: for, matter is a perpetual motion, that is always dividing and composing; so that not any creature can be eternally one and the same: for, if there were no dissolvings, and alterings, there would be no varieties of particulars; for, though the kinds and sorts may last, yet not the particulars. but, mistake me not, i do not say those figures are lost, or annihilated in nature; but only, their society is dissolved, or divided in nature. but this is to be noted, that some creatures are sooner produced and perfected, than others; and again, some creatures are sooner decayed, or dissolved. chap. iii. of productions in general. there are so many different composed parts, and so much of variety of action in every several part of one creature, as 'tis impossible for human perception to perceive them; nay, not every corporeal motion of one creature, doth perceive all the varieties of the same society; and, by the several actions, not only of several parts, but of one and the same parts, cause such obscurity, as not any creature can tell, not only how they were produced, but, not how they consist: but, by reason every part knows his own work, there is order and method: for example, in a human creature, those parts that produce, or nourish the bones, those of the sinews, those of the veins, those of the flesh, those of the brains, and the like, know all their several works, and consider not each several composed part, but what belongs to themselves; the like, i believe, in vegetables, minerals, or elements. but mistake me not; for, i do not say, those corporeal motions in those particulars, are bound to those particular works, as, that they cannot change, or alter their actions if they will, and many times do: as some creatures dissolve before they are perfect, or quite finished; and some as soon as finished; and some after some short time after they are finished; and some continue long, as we may perceive by many creatures that dye, which i name dissolving in several ages; but, untimely dissolutions, proceed rather from some particular irregularities of some particular parts, than by a general agreement. chap. iv. of productions in general. the reason that all creatures are produced by the ways of production, as one creature to be composed out of other creatures, is, that nature is but one matter, and that all her parts are united as one material body, having no additions, or diminutions; no new creations, or annihilations: but, were not nature one and the same, but that her parts were of different natures; yet, creatures must be produced by creatures, that is, composed figures, as a beast, a tree, a stone, water, &c. must be composed of _parts_, not a _single part_: for, a single part cannot produce composed figures; nor can a single part produce another single part; for, matter cannot create matter; nor can one part produce another part out of it self: wherefore, all natural creatures are produced by the consent and agreement of many self-moving parts, or corporeal motions, which work to a particular design, as to associate into particular kinds and sorts of creatures. chap. v. of productions in general. as i said in my former chapter, that all creatures are produced, or composed by the agreement and consent of particular parts; yet some creatures are composed of more, and some of fewer parts: neither are all creatures produced, or composed after one and the same manner; but some after one manner, and some after another manner: indeed, there are divers manners of productions, both of those we name natural, and those we name _artificial_; but i only treat of natural productions, which are so various, that it is a wonder if any two creatures are just alike; by which we may perceive, that not only in several kinds and sorts, but in particulars of every kind, or sort, there is some difference, so as to be distinguished from each other, and yet the species of some creatures are like to their kind, and sort, but not all; and the reason that most creatures are in _species_, according to their sort, and kind, is not only, that nature's wisdom orders and regulates her corporeal figurative motions, into kinds and sorts of societies and conjunctions; but, those societies cause a perceptive acquaintance, and an united love, and good liking of the compositions, or productions: and not only a love to their figurative compositions, but to all that are of the same sort, or kind; and especially, their being accustom'd to actions proper to their figurative compositions, is the cause that those parts, that divide from the producers, begin a new society, and, by degrees, produce the like creature; which is the cause that animals and vegetables produce according to their likeness. the same may be amongst minerals and elements, for all we can know. but yet, some creatures of one and the same sort, are not produced after one and the same manner: as for example, one and the same sort of vegetables, may be produced after several manners, and yet, in the effect, be the same, as when vegetables are sowed, planted, engrafted; as also, seeds, roots, and the like, they are several manners, or ways of productions, and yet will produce the same sort of vegetable: but, there will be much alterations in replanting, which is occasioned by the change of associating parts, and parties; but as for the several productions of several kinds and sorts, they are very different; as for example, animals are not produced as vegetables, or vegetables as minerals, nor minerals as any of the rest: nor are all animals produced alike, nor minerals, or vegetables; but after many different manners, or ways. neither are all productions like their producers; for, some are so far from resembling their figurative society, that they produce another kind, or sort of composed figures; as for example, maggots out of cheese, other worms out of roots, fruits, and the like: but these sorts of creatures, man names _insects_; but yet they are animal creatures, as well as others. chap. vi. of productions in general. all creatures are produced, and producers; and all these productions partake more or less of the producers; and are necessitated so to do, because there cannot be any thing new in nature: for, whatsoever is produced, is of the same matter; nay, every particular creature hath its particular parts: for, not any one creature can be produced of any other parts than what produced it; neither can the same producer produce one and the same double, (as i may say to express my self:) for, though the same producers may produce the like, yet not the same: for, every thing produced, hath its own corporeal figurative motions; but this might be, if nature was not so full of variety: for, if all those corporeal motions, or self-moving parts, did associate in the like manner, and were the very same parts, and move in the very same manner; the same production, or creature, might be produced after it was dissolved; but, by reason the self-moving parts of nature are always dividing and composing _from_, and _to_ parts, it would be very difficult, if not impossible. chap. vii. of productions in general. as there are productions, or compositions, made by the sensitive corporeal motions, so there are of the rational corporeal motions, which are composed figures of the mind: and the reason the rational productions are more various, as also more numerous, is, that the rational is more loose, free, and so more agil than the sensitive; which is also the reason that the rational productions require not such degrees of time, as the sensitive. but i shall treat more upon this subject, when i treat of that animal we name _man_. chap. vii. _lastly_, of productions in general. though all creatures are made by the several associations of self-moving parts, or (as the learned name them) _corporeal motions_; yet, there are infinite varieties of corporeal figurative motions, and so infinite several manners and ways of productions; as also, infinite varieties of figurative motions in every produced creature: also, there is variety in the difference of time, of several productions, and of their consistency and dissolution: for, some creatures are produced in few hours, others not in many years. again, some continue not a day; others, numbers of years. but this is to be noted, that according to the regularity, or irregularity of the associating motions, their productions are more or less perfect. also, this is to be noted, that there are rational productions, as well as sensitive: for, though all creatures are composed both of sensitive and rational parts, yet the rational parts move after another manner. chap. viii. productions must partake of some parts of their producers. no animal, or vegetable, could be produced, but by such, or such particular producers; neither could an animal, or vegetable, be produced without some corporeal motions of their producers; that is, some of the producers self-moving parts; otherwise the like actions might produce, not only the like creatures, but the same creatures, which is impossible: wherefore, the things produced, are part of the producers; for, no particular creature could be produced, but by such particular producers. but this is to be noted, that all sorts of creatures are produced by more, or fewer, producers. also, the first producers are but the first founders of the things produced, but not the only builders: for, there are many several sorts of corporeal motions, that are the builders; for, no creature can subsist, or consist, by it self, but must assist, and be assisted: yet, there are some differences in all productions, although of the same producers; otherwise all the off-springs of one and the same producer, would be alike: and though, sometimes, their several off-springs may be so alike, as hardly to be distinguished; yet, that is so seldom, as it appears as a wonder; but there is a property in all productions, as, for the produced to belong as a right and property to the _producer_. chap. ix. of resemblances of several off-springs, or producers. there are numerous kinds and sorts of productions, and infinite manners and ways, in the actions of productions; which is the cause that the off-springs of the same producers, are not so just alike, but that they are distinguishable; but yet there may not only be resemblances between particular off-springs of the same producers, as also of the same sort; but, of different sorts of creatures: but the actions of all productions that are according to their own _species_, are imitating actions, but not bare imitations, as by an incorporeal motion; for if so, then a covetous woman, that loves gold, might produce a wedg of gold instead of a child; also, _virgins_ might be as fruitful as _married wives_. chap. x. of the several appearances of the exterior parts of one creature. every altered action of the exterior parts, causes an altered appearance: as for example, a man, or the like creature, doth not appear when he is old, as when he was young; nor when he is sick, as when he is well in health; no, nor when he is cold, as when he is hot. nor do they appear in several passions alike: for, though man can best perceive the alteration of his own kind, or sort; yet, other creatures have several appearances, as well as man; some of which, man may perceive, though not all, being of a different sort. and not only animals, but vegetables, and elements, have altered appearances, and many that are subject to man's perception. the fourth part. chap. i. of animal productions; and of the differences between productions, and transformations. i understand productions to be between particulars; as, some particular creatures to produce other particular creatures; but not to transform from one sort of creature, into another sort of creature, as cheese into maggots, and fruit into worms, &c. which, in some manner, is like metamorphosing. so by transformation, the intellectual nature, as well as the exterior form, is transform'd: whereas production transforms only the exterior form, but not the intellectual nature; which is the cause that such transformations cannot return into their former state; as a worm to be a fruit, or a maggot a cheese again, as formerly. hence i perceive, that all sorts of fowls are partly produced, and partly transformed: for, though an egg be produced, yet a chicken is but a transformed egg. chap. ii. of different figurative motions in man's production. all creatures are produced by degrees; which proves, that not any creature is produced, in perfection, by one act, or figurative motion: for, though the producers are the first founders, yet not the builders. but, as for animal creatures, there be some sorts that are composed of many different figurative motions; amongst which sorts, is mankind, who has very different figurative parts, as bones, sinews, nerves, muscles, veins, flesh, skin, and marrow, blood, choler, flegm, melancholy, and the like; also, head, breast, neck, arms, hands, body, belly, thighs, leggs, feet, &c. also, brains, lungs, stomack, heart, liver, midriff, kidnies, bladder, guts, and the like; and all these have several actions, yet all agree as one, according to the property of that sort of creature named man. chap. iii. of the quickning of a child, or any other sort of animal creatures. the reason that a woman, or such like animal, doth not feel her child so soon as it is produced, is, that the child cannot have an animal motion, until it hath an animal nature, that is, until it be perfectly an animal creature; and as soon as it is a perfect child, she feels it to move, according to its nature: but it is only the sensitive parts of the child that are felt by the mother, not the rational; because those parts are as the designers, not the builders; and therefore, being not the labouring parts, are not the sensible parts. but it is to be noted, that, according to the regularity, or irregularity of the figurative motions, the child is _well shaped_, or <strong>mishaped</strong>. chap. iv. of the birth of a child. the reason why a child, or such like animal creature, stays no longer in the mother's body, than to such a certain time, is, that a child is not perfect before that time, and would be too big after that time; and so big, that it would not have room enough; and therefore it strives and labours for liberty. chap. v. of mischances, or miscarriages of breeding creatures. when a mare, doe, hind, or the like animal, cast their young, or a woman miscarries of her child, the mischance proceeds either through the irregularities of the corporeal motions, or parts of the child; or through some irregularity of the parts of the mother; or else of both mother and child. if the irregularities be of the parts of the child, those parts divide from the mother, through their irregularity: but, if the irregularity be in the parts of the mother, then the mother divides in some manner from the child; and if there be a distemper in both of them, the child and mother divide from each other: but, such mischances are at different times, some sooner, and some later. as for false conceptions, they are occasioned through the irregularities of conception. chap. vi. of the encrease of growth, and strength of mankind, or such like creatures. the reason most animals, especially human creatures, are weak whilst they are infants, and that their strength and growth encreases by degrees, is, that a child hath not so many parts, as when he is a youth; nor so many parts when he is a youth, as when he is a man: for, after the child is parted from the mother, it is nourished by other creatures, as the mother was, and the child by the mother; and according as the nourishing parts be regular, or irregular, so is the child, youth, or man, weaker, or stronger; healthful, or diseased; and when the figurative motions move (as i may say for expression sake) curiously, the body is neatly shaped, and is, as we say, beautiful. but this is to be noted, that 'tis not greatness, or bulk of body, makes a body perfect; for, there are several sizes of every sort, or kind of creatures; as also, in every particular kind, or sort; and every several size may be as perfect, one, as the other: but, i mean the number of parts, according to the proper size. chap. vii. of the several properties of the several exterior shapes of several sorts of animals. the several exterior shapes of creatures, cause several properties, as running, jumping, hopping, leaping, climbing, galloping, trotting, ambling, turning, winding, and rowling; also creeping, crawling, flying, soaring or towring; swimming, diving, digging, stinging or piercing; pressing, spinning, weaving, twisting, printing, carving, breaking, drawing, driving, bearing, carrying, holding, griping or grasping, infolding, and millions of the like. also, the exterior shapes cause defences, as horns, claws, teeth, bills, talons, finns, _&c._ likewise, the exterior shapes cause offences, and give offences: as also, the different sorts of exterior shapes, cause different exterior perceptions. chap. viii. of the dividing and uniting parts of a particular creature. those parts (as i have said) that were the first founders of an animal, or other sort of creature, may not be constant inhabitants: for, though the society may remain, the particular parts may remove: also, all particular societies of one kind, or sort, may not continue the like time; but some may dissolve sooner than others. also, some alter by degrees, others of a sudden; but, of those societies that continue, the particular parts remove, and other particular parts unite; so, as some parts _were_ of the society, so some other parts are of the society, and _will be_ of the society: but, when the form, frame, and order of the society begins to alter, then that particular creature begins to decay. but this is to be noted, that those particular creatures that dye in their childhood, or youth, were never a full and regular society; and the dissolving of a society, whether it be a full, or but a forming society, man names _death_. also, this is to be noted, that the nourishing motion of food, is the uniting motion; and the cleansing, or evacuating motions, are the dividing corporeal motions. likewise it is to be noted, that a society requires a longer time of uniting than of dividing; by reason uniting requires assistance of foreign parts, whereas dividings are only a dividing of home-parts. also, a particular creature, or society, is longer in dividing its parts, than in altering its actions; because a dispersing action is required in division, but not in alteration of actions. the fifth part. chap. i. of man. now i have discoursed, in the former parts, after a general manner, of _animals_: i will, in the following chapters, speak more particularly of that sort we name _mankind_; who believe (being ignorant of the nature of other creatures) that they are the most knowing of all creatures; and yet a _whole man_ (as i may say for expression-sake) doth not know all the figurative motions belonging either to his mind, or body: for, he doth not generally know every particular action of his corporeal motions, as, how he was framed, or formed, or perfected. nor doth he know every particular motion that occasions his present consistence, or being: nor every particular digestive, or nourishing motion: nor, when he is sick, the particular irregular motion that causes his sickness. nor do the rational motions in the head, know always the figurative actions of those of the heel. in short, (as i said) man doth not generally know every particular part, or corporeal motion, either of mind, or body: which proves, man's natural soul is not inalterable, or individable, and uncompoundable. chap. ii. of the variety of man's natural motions. there is abundance of varieties of figurative motions in man: as, first, there are several figurative motions of the form and frame of man, as of his innate, interior, and exterior figurative parts. also, there are several figures of his several perceptions, conceptions, appetite, digestions, reparations, and the like. there are also several figures of several postures of his several parts; and a difference of his figurative motions, or parts, from other creatures; all which are numberless: and yet all these different actions are proper to the nature of _man_. chap. iii. of man's shape and speech. the shape of man's sensitive body, is, in some manner, of a mixt form: but, he is singular in this, that he is of an upright and straight shape; of which, no other animal but man is: which shape makes him not only fit, proper, easie and free, for all exterior actions; but also for speech: for being streight, as in a straight and direct line from the head to the feet, so as his nose, mouth, throat, neck, chest, stomack, belly, thighs, and leggs, are from a straight line: also, his organ-pipes, nerves, sinews, and joynts, are in a straight and equal posture to each other; which is the cause, man's tongue, and organs, are more apt for speech than those of any other creature; which makes him more apt to imitate any other creature's voyces, or sounds: whereas other animal creatures, by reason of their bending shapes, and crooked organs, are not apt for speech; neither (in my opinion) have other animals so melodious a sound, or voice, as man: for, though some sorts of birds voices are sweet, yet they are weak, and faint; and beasts voices are harsh, and rude: but of all other animals, besides man, birds are the most apt for speech; by reason they are more of an upright shape, than beasts, or any other sorts of animal creatures, as fish, and the like; for, birds are of a straight and upright shape, as from their breasts, to their heads; but, being not so straight as man; causes birds to speak uneasily, and constrainedly: man's shape is so ingeniously contrived, that he is fit and proper for more several sorts of exterior actions, than any other animal creature; which is the cause he seems as lord and sovereign of other animal creatures. chap. iv. of the several figurative parts of human creatures. the manner of man's composition, or form, is of different figurative parts; whereof some of those parts seem the supreme, or (as i may say) fundamental parts; as the head, chest, lungs, stomack, heart, liver, spleen, bowels, reins, kidnies, gaul, and many more: also, those parts have other figurative parts belonging or adjoining to them, as the head, scull, brains, _pia-mater, dura-mater_, forehead, nose, eyes, cheeks, ears, mouth, tongue, and several figurative parts belonging to those; so of the rest of the parts, as the arms, hands, fingers, leggs, feet, toes, and the like: all which different parts, have different sorts of perceptions; and yet (as i formerly said) their perceptions are united: for, though all the parts of the human body have different perceptions; yet those different perceptions unite in a general perception, both for the subsistence, consistence, and use of the whole man: but, concerning particulars, not only the several composed figurative parts, have several sorts of perceptions; but every part hath variety of perceptions, occasioned by variety of objects. chap. v. of the several perceptions amongst the several parts of man. there being infinite several corporeal figurative motions, or actions of nature, there must of necessity be infinite several self-knowledges and perceptions: but i shall only, in this part of my book, treat of the perception proper to mankind: and first, of the several and different perceptions, proper for the several and different parts: for, though every part and particle of a man's body, is perceptive; yet, every particular part of a man, is not generally perceived; for, the interior parts do not generally perceive the exterior; nor the exterior, generally or perfectly, the interior; and yet, both interior and exterior corporeal motions, agree as one society; for, every part, or corporeal motion, knows its own office; like as officers in a common-wealth, although they may not be acquainted with each other, yet they know their employments: so every particular man in a common-wealth, knows his own employment, although he knows not every man in the common-wealth. the same do the parts of a man's body, and mind. but, if there be any irregularity, or disorder in a common-wealth, every particular is disturbed, perceiving a disorder in the common-wealth. the same amongst the parts of a man's body; and yet many of those parts do not know the particular cause of that general disturbance. as for the disorders, they may proceed from some irregularities; but for peace, there must be a general agreement, that is, every part must be regular. chap. vi. of divided and composed perceptions. as i have formerly said, there is in nature both divided and composed perceptions; and for proof, i will mention man's exterior perceptions; as for example, man hath a composed perception of seeing, hearing, smelling, tasting, and touching; whereof every several sort is composed, though after different manners, or ways; and yet are divided, being several sorts of perceptions, and not all one perception. yet again, they are all composed, being united as proper perceptions of one man; and not only so, but united to perceive the different parts of one object: for, as perceptions are composed of parts, so are objects; and as there are different objects, so there are different perceptions; but it is not possible for a man to know all the several sorts of perceptions proper to every composed part of his body or mind, much less of others. chap. vii. of the ignorances of the several perceptive organs. as i said, that every several composed perception, was united to the proper use of their whole society, as one man; yet, every several perceptive organ of man is ignorant of each other; as the perception of sight is ignorant of that of hearing; the perception of hearing, is ignorant of the perception of seeing; and the perception of smelling is ignorant of the perceptions of the other two, and those of scent, and the same of tasting, and touching: also, every perception of every particular organ, is different; but some sorts of human perceptions require some distance between them and the object: as for example, the perception of sight requires certain distances, as also magnitudes; whereas the perception of touch requires a joyning-object, or part. but this is to be noted, that although these several organs are not perfectly, or throughly acquainted; yet in the perception of the several parts of one object, they do all agree to make their several perceptions, as it were by one act, at one point of time. chap. viii. of the particular and general perceptions of the exterior parts of human creatures. there is amongst the exterior perceptions of human creatures, both particular sorts of perceptions, and general perceptions: for, though none of the exterior parts, or organs, have the sense of seeing, but the eyes; of hearing, but the ears; of smelling, but the nose; of tasting, but the mouth: yet all the exterior parts have the perception of touching; and the reason is, that all the exterior parts are full of pores, or at least, of such composed parts, that are the sensible organs of touching: yet, those several parts have several touches; not only because they have several parts, but because those organs of touching, are differently composed. but this is to be noted, that every several part hath perception of the other parts of their society, as they have of foreign parts; and, as the sensitive, so the rational parts have such particular and general perceptions. but it is to be noted, that the rational parts, are parts of the same organs. chap. ix. of the exterior sensitive organs of human creatures. as for the manner, or ways, of all the several sorts, and particular perceptions, made by the different composed parts of human creatures; it is impossible, for a human creature, to know any otherwise, but in part: for, being composed of parts, into parties, he can have but a parted knowledg, and a parted perception of himself: for, every different composed part of his body, have different sorts of self-knowledg, as also, different sorts of perceptions; but yet, the manner and way of some human perceptions, may probably be imagined, especially those of the exterior parts, man names the _sensitive organs_; which parts (in my opinion) have their perceptive actions, after the manner of patterning, or picturing the exterior form, or frame, of foreign objects: as for example, the present object is a candle; the human organ of sight pictures the flame, light, week, or snuff, the tallow, the colour, and the dimension of the candle; the ear patterns out the sparkling noise; the nose patterns out the scent of the candle; and the tongue may pattern out the tast of the candle: but, so soon as the object is removed, the figure of the candle is altered into the present object, or as much of one present object, as is subject to human perception. thus the several parts or properties, may be patterned out by the several organs. also, every altered action, of one and the same organ, are altered perceptions; so as there may be numbers of several pictures or patterns made by the sensitive actions of one organ; i will not say, by one act; yet there may be much variety in one action. but this is to be noted, that the object is not the _cause_ of perception, but is only the _occasion_: for, the sensitive organs can make such like figurative actions, were there no object present; which proves, that the object is not the cause of the perception. also, when as the sensitive parts of the sensitive organs, are irregular, they will make false perceptions of present objects; wherefore the object is not the cause. but one thing i desire, not to be mistaken in; for i do not say, that all the parts belonging to any of the particular organs, move only in one sort or kind of perception; but i say, some of the parts of the organ, move to such, or such perception: for, all the actions of the ears, are not only hearing; and all the actions of the eye, seeing; and all the actions of the nose, smelling; and all the actions of the mouth, tasting; but, they have other sorts of actions: yet, all the sorts of every organ, are according to the property of their figurative composition. chap. x. of the rational parts of the human organs. as for the rational parts of the human organs, they move according to the sensitive parts, which is, to move according to the figures of foreign objects; and their actions are (if regular) at the same point of time, with the sensitive: but, though their actions are alike, yet there is a difference in their degree; for, the figure of an object in the mind, is far more pure than the figure in the sense. but, to prove that the rational (if regular) moves with the sense, is, that all the several sensitive perceptions of the sensitive organs, (as all the several sights, sounds, scents, tasts, and touches) are thoughts of the same. chap. xi. of the difference between the human conception, and perception. there are some differences between perception, and conception: for, perception doth properly belong to present objects; whereas conceptions have no such strict dependency: but, conceptions are not proper to the sensitive organs, or parts of a human creature; wherefore, the sensitive never move in the manner of conception, but after an irregular manner; as when a human creature is in some violent passion, mad, weak, or the like distempers. but this is to be noted, that all sorts of fancies, imaginations, _&c._ whether sensitive, or rational, are after the manner of conceptions, that is, do move by rote, and not by example. also, it is to be noted, that the rational parts can move in more various figurative actions than the sensitive; which is the cause that a human creature hath more conceptions than perceptions; so that the mind can please it self with more variety of thoughts than the sensitive with variety of objects: for variety of objects consists of foreign parts; whereas variety of conceptions consists only of their own parts: also, the sensitive parts are sooner satisfied with the perception of particular objects, than the mind with particular remembrances. chap. xii. of the several varieties of actions of human creatures. to speak of all the several actions of the sensitive and rational parts of one creature, is not possible, being numberless: but, some of those that are most notable, i will mention, as, respirations, digestions, nourishments, appetites, satiety, aversions, conceptions, opinions, fancies, passions, memory, remembrance, reasoning, examining, considering, observing, distinguishing, contriving, arguing, approving, disapproving, discoveries, arts, sciences. the exterior actions are, walking, running, dancing, turning, tumbling, bearing, carrying, holding, striking, trembling, sighing, groaning, weeping, frowning, laughing, speaking, singing and whistling: as for postures, they cannot be well described; only, standing, sitting, and lying. chap. xiii. of the manner of information between the rational and sensitive parts. the manner of information amongst the self-moving parts of a human creature, is after divers and several manners, or ways, amongst the several parts: but, the manner of information between the sensitive and rational parts, is, for the most part, by imitation; as, imitating each other's actions: as for example, the rational parts invent some sciences; the sensitive endeavour to put those sciences into an art. if the rational perceive the sensitive actions are not just, according to that science, they inform the sensitive; then the sensitive parts endeavour to work, according to the directions of the rational: but, if there be some obstruction or hindrance, then the rational and sensitive agree to declare their design, and to require assistance of other associates, which are other men; as also, other creatures. as for the several manners and informations between man and man, they are so ordinary, i shall not need to mention them. chap. xiv. of irregularities and regularities of the self-moving parts of human creatures. nature being poised, there must of necessity be irregularities, as well as regularities, both of the rational and sensitive parts; but when the rational are irregular, and the sensitive regular, the sensitive endeavour to rectifie the errors of the rational. and if the sensitive be irregular, and the rational regular, the rational do endeavour to rectifie the errors of the sensitive: for, the particular parts of a society, are very much assistant to each other; as we may observe by the exterior parts of human bodies; the hands endeavour to assist any part in distress; the leggs will run, the eyes will watch, the ears will listen, for any advantage to the society; but when there is a general irregularity, then the society falls to ruine. chap. xv. of the agreeing, or disagreeing, of the sensitive and rational parts of human creatures. there is, for the most part, a general agreement between the rational and sensitive parts of human creatures; not only in their particular, but general actions; only the rational are the designing-parts; and the sensitive, the labouring parts: as for proof, the mind designs to go to such, or such foreign parts, or places; upon which design the sensitive parts will labour to execute the mind's intention, so as the whole sensitive body labours to go to the designed place, without the mind's further concern: for, the mind takes no notice of every action of the sensitive parts; neither of those of the eyes, ears; or of the leggs, or feet; nor of their perceptions: for, many times, the mind is busied in some conception, imagination, fancy, or the like; and yet the sensitive parts execute the mind's design exactly. but, for better proof, when as the sensitive parts are sick, weak, or defective, through some irregularities, the sensitive parts cannot execute the mind's design: also, when the sensitive parts are careless, they oft mistake their way; or when they are irregularly opposed, or busied about some appetite, they will not obey the mind's desire; all which are different degrees of parts. but, as it is amongst the particular parts of a society; so, many times, between several societies; for, sometimes, the sensitive parts of two men will take no notice of each other: as for example, when two men speak together, one man regards not what the other says; so many times, the sensitive parts regard not the propositions of the rational; but then the sensitive is not perfectly regular. chap. xvi. of the power of the rational; or rather, of the indulgency of the sensitive. the rational corporeal motions, being the purest, most free, and so most active, have great power over the sensitive; as to perswade, or command them to obedience: as for example, when a man is studying about some inventions of poetical fancies, or the like; though the sensitive corporeal motions, in the sensitive organs, desire to desist from patterning of objects, and would move towards sleep; yet the rational will not suffer them, but causes them to work, viz. to write, or to read, or do some other labour: also, when the rational mind is merry, it will cause the leggs to dance, the organs of the voice to sing, the mouth to speak, to eat, to drink, and the like: if the mind moves to sadness, it causes the eyes to weep, the lungs to sigh, the mouth to speak words of complaint. thus the rational corporeal motions of the mind, will occasion the senses to watch, to work, or to sport and play. but mistake me not; for i do not mean, the senses are bound to obey the rational designs; for, the sensitive corporeal motions, have as much freedom of self-moving, as the rational: for, the command of the rational, and the obedience of the sensitive, is rather an agreement, than a constraint: for, in many cases, the sensitive will not agree, and so not obey: also, in many cases, the rational submits to the sensitive: also, the rational sometimes will be irregular; and, on the other side, sometimes the sensitive will be irregular, and the rational regular; and sometimes both irregular. chap. xvii. of human appetites and passions. the sensitive appetites, and the rational passions do so resemble each other, as they would puzzle the most wise philosopher to distinguish them; and there is not only a resemblance, but, for the most part, a sympathetical agreement between the appetites, and the passions; which strong conjunction, doth often occasion disturbances to the whole life of man; with endless desires, unsatiable appetites, violent passions, unquiet humors, grief, pain, sadness, sickness, and the like; through which, man seems to be more restless, than any other creature: but, whether the cause be in the manner, or form of man's composition, or occasioned by some irregularities; i will leave to those who are wiser than i, to judg. but this is to be noted, that the more changes and alterations the rational and sensitive motions make, the more variety of passions and appetites the man hath: also, the quicker the motions are, the sharper appetite, and the quicker wit, man hath. but, as all the human senses are not bound to one organ; so all knowledges are not bound to one sense, no more than all the parts of matter to the composition of one particular creature: but, by some of the rational and sensitive actions, we may perceive the difference of some of the sensitive and rational actions; as, sensitive pain, rational grief; sensitive pleasure, rational delight; sensitive appetite, rational desire; which are sympathetical actions of the rational and sensitive parts: also, through sympathy, rational passions will occasion sensitive appetites; and appetites, the like passions. chap. xviii. of the rational actions of the head and heart of human creatures. as i formerly said, in every figurative part of a human creature, the actions are different, according to the property of their different composers; so that the motions of the heart are different to the motions of the head, and of the other several parts: but, as for the motions of the head, they are (in my opinion) more after the manner of emboss'd figures; and those of the heart, more after the manner of flat figures; like painting, printing, engraving, _&c._ for, if we observe, the thoughts in our heads are different from the thoughts in our hearts. i only name these two parts, by reason they seem to sympathize, or to agree, more particularly to each other's actions, than some of the other parts of human creatures. chap. xix. of passions and imaginations. some sorts of passions seem to be in the heart; as, love, hate, grief, joy, fear, and the like; and all imaginations, fancies, opinions, inventions, _&c._ in the head. but, mistake me not, i do not say, that none of the other parts of a man have not passions and conceptions: but, i say, they are not after the same manner, or way, as in the heart, or head: as for example, every part of a man's body is sensible, yet not after one and the same manner: for, every part of a man's body hath different perceptions, as i have formerly declared, and yet may agree in general actions: but, unless the several composed parts of a human creature, had not several perceptive actions, it were impossible to make a general perception, either amongst the several parts of their own society, or of foreign objects. but, it is impossible for me to describe the different manners and ways of the particular parts, or the different actions of any one part: for, what man can describe the different perceptive actions of that composed part, the eye, and so of the rest of the parts. chap. xx. that associations, divisions, and alterations, cause several effects. the rational and sensitive corporeal motions, are the perceptive parts of nature; and that which causes acquaintance amongst some parts, is their uniting and association: that which loses acquaintance of other parts, is their divisions and alterations: for, as self-compositions cause particular knowledges, or acquaintances: so self-divisions cause particular ignorances, or forgetfulnesses: for, as all kinds and sorts of creatures are produced, nourished, and encreased by the association of parts; so are all kinds and sorts of perceptions; and according as their associations, or their compositions do last, so doth their acquaintance; which is the cause, that the observations and experiences of several and particular creatures, such as men, in several and particular ages, joyned as into one man or age, causes strong and long-liv'd opinions, subtile and ingenious inventions, happy and profitable advantages; as also, probable conjectures, and many truths, of many causes and effects: whereas, the divisions of particular societies, causes what we name death, ignorance, forgetfulness, obscurity of particular creatures, and of perceptive knowledges; so that as particular perceptive knowledges do alter and change, so do particular creatures: for, though the kinds and sorts last, yet the particulars do not. chap. xxi. of the differences between self-love, and passionate love. self-love, is like self-knowledg, which is an innate nature; and therefore is not that love man names passionate love: for, _passionate love_ belongs to several parts; so that the several parts of one society, as one creature, have both passionate love, and self-love, as being sympathetically united in one society: also, not only the parts of one and the same society, may have passionate love to each other; but, between several societies; and not only several societies of one sort, but of different sorts. the sixth part. chap. i. of the motions of some parts of the mind; and of forrein objects. notions, imaginations, conceptions, and the like, are such actions of the mind, as concern not forrein objects: and some notions, imaginations, or conceptions of one man, may be like to another man, or many men. also, the mind of one man may move in the like figurative actions, as the sensitive actions of other sorts of creatures; and that, man names _understanding_: and if those conceptions be afterwards produced, man names them _prudence_, or _fore-sight_; but if those parts move in such inventions as are capable to be put into arts, man names that, _ingenuity_: but, if not capable to be put into the practice of arts, man names it, _sciences_: if those motions be so subtile, that the sensitive cannot imitate them, man names them, _fancies_: but, when those rational parts move promiscuously, as partly after their own inventions, and partly after the manner of forrein or outward objects; man names them, _conjectures_, or _probabilities_: and when there are very many several figurative, rational motions, then man says, _the mind is full of thoughts_: when those rational figurative motions, are of many and different objects, man names them, _experiences_, or _learning_: but, when there are but few different sorts of such figurative motions, man names them _ignorances_. chap. ii. of the motions of some parts of the mind. when the rational figurative corporeal motions of an human creature, take no notice of forrein objects, man nameth that, _musing_, or _contemplating_. and, when the rational parts repeat some former actions, man names that, _remembrances_. but, when those parts alter those repetitions, man names that, _forgetfulness_. and, when those rational parts move, according to a present object, man names it, _memory_. and when those parts divide in divers sorts of actions, man names it, _arguing_, or _disputing in the mind_. and when those divers sorts of actions are at some strife, man names it, _a contradicting of himself_. and if there be a weak strife, man names it, _consideration_. but, when those different figurative motions move of one accord, and sympathetically, this man names, _discretion_. but, when those different sorts of actions move sympathetically, and continue in that manner of action, without any alteration, man names it, _belief, faith_, or _obstinacy_. and when those parts make often changes, as altering their motions, man names it _inconstancy_. when their rational parts move slowly, orderly, equally, and sympathetically, man names it _sobriety_. when all the parts of the mind move regularly, and sympathetically, man names it, _wisdom_. when some parts move partly regularly, and partly irregularly, man names that, _foolishness_, and _simplicity_. when they move generally irregularly, man names it _madness_. chap. iii. of the motions of human passions, and appetites; as also, of the motions of the rational and sensitive parts, towards forrein objects. when some of the rational parts move sympathetically, to some of the sensitive perceptions; and those sensitive parts sympathize to the object, it is _love_. if they move antipathetically to the object, it is _hate_. when those rational and sensitive motions, make many and quick repetitions of those sympathetical actions, it is _desire_ and _appetite_. when those parts move variously, (as concerning the object) but yet sympathetically (concerning their own parts) it is _inconstancy_. when those motions move cross towards the object, and are perturbed, it is _anger_. but when those perturbed motions are in confusion, it is _fear_. when the rational motions are partly sympathetical, and partly antipathetical, it is _hope_, and _doubt_. and if there be more sympathetical motions than antipathetical, there is more _hope_ than _doubt_. if more antipathetical than sympathetical, then more _doubt_ than _hope_. if those rational motions move after a dilating manner, it is _joy_. if after a contracting manner, it is _grief_. when those parts move partly after a contracting, and partly after an attracting manner, as attracting from the object, it is _covetousness_. but, if those motions are sympathetical to the object, and move after a dilating manner towards the object, it is _generosity_. if those motions are sympathetical to the object, and move after the manner of a contraction, it is _pity_ or _compassion_. if those motions move antipathetically towards the object, yet after a dilating manner, it is _pride_. when those motions move sympathetically towards the object, after a dilating manner, it is _admiration_. if the dilating action is not extream, it is only _approving_. if those motions are antipathetical towards the object, and are after the manner of an extream contraction, it is _horror_. but, if those actions are not so extraordinary as to be extream, it is only _disapproving, despising, rejecting_, or _scorning_. if the rational parts move carelesly towards forrein objects, as also partly antipathetically, man nameth it, _ill-nature_. but, if sympathetically and industriously, man nameth it, _good-nature_. but this is to be noted, that there are many sorts of motions of one and the same kind; and many several particular motions, of one sort of motion; which causes some difference in the effects: but, they are so nearly related, that it requires a more subtile observation than i have, to distinguish them. chap. iv. of the repetitions of the sensitive and rational actions. both the rational and sensitive corporeal motions, make often repetitions of one and the same actions: the sensitive repetitions, man nameth, _custom_. the rational repetitions, man nameth, _remembrances_: for, repetitions cause a facility amongst the sensitive parts; but yet, in some repeating actions, the senses seem to be tired, being naturally delighted in variety. also, by the rational repetitions, the mind is either delighted, or displeased; and sometimes, partly pleased, and partly displeased: for, the mind is as much pleased, or displeased in the absence of an object, as in the presence; only the pleasure, and displeasure of the senses, is not joyned with the rational: for, the sense, if regular, makes the most perfect copies when the object is present: but, the rational can make as perfect copies in the absence, as in the presence of the object; which is the cause that the mind is as much delighted, or grieved, in the absence of an object, as with the presence: as for example, a man is as much grieved when he knows his friend is wounded, or dead, as if he had seen his wounds, or had seen him dead: for, the picture of the dead friend, is in the mind of the living friend; and if the dead friend was before his eyes, he could but have his picture in his mind; which is the same for an absent friend alive; only, as i said, there is wanting the sensitive perception of the absent object: and certainly, the parts of the mind have greater advantage than the sensitive parts; for, the mind can enjoy that which is not subject to the sense; as those things man names, _castles in the air_, or _poetical fancies_; which is the reason man can enjoy worlds of its own making, without the assistance of the sensitive parts; and can govern and command those worlds; as also, dissolve and compose several worlds, as he pleases: but certainly, as the pleasures of the rational parts are beyond those of the sensitive, so are their troubles. chap. v. of the passionate love, and sympathetical endeavours, amongst the associate parts of a human creature. in every regular human society, there is a passionate love amongst the associated parts, like fellow-students of one colledg, or fellow-servants in one house, or brethren in one family, or subjects in one nation, or communicants in one church: so the self-moving parts of a human creature, being associated, love one another, and therefore do endeavour to keep their society from dissolving. but perceiving, by the example of the lives of the same sort of creatures, that the property of their nature is such, that they must dissolve in a short time, this causes these human sorts of creatures, (being very ingenuous) to endeavour an after-life: but, perceiving again, that their after-life cannot be the same as the present life is, they endeavour (since they cannot keep their own society from dissolving) that their society may remain in remembrance amongst the particular and general societies of the same sort of creatures, which we name mankind: and this design causes all the sensitive and rational parts, in one society, to be industrious, to leave some mark for a lasting remembrance, amongst their fellow-creatures: which general remembrance, man calls _fame_; for which _fame_, the rational parts are industrious to design the manner and way, and the sensitive parts are industrious to put those designs in execution; as, their inventions, into arts or sciences; or to cause their heroick or prudent, generous or pious actions; their learning, or witty fancies, or subtile conceptions, or their industrious observations, or their ingenious inventions, to be set in print; or their exterior effigies to be cast, cut, or engraven in brass, or stone, or to be painted; or they endeavour to build houses, or cut rivers, to bear their names; and millions of other marks, for remembrance, they are industrious to leave to the perception of after-ages: and many men are so desirous of this after-life, that they would willingly quit their present life, by reason of its shortness, to gain this after-life, because of the probability of a long continuance; and not only to live so in many several ages, but in many several nations. and amongst the number of those that prefer a long after-life, before a short present life, i am one. but, some men dispute against these desires, saying, that _it doth a man no good to be remembred when he is dead_. i answer: it is very pleasing, whilst as man lives, to have in his mind, or in his sense, the effigies of the person, and of the good actions of his friend, although he cannot have his present company. also, it is very pleasant to any body to believe, that the effigies either of his own person, or actions, or both, are in the mind of his friend, when he is absent from him; and, in this case, absence and death are much alike. but, in short, god lives no other ways amongst his creatures, but in their rational thoughts, and sensitive worship. chap. vi. of acquaintance. as there are perceptive acquaintances amongst the parts of a human creature; so there is a perceptive acquaintance between, or amongst the human sorts of creatures. but, mistake me not; for i do not say, men only are acquainted with each other; for, there is not only an acquaintance amongst every particular sort, as between one and the same sort of creatures, but there are some acquaintances between some sorts of different kinds: as for example, between some sorts of beasts, and men; as also, some sorts of birds, and men, which understand each other, i will not say, so well as man and man; but so well, as to understand each other's passions: but certainly, every particular sort of creatures, of one and the same kind, understand each other, as well as men understand one another; and yet, for all that, they may be unacquainted: for, acquaintance proceeds from association; so that, some men, and some beasts, by association, may be acquainted with each other; when as some men, not associating, are meer strangers. the truth is, acquaintance belongs rather to particularities, than generalities. chap. vii. of the effects of forrein objects of the sensitive body; and of the rational mind of a human creature. according as the rational parts are affected, or disaffected with forrein objects, the sensitive is apt to express the like affections, or disaffections: for, most forrein objects occasion either pleasure and delight, or displeasure and dislike: but, the effects of forrein objects are very many, and, many times very different; as, some objects of devotion, occasion a fear, or superstition, and repentance in the mind; and the mind occasions the sensitive parts to several actions, as, praying, acknowledging faults, begging pardon, making vows, imploring mercy, and the like, in words: also, the body bows, the knees bend, the eyes weep, the hands hold up, and many the like devout actions. other sorts of objects occasion pity and compassion in the mind, which occasions the sensitive parts to attend the sick, relieve the poor, help the distressed, and many more actions of compassion. other sorts of forrein objects, occasion the rational mind to be dull and melancholy; and then the sensitive parts are dull, making no variety of appetites, or regard forrein objects. other sorts of objects occasion the mind to be vain and ambitious, and often to be proud; and those occasion the sensitive actions to be adventurous and bold; the countenance of the face, scornful; the garb of the body, stately; the words, vaunting, boasting, or bragging. other objects occasion the mind to be furious; and then the sensitive actions are, cursing words, frowning countenances, the leggs stamping, the hands and arms fighting, and the whole body in a furious posture. other sorts of objects occasion the mind to a passionate love; and then the sensitive actions are, flattering, professing, protesting in words, the countenance smiling, the eyes glancing; also, the body bows, the leggs scrape, the mouth kisses: also, the hands mend their garments, and do many of the like amorous actions. other objects occasion the mind to valour; and then the sensitive actions are, daring, encouraging, or animating. other objects occasion the mind to mirth, or cheerfulness; and they occasion the sensitive actions of the voice, to sing, or laugh; the words to be jesting, the hands to be toying, the leggs to be dancing. other sorts of objects occasion the mind to be prudent; and then the sensitive actions, are sparing or frugal. other sorts of objects occasion the mind to be envious, or malicious; and then the sensitive actions are mischievous. there are great numbers of occasional actions, but these are sufficient to prove, _that sense and reason understand each other's actions or designs_. chap. viii. of the advantage and disadvantage of the encounters of several creatures. there is a strong sympathy between the rational and sensitive parts, in one and the same society, or creature: not only for their consistency, subsistency, use, ease, pleasure, and delight; but, for their safety, guard, and defence: as for example, when one creature assaults another, then all the powers, faculties, properties, ingenuities, agilities, proportions, and shape, of the parts of the assaulted, unite against the assaulter, in the defence of every particular part of their whole society; in which encounter, the rational advises, and the sensitive labours. but this is to be noted concerning advantage and disadvantage in such encounters, that some sorts of creatures have their advantage in the exterior shape, others meerly in the number of parts; others in the agility of their parts, and some by the ingenuity of their parts: but, for the most part, the greater number have advantage over the less, if the greater number of parts be as regular, and as ingenious as the less number: but, if the less number be more regular, and more ingenious than the greater, then 'tis a hundred to one but the less number of parts have the advantage. chap. ix. that all human creatures have the like kinds and sorts of properties. all human creatures have the like kinds and sorts of properties, faculties, respirations, and perceptions; unless some irregularities in the production, occasion some imperfections, or some misfortunes, in some time of his age: yet, no man knows what another man perceives, but by guess, or information of the party: but, as i said, if they have have no imperfections, all human creatures have like properties, faculties, and perceptions: as for example, all human eyes may see one and the same object alike; or hear the same tune, or sound; and so of the rest of the senses. they have also the like respirations, digestions, appetites; and the like may be said of all the properties belonging to a human creature. but, as one human creature doth not know what another human creature knows, but by confederacy; so, no part of the body, or mind of a man, knows each part's perceptive knowledg, but by confederacy: so that, there is as much ignorance amongst the parts of nature, as knowledg. but this is to be noted, that there are several manners and ways of intelligences, not only between several sorts of creatures, or amongst particulars of one sort of creatures; but, amongst the several parts of one and the same creature. chap. x. of the irregularity of the sensitive, and of the rational corporeal motions. as i have often mentioned, and do here again repeat, that the rational and sensitive parts of one society, or creature, do understand, as perceiving each other's self-moving parts; and the proof is, that, sometimes, the human sense is regular, and the human reason irregular; and sometimes the reason regular, and the sense irregular: but, in these differences, the regular parts endeavour to reform the irregular; which causes, many times, repetitions of one and the same actions, and examinations; as, sometimes the reason examines the sense; and sometimes the sense, the reason: and sometimes the sense and reason do examine the object; for, sometimes an object will delude both the sense and reason; and sometimes the sense and reason are but partly mistaken: as for example, a fired end of a stick, by a swift exterior circular motion, appears a circle of fire, in which they are not deceived: for, by the exterior motion, the fired end is a circle; but they are mistaken, to conceive the exterior figurative action to be the proper natural figure: but when one man mistakes another, that is some small error, both of the sense and reason. also, when one man cannot readily remember another man, with whom he had formerly been acquainted, it is an error; and such small errors, the sense and reason do soon rectifie: but in causes of high irregularities, as in madness, sickness, and the like, there is a great bustle amongst the parts of a human creature; so as those disturbances cause unnecessary fears, grief, anger, and strange imaginations. chap. xi. of the knowledg between the sensitive organs of a human creature. the sensitive organs are only ignorant of each other, as they are of forrein objects: for, as all the parts of forrein objects, are not subject to one sensitive organ; so all the sensitive organs are not subject to each sensitive organ of a human creature: yet, in the perceptive actions of forrein objects, they do so agree, that they make an united knowledg: thus we may be particularly ignorant one way, and yet have a general knowledg another way. chap. xii. of human perception, or defects of a human creature. it is not the great quantity of brain, that makes a man wise; nor a little quantity, that makes a man foolish: but, the irregular, or regular rational corporeal motions of the head, heart, and the rest of the parts, that causes dull understandings, short memories, weak judgments, violent passions, extravagant imaginations, wild fancies, and the like. the same must be said of the sensitive irregular corporeal motions, which make weakness, pain, sickness, disordered appetites, and perturbed perceptions, and the like: for, nature poysing her actions by opposites, there must needs be irregularities, as well as regularities; which is the cause that seldom any creature is so exact, but there is some exception. but, when the sensitive and rational corporeal motions are regular, and move sympathetically, then the body is healthful and strong, the mind in peace and quiet, understands well, and is judicious: and, in short, there are perfect perceptions, proper digestions, easie respirations, regular passions, temperate appetites. but when the rational corporeal motions are curious in their change of actions, there are subtile conceptions, and elevated fancies: and when the sensitive corporeal motions move with curiosity, (as i may say) then there are perfect senses, exact proportions, equal temperaments; and that, man calls _beauty_. chap. xiii. of natural fools. there is great difference between a natural fool, and a mad man: for, madness is a disease, but a natural fool is a defect; which defect was some error in his production, that is, in the form and frame either of the mind, or sense, or both; for, the sense may be a natural fool as well as the reason; as we may observe in those sorts of fools whom we name _changelings_, whose body is not only deformed, but all the postures of the body are defective, and appear as so many fools: but sometimes, only some parts are fools; as for example; if a man be born blind, then only his eyes are fools; if deaf, then only his ears are fools, which occasions his dumbness; ears being the informing parts, to speak; and wanting those informations, he cannot speak a language. also, if a man is born lame, his leggs are fools; that is, those parts have no knowledg of such properties that belong to such parts; but the sensitive parts may be wise, as being knowing; and the rational parts may be defective; which defects, man names _irrational_. but this is to be noted, that there may be natural and accidental fools, by some extraordinary frights, or by extraordinary sickness, or through the defects of old age. as for the errors of production, they are incurable; as also, those of old age; the first being an error in the very foundation, and the other a decay of the whole frame of the building: for, after a human creature is brought to that perfection, as to be, as we may say, at full growth and strength, at the prime of his age; the human motions, and the very nature of man, after that time, begins to decay; for then the human motions begin to move rather to the dissolution, than to the continuance; although some men last to very old age, by reason the unity of their society is regular and orderly, and moves so sympathetically, as to commit few or no disorders, or irregularities; and such old men are, for the most part, healthful, and very wise, through long experience; and their society having got a habit of regularity, is not apt to be disturbed by forrein parts. but this is to be noted, that sometimes the sensitive body decays, before the rational mind; and sometimes the rational mind, before the sensitive body. also, this is to be noted, that when the body is defective, but not the mind; then the mind is very industrious to find out inventions of art, to help the defects that are natural. but pray mistake me not; for i do not say, that _all_ deformities, or defects, but only _some_ particular sorts of deformity, or defects, are foolish. the seventh part. chap. i. of the sensitive actions of sleeping and waking. the sensitive and rational corporeal figurative motions, are the cause of infinite varieties: for, though repetitions make no varieties; yet, every altered action is a variety: also, different actions, make different effects; opposite actions, opposite effects; not only of the actions of the several self-moving parts, or corporeal motions, but of the same parts: as for example, the same parts, or corporeal motions, may move from that, man names _life_, to that which man names _death_; or, from health to sickness, from ease to pain, from memory to forgetfulness, from forgetfulness to remembrance, from love to hate, from grief to joy, from irregularity to regularity; or, from regularity to irregularity, and the like; and from one perception to another: for, though all actions are perceptive, yet there are several kinds, several sorts, and several particular perceptions: but, amongst the several corporeal motions of animal, or human kind, there are the opposite motions of what we name _waking_, and _sleeping_; the difference is, that waking-actions are, most commonly, actions of imitation, especially of the sensitive parts; and are more the exterior, than the interior actions of a human creature. but, the actions of sleep, are the alterations of the exterior corporeal motions, moving more interiorly, as it were inwardly, and voluntarily: as for example, the optick corporeal motions, in waking-actions, work, or move, according to the outward object: but, in sleeping-actions, they move by rote, or without examples; also, as i said, they move, as it were, inwardly; like as a man should turn himself inward, or outward, of a door, without removing from the door, or out of the place he stood in. chap. ii. of sleeping. although the rational and sensitive corporeal motions, can never be tired, or weary of moving or acting, by reason it is their nature to be a perpetual corporeal motion; yet they may be weary, or tired with particular actions. also, it is easier and more delightful, to move by rote, than to take copies, or patterns; which is the reason that sleep is easie and gentle, if the corporeal motions be regular; but if they be irregular, sleep is perturbed. but this is to be noted, that the corporeal motions delight in varieties so well, that, many times, many and various objects will cause the sensitive and rational corporeal motions in a man, to retard their actions of sleep; and, oft-times, want of variety of forrein or outward objects, will occasion the action of sleep; or else musing and contemplating actions. also, it is to be noted, that if some parts of the body, or mind, be distempered with irregularities, it occasions such disturbances to the whole, as hinders that repose; but if the regular parts endeavour not to be disturbed with the irregular; and the irregulars do disturb the regular; then it occasions that which man names, _half-sleeps_, or _slumbers_, or _drowsiness_. and if the regular corporeal motions get the better, (as many times they do) then we say, sleep hath been the occasion of the cure; and it oft proves so. and it is a common saying, _that a good sleep will settle the spirits_, or ease the pains; that is, when the regular corporeal motions have had the better of the irregular. chap. iii. of human dreams. there are several kinds, sorts, and particulars of corporeal irregularities, as well as of regularities; and amongst the infinite kinds, sorts, and particulars, there is that of human dreams; for, the exterior corporeal motions in waking-actions, do copy or pattern outward objects; whereas, in actions of sleep, they act by rote, which, for the most part, is erronious, making mixt figures of several objects; as, partly like a beast; and partly, like a bird, or fish; nay, sometimes, partly like an animal, and partly like a vegetable; and millions of the like extravagancies; yet, many times, dreams will be as exact as if a man was awake, and the objects before him; but, those actions by rote, are more often false than true: but, if the self-moving parts move after their own inventions, and not after the manner of copying; or, if they move not after the manner of human perception, then a man is as ignorant of his dreams, or any human perception, as if he was in a swound; and then he says, he did not dream; and, that such sleeps are like death. chap. iv. of the actions of dreams. when the figures of those friends and acquaintants that have been dead a long time, are made in our sleep, we never, or seldom question the truth of their being alive, though we often question them how they came to be alive: and the reason that we make no doubt of their being alive, is, that those corporeal motions of sleep, make the same pattern of that object in sleep, as when that object was present, and patterned awake; so as the picture in sleep seems to be the original awake: and until such times that the corporeal motions alter their sleeping-actions to waking-actions, the truth is not known. though sleeping and dreaming, is somewhat after the manner of forgetfulness and remembrance; yet, perfect dreams are as perceptive as waking-patterns of present objects; which proves, that both the sensitive and rational motions, have sleeping actions; but both the sensitive and rational corporeal actions in sleep, moving partly by rote, and partly voluntarily, or by invention, make walking-woods, or woodden men; or make warrs and battels, where some figures of men are kill'd, or wounded, others have victory: they also make thieves, murderers, falling houses, great fires, floods, tempests, high mountains, great precipices; and sometimes pleasant dreams of lovers, marriage, dancing, banquetting, and the like: and the passions in dreams are as real, as in waking actions. chap. v. whether the interior parts of a human creature, do sleep. the parts of my mind were in dispute, whether the interior parts of a human creature, had sleeping and waking actions? the major part was of opinion, that sleep was not proper to those human parts, because the interior motions were not like the exterior. the opinion of the minor part was, that change of action, is like ease after labour; and therefore it was probable, the interior parts had sleeping and waking actions. the opinion of the major parts, was, that if those parts, as also the food received into the body, had sleeping actions, the body could not be nourished; for, the meat would not be digested into the like parts of the body, by reason sleeping actions were not such sorts of actions. the opinion of the minor parts was, that the sleeping actions were nourishing actions, and therefore were most proper for the interior parts; and, for proof, the whole human body becomes faint and weak, when they are hindred, either by some interior irregularity, or through some exterior occasion, from their sleeping actions. the opinion of the major part, was, that sleeping actions are actions of rote, and not such altering actions as digesting actions, and nourishing actions, which are uniting actions. besides, that the reason why the interior actions are not sleeping actions, was, that when the exterior parts move in the actions of sleep, the interior parts move when the exterior are awake; as may be observed by the human pulse, and human respiration; and by many other observations which may be brought. chap. vi. whether all the creatures in nature, have sleeping and waking actions. some may ask this question, _whether all creatures have sleeping actions?_ i answer, that though sleeping actions are proper to human creatures, as also, to most animal creatures; yet, such actions may not any ways be proper to other kinds and sorts of creatures: and if (as in all probability it is) that the exterior parts of a human creature have no such sleeping actions, it is probable that other kinds and sorts of creatures move not at any time, in such sorts of actions. but some may say, _that if nature is poysed, all creatures must have sleeping actions, as well as waking actions_. i answer, that though nature's actions are poysed, yet that doth not hinder the variety of nature's actions, so as to tye nature to particular actions: as for example, the exterior parts of animals have both sleeping and waking actions; yet that doth not prove, that therefore all the parts or creatures in nature, must have sleeping and waking actions. the same may be said of all the actions of an animal creature, or of a human creature; nay, of all the creatures of the world: for, several kinds and sorts of creatures, have several kinds and sorts of properties: wherefore, if there be other kinds and sorts of worlds besides this, 'tis probable that those worlds, and all the parts, or several kinds and sorts of creatures there, have different properties and actions, from those of this world; so that though nature's actions are poysed and balanced, yet they are poysed and balanced after different manners and ways. chap. vii. of human death. _death_ is not only a general alteration of the sensitive and rational motions, but a general dissolution of their society. and as there are degrees of time in productions, so in dissolutions. and as there are degrees to perfection, as from infancy to manhood; so there are degrees from manhood to old age. but, as i said, _death_ is a general dissolution, which makes a human creature to be no more: yet, some parts do not dissolve so soon as others; as for example, human bones; but, though the form or frame of bones is not dissolved; yet the properties: of those bones are altered. the same when a human creature is kept by art from dissolving, so as the form, or frame, or shape may continue; but all the properties are quite altered; though the exterior shape of such bodies doth appear somewhat like a man, yet that shape is not a man. chap. viii. of the heat of human life, and the cold of human death. there are not only several sorts of properties belonging to several sorts of creatures, but several sorts of properties belonging to one and the same sort of creature; and amongst the several sorts of human properties, human heat is one, which man names _natural heat_: but, when there is a general alteration of the human properties, there is that alteration of the property as well of his natural, as human heat: but, natural heat is not the cause of human life, though human life is the cause of that natural heat: so that, when human life is altered or dissolved, human heat is altered or dissolved: and as death is opposite actions to that man names _life_; so cold is opposite actions to that man names _heat_. chap. ix. of the last act of human life. the reason some human creatures dye in more pain than others, is, that the motions of some human creatures are in strife, because some would continue their accustomed actions, others would alter their accustomed actions; which strife causes irregularities, and those irregularities cause differences, or difficulties, which causes pain: but certainly, the last act of human life is easie; not only that the expulsive actions of human respirations, are more easie than the attracting actions; but, that in the last act of human life, all the motions do generally agree in one action. chap. x. whether a human creature hath knowledg in death, or not? some may ask the question, _whether a dead man hath any knowledg or perception?_ i answer, that a dead man hath not a human knowledg or perception; yet all, and every part, hath knowledg and perception: but, by reason there is a general alteration of the actions of the parts of a human creature, there cannot possibly be a human knowledg or perception. but some may say, that a man in a swound hath a general alteration of human actions; and yet those parts of a human creature do often repeat those former actions, and then a man is as he was before he was in that swound. i answer, that the reason why a man in a swound hath not the same knowledg as when he is not in a swound, is, that the human motions are not generally altered, but only are generally irregular; which makes such a disturbance, that no part can move so regularly, as to make proper perceptions; as in some sorts of distempers, a man may be like a natural fool; in others, he may be mad; and is subject to many several distempers, which cause several effects: but a human swound is somewhat like sleeping without dreaming; that is, the exterior senses do not move to human exterior perception. chap. xi. whether a creature may be new formed, after a general dissolution. some may ask the question, _whether a human creature, or any other creature, after their natural properties are quite altered, can be repeated, and rechanged, to those properties that formerly were?_ i answer, yes, in case none of the fundamental figurative parts be dissolved. but some may ask, _that if those dissolved parts were so inclosed in other bodies, that none of them could easily disperse or wander; whether they might not joyn into the same form and figure again, and have the same properties?_ i answer, i cannot tell well how to judg; but i am of the opinion, they cannot: for, it is the property of all such productions, to be performed by degrees, and that there should be a dividing and uniting of parts, as an intercourse of home and forrein parts; and so there is requir'd all the same parts, and every part of the same society, or that had any adjoining actions with that particular creature; as all those parts, or corporeal motions, that had been from the first time of production, to the last of the dissolving; and that could not be done without a confusion in nature. but some may say, _that although the same creature could not be produced after the same manner, nor return to the degree of his infancy, and pass the degrees from his infancy, to some degree of age; yet, those parts that are together, might so joyn, and move, in the same manner, as to be the same creature it was before its dissolution?_ i answer, it may not be impossible: but yet, it is very improbable, that such numerous sorts of motions, after so general an alteration, should so generally agree in an unnatural action. chap. xii. of foreknowledg. i have had some disputes amongst the parts of my mind, _whether nature hath foreknowledg?_ the opinion of the minor parts was, that nature had foreknowledg, by reason all that was material, was part of her self; and those self-parts having self-motion, she might foreknow what she would act, and so what they should know. the opinion of the major parts was, that by reason every part had self-motion, and natural free-will, nature could not foreknow how they would move, although she might know how they have moved, or how they do move. after this dispute was ended, then there was a dispute, _whether the particular parts had a foreknowledg of self-knowledg?_ the opinion of the minor parts was, that since every part in nature had self-motion, and natural free-will, every part could know how they should move, and so what they should know. the opinion of the major parts was, that first, the self-knowledg did alter according to self-action, amongst the self-moving parts: but, the self-knowledg of the inanimate parts, did alter according to the actions of the sensitive self-moving parts; and the perceptive actions of the self-moving parts, were according to the form and actions of the objects: so that foreknowledg of forein parts, or creatures, could not be: and for foreknowledg of self-knowledg of the self-moving parts, there were so many occasional actions, that it was impossible the self-moving parts could know how they should move, by reason that no part had an absolute power, although they were self-moving, and had a natural free-will: which proves, that prophesies are somewhat of the nature of dreams, whereof some may prove true by chance; but, for the most part, they are false. the eighth part. chap. i. of the irregularity of nature's parts. some may make this question, that, _if nature were self-moving, and had free-will, it is probable that she would never move her parts so irregularly, as to put her self to pain._ i answer, first, that nature's parts move themselves, and are not moved by any agent. secondly, though nature's parts are self-moving, and self-knowing, yet they have not an infinite or uncontrolable power; for, several parts, and parties, oppose, and oft-times obstruct each other; so that many times they are forced to move, and they may not when they would. thirdly, some parts may occasion other parts to be irregular, and keep themselves in a regular posture. lastly, nature's fundamental actions are so poysed, that irregular actions are as natural as regular. chap. ii. of the human parts of a human creature. the form of man's exterior and interior parts, are so different, and so numerous; that i cannot describe them, by reason i am not so learned to know them: but, some parts of a human creature, man names _vital_; because, the least disturbance of any of those parts, endangers the human life: and if any of those vital parts are diminished, i doubt whether they can be restored; but if some of those parts can be restored, i doubt all cannot. the vital parts are, the heart, liver, lungs, stomack, kidneys, bladder, gaul, guts, brains, radical humours, or vital spirits; and others which i know not of. but this is to be noted, that man is composed of rare and solid parts, of which there are more and less solid, more and less rare; as also, different sorts of solid, and different sorts of rare: also, different sorts of soft and hard parts; likewise, of fixt and loose parts; also, of swift and slow parts. i mean by fixt, those that are more firmly united. chap. iii. of human humours. _humours_ are such parts, that some of them may be divided from the whole body, without danger to the whole body; so that they are somewhat like excremental parts, which excremental parts, are the superfluous parts: for, though the humours be so necessary, that the body could not well subsist without them; yet, a superfluity of them is as dangerous, (if not more) as a scarcity. but there are many sorts of humours belonging to a human creature, although man names but four, according to the four elements, _viz. flegm, choler, melancholy_, and _blood_: but, in my opinion, there are not only several sorts of _choler, flegm, melancholy_, and _blood_; but other sorts that are none of these four. chap. iv. of blood. i have heard, that the opinions of the most learned men, are, that all animal creatures have blood, or at least, such juyces that are in lieu of blood; which blood, or juyces, move circularly: for my part, i am too ignorant to dispute with learned men; but yet i am confident, a _moth_ (which is a sort of worm, or fly, that eats cloth) hath no blood, no, nor any juyce; for, so soon as it is touched, it dissolves straight to a dry dust, or like ashes. and there are many other animals, or insects, that have no appearance of blood; therefore the life of an animal doth not consist of blood: and as for the circulation of blood, there are many animal creatures that have not proper vessels, as veins and arteries, or any such gutters, for their blood, or juyce, to circulate through. but, say the blood of man, or of such like animal, doth circulate; then it is to be studied, whether the several parts of the blood do intermix with each other, as it flows; or, whether it flows as water seems to do; where the following parts may be as great strangers to the leading parts, as in a crowd of people, where some of those behind, do not know those that are before: but, if the blood doth not intermix as it flows, then it will be very difficult for a chyrurgion, or physician, to find where the ill blood runs: besides, if the blood be continually flowing, when a sick man is to be let blood, before the vein is opened, the bad blood may be past that part, or vein, and so only the good blood will be let out; and then the man may become worse than if he had not been let blood. chap. v. of the radical humours, or parts. there are many parts in a human body, that are as the foundation of a house; and being the foundation, if any of those parts be removed or decayed, the house immediately falls to ruine. these fundamental parts, are those we name the _vital parts_; amongst which are those parts we name the _vital_ and _radical spirits_, which are the oyl and flame of a human creature, causing the body to have that we name a _natural heat_, and a _radical moisture_. but it is to be noted, that these parts, or corporeal motions, are not like gross oyl, or flame: for, i believe, there are more differences between those flames, and ordinary flames, than between the light of the sun, and the flame of a tallow candle; and as much difference between this oyl, and the greasie oyl, as between the purest essence, and lamp-oyl. but, these vital parts are as necessary to the human life, as the solid vital parts, viz. the heart, liver, lungs, brains, and the like. chap. vi. of expelling malignant disorders in a human creature. expelling of poyson, or any malignity in the body, is, when that malignity hath not got, or is not setled into the vital parts; so that the regular motions of the vital parts, and other parts of the body, endeavour to defend themselves from the forrein malignancies; which if they do, then the malignant motions do dilate to the exterior parts, and issue out of those exterior passages, at least, through some; as, either by the way of purging, vomiting, sweating, or transpiration, which is a breathing through the pores, or other passages. after the same manner is the expelling of surfeits, or superfluities of natural humours: but, if the malignity or surfeit, superfluity or superfluous humours, have the better, (as i may say) then those irregular motions, by their disturbances, cause the regular motions to be irregular, and to follow the mode; which is, to imitate strangers, or the most powerful; the most fantastical, or the most debauch'd: for it is, many times, amongst the interior motions of the body, as with the exterior actions of men. chap. vii. of human digestions and evacuations. to treat of the several particular digestive actions of a human creature, is impossible: for, not only every part of food hath a several manner of digestive action; but, every action in transpiration, is a sort of digestion and evacuation: so that, though every sort of digestion and evacuation, may be ghest at; yet, every particular is not so known, that it can be described. but this is to be noted, that there is no creature that hath digestive motions, but hath evacuating motions; which actions, although they are but dividing, and uniting; yet they are such different manners and ways of uniting and dividing, that the most observing man cannot particularly know them, and so not express them: but, the uniting actions, if regular, are the nourishing actions; the dividing actions, if regular, are the cleansing actions: but if irregular, the uniting actions are the obstructive actions; and the dividing actions, the destructive actions. chap. viii. of diseases in general. there are many sorts of human diseases; yet, all sorts of diseases are irregular corporeal motions; but, every sort of motion is of a different figure: so that, several diseases are different irregular figurative motions; and according as the figurative motions vary, so do the diseases: but, as there are human diseases, so there are human defects; which defects (if they be those which man names _natural_) cannot be rectified by any human means. also, there are human decays, and old age; which, although they cannot be prevented, or avoided; yet, they may, by good order, and wise observations, be retarded: but there are not only numerous sorts of diseases, but every particular it self, and every particular sort, are more or less different; insomuch, that seldom a disease of one and the same sort, is just alike, but there are some differences; as in men, who though they be all of one sort of animal-kind, yet seldom any two men are just alike: and the same may be said of diseases both of body and mind; as for example, concerning irregular minds, as in mad-men; although all mad-men are mad, yet not mad alike; though they all have the disease either of sensitive or rational madness, or are both sensitively and rationally mad. also, this is to be noted, that as several diseases may be produced from several causes, so several diseases from one: cause, and one disease from several causes; which is the cause that a physician ought to be a long and subtile observer and practiser, before he can arrive to that experience which belongs to a good physician. chap. ix. of the fundamental diseases. there are numerous sorts of diseases, to which human creatures are subject; and yet there are but few fundamental maladies; which are these as follow; pain, sickness, weakness, dizziness, numbness, deadness, madness, fainting and swounding; of which one is particular, the rest are general: the particular is sickness, to which no parts of the body are subject, but the stomack: for, though any parts of the body may have pain, numbness, dizziness, weakness, or madness; yet in no part can be that which we name sickness, but the stomack. as for dizziness, the effects are general, as may be observed in some drunken men: for, many times, the head will be in good temper, when the leggs (i cannot say, are dizzie, yet) will be so drunk, as neither to go or stand; and many times the tongue will be so drunk, as not to speak plain, when all the rest of the body is well temper'd; at least so well, as not to be any ways perceived, but by the tripping of their speech: but, as i said, no part is subject to be sick, but the stomack: and though there are numerous sorts of pains to which every part is subject, and every several part hath a several pain; yet they are still pain. but some may say, _that there are also several sorts of sicknesses_. i grant it; but yet those several sorts of sicknesses, belong only to the stomack, and to no other part of the body. the ninth part. chap. i. of sickness. to go on as orderly as i can, i will treat of the fundamental diseases, and first of _sickness_, by reason it is the most particular disease: for though, as i have said, no part of a human creature is subject to that disease, (namely, _sickness_) but the stomack; yet, there are different sorts of sicknesses of the stomack; as for example, some sorts of sickness is like the flowing and ebbing of the sea: for, the humours of the stomack agitate in that manner, as, if the flowing motions flow upwards, it occasions vomiting; if downwards, purging: if the humours divide, as, partly to flow upwards, and partly downwards, it occasions both vomiting and purging. but the question is, _whether it is the motion of the humours, that occasions the stomack to be sick; or the sickness of the stomack, that occasions the humours to flow?_ i answer: that 'tis probable, that sometimes the flowing of the humours causes the stomack to be sick; and sometimes the sickness of the stomack occasions the humours to flow; and sometimes the stomack will be sick without the flowing of humours, as when the stomack is empty; and sometimes the humours will flow, without any disturbance to the stomack; and sometimes both the humours and the stomack do jointly agree in irregularities: but, as i said, there are several sorts of sicknesses of the stomack, or at least, that sickness doth produce several sorts of effects; as, for example, some sorts of sickness will occasion faint and cold sweats; which sick motion is not flowing up or down of the humours; but it is a cold dilatation, or rarifying, after a breathing manner; also expelling of those rarified parts through the pores: other sorts of motions of the humours, are like boyling motions, viz. bubling motions; which occasion steaming or watry vapours, to ascend to the head; which vapours are apt to cloud the perception of sight. other sorts of sick motions, are circular, and those cause a swimming, or a dizzie motion in the head, and sometimes a staggering motion in the leggs. other sorts of sick motions are occasioned through tough and clammy humours, the motion of which humours, is a winding or turning in such a manner, that it removes not from its center; and until such time as that turning or winding motions alter, or the humour is cast out of the stomack, the patient finds little or no ease. chap. ii. of pain. as i said, no part is subject to be sick, but the stomack; but every several part of a human creature, is subject to pain; and not only so, but every particular part is subject to several sorts of pain; and every several sort of pain, hath a several figurative motion: but to know the different figurative motions, will require a subtile observation: for, though those painful parts, know their own figurative motions; yet, the whole creature (suppose _man_) doth not know them. but it may be observed, whether they are caused by irregular contractions or attractions, dilatations or retentions, expulsions or irregular pressures and re-actions, or irregular transformations, or the like; and by those observations, one may apply, or endeavour to apply proper remedies: but all pain proceeds from irregular and perturbed motions. chap. iii. of dizziness. i cannot say, _dizziness_ belongs only to the head of an animal creature, because we may observe, by irregular drinkers, that sometimes the leggs will seem more drunk than their heads; and sometimes all the parts of their body will seem to be temperate, as being regular, but only the tongue seems to be drunk: for, staggering of the leggs, and a staggering of the tongue, or the like, in a drunken distemper, is a sort of dizziness, although not such a sort as that which belongs to the head; so that, when a man is dead-drunk, we may say, that every part of the body is _dizzily drunk_. but mistake me not; for i do not mean, that all sorts of dizzinesses proceed from drinking; i only bring drunkenness for an example: but, the effects of dizziness of the head, and other parts of the body, proceed from different causes; for, some proceed from wind, not wine; others from vapour; some from the perception of some forrein object; and numbers of the like examples may be found. but this is to be noted, that all such sorts of swimming and dizziness in the head, are produced from circular figurative motions. also it is to be noted, that many times the rational corporeal motions are irregular with the sensitive, but not always: for, sometimes in these and the like distempers, the sensitive will be irregular, and the rational regular; but, for the most part, the rational is so compliant with the sensitive, as to be regular, or irregular, as the sensitive is. chap. iv. of the brain seeming to turn round in the head. when the human brain seems to turn round, the cause is, that some vapours do move in a circular figure, which causes the head to be dizzy; as when a man turns round, not only his head will be dizzy, but all the exterior parts of his body; insomuch that some, by often turning round, will fall down; but if, before they fall, they turn the contrary way, they will be free from that dizziness: the reason of which is, that, by turning the contrary way, the body is brought to the same posture it was before; as, when a man hath travell'd some way, and returns the same way back, he returns to the place where first he began his journey. chap. v. of weakness. there are many sorts of _weakness_; some weakness proceeds from age; others, through want of food; others are occasioned by oppression; others, by disorders and irregularities; and so many other sorts, that it would be too tedious to repeat them, could i know them: but, such sorts of weakness, as human creatures are subject to, after some disease or sickness, are somewhat like weariness after a laborious or over-hard action; as, when a man hath run fast, or laboured hard, he fetches his breath short and thick; and as most of the sensitive actions are by degrees, so is a returning to health after sickness: but, all irregularities are laborious. chap. vi. of swounding. the cause why a man in _swound_, is, for a time, as if he were dead; is, an irregularity amongst some of the interior corporeal motions, which causes an irregularity of the exterior corporeal motions, and so a general irregularity; which is the cause that a man appears as if he were dead. but some may say, _a man in a swound is void of all motion_. i answer: that cannot be: for, if the man was really dead, yet his parts are moving, though they move not according to the property or nature of a living man: but, if the body had not consistent motions, and the parts did not hold together, it would be dissolved in a moment; and when the parts do divide, they must divide by self-motion: but, in a man in a swound, some of his corporeal motions are only altered from the property and nature of a living man; i say, some of his corporeal motions, not all: neither do those motions quite alter from the nature of a living man, so as the alterations of the fundamental motions do: but they are so alter'd, as language may be alter'd, viz. from _hebrew_ to _greek, latin, french, spanish, english,_ and many others; and although they are all but languages, yet they are several languages or speeches; so the alteration of the corporeal motions of a man in a swound, is but as the altering of one sort of language to another; as put the case, _english_ were the natural language or speech, then all other languages were unknown to him that knows no other than his natural: so a man in a swound is ignorant of those motions in the swound: but, when those motions return to the nature of a living man, he hath the same knowledg he had before. thus human ignorance, and human knowledg, may be occasioned by the alterations of the corporeal motions. the truth is, that swounding and reviving, is like forgetfulness and remembrance, that is, alteration and repetition, or exchange of the same actions. chap. vii. of numb and dead palsies, or gangren's. as for _numb_ and _dead palsies_, they proceed not only from disordered and irregular motions, but from such figurative motions as are quite different from the nature of the creature: for, though it be natural for a man to dye; yet the figurative motions of _death_ are quite different from the figurative motions of life; so in respect to that which man names life, that which man names death, is unnatural: but, as there are several sorts of that man names _life_, or _lives_; so there are several sorts of those corporeal motions, man names _death_: but, _dead palsies_ of some parts of a man's body, are not like those of a man when he is, as we say, _quite dead_; for, those are not only such sorts of motions that are quite, or absolutely different from the life of the man, or such like creature; but such as dissolve the whole frame, or figure of the creature: but, the motions of a _dead palsie_, are not dissolving motions, although they are different from the natural living motions of a man. the same, in some manner, are _numb palsies_; only the motions of _numb palsies_ are not so absolutely different from the natural living motions; but have more irregularities, than perfect alterations. as for that sort of numbness we name _sleepy numbness_, it is occasioned through some obstruction that hinders and stops the exterior sensitive perception. as, when the eyes are shut, or blinded, or the ears stopt, or the nostrils; the sensitive figurative motions of those sensitive organs, cannot make perceptions of forrein objects: so, when the pores of the flesh, which are the perceptive organs of forrein touches, are stopt, either by too heavy burthens or pressings, or tying some parts so hard, as to close the exterior organs, (_viz_. the pores) they cannot make such perceptions as belong to touch: but, when those hinderances are removed, then the sensitive perception of touch, is, in a short time, as perfect as before. as for _gangren's_, although they are somewhat like _dead palsies_, yet they are more like those sorts of dead corporeal motions, that dissolve the frame and form of a creature: for, _gangren's_ dissolve the frame and form of the diseased part; and the like do all those corporeal motions that cause rottenness, or parts to divide and separate after a rotten manner. chap. viii. of madness. there are several sorts of that distemper named _madness_; but they all proceed through the irregularities, either of the rational, or the sensitive parts; and sometimes from the irregularities both of sense and reason: but these irregularities are not such as are quite different from the nature or property of a human creature, but are only such irregularities as make false perceptions of forrein objects, or else make strange conceptions; or move after the manner of dreams in waking-actions; which is not according to the perception of present objects: as for example, the sensitive motions of the exterior parts, make several pictures on the outside of the organs; when as no such object is present; and that is the reason mad-men see strange and unusual sights, hear strange and unusual sounds, have strange and unusual tasts and touch: but, when the irregularities are only amongst the rational parts, then those that are so diseased, have violent passions, strange conceptions, wild fancies, various opinions, dangerous designs, strong resolutions, broken memories, imperfect remembrances, and the like. but, when both the sensitive and rational are sympathetically disorderly; then the mad-men will talk extravagantly, or laugh, sing, sigh, weep, tremble, complain, &c. without cause. chap. ix. the sensitive and rational parts may be distinctly mad. the senses may be irregularly mad, and not the reason; and the reason may be irregularly mad, and not the sense; and, both sense and reason may be both sympathetically mad: and, an evident proof that there is a rational and sensitive madness, is, that those whose rational parts are regular, and only some of the sensitive irregular, will speak soberly, and declare to their friends, how some of their senses are distemper'd, and how they see strange and unusual sights, hear unusual sounds, smell unusual sents, feel unusual touches, and desire some remedy for their distempers. also, it may be observed, that sometimes the rational parts are madly distemper'd, and not the sensitive; as when the sensitive parts make no false perceptions, but only the rational; and then only the mind is out of order, and is extravagant, and not the senses: but, when the senses and reason are madly irregular, then the diseased man is that we name, _outragiously mad_. chap. x. the parts of the head are not only subject to madness, but also the other parts of the body. _madness_ is not only in the head, but in other parts of the body: as for example, some will feel unusual touches in their hands, and several other parts of their body. we may also observe by the several and strange postures of mad-men, that the several parts of the body are madly distemper'd. and it is to be noted, that sometimes some parts of the body are mad, and not the other; as, sometimes only the eyes, sometimes only the ears; and so of the rest of the organs, and of the rest of the parts of the body; one part only being mad, and the rest in good order. moreover, it is to be noted, that some are not continually mad, but only mad by fits, or at certain times; and those fits, or certain times of disorders, proceed from a custom or habit of the rational or sensitive motions, to move irregularly at such times; and a proof that all the parts are subject to the distemper of _madness_, is, that every part of the body of those sorts of mad-men that believe their bodies to be glass, moves in a careful and wary motion, for fear of breaking in pieces: neither are the exterior parts only subject to the distemper of madness, but the interior parts; as may be observed, when the whole body will tremble through a mad fear, and the heart will beat disorderly, and the stomack will many times be sick. chap. xi. the rational and sensitive parts of a human creature, are apt to disturb each other. although the rational and sensitive corporeal motions, may, and do sometimes disagree; yet, for the most part, there is such a sympathetical agreement between the sensitive and rational corporeal motions of one society, (viz. of one creature) as they often disturb each other: as for example, if the rational motions are so irregular, as to make imaginary fears, or fearful imaginations, these fearful imaginations cause the sensitive corporeal motions, to move according to the irregularities of the rational; which is the cause, in such fears, that a man seems to see strange and unusual objects, to hear strange and unusual sounds, to smell unusual sents, to feel unusual touches, and to be carried to unusual places; not that there are such objects, but the irregular senses make such pictures in the sensitive organs; and the whole body may, through the strength of the irregular motions, move strangely to unusual places: as for example, a mad-man, in a strong mad fit, will be as strong as ten men; whereas, when the mad fit is over, he seems weaker than usually, or regularly, he uses to be; not that the self-moving parts of nature are capable of being weaker, or stronger, than naturally they are: but having liberty to move as they will, they may move stronger, or weaker, swifter or slower, regularly or irregularly, as they please; nor doth nature commonly use force. but this is to be noted, that there being a general agreement amongst the particular parts, they are more forcible than when those parts are divided into factions and parties: so that in a general irregular commotion or action, all the sensitive parts of the body of a man, agree to move with an extraordinary force, after an unusual manner; provided it be not different from the property and nature of their compositions; that is, not different from the property and nature of a man. but this is likewise to be noted, that in a general agreement, man may have other properties, than when the whole body is governed by parts, as it is usual when the body is regular, and that every part moves in his proper sphere, as i may say, (for example) the head, heart, lungs, stomack, liver, and so the rest, where each part doth move in several sorts of actions. the like may also may be said of the parts of the leggs and hands, which are different sorts of actions; yet all move to the use and benefit of the whole body: but, if the corporeal motions in the hands, and so in the leggs, be irregular, they will not help the rest of the parts; and so, in short, the same happens in all the parts of the body, whereof some parts may be regular, and others irregular; and sometimes all may be irregular. but, to conclude this chapter, the body may have unusual force and properties; as when a man says, he was carried and flung into a ditch, or some place distant; and that he was pinch't, and did see strange sights, heard strange sounds, smelt strange scents; all which may very well be caused by the irregular motions, either by a general irregularity, or by some particular irregularity; and the truth is, the particular corporeal motions, know not the power of the general, until they unite by a general agreement; and sometimes there may be such commotions in the body of a man, as in a common-wealth, where many times there is a general uproar and confusion, and none know the cause, or who began it. but this is to be noted, that if the sensitive motions begin the disorder, then they cause the rational to be so disordered, as they can neither advise wisely, or direct orderly, or perswade effectually. chap. xii. of diseases produced by conceit. as there are numerous sorts of _diseases_, so there are numerous manners or ways of the production of diseases; and those diseases that are produced by _conceit_, are first occasioned by the rational corporeal figurative motions: for, though every several conceit, or imagination, is a several rational corporeal figurative motion; yet, every conceit or imagination doth not produce a sensitive effect: but in those that do produce a sensitive effect, it is the conceit or imagination of some sorts of diseases; but in most of those sorts that are dangerous to life, or causes deformity: the reason is, that as all the parts of nature are self-knowing, so they are self-loving: also, regular societies beget an united love, by regular agreements, which cause a rational fear of a disuniting, or dissolving; and that is the reason, that upon the perception of such a disease, the rational, through some disorder, figures that disease; and the sensitive corporeal motions, take a pattern from the rational, and so the disease is produced. the tenth part. chap. i. of fevers. some are of opinion, that all, or, at least, most diseases, are accompanied, more or less, with a _feverous distemper_: if so, then we may say, a _fever_ is the _fundamental disease_: but, whether that opinion is true, or no, i know not; but i observe, there are many sorts of fevers, and so there are of all other diseases or distempers: for, every alteration, or difference, of one and the same kind of disease, is a several sort. as for fevers, i have observed, there are fevers in the blood, or humours, and not in any of the vital parts; and those are ordinary burning-fevers: and there are other sorts of fevers that are in the vital parts, and all other parts of the body, and those are _malignant fevers_; and there are some sorts of fevers which are in the radical humours, and those are _hectick fevers_; and there are other sorts of fevers that are in those parts, which we name the _spiritous parts_. also, all _consumptions_ are accompanied with a feverish distemper: but, what the several figurative motions are of these several sorts of fevers, i cannot tell. chap. ii. of the plague. there are two visible sorts of the disease named the _plague_: the weaker sort is that which produces swellings, or inflamed or corrupted sores, which are accompanied with a fever. the other sort is that which is named the _spotted plague_. the first sort is sometimes curable; but the second is incurable; at least, no remedy as yet hath been found. the truth is, the _spotted plague_ is a _gangrene_, but is somewhat different from other sorts of _gangren's_; for this begins amongst the vital parts, and, by an infection, spreads to the extream parts; and not only so, but to forrein parts; which makes not only a general infection amongst all the several parts of the body, but the infection spreads it self to other bodies. and whereas other sorts of _gangren's_ begin outwardly, and pierce inwardly; the _plaguy gangrene_ begins inwardly, and pierces outwardly: so as the difference (as i said) is, that the ordinary sort of _gangren's_ infect the next adjoining parts of the body, by moderate degrees; whereas the _plaguy gangrene_ infects not only the adjoining parts of the same body, and that suddenly, but infects forreign bodies. also, the ordinary _gangren's_ may be stopped from their infection, by taking off the parts infected, or diseased. but the _plaguy gangrene_ can no ways be stopped, because the vital parts cannot be separated from the rest of the parts, without a total ruine: besides, it pierces and spreads more suddenly, than remedies can be applyed. but, whether there are applications of preventions, i know not; for, those studies belong more to the _physicians_, than to a _natural philosopher_. as for the diseases we name the _purples_, and the _spotted fever_, they are of the same kind, or kindred, although not of the same sort, as _measles_, and the _small-pox_. but this is to be noted, that infection is an act of imitation: for, one part cannot give another part a disease, but only that some imitate the same sorts of irregular actions of other parts; of which some are near adjoining imitators, and some occasion a general mode. chap. iii. of the small-pox, and measles. the _small-pox_ is somewhat like the _sore-plague_, not only by being infectious, as both sorts of plagues are; but, by being of a corrupt nature, as the sore-plague is; only the _small-pox_ is innumerable, or very many small sores; whereas the _sore-plague_ is but one or two great sores. also, the _small-pox_ and _sore-plague_, are alike in this, that if they rise and break, or if they fall not flat, but remain until they be dry and scabbed, the patient lives: but, if they fall flat, and neither break, nor are scabbed, the patient is in danger to dye. also, it is to be noted, that this disease is sometimes accompanied with a feverish distemper; i say, sometimes, not always; and that is the cause that many dye, either with too hot, or too cooling applications: for, in a feverish distemper, hot cordials are poyson; and when there is no fever, cooling remedies are _opium_: the like for letting blood; for if the disease be accompanied with a fever, and the fever be not abated by letting blood, 'tis probable the fever, joyned with the pox, will destroy the patient: and if no fever, and yet loose blood, the pox hath not sufficient moisture to dilate, nor a sufficient natural vapour to breathe, or respirate; so as the life of the patient is choaked or stifled with the contracted corruptions. as for measles, though they are of the same kind, yet not of the same sort; for they are rather small risings, than corrupted sores, and so are less dangerous. chap. iv. of the intermission of fevers or agues. _agues_ have several sorts of distempers, and those quite opposite to each other, as cold and shaking, hot and burning, besides sweating: also, there are several times of intermissions; as some are every-day agues, some third-day agues, and some _quartan_ agues; and some patient may be thus distempered, many times, in the compass of four and twenty hours: but those are rather of the nature of intermitting fevers, than of perfect agues. also, in agues, there is many times a difference of the hot and cold fits: for sometimes the cold fits will be long, and the hot short; other times, the hot fits will be long, and the cold fits short; other times, much of an equal degree: but, most intermitting fevers and agues, proceed either from ill-digestive motions, or from a superfluity of cold and hot motions, or an irregularity of the cold, hot, dry; or moist motions, where each sort strives and struggles with each other. but, to make a comparison, agues are somewhat like several sorts of weather, as freezing and thawing, cloudy or rainy, or fair and sun-shining days: or like the four seasons of the year, where the cold fits are like _winter_, cold and windy; the hot fits like _summer_, hot and dry; the sweating fits like _autumn_, warm and moist; and, when the fit is past, like the _spring_. but, to conclude, the chief cause of agues, is, irregular digestions, that make half-concocted humours; and according as these half-concocted humours digest, the patient hath his aguish distempers, where some are every day, others every second day, some every third day, and some _quartans_: but, by reason those half-concocted humours, are of several sorts of humors, some cold, some hot, some cold and dry, some hot and dry, or hot and moist; and those different sorts, raw, or but half-concocted humours; they occasion such disorder, not only by an unnatural manner of digestion, as not to be either timely, or regular, by degrees; but, those several sorts of raw humours, strive and struggle with each other for power or supremacy: but, according as those different raw humours concoct, the fits are longer or shorter: also, according to the quantity of those raw humours, and according as those humours are a gathering, or breeding, so are the times of those fits and intermissions. but here is to be noted, that some agues may be occasioned from some particular irregular digestions; others from a general irregular digestion, some from some obscure parts, others from ordinary humours. chap. v. of consumptions. there are many sorts of _consumptions_; as, some are consumptions of the vital parts, as the liver, lungs, kidneys, or the like parts: others, a consumption of the radical parts: others a consumption of the spiritous parts: other consumptions are only of the flesh; which, in my opinion, is the only curable consumption. but, all consumptions, are not only an alteration, but a wasting and dis-uniting of the fundamental parts; only those consuming parts do, as it were, steal away by degrees; and so, by degrees, the society of a human creature is dissolved. chap. vi. of dropsies. _dropsies_ proceed from several causes; as, some from a decay of some of the vital parts; others through a superfluity of indigested humours; some from a supernatural driness of some parts; others through a superfluity of nourishing motions; some, through some obstructions; others, through an excess of moist dyet: but, all dropsies proceed not only from irregular motions, but from such a particular irregularity, as all the motions endeavour to be of one mode, (as i may say) that is, to move after the manner of those sorts of motions which are the innate nature of water, and are some sorts of circular dilatations: but, by these actions, the human society endeavours to make a deluge, and to turn from the nature of blood and flesh, to the nature of water. chap. vii. of sweating. all _sweating-diseases_ are somewhat of the nature of dropsies; but they are (at least, seem to be) more exterior, than interior dropsies: but, though there be sweating-diseases which are irregular; yet, regular sweating is as proper as regular breathing; and so healthful, that sweating extraordinary, in some diseases, occasions a cure: for, sweating is a sort of purging; so that the evacuation of sweat, through the pores, is as necessary as other sorts of evacuation, as breathing, urine, siege, spitting, purging through the nose, and the like. but, excess of sweating, is like other sorts of fluxes, of which, some will scowr to death; others vomit to death; and others the like fluxes will occasion death; the like is of sweating: so that the _sweating-sickness_ is but like a _fluxive-sickness_. but, as i said, regular sweating is as necessary as other ordinary evacuations: and as some are apt to be restringent, others laxative; and sometimes one and the same man will be laxative, other times, costive; so are men concerning sweating: and as some men take medicines to purge by stool, or vomits, or urine; so they take medicines to purge by sweating. and, as man hath several sorts of excremental humours, so, several sorts of sweats; as, clammy sweats, cold sweats, hot sweats, and faint sweats: and, as all excess of other sorts of purgings, causes a man to be weak and faint; so doth sweating. chap. viii. of covghs. there are many several sorts of _coughs_, proceeding from several causes; as, some coughs proceed from a superfluity of moisture; others from an unnatural heat; others from a corruption of humors; others from a decay of the vital parts; others from sudden colds upon hot distempers: some are caused by an interior wind; some coughs proceed from salt humors, bitter, sharp, and sweet: some coughs proceed from flegm, which flegm ariseth like a scum in a pot, when meat is boiling on a fire: for when the stomack is distemperedly hot, the humors in the stomack boyl as liquid substances on the fire; those boiling motions bearing up the gross humors beyond the mouth of the stomack, and, causing a dispute between the breath and humors, produce the effect of straining, or reaching upwards towards the mouth, much like the nature and motions of vomiting: but, by reason those motions are not so strong in coughing, as in vomiting, the coughing motions bring up only pieces or parts of superfluous flegm, or gross spittle. the like for corrupt humors. other coughs proceed from unnatural or distempered heats; which heats cause unnecessary vapours, and those vapours ascending up from the bowels, or stomack, to the head, and finding a depression, are converted or changed into a watry substance; which watry substance falls down, like mizling or small rain, or in bigger drops, through the passage of the throat and wind-pipe: which being opprest, and the breath hindered, causes a strife; which striving, is a straining; like as when crumbs of bread, or drops of drink, go not rightly through the throat, but trouble and obstruct the wind-pipe, or when any such matter sticks in the passage of the throat: for, when any part of the body is obstructed, it endeavours to release it self from those obstructions: also, when the vapour that arises, arises in very thin and rarified vapour, that rarified vapour thickens or condenses not so suddenly, being farther from the degree of water; but when condensed into water, it falls down by drops; which drops trickling down the throat, (like as tears from the eyes trickle down the cheeks of the face) the cough is not so violent, but more frequent: but if the rheum be salt or sharp, that trickles down the throat, it causes a gentle or soft smart, which is much like the touch of tickling or itching, which provokes a faint or weak strain or cough. also, wind will provoke to strain or cough: the motion of wind is like as if hair should tickle the nose. or, wind will cause a tickling in the nose, which causes the effect of sneezing: for, sneezing is nothing but a cough through the nose; i may say, it is a nose-cough. and hickops are but stomach-coughs, wind causing the stomack to strain. also, the guts have coughs, which are caused by the wind, which makes a strife in the guts and bowels. other coughs are produced from decayed parts: for, when any part is corrupted, it becomes less solid than naturally it should be: as for example, the flesh of the body, when corrupted, becomes from dense flesh, to a slimy substance; thence, into a watry substance, which falls into parts, or changes from flesh, into a mixt corrupted matter, which falls into parts. the several mixtures, or distempered substances, and irregular motions, causes division of the composed parts; but in the time of dissolving, and divisions of any part, there is a strife which causes pain: and if the strife be in the lungs, it causes coughs, by obstructing the breath: but, some coughs proceed from vapours and winds, arising from the decayed interior parts, sending up vapours from the dissolving substance, which causeth coughs; and some coughs cause decays of the prime interior parts: for, when there falls from the head a constant distillation, this distillation is like dropping water, which will penetrate or divide stone; and more easily will dropping or drilling water do it, as rheum, will corrupt spongy matter as flesh is: but, according as the rheum is fresh, salt, or sharp, the parts are a longer or shorter time decaying: for, salt and sharp is corroding; and, by the corroding motions, ulcerates those parts the salt rheums fall on, which destroys them soon. as for _chin-cough_, 'tis a wind or vapour arising from the lungs, through the wind-pipe; and as long as the wind or vapour ascends, the patient cannot draw in reviving air or breath, but coughs violently and incessantly, until it faint away, or have no strength left; and with straining, will be as if it were choaked or strangled, and become black in the face, and, after the cough is past, recover again; but some dye of these sorts of coughs. chap. ix. of gangren's. _gangren's_ are of the nature of the _plague_; and they are of two sorts, as the _plague_ is; the one more sudden and deadly than the other: the only difference of their insecting qualities, is, that _gangren's_ spread by insecting still the next, or neighbouring parts; whereas plagues infect forrein, as much as home-parts. also, the deadly sort of _gangren's_, infect (as i may say) from the circumference towards the center: when as the deadly sorts of plague, infect from the center, towards the circumference. but, that sort of _gangrene_ that is the weaker sort, infects only the next adjoining parts, by degrees, and after a spreading manner, rather than after a piercing manner. but some may object, that _plagues_ and _gangren's_ are produced from different causes; as for example, extream cold will cause _gangren's_; and extream heat causes _plagues_. i answer, that two opposite causes may produce like effects, for which may be brought numerous examples. chap. x. of cancers and fistula's. _cancers_ and _fistula's_ are somewhat alike, in that they are both produced from salt, or sharp corroding motions: but in this they differ, that cancers keep their center, and spread in streams; whereas _fistula's_ will run from place to place: for if it be stopt in one place, it is apt to remove and break out in another. yet _cancers_ are somewhat like _gangren's_, in infecting adjoining parts; so that unless a _cancer_ be in such a place as can be divided from the sound parts, it destroys the human life, by eating (as i may say) the sound parts of the body, as all corroding, and sharp or salt diseases do. chap. xi. of the govt. as for the disease named the _gout_, i never heard but of two sorts; the _fixt_, and the _running gout_: but, mistake me not, i mean _fixt_ for _place_, not _time_. the _fixt_ proceeds from hot, sharp, or salt motions: the _running gout_ from cold, sharp motions; but, both sorts are intermitting diseases, and very painful; and i have heard those that have had the _fixt gout_, say, that the pain of the fixt gout, is somewhat like the tooth-ach: but, all gouts are occasioned by irregular pressures and re-actions. as for that sort that is named the windy gout, it is rather a sciatica, than a gout. chap. xii. of the stone. of the disease of the _stone_ in human creatures, there are many sorts: for, though the _stone_ of the _bladder_, of the _kidneys_, and in the _gaul_, be all of one kind of disease called the _stone_, yet they are of different sorts: but, whether the disease of the _stone_ be produced of hot or cold motions, i cannot judg: but 'tis probable, some are produced of hot motions, others of cold; and perchance, others of such sorts of motions as are neither perfectly hot, nor cold: for, the _stone_ is produced, as all other creatures, by such or such sorts of figurative motions. here is to be noted, that some of the humours of the body may alter their motion, and turn from being flegm, choler, or the like, to be _stone_; and so from being a rare, moist, or loose body, to be a dry, densed, hard, or fixt body. but certainly, the _stone_ of the _bladder_, _kidneys_ and _gaul_, are of several sorts, as being produced by several sorts of figurative motions; as also, according to the properties and forms of those several parts of the body they are produced in: for, as several sorts of soyls, or parts of the earth, produce several sorts of minerals; so several parts of the body, several sorts of the disease of the _stone_: and, as there are several sorts of stones in the several parts of the earth; so, no doubt, there may not only be several sorts of stone in several parts, but several sorts in one and the same part; at least, in the like parts of several men. chap. xii. of apoplexies, and lethargies. _apoplexies, lethargies_, and the like diseases, are produced by some decay of the vital spirits, or by obstructions, as being obstructed by some superfluities, or through the irregularities of some sorts of motions, which occasion some passages to close, that should be open. but mistake me not, i do not mean empty passages; for there is no such thing (in my opinion) in nature: but, i mean an open passage for a frequent course and recourse of parts. but an _apoplexy_ is somewhat of the nature of a _dead-palsie_; and a _lethargy_, of a numb-palsie; but i have heard, that the opinion of learned men is, that some sorts of vaporous pains are the fore-runners of _apoplexies_ and _palsies_: but, in my opinion, though a man may have two diseases at once; yet surely, where vapour can pass, there cannot be an absolute stoppage. chap. xiii. of epilepsies. _epilepsies_, or that we name the _falling-sickness_, is of the nature of swounding or fainting fits: but there are two visible sorts; the one is, that only the head is affected, and not the other parts of the body; and for proof, those that are thus distempered only in the head, all the other parts will struggle and strive to help or assist the affected or afflicted parts, and those parts of the head that are not irregular, as may be observed by their motions; but, by the means of some other parts, there will also be striving and strugling, as may be observed by foaming through the mouth. the other sort is like ordinary swounding-fits, where all the parts of the body seem, for a time, to be dead. but this is to be observed, that those that are thus diseased, have certain times of intermissions, as if the corporeal motions did keep a decorum in being irregular. but some have had _epilepsies_ from their birth; which proves, that their productive motions was irregular. chap. xiv. of convulsions, and cramps. _convulsions_ and _cramps_ are somewhat alike; and both, in my opinion, proceed from cold contractions: but, _cramps_ are caused by the contractions of the _capillary_ veins, or small _fibers_, rather than of the nerves and sinews: for, those contractions, if violent, are _convulsions_: so that cramps are contractions of the small fibers; and convulsions are contractions of the nerves and sinews. but the reason (i believe) that these diseases proceed from cold contractions, is, that hot remedies produce, for the most part, perfect cures; but, they must be such sorts of hot remedies, that are of a dilating or extenuating nature; and not such whose properties are hot and dry, or contracting: also, the applications must be according to the strength of the disease. chap. xv. of cholicks. _cholicks_ are like _cramps_ or _convulsions_; or _convulsions_ and _cramps_, like _cholicks_: for, as _convulsions_ are contractions of the nerves and sinews; and _cramps_, contractions of the small _fibers_: so _cholicks_ are a contracting of the gutts: and, for proof, so soon as the contracting motions alter, and are turn'd to dilating or expelling actions, the patient is at ease. but, there are several causes that produce the _cholick_: for, some _cholicks_ are produced by hot and sharp motions, as _bilious cholicks_; others from cold and sharp motions, as _splenetick cholicks_; others from crude and raw humours; some from hot winds; some from cold winds. the same some sorts of _convulsions_ and _cramps_ may be: but, though these several _cholicks_ may proceed from several causes; yet, they all agree in this, to be contractions: for, as i said, when those corporeal motions alter their actions to dilatation or expulsion, the patient is at ease. but, those _cholicks_ that proceed from hot and sharp motions, are the most painful and dangerous, by reason they are, for the most part, more strong and stubborn. as for _cholicks_ in the stomack, they are caused by the same sorts of motions that cause some sorts of contractions: but, those sorts of _cholick_ contractions, are after the manner of wreathing, or wringing contractions. the same in convulsive-contractions. chap. xvi. of shaking palsies. _shaking palsies_ proceed from a slackness of the nerves, or sinew strings, as may be observed by those that hold or lay any heavy weight upon the arms, hands or leggs: for, when the burdens are removed, those limbs will be apt to tremble and shake so much, for a short time, (until they have recovered their former strength) that the leggs cannot go, or stand steadily; nor the arms, or hands, do any thing without shaking. the reason of these sorts of slackness, is, that heavy burdens occasion the nerves and sinews to extend beyond their order; and being stretched, they become more slack, and loose, by how much they were stretched, or extended; until such time as they contract again into their proper posture: and the reason that old age is subject to _shaking-palsies_, is, that the frame of their whole body is looser and slacker, than when it was young: as in a decayed house, every material is looser than when it was first built; but yet, sometimes an old shaking house will continue a great while, with some repairs: so old shaking men, with care, and good dyet, will continue a great time. but this is to be noted, that trembling is a kind of a _shaking-palsie_, although of another sort; and so is weakness after sickness: but, these sorts are occasioned, as when a house shakes in a great wind, or storm; and not through any fundamental decay. chap. xvii. of the muther, spleen, and scurvy. as for those diseases that are named the _fits of the muther_, the _spleen_, the _scurvy_, and the like; although they are the most general diseases, especially amongst the females; yet, each particular sort is so various, and hath such different effects, that, i observe, they puzzle the most learned men to find out their jugling, intricate, and uncertain actions. but this is to be observed, that the richest sorts of persons are most apt to these sorts of diseases; which proves, that idleness and luxury is the occasion. chap. xviii. of food, or digestions. as i have said, _digestions_ are so numerous, and so obscure, that the most learned men know not how food is converted and distributed to all the parts of the body: which obscurity occasions many arguments, and much dispute amongst the learned; but, in my opinion, it is not the parts of the human body, that do digest the food, although they may be an occasion (through their own regularities, or irregularities) to cause good or bad digestions: but, the parts of the food, do digest themselves; that is, alter their actions to the property and nature of a human body: so that digestive parts are only additional parts; and, if those nourishing motions be regular,they distribute their several parts, and joyn their several parts, to those several parts of the body that require addition. also, the digestive motions are according to the nature or property of each several part of the human body, as for example, those digestive parts alter into blood, flesh, fat, marrow, brains, humors, and so into any other figurative parts of the sensitive body. the same may be said of the rational parts of the mind: but, if those digestive parts be irregular, they will cause a disorder in a well-ordered body: and, if the parts of the body be irregular, they will occasion a disorder amongst the digestive parts: but, according to the regularities and irregularities of the digestive parts, is the body more or less nourished. but this is to be noted, that according to the superfluity or scarcity of those digestive parts, the body is opprest, or starved. chap. xix. of surfeits. _surfeits_ are occasioned after different manners: for, though many surfeits proceed from those parts that are received into the body; yet, some are occasioned through often repetitions of one and the same actions: as for example, the eyes may surfeit with too often viewing one object; the ears, with often hearing one sound; the nose, with smelling one sent; the tongue, with one tast. the same is to be said of the rational actions; which surfeits, occasion an aversion to such or such particulars: but, for those surfeits that proceed from the parts that are received into the body, they are either through the _quantity_ that oppresses the nature of the body; or, through the _quality_ of those parts, being not agreeable to the nature of the body; or, through their irregularities, that occasion the like irregularities in the body: and sometimes, the fault is through the irregularities of the body, that hinder those received parts, or obstruct their regular digestions; and sometimes, the fault is both of the parts of the body, and those of the food: but, the surfeits of those parts that receive not food, are caused through the often repetition of one and the same action. chap. xx. of natural evacuations, or purgings. there are many sorts, and several ways or means of purging actions; whereof some we name _natural_, which purge the excremental parts; and such natural purgings, are only of such parts as are no ways useful to the body; or of those that are not willing to convert themselves into the nature and property of the substantial parts. there must of necessity be purging actions, as well as digestive actions; because, no creature can subsist singly of it self, but all creatures subsist each by other; so that, there must be dividing actions, as well as uniting actions; only, several sorts of creatures, have several sorts of nourishments and evacuations. but this is to be noted, in the human nourishments and evacuations, that, through their irregularities, some men may nourish too much, and others purge too much; and some may nourish too little, and some may purge too little. the irregularities concerning nourishments, are amongst the adjoining parts; the errors concerning purging, are amongst the dividing parts. chap. xxi. of purging druggs. there are many sorts of _druggs_, whereof some are beneficial, by assisting those particular parts of the body that are oppressed and offended, either by superfluous humours, or malignant humours: but, there are some sorts of druggs that are as malicious to the human life, as the assistant druggs are friendly. several sorts of druggs, have several sorts of actions, which causes several effects; as, some druggs work by siege; others, by urine; some, by vomit; others, by spitting; others, by sweating; some cause sleep; some are hot, others are cold; some dry, others moist. but this is to be noted, that 'tis not the motions of the druggs, but the motion of the humours, which the druggs occasion to flow; and not only to flow, but to flow after such or such a manner and way. the actions of druggs, are like the actions of hounds, or hawks, that flye at a particular bird, or run after a particular beast of their own kind, although of a different sort: the only difference is, that druggs are not only of a different sort, but of a different kind from animal kind; at least, from human sort. chap. xxii. of the various humours of druggs. the reason, one and the same quantity or dose of one and the same sort of purging-druggs or medicine, will often work differently in several human bodies; as also, differently in one and the same body, at several times of taking the same sorts of medicines; is, that several parts of one and the same sort, may be differently humoured: as, some to be duller and slower than others; and some to be more active than others. also, some parts may be ill-natured, and cause factions amongst the parts of the body; whereas others will endeavour to rectifie disorders, or factions. and sometimes both the druggs, and the body, falls out; and then there is a dangerous strife; the body striving to expel the physick, and the physick endeavouring to stay in the body, to do the body some mischief. also, some parts of one and the same sort, may be so irregular, as to hunt not only the superfluous humours, or the malignant humors, but all sorts of flowing parts; which may cause so great and general disorder, as may endanger human life. chap. xxiii. of cordials. there are many sorts of _cordials_: for, i take every beneficial remedy to be a cordial: but, many of the vulgar believe, that there is no cordial but brandy, or such like strong-waters; at least, they believe all such remedies that are virtually hot, to be cordials: but, when they take too much of such cordials, either in sickness, or health, they will, in some time, find them as bad as poyson. but, all such applications as are named _cordials_, are not hot: for, some are cool, at least, of a temperate degree. and as there are regular and irregular corporeal motions; so there are sympathetical, and antipathetical motions; and yet both sorts may be regular. also, there is a neutral sort, that has neither sympathy nor antipathy, but is indifferent. but in disputes between two different parties, a third may come in to the assistance of one side, more out of hate to the opposite, than love to the assisted. the same may cordials, or such like applications, do, when the corporeal motions of human life are in disorder, and at variance: for, oftentimes there is as great a mutiny and disorder amongst the corporeal motions, both in the mind and body of a man, as in a publick state in time of rebellion: but, all assistant cordials, endeavour to assist the regular parts of the body, and to perswade the irregular parts. as for poysons, they are like forrein warr, that endeavours to destroy a peaceable government. chap. xxiv. of the different actions of the several sensitive parts of a human creature. some parts of a human creature will be regular, and some irregular: as, some of the sensitive parts will be regular, and some irregular; that is, some parts will be painful, or sick, others well: some parts will make false perceptions; others, true perceptions: some parts be temperate; others, intemperate: some parts be madd, other parts sober: some parts be wise; others, foolish: and the same is to be said of the rational motions. but, in a regular society, every part and particle of the body, is regularly agreeable, and sympathetical. chap. xxv. of the antipathy of some human creatures, to some forrein objects. as i have often said, there is often both sympathy and antipathy between the parts of some particular human and forrein objects; in so much, that some will occasion such a general disturbance, as will cause a general alteration, viz. cause a man to swound, or at least, to be very faint, or sick: as for example, some will swound at some sorts of sounds, some sorts of scents, some sorts of tast, some sorts of touches, and some sorts of sights. again, on the other side, some human creatures will so sympathize with some sorts of forrein objects, as some will long for that, another will swound to have. chap. xxvi. of the effects of forrein objects, on the human mind. as there is often antipathy of the parts of a human creature, to forrein objects; so there are often sympathetical effects produced from forrein objects, with the parts of a human creature. as for example, a timely, kind, and discreet discourse from a friend, will compose or quiet his troubled mind: likewise, an untimely, unkind, hasty, malicious, false, or sudden discourse, will often disorder a well-temper'd, or regular mind, the mind imitating the smooth or harsh strains of the object: and the same effects hath musick, on the minds of many human creatures. chap. xxvii. of contemplation. human _contemplation_, is a conversation amongst some of the rational parts of the human mind; which parts, not regarding present objects, move either in devout notions, or vain fancies, remembrances, inventions, contrivancies, designs, or the like. but the question is, whether the sensitive parts of a human society, do, at any time, contemplate? i answer, that some of the sensitive parts are so sociable, that they are, for the most part, agreeable to the rational: for, in deep contemplations, some of the sensitive parts do not take notice of forrein objects, but of the rational actions. also, if the contemplations be in devout notions, the sensitive parts express devotion by their actions, as i have formerly mentioned. also, when the rational parts move in actions of desire, straight the sensitive move in sympathetical appetites: wherefore, if the society be regular, the sensitive and rational parts are agreeable and sociable. chap. xxviii. of injecting of the blood of one animal, into the veins of another animal. to put blood of one animal, into another animal; as for example, some ounces of blood taken, by some art, out of a dogg's veins, and, by some art, put into a man's veins, may very easily be done by _injection_; and certainly, may as readily convert it self to the nature of human blood, as roots, herbs, fruit, and the like food; and probably, will more aptly be transformed into human flesh, than hogg's blood, mixt with many ingredients, and then put into gutts, and boyled, (an ordinary food amongst country people;) but blood being a loose humourish part, may encrease or diminish, as the other humors, viz. _flegm, choler_, and _melancholy_, are apt to do. but this is to be observed, that by reason blood is the most flowing humor, and of much more, or greater quantity than all the rest of the humours, it is apt (if regular) to cause, not only more frequent, but a more general disturbance. the eleventh part. chap. i. of the different knowledges, in different kinds and sorts of creatures. if there be not infinite kinds, yet, it is probable, there are infinite several sorts; at least, infinite particular creatures, in every particular kind and sort; and the corporeal motions moving after a different manner, is the cause there are different knowledges, in different creatures; yet, none can be said to be _least knowing_, or _most knowing_: for, there is (in my opinion) no such thing as _least_ and _most_, in nature: for, several kinds and sorts of knowledges, make not knowledg to be more, or less; but only, they are different knowledges proper to their kind, (as, animal-kind, vegetable-kind, mineral-kind, elemental-kind) and are also different knowledges in several sorts: as for example, man may have a different knowledg from beasts, birds, fish, flies, worms, or the like; and yet be no wiser than those sorts of animal-kinds. the same happens between the several knowledges of vegetables, minerals, and elements: but, because one creature doth not know what another creature knows, thence arises the opinion of _insensibility_, and _irrationability_, that some creatures have of others. but there is to be noted, that nature is so regular, or wise, in her actions, that the _species_ and knowledg of every particular kind, is kept in an even, or equal balance: for example, the death or birth of animals, doth neither add or diminish from, or to the knowledg of the kind, or rather the sort. also, an animal can have no knowledg, but such as is proper to the _species_ of his figure: but, if there be a creature of a mixt _species_, or figure, then their knowledg is according to their mixt form: for, the corporeal motions of every creature, move according to the form, frame, or _species_ of their society: but, there is not only different knowledges, in different kinds and sorts of creatures; but, there are different knowledges in the different parts of one and the same; as, the different senses of seeing, hearing, smelling, tasting, and touching, have not only different knowledges in different sensitive organs, but in one sense, they have several perceptive knowledges: and though the different sensitive organs of a human creature, are ignorant of each other; yet, each sense is as knowing as another. the same (no question) is amongst all the creatures in nature. chap. ii. of the variety of self-actions in particular creatures. there are numerous varieties of figurative motions in some creatures; and in others, very few, in comparison: but, the occasion of that, is the manner of the frame and form of a creature: for, some creatures that are but small, have much more variety of figurative motions, than others that are very bigg and large creatures: so that, it is not only the quantity of matter, or number of parts, but the several changes of motion, by the variety of their active parts, that is the cause of it: for, nature is not only an infinite body, but, being self-moving, causes infinite variety, by the altered actions of her parts; every altered action, causing both an altered self-knowledg, and an altered perceptive knowledg. chap. iii. of the variety of corporeal motion, of one and the same sort or kind of motion. there is infinite variety of motion of the same sorts and kinds of motions; as for example, of dilatations, or extensions, expulsions, attractions, contractions, retentions, digestions, respirations: there is also varieties of densities, rarities, gravities, levities, measures, sizes, agilness, slowness, strength, weakness, times, seasons, growths, decays, lives, deaths, conceptions, perceptions, passions, appetites, sympathies, antipathies, and millions the like kinds, or sorts. chap. iv. of the variety of particular creatures. nature is so delighted with _variety_, that seldom two creatures (although of the same sort, nay, from the same producers) are just alike; and yet human perception cannot perceive above four kinds of creatures, viz. _animals, vegetables, minerals_, and _elements_: but, the several sorts seem to be very numerous; and the varieties of the several particulars, infinite: but, nature is necessitated to divide her creatures into kinds and sorts, to keep order and method: for, there may be numerous varieties of sorts; as for example, many several worlds, and infinite varieties of particulars in those _worlds_: for, worlds may differ from each other, as much as several sorts of animals, vegetables, minerals, or elements; and yet be all of that sort we name worlds: but, as for the infinite varieties of nature, we may say, that every part of nature is infinite, in some sort; because every part of nature is a perpetual motion, and makes infinite varieties, by change or alteration of action: but, there is so much variety of the several shapes, figures, forms, and sizes, as, bigger, and less; as also, several sorts of heats, colds, droughts, moistures, fires, airs, waters, earths, animals, vegetables, and minerals, as are not to be expressed. chap. v. of dividing, and rejoyning, or altering exterior figurative motions. the interior and exterior figurative motions of some sorts of creatures, are so united by their sympathetical actions, as they cannot be separated without a total dissolution; and some cannot be altered without a dissolution; and other figurative motions may separate, and unite again; and others, if separate, cannot unite again, as they were before: as for example, the exterior parts of a human creature, if once divided, cannot be rejoyned; when as some sorts of worms may be divided, and if those divided parts meet, can rejoyn, as before. also, some figurative motions of different sorts, and so different, that they are opposite, may unite in agreement, in one composition, or creature; yet, when the very same sorts of figurative motions, are not so united, they are, as it were, deadly enemies. chap. vi. of different figurative motions in particular creatures. there are many creatures that are composed of very opposite figurative motions; as for example, some parts of fire and water; also, all cordials, vitriols, and the like waters; also, iron and stone, and infinite the like: but, that which is composed of the most different figurative motions, is _quick-silver_, which is exteriorly cold, soft, fluid, agil, and heavy: also, divisible, and rejoynable; and yet so retentive of its innate nature, that although it can be rarified, yet not easily dissolved; at least, not that human creatures can perceive; for, it hath puzled the best _chymists_. chap. vii. of the alterations of exterior and innate figurative motions of several sorts of creatures. the form of several creatures, is after several manners and ways, which causes several natures or properties: as for example, the exterior and innate corporeal motions of some creatures, depend so much on each other, that the least alteration of the one, causes a dissolution of the whole creature; whereas the exterior corporeal motions of other sorts of creatures, can change and rechange their actions, without the least disturbance to the innate figurative motions: in other sorts the innate motions shall be quite altered, but their exterior motions be in some manner consistent: as for proof, fire is of that nature, that both the exterior and innate motions, are of one and the same sort; so that the alteration of the one, causeth a dissolution of the other; that is, fire loses the property of fire, and is altered from being fire. on the other side, the exterior figurative motions of water, can change and rechange, without any disturbance to the innate nature: but, though the alteration of the innate figurative motions of all creatures, must of necessity alter the life and knowledg of that creature; yet there may be such consistent motions amongst the exterior parts of some sorts of creatures, that they will keep their exterior form: as for example, a tree that is cut down, or into pieces, when those pieces are withered, and, as we say, dead; yet, they remain of the figure of wood. also, a dead beast doth not alter the figure of flesh or bones, presently. also, a dead man doth not presently dissolve from the figure of man; and some, by the art of embalming, will occasion the remaining figurative motions of the dead man to continue, so that those sorts of motions, that are the frame and form, are not quite altered: but yet, those exterior forms are so altered, that they are not such as those by which we name a _living man_. the same of flyes, or the like, intomb'd in _amber_: but by this we may perceive, that the innate figurative motions may be quite altered, and yet the exterior figurative consistent motions, do, in some manner, keep in the figure, form, or frame of their society. the truth is, (in my opinion) that all the parts that remain undissolved, have quite altered their animal actions; but only the consistent actions, of the form of their society, remains, so as to have a resemblance of their frame or form. chap. viii. of local motion. all corporeal motion is _local_; but only they are different local motions: and some sorts or kinds, have advantage of others, and some have power over others, as, in a manner, to inforce them to alter their figurative motions; as for example, when one creature doth destroy another, those that are the _destroyers_, occasion those that we name the _destroyed_, to dissolve their unity, and to alter their actions: for, they cannot annihilate their actions; nor can they give or take away the power of self-motions; but, as i said, some corporeal motions can occasion other corporeal motions to move so, or so. but this is to be noted, that several sorts of creatures have a mixture of several sorts of figurative motions; as for example, there are flying fish, and swimming beasts; also, there are some creatures that are partly beasts, and partly fish, as _otters_, and many others; also, a _mule_ is partly a horse, and an ass; a _batt_ is partly a mouse, and a bird; an _owle_ is partly a cat and a bird; and numerous other creatures there are, that are partly of one sort, and partly of another. chap. ix. of several manners, or ways of advantages, or disadvantages. not only the manner, form, frame, or shape of particular creatures; but also, the regularity or irregularity of the corporeal motions of particular creatures, doth cause that which man names _strength_ or _weakness, obedience_ or _disobedience_, _advantages_ or _disadvantages_ of power and authority, or the like: as for example, a greater number will overpower a lesse: for, though there be no differences (as being no degrees) of self-strength amongst the self-moving parts, or corporeal motions; yet, there may be stronger and weaker compositions, or associations; and a greater number of corporeal motions, makes a stronger party: but, if the greater party be irregular, and the lesser party be regular, a hundred to one, but the weaker party is victorious. also, the manner of the corporeal motions; as, a diving-motion may get the better of a swimming-motion; and, in some cases, the swimming, the better of the diving. jumping may have the advantage over running; and, in other cases, running, over jumping. also, creeping may have the advantage over flying; and, in other cases, flying, over creeping. a cross motion may have the advantage over a straight; and, in other cases, a straight, over a cross. so it may be said, of turning and lifting, of contracting and dilating motions. and many the like examples may be had; but, as i have often said, there is much advantage and disadvantage in the manner and way of the composed form and figure of creatures. chap. x. of the actions of some sorts of creatures, over others. some sorts of creatures are more exteriorly active, than other sorts; and some more interiorly active; some more rare, some more dense, and the like: also, some dense creatures are more active than the rare; and some rare, are more active than other sorts that are dense. also, some creatures that are rare, have advantage of some that are dense; and some that are dense, over some sorts that are rare; some leight bodies, over some heavy bodies; and some heavy bodies, over some sorts of leight bodies. also, several sorts of exterior motions, of several sorts of creatures, have advantage and disadvantage of each other; as for example, springs of water, and air, will make passages, and so divide hard strong rocks. and, on the other side, a straw will divide parts of water; and a small flye, will divide parts of the air: but, mistake me not, i mean, that they occasion the airy or watry parts, to divide. chap. xi. of glassie bodies. tis impossible, as i have said, to describe the infinite corporeal figurative motions: but, amongst those creatures that are subject to human perception, there are some that resemble each other, and yet are of different natures; as for example, _black ebony_, and _black marble_, they are both glassie, smooth, and black; yet, one is stone, the other wood. also, there be many light and shining bodies, that are of different natures; as for example, metal is a bright shining body; and divers sorts of stones, are bright shining bodies: also, clear water is a bright shining body; yet, the metal and stones are minerals, and water is an element. indeed, most bodies are of a glassie hue, or, as i may say, complexion; as may be observed in most vegetables; as also, skins, feathers, scales, and the like. but some may say, that _glassiness is made by the brightness of the light that shines upon them_. i answer: if so, then the ordinary earth would have the like glassiness: but, we perceive the earth to appear dull in the clearest sun-shining day: wherefore, it is not the light, but the nature of their own bodies. besides, every body hath not one and the same sort of glassiness, but some are very different: 'tis true, some sorts of bodies do not appear glassie, or shining, until they be polished: but, as for such sorts of shining bodies that appear in the dark, there is not many of them perceiv'd by us, besides the moon and starrs; but yet some there are, as fire; but that is an element. there are also glow-worms tayles, cats eyes, rotten wood, and such like shining-bodies. chap. xii. of metamorphoses, or transformations of animals and vegetables. there are some creatures that cannot be metamorphosed: as for example, animals and vegetables, at least, most of those sorts, by reason they are composed of many several and different figurative motions; and i understand _metamorphose_, to be a change and alteration of the exterior form, but not any change or alteration of the interior or intellectual nature: and how can there be a general change of the exterior form or shape of a human creature, or such like animal, when the different figurative motions of his different compositions, are, for the most part, ignorant of each others particular actions? besides, as animals and vegetables require degrees of time for their productions, as also, for their perfections; so, some time is requir'd for their alterations: but, a sudden alteration amongst different figurative motions, would cause such a confusion, that it would cause a dissolution of the _whole_ creature, especially in actions that are not natural, as being improper to their kind, or sort: the same of vegetables, which have many different figurative motions. this considered, i cannot chuse but wonder, that wise men should believe (as some do) the change or transformation of witches, into many sorts of creatures. chap. xiii. of the life and death of several creatures. that which man names _life_, and _death_, (which are some sorts of compositions and divisions of parts of creatures) is very different, in different kinds and sorts of creatures, as also, in one and the same sort: as for example, some vegetables are old and decrepit in a day; others are not in perfection, or in their prime, in less than a hundred years. the same may be said of animal kinds. a _silk-worm_ is no sooner born, but dyes; when as other animals may live a hundred years. as for minerals, tinn and lead seem but of a short life, to gold; as a worm to an elephant, or a tulip to an oak for lasting; and 'tis probable, the several productions of the planets and fixed starrs, may be as far more lasting, than the parts of gold more lasting than a flye: for, if a composed creature were a million of years producing, or millions of years dissolving, it were nothing to eternity: but, those produced motions that make vegetables, minerals, elements, and the like, the subtilest philosopher, or chymist, in nature, can never perceive, or find out; because, human perception is not so subtile, as to perceive that which man names natural productions: for, though all the corporeal motions in nature are perceptive; yet, every perceptive part doth not perceive all the actions in nature: for, though every different corporeal motion, is a different perception; yet, there are more objects than any one creature can perceive: also, every particular kind or sort of creatures, have different perceptions, occasioned by the frame and form of their compositions, or unities of their parts: so as the perceptions of animals, are not like the perceptions of vegetables; nor vegetables, like the perceptions of minerals; nor minerals, like the perceptions of elements: for, though all these several kinds and sorts, be perceptive; yet, not after one and the same way, or manner of perception: but, as there is infinite variety of corporeal motions, so there are infinite varieties of perceptions: for, infinite self-moving matter, hath infinite varieties of actions. but, to return to the discourse of the productions and dissolutions of creatures; the reason, that some creatures last longer than others, is, that some forms or frames of their composition, are of a more lasting figure. but this is to be observed, that the figures that are most solid, are more lasting than those that are more slack and loose: but mistake me not; i say, _for the most part_, they are more lasting. also, this is to be noted, that some compositions require more labour; some, more curiosity; and some are more full of variety, than others. chap. xiv. of circles. a _circle_ is a round figure, without end; which figure can more easily and aptly alter the exterior form, than any other figure. for example, a circular line may be drawn many several ways, into different and several sorts of figures, without breaking the circle: also, it may be contracted or extended into a less or wider compass; and drawn or formed into many several sorts of figures, or works; as, into a square, or triangle, or oval, or cylinder, or like several sorts of flowers, and never dissolve the circular line. but this is to be noted, that there may be several sorts of circular lines; as, some broad, some narrow, some round, some flat, some ragged or twisted, some smooth, some pointed, some edged, and numbers of the like; and yet the compass be exactly round. but some may say, that, _when a circle is drawn into several works, it is not a circle: as for example, when a circle is squared, it is not a circle, but a square._ i answer: it is a circle squar'd, but not a circle broken, or divided: for, the interior nature is not dissolved, although the exterior figure is altered: it is a natural circle, although it should be put into a mathematical square. but, to conclude this chapter, i say, that all such sorts of figures that are (like circular lines) of one piece, may change and rechange their exterior figures, or shapes, without any alterations of their interior properties. chap. xv. human creatures cannot so probably treat of other sorts of creatures, as of their own. to treat of the productions of vegetables, minerals, and elements, is not so easie a task, as to treat of animals; and, amongst animals, the most easie task is, to treat of human productions; by reason one human creature may more probably guess at the nature of all human creatures (being of the same nature) than he can of other kinds of other kinds of creatures, that are of another nature. but, mistake me not, i mean not of another nature, being not of the same kind of creature, but concerning vegetables, minerals, and elements. the elements may more easily be treated of, than the other two kinds: for, though there be numerous sorts of them, at least, numerous several particulars; yet, not so many several sorts, as of vegetables: and though minerals are not, as to my knowledg, so numerous as vegetables; yet, they are of more, or at least, of as many sorts as elements are. but, by reason i am unlearned, i shall only give my opinion of the productions of some sorts; in which, i fear, i shall rather discover my ignorance, than the truth of their productions. but, i hope my _readers_ will not find fault with my endeavour, though they may find fault with my little experience, and want of learning. the twelfth part. chap. i. of the equality of elements. as for the four elements, _fire, air, water_, and _earth_; they subsist, as all other creatures, which subsist by each other: but, in my opinion, there should be an equality of the four elements, to balance the world: for, if one sort should superabound, it would occasion such an irregularity, that would cause a dissolution of this world; as, when some particular humour in man's body superabounds, or there is a scarcity of some humours, it causes such irregularities, that do, many times, occasion his destruction. the same may be said of the four elements of the world: as for example, if there were not a sufficient quantity of elemental air, the elemental fire would go out; and if not a sufficient quantity of elemental fire, the air would corrupt: also, if there were not a sufficient quantity of elemental water, the elemental fire would burn the earth; and if there were not a sufficient quantity of earth, there would not be a solid and firm foundation for the creatures of the earth: for, if there were not density, as well as rarity; and levity, as well as gravity; nature would run into extreams. chap. ii. of several tempers. heat doth not make drought: for, there is a _temper_ of hot and moist. nor cold doth not make drought: for, there is a _temper_ of cold and moist. neither doth heat make moisture: for, there is a _temper_ of hot and dry. nor doth cold make moisture: for, there is a _temper_ of cold and dry. but, such or such sorts of corporeal figurative motions, make hot, cold, moist, dry; hot and dry, hot and moist; cold and dry, cold and moist; and, as those figurative motions alter their actions, those _tempers_ are altered: the like happens in all creatures. but this is to be observed, that there is some opposite or contrary _tempers_, which have a likeness of motion: as for example, a moist heat, and a moist cold, have a likeness or resemblance of moistness; and the same is in dry heats and cold: but surely, most sorts of moistures, are some sorts of dilative motions; and most droughts, are some sorts of contractive motions: but, there are several sorts of dilatations, contractions, retentions, expulsions, and the like: for, there are cold contractions, hot contractions; cold dilatations, hot dilatations; hot retentions, cold retentions; and so of digestions, expulsions, and the like: but, as i said, moist heats, and moist colds, seem of a dilative nature; as dry, of a contractive nature. but, all cold and heat, or dry and moist, may be made by one and the same corporeal motions: for, though the actions may vary, the parts may be the same: yea, the like actions may be in different parts. but, no part is bound to any particular action, having a free liberty of self-motion. but, concerning hot and cold, and the like actions, i observe, that extream heat, and extream cold, is of a like power, or degree: neither can i perceive the hot motions to be quicker than cold: for water, in little quantity, shall as suddenly freeze, as any leight fewel or straw, burn: and animals will as soon freeze to death, as be burned to death: and cold is as powerful at the poles, as heat in the _torrid zone_. and 'tis to be observed, that freezing is as quick and sudden, as thawing: but sometimes, nay very often, cold and hot motions will dispute for power; and some sorts of hot, with other sorts. the like disputes are amongst several sorts of cold motions; dry with moist, dry with dry, moist with moist. and the like disputes are also often amongst all creatures. as for density, it doth not make gravity: for, there may be dense bodies, that are not grave; as for example, feathers, and snow. neither doth gravity make density: for, a quantity of air hath some weight, and yet is not dense. but mistake me not; for, i mean by _grave, heavy_; and not for the effects of ascending, and descending: for feathers, though dense, are more apt to ascend, than descend; and snow, to descend. also, all sorts of fluidity, do not cause moist, liquid, or wet; nor all extenuations, cause light: but, they are such and such sorts of fluidities and extenuations, that cause such and such effects. and so for heats, colds, droughts, moistures, rarities. the same for gravities, levities, and the like. so that, creatures are rare, fluid, moist, wet, dry, dense, hard, soft, leight, heavy, and the like, according to their figurative motions. chap. iii. of the change and rechange; and of dividing and ioyning of the parts of the elements. of all creatures subject to human perception, the elements are most apt to transform, _viz._ to _change_ and _rechange_; also, to _divide_ and _ioyn_ their parts, without altering their innate nature and property. the reason is, because the innate figurative motions of elements, are not so different as those of animals and vegetables, whose compositions are of many different figurative motions; in so much, that disjoining any part of animals, or vegetables, they cannot be joined again, as they were before; at least, it is not commonly done: but, the nature and property of the elements, is, that every part and particle are of one innate figurative motion; so that the least grain of dust, or the least drop of water, or the least spark of fire, is of the same innate nature, property, and figurative motions, as the whole element; when as, of animals, and vegetables, almost, every part and particle is of a different figurative motion. chap. iv. of the innate figurative motions of earth. there are many sorts of _earth_, yet all sorts are of the same kind; that is, they are all earth: but (in my opinion) the prime figurative motions of earth, are circles; but not dilated circles, but contracted circles: neither are those circles smooth, but rugged; which is the cause that earth is dull, or dim, and is easily divided into dusty parts: for all, or at least, most bodies that are smooth, are more apt to joyn, than divide; and have a glassie hew or complexion; which is occasioned by the smoothness, and the smoothness occasioned by the evenness of parts, being without intervals: but, according as these sorts of circular motions are more or less contracted, and more or less rugged, they cause several sorts of earth. chap. v. of the figurative motions of air. there are many sorts of _airs_, as there is of other creatures, of one and the same kind: but, for elemental air, is composed of very rare, figurative motions; and the innate motions, i conceive to be somewhat of the nature of water, viz. circular figurative motions, only of a more dilating property; which causes air, not to be wet, but extraordinary rare; which again causes it to be somewhat of the nature of light: for, the rarity occasions air to be very searching and penetrating; also, dividable and compoundable: but, the rarity of air, is the cause that it is not subject to some sorts of human perception; but yet, not so rare, as not to be subject to human respirations; which is one sort of human perception: for, all parts of all creatures, are perceptive one way, or another: but, as i said, there are many sorts of air; as, some cold, some hot; some dry, some moist; some sharp; some corrupt, some pure, some gross; and numbers more: but, many of these sorts are rather metamorphosed vapours, and waters, than pure elemental air: for, the pure elemental air, is, in my opinion, more searching and penetrating, than light; by reason light may be more easily eclipsed, or stopt; when as air will search every pore, and every creature, to get entrance. chap. vi. of the innate figurative motion of fire. the innate figurative motions of elemental fire, seem the most difficult to human perception, and conception: for, by the agilness, it seems to be more pure than the other sorts of elements; yet, by the light, or visibleness, it seems more gross than air; but, by the dilating property, it seems to be more rare than air, at least, as rare as air. by the glassie or shining property, it seems to be of smooth and even parts: also, by the piercing and wounding property, fire seems to be composed of sharp-pointed figurative motions: wherefore, the innate figurative motions of fire, are, pure, rare, smooth, sharp points, which can move in circles, squares, triangles, parallels, or any other sorts of exterior figures, without an alteration of its interior nature; as may be observed by many sorts of fuels: as also, it can contract and dilate its parts, without any alteration of its innate property. chap. vii. of the productions of elemental fire. it is to be observed, that points of fire are more numerous, and more suddenly propagating, than any other element, or any other creature that is subject to human perception. but, sparks of fire, resemble the seeds of vegetables, in this, that as vegetables will not encrease in all sorts of soyles, alike; neither will the points of fire, in all sorts of fuel, alike. and, as vegetables produce different effects in several soyls; so doth fire on several fuels: as for example, the seeds of vegetables do not work the same effect in a birds crop, as in the earth: for, there they encrease the bird by digestion; but, in the ground, they encrease their own issue (as i may say): so fire, in some fuels, doth destroy it self, and occasions the fuel to be more consumed; when as, in other sorts of fuel, fire encreases extreamly. but fire, as all other creatures, cannot subsist single of it self, but must have food and respiration; which proves, fire is not an immaterial motion. also, fire hath enemies, as well as friends; and some are deadly, namely, water, or watry liquors. also, fire is forced to comply with the figurative motions of those creatures it is joyned to: for, all fuels will not burn, or alter, alike. chap. viii. of flame. _flame_ is the rarest part of fire: and though the fuel of flame be of a vaporous and smoaky substance; yet surely, there are pure flames, which are perfect fires: and, for proof, we may observe, that flame will dilate and run, as it were, to catch smoak: but, when the smoak is above the flame, if it be higher than the flame can extend, it contracts back to the fiery body. but, flame doth somewhat resemble that we name _natural light_: but yet, in my opinion, light is not flame; nor hath it any fiery property, although it be such a sort of extenuating or dilating actions, as flame hath. chap. ix. of the two sorts of fire most different. there are many sorts of fires: but two sorts are most opposite; that is, the hot, glowing, burning, bright, shining fire; and that sort of fire we name a _dead, dull fire_; as, vitriol fires, cordial fires, corrosive fires, feverish fires, and numerous other sorts; and every several sort, hath some several property: as for example, there is greater difference between the fiery property of oyl, and the fiery property of vitriol: for, oyl is neither exteriorly hot, nor burning; whereas vitriol is exteriorly burning, though not exteriorly hot: but, the difference of these sorts of fires, is, that the actions of elemental fire, are to ascend, rather than to descend: and the dull, dead fire, is rather apt to descend, than ascend; that is, to pierce, or dilate, either upwards, or downwards: but, they are both of dilating and dividing natures. but this is to be noted, that all sorts of heats, or hotness, are not fire. also it is to be noted, that all fires are not shining. chap. x. of dead or dull fires. of _dull, dead fires_, some sorts seem to be of a mixt sort: as for example, vitriol, and the like, seem to be exteriorly, of the figurative motions of fire; and interiorly, of the figurative motions of water, or of watry liquors: and oyl is of fiery figurative motions, interiorly; and of liquid figurative motions, exteriorly; which is the cause that the fiery properties of oyl cannot be altered, without a total dissolution of their natures. but, such sorts whose fiery figurative motions are exterior, as being not their innate nature, may be divided from those other natural parts they were joyned to, without altering their innate nature. chap. xi. of the occasional actions of fire. all creatures have not only innate figurative motions that cause them to be such or such a sort of creature; but, they have such and such actions, that cause such and such effects: also, every creature is occasioned to particular actions, by forrein objects; many times to improper actions, and sometimes to ruinous actions, even to the dissolution of their nature: and, of all creatures, fire is the most ready to occasion the most mischief; at least, disorders: for, where it can get entrance, it seldom fails of causing such a disturbance, as occasions a ruine. the reason is, that most creatures are porous: for, all creatures, subsisting by each other, must of necessity have _egress_ and _regress_, being composed of interior and exterior corporeal motions. and fire, being the sharpest figurative motion, is apt to enter into the smallest pores. but some may ask, _whether fire is porous it self?_ i answer: that having respiration, it is a sufficient proof that it is porous: for, fire dyes if it hath not air. but some may say, _how can a point be porous?_ i answer, that a point is composed of parts, and therefore may very well be porous: for, there is no such thing as a single part in nature, and therefore, not a single point. also, some may say, _if there be pores in nature, there may be vacuum_. i answer, that, in my opinion, there is not; because there is no empty pores in nature: pores signifying only an _egress_ and _regress_ of parts. chap. xii. fire hath not the property to change and rechange. of all the elemental creatures, _fire_ is the least subject to change: for, though it be apt to occasion other creatures to alter; yet it keeps close to its own properties, and proper actions: for, it cannot change, and rechange, as water can. also, natural air is not apt to change and rechange, as water: for, though it can (as all the elements) divide and join its parts, without altering the property of its nature: yet, it cannot readily alter, and alter again, its natural properties, as water can. the truth is, water and fire, are opposite in all their properties: but, as fire is, of all the elements, the furthest from altering: so water is, of all the elements, the most subject to alter: for, all circular figures are apt to variety. chap. xiii. of the innate figurative motions of water. the nature of _water_ is, rare, fluid, moist, liquid, wet, glutinous, and glassie. likewise, water is apt to divide and unite its parts, most of which properties are caused by several sorts of dilatations, or extenuations: but, the interior, or innate figure of water, is a circular line. but yet, it is to be observed, that there are many several sorts of waters, as there are many several sorts of airs, fires, and earths, and so of all creatures: for, some waters are more rare than others, some more leight, and some more heavy; some more clear, and some more dull; some salt, some sharp; some bitter, some more fresh, or sweet; some have cold effects, some hot effects: all which is caused by the several figurative motions of several sorts of waters: but, the nature of water is such, as it can easily alter, or change, and rechange, and yet keep its interior, or innate nature or figure. but this is also to be observed, that the dilating or extenuating circle of water, is of a middle degree, as between two extreams. chap. xiv. the nature or property of water. wetness, which is the interior or innate property, or nature of water, is, in my opinion, caused by some sort of dilatations or extenuations. as, all droughts, or dryness, are caused by some sorts of contractions; so, all moistures, liquors, and wets, by dilatations: yet, those extenuations, or dilatations, that cause wet, must be of such a sort of dilatations, as are proper to wet; _viz._ such a sort of extenuations, as are circular extenuations; which do dilate, or extenuate, in a smooth, equal dilatation, from the center, to the circumference; which extenuations, or dilatations, are of a middle degree; for otherwise, the figure of water might be extended beyond the degree of wet; or, not extended to the degree of wet. and it is to be observed, that there is such a degree as only causes moistness, and another to cause liquidness, the third to cause wetness: for, though moistness and liquidness are in the way of wetness; yet, they are not that which we name wet: also, all that is soft, or smooth, is not wet; nor is all that is liquid, or flowing, wet: for, some sorts of air are liquid and flowing, but not wet: nay, flame is liquid and flowing, but yet quite opposite from wet. dust is flowing, but neither liquid or wet, in its nature. and hair and feathers are soft and smooth, but neither liquid, nor wet. but, as i said, water is of such a nature, as to have the properties of soft, smooth, moist, liquid, and wet; and is also of such flowing properties, caused by such a sort of extenuating circles as are of a middle or mean degree: but yet, there are many several sorts of liquors, and wets, as we may perceive in fruit, herbs, and the like: but, all sorts of wets, and liquors, are of a watry kind, though of a different sort. but, as i have said, all things that are fluid, are not wet; as, melted metal, flame, light, and the like, are fluid, but not wet: and smoak and oyl are of another sort of liquidness, than water, or juyce; but yet they are not wet: and that which causes the difference of different sorts of waters, and watry liquors, are the differences of the watry circular lines; as, some are edged, some are pointed, some are twisted, some are braided, some are flat, some are round, some ruff, some smooth; and so after divers several forms or figures: and yet are perfect circles, and of some such a degree of extenuations or dilatations. chap. xv. of the alteration of the exterior figurative motion of water. as i formerly said, the figurative motions of the innate nature of water, is a sort of extenuating; as being an equal, smooth circle: which is the cause water is rare, fluid, moist, liquid, and wet. but, the exterior figurative motions of the watry circle, may be edged, pointed, sharp, blunt, flat, round, smooth, ruff, or the like; which may be either divided, or altered, without any alteration of the innate nature, or property: as for example, salt-water may be made fresh, or the salt parts divided from the watry circle: the like of other sorts of waters; and yet the nature of water remains. chap. xvi. of oyl, and vitriol. the exterior figurative motions of _oyl_, are so much like those of _water_, as, to be fluid, smooth, soft, moist, and liquid, although not perfectly wet: but, the interior figurative motions of oyl, are of that sort of fire, that we name a _dull, dead fire_: and the difference between _salt waters_, _vitriol_ or the like, and _oyl_, is, that the exterior figurative motions of _vitriol_ and _salt waters_, are of a sort of fire; whereas it is the interior figurative motions of oyl, or the like, that are of those sorts of fire; and that is the reason that the fiery motions of oyl cannot be altered, as the fiery motions of _vitriol_ may. but this is to be noted, that although the interior figurative motions of oyl, are of such a sort of fiery motions; yet, not just like those of _vitriol_; and are not burning, corroding, or wounding, as _vitriols, corrosives_, and the like, are: for, those are somewhat more of the nature of bright shining fires, than oyls. chap. xvii. of mineral and sulphureous waters. in _sulphureous_ and _mineral waters_, the _sulphureous_ and _mineral_ corporeal motions, are exterior, and not interior, like salt waters: but, there are several sorts of such waters; also, some are occasionally, others naturally so affected: for, some waters running through sulphureous, or mineral mines, gather, like a rowling stone, some of the loose parts of gravel, or sand; which, as they stick or cleave to the rowling stone; so they do to the running waters; as we may perceive by those waters that spring out of chalk, clay, or lime grounds, which will have some tinctures of the lime, chalk, or clay; and the same happens to minerals. but, some are naturally sulphureous; as for example, some sorts of hot baths are as naturally sulphureous, as the sea-water is salt: but, all those effects of minerals, sulphurs, and the like, are dividable from, and also may be joyn'd to, the body of water, without any disturbance to the nature of water; as may be proved by salt-water, which will cause fresh meat to be salt; and salt meat will cause fresh-water to be salt. as for hot baths, those have hot figurative motions, but not burning: and the moist, liquid, and wet nature of water, makes it apt to joyn, and divide, to, and from other sorts of motions; as also, to and from its own sort. chap. xviii. the cause of the ebbing and flowing of the sea. the nature of water is to flow; so that all sorts of waters will flow, if they be not obstructed: but it is not the nature of water, to ebb. neither can water flow beyond the power of its quantity: for, a little water will not flow so far as a great one. but, i do not mean by flowing, the falling of water from some descent; but, to flow upon a level: for, as i have said, all waters do naturally flow, if they be not obstructed; but, few sorts of water, besides sea-water, ebbs. as for the exterior figurative motions of water, in the action of flowing, they are an oval, or a half circle, or a half moon; where the middle parts of the half moon, or circle, are fuller than the two ends. also, the figure of a half moon, or half circle, is concave on the inside, and convex on the outside of the circle: but, these figurative motions, in a great quantity of water, are bigg and full, which we name _waves of water_; which waves flowing fast upon each other, presses each other forward, until such time as the half circle divides: for, when the bow of the half circle is over-bent, or stretched, it divides into the middle, which is most extended: and when a half circle (which is a whole wave of water) is divided, the divided parts fall equally back on each side of the flowing waves: so, every wave dividing, after that manner, in the full extension, it causes the motion of ebbing, that is, to flow back, as it flow'd forward: for, the divided parts falling back, and joining as they meet, makes the head of the half circle, where the ends of the half circle were; and the convex, where the concave was; by which action, the ebbing parts are become the flowing parts. and the reason that it ebbs and flows by degrees, is, that the flowing half circles require so much time to be at the utmost extension. also, every wave, or half circle, divides not all at one time, but one after another: for, two bodies cannot be in one place at one point of time; and until the second, third, and so the rest, flow as far as the first, they are not at their full extension. and thus the sea, or such a great body of water, must flow, and ebb, as being its nature to flow; and the flowing figure, being over-extended, by endeavouring to flow beyond its power, causes a dividing of the extended parts, which is the cause of the ebbing. but, whether this opinion of mine, be as probable as any of the former opinions concerning the ebbing and flowing of the sea, i cannot judg: but i would not be mistaken; for the flowing of the water, is according to its quantity; for, the further it flows, the fainter, or weaker it is. chap. xix. of overflows. as for _overflows_, there be many; and many more would be, if the waters were not hindred and obstructed by man's inventions. but, some overflows are very uncertain and irregular; others, certain and regular, as, the flowing of _nilus_ in _egypt_: but as for the distance of time of its flowing, it may proceed from the far journey of those flowing-waters: and, the time of its ebbing, may be attributed to the great quantity of water; so that the great quantity of water, will cause a longer or a shorter time in the flowing or ebbing; and certainly the waters are as long a flowing back, as flowing forward. as for spring tides, they are only in such a time when there is a naturall issue of a greater quantity of water: so that spring-tides are but once a month, and single-tides in so many hours: but, many several occasions, may make the tides to be more or less full. as for double-tides, they are occasioned through the irregular dividing of the half-circle; as, when they divide not orderly, but faster than they orderly should do; which, falling back in a crowd, and being, by that means, obstructed, so that they cannot get forward, they are necessitated to flow, where they ebb'd. the reason the tides flow through streams of running-waters, is, that the tide is stronger than the stream: but, if the stream and tides pass through each other, then the tide and stream are somewhat like duellers together, which make passes and passages for their conveniency. chap. xx. of the figure of ice and snow. a circle may not only extend and contract it self without dividing; but may draw it self into many several figures, as squares, or triangles: as also, into many other figures mix'd of squares, triangles, cubes, or the like; being partly one, and partly, another; and into other several ways, and after several manners; which is the reason, water may appear in many several postures of snow, ice, hail, frost, and the like: and, in my opinion, when the water-circle is triangular, it is snow; when the circle is square, it is ice: as for hail, they are but small pieces of ice; that is, small parts, or few drops of water, changed into ice; and those several parts moving after several manners, make the exterior figures, after several shapes; as, great bodies of ice will be of many several shapes, occasioned by many or fewer parts, and by the several postures of those parts: but, such figures, though they are of ice, yet, are not the innate figures of ice. the same is to be said of snow. but, the reason of these my opinions concerning the figures of ice and snow, is, that snow is leighter than the water it self; and ice is heavier, at least, as heavy. and the reason snow is so leight, is, that a triangular figure hath no poyse, being an odd figure; whereas a square is poysed by even and equal lines, and just number of points, as, two to two: but, a triangle is two to one. also, a circle is a poysed figure, as being equal every way, from the center to the circumference; and from the circumference to the center, all the lines drawing to one point. but, mistake me not; for i treat (concerning the figures of snow and ice) only of those figures that cause water to be snow or ice; and not of the exterior figures of snow and ice, which are occasioned by the order or disorder of adjoining parts: for, several parts of water, may order themselves into numerous several figures, which concern not the nature of water, as it is water, snow, or ice: as for example, many men in a battel, or upon ceremony, joyn into many several figures or forms; which figures or forms, are of no concern to their innate nature. also, the several figures or forms of several houses, or several sorts of building in one house, are of no concern to the innate nature of the materials. the like for the exterior figures of ice and snow; and therefore _microscopes_ may deceive the artist, who may take the exterior for the interior figure; but there may be great difference between them. chap. xxi. of the change and rechange of water. _water_ being of a circular figurative motion, is, as it were, but one part, having no divisions; and therefore can more easily change and rechange it self into several postures, viz. into the posture of a triangle, or square; or can be dilated or extended into a larger compass, or contracted into a lesser compass; which is the cause it can turn into vapour and vaporous air; or into slime, or into some grosser figure: for example, water can extend it self beyond the proper degrees of water, into the degree of vapour; and the circle, extending further than the degree of a vaporous circle, is extended into a vaporous air; and if the vaporous airy circle be extreamly extended, it becomes so small, as it becomes to be a sharp edg, and so, in a degree, next to fire; at least, to have a hot effect: but, if it extends further than an edg, the circle breaks into flashes of fire, like lightning, which is a flowing flame: for, being produced from water, it hath the property of flowing, or streaming, as water hath, as we may perceive by the effects of some few parts of water flung on a bright fire; for those few drops of water being not enough to quench the fire, straight dilate so extreamly, that they break into a flame; or else cause the fire to be more brisk and bright: and as the water-circle can be turned into vapour, air, and flame, by extension; so, it can be turned into snow, hail, or ice, by contraction. chap. xxii. of water quenching fire; and fire evaporating water. there is such an antipathy betwixt _water_ and _fire_, (i mean bright shining fire) that they never meet body to body, but fire is in danger to be quenched out, if there be a sufficient quantity of water. but it is to be observed, that it is not the actual coldness of water, that quenches out fire; for, scalding-water will quench out fire: wherefore, it is the wetness that quenches out fire; which wetness choaks the fire, as a man that is drown'd: for, water being not fit for man's respiration, because it is too thick, choaks and smuthers him; and the same doth water to fire: for, though air is of a proper temper for respiration, both to some sorts of animals, such as man; as also, to fire: yet, water is not: which is most proper for other sorts of animals, namely, fish; as also, for some sorts of animals that are of a mixt kind or sort, partly fish, and partly flesh: to which sort of creatures, both air and water are both equally proper for their respiration; or, their respiration equal to either: for certainly, all sorts of creatures have respiration, by reason all creatures subsist by each other; i say, _by each other_, not _of each other_. but, there are many several sorts and kinds of respirations; as concerning water and fire, though a sufficient quantity of water, to fire, doth always choak, smuther, or quench out the fire's life, if joyn'd body to body; yet, when there is another body between those two bodies, water is in danger to be infected with the fire's heat; the fire first infecting the body next to it; and that body infecting the water: by which infection, water is consumed, either by a languishing hectick fever; or, by a raging boyling fever; and the life of water evaporates away. chap. xxiii. of inflamable liquors. there are many bodies of mixt natures; as for example, wine, and all strong liquors, are partly of a watry nature, and partly of a fiery nature; but, 'tis of that sort we name a _dead_, or _dull fire_: but, being of such a mixt nature, they are both apt to quench bright fire, as also, apt to burn or flame; so that such sorts are both inflamable, and quenchable. but, some have more of the fiery nature; and others more of the watry nature; and, by those effects, we may perceive, that not only different, but opposite figurative motions, do well agree in one society. chap. xxiv. of thunder. i observe, that all tempestuous sounds have some resemblances to the flowing of waters, either in great and ruffling waves; or, when the waters flow in such sort, as to break in pieces against hard and rugged rocks; or run down great precipices, or against some obstruction. and the like sound hath the blowings of wind, or the clappings of thunder; which causes me to be of opinion, that thunder is occasioned by a discord amongst some water-circles in the higher region; which, pressing and beating upon each other in a confused manner, cause a confused sound, by reason all circles are concave within the bow, and convex without; which is a hollow figure, although no vacuum: which hollow figure, causes quick repetitions and replies; which replies and repetitions, we name rebounds but, replies are not rebounds; for, rebounds are pressures and re-actions; whereas repetitions are without pressure, but re-action is not: and, replies are of several parts; as, one part to reply to another. but for _thunder_, it is occasioned both by pressures and re-actions; as also, replies of extended water-circles, which make a kind or sort of confusion, and so a confused sound, which we name _horrid_; and, according to their discord, the sound is more or less terrifying, or violent. but this is to be noted, that as _thunder_ is caused by undivided or broken circles; so _lightning_ is caused by broken or divided circles, that are extended beyond the power of the nature of the water-circle; and when the circle is extreamly extended, it divides it self into a straight line, and becomes a flowing flame. chap. xxv. of vapour, smoak, wind, and clouds. _vapour_ and _smoak_ are both fluid bodies: but, smoak is more of the nature of oyl, than water; and vapour more of the nature of water, than oyl; they are dividable: and may be join'd, as other elements: also, they are of a metamorphosing nature, as to change and rechange; but, when they are metamorphosed into the form of air, that air is a gross air, and is, as we say, a corruptible air. as for vapour, it is apt to turn into wind: for, when it is rarified beyond the nature of vapour, and not so much as into the nature of air, it turns into some sorts of wind. i say, some sorts: and certainly, the strongest winds are made of the grossest vapours. as for smoak, it is apt to turn into some sorts of lightning; i say, apt: for, both vapour and smoak can turn into many sorts of metamorphosed elements. as for wind, it proceeds either from rarified vapour, or contracted air. and there are many sorts of vapours, smoaks, and winds; all which sorts of vapours and smoaks, are apt to ascend: but, wind is of a more level action. as for clouds, they cannot be composed of a natural air; because natural air is too rare a body to make clouds. wherefore, clouds are composed of vapour and smoak: for, when vapour and smoak ascends up high without transformation, they gather into clouds, some higher, some lower, according to their purity: for, the purer sort (as i may say for expression-sake) ascends the highest, as being the most agil. but, concerning the figurative motions of vapour and smoak, they are circles; but of winds, they are broken parts of circular vapours: for, when the vaporous circle is extended beyond its nature of vapour, the circumference of the circle breaks into perturbed parts; and if the parts be small, the wind is, in our perception, sharp, pricking, and piercing: but, if the parts are not so small, then the wind is strong and pressing: but wind, being rarified vapour, is so like air, as it is not perceived by human sight, though it be perceived by human touch. but, as there are hot vapours, cold vapours, sharp vapours, moist vapours, dry vapours, subtil vapours, and the like; so there is such sorts of winds. but, pray do not mistake me, when i say, that some sorts of winds are broken and perturbed circles, as if i meant, such as those of lightning: for, those of lightning, are extended beyond the degree of air; and those of vapours, are not extended to the degree of air: also, those of lightning, are not perturbed; and those of wind, are perturbed. again, those of lightning, flow in streams of smooth, small, even lines; those of wind, in disordered parts and fragments. chap. xxvi. of wind. _wind_ and _fire_ have some resemblance in some of their particular actions: as for example, wind and fire endeavour the disturbance of other creatures, occasioning a separating and disjoining of parts. also, wind is both an enemy and friend to fire: for wind, in some sorts of its actions, will assist fire; and in other actions, dissipates fire, nay, blows it out: but certainly, the powerful forces of wind, proceed not so much from solidity, as agility: for, soft, weak, quick motions, are far more powerful, than strong, slow motions; because, quick replies are of great force, as allowing no time of respit. but this is to be observed, that wind hath some watry effects: for, the further water flows, the weaker and fainter it is: so the wind, the further it blows, the weaker and fainter it is. but this is to be observed, that according to the agilness or slowness of the corporeal motions; or, according to the number; or, according to the manner of the compositions, or joynings, or divisions; or, according to the regularity or irregularity of the corporeal figurative motions, so are the effects. chap. xxvii. of light. water, air, fire, and light; are all rare and fluid creatures; but they are of different sorts of rarities and fluities: and, though light seems to be extreamly rare and fluid; yet, light is not so rare and fluid, as pure air is, because it is subject to that sort of human perception we name _sight_; but yet, it is not subject to any of the other perceptions: and, pure air is only subject to the perception of respiration, which seems to be a more subtil perception than sight; and that occasions me to believe, that air is more rare and pure, than light: but howsoever, i conceive the figurative motions of light, to be extraordinary even, smooth, agil lines of corporeal motions: but, as i said before, there are many sorts of lights that are not elemental lights; as, glow-worms tails, cats eyes, rotten wood, fish bones, and that human light which is made in dreams, and infinite other lights, not subject to our perception: which proves, that light may be without heat. but, whether the light of the sun, which we name _natural light_, is naturally hot, may be a dispute: for, many times, the night is hotter than the day. chap. xxviii. of darkness. the figurative motions of _light_ and _darkness_, are quite opposite; and the figurative motions of colours, are as a mean between both, being partly of the nature of both: but, as the figurative motions of light, in my opinion, are rare, straight, equal, even, smooth figurative motions: those of darkness are uneven, ruff, or rugged, and more dense. indeed, there is as much difference between light and darkness, as between earth and water; or rather, between water and fire; because each is an enemy to other; and, being opposite, they endeavour to out-power each other. but this is to be noted, that darkness is as visible to human perception, as light; although the nature of darkness is, to obscure all other objects besides it self: but, if darkness could not be perceived, the optick perception could not know when it is dark; nay, particular dark figurative motions, are as visible in a general light, as any other object; which could not be, if darkness was only a privation of light, as the opinions of many learned men are: but, as i said before, darkness is of a quite different figurative motion, from light; so different, that it is just opposite: for, as the property of light is to divulge objects; so, the property of darkness is to obscure them: but, mistake me not; i mean, that light and darkness have such properties to our perception: but, whether it is so to all perceptions, is more than i know, or is, as i believe, known to any other human creature. chap. xxix. of colours. as for _colour_, it is the same with body: for surely, there is no such thing in nature, as a colourless body, were it as small as an atom; nor no such thing as a figureless body; or such a thing as a placeless body: so that matter, colour, figure, and place, is but one thing, as one and the same body: but matter, being self-moving, causes varieties of figurative actions, by various changes. as for colours, they are only several corporeal figurative motions; and as there are several sorts of creatures, so there are several sorts of colours: but, as there are those, man names artificial creatures; so there are artificial colours. but, though to describe the several species of all the several sorts of colours, be impossible; yet we may observe, that there is more variety of colours amongst vegetables and animals, than amongst minerals and elements: for, though the rain-bow is of many fine colours; yet, the rain-bow hath not so much variety, as many particular vegetables, or animals have; but every several colour, is a several figurative motion; and the brighter the colours are, the smoother and evener are the figurative motions. and as for shadows of colours, they are caused when one sort of figurative motions is as the foundation: for example, if the fundamental figurative motion, be a deep blew, or red, or the like, then all the variations of other colours have a tincture. but, in short, all shadows have a ground of some sort of dark figurative motions. but, the opinions of many learned men, are, that all colours are made by the several positions of light, and are not inherent in any creature; of which opinion i am not: for, if that were so, every creature would be of many several colours; neither would any creature produce after their own _species_: for, a parrot would not produce so fine a bird as her self; neither would any creature appear of one and the same colour, but their colour would change according to the positions of light; and in a dark day, in my opinion, all fine coloured birds, would appear like crows; and fine coloured flowers, appear like the herb named night-shade; which is not so. i do not say, that several positions of light may not cause colours; but i say, the position of light is not the maker of all colours; for, _dyers_ cannot cause several colours by the positions of light. chap. xxx. of the exterior motions of the planets. by the _exterior motions of the planets_, we may believe their exterior shape is spherical: for, it is to be observed, that all exterior actions are according to their exterior shape: but, by reason vegetables and minerals have not such sorts of exterior motions or actions, as animals; some men are of opinion, they have not sensitive life; which opinion proceeds from a shallow consideration: neither do they believe the elements are sensible, although they visibly perceive their progressive motions; and yet believe all sorts of animals to have sense, only because they have progressive motions. chap. xxxi. of the sun, and planets, and seasons. the sun, moon, planets, and all those glittering starrs we see, are several sorts of that man names _elemental creatures_: but man, having not an infinite perception, cannot have an infinite perceptive knowledg: for, though the rational perception is more subtil than the sensitive; yet, the particular parts cannot perceive much further than the exterior parts of objects: but, human sense and reason cannot perceive what the sun, moon, and starrs are; as, whether solid, or rare; or, whether the sun be a body of fire; or the moon, a body of water, or earth; or, whether the fixed starrs be all several suns; or, whether they be other kinds or sorts of worlds. but certainly, all creatures do subsist by each other, because nature seems to be an infinite united body, without _vacuum_. as for the several seasons of the year, they are divided into four parts: but the several changes and tempers of the four seasons, are so various, altering every moment, as it would be an endless work, nay, impossible, for one creature to perform: for, though the _almanack-makers_ pretend to foreknow all the variations of the elements; yet, they can tell no more than just what is the constant and set-motions; but not the variations of every hour, or minute; neither can they tell any thing, more than their exterior motions. chap. xxxii. of air corrupting dead bodies. some are of opinion, _that air is a corrupter, and so a dissolver of all dead creatures, and yet is the preserver of all living creatures._ if so, air hath an infinite power: but, all the reason i can perceive for this opinion, is, that man perceives, that when any raw (or that we name _dead_) _flesh_, is kept from the air, it will not stink, or corrupt, so soon as when it is in the air: but yet it is well known, that extream cold air will keep flesh from corrupting. another reason is, that a flye entomb'd in _amber_, being kept from air, the flye remains in her exterior shape as perfectly as if she were alive. i answer, the cause of that may be, that the figurative motions of _amber_, may sympathize with the exterior consistent motions of the fly, which may cause the exterior shape of the flye to continue, although the innate nature be altered. but air is, as all other creatures are, both beneficial, and hurtful to each other; for nature is poysed with opposites: for we may perceive, that several creatures are both beneficial and hurtful to each other: as for example, a bear kills a man; and, on the other side, a bear's skin will cure a man of some disease. also, a _wild-boar_ will kill a man; and the _boar_'s flesh will nourish a man. fire will burn a man, and preserve a man; and millions of such examples may be proposed. the same may be said of air, which may occasion good or evil to other creatures; as, the _amber_ may occasion the death of a fly; and, on the other side, may occasion the preservation, or continuation of the fly's exterior figure, or form: but, nature being without _vacuum_, all her parts must be be joined; and her actions being poysed, there must be both sympathetical, and antipathetical actions, amongst all creatures. the thirteenth part. chap. i. of the innate figurative motions of metals. all sorts of _metals_, in my opinion, are of some sorts of circular motions; but not like that sort, that is water: for, the water-circle doth extend outward, from the center; whereas, in my opinion, the circular motion of metal, draws inward, from the circumference. also, in my opinion, the circular motions are dense, flat, edged, even, and smooth; for, all bright and glassie bodies are smooth: and, though edges are wounding figures; yet, edges are rather of the nature of a line, than of a point. again, all motions that tend to a center, are more fixt than those that extend to a circumference: but, it is according to the degree of their extensions, that those creatures are more or less fixt; which is the cause that some sorts of metals are more fixt than others; and that causes gold to be the most fixt of all other sorts of metals; and seems to be too strong for the effects of fire. but this is to be noted, that some metals are more near related to some sort, than other: as for example, there is no lead, without some silver; so that silver seems to be but a well-digested lead. and certainly, copper hath some near relation to gold, although not so near related, as lead is to silver. chap. ii. of the melting of metals. _metals_ may be occasioned, by fire, to slack their retentive motions, by which they become fluid; and as soon as they are quit of their enemy, _fire_, the figurative motions of metal return to their proper order: and this is the reason that occasions metal to melt, which is, to flow: but yet, the flowing motion is but like the exterior, and not the innate actions of: for, the melting actions do not alter the innate actions; that is, they do not alter from the nature of being metal: but, if the exterior nature be occasioned, by the excess of those exterior actions, to alter their retentive actions, then metal turns to that we name _dross_; and as much as metal loses of its weight, so much of the metal dissolves; that is, so much of those innate motions are quite altered: but, gold hath such an innate retentiveness, that though fire may cause an extream alteration of the exterior actions; yet, it cannot alter the interior motions. the like is of quick-silver. and yet gold is not a god, to be unalterable, though man knows not the way, and fire has not the power to alter the innate nature of gold. chap. iii. of burning, melting, boyling, and evaporating. _burning, melting, boyling_, and _evaporating_, are, for the most part, occasioned by fire, or somewhat that is, in effect, hot: i say, _occasioned_, by reason they are not the actions of fire, but the actions of those bodies that melts, boyls, evaporates, or burns; which being near, or joyned to fire, are occasioned so to do: as for example, put several sorts of creatures, or things, into a fire, and they shall not burn alike: for, leather and metal do not burn alike; for metal flows, and leather shrinks up, and water evaporates, and wood converts it self, as it were, into fire; which other things do not; which proves, that all parts act their own actions. for, though some corporeal motions may occasion other corporeal motions to act after such or such a manner; yet, one part cannot have another part's motion, because matter can neither give nor take motion. chap. iv. of stone. all minerals seem to be some kinds of dense and retentive motions: but yet, those kinds of dense and retentive motions, seem to be of several sorts; which is the cause of several sorts of minerals, and of several sorts of stones and metals. also, every several sort, hath several sorts of properties: but, in my opinion, some sorts are caused by hot contractions and retentions; others, by cold contractions and retentions; as also, by hot or cold densations: and the reason why i believe so, is, that i observe that many artificial stones are produced by heat: but ice, which is but in the first degree of a cold density, seems somewhat like transparent stones; so that several sorts of stones, are produced by several sorts of cold and hot contractions and densations. chap. v. of the loadstone. as for the _loadstone_, it is not more wonderful in attracting iron, than beauty, which admirably attracts the optick perception of human creatures: and who knows, but the north and south air may be the most proper air for the respiration of the _loadstone_; and, that iron may be the most proper food for it. but, by reason there hath been so many learned men puzled in their opinions concerning the several effects of the _loadstone_, i dare not venture to treat of the nature, and natural effects of that mineral; neither have i had much experience of it: but i observe, that iron, and some sorts of stone, are nearly allied; for, there is not any iron, but what is growing, or is intermixt and united in some sorts of stone, as that which we call _iron-stone_. wherefore, it is no wonder if the _loadstone_, and iron, should be apt to embrace one another. chap. vi. of bodies, apt to ascend or descend. there are so many several causes that occasion some sorts of creatures to be apt to _ascend_, and others to _descend_, as they are neither known, or can be conceived by one finite creature: for, it is not rarity or density, that causes levity and gravity; but, the frame or form of a creature's exterior shape, or parts. as for example: a flake of snow is as rare as a downy feather; yet, the feather is apt to ascend, and the flake of snow to descend. also dust, that is hard and dense, is apt to ascend; and water, that is soft and rare, is more apt to descend. again, a bird, that is both a bigger, and a more dense creature by much, than a small worm; yet, a bird can flye up into the air, when as a leight worm cannot ascend, or flye, having not such a sort of shape. also, a great heavy ship, as big as an ordinary house, fraughted with iron, will swim upon the face of the water; when as a small bullet, no bigger than a _hasle-nut_, will sink to the bottom of the sea. a great bodied bird will flye up into the air; when as a small worm lies on the earth, with a slow kind of crawling, and cannot ascend. all which is caused by the manner of their shapes, and not the matter of gravity and levity. chap. vii. why heavy bodies descend more forcibly than leight bodies ascend. although the manner of the shape of several creatures, is the chief cause of their _ascent_, and _descent_; yet, gravity and levity, doth occasion more or less agility: for, a heavy body shall descend with more force, than a leight body ascend: and the reason is, not only that there may be more parts in a heavy body, than a leight; but, that in a descent, every corporeal motion seems to press upon each other; which doubles and trebles the strength, weight, and force, as we may perceive in the ascending and descending of the flight of birds, especially of hawks; of which, the weight of the body is some hindrance to the ascent, but an advantage to the descent: but yet, the shape of the bird hath some advantage by the weight, in such sort, that the weight doth not so much hinder the ascent, as it doth assist the descent. chap. viii. of several sorts of densities and rarities, gravities and levities. there are different sorts of densities and rarities, softness and hardness, levities and gravities: as for example, the density of earth is not like the density of stone; nor the density of stone, like the density of metal: nor are all the parts of the earth dense alike; nor all stones, nor all metals; as we may perceive in clay, sand, chalk, and lime-grounds. also, we may perceive difference between lead, tynne, copper, iron, silver, and gold; and between marble, alablaster, walling-stone, diamonds, crystals, and the like: and so much difference there is between one and the same kind, that some particulars of one sort, shall more resemble another kind, than their own: as for example, gold and diamonds resemble each other's nature, more than lead doth gold; or diamonds, crystal; i say, in their densities. also, there is a great difference of the rarity, gravity, and levity of several sorts of waters, and of several sorts of air. chap. ix. of vegetables. _vegetables_ are of numerous sorts, and every sort of very different natures: as for example, some are reviving cordials; others, deadly poyson; some are purgers, others are nourishers: some have hot effects, some cold; some dry, some moist; some bear fruit, some bears no fruit; some appear all the year young; others appear but part of the year young, and part old; some are many years a producing; others are produced in few hours; some will last many hundred years; others will decay in the compass of few hours: some seem to dye one part of the year, and revive again in another part of the year: some rot and consume in the earth, after such a time; and will continue in perfection, if parted from the earth. others will wither and decay, as soon as parted from the earth. some are of a dense nature, some of a rare nature; some grow deep into the earth; others grow high out of the earth; some will only produce in dry soyls, some in moist: some will produce only in water, as we may perceive by some ponds; others on houses of brick or stone. also, some grow out of stone; as, many stones will have a green moss: some are produced by sowing their seed into the earth; others, by setting their roots, or slips, into the earth: others again, by joyning or engrafting one plant into another: so that there is much variety of vegetables, and those of such different natures, that they are not only different sorts, but are variety of effects of one and the same sort; and it requires not only the study of one human creature, or many human creatures; but, of all the human creatures in all nations and ages, to know them; which is the reason, that those that have writ of the natures of herbs, flowers, roots, and fruits, may be much mistaken. but i, living more constantly in my study, than in my garden, shall not venture to treat much of the particular natures, and natural effects of vegetables. chap. x. of the production of vegetables. tis no wonder, that some sorts of _vegetables_ are produced out of stone or brick, (as some that will grow on the top of houses) by reason that brick is made of earth, and stone is generated in the bowels of the earth; which shows they are of an earthly nature or substance. neither is it a wonder that vegetables will grow upon some sorts of water, by reason some sorts of waters may be mixt with some parts of earth. but, i have been credibly informed, that a man whose legg had been cut, and a seed of an oat being gotten into the wound by chance, the oat did sprout out into a green blade of grass: which proves, that vegetables may be produced in several soyls. but 'tis probable, that though many sorts of vegetables may sprout, as barly in water; yet, they cannot produce any of the off-spring of the same sort or kind. but, my thoughts are, at this present, in some dispute; as, whether the earth is a part of the production of vegetables, as being the breeder? or, whether the earth is only parts of respiration, and not parts of production; and so, rather breathing-parts, than breeding-parts, as water to fishes? but, if so, then every particular seed must encrease, not only by a bare transformation of their parts into the first form of production; but, by division of their united parts, must produce many other societies of the same sort; as religious orders, where one convent divides into many convents of the same order; which occasions a numerous encrease. so the several parts of one seed, may divide into many seeds of the same sort, as being of the same _species_; but then, every part of that seed, must be encreased by additional parts; which must be, by nourishing parts: which nourishing parts are, in all probability, earthy parts; or, at least, partly of earthy parts; and partly, of some of the other elemental parts: but, as i have often said, all creatures in nature are assisted, and do subsist, by each other. chap. xi. of replanting vegetables. _replanting of vegetables_, many times, occasions great alterations; in so much as a vegetable, by often replanting, will be so altered, as to appear of another sort of vegetable: the reason is, that several sorts, or parts of soyls, may occasion other sorts of actions, and orders, in one and the same society. but this is to be noted in the lives of many animals, that several sorts of food, make great alterations in their temper and shape; though not to alter their species, yet so as to cause them to appear worse or better: but, this is most visible amongst human creatures, whom some sorts of food will make weak, sick, faint, lean, pale, old, and withered: other sorts of food will make them strong, and healthy, fat, fair, smooth, and ruddy. so some sorts of soyls will cause some vegetables to be larger, brighter, smoother, sweeter, and of more various and glorious colours. chap. xii. of artificial things. _artificial things_, are natural corporeal figurative motions: for, all artificial things are produced by several produced creatures. but, the differences of those productions we name _natural_ and _artificial_, are, that the natural are produced from the producer's own parts; whereas the artificial are produced by composing, or joyning, or mixing several forrein parts; and not any of the particular parts of their composed society: for, artificial things are not produced as animals, vegetables, minerals, or the like: but only, they are certain seral mixtures of some of the divided, or dead parts, as i may say, of minerals, vegetables, elements, and the like. but this is to be noted, that all, or at least, most, are but copied, and not originals. but some may ask, _whether artificial productions have sense, reason, and perception?_ i answer: that if all the rational and sensitive parts of nature, are perceptive, and that no part is without perception; then all artificial productions are perceptive. chap. xiii. of several kinds and sorts of species. according to my opinion, though the _species_ of this world, and all the several kinds and sorts of _species_ in this world, do always continue; yet, the particular parts of one and the same kind or sort of _species_, do not continue: for, the particular parts are perpetually altering their figurative actions. but, by reason some parts compose or unite, as well as some parts dissolve or disunite; all kinds and sorts of _species_, will, and must last so long as nature lasts. but mistake me not, i mean such kinds and sorts of _species_ as we name natural, that is, the fundamental species; but not such _species_, as we name artificial. chap. xiv. of different worlds. tis probable if nature be infinite, there are several kinds and sorts of those species, societies, or creatures, we name _worlds_; which may be so different from the frame, form, species, and properties of this world, and the creatures of this world, as not to be any ways like this world, or the creatures in this world. but mistake me not, i do not mean, not like this world, as it is material and self-moving; but, not of the same species, or properties: as for example, that they have not such kind of creatures, or their properties, as light, darkness, heat, cold, dry, wet, soft, hard, leight, heavy, and the like. but some may say, _that is impossible: for, there can be no world, but must be either light or dark, hot or cold, dry or wet, soft or hard, heavy or leight; and the like_. i answer, that though those effects may be generally beneficial to most of the creatures in this world; yet, not to all the parts of the world: as for example, though light is beneficial to the eyes of animals; yet, to no other part of an animal creature. and, though darkness is obstructive to the eyes of animals; yet, to no other parts of an animal creature. also, air is no proper object for any of the human parts, but respiration. so cold and heat, are no proper objects for any part of a human creature, but only the pores, which are the organs of touch. the like may be said for hard and soft, dry and wet: and since they are not fundamental actions of nature, but particular, i cannot believe, but that there may be such worlds, or creatures, as may have no use of light, darkness, and the like: for, if some parts of this world need them not, nor are any ways beneficial to them, (as i formerly proved) surely a whole world may be, and subsist without them: for these properties, though they may be proper for the form or species of this world; yet, they may be no ways proper for the species of another kind or sort of world: as for example, the properties of a human creature are quite different from other kinds of creatures; the like may be of different worlds: but, in all material worlds, there are self-moving parts, which is the cause there is self-joyning, uniting, and composing; self dividing, or dissolving; self-regularities, and self-irregularities: also, there is perception amongst the parts or creatures of nature; and what worlds or creatures soever are in nature, they have sense and reason, life and knowledg: but, for light and darkness, hot and cold, soft and hard, leight and heavy, dry and wet, and the like; they are all but particular actions of particular corporeal species, or creatures, which are finite, and not infinite: and certainly, there may be, in nature, other worlds as full of varieties, and as glorious and beautiful as this world; and are, and may be more glorious or beautiful, as also, more full of variety than this world, and yet be quite different in all kinds and sorts, from this world: for, this is to be noted, that the different kinds and sorts of species, or creatures, do not make particulars more or less perfect, but according to their kind. and one thing i desire, that my _readers_ would not mistake my meaning, when i say, _the parts dissolve_: for, i do not mean, that matter dissolves; but, that their particular societies dissolve. appendix to the grounds of natural philosophy. the first part. chap. i. whether there can be a substance, that is not a body. what a _substance_, that is not _body_, can be, (as i writ in the first chapter of this book) i cannot imagine; nor, that there is any thing between _something_ and _nothing_. but, some may say, _that spiritual substances are so_. i answer: that spirits must be either material, or immaterial: for, it is impossible for a thing to be between body and no body. others may say, _there may be a substance, that is not a natural substance; but, some sort of substance that is far more pure than the purest natural substance_. i answer: were it never so pure, it would be in the list or circle of body: and certainly, the purest substance, must have the properties of body, as, to be divisible, and capable to be united and compounded; and being divisible and compoundable, it would have the same properties that grosser parts have: but, if there be any difference, certainly the purest substance would be more apt to divide and unite, or compound, than the grosser sort. but, as to those sorts of substance, which some learned men have imagined; in my opinion, they are but the same sort of substance that the vulgar call, _thoughts_, and i name, the _rational parts_; which, questionless, are as truly body, as the grossest parts in nature: but, most human creatures are so troubled with the thoughts of dissolving, and dis-uniting, that they turn fancies and imaginations, into spirits, or spiritual substances; as if all the other parts of their bodies, should become rational parts; that is, that all their parts should turn into such parts as thoughts, which i name, the _rational parts_. but that opinion is impossible: for, nature cannot alter the nature of any part; nor can any part alter its own nature; neither can the rational parts be divided from the sensitive and inanimate parts, by reason those three sorts constitute but one body, as being parts of one body. but, put the case that the rational parts might divide and subsist without the sensitive and inanimate parts; yet, as i said, they must of necessity have the properties and nature of a body, which is, to be divisible, and capable to be united, and so to be parts: for, it is impossible for a body, were it the most pure, to be indivisible. chap. ii. of an immaterial. i cannot conceive how an immaterial can be in nature: for, first, an immaterial cannot, in my opinion, be naturally created; nor can i conceive how an immaterial can produce particular immaterial souls, spirits, or the like. wherefore, an immaterial, in my opinion, must be some uncreated being; which can be no other than god alone. wherefore, created spirits, and spiritual souls, are some other thing than an immaterial: for surely, if there were any other immaterial beings, besides the omnipotent god, those would be so near the divine essence of god, as to be petty gods; and numerous petty gods, would, almost, make the power of an infinite god. but, god is omnipotent, and only god. chap. iii. whether an immaterial be perceivable. whatsoever is corporeal, is perceivable; that is, may be perceived in some manner or other, by reason it hath a corporeal being: but, what being an immaterial hath, no corporeal can perceive. wherefore, no part in nature can perceive an immaterial, because it is impossible to have a perception of that, which is not to be perceived, as not being an object fit and proper for corporeal perception. in truth, an immaterial is no object, because no body. but some may say, that, _a corporeal may have a conception, although not a perception, of an immaterial_. i answer, that, surely, there is an innate notion of god, in all the parts of nature; but not a perfect knowledg: for if there was, there would not be so many several opinions, and religions, amongst one kind, or rather, sort of creatures, as mankind, as there are; insomuch, that there are but few of one and the same opinion, or religion: but yet, that innate notion of god, being in all the parts of nature, god is infinitely and eternally worshipped and adored, although after several manners and ways; yet, all manners and ways, are joyned in one worship, because the parts of nature are joyned into one body. chap. iv. of the differences between god, and nature. god is an eternal creator; nature, his eternal creature. god, an eternal master: nature, god's eternal servant. god is an infinite and eternal immaterial being: nature, an infinite corporeal being. god is immovable, and immutable: nature, moving, and mutable. god is eternal, indivisible, and of an incompoundable being: nature, eternally divisible and compoundable. god, eternally perfect: nature, eternally imperfect. god, eternally inalterable: nature eternally alterable. god, without error: nature, full of irregularities. god knows exactly, or perfectly, nature: nature doth not perfectly know god. god is infinitely and eternally worshipped: nature is the eternal and infinite worshipper. chap. v. all the parts of nature worship god. all creatures (as i have said) have an innate notion of god; and as they have a notion of god, so they have a notion to worship god: but, by reason nature is composed of parts; so is the infinite worship to god: and, as several parts are dividing and uniting after several kinds, sorts, manners and ways; so is their worship to god: but, the several manners and ways of worship, make not the worship to god less: for certainly, all creatures worship and adore god; as we may perceive by the holy scripture, where it says, _let the heavens, earth, and all that therein is, praise god._ but 'tis probable, that some of the parts being creatures of nature, may have a fuller notion of god than others; which may cause some creatures to be more pious and devout, than others: but, the irregularity of nature, is the cause of sin. chap. vi. whether god's decrees are limited. in my opinion, though god is inalterable, yet no ways bounded or limited: for, though god's decrees are fixt, yet, they are not bound: but, as god hath an infinite knowledg, he hath also an infinite fore-knowledg; and so, fore-knows nature's actions, and what he will please to decree nature to do: so that, god knows what nature can act, and what she will act; as also, what he will decree: and this is the cause, that some of the creature's or parts of nature, especially man, do believe _predestination_. but surely, god hath an omnipotent divine power, which is no ways limited: for god, being above the nature of nature, cannot have the actions of nature, because god cannot make himself no god; neither can he make himself more than what he is, he being the all-powerful, omnipotent, infinite, and everlasting being. chap. vii. of god's decrees concerning the particular parts of nature. though nature's parts have free-will, of self-motion; yet, they have not free-will to oppose _god's decrees_: for, if some parts cannot oppose other parts, being over-power'd, it is probable, that the parts of nature cannot oppose the all-powerful decrees of _god_. but, if it please the all-powerful _god_ to permit the parts of nature to act as they please, according to their own natural will; and, upon condition, if they act so, they shall have such rewards as nature may be capable to receive; or such punishments as nature is capable of; then the omnipotent _god_ doth not predestinate those rewards, or punishments, any otherwise than the parts of nature do cause by their own actions. thus all corporeal actions, belong to corporeal parts; but, the rewards and punishments, to _god_ alone: but, what those punishments and blessings are, no particular creature is capable to know: for, though a particular creature knows there is a _god_; yet, not what _god_ is: so, although particular creatures know there are rewards and punishments; yet, not what those rewards and punishments are. but mistake me not; for i mean the general rewards and punishments to all creatures: but 'tis probable, that _god_ might decree nature, and her parts, to make other sorts of worlds, besides this world; of which worlds, this may be as ignorant, as a particular human creature is of _god_. and therefore, it is not probable (since we cannot possibly know all the parts of nature, of which we are parts) that we should know the decrees of _god_, or the manners and ways of worship, amongst all kinds and sorts of creatures. chap. viii. of the ten commandments. in my opinion, the notions man hath of _god's commands_ concerning their behaviour and actions to himself, and their fellow-creatures, is the very same that moses writ, and presented to all those of whom he was head and governour. but, mistake me not, i mean only the _ten commandments_; which commandments are a sufficient rule for all human creatures: and certainly, _god_ had decreed, that moses should be a wise man, and should publish these wise commands. but, the interpretation of the law must be such, as not to make it no such law: but, by reason nature is as much irregular, as regular, human notions are also irregular, as much as regular; which causes great variety of religions: and their actions being also irregular, is the cause that the practise of human creatures is irregular; and that occasions irregular devotions, and is the cause of sin. chap. ix. of several religions. concerning _the several religions_, and several opinions in religions, which are like several kinds and sorts; the question is, _whether all mankind could be perswaded to be of one religion, or opinion?_ the opinion of the minor part of my thoughts, was, that all men might be perswaded. and, the opinion of the major part of my thoughts, was, that nature, being divisible and compoundable, and having free-will, as well as self-motion; and being irregular, as well as regular; as also, variable, taking delight in variety; it was impossible for all mankind to be of one _religion_, or _opinion_. the opinion of the minor part of my thoughts, ,$ was, that the grace of god could perswade all men to one opinion. the major part of my thoughts was of opinion, that god might decree or command nature: but, to alter nature's nature, could not be done, unless god, by his decree, would annihilate this nature, and create another nature, and such a nature as was not like this nature: for, it is the nature of this material nature, to be alterable; as also, to be irregular, as well as regular; and, being regular, and irregular, was a fit and proper subject for god's justice, and mercies; punishments, and rewards. chap. x. of rules and prescriptions. as saint paul said, _we could not know sin, but by the law_; so, we could not know what punishment we could or should suffer, but by the law; not only moral, but divine law. but, some may ask, _what is law?_ i answer: law is, limited prescriptions and rules. but, some may ask, _whether all creatures in nature, have prescriptions and rules?_ i answer: that, for any thing man can know to the contrary, all creatures may have some natural rules: but, every creature may chuse whether they will follow those rules; i mean, such rules as they are capable to follow or practise: for, several kinds and sorts of creatures, cannot possibly follow one and the same prescription and rule. wherefore, divine prescriptions and rules, must be, according to the sorts and kinds of creatures; and yet, all creatures may have a notion, and so an adoration of god, by reason all the parts in nature, have notions of god. but, concerning particular worships, those must be prescriptions and rules; or else, they are according to every particular creature's conception or choice. chap. xi. sins and punishments, are material. as all sins are material, so are punishments: for, material creatures, cannot have immaterial sins; nor can material creatures be capable of immaterial punishments; which may be proved out of the sacred scripture: for, all the punishments that are declared to be in hell, are material tortures: nay, hell it self is described to be material; and not only hell, but heaven, is described to be material. but, whether angels, and devils, are material, that is not declared: for, though they are named spirits, yet we know not whether those spirits be immaterial. but, considering that hell and heaven is described to be material, it is probable, spirits are also material: nay, our blessed saviour christ, who is in heaven, with god the father, hath a material body; and in that body will come attended by all the hosts of heaven, to judg the quick and the dead; which quick and dead, are the material parts of nature: which could not be actually judged and punished, but by a material body, as christ hath. but, pray mistake me not; i say, they could not be actually judged and punished; that is, not according to nature, as material actions: for, i do not mean here, divine and immaterial decrees. but christ, being partly divine, and partly natural; may be both a divine and natural judg. chap. xii. of human conscience. the human notions of god, man calls _conscience_: but, by reason that nature is full of varieties, as having self-moving parts; human creatures have different notions, and so different consciences, which cause different opinions and devotions: but, nature being as much compoundable as dividable, it causes unity of some, as also, divisions of other opinions, which is the cause of several religions: which religions, are several communities and divisions. but, as for conscience, and holy notions, they being natural, cannot be altered by force, without a free-will: so that the several societies, or communicants, commit an error, if not a sin, to endeavour to compel their brethren to any particular opinion: and, to prove it is an error, or sin, the more earnest the _compellers_ are, the more do the _compelled_ resist; which hath been the cause of many martyrs. but surely, all christians should follow the example of christ, who was like a meek lamb, not a raging lyon: neither did christ command his apostles to persecute; but, to suffer persecution patiently. wherefore, _liberty of conscience_ may be allowed, conditionally, it be no ways a prejudice to the peaceable government of the state or kingdom. the second part. chap. i. whether it is possible there could be worlds consisting only of the rational parts, and others only of the sensitive parts. the parts of my mind did argue amongst themselves, _whether there might not be several kinds and sorts of worlds in infinite nature?_ and they all agreed, that probably there might be several kinds and sorts of worlds. but afterwards, the opinion of the major parts of my mind, was, that it is not possible: for, though the rational parts of nature move free, without burdens of the inanimate parts; yet, being parts of the same body, (viz. of the body of nature) they could not be divided from the sensitive and inanimate parts; nor the sensitive and inanimate parts, from the rational. the opinion of the minor parts of my mind, was, that a composed world, of either degree, was not a division from the infinite body of nature, though they might divide so much, as to compose a world meerly of their own degree. the major's opinion was, that it was impossible; because the three degrees, rational, sensitive, and inanimate, were naturally joyned as one body, or part. the minor's opinion was, that a world might be naturally composed only of rational parts, as a human mind is only composed of rational parts; or, as the rational parts of a human creature, could compose themselves into several forms, _viz._ into several sorts and kinds of worlds, without the assistance of the sensitive or the inanimate parts: for, they fancy worlds which are composed in human minds, without the assistance of the sensitive. the major part agreed, that the rational corporeal actions, were free; and all their architectors were of their own degree: but yet, they were so joyned in every part and particle, to the sensitive and inanimate, as they could not separate from these two degrees: for, though they could divide and unite from, and to particulars, as either of their own degrees, or the other degrees; yet, the three degrees being but as one united body, they could not so divide, as not to be joyned to the other degrees: for, it was impossible for a body to divide it self from it self. after this argument, there followed another; _that, if it were possible there could be a world composed only of the rational parts, without the other two degrees; whether that world would be a happy world?_ the major part's opinion was, that, were it possible there could be such unnatural divisions, those divide parts would be very unhappy: for, the rational parts would be much unsatisfied without the sensitive; and the sensitive very dull without the rational: also, the sensitive architectors would be very irregular, wanting their designing parts, which are the rational parts. upon which argument, all the parts of my mind agreed in this opinion, that the sensitive was so sociable to the rational, and the rational so assisting to the sensitive, and the inanimate parts so necessary to the sensitive architectors, that they would not divide from each other, if they could. chap. ii. of irregular and regular worlds. some parts of my mind were of opinion, _that there might be a world composed only of irregularities; and another, only of regularities: and some, that were partly composed of the one, and the other._ the minor part's opinion was, that all worlds were composed partly of the one, and partly of the other; because all nature's actions were poysed with opposites, or contraries: wherefore, there could not be a world only of irregularities, and another of regularities. the major part's opinion, was, that nature's actions were as much poysed by the contrary actions of two worlds, as by the contrary actions of the parts of one world, or one creature: as for example, the peace and trouble, health and sickness, pain and ease, and the like, of one human creature; and so of the contrary natures of several kinds and sorts of creatures of one and the same world. after which discourse, they generally agreed, there might be regular and irregular worlds; the one sort to be such happy worlds, as that they might be named blessed worlds; the other so miserable worlds, as might be named cursed worlds. chap. iii. whether there be egress and regress between the parts of several worlds. there arose a third argument, _viz. whether it was possible for some of the creatures of several worlds, to remove, so as to remove out of one world, into another?_ the major part's opinion was, that it was possible for some creatures: for, if some particular creatures could move all over the world, of which they were a part, they might divide from the parts of the world they were of, and joyn with the parts of another world. the minor part's opinion was, that they might travel all over the world they were part of, but not to joyn with the parts of another world, to which they belong not. the major's opinion was, that every part and particle, belonged to the infinite body of nature, and therefore not any part could account it self not of the infinite body; and being so, then every part of nature may joyn, and divide from and to particular parts, as they please, if there were not obstructions and hindrances, and some parts did not obstruct other parts: wherefore, if there were not obstructions, there might be egress and regress amongst the particular parts of several worlds. the minor's opinion was, that if it could be according to the major's opinion, it would cause an infinite confusion in infinite nature: for, every creature of every world, was composed according to the nature and compositions of the world they were of: wherefore, the products of one kind or sort of worlds, would not be sutable, agreeable, and regular, to the productions of another kind. the major part's opinion was, that it was impossible, since nature is one united body, without _vacuum_, but that the parts of all worlds must have egress and regress. chap. iv. whether the parts of one and the same society, could, after their dissolution, meet and unite. the fifth argument, was partly of the same subject, _viz. whether the particular parts of a creature, (such as a human creature is) could travel out of one world into another, after the dissolution of his human life?_ the major part's opinion was, that they could. the minor's opinion was, they could not; because the particular parts so divided and joyned to and from other particular parts and societies, as it was impossible, if they would, so to agree, as to divide from those parts and societies they are joyned to, and from those they must joyn with, to meet in another world, and joyn as they would, in the same society they were of, when the whole society is dissolved. neither can parts divide and joyn, as they would: for, though self-moving parts have a free-will to move; yet, being subject to obstructions, they must move as they can: for, no particular part hath an absolute power. wherefore, the dispersed parts of a dissolved society, cannot meet and joyn as they would. besides, every part is as much affected to one sort, kind, or particular, they are parts of, as to another. besides, the knowledg of every part alters, according as their actions alter: so that the parts of one and the same society, after division, have no more knowledg of that society. chap. v. whether, if a creature being dissolved, and could unite again, would be the same. the sixth argument was, _that, put the case it were possible all the several parts belonging to one and the same society; as for example, to one human creature, after his human life was dissolved, and his parts dispersed, and afterwards, all those parts meeting and uniting; whether that human creature would be the same?_ the minor part's opinion was, that it could not be the same society: for, every creature was according to the nature of their kind or sort; and so according to the form and magnitude of one of their kind or sort. the major part's opinion was, that though the nature of every particular creature had such forms, shapes, and properties, as was natural to that sort of creatures they were of; yet, the magnitude of particular creatures of one and the same sort, might be very different. the minor part's opinion was, that if all the parts of one society, as for example, a man, from the first time of his production, to the time of his dissolution, should, after division, come to meet and unite; that man, or any other creature, would be a monstrous creature, as having more parts than was agreeable to the nature of his kind. the major part's opinion was, that though the society, viz. the man, would be a society of greater magnitude; yet, not any ways different from the nature of his kind. chap. vi. of the resurrection of human kind. the seventh argument, was, _whether all the particular parts of every human creature, at the time of the resurrection, be, to meet and joyn, as being of one and the same society?_ the minor part's opinion was, they shall not: for, if all those parts that had been of the same body and mind of one man, from his first production, to the last of his dissolution; or, from his birth, to the time of his death, (supposing him to have liv'd long) should meet and joyn, as one society, that is, as one man; that man, at the time of his resurrection, would be a gyant; and if so, then old men would be gyants; and young children, dwarfs. the major part's opinion was, that, if it was not so, then every particular human society would be imperfect at the time of their resurrection: for, if they should only rise with some of their parts, as (for example) when they were in the strength of their age, then all those parts that had been either before, or after that time, would be unjustly dealt with, especially if man be the best product in nature. besides, if a dead child did rise a man, as at his most perfect age, it could not be said, he rises according to a natural man, having more parts than by nature he ever had; and an old man, fewer parts than naturally he hath had: so, what by adding and diminishing the parts of particular men, it would not cause only injustice; but, not any particular human creature, would be the same he was. chap. vii. of the dissolution of a world. the eighth argument was, _that when all human creatures that were dissolved, should rise, whether the world they were of, should not be dissolved?_ all the parts of my mind agreed, that when all the human creatures that had been dissolved, should rise, the whole world, besides themselves, must also dissolve, by reason they were parts of the world: for, when all those numerous dissolved and dispersed parts, did meet and joyn, the world wanting those parts, could not subsist: for, the frame, form, and uniformity of the world, consisted of parts; and those parts that have been of the human kind, are, at several times, of other kinds and sorts of creatures, as other sorts and kinds are of human kind; and all the sorts and kinds, are parts of the world: so that the world cannot subsist, if any kind or sort of creatures, that had been from the first time of the creation, should be united; i mean, into one and the same sort or kind of creatures; as it would be, if all those that are quick, and those that have been _dissolved_, (that is, have been dead) should be alive at one time. chap. viii. of a new heaven, and a new earth. the ninth argument was, _that if a world could be dissolved, and that the human creatures should rise, and reunite; what world should they reside in?_ all the parts of my thoughts generally agreed, that the omnipotent god would command the parts of his servant nature, to compose other worlds for them, into which worlds they should be separated; the good should go into a blessed world; the bad, into a cursed world: and the sacred scripture declares, that there shall be a _new heaven_, and a _new earth_; which, in their opinion, was a heaven and a hell, for the blessed and cursed human kind of this world. chap. ix. whether there shall be a material heaven and hell. the tenth argument was, _whether the heaven and hell that are to be produced for the blessed and cursed, shall be material?_ the minor part's opinion was, that they shall not be material. the major parts were of opinion, they shall be material, by reason all those creatures that did rise, were material; and being material, could not be sensible either of immaterial blessings, or punishments: neither could an immaterial world, be a fit or proper residence for material bodies, were those bodies of the purest substance. but, whether this material heaven and hell, shall be like other material worlds, the parts of my mind could not agree, and so not give their judgment. but, in this they all agreed, that the material heaven and hell, shall not have any other animal creatures, than those that were of human kind, and those not produced, but raised from death. but when they came to argue, whether there might be elements, minerals, and vegetables, they could not agree; but some did argue, and offer to make proof, that there might be mynes of gold, and rocks of diamonds, rubies, and the like; all which, were minerals. also, some were of opinion, there were elements: for, darkness and light, are elemental effects: and, if hell was a world of darkness; and heaven, a world of light; it was probable there were elements. chap. x. concerning the ioys or torments of the blessed and cursed, after they are in heaven, or hell. as for the _ioys of heaven_, and the _torments of hell_, all the parts of my mind agreed, they could not conceive any more probably, than those they had formerly conceived: which former conceptions they had occasioned the sensitive parts to declare; and having been formerly divulged in the book of my _orations_, their opinion was, _that it would be a superfluous work to cause them to be repeated in this book._ but, the ground or foundation of those conceptions, is, that god may decree, _that both the sensitive and rational parts of those that are restored to life, should move in variety of perceptions, or conceptions, without variety of objects: and, that those creatures_ (viz. _human creatures) that are raised from death to life, should subsist without any forrein matter, but should be always the same in body and mind, without any traffick, egress, or regress of forrein parts_. and the proof, that the sensitive and rational parts of human creatures, may make perceptions, or rather conceptions, without forrein objects, is, _that many men in this world have had conceptions, both amongst the rational and sensitive, which man names visions, or imaginations; whereof some have been pleasing and delightful; others, displeasing, and dreadful_. the third part. the preamble. the parts of my mind, after some time of respite from _philosophical arguments_, delighting in such harmless pastimes; did begin to argue about a _regular_ and _irregular world_; having formerly agreed, there might be such worlds in nature; and that the regular worlds, were happy worlds; the irregular, miserable worlds. but, there was some division amongst the parts of my mind, concerning the choice of their arguments; as, whether to argue, first, of the particular parts of the regular, or of the irregular world. but, at last, they agreed to argue, first, of the regular world. but, pray mistake not these arguments; for they are not arguments of such worlds as are for the reception of the blessed and cursed humans, after their resurrections: but, such as these worlds we are of, only freely regular, or irregular. also, though i treat but only of one regular world, and one irregular world; yet, my opinion is, there may be a great many irregular worlds, and a great many regular worlds, of several kinds and sorts: but, these i shall treat of, are such as are somewhat like this world we are of. chap. i. of the happy and miserable worlds. the first argument was, _whether there might not be such worlds in nature, as were in no kind or sort like this world we are of?_ they all agreed, that it was probable there was. the second argument was, _whether it was probable that the happy and miserable worlds were, in any kind, like this we are of._ they all agreed, it was probable that this world was somewhat like both one, and the other; and so, both those were somewhat like this: for, as the _happy world_ was no ways irregular; and the _miserable world_ no ways regular: so this world we are of, was partly irregular, and partly regular; and so it was a _purgatory world_. chap. ii. whether there be such kinds and sorts of creatures in the happy and blessed world, as in this world. the third argument was, _whether it was probable, the happy and miserable worlds, had animal, vegetable, mineral, and elemental kinds?_ they agreed, it was probable there were such kinds: but yet, those kinds, and particular sorts of those kinds, might be different from those of this world. the fourth argument was, _whether there was human sorts of creatures in those worlds._ they all agreed, there was. chap. iii. of the births and deaths of the heavenly world. the fifth argument was, _whether there could be births and deaths in the happy world?_ some parts of my mind were of opinion, that if there was so regular a world, as that there were no irregularities in it, there could not be _deaths_: for, death was a dissolution; and if there was no death, there could be no birth, or production: for, if any particular sort of creatures should encrease, and never dissolve, they would become infinite; which every particular kind or sort of creatures, may be, for time, and be eternal; as also, be infinite for number; because, as some dissolve, others are produced. and so, if particular sorts or kinds of creatures, be eternal; the particular production and dissolution, is infinite: but, if any sort, or kind, should encrease, without decrease, not any particular world could contain them: as for example, if all the human creatures that have been produced from our father _adam_, (which hath not been above six thousand years) should be alive, this world could not contain them; much less, if this world, and the human sorts of creatures, had been of a longer date. and besides, if there should be a greater encrease, by the number of human creatures: in truth, the numerous encrease, would have caused mankind, in the space of six thousand years, to be almost infinite. but, the minor parts of my mind was of opinion, that then the _happy world_ could not be so perfectly regular, if there was death. the major part's opinion was, that some sorts of deaths were as regular, as the most regular births: for, though diseases were caused by irregular actions, yet, death was not: for, as it is not irregular, to be old; so it is not irregular, to dye. but, this argument broke off for that time. chap. iv. whether those creatures could be named blessed, that are subject to dye. the sixth argument was, _whether those creatures could be called_ blessed, _or_ _happy, that are subject to dye?_ the major parts of my mind was of opinion, that, if death was as free from irregularities, as birth; then it was as happy to dye, as to be born. the minor parts were of opinion, that though dissolution might be as regular as composition; yet, it was an unhappiness for every particular society, to be dissolved. the major part's opinion, was, that though the particular societies were dissolved; yet, by reason the general society of the kind, did continue, it was not so much unhappiness; considering, particular parts, or creatures, did make the general society; and not, the general, the particular societies: so that, the parts of the particulars, remained in the general, as in the kind of sort. the minor parts were of opinion, that the particulars of the same kind or sort, (as _mankind_) did contribute but little to the general: for, other sorts of creatures did contribute more than they; only mankind was the occasion, or contributor of the first foundation, but no more: but, the other parts or creatures of the world, did contribute more to their kind, than the creatures of the same kind did: and, as other kinds, and sorts, did contribute to mankind; so mankind, to other kinds or sorts: for, all kinds and sorts, did contribute to the subsistance and assistance of each other. the major part's opinion was, that if all the parts of a world did assist each other, then death could be no unhappiness, especially in the regular world; by reason all creatures in that world, of what kind or sort soever, was perfect and regular: so that, though the particular human creatures did dissolve from being humans; yet, their parts could not be unhappy, when they did unite into other kinds, and sorts, or particular societies: for, those other sorts and kinds of creatures, might be as happy as human creatures. chap. v. of the productions of the creatures of the regular world. the seventh argument was, of productions of the creatures of the regular world, _viz. whether their productions were frequent, or not?_ the minor part's opinion, was, that they were frequent. the major part's opinion, was, that they were not _frequent_, or _numerous_, by reason the world was regular, and so all the productions or generations, were regular; but could not exceed such a number as was, regularly, sufficient for a world, of such a dimension as the regular world; and according to the dimensions, must the society or creatures be, let them be large or little. chap. vi. whether the creatures in the blessed world, do feed, and evacuate. the eighth argument, was, _whether the blessed humans, in the happy world, did eat, and evacuate?_ they agreed, that, if they did feed, they must evacuate. then there was a dispute, _whether those happy creatures did eat?_ they all agreed, that, if they were natural human creatures, they had natural appetites: but, by reason there were no irregularities in this world, the human creatures had not any irregular appetites, nor irregular digestions; irregular passions, or irregular pastimes. then there arose a dispute, _whether those blessed creatures did sleep?_ some were of opinion, they did not sleep: for, sleep was occasioned through a weariness of the sensitive organs, making perceptions of forrein objects; and all weariness, or tiredness, was irregular. the major part of my mind, was of a contrary opinion; because the delight of nature, is in variety: and therefore, regular sleeps were delightful. the minor was of opinion, that sleep was like death, and therefore it could not be happy. but, at last, they did conclude, that sleep, being a soft and quiet repose, (as being retired from all actions concerning forrein parts, and had only actions at home, and of private affairs; and that all the parts of body and mind, were then most sociable amongst themselves) that the blessed humans did sleep. chap. vii. of the animals, and of the food of the humans of the happy world. the ninth argument, was, _whether there were all sorts of animals in the regular world?_ all the parts of my mind agreed, that if there were such creatures as human creatures, it was probable there was other animal creatures: but, by reason there was no irregularities, there could not be cruel or ravenous animal creatures: for, a lyon, leopard, or wolf, in that world, would be as harmless as a sheep in this; and all kites, hawks, and the like ravenous birds, would be as harmless as those birds that only feed on the berries, and fruit of the earth. chap. viii. whether it is not irregular, for one creature to feed on another. the tenth argument was, _whether it was not irregular, for one creature to feed on another?_ some were of opinion, that it was natural for one creature to subsist by another, and to assist each other; but not cruelly to destroy each other. upon this argument, the parts of my mind divided into a minor and a major part. the minor part's opinion, was, that, since all the creatures in nature, had life; then, all creatures that did feed, did destroy each other's life. the major part's opinion, was, that they might be assisted by the lives of other creatures, and not destroy their lives: for, life could not be destroyed, though lives might be occasionally alter'd: but, some creatures may assist other creatures, without destruction or dissolution of their society: as for example, the fruits and leaves of vegetables, are but the humorous parts of vegetables, because they are divisible, and can encrease and decrease, without any dissolution of their society; that is, without the dissolution of the plant. also, milk of animals, is a superfluous humor of animals: and, to prove it to be a superfluous humor, i alledg, that much of it oppresses an animal. the same i say of the fruits and leaves of many sorts of vegetable creatures. besides, it is natural for such sorts of creatures to have their fruits and leaves to divide from the stock. the minor part's opinion, was, that the milk of animals, and the fruits of vegetables, and the herbs of the earth, had as much life as their producers. the major part's opinion, was, that though they had as much life as their producers; yet, it was natural for such off-springs to change and alter their lives, by being united to other sorts of creatures: as for example, an animal eats fruit and herbs; and those fruits and herbs convert themselves into the nature of those animals that feed of them. the same is of milk, eggs, and the like; out of which, a condition of life is endeavoured for: and, for proof, such sorts of creatures account an animal life the best; and therefore, all such superfluous parts of creatures, endeavour to unite into an animal society; as we may perceive, that fruits and herbs, are apt to turn into worms, and flies; and some parts of milk, as cheese, will turn into maggots; so that when animals feed of such meats, they occasion those parts they feed on, to a more easie transformation; and not only such creatures, but humans also, desire a better change: for, what human would not be a glorious sun, or starr? after which discourse, all the parts of my mind agreed unanimously, that animals, and so human creatures, might feed on such sorts of food, as aforesaid; but not on such food as is an united society: for, the root and foundation of any kind and sort of creature, ought not to be destroyed. chap. ix. of the continuance of life in the regular world. the opinion of the parts of my mind, was, that, it was probable, that all societies in the regular world, (that is, all such parts of nature as are united into particular creatures) are of long life, by reason there are no irregularities to destroy them, before their natural time. but then a dispute was raised amongst the parts of my mind, concerning the natural time, that is, the proper time of the lives of those creatures: for, all creatures were not of the same time of production; nor, after their production, of the same time of continuance. but the parts of my mind concluded, that though they could not judg by observation of any creature, no, not of their own sort; yet, they did believe they could judg better of human creatures, as being, at that time, of a human society, than of any other: but, by reason they were of this world (that is, irregular in part) they did believe they might very much err in their judgment, concerning the continuance of human lives, in the _happy world_. but, after much debate, they concluded, that a human creature, in the regular world, might last as long as the productions did not oppress or burden that world, (for that would be irregular) but how long a time that might be, they could not possibly conceive or imagine. chap. ix. of the excellency and happiness of the creatures of the regular world. the parts of my mind could not possibly, being parts of a purgatory world, conceive the happy condition of all creatures in the regular world; but only, conceiving there was no irregularities, they did also conceive, that all creatures there, must be in perfection; and that the elemental creatures were purer, without drossie mixtures; so that their earth must needs be so fruitful, that it produces all sorts of excellent vegetables, without the help of art; and their minerals as pure, as all sorts of stone that are transparent, and as hard as diamonds; the gold and silver, more pure than that which is refined in our world. the truth is, that, in their opinions, the meanest sorts of metal in the regular world, were more pure than the richest sort in this world: so that then, their richest metal must be as far beyond ours, as our gold is beyond our iron, or lead. as for the elemental waters in the regular world, they must be extraordinary smooth, clear, flowing, fresh, and sweet; and the elemental air only, a most pure, clear, and glorious light; so that there could be no need of a sun: and, by reason all the air was a light, there could be no darkness; and so, no need of a moon, or starrs. the elemental fire, although it was hot, yet it was not burning. also, there could neither be scorching heats, nor freezing colds, storms, nor tempest: for, all excess is irregular. neither could there be clouds, because no vapours. but, not to be tedious; it was my mind's opinion, that all the parts of the happy world, being regular, they could not obstruct each other's designs or actions; which might be a cause, that both the sensitive and rational parts may not only make their societies more curious, and their perceptions more perfect; but their perceptions more subtile: for, all the actions of that world being regular, must needs be exact and perfect; in so much, that every creature is a perfect object to each other; and so every creature must have, in some sort, a perfect knowledg of each other. chap. xi. of human creatures in the regular world. the opinion of my mind, was, that the _happy world_, having no irregularities, all creatures must needs be excellent, and most perfect, according to their kind and sort; amongst which, are human creatures, whose kinds, or sorts, being of the best, must be more excellent than the rest, being exactly formed, and beautifully produced: there being, also, no irregularities, human creatures cannot be subject to pains, sickness, aversions, or the like; or, to trepidations, or troubles; neither can their appetites, or passions, be irregular: wherefore, their understanding is more clear, their judgments more poysed: and by reason their food is pure, it must be delicious, as being most tastable: also, it must be wholsome, and nourishing; which occasions the parts of body and mind, to be more lively and pleasant. chap. xii. of the happiness of human creatures in the material world. the happiness that human creatures have in the _regular world_, is, that they are free from any kind or sort of disturbance, by reason there are no irregular actions; and so, no pride, ambition, faction, malice, envy, suspition, jealousie, spight, anger, covetousness, hatred, or the like; all which, are irregular actions among the rational parts: which occasions treachery, slander, false accusations, quarrels, divisions, warr, and destruction; which proceeds from the irregularities of the sensitive parts, occasioned by the rational, by reason the sense executes the mind's designs: but, there are no plots or intrigues, neither in their state, nor upon their stage; because, though they may act the parts of harmless pleasures; yet, not of deceitful designs: for, all human creatures, live in the regular world, so united, that all the particular human societies, (which are particular human creatures) live as if they were but one soul, and body; that is, as if they were but one part, or particular creature. as for their pleasures, and pleasant pastimes; in my opinion, they are such, as not any creature can express, unless they were of that world, or heaven: for, all kinds and sorts of creatures, and all their properties or associations, in this world we are of, are mixt; as, partly irregular; and partly, regular; and so it is but a _purgatory-world_. but surely, all human creatures of that world, are so pleasant and delightful to each other, as to cause a general happiness. the fourth part. chap. i. of the irregular world. after the arguments and opinions amongst the parts of my mind, concerning a regular world; their discourse was, of an _irregular world_: upon which they all agreed, that if there was a world that was not in any kind or sort, irregular; there must be a world that was not in any kind or sort, regular. but, to conceive those irregularities that are in the irregular world, is impossible; much less, to express them: for, it is more difficult to express irregularities, than regularities: and what human creature of this world, can express a particular confusion, much less a world of confusions? which i will, however, endeavour to declare, according to the philosophical opinions of the parts of my mind. chap. ii. of the productions and dissolutions of the creatures of the irregular world. according to the actions of nature, all creatures are produced by the associations of parts, into particular societies, which we name, _particular creatures_: but, the productions of the parts of the irregular world, are so irregular, that all creatures of that world are monstrous: neither can there be any orderly or distinct kinds and sorts; by reason that order and distinction, are regularities. wherefore, every particular creature of that world, hath a monstrous and different form; insomuch, that all the several particulars are affrighted at the perception of each other: yet, being parts of nature, they must associate; but, their associations are after a confused and perturbed manner, much after the manner of whirlwinds, or _aetherial globes_, wherein can neither be order, nor method: and, after the same manner as they are produced, so are they dissolved: so that, their _births_ and _deaths_ are _storms_, and their _lives_ are _torments_. chap. iii. of animals, and of humans, in the irregular world. it has been declared in the former chapter, _that there was not any perfect kind or sort of creatures in the irregular world_: for, though there be such creatures as we name animals; and amongst animals, humans: yet, they are so monstrous, that, being of confused shapes, or forms, none of those animal creatures can be said to be of such, or such a sort; because they are of different disordered forms. also, they cannot be said to be of a perfect animal-kind, or any kind; by reason of the variety of their forms: for, those that are of the nature of animals, especially of humans, are the most miserable and unhappy of all the creatures of that world; and the misery is, that death doth not help them: for, nature being a perpetual motion, there is no rest either alive or dead. in this world, it's true, some societies (_viz._ some creatures) may, sometimes, after their dissolutions, be united into more happy societies, or forms; which, in the irregular world, is impossible; because all forms, creatures, or societies, are miserable: so that, after dissolution, those dispersed parts cannot joyn to any other society, but what is as bad as the former; and so those creatures may dissolve out of one misery, and unite into another; but cannot be released from misery. page chap. iv. of objects, and perceptions. the opinions amongst the parts of my mind, were, that in the unhappy, or miserable world, all the actions of that world, being irregular, it must needs be, that all sorts of perceptions of that world, must also be irregular: not only because the objects are all irregular; but, the perceptive actions are so too; in such manner, that, what with the irregularity of the objects, and the irregularity of the perceptions, it must, of necessity, cause a horrid confusion, both of the sensitive and rational parts of all creatures of that world, in so much, that not only several creatures may appear as several devils to each other; but, one and the same creature may appear, both to the sense and reason, like several devils, at several times. chap. v. the description of the globe of the irregular world. the opinion of my mind was, that the globe of the irregular world was so irregular, that it was a horrid world: for though, being a world, it might be somewhat like other worlds, both globous, and a society of it self, by its own parts; and therefore might have that which we name _earth, air, water_, and _fire_: but, for sun-light, moon-light, starr-light, and the like, they are not parts of the world they appear to; and are worlds of themselves. but, there can be no such appearances in the irregular world: for, the irregularities do obstruct all such appearances; and the elemental parts (if i may name them so) are as irregular, and therefore as horrid as can be: so that it is probable, that the elemental fire is not a bright shining fire, but a dull, dead fire, which hath the effects of a strong corrosive fire, which never actually heats, but actually burns; so that some creatures may both freeze and burn at once. as for the earth of that world, it is probable that it is like corrupted sores, by reason all corruptions are produced by irregular motions; from which corruptions, may proceed such stinking foggs, as may be as far beyond the scent of brimstone, or any the worst of scents that are in this world, as _spanish_ or _roman_ perfumes, or essences, are beyond the scent of carion, or _assafoetida_; which causes all creatures (of airy substances) that breathe, to be so infected, as to appear like poysoned bodies. as for their elemental water, 'tis probable, that it is as black as ink, as bitter as gaul, as sharp as _aquafortis_, and as salt as brine, mixt irregularly together, by reason the waters there, must needs be very troubled waters. as for the elemental air, i shall declare the opinion of my rational parts, in the following chapter. chap. vi. of the elemental air, and light of the irregular world. 'tis probable, that the elemental air of the irregular world, is neither perfectly dark, nor perfectly light; for, either would be, in some part or kind, a perfection or regularity: but, being irregular, it must be a perturbed air; and, being perturbed, it is probable it produces several colours. but, mistake me not, i do not mean such colours as are made by perturbed light; but, such as are made by perturbed air: and, through the excess of irregularities, may be horrid colours; and, by reason of the _aetherial_ whirling motions, which are circular motions, the air may be of the colour of blood, a very horrid colour to some sorts of creatures: but 'tis probable, this bloody colour is not of a pure bloody colour, but of a corrupted bloody colour: and so the light of the irregular world, may, probably, be of a corrupt bloody colour: but, by the several irregular motions, it may be, at several times, of several corrupted bloody colours: and by reason there are no intermissions of _air_, there can be no intermissions of this _light_, in the irregular world. page chap. vii. of storms, and tempests, in the irregular world. as for _storms_, and _tempests_, and such irregular weather, 'tis probable there are continual winds and thunders, caused by the disturbance of the air; and those storms and tempests, being irregular, must needs be violent, and therefore very horrid. there may also be lightnings, but they are not such as those that are of a fiery colour; but such as are like the colour of fire and blood mixt together. as for rain, being occasioned by the vapours from the earth and waters, it is according as those vapours gather into clouds: but, when there is thunder, it must needs be violent. chap. viii. of the several seasons, or rather, of the several tempers in the irregular world. as for _several seasons_; there can be no constant season, because there is no regularity; but rather, a great irregularity, and violence, in all tempers and seasons; for there is no mean degree: and surely, their freezing is as sharp and corroding, as their corrosive-burnings; and it is probable, that the ice and snow in that world, are not as in this world, _viz._ the ice to be clear, and the snow white; because there the water is a troubled, and black water; so that the snow is black, and the ice also black; not clear, or like black polished marble; but 'tis probable, that the snow is like black wool; and the ice, like unpolished black stone; not for solidity, but for colour and roughness. chap. ix. the conclusion of the irregular and unhappy or cursed world. i have declared in my former chapter, concerning the _irregular world_, that there could not be any exact, or perfect kind or sort, because of the irregularities; not that there is not animal, vegetable, mineral, and elemental actions, and so not such creatures; but, by reason of the irregularities, they are strangely mixt and disordered, so that every particular seems to be of a different kind, or sort, being not any ways like each other; and yet, may have the nature of such kinds, and sorts, by reason they are natural creatures, although irregularly natural: but, those irregular natural creatures, cannot chuse, by the former descriptions, but be unhappy, having, in no sort or kind, pleasure, or ease: and for such creatures that have such perceptions as are any way like ours, they are most miserable: for, by the sense of touch, they freeze and burn: by the sense of tast, they have nauseousness, and hunger, being not satisfied: by the sense of scent, they are suffocated, by reason of irregular respiration: by the sense of hearing, and sense of seeing, they have all the horrid sounds and sights, that can be in nature: the rational parts are, as if they were all distracted or mad; and the sensitive parts tormented with pains, aversions, sicknesses, and deformities; all which is caused through the irregular actions of the parts of the irregular world; so that the actions of all sorts of creatures, are violent, and irregular. but, to conclude: as all the creatures of our world, were made for the benefit of human creatures; so, 'tis probable, all the creatures of the irregular world, were produced for the torment and confusion of human creatures in that world. the fifth part, being divided into fifteen sections. concerning restoring-beds, or wombs. i. at the latter end of my _philosophical conceptions_, the parts of my mind grew sad, to think of the dissolving of their society: for, the parts of my mind are so friendly, that although they do often dispute and argue for recreation and delight-sake; yet, they were never so irregular, as to divide into parties, like factious fellows, or unnatural brethren: which was the reason that they were sad, to think their kind society should dissolve, and that their parts should be dispersed and united to other societies, which might not be so friendly as they were. and, after many several thoughts, (which are several rational discourses: for, thoughts are the language of the mind) they fell into a discourse of _restoring beds_, or _wombs_, viz. _whether there might not be restoring beds, as well as producing beds, or breeding beds_. and, to argue the case, they agreed to divide into minor and major parts. ii. the major parts of my mind were of opinion, that there are beds, or wombs, of restoration, as well as beds of production: for, if nature's actions be poysed, there must be one, as well as the other. the minor part's opinion, was, that, as all creatures were produced, so all creatures were subject to dissolve: so that, the poyse of nature's productions, was nature's dissolutions, and not restorations. the major part's opinion, was, that there are restorations in nature: for, as some dissolved, others united in every kind and sort of creature, which was a restoration to the kinds and sorts of creatures. the minor part's opinion, was, that though every sort and kind of creatures, continued as the species of each sort and kind; yet, they did not continue by such restorations as they were arguing about: for though, when some creatures dye, others of the same sort or _species_, are born or bred; yet, they are produced, not restored: for, they conceived, that restoration was a reviving and re-uniting the parts of a dissolved society or creature; which restoration was not natural, at least, not usual. the major part's opinion, was, that restoration was natural, and usual: for, there were many things, or creatures, restored, in some sort, after they were dead. the minor part's opinion, was, that some creatures might be restored from some infirmities, or decays; but, they could not be restored after they were dissolved, and their parts dispersed. the major part's opinion, was, that if the roots, seeds, or springs of a society, or creature, were not dissolved and dispersed, those creatures might be restored to their former condition of life, if they were put, or received, into the restoring beds: as for example, a dry and withered rood of some vegetable, although the parts of that vegetable be, as we say, dead; yet, they are often restored by the means of some arts: also, dead sprigs will, by art, receive new life. the minor part's opinion, was, that if there were such actions of nature, as restoring actions; yet, they could not be the poysing actions, nor the artificial actions: for, not any dead creature can be restored by art. iii. some of the gravest parts of my mind, made this following discourse to some other parts of my mind. _dear associates_, there hath been many human societies, that have perswaded themselves, that there are such restoring actions of nature, which will restore, not only a dead, but a dispersed society; by reason they have observed, that vegetables seem to dye in one season, and to revive in another: as also, that the artificial actions of human creatures, can produce several artificial effects, that resemble those we name _natural_; which hath occasioned many human creatures to wast their time and estates, with fire and furnace, cruelly torturing the productions of nature, to make their experiments. also, they trouble themselves with poring and peeping through telescopes, microscopes, and the like toyish arts, which neither get profit, nor improve their understanding: for, all such arts prove rather ignorant follies, than wise considerations; art being so weak and defective, that it cannot so much assist, as it doth hinder nature: but, there is as much difference between art and nature, as between a statue and a man; and yet artists believe they can perfect what by nature is defective; so that they can rectifie nature's irregularities; and do excuse some of their artificial actions, saying, they only endeavour to hasten the actions of nature: as if nature were slower than art, because a carver can cut a figure or statue of a man, having all his materials ready at hand, before a child can be finished in the breeding-bed. but, art being the sporting and toyish actions of nature, we will not consider them at this time. but, _dear associates_, if there be any such things in nature, as _restoring-beds_, which most of our society are willing to believe; yet, those beds cannot possibly be _artificial_, but must be _natural beds_. nor can any one particular sort of bed, be a general restorer: for, every several sort or kind, requires a bed, or womb, that is proper for their sorts or kinds: so that, there must be as many sorts, at least, and kinds of beds, as there are kinds of creatures: but, what those wombs or beds are, we human creatures do not know; nor do we know whether there be any such things in this world: but, if there be such things in this world, we cannot conceive where they are. iv. after the former discourse, the parts of my mind were a little sad: but, after many and frequent disputes and arguments, they all agreed, that there are restoring beds, or wombs, in nature: but that to describe their conceptions of those restoring beds, was only to describe opinions, but not known truths: and their opinions were, that those beds are as lasting as gold, or quick-silver: for, though they may be occasioned to alter their exterior form; yet, not their interior or innate nature. but, mistake not my mind's opinion: for, their opinion is not, that those beds are gold, or quicksilver: for, their opinion was, that neither gold, or quicksilver, were restorers of life: but, if they were restorers, they could restore no other creatures, but only dead metals, by reason several creatures require several restoring beds proper to their sorts or kinds: so that a mineral kind or sort, could not restore an animal kind or sort; because there was no such thing in nature, as the elixir, or philosophers-stone, which the chymists believe to be some deity, that can restore all sorts and kinds. v. as it has formerly been declared, the parts of my mind were generally of opinion, that it was, at least, probable, there were such things in nature as _restoring-beds_, or _wombs_. the next opinion was, that these beds were of several kinds or sorts, viz. animal, vegetable, mineral, and elemental: so that every kind or sort, is a general restorer of the lives of their kind or sort. as for example, an animal _restoring-bed_, may restore any dead animal, to his former animal life, in case the animal roots or seeds, (which we name, the _vital parts_) were not divided and dispersed, but inclosed, or inurned, so that no other animal could come to feed on those roots and seeds of the dead animal body; and in case the body was so closely kept, though dead many years, if it was put into a _restoring-bed_, that animal creature would reunite to the former animal life and form. but then there arose this argument, _that if the bodies of the dead animals, did corrupt and dissolve of themselves, as most dead animal bodies do; whether, after their dissolution, they could be restored?_ the minor part's opinion was, that those dissolved bodies, being dissolved, or divided, and their parts out of their places, could not be restored. the major part's opinion, was, they might be restored; first, because, though the parts may be divided; yet, they were not annihilated. the next, that those divided parts were not so separated and dispersed, as to be united to other societies: wherefore, if all those dead animal parts were put into a _restoring-womb_, or _bed_; the bed would occasion those parts to place themselves into their proper order and form. vi. after the former discourse, some of the parts of my mind were sad, to think, that those that had been embowelled, were made incapable of ever being restored; and, that it was a greater cruelty to murder a dead man, and to rob him of his interior parts; than to murder a living man, and yet suffer his whole body to lye peaceably in the urn, or grave. but, the other parts endeavouring to comfort those sad parts, made this argument, viz. _whether it might not probably be, that the bones or carcase of a human creature, were the root of human life? and if so, then if all the parts were dissolved, and none were left undissolved, but the bare carcase; they might be restored to life._ the sad part's opinion, was, that it was impossible they could be restored, by reason the roots of human life, were those we name the _vital parts_; and those being divided from the carcase, and dispersed, and united unto other societies, could not meet and joyn into their former state of life, or society, so as to be the same man. the comforting parts were of opinion, it was not probable that the fleshy and spungy parts, being the branches of human life, could also be the roots. wherefore, in all probability, the bones were the roots; and the bones being the roots, if the bare carcase of a man should be put into a restoring bed, all the fleshy and spungy parts, both those that were the exterior, and those that were interior, would spring and encrease to their full maturity. the sad part's opinion, was, that if the bones were the roots; and that, from the roots, all the exterior and interior parts, belonging to a human creature, should spring, and so encrease to full maturity; yet, those branches would not be the same they were, viz. the same parts of the same man; and besides, those branches would rather be new productions, than restorations. the comforting part's opinion, was, that though the branches were new, the carcase, as the root, being the same, the man would be the same: for, though the spungy and fleshy parts, divide and unite from home, and to forrein parts; yet, the man is the same: and to prove that the bony parts are the roots of human life, doth it not happen, that if the flesh be cut from the bone, and the bone be left bare; yet, in time, the bone produces new flesh: but, if any bone be separated from the body, that bone cannot be restored; nor can a new bone spring forth, nor can the divided bone be joyned or knit to the body, as it was before: for, although a broken bone may be set; yet, a divided bone cannot be rejoyned: all which arguments, were a sufficient proof, that the bones were the roots of life. the sad part's argument, was, that it was well known, that if any of the vital parts of a human creature, as the liver, lungs, heart, kidneys, and the like, were decayed, pierced, or wounded, the human creature dyed, by reason those parts are incurable. the comforting parts were of opinion, that there were many less causes which did often occasion human death; yet, those causes were not the roots of life: nor were those parts the roots of life, although those parts which we name _vital_, were the chief branches of human life. but, at last, they all agreed in this opinion, that the _bones_, were the roots; the _marrow_, the sapp, and the _vitals_, the chief branches of life. also, they agreed, that when an human life was restored, the bones did first fill with some oylie juyces; and from the bones, and the sap or juyce of the bones, did all the parts belonging to a human creature, spring forth, and grow up to maturity: and certainly, _not to disturb_ the _bones of the dead_, was a holy and religious charge to human creatures. vii. after the pacifying the sad parts of my mind, their argument was, _that, supposing creatures could be restored; whether they should be restored as when they were first produced; or, as when they were at the perfection of their age; or, as when they were at old age?_ but, after many disputes, they all agreed, that those that should be restored, should be restored to that degree of age and strength, which is the most perfect: and, as all productions arrived towards perfection by degrees; so those that were restored, should return to perfection by degrees, if they were past the perfect time of their age: and those that were not arrived to their perfection, before they dyed, should arrive to it, however, as those that had it: so that, both _youth_ and _age_, shall meet in perfection: for, as the one encreases, as it were, forward; so the other return to their strength and perfection of their past age. viii. after the former opinions, the parts of my mind were somewhat puzled in their _arguments_ concerning the degrees of the restoring times; as, _whether restoration was done by a general act, or by degrees?_ the most doubting part's opinion, was, that it was not natural to restore, although it was natural to produce; and, that all natural productions, were by degrees: but, for restorations, (being not natural productions) they could not be done by degrees: and therefore the action of restoration, was but as one action, although of many parts. the believing parts of my mind were of opinion, that all nature's actions, being by degrees, all restorations were also by degrees. the doubting part's opinion was, that there were some actions that had no degrees: for, one action might signifie a thousand. the other part's opinion was, that a thousand actions, or degrees, were in the figure of one. the doubting parts were of opinion, that it was impossible. but, at last, they agreed, that the restoring actions were by degrees. ix. the parts of my mind were divided into minor and major parts, about the time or degrees of restoration of human creatures. the minor's opinion was, that the restoring actions of nature, were so much quicker than the producing actions, that a human creature might be restored in a months time; whereas the production of a human creature was in ten months: for, though a human creature may quicken at three months time; yet, it was not fully ripe for birth, before the time of ten months. the major part's opinion was, that restoration was according as the creature was dissolved: for, a man that was newly dead; or not so long dead, that his parts were not yet divided; that man might be restored to life in an hour's time, or less: but, if all the parts, excepting the bare carcase, were dissolved, there would require as long a time in restoring, as in producing. the minor's opinion, was, that the restoring-time, was no longer than the time of quickning. the major part's opinion, was, that though the exterior form or frame of a child, might be before the quickning; yet, it was not a perfect animal, until it was quick: and although it might be a perfect animal when it was quick; yet, not ripe, that is, not at the full perfection of a human creature. as it is with fruits: for, a green plumb is not like a ripe plumb; but, any green fruit, is like a dead fruit, in comparison of a ripe fruit. at last, the parts of my mind did agree, that if a human creature was dissolved, excepting the bare carcase; it would require ten months time ere it could perfectly be restored: for, the springing parts would require so long a time ere they could come to full maturity. x. the question being stated, _whether the restoring-bed, was a fleshy bed_; all the parts of my mind, after many disputes, agreed, that it could not be a fleshy bed, by reason the nature of flesh is so corruptible, dissolvable, and easie to be dissolved, that it could not possibly be of such a lasting nature, as is required for _restoring-beds_. but yet, they agreed, they were like flesh, for softness, or spunginess; as also, for colour. also, they agreed, that the animal _restoring-bed_, was of such a nature or property, that it could dilate and contract, as it had occasion; in so much, that it could contract to the compass of the smallest, or extend to the magnitude of the largest animal. also, they did agree, that it was somewhat like the stomack of a human creature, or of the like animal, that could open and shut the orifice; and that when an animal creature was put into the _restoring-bed_, it would immediately inclose the animal: and when it had caused a perfect restoration, the _restoring-bed_ would open it self, and deliver it to its own liberty. xi. another question amongst the parts of my mind concerning _restoring-beds_, or _wombs_, was: _that in case there were such restoring-beds in nature, as in all probability there were; where could those restoring-beds be?_ viz. _whether there were any in this world? if not in this world, in any other world?_ the minor parts were of opinion, there were none in this world; but, that there were some in other worlds. the major part's opinion, was, that there were such beds; but, that human creatures would not know them, though they could perceive them: nor, if they could perceive them, could they tell how to make use of them. at last they all agreed, that those _restoring-beds_ were in the center of the world: but, where the center is, no human creature, no, not the most subtile and _learned mathematicians, geometricians_, or _astrologers_, could, with their most laborious arts, and subtile observations, know; and therefore, unless by a special decree from god, no such restoration can be made. xii. the parts of my mind were very studious to conceive where the center of the world was: some of the parts of my mind was of opinion, that there were four centers, _viz._ a center in the earth, a center in the air, a center in the sea, and a center in the element of fire. upon which opinion, the parts of my mind divided into minor and major parts. the minor parts were of opinion, that there were centers in all the four elemental parts; and that the _restoring-beds_, were only of four kinds: but yet, there might be many several sorts of each particular kind; and that each particular kind, with all the several sorts, was produced in each particular elemental center. the major part was of opinion, that there might be infinite centers, if there were infinite worlds: also, there might be many centers in this world; for, every round globe hath a center. but, their opinion concerning the _restoring-beds_, was, that they were in the center of the globe of our whole world, and not of any of the parts of the world: for, the _air_ could have but an uncertain center; neither could the _water_ have a very solid center; and the _earth_ was too solid to have a center, consisting of the four kinds of elements: neither could the elemental _fire_ have such a center, as to breed such different kinds and sorts of _beds_, as the _restoring-beds_ are, because many of them are quite of a different nature from the nature of elemental fire: wherefore, it must be the center of the world, which must consist of all the elemental kinds. xiii. after the former _argument_, the parts of my mind were very studious in conceiving, where the center of the whole universe of this our world, might be: at last they all agreed, it was the _sea_, which is the watry element: for, the _sea_ is inclosed with the _airy, fiery_, and _earthy_ parts of the universe, and therefore must be the center. and, though the sea was the center of the world; yet, there was a center of the sea: so that, there was a center in a center; in which center, were the _restoring-beds_. xiv. after the former conceptions, the parts of my mind were very studious, to conceive where the center's center might be. but, they could not possibly conceive it, by reason they could not possibly imagine how large, and of what compass the sea may be of: for they did verily believe, that the utmost extension of the sea, is not, as yet, known to human-kind: for, that circle about which the ships of _cavendishe_, and _drake_, did swim, might be, in comparison to the whole body of the sea, but such a circle as a boy may occasion, with throwing a small stone, or such like thing, into a pond of water. xv. the last conception of my mind, concerning _restoring-beds_, was, that the parts of my mind did conceive, that the center of the whole universe, was the sea; and in the center of the sea, was a small island; and in the center of the island, was a creature, like (in the outward form) to a great and high rock: not that this rock was stone; but, it was of such a nature, (by the natural compositions of parts) that it was compounded of parts of all the principal kinds and sorts of the creatures of this world, viz. of _elemental, animal, mineral_, and _vegetable_ kinds: and, being of such a nature, did produce, out of it self, all kinds and sorts of _restoring-beds_; whereof, some sorts were so loose, that they only hung by strings, or nerves: others stuck close. some were produced at the top, or upper parts: others were produced out of the middle parts; and some were produced from the lower parts, or at the bottom. in short, the opinion of the parts of my mind, was, that this rocky creature was all covered with its own productions; which productions were of all kinds and sorts: not that they were numerous; but, various productions: also, that these various productions, were _restoring-beds_: for, the nature of this rocky creature, is as lasting as the sun, or other planets; which was the reason that those productions are not subject to decay, as other productions are: nor can they produce new creatures; but only restore former creatures; as, those that had been produced, and were partly dissolved. the conclusion. after the wisest parts of my mind had ended their _arguments_, there being some of the dullest, and the most unbelieving, or rather, strange parts of my mind, that had retired into the _glandula_ of my brain, which is a kind of a kernel; which they made use of, instead of a pulpit: out of which, they declared their opinions, thus: _dear associates_, we, that were not parties of your disputations, or argumentations, concerning _restoring-beds_; being retired into the _glandula_ of the brain, where we have been informed by the nerves, and sensitive spirits, of your wise opinions, and subtile arguments, considering that your conclusion was as improbable, if not as impossible, as the chymical _philosophers-stone_, or _elixir_; we desire you (being parts of one and the same society) not to trouble the whole society, in the search of that, which, if it was in nature, will never be found. but to prevent, that your painful studies, and witty arguments, be not buried in oblivion; we advise you, to perswade the sensitive parts of our society, to record them, so that they may be divulged to all the societies of our own kind or sort of creatures; as _chymists_ do, who, after they have wasted their times and estates, to gain the _philosophers-stone_, or _elixir_; write books to teach it to the sons of art: which is impossible, at least, very improbable, ever to be learn'd, there being no such art in nature: but, were it possible such an art was to be obtained; yet, when obtained, the artist would never divulge it in print. but, those great practitioners, finding, after much loss and pains, nothing but despair, write books of that art; which, instead of the _elixir_, did produce _despair_; which again, though produced by art, did produce, naturally, that vice, named _malice_; and _malice_, being a pregnant seed, sowed upon the fertile ground of their writings, produces so much mischief, that many men of good estates, have been undone, in following their rules in _chymistry_: and if your books should be as succesful as _chymistry_ hath been (i dare not say, among _fools_; but) amongst credulous men; your books will cause as much mischief as theirs have done; not by the ways of _fire_, but by the ways of _water_: for, your books send men to sea, a much cooler element than _fire_; but, more dangerous than _chymical fire_, unless _chymical fire_ be _hell-fire_. upon which discourse, the rest of my thoughts were very angry, and pull'd them out of their pulpit, the _glandula_; and not only so, but put them out of their society, believing they were a factious party, which, in time, might cause the society's dissolution. finis. images of public domain material from the google print project.) the energy system of matter the energy system of matter a deduction from terrestrial energy phenomena by james weir _with diagrams_ longmans, green and co. paternoster row, london new york bombay, and calcutta all rights reserved preface an intimate study of natural phenomena and a lengthened experience in physical research have resulted in the formation of certain generalisations and deductions which i now present in this volume. i have reached the conclusion that every physical phenomenon is due to the operation of energy transformations or energy transmissions embodied in material, and takes place under the action or influence of incepting energy fields. in any instance the precise nature of the phenomena is dependent on the peculiar form of energy actively engaged, on the nature of the material to which this energy is applied, and on the nature of the incepting field which influences the process. in the course of the work several concrete cases are discussed, in which these features of energy are illustrated and explained by the use of simple experimental apparatus. it is hoped that, by this means, the distinctive differences which exist in the manifestations of energy, in its transformation, in its transmission, and in its incepting forms will be rendered clear to the reader. i have to express my indebtedness to mr. james affleck, b.sc., for his assistance in the preparation of this work for publication. james weir. over courance, lockerbie, scotland. contents page introduction part i general statement . advantages of general view of natural operations . separate mass in space . advent of energy--distortional effects . the gravitation field . limits of rotational energy--disruptional phenomena . passive function and general nature of gravitation field . limit of gravitation transformation . interactions of two planetary bodies--equilibrium phenomena . axial energy--secondary processes . mechanism of energy return . review of cosmical system--general function of energy . review of cosmical system--natural conditions part ii principles of inception . illustrative secondary processes . incepting energy influences . cohesion as an incepting influence . terrestrial gravitation as an incepting influence . the gravitation field . the thermal field . the luminous field . transformations--upward movement of a mass against gravity . transformations--the simple pendulum . statical energy conditions . transformations of the moving pendulum--energy of motion to energy of position and vice versa . transformations of the moving pendulum--frictional transformation at the bearing surfaces . stability of energy systems . the pendulum as a conservative system . some phenomena of transmission processes--transmission of heat energy by solid material . some phenomena of transmission processes--transmission by flexible band or cord . some phenomena of transmission processes--transmission of energy to air masses . energy machines and energy transmission . identification of forms of energy . complete secondary cyclical operation part iii terrestrial conditions . gaseous expansion . gravitational equilibrium of gases . total energy of gaseous substances . comparative altitudes of planetary atmospheres . reactions of composite atmosphere . description of terrestrial case . relative physical conditions of atmospheric constituents . transmission of energy from aqueous vapour to air masses . terrestrial energy return . experimental analogy and demonstration of the general mechanism of energy transformation and return in the atmospheric cycle . application of pendulum principles . extension of pendulum principles to terrestrial phenomena . concluding review of terrestrial conditions--effects of influx of energy the energy system of matter introduction the main principles on which the present work is founded were broadly outlined in the author's _terrestrial energy_ in , and also in a later paper in . the views then expressed have since been amply verified by the course of events. in the march of progress, the forward strides of science have been of gigantic proportions. its triumphs, however, have been in the realm, not of speculation or faith, but of experiment and fact. while, on the one hand, the careful and systematic examination and co-ordination of experimental facts has ever been leading to results of real practical value, on the other, the task of the theorists, in their efforts to explain phenomena on speculative grounds, has become increasingly severe, and the results obtained have been decreasingly satisfactory. day by day it becomes more evident that not one of the many existing theories is adequate to the explanation of the known phenomena: but, in spite of this obvious fact, attempts are still constantly being made, even by most eminent men, to rule the results of experimental science into line with this or that accepted theory. the contradictions are many and glaring, but speculative methods are still rampant. they have become the fashion, or rather the fetish, of modern science. it would seem that no experimental result can be of any value until it is deductively accommodated to some preconceived hypothesis, until it is embodied and under the sway of what is practically scientific dogma. these methods have permeated all branches of science more or less, but in no sphere has the tendency to indulge in speculation been more pronounced than in that which deals with energetics. in no sphere, also, have the consequences of such indulgence been more disastrous. for the most part, the current conceptions of energy processes are crude, fanciful, and inconsistent with nature. they require for their support--in fact, for their very existence--the acceptance of equally fantastic conceptions of mythical substances or ethereal media of whose real existence there is absolutely no experimental evidence. on the assumed properties or motions of such media are based the many inconsistent and useless attempts to explain phenomena. but, as already pointed out, nature has unmistakably indicated the true path of progress to be that of experimental investigation. in the use of this method only phenomena can be employed, and any hypothesis which may be formulated as the result of research on these lines is of scientific value only in so far as it is the correct expression of the actual facts observed. by this method of holding close to nature reliable working hypotheses can, if necessary, be formed, and real progress made. it is undeniably the method of true science. in recent years much attention has been devoted to certain speculative theories with respect to the origin and ultimate nature of matter and energy. such hypotheses, emanating as they do from prominent workers, and fostered by the inherent imaginative tendency of the human mind, have gained considerable standing. but it is surely unnecessary to point out that all questions relating to origins are essentially outside the pale of true science. any hypotheses which may be thus formulated have not the support of experimental facts in their conclusions; they belong rather to the realm of speculative philosophy than to that of science. in the total absence of confirmatory phenomena, such theories can, at best, only be regarded as plausible speculations, to be accepted, it may be, without argument, and ranking in interest in the order of their plausibility. of modern research into the ultimate constitution of matter little requires to be said. it is largely founded on certain radio-active and electrical phenomena which, in themselves, contribute little information. but aided by speculative methods and the use of preconceived ethereal hypotheses, various elaborate theories have been formulated, explaining matter and its properties entirely in terms of ethereal motions. such conceptions in their proper sphere--namely, that of metaphysics--would be no doubt of interest, but when advanced as a scientific proposition or solution they border on the ridiculous. in the absence of phenomena bearing on the subject, it would seem that the last resort of the modern scientist lies in terminology. to the average seeker after truth, however, the term "matter," as applied to the material world, will still convey as much meaning as the more elaborate scientific definitions. it is not the purpose of this work to add another thread to the already tangled skein of scientific theory. it is written, rather, with the conviction, that it is impossible ever to get really behind or beyond phenomena; in the belief that the complete description of any natural process is simply the complete description of the associated phenomena, which may always be observed and co-ordinated but never ultimately explained. phenomena must ever be accepted simply as phenomena--as the inscrutable manifestations of nature. by induction from phenomena it is indeed possible to rise to working hypotheses, and thence, it may be, to general conceptions of nature's order, and as already pointed out, it is to this method, of accepting phenomena, and of reasoning only from experimental facts, that all the advances of modern science are due. on the other hand, it is the neglect of this method--the departure, as it were, from nature--which has led to the introduction into the scientific thought of the day of the various ethereal media with their extreme and contradictory properties. the use of such devices really amounts to an admission of direct ignorance of phenomena. they are, in reality, an attempt to explain natural operations by a highly artificial method, and, having no basis in fact, their whole tendency is to proceed, in ever-increasing degree, from one absurdity to another. it is quite possible to gain a perfectly true and an absolutely reliable knowledge of the properties of matter and energy, and the part which each plays, without resorting to speculative aids. all that is required is simply accurate and complete observation at first hand. the field of research is wide; all nature forms the laboratory. by this method every result achieved may be tested and verified, not by its concurrence with any approved theory, however plausible, but by direct reference to phenomena. the verdict of nature will be the final judgment on every scheme. it is on these principles, allied with the great generalisations with respect to the conservation of matter and energy, that this work is founded. as the result of a long, varied, and intimate acquaintance with nature, and much experimental research in many spheres, the author has reached the conclusion, already foreshadowed in _terrestrial energy_, that the great principle of energy conservation is true, not only in the universal and generally accepted sense, but also in a particular sense with respect to all really separate bodies, such as planetary masses in space. each of these bodies, therefore, forms within itself a completely conservative energy system. this conclusion obviously involves the complete denial of the transmission of energy in any form across interplanetary space, and the author, in this volume, now seeks to verify the conclusion by the direct experimental evidence of terrestrial phenomena. under present-day conditions in science, the acceptance of the ordinary doctrine of transmission across space involves likewise the acceptance of the existence of an ethereal substance which pervades all space and forms the medium by which such transmission is carried out. the properties of this medium are, of course, precisely adapted to its assumed function of transmission. these properties it is not necessary to discuss, for when the existence of the transmission itself has been finally disproved, the necessity for the transmitting medium clearly vanishes. part i general statement . _advantages of general view of natural operations_ the object of this statement is to outline and illustrate, in simple fashion, a broad and general conception of the operation and interaction of matter and energy in natural phenomena. such a conception may be of value to the student of nature, in several ways. in modern times the general tendency of scientific work is ever towards specialisation, with its corresponding narrowness of view. a broad outlook on nature is thus eminently desirable. it enables the observer to perceive to some extent the links uniting the apparently most insignificant of natural processes to those of seemingly greater magnitude and importance. in this way a valuable idea of the natural world as a whole may be gained, which will, in turn, tend generally to clarify the aspect of particular operations. a broad general view of nature also leads to the appreciation of the full significance of the great doctrines of the conservation of matter and energy. by its means the complete verification of these doctrines, which appears to be beyond human experiment, may be traced on the face of nature throughout the endless chain of natural processes. such a view also leads to a firm grasp of the essential nature and qualities of energy itself so far as they are revealed by its general function in phenomena. . _separate mass in space_ in the scheme now to be outlined, matter and energy are postulated at the commencement without reference to their ultimate origin or inherent nature. they are accepted, in their diverse forms, precisely as they are familiar from ordinary terrestrial experience and phenomena. for the purpose of general illustration the reader is asked to conceive a mass of heterogeneous matter, concentrated round a given point in space, forming a single body. this mass is assumed to be assembled and to obtain its coherent form in virtue of that universal and inherent property of matter, namely, gravitative or central attraction. this property is independent of precise energy conditions, its outward manifestation being found simply in the persistent tendency of matter on all occasions to press or force itself into the least possible space. in the absence of all disturbing influences, therefore, the configuration of this mass of matter, assumed assembled round the given point, would naturally, under the influence of this gravitative tendency, resolve itself into that of a perfect sphere. the precise magnitude or dimensions of the spherical body thus constituted are of little moment in the discussion, but, for illustrative purposes, it may, in the meantime, be assumed that in mass it is equivalent to our known solar system. it is also assumed to be completely devoid of energy, and as a mass to be under the influence of no external constraint. under these conditions, the spherical body may obviously be assumed as stationary in space, or otherwise as moving with perfectly uniform velocity along a precisely linear path. either conception is justifiable. the body has no relative motion, and since it is absolutely unconstrained no force could be applied to it and no energy expenditure would be required for its linear movement. . _advent of energy--distortional effects_ nature, however, does not furnish us with any celestial or other body fulfilling such conditions. absolutely linear motion is unknown, and matter is never found divorced from energy. to complete the system, therefore, the latter factor is required, and, with the advent of energy to the mass, its prototype may be found in the natural world. this energy is assumed to be communicated in that form which we shall term "work" energy (§§ , ) and which, as a form of energy, will be fully dealt with later. this "work" energy is assumed to be manifested, in the first place, as energy of motion. as already pointed out, no expenditure of energy can be associated with a linear motion of the mass, since that motion is under no restraint, but in virtue of the initial central attraction or gravitative strain, the form of energy first communicated may be that of kinetic energy of rotation. its transmission to the mass will cause the latter to revolve about some axis of symmetry within itself. each particle of the mass thus pursues a circular path with reference to that axis, and has a velocity directly proportional to its radial displacement from it. this energised rotating spherical mass is thus the primal conception of the energy scheme now to be outlined. it will be readily seen that, as a primal conception, it is essentially and entirely natural; so much so, indeed, that any one familiar with rotatory motion might readily predict from ordinary experience the resulting phenomena on which the scheme is, more or less, based. when energy is applied to the mass, the first phenomenon of note will be that, as the mass rotates, it departs from its originally spherical shape. by the action of what is usually termed centrifugal force, the rotating body will be distorted; it will be flattened at the polar or regions of lowest velocity situated at the extremities of the axis of rotation, and it will be correspondingly distended at the equatorial or regions of highest velocity. the spherical body will, in fact, assume a more or less discoidal form according to the amount of energy applied to it; there will be a redistribution of the original spherical matter; certain portions of the mass will be forced into new positions more remote from the central axis of rotation. . _the gravitation field_ these phenomena of motion are the outward evidence of certain energy processes. the distortional movement of the material is carried out against the action and within the field of certain forces which exist in the mass of material in virtue of its gravitative or cohesive qualities. it is carried out also in virtue of the application of energy to the sphere, which energy has been, as it were, transformed or worked down, in the distortional movement, against the restraining action of this gravitation field or influence. the outward displacement of the material from the central axis is thus coincident with a gain of energy to the mass, this gain of energy being, of course, at the expense of, and by the direct transformation of, the originally applied energy. it is stored in the distorted material as energy of position, potential energy, or energy of displacement relative to the central axis. but, in the distortive movement, the mass will also gain energy in other forms. the movement of one portion of its material relative to another will give rise (since it is carried out under the gravitational influence) to a fractional process in which, as we know from terrestrial experience, heat and electrical energy will make their appearance. these forms of energy will give rise, in their turn, to all the phenomena usually attendant on their application to material. as already pointed out also, the whole mass gains, in varying degree, energy of motion or kinetic energy. it would appear, then, that although energy was nominally applied to the mass in one form only, yet by its characteristic property of transformation it has in reality manifested itself in several entirely different forms. it is important to note the part played in these transformation processes by the gravitation field or influence. its action really reveals one of the vital working principles of energetics. this principle may be generally stated thus:-- every transformation of energy is carried out by the action of energised matter in the lines or field of an incepting energy influence. in the particular case we have just considered, the incepting field is simply the inherent gravitative property of the energised mass. this property is manifested as an attractive force between portions of matter. this, however, is not of necessity the only aspect of an incepting influence. in the course of this work various instances of transformation will be presented in which the incepting influence functions in a guise entirely different. it is important to note that the incepting influence itself is in no way changed, altered, or transformed during the process of transformation which it influences. . _limits of rotational energy. disruptional phenomena_ it is clear that the material at different parts of the rotating spheroid will be energised to varying degrees. since the linear velocity of the material in the equatorial regions of the spheroid is greater than that of the material about the poles, the energy of motion of the former will exceed that of the latter, the difference becoming greater as the mass is increasingly energised and assumes more and more the discoidal form. the question now arises as to how far this process of energising the material mass may be carried. what are its limits? the capacity of the rotating body for energy clearly depends on the amount of work which may be spent on its material in distorting it against the influence of the gravitative attraction. the amount is again dependent on the strength of this attraction. but the value of the gravitative attraction or gravitation field is, by the law of gravitation, in direct proportion to the quantity of material or matter present, and hence the capacity of the body for energy depends on its mass or on the quantity of matter which composes it. now if energy be impressed on this mass beyond its capacity a new order of phenomena appears. distortion will be followed by disruption and disintegration. by the action of the disruptive forces a portion of the primary material will be projected into space as a planetary body. the manner of formation of such a secondary body is perhaps best illustrated by reference to the commonplace yet beautiful and suggestive phenomenon of the separation of a drop of water or other viscous fluid under the action of gravitation. in this process, during the first downward movement of the drop, it is united to its source by a portion of attenuated material which is finally ruptured, one part moving downwards and being embodied in the drop whilst the remainder springs upwards towards the source. in the process of formation of the planetary body we are confronted with an order of phenomena of somewhat the same nature. the planetary orb which is hurled into space is formed in a manner similar to the drop of viscous fluid, and under the action of forces of the same general nature. one of these forces is the bond of gravitative attraction between planet and primary which is never severed, and when complete separation of the two masses finally occurs, the incessant combination of this force with the tangential force of disruption acting on the planet will compel it into a fixed orbit, which it will pursue around the central axis. when all material links have thus been severed, the two bodies will then be absolutely separate masses in space. the term "separate" is here used in its most rigid and absolute sense. no material connection of any kind whatever exists, either directly or indirectly, between the two masses. each one is completely isolated from the other by interplanetary space, and in reality, so far as material connection is concerned, each one might be the sole occupant of that space. this conception of separate masses in space is of great importance to the author's scheme, but, at the same time, the condition is one which cannot be illustrated by any terrestrial experimental contrivance. it will be obvious that such a device, as might naturally be conceived, of isolating two bodies by placing them in an exhausted vessel or vacuous space, by no means complies with the full conditions of true separation portrayed above, because some material connection must always exist between the enclosed bodies and the containing vessel. this aspect is more fully treated later (§ ). the condition of truly separate masses is, in fact, purely a celestial one. no means whatever are existent whereby such a condition may be faithfully reproduced in a terrestrial environment. in their separate condition the primary and planetary mass will each possess a definite and unvarying amount of energy. it is to be noted also, that since the original mass of the primary body has been diminished by the mass of the planet cast off, the capacity for energy of the primary will now be diminished in a corresponding degree. any further increment of energy to the primary in any form has now, however, no direct influence on the energy of the planet, which must maintain its position of complete isolation in its orbit. but although thus separate and distinct from the primal mass in every material respect, the planet is ever linked to it by the invisible bond of gravitation, and every movement made by the planet in approaching or receding from the primary is made in the field or influence of this attraction. in accordance, therefore, with the general principle already enunciated (§ ), these actions or movements of the energised planetary mass, being made in the field of the incepting gravitative influence, will be accompanied by transformations, and thus the energy of the planet, although unvarying in its totality, may vary in its form or distribution with the inward or outward movement of the planet in its orbital path. as the planet recedes from the primary it gains energy of position, but this gain is obtained solely at the expense, and by the direct transformation of its own orbital energy of motion. its velocity in its orbit must, therefore, decrease as it recedes from the central axis of the system, and increase as it approaches that axis. thus from energy considerations alone it is clear that, if the planetary orbit is not precisely circular, the velocity of the planet must vary at different points of its path. . _passive function and general nature of gravitation field_ from the phenomena described above, it will be observed that, in the energy processes of transformation occurring in both primary and planet, the function of the gravitation field or influence is entirely passive in nature. the field is, in truth, the persistent moving or directing power behind the energy processes, the incepting energy influence or agency which determines the nature of the transformation in each case without being, in any way, actively engaged in it. in accelerating or retarding the transformation process it has thus absolutely no effect. these features are controlled by other factors. neither does this incepting agency affect, in any way, the limits of the transformation process, these limits being prescribed by the physical or energy qualities of the acting materials. in general nature the gravitation field appears to be simply an energy influence--a peculiar manifestation of certain passive qualities of energy. this aspect will, however, become clearer to the reader when the properties of gravitation are studied in conjunction with those of other incepting energy influences (§§ , , ). . _limit of gravitation transformation_ in the case of a planetary body, there is a real limit to the extent of the transformation of its orbital energy of motion under the influence of the gravitation field. as the orbit of the planet widens, and its mean distance from the primary becomes greater, its velocity in its orbital path must correspondingly decrease. as already pointed out (§ ), this decrease is simply the result of the orbital energy of motion being transformed or worked down into energy of position. but since this orbital energy is strictly limited in amount, a point must ultimately be reached where it would be transformed in its entirety into energy of position. when this limiting condition is attained, the planet clearly could have no orbital motion; it would be instantaneously at rest in somewhat the same way as a projectile from the earth's surface is at rest at the summit of its flight in virtue of the complete transformation of its energy of motion into energy of position. in this limiting condition, also, the energy of position of the planet would be the maximum possible, and its orbital energy zero. the scope of the planetary orbital path is thus rigidly determined by the planetary energy properties. assuming the reduction of gravity with distance to follow the usual law of inverse squares, the value of the displacement of the planet from the central axis when in this stationary or limiting position may be readily calculated if the various constants are known. in any given case it is obvious that this limiting displacement must be a finite quantity, since the planetary orbital energy which is being worked down is itself finite in amount. . _interactions of two planetary bodies--equilibrium phenomena_ up to the present point, the cosmical system has been assumed to be composed of one planetary body only in addition to the primary mass. it is clear, however, that by repetition of the process already described, the system could readily evolve more than one planet; it might, in fact, have several planetary masses originating in the same primary, each endowed with a definite modicum of energy, and each pursuing a persistent orbit round the central axis of the system. since the mass of the primary decreases as each successive planet is cast off, its gravitative attractive powers will also decrease, and with every such decline in the central restraining force the orbits of the previously constituted planets will naturally widen. by the formation in this way of a series of planetary masses, the material of the original primary body would be as it were distributed over a larger area or space, and this separation would be accompanied by a corresponding decrease in the gravitative attraction between the several masses. if the distributive or disruptive process were carried to its limit by the continuous application of rotatory energy to each separate unit of the system, this limit would be dependent on the capacity of the system for energy. as is shown later (§ ), this capacity would be determined by the mass of the system. for simplicity, let us consider the case in which there are two planetary bodies only in the system in addition to the primary. in virtue of the gravitative attraction or gravitation field between the two, they will mutually attract one another in their motion, and each will, in consequence, be deflected more or less out of that orbital path which it would normally pursue in the absence of the other. this attraction will naturally be greatest when the planets are in the closest proximity; the planet having the widest orbit will then be drawn inwards towards the central axis, the other will be drawn outwards. the distance moved in this way by each will depend on its mass, and on the forces brought to bear on it by the combined action of the two remaining masses of the system. moving thus in different directions, the motion of each planet is carried out in the lines of the gravitation field between the two. one planet, therefore, gains and the other loses energy of position with respect to the central axis of the system. the one planet can thus influence, to some extent, the energy properties of the other, although there is absolutely no direct energy communication between the two; as shown hereafter, the whole action and the energy change will be due simply to the motion carried out in the field of the incepting gravitation influence. it is clear, however, that this influence is exerted on the distribution of the energy, on the form in which it is manifested, and in no way affects the energy totality of either planet. each, as before, remains a separate system with conservative energy properties. that planet which loses energy of position gains energy of motion, and is correspondingly accelerated in its orbital path; the other, in gaining energy of position, does so at the expense of its own energy of motion, and is retarded accordingly. the action is really very simple in nature when viewed from a purely energy standpoint. it has been dealt with in some detail in order to emphasise the fact that there is absolutely nothing in the nature of a transmission of energy between one planet and the other. taking a superficial view of the operation, it might be inferred that, as the planets approach one another, energy of motion (or energy of position) is transmitted from one to the other, causing one to retard and the other to accelerate its movement, but a real knowledge of the energy conditions shows that the phenomenon is rather one of a simple restoration of equilibrium, a redistribution or transformation of the intrinsic energy of each to suit these altering conditions. each planet is, in the truest sense, a separate mass in space. . _axial energy--secondary processes_ passing now to another aspect of the energy condition of a planetary body, let the planet be assumed to be endowed with axial energy or energy of rotation, so that, while pursuing its orbital path in space, it also rotates with uniform angular velocity about an axis within itself. what will be the effect of the primary mass on the planet under these new energy conditions? we conceive that the effect is again purely one of transformation. in this process the primary mass functions once more as an entirely passive or incepting agent, which, while exerting a continuous transforming influence on the planet, does not affect in any way the inherent energy properties of the latter. up to the present point we have only dealt with one incepting influence in transformation processes, namely, that of gravitation, which has always been manifested as an attractive force. it is not to be supposed, however, that this is the only aspect in which incepting influences may be presented. although attractive force is certainly an aspect of some incepting influences, it is not a distinctive feature of incepting influences generally. in many cases, the aspect of force, in the sense of attraction or repulsion, is entirely awanting. in the new order of transformations which come into play in virtue of the rotatory motion of a planetary mass in the field of its primary, we shall find other incepting influences in action entirely different in nature from the gravitation influence, but, nevertheless, arising from the same primary mass in a similar way. now the application of energy to the planet, causing it to rotate in the lines or under the influence of these incepting fields of the primary, brings into existence on the planet an entirely new order of phenomena. so long as the planet had no axial motion of rotation, some of the incepting influences of the primary were compelled, as it were, to inaction; but with the advent of axial energy the conditions are at once favourable to their action, and to the detection of their transforming effects. in accordance with the general principle already enunciated (§ ), the action of the planetary energised material in the lines of the various incepting fields of the primary is productive of energy transformations. the active energy of these transformations is the axial or rotatory energy of the planet itself, and, in virtue of these transformations, certain other forms of energy will be manifested on the planet and associated with the various forms of planetary material. these manifestations of energy, in fact, constitute planetary phenomena. since the action or movement of the rotating material of the planet through the incepting fields of the primary is most pronounced in the equatorial or regions of highest linear velocity, and least in the regions of low velocity adjoining the poles of rotation, the transforming effect may naturally be expected to decrease in intensity from equator to poles. planetary energy phenomena will thus vary according to the location of the acting material. it will be clear, also, that each incepting agency or influence associated with the primary mass will give rise to its own peculiar transformations of axial energy on the planetary surface. these leading or primary transformations of axial energy, in which the incepting influence is associated with the primary mass only, we term primary processes. but it is evident that the various forms of energy thus set free on the planet as a result of the primary processes will be communicated to, and will operate on, the different forms of planetary material, and will give rise to further or secondary transformations of energy, in which the incepting agency is embodied in or associated with planetary material only. the exact nature of these secondary transformations will vary according to the circumstances in which they take place. each of them, however, as indicated above, will be, in itself, carried out in virtue of some action of the energised planetary material in the lines or field of what we might term a secondary incepting influence. the latter, however, must not be confused with the influences of the primary. it is essentially a planetary phenomenon, an aspect of planetary energy; it is associated with the physical or material machine by means of which the secondary process of transformation is carried out. the nature of this secondary influence will determine the nature of the secondary transformation in each case. its precise extent may be limited by other considerations (§ ). as an example, assume a portion of the axial energy to be primarily transformed into heat in virtue of the planet's rotation in the field of an independent thermal incepting influence exerted by the primary. to the action of this agency, which we might term the thermal field, we assume are due all primary heating phenomena of planetary material. now the secondary transformations will take place when the heat energy thus manifested is applied to some form of matter. it is obvious, however, that this application might be carried out in various ways. heat may be devoted to the expansion of a solid against its cohesive forces. it may be expended against the elastic forces of a gas, or it may be worked down against chemical or electrical forces. in every case a transformation of energy will result, varying in nature according to the peculiar conditions under which it is carried out. in this or a similar fashion each primary incepting influence may give rise to a series of secondary actions more or less complex in nature. these secondary transformation processes, allied with other processes of transmission, will, in fact, constitute the visible phenomena of the planet, and in their variety will exactly correspond to these phenomena. with regard to the gravitation field, its general influence on the rotating mass may be readily predicted. the material on that part of the planetary surface which is nearest to or happens to face the primary in rotation is, during the short time it occupies that position, subjected to a greater attractive influence than the remainder which is more remote from the primary. it will, in consequence, tend to be more or less distorted or elevated above its normal position on the planetary surface. this distorting effect will vary in degree according to the nature of the material, whether solid, liquid, or gaseous, but the general effect of the distortional movement, combined with the rotatory motion of the planet, will be to produce a tidal action or a periodical rise and fall of the more fluid material distributed over the planetary surface. the distortion will, of course, be accompanied by energy processes in which axial energy will be transformed into heat and other forms, which will finally operate in the secondary processes exactly as in previous cases. . _mechanism of energy return_ but the question now arises, as to how this continuous transformation of the axial energy can be consistent with that condition of uniformity of rotation of the planet which was originally assumed. if the total energy of the planetary mass is limited, and if it can receive no increment of energy from any external source, it is clear that the axial energy transformed must, by some process, be continuously returned to its original form. some process or mechanism is evidently necessary to carry out this operation. this mechanism we conceive to be provided by certain portions of the material of the planet, principally the gaseous matter which resides on its surface, completely enveloping it, and extending outwards into space (§ ). in other words, the atmosphere of the planet forms the machine or material agency by which this return of the transformed axial energy is carried out. it has already been pointed out (§ ) how the working energy of every secondary transformation is derived from the original axial energy of the planet itself. each of these secondary transformations, however, forms but one link of one cyclical chain of secondary transformations, in which a definite quantity of energy, initially in the axial form, passes, in these secondary operations, through various other forms, by different processes and through the medium of different material machines, until it is eventually absorbed into the atmosphere of the planet. these complete series of cyclical operations, by which the various portions of axial energy are carried to the atmosphere, may in some cases be of a very simple nature, and may be continuously repeated over very short intervals of time; in other cases, the cycle may seem obscure and complicated, and its complete operation spread over very long periods, but in all cases the final result is the same. the axial energy abstracted, sooner or later, recurs to the atmospheric machine. by its action in this machine, great masses of gaseous material are elevated from the surface of the planet against the attractive force of gravitation; the energy will thus now appear in the form of potential energy or energy of position. by a subsequent movement of these gaseous masses over the surface of the planet from the regions of high velocity towards the poles, combined with a movement of descent to lower levels, the energy of position with which they were endowed is returned once more in the original axial form. this, roughly, constitutes the working of the planetary atmospheric machine, which, while in itself completely reversible and self-contained, forms also at the same time the source and the sink of all the energy working in the secondary transformations. in the ceaseless rounds of these transformations which form planetary phenomena it links together the initial and concluding stages of each series by a reversible process. energy is thus stored and restored continuously. the planet thus neither gains nor loses energy of axial motion; so far as its energy properties are concerned, it is entirely independent of every external influence. its uniformity of rotation is absolutely maintained. each planet of the system will, in the same way, be an independent and conservative unit. . _review of cosmical system--general function of energy_ reviewing the system as a whole, the important part played by energy in its constitution is readily perceived. the source of the energy which operates in all parts of the system is found in that energy originally applied (§ ). when the system is finally constituted, this energy is found distributed amongst the planets, each of which has received its share, and each of which is thereby linked to the primary by its influence. it is part of this same energy which undergoes transformation in virtue of the orbital movements of the planets in the field of the gravitative influence. again, it is found in the form of planetary axial energy, and thence, under the influence of various incepting agencies, it passes in various forms through the whole gamut of planetary phenomena, and finally functions in the atmospheric machine. every phenomenon of the system, great or small, is, in fact, but the external evidence either of the transformation or of the transmission of this energy--the outward manifestation of its changed or changing forms. its presence, which always implies its transformation (§ ), is the simple primary condition attached to every operation. the primal mass originally responded to the application of energy by the presentation of phenomena. every material portion of the system will similarly respond according to circumstances. energy is, in fact, the working spirit of the whole cosmical scheme. it is the influence linking every operation of the system to the original transformations at the central axis, so that all may be combined into one complete and consistent whole. it is to be noted, however, that although they have a common origin the orbital energy of each planetary mass is entirely distinct from its energy of axial rotation, and is not interchangeable therewith. the transformation of the one form of energy in no way affects the totality of the other. the disruption of the primary mass furnishes a view of what is virtually the birth of gravitation as it is conceived to exist between separate bodies. it may now be pointed out that the attractive influence of gravitation is, in reality, but one of the many manifestations of energy of the system. it is not, however, an active manifestation of the working energy, but rather an aspect of energy as it is related to the properties of matter. we have absolutely no experimental experience of matter devoid of energy. gravitation might readily be termed an energy property of matter, entirely passive in nature, and requiring the advent of some other form in order that it may exercise its function as an incepting agency. from a general consideration of the features of this system, in which every phenomenon is an energy phenomenon, it seems feasible to conclude also that every property of matter is likewise an energy property. it is certain, indeed, that no reasonable or natural concept of either matter or energy is possible if the two be dissociated. the system also presents a direct and clear illustration of the principles of conservation in the working of the whole, and also in each planetary unit. . _natural conditions_ it will be noted that, up to the present point, the cosmical system has been discussed from a purely abstract point of view. this method has been adopted for a definite reason. although able, at all points, to bring more or less direct evidence from nature, the author has no desire that his scheme should be regarded in any way as an attempt to originate or describe a system of creation. the object has been, by general reasoning from already accepted properties of matter and energy, to arrive at a true conception of a possible natural order of phenomena. it is obvious, however, that the solar system forms the prototype of the system described above. the motion of the earth and other planets is continuously occurring under the influence of gravitation, thermal, luminous, and other incepting fields which link them to the central mass, the sun. as a result of the action of such fields, energy transformations arise which form the visible phenomena of the system in all its parts, each transformation, whether associated with animate or inanimate matter, being carried out through the medium of some arrangement of matter hereafter referred to as a material machine. the conditions are precisely as laid down above. the system is dominated, in its separate units, and as a whole, by the great principle of the conservation of energy. each planetary mass, as it revolves in space, is, so far as its energy properties are concerned, an absolutely conservative unit of that system. at the same time, however, each planetary mass remains absolutely dependent on the primary for those great controlling or incepting influences which determine the transformation of its inherent energy. in the special case of the earth, which will be dealt with in some detail, it is the object of this work to show that its property of complete energy conservation is amply verified by terrestrial phenomena. the extension of the principle from the earth to the whole planetary system has been made on precisely the same grounds as newton extended the observed phenomena to his famous generalisation with respect to gravitation. part ii principles of inception . _illustrative secondary processes_ in this part of the work, an attempt will be made to place before the reader some of the purely terrestrial and other evidential phenomena on which the conclusions of the preceding general statement are founded. the complete and absolute verification of that statement is obviously beyond experimental device. bound, as we are, within the confines of one planet, and unable to communicate with the others, we can have no direct experimental acquaintance with really separate bodies (§ ) in space. but, if from purely terrestrial experience we can have no direct proofs on such matters, we have strong evidential conclusions which cannot be gainsayed. if the same kind of energy operates throughout the solar system, the experimental knowledge of its properties gained in one field of research is valuable, and may be readily utilised in another. the phenomena which are available to us for study are, of course, simply the ordinary energy processes of the earth--those operations which in the foregoing statement have been described as secondary energy processes. their variety is infinite, and the author has accordingly selected merely a few typical examples to illustrate the salient points of the scheme. the energy acting in these secondary processes is, in every case, derived, either directly or indirectly, from the energy of rotation or axial energy of the earth. in themselves, the processes may be either energy transformations or energy transmissions or a combination of both these operations. when the action involves the bodily movement of material mass in space, the dynamical energy thus manifested, and which may be transmitted by the movement of this material, is termed mechanical or "work" energy (§ ); when the energy active in the process is manifested as heat, chemical, or electrical energy, we apply to it the term "molecular" energy. the significance of these terms is readily seen. the operation of mechanical or "work" energy on a mass of material may readily proceed without any permanent alteration in the internal arrangement or general structure of that mass. mechanical or "work" energy is dissociated from any molecular action. on the other hand, the application of such forms of energy as heat or electrical energy to material leads to distinctly molecular or internal effects, in which some alteration in the constitution of the body affected may ensue. hence the use of the terms, which of course is completely arbitrary. the principal object of this part of the work is to illustrate clearly the general nature, the working, and the limits of secondary processes. for this purpose, the author has found it best to refer to certain more or less mechanical contrivances. the apparatus made use of is merely that utilised in everyday work for experimental or other useful purposes. it is essentially of a very simple nature; no originality is claimed for it, and no apology is offered for the apparent simplicity of the particular energy operations chosen for discussion. in fact, this feature has rather led to their selection. in scientific circles to-day, familiarity with the more common instances of energy operations is apt to engender the belief that these processes are completely understood. there is no greater fallacy. in many cases, no doubt, the superficial phenomena are well known, but in even the simplest instances the mechanism or ultimate nature of the process remains unknown. a free and somewhat loose method of applying scientific terms is frequently the cloak which hides the ignorance of the observer. no attempt will here be made to go beyond the simple phenomena. the object in view is simply to describe such phenomena, to emphasise and explain certain aspects of already well-known facts, which, up to the present, have been neglected. in some of the operations now to be described, mechanical or "work" energy is the active agent, and material masses are thereby caused to execute various movements in the lines or field of restraining influences. for ordinary experimental convenience, the material thus moved must of necessity be matter in the solid form. the illustrative value of our experimental devices, however, will be very distinctly improved if it be borne in mind that the operations of mechanical energy are not restricted to solids only, but that the various processes of transformation and transmission here illustrated by the motions of solid bodies may, in other circumstances, be carried out in a precisely similar fashion by the movements of liquids or even of gases. the restrictions imposed in the method of illustration are simply those due to the limitations of human experimental contrivance. natural operations exhibit apparatus of a different type. by the movements of solid materials a convenient means of illustration is provided, but it is to be emphasised that, so far as the operations of mechanical energy are concerned, the precise form or nature of the material moved, whether it be solid, liquid, or gaseous, is of no consequence. to raise one pound of lead through a given distance against the gravitative attraction of the earth requires no greater expenditure of energy than to raise one pound of hydrogen gas through the same distance. the same principle holds in all operations involving mechanical energy. another point of some importance which will be revealed by the study of secondary operations is that every energy process has in some manner definite energy limits imposed upon it. in the workings of mechanical or "work" energy it is the mass value of the moving material which, in this respect, is important. the mass, in fact, is the real governing factor of the whole process (§ ). it determines the maximum amount of energy which can be applied to the material, and thus controls the extent of the energy operation. but in actions involving the molecular energies, the operation may be limited by other considerations altogether. for example, the application of heat to a solid body gives rise to certain energy processes (§ ). these processes may proceed to a certain degree with increase of temperature, but a point will finally be attained where change of state of the heated material takes place. this is the limiting point of this particular operation. when change of state occurs, the phenomena will assume an entirely different aspect. the first set of energy processes will now be replaced by a set of operations absolutely different in nature, themselves limited in extent, but by entirely different causes. the first operation must thus terminate when the new order appears. in this manner each process in which the applied energy is worked will be confined within certain limiting boundaries. in any chain of energy operations each link will thus have, as it were, a definite length. in chemical reactions, the limits may be imposed in various ways according to the precise nature of the action. chemical combination, and chemical disruption, must be looked on as operations which involve not only the transformation of energy but also the transformation of matter. in most cases, chemical reactions result in the appearance of matter in an entirely new form--in the appearance, in fact, of actually different material, with physical and energy properties absolutely distinct from those of the reacting constituents. this appearance of matter in the new form is usually the evidence of the termination, not only of the particular chemical process, but also of the energy process associated with it. transformation of energy may thus be limited by transformation of matter. examples of the limiting features of energy operations could readily be multiplied. even a cursory examination of most natural operations will reveal the existence of such limits. in no case do we find in nature any body, or any energy system, to which energy may be applied in unlimited amount, but in every case, rigid energy limits are imposed, and, if these limits are exceeded, the whole energy character of the body or system is completely changed. . _incepting energy influences_ in experimental and in physical work generally, it has been customary, in describing any simple process of energy transformation, to take account only of those energies or those forms of energy which play an active part in the process--the energy in its initial or applied form and the energy in its transformed or final form. this method, however, requires enlarging so as to include another feature of energy transformation, a feature hitherto completely overlooked, namely, that of incepting energy. now, this conception of incepting energy, or of energy as an incepting influence, is of such vital importance to the author's scheme, that it is necessary here, at the very outset, to deal with it in some detail. to obtain some idea of the general nature of these influences, it will be necessary to describe and review a few simple instances of energy transformation. one of the most illuminating for this purpose is perhaps the familiar process of dynamo-electric transformation. a spherical mass a (fig. ) of copper is caused to rotate about its central axis in the magnetic field in the neighbourhood of a long and powerful electro-magnet. in such circumstances, certain well-known transformations of energy will take place. the energy transformed is that dynamical or "work" energy which is being applied to the spherical mass by the external prime mover causing it to rotate. as a result of this motion in the magnetic field, an electrical action takes place; eddy currents are generated in the spherical mass, and the energy originally applied is, through the medium of the electrical process, finally converted into heat and other energy forms. the external evidence of the process will be the rise in temperature and corresponding expansion of the rotating mass. [illustration: fig. ] such is the energy transformation. let us now review the conditions under which it takes place. passing over the features of the "work" energy applied and the energy produced in the transformation, it is evident that the primary and essential condition of the whole process is the presence of the magnetic field. in the absence of this influence, every other condition of this particular energy operation might have been fulfilled without result. the magnetic field is, in reality, the determining agency of the process. but this field of magnetic force is itself an energy influence. its existence implies the presence of energy; it is the external manifestation of that energy (usually described as stored in the field) which is returned, as shown by the spark, when the exciting circuit of the electro-magnet is broken. the transformation of the dynamical or "work" energy (§ ) applied to the rotating sphere is thus carried out by the direct agency, under the power, or within the field of this magnetic energy influence, to which, accordingly, we apply the expression, incepting energy influence, or incepting energy. there are several points to be noted with regard to these phenomena of inception. in the first place, it is clear that the energy which thus constitutes the magnetic field plays no active part in the main process of transformation: during the operation it neither varies in value nor in nature: it is entirely a passive agent. neither is any continuous expenditure of energy required for the maintenance of this incepting influence. it is true that the magnetic field is primarily due to a circulatory current in the coils or winding of the electro-magnet, but after the initial expenditure of energy in establishing that field is incurred, the continuous expenditure of energy during the flow of the current is devoted to simply heating the coils. a continuous heat transformation is thus in progress. the magnetic energy influence, although closely associated with this heat transformation, yet represents in itself a distinct and separate energy feature. this last point is, perhaps, made more clear if it be assumed that, without altering the system in any way, the electro-magnet is replaced by a permanent magnet of precisely the same dimensions and magnetic power. there would then be no energy expenditure whatever for excitation, but nevertheless, the main transformation would take place in precisely the same manner and to exactly the same degree as before. the incepting energy influence is found in the residual magnetism. if an iron ball or sphere were substituted, in the experiment, for the copper one, the phenomena observed on its rotation would be of an exactly similar nature to those described above. there is, however, one point of difference. since the iron is magnetic, the magnet pole will now exert an attractive force on the iron mass, and if the latter were in close proximity to the pole (fig. ), a considerable expenditure of energy might be required to separate the two. it is evident, then, that in the case of iron and the magnetic metals, this magnetic influence is such that an expenditure of energy is required, not only to cause these materials to move in rotation so as to cut the lines of the field of the magnetic influence, but also to cause them to move outwards from the seat of the influence _along_ the lines of the field. the movements, indeed, involve transformations of energy totally different in nature. assuming the energy to be obtained, in both cases, from the same external source, it is, in the first instance, converted by rotatory motion in the field into electrical and heat energy, whereas, in the second case, by the outward motion of displacement from the pole, it is transformed and associated with the mass in the form of energy of position or energy of displacement relative to the pole. since the attractive force between the iron mass and the pole may be assumed to diminish according to a well-known law, the energy transformation per unit displacement will also diminish at the same rate. the precise nature and extent of the influence of the incepting agent thus depend on the essential qualities of the energised material under its power. in this case, the magnetic metals, such as iron, provide phenomena of attraction which are notably absent in the case of the dia-magnetic metals such as copper. other substances, such as wood, appear to be absolutely unaffected by any movement in the magnetic field. the precise energy condition of the materials in the field of the incepting influence is also an important point. the incepting energy might be regarded as acting, not on the material itself, but rather on the energy associated with that material. from the phenomena already considered, it is clear that before the incepting influence of magnetism can act on the copper ball, the latter must be endowed with energy of rotation. it is on this energy, then, that the incepting influence exerts its transforming power. it would be useless to energise the copper ball, say by raising it to a high temperature, and then place it at rest in the magnetic field; the magnetic energy influence would not operate on the heat energy, and consequently, no transformation would ensue. it is easy to conceive, also, that in the course of an energy transformation, the material may attain an energy condition in which the incepting influence no longer affects it. take once more the case of the iron ball. it is well known that, at a high temperature, iron becomes non-magnetic. it would follow, then, that if the rotational transformation in the magnetic field could be carried out to the requisite degree, so that, by the continuous application of that heat energy which is the final product of the process, the ball had attained this temperature, then the other transformation consequent on the displacement of the ball from the attracting pole could not take place. no change has really occurred in the incepting energy conditions. they are still continuous and persistent, but the energy changes in the material itself have carried it, to a certain degree, beyond the influence of these conditions. . _cohesion as an incepting influence_ other aspects of incepting energy may be derived from the examples cited above. returning to the case of the rotating copper sphere, let it be assumed that in consequence of its rotation in the magnetic field it is raised from a low to a high temperature. due to the heating effect alone, the mass will expand or increase in volume. this increase is the evidence of a definite energy process by which certain particles or portions of the mass have in distortion gained energy of position--energy of separation--or potential energy relative to the centre of the sphere. in fact, if the mass were allowed to cool back to its normal condition, this energy might by a suitable arrangement be made available for some form of external work. it is obvious, however, that this new energy of position or separation which has accrued to the mass in its heated condition has in reality been obtained by the transformation of the "work" energy originally applied. the abnormal displacement of certain particles or portions of the mass from the centre of the sphere is simply the external evidence of their increased energy. now this displacement, or strain, due to the heat expansion, is carried out against the action of certain cohesive forces or stresses existing between the particles throughout the mass. these cohesive forces are, in fact, the agency which determines this transformation of heat into energy of position. their existence is essential to the process. but these cohesive forces are simply the external manifestation of that energy by virtue of which the mass tends to maintain its coherent form. they are the symbol of that energy which might be termed the cohesion energy of the mass--they are, in fact, the symbol of the incepting energy influence of the transformation. this incepting energy influence of cohesion is one which holds sway throughout all solid material. it is, therefore, found in action in every movement involving the internal displacement or distortion of matter. it is a property of matter, and accordingly it is found to vary not only with the material, but also with the precise physical condition or the energy state of the material with which it is associated. in this respect, it differs entirely from the preceding magnetic influence. the latter, we have seen, has no direct association with the copper ball, or with the material which is the actual venue of the transformation. as an energy influence, it is itself persistent, and unaffected by the energy state of that material. on the other hand, the cohesion energy, being purely a property of the material which is the habitat of the energy process, is directly affected by its energy state. this point will be clearer by reference to the actual phenomena of the heat transformation. as the process proceeds, the temperature of the mass as the expansion increases will rise higher and higher, until, at a certain point, the solid material is so energised that change of state ensues. at this, the melting-point of the material, liquefaction takes place, and its cohesive properties almost vanish. in this fashion, then, a limit is clearly imposed on the process of heat transformation in the solid body--a limit defined by the cohesive or physical properties of the particular material. in this limiting power lies the difference between cohesion and magnetism as incepting influences. looking at the whole dynamo-electric transformation in a general way, it will be clear that the magnetic influence in no way limits or affects the amount of dynamical or "work" energy which may be applied to the rotating sphere. this amount is limited simply by the cohesive properties of the material mass in rotation. the magnetic influence might, in fact, be regarded as the primary or inducing factor in the system, and the cohesion influence as the secondary or limiting factor. . _terrestrial gravitation as an incepting influence_ the attractive influence of gravitation appears as an incepting agency in terrestrial as well as in celestial phenomena. in fact, of all the agencies which incept energy transformations on the earth, gravitation, in one form or another, is the most universal and the most important. gravitation being a property of all matter, no mundane body, animate or inanimate, is exempt from its all-pervading influence, and every movement of energised matter within the field of that influence leads inevitably to energy transformation. let us take a concrete illustration. a block of solid material is supported on a horizontal table. by means of a cord attached, energy is applied to the block from an external source, so that it slides over the surface of the table. as a result of this motion and the associated frictional process, heat energy will make its appearance at the sliding surfaces of contact. this heat energy is obviously obtained by the transformation of that energy originally applied to the block from the external source. what is the incepting influence in this process of transformation? the incepting influence is clearly the gravitative attraction of the earth operating between the moving block and the table. the frictional process, it is well known, is dependent in extent or degree on the pressure between the surfaces in contact. this pressure is, of course, due to the gravitative attraction of the earth on the mass of the block. if it be removed, say by supporting the block from above, the heat-transformation process at the surfaces at once terminates. gravity, then, is the primary incepting influence of the process. the effect of gravitation in transformation has apparently been eliminated by supporting the block from above and removing the pressure between block and table. it is not really so, however, because the pressure due to the gravitative attraction of the earth on the block has in reality only been transferred to this new point of support, and if a movement of the block is carried out it will be found that the heat transformation has been also transferred to that point. but there are also other influences at work in the process. the extent of the heat transformation depends, not only on the pressure, but also on the nature of the surfaces in contact. it is evident, that in the sliding movement the materials in the neighbourhood of the surfaces in contact will be more or less strained or distorted. this distortion is carried out in the lines of the cohesive forces of the materials, and is the real mechanism of the transformation of the applied work energy into heat. it is obvious that the nature of the surfaces in contact must influence the degree of distortion, that is, whether they are rough or smooth; the cohesive qualities of the materials in contact will depend also on the nature of these materials, and the extent of the heat transformation will be limited by these cohesive properties in precisely the same way as described for other examples (§ ). the function of gravitation in this transformation is, obviously, again quite passive in nature, and is in no way influenced by the extent of the process. gravitation is, as it were, only the agency whereby the acting energy is brought into communication with the cohesive forces of the sliding materials. a little reflection will convey to the reader the vast extent of this influence of gravitation in frictional phenomena, and the important place occupied by such phenomena in the economy of nature. from the leaf which falls from the tree to the mighty tidal motions of air, earth, and sea due to the gravitative effects of the sun and moon, all movements of terrestrial material are alike subject to the influence of terrestrial gravitation, and will give rise to corresponding heat processes. these heat processes are continually in evidence in natural phenomena; the effect of their action is seen alike on the earth's surface and in its interior (internal heating). of the energy operating in them we do not propose to say anything further at this stage, except that it is largely communicated to the atmospheric air masses. . _the gravitation field_ the foregoing examples of transformation serve to place before the reader some idea of the general nature and function of an incepting energy influence. but for the broadest aspects of the latter agencies it is necessary to revert once more to celestial phenomena. as already indicated in the general statement, the primary transformations of planetary axial energy are stimulated by certain agencies inherent to, and arising from, the central mass of the system. these energy agencies or effects operate through space, and are entirely passive in nature. they are in no way associated with energy transmission; they are merely the determining causes of the energy-transforming processes which they induce, and do not in the least affect the conservative energy properties of the planetary masses over which their influence is cast. of the precise number and nature of such influences thus exerted by the primary mass we can say nothing. the energy transformations which are the direct result of their action are so extensive and so varied in character that we would hesitate to place any limit on the number of the influences at work. some of these influences, however, being associated with the phenomena of everyday experience, are more readily detected in action than others and more accessible to study. it is to these that we naturally turn in order to gain general ideas for application to more obscure cases. of the many incepting influences, therefore, which may emanate from the primary mass there are three only which will be dealt with here. each exerts a profound action on the planetary system, and each may be readily studied and its working verified by the observation of common phenomena. these influences are respectively the gravitation, the thermal, and the luminous fields. the general nature and properties of the gravitation field have to some extent been already foreshadowed (§§ , , ). other examples will be dealt with later, and it is unnecessary to go into further detail here. the different aspects, however, in which the influence has been presented may be pointed out. firstly, in the separate body in space, as an inherent property of matter (§ ); secondly, as an attractive influence exerted across space between primary and planet, both absolutely separate bodies (§ ); and thirdly, as a purely planetary or secondary incepting influence (§ ). in every case alike we find its function to be of an entirely passive nature. its most powerful effect on planetary material is perhaps manifested in the tidal actions (§ ). with respect to these movements, it may be pointed out that the planetary material periodically raised from the surface is itself elevated against the inherent planetary gravitative forces, and also, to a certain extent, against the cohesive forces of planetary material. each of these resisting influences functions as an incepting agency, and thus the elevation of the mass involves a transformation of energy (§ ). the source of the energy thus transformed is the axial energy of the planet, and the new forms in which it is manifested are energy of position or potential energy relative to the planetary surface, and heat energy. on the return of the material to its normal position, its energy of position, due to its elevation, will be returned in its original form of axial energy. in the case of the heat transformation, however, it is to be noted that this process will take place both as the material is elevated and also as it sinks once more to its normal position. the heat transformation thus operates continuously throughout the entire movement. the upraising of the material in the tidal action is brought about entirely at the expense of inherent planetary axial energy. the gravitative and cohesive properties of the planetary material make such a transformation process possible. it is in virtue of these properties that energy may be applied to or expended on the material in this way. the tidal action on the planetary surface is, in fact, simply a huge secondary process in which axial energy is converted into heat. the primary incepting power is clearly gravitation. of the aspect of gravitation as a purely planetary influence (§ ) little requires to be said. the phenomena are so prominent and familiar that the reader may be left to multiply instances for himself. . _the thermal field_ the thermal field which is induced by and emanates from the primary mass differs from the gravitation field in that, so far as we know, it is unaccompanied by any manifestation of force, attractive or otherwise. its action on the rotating planetary mass may be compared to that of the electro-magnet on the rotating copper sphere (§ ); the electro-magnet exerts no force on the sphere, but an energy expenditure is, nevertheless, required to rotate the latter through the field of the magnetic influence. to this thermal field, then, in which the planets rotate, we ascribe all primary planetary heating phenomena. the mode of action of the thermal field appears to be similar to that of other incepting influences. by its agency the energy of axial rotation of planetary material is directly converted into the heat form. as already shown (§ ), heat energy may be developed in planetary material as a result of the action of other incepting agencies, such as gravitation. these processes are, however, more or less indirect in nature. but the operation due to the thermal field is a direct one. the heat energy is derived from the direct transformation of planetary axial energy of rotation without passing through any intermediate forms. in common parlance, the thermal field is the agency whereby the primary mass heats the planetary system. no idea of transmission, however, is here implied in such phraseology; the heating effect produced on any planetary mass is entirely the result of the transformation of its own energy; the thermal field is purely and simply the incepting influence of the process. now, in virtue of the configuration of the rotating planetary masses, their material in equatorial regions is much more highly energised than the material in the neighbourhood of the poles, and will, accordingly, move with much greater linear velocity through the thermal field. the heat transformation will vary accordingly. it will be much more pronounced at the equator than at the poles, and a wide difference in temperature will be maintained between the two regions. the thermal field, also, does not necessarily produce the same heating effect on all planetary material alike. some materials appear to be peculiarly susceptible--others much less so. this we may verify from terrestrial experience. investigation shows the opaque substances to be generally most susceptible, and the transparent materials, such as glass, rock-salt, tourmaline, &c. almost insusceptible, to the heating effect of the sun. the influence of the thermal field can, in fact, operate through the latter materials. a still more striking and important phenomenon may be observed in the varying action of the thermal field on matter in its different forms. it has been already pointed out that, in the course of transformation in the field of an incepting influence, a material may attain a certain energy state in which it is no longer susceptible to that influence. this has been exemplified in the case of the iron ball (§ ) and a phenomenon of the same general nature is revealed in the celestial transformation. a piece of solid material of low melting-point is brought from the polar regions of the earth to the equator. due to the more rapid movement across the sun's thermal field, and the consequent increased action of that field, a transformation of the axial energy of rotation of the body takes place, whereby it is heated and finally liquefied. in the liquid state the material is still susceptible to the thermal field, and the transformation process accordingly proceeds until the material finally assumes the gaseous form. at this point, however, it is found that the operation is suspended; the material, in assuming the gaseous state, has now attained a condition (§ ) in which the thermal field has no further incepting or transforming influence upon it. no transformation of its axial energy into the heat form is now possible by this means; indeed, so far as the _direct_ heating effect of the sun is concerned, the free gaseous material on the planetary surface is entirely unaffected. all the evidence of nature points to the conclusion that all gaseous material is absolutely transparent to the _direct_ thermal influence of the sun. matter in the gaseous form reaches, as it were, an ultimate or limiting condition in this respect. this fact, that energised material in the gaseous form is not susceptible to the thermal field, is of very great importance in the general economy of nature. it is, in reality, the means whereby the great primary process of the transformation of the axial energy of the earth into the heat form is limited in extent. as will be explained later, it is the device whereby the planetary energy stability is conserved. it will be apparent, of course, that heat energy may be readily applied to gaseous masses by other means, such as conduction or radiation from purely terrestrial sources. the point which we wish here to emphasise is, simply, that gaseous material endowed with axial energy on the planetary surface cannot have this axial energy directly transformed into heat through the instrumentality of the thermal field of the primary. . _the luminous field_ the planetary bodies are indebted to the primary mass not only for heat phenomena, but also for the phenomena of light. these light phenomena are due to a separate and distinct energy influence (or influences) which we term the luminous field. the mode of action of the luminous field is similar to that of other incepting influences. it operates from the primary, and is entirely passive in nature. like the thermal field, it does not appear to be accompanied by any manifestation of physical stress or force, except, indeed, the experimental demonstrations of the "pressure of light" can be regarded as such. in any case, this in no way affects the general action of light as an incepting agency. its action on energised planetary material gives rise to certain transformations of energy, transformations exclusive and peculiar to its own influence. we will refer to terrestrial phenomena for illustrations of its working. perhaps the commonest example of transformation in which the luminous field appears as the incepting agency is seen in the growth of plant life on the surface of the earth. the growth and development of vegetation and plants generally is the outward evidence of certain energy transformations. the processes of growth, however, are of such a complex nature that it is impossible to state the governing energy conditions in their entirety, but, considering them merely in general fashion, it may be said that energy in various forms (potential, chemical, &c.) is stored in the tissues of the growing material. now the source of this energy is the axial energy of the earth, and, as stated above, the luminous field is an incepting factor (there may be others) in the process of transformation, a factor whereby this axial energy is converted into certain new forms. it is well known that, amongst the factors which influence the growth of vegetation, one of the most potent is that of light. the presence of sunlight is one of the essential conditions for the successful working of certain transformations of plant life, and these transformations vary not only in degree but in nature, according to the variation of the imposed light in intensity and quality. some of the processes of growth are no doubt chemical in nature. here, again, light may be readily conceived to have a direct determining influence upon them, exactly as in the cases of its well-known action in chemical phenomena--for instance, as in photography. other examples will readily occur to the reader. one of the most interesting is the action of light on the eye itself. it may be pointed out indeed that light is, first and foremost, a phenomenon of vision. whatever may be its intrinsic nature, it is primarily an influence affecting the eye. but the action of seeing, like all other forms of human activity, involves a certain expenditure of bodily energy. this energy is, of course, primarily derived from the axial energy of the earth through the medium of plant and animal life and the physico-chemical processes of the body itself. its presence in one form or another is, in fact, essential to all the phenomena of life. the action of seeing accordingly involves the transformation of a certain modicum of this energy, and the influence which incepts this transformation is the luminous field which originates in and emanates from the central mass of the system, the sun. in a similar way, planetary material under certain conditions may become the source of an incepting luminous field. it is this light influence or luminous field which, in common parlance, is said to enter the eye. in that organ, then, is found the mechanism or machine (§ ), a complicated one, no doubt, whereby this process of transformation is carried out which makes the light influence perceptible to the senses. of the precise nature of the action little can be said. the theme is rather one for a treatise on physiology. it may be pointed out, however, with regard to the process of transformation, that dewar has already demonstrated the fact that when light falls on the retina of the eye, an electric current is set up in the optic nerve. the energy associated with this current is, of course, obtained at the expense of the bodily energy of the observer, and this energy, after passing, it may be, through a large number of transformation processes, will finally be returned to the source from which it was originally derived, namely, the axial energy of the earth. the luminous field, also, like the thermal field, has no transforming effect whatever on the energy of certain substances. it may pass completely through some and be reflected by others without any sign of energy transformation. its properties are, in fact, simply the properties of light, and must be accepted simply as phenomena. now, it is very important, in studying matters of this kind, to realise that it is impossible ever to get beyond or behind phenomena. it may be pointed out that in no sphere of physics has the influence of so-called explanatory mechanical hypotheses been stronger than in that dealing with the properties of light. new theories are being expounded almost daily in attempts to explain or dissect simple phenomena. but it may be asked, in what does our really useful knowledge of light consist? simply in our knowledge of phenomena. beyond this, one cannot go. we may attempt to explain phenomena, but to create for this purpose elastic ethereal media or substances without direct evidential phenomena in support is not to advance real knowledge. there are certain properties peculiar to the luminous as to all other incepting fields, certain conditions under which each respectively will act, and the true method of gaining real insight into these agencies is by the study of these actual properties (or phenomena) and conditions, and not by attempts to ultimately explain them. it will be evident that in most cases of natural energy operations there is more than one energy influence in action. as a rule there are several. in a growing plant, for example, we have the thermal, luminous, gravitation, and cohesive influences all in operation at the same time, each performing its peculiar function in transformation, each contributing its own peculiar energy phenomena to the whole. this feature adds somewhat to the complexity of natural operations and to the difficulties in the precise description of the various phenomena with which they are associated. . _transformations--upward movement of a mass against gravity_ when the significance of energy inception and the characteristic properties of the various agencies have been grasped, it becomes much easier to deal with certain other aspects of energy processes. to illustrate these aspects it is, therefore, now proposed to discuss a few simple secondary operations embodied in experimental apparatus. a few examples of the operations of transformation and transmission of energy will be considered. the object in view is to show the general nature of these processes, and more especially the limits imposed upon them by the various factors or properties of the material machines in which they are of necessity embodied. the reader is asked to bear in mind also the observations already made (§ ) with respect to experimental apparatus generally. the first operation for discussion is that of the upward movement of a mass of material against the gravitative attraction of the earth. this movement involves one of the most simple and at the same time one of the most important of secondary energy processes. as a concrete illustration, consider the case of a body projected vertically upwards with great velocity from the surface of the earth. the phenomena of its motion will be somewhat as follows:--as the body recedes from the earth's surface in its upward flight, its velocity suffers a continuous decrease, and an altitude is finally attained where this velocity becomes zero. the projectile, at this point, is instantaneously at rest. its motion then changes; it commences to fall, and to proceed once more towards the starting-point with continuously increasing velocity. neglecting the effect of the air (§ ) and the rotational movement of the earth, it may be assumed that the retardation of the projectile in its upward flight is numerically equal to its acceleration in its downward flight, and that it finally returns to the starting-point with velocity numerically equal to the initial velocity of projection. the process then obviously involves a complete transformation and return of energy. at the earth's surface, where its flight commences and terminates, the body is possessed of energy of motion to a very high degree. at the highest point of flight, this form of energy has entirely vanished; the body is at rest. its energy properties are then represented by its position of displacement from the earth's surface; its energy of motion in disappearing has assumed this form of energy of position, energy of separation, or potential energy. the moving body has thus been the mechanism of an energy transformation. at each stage of its upward progress, a definite modicum of its original energy of motion is converted into energy of position. between the extreme points of its flight, the energy of the body is compounded of these two forms, one of which is increasing at the expense of the other. when the summit of flight is reached the conversion into energy of position is complete. in the downward motion, the action is completely reversed, and when the body reaches the starting-point its energy of position has again been completely transformed into energy of motion. it might be well to draw attention here to the fact, often overlooked, that this energy of position gained by the rising mass is, in reality, a form of energy, separate and distinct, brought into existence by the transformation and disappearance of the energy of the moving mass. energy of position is as truly a form of energy as heat or kinetic energy. the transformation here depicted is clearly a simple process, yet we know absolutely nothing of its ultimate nature, of the why or wherefore of the operation. our knowledge is confined to the circumstances and conditions under which it takes place. let us now, therefore, deal with these conditions. the transformation is clearly carried out in virtue of the movement of the body in the lines or field of an incepting influence. this influence is that of gravitation, which links the body continually to the earth. now the function of gravitation in this process, as in others already described, is that of a completely passive incepting agent. the active energy which suffers change in the process is clearly the original work energy (§ ) communicated to the projected body. the whole process is, in fact, a purely mechanical operation, and as in the case of other processes involving mechanical energy, it is limited by the mass value of the moving material. it is clear that the greater the amount of energy communicated to the projectile at the starting-point, the greater will be the altitude it will attain in its flight. the amount of energy, however, which can thus be communicated is dependent on the maximum force which can be applied to the projectile. but the maximum force which can be applied to any body depends entirely on the resistance offered by that body, and in this case the resisting force is the gravitative attraction of the earth on the projectile, which attraction is again a direct function of its mass. the greater the mass, the greater the gravitative force, and the greater the possibility of transformation. the ultimate limit of the process would be reached if the projected mass were so great as to equal half the mass of the earth. in such circumstances, the earth being assumed to be divided into two equal masses, the maximum limiting value of the gravitative attraction would clearly be attained. any increase of the one mass over the other would again lead, however, to a diminution in the attractive force and a corresponding decrease in the energy limit for transformation. the precise manner in which the operations of mechanical energy are limited by the mass will now be clear. the principle is quite general, and applicable to all moving bodies. mass is ever a direct measure of energy capacity. a graphical method of representing energy transformations of this kind, by a system of co-ordinates, would enable the reader to appreciate more fully the quantitative relations of the forms of energy involved and also their various limits. . _the simple pendulum_ the remaining operations of transformation for discussion are embodied in the following simple apparatus. a spherical metallic mass m (fig. ) is supported by a rod p which is rigidly connected to a horizontal spindle hs as shown. [illustration: fig. ] the spindle is supported and free to revolve in the bearings b{ } and b{ } which form part of the supporting framework v resting on the ground; the bearing surfaces at b{ } and b{ } are lubricated, and the mass m is free to perform, in a vertical plane, complete revolutions about the axis through the centre of the spindle. in carrying out this motion its path will be circular, as shown at dcfe; the whole arrangement is merely an adaptation of the simple pendulum. as constituted, the apparatus may form the seat of certain energy operations. some of these will only take place with the application of energy of motion to the pendulum from an external source, thereby causing it to vibrate or to rotate: others, again, might be said to be inherent to the apparatus, since they arise naturally from its construction and configuration. we shall deal with the latter first. . _statical energy conditions_ the pendulum with its spindle has a definite mass value, and, assuming it to be at rest in the bearings b{ } and b{ }, it is acted upon by gravitation, or in other words, it is under the influence or within the field of the gravitative attraction of the earth's mass upon it. the effect of this field is directly proportional to the mass of the pendulum and spindle, and to its action is due that bearing pressure which is transmitted through the lubricant to the bearing surfaces and thence to the supporting arms n{ } and n{ } of the framework. bearings and columns alike are thus subjected to a downward thrust or pressure. being of elastic material, they will be more or less distorted. this distortion will proceed until the downward forces are balanced by the upward or reactive forces called into play in virtue of the cohesive properties of the strained material. corresponding to a slight downward movement of the pendulum and spindle in thus straining or compressing them, the supporting columns will be decreased in length. this downward movement is the external evidence of certain energy operations. in virtue of their elevation above the earth's surface, the pendulum and spindle possess, to a certain degree, energy of position, and any free downward movement would lead to the transformation of this energy into energy of motion (§ ). but the downward motion of pendulum and spindle is not free. it is made against the resistance of the material of the supporting columns, and the energy of position, instead of assuming the form of energy of motion, is simply worked down or transformed against the opposing cohesive forces of the supporting materials. this energy, therefore, now resides in these materials in the form of energy of strain or distortion. in general nature, this strain energy is akin to energy of position (§ ). certain portions of the material of the columns have been forced into new positions against the internal forces of cohesion which are ever tending to preserve the original configuration of the columns. this movement of material in the field of the cohesive influence involves the transformation of energy (§ ), and the external evidence of the energy process is simply the strained or distorted condition of the material. if the latter be released, and allowed to resume its natural form once more, this stored energy of strain would be entirely given up. in reality, the material can be said to play the part of a machine or mechanism for the energy process of storage and restoration. no energy process, in fact, ever takes place unless associated with matter in some form. the supporting arms, in this case, form the material factor or agency in the energy operation. all such energy machines, also, are limited in the extent of their operation, by the qualities of the material factors. in this particular case, the energy compass of the machine is restricted by certain physical properties of the material, by the maximum value of these cohesive or elastic forces called into play in distortion. these forces are themselves the evidence of energy, of that energy by virtue of which the material possesses and maintains its coherent form. in this case this energy is also the factor controlling the transformation, and appears as a separate and distinct incepting agency. if the process is to be a reversible one, so that the energy originally stored in the material as strain energy or energy of distortion may be completely returned, the material must not be stressed beyond a certain point. only a limited amount of work can be applied to it, only a limited amount of energy can be stored in it. too much energy applied--too great a weight on the supporting columns--gives rise to permanent distortion or crushing, and an entirely new order of phenomena. this energy limit for reversibility is then imposed by the cohesive properties of the material or by its elastic limits. up to this point energy stored in the material may be returned--the process is reversible in nature--but above this elastic limit any energy applied must operate in an entirely different manner. a little consideration will show also, that the state of distortion, or energy strain, is not confined to the material of the supporting columns alone. action and reaction are equal. the same stresses are applied to the spindle through the medium of bearings and lubricant. in fact, every material substance of which the pendulum machine is built up is thus, more or less, strained against internal forces; all possess, more or less, cohesion or strain energy. it will be evident, also, that this condition is not peculiar to this or any other form of apparatus. it is the energy state or condition of every structure, either natural or artificial, which is built up of ordinary material, and which, on the earth's surface, is subjected to the influence of the gravitation field. this cohesion or strain energy is one of the forms in which energy is most widely distributed throughout material. in reviewing the statical condition of the above apparatus, the pendulum itself has been assumed to be hanging vertically at rest under the influence of gravitation. if energy be now applied to the system from some external source so that the pendulum is caused to vibrate, or to rotate about the axis of suspension, a new set of energy processes make their appearance. the movement of the pendulum mass, in its circular path around the central axis, is productive of certain energy reactions, as follows:-- _a._ a transformation of energy of motion into energy of position and vice versa. _b._ a frictional transformation at the bearing surfaces. these processes will each be in continuous operation so long as the motion of the pendulum is maintained. their general nature is quite independent of the extent of that motion, whether it be merely vibratory through a small arc, or completely rotatory about the central axis. in the articles which immediately follow, the processes will be treated separately. . _transformations of the moving pendulum--a. energy of motion to energy of position and vice versa_ in this simple transformation the motion of the pendulum about the axis of suspension may be either vibratory or circular, according to the amount of energy externally applied. in each case, every periodic movement of the apparatus illustrates the whole energy operation. the general conditions of the process are almost identical with those in the case of the upward movement of a mass against gravity (§ ). gravitation is the incepting energy influence of the operation. if the pendulum simply vibrates through a small arc, then, at the highest points of its flight, it is instantaneously at rest. its energy of motion is here, therefore, zero; its energy of position is a maximum. at the lowest point of its flight, the conditions are exactly reversed. here its energy of motion is a maximum, while its energy of position passes through a minimum value. the same general conditions hold when the pendulum performs complete revolutions about the central axis. if the energy of motion applied is just sufficient to raise it to the highest point e (fig. ), the mass will there again be instantaneously at rest with maximum energy of position. as the mass falls downwards in completing the circular movement, its energy of position once more assumes the kinetic form, and reaches its maximum value at c (fig. ), the lowest position. the moving pendulum mass, so far as its energy properties are concerned, behaves in precisely the same manner as a body vertically projected in the field of the gravitative attraction (§ ). this simple energy operation of the pendulum is perhaps one of the most familiar of energy processes. by its means, however, it is possible to illustrate certain general features of energy reactions of great importance to the author's scheme. the energy processes of the pendulum system are carried out through the medium of the material pendulum machine, and are limited, both in nature and degree, by the properties of that machine. as the pendulum vibrates, the transformation of energy of motion to energy of position or vice versa is an example of a reversible energy operation. the energy active in this operation continually alternates between two forms of energy: transformation is continually followed by a corresponding return. neglecting in the meantime all frictional and other effects, we will assume complete reversibility, or that the energy of motion of the pendulum, after passing completely into the form of energy of position at the highest point, is again completely returned, in its original form, in the descent. now, for any given pendulum, the amount of energy which can thus operate in the system depends on two factors, namely, the mass of the pendulum and the vertical height through which it rises in vibration. if the mass is fixed, then the maximum amount of energy will be operating in the reversible cycle when the pendulum is performing complete revolutions round its axis of suspension. the maximum height through which the pendulum can rise, or the maximum amount of energy of position which the system can acquire, is thus dependent on the length of the pendulum arm. these two factors, then, the mass and the length of the pendulum arm, are simply properties of this pendulum machine, properties by which its energy compass is restricted. let us now examine these limiting factors more minutely. it is obvious that energy could readily be applied to the pendulum system in such a degree as to cause it to rotate with considerable angular velocity about the axis of suspension. now the motion of the pendulum mass in the lines of the gravitation field, although productive of the same transformation process, differs from that of a body moving vertically upward in that, while the latter has a linear movement, the former is constrained into a circular path. this restraint is imposed in virtue of the cohesive properties of the material of the pendulum arm, and it is the presence of this restraining influence that really distinguishes the pendulum machine from the machine in which the moving mass is constrained by gravity alone (§ ). it has been shown that the energy capacity of a body projected vertically against gravity is limited by its mass only; the energy capacity of the pendulum machine may be likewise limited by its mass, but the additional restraining factor of cohesion also imposes another limit. in the course of rotation, energy is stored in the material of the pendulum against the internal forces of cohesion. the action is simply that of what is usually termed centrifugal force. as the velocity increases, the pendulum arm lengthens correspondingly until the elastic limit of the material in tension is reached. at this point, the pendulum may be said to have reached the maximum length at which it can operate in that reversible process of transformation in which energy of motion is converted into energy of position. the amount of energy which would now be working in that process may be termed the limiting energy for reversibility. this limiting energy is the absolute maximum amount of energy which can operate in the reversible cycle. it is coincident with the maximum length of the pendulum arm in distortion. when the stress in the material of that arm reaches the elastic limit, it is clear that the transformation against cohesion will also have attained its limiting value for reversibility. this transformation, if the velocity of the pendulum is constant, is of the nature of a storage of energy. so long as the velocity is constant the energy stored is constant. if the elastic limiting stress of the material has not been exceeded, this energy--neglecting certain minor processes (§§ , )--will be returned in its original form as the velocity decreases. if, however, the material be stressed beyond its elastic powers, the excess energy applied will simply lead to permanent distortion or disruption of the pendulum arm, and to a complete breakdown and change in the character of the machine and the associated energy processes (§ ). the physical properties of the material thus limit the energy capacity of the machine. this limiting feature, as already indicated, is not peculiar to the pendulum machine alone. every energy process embodied in a material machine is limited in a similar fashion by the peculiar properties of the acting materials. every reversible process is carried out within limits thus clearly defined. nature presents no exception to this rule, no example of a reversible energy system on which energy may be impressed in unlimited amount. on the contrary, all the evidence points to limitation of the strictest order in such processes. . _transformations of the moving pendulum--b. frictional transformation at the bearing surfaces_ the motion of the pendulum, whether it be completely rotatory or merely vibratory in nature, invariably gives rise to heating at the bearings or supporting points. since the heating effect is only evident when motion is taking place, and since the heat can only make its appearance as the result of some energy process, it would appear that this persistent heat phenomenon is the result of a transformation of the original energy of motion of the pendulum. the general energy conditions of the apparatus already adverted to (§ ) still hold, and the lubricating oil employed in the apparatus being assumed to have sufficient capillarity or adhesive power to separate the metallic surfaces of bearings and journals at all velocities, then every action of the spindle on the bearings must be transmitted through the lubricant. the latter is, therefore, strained or distorted against the internal cohesive or viscous forces of its material. the general effect of the rotatory motion of the spindle will be to produce a motion of the material of the lubricant in the field of these incepting forces. to this motion the heat transformation is primarily due. other conditions being the same, the extent of the transformation taking place, in any given case, is dependent on the physical properties of the lubricant, such as its viscosity, its cohesive or capillary power, always provided that the metallic surfaces are separated, so that the action is really carried out in the lines or field of the internal cohesive forces of the lubricant. in itself, this transformation is not a reversible process; no mechanism appears by which this heat energy evolved at the bearing surfaces could be returned once more to its original form of energy of motion. it may be, in fact, communicated by conduction to the metallic masses of the bearings, and thence, by conduction and radiation, to the air masses surrounding the apparatus. its action in these masses is dealt with below (§ ). the operation of bearing friction, though in itself not a reversible process, really forms one link of a complete chain (§ ) of secondary operations (transmissions and transformations) which together form a comprehensive and complete cyclical energy process (§ ). when no lubricant is used in the apparatus, so that the metallic surfaces of bearings and journals are in contact, the heat process is of a precisely similar nature to that described above (see also § ). distortion of the metals in contact takes place in the surface regions, so that the material is strained against its internal cohesive forces. the transformation will thus depend on the physical properties of these metals, and will be limited by these properties. different metallic or other combinations will consequently give rise to quite different results with respect to the amounts of heat energy evolved. . _stability of energy systems_ the ratio of the maximum or limiting energy for reversibility to the total energy of the system may vary in value. if the pendulum vibrates only through a very small arc, then, neglecting the minor processes (§§ , ), practically the whole energy of the system operates in the reversible transformation. this condition is maintained as the length of the arc of vibration increases, until the pendulum is just performing complete revolutions about the central axis. after this, the ratio will alter in value, because the greater part of any further increment of energy does not enter into the reversible cyclical process, but merely goes to increase the velocity of rotation and the total energy of the system. the small amount of energy which thus enters the reversible cycle as the velocity increases, does so in virtue of the increasing length of the pendulum arm in distortion. to produce even a slight distortion of the arm, a large amount of energy will require to be applied to and stored in the system, and thus, at high velocities of rotation, the energy which operates in the reversible cycle, even at its limiting value, may form only a very small proportion of the total energy of the system. at low velocities or low values of the total energy, say when the pendulum is not performing complete rotations, practically the whole energy of the system is working in the reversible cycle; but, in these circumstances, it is clear that the total energy of the system, which, in this case, is all working in the reversible process, is much less than the maximum or limiting amount of energy which might so work in that process. under these conditions, when the total energy of the system is less than the limiting value for reversibility, so that this total energy in its entirety is free to take part in the reversible process, then the energy system may be termed stable with respect to that process. stability, in an energy system, thus implies that the operation considered is not being, as it were, carried out at full energy capacity, but within certain reversible energy limits. we have emphasised this point in order to draw attention to the fact that the great reversible processes which are presented to our notice in natural phenomena are all eminently stable in character. perhaps the most striking example of a natural reversible process is found in the working of the terrestrial atmospheric machine (§§ , ). the energy in this case is limited by the mass, but in actual operation its amount is well within the maximum limiting value. the machine, in fact, is stable in nature. other natural operations, such as the orbital movements of planetary masses, (§ ) illustrate the same conditions. nature, although apparently prodigal of energy in its totality, yet rigidly defines the bounding limits of her active operations. . _the pendulum as a conservative system_ under certain conditions the reversible energy cycle produces an important effect on the rotatory motion of the pendulum. for the purpose of illustration, let it be assumed that the pendulum is an isolated and conservative system endowed with a definite amount of rotatory energy. in its circular movement, the upward motion of the pendulum mass is accompanied by a gain in its energy of position. this gain is, in the given circumstances, obtained solely at the expense of its inherent rotatory energy, which, accordingly, suffers a corresponding decrease. the manifestation of this decrease will be simply a retardation of the pendulum's rotatory motion. its angular velocity will, therefore, decrease until the highest altitude e (fig. ) is attained. after this, on the downward path, the process will be reversed. acceleration will take place from the highest to the lowest point of flight, and the energy stored as energy of position will be completely returned in its original form of energy of motion. the effect of the working of the reversible cycle, then, on the rotatory system, under the given conditions, is simply to produce alternately a retardation and a corresponding acceleration. now, it is to be particularly noted that these changes in the velocity of the system are produced, not by any abstraction from or return of energy to the system, which is itself conservative, but simply in consequence of the transformation and re-transformation of a certain portion of its inherent rotatory energy in the working of a reversible process embodied in the system. the same features may be observed in other systems where the conditions are somewhat similar. in the natural world, we find processes of the same general nature in constant operation. when any mass of material is elevated from the surface of a rotating planetary body against the gravitative attraction, it thereby gains energy of position (§ ). this energy, on the body's return to the surface in the course of its cycle, reappears in the form of energy of motion. now the material mass, in rising from the planetary surface, is not, in reality, separated from the planet. the atmosphere of the planet forms an integral portion of its material, partakes of its rotatory motion, and is bound to the solid core by the mutual gravitative forces. any mass, then, on the solid surface of a planet is, in reality, in the planetary interior, and the rising of such a mass from that surface does not imply any actual separative process, but simply the radial movement, or displacement of a portion of the planetary material from the central axis. if the energy expended in the upraisal of the mass is derived at the expense of the inherent rotatory energy of the planet, as it would be if the latter were a strictly conservative energy system, then the raising of this portion of planetary material from the surface would have a retarding effect on the planetary motion of rotation. but if, on the other hand, the energy of such a mass as it fell towards the planetary surface were converted once more into its original form of energy of axial motion, exactly equivalent in amount to its energy of position, it is evident that the process would be productive of an accelerating effect on the planetary motion of rotation, which would in magnitude exactly balance the previous retardation. in such a process it is evident that energy neither enters nor leaves the planet. it simply works in an energy machine embodied in planetary material. this point will be more fully illustrated later. the reader will readily see the resemblance of a system of this nature to that which has already been illustrated by the rotating pendulum. in the meantime, it may be pointed out that matter displaced from the planetary surface need not necessarily be matter in the solid form. all the operations mentioned above could be quite readily--in fact, more readily--carried out by the movements of gaseous material, which is admirably adapted for every kind of rising, falling, or flowing motion relative to the planetary surface (§ ). . _some phenomena of transmission processes--transmission of heat energy by solid material_ the pendulum machine described above furnishes certain outstanding examples of the operation of energy transformation. it will be noted, however, that it also portrays certain processes of energy transmission. in this respect it is not peculiar. most of the material machines in which energy operates will furnish examples of both energy transmissions and energy transformations. in some instances, the predominant operation seems to be transformation, in others, transmission; and the machines may be classified accordingly. it is, however, largely a matter of terminology, since both operations are usually found closely associated in one and the same machine. the apparatus now to be considered is designed primarily to illustrate the operative features of certain energy transmissions, but the description of the machines with their allied phenomena will show that energy transformations also play a very important part in their constitution and working. a cylindrical metallic bar about twelve inches long, say, and one inch in diameter, is placed with its ends immersed in water in two separate vessels, a and b, somewhat as shown. [illustration: fig. ] by the application of heat energy, the temperature of the water in the vessel a is raised to a point say ° f. above that of b, and steadily maintained at that point. it is assumed that b is also kept at the constant lower temperature. in these circumstances, a transmission of heat energy takes place from a to b through the metallic bar. when the steady temperature condition is reached, the transmission will be continuous and uniform; the rate at which it is carried out will be determined by the length of the bar, by the material of which it is composed, and by the temperature difference maintained between its ends. now what has really happened is that by a combination of phenomena the bar has been converted into a machine for the transmission of heat energy. a full description of these phenomena is, in reality, the description of this machine, and vice versa. let us, therefore, now try to outline some of these phenomena. the first feature of note is the gradient of temperature which exists between the ends of the bar. further research is necessary regarding the real nature of this gradient--it appears to differ greatly in different materials--but the existence of such a gradient is one of the main features of the energy machine, one of the essential conditions of the transmission process. another feature is that of the expansive motion of the bar itself. the expansion of the bar due to the heating varies in value along its length, from a maximum at the hot end to a minimum at the cool end. the expansion, also, is the evidence of a transformation of energy. the bar has been constrained into its new form against the action of the internal molecular or cohesive forces of its material (§ ). the energy employed and transformed in producing the expansion is a part of the original heat energy applied to the bar, and before any transmission of this heat energy takes place between its extreme ends, a definite modicum of the applied energy has to be completely transformed for the sole purpose of producing this distortive movement or expansion against cohesion. this preliminary straining of the bar is, in fact, a part of the process of building up or constituting the energy transmission machine, and must be completely carried out before any transmission can take place. it is clear, then, that concurrent with the gradient of temperature, there also exists, along the bar, what might be termed a gradient of energy stored against cohesion, and that both are characteristic and essential features of this particular energy machine. a point of some importance to note is the permanency of these features. once the machine has been constituted with a constant temperature difference, the transmission of energy will take place continuously and at a uniform rate. but no further transformation against cohesion takes place; no further expenditure of energy against the internal forces of the material is necessary. neglecting certain losses due to possible external conditions, the whole energy applied to the machine at the one end is transmitted in its entirety to the other, without influencing in any way either the temperature or the energy gradient. such is the general constitution of this machine for energy transmission. its material foundation is, indeed, the metallic bar, but the temperature and energy gradients may be termed the true determining factors of its operation. as already indicated, the magnitude of the transformation is dependent on the temperature difference between the ends of the bar. but this applies only within certain limits. with respect to the cool end, the temperature may be as low as we please--so far as we know, the limit is absolute zero of temperature; but with the hot end, the case is entirely different, because here the limit is very strictly imposed by the melting-point of the material of the bar. when this melting temperature is attained, the melting of the bar indicates, simply, that the heat energy stored or transformed against the cohesive forces of the material has reached its limiting value; change of state of the material is taking place, and the machine is thereby being destroyed. it is evident, then, that the energy which is actually being transmitted has itself no effect whatever in restricting the action or scope of the transmission machine. it is, in reality, the residual energy stored against the cohesive forces which imposes the limits on the working. it is the maximum energy which can be transformed in the field of the cohesive forces of the material which determines the power of that material as a transmitting agent. this maximum will, of course, be different for different materials according to their physical constitution. it is attained in this machine in each case when melting of the bar takes place. . _some phenomena of transmission processes--transmission by flexible band or cord_ this method is often adopted when energy of motion, or mechanical energy, is required to be transmitted from one point to another. for illustration, consider the case of two parallel spindles or shafts, a and b (fig. ), each having a pulley securely keyed upon it. spindle a is connected to a source of of mechanical energy, and it is desired to transmit this energy across the intervening space to spindle b. [illustration: fig. ] this, of course, might be accomplished in various ways, but one of the most simple, and, at the same time, one of the most efficient, is the direct drive by means of a flexible band or cord. the band is placed tightly round, and adheres closely to both pulleys; the coefficient of friction between band and pulleys may, in the first instance, be assumed to be sufficiently great to prevent slipping of the band up to the highest stress which it is capable of sustaining in normal working. connected in this fashion, the spindles will rotate in unison, and mechanical energy, if applied at a, may be directly transmitted to b. the material operator in the transmission is the connecting flexible band, and associated with this material are certain energy processes which are also essential features of the energy machine. when transmission of energy is taking place, a definite tension or stress exists in the connecting band, and neglecting certain inevitable losses due to bearing friction (§ ) and windage (§ ), practically the whole of the mechanical or work energy communicated to the one spindle is transmitted to the other. now the true method of studying this or any energy process is simply to describe the constitution and principal features of the machine by which it is carried out. these are found in the phenomena of transmission. one of the most important is the peculiar state of strain or tension existing in the connecting band. this, as already indicated, is an absolutely essential condition of the whole operation. no transmission is possible without some stress or pull in the band. this pull is exerted against the cohesive forces of the material of the band, so that before transmission takes place it is distorted and a definite amount of the originally applied work energy is expended in straining it against these forces. this energy is accordingly stored in the form of strain energy or energy of separation (§ ), and, if the velocity is uniform, the magnitude of the transmission is proportional to this pull in the band, or to the quantity of energy thus stored against the internal forces of its material. but, in every case, a limit to this amount of energy is clearly imposed by the strength of the band. the latter must not be strained beyond its limiting elastic stress. so long as energy is being transmitted, a certain transformation and return of energy of strain or separation is taking place in virtue of the differing values of the tensions in the two sides of the band; and if the latter were stressed beyond the elastic limit, permanent distortion or disruption of the material would take place. under such conditions, the reversible energy process, involving storage and restoration of strain energy as the band passes round the pulleys, would be impossible, and the energy transmission machine would be completely disorganised. the magnitude of the energy operation is thus limited by the physical properties of the connecting band. another important feature of this energy transmission machine is the velocity, or rather the kinetic energy, of the band. the magnitude of the transmission process is directly proportional to this velocity, and is, therefore, also a function of the kinetic energy. at any given rate of transmission, this kinetic energy, like the energy stored against the cohesive influence, will be constant in amount, and like that energy also, will have been obtained at the expense of the originally applied energy. this kinetic energy is an important feature in the constitution of the transmission machine. as in the case of the strain energy, its maximum value is strictly limited, and thus imposes a limit on the general operation of the machine. for, at very high velocities, owing to the action of centrifugal force, it is not possible to keep the band in close contact with the surface of the pulleys. when the speed rises above a certain limit, although the energy actually being transmitted may not have attained the maximum value possible at lower speeds with greater tension in the band, the latter will, in virtue of the strain imposed by centrifugal action, be forced radially outwards from the pulley. the coefficient of friction will be thereby reduced; slipping will ensue, and the transmission may cease either in whole or in part. in this way the velocity or kinetic energy limit is imposed. the machine for energy transmission may thus be limited in its operation by two different factors. the precise way in which the limit will be applied in any given case will, of course, depend on the circumstances of working. . _some phenomena of transmission processes--transmission of energy to air masses_ the movement of the pendulum (§ ) is accompanied by a certain transmission of energy to the surrounding medium. when this medium is a gaseous one such as air, the amount of energy thus transmitted is relatively small. the process, however, has a real existence. to illustrate its general nature, let it be assumed that the motion of the pendulum is carried out, not in air, but in a highly viscous fluid, say a heavy oil. obviously, a pendulum falling from its highest position to its lowest, in such a medium would transmit its energy almost in its entirety to the medium, and would reach its lowest position almost devoid of energy of motion. the energy of position with which it was originally endowed would thus be transformed and transmitted to the surrounding medium. the agent by which the transmission is carried out is the moving material of the pendulum, which, as it passes through the fluid, distorts that fluid in the lines or field of its internal cohesive or viscous forces which offer a continuous resistance to the motion. as the pendulum passes down through the liquid, the succeeding layers of the latter are thus alternately distorted and released. the distortive movement takes place in virtue of the communication of energy from the moving pendulum to the liquid, and during the movement energy is stored in the fluid as energy of strain and as kinetic energy. at the same time, a transformation of the applied energy into heat takes place in the distorted material. the release of this material from strain, and its movement back towards its original state, is also accompanied by a similar transformation, in which the stored strain energy is, in turn, converted into the heat form. the whole operation is similar in nature to that frictional process already described (§ ) in the case of a body moving on a rough horizontal table. the final action of the heat energy thus communicated to the fluid is to expand the latter against the internal cohesive or viscous forces of its material, and also against the gravitative attraction of the earth. now when the pendulum moves in air, the action taking place is of the same nature, and the final result is the same as in oil. it differs merely in degree. compared with the oil, the air masses offer only a slight resistance to the motion, and thus only an exceedingly small part of the pendulum's energy is transmitted to them. the pendulum, however, does set the surrounding air masses in motion, and by a process similar in nature to that in the oil, a modicum of the energy of the falling pendulum is converted into heat, and thence by the expansion of the air into energy of position. in the downward motion from rest, the first stage of the process is a transformation peculiar to the pendulum itself, namely, energy of position into energy of motion. the transmission to the fluid is a necessary secondary result. it is important to note that this transmission is carried out in virtue of the actual movement of the material of the pendulum, and that the energy transmitted is in reality mechanical or work energy (§ ). this mechanical or work energy, then actually leaves or is transmitted from the pendulum system, and is finally absorbed by the surrounding air masses in the form of energy of position. considered as a whole, there is evidently no aspect of reversibility about the operation, but it will be shown later (§ ) that with the introduction of other factors, it really forms part of a comprehensive cyclical process. it is itself a process of direct transmission. it is carried out by means of a definite material machine which embodies certain energy transformations, and which is strictly limited in the extent of its operations by certain physical factors. these factors are the cohesive properties of the moving pendulum mass and the fluid with which it is in contact (§ ). it is clear, also, that in an apparatus in which the motion is carried out in oil, any heat energy communicated to the oil would inevitably find its way to the surrounding air masses by conduction and radiation. the final result of the pendulum's motion would therefore be the same in this case as in air; the heat energy would, when communicated to the surrounding air masses, cause an expansive movement against gravity. . _energy machines and energy transmission_ [illustration: fig. ] the various examples of energy transformation and transmission which have been discussed above (§§ - ) will suffice to show the essential differences which exist in the general nature of these operations. but they will also serve another purpose in portraying one striking and important aspect in which these processes are alike. from the descriptions given above, it will be amply evident that each of these processes, whether transformation or transmission, requires as an essential condition of its existence, the presence of a certain arrangement of matter; each process is of necessity associated with and embodied in a definite physical and material machine. this material machine is simply the contrivance provided by nature to carry out the energy operation. it differs in construction and in character for different processes, but in every case there must be in its constitution some material substance, perceptible to the senses, with which the acting energy is intimately associated. this fact is but another aspect of the principle that energy is never found dissociated from matter (§ ). in every energy machine, the material substance or operator forms the real foundation or basis of the energy operation, but besides this there are also always other phenomena of a secondary nature, totally different, it may be, from the main energy operation, which combine with that operation to constitute the whole. these subsidiary energy phenomena are the incepting factors, and are most important characteristics. their presence is just as essential in energy transmission as it is in energy transformation. as demonstrated above, they are usually associated with the physical peculiarities of the basis or acting material of the energy machine, and their peculiar function is to conserve or limit the extent of its action. a complete description of these phenomena, in any given case, would not only be equivalent to a complete description of the machine, but would also serve as a complete description of the main energy operation embodied in that machine. sometimes, however, the description of the machine is a matter of extreme difficulty, and may be, in fact, impossible owing to the lack of a full knowledge of the intimate phenomena concerned. an illustrative example of this is provided by the familiar phenomenon of heat radiation. take the case of two isolated solid bodies a and b (fig. ) in close proximity on the earth's surface. if the body a at a high temperature be sufficiently near to b at a lower temperature, a transmission of energy takes place from a to b. this transmission is usually attributed to "radiation," but, after all, the use of the term "radiation" is merely a descriptive device which hides our ignorance of the operation. it is known that a transmission takes place, but the intimate phenomena are not known, and, accordingly, it is impossible to describe the machine or mechanism by which it is carried out. from general considerations, however, it appears that the material basis of this machine is to be found in the air medium which surrounds the two bodies. experiment shows, indeed, that if this intervening material medium of air be even partially withdrawn or removed, the transmission is immensely reduced in amount. in fact, this latter phenomenon is largely taken advantage of in the so-called vacuum flasks or other devices to maintain bodies at a temperature either above or below that of the external surrounding bodies. the device adopted is, simply, as far as practicable to withdraw all material connection between the body which it is desired to isolate thermally and its surroundings. but it is clearly impossible to isolate completely any terrestrial body in this way. there must be some material connection remaining. as already pointed out (§ ), we have no experimental experience of really separate bodies or of an absolute vacuum. it is to be noted that any vacuous space which we can experimentally arrange does not even approximately reproduce the conditions of true separation prevailing in interplanetary space. any arrangement of separate bodies which might thus be contrived is necessarily entirely surrounded or enclosed by terrestrial material which, in virtue of its stressed condition, constitutes an energy machine of the same nature as those already described (§ ). even although the air could be absolutely exhausted from a vessel, it is still quite impossible to enclose any body permanently within that vessel without some material connection between the body and the enclosing walls. if for example, as shown in fig. , cc represents a spherical vessel, completely exhausted, and having two bodies, a and b at different temperatures, in its interior, it is obvious that if these bodies are to maintain continuously their relative positions of separation, each must be united by some material connection to the containing vessel. but when such a connection is made, say as shown at d and e (fig. ), it is clear that a and b are no longer separate bodies in the fullest sense of the word, but are now in direct communication with one another through the supports at d and e and the enclosing sides of the vessel cc. the practicable conditions are thus far from those of separate bodies in a complete vacuum. it would seem, indeed, to be beyond human experimental contrivance to reproduce such conditions in their entirety. so far as these conditions can be achieved, however, and judging solely by the experimental results already attained with respect to the effect of exhaustion on radiation, it may be quite justly averred that, if the conditions portrayed in fig. could be realised, no transmission of energy would take place between two bodies, such as a and b, completely isolated from one another in a vacuous space. it appears, in fact, to be a quite reasonable and logical deduction from the experimental evidence that the energy operation of transmission of heat from one body to another by radiation is dependent on the existence between these bodies of a real and material substance which forms in some way (at present unknown) the transmitting medium or machine. the difficulty which arises in the description of this machine is due, as already explained above, simply to lack of knowledge of the intimate phenomena of its working. many other energy processes will, no doubt, occur to the reader in which the same difficulty presents itself, due to the same cause. [illustration: fig. ] in dealing with terrestrial operations generally, and particularly when transmission processes are under consideration, it is important to recognise clearly the precise nature of these operations and the peculiar conditions under which they work. it must ever be borne in mind that the terrestrial atmosphere is a real and material portion of the earth's mass, extending from the surface for a limited distance into space (§ ), and whatever its condition of gaseous tenuity, completely occupying that space in the manner peculiar to a gaseous substance. when the whole mass of the planet, including the atmosphere, is taken into consideration, it is readily seen that all energy operations embodied in or associated with material on what is usually termed the surface of the earth take place at the bottom of this atmospheric ocean, or, in reality, in the interior of the earth. the operations themselves are the manifestations of purely terrestrial energy, which, by its working in various devices or arrangements of material is being transformed and transmitted from one form of matter to another. as will be fully demonstrated later (part iii.), the nature of the terrestrial energy system makes it impossible for this energy ever to escape beyond the confines of the planetary atmospheric envelope. these are briefly the general conditions under which the study of terrestrial or secondary energy operations is of necessity conducted, and it is specially important to notice these conditions when it is sought to apply the results of experimental work to the discussion of celestial phenomena. it must ever be borne in mind that even the direct observation of the latter must always be carried out through the encircling planetary atmospheric material. [illustration: fig. ] in this portion of the work it is proposed to investigate in the light of known phenomena the possibility of energy transmission between separate masses. as explained above, the term separate is here meant to convey the idea of perfect isolation, and the only masses in nature which truly satisfy this condition are the celestial and planetary bodies, separated as they are from one another by interplanetary space and in virtue of their energised condition (§ ). since this state of separation cannot be experimentally realised under terrestrial conditions, it is obvious, therefore, that no purely terrestrial energy process can be advanced either as direct verification or direct disproof of a transmission of energy between such truly separate masses as the celestial bodies. but as we are unable to experiment directly on these bodies themselves or across interplanetary space, we are forced of necessity to rely, for experimental facts and conclusions, on the terrestrial energy phenomena to which access is possible. as already indicated in the general statement (§ ), the same energy is bestowed on all parts of the cosmical system, and by the close observation of the phenomena of its action in familiar operations the truest guidance may be obtained as to its general nature and working. in such investigations, however, only the actual phenomena of the operation are of scientific or informative value. there is no gain to real knowledge in assuming, say in the examination of the phenomena of magnetic attraction between two bodies, that the one is urged towards the other by stresses in an intervening ethereal medium, when absolutely no phenomenal evidence of the existence of such a medium is available. it may be urged that the conception of an ethereal medium is adapted to the explanation of phenomena, and appears in many instances to fulfil this function. but as already pointed out (see introduction), it is absolutely impossible to explain phenomena. so-called explanations must ever resolve themselves simply into revelations of further phenomena. while the value of true working hypotheses cannot be denied, it is surely evident that such hypotheses, unless they embody and are under the limitation of controlling facts, are not only useless, but, from the misleading ideas they are apt to convey, may even be dangerous factors in the search for truth. now, if all speculative ideas or hypotheses are banished from the mind, and reliance is placed solely on the evidential phenomena of nature, the study of terrestrial energy operations leads inevitably to certain conclusions on the question of energy transmission. in the first place, it must lead to the denial of what has been virtually the great primary assumption of modern science, namely, that a mass of material at a high temperature isolated in interplanetary space would radiate heat in all directions through that space. such a conception is unsupported by our experimental or real knowledge of radiation. the fact that heat radiation takes place from a hot to a cold body in whatever direction the latter is placed relatively to the former, does not justify the assumption that such radiation takes place in all directions in the absence of a cold body. and since there is absolutely no manifestation of any real material medium occupying interplanetary space, no sign of the material agency or machine which the results of direct experiment have led us to conclude is a necessity for the transmission process of heat radiation, the whole conception must be regarded as at least doubtful. even with our limited knowledge of radiation, the doctrine of heat radiation through space stands controverted by ordinary experimental experience. with this doctrine must fall also the allied conception of the transmission of heat energy by radiation from the sun to the earth. it is to be noted, however, that only the actual transmission of heat energy from the sun to the earth is inadmissible; the _heating effect_ of the sun on the earth, which leads to the manifestation of terrestrial energy in the heat form, is a continuous operation readily explained in the light of the general principle of energy transformation already enunciated (§ ). with respect to other possible processes of energy transmission between the sun and the earth or across interplanetary space, the same general methods of experimental investigation must be adopted. the transmission of energy under terrestrial conditions is carried out in many different forms and by the working of a large variety of machines. in every case, no matter in what form the energy is transmitted, that energy must be associated with a definite arrangement of terrestrial material constituting the transmission machine. each energy process of transmission has its own peculiar conditions of operation which must be completely satisfied. by the study of these conditions and the allied phenomena it is possible to gain a real knowledge of the precise circumstances in which the process can be carried out. now let us apply the knowledge of transmission processes thus gained to the general celestial case, to the question of energy transmission between truly separate bodies, and particularly to the case of the sun and the earth. do we find in this case any evidence of the presence of a machine for energy transmission? it is impossible, within the limits of this work, to deal with all the forms in which energy may be transmitted, but let the reader review any instance of the transmission of energy under terrestrial conditions, or any energy-transmission machine with which he is familiar, noting particularly the essential phenomena and material arrangements, and let him ask himself if there is any evidence of the existence of a machine of this kind in operation between the sun and the earth or across interplanetary space. we venture to assert that the answer must be in the negative. the real knowledge of terrestrial processes of energy transmission at command, on which all our deductions must be based, does not warrant in the slightest degree the assumption of transmission between the sun and the earth. the most plausible of such assumptions is undoubtedly that which attributes transmission to heat radiation, but this has already been shown to be at variance with well-known facts. the question of light transmission will offer no difficulty if it be borne in mind that light is not in itself a form of energy, but merely a manifestation of energy as an incepting influence, which like other incepting influences of a similar nature, can readily operate across either vacuous or interplanetary space (§ ). on these general considerations, deduced from the observation of terrestrial phenomena, allied with the conception of energy machines and separate masses in space, the author bases one aspect of the denial of energy transmission between celestial masses. the doctrine of transmission cannot be sustained in the face of legitimate scientific deduction from natural phenomena. in the later parts of this work, and from a more positive point of view, the denial is completely justified. . _identification of forms of energy_ before leaving the question of energy transmission, there are still one or two interesting features to be considered. although energy, as already pointed out, is ever found associated with matter, this association does not, in itself, always furnish phenomena sufficient to distinguish the precise phase in which the energy may be manifested. some means must, as a rule, be adopted to isolate and identify the various forms. now one of the most interesting and important features of the process of energy transmission is that it usually provides the direct means for the identification of the acting energy. energy, as it were, in movement, in the process of transmission, is capable of being detected in its different phases and recognised in each. the phenomena of transmission usually serve, either directly or indirectly, to portray the precise nature of the energy taking part in the operation. one of the most direct instances of this is provided by the transmission of heat energy. for illustrative purposes, let it be assumed that a body a, possessed of heat energy to an exceedingly high degree, is isolated within a spherical glass vessel cc, somewhat as already shown (fig. ). if it be assumed that the space within cc is a perfect vacuum, and that no material connection exists between the walls of the vessel and the body a, the latter is completely isolated, and no means whatever are available for the detection of its heat qualities (§ ). it may seem that, if the temperature of the body a were sufficiently high, its energy state might be detected, and in a manner estimated, by its effect on the eye or by its luminous properties, but we take this opportunity of pointing out that luminosity is not invariably associated with high temperature. on the contrary, many bodies are found in nature, both animate and inanimate, which are luminous and affect the eye at comparatively low temperatures. how then is the energy condition of the body to be definitely ascertained? the only means whereby it is possible to identify the energy of the body is by transmitting a portion of that energy to some other body and observing the resultant phenomena. suppose, then, another body, such as b (fig. ), at a lower temperature than a, is brought into contact with a, so that a transmission of heat energy ensues between the two. the phenomena which would result in such circumstances will be exactly as already described in the case of the transmission of energy through a solid (§ ). amongst other manifestations it would be noticeable that the material of b was expanded against its inherent cohesive forces. now if, instead of a spherical body such as b, a mercurial thermometer were utilised, the phenomena would be of precisely the same nature. a definite portion of the heat energy would be transmitted to the thermometer, and would produce expansion of the contained fluid. by the amount of this expansion it becomes possible to estimate the energy condition and properties of the body a, relative to its surroundings or to certain generally accepted standard conditions. thermometric measurement is, in fact, merely the employment of a process of energy transmission for the purpose of identifying and estimating the heat-energy properties of material substances. in everyday life, rough ideas of heat energy are constantly being obtained by the aid of the senses. this method is, however, only another aspect of transmission, for it will be clear that the sensations of heat and cold are, in themselves, but the evidence of the transformation of heat energy to or from the body. the process of energy transmission by a flexible band or cord (§ ) also provides evidence leading to the identification of the peculiar form of energy which is being transmitted. at first sight, it would appear as if this energy were simply energy of motion or kinetic energy. a little reflection, however, on the general conditions of the process must dispel this idea, for it is clear that the precise nature of the energy transmitted has no real connection with the kinetic properties of the system. the latter, truly, influence the rate of transmission and impose certain limits, but evidently, if the pull in the band increases without any increase in its velocity, the actual amount of energy transmitted by the system would increase without altering in any respect the kinetic properties. it becomes necessary, then, to distinguish clearly the energy inherent to, or as it were, latent in the system, from the energy actually transmitted by the system, to recognise the difference between the energy transmitted by moving material and the energy of that material. in this special instance, to identify the form of energy transmitted it must of necessity be associated with the peculiar phenomena of transmission. now the energy is evidently transmitted by the movement of the connecting belt or band. before any transmission can take place, however, a certain amount of energy must be stored in the moving system, partly as cohesion or strain energy and partly as energy of motion or kinetic energy. it is this preliminary storage of energy which, in reality, constitutes the transmission machine, and for a given rate of transmission, the energy thus stored will be constant in value. it is obtained at the expense of the applied energy, and, neglecting certain minor processes, will be returned (or transmitted) in its entirety when the moving system once more comes to rest. this stored energy, in fact, works in a reversible process. but when the transmission machine is once constituted, the energy transmitted is then that energy which is being continually applied at the spindle a (fig. ) and as continually withdrawn at the spindle b. it must be emphasised that the energy thus transmitted is absolutely different from the kinetic or other energy associated with the moving material of the system. it is the function of this energised material of the band to transmit the energy from a to b, but this is the only relationship which the transmitted energy bears to the material. the energy thus transmitted by the moving material we term mechanical or work energy. we may thus define mechanical or work energy as "_that form of energy transmitted by matter in motion_." the idea of work is usually associated with that of a force acting through a certain distance. the form of energy referred to above as work energy is, in the same way, always associated with the idea of a thrust or of a pressure of some kind acting on moving material. work energy thus bears two aspects, which really correspond to the familiar product of pressure and volume. both aspects are manifested in transmission. since work energy is invariably transmitted by matter in motion, every machine for its transmission must possess energy of motion as one of its essential features. as shown above (see also § ), this energy of motion is really obtained at the expense of the originally applied work energy, and as it remains unaltered in value during the progress of a uniform transmission, it may be regarded as simply transformed work energy, stored or latent in the system, which will be returned in its entirety and in its original form at the termination of the operation. the energy stored against cohesion or other forces may be regarded in the same way. it is really the manifestation of the pressure or thrust aspect of the work energy, just as the kinetic energy is the manifestation of the translational or velocity aspect. our definition of work energy given above enables us to recognise its operation in many familiar processes. take the case of a gas at high pressure confined in a cylinder behind a movable piston. we can at once say that the energy of the gas is work energy because this energy may quite clearly be transmitted from the gas by the movement of the piston. if the latter form part of a steam-engine mechanism of rods and crank, the energy may, by the motion of this mechanism, be transmitted to the crank shaft, and there utilised. this is eminently a case in which energy is _transmitted_ by matter in motion. the moving material comprises the piston, piston-rod, and connecting-rod, which are, one and all, endowed with both cohesive and kinetic energy qualities, and form together the transmission machine. so long as the piston is at rest only one aspect of the work energy of the gas is apparent, namely, the pressure aspect, but immediately motion and transmission take place, both aspects are presented. the work energy of the gas, obtained in the boiler by a _transformation_ of heat energy is thus, by matter in motion, transmitted and made available at the crank shaft. the shaft itself is also commonly utilised for the further transmission of the work energy applied. by the application of the energy at the crank, it is thrown into a state of strain, and at the same time is endowed with kinetic energy of rotation. it thus forms a machine for transmission, and the work energy applied at one point of the shaft may be withdrawn at another point more remote. the transmission is, in reality, effected by the movement of the material of the shaft. so long as the shaft is stationary, it is clear that no actual transmission can be carried out, no matter how great may be the strain imposed. if our engine mechanism were, by a change in design, adapted to the use of a liquid substance as the working material instead of a gas, it is clear that no change would be effected in the general conditions. the energy of a liquid under pressure is again simply work energy, and it would be transmitted by the moving mechanism in precisely the same manner. from the foregoing, it will now be evident to the reader that the energy originally applied to the primary mass (§ ) of our cosmical system must be work energy. it is this form of energy also which is inherent to each unit of the planetary system associated with the primary. in this system it is of course presented outwardly in the two phases of kinetic energy and energy of strain or distortion. it is apparent, also, that work energy could be transmitted from the primary mass to the separate planets on one condition only, that is, by the movement of some material substance connecting each planet to the primary. since no such material connection is admitted, the transmission of work energy is clearly impossible. . _complete secondary cyclical operation_ a general outline of the conditions of working and the relationships of secondary processes has already been given in the general statement (§ ), but it still remains to indicate, in a broad way, the general methods whereby these operations are linked to the atmospheric machine. in the example of the simple pendulum, it has been pointed out that the energy processes giving rise to heating at the bearing surfaces and transmission of energy to the air masses are not directly reversible processes, but really form part of a more extensive cyclical operation, in itself, however, complete and self-contained. this cyclical operation may be regarded as a typical illustration of the manner in which separate processes of energy transmission or transformation, such as already described, are combined or united in a continuous chain forming a complete whole. it has been assumed, in all the experiments with the pendulum, that the operating energy is initially communicated from an outside source, say the hand of the observer. this energy is, therefore, the acting energy which must be traced through all its various phases from its origin to its final destination. at the outset, it may be pointed out that this energy, applied by hand, is obtained from the original rotational energy of the earth by certain definite energy processes. due to the influences of various incepting fields which emanate from the sun (§§ - ), a portion of the earth's rotational energy is transformed into that form of plant energy which is stored in plant tissue, and which, by the physico-chemical processes of digestion, is in turn converted into heat and the various other forms of energy associated with the human frame. this, then, is the origin of the energy communicated to the pendulum. its progress through that machine has already been described in detail (§§ - ). the transformation of energy of motion to energy of position which takes place is in itself a reversible process and may in the meantime be neglected. but the final result of the operations, at the bearing surfaces and in the air masses surrounding the moving pendulum, was shown to be, in each case, that heat energy was communicated to these air masses. the effect of the heat energy thus impressed, is to cause the expansion of the air and its elevation from the surface of the earth in the lines or field of the gravitative attraction, so that this heat energy is transformed, and resides in the air masses as energy of position. the energy then, originally drawn from the rotational energy of the earth, has thus worked through the pendulum machine, and is now stored in the air masses in this form of energy of position. to make the process complete and cyclical this energy must now, therefore, be returned once more to the earth in its original rotational form. this final step is carried out in the atmospheric machine (§ ). in this machine, therefore, the energy of position possessed by the air masses is, in their descent to their original positions at lower levels, transformed once more into axial or rotational energy. in this fashion this series of secondary processes, involving both transformations and transmissions, is linked to the great atmospheric process. the amount of energy which operates through the particular chain of processes we have described is, of course, exceedingly small, but in this or a similar manner all secondary operations, great or small, are associated with the atmospheric machine. instances could readily be multiplied, but a little reflection will show how almost every energy operation, no matter what may be its nature, whether physical, chemical, or electrical, leads inevitably to the communication of energy to the atmospheric air masses and to their consequent upraisal. it is interesting to note the infallible tendency of energy to revert to its original form of axial energy, or energy of rotation, by means of the air machine. all nature bears witness to this tendency, and although the path of energy through the maze of terrestrial transformation often appears tortuous and uncertain, its final destination is always sure. the secondary operations are thus interlinked into one great whole by their association in the terrestrial energy cycle. many of these secondary operations are of short duration; others extend over long periods of time. energy, in some cases, appears to slumber, as in the coal seams of the earth, until an appropriate stimulus is applied, when it enters into active operation once more. the cyclical operations are thus long or short according to the duration of their constituent secondary energy processes. but the balance of nature is ever preserved. axial energy, transformed by the working of one cyclical process, is being as continuously returned by the simultaneous operation of others. part iii terrestrial conditions . _gaseous expansion_ before proceeding to the general description of the atmospheric machine (§ ), it is desirable to consider one or two features of gaseous reaction which have a somewhat important bearing on its working. let it be assumed that a mass of gaseous material is confined within the lower portion of a narrow tube abcd (fig. ) assumed to be thermally non-conducting; the upper portion of the tube is in free communication with the atmosphere. the gas within the tube is assumed to be isolated from the atmosphere by a movable piston ef, free to move vertically in the tube, and for the purpose of illustration, assumed also frictionless and weightless. with these assumptions, the pressure on the confined gas will simply be that due to the atmosphere. if heat energy be now applied to the gas, its temperature will rise and expansion will ensue. this expansion will be carried out at constant atmospheric pressure; the gaseous material, as it expands, must lift with it the whole of the superimposed atmospheric column against the downward attractive force of the earth's gravitation on that column. work is thus done by the expanding gas, and in consequence of this work done, a definite quantity of atmospheric material gains energy of position or potential energy relative to the earth's surface. at the same time, the rise of temperature of the gas will indicate an accession of heat energy to its mass. these familiar phenomena of expansion under constant pressure serve to illustrate the important fact that, when heat energy is applied to a gaseous mass, it really manifests itself therein in two aspects, namely, heat energy and work energy. the increment of heat energy is indicated by the increase in temperature, the increment of work energy by the increase in pressure. in the example just quoted, however, there is no increase in pressure, because the work energy, as rapidly as it is applied to the gas, is transformed or worked down in displacing the atmospheric column resting on the upper side of the moving piston. the energy applied, in the form of heat from the outside source, has in reality been introduced into a definite energy machine, a machine in this case adapted for the complete transformation of work energy into energy of position. as already indicated, when the expansive movement is completed, the volume and temperature of the gaseous mass are both increased but the pressure remains unaltered. while the increase in temperature is the measure and index of a definite increase in the heat energy of the gas, it is important to note that, so far as its work energy is concerned, the gas is finally in precisely the same condition as at the commencement of the operation. work energy has been, by the working of this energy machine, as it were passed through the gaseous mass into the surrounding atmosphere. the pressure of the gas is the true index of its work energy properties. so long as the pressure remains unaltered, the inherent work energy of the material remains absolutely unaffected. a brief consideration of the nature of work energy as already portrayed (§ ) will make this clear. work energy has been defined as "_that form of energy transmitted by matter in motion_," and it is clear that pressure is the essential factor in any transmission of this nature. temperature has no direct bearing on it whatever. it is common knowledge, however, that in the application of heat energy to a gaseous substance, the two aspects of pressure and temperature cannot be really dissociated. they are mutually dependent. any increment of heat energy to the gas is accompanied by an increment of work energy, and vice versa. the precise mode of action of the work energy will, of course, depend on the general circumstances of the energy machine in which it operates. in the case just considered the work energy does not finally reside in the gaseous mass itself, but, by the working of the machine, is communicated to the atmosphere. if, on the other hand, heat energy were applied in the same fashion to a mass of gas in a completely enclosed vessel, that is to say at constant volume instead of at constant pressure, the general phenomena are merely altered in degree according to the change in the precise nature of the energy machine. in the former case, the nature of the energy machine was such that the work energy communicated was expended in its entirety against gravitation. under what is usually termed constant volume conditions, only a portion of the total work energy communicated is transformed, and the transformation of this portion is carried out, not against gravitation, but against the cohesive forces of the material of the enclosing vessel which restrains the expansion. no matter how great may be the elastic properties of this material, it will be distorted, more or less, by the application of work energy. this distortional movement is the external evidence of the energy process of transformation. energy is stored in the material against the forces of cohesion (§ ). but the energy thus stored is only a small proportion of the total work energy which accrues to the gas in the heating process. the remainder is stored in the gas itself, and the evidence of such storage is found simply in the increase of pressure. different energy machines thus offer different facilities for the transformation or the storage of the applied energy. in every case where the work energy applied has no opportunity of expending itself, its presence will be indicated by an increase in the pressure or work function of the gas. [illustration: fig. ] the principles which underlie the above phenomena can readily be applied to other cases of gaseous expansion. it is a matter of common experience that if a given mass of gaseous material be introduced into a vessel which has been exhausted by an air-pump or other device for the production of a vacuum, the whole space within the vessel is instantly permeated by the gas, which will expand until its volume is precisely that of the containing vessel. further phenomena of the operation are that the expanding gas suffers a decrease in temperature and pressure corresponding to the degree or ratio of the expansion. before the expansive process took place the gaseous mass, as indicated by its initial temperature and pressure, is endowed with a definite quantity of energy in the form of heat and work energy. after expansion, these quantities are diminished, as indicated by its final and lower temperature and pressure. the operation of expansion has thus involved an expenditure of energy. this expenditure takes place in virtue of the movement of the gaseous material (§ ). it is obvious that if the volume of the whole is to be increased, each portion of the expanding gas requires to move relatively to the remainder. this movement is carried out in the lines of the earth's gravitative attraction, and to a certain extent over the surface of the containing vessel. in some respects, it thus corresponds simply to the movement of a body over the earth's surface (§ ). it is also carried out against the viscous or frictional forces existing throughout the gaseous material itself (§ ). assuming no influx of energy from without, the energy expended in the movement of the gaseous material must be obtained at the expense of the inherent heat and work energy of the gas, and these two functions will decrease simultaneously. the heat and work energy of the gas or its inherent energy is thus taken to provide the energy necessary for the expansive movement. this energy, however, does not leave the gas, but still resides therein in a form akin to that of energy of position or separation. it will be clear also, that the reverse operation cannot, in this case, be carried out; the gas cannot move back to its original volume in the same fashion as it expanded into the vacuum, so that the energy utilised in this way for separation cannot be directly returned. the expansion of the gas has been assumed above to take place into a vacuous space, but a little consideration will show that this condition cannot be properly or even approximately fulfilled under ordinary experimental conditions. the smallest quantity of gas introduced into the exhausted vessel will at once completely fill the vacuous space, and, on this account, the whole expansion of the gas does not in reality take place _in vacuo_ at all. to study the action of the gas under the latter conditions, it is necessary to look on the operation of expansion in a more general way, which might be presented as follows. . _gravitational equilibrium of gases_ consider a planetary body, in general nature similar to the earth, but, unlike the earth, possessing no atmosphere whatever. the space surrounding such a celestial mass may then be considered as a perfect vacuum. now let it be further assumed that in virtue of some change in the conditions, a portion of the material of the planetary mass is volatilised and a mass of gas thereby liberated over its surface. the gas is assumed to correspond in temperature to that portion of the planet's surface with which it is in contact. it is clear that, in the circumstances, the gas, in virtue of its elastic and energetic properties, will expand in all directions. it will completely envelop the planet, and it will also move radially outwards into space. in these respects, its expansion will correspond to that of a gas introduced into a vacuous space of unlimited extent. the question now arises as to the nature of the action of the gaseous substance in these circumstances. it is clear that the radial or outward movement of the gas from the planetary surface is made directly against the gravitative attraction of the planet on the gaseous mass. in other words, matter or material is being moved in the lines or field of this gravitative force. this movement, accordingly, will be productive of an energy transformation (§ ). in its initial or surface condition each portion of the gaseous mass is possessed of a perfectly definite amount of energy indicated by and dependent on that condition. as it moves upwards from the surface, it does work against gravity in the raising of its own mass. but as the mass is thus raised, it is gaining energy of position (§ ), and as it has absolutely no communication with any external source of energy in its ascent, the energy of position thus gained can only be obtained at the expense of its initial inherent heat and work energy. the operation is, in fact, a simple transformation of this inherent energy into energy of position, a transformation in which gravity is the incepting agency. the external evidence of transformation will be a fall in temperature of the material. since the action is exactly similar for all ascending particles, it is evident that as the altitude of the gaseous mass increases the temperature will correspondingly diminish. this diminution will proceed so long as the gaseous particles continue to ascend, and until an elevation is finally attained at which their inherent energy is entirely converted into energy of position. the expansion of the gas, and the associated transformation of energy, thus leads to the erection of a gaseous column in space, the temperature of which steadily diminishes from the base to the summit. at the latter elevation, the inherent energy of the gaseous particles which attain to it is completely transformed or worked out against gravity in the ascent; the energy possessed by the gas at this elevation is, therefore, entirely energy of position; the energy properties of heat and work have entirely vanished, and the temperature will, therefore, at this elevation, be absolute zero. it is important to note also that in the building of such a column or gaseous spherical envelope round the planet, the total energy of any gaseous particle of that column will remain unchanged throughout the process. no matter where the particle may be situated in the column, its total energy must always be expressed by its heat and work energy properties together with its energy of position. this sum is always a constant quantity. for if the particle descends from a higher to a lower altitude, its total energy is still unchanged, because a definite transformation of its energy of position takes place corresponding to its fall, and this transformed energy duly appears in its original form of heat and work energy in accordance with the decreased altitude of the particle. since the temperature of the column remains unchanged at the base surface and only decreases in the ascent, it is clear that the entire heat and work energy of the originally liberated gaseous mass is not expended in the movement against gravity. every gaseous particle--excepting those on the absolute outer surface of the gaseous envelope--has still the property of temperature. it is evident, therefore, that in the constitution of the column, only a portion of the total original heat and work energy of the gaseous substance is transformed into energy of position. the space into which the gas expands has been referred to as unlimited in extent. but although in one sense it may be correctly described thus, yet in another, and perhaps in a truer sense, the space is very strictly limited. it is true there is no enclosing vessel or bounding surface, but nevertheless the expansion of the gas is restrained in two ways or limited by two factors. the position of the bounding surface of the spherical gaseous envelope depends, in the first place, on the original energy of the gas as deduced from its initial temperature and its other physical properties, and secondly on the value of the gravitative attraction exerted on the gas by the planetary body. looking at the first factor, it is obvious that since the gaseous mass initially possesses only a limited amount of energy, and since only a certain portion of this energy is really available for the transformation, the whole process is thereby limited in extent. the complete transformation and disappearance of that available portion of the gaseous energy in the process of erection of the atmospheric column will correspond to a definite and limited increase of energy of position of gaseous material. since the energy of position is thus restricted in its totality, and the mass of material for elevation is constant, the height of the column or the boundary of expansion of the gas is likewise rigidly defined. in this fashion, the energy properties of the gaseous material limit the expansive process. looking at the operation from another standpoint, it is clear that the maximum height of the spherical gaseous envelope must also be dependent on the resistance against which the upward movement of the gas is carried out, that is, on the value of the gravitative attraction. the expenditure of energy in the ascent varies directly as the opposing force; if this force be increased the ultimate height must decrease, and vice versa. each particle might be regarded as moving in the ascent against the action of an invisible spring, stretching it so that with increase of altitude more and more of the energy of the particle is transformed or stored in the spring in the extension. when the particle descends to its original position, the operation is reversed; the spring is now contracting, and yielding up the stored energy to the particle in the contraction. the action of the spring would here be merely that of an apparatus for the storage and return of energy. in the case of the gaseous mass, we conceive the action of gravitation to be exactly analogous to that of a spring offering an approximately constant resistance to extension. (the value of gravity is assumed approximately constant, and independent of the particle's displacement.) the energy stored or transformed in the ascension against gravity is returned on the descent in a precisely similar fashion. the operation is a completely reversible one. the range of motion of the gaseous mass or the ultimate height of the gaseous column will thus depend on the value of the opposing attractive force controlling the motion or, in other words, on the value of gravity. this value is of course defined by the relative mass of the planet (§ ). it is evident that the spherical envelope which would thus enwrap the planetary mass possesses certain peculiar properties which are not associated with gaseous masses under ordinary experimental conditions. it by no means corresponds to any ordinary body of gaseous material, having a homogeneous constitution and a precise and determinate pressure and temperature throughout. on the contrary, its properties are somewhat complex. throughout the gaseous envelope the physical condition of the substance is continually changing with change of altitude. the extremes are found at the inner and outer bounding surfaces. at any given level, the gaseous pressure is simply the result of the attractive action of gravitation on the mass of gaseous material above that level--or, more simply, to the weight of material above that level. there is, of course, a certain decrease in the value of the gravitative attraction with increase of altitude, but within the limits of atmospheric height obtained by ordinary gaseous substances (§ ) this decrease may be neglected, and the weight of unit mass of the material assumed constant at different levels. increase of atmospheric altitude is thus accompanied by decrease in atmospheric pressure. but decrease in pressure must be accompanied by a corresponding decrease in density of the gas, so that, if uniform temperature were for the time being assumed, it would be necessary at the higher levels to rise through a greater distance to experience the same decrease in pressure than at the lower levels. in fact, given uniform conditions of temperature, if different altitudes were taken in arithmetical progression the respective pressures and densities would diminish in geometrical progression. but we have seen that the energy conditions absolutely preclude the condition of uniformity of temperature, and accordingly, the decreasing pressure and density must be counteracted to some extent at least by the decreasing temperature. the conditions are somewhat complex; but the general effect of the decreasing temperature factor would seem to be by increasing the density to cause the available gaseous energy to be completely worked down at a somewhat lower level than otherwise, and thus to lessen to some degree the height of the gaseous envelope. it is to be noted that a gaseous column or atmosphere of this nature would be in a state of complete equilibrium under the action of the gravitative attraction--provided there were no external disturbing influences. the peculiar feature of such a column is that the total energy of unit mass of its material, wherever that mass may be situated, is a constant quantity. in virtue of this property, the equilibrium of the column might be termed neutral or statical equilibrium. the gas may then be described as in the neutral or statical condition. this statical condition of equilibrium of a gas is of course a purely hypothetical one. it has been described in order to introduce certain ideas which are essential to the discussion of energy changes and reactions of gases in the lines of gravitational forces. these reactions will now be dealt with. . _total energy of gaseous substances_ since the maximum height of a planetary atmosphere is dependent on the total energy of the gaseous substance or substances of which it is composed, it becomes necessary, in determining this height, to estimate this total energy. this, however, is a matter of some difficulty. by the total energy is here meant the entire energy possessed by the substance, that energy which it would yield up in cooling from its given condition down to absolute zero of temperature. on examination of the recorded properties of the various gaseous substances familiar to us, it will be found that in no single instance are the particulars available for anything more than an exceedingly rough estimate of this total energy. each substance, in proceeding from the gaseous condition towards absolute zero, passes through many physical phases. in most cases, there is a lack of experimental phenomena or data of any kind relating to certain of these phases; the necessary information on certain points, such as the values and variations of latent and specific heats and other physical quantities, is, in the meantime, not accessible. experimental research in regions of low temperature may be said to be in its infancy, and the properties of matter in these regions are accordingly more or less unknown. the researches of mendeleef and others tend to show, also, that the comparatively simple laws successfully applied to gases under normal conditions are entirely departed from at very low temperatures. in view of these facts, it is necessary, in attempting to estimate, by ordinary methods, the total energy of any substance, to bear in mind that the quantity finally obtained may only be a rough approximation to the true value. these approximations, however, although of little value as precise measurements, may be of very great importance for certain general comparative purposes. keeping in view these general considerations, it is now proposed to estimate, under ordinary terrestrial atmospheric conditions, the total energy properties of the three gaseous substances, oxygen, nitrogen, and aqueous vapour. the information relative to the energy calculation which is in the meantime available is shown below in tabular form. as far as possible all the heat and other energy properties of each substance as it cools to absolute zero have been taken into account. _table of properties_ +--------+---------+----------+----------+-------------+-------+---------+ | i | ii | iii | iv | v | vi | vii | +--------+---------+----------+----------+-------------+-------+---------+ | |specific | evaporation | | | | | | heat at |temperature of liquid| approximate | latent| vapour | | gas | constant| at atmospheric | latent heat |heat of|pressure.| | |pressure.| pressure. |of gas ° f.|liquid.| ° f. | | | | °f. °f. (abs.)| | | | +--------+---------+----------+----------+-------------+-------+---------+ |oxygen | · | - | | | ... | ... | +--------+---------+----------+----------+-------------+-------+---------+ |nitrogen| · | - | | | ... | ... | +--------+---------+----------+----------+-------------+-------+---------+ |aqueous | | | | | | | |vapour | · | | | | | · | +--------+---------+----------+----------+-------------+-------+---------+ since no reliable data can be obtained with regard to the values and variations of specific heats at extremely low temperatures, they are assumed for the purpose of our calculation to be in each case that of the gas, and to be constant under all conditions. latent heats are utilised in every case when available. with these reservations, the total energy, referred to absolute zero, of one pound of oxygen gas at normal temperature of ° f. or ° f. (abs.) will be approximately ( Ã� · ) + = thermal units fahrenheit. this in work units is roughly equivalent to Ã� = , ft. lbs. adopting the same method with nitrogen gas, its energy at the same initial temperature will be, per unit mass, , ft. lbs. there is thus a somewhat close resemblance, not only in the general temperature conditions but also in the energy conditions, of the two gases oxygen and nitrogen. it will be readily seen, however, that under the same conditions the energy state of aqueous vapour differs very considerably from either, for by the same method as before the energy per pound of aqueous vapour is equal to {( Ã� · ) + + } Ã� = , , ft. lbs. under ordinary terrestrial atmospheric conditions, the energy of aqueous vapour per unit mass is thus nearly seven times as great as that of either oxygen or nitrogen gas. it is to be observed, also, that three-fourths of this energy of the vapour under the given conditions is present in the form of latent energy of the gas, or what we have already termed work energy. the values of the various temperatures and other physical features, which we have included in the table of properties above, and which will be utilised throughout this discussion, are merely those in everyday use in scientific work. they form simply the accessible information on the respective materials. they are the records of phenomena, and on these phenomena are based our energy calculations. further research may reveal the true values of other factors which up to the present we have been forced to assume, and so lead to more accurate computation of the energy in each case. such investigation, however, is unlikely to affect in any way the general object of this part of the work, which is simply to portray in an approximate manner the relative energy properties of the three gaseous substances under certain assumed conditions. . _comparative altitudes of planetary atmospheres_ the total energy of equal masses of the gases oxygen, nitrogen, and aqueous vapour, as estimated by the method above, are respectively in the ratios : · : · referring back once more to the phenomena described with reference to the gravitational equilibrium of a gas, let it be assumed that the gaseous substance liberated on the surface of the planetary body is oxygen, and that the planetary body itself is of approximately the same constitution and dimensions as the earth. the oxygen gas thus liberated will expand against gravity, and envelop the planet in the manner already described (§ ). now the total energy of a mass of one pound of oxygen has been estimated under certain assumptions (§ ) to be , ft. lbs. the value of the gravitative attraction of the planet on this mass is the same as under ordinary terrestrial conditions, so that if the entire energy of one pound of the gas were utilised in raising itself against gravity, the height through which this mass would be raised, and at which the material would attain the level of absolute zero of temperature, assuming gravity constant with increasing altitude, would be simply , ft. or approximately miles. the whole energy would not, of course, be expended in the expansive movement; only the outermost surface material of the planetary gaseous envelope attains to absolute zero of temperature. in estimating the altitude of this surface, however, the precise mass of gaseous substance assumed for the purpose of calculation is of little or no importance. whatever may be the value of the mass assumed, its total energy and the gravitative attraction of the planetary body on it are both alike entirely and directly dependent on that mass value. it is therefore clear that no matter how the mass under consideration be diminished, the height at which its energy would be completely worked down, and at which its temperature would be absolute zero, is the same, namely miles. at the planet's surface, the total energy of an infinitesimally small portion of the gaseous mass is proportional to that mass. this amount of energy is, however, all that is available for transformation against gravitation in the ascent. but at the same time, the gravitative force on the particle, that force which resists its upward movement, is proportionately small corresponding to the small mass, so that the particle will in reality require to rise to the same altitude of miles in order to completely transform its energy and attain absolute zero of temperature. when the expansive process is completed, the outer surface of the spherical gaseous envelope surrounding the planet is then formed of matter in this condition of absolute zero; this height of miles is then the altitude or depth of the statical atmospheric column at a point on the planetary surface where the temperature is ° f. it is to be particularly noted that this height is entirely dependent on the gravitation, temperature, and energy conditions assumed. with respect, also, to the assumption made above, of constant gravitation with increasing altitude, the variation in the value of gravity within the height limits in which the gas operates is so slight, that the energy of the expanding substance is completely worked down long before the variation appreciably affects the estimated altitude of absolute zero. in any case, bearing in mind the approximate nature of the estimate of the energy of the gases themselves, the variation of gravity is evidently a factor of little moment in our scheme of comparison. knowing the maximum height to be miles, a uniform temperature gradient from the planetary surface to the outermost surface of the atmospheric material may be readily calculated. in the case of oxygen, the decrease of temperature with altitude will be at the rate of ° f. per mile, or ° f. per ft. if the planetary atmosphere were composed of nitrogen instead of oxygen, the height of the statical atmospheric column under the given conditions would then be approximately Ã� · = miles, and the gradient of temperature · ° f. per mile. in the case of aqueous vapour, which is possessed of much more powerful energy properties than either oxygen or nitrogen, the height of the statical column, to correspond to the energy of the material, is no less than miles and the temperature gradient only · ° f. per mile. each of the gases, then, if separately associated with the planetary body, would form an atmosphere around it depending in height on the peculiar energy properties of the gas. a point to be observed is that the actual or total mass of any gas thus liberated at the planet's surface has no bearing on the ultimate height of the atmosphere which it would constitute. when the expansive motion is completed, the density properties of the atmosphere would of course depend on the initial mass of gas liberated, but for any given value of gravity it is the energy properties of the gas per unit mass, or what might be termed its specific energy properties, which really determine the height of its atmosphere. . _reactions of composite atmosphere_ it is now possible to deal with the case in which not only one gas but several gases are initially liberated on the planetary surface. since the gases are different, then at the given surface temperature of the planet they possess different amounts of heat energy, and for each gas considered statically, the temperature-altitude gradient will be different from any of the others. the limiting height of the gaseous column for each gas, considered separately, will also depend on the total energy of that gas per unit mass, at the surface temperature. but it is evident that in a composite atmosphere, the separate statical conditions of several gases could not be maintained. in such a mixture, separate temperature-altitude gradients would be impossible. absolute zero of temperature could clearly not be attained at more than one altitude, and it is evident that the temperature-altitude gradient of the mixture must, in some way, settle down to a definite value, and absolute zero of temperature must occur at some determinate height. this can only be brought about by energy exchanges and reactions between the atmospheric constituents. when these reactions have taken place, the atmosphere as a whole will have attained a condition analogous to that of statical equilibrium (§ ). each of its constituents, however, will have decidedly departed from this latter condition. in the course of the mutual energy reactions, some will lose a portion of their energy. others will gain at their expense. all are in equilibrium as constituents of the composite atmosphere, but none approach the condition of statical equilibrium peculiar to an atmosphere composed of one gas only (§ ). the precise energy operations which would thus take place in any composite atmosphere would of course depend in nature and extent on the physical properties of the reacting constituents. if the latter were closely alike in general properties, the energy changes are likely to be small. a strong divergence in energy properties will give rise to more powerful reactions. a concrete instance will perhaps make this more clear. let it be assumed in the first place that the planetary atmosphere is composed of the two gases oxygen and nitrogen. from previous considerations, it will be clear that the natural decrease of temperature of nitrogen gas with increase of altitude is, in virtue of its slightly superior energy qualities, correspondingly slower than that of oxygen. the approximate rates are · ° f. and ° f. per mile respectively. the tendency of the nitrogen is therefore to transmit a portion of its energy to the oxygen. such a transmission, however, would increase the height of the oxygen column and correspondingly decrease the height of the nitrogen. when the balance is finally obtained, the height of the atmospheric column does not correspond to the energy properties of either gas, but to those of the combination. in the case of these two materials, oxygen and nitrogen, the energy reactions necessary to produce the condition of equilibrium are comparatively small in magnitude on account of the somewhat close resemblance in the energy properties of the two substances. on this account, therefore, the two gases might readily be assumed to behave as one gas composing the planetary atmosphere. but what, then, will be the effect of introducing a quantity of aqueous vapour into an atmosphere this nature? the general phenomena will be of the same order as before, but of much greater magnitude. from the approximate figures obtained (§§ , ), the inherent energy of aqueous vapour per unit mass is seen to be, under the same conditions, enormously greater than that of the other two gases. in statical equilibrium (§ ), the altitude of the gaseous column formed by aqueous vapour is almost seven times as great as that of the oxygen or nitrogen with which, in the composite atmosphere, it would be intermixed. in the given circumstances, then, aqueous vapour would be forced by these conditions to give up a very large portion of its energy to the other atmospheric constituents. the latter would thus be still further expanded against gravity; the aqueous vapour itself would suffer a loss of energy equivalent to the work transmitted from it. it is therefore clear that in a composite atmosphere formed in the manner described, any gas possessed of energy properties superior to the other constituents is forced of necessity to transmit energy to these constituents. this phenomenon is merely a consequence of the natural disposition of the atmospheric gaseous substances towards a condition of equilibrium with more or less uniform temperature gradation. the greater the inherent energy qualities of any one constituent relative to the others, the greater will be the quantity of energy transmitted from it in this way. . _description of terrestrial case_ bearing in mind the general considerations which have been advanced above with respect to planetary atmospheres, it is now possible to place before the reader a general descriptive outline of the circumstances and operation of an atmospheric machine in actual working. the machine to be described is that associated with the earth. in the earth is found an example of a planetary body of spheroidal form pursuing a clearly defined orbit in space and at the same time rotating with absolutely uniform velocity about a central axis within itself. the structural details of its surface and the general distribution of material thereon will be more or less familiar to the reader, and it is not, therefore, proposed to dwell on these features here. attention may be drawn, however, to the fact that a very large proportion of the surface of the earth is a liquid surface. of all the material familiar to us from terrestrial experience there is none which enters into the composition of the earth's crust in so large a proportion as water. in the free state, or in combination with other material, water is found everywhere. in the liquid condition it is widely distributed. although the liquid or sea surface of the planet extends over a large part of the whole, the real water surface, that is, the _wetted_ surface, if we except perhaps a few desert regions, may be said to comprise practically the entire surface area of the planet. and water is found not only on the earth's crust but throughout the gaseous atmospheric envelope. the researches of modern chemistry have revealed the fact that the atmosphere by which the earth is surrounded is not only a mixture of gases, but an exceedingly complex mixture. the relative proportions of the rarer gases present are, however, exceedingly small, and their properties correspondingly obscure. taken broadly, the atmosphere may be said to be composed of air and water (in the form of aqueous vapour) in varying proportion. the former constituent exists as a mixture of oxygen and nitrogen gases of fairly constant proportion over the entire surface of the globe. the latter is present in varying amount at different points according to local conditions. this mixture of gaseous substances, forming the terrestrial atmosphere, resides on the surface of the planet and forms, as already described (§ ), a column or envelope completely surrounding it; the quantity of gaseous material thus heaped up on the planetary surface is such that it exerts almost uniformly over that surface the ordinary atmospheric pressure of approximately · lb. per sq. inch. it is advisable, also, at this stage to point out and emphasise the fact that the planetary atmosphere must be regarded as essentially a material portion of the planet itself. although the atmosphere forms a movable shell or envelope, and is composed of purely gaseous material, it will still partake of the same complete orbital and rotatory axial motion as the solid core, and will also be subjected to the same external and internal influences of gravitation. such are the general planetary conditions. let us now turn to the particular phenomena of axial revolution. in virtue of the unvarying rotatory movement of the planetary mass in the lines of the various incepting fields of its primary the sun, transformations of the axial or mechanical energy of the planet will be in continuous operation (§§ - ). although the gaseous atmospheric envelope of the planet partakes of this general rotatory motion under the influence of the incepting fields, the latter have apparently no action upon it. the sun's influence penetrates, as it were, the atmospheric veil, and operating on the solid and liquid material below, provokes the numerous and varied transformations of planetary energy which constitute planetary phenomena. at the equatorial band, where the velocity or axial energy properties of the surface material is greatest, these effects of transformation will naturally be most pronounced. in the polar regions of low velocity they will be less evident. one of the most important of these transforming effects may be termed the heating action of the primary on the planet--a process which takes place in greater or less degree over the entire planetary surface, and which is the result of the direct transformation of axial energy into the form of heat (§ ). in virtue of this heat transformation, or heating effect of the sun, the temperature of material on the earth's surface is maintained in varying values from regions of high velocity to those of low--from equator to poles--according to latitude or according to the displacement of that material, in rotation, from the central axis. owing to the irregular distribution of matter on the earth's surface, and other causes to be referred to later, this variation in temperature is not necessarily uniform with the latitude. this heating effect of the sun on the earth will provoke on the terrestrial surface all the familiar secondary processes (§ ) associated with the heating of material. most of these processes, in combination with the operations of radiation and conduction, will lead either directly or indirectly to the communication of energy to the atmospheric masses (§ ). closely associated with the heat transformation, there is also in operation another energy process of great importance. this process is one of evaporative transformation. reference has already been made to the vast extent of the liquid or wetted surface of the earth. this surface forms the seat of evaporation, and the action of the sun's incepting influence on the liquid of this surface is to induce a direct transformation of the earth's axial or mechanical energy into the elastic energy of a gas, or in other words into the form of work energy. by this process, therefore, water is converted into aqueous vapour. immediately the substance attains the latter or gaseous state it becomes unaffected by, or transparent to, the incepting influence of the sun (§ ). and the action of evaporation is not restricted in locality to the earth's surface only. it may proceed throughout the atmosphere. wherever condensation of aqueous vapour takes place and water particles are thereby suspended in the atmosphere, these particles are immediately susceptible to the sun's incepting field, and if the conditions are otherwise favourable, re-evaporation will at once ensue. like the ordinary heating action also, that of transformation will take place with greater intensity in equatorial than in polar regions. these two planetary secondary processes, of heating and evaporation, are of vital importance to the working of the atmospheric machine. but, as already pointed out elsewhere (§§ , ), every secondary operation is in some fashion linked to that machine. other incepting influences, such as light, are in action on the planet, and produce transformations peculiar to themselves. these, in the meantime, will not be considered except to point out that in every case the energy active in them is the axial energy of the earth itself operating under the direct incepting influence of the sun. the general conditions of planetary revolution and transformation are thus intimately associated with the operation of the atmospheric machine. in this machine is embodied a huge energy process, in the working of which the axial energy of the earth passes through a series of energy changes which, in combination, form a complete cyclical operation. in their perhaps most natural sequence these processes are as follows:-- . the direct transformation of terrestrial axial energy into the work energy of aqueous vapour. . the direct transmission of the work energy of aqueous vapour to the general atmospheric masses, and the consequent elevation of these masses from the earth's surface against gravity. . the descent of the atmospheric air masses in their movement towards regions of low velocity, and the return in the descent of the initially transformed axial energy to its original form. the first of these processes is carried out through the medium of the aqueous material of the earth. it is simply the evaporative transformation referred to above. by that evaporative process a portion of the energy of motion or axial energy of the earth is directly communicated or passed into the aqueous material. its form, in that material, is that of work energy, or the elastic energy of aqueous vapour, and, as already pointed out, this process of evaporative transformation reaches its greatest intensity in equatorial or regions of highest velocity. in these regions also, in virtue of the working of the heat process already referred to above, the temperature conditions are eminently favourable to the presence of large quantities of aqueous vapour. the tension or pressure of the vapour, which really depends on the quantity of gaseous material present, is directly proportional to the temperature, so that in equatorial regions not only is the general action of transformation in the aqueous material most intense, but the surrounding temperature conditions in these regions are such as to favour the continuous presence of large quantities of the aqueous vapour which is the direct product of the action of transformation. the equatorial regions of the earth, or the regions of high velocity, are thus eminently adapted, by the natural conditions, to be the seat of the most powerful transformations of axial energy. as already pointed out, however, these same transformations take place over the entire terrestrial surface in varying degree and intensity according to the locality and the temperature or other conditions which may prevail. now this transformation of axial energy which takes place through the medium of the evaporative process is a continuous operation. the energy involved, which passes into the aqueous vapour, augmented by the energy of other secondary processes (§ ), is the energy which is applied to the atmospheric air masses in the second stage of the working of the atmospheric machine. before proceeding to the description of this stage, however, it is absolutely necessary to point out certain very important facts with reference to the energy condition of the atmospheric constituents in the peculiar circumstances of their normal working. . _relative physical conditions of atmospheric constituents_ it will be evident that no matter where the evaporation of the aqueous material takes place, it must be carried out at the temperature corresponding to that location, and since the aqueous vapour itself is not superheated in any way (being transparent to the sun's influence), the axial energy transformed and the work energy stored in the material per unit mass, will be simply equivalent to the latent heat of aqueous vapour under the temperature conditions which prevail. in virtue of the relatively high value of this latent heat under ordinary conditions, the gas may be regarded as comparatively a very highly energised substance. it is clear, however, that since the gas is working at its precise temperature of evaporation, the maximum amount of energy which it can possibly yield up at that temperature is simply this latent heat of evaporation, and if this energy be by any means withdrawn, either in whole or in part, then condensation corresponding to the energy withdrawal will at once ensue. the condition of the aqueous vapour is in fact that of a true vapour, or of a gaseous substance operating exactly at its evaporation temperature, and unable to sustain even the slightest abstraction of energy without an equivalent condensation. no matter in what manner the abstraction is carried out, whether by the direct transmission of heat from the substance or by the expansion of the gas against gravity, the result is the same; part of the gaseous material returns to the liquid form. in the case of the more stable or permanent constituents of the atmosphere, namely oxygen and nitrogen, their physical conditions are entirely different from that of the aqueous vapour. examination of the table of properties (p. ) shows that the evaporation temperatures of these two substances under ordinary conditions of atmospheric pressure are as low as - ° f. and - ° f. respectively. at an ordinary atmospheric temperature of say ° f. these two gases are therefore so far above their evaporation temperature that they are in the condition of what might be termed true gaseous substances. although only at a temperature of ° f., they may be truly described as highly superheated gases, and it is evident that they may be readily cooled from ° f. through wide ranges of temperature, without any danger of their condensation or liquefaction. oxygen and nitrogen gases thus present in their physical condition and qualities a strong contrast to aqueous vapour, and it is this difference in properties, particularly the difference in evaporation temperatures, which is of vital importance in the working of the atmospheric machine. the two gases oxygen and nitrogen are, however, so closely alike in their general energy properties that, in the meantime, the atmospheric mixture of the two can be conveniently assumed to act simply as one gas--atmospheric air. from these considerations of the ordinary atmospheric physical properties of air and aqueous vapour it may be readily seen how each is eminently adapted to its function in the atmospheric process. the peculiar duty of the aqueous vapour is the absorption and transmission of energy. its relatively enormous capacity for energy, the high value of its latent heat at all ordinary atmospheric temperatures, and the fact that it must always operate precisely at its evaporation temperature makes it admirably suited for both functions. thus, in virtue of its peculiar physical properties, it forms an admirable agent for the storage of energy and for its transmission to the surrounding air masses. the low temperature of evaporation of these air masses ensures their permanency in the gaseous state. they are thus perfectly adapted for expansive and other movements, for the conversion of their energy against gravity into energy of position, or for any other reactions involving temperature change without condensation. . _transmission of energy from aqueous vapour to air masses_ the working of the second or transmission stage of the atmospheric machine involves certain energy operations in which gravitation is the incepting factor or agency. let it be assumed that a mass of aqueous vapour liberated at its surface of evaporation by the transformation of axial energy, expands upwards against the gravitative attraction of the earth (§§ , ). as the gaseous particles ascend and thus gain energy of position, they do work against gravity. this work is done at the expense of their latent energy. since the aqueous material is always working precisely at its evaporation temperature, this gain in energy of position and consequent loss of latent energy will be accompanied by an equivalent condensation and conversion of the rising vapour into the liquid form. this condensation will thus be the direct evidence and measure of work done by the aqueous material against the gravitational forces, and the energy expended or worked down in this way may now, accordingly, be regarded as stored in the condensed material or liquid particles in virtue of their new and exalted position above the earth's surface. it is this energy which is finally transmitted to the atmospheric air masses. the transmission process is carried out in the downward movement of the liquid particles. the latter, in their exalted positions, are at a low temperature corresponding to that position--that is, corresponding to the work done--and provided no energy were transmitted from them to the surrounding air masses, their temperature would gradually rise during the descent by the transformation of this energy of position. in fact the phenomena of descent, supposing no transmission of energy from the aqueous material, would simply be the reverse of the phenomena of ascent. since, however, the energy of position which the liquid particles possess is transmitted from them to the atmospheric masses, then it follows that this natural increase in their temperature would not occur in the descent. a new order of phenomena would now appear. since the evaporative process is a continuous one, the liquid particles in their downward movement must be in intimate contact with rising gaseous material, and these liquid particles will, accordingly, at each stage of the descent, absorb from this rising material the whole energy necessary to raise their temperature to the values corresponding to their decreasing elevation. in virtue of this absorption of energy then, from the rising material, these liquid particles are enabled to reach the level of evaporation at the precise temperature of that level. now, considering the process as a whole, it will be readily seen that for any given mass of aqueous material thus elevated from and returned to a surface of evaporation, there must be a definite expenditure of energy (axial energy) at that surface. since the material always regains the surface at the precise temperature of evaporation, this expenditure is obviously, in total, equal to the latent heat of aqueous vapour at the surface temperature. it may be divided into two parts. one portion of the axial energy--the transmitted portion--is utilised in the elevation of the material against gravity; the remainder is expended, as explained above, in the heating of the returning material. the whole operation takes place between two precise temperatures, a higher temperature, which is that of the surface of evaporation, and a lower temperature, corresponding to the work done, and so related to the higher that the whole of the energy expended by the working aqueous substance--in heating the returning material and in transmitted work--is exactly equivalent to the latent heat of aqueous vapour at the high or surface temperature. but, as will be demonstrated later, the whole energy transmitted from the aqueous material to the air masses is finally returned in its entirety as axial energy, and is thus once more made available in the evaporative transformation process. the energy expended in raising the temperature of the working material returning to the surface of evaporation is obviously returned with that material. both portions of the original expenditure are thus returned to the source in different ways. the whole operation is, in fact, completely cyclical in nature; we are in reality describing "nature's perfect engine," which is completely reversible and which has the highest possible efficiency.[ ] although the higher temperature at the evaporation surface may vary with different locations of that surface, in every case the lower temperature is so related to it as to make the total expenditure precisely equal to the latent heat at that evaporation temperature.[ ] it must be borne in mind also, that all the condensed material in the upper strata of the atmosphere must not of necessity return to the planetary liquid surface. on the contrary, immediately condensation of the aqueous vapour takes place and the material leaves the gaseous state, no matter where that material is situated, it is once more susceptible to the incepting influences of the sun. re-evaporation may thus readily take place even at high altitudes, and complete cyclical operations may be carried out there. these operations will, however, be carried out in every case between precise temperature limits as explained above. [ ] the conception of "nature's perfect engine" was originally arrived at by the author from consideration of the phenomena of the steam-engine. the following extract from the "review" of his work ( ) illustrates the various stages which finally lead to that conclusion:-- "my first steps in the right direction came about thus. i had always been working with a cylinder and piston, and could make no progress, till at length it struck me to make my cylinder high enough to do without a piston--that is, to leave the steam to itself and observe its behaviour when left to work against gravity. the first thing i had to settle was the height of my cylinder. and i found, by calculation from regnault's experiments that it would require to be very high, and that the exact height would depend on the temperature of the water in the boiler which was the bottom of this ideal cylinder. now, at any ordinary temperature the height was so great that it was impossible to get known material to support its own weight, and i did not wish to use a hypothetical substance in the construction of this engine. finally, the only course left me was to abolish the cylinder as i had done the piston. i then discovered that the engine i had been trying to evolve--the perfect engine--was not the ideal thing i had been groping after but an actual reality, in full working order, its operations taking place every day before my eyes. "every natural phenomenon fitted in exactly; it had its function to perform, and the performance of its function constituted the phenomenon. let me trace the analogy in a few of its details. the sea corresponds to the boiler; its cylinder surrounds the earth; it has for its fuel the axial energy of the earth; it has no condenser because it has no exhaust; the work it performs is all expended in producing the fuel. every operation in the cycle is but an energy transformation, and these various transformations constitute the visible life of the world." [ ] for definite numerical examples see the author's _terrestrial energy_ (chap. .). it will be evident, from a general consideration of this process of transmission of energy from the aqueous vapour, that relatively large quantities of that vapour are not required in the atmosphere for the working of the gaseous machine. the peculiar property of ready condensation of the aqueous vapour makes the evaporative process a continuous one, and the highly energised aqueous material, although only present in comparatively small amount, contributes a continuous flow of energy, and is thus able to steadily convey a very large quantity to the atmospheric masses. for the same reason, the greater part of the energy transmission from the aqueous vapour to the air will take place at comparatively low altitudes and between reasonably high temperatures. the working of any evaporative cycle may also be spread over very large terrestrial areas by the free movement of the acting material. aqueous vapour rising in equatorial regions may finally return to the earth in the form of ice-crystals at the poles. in every complete cycle, however, the total expenditure per unit mass of material initially evaporated is always the latent heat at the higher or evaporation temperature; in the final or return stages of the cycle, any energy not transmitted to the air masses is devoted to the heating of returning aqueous material. referring again to the transmitted energy, and speaking in the broadest fashion, the function of the aqueous vapour in the atmosphere may be likened to that of the steam in the cylinder of a steam-engine. in both cases the aqueous material works in a definite machine for energy transmission. in the case of the steam-engine work energy is transmitted (§ ) from the steam through the medium of the moving piston and rotating shaft, and thence may be further diverted to useful purposes. in the planetary atmospheric machine the work energy of aqueous vapour is likewise transmitted by the agency of the moving air masses, not to any external agent, but back once more to its original source, which is the planetary axial energy. in neither case are we able to explain the precise nature of the transmission process in its ultimate details. we cannot say _how_ the steam transmits its work energy by the moving piston, nor yet by what agency the elevated particles of aqueous material transmit their energy to the air masses. our knowledge is confined entirely to the phenomena, and, fortunately, these are in some degree accessible. nature presents direct evidence that such transmissions actually take place. this evidence is to be found, in both cases, in the condensation of the aqueous material which sustains the loss of its work energy. in the engine cylinder condensation takes place due to work being transmitted from the steam; in the atmosphere the visible phenomena of condensation are likewise the ever present evidence of the transmission of work energy from the aqueous vapour to the air masses. in virtue of this accession of energy these masses will, accordingly, be expanded upwards against the gravitational attractive forces. this upward movement, being made entirely at the expense of energy communicated from the aqueous vapour, is not accompanied by the normal fall of temperature due to the expansion of the air. planetary axial energy, originally absorbed by the aqueous vapour, in the work form, has been transferred to the air masses in the same form, and is now, after the expansive movement, resident in these masses in the form of energy of position. it is the function of the atmospheric machine in its final stage to return this energy in the original axial form. . _terrestrial energy return_ let it be assumed that an atmospheric mass has been raised, by the transmission of work energy, to a high altitude in the equatorial regions of the earth. the assumption of locality is made merely for illustrative purposes; it will be evident to the reader that the transmission of work energy to the atmospheric masses and their consequent elevation will be continuously proceeding, more or less, over the whole planetary surface. to replace the gaseous material thus raised, a corresponding mass of air will move at a lower level, towards the equator from the more temperate zones adjoining. a circulatory motion will thus be set up in the atmosphere. in the upper regions the elevated and energised air masses move towards the poles; at lower levels the replacing masses move towards the equator, and in their passage may be operated on by the aqueous vapour which they encounter, energised, and raised to higher levels. the movement will be continuous. in their transference from equatorial towards polar regions, the atmospheric masses are leaving the surfaces or regions of high linear velocity for those of low, and must in consequence lose or return in the passage a portion of that natural energy of motion which they possess in virtue of their high linear velocity at the equator. but on the other hand, the replacing air masses, which are travelling in the opposite direction from poles to equator, must gain or absorb a corresponding amount of energy. the one operation thus balances the other, and the planetary equilibrium is in no way disturbed. but the atmospheric masses which are moving from the equator in the polar direction will possess, in addition, that energy of position which has been communicated to them through the medium of the aqueous vapour and by the working of the second stage of the atmospheric machine. these masses, in the circulatory polar movements, move downwards towards the planetary surface. in this downward motion (as in the downward motion of a pendulum mass vibrating under the action of gravitation) the energy of position of the air mass is converted once more into energy of motion--that is, into its original form of axial energy of rotation. in equatorial regions the really important energy property of the atmospheric mass was indicated by its elevation or its energy of position. in the descent this energy is thus entirely transformed, and reverts once more to its original form of energy of rotation. the continual transformation of axial energy by the aqueous vapour, and the conversion of that energy by the upward movement of the air masses into energy of position, naturally tends to produce a retardative effect on the motion of revolution of the earth. but this retardative effect is in turn completely neutralised or balanced by the corresponding accelerative effect due to the equally continuous return as the energy of the air masses reverts in the continuous polar movement to its original axial form. speaking generally, the equatorial regions, or the regions of high velocity, are the location of the most powerful transformation or abstraction of axial energy by the aqueous vapour. conversely, the polar or regions of low velocity are the location of the greatest return of energy by the air. as no energy return is possible unless by the transference of the atmospheric material from regions of high to regions of low velocity, the configuration of the planet in rotation must conform to this condition. the spheroidal form of the earth is thus exquisitely adapted to the working of the atmospheric machine. as already pointed out, however, the energising and raising of atmospheric masses is by no means confined to equatorial regions, but takes place more or less over the whole planetary surface. the same applies to the energy return. the complete cycle may be carried out in temperate zones; gaseous masses, also, leaving equatorial regions at high altitudes do not necessarily reach the polar regions, but may attain their lowest levels at intermediate points. neither do such masses necessarily proceed to the regions of low velocity by purely linear paths. on the contrary, they may and do move both towards the poles and downwards by circuitous and even vortical paths. in fact, as will be readily apparent, their precise path is of absolutely no moment in the consideration of energy return. it might naturally be expected that such movements of the atmospheric air masses as have been described above would give rise to great atmospheric disturbance over the earth's surface, and that the transfer of gaseous material from pole to equator and vice versa would be productive of violent storms of wind. such storms, however, are phenomena of somewhat rare occurrence; the atmosphere, on the whole, appears to be in a state of comparative tranquillity. this serenity of the atmosphere is, however, confined to the lower strata, and may be ascribed to an inherent stability possessed by the air mass as a whole in virtue of the accession of energy to it at high levels. as already explained, the transfer of energy from the vapour to the air masses is accomplished at comparatively low altitudes, and when this reaction is taking place the whole tendency of the energised material is to move upwards. in so moving it tends to leave behind it the condensed aqueous vapour, and would, therefore, rise to the higher altitudes in a comparatively dry condition. this dryness is accentuated by the further loss of aqueous vapour by condensation as the air moves toward regions of low velocity. that air which actually attains to the poles will be practically dry, and having also returned, in its entirety, the surplus energy obtained from the aqueous vapour, it will be in this region practically in the condition of statical equilibrium of a gas against gravity (§ ). but the general state of the atmosphere in other regions where a transference of energy from the aqueous vapour has taken or is taking place is very different from this condition of natural statical equilibrium which is approached at the poles. in the lower strata of the atmosphere the condition in some cases may approximate to the latter, but in the upper strata it is possessed of energy qualities quite abnormal to statical equilibrium. its condition is rather one of the nature of stable equilibrium. it is in a condition similar to that of a liquid heated in its upper layers; there is absolutely no tendency to a direct or vertical downward circulation. in statical equilibrium, any downward movement of an air mass would simply be accompanied by the natural rise in temperature corresponding to the transformation of its energy of position, but in this condition of stable equilibrium any motion downwards must involve, not only this natural temperature rise, but also a return, either in whole or in part, of the energy absorbed from the aqueous vapour. the natural conditions are therefore against any direct vertical return. these conditions, however, favour in every respect the circulatory motion of the highly energised upper air masses towards regions of low velocity. all circumstances combine, in fact, to confine the more powerfully energised and highly mobile air masses to high altitudes. in the lower atmosphere, owing to the continuous action of the aqueous vapour on the air masses moving from regions of low to those of high velocity, the circulation tends largely to be a vertical one, so that this locality is on the whole preserved in comparative tranquillity. it may happen, however, that owing to changes in the distribution of aqueous vapour, or other causes, this natural stability of the atmosphere may be disturbed over certain regions of the earth's surface. the circumstances will then favour a direct or more or less vertical return of the energy of the air masses in the neighbourhood of these regions. this return will then take the form of violent storms of wind, usually of a cyclonic nature, and affording direct evidence of the tendency of the air masses to pursue vortical paths in their movement towards lower levels. under normal conditions, however, the operation of the atmospheric machine is smooth and continuous. the earth's axial energy, under the sun's incepting influence, steadily flows at all parts of the earth's surface through the aqueous vapour into the atmospheric masses, and the latter, rising from the terrestrial surface, with a motion somewhat like that of a column of smoke, spread out and speed towards regions of lower velocity, and travelling by devious and lengthened paths towards the surface, steadily return the abstracted energy in its original form. every operation is exactly balanced; energy expenditure and energy return are complementary; the terrestrial atmospheric machine as a whole works without jar or discontinuity, and the earth's motion of rotation is maintained with absolute uniformity. like every other energy machine, the atmospheric machine has clearly-defined energy limits. the total quantity of energy in operation is strictly limited by the mass of the acting materials. it is well, also, to note the purely mechanical nature of the machine. every operation is in reality the operation of mechanical energy, and involves the movement of matter in some way or other relative to the earth's surface and under the incepting action of the earth's gravitation (§§ , ). the moving gaseous masses have as real an existence as masses of lead or other solid material, and require as real an expenditure of energy to move them relative to the terrestrial surface (§ ). this aspect of the planetary machine will be more fully treated later. throughout this description we have constantly assumed the atmospheric mixture of oxygen and nitrogen to act as one gas, and at ordinary temperatures the respective energy properties of the two substances (§ ) make this assumption justifiable. both gases are then working far above their respective evaporation temperatures. but, in the higher regions of the atmosphere, where very low temperatures prevail, a point or altitude will be reached where the temperature corresponds to the evaporation or condensation temperature of one of the gases. since oxygen appears to have the highest temperature of evaporation (see table of properties, p. ), it would naturally be the first to condense in the ascent. but immediately condensation takes place, the material will become susceptible to the incepting influence of the sun, and working as it does at its temperature of evaporation it will convey its energy to the surrounding nitrogen in precisely the same fashion as the aqueous vapour conveys the energy to the aerial mixture in the lower atmosphere. the whole action is made possible simply by the difference existing in the respective evaporation temperatures of the two gases. it will give rise to another cyclical atmospheric energy process exactly as already described for lower altitudes. axial energy of rotation will be communicated to the nitrogen by the working material, which is now the oxygen, and by the movement of the nitrogen masses towards regions of low velocity, this transmitted energy will be finally returned to its original axial form. it has been already explained (§§ , ) how all terrestrial energy processes, also, great or small, are sooner or later linked to the general atmospheric machine. the latter, therefore, presents in every phase of its working completely closed energy circuits. in no aspect of its operation can we find any evidence of, or indeed any necessity for, an energy transmission either to or from any external body or agent such as the sun. every phenomenon of nature is, in fact, a direct denial of such transmission. the student of terrestrial phenomena will readily find continuous and ample evidence in nature of the working of the atmospheric machine. in the rising vapour and the falling rain he will recognise the visible signs of the operation of that great secondary process of transmission by which the inherent axial energy of the earth is communicated to the air masses. the movements of bodies, animate and inanimate, on the earth's surface, the phenomena of growth and decay, and in fact almost every experience of everyday life, will reveal to him the persistent tendency of the energy of secondary processes to revert to the atmospheric machine. and in the winds that traverse the face of the globe he will also witness the mechanism of that energy return which completes the atmospheric cyclical process. it may be pointed out here also that the terrestrial cyclical energy processes are not necessarily all embodied in the atmosphere. the author has reason to believe, and phenomenal evidence is not awanting to show, that the circulatory motions of the atmosphere are in some degree reproduced in the sea. the reader will readily perceive that as regards stability the water composing the sea is in precisely the same condition as the atmosphere, namely, that of a liquid heated in its upper strata, and any circulatory motion of the water must therefore be accompanied by corresponding transformations of energy. that such a circulatory motion takes place is undoubted, and in the moving mass of sea-water we have therefore a perfectly reversible energy machine of the same general nature as the atmospheric machine, but working at a very much slower rate. it is not beyond the limits of legitimate scientific deduction to trace also the working of a similar machine in the solid material of the earth. the latter is, after all, but an agglomeration of loose material bound by the force of gravitation into coherent form. by the action of various erosive agencies a movement of solid material is continually taking place over the earth's surface. the material thus transported, it may be, from mountain chains, and deposited on the sea-bed, causes a disturbance of that gravitational equilibrium which defines the exact form of the earth. the forces tending to maintain this equilibrium are so enormous compared with the cohesive forces of the material forming the earth that readjustment continuously takes place, as evidenced by the tremors observed in the earth's crust. where the structure of the latter is of such a nature as to offer great resistance to the gravitational forces, the readjustment may take the form of an earthquake. geological evidence, as a whole, strongly points to a continuous kneading and flow of terrestrial material. the structure of igneous rocks, also, is exactly that which would be produced from alluvial deposits subjected during these cyclical movements to the enormous pressure and consequent heating caused by superimposed material. the occurrence of coal in polar regions, and of glacial residue in the tropics, may be regarded as further corroborative evidence. from this point of view also, it becomes unnecessary to postulate a genesis for the earth, as every known geological formation is shown to be capable of production under present conditions in nature, and in fact to be in actual process of production at all times. . _experimental analogy and demonstration of the general mechanism of energy transformation and return in the atmospheric cycle_ in the preceding articles, the atmospheric machine has been regarded more or less from the purely physical point of view. the purpose of this demonstration is now to place before the reader what might be termed the mechanical aspects of the machine; to give an outline, using simple experimental analogies, of its nature and operation when considered purely and simply as a mechanism for the transformation and return of mechanical energy. familiar apparatus is used in illustration. in all cases, it is merely some adaptation of the simple pendulum (§ ). its minute structural details are really of slight importance in the discussion, and have accordingly been ignored, but the apparatus generally, and the energy operations embodied therein, are so familiar to physicists and engineers that the experimental results illustrated can be readily verified by everyday experience. it is of great importance, also, in considering these results, to bear in mind the principles already enunciated (§§ , ) with reference to the operation of mechanical energy on the various forms of matter. the general working conditions of energy systems with respect to energy limits, stability, and reversibility (§ ) should also be kept in view. as an introductory step we shall review first a simple system of rotating pendulums. two simple pendulums cm and dm{ } (fig. ) are mounted by means of a circular collar cd upon a vertical spindle ab, which is supported at a and b and free to rotate. when the central spindle ab is at rest the pendulums hang vertically; when energy is applied to the system, and ab is thereby caused to rotate, the spherical masses m and m{ } will rise by circular paths about c and d. this upward movement, considered apart from the centrifugal influence producing it, corresponds in itself to the upward movement of the simple pendulum (§ ) against gravity. it is representative of a definite transformation, namely, the transformation of the work energy originally applied to the system and manifested in its rotary motion, into energy of position. the movements of the rotating pendulums will also be accompanied by other energy operations associated with bearing friction and windage (§§ , ), but these operations being part of a separate and complete cyclical energy process (§ ), they will in this case be neglected. [illustration: fig. ] it will be readily seen, however, that the working of this rotating pendulum machine, when considered as a whole, is of a nature somewhat different from that of the simple pendulum machine in that the energy of position of the former (as measured by the vertical displacement of m and m{ } in rotation) and its energy of rotation must increase concurrently, and also in that the absolute maximum value of this energy of position will be attained when the pendulum masses reach merely the horizontal level hl in rotation. the machines are alike, however, in this respect, that the transformation of energy of motion into energy of position is in each case a completely reversible process. in the working of the rotating pendulums the limiting amount of energy which can operate in this reversible process is dependent on and rigidly defined by the maximum length of the pendulum arms; the longer the arms, the greater is the possible height through which the masses at their extremities must rise to attain the horizontal position in rotation. it will be clear also that it is not possible for the whole energy of the rotating system to work in the reversible process as in the case of the simple pendulum. as the pendulum masses rise, the ratio of the limiting energy for reversibility to the total energy of the system becomes in fact smaller and smaller, until at the horizontal or position of maximum energy it reaches a minimum value. this is merely an aspect of the experimental fact that, as the pendulum masses approach the ultimate horizontal position, a much greater increment of energy to the system is necessary for their elevation through a given vertical distance than at the lower levels. a larger proportion of the applied energy is, in fact, stored in the material of the system in the form of energy of strain or distortion. the two points which this system is designed to illustrate, and which it is desirable to emphasise, are thus as follows. firstly, as the whole system rotates, the movement of the pendulum masses m and m{ } from the lower to the higher levels, or from the regions of low to those of higher velocity, is productive of a transformation of the rotatory energy of the system into energy of position--a transformation of the same nature as in the case of the simple pendulum system. neglecting the minor transformations (§§ , ), this energy process is a reversible one, and consequently, the return of the masses from the higher to the lower positions will be accompanied by the complete return of the transformed energy in its original form of energy of rotation. secondly, the maximum amount of energy which can work in this reversible process is always less than the total energy of the system. the latter, therefore, conforms to the general condition of stability (§ ). but this arrangement of rotating pendulums may be extended so as to include other features. to eliminate or in a manner replace the influence of gravitation, and to preserve the energy of position of the system--relative to the earth's surface--at a constant value, the pendulum arms may be assumed to be duplicated or extended to the points k and r (fig. ) respectively, where pendulum masses equal to m and m{ } are attached. the arms mk and m{ }r are thus continuous. each arm is assumed to be pivoted at its middle point about a horizontal axis through n, and as the lower masses m and m{ } rise in the course of the rotatory movement about ab the upper masses k and r will fall by corresponding amounts. the total energy of position of the system--referred to the earth's surface--thus remains constant whatever may be the position of the masses in the system. the restraining influence on the movement of the masses, formerly exercised by gravitation, is now furnished by means of a central spring f. a collar cd, connected as shown to the pendulum arms, slides on the spindle ab and compresses this spring as the masses move towards the horizontal level hl. as the masses return towards a and b the spring is released. [illustration: fig. ] if energy be applied to the system, so that it is caused to rotate about the central axis ab, the pendulum masses will tend to move outwards from that axis. their movement may be said to be carried out over the surface of an imaginary sphere with centre on ab at n. the motion of the masses, as the velocity of rotation increases, is from the region of lower peripheral velocity, in the vicinity of the axis ab, to the regions of higher velocity, in the neighbourhood of h and l. this outward movement from the central axis towards h and l is representative of a transformation of energy of an exactly similar nature to that described above in the simple case. part of the original energy of rotation of the system is now stored in the pendulum masses in virtue of their new position of displacement. but in this case, the movement is made, not against gravity, but against the central spring f. the energy, then, which in the former case might be said to be stored against gravitation (acting as an invisible spring) is in this case stored in the form of energy of strain or cohesion (§ ) in the central spring, which thus as it were takes the place of gravitation in the system. as in the previous case also, the operation is a reversible energy process. if the pendulum masses move in the opposite direction from the regions of higher velocity to those of lower velocity, the energy stored in the spring will be returned to the system in its original form of energy of motion. a vibratory motion of the pendulums to and from the central axis would thus be productive of an alternate storage and return of energy. it is obvious also, that due to the action of centrifugal force, the pendulum masses would tend to move radially outwards on the arms as they move towards the regions of highest velocity. let this radial movement be carried out against the action of four radial springs s{ }, s{ }, s{ }, s{ }, as shown (fig. ). in virtue of the radial movement of the masses, these springs will be compressed and energy stored in them in the form of energy of strain or cohesion (§ ). the radial movement implies also that the masses will be elevated from the surface of the imaginary sphere over which they are assumed to move. the elevation from this surface will be greatest in the regions of high velocity in the neighbourhood of h and l, and least at a and b. as the masses move, therefore, from h and l towards the axis ab, they will also move inwards on the pendulum arms, relieving the springs, so that the energy stored in them is free to be returned to the system in its original form of energy of rotation. every movement of the masses from the central axis outwards against the springs is thus made at the expense of the original energy of motion, and every movement inwards provokes a corresponding return of that energy to the system. every movement also against the springs forms part of a reversible operation. the sum total of the energy which works in these reversible operations is always less than the complete energy of the rotatory system, and the latter is always stable (§ ), with respect to its energy properties. let it now be assumed that the complete system as described is possessed of a precise and limited amount of energy of rotation, and that with the pendulum masses in an intermediate position as shown (fig. ) it is rotating with uniform angular velocity. the condition of the rotatory system might now be described as that of equilibrium. a definite amount of its original rotatory energy is now stored in the central spring and also in the radial springs. if now, without alteration in the intrinsic rotatory energy of the system, the pendulum masses were to execute a vibratory or pendulum motion about the position of equilibrium so that they move alternately to and from the central axis, then as they move inwards towards that axis the energy stored in the springs would be returned to the system in the original form of energy of rotation. this inward motion would, accordingly, produce acceleration. but, in the outward movement from the position of equilibrium, retardation would ensue on account of energy of motion being withdrawn from the system and stored in the springs. [illustration: fig. ] under the given conditions, then, any vibratory motion of the pendulum masses to and from the central axis would be accompanied by alternate retardation and acceleration of the moving system. the storage of energy in the springs (central and radial) produces retardation, the restoration of this energy gives rise to a corresponding acceleration. the angular velocity of the system would rise and fall accordingly. these are the natural conditions of working of the system. as already pointed out, the motion of the pendulum masses may be regarded as executed over the surface of an imaginary sphere. their motion against the radial springs would therefore correspond to a displacement outwards or upwards from the spherical surface. a definite part of the effect of retardation is, of course, due to this outward or radial displacement of the masses. assuming still the property of constancy of energy of rotation, let it now be supposed that in such a vibratory movement of the pendulum masses as described above, the energy required merely for the displacement of the masses _against the radial springs_ is not withdrawn from and obtained at the expense of the original rotatory energy of the system, but is obtained from some energy agency, completely external to the system, and to which energy cannot be returned. the retardation, normally due to the outward displacement of the masses against the radial springs, would not then take place. but the energy is, nevertheless, stored in the springs. it now, therefore, forms part of the energy of the system, and consequently, on the returning or inward movement of vibration of the masses towards the central axis, this energy, received from the external source, would pass directly from the springs to the rotational energy of the system. it is clear, then, that while the introduction of energy in this fashion from an external source has in part eliminated the effect of retardation, the accelerating effect must still operate as before. each vibratory movement of the pendulum system, under the given conditions, will lead to a definite increase in its energy of rotation by the amount stored in the radial springs. if the vibratory movement is continuous, the rotatory velocity of the system will steadily increase in value. energy once stored in the radial springs can only be released by the return movement of the masses and _in the form of energy of rotation_; the nature of the mechanical machine is, in fact, such that if any incremental energy is applied to the displacement of the masses against the radial springs, it can only be returned in this form of energy of motion. these features of this experimental system are of vital importance to the author's scheme. they may be illustrated more completely, however, and in a form more suitable for their most general application, by the hypothetical system now to be described. this system is, of course, devised for purely illustrative purposes, but the general principles of working of pendulum systems and of energy return, as demonstrated above, will be assumed. . _application of pendulum principles_ the movements of the pendulum masses described in the previous article have been regarded as carried out over the surface of an imaginary sphere. let us now proceed to consider the phenomena of a similar movement of material over the surface of an actual spherical mass. the precise dimensions of the sphere are of little moment in the discussion, but for the purpose of illustration, its mass and general outline may be assumed to correspond to that of the earth or other planetary body. this spherical mass a (fig. ) rotates with uniform angular velocity about an axis ns through its centre. associated with the rotating sphere are four auxiliary spherical masses, m{ }, m{ }, m{ }, m{ }, also of solid material, which are assumed to be placed symmetrically round its circumference as shown. these masses form an inherent part of the spherical system; they are assumed to be united to the main body of material by the attractive force of gravitation in precisely the same fashion as the atmosphere or other surface material of a planet is united to its inner core (§ ); they will therefore partake completely of the rotatory motion of the sphere about its axis ns, moving in paths similar to those of the rotating pendulum masses already described (§ ). the restraining action of the pendulum arms is, however, replaced in this celestial case by the action of gravitation, which is the central force or influence of the system. opposite masses are thus only united through the attractive influence of the material of the sphere. the place of the springs, both central and radial, in our pendulum system is now taken by this centripetal force of gravitative attraction, which therefore forms the restraining influence or determining factor in all the associated energy processes. while the auxiliary masses m{ } m{ }, &c., partake of the general motion of revolution of the main spherical mass about ns, they may also be assumed to revolve simultaneously about the axis we, perpendicular to ns, and also passing through the centre of the sphere. each of these masses will thus have a peculiar motion, a definite velocity over the surface of the sphere from pole to pole--about the axis we--combined with a velocity of rotation about the central axis ns. the value of the latter velocity is, at any instant, directly proportional to the radius of the circle of latitude of the point on the surface of the sphere where the mass happens to be situated at that instant in its rotatory motion from pole to pole; this velocity accordingly diminishes as the mass withdraws from the equator, and becomes zero when it actually reaches the poles of rotation at n and s; and the energy of each mass in motion, since its linear velocity is thus constantly varying, will be itself a continuously varying quantity, increasing or diminishing accordingly as the mass is moving to or from the equatorial regions, attaining its maximum value at the equator and its minimum value at the poles. now, since the masses thus moving are assumed to be a material and inherent portion of the spherical system, the source of the energy which is thus alternately supplied to and returned by them is the original energy of motion of the system; this original energy being assumed strictly limited in amount, the increase of the energy of each mass as it moves towards the equator will, therefore, be productive of a retardative effect on the revolution of the system as a whole. but, in a precisely similar manner, the energy thus gained by the mass would be fully returned on its movement towards the pole, and an accelerative effect would be produced corresponding to the original retardation. in the arrangement shown (fig. ), the moving masses are assumed to be situated at the extremities of diameters at right angles. with this symmetrical distribution, the transformation and return of energy would take place concurrently. retardation is continually balanced by acceleration, and the motion of the sphere would, therefore, be approximately uniform about the central axis of rotation. it will be clear that the movements thus described of the masses will be very similar in nature to those of the pendulum masses in the experimental system previously discussed. the fact that the motion of the auxiliary masses over the surface of the sphere is assumed to be completely circular and not vibratory, as in the pendulum case, has no bearing on the general energy phenomena. these are readily seen to be identical in nature with those of the simpler system. in each case every movement of the masses implies either an expenditure of energy or a return, accordingly as the direction of that movement is to or from the regions of high velocity. [illustration: fig. ] the paths of the moving auxiliary masses have been considered, so far, only as parallel to the surface of the sphere, but the general energy conditions are in no way altered if they are assumed to have in addition some motion normal to that surface; if, for example, they are repelled from the surface as they approach the equatorial regions, and return towards it once more as they approach the poles. such a movement of the masses normal to the spherical surface really corresponds to the movement against the radial springs in the pendulum system; it would now be made against the attractive or restraining influence of gravitation, and a definite expenditure of energy would thus necessarily be required to produce the displacement. energy, formerly stored in the springs, corresponds now to energy stored as energy of position (§ ) against gravitation. if this energy is obtained at the expense of the inherent rotatory energy of the sphere, then its conversion in this fashion into energy of position will again be productive of a definite retardative effect on the revolution of the system. it is clear, however, that if each mass descends to the surface level once more in moving towards the poles, then in this operation its energy of position, originally obtained at the expense of the rotatory energy of the sphere, will be gradually but completely returned to that source. in a balanced system, such as we have assumed above, the descent of one mass in rotation would be accompanied by the elevation of another at a different point; the abstraction and return of the energy of rotation would then be equivalent, and would not affect the primary condition of uniformity of rotation of the system. in the circumstances assumed, the whole energy process which takes place in the movement of the masses from poles to equator and normal to the spherical surface would obviously be of a cyclical nature and completely reversible. it would be the working of mechanical energy in a definite material machine, and in accordance with the principles already outlined (§ ) the maximum amount of energy which can operate in this machine is strictly limited by the mass of the material involved in the movement. the energy machine has thus a definite capacity, and as the maximum energy operating in the reversible cycle is assumed to be within this limit, the machine would be completely stable in nature (§ ). the movements of the auxiliary masses have hitherto also been considered as taking place over somewhat restricted paths, but this convention is one which can readily be dispensed with. the general direction of motion of the masses must of course be from equator to pole or vice versa; but it is quite obvious that the exact paths pursued by the masses in this general motion is of no moment in the consideration of energy return, nor yet the precise region in which they may happen to be restored once more to the surface level. whatever may be its position at any instant, each mass is possessed of a definite amount of energy corresponding to that position; this amount will always be equal to the total energy abstracted by that mass, less the energy returned. the nature of the energy system is, however, such that the various energy phases of the different masses will be completely co-ordinated. since the essential feature of the system is its property of uniformity of rotation, any return of energy in the rotational form at any part of the system--due to the descent of material--produces a definite accelerating effect on the system, which effect is, however, at once neutralised or absorbed by a corresponding retardative effect due to that energy which must be extracted from the system in equivalent amount and devoted to the upraising of material at a different point. for simplicity in illustration only four masses have been considered in motion over the surface of the sphere, but it will be clear that the number which may so operate is really limited only by the dimensions of the system. the spherical surface might be completely covered with moving material, not necessarily of spherical form, not necessarily even material in the solid form (§ ), which would rise and fall relative to the surface and flow to and from the poles exactly in the fashion already illustrated by the moving masses. the capacity of the reversible energy machine--which depends on the mass--would be altered in this case, but not the general nature of the machine itself. if the system were energised to the requisite degree, every energy operation could be carried out as before. as already pointed out, the dominating feature of a spherical system such as we have just described would be essentially its property of energy conservation manifested by its uniformity of rotation. all its operations could be carried out independently of the direct action of any external energy influences. for if it be assumed that the energy gained by the auxiliary moving surface material _in virtue of its displacement normal to the spherical surface_ be derived, not from the inherent rotational energy of the sphere itself, but by an influx of energy from some source completely external to the system, then since there has been no energy abstraction there will be no retardative effect on the revolution due to the upraising of this material. but the influx of energy thus stored in the material must of necessity work through the energy machine. in the movement towards the poles this energy would therefore be applied to the system in the form of energy of rotation, and would produce a definite accelerative effect. if the influx of energy were continuous, and no means were existent for a corresponding efflux, the rotatory velocity of the system would steadily increase. the phenomena would be of precisely the same nature as those already alluded to in the case of the system of rotating pendulums (§ ). acceleration would take place without corresponding retardation. a direct contribution would be continuously made to the rotatory energy of the system, and would under the given conditions be manifested by an increase in its velocity of revolution. . _extension of pendulum principles to terrestrial phenomena_ the energy phenomena illustrated by the experimental devices above are to be observed, in their aspects of greatest perfection, in the natural world. in the earth, united to its encircling atmosphere by the invisible bond of gravitation, we find the prototype of the hypothetical system just described. its uniformity of rotation is an established fact of centuries, and over its spheroidal surface we have, corresponding to the motion of our illustrative spherical masses, the movement of enormous quantities of atmospheric air in the general directions from equatorial to polar regions and vice versa. this circulatory movement, and the internal energy reactions which it involves, have been already fully dealt with (§ ); we have now to consider it in a somewhat more comprehensive fashion, in the light of the pendulum systems described above. as already explained (§ ), the operation of mechanical energy is not confined to solid and liquid masses only, but may likewise be manifested by the movements of gaseous masses. the terrestrial atmospheric machine provides an outstanding example. in its working conditions, and in the general nature of the energy operations involved, the terrestrial atmospheric machine is very clearly represented by the rotating pendulum system (§ ). the analogy is still closer in the case of the hypothetical system just described. the actual terrestrial energy machine differs from both only in that the energy processes, which they illustrate by the movements of solid material, are carried out in the course of its working by the motion of gaseous masses. it is obvious, however, that this in no way affects the inherent nature of the energy processes themselves. they are carried out quite as completely and efficiently--in fact, more completely and more efficiently--by the motions of gaseous as by the motions of solid material. the atmospheric circulation, then, may be readily regarded as the movement, over the terrestrial surface, of gaseous masses which absorb and return energy in regions of high and low velocity exactly in the fashion explained above for solid material. in their movement from polar towards equatorial regions these masses, by the action of the aqueous vapour (§ ), absorb energy (axial energy) and expand upwards against gravity. here we have an energy operation identical in nature with that embodied in the movements of a pendulum mass simultaneously over a spherical surface and against radial springs as in the system of rotating pendulums, or identical with the equatorial and radial movement of the auxiliary masses in the hypothetical system. the return movement of the aerial masses over the terrestrial surface in the opposite direction from equatorial to polar regions provides also exactly the same phenomena of energy return as the return movement of the masses in our illustrative systems. these systems, in fact, portray the general operation of mechanical energy precisely as it occurs in the terrestrial atmospheric machine. but obviously they cannot illustrate the natural conditions in their entirety. the passage or flow of the atmospheric air masses over the earth's surface is a movement of an exceedingly complex nature, impossible to illustrate by experimental apparatus. and indeed, such illustration is quite unnecessary. as already pointed out (§ ), no matter what may be the precise path of an aerial mass in its movement towards the planetary surface the final energy return is the same. sooner or later its energy of position is restored in the original axial form. the terrestrial atmospheric machine will be thus readily recognised as essentially a material mechanical machine corresponding in general nature to the illustrative examples described above. the combination of its various energy processes is embodied in a complete cyclical and reversible operation. its energy capacity, as in the simpler cases, is strictly limited by the total mass of the operating material. the active or working energy is well within the limit for reversibility (§ ), and the machine is therefore essentially stable in nature. the continuous abstraction of axial energy by the aqueous vapour is balanced by an equally continuous return from the air masses, and the system, so far as its energy properties are concerned, is absolutely conservative. energy transmission from or to any external source is neither admissible nor necessary for its working. . _concluding review of terrestrial conditions--effects of influx of energy_ the aspect of the earth as a separate mass in space, and its energy relationship to its primary the sun and to the associated planetary masses of the solar system have been broadly presented in the general statement (§§ - ). in that statement, based entirely on the universally accepted properties of matter and energy, an order of phenomena is described which is in strict accordance with observed natural conditions, and which portrays the earth and the other planetary bodies, so far as their material or energy properties are concerned, as absolutely isolated masses in space. the scientific verification of this position must of necessity be founded on the terrestrial observation of phenomena. so far as the orbital movements of the planet are concerned these are admittedly orderly; each planetary mass wheels its flight through space with unvarying regularity; the energy processes, also, associated with the variations of planetary orbital path, and which attain limiting conditions at perihelion and aphelion, are readily acknowledged to be reversible and cyclical in nature. in fact, even a slight observation of the movements of celestial masses inevitably leads to the conviction that the great energy processes of the solar system are inherently cyclical in nature, that every movement of its material and every manifestation of its energy is part of some complete operation. the whole appears to be but the natural or material embodiment of the great principle of energy conservation. it has been one of the objects of this work to show that the cyclical nature of the energy operations of the solar system is not confined only to the more prominent energy phenomena, but that it penetrates and is exhibited in the working of even the most insignificant planetary processes. each one of the latter in reality forms part of an unbroken series or chain of energy phenomena. each planet forms in itself a complete, perfect, and self-contained energy system. every manifestation of planetary energy, great or small, whether associated with animate or inanimate matter, is but one phase or aspect of that energy as it pursues its cyclical path. it is a somewhat remarkable fact that in this age of scientific reason the observation of the strictly orderly arrangement of phenomena in the solar system as a whole should not have led to some idea in the minds of philosophical workers of a similar order of phenomena in its separate parts, but the explanation lies generally in the continual attempts to bring natural phenomena into line with certain preconceived hypotheses, and more particularly to the almost universal acceptance of the doctrine of the direct transmission of energy from the sun to the earth and the final rejection or radiation of this energy into space. there is no denying the eminent plausibility of this doctrine. the evidence of nature _prima facie_ may even appear to completely substantiate it. but we would submit that the general circumstances in which this doctrine is now so readily accepted are very similar to those which prevailed in more ancient times, when the revolution of the sun and stars round the earth was the universal tenet of natural philosophy. this conception, allied to the belief that the sole function of the celestial bodies was to provide light and heat to the terrestrial mass, appeared to be in strict accordance with observed phenomena, and held undisturbed possession of the minds of men for centuries, until it was finally demolished by copernicus as the result of simple and accurate observation of and deduction from natural phenomena. at the present time, the somewhat venerable belief in the transmission of energy in various forms from the sun to the earth appears at first sight to be supported by actual facts. but a more rigid scrutiny of the evidence and of the mental processes must inevitably lead the unbiassed mind to the conclusion that this belief has no real foundation on truly scientific observation, but is entirely unsupported by natural phenomena. every operation of nature, in fact, when considered in its true relationships is an absolute denial of the whole conception. like its predecessor relating to the motion of the sun and stars round the earth, the doctrine of energy transmission between separate masses in space such as the sun and the earth cannot be sustained in the face of scientific observation. this doctrine is found on investigation to be supported not by phenomena but by the conception of an elastic ethereal medium, of whose existence there is absolutely no evidential proof, and the necessity for which disappears along with the hypothesis it supports. it is, however, not proposed to discuss in any detail either the supposed transmission of energy from the sun to the planets or the arbitrary properties of the transmitting medium, but rather to adopt a more positive method of criticism by summarising briefly the evidential phenomena which show the cyclical nature of the whole terrestrial energy process, and which remove the basis of belief in such a transmission. to recapitulate the more general conditions, we find the earth, alike with other planetary masses, pursuing a defined orbital path, and rotating with uniform angular velocity in the lines or under the influence of the gravitation, thermal, luminous, and other incepting fields (§§ , , ) which originate in the sun. its axial rotation, in these circumstances, gives rise to all the secondary transformations (§ ) of terrestrial axial energy, which in their operation provide the varied panorama of terrestrial phenomena. terrestrial axial energy is thus diverted into terrestrial secondary processes. each of these processes is found to be united to or embodied in a definite material machine (§§ - ), and is, accordingly, limited in nature and extent by the physical properties and incepting factors associated with the materials of which the machine is composed. by ordinary methods of transmission, energy may pass from one material to another, that is to say from one machine to another, and by this means definite chains of energy processes are constituted, through which, therefore, passes the axial energy originally transformed by the action of the sun. these series or chains of energy processes are also found to be one and all linked at some stage of their progress to the general atmospheric machine (§ ). the energy operating in them is, in every case, after many or few vicissitudes according to the nature of the intermediate operations, communicated to the gaseous atmospheric material. by the movement of this material in the working of the atmospheric machine (§ ) the energy is finally returned in its original form of axial energy of rotation. the sun's action is thus in a manner to force the inherent rotatory energy of the planet into the cyclical secondary operations, all of which converge alike towards the general atmospheric mechanism of return. the passage of the energy through the complete secondary operations, and its re-conversion into its original axial form, may be rapid or slow according to circumstances. in equatorial regions, where the influence of the sun's incepting fields is most intense, we find that the inherent planetary axial energy is communicated with great rapidity through the medium of the aqueous vapour to the air masses. by the movement of the latter it may be just as rapidly returned, and the whole operation completed in a comparatively short interval of time. in the same equatorial regions, the transformations of axial energy which are manifested in plant life attain their greatest perfection and vigour. but in this case the complete return of the operating energy may be very slow. the stored energy of tropical vegetation may still in great part remain in the bosom of the earth, awaiting an appropriate stimulus to be communicated to more active material for the concluding stages of that cyclical process which had its commencement in the absorption of axial energy into plant tissue. the duration of the complete secondary operation has, however, absolutely no bearing on the conservative energy properties of the planet. in this respect, the system is perfectly balanced. every transformation or absorption of rotatory energy, great or small, for long or short periods of time, is counteracted by a corresponding return. absolute uniformity of planetary axial rotation is thus steadily maintained. it is scarcely necessary at this stage to point out that the verification of this description of natural operations lies simply and entirely in the observation of nature's working at first hand. the description is based on no theory and obscured by no preconceived ideas, it is founded entirely on direct experimental evidence. the field of study and of verification is not restricted, but comprises the whole realm of natural phenomena. in a lifetime of observation the author has failed to discern a single contradictory phenomenon; every natural operation is in reality a direct confirmation. the conception of energy, working only through the medium of definite material machines with their incepting and limiting agencies, is one which is of great value not only in natural philosophy but also in practical life. by its means it is possible in many cases to co-ordinate phenomena, apparently antagonistic, but in reality only different phases of energy machines. it aids materially also in the obtaining of a true grasp of the inexorable principle of energy conservation and its application to natural conditions, and it emphasises the indefensible nature of such ideas as the radiation of energy into _space_. it will be evident that in a planetary system such as described above there is no room for any transmission of energy to the system from an external source. the nature of the system is, in fact, such that a transmission of this kind is entirely unnecessary. as already demonstrated, every phenomenon and every energy operation can be carried out independently of any such transmission. for the purpose of illustration, however, it may be assumed that such a communication of energy does take place; that according to the accepted doctrines of modern science the sun pours energy in a continuous stream into the terrestrial system. now, no matter in what form this energy is communicated, it is clear that once it is associated with or attached to the various planetary materials it is, as it were, incorporated or embodied in the planetary energy machines, and must of necessity work through the secondary energy operations. but these operations have been shown to be naturally and irresistibly connected to the general atmospheric machine. into this machine, then, the incremental energy must be carried, and it will be there directly converted into the form of axial energy of rotation. once the incremental energy is actually in the planet, once it is actually communicated to planetary material, the nature of the system absolutely forbids its escape. the effect of a direct and continuous influx such as we have assumed would inevitably be an increase in the angular velocity of the system. this effect has already been verified from an experimental point of view by consideration of the phenomena of a rotating pendulum system (§§ , ). whilst the influx of energy proceeds, then in virtue of the increasing velocity of the planetary material in the lines of the various incepting fields of the sun, all terrestrial phenomena involving the transformation of rotary or axial energy would be increased in magnitude and intensified in degree. the planet would thus rapidly attain an unstable condition; its material would soon become energised beyond its normal capacity, and the natural stability (§ ) of its constituent energy machines would be destroyed; the system as a whole would steadily proceed towards disruption. but, happily, nature presents no evidence of such a course of events. the earth spins on its axis with quiet and persistent regularity; the unvarying uniformity of its motion of axial rotation has been verified by the observations of generations of philosophers. its temperature gradations show no evidence of change or decay in its essential heat qualities, and the recurrence of natural phenomena is maintained without visible sign of increase either in their intensity or multiplicity. the finger of nature ever points to closed energy circuits, to the earth as a complete and conservative system in which energy, mutable to the highest degree with respect to its plurality of form, attains to the perfection of permanence in its essential character and amount. printed by ballantyne, hanson & co. edinburgh & london transcriber's note: in this plain-text version, numerical subscripts have been transcribed within {} brackets, such as: m{ }. obvious typographical errors from the original printed version of this book have been corrected without comment. footnotes in the plain-text version have been placed at the end of the paragraph in which the footnote tag appears. illustrations have been moved to the nearest paragraph break. trinity site by the u.s. department of energy national atomic museum, albuquerque, new mexico contents: the first atomic test. jumbo. schmidt-mcdonald ranch house. notes. bibliography. the national atomic museum. the first atomic test on monday morning july , , the world was changed forever when the first atomic bomb was tested in an isolated area of the new mexico desert. conducted in the final month of world war ii by the top-secret manhattan engineer district, this test was code named trinity. the trinity test took place on the alamogordo bombing and gunnery range, about miles south of the manhattan project's headquarters at los alamos, new mexico. today this , square mile range, partly located in the desolate jornada del muerto valley, is named the white sands missile range and is actively used for non-nuclear weapons testing. before the war the range was mostly public and private grazing land that had always been sparsely populated. during the war it was even more lonely and deserted because the ranchers had agreed to vacate their homes in january . they left because the war department wanted the land to use as an artillery and bombing practice area. in september , a remote by square mile portion of the north-east corner of the bombing range was set aside for the manhattan project and the trinity test by the military. the selection of this remote location in the jornada del muerto valley for the trinity test was from an initial list of eight possible test sites. besides the jornada, three of the other seven sites were also located in new mexico: the tularosa basin near alamogordo, the lava beds (now the el malpais national monument) south of grants, and an area southwest of cuba and north of thoreau. other possible sites not located in new mexico were: an army training area north of blythe, california, in the mojave desert; san nicolas island (one of the channel islands) off the coast of southern california; and on padre island south of corpus christi, texas, in the gulf of mexico. the last choice for the test was in the beautiful san luis valley of south-central colorado, near today's great sand dunes national monument. based on a number of criteria that included availability, distance from los alamos, good weather, few or no settlements, and that no indian land would be used, the choices for the test site were narrowed down to two in the summer of . first choice was the military training area in southern california. the second choice, was the jornada del muerto valley in new mexico. the final site selection was made in late august by major general leslie r. groves, the military head of the manhattan project. when general groves discovered that in order to use the california location he would need the permission of its commander, general george patton, groves quickly decided on the second choice, the jornada del muerto. this was because general groves did not want anything to do with the flamboyant patton, who groves had once described as "the most disagreeable man i had ever met."[ ] despite being second choice the remote jornada was a good location for the test, because it provided isolation for secrecy and safety, was only miles south of los alamos, and was already under military control. plus, the jornada enjoyed relatively good weather. the history of the jornada is in itself quite fascinating, since it was given its name by the spanish conquerors of new mexico. the jornada was a short cut on the camino real, the king's highway that linked old mexico to santa fe, the capital of new mexico. the camino real went north from mexico city till it joined the rio grande near present day el paso, texas. then the trail followed the river valley further north to a point where the river curved to the west, and its valley narrowed and became impassable for the supply wagons. to avoid this obstacle, the wagons took the dubious detour north across the jornada del muerto. sixty miles of desert, very little water, and numerous hostile apaches. hence the name jornada del muerto, which is often translated as the journey of death or as the route of the dead man. it is also interesting to note that in the late th century, the spanish considered their province of new mexico to include most of north america west of the mississippi! the origin of the code name trinity for the test site is also interesting, but the true source is unknown. one popular account attributes the name to j. robert oppenheimer, the scientific head of the manhattan project. according to this version, the well read oppenheimer based the name trinity on the fourteenth holy sonnet by john donne, a th century english poet and sermon writer. the sonnet started, "batter my heart, three-personed god."[ ] another version of the name's origin comes from university of new mexico historian ferenc m. szasz. in his book, the day the sun rose twice, szasz quotes robert w. henderson head of the engineering group in the explosives division of the manhattan project. henderson told szasz that the name trinity came from major w. a. (lex) stevens. according to henderson, he and stevens were at the test site discussing the best way to haul jumbo (see below) the thirty miles from the closest railway siding to the test site. "a devout roman catholic, stevens observed that the railroad siding was called 'pope's siding.' he [then] remarked that the pope had special access to the trinity, and that the scientists would need all the help they could get to move the ton jumbo to its proper spot."[ ] the trinity test was originally set for july , . however, final preparations for the test, which included the assembly of the bomb's plutonium core, did not begin in earnest until thursday, july . the abandoned george mcdonald ranch house located two miles south of the test site served as the assembly point for the device's core. after assembly, the plutonium core was transported to trinity site to be inserted into the thing or gadget as the atomic device was called. but, on the first attempt to insert the core it stuck! after letting the temperatures of the core and the gadget equalize, the core fit perfectly to the great relief of all present. the completed device was raised to the top of a -foot steel tower on saturday, july . during this process workers piled up mattresses beneath the gadget to cushion a possible fall. when the bomb reached the top of the tower without mishap, installation of the explosive detonators began. the -foot tower (a surplus forest service fire-watch tower) was designated point zero. ground zero was at the base of the tower. as a result of all the anxiety surrounding the possibility of a failure of the test, a verse by an unknown author circulated around los alamos. it read: from this crude lab that spawned a dud. their necks to truman's ax uncurled lo, the embattled savants stood, and fired the flop heard round the world.[ ] a betting pool was also started by scientists at los alamos on the possible yield of the trinity test. yields from , tons of tnt to zero were selected by the various bettors. the nobel prize-winning ( ) physicist enrico fermi was willing to bet anyone that the test would wipe out all life on earth, with special odds on the mere destruction of the entire state of new mexico! meanwhile back at the test site, technicians installed seismographic and photographic equipment at varying distances from the tower. other instruments were set up for recording radioactivity, temperature, air pressure, and similar data needed by the project scientists. according to lansing lamont in his book day of trinity, life at trinity could at times be very exciting. one afternoon while scientists were busily setting up test instruments in the desert, the tail gunner of a low flying b- bomber spotted some grazing antelopes and opened up with his twin. -caliber machine guns. "a dozen scientists,... under the plane and out of the gunner's line of vision, dropped their instruments and hugged the ground in terror as the bullets thudded about them."[ ] later a number of these scientists threatened to quit the project. workers built three observation points . miles ( , yards), north, south, and west of ground zero. code named able, baker, and pittsburgh, these heavily-built wooden bunkers were reinforced with concrete, and covered with earth. the bunker designated baker or south , served as the control center for the test. this is where head scientist j. robert oppenheimer would be for the test. a fourth observation point was the test's base camp, (the abandoned dave mcdonald ranch) located about ten miles southwest of ground zero. the primary observation point was on compania hill, located about miles to the northwest of trinity near today's stallion range gate, off nm . the test was originally scheduled for a.m., monday july , but was postponed to : due to a severe thunderstorm that would have increased the amount of radioactive fallout, and have interfered with the test results. the rain finally stopped and at : : a.m. mountain war time, the device exploded successfully and the atomic age was born. the nuclear blast created a flash of light brighter than a dozen suns. the light was seen over the entire state of new mexico and in parts of arizona, texas, and mexico. the resultant mushroom cloud rose to over , feet within minutes, and the heat of the explosion was , times hotter than the surface of the sun! at ten miles away, this heat was described as like standing directly in front of a roaring fireplace. every living thing within a mile of the tower was obliterated. the power of the bomb was estimated to be equal to , tons of tnt, or equivalent to the bomb load of , b- , superfortresses! after witnessing the awesome blast, oppenheimer quoted a line from a sacred hindu text, the bhagavad-gita: he said: "i am become death, the shatterer of worlds."[ ] in los alamos miles to the north, a group of scientists' wives who had stayed up all night for the not so secret test, saw the light and heard the distant sound. one wife, jane wilson, described it this way, "then it came. the blinding light [no] one had ever seen. the trees, illuminated, leaping out. the mountains flashing into life. later, the long slow rumble. something had happened, all right, for good or ill."[ ] general groves' deputy commander, brigadier general t. f. farrell, described the explosion in great detail: "the effects could well be called unprecedented, magnificent, beautiful, stupendous, and terrifying. no man-made phenomenon of such tremendous power had ever occurred before. the lighting effects beggared description. the whole country was lighted by a searing light with the intensity many times that of the midday sun. it was golden, purple, violet, gray, and blue. it lighted every peak, crevasse and ridge of the nearby mountain range with a clarity and beauty that cannot be described but must be seen to be imagined..."[ ] immediately after the test a sherman m- tank, equipped with its own air supply, and lined with two inches of lead went out to explore the site. the lead lining added tons to the tank's weight, but was necessary to protect its occupants from the radiation levels at ground zero. the tank's passengers found that the -foot steel tower had virtually disappeared, with only the metal and concrete stumps of its four legs remaining. surrounding ground zero was a crater almost , feet across and about ten feet deep in places. desert sand around the tower had been fused by the intense heat of the blast into a jade colored glass. this atomic glass was given the name atomsite, but the name was later changed to trinitite. due to the intense secrecy surrounding the test, no accurate information of what happened was released to the public until after the second atomic bomb had been dropped on japan. however, many people in new mexico were well aware that something extraordinary had happened the morning of july , . the blinding flash of light, followed by the shock wave had made a vivid impression on people who lived within a radius of miles of ground zero. windows were shattered miles away in silver city, and residents of albuquerque saw the bright light of the explosion on the southern horizon and felt the tremor of the shock waves moments later. the true story of the trinity test first became known to the public on august , . this is when the world's second nuclear bomb, nicknamed little boy, exploded , feet over hiroshima, japan, destroying a large portion of the city and killing an estimated , to , of its inhabitants. three days later on august , a third atomic bomb devastated the city of nagasaki and killed approximately , more japanese. the nagasaki weapon was a plutonium bomb, similar to the trinity device, and it was nicknamed fat man. on tuesday august , at p.m. eastern war time, president truman made a brief formal announcement that japan had finally surrendered and world war ii was over after almost six years and million deaths! on sunday, september , , trinity site was opened to the press for the first time. this was mainly to dispel rumors of lingering high radiation levels there, as well as in hiroshima and nagasaki. led by general groves and oppenheimer, this widely publicized visit made trinity front page news all over the country. trinity site was later encircled with more than a mile of chain link fencing and posted with signs warning of radioactivity. in the early s most of the remaining trinitite in the crater was bulldozed into a underground concrete bunker near trinity. also at this time the crater was back filled with new soil. in the trinitite was removed from the bunker, packed into -gallon drums, and loaded into trucks belonging to the atomic energy commission (the successor of the manhattan project). trinity site remained off-limits to military and civilian personnel of the range and closed to the public for many years, despite attempts immediately after the war to turn trinity into a national monument. in about people attended the first trinity site open house sponsored by the alamogordo chamber of commerce and the missile range. two years later, a small group from tularosa, nm visited the site on the th anniversary of the explosion to conduct a religious service and pray for peace. regular visits have been made annually in recent years on the first saturday in october instead of the anniversary date of july , to avoid the desert heat. later trinity site was opened one additional day on the first saturday in april. the site remains closed to the public except for these two days, because it lies within the impact areas for missiles fired into the northern part of the range. in , range officials erected a modest monument at ground zero. built of black lava rock, this monument serves as a permanent marker for the site and as a reminder of the momentous event that occurred there. on the monument is a plain metal plaque with this simple inscription: "trinity site where the world's first nuclear device was exploded on july , ." during the annual tour in , a second plaque was added below the first by the national park service, designating trinity site a national historic landmark. this plaque reads, "this site possesses national significance in commemorating the history of the u.s.a." jumbo lying next to the entrance of the chain link fence that still surrounds trinity site are the rusty remains of jumbo. jumbo was the code name for the -ton thermos shaped steel and concrete container designed to hold the precious plutonium core of the trinity device in case of a nuclear mis-fire. built by the babcock and wilcox company of barberton, ohio, jumbo was feet long, feet, inches in diameter, and with steel walls up to inches thick. the idea of using some kind of container for the trinity device was based on the fact that plutonium was extremely expensive and very difficult to produce. so, much thought went into a way of containing the lb. plutonium core of the bomb, in case the , lbs. of conventional high explosives surrounding the core exploded without setting off a nuclear blast, and in the process scattering the costly plutonium (about million dollars worth) across the dessert. after extensive research and testing of other potential containment ideas, the concept of jumbo was decided on in the late summer of . however, by the spring of , after jumbo had already been built and transported (with great difficulty) to the trinity site by the eichleay corporation of pittsburgh, it was decided not to explode the trinity device inside of jumbo after all. there were several reasons for this new decision: first, plutonium had become more readily (relatively) available; second, the project scientists decided that the trinity device would probably work as planned; and last, the scientists realized that if jumbo were used it would adversely affect the test results, and add tons of highly radioactive material to the atmosphere. not knowing what else to do with the massive million dollar jumbo, it was decided to suspend it from a steel tower yards from ground zero to see how it would withstand the trinity test. jumbo survived the approximately kiloton trinity blast undamaged, but its supporting -foot tall steel tower was flattened. two years later, in an attempt to destroy the unused jumbo before it and its million dollar cost came to the attention of a congressional investigating committee, manhattan project director general groves ordered two junior officers from the special weapons division at sandia army base in albuquerque to test jumbo. the army officers placed eight -pound conventional bombs in the bottom of jumbo. since the bombs were on the bottom of jumbo, and not the center (the correct position), the resultant explosion blew both ends off jumbo. unable to totally destroy jumbo, the army then buried it in the desert near trinity site. it was not until the early s that the impressive remains of jumbo, still weighing over tons, were moved to their present location. schmidt-mcdonald ranch house the schmidt-mcdonald ranch house is located two miles south of ground zero. the property encompasses about three acres and consists of the main house and assorted outbuildings. the house, surrounded by a low stone wall, was built in by franz schmidt, a german immigrant and homesteader. in the s schmidt sold the ranch to george mcdonald and moved to florida. the ranch house is a one-story, , square-foot adobe (mud bricks) building. an ice house is located on the west side along with an '- " deep underground cistern. a by . foot stone addition, which included a modern bathroom, was added onto the north side in the s. east of the house there is a large, divided concrete water storage tank and a windmill. south of the windmill are the remains of a bunkhouse, and a barn which also served as a garage. further to the east are corrals and holding pens for livestock. the mcdonalds vacated their ranch house and their thousands of acres of marginal range land in early when it became part of the alamogordo bombing and gunnery range. the old house remained empty until manhattan project personnel arrived in . then a spacious room in the northeast corner of the house was selected by the project personnel for the assembly of the plutonium core of the trinity device. workmen installed work benches, tables, and other equipment in this large room. to keep the desert dust and sand out, the room's windows and cracks were covered with plastic and sealed with tape. the core of the bomb consisted of two hemispheres of plutonium, (pu- ), and an initiator. according to reports, while scientists assembled the initiator and the pu- hemispheres, jeeps were positioned outside with their engines running for a quick getaway if needed. detection devices were used to monitor radiation levels in the room, and when fully assembled the core was warm to the touch. the completed core was later transported the two miles to ground zero, inserted into the bomb assembly, and raised to the top of the tower. the trinity explosion on monday morning, july , did not significantly damage the mcdonald house. even though most of the windows were blown out, and the chimney was blown over, the main structure survived intact. years of rain water dripping through holes in the metal roof did much more damage to the mud brick walls than the bomb did. the nearby barn did not fare as well. the trinity test blew part of its roof off, and the roof has since totally collapsed. the ranch house stood empty and deteriorating for years until when the us army stabilized it to prevent any further damage. the next year, the department of energy and the army provided funds for the national park service to completely restore the house to the way it appeared in july, . when the work was completed, the house with many photo displays on trinity was opened to the public for the first time in october during the semi-annual tour. the schmidt-mcdonald ranch house is part of the trinity national historic landmark. footnotes [footnote : szasz, ferenc. the day the sun rose twice. albuquerque: university of new mexico press, . p. .] [footnote : hayward, john, ed. john donne: complete poetry and selected prose. new york: random house, inc., . p. .] [footnote : szasz, the day the sun rose twice, p. .] [footnote : wyden, peter. day one: before hiroshima and after. new york: simon and schuster, . p. .] [footnote : lamont, lansing. day of trinity. new york: atheneum, . p. - .] [footnote : kunetka, james w. city of fire: los alamos and the atomic age, - . albuquerque: university of new mexico press, . p. .] [footnote : wilson, jane s. and charlotte serber, eds. standing by and making do: women in wartime los alamos. los alamos: los alamos historical society, . p. x, xi.] [footnote : brown, anthony cave, and charles b. macdonald. the secret history of the atomic bomb. new york: dell, . p. .] bibliography bainbridge, kenneth t. trinity. los alamos: los alamos scientific laboratory, (la- -h), . brown, anthony cave, and charles b. macdonald. the secret history of the atomic bomb. new york: dell, . compton, arthur holly. atomic quest: a personal quest. new york: oxford university press, . fanton, jonathan f., stoff, michael b. and williams, r. hal editors. the manhattan project: a documentary introduction to the atomic age. philadelphia: temple university press, . feis, herbert. japan subdued: the atomic bomb and the end of the war in the pacific. princeton: princeton university press, . groves, leslie r. now it can be told: the story of the manhattan project. new york: da capo press, . hersey, john. hiroshima. new york: alfred a. knopf, . jette, eleanor. inside box . los alamos: los alamos historical society, . kunetka, james w. city of fire: los alamos and the atomic age, - . albuquerque; university of new mexico press, . lamont, lansing. day of trinity. new york: athenaeum, . rhodes, richard. the making of the atomic bomb. new york: simon and schuster, . skates, john ray. the invasion of japan: alternative to the bomb. columbia; university of south carolina press, . smyth, henry dewolf. atomic energy for military purposes. princeton: princeton university press, . szasz, ferenc. the day the sun rose twice. albuquerque: university of new mexico press, . tibbets, paul w. flight of the enola gay. reynoldsburg, ohio: buckeye aviation book company, . williams, robert c. klaus fuchs, atom spy. cambridge, massachusetts: harvard university press, . wilson, jane s. and serber, charlotte, eds. standing by and making do: women in wartime los alamos. los alamos: los alamos historical society, . wyden, peter. day one: before hiroshima and after. new york: simon and schuster, . the national atomic museum, kirtland air force base, albuquerque, new mexico since its opening in , the objective of the national atomic museum has been to provide a readily accessible repository of educational materials, and information on the atomic age. in addition, the museum's goal is to preserve, interpret, and exhibit to the public memorabilia of this age. in late the museum was chartered by congress as the united states' only official atomic museum. prominently featured in the museum's high bay is the story of the manhattan engineer district, the unprecedented . billion dollar scientific-engineering project that was centered in new mexico during world war ii. the manhattan project as it was more commonly called, developed, built, and tested the world's first atomic bomb in new mexico. this display also includes casings similar to the only atomic bombs ever used in warfare. dropped on the japanese cities of hiroshima and nagasaki, these two bombs helped bring world war ii to an end in mid-august . the story of the manhattan project's three secret cities, hanford, washington, los alamos, new mexico, and oak ridge, tennessee, is also presented in this area. a portion of the museum, the low bay, is devoted to exhibits on the research, development, and use of various forms of nuclear energy. historical and other traveling exhibits are also displayed in this area. also found in the low bay is the museum's store, which is operated by the museum's foundation. adjacent to the low bay is the theater. the featured film is david wolpers classic production, ten seconds that shook the world. this excellent film is a -minute documentary on the manhattan project. other films relating to the history of the atomic age are available for viewing and checkout from the library. next to the theater is the library/department of energy public reading room, containing government documents that are available to the public for in-library research. the library also has many nuclear related books available for reference and checkout. located around the outside of the museum are a number of large exhibits. these include the boeing b- b jet bomber that dropped the united states' last air burst h-bomb in , and a -mm ( inches) atomic cannon, once america's most powerful field artillery. also found in this area is a navy ta- c (a modified a- b) corsair ii fighter-bomber, a veteran of the vietnam war. many other nuclear weapons systems, rockets, and missiles are found in this area. in front of the museum are a pair of navy terrier missiles. the terrier was the navy's first operational surface to air missile. to the south of the museum, next to the visitors parking lot, is a republic f- d thunderchief fighter-bomber. further south is a world war ii boeing b- superfortress. this plane is similar to the b- 's, enola gay and bockscar that dropped the atomic bombs on japan. the national atomic museum, is open a.m. to p.m. daily, except for new years day, easter, thanksgiving, and christmas. the museum is located at wyoming blvd. se, on kirtland air force base, albuquerque, new mexico. guided tours for groups are available by calling ( ) - in advance. admission and tours are free, and cameras are always welcome! generously made available by internet archive (https://archive.org) note: project gutenberg also has an html version of this file which includes the original illustrations. see -h.htm or -h.zip: (http://www.gutenberg.org/files/ / -h/ -h.htm) or (http://www.gutenberg.org/files/ / -h.zip) images of the original pages are available through internet archive. see https://archive.org/details/thunderlightning flamuoft thunder and lightning * * * * * medium vo, cloth extra, _s._ _d._ flammarion's popular astronomy translated from the french by j. ellard gore, f.r.a.s. with plates and illustrations. "the six books into which the book is divided give a very lucid and accurate description of the knowledge which has been acquired of the moving bodies of space, both as respects their motions and physical constitutions. of the translation we can only speak in terms of praise. not only does it well represent the original, but mr. gore has added useful notes for the purpose of bringing the information up to date, and has also increased the number--already very considerable--of the excellent illustrations, so that the work is likely to become as popular in england as it has been in france."--athenÆum. "the work which mr. j. e. gore has translated into english has made for itself a name and reputation in france ... and has gone into general circulation to the number of a hundred thousand copies. this last fact is proof how well within the bounds of possibility it is to make the latest discoveries of science comprehensible and fascinating to the common mind. m. flammarion has attained this triumph through the grasp of his knowledge, the lucidity of his style, and his power of bringing home the most stupendous and complicated of the things revealed to us in the depths of space. m. flammarion's pages should find almost as great acceptance in this country as in his own. simplicity of arrangement and of statement are part of his charm and of his success."--scotsman. "m. flammarion's latest volume, if it does not displace its english rivals, may well take a high place in the rank to which they belong. it is full, lucid, and, thanks to mr. gore's careful revision, well up to date.... mr. gore's edition is so carefully brought abreast of the latest discoveries that the english student may now congratulate himself on being in an even better position than the countrymen of m. flammarion."--daily chronicle. "young students of astronomy who wish to obtain a general idea of the most wonderful and fascinating of all sciences will find precisely what they seek in m. flammarion's eloquent and poetic chapters.... there are many illustrations in this able and attractive treatise."--speaker. "it is a fascinating work, extending to nearly seven hundred pages, and dealing in popular language with some of the most interesting of the discoveries and speculations of astronomers."--daily news. "m. flammarion is a sound practical astronomer; he has rendered good and laborious service to the science, and he possesses a valuable faculty of popular exposition.... the volume is profusely and well illustrated, some of the best plates making here their first appearance."--saturday review. "a high place must be accorded to flammarion's 'popular astronomy.' never before has the science of the heavens been treated with such fulness and interest as in this fascinating book; for flammarion is a man of letters as well as a man of science--a man of letters, too, endowed with the wondrous gifts of lucidity and charm which distinguish the best french writers.... flammarion's book is much more absorbing than most novels, more romantic than most romances, more poetic than most poems, yet strictly and scientifically accurate."--ludgate monthly. "it must be confessed that m. flammarion not only arrests the attention, but assists the reader to grasp astronomical theories--a task in which less popular writers often fail when they make the attempt."--literary world. "the book is a most fascinating one, and holds the reader from start to finish.... as a manual for those who wish to obtain a good general knowledge of astronomy this work will be found unsurpassed."--science gossip. london: chatto & windus, st. martin's lane, w.c. * * * * * thunder and lightning by camille flammarion translated by walter mostyn [illustration] with illustrations london chatto & windus printed by william clowes and sons, limited, london and beccles. contents chapter page i. the victims of lightning ii. atmospheric electricity and storm-clouds iii. the flash and the sound iv. fireballs v. the effects of lightning on mankind vi. the effects of lightning on animals vii. the effects of lightning on trees and plants viii. the effects of lightning on metals, objects, houses, etc. ix. lightning conductors x. pictures made by lightning thunder and lightning chapter i the victims of lightning it would be an interesting thing to make a careful study once a year, towards the end of the summer, of the habits and customs of thunder and lightning. perhaps in this way we should succeed one day in determining the still mysterious nature of these elusive forces. i, for my part, have been engaged upon the task for many years past. it has produced a big accumulation of records, and in this volume i can find room but for a _résumé_ of them, as varied as possible. in my first chapter i shall present a few characteristic examples, just to give my readers some hint of this variety. not to go too far back, let us begin with a harmless--i might almost say playful--fireball performance, of which m. schnaufer, professor at marseilles, has given me the particulars. in october, , the fireball in question made its appearance in a room and advanced towards a young girl who was seated at the table, her feet hanging down without touching the floor. the luminous globe moved along the floor in the girl's direction, began to rise quite near her and then round and round her, spiral fashion, darted off towards a hole in the chimney--a hole made for the stove-pipe, and closed up with glued paper--made its way up the chimney, and, on emerging into the open air, gave out upon the roof an appalling crash which shook the entire house. it was a case of coming in like a lamb and going out like a lion! a similar occurrence is recorded as having been observed in paris, on july , , in a tailor's room, including the same curious detail of the departure through the hole in the chimney, closed up with paper. it was in the rue saint jacques, near the val de grâce. the fireball burst into the room from the chimney, knocking over the paper guard in front of the fireplace. in appearance it suggested a young cat, gathered up into a ball, as it were, and moving along without using its paws. it approached the tailor's legs as though to play with them. the tailor moved them away to avoid the contact, of which he naturally was in terror. after some seconds, the globe of fire rose vertically to the height of the man's face as he sat, and he, to save himself, leaned quickly back and fell over. the fireball continued to rise, and made its way towards a hole which had been made at the top of the chimney for the insertion of a stove-pipe in the winter, but which, as the tailor put it afterwards, "the fireball couldn't see," because it was closed up with paper. the ball stripped off the paper neatly, entered the chimney quite quietly, and having risen to the summit, produced a tremendous explosion, which sent the chimney-top flying, and scattered it in bits all over the neighbouring courtyard and surrounding roofs. there we have a unique occurrence, recorded for us by babinet and arago, and of which i have given here the exact particulars. in both these cases we have to note the attraction of the hole in the chimney and the explosion of the thunderbolt on getting to the top. but it is not easy to detect the law underlying these phenomena. in one of the latest volumes of the association française a somewhat similar case is dealt with. "a violent storm," says the writer, m. wander, "had descended upon the commune of beugnon (deux-sèvres). i happened to be passing through a farm, in which were two children of about twelve and thirteen. these children were taking refuge from the rain under the door of a stable, in which were twenty-five oxen. in front of them extended a courtyard, sloping downwards towards a large pond, twenty or thirty yards away, beside which grew a poplar-tree. suddenly there appeared a globe of fire, of the size of an apple, near the top of the poplar. we saw it descend, branch by branch, and then down the trunk. it moved along the courtyard _very slowly_, seeming almost to pick its way between the pools of water, and came up to the door where stood the children. one of them was bold enough to touch it with his foot. immediately a terrible crash shook the entire farm to its foundations, the two children were thrown to the ground uninjured, but eleven of the animals in the stable were killed!" who is to explain these anomalies? the child who touched the fireball escapes with a fright, and a few feet behind him eleven animals out of twenty-five perish on the spot! during the storm which broke out at the town of gray, on july , , my friend m. vannesson, president of the tribunal, saw a fireball of from thirty to forty centimetres in diameter, which exploded on the corner of a roof, cutting clean off the end portion of the central beam to the length of about half a yard (like a bundle of matches, but without setting it on fire), scattering the splinters over the upper story and loosening the plaster upon the walls below. it then _rebounded_ on the roofing of a little outside staircase, made a hole in it, smashing and sending flying the slates, came down upon the road, and rolling right in the midst of some passers-by--who, like the child in the farm, escaped with a fright--disappeared. my learned fellow-member of the astronomical society of france, dr. bougon, has discovered an account of one of the most remarkable fireballs ever recorded in _la gloire des confesseurs_, a work written by gregory of tours, the twentieth bishop of that town. on the dedication day of an oratory which he had constructed in one of the outer buildings of the episcopal palace, all the participants in the procession from the cathedral, while approaching the oratory with the sacred relics and singing the litanies, saw a globe of fire, so intensely brilliant that their eyes were dazzled, and they could scarcely keep them open. seized with terror, priests, deacons, sub-deacons, choristers, together with the distinguished citizens of the town, who were carrying the relics upon their shoulders, all with one accord threw themselves on the ground, face downward. then gregory, remembering that on the occasion of the death of st. martin, some of whose bones were among the relics being carried from the cathedral, a globe of fire was said to have been observed to leave the saint's head and ascend heavenwards, believed himself to be in the presence of a miracle, vouchsafed as evidence at once of st. martin's sanctity and the genuineness of his relics. this globe of fire did no damage and burnt nothing. _discurrebat autem per totam cellulam_, tanquam fulgur, _globus igneus_. there is to be seen at the louvre a picture by eustache lesueur, entitled "la messe de saint martin," which seemed to me at first to illustrate this narrative, but the spectators are shown in silent wonder instead of being prostrated as in the story. moreover, gregory of tours tells us in his life of st. martin, that one day during mass a globe of fire was seen to appear above the head of the bishop, and then to rise heavenwards, to the great edification of the devout. it was this "miracle," evidently, that lesueur intended to represent. here is another case of a remarkably harmless fireball which is often cited. the abbé spallanzani it is who tells the story. on august , , a young peasant woman was in a field during a storm, when suddenly there appeared at her feet a globe of fire of about the size of a billiard ball. slipping along the ground, this little fireball reached her feet, caressed them, as it were, made its way up under her clothes, and issued again from the middle of her bodice, and, still keeping its globular form, darted off into the air and exploded noisily. when it got under her petticoats, they blew out like an umbrella, and she fell back. two witnesses of the scene ran to her assistance, but she was unhurt. a medical examination revealed only a slight erosion of the skin, extending from the right knee to the middle of her breast; her chemise had been torn in two along the same line, and there was a hole through her bodice where the thunderbolt had got out. in the "memoirs of du bellay" the following very curious narrative is to be found. in all probability it is a fireball that is in question:-- "on march , , diane of france, illegitimate daughter of henri ii., then the dauphin, married françois de montmorency. on the night of their wedding, an oscillating flame came into their bedroom through the window, went from corner to corner, and finally to the nuptial bed, where it burnt diane's hair and night attire. it did them no other harm, but their terror can be imagined." perhaps it may be as well to take with a pinch of salt the statement that the lady's attire was burnt in this way without harm to her person, yet there are other authentic stories of a similar kind almost as curious. in , at linguy (eure-et-loire), a man and his wife were sleeping quietly, when suddenly a terrible crash made them jump out of bed. they thought their last hour had come. the chimney, broken to pieces, had fallen in and its wreckage filled the room, the gable-end was put out and the roof threatened to come down. the effects of the thunderbolt in the room itself were less alarming than its effects outside, but were very curious. for instance, bricks from one wall had been dashed horizontally against the wall opposite, with such extraordinary force that they were to be seen imbedded in it up above a dresser upon which pots and pans, etc., were ranged, and within a few inches of the ceiling, while the windows of the room had been smashed into bits, and a looking-glass, detached from the wall, stood on end whole and entire upon the floor, delicately balanced. a chair near the bed, upon which articles of clothing had been placed, had been spirited away to a spot near the door. a small lamp and a box of matches were to be found undamaged upon the floor. an old gun, suspended from a beam, was violently shaken and had lost its ramrod. the thunderbolt actually frolicked over the bed, leaving its occupants more dead than alive from terror but quite unhurt. it passed within a few inches of their heads and passed through a fissure in a partition into an adjoining dairy, where it carried a whole row of milk-cans, full of milk, from one side of the room to another, breaking the lids but not upsetting a single can. it broke four plates out of a pile of a dozen, leaving the remaining eight intact. it carried away the tap from a small barrel of wine, which emptied itself in consequence. it ended by passing out through the window without further breakage, leaving the husband and wife unscathed but panic-stricken. one of the strangest tricks to which lightning is addicted is that of undressing its victims. it displays much more skill and cleverness in such diversions than is to be found in animals or even in many human beings. here is one of the most curious instances of this on record, as narrated by morand:-- "a woman in man's costume. a storm suddenly comes on. a flash of lightning strikes her, carries off and destroys her clothes and boots. she is left stark naked, and she has to be wrapped up in a cloth and taken thus to the neighbouring village." in , at courcelles-les-sens, mlles. philomène escalbert, aged , adèle delauffre, aged , and madame léonie legère, aged , were standing round a reaping-machine, when a flash of lightning struck madame legère and killed her on the spot. the two young girls were stripped to the skin, even their boots being torn from their feet. otherwise they were left safe and sound--and astonished. on october , , seven persons took refuge during a storm under a huge ash-tree near the village of bonello, in the commune of perret (côtes-du-nord), when suddenly the tree was struck by lightning, and one of them--a woman--was killed. the six others were knocked to the ground without being seriously damaged. the clothes of the woman who had been killed were torn into shreds, many of which were found clinging to the branches of the tree. on may , , a farmer at ardillats was tilling the ground with his two oxen, not far from his dwelling-place, about four in the afternoon. the air was close and heavy, and the sky covered with black clouds. suddenly there was a great thunderclap, and a flash of lightning struck both man and beasts dead on the spot. the man was found stripped to the skin, and his boots had been carried thirty yards away. in july, , at epervans (saône-et-loire), a young man named petiot, who was mowing in a meadow, was struck dead by lightning while lighting a cigarette, and left in a state of complete nakedness. on august , , a man was struck by lightning near vallerois (haute-saône), and stripped naked. all that could be found afterwards of his clothes was a shirt-sleeve, a few other shreds, and some pieces of his hobnailed boots. ten minutes after he was struck he regained consciousness, opened his eyes, complained of the cold, and inquired how he happened to be naked. there is no telling what lightning will not do. sometimes it will snatch things out of your hand and carry them right away. there is a case of a mug being thus spirited away from a man, who had just been drinking out of it, and deposited undamaged in a courtyard near--the man himself suffering no injury. a youth of eighteen, holding up a missal from which he is singing, has it torn out of his hands and destroyed. a whip is whisked out of a rider's hand. two ladies, quietly knitting, have their knitting-needles stolen. a girl was sitting at her sewing-machine, a pair of scissors in her hand; a flash of lightning, and her scissors are gone and she is sitting _on_ the sewing-machine. a farmer's labourer is carrying a pitchfork on his shoulder; the lightning seizes it, carries it off fifty yards or so, and twists its two prongs into corkscrews. on july , , at gien (nievre), a woman while sprinkling her house with holy water during a storm, saw her holy-water bottle smashed actually in her fingers by the lightning, which at the same time smashed up the tiled pavement of the room. in a church at dancé (loire) during vespers, one day in june, , a flash of lightning killed the priest and all the congregation, knocked over the monstrance on the altar, and buried the host in a heap of _débris_. on june , , the cupola of the javisy observatory, which was not then provided with a lightning conductor, was struck by lightning. an enormous piece of oak from _un angle de construction_ was torn to shreds, and one splinter was lodged in the hinge of a window behind the _pivot_, in the part between the _pivot_ and the frame, hardly a twenty-fifth of an inch apart, and this without breaking the glass. in other cases lightning has been known to split men in two, almost as with a huge axe. on january , , this happened to a miller's assistant in a windmill at groix. the lightning struck him, and split him from his head downwards in two. in the course of july, , four inhabitants of heiltz-le-maurupt, near vitry-le-françoise, took refuge under trees during a storm, three of them under a poplar, and the fourth under a willow, against which doubtless he leaned. in a few minutes this one was struck by lightning. a bright flame was observed to be issuing from his clothes, but he remained standing, and seemed unconscious of what had happened. "you're on fire! you're on fire!" exclaimed his friends. getting no reply, they went up to where he was, and found to their horror that he was a corpse. a clergyman named butler was a witness of the following incident, which took place at everdon. ten harvest-men took refuge under a lodge on the approach of a storm. there was a thunderclap, and in a moment four of them were killed by lightning. one of them was found dead, still holding between finger and thumb a pinch of snuff he had been in the act of taking. a second had one hand upon the head of a small dog, also killed, and still sitting upon his knees, and in the other hand a piece of bread; a third was sitting, his eyes open, facing in the direction from which the storm came. at castellane, in august, , during a violent storm, a flock of sheep was struck by lightning while crossing the mountain of peyresy. seventy-five of them were killed. the shepherd escaped. the sheep probably were all wet from the rain, and clinging together in one great mass. in the same month a pond at vauxdîmes (côte-d'or) was demolished, and all the fish in it killed. quite recently, a young man at franxault (côte-d'or) was killed by lightning on his way home from work. all the nails were found to have been torn out of his shoes, and the links of his silver watch-chain were all moulded together. to fuse silver in this way a heat of degrees is needed! on july , , at buffon (côte-d'or), a woman had one of her earrings melted in the same way, but she was not killed. on the same day at void (meuse) two workmen, who had taken shelter under a willow, were thrown a distance of four yards without being killed. on august , of the same year, at chanvres (yonne), a vine-dresser was struck by lightning and killed, but his heart continued to beat for thirty hours. dr. gaultier de claubry was struck by lightning, with the extraordinary result that his beard was taken off him, roots and all, so that it never grew again. at fresneaux (oise), a young girl of twenty, mlle. laure leloup, had her head shorn by lightning. a wide furrow was to be traced on the crown of her head, caused by the electric fluid. her hair was removed right down to the skin as though by a razor. on september , , a flash of lightning lit up all the electric lamps in the prefecture of lyon. really it is extraordinary the queer things lightning will do! death in one case, an innocent practical joke in another! i have hundreds of quaint records before me. impossible to deduce any kind of law from them all. you are tempted to believe that the electric current has a brain. a young woman was picking cherries off a rather tall cherry-tree. a young man stood underneath. the young woman was struck by lightning, and fell dead. this was in july, . in september, , at remaines, near ramerupt (aube), a certain m. finot, an innkeeper, was standing on his doorstep looking out at a storm, when a flash of lightning followed by a thunderclap sent him flying back into the hall. he remained unconscious for a time, and his sight was affected for ten hours. the extraordinary thing, however, in his case was that he had been a victim of rheumatism until then, and walked with difficulty and only with a stick, and that ever since this occurrence he has been able to do without the stick, and to pursue his avocations quite comfortably. he feels that he has no reason to regret his experience, though he is not anxious to go through anything of the kind again. this kind of electrical phenomenon might be catalogued under the title "medicinal lightning." now for a case of "judicial lightning." on july , , a negro named norris was hanged in the state of kentucky for having killed a mulatto, a fellow-workman of his. at the moment of his setting foot upon the scaffold, there was a terrible clap of thunder, and the condemned man was struck dead by lightning. the sheriff was so much moved by the occurrence that he resigned his office. let us wind up this little collection of strange cases with another occurrence reported from the united states. an immense grange had been built by a man named abner millikan, an ardent republican, who adorned the front walls of his farm with portraits of mackinley and hobart. during a violent storm that broke out, the building was struck by lightning several times, and it looked as though it were enveloped in great sheets of flame. millikan, who had been at some distance from the spot, rushed thither much alarmed, and found to his relief that no damage had been done. the portraits alone had been destroyed, and--here is the strange detail--the lightning had traced the politicians' features upon the wall. certainly lightning plays queer pranks. and i have said nothing yet of the photographs lightning sometimes takes. _pranks_ they seem to us, but we may be sure there is some method in their mischievousness. it is the same with women. women in their caprices are but obeying some law of nature. they are not so capricious as they seem. these strange facts teach us, anyway, not for the first time, that our knowledge of the universe is still very incomplete, and that its study is worth following up in all its chapters. we may be certain that electricity exercises a much more important influence in nature than is generally supposed, and that it plays a _rôle_ in our own lives which is still practically unrecognized. in the oppression we feel before the coming of a storm, and the sense of relief we experience when it has passed, we have an instance of the way in which physical and moral influences are apt to blend or overlap. chapter ii atmospheric electricity and storm-clouds with such strange facts before us--facts the strangeness and diversity of which baffle all hypotheses and forbid all definite conclusions--we can but keep adding to our observations and accumulating other facts which may tend to elucidate the mystery. the terrible ravages caused every year by lightning make it necessary for us to find some means of preventing the recurrence of certain memorable catastrophes. it is only in the actual investigation of the phenomenon, in the study of all its smallest manifestations, that we can hope to discover the methods of the mysterious power. from the earliest times mankind has devoted much thought to the subject. if we glance back towards past centuries we find that thunder and lightning have ever been regarded as a terrible agent of the will of the powers above. the strongest and subtlest brains of antiquity, anaxagoras, aristotle, seneca, were unable to form any kind of reasonable view regarding the fantastic phenomena resulting from the force of nature and held so mysterious to us moderns. thunder and lightning were generally believed by them to be due to emanations from the earth or to vapours contained in the air. the etruscans, who flourished fifteen hundred years before christ, and who were much given to the study of nature, are said to have observed the tendency of lightning to make for points, but no theory upon the subject has come down to us from them. electricity for the ancients was an unplumbed ocean, whose slightest fluctuations affected them in ways they could not understand. in vain they appealed to their gods to help them to explain the enigma. olympus turned a deaf ear to their prayers. their imagination exhausted itself in researches into the nature of such things as amber, in which they recognized the curious attribute of attraction and repulsion for objects of slight weight. the poets attributed it to the tears of phaëton's sisters, lamenting over the dreams of eridan. certain naturalists regarded it as a kind of gum issuing from trees during the dry days. no one gave any thought to electricity, by whose subtle fluid the earth and everything upon it is penetrated and enveloped. the superstitions connected with lightning would furnish forth material in themselves for a very curious volume of stories--half comic, half tragic. with the romans the fall of a thunderbolt was always taken as an omen. in the reign of domitian, thunder was to be heard once so constantly during a period of eight months that the tyrant, frightened by the bombardment from on high, at last cried out in his terror: "let the blow come, then, where it will!" the stroke fell upon the capitol, and upon the temple of the flavian family, as well as upon the emperor's palace and the very room in which he slept. the inscription beneath the triumphal statue was even torn away by the tempest and thrown into a neighbouring garden. otto de guérike, burgomaster of magdeburg and inventor of the air-pump, was the first person to discover the means of producing the electric spark, about . about the same time, dr. wall, while watching electricity being released from a roll of amber, noticed a spark and a sudden sharp report, suggestive of a minute flash of lightning, followed by a minute peal of thunder. the analogy was striking. this discovery opened out a new horizon to physicists, and almost immediately the feeble electric light produced by the hand of man came to be associated with the monstrous sheaves of fire let loose in space by unknown forces. l'abbé nollet, considered in the france of his time as an oracle in regard to natural philosophy, expressed himself as follows upon this subject:--"if some one, after comparing the phenomena, were to undertake to prove that thunder is in the hands of nature what electricity is in ours, that those electrical wonders with which we are now able to make so much play are petty imitations of those great lightning effects which frighten us; that both result from the same mechanism; and if he could make it evident that a cloud produced by the action of the winds, by heat, and by the mingling of exhalations, bears the same relation to a terrestrial object as an electrified body bears to an unelectrified body in its close proximity, i admit that the idea, if well worked out, would captivate me greatly; and, to work it out, how many plausible arguments there are at the disposal of a man who is properly versed in electricity!" the invention of the leyden jar in , and franklin's brilliant investigations, make these conjectures the more probable. since then electricity has gone ahead and become one of the most important branches of modern natural philosophy. when franklin demonstrated that the air is in a permanent condition of electrification even when the sky is clear, people began to study not thunder alone but the general electrical state of the atmosphere. and ever since meteorological observatories have made it a practice to register every day the degree and nature of atmospheric electricity by the use of very ingenious instruments. but the records obtained up till now leave us in doubt upon many points. the subject is still full of new surprises. whence come those masses of electricity which move about in the clouds, sometimes escaping from them in thunderclaps and causing such tremendous ravages upon this earth of ours? the evaporation of the sea is one of their principal causes. the atmosphere is continually impregnated with electric effluvia which flow silently through the soil through the medium of all bodies, organized or not, attached to the earth's surface. plants afford an especially welcome pathway to this fluid. the green leaves you see rustling in the wind are often being traversed by electrical currents, luckily harmless, of precisely the same nature as those of the deadly lightning. on the other hand, the earth itself emits a certain quantity of electricity, and it is from the attraction exerted by these two fluids upon each other that thunder comes into existence. to put it in another way, thunder is a sudden striking of a balance between two different masses of electricity. minute researches have established the fact that in ordinary conditions the terrestrial globe is charged with resinous, or negative electricity, while the atmosphere holds in suspension _vitrée_, or positive electricity. in two words, our planet and its aerial envelope are two great reservoirs of electricity, between which take place continual exchanges which play a _rôle_ in the life of plants and animals complementary to that which is played by warmth and moisture. the aurora borealis, which sometimes illumines, with a brilliancy as of fairyland, the darkness of night in the arctic and all the regions of the north, finds its explanation in the same phenomenon. it also is a striking of a balance, silent but visible, between two opposing tensions of the atmosphere and the earth; thus the apparition of the aurora borealis in sweden or norway is accompanied by electric currents moving through the earth to a distance sufficiently great to cause the magnetic needle to record the occurrence in the paris observatory. indeed, the electricity which pervades the earth, silently and invisibly, is identical with that which moves in the heights of the enveloping atmosphere, and, whether it be positive or negative, its essential unity remains the same, these qualities serving only to indicate a point, more or less in common, between the different charges. the heights of the atmosphere are more powerfully electrified than the surface of the globe, and the degree of electricity increases in the atmosphere with the distance from the earth. atmospheric electricity undergoes, like warmth, and like atmospheric pressure, a double fluctuation, yearly and daily, as well as accidental fluctuations more considerable than the regular ones. the maximum comes between six and seven in the morning in summer, and between ten and twelve in winter; the minimum comes between five and six in the afternoon in summer, and about three in the afternoon in winter. there is a second maximum at sunset, followed by a diminution during the night until sunrise. this fluctuation is connected with that of the hygrometric condition of the air. in the annual fluctuation the maximum comes in january, and the minimum in july; it is due to the great atmospherical circulation; the winter is the time when the equatorial currents are most active in our hemisphere, and when the aurora borealis is to be seen most often. on the other hand, the water of oceans and rivers is continually evaporating under the influence of solar heat, and rises into the atmosphere, where it remains in the form of an invisible gaseous vapour. soon it becomes cold again, and, in the process of condensation, transparent gaseous molecules become transformed into minute drops, which accumulate into a cloud. generally speaking, clouds are, like the atmosphere, charged with positive electricity. sometimes, however, there are negative clouds. you may frequently see, on the summits of mountains, clouds which seem to cling to the peaks for a while, as though drawn to them by some force of attraction, and then move away to follow the general direction of the winds. it often happens that in this case the clouds have lost their positive electricity in thus coming in contact with the mountains, and have derived from them in its place the negative electricity which, instead of holding them, has a tendency to drive them off. a mass of clouds lying between the negative earth and a mass of positive clouds above is almost neutral; the positive electricity accumulates towards its lower surface, and the first drops of rain will make it disappear. this mass will, from that moment, become like the surface of the soil--that is to say, it will become negative under the influence of the mass above it, endowed with a strong positive tendency. the cloud remains suspended in space until the moment when, under the influence of the ambient medium, it dissolves in rain. the causes of the instability of clouds are very numerous. my readers are aware that the atmosphere is being constantly agitated by vast currents which pass from the equator to the poles, and from which the different winds result. the clouds take part in this universal whirl of atmospheric waves. transported from one point to another--often far beyond the region where they came into existence--subjected to every vicissitude of atmosphere, and blown about by contrary currents, they follow the gigantic movements which take the form sometimes of cyclones and tempests. under the influence of warmth, and probably also by its transformation, these movements engender great masses of electricity, and presently, when the clouds have become saturated with it, the electricity breaks out, and there is a thunderstorm. the electric fluid, escaped from the cloud in which it has been imprisoned, flies to unite itself, either with the negative electricity stored in the surface of the earth, or else with the electricity in other neighbouring clouds. almost always the cloud torn open by the electric discharge dissolves in rain or hail. thus a storm is the outcome of violent movements produced by the force of electricity when this has reached its maximum of intensity. thunderstorms are generally heralded by certain premonitory signs. the barometer goes down steadily. the air, calm and heavy, is pervaded by a bitter sulphurous odour. the heat is stifling. an abnormal silence reigns over the land. all this has a remarkable effect upon certain organisms, and produces nervous complaints, a buzzing in the ears, a sense of painful oppression, a sort of good-for-nothingness that we combat in vain. in most cases storms come to us in france ready made, so to speak, from the sea, borne in by the currents from the south-west; they are the off-shoots of the cyclones, and are born in the tropics, moving in lines from the south-west to the north-east. ordinarily they lose part of their strength _en route_ and come to an end suddenly with us. there are, of course, home-made storms also, so to speak, especially in france during our hot summers, when the sun is shining all the day, and thus promoting the rapid evaporation of our seas and rivers. the air is charged with a heavy mist which veils the horizon; the barometer is going down, the thermometer going up. the sun looks leaden though there are no clouds. when it approaches the meridian and its rays are most scorching, columns of vapour ascend and become condensed into the light clouds termed _cirri_. at the end of some hours these clouds become attracted to each other, descend a little, and become grouped together into what look like great masses of cotton-wool. these are termed _cumuli_. presently a small grey cloud joins the others. it looks innocent and harmless, but very often this is the beginning of the battle. first there ensues, perhaps, a discharge or two of lightning without casualties, but soon the bombardment becomes general, and long blinding fusillades flash through space. the heavens, darkened over, seem to have sunk quite low, and to have become a great black mass, from which the lightning escapes in sudden jets. rain and hail pelt down upon the earth to an accompaniment of the rumbling of thunder. confusion has fallen upon the entire universe. then, finally, the fight comes to a close. the clouds disperse and allow us to see once again a wide expanse of sunlit blue. the birds, their hearts freed again from terror, begin to sing again. flowers and foliage and soil, refreshed by the rain, give out sweet perfumes. an immense joy takes the place of the sense of melancholy and oppression. it is good to see the sun again! alas, though, there are grim realities to be faced presently. the hailstones have destroyed the crops and begotten famine--the lightning has sown death and plunged whole families into mourning. it is with these misfortunes before us that we make up our minds to do what in us lies to diminish the destructiveness of this terrible force. how are storm-clouds to be detected? generally speaking, their shape is very clearly defined, and they have a look of solidity about them. their lower surface is often unbroken, presenting a level plain from which there rise huge ragged protuberances like great plumes. sometimes, on the other hand, they have great projections underneath, trailing quite near the ground. storm-clouds move generally in large numbers, and are generally composed of two separate masses, differently electrified--the lower one giving out negative electricity, the higher positive electricity. the flashes of lightning occur generally between these two masses, though also, less frequently, between the lower mass and the earth. it may be said that, generally speaking, storms are the result of the meeting of two masses of clouds differently electrified. for long, physicists refused to admit the validity of any other theory, and combated in particular the idea that lightning could issue from a single isolated cloud. this has, however, been established now as a fact, and in such cases the flashes have always, of course, taken place between the cloud and the earth. marcorelle, of toulouse, reports that on september , , the sky being then pure and cloudless but for one round speck, there was suddenly a thunderclap and a flash which killed a woman on the spot, burning her breast but doing no damage to her clothes. here is another interesting case. two priests of the cathedral of lombey, who were standing in the area of their chapter-house, busy winnowing, saw a small cloud approaching them little by little. when it was immediately above them a flash of lightning broke out and struck a tree just beside them, splitting it from top to bottom. they heard no thunderclap. the weather was quite fine. there was no wind, and this was the only cloud in the sky. storms are far more prevalent in some countries than in others. according to pliny, thunder was unknown in egypt, and, according to plutarch, in abyssinia. this could not be said now, however, perhaps because these lands have grown unworthy of their exemption. it might be said, however, of peru, whose pure and limpid skies are never troubled by tempest. _jupiter tonans_ must be a myth indeed to a people who know nothing of thunderclaps or wet days. storms diminish in number in high latitudes, but there are local conditions which affect their distribution. then they are particularly frequent in countries that are thickly wooded and in mountainous districts. arago came to the conclusion, after a considerable number of observations, that, out in the open sea or among islands, there is no thunder in the north beyond the th degree of latitude. this is not absolutely so, but it is a fact that storms are very much rarer in the polar regions. they become more and more frequent towards the equator, and are very numerous in the tropics. on either side of the equator storms come year after year with remarkable regularity in the wet season, and at the time of the monsoons. at guadeloupe and martinique there is never any thunder in december, january, february, or march. in temperate climates there are scarcely any storms in winter; they begin in the spring, and attain their maximum of intensity in the heat of summer. in italy there are thunderstorms at almost all times of the year. in greece they come chiefly in spring and autumn. it is noticeable that in all latitudes they come most often in the afternoon. chapter iii the flash and the sound the romans attributed a mysterious influence to each manifestation of electricity. they divided lightning into individual and family lightning, lightning of advice, monitory, explanatory, expostulatory, confirmatory, auxiliary, disagreeable, perfidious, pestiferous, menacing, murderous, etc., etc. they adapted it to every taste and circumstance, but modern science has come to put order into this capharnaum. when a cloud is superabundantly charged with electricity, this electricity, which is compressed in the cloudy envelope, tries to escape in order to join the electricity accumulated either in another cloud or on the ground. an electric deflagration ensues, and a long ignited dart precipitates itself into space, showing us on a large scale what our experience of physics has taught us in a small way in our laboratories. this luminous and often dazzling trail constitutes lightning. lightning is not always the same, and in order to classify the different forms it takes more easily, it can be divided into three groups--diffused lightning, linear lightning, and fireballs. this last is the most curious of the three. the variety and eccentricity of fireballs are celebrated in the history of lightning, and i propose to devote the following chapter to their vagaries. diffused lightning is the commonest of all. you can count hundreds of flashes on a stormy night. occasionally they succeed one another with such rapidity that the sky is momentarily entirely illumined with a fantastic brightness. at these times great sombre clouds can be seen surging from the darkness of the night, to shine suddenly with an ephemeral brightness of a diffused red, blue, or violet tinge. their irregular shapes, with their jagged edges of light, are visible against the dark background of the heavens, and the thunder growls monotonously. whether the exchange of electricity is produced on a vast stretch between two rows of clouds, or whether it is manifested by a long thin spark launched like an arrow and veiled by the curtain of clouds, all that can be seen is a strange light, vague, diaphanous, instantaneous, which sometimes spreads itself like a sheet of fire all over the horizon. it is diffused lightning which gives us the finest storm effects on those heavy summer evenings when the air is breathless and saturated with electricity. suddenly the clouds are illumined, nebulous veils of light on which can be seen, in sombre fantastic, fugitive vision, the outlines of the trees, houses, and other landmarks. then, all at once, heaven and earth fall back into a darkness deeper than before, owing to the contrast. linear lightning is more terrible. it is regarded by astronomers as the most perfect form of destructive lightning. it is a strong flash--a thin trail of light--very clear, and extraordinarily rapid, which shoots from an electric cloud to the earth, or from one cloud to another. like a supple and undulating serpent of fire, it twists itself luminously into space, spreading itself menacingly in the heavens with its long spirals of light. sometimes--in a hurry, no doubt, to reach its prey--it effects its passage in a straight line, but as a rule it follows a sinuous track, and forms itself into a zigzag at an obtuse angle. the different forms which this lightning takes are no doubt attributable to various causes. one of the chief of these seems to be the unequal distribution of humidity in the air, which renders it a more or less good conductor. in fact, fulminic matter is strongly attracted towards damp regions, and goes quickly from one point to another, guided in its chosen way by the hygrometrical conditions of the atmosphere; and it is these constant changes of direction which determine the meanderings of its course. thus the lightning would trace a sort of plan of the hygrometrical state of the air for a certain portion of the atmosphere. for it, the short road is hardly ever the straight line. on the other hand, the variability of the overloading of electricity has something to say to the form it takes. sometimes lightning forms itself into two or three branches, and becomes forked lightning. or it even divides itself into a number of points from a principal branch, out of which a great many sparks burst forth. these incandescent sheaves move through space with extraordinary agility. it has not been possible to measure their speed with absolute accuracy, but their rapidity is such that their transit appears to be instantaneous. the latest researches seem to have proved that their speed is superior to that of light, which is , kilometres a second. lightning is not always of a dazzling whiteness, it is often yellow, red, blue, violet, or green. its colour depends on the quantity of the electricity thrown on the atmosphere by the discharge; on the density of the air at the time of the passage of the ignited matter; on its hygrometrical state, and on the substances which it contains during suspension. it has been remarked in the study of physics that the electric spark is white in the open air, but that it gets a violet tinge in the vacuum of a pneumatic machine. this proves that violet lightning comes from the far-off regions of the atmosphere. it traverses a bed of rarified air, and shows the great height of the storm-clouds from which it emanates. the fulminating spark is so fugitive that it is difficult to form an idea of its length. one could easily take it to be a yard or so long, so illusory and deceptive are our impressions. as a matter of fact, it is proved that flashes of lightning cover a distance of several kilometres. there are various methods to which one can have recourse in these scientific researches. the first, which gives the length of horizontal lightning, is based on a minute comparison between the trajectory described by the meteor and the known distance of the terrestrial points between which it travels. in order to gauge the extent of vertical lightning, you must estimate approximately the height of the clouds from which it comes, based on the irregularities of the earth of which the height is known. but there is a still simpler method for approximate measurement within the reach of every one. it consists in multiplying (the number of yards traversed by sound in a second) by the number of seconds during which the thunder lasts. these methods all give the same result, and prove that lightning is often , , and kilometres in length. the greatest length proved up to the present has been kilometres. when one thinks of the instantaneousness of these flashes, one marvels at their incomparable agility, and we can only be lost in admiration of the magic force of the heavenly sling, which is capable of hurling these whole rivers of fire to roll in their sinuous course right through space, and in a space of time almost inappreciable to our senses. yet, in spite of the extreme rapidity of the lightning, it has been possible to determine that these meteors do not last the thousandth part of a second. to prove this, we take a circle of cardboard, divided from the middle into black and white sections. this circle can be turned like a wheel almost as quickly as one can wish. we know that luminous impressions remain on the retina the tenth part of a second; thus, if we imitate the childish game of turning a lighted coal--if the turn is made in the tenth of a second, each successive position of the coal remaining impressed on the retina for the same length of time--we have a continuous circle. in turning our cardboard wheel with the black and white spokes, if each spoke passes before our eyes in less than the tenth of a second, we can no longer distinguish between the sections, but can only see a grey circle. but we can make it rotate a hundred turns or more in a second; this being done, if we continue to observe the circle, we can no longer see the lines, they succeed each other more quickly in our eyes than the impression they produce. but if the circle turns before us in the darkness, and it is suddenly lighted up and as suddenly darkened, the impression produced on our eyes by each of the sections would last less than the tenth of a second, and the circle would appear to us as if it were stationary. in applying a calculated rotation to this contrivance, charles wheatstone has proved that some lightning does not last the thousandth part of a second. this measure is probably a minimum; in the majority of cases the duration of lightning is longer than this. often during the hot, transparent summer nights, we see a considerable number of flashes, which furrow the firmament with their gentle, bluish light. these fugitive gleams remind us in the sky of the will-o'-the-wisps, which come forth silently from marshy ground. the atmosphere is pure; there are no apparent traces of a storm, and yet the sky is glistening with thousands of small flames. the flashes succeed one another almost without interruption. these electric sparks are known as heat-lightning, but this is quite inaccurate, and has no meaning in the language of modern science. in a great number of cases an astronomer would be able to discover certain characteristic signs indicating that a storm is taking place under the horizon at a very great distance from the point of observation. it is only at the moment when the sky is lighted up that one can see the ridge of clouds lying low on the horizon. at other times there is no sign of a storm, as far as the eye can see. the atmosphere is quite clear, and yet the sky is swept with a number of electric flashes. but afterwards you hear that a violent storm has devastated the region over which the gleams have appeared, and that it is to this that they are attributable. they are only reflected lights. a sailor tells us that once when he was out at sea, more than kilometres from lima, he saw a number of bright flashes, without any thunder, to the east and north-east of the horizon. the weather was perfect, and the sky absolutely serene. now we know that storms, and the electric phenomena which they produce, are unknown upon that coast; but this immunity does not extend for more than kilometres to the interior of this country, so that this lightning which was observed at sea, kilometres from the shore, must have taken place more than kilometres away. one of our correspondents, m. soleyre of constantine, sent us word, in , of an interesting case of lightning without thunder. "in august," he says, "i noticed it in the valley of the arve above salambes; when i came back to algiers i saw it again on september , and on october . "it was not sheet lightning, but ordinary lightning concentrated in very thin lines. this lasted long, and was very near. another thing, there was no hail. this is not very rare in algiers." on september , , i happened to be in geneva at about p.m. the weather was heavy but very fine. i noticed a good deal of lightning on the south-west of the horizon. it went on almost without interruption above the savoy alps. each flash illuminated at the same time the ridge of the mountains and the fringed edge of the great sombre clouds lying low on the horizon. this lightning was silent; the noise of the thunder did not reach geneva. the next day i learnt that a terrible storm had devastated the neighbourhood of chambéry and aix-les-bains. moreover, apart from storms, there have been other records of this lighting up of the sky being observed at great distances. thus, in , a service of luminous communications was established on mount brocken in the hartz mountains in order to determine the differences of longitude. the combustion of to grammes of powder, burnt in the open air, for each of the signals, produced a light which was observed by astronomers stationed on mount kenlenberg, although they were kilometres from brocken, which is itself invisible from kenlenberg. on certain fête-days, july , for example, when the principal monuments in paris are illuminated, at a distance of and kilometres we can see a sort of luminous vapour which floats above the town and reflects the lights of the boulevards, although the lights themselves are invisible from the point of observation. here is another example which any parisian can verify: the captive balloon of the aërodrome at porte-maillot, which soars some hundreds of yards above paris during the spring and summer, as seen from the dark paths of the bois de boulogne, appears against the azure of the sky like a magnificent globe bathed in light, resembling an enormous moon. well, this gentle, pale light is only the reflection of the lights of paris which are invisible from the bois de boulogne. the earth and all the planets which are dark in themselves, shine in space lighted up by the sun. the silent lightning which flashes in the sky is only the reflection of a distant storm. whether on account of the spherical shape of the earth or on account of the irregularities of the land, the clouds are invisible, but the effluvium which escapes from them can be seen at a great distance. these poetic and ephemeral flames which glide through the sky, appeal to the imagination of the dreamer, and yet they are quite as terrible as the flashes which are accompanied by thunder. if the noise which accompanies these is not perceptible, it is because the sound of the thunder does not carry far, and has been lost in space before reaching us. it is the same with the silent lightning which gleams in a stormy sky. this phenomenon is particularly frequent in the antilles. either the storm breaks too far from the observer, or the discharge has taken place between two beds of clouds, the lower of which intercepts the waves of sound without preventing the escape of the electric spark, and the thunder is not heard. as a rule we imagine that lightning always descends, that it comes to us from the higher celestial regions to be lost in the common reservoir. but this is quite inaccurate. lightning sometimes ascends. sometimes it descends and reascends. that is to say, after it reaches the ground, either there is no attraction there, or a stronger force draws it back to the aerial regions, and it flies back to the clouds whence it came. as a rule we only fear the direct lightning. this is a great mistake. there are many cases of lightning striking from a distance. for example, at the end of may, , an english coastguard was making his rounds on the coast of one of the shetland isles, when a flash of lightning passed near him, striking a great rock. the unfortunate man was completely blinded, and plunged into darkness thus suddenly, he would inevitably have fallen down an abyss, if his companions, attracted by his cries, had not come to the rescue and taken him home. here is another case:-- on september , , a terrible storm burst over versailles, accompanied by a great deal of thunder and lightning. at the moment when the lightning struck galli's farm, an old man who was in a street in versailles, at a distance of two kilometres from the farm, suddenly felt a violent shock, accompanied by a feeling of oppression and giddiness and a semi-paralysis of the tongue and the whole of his left side. next morning this had passed away, but in the evening at the same time as the shock had occurred, he felt similar sensations of fainting, and it was the same to the end of the week. it would be well to remark here that at the moment of the accident, m. b---- happened to be near the wall of a house, not far from the metallic tube which conducted the rain-water into the level of the pavement. the following phenomenon, to which we have already alluded, is no less curious:-- on july , , at about o'clock in the evening, at gien-sur-cure (nièvre), the thunder had been growling violently for some time, when all of a sudden the lightning struck a thatched house, which it set on fire. at the same time a woman who was in a house ten yards away, felt a shock, and saw the tiled floor rise beneath her. her two sabots were broken on her feet, and a bottle of holy water with which she was blessing the house was broken in her hand, only the neck remained in her fingers. she herself suffered nothing but the shock. nineteen of the tiles were flung in all directions. here is another very remarkable case of _ascending lightning_, published in the _comptes rendus_ of the academie des sciences:-- at porto-alegre, on june , , at a.m., during a violent storm, on the property of m. laranja e oliveira, at brazil, a servant was entering the house; he was about ten yards away, when a flash of lightning illuminated it; at the same moment he felt a great tingling in the flesh of his feet, then in his legs, then all over his body, and finally in his head, on which the hair stood on end to such an extent that _he was obliged to hold his hat on_ in order to prevent its falling off. at the same time, a white flame burst from the ground about two yards in front of him, accompanied by a shower of sparks. terrified by such a phenomenon, which he attributed to souls from another world, he thought he was petrified to the spot; finally, he ran away. anything metallic which he had about him at the time of this occurrence became magnetized. a key which was in his pocket remained magnetic for two days. thus, as well as the ordinary fulguration, in which the lightning (which we imagine descends from the clouds) acts directly on the body, and the lightning which strikes indirectly, there are other electric shocks which can be experienced by men and animals. notable among these is the _striking from the earth_, commonly known as _choc de retour_, and which is in reality only an instance of the ascending current, or of lightning striking from a distance. we must also describe the striking _by a man who has been struck_. the abbé richard, in his _histoire de l'air_, tells the following story:-- in the neighbourhood of the village of rumigny, in picardy, on august , , at six o'clock in the morning, there was a sudden irruption of fulminating matter from the bosom of the earth in such quantities as to produce the most violent results. the sky was cloudy, and looked like a storm. a young farmer and his wife were following, at some distance, a vehicle drawn by four horses. suddenly the driver of this, _without seeing the lightning or hearing the thunder_, was thrown to the earth. his four horses were stretched dead on the ground near the carriage. there was a _smoking hole_ in the ground, from which the effluvium came forth and killed the young man and his wife at ten paces off and separated from each other by twenty paces. the current also knocked down, at a hundred paces, the father of the young man in the same fashion as it had done the driver, but without injuring one or the other. the bodies showed no signs of a wound, only a considerable swelling and a great deformity of the features. the woman, who was young and pretty, became hideous; the whole of her body as well as that of her husband was absolutely yellow. the four horses had their intestines drawn from their bodies. they were all thrown on the same side. the man's hat was pierced and his hair burnt, but he had no bruise on his head. this account, in which we must not be surprised to find the ideas and language of the time (let us observe in passing that the man who was struck did not hear the thunder, and had not even time to see the lightning of which he was the victim)--this account, i say, gives us an instance of ascending lightning. here is another. the traveller brydone gives the following example, which he himself observed:-- on july , , a storm burst near coldstream between and a.m. a woman who was cutting hay on the banks of the tweed _fell backwards_. she at once called to her companions, and said she had just received a violent blow on her foot for which she could in no way account. at the time there was no thunder or lightning in the sky. the shepherd of a farm at lennel hill saw a sheep fall near him, which a few minutes before appeared to be in perfect health; he found it stone dead. the storm then appeared very far off. two carts laden with coal, and each driven by a young driver seated on a small seat in front, crossed over the tweed. they had just climbed a small hill near the banks of this river when they heard a great detonation round about, similar to that which would be produced by the discharge of several guns. at the same instant the driver of the second cart saw the first, with his companion and the two horses, fall to the ground. _the driver and horses were stone dead. the ground was pierced with three circular holes at the very spot where the wheels had touched it when the accident happened._ half an hour after this event the holes emitted an odour which brydone compared to that of ether. the two circular iron bands which covered the felloes of the wheels showed evident signs of fusion in the two spots which rested on the ground at the moment of the detonation, and in no other place. the skin of the horses had been burnt, particularly about the legs and under the stomach. the body of the driver had marks of burning here and there. his clothes, his shirt, and, above all, his hat, were reduced to shreds, and gave out a strong smell. orioli gives an example of two men who were surprised by a violent storm near the village of benvenide. they lay down on the ground to let the meteor pass. some moments later one of them got up feeling very tired, but the other was dead. the bones of the latter were so soft that it was easy to bend them; his whole body was of the consistency of paste. the tongue had been torn from the roots, and no one knew what had become of it. now, just as the earth can strike, so can the human body become fulminating and act like lightning. after having been struck, it can effectively acquire the power to strike in its turn. for instance, on june , , a man named barri was killed by lightning near the jardin des plantes, in paris, and his body lay for some time exposed to a beating rain. after the storm had passed, two soldiers from the neighbouring guard-house tried to remove the body, and each received a violent blow when they touched it. they got off with a shock, perhaps because the body had been drenched with rain, which acted as a conductor to the electricity, and thus it had had time to lose a part of the fluid. what a mysterious world is that of atmospheric electricity! it is truly the new world for the scientific mind--a mine, fruitful in unknown and even unsuspected marvels, which is perpetually disclosing its riches for our admiration. one of our most valued collaborators in our researches on the nature of lightning is photography. faithfully and unhesitatingly it registers an indestructible document of the fugitive lightning, which imprints itself on the sensitized plate, and the astronomer can afterwards examine the smallest details of the sudden apparition comfortably and at his leisure. we have already a considerable number of plates of the outline of the lightning in flight. an examination of these electric pictures is very interesting. who knows whether, later on, when phonography is brought to perfection, it will not also register the noisy accompaniment to the electric flash? then, with the help of the cinematograph, we could have dramatic representations of sensational storms. while the photograph unrolls all the phases of the lightning, from its emerging from the cloud to its fall to earth, before the gaze of the spectators, the phonograph will repeat the sonorous accents of the terrorizing voice of thunder. thunder, as all the world knows, is the noise which accompanies lightning. it is produced when a change of electricity--a neutralization--takes place between two points more or less distant. the causes which provoke it are still somewhat of a mystery. the luminous rocket which flings itself precipitately from a cloud saturated with electricity, spreads itself like a trail of flames in the atmosphere where an infinity of invisible molecules are floating; these it repels. the passage of this whirlwind of fire in a centre which is greatly compressed produces a momentary void into which the surrounding air at once rushes, and it is the same all the way along the route followed by lightning. in all probability the equilibrium of the atmosphere, which is momentarily disturbed by the intrusion of the ignited matter, hastily re-establishes itself by a rush of the air which the lightning has ejected, and which is swallowed up with a crash in the opening which has been made. it is, on a large scale, a similar phenomenon to that which is produced by opening a case which has been hermetically sealed. the air rushing in makes a dull noise. pouillet objects to this very natural explanation on the ground that the flight of a cannon-ball ought to produce a similar noise. but this objection errs in its basis, because, as regards velocity, a cannon-ball is as a tortoise as compared with the arrow of lightning, and as regards size, who can compare a few grammes of powder to the torrents of fire launched into space by the prodigious force of electricity? the lightning discharge produces a violent concussion in the cloud, and very often a shower of rain immediately follows it. the electric conditions of the different clouds which make a storm being separately liable the one to the other, the discharge of one must lead to that of several others more or less distant. in all cases the noise is caused by the expansion of the air where a more or less partial void has been made. it is the same with firearms, _crève-vessie_, etc. one of the chief characteristics of thunder is the rolling, which is often prolonged, and reverberates on the sides of steep mountains. this voice, with its lugubrious tone, becomes grave and sometimes sinister in the revolution of space--this voice, celestial and infernal, seems to momentarily dominate the world, while the clouds are enveloped with a thousand diabolical flames. sometimes it rings in the air with fierce calls, at others it spreads itself in dull, languorous complaints. nevertheless, the intenseness of thunder undergoes a thousand fluctuations, and presents astonishing variations. generally it strikes and frightens, but the curious thing is that, for the ear, in reality it is less strong than the crinkling noise of a piece of paper torn close to it. often, too, it may be compared to the discharge of firearms, a pistol or a cannon. thus, when the lightning penetrated volney's apartments at naples, the people present, among whom was saussure, had the impression of a pistol-shot in the next room. there is a case given of m. and mme. boddington, who were seated on the back seat of their coach in order to enjoy the view of the country, and had given the inside seats to two servants. suddenly there was a flash of lightning, which struck m. and mme. boddington and flung the postillion to a great distance. the servants were untouched, and escaped with a fright. when they got over their terror, one of them said that a very brilliant flash of lightning had been immediately followed by a noise similar to that of a heavily charged musket. he thought some one had shot the horses. his fright had stunned him so that he hardly knew what had happened. at other times thunder is accompanied by a whistling noise, but as a rule it is the rolling which predominates. we ask ourselves to what it is due that this rolling lasts so long. there are several causes. the first is due to the length of lightning and the difference in speed between sound and light. let us suppose, for example, a flash of lightning, ae, , metres long. the observer stationed at o, underneath extremity e of the lightning (which is one kilometre high), will see the lightning in its full length in one indivisible instant. the sound will form itself also at the same instant all along the line of lightning, but the sound-waves will only reach the ear of the observer at different times. that which starts at point e, the nearest, will arrive in seconds, sound travelling about metres a second. that which is formed at the same moment at point d, yards from point o, will take double the time to arrive. that which comes from point c will not arrive for seconds. the sound formed at b will not arrive until the time necessary to cover kilometres--that is to say, not for seconds--and the sound formed at a will only reach after seconds. thus the rolling will have lasted more than half a minute from start to finish. [illustration: diagram explaining the duration of the sound of thunder.] and if, which is very often the case, the astronomer is not exactly under one of the extremities of the lightning, but at some other point in its course, he first hears a clap, then an increased noise, then a diminution. in fact, in this case, the sound which leaves point d just overhead, which is metres off, arrives alone in seconds, but the sounds formed from d to e on one side, and from c to d on the other, arrive at the same time, having joined each other, taking seconds, which is the necessary time to come from to metres. the sounds beyond c arrive and depart according to distance, as in the preceding example, and the thunder has lasted seconds instead of seconds. [illustration: commencement, augmentation, and diminution of the intensity of thunder.] i must add that lightning is never straight, but always crooked. the length of time the thunder rolls has nothing to do with the distance of the cloud where the phenomenon begins. it is proportionate to the length of the lightning with which it is associated. the rolling is often still more prolonged by a succession of small discharges, which follow each other very rapidly between the stormy clouds; by the zigzags and ramifications of the lightning caused by the hygrometrical diversity of the different beds of air; by the echoes repeated by the mountains, the earth, the water, and the clouds themselves--to all which must be added also the interferences caused by the encounter of the different systems of sound-waves. its duration is extremely variable, however; it rarely exceeds seconds, though the noise may sometimes seem to last much longer, so that an observation of this kind may have any value--one must take into consideration the echo, and isolate a single clap from the series of discharges which take place in the bosom of the storm. the longest verified duration of a single discharge is seconds. that is tremendous if we think of the instantaneousness of the lightning, and reflect that the flash and the sound are produced in reality at the same moment, that they are dependent the one on the other, and that in their various manifestations there is only the difference of motion. sound moves like a tortoise behind the swift lightning, whose vibrations spread in the air with inconceivable rapidity. hence these seconds correspond to a flash of lightning more than kilometres in length, but we know that there are even longer ones. i have already said that we can calculate the distance of the celestial cannon from which the fulminating discharge comes by counting the number of seconds which separate the apparition of the lightning from the first growls of the thunder. thus the longest interval that has been proved between the appearance of the lightning and the noise it produces is kilometres. this, however, is a maximum. numerous observations have proved that thunder is never heard beyond or perhaps kilometres. lightning pierces the cloudy veil, but the voice of thunder does not carry so far. in this the great jupiter shows himself inferior to the ingenuity of human pigmies, whose destructive and barbarous art has been able to invent infernal machines the noise of which can be heard much further. cannon can easily be heard at a distance of kilometres. sometimes, in sieges and big battles the cannonades can be heard muttering lugubriously more than kilometres away. during the siege of paris, krupp's cannon--that most expeditious of all vehicles of civilization in the eyes of the statesmen of this planet!--could be heard as far as dieppe, kilometres away, during the nights when they were bombarding. the cannonade of march , , which crowned the first empire, as it crowned the second, was heard between lisieux and caen, a distance of kilometres. arago even alleges that the cannon at waterloo could be heard as far as creil, which is kilometres away. thus man's thunder can be heard at a greater distance than that of nature. it is true that it is incomparably more vicious, and that it has a great many more victims. in its natural state, if we might explain it thus--left to itself--it comes directly to us from the high regions of the atmosphere, and is the most terrible of aerial messengers--a subtle messenger, malicious and violent, it is the terror of the human race. but ruled by the genius of man, it becomes a powerful agent towards modern civilization, and we cannot sufficiently admire its many advantages. if we could tame lightning and guide it safely, its services would probably become innumerable. lightning as man's right hand! why not? was it not the auxiliary of the gods in the dark ages? to-day, is it not regarded by astronomers as one of the most important forces of nature? why should it not be the collaborator of man's intelligence to-morrow? chapter iv fireballs here we penetrate into what is, perhaps, the most mysterious, and certainly the least understood domain of thunder and lightning. among all the electrical phenomena to be observed in the atmosphere, there is nothing stranger than those fireballs of which we have already spoken, and which in form and size recall the electric lights in our paris boulevards. curious the contrast between electricity tamed and civilized and electricity running wild! between the arc lights fulfilling their peaceful and useful function as substitutes for the sun, and these dread engines of destruction sowing death and havoc! it is not long since the existence of these fireballs has been acknowledged by scientists as an actual fact. until quite recent times they were regarded as the figment of excited imaginations, and wise men smiled at the wild stories of their ravages. their reality has now been established, however, beyond the possibility of doubt. in shape they are not always quite spherical, though this is their normal appearance; and although their contours are usually clearly defined, they are sometimes encircled by a kind of luminous vapour, such as we often see encircling the moon. sometimes they are furnished with a red flame like a fuse that has been lit. sometimes their course is simply that of a falling star. sometimes they leave behind them a luminous trail which remains visible long after they themselves have disappeared. they have been described as looking like a crouching kitten, an iron bar, a large orange--so harmless apparently, that you were tempted to put out your hand to catch it. there is record of one being seen as large as a millstone. one remarkable thing about them is the slowness with which they move, and which sometimes enables their course to be watched for several minutes. in our first chapter we gave several instances of the occurrence of fireballs. let us look at some more. here is one taken from arago's learned treatise upon thunder. the record is from the pen of batti, a marine painter in the service of the empress of austria and resident at milan. "in the month of june, , i was staying at the hôtel de l'agnello in a room on the second floor, overlooking the corso dei servi. it was about six in the afternoon. the rain was coming down in torrents, and the darkest rooms were lit up by the lightning flashes better than our rooms generally are by gas. thunder broke out every now and again with appalling violence. the windows of the houses were closed, and the streets were deserted, for, as i have said, there was a steady downpour, and the main road was turned into a torrent. i was sitting quietly smoking, and looking out at the rain, which an occasional ray of sunlight set flashing like threads of gold, when i suddenly heard voices in the street calling out 'guarda, guarda!'--'look, look!' and at the same moment a clatter of hob-nailed boots. after half an hour of absolute silence, this noise attracted my attention. i ran to the window, and looking to the right, in the direction of the clamour, i saw a fireball making its way down the middle of the road on a level with my window, in a noticeably oblique direction, not horizontally. eight or ten persons, continuing to call out 'guarda, guarda!' kept pace with it, walking down the street, stepping out quickly. the meteor passed my window, and i had to turn to the left to see what would be the end of its caprice. after a moment, fearing to lose sight of it behind some houses which jutted out beyond my hotel, i went quickly downstairs and into the street, and was in time to see it again and to join those who were following its course. it was still going slowly, but it was now higher up, and was still ascending--so much so that after a few minutes it hit the cross upon the clock tower of the chiesa dei servi and disappeared. its disappearance was accompanied by a dull report like that of a big cannon twenty miles away when the wind carries the sound. "to give an idea of the size and colour of this globe of fire, i can only compare it to the appearance of the moon as one may see it sometimes rising above the alps on a clear night in winter, and as i myself have seen it at innsbrück--that is to say, of a reddish yellow, with patches on it almost of red. the difference was that you could not see the contours of the meteor distinctly as you could the moon, and that it seemed to be enveloped in a luminous atmosphere of indefinite extent." this fireball was an innocuous one. we may take next, by way of contrast, the case of one which wreaked terrible damage and loss of life. on july , , at about three o'clock in the afternoon, a fireball of about the size of a cannon-ball, fell in a great hall at feltri (marche trevisane) in which six hundred people were seated, wounded seventy and killed ten, putting out all the lights. on july , , about eleven o'clock in the morning, a fireball penetrated into the church of chateauneuf-les-moustiers (basses-alpes) just as the bell was ringing and a large congregation had taken their seats. nine persons were killed on the spot and eighty-two others were wounded. all the dogs that had got into the church were killed. a woman who was in a hut on a neighbouring hill saw three fireballs descend that day, and made sure they would reduce the village to ashes. müsschenbroek recounts the following incident which took place at solingen in . m. pyl, the pastor at duytsbourg, was preaching one sunday, when in the middle of a storm a fireball fell into the church through the clock tower and exploded. the sanctuary was set on fire and became thick with smoke. three persons were killed and more than a hundred were wounded. from the _bulletin_ of the société astronomique de france the following narrative contributed to it by mlle. de soubbotine, a member of this society, has been taken:-- "a terrible storm broke out at ouralsk on may , . it was a _fête_ day and the streets were thronged with people. towards five in the afternoon some young men and girls, twenty-one in all, had taken refuge in the vestibule of a house, and a girl of seventeen, mlle. k., had sat down on the threshold, her back turned towards the street. suddenly there was a violent clap of thunder, and in front of the door there appeared a dazzlingly brilliant ball of fire, gradually descending towards where they were all grouped. after touching mlle. k.'s head, who bowed down at once, the fireball fell on the ground in the middle of the party, made a circuit of it, then forcing its way into the room of the master of the house, whose boots it touched and singed, it wreaked havoc with the apartment, broke through the wall into a stove in the adjoining room, smashed the stove-pipe, and carried it off with such violence that it was dashed against the opposite wall, and went out through the broken window. "after the first feeling of fright, this is what transpired. the door near which mlle. k. was seated had been thrown back into the court, and in the ceiling there were two holes of about centimetres each. "the young girl, still seated with her head bowed down, looked as though she were asleep. some of the people were walking in the courtyard, having seen and heard nothing, and the others were all lying in the vestibule in a dead faint. mlle. k. was dead. the fireball had struck her on the nape of her neck and had proceeded down her back and left hip, leaving a black mark all along. there was a sore on one hand, with some blood on it, and one of her shoes was torn completely off, and there was a small hole in one of the stockings. "all the victims became deaf." on september , , at about two in the afternoon, in the course of a violent storm, a fireball came down the chimney into a room in a house in the village of salagnac (creuse). a child and three women who were in the room suffered no harm from it. then it rolled into the middle of the kitchen, and passed near the feet of a young peasant who was standing in it. after which it went into an adjoining room, and disappeared without leaving any trace. the women tried to persuade the man to go in and see whether he could not stamp it out, but he had once allowed himself to be electrified in paris, and thought it prudent to refrain. in a little stable hard by, it was found afterwards that the fireball had killed a pig. it had gone through the straw without setting fire to it. on july , , a new form of fireball made its appearance in the commune of hécourt (oise). it was of the size of an egg, and it was seen burning upon a bed. efforts were made in vain to extinguish it, and presently the entire house, together with the neighbouring dwellings and barns, became a prey to the flames. on october , , at . p.m., during a violent storm, a globe of fire of the size of a small apple was seen coming into a ground-floor room in a house at constantinople through an open window, the family being at table in this room at the time. it first played round a gas-jet, then, moving towards the table, it passed between two guests, went round a lamp hanging over the centre of the table, and then precipitated itself into the street, where it exploded with an appalling crash, but without having caused any damage or hurt anybody. not far from the scene of this phenomenon there are a number of buildings provided with lightning conductors. the fireball left no trace of smell behind it. here is another curious narrative of a fireball. a party of five women took refuge during a storm in the entrance to a house in order to escape from the rain and the lightning. they had scarcely gained the doorway when there was a tremendous thunderclap which sent them flying backward--and two girls who had joined them--knocked senseless by lightning in the form of a fireball. one of the girls remained unconscious for a long time; all the others were more or less seriously injured, but all recovered. the strangest circumstance in connection with this affair, however, still remains to be told. [illustration: singular case of three fireballs observed in paris on june , , by m. h. rudaux. they were seen to descend in this way upon the lightning conductor above the palais royal electric-power station. this engraving, after a sketch made at the time by m. rudaux, appeared in _la science illustrée_, for august, .] on the same side of the street as the passage, in a neighbouring house, nine or ten yards away, in a ground-floor room of which the door was shut, a young woman was working at a sewing-machine. at the moment of the thunderclap, she experienced a violent shock throughout her whole body, and a fierce burning sensation in the hollow of her back. it was found afterwards that between the shoulder-blades and also on her leg, she had been badly scorched, but the wounds quickly healed. now, in the room of this victim, no trace was to be found of the passing of the fireball, neither on the ceiling, nor on the floor, nor on the walls. there was absolutely nothing to show how the electric fluid could have made its way in from the spot in which the fireball had exploded in the neighbouring house, separated from it by two thick walls. mysterious, is it not? the fireball seems to dwindle out of sight. in some cases, it seems to reduce itself into vapour in order to pass from one place to another. with animals these fireballs seem deadlier and more merciless than with human beings. thus, on february , , a thunderstorm descended upon a farm in the commune of chapelle-largeau (deux-sèvres), and the circumstances attending its explosion are too remarkable to be overlooked. after a tremendous thunderclap, a young man who was standing near the farm saw an immense fireball touch the ground at his feet, but it did him no damage, but passed, still harmlessly, through a room in the farmhouse in which there were nine persons. the only effect it produced was the flaring up of some matches upon the chimney-piece. it proceeded towards the stables, which were divided into two compartments. in one there were two cows and two oxen: the first cow, to the right of the entrance, was killed, the second was uninjured; the first ox was killed, the second was uninjured. the same effect was found to have been produced in the other compartment, in which there were four cows; the first and the third were killed, the second and fourth were spared: the odd numbers taken and the even numbers left. similar freaks have been recorded in connection with piles of plates struck by lightning--holes being found in alternate plates. how are these things to be explained? the following story is very extraordinary, though it does not help to clear up the mystery of lightning's strange ways:-- on august , , about ten in the morning, in the midst of a storm of wind and rain, several persons saw descending to the ground a whitish-coloured globe of about an inch and a half in diameter, which, on touching the ground, split into two smaller globes. these rose at once to the height of the chimneys on the houses close by and disappeared. one went down a chimney, crossed a room in which were a man and a child, without harming them, and went through the floor, perforating a brick with a clean round hole of about the size of a franc. under this room there was a sheepfold. the shepherd's son, seated at the doorway, suddenly saw a bright light shining over the flock of sheep, while the lambs were jumping about in alarm. when he went up to them, he was startled to discover that five sheep had been killed. they bore no trace of burning, or of wound of any kind, but about their lips was a sort of foam, slightly pink in colour. in the adjoining house, the second fireball had also gone down a chimney, and had exploded in the kitchen, causing great damage. in , a young farmer was working on a plot of ground, two or three miles from montfort-l'amaury. a storm breaking out, he stood up against his horses to take refuge from the rain; moving away a few yards in order to get his whip, there was seen, when he returned, a ball of fire almost touching the ear of one of his horses. a moment later it exploded with a deafening noise. the two horses fell--one of them unable to get up again. the farmer himself was dashed to pieces. on other occasions the meteor is hardly more devastating than the ordinary bomb. on april , at lanxade, near bergerac, a storm had been raging already for some hours, when suddenly--simultaneously with a small thunderclap--a ball of fire, of the size of the opening of a sack of corn, fell slowly on one of the banks of the dordogne, spoiling some fruit trees, and then crossing the river, it raised a waterspout several yards high as it went. it disappeared finally on the other side of a field of corn. on november , , a very curious instance of a fireball was noticed on the atlantic. it was at midnight, near cape race. an enormous fireball was seen to rise slowly out of the sea to the height of sixteen or seventeen metres. it travelled against the wind, and came quite near the vessel from which it was being watched. then it turned towards the south-east and disappeared. the apparition lasted about five minutes. in july, , in the course of a violent storm, and immediately after a loud peal of thunder, a fireball of about the size of a toy balloon was seen to make its appearance suddenly in the rue veron at montmartre. after moving along, just above the ground, in front of a wine-merchant's shop, it exploded like a bomb, most fortunately without hurting any one, or doing any damage. the little village of candes, situated by the confluence of the vienne and the loire, was the scene of the appearance of a fireball in june, . three persons were sitting in the verandah of a house during a storm, when they suddenly saw a fireball travelling past them through the air for a distance of thirty yards or so. then it exploded with a loud noise, striking sparks from the ironworks of the verandah. at the same moment, the servants saw another fireball cross a garden at the other side of the house, and drop into a small pond. a gardener was knocked over, but not hurt. on march , , m. dandois, professor of surgery at the university of louvain, went to the neighbouring town of linden, by railway, to see a patient. on his return, on foot, the sky suddenly so darkened over, that he made for the nearest dwelling-place, avoiding, as he did so, the telegraph poles along the road. suddenly a ball of fire came against him and threw him over a ditch into a field, where he lay unconscious. a quarter of an hour later, having regained his senses and finding himself undamaged save for a numbness in one arm and one leg, the doctor set out again, congratulating himself on the fact that his umbrella had acted as a sort of portable lightning conductor, for the steels were all twisted, and showed signs of having borne the brunt of the fray. had the handle been of steel also, the electric current would have run down it into his hand, doubtless, and killed him. on another occasion a fireball fell upon the door of a house, pushed it violently open, and made its way into the kitchen. at the sight of this strange visitor, the cook bolted from the room. a sempstress, who was at work near the window, received a small burn on her forehead, of about the size of half a franc, with a slight weal a couple of inches long--like the tail of a comet. after bursting, the fireball made its way up the chimney, from which it removed a mass of soot, smelling somewhat of sulphur. here is an instance more curious still-- a violent storm was raging near marseilles, when seven persons, seated together in the ground-floor drawing-room of a country house, saw a fireball as big as a plate appear in their midst. it directed its course towards a young girl of eighteen, who, frightened out of her life, had fallen on her knees. touching her shoes, it rebounded to the ceiling, then came down to her feet again, and so on two or three times, with mysterious regularity, the girl experiencing, it seems, no other sensation than that of a slight cramp in her legs. eventually the fireball made its exit from the room through a keyhole! the girl could not get up at once after it had gone. for a fortnight or so she could not walk without assistance, and it was two years before she got over a liability to sudden weakness in her legs, causing her suddenly to fall. it is strange to reflect that these diminutive fireballs, produced by the actual atmosphere we breathe, are less understood by us than that enormous globe which we call the sun, and to which is due the flowering of the entire life of our planet. if we are still in doubt as to the nature of the sun's spots, at least we have been able to analyse its own elements. and we know its dimensions, its weight, its distance from us, its rate of rotation, etc., etc. yet these electric spheres that make their escape from the clouds in times of storm, baffle our investigations altogether. according to records which seem authentic, fireballs have been seen actually to come into existence upon the surface of a ceiling, at the mouth of a well, and upon the flagstones of a church. in , at the château of fosdinaro, in the neighbourhood of massa carrara, in the course of a storm and heavy downpour of rain, there was seen to appear suddenly upon the ground a very vivid flame, white and blue in colour. it seemed to flare fiercely, but did not move apparently from the one spot, and after growing quickly in volume it suddenly disappeared. simultaneously with its going, one of the observers felt a curious sort of tickling behind his shoulder, moving upwards; several bits of plaster from the ceiling under which he stood fell upon his head, and there was a sudden crash quite unlike an ordinary thunderclap. in , on the nd of july, at about three in the afternoon, the abbé richard happened to be in the church of st. michel at dijon during a storm. "suddenly," he tells us, "i saw between two pillars of the nave a bright red flame floating in the air about three feet above the floor. presently it rose to a height of twelve or fifteen feet, increasing in volume. then, after having moved some yards to one side, while still rising diagonally to the height almost of the woodwork of the organ, it disappeared at last with an explosion like the report of a cannon." on july , , a violent storm broke out in boulogne, and the tower of a convent was struck by a fireball. it was of great size, and was seen to emerge from one of the sewers of the town and to move along the surface of the road until it hit against this tower, of which a part subsided. no one was hurt. a nun affirmed that some years before she had seen just such another fireball emerge from the same spot and precipitate itself with a crash against the summit of the tower without doing any damage. in the middle of a violent storm, dr. gardons saw several fireballs flying in different directions, not far from the ground, making a crackling sort of noise. one of them was seen by witnesses to come out of an excavation full of stagnant water. they killed one man, several animals, and did much damage to the trees and houses in the vicinity. in february, , at presbourg, a blue, conical flame escaped suddenly with a detonating noise from a brasier, breaking it to pieces, and scattering the glowing cinders all around. it then went twisting about the room, burnt the face and hands of a child, escaped partly through the window, partly through the door, broke into a thousand pieces a second brasier in another room, and disappeared finally up a chimney, carrying up with it and discharging from the chimney-top into the street several hams which had been hung under the chimney-piece. for several days afterwards the atmosphere of the house retained a smell of sulphur. in some cases, fireballs have been seen to come down from the sky apparently, and then, after almost reaching but not actually touching the ground, to ascend again. thus on a hot day in summer , m. hapoule, a landed proprietor in the department of the moselle, standing in front of the entrance to his stables under the shelter of a porch during a storm, saw a fireball about the size of an orange moving in the direction of a dung-heap not far from him. but instead of going right into it, it stopped about a yard off, and changing its route, it went off at an angle, keeping the same level for some distance, when it suddenly seemed to change its mind again, and rose perpendicularly till it disappeared in the clouds. these sudden changes, as we have seen, are strangely characteristic of the habits of fireballs. the garde champêtre of the village of lalande de libourne (gironde) was traversing the country one evening about half-past ten, engaged in organizing a _garde de surveillance_, when he suddenly found himself surrounded by a bright and penetrating light. astonished, he looked behind him, and saw a fireball, just broken loose from a cloud, descending quickly to the ground. the light vanished presently, but he made his way towards where the fireball seemed to be falling. when he had gone about two hundred yards, he saw another brilliant light breaking out from the top of a tree and spreading itself into a sheaf of rays, every point of which seemed to emit electric sparks. at the end of a quarter of an hour the light became weaker, and then disappeared. the tree was afterwards cut down, and it was found that the lightning had gone down the centre to a distance of three yards, and had then passed down outside to the soil, leaving trace of a semi-circular route; and finally, after rising again on the opposite side of the tree to a height of four yards, tearing off two narrow strips of bark, had disappeared. at the foot of the tree a small hole, about an inch and a quarter in diameter, retained a certain degree of warmth for an hour and a half afterwards. fireballs often keep within the frontiers of cloud-land. they may be seen passing sometimes from one cloud to another in the high regions of the atmosphere. on september , , at seven in the evening, m. louis ordinaire saw a fireball leave a cloud at the zenith--the sky being very much lowering at the time--and go towards another. it was of a reddish-yellow and extremely brilliant, lighting up the ground with a bright radiance. he was able to follow its movements for at least a minute, and then saw it disappear into the second cloud. there was an explosion followed by a dull sound like the firing off of a cannon in the distance. after a violent storm which broke out near wakefield on march , , there remained only two clouds in the sky, just above the horizon. balls of fire were observed gliding from the higher of the two into the lower, like falling stars. in high mountainous districts--in the alps, for instance--you may often look down from above upon a storm. it is fascinating thus to watch the grandiose spectacle of the elements at war. here from the pen of père lozeran du fesch is a striking picture of such a scene-- "it was on the nd of september, , about three o'clock in the afternoon. a traveller was making his way down towards vic from the summit of cantal, accompanied by a guide. "the weather was calm and very warm, but down below, about the middle of the mountain, a vast sea of mist stretched out in wavelike clouds. "these clouds were furrowed continually by lightning flashes, some going quite straight, some zigzag, some taking the shape of fireballs. when the two men came near this region of clouds, the mist grew so thick they could hardly see the bridles of their horses. "the air became gradually more cold and the darkness more dense as they proceeded downwards. now they were in the midst of the fireballs flying in every direction all round them, revolving as they went, reddish in colour, like saffron lit up. "they were of all sizes--some quite small on their first appearance, seeming to grow immensely in volume in a few moments. drops of rain fell when they passed. up to this point the sight had been curious but not terrifying, but suddenly now, one of these fireballs, about two feet in diameter, burst open near the traveller and emitted streams of a bright and beautiful light in every direction, and there was a dull report followed by a tremendous crash. the two men were much shaken and the air all round them seemed polluted. after a minute or two, however, all trace of the explosion had been dissipated, and they proceeded on their way." on january , , near merlan, about six in the afternoon, a fireball burst above the heads of two men, enveloping them in a bluish light, without hurting them or even damaging their clothes, but giving them a momentary thrill as from an electric battery. it left no traces of any kind, not even a smell. mr. g. m. ryan records an instance which he witnessed at karachi in scinde. while in his drawing-room one day with two friends who were taking refuge from a storm, he rose from his chair and went to the door to open it, the windows as well as the door being shut at the time. returning, he saw in the air and between his friends, a ball of fire of about the size of a full moon. at the same time there was a terrible clap of thunder. two of the spectators were slightly wounded; one felt a sharp pain on the left side of the face, the other, a sensation in one arm with a feeling as if his hair were burning. there was a strong smell of sulphur. in the next room there were two rifles in a case; one was intact but the other was broken, and there was a hole in the wall at the point where the muzzle leant against it, and there were two holes in the same wall a story higher. on sunday, august , , several people were assembled in a room in the château of the baron de france at maintenay (pas-de-calais), when there was a violent storm raging over the country. suddenly there appeared in the midst of the eleven people who were there, a globe of blue fire about the size of an infant's head, which quietly crossed the room, touching four people on its way. none of them were injured. an awful explosion was heard at the moment when the electric ball disappeared through an open door in front of the great staircase. on august , , a fireball struck the house of a mr. david sutton, not far from newcastle-on-tyne. eight people were having tea in the drawing-room when a violent clap of thunder knocked down the chimney. immediately after they saw on the ground, at the door opposite the fireplace, the brilliant visitor which announced itself in the sonorous voice of jupiter the thunderer. it remained discreetly at the entrance of the room, no doubt waiting for the sign to advance. no one making a move, it came into the middle of the room, and there burst with a crash, throwing out fiery grains like aeroliths. the spectacle must have been magnificent--but, we must acknowledge, rather disquieting. on september , , at besançon, a voluminous fireball crossed over a corn-shop and the ward of a hospital full of nurses and children. this time again the lightning was merciful--it spared nurses and children, and went and drowned itself in the doubs. nearly thirty years before, in july, , it showed the same regard for an honest german peasant woman. she was occupied in the kitchen superintending the family meal, when, after a terrible clap of thunder, she saw a fireball the size of a fist come down the chimney, pass between her feet without hurting her, and continue on its course without burning or even upsetting the spinning-wheel and other objects on the floor. much frightened, the young woman tried to escape; she threw herself towards the door and opened it, when the fireball at once followed her, played about her feet, went into the next room, which opened out-of-doors, crossed it, and through the door into the yard. it went round the yard, entered a barn by an open door, climbed the wall opposite, and reaching the edge of the roof, burst with such a terrific noise that the peasant woman fainted. the barn at once took fire and was reduced to cinders. towards the middle of the last century, march , , the steeple of crailsheim was set on fire by lightning. the guardian's daughter, aged twenty years, was at this moment in her room and had her back turned to the window, when her young brother saw a fireball enter by the window-sill and descend on to his sister's back, giving her a sudden shock all over her body. the young girl then saw at her feet a quantity of small flames, which went towards the kitchen, the door of which had been opened, and set fire to a pile of mossy wood. there was no further damage than this attempt at incendiarism, which was easily extinguished. occasionally a fireball seems to take a malignant pleasure in hurling itself like a fury against lightning conductors; but instead of quietly impaling itself like the linear lightning, and breathing its last sigh in a prolonged roar, it struggles, and comes forth victorious from this curious contest. there are many cases of fireballs playing about the lightning conductors without being caught. in , a fireball shot from the clouds on to the point of the lightning conductor on the observatory of padua. the conductor, which consisted of an iron chain, was broken at its junction with the stem. however, it sent on the discharge. some years later, in , a huge ball of lightning struck one of the two conductors on the house of m. haller at villiers la garenne. this conductor was much injured by the audacious assailant, and so was the framework of the house; the keen fluid had damaged the metallic gutters. at this point i must add that lightning conductors are of recent creation. nor would it be surprising if there were defective ones which could not assure an efficient protection. however, much later, on december , , the same phenomenon was observed at the château of bortyvon, near vire. there, again, the fireball, ignoring the danger to which it was exposing itself, flung itself on a lightning conductor placed in the centre of the château. it was spared, but the château suffered greatly. the electric ball descended from both sides of the metallic stem, causing a great deal of damage along its path. on touching the ground it expanded, and many persons affirm that they saw what was like a huge cask of fire rolling along the ground. in truth, ball lightning seems in a certain measure to escape the influence of lightning conductors. on september , , towards ten o'clock in the evening, m. laurence rotch, director of the observatory of blue hill (u.s.), happening to be in paris, made the following curious observation from the rond-point of the champs elysées. looking in the direction of the eiffel tower, he saw the summit of the edifice struck by white lightning coming from the zenith. at the same moment a fireball, less dazzling than the lightning, slowly descended from the summit to the second platform. it appeared to be about one yard in diameter, and to be situated in the middle of the tower, taking less than two seconds to cover a distance of about yards. then it disappeared. the next day the observer ascertained, on visiting the tower, that it had actually been struck by lightning twice on the previous day. it is to be noted that the meteor did not follow the conductor; but, after all, is not the whole tower itself the most powerful conductor imaginable? would not the enormous masses of iron used in its construction neutralize the attraction of the thin metallic rods, effectual for the protection of ordinary buildings, but incapable, one would think, of competing with the attractive force of this immense metallic framework? here are some cases where globular lightning has struck bells or telegraph wires, which it has followed with docility. several times it has been seen poised like a bird on a telegraph wire near a railway-station, and has then quietly disappeared. [illustration: the eiffel tower as a colossal lightning conductor. photograph taken june , , at . p.m., by m. g. loppé. published in the _bulletin de la société astronomique de france_ (may, )] we see that it is not absolutely inimical to points, nor to metals, but it prefers its independence, and he must get up early who would catch it in a snare. it is an anarchist--it acknowledges no rule. but we must confess that if spheroidal lightning seems particularly capricious, it is because we are still ignorant of the laws which guide it. our ignorance alone is the cause of the mystery. we try to discover the enigma in the silence of the laboratories, where physicians question science without ceasing; we try to reproduce fireballs artificially, but the problem is complicated, and its solution presents enormous difficulties. hypotheses are not wanting. some years ago, m. stéphane leduc recorded an interesting experiment, producing a moving globular spark. when two very fine and highly polished metallic points, each in affinity with one of the poles of an electro-static machine, rest perpendicularly on the sensitive face of a gelatine bromide of silver photographic plate, which is placed on a metallic leaf, the two points being to centimetres the one from the other, an effluvium is produced round the positive point, while at the negative point a luminous globule is formed. when this globule has reached a sufficient size, you can see it detach itself from the point, which ceases to be luminous, begin to move forward slowly on the plate, make a few curves, and then set off for the positive point; when it reaches this, the effluvium is extinguished, all luminous phenomenon ceases, and the machine acts as if its two poles were united by a conductor. the speed with which the luminous globe moves is very slight. it takes from one to four minutes to cover a distance of to centimetres. sometimes, before reaching the positive point, the globe bursts into two or more luminous globules, which individually continue their journey towards the positive point. on developing the plate, you will find traced on it the route followed by the globule, the point of explosion, the routes resulting from the division, the effluvium round the positive point. also, if you stop the experiment before the arrival of the globule at the positive point, the photograph will only give the route to that point. the globule makes its course the conductor. if during its journey you were to throw powder on the plate--sulphur, for example--the course it followed will be marked by a line of little aigrettes, looking like a luminous rosary. of all the known electric phenomena, this is the most analogous with globular lightning. [illustration: photograph of the positive pole of an electric spark.] [illustration: photograph of the negative pole of an electric spark.] but the really complicated part of the question is when ball lightning loses part of its fluidity and becomes a semi-solid body, as in the following instance:-- on april , , a storm burst over mortrée (orne), and the lightning literally chopped the telegraph wire on the route to argentan for a distance of yards. the pieces were so calcinated that they might have been under the fire of a forge; some of the longer ones were bent and their sections welded together. the lightning entered by the door of a stable in the form of a fireball, and came near a person who was preparing to milk a cow; then it _passed between the legs of the animal_, and disappeared without causing any damage. the terrified cow raised itself on its hind legs with frantic bellowing, and its master ran away, frightened out of his wits, but there was no harm done. the inexplicable phenomenon was that at the precise moment when the lightning crossed the stable, a great quantity of incandescent stones fell before a neighbouring house. "some of these fragments, of the size of nuts," wrote the minister of post and telegraphs at the academy, "are of a not very thick material, of a greyish-white, and easily broken by the fingers, giving forth a characteristic odour of sulphur. the others, which are smaller, are exactly like coke. "it would perhaps be useful to say here, that during this storm the thunderclaps were not preceded by the ordinary muttering, they burst quickly like the discharge of musketry, and succeeded one another at short intervals. hail fell in abundance, and the temperature was very low." it is only by a semblance of disbelief that one can get the peasants to tell us the stories of what they pretend to have seen of the fall of aeroliths during storms. they have christened the uranoliths "thunder-stones." these substances have evidently no relation to uranoliths, but they prove none the less that ponderable matter may accompany the fall of lightning. here are two more examples-- in the month of august, , a storm burst over sotteville (seine-inférieure); lightning furrowed the sky, the thunder muttered, and the rain fell in torrents. suddenly, in the rue pierre corneille, several small balls, about the size of a common pea, were seen to fall; these burned on touching the ground, sending out a little violet flame. people counted more than twenty, and one of the spectators, on putting her foot on one of them, produced a fresh flame. they left no trace on the ground. on august , , in paris, during a rather violent storm, in broad daylight, m. a. trécul, of the institute, saw a very brilliant voluminous body, yellowish-white, and rather long in shape, being apparently to centimetres in length, by about in width, with slightly conical ends. this body was only visible for a few seconds; it seemed to disappear and re-enter a cloud, but in departing--and this is the chief point--it dropped a little substance, which fell vertically like a heavy body under the sole influence of gravity. it left a trail of light behind it, at the edges of which could be seen sparks, or rather red globules, because their light did not flash. near the falling substance the luminous trail was almost vertical, while in the further part it was sinuous. the small substance divided in falling, and the light went out soon after, when it was on the point of reaching the tops of the houses. when it was disappearing, and at the moment of the division, no noise was heard, although the cloud was not far away. this fact incontestably proves the presence of ponderable matter in clouds, which is not violently projected by an explosion in the bolis, nor accompanied by a noisy electric discharge. we are still far from understanding the interesting problem of the formation and nature of ball lightning. instead of denying it, men of science ought to study it, because it is certainly one of the most remarkable of the curiosities of atmospheric electricity. we must begin by finding out the exact facts, which are extraordinary enough to captivate our attention. the theories will follow. chapter v the effects of lightning on mankind the destructive work of lightning in every form is immense. a formidable and invisible world skirts the earth--an enchanted world, more wonderful than any eastern legend--an unknown ocean, whose immaterial presence is constantly brought before us by the most fearful electric conflagrations. even to-day the brilliancy of lightning hides itself from us in the darkness of impenetrable mystery. but we feel that there is an immeasurable power, an unimaginable force which rules us. we are, in fact, but puny beings in comparison with this magic force, and the ancients were wise when they made the king of the gods responsible for the actions of lightning. he alone in his splendour and sovereignty could exercise such an empire over our modest planet--above all, over man's imagination. science slowly follows the centuries in their ascending march towards progress. at present our knowledge of ball lightning is limited, and we have only the principal facts of nature to contribute to the elucidation of the problem. in increasing our observations, and in comparing those which are analogous, we may hope, if not to arrive at an immediate conclusion, at least to help in the work of discovering what laws govern this subtle and imponderable fluid. here it will strike a man dead without leaving a trace; there it will only attack the clothes and insinuate itself as far as the skin without even grazing it. it will burn the lining of a garment, and leave the material of which it is made intact. sometimes it profits by the bewilderment caused by its dazzling light to entirely undress a person, and leave him naked and inanimate, but with no external wound, not even a scratch. we find as many peculiarities as facts. some of the actions of lightning remind one of the fantastic stories of hoffmann and edgar poe, but nature is more wonderful than the imagination of man, and lightning remains supreme in its phantasmagoria. thunder seems to play with the ignorance of man; its crimes and jests would have been ascribed to the devil in olden days. we submit to the effects without being able to determine the cause which directs them. it would seem as if lightning were a subtle being--a medium between the unconscious force which lives in plants and the conscious force in animals. it is like an elemental spirit, keen, capricious, malicious or stupid, far-seeing or blind, wilful or indifferent, passing from one extreme to another, and of a unique and terrifying character. we see it twisting into space, moving with astonishing dexterity among men, appearing and disappearing with the rapidity ... of lightning ... it is impossible to define its nature. at all events, it is a great mistake to trifle with it. it means running great risks. it resents being interfered with, and those who try to probe into its domain are generally rather cruelly put in their place. it was an indiscretion of this kind which cost dr. richmann his life. he had fixed an insulated iron rod from the roof of his house to his laboratory; this conducted the atmospheric electricity to him, and he measured its intensity every day. on august , , in the middle of a violent storm, he was keeping at a distance from the rod in order to avoid the powerful sparks, and was waiting for the time to measure it, when, his engraver entering suddenly, he took a few steps towards him which brought him too near the conductor. a globe of blue fire, the size of a fist, struck him on the head and stretched him stone dead. this beginning to the study of physics was hardly encouraging. the visitations of lightning are so numerous that it would naturally be impossible to describe them all in this small collection. we must, therefore, choose among them, but here we encounter a great difficulty. among the thousands of _tours de force_ and of dexterity accomplished by lightning, which should we take and which leave? the selection is very difficult, as it means leaving out a large number of curious examples with a good many very interesting observations. we will choose the most important--those of which the authenticity appears incontestable, and which contain the most precise details. we will group together those among them which present points of resemblance. this approximate classification will give us a sufficiently complete picture for the harmony of this study. * * * * * one of the most astonishing actions of lightning is certainly that of leaving the victim in the very attitude in which he was surprised by death. cardan gives an extraordinary example of this kind. in the course of a violent storm, eight reapers, who were taking their meal under an oak, were struck, all eight of them, by the same flash of lightning, the noise of which could be heard a long way off. when the passers-by approached to see what had happened, the reapers thus suddenly petrified by death, appeared to be continuing their peaceful meal. one held his glass, another was carrying the bread to his mouth, a third had his hand on the dish. death had seized them all in the position which they occupied when the explosion occurred. we hear of many similar cases to this. here is one of a young woman who no doubt was struck by lightning in the position in which she was found after the accident. it was during a violent storm on july , ; she was alone in the house at saint-romain-les-atheux (loire), and outside the thunder rolled fearfully. when her parents came back from the fields, they found a sad sight. the young woman had been killed by lightning. they found her kneeling in a corner of the room with her head buried in her hands; she had no trace of a wound. her child of four months, who was in bed in the same room, was only lightly touched. quite recently, on may , , at charolles (saône-et-loire), a certain mlle. moreau, who lived at lesmes, was waiting for the end of a storm in a grocer's shop where she had been making some purchases. several people were gathered round the fireplace. they felt a great movement following a violent clap of thunder. the sensation having passed, every one prepared to go. mlle. moreau alone remained seated, and did not move. she had been struck by the fluid, which had made a hole under her right ear and come out by the left! the petrifying action of the electric fluid is so rapid that horsemen who have been struck have remained on horseback and been carried a long way from the place of the accident without being unsaddled. according to abbé richard, towards the end of the eighteenth century, the procurator of the seminary of troyes was coming home on horseback when he was struck by lightning. a brother who followed him, not perceiving this, thought that he was asleep when he saw him reeling. when he tried to awaken him, he found he was dead. the following observation is very remarkable on account of the special attitudes preserved by the bodies which had been struck:-- a vessel which was at port mahon was struck at the time when the crew were dispersed over the yards to furl the sails. fifteen sailors who were scattered on the bowsprit were killed or burned in the twinkling of an eye. some were thrown into the water; others, bent dead across the yard-arm, remained in the position they had occupied before the accident. often the corpses of people who have been struck have been found either sitting or standing. at the approach of a storm a vine-dresser was seated under a nut tree which was planted near a hedge: soon afterwards, when it had ceased raining and the thunder was quiet, his two sisters, who had been taking shelter under the hedge, saw him sitting, and called to him to go back to work, but he did not reply; on going up to him, they found him dead. in , in the neighbourhood of asti, a priest who was struck while dining remained in his place. in , a ship was struck at about four o'clock in the morning, not far from saint-pierre. at daybreak a sailor was found sitting stone dead at the bow of the ship, with his eyes open and the whole body in such a natural attitude that he seemed to be alive. he had suffered no injury either external or internal. dr. boudin describes a still more surprising case. a woman was struck while she was in the act of plucking a poppy. the body was found standing, only slightly bent and with the flower still in her hand. it is hard to understand how a human body could remain standing, slightly bent, without a support to prevent its falling. this case is a contradiction to all the laws of equilibrium. but with such a fantastic agent as that with which we are dealing, nothing is surprising--we may expect anything. thus-- on august , , lightning struck the entrance pavilion of the prince eugene barracks in paris just when the soldiers were going to bed. all those who were lying down suddenly found themselves standing, and those who were standing were thrown on the ground. in the preceding examples the victims struck dead are not disfigured by the fulgurant force. they preserve a deceptive appearance of life. the catastrophe is so sudden that the face has no time to assume a sad expression. no contraction of the muscles reveals a transition in the passage of life and death. the eyes and mouth are open as though in a state of watching. when the colour of the flesh is preserved, the illusion is complete. but when we approach these statues of flesh--so lately animated with vital fire, now mummified by celestial fire--we are surprised on touching them to find that they crumble to ashes. the garments are intact, the body presents no difference, it keeps the attitude it had at the supreme moment, but it is entirely burnt, consumed. thus-- at vic-sur-aisne (aisne) in , in the middle of a violent storm, three soldiers took shelter under a lime tree. lightning struck them all dead at one blow. all the same, they all three remained standing in their original positions as though they had not been touched by the electric fluid: their clothes were intact! after the storm some passers-by noticed them, spoke to them without receiving an answer, and went up to touch them, when they fell pulverized into a heap of ashes. this experience is not unique, and even the ancients remarked that people who were struck crumbled to dust. here is a similar case, no less curious-- on june , , at rodez, a shepherd named desmazes, seeing that a storm was threatening, collected his beasts and drove them quickly towards the farm. when he was just there, he was struck by lightning. his body, which was completely incinerated, preserved a natural appearance. it is by this complete incineration and the probable volatilization of the cinders that certain authors explain the sudden disappearance of some of those who have been struck. legend attributes the mysterious death of romulus to a similar cause. according to livy, the illustrious founder of rome was reviewing his army in a plain near the marsh of capra. suddenly a storm accompanied by violent claps of thunder enveloped the king in a cloud so thick that it hid him from sight. from that moment romulus was seen no more on earth. it is true, livy adds, that some of the witnesses suspected the senators of having torn him to pieces: kings have sometimes been subject to all kinds of surprises on the part of their "courtiers." in most cases the electric matter produces burns more or less severe. these, when they do not attack the whole organism as in the preceding examples, are localized to certain parts of the body. sometimes they are quite superficial and only attack the epidermis. often without absolute carbonization, they penetrate deep into the flesh and cause death after the most fearful suffering. here are some examples of different sorts of burns-- in , in the rue pigalle in paris, a man had his eyes burnt by lightning. a young soldier of the th battalion of chasseurs was armed, mounting guard at the col de soda. it was in the month of july, . suddenly he was surrounded by the dazzling glare of lightning, which was almost immediately succeeded by an awful explosion of thunder. the sentinel, leaving his arms, fell backwards screaming. people ran to him, and saw that the fluid, attracted by the point of the bayonet, had struck it, and, gliding down, the metal had burnt his feet rather severely. at malines, in belgium, a mill was reduced to splinters by the fire of heaven. the miller and two of his customers were there at the time of the accident. not one of the three men was killed, but the miller was seriously burnt in the head, on the chin and the cheeks. he was deaf and blind for twenty-four hours. one of the others was burned in the hands. on june , , at about six in the evening, during a bad storm, five farmers were crossing the champ de gentillerie near saint-servan, in order to take shelter. three of them were walking abreast, the two others, of whom one was leading an ass, were some paces behind, when suddenly the five men and the ass were thrown on the ground by a violent clap of thunder. three of the farmers, recovering their consciousness after the shock, observed that their two companions were struck; the head of one was carbonized, and the left side of the other was burnt as though by a red-hot iron. another phenomenon, no less appalling-- a woman who was struck had her leg so horribly burnt that, on removing the stocking, some particles of flesh adhered to it. from the knee to the end of the foot the skin was black as though carbonized, and the whole surface was covered with a species of blister full of a sero-purulent liquid. the burns were very serious but not mortal, and were localized in the leg. lightning also sometimes produces wounds which are more or less severe. it perforates the bones. the injuries it causes are similar to those inflicted by firearms. it can also cause partial or total paralysis, the loss of speech or sight, temporary or permanent. its action is manifold on the human organism. a more extraordinary phenomenon still is that people who are struck show no sign of the slightest injury on a minute medical examination. the ancients remarked this, as we see in the charming passage from plutarch: "lightning struck them dead without leaving any mark on the bodies nor any wound or burn--their souls fled from their bodies in fright, like a bird which escapes from its cage." we have already spoken of the smell of fulminated air and of ozone. in some cases there is more than that. on june , , lightning struck a low house at moulins in the course of a violent storm. the fluid, eccentric as usual, attacked the outer chimney, the bricks of which were loose and projected slightly. it broke some tiles on the roof, the length of one rafter, and inside the corn-loft it broke the wooden handle of an iron rake to splinters. on the ground floor, bricks were both loosened and torn out near where the pipe of the stove went into the wall of the chimney-piece. a dozen plates were broken in a cupboard to the left of the hearth, and a woman who happened to be near it at the time of the explosion, said she had felt her legs warmed by the burning air which came from the cupboard. the room was afterwards filled with a thick infected smoke, a veritable poison. sometimes the victims are nearly asphyxiated by the fulminic effluvium, and only owe their preservation to the extreme care which is lavished on them. very often the bodies and the clothes of people who have been struck give forth a nauseous smell--generally similar to that of burning sulphur. in the month of august, , a woman who had been struck at montoulieu, in the champ descubert quarter, had her skull perforated as though a big ball had passed through it, and her burnt clothes gave forth insupportable emanations. dr. minonzio relates how three persons were wounded by lightning on board the austrian frigate _the medée_. "i remember," he says, "the sensation which was caused in the locality by the stench which came from the bodies and clothes of these people who were struck--a stench nearly as offensive as that of burnt sulphur mingled with empyreumatical oil." one of the most frequent and good-natured effects of lightning on man is to shave his hair and beard, to scorch them, or even to depilate the whole body. generally the victim may consider himself lucky if he leaves a handful of hair as a ransom to the lightning, and escapes with a fright. there is even a case given of a young girl of twenty who had her hair cut as though by a razor, without perceiving it or feeling the least shock. on may , , two men who were in a windmill were struck by lightning. they were both struck deaf, and the hair and beard and eyebrows of one were burnt. in addition to this, their clothes crumbled to the touch. a man, who must have been very hairy, was struck by lightning near aix. the electric current raised the hairs of his body in ridges from the breast to the feet, rolled them into pellets, and incrusted them deeply in the calf of the leg. very often the injury to the hair, instead of being spread all over the body, limits itself to certain places where it is thicker or damper on the body of a man, and more especially on that of a woman. here are some curious examples. in dr. sestier's learned work, vol. ii. p. , we read the following case observed at montpellier:-- "accidit apud monspelienses ut fulmen cadens in domum vicarii generalis de grassi pudendum puellae ancillae pilos abraserit ut bartassius in muliere sibi familiari olim factum fuisse." toaldo richard has described similar experiences, and d'hombres firmas has described several others:-- a number of people were assembled at mas-lacoste, near nîmes, when lightning penetrated to where they were. a girl of twenty-six was thrown over and became unconscious; when she came to her senses, she could hardly support herself or walk, and felt a great deal of pain in the centre of her body. when she was alone with her friends, they examined her, and they saw "non sine miratione pudendum perustum ruberrimum, labia tumefacta pilos deficientes usque ad bulbum punctosque nigros pro pilis, inde cutim rugosissimam; ejus referunt amicae primum barbatissimam et hoc facto semper imberbem esse." lightning is indeed a joker, but so it has always been. in most cases the hair grows again, but sometimes the system is completely destroyed, and the victim must either wear a wig or go bald. we have already spoken further back of the case of dr. gaultier, of claubry, who was struck one day by globular lightning, near blois, and had his beard shaved off and destroyed for ever; it never grew again. he nearly died of a curious malady, his head swelled to the size of a metre and a half in circumference. we also hear of corpses of people who have been struck, which show no other injury than a complete or partial epilation. for example, a woman who was struck in the road had the hair completely pulled out of the top of her head. on july , , a farm servant, pierre roux, was killed while in the act of loading a waggon of hay. the only trace the lightning left behind it was to completely scorch the beard of its victim. now, here is a case the complete opposite of the preceding ones and still more curious, in which the capricious and fantastic lightning attacked the epidermis without burning the hair which covered it. at dampierre thunder broke over a house belonging to m. saumois. his arm, one leg, and the left side of his body were burnt, and the extraordinary thing is that the skin of the arm was burnt leaving the hair intact. a little further on we shall have cases where the lightning has proved salutary in certain forms of illness. generally the people who are struck fall at once without a struggle. it has been proved by a great number of observations that the man who has been struck by lightning so as to lose consciousness immediately falls without having seen, heard, or felt anything. this is easy to believe, since electricity is animated by a movement much quicker than that of light, and still more so than that of sound. the eye and ear are paralyzed before the lightning and the thunder could have made an impression on them; so much so that the victims, when they recover themselves, are unable to explain what has happened to them. people struck by lightning nearly always sink on the very spot where they have been struck. besides this, we have already remarked several cases where the people struck have preserved the exact positions they had at the moment of the catastrophe. but, on the other hand, we can quote some examples, rarer, but diametrically opposed to these. on july , , lightning struck an oak near triel (seine-et-oise), and also struck two quarrymen, father and son. this last was killed dead, raised, and transported twenty-three yards away. the surgeon brillouet was surprised by a storm near chantilly, and was raised by the lightning and deposited twenty-five paces from where he had been. on august , , at namur (belgium), a man was flung ten yards from the tree under which he had been struck by lightning. the following notice was in the papers in august, :-- "brousses-et-villaret (aude), august . during the storm which burst over that region the lightning killed two cows belonging to m. bouchère. it also struck, but without hurting him, a young man of twenty-three years of age, bernard robart, artilleryman, who was taking a holiday. he was walking to a neighbouring farm when he was suddenly carried through the air for fifty yards. he got up again without any hurt, only he was dazzled by the lightning which had flashed before his eyes." on writing to the victim to verify this fact, i received the following answer:-- "i have the honour to inform you that the article relating to the incident which happened to me during the lightning, on the th, is absolutely true. "i was on leave at brousses, canton of saissac (aude). i left my uncle's house at about p.m. there had been a heavy storm. the rain had nearly stopped for about two or three minutes, but it still fell a little. there had been a good deal of thunder during the storm. i was sleeping at home, the house being about two hundred yards away. it was very dark, and seeing that the rain was going to begin again with violence, i started to run. i went very quick. i was crossing the place, and when i arrived in front of m. combes' house, i suddenly felt myself stopped, and without being able to explain how, i found myself in the same instant at the other side of the place, lying on the ground against the wall of m. maistre's house. i was astounded; i waited a good minute without knowing where i was. when i got home i felt a severe pain in the right knee, and i perceived that my trousers were torn and that i had a big scar on my knee, and that my hands were slightly scorched. it must have happened against the wall where there were some loose stones. i was transported about fifty yards, and i cannot tell you if it thundered at the same time, but there had been a big clap about a minute before. two people who were leaving m. combes' house were witness of the fact. the lightning penetrated into m. bouchère's stables two hundred yards away, and killed two cows and broke the leg of another. as it went in it broke the cover of the doorway, which was of freestone, in two, and knocked over a chair and seven or eight bottles which were on a shelf. "believe me, etc., "bernard robert, "artilleryman, fort saint nicholas, "marseilles." thus we have several examples of people being transported , , and yards from the point where the lightning has struck them. sometimes the bodies of people who are struck are as stiff as iron and retain their stiffness. on june , , a waggoner, thirty-five years of age, was struck in paris. the next day dr. sestier saw his corpse at the morgue: it was perfectly stiff. the next day, forty-four hours after the death, this stiffness was still most marked. some years ago, in the commune of hectomare (eure), lightning struck a man named delabarre, who was holding a piece of bread in his hand. the contractility of the nerves was so strong that it could not be taken from him. on the other hand, the bodies very often remain flexible after death, as in life. on september , , a violent storm burst over eastbourne. a coachman and footman were killed. "although the bodies remained from sunday to tuesday unburied," remarked an observer, "all their limbs were as flexible as those of living people." sometimes the corpses soften and decompose rapidly, leaving an unbearable odour. on june , , lightning killed a lady in a ballroom at fribourg. the corpse rapidly gave forth a curious odour of putrefaction. the doctor could hardly examine it without fainting. the inhabitants of the house were obliged to go away thirty-six hours after the death, the odour was so penetrating. it was with difficulty that they were able to put the fetid corpse into a coffin. it fell to pieces. the flabbiness often observed in the bodies of people who are struck is due, no doubt, to the fact that in the case of enormous discharge, the stiffness of a dead body develops so quickly, and is of such a short duration, that it may escape observation. numbers of experiments made on animals justify this hypothesis. nevertheless, in the majority of cases, bodies which have been struck decompose rapidly, which explains quite naturally the softness of bodies killed by lightning. the colouration of these presents numerous varieties; sometimes the face is of a corpse-like paleness, at others it preserves its natural colour. in many cases, the face is livid, red, violet, violet-bronze, black, yellow, and even covered over with brown or blue spots. the colouration of the face may be extended over all or nearly all the body. the eight reapers who were killed under an oak, quoted by cardan in our first example, were quite black. that the subtle fluid accumulated in great masses in the clouds should kill a man, deprive him of movement, annihilate his faculties, or slightly wound him--this ought not to astonish us when we contemplate the marvellous results and the prodigies of strength accomplished by the much more feeble electricity of our laboratories. but the extraordinary point about lightning is its variety of action. why does it not invariably kill those it strikes? and why does it sometimes not even wound them? there are inexplicable subtleties in the world. one knows of many examples of people who are struck whose garments remain absolutely intact. the imponderable fluid insinuates itself through the garments, leaving no trace of its passage, and may cause grave disorders in the body of a man without any exterior mark to reveal it to the most perspicacious observer. we hear of the case of a man who had nearly the whole of his right side burnt from the arm to the foot, as though it had been for a long time too near a quick fire, but his shirt, his pants, and the rest of his clothes were untouched by the fire. the abbé pinel gives the case of a man who, amongst other injuries, had his right foot very badly lacerated, while the left was untouched; the right sabot was untouched, and the left was broken. on june , , at bellenghise, near saint-quentin, a lady was killed under a tree: she had deep marks of burning on the breast and stomach, but her clothes remained intact. lightning is very mystifying. th. neale cites a case where the hands were burnt to the bone in gloves which remained intact! at other times, garments, even those nearest the skin, are perforated, burnt, and torn, without the surface of the skin being injured. thus the boot of a man who had been struck was so torn that it was reduced to ashes, while there was no trace of a wound on the foot. an extraordinary case in point happened at vabreas (vaucluse) in july, . a peasant was in the fields when there was a violent clap of thunder. the electric fluid struck his head, shaved the left side, and completely burnt his hat. then, continuing its route, it tore his garments, penetrated the length of his legs, and tore his trousers from top to bottom. finally, it transported the unfortunate man, nearly naked, six or seven yards from his original place, and laid him on his stomach on a bush with his head hanging over the edge of a river. sometimes, when the garments are seriously injured, we find slight injuries under the skin which do not always correspond to the places where the garment is most seriously affected. lichtenberg quotes the case of a man who had his clothes cut as by the point of a knife from the shoulders to the feet, without the sign of a wound except a small sore on the foot under the buckle of the shoe. according to howard, a man had his clothes torn to atoms without showing any trace of the action of the electric fluid on the surface of his body, except a light mark on the forehead. sometimes, as we have already said, the inner garments are burnt while the outer ones are respected. a woman had her chemise scorched by the fire of heaven, while her dress and petticoats were spared. on june , , lightning fell at poitiers in a yard where a young cooper was working. it went under his right foot, burning his shoe, passed between his stocking and leg, singed the stocking without wounding the leg, burned the lining of his trousers, raised the epidermis of the abdomen, tore off a brass button which fastened his garment, and went off to twist a carpenter round in a neighbouring lane. neither one nor the other felt the effects of this stroke of lightning. finally, the clothes, above all the shoes, are unsewn carefully and without a tear, as though by the hand of a clever workman. here are two cases in a thousand-- on june , , at grange forestière, near petit-creusot, a man had his trousers unsewn from top to bottom and his shoes taken off. in the department of eure-et-loire, some peasants were engaged in binding sheaves, and their daughter, aged nine, was playing near them when a storm broke with great violence. "let us go in, i am frightened," she cried, running to take refuge between her parents. "we will go in immediately, but we must finish binding before the rain comes on." "then i will beg of the good god to keep the thunder from us." "do." and while the father and mother continued their work, the child went down on her knees, and with her hands over her eyes commenced her prayer. suddenly, without hearing or seeing anything, the father felt the straw move under his feet; he turned mechanically, and gave a great cry on seeing his little daughter stretched motionless on the ground. she was dead. her little corset was unsewn and her chemise burnt. but of all the fantastic actions of lightning, the most extraordinary and incomprehensible is the mania it has for undressing its victims, and leaving them dead or fainting in the primitive costume of our first parents--or in a dress too simple to be allowed by our civilized customs. this deplorable and quite inexplicable habit has given lightning a large scientific _dossier_, from which we have already cited examples in the first chapter, and from which we will again extract some fragments. near angers, on may , , a farm lad named rousteau, aged twenty-three, was killed by lightning in the middle of the fields. the corpse was found nearly naked. on june , , at pradettes (ariège), the mayor was unfortunate enough to take shelter under a very high poplar. soon after he had done so, there was a burst of lightning which split the tree and struck him. in one of its diabolical freaks it entirely undressed him, throwing his various garments round about him, reduced to rags, with the exception of one shoe. in june, , at saint-laurent-la-gatine, thunder broke over m. fromentin while he was working a plough drawn by three horses. lightning killed the leader, and completely undressed m. fromentin after burning his hat. the same day, at limoges, a farm servant named barcelot was struck under an oak. his corpse was completely naked and he had a severe wound on his left side. on august of the same year, a violent storm burst over the isle of re. a farmer, who was on his way to the station at finaud, was struck fifty yards from his own house. the lightning removed all his clothes. in the keeper of the commune of saint-cyr-en-val, near orleans, was struck while on his rounds; the fluid deprived him of his clothes and removed all the nails from one of his shoes. on july , at aseras, near nice, during a violent storm with hailstones grammes in weight, a mme. blanc was on her way to meet a servant who was in the fields. she had only taken a few steps when she was struck by lightning and completely undressed. her body was uninjured, but the poor woman became dumb. how fantastic and extravagant it is! it is impossible to assign any rule to the capricious advance of lightning. how are we to explain the following facts of nature? one night in april, at about p.m., near ajaccio, a peasant named j. b. pantaloni was leaving the fields and hurrying home to escape from a storm. he had hardly reached his house when it was set on fire by an electric discharge, and the unfortunate man was killed dead and carbonized. at the same time his two sons and a daughter, who were in the same room, were completely undressed and their garments disappeared. these last were not hurt in any way. very often clothes, which have been torn and tattered, are taken a long way off. on october , , seven people were seeking shelter under an enormous beech near the village of bonello in the commune of perret (côtes-du-nord), when, suddenly, lightning struck the tree and killed one of them. the six others were thrown to the ground without being much hurt. the clothes of the one who had been struck were reduced to tatters; several of these were found hanging on the branches of the tree. one day a workman was sheltering under the shed of a kiosque in which there were five men playing cards. he was grazed by lightning. the fluid, after having passed between the players without hurting them, left the kiosque, and removed a shoe from the poor workman, who was petrified with fright. they searched for the shoe which had been confiscated by the fulminant matter, but in vain. moreover, lightning seems to have a special predilection for shoes; it seldom respects them, even when it spares the other garments. sabots, shoes, and even boots are removed, unsewn, un-nailed, cut to pieces, and thrown far away with extraordinary violence. very often the discharge penetrates into the human body by the head and leaves it by the feet. during a violent storm (june , ) a workman was passing near the jardin des plantes, when he felt a great oppression on his stomach. he was then knocked down roughly by an irresistible force, and deprived of the use of his senses at the moment of the fall. he was picked up and taken home, and on being examined, his body bore no trace of a wound, and he escaped with a fright. but some days after, when he had recovered from the shock, he remembered that he had worn boots at the time of the accident. these had disappeared, the lightning had stolen them from him, though it acted from a distance. the boots were found in the street, and the soles had the nails completely removed, although they were screwed in and the boots were nearly new. on may , , at villemontoire (aisne), a workman was killed on a hay-cock, his clothes were reduced to fragments, and his shoes were not to be found. two other workmen were wounded, and the cock was set on fire. on may , , lightning broke over the commune of chapelle-en-blezy (haute-marne). a young shepherd, who was watching his flock in the fields, was knocked over by the fluid and lost consciousness. when he came to himself he found that his sabots and cap had disappeared. arago states that a workman was struck under a pavilion, and that the pieces of his hat were found embedded in the ceiling. biot gives the case of a hat which was flung ten paces without a breath of wind. we could multiply these very curious observations, but we must restrain ourselves so as to remain within the limits of this little book. did i not say just now that lightning has sometimes--though very rarely--exercised a beneficial influence on sick people it strikes? yes; we hear of several cases where thunder has shown itself a rival to the noblest disciples of esculapius, and where it has worked veritable miracles. for instance, a person who had been paralyzed thirty-eight years, suddenly, at the age of forty-four, recovered the use of her legs, after a stroke of lightning. a paralytic had been taking the curative waters of tunbridge wells for twenty years, when the spark touched him and cured him of his terrible infirmity. lightning has sometimes worked marvels on the blind, deaf, and dumb, to whom it restores sight, hearing, and speech. a man who had the whole of his left side paralyzed from infancy was struck in his room on august , . he lost consciousness for twenty minutes, but after some days he gradually and permanently recovered the use of his limbs. a weakness of the right eye also disappeared, and the invalid could write without spectacles. on the other hand, he became deaf. indeed, if we are to believe stories which appear to be authentic, a cold, a tumour, and rheumatism have been cured by lightning. we have given an example in our first chapter. it is impossible to explain in what manner the subtle fluid accomplishes these wonderful cures. are they to be attributed to the shock, to a general upheaval which brings back the circulation to its normal course? or are we to attribute to the electric substance--still unknown to physicians and physiologists--an action capable of overcoming the most inveterate evils? the science of therapeutics already makes excellent use of the electricity of the machines. can we, then, marvel much that lightning should rival our feeble electric resources? no! what a number of services might it not render if it were not for its mad independence! what an amount of lost power there is in the gleam of lightning! as a matter of fact, we owe no gratitude to lightning. there are too many miseries for a few happy results. the balance is really too unequal. some lightning strokes have proved veritable disasters, on account of the number of the victims and the havoc which has been caused. the most extraordinary of these are the following:-- on a feast-day lightning penetrated into a church near carpentras. fifty people were killed or wounded or rendered imbecile. on july , , lightning struck a church at seidenburg, near zittau, during the service; forty-eight people were killed or wounded. on june , , lightning struck the church of villars-le-terroy, when its bells were being rung; it killed eleven people, and wounded thirteen. on board the sloop _sapho_, in february, , six men were killed by a stroke of lightning and fourteen seriously wounded. on board the ship _repulse_, near the shores of catalonia, on april , , lightning killed eight men in the rigging and wounded nine, of whom several succumbed. on july , , three hundred people were assembled in the church at grosshad, a small village, two miles from düren, when lightning struck it; one hundred people were wounded, thirty of them seriously. six were killed, and they were six hardy men. early in july, , lightning fell on the territory of coray (finisterre) in a warren where sixteen people were weeding. six men and a child were killed by the same stroke, and three others were severely wounded. several were stripped naked, their garments being scattered in rags over the ground; their shoes were cut to pieces and all broken. a curious point is that the workers were struck at a distance of yards from each other. on july , , at mount pleasant (tennessee, u.s.a.), lightning killed nine people who were taking refuge under an oak during a storm. these formed part of a procession which was conducting a negress to her last home. here is another very curious and complex case-- on the last sunday in june, , during vespers, lightning struck a church at dancé, canton of saint-germain-laval (loire). a deathlike silence succeeded the noise of the explosion, then a cry was heard, then a hundred more. the curé, who thought that he alone had received the whole electric discharge--and was in reality unhurt--left his place, where he was enveloped in a cloud of dust and smoke, and spoke to his parishioners from the communion rails, to reassure them. "it is nothing," he said. "keep your places; there is no harm done." he was mistaken; twenty-five to thirty people had been more or less struck. four were carried away unconscious, but the worst treated of all was the treasurer. in raising him they perceived that his eyes were open, but dull and veiled, and he gave no sign of life. his clothes were burnt, and his shoes, which were torn and full of blood, were removed from his feet. the monstrance, which had been exposed, had been thrown down on the ground, and was battered and pierced in the stem, and the host had disappeared. the priest searched for it for a long time, and finally discovered it on the altar in the middle of the corporal, on a thick bed of rubbish. three or four yards of the wainscoting of the choir had burnt into atoms. outside, the arrow of the belfry had been carried off, and its slates were scattered about in the neighbouring fields. on june , , lightning struck, the church of pineiro (province of orense, spain) during a funeral. there were twenty-five dead and thirty-five severely wounded. these are cases of destruction on a large scale, but we can give parallel cases where the terrible fluid seems only to amuse itself. in fact, some people appear to enjoy the privilege of particularly attracting lightning, and of frequently receiving its visits without suffering much from its reiterated attacks. they say that mithridates was twice touched by lightning. the first stroke was when he was in his cradle, his swaddling clothes were singed, and the scar of a burn which he received on his forehead was covered with hair afterwards. according to the abbé richard, a lady, who lived in a château on an elevation near bourgogne, saw the lightning several times enter her room, divide itself into sparks of different sizes, of which the greater part attached themselves to her clothes without burning them, and left livid traces on her arms and even on her thighs. she said, when speaking on this subject, that thunder had never done her more harm than to whip her two or three times, though it fell pretty often on her château. there seems to be a sort of relative immunity in women and children. these are seldom struck. we have even several examples of children remaining safe and sound in the arms of their mothers who are struck. fracastor's mother had her child to her breast when she was struck by lightning. the child itself was spared. in august, , at georgetown (essex), mrs. russel, wife of the protestant minister, was killed by lightning, while a small child which she had in her arms was unhurt. it would seem as if lightning pitied the feeble--the women and children. we hear of cases where people were struck several times during the same storm without succumbing to its manifold attacks. "in two similar situations," says arago, "one man, according to the nature of his constitution, runs more risk than another. there are some exceptional people who are not conductors to the fulminating matter, and who neither receive nor pass on a shock. as a rule, they must be ranked among the non-conductor bodies that the lightning respects, or, at least, that it strikes rarely. such decided differences could not exist without there being finer shades. thus each degree of conductibility corresponds during the time of a storm to a certain degree of danger. the man who conducts like a metal will be struck as often as a metal, while the man who cuts off the communication in the chain, will have almost as little to fear as if he were made of glass or resin. between these limits there will be found individuals whom lightning might strike as it would strike wood, stones, etc. thus, in the phenomena of lightning, everything does not depend on the place that a man may occupy; his physical constitution will have something to do with it." the phantasmagoria of lightning leaves us perplexed. all these observations are extraordinary and very disconcerting. the facts contradict each other, and lead us to no actual conclusion. the _gazette de cologne_ gave the following case in june, :-- at czempin, a young girl of eighteen was struck by lightning while she was working near a hearth. she remained unconscious, in spite of all the efforts made to revive her. at last, acting on the advice of an old man, they placed her in a freshly dug ditch, and covered her body with earth, but in such a way as to avoid stifling her. after some hours she recovered consciousness, and was restored to health. sometimes lightning amuses itself nicely and innocently. it mixes in the society of men without doing them harm, or leaving any remembrance but a great fear. one day lightning entered by the chimney into the middle of a lively dance at m. van gestien's, the innkeeper at flone (belgium). at the sight of it the dancers were petrified with terror, and not one could try and escape. but they misunderstood the intentions of the lightning, which were of the most straightforward; it only wanted to be a spoil-sport. it also had the good taste to depart quietly. after the first excitement a profound stupefaction seized hold of the persons present; they were all transformed into niggers. the lightning had swept the chimney, and cast the soot into the ball-room, powdering all the faces and toilettes. lightning might be the daughter of goblins rather than a messenger from olympus. the following facts might confirm this impression:-- at bayonne, on june , , lightning knocked over a gas-burner, and threw a person down, after making her turn round three times. a family of twelve were gathered together at a table, sixty yards, at least, from the point where it burst. they were all knocked down, but without sustaining any injury. during a violent storm, lightning entered a country house by the chimney; it lifted two big stones from the hearth, and carried them over to near the head of a child who was asleep, and placed one on each side, without grazing it or hurting the child. and this same lightning, whose almost maternal delicacy is quite exquisite, entered another time, also by the chimney, into a house, hit a man savagely on the head, wounded him severely, and left him dead in the middle of a pool of blood. then it took a quantity of this blood which was accumulated round his head, and went and stuck it on the ceiling of the higher story. a child who was present at this tragic scene was unhurt. in august, , an electric spark penetrated into a house in the village of porri, near ajaccio, and started to make the tour of the property. first it visited the second-floor rooms, without doing much damage there; then it went down to the first floor, where there were two young girls, turned them round, and burnt their legs. it continued on its course as far as the cellar, where its dazzling brightness terrified three young children who had taken refuge there. it spared two, but burned the third rather severely. let us finish this series of electric pictures, which depict--sometimes in a very tragic manner--the different modes of activity of one of the grandest of nature's phenomena, by two facts, the strangeness of which surpasses everything that one can imagine. pliny gives the case of a roman lady, who, having been struck by lightning during her confinement, had a stillborn child, without herself suffering the least harm. another:-- the abbé richard, in his _histoire de l'air_, gives a more extraordinary case still. at altenbourg, in saxony, in july, , lightning struck a woman who was expecting her confinement. she was delivered some hours afterwards of a child who was half burnt, and whose body was all black. the mother recovered her health. we can neither define nor delimit the power of lightning. sometimes merciful, often cruel, it constitutes in the universality of its actions, one of nature's most terrible scourges. chapter vi the effects of lightning on animals animals, even more than mankind, attract the fire of heaven. lightning has a certain regard for human beings, which it seems to lose entirely when it is a case of the humble and faithful servants that nature has given us. and, between ourselves, thunder is not always as absurd as it appears. its proceedings are sometimes even very tactful. though it may often strike innocent victims blindly and ferociously; yet it seems at times to show a certain amount of intelligence. thus we find among our many examples a strange fact, which will serve to reconcile our thoughts a little to thunder. on june , , in kentucky state, we have already cited the case of the nigger norris, who was going to be hanged for the murder of a mulatto companion, and who, just as he was putting his foot on the fatal platform, was struck by lightning, and thus spared the sheriff the trouble of hurling him into eternity. here was a case where thunder was full of justice, and we cannot praise it too much. arago gives another case where a chief of brigands was shut up in a bavarian prison, together with his accomplices. no doubt he was encouraging their arrogance by his blasphemies--the stone to which he was attached acting as a tribune for him--when he was suddenly struck by lightning while haranguing his disciples. he fell dead. the iron manacles had brought on the disaster, but the brigands did not stop to think of this natural cause; they were just as terrified as if the iron had not been there, and the lightning had chosen its victim with intelligence. here is another instance-- the favourite of a prince had obtained from him a written recognition of her son. she counted on this to give trouble to the state after the death of her benefactor. she enclosed it carefully in a chest, and went and buried it deep in a wood, hoping to render all search useless, if the prince should change his mind. but behold, the lightning intervened; the tree was struck, and the open chest was thrown on the highway, where it was found by a peasant. animals are worse treated than men, but better than plants and inorganic bodies. what are the causes of this difference? can we attribute it to physical predisposition? but this has not yet been proved. experience shows that sparks directed on the vertebral column are particularly dangerous. now, the backs of quadrupeds are greatly exposed to mortal strokes from the celestial fire. their fur or their plumage, which form an intrinsic part of their bodies, put them more or less in the situation of a man who, to protect himself from inclemency, should envelop himself in his hair, supposing this to be long enough and rich enough to cover him decently. animals rarely survive when struck. when they do not die on the spot, they succumb soon after to their wounds. the ancients have remarked on this. "man," says pliny, "is the only animal that lightning does not always kill; all the others die on the spot. it is a prerogative granted to him by nature, though so many animals surpass him in strength." and, further on, he adds that amongst birds the eagle is never struck. this has given it the name of _porte foudre_. but these assertions are slightly exaggerated, and we can quote a certain number of examples of animals which have resisted the baneful influence of the electric current. in , a horse was touched by lightning, which was certainly attracted by the iron of his shoe. it traced two deep trails right along the animal's leg, where the skin was abrased, and appeared as though it were cauterized. these two lines joined together at the fold of the ham, and then formed a single furrow, all sign of which was lost in the abdominal region. the rest of the body was unhurt, and the animal sustained no further harm after being struck than it would have done if an incompetent veterinary surgeon had fired him too severely. on july , , at castres, ten persons and nine horses were struck by lightning; all survived the accident. on june , , in the grand duchy of luxembourg, three cows and a little girl, who was in charge of them, were knocked over by a violent shock. the child and the beasts soon got up. only an ox was killed some distance from there. very often horses are stunned by the discharge on animals which are killed, but after a time they recover. this phenomenon has also been observed in other animals. for instance, five or six pigs which were in a cage in the prow of a ship were killed by an electric discharge, whilst others which were only separated from them by a cloth were saved. but the cases are rare in which animals do not succumb to lightning. they nearly always perish. at present we will only discuss animals as a body, equal, or superior to man. the others, the smaller ones, offer a still more convincing generality. all animals seem to be greatly exposed to the wrath of jupiter; nevertheless, some species appear to be peculiarly sensitive to lightning--the gentle sheep, for example, which huddle together fraternally during a storm, and fall in a mass, struck by the fire of heaven. i have before me a list of animals which have been struck. there are some of every kind. we might divide them thus-- several hundred rams, sheep, and ewes. horses, mares, and colts. oxen, cows, or bulls. dogs. asses. goats. cats. mules. pigs. hare. squirrel. a prodigious quantity of geese, chickens, pigeons, and small birds. fish also contribute a respectable contingent to lightning. as a rule, large groups of animals are dangerous when there is thunder, as they seem to exercise a strong attractive influence to the electric fluid. often entire herds are destroyed by lightning. dr. boudin gives the following example:-- on may , , at about . p.m., hubert wera, a shepherd who was surprised in the fields when a storm overtook him, was hurrying home with his flock. on coming to a narrow and difficult road, the sheep formed themselves into two groups. the shepherd took shelter behind a bush, when a terrible clap of thunder was heard. lightning struck him and his flock. the unfortunate man was struck on the top of his head. all his hair had been taken from the nape of his neck, and the electric fluid had traced a ridge on his forehead, his face, and breast. his body was quite naked; all his clothes were reduced to rags. moreover, there was no trace of blood. the iron of his crook had been detached from the handle and thrown several yards away, and the handle itself was broken to pieces. a small metal crucifix and a scapular belonging to the unfortunate shepherd were found fifteen yards away. of the flock of sheep, were killed. they were covered with blood, and their wounds were as varied as they were peculiar. some had their heads chopped, others had them pierced from side to side, others had their legs fractured. as to the dog, he was not to be found. on may , , near fehrbellin (prussian states), one clap of thunder killed a shepherd and sheep. on june , , thunder killed hairy beasts in a field at gulpin (limbourg). at prades, on july , , sheep were struck at one blow. during a violent storm which burst over montmaur in the isère, lightning struck a flock of sheep, and killed . in the month of april, , thunder burst over a sheepfold in which there were sheep. fifty of these were found entirely carbonized, the thirty others were covered with sores, on the head, in the eyes, and on the back, and half asphyxiated by the fulminant fluid. the poor sheep were all cowering together. on august , , a flock of sheep were carbonized, and cattle of every kind were struck. at limoges, on july , , cows or oxen were struck by the spark. they were all joined together by an iron chain. on june , , near hayengen (wurtemberg), a shepherd and sheep out of , were struck in the open field. lastly, according to abbadie, a storm in ethiopia killed in one single stroke, goats and their shepherd. these figures are, i think, sufficiently eloquent, and if it were not for fear of fatiguing my readers, who might become bored, we could add a great many similar examples to this list. but it would be superfluous to expatiate further on the dangers incurred in a storm by large agglomerations of animals. in their terror, beasts, particularly sheep, press closely together, and are soaked by the rain. in this way they offer a large surface, which absolutely conducts the lightning. also the column of vapour which rises from these living masses, affords an excellent passage for the fluid to pass through while crossing over the bodies of the poor beasts. it would be better to disperse the flock, rather than form a compact group of them, during a storm. one sometimes wonders also what would be the effect of lightning on animals arranged in a file. would it act the same with atmospheric electricity as with that in our laboratories? would the influence of the electric matter be more dangerous in the extremities than in the middle? when lightning meets a metallic bar, it does no harm except on entering and departing. on the other hand, when several people form a chain, holding hands, if the first touches the body of a leyden electrical jar and the last touches the top, the whole circle will instantaneously receive a shock. only those in the middle receive a less violent one than those who touch the jar. well, the discharges from the clouds produce similar effects on men and animals. arago supports this by the following facts:-- at flavigny (côte-d'or), five horses were in a stable when the lightning penetrated. the two first and the two last perished, the fifth, which was in the middle, was unhurt. one day lightning fell on an open field on five horses in a line and killed the first and last; the three others were spared. but we should require a much larger number of proofs before we could be sure of this. in certain cases, lightning, always fantastic and extraordinary, seems to make a fastidious choice of its victims. it kills one, spares another, strikes a third, does good to a fourth--what a strange game! how fantastic! madame la comtesse mycielska, of the duchy of posen, wrote to me recently-- "during a storm which took place in the month of august, , lightning entered by a half-open door into a stable where there were twenty cows, and killed ten. beginning with that which was nearest the door, the second was spared, the third killed, the fourth was uninjured, and so on. all the uneven numbers were killed, the others were not even burnt. the shepherd who was in the stable at the time of the shock, got up unhurt. the lightning did not burn the building, although the stable was full of straw." we have given a similar case in the chapter on fireballs. _a propos_ of this, m. elisee duval, of criquetot l'esneval (seine-inférieure), relates a very remarkable case. on june , , lightning fell on the telegraph poles of havre and Étretat. a dozen were thrown over, and the curious part is that every second one was knocked down. here is a more extraordinary case still. we were not aware that thunder could distinguish between colours, and that it has its preferences amongst them. well, we need no longer be surprised at this. here we have a case where the fluid declares itself distinctly in favour of black. it was at lapleau in corrèze. one day thunder fell on a grange full of hay and straw, and covered with thatch, without setting it on fire. then it went to the sheepfold and killed seven black sheep, and left the white alone. this choice is categorical, and people who fear lightning might follow this example by wearing long white garments in a storm. unfortunately, lightning is so eccentric and uncertain, that we must not defy it; it is not to be trusted. who can explain why it sometimes glides into a stable full of cows without injuring one? this extraordinary thing happened in the commune of grignicourt (marne). after a great clap of thunder, all the cows that were in a shed became unfastened, without one of them being hurt. there, again, the lightning only seemed to want to make itself useful. if, in some cases, by a providential chance, cattle have been saved, it is none the less true that an animal very rarely survives a discharge which has caused the death of a human being. but, as there is no rule without exceptions, we will give the following:-- the sky was dark and lowering, and a shepherd, seeing that there was about to be a storm, ran to his flock to drive it to the shed. just at the same moment, lightning burst and knocked him down, together with thirty sheep. the beasts all got up soon, but the poor shepherd was dead. on another occasion, on june , , a shepherd was killed by lightning, and the remarkable thing was that only one sheep out of the hundred of the flock was struck. on june , , thunder entered a sheepfold, containing one hundred sheep. only four perished. one of them was marked on the back with a cross, formed of two rectilinear grooves, penetrating to the skin; only the wool was removed. sometimes, but very rarely, men and animals survive the discharge. thus, dr. brillouet's horse was thrown into a ditch, and remained there without moving for three-quarters of an hour, after which he was able to get up. later on he became very feeble in the legs. very often the same stroke kills men and animals simultaneously. we have already given several cases of this kind. here are some more-- a terrible storm burst at la salvetat, on august , . a shepherd and his flock, composed of twenty-three sheep, were all killed by lightning. on june , , a young boy, fifteen years of age, living at montagnat (ain), was struck while fastening oxen to the door of a stable; an ox was also killed. at lagraulière (corrèze), on august , , three girls were looking after their flocks. a violent storm burst at about five o'clock, and the thunder growled terribly. the shepherdesses, taken by surprise, had no time to take their flocks in. the two first took shelter under a big chestnut, the third under an oak twenty-five yards away from them. suddenly lightning struck the chestnut and enveloped the two little refugees. they fell dead. the third fainted, half asphyxiated by the smell of the sulphur. the clothes of the two unfortunate girls who had been struck were burnt, their sabots were broken. near them there were five sheep, a pig, and a she-ass, which had also been killed by the fluid. the shepherdess's dog had been cut in two. sometimes, also, the clap of thunder, when striking men and animals, proves more murderous for the latter than the former, who, however, have sometimes succumbed. a diligence was slowly mounting an incline, when suddenly a stroke of lightning interrupted its ascent. an electric ball burst over the heads of the horses, and threw the whole five down, stone dead. the postillion was struck, but not one other person was touched, though the carriage was full of women and children. there is one peculiarity about this incident which ought to attract our attention--the terrible meteor was not accompanied by any emission of light, nor followed by any reverberation of sound. in june, , at about two in the afternoon, a farmer at grange-forestière was trying a couple of oxen, which he had just bought at the fair, in a field. lightning knocked over the man and the animals. some hours after, the poor farmer was picked up in a pitiable plight. his hair was burnt in parts, also hair on his chest, he was quite deaf, and in a state of absolute prostration. his trousers were unsewn from top to bottom in all four stitchings, his hat was riddled with holes, and his shoes torn off. all the same, he survived the accident. the oxen were killed on the spot. in fact, as we have already said, when the spark strikes men and animals at the same time, only the former can resist the shock. in june, , thunder burst over a flock of sheep in the commune of saint-leger-la-montagne (haute vienne); seventy-eight sheep and two watch-dogs were killed on the spot. a woman who was looking after the flock was slightly touched. on september , , lightning struck a labourer who was driving near sainte menehould. his two horses were killed; the man escaped with a temporary deafness. in august, , two out of four oxen were killed, the third was paralyzed on the left side. as to the farmer, he came off with a numbness of the left leg. very often a man feels nothing, not even a shock, while the animals beside him fall dead. here are some facts-- on february , , a herd of pigs were surprised by a water-spout near liége. one hundred and fifty of these animals perished by the action of the electric fluid, their guides felt nothing. in , lightning fell on the abbey of noirmoutiers, near tours, and killed twenty-two horses without doing any harm to monks, whose refectory it visited and upset the bottles containing their ration of wine. in the year ix., lightning killed a horse and a mule near chartres, sparing the miller who conducted them. on july , , four cows were going along a road, when suddenly they were pushed and thrown roughly to the edge of the road. the old drover who was with them felt nothing except the sensation of a strong and very characteristic odour which he could not define. in , a fulgurant discharge took place near mr. cowen's and killed his dog beside him, without doing him any harm. in august, , lightning penetrated into a cart-shed, where twelve chickens were taking shelter. the poor things were killed, but a lady who was feeding them was unhurt. one often asks if lightning strikes birds in flight. this question, so often put, would seem to find an answer in the following facts:-- a lady was looking out of her window, when there was a flash of lightning, accompanied by a great clap of thunder. at the same time she noticed on the grass a dead gull which she had not seen before. the people who picked up the bird, affirmed that they found it still hot, and they added that there was a strong smell of sulphur. examples of this kind are rare: we have two more-- one day, mr. w. murdochs with two friends was looking on at a very violent storm, which spread itself over the valley of the ayr. just then his dog dislodged a flock of ducks which had been sheltered behind an old building. one of the birds began to fly, and as it was cutting through the air, it was struck by lightning and killed as though by a gun. during a storm in the united states, mr. burch saw a flock of wild geese flying by. suddenly there was a flash of lightning which threw the flock into disorder; six birds fell dead to the ground. one would have thought that the absence of all communication with the ground ought to protect the graceful winged tribe from lightning; but no, the poor birds have received no mercy from this terrible adversary. all the same, lightning is less redoubtable for them than the sportsman's gun. it is very seldom that the kings of the air are the victims of the fire of heaven, but they have another enemy, barbarous, unpardonable man. yes, the little earthly jupiters are infinitely more terrible for the bird-world than the giant of the gods. they are rarely softened by the seductive grace, the elegance, and the delightful twittering of the charming inhabitants of space. in truth, one of the reasons why birds are so rarely struck in their flight is that they foresee the storm, and have the prudence to take shelter before it bursts. amongst birds, sparrows are those which suffer most from the electric fluid. we sometimes find them hanging by their shrivelled claws from telegraph wires or from the branches of trees. but this latter is rather rare. they generally nest high in the trees, and lightning affects the branches much less than the trunk. we also hear of little caged birds being killed in their iron prison. one day a canary was in a cage with five others and was killed; the rest were unhurt. the spark was attracted by the metallic bars, and struck the canary, which was no doubt resting on iron. fishes in their dark dwellings are no more privileged than other animals. they also frequently receive visits from the lightning, and their sad fate has often proved how dangerous it is to remain near a pool or pond during thunder. moreover, why are we recommended always to put the conductor into a well, damp earth, or even into a small pond? it is because water conducts the electric substance admirably. we can understand that a vast space of liquid would be a good refuge for lightning, when, after having made several victims on earth, and fearing the vengeance of the conductors, it hurls itself into the water. more often it drowns itself, and in this it follows the example of the immortal gribouille; but enough of that. the logic of lightning is still contestable. however that may be, many examples show us the dangers to which the denizens of rivers, and of the liquid element generally, are exposed. not only are fishermen and sailors unanimous in attesting to the ravages wrought by lightning, but the history of electricity has preserved the recollection of memorable disasters, of veritable hecatombs of fish, which they attribute to the fire of heaven. arago recounts that on september , , lightning fell on the doubs and killed all the pike and trout which were in the river. the water was soon covered with their corpses which floated, stomach upwards. a century before, during the year , the lake in the subterranean part of zirknitz was the theatre of a similar event, even more terrible, on account of the number of victims. the inhabitants of the neighbourhood collected such a number of fish that were struck, that they were able to fill eighteen carts. in , during a violent storm at night, the electric discharge fell on a little fish-pond in which a number of fish sported. the next morning they were all found floating dead on the surface of the water. they had the appearance of boiled fish, and their flesh fell to pieces on being touched, just as it would if it had been cooked. there was no injury to be seen, external or internal. the scales and the swimming bladder, which was full of air, had been preserved. the water of the pond remained troubled and muddy the day after the storm, as though the agitation of the tempest had been quite recent. here is an observation, similar to the last:-- in , lightning fell on two poplars near ignon, in the territory of saulx-le-duc (côte-d'or). a neighbouring pond, which measured yards in length by in width, was also struck. the owner states that all the fish, to the number of about a thousand, were killed. another more curious case still:-- one day the fish in an aquarium placed in a drawing-room were struck. they were all found lying dead on the floor. the glass which formed the bottom of the vessel was twisted and coated with a thick bed of yellowish substance. if we study the effects of lightning on animals from the point of view of the injuries which it produces, we can make some very interesting remarks. more often the hair of animals is injured or burnt. sometimes the spark acts on the skin over a large surface of the body of the animal. thus, two horses had their hair singed nearly all over their bodies, and more particularly on the leg and under the stomach. at other times the hair is only burnt in certain places. lightning struck a young four-year-old ox which was red with white spots. it burnt and removed all the white spots and left the red hair. but generally we find one or more furrows of different kinds. the skin is seldom intact under injured hair. it is nearly always more or less burnt. and one often notices extravasations of blood which correspond to the injuries of the epidermis, in the subcutaneous cellular tissue. in some cases, the fulminant fluid only attacks the colour of the hair of the animal. the fracture of the bones or the ablation of a limb is often observed on animals which have been struck. in , a violent storm broke near nimegue, and several oxen were killed in the meadows and their bones were broken. in the month of may, , in the marche de priegnitz, eight sheep were struck. they could not be used as food, because all their bones had been broken as though in a mortar, and the fragments were intermingled in the flesh. these, however, remained intact. we have seen in the preceding chapter that fulguration often leaves no particular sign on men who are struck. it is the same with animals. the electric fluid entirely absorbs the source of life and only leaves insignificant traces of its passage. sometimes even we can find no exterior injury. on july , , near hamburg, lightning killed two horses in their stable. they showed no exterior trace of a burn, though both had a rupture of the auricles. in the month of september, , at ogenne, two cows and a heifer were struck in their stable; no exterior wound was to be found on their bodies. another observation is given by the abbé chapsal in his remarkable description of the effects of lightning. a pig fell dead, struck by a clap of thunder, and no indication could be found of the electric passage. we see that lightning does not always make a great distinction between the blows which it inflicts on men and those which it inflicts on animals. sometimes, also, the corpses of beasts which are struck are completely incinerated. at the first sight, the body appears intact, but when you touch it, it falls to pieces. at clermont (oise) on june , , several animals were entirely carbonized in their stable. we have also heard of animals being transported by the meteor a long way from the place of the catastrophe. others have suffered from grave nervous troubles, following on the strokes of lightning which they have received. sometimes partial or total paralysis results. thus, a cow which had been struck by lightning, was knocked over, and remained a quarter of an hour motionless, after which it was seized with violent convulsions, then it got up quickly looking terrified. here is a case of a severe shock which brought on an access of delirium. in the course of a terrible storm on september , , a butcher, accompanied by a dog, took refuge under a beech at the edge of the road. suddenly lightning fell on the tree and struck the dog, which became mad, and threw itself on its master, bit him in the thigh, and only let go when the butcher dragged the animal with him into a neighbouring house and cut his tail. the dog died in the night. there are some examples of injuries wrought on animals which are barely perceptible. for instance, when it makes a transparent horn, opaque, and when it burns the mucous membrane of the nose. on the other hand, the foetus which sleeps under the frail covering of the egg, is exposed to the pitiless blows of the most terrible meteor, as is the baby in its mother's womb. chickens have often been struck before they ever saw the light of day. often the noise of thunder, and the fear which results from it, causes the miscarriage of hinds, and particularly of lambs. an animal which has been struck generally sinks instantly, without a struggle. all the same, we hear of the case of a horse which was struck by the flame, and which struggled for a long time against an inevitable death. the corpses of animals, like those of men, are sometimes very rigid; at others they are soft and flaccid, and decompose rapidly. thus all the sheep of a flock which were together under a tree in scotland, were killed by a great clap of thunder. the next morning the owner, wishing to get some advantage out of their remains, sent his men to skin them, but the bodies were already in such a state of decomposition, and the stench was so abominable, that it was impossible for the servants to execute his orders. they hurried to bury the sheep in their skins. on september , , at about p.m., lightning fell on a house in the village of salagnac (creuse). amongst other accidents it killed a pig in a stable; three hours after the body was completely decomposed. when animals are killed, not by the atmospheric fluid, but by the lightning of our machines, decomposition always comes on very rapidly. brown sequard made the following very curious experiment on this subject:-- he took the hearts away from five rabbits of the same kind, the same age, and about the same strength. he put one aside without touching it, and he submitted the four others to the passage of an electric current, of a different strength for each animal. here are the different results obtained-- the first animal became rigid after ten hours, and its rigidity, which was excessively marked, lasted eight days. the rigidity of the four others was feebler, and lasted a shorter time in proportion to the strength of the electric current. thus, the one which received the weakest current, became rigid at the end of seven hours, and this lasted six days. the one which received the strongest current became rigid in seven minutes, and its body softened after a quarter of an hour. this experiment explains the absence, or the shortness in duration, of corpselike stiffness in subjects which have been subjected to the terrible discharge of lightning. animals are not only the frequent victims of lightning, but, as this experiment shows, they are still oftener the martyrs of science. laboratories are sometimes transformed into small cemeteries, where lie poor guineapigs, frogs which have been quartered, and mutilated rabbits. but what is the ordinary lot of these last when science spares them? the chief point is not to let the innocent victims suffer. can we eat with impunity the flesh of animals which have been struck? several people say yes, many say no. both are right. putting aside the question of the rapid putrefaction to which these bodies are nearly always subjected, the flesh of animals killed by fulguration has often been found unhealthy and uneatable. a veterinary surgeon who was commissioned to examine the bodies of two cows and an ox which had been struck in a stable, declared that their flesh could not be eaten without danger. on the other hand, franklin recounts how some people ate fowls which had been killed by the electric spark--"this funny little lightning"--and cooked immediately after death. the flesh of these capons was excellent and particularly tender, and the illustrious inventor of lightning-conductors concluded by proposing that we should follow this proceeding in order to ensure our fresh meat being as clean as possible when served at table. we think, however, that it is more prudent to sacrifice the meat which has been struck, as it has been proved that in certain cases the decomposition is very rapid. up to now we have seen all animals, man included, as victims of lightning: it is the general rule. nevertheless, we often meet beings in this world, men, animals, or plants, which try to distinguish themselves from others by some sort of originality. this appears to be the case with the electric fish, whose existence seems to be dedicated to the worship of jupiter. these curious fish have received the gift from nature of being able to hurl lightning to a certain distance. this is how they set to work. a little fish in search of food goes too near this terrible enemy, who at once sets his living tail in motion. fascinating it with his eye, he renders it immovable, and lets fly repeated discharges to it. after a minute, the poor fish is overcome, and allows itself to be snapped up by its pitiless adversary without resistance. certain rivers in asia and africa and the depths of the pacific ocean, in which these curious animals live, are often the scenes of terrible dramas, caused by the presence of these lightning fish, which are divided into five species: the tetrodon, the trichiure, the silurus, the raie torpille (cramp-fish), and the gymnote (electric eel). these aquatic lightnings work terrible havoc among the inhabitants of neptune's kingdom. they use their influence over men as well as fish. if you touch a torpille, you feel a shock strong enough to benumb and paralyze the arm for some minutes. a curious experiment was tried: eight people formed a chain, and one of them, with a piece of metallic wire, touched the back of a torpille which had been imported. they all felt the shock. if thunder had elected to be domiciled anywhere but in its own clouds, it would seem as if it would be in the organism of these curious fish. unfortunately, in our international relations, humanity has invented a much more dangerous torpille (torpedo)! chapter vii the effects of lightning on trees and plants nearly two thousand years ago, pliny wrote, "as regards products of the earth, lightning never strikes the bay tree." and this is why the roman emperors, in fear always of the fire of heaven, crowned themselves with laurels. this belief was almost universal in ancient times, and survived for many centuries. but every new century has proclaimed the immunity from lightning of some one member of the vegetable world, though impartial research has now established the fact that there is no such absolute privilege. if certain trees are rarely struck, that is, perhaps, due less to its species than to its size, its hygrometrical condition, and to other influences which it is still difficult to specify; for lightning, as we have seen, has capricious habits which we have not yet succeeded in explaining. thus the bay tree has lost its proud position in this respect, and has had to take its place amongst the ordinary run of trees, subject to the unjust anger of jupiter. many bay trees of some size have been seen to fall victims to the electric fluid. the fig tree, the mulberry tree, and the peach tree have also been reputed to enjoy safety, but this also is not the case. there is an instance on record of a fig tree being struck by lightning and completely withered, and another of a mulberry-tree, eighty years old, being partly destroyed. in our own days, the beech is believed to go uninjured. in the state of tennessee, in the united states, the opinion is so deeply rooted that beech tree plantations are often resorted to as a refuge in times of storm. but it would be a mistake to place too much trust in them. there are records of beech trees being struck by lightning and destroyed, just like bay trees, fig trees, and the rest. in , an old beech tree was struck in the forest of villers-cotterets. this venerable patriarch was more than three hundred years old. of its upper branches, which were wide and strong, four of the finest were destroyed; a fifth, stripped of its bark to a great extent, was not torn off the trunk. the trunk was split where the other four branches were torn from it. the interior of it was blackened and slightly carbonized. on july , , at chéfresne, canton de percy (manche), an oak and an ash were struck by lightning within five minutes of each other. on august , , at haute-croix, in brabant, an ash was struck and destroyed. on august , in the same year, an ash was struck also at namur. the box tree and the virginian creeper used to be regarded as safeguards against lightning. the same virtue was attributed to the house-leek, a thick herbaceous plant, which grows usually upon walls and roofs, and which the germans call donnerblatt or donnerbarb, thunder-leaf or thunder-beard. according to some authors, again, lightning never strikes resinous trees, such as pines or firs. but this also is disproved by the facts, especially in regard to firs. among the many particulars i have collected of recent years, is the following list of sixty-five different kinds of trees, with the record of the number of times each species has been struck by lightning within a given period:-- oaks. poplars. elms. walnut trees. firs. willows. pine trees. ash trees. beech trees. pear trees. cherry trees. chestnut trees. catalpas. lime trees. apple trees. mountain ash. mulberry tree. alder. laburnum. acacia. pseudo-acacia. fig tree. orange tree olive tree. birch. maple. height obviously accounts for a good deal. it is incontestable that, in the case of a clump of trees standing in the middle of a plain, lightning will in most cases pick out the tallest. but this is not an absolute rule. the isolation of trees, their qualities as conductors, the degree of moisture in the soil in which they are rooted, their distance from the storm clouds, the character of their foliage and of their roots--all these things are important factors. numerous experiments have been made with a view to ascertaining the amount of resistance offered to the electric spark by different kinds of wood. similar pieces of beech and oak have been exposed lengthwise to the electric spark given out by one of holtz's machines, with the result that the oak wood was pierced by the electric fluid after one or two revolutions of the machine, whereas for the beech wood a dozen or twenty were needed. black poplar wood and willow offer a moderate resistance: a few revolutions suffice to penetrate them. in all instances the susceptibility of the wood depends on the sap. it has been proved by analysis that the woods which contain starch with but little oil, such as the oak, poplar, willow, maple, elm, and ash, offer much less resistance to the electric current than those trees which are richer in fatty matter, as the beech tree, walnut tree, lime tree, birch tree, and so on. these conclusions are corroborated by the case of the pine tree, the wood of which has a great quantity of oil in winter, but in summer lacks it as much as those trees which contain more starch. experiments have proved that in summer this wood is quite as likely to serve as a conductor as the oak; while in winter its resistance to the electric spark equals that of the beech and other trees which are rarely struck by lightning. decayed trees are excellent conductors of electricity; those in full vigour being much more rarely struck. in any case, it has been proved that the effects of lightning are particularly severe in the vegetable world. it has been pointed out elsewhere in this little book to what dangers those persons are exposed who take shelter beneath the trees during a thunderstorm; there are innumerable examples of the imprudence of taking refuge from the rain under thick foliage, people having been killed by a fireball--for lightning does not always take the trouble to make a selection, sparing neither the protector nor the protected. we shall give some more instances, chosen from a considerable number of similar observations. in , ten reapers, surprised by drops of rain and distant rumbling of thunder, left their work and took refuge beneath a big walnut tree. but one of them having questioned the security of this retreat, all immediately fled in the direction of a neighbouring wood, except one young girl of fourteen years. several who returned to advise her to follow them, saw her smilingly throw her arms round the trunk of the tree, and almost at once fall backwards, her arms extended. she was dead. on the nd of august in the same year, four labourers, returning from work, were overtaken by a thunderstorm. three of them stopped under an elm, the fourth prudently continued on his way. well it was for him. several minutes later, the lightning struck the tree, killing two of the labourers outright, and grievously wounding the third. the latter was found almost completely naked; his garments, burnt and tattered, were scattered round him. when he came to himself, he was in such a violent delirium that it was necessary for several men to bring the unfortunate victim to his home, where he died shortly afterwards in the most horrible agony. about six o'clock, on the rd of june, seven men employed on the farm of puy-crouel, were working in a field of beet-root. overcome with the heat, they went into the shade of a walnut tree. all at once, a flash of lightning illumined the sky; the seven workmen were thrown down, one of them being hurled several yards away. three of them were able to get up and go to the farm, the others were severely burnt, and half asphyxiated. one of the victims had his back skinned the whole length of the vertebral column; the other had his face scratched, as if torn by fingernails. all had lost their memory. the walnut tree under which they had sheltered was cleft from top to bottom. here is another example no less terrible-- seven children, belonging to ahrens, were caught in a thunderstorm as they were coming home from the fields, and took shelter under a tree. the lightning killed the seven little people. another time, four young men taking refuge under an oak, were struck and thrown down. one of them was killed instantly, his companions were cruelly injured. on the th of july, in belgium, a woman gathering cherries was killed on a tree which attracted the fluid. a young man standing beneath it was paralyzed. we might multiply these tragic tales; each year a number of similar cases happen. the imprudence of human beings is truly incorrigible! everybody, however feeble his instinct of self-preservation, should flee the vicinity of trees during a thunderstorm, and allow himself to be drenched on the road, rather than offer his life as a too generous burnt-offering to the lightning, for the oak's robust trunk, or that of the poplar, elegantly plumed with its graceful foliage, may be the altar on which the sacrifices in honour of jupiter are made. the wood of trees is not so good a conductor of electricity as the human body. for this reason, a person leaning against a tree receives the full discharge; at times the tree is splintered, because it did not serve as a perfect conductor. yet the conductive power of certain species is so remarkable, that the neighbourhood of particular trees may be regarded as a protection against lightning (this, however, without coming in contact with them!). the tips of the branches pointing towards the clouds, and the moisture they receive, undoubtedly influence the electricity of the atmosphere; and, moreover, by means of these graceful branches, an inaudible but continual exchange is effected between the electricity of the earth and sky, thus holding the balance between two opposite charges. colladon asserts that poplars planted near houses may, in favourable conditions, act as lightning conductors, on account of their height and powers of conducting. he adds that it is necessary to take other circumstances regarding the situation of the dwelling into account, which are not always easy to define. their protection of the neighbourhood is not constantly the same. for it to be effectual, the foliage should be very low, and they should be at least two metres distant from the roof and walls. their roots, too, should be in a damp soil, and metal should not enter largely into the construction of the neighbouring houses. in these conditions, poplars may fulfil the useful functions of lightning conductors. at times, during a storm, several trees are struck by the same flash. for instance, on may , , in belgium, three poplars were blasted by a single thunderbolt. on the other hand, trees planted in lines are sometimes struck alternately. a case occurred where the lightning seemed to have taken aim and touched all the odd numbers in a row without striking the others. certain plantations act on the fluid with an extraordinary intensity. at lovenjoul, in belgium, a wood of undergrowth and big trees, planted in marshy ground, seems to possess this singular privilege, and the agriculturists of the country declare that no storm ever passes their way without lightning falling there. in the middle of this wood one can count seven oaks, near to one another, struck by it. not far off, a huge ash, and a little farther away two poplars, likewise blasted. all the trees have not been struck in the same way; some are scorched or stripped of their leaves; the others have their trunks perforated, or split in different parts. usually trees are cleft from top to bottom; in some cases the furrow is horizontal or perpendicular in the direction of the branches. pieces of bark or of wood are sometimes torn off lengthwise, and only adhere to the trunk in strips here and there. but that does not prove conclusively that the lightning struck upwards from the ground; it may have rebounded (?) after striking from above. certain effects, however, can only be explained by an ascending movement of the fluid. the following cases for example:-- "during the summer of , two men were sheltering under a tree at tancon, beaujolais, when they were struck by lightning. one of them was killed on the spot, the other felt no ill effects other than momentary suffocation. their horses were caught up to the top of the tree. an iron ring which bound the wooden shoe belonging to one of the men, was found hanging from a high branch of the same tree. now, at a little distance, there was a tree which had also suffered greatly by the passage of the electric fluid. in the soil at its base a round hole was to be seen, shaped like a funnel. directly above it the bark had been loosened and slit into slender thongs. as for the tree beneath which the men had sheltered, it also had half its bark off, and long splinters were to be seen hanging only by the upper parts. on one side of the tree the leaves were withered, on the other they were still quite green." in this most remarkable instance the lightning had come out of the ground. in the cleft of a willow tree blasted by lightning its roots were found. besides, the soil is often undulating, and thrown up around trees which have been struck. vegetables do not always succumb, any more than men, to these attacks. they may be lightly struck in a vital part, in which case they recover from their wounds. very often they are merely stripped of their natural garments, in other words, of their bark and foliage. this is one of those superficial injuries to which they are most subject. the following is an example of this kind of fulguration:-- on july , , two oaks were struck at brampton. the larger measured about ten feet around the base. they were both split asunder, and the bark peeled off from the summit to the soil, a length of twenty-eight feet. completely detached from the trunk, it hung in long strips from the top. boussingault witnessed the destruction by lightning of a wild pear tree at lamperlasch, near beekelleronn. at the moment of the explosion an enormous column of vapour arose, like smoke coming out of a chimney when fresh coal has been put on the fire. the lightning flashed in all directions, great branches gave way, and when the vapour cleared off, there stood the pear tree, its trunk a dazzling white: the lightning had taken the bark completely off. sometimes the bark is only partially stripped off one side, or left on, in more or less regular bands, either on the trunk or on the branches. during a violent storm at juvisy, on may , , an elm five hundred metres distant from the observatory was struck by lightning, which took the bark off lengthwise in a strip, four centimetres wide and five centimetres deep. this band of bark was cut clean off. there was no trace of burning. sometimes only the mosses and lichens are whisked off the sides of the trees, which escape with light scratches. two great oaks which had been struck by fireballs, only bore traces of two punctures which might have been made by small shot. moreover, it is not uncommon to see the bark riddled with a multitude of little holes, like those made by worms. two men were struck by lightning near casal maggiore on august , , beneath an elm tree. one of them had his elbow on the tree at the moment, and amongst other injuries were a number of little holes in the arm. there was a twist in the tree at the part where the elbow rested, and a hole penetrated the centre of it to the core of the wood. the surrounding bark looked as if it had been mite-eaten. several scars started from this point and ascended almost perpendicularly towards the top of the trunk. there was no damage done to the branches. lightning cut through a chestnut tree, five metres high, on the roadside at foulain (haute-marne), burning several leaves, then struck some water-pipes at a depth of a metre and a half, and finally passed into the dike through two holes a metre deep by a decimetre in diameter. the bark is often reduced to thin splinters scattered on the soil, or hanging from the neighbouring trees, or even thrown to a considerable distance. on june , a fireball fell near jare (landes) on a pine tree, which it shivered into a myriad slender strips, about metres long, many of which were caught on the branches of pines within a distance of metres. only a stump, - / metres in height, remained standing. at the same time three other pines, which stood and metres away from the first, were destroyed. the bark had been stripped off each, but only as far as the incision made for extracting the resin. furrows, of varying width, and running in different directions, may at times be seen on trees, some short, others reaching to the top of the tree, and occasionally to the roots. these marks show the passing of the lightning. sir john clark has seen a huge oak in cumberland, at least feet high and feet in diameter, from which the lightning had stripped a piece of bark, about centimetres wide and centimetres thick, the whole length of the trunk in a straight line. the furrow is not always single, it may be double, and either stretch in two parallel lines or diverge. the chevalier de louville observed in the park of the castle at nevers, a tree struck at the top of the trunk by lightning which, dividing in three shafts, hollowed three furrows that might have been made by three rifle shots fired towards the roots. these three furrows followed the irregularities of the trunk, always slipping, gliding between the wood and the bark, and curiously enough the former was not burnt. but these bands are not invariably straight either; in the above example they followed the caprices of the vegetable body. they are to be found oblique in certain cases, but more often they surround the trunk in long spirals of varying width, showing that the lightning clasped the tree in the form of a serpent of fire. here is an example:-- during a violent storm on july , , a poplar was blasted on the road through the forest of moladier, metres north-west of the castle of valliere. the tree was metres high, and in full leaf from base to summit; it was struck halfway up by the discharge, and a spiral furrow centimetres wide twisted round the trunk to the ground. i noted a similar case, august , . lightning struck one of the highest trees in the park at juvisy, a magnificent ash, stripping off and destroying the bark where the electric fluid curved round and round down the full length of the trunk, which was shattered by the meteor a few metres above the roots. enormous fragments lay all round the trunk, some hurled to such a distance that it was obvious the explosive force of the phenomenon must have been of extraordinary violence. i was able to trace the course of the lightning to the foot of the tree, along its roots to a great depth, by a black furrow. the tree is not dead. the ivy which clung to it is dead. the vast and splendid forest of saint germain often witnesses the presence of the lightning, and the magnificent trees which adorn and beautify this charming and celebrated place are, unfortunately, too often the victims of these inopportune visitations. lightning has no respect for old memories. it demolished with a single flash a superb giant whose long branches, laden with perfumed leaves, had given shade to many generations. the splendid tree, which had survived the severity of several centuries, fell beneath the arrow of the pernicious fluid. such was the fate of an oak near l'etoile du grand-veneur. struck on the top, its upper branches were violently torn off.... a spiral furrow beginning at the top ended within a metre of the ground. but, wonderful to relate, the whole mass of the tree appeared to have been twisted mightily by a force which worked with so much power that the tree could never regain its original position. the fibre, instead of growing vertically, followed the furrow made by the lightning, and became twisted like a corkscrew. there exist certain singular trees, the fibre of which grows in spiral fashion, and is called twisted wood by carpenters and cabinet-makers. pines and firs in mountainous countries are fairly often affected in this curious fashion. one can no more account for it than one can define the cause of the curved form of some flashes of lightning. one does not know exactly if they should be attributed to their following the direction taken by the fibre, or whether, on the contrary, the tree had been struck in its infancy by a spiral flash, and, submitting to that influence, continued to grow up corkscrew fashion. it is most probable that the fall of the thunder-ball on the trees in this manner is governed by the laws of electricity. we may even note casually that traces of similar spirals have been remarked on objects as well as on the dead bodies of those struck by lightning, thus preserving the ceraunic likeness of the mortal blow. other observers, besides, have declared that they saw distinctly the spiral lightning flash through the atmosphere. but these observations would need to be confirmed by photographs of indisputable accuracy. in these circumstances, as in many others, the dark room is worth a hundred human eyes! in some cases the curved furrow turns several times. for instance, in may, , grebel saw an alder nearly twenty metres high struck by lightning on the right bank of the elster below zeitz. on the lower part of its trunk were two spiral bands which had carried away bark and sap-wood, leaving no trace of combustion. the depth and width of the twist are very variable; at times the furrow is deeper in the veined parts than at the edges; again it reaches the core. two oaks were struck in june, , in the park at thornden. one was marked with a spiral for a length of forty feet to within a little distance of the ground. the band was five inches wide, but became narrower as it descended, and was finally no more than two inches wide. the wood was incised and even torn in places, but the branches were not hurt. the rest of the bark seemed to have been riddled by small shot. all the injuries of which we have spoken (excoriation, stripping off the bark, furrows), are not necessarily mortal. but there are other more serious wounds from which the tree rarely recovers. we allude to deep fissures and breaks produced by lightning. when the fracture touches only a portion of the topmost part of the tree, the result of the accident is not necessarily fatal. but this is not always so. on may , , a poplar was split in two by lightning at montigny-sur-loing. one half to the full height continued standing. the other half was chopped up in small fragments and thrown to a distance of a hundred metres. these pieces, which were brought to me by m. fouché, are so dried up and fibrous that they might be taken for hemp instead of wood. in the majority of these cases the tree is split from top to bottom. on july , , in belgium, a poplar, the biggest of a group of trees of the same species, was struck and split down its full length. in the month of august, , on the side of the road from ville-d'avray to versailles, a poplar of about twenty years old was cleft from the topmost bough to its roots; one half remained in its place, the other fell on the road. a black line, about a millimetre wide, ran down the centre of the tree. sometimes the tree is divided into several parts by vertical fissures. for example, in , near vicence, a pear tree, three feet in diameter, was split into four parts, from the top down. how often one has remarked great tree trunks in the forests, decayed and desolate, standing sadly, like poor headless bodies? very often lightning has been the executioner of these trees. in the month of may, , in the forest of fontainebleau, a magnificent oak, about two metres in circumference, was completely decapitated by lightning; its branches fell on the ground. the part of the trunk left standing was barked to the roots and splintered into fragments of varying sizes. they were scattered on the ground or hung from the branches of the surrounding trees. several pieces of considerable size were hurled more than thirty metres away, much to the injury of the bark of the trees which they struck. in numerous cases, the tree struck by lightning is broken in several places, and fragments of it thrown far and wide. on july , , at the farm of etiefs, near rouvres, canton of auberive (haute-marne), lightning struck an italian poplar, sixty years old, thirty metres high, and three metres round at a height of one metre from the ground, splintering off enough wood to make a heap sixty-five centimetres round, and fifty centimetres high. an ash was struck by lightning on july , , on the road to clermont. this tree, ten metres high, was broken at a point - / metres from the ground, and the crown, still hanging by a shred from the trunk, lay on the embankment. the violence of the explosion threw pieces thirty centimetres wide and - / metres in length, into a field from twenty-five to thirty metres off. on july , , in belgium, a willow was reduced to a heap of atoms on the ground. in march, , at plymouth, a fir more than a hundred feet high and forty feet in circumference, the admiration of the countryside, disappeared, literally shattered into bits. some fragments were thrown two hundred and fifty metres away. one of the most curious effects of lightning is to divide the interior of the tree into concentric layers, fitting them perfectly one into the other, but at the same time separating them with extraordinary precision. _arbres roulés_ (thus are the trees called which are victims of this odd phenomenon), as a rule, do not show any injury on the outside. but the body, dissected by the electric fluid, soon succumbs. an oak, twenty-five metres high, having been struck on august , , was opened to be examined carefully, and it was stated that the concentric layers were as detached from one another as the tubes of an opera-glass. the fireball sometimes hollows a canal through the centre of the trees from the top to the bottom, the sides of which are burnt black. the following is a curious example:-- in june, , at moisselles, lightning fell upon a great elm, and striking against an enormous knob, rebounded on to a neighbouring elm half its own height, pierced it through and through, shivering it to tatters; the trunk was burst open to the roots, it looked as if it had been bored through from one end to the other by a red-hot bullet that blackened and charred it. does it not seem as if the lightning plays with the lives of the trees as with man? it threatens, changes, apparently spares, returns to the charge and finally annihilates. and this sport is accompanied, at times, by inconceivable effects. but records are still more eloquent than reflections: nature, in her own mute speech, tells us of a thousand marvels. is not the following phenomenon enough to make lightning more mysterious in its fantastic and varied mode of action? on the th of april, lightning struck an oak in the forest of vibraye (sarthe), cut this tree, measuring a metre and a half in circumference, at two-thirds of its height, pulverized the lower parts, strewed the shreds over a circuit of fifty metres, and planted the upper part exactly on the spot from which the trunk had been snatched, with all the rapidity of a flash. moreover, the annual concentric circles were separated by the sudden drying up of the sap so effectually, that, the strips only remained welded together where the knots made too great an obstacle to their separation. how was the lightning able to plant in the earth, with such inconceivable rapidity, the top of the tree where the roots had been? this is something which no one can explain. it alone is capable of creating such situations. but it has done better still! two years later, in , it took the opportunity of playing a good trick on two trees of different species, an english oak and a forest pine, which, without race jealousy, fraternized in the forest of pont-de-bussière (haute-vienne). these two trees were about ten yards apart, and were simultaneously hit by the explosive matter, and in the twinkling of an eye, their leaves were changed. the pine needles found themselves on the oak, and the leaves of the oak went to brighten the austerity of the pine with their delicate verdure. there was nothing commonplace about the metamorphosis. accordingly all the inhabitants went in crowds to the scene of this miracle to contemplate the unusual spectacle of a pine-oak and an oak-pine. and the unexpected happened: both trees appeared to thrive very well in these new conditions: the pine continued to be agreeably adorned with its festival foliage, whilst the oak agreed perfectly with the sombre needles of the pine. after such marvels, my readers will not be surprised to learn that lightning sometimes shatters the living wood, or decayed wood, into a thousand morsels without setting it on fire. for instance, a bundle of faggots lying on the hearth has been reduced to atoms by lightning, without any trace of combustion being visible. a fireball fell on a sheaf of barley in the open field without setting it on fire, and buried itself in the ground without doing other further damage. in certain cases the electric fluid chars wood at varying depths: the blackened layer is often very slight; sometimes, on the contrary, combustion is complete. as for the leaves, they are unhurt as a rule. when they are attacked they shrivel up; an autumnal shade takes the place of their charming green tints; they turn brown and dry up quickly. one of the trees in the champs-Élysées having been struck, it was proved that all round it the ground was full of little holes. in two or three places the bark was raised from beneath; the leaves were yellow and shrivelled up as parchment would be by the fire; the upper part remained green. everything seemed to prove that the lightning came out of the ground. at other times the same effect may be observed on the leaves, when the trunk and roots are apparently uninjured. it is not unusual to see the tree instantly stripped of its leaves as if by some mysterious power. the lightning acts also on the roots, as we have seen in the preceding examples. they have been seen uncovered where the ground was much disturbed, torn in strips, or cleft in more or less regular pieces. we see that lightning does not make more ado about exhaling its baleful breath on the life of plants, than on animals and men. and moreover, that it often strikes these latter with sudden death without leaving a trace of its passing, just as sometimes it strikes the trees without leaving any exterior injury. now and then life is not completely extinguished, and little by little the tree recovers its health. often the vitality is not changed, one sees the tree which was struck bear fruit as before the catastrophe. has it not been asserted that lightning may exert a benign influence on vegetation? this was the opinion of the ancients. _a propos_ of this, pliny said, "that thunder is rarely heard in winter, and that the great fertility of the soil is due to the frequency of thunder and rain in spring; for the countries where it rains often and in good earnest during the spring, as in the island of sicily, produce many and excellent fruits." it has been proved in our times that the ancients were right in extolling rainwater as nourishment for the products of the earth, and science has discovered the cause to be the presence of great quantities of nitrogen and ammonia in the thunder-rain and in hail. perhaps electricity has a similar effect. in the neighbourhood of castres, on april , , an old poplar was stripped of its bark in several places. now, shortly afterwards it burst into leaf, although the neighbouring poplars were much later than it. the ravages caused in the fields by the electric meteor to forage and vegetables are sometimes considerable. this is especially so with grass when cut, to haycocks, ricks of straw, barley, etc. we have a collection of records of men or animals who, when leaning against haystacks, were struck. as a rule the haystack is burnt; sometimes, however, the grass is simply scattered and thrown to a distance. in , a very curious occurrence was observed at vayres (haute-vienne). the lightning struck a field of potatoes at the village of puytreuillard; some of the stalks were burnt to cinders; but most remarkable of all, _the potatoes were done to a turn_, just as if they had been cooked beneath hot ashes. a belief which was very general in ancient times and derived without doubt from a recollection of the circumstances which were said to accompany the birth of bacchus, gave the vine the privilege of protecting the neighbourhood from the fatal effects of lightning. but this again is only a legend. the following observation proves it:-- on july , , at chanvres (yonne), fifty vine-stocks were frizzled up by lightning. it used to be supposed, too, that the electric fluid held the lily in particular respect. but here is a note which shows us that the white flower is visited by the burning flashes. during a violent thunderstorm on june , , at montmorin (haute-garonne)----but let m. larroque, who witnessed the curious phenomenon, describe it: "in a clump of lilies in my garden," says he, "i see the highest of them surrounded by a violet glimmer, which formed an aureola round the corolla. this glimmer lasted for eight or ten seconds. as soon as it disappeared, i went close to the lily, which, to my great surprise, i found had been deprived of its pollen, while the surrounding flowers were laden with it. so the electric fluid must have scattered or carried off the pollen." when jupiter thunders, he still seems to dominate our world, as in the days when the graceful legends of mythology flourished. and not only does he work above ground, but, contrary to the belief of the ancients, his influence extends beneath the soil. a great number of men were working in the mines at himmelsfurth on july , . they were, as often happens, working at various points along the vein of metal, and never dreaming of the events which might take place on the surface of the ground. all at once they were conscious of several very violent shocks, given in the oddest and most extravagant fashion. some felt the shock in their backs, while their neighbours received them on their arms or legs. they might have been shaken by a mysterious invisible hand, stretching now up from below, now from above, now from the sides of the galleries. one of the miners found himself hurled against the wall, two others, whose backs were turned, almost came to blows, each believing that his mate had thumped him. the real culprit was the thunder, of whom they might well demand an explanation of these strange proceedings. here is another example which bears out the foregoing:-- on the th of may, the watchman on guard at the pit mouth of one of the principal mines at freyberg, perceived an electric glimmer run along the wire rope going to the bottom of the mines, and used by the miners to exchange signals with the men employed in working the lifts. suddenly all the pits were brilliantly lit up. at the same moment the watchman saw a clear vivid flame shoot out at the other end of the chain. on this occasion the lightning behaved with due discretion, and shone through the mine without giving any one the slightest shock. in vain the monster tiberius, and the infamous caligula, sought a subterranean refuge from lightning. their impure consciences, laden with crimes, dreaded the chastisement of heaven. by fleeing from the lightning flash, they believed themselves saved from death. lightning dogs our footsteps, and works even when the criminals believe themselves in safety. it is conceivable that the ancients should have dreaded it as an instrument of celestial justice. usually lightning strikes the ground with a vertical stroke, but at times obliquely, when it traces long, horizontal lines. often the ground may be seen turned up at the foot of trees which have been struck, the sod is torn, and stones thrown to a great distance. sometimes, too, an excavation may be seen in the ground near the object struck, of varying breadth and height. this opening may be like a funnel or hemispherical. in a case observed on june , , at côte (haute-saône), a circular hole, having a depth of · metres, has been seen in a dyke on the declivity of the road, below a coach which was not struck. occasionally the hole is but the beginning of a canal, hollowed rather deeply and perpendicularly in the ground, the sides of which serve as a sheath to the fulgurite. but before treating of fulgurite tubes, which constitute the most curious phenomena in the world connected with lightning, we shall discuss certain remarkable effects observed on the surface of the ground. falling on solid rocks, the electric spark can break them, cut them, or pierce them in one or more places. often instead of spoiling or cutting off pieces of the stone, it covers the surface instantaneously with a vitreous coat, having blisters of various colours. this vitrification is often to be seen on mountains. de saussure found rocks of schistous amphiboles covered with vitreous bubbles, like those seen on tiles where struck by lightning. humboldt made similar observations on porphyritic rocks at névada de toluca, in mexico, and ramond, at the sanadoire rock in puy-de-dôme. in these cases, the spark, on reaching the surface, melts it more or less completely over a varying extent, and this fusion, worked upon by an extraordinary heat, produces a coat having a peculiar appearance, but in which microscopical analysis finds the elements of the body it covers. thus the vitreous layer deposited over chalk is of chalky origin; that covering granite is of the nature of granite, etc. this does not apply to certain deposits found on rocks, and even on trees, which have been struck by lightning, and which are of very different origin. whilst the former is only the stone in a fused or vitrified condition, the latter is caused by the presence of foreign bodies, some fragments of which have been detached by the ray and travel with it. this transport of solid substances by lightning has often been observed. here are two examples of this strange phenomenon:-- on july , , at luchon, on the bigorre road a passer-by saw lightning fall twenty yards away from him. recovered from the shock, he went out of curiosity to look at the result, and saw the wall at the edge of the road, the schistous and chalky rocks, even the trees themselves, coated over with layers of brown. it was certainly a case of the lightning having effected a deposit. this latter was very curious. lines could be traced on it with the finger-nail, it fell to powder under slight pressure, became soft with gentle rubbing, caught fire from a candle, and then gave off a resinous odour with much smoke. what is this resinous matter? that is what no one yet can say. in the month of july, , on the day following a violent thunderstorm which had struck the telegraph-office in the station of savigny-sur-orge, i myself picked up a little black powder off the telegraph poles, which had been left by the lightning, and which had a sulphurous smell. the production of this ponderable matter has often been attributed to bolides, but direct observation proves beyond a doubt that the electricity carries various solid substances found on earth after a storm. lightning is truly the most venerable of glass-makers. long before the most remote peoples of antiquity appeared, whose glasswares encrusted with marvellous iridescent tones by the passing of the centuries, are unearthed by scientific excavations, and displayed in national collections; long before man could have learnt to make use of the resources of nature, lightning, burrowing in the sand, there fashioned tubes of glass that hold the hues of the opal, and are called fulgurites. the ancients seem to have known of these fulgurite tubes, but we owe the first precise description and the first specimen of these extraordinary vitrifactions to hermann, a pastor at massel in silesia. his fulgurite, found in , is in the dresden museum. since this discovery, fulgurites have often been sought for and found. the tubes, contracted at one end, and ending in a point, are to be seen in sandy soils. their diameter varies from to millimetres, and the thickness of their sides from half to millimetres. as to the length, it sometimes exceeds metres. vitrified inside, they are covered outside with grains of sand agglutinated and apparently rounded as if they had been subjected to a beginning of fusion. the colour depends on the nature of the sand in which they have been formed. where the sand is ferruginous the fulgurite takes a yellowish hue, but if the sand is very clean, it is almost colourless or even white. as a rule, the fulgurites penetrate the ground vertically, nevertheless, they have been found in an oblique position. at times, also, they are sinuous, twisted, or even zigzag if they have met with pebbles of considerable size. it is not uncommon for the fulgurite tube to divide in two or three branches, each of which gives birth to little lateral branches of or centimetres long, and ending in points. there are also solid fulgurites and foliated fulgurites. the former, no doubt, had a canal originally, which has been stopped up by matter in fusion. the latter, instead of being stretched out in cylindrical form, are composed of slender layers like the leaves of a book. the scientific museum at the observatory of juvisy possesses a very curious fulgurite which was offered to me some years ago by m. bernard d'attanoux, and found by him in sahara. it is not a tube ending in a point. the lightning penetrating the sand, vitrified it on its passage, and branched irregularly in three principal directions. one might say it was slag formed by the juxtaposition, irregular and crumpled, of three blades of vitrified sand, which would be pressed together by leaving a narrow opening to their central vertical axis. this fulgurite, which is extremely light, measures six centimetres in length. it was found in the sand of grand-erg, at a depth of several centimetres. it has been found possible to produce miniature fulgurites by means of our electrical machines. by adding ordinary salt to the sand, and directing a strong current into it, complete vitrification of a tube of several millimetres is obtained. chapter viii the effects of lightning on metals, objects, houses, etc. when lightning strikes the earth, it makes straight for metals. their perfect conducting powers place them in the first rank of conductors, and the innumerable cases of lightning with which they are associated have gained them a certain celebrity in the annals of thunder. we know, indeed, the preference of the spark for metals; we know it nurses a veritable passion for nails, wire, bell-pulls, that it dotes on rain-spouts, leaden pipes, and telegraph wires, that it is very feminine in its adoration of jewels, which it sublimates sometimes with a truly fantastic dexterity. now and then lightning deviates from its path, and performs acrobatic feats, elfin capers to reach the objects it covets. on april , , it struck the church of brexton, springing on the cross of the steeple at first and running down the stem, but, arrived at the masonry which supported it, broke it into pieces; then with one bound it fell upon a second conductor, whose support was also broken. finally, it struck a third conductor much lower down. the fluid often searches for metals hidden beneath non-conductors, which it breaks or pierces. it avoids the mattress to pursue the iron of the bed, glances off the windows to glide over the curtain-rods, or the lead of the sash. it has been seen to penetrate thick walls to reach the iron safes hidden behind them. we have already mentioned the case of the woman who, without having been killed, had her ear-ring split. well, we have a certain number of similar examples to that. on june , , in a boarding-school for young ladies, at bordeaux, a gold chain, worn by one of the young ladies, was melted by the lightning, which left a black indented line in its place, which, however, soon passed off. the lady was struck, but recovered consciousness within a few hours, being none the worse. her slender chain, worn in three rows round her neck, had been cut into five pieces. some of the fragments showed signs of fusion, and had been carried to a distance. other examples, in which the consequences were more dramatic, will show ladies the dangers of a love of adornment. on september , , during a violent thunderstorm which burst over the region of narbonne, a fireball fell in the domain of castelou. a young girl of fourteen was fatally struck by the meteor. the gold chain which she wore round her throat was completely evaporated. there was not a trace of it to be found. it is not unusual to see gold chains broken, melted, partially or completely, in the pocket which had held them. thus, lightning melted a watch and chain into a single lump in the pocket of a man killed on board a passenger boat. bracelets, hairpins, and even precious stones are sometimes very strangely altered. as for watches, without speaking of the magnetization observed after a violent electrical discharge, it has been remarked that the movement became slower. in some cases they stopped short, and marked the exact instant when the lightning stopped them. when the ship _eagle_ was struck by lightning, none of the passengers were injured, but all their watches stopped at the moment the shock took place. at other times there are peculiarities in the works which are absolutely inexplicable. the following observation, related by biot, is a curious case in point. a young man was slightly struck by lightning in the street of grenelle-saint-germain. his watch was in no wise hurt outside, but, although it was only a quarter-past eleven, the hands pointed to a quarter to five. convinced that it was in need of repair, the young man placed it on his table, intending to take it to the watchmaker; but next day, thinking he would wind it up to make sure of the extent of the damage, he saw, to his amazement, the hands moved and kept regular time. in some instances the case of the watch is seriously injured, while the works are none the worse. a man wore a watch with a double cap attached to a gold chain. the chain was broken, some of the links soldered together. the cap had been perforated, and the gold spilt in his pocket. the watch itself had not been altered. but if lightning sometimes stops the works of watches, it also produces the contrary effect. beyer relates that a flash of lightning, having entered a room and broken the corner of a glass, set a watch going which had been stopped for a long time. i find the following note amongst my papers: "m. coulvier-gravier, director of the meteoric observatory of the palace of luxembourg, told me yesterday that on sunday, april , at . in the evening, a watch (wound up), which had stopped a week previously, went on at the moment lightning struck the lightning conductor on the luxembourg above these rooms." often enough the case is badly injured: the polish is rubbed off the metal, it is melted, bored through, and even dented, without any trace of fusion. a case of the latter is rare. here is an example, however. in the month of june, , a man from aigremont having been killed by lightning, his silver watch was found in his watch-pocket completely smashed. indeed, one of the most common effects of lightning on watches is the magnetization to which the various pieces of steel are subjected. we have a considerable number of records concerning these magnetic properties. in one case the balance had its poles so well pointed that, when placed on a raft, it served as a compass. we may observe, by the way, that clocks and chronometers are sometimes as much injured also by the spark. it often gives an energetic twist to the needles, or to the spring for regulating the strokes, or it even melts the wheel-works, either partially or completely. it is difficult to form any idea of the various operations of lightning; here it hurls itself down like a fiery torrent, there it makes itself so tiny that it can pass through the smallest apertures. does it not even slip under women's corsets, melting the busks and the little knobs which serve to hook them. it even attacks the various metal articles which set off our garments, even to the shoe-buckles, buttons, etc. keys are, as a rule, very ill-treated by the fire of heaven: they are twisted, flattened, melted or soldered to the ring from which they hang. on may , , a man living at troyes returned to his house while a violent storm was raging. the moment he put his key into the lock, the white gleam of a dazzling flash of lightning surrounded him, the ring holding his keys was broken in his hand, and they were scattered on the threshold. at times, too, scissors, needles, etc., are snatched out of the hands of the workers, and carried some distance off when they are not reduced to vapour. at saint-dizier (haute-marne) in july, , lightning fell on the workshop of m. penon, a chain-maker. five or six workmen were finishing their work or getting ready to leave. entering by the window near which m. penon--who was absent at the time--usually worked, the fluid grazed the bellows which were opposite, and caught up a piece of it, which one would have thought was cut off with a knife. turning to the left, and passing behind a chain-maker, who felt a violent shock, it passed to a heap of chains which it did not damage much. all the links in a chain of about a metre long were, however, soldered together; the whole chain seemed to be galvanized, and the soldering was not easily broken by hand. pieces of iron which had been cut and prepared for the manufacture, were found twisted and soldered together in the same way. finally the lightning snatched the iron hoops from a tub, and, returning the same way, broke a piece of wood from a board, so as to go through the lower part of a partition, the masonry of which was carried away for a length of fifty centimetres. very often lightning rivals the most skilful cabinet-makers: iron or copper nails are pulled out of a piece of furniture with a most amazing skill, without doing any harm to the material they kept in place. ordinarily they are thrown far away. here are two examples of this curious phenomenon:-- on september , , lightning penetrated a house at campbeltown; the copper nails in the chairs were pulled out very precisely, without the stuff being spoiled. some were conveyed to the corner of a box standing at the opposite end of the room, others were so solidly fixed in the partitions, that it was only with great difficulty that they were pulled out (howar). at another time, close to marseilles, lightning slipped into a drawing-room, one might say, like a robber, one evening, and pilfered all the nails out of a couch covered with satin. then it departed by the chimney through which it had entered. as for the nails, they were found, two years afterwards, under a tile! locks, screws, door-knobs are frequently pulled out by the fluid. sometimes metal objects of much larger size, such as forks or agricultural instruments, share the same fate. violently torn out of the hands of their owners, they start upon an aerial voyage, borne on the incandescent wings of the wrathful lightning. workers in the fields have often been warned of the dangers to which they expose themselves beneath a thundery sky, by carrying their implements with the point in the air. each year the same accidents occur in precisely similar circumstances. the electric fluid, invited by the metal point which acts like a little lightning conductor, darts from the clouds upon this centre of attraction, and runs into the ordinary reservoir by the intermedial body of the man, who plays the _rôle_ of conductor. two labourers were spreading manure in a field, when a storm came on. it was at the beginning of may, . obliged to give up work, they were thinking of returning home. each carried an american fork over his shoulder. they had come within metres of the village, when a formidable burst of flame took place over their heads. instantly the two labourers fell, never to rise again. in i made notes of several cases of this kind, from which i shall quote the two following:-- on june , a labourer from the hamlet of pair, commune of taintrux (vosges), aged forty, was sharpening a scythe in an orchard close to his house. suddenly a terrific clap of thunder was heard, and the unfortunate man fell down stone dead. on the following day, in the same region, at uzemain, not far from epinal, a young man, twenty-eight years of age, went to get grass in the country. all at once he was struck by lightning, and his horse, which he was holding by the bridle, as well. the poor fellow had been guilty of the imprudence of putting his scythe on the cart with its point in the air. on may , in the vosges, the lightning fell on a labourer, cyrille bégin, who was driving a cart to which were yoked four horses. the unhappy man was struck, as well as two of the horses. some authorities have attributed a doubly preservative influence to umbrellas. the first is undoubtedly to shelter us from the rain; the second, more doubtful, is the gift of preserving us to a certain extent from the strokes of the terrible meteor. silk, having the property of a veritable repulsion to lightning, one might really believe that umbrellas, whose covers are often made of this fabric, are protectors against the fire of heaven. but the records which we possess are not conclusive; if, now and then, the discharge becomes distributed by means of the ribs, it also very often happens that it runs along the metal parts of the handle to whatever pieces of metal may be on the person, finally striking the soil through the human body. on july , , in the province of liége, a man and a woman sheltering under the same umbrella were struck by lightning. the man was killed instantly. his garments were in tatters, and the soles torn from his shoes. his pipe was thrown twenty yards away, as well as the artificial flowers in his companion's hat. the latter, who was carrying the umbrella, was stunned. at a season when, as a rule, thunder is not dreaded--december , , to wit--two men, who were walking on either side of a schoolboy holding an umbrella, were killed by lightning. the child was merely thrown down, and got off with a few trifling wounds. in each of these cases, the person who carried the umbrella suffered less from the electric discharge, but did not escape altogether, nevertheless. it may be remarked, also, that the chief victims were just under the points of the frame, and that in all probability the electricity passed through these points. the fusion of metals is one of the lightning's most ordinary performances; it has occurred at times in considerable quantities. on april , , a fulminant discharge struck the windmill at great marton, in lancashire. a thick iron chain, used for hoisting up the corn, must have been, if not actually melted, at any rate considerably softened. indeed, the links were dragged downwards by the weight of the lower part, and meeting, became soldered in such a way that, after the stroke of lightning, the chain was a veritable bar of iron. how, one asks, can this truly formidable fusion take place during the swift passage of the electric spark, which disappears, it may well be said, "with lightning speed." what magic force gives the fiery bolt from the sky the power to transform the atmosphere into a veritable forge, in which kilos of metal are melted in the thousandth part of a second! great leaden pipes melt like a lump of sugar in a glass of water, letting the contents escape. in paris, june , , lightning broke tempestuously into a kitchen, and, melting the gaspipes, set fire to the place. on another occasion, the meteor breaking into the workshop of a locksmith, files and other tools hanging from a rack on the wall were soldered to the nails with which the iron ferrules of their handles came in contact, and were with difficulty pulled apart. a house at dorking, sussex, received a visit from lightning on july , . nails, bolts, and divers small objects were soldered together in groups of six, seven, eight, or ten, just as if they had been thrown into a crucible. "money melts, leaving the purse uninjured," says seneca. "the sword-blade liquifies, while the scabbard remains intact. the iron in the javelin runs down the handle, which is none the worse." we could add other examples, quite as unheard of, as those enumerated by the preceptor of nero. a hat-wire melted into nothing, though the paper in which it was wrapped was not burnt. knives and forks were melted without the least injury being done to the linen which enveloped them, by the presence of the fluid. these proceedings give proof of exquisitely delicate feelings; it is a pity the lightning does not always behave in the same way. wires, and particularly bell-wires, make the most agreeable playthings for the lightning, judging from the frequency with which they are struck. sometimes, in the middle of a fearful thunderstorm, the doorbell is violently rung; the porter rushes to open the door for the impatient visitor, only to receive a shock of lightning by way of salvo. the mysterious hand which pulled the bell is already far away; but it has left its impress on the bell, and the guiding ray follows the metal wire in all its windings, passing through holes no bigger than the head of a pin. the wires are often melted into globules, and scattered around in all directions. the abbé richard has seen globules from a bell-wire fall into coffee cups, and become embedded in the porcelain, without the latter being any the worse. metal wires supporting espaliers and vines are often compromising to the safety of their neighbourhood, especially when they are against a house. without renouncing the succulent peach, or the golden chasselas grapes, propped on espaliers, we ought to see that they are so arranged that they do not act as lightning-conductors to our habitation. in august, , in a farm amongst the mountains near lyons, lightning fell at a distance of about fifteen metres from a dwelling where there were four people; the meteor, conducted by the wire supporting a vine on a trellis, followed it into the house, and knocked the four people down. one could almost believe that lightning takes a certain pleasure in looking at its diaphanous and fugitive form in the mirrors hung as ornaments in our drawing-rooms. in , a very coquettish flash of lightning rushed to a mirror, breaking more than ten openings in the gilt frame. then it evaporated the gilding, spreading it over the surface of the glass, while on the silvered back the evaporation of this latter metal produced the most beautiful electric traceries. sometimes the tinfoil or pieces of melted glass are thrown to a great distance; and at times the fusion of the glass is so complete that the _débris_ hangs down like little stalactites. as for the gilding of the frames, it is often carefully removed by the lightning to a distance, and applied to the gilding of objects which were never intended to receive this style of decoration. it is just the same with the gilding on clocks, cornices, church ornaments, etc. there are innumerable examples coming under this category. here are a few:-- on march , at naples, lightning flashed through the rooms of lord tylney, who was holding a reception that evening. more than five hundred were present; without any person being injured, the lightning took the gilding clean off cornices, curtain-poles, couches, and door-posts; then it shook its booty in a fine gold dust over the guests and the floor. on june , , lightning struck the steeple of philippshofen in bohemia, and went off with the gold of the clock, to gild the lead in the chapel window. in , it went into the church of the academical college in vienna, and took the gold from the cornice of one of the altar pillars to put it on a silver vase. it seems difficult for lightning to resist the attraction of gildings. it was reported that when a house in the rue plumet in paris was struck in , among several frames hanging in a room, the spark only touched one which was gilt. none of the others were struck. in spite of this extraordinarily independent behaviour, lightning has not so much liberty of action as we might be tempted to believe; it obeys certain laws which are not yet defined, and its gestures, although apparently wild and capricious, are not the result of fortuitous circumstances. to allude to it as chance may serve as a refuge from ignorance, but it does not, any more than we can, explain the extraordinary phenomena. why are certain organic or non-organic bodies visited repeatedly by lightning? we need not have recourse to magic to explain. it is simply because they serve as favourable conductors for the fluid. one of the best-known examples of this kind is that of the church of antrasme. it was struck by lightning in . it melted the gold of the picture frames adorning the sanctuary, blackened the edges of the niches in which the images stood, scorched the pewter vases enclosed in a press in the sacristy; then, lastly, it made two very neat holes at the end of a side chapel, by which it took its departure. the traces of this disaster were removed with all haste, but twelve years later, on june , , the lightning returned to the charge. it penetrated the church for the second time, but the most remarkable fact is, that it worked havoc similar to that done on its first visit. again the sacred picture-frames were despoiled of their gilding, the niches of the saints blackened, the pewter vases scorched, and the two holes in the chapel reopened. what demon guided the lightning in these scenes of pillage? the end of the story gives us the clue. soon after the catastrophe the use of the lightning-conductor became general throughout the whole world. the church was put under the protection of a rod of iron, after the principles of franklin, and ever since lightning allows the faithful to pray in peace within the sanctuary, and has never returned to profane the church at antrasme. such incidents are of fairly frequent occurrence; they give us a chance of understanding the supposed preferences of lightning. in the last chapter we shall see curious cases of "galvanoplasty," of the nature of the following: amongst others, that of a piece of gold in a purse, which was silvered over with silver taken out of another part of the purse, through the leather of the compartment. what a trick of prestidigitation! on our music-hall stages this turn would have a great success. but our last word has not been said about lightning. just a few more. one of the most curious effects produced on metals, is the magnetic polarity communicated to objects in steel and iron, no matter what they be. we have already quoted a remarkable case, that of the ascending lightning. a tailor was slightly touched by the spark; the day after the accident he found his needles were magnetized: they clung closely to each other as they were taken out of the case. another case of magnetization has been recorded, where certain objects, which were struck by lightning, had power to raise three times their own weight. this magnetization is almost always temporary. examples are known, however, where objects preserved the magnetic powers that they acquired in the moment of the shock. and one can understand the terror inspired by lightning in uncultured minds, when, after the passing of the meteor, they see common things suddenly animated by a fantastic vitality, fine needles attract and raise very much larger bodies than themselves, and impart a feverish agitation to any pieces of steel or iron that may be placed near them. what a lively impression these curious phenomena must have made on the minds of men in the days when sorcery was in fashion, and when lightning was, according to the belief then popular, at the service of heaven and hell! but, nowadays, sorcery is fallen into disuse; the magnetization of metal bodies, even when the result of lightning, is something too well known to be attributed to any connection with satan. and yet the gambols of electricity are truly extraordinary. in the month of june, , the electric fluid penetrated into a butcher's shop, quite calmly followed the iron bars from which the quarters of meat were hanging. from one of the hooks a whole ox was suspended. all at once the skinned carcase was galvanized by the electric current, and during several instants it was seen convulsed by the most frightful contortions. again, on june , , a concierge in the avenue de clichy was sweeping his courtyard when the lightning broke at one metre above his head. the poor man escaped with the fright. the fluid ran up the leaden pipes and entered a room, where it broke the mirrors and a clock, injured the ceiling, and got off by breaking the panes in the window. on the upper storey it got into a lodging occupied by two old women, where it caused the following damage: one of the women was holding a bowl of milk, the bottom of the bowl was cracked and the milk spilled on the floor; some money which was in a wooden bowl disappeared and could not be found. the clock was stopped at half-past six, the pendulum unhooked; and a hole made in a glass globe the size of a five-shilling piece. finally, a woman in bed on the same landing saw the bed split in two by the lightning, which disappeared in the wall. none of these persons were injured. as a general rule, indeed, when lightning breaks into houses, although it often does a great deal of harm, it almost always spares the people who may be there. one is safer there than anywhere else. sometimes the walls are pierced or merely hollowed. this perforation of the walls is one of the most common effects of the meteor on buildings. the thickness of the perforated walls is very variable. at the castle of clermont, in beauvaisis, there was a formidable old wall, built in the time of the romans, so tradition has it, which was ten feet thick, and the cement was as hard as stone, so that it was almost impossible to break it. "one day," says nollet, "a flash of lightning struck it, and instantly a hole, two feet deep and equally wide, was made in it, the _débris_ being thrown more than fifty feet away." on june , , at louvemont (haute-marne), the wall of a bakehouse, fifty-five centimetres wide, was broken in by lightning. the church at lugdivan was struck by lightning in . two furrows like those made by a plough were to be seen on the wall. one of the most dreadful acts of which lightning is capable is that of hurling considerable masses of stone and rock, broken or intact, to great distances. we have numerous examples of this terrible phenomenon. here are a few:-- on august , , thunder burst over the belfry of maison-ponthieu. the explosion scattered the slates and beams of the roof, and shot a stone, measuring thirty-five centimetres, to a distance of twenty metres. rough stones, weighing more than forty pounds, were torn up and thrown almost horizontally as far as an opposite wall thirty feet away. at fuzie-en-fetlar, in scotland, towards the end of the eighteenth century, lightning broke, in about two seconds at most, a mica-schist rock of one hundred and fifty feet long, by ten broad, and in some parts four feet thick; this it split into great pieces. one, measuring twenty-six feet long by ten broad, and four in thickness, fell on the ground twenty centimetres off. enormous stones are thrown, at times, in different directions. in lightning struck the belfry of breag church in cornwall, broke the stone pinnacle of the edifice, and threw one of the stones, weighing at least a hundred-weight and a half, on the roof of the apsis, in a southerly direction, fifty-five metres away. in a northerly direction another huge stone was found at metres or so from the belfry; and a third, still larger, to the south-east of the church. in certain cases the lightning unites a fantastic skill with this excessive brutality. for instance, a wall has been removed intact without being broken in any part. here is a record of one such extraordinary occurrence:-- on august , , at swinton, near manchester, during a deluge of rain, the lightning all at once filled a brick building, in which coal was stored, full of pestilential, sulphureous vapour. above it was a cistern half full. suddenly the edifice, the walls of which measured thirty centimetres in thickness, were torn out of the ground, the foundations being sixty centimetres deep, and was transported in an upright position to a distance of ten metres. the weight of this mass, so oddly and so rapidly moved by lightning, was estimated at ten thousand kilograms. in many cases, on the contrary, the subtle fluid has pulverized a hard stone on the spot and reduced it to powder. tiles and slates are very often torn off the roofs: the lightning makes them fly through the air. sometimes it is content to perforate them with a multitude of little holes. as for chimneys, they are generally very ill-treated by the meteor. the blows of which they are victims are to be accounted for easily, for they offer perfect powers of conducting to the fulminant matter, firstly, because of their prominence on the summit of the building, especially when they are surmounted by a vane. again, the flue is often in cast-iron, and if it is bricked it is supported by bars of iron. the surface of the interior is covered with a layer of soot, an excellent conductor, and a stove-pipe often opens into it. then, too, the hearth and its surroundings are more or less made of metal. finally, the column of smoke and of hot, damp air rising into the air, shows the lightning the way. the latter often accepts this invitation, and very frequently gets into a house by the chimney, where everything seems ready for its reception. rafters and doors are sometimes bored through with one or two holes by the spark, and split or furrowed more or less deeply. a curious fact is that it is rare to find the slightest trace of combustion round them. in the month of august, , lightning struck the belfry of the church at abrest (allier), carrying off part of the roof. it destroyed the walls of the porch, and in both sides of the swing doors bored two holes, each as big as a pigeon's egg, and as symmetrically as if they had been made by the hand of man. the cleavage of beams is amongst the most extraordinary injuries to be observed on woodwork. lightning works with wrought wood just as it does when the tree is in full sap: it reduces it to rags, and follows the direction of the fibres. with what crimes lightning is charged! when it is a question of robbing a house, it spares nothing in its way. the window-panes fly in pieces, and sometimes are thrown a long way off. often they are melted and disappear totally. in july, , at campo sampiero castello (padua), lightning struck a building full of hay; the windows had glass in them, and the panes were melted _without the hay catching fire_! a still more astonishing phenomenon is that of the total disappearance of the glass panes, observed at the castle of upsal, on august , . lightning visited this edifice and then took flight, carrying off sixteen panes out of a window. not the smallest fragment of them was ever found. perhaps, as often happens, terrific heat was generated, and the glass evaporated. if we follow the track of lightning through rooms, very singular effects may be seen on the furniture. chests of drawers and wardrobes are gutted, and the contents pulled out and strewn about the room. in the middle of august, , a house at francines, near limoges, was struck by lightning. it fell in a room where the master of the house was in bed. he felt a terrific shock, and saw his eiderdown pierced through and through by the perfidious fluid, and a chest of drawers with all its contents broken. continuing on its way, the lightning demolished the door and entered another room. a man who was asleep in it was killed. his wife by his side and his little girl felt nothing, but _a pillow on which one of them had her head was thrown to a distance_. finally, the meteor went through the floor, broke a large clock on the ground floor, setting fire to everything on its way. on june , , a fulminant ray fell on the church of cussy-la-colonne (côte d'or). to start, it turned the clock tower upside down, broke a clock, then opened a cupboard in the sacristy in which there were various articles, and broke them all. in april, , lightning did great damage in the church at montredon (tarn). it demolished the steeple to an extent of three metres, several bells, and carried the enormous iron bar which supported them a long way off. the roof of the church was burst in and the tiles were pulverized in several places by the falling masonry. in the interior a bench was broken, an image of christ reduced to powder, and a metal image of st. peter twisted. we may remark, by the way, that churches are very often struck by lightning, doubtless owing to the height of the steeple above the edifice. we have innumerable notes about ruined steeples, turrets knocked off, the plundering of priestly objects. sculptures and pictures adorning the sanctuary are often destroyed, and the altar itself shattered. cases of priests struck while officiating are not uncommon. as for the faithful killed while at church, they may be counted by the hundred. without wishing to call lightning a miscreant or an infidel, one is obliged to confess it fails in respect for holy places. however, the quips and cranks of lightning observed in dwellings are no less varied and curious. here are some remarkable accounts:-- one night during a terrible thunderstorm, lightning came down the chimney of a room where two people were asleep. the husband awoke with a start and, believing the house to be on fire, groped his way to the mantelpiece to get a candle, but was stopped by a heap of rubbish. everything, in fact, of which the chimney had once consisted was heaped up in the middle of the room. the mantelshelf, violently torn off, had been partly melted, the clock had had the door of the case pulled off, and all the window-panes were broken. on the lower storey, another clock was similarly demolished, the floor was torn up and the tiles thrown with such force against the ceiling that there were splinters sticking all over it. in the month of april, , at bure (luxembourg), the thunder, which had been rumbling for some time, suddenly crashed down all at once about midnight with the most appalling violence, so that the ground seemed to tremble and the houses rock on their foundations. all the inhabitants, aroused in terror; instinctively several of them sprang out of bed, thinking that their dwellings must be annihilated. every one had the presentiment of disaster, which was only too real: the fluid had just struck the house of a poor workman, and left a scene of frightful destruction behind it. the roof had been carried off, the chimney destroyed, the windows reduced, so to speak, to atoms, the principal door smashed and hurled to a distance; of the furniture there was nothing left but shapeless wreckage. but what was most extraordinary is, that this catastrophe only cost the life of one person, while all that were in the house might well have been killed. three children, sleeping in an upper storey, found themselves thrown outside the house without knowing how they got there, but _safe and sound_, though the bed was broken to pieces. the father and mother were asleep on the ground floor, with two little children, one of whom was at the breast. this latter was flung out of his cradle and thrown against the wall, without being hurt. at this moment the mother sprang out of bed to succour those dear to her, but while the poor woman was in the act of lighting a candle, the lightning struck her lifeless on the floor. the husband, who was in the bed with another child, only felt a severe shaking. the lightning, having accomplished its work of destruction, finally broke an opening in the lower part of the wall, went into the stable adjoining the house, and there killed the only cow that was in it. in the month of august, , at liége, rue du calvaire, at the point where the mountain of st. laurent is highest, lightning first of all struck two earthenware chimney-pots which were higher than the roofs. one of these pots was thrown to the ground and broken, the other disappeared. then the electric spark ripped off a great part of the roof. all the tiles were scattered round the house. a young servant slept in a garret under the roof; the lightning penetrated into the garret through a little hole in the wall just above the head of the maid's bed; the latter was flung into the middle of the room without the slightest bruise, though the wood of the bed was bored through in two places. from there, the spark going through the wall again, went down to the ground floor, following the gutter pipe, which it broke. it re-entered the house by making a little hole in the wall, pulled off the plaster which was round two nails holding up a mirror, broke part of the frame; again left the room, entered a little room adjoining where six people were sleeping--the father, the mother, and four young children; pierced the wall to enter a locksmith's, scattered all the tools, tore out a drawer, broke it into a thousand pieces, and threw the contents on the floor, broke all the panes of glass; again went through the wall, went to a hutch with a rabbit in it, killed the animal, and at last went into the garden, where it dug a double trench several feet long. the house was occupied by two families of ten persons, none of whom were struck. terrified by the report they rose instantly; the smell of smoke filling all the rooms told them of the danger they had just escaped. on another occasion one sees the woodwork of the chimney burnt, as well as a press, a looking-glass, and a clock badly injured by the lightning; which before retiring, and by way of being a good joke, turns a felt hat upside down, and unscrews the andirons. examples of this kind are very numerous. we constantly speak of the caprices of the lightning, but what name could one give to anything so burlesque or incomprehensible as the following:-- in the month of july, , lightning fell in the village of boulens, on a cottage almost covered with thatch. entering through the chimney, which it destroyed, it first threw down a rack, pulling out the hinge which held it up, and making in the place of the said hinge a hole right through the wall. afterwards it lifted a pot and the lid from the hearth over to the middle of the floor, tearing up some tiles as it went. it broke the latch of the hall door, as well as the key which was in the lock; this latter was found afterwards in a wooden shoe which was under the sideboard. two canes that were beside the mantelpiece, were laid on the said mantelpiece as if placed there by hand. a meat chopper and a copper basin used for ladling water out of a pail, and attached to either end of the stove, were likewise thrown into the middle of the room. but the oddest part was that these two articles were fastened together, the twine which served to hang up the chopper being rolled round the handle of the basin. finally, the flash divided, and zigzagged off, one part carrying off a piece of the oak jamb of the hall door, the other part piercing a hole above the stove in a mud wall. through this it threw fragments of laths and mortar into a window eleven metres off, near which two people were sleeping. this little dance, in which so many and various articles took part, does not lack piquancy! this is how lightning joins in the national fête of france! on july , , in the village of tourettes (vaucluse), lightning struck a house, carrying off a corner of the roof. it knocked off the lower part of the roof, and broke through a wall at least fifty centimetres thick. in a press built half into the wall, and in which there were about fifteen bottles containing various kinds of liqueur, only one bottle of spirits was broken, and this was done in such a manner that no trace could ever be found either of the glass or the liquid. from thence it sprang to the pictures hanging above the head of a little girl of five, who was sound asleep. three pictures were torn from their frames, engravings and mirrors were ground to powder, but the child was not hurt. then the electric current made an opening in the ceiling, which was about forty-five centimetres thick, broke a great many tiles as it left the house, but soon returned by way of the chimney, three parts of which it demolished. then it explored the kitchen on the ground floor, where there were three men by a fire. one, standing up, was thrown violently against the opposite wall; another was hurled against the door; the third, seated, was raised from his chair to a height of at least fifty centimetres, and then dropped. to crown all, the spark tore away half the butt-end of a gun, and carried it into the next room, where there were eleven people who got off with nothing worse than the fright. then going up the chimney, it exploded at a height of · metre, throwing bits of plaster and of the pothanger in all directions. what frantic and almost childlike fury! yet somewhere else the very brother of this ray may caress the little head of a sleeping child, and not do it the slightest harm; may scoop a hole in the little cot, and then depart quietly without giving any further cause for talk. or this same lightning, terrible and ungovernable at times, will snatch something out of a person's hand with so much dexterity, one might almost say delicacy, that one would hardly dare to reproach him with his lack of ceremony. at perpignan on august , , lightning fell on the mountains of nyer, near olette. twenty-five out of a flock of sheep were struck. the shepherd was enveloped by a flash, yet escaped, but the knife he was holding in his hand disappeared--and likewise his dog. another time it fell on a house at beaumont (puy-de-dôme), flashed through every part of it, blew up the stone staircase, and did considerable damage. it grazed a woman who was sitting with a cup in her hand, but she was not hurt, though the cup was rudely torn out of her hands. in july, , a labourer was in the act of mowing, when lightning coming on unawares, stole his scythe and threw it metres away. the man was not in the least hurt. the following example is truly amazing from this point of view. a woman was busy milking a cow, when suddenly she saw a tongue of fire shoot into the stable and round it, pass between a cow and the wall at a place where there was not more than or centimetres of space, and finally go out of the door without leaving any marks, or hurting any living thing. very often lightning contents itself with making a frightful hubbub, and breaking any china or glass it may come across. in july, , thunder burst over a house at langres. it was at breakfast-time. the fluid came down the chimney, which it swept thoroughly, came near the table, ran between the legs of an astounded guest, and then knocked a hole as big as a shilling in the neck of a bottle which was being filled at the pump. then it took itself off to the courtyard, which it swept clean, and disappeared without hurting any of the witnesses of this strange phenomenon. on august , , two women were in the dining-room of their house at confolens, when lightning broke a pane of glass in the window, and passing within a few metres of them, went through the kitchen, and disappeared through the wall, after having broken several cooking utensils and the mantelpiece into atoms. at port-de-bouc, on august , , lightning struck the custom house, went into the room of one of the officials, and cut clean in two a china vase, which was on the mantelpiece, without separating the pieces. several days later, on august , the mysterious fluid came to disturb the peaceful repast of two honest labourers. having taken refuge from the storm in a hut, they had set out their provisions for breakfast. all at once the thunderstorm burst into the humble dining-room, snatched up the bread, cheese, etc., overturned the bottles and other articles, covered everything with straw, as if by a violent gust of wind. the labourers felt nothing but stupefaction. was not it a veritable farce? in another place it bursts open a cupboard, throwing the door away, and damaging the crockery in the most systematic fashion: it breaks the first plate, leaves the second intact, cracks the next, spares the fourth, and so on to the bottom of the pile. then its task finished, it becomes quite diminutive, like some little gnome out of a fairy story, and flees through the keyhole, but without making the key spring out of the lock. on august , , at chaumont, lightning, having played havoc in a house in various ways, espied a pile of plates in a cupboard, china and earthenware plates being mixed, it broke all the china ones, leaving the others untouched. why this preference? the lightning does not explain. it is for us to find out. on may , , at tillieu-sous-aire (eure), during a thunderstorm, a number of china plates were filled with a kind of sticky water. the earthenware plates beside them were not even wet. i received a little flask of this water sent me by the parish priest, but analysis revealed nothing unusual. the following case gives a formal denial to the ancient prejudice which attributes a cabalistic influence to the number thirteen. there were thirteen people in the dining-room of a house at langonar while the thunder rumbled outside. suddenly a flash of lightning struck a plate in the middle of the table, threw dishes, glasses, plates, knives, and forks in all directions--in a word, cleared the table, not forgetting the tablecloth. none of the thirteen guests were touched. it sometimes happens, indeed, that glasses or bottles are altogether or partly melted. boyle gives a very curious instance of the kind. two large drinking glasses were side by side on a table. they were exactly alike. lightning seemed to pass between them, yet neither was broken; one was slightly distorted, however, and the other so much bent by an instantaneous softening that it could hardly stand. when firearms are struck by lightning, their injuries are often of the most varied kind. sometimes the wood, particularly of the butt-end, is split, or broken to pieces, the metal parts torn out, or thrown right away. on july , , the meteor struck a sentry-box at fort nicolai, breslau, and pierced the top to get at the sentry and his gun. the barrel was blackened; the butt-end broken and thrown to a distance. the shot had been discharged and pierced the roof of the sentry-box. the man got off with a few scratches. however, firearms when carried by men appear to attract the lightning. soldiers are often enough struck when in the exercise of their calling, when they are carrying arms. but, curiously enough, many cases are known in which lightning has struck a loaded gun, melting the bullet and part of the barrel, without setting fire to the powder. thus, at prefling, lightning penetrated the room of a gamekeeper, yet none of the many firearms hanging up went off. the wall was damaged between each rifle. one was standing in a corner of the room; the wall was injured on a level with the lower end, and above it a hole was to be seen in the woodwork. on june , , near nimburg, lightning burst into the house of a horse-keeper, where it struck a loaded carbine leaning against a wall on the ground floor. the muzzle was slightly melted by the spark, which ran along the barrel to the trigger, and which it soldered together in parts. there were five bullets melted and soldered together in the magazine and the wads much scorched. however, incredible as it may seem, there was no explosion. in another case the lightning went the whole length of a rifle, both inside and outside, leaving a direct line of fusion, and yet, incredible though it may seem, no shot was fired though the fusion reached the powder. these phenomena appear quite extraordinary, and altogether incompatible with the usual theory of the combustibleness of gunpowder. to what cause can the invulnerability of the explosive matter be due? doubtless to the quickness of the lightning, which does not leave the powder time enough to ignite. powder magazines are frequently struck by lightning, and this subject is one of very great interest; they are not always blown up, in spite of the vast quantities of explosive materials which they contain. here are some examples which go to prove this statement:-- on november , , lightning fell near rouen on the maromme powder magazine, and split one of the beams of the roof. two barrels of powder were reduced to atoms without exploding. the magazine contained eight hundred of these barrels. can it be that man's thunder can repulse that of jupiter? not always, as numerous examples prove the contrary. the following observations are extracted from a collection of similar facts:-- lightning struck the tower of st. nazaire, brecia, on august , . it stood above an underground magazine containing a million kilogrammes of powder belonging to the republic of venice. the whole edifice was blown up, the stones falling in showers. part of the town was thrown down; three thousand people perishing. at four o'clock in the afternoon of october , , lightning penetrated the vaults of the church of st jean, at rhodes, setting fire to an enormous quantity of powder. four or five thousand people lost their lives in the catastrophe. the power of lightning is immeasurable. well, it sometimes enjoys itself after the following manner:-- in it lit a candle which had just been put out. the person who held it was not struck, but the shock sent him to sleep for four days; then he awoke, only to go mad, and then slept for seven consecutive days. at harbourg it put out all the lights at a ball; the room was plunged in darkness, and filled with thick and fetid vapour. many a time, too, has a fire, burning brightly in a grate, been suddenly extinguished by lightning; and the same thing has happened with pottery and tile-making furnaces. as a rule, it is extremely difficult to re-light candles or fires thus extinguished. in some instances it takes on itself to light the gas. on august , , near the observatory in paris, rue leclerc, towards the corner of the boulevard saint jacques, a gas jet was lit by lightning. the latter was twenty centimetres from a long gutter, and was in the gap, so to speak, of an electric circuit formed by it and the damp wall communicating with the ground. a violent explosion took place at the moment the gas caught alight, the gas meter, on the wall two metres above it, was dislodged, when a second explosion was heard. the thunderclap was truly terrific, and immediately followed the lightning flash. the chronometer in the meteorological bureau in the observatory was stopped suddenly. the keeper of the square of the luxembourg saw a ball of red fire explode with great violence, and scatter in all directions. the plate belonging to the pères was, according to m. de fonvielle, broken to a thousand pieces, and the outer part of an iron bar was volatilized. there were no deaths or injuries to record, although several people were thrown down by the shock. sometimes great disasters are indirectly caused by lightning. thus in july, , it set fire to an old house at muda, paluzzo. under other circumstances, the accident might have been insignificant. but, fanned by a violent wind, the flames increased, and, approaching nearer and nearer, burned a hundred houses, or in other words, the whole village. a similar catastrophe took place at the village of ochres, in dauphine, on august , . lightning set fire to twenty thatched cottages, which, out of thirty-two composing the village, were in ashes within less than an hour. three persons were burnt alive, and four others seriously injured. on august , , lightning struck the village of saint innocent, at three o'clock in the morning. seven houses were totally burnt, and three women perished in the flames. a fire caused by lightning burst out on june , , at perrigny, near pontailler (côte d'or). seventeen houses were burnt, and seventy-eight people found themselves homeless. sometimes these disasters attain terrifying proportions. during an awful thunderstorm, the electric spark set fire to eighteen parishes in belgium; ruin spread over an area of kilometres. but could anything be more dreadful than the fate of certain ships that have been struck by lightning? here is the case of one which was literally cut in two. on august , , the ship _moses_, on her passage from ibraila to queenstown, was overtaken in sight of malta by a violent thunderstorm. towards midnight lightning struck the mainmast, and then downwards along it to the hold, cutting the vessel in two. she filled immediately. crew and passengers were lost. captain pearson was on the bridge, and had just time to catch a floating spar, which supported him during seventeen hours. the ship sank in three minutes. at the commencement of last century, the ship _royal charlotte_ being in diamond harbour, on the hoogley, was struck by lightning and blown into a thousand pieces, through the explosion of her powder magazine. the report was heard a great distance off, and the shock was felt for miles around. the form and position of the masts exposes them particularly to the attacks of the dread meteor. several examples are known of sailors being struck by the electric current while aloft in the rigging, and even being thrown from there into the sea. on august , , the steamer _numidie_, sailing from bone, was struck by lightning. the fluid fell on the mizzen-mast, and went down the standing jib, to which the second officer was clinging. the unfortunate man had had both his hands paralyzed and fallen; but if he had fallen on the outside of the draille, death would have been inevitable. the _rodney_ was under weigh before syracuse when it was struck. this was on december , . the top-gallant-mast went first; it weighed eight hundred pounds, and such was the violence of the stroke that it was instantly reduced to shavings, which hung the whole length of the vessel, like rubbish in a carpenter's shop. the topmast was very much damaged and shattered here and there. as for the mainmast, with its ironwork weighing more than a ton, it was wrecked for a length of some seventeen metres. at times the masts are split from top to bottom, broken or cut transversely in fragments, and flung to a distance. sometimes they are planed, like the beams and trees of which we have already spoken. the _blake_ was struck by lightning in . the top-gallant-mast was in green pine, which was split into long fibres in every direction, like branches of a tree. it is not unusual for lightning to creep into the heart of a mast and do it all kinds of injuries, without in any way hurting the outside; in a word, there may be single or double furrows, longitudinal or zigzag, sometimes curved, and of varying depth. sometimes also, the electric current, far more powerful than the blast of the wind, seizes the rigging and carries it off. this phenomenon was observed on the _clenker_, december , ; the topmast and sails were torn off and thrown into the water. neither are the sails spared by the terrible meteor; they are torn, riddled with holes, or set on fire. but as a rule the yards are spared. one of the most frightful effects of thunder on ships is fire, which it drives from one part of the vessel to another. under ordinary circumstances it is usually local, and easily extinguished; but when it seizes on various parts of the ship at once, as when struck by lightning, then destruction becomes inevitable. in the _king george_ from bombay was sailing up the river at canton, when an electric spark, followed by a violent clap of thunder, grazed the mizzen-mast, and disappeared in the hold, after killing seven men. seven hours later it was discovered with consternation that the hold, full of an inflammable cargo, was on fire. it spread rapidly over the whole ship, which it burned to the water's edge. the ship _bayfield_ from liverpool was struck by lightning november , . instantly the deck was seen covered with globes of fire and large sparks, which set fire to the vessel. as it threatened the powder-magazine, the captain decided to abandon the ship. a rush was made for the boats, but as only thirty pounds of bread could be saved, many perished of hunger and thirst. often, indeed, the explosion of the powder-magazine makes the catastrophe even more terrible. thus, in , the english vessel the _resistance_, was blown up in the straits of malacca. only two or three of the crew were saved. but lightning plays more tricks with the compass than with anything else when it visits a ship. the vibrating, quivering, magnetic needle is often paralyzed by the electric current; sometimes its poles are reversed, or the points, disturbed by the passage of the spark, deviate, and no longer responding to the magnetic pole, mislead and move hither and thither. sometimes they even lose all their magnetic properties. these changes in the compass often lead to disastrous consequences. many cases are known of ships being steered to destruction through the deviation of the compass. arago tells of a genoese ship which, about the year , sailing for marseilles, was struck a little way off algiers. the needles of the compasses all made half a revolution, although the instruments did not appear to be injured, and the vessel was wrecked on the coast when the pilot believed he could round the cape to the north. this may account for the total disappearance of certain ships. some ships, like certain individuals and certain trees, appear in particular to attract the electric fluid. we have many records of vessels struck several times in the course of a single electric storm. here are a few:-- on august , , the _malacca_ was struck repeatedly. in , the _competitor_ was struck twice within an hour. at the beginning of december, , between mahon and malta, the ship of a russian admiral was struck three times in a single night. on january , , in the straits of corfu, the _madagascar_ received five destructive discharges in two hours. we could add many others to this list. but enough. and yet we have not said the last word on the subject. we have to discuss the interchange of sympathetic currents, and those which are the reverse, taking place between the electricity of the skies and that of the telegraph. lightning often comes incognito to visit the earth's surface, or even the depths of the ocean. these little excursions to our terrestrial dominions usually pass unperceived; however, in certain cases the telegraph wires commit the indiscretion of revealing them. on the other hand, we know that the wires entrusted with carrying our thoughts round the world, are almost inconceivably sensitive. without being conscious of the fact, they are in correspondence with the sun, millions of kilometres away, and any agitation on the surface of this luminary may cause them indescribable agitation, as we witnessed at the close of the year . during the formidable magnetic tempest of the st october, telegraphic and telephonic communication were interrupted in many parts of the world. in fact, the phenomenon was observed all over the surface of the globe. from nine o'clock in the morning, till four in the afternoon, the old world and the new were strangers to one another. not a word nor a thought crossed the ocean; the submarine cables were paralyzed on account of solar disturbances. in france, communication between the principal towns and the frontiers was interrupted. during this time the sun was in a condition of violent agitation, and its surface vibrated with intense heat. in such times the subtle fluid profits by the confusion to glide noiselessly along the paths which are open to it. but he does not always wait for these favourable opportunities. let a thunder-cloud pass over the telegraph wires, either noiselessly or hurling petards in all directions, the line will be affected. the fluid imprisoned in the sky will act by induction on the electricity of the wires which will result in the vibration of the latter, accompanied sometimes by a flash of lightning. these phenomena may cause grave accidents to the telegraph clerks, unless they are on their guard against the treachery of the lightning. these mute discharges happen frequently, but the spark strikes the telegraph wires often, too, as well as the apparatus in the office. all sorts of accidents result from these repeated attacks. we know, for instance, how the birds fall victims to the lightning when they alight on the telegraph wires after a thunderstorm; they are often found dead hanging by their claws. but the fluid acts on man also, through the medium of the wires. thus, on april , , a telegraph clerk was engaged with several other employees repairing some telegraph wires in the station at pontarlier, when all at once they felt, at the knee-joints more particularly, a violent shock which made them bend their legs as if they had been struck with a stick; one of them was even thrown down. no doubt the fluid reached the wires, which in those remote parts was in charge of the clerks. on september , , during a violent thunderstorm, two telegraph poles were thrown down at zara in dalmatia. two hours later, as they were being set up again, a couple of artillerymen, having seized the wire, felt slight electric shocks, then suddenly found themselves flat on the ground. both had their hands burnt; one indeed, gave no sign of life; the other, in trying to raise himself up, fell back as soon as his arm came in contact with that of one of his comrades, who ran to his assistance on hearing him cry for help. the latter thrown down in turn, felt his nerves tingle, and giddiness seize him, with singing in his ears. when his arm was uncovered, there was a superficial burn just on the spot where he had been touched. on may , , lightning fell on the road from bastogne to houffalize (luxembourg), attracted by the telegraph wire, which it destroyed for about a kilometre. at a certain part, and over a length of about twenty metres, the wire was cut in small pieces, three or four centimetres long, which were scattered over the ground, and were as black and as fragile as charcoal. the poles which supported them, and several poplars planted on the same side of the road, were more or less damaged. it has been observed that trees planted on the same side as a telegraph line were sometimes blasted on a level with the wires. it is the same with houses near the copper threads along which human thoughts take wing. thus, at chateauneuf-martignes, on august , , lightning destroyed the telegraph poles on the outskirts of the railway-station. a severe shock, like an electrical discharge, was felt at the same moment by two people who were in bed, not far from where the wire was fixed in the wall of the house, which was a very low one. the same phenomenon had been felt there already. in the railway-stations, as well as in the telegraph and telephone offices, curious results of the spark passing at a certain distance, or even in the immediate neighbourhood, are sometimes observed. on may , , towards five o'clock, the sky looking overcast, the station-master at havre warned his colleague at beuzeville that it would be well to put his apparatus in connection with the ground. beuzeville is twenty-five kilometres away from havre, and at the former station the weather then did not look at all threatening. but clouds soon piled up, driven before a violent wind. suddenly three awful peals of thunder succeeded each other in quick succession. with the last, lightning struck a farm about a kilometre from the station, and at the same moment a globe of fire of a reddish brown, and apparently about the size of a small bomb-shell, rose as if out of a clump of trees. it glided through the air like an aerolite, and leaving behind it a train of light. at a hundred metres or so from the station, it alighted like a bird on the telegraph wires, then disappeared with the rapidity of lightning, leaving no trace of its passage, either on the wires or the station. but at beuzeville several interesting phenomena were observed. firstly, the needles turned rapidly, with a grating noise like that of a turnspit suddenly running down, or like a grindstone sharpening iron, which emits sparks. a great number, indeed, flew out of the apparatus. one of the needles, that on the rouen side, went out of order; all the screws on that part of the instrument were unscrewed, and on the copper dial near the axis of the needle, there was a hole through which one could pass a grain of corn. the instruments at havre were unaffected. the needle remained as usual, also the dial, screws, and so on. one of our correspondents has sent me the following very interesting communication:-- "on june , , having rung up at the central telephone-office at st. pierre, martinique, a harsh noise was heard, which was almost immediately succeeded by the appearance of a ball of fire, having an apparent diameter of twenty centimetres, and the brilliancy of an electric light of twenty candle power. this voluminous globe followed the telephone wire towards the instrument. arrived near the receiver, it burst with a terrific explosion. the witness of this phenomenon felt a severe shock, and dizziness. recovered from his stupefaction, he noted the following facts: the telephone apparatus was completely burnt, the relay of morse's installation was slightly damaged. the electrical tension must have been enormous, for the wire of the bobbins was, to a great extent, melted." this latter effect, however, occurs very frequently. not only does the lightning melt and break the telegraph wires, but it injures the poles which support them. these are sometimes broken, split, thrown down, burst, or splintered, sometimes into threads or shavings. poles which have been blasted are often to be seen alternating with others which are uninjured. thus, on the line from philadelphia to new york, during a great storm, every alternate pole up to eight was broken or thrown down; the odd numbers were uninjured. we have mentioned a similar case already. there are several accounts, too, of lightning in pursuit of trains. on june , , travellers by train from carhaix to morlaix, between sorignac and le cloistre, saw lightning follow the train over a course of six kilometres, breaking or splitting several telegraph poles. this feat has been observed more than once. the train is escorted by lightning flashes which succeed each other almost without cessation, and the travellers seem to be whirled through an ocean of flame. lightning rarely strikes the carriages; only on one occasion did it actually wreck one, by breaking a wheel. the mutilated coach, however, continued to hobble along until the injury was discovered. generally the fluid is content to wander about the rails, to the great terror of the passengers who witness this display of rather alarming magic. it spreads itself over masses of iron, as for instance the roofs and balconies in paris, without striking any particular point. the danger would be greater to a cyclist on a road. in the suburbs of brussels, on july , , a cyclist named jean ollivier, aged twenty-one years, was riding during a violent storm, when suddenly he was struck and killed on the spot. we shall end this description of the whims and caprices of lightning by a notice of the blasting of a german military balloon. it happened in june, . the aeronaut, whose car was steered by a sub-lieutenant, was held captive, and soared at a height of about metres above the fortifications at lechfeld, near ingolstadt. all at once the aerial skiff was touched by an electric spark, caught fire, and began to descend, slowly at first, then swiftly. the aeronaut had the good luck to get off with a broken thigh. the five assistants, who worked the windlass and the telephone, also received shocks transmitted through the metal wires of the cable. they fell unconscious, but were quickly restored. this phenomenon, which is excessively rare, fittingly closes this odd collection of stories, fantastically illustrated by lightning. a communication from berlin also mentions that the captive balloon of the battalion of aeronauts was struck by lightning on the exercise ground at senne. two under-officers and a private were wounded by the explosion. chapter ix lightning conductors until comparatively recent times, as we have seen, all that was known about thunderstorms was that they occurred pretty well all over the world, and generally in either spring or summer. while efforts were being made on our old continent to establish by long and ingenious dissertations the exact degrees of relationship between lightning and the sparks given out by machines, in america practical experiments were being set about towards solving the problems of electricity. franklin it was who hit upon the idea of extracting electricity from the clouds for the purpose of investigation. this man of immortal genius, who by his achievements in science, his noble character, and his devotion to his country, has won the admiration and gratitude of posterity, was of humble origin. the son of a soap manufacturer in a small way of business, benjamin franklin was born at boston in . his parents had intended him to go in for science. he was successively an apprentice to a candle manufacturer, a journeyman printer, the head of a big printing firm in philadelphia, deputy to congress, an ambassador, and finally president of the assembly of the states of pennsylvania. his political record was a great one. no one ever rendered greater services to his country than the diplomatist who signed the peace of , and insured the independence of the united states. it was towards the age of forty that franklin began his study of electricity. here is his own account of the memorable experiments to which he owed the greater part of his immense fame:-- "in i met at boston a certain dr. spence, who came from scotland. he performed some electrical experiments before me. they were not very perfect, as he was not a man of great ability; but as the subject was new to me they surprised me and interested me in an equal degree. shortly after my return to philadelphia, our librarian received as a gift from pierre collinson, a member of the royal society of london, a tube of glass, together with certain written instructions as to the way in which it should be used for experiments. i seized eagerly on the chance of reproducing what i had seen done at boston, and with practice i acquired a great facility in performing the experiments indicated to us from england and in devising other ones. i say 'with practice,' because many people came to my house to witness these marvels." after making several discoveries in regard to electricity, franklin took it into his head to extract the fluid direct from the clouds. he had established the fact that a stem of pointed metal, placed at a great height--on the summit of a building, for instance--served as an attraction to lightning and guided it into the way prepared for it. he had been looking eagerly to the erection of a clock-tower which was being built at this time at philadelphia; but, tired of waiting and anxious to carry out experiments which should solve all doubts, he had recourse to a more expeditious instrument, and one, as events proved, not less efficacious, for getting into touch with the region of thunder--a kite such as children play with. he prepared two sticks in the form of a cross, with a silk handkerchief stretched upon them, and with a string attached of suitable length, and set forth on his mission the first time there was a storm. he was accompanied only by his son. fearing the ridicule that is showered upon failure, he did not take any one else into his confidence. the kite was set flying. a cloud which looked promising passed without result. others followed, and the excitement with which they were awaited can be imagined. at first there was no spark and no sign of electricity. presently some filaments of the string began to move, as though they had been pushed out, and a slight rustling could be heard. franklin now touched the end of the string with his finger, and instantly a spark was given out, followed quickly by others. thus for the first time the genius of man may be said to have come to grips with lightning, and begun to learn the secret of its existence. this experiment took place in june, , and made an immense sensation throughout the world, and was repeated in other countries, always with the same success. a french magistrate, named de romas, making use of franklin's idea as soon as it was known in france, took it into his head to use a kite with raised cross-bars, and in june, , before the full results of franklin's experiments were made public, secured still more remarkable signs of electricity, having inserted a thread of metal throughout the whole length of the string, which was metres. later, in , de romas repeating his experiments during a storm, secured sparks of a surprising size. "imagine before you," he said, "lances of fire nine or ten feet in length and an inch thick, and making as much noise as pistol shots. in less than an hour i had certainly thirty lances of this length, without reckoning a thousand shorter ones of seven feet and under." numbers of people, ladies among them, were present at these experiments. they were not without danger, as may be imagined; de romas was once knocked over by an unusually heavy discharge, but without being seriously hurt. franklin was the first to turn his experiments to practical account, attaching lightning-conductors to public and private buildings for their protection, and achieving marvellous results; the lightning being caught by the metallic stem and following it obediently into the ground. from this time, lightning-conductors came into almost universal use, and their value was not long in being generally recognized. curiously enough, france, which had been ahead of all other countries in the study of electricity, was not one of the earliest to go in for lightning-conductors. there were, indeed, signs of strong hostility to their introduction. it was held even that they went against the designs of providence. in , the abbé poncelet, in his work entitled "la nature dans la formation du tonnerre et la reproduction des êtres vivants," in which he sets out to demonstrate that the force which produces lightning is the same as that which causes the earth to fructify, makes a strong protest against the construction of lightning-conductors. in , nevertheless, at the reiterated request of le roi, a member of the academie des sciences, and friend and admirer of franklin, the louvre was endowed with the first lightning-conductor put up on a public building in france. soon afterwards they became common. in the academie des sciences drew up the first set of rules for the construction of lightning-conductors. it was revised and corrected in , in accordance with the various improvements that had been introduced up till then, and it has been further added to in , , and . these instructions point out that the most important metallic portions of the building should be placed in communication with the conductor, and this should sink into a well. conductors that are not perfectly constructed are a source of danger, instead of being a protection, for the electric current is apt, instead of running down into the earth, to make for any kind of metallic substance, and cause great havoc. the conductor ought really to communicate with a large body of water--a body of water of greater extent than the storm cloud from which the lightning comes. when the flow is insufficient, the water itself is apt to become electrically charged. it is dangerous to bury the conductor in merely damp soil; first, because one generally does not know whether there is enough of this soil; secondly, because one cannot be sure that the humidity will be sufficient at times of great drought--the very times when storms are most to be feared. failing a river or great pond, the conductor should be put into wells issuing or having their source in inexhaustible supplies of water deep down in the soil. in his table of statistics showing the number of cases in which lightning has struck either lightning-conductors, or buildings, or ships furnished with conductors, quebelet gives a hundred and sixty-eight cases in which the conductor has been struck, and in only twenty-seven instances of these (one-sixth of the whole) have the conductors, from some grave flaw in their construction, failed to fulfil their office. these results are the best proof possible of the efficacy of conductors, and the best answer to those who decry them. the area of protection covered by the conductor is not so great as is generally supposed. it is limited to a distance about three or four times the length of the conductor above the roof. thus a conductor standing out five yards will protect an area stretching only about fifteen or twenty yards away. this depends also to some extent upon the nature of the place and the materials of which the house is constructed. buildings are often struck by lightning because the number of conductors has been insufficient for the extent of the edifice to be protected. to remedy this defect, conductors are made with a number of separate stems--veritable wire traps in which to catch the lightning. this system, the invention of a belgian physicist, m. melsens, decreases considerably the risks of destruction, and is much more economical than the erection of a number of separate conductors. a conductor of this kind has been installed on the hotel de ville at brussels, which has been well protected from lightning ever since, whereas previously this building had been struck by lightning several times in spite of the single conductors with which it was supplied. the metallic trellis is in communication with the sewers. the slaughter-houses of la villette, the hôtel evigné, and other buildings in paris, are provided with similar defences. the eiffel tower boasts several such multiplex conductors. it has often been struck by lightning, but no one who has happened to be up it at the time has ever suffered any damage therefrom. the lightning strikes the conductor sometimes from out the actual cloud--curious photographs have been taken of this. the eiffel tower is in itself a gigantic lightning-conductor. portable conductors have been invented from time to time--silk umbrellas without iron ribs, and clothes of indiarubber and such-like; but they have all been childish things. * * * * * without allowing one's self to get lightning, so to speak, on the brain, it is well to take certain precautions during a storm. the first and principal one is not to get under a tree. the second is to give a wide berth to telegraph posts, so as to avoid contact with the sparks that may issue from them. movements of the air having the effect of preparing an excellent route for the fluid, it is well not to run in a storm. it is well also not to ring a bell. it is well, also, to avoid being in the neighbourhood of animals, in view of their attraction for lightning. in houses, doors and windows should be closed in order to avoid draughts. it is well to keep away from the chimney, too, as well as from metallic objects. but lightning always has its caprices. it is this that makes its study so interesting. chapter x pictures made by lightning in this last chapter i would like to group together a series of instances of pictures made by lightning, some of them very curious and attributed, it would seem, to flashes of a special character, which we may perhaps term ceraunic rays, from _keraunos_, lightning. these instances are of great variety, and doubtless admit of many different explanations. here, then, is a selection worth looking into. in this case, as in so many others, it is extremely difficult to get at the exact truth. generally speaking, it is from the newspapers that we get the facts--more or less accurately observed, more or less accurately recorded. i have made great efforts to inform myself personally as to the incidents whenever this has been practicable. the _petit marseillais_ of june , , published the following:-- "a correspondent writes to us from pertius, june :-- "'in the course of the storm here yesterday, two day-labourers of our town, jean sasier and joseph elisson, took refuge in a cabin constructed of reeds. they were standing at the entrance when they were struck by lightning and thrown violently to the ground. elisson, who was not much hurt, soon recovered his senses and called for help. people ran up at once and carried the two men to where they live, where all necessary attention was given to them. "'sasier's condition, though serious enough by reason of a burn on his right side, is not causing anxiety. the curious part of the incident is the effect the electric fluid has produced upon elisson. the lightning cut open one of his boots and tore his trousers; but over and above this, like a tattooer making use of photography, _it reproduced admirably_ on the artisan's body a representation of a pine tree, of a poplar, and of the handle of his watch. it is an undoubted case of photography through opaque materials; most luckily the sensitive plate--elisson's body--merely took the impression and received no injury.'" on reading this narrative, i wrote to the mayor of the commune of pertius to ask him for confirmation of it, and for a photograph, if possible, of the picture on elisson's body. by a fortunate circumstance, the mayor happened to be the doctor who had attended the victim. here is his reply:-- "m. joseph elisson, of pertius, aged about thirty-eight, was struck by lightning on june . called to attend to him at about two in the afternoon, i found some superficial burns forming a trail, which began near the teat of his left breast, at the level of his waistcoat pocket, in which there was a watch (which had not stopped), and went down towards the navel, then turned boldly to the right towards the iliac spine and down the outer side of his right leg as far as the ankle, at the level of which his boot, made of strong leather, had been split open. "to the right, a little outside the vertical line passing the teat, there was imprinted in vivid red--the red of the burn--a picture of a tree. the foot of it was on a level with the edge of the ribs, the top went slightly above the teat. this picture was absolutely vertical. its outlines stood out very distinctly from the white skin. it was composed of bold, clearly defined lines, about a demimillimetre in width. neither waistcoat nor shirt were burnt or marked in any way to correspond with it. other representations of tree branches were reproduced higher up on the breast, but not so distinctly in the midst of a uniform redness. not having by me my camera, i made a sketch of the tree, which was marvellously distinct, leaving the taking of a photograph until next day. next day, when i returned with my camera, the picture was still clearly visible, but it had faded a good deal, lost in the colour of the skin, and no longer to be reproduced by photography. i regretted bitterly not having taken it the day before. i regret this all the more now that you have done me the honour of writing to me on the subject, and i am glad to be able to send you my sketch of the picture, which is correct as to dimensions, and which represents what i saw as accurately as i could make it. "dr. g. tournatoire." here is a facsimile of the sketch enclosed by the doctor. [illustration] it is somewhat like the shape of a poplar. there is nothing to suggest that we have here a case of swollen veins, or arteries made conspicuous by a flow of blood, nor of a tree-like form due to blood vessels, in which the blood has taken on a more or less marked aspect. on the other hand, it is certainly not much easier to recognize in it a photograph of a more or less distant tree. in this state of uncertainty on the subject, i wrote again to dr. tournatoire, and begged of him to go to the scene of the incident and to make a plan of the ground, and take a photograph of the view. here is the doctor's reply:-- "this plan can be reproduced by a few typographical lines in such a way as to show clearly how things happened-- [illustration] "the square represents the cabin in which the two workmen took refuge. they were sitting almost opposite each other on the seats marked a and b. there is a flash of lightning. one of the men is knocked over, and bears on his right side a picture of the poplar p, which stands one hundred metres away, and is visible by a through the door o, of which the width is one metre. behind this poplar stands a big pine, a branch of which is also depicted on the man's body. by this same stroke of lightning the other worker, seated at b, is thrown out of the cabin three metres by an opening d, about forty centimetres wide. the two men are alive, and have come out of it with a few days' rest. they saw nothing, heard nothing, and can remember nothing." in the photographs taken by dr. tournatoire it is not clear which is the tree, for the poplar, p, does not stand out alone. as the cabin stands under the shade of a pine tree, one is disposed to ask whether the lightning did not strike this pine? but judging by the position of the trees relatively to the man struck, the most likely hypothesis is that the electric discharge came from the point p towards a, and that the poplar as well as the adjacent pine formed a sort of screen, and reproduced their reflection by the agency of some unknown constituent of these ceraunic rays, which enable them to photograph things in this way through clothes on to the human body. this assuredly is a more extraordinary effect than those obtained by the cathodic rays or anti-cathodic rays, of which science seems equally unable to give any explanation. let us proceed with our studies. it is important above all never to take the newspaper narratives on trust without verification. in the month of june, , the following appeared in the newspapers:-- "photographic lightning.--a _chasseur_ of the th battalion, in barracks at remiremont, was struck by lightning. he was standing upon a mound not far from a grove of pines, in the midst of ferns. curious to relate, on the occasion of recording the fact of the man's being dead, it was discovered that his body was covered with punctures imprinted on it by the lightning, and representing the nature and aspect of the branches and plants all round him at the time when he was struck." i wrote at once to the _chef de bataillon_ for a precise confirmation of this, and received the following reply:-- "m. le commandant joppé, commanding officer of the th battalion of chasseurs, has handed me your letter of june th, and asked me to reply to it. "it is the case that a _chasseur_ of the battalion was struck by lightning on the afternoon of june , but it is quite untrue that there was found on his body a photograph of the trees adjoining the spot of the accident. the man's clothes were not affected in any way, and the only traces left by the passage of the lightning, consisted in some slight irregularly shaped burns on the upper part of the hollow of the right temple of linear formation, save for one circular burn measuring from three to four millimetres in diameter, and depressing the skin into the shape of a saucer. there was no lesion on the whole surface of the body. "mauny, "surgeon-major th battalion of chasseurs." this reply was covered by a note which ran as follows:-- "i thought it desirable that the reply to your letter of the th inst. should be made by the surgeon-major of my battalion in order that it might be the more scientifically accurate and authoritative. "joppÉ, "chef de bataillon breveté, "commanding officer of the th battalion of the chasseurs des vosges." clearly, the student of natural phenomena cannot take too many precautions. and yet ... an officer of high rank confided to me recently that "surgeon-majors hardly ever take the trouble to examine bodies thoroughly," and that it is possible that in this case "the examination may have been very superficial." if this be a general rule, there must have been an exception here, as also in the fifth case, to which we shall be coming just now. the problem is far from being solved, and we can but seek to study it as set forth in a number of instances. here is a third case:-- on sunday, august , , a certain number of riflemen were practising at the charbonnières range, near the village of le pont, in the valley of lalle joux (canton de vaud, suisse), five targets out of six were in use. the targets, distant metres from the firing-line, are placed to the side of a grove of pines. between stretches an undulating rock-strewn meadow-land. only the butts are provided with a lightning-conductor, and in it are five markers. _there are six telephone wires along the line of fire_; they come as far as the stand and go down to within about centimetres of the scorers' seats. _each target has its own telephone bell._ the weather was not stormy, though the sky was somewhat overcast. firing was going on. at about . there was a clap of thunder, and lightning struck the electric wires. in the stand twenty-eight men, riflemen, scorers, and spectators, are thrown to the ground in every direction and in every position. some are quite inert, apparently dead; others, looking as though they were asphyxiated, give out a painful rattling noise from their throats. at the bar, to the side of the stand, no one felt anything--it was not even noticed that there had been a violent stroke of lightning. a kilometre away the band, which had been giving a concert in front of the hotel de la triute on the bridge, continued to play. presently a man reaches them with the report that twenty of the riflemen have been killed. consternation becomes general, and relief parties are organized. fortunately the damage done was much less than was supposed. let us describe the men actually engaged in firing, and placed in a line, a, b, c, d, and e, their scorers are behind them. let us give our attention, first, to these men and their scorers. presently we shall come to the others awaiting their turn, then to the spectators, and finally to the markers at the targets. the men were firing, either kneeling or lying. a remained in position, kneeling "like a statue," unable to move. he turned right over as soon as he was touched. killed. b had _a pine tree depicted on his breast_--upside down, its roots indicated by some outlines up at the top; the picture was of a brownish rather than a blueish shade. it was suggested that it resembled a pine, because of a pine being ten metres away from the stand, but really it was more like a branch of fern. killed. c felt hardly anything apart from a certain heaviness in his limbs when picking himself up. d had some slight burns, which were healed within two or three days. e was holding his rifle with the barrel vertically. he found himself about - / metres from his position on the ground, with a stone in his hands; his rifle had been bent in two below the trigger. a's scorer held the pear-shaped handle of the electric bell between his fingers, his elbows upon the table. saw nothing, heard nothing. felt himself suddenly bent up double, his face buried in the gravel; lost consciousness while he was being carried away; when he came to himself again he began to ramble in his speech; his pencil was broken lengthwise into four pieces. the wire of his bell had been electrified. by way of wound, he had a picture of a pine branch on his back; water issued from it as from a blister; there were no traces of blood. the picture disappeared at the end of two days. for a certain time the young man had a pain in his loins; he still limps somewhat; probably what he now suffers from is sciatic lumbago, the result of partial and temporary paralysis. b's scorer had only some insignificant burns. c's scorer only came to himself twenty minutes later as the result of artificial respiration. at the moment of the lightning he was pressing the electric button with his thumb in order to give the signal, "change the target;" he had a small hole in his thumb. this burn bled later, and the wound took four weeks to heal. he had also some burns on his legs. d's scorer was holding the handle of the bell against his left cheek, on a level with his eye. the handle, made of wood, burst. the sight of his left eye is still affected, being very weak--the retina was probably torn away. the day after the accident, the young man's face became all inflamed, especially the part round the eyes. these were quite hidden. this inflammation, of a bluish tint, is due to the dilation of the small veins or of the capillaries. dr. yersin, who attended several of the victims, attributes this dilation to a paralysis of the vasomotor nerves, "_which would also explain the tree-like form of the pictures seen upon the skin_," and the transudation (?) of water across the small blood-vessels. e's scorer had time to see the men on his left fall, in a green or violet light. he had heard a general death-rattle-like chorus, "aôôô"--then, before he could make out what was happening, he found himself driven up against the wall of the stand. he had a wound under his feet; his thumb torn also, probably in trying to hold himself up against the wall. behind the scorers were a dozen other riflemen and some spectators. to the left the electric current left intact the rifles standing on the rack. quite near this, a man awaiting his turn fell, clinging on to the neck of one of his comrades, also struck. later he found his purse in the middle of the stand. in the case of several of the spectators, the burns were to be found in separate sores. one had his hair burnt on one spot of about the size of a five franc piece; others, who had burns upon their feet and legs, are under the impression they saw a small blue flame at the tips of their shoes. the general feeling was at first merely that of stupefaction. terror did not come until afterwards. "those who did not lose all consciousness were half stunned." a young boy was noticed jammed up against the wall, incapable of moving, but bewailing his inability to get to his father, who lay dead upon the ground. two men took flight without throwing aside their guns; another ran as far as the village, and some hours afterwards he was found asleep in a house "to which there was nothing to take him." one young spectator, a stranger to the neighbourhood, was seized with a partial paralysis of the brain; he could not keep his balance when walking, and when questioned he would recite the names of stations on a swiss railway. he is better now. this event will not be soon forgotten in the joux valley. but dr. yersin's explanation of the ceraunic pictures does not seem to me to be justified. here is a fourth instance, given me long ago by one of the most learned physicists of last century, hoin, of the institute. "i am going to tell you," he wrote to me in july, , "of a stroke of lightning which was very curious in its effects. it occurred at midday on june , at bergheim, a village situated to the north of logelbach at the foot of the vosges. it struck two travellers who had taken refuge under the tree and knocked them over senseless--one of them was lifted to a height of more than a yard and thrown upon his back. it was thought they must be dead, but thanks to the attention given to them at once, they were brought to themselves, and they are now out of danger. but here is the strange feature of the accident. both travellers have on their backs, extending down to their thighs, _the imprint, as though by photography, of the leaves of a lime tree_; according to the statement of the mayor, m. radat, _the most skilful draughtsman could not have done it better_." here is a fifth instance which i find among my records. the incident happened at chambéry, may , . in the course of a violent storm, a soldier of the th regiment was struck by lightning underneath a chestnut tree. in a memorandum drawn up on june by a learned doctor of chambéry, an eyewitness of the occurrence, the following facts are recorded:-- "the man who was killed had been standing in the centre of a group of eight soldiers, who had their guns in their hands, without bayonets. struck in the region of the heart, he did not succumb for about a quarter of an hour, after saying a few words. the corpse bore an oval plate, so to speak, of about to centimetres in length, by to in width, occupying largely the precordial region, and presenting the parchment-like aspect of a vesicatory that had become rapidly dried up. the clothes were neither torn nor burnt. "two hours after his death an examination of the body resulted in the discovery of a phenomenon already recorded by several observers--the reproduction of photo-electric pictures. "on the right shoulder were three bunches of leaves of a more or less deep reddish violet hue, reproduced minutely with the most absolute photographic precision. the first, situated on the lower part of the inside of the forearm, represented a long branch of leaves like those of a chestnut tree; the second, which seemed to be formed by two or three such branches twisted together, was in the middle of the outside of the arm; and the third, in the middle of the shoulder, larger and rounder, showed only some leaves and small branches at the top and at the borders, the centre presenting a red stain diminishing towards the circumference. the body, when dissected, bore no sign of any interior lesion." here is a sixth instance:-- in june, , a trappist was struck by lightning at the monastery of scourmant, near chimay in belgium. it was the afternoon, and the monks were busy mowing. the storm coming along obliged them to seek shelter. one of them, who was following the mowing machine worked by two horses, directed it towards an ironwork enclosure, and knelt down beside this trellis. there was a terrible thunderclap, the horses bolted in their fright, and the monk remained with his face to the ground. the others, who saw him fall, ran up to his assistance, only to find him dead. the medical attendant of the monastery, sent for at once, discovered on the body two large and deep burns, identical in shape, and placed symmetrically to each side of the breast; he pointed out also to those who were present a white spot under the left armpit, _presenting a very distinct picture of the trunk of a tree with branches on it_. out of these six cases, five may be taken as fully authenticated. dr. lebigne, mayor of nibelle (loiret), published the following narrative in the _moniteur_ of september , . "on sunday, september , , at about . a.m., three men were busy gathering pears about metres out of nibelle, when the pear tree was struck by lightning, and was distorted from top to bottom in the form of a screw; the lightning carried away the bark and about a centimetre of the wood beneath; then quitting the tree, it struck the head of one of the workmen, who was eating some bread at the time, and killed him, as well as a dog by his side. the head was burnt behind from top to bottom and was impregnated with a strong smell of sulphur. "the two other workmen who were on the tree were thrown to the ground, and remained for some time senseless. when they came to themselves they could not move their legs. they were taken to their homes, and it was found that both had been in contact with the electric fluid. the astonishing thing about them is, that _one of them had the branches and leaves of the pear tree clearly imprinted on his breast as though by daguerreotype_. the terrible photographer had been merciful, however, for that evening both the men were up again and able to walk." the _comptes rendus_ of the academie des sciences for the year (xvi. p. ) records that in july, , in the department of indre-et-loire, a magistrate and a miller's boy were struck by lightning in the vicinity of a poplar. it was found that both of them had on their breasts stains exactly like the leaves of a poplar. these marks, in the case of the magistrate, went away gradually as the blood began to circulate again. in the case of the boy, who was killed on the spot, they had faded somewhat by the day after, when decomposition had begun to set in. _a propos_ of this case, very similar to those preceding, arago recalled the fact that in leroy, a member of the academie des sciences, declared that franklin had several times told him how a man who was standing at a door during a storm had seen a tree struck by lightning opposite him, and that a representation of this tree was found imprinted upon his breast. arago recalled, too, in this connection a report made to the old academy in august , , by bossuet and leroy, in which there was question of a man killed by lightning on may , , in the collegiate school of riom in auvergne; in this case the electric fluid had entered by the heel and gone out by the head, leaving on the body singular marks, described in the report. it was thought that the lightning on its way, through having forced the blood into all the vessels in the skin, must have made all the ramifications of these vessels sensitive to impressions from without. extraordinary though this may appear, they go on to say, it is not new; père beccaria cites a similar case; and franklin's case is cited here also as analogous. besile, the author of the record of the riom case, "did not hesitate," he tells us, "to attribute the effect to an eruption of the blood in the vessels of the skin, producing a result similar to that of an injection." the statement in the _comptes rendus_ is entitled "strange appearance of ecchymoses formed by lightning upon the skin of two persons." that is just the question. was there in these pictures nothing but ecchymoses--infiltrations of the blood into the cellular tissue? perhaps that may be so in some cases, but not in all. photography, the photo-electric pictures produced in the laboratories of physicists, moses's _figures_(?), the lichtenberg flowers, cathodic rays, rontgen rays, radiography-all these things open new horizons for us. and even if we do not find any explanation satisfactory, we should not be justified in accepting the first offered to us as being so, if in fact it be not. here are four interesting cases recorded by poey in his "rélation historique des images photo-electriques de la foudre." mme. morosa, of lugano, seated near a window during a storm, experienced a shock from which she is not stated to have suffered any ill effects; but a flower, which had stood in the route of the electric current, was found perfectly _drawn_ on her leg, and this picture lasted the rest of her days. in august, , a young girl in the united states of america was standing at a window facing a nut tree at the moment of a dazzling flash of lightning; a complete picture of the tree was reproduced on her body. in september, , a peasant woman of seine-et-marne, who was minding a cow, was struck by lightning under a tree. the cow was killed, and the woman was thrown on the ground insensible. she was, however, soon revived. in loosening her clothes to attend to her, the people who came to her assistance found perfectly reproduced on her breast _a picture of the cow_. on august , , a mill at lappion (aisne), belonging to m. carlier, was struck by lightning. on the back of a woman of forty-four, who was also struck, the lightning left a reproduction (of a reddish hue) of a tree--_trunk_, _branches_, _foliage_, _and all_. her clothes bore no trace of the passage of the lightning. unless we are to suppose that all these have been inaccurately observed, it seems to me that we must admit that there is something else besides ecchymoses, something else besides the workings of veins and arteries in these pictures wrought by lightning. certain of these tree-like pictures resemble the patterns we get in photographing electrical discharges upon sensitized plates. might they not be produced by this discharge upon the surface of the body--or by the emission of electricity from the body struck? the pictures we shall now hear of, to be distinguished from those already dealt with, are easier to explain, and about their genuineness there can be no doubt. in the summer of , a doctor from the neighbourhood of vienna, dr. derendinger, was returning home by train. on getting out at the station he found that he had not got his purse on him--some one probably had stolen it. this purse was made of tortoise-shell, and had on one side of it a steel plate marked with the doctor's monogram--two d's intermingled. some time after, the doctor was called to attend to a stranger who had been found lying insensible under a tree, having been struck by lightning. the first thing that he noticed on examining the man's body was that on his thigh there was a reproduction, as though by photography, of his own monogram. his astonishment may be imagined. he succeeded in reviving the stranger, who was taken to a hospital. the doctor remarked that in his clothes his lost tortoise-shell purse would probably be found. so it proved. the individual struck by lightning was the thief. the electric fluid had been attracted by the steel plate, and had imprinted the monogram upon the man's body. in this case we are set thinking of electro-metallurgy, all the more because there are a number of other instances which certainly belong to this category. thus, for instance, on july , , at nantes, a stranger near the pont de l'erdre, on the quai flesselles, was enveloped by a flash of lightning, but proceeded on his way without experiencing any ill effects. he had on him a purse containing two pieces of silver in one compartment and a ten-franc piece in gold in another. on taking out his purse he found that a coating of silver taken from one of the silver pieces--a franc--had been transferred to both sides of the ten-franc piece. the franc, slightly thinner, especially over the moustache of napoleon iii., was in parts slightly bluish. this transference of the silver on to gold was made through the skin of the partition of the compartments![ ] another case. in gilbert's "annalen der physik" ( ) we read that a flash of lightning struck the tower of a chapel near dresden and took the gilt off the framework of the clock and transferred it to the leaden runs of the window-panes in such a way as to leave no sign of how these had been gilded. in these cases the analogy with galvano electro-metallurgy is evident. but in the earlier cases this was not so; the trees contained no metallic element. it was not a case of transference. they seem to have been photographed by the ceraunic rays. on october , , a young man was killed by lightning. the corpse bore in the middle of the right shoulder six rings of flesh-colour, which seemed the more distinct in that the rest of the man's own skin was very dark. these rings, overlapping each other, were of different sizes, corresponding exactly with those of the gold coins which he had on him on the right side of his belt, as the public official who examined his body and all the witnesses were able to testify. this makes us think of radiography. a correspondent of poey, the astronomer, told him that he had known a trinidad lady who had been struck by lightning in her youth and on whose stomach the lightning had imprinted a metallic comb which she carried in her apron. in these instances there was some kind of contact of the objects with the persons struck. here are others in which the objects reproduced are further removed, but still of metallic substance and still reminding us therefore of electro-metallurgy. in september, , the brigantine _le buon-servo_, at anchor in the bay of armiro, was struck by lightning. a sailor seated at the foot of the mizzen-mast was killed. on his back was found a light yellow and black mark, beginning at his neck and going down to his loins, where there was discovered an exact reproduction, in facsimile, of a horseshoe nailed to the mast. the mizzen-mast of another brigantine was struck by lightning in the roadstead of zaube. under the left breast of a sailor who was killed was found imprinted the number , which his mates all declared was not there before. these two figures, large and well formed, with a full stop between them, were identical with the same numbers in metal affixed to the rigging of the ship, and placed between the mast and the sailor's bunk, in which he was lying asleep when struck. may it not have been a tattoo-mark in spite of what his companions declared? m. josé maria dau, of havana, records that in , in the province of candaleria, in cuba, there was found on the right ear and on the right side of the neck of a young man struck by lightning, the reproduction of a horseshoe, which had been nailed up at a short distance from him against a window. these various records lead us to the reflection: first, that ceraunography should form a new branch of physics, well meriting study; secondly, that the facts set forth are sufficiently inverse in their nature to show us that we have before us several quite distinct specimens of phenomena. however, these matters have been a subject for study long before our day. a priest, p. lamy, of the congregation of saint maur, published in an excellent little work,[ ] informed by the most lucid common sense upon the curious effects of lightning--then a text for the most superstitious commentaries. voltaire could not have reasoned the thing out better. he deals with two very extraordinary cases among others. the first had for scene the abbey of saint médard, at soissons, on april , . a flash of lightning struck the tower of the abbey, went into the clock, penetrated a wall eight feet thick, by a hole conducting an iron rod _à l'aiguille de cadran_, detached two planks, four feet high, and threw them to the extreme end of the dormitory, followed a brass wire stretched along the whole length of the wall, setting fire to it and spreading it out like a ribbon painted to represent a furrow of flames. here is the author's own description:-- "the most surprising effect, and one which has aroused the curiosity of an immense number of people, is a kind of frieze of all kinds of colours extending along the wall of the dormitory and just above the doors. "the depth of this frieze is about two feet; its length is almost equal to that of the dormitory; the designs upon it are of flames darting up and down from a kind of wide band which occupies the centre of the frieze throughout its length. "i have had a portion of this frieze copied, so as to give the reader an idea of it, but it must be admitted that it is difficult to suggest the variety of _nuances_ in the original. some people declare that in the midst of all the colours in the flames, faces of men may be descried as well as of marmosets and demons; but those who are less richly endowed with imagination can see nothing of all this." on p. is a copy of the design by p. lamy. at this period, physicists were of the belief that lightning was "an exhalation of nitre and sulphur," acting something after the fashion of powder, and able to burn up or throw over everything encountered on its route. in this girdle traced by the lightning, the author sees a scattering of all the constituents of the brass wire, transformed into all kinds of colours due to the dilation of the copper, melted and vaporized over the width of two feet, the colours, in which yellow predominates, varying according to the thickness and the inequalities of the "projection." [illustration] the second case examined into by p. lamy, was that of what happened in the church of sauveur at lagny, when it was struck by lightning on july , . this is one of the most astounding in the entire history of the subject. let us see what our author has to tell us:-- "if we were to look for some excuse for the strangeness and diversity of the people's sayings and doings in connection with the lagny case of lightning, we should assuredly find it in the extraordinary nature of the case itself. "for what would naturally be the effect upon minds accustomed to see mysteries in the most transparently natural events, minds whose philosophy never goes beyond the senses, when they learn-- " . that the lightning had not only descended upon the clock-tower of the church, and carried off the slates from its roof, but had struck and overthrown nearly fifty persons inside the edifice, and wrought great havoc on the high altar. " . that it knocked over and broke the pedestal on which the figure of christ was raised to the level of the altar-screen, though this figure remained miraculously suspended in the same place--for this is what is reported. " . that it carried off the curtain covering the panels of the altar and threw it to the ground without breaking or melting any of its rings, which were made only of copper, and without displacing the rod above the ring-bolts on which they hung. " . that it upset the oil-lamp burning before the high altar. " . that it broke into two pieces the stone upon which the priest consecrates the host. " . that it tore into four pieces the card on which the canon of the mass was printed. " . that it tore the altar-cloth and the cloth which was over it--both of them in an extraordinary way, namely, in the form of a cross of st. anthony. " . that the high altar was seen to be burning. " . that it burnt a part of the communion-cloth and of the tabernacle, upon which it formed several black waves. " . finally that it imprinted upon the altar-cloth the sacred words of the consecration, beginning with _qui pridie quam pateretur_, and going down to _hæc quotiescumque feceretis in mei memoriam facietis_, inclusive; only omitting those which are usually set forth in special characters, namely, _hoc est corpus meum_; _et hic est sanguis meus_. "what, i repeat, can you expect unphilosophical minds to make of so astonishing an affair as this? how account for the choice, the discernment, and the mysterious preference for some words over others. which shall we consider the privileged words--those taken or those left? what is one to think of the extraordinary way in which the figure of the saviour was left hanging? and of that strange imprint of the cross? how resist all the thousand delusions and uncertainties and fears the entire thing calls forth? "i wonder whether the unfortunate balthasar, when his eyes beheld the terrible sight of the unknown hand inscribing upon the walls of his banqueting-room the announcement of his doom, can have been a prey to a greater variety of fears and tremors than those who witnessed or who even heard of the effects of the lightning at lagny. for no doubt was felt that they were the outcome of supernatural forces--spirits alone could have worked these marvels; it was a question only whether they were the work of evil spirits or good. some believed them to be the work of good spirits, deducing this from the omission of the words, _hoc est corpus_, etc., which they set down to a spirit of reverence for the sacred mystery. "others believed them to be the work of evil spirits, but here again there were different theories. some held that bad spirits had perpetrated these things out of sheer wickedness, wilfully profaning the holy objects and suppressing out of contempt, or some other evil design, the words so essential to the mystery; others held that mere imps had been at work, actuated more by mischief than sinfulness, and wishing only to give amusement to themselves and others by the quaintness of their pranks. i myself do not share any of those theories." lamy's narrative proceeds to an examination of all the effects recorded, which he explains in the simplest way in the world, without having to have recourse to any occult causes. he comes, finally, to the last of all and the most extraordinary. "not wishing to put trust in anything but my own eyes, i went to the church myself, and the effects of the lightning i saw there repaid me for the trouble. "i examined carefully the new imprint on the cloth. i found it very clear and fine, the letters well finished, but the ink a little indistinct, perhaps i should say faded. as m. le curé de saint-sauveur (who was kind enough to show me everything) assured me that at the moment of the lightning the three-leaved card which contains the canon of the mass lay between the altar-cloth and the small mat upon the stone on which the consecration takes place, folded in such a way that the printed side was next to the altar-cloth, i compared the characters printed by the lightning with the original lettering, and found that they corresponded exactly, except that they went from right to left, backwards, so that they had to be read with the help of a mirror, or else through the cloth from behind. "i observed that the words which the lightning had not printed on the cloth, but had omitted, were done in red letters on the card, and were no more favoured nor ill-used than certain other marks without any significance also printed in red upon the card, and leaving no trace upon the altar-cloth." the author proceeds to explain the so-called mystery, ascribing it to the difference between the two inks--the thick black ink and the thin red ink. he examines also into the other phenomena, explaining them in the same way, like the sagacious and enlightened observer he was. it is clear, then, that the study of the phenomena of lightning is no new thing, and that it has been followed conscientiously for many centuries. in the case of the canon of the mass printed by the lightning at lagny, the reproduction was by contact and pressure--it was not a case of reproducing distant objects as though by photography. here is another case hardly less remarkable. the narrative is from the pen of isaac casaubon, in his _adversaria_:-- "on a summer's day, about , while divine service was in progress in the cathedral at wells, two or three thunderclaps were heard, of so terrible a nature that the whole congregation threw themselves down on the ground. lightning followed at once, but no one was hurt. the astonishing thing about the affair lies in the fact that crosses were afterwards found to have been imprinted upon the bodies of some of those present at the service. the bishop of wells assured the bishop of ely that his wife told him she had a cross thus imprinted upon her; and that on his being incredulous, she had shown it to him, and that he himself found afterwards that he, too, was thus adorned--on his arm, if i remember right. some had it on their breast, some on their shoulders. it is from the bishop of ely i have these facts, which he tells me are well authenticated." what shall we say now of the photographing of a landscape on the inside of the skin of sheep which had been struck by lightning? the record of this seems well authenticated. in , near the village of combe-hay, four miles from bath, there was a wood composed largely of oaks and nut trees. in the middle of it was a field, about fifty yards long, in which six sheep were struck dead by lightning. when skinned, there was discovered on them, on the inside of the skin, a facsimile of part of the adjacent landscape. these skins were exhibited at bath. this record was communicated by james shaw to the meteorological society of london at its session of march, . here are his own words:-- "i may add that the small field and its surrounding wood were familiar to me and my schoolmates, and that when the skins were shown to us we at once identified the local scenery so wonderfully represented." andrès poey tells us of these other curious cases:-- in the province of sibacoa, cuba, in august, , lightning imprinted on the trunk of a big tree a picture of a bent nail, which was to be found, bent in the opposite direction, embedded in one of the upper branches. on july , , in a plantation at st. vincent in cuba, a palm tree was struck by lightning, and engraved on its dried leaves was a picture of pine trees which surrounded it at a distance of nearly yards. dr. sestier tells us that after the meeting of the american association, a person was killed by lightning while standing up near a whitewashed wall, and that his silhouette was fixed upon the wall in a dark colour. with such facts before us, we seem bound to believe in the existence of some kind of especial rays, ceraunic rays, emitted by lightning, and capable of photographing alike on the skin of human beings, animals, and plants, more or less distinct pictures of objects far and near. decidedly, we have much to learn in this as well as in all the other branches of knowledge. footnotes: [ ] academie des sciences, août , . [ ] "conjectures physiques sur les plus extraordinaires effets du tonnerre." paris, mdcxcvi. printed by william clowes and sons, limited, london and beccles. [illustration] * * * * * transcriber's note: obvious typographical errors were corrected. duplicate title at the beginning was deleted. hyphenation and accent variants were changed to the most frequently used in the original. those occurring equally were retained (for example: rainwater and rain-water). note: project gutenberg also has an html version of this file which includes the original illustrations. see -h.htm or -h.zip: (http://www.gutenberg.net/dirs/ / / / / / -h/ -h.htm) or (http://www.gutenberg.net/dirs/ / / / / / -h.zip) u. s. service series. the boy with the u. s. weather men by francis rolt-wheeler with seventy-two illustrations from photographs [illustration: publisher's logo] boston lothrop, lee & shepard co. [illustration: the funnel of death. photograph of a tornado in kansas, taken less than a minute before it struck the point where the camera had stood. (this is one of the best tornado photographs in the world and has not been retouched.) _courtesy of geo. s. bliss, u.s. weather bureau, philadelphia, pa._] published, september, copyright, by lothrop, lee & shepard co. all rights reserved the boy with the u. s. weather men norwood press berwick & smith co norwood, mass. u. s. a. preface the savage fury of the tempest and the burning splendor of the sun in all ages have stirred the human race to fear and wonder. all the great stories and legends of the world began as weather stories. the lightnings were the thunderbolts of jove, the thunder was the rolling of celestial chariot-wheels, and the rains of spring were a goddess weeping for her daughter, nature, held a captive in the icy prison of winter. we know a great deal more about the forces of the weather than the ancients did, yet we know but little still. the hurricane does not come unheralded to our shores, the freezing grip of a cold wave is forecast in time to enable us to fight it, the lightning is tamed by the metal finger we thrust upward to the sky. but the tornado sweeps its funnel of death over our cities in spite of all we do, the cloudburst falls where it will, and rivers rush to flood with the melting of the snows upon the distant mountains. there is no battle greater than the battle with the weather, which is both our enemy and our ally. death and disaster are the price we pay for ignorance. great victories have been won by knowledge. galveston's sea-wall dared and defeated the hurricane, the levees of the mississippi have held captive many a flood, and our myriad spears of defence have snatched at the power of the lightning flash and hurled it harmlessly to the ground. we are not slaves to the demons of the weather, now--not as we once were. the united states weather bureau, day by day, draws closer and closer the chains which bind the untrammeled violence of sun and storm. high, high in the atmosphere, is a world all unexplored, where no man can dwell; where, as yet, no human-made instrument has reached. this unknown world calls for explorers, it calls for adventure, it calls for daring and patient work. it is for man to tame the forces of the sky, and tame them he must and will. to show how much the weather bureau is accomplishing, to depict the marvels of its work, to portray the ruthless ferocity of the forces as yet uncontrolled and to reveal the gripping fascination of this work, in which every american boy may join, is the aim and purpose of the author. contents chapter i page adrift on the flooded river chapter ii the home of the rain chapter iii putting the sun to work chapter iv the massacre of an army chapter v the runaway kite chapter vi defeating the frost chapter vii clearing an innocent man chapter viii in the whirl of a tornado chapter ix the trail of the hurricane chapter x struck by lightning illustrations the funnel of death _frontispiece_ facing page futen, god of the winds there, before the flood, stood anton's house the edge of a tornado's whirl in the path of the lightning in the path of the tornado facing a climb on snow-shoes twenty-five-foot drift a mile long forest ranger in idaho observer among the quaking aspens no peak too lofty for a weather station wall and upright sun-dials the first line of defence against the tempest solar halo seen in the united states solar halo seen in russia the dust that makes red sunsets an army destroyed by weather types of upper clouds types of lower clouds types of rain clouds kite-flying--the new way kite-flying--the old way the explorer of the upper air snow-flakes from the upper regions of the air snow-flakes from the middle regions of the air snow-flakes from the lower regions of the air ringing the frost alarm fighting frost in an orchard--night fighting frost in an orchard--dawn bucking a snow-drift clear the way! measuring the blizzard's rage signals on delaware breakwater signal tower for storm warnings thermometers and rain-gauge pencil drawings of tornado in dakota true tornado forming in advance of a dust whirl tornado dropping towards ground tornado wrecking a farm tornado whirling sidewise galveston causeway before and after the hurricane shot from the gun of a hurricane scale of winds, illustrated by clipper ships branch lightning and multiple flash eiffel tower struck by lightning lightning flash striking building mules carried in the air three miles from their stable grand piano picked up by a tornado and dropped in a cow-pasture the boy with the u. s. weather men chapter i adrift on the flooded river "what is it, rex, old boy? what are you after? somebody else in trouble, eh?" ross looked down through the pouring rain at his airedale, who was pulling at his trouser leg with sharp, determined jerks. the dog looked far more like a seal than a terrier, his hair dripping water at every point, while a cascade streamed from his tail. the boy was every whit as wet. here and there, through the slanting lines of rain, could be seen the smoky gleams of camp-fires, around which, shivering, gathered the hundreds of people who had been rendered homeless by the flooded mississippi. the lad turned to his father, who was bandaging a child's wrist, which had been broken during the work of rescue. "it looks as if i ought to go, father," he suggested, "that's if you don't mind. by the way rex is going on, there's something up, for sure." "go ahead, then, son," his father agreed, "the dog's got sense enough for a dozen. watch out for yourself, though, and don't get foolhardy," he added warningly, as the lad disappeared in the darkness; "you've got to be right careful when the mississippi's in flood." "i'll watch out," ross answered reassuringly, as he started off with the dog, and, a moment later, the glow of the camp-fire was blotted out in the falling rain. "this is your hike, rex," announced the lad; "you lead and i'll follow." the airedale cocked up one ear on hearing his young master's voice, then, putting his head knowingly on one side, as if he understood every word that had been said, he trotted to the front and splashed through the pools of mud and water, his stump of a tail wagging with evident satisfaction. ross was used to all kinds of weather, but a downpour such as this he had never seen before. the rain fell steadily and relentlessly, with never a pause between. the night was too dark to see clearly, as the sheets of water were swept before the wind, but their force was terrific. several times the boy had to turn his back to the driving storm and gasp, in order to get his breath. "where are you going, old boy?" again queried ross. the terrier paused, shook himself so that the drops flew in all directions, looked up in his master's face, gave a short sharp bark and trotted on. ross leaned down, patted the dog, and followed. by some instinct of his own, the terrier was keeping to a submerged road, though how he managed to remain on it was beyond the lad's comprehension, for the night was as dark as a wolf's throat and the path was under water half the time. suddenly the dog stopped and looked back as though for guidance. before them was a swirl of water. in the darkness it was impossible to say how deep the wash-out might be, or how wide. ross hesitated. his father had warned him against foolhardiness, and here he was facing the crossing of a swift current of unknown depth on a pitch-black night. should he venture? rex barked, a short excited "yap" of urgency. "i'll go as far as i can wade, anyhow," said ross in response; "maybe it isn't so deep after all. i'm not particularly anxious to have to swim." the terrier watched his master, and as soon as the boy started to cross the wash-out in the road-bed, the dog plunged in. the current swept him down rapidly, but rex was a powerful swimmer and the lad had little fear for him. it took all his own strength to keep him from being swept off his feet, but the break in the road was not more than six yards across, and the boy was soon safe on the other side. he whistled shrilly and a moment or two later, rex came bounding up and jumped on his master with clumsy delight. then, with another cock of his head, as though to make sure of himself, he took up his position in front of the lad and trotted ahead. how it rained! the water had gone down ross's neck and inside his shoes, so that they sloshed and gurgled with each step. little rills of water trickled coldly down his back and legs. the wind was dropping, so that the rain drove less in slanting sheets, but it seemed to pelt down all the more heavily for that. even in the darkness, ross could see the plops, where the drops fell, standing up from the surface of the flooded water like so many spiny warts. it was lonely, even with rex for company, so dark and so wet was the night, and ross was glad when the glow of a fire in the distance told him that he was approaching an encampment, probably, he thought, that of another group of settlers who had been driven from their flooded houses and were shivering, homeless, in the night. when he arrived near enough to take in a full view of the scene, however, he found it very different from what he expected. true, there was a large camp-fire burning, such as the one he had left, and around it were gathered a number of women and children, cold, hungry and wet. a rough, lean-to tent, made of a sheet of tarpaulin, had been stretched in order to try to keep off the worst of the downpour, but no shelter availed. a few steps farther, on the river bank, was a scene of excitement and commotion. a large gasoline torch flared into the night, defying the efforts of the storm to extinguish it, and by the light of this torch, scores of men were working busily, almost crazily, repairing a cave-in that threatened every moment to make a new break in the levee. "who's that? another man?" rang out a clear, strong voice, as ross came near. "good! we need men badly, right now." "it's me, mr. levin," answered the boy promptly, as he recognized the voice, and hurried into the circle of light, "it's me, ross planford." "howdy, ross," came the greeting in reply, "all your folks safe?" "yes, sir," the boy answered. "it was a narrow shave, though. rex got us out just in time." "good dog, that," was the terse comment. "i always did like airedales. well, ross, it's time you got busy. bring me a pile of empty bags from dave's sugar-mill, there." "yes, sir," answered the lad, and darted off towards the factory. rex followed at his heels, and when, staggering back with his load, ross dropped one of the empty bags, the terrier picked it up and came trotting after, carrying it in his teeth. "i dropped one, mr. levin," said the boy, "i'll go right back for it." "you don't need to," replied the weather forecaster, "your pup retrieved it for you. see?" and he held up the missing bag. the engineer in charge of this section of the mississippi, whose duty it was to guard the artificial banks or "levees" of the river, was working on the main break in the levee, with a huge gang of men. in this crisis, one of the planters, who formerly had been the local weather bureau official, had offered to take charge of the new threatened source of danger. at his request, ross busied himself for some time in bringing empty bags, which were then filled up with sand and dumped into the cave-in. being in bags, the washing action of the water could not carry away the sand, and the gradually crumbling bank again was made firm. after a while, however, ross again felt the dog tugging at his trouser leg and he realized that the mission on which he had started had been forgotten in the excitement of mending the crack in the levee. "that's right, i was forgetting," said ross aloud, and he appealed to his friend the forecaster. "mr. levin," he said, "can you spare me for a bit? i left father's camp because we thought there was something wrong. rex kept on tugging at my leg, as though he wanted to lead me somewhere. he's worrying again, now. do you mind if i go ahead and see?" "not a bit," was the hearty answer, "a dog doesn't generally go on like that without some reason of his own. i'll send one of the roustabouts with you, if you like?" "no, thanks, sir," the lad answered, "if i really need help i'll come back and ask for it. right now, i just want to find out what it is that's bothering rex." "off with you, then," said the other, kindly, "but go easy. oh, and ross!" he added, "if you're going down stream, just keep your eye on the levee, won't you? if you see any signs of trouble, get back on the double-quick. don't try any of that story-book business about sitting down with your back to a hole in the bank. that sort of thing may be all very well in holland but it wouldn't work with the mississippi." ross grinned, remembering the story. "all right, mr. levin," he answered, "if i see anything that looks like trouble, i'll come right back and report." for a short distance down the river, rex led the boy along the levee, then he branched away from the river bank towards a large stretch of low-lying land. this was familiar territory to ross, for one of his best chums, a little crippled lad, lived in a house in the hollow. "i hope anton got out all right!" suddenly exclaimed ross, half aloud, as the thought swept over him of the plight in which his chum might have been. this fear became more poignant when, as rex reached the path that led up to anton's house, he turned up it, half trotting and half splashing his way through. ross followed him closely, breaking into a run himself, as the dog galloped ahead. there was a slight rise of the ground, near the wood below which lay the house, and from this shallow ridge the rain ran off in muddy gullies that were miniature torrents. this ridge reached, ross looked down over the hollow toward the house. the entire plantation was a sheet of water, and, in the middle, still stood the house, the water half-way up its first story. rex set his forelegs firmly on the ground and barked fiercely, with loud, explosive barks that rang through the storm like the successive discharges from a small cannon. then, out of the rain, faintly through the distance, a shout was heard. it sounded like a boy's voice. "it's anton!" cried ross. "he's been left behind! and that house is apt to go to pieces any minute!" the first thought that sped across his mind, as he peered through the darkness to the dim outlines of the white house, was to hurry back to the forecaster for help. even as this thought came to him, however, ross realized that such action might be of little use. already the waters of the flood, swirling around the house, undermined it every moment, and it would take a long time to portage a boat all the way from the levee to the hollow, now in the wild sweep of the torrent. then ross remembered that, a couple of years before, when a wet summer had caused a considerable quantity of water to gather in the hollow, forming a small lake, anton and he, together with the rest of the boys, had built a rough boat. they had played the whole story of "treasure island" in this craft, anton, with his crutch, taking the part of long john silver. the boat was a rough affair, as he remembered it, something like an ancient coracle, but it had been water-tight, at least. perhaps it would be sea-worthy, still. at least, it was worth a trial. turning his back on the building that was islanded by the flood, ross raced as fast as he could to the little block-house on the ridge that the boys had built two years before, near which he hoped to find the boat. twice he stumbled over a root in the darkness and fell headlong into the mud and water. still, as he could not be any wetter than he was already and as he did not hurt himself, a few falls were no great matter. on the ridge, fast to the block-house, to which level the water had not yet reached, ross found the boat. moreover, to his great delight, he saw that anton had been patching it up, so that it was now more serviceable than ever. it was a different matter, punting this home-made boat around the waters of a pond on a calm summer's day, and striking out with it in a blinding storm across the flooding lowlands of the mississippi river. again his father's warning not to be foolhardy, came to ross's remembrance, and, together with it, the weather bureau man's caution. none the less, the boy knew well that his father would never bid him hold back from a piece of work that was dangerous or difficult when life was at stake. the boat was half full of water from the pouring rain. ross bailed it out with a cocoanut-shell to which a handle had been affixed, evidently a home-made bailer of anton's manufacture, and, as soon as it was clear of water, dragged it to the border of the current and launched it. the craft floated crankily, it was true, but it floated, and, so far as the boy could tell, it seemed fairly water-tight. jumping out again, ross swung himself into the water and shoved the boat along beside him. he saw the value of wading as far as possible, for he knew that, as long as his feet were on the bottom, he could govern his direction. to what extent he might be able to stem the current by the use of oars in a boat of that character, he did not know. rex, however, was convinced that the boat had been secured expressly for him, and, as soon as ross came near enough to the shore, the dog bounded through the shallow water in long leaps, swimming the last few feet, and put his paws on the gunwale. ross picked up the terrier and heaved him into the boat. rex gave a snort of satisfaction, shook himself so that he sent a trundling spray of water clear in his master's face and then took his post in the bow of the boat and set himself to barking with all his might and main. it seemed almost as though he really knew that he was at the head of a rescue expedition and wanted to convey the information. when at last rex ceased barking, which was not for some minutes, ross gave a shout. instantly, at one of the upper windows, something white appeared. in the darkness the boy could not tell what it might be, but he guessed, and rightly, that it was anton's shirt, and he heard again, though faintly, the answering call across the river. "keep up your nerve, anton," he yelled, through the storm, "i'll be over there in a minute." faintly, again, came the answering cry, "hello, ross! is that you? i wondered who it was that was coming." the slow progress made by shoving the boat along, however, was not at all to rex's liking. he turned and looked at his master doubtfully, then barked again. to his disgust, in turn, the boy found that the slope of the hollow curved away from the house a great deal. he was tempted, time after time, to jump into the boat and pull straight across, but he knew that if the force of the current drifted him below the house, he could never hope to go upstream against it. his only chance was to make sure that he could reach the middle of the torrent above the house and drift right down upon it. a few yards' extra leeway would enable him to steer his cranky craft to the desired spot. so, though it seemed to him as if he were going away from anton, and though, indeed, he was now so far away that the crippled boy's shouts no longer could be heard, ross stuck to his intentions, and, still wading, pushed the little craft up-stream. rex protested vigorously. he ran back from the bow and looked into ross's face with a reproachful and almost angry bark, as much as to say: "you silly! can't you tell what i brought you here for?" the boy knew better than the dog. "lie down!" he ordered sharply. rex, understanding in a doggish way that he was in the wrong somewhere, went back to his post in the bow, where he stood dejectedly, his tail no longer at the jaunty angle than it had been before. at last ross felt that he had reached a point high enough up the flooded bank to justify him in the attempt to get across. he jumped into the home-made skiff, and, setting his strength to the clumsy oars, began to pull with all his might. he had not over-estimated the force of the current. as the light craft got into the swirl, the black water caught it like a feather. ross pulled with all his might, but the banks slipped by as though he were in tow of one of the river steamboats. never had the boy tugged at a pair of oars as he did now, and never had he so wished for a good boat and for real oars. he was only two-thirds of the distance across to the house when it came into sight, only a little distance below him. he would not reach it! with the energy of despair, ross tugged on his oars, every muscle of his body tense with the strain. rex, divining the struggle, stood silent, not looking forward over the bow as he had been doing, but watching his master as he toiled with his oars. then, out from the darkness, shot the long black menace of a floating tree trunk. straight for the boat it sped. from the window, now close at hand, came a cry: "look out, ross! look out!" ross saw the danger. he knew, if he backed water, or halted long enough to let the tree go by, he would infallibly be swept past the house and all hope of rescuing anton would be gone. he saw, too, that if the tree struck the frail boat, it would sink it as a battleship's ram sinks a fishing-boat in a fog at sea. he might win through, but if it struck-- the oars creaked with the sudden strain thrown on them. on came the tree, but, just as it was about to strike the boat, it checked and turned half over, as the projecting stump of a broken bough caught on the ground below. for an instant, only, the tree halted and began to swing. the halt gave a moment's respite, one more chance for an extra pull with the oars. the big log, thus poised, made a backwater eddy on the surface of the river, checking the force of the current. ross reached back for another stroke, with every ounce of his muscle behind it. the tree turned over sullenly and charged down the river anew. yet that brief pause, that second of delay, that back-water ripple as the log hung in suspension, had given ross just the advantage that was needed. the branches of the upper part of the tree swept round, one of them catching the stern of the boat and almost pulling it under. peril had been near, but victory was nearer. the bow of the boat touched the wall of the house. the current, swirling around the rocking walls, carried the boat to the lee of the house, and, as it spun round, ross leaped on to the porch, chest-deep in water, and took a quick turn with the boat's painter around the corner post of the porch. the torrent took his feet from under him, and swept him down-stream, floating, but ross held a firm grip on the rope and dragged himself back. there, clasping the post tightly, he got back his breath. after a moment's groping he found the railing of the porch. by standing on this and holding fast to the corner post, he was, for the moment, out of danger. he had reached the house, but how was anton to be rescued? the crippled boy was on the second story and the upper window could not be reached from the boat, even if the boat could have been held in place directly under it. fortunately, ross knew the arrangements of his chum's house as well as he did those of his own. stepping gingerly along the porch railing, he came close to the window of the sitting room. the glass was still in the window frame, but as the front door was swinging wide open, though partly choked with débris, ross knew that the sitting room must be full of water. he kicked the glass out and then, with a heavier kick, broke away the middle part of the window-sash. the water did not come quite to the top of the window frame, sure evidence that there was room for air between the water and the ceiling. taking a long breath, but with his heart knocking against his ribs, ross dived through the broken window. it is one thing to be able to swim and dive, it is another to plunge through a splintered window-frame into a dark house in the middle of the night, with a flood roaring on all sides. was the door into the hall open? on that, success depended. the boy turned sharply to the left as he came up to the surface and took breath. his hand struck the top of the door jamb. the door was open, but the casing was only three inches above the water. ross dived again through the door, and, under water, turned to the right. one swimming stroke brought him to the staircase and he rushed up the few steps at the top to the room above. there, by the light of a single candle, he saw anton, his eager eyes shining out of his pale face. the crippled boy hobbled across the room on his crutch and grasped his chum tightly by the shoulder. he was trembling like an aspen-leaf in the wind. "scared, anton?" said ross. "i'm not surprised. you've a good right to be." "i wasn't so scared," the younger lad replied, with the characteristic desire of a boy not to be thought cowardly, "i just got to wondering, that was all." "wondering if any one was going to come for you?" "yes." "how did you get left behind, anyhow?" queried ross. "oh, it was my own fault, all right," the crippled lad replied. "it was all because of the dog. you know, ross, lassie had pups, last monday." "no, i didn't know about it," responded the older boy. "why didn't you tell a fellow?" "i haven't seen you since," anton explained. "well, when the levee broke and the water commenced to come into the house, dad and uncle jack went and got the two boats we always keep on the river. dad picked me up and carried me down on to the porch. i heard him call to uncle jack: "'you go ahead and get clara; i've got anton safe with me.'" "then you were with him, weren't you?" queried ross. "sure i was. just as i was getting into the boat, though, i thought of lassie and her puppies and i went back to get them. i called to dad and said: "'i'm just going to fetch lassie, dad, and i'll go in uncle jack's boat.' "so, dad, he called to uncle, saying that i was to go with him. his boat was pretty well crowded up, too. back i went to get lassie. as soon as i'd picked up the pups, lassie was willing enough to come along. the water was running over the floor and made it slippery. my crutch slithered on the wet wood and i tumbled down. it was pretty dark, and i had a job finding the four puppies again. when i did gather 'em up and started for the porch again, uncle jack was gone." "without you?" "he thought i was with dad, and i suppose dad was sure i was with uncle jack." "they ought to have found out and come back after you as soon as they got together." "i thought of that," the crippled lad answered, "and that's what i expected would happen. i suppose, though, they didn't land at the same place, and so each bunch thinks i'm with the other and isn't doing any worrying." "it's a mighty awkward mix-up," declared ross. "there's no saying what might have happened to you if rex hadn't been on the job." "was it rex who brought you here?" "it sure was," ross replied, and he described how the terrier had pulled him by the leg and insisted on his coming over to the house in the hollow. "where's rex now," queried anton, "down in our old boat?" "yes, he's down there, keeping watch, good old scout," answered ross. "he ought to be satisfied now, he certainly made fuss enough to bring me here. but, look here, anton, how are we going to get you out? you don't swim." "no," answered his chum mournfully, "i can't swim." "if there was room enough down that stair," said ross, thoughtfully, "i could take you on my back, but we'd never get through that door, and the window would be even worse." "i'd been thinking of that," anton answered. "i wondered how dad would get me when he found out that i wasn't with uncle jack and came for me. so i made a long rope out of strips of my sheets." "what's the good of that?" "well," said the younger boy, "i was wondering if i couldn't get out of the window. my arms are awful strong, you know, ross." "yes," the other agreed, "you've plenty of muscle there." "i thought if i could drop that line out of the window, dad could grab it and hold the boat there. then i could chuck down lassie and the pups in a basket--i've got the basket--and slide down the rope of sheets into the boat." ross thought for a minute. "i don't see why we couldn't do that now," he said. "suppose we tied a piece of wood to the end of this rope of sheets, so that it would float, the current would curl it around the corner of the house so that i could get hold of it from the boat. if your end of the line was made fast up here, i could hand over hand the boat right under your window, the way you say. why, i could get you out without any trouble at all! let's see how it goes." suiting the action to the word, ross tied one end of the line of sheets around the hinge of the door, passed it through the window, and, to the other end, tied a spare crutch. then he leaned out of the window and watched it. the current snatched the crutch down and, as ross expected, swung it around the corner of the house. "fine," said the lad. "we can work that all right. i'll have you out of here in two shakes, anton. where are the pups?" anton pointed to the bed, on which a basket was lying. "aren't they dandies?" he said. ross took the candle over and picked up one of the pups. lassie growled in a low voice. "all right, lassie," said ross, "you ought to know me." he bent down and patted her. the dog smelt his hand and whacked her tail on the floor in token of recognition, but growled again, nevertheless. "i won't hurt your pup," declared ross, putting the blind little creature back in the basket. "nicely marked, anton," he said, "they look great. but we've got to get busy." he went to the head of the staircase and stared down. "it doesn't look a bit nice," he declared, "i sort of hate to go through there again." "why do you?" queried anton. "you could go down the line and reach the boat that way." "that's an idea," declared ross thoughtfully, then he shook his head. "no," he said, "my weight would swing the crutch out clear away from the house. i'd better go down the way i came up. i can always get back, anyway." he ran down the staircase until the water reached to his chest and then struck out. the water had risen slightly, but he got through the door without any trouble. passing through the window he was not so lucky, for a projecting splinter of glass scraped him as he dived through, making a long but shallow cut in the upper part of his arm. rex welcomed him back with short joyful barks. "i'm not a bit sure," said ross as he patted the dog, "whether it was anton or the pups that you wanted me to rescue, eh? which was it?" for answer rex only wagged his tail and jumped up on his young master. "down, rex, down," ordered ross, "this boat's too cranky for that sort of thing. now, where's that crutch?" in the darkness and the pouring rain it was hard to distinguish anything, but the white gleam of the sheets showed where the crutch was floating. "out of reach," muttered ross in disgust. "just my luck! how am i going to get it?" it was a problem. the crutch was floating on the current above twelve feet beyond the reach of the boat's painter, let out to its utmost length. by stretching out with one of the oars, ross was about four feet short. just four feet, but so far as success was concerned, it might as well have been four miles. if he jumped from the boat and swam for it, there was always a chance that the current would pluck him down before he could grasp the line, and then he would not only be in danger himself, but he would have lost all chance of saving his crippled friend. as long as he stayed either with the boat or with the house, there was a chance. it would be foolhardy to lose connection with both. then a brilliant idea struck him. suppose he tied the painter of the boat under his arms, loosed the boat from the post and jumped into the water. he ought to reach the floating line before the current had taken up the slack of the boat's painter. if he left loose a long enough end, with a loop knot, he could fasten the rope from the boat to the line of sheets, and the boat would be made fast. the loop knot would unfasten itself and he could easily clamber into the boat, from the stern, since it was fastened to the line coming out from anton's window. then he could haul up the boat, hand over hand, as agreed upon, take anton and the puppies aboard and strike out straight for the shore. no sooner was the idea conceived than ross proceeded to put it into action. slipping the line around his arms, once, he tied a loop knot in front of his chest, where it would be easy to reach, leaving about three feet of rope hanging, untied the painter and shoved off the boat. the instant that the boat felt the current it yawed around, but, at the same moment, ross jumped out and forward with all his might. the action sent the boat down-stream all the quicker, but in a second's time, ross had grasped the floating crutch and had taken a turn with the loose end of the rope around it. he was not an instant too soon, for a sharp tug at his chest, followed by a sudden release of the weight, told him that the loop knot had untied itself, as he hoped it would. holding on to the sheet line with one hand, he rapidly passed the rope once under and through. ross had not learned his knots from the mississippi sailors for nothing, and as the boat came to the end of its tether and jerked on the line, the boy had the satisfaction of seeing the knot tighten. with the strain off, it was easy to take another half-hitch around the line, and the knot was secure beyond peradventure. he climbed aboard, raised a cheery cry to anton, and commenced to pull the boat hand over hand along the line of sheets. it was only a moment before the little craft was bobbing on the flood, immediately beneath the window. "let's have the puppies first," cried ross. anton's head disappeared from the window, and reappeared in a moment. "catch!" he cried and held out the basket. ross balanced himself as best he could and caught the falling basket. it was not more than a five feet drop and the basket landed squarely in his arms. he placed it in the boat. loud barking overhead announced that lassie was displeased and worried over the sudden departure of her offspring. "how am i going to get lassie out?" queried anton. "i'd never thought of that. she'll strangle if i let her down by the collar." "that's easy," ross called back. "tie a bit of string to her collar, chuck me the end of the string, and then throw her into the water. it won't hurt her, and i can easily haul her aboard." "all right, then," the other answered, "get the boat out of the way." "chuck me down the end of the string first," warned ross, and, as he spoke, a ball of stout twine fell in the boat. "out with her now," he continued, slackening away on the line, so that the boat was no longer directly out of the window. there was a moment's pause and then the big dog appeared in the opening, struggling in anton's strong, if clumsy, grasp. she clawed at the window-sill, not understanding what was happening, but anton gave her a push, and half turning as she fell, lassie struck the water all of a heap. the instant she was afloat, however, her natural swimming instincts asserted themselves and she started for the shore. "here, lassie!" called ross, with a whistle, and pulled gently on the string that was fastened to her collar. the dog felt the pull and turned around, swimming directly for the boat. ross stooped down and lifted her in. the mother immediately smelt the puppies and scrambled along the bottom of the boat to the basket. she smelt her children, nosed them over, one by one, then, satisfied that everything was all right, muzzled against rex, and lay down contentedly. this feat accomplished, ross pulled the boat under the window again. "now, anton," he called, "it's your turn." "all right," the younger lad replied, "i'm coming." ross heard him drag a chair to the window, to make it easier for him to clamber out. just at that instant, there came a cracking from the front of the house, the corner-post of the porch, to which the boat had been fastened less than five minutes before, fell with a crash and the front of the house crumbled. there was a moment's pause, and then the whole structure keeled over, away from the boat, and with a rending and cracking of timbers, broke from its foundation. over and over it heeled, and it looked as though it would go to pieces. from the window overhead came a scream of terror. realizing that anton could never save himself, if the house were collapsing, ross leaped for the rope of linen that was hanging out of the window and went up it like a monkey. the chair on which anton had climbed, to get out of the window, had slid to the far end of the room and fallen on the sloping floor, the lower edge of which was now in the water, and the crippled lad was pinned down and unable to get out. the candle had been thrown down on the table and fire was beginning to lick some paper that had not slipped to the floor. ross dashed in, grabbed anton by the arm, picked him up with the "firemen's carry" and staggered up the sloping floor to the window. had the boat suffered in the careening of the house? the line, made of linen sheets, still was taut, and ross, peering out of the window, saw to his great delight that the boat was still there with all its passengers safe, rex, lassie, and the puppies. a lurch almost threw ross upon his face and the whole house swayed as though with a violent earthquake. the next instant, a sense of motion beneath them told the boys that the house was afloat. "the house has gone, the house has gone! what are we going to do?" cried the crippled boy. "that's all right, anton," the older lad said consolingly, "things aren't so bad. see, it's beginning to get daylight." "but," said the younger boy, "the house is floating down to pirate's cave, that gully where the big rocks are. if we run up against those, the house'll be smashed to bits, sure." ross thought for a moment and saw that his chum was right. "guess we'll have to take to the boat after all, anton," he said, "it's a good thing the house got on a level keel again, when she came afloat." action was needed and that immediately. ross climbed half-way through the window. "i've got to get that boat up here in a hurry," he said, "the current's swift enough, when you're in that small boat, but this house doesn't float down so fast. it's a mile, anyway, to the gully." so saying, he swung himself out of the window, went down the linen rope and dropped into the water. hand over hand, again, up the rope came the boat until once more it was under the window. meanwhile, by heroic exertions, anton had swung himself up on the window-sill. as the boat came beneath him, the crippled lad swung out on the rope and proceeded to climb down into the boat. he was not a moment too soon. while ross had been bringing the boat to place, the speed of the current had increased and the house, like a clumsy noah's ark, began to sweep swiftly towards the gully of which anton had spoken. "quick, anton," said ross, as the smaller lad hesitated, "we've got to be quick." he cut the boat loose. in spite of his blunt words, it was with the greatest gentleness that ross handed the lad to a seat in the rough craft where they had played pirates during the preceding summer, and settled down to his oars. lassie, finding her master safe in the boat, came and laid her head on his knee, while the shore went slipping by. here and there a barn still stood, the tops of the trees showed above the flood, but all the ground was hidden and the torrent was running like a mill-race. little by little, ross edged the boat towards the shore, not trying to stem the current but rowing diagonally across it. only a few hundred yards separated the house from the gorge which the boys knew as pirates cave. by this time the boat had reached the higher portion of the hollow, where the current slackened. a few strong strokes of the oars and the boat grounded, safely. at that instant the slight lightening of the rain-filled skies showed that, behind the clouds, the sun had risen. the boys turned to look at the house which had been anton's refuge, and which so nearly had been his tomb. as they looked, the structure struck against the uppermost of the rocks with a crash and collapsed as though made of matchwood, while, a second after, into the medley of boards and timbers some uprooted trees came crashing. "you wouldn't have stood much chance there, anton," said ross. the crippled lad put his hand on the older boy's shoulder, with as close an approach to a gesture of affection as boy nature would permit. "i guess i'd have been a goner," he answered, "but for you." chapter ii the home of the rain the gray morning broke over the desolate scene, and anton, hollow-eyed and exhausted, looked at the muddy waters rushing savagely over the place where his home had stood. by the tops of the trees, only, was he able to trace the outline of the fields he had known all his boyhood. "do you suppose it'll ever dry up, ross?" he asked. "of course it will, anton," the older lad said, reassuringly, "you'll see. in a week or two all this water'll run off and you'll forget that the old place ever looked like this." the crippled lad shook his head, as though in doubt. "my books have gone," he said mournfully. the tones were quiet, but a tragedy lay beneath the words, and no one knew better than ross how largely his chum's life lay in the world revealed in his tiny library. the flood would pass away and the fertility of summer would hide every trace of the disaster, but for anton's loss there was no such swift remedy. his books were his closest friends, and now, at one stroke, he was bereft of all of them. "come," said ross, to change the current of his chum's thoughts, "we'll have to make a start. where do you suppose your folks are?" the younger lad turned to his friend with the quick responsiveness and willing resignation often found among those who have suffered a great deal or who are handicapped in life's race. "i haven't the least idea," he said, "they might have gone over to the other shore." "yes," agreed ross, thoughtfully, "that's likely. they'd certainly have more chance of finding help and grub over there. and, talking of grub, anton, aren't you hungry?" "starving," admitted the younger lad. "then i tell you what, we'd better go and hunt up levin." "the chap who used to be with the weather bureau, you mean?" anton asked. "yes." "don't you think that i ought to try to find father first?" queried the younger lad, hesitatingly. "he might be worrying." "it's because of your folks that i think we ought to go first to the camp," explained ross. "we couldn't possibly row right across the flood to the other shore. we've had trouble enough getting as far as this. besides, anton, even if we did get over, we wouldn't know where to look for your people. there's a chance that levin may have heard from them, and if he hasn't, he might send some one with a message. we couldn't do much searching, anyway." in truth, the boys were utterly exhausted. the only member of the party who seemed in high spirits was rex. he frisked about and jumped on the two boys, his tail sticking straight up in the air, as though he were convinced that it was solely through his exertions that lassie and the puppies had been rescued. ross slung the basket, with its living freight, across his shoulders and started off. lassie watched this elevation of her children with manifest uneasiness, but as her master seemed satisfied, there was nothing for her to do but to follow behind, which she did with her nose as close to the basket as possible. nerve-frazzled and tired out, anton pegged away behind. the heavy downpour of rain, which had not ceased for a day and a night, and which had followed upon the heavy rains of the week before, had made the ground as soft as a bog. the crippled lad's crutch sank in so deeply at every step that it was only with great pain that he could keep up at all. still, he struggled along bravely. ross, turning to see how his chum was faring, caught the boy's tense and haggard look, and understood. "look here, anton," he said, at once, "we'll never get anywhere this way. you get into the boat and i'll tow you." "but you can't, you're just about all in," protested the younger boy. "you can't tow the boat with me in it, all the way." "got to!" declared ross abruptly. "it's a sure thing that you're not able to walk there with the ground in this sodden condition. anyway, i won't have to carry the puppies." thankful but still protesting, anton got into the boat and the journey began anew. it was a weary way. ross staggered forward, half-blind with sleep, wading knee-deep, sometimes waist-deep, in the water. the rain had stopped, but the sky was heavy and the clouds hung low. twice anton had to jerk on the tow-rope to jolt ross awake, for, unnoticing, he was heading for deep water. even near the shore the torrent was full of floating debris. the bodies of horses and cattle drifting down the stream told of many impoverished farms and the flotsam was eloquent of wrecked and demolished houses and indicative of suffering. when, after an hour's toil, rescuer and rescued reached the drier land that sloped up to the levee, it was hard to tell which was the more exhausted. to the last, however, ross refused to let his chum bear the burden of the puppies, and he lurched up the road to the place where he had left the gang at work on the cave-in, not so many hours before. it seemed weeks ago. the weather man was still at work. he had been up all night, also, but he greeted the lad cheerily as he came in sight. "hello, boss!" he called, then, as the boy's exhausted state became more evident, "what have you been doing? has anything happened?" "anton was marooned," answered ross in the dull, listless voice of extreme fatigue. "marooned? you mean he was caught by the flood?" as though in answer, anton, toiling heavily and wearily on his crutch, came in sight. "yes," said ross, in the same tone, "he was left behind." "how was that?" the weather man asked sharply. "it wasn't anybody's fault, mr. levin," replied anton, who had heard the last two sentences as he came up, "father thought i'd gone with uncle jack, and uncle jack thought i'd gone with father." "you're not hurt?" "no, sir," the crippled lad answered, "not a bit. ross is, though. he cut his arm diving through the window." the forecaster turned swiftly to the older boy and began examining the injury. "is the house still standing?" he asked. "no, sir," the boy answered, "it's all in bits down by jackson's gully." the weather expert nodded. he knew the lay of the land and had expected the water from the flooded hollow to pour down towards the entrance to the gully. "how did you get out, then?" he asked. anton burst into a glowing account of his rescue in the little boat which the boys had made for their pirate adventures of two years before. even the excitement of the story, however, was not strong enough to keep his overtaxed frame from showing signs of a breakdown and the weather man cut the story short. "i'm going to breakfast later," he said curtly, "but not for a couple of hours. you two had better take a rest now. here, sam," he called to one of the negroes, "bring me a bucket of coffee from your camp-kettle, and fetch some corn-pone. quick now, these boys are famished." "yas, suh! yas, suh!" came the reply, and, a moment later, a bucket of coffee and some corn-bread and molasses were brought. despite their hunger, neither ross nor anton could eat more than a few mouthfuls, and the hot drink was the last straw to their sleepiness. ross fell asleep with an unfinished piece of corn-pone in his hand, and anton's head was nodding. "ain' no more weight than a babby, mister levin," said the laborer, as he picked up the little crippled lad and carried him to a tiny open shed near by, which was the only dry spot to be found in the neighborhood. very tenderly he laid the boy down on a pile of clothes that had been salvaged while the forecaster put his overcoat over ross and laid him beside his chum. "there," said the weather man, "let them sleep a while. they'll be ready for a real breakfast in a couple of hours." though hungry himself, the forecaster waited for three hours before awakening the lads. anton, by nature a light sleeper, awoke easily and was refreshed, but the awakening of ross was a real task. he had been on a severe strain for twenty-seven hours and nature demanded sleep. at last, however, he was roused and after he had plunged his head in a pail of cold water, he felt as full of ginger as ever and ready to start on rescue work all over again. "i'm just going to breakfast," the forecaster announced. "do you want to go along?" "do i? i should say i did! but i'm afraid, sir, that anton and i will eat up everything in sight." "you don't need to worry about that," the forecaster replied, "my men have been hauling supplies all night. why, ross, there are over two thousand people homeless this morning, right around this district. they've all got to eat breakfast, too, so you see even your best efforts won't seriously decrease the supply." "i'm not so sure about that, sir," ross said laughing, "right now i feel as though i could eat all you've gathered for the entire two thousand." "come and try, then," the weather man said, smiling. then, turning to anton, he continued, "likely enough, some of your people will be at the big tent that's been put up. if they're not there, i'll send out a couple of the boys on horseback to cover both sides of the flooded area and pass the word that you're safe." he turned to the older boy. "i've already sent word to your father, ross." the boys thanked him and started down the levee. owing to the continuous work of the night, the cave-in had gradually been filled up, averting a break at this point. the river, turbid and swollen, was swirling by, not more than three feet below the top of the levee. "is the water going down yet, mr. levin?" asked ross. "it looks as though the rain were over." "yes," answered the forecaster, "the rain is over, but the water's not going down yet. it's rising. i'm fairly sure that there won't be any more rain for a few days, fortunately, but i heard from greenville this morning that the river was still rising. we can stand another nine or ten inches, but a foot would be serious. of course, the break that flooded out jackson's hollow, where your place was, anton, is relieving the pressure a little. we've been lucky here. i haven't heard of any loss of life so far. it's a nasty flood, but when the rainfall last week was reported as being so heavy, i knew we couldn't escape trouble." "is it just the rain that makes floods?" anton asked. "just rain," was the laconic answer. "why is it," asked the younger boy, "that there's more rain one year than another?" "if i could tell you that," the old weather forecaster replied, "i'd be the cleverest meteorologist in the world." "but doesn't anybody know why it rains?" "certainly, we know why it rains." "why, mr. levin?" the forecaster pushed back his hat from his forehead and looked quizzically at the white-faced lad. "you really want to know why rain comes? very well, anton, i'll try to tell you. stop me, though, if you don't quite understand. "the earth goes whirling about in space, revolving around the sun, as you know, and it has, like a sort of skin around it, an envelope of air. this air is kept from flying off by the force of gravity. you know what that is?" "yes, sir," the cripple answered, "it's what makes a stone fall to the ground." "exactly. now the air is made up of little particles or molecules, like the stone, only, of course, not so heavy. they're heavy enough, though. how much weight of air do you suppose you're carrying, anton?" the boy looked puzzled. "i don't quite see what you mean, sir," he answered. "suppose you had a pea on your head, it wouldn't be heavy to carry, would it?" "why, no," answered the lad, laughing. "supposing you had a basket of peas, the basket being only about as big round as your head, but six feet high, that would make quite a load, wouldn't it?" "i don't believe i could carry it," was the answer. "and if the basket were sixty feet high, as high as a barn?" "i'd be squashed under it." "and if it were six miles high!" "why," answered anton, "a basket six miles high, even if you filled it up with cotton fluff, would weigh tons and tons!" "well, my boy," said the weather forecaster, "you're carrying on the top of your head a column of air, not only six, but sixty miles high, yes, and more than that! you don't notice it, of course, because you're used to it, and your body is made to accommodate itself to that weight by your tissues being full of air at the same pressure. just the same, not counting the weight which presses on your whole body, amounting to about seventeen tons, you're carrying on your head, at this minute, a weight of over six hundred pounds." "six hundred pounds! as much as if i were carrying three heavy men sitting on my head!" "every bit of it, and more, under certain conditions of the atmosphere. this depends mainly on the circulation of the winds, especially those great movements a thousand miles in diameter known as 'lows' and 'highs' or cyclones and anti-cyclones. in the united states, an anti-cyclone generally means fair weather, and in an anti-cyclone the barometric column rises. that's why a barometer helps to foretell weather some time in advance; it responds to the vast movements of the atmosphere rather than to local conditions. "of course, anton, at sixty miles up, the air is so thin that it has hardly any weight. indeed, we wouldn't know there was any air at that height but for the trail that shooting stars leave. a meteor glows because of friction, and in a vacuum there is no friction. therefore there must be air at the vast heights where shooting stars are first seen." "could an aeroplane get up there?" the forecaster shook his head. "never," he answered. "even six miles up, the air would be too thin to sustain the weight of an aeroplane unless the machine were flying at terrific velocity, and besides, at that height, there wouldn't be enough air for an aviator to breathe. at that, anton, you can see for yourself that if the air is saturated with water vapor--and the cloud-bearing atmosphere is eight or ten miles thick--there is room for a lot of water." "it's evaporation that puts water into the air, isn't it, sir?" asked ross. "exactly. the sun is shining on some part of the earth all the time. there's never a second, day or night, that water is not being evaporated from the seas, from lakes, from rivers and from the earth itself. all the water that is taken up must fall somewhere, and all the rain that falls means that the atmosphere must fill itself with water vapor again. it's a continuous performance, and the water which is being evaporated into the air falls to the earth, sooner or later, as rain, hail, or snow." "if it's all so regular," said anton thoughtfully, "i don't see why we don't get the same amount of rain every day, or at least every season." "it isn't regular at all," the weather forecaster explained. "if climatic conditions were regular, we could forecast the weather several years in advance, instead of only a few days. there are a thousand complicating factors. land and sea are irregularly divided, and as there is more evaporation from the sea than the land, every little curve in a coast line means a disturbance of regularity. then, anton, remember, while the earth is almost a globe it is not perfectly round, so that every variation from the regular curve disturbs the air currents. moreover, the motions of the earth are very complicated. sometimes it is nearer the sun than at other times. it wobbles slightly on its axis. it is inclined to the plane of the ecliptic, causing the seasons, and that brings a new set of factors into the problem. a mountain range or a desert will modify the atmosphere, even the difference between a forest and a prairie is noticeable." "suppose you could figure all those things out, couldn't you foretell the weather, then?" the forecaster shook his head. "suppose you had a thousand marbles of different colors," he said, "and you dropped them from the top of a house to the hard ground below, a rough and rocky piece of ground, could you ever figure out what kind of a pattern they would make? you might measure the size of the marbles and compute how many times they would strike against each other in falling, meantime figuring the angles of direction that each collision would produce. you might measure the resistance of the ground and the elasticity of the marbles and estimate the manner in which they would bounce after striking the ground and the distance to which they would roll. after you had done all that, you might have the right to expect that you would know the pattern that the marbles would make as they lay scattered on the ground. but you would be wrong, for if you dropped those marbles a thousand, yes, a million times, the pattern would be different each time. after tens of billions of experiments you might be able to find the proportion of patterns, but the result would never be of practical use. "it's the same way with the weather. we know well enough how to do the things that would enable us to prophesy a long time in advance what the weather is going to be, but the problem approaches impossibility because there are too many factors that enter into the calculation. we're learning all the time, but it's a big piece of work and needs big men to do it. that's why, anton, i can't tell you why this particular district had more rain this year than it has had for several seasons past." anton, pegging away on his crutch beside the forecaster, looked up at him with an added eagerness in his eyes. "and yet all those things are going on, right where i can see them!" he exclaimed. "yes," the weather man answered. "some men can explore distant countries, and we envy them; some men can explore the greatest and the smallest things in the world with marvellous scientific instruments and we envy them, too; but every day and all day, and every night and all night, we are surrounded by the world of the weather, less explored, less known than even the most remote corner of the earth. why, anton, if you could simply follow all the various causes that brought about this flood that made you homeless, you would have a story of adventure that would make the most daring explorer green with envy." "but you do predict floods and rains, mr. levin," ross put in. "father told me, a week ago, that warnings for this flood had been sent out by the weather bureau." "yes, indeed," the weather man answered. "i should say that weather warnings issued by the bureau save half a billion dollars to the country every year and prevent the loss of hundreds of lives. all those are short-range predictions. very few of them cover much more than a week in advance, except, perhaps, a west indian hurricane which has been reported from the antilles, or a flood on the mississippi which is caused by heavy rains in the upper reaches of the streams flowing into it." "well, that's prophesying, isn't it?" "yes, and no," was the reply. "it's predicting, and it's due to observation. if a storm is moving eastward, with a heavy rainfall, and we've had telegraphic dispatches from all the towns in the west through which it has passed, it's not hard to figure the speed at which the storm is traveling, and it's a sure prediction to tell a city to the eastward of that storm that rain can be expected at about a certain date. or, if there's a high flood wave at st. louis, and we know the speed of the mississippi current, we can notify greenville, vicksburg, and new orleans at what time the trouble is likely to come to them. if no more rain is falling at greenville and the river is going down there, we can notify vicksburg that the flood danger is passing away. that's the observational end of the work, and in that line, the weather bureau of the united states is the best in the world." the weather expert was proceeding to explain in detail the manner of collecting these observations, when suddenly anton clutched him by the arm. "what's that, mr. levin?" he cried. the forecaster looked ahead, then glanced down at the boy with a smile. "what does it look like?" he asked. "why," said anton, "it looks like a circus tent; you know, the one that was here the week before last." "it is the circus tent," the forecaster replied. "when i found that there were a couple of thousand people to be fed and looked after, the only shelter i could think of, that was big enough, was the circus tent. so, late last night, i sent a wagon up there, asking for the loan of the tent for a day or two. and what do you suppose the circus folk did?" "sent it?" "they sent it, with two of their wagons, a lot of food, their cooking kit, and the two cooks who travel with the circus. what's more, anton, you remember those two clowns in the show who were so funny?" "you bet i do!" exclaimed the lad, his eyes shining. "they volunteered to come down and help as waiters. they're doing it, too, and it's a right good thing, for every one around in the place is roaring with laughter half the time. folks work a lot better when they're cheerful." a perfect gale of merriment, which greeted the boys as they neared the tent, showed the truth of the forecaster's statement. he had greatly understated the work of the circus. nearly all the performers were there, busily helping the distressed. "they're a right kindly folk, the circus people, as a rule," remarked the forecaster. "are they all here?" queried anton. "goliath, the strong man, the flying squirrel brothers, androcles, the lion tamer, princess tiny and the rest?" "yes, most of them," the forecaster answered. "goliath is in charge of one of the gangs i've got at work on the river front, and the darkies are so proud of being under him that they're working like fury. the flying squirrel brothers--cracker-jack mechanics, both of them--have been fixing up some tackle and machinery that we needed, but i think androcles stayed back with his lions. i suppose he thought the lions wouldn't do us any good. but if you're not too hungry to wait just for a second--" he paused. "what?" queried anton excitedly. "yes, there they are!" the forecaster answered, gazing along the levee. both boys followed his glance. vast, bulky shadows stood outlined against the distant arkansas shore and the clearing sky. unreal they seemed, until it was evident that they were moving. there, shuffling along with that heavy rolling gait which is unlike that of any other animal in the world, came two colossal elephants. anton shrieked with delight. "elephants! real elephants!" he cried. "oh, mr. levin, i haven't ever seen an elephant quite close." he started off up the levee, but the forecaster called him back. "have your breakfast first, anton," he said; "you've got all day to look at the elephants. they're the best workers i've got. i'd like to have a gang of them at work on the levee all the time." this sentiment was not shared by rex. at the first sight of the huge creatures, lassie had given a low growl. rex stood silent, with a stillness that ross knew to be ominous, and just as the forecaster finished speaking, with an angry growl, he started off to do battle against the elephants. it was a sight to see him, with his hair bristling, rushing forward to dispute the passage of these huge brutes who dared to approach the vicinity of lassie and the puppies. only the sharp commands of ross availed to bring him back, and throughout breakfast he lay well in advance of the tent, watching, and growling loudly every time the elephants passed, dragging the flat sleds loaded with sand bags to the cave-in a few hundred yards beyond. "i've been wondering," began anton, using the expression most often on his lips, "why there are so many floods on the mississippi. why is it? lots of rivers i know don't have these awful floods every year." "i've wondered, too," said ross. the weather man looked at the two boys, then took a cigar out of his pocket. "i can't stay away from the levee very long," he said, "but i need a cigar after breakfast, anyway, and i'll tell you why the mississippi is one of the worst flood rivers in the world and why the safeguarding of the mississippi is the biggest piece of work to be done in the united states. it's a bigger piece of work than the panama canal, and a more difficult piece of work. it means millions of dollars every year to the people of the united states." "why is it such a hard job?" "the mississippi river," the forecaster began, "is two and a half thousand miles in length; the longest river in the world." "longer than the amazon?" asked anton. "yes, a great deal. besides, it is navigable for nearly two thousand miles, clear from st. paul, minnesota, to the gulf. it drains two-fifths of the area of the united states. to put it another way, all the rain and snow that falls between new york state and montana sooner or later makes its way into the mississippi river, except for the rain that is used up by plants and animals or that is evaporated before it reaches the river or that drains by underground seepage to the ocean. so you see what a vast amount of water it must carry. now, boys," he continued, "what kind of banks has the river around here, rock or earth?" "mud!" answered ross, tersely. "right," the forecaster agreed, "and it is mud nearly all the way along. but do you know what mud is?" this was rather a poser, but finally anton said slowly, "it's a mixture of earth and water, isn't it?" the forecaster looked shrewdly at the boy. "you've hit it just right," he said, "mud is earth or soil that has been washed down by the river. that's what makes the bottom of the river so irregular and why it's always shifting. you can see for yourselves, boys, that if the bottom of the mississippi is just made of light mud, light enough to be carried down as muddy water for hundreds of miles, any little change in the current of the river will stir up that mud again and scoop out a hole. if it happens to be near a bank, the bank will be eaten away and, naturally, will cave right in." "about how much mud does the mississippi carry down, mr. levin?" anton asked. "in flood time, as much as a thousand tons a minute will be carried past here." ross whistled. "a thousand tons a minute!" he exclaimed. "why, i should think that would fill up the river in no time." "it would," the weather man answered, "if the river stood still. in flood time, however, the water is flowing rapidly and takes the mud clear down to the delta. that's why there is always so much new land being made at the mouth of the river. you could buy a piece of land under water now, ross, if you wanted, and be quite sure that in twenty years' time there would be land there for a farm." "but a thousand tons a minute!" the boy repeated, "that seems huge!" "it is pretty big," the forecaster agreed, "but i'll show you where it comes from. you know, boys, generally the land slopes down in the direction of the river, doesn't it?" "yes," assented the two boys, "it's supposed to. but it doesn't here. the lie of the land is away from the river." "that's just exactly the point," the forecaster declared. "the banks of the mississippi range in height from about twenty to forty feet above extreme low water. as the river, in times of flood, rises as high as forty to fifty feet above low water, unless there were levees, the river would overflow its banks every spring or flood time." "it does, quite often, even yet," commented ross, looking on the flooded scene around him. "well," said the weather man, "the present levee system only dates back to the end of the civil war, although there were levees built during the first settlement of new orleans, two centuries ago. remember, though, that the mississippi has been flowing down its present bed for several hundred thousand years, with a flood every spring, so that the overflow has had its effect. of course, before the land was broken up by farming, there wasn't as much earth carried down into the river to make mud as there is now. "when the mississippi river, with its heavy sediment, overflows the banks into the swamps, it's easy to see that the current will be slower in the flooded area than in the main bed of the river." "of course," agreed ross, "but what has that got to do with it?" "a great deal," the forecaster replied succinctly. "the faster a river flows, the more sediment it can carry without allowing it to drop to the bottom; the slower it flows, the more readily is the sediment dropped. if you put some mud in a glass of water and keep stirring it with a spoon, the mud will never sink to the bottom. even if you let it stand perfectly still, it will take several days before the finest particles sink to the bottom of the glass and the water becomes clear." "yes," agreed anton, "i've often wondered why." "well," the weather man continued, "if you look closely at the mud in the bottom of the glass, you'll see that the bigger particles are at the bottom and then those a little smaller and so on up, until your top layer is made of a mud composed of particles so fine that you'd have to get a microscope to see them." "i don't quite see why," said ross. "i know bigger things are heavier, but why should a big bit of earth sink more quickly than a small bit, when they're both made of just exactly the same stuff?" the weather man looked at him. "some of these days," he said, "remind me to talk to you about sunlight and dust, and i'll tell you a heap of things you don't know. right now, get this idea in your head. the larger a piece of matter is, the smaller is the surface in proportion to the bulk. a feather of swan's down will float in a high wind, but if you roll that feather into a ball, it will fall. why? you haven't made it any heavier. you've only reduced the amount of surface which was borne up by the air. it's the same way with mud, the bigger pieces sink first because they have less surface in proportion to their weight." "yes," answered ross, "i can see that now." "very good, then," the forecaster continued, "when the mississippi overflowed its banks and the water got out of the current of the main stream, so that it flowed more gently, the sediment began to fall, the larger pieces first and those that were finer until it was only at the most distant point from the river that the finest mud settled. this has gone on, year after year, for thousands of years. "therefore, you see, the lands nearest the river are higher than those farther away. in two big basins, the st. francis and the yazoo basins, the slope and the drainage is away from the river, instead of towards it." "in that case, then," said anton thoughtfully, "the mississippi runs in a groove on the top of a hill." "that's it exactly," the forecaster said, "and some of the most fertile fields lie in the lowlands made of the fine mud at the bottom of this hill. it's just like that hollow where your house was, anton. the flood hasn't done much damage south of here because all the waters poured down into that fine plantation land where your place was located." "what i don't see," said ross, "is why the government doesn't build a really high levee all the way along the river. i don't mean just a few feet higher, but a regular wall 'way higher than the river ever goes. i mean a regular stone wall, twice as high as any levee that we've got now. i should think that would make the river behave." "it would and it wouldn't," replied the weather man. "what are you going to build that wall on? on the ground?" "i suppose so," said ross. "i hadn't thought much about that." "indeed you hadn't," his friend replied. "you've got to remember, ross, that the mississippi doesn't run in a straight line; it bends and twists like a snake. in the bends the current strikes on the outwardly curving bank, and, as you know, the water is always deep there. this causes a rapid caving and erosion of the bank. at the foot of each bend, the main flow crosses to the other side, where it strikes the bank which has become concave there, and eats into that bank just as, a few hundred yards higher, it has been eating into the opposite one." "i know you've always got to pilot a boat first on one side of the river and then on the other," said ross thoughtfully. "you have. and, if you remember, you'll see that it is generally on the side nearest to the concave shore that the boats pass." "yes," agreed ross thoughtfully, "i guess it is." "now, you can easily see," the forecaster continued, "that the river might keep its own channel clean if it flowed straight down with a current of equal strength. but, as the current crosses from side to side, it slackens speed at each of these crossings. therefore, as the current becomes slower, it drops some of the heavier particles of sand or mud, forming a bar at every bend, sometimes so high as to prevent navigation." "that's what the dredges are for, isn't it?" asked ross. "yes. the government has twelve large dredges at work all the time, keeping the navigation channel open." "i don't see, yet, why the stone wall idea wouldn't work," protested ross. "i'm just showing you," was the reply. "if you built your heavy wall on the bank, the water would strike the concave bank at one of these crossings, eat away the earth under the wall and your wall would topple in. then the current would cross the stream, undermine the bank on the other side and your masonry would crumble there, too. so much for the wall." "suppose you sunk that wall, away down deep, below the level of the bottom of the river?" suggested ross. "that might work," the expert replied, "but it would cost more money than the united states could afford to spend. besides, ross, where would you build this wall? right on the bank?" "of course." "but the mississippi is half a mile wide at some places and three miles wide at others. if the river were absolutely walled in, you'd have swift currents at one place and slow in another. then your channel would fill up in the wide places and you'd be as badly off as before." "make it all the same width, then," said ross. "build two-thirds of the whole two thousand miles by some underwater system, constructing the wall under water? if you had ever read of the difficulty of building one lighthouse foundation, my boy, you wouldn't talk so glibly about building huge retaining masses of masonry under water." "suppose it were done, that way, mr. levin," put in anton, "would that settle it all?" "you mean--suppose there was a high masonry wall, making a canal equal in width and height from st. louis to the gulf, would that turn the mississippi into a permanent ship channel? is that what you mean?" "yes." "no, it wouldn't," the expert replied. "what are you going to do with all the little streams that flow into the mississippi? think for a minute, boys. you can see that wherever you narrow the banks, the river channel has got to be made deeper to accommodate the water, hasn't it?" "yes," both boys agreed, "it has." "in other words, suppose that before you put up this huge masonry wall, the flood crest was fifty feet at memphis, then, after the wall was built, the flood crest would be seventy-five or a hundred." "suppose it were," said ross, "the wall would hold it in." "so you think. there are the tributaries to consider. take the yazoo, for example. it flows into the main river until the mississippi reaches the fifty-foot flood level. if you raise the flood level of the mississippi to seventy-five feet, the water in the main river will be twenty-five feet higher than the water which used to run into it at the fifty-foot level, won't it?" ross whistled. "i see where you're coming to," he said; "i'd never have thought of that. go ahead, mr. levin." "with the water in the main stream twenty-five feet higher than in the tributary, due to your retaining wall, boy, instead of the water in the yazoo river flowing into the mississippi, all the water above the fifty-foot level in the mississippi would flow into the yazoo. the yazoo couldn't hold the water, and as the stream backed up, it would overflow its banks. all the low valleys would be flooded in exactly the same way that they were before, only, instead of the floods coming directly through a break in the levee or over the banks of the mississippi, they would come over the banks of the yazoo. that would be true of every small river that flows into the mississippi, and there are scores of them." "what can be done, mr. levin?" "there's only one thing to do," the weather man answered, "and that's to build up the levee system, year after year, steadily and without pause, making allowances for the tributaries flowing into the mississippi and paying especial heed to the rainfall that may be expected in the basin. wherever possible, forestry must be undertaken to keep the slopes from erosion. reservoirs might be built with great profit, from which water could be let down during the low water periods. "when the river channels are accurately adjusted to the amount of rainfall in the river basin, destructive floods will be averted. we can never expect that the mississippi will be absolutely put in harness. the basin is too huge, the amount of water that has to be carried down is too great. permanent dredging and permanent levee construction and repair will always be necessary, and a close co-ordination between the weather bureau and the government and state engineers is a first need in the problem." "just how does the weather bureau come in," asked ross, "the rainfall?" "it isn't only the rainfall of the few days in advance," the forecaster answered, "it's the rain that has fallen before and the rain that's going to fall. if there should be twelve inches of rainfall after a long drought throughout the mississippi basin, it would make comparatively little difference, for all the rain that fell on the dry ground would be sucked up by it and only a very little would flow into the rivers and streams that feed the mississippi. "on the other hand, if there had been slight but frequent rains for weeks and weeks, those twelve inches of water would make an entirely different story. no one, except the weather bureau, would have kept track of the amount of rain that had fallen. "if the ground has been steadily soaked, even by light occasional showers, twelve inches more of rain cannot soak in. therefore, the entire amount of rain will flow directly into the stream channels and thus into the mississippi. flood warnings will be sent out, the height of the flood crest can be estimated, the length of the period of the danger will be known in advance and the proper preparations can be made. if further rain is threatened, that information can be sent out, also, and the entire mississippi valley is completely prepared. that's the true preparedness, my boy, being ready for the foe that you know will come. stupidity or cowardice are the only causes for not being willing and ready to help in time of danger." "what can a chap do?" asked ross, aflame with eagerness. the forecaster looked at him thoughtfully, but before he answered, anton piped in, with a plaintive note in his voice: "is there anything that i could do?" in spite of himself, the forecaster's glance fell on the crutch. anton's intent gaze followed the look and he flushed. a sudden silence fell, the silence of an abiding tragedy from which all eyes are always turned, the tragedy of the disabled. "yes," he said with grave quietness, "there's a great deal that you can do." the crippled lad regarded him steadily. the steady rushing of the mississippi in flood could be heard near by with its thousand miles of menace. "we need work," the weather expert said, at last, "work with the heart behind it. even now, the united states weather bureau has over four thousand co-operative observers, who work without pay, who work with their hearts behind their duties. still, this is all too few." anton's gaze never wavered, but a question crept into his eyes. "yes," answered the forecaster, "you can be one. i know your father well, and i'm sure that he will be guaranty for the instruments. the work of making and recording observations will be yours. never late, never forgetting, never swerving from your duty, your post at the rain-gauge and the barometer will be as honorable and responsible a post as the soldier's at sentry-post or behind the gun." the lad's eyes glowed more deeply. "storms, frosts, and droughts will be your enemies," the forecaster continued, "and they never sleep and never give quarter. the lighthouse-keeper who lets his light go out and permits a ship to go unwarned to wreck upon the rocks is not more guilty than the weather observer who allows disaster to sweep, unwarned, upon his district. it is a trust, anton. can you and will you take it?" the sun broke through the clouds, lighting up the yellow wood of the crutch and turning it into gold. it caught the boy's eye, but with a new significance. no longer would it stand between him and his future. there was something he could do for his country, as well as though he were the strongest and best-built lad in all the neighborhood. life, with its promises of work, opened before him. "i'll take the trust," he answered simply. chapter iii putting the sun to work "fo' the land's sake, mistah anton, what fo' yo' puttin' up that pole on the grass?" "so that i can find the sun, dan'l," the crippled lad answered cheerily, as he held upright the pole, while ross began to fill in the deep hole that the two boys had spent the morning in making. "yo' don't need no pole to find the sun," the old darky answered; "why, yonder's the sun, right up over yo' head." "is it right over my head, dan'l?" the boy asked. the negro, an old family servant, put his hand above his eyes and squinted at the sky. "not right over," he corrected himself, "but mighty near it." "how near?" dan'l looked at the boy with a puzzled air. "ah don't jest know how near," he answered. "that's the idea, exactly," anton rejoined, "i want to know how near." "is this hyar another of your contraptions to tell what the weather's goin' to be like the year after next?" the plantation hand queried, taking advantage of his position as an old family appanage. the instruments had been a point of discussion all summer, for dan'l prided himself on being a weather prophet, though he based most of his predictions on the behavior of the animals and birds around the farm. "this is to tell time, not weather, dan'l," anton answered, "but we'll use it for weather, too." the darky shook his head. "ah don't hold with none o' them glass things with silver runnin' up an' down in their insides, what you calls 'fermometers," he declared, "they're not nateral. ah believe in signs. when, in the evenin', a rooster crows like he's done goin' to bust, ah knows sho' it's goin' to rain befo' mornin'." he ambled up to ross, who was busily shovelling in the earth. "hyar, mist' ross," he said, "let me do that for yo'. yo' ought to ask old dan'l when yo' got a job like that." "that's all right," the older boy answered, readily yielding up the spade, however, and wiping the perspiration from his brow, "it is pretty hot, though." "yo' got no call to be workin' right near noon," the negro protested, "that's not fo' white folks. fust thing yo' know, yo'll be havin' a sunstroke." he shoveled vigorously as he talked, tamping the earth down hard. "it's sho' goin' to be a hot summer," he said, "yo' only find the field-mouse nests where the shadder's thickest. thar," he continued, patting down the earth level with his spade, "that's done now. yas, suh, it's hot." he wiped the perspiration from his forehead with the back of his hand. "you bet the sun's hot," the boy agreed, "but mr. levin told me the other day that we only get a two-billionth part of the heat put out by the sun. did you know that, ross? the sun has heat enough to warm two billion earths as big as this one. even at that, dan'l, the amount of heat we get from the sun would make thirty-seven billion tons of freezing water boil in one minute." the negro's jaw dropped. "yo' not fooling?" he said. "not a bit." "ah's hot," he said. "ah's goin' to boil, soon." "cheer up, dan'l. you'll cool off tonight," suggested the older lad. "nearly everything that takes in heat has to give it out again. the earth, the sea and the dust in the air, all gradually let out some of the heat during the night. if it wasn't for that, everything would stay at the same temperature all night long. that's why it's always colder an hour before dawn than an hour after sunset. "see, dan'l, the earth and the air which take in heat easily and give off heat easily, by the end of the night, have got rid of a lot of their heat. at sea, though, where the water lets go its heat less easily, it is never as cold as on land. the thermometer shows when it's hot and when it's cold." "ah don't hold with none o' them fermometers," the old darky repeated. "that's because you don't understand them," the crippled lad replied. "it's dead easy, though. you see, dan'l, when a thing is hot it gets bigger and when it's cold it gets smaller, that is, most things do." "ah don't see that, nohow," the negro answered. "a red hot stove is just 'zackly the same size as when the fire's out." "no, it isn't, as a matter of fact," the lad replied, "but you can't always see the difference. iron does get bigger as it gets hot. you've seen the steel rails on railroad tracks, haven't you, dan'l." "sho'." "did you ever notice that there's a little crack between each rail? in winter, the crack is quite wide." the negro thought for a moment. "is that the crack that makes a train bump?" "yes, that's it. now, dan'l, on a hot day in summer, you can't see any crack there at all, the rail has expanded or got bigger, and filled it up. on a frosty day in winter, there's a big crack, so big that you could drop a lead pencil between the ends of the rails. that's the difference of expansion on a steel rail between winter and summer." "that's powehful little!" "it's quite a good deal. i'll show you. suppose, dan'l, you had a small rubber ball filled with ink and there was a pipe out of the ball sticking straight up in the air, and suppose you put that little rubber ball in the crack between the rails." "yes?" "then, on a cold day, the rubber ball would have room enough. it wouldn't be squeezed and all the ink would stay in it. on a hot day, as the end of rails came together, they would squeeze the ball and the ink would squirt up. as there wouldn't be anywhere for it to go except through the tube, it would shoot up the tube, wouldn't it?" "sho'." "so that you could tell, by the height of the ink in the tube, how much the rails had come closer together, or expanded. as the only thing that would make them expand would be the heat, you could measure the heat that way, couldn't you?" "ah reckon yo' could." "that's what a thermometer does, dan'l. the little bulb at the bottom contains something that's easily swelled by the heat. in a hot climate, quicksilver is used, because it doesn't boil except at a heat much greater than the air ever gets, though it freezes easily; in a cold climate, they use alcohol because it doesn't freeze except at a degree of cold much colder than the atmosphere ever gets, though it boils easily." "yo' fermometer's got blood in it!" "no, the alcohol is colored, so that you can see it easily, dan'l, that's all. the quicksilver, or the alcohol, is put into a little bulb and up from this bulb there runs a tube. that tube is awfully thin, sometimes a hundred times thinner than a hair. when a tube is as thin as that, even a tiny amount of expansion or contraction will make the quicksilver run up the tube or down. if you watch that thermometer i've got in that white shelter over there, dan'l, you can easily tell when it's hotter and colder. it's nearly always hotter around noon." "it's sho' mighty near noon now," dan'l declared. "how do you know?" "ah can tell that fo' sho', yas, suh!" "how, dan'l?" "by mah own fermometer, mist' ross, an' that's mah inside. right about five minutes befo' noon, thar's a little knock that says 'tap, tap,' dan'l, yo're hungry.' an' that knockin's always right, mistah ross. ah sho' is hungry right at that hyar time." "it hasn't knocked yet, dan'l, has it?" the darky looked thoughtful. "ah hasn't felt it," he answered, "but ah's got a feelin' that ah can expect it now 'most any minute." "well," the younger lad answered, watching the black shadow of the pole as it stretched along the ground almost to his feet, "we'll find out how near right your inside is." he took a piece of steel tape from his pocket and handed it to his chum. "how long is it, ross?" he asked. he bent down eagerly and watched the measuring of the shadow. "four feet, six inches," the older lad announced. the negro looked at the shadow a moment and then burst into a hearty laugh. "what is it, dan'l?" "why, mistah ross, it ain't no use for yo' to measure that! yo' done forgot that a shadder don't stay still." "why not?" "a shadder keeps movin' round. yo' ought to have thought o' that," he added seriously. "we thought of it, all right, dan'l," anton answered. "see, the line of the shadow's already on one side of the tape. try it again, ross!" "four feet, five inches and three quarters," came the reply. "what fo' makes that shorter?" queried the negro. "dan'l," said the younger boy, reprovingly, "why don't you use that thick head of yours a little? when you get up in the morning, isn't your shadow longer than it is in the middle of the day?" "sho', it stretches away off yonder!" "and in the evening?" "jest as far." "and around noon-time?" "it's right short." "then," said the crippled lad, "don't you see that if we measure where the morning shadow stops growing shorter and the afternoon shadow begins growing longer, that'll be the middle of the day?" the darky slapped the side of his leg with a resounding smack. "who'd have thought o' that, now?" he said. "it sho' does look like you was right." ross bent down and measured the shadow. "i think we'd better put in a peg to mark it," he said, looking up; "it doesn't seem to be changing so much. i can only make it five and five-eighths, now." anton stuck a sharpened peg in the ground and took out the little silver watch that had been given him on his birthday. "it's not nearly twelve o'clock by my watch yet," he said. "that's standard time," ross reminded him; "don't forget that we're not right on the line of standard time here, anton. that's new orleans time you've got, not sun time." "is thar more'n one kind of time?" the darky asked. "ain't time, jest time, all over?" "i should say not!" declared both boys at once, "it's never the same true time at any two places in the world." "that is," corrected ross, "unless they happen to be due north and south." "yo' makin' a joke of me, mistah anton," declared dan'l. "not a bit of it," replied anton. "i'll show you just why. the sun rises in the east, doesn't it?" "sho'." "so, if you walked a long way east, you'd see the sun quicker, wouldn't you?" "ah s'pose ah would," the darky responded hesitatingly. "and your watch would show that the sun rose earlier." "sho'!" "so noon would come sooner, too. and if you walked west, it would be longer before the sun rose and noon would be later, that is, figured by your watch." "ah declah ah never thought o' that!" "so, you see, every place has a different time." "but," the darky protested, "it's the same time when ah goes to vicksburg." "certainly," the lad answered, "and if you went away to texas it would seem the same, but it really wouldn't be. the clocks change four times in the united states, don't they, ross?" "yes, four times," the older lad agreed. "east of a line running through buffalo, wheeling, asheville and atlanta, time is called 'eastern time.' everything west of that line is really an hour later, so the clock has to be put back an hour. if a train comes from the east into the station at wheeling, at ten o'clock in the morning, and only stays in the depot five minutes, the timetable shows that it left at five minutes past nine." "what-all happens to that yar hour?" asked dan'l. "it's just lost," ross declared. "that standard of time, which is called 'central time,' reaches clear across to the middle of the dakotas, and the eastern boundaries of colorado, and new mexico. there you lose another hour, 'mountain time' extending as far as the ridge of the rockies. from there to the pacific coast, it's called 'pacific time' and is another hour later. "you see, dan'l," he continued, "when it's noon in washington and new york, it's eleven o'clock in chicago, st. louis and new orleans; ten o'clock in butte, cheyenne and denver; and nine o'clock in spokane, san francisco and los angeles." "who-all fixed it up that way?" "the railways," ross answered, "but the various states have o.k.ed it. you've got to arrange the setting of time in some definite way for the handling of railroads and telegraphs and things of that sort. it seems funny, dan'l, but if you send a telegram here to a friend in san francisco, he'll get it, according to his watch, nearly two hours before you sent it." ross stooped down as he spoke, and again measured the shadow of the pole, as it lay stretched out like a black line across the grass. "it's just the same!" he cried. "it's noon now!" anton promptly set his watch right by the sun. "there's mr. levin coming," he announced, "let's show him that his watch is wrong. he's always so exact." the boys came up to him, but before they could put their question, the weather man spoke. "well, boys," he said, "what are you after? putting up a flag-pole? it's a little short, isn't it?" "no, mr. levin," anton answered, "that isn't a flag-pole, it's a new clock, and one that's always right!" "how have you been making it?" the forecaster asked, immediately interested. anton described the principles that the boys had used and especially the means adopted to ensure that the pole should be upright. "why don't you fix it so that you won't have to measure the length of the shadow every day?" queried the forecaster. "it's quite easy when you know how." "won't you show us?" responded anton. "certainly," the old weather man answered, getting out of his buggy. "i see," he continued, "you've got hold of the idea that when the sun casts the shortest shadow it must be true noon, because the sun is half-way between the longest shadow and the shortest. that means, of course, that the sun is at the meridian." "yes, sir." "it would be much the same thing, wouldn't it, if you measured half the distance between the points on the horizon where the sun rose and the sun set?" ross thought for a moment. "yes," he said, "i suppose it would. but is that always the same?" "how can it be anything else?" the forecaster asked. "in winter the day is short and in summer it is long, but the meridian plane is always the same--that is, excepting for certain very small astronomical variations which would make no difference to you in the matter of measuring time. let's get the meridian plane, first. dan'l, do you suppose there's a pail of whitewash in the barn?" "yas, suh," the darky replied, "ah knows there is." "go ahead and get it then," the observer asked, "and let me have a piece of string." he fastened the string to the bottom of the pole and awaited the return of dan'l with the whitewash. in a moment the old negro came back with the pail. "now," said the forecaster, "i'm going to hold this string right at the end, and, holding it tightly, walk around the pole. what kind of a figure will that make?" "a circle," answered the two boys. "right. dan'l, you take the brush and whitewash a narrow line right behind my hand as i move the string round." dan'l stooped down and rapidly painted in the circle, as the forecaster moved the string. "next," said the weather man, "we'll make another circle, a little closer in." "at any special distance, sir?" asked anton. "no," was the reply. "it doesn't matter. any distance at all will do." a second, and again a third circle was thus made. "tie a piece of rope around the pole," was the next direction, "as high as you can reach." this only took a minute. "now, boys," the forecaster said, "all that you have to do is to watch when the shadow of the rope crosses those three circles. put in a peg this evening when it crosses the inside one, then the middle and then the outside. to-morrow morning, mark with pegs the place where the shadow crosses the same circles on the other side, only, of course, it will cross the outer one first." "then what shall we do, sir?" asked anton. "have you a long straight board?" he asked in reply. "plenty of them," the younger lad answered. "good. well then, to-morrow morning lay that board so that its edge touches the two points where the shadow of the rope on the pole crossed the outer circle and let dan'l whitewash a straight line joining the two points. do the same with the second and with the inside circles." "yes?" queried the lad eagerly, "and then?" "you'll have three parallel lines," the forecaster said, "the outer one longer and the next two shorter. bisect those lines. do you know how to do that?" the younger lad shook his head. "only by measuring with a bit of string and doubling the string," he said. the forecaster took a pencil and an envelope out of his pocket. "it's quite simple," he explained. "fasten a string to the peg at one end of the line you want to divide in half. stretch the string along the line till you come to the end of this line. then make a circle. do the same thing from the other end of the line. that will give you two circles crossing one another. with the board, draw a straight line joining the points where the circles cross. "to be exact, bisect the line on the middle and on the inner circles in the same way. you'll find they all come out the same. the bisecting line, reaching from the pole, and crossing the bisected lines is called the plane of the meridian. if i were you, i'd make that line a permanent mark by pressing into the ground a row of stones, or those white clay marbles. then the rain can destroy the other whitewash lines, without doing any harm, because you've got what you were after." "but how is that going to show the time?" queried ross. "because," said the forecaster with a smile, "whenever the shadow of the pole lies along the line of white marbles, which marks the meridian plane, it is exactly twelve o'clock by sun time." "without any measuring as to length?" "without any measuring at all." "that ain't no clock, mistah levin," the darky announced in a superior way. "ah don't hold with no clock like that." "why not, dan'l?" "ah gets hungry other times besides noon," he said. "ah'd only eat once a day by that clock. no, suh, ah wants a clock that tells every hour o' the day, not jest noon-time. "ah got another clock that don't never need no mending, not in summer-time," continued dan'l. "my marigolds open at seven sharp every mornin' an' wink their eyes at me an' say 'dan'l, yo're hungry,' and ah sho' is. an' jest before six o'clock in the evenin', the white moon-flowers say, 'dan'l, time fo' supper and yo' little white bed.' an' dey's right, too. don't need no sun-clocks." "i'm like dan'l," put in anton, "i'd like to be able to tell every hour, not just twelve o'clock only!" "well," the forecaster answered cheerfully, "you can make your sun-clock that way if you like." "can we, sir?" asked anton. "how?" "by using your pole as the style or upright of a sun-dial. before clocks were invented, people told the time by sun-dials, and there was a whole science of sun-dials, called gnomonics. it was quite a difficult mathematical science. even after clocks and watches came into use, sun-dials continued to be used as time-pieces, because watches and clocks were expensive and there were few mechanics who could mend them." "i've been wondering--" began anton. "let's make a sun-dial here, mr. levin?" asked ross, finishing anton's sentence. "we can, can't we?" "certainly. you can make a sun-dial anywhere. if you had to do it without a watch, you might find it a little difficult, of course, but it can be done. for example, i can tell you off-hand that for this latitude here, the angle between noon and eleven o'clock, is a little over nine degrees, while it is nearly ten degrees at new york. "since you've got a watch, however, it's quite easy. your meridian line marks twelve o'clock, and a line drawn at right angles to it, from the base of the pole, inclined to an angle corresponding to the latitude, will mark six o'clock, morning and evening. if you'll put in a peg on the circle that dan'l whitewashed, exactly at the place where the shadow touches when it is one o'clock, two o'clock and so forth on your watch, the watch having been made to agree with the shadow at noon, your sun-dial will be right all the year, round. you don't need to mark anything earlier than four in the morning or later than eight in the evening, as even on the longest day, here, the sun does not rise before that time nor set after it. you don't have to get up before six o'clock to mark the hours, as the lines are the extension of the four and five lines of the afternoon." "let's do it!" cried anton. "we'll make a clock with white stones, just that way! couldn't i divide it up into five minute distances, like a regular clock, mr. levin?" "yes," the forecaster answered, "if your circle is big enough. and if you wanted to do the thing in the way that it used to be done, you could have a little motto running all around the circle, just picked out in white stones." "what kind of a motto, sir?" "all kinds were used," the other answered, "i remember one that read 'pass on'; another 'do not linger'; but the one i like best is the old latin one which ran 'i count only the bright hours.' i suppose you've heard the story of the american sun-dial motto?" "no, sir," said both boys together. "you knew that the sun-dial is one of the official emblems of the united states?" "i never heard of it," ross exclaimed. "it is. it was used on some of the earliest american coins. last century, in london, one of the courts of justice, known as the inner temple, gave an order to a sun-dial maker to put up a dial. he asked for a motto, and was told to come the next day for it. next day it was not ready, nor the day after. still the dial-maker persisted. at last, one day, in making his request, he interrupted an important meeting, and the chairman turned to him quite impatiently and said: "'sirrah! begone about your business!' "'a very good motto,' said the dial-maker, not realizing that the command was meant personally for him, and he engraved the words on the dial. when the lawyers of the inner temple saw the motto, they agreed that nothing could be better, though it had never been intended. "when our first coinage was discussed, benjamin franklin was on the committee and he suggested that a sun-dial should be used. as, however, the coinage would go to the people instead of the people going to the sun-dial, he suggested the old motto with a change. this motto read: "'mind your business!'" "that's good, too," exclaimed anton. "very good. so that phrase was engraved on the american coinage, and on some money that was issued by the state of new york, over a century ago. you could use whichever motto you liked best." "i'll use the american one!" declared anton enthusiastically. "i've a lot of those marbles. i'm going right off now to see if i haven't enough." he shifted his crutch to a more comfortable position under his arms and pegged across the yard to the house as hard as he could go. "i've noticed," said the forecaster, as he looked after the limping boy, "that anton seems a lot happier since the flood. he used to be such a mournful little fellow." "it's this weather work you started him on," the boy answered. "it means a lot to him." "ross," said the weather expert, "i've been thinking a good deal about anton and about all the rest of you boys in this neighborhood. issaquena county is over ninety per cent colored and there aren't very many of you white boys, but the dozen or so that are here seem to me to be mighty good american stuff." "they're a dandy lot," ross agreed. "have any of you boys thought at all about what's going to happen to anton, when he grows up? his father hasn't money enough to send him to college, or anything like that, especially since he lost so much by the flood, and, being a cripple, anton's not going to have much of a chance on the plantation." "i hadn't thought of it," ross answered, "but it does seem as if he were up against it, doesn't it?" "why don't you boys make it easy for him?" "how, mr. levin? we would in a minute, any of us. everybody likes anton." "look here," said the weather man, putting his hand on ross's shoulder, "i know from experience that when you suggest something worth doing to a bunch of american boys, they're mighty apt to go ahead with it. now, as you said yourself, anton seems to have a real interest in these weather observations. his father tells me he's never two minutes late in taking them. making this sun-dial is another example of the same thing. what i'm thinking is this--why couldn't anton be taken in hand and taught to fit himself for the weather bureau? i'll teach him mathematics as my share, but you boys will have to do your bit." "what could we do?" "suppose--of course, without letting anton know why you're doing it--suppose you boys got together and took up this weather plan as a sort of outdoor club. you could meet here at anton's place. if all his chums were interested and having a natural earnestness, i'm sure he'd work like fury at it. it would give him a real chance, and, what's more, i believe you chaps would like doing it." "make a weather bureau of our own, mr. levin? i think it would be great!" "i think myself that you'd get a lot of fun out of it," said the forecaster, "but the real idea is that you'd be helping anton, yes, helping him more even than when you rescued him from the drifting house during the flood, because you'd be giving him a start in life. it's a piece of work that's worth the doing, ross." "it's a bully scheme, sir," agreed the boy, waving his hand to another lad who was coming up the road. "i'm game to do all i can." "you'll have a good deal to do," the weather man warned him. "i know you're practically the leader of the neighborhood and the boys follow you. i've spoken to a few of the fellows and asked them to meet me here this afternoon, but i wanted to see you first. i've just come from your house and they said you were over here. it's got to be a boys' deal, through and through." ross thought for a moment. "you said, sir, we oughtn't to let anton know. i think, perhaps, we ought to keep it dark. but i'd like to talk to bob portlett about it, if you don't mind. he doesn't talk much, but the chaps put a lot of stock in what he says. bob and i are pretty thick, you know." "of course, talk things over with him. i spoke to him about it yesterday. you two go into executive session, while i go up to the house a minute." he nodded to bob and strode off across the yard. "levin been talking to you about anton, bob?" ross asked, as soon as the forecaster was out of hearing. "yes," answered bob, in his abrupt way. "he said you knew all about it." "he only sprung it on me just a few minutes ago," ross rejoined, "but i think it's a dandy idea," and he proceeded to relate to his friend the outline of the plan. when he had finished, bob nodded his head. "count me in," he said, "i'll do anything for anton." "what'll you do?" "wireless," was the brief reply. "what's that got to do with weather?" "a lot. i got my new big sending apparatus yesterday and i've got a transmitting license." "have you?" said ross in surprise. "i thought they were so awfully hard to get. don't you have to pass an examination, or something?" "yes. i passed it. i've still got the small apparatus i used to have, the one you know. i'll give that to anton, teach him to work it. he can send me his observations and i'll transmit. i've a lot of amateur stations on my string. how's that?" "fine!" declared ross enthusiastically, "it would keep the observations up to scratch if the chaps knew they were going to be used. who else do you think would join in?" one by one the two lads discussed the other boys in the neighborhood. meanwhile, many of them had arrived and were clustering around mr. levin and anton, asking innumerable questions about the new sun-dial. dan'l was giving out information freely, and one of the puppies had taken exception to the whitewash line and was barking at it with high puppy-toned barks. presently ross caught the forecaster's eye, and came over and joined the group. "i've just been telling the fellows, ross," said the weather man, speaking as though the lad knew nothing about it, "that we've a good chance in this county to give a hand to the weather bureau. i'm out of the work, now, of course, but my heart's in it yet, and i'd like to see issaquena county put on the map. we haven't got an observer's station in the entire county. weather's the most important thing in the world and we've only just begun to learn how wonderful it is. "every one of you boys has seen what it means when the mississippi gets in flood, and most of you could guess what would have happened last spring if the weather bureau hadn't given any warnings. as it was, nobody was drowned, all the way down the river. in the johnstown flood, just because it was a case in which no warning could be given, over two thousand people were killed. "think of it, boys, if we could get together and map out the weather in every square mile of this county, we could make this district the best kept and most famous meteorological centre in the world! "i know, sometimes, it seems as if we were a good deal out of things, here. there's not a town of any size in the county, one day's a good deal like another, and we're apt to think of places like new york, chicago, new orleans and san francisco as being the fighting centres of the nation's life. "yet, right here, right over our heads, the never-ending battle of the weather goes on, with its brigades of warring clouds, its wind-cavalry and its artillery of storm. the sky holds more secrets than the city does and there's a lot of adventurous work to be done. which of you is game to do it? who'll volunteer?" an excited babble of answers greeted him. "i will, mr. levin!" cried one. "sure!" said another. "put me down for it," proclaimed a third, voicing the general sentiment. ross brought the matter to a point. "the way i feel about it," he said, "i reckon we'd all like to tackle something like that. and, i tell you, chaps, it would be bully for us to have a club-house of our own." "a club-house!" cried one. "yes," said ross, "anton's father is ready to give us the old barn. he says we can fix it up any way we like." "all for our own?" "yes, to do anything we like with. mr. levin has given me some bully ideas about things we can do, and bob's thought up a scheme that's just great!" and he proceeded to explain the lad's offer of wireless. the enthusiasm of the boys was rapidly growing. with the forecaster behind him, with anton's rain-gauge, with the new sun-dial staring them in the face, with bob's plan for the wireless plant, with a club-house of their own and the admitted leadership of ross, the whole group was swinging into line. "tell you what i'll do for my share, fellows," said another of the boys. "you know that printing-press of mine?" "you mean the one you printed the pirate flags on, fred?" queried ross, referring to the treasure island period when the boat was made. "yes. ever since dad found that he had to use the shed i used to keep the press in, i haven't had much chance to get at it. i'll ship the press over here, if there'd be room for it in our club-house," the words were said with great pride. "we could print a little weekly paper. i wanted to do that last year, but dad said that he didn't want me to print nothing but gossip, and there didn't seem anything else to write. if we really had some stuff worth reading, like weather news, i'm sure i could make it go. enough, anyhow, to pay for paper and mailing." "you think we ought to issue a regular weather bulletin," said ross. "that's a good notion, fred." "i'll let you have some of my stories," said one. "or fatty's jokes," suggested another, dodging a nudge of the elbow from his neighbor. "a weather bulletin would be a good thing," the forecaster said, approvingly. "what could the rest of us do?" asked an alert youngster. "i haven't a printing-press, or a wireless apparatus or anything else." "nor have i," said two or three voices. the forecaster looked quickly at ross. this was a crucial point. it was anton who answered. "you've got plenty of wind at your place, lee, haven't you?" he asked. the lad laughed. "pop says it's the windiest place in the county," he answered, "poked right up there on the top of that knoll." "you ought to be the official wind-measurer," the crippled lad declared. "there is a way to measure wind, isn't there, mr. levin?" "certainly," the forecaster answered, "it's a very necessary thing to do, too." "pete's camera!" interjected the laconic bob. "what's the good of that?" broke in its owner. "you can't snap-shot the wind, at least not that i've ever seen." "clouds!" said bob. "that's right," agreed anton, "you could photograph the clouds, pete. suppose you took a snap-shot of the sky every day, at the same time, for a year, it would make a peach of a series." "the bureau at washington would be glad of a series like that," put in the forecaster. "so far as that's concerned, pete, i'd buy a daily print for my own use. i couldn't pay much, of course, but enough to meet the cost of materials." pete brightened up. "i'll do that, quicker'n a wink," he said. "i've snapshotted about everything else around here, but i never thought of the sky." "you could tackle eclipses and halos and rainbows and lightning--all sorts of things," suggested anton. "right-o!" answered pete, "you can put me down as official photographer." "i don't see," said one of the smaller lads, "where that rain-gauge is so hard to make. i'll make one and put it up at my place." "dad's got an old barometer," suggested another, "that he used to have when he was a steamboat skipper. i'm sure he'd let me have it. it's in the attic now, where nobody looks at it." "some of us might measure the amount of sunshine," said ross. "isn't there some way of doing that, mr. levin?" "indeed there is," the forecaster replied. "why, in some places, they run machinery by sunshine. there is a big solar engine at pasadena, in california, where they pump water and irrigate an orchard just by an arrangement of mirrors. even a small one would run quite a good-sized engine." "gimme that! oh, gimme that!" burst in another of the boys, who had been passive theretofore but who was absorbed in mechanics. "i'll be tickled to have an engine run by sunshine." the weather forecaster looked around with a smile at the enthusiastic group. "it seems to me," he said, "that with an official photographer, an official wind-measurer, an official sunshine recorder, an official wireless station, a club-house and an editor with an official publication, 'the mississippi league of the weather' is mighty well launched on its way. "now, i'm going to have the fun of making the first motion. i move you, mr. chairman, that the league come into the house and hold its first official feast!" chapter iv the massacre of an army "where's the boss?" queried a strange voice, one afternoon. the entire mechanical staff of the _issaquena county weather herald_, consisting of fred lang, publisher and editor-in-chief, aged fifteen, and a general assistant with the blackest face and the whitest teeth in the county, aged seventy, named dan'l, turned at the question. "why?" asked fred. the stranger stepped into the office of the _herald_. "i'd be wishful to see the foreman," he said, with a twinkle in his eye, "that's if he's not too busy." fred grinned in response. "i guess i'm the foreman," he said. "i'm lookin' for a job," the new-comer explained. "what kind of a job?" "any kind of a job in a printin' shop," the irishman replied. "i'm an old-timer. there's nothin' about printin' i don't know." "have you seen a copy of our paper?" asked fred. "i have so," was the reply, "i've got it with me, right here." he pulled from his pocket the latest number of the little four-page sheet. "'tis an illigant publication," he went on, "but i'm thinkin' that you're in sore need of a printer." "does it look so bad?" queried the "foreman." "the worst of it is, i don't know how to make it any better." "i'm not saying that it's bad, but there's a deal to be learnt about printin'," the journeyman declared. "i'm thinkin' your compositor hasn't had overmuch experience." "he hasn't," the boy admitted. "i'm him. dan'l helps me all he can, but since he can't read, it makes it bad." "give me the job," said the irishman, "an' i'll make the paper look right." "i can't," fred replied. "the subscriptions hardly pay for the paper and the ink. i give dan'l thirty cents a week for wages to run the press and it's hard to scrape up that much, because mr. levin says i mustn't pay out a cent that the _herald_ hasn't actually earned. what wages do you want?" "three dollars a day when i'm workin'," the journeyman printer replied, "an' the good green grass to sleep on and a hunk of corn-bread to eat when i'm not." the young editor looked at the journeyman printer with a sudden eagerness. "i've got four dollars and a half saved up," he said, "that's a day and a half's wages. will you teach me all about printing in a day and a half? that isn't office money, that's my own, but, you see, it's for me." "i'll teach ye for nothin'," said the irishman, pleased at the boy's pluck, "if ye'll give me a bite to eat an' a place to sleep." fred shook his head. "no," he said, "mr. levin won't let any of us boys take something for nothing. i'd sooner pay. it would be great if you could get out this week's number for us, and let me see how you do it. i'd learn a heap that way, and it would be just the stuff i want to know. then the number you got out we could use for something to go by. but you'll have to do it in a day and a half, because that's all the money i've got. can you?" "i can that," the printer answered, "an' i'll pay for my board out of it, so that you won't be spending all your money." "can't do that either," said the boy, "because that would make it anton's dad's money, not mine. if you want to pay him, all right." the irishman stripped off his coat and rolled up his sleeves. "i'll be lookin' to see what fonts o' type ye have in the shop," he declared, and examined the forms which were lying on the rough table. "did anny one ever show you annything about printin'?" he asked presently. "no," said the boy, "i got this printing-press from a chap whose brother used to run it. the fellow who owned it was going to show me how it worked, but he went away and hasn't come back." "watch me a while," the journeyman responded and began to unlock the forms that had stood since the issue of the week before. it was a revelation to the boy to see how the trained fingers of the printer sorted, classified, and arranged the type. talking steadily, in his irish fashion, the journeyman explained how the type should be set up, showed that they had been using twice as much ink as necessary, warned them against pinching the type too closely, explaining that this "put the letters off their feet," and, by altering the arrangement of the sheet, improved its appearance a thousandfold. these routine matters were quickly adjusted, and then the printer asked for the copy which was to fill the first page. "it's just got here," the young editor answered. "i haven't looked over it yet, but i guess it's all right. i had a wireless yesterday that one of our chaps was sending in a corking description of a sunset, or rather a sort of description of all the sunsets in the last month. here it is." he handed the pages of boyish handwriting to the journeyman, who looked over them hastily. "'tis fine stuff, entirely," he said in surprise. "i'd be wishful to take some copies of the paper for myself. listen to this now!" and, turning the sheets, the enthusiastic irishman read aloud: "'sunsets all look different, but when you write down what you see, one right after the other, they seem to be quite alike, that is, when the sky is clear. when the sun begins to set, and there are not many clouds, the lowest part of the sky is more different from the rest of it than in daytime. in the west--at the side of the setting sun--the sky looks white, changing to yellow. in the north and south, it is a dull yellow, which gets yellower. in the east, it is a dirty yellow, which changes slowly into a dull purple. all these yellows are duller at the horizon than a little way above. the purple in the east looks gray at the sky-line but shades into blue, higher up.' "'tis an illigant style the boy has," declared the journeyman, and continued: "'just as soon as the sun begins to drop below the horizon, an ash-colored plate (the shadow of the earth) begins to creep up the eastern sky, covering part of the purple bit and making it look like a purple rainbow. soon the shadow covers all the purple light in the east. "'in the west, where the sun is setting, the colors are all different. the whitish light spreads quite a long way up into the blue, but when the sun comes close to the horizon, this turns to yellow, lighter higher up and darker lower down. it is sometimes reddish at the horizon line, and the clouds are turned to pink. "'after the sun has really gone down, the yellow gets darker, changing into orange, sometimes, while the white spot spreads sideways and its upper edge marks off the brighter from the darker bits of the sky. "'in the darker part of the sky, at about quarter way up, a purple glow suddenly appears. it grows bigger quickly, making a circle, the lower edge of which looks as though it slipped behind the yellow strip. this purple spot in the west comes just as the purple rainbow in the east is dying out, and as the western purple spot grows it gets brighter, so that there is a time, after the sun sets, when it seems brighter than it did before.'" "that's queer," interrupted fred. the printer thought for a moment. "it's right, bedad," he said, "i've noticed it meself." he continued reading: "'sometimes there are dark blue and greenish stripes running down to the sun and these stripes shoot a long way up into the sky. "'if there are any clouds, they seem to be generally light yellow to begin with, changing to pink and rose, then red and dark orange. i couldn't find any system in the color of the clouds, perhaps because they are at different heights. "'a few times i've seen a sort of second faint purple arch or bow in the east, but by that time it's dark. in the west, though, the second arch is quite clear. as the first western purple arch sinks to the horizon, following the sun, a green stretch, ever so green, shows up, and above it is a second arch of bright light, with a purple arch above that. when this last one sinks, it is quite dark.'" mr. levin, as was his habit on saturday afternoon, had come over to the league's club-house, and he had entered during the reading, followed by his usual bevy of boys; rex, lassie, and four roly-poly puppies, now able to run around on unsteady legs, bringing up the rear. "that's a mighty accurate description of sunset colors," the forecaster commented; "whoever did that, deserves a lot of credit. hello! have you enlarged your staff, fred?" he continued, as he noted the printer and realized, at a glance, that the little shed had already assumed a more business-like look. the editor-in-chief explained the bargain he had made and the weather man nodded his head approvingly. "that's the best way i know to spend your savings," he said, "using them to learn something. i'm glad you're going to have this issue properly printed, too, because that sunset article is about the best you've had, so far. if i don't miss my guess, a good many people will keep that number as a sort of reference for the colors of sunset. who wrote it?" "i did, sir," said one of the boys who had come in with him. "good work," the forecaster commented. "do you happen to know, though, bert, what makes the colors of sunset? why doesn't it just gradually get dark as the sun goes down?" "i don't know," the boy replied. "i tried to explain it the other day and i found i hadn't the least idea why, myself. i asked father, but he didn't know either." "yet it's quite simple," the weather man answered, "and if you boys are going to be real meteorologists, you ought to know the reasons for things. first of all, why is the sky blue?" there was a gasp of astonishment, followed by silence. "sure, 'tis the air that's blue," hazarded the printer. "that doesn't help much," the forecaster said, "though perhaps it does, a little. why is the air blue?" the irishman shook his head. "why is annything blue?" he asked. "that's just what i'm going to tell you," the weather man answered, "and you want to listen carefully, boys, because the colors of the sunset depend a great deal on the weather. you can foretell weather from the sunset." "yo' sho' can," interrupted dan'l. "don't yo' remember mammy's old rhyme: "evenin' red an' mornin' gray certain signs of a beautiful day; evenin' gray an' mornin' red, sends a nigger wet to bed." "all those old rhymes are fakes, though, dan'l," declared anton, with the importance of his newly acquired weather knowledge. "easy there, easy there!" warned the forecaster. "not so fast. a good many of those old rhymes are mighty good weather forecasts. that one is, for example." "you mean, sir, that a red sunset and a gray sunrise really tell that the weather is going to be fine?" "yes, to a great extent, they do." "why, mr. levin?" "because they show the state of the atmosphere, boys. rain can't fall unless there is dust. every little drop of rain has a grain of dust in the middle. the colors of the sunset, too, are due partly to dust. not only that, but colors of the sunset vary as the particles of dust which reflect the rays of light, are enveloped by water vapor. "a piece of dust, without an envelope of water, is smaller than one with a little wetness around it. when more water vapor gathers around the piece of dust, the drop becomes bigger. when the sunset is red, it is a sign that it is shining on very small bits of dust, or that the condensation of water vapor into rain has not advanced very far. if, however, the sunset sky is gray, that means that the upper air is saturated, that it has all the water it can hold, and, of course, rain is likely to come soon." "i should think, then," said anton, "that gray in the morning would be a bad sign, too." "it's not, though," the forecaster replied; "the proverb is right there, as well. a gray sky in the morning means that the air is filled with water drops which are large enough to reflect light of every color. while this is the same as the gray of evening, the processes that led to the forming of these drops is quite different. in the day the dust is heated and the forming of the droplets in the afternoon is due to cooling. in the night, the condensation is caused by loss of heat through radiation. radiation shows that the air above must be dry. therefore a gray morning means a dry air above the water drops, and this means a fine day, for the droplets will soon be evaporated by the rising sun. the red morning sky declares that the dust particles have been protected from radiation by a blanket of overlying moisture, the air, therefore, is saturated to great heights and rain is probable. so you see, anton, mammy's rhyme is right." "what fo' yo' talk to me against signs," declared dan'l, putting out his chest and strutting; "ah done told yo' them signs am pow'ful good." "but the sunset colors, sir?" the author of the article asked. "you said they were due to dust. just how, sir?" "yes, to dust, plain ordinary dust, but dust of the lightest kind," was the reply. "if you could go up in the air a hundred miles, the sky above you in the middle of the day would be jet black and the sun would shine down on you like a great bright-blue ball, without any white glare around it at all." "then it's a blue sun that makes a blue sky!" cried fred. "don't go so fast," the forecaster warned him. "i want you to think of the sky, first. it's a dead black, a hundred miles up. now, at a hundred miles up, the air is so thin that there's little or no dust, but as you gradually come down and the air becomes denser, it begins to be able to buoy up some dust. boys," he said, breaking off suddenly, "why does a stick float in water when it falls in air?" "because water is denser than air?" guessed ross. "exactly. and why does a bar of iron sink through water and not through earth?" "because the particles of earth won't move aside as easily as the particles of water, i suppose." "not quite, but something that way. so, you see, as the air gets gradually denser it becomes gradually more able to support particles of dust, light ones at first, then heavier and heavier, until near the earth big pieces of dust can be carried in the air. you know how big some of them are when you happen to get a grain in your eye! viscosity has a lot to do with it, too. "the light of the sun is a bluish-white, like some of the blue stars. white, as you remember from the rainbow, is just a mixture of all sorts of colors and the different colors are created by waves of light, some being shorter and others longer. a long wave, like the red, will pass around a tiny piece of dust, but a short wave, like the blue, will be stopped by it, and scattered, sometimes polarized, as it is called, or turned into one plane." "i don't think i quite see that," said anton. "it's a little complicated," the weather man answered, "but maybe i can give you an idea of it. suppose you were on a big steamboat in a choppy sea. as the steamer's length would extend over several of these waves, none of them would be big enough to make the vessel heave. if you were on that same choppy sea in a small canoe, you would be tossed in every direction. now, if you think of the long red wave of light as a steamer and the blue as a canoe, you can see that in a ripple of small particles of dust the blue is going to be more affected than the red. in other words, the blue will be scattered. it will be diffused all over the sky and the light that comes through will be less blue." "then i should think the sun would look red," said anton. "it does," the forecaster explained, "when there's a fog, which simply means, when there's more obstruction in the air. sunlight is never white, as you know, it's yellow-white and the golden effect is due to dust. it's the same way at sunset. then the rays of the sun which reach you pass through a larger amount of air, because you're looking at them from an angle, so they have to strike more grains of dust, and more of the blue rays are scattered. then, too, when the sun, at sunset is, to you, shining obliquely on the atmosphere, it is passing through several layers of air and these bend the rays differently." "i still don't see," said the author of the sunset-color article, "why there should be so much pink, or rose-color, and why the clouds should generally be pink." "there's not much pink in a clear sky," the forecaster answered, "and as for the pink clouds, you've never seen them in the west when the sun was still above the horizon, have you?" "no--no," said the other, "i don't think so. the pink generally comes after the sun had disappeared." "scientifically, of course," the weather man said, "the sun has gone below the horizon at least two minutes before you see it disappear. you're looking at a sun that isn't there at all. that's due to refraction. the reason of the pink glow is that when you see it, the earth and the air for several thousand feet above you are in the shadow of the edge of the earth. the sun, therefore, is not shining on the thicker dust of the lower part of the air, but the finer dust of the upper part, the particles of which are small and more uniform in shape. "the glow is of a rose-color because the particles are of the size to diffuse the rays of this wave-length. that's why rose colors appear in the east, before the west, and why the color lasts in the sky, which may be reflected on dust twelve miles high, after it has disappeared from the upper clouds, which are not more than eight miles high." "'tis the illigant hand ye are at explainin'," put in the irishman, "but i c'n remember, when i was learnin' me trade, about thirty-four years ago, the sunsets were much finer than annything i've seen since. we don't have such sunsets now as when i was a boy." "they were sho' brighter," agreed dan'l. "ah can remember when the skies used to look like they was all burning up. ah thought the end of the world was a-comin', sho'!" "thirty-four years," said the forecaster thoughtfully; "that would be in , wouldn't it? why, of course, mike," he continued; "that was during the period of the famous krakatoa sunsets." "an' what's a kraker-something sunset?" the printer asked. "krakatoa," the weather man explained, "was a volcano, near java. in august, , one of the most violent eruptions in the history of the world occurred. half the island was blown up in the air, and, where a mountain had stood, the ocean rolled a thousand feet deep. "the vibrations in the air were so terrific as to break windows and overturn frame houses over a hundred miles away, and the pressure wave, like some huge blast of wind, traveled round the world three times before it died down. the huge sea-waves caused by the eruption and the engulfing of the island, swept across the oceans, destroying the coasts for hundreds of leagues around. over thirty thousand people were drowned. "pumice and ashes fell over the sea so thickly that within three miles of the island you could walk on them, and even five hundred miles away, the ashes formed a scum on the surface of the sea. the finer dust and the icy particles from the condensed vapor reached extreme heights in the air. these dust particles spread all round the world, completing the circuit in fifteen days. "the sunsets were extraordinarily red, because, in the very thin air of great heights, there was an unusual amount of dust which had been forced there by the great volcanic outburst. it took three years for this dust gradually to settle into the lower air, and this made the sunsets that pat speaks of. the great eruption of mont pelé in created unusually beautiful sunsets in america for a couple of months afterward, but, of course, this was not to be compared to the krakatoa eruption. "it's curious, though, boys," he said, "that bert, here, should have been writing this article on sunsets, because it happens that i've got something here quite important to show you." walking to the table, he took a large home-made portfolio from under his arm and spread it out. he untied it, threw open the cover and stepped back to let the boys look. they crowded round. "oh--oh!" said one. "isn't that bully!" the forecaster turned over a second picture. this was greeted with cries of delight, and one of the lads added: "i saw a sunset exactly like that only a week ago!" the forecaster bent down and looked at a pencilled note underneath the vivid chalk drawing. "it is dated just a week ago," he said. "i didn't know you drew with chalks!" said ross. for answer, the forecaster smiled and turned to another one. the first few had been a little crude, but it was evident that they improved as the series went on. all of them, in a curious way, possessed the faculty of giving a real impression of the sunset. "so you like them," the weather man said, when the whole series had been examined. "they're dandies," declared ross, and fred added: "i wish we could use them as colored plates in the _review_." "who do you suppose drew them?" the weather man asked. "didn't you?" queried several of the boys together. the forecaster shook his head. "one of the boys?" asked ross. again the forecaster made a negative gesture. "a boy drew them," he said, "but not a member of the mississippi league of the weather." "who was it, mr. levin?" pleaded anton. "cæsar," he answered, "down on mcdowell's place." "cæsar!" exclaimed fred; "it couldn't be. why, he's--" he checked himself just in time, remembering that dan'l was close by. "yes, he's colored," the forecaster agreed. "but don't you think he can draw?" "he surely can." it was on the point of anton's tongue to suggest that the colored artist should be admitted to the membership of the club, but, so far, its membership had been confined to the white boys, largely in deference to the feelings of the older people of the neighborhood, many of whom remembered the difficulties that followed the reconstruction period after the civil war. anton looked a little troubled. "do you think we ought to get mixed up in a thing like this?" he asked. the forecaster glanced at him. "you mean because cæsar is a negro?" "yes, sir," the crippled lad replied. "i don't want to persuade you one way or the other," the weather man replied, "but i can tell you how i feel about it. i don't see that it matters very much what point of view a fellow has on the color question, we're all agreed that the darkies should be given every chance. you certainly can't harm yourself by helping any one, no matter who it is that you help." "sure," ross agreed. "and even if the person you help is never going to be able to do you any good, why, that's all the more reason for helping, isn't it?" "yes," admitted anton. "all right, then. supposing some of the older people here do feel that it's necessary to draw the color line closely; well, i don't see that it wouldn't be a good thing for us to strike out a little. the color line is there, and it's going to stay there. but the most unreconstructed man in the district--even colonel grattan, for example--will do everything possible to better the condition of the negroes. i think it's the absolute duty of every american boy to help every other american boy when he gets the chance, whether his skin is white or black." "yes," said the laconic bob. anton brightened up, for he was anxious to help cæsar. "what do you suppose we can do?" he asked. "i'd rather put it up to you boys," said the forecaster. "this is your affair, after all." anton turned to ross. "haven't you some scheme?" he asked. ross shook his head. "i haven't thought one out. how about it, bob?" "deacon paul," was the abrupt reply. "yes," said ross, "old paul will do pretty nearly anything for me, because dad was so good to his father when he was a slave. but i don't quite see what he can do?" "i do be thinkin'," said the irishman, "if i might be so bold as to make a suggestion, that there's no reason why you boys shouldn't use a colored lad's work. he's only a contributor, annyway. when a paper takes a story or a picture from a man, it doesn't ask who his parents were. why don't ye make some color plates and give them as premiums for subscriptions?" the weather forecaster laughed aloud. "that's a good business idea, pat," he said. "some of the colored planters and farmers are fairly progressive here, and a premium of a colored lad's work might be a good scheme." "but i can't make colored plates!" protested fred. "no," said pat, "you can't, an' that's a fact. i was forgettin' that this wasn't a regular shop." "how could we get them made?" asked anton. "do you suppose the weather bureau in washington would make them for us and let us have a few copies?" "no," said the forecaster decidedly, "i know the bureau wouldn't. they've a hard enough job doing their work on their present appropriations, as it is, and if they were going to spend money on sunset pictures, anton, such would be done by some big artist, in consultation with trained meteorologists." "i've been wondering," began anton, and paused. "go ahead," urged ross. "couldn't we interest some one else to do them, just to help the thing along?" "one of the big negro colleges has a lithographing plant," the forecaster said thoughtfully; "they might be interested in it, if the matter were put before them the right way. i don't suppose they'd give any money, but they might make plates for you at cost and you could sell them here at enough to cover the expense. bob has the right idea. "talk it over with deacon paul, the colored minister; he's closely in touch with all the progressive work among the negroes. i think you'll find it can be arranged, because there's a right fine spirit among the negroes. they're trying hard to improve themselves. "i believe you could interest them, too, by showing that the study of the weather, even in sunsets, is a patriotic duty. the negroes are mighty loyal." "mr. levin!" exclaimed one of the boys, "what has a sunset got to do with patriotism?" "they do look pretty far apart, don't they?" replied the weather expert, with a smile. "yet one of the great tragedies of military history, one which led to the death of hundreds of thousands of men and changed the map of the world, was due to a failure to study the colors of a sunset." "what was that, mr. levin? won't you tell us the story?" pleaded anton. "very well," the forecaster agreed; "maybe it'll show you how important to the world everything is that is connected with the weather. "i was telling you about krakatoa and its eruption and how the outburst had caused red sunsets that lasted for three years. now, if you think for a moment, you'll see that any one who observed a period of unusually red sunsets and knew the cause of them would know that there had been a big volcanic eruption somewhere." "of course." "now, boys," said the forecaster, "suppose that the upper air were unusually full of dust, what effect do you suppose that would have on the temperature?" for a moment no one spoke, then anton piped up: "i've been wondering," he said, "if the dust wouldn't shut out some of the sunlight and make the earth colder." the weather forecaster gave the boy a shrewd look. "we're going to make a real weather man out of you, anton," he said. "as a matter of fact, it does, though, of course, not to such a very noticeable extent. indeed, it's only quite recently that we've been working out the relations between volcanic eruptions and weather. they're striking, though, and while it may be a little too early to say that the one causes the other, volcanic action has a big influence. "the krakatoa eruption, as i said, produced a dust cloud in the upper parts of the air, which not only created red sunsets, but which kept so permanent a haze over the sky that the sun was surrounded by a reddish brown circle, known as 'bishop's ring,' during most of that time. this circle showed the existence of a dust cloud, through which the sunlight had to pass. as a result, the amount of sunlight was diminished. when the sunlight is less, the crops are poorer, for it needs the entire force of the sun to ripen them, and the three years following the eruption of krakatoa are known to history as 'the poverty years.' the still more famous 'year without a summer,' which was the year , followed the eruption of tombora, the autumn before." "that seems to cinch it, mr. levin," said ross. "it isn't sure," was the reply, "but it seems that way. famines have a tremendous effect on the world's history. the great french revolution, one of the greatest events in modern history, was brought to a head by a famine. this was the 'three year freeze' of - ." "did that follow a volcanic eruption, sir?" asked anton. "it followed the greatest eruption in the history of the world, that of asama, in japan, in the year . in that eruption, fifty-six thousand people were killed and the entire atmosphere of the earth was shaken. like krakatoa, you see, boys, it took three years for the dust to settle down." "but what has that got to do with the army, sir?" fred asked. "i was just coming to that," the forecaster replied. "if napoleon had known as much about the weather as we do now, boys, the world's history might have been very different. there had been some marvellous sunsets during the years of and and the spring of , but none of the scientists of that time thought of observing them or finding any significance in them, nor did any of them imagine that such could have any effect on the weather. before napoleon started on his march for russia, which was begun in june, he asked the french meteorologists at what time the russian winter usually began. they told him that if he could begin his return by the middle of november, his army could get safely out of russia before the winter set in. "but, boys, the three years before that campaign had been three years of eruptions. st. george, in the west indies, erupted in ; etna, the great volcano of sicily, had an eruption in ; and la soufrière, which broke loose again in your lifetime, boys, erupted in . as a result, the upper air was full of dust, and the middle air was even more filled, for while these eruptions were not as powerful as asama and krakatoa, there had been a continual replenishment of the stores of volcanic dust. "so napoleon and his army started off. the great march into russia began with an army of four hundred and fifty thousand men, in torrid summer heat. the crops were still green, for the spring had been late and the summer most unseasonable. as a result, there was not enough food for the horses and terrible epidemics of disease broke out among them. napoleon was always especially strong in cavalry, over eighty thousand of his troops being mounted. when, to this, is added the twenty thousand horses needed for officers and for the artillery, it is easy to see that the lack of forage seriously handicapped the army. it is by no means easy to feed a hundred thousand horses. before the army had advanced more than ten days' march, one-fourth of the horses had died. "the russians, thoroughly realizing that their strongest ally was distance, retreated, without giving battle. napoleon's army marched on. the cossacks, with their well fed horses, constantly circled round the french army and cut to pieces the small detachments in the van and in the rear-guard. the french cavalry, with their horses dead, dying or out of condition, could not pursue. meanwhile the army, under the burning heat of the short summer which had known no spring, marched on. "into that huge wilderness, over the marshes and plains, the army marched. always before it lay a land bare and dumb. the vast russian army could never be found. in endless succession the french crossed plains on which the grass grew, thin and bare, splendid for the grazing of cattle, but utterly insufficient for a hundred thousand horses, now reduced to seventy thousand. ahead of the soldiers, every day, the sun rose red upon an empty land, every night it set, red, behind them, upon a land equally bare and empty. day after day they marched through this land without food, unmolested by the russians, who knew well that lack of forage and interminable marching was defeating the great napoleon better than they could upon the battlefield, and without the sacrifice of a single russian soldier. weather, boys, always weather, is the greatest ally or the greatest enemy in the entire history of war. "at last the army saw in the distance a long black line. every effort of napoleon to persuade the russians to attack him had failed, the russian army steadily withdrew. but when the long black line of smolensk appeared, hope was restored to the french army. at last they would meet the russians on equal terms and decide the campaign against guns and bayonets instead of against leagues and starvation! on napoleon marched and at last found himself before the town of smolensk. the french army, now only four hundred thousand strong, was yet an unwieldy force to handle. it took two days for the various groups to form into positions and then they charged the town. "the soldiers fighting them had fled. everybody had fled. the city was utterly deserted, sad and silent as a grave-yard. there was nothing there to eat. the russians had destroyed everything. there was not a handful of oats, not a loaf of bread. the french victory had gained for them only an empty city and an empty land. it was now the end of august, and moscow was a long way away. "the march continued. before them, the sun rose red through the volcanic dust every morning and set red every night. had there been a meteorologist present able to warn napoleon, even then, the army could have retreated safely. but the army went on and on, into the land that the russians themselves had swept bare and left empty. villages and towns were passed, each deserted, as smolensk had been. what the people could not carry away they had burned. the fields were scorching and black. smoke filled the air. for three weeks more, well into september, the french army toiled forward, steadily growing hungrier and leaner, losing horses and men all along the line of march. "at last the russians made a stand. the desperate conditions of the march had divided the french army into scattered portions, and when, quite suddenly, the russian troops confronted them, only a hundred and twenty-eight thousand men were available, the others straggling behind. the russians had a hundred thousand men, but the french superiority was not enough for them to secure a final victory. the great battle of borodino began before sunrise, and the setting sun, red as always, sank too early to see its end. when night fell on the scene, thirty-eight thousand russians had fallen and only twenty-five thousand french, but it acted almost as a defeat upon the french, accustomed as they were to sweeping victories. "the red sun next morning rose on the french army, eager to continue the battle. but in the night the russians had fallen back again, and, before the french, the road to moscow lay open. open, indeed, but burned black and desolate as before. seven more days of marching, with hungry stomachs and famished horses and then, moscow! the goal of the french! the army beheld the city it had come so far to conquer. the red sun of the seventh day found the spires of the kremlin in sight. again the french were sure of victory. "moscow was as clean swept as the smallest village on the road. everything had been carried off or destroyed. moscow lies far to the north and the days began to grow perilously short. napoleon sought to make terms with the russians, but met with nothing but delays. the russians were waiting for the approach of their great ally, the winter. "in all moscow there was no food and forage. all the people had gone. napoleon did not dare to bring his whole army into the city. there was nothing to eat. they camped at various distances outside, tightening their belts for hunger. meantime the russians, constantly retreating and moving the provisions back with them, were steadily growing stronger in position and men. "the rapidly shortening days meant long cold nights. the soldiers in moscow made camp-fires of the costly pieces of furniture that remained in the palaces, but those who were encamped on the plains outside had no fire at all in the long hours of darkness. many of them, too, were from the south of france, unaccustomed to the cold, and, besides, were equipped for a summer campaign, not garbed in the heavy clothing of the russian troops. in that country which had been abandoned for purposes of war, there was not even wood enough to light the fires for cooking. ever the days grew shorter and the red sunrises and the red sunsets--which would have meant so much had any one understood--continued. "then into the city came fire! in the middle of the night, at a dozen different points, moscow was set aflame by the russians. a great wave of fire started from all quarters at the same time, swept over the city, for the russians had waited for the moment when the wind was high and the night was cold. houses and palaces flared upward in the conflagration, then sank to smoking ashes, for almost the entire city was built of wood. "all in a jumble--infantry, cavalry and artillery--the french got away, the flames howling so closely after them that the backs of their necks were singed. suddenly they found themselves in the midst of a tremendous rush of water and ice. on one side, to windward, the russians had started the fire, on the other, where there was a possible escape from its fury, they had turned the river into the streets. the french were caught between the two. some of the horses, fairly maddened, turned backward and plunged with their riders into the flames. for an instant, horse and man would flare up like tow and then there would be a black twisting thing that dwindled to nothing in the blaze. out from the burning city, in wild and utter retreat, flew the french grand army, out to a land without food, without forage, without inhabitants, and the nearest help a thousand miles away. "then came the snow. no longer was the red sunrise before them, but behind them. the victorious march was a defeat. black-gray clouds came over the sky and obscured the sun. at first the snow was to the ankles, then to the calves, and then to the knees. the wind was bitterly cold and the men ill-clad. it froze the french to their marrow. every few minutes a soldier dropped from starvation, cold and exhaustion. the russians did not appear. there was no need. they had a new ally--the wolves! no one could stop to pick up an exhausted soldier; it was all that any man could do to keep up himself. half the officers were on foot. the cannons were abandoned. when a horse died, the regiment ate him and staggered on. "the cossacks now began to add their terrors to those of the wolves. if a small detachment straggled out of the blinding snow, unseen until that time would come a rush of the furious and valiant horsemen of the steppes, and the detachment, hungry and exhausted, would be cut to pieces. they fought with heroic courage, but no man can fight the weather. "smolensk was reached on the return march, with the wreck of the french army, now only fifty thousand strong. the skeletons of four hundred thousand men lay on the russian plains. near a place called krasnoi, the russian army suddenly appeared and a battle was fought. napoleon commanded with his old-time mastery and succeeded in breaking through the russian lines, but he had to leave marshal ney with six thousand men behind him. ney performed wonders, and with his tiny force also broke through the russian army, but when the french resumed their flight, ney had only eight hundred men. the rear-guard alone lost five thousand at that place. "the french army had now reached the marshes, but the weather was fighting for russia. just at this time, a sudden and unexpected thaw set in, making the marsh a morass. the russians, well-provisioned, circled around the french army, and again came in front of them at a river called the beresina. waist-deep in that icy current, with masses of floating ice being carried down by the sudden thaw, with a huge russian army on the opposite bank, the french soldiers fought for their homeward way. winter was before, winter behind, the russians on the barrier. yet the french fought on and crossed the beresina with marvellous courage, the russian strategy, meanwhile, sacrificing comparatively few men. the beresina was crossed, but when the russians were finally swept aside and the french passed through, less than nine thousand men answered the roll call. forty thousand had been lost between smolensk and the beresina. "the thaw was followed by another terrible period of cold. the retreat of the army became a fearful rout. napoleon, himself, fell a victim to the panic, and deserting his troops to murat, spurred for france, reaching paris after a ride of three hundred and twelve hours. the routed and disorganized french army straggled back to germany, to austria and to france. when christmas day that year came down over europe, less than five thousand men were alive of the four hundred and fifty thousand who had started six months before to carry the eagles of napoleon over russia. it was the most splendid campaign and the most spectacular rout in history, and the foe who fought the battles that defeated the great emperor was--the weather." chapter v the runaway kite the sunset pictures made a better showing as lithographs than even their young creator could have hoped, and the _issaquena county weather review_ became a source of personal pride to every one in the neighborhood. the farmers and planters vied with each other in giving information of weather happenings and the little publication was never short of "copy." "dan'l," said fred to his chief assistant, one day, "i'm going to print an article on 'weather superstitions.'" "yas, suh," said the darky, wondering what was coming. "and you're going to write it." "ah write it? sho', now, you'se jokin', mistah fred. ah can't even write my own name." "i know that. you don't need to write, dan'l. you're going to collect every rhyme and proverb and saying about the weather you can hunt up in the neighborhood. get mammy crockett to tell you all she knows. then you must repeat it to me. i'll write it down word for word, and it'll be your article." "if yo' wrote it down, it wouldn't be mine," objected dan'l. "oh, yes, it would," the editor-in-chief assured him, "some of the greatest authors in the world dictate their books." so dan'l went all around the neighborhood, announcing that he was a "sho' enough autho' now," and so full of delight that there was no holding him in at all. he proved a good collector of superstitions, moreover, and when at last the article came out in the _review_, it was so complete and so original that it was reprinted in one of the big folk-lore magazines. the visit of the journeyman printer had been of great value. fred had been shown just how the work should be done and his pride was involved in keeping the paper up to the standard. moreover, the irishman had secured a large box of discarded type from a printing firm in vicksburg, and had forwarded this to the boys. fred returned the courtesy by mailing mike a copy of the _review_ regularly, and mike occasionally sent a package of the printing trade magazines that he found lying around the shop. fred picked up many hints from these and thus secured quite a good start in his knowledge of the printing trade. the "official photographer" had been equally successful. one day, while up on the levee trying to take a satisfactory picture of an elusive "mackerel sky," which was changing from moment to moment, he met a stranger. this stranger was sitting on a log that projected into the river, holding a rod and line, and landing fish with an accustomed skill. "what in blazes are you trying to photograph?" he said after a while, as he watched the lad focussing his camera earthwards on what looked like a piece of black glass, which projected from the stand. "clouds, sir," answered ralph. "when i try to photograph clouds i look at the sky, not on the ground," the stranger remarked. "what's that contrivance you've got on your camera stand, anyway?" "it's just a broken piece of looking glass," said the boy, "but i painted it on the back with black enamel." "what for?" "so that i could get at the clouds easier, sir," the boy replied. "i read how to do that in a book i've got." "i don't see why black glass should make any difference," said the fisherman, getting up from the log and coming over to where the boy was standing. "it does, sir. if you look on the glass," said ralph, "you'll see. the clouds are ever so much sharper." the stranger looked in. even the fleecy white clouds, scarcely visible in the blue sky overhead, stood out a clear white against the blackness of the mirror. the blue sky was not reflected in the glass. "that's queer," said the stranger, "the blue hardly shows at all. i wonder why?" "it said in the book," ralph explained, "that the blue didn't show up so much because it was partly polarized. i couldn't quite understand what that meant. as far as i could make out, the blue color of the sky is due to waves that are scattered sideways instead of coming straight down like the white light does." "i suppose it is polarized," said the fisherman, "but it hadn't ever occurred to me that the sky wouldn't be reflected in a black mirror. you're right, though. the clouds do stand out well! you ought to be able to get some good pictures from your mirror." "i have got a lot, sir," said ralph. "i've made three cloud photographs every day, rain or shine, for over two months now." "every day?" "yes, sir, before breakfast, after dinner, and just before i begin my evening chores." "what's the idea of that?" finding a ready listener, ralph plunged into the story of the mississippi weather league and of his crippled friend, anton. "it's a mighty useful piece of work," the fisherman commented, when the lad had finished, "and i'm especially interested in these cloud photographs of yours. i need some. have you any prints of them?" "yes, sir," was the reply, "heaps." "if they're really any good, i might be able to use a few," the fisherman continued. "i'm writing a series of articles for an outdoor magazine and i want some mississippi river pictures pretty badly. mine haven't come out particularly well." "i'll show you all i've got," eagerly replied ralph, and, a little later, he took the stranger home with him. there he displayed, not only his cloud photographs, but also all the snap-shots he had made with his camera during the three years he had owned it. the magazine writer was highly delighted, for many of the pictures were exactly what he needed, and when he went away he took with him thirty photographs, for which he paid ralph, as he said, the "regular price" of three dollars apiece. "that's what they'd have to pay if they bought them from any of the news photo houses," he remarked, "and you might as well get the same." to ralph this ninety dollars was a fortune. he offered to turn the entire sum over to the league, or at least that part of it which had been paid for the cloud photographs. ross vetoed this offer, on the ground that the league itself had not earned the money. instead, ralph put away some of the cash and with the rest he bought a new lens for his camera. with this lens he was able to take cloud pictures even better than his former ones. a few weeks later, at the next monthly feast of the league, ralph came proudly forward with a collection of over one hundred cloud photographs. "i don't see, fellows," he said, "why we all couldn't have a shot at observing the clouds. i was talking to anton the other day, and he didn't seem to know anything about the names of the clouds at all. i dug 'em up from a book i've got at home. i was thinking that it would be rather jolly if each member of the league had a set of cloud photographs for himself, with the right names of the clouds and all that sort of thing on the back. it isn't much trouble to make prints." "i'd like to have a set, ralph," said ross promptly. "i hate to feel like a dub and not know about the clouds. it's like not knowing any of the stars." "there certainly ought to be a set in the office of the _review_," declared its editor-in-chief. "i've been wondering," began anton, "whether mr. levin wouldn't pick out the best ones and tell us exactly what they are. i had an awful job trying to get ralph to bring his pictures to-day; he said he wanted to wait until he had perfect ones." "you'll wait a long time, my boy," the forecaster put in, "if you wait until you have a perfect set. i don't know of such a set anywhere in the world. clayden, in england, has got some fine examples--" "it's his book i've got," interrupted ralph. "there are a few good pictures in that," the weather expert said. "loisel, in france, has some good examples and our own weather bureau has done quite a little cloud work. but those i've seen of yours, ralph, are quite good. if you like, i'll go over them for you and pick out the ones that are the most characteristic. your plan to give a set to each of the boys is quite worth while. let's see the pictures, ralph." the "official photographer" pulled out, from a bulging inside pocket, a large bundle of photographic prints and spread them on the table. the collection included both the pictures ralph had taken with his new lens and some of the old ones intensified in the way that his visitor had showed him. they made a striking contrast, in their vivid black and white, to the cloud pictures, printed in a pale blue, issued by the weather bureau. "i think ralph's pictures are away ahead of the weather bureau ones," declared fred. the forecaster shook his head. "some of them are prettier pictures," he said, "but the weather bureau sheet is chosen to help observers classify the clouds. if you notice that blue sheet of cloud forms that washington has issued, you'll notice that they are very carefully selected and that you really can tell the various types of cloud from them. at the same time, clouds are hard to classify, because, at any given time, you're looking at a stretch of sky--counting the separate layers of cloud--several hundred square miles in extent, and, generally, there are many different types of cloud in the sky at the same time." "how many kinds of clouds does the weather bureau name?" asked anton. "ten," was the reply. "there are lots of variations in those main groups, but that's enough to begin on. the general idea of the classification is by the heights of clouds, the cirrus group being the highest, from about six to ten miles, the alto group, ranging from two to six miles, and the cumulus and stratus groups below that. here," he continued, picking out a photograph that showed only a few faint specks of white, "is a true cirrus. it is the highest of the clouds, and, as you can see from the photograph, it is delicate and fibrous. this one, that looks like the ghosts of feathers, is another form. "cirrus clouds always appear to move slowly, because they're so high up. as a matter of fact, they fly along at the rate of from one hundred to two hundred miles an hour, and generally in an easterly direction. this photo that looks as if the clouds were a whole pile of spiders' webs, all mixed up, is the second class of clouds, known as cirro-stratus. did you happen to notice, ralph, whether there was a halo round the sun when you took this?" "yes, sir, there was," the boy answered, "but it hasn't showed up on the plate. i've got some halo pictures at home, but i didn't think of bringing them along. i just brought my cloud stuff this time." "well," said the forecaster, "suppose you put one of those in here as an example of cirro-stratus. there couldn't be a halo without it. all the upper clouds are made of ice crystals and it is the refraction of the sunlight through these ice crystals that forms most halos. by the way, boys, don't confuse a halo with a corona. they're quite easy to tell apart, because a halo, unless it is one of the unusual white ones, always has red as the inside color and a corona always has the red on the outside." "how can i tell them apart on a photograph plate, sir?" asked ralph. "that doesn't show any colors." "by their distance from the sun," the meteorologist replied. "halos are seldom seen except at distances of about twenty-two degrees and forty-six degrees from the sun. there are lots of others, but they are rare. you'll soon learn to catch those distances by eye. coronas are usually much smaller. "i think one of the most striking forms of cirro-stratus is the polar 'band,' which stretches from one side of the sky to the other, like a wide white road." "ah knows that one, mistah levin," put in dan'l. "noah, he done stretch that road for the animals to get out of the ark." the forecaster glanced at the aged darky. "you certainly did manage to pick up a lot of queer superstitions in that article of yours, dan'l. i've heard that cloud called a noah's ark cloud, but i never knew why." "yas, suh; oh, yas, suh," dan'l repeated earnestly, "noah, he done make that cloud, jest like the rainbow was made to convince noah that there weren't goin' to be no more floods." "a high cirro-stratus which looks as if some cream had been poured on the blue sky and hadn't mixed properly yet," the forecaster continued, "is cirro-nebula. it's very hard to photograph, and even when you do get it on a plate, it doesn't look like much. "now the third one in the classification is very familiar. that's the well-known mackerel sky. what's the rhyme about that, dan'l?" proud at being thus appealed to, the darky quoted triumphantly: "mackerel scales and mares' tails, make lofty ships carry low sails." "that's correct," said the weather expert, "because those clouds foretell wind. sometimes the cloud flakes are less solid and look like the foam in the wake of a steamer. "beneath them come the alto clouds, which are made up of drops of moisture instead of crystals of ice. the fourth class, called alto-stratus, is a thick sheet of gray or bluish color, sometimes thin enough to let the sun shine through. when lower and in heavy roundish masses it's called alto-cumulus, which is the fifth on the list, and when it is lower still and looks like a lot of great blue-gray footballs wedged closely together it is known as strato-cumulus." he shuffled the prints rapidly, selecting types of clouds as he did so, and pencilling on the back the character of the cloud. "then comes the cumulus, the big round cloud, that looks like masses of fluffy cotton wool piled on top of each other. these are the 'woolpack clouds,' which, in summer time, throw deep shadows on the grass. it is this cloud which, when it comes between you and the sun, gives rise to the old saying that 'every cloud has its silver lining.'" "those aren't the thunder clouds, sir, are they?" the photographer asked. "no," the forecaster answered. "the thunderstorm clouds are called cumulo-nimbus. they're heavy masses of cloud rising in the forms of mountains or towers. isn't there a rhyme about clouds and towers, dan'l?" "yas, suh, there's a rhyme," the old darky replied, and he quoted: "when clouds resemble domes an' towers the earth is wet with frequent showers." "that, boys," the weather expert said, "is another true proverb, because the description applies to thunderstorm clouds, when the rain is likely to fall in frequent showers." "it doesn't look like a regular rainy sky, though, mr. levin," said anton. "i thought rainy skies were usually heavy and gray." "they are," the forecaster answered, "and the weather bureau gives all the rain clouds the general name of nimbus, which simply means a thick layer of dark clouds, without shape and with ragged edges, through which rain or snow falls steadily. sometimes, when there is a powerful wind in the cloud layer, the lower edges of the clouds are broken apart, or loose clouds are seen traveling fast under the overlying gray. sailors call this scud." "mr. levin, suh," broke in dan'l, "ah knows a rhyme for scud, too," and he quoted: "scud above and scud below shows there's goin' to be a blow." "well," said the forecaster, hesitating, "that's not quite as good as some of the others, because you don't see scud until the wind has already come. as a whole, though, it's right, because it implies that the atmospheric currents are powerful, and if the rain disappears, a wind is likely to follow. i noticed you missed the rhyme about the rain before the wind, in your article, dan'l," he continued. "yas, suh!" the darky answered, "ah don't know that one." "it runs like this," the forecaster answered: "when the rain comes before the wind, be sure to take your topsails in, when the wind comes before the rain, you can put them on again." "that's a good one, too, because high winds and steady rain seldom go together. "the last type of clouds, which is number ten in the weather bureau classification, is called stratus. it really looks like a lifted fog, which sometimes it is. indeed, there is no essential difference between clouds and fogs, anyway, except that fogs are formed at the surface and clouds above it." "all clouds are fogs, sir?" said anton, in a surprised voice. "yes, my boy. clouds are visible water vapor. their visibility depends largely on condensation, just as rain depends largely on the dew-point." "what's the dew-point, sir?" "the dew-point," the forecaster explained, "is the temperature at which the air becomes so full of vapor that it can't hold any more without letting it down as rain or snow. it's never the same any two days in succession, because the air can hold more water vapor when it is warm than when it is cold." "is that why muggy days are so uncomfortable?" asked ross. "yes. when the air is full of water vapor, it hasn't the same readiness to absorb it. when you perspire on a dry, hot, windy day, the air absorbs it right away, but on a day that's humid or muggy, the air can't hold any more, so it doesn't evaporate and the perspiration trickles down your back and into your eyes. a moist climate feels hotter in the summer and colder in the winter than a dry one, although, in reality, it isn't as hot or as cold. every moist climate is a cloudy climate, and ireland--which is called the green or emerald isle because there's so much rain that none of the vegetation ever dries up--has some of the most beautiful clouds in the world." "is there any place in the united states without clouds?" asked ralph. "there's no place in the world that's absolutely cloudless," was the answer, "but clouds in some deserts are few and far between. there's one well known hotel, in the southwest, that advertises 'free board every day that the sun doesn't shine.' it's a safe offer, too, for last year they only lost two days on it. there are some clouds there, but not such as to obscure the sun. "in a cloudless country, boys, there are great extremes of temperature, as much as forty to fifty degrees between noon and midnight. you'll get sunstroke in the early part of the afternoon and shiver under blankets in the evening. that's because there are no protecting layers of clouds to equalize the radiation. the air, especially high up, is very cold. don't forget that the upper clouds are all made of ice crystals." "i've been wondering," said anton, "how you can find out that it's so cold high up in the air if no one can live up there?" "balloonists have often passed through clouds of ice crystals and snow," the forecaster answered, "though, of course, they've not been as high as the upper clouds. many observations have been made by releasing small sounding balloons with an instrument attached, letting them go as high as they could, until they burst and fell to the ground. but much of our upper-air exploring has also been done with kites." "kites? like franklin's?" "not quite," said the forecaster; "our weather kites aren't built like that. they look more like a box. i'm expecting one here, every day." "here?" "yes, boys," the forecaster answered, "right here. there's a young chap i know who used to work with william a. eddy, of new jersey, the father of scientific kite-flying in this country. i wrote to young osborne, and sent him a copy of the _issaquena county weather review_, the one with the sunset articles and pictures in it." "osborne, sir!" ejaculated the editor-in-chief, "i got his subscription just a week ago." "did you?" said the forecaster, interested. "that's nice of him! he wrote to me that he was constantly improving his kite models and that he had a couple of old ones which he now seldom flew. he sent me their records, too, so i know they must be good kites. he wanted to know if the mississippi league of the weather wouldn't do some kite-flying and send him records of the observations." "would we?" cried the enthusiastic monroe. "i should say we would!" "it means quite a bit of trouble," the weather expert warned them; "scientific kite-flying needs machinery." "why, sir?" asked ross. "can't we do it by hand?" "no," was the reply, "you can't. how would you reel the kite home? it's a very different thing sending up a japanese paper kite on a string a few hundred feet in the air, and making an ascent of a couple of miles with a weather kite. for one thing, the weather kite is flown with wire and an especially strong kind of wire at that." "where will we get the wire?" "i've advanced the money for it," the forecaster answered, "and for the shipment of the kites. i thought, perhaps, after a while, we might hold a kite contest and charge an admission fee, because, as you know, i think the league should be on a self-supporting basis. i'll render you a bill, then, and you can pay me." "thanks ever so much, sir," said ross. "that's fine. we'll do it. but who's to have charge of the kite-flying?" "that's your affair," the forecaster answered. "i've nothing to do with the inner workings of the league." "i've been wondering," said anton, "if tom oughtn't to do it. he's our wind expert." tom flushed with pleasure at the suggestion. "i haven't done much on the wind stuff," he admitted; "there didn't seem anything to do but to take measurements and things." "i seem to remember reading them weekly in the _review_," the forecaster remarked. "oh, i've done it all regularly enough, but it didn't seem to be of much use," the boy said. "you'll find that it will be of a great deal of use in the league's kite work," the weather expert rejoined. "i think anton's right," put in ross. "hands up those who think tom ought to do it." every hand shot up in the air. tom shuffled his feet on the ground and squirmed uneasily. "all right," he said, "i'll try. you'll tell us what to do, mr. levin." the next few weeks were busy ones for the mississippi league of the weather. the building of the kite reel, more than anything else, gave the boys a sense of the power of the new force that they were going to handle. the _weather review_ announced the expected arrival of the two kites, and the interest of the neighborhood was aroused. not since the days of the civil war had anything given the farmers of the district as much to talk about as did the weekly issues of the _issaquena county weather review_, and the people of the county took the keenest interest in all the doings of the league. fred had been anxious to make the paper bigger and more important, as soon as it became flourishing, but he was held back in this by the conservative and laconic bob. the wireless expert showed him that as long as the paper was kept small and easy to get out, it could be kept good. as a result, everything had to be condensed, and every bit of the little sheet was interesting. twice the _review_ was quoted in important meteorological journals and various weather periodicals were sent as exchanges to the office. it meant a lot of work for the editor-in-chief, but fred's father, realizing that the post was an excellent training for his son, released him from all his saturday chores. at last the word came that the kites had actually arrived. a farm wagon was sent in to fetch the wooden cases, and that wagon, when it drove into town, had every member of the league on board, all excited and chattering like so many magpies. rex and lassie, the pair of four-legged members of the league, also came along to give dignity to the occasion. permission had been secured from tom's father to use part of the pasture as a kite-flying station, and, bright and early the next saturday, the league gathered at the wind-measurer's home to see the cases unpacked. mr. levin also came, to give advice and suggestions. "what's the direction of the wind, tom?" he asked. the boy glanced up at his home-made weather-vane, which had been adjusted so that it was right to the fraction of a degree. "south-southeast, sir," he said. "is it steady or veering?" the weather expert continued. he was anxious that tom should feel the importance of his wind observations. "what was it this morning?" "i'll see, sir," said tom, and hurried into the house for his book on wind observations, which he had kept faithfully, though, in all the five months of the league's work, there had been no opportunity to make use of them. "it was south--a quarter--east this morning," he answered quite importantly. "and what is the present velocity?" came the next query. tom ran up the short ladder to the dial of his robinson anemometer or wind-measurer. this consisted of four cup-shaped pieces of metal fastened to four arms at right angles to each other, and set horizontally in a socket. the force of the wind on the open cup-shaped sides was so much stronger than on the convex or rounded sides that the anemometer whirled around quite rapidly. "say," said one of the boys as he watched tom, "i didn't know he had all this down so pat! it's great!" "fourteen miles an hour, sir," said tom, as he ran down the ladder, "by the anemometer dial." "well," the forecaster replied, "fourteen miles an hour is a good enough breeze for kite-flying. how about it, boys? shall we try a flight to-day?" "oh, let's!" the boys exclaimed. "very well," said the forecaster, "we'll put the kites together. have any of you ever seen a weather kite?" he queried. "i've seen a picture of one, sir," said fred. "i saw it in one of the weather bureau booklets. it looked like a box with the ends knocked out. are these like that?" "yes," the weather expert replied, "all over the world the hargrave or box kite is used. there's a little difference in the methods of bracing the frames, but the principle of them all is the same." "are they the best kites for lifting, sir?" asked anton. "i saw a picture, once, of a man being carried along the ground by a kite, but it didn't look like this. it was like a lot of little triangles all piled one on top of the other." "that's a different kind," the forecaster answered, "it's called a tetrahedral kite, and was invented by dr. alexander graham bell. they will lift a man quite easily. owing to the form of construction, they're much heavier and harder to handle and they won't go up as high. the box kites fly higher and more easily. they'll go up even in the lightest wind, and that's quite important, boys, because you must remember that sometimes there's quite a strong wind in the upper layers of the air when there's only a zephyr below. as you see, boys, this kite consists simply of four long sticks arranged in a square, with one third of the length at either end covered with a specially treated and tightly stretched muslin." he was working rapidly as he talked, and, before long the kite was assembled, the wire attached and wound on the reel and all was ready for launching. "will that wire hold it, sir?" asked ross, as he noted the extremely fine line that the forecaster was using. "certainly, it's piano wire. it's only a thirty-second part of an inch in diameter, but it will stand a pull of nearly three hundred pounds. that's more than you could pull. more even than monroe could pull, and he's the strongest of you." "couldn't i hold one of those small kites, sir?" asked monroe. "yes," the forecaster said, "you could with a well-made hand reel, and if the wind wasn't too strong. but your arms would soon give out. of course, the pull of a kite depends on the amount of square feet of sail area. anton," he added, turning to the crippled lad, "you're the mathematician of the league, measure that kite and tell us how many square feet of sail area it has." anton took a foot rule from his pocket and measured the kite rapidly. "a trifle over thirty-six feet, sir," he said. "i can give you the fractions, if you like." "no, that's near enough," said the forecaster. "thirty-six feet of sail area in a fourteen mile wind will lift nearly twenty pounds of wire and, probably, will have a pull of about sixty pounds. i don't think you'd care to stand a sixty-pound drag very long, monroe. we'll let our new reel do the work." "about how high could we make this kite go, sir?" asked tom. "does that depend on the wind?" "no," the forecaster answered, "it depends on the sail area of the kite and the weight of the wire. ten square feet of sail area will lift three pounds or, a thousand feet of wire. there are over five thousand feet to a mile, and a kite usually ascends at about an angle of forty-five degrees. so, if you allow for sag and so forth, you'd have to put out eight or nine thousand feet of wire to reach a mile, wouldn't you?" "yes," said tom, "i guess that's how it would go." "it's an awful lot of line," commented fred. "therefore," said the forecaster, "if ten square feet will lift a thousand feet of wire, for eight thousand feet, you'd need eighty square feet of sail area." "then even the two of these together aren't big enough to go up a mile!" cried tom. "a mile is pretty high, my boy," said the forecaster; "you've never seen a kite go up a quarter as far." "what's the highest flight that ever was made?" queried tom. "america holds the world's record," was the answer. "the united states weather bureau sent up a string of kites at mount weather, in virginia, that ascended higher than four miles and a quarter, , feet above the reel, to be exact." "how many kites did they use?" tom asked. "eight," the forecaster answered, "with a lifting surface of five hundred and forty-four square feet of sail area. there wouldn't have been much chance for you, monroe, if you'd tried to hold that bunch in your hand. the kites would have picked you off the ground and whisked away with you like a piece of rag tied to the tail of a japanese kite. there," he concluded as he stepped back, "i think we're ready now. tom, how's the wind?" the official wind-measurer ran up the ladder to his dial, calculated rapidly and answered: "freshening, sir. it's about seventeen miles an hour, now." "that's all right," the weather expert declared. "tom, you start her off." "what do i do, sir?" asked the boy. "just toss the kite in the air," the forecaster answered. "don't i have to run with it?" "not a step, except when the wind is very light. off with you!" tom carried the kite about a hundred feet, the line paying out as he went, and waited the word. the boys clustered around the reel excitedly. monroe went along with tom. rex also wanted to follow, but as ross was afraid that he might jump at the kite and tear it with his teeth, though in play, he called the terrier back. "ross," then said the forecaster, "you take the time of the flight, and anton, i think you'd better watch the reel and see that the line doesn't foul." the excitement of the boys grew intense. the box kite looked so unlike any of the kites that they had flown that some wondered whether it would go up in the air. fred, in his capacity as editor, having seen a picture of a box-kite up in the air, was quite arrogant in his assurances that it would really fly. "are you ready?" the forecaster said, watching the whirling anemometer. "throw!" at the word, tom gave the kite a light toss in the air, against the direction of the wind, as indicated. the kite swayed from side to side, but having four surfaces to the wind, did not swoop and dive like the flat kites. only half a dozen times did it dart from side to side, then the current of the wind caught it at the right angle and it began to climb up into the air. tom waved his cap at it with an excited cheer, in which all the boys joined. the first kite-flight of the league was on! smaller and smaller grew the kite, climbing until it was almost out of sight. the rattle of the reel, as the wire ran out, was music in the boys' ears. when the half-mile mark on the wire was passed, the forecaster said: "i think that's enough for a first flight, boys. better pull her in." some of the boys begged that the kite might be allowed to go up a little higher, but the home-made reel was a trifle rickety and would need strengthening. winding the reel by hand took quite a long time, but the kite came to the ground, safely, unharmed. from that time on, kite-flying became a passion with the boys. the official measurements of the weather bureau kites were secured, together with diagrams showing exactly how the kites were to be built. before a month was over, every member had a kite, and, as kite-races were to be held, every boy had to build his kite himself, absolutely without any outside help. it was nothing less than amazing to see how these kites, all built on the same pattern by different boys, behaved differently. it seemed almost as if the characters of the boys appeared in their kites. bob's was the slowest and most powerful, anton's the fastest but behaved poorly in a strong wind, monroe's was absolutely useless in a zephyr. tom, who up to that time, had felt that his share in the work of the league was extremely small, now found himself of great importance. he thought of kites in every spare minute of the day and dreamed of kites at night. his father had to forbid the mention of the word "kite" at meal-times. the lad made fliers of every shape and pattern, and his kites were usually so stable that it was upon his model that the meteorograph was fastened which registered the pressure, humidity and temperature of the air and the velocity of the wind, according to the request of the young fellow who had sent the league the two first kites. the _issaquena county weather review_ was compelled to run a regular weekly feature of "kite records" and few were the weeks without a flight. at last came the fateful saturday, the last saturday in october, the day set for the kite races. many of the boys had made new kites for the occasion and all had overhauled them. secret practice flights had been made and the rivalry was keen. what was the wind going to be like? would the day be fine? it was hinted that tom had some special secret, but what it was no one knew, unless, perhaps, the forecaster. the event had been quite widely advertised--had it not appeared in the _review_!--and the neighborhood gathered as though to a country fair. the roped inclosure was full of people and the dimes which rattled into the dried gourd more than paid up the club's indebtedness for the wire and the shipment of the kites. there were all kinds of races, races for speed, to see whose kite would reach a certain height the soonest; races for steadiness; races for altitude. anton created great excitement by sending up one of the puppies in a basket attached to a parachute fastened to a kite which was released when he pulled a string. it was a big parachute and a small puppy, so that no one feared for the pup's safety. ross then came forward with his big kite. it could not be entered in the races, because all the kites for racing had been of standard size. "what are those little balls?" one of the boys asked, pointing to bundles covered with paper and attached to a leading string, which were fastened at fifty-foot intervals to the leading wire. "you'll see," said ross, and up went the big kite. it flew steadily and well and when a couple of hundred yards above the ground, he made it fast to one of the stakes. then, while every one watched, he gave the leading string a sharp tug, and then a succession of pulls, breaking loose each of the little bundles attached to the leading wire. and, as the people looked, first one and then another american flag burst out of its covering, the lowermost and largest bundle being a big stars and stripes that floated out gallantly above the kite-ground. "now," said ross, turning to the kite-master, as the boys had begun to call tom, "out with your secret! what is it?" tom turned to the forecaster. "is it all right for to-day?" he asked. the weather expert looked keenly at the sky, glanced at the weather-vane and the whirling anemometer, and nodded his head. "i think so," he said. "the weather's a little gusty, but this is the time to try. nothing venture, nothing have!" at the word, tom ran off into the house. the boys watched him, wondering what new contrivance the kite-master was going to produce. he reappeared in a moment, carrying with him a new kite, a little larger than the others, but of the same usual pattern. this was not particularly exciting. he laid the kite down on the ground and ran into the house again. in a moment, he was out again with another. "going to fly them tandem?" asked ross. tom did not answer. he laid that one on the ground and returned into the house again. "do you suppose he's got three?" anton asked. this was amazing riches, three kites. all the boys knew what a tremendous amount of careful and exacting work went into the making of even one of them. out darted tom and laid a third and then a fourth kite on the ground. the four great kites, each of them with the forward part white and the rear section painted black, made a noble showing in the afternoon sun. ralph, with his ever-ready camera, stepped forward. "wait a minute," said tom, "i've got another one," and he darted into the house to get it. he returned a moment later with a fifth kite, similar in every detail to the other four and then, readily enough, posed beside the kites for his picture. overhead flew the stars and stripes. "i want that for the _review_," said fred. "what are you going to do, tom?" asked ross. tom hesitated a moment and then announced: "i'm going to try for a world's record!" the audacity of this startled the boys for a moment, and then a shout went up, while word was passed around the crowd that issaquena county was going to try for the kite record of the world. the first kite, which no one but tom and the forecaster had yet seen in flight, took the air and was off. tom gave it four hundred feet of line and then fastened his second kite, which he let run up until eight hundred feet more of the line was out. the wind was now stronger, registering twenty-two miles an hour. the three lower kites were run in tandem, about two hundred feet of line apart. when the last of the five kites was still on the ground, the topmost one was out of sight, and the kites were carrying only a fraction of the weight of wire that their lifting surface could bear. "i'm afraid of it, sir," said tom, his finger on the wire that was running from the reel, "it doesn't feel right." "probably your lower kite is in gusts," the forecaster answered. "let her go up, there may be calmer wind higher. fasten on your three small ones, now, tom; you might as well have all the sail area that you can." the eighth kite was started on its journey upwards. only those with the strongest eyes now could see the second group of three, the first pair was far out of sight. with anton carefully measuring the angle of altitude and giving tom the figures in a low voice, tom, watching the registering apparatus on the reel, suddenly announced: "two miles up!" the reel rattled merrily as the line was paid out, the brake keeping it at exactly a uniform pressure under tom's skillful guiding. "two miles and a half!" the crowd began to press around the reel. nothing was visible in the air, now, nothing but a thin piece of wire leading up into the sky. had no one known that the kites were there, high above the clouds, it would have seemed like black magic. some of the superstitious negroes began to mutter among themselves. "three miles!" the boys yelled in delight. "up with her, tom!" cried fred. "it's the amateur world's record!" announced the forecaster. the words were scarcely out of his lips when there came a sudden sharp crack. the kite-wire snapped close by the reel and as it curled on itself the coils appeared to run up into the sky. "gone! my kites are gone!" cried tom, and a perfect howl of disappointment went up from all the boys. "gone!" cried the forecaster, "of course they're gone, but we're going after them!" throwing himself on the back of an old mule which a darky had ridden to the kite ground, he started full tilt after the disappearing wire, the whole membership of the league streaming at his heels. chapter vi defeating the frost out across fields and woods, the forecaster leading on the old mule, the boys followed the direction of the kite. bob's pocket compass held them true to their course and tom's keen sense told of any shift of the wind. the boys ran fast, the mule ran faster, and lassie and rex ran faster still. only anton, the crippled lad, had stayed behind. midway up the first hill, fatty dropped out. his intentions were good, but he was no match for the others in running. monroe, the athlete of the group, was swinging along in light springy strides; bob, the silent, ran heavily and mechanically; while tom, eager for the recovery of his kites, kept to the front with the other two. the forecaster checked his mule and let the boys come up to him. "it's no use trying to outrace the kites, boys," he said, "they're dropping in any case. but as they were three miles up, they were also three miles to leeward, and as they won't fall like a stone but float down gently, it'll be another mile or two at least before they strike ground. so you've a five mile run ahead of you and you'd better settle down into a jog trot, for you can never keep up this pace." the faces of the boys fell at the thought of a five mile run, for while they were all strong and vigorous, cross-country running was not one of their regular sports. ross turned to the younger boys of the party, calling them by name. "you'd better drop out," he said kindly; "you won't be able to keep it up and there's no use getting yourselves worn out and then having to walk back, half dead. fred," he continued, turning to the editor-in-chief, "you'd better quit, too." "not much," answered fred, "i've got to write this up for the _review_." the forecaster smiled. he liked pluck. "all right, my boy," he said, "come along, if you want to. still, i think ross is right." over fields and woods they ran, but it was an hour before bob, lean, wiry and silent, pointed to the sky. "kite!" he said. the weather expert pulled up the mule and drew out his field glasses. "yes," he said, "that's the string of kites, sure enough. but they're going up, boys, not coming down." "going up, sir?" exclaimed tom. "they couldn't be! they must be coming down. all the kites were out of sight when the wire broke." "they have come down, of course," the forecaster replied, "but they're certainly going up now. and, what's more, they're going up fast." "but they can't be!" the boy protested. "the wire isn't holding on to anything." "how do you know?" the meteorologist rejoined. "perhaps the wire has got foul of something. i remember, once, how eddy of bayonne had a string of nine kites get away from him. they crossed the water between new jersey and staten island. the owner had to take a train and then a small boat after them. on staten island he took another train and then a street car, and another street car, all the time hanging out of the window, to keep track of the fugitives, which were sailing away merrily." "chasing a kite with a train and a street-car sounds funny," puffed tom. "on staten island," the forecaster continued, "the wire caught in a telegraph post, and, of course, as soon as the wire held, the kites took the proper angle to the wind and shot up in the air again. before eddy could reach the place, the wire chafed through and broke again, but the kites had risen another mile or more. falling diagonally, they crossed the lower end of new york bay toward long island. eddy had to take a ferry boat, next, to chase the runaways. he crossed to new york and took the elevated railroad to brooklyn. an hour later, he caught sight of the kites again. one of the groups had reached the ground and dragged. that sent the other six up in the air again. they flew over the whole of brooklyn and fell again, finally entangling themselves in a telephone wire. when the owner finally reached them, after a chase of thirty miles, in two states, three of the kites, still undamaged, were flying safely in the air, never having come to ground at all." "i hope mine aren't smashed," tom said eagerly. the story had given him hopes. on the boys pounded. fred was at the end of his strength. ross, himself, was almost done out, but he felt that, as head of the league, he ought to go on. seeing, however, that the editor-in-chief might really hurt himself unless he gave in, ross decided to stop. he knew that fred would give up if he did. "i've had enough, fred," he said at last. "let the other three go ahead. we can't hope to beat monroe." the editor stopped, willingly enough. he looked a little longingly at the other three, as they ran on. "i'd have liked to be there, so as to write it up," he announced wistfully. "you can't be everywhere, fred," ross answered, and the two boys turned homewards. monroe, bob, and tom, with monroe leading, swung on their way. twenty minutes more passed. tom's heart was beating like a trip-hammer and there was a drawn look about his face which showed that he was nearly done. bob, who had not uttered a word since he first saw the kite, and who had not varied his pace by a fraction since he began, was jogging along as though he were a machine. monroe still ran springily and with the jauntiness which betokened the practised runner. then, suddenly, the forecaster pointed ahead. "there's something caught in that tree!" he said. in another minute the kite wire could be seen. it had hooked its coils into a bale of barbed wire, and in trying to lift this had entangled the bale in the branches. as though he were starting for a hundred yard dash, monroe sped ahead. grimly, bob tried to catch up to him, but it was like a bull-dog chasing a deer. tom, his face in the tense grin of exhaustion, struggled bravely, but dropped behind step by step. monroe was within fifty feet of the tree when a sudden thought struck him. he slowed down, and as bob caught up to him, said in a low voice: "tom's made a great run! let him be the first to get there." bob nodded. as the pace slowed down, tom, his gait a little staggering, caught up with the other two and passed them. he reached the tree first and looked up. "my kites!" he cried. "and i got the amateur record!" and he collapsed on the ground at the foot of the tree, worn out but supremely happy. with the approach of winter, kite-flying became less popular as a sport, but two or three times a month tom sent up one of his kites with the meteorograph, and the observations were faithfully forwarded to osborne, whose original gift of the two kites had been the stimulus to the mississippi league of the weather. the first few flakes of snow turned the attention of the boys to an entirely new line of weather observations. many and many a time had the boys noticed the strange shapes of snow-flakes, but without paying much attention to them. on the first saturday after the light snow-fall, however, three different boys brought in rough drawings of star-like and feather-like snow forms that they had noticed. "i've been wondering," said anton, thoughtfully, "what makes snow-flakes take those shapes? hail comes down in lumps, and rain-drops must be round, because when you see the first heavy drops of a shower they make round blobs on the ground with pointed splashes at the side." "a snow-flake," the meteorologist replied, "is a collection of icy crystals. if you could look at one under the microscope, anton, you'd see that every little projection that goes to make up the shape of the flake, is a six-sided crystal. you've eaten barley-sugar from a string some time, haven't you?" "sure!" said several of the boys, and one added, "mother often makes it." "how does she make it?" queried the forecaster. "melts up some sugar and water and, as when it begins to cool off, she hangs a string in the middle of the pot and the sugar settles on that." "it settles in regular shapes, doesn't it?" "yes." "well, those are crystals. when water cools into ice, boys, it does the same thing. haven't you sometimes seen, after a cold night, a lot of needles shooting out from the sides from a puddle?" "yes, sir, often." "those are all six-sided crystals. frost on the window pane is made in the same way. all those designs that look like lace work or trees or ferns are six-sided crystals produced by water-vapor, in the air, cooling and crystallizing on the cold glass. ice crystals grow from each other quite readily. this is called twinning." "but why are they always so regular?" the forecaster shook his head. "you're always expecting everything to be regular, ross," he said. "they're not regular at all. there are thousands of different forms. the united states is fortunate in having one man who's the world's expert on snow crystals, and he examines and photographs thousands every year and adds, perhaps, two or three new examples each season." "who's that, sir?" asked fred. "wilson a. bentley, of jericho, vermont," the forecaster answered. "he's made thousands of photographs of snow crystals through a microscope. what's more, he's done it for the love of the work. why don't you send him a copy of the _review_, fred? i'm sure he'd like to see it. perhaps he might send you some prints of his snow crystals. he'd appreciate a plate of cæsar's sunsets and ralph's clouds, i'm sure." "i'll send them to him right away," the editor answered. "why is it," queried anton, "that when snow-flakes fall slowly and only a few of them at a time, they are big, but when there's a heavy snow-storm the flakes are small?" "because they are manufactured in different layers of the air," the forecaster answered, "in the upper air, eight or ten miles up, where the faintest cirrus clouds are, they are not flakes at all, but tiny needle-like crystals, called spicules. in the depth of the arctic winter, near the north pole and especially on the greenland ice-cap--one of the coldest regions of the world--the wind is full of these spicules, which one can't very well call snow. "snow-flakes that come from the cold regions of the air, three or four miles high, generally have a solid form. all, of course, show the six-sided form of the snow crystals. being smaller and heavier in proportion to their surface they fall more quickly. in the layers of the atmosphere, one or two miles high, where the air is not as cold and where the content of water vapor is higher, the flakes have more opportunity to grow as they slowly sink through the air. snow-flakes that have been formed only a short distance above the ground become large and feathery, the kind of which northern peoples say that 'the old woman of the sky is plucking her geese.'" "i suppose, in the northern part of the country, sir," ralph suggested, "snow has to be measured, as well as rain." "certainly," the forecaster answered, "otherwise we wouldn't be able to tell the precipitation of a region at all. there is a regular instrument for it, called a shielded snow-gauge. this is like a rain-gauge, boys, only it stands ten or twenty feet above the ground, to avoid surface drifting. the snow caught is melted and expressed as so many inches of precipitation. sometimes the depth of snow is measured by thrusting a measuring stick down to the ground. "of course, that's not nearly all that the weather bureau has to do with snow. in the northern states, especially of the pacific coast, snow surveys are of great importance. the weather bureau often sends men to determine the amount of snow that has fallen over a given area, in order to find out how much water may be expected. this is needed in flood forecasts and irrigation projects. some of our men, boys, can tell you thrilling tales of their expeditions on snow-shoes up snow-covered slopes where there is never a trail. "railroads whose tracks run through the regions of heaviest snowfall build snowsheds to keep their lines from being buried in avalanches, and these sheds are built to withstand pressures calculated by the weather bureau. where drifting occurs and the railroad tracks are being covered with the drifting snow, it is the combined snow and wind records of the weather bureau which form the basis for the work of the rotary-snow-plow. "even so, boys, the value of the work of the weather bureau in snow surveys is very small compared with the importance of frost warnings. these save the country tens of millions of dollars every year, especially in the fruit sections." "you mean by smoking them?" queried ross. "father heard about that a couple of years ago and bought a lot of fire-pots for his orchard." "how did he succeed?" asked the forecaster. "he didn't succeed at all," the boy answered. "there were only two bad frosts that spring, and both times the evening before had been so warm that no one suspected that there would be frost before morning. the one night that he did start the fires, it turned warm towards midnight and we wouldn't have needed the fires any way. old jed tighe, who's got the biggest fruit farm here, has made fun of father's fire-pots ever since." "now, if your father had received the weather bureau's frost warnings in advance," the forecaster said, "he wouldn't have wasted fuel on the night that there wasn't a frost and he wouldn't have let his crop freeze on the nights that the temperature really did drop below the danger point. for example, boys, if the league of the weather had been in existence at that time and could have given good frost warnings, all that crop would have been saved, wouldn't it?" "yes, sir," said the boys, "it would." "of course," the forecaster continued, "a really progressive fruit-grower ought to make himself partly independent of the weather bureau. he can put up frost-alarm thermometers." "what are they, sir?" asked anton. "they're thermometers with an electrical attachment, something on the principle of the thermostat, which you see nowadays in big buildings. a thermostat is electrically connected with a tiny lever, and when the air of a room gets to a certain heat, the increasing temperature operates a lever and closes the steam pipe which brings the heat. when the temperature falls below a certain point, the lever is released and the steam rises again. the same principle is used as a fire alarm. when the air inside a building rises to a point hotter than it could naturally do, it operates a lever which rings an alarm bell. the frost thermometer acts exactly on the same principle. when the temperature of the air, near a fruit orchard, falls to within three or four degrees of the point at which the fruit will be harmed, the fall of the mercury breaks an electric circuit which starts an alarm bell ringing in the owner's house, perhaps a half mile away." "i've been wondering," began anton in his meditative way, "whether it wouldn't cost more to heat all the out-of-doors than it would be to lose some of the fruit." "you haven't got the idea of it at all," the weather expert said briskly. "it's got nothing to do with heating the whole of out-of-doors." "then what are the fires for?" "just to heat a very small section of the air on the ground. don't forget, boys, that a fruit tree ten feet high may have all the fruit on its lower branches, up to five or six feet, absolutely killed off, while the top branches are unharmed." "how's that?" queried ross in surprise. "i thought frost came down from on top, and that the higher up you went the colder it would be." "not at all," the weather expert answered. "frost comes from down below. when the air is still and clear, the earth loses heat by radiation. the heat goes up and up and through the air to higher levels, the cold earth cooling the air below. therefore, on a frosty night, in a region where frosts are rare, or at a time of year when frosts are few, a still clear night will cause a belt of cold air perhaps only a few inches in depth, perhaps ten or twenty feet in height, this belt being several degrees colder than the air overhead. "now, ross, you can see that to light huge fires, with the intention of warming up all the air, would be foolish and unnecessary. all that is needful is to heat this lower cold belt of air, a few feet in depth, and only to heat it the three or four degrees necessary to bring it to the warmth of the air above." "but suppose a wind comes up and blows the heat away?" asked anton. the forecaster smiled at the question. "if a wind comes up," he answered, "you wouldn't need to use any heat at all, because the wind would mix the warmer air overhead with the cooler air below and there couldn't be any killing frost." "but doesn't it cost an awful lot?" "it costs less than to lose your crop," the weather expert replied. "usually you can figure that a frosty night will take a gallon of oil per tree, or from twenty to twenty-five cents. in a fruit growing section a grower is unlikely to have more than four or five still, freezing nights a year when his crop may be ruined by frost, so that he will spend a dollar or so per tree in protecting his orchard. as there are few fruit trees which bring in a profit of less than ten dollars during the season, and some a great deal more--according to the nature of the crop--the proportionate expense of heating is small compared with the amount of fruit saved." "then you think that heating an orchard will save the fruit?" "absolutely without any question," the weather expert answered. "and, if the fruit-grower will keep in close touch with the weather bureau, he will know when precautions are necessary. of course, boys, it's especially important for this work that there are a number of co-operative observers, because frost is not a widespread general phenomenon. you could have a fearful killing frost down in the hollow where anton's house is, or in the low ground near your house, ross, and still tom's place, on that little hill, would be quite safe. one of the things that the league of the weather ought to be able to do this winter and spring is to see that frost is fought. even when your fathers haven't got regular oil-pots, boys, a few smudges with heavy smoke, drifting over the orchards or the truck fields, if started early enough in the evening may check a freeze." "why, sir?" asked ross. "smoke isn't hot." "no, my boy. but you remember that i told you that the cold was caused by the radiation of heat from the earth escaping into the air and through it. if there's a steady layer of smoke, like a blanket, floating across the land, the heat radiating from the earth will not have a chance to escape to the upper air. it will stay in the lower layer of the air and thus keep it from dropping to the killing temperatures of a true freeze. that's what the indians of the pueblos used to do." in the mild winters and early springs of issaquena county, there seemed little reason for the boys of the league to trouble themselves with frost warnings, but, at the forecaster's urgency, the boys kept wide awake for it. it happened, though, that the lads had talked so much about their frost protection plans that several of the farmers decided to get some oil-burning fire-pots for use that spring, in the event of a freeze. jed tighe, however, one of the few people of the neighborhood who had shown but a perfunctory interest in the league, laughed to scorn the idea of buying the fire-pots, as fred had suggested in a recent issue of the _review_. even jed tighe read the little sheet every week, in spite of his alleged scornfulness. one afternoon, when ross was over at the club-house, where he spent so much of his spare time, anton pointed out that the conditions were ripe for a killing frost. "the hottest to-day was sixty-two degrees," he said, "and you remember mr. levin told us that one wasn't ever safe unless the maximum was sixty-four. there's not a cloud in the sky anywhere and there's practically no wind, and what there is, tom told me over bob's wireless, is from the northwest, and that's the worst quarter. i was just going to take the dew-point when you came in." "let's do it now, anton," said ross. "got the cup?" for answer the crippled lad took down from the shelf a small tin mug. it was already bright and shining, but he polished it until it looked like silver. "i've got the jug of ice-water ready," he said. pouring some tap water into the cup, and filling it about one third full, he began to stir it round and round with a thermometer. the mercury in the tube quickly dropped, until it read °, showing the temperature of the water. "now, ross," said anton, "pour in the ice-water slowly." ross picked up the pitcher and began to let the water trickle in a tiny stream into the bright tin cup. anton went on stirring. steadily the mercury descended in the tube as the water in the cup grew colder and colder. ross poured in more and more slowly. then suddenly, quite suddenly, while both boys were watching, the brightness of the tin cup clouded over, as though with a sudden fog. anton drew out the thermometer and looked at it. "the dew-point's only thirty-four," he cried, "and as we've got to figure frost at three or four degrees lower, it'll be so cold that there won't be any fog to stop a freeze. ross, it's just the night for a killing frost. what do you think we'd better do?" the older lad hesitated. "if you don't mind, anton," he said, "i'll stay to supper, and we'll see what your night observations say." by evening the threats of a frost were even more definite and the two boys consulted what had best be done. "i can easily get father to start his fire-pots," said ross, "we got them all fixed up this winter. bob's dad has got some fruit, and we can warn him by wireless, and we could get a lot of the fellows together. i don't want to make a mistake, though. if we suggest that the fire-pots ought to be started and then it doesn't freeze, we'll hurt the league a lot more than we'll help it." "i wish we could talk it over with mr. levin," said anton, "but he's down with one of his sick spells and we oughtn't to disturb him. whatever we do, we've got to do it on our own." "let's get bob here," suggested ross, "he's got a steady head." "and fred," anton added, "he's read all the weather bureau stuff on frosts, i know. he's been writing his articles for the _review_ from them." "all right," said ross, "i'll slip over and call for fred and you get bob on the wireless and ask him to come over here." an hour later, the four boys were poring over the weather maps, comparing notes and observations and trying to decide whether they ought to do anything. fred, always ready to take up something new, was for plunging ahead, on the chance that there might be frost, but doubted whether a frost was likely. ross, as head of the league, was a little timid and afraid to make a serious mistake. anton was firmly convinced that a killing frost would come before morning. bob settled it. "better for the league to be laughed at than chance having the crops ruined," he said. this turned the scale, and from a discussion of the advisability of frost warning, the question turned to the best way of letting people know. it was decided that bob should return to his wireless, get as many of his connected operators in touch as possible and get them to warn their districts. fred, who had persuaded his father to install a 'phone, was to get in touch with the few farmers in the district who had telephones and ask them to spread the warning. anton was to borrow his father's buggy and drive to points not reached in any other way, and ross was to go on his pony. by this means, the county would be fairly well covered. the boys were just separating, when bob stopped. "jed tighe!" he said. "oh, let the old skinflint go," said fred, "there isn't any way of reaching him, any way." "that doesn't seem quite fair," said ross, dubiously, "he's got more fruit than anybody else." "it isn't fair," said bob. "i've been wondering," said anton, "if we oughtn't to notify jed tighe somehow." "we've got to," said bob. "and only get rowed at for our pains," declared fred. this was so likely that all the boys felt the truth of the remark and there was a moment's silence. "play square," said bob. "jed tighe has never done anything to help the league," said fred. "i don't see why we should do anything to help him." "well," said ross, "we can't take that stand. any chap that needs help ought to be warned. if you saw his house on fire, fred, you wouldn't hesitate to tell jed tighe, would you?" "no," answered the editor doubtfully, "i wouldn't, but this seems different, some way. we might be making fools of ourselves and he'd have the laugh on us for ever." "better be laughed at for trying to help than blamed for not trying," repeated bob. this was unanswerable and to ross was deputed the dubious pleasure of notifying the hard old farmer. as the boys separated, anton looked at his watch. "it's going to be all hours before you get home to your own place, ross," said anton, "it would be a shame if your fruit ran a risk by your being late. your dad hasn't got a 'phone." "that's easily fixed," said ross. he went to the door and whistled. rex came bounding up. ross went to the table and scribbled on a piece of paper: "frost to-night! light the pots!" this he fastened securely to the airedale's collar. "home! rex!" he said. the terrier looked up in his master's face to make sure that it was an order, and not a game, and evidently being satisfied, started down the road at a long sweeping trot. about a hundred yards away he stopped and turned round to look. ross was expecting this, so raised his arm and pointed. quite satisfied, rex swung round to the road again and galloped out of sight. the boys separated at once, bob to his wireless outfit, fred to his 'phone. anton, however, did not get in the buggy, as arranged. instead, his father, knowing that the lad was frail, packed him off to bed and drove in the buggy himself, warning all his neighbors. ross, on his little pony, riding like another paul revere, covered many miles. it was well on towards midnight when he reached jed tighe's house. the dogs broke out into a furious barking, and, wakened by their tumult, the old farmer with his thin scraggly beard, came to the door. "what do you want, coming to my house at this hour of the night?" he began, not recognizing his visitor. "it's me, ross planford," the boy answered. "i came to tell you that it's going to freeze tonight." "that's a nice reason for getting a man out of his bed! besides, it ain't so. there's never been a frost in this county later'n april ." he snapped his fingers at the boy. "that's how much you know about it." ross found it hard to keep down his temper at this discourtesy. "it's going to freeze, just the same," he retorted. "well, let it freeze, and you, too." the old farmer began to close the door. "but your fruit'll all be frosted!" "save it yourself, then," snapped jed tighe and slammed the door. ross dug his heels into his pony and started for home. the ride had taken him six miles out of his way and he was anxious to get home to make sure rex had delivered his message. still, as he rode, his pony's hoofs seemed to beat out the message: "save it yourself, then!" why should he? again-- why shouldn't he? the gallop came down to a trot and then to a walk, as ross brooded over what he should do. as it chanced, his path lay near one of the younger members of the league, who had bought a small wireless outfit, similar to that of anton's. ross reined in. as at jed tighe's, the hounds announced his arrival and the farmer poked his head out of the window. he recognized the boy at once. "what's up, ross?" he asked. "anything wrong?" "there's a killing freeze coming tonight, mr. lovell," the boy answered. "we're warning every one with fruit trees to start a smudge going. and, mr. lovell, can i use the wireless for a minute?" "of course. much obliged for the tip, my boy, i'll get right up and attend to things. of course, i don't know as it'll do any good, if it's a goin' to freeze; to my way o' thinkin' it's goin' to freeze and nothin'll stop it. but no one can say that tim lovell was too lazy to try an' save his crops." ross tied his pony and hurried up to his friend's room. in a minute the wireless was buzzing and presently, back came the answering buzz. georgie sat up in bed and listened. "i'll go with you to jed tighe's," he said, "that is, if father'll let me." "try it," said ross, "if he will, you can jump on the pony behind me." permission was readily granted, for the farmer was grateful for his own warning, and in less than ten minutes' time the two boys were galloping back along the frosty road to the old skinflint's place. "aren't you going to tell him about the frost?" asked george, as ross turned his pony off on the windward side of the orchard. "i have told him," answered ross, and he related the story of the meeting, gathering together dry twigs and branches as he talked. george waxed indignant. "i'd let him go to grass!" he said. "that's what i thought at first," ross replied, "but if you saw a chap drowning, you'd jump in and save him without waiting to find out whether he was delirious and didn't want to be saved." "of course," george answered, "any fellow would jump in." "that's what we're doing, we're jumping in." minutes were precious and the two boys worked with all their might, gathering piles of twigs and dry sticks. there was a heap of straw and stable manure a field or two away, and ross rolled several wheelbarrow loads of it across the fields. after two hours' work, the boys had a row of little piles of fuel, covering one quarter of the length of the orchard. "you light the first one, georgie," said ross, wanting to give the younger lad the honor, for he had worked pluckily and hard. the lad went down and touched a match to the first pile. it blazed up merrily, and just as the smoke began to rise, the wheels of a buggy were heard along the road. a moment later bob jumped out. "hello!" was all he said. he cast one glance at the piles and commenced to work with a will. presently a shout was heard and ralph, the photographer, appeared on his wheel. "there's a bunch more coming," he said, and he, too, set to work. "frost!" said bob suddenly, as he pointed to a small glistening crystal of hoar frost on a blade of grass. the boys cheered. their prophecies were justified, and they plugged at the work harder than ever. bob, who feared neither jed tighe's tongue, nor anything else, opened the farmer's stable, harnessed and hitched up a team, and commenced to draw the manure and straw to the edge of the orchard. it was now three o'clock and the frost was beginning to form rapidly. "we can't save the rest of it," said ross, as he looked longingly at the far quarter of the orchard; "we've got all we can do to keep going what we've got." four o'clock and five o'clock passed. the sun rose. promptly at five-thirty, his regular hour, old jed tighe got up and walked to the window to see what kind of a day it was. he rubbed his eyes and looked again, astonished. there, on his land, using his team of horses, was a group of eight boys, their forms only occasionally seen through the blanket of smoke which drifted sluggishly over and through the trees of his orchard. the ground was white with hoar frost and the lower branches of the trees in the yard had frost crystals on them. the farmer dressed hurriedly and went out. a dead silence fell along the boys as the tall spare form of the farmer was seen approaching. georgie and some of the younger ones shrank back. ross stood his ground. bob lounged forward. jed tighe said never a word. he cast a shrewd glance at the fruit trees in the orchard which had been nearest to the fires and the smudges, and then, still silently, walked down the entire line of the fires until the end of it, and beyond. on the unprotected stretch, the frost lay thick. he stood thoughtfully a moment and then walked back up the line, more slowly, until he came to where ross stood, watching him. "so you did save it, eh?" "yes, mr. tighe," the boy said, "i did." "and i suppose you think i told you to?" "yes, you did." "i'm not any fonder of being made to look like a fool than most men are," the farmer said, "but i'm fair." he turned on his heel and started to walk away. over his shoulder he snapped: "twenty-five per cent of the value of the difference between the fruit on the protected and the unprotected parts of my ground goes to the league. and i'll let my boy, bill, join you." chapter vii clearing an innocent man the saving of jed tighe's crop did more to establish the reputation of the mississippi league of the weather than anything which the boys had done since the league was organized. although jed tighe was stern by nature, he was thoroughly fair. he had no hesitation in placing the credit where it belonged, and the boys soon found that they had no stronger ally than the hard-spoken old farmer. even his friendship, however, did not prepare the boys for the farmer's sudden arrival at their club-house, on a saturday afternoon, two weeks later. he drove up in a ramshackle old buggy, driving two of the finest horses in the county. skinflint though he was, he loved horses. he came into the club-house and eyed the boys standing around the table. "i'm going to ship some potatoes to chicago," he said abruptly, without any preface. "i want to know whether they'll be safe from freezing on the way." there was a moment's dead silence. the boys had not bargained for such a point-blank demand for help, and it took them off their feet. one looked at the other and several shuffled uncomfortably. the forecaster watched the lads keenly, interested to see how they would face the issue. ross spoke first. "well, mr. tighe," he said hesitatingly, "we haven't done any figuring on the weather outside this neighborhood, as yet." this cautious attitude did not appeal to fred, who always wanted to plunge in head first. "sure we can, ross!" he declared. the president of the league looked inquiringly at his mainstay, the silent bob, and, in answer to his unspoken question, the other nodded. "we could try it, of course, if you wanted us to," agreed ross. "ain't i asking you to?" said their visitor, sharply. "but suppose we don't get it just right?" ross queried. "that's the chance i'm taking," the farmer replied. "but there's no doubt that you know a lot more about it than i do, and your guess is likely to be nearer than mine. those potatoes have just got to go to chicago some time next week, anyway." "it's a new stunt for the league," said ross again, hesitating, but the editor-in-chief broke in impatiently. "we might as well tell what we know," he said. "we do know that there's a cold wave on the way." "there is? how cold?" the farmer asked, with a sudden quickening of interest. "cold enough to freeze potatoes, at any rate," assured fred. "i was looking at the weather map only about an hour ago. oh, it's going to be cold, all right." "how do you know?" jed tighe demanded. "if i'm goin' to act on what you boys say, i'd like to know how you find out." "i've been wondering," put in anton thoughtfully, "if it wouldn't be a good idea to have mr. tighe go over the map with us. he might be interested in figuring it out, and then if we didn't hit it just right, he'd know we'd done our best, anyway." "well," rejoined the farmer grimly, "if i've got to hand you over some of my crop this fall, i might as well find out what sort of project i'm supporting. i really would like to see how you find out. you boys certainly made good on that frost business the other night." from a hook over the compositor's "case," fred reached down a sheaf of the daily weather reports, and laid those for the last three days on the table in front of anton. the forecaster stood by to help the crippled lad and to correct him if he made any mistakes in his explanations. "all our weather in the united states," the boy began, explanatorily, "comes from the west." "why?" snapped back jed tighe. the forecaster smiled. he realized that the question went to the very root of weather knowledge. the query was a poser to anton. he stammered. "i know it does," he said, "but just why, i--i--" "you'll have to begin at the beginning, anton," put in the forecaster quietly. "if mr. tighe really wants to know, you can't take anything for granted. explain to him the circulation of the atmosphere, just the way i taught it to you during the winter." the crippled lad's face brightened. he knew, now, how to proceed. "all changes of weather, mr. tighe," he said, "happen because of the winds, and all the changes of winds are due to the differences in heat at various parts of the globe, especially at the equator, where it is always hot, and at the poles, where it is cold nearly all the year round." "you mean to say that the weather at the north pole and at the equator has anything to do with our weather here?" "everything," anton answered, nodding his head. "the heat of the sun is what causes weather changes, because winds are due to the heating of the air, and the sun is the only thing that heats the air. at the equator, where the sun shines nearly overhead all the year round, the air gets to be very hot. hot air expands, and as it gets bigger, it displaces the cold air above it. gravity pulls down the colder air on both sides of this belt of rising hot air, and the down-flowing cold air on both sides blows in toward the equator under the warm air, where the heat of the sun warms it again, and, in turn, it rises. this is going on all the time and is one of the chief things that starts the winds blowing." "but winds don't always blow the same way," said the farmer; "you talk as if they did." "some of them do," anton replied. "there are lots of places where the winds hardly change, at all, but always blow in the same direction. you read of sailing ships taking the 'trade winds' when coming from europe to america. those are all easterly winds and blow towards the american coasts all the year round." "i don't see how they can," the other objected. "they do, mr. tighe," the forecaster interrupted, endorsing anton's statements; "the trade winds are the downflowing currents of cold air that anton spoke of, which come down at either side of the equatorial belt to replace the warm air which is rising. the trade winds, however, form only a narrow belt and blow only near the surface of the earth. above them, you can see the lighter clouds blowing eastward with a westerly wind, so that, quite often, in the trade winds, you can look overhead and see two layers of clouds driving in opposite directions." "you mean to say that there are different layers of wind?" queried the farmer. "sure," put in ralph, the cloud expert, "i've got photographs that show that up clearly. you've seen clouds going at different rates, haven't you, mr. tighe, some fast and some slowly?" the other nodded and turned to the forecaster, who continued. "there are always several layers of wind, and, except above the equatorial belt," he said, "the direction of the upper air winds is generally towards the east." "how can you tell that?" "by the clouds, or by kites and balloons. but we don't even need to do this, because there are a few places that rise above the lower layers of the trade winds. thus, the peak of teneriffe, which is in the trade-wind belt, has a continuous easterly wind on its lower slopes and a continuous westerly wind right at the summit. "this gives three belts of weather in the tropical and sub-tropical zones. the first of these is a light up-flowing east wind on or near the equator--it shifts a little to the north or south with the change of the seasons; a belt of heavy rains and calm, the rains being due to the warm, moist, uprising air cooling by expansion so that the moisture is condensed--this region is known to sailors as the 'doldrums' and many a sailing-vessel has been held for weeks there, without enough wind to carry her the few miles necessary to get into the next belt of winds; outside this, come the downflowing easterly currents, known as the trade winds, which form a belt between the tropics and the temperate zones. beyond this--to the north and south of the tropical zones--come the prevailing belts of strong west winds, which stretch almost to the poles. "the united states is in this west-wind zone and the strength and regularity of the eastward movement of the weather is because both the winds of the surface and of the upper air blow in the same direction. naturally, the same conditions are repeated on the other side of the equator. in the southern hemisphere the land masses are not so large and the regularity of the winds is less disturbed. there, the west winds are so strong that certain latitudes are known as the 'roaring forties.' these 'forties' correspond in latitude to the northern third of the united states. chicago and new york are both in the 'roaring forties' of the northern hemisphere." "the way you tell it, it sounds all right," the farmer objected, "but from my experience, winds blow from all over the place." "locally, perhaps, they seem to," the weather expert responded, "but if you watched them closely, you'd find that about seventy per cent of the winds come from a westerly direction." "they do here, for a fact," put in tom, who, as official wind-measurer of the league, had been following the explanation with the keenest attention. "i've noticed that in my kite-flying. the winds are from the southwest or from the northwest nearly all the time." "you mean both in summer and winter?" "yes," answered tom, "they're more from the northwest in winter, i think, but they're generally westerly." "if the winds are due to the position of the equator and the poles," the old farmer said shrewdly, "i don't see why summer and winter ought to make any difference." "that," said the forecaster, "is due to an entirely different set of conditions. it's due to the difference in radiation. there's much greater change in temperature over the land than over the sea. take an island like bermuda, for example. from the hottest day in summer to the coldest day in winter there isn't a change of more than forty degrees, because bermuda is surrounded by water and is near warm ocean currents. in arizona, on the other hand, there's a change of as much as fifty degrees of temperature in a single day. that is because land absorbs heat quickly and lets it go equally quickly. the interior of a continent in summer time heats and expands the air in the same way that the air is heated over the equator, and, in the same manner, sets in motion another system of winds, for cold air comes rushing down from all sides and forces up the rising warm air. "take asia, for example, where the continental mass is large and the plateaus high. the interior becomes so hot that the air is sent up like the draught in a big chimney, and cool winds from the sea blow toward the interior from all sides in the summer time, and away from it, to all sides, in the winter time. that's what causes the famous indian monsoons, which blow steadily to the north-east for the six months of summer and just as steadily to the south-west for the six months of winter. the native boats, there, are built on purpose for the monsoon, so that they can only sail with a fair wind and they make one round trip a year, going south with the monsoon in winter and returning with the summer monsoon." the old farmer scratched his head. "there's more to this than i thought," he said; "i always supposed that winds just happened." "no, indeed," the forecaster answered, "every place in the world has its own system of winds, though in some parts there are so many variations that it isn't always easy to distinguish between the regular and the irregular currents. in the united states the surface winds are very irregular, for we live in one of the stormiest regions of the entire world. still, that doesn't alter the general rule that all our weather comes from the west." "and yet," said the farmer, in a puzzled manner, "i don't see why it comes from the west." "i think i can explain it to you," the weather expert replied. "you know that when water is running down a hole at the bottom of a basin, if it is in motion it doesn't go down straight but with a circular movement, finally making a whirlpool?" "of course," the farmer said. "so does air," the forecaster rejoined. "there is something the same sort of a whirl at the poles. the prevailing westerly winds of the united states are due to this circumpolar whirl, though modified and altered by the changes of the seasons, the differences of heat between day and night, the radiation from the land, the irregularity of the coastline, the currents of the ocean and a thousand other factors. each of these the weather man has to study when he makes a forecast, but, in the united states, his work is aided by the fact that weather always travels eastward and that the storm follows regular tracks, sharply outlined, like indian trails across the country." "roads in the air?" queried fred. "yes, my boy," the forecaster answered, "regular roads in the air. there used to be an old saying: 'american weather is made at medicine hat.' in a sense this was true, for about sixty per cent of the storm areas--'lows' or region of low barometric pressure--come from the canadian northwest. the st. lawrence valley is the outlet for our storms. you know the saying about the st. lawrence, don't you?" "no, tell us, mr. levin," begged fred, always eager for some weather saying which he could put into the _review_. "up there," the forecaster rejoined, "they say that when a stranger complains about the weather, a native will reply, 'don't mind this, we'll have another sample along in about five minutes.' and, sure enough, they do. the st. lawrence valley is a magnet for weather changes and has, perhaps, more storms than any other valley in the world." "you spoke of the 'roads in the air,' sir," put in ross, "how many are there?" "five regular trails," the forecaster answered. "the northernmost one begins at the canadian northwest, runs along the international boundary, crosses the lake region and disappears up the st. lawrence valley. the second starts at the same point in the canadian northwest, travels southeast to the lower mississippi valley--a little north of where we are now, boys--curves up to the ohio valley and also escapes by the st. lawrence route. "a third storm track strikes into the pacific coast a little north of san francisco and runs east and a little south until it joins the ohio valley and st. lawrence track. a fourth develops in the southwestern states and runs along texas and the gulf states to the florida coast, where it curves northward along the atlantic coast, though a few storms take a sharp turn in the mississippi valley and go ohiowards. the fifth storm track is that of the west indian hurricanes, which whirl around the west indies and enter the united states south of cape hatteras or from the gulf of mexico and pass north or northeastward. a few of these hurricanes--like the famous galveston type--sweep westwards a long way before the northward movement sets in. this type also goes to the st. lawrence valley. "these five tracks are clearly marked, but as such areas are a thousand miles across, it follows that the country for five hundred miles on either side of the lines has its weather governed by them. knowing these tracks is of great importance in forecasting weather, because, while you cannot always tell exactly what a storm is going to do, you definitely know some of the things that it will never do." "what sort of things, sir?" asked fred. "well, my boy," the forecaster answered, "if there's an area of low pressure in dakota, we know that it won't strike california; if there's one in new york, we know that maryland is safe. a storm will never go down the mississippi, nor up the st. lawrence, but will always travel up the mississippi and down the st. lawrence." "there does seem to be something regular about it," the farmer remarked, his interest growing, as the forecaster took his pencil and sketched out, across the map of the united states, the five great storm tracks. "that's all right for storms, maybe. but how about a cold wave? fred, here, said that a cold wave was coming. can you figure that out in the same way?" "certainly," the weather expert answered. "as a matter of fact, it is comparatively easy. a cold wave is simply a fall of temperature caused by the cold air from the upper atmosphere sweeping downwards after a cyclone of low pressure has passed." "a cyclone?" ejaculated ross, in surprise. "is there always a cyclone before a cold wave?" "always," the forecaster answered, "but, unless i'm mistaken, ross, you're using the word 'cyclone' in the wrong sense. most people do. i suppose you think a cyclone is some kind of a whirlwind, a particularly violent storm, eh?" "yes, sir," said ross, "that's what i thought." "well, anton can tell you better than that," the weather expert rejoined. "tell him what a cyclone is, anton." "so far as i can make out," the crippled lad answered, "a cyclone is a whirl in the air, generally from five hundred to a thousand miles across, in the middle of which the barometer is very low, and on the edge of which the barometer rises. it always has winds that blow spirally inwards, those in the united states whirling in a direction opposite to the movement of the hands of a clock. "so you see, ross, to the east of a 'low' or ahead of it, the winds are southeasterly, to the north they are northeasterly, to the west, or behind it, they are northwesterly, and to the south, they are southeasterly, all curving into the centre and shifting as the 'low' advances. as these 'lows' travel along the storm track at an average rate of four hundred miles a day, as mountains interfere, and as the shape of a 'low' in america isn't quite round, but looks like a sort of crooked oval, it takes close figuring to find out what the wind is going to do." "and where does the cold wave come in?" persisted the farmer. "that comes after the cyclone," explained anton. "a 'low' means that the pressure of the atmosphere is less than usual, and, consequently, doesn't press the mercury up so far in the barometer. the air weighs less, that shows that it must be expanding. the winds in front blowing into a 'low' are generally warm winds. when a 'low' is traveling fast, with a 'high' or 'anti-cyclone' behind, the colder winds come rushing forward to take the place of the rising warm air and they bring colder weather with them. the freeze comes during the early clearing weather of a 'high,' before the anti-cyclonic winds--which blow in the opposite direction, the way of the hands of a clock--have had a chance to steady down." "then," said the farmer shrewdly, "if you get reports of wind and of barometer from points to the west and northwest, you can tell when a cold wave in on the way. is that it?" "exactly," the forecaster replied. "we cannot always tell, of course, when the weather is going to be a little colder or a little warmer, but a cold wave, serious enough to damage crops and property, can always be foretold. remember your storm tracks again. in this county, in the state of mississippi, we are very unlikely to get a freeze, unless there is a rapidly moving 'low' passing up towards the ohio and st. lawrence valleys followed by an equally energetic 'high' plunging down from the canadian northwest." "and can you always tell what the weather is like, all over the country?" "yes, indeed," the forecaster answered. "there are two hundred official stations scattered all over the united states and the west indies, each one carefully selected because its site is a key station to weather changes. twice a day, exactly at eight o'clock in the morning and eight o'clock in the evening, the observations are taken at each station." "and have they all got rain gauges like mine?" asked anton. "yes, all of them." "and wind-measurers, like my anemometer?" queried tom. "yes," the forecaster agreed with a smile, "and some of them have devices that make a continuous record of wind velocity." "and barometers like mine?" put in one of the younger boys, not to be outdone. "various forms of barometers, and barographs, and thermographs, and sunshine recorders and all sorts of things. some of them even have seismographs, which tell of every tiny little earthquake, that may be going on all over the world. you know, boys, there's hardly an hour of the day that there isn't a small earthquake, somewhere, and there are really quite sizeable earthquakes at least once a month. a well-equipped weather office is quite a complicated affair, and it takes well-trained men to conduct the observations and interpret them properly." "all those observations are sent to washington, aren't they, sir?" queried anton. "just as i send mine every night to bob, for him to transmit by wireless." "just the same way," the forecaster answered, "except that they're all sent in cipher, of course. once in a while the cipher results in some queer combinations. the regular routine requires that an observer send the temperature, the barometric pressure of the atmosphere, the amount of rain or snow, the direction and force of the wind, the state of the weather, the types of clouds and the highest and lowest temperature since the last observation. i remember once, while at the milwaukee station, we got the following message from la crosse, wisconsin: "'cross all my ink frozen' "it so happened that we had charlie cross working at that station at that time, but the message did not apply to him, nor, for that matter, to his ink. on second consideration and reading, the message read very differently. 'cross' was the code name of the station; 'all' meant that his barometer read . and that his morning temperature was zero; 'my' conveyed the information that his sky was clear, the wind from the south and that his minimum temperature for the night was zero; 'ink' informed us that the wind velocity at the station was six miles an hour and that he could not add the usual height of the water in the mississippi as the river was 'frozen.' similar code messages are sent in twice a day from each of the two hundred stations. "so you see, mr. tighe, if all these various observations combine to describe a certain weather type, if we can check up the accuracy by comparison with stations to the north, south, east and west, and if all these combine to produce a certain definite picture, our weather forecast can be made with tolerable certainty. as an absolute matter of fact, during the past six years, the exact percentage of accurate forecasts is eighty-two per cent, and of the eighteen per cent remaining, eleven were partly right. that leaves a very small proportion of mistakes in weather forecasting. now, let us take in detail the cold wave which fred, quite rightly, said was on its way here. "here is the weather map of the day before yesterday." he placed it on the table in front of the old farmer. "you will notice two sets of curved lines, solid lines and dotted lines. the solid lines are called 'isobars' and they follow the course of places which have the same barometric pressure. the dotted lines are called 'isotherms' and they follow the lines of places having the same temperature. these maps are never twice the same. the weather bureau does not possess on its books the record of any two days when the weather was duplicated over the united states." "you mean that every day's weather map is different?" "as different as every human face," the forecaster replied, "and to those of us who have done much forecasting, it is as easy to see from the map when the weather is going to be peaceful or stormy as it is to tell whether a man is smiling or scowling. but let us look at these three charts closely, and you will see just why fred was right. "at eight o'clock in the morning, the day before yesterday, there was a well-defined 'low' with a barometer of . just east of salt lake city, driving warmer weather before it. issaquena county was just recovering from the effects of a 'high,' which, as you can see on the map, was disappearing by its favorite route, the st. lawrence valley. what was your temperature here the day before yesterday, anton?" "thirty-six degrees, sir," the crippled lad answered, rapidly consulting his week's record, which was hanging on the wall. "fairly cold, you see. and the wind, tom?" tom pulled out a note-book from his pocket. "north-east, sir," he said. "very good. now, mr. tighe, you can see from the map that the barometric pressure, the isobar, running through this part of the country shows a barometric pressure of . . from what anton told you, it is easy to see that, the day before yesterday, issaquena county was still in the grip of the tail end of a 'high,' with a high barometric pressure--five points above the low in salt lake city--with a cold temperature, and with a wind blowing outwards from the 'high' or anti-cyclone. is that clear?" "clear as well water," the farmer declared. "now," said the forecaster, "let us look at yesterday's map for eight o'clock in the morning. here, just over the canadian border, right at medicine hat--as though to make good the old proverb--is a vigorous 'high,' with a barometer of . , with a temperature of ° below zero and with the winds blowing outward from the centre. the 'low,' which the day before yesterday was central over salt lake city, yesterday was central over oklahoma city. it has, therefore, traveled over five hundred miles in the day. on all sides of the 'low' there is rain, and you remember how it rained here, yesterday morning, early?" "indeed i do," said jed tighe. "i didn't get out on the land until nearly eleven o'clock." "now what was the temperature here yesterday morning, anton?" the forecaster queried. "forty-six degrees," answered anton promptly, for he had been expecting the question. "ten degrees warmer, you see, mr. tighe, as the 'low' came nearer. and what was the wind, tom?" "south-south-east," the lad answered, his note-book in hand. "showing," the forecaster explained, "that during the twenty-four hours, issaquena county had lost the effect of the 'high,' which has disappeared from the map, and was fully in the grip of the oncoming 'low.' now, if you look at the map, mr. tighe, you'll see that the isobar for this region shows a barometer pressure of . , a terrific drop of four points in twenty-four hours. no wonder it rained!" the farmer bent over the map, his eyes glued on the lines which suddenly seemed to spring into life before him. "down over the country comes this 'low,' at the rate of five hundred miles a day, with rain and moist winds accompanying it, and sharp on its heels, racing from the north, comes the cold 'high' which we have just seen forming at medicine hat. the cold wave is fully organized and is on its way." he laid the third map on the table. "here is the situation at eight o'clock this morning," he said. "the 'low' or storm, has swung at right angles, following the preferred ohio and st. lawrence valley route. it left toledo early this morning and at eight o'clock was raging over the great lakes, with its centre north of buffalo. it is speeding up, you see, having traveled eight hundred miles since yesterday. the cold wave 'high' from medicine hat has traveled along its usual track and is now central over kansas, with clear skies and a drop of thirty degrees in temperature. there was a severe freeze in kansas last night, with zero temperatures, and freezing point was touched on the mexican border." "whew," whistled the farmer, "and is that on its way here?" "it is," the forecaster answered. "your temperature?" he continued, turning to the boy. "thirty-seven," anton answered. "going down rapidly, you see. the wind, tom?" "northwest." "blowing outwards from the rapidly approaching 'high.'" "what's the barometer?" asked the farmer, who was quickly grasping the manner of reading a weather map. "it has gone up again to . . the cold wave is coming fast. since dodge city, kansas, is about five hundred miles from here, and since the 'high' is traveling at about seven hundred miles a day, and as, moreover, there is generally a slight slowing up as it makes the turn, the centre of the 'high' ought to strike us here about six o'clock tomorrow morning. the cold wave, however, is in advance of the centre, so mr. tighe, you need to be prepared for a cold wave tonight. "if you ship your potatoes this afternoon, as you planned to do, they would meet severe weather and might get frozen. if you ship them tomorrow, you might be safe, but you couldn't be sure, because the 'high' is turning northwards and therefore its eastward distance is not so great. if you ship them on monday you would be safe, but even then you could not ship them to new york, for a fast train might overtake the tail of the cold wave. on tuesday you can safely ship them to any part of the united states." the farmer stepped back from the table and his eye roved over the boys. "and was that the way that you lads figured out that my fruit was likely to be frozen?" he asked. "yes, sir," said anton, "that was how." "it's a marvel," the farmer declared. "i don't see why more people don't use these weather maps." "hundreds of thousands of people do," the forecaster replied. "you'd be surprised, mr. tighe, if you knew how big business firms all over the country study these changes of weather. heating and lighting plants of great cities study conditions of cold and of darkness. municipal systems, with exposed water mains, take precautions against frost. large stockyards, like those of chicago, drain their water pipes. gasoline engines are drained. street railway companies are supposed to turn more heat into their cars. natural gas companies are required to put on a greater pressure. dredging of sand and gravel is suspended. piles of iron ore, lying on wharves, are placed in the holds of vessels to keep the ore from freezing solid. "take ordinary questions of trade, which we all know well. wholesalers distribute stocks of cold-weather goods to retailers when a cold spell is forecast, and wideawake retailers make special provisions for it. advertising managers of big department stores, who prepare their advertisements for the daily papers, the day before, study weather reports very carefully. you can go into an ad-writer's office, with the sun shining in at his window, and find that he is writing display of umbrellas and rubbers. the explanation is the weather map, which is lying on his desk. everywhere you go, you'll find that the really big business organizations study the weather reports as closely as a stock-broker studies the wall street reports." the farmer stared at the forecaster. "why," he said, in astonishment, "i never had any idea that the weather forecast was so important. i just thought people read it to know whether it was going to rain, whether they should take an umbrella or not." "rain forecasts," the weather expert rejoined, "may be useful for one's personal comfort, but their importance is nation-wide. until a few years ago, one-eighth of the value of the entire raisin crop was lost every year by occasional showers while the fruit was drying. the weather bureau established a special service to take care of this region and for five years there has not been a single non-avoidable loss. berries are picked before rain. vegetables which are dug before rain, stand shipment better than those dug afterwards. in the alfalfa region, rain forecasts are all-important, since the hay can be baled in the field when it is dry but not when it is wet. "every kind of brick, cement, and lime manufacture has got to be protected from the rain, and twenty-four hours' notice enables all such factories to protect their product. contractors for outdoor work make their estimates and contracts on the basis of weather forecasts, railroad companies provide against washouts, and irrigation companies control their output of water according to the expected rainfall." "this is great stuff," said fred, under his breath to ross. "i'm going to run this in the _review_!" "snow warnings," the forecaster went on, "are of equal value. all over the western country, where the snows are apt to be heavy, the tonnage of passenger and freight trains is made up in accordance with the expected weather, and the snow-fighting equipment is prepared. on the great western ranches, stock is hurried from the open range either to constructed shelters or to naturally protected gullies, on notice of blizzards, northers and heavy snows. this is especially necessary on sheep ranches. twenty-four hours' notice of a heavy snow-storm saves the country at least half a million dollars in stock loss and property damage. "storm warnings, perhaps, are even more important. hundreds of lives are saved, every year, by vessels remaining in port when a storm or hurricane is expected. a recent storm on the great lakes was forecast as being so severe that scarcely any vessels left port. many ships, undoubtedly, would have foundered, had they been out in the gale. yet, aside from the weather map, there was no local indication that bad weather was brewing. when storm warnings are issued, fishermen take steps to protect their boats and nets and a fisherman's boat and net is his whole livelihood. lumbermen make their booms of logs secure. rice-planters flood their crops to prevent the breaking of the brittle straw by the wind. wherever construction work is proceeding, and a wind of unusual force is forecast, builders and engineers make doubly secure that which is already constructed, instead of proceeding with outlying portions of the structure. "in short, mr. tighe, there is scarcely a business in the country which would not be benefited by a close study of weather conditions. the difference between a careful man and a careless one is the difference between a man who thinks in advance and a man who does not think until some condition of grave difficulty is thrust upon him. weather is, to this day, and will ever remain, one of the most potent factors in human welfare, and a man cannot plan for the weather in advance, unless he has a weather forecast." the farmer brought his fist down on the table with a thump. "tell me, then," he said, "since all the big business firms in the country use the weather bureau so completely, why do people laugh at the weather man?" "that's very simply answered," the forecaster replied, "it's because every one is not a wide-awake business firm. ask a commission merchant, whose business depends upon his receiving his produce in good condition, whether the weather bureau warnings are profitable or no? ask a fruit merchant, who knows that a difference of twenty degrees in temperature during shipment spells either profit or disaster! ask a shipowner on the great lakes or the captain of a trading schooner in the gulf! these men will tell you that their lives and their fortunes hang on their careful understanding of the weather. but if you ask some one who merely wants to know whether or not to wear new clothes or whether it will be safe to have a picnic on a certain afternoon--then, indeed, unless the weather is of the particular pattern that they prefer, you are apt to hear that 'the weather man is always wrong.' "there's another reason, too," he admitted, "and that is that local conditions may differ from regional conditions. i've shown you that there's a cold wave coming, and that over this section the temperature may drop twenty degrees. but suppose your thermometer, mr. tighe, is near the slope of a hill, which starts a small current of air moving, just enough to keep the air well mixed, then your thermometer may not register a fall of more than ten degrees, and you'll accuse me of being an alarmist. none the less, in a valley a quarter of a mile from your thermometer, the temperature may have dropped twenty-five degrees and for a hundred miles in every direction, the average temperature will be equally low. "suppose, over a section as large as the gulf states, or new england, the weather bureau announces a forecast of showers. there might be stretches of fifty miles square in which never a drop of rain fell, and people in a hundred towns would take their umbrellas needlessly. yet, in six hundred other towns in that region, there would be showers. "naturally, the weather bureau could give a much more detailed, though not necessarily a more accurate report, if, instead of having stations, we had two thousand, and if the appropriations of the bureau were multiplied by ten, so that there might be a larger force to interpret and explain the observations that have been recorded. still, we're all proud of the weather bureau and its work, and if you watch it closely, mr. tighe, you won't find us far out. just as a test of it, keep your potatoes in your root-house until tuesday and watch the thermometer for the next two days." "i'll do that," said the farmer, "and i'm much obliged." he took his hat. "any of you boys coming my way?" he asked. this was an unheard-of geniality on the part of jed tighe, but two of the boys jumped at the offer. the last words that the forecaster heard were in the farmer's voice, as he drove off: "about that weather map, now--" mr. levin nodded to the two boys and strolled across the sun-dial lawn to his own buggy, well satisfied that another convert to the weather bureau work had been made. about ten days after this meeting, after supper, just as anton was going to bed, his father came in with a grave face. "i'm afraid dan'l's in a peck of trouble," he said. "why, father?" asked the crippled lad. "he's accused of having shot carl lindstrom," was the startling reply. "but he couldn't!" declared anton, jumping at once to the defence of the darky. "well," his father said, "it looks a little black for him. i don't mean, of course, that there's anything purposed, but it looks as if dan'l had been careless with his gun. carl was shot in the leg this evening, just as we heard. now it appears that, about the same time, dan'l was seen walking with his gun and his two old hounds at his heels, coming from that direction along the levee." "oh, i'm sure it can't be dan'l," said anton. "where is he?" "in his cabin, under arrest," his father said. "the sheriff's there. dan'l seems quite excited about it and he said he wouldn't move until he saw you." "sure," said anton, reaching out for his crutch. "i know well enough he didn't do it, though." he hurried across the sun-dial to the negro's quarters. it was a poignant scene that anton faced when he reached the hut. dan'l was sitting on the bed, in shirt and trousers, evidently having just been awakened from sleep. the sheriff, tall and rangy, showed little interest in the affair. to him it was a clear case. the man had been shot. the negro had been seen in the neighborhood with a gun. what more proof could any one want? the brother of the man who had been shot, a nervous, excitable chap, was there and wanted to lynch dan'l immediately. one of the sheriff's men, keen and watchful, stood beside his prisoner, his hand on the negro's shoulder. "ah never done it, mistah anton," said dan'l, as the boy came in, "ah never done nothin'!" "i've brought anton, dan'l," said the father, quietly, "but it doesn't do you any good to say anything. they'll only make use of everything you say." "ah've got nothin' to say," the darky declared. "ah jes' went after some rabbit an' come home. ah've been in my bed since a little after sundown." "you couldn't ha' been," declared the sheriff, "'cause the injured man wa'n't shot till it was nigh dark." "what time was the shooting?" asked anton. "between a quarter and a half after eight," the sheriff replied coolly, "we know that much fo' sure, any way. and dan'l can't show an alibi. he says he was in bed. his bed can't give evidence in court. yo' didn't see him, anton?" "no," the boy answered, "i haven't been out of the house since seven o'clock except just to my rain-gauge." "well," said the sheriff, yawning, "that's yo' last chance, dan'l. if anton had seen yo', there'd have been a witness. but yo' ain't got none and ole lindstrom, here, declares that he seen yo' jes' afore it got dark." "ah've done nothin'!" the darky declared. the sheriff kicked the darky's tattered boots across the floor, not unkindly. "hyar," he said, "put yo' shoes on. carl ain't goin' to die, and the jedge won't do much to yo'." "ah never done nothin'," the negro protested, but he leant down as he was told, and started to put on his shoes. one of the shoes had slid close to anton's feet, almost knocking the crutch out of his hand, and the lad's glance fell on it. he started. "what time did you say the shooting was done, mr. abner?" he asked. "between a quarter and a half after eight," the sheriff replied. with a sudden excitement in his voice, anton turned to the negro. "how many pairs of shoes have you got?" he asked. dan'l caught the tension in his voice. "two pair, mistah anton," he said. "which did you wear this afternoon?" "these hyar." "and where are the others?" "in yonder corner." anton limped across the room and brought out the second pair of shoes. the leather was all dry and wrinkled. they had evidently not been used for a long time. "he's right, mr. abner," he said, "he wore those shoes." the sheriff, divining by the excitement in the boy's voice that there was a hidden purpose in these remarks, took up the second pair of shoes and looked at them. "yes, that's sho'," he answered, "he didn't wear these hyar!" "then he wore those," said anton. "well, what if he did?" "look at your shoes," said the boy. "well?" queried the sheriff, looking down at his boots. "they're muddy, aren't they?" persisted the boy. "right muddy," the sheriff agreed. "and bill's shoes are muddy, too." there was no doubt of that, either. "well?" said the sheriff, questioningly. for answer anton held out dan'l's other shoe, the one he had been holding in his hand. "this isn't muddy," he said. "what's more, it's got dust on it, dust in all the cracks. you can see it hasn't been cleaned for a long time, probably never since it was given him." "well?" repeated the sheriff, still uncomprehending. "lindstrom's place is more'n a mile from here," declared anton, his heart beating hard. "jest a mile," said ole lindstrom. "and you say the shooting was before half-past eight?" "it sho' was," the sheriff answered, "it was jest a little after half-past eight that carl was carried home." "then," declared anton, in a quiet way that carried conviction, "dan'l didn't do it, and i can prove it." "mistah anton! mistah anton!" the darky cried. "quiet! you!" said the man who was holding the prisoner. "what do you mean, anton?" the boy's father asked him. "it's quite easy," the boy declared. "if the shooting was done before half-past eight, it was done just about the time that the rain began. it would take dan'l--if he'd done it--all of twenty minutes to walk from lindstrom's place here. it rained heavily, if you like i can give you the amount of rain in tenths of an inch, and twenty minutes of walking in that rain would make him wet through. by the time it had rained five minutes, the ground would be muddy. but see, mr. abner, the soles of the shoes are quite dry. and, dan'l's clothes are quite dry." he picked up the gun that stood leaning in the corner. "the gun's dry, it hasn't been cleaned and there's no rust on it. dan'l hasn't got two sets of rough clothes and he sure hasn't got two guns. doesn't that prove he couldn't have been out after the rain started?" the sheriff looked a little dubious. "yo' sho' put up a good argument for yo' nigger," he said, "but yo' boys' foolin' about weather ain't evidence. that don't go in court, yo' know." "you're a little wrong there, mr. abner," said anton's father. "this is an official co-operative observer's station of the weather bureau. by a decision of the supreme court, our records have got to be accepted as evidence. there's a ruling to that effect." "there is, eh?" said the sheriff. "first i ever knew of it. but if yo' say so, why, of course, it's so. but how can you-all tell when the rain began?" "when rain comes down unusually hard," the boy answered, "the weather bureau likes to keep a record of the amount of precipitation in five minute intervals. my big record-book is in the house, but here are the notes i made," and he took a little note-book from the pocket of his shirt. "'rain began, . ,'" he read, "'first five minutes three-tenths of an inch, second five minutes four-tenths, third five minutes, three-tenths,'" he stopped and held the book open, "it began to get less, then, and i didn't need to keep the record any longer. but you can see, mr. abner, that it was impossible for dan'l to have left the lindstroms' and reached here before the rain came, and just as impossible for him to have come through the rain with dry clothes and dusty shoes." "an' the courts have a ruling that weather records is evidence?" "from an official station such as this, yes!" anton's father declared. "evidence as to weather is a factor in a great variety of cases. civil cases are largely personal injury, damage to perishable goods by freezing or rain and loss by fire. the criminal cases are usually confined to murder trials. "when accidents occur by reason of a street car running into somebody or something, the question arises as to whether the rails were so slippery that the car could not be stopped. this fact is, of course, important in an action for damages. a slippery rail can be caused by 'sweating' but it is generally due to recent rain. the relative humidity may be such as to prevent the drying up of the rail. "an observer was called in a case where it was alleged that the plaintiff had been injured by being pitched through the open window of a car. it was claimed that she was trying to shut the window on account of the raw, cold weather and, as the car reached a curve, she was suddenly thrown headlong into the street. the weather record showed that it was a warm and sunny day. "the question as to whether a sidewalk was sufficiently slippery to make it dangerous for travel frequently comes up in court. one such case, i remember, was that of a man who asked damages from a jitney driver for starting his bus before he had alighted. the driver declared that the passenger slipped and fell on the ice in the gutter, several feet away from the bus. the plaintiff declared that it was a warm day and that there was no ice. the weather record showed rain the day before, with a severe frost during the night, precisely the conditions to support the jitney-driver's story. "many accidents are alleged to have been due to fog. the weather expert is called upon to testify to the degree of visibility permitted by atmospheric conditions. one man who was accused of murder and who undoubtedly would have been convicted, was positively identified by the wife of the murdered man, the woman declaring that she saw him at a certain hour of the evening passing in front of the house. the weather records showed conclusively that, at that hour, owing to the excessive cloudiness of the atmosphere, it would have been impossible for the woman to identify the suspect, even at half the distance. "wind records are often very important. in april, , a severe storm moved over the middle western states, and, at one place in indiana, it developed such velocity as to start in motion an empty box car standing on a railway siding. it was carried on to the main track, the derailing switch not being turned, and ran for two miles before the wind, the grade being slightly up-hill. it finally collided with a passenger train and several persons were killed. the railroad company produced the weather records to show that a storm of such violence was outside the common run of events, seeking thereby to lessen the amounts awarded for damages. "this direction of the wind often is called into requisition. a suit for many thousand dollars was brought by the owners of some property in chicago, against a railroad company, the property-owners alleging that a fire which had destroyed some of the buildings had originated from sparks from a locomotive. the weather bureau records, however, showed that there was a brisk wind blowing directly from the property to the railroad. of course, all damages incurred in storms of unusual severity, such as the st. louis tornado or the galveston flood, would be ignored in a court of law, as they would come under the head of unavoidable happenings of 'the act of providence,' a well-known legal phrase. in all matters connected with events in which the weather is a possible factor, the weather bureau observer has a place and a part, and the united states supreme court, as long as thirty-five years ago, ruled that weather records were competent evidence." "i reckon yo' is wrong, mr. lindstrom," said the sheriff, turning to the brother of the wounded man. "ef the weather records goes as read, this hyar's a powerful bit of evidence. look at them shoes!" "i'm satisfied," the other remarked gloomily, "i reckon the boy's right. but i'd have sworn that it was him i saw. all right, sheriff, i'll withdraw the charge." "let him go, bill," said the sheriff, nodding to his assistant. "that's a mighty narrow escape fo' yo' nigger," he continued, "i thought it was yo' myself, for sho'." for a moment dan'l did not understand. then it flashed over him. "ah's free! ah's free!" he cried, and fell on his knees on the floor. chapter viii in the whirl of a tornado the success of the weather forecasts which had been put out in the weekly _review_ and the saving of jed tighe's crop had given the league a high standing among the farmers of the neighborhood, but when the story became noised abroad how anton had saved dan'l from unjust arrest, every darky in the neighborhood became its devoted slave. dan'l himself racked his brains for some way to show his appreciation, but none occurred to him. he could not be any more faithful and loyal than he had been in the past. a dozen plans occurred to him, all to be set aside as useless. he wanted to do something that really would help the league. what was there that he could do? as in all cases of difficulty, he decided to go to blind mammy for advice. the conference in the old fortune-teller's cabin was a long one, but when dan'l came out, he carried a huge bundle in his arms and his black face shone with triumph. as spring advanced, kite-flying resumed its former sway among the boys and tom's place became again a centre of attraction. assiduous as he had been before, dan'l had redoubled his attentions, and he was seldom found far distant from anton's side. one saturday, however, he did not appear at the kite-ground until well on in the afternoon, and when he did come, he was carrying something big in his arms, and stepping along as gingerly as if the burden were a baby. "what on earth have you got there, dan'l?" asked tom. "ah done got somethin' fo' the league," the darky answered, and, coming up to the midst of the group, which was gathered around the kite-reel, he lowered the burden gently, very gently, to the ground. "what is it?" asked anton. dan'l looked around. there was triumph in his glance. he was evidently very proud of himself. "ah's made a discovery," he said. "mistah fred, yo'-all wants to take notes of what i say, so's yo' can print it in the _review_." to humor the old darky, the editor-in-chief took out his pencil and note-book and waited for the story. "ah was down in ol' mammy lee's cabin the other day," he began, "becase ah wanted to talk to mammy about somethin'." "went to have your fortune told, i suppose," put in tom. "no, mistah tom, no, ah done hold with no tellin' of fortunes, but mammy she knows a heap an' can see more with her eyes shut than most folks with them open. it was a mighty hot day an' the sun was a shinin' hot. ef it hadn't been that the sun was a shinin' so hot, ah wouldn't have this story to tell yo'." he paused for effect and the boys drew closer. dan'l was a famous story-teller and his tales were always popular among the boys. "ah was standing in mammy's cabin," he continued. "she was a sittin' in her old rockin' chair in the sun right near that little table where she keeps the big glass ball for tellin' fortunes." "you mean her crystal?" put in the forecaster. "yas, suh, mistah levin, her crystal. mammy has two, the little one, what she uses all the time an' the big one, which she doesn't use no mo'. ah was a sittin' on the other side o' the table, right by the window, an' my hand was on the table. by and by, ah felt my hand burnin' as though some one had laid a match on it. ah pulled away my hand but thar wa'n't nothin' thar. ah thought it queer, but ah didn't say nothin' and went on talkin'. by and by, leanin' forward to say some thin' mo' to mammy, ah put my hand on the table again, an' suddenly, the back of my hand began to burn as if de devil was standin' on it. "ah looked, an' ah looked again, but thar wasn't nothin' thar but jes' a spot o' sunshine, jes' so bright. an'it sho' was burning hot. ah took my hand away an' looked at the table. yas, suh, it was burnin' hot. it's an ol' table and in a sort o' ring jes' exactly the same shape as the ring o' white stones that mistah anton put round his sun clock, thar was a burned groove in the table. no wonder my hand got hot. if ah'd have left it there, there'd have been a hole burned right through my hand. yas, suh. "ah spoke to mammy about it, and mammy she says to me that in summer time, when it's very hot, she has to throw a cloth over the crystal to keep it from settin' the table on fire. in winter and in cloudy weather thar ain't no heat at all. so ah says to myself: "'dan'l, if a bright sun burns the table and a half-bright day scorches the table an' a dull day don't do nothin' to the table, why couldn't some kind o' record be made o' the amount o' sunshine? mistah anton, he likes most everythin' like that, an' ah'm goin' to talk to him about it." "but you never did, dan'l," put in anton, not giving much belief to the darky's story. "ah 'sperimented all by myself first," dan'l answered. "ah took a piece of cardboard, the shiny kind, an' i cut out a piece like the shape of the new moon an' laid it on mammy's table. sho's yo' born, mist' anton, that spot of light from the crystal jes' started to scorch that cardboard. when the sun was bright it burned it a real dark brown, when thar was a cloud over the sun, it didn't burn it at all. when the sun had a little cloud it jes' burned that cardboard a light brown. ah'll show yo'." he pulled from inside his shirt a piece of cardboard. it was marked with the hours of the day and, as he had said, in places it had been burned dark brown and in others a light brown. at one spot, there was about an inch where the cardboard was perfectly white, and opposite this, dan'l had got his son to write in sprawling letters, "cloud here." the cardboard passed quickly from hand to hand. "but this is great!" cried fred. "i wonder if mammy wouldn't keep a regular record for us!" with a pompous air, dan'l stretched out his hand and made a clean sweep around him. then he reached down for the package at his feet and commenced unwrapping from around it the newspapers in which it was hidden. as, with a flourish, he pulled away the last piece of paper, there was a gasp of admiration from all the boys. there, on the ground before them, was the huge globe of crystal, clear, shining and flawless. "how did you get it, dan'l?" cried the boys. "ah bought it," the darky replied. "leastways, a lot of us got together an' bought it fo' a present to the league. deacon brown he arranged it all, when mistah levin said to us that the crystal would really work right." "mr. levin!" cried anton. "then you've known all about this, and never told us!" "it was dan'l's secret," the forecaster answered. "do you suppose i'd rob him of the fun of telling you? he's right. dan'l's worked out, all by himself, the principle of the campbell-stokes sunshine recorder, and i think there's a lot of credit coming to him." anton leaned down and tried to pick up the globe, but it was too heavy for him. monroe raised it for examination. it was a beautiful crystal, almost two feet in diameter and without a scratch. "what a corker!" cried tom. "where will you put it, boys?" asked the forecaster. there was a moment's pause and then bob said: "club-house." "yes," the forecaster agreed, "i think that's best, because i know dan'l really would like to see it a part of anton's outfit. besides, boys, anton's going to do some work this summer on sunshine measuring and the relation of sun-spots to the weather, and he'll need a recorder just like this." "have sun-spots anything to do with the weather, sir?" asked ross, in surprise. "yes," the forecaster answered, "it seems quite possible that they have, though to what extent we don't quite know. there's a big field of original work, there, and we've only just found out about it. it's rather a pitiful story, boys, but the man who blazed the trail to that new knowledge, died just two months before the world knew about him." "who was that, sir?" asked anton. "veeder," was the answer. "dr. major albert veeder, who lived and died, an almost unknown country doctor in the little town of lyons, n. y. without any money of his own, he worked hard on meteorology, especially studying auroras and sun-spots. more than any man who ever lived, he tried to show to what an extent the weather of the earth is modified by changes in the sun, chiefly by intensifying the pressure of the anticyclonic areas. "now, boys, for the discovery. "in january, , one of the best-known american meteorologists sent to a brother scientist a postal card which called attention to a recently published article which appeared to be of a good deal of importance. by a curious coincidence, the other scientist had that very day been reading an article published twenty years before in an obscure local scientific magazine, written by dr. veeder. "the two meteorologists, struck by the originality of the ideas and the evidence of the vast amount of work that lay behind them, wrote to dr. veeder at his home in the little new york state town. the recognition that had so long been delayed was on its way. a black-bordered letter came in reply. dr. veeder had died two months before!" a sharp indrawing of the breath told of the boys' interest. "dr. veeder's family at once forwarded the papers, published and unpublished, of the unknown country doctor. these revealed that, as early as twenty years before his death, he had made discoveries of vast importance to meteorology and astronomy. he wrote time and again to the weather bureau, begging us to give his hypothesis a trial." "and didn't you?" asked fred. the forecaster shook his head. "we couldn't," he answered. "we had no funds for special research and dr. veeder's ideas were so far ahead of his time that, then, they seemed visionary. now, twenty years later, when a great deal of similar work has been done in europe and in this country, we see that dr. veeder was a real pioneer, although, of course, many of his conclusions are still doubtful. yet, in poverty, in discouragement, in the turmoil of a busy life, he continued his work for fifteen years, then reluctantly abandoned it, despairing of support and opportunity. yet he leaves a debt that science can never repay. such men may be everywhere; one of you boys may be the meteorologist of the coming generation. veeder may be dead but his work lives after him." the weather expert picked up the great glass crystal which monroe had replaced upon the ground. "we will go on with veeder's work ourselves," he repeated, "so far as we can. veeder showed us that sun-spots and changes in the sun are closely followed by changes on the earth, and he suggested that this is caused by some agency other than heat. from that we shall go on. let us do some sun-study. it is symbolic, to me, that a crystal once used for the superstition of crystal-gazing, should become a tool for scientific research." he raised the crystal to shoulder height. "here's to veeder!" he shouted. "and to dan'l!" the cheers were given with a vim. interesting as the work of the league had been to the boys during its first summer, when all were learning of the ways to read the weather, this second summer became tenfold more exciting, when every lad realized that he was part of a group striving to advance along the lines laid down by veeder. the money which jed tighe handed over to the league as its fair share of having saved his fruit crop, was spent in the purchase of a telescope for studying the sun and for various other scientific instruments, and, as the forecaster had foretold, issaquena county began to take its place as one of the most efficiently organized meteorological regions of the united states. the summer was passing on. the year and a half that had elapsed since the flood, a year and a half of constant association with the forecaster, and still more, of constant association with work that was worth while, had developed the boys of the league and given them a new grip on life. one saturday, ross came over early in the morning to help anton with some of his sunshine experiment work. the crippled lad had definitely settled down to the study of meteorology and spent all his time either at his instruments or at his books. under the forecaster's teaching, he was becoming thoroughly proficient, and the fact that the lad was a natural-born mathematician stood him in a good stead. he was no longer merely a crippled lad, with scarcely a chance before him, he was making a place for himself in the community and there was no doubt that he would make a place for himself in life. this morning, as anton came out of the club-house to meet his friend, ross looked at him and thought how wisely the forecaster had done in suggesting the formation of the league. "bad weather coming, isn't there, anton?" ross asked, as they strolled into the club-house together. "thunderstorms, i expect," the other answered, glancing carelessly at the weather map. "there's a big 'low' over illinois, with colder weather coming." "i'm glad it's going to be cool," said ross, mopping his forehead, "to-day is something fierce." "yes, it's hot," agreed anton, and turned the subject to some of his recent work on sun-spots and the weather. he had become an absolute convert to dr. veeder's theories, and the dream of the boy's life was to be able to take a part in the most fascinating of all weather problems--long-range forecasting. "it would be great, ross," he said, "if we could tell a year in advance what kind of weather we were going to have, so that farmers would know exactly just what kind of crops to plant and when!" "yes," ross agreed, but uneasily, for he was watching the sky steadily, "but do you think we'll ever be able to do it?" "i don't think we'll ever be able to tell exactly," replied anton, "but i'm sure the time's coming when we're going to be able to get a general idea. if we can just find out enough about the sun's influence on our weather and enough about the big changes in the sun, we ought to be able to foretell something. there's no doubt that weather does go in cycles." "i don't see that," said ross. "i think it's changing all the time. you always hear people say that the winters aren't nearly as cold as they used to be." "that's all bosh," anton declared. "mr. levin and i were talking over that just the other day. there hasn't been any change of weather. the winters to-day average the same that they did fifty years ago. there's some sort of an eleven-year cycle in rainfall, and there's a variation in temperature that seems to swing around about once in every thirty-seven or thirty-eight years, but the differences are so small that only weather bureau records can prove them. the weather isn't any hotter or any colder than it used to be, it's just about the same." but ross was not listening. his eyes were fixed on the horizon. "anton," he said, "i wish you'd come here a minute." struck by his companion's tone, the younger lad looked up and, grasping his crutch, limped to the door. he took a glance at the sky and whistled in a low and thoughtful way. "look at those clouds to the north-west," said ross. then, pointing to the south-west quarter, "and look at them there!" anton looked, his eyes dilating. in the north-west, swarthy, curling wreaths of vapor that seemed as though they rose from a monstrous burning straw-stack writhed their way upward to a great height, the upper portion seeming to tremble threateningly, as though there were a shaking fist within the swirl, hidden by clouds. the column was smoky and threatening, yet a whitish light came from beneath it suggesting phosphorescent vapors. to the south-west were clouds of a different character, darker and more compact. they were not blacker than many clouds preceding a heavy rainstorm, but they had an uneasy motion. from these came no whitish phosphorescent light; instead, there was a greenish glitter, like a snake's eyes seen in the dark. there was something evil and sinister about them. the air was reverberant, sounds could be heard to a great distance. the farm animals were unquiet and moved restlessly. anton wiped the perspiration from his forehead with the back of his hand. he glanced up at the weather-vane. "it ought to pass to the east of us," he said. ross also looked at the weather-vane, and then at the advancing cloud. he knew that nearly all such storms traveled to the north-east. "it may pass us," he said, "but sometimes they swing north." "i know it," anton answered, and fell silent, watching the coming of the storm. in the distance a faint moaning was heard. the two huge cloud masses from the two quarters of the sky, as though advancing to give battle, hurled themselves toward each other, the whitish cloud of the north-west towering above the sinister black cloud of the south-west. for a moment, almost as if they paused, a strip of blue sky could be seen between them, then with a sudden rush, the two collided. so solid seemed the masses of the clouds that both boys started, expecting a clap of thunder. yet never a flash of lightning appeared nor was there any sound. in the whirl of the two meeting clouds there was a minute of confusion, and then, slowly, a long funnel, like a black finger, began to reach towards the earth. both boys saw it at the same time. "a tornado!" cried anton. "let's get to the cellar!" cried ross, and started to run, but anton grasped him by the shoulder. "no," he said, "we're safe here; it'll pass to the east over the farm lands and won't hit anybody." in a few seconds ross saw that the crippled lad was right, and, themselves safe, the boys watched the passing of the tornado. "it's going about thirty miles an hour," said anton, figuring rapidly, "and it's all of fifteen miles away. there won't be much left of it by the time it passes here. we don't need to worry." reassured, ross turned to his companion, and asked: "what makes tornadoes, anton?" "a quick current of warm air going up in a thunderhead cloud," he said, "which takes a spinning motion from the general whirl of the cyclone to which it belongs. it has a whirling vortex, from the outside to the inside, and its speed gets higher toward the middle. the speed of the inside of a tornado has never been figured out, but it has been estimated at eight hundred miles an hour, or sixteen times as fast as a train." "eight hundred miles an hour!" ross repeated. "but how did they find that out?" "not by any instrument," said anton; "there isn't anything made that a tornado wouldn't level to the ground. but you can figure that from the size and weight of objects lifted and from the effects of tornadoes. anyhow, the inside of a tornado is like a vacuum, the pressure is so low. "i remember reading in a tornado account of a storm in new england where the funnel passed within twenty yards of a house. it was exactly as if a house filled with air were suddenly plunged into a vacuum. all the windows were blown out, the walls bulged, furniture flew out of the windows and corks were drawn from empty bottles by the air inside trying to get out to fill the vacuum in the tornado." "that's a wonder," ejaculated ross. "but we're not going to get anything like that this time." as the boys were talking, the distant tornado suddenly raised itself from the ground and seemed to be drawn up in the clouds again. the danger from the funnel was over. a few minutes afterwards, there came a clap of thunder and the rain commenced to fall in torrents. it rained for less than a minute, however, then was followed by a few hailstones as large as walnuts. the hail stopped as suddenly as it had begun. yet, though the funnel cloud had been withdrawn again into the sky, though the rain and hail had ceased, the two boys did not move from the doorway of the club-house. the sky was pressing down heavily and in the masses of clouds that seemed to be moving in every direction, the whitish luminous cloud and the greenish black cloud could both be traced. this was no puny battle of the elements, but a veritable war. then, absolutely without warning, as suddenly as though some malevolent demon had picked them out for destruction, from the low-lying bank of clouds that was advancing, a long black swaying clutch thrust at them from the clouds. for a second or two the funnel swayed as though there were eyes in its tip and then snatched at the earth with a roar and crash like a thousand trains in collision. while one could count three, the lads watched, panic-stricken, then anton shouted: "run north-west, ross! north-west!" like a flash the forecaster's advice in the event of the approach of a tornado recurred to the boy's mind, and he sprang into a full run. ten yards, perhaps, he ran, then cast a glance over his shoulder to see if anton were following. he saw the younger lad huddling down by the south-western corner of the club-house. ross colored with shame. for one second he had forgotten anton's crippled condition. he whirled on his heel with a speed scarcely less than that of the approaching tornado and darted back for his friend. a dozen strides took him back and he reached down for the younger lad. as he did so, with the corner of his eye, he saw the tornado touch a neighbor's barn. the moaning suddenly swelled into a vicious and snapping roar. the point of the tornado enlarged, as it became filled with the débris of the barn, and ross fancied he could hear the squealing of the mangled horses. out from the upper part of the wild whirl, high in the sky, a black spot flew. thrown at a tangent, it fell, growing larger and more bat-like as it fluttered down, striking the earth with a crash. it was the roof of the barn. all this had happened in the fraction of a second that had elapsed while ross was picking up the crippled lad, and by the time that he had flung him across his shoulder, the tornado had passed over the neighbor's farm and there was nothing left of the barn but a black bare spot. before the out-flung roof had struck the ground, ross was running from the track of the swiftly-moving destruction, with his chum on his shoulder. the boy knew well that in ninety seconds or less, the tornado would be upon them, and while it swayed with a malicious eagerness from one side to the other, as though seeking for its prey, there was no doubt that it was rushing straight at them. second by second, the moaning grew louder, with an uncanny sucking sound as though the monster were licking its lips over the destruction yet to come. the air grew more oppressive and more still. twenty yards from the club-house, ross found dan'l crouching on the ground, quivering with fright. "mistah anton, mistah anton," he cried, "we's all goin' to be killed!" "run, dan'l!" cried ross, as he sped past. "run north-west! follow us!" white with terror, the aged negro rose and started to run, but before he had gone two yards, his steps slowed down. "thar's mammy," he said, aloud. "ah can't leave mammy, nohow. thar's no one to look after her." he turned back with unsteady steps, hurrying towards the negro quarters, almost facing the approaching finger that seemed to point at him as he ran. ross never looked back. his terror and the terrific heat of the air choked his breathing and he gasped as he ran. a sudden swirl of air clutched at his feet. he stumbled and almost fell. the crippled boy's crutch slipped to the ground. anton slid to the earth and a second swirl picked ross's feet from under him and threw him to the ground. then, with a roar and a confusion which stunned the senses, the thing struck! a legion of hands tugged at them. the earth rose up in a cloud of dust around them. towards them the tornado swerved, then away, just a fraction out of its course, and swung back again towards them. as in a dream, ross saw the crutch, which had slipped out of anton's grasp, not five yards from where they lay, move restlessly, then, touched by an unseen hand, rise up. while two heart-beats lasted, the crutch stood still and perfectly upright, and then flew straight upwards into the all-devouring maw. the black-green fury snatched at the waiting world. with a roar like that of crashing universes, it swept by the boys and swung into the farm building. a hay-stack disappeared into the vortex like a puff of smoke. with a crash of glass, the tornado swept by the corner of the house, and with one wild last shriek was gone. gasping, ross sat up. across the fields the cloud swept, the long black finger still touching the ground and still bringing wreck and destruction in its wake. ross gently raised the younger boy, who was only half-conscious from the din and tumult, for the tornado had passed within a few yards of them. they had scarcely walked a dozen yards when the scene of destruction met them full view. every window in the house had been shattered and the garden was strewn with broken glass. the buggy, which had been standing before the door, was nowhere to be seen, but one wheel impaled in a tree twenty yards away, told the story. the upright of the sun-dial was gone, snapped off at the ground as though it had been a reed. the club-house remained intact. the track of the tornado was not more than forty feet wide, but where it had passed, the ground was swept clean and bare. only one thing remained, and that, by one of the freaks of the tornado, was the pedestal and the large globe of crystal. it had not even been fastened down; it had passed through the centre of the tornado and yet it stood there as unwinking as the sun itself. stood there all by itself, sharply gleaming against the black ground-- what was that lying on the farther side of it? "go back, anton, go back!" said ross, hoarsely. but anton had seen it, too. he shook his head. haltingly, step by step, the two boys advanced, anton's hand on ross's shoulder, to the figure lying on the ground beyond the sun-dial, motionless and oh, so still. behind the fast-flying clouds the sun shone out, shone clear and strong on the crystal, standing on its pedestal, and the gleam, passing through, fell full on the face of the man. "dan'l! dan'l!" the crippled lad cried, and dropped to the ground beside him. he was not hurt. he would never be hurt any more. ross looked down at the faithful old darky, who, despite his terror and in the teeth of certain death, had turned back to try to save the aged blind woman in the negro quarters. the tornado had dealt kindly with him. his ragged clothing fluttered in the wind, but his kind old face was peaceful. the sunlight, gleaming through the crystal, made a halo of light around the negro's head. "don't!" said ross, laying his hand on anton's shoulder. "there's mighty few of us that'll ever get the chance to die like dan'l." chapter ix the trail of the hurricane "two o'clock, tuesday morning, august the seventeenth, nineteen hundred and fifteen! "slowly down and across the white, faintly ruled paper wrapped about the revolving drum, i watched the long-shanked, awkward pen of the barograph in our weather bureau station at galveston. in the jerky, scrawling fashion of a child writing his first copy on a slate, i saw the pen gradually draw what looked like a rough profile map--a long declining plateau, a steep and then a steeper slope, a jagged ugly valley-- "the valley of the shadow of death!" the boys clustered closer round the speaker, the man who had seen and lived through, the galveston hurricane. "we knew well, the three of us in the weather bureau," he went on, "that descending zig-zag line meant that the hurricane, then beginning to rage over our heads, would increase in fury and in ruin, until the other wall of that strangely-drawn valley should begin to form under the halting pen. thus we watched and waited. "'read the wind velocity,' my chief said to me. "i focused a glass on the recorder, holding a lantern in my other hand. "'ninety miles an hour, sir,' i said. "'it'll be a good deal more than that,' he answered. 'i only hope we don't have a repetition of .'" "that was the worst ever, wasn't it, sir?" asked anton. "it was the most destructive storm that the united states ever saw," the galveston weather observer answered, "but, as a storm, it wasn't nearly as violent as the one we've just been through." the speaker, who had his arm in a sling and who was still frail and weak from the injuries he had received during the hurricane, looked round at the boys. being the forecaster's nephew, he had come to his uncle's house to recuperate and the work of the league had fired his imagination. "tell them of the storm first," said the forecaster. "you tell them, uncle," his nephew replied; "you remember that better than i do, and then i'll tell the boys my adventures in last week's storm." "yes," put in fred, "you tell us, mr. levin." "very well," said the founder of the league, and he began: "i suppose, measured by the loss of life and property, the galveston hurricane of was the worst catastrophe that wind and water has ever brought to america. on galveston island alone, over six thousand people were killed, and five thousand more in the inland coast country. the ruin and loss of life was caused by a storm wave, which swept in from the gulf in advance of the hurricane's vortex. this wave, four feet in depth, struck the already submerged island with almost irresistible force and entirely destroyed the city for ten blocks inland. over five hundred city blocks were ravaged and two hundred blocks were laid level to the ground. three thousand three hundred and thirty-six houses were destroyed." "where did it begin, sir?" asked anton. "in the west indies?" "undoubtedly," the forecaster answered, "but, unlike last week's storm, we knew very little about it, before it came. three days before the hurricane struck galveston, storm warnings were hoisted, although, at that time, advices from cuba showed that it had developed but little force. by the next afternoon it was beginning to wake up to true hurricane strength and the steamer _louisiana_ almost foundered in the middle of the gulf. "in galveston, our barometer commenced falling that afternoon, and by next morning the situation began to look serious. the barometer was still falling steadily and high cirrus clouds of the mares'-tails variety, that always run in advance of the hurricane, were clearly marked. "that afternoon over the waters of the gulf came the long low swell, each wave one to five minutes apart, which is the sure sign of trouble. though the wind was from the north and north-west, the swell from the south-east steadily increased and the tide began to rise. before mid-night, the weather bureau had sent warnings to the newspapers to urge special precautions for the next day, as a rising tide and possible hurricane threatened disaster. at breakfast, the next morning, every one in galveston read these warnings, none too soon, for at nine o'clock, the edge of the storm struck the city. "the wind was steadily rising, and shifting by gusts at five minute intervals, until one o'clock in the afternoon, when it reached storm velocity. after that, it began to increase in fury. every subscriber of the telephone company was warned personally from the weather bureau. hundreds of people who could not be reached by telephone besieged the weather bureau, seeking advice. dr. cline, the chief of the station, who had been directing all precautionary measures since five o'clock in the morning, went to his home for lunch at half-past three o'clock that historic afternoon. the wind was then blowing fifty miles an hour. "'i reached home,' wrote dr. cline, 'and found the water around my residence waist-deep. at once, i went to work assisting people, who were not securely located, into my residence, which, being large and very strongly built, i thought could weather wind and tide. about : p. m., one of the other weather observers, who had been on duty since the previous midnight, reached my residence, where he found the water neck deep. he informed me that the barometer had fallen below . , that no further messages could be got off to washington, or anywhere else, as all the wires were down, and that he had advised every one whom he could see, to go to the center of the city; also, he thought that we had better make an attempt in this direction. "'the roofs of houses and timbers, however, were flying through the streets as if they were paper, and it appeared suicidal to attempt a journey through the flying timbers. just at this time, the anemometer in the weather bureau office registered one hundred miles an hour and blew away soon after. in the next hour the wind rose to a velocity of one hundred and twenty miles an hour. many people were killed by flying timbers, about this time, while endeavoring to escape to town. "'the water rose at a steady rate from p. m., until about : p. m., when there was a sudden rise of four feet in as many seconds. (hundreds of people, undoubtedly, were killed and drowned during those four seconds.) i was standing at my front door, which was partly open, watching the water, which was flowing with great rapidity from east to west. the water at this time was about eight inches deep in my residence, and the sudden rise of four feet brought it to my neck before i could change my position. the tide rose in the next hour nearly five feet additional, making a total tide in that locality of about twenty feet. "'by p. m. a number of houses had drifted up and lodged to the east and south-east of my residence, and these, with the force of the waves, acted as a battering ram against which it was impossible for any building to stand for any length of time. at : p. m. my residence went down, with about fifty persons who had sought it for safety, and all but eighteen were hurled into eternity. among the lost was my wife, who never rose above the water after the wreck of the building. "'i was nearly drowned and became unconscious, but recovered through being crushed by the timbers and found myself clinging to my youngest child, who had gone down with myself and my wife. mr. j. l. cline joined me five minutes later with my other two children, and together with a woman and child whom we had picked up from the raging waters, we drifted for three hours, landing three hundred yards from where we started. there were two hours that we did not see a house or any person, and from the swell we inferred that we were drifting to sea, which, in view of the north-east wind that then was blowing, was more than probable. during the last hour that we were drifting, which was with south-east and south winds, the wreckage on which we were floating knocked several residences to pieces. when we landed about : p. m. by climbing over floating debris, the water had fallen four feet. it continued falling, and on the following morning the gulf was nearly normal. "'while we were drifting, we had to protect ourselves from flying timbers by holding planks between us and the wind, and with this protection we were frequently knocked great distances. many persons were killed on top of the drifting debris by flying timbers, after they had successfully escaped from their wrecked homes. in order to keep on the top of the floating masses of wrecked buildings, one had to be constantly on the look-out and continually climbing from drift to drift. hundreds of people had similar experiences.' "fearful as was the disaster," the forecaster continued, "it would have been incalculably worse had it not been for the weather bureau warnings. hundreds of people were saved by retiring to the upper portion of the town during the afternoon of the hurricane and no amount of foreknowledge could have told the sudden four-foot rise in the gulf. galveston learned her lesson, too, as was shown in the recent hurricane." "i don't understand those hurricanes a bit," declared fred, "they don't seem to act like tornadoes, and instead of coming from the west, like all the rest of our weather, they come up from the south-east. how is that, mr. levin?" "the west indian hurricanes," the forecaster replied, "are storms which are also called tropical 'cyclones' and which in the china sea are known as 'typhoons,' and the fearful stories that one has read of the typhoon in the china seas applies equally to the hurricanes that strike our gulf coasts. "like all other tropical cyclones, the west indian hurricanes are formed by an upward rising current of air over a moist heated area. there are five cradles of such storms. one is over the pacific ocean south-east of asia and gives the coast of china, the philippine islands and japan the typhoon. a second and a third are in the north and the south parts of the indian ocean. a fourth, which is less frequent, is found east of australia. "the cradle of the west indian hurricanes is in the north atlantic, about six to eight degrees north of the equator and from two hundred to a thousand miles east of the west indies. these hurricanes, when first seen, are quite small but they increase in size and in motion as they come westward. most of them, when they reach the lesser antilles--where uncle sam's new islands lie, the virgin islands--also increase in whirlwind character, and turn northwestward, skirting the northern edge of porto rico. this is the mean track. about seventy-five per cent of them pass over a regular storm trail between bermuda and charleston, most of these coming close to the coast and sweeping circularly away from the land at cape hatteras. at the latitude of new york, the curve has taken them half way round the circle and they disappear as violent westerly gales, though beginning as easterly hurricanes. "as you will have noticed, nearly all these storms come in the autumn. that is because the cradle of the hurricane is the doldrums, and in august and september, the atlantic doldrums are at their furthest north. the chinese typhoons are most frequent in the same months of the year, from the same cause." "and this last one, sir," tom asked, "the one that blew down my anemometer last week and which smashed up the old windmill, was it just like the hurricane of ?" "i think i'll let my nephew tell you about that," was the reply; "he was in the thick of it, and the people of galveston gave him a medal for bravery in connection with it, so he ought to be the one to speak." "gee, did you get a medal!" exclaimed fred. "do let's have a look at it." the young weather observer shook his head. "i haven't got it with me," he said, a little embarrassed. "but if you chaps want to hear about the hurricane, i guess, perhaps, i can do that." he smiled. "i don't know that i've anything quite as thrilling as dr. cline's drift to sea, but one really astonishing thing did happen. i'll tell you about it." "tell us the whole thing," said anton, "how the storm started and when you first got hold of it and what you did, and why they gave you the medals and--oh, everything!" "all right," the young observer answered, and nursing his broken arm with his other hand, he began: "we first heard about the hurricane on the morning of august th, where it had been seen between the islands of barbados and dominica. a little before ten o'clock that morning, storm warnings were sent to all west indian stations. it came as a good deal of a surprise to us at galveston because there had been none of the signs which usually go before a bad tropical disturbance. at two o'clock in the afternoon of that day, notice of the approach of a storm was sent to all atlantic and gulf stations of the weather bureau and the report was sent out by the wireless naval station at arlington, virginia. "on the morning of the eleventh, the storm was south of the island of st. croix, with a hurricane strength wind of sixty miles an hour at porto rico. on the twelfth, it was central off haiti, and by the next morning was ravaging jamaica. hurricane warnings were sent out by the bureau for key west and miami. on the fourteenth, the hurricane was central off the isle of pines, cuba, and on the fifteenth, was central in the gulf, gathering force steadily. all vessels were urged to remain in port. as a result of this warning, shipping scheduled to sail and valued at forty-five million dollars remained in harbor until after the hurricane had passed. had they sailed, few of these ships would have lived. hurricane warnings were ordered as far west as brownsville, texas. on monday, august th, the storm approached the coast, and, in our office in galveston, its menace began to make itself felt. "over the glassy surface of the gulf there came a long, low swell, smooth and deep, the waves several minutes apart. those who saw the swell remembered the disaster of fifteen years before, when eleven thousand lives were lost. true, the great sea-wall had since been built to protect the town, but would it stand? man against the hurricane--which would win? "in the sky, which was a weak, watery blue, appeared the ice-plumes of the cirro-stratus clouds, the true mares'-tails, flung out across the vault, their ends stretching to the centre of the storm. at the horizon, a wicked, dull glare gave threat of the typhoon's approach. all as yet was soundless, only the far-flung clouds told of the fury which was hurling them ahead of the circling hurricane below. "then! a low, whirring whistle of the wind. not like the moan of an approaching tornado is this wind, but like the high-pitched note of an engine running smoothly at high speed. characteristic and peculiar, boys, is that heralding wind, with a throbbing note in its character. that day, too, came the white squalls, lasting a minute or two each, with puffs of furious wind and a bucketful of rain, like bombs fired in advance of the hurricane by some huge æolian howitzer. steadily the whir of the advancing wind became louder, steady, without gusts, and more and more frequent became the white squalls. "up, up and ever up came the sea, forced by the iron hand of the grim wind-tyrant behind. the swells came faster and the tide rose. against the sea-wall the billows fell back, baffled, but, inch by inch, the waters of the gulf rose against the city. man's hereditary enemy, the ocean, prepared itself for attack. inch by inch the water gained, wound its sinuous way through the channel in the bay, backed into nook and cove and, long before the storm came, swirled a foot deep over land which never before in the city's history had been under water, even in the great storm of . "all day long, since midnight of the day before, three of us, up in the weather bureau, kept watch by our instruments, at the telegraph wire and the telephone. we had the men of galveston to deal with, men who were not afraid of danger, men who knew well what the word 'hurricane' meant. all through that day an army was organized, an army of men that rested neither for food nor sleep, warning those who were in the path of danger, leading the women and children to safety, carrying the old and sick upon their shoulders from regions where death was threatening. "our chief, at the weather office, summoned volunteers with motor-cycles and these men went to every corner of the city with the news of the approaching disaster. through the streets rode these paul reveres, carrying the cry of the warning, and on that sunday not one house in the entire city of galveston was left unwarned. the city had lost six thousand lives in the hurricane of . it was not to be caught napping a second time. "at seabrook, texas, across the bay, professor stearns, a co-operative observer, personally visited every house in that section on sunday, the fifteenth, and again on monday. before the hurricane, eighty-eight houses stood there; after the hurricane, there were three. yet every one was saved, except two people, who had laughed at the weather warnings. "steadily the sea rose, all day monday, and equally steadily the wind increased. the fire department joined in the work of protection. the police joined in the work of saving. as yet the hurricane had not come, but, through the weather bureau warnings, no one was allowed to pass into a fool's paradise of security. "the summer evening came on with the whistling whir of the wind changing its note to an angry rage. in our little office at the top of the building, it looked as though we should be blown away. but there was too much to do for any man to leave. still, had it not been for the thoughtfulness of one friend, none of us would have had anything to eat. we did not have a let-up of any kind for fifty-six hours. "a wall of water swept towards the island, and before it became too dark to observe, in the early twilight one could see the wind-lashed waters of the bay begin to heap themselves into broken and irregular waves, each striving to overtop the other in their plunge upon the city. they broke, indeed, into the back door of the city, and then, with a suddenness that seemed to rock the very foundations of the earth, the wind struck us, in three nerve-racking blasts. "with the savagery of the elements at their worst, the registering-pen of the anemometer in our office began to write its message. raging in fury, the tempest leaped to eighty miles an hour, to a hundred miles an hour, to a hundred and twenty miles an hour. the air in the middle of a hurricane is estimated to have the weight of half a million ocean liners and four hundred and seventy-three million horsepower. imagine a weight of several billion tons being hurled with five hundred million horsepower at a speed of two miles a minute! that, boys, was the storm that plucked at our little office in the sky, and that was the force which picked up the billows of the sea and hurled them at the seawall built by the hands of man. "at the signal given by the titanic winds, the waves drove in from the gulf and from the bay and smashed into a thousand pieces the houses of the lower section of the city. but the wind and the waves found nothing on which to wreak their vengeance except the empty shells of houses. without our warnings, thousands of people would have been there and thousands of lives lost. but the hurricane was foiled of its prey, because of the writing of the little instruments at the top of the weather bureau tower. "when the storm was at its height, our anemometer blew away. when she went, the wind was howling cheerfully along at seventy-five miles an hour. the chap who was with me, a plucky fellow, suggested that we should go up on the roof and put up a new one. i thought myself that if we went up there, we'd be carried off like a couple of straws. but i wasn't going to have him think that i was scared. so up we went. my word, boys, but it was blowing! we worked for half an hour when the gale got under my coat and blew it open like a sail. in a fraction of a second i was being driven breathless to the parapet. "through the storm i heard a faint voice crying: "'take it off!' "i tore the coat off and it flew up in the air like a crow, but it was almost too late. i was thrown against the parapet like a bullet. my shirt-sleeve tore and flew to ribbons, and i became conscious that my arm was hurting horribly. i fought my way back against the wind over to the roof and helped the other chap with the anemometer, which had nearly been erected when the wind caught me, and we got down the trap-door to the office of the bureau. then i keeled over. my arm was broken. my partner fixed it up as best he could." "and you went on working?" asked fred. "naturally," the young observer answered. "i wasn't going to give in just because of a broken arm. besides, there was work to do, work worth doing. "far out to sea, meanwhile, was occurring one of the strangest stories of the sea. the annals of the ocean hold many thrilling escapes, but none, perhaps, more startling than that of the stranding of the three-masted schooner _allison doura_, which passed through the eye of the galveston hurricane. obed quayle, a cape cod sailor who was one of the men on board, told me the story. "'we were six days out of progresso, mexico,' he said, 'with a cargo of bales of sisal. the weather had been fair, with a goodish bit of head winds, but we reckoned to make mobile on sunday, the fifteenth. on friday the weather began to look dirty and there was a long rollin' swell from the eastward that i thought was going to yank the booms out of her. "'at eight bells of the second watch, the wind shifted, and any one could see with half an eye that there was trouble brewin'. the sea smelt of a storm. we made everything snug alow and aloft, put in double reefs and lay by. "'at two bells of the afternoon watch, the gale struck us, and it struck us hard. captain evans wood, the skipper, a mighty good seaman, handled the craft well, but our foretopmast was snapped right out before the gale had been on us an hour. "'the jib-boom, too, went with the crash and the nasty mess of timber and shrouds, floatin' to leeward, began to hammer at our hull in an ugly fashion. a couple of us got at the wreckage as best we could, but before we had cut it adrift, the _allison doura_ had sprung a leak and four of us went to the pumps. "'while we were workin' at the wreckage of the foremast, the schooner was pooped and the wheel was carried away. bill higgins, a young fellow who was at the wheel, was swept against the rail and had his head split open. "'i've seen some bad weather in my time, but never just in that way. with the mizzen boom we rigged up a fore jury-mast and made shift to hoist a storm staysail to give us steerin' way and rigged up a tiller for steerin'. the wind was whistling like all possessed. it was askin' more than any vessel had a right to stand, and around midnight the fore staysail was blown clean out of the bolt ropes and she lost steerage way again. we couldn't hold her to the wind. "'with losin' steerage way so much and without bein' able to hold her up to the wind at all, we couldn't run out of the storm. the gale drove us in and in to the centre of the hurricane. somewhere around dawn on sunday mornin' the wind decided to show us what it really could do. we were runnin' before the wind with a triple-reefed mainsail and not another stitch. "why weren't we under bare poles," you asks? because there was a sea chasin' after us with every wave looking like a whale out of water. we weren't lookin' to get pooped, any more than we had to. the mainmast went with a crash. "'that left it nasty. the mizzen-mast, bein' the only one left standin', took her down by the stern and the waves runnin' along behind slapped us in the quarters good and proper. the skipper he give us orders to cut away the mizzen-mast, to lighten her. "'it didn't take much cuttin' neither. the axes hadn't more than gotten through one of the weather shrouds, when the gale took the mast and chucked it over the side. that left us with the fore jury-mast that we'd rigged up, but not a stitch of canvas. the ship was as naked as a nigger baby in the cannibal islands. "'we did our best with it, of course, and dug up a stretch of storm canvas about the size of a leg-o'-mutton sail and lashed that to the jury foremast and the stump of the bowsprit. with that gale cuttin' off our ears, it was all the sail she could carry. bill, we had him lashed near the tiller we'd rigged up, not havin' a wheel, and by-n-by, most of us was wishin' we was lashed. but the old hooker stood up under it well, and though she was buried nearly all the time, her nose came right out of the green. "'we'd have done anything in the world to beat north-east, for we knew the hurricane was goin' to the north-westward, but we couldn't do anything but run before the wind in our crippled state and the wind was blowin' north-east. it was shifting northerly and then westerly and we all knew that we were bein' driven into the very middle of the storm. the gale grew fiercer and fiercer, the sea was lashed to a mass of foam and in the shriekin' of the hurricane we couldn't tell, half the time, whether we were under water or above it. "'bill, with his broken head, stayed put at the tiller, the skipper never went below, cookie tried to get some grub and the other four of us were lashed to the pumps. it was rainin' in torrents, too, but that didn't make any difference, for there was so much water that you couldn't tell whether it was the waves or the spray or the rain that was drownin' you; all we knew was that we were gaspin' for breath in an atmosphere that seemed about half air and half water. "'then, quite suddenly, the wind died down, and the rain fell from the sky as though the sea had been picked up and were bein' tilted over the ship. the clouds, racin' by and so low that they seemed almost as if you could reach up and touch them, flew overhead so fast that you couldn't believe it was a real sky you were lookin' at. it seemed like a painted piece of metal driving across the sky on an aeroplane. it fairly made me giddy to watch them. the winds died down, and suddenly became quite calm. "'i've seen some seas, too, in my time, but never nothin' like this. waves, no matter how high, i've been used to all my life. i've seen seas over the banks of newfoundland that would look like a mountain, but waves like those in the eye of the hurricane i never saw before and i never hope to see again. they came from the east and the west, from the north and from the south. they met in the middle and struck each other, making whirlpools that set the schooner spinnin', they rose up and fought against each other, they swerved and leaped and jumped. one end of the schooner was yanked this way and a wave would come along and yank it to the other, cross currents pitched her nose down, and while her bow was down, another would slap her in the stern. "'we was all lashed to the pump wheels. we were bruised and battered and sore. i never thought we'd get out of it. and, steadily, while lyin' almost without enough wind to fill our one small sail, we were pitched and tossed and shaken as a terrier shakes a rat. how the timbers of the ship ever held together, i don't know. we sprung another leak and while, before, we had been able to have ten minutes' spell in every hour, now we not only had to keep pumping steadily, but we had to keep those handles going at a swingin' pace. cookie came and gave us a hand at the pumps and started some of the old chanties. the sun came out and shone clear above us and all the clouds disappeared. you might have thought it was a warm, mild day in summer, only for the orange-colored ring all round the sky and that boiling spot of a sea. we went on pumpin'. "'it got so quiet in the eye of the hurricane that i felt as if i wanted to scream, and when cookie stopped singin' for five minutes, i could see the glare of madness comin' into the men's eyes. for all i know, it may have been in my own. bill was the first to go. he dropped the tiller and came shriekin' along the deck with his sheath knife, yellin' for the wind to begin again. the skipper drew a revolver, ready to shoot him if necessary. but i saw bill was comin' for me, and before he could reach me with his knife, i got him one in the right on the point of the jaw. one of the other men went to the tiller, while cookie and the skipper lashed bill fast to the stump of one of the masts, standin' him upright, so that when he came to, he wouldn't be able to hurt any one. "'the other men at the pumps began to talk wildly. we hadn't no water. our deck-casks had been carried away, with all our boats and everything movable, and we couldn't get at the tanks below, because we couldn't open the hatches. they was battened tight and if you so much as lifted a corner of the tarpaulin, the whole gulf of mexico would tumble in and there would be the end of us. "'one of the chaps, however, insisted on scoopin' up with his hands the briny water that flowed from the pumps. it was mixed with bilge water and smelt horribly. he went mad, too. but we couldn't afford to lose any man's work and we lashed his hands to the pump handle. he went mad in a happy fashion and pumped wildly, singin' and talkin' in a way that made your heart curdle to hear it. still, he pumped. the clouds began to form again round us, the same racin' clouds, the orange rim came nearer and we knew that we were once again approachin' the edge of the hurricane. there happened to be a little food in the galley and a scrap was given to each man. if we were going under, there was no need to drown hungry. so, faintly, but with quickenin' loudness, the whirring roar of the hurricane rose into a shriek and the fury hit us again. "'i suppose i went on pumpin', i suppose we all went on pumpin', for the vessel stayed afloat, but what happened after we passed into the hurricane again, i can't tell you. i was deafened, stunned, blinded. i think i must have gone mad, too. our trysail blew out right away, and the tiller that we had rigged up went as well. the bulwarks were laid flat with the deck. the skipper and one of the men were lashed to the stump of the mizzen mast, bill, who had come to again and was ravin', was lashed to the jury foremast, and the other four of us were lashed to the pumps. "'whether i pumped for a day, a week, or a century, i'll never tell you. it seemed to me that i had been drivin' round that pump wheel for thousands and thousands of years. i remember that i thought that i was dead and that i had been sentenced to turn the wheel of a ship's pump forever. on saturday afternoon i started my trick at the pumps, and maybe half a dozen times before midnight, i had ten minutes' spell. on sunday i never left the handles and the last bite i had to eat was in the evenin'. all day monday the four of us, lashed to the pumps, had never a stop, nor a bite to eat, nor a drop to drink. we laughed; how we laughed! i must have laughed for hours. we would have killed each other to stop, but the skipper had lashed our wrists to the pump handles. did we stop? no one could ever tell. did we pump without stoppin'? no one could ever tell that, either. once in a while my brain cleared, and i saw the skipper, sagged, unconscious, dead, i thought, by the mizzen mast, and i heard the ravin's of bill, lashed to the fore. "'in the night, i suddenly saw the lights of a town. it was galveston, and we were drivin' right on for it. i was so glad that i sang and shouted. at last, at last we were goin' to be wrecked. then, perhaps, there would be rest, unless indeed i were already dead and pumpin' forever. we drove on and on, while i shouted--and went on pumpin'. "'a sea picked us up and threw us at the sea-wall, the seventeen foot high sea-wall. just before we struck, i saw the captain move and look up. the schooner was thrown out of the water, as a porpoise jumps, vaulted the sea-wall and came to solid ground with a crash that broke every timber. we landed stern first, and the wave that followed us tore off our bow and foredeck and threw them clear over the vessel. the foredeck was found, after the storm, a hundred yards southeast of the maindeck. the bow was found eight blocks away, in the centre of the business district of the city. "'we stopped pumpin'. there weren't any pumps any more. of the seven of us, five were unconscious when a rescuin' party reached us, through the hurricane, four hours later. two of us were crippled for life, and it was many a long day before bill was free from the madness which had begun with the crack on the head when the wheel was swept away. "'daylight of tuesday found me in bed, with an army surgeon straightenin' out my broken bones. the hurricane still raged over galveston. we had been derelict for two days and a half, at the pumps for fifty-seven hours, without food or water for forty hours, yet not a man was lost. no other dismasted vessel has ever lived through the eye of a hurricane and been tossed over a sea-wall into the business streets of a city. yet seven of us, all americans, still live to tell the tale.'" the young observer paused and looked at the boys. they were all very still. "and the beach," the young observer continued, "that once white beach with its stretches of sand, what did that look like, beyond the engineers' parade ground, where the wrecked schooner lay? mis-shapen, distorted, blotched, drabbled and crimsoned, it spread away to the horizons, east and west, its scars showing under the rays of the sun which shone out from the mares' tails of the departing hurricane. part of it had disappeared under the waters, now rapidly subsiding. the great causeway was a mass of ruins, but the sea-wall, the two-million dollar sea-wall, stood with its front to the ocean, grimly defiant still, the conqueror against the rage of the tempest, and an unwrecked galveston shone triumphant. "but i should do the hurricane a grave injustice," he continued, "to leave you boys with the story of galveston alone. its terrors were far more widespread than that. on my way here from galveston, i saw the ravages of the storm inland. everywhere on the flat prairie near texas city were ruined houses and outbuildings, many of them absolutely abandoned, others still with a corner occupied by their ruined owners. trees were broken short off or up-rooted and lying prostrate. the hurricane which had been foiled of the slaughter which had been granted to its predecessor fifteen years before, had swept on, mile after mile, for hundreds of miles, slaying and wrecking as it went. acres of pear orchards were stripped as though the giant of the winds had drawn each separate branch through his clenched fists. for twenty miles inland the prairie grass lay prostrate. twelve miles from the shore i saw a fishing schooner there, her masts still standing, and near it lay a child's rocking-horse, a cradle, a boy's baseball-bat and a five hundred pound bale of cotton. "not fifty yards from the hastily relaid railway track, i saw a strange example of the fury of the waves and wind. on the floor of the first story of a negro shack, without a scrap of furniture around it, with no wreckage or piece of wood to be seen in any direction, a rude cabin indeed, was a large grand piano, its boards warped by the water and the sun, but otherwise uninjured. from what house in galveston had this floated, to find a resting-place on the floor of an un-roofed and un-walled negro's cabin? around it was not a sign of wreckage save the bodies of scores of drowned horses and cattle and, among them, many human forms. "no census will ever tell how many were killed in that stretch of prairie between galveston and texas city. years hence men will stumble over human bones on that grassy plain and give burial to some victim of the greatest storm that ever visited american shores. yet, withal, that the hurricane of claimed six hundred victims instead of tens of thousands was due alone to the warnings of the weather bureau, to the heroism of the men and women of galveston and to the craft, skill and honesty of the men who built the great sea-wall." chapter x struck by lightning there was but little further interest in kite-flying that afternoon, when the young observer ended his story of the galveston hurricane. the boys had been brought close to danger and they crowded around the stranger with questions concerning the hurricane. the lads were all the more thrilled by reason of the fact that the sky was becoming dark and ominous, and that, even while the stranger spoke, the clouds grew more threatening. "there might be a hurricane coming now," said the youngest of the group, looking fearfully at the sky. "no," answered the observer, "that's nothing but a thunderstorm. you'll never forget the look of the hurricane as it comes near, if you've seen it once." "nor a tornado," put in ross, and he told of dan'l's death and of his narrow escape with anton. "i was in the st. louis tornado," the observer rejoined, and in turn he told of the devastation that had struck the city in .[ ] [footnote : while this book was in press a most destructive series of tornadoes visited the united states, illinois especially suffering. hundreds of deaths were recorded.] meantime the thunderstorm was drawing closer and the thunder and the lightning grew gradually nearer. "do you suppose, sir," asked tom, "that it would be safe to send up the kite? i've been listening to the hurricane story, and haven't taken the weekly observation yet. franklin sent up a kite in a storm." "it might be safe, but i wouldn't advise it," answered the forecaster. "franklin did it deliberately, for a different purpose, and it was because of his experiment with a kite that we first found out about lightning." "yes," answered tom, who knew the story well, "and he collected sparks from the string. but that was a silk string, mr. levin. i should think this piano wire would be much worse." "why?" asked the forecaster. "on the contrary, it would act as a lightning-rod. your kite reel is of metal and fastened to the ground. wire is a much better conductor of electricity than the body, so that there's less likelihood of your being struck." "is it the difference between a good conductor and a bad one that makes people put up lightning-rods?" asked fred. "certainly. all that a lightning-rod does is to convey to the ground the electricity that is about to strike a building. that's the whole system of lightning protection. i can explain it to you fairly well by trees. you know in fairy tales that some trees are supposed to be wicked and other trees are supposed to be good?" "yes, sir," put in anton, "dan'l used to talk about that. he always used to say that the oak tree was a black witch tree and that the beech tree and the alder tree were white witches." "like nearly all folk-lore," replied the forecaster, "there's a mighty good reason for that superstition. folk-lore, after all, is merely keen observation reduced to a saying or a story. it is true that the oak-tree is a black witch so far as lightning is concerned and that the beech and alder are white witches. the proportion of trees struck by lightning has often been counted and for every fifty-four oaks struck, only one beech, or birch, or maple or alder is struck. elms are fairly dangerous, being forty to the beech's one, and pines are less so, their ratio being fifteen. not only this, boys, but a good deal depends on the way in which a tree is struck. an oak-tree may be riven into splinters, showing the terrible resistance that it gives to the stroke. a beech-tree, usually, is killed outright, yet shows but little outward injury. the oak has resisted the current, it is a bad conductor; the beech has allowed the current to flow directly to the ground. "so, boys, if you are in a mixed forest and stand beneath a tree, the figures show that you are fifty-four times as likely to be struck with lightning when standing beneath an oak, instead of a beech. not only that, but if the oak be struck, the lightning may jump from the tree to you more surely than it would from a beech-tree. "it's surprising," he went on, "but even trees of closely related character show very different effects of lightning. 'nothing but lightning,' writes john muir, 'hurts the sequoia or big tree. it lives on through indefinite thousands of years, until burned, blown down or undermined, or shattered by some tremendous lightning stroke. no ordinary bolt ever hurts the sequoia. i have seen silver firs split into long peeled rails radiating like spokes of a wheel from a hole in the ground where the tree stood. but the sequoia, instead of being split and shivered, usually has forty to fifty feet of its brash knotty top smashed off in short chunks, about the size of cord-wood, the rosy-red ruins covering the ground in a circle one hundred feet wide or more. "'i never saw any that had been cut down to the ground, or even to below the branches, except one about twelve feet in diameter, the greater part of which was smashed to fragments. all the very old sequoias have lost their heads by lightning. all things come to him who waits, but of all living things, sequoia is perhaps the only one able to wait long enough to make sure of being struck by lightning. thousands of years it stands ready and waiting, offering its head to every passing cloud, as if inviting its fate, praying for heaven's fire as a blessing, and when, at last, the old head is off, another of the same shape immediately grows on.'" "and then, i suppose," said fred, "it will never be struck again. lightning never strikes twice in the same place." "oh, yes, it does," said the forecaster. "that's all nonsense. take the eiffel tower in paris, for example. that's struck nearly every time there's a thunderstorm. but lightning can't hurt the eiffel tower because practically the entire building is a lightning-rod and it has been very carefully grounded into deep wells, a long way below the ground." "i've been wondering," said anton thoughtfully, with his characteristic opening, "just how a thunder-and-lightning storm happens. you promised to explain it to me, mr. levin," he continued, "and you never have." "very good," said the forecaster, briskly, "i'll explain it now. and you couldn't have picked a better day for your question, anton, because we can see the tail end of that thunderstorm going off to the east, and, if i'm not mistaken, there's another one coming up to the south-west. do you see that layer of cirro-stratus clouds?" "yes, sir." "and do you notice those festoons of cloud, slowly coming down and dissolving--you see there's one small one there, and another one a little larger, behind?" "sure!" "well, those are the heralds of a thunderstorm. we've only seen those since my nephew began talking about the hurricane, about an hour ago. away off on the horizon, though, you can see a bigger bunch of those festoons, dropping from the five-mile height of the cirro-stratus and condensing away down lower. this heat that we're now feeling will diminish, just as soon as that cloud covers the sun, not because the sun is hidden, but because of a change of wind." "but the storm's coming up at right angles to the wind," said tom, "the wind's a little east of south." "it'll blow from the north-east presently," declared the forecaster oracularly. "directly opposite to the storm?" ejaculated the kite expert in surprise. "certainly," was the answer, "that's a part of the thunderstorm formation. you can see now," he continued, "how the thunder heads of cumulo-nimbus are beginning to show, leaden in color below, with the white billowy tops. they're very thick, those masses of cloud, perhaps two miles thick, and the gray rain curtain trails along behind them. well, tom, what is it?" he added, turning to the boy, who was claiming his attention. "the wind's shifting," answered the lad. "to the eastward? of course. it'll be north-east in a minute or two, as i told you. it's got to be." "but why, sir?" asked tom. "i don't see why a surface wind should have to blow up against a storm." "that," said the forecaster, "is quite easy. if the rain is falling, it brings down a mass of cold air with it, displacing the warm air that lies before the advancing storm. the warm air is driven forward, but, at the same time, the descending cold air requires warm air to replace it in its turn, and the warm air, therefore, curves backward and flows into the upper portion of the storm cloud, where its moisture is condensed as rain. so, my boy, a little distance in advance of a thunderstorm there are three currents of air, an upper current of cold air, traveling in the same direction as the storm, and driving the cirrus clouds before it; a current of warm air, going in the opposite direction to the storm and pouring a torrent of warm air into the cloud; and the cold squall, which drives out from under the thunder-cloud and which comes in violent gusts." "but i thought," said fred, "that thunder and lightning came from two clouds banging together. if most of the thunder storms travel from the west, where does this banging come in?" "it doesn't come in at all," the forecaster replied; "thunder and lightning do not result from clouds striking each other. it's not quite so simple as that. "the lower air is full of positive electricity just as the surface of the earth is charged with negative electricity. as you know, boys, rain is formed by a lot of little drops of moisture combining to form one large drop, which, when it is heavy enough, falls to the ground. now the surface of every drop of moisture is charged with electricity. when these drops come together to make one big drop, the surface of the big drop is proportionately much smaller than the combined surfaces of all the small drops. there isn't room enough on the surface of the big drop to hold all the electricity that existed on the surface of the larger number of smaller drops and, therefore, a great deal of electricity is set free. "only a few flashes of lightning reach the earth. most lightning-flashes occur between two cloud masses in the body of the thunder-cloud. photographs of these show them to consist of scores of fine branches which jump from one cloud to the other, the flash being strong or weak according to the distance to be jumped. you can see that a very faint flash could jump a distance of an inch, but that it would take a stronger current to jump a yard, and that a terrific force of electricity must have accumulated before the current is strong enough to break down the resistance of the non-conducting air and jump a quarter of a mile. when lightning is attracted by the earth, it means that the air between the thunder-cloud and the earth is being subjected to a constant strain, and the weakest place gives way first. the weakest place, generally, is the place when the jump is shortest and there is a good conductor available. "one of the reasons that buildings and trees are struck by lightning is because they project up into the air, and according to their height, they remove a corresponding amount of the poorly conducting air. if the lower edge of a thunder-cloud is two thousand five hundred feet above the air, and the spire of a church is five hundred feet high, it follows that it is easier for a flash to jump two thousand feet than two thousand five hundred. so when the electricity-bearing cloud comes over the church spire the flash will leap to the church, five hundred feet of obstacle being removed. the highest building, therefore, is usually struck first, or the highest tree in a forest. "a lightning-rod or conductor is the best preventive against the destruction of a building by lightning, if the rod sticks up in the air above the building, even a couple of feet. the current will more readily strike the lightning-rod. as these are made of metal--copper or iron, generally--which are extremely good conductors, the current flows through them to the ground without harming the building. "the big lightning flashes that you see, boys, aren't always a single flash, but often a whole series of flashes, which occasionally run up as well as down. the resistance of the air being broken down, makes a path for the electrical discharge, so that the conductor does not have to stand the entire strain of the cloud at once, but only in a series of discharges. photographs of lightning flashes show these very clearly." "i've never done any lightning photography," said ralph disgustedly, "i'd never thought of it." "you try it," said the forecaster, "and you'll find that there are no two flashes of lightning that look alike. some of them are several miles long. one thing you will notice at once, ralph, and that is that lightning is never zigzag, the way you see it in pictures, but runs in an irregular line, winding a little like a river-course." "how about sheet-lightning?" asked ralph. "that's just the same as any other kind of lightning," was the reply, "except that it doesn't come to the earth or is so distant that the earth flash is not visible. it is generally due to discharges between upper and lower clouds, and the lower clouds are illuminated by the lightning. heat-lightning, as it is called, is pretty much the same thing." "father told me once," said fred, "that during a thunder-storm, a ball of fire came down on the chimney and rolled all around the room like a bubble of quicksilver and then struck a shovel that was standing in the corner, when it blew up with a bang. what was that, mr. levin?" "that's globe, or ball lightning," was the reply. "there have been some very curious freaks done with these electric balls. one of them, in a baker's shop at paris, jumped into an open oven door and exploded, giving off so much heat that a pan of biscuits was baked in the fraction of a second. at least, so flammarion tells the story, though it sounds a bit queer." "but what's the cause of ball-lightning?" "we don't know," answered the forecaster, simply. "a couple of days before the galveston hurricane," put in the young observer, "i noticed two or three examples of st. elmo's fires, and even had them from my fingers." "what are st. elmo's fires?" queried fred. "corpse candles, they used to be called," the young observer answered, "or st. john's fires. they are brush-like discharges of electricity, being discharged from the earth towards the sky, and generally gather on elevated points, such as the masts of ships, the tips of trees or the iron railings around a roof. it was on the top of the weather bureau building in galveston that i saw them, just the other day. they look like a bluish flame, and give a crackling sound. i had my hand on the rail and was reaching up with the other hand towards the anemometer when i noticed from my third and little fingers two blue flames burning. it looked exactly as if my hand were alight." "weren't you afraid of being killed?" the boy asked. "no," said the observer, "that's not the way that one gets killed with lightning. the st. elmo's fire is a very weak electric discharge. my fingers tingled a little, that was all." "but do many people get killed with lightning?" queried ross. "i thought that it was really quite rare." "not as rare as you would think," the forecaster answered. "about five hundred people are killed by lightning every year in the united states and there is an annual property loss of eight million dollars." "is that high as compared with other countries?" anton asked. "yes," the forecaster replied, "more people are killed by lightning in the western states than in any place in the world. in the dakotas, out of every million deaths twenty-seven are due to lightning; in missouri, twenty-one. in hungary sixteen out of every million deaths are due to lightning; in the united states as a whole, ten; in germany, six; in england, four; in france and sweden, three, and in belgium, two. the greatest number of deaths by lightning are on the plains, the fewest in the cities." "i should think lightning would be much worse in the city," said ross, "because if a building is struck with a lot of people in it, they'd all be killed." the forecaster shook his head. "not at all," he said. "last year, for example, a church was struck by lightning on a sunday morning, during a religious service. there were three hundred people in the building. it was a bolt of unusual force, which practically wrecked the church. only six people were killed by lightning, thirty were injured from the falling timbers, seventy were made unconscious by shock, and two hundred were absolutely uninjured. "the largest number of persons killed by lightning at any one time in america was in an amusement park in chicago. eleven people had huddled into a zinc-lined hut under a pier, for protection from the rain. the lightning struck the pier and jumped to the hut. if the hut had touched the wet sand, none of them would have been hurt, but the hut was on posts a couple of inches above the beach. the lightning could not escape to the ground and it spread from the zinc sides, killing every one there. a piece of wire a sixteenth of an inch thick and six inches long, running from the hut into the ground, might have saved every life." in the distance a flash of lightning followed by a low rumble of thunder told of the nearer approach of the storm. the galveston observer took his watch from his pocket and counted the seconds between the flash and the thunder. "fifty seconds!" he continued. "the front of the storm is still ten miles away." "do you reckon five seconds to a mile between the lightning and the thunder?" asked anton. "yes," the observer replied, "light travels so fast that for something as near as a lightning flash, you can reckon it as instantaneous, while sound only travels at a little more than a thousand feet a second." "but why does thunder make a noise?" asked fred. "you told me the clouds didn't bang together." "they don't," the forecaster answered. "thunder is caused by the electric discharge. you've heard bob's big wireless outfit crackle, when he sends out a spark, haven't you?" "sure," said fred, "you can hardly hear yourself talk, when bob's got his wireless busy." "and why does that crackle? do you know, bob?" he asked, turning to the wireless expert. "no," answered the boy. "you've often heard the crackling of a near-by thunder compared to an irregular volley of rifles, haven't you?" "yes." "naturally, because that's exactly what it is. a rifle shot is an explosion caused by the firing of a powder, which, in turn, means the expansion of the powder into gases, the force of that expansion driving forward the bullet. sound, as you know, is a series of air vibrations. the explosion wave sets up a series of these vibrations, by compressing the air in front of it. "lightning does the same thing. when a lightning flash breaks down the resistance of the air, and passes through a channel of air, it heats the air suddenly to a temperature of two or three thousand degrees, causing a terrific expansion along the entire length of the flash and starting an explosion wave. this compresses the air on all sides and sets sound vibrations in action. as soon as the flash is discharged, the air rushes back to fill the partial vacuum that the heating by electricity has caused, adding force to the vibrations. "that's why you hear the crackle of near-by thunder. you are near enough to hear the explosions made by all the little side-branches of the lightning flash--you can hear the same sometimes when you comb your hair or rub a cat's fur--while the big crashes are due to your hearing, all at once, the main wave of sound set in action by the flash jumping from the cloud to the earth or from one cloud to another. "the rumble of the thunder--which used to be thought the rolling of a chariot in the sky, is due to the different distances of various portions of the discharge, to the echo of the explosions from the projecting hills and valleys of the cloud forms, and to the irregular shape of the earth, when the sound waves strike the ground." "hail is electric, too, isn't it?" said anton. "in a hail-storm the other day i noticed that the hail jumped up a lot higher from an old piece of iron that lay on the ground than from a stone right beside it. i tried the iron and the stone with a marble, after the storm was over, and the marble bounced higher from the stone. i figured that there must be some kind of electric repulsion and that the hail must be electrified." "it is, very often," the forecaster answered. "in some very violent electric storms, you'll see hail jump up as if it were alive, when it strikes the earth. of course, boys, there's some slight elasticity in a hail-stone, too, because a good many of them are made like an onion or a pearl, with a number of layers round each other." "but why in the world should a hail-stone be made like an onion?" said fred, with a puzzled stare. "isn't hail just frozen rain?" "no," answered the forecaster, "frozen rain is sleet, which is never seen in summer. it is caused by the rain in the upper air falling through a cold layer of surface air and becoming frozen on the way. sleet is ice, and transparent. "hail never falls in winter, only in summer, and almost always in connection with a thunderstorm. it is made by drops of moisture, like very fine rain, being carried by the strong upward currents of a thunderstorm to altitudes where the air is very cold, there becoming coated with a layer of snow, and becoming heavier, falling through the less active upward currents on the edge of a storm. as these snow-covered frozen raindrops fall through the clouds, they grow bigger, because on their cold snow surfaces the moisture condenses and is frozen to a skin of ice. at the base of the cloud, they are often sucked in by the upward current and carried up again for another layer of snow, falling again through the clouds and being covered with another skin of ice. this may happen a dozen or a hundred times, the hailstones growing in size with every successive layer of snow and ice, until at last they become so heavy that they can no longer be carried up by the ascending currents, and fall to the ground." "no wonder hailstones sometimes get so big!" exclaimed fred. "i've seen them as big as pigeon's eggs. i never could understand it." "i've seen hailstones that weighed more than half a pound," the forecaster answered. "not so very long ago, two ranchers and six hundred head of cattle were killed by hail in one texas storm. not a single animal was left alive. the loss from hail in our western states is so large that most of the progressive farmers pay heavy hail insurance. jagged bits of hail the size of a child's fist are not at all uncommon. if i'm not mistaken," he continued, "we may have some hail this afternoon, but nothing like that. this county isn't in the regular hail-belt." during the description of the storm, tom had been reeling in his kite and after the week's observations had been duly made and recorded, the boys prepared to scatter. before they left, the forecaster turned to them, his hand on anton's shoulder. "i think you boys ought to know," he said, "that i received a letter the other day from the chief of the weather bureau. he's going down to new orleans next month, and has promised to drop off here and spend the night with me. we were chums at college. he ought to meet the mississippi league of the weather." an excited cheer went up from the boys. "and what's more," the forecaster went on, "i can tell you this--that just as soon as anton is old enough, there will be a place waiting for him in the bureau. he knows almost enough now to pass the civil service exam, and in a couple of years he'll be as well equipped to enter the service as any of the boys that are going in. i miss my guess if we don't find out, some day, that issaquena county has given to the united states one of the best meteorologists of the next generation." "three cheers for anton!" shouted fred. they were given heartily and the boys separated in groups, excitedly discussing what they ought to do to prepare for the visit of the chief of the weather bureau. anton and ross drove home to anton's place together, ross driving and the crippled lad, with his eyes glowing with enthusiasm, talking about the work he intended to do in the ranks of the weather bureau. meanwhile, the storm grew nearer and nearer. the thunder, which had been rolling menacingly, now came with shorter and sharper claps. "i wonder if we'll get home before the rain," said ross and leaned forward to slap the pony with the reins. at the instant that he leaned forward there was a blinding flash of light, and, almost simultaneously, a terrific crash. for a second anton was stunned, and then, as the frightened pony started to bolt, he saw he was alone. ross was gone. the crippled lad cast a frightened glance over his shoulder and saw his chum lying on the ground beside the roadway, stripped to the skin. some pieces of his clothing, burning and smouldering, lay a few feet away. grabbing the reins, anton managed to pull the pony down to a walk and scrambled out, awkwardly, with the crutch, but rapidly. the lightning, as so often happens, had snatched every stitch of ross's clothes from him. there was not a mark of a burn on the boy's body, but he lay deathly still, with his arm cramped under him. anton, exerting all his strength, rolled his chum over on his back. then, kneeling across him, as best he could with his lame leg, he took ross's wrists, jerked his arms to their full length, brought the wrists back upon the chest and pressed. again he stretched the arms out, again brought them back, and pressed. again, and again and again. time passed and the perspiration stood out on the crippled lad's forehead and trickled down into his eyes and the corners of his mouth. yet he did not pause for a second. he stretched the arms out, brought them in and pressed down upon the chest. again and again and again. fifteen minutes passed, and there was no sign. probably further work was of no use, but anton persisted. he could not stop, as long as there was a chance. out, in again, and pressure on the chest. a clatter of approaching wheels caused anton to look up. it was the buggy, with his father whipping the pony to full speed, returning along the road to find out what accident had happened. anton shouted, but did not stop. out, in again, and pressure on the chest. the buggy stopped and his father jumped out. "who is it?" he asked. "ross," answered anton, "struck by lightning!" "dead?" queried his father. "he can't be!" declared anton passionately, and went on with his artificial respiration. "let me do that a while," said his father. "wait!" cried anton. he thought he saw an eyelid flutter. out, in again, and pressure on the chest. "he's coming to!" the man declared. yes, that was a movement. the lips parted. there was a faint heave of the chest, and anton's father, stooping down, felt a slight trembling of the boy's heart. it fluttered, hesitated, stopped; then trembled again, and struck into a low soft throb, irregular indeed, but still a definite throb. out, in again, and pressure on the chest. for five minutes more anton continued his artificial respiration, silently, and then ross opened his eyes. "what's wrong?" he asked, faintly. "you've had a lightning shock," answered anton. "i thought you were dead," put in the lad's father, "but it looks as though anton had pulled you through." ross smiled at his chum. "bully for you, old boy," he said weakly, "the sea-wall licked the hurricane and you've licked the lightning-flash!" the end u. s. service series by francis rolt-wheeler illustrations from photographs taken in work for u. s. government large mo cloth $ . each, net "there are no better books for boys than francis rolt-wheeler's 'u. s. service series.'"--_chicago record-herald_. the boy with the u. s. survey [illustration: cover of _the boy with the u. s. survey_] this story describes the thrilling adventures of members of the u. s. geological survey, graphically woven into a stirring narrative that both pleases and instructs. the author enjoys an intimate acquaintance with the chiefs of the various bureaus in washington, and is able to obtain at first hand the material for his books. "there is abundant charm and vigor in the narrative which is sure to please the boy readers and will do much toward stimulating their patriotism by making them alive to the needs of conservation of the vast resources of their country."--_chicago news_. the boy with the u. s. foresters the life of a typical boy is followed in all its adventurous detail--the mighty representative of our country's government, though young in years--a youthful monarch in a vast domain of forest. replete with information, alive with adventure, and inciting patriotism at every step, this handsome book is one to be instantly appreciated. "it is a fascinating romance of real life in our country, and will prove a great pleasure and inspiration to the boys who read it."--_the continent, chicago_. the boy with the u. s. census through the experiences of a bright american boy, the author shows how the necessary information is gathered. the securing of this often involves hardship and peril, requiring journeys by dog-team in the frozen north and by launch in the alligator-filled everglades of florida, while the enumerator whose work lies among the dangerous criminal classes of the greater cities must take his life in his own hands. "every young man should read this story from cover to cover, thereby getting a clear conception of conditions as they exist to-day, for such knowledge will have a clean, invigorating and healthy influence on the young growing and thinking mind."--_boston globe_. the boy with the u. s. fisheries [illustration: cover of _the boy with the u. s. fisheries_] with a bright, active american youth as a hero, is told the story of the fisheries, which in their actual importance dwarf every other human industry. the book does not lack thrilling scenes. the far aleutian islands have witnessed more desperate sea-fighting than has occurred elsewhere since the days of the spanish buccaneers, and pirate craft, which the u. s. fisheries must watch, rifle in hand, are prowling in the behring sea to-day. the fish-farms of the united states are as interesting as they are immense in their scope. "one of the best books for boys of all ages, so attractively written and illustrated as to fascinate the reader into staying up until all hours to finish it."--_philadelphia despatch_. the boy with the u. s. indians [illustration: cover of _the boy with the u. s. indians_] this book tells all about the indian as he really was and is; the menominee in his birch-bark canoe; the iroquois in his wigwam in the forest; the sioux of the plains upon his war-pony; the apache, cruel and unyielding as his arid desert; the pueblo indians, with remains of ancient spanish civilization lurking in the fastnesses of their massed communal dwellings; the tlingit of the pacific coast, with his totem-poles. with a typical bright american youth as a central figure, a good idea of a great field of national activity is given, and made thrilling in its human side by the heroism demanded by the little-known adventures of those who do the work of "uncle sam." "an exceedingly interesting indian story, because it is true, and not merely a dramatic and picturesque incident of indian life."--_n. y. times_. "it tells the indian's story in a way that will fascinate the youngster."--_rochester herald_. the boy with the u. s. explorers the hero saves the farm in kansas, which his father is not able to keep up, through a visit to washington which results in making the place a kind of temporary experiment station. wonderful facts of plant and animal life are brought out, and the boy wins a trip around the world with his friend, the agent. this involves many adventures, while exploring the chinese country for the bureau of agriculture. "boys will be delighted with this story, which is one that inspires the readers with the ideals of industry, thrift and uprightness of conduct."--_argus-leader, portland, me._ the boy with the u. s. life savers [illustration: cover of _the boy with the u. s. life savers_] the billows surge and thunder through this book, heroism and the gallant facing of peril are wrought into its very fabric, and the coast guard has endorsed its accuracy. the stories of the rescue of the engineer trapped on a burning ship, and the pluck of the men who built the smith's point lighthouse are told so vividly that it is hard to keep from cheering aloud. "this is an ideal book for boys because it is natural, inspiring, and of unfailing interest from cover to cover."--_marine journal._ the boy with the u. s. mail how much do you know of the working of the vast and wonderful post office department? the officials of this department have, as in the case of all other departments covered in this series, extended their courtesy to dr. rolt-wheeler to enable him to tell us about one of the most interesting forms of uncle sam's care for us. "stamp collecting, carrier pigeons, aeroplanes, detectives, hold-ups, tales of the overland trail and the pony express, indians, buffalo bill--what boy would not be delighted with a book in which all these fascinating things are to be found?"--_universalist leader._ _for sale by all booksellers or sent postpaid on receipt of price by the publishers_ lothrop, lee & shepard co., boston [illustration: futen, god of the winds. japanese conception of the origin of storms, which come from the bag on the demon's back.] [illustration: there, before the flood, stood anton's house. overflowed lands in the mississippi valley, where scores of lives are lost when the rising waters break down a levee. _courtesy of u. s. weather bureau._] [illustration: the edge of a tornado's whirl. note the house in the background unharmed, and the house next to it spun around like a top. _courtesy of u. s. weather bureau._] [illustration: in the path of the lightning. _courtesy of u.s. weather bureau._] [illustration: in the path of the tornado. a farm-house, with farm buildings in a copse of trees stood here; the buggy, after a flight through the air, was dropped, little injured. _courtesy of t. b. jennings, u. s. weather bureau, topeka, kans._] [illustration: facing a climb on show-shoes.] [illustration: twenty-five foot drift a mile long.] [illustration: forest ranger in idaho. [illustration: observer among the quaking aspens. snow survey work. _courtesy of u. s. weather bureau and of j. cecil alter._] [illustration: no peak too lofty for a weather station. taking recording instruments up a mountainside where there has never been a trail. _copyright by j. cecil alter, u. s. weather bureau, cheyenne, wyo._] [illustration: wall sun-dial at santa barbara, cal., on old spanish mission.] [illustration: sun-dial at hillside, n.y., duplicate of that of sir walter scott at abbotsford.] [illustration: the first line of defence against the tempest. headquarters of the u. s. weather bureau, at washington, d. c., where every wind and cloud that passes over the united states is chronicled and watched; the greatest forecast office in the world. _courtesy of u. s. weather bureau._] [illustration: solar halo seen in the united states. this halo was seen over a wide area, and was especially bright in virginia. _courtesy of scientific american._] [illustration: solar halo seen in russia. (by permission from camille flammarion's "meteorologie.")] [illustration: the dust that makes red sunsets. the volcanic eruption of lassen peak, cal., on october , , taken at intervals, the first three photos five minutes apart, the fourth, ten minutes later, showing the beginning of the second cloud. _copyright by chester mullen. courtesy of u. s. weather bureau._] [illustration: an army destroyed by weather. napoleon deserting his troops during the retreat from moscow, when the emperor defied the winter, and left a quarter of a million men dead on the snows of russia.] [illustration: _cirrus implexus_ _alto-strato-cumulus_ types of upper clouds.] [illustration: _cumulus_ _stratus_ types of lower clouds.] [illustration: _cumulo-nimbus_ _nimbus_ types of rain clouds.] [illustration: kite-flying--the new way. _courtesy of u. s. weather bureau._] [illustration: kite-flying--the old way. benjamin franklin performing his famous experiment, whereby he proved that a flash of lightning was an electric discharge.] [illustration: the explorer of the upper air. weather box kite being released at the drexel aerological station, with equipment to tell altitude, pressure of atmosphere, velocity of wind, and temperature, in a continuous record. _courtesy of u.s. weather bureau._] [illustration: snow-flakes from the upper regions of the air.] [illustration: snow-flakes from the middle regions of the air.] [illustration: snow-flakes from the lower regions of the air. note the gradual progression from solid to feathery forms, and especially that every elaboration maintains the six-pointed crystal type. _courtesy of j. wilson bentley._] [illustration: ringing the frost alarm! thermometer with electric attachment which wakes the neighborhood when the grip of a cold wave menaces ruin to a fruit crop. _courtesy of u.s. weather bureau._] [illustration: fighting frost in an orchard--night.] [illustration: fighting frost in an orchard--dawn. the pall of smoke prevents evaporation and keeps the air near the ground from freezing temperatures. _copyright by j. cecil alter, u.s. weather bureau, cheyenne, wyo._] [illustration: bucking a snow drift.] [illustration: clear the way! even an avalanche cannot stop man, backed with the resources of modern snow-fighting machinery. _courtesy of northern pacific railway co._] [illustration: measuring the blizzard's rage. shielded snow gauge in the northwest to register the amount of snow-fall. _courtesy of u. s. weather bureau._] [illustration: signals on delaware breakwater. _courtesy of geo. s. bliss, u.s. weather bureau, philadelphia, pa._] [illustration: signal tower for storm warnings. flags used by day, lanterns by night. _courtesy of u.s. weather bureau._] [illustration: thermometers and rain-gauge. instruments in shelter, as supplied to each co-operative observer. _courtesy of the u.s. weather bureau._] [illustration: pencil drawings of tornado in dakota. for many years this was an authoritative series of pictures, and shows:--(a) tornado becoming a waterspout;--(b) tornado wrecking a farmhouse and barn, nothing but fragmentary timbers being thrown out;--(c) tornado funnel rising from the ground;--(d) successive funnel formations, with a second whirl reaching ground and sucking up a pillar of dust. _copyright by sam w. glenn, courtesy of u. s. signal service._] [illustration: true tornado forming in advance of a dust whirl. _courtesy of u.s. weather bureau._] [illustration: tornado dropping towards ground. _courtesy of t. b. jennings, u.s. weather bureau, topeka, kans._] [illustration: tornado wrecking a farm. whirl had been in action for ten minutes when photo was taken. _courtesy of t. b. jennings, u.s. weather bureau, topeka, kans._] [illustration: tornado whirling sidewise. the swaying motion of the funnel cloud makes the path of escape uncertain. _courtesy of u.s. weather bureau._] [illustration: galveston causeway before the hurricane.] [illustration: galveston causeway after the hurricane. the sea-wall saved the greater part of galveston in the hurricane of , but the causeway was exposed to the full fury of wind and water. _courtesy of i. r. tannehill, u. s. weather bureau, galveston, tex._] [illustration: shot from the gun of a hurricane. thin strips of weather-boarding driven through a porch post, a marvellous example of the force of a hundred-mile-an-hour wind. straws have been driven into brick walls in similar fashion. (upper photo taken from across the street; lower photo at close range.) _courtesy of i. r. tannehill, u. s. weather bureau, galveston, tex._] [illustration: . zephyr] [illustration: . light breeze] [illustration: . fresh breeze] [illustration: . moderate wind] [illustration: . strong wind] [illustration: . gale] [illustration: . full gale] [illustration: . storm] [illustration: . hurricane scale of winds illustrated by reduction of sail on american clipper ships. _note._-- this is a combined scale with average wind velocities as follows:--( ) calm;--( ) miles an hour;--( ) miles an hour;--( ) miles an hour;--( ) miles an hour;--( ) miles an hour;--( ) miles an hour;--( ) miles an hour;--( ) miles an hour;--( ) miles an hour or more.] [illustration: branch lightning. _copyright by h. e. clark, indianapolis. courtesy of u.s. weather bureau._] [illustration: multiple flash. _courtesy of general electric co., schenectady, n. y._] [illustration: eiffel tower struck by lightning. unusual example of attraction of electric discharge. this great structure in paris is struck in almost every thunderstorm.] [illustration: lightning flash striking building. single disruptive discharge of great intensity, at greensboro, n.c. this non-branched form is rare. _courtesy of gen. electric co., schenectady, n. y._] [illustration: carried in the air three miles from their stable.] [illustration: grand piano picked up by a tornado and dropped in a cow-pasture. _courtesy of t. b. jennings, u.s. weather bureau, topeka, kans._] [illustration: effect of heat. frontispiece.] curiosities of heat. by rev. lyman b. tefft. philadelphia: the bible and publication society, arch street. entered according to act of congress, in the year , by the bible and publication society, in the office of the librarian of congress, at washington. westcott & thomson, stereotypers, philada. contents. page chapter i. mr. wilton's bible class chapter ii. new thoughts for the scholars chapter iii. a difficult question chapter iv. heat a gift of god chapter v. conveyance and varieties of heat chapter vi. management and sources of heat chapter vii. preservation and distribution of heat chapter viii. modification of temperature chapter ix. the ministry of suffering chapter x. transportation of heat chapter xi. an effective sermon chapter xii. transfer of heat in space chapter xiii. ocean currents and icebergs chapter xiv. combustion.--coal-beds chapter xv. economy of heat chapter xvi. a day of joy and gladness curiosities of heat. chapter i. mr. wilton's bible class. "the book of nature is my bible. i agree with old cicero: i count nature the best guide, and follow her as if she were a god, and wish for no other." these were the words of mr. hume, an infidel, spoken in the village store. it was monday evening. by some strange freak, or led by a divine impulse, he had determined, the previous sunday afternoon, to go to church and hear what the minister had to say. so the christian people were all surprised to see mr. hume walk into their assembly--a thing which had not been seen before in a twelvemonth. mr. hume did not shun the church from a dislike of the minister. he believed mr. wilton to be a good man, and he knew him to be kind and earnest, well instructed in every kind of knowledge and mighty in the scriptures. he kept aloof because he hated the bible. he had been instructed in the scriptures when a boy, and many bible truths still clung to his memory which he would have been glad to banish. he could not forget those stirring words which have come down to us from the lord jesus, and from prophets and apostles, and they sorely troubled his conscience. he counted the bible an enemy, and determined that he would not believe it. at that time there was an increasing religious interest in the church. mr. wilton had seen many an eye grow tearful as he unfolded the love of christ and urged upon his hearers the claims of the exalted redeemer. he found an increasing readiness to listen when he talked with the young people of his congregation. the prayer-meetings were filling up, and becoming more interesting and solemn. the impenitent dropped in to these meetings more frequently than was their wont. mr. wilton himself felt the power of christ coming upon him and girding him as if for some great spiritual conflict. his heart was filled with an unspeakable yearning to see sinners converted and christ glorified. he seemed to himself to work without fatigue. his sermons came to him as if by inspiration of the holy spirit. he felt a new sense of his call from god to preach the gospel to men, and spoke as an ambassador of christ, praying men tenderly, persuadingly, to be reconciled to god, yet as one that has a right to speak, and the authority to announce to man the conditions of salvation. a few of the spiritual-minded saw this little cloud rising, but the people in general knew nothing of it. least of all did mr. hume suspect such an undercurrent of religious interest; yet for some reason, he hardly knew what, he felt inclined to go to church. that afternoon the preacher spoke as if his soul were awed, yet lifted to heavenly heights, by the presence of god and christ. reading as his text the words, "thou thoughtest that i was altogether such an one as thyself" (ps. l. ), he showed, first, the false notions which men form of god, and then unfolded, with great power and pungency, the scripture revelation of the one infinite, personal, living, holy, just, and gracious jehovah. this was the very theme which mr. hume wished most of all not to hear. that very name, jehovah, of all the names applied to god, was most disagreeable; it suggested the idea of the living god who manifested himself in olden time and wrought wonders before the eyes of men. but the infidel, with his active mind, could not help listening, nor could he loosen his conscience from the grasp of the truth. yet he could fight against it, and this he did, determined that he would not believe in such a god--a god who held him accountable, and would bring him into judgment in the last great day. in this state of mind he dropped into deacon gregory's store. deacon gregory was accustomed to obey paul's injunction to timothy: "be instant in season, out of season; reprove, rebuke, exhort with all long suffering and doctrine." having taken mr. hume's orders for groceries, he said, "i was glad to see you at church yesterday, mr. hume. how were you interested in the sermon?" "i like mr. wilton," answered mr. hume; "i think him a very earnest and good man." "but were you not interested and pleased with the discourse? it seems to me that i shall never lose the impression of god's existence and character which that discourse made upon me. i almost felt that mr. wilton spoke from inspiration." "i suppose he was inspired just as much as the writers of that book which men call 'the bible.'" "but can you wholly get rid of the conviction that the bible is the word of god, written by holy men inspired by the holy spirit?" "you know, deacon gregory, that i do not believe what you profess to believe. the book of nature is my bible. i agree with old cicero: i count nature the best guide, and follow her as if she were a god, and wish no other." deacon gregory had never read cicero, and of course did not attempt to show, as he might otherwise have done, that cicero did not mean to deny the existence of a living, personal god, who governs the world. "but," said he, "does not the book of nature--your bible, as you call it--have something to say of god? does it not speak of an infinitely wise and good creator and governor? do not the works of nature tell of the same god whose being and character were preached to us yesterday from the holy scriptures?" "nature has never spoken to me of any god except herself. what need is there of a creator? who can prove that the universe did not exist from eternity? nature has her laws of development, and under those laws all the operations of nature go on. you had better read darwin. if one must find the character of god in nature, he may as well picture an evil creator and governor as one that is good and righteous. does nature punish those whom you call the wicked? does nature reward the righteous? do not the laws of nature bring suffering to the good and the bad alike, and happiness also to all classes of men? would you, if you had power, create a world like this--a world in which danger, pain, and death, in every shape, lie in ambush against its inhabitants every hour of their poor existence? but i must go." pausing a moment, however, as if reluctant to go, with a voice sad and almost tremulous, which revealed a great deal more of his heart than he designed to express, he added: "god knows, deacon, if there be a god, how i wish i knew the truth about these matters. the world and myself are to me great and dreadful mysteries." "'he that will do his will shall know of the doctrine,'" answered deacon gregory; and inviting him to come to church again, they separated. this conversation with the pious deacon, though he had himself done most of the talking and had his say almost unopposed, did not tend at all to bring rest to mr. hume's conscience. he saw that the deacon's faith in god did for him more than belief in nature and worship at the altar of science could do for unbelievers. he felt also that he had spoken a little too freely, especially in revealing, at the last, his unrest of spirit from the want of fixed convictions in regard to religious truth. deacon gregory, by the sincerity and manliness of his address, was accustomed to draw out the hearts of men so that they expressed them more freely than they designed. upon a bench in a shaded corner of the store sat a lad of sixteen or seventeen years, unnoticed for the time being by either mr. hume or deacon gregory. his name was ansel, and he was the son of the senior deacon of the church. he was in the village academy, and had there been nearly fitted for college. he stood at the head of his class, and, with his sharp intelligence, his impetuous energy, and high ambition, every one was predicting for him a distinguished life. he had grown up thus far in the bosom of a family where piety was no pretence. earnest prayer had gone up for him by day and by night. he had been well trained in the sunday-school, and for a year had been a member of the small class of young men taught by mr. wilton. he had always shown a ready interest in all bible studies and a quick understanding of scripture doctrine, so that some thought him not far from the kingdom of god. but deacon arnold little thought what was in the heart of his son. he might have known, for to read his son's heart he had only to recall his own early manhood. for years he had hung trembling upon the brink of ruin, swept, at times, by his self-will and turbulent youthful passions, to the very verge of the precipice, and had been preserved only by singular grace from falling over. now ansel was following in his father's early footsteps--self-willed, and stubborn against the spirit of god, and, at times, almost persuaded to cast off all religious restraint, that he might carve out his worldly fortunes untrammeled by religious or conscientious scruples. he had rarely heard infidel sentiments expressed, but the little that he had heard had attracted him, and had encouraged him to give loose reins to his own unbelieving disposition. it had not escaped his notice that the two or three men whom he had heard spoken of as infidels were among the most respectable and shrewdest business-men in the village. the idea, moreover, of rejecting all authoritative doctrine, and believing whatever should please him, carried with it so free and independent an air, and harmonized so well with his natural disposition, that he easily drifted in the direction of unbelief. sitting this evening unobserved, he drank in every word which mr. hume uttered. some of the notions thrown out were quite new to him. "the book of nature my bible"--"nature reveals no god but her own laws"--"no proof that the matter of the universe has not existed from eternity uncreated"--"nature has her laws of development"--"no need of a god to govern the world,"--these were seed-thoughts in ansel's mind. he had before thought of the only alternative to be set over against belief in the sacred scriptures as simply unbelief--bare, blank denial of their truth. he had not dreamed of building up a set of proud, rationalistic notions, and denying the truths of religion in the character of a young philosopher. he kept his thoughts to himself, and turned them over and over in his mind during the week, and when again he met his pastor in the bible class his head was full of his new notions. the lesson went on, however, and closed as usual. it so happened that this was the last in a series of lessons upon the gospel of john. it was necessary, therefore, that another course of lessons should be decided upon. mr. wilton proposed the question to the class: "what shall be our next course of lessons? would you like to study one of the epistles--the epistle to the romans or that to the hebrews?" and he briefly stated the subject discussed in these epistles of paul. "perhaps," he continued, "you would prefer to study one of the historic books of the old testament?" the class had no opinion. they wavered between an epistle and a historic book and topical lessons which should confine them to no one book of the bible. then ansel spoke up: "mr. wilton, why can we not study something which we know to be true?" ansel meant to be very cautious as well as very respectful, and did not design to commit himself by suggesting his own thoughts. he was respectful, but in the confusion of the moment he had brought out the very thoughts which he meant to conceal. mr. wilton was startled, though he did not fully understand the drift of ansel's question. "what do you mean, ansel?" he asked; "do you think genesis less trustworthy than the epistle of paul?" ansel saw that he had committed himself and must now make the best of his situation. he therefore answered cautiously: "some persons, i have heard said, do not believe the bible to be inspired, and they say that we have no evidence that it is true." "what have you been reading, ansel, that has put such thoughts into your mind?" "i have never read a book that said anything against the bible." "but what did you mean? do you wish to study the evidences of the truth and inspiration of the holy scriptures?" "i should indeed like a course of lessons upon that subject, but that was not quite what i was thinking of." "what book can you find which is true if the bible is not true?" "i do not know, sir, but i heard mr. hume say that the book of nature is his bible, and that we do not need any other, and that, whether the bible be true or not, we know that the teachings of nature must be true." "but we should find," said mr. wilton, "that the teachings of nature and the bible would perfectly agree. did mr. hume say that what he calls 'the book of nature' contradicts the sacred scriptures?" now that ansel could give the thoughts which filled his mind, not as his own, but as mr. hume's, he showed no farther hesitation in speaking. "yes, sir," he answered; "he said that nature teaches us that there is no god, because there is no need of any. he said that we cannot prove that god created the universe, but that matter has existed from eternity uncreated, and that all the changes in nature go on by certain laws of development, and that a certain mr. darwin had written a book and proved this." the reader will notice that in the report of mr. hume's language the scholar went somewhat farther than his master had done. mr. wilton was well acquainted with the present shape of scientific infidelity, and saw that ansel's statements were somewhat exaggerated, but he understood in a moment the drift of ansel's thoughts, though he could not tell as yet how deep and fixed an impression had been made upon his mind. but he did not care to probe ansel's conscience just then and there, in order to learn the exact state of the case. "if i understand you, then," he said, "you would like a course of lessons in the teachings of nature?" "of course, i did not suppose that you would allow us to have a course of lessons in the works of nature instead of the bible." "but if i were willing to give you a course of lessons showing the footprints of the creator, so to speak, in the physical world, how would it please you?" "i should like it very much." "how would such a plan please the other members of the class?" the idea was entirely new; no one of them had ever dreamed of studying in a bible class anything except the bible; but young people are not averse to novelties, and they readily gave their assent. yet i should do the class injustice by leaving the impression that they were influenced simply by the love of something new. they were of just that age when one hardly knows whether to call them lads or young men; they had been well instructed, and were just beginning to think independently. they were rapidly becoming conscious of their own mental power, and were eager to try their strength upon every line of thought. their own weakness they had hardly begun to learn. perhaps they were all the more ready to undertake such a course of study because they knew nothing of the difficulties attending it. the tinkling of the superintendent's bell warned them to close their conversation. "we have not time to-day," said mr. wilton, "to fix on the particular line of study which we shall follow. of course we cannot examine all the works of nature, and study every science, and trace the footprints of the creator in every place where he has walked; we must fix on some small part of the works of god, and direct our attention closely to that. we shall find this course more profitable than roaming carelessly over a much larger space. our next lesson will have to be a general one--a kind of preface to what shall come after. in the mean while, you can be collecting your thoughts upon the subject, and calling to mind anything that you have read bearing upon the handiwork of god manifest in nature." the school closed, and as the scholars pass out let me introduce to you the members of the pastor's class. this class was small for several reasons. the church to which mr. wilton preached was not the popular church. the fashionable people and all who loved popularity and drifted with the tide went to another church. careless, thoughtless young people naturally went with the crowd, and of those who attended his church some did not care to join his class. he was too much in earnest to please them. he made religion a reality, and his instruction compelled them to think, and of course those who did not like to think were not well pleased with him. but there were a few of the young men who were greatly interested in his instructions. they were earnest readers of instructive books; they liked conversation which called out thought; they were most of all pleased with questions and themes which gave them new ideas. indeed, in the community, there were two classes of persons who held mr. wilton in the highest esteem and regard: one of these was composed of men and women of earnest, intelligent piety, experienced christians; the other, of those who were not christians, but who respected sincerity and disinterested godliness, and liked sermons filled with meat and marrow. thus, at the present time, we find his class composed of but three young men. with ansel you are already acquainted. the second is peter thornton, the son of a master-carpenter. he was frank, outspoken, quick in the acquisition of almost every kind of knowledge, but very little given to silent reflection. he listened to his pastor's instruction as he would go to a well-filled library, to draw out its stores of information. morals and moralizing he did not like. he was not pious, and gave no indication of serious impressions. the third was samuel ledyard, the son of a poor widow. by painful industry and economy his pious mother was giving him the best advantages for education which the village afforded, praying the lord to give him a part in the blessed work of preaching the gospel and winning sinners to christ and salvation. when but twelve years of age he gave himself to christ, and had been trying faithfully to follow his lord. the long winter evenings were spent in reading books of history and science--books fitted to furnish and strengthen his mind--and long ere the light dimmed the morning star he was poring over his bible, alternately reading the word and praying that his mind might be opened to understand the truth in its beauty and greatness, and that the truth might be wrought in him by vital experiences. with such habits it was no wonder that he grew in grace--it was no wonder that he grew in all manly qualities. he was silent, meditative, and retiring, as gentle in his ways as a quiet girl, yet all who knew him recognized in him a singular weight and worth of character. those to whom the lord revealed his secrets began to say that samuel was appointed of god to preach the gospel, and his mother felt the assurance growing strong in her heart that her prayer was granted, and that the lord was preparing her only son and only child for a place in the gospel ministry. if only she might train up a son to such a work, and when she should go to her rest leave in her place a man working for christ in his harvest-field, gathering sheaves unto everlasting life, she felt that her cup would be full. she was ready to say with simeon: "now lettest thou thy servant depart in peace, for mine eyes have seen thy salvation." how unlike she was to those mothers who lay all hindrances in the way of their sons entering the work of the christian ministry, willing that they should do anything but this! and how different from those who declare that their daughters shall never wed ministers of the gospel, teaching them to despise the service of a pastor's wife! how often god gives over such sons and daughters--children consecrated from their birth to worldliness--to be entangled and lost in worldly snares! as such mothers sow, thus also do they reap. these were the three lads, just growing into young manhood, at this time under the instruction of mr. wilton. he was not ashamed of his class, though it was small. as he saw them expanding in thought and taking shape under his hand, he felt that in them he was perpetuating his influence in coming generations. he believed that in one or more of them he should preach the gospel after his body was sleeping in the earth awaiting the resurrection. i trust the kind reader will be interested in following the course of study through which their pastor shall lead them. chapter ii. new thoughts for the scholars. the little class which has been introduced to the reader came together the next lord's day interested and expectant, yet not knowing what to expect. they had chosen a course of study, yet they could not tell what that course was to be. they had tried to think of something definite about it, but could fix their minds upon nothing. in fact, the whole subject was new, and they could not decide where or how to take hold of it. they came together, therefore, with no more knowledge of the subject than when they separated. mr. wilton himself came before his class in a state of doubt. he had given the subject many hours of thought, and had carried it to his closet and besought the guidance of the holy spirit, for he believed the divine spirit to be the best guide in understanding the works as well as the word of god. he felt that his prayer had been heard and answered. he was prepared, therefore, to speak with the force of clear understanding and positive convictions. but the precise line of study he had left to be determined by circumstances, perhaps by the previous studies of his class in their academic course. this was to be decided by further consultation. "since no lesson was assigned upon which you could prepare yourselves," mr. wilton said, after the opening exercises of the school were finished, "i shall spend the half hour to-day in a kind of conversational lecture. you may call this the preface or introduction to the lessons which will follow. i shall try to make plain some general principles which we must keep in mind, whatever department of god's works we shall attempt to examine. i wish you to feel entirely free to interrupt me at any time, and ask any question or present any objection which may strike your minds. we must, if possible, have no prowling bands of enemies in the rear. i wish to make everything as plain as the case will admit. "one thing let me remind you of in the beginning: i shall not try to prove to you that there is a god. i shall not try to prove that the world had a creator. there are some things which men do not believe merely on account of good evidence, nor disbelieve for want of proof. men believe in their own existence, but not from a course of argument. most men believe in the real existence of the outward world--the earth, the hills, the rivers, the trees, everything which we see and hear and feel--but not on account of proof. here and there a strange man is found who professes to disbelieve the real existence of all material things, but he disbelieves not for want of proof. men believe that their sight and hearing and touch do not deceive them, but their confidence in them is not the result of a course of reasoning. to believe in our own existence, and in the existence of the world outside of us, and in the truthfulness of our senses, is natural; to disbelieve these things is unnatural: it shows a state of disordered mental action. when such disbelief is not practically corrected by a man's understanding he is counted insane, and is treated accordingly. "belief in the existence of god is also a natural belief. a denial of god's existence shows, not disordered mental action, but a disordered moral and spiritual state. it shows the absence of that spiritual faculty by which we receive spiritual impressions, and are brought into contact with the spiritual world, and hold intercourse with god and christ and the holy spirit. men must be convinced of the existence of god through their conscience, their moral and spiritual nature. do not misunderstand me. i do not say that good evidence cannot be brought to prove to one's reason the existence of god, but god has not left his existence to be _proved_: he has _revealed_ himself to men's consciences and to their faith; and those in whom conscience and faith are well developed, sound, and right do not need an elaborate argument to prove the divine existence. i shall simply try to show that the works of creation exhibit the wisdom and goodness of god. if any man, looking at such indications of wisdom and kindness, can believe that it all comes by chance or is the work of some evil agency, and that no being of boundless intelligence, wisdom, power, and goodness has anything to do with the making and governing the world, he certainly shows great prejudice: he does not want to recognize god's existence. he must be one of those spoken of by the psalmist who say, 'no god.' "during my recent visit to greenville i visited a mill, the largest of its kind in the country. in one room was a machine, something like a huge straw-cutter, working with great power. in another room was a large steam boiler hung upon a shaft and made slowly to revolve while filled with steam. in a third room were large oval tanks, or cisterns, which might be filled with water. across each tank was a heavy shaft carrying a drum set with steel blades, and as the drum revolved these blades passed other blades in the bottom of the tank, cutting whatever came between like scissors. in a fourth room were certain long and complicated machines. each machine was composed mostly of rollers. there were large rollers and small rollers, solid rollers of enormous weight, and hollow rollers to be heated by steam within. over and around a portion of these rollers passed a broad wirecloth belt. over others passed a like belt of felted cloth. with these machines before you, could you tell me whether the inventor were a wise and skillful machinist?" "how could we tell," asked peter, "without knowing what kind of work the machine was designed to do?" "you could not tell," answered mr. wilton; "you would need to know both what the machine was designed to do and all the processes by which the work was to be carried on. this brings out the first point which i wish you to fix in mind. it is this: to judge of the wisdom of any contrivance, we must understand the purpose, or object, which the inventor had in view; we must understand the work to be accomplished, and also the difficulties to be overcome. an ordinary locomotive steam-engine is admirably fitted to run on iron rails, but he would be a foolish man who should purchase such an engine to draw a train of loaded wagons over a common road of earth. on such a road it could not even move itself. it is good for that for which it was made, and for nothing else. how would you apply this principle to the subject we are now considering? you may answer, samuel." "i think you mean," said samuel, "that, in order to judge of the wisdom and goodness of god in creating and governing this world, we must know the object he had in view in making such a world." "that is my meaning, and i am glad that you understand me so perfectly. if this world were created with no other object than to be the grazing-field for herds of cattle, which see no difference between the beauty of the violet and the dull shapelessness of the cold earth upon which it grows, and never lift their eyes above the horizon, then all the beauty of earth and sky would be useless; there would be no wisdom or goodness in the creation of this beauty. there would be no wisdom or goodness in laying up in store beds of coal, buried deep beneath the surface of the earth, if god designed the world to be inhabited only by savages too rude and ignorant ever to mine it, and turn it to some practical use. "but let me give you another illustration, which can better be applied to the condition of things in this world. just in the outskirts of one of our inland cities i once saw a large and elegant building, whether a private dwelling or a public institution i could not at first tell. it stood high and airy, commanding the most pleasing prospect that all the region presented. we will follow a visitor as he goes to examine that noble establishment. "as he comes nearer, he sees that the edifice is simple and classic in its style and chaste in its architectural adornment. it is a pleasure for the eye to rest upon its graceful symmetry. but in place of the light and graceful fence which he expects to find enclosing its grounds, he sees a stockade strong and high. the janitor turns the heavy key, the rusty bolt flies back, and the visitor enters the enclosure. within the stockade he finds a portion of the ground laid out with taste and cultivated with choice and beautiful flowers; another part is devoted to the culture of garden vegetables. he finds workshops also for the manufacture of pails and tubs, brooms and mattresses. the visitor is ushered into the mansion itself. he finds everything more than comfortable; the rooms are heated from furnaces below; every part is perfectly ventilated; the windows command a view of the country around which must please the most cultivated eye; a school-room is provided with all needed apparatus for the most thorough instruction. 'surely,' says the visitor, 'the founder of this institution must have been both wise and good. he must have loved the young in order to study and supply all their needs so completely.' but some things strike the visitor painfully. the windows are grated with iron, and some of the rooms are almost like prison cells. 'can it be possible,' he thinks within himself, 'that the young need to be confined by a stockade in so pleasant a place and shut in by grates of iron for the enjoyment of such advantages?' the master as he teaches his pupils seems as kind and gentle as a mother, yet there is a firmness and authority in his tones and a rigidity in his training, as if his government were kept braced against a mutinous spirit. the means of punishment also are provided, and, when occasion requires, stern chastisement is employed. all this seems to the visitor like an enigma. the institution appears to him like a bundle of contradictions. a father could not have provided a pleasanter home or larger advantages for his children, but fathers do not commonly surround their homes with stockades, and cover their windows with bars of iron, and train their obedient children with a hand of such firm, unyielding force. 'pray, sir,' he says to the master, 'what is this strange contradictory institution?' 'it is the state reform school,' the master answers. 'and who are these lads and young men for whom all this work and wisdom is expended?' 'they are those who have taken the first steps in crime, but have not as yet become hardened and fixed in wickedness, and are sent here with the hope of overcoming their vicious propensities and training them to virtue and an honorable manhood.' "everything is now made plain. the need of the stockade, and the grated windows, and the rigid government, as well as of the pure air, the garniture of beauty, and the kind loving care, is manifest. it is a place unsuited to a family of obedient children, and equally unsuitable as a place of confinement for confirmed criminals, shut up, not for reform, but for punishment. it is wisely adapted to the work designed to be accomplished, and to no other. "in like manner, if we would judge of the wisdom and goodness of god in the creation and government of this world, we must understand the use for which the world was designed. is this plain to you, ansel, and does it seem reasonable?" "yes, sir; i think i understand it, and i can see no objection to the principle. i think even mr. hume could find no fault with that. but how shall we know the object for which god made and governs the world?" "that is the next point to be considered. perhaps you will tell us what seems to you to be that object? young people sometimes have thoughts and opinions upon the greatest questions." "i have never formed an opinion of my own," ansel replied, "but i have always heard it said that god designed to show how perfect and good and beautiful a world he could make. but many things in the world seem to me neither perfect, nor good, nor beautiful." "why, ansel!" exclaimed samuel; "the bible says that 'god saw everything that he had made, and behold it was very good.'" "and, mr. wilton," asked peter, "does not the bible say that 'god created all things for his own glory'?" "before answering any of these questions, let me ask samuel a question. what do you understand to be the meaning of the words you quoted from the last verse of the first chapter of genesis?--'god saw everything that he had made, and behold it was very good.'" "i suppose it means," answered samuel, "that god made everything just as good and beautiful as it can be, so that any change must be a change for the worse. the lecturer last winter said that if men could entirely destroy any one of the most troublesome species of insects, their destruction would be a great loss to the world, and that if a single atom of matter belonging to the earth were annihilated, it might throw the solar system out of balance, so that it would finally be destroyed." "i remember," said mr. wilton, "that some lecturer last winter made statements of that kind, and i have heard other people declare that the least possible change in the world would be injurious, if not destructive, to the interests of man, and that the most troublesome beasts and insects and the most loathsome reptiles are necessary to human happiness. does that seem to you to be true, samuel?" "i have always tried to believe it, because i thought i ought to believe it. it has seemed to me to be dishonoring god to believe that he did not make the best possible world." "you are right in trying to believe what seems to be right and true, even though difficulties do lie in the way. difficulties do not by any means show that an opinion is false. we must certainly believe that god made this world perfect for the object which he had in view in making it. but not a few skeptics deny the existence of a good, wise, righteous creator and governor, because they have a wrong idea of the end for which the world was created, and, consequently, a wrong idea of that in which its perfection must consist. let me ask you a few questions which will lead your minds in the right direction. do not men produce by cultivation better fruits and vegetables than nature ever grows when left to herself?" "yes, sir," said ansel; "the peach and apple and potato have been brought up to their present state of excellence by great care and exertion. originally, they were almost worthless." "and not only that," said mr. wilton, "but when once that careful culture is relaxed they begin to return to their former badness. again, do we not improve upon nature by drainage and improve upon the climate by irrigation?--in fact, do not men by drainage and irrigation and all manner of culture greatly improve the natural climate of a country?" "i think that is true," said ansel. "i never thought of that before," said peter. "moreover, do you not suppose that heaven will be more beautiful than the earth, and that a thousand troublesome things besides sin--loathsome sights, discordant and jarring noises, disgusting and nauseous odors--will be absent from that 'better land'?" "and _i_ never thought of that before," said samuel. "i am sure that many unpleasant things besides those which sin has brought into the world will not be found in heaven. i see that this world might be changed and not be made worse for holy beings to live in." "the world is very good," said mr. wilton, "for the purpose for which it was created, but we need not look upon it as designed for a specimen of the most beautiful, pleasant, and desirable world which the creator could produce." "but you have not told us," said peter, "what the bible means when it says that god created all things for his own glory. does it not mean that he made the world so good and perfect that all creatures ought to praise him on account of it?" "we ought," said mr. wilton, "to praise god for the wisdom and goodness displayed in the works of creation. that is the teaching of the bible in many places; it is also the sentiment of the bible that god created the world and carries on all things for his own glory, but it nowhere uses the exact language which you have employed. in isa. xliii. , speaking of 'every one that is called by my name,' the lord says, 'i have created him for my glory.' in prov. xvi. it is written, 'the lord hath made all things for himself; yea, even the wicked for the day of evil;' and the four and twenty elders fell before the throne of god saying: 'thou art worthy, o lord, to receive glory and honor and power; for thou hast created all things, and for thy pleasure they are'--that is, exist--'and were created.' i might quote other texts of similar meaning. we are taught also that our first and supreme aim in all our conduct should be the glory of god. 'whatever ye do, do it all to the glory of god.' but here two questions arise: what is the glory of god? and, what is it for god to glorify himself by his works of creation and government? who will tell us?" all were silent, and mr. wilton went on speaking: "the word glory means, first and literally, a halo of light. the glory of god is the radiance, or halo, so to speak, of his infinite attributes and holy character. god glorifies himself when he reveals himself, and makes known his character, and causes the uncreated splendor of his attributes to break forth, so that his creatures recognize them and adore him. this, you see, is very different from the idea of glory among ambitious men. god glorified himself in the creation of the physical world, because from that creation his wisdom, power, and goodness are manifest. he glorified himself in the creation of angels and men, because they were created in the image of god and are finite pictures, so to speak, of the infinite creator--a revelation of his spiritual being and personality. he glorifies himself in his government of the world, because his administration of affairs exhibits his justice, mercy, and holiness. this is what we mean by the glory of god and his working all things for his own glory. this is somewhat difficult for persons of your age, so we will leave it and return to the exact subject of discussion. admitting that god created the world and governs it for his own glory--that is, to reveal himself--for what specific purpose did he design this earth?" "i don't know," said peter, "that we understand what you mean by 'specific purpose.'" "very well, then," said mr. wilton; "i will suggest the answer. does the world seem as if fitted up to be the dwelling-place of holy beings?" "i have never thought of the question before," said ansel; "but it seems to me that many things in this world would give pain even to angels if they lived here with bodies like ours." "i agree with you, ansel. if men were sinless and holy as the angels of heaven, many things in this world would bring them distress. but does it seem reasonable that the world was designed merely as a place of punishment for men by reason of their wickedness?" "some men are not wicked," replied samuel. "there have always been men willing to die rather than disobey god. surely, god does not punish such men. and many beautiful and pleasant things are found in the world--arrangements plainly designed for the welfare and happiness of men." "i think you are right, samuel. but, without asking further questions, i will give you the conclusions to which my study upon this subject has brought me, and some of the reasons for those conclusions. "this world was made chiefly as the dwelling-place of man. the world was not planned merely as the abode of brute animals. men are nobler than the brutes. men have permanent interests and advantages. aside from the glory of god, men are an end unto themselves. to become and be _men_ is the noblest object of human life, but the animal tribes exist for the use and benefit of others. to be an end to itself, a creature must be immortal; but the brutes exist for the use and advantage of man, live out their transient life, and exist no more. this is the view presented in the sacred scriptures. god gave to man lordship over the earth--not only over the soil to subdue it, and over the great forces of nature to bring them into subjection for human advantage, but also over the brute creation, 'over the fish of the sea, the fowls of the air, and every living thing that moveth upon the earth.' i conclude also that god did not prepare this world as a prison-house and place of punishment for rebels against his government. too many pleasant things abound for me to believe that. the pleasant breezy air, the glorious sunlight, the refreshing showers, the treasures of mineral wealth stored up in the earth, the fertile land and golden wheat, the beauty spread over all nature, the sweet consciousness of existence, so that just to live and act is joy, and the comfort and hope of immortal pleasure enjoyed by truly christian men,--all these things, and many more, assure me that not the subtle shrewdness of a tormentor nor the unmingled justice of an inexorable judge, but the heart of a kind and loving father, planned our earthly dwelling-place. you said, samuel, with truth, that there are many pious men in the world who are dear to god, and paul says, 'we know that all things work together for good to them that love god.' for those dear ones christ has such love that he counts everything--whether good or bad--that is done to them as if done to himself. 'inasmuch,' he says, 'as ye have done it unto one of the least of these my brethren, ye have done it unto me.' moreover, jesus said: 'for god so loved the world, that he sent his only begotten son, that whosoever believeth in him should not perish, but have everlasting life.' from these words of jesus we see that there is love manifested in the dealings of god with the inhabitants of our world. were it not so, there would nothing remain but a 'fearful looking-for of judgment and fiery indignation, which shall devour the adversaries.' "on the other hand, i conclude that god made the world as the dwelling-place, not of obedient, holy children, but of those who are disobedient, fallen, and alienated. these disobedient and alienated ones he holds under discipline and chastisement, in order to keep their wickedness in check, to recover them from their sins, and train them up in virtue and holiness, or to remove from the obstinate and incorrigible all excuse for their sins and all plea against their final condemnation. in doing this he glorifies himself by manifesting his wisdom, goodness, mercy, and holiness. "this opinion seems probable from the fact that this is the purpose for which god has actually used and is now using the world. here he keeps and governs the human race. this race is made up neither of holy beings nor of hopeless reprobates. they are the creatures of god; fallen indeed, yet loved; sinful, but objects of divine compassion; deserving of righteous wrath, but the recipients of the offers of salvation through christ. even penitent believers in christ and devoted servants of god are not free from evil propensities, but need to be kept under constant training and discipline. this is the use to which the creator has actually put the world. is it not reasonable to believe that he designed it for their use? ought we to believe that god planned the world for an object for which it never has been and never will be employed? "if sin were removed from the world, the chief part of human suffering would be removed. this no man can deny. wars would cease; the want, disease, and woe resulting from selfishness, idleness, and vice would disappear, and nothing would stand between man and his maker. what new life and joy would fill the world if free communication were restored between man and god, and the divine smile were again to enlighten the world! it would seem that heaven had enlarged her borders to embrace this earthly ball. but the fact would still remain that this physical world is unfitted to be the dwelling-place of sinless beings. the constitution of the world would bring upon them pains and evils which would seem a most unworthy heritage for loving and obedient children of our heavenly father. let sin be taken away, and wearisome toil in subduing the earth would remain. the soil of the earth is hard and clogged with stones, and clammy with stagnant waters, and sown well with the seeds of noxious weeds, and overgrown with thorns and thistles. endless watchfulness and toil is the price of a livelihood. with the sweat of his face man must eat his bread. an army of enemies have pre-empted the soil which man must till. this state of things the word of god refers to sin: 'cursed is the ground for thy sake; in sorrow shalt thou eat of it all the days of thy life. thorns also and thistles shall it bring forth to thee.' the necessity of toiling as we do now for our daily bread, god denounced upon man as a curse on account of sin. we cannot, therefore, regard this as a suitable condition for sinless beings. "this burden of toil is lightened by the progress of modern sciences and inventions much less than some men think. every step of progress has been made by the sacrifice of hecatombs of human lives. from our laboratories and workshops products of human skill, rich and rare, are sent forth; but what are they but smelted and hammered and graven and woven human bones and sinews, the health and life of men? no means have been discovered by which the most necessary processes of the arts can be made otherwise than dangerous to health. only when thousands of miserable workmen had perished was sir humphrey davy's safety-lamp invented; and now the danger, to say nothing of the hard toil, of the collier's life is only lessened, but not removed. still, our furnaces roar and the whole tide of civilization goes on by the health-destroying servitude of men, buried alive as it were in the dark bosom of the earth. would that seem to be a fitting employment for the sinless children of the all-loving father? employés in many kinds of manufacture slowly sink under the accumulated evils of daily toil, and no means of making their employments healthful have been discovered. the friction-match, which has become so nearly a necessity, is made by a process so destructive to health that only a certain class of laborers can be prevailed upon to do the work. i might go on to speak of other painful circumstances in which men find themselves by the almost antagonistic attitude of nature. but if we reject these dangerous processes of manufacture and art, we go back at once to the wooden plough, the distaff and tinder-box of primitive times, and also to primitive poverty and primitive toil, and, i may also add, to primitive exposure to the hostile and pitiless forces and inclemencies of nature. purge the earth of sin, and wearisome toil would still remain. nature must be nursed and cultivated or she yields no bread. her hostile attitude must be overcome; the thorns and thistles must be rooted out; and every step of progress, won by suffering, must be held by painful work and watchfulness; otherwise nature returns to the wild and savage state. relax the culture of the choicest fruit, and it begins to deteriorate; leave the best-blooded breed of cattle to itself, and it returns again to the level of native, uncultured stock. "the inhabitants of this world are also liable at all times to diseases and destructive accidents. this condition of things could not be changed without changing the entire structure and plan of the world. is that a fit dwelling-place for a sinless being where chilling winds one day shrivel his skin and fill his bones with rheumatic pains, and the next, sweltering heats pervade all his system with languid lassitude--where miasma lies in wait unseen to poison his blood, kindle the malignant fever, and bring him to the shades of death, and every form of accident crouches in ambush, ready to spring upon his victim unawares and tear him limb from limb? we cannot see that the absence of sin would dissipate this liability to disease and the danger of accidents. nay, this liability and danger are written upon the very constitution of the human body. the finger of god has engraved it upon every muscle and bone and life-cell. the creator gave the body that wonderful power called the _vis medicatrix_--the power of recovering from injuries and repairing damage done to itself. pull a leg from a grasshopper and another grows in its place. by this we know that the creator understood the liability of this little insect to lose a limb, and prepared him for it. in like manner the power in man's body to heal a wound or join a broken bone gives us to understand that the creator expected man to live in the midst of danger. the precaution proves the risk. "these accidents are such as no possible carefulness could guard against. to say nothing of the fact that all our knowledge of these perils comes from a painful experience of danger and death, what care, even after ages of sad experience, could ward off the thunderbolt? what carefulness could guard against the tornado on the land, or the hurricane and the cyclone upon the sea? who should stand sentinel against the unseen poison borne upon the wings of the wind? what power should save him from the bursting of the volcano and the jaws of the earthquake? what care could give him knowledge of the qualities of all natural substances, that he might avoid their dangerous properties? we can suppose a divine care over man that should do all this and save men from harm, but it would be a providence superseding all human knowledge and exertion--it must be a providence to which the human race is now a stranger; miracles would then be the rule, and the undisturbed course of nature the exception. "if, however, we suppose that god designed the world as a training-school, so to speak, of fallen beings, such as the word of god declares the human race to be, all is plain, everything is suitable and harmonious. we can see the fitness of at least the chief outlines of man's earthly condition, and can perceive god's wisdom and goodness in the constitution of the world. "the pain and woe-producing agencies of nature are seen to be not at all contradictory to goodness, but on the other hand eminently wise and righteous. the whole sum of human misery expresses god's displeasure at sin. by their sufferings men learn how abhorrent is sin in god's sight. by the consequences of evil-doing they learn not to transgress. as none are free from the taint of depravity, none are free from pains. the necessity of labor--one of the elements of the primal curse--is a check to sin on the part of the vicious, and a discipline and trial to virtue on the part of the penitent. the multiform trials of life--which can indeed be borne well only by the grace of god--while they teach the evil of sin and keep the heart chastened and subdued, nourish heroic and dauntless virtue in the faithful. 'daily cares' become 'a heavenly discipline.' dangers and calamities startle the stupid conscience, and keep alive the sense of responsibility to god on the part of the wicked; they quicken the sense of weakness and dependence in the believing and educate their faith in god. the more sudden and overwhelming these evils, and the more these dangers are placed beyond the possibility of being warded off by human care, the more do they awaken in men a sense of the divine presence and of responsibility to god. "but would not all these natural agencies subserve essentially the same ends in the discipline of unfallen and sinless beings? by no means. if sufferings came upon a sinless being, he could not feel that they came as chastisements; he could not feel them to be deserved. they would be to him a 'curse causeless,' and hence would bring no advantage. he could only cry out in astonishment, 'father, why am i, thine obedient son, thus smitten?' calamity falling upon the innocent would be an anomaly in the universe. but now the sufferer, pierced through and through with a sense of ill desert, meekly bows his head, murmuring, 'father, all thy judgments are just and right.' "one very important feature of the world we live in is its moral symbolism. the world is full of most suggestive symbols and emblems of moral good and evil. there are all beautiful and glorious things, to stand as types of goodness, truth, and righteousness; there are all loathsome, malignant, and hideous things, to serve as the types of folly and wickedness. was it merely an accident that the dove was fitted to become the emblem of purity and of the holy spirit? the lamb, to be the emblem of gentleness, of christ the gentle sufferer, and of his suffering people? the ant, to be the type of prudent industry? the horse, of spirit and daring? and the lion, of strength and regal state? was it only an accident that prepared cruel beasts and disgusting, poisonous reptiles as the types of evil passions and sins--that made the venom of the viper, the cunning of the fox, the blood-thirstiness of the wolf, the folly of the ape, and the filth of the swine, symbols of foul, subtle, malignant sin and folly? nature is full of these emblems. the palm tree with its crown of glory, the cedar of lebanon, the fading flower and withering grass, the early dew and the morning mist, the thorn hidden among the leaves of the fragrant rose, poisons sweet to the taste, and medicines bitter as gall,--how all these natural things preach to men sermons concerning spiritual verities! there is no virtue or grace which is not commended to man by its image of beauty in the animal tribes; there is no vice against which men are not warned by its loathsome, disgusting form shadowed out in the instinctive baseness of irresponsible brutes. "thus we find earth, air, and sky to be full of silent voices proclaiming in the ears of man that which he most of all needs to remember. these types and symbols of virtue and vice are specially needed by fallen beings. they seem fitted for beings whose spiritual eyes are blinded and all their spiritual senses blunted--beings with whom there is no longer 'open vision' of spiritual realities. these pictures of evil are most impressive to men who see in them the reflection of their own base passions. how the fetid goat and the swine wallowing in the mire speak to the lecherous man and the drunkard! in a world of sinless beings these mimic vices would seem rather to mar god's handiwork. "set the human race, fallen as it is, in a world where the patience of daily industrious toil would not be needed, and the race would rot with putrid, festering vice. remove all danger, and men would forget and deny that the creator holds them responsible. let no evil consequences follow evil-doing, and men would cease to make a distinction between right and wrong. take away death, and they would deny the existence of a spiritual world. but in this world god has hedged men around with checks and penalties and painful discipline, such as are of use only in dealing with sinners. "i conclude, therefore, that god prepared this world as it now is as a place of discipline for a fallen race. this is the use to which he has devoted it in the past; and when there is no longer need of such a world for the discipline of men, we learn from the word of god that a 'new heaven and a new earth' shall be provided. this world is thus declared to be an unfit abode for the glorified saints. to judge, then, of the wisdom and goodness of god in the works of nature, we must keep in mind the object for which the creator prepared the world. ansel, tell us how this strikes you." "i never thought of it in this way before," he answered; "indeed i have thought very little of this subject, but--" tinkle, tinkle went the bell upon the superintendent's desk. this was the second time the superintendent had struck his bell, but mr. wilton had been so intent upon his subject that he did not hear the first ringing. the school was dismissed, but mr. wilton remained with his class to fix upon the particular department of nature which they would study. he found that all were studying natural philosophy, and had recently gone over the subject of heat. at his recommendation, therefore, they agreed to examine, as a specimen of god's works, his management of heat in the world. mr. wilton requested them to review the subject during the week, and be prepared to state and apply the general principles touching the nature, phenomena, and laws of heat which they had already learned. this work they will enter upon next lord's day. chapter iii. a difficult question. during the week, ansel, peter, and samuel were busy reviewing and fixing in memory what they had already learned of the nature and laws of heat. they were not only interested in the new line of study, and desirous of pleasing mr. wilton, but they also felt that their scholarship was to be tested, and each one was ambitious of standing equal to the best. ansel, of course, was busy and ambitious. the lesson was coming somewhat upon his own ground, and he felt in no wise unwilling to show how well he had mastered the subject. he entered upon it with feelings a little different, however, from his anticipations. the explanation which mr. wilton had given of the purpose of the creator in making such a world seemed to him very reasonable. he could make no objection to it. but that explanation had taken away at one sweep a whole store of objections to god's goodness which he was waiting to bring out as soon as a good opportunity was presented. a world designed for the dwelling-place of sinners--sinners not already given over and doomed to final wrath, but to be recovered from sin and trained in virtue and holiness, or, if incorrigible, to be held in check and used as helps in the discipline of the righteous--he plainly saw must be as unlike a world fitted up for holy beings as a reform school is different from a home for kind and obedient children. those arrangements which he had thought the most painful and objectionable might, after all, be the wisest and best. he did not see where to put in a reasonable objection to mr. wilton's unexpected argument, yet he did not feel quite satisfied to confess to himself that he was so soon and so easily defeated. in this state of mind, on saturday morning he met mr. hume upon the street. "good-morning, ansel," said mr. hume. "good-morning," returned ansel. "i hear," said mr. hume, "that you have given up studying the bible in your bible class, and have begun the study of natural philosophy. is that so?" "not quite true, mr. hume. we are to examine some department of the works of nature, and see what indications appear of the creator's wisdom and goodness." "that is a little different from the report which came to me. but what did you learn last sunday?" "mr. wilton told us that in order to judge of the wisdom and goodness of god in any of the affairs of this world we must consider the object for which that arrangement was designed. he said that if a man examine a cotton-gin, supposing it to be a threshing-machine, he would be likely to pronounce it a foolish and worthless contrivance; and that the fine edge of a razor would be worse than useless upon the cutter of a breaking-up plough. he told us that the earth was not prepared as the dwelling-place of sinless beings, but as a place of discipline for the fallen human race, and that we ought not to look upon it as the choicest specimen of workmanship which the creator could construct." "i have heard that mr. wilton believes something of that kind. ansel, have you studied geology?" "i have read a little upon that subject and have heard some lectures." "can you tell me, then, whether or not the natural laws which prevailed on the earth ages and ages ago, before the earth was fit for men to live upon it, are the same as those which have been in operation in these later ages, since men have inhabited it?" "i suppose that the same laws have prevailed from the beginning of the geologic periods. i think that geology makes that very evident." "if that were not so," said mr. hume, "the past history of the globe would be a riddle to us; it would be confusion worse confounded. in regard to those early ages we could not reason from cause to effect, for we should know nothing of the forces and principles then in existence. in geologic studies we judge the past from the present, and if that be not a trustworthy method of reasoning, all the conclusions of geologists are as worthless as dreams. have you any reason to suppose, from what you have read on this subject, that a curse changed the character of the earth as a dwelling-place for man some six thousand years ago? is it true, as milton says, that then 'the sun had _first_ his precept so to move, so shine, as might affect the earth with cold and heat scarce tolerable, and from the north call decrepit winter--from the south to bring solstitial summer's heat'? did the creator then 'bid his angels turn askance the poles of earth twice ten degrees and more from the sun's axle'? or was death then first introduced among the brute creation, as milton fancies?-- 'but discord first, daughter of sin, among the irrational death introduced through fierce antipathy; beast now with beast 'gan war, and fowl with fowl, and fish with fish; to graze the herb all leaving, devoured each other.'" "animals must have died," said ansel, "for their remains lie imbedded in rock which certainly existed before man lived on the earth." "i wish you would ask mr. wilton one question for me." "i am willing to ask him any proper question, and i suppose you would not wish me to ask any other." "i certainly would not. will you ask him how it was possible for man not to sin and fall if god created the world for a sinful race myriads of ages before man was brought into existence? it would seem that if man had remained obedient he could not have lived pleasantly in a world prepared for sinners, and at the same time, by man's obedience, all the creator's plans touching this world would have been dislocated and disappointed." "i will ask him, sir," said ansel, "at the first good opportunity." this good opportunity occurred sooner than ansel expected, for, before entering upon the proposed lesson the next lord's day, mr. wilton said to the class: "i wish in these lessons to advance carefully and safely, and, as far as possible, have everything well understood. for that reason i invite you to speak freely of any difficulties or objections which may suggest themselves to your own minds or which you may hear presented by others. at the close of the last lesson the views which i had presented to you seemed very reasonable, but it is possible that, as you have thought upon the subject during the week, objections may have arisen in your minds. if so, i should be glad to hear them now." "there are many things," said peter, "of which i cannot see the use, even if we suppose that the earth was designed as the dwelling-place of sinners." "it would be very surprising indeed if you could unravel all the mysteries of creation in a week's time. wiser men than any of us have spent a lifetime in searching out the meaning of god's works, and died still in the dark upon many points. we need not expect to unravel and understand all the deep, complex, and delicately-interwoven contrivances in a world so vast and curious as this. the world is a great mystery--mysterious as a whole, and mysterious in all its parts--upon any supposition. but the explanation which i gave of its design furnishes a sufficient reason for the great outline of creation. this gives a reason for the pains and miseries which dog man at every step. this gives a reason for the earth's being left rugged and sluggish, bringing forth thorns and thistles, and requiring to be subdued by patient industry. it shows a ground for the necessity of exhausting toil under a frowning sky and mid miasmatic airs--for the liability to diseases and accidents, and the hard necessity of death. these great elements of divine providence are not stripped of their halo of mystery, but with this explanation they are seen to form a harmonious whole for the accomplishment of a great and glorious purpose." mr. wilton paused. then ansel said, "mr. hume wished me to ask you a question." "very well, i should be glad to hear it. i hope, indeed, that he sends his question from interest in the subject, and not with the design of perplexing us. i wish also that he were here to ask the question and hear the answer for himself. but what is the question?" "he wished me to ask how it was possible for man not to sin and fall if god placed him in a world prepared for a race of sinners and unfitted for a sinless race. he said that in such a case, if man had remained obedient, the plans of god would have been disarranged." "what answer did you try to give him, ansel?" "i did not try to make any explanation. it seemed to me a very great objection. i did not see how such a course was consistent with god's righteousness." "and you are not the first person who has objected to this as a great inconsistency. i am afraid the discussion will take more time than we ought to spare, but now that the question has been asked and the objection presented, i must take time to answer it, even if it consume the whole half hour. "in considering this subject, as well as many others, we need to remember that the existence of difficulties is no objection to a principle or a fact. difficulties wholly inexplicable by man attend facts and principles which must be true. a fact may be incomprehensible, though undeniable. the great doctor johnson said, 'there are insuperable objections against a plenum, and insuperable objections against a vacuum, yet one of these must be true.' what did he mean by that, samuel?" "he meant, i suppose, that we could not explain the possibility that any space should be wholly empty of matter, and could no more explain the possibility that any space should be filled with matter, but that all space must be filled, or else there must be empty space. whether we can explain the possibility or not, one of them must be true." "that is right. the same is true of many other facts besides a plenum and a vacuum. we cannot conceive of infinite space; we cannot conceive that space should not be infinite, but bounded. we cannot conceive of the creation of the world from nothing, and no more can we conceive of its eternal existence. the truth is that the mind of man cannot grasp such subjects so as to reason upon them correctly. no sooner do we attempt to reason about the infinite things of god than we run into absurdities and reach the most contradictory conclusions. and in this respect it makes no difference with what principle or proposition we start if it only contain some infinite element. let me give you a simple illustration from geometry--an illustration which, very likely, is familiar to you: the larger a circle, the less is the curvature of the line which bounds it; that is, the more nearly does that line approach a straight line. an infinite circle must be bounded by a straight line, because with any degree of curvature the circle would be less than infinite. but a straight line cannot bound a circle. the attempt to reason about an infinite circle brings us at once to the most palpable absurdities and contradictions. or take this illustration: the whole of a thing is greater than any of its parts. but divide a line of infinite length in the middle, and each part is infinite. we reach the conclusion either that the half is equal to the whole or that other wholly incomprehensible proposition, that one infinity is twice as great as another infinity. i have made these statements to show you that the existence of difficulties does not indicate, much less prove, that a fact is not real and true. "mr. hume thinks the fact that the earth existed in its present condition before men sinned an insuperable objection to the view that this world was prepared as a place for the discipline of a fallen race. but let us look at the other side, and see if equal objections do not exist. the creator foresaw the fall of man; is there no objection to the supposition that, knowing that man would sin, god made no provision for it? on the one supposition he foresees the evil and makes no provision; on the other, he foresees it and provides for the catastrophe. the former supposition certainly involves the greater difficulties. "the objector may reply that the plan of god, by embracing the fall of man and including it as one of its essential elements, made that fall necessary. but why should not god embrace in his plan that great event, the fall of man, which he foresaw in the future? would it have been wiser and better to leave out of account that most stupendous fact in the history of the human race? this same objection, which mr. hume and many others have brought forward, lies with equal force against the great central fact of the gospel, the death of christ. god's plan touching this world included the incarnation and death of his son. jesus, the 'lamb of god,' is spoken of as 'slain from the foundation of the world.' rev. xiii. . but the incarnation and death of christ presuppose the apostasy of the human race. did this plan touching christ make the apostasy of man a necessity? if preparing a world--fallen, so to speak, beforehand--for a race which god foresaw would fall, be inconsistent with his righteousness, it must be equally inconsistent to prepare a saviour beforehand for that same race. "again, the divine plan touching the death of his son included his betrayal by judas and his crucifixion by the jews. if judas had known that god had poised the salvation of man upon the pivot of his treachery, he would doubtless have argued as mr. hume and others are accustomed to do. but did god's plan excuse his treason against his lord? his own conscience, piercing and rending his soul with remorse, drove him to self-destruction, and christ confirmed the sentence of his conscience and called him the 'son of perdition.' the fact that god weaves the foreseen crimes of men into his plans is no palliation of their guilt. "would it be wise and well to take no account of foreseen events? jesus has gone to prepare mansions for those who will, as he foresees, believe in him: why not make provision for foreseen evils also? our civil government, knowing the liability to crime among men--a liability which the experience of man has shown to be a practical certainty--makes provision for those crimes by maintaining a police, reform schools, prisons, and armies. the governor of the universe, knowing the liability of man to sin and fall--a liability which by his foreknowledge was to him a certainty--made provision for that foreseen apostasy. he made provision, both by the creation of a world suited to a sinful race kept under a probation of mercy, and by appointing a redeemer, the 'lamb of god,' slain, in the eternal purpose, before the foundation of the world. if mr. hume's objection has force at all, it has force against every wise provision of god to meet the consequences of man's foreseen wickedness. it is wise, forsooth, on man's part, to foresee coming evil and prepare for it; but if god do this, men count it worse than folly: they declare it to be an endorsement of the evil! so foolishly do men reason about the high things of god! my answer to mr. hume, then, has four parts: " . the existence of unexplainable difficulties does not disprove the truth and reality of any fact or principle. " . the supposition that god made provision for the present apostasy of the human race is burdened with fewer and smaller difficulties than its denial. " . the word of god declares that he did make provision for the fall of man by the pre-appointment of a redeemer. " . that style of reasoning which seeks to justify or palliate man's first sin because god prepared this world for a fallen race would palliate and justify all wickedness, because the sins of men are woven into every figure of the web of divine providence. not the treason of judas alone, but the whole sum of man's evil-doing, is embraced in the far-reaching plan of god. how this magnifies the wisdom of god! he binds together in one bundle his own righteousness and the sins of men, in a most intricate interlacing, yet without blending the two and without staining the glory of his holiness. "i hope i have made this plain. do you think, ansel, that you can repeat the substance of this answer to mr. hume?" "i will try, sir, if he asks." "you will all notice," added mr. wilton, "that i have not denied that there is a deep mystery in this preparation for the sins of men not yet created, and that i have not attempted to explain this mystery. i have only tried to show that the admission of the view i have given you is more satisfactory to reason than its denial, and that the mysteries of this view are not unreasonable and self-contradictory, for the greatest mysteries are often the most reasonable things in the world. "my introduction has become much longer than i designed, but now let us turn our attention to the subject of the lesson. "to aid us in understanding god's wise arrangements in the management of heat, we need, first, to consider what heat is and to review the laws of its action. without this, we could look on and wonder at god's working in nature, but could not explain that which we saw. "ansel, will you state the theories which have been held touching the nature of heat?" "i will do it as well as i can. the ancient philosophers supposed fire to be one of the four elements of which all bodies were composed. the three other elements were earth, air, and water. these four elements were mingled in various proportions. of these, fire was esteemed the purest and most ethereal; this constantly tended upward to the empyrean, the highest heaven, where the element of fire and light was supposed to exist unmingled and pure. in the seventeenth century, beccher and stahl, two german chemists, brought forward what is known as the _phlogistic hypothesis_. they supposed that every combustible body held in composition a pure, ethereal substance which they called _phlogiston_, a greek word which signifies _burned_, and that in combustion this phlogiston escaped. flame was supposed to be this escaping phlogiston. these were the notions held about fire and combustion, but they are hardly worthy to be called theories of heat. the discovery of oxygen by dr. priestley of england, in , and the introduction of the balance by lavoisier of france, joined with the ever-enlarging circle of facts to be explained, rendered the phlogistic hypothesis untenable, and it was thrown aside. "until a few years since the _caloric_ theory was generally received. according to this theory, heat is a _substance_, a subtle ether, diffused through all bodies and surrounding their atoms. this ether has been supposed to have a strong attraction for the atoms of every other substance, while between its own atoms a strong repulsion exists. in solid bodies each atom of matter, or in compound bodies each cluster of atoms, has been supposed to be surrounded by a little atmosphere, so to speak, of caloric, which prevented the atoms from coming into absolute contact. according to this theory, heat expands bodies by increasing and deepening these minute atmospheres, thus pressing the atoms farther from each other." "you need not explain this theory farther," said mr. wilton; "we have hardly time to go into the history of theories. tell us the latest received theory." "the theory now commonly believed is called the _mechanical_ or _dynamic_ theory. according to this theory, the essence of heat is _motion_. a hot body is one whose atoms are in a state of rapid and intense motion or vibration; and the sensation of heat on touching a hot body arises from the impact, or rapid blows, of the agitated atoms, communicating the same atomic vibration to the flesh and nerves of the hand." "very well stated, ansel. this is the theory now more commonly received. the caloric theory, like the crude notions of the old greek philosophers about fire, and like the phlogistic hypothesis, has been rejected because it failed to explain the phenomena of heat. whether the dynamic theory is destined to share the same fate remains to be seen. it seems, however, to have a better foundation than its predecessors. the dynamic theory, though recently made popular, is by no means a recent conception. it was advocated by such men as bacon, newton, rumford, davy, locke, and others. locke, the distinguished intellectual philosopher who lived in the latter half of the seventeenth century (born , died ), said, 'heat is a very brisk agitation of the insensible parts of an object, which produces in us that sensation from which we denominate the object hot, so that what in our sensations is heat in the object is nothing but motion.' benjamin thompson, an american gentleman who went to europe in the time of our revolution, and for his scientific fame was made count rumford, and became the founder of the royal institution of england, declared that he could form no conception of the nature of heat generated by friction unless it were motion. "a beautiful generalization has been made to show how well this idea of heat harmonizes with the entire plan of the universe. in the whole boundless universe each system of worlds, like our solar system, may be regarded as a molecule, or complex atom. these cosmical molecules, or complex atoms of the universe, are in motion through unmeasured space. in these systems of worlds the planets, with their satellites, are the molecules, and they are in motion--indeed, they commonly have several motions. our earth, for example, rotates upon its axis once each day; it revolves in its orbit around the sun once each year, and the axis of the earth has a slow wabbling motion which produces the precession of the equinoxes, requiring , years for a complete revolution. the earth also is made up of parts, and all these are in ceaseless motion. as said the old greek philosopher, 'all things flow'--that is, everything is in a state of change. solomon has well described this perpetual movement and change: 'one generation passeth away, and another generation cometh. the sun also ariseth, and the sun goeth down, and hasteth to his place whence he arose. the wind goeth toward the south, and turneth about unto the north. it whirleth about continually, and the wind returneth according to his circuits. all the rivers run into the sea; yet the sea is not full; unto the place from whence the rivers come, thither do they return again. all things are full of labor; man cannot utter it. the eye is not satisfied with seeing, nor the ear filled with hearing. the thing that hath been, it is that which shall be; and that which is done is that which shall be done, and there is no new thing under the sun.' eccles. i. - . it is certainly in harmony with this universal movement that the atoms of matter, though they seem so closely packed, should in their inconceivable smallness through inconceivably minute spaces vibrate, or rotate, or revolve through an orbit, never at rest. intensity of heat we may think of as intensity of this atomic motion--a wider swing, so to speak, in their vibration or revolution. this, of course, requires a wider separation of the atoms and a consequent expansion of bodies. a feebler atomic motion permits the atoms to approach each other. in this manner we explain the enlargement of bodies by heat and their contraction by decrease of temperature. 'the ideas of the best-informed philosophers are as yet uncertain regarding the exact nature of the motion of heat, but the great point at present is to regard it as a motion of some kind, leaving its more precise character to be dealt with in future investigation.' this is the most we can do at present." "what is the evidence," asked samuel, "that the dynamic theory of heat is true?" "the evidence that any theory is true is its ability to explain the facts or phenomena with which it has to do. if it explains all the facts and contradicts no known principles, it is regarded as true, or at least no objection can be made to it. let me illustrate. astronomers had long inquired what force or law controlled the movements of the heavenly bodies. at length newton answered, a force of attraction between bodies which decreases in proportion as the square of the distance between them increases. this explanation has been found sufficient to explain all the known facts in the working of the heavenly bodies. upon the basis of this theory astronomers calculate the positions of planets and comets for years and centuries to come. "this theory led to the discovery of the planet neptune, the last discovered of the primary planets. for thirty years irregularities in the motion of uranus had been noticed. these variations were so slight that if another planet had revolved in the proper orbit of uranus they would have seemed to the naked eye, throughout their course, one and the same star. this slight irregularity of motion was so nicely measured that the place of the unseen planet which caused it was almost exactly calculated from the estimated force and direction of its attraction. this theory of a universal attraction of gravitation so well explains all the facts in the case, and has become so universally received, that we are liable to forget that, after all, it is nothing but a theory. "our idea of the structure of the solar system was at first only a theory. the astronomer does not see the planets revolving in regular circles through the heavens and moving around the sun. he only sees the shining points moving back and forth upon the concave vault, doubling and crossing their tracks apparently in the greatest disorder. how shall their motions be explained? astronomers have found that the motions of planets revolving around a central sun, when seen from one of the planets, must present just these apparent irregularities. this explanation is so full and complete that it is now counted not a theory, but an established fact. the same may be said of the shape of the earth. "the dynamic theory of heat explains the phenomena of heat better than any other explanation that has been proposed. it explains the radiation of heat from the sun or from any other hot body: vibrations or impulses are propagated through that ether which is supposed to fill all space. it explains the conduction of heat through solid bodies in the same manner. it explains the expansion of bodies: the atomic motion forces the atoms of bodies farther apart. it explains the production of heat by friction or collision, which no other theory is able to do: the shock of the collision generates this atomic vibration. it explains the production of heat by combustion: the atoms of oxygen and carbon or hydrogen dash against each other and generate heat by the collision. this theory explains the transmutation of motion, or living force, and electricity, into heat, and the transmutation of heat into electric or mechanical force. these points will come up again, and i now only refer to them in answering samuel's question. the dynamic theory explains the phenomena of heat and its relations to force, light, and electricity exceedingly well, and for this reason men look upon it with favor and count it as probably true. if in the progress of scientific investigation it shall be found to explain all the new facts discovered and meet well all the demands made upon it, it will at length be received as an admitted principle in physical science. the _wave_ theory of light and the _vibratory_ theory of sound may be looked upon as thus established. "at our next lesson we shall take a rapid review of the effects and laws of heat." chapter iv. heat a gift of god. the class is again promptly in place and ready for work. "as i announced a week ago," said mr. wilton, "we will to-day take a rapid review of the effects and laws of heat. will you tell us, peter, the first and chief of these effects?" "yes, sir: combustion." "what is combustion?" "commonly the rapid union of oxygen with some combustible substance, attended with the evolution of heat." "was your answer correct, then?" "no, sir," said peter, blushing; "i spoke before i thought." "will you correct your answer?" "the first and chief effect of heat is expansion." "that is right. our sensation of heat is of course only a _sensation_--merely the _feeling_ which results from the effects of heat upon our nerves--but the chief physical effect of heat is the expansion of bodies. the chemical qualities of bodies are not changed: they are not made either heavier or lighter. a sufficiently high temperature renders bodies luminous, and then we call them red hot or white hot. solid bodies begin to be luminous at a temperature of about one thousand degrees. but the one invariable effect of heat, with two or three apparent exceptions, is expansion. you may mention, samuel, some familiar illustrations of the effect of heat in expanding bodies." "the blacksmith heats the wagon-tire in order that it may easily slip over the wheel. if a kettle be filled with cold water, by heating it the water is expanded and runs over. i have noticed that the spaces between the ends of the successive iron rails upon the railroad are larger in winter than in summer, showing that the rails are shorter in winter than in summer. while skating during the cold winter evenings upon the mill-pond, i have seen cracks in the thick ice start and run across the mill-pond with a roar almost like thunder. the ice was contracted by the cold till it could no longer fill the whole space between the banks, and being frozen fast to the banks, it was torn asunder. the mercury in the tube of a thermometer is constantly expanding or contracting by every change of temperature." "yes, those are all good illustrations, and we might go on to mention others equally good by the score. in cold countries, during the intense cold of winter, the surface of the earth cracks by shrinkage, just as you have seen the ice upon the mill-pond torn in two. the britannia iron tubular bridge over the st. lawrence at montreal rises and falls two and one-half inches on account of greater expansion of the upper surface when exposed to the heat of the sun, while a loaded freight train causes a depression of but one-fourth of an inch. a few years since, in order to make some philosophical experiments connected with the rotation of the earth upon its axis, a ball was suspended by a wire in the interior of bunker hill monument. by this means it was accidentally discovered that the heat of the sun, expanding the sides of the monument exposed to its rays, caused the whole monument to sway back and forth daily." here ansel raised his hand. "what is it, ansel?" "i was going to mention the belief of geologists that the mountain ranges were thrown up by the contracting of the earth's crust on account of cooling." "that is an illustration of contraction by loss of heat on an enormous scale. the materials which form our globe may have existed in the beginning in a nebulous or gaseous state. there is certainly very good reason for believing that the earth was once in a fluid state, the whole of its substance molten by intense heat. it is certain that the interior is now hot, and portions of it molten. it is by very many believed that the whole interior is molten. the crust of the earth may have been formed by cooling. if after an outer crust had been formed, and its temperature had fallen so low as to become nearly stationary, the interior mass continued to cool, the molten mass would tend to sink away from the crust and the crust would sink in upon it by wrinkling. thus mountains may have been formed. along the line of fracture the easiest vents would be formed for volcanoes. but this carries us somewhat aside from our subject, and as the expansion of bodies by heat has been sufficiently illustrated, we will leave it. will some one now state the manner in which the dynamic theory of heat explains this expansion?" samuel answered: "i think you have already given us the explanation." "i have briefly referred to it, but you may give it again." "the atomic motion which is supposed to constitute what we call heat, whatever that motion be, whether a vibration or rotation or revolution, requires that the atoms of bodies shall not be packed in absolute contact, and the more intense the agitation or the wider the swing of the vibration or revolution, the greater must be their separation. hence heat expands bodies by thrusting their atoms farther apart." "that will do," said mr. wilton. "let us look now at some of the secondary effects of heat. you may mention some of them, ansel." "heat relaxes or overpowers the cohesive attraction of bodies." "what is cohesive attraction?" "it is that force which binds together the atoms of matter in simple substances, that is, bodies like iron or copper or silver, composed of but one kind of substance, or in compound bodies it is the force which unites the compound molecules of matter." "give us now some illustrations of the effect of heat in overcoming cohesive attraction." "the blacksmith heats his iron in order to overcome its cohesive attraction and render it soft, that he may easily hammer it. the founder heats his metal till its cohesion is so far destroyed that it becomes fluid and can be poured into the mould. heat relaxes the cohesive force of ice and changes it to water, and by farther heating its cohesion is entirely overcome and the water is changed to a gas." "we use heat also in cooking our food," spoke up peter: "is it not because heat destroys the cohesive attraction, and thus softens it?" "if that were the only effect of heat upon food," said mr. wilton, "we should be obliged to eat our food hot, for as soon as it cooled the cohesion would return and the food would be raw again. the operation of heat in cooking is various, and part of the effect is commonly to be ascribed to the water in which the food is cooked or to that which is contained in it. by the combined agency of heat and water starch swells to twenty or thirty times its original bulk and the minute starch grains burst open. in cooking potatoes the starch of the potato absorbs a portion of the water that is in it, and thus renders it dry and mealy. the action of heat and water upon rice, wheat, and other grains is similar to their operation upon starch. in the baking of bread the starch is converted into gum. in boiling flesh the effect is partly due to the solvent powers of water: the juices of the flesh are extracted, the gelatin is dissolved, the fat is liquefied, and the cells in which the fatty matter is held more or less burst, the albumen is solidified, and by long boiling the texture and fibre of the flesh are destroyed. the albumen of an egg, that is, the white, coagulates by heat. but in most of these processes the action of heat cannot be separated from that of water. "but there is another effect of heat very important both in nature and in the arts. what is that?" "the quickening of chemical affinity," answered samuel. "that is right: heat is necessary for the operation of chemical affinity. perhaps this is only a weakening of the cohesive force, thus allowing the chemical attractions to assert their strength. but the fact is that, while in many cases the chemical affinities act with great energy at ordinary temperatures, in other cases they slumber, however closely the substances are brought into contact, till their temperature is raised. samuel, you may mention some illustrations of this principle." "a few months ago i visited hazard's powder mills, in enfield, connecticut, and there learned how gunpowder is made. the charcoal, the sulphur, and the nitre are first finely pulverized, then ground together for hours till thoroughly mixed, and afterward pressed together. this mass is then broken into grains and the grains polished. but though these elements are brought into so close contact, yet they do not combine and explode till heat is applied. the same is true of the combustion of wood and coal. the carbon and the hydrogen of the fuel are constantly surrounded with the oxygen of the air, but they do not take fire and burn, that is, they do not combine with the oxygen, till they are raised to a red heat, or perhaps even to a higher temperature. if a stove filled with burning coal be cooled down to a low temperature by applying ice, the combustion will cease, the fire will go out. our teacher at the academy on one occasion heated a steel watch-spring red hot and plunged it into a jar of oxygen, and the steel spring began quickly to burn with great fury." "you have given us good illustrations, samuel, and that which is true of carbon and hydrogen and oxygen is true of substances in general. the effect of heat in producing chemical changes is very important everywhere. it is seen not only in the chemist's laboratory and in the artisan's shop, but also in the laboratory of nature. plant a grain of corn in midwinter: why does it not germinate and grow? nothing is needed but the requisite heat to quicken the chemical affinities into action. earth and air furnish the needed material for the growth of forest trees in winter as well as in summer, but the cold holds in check the chemical forces and prevents the requisite chemical combinations. no sooner does the sun quicken that atomic vibration or revolution which we call heat than vegetable growth begins. heat is necessary for those chemical changes by which food is digested in the stomach and the processes of nutrition carried on in every part of the body. if a man finish his dinner with ice cream or ice water, the process of digestion is delayed till the contents of the stomach recover their proper temperature. this is one chief reason why warm, comfortable clothing is so very important, especially for children. all the vital processes are chemical processes: they are carried on through chemical affinities. unless the body be kept at a suitable temperature, these processes are feeble and imperfect, nutrition and vital combustion are hindered, and diseases are engendered. "these, then, are the chief effects of heat. it expands bodies, weakens cohesive attraction, and quickens the chemical affinities into activity." ansel again raised his hand. "what do you wish?" "will you please tell us, mr. wilton, how this weakening of cohesive attraction is explained upon the dynamic theory of heat?" "i will do so with pleasure. the increased atomic motion in the heated body throws the atoms farther apart, as we have already learned, and by this increase of distance their attraction is diminished. if the earth were twice its present distance from the sun, their attraction for each other would be four times less than it now is; if its distance were three times as great, their attraction for each other would be nine times less. the attraction of gravitation diminishes in proportion as the square of the distance through which it must act increases. perhaps cohesive attraction diminishes according to the same law, though the spaces are so small that this cannot be demonstrated, but it is certainly weakened by the expansion of bodies through the agency of heat." here peter raised his hand. "what will you say, peter?" "do not men heat and burn bricks, not to soften them, but to harden them?" "that is true," said mr. wilton; "but in this there is a process of drying as well as of heating, and the hardening is due chiefly to the complete drying by the intense heat. too great heat will melt bricks while in the process of burning. i once heard a brick-burner say that he could melt the brick around the arches in his kiln in half an hour, if he pleased to put in fuel and let the fire burn. indeed, almost every known solid substance has been fused by heat. whether carbon has ever been melted is an unsettled question." "i would like to inquire," said samuel, "why water will not burn. is it because it evaporates before it reaches a sufficiently high temperature?" "this is a little aside from our subject, but the incombustibility of water is a provision of the creator so very important that we will stop to notice it. i think, however, that by a little thought you yourself can answer the question. tell me again what combustion is." "combustion is commonly the combining of oxygen with some other substance called a combustible. the rusting of iron and the decay of organic bodies are forms of slow combustion." "now tell us the composition of water." "water is composed of oxygen and hydrogen--eight parts of oxygen to one of hydrogen, by weight, or two parts of hydrogen to one of oxygen, by measure." "how is water formed from these two gases? are they mixed together as oxygen and nitrogen are mingled in the air, or are they chemically united?" "they are chemically united: they are burned together. when hydrogen burns, the product is water." "water is then a _product_ of _combustion_. can you not now tell why water is incombustible?" "i think i now see the reason. the oxygen, being itself the supporter of combustion, will not burn, and the hydrogen has been already once burned in the formation of water." "and that which is true of water is true, in a greater or less degree, of other products of combustion. the burning of charcoal produces carbonic acid, and carbonic acid will not burn because it is the production of combustion. a candle is extinguished by it as quickly as by water. by a recent invention carbonic acid is used to extinguish conflagrations. the carbon has once united with oxygen, and a second combination with an additional amount, or, as a chemist would say, with another equivalent, of oxygen is much more difficult." "i think," said samuel, "i now understand why water will not burn, but will you please also to tell us why water puts out fire better than almost anything else?" "in order to extinguish fire one of two things must be done: either the supply of oxygen must be cut off or the combustible must be cooled down to a temperature below the burning point, when the combustion will cease of itself. when we shut the draught of an air-tight stove, we check the combustion by shutting off the full supply of oxygen. if we could wholly prevent the access of oxygen to the fuel, the fire would at once be extinguished. if oxygen should then be admitted again before the fuel had cooled down below the burning point, combustion would at once begin again. a blazing brand is extinguished by being thrust into ashes, because it is shut away from oxygen. in the same way we extinguish the flame of a candle with a tin extinguisher. on the other hand, fires often go out because the necessary temperature is not maintained. water puts out fire in both these ways, but especially by the second. water poured in torrents from a fire engine upon a fire forms a film of water, and the burning material shuts out the oxygen. but the water acts chiefly by lowering the temperature. no other known substance except hydrogen gas requires so much heat to raise it through a given number of degrees of temperature as water. as much heat is required to heat one pound of water as thirty pounds of mercury. hence, water poured upon burning timber cools it to so low a temperature that it ceases to burn. "in addition to this, we may notice that wood saturated with water cannot be heated above the boiling point of water till the water is evaporated. as fast as the wood and the water rise or tend to rise above two hundred and twelve degrees, the water changes into steam and carries away the additional heat. the consumption of heat in the formation of vapor we must look at more carefully in a future lesson. we will suppose that a house is in flames. a fire engine throws a stream of cold water into the midst of the conflagration. the cold water, dashing against the burning wood, cools the heated surface; it is absorbed into the pores of the wood and hinders its rapid heating; a portion of the water, being changed into steam, carries off the heat; the steam, mingling with the flame, lowers the temperature of the burning gas, and in proportion as steam fills the surrounding space oxygen is driven away. a burning coal mine in england was once extinguished by forcing steam into it, thus driving out the air which supported the combustion and cooling down the burning coal. "the advantages which men receive from these agencies of heat are so manifest that we cannot help noticing them. i do not refer to the comfort of a pleasant temperature, nor the impossibility of living in a temperature extremely low, but to all those processes by which man subdues nature, provides for himself food, clothing, and dwelling-places, and builds up civilization. heat is that force which enables man to accomplish his ends. heat brings the iron from the native ore, and heat renders it malleable and plastic to be shaped for man's uses. heat quickens the chemical affinities and renders the arts of civilized life a possibility. heat brings together oxygen and carbon in ten thousand furnaces, and the heat engendered by the combustion, changed to force, drives the ponderous or nimble machinery which carries on the work of the world. heat quickens the chemical affinities and causes the wheat to grow; heat prepares the wheat for man's food; and by the aid of heat that food is changed in man's body, nutrition goes on, the body is built up, waste matter is removed, and all the vital processes are supported. without these agencies of heat--softening and subduing stubborn matter on the one side, and quickening its forces on the other--man could not exist. "let me remind you that these agencies of heat are of god's devising. if the operations of heat are beneficent to man, it is because god wished to bless his creatures. i am not much given to moralizing, but when i see how completely these simple effects of heat meet man's wants, i cannot help remembering and admiring the wisdom of the great designer. it is _god_ and not blind, unconscious nature that is working." "this reminds me," said samuel, "of the tradition in greek mythology that prometheus stole fire from jupiter and brought it down to man in a reed as a precious treasure. it seems to me like a gift from heaven." "this mythological tradition has, however, one falsehood: there was no need that men should steal fire from the gods; god freely gave it. heat is indeed a gift from heaven." chapter v. conveyance and varieties of heat. "to-day we review the modes in which heat passes or is conveyed from place to place. it is evident that if heat were confined to the very place or point where it is generated, it could subserve none of those uses to which it is now applied in the economy of nature or in the works and arts of man. but heat passes from place to place with great facility, and by one method, with the speed of light, it tends to diffuse itself evenly through all; it seeks an equilibrium. the modes of its diffusion, or conveyance, are three in number. ansel may name them." "heat passes from place to place and from body to body by 'conduction,' by 'radiation,' and by 'convection.'" "what is meant, ansel, by the 'conduction' of heat?" "the passing of heat from atom to atom and from particle to particle through a body is called conduction." "that is right. i will call upon peter to give some illustrations of the conduction of heat." "the examples are so many," peter answered, "that i hardly know what to mention first. if i hold a pin in the flame of a lamp, the part of the pin that touches the flame is first heated, but soon the heat runs along the whole length of the pin and burns my fingers. the parts of a stove which touch the fire are first heated, and from them the heat spreads through the whole stove. a pine-wood shaving, kindled at one end, is heated by conduction, but the heat passes through it very little faster than the flame follows. heat escapes from our bodies by being slowly conducted through our clothing. there is no end to the examples of conduction which one might give." "we must not think of the conduction of heat," said mr. wilton, "as if it were a fluid slowly absorbed by a porous body, as water poured upon the ground soaks into it, or as water percolates through a lump of sugar and moistens the whole of it. we must remember that the transfer of heat is not a transfer of any substance, but a transfer of motion. one atom is set in motion, and strikes against another atom and sets that in motion, and thus motion is communicated from atom to atom and from molecule to molecule through the whole mass of matter till every atom is agitated with the heat vibrations. do all bodies conduct heat with equal rapidity?" "no, sir," replied ansel; "there is the greatest possible difference. some substances are called good conductors, because heat permeates them so readily and rapidly; others conduct heat very slowly, and are called poor conductors or bad conductors." "that is right. every child soon learns by experience to make a practical distinction of this kind. he very soon understands that he can hold a stick of wood without burning his hand, even though it be blazing at the other end, but that when a piece of iron is red hot at one end he must not take hold of it at the other. the child very soon learns to know the different feeling of a cotton night-gown from one of flannel, and the difference in apparent warmth between a linen pillow-case and a woolen blanket. after a room has been heated for a considerable time the various objects in it all become of the same temperature, and the same is true in a cold room; but how great the difference in the sensations produced by touching the oil-cloth and a woolen carpet in a cold room! good conductors of heat, if hot, feel very hot; or if cold, feel very cold; while poor conductors make a much less decided impression. why is this, samuel?" "the good conductors receive heat or part with it very readily. if the good conductor be hotter than our bodies, it imparts its heat rapidly to our hand, and because we receive heat rapidly from it, it feels to us very hot. or if it be colder than our bodies, it takes heat from our hands very rapidly, and gives the impression of being very cold. poor conductors impart heat to the skin or take it away more slowly, and hence feel as if their temperature were more nearly like that of the body." "the conducting qualities of bodies," said mr. wilton, "seem to depend chiefly upon their structure or the arrangement of their atoms. bodies which are compact and solid in their structure convey heat more rapidly than those which are loose and porous. hence solids are better conductors than fluids, and fluids are better conductors than gases, and among solids the metals are better conductors than organized bodies, like wood or flesh, and better than the loose and porous minerals. in bodies of loose, porous, or fibrous texture, the continuity of the conductory substance is constantly broken. the particles in a mass of sawdust touch only at a few points, leaving frequent spaces. in woolen and cotton fabrics the points of junction of the fibres are very few, comparatively. for this reason the motion is not readily communicated from atom to atom. "the crystalline arrangement of atoms has an influence upon conduction of heat. heat is conducted more rapidly in a direction parallel with the axis of crystallization than across that axis. wood conducts heat more rapidly in the direction of the grain. this arrangement seems to be well adapted for keeping trees warm in winter. their roots reach down into the earth, which remains warm in the coldest weather. this heat of the earth travels along the fibres up through the tree, while the heat conducted across the fibres escapes much more slowly into the open air. the bark also, being a very bad conductor, hinders the escape of heat. of metals, silver is the best conductor. i will give you a brief table which will show the great difference in the conducting qualities of some of the metals. counting the conducting qualities of silver as , the table is: 'silver, ; gold, ; copper, ; iron, ; platinum, ; german silver, ; bismuth, .'--_youmans._ "what is the second method by which heat passes from place to place?" "it is radiated," replied ansel. "and what is radiation?" "it is motion in straight lines or rays diverging from a centre. from a hot body heat is passing off in straight lines in every direction. as a lamp radiates light, so does a hot body radiate heat." "radiant heat," said mr. wilton, "moves with the same velocity as light, that is, one hundred and ninety-two thousand miles per second. it also follows the same general principles as light in all its motions. it is absorbed, reflected, or transmitted in the same manner as light. and this is true of either luminous heat--that is, heat radiated from a body which is red hot--or obscure, or dark heat. "as there are good and poor conductors, so there are good and bad radiators of heat. the radiation of heat depends upon three conditions: " . upon the temperature of the body. the higher the temperature, the more rapid and energetic is its radiation. " . upon the surface of the radiating body. a dull, rough surface radiates heat more rapidly than a surface bright and polished. " . upon the substance of the radiating surface. with surfaces equally smooth and bright, some substances radiate heat much better than others. a surface of varnish radiates heat much more powerfully than a surface of gold or silver. "ansel, you may, if you can, explain the radiation of heat." "i can give no other explanation than that radiation is conduction through that subtle ether which is supposed to pervade all space." "very well; perhaps that is as good an explanation as can be given. but it seems rather like the propagation of an impulse than the spreading of atomic vibrations in every direction. the motion is propagated in straight lines. if it be conduction, it must be carried on by different vibrations from those of ponderable substances. heat, light, and electricity are supposed to be all propagated through the same theoretical ether. sir isaac newton estimated the density of the ether as seventy thousand times less than the density of our atmosphere, and its elasticity in proportion to its density as four hundred and ninety millions times greater. but the very existence of this universally-diffused ether is a supposition made to account for the phenomena of light, heat, and electricity; and, of course, all its qualities must be theoretical also. radiation is believed to be the propagation of a motion or impulse through an inconceivably rare and elastic ether. "peter, what is the third method by which heat passes from place to place?" "convection," was his reply. "what is meant by convection of heat?" "the conveyance of heat by carrying a heated body. if i remove a hot iron or a kettle of hot water, i must of course carry the heat which it contains." "a very good illustration of the convection of heat," said mr. wilton, "is seen in the common method of heating water. the heat is applied at the bottom of the vessel containing the water; as fast as the water at the bottom next the fire is heated, it rises and carries the heat to the top; cold water comes to take its place, and this in turn is heated and rises and carries heat to the top. this process is carried on till all the water comes to the same temperature. thus water is heated by convection of heat. "a grander illustration is seen in winds and ocean currents. warm winds carry heat enough to warm a continent, and the mighty ocean currents are still more efficient in transferring heat from one part of the earth to another. "another point we need to understand. when radiant heat falls upon a body, what becomes of it?" "it is disposed of," answered samuel, "in one of three ways: it may be reflected according to the same principles by which light is reflected; or it may be transmitted, that is, pass through the body; or it may be absorbed, that is, stop in it." "very well stated, samuel. in regard to reflection i need to say very little. you know how light is reflected from a polished surface, such as a lamp reflector: heat is reflected in the same manner. one fact you must bear in mind touching reflected heat: it does not heat the reflecting body. "there is no need of telling you that light passes through certain substances. it passes through gases and through some liquids and some solids. the best of glass, though it is so solid, interposes very little hindrance to the passage of light. heat in like manner radiates through certain solids. luminous heat is radiated through glass. rock-salt transmits dark heat also. a plate of alum permits light to pass, but stops both luminous heat and dark heat. remember that transmitted heat, as was said of reflected heat, does not heat the body through which it passes. i have seen boys make burning-glasses of ice. the heat passes through them and burns that upon which it is concentrated, while the ice itself through which the heat passes is not melted. "if a body have a good radiating surface, that is, if its surface be dull and rough, the heat which falls upon it will be mostly absorbed. the reflecting and absorbing qualities hold an inverse ratio to each other; the better the reflecting qualities, the worse the absorbing, and the worse the reflecting, the better the absorbing. heat which is absorbed by a body commonly raises its temperature, and remains in the body till it is slowly radiated or is conducted away by the air or other bodies which come in contact with it. "what is that heat called, ansel, which is absorbed by a body with no rise of temperature?" "it is called _latent_ heat." "that is the old and common expression, but what is meant by latent heat?" "the word _latent_ signifies _lying hidden_ or _concealed_. latent heat, as you suggested in your first question, is that heat which a body receives without showing it by a change of temperature." "that name 'latent heat,'" said mr. wilton, "expresses the opinion of those who invented it; they supposed that heat was in some manner hidden in certain bodies. we must not suppose, however, that this latent heat continues to exist in bodies as heat; latent heat is that heat which is converted into force or some other motion than the atomic heat vibrations, and is employed otherwise than in raising the temperature. you will understand this best by an illustration. "take one hundred pounds of ice at the temperature of thirty-two degrees, that is, as warm as is possible without melting. that one hundred pounds of ice will absorb heat which would raise one hundred pounds of ice water through one hundred and forty degrees, and by receiving that heat it is melted, but the water produced has the temperature of thirty-two degrees. it has received one hundred and forty degrees of heat, but its temperature is not raised a single degree. this one hundred and forty degrees of heat has been transmuted into force and employed in overcoming the crystalline attraction of the atoms of water. "let that ice water at thirty-two degrees of temperature receive one hundred and eighty degrees of heat, and the water rises to two hundred and twelve degrees, the temperature of boiling. but whatever additional heat is absorbed brings no increase of temperature, but transforms the water into steam. it is employed in overcoming the cohesive attraction of the molecules of water and changing the liquid to a gas. about one thousand degrees of heat is thus expended, but the steam which is produced has only the temperature of two hundred and twelve degrees. if the process be reversed, the steam gives up, as it is said, the one thousand degrees of heat in returning to the condition of water and the one hundred and forty degrees in resuming the crystalline structure of ice. the heat which was employed as force in overcoming the atomic and molecular attractions is transmuted again to heat, and shows itself in raising the temperature. and that which is true of water is true of any other substance in changing its form from a solid to a liquid or from a liquid to a gas, or the opposite. in an amount different for each kind of matter, in all these changes of condition, heat is transmuted to force or force to heat. "these transmutations are going on ceaselessly in the operations of nature, and without understanding them we cannot appreciate the wonderful operations of heat in the world. the heat of the sun beams upon the ocean; the greater part of that heat is expended as force in overcoming the molecular attraction of water, thus converting it to vapor, and in raising that vapor to the higher regions of the atmosphere. this heat-force, or, as we might call it, 'sunpower,' expended upon the earth, amounts to thousands of millions of horse-power daily. [illustration: transmutation of heat. page .] "examples of the transmutation of force into heat abound everywhere. a boy strikes his heel upon the stone pavement; from the point of contact between the stone and the steel points in his boot heel sparks of fire fly out. force is changed to heat so intense that particles of steel are set on fire. savages who have no better methods of kindling fire rub dry wood together till the sticks ignite. the force expended in overcoming the friction is changed to heat. in the combustion of coal beneath the steam boiler we see both processes going on. the atoms of carbon dash against the atoms of oxygen, and the force of the collision generates the heat of the combustion. this heat, born thus of force, is again transmuted to force, and drives the engine and the machinery attached. in our study of god's management of heat we shall constantly meet with these changes. you will need, therefore, to study carefully this subject of latent heat. "dr. joule, of manchester, england, has discovered the ratio between heat and force, that is, the amount of force which by transmutation produces any given amount of heat. the force of a one-pound weight which has fallen one foot is taken as the unit of force, and the amount of heat which is required to raise one pound of water one degree is taken as the unit of heat. by many and various careful experiments, dr. joule demonstrated that units of force are the equivalent of one unit of heat. a pound weight falling feet, or pounds falling one foot, and then arrested, produces heat sufficient to raise one pound of water one degree. the result is the same whatever the method by which the force is expended. if water be agitated or shaken, if sticks of wood or iron plates be rubbed together, if an anvil be struck with a hammer, or if a bar of iron or copper be moved back and forth between the poles of an electromagnet, the force expended is changed to heat. you must remember, however, that force becomes heat only so far as the force is actually expended, or used up so that it no longer exists as force. "these conclusions are supported by other beautiful experiments. 'an electric current which, by resistance in passing through an imperfect conductor, produces heat sufficient to raise one pound of water one degree, sets free an amount of hydrogen which, when burned, raises exactly one pound of water one degree. again, the same amount of electricity will produce an attractive magnetic force by which a weight of pounds may be raised one foot high.'--_youmans._ we conclude from experiments like these that heat, mechanical force, and electricity are interchangeable forces; they may be transmuted the one into another. "by this principle of the transmutation of heat and mechanical force we explain the production of heat by compression and the loss of heat by expansion. samuel, you may state the fact upon this point." "if any substance be suddenly compressed," answered samuel, "heat appears; if it be expanded, cold is produced. since gases expand or yield to pressure so readily, they furnish the best illustration of this principle." "the suddenness of the compression or expansion," said mr. wilton, "is a matter of no consequence. the effect is the same whether the operation be sudden or slow, but if the compression or expansion be slow, the heat or cold generated is less apparent; the heat is dissipated as fast as produced and the colder gas is warmed by the vessel which contains it. ansel, how shall we explain this?" "i cannot explain it, sir." "the explanation is very simple," said mr. wilton. "mechanical force is employed in the compression of the gas; the force is expended and used up upon the gas, and appears again in the form of atomic heat motion. in the expansion of gases the operation is just the reverse; the atomic heat motion is expended in producing expansion, and hence disappears as heat. the general principle is that no force can be expended in two ways at the same time. "one other point we must notice to-day, that is, _specific heat_. what is understood, ansel, by this term, specific heat?" "the relative amount of heat which different substances require to raise their temperature through any given number of degrees." "that is right. i think that you all must have noticed that it requires much more heat to raise the temperature of some bodies than others. what an amount of heat is required to raise the temperature of water! that heat which will raise one pound of water one degree will cause an equal increase of temperature in five pounds of sulphur, or four pounds of air, or nine pounds of iron, or eleven pounds of copper, or thirty pounds of mercury, lead, or gold. this is what is meant by saying that one substance has a greater capacity for heat than another. the specific heat of water is greater than that of any other known substance except hydrogen gas. this fact, taken in connection with its great specific latent heat and its poor conducting qualities, renders it exceedingly important in regulating climate and moderating extremes of temperature; of this you will be reminded very often as our lessons go on. "no law or principle determining the specific heat of the various elements and explaining the different capacities for heat has as yet been discovered. it has been suggested that specific heat depends upon the number of atoms, that it holds an inverse ratio to their combining numbers, or, what is the same thing, a direct ratio to the number of atoms. this would harmonize well with the dynamic theory of heat, but the harmony between the specific heat of substances and the number of atoms is not sufficiently uniform to establish this supposition. "this completes our review of first principles. i hope that this not very entertaining review of your academic studies has not wearied you of the very word _heat_ and worn out your interest in examining god's management of heat before making a beginning." "i think," said samuel, "that we are not in the habit of becoming disgusted with our studies." "you may expect," continued mr. wilton, "if the past has been interesting to you, that the lessons to come will prove more interesting still. next week we shall consider the abundant provision which the creator has made for warming the earth." and let me say to you, patient reader, that if i had known that you were as familiar with the laws and principles of heat as ansel, peter, and samuel seem to have been, this and the preceeding chapter would not have been written. however dull this review may have seemed to you, it was needful, perhaps, for others, that they might understand the wonderful works of god which we shall now proceed to examine. and, reader, do not forget that heat itself, that subtle motion and mighty force, with all its laws and principles, is one of god's works. already have we been looking at the creator's handiwork. already have we been trying to trace out the thoughts of god as they are written in the "bible of nature." the thoughts of god are great and wonderful. it has been useful and interesting to read thus far in this book written with the finger of the creator of worlds and of man, even if we turn not another page. chapter vi. management and sources of heat. while the lessons which have been reported were going on, the religious interest in the church was deepening. mr. wilton did not cease to make his sermons instructive, but, in addition to the instruction, he made them more and more pungent and persuasive. he aimed to gather up the impressions and convictions already wrought in the minds of his hearers and combine them for united and immediate effect. he believed that this was to be a reaping-time. mr. hume was becoming interested, not because he had been at church, for he had not been there, but the holy spirit of god was working upon his heart. he was becoming uneasy in his unbelief. for some reason, he knew not why, his opinions were becoming more and more unsettled. he did not like to go to the house of god; his self-will and pride of consistency rebelled against the thought of hearing and believing the gospel; but he was restless and discontented away from the place of worship. his associations with his infidel comrades grew distasteful. his sundays were days of distress: with his attention relieved from business cares, thoughts of god and eternity pressed upon him, and he could not escape them. at length he determined to go and hear mr. wilton again: perhaps he should hear something which he could so positively reject as to set his mind at rest. he went, accordingly, the next lord's day, and heard a very impressive sermon. the text for the forenoon was ps. lxvi. : "come and see the works of god: he is terrible in his doing toward the children of men." the sermon gave first a brief and rapid review of some striking displays of god's displeasure at the sins of men: that ancient world of men whose "thoughts were only evil continually" he overwhelmed with the flood; he burned with fire from heaven sodom and gomorrah, zeboim and admah, those lascivious and festering cities of the plain; he sent his torturing and consuming plagues upon the egyptians, and sunk the army of pharoah like a stone in the deep waters of the red sea: "they sank as lead in the mighty waters;" he caused the earth to open and receive korah and his adherents, and bade his angel in "one night" to touch with death the thousands of sennacherib's army. this record of divine wrath against evil-doers has startled the consciences of wicked men, and will continue to startle them so long as the ungodly live upon the earth. it is easy for unbelievers to call the word of god a record of fabulous wonders, but that record lives and will live, and its words assert their divinity by touching and burning the consciences of men as if they were tongues of fire. "but to the thoughtful man," said mr. wilton, "there is a manifestation of god's displeasure at sin even more impressive than these miraculous judgments. the creator has built his wrath against sin into the very fabric of the universe; he has written it upon the very atoms and elements of matter and of mind, and graved it upon the 'nature of things.' the forces of nature are all instinct with holy wrath against ungodliness. evil doing works out evil consequences by the regular course of nature. babylon, nineveh, and tyre were great and prosperous, and as mighty in wickedness as in commerce and war. in the height of their prosperity god denounced upon them disaster and desolation, and by the natural processes of evil their decay and destruction came upon them. no miracle broke the harmony of their mighty march to decay and the silence of death. great nations have perished, but not till they became corrupt. rome fell, but luxury first gendered luxuriant vices, and vices enervated her hardihood and undermined the defences of her courage. no righteous nation ever perished. no nation ever fell into decay till ripe in sin and ready for moral putrefaction. but against wicked and corrupt nations wars and desolations are determined, and the end thereof is with a flood. the very forces of nature seem allied in firm compact with the laws of god, ready with resistless hand to avenge their transgression and to visit evil upon evil-doers. this steady march of all the forces of the world in bringing decay and wretchedness upon sinners is more impressive than any single desultory example of avenging wrath. "but perhaps an unbeliever replies, 'not so; there is a natural law of development, decay, and death, apart from sin. trees grow up, become old, and die. men pass from childhood up to manhood, and from manhood down to second childhood, and return to the dust whence they came. by a like principle, nations pass through similar changes of development, decay, and desolation. but in all this there is no manifestation of divine favor or disfavor.' "this is narrow and false reasoning. if a single great city had become corrupt while all the world beside remained righteous, and god had denounced his displeasure upon it and had executed his wrath by sudden and tremendous judgment, that one city standing out in single and solitary ungodliness and desolation, who would deny, who could deny, that the fate of that unhappy city was a manifestation of divine displeasure? if a second example were made of a second ungodly city, would the expression of divine wrath be weakened? nay; every man would say that it is made stronger. what if a third example be made of a third city? what if every wicked city is made an example? what if god embody his displeasure at evil-doing in the structure of the world, and give to the very atoms of matter and the elements of mind such natures that by the working of their own proper forces, without a miracle, they shall bring pain and evil, decay and death, upon the ungodly? what is this but writing his wrath against sin upon the earth and sky, upon matter and the consciences of men, declaring by this that till the heavens and the earth and the spirits of men be no more he will never withdraw his indignation? this is what god has done. the wicked man sets in motion the machinery which works out his own everlasting undoing. his own hand sows the seeds of death, and as those seeds germinate they strike their roots into his corruptions and draw their nourishment from his evil life. thus do sinners go on 'treasuring up wrath against the day of wrath and revelation of the righteous judgments of god.' "but remember that god has not left the world in these later ages without the testimony of wrathful judgments which ought to startle and alarm the consciences of the wicked like the fires of sodom. let me give you what i suppose to be a true record of the fate which befell a band of bold blasphemers. in that uprising of infidelity which took place near the close of the last century there was formed at newburg, n. y., through the influence of a man known as 'blind palmer,' an association of infidels under the name of the druidical society. the object of the society was to uproot and destroy revealed religion. in pursuit of this object they descended to the most blasphemous mockery. at one of their meetings they burned the bible, baptized a cat, partook of the bread and wine as appointed for the ordinance of the lord's supper, and gave the elements to a dog. then the wrath of god broke out upon them. 'on the evening of that very day he who had administered the mock sacrament was attacked with a violent inflammatory disease; his inflamed eyeballs were protruded from their sockets; his tongue was swollen, and he died before morning in great bodily and mental agony. dr. h----, another of the same party, was found dead in his bed the next morning. d---- d----, a printer who was present, three days after fell in a fit, and died immediately. in a few days three others were drowned. within five years from the time the druidical society was organized all the thirty-six original members--actors in the blasphemous ceremonies spoken of--died in some strange or unnatural manner. two were starved to death, seven were drowned, eight were shot, five committed suicide, seven died on the gallows, one was frozen to death, and three died, the record says, _accidentally_.' be sure of this: god has not left the world nor forgotten his judgments against his enemies, neither is he tied up and hampered by the laws of nature. 'god is angry with the wicked every day. if he turn not, he will whet his sword: he hath bent his bow and made it ready. he hath also prepared for him the instruments of death.' "but remember, also, that god does not limit his expression of wrath to these natural agencies. the smile of god beams direct upon the soul as the warm rays of the sun fall upon the cold earth, and the frown of god throws a shadow which darkens the soul with the gloom of eternal death." this discourse stirred the mind of mr. hume in a wonderful manner. the story of god's judgments upon wicked men and dissolute cities he had read many a time in his boyhood, but the rapid review of them by mr. wilton seemed to bring them up with a lifelike vividness. and that view of the forces of nature, as allied with the moral laws of god to work out wrath upon evil-doers, was new to him, but his own mind quick as thought suggested many more illustrations than mr. wilton had time to give. he remembered that all manner of vices--drunkenness, lust, devotion to gay, sensual pleasures--bring ruin to men. he had noticed that the saddest faces are those of worn-out lovers of pleasure, and he knew that lovers of pleasure are very quickly worn out--that five years of sensuality will waste the powers of life more than fifty years of good work. he knew also that infidels and blasphemers, whatever else they might be, were unhappy men, and died joyless, foreboding deaths. he was not exactly angry, but his heart rebelled against thus being held by the mighty power of god, willing or unwilling, and against the thought that even nature herself had conspired against him. it seemed to him hard that he was born into such a world, and that there was no escape from it. he did not consider at the moment that god and his works were against him only because he was against god, and that by submitting to god in loving obedience all the forces of god's world and god's providential government would turn in his favor--"that all things work together for good to them that love god." at length better thoughts came to him. "i must know," he said to himself, "whether these things are so. i have never examined the subject to discover the truth, but have tried to find reasons for disbelieving the bible and denying the gospel. i ought to look at the other side. if nature and nature's god have blessings in store for the willing and the obedient, why should not i know this and receive my share?" under the impulse of thoughts like these he formed the sudden resolution to join mr. wilton's bible class--that is, if he would receive him willingly, of which he had no small doubt. coming directly forward at the proper time, he said to mr. wilton: "i have learned what your class is studying, and should like, i hardly know why, to join your class for a few sundays, if you are entirely willing." mr. wilton, of course, did not know the exact state of mr. hume's mind; he did not know but that he came with a contentious spirit to bring up objections and propose hard questions; but he felt certain that, whatever his state of mind, the spirit of god was bringing him to take this step. he had prayed for him; in prayer his soul had travailed in pain for him; and he felt that by way of the throne of grace he had obtained a hold upon mr. hume--that the holy spirit had bound a cord between them which could not be broken. he believed, therefore, that, whether he came penitent or angry, good would result from his coming. he gave him, therefore, a hearty welcome. "i am not only willing," he said, "but very glad, to have you come; and as i know that you have kept yourself informed of the latest phases of modern science, i hope we shall have your help in unfolding the subject which we are engaged in studying. i think you will be able to do us good." "your kind welcome ought certainly to incline me to do anything which i can to help the interest of your study, but i only ask the privilege of sitting with your class as a silent listener." the sunday-school opened as usual, and the classes entered upon their work. "you have come in, mr. hume, at just the proper point in the progress of our lessons," said mr. wilton. "we have been preparing the way by a brief review of the laws of heat. we have gone over the effects of heat; the conduction, radiation, and convection of heat; thermal reflection, absorption, and transmission; specific and latent heat. we have tried to form a conception of the existence and operations of heat according to the dynamic theory that heat is a mode of atomic motion. this review would have had little interest to you. we are now prepared to look at the goodness and wisdom of god in the management of heat. we are not trying to prove the existence of a creator and governor--we are only looking at the mighty and wise works of that god in whom we already believe. we shall find the works of god planned and wrought out with wondrous skill, and that wonderful skill is employed in the interest of goodness. god has planned and wrought for the benefit of his creatures. his wisdom and goodness are exhibited on the grandest scale and in gigantic proportions. this is all that is needed practically to demonstrate the existence of god. a good conscience does the rest. being once assured that there is a creator, a good conscience leaps to the conclusion that we ought to obey and serve him. nay, the very work and existence of a conscience implies a divine lawgiver and ruler. to a good conscience a god is a necessity. but as we are not now attempting to show that there is a god, but to study his works, we will pass this point. "with respect to the subject before us, let us first notice that heat is a necessity to the world and to man, and that god has made ample provision for that need. what the condition of the world would be without heat we can only conjecture. in the polar regions a natural temperature of seventy degrees below zero has been observed. at this temperature all the water upon the globe would turn to ice hard as adamant; all vegetation would cease, and with the disappearance of vegetable life all animal life must perish. the whole earth would be a frozen, lifeless, silent waste in the midst of silent space. some lines in byron's picture of universal darkness would fitly describe the state of the world: 'the waves are dead, the tides are in their grave, the winds are withered in the stagnant air, and the clouds are perished.' this description would be no figure, for motion as well as life depends upon heat. yet seventy degrees below zero is but the beginning of cold. 'by mixing liquid protoxide of nitrogen with bisulphate of carbon in a vacuum, m. natterer produced a temperature of two hundred and twenty degrees below zero.' at this temperature some of the so-called permanent gases--as carbonic acid, chlorine, and ammonia--can be compressed into liquids, and it is believed that in the complete absence of all heat all the gases would become solids. but by the agency of heat the world teems with active life. vegetation clothes the earth with a garment of beauty; and earth, air, and sea swarm with living creatures full of enjoyment. this great need of the world is bountifully supplied. the power and wisdom of god are employed in producing happiness. "this, however, is but a part of the benefit which heat confers upon the world. the chief inhabitant of the earth is man, and man was created for something higher than bare existence. he was created for civilization and culture. the savage state is not, as some self-styled philosophers dream, the natural state of man. nothing is so much against nature. the natural state is that condition in which he attains the fullest development. let a brute be placed in so unfavorable conditions that his growth is dwarfed and his natural instincts are not called into exercise, and no one would look upon that as a natural state. but man, wild, uncultured, undeveloped, is spoken of as being in his natural state. there could be no greater mistake. culture and civilization are according to nature, but culture and civilization require that man should get the mastery of nature and subdue her forces. till man gets the victory over the forces of this rough world, he spends a precarious existence in a hard struggle to gain a meagre support for his animal life. but when once science brings art, and the mastery of nature is gained, man can rise into culture and beauty. opportunity is given for development. he blossoms into greatness and strength. ideal and spiritual ends take the place of mere subsistence. "but by what agency does man achieve the mastery of nature? by the agency of heat. by the aid of heat man subdues the world. heat brings the lustrous metal from its native ore; heat fashions the metal into a thousand shapes for the use of men; heat reigns as king in the curious processes of the chemist's laboratory, and the laboratory is the mother of all those modern arts which bless and beautify human life. by heat man prepares his food; by heat he drives his machinery; by heat he outstrips the flight of the winds; by heat he turns winter into summer and in his own dwelling makes for himself a perpetual springtime. for these purposes of human comfort and culture, god has provided generous stores of heat and placed them under man's control. he has placed in man's hands the means by which he can generate a heat which devours the hardest metals like stubble and a cold greater by far than nature ever produces. we see that the creator has provided for man as a being susceptible of culture and development, as a being of soul and sentiment, of spirit and aspiration. god has fitted the world to be the dwelling-place of spiritual beings like man." "i beg your pardon," said mr. hume at this point, "that the first word i speak in your class should be a question which amounts to an objection." "i shall be glad," said mr. wilton, "to hear your question, even though it be an objection. i will also answer it if i can." "i wished to ask why it is, if god designed to provide for man's wants, that man can supply his wants, especially his higher wants--the wants of his intellectual and spiritual nature--only with the greatest difficulty and toil? the brutes supply their need with comparative ease, but man with boundless thought and labor." "your question is an important one, and deserves an answer. for myself, i look upon the fact to which you refer as one of the many points in which this world is adapted to human needs. man is put in a condition which requires boundless thought and toil for the supply of his higher wants just because he possesses a nobler nature and such thought and exertion are needed for its development. which is the more desirable condition for a young man to be placed in--one in which his every wish is anticipated and his every aspiration is gratified without exertion on his own part, or one in which opportunity and means are furnished for self-help, one in which he can supply his wants and satisfy his aspirations only by the exercise of his best abilities? which will encourage the larger manliness and nurture the higher culture and strength? he who has no need for exertion rises at best only to a soft and feeble luxury, without mental vigor or moral force. what does man need besides scope and reward for exertion? effort and struggle are necessities of our nature. this is especially true of man's higher faculties. human greatness and goodness are not created by a word: they must be developed by exertion. for this reason god has made exertion necessary, and as much more necessary with man than with the brutes as his culture is more the result of voluntary, intelligent exertion. does this explanation seem to you satisfactory, mr. hume?" "i have no fault to find with it; i must think of it." "very well, then; if no other one has a question to ask, we will look at another subject. we will survey the storehouses of heat which god has prepared for warming the earth. samuel, you may name the first great source of heat." "i think, sir, that the sun is the chief source of heat." "we certainly receive the larger part of our heat from the sun. no one can doubt this. so much of our heat comes from the sun that the temperature of the earth varies according to the sun's heat, as if that were the only supply. if but a fleecy cloud pass between the sun and the earth, we feel a decided change of temperature. a few hours less of sunshine each day, and a few degrees more of inclination to the sun's rays, change summer to winter and make the difference between the torrid and the frigid zones. withdraw the heat of the sun altogether, and the whole world would become a desert of frozen death." "what is the cause of the sun's heat?" asked peter. "you have asked a question which i cannot answer, and which no man can answer. the most careful and patient observations have been made to discover if possible the constitution of the sun; learned and curious conjectures have been brought forward to explain the source of its heat; but the positive results have not been very large. it is certain that the sun is a globe revolving upon its axis in a period of twenty-five days, nine hours, and thirty-six minutes. this is known by the motion of dark spots upon its surface. the appearance of the sun as seen through a telescope is that of a globe of fire, its surface often in a state of violent agitation and flecked here and there with dark, irregular, changeable spots. these spots are sometimes of enormous dimensions--thirty thousand or fifty thousand miles in diameter. they present a dark centre with a narrow border or penumbra of lighter shade. to account for these spots, it has been conjectured that the body of the sun is dark, but surrounded by a double envelope of clouds, the outer layer of which is intensely luminous. openings in such enveloping clouds would present an appearance like the spots upon the sun. according to this supposition, the heat and light of the sun proceed, not from the body of the sun, but from this luminous enveloping cloud. but granting that this supposition is true, it gives no explanation of the origin of the sun's heat. laplace conjectured that the sun is a globe of fire in a state of violent, explosive conflagration, and that the spots are enormous crater-like caverns in its surface. newton conjectured that comets falling into the sun and being consumed feed the solar fires and maintain its temperature. the reception of the dynamic theory of heat has led to the revival, in a modified form, of this conjecture of newton. it is suggested that meteors or meteoric matter falling into the sun generates its heat by the force of concussion. to show that the intense heat of the sun might be thus generated, elaborate calculations have been made. it has been demonstrated that if the sun were a solid mass of anthracite coal, its combustion would maintain its heat at its present rate of emission only five thousand years, while the falling of the planet jupiter into the sun would generate an equal amount of heat for thirty-five thousand years. a lump of coal falling from the earth to the sun would produce three thousand times more heat by the concussion than by its combustion. "the nearest approach that has been made, of an exact and scientific kind, toward determining the constitution of the sun's surface has resulted from an examination of the _solar spectrum_. a ray of light, by passing through a triangular prism of glass, is, as you know, divided into its elements, or constituent colors. the ray of light is spread out like a half-open fan. this divided and expanded ray, thrown upon a screen, is called the spectrum. an examination of the solar spectrum by a microscope shows certain fine dark lines across it. the lines are invariably the same in their position and grouping. the spectrum of the stellar light is found to differ from that of the solar light, and the light of one star differs from that of another star. light from incandescent metallic vapors gives bright lines across the spectrum. each metal has its own number, position, grouping, and color of these spectral lines. by comparing the solar spectrum with the spectra of the various metals--the processes are curious and the explanation difficult to be understood--corresponding lines are discovered, and the conclusion is reached that the sun's atmosphere contains the vapors of several of our well-known metals, as iron, nickel, sodium, potassium, and others. this is a most curious and marvelous scientific feat, to make an approximate chemical analysis of the sun and stars by means of their light. the conclusions, however, seem trustworthy. "can you tell us, ansel, whether the earth receives heat from the moon and stars?" "i cannot, sir." "i should be glad, mr. hume, to have you instruct us upon this point." "in regard to the fixed stars," answered mr. hume, "counting them as the remote suns of other planetary systems, we must believe that they radiate more or less heat upon the earth; some indeed have extravagantly maintained that we receive from them nearly as much heat as from the sun. the heat received from them is so small that we perceive no difference whether they be hidden, or shine with their utmost brilliancy. i do not know that investigations have been made to determine scientifically their exact thermal influence upon the earth. but little more can be said about the heat of the moon. the light of the full moon, concentrated by a two-foot burning-glass and thrown upon the bulb of the most delicate thermometer, produces no perceptible effect. by means of the electroscope or galvanometer, it is said, however, that the moon's heat has been detected. at a late scientific convention held in chicago, prof. elias loomis read a paper, in which he stated that mr. harrison of england, by a comparison of observations made for sixteen years at greenwich, nine years at oxford, and sixteen years at berlin, has discovered that the moon exerts a sensible influence upon the temperature of the earth, the highest temperature occurring from six to nine days after the new moon and the lowest about four days after the full moon. the conclusion, the opposite of what we should naturally expect--the higher temperature occurring when the enlightened face of the moon is turned from the earth--was explained by supposing the moon's heat to be dark heat which would be absorbed by the vapors and the clouds, and thus tend to warm and dissipate them. by the dispersion of the clouds, the radiation of heat from the earth's surface would go on more rapidly and the temperature would fall. according to this explanation, the lunar heat reduces instead of raising the temperature of the earth. the difference of temperature due to the moon's influence mr. harrison believed to be two and a half degrees. upon extending his calculations through forty-three years of observations made at greenwich, he found the difference reduced to about one degree. as for myself, i confess myself still a skeptic touching the supposed influence of the moon upon temperature." "upon that subject, i think," said mr. wilton, "that we must wait patiently for more light. the popular superstitions which refer sickness and health, and every kind of good or evil fortune, to the benign or malignant influence of the moon, we, of course, must reject. samuel, will you name the second chief source of heat?" "i am obliged to answer as ansel answered just now--i cannot tell. the enormous amount of wood and coal burned amounts to something, but this can have very little effect upon the temperature of the earth." "the second great store of heat is the internal heat of the earth," said mr. wilton. "the importance of this store of heat we can easily understand by considering that the earth is a mass of molten mineral matter cooled and hardened upon the surface. the crust upon which we live is warmed from beneath by an ocean, or rather a globe, a world, of glowing molten rock. deep excavations have been made in mining operations, and artesian wells have been bored to still greater depths--as deep as two thousand, three thousand, or thirty-five hundred feet. the heat of the sun penetrates not more than seventy-five or a hundred feet; below that depth the temperature of the earth remains the same throughout the year. below the point of constant temperature the heat of the earth is found to increase regularly and constantly. the rate of increase varies in different regions, but the average rate is about one degree of temperature for each fifty or sixty feet of descent. from this rate of increase it is easy to calculate the temperature at any given depth. at a depth of less than two miles water would boil. at twelve miles in depth the rock becomes incandescent. at twenty-two miles silver melts, at twenty-four miles gold melts, and at thirty-five miles cast iron becomes liquid. volcanic eruptions also demonstrate the existence of immense masses of molten rock in the interior of the earth; and we can account for the existence of volcanoes only by supposing that they now communicate or once communicated with the deep interior heat of the earth. the thickness of the earth's crust is, however, a matter of conjecture. the melting point of different substances rises as the pressure upon them increases, and as the density of the rock increases its conducting power becomes greater. the crust of the earth, therefore, may be fifty miles in thickness, or it may be one hundred miles or two hundred or three hundred miles. the effect of this internal heat in maintaining the temperature of the earth must be very great." "i want to ask," said peter, "how this internal heat came to exist, and how it is maintained?" "this, like your former question, is altogether beyond our knowledge. all that we certainly know is that god made it thus. the process of creation, if indeed god did not create the earth by a word, without a process, is a matter of sublimest and most venturesome conjecture. according to the opinion of some, the elements of which the earth is composed were created separate and uncombined, and were suffered afterward to unite by their chemical affinities. this chemical combination would be nothing else than a tremendous conflagration, and the result would be the most intense heat of which we can form a conception. others have dreamed of a 'fire-mist' created of god and by some means condensed into worlds. the temperature of the earth is maintained, so far as we know, only by the poor conducting quality of the enveloping crust preventing its cooling. at the present rate of radiation, millions of years would be required to render the change of temperature perceptible. "what is the third great natural source of heat? i will ask mr. hume." "mechanical action, or force transmuted to heat." "will you please explain this?" "strictly speaking," said mr. hume, "this is not to be counted an original source of heat. but heat is used in the production of winds and waves, the flow of rivers, and all the ceaseless activities of the world, and this force reappears from time to time transmuted again to heat. whenever in the friction of air and of water, in the dashing of matter against matter and force against force, motion and force seem to be lost, heat is produced. the water of the sea after long storms is said to be sensibly warmed. we can appreciate the amount of heat generated in this manner only by considering in how many thousand ways force is meeting force and motion is destroyed. all this lost motion--lost as sensible motion--reappears as atomic motion, that is, as heat. such heat has been applied to artificial uses. heat generated by the friction of iron plates ground together has been used for heating buildings." "and this transmutation of living force and heat," added mr. wilton, "is but one of many illustrations of god's economy in the management of heat. nothing is wasted. the voices of nature all echo the words of jesus: 'gather up the fragments, that nothing be lost.' "the fourth source of heat is chemical action. what is the chief form of this which is used for the production of heat? samuel may tell us." "combustion, i think, sir." "that is right; and the most common form of combustion is the combination of carbon with oxygen. this is commonly employed, not because it generates the most intense heat, but because carbon exists so abundantly, and is the most available and the cheapest. the most common form of carbon is wood and coal. this is that storehouse of heat which god has placed in man's keeping. without this the larger part of the earth's surface would be uninhabitable. this renders culture and civilization possible. without it the arts could have no existence. the key of this storehouse of heat god has given to man, so that he may enter in and use its treasures at his pleasure. in the finer arts where very great heat is required, hydrogen is used in place of carbon. jets of oxygen and hydrogen gas thrown together constitute what is called the oxy-hydrogen blowpipe, and generate the intensest heat which can be produced by man. "another source of heat not often mentioned is electrical force. this, like mechanical force, may be transmuted into heat. an electric current sent through an insufficient or poor conductor heats it, and, if the current be sufficiently strong, consumes it. thus lightning-rods are sometimes melted and buildings set on fire. "these, then, are the natural reservoirs of heat: , the sun and other heavenly bodies; , the internal heat of the earth; , living force, or motion; , chemical action; , electric force. "we can hardly over-estimate the abundance of these natural supplies of heat. the world is warmed on the most munificent scale. the earth receives from the sun heat sufficient to boil three hundred cubic miles of ice water per hour, and the whole sum of the sun's heat would boil , , , cubic miles of ice water in the same time, that is, the heat radiated by the sun would boil a mass of ice water of the size of our globe in twenty-five minutes. "the amount of carbon provided by the creator is enormous beyond conception. vast regions of country are covered with dense forests, but the fuel from the forests is but a handful in comparison with the fuel stored up in coal-beds below the surface of the earth. mr. mitchel estimated the extent of the coal-beds of a portion of europe as follows: great britain, , square miles; spain, ; france, ; belgium, . mr. r. c. taylor has made a like estimate for north america, giving to british america , and to the united states , square miles. "these estimates, you will notice, say nothing of asia, africa, south america, or the islands of the sea, and include only the smaller part of europe. in the united states, also, new coal-fields are constantly discovered. the supply of carbon for fuel seems exhaustless. in the british islands about , , tons of coal are mined annually. at this rate the known supply would last for a thousand years. in the united states the supply has no known limit. "you will keep in mind that this supply of heat is also a supply of mechanical force. the coal-fields are an exhaustless storehouse of heat and power. they warm the dwellings of man and drive millions of engines working with the strength of titans for human welfare. "in this bountiful supply of heat to warm the earth and serve human needs must we not see a kind design on the part of the creator? god has provided that which the world needs. he has provided without stint or limit. the general heating of the globe he accomplishes by his own power. he has provided for human culture, development, and happiness by placing stores of heat under man's control. he has furnished scope and means and encouragement for achieving greatness and goodness. he has put man in the condition which a wise father would desire for his son. "in our next lesson we will look at the preservation and distribution of heat, some of the primary elements and arrangements upon which the temperature of the earth depends." chapter vii. preservation and distribution of heat. another lord's day comes, and the members of the class are, as usual, all in their places. they find the subject increasing in interest after leaving the review of the laws and principles of heat. "a week ago," said mr. wilton, "we looked at the chief sources of heat. these are the sun, the internal heat of the earth, chemical action, in which combustion is most important, electrical action, and mechanical action, or 'living force.' the amount of heat furnished from these sources is above all comprehension. the creator seems bountiful even to prodigality in supplying heat for the needs of the world and the uses of man. but with all this largeness of supply the provision would prove wholly inadequate if it were not prudently husbanded and all the avenues of waste carefully closed. men of ample incomes sometimes come to want from too free expenditure. their incomes are large, but their expenses are larger. so it would prove in respect to heat if nature were not as prudent in saving as she is bountiful in providing. will some one mention some of the general methods by which the waste of heat is prevented?" no one answered. mr. hume did not think it best to put himself forward in answering questions, and therefore answered only when personally addressed. the others were silent because they had nothing to say. "i see that i shall have to suggest the answer. ansel, what part of the atmosphere is warmest?" "the bottom, i suppose, for the higher a man goes up upon the lofty mountains or in a balloon, the colder he finds the air." "that is right; and we need to ascend only about three miles, even in the tropics, to reach the region of perpetual snow, while in the polar regions the line of perpetual freezing comes down to the sea level. what would be the effect, ansel, if the atmosphere were as warm, or warmer, at the top than at the surface of the earth? how would that affect the rate of radiation from the earth?" "it must, of course, increase the radiation very much. with the temperature twenty or fifty or seventy degrees below zero, the radiation must be very little." "by some means, then, the atmosphere is kept warm at the surface of the earth and cold in the higher regions, and in this manner the radiation of heat into open space is prevented. this is accomplished notwithstanding that the top of the atmosphere is nearer the chief source of heat, the sun. this would be no very easy problem if its solution were left to human ingenuity. the explanation is very simple, however, when once suggested. the atmosphere is diathermic, that is, it permits the luminous heat from the sun to pass directly through it without heating the air, but the solid earth stops the heat by absorption, and is warmed by it. the warm surface of the earth imparts, in turn, its heat to the atmosphere resting upon it. this warm air, being expanded by the heat received, becomes lighter than the cold air around, and rises, or rather is forced, upward by the greater weight of the colder air. but as it rises and the pressure of the air is diminished it expands still further. by this expansion its sensible heat becomes latent, that is, the heat is transmuted into force, and, as force, is incapable of being radiated. in this manner radiation from the upper surface of the atmosphere is greatly hindered and waste of heat is in a good degree prevented. "in respect to this heating of the atmosphere from the surface of the earth, a layer of clouds sometimes forms a kind of second surface which receives the sun's rays and warms the air above. a few years ago i saw a balloon ascension in providence, r. i. the day was bleak and chilly, and the sky entirely covered with clouds. the aeronauts were expecting a chilly voyage. the balloon shot like an arrow toward the zenith, and in five minutes was completely hidden by the clouds. but to the surprise of the voyagers of the sky, on passing through the clouds their thermometer rose ten degrees. this, doubtless, must very often be the case. the air above the clouds must often be warmer than that below. "i think you all must have noticed illustrations of this principle on a small scale. have you not seen that snow and ice often melt around straws and sticks, the snow or ice remaining still frozen at a little distance, as if the sticks and straws were warm and had melted them? have you not seen a dark-colored board covered with ice, and the ice remain firm till the sun shone upon it, and then the ice melt upon the under surface, leaving the upper surface unaffected?" "i have seen such things a great many times," said peter, "and wondered what the reason was." "the reason is that ice is _diathermic_. heat passes through the ice without warming; but when the rays of heat fall upon the stick or stone or board, the heat is absorbed, the dark body is heated and in turn warms and melts the ice. in the same manner the atmosphere is warmed. the heat-rays of the sun pass through the atmosphere and fall upon the surface of the earth; the earth is warmed, and in turn warms the air resting upon it. "the gases and watery vapor contained in the air also hinder the radiation of heat from the earth. pure atmospheric air is perfectly diathermic to both luminous and dark heat, and vapors and gases are also diathermic to luminous heat. but to dark heat some of the gases are almost impenetrable. ammonia stops dark heat almost completely. in a smaller degree watery vapor does the same. gases and vapors thus serve as blankets to keep the earth warm. the heat of the sun, being luminous heat, penetrates the atmosphere with its vapors and foreign gases, and falls upon the earth almost without loss, but, being absorbed by the earth, it becomes dark heat, and cannot be radiated back through the same gases and vapors. vapor serves thus as a valve: it admits the heat of the sun to the surface of the earth, but prevents its escape. prof. youmans calls watery vapor the barb of heat; it catches the heat of the sun and holds it fast. "who can sufficiently admire the simplicity of these arrangements for preventing the radiation of heat into the stellar regions?--and their efficiency is no less admirable than their simplicity. arrangements like these show that the creator had a definite object in view, and that object is benevolent. for the advantage and enjoyment of the inhabitants of this world these arrangements were made. "we ought at this point to look at those adjustments by which the earth receives just the amount of heat needed to maintain the requisite temperature. the importance of maintaining some certain average temperature cannot be over-estimated. every animal and plant has its own _habitat_--that is, its natural dwelling-place or location--outside of which it perishes or maintains a stunted and precarious life. the habitat of animals and plants depends in a very great degree upon temperature. what a panorama would be seen if we could fly like a bird from the equator to the poles, and look down upon the ever-changing animal and vegetable life as we pass! how the luxuriant vegetation and flaunting colors of the tropics would shade off into the scantier vegetable life and more sober hues of the temperate zones, and these in turn die out and disappear in polar barrenness! we should see the lion and tiger give place to the bear and the wolf, the elephant and camel to the ox and horse, and these to the white bear and reindeer. this sublime panorama we see, in miniature, in ascending lofty mountains in the tropics. around the base of the mountain flourish the rich and various productions of the torrid zone; a few thousand feet of elevation bring us among the productions of the temperate zones. the most valuable fruits and grains thrive. then vegetation becomes scanty and stunted, and at last disappears. the top of mt. washington, feet high, in latitude forty-four degrees, is as bare of trees and plants and every form of vegetation as the north pole. "the fitting temperature is almost as necessary to the animal tribes as to vegetable life. animals which are native to the tropics do not thrive in colder countries, or if the difference of temperature be very great, they perish. a change from a cold to a warm region is equally disastrous. man indeed transfers animals from their natural habitat by protecting them from the extremes of temperature, but this is, of course, no exception to the general principle of which i am speaking. a change of only a few degrees in the mean annual temperature would render this earth a hard place for even the human race to subsist. but the temperature of the earth depends upon many a wise adjustment--how many, we cannot tell. will you tell us, samuel, the first adjustment or arrangement upon which the temperature of the earth depends?" "it must depend chiefly i think upon the intensity of the sun's heat." "whether or not that be the chief adjustment by which the right temperature is secured, it is at least a very important item. the intensity of the sun's heat must, of course, be considered in connection with its distance from the earth. the distance of the sun is no less important than the power of his rays; indeed, in one sense, it is more important, for if the intensity of the sun's heat were doubled, the temperature of the earth would be increased only twofold; whereas, if the earth were brought to one-half its present distance from the sun, the heat would be increased four times. heat being one of the radiant forces, its intensity diminishes in proportion to the square of the distance through which it acts. if the earth were , , of miles from the sun instead of , , , as it now is, the force of the sun's rays would be diminished fourfold. the creator has so fixed the distance of the earth and sun, and the power of the sun's heat, as to give to this world a temperature suited to its various inhabitants. "the temperature of the earth has also some dependence upon our atmosphere. can you tell us, ansel, how the temperature of the earth is affected by the atmosphere?" "you have already told us that the atmosphere is _diathermic_, allowing the heat of the sun to fall upon the earth almost undiminished in force. if the air were so constituted as to intercept the sun's rays, it is plain that the earth would receive less heat." "this adaptation of our atmosphere to transmit the sun's rays," said mr. wilton, "is more subtle than it appears at first sight. it is not merely a matter of depth and density, though those are important considerations, nor is it merely a question of the elements of which the atmosphere is composed. simple gases are _diathermic_. the atmosphere is therefore made up of two simple gases, oxygen and nitrogen, not chemically combined, but mixed together. compound gases intercept the passage of heat. ammonia, composed of hydrogen and nitrogen chemically united, almost wholly stops it. even ozone, which is nothing but oxygen in a changed or _allotropic_ state, is not _diathermic_. the _diathermic_ quality of the air depends, then, not only upon the fact that it is composed of simple elements mingled, but not chemically joined, but also upon the _state_, or _condition_, of those simple elements. "another point deserves attention. oxygen is an element having a wide range of very strong and active affinities. it is ready to unite with every known substance, fluorine excepted. what if some other equally active element were mingled with oxygen to form the atmosphere? what if, in place of nitrogen, vapor of sulphur were substituted? what if hydrogen were put in the place of nitrogen? the two elements would combine in sudden combustion or explosion, and the atmosphere itself would perish. but nitrogen is a substance so sluggish and inert that it can be brought into union with oxygen only by indirect processes. because the air is composed of one so inert element as nitrogen, the atmosphere is preserved, and, what is almost as important, it is kept, as it now is, composed of simple elements, and hence _diathermic_. if our atmosphere were a compound gas, the world would perish with cold. "the temperature of the earth depends also upon certain qualities of the earth's surface. i should be glad to have mr. hume explain this." "i suppose," answered mr. hume, "that you refer to the qualities of the earth as an absorbent and conductor of heat. the earth must needs have the capacity of receiving and retaining the heat which falls upon it from the sun. if the earth's surface were polished and brilliant, the heat of the sun would be reflected into space as from the surface of a mirror, and very small advantage would the earth receive from the solar heat. a dark soil absorbs heat more readily than a soil of lighter color, and a wet soil, on account of the high specific heat of water, requires more heat to raise its temperature than a dry soil. the mineral elements of the soil and its compactness or porosity also help to make up its capacity for receiving and retaining heat. the color and constitution of the soil sometimes go far toward making the climate of a region. the conducting qualities of the earth's crust in its profoundest depths also must be taken into account. if the crust of the earth were composed of silver, or any other substance of like conducting quality, and the interior of the earth were molten rock, as it now is, the interior heat would be so rapidly conducted to the surface that everything upon the earth would be consumed." "upon so many circumstances wisely adjusted and nicely blended," said mr. wilton, "does the temperature of the earth depend. the intensity of the sun's heat, the dimensions of the earth's orbit, the constitution of our atmosphere in the subtlest qualities and relations of its elements, and the material, structure, and color of the earth's crust,--on all these and many other things which i cannot stop to mention depends the temperature needful for the well-being of the inhabitants of this globe. i beg your pardon, mr. hume, but allow me to ask whether such a combination of agencies and conditions, uniting to work out good for man, does not seem to you quite superhuman and worthy of a wise and good creator?" "i cannot deny it, sir," he replied; "i am not prepared to make any objections. there are many things painful to man in the vicissitudes of heat and cold, and if i were to make a world, i suppose i should leave them out, or perhaps make the world upon a very different plan. but i am not prepared to affirm that any changes which i could make would be improvements, though i have thought until recently that more of knowledge and power, and perhaps more of chance, too, than of wisdom and goodness, were displayed in the works of nature. but i must confess my opinion has been much modified." "i think your change of mind is in the right direction, and i am glad that it is so. we learn the secrets of nature and appreciate her spirit much better when we come as reverent questioners than when we come with preconceived notions and a patronizing air. i can well understand your feelings and state, for i myself have traveled over the same ground. my eyes were once dazzled with the glories of science; i worshiped at the shrine of natural laws. but i have learned that god is greater than nature, the creator is mightier than the creation. nature has no mind or purpose apart from the plan and will of the supreme architect and ruler, and this inner plan and purpose of nature is seen only in the government and discipline of our sinful race. i shall greatly rejoice for you and with you if you shall go on to the same end which i have reached." "i shall much rejoice if i reach some satisfactory and peaceful conclusion." "to understand the management of heat," said mr. wilton, "we must take note of the differences and fluctuations of terrestrial temperature. the sources of heat are constant. the sun sends out its flood of heat uninterrupted and changeless for ever. the internal fires of the earth give an even inward heat. mechanical and chemical agencies are active everywhere. these sources of heat do not fluctuate, flaming up and dying away, yet temperature is the most variable of all inconstant things. in passing from equator to pole we go from torrid to frigid, from everlasting summer to everlasting winter. and not only this, but in the same region the temperature never remains the same for even twenty-four hours. the thermometer may pass from forty degrees above to thirty below zero in a very few hours. we must first consider the agencies by which these inequalities are produced. ansel may mention the first of these." "the shape of the earth," said ansel. "how does the form of the earth operate to produce inequality of temperature?" "the earth is a sphere, and the rays of the sun fall upon it in nearly parallel lines. upon the centre of the hemisphere which is turned toward the sun the rays fall perpendicularly, the sun is directly over head, while toward the edges of the hemisphere, on account of the curvature of the earth's surface, the rays fall more and more slanting, as if the sun were sinking toward the horizon." "what is that inequality of temperature which is produced by the shape of the earth?" "the five zones," answered peter. "this subject is so well understood," said mr. wilton, "that i need not spend time in explaining it. every boy knows the difference between setting his wet slate before the fire to dry so that the heat will fall squarely and perpendicularly upon it and placing it edgewise to the fire. upon the torrid zone the sun shines perpendicularly, upon the temperate zones obliquely, and upon the frigid zones still more obliquely, and during a part of the year the sun is entirely hidden. in proportion as the rays of heat fall obliquely, any given amount of heat is spread, so to speak, over a larger surface, and the larger the space over which it is spread, the feebler it becomes. what is another cause of inequality of temperature?" no one answered. "samuel, what is the cause of day and night?" "the turning of the earth upon its axis." "and the rotation of the earth upon its axis," continued mr. wilton, "brings not only an alternation of light and darkness, but also of heat and cold. the heat of the sun is withdrawn along with the light. the heat of the sun is not withdrawn from the earth, but one-half of the earth's surface is constantly turned away from its influence. this must produce a daily change of temperature. this diurnal fluctuation of temperature may be very small or it may amount to seventy or eighty degrees. samuel, what is a third cause of unequal temperature?" "the inclined position of the earth's axis and the revolution of the earth around the sun cause the change of seasons." "if it were not for this, the earth would still have her zones of seasons; a part of the earth would have endless summer, a part endless spring, and the rest unbroken winter, but the alternation of seasons at the same place would be unknown. the axis of the earth is now inclined about twenty-three degrees, twenty-seven minutes, twenty-three seconds to the plane of the earth's orbit, and as this axis maintains constantly the same position, being parallel in one part of the earth's orbit to its position in any other part of its orbit, during one part of the year the north pole is turned twenty-three and a half degrees toward the sun, while in the opposite part of the year the south pole is in like manner brought into the light and heat. this causes the sun to appear to move to and fro, north and south, twenty-three degrees, twenty-seven minutes, and twenty-three seconds from the equator in either direction. the tropics, or turning-places, mark the limits of the sun's northern and southern journey. everywhere between the tropics the sun, at some period of the year, passes through the zenith, that is, exactly overhead at noon. north and south of the tropics the sun seems to rise higher in summer and to sink lower in winter. in summer the sun at midday is about forty-seven degrees nearer the zenith than in winter. within the polar circles, which are the same distance from the poles as the tropics from the equator, the heat of the sun is entirely withdrawn during a portion of the year, and during another portion of about equal length the sun does not set. the extremes of temperature, caused by the inclination of the earth's axis and its revolution around the sun, are very great. in the northern part of minnesota, the temperature rises in summer to one hundred degrees, and in winter sinks to fifty degrees below zero, giving thus an alternation of one hundred and fifty degrees. "in this connection you may also remember that the sun is nearer the earth in one part of its orbit than in another part. this difference amounts to about , , miles. the sun also remains eight days longer north of the equator than south of it. our summer, therefore, is eight days longer than the summer of the southern hemispheres, and our winters are correspondingly shorter. these differences tend, however, to balance each other, for while the southern summer is shorter, the sun at that time is nearer, and while our summer is longer, the sun is more distant. peter, you may explain to us the effect upon temperature caused by the division of the earth's surface into land and water." "i learned while studying physical geography that the temperature is more even upon the sea than upon the land. but why, i do not know." "the smooth surface of the sea reflects heat better than the rough land: for this reason, a larger proportion of the heat which falls upon the sea is not absorbed, but reflected and lost, so far as the temperature of this world is concerned. water is also a very poor conductor of heat, and has withal a very high specific heat. for these reasons the sea receives and parts with heat more slowly than the land, and its absorption or radiation causes a smaller variation of temperature. the result is, therefore, that the sea is cooler in summer and warmer in winter than the land, and the average ocean temperature is lower than the mean continental temperature. the land receives heat more readily and parts with it more rapidly; the fluctuations of temperature must therefore be greater. hence, the interiors of the continents have much greater extremes of temperature than the sea-board. but of the influence of water in equalizing temperature i shall have occasion to speak again more at length, and will pass it by for the present. what effect, peter, has the unevenness of the earth's surface upon temperature?" "the higher we ascend upon mountains, the colder we find it." "that is, peter, the greater the elevation of any place or country above the sea level, the lower the temperature. almost the whole surface of the earth is an alternation of mountain and hill, valley and plain. one continent has a very much greater mean elevation than another. one region or tract of country lies sloping toward the sun, another is inclined from it. the effect in the one case is the same as if the sun were brought more nearly overhead; in the other case, the sun is depressed toward the horizon. it is all the same as if the region of country were brought nearer the equator or removed farther from it. the effects of the curvature of the earth are obviated or exaggerated. do clouds tend to produce inequalities of temperature?" "i think they must do this," answered samuel. "clouds cover one portion of the earth's surface and shut out the heat of the sun, while other portions are well exposed to the sun's rays." "that is right, samuel. does any one think of another cause of inequality of temperature?" there was a pause. then mr. hume answered: "considering the unmeasured cycles of the past, the gradual cooling of the earth has brought a great change of temperature." "and this change," continued mr. wilton, "has been very important for the welfare of the human race. at the present temperature of the earth, the coal-beds, so necessary for the culture and progress of the race, could hardly have been formed, and at the temperature of the carboniferous periods, when the coal-beds were deposited, the human race could with difficulty have survived. the high temperature required to prepare the earth for man is now no longer needed, but would prove destructive. and this great change of temperature was doubtless caused by the cooling of the earth. "the result of all these agencies--the shape of the earth, its daily and yearly motions, the inclination of its axis, the eccentricity of its orbit, the division of its surface into land and water, the varying elevation of its surface, and the clouds and storms that hide the sun--is that we have great extremes and rapid transitions of heat and cold, and every variety of climate. these changes of temperature are often painful and, unless guarded against, dangerous. yet, taken as a whole, can one doubt that variety of climate and change of temperature are of advantage to man? what weariness and lassitude a changeless temperature would bring! how the cooler air of the night comes as a tonic after the relaxation of the heated noonday! who can estimate the value of our northern winters, not alone in building up a vigorous and nervous physical frame, but in helping the culture of men and nurturing the domestic virtues? we might almost say that her winter evenings have been the making of new england. but periods of heat are needed for bringing fruit and grain to ripeness. what variety and richness of productions for the use of man the different zones furnish! the supply of man's wants would be comparatively meagre if we had but one zone, even though we had our choice of the zones. but every zone is necessary for the perfection of the temperate zones. that we may have the warmth of summer in the temperate zones we must have the torrid zone. that we may have the tonic cold of the temperate zones we must needs have the severity of polar winters. i do not mean that the creator could not devise a world that should not have these painful extremes, yet enjoy the advantages of the temperate regions. but that would plainly require a world constituted upon principles very unlike those which now prevail. with god this is doubtless possible, but the mode is to us inconceivable. but we can easily see that by the present arrangement of things god has secured many great advantages for man--how many and how great, we can hardly understand--and the apparent disadvantages we cannot positively affirm to be real evils. we can safely declare that this world is well adapted to man's necessities. but these inequalities of temperature are modified and softened by a most comprehensive and beneficent system of agencies by which the extremes are prevented from becoming destructive. in this system of compensating agencies two great divine ideas are clearly developed, economy in the expenditure of heat and benevolence toward man. upon this subject we are now prepared to enter." chapter viii. modification of temperature. resuming the subject where it was left the previous lord's day, mr. wilton said: "we saw at our last session that the most prominent and permanent features of the earth tend to produce differences and great extremes of temperature. these variations of temperature within due limits must be regarded as beneficial, if not absolutely essential, to the well-being of the human race. the different zones give the world a richer and more varied supply of food, and finer and more varied plants and animals. the change of seasons gives variety in the experience of life; the warmth of summer ripens the fruit and grain, and the cold of winter tones up the physical strength; nay, the winter's frost is a natural subsoiler, loosening up the hard earth and promoting vegetable growth. as for man's higher interests, no one can tell how much the world is indebted to winter evenings, to a period of darkness longer than is needed for sleep, and a period of cold during which the work of husbandry may largely cease. learning, the domestic virtues, and religion are greatly indebted to our winters. but were these agencies which tend to produce inequality of temperature suffered to operate without counteracting influences, the extremes of heat and cold would cease to be genial and healthful, and become destructive. we are now to begin the consideration of those counteracting agencies by which the extremes of temperature are moderated. "let us look first at the daily fluctuation of temperature caused by the revolution of the earth upon its axis. the rotation of the earth brings every place by turns under the influence of the sun's rays, and in turn withdraws it from the heat of the sun, thus producing a daily change of temperature. how is this diurnal change of temperature alleviated?" this was addressed to all, but no one answered. "mr. hume, i should be glad to have you suggest the answer." "there are two chief agencies," mr. hume replied--"first, the absorption of heat during the day and the radiation of that heat during the night; and, secondly, the formation of watery vapor during the day and the deposition of dew by night." "the first of these agencies," said mr. wilton, "is so plain that very little explanation need be made. during the day, while the sun is shining and the temperature is rising, the surface of the earth, the rocks, the trees, and all things are absorbing heat. this heat is, so to speak, laid up in store, ready for use in time of need. in due time the sun sinks low and sets behind the horizon; the supply of heat is cut off and the temperature begins to fall. then all those objects which during the day were laying up heat in store begin to radiate heat into the air, and by their contact with it keep up its warmth. commonly, the temperature falls so low that bodies radiate more heat than they absorb before the setting of the sun. in this process water plays a very conspicuous part. you will call to mind what was said before about the large specific heat of water. by means of this, water is able to store up heat in large amounts--larger in proportion to its weight than any other substance except hydrogen gas. the heat that is stored up during the day is given off by contact with the air and by radiation during the night. "but water plays a still more important part in moderating the daily fluctuations of temperature by the process of evaporation and the formation of dew. call to mind what was said of the formation of vapor when we were speaking of latent heat. heat water to two hundred and twelve degrees--the boiling point: it must still be heated a long time before it evaporates. boiling water must receive five and a half times more heat to give it the form of vapor than to raise it from the freezing to the boiling point; that is, about one thousand degrees of heat are required to turn boiling hot water to vapor. the same amount of heat is required for the formation of vapor whatever the temperature of the water from which the vapor rises. there is only this difference--vapor from cold water is cold, while vapor from hot water is hot. evaporation goes on more rapidly in proportion as the temperature rises, but vapor is formed at all temperatures. evaporation goes on from ice. the alpine glaciers, or rivers of ice, sink away several feet by evaporation from their surface during their slow course of many years down the mountain ravines. this process of evaporation goes on, i say, during the day, and in the formation of vapor an amount of heat which would raise an equal weight of water through one thousand degrees of temperature is used up. "this vapor which is formed is not supported _by the_ air, as men commonly suppose. it is true that clouds are held up by the atmosphere, but clouds are condensed vapor--minute globules of water floating in the air. vapor is invisible. you must have noticed that steam is invisible till it is condensed by contact with the colder air. vapor rests upon the earth and supports itself by its own elastic force, just as the atmosphere supports itself. the presence of air makes no difference with the formation of vapor, except that in a vacuum vapor forms very much more rapidly, because no air stands in its way. but at any given temperature, in the air or in a vacuum, the same amount of vapor rises in due time, and the same amount can support itself. vapor seems to circulate between the atoms of air, as sand fills the spaces between marbles. at the temperature of four degrees below zero vapor equal to two-thirds of an inch of water can be formed and support itself by its elasticity; that is, the elastic force of vapor at four degrees below zero is equal to two-fifths of an ounce per square inch; at thirty-six degrees vapor equal to two and two-thirds inches of water can support itself; at eighty degrees vapor equal to thirteen inches of water can exist; at one hundred and seventy-nine degrees, seventeen feet; and at two hundred and twelve degrees nearly thirty-four feet; that is, vapor at two hundred and twelve degrees has an elastic force of fifteen pounds to the square inch. let us suppose that at sunrise the air has a temperature of thirty-six degrees, and that as much vapor is already formed as can sustain itself at that temperature. as the sun sheds down his rays the temperature rises and more vapor is formed. we will suppose that half an inch of water is evaporated. some of this vapor will be carried by ascending currents of air into the higher regions and condensed into clouds, some will be carried by winds into drier and warmer regions, yet the amount of vapor will increase during the day. we will suppose that during the night the temperature falls again to thirty-six degrees; all the excess of vapor above two inches and two-thirds of water will be condensed and become dew or fog, and in this condensation the thousand degrees of heat absorbed in the formation of the vapor will be given out again. if vapor equal to one inch of water be condensed, heat is set free sufficient to boil a sheet of ice water, five and a half inches in thickness, extending over the whole region; that is, it would be all the same as if a fire were kindled on every square rod of land hot enough to boil during the night more than twenty barrels of ice water. in this illustration i have supposed a larger condensation than commonly takes place, but very much less than is conceivable. suppose that the temperature is eighty degrees, and that, as is possible, more than one foot of water exists in the state of vapor. let the temperature fall to thirty-six degrees, and full ten inches of water must be condensed, setting free heat which would boil four and a half feet of ice water. so large a condensation as this never takes place in twelve hours, partly because the full amount of vapor which might be formed is never actually produced, and partly because the condensation of but a small part of this vapor would check the fall of temperature and prevent farther condensation. the supposition that i have made shows the possibilities of this method of moderating extremes of heat and cold. were it not for these processes, our days would be much warmer and our nights much cooler than they now are. by the formation of vapor the excess of heat during the day is stored up in a latent form; that is, it is used, not as heat, but as force, and is employed in bringing the atoms of water into new relationship; during the night the vapor returns to its former state as water, and the heat-force again becomes sensible heat. thus the day is cooled and the night made warmer. "ansel, have you ever heard the 'dew point' spoken of?" "yes, sir, i have." "do you know what is meant by it?" "that point or degree of temperature at which dew begins to be formed." "upon what does the dew point depend?" "upon the amount of vapor in the air." "that is right, ansel. if at any time the full possible amount of vapor should exist, any diminution of the temperature must, of course, cause dew to be deposited. do you know, ansel, how to ascertain the dew point at any time?" "no, sir, i do not." "there is a beautiful instrument known as daniell's hygrometer which shows the dew point as a thermometer shows the temperature. but any one can easily determine the dew point without a special instrument for that purpose. pour warm water into a glass pitcher or goblet whose outer surface has been wiped perfectly dry, and polished. into this set a common thermometer. cool down this warm water by dropping into it small pieces of ice, and notice carefully when the polished glass begins to be dimmed as if it had been breathed upon. when that begins to take place the thermometer will show the dew point. in this manner we can determine the amount of vapor in the air, and by estimating the probable temperature of the night judge of the probability that dew will fall." "i have noticed some things," said peter, "about the formation of dew which i do not understand, and i wish very much to ask about them." "i should be glad to hear your questions, and will answer them if i can." "i have noticed that dew falls on clear nights, but not very often on cloudy nights. i don't see why that is so." "have you ever noticed whether cloudy nights or clear nights are the warmer?" "cloudy nights are commonly warmer, i think, but i never could see the reason for that, either." "can you tell why a newspaper spread over a tomato vine keeps the frost from the vine?" "because the frost comes upon the paper instead of the vine, of course." "but why do you say, of course? why does not the dew--for frost is nothing but dew frozen as it forms--come upon the under side of the paper?" "how could the dew fall upon the under side?" "that is just the point which we need first of all to understand. men commonly speak of dew as if it fell. i don't know but i have spoken of the falling of dew in this lesson. but dew does not fall at all. the vapor simply touches some cold object, and is condensed upon it. the vapor by its elasticity presses against the cold body, and the process of condensation continues until either the body is warmed by the heat set free so that its temperature rises above the dew point, or till the vapor is so far exhausted that the dew point falls below the existing temperature. dew is formed upon the upper surface and not upon the under, because the upper surface is cool and the under surface is warmer. beneath the paper spread over the tomato vine, the earth is radiating heat and the paper is radiating it back again. if the paper were not there, the heat would be radiated into space and not returned again. the vine would soon radiate away its little store of heat, its temperature would sink, below the dew point, and dew or frost would be deposited upon it. the under surfaces of objects are kept warm by the radiation from the earth. in the same manner clouds are wrapped around the earth and keep it warm by radiating back its radiant heat. dew is not formed on cloudy nights, because they are warmer: the clouds throw back the heat which otherwise would be lost in open space." "i never knew before," said peter, "that clouds were of any great use except to send down rain." "we shall see in the course of our lessons that clouds are of very great use in warming the earth in other ways, as well as by serving as blankets and radiating back the heat which otherwise would escape." "i wanted also to ask why dew falls--i mean, is formed--on grass and leaves of plants while stones are dry." "i will answer your question by asking another. did you ever see barefoot boys running in the cold dew stop and stand upon a stone or rock to get their feet warm?" "oh yes, sir; i have done it myself." "why did you stand upon a rock?" "because i had learned that the rocks would be warm." "i think that answers your question. the rocks and stones are warmer than the grass and the leaves. the blades of grass and the leaves are thin and pointed and rough, and have a very large radiating surface. they have but little heat, and that little they part with rapidly. the rocks and stones, on the other hand, are bulky, and contain a much larger store of heat, their radiating surface is comparatively small; only one side is exposed, the other being covered by the warm earth, from which they are drinking in heat almost as rapidly as they lose it. they therefore do not lose heat enough to sink their temperature to the dew point. "so much, then, for the means employed to moderate the changes of temperature from day to night and from night to day. but upon the sea-coast and upon certain islands of the sea another agency is employed. will some one suggest what this agency is?" no one else answered, and finally mr. hume said: "i suppose, of course, that you refer to the land and sea breezes?" "this is what i had in mind. during the day the land is warmer than the sea, and the breeze from the sea blowing upon the land cools the air. during the night the land radiates its heat more rapidly than the water, and soon the sea becomes the warmer. then a breeze springs up in the opposite direction; the cooler air of the land flows out upon the sea. by this means the air upon the land and the air upon the sea are daily commingled, thus securing a more even temperature upon the land. this softens the extremes of daily temperature. i make only this brief reference to the land and sea breezes, because in another connection we shall examine the general subject of winds and their influence in the equalization of temperature. "the result of all these influences is that the changes of temperature from day to night and from night to day, while not inconsiderable, are by no means destructive, and in many cases are no greater than is refreshing and agreeable. these agencies remind us every day of the wise provision of the creator for the well-being of his creatures. 'day unto day uttereth speech and night unto night showeth knowledge. there is no speech nor language where their voice is not heard.' this care for the earthly well-being of men is but a type of his care for their spiritual happiness. the plan of salvation, and the ways of divine providence working in accordance therewith, are more wonderful both in their means and their end than the greatest of the works of nature. if while we study the natural we forget the supernatural, we commit the greatest mistake: we pass by the greater to examine the less. the natural is valuable only as it leads to the spiritual." chapter ix. the ministry of suffering. "you must know, mr. wilton," said mr. hume, "that my mind is full of objections, whether i speak them out or keep silence. i have looked so long upon one side only that i find it hard to look upon other sides also; and if there be a satan, as the bible teaches, i think he must be marshaling all his legions to overwhelm me by the force of his impetuous assaults. i cannot disguise the fact--i do not attempt to disguise it--that my mind is not at ease. it used to be at rest, at least comparatively so--not happy, yet not agitated and distressed. my heart was not satisfied, but i believed that my position of unbelief was logically impregnable. but i confess it, my unbelief has of late been shaken. i am no longer contented. how i came into this state, i do not know. i am certain that my present unrest was not produced by the force of arguments which i had heard. it seems to me as if it sprang up uncaused. the old arguments which i have thought impregnable do not now satisfy me. why, i cannot tell. i think this statement is due to you to explain my position in your bible class, and also to prepare the way for a question which i wish very much to propose. i have no objections to make to the marks of wisdom and benevolent design seen in the works of creation which i cannot myself answer and remove. good-will and goodness to the inhabitants of the earth lie on the very surface of things; or, if i go beneath the surface, i find them no less manifest in the profoundest and subtlest arrangements of the universe. if i say, 'this is all the work of chance,' my very language is self-contradictory and looks me out of countenance, for the very idea of chance is the opposite of wise and orderly arrangement. the difference between design and chance is that the one works by orderly arrangements adapted to the accomplishment of a foreseen end, while the other shows itself in chaotic disorder, with no adaptation to the accomplishment of a purpose. to say that a universe like this, filled in every part with order and beauty, with subtle and unseen elements and agencies working out into the boldest relief in the accomplishment of beneficent ends, all minute elements blending in the sublime sweep of the universal plan,--to say that such a universe is the work of chance is to use language without meaning. "if i deny a providential plan in the creation and government of the world, and attribute to brute matter a nature that, by its own inherent force, spontaneously develops into all these contrivances of use and beauty, i see that the wisdom of the whole universe is concentrated in the nature of matter, and, if it be possible, infinite subtlety of design is doubly manifest. to create a machine which, upon its elements being thrown into an indiscriminate pile, shall arrange itself, adapt part to part, and set itself in motion; which shall repair all its breaks, produce other machines as curious as itself, and thus reproduce itself and perpetuate its existence for ever--that would certainly be the acme of intelligent design. "or if i go farther and deny a creator, ascribing to the universe an eternal, uncreated existence, i see that i only entangle myself in a complication of difficulties. i find myself standing face to face with the best-established facts of geology. if the fact that the animal tribes which inhabit the earth, and especially the human race, had a beginning be not well established, then no fact in geological science can be reckoned as fixed. geology has overturned the idea of an infinite series of generations of animals and men. nor do i see that i gain any advantage or give any explanation of the universe by attributing to matter everything which others refer to an intelligent and almighty creator. the distinction between mind and matter is that mind is endowed with intelligence and will, while matter has neither intelligence nor will, but only blind forces, blind attractions and repulsions. if i attribute the order, beauty, design, and benevolence of the universe to mere matter, i clothe matter with the attributes of spirit. in fact, i only set up another god and ascribe to the universe a true divinity. i make myself a kind of pantheist, investing all matter with the attributes of mind and spirit. all this i have pondered over for many a day, and i cannot deny that a belief in an intelligent creator of the universe is logically more satisfactory. but there is one question which confronts me at every turn. i suppose that i might at length work out an answer for myself and that i should now see the explanation if all my thinking for so many years had not been upon the other side." "i am afraid that i shall not be able to give you satisfaction," replied mr. wilton, "but i shall be glad to hear your question. i can at least appreciate your state, and sympathize with you in your groping and struggling. i am glad that you are walking the road you have just described. you say that you do not know what has brought you to your present state. i can easily tell you: your experience at this point is not singular; i think the holy spirit of god has been leading you and has brought you to your present position. i trust in god that he will lead you still farther. you have great cause for thankfulness and great cause for trembling. let me caution you: be careful how you treat the divine spirit; walk softly; be honest, sincere, and simple-hearted as a little child. 'except a man become as a little child, he cannot see the kingdom of god.' above all things, be sincere and straightforward. deal truly and frankly with the spirit. if you will only be honest and frank,--honest and frank to yourself, honest and frank to all men, honest and frank with god,--god will soon give you cause to praise him and love him for ever and ever. but what is the question which you wished to propose?" "my difficulty is this: along with the many arrangements for conferring enjoyment and promoting the well-being of man are other arrangements for suffering. man is made as capable of suffering as of enjoyment, and there are appliances provided which are certain to inflict that pain of which man is capable. how is this provision for suffering in man and in all sentient creatures consistent with the benevolence elsewhere shown? how are we to combine these two sets of arrangements in our thinking?" "a full unfolding of the ministry of pain in the good providence of god would lead us entirely aside from our course of study." "but for me," said mr. hume, earnestly, "it would be not at all aside; for if i can once see that the provision for suffering made in the constitution of man and of nature is not repugnant to the idea of a wise and good creator and disposer of human affairs, i will admit whatever you shall have to say afterward, and i shall feel that the gospel of christ comes to man and comes to me with a moral force which ought not to be resisted. i know that i have no right to come into your class and ask you to turn aside from your course of study, and the gospel certainly owes nothing to me, yet i do hope you will give the opinions which you hold upon this subject, if you have formed any positive opinions." "i am sure," exclaimed peter, "that we shall all be very glad to have you spend the time of this lesson in speaking of this subject." "but how would it please you if my talk upon the ministry of pain should prove to be very much like a sermon?" "i think we like your sermons. i know that we were never so much interested in them as now." "very well, then; i will give you, as mr. hume says, some of my conclusions touching this matter of pain and suffering; and if my opinions are not satisfactory or do not cover the facts in the case, it will not be because i have given the subject little thought or have had little experience of suffering. the lord has led me by a rugged road; he has given me tears to drink and mingled my cup with weeping. but for this i thank him, and i expect, when i shall look back from the life to come upon my earthly course, to see my days of pain and grief shining more brightly than the hours of radiant sunshine. "first of all, then, i believe that with the clear exhibition of benevolent design which we see in this world we ought not to doubt the goodness of the creator, even if we can give no rational explanation of the suffering which abounds. we ought not to believe, we cannot believe, that the creator's own attributes are self-contradictory and antagonistic, that the same infinite being is both good and evil, partly benevolent and partly malignant. if god is good at all, he is wholly good. nor can we believe that a good being and an evil being--god and satan--hold joint sway over the universe and co-operated in the work of creation, and that the good is to be ascribed to the one, and the pain and suffering to the other. whether we can explain it or not, we must believe that there is a good reason for the existence of suffering; unless, indeed, we count the infliction of pain the chief end of the creation, and refer the happiness which men enjoy to some incidental arrangements not contemplated as important in the work of creation. but no sane man can think that this world is the work of a demon seeking to fill the earth with groans and wretchedness. our consciences and our reason alike require us to believe in the supremacy of goodness. "in presenting my views, i of course cannot attempt to prove everything from the beginning: i must take some things for granted between us. we must start with the admission that there is a god, and that he is a righteous, moral governor. we must at least believe what paul declares to be needful: 'he that cometh to god must believe that he is, and that he is a rewarder of them that diligently seek him.' we must also believe our own consciences when they testify that men are responsible, free moral actors, and that sin and guilt are not false notions arising from diseased and morbid mental conditions, but realities, true ideas which arise in the mind when it works as god designed. do you freely admit these points of belief, mr. hume?" "yes, sir; i could not ask you to prove every point touched upon in the argument, for that would require half a score of volumes, nor will i deny the testimony of my own conscience that there is a god, and that men are rightly responsible to him." "starting, then, with these fundamental principles, we will look first at the provision made for physical pain. men and, i suppose, all living creatures are created with the capacity of suffering. the same nerves of sensation which if excited naturally give rise to pleasure may be excited unnaturally and inflict pain. but why not endow living creatures with nerves of sensation which could experience pleasure, but could not feel pain? is this possible? perhaps so, but no man can affirm it with certainty. i do not think that any man can clearly conceive such a thing. to us the capacity of enjoying and that of suffering seem inseparable. but there is no need of insisting upon this point, for the capacity of feeling pain is a most benevolent provision of the creator for the benefit of living creatures. it is designed to save life and limb. pain is the sentinel set to guard the outposts of the citadel of life. if there were no pain, men would thrust their hands into flames without knowing it. they would indulge in all manner of destructive excesses, and no sufferings would warn them of danger. they would drink poison, and no pain would bid them make haste to take the antidote. tear men limb from limb, hew them in pieces with the sword, and no painful sensations would rouse them to self-defence. without this benevolent provision of pain the race of man could hardly be saved from extinction. how much more would this be true of the animal tribes, which are wholly dependent on instinct for guidance and impulse to action! we accordingly find pain possible in those parts of the body where pain can subserve the purpose of protection; elsewhere no provision is made for pain. nerves of sensation abound in those parts which require especial care or are especially exposed. the skin is exposed, therefore the skin is well supplied with nerves. the parts beneath the skin are less exposed, and are injured only by first wounding the skin; they are therefore less sensitive. the heart, though so very important, is almost insensible to pain, because the capacity of suffering at that point would confer no protection. the eye is delicate and requires the greatest care, and to secure that needed care the creator has made it delicately susceptible of pain. the sole of the foot, as its work demanded, was made capable of bearing the roughest usage, and hence the sole of the foot is but little supplied with nerves of sensation. still farther, when on account of injury any part of the body requires unwonted care, provision is made that the injured part shall become especially sensitive. a bone when well and sound may be cut or sawed almost without pain, but when the bone is injured it becomes inflamed and feels pain most keenly. when a limb for the sake of its own safety ought to be kept quiet, nature makes it painful to move it. for the benevolent object of preserving life and guarding the well-being of living creatures pain is given. the provision for pain shows the presence of danger, the liability of receiving injury, and the kind design of putting men on their guard. it is the automatic guardian of our happiness. this is all that i have to say about bodily pain. "mental suffering and pain of conscience are designed, first of all, to subserve the same purpose. the sense of guilt when a man commits a wicked act is designed, first, to lead him to repentance. it is the divine alarum placed within the soul to remind men that they have done evil and received moral damage which must be repaired. it is the moral goad which pricks men to warn them to turn from wickedness. if evil-doing were as pleasant as well-doing, men would see no difference between right and wrong; all moral ideas would be subverted and the glory and beauty of man would be trailed in the dust. "but a guilty conscience continues to trouble wicked men after the day of repentance has passed; remorse indeed seems to rise up with preternatural power when mercy has withdrawn for ever from the sight of hope. what is the meaning of this? it means that which we admitted in the beginning, that sinners are guilty in god's sight, that guilt is a real thing and deserves punishment, and that god, the holy and righteous king of men, does actually punish the guilty. god is holy and abhors sin. remorse of conscience is the shadow of the creator's frown, the voice of his eternal indignation echoing and re-echoing in the soul of man. it is the divine wrath penetrating the human spirit and making itself felt. as the holy god abhors sin for ever, the wicked must expect to feel that abhorrence for ever. he who puts himself into a rebellious position toward his creator must stand in that unnatural attitude guilty and suffering. we can conceive that this should be otherwise only by subverting the foundations of the moral world. beings created in the image of god, created with a conscience and moral affections, created with moral freedom, can attain blessedness only by aspiring to heavenly things and becoming god-like. if they break away from the divine will and order, they must suffer the divine frown, they must feel that frown. how can god make his frown felt except by looking pain, so to speak, into the sinner's conscience? "but this whole subject of pain and suffering derives a double significance from the fact that the human race is a fallen race, alienated from god by wicked works, yet under a merciful dispensation in which they are called to return to obedience. there is no moral quality good and beautiful to our eyes or pleasing to god in which men are not altogether lacking, and what is still worse, men grow in evil; their last state is worse than the first. there is no healing power in the man which can renovate his heart and bring him back to holiness. it would seem as if some satanic power were hurrying the human race along the road to ruin. if men are to be saved, it must be by a force of renovation outside of themselves, which shall reverse the evil bias of their nature. you say that the world seems fitted to develop man's capacity for suffering, and that this appears to be as much a part of the divine plan as the impartation of happiness. what, think you, would be the result if the human race were planted in a world where nothing could give pain, where everything would afford gratification? what, mr. hume, do you think the effect would be upon creatures such as we all know men to be?" "i hardly dare answer with the little thought i have given to the subject. i would rather listen than speak." "i have noticed," exclaimed ansel, "that those boys who have everything done to suit them at home are the most unmanageable in school and the most disagreeable to play with." "picture to yourselves," continued mr. wilton, "a man who from childhood should have nothing to suffer, no pain or weariness or hardness to bear. from childhood he has no bodily pain, and the comforts of life are so carefully and bountifully provided that he receives no unpleasant sensation. winter never chills him, summer never heats him. his slightest wants are all anticipated. all his sensations are pleasure. let the same be true of his mind. his will is never crossed; whatever he wishes is given him; there is no call for self-denial or self-control or abstinence or patience. he feels no pressure of need spurring him to exertion. his whole life is enjoyment. his very body would grow up, not strengthened and compacted for exertion, but fitted only for the softness of indolence and ease. his will would be the selfishness of self-will rather than an intelligent, reasonable self-control. there would be no tenderness and power of love, no endurance and patience in labor, no strength of moral purpose under temptation, no self-denial and self-sacrifice of love for the good of others or for the attainment of a higher blessedness, no faith in god nurtured in darkness and trial. we should have a mushroom growth of luxurious tastes and indolent ease, impulsiveness and impatience, strength only in selfish, passionate self-will and rampant, luxuriant vices. no other result would be possible with creatures like us. strength is developed only under circumstances which call for the exercise of strength. a certain hardness and hardihood of living is needed to develop a manly body. resolute intellectual exertion in the face of difficulties is demanded to educate the mental faculties. an earthly life not wholly satisfactory is needed to awaken in faithless men a longing for a better land. we may look upon the sufferings of this world, taken as a whole, as an expression of god's displeasure at sin. how very much is such an expression needed! if life were nothing but pleasure, how completely men would forget sin and duty and god and heaven! all the varied experiences of joy and sorrow, of good and ill, of trial and triumph, are needed for man's spiritual discipline. i think you will bear me witness that the noblest, sweetest, most beautiful characters are found in those who have drunk the cup of sorrow to the dregs." "i cannot deny it, mr. wilton. there is old deacon smith. we all know something of his history, i suppose. he was a poor boy; when he was twelve years of age his father died, and his mother died four years later. but he worked his way, first to a good education, and then to an honorable position and ample fortune. then the dishonesty of a partner brought him back to poverty too late in life for him to recover himself. now in his old age he works for a small salary in the office of another. but he is as cheerful and as grateful as if he had all that heart could wish, and had never in his life suffered a pang. i think he verily believes that everything which has befallen him has been an expression of god's love for him. he sheds no tears except for the griefs of others. i think he truly rejoices with those that rejoice and weeps with those that weep. as for faith in god, i suppose he would go into a lion's den as calmly as did daniel. if every professor of religion were like him, i am sure that nobody could say a word against the gospel. i freely confess that deacon smith's character has affected me more than all the arguments i have heard in favor of christianity." "as to that, mr. hume," replied mr. wilton, "we have both of us, doubtless, seen men who would hate a man the more bitterly in proportion as he should show himself christlike. and as to every church-member being like deacon smith, we could hardly expect such a character to be nurtured in a day or a year. deacon smith has become what he is by a lifetime of severest spiritual discipline and patient endeavor. such characters are wrought out only by a discipline of every form of trial. this world is constituted as it is for the purpose of giving just such a discipline of effort and patience. "this explanation brings us, however, only to the vestibule of the great mystery of suffering in the work of recovering man from the fall. the captain of our salvation, who put himself in man's place and took upon himself all human conditions, was made perfect through suffering. the full preparation for his work as the saviour of man called for a discipline of pain. i shall not attempt to explain this experience of christ, but salvation brings the believer into a state of profoundest and most mysterious union with christ. the believer must walk in the footsteps of jesus. as christ first came into a condition of sympathy with man, so must man come into a condition of sympathy with him. the believer must share and repeat, in a feebler way, of course, the experiences of the lord jesus. he must fill up that which is behind of the sufferings of his saviour. by this union with christ in the discipline of pain the christian is prepared for a union of blessedness. 'if we suffer, we shall also reign with him.' how broad and deep this union of the believer with christ may be, i cannot tell. i am not able to measure this idea. it seems to me like one of god's infinite thoughts, revealed in its dimness to overawe the souls of men by its shadowy sublimity--seen only enough to suggest how much vaster is that which remains unseen--an iceberg, one part standing out and nine parts sunk in the unfathomed sea. it is a thought to be felt and experienced rather than weighed and measured by human logic. this is all that i have to say upon this subject. do these views commend themselves to you, mr. hume?" "i do not know," was the reply; "i want to revolve the subject in my own mind. i have received some new ideas, but i judge that a man needs experience in this matter as well as thinking. if i had deacon smith's experience of life, i could form a better opinion. as much as this i can see to be true--that provision for bodily pain is a safeguard to the happiness and life of men, and that a world which should anticipate every human want, leaving nothing to be struggled after and nothing to be endured, would have a disastrous influence upon human character. i will admit that the provision for pain is wise and good." "one other point," continued mr. wilton, "we ought to notice before leaving this subject. the word of god says, 'we know that all things work together for good to them which love god,' but it says no such thing of those who do not love him. the afflictions of this life work out for the righteous 'a far more exceeding and eternal weight of glory.' the ministry of pain is a ministry of love only to those who submit to christ. to those who kick at god's mercies the best blessings turn to evils and curses; to the faithful in christ the greatest griefs and calamities become choice blessings. a submissive heart and the agency of the holy spirit are needed to sanctify pain. it is a great mistake to think that all men are made better by afflictions. only the few get good from the discipline of life. with many persons troubles only stir up the worst passions till they rage like caged tigers." "this last remark, mr. wilton, has thrown a flood of light upon this subject. but it seems strange to me to find myself saying this. i see how it is that so large a part of the pains of life is found in the end to accomplish no good. the evil remains evil. do you think that my long trial of doubt and unrest and pain of heart can ever be blessed to my good?" "that it can be so blessed to your good and to the good of many others i have no doubt; but whether it will be, i cannot tell. that depends upon yourself, upon your coming through christ to god as your heavenly father. it is my earnest prayer that from your unrest of spirit deep peace in christ may break forth; and many others unite in the same." "i certainly hope," said mr. hume, "that my life may not come to nothing. it seems as if something better than a few years of mingled pain and pleasure, overshadowed by most painful doubt and darkness and followed by a plunge into nothingness, must be possible for me." "god give you grace," said mr. wilton, earnestly, "to forget the things which are behind, and reach out your hands toward the worthiest destiny! but remember that there is a destiny more terrible than to cease to be, there is a death deeper and darker than the grave. 'there is a death whose pang outlasts the fleeting breath; oh, what eternal terrors hang around the second death!'" mr. wilton did not think it best to attempt to draw out mr. hume farther at that time. he saw that he appeared to be under the guidance of the holy spirit, and hoped that he would soon experience the new birth by which old things pass away and all things become new. he knew that time is an element even in the operations of the spirit, and he feared to shake the bough too roughly lest the fruit should fall untimely only to wither in his hand. happily, the superintendent's bell brought the conversation at that point to a natural conclusion. chapter x. transportation of heat. "to-day we come to that subject which we should have looked at a week ago, if that i hope not unprofitable discussion of the uses of trials and the ministry of pain had not prevented. we must now examine the arrangement for softening the rigors of winter and toning down the heat of summer. the general principle is that in summer the earth receives an excess of heat, while in winter the opposite is true. these extremes are mitigated by transferring heat from summer to winter. how is this accomplished? any one who has thoughts upon this subject may answer." "i have some thoughts," said ansel, "but whether right or wrong, i cannot tell. i should think heat might be carried from summer to winter in the same way as from day to night." "what are some of those means for transferring heat which seem to you to operate the same in the annual as in the daily changes of temperature?" "one is the absorption and radiation of heat, and another is the evaporation of water and the condensation of vapor." "you are right," said mr. wilton. "the effect of these operations in the equalization of the annual extremes of heat is in no wise different from their effect upon the temperature of day and night, but from summer to winter their effect is vaster and more impressive. during the summer, sea and land, and 'all that in them is,' are receiving heat and rising in temperature. the heat of summer penetrates and warms the earth nearly a hundred feet in depth. into the sea heat penetrates still deeper. how vast the amount of heat required to warm the whole surface of the earth and sea to such depths! by withdrawing so much heat from active use the intensity of the summer temperature is softened. during the colder months the land and sea slowly radiate their heat. we can hardly over-estimate the effect of this alternate absorption and radiation of heat. so great is the effect of this stored up heat that the sea and the great lakes never freeze even in the coldest winter weather, except in the polar regions, and the temperature must fall far below freezing and continue for a long time below the freezing point before the earth begins to freeze. the great bodies of water, remaining always at a temperature above thirty-two degrees, are especially important in warming the wintry air. in the coldest weather they seem like steaming caldrons throwing up their warm vapor. it is the absorption and radiation of heat alone which prevent the temperature of the atmosphere from rising or falling suddenly to the highest or lowest point possible. the sun breaks forth in all its splendor at noonday in summer: what if the sun were to remain stationary, shining thus in his strength for days and months? everything would be consumed with heat. but why do not the glowing rays of the sun raise the temperature at once to the highest possible point? because the earth and sea and every object upon the earth absorb the heat, storing it up and holding it in reserve. on the other hand, when the sun sets and his heat is withdrawn, why does not the temperature fall suddenly to the lowest possible point? because the heat held in store is slowly radiated and the change of temperature rendered gradual. "in this work of absorbing and radiating heat every object, earth, air, and sea, does its appropriate share. but water stands chief, and performs the largest service. its high specific heat enables it to hold in store the largest calorific treasure, and causes it to change its temperature more slowly. "the formation and condensation of vapor also operate in the same manner as in the transitions of day and night. during the summer the higher average temperature makes it possible for a much larger amount of vapor to be formed than in winter. you remember that at eighty degrees vapor equal to thirteen inches of water can sustain itself, while at thirty-six degrees the elastic force of vapor is equal to the pressure of only two inches and two-fifths of water, and at four degrees to three-fifths of an inch. if the mean summer temperature at any place were eighty degrees, it would be possible for more than one foot of water to be held in the form of vapor. in the formation of this vapor heat would be consumed sufficient to boil more than five and a half feet of ice water. if the mean winter temperature at the same place be thirty-six degrees, more than three-fourths of this vapor must be condensed and give out its latent heat to warm the air. it is not to be supposed that the full amount of vapor which can support itself does commonly exist, but the difference between the average amount of vapor in summer and in winter must be very great. i suppose this difference often amounts to four or six inches of water. if we suppose it to be four inches, an amount of heat is transferred from summer to winter sufficient to boil twenty-two inches of ice water. in estimating the effect of this we must consider that this heat is not given out gradually and regularly for three months, but whenever there is a sudden fall of temperature vapor is condensed, latent heat becomes sensible, and the suddenness and intensity of the fall are diminished. we need also to bear in mind that every open body of water is sending up its clouds of vapor constantly. the open lakes, and especially the sea, are like a seething caldron; and thus immensely more vapor is condensed during the winter months than is brought over from summer to winter. much of the vapor formed in winter is to be set to the account of summer, for it is the summer's heat absorbed by the water, which maintains its temperature and enables it to throw up such clouds of vapor, even in midwinter. but this comes in more properly at another place, and we will leave it for the present. "there is another transition experienced by water by which heat treasured up in summer is made available for softening the rigors of winter. who will suggest it?" "it is the freezing of water," said mr. hume. "in the process of crystallization one hundred and forty degrees of latent heat become sensible." "and this," continued mr. wilton, "is no inconsiderable matter. every pound of water frozen upon the surface of our lakes and rivers, every pound of water frozen in the wet earth, every pound of water frozen as snow or sleet in the air, gives out as much heat as would boil an equal amount of water at seventy-two degrees. have you never heard of setting tubs of water in cellars to keep vegetables from freezing?" "i have," replied peter. "i visited my grandfather two years ago, and his cellar sometimes froze. i asked him why he put tubs of water in his cellar, but he could not tell me, only he said that he knew that tubs of water in his cellar did keep his vegetables from being nipped with the frost." "can you tell us, peter, why tubs of water set in a cellar should have this effect?" "i suppose that when the water begins to freeze it begins to give out its latent heat." "that is one part of the reason. the water is drawn from the well at perhaps fifty degrees; it must lose eighteen degrees of heat before it begins to freeze, and all the heat which the water loses the air of the cellar gains. and then, as you said, as soon as the water begins to freeze latent heat begins to become sensible. every pound of water frozen sets free heat enough to raise a pound of water through one hundred and forty degrees. but why do not the vegetables begin to freeze as soon as the water?" "i don't know." "water holding salt or other minerals in solution freezes at a lower temperature than pure water. for this reason the juices of vegetables and fruits and the sap of trees may be cooled below thirty-two degrees without freezing. on this account the water set in cellars tends to prevent vegetables from freezing; the water begins to freeze at thirty-two degrees, while potatoes and turnips may be cooled a little lower than thirty-two degrees without harm. in this manner the buds of trees are sometimes warmed and protected by the coating of ice which forms around them. the drops of water, falling through the sleety air, touch upon the twigs of trees and freeze upon them, an icy coat embracing them all around. in freezing, the water gives out one hundred and forty degrees of heat, a part of which goes to the air and a part to the twig." "this reminds me," said ansel, "of what the irishman said on being told that snow contains heat, that 'it would be a blessed thing for the poor if one could tell how many snowballs it would take to boil a tea-kettle.'" "it might be difficult to use snowballs to boil the tea-kettle, but the heat given out in the formation of the snowflakes is doubtless employed quite as usefully for the poor as if used in preparing their tea. you have all noticed that before a snow-storm, or perhaps during the early part of the storm, the temperature generally becomes milder, and you have often heard the remark, 'it is too cold to snow.' men have learned that the coming of a snow-storm is attended by a warming of the air. this popular impression is philosophical, yet few understand its philosophy. a foot of snow falls, equal to two or three inches of water. in the condensation of the vapor which formed this snow one thousand degrees of latent heat become sensible, and then in the congelation of the clouds into snowflakes one hundred and forty degrees of heat are evolved. this softening of the rigors of winter is, i think, as great a blessing to the poor as the heating of the tea-kettle. let us make an estimate of the amount of heat set free in the production of one great snow-storm. two feet of snow falls, equal, we will suppose, to five inches of water. in the condensation of the watery vapor one thousand degrees of heat are evolved, and in the congelation one hundred and forty degrees--an amount of heat which would boil three feet of cold spring water. in every square mile there are , , square feet, and a square mile of water three feet in depth would contain , , cubic feet. the production of such a snow-storm sets free for every square mile of surface heat which would boil more than , , of cubic feet of spring water. such a storm sometimes extends over a region of country a thousand miles square, that is, over a million of square miles. in the production of one such storm--a very heavy and extensive storm, i have supposed--heat is generated which would boil eighty millions of millions ( , , , , ) of cubic feet of spring water--an amount altogether too vast for our comprehension. to accomplish this result by combustion would require more than , , of tons of anthracite coal--an amount at least three times as great as the yearly product of all the coal-mines of the world. and this is but one heavy storm. the amount of rainfall in the united states may be thirty-six inches or forty or forty-five inches. supposing the average rainfall of the whole earth to be twenty-four inches--an estimate very far below the truth--we have this result: there are, in round numbers, two hundred millions of square miles of surface, more than five and a half quadrillions ( , , , , , ) of square feet and more than eleven quadrillions of cubic feet of water. the condensation of this amount of vapor would boil more than sixty quadrillions of cubic feet of ice water. one pound of anthracite coal burned under the most favorable circumstances will boil sixty pounds of ice water. to boil sixty quadrillions of cubic feet of ice water would require sixty quadrillions of pounds of coal--thirty billions of tons--not less than twenty-five tons to every inhabitant of the globe. at this rate a very few years would exhaust the coal-fields of the world. calculations like these are useful in showing upon how stupendous a scale the creator carries on his operations. but we must remember that these works are carried on, not to amaze men, but to benefit them. the works go on silently and unseen, challenging no attention from fools, receiving no thought except from the patient student of nature, and eliciting no thankful recognition save from a few reverent worshipers. "but i have been led away from a point which i had in mind. while considering the effect of heat in expanding bodies, i reminded you that water presents a marked peculiarity, and promised to speak of it more fully. this is the place for us to look at this singular and beautiful peculiarity of water. what is the general principle touching the effect of heat upon bodies?" "heat expands bodies and cold contracts them," answered ansel. "water both illustrates this rule and presents some very interesting apparent exceptions. it contracts by cold like other bodies till it reaches the temperature of thirty-nine and a half degrees; it then begins to expand, and expands regularly till it falls to thirty-two degrees; at that point it freezes, and in freezing it expands at once about one-ninth of its bulk. if the cooling process be continued, the ice produced contracts like any other solid. this peculiarity of the interrupted and unequal expansion of water is of the utmost importance in the affairs of our world. consider the result if the water were to contract by cold as do other bodies down to the freezing point and below it. water is cooled from the top by contact with the cold air. as the upper film of water cooled it would sink and a new stratum be brought to the surface; that in turn would be cooled and sink, and thus the cooling process would go on with the utmost rapidity till the whole body of water should be reduced to the freezing temperature. then congelation would begin, and the first particles of ice formed would sink to the bottom, and as fast as the water became frozen at the top the ice would sink. in this manner a solid body of ice would be formed at the bottom of our lakes and rivers, while the surface would remain unfrozen in contact with the cold air till the whole body of water became a compact mass of ice. great lakes turned to solid ice would not be thawed during the whole of the summer, for the water warmed from the top would not sink, but would form a warm stratum of water upon the surface, while, below, the solid ice would lie hardly feeling the summer heat. nay, more; in the higher latitudes it would seem as if the very ocean must be turned to solid ice, never to be melted till the end of time. by the singular expansion of water below thirty-nine and half degrees and its great expansion in congelation, these disastrous consequences are prevented. our lakes are cooled even in winter only to thirty-nine and a half degrees; below this temperature the colder water is lighter and remains upon the surface; ice floats upon the surface. the top becomes ice, but the great mass of the water remains at thirty-nine degrees, and the inhabitants of the waters live on unharmed. spring comes, and the ice, being upon the surface, is soon melted, and the unbound waves begin again to ripple forth their unconscious joy." "do you look upon this irregular expansion and contraction of water," asked mr. hume, "as a real exception to the rule that heat expands bodies?" "not at all. in freezing, a new force comes in and asserts itself--the force of crystallization; or, more exactly, as the force of heat diminishes the force of crystallization becomes predominant, and throws the atoms into new positions and new relationships. to this new arrangement of atoms is due the expansion in freezing. ice contracts and expands by cold and heat the same as any other solid. the attraction of crystallization begins, doubtless, to throw the atoms into their new and crystalline arrangement at the temperature of thirty-nine and a half degrees. "we must remember that the heat which is set free in the condensation of vapor and in the freezing of water is absorbed in the formation of vapor and the melting of water. as much heat is taken from summer as is conferred upon winter. the summer is cooled as much as winter is warmed. the formation of vapor is a cooling process. water is prevented from rising above the boiling point by the formation of vapor. perspiration cools us by the evaporation to which it gives rise from the whole surface of our bodies. and the higher the temperature, the more rapid the evaporation, and the more vigorous the cooling process. "we might look at other appliances for transferring heat from summer to winter, but they belong in principle to another department. we have now looked at some of the means for transferring heat in time. the heat is treasured up at the heated noonday, to be brought out for use during the cool hours of night; it is garnered from the excessive heats of summer to supply the deficiencies of winter. it is laid up in store to-day to be expended at any future time when needed. the transfer is a transfer not in space, but in time. we must hereafter examine those arrangements by which heat is transported through space. some of these arrangements exert an influence upon day and night and upon summer and winter, and thus throw further light upon the subjects already discussed. already more than once topics have been suggested and their full consideration put off till some more fitting time. in our next lesson we must begin the examination of these new principles. we have before spoken of the vicissitudes of days and seasons and years. we shall now have to do with the vicissitudes of zones and lands and seas, of deserts and mountain ranges. the elements become vaster, the stage is broader, and the movements more sublime. "i am glad that you are so well interested in these great and beautiful works of god's wisdom and power, but i hope that you do not forget that the crucified christ is pre-eminently the power of god and the wisdom of god. these natural works are but the husk of which salvation from sin by christ is the kernel. these outward things are wonderful and beautiful for the setting, but the gem, the royal precious stone, the koh-i-noor, the 'mountain of light,' for which the setting was made, is the true knowledge of the true god and of his son jesus christ. during the past few weeks you have heard others asking, 'what shall we do to be saved?' i should be greatly guilty if i allowed you to think earth, air, and sea, with all their silent and solemn movements, more important than our spiritual attitude toward god the father and christ the saviour. are you, samuel, in your interest in studying nature, forgetting christ and the souls of men?" "i hope not, and i think not. during the three years since my baptism i have never felt so much my obligation to christ as now. i never felt before so deep a desire that my friends should repent and believe in jesus. i think the love of christ constrains me. i have not felt before that my work was very important; i have been expecting to work more earnestly by and by; but lately i have felt that christ gives me something to do now for which he holds me responsible." "what have you tried to do for christ?" "i have been praying for some of my young friends, and especially for ansel and peter. and then i felt that i must talk with them as well as pray for them." "and can you, my young friends, be careless about your own salvation while samuel is so anxious for you? are you contented to live 'having no hope and without god in the world'? is your happiness here and hereafter more important to samuel than to yourselves?" "we are interested," said ansel. "we have been talking together about being christians, but we don't know what to do." "they said," broke in samuel, "that they wished i would ask you to preach a sermon and tell them what they must do to be saved. they wished to go on with these lessons, but they thought that perhaps you would be willing to preach a sermon just upon that subject." "you know that i often speak of that subject, and when persons have come to the inquiry-meeting i have told them what they must do. but i know that there must needs be 'line upon line.' if ansel and peter wish it, i will devote a sermon to the subject, and make it as plain as i can. hardly anything gives me more pleasure than to explain the way of salvation when i know that my hearers are interested." "we do wish to have you preach upon that subject, and i am sure that you will have a great many interested hearers besides ansel and myself." "but, samuel, did you not pray for mr. hume also, and talk with him?" "i prayed for him, but i was afraid to speak with him. i have tried to pray for him a double portion because i could not speak with him." tears gathered in mr. hume's eyes; the thought came to him that his unbelief had raised a barrier between himself and both god and his people. this pious young man was afraid to come to him lest he should meet the scornful arguments and cold derision of a proud unbeliever. he felt humbled--he, a subtle, well-read unbeliever, and samuel a pious lad yearning for the salvation of his soul, but daring only to pray in secret for him. "have not you, mr. hume, been treating christ and the holy spirit as samuel feared that you would treat him?" "perhaps so," he answered. "i am sorry that samuel did not come to me freely. i think he need not be afraid of me now. i also hope you will preach the sermon which ansel and peter wish to hear." mr. wilton assured them that he would do as they wished unless the spirit clearly drew him to some other subject. "i always look," he said, "to the holy spirit for direction in my preaching. 'when he, the spirit of truth, is come,' said jesus, 'he will lead you into all truth.' this was fulfilled pre-eminently, i suppose, in the inspired men who laid the foundation of the church, but the spirit still dwells in believers and leads those who love and follow christ. the preacher of the gospel can do nothing without the power of the spirit of god." and i, kind reader, will give you the outline of the sermon if the spirit bids him preach it. chapter xi. an effective sermon. mr. wilton preached the sermon spoken of at the close of the last chapter the next lord's day morning. the more he thought upon the matter and inquired the mind of the spirit, the more he felt that for a purpose the spirit was calling him to unfold again the authority of god and the conditions of salvation. he gave notice of his subject, and invited all good men to pray that he might be able, like a good and wise steward of the mysteries of grace, to bring forth out of the treasure-house things new and old, and that the word might prove as a nail fastened in a sure place by the master of assemblies. much prayer was offered, and the people came together in a spirit of unwonted solemnity and earnestness. mr. wilton prayed to the glorified redeemer for his blessing: "o thou exalted christ, we assemble in thy name and by thine authority. thou hast bidden us not forsake the assembling of ourselves together for thy worship and the preaching of thy gospel. by thy grace we enjoy another of these sacred days. by thy death thou didst purchase for thy people eternal redemption. thou hast wrought out for them a great and glorious salvation. for thy great love wherewith thou hast loved us thou didst empty thyself of divine glories, and madest thyself a servant among servants, and didst suffer in the garden, and die upon the cross, and enter the grave. now thou art exalted at the right hand of the father, a prince and a saviour, to give repentance and remission of sins. o thou that judgest men, thy justice is great and glorious as thy mercies. years ago we tested thy love, years ago we felt the shadow of thy wrath; our guilt made us afraid and we cried unto thee, and thou forgavest our sins, and didst shed abroad thy peace in our hearts. in these recent days thou hast brought other sinners to feel their guilt. they have seen thee upon the cross, and have been smitten with anguish, and have repented, and thou hast received them. others are bowed down; they mourn; they feel themselves poor and needy; they confess thy justice; they feel the need of thy salvation; they walk in darkness; they grope and find no light; they look unto thee from a distance; but they do not come to thee, they do not follow thee. wilt thou not draw them to thyself? wilt thou not bow their pride of heart and turn their wills and make their hearts tender, gentle, and believing? wilt thou not smite the rock, and cause the waters of penitent grief to flow? lay thy cross, o jesus, upon their shoulders and upon their hearts, that they may bear it after thee and share thy glory. open thou their eyes that they may see eternal destinies and look upon thy divine glories, thy beauty, and thy tenderness. let them follow thee and trust in thee, strengthened and comforted by thy rod and thy staff. o christ, for thine eternal love with which thou hast loved us, reach down thine arm mighty to save and lift us up. lord, save or we perish. and speak thou by thy servant to-day, and cause all that hear to recognize the message not as his, but as thine." he read as his text acts xvi. : "sirs, what must i do to be saved?" he briefly recited the arrest, imprisonment, and release of paul and silas. "the salvation for which the jailer cried out was not deliverance from the dangers of the earthquake, nor from the displeasure of the roman governor. this was the bitter cry of a soul sinking under a load of guilt and trembling at the thought of god's impending wrath. some of you can appreciate his feelings and his fears. your sins against god and christ and the holy spirit have risen up before you; they stare you in the face; they condemn you. you feel your guilt--not a light and trifling fault, but guilt deep and dark, such as creatures made in the image of god incur by rebellion against the blessed and holy creator. the holy spirit has recited the divine law in your ears. your consciences have heard that voice and echoed its condemnation. you desire to escape that divine displeasure; you desire to have the fires of guilt that burn in your consciences quenched. you cry out, 'men and brethren, what shall we do?' the answer must be drawn from many parts of the holy scriptures. "understand, in the first place, that you are not to be saved by searching out some plan of salvation for yourselves. ask for the old paths. 'he that entereth not by the door, but climbeth up some other way, the same is a thief and a robber.' 'other foundation can no man lay than that is laid.' 'there is but one name given under heaven among men by which we must be saved.' "understand also that it is useless to attempt to save yourselves by making yourselves righteous. you have tried, i doubt not, to make yourselves better. perhaps you have resolved that you would not come to christ till you can present yourselves in some degree worthy of his care. have you succeeded in getting rid of your sins? can you blot out your past sins? can you erase the record which stands written in the book of remembrance on high? the law of god written in this bible condemns you; god condemns you; you are condemned already for not believing in the name of god's only begotten son, the lord jesus from heaven. can you change that condemnation by your feeble, fickle resolutions to reform? 'can the ethiopian change his skin or the leopard his spots? then may ye also do good that are accustomed to do evil.' "be assured also that it does not belong to you to change your own hearts. 'ye must be born again;' 'except a man be born again he cannot see the kingdom of god.' but that second birth comes not of blood, nor of the will of the flesh, nor of the will of man, but of god. 'ye must be born again, but ye must be born of the spirit.' notice that the word _saved_ is in the passive voice. sinners do not save themselves; they must be saved by another; they must be saved by one able to save, by one almighty to save, from the wrath of god and from sin, by one able to do for those who trust in him all that they need to have done in order to make their salvation complete and glorious. christ is able to do this. the crucified and risen christ is exalted a prince and saviour, to give repentance and remission of sins. the word of god says, 'to give,' and he rejoices to give. "on one point we must pause and dwell with special clearness. every anxious sinner must not only feel his guilty and lost condition, but he should also thoroughly understand what he means when he asks what he must do to be saved. he should see to it that he wants that salvation which jesus gives. "in the scriptures the sinner who would be saved is called upon to return to god. he has gone astray. he must retrace his steps. what is meant by this? i mean that man's sin consisted at first and consists to-day in saying, 'i will,' and 'i will not,' in opposition to the will and command of god. god said, 'thou shalt not;' man said, 'i will.' god says, 'thou shalt;' sinners say, 'i will not.' if a sinner is to be saved from sin, this opposition must cease. when god says, 'thou shalt not,' the sinner must reply, 'i will not,' and when god says, 'thou shalt,' the sinner must answer, 'i will.' the sinner's 'will' and 'will not' must agree with god's 'shall' and 'shall not.' in place of your self-will you must put god's will; that is, repentance, a turning about, a returning to god. but remember, salvation, if it be real and thorough, is not submission for an hour, a day, or a year, but submission for ever and ever. it is submission without condition and without limits. "the sinner says, 'this is a hard saying,' this utter and boundless denial of self-will and selfishness. but is it hard that the creature should yield to the creator, that ignorance should yield to wisdom, that selfishness should yield to love, that sin should yield to holiness, that poor, lost, wretched, fallen man should yield to the eternal and ever-blessed god? it is only by yielding that his will is brought into sweet harmony with the will of god, and that he can be a sharer of the divine blessedness. "your views on this point should be clear and distinct. if you wish only to be saved from the penalty of your sins, you do not desire the salvation which jesus gives. he saves his people, not in their sins, but from their sins. if, however, you really wish for his full and glorious salvation, you will desire that your will may be wholly subdued to the will of god. you will be found ready to unite in the memorable prayer of the lord jesus, 'not my will but thine be done.' salvation implies the giving up of self-will and a reverent submission to the will of god. "other sinful passions oppose the grace of god, but chiefly as helpers and supporters of self-will. pride and vanity strengthen self-will. turbulent fleshly lusts urge on and back up self-will. fear of man, fear of danger, and unbelief are but props of self-will. when 'my lord will-be-will' submits, the town of mansoul returns to her rightful allegiance. "the question at issue between god and the sinner, the question of self-will or submission, is often contested around the performance of some single definite duty. the holy spirit often presents to the convicted sinner's conscience some single duty and presses its performance. that duty is a test of the feelings and desires of the sinner's heart. so the spirit understands it, so the sinner often understands it. as, in the garden of eden, god gave to adam a test command, so does he now press upon the conscience of convicted sinners test duties to show them what they are. that which is required may be important, exceedingly important, in and of itself, or it may be in itself of very little consequence, but in every case the duty is all-important and its performance absolutely essential, because the spirit has laid it upon the sinner's conscience. it will show whether he wishes for salvation from sin or not. "i used to hear a christian relate an experience like this. while the spirit of god was striving with him and conviction of sin was heavy upon him, he felt a clear impression that he ought to go to his barn, and there at one certain place upon the hay-mow kneel and pray. his self-will rose in rebellion, chiefly, it would seem, because it was laid upon his conscience as a duty. but his distress grew upon him. he went to his barn and stood at another place and tried to pray, but no light or peace came; his sense of his sins grew heavier. how could it be otherwise? he went to the spot where he thought that he ought to go, and stood and prayed. still no peace came, but increasing sense of sin. at length he thought, 'why should i not? why not give up my own will? why not pray that god's will may be done?' he yielded, he kneeled at the place where he had thought he ought to kneel, and there he first felt peace before god. this was a singular experience. perhaps a man more intelligent and better taught in the sacred scriptures would never have such a thing pressed upon his conscience. but the battle of self-will is commonly fought around some single definite duty. that duty may be a confession of wrong done to a neighbor, or conversation with an impenitent associate, or a public confession of sin before the great congregation. whatever it may be, it shows the sinner his heart and leads him to decide to follow his own will just as he had always been accustomed to do, or it will lead him to pray earnestly that he may be enabled in everything to bow his will to the will of god. he will want the full salvation which jesus in his grace brings men--salvation from the penalty of sin and deliverance from its power. "i draw no bow at a venture and speak not doubtfully when i say some of you are standing face to face with duties pressed upon you by the holy spirit. your self-will, supported by pride, and fear of man, and unbelief, and satanic temptation, refuses to yield. the yoke of christ seems to you like bondage. the cross is supremely heavy. you draw back from it, and refuse to bear it. i cannot take away the cross which the spirit bids you bear. i dare not do it; i will not do it. as the messenger of christ, i repeat the voice of the spirit and lay the duty, whatsoever it may be, upon your consciences. do you really and honestly wish to be saved from sin? then you will yield to the spirit's kind and gracious movings; you will yield humbly but heartily. if, however, you want something else than the salvation which jesus gives, what can you expect but perplexity, difficulty, darkness? i beseech of you, deal truly and faithfully with yourselves on this point. "to those who wish really to be saved i have good news to proclaim. there is a saviour such as you need. trust in jesus as your saviour. place the whole work of your salvation in his gracious hands. christ saves sinners just such as you are. the faith which you are but to exercise is nothing else than your confidence, by which you entrust yourselves to him. faith has no saving virtue in itself, but it is the hand by which the sinner takes hold of christ. with this duty few of you will have any great difficulty. when once you wish to be saved from sin and are ready to submit to the will of christ, you will have no reluctance to take him for your saviour. you believe that christ is a divine saviour. if saved at all, you expect to be saved by him who died on calvary. hardly for the world would you resign your opportunity of coming to christ and receiving his grace. you believe that jesus is the christ, the son of the living god, the great sacrifice for sin. it remains that you should gladly accept what he offers and follow him as loving, trusting disciples. "follow the spirit, and you will be led to jesus and will come speedily to the joy of salvation; resist the spirit, and you grope in boundless darkness and fall upon the dark mountains. "in the holy scriptures the question of the text is asked and answered many times. hardly any two answers are alike. are there different conditions and different duties required of different men? by no means. but the holy spirit adapted the answer to the different spiritual states of the various inquirers. the answer is made to each questioner's heart. a self-righteous young man came to jesus asking, 'good master, what good thing shall i do that i may inherit everlasting life?' jesus answered, 'keep the commandments: thou shalt do no murder; thou shalt not commit adultery; thou shalt not steal; thou shalt not bear false witness; honor thy father and thy mother; and thou shalt love thy neighbor as thyself.' the young man answered, 'all these have i kept from my youth up; what lack i yet?' jesus said, 'if thou wilt be perfect, go sell that thou hast, and give to the poor, and thou shalt have treasure in heaven, and come, follow me.' the young man went away sorrowful. jesus knew his self-righteousness, and gave him answers which opened that young man's eyes to see himself. he gave him a test command, and the young man's revulsion from that duty showed that, notwithstanding his self-confident claim to righteousness, his riches filled all his heart. if your hearts are filled with the love of the world, you must put your possessions out of your hearts and follow jesus. "nicodemus also came making the same inquiry. he must have asked something like this, for jesus answered such a question. 'ye must be born again; ye must be born of the spirit,' said jesus. nicodemus was looking for a legal salvation by outward formal services, but christ gave him to understand that salvation involves a great spiritual renovation wrought by the holy spirit, by which men old in sin become new creatures and enter the kingdom of god as little children. he taught him thus that salvation was only from god. if any of you are looking for a cloak of self-righteous religious duties which you can put on, be assured that true religion springs from a work of god wrought in the heart. you must be born again by the power of the holy spirit. you must become new creatures in christ jesus. "on the day of pentecost the great company of men 'out of every country under the whole heaven,' while listening to peter's pungent address, cried out, 'men and brethren, what shall we do?' 'repent and be baptized, every one of you, in the name of the lord jesus, for the remission of sins,' answered peter. here were men who had a hand in crucifying christ, or if they had no active share in that deed of darkness, they had consented to his death; they were partakers of the crime; very likely they had cried, 'crucify him, crucify him.' they saw their sin, and were pricked in the heart. well might they repent of their rejection and crucifixion of their promised saviour, the son of god, from heaven. others were devout men who had come to jerusalem to worship. like simeon they may have waited long for the consolation of israel. how easy for them to enroll themselves among the followers of christ! all alike are commanded after repentance to put on christ by baptism. that burial with christ was the symbol of their dying and living again--of their dying unto sin and living again unto god. the same duties are enjoined upon you. repent of your long rejection of the grace of god and his son jesus christ, and before god and men devote yourselves to his service by a public confession of christ in baptism. "the jailer of philippi was taken in the midst of his sins. he was holding the servants of christ in his dungeon. he knew for what offence they had been seized, and he made himself a partner in the crime of persecuting them by the zest with which he thrust them into the inner prison and made their feet fast in the stocks. his conscience was ill at ease. then came the earthquake's shock, and he felt as if called to stand face to face with his judge. his soul was pierced through and through with a sense of guilt. 'what must i do to be saved?' he cried in the bitterness of his conviction. 'believe on the lord jesus christ, and thou shalt be saved,' answered paul. this is the answer to all of you who are well convicted of sin and have given up all self-righteous hopes. christ saves you. look to christ, ask christ; whosoever comes to him he will in no wise cast out. will you not come to him? will you not trust his promises and commit yourselves to his hands to be saved? he waits to bless you. he delights to be gracious. to save sinners he lived among men, and died and has ascended. his hands are full of gifts. he comes to you, and stands and knocks at the door of your hearts. will you bolt the door? there is joy in heaven over repenting sinners. this alone of all earthly transactions carries joy to christ and the angels. accept of christ, and earth and heaven will throb with a common joy." these words were listened to with most earnest attention, for at that time christ and heaven were realities in the minds of men, and salvation was a living issue. mr. wilton spoke as an earnest man, without cant or circumlocution, pressing upon men of thought and conscience the great concerns of eternity. the full result of this discourse will be known only when the opening of the books at the last day shall reveal it, but the beginning of the result was seen in the evening prayer-meeting. when the invitation was given for anxious persons to make known their feelings, both ansel and peter arose, and confessing in few words that the spirit of god had been striving with them, and that they had been resisting the spirit, said that now they were determined to resist no more, and asked christians to pray for them that they might be able to submit fully to the lord jesus and trust entirely in him. then there was a pause. mr. wilton was just on the point of rising to close the meeting when mr. hume rose to his feet. after a sudden start of surprise, a deep hush passed over the congregation, and in the midst of deepest silence mr. hume said: "i have been more than merely an impenitent man: i have been an unbeliever; i have been an infidel. i have not only tried to disbelieve the holy scripture, but i have actually disbelieved. i have thought myself wiser than the word of god. i do not mean that i have enjoyed peace, that my conscience has been at rest, and that i have been happy in my unbelief. three months ago i began to grow more than usually discontented with myself. questions which i counted settled and put to rest for ever came back to trouble me. a hundred times a day the questions came, what if there be a god who holds me responsible? what if there be a future life and a judgment day? what if christ be the son of god? why such questions should haunt me day and night i could not tell. i have learned to believe that the spirit of god was speaking to me. this restlessness brought me to the church for half a day. if my object was to gain rest in unbelief, i could not have done worse. my old arguments were unavailing to break the force of the truths preached. the questions which had been sounding in my ears and echoing in my heart began to change to solemn affirmations: 'there is a god;' 'there is a day of judgment;' 'appointed unto man once to die, and after that the judgment;' 'christ is risen.' texts of scripture learned in my boyhood and forgotten long years ago came back fresh to my memory. but i will not stop to rehearse to you all my struggles of mind for two months past. for a few weeks you have seen me here. i determined that i would try to find christ if he manifests himself to men in these latter days. for two weeks i have tried to pray, but i have found no satisfaction. christ has not manifested himself. my darkness has grown deeper and deeper. i have sometimes almost determined to abandon all thought of christ and throw myself back again upon my former unbelief. but i could not lay down the subject. "since i began to try to pray i have felt, faintly at first, like the whisper of a suggestion, but becoming clearer and stronger, like a voice from heaven, that i must in this congregation confess my former state and the feelings which i have had. it seemed to me that i could not do this. it seemed easier to die than to stand up here and confess that my belief, which i had pressed upon others and had boasted of as better than the gospel, had given me no peace. to-day i have been made to understand that the spirit of god has set me face to face with this confession. i have seen what it means to be saved--that my self-will must die or i must bid adieu to christ and hope. i cannot live and die hopeless. i cannot rest my head upon unbelief. i confess to you that all my thoughts have been wrong. my beliefs and my unbelief have done me no good. my whole life has been enmity and opposition to the holy spirit. i will try to oppose the spirit no more. i know not what the spirit may lay upon me, i know not how soon i may break my resolution, but i now feel that i want to be saved from sin, and cannot do otherwise than follow the spirit though i dwell in darkness for ever. if christ reject me i cannot complain, but if you think there is hope for one who has so despised the grace of god, i entreat you to pray for me." it is needless to say that from scores of family altars and closets supplications went up to god that night for the salvation of mr. hume and ansel and peter, and men prayed especially that mr. hume, who for years had been such a tower of strength to the ungodly and the dread of christians, might be saved for the glory of christ and the confounding of unbelievers. those prayers were heard. when the report of that meeting and that confession went out through the community, unbelievers were silent. it was as if the god of battles had emptied his quiver into the hearts of his enemies. chapter xii. transfer of heat in space. "we now turn our attention," said mr. wilton, "to a new theme. in the vicissitudes of day and night and of summer and winter heat is transferred _in time_. we now are to look at the arrangements by which heat is transferred _in space_. but since the transfer of heat in space requires more or less of time, the means employed are such as suffice to accomplish both objects. heat is treasured up and carried away to distant regions, and delivered up for use as occasion demands. "in a previous lesson the inclination of the earth's axis was spoken of. by this means the northern hemisphere of the earth is turned somewhat toward the sun during one half of the year, and receives a correspondingly larger portion of heat, while during the other half of the year the southern hemisphere is turned toward the sun and is warmed. this inclination of the earth's axis to the plane of its orbit gives us the change of seasons. "the change of seasons is manifestly designed for the welfare of man. along with the genial warmth of summer, fruits and grains and the comforts of life are carried far toward the poles, into regions which otherwise would be desolate with perpetual frost. but these extremes need to be softened; otherwise, the violence of the changes would prove destructive rather than beneficent. the severity of these annual changes of temperature is ameliorated by some of the grandest movements and arrangements upon our globe. these arrangements we have in a very imperfect way already examined. "but there are other inequalities of temperature besides those of day and night, summer and winter. passing from the equator toward the poles, every degree of the earth's surface passed over causes the sun to sink one degree from the zenith toward the horizon, and gives a corresponding lower temperature, till within the polar circles for a part of the year the sun is entirely hidden and winter reigns without a rival. the temperature of the sea differs from the temperature of the land; the sun comes nearer to one hemisphere than the other, and remains longer north of the equator than south. these and many other differences upon the earth give to different parts of the world every possible variety of temperature and climate. these differences of temperature upon sea and land, from zone to zone and from hemisphere to hemisphere, are equalized or ameliorated by many agencies, but chiefly by a transfer of heat in space, a transfer of heat from place to place. "i do not need to tell you that while we in the northern hemisphere are enjoying the warmth of summer the southern hemisphere is enduring the severities of winter, and in turn, when winter comes to us, summer smiles upon the nations that live south of the equator. you also remember that the orbit of the earth is not an exact circle, but an ellipse, that is, what is sometimes called in common language a long circle. for this reason the earth is three millions of miles nearer the sun in one part of its orbit than when in another part. can you tell us, peter, at what season of year the earth is nearer the sun?" "in midwinter, or about the first of january. i have always remembered it because it seemed so strange to me, when i learned it, that the sun should be nearest the earth at the coldest season of the year." "yes, one is reminded by it of the humorous argument that the sun must emit cold instead of heat, because when we are at the point of the earth's orbit which is nearest the sun it is winter, and the higher one ascends upon mountains toward the sun, the colder he finds it. but this nearness of the sun while south of the equator would naturally give the southern hemisphere a warmer summer than the northern. for this there is a beautiful compensation. the earth passes through her orbit more rapidly when nearer the sun, and that half of her orbit is also smaller, so that, as the result of this, the sun remains north of the equator about eight days longer than in the southern hemisphere. the sun is nearer while in the southern hemisphere, but the summer is shorter. that which the southern hemisphere gains in distance it loses in time, and that which the northern loses in distance it gains in time. "the nearness of the sun while south of the equator, the shortness of the summer, and the corresponding distance of the sun and length of the winter would tend to give the southern hemisphere great extremes of heat and cold, a short and hot summer and a long and cold winter. for this also there is a most interesting compensation in the comparative amount of land and water north and south of the equator. much more than one-half of the dry land lies in the northern hemisphere. this would tend to give the northern hemisphere extremes of heat and cold. south of the equator there is comparatively little land and much water, which tends to give the southern hemisphere evenness of temperature. the inequalities of the earth's orbit and the earth's motion in its orbit we find counterbalanced by the arrangement of land and water upon the earth's surface. "in connection with this we may notice still another compensation in the elevation of the lands by which the burning heat of the torrid zone and the rigors of the colder zones are more or less diminished. the greater the elevation of any region of country, the cooler must be its climate. physical geographers like baron von humboldt and guyot have made calculations which show that those grand divisions of the earth which lie in the hot regions of the earth are most elevated above the sea level. south america lies higher than north america, asia is more elevated than europe, and africa is more elevated than asia. the continents rise as they approach the equator and sink toward the sea level as they come nearer the poles. as these colder lands approach the water level their valleys sink beneath the sea, their coast lines become deeply indented with bays and gulfs, and lakes abound. thus the warmer waters of the sea are interspersed among the cooler lands, and the temperature of the lands is raised. the very elevation of the continents and the configuration of the lands have a providential relation to the temperature and climate of the world. we cannot suppose that arrangements like these, so aptly fitted to the needs of man, came by chance. in the unmeasured ages past, while this earth was in preparation for man, god had the beneficent _end_ in view; nay, in the very beginning, the whole plan and its beautiful completion was had clearly in mind. millions of ages ago the great creator tenderly considered the comfort and well-being of the human race, the latest born of his creatures, in these last ages. "as a general statement, the torrid zone receives an excess of heat, while the frigid zones receive too little, and the temperate zones, lying between, receive, at different times and places, sometimes too little and sometimes too much. the providential arrangements for equalizing temperature are, then, chiefly arrangements for conveying heat from the overheated tropical regions and scattering it over the temperate and polar regions. first among these means we will notice the _trade-winds_, or, as for the sake of brevity they are often called, 'the trades.' will you tell us, samuel, how winds are caused?" "the air is heated at some place and expands; it becomes lighter and rises, while the colder air around rushes in to fill its place." "you use the words which are commonly employed in explaining the origin of winds, and very likely your idea is right, but the language needs a little correction. the warm air does not rise of its own accord, so to speak, but is pressed upward. the warm air is expanded; it presses outward and upward; the same weight of warm air occupies more space than cold air; the warm air rises and overtops the surrounding air, and then flows off in order to reach the common level. the column of warm air is lighter than the cooler air, and cannot balance it; consequently, the cold air sinks down, pressing the warm air upward. in this manner an ascending current of warm air is formed, and also currents of cold air flowing from every direction toward the warm centre. these currents continue until the temperature of the air is equalized. "the atmosphere is commonly believed to be forty-five or fifty miles in height, though some men have estimated its height as very much less than this, while others believe it to be six or seven hundred miles in height. are we to suppose that the column of heated air reaches to the top of the atmosphere?" "i think not," answered mr. hume. "the rarefaction of the lower part of the column renders the whole column lighter than the air around, and the warm air, as we know by the movements of the clouds, after rising a little way, spreads off in every direction, forming upper currents corresponding to the currents below, but moving in the opposite direction." "only a few days ago," remarked peter, "i saw in the same part of the sky clouds moving in exactly opposite directions, and others which seemed to be standing still. i knew how one layer of clouds might be moving north and another layer moving south, but i did not understand why some should be standing still." "do you imagine, peter, that the upper and lower currents of air, moving in opposite directions, come sharply together, the one sliding against the other?" "i think not," said peter. "supposing, then, as is certainly true, that a stratum of still air lies between the upper and lower winds, does not that explain how certain clouds might be standing still while the others were moving?" "i might have thought of that myself." "but how does this carry heat from the warmer region to the colder regions around?" asked ansel. "i see how the colder air coming in would cool the warm region, and how the warm ascending air would carry away the excess of heat, but how do the cooler regions get the advantage of this heat?" "that is just what i was on the point of explaining. do you remember what was said about the production of cold by expansion and of heat by compression?" "i remember that if air be rarefied by removing pressure from it, its temperature falls: i think you said that a part of its sensible heat becomes latent; and if air be compressed, its temperature rises. i have seen experiments with the air pump and condenser to prove this." "that principle explains the transfer of heat by winds. if the heated air rose to the upper regions, and there radiated its heat, nothing would be gained; the heat would be simply radiated into space. but as the warm air rises pressure is more and more removed from it; it expands; its sensible heat becomes latent and is thus kept from radiation; its temperature falls, but not from loss of heat. this rarefied air forms the upper current flowing away from the heated centre. in due time this air must come to the surface of the earth again. whenever this takes place the air is brought again under pressure; it is compressed, and its latent heat becomes again sensible. heat is thus transferred from the warmer region to the colder in a latent condition, so that it cannot be lost. we must now apply this to the trade-winds. what are the trade-winds, mr. hume?" "they are regular winds blowing from a little north and south of the tropics of cancer and capricorn south-west and north-west toward the equator." "these winds are called _trade-winds_," continued mr. wilton, "on account of their great advantage to trade or commerce. the regular and steady sweep of these winds bears the merchantmen rapidly and safely on their way. the formation of 'the trades' is easily explained. by the intense heat of the sun under the equator the air is greatly expanded and rarefied; the heated air rises along the whole line of the equator; from both sides the cooler air presses in, is heated, and rises; thus steady winds are formed from the tropics, or a little beyond the tropics, toward the equator. if the earth had no rotation upon its axis, these winds would blow directly toward the equator, exactly south and north. the rotation of the earth gives the trade-winds their oblique, south-west and north-west direction. suppose that a single particle of air at the tropic of cancer starts upon its journey toward the equator. at its starting it has the same motion eastward as the surface of the earth at that place, that is, about nine hundred and fifty miles per hour. but as it moves on southward the degrees of longitude become longer and the motion of the earth's surface becomes more rapid, till at the equator its motion is one thousand and forty miles per hour. but the particle of air we are watching is not fastened to the earth's surface, and as the earth moves more rapidly the nearer we come to the equator, the particle of air falls behind, that is, the air moves southward and eastward, but the earth moves eastward more rapidly than the air, so that the air falls behind and seems to be moving westward. the result is that the air upon the earth's surface moves south-west. that which takes place with a single particle takes place with the whole body of the air, and that which takes place north of the equator takes place south of it also, producing north-west winds. on reaching the equator the winds from the north and the south meet and stop, forming the equatorial calms, and mingling together, they rise into the higher regions. in rising, the air bears away heat from the torrid zone, and this heat, rendered latent by the expansion of the air, is carried north and south by the upper currents as far as the limits of 'the trades.' in due time these upper currents descend and their latent becomes sensible heat, and is used in raising the temperature. mr. hume, can you suggest any method by which we can estimate the amount of heat which is carried north and south by the return trades?" "i know of no method, except to estimate the amount of heat necessary to raise that flood of air which pours in from the temperate zones to the equatorial heat. that immense amount of heat must, nearly all of it, be carried away to the temperate regions." "this is the general explanation of the trade-winds. you must understand, however, that, in certain regions and under certain conditions, the trades are liable to interruption or change of direction. desert regions within or near the tropics give rise to local winds which overpower the trades. in southern asia, while the sun is north of the equator, the land becomes so much hotter than the sea under the equator that the trade-wind is overpowered and reversed, forming a wind which blows to the north-east instead of the south-west. but this is only a beautiful flexure, so to speak, of a general arrangement for the greater advantage of a particular region. by this means the summer winds of southern asia come from the sea. northern winds would have been dry. prevailing northern winds would have made the whole of southern asia a desert; but the south-west monsoons come from the indian ocean laden with vapor, and render southern asia a very garden for fertility. "the next great agency for equalizing temperature between the torrid and temperature zones is the formation and condensation of vapor. this comes in here, because it depends for its efficiency upon the agency of winds. more than once this method of conveying heat from place to place has been hinted at, but deferred till we came to the proper place to speak of winds. "the trade-winds, passing over from a colder to a warmer climate, are constantly accumulating vapor. under the equator the annual evaporation from the surface of the ocean is set down at fifteen feet, or half an inch daily. the formation of this vapor consumes heat which would boil more than eighty feet of ice water. the vapor thus formed is borne upward by the ascending current of heated air. on reaching the higher regions a portion of it is condensed and forms a belt of clouds around the earth. this belt of clouds along the equator is known as the 'cloud-ring.' this cloud-ring shields the belt of calms from the burning rays of the sun and sends down almost incessant rains. but does not that condensation which forms the cloud-ring set free latent heat, and thus intensify the great heat of the equator? latent heat becomes sensible, but it is given out into the ascending current of air, and serves only to give it another lift till by expansion of the air it again becomes latent. the heat is simply transferred from the vapor to the air. the vapor which remains uncondensed is borne away on the wings of the return 'trades' to the south and to the north, and in due time is condensed and returns to the earth as rain; the heat which is given out by its condensation, wherever and whenever it is condensed, is given over as latent heat to the keeping of the air, and is passed back for use whenever the air descends to the earth. "vapor gathered from sea or land is everywhere exerting this equalizing influence upon temperature. does the temperature rise in any place? vapor is formed. every moist body begins to give up its moisture, and the excess of heat is employed in turning this water into vapor. this is the method by which perspiration cools man or beast; whether it be insensible perspiration from the invisible pores of the skin, or perspiration standing in beady drops upon the face of the toiling laborer, vapor is formed and heat is carried away. have you not noticed on close, muggy days when nothing dries, showing that very little vapor is forming, that perspiration seems to have no cooling effect? it oozes from the skin, but does not evaporate, and hence does not carry off the surplus heat. animals like dogs and oxen, that do not become wet with perspiration, do not bear heat well; they soon pant and loll, attempting to get rid of the excessive heat through the moist breath and open mouth. "the sum-total of heat transferred by this agency is too great for comprehension. look at the amazon rolling to the ocean a flood broad as an arm of the sea. that great river is brought from the atlantic ocean on the shoulders of the trade-wind. as the vapor is slowly lifted by the rise of the land from the sea level to the summits of the andes, it is condensed, and falls as rain. well is it for south america that the andes were thrown up on the western coast, for the winds west of the mountains are dry as a pressed sponge, and the most of that narrow slope is barren and desolate. south america would be a desert if the andes ran along the eastern coast. look at the mississippi, and the great rivers of europe, and the matchless rivers of southern asia. all the rivers of the world represent only the _wastage_ of the rain which falls upon the land after supplying the wants of the vegetable kingdom and keeping the lands moist. all this water is lifted into the air by heat, and every movement of vapor is a movement of heat. every particle of vapor goes freighted with heat. every cloud driven across the sky represents the transfer of heat, and every transfer is in the direction of equalization. everywhere the tendency is to equilibrium. nature has no processes for transferring heat from colder to warmer regions. "we may form a conception of the amount of heat transferred by the agency of vapor by estimating the amount of heat-force required to evaporate the water which forms our rain-clouds and lift them into the upper regions. according to a calculation of mr. allen, late of providence, to evaporate one-eighth of an inch of water daily from that belt of the surface of the earth lying within the tropics, and raise it five thousand feet high, requires , , , horse-power, or one hundred and thirty times the effective force of the whole human race, reckoning it at , , able-bodied men. but the actual evaporation from the sea within the tropics is believed to be about half an inch daily--four times as great as mr. allen's supposition. "i see, however, that our time is nearly exhausted, and i wish before closing to revert to that more important theme upon which i spoke this forenoon. i do not know how the truths preached interested or affected you, nor do i now wish to have you tell me. i wish only to say that, as the sermon was preached at your request, i hope it proved applicable to you, and that you will give the truths presented earnest attention. consider them well, and make your conclusions known this evening." the conclusion which the evening made known, you, reader, have already learned. chapter xiii. ocean currents and icebergs. a week has passed since mr. hume made his frank confession. he went home no lighter of heart than before, yet he felt in some respects different, for he had attempted to do what was right in the sight of god. but he did not feel the joy of sins forgiven. he had not looked upon christ as a saviour for himself. he felt that god had distinctly set life and death before him. his doubts were gone; the spiritual world was a reality; christ stood at his right hand and satan at his left; he stood where the path of destiny divided, the one path leading up to heavenly seats with christ, the other leading down to darkness and despair. a voice seemed to be whispering in his ears, "this is the last call." he went to his chamber determined, if possible, to settle the question of life or death before he left the place and before he slept. he took his bible, and on his knees turned and read the psalms at random. but the cloud of darkness only gathered deeper. the words of david's penitential psalm caught his eye: "against thee, thee only, have i sinned, and done this evil in thy sight." he felt that these words of david were true in his case also. all his long impenitence and bold unbelief had been against god. by night and by day, for many a long year, before the sleepless eye of god, he had lifted up his hand, almost defying the holy one, yet the lightning of god had not smitten him. he wondered as much at the long-suffering of god as at his own dreadful daring of the divine wrath. he had been taught better things; he was trained to know the scriptures and to go reverently to the house of god, but he had turned from christ and hope. he read on: "deliver me from blood-guiltiness, o god, thou god of my salvation." he felt that this belonged to himself more than to david. david had shed the blood of natural life, but he had destroyed the souls of men. he had stood chief among unbelievers. he had led young men into infidelity. he had seen them drink in his unbelief like water, throw off all restraint, and rush headlong to ruin. he had wrought a work of evil which he could never undo, and for which he could make no atonement. what was a confession in comparison with the ruin he had caused? what could his confession do for the young men already, perhaps, among the lost through his influence? could his late repentance call them back to life and hope? would god forgive and raise to heavenly heights a man who had dragged others down to hell? would it be possible that christ should fill his soul with blessedness while his victims were drinking the wine of the wrath of god? a deep horror seized him. the darkness of eternal death seemed to enfold him. must he, then, after having caught a glimpse of life and joy, be cut off from hope and be driven from god for ever? this would be just, but he felt that he could not endure it. "o thou great and holy god," he prayed, "i will ascribe righteousness to thee though thy righteous wrath shall sink me to hell; but, o thou merciful god, my soul cannot endure thy justice. the foretaste of thy wrath fills me with the pangs of eternal death. o god, have mercy upon me. o god, blot out my transgressions. create in me a clean heart, and renew a right spirit within me. o christ, whom i have despised, cast me not from thy presence. help me to submit to thee. help me to follow thee. spare me that i may undo something of that which i have done against thy glory and the souls of men. o jesus, i can do nothing to save myself. o lord, have mercy on me, the chief of sinners." he read the invitations and promises of christ, and prayed again. again he read and again he prayed. little by little the promises of christ stirred a feeble faith in his heart; he felt that there was still hope for him, and with the determination to cast himself upon the sure mercies of christ and to devote himself to his service, he threw himself upon his bed, and being wearied almost to exhaustion, soon fell asleep. when he awoke it was broad daylight. he had slept a sweet, refreshing sleep. but he was refreshed not merely in body. he woke to a new world. his heart was filled with sweet thankfulness. "how beautiful," he said, "is god's world! i never saw it so before, but the earth and sky seem clothed in glory. but most wonderful of all is god's goodness to me. i have rebelled against him all my life, yet he has loved me and sought for my salvation, and now the sunlight of his love has broken through the thick clouds of my sin, and a day of hope and joy has dawned upon my life. christ has indeed revealed himself. blessed be his holy name for ever and ever! what shall i render unto the lord for all his benefits? i will take the cup of salvation and call upon the name of the lord. i will pay my vows now in presence of all his people. i will teach transgressors thy ways, and sinners shall be converted unto thee." all this was known to the people, for during the week mr. hume had spoken of it in private and in public. he had told it to mr. wilton, and they had rejoiced together. ansel and peter had also regularly presented themselves at every meeting as anxious inquirers desiring the grace of god. peter had also on his knees said from the heart, "here, lord, i give myself away," and had received the assurance that his sins were forgiven. the spirit of god witnessed with his spirit that he was born of god. he began at once to use all his influence to bring his young friends to jesus. the addition of two such workers as mr. hume and peter, each moving in his own circle of acquaintances, gave a fresh impulse to the religious interest, which was now becoming deep and pervasive. especially had mr. hume's conversion, so clear and positive, confounded those who had sat "in the seat of the scornful," and many came in now for the first time to see for themselves what it could be that had mastered their cold, clear-headed leader in unbelief. but ansel still walked in darkness. he had talked with mr. wilton, but no light had entered his mind. he said that he thought he had submitted in all things to the will of god. he was becoming impatient that christ had not come to him as to others. this was their condition as they came together upon the lord's day. they all understood each other, and had no need now to ask questions or make explanations. mr. wilton believed that the study of god's works would not interrupt the working of the holy spirit, and therefore went on with his lesson as usual. "we have already spoken of the transfer of heat from the torrid to the temperate and frigid zones by the agency of winds and watery vapor. these carry heat chiefly in a latent condition. but great movements of heat take place in a sensible state. in this transfer of heat, also, water is the great carrier. the winds and vapor go freighted with latent heat above, and the waters and wind go freighted with sensible heat below. we will first examine the operation of the ocean currents. "not only do rivers run through the lands and hasten to the sea, but in the midst of the oceans rivers are flowing in comparison with which the mississippi, the amazon, and the yang-tse-kiang are rippling brooklets. the earth is belted by these ocean streams traversing the seas. an ocean current, called the gulf stream, issues from the gulf of mexico between the florida coast and the bahama islands. it flows northward off the coast of the united states, gradually increasing in breadth and spreading over the atlantic ocean. it is deflected by the new england coast and the great shoals off newfoundland, called the grand banks, or else by another current flowing southward from baffin's bay, and strikes across the north atlantic, bathing the shores of the british islands and reaching even to iceland. "the general outline of the ocean currents is this: issuing from the south pacific, a current flowing eastward splits upon cape horn. the western portion, called humboldt's current, flows northward along the western coast of south america, and is swallowed up and lost in the great equatorial current of the pacific. this is a broad current flowing westward and covering the entire space between the tropics. striking upon the eastern shores of asia, this equatorial current divides, one part flowing northward along the coast of asia, the other finding its way through the many islands, sweeping across the indian ocean, and flowing down the eastern shore of africa on each side of madagascar. doubling the cape of good hope, the current continues in a north-westerly direction across the atlantic. striking upon cape st. roque, this current again divides; a part flows south and a part pours into the caribbean sea. from the caribbean sea it issues as the gulf stream, of which i have already spoken. this gulf stream impinges upon the western coast of europe, and pours partly into the north sea and partly flows south off the western coast of africa, completing thus the circuit of the atlantic. the currents of the indian and of the great southern oceans are as yet very imperfectly understood. of all the ocean streams the gulf stream is most famous and best understood. i shall therefore use this as an illustration of the agency of ocean currents in conveying heat and modifying climate. "the waters of the caribbean sea are heated by the tropic sun to eighty-eight degrees. from these heated waters the gulf stream issues salter and warmer, and of a deeper blue, than the waters of the surrounding sea. its greatest velocity as it issues from the gulf is a little more than three miles per hour. as it flows northward its velocity diminishes, its breadth becomes greater, and its depth less. it covers thus with its warm waters a broad belt of the atlantic ocean, and extends its influence to the most northern part of europe. you can judge of the amount of heat which is removed from the tropics when i tell you that the unmeasured flood of the gulf stream would swallow up three thousand rivers like the mississippi. this one ocean stream is many times greater than all the rivers of the world. we feel the warmth of the gulf stream with every wind that blows from the sea. to this the british isles owe their mild, moist climate and perennial greenness, and by its influence a winter in iceland, upon the arctic circle, is no more rigorous than a winter in montreal, twenty-one degrees nearer the equator. but what is the gulf stream, though it be fifty fold greater than all the rivers of the world, in comparison with the whole sum of the ocean streams? upper currents and under currents fill the sea. they meet the explorers of the sea everywhere. the navigator drops his measuring line, and finds it swept away and drawn out by unseen currents. all these movements of the waters are in favor of the equalization of temperature. the cooler waters of the frigid and temperate zones are mingled with the heated waters of the tropics and exchanged for the equatorial waters. the transfer of heat would not be greater if broad rivers of molten lava were flowing from the equator to the poles. "another agency for the transfer of heat is the movement of ice, and especially of icebergs." "will you not tell us," said samuel, "how these ocean currents are produced? i can understand how winds are formed, but i do not see that these streams in the sea could be formed in the same way." "i designed to speak of this, but for the moment it had slipped from my mind: i am glad that you called my attention to it. i do not expect, however, to give a full and satisfactory account of their origin. if i should do this, i should succeed where every other man has failed. i shall not attempt a full explanation. by some means or other, the waters of the ocean are thrown out of equilibrium, and these currents are plainly an effort to restore the balance or equilibrium of the waters. many influences and agencies conspire to disturb the equilibrium of the sea. the attractions of the sun and moon are constantly counteracting the attraction of the earth and lifting the waters, so to speak, above their natural level. the tides produced by these attractions of the sun and moon are the immediate cause of some of the minor local currents. the winds set the waters in motion, tending to pile them up in one place and leave the sea below its natural level at another. the effect of strong winds in piling up the waters, even upon our great lakes, is very considerable. a heavy east wind upon lake erie has been known to drive the waters toward the western end of the lake so much as to leave niagara river above the falls almost dry. on the other hand, a heavy west wind drives the waters eastward, and produces almost a flood in the river. the influence of constant winds like the 'trades' acting upon an immense expanse of water must be very much greater. unequal evaporation tends to destroy the balance of the waters. in the colder regions the evaporation is very little, while within the tropics it amounts to about half an inch daily, or fifteen feet per annum. the head of the red sea is two feet lower than its mouth on account of evaporation. this unequal evaporation causes also an unequal saltness, and consequently an unequal weight. the fresher and lighter water cannot balance an equal bulk of salter and heavier water. when once currents are started the revolution of the earth upon its axis would affect them, just as the rotation of the earth affects the trade-winds. now, all these various agencies, and perhaps many others, combine their influence to destroy the equilibrium of the waters of the ocean. they unite and interweave their influence in a thousand ways beyond all human calculation. the result is the ocean currents. but how much is due to one cause and how much to another in the present state of knowledge no man can tell. only for a few years have the phenomena of ocean currents been made the object of scientific observation and research. but the effect of ocean currents in modifying climate is well understood, and the modification of climate means nothing else than the transfer of heat. this is all that i have to say of the rivers of the sea, and if there are no more questions, we will now look at the movement of heat caused by icebergs." no question was asked, and mr. wilton continued: "in polar regions there must be an immense formation of ice. except in the oceans, the movements of water are chiefly movements of water in the condition of ice. only for a small part of the year could water exist unfrozen. immense regions of the antarctic continent seem to be covered with one broad glacier. the ice pushes down into the sea until, undermined by the dashing of the waves, it breaks off, and enormous fragments are launched upon the deep waters. sir james ross saw in the southern ocean a chain of such icebergs extending as far as the eye could reach from the mast-head, many of them from one hundred feet to one hundred and eighty feet in height and miles across. captain d'urville saw one thirteen miles long and one hundred feet high. its bulk was so vast that though the waves were dashing against it not a tremor was perceptible. astronomic observations could be made from it as if it were solid rock rooted in the heart of the earth. in the same manner icebergs are formed in the northern ocean also. how much heat is given out in the freezing of water?" "about one hundred and forty degrees," answered peter. "in the formation of icebergs, then, heat is given out nearly sufficient to boil an equal quantity of cold water. the icebergs float away toward the equator. they come down from baffin's bay till they meet the gulf stream off newfoundland. in the southern hemisphere they come ten degrees nearer the equator. as they float toward the tropics they slowly melt, and in their melting they exact from the air and the sea where they melt the same amount of heat which they gave up in their freezing. if they melted at the same place where they froze, there would be no transfer of heat. but they are formed in the polar regions; they give out their heat in the frigid zone, while they melt and absorb a like amount of heat from the temperate zones. in this manner the polar regions are exchanging with the temperate zones ice for water. they borrow water, rob it of its latent heat, and send it back in the form of ice. the temperate zones supply the needed heat and bring the ice back to the form of water, when the polar regions again borrow it, seize upon its heat, and again send it back in the form of ice mountains. the effect is the same as if thousands of railroad trains were transporting water to the frigid zones, leaving it there to freeze and give up its one hundred and forty degrees of latent heat, and bringing it back in the form of ice. let us estimate the bulk of one such iceberg as that seen by captain d'urville. it was thirteen miles long and one hundred feet high, and we will suppose that it was four miles broad. standing out from the water one hundred feet, it must have sunk at least eight hundred feet below the surface. this would give us the enormous bulk of ( , , , , ) one trillion three hundred and four billions seven hundred and nine millions one hundred and twenty thousand cubic feet of ice. the burning of one pound of coal will generate heat sufficient to melt about five and a half cubic feet of ice. to melt one such iceberg would require more than one hundred and eighteen millions of tons of anthracite coal. this is the amount of heat given out in the polar region by its freezing. this is the amount of heat transported from the warmer to the colder regions. but what is one iceberg to the thousands which drift yearly from the frigid zones toward the tropics? "but even this hardly represents the entire transfer of heat by the agency of icebergs. the icebergs are formed from the snows of polar storms, and these are formed from the condensation and freezing of vapors. in the process of condensation one thousand degrees of heat are given out. every iceberg _represents_ a transfer of heat sufficient to boil more than six times its weight of ice water. "one marked illustration of the effect of icebergs we ought to notice. down through baffin's bay icebergs are constantly floating. they are borne on southward till, in the still waters of the grand banks, between the polar current and the gulf stream, they float around and melt and disappear. to these melting icebergs the chilliness and unfailing fogs of the grand banks are due; and not only this, but the very existence of the banks is supposed to be due to the deposit of sediment, sand, earth, and stone brought by polar ice. "i have spoken only of the polar glaciers and the icebergs formed by their pushing off into the sea. but the same transfer of heat is taking place, on a very much smaller scale and within narrow limits, by the glaciers of the alps and every other mountain glacier. the glaciers are nothing else than rivers of ice. snow falls upon the mountain tops and valleys of the mountain sides from age to age. the snow slowly changes to the structure of ice, and by its enormous weight flows down through the gorges of the mountain sides, till in the warmer vales below it melts and disappears. we have not time to go into a full examination of all the interesting phenomena of glaciers, but this one point you will notice and remember: these rivers of ice--for they flow like rivers--cool the valleys and tend to warm the mountain tops; of course upon the tops of the mountains there can be no accumulation of heat, because, standing out into the eternal coldness of space, and swept by winds for ever, and exposed by the thinness of the air to a rapidity of evaporation unknown at the sea level, heat is caught up and borne away in a moment. [illustration: transportation of heat. page .] "this closes this department of our theme. i might have gone much more into details and given you great stores of particular facts and figures, but they would have added nothing to your understanding of the subject, and we can hardly afford to devote our lord's day to mastering the details of the natural sciences. we have now looked at some of the methods by which the extremes of heat and cold, in day and night, in summer and winter, and in the tropics and polar regions, are mitigated. the same principles operate upon the smallest and upon the largest scale. if there is need for me to attempt in a formal way to awaken in you admiration for the wisdom and goodness of god shown in all these beneficent arrangements for equalizing temperature, our study has been largely in vain. we have only to remember that all these contrivances are the lord's designs. he created the world; he endowed matter with its qualities and forces, and he gave it these qualities and forces for the purpose of using it as he has used it. he planned all those contrivances by which he secures the comfort and the good of man, and the fact that these natural agencies are fitted for moral uses in recovering sinners to holiness and blessedness is but the culmination of its adaptation to the uses of man. "this, however, does not complete our course of study. a few other points will demand our attention for two or three more lessons. but while we go on with our studies of nature, remember that the physical was created for the sake of the spiritual; the spiritual is more important. let us not subvert the divine order and sink the high purpose of the creation to mere material agencies and contrivances. to know god is greater and better than to understand nature. that we might know and enjoy and glorify the creator was the object of our creation. we cannot express it in better language than that employed in the old catechism: 'the chief end of man is to glorify god and enjoy him for ever.' that term 'for ever' includes the present life as well as the future. we ought to know, enjoy, and glorify god to-day. i hope that another week may find ansel with some happy experience in this matter." chapter xiv. combustion.--coal-beds. another lord's day comes, and no change has taken place with the class which calls for mention. ansel still walks in darkness, ready indeed on every occasion to manifest his concern for the salvation of his soul, diligent in reading the scriptures, frequent in prayer, and giving yet no indication of a flagging of his avowed purpose to follow christ, but he receives no comfort and peace. a painful and distressed interest is becoming more and more concentrated upon him. what will be the end of his groping in darkness? this cannot last always. unless the hindrance, whatever it be, which prevents the exercise of faith, be seen and removed, ansel will probably soon go back to his former careless state, and, it may be, become tenfold more obdurate than before. he will be likely, on the one hand, to become self-righteous from his supposed effort to come to jesus, and, on the other, discouraged and despairing, feeling that for him effort is vain and salvation unattainable. while he remains in this state the very lapse of time is dangerous. all feel concerned for him, but no questions are asked, and the lesson goes on as usual. "the method of transferring heat which we are now to examine is wholly different in principle from any which we have as yet considered. i refer to the production of heat by combustion. the transfer of heat by combustion cannot be compared for vastness with those great movements of heat which have before claimed our attention, yet for the comfort and well-being of the human race combustion is exceedingly important. without that command of heat which combustion gives, man could not rise at best above the savage state, and in fact could hardly exist upon the earth. we smile at the grecian myth that prometheus stole fire from the gods and brought it to men in his reed staff, but fire is certainly worthy of being counted one of god's great gifts. but whence comes the heat of combustion? is it a new and original generation of heat, or is it merely a transfer? will some one explain this?" "i don't think that i can tell," said samuel. "i remember the principles you have given us about the nature and production of heat, but i do not know how to apply them to combustion." "i did not suppose that you would be able to explain all the phenomena of nature at sight, yet the production of heat by combustion is not difficult to be understood. the burning of wood and coal is chiefly the union of oxygen with carbon. the oxygen of the air unites with the carbon of the combustible. the attractive force between oxygen and carbon is very strong. when they unite, the atoms of oxygen dash against the atoms of carbon with great violence. as they dash one upon another their motion is lost, but by the laws of transmutation of forces that lost motion reappears as heat; that is, the motion of the atoms as they fall the one against the other is changed to that vibration of the atoms which we call heat. the atoms of carbon, in their separation from oxygen, may be compared to weights suspended, ready to fall. let once the cord be cut, and the weight falls and dashes against the earth; its motion in falling is lost, and reappears as heat. so carbon is suspended, so to speak, waiting to unite with oxygen. but how is the weight raised? how is carbon brought into this state of suspense, waiting to dash upon oxygen and develop heat? that is not its natural state. "carbonic acid is found everywhere mingled in small proportions with the atmosphere. this carbonic acid is nothing else than carbon and oxygen united in the proportion of one atom of carbon to two atoms of oxygen. this is the natural state of carbon. this carbonic acid is the food of plants; it is this which supports all vegetable growth. the carbonic acid is absorbed by the leaves of plants and trees, and in the hidden laboratory of the leaf, by what process is one of the undiscovered secrets of nature, the carbon is separated from the oxygen, the oxygen is discharged through the pores of the leaf, and the carbon is carried into the circulation to build up the fabric of the woody fibre. that which the most skillful chemist in the world cannot do, except by indirect processes and at a high temperature, the leaves are doing directly at the ordinary temperature. vegetable growth is a deoxidizing process. to accomplish this an enormous force is requisite. to separate carbon and oxygen, a force is demanded which is able to overcome their powerful attraction. how shall we estimate the strength of this force? in order that they may unite, as in the explosion of gunpowder, solid rocks are torn asunder. the attraction of carbon and oxygen is strong enough to tear great rocks in twain. it is this attraction which sends the cannon ball and the shell like meteors of death upon their errands of destruction. this great force must be overcome; carbon must be separated from oxygen and built into trees. this is the lifting up of the weight. but whence comes the force necessary to accomplish this? from the sunbeam. the heat of the summer's sun, employed as force, is used to deoxidize carbonic acid. heat is used, and used up, in lifting the weight which in its fall shall generate again a like amount of heat. the combustion of wood produces the same amount of heat as was needful to separate its carbon from the carbonic acid of the air. vegetable growth is thus a cooling process; heat is withdrawn from use as heat, and is employed as force. as force it has nothing to do with temperature. the summer's heat, employed in vegetable growth, reappears in the blazing billets of the kitchen fire. heat is condensed and solidified, so to speak, and placed under man's control. in this solidified form heat may be laid up in store or transported at pleasure. "the grandest application of this principle is seen in the formation of the coal-beds. at some early period in the unmeasured ages past, the temperature of the earth must have been much higher than it now is; the air was filled with moisture, and carbonic acid abounded. as a consequence, there was an enormous vegetable growth. this, as we have seen, is a heat-consuming process. the heat is withdrawn from the air and employed in deoxidizing the carbonic acid. this vast vegetable growth--enormous ferns and coniferous trees--fell, and was swept by rivers or by floods into valleys, or the beds of lakes, or the sea; the sediment of the waters covered it, and there, shut up from the air and subjected to a heavy pressure, this vegetable mass underwent a slow transformation. peter, have you ever seen a coal-pit? i do not mean a coal _mine_, but that which charcoal-burners call a coal-pit." "i have seen them many a time." "tell us, then, how wood is burned to coal without being burned up." "the wood is set on end, closely packed in the shape of a mound, and then covered with earth. fire is kindled in the middle of the pile, and just enough air admitted through air-holes at the bottom to keep up a slow burning. it burns just fast enough to heat and dry the wood without burning it up." "the same process," said mr. wilton, "went on in the formation of the coal-beds, but very much more slowly. under the pressure of earth and water the vegetable deposits lie smouldering, not for a few days, but probably for ages, till nothing but the carbon remains, and that pressed into a solid mass heavy as stone. veins of coal are found interspersed with layers of earth and rock, layer above layer, and these layers are commonly not level, but more or less inclined and sometimes broken. this shows that a deposit of driftwood was made, then a deposit of sand or clay, then another deposit of vegetable material and another layer of earth. at length, by internal convulsions, the whole surface was raised from beneath the waters, and in due time the coal-veins were laid open, and the coal brought out for the use of man. then the force so long pent up and held in suspense is set free; the stored-up heat of the geologic ages is brought out for use. the excess of heat in that ancient period is handed down to these later times. how sublime this transfer of heat! it carries us back, in imagination, to the 'heroic ages,' so to speak, of the history of creation. by other methods heat is treasured up for a day or a year: by this method it is kept in store for myriads of ages. we see that the same natural forces were working in those early ages as to-day, and the same benevolent creator was arranging the affairs of the world for man's advantage. the sunbeam which streamed upon the earth long ages before man was created is to-day smelting ores, driving machinery, dragging ponderous trains of loaded cars, and ploughing the seas with freighted keels. this seems like a fairy-story or a dream, but instead of that it is the soberest of philosophic and scientific truth. "we ought also to notice the internal heat of the earth. this has been handed down from the day of creation, it would seem, till the present. no new principle is seen in the earth's internal fires, but a sublime illustration of the storing up of heat in a hot body and its slow radiation. "the origin of the internal heat of the earth we can only conjecture. perhaps god created the various elements separate, uncombined, and allowed them then to combine according to their natural affinities. this sublime conflagration of all the elements of the earth would generate the highest temperature which could be produced by combustion. the elements would melt with fervent heat; everything which could be vaporized by heat would be turned to vapor. then radiation of heat would begin. vapors would sink to fluids and fluids turn to solids; a hard crust would be formed on the surface of the globe through which the heat of the still molten mass within would be slowly conducted and escape. upon this internal heat the earth depends in no small degree for its temperature. the heat generated perhaps upon the day of creation helps now to render the earth habitable. "that the earth was once in a fluid state and has lost a portion of its heat by radiation is indicated by several facts. it is one of the received beliefs among geologists that at some period in the past the temperature of the earth was much higher than it now is. the animals and plants which flourished during the ages when the coal-fields were deposited show that sea and land were warmer than at present. it is believed that the change of temperature has taken place on account of the cooling of the earth from radiation. the rate of radiation is so slow, however, that no farther sensible change of temperature can take place for thousands of generations. "the form of the earth also indicates that it was once fluid. the earth is an oblate spheroid, a flattened sphere, and has that degree of flatness which a fluid mass would assume if revolving at its present rate. the earth swells at the equator and rises thirteen or fourteen miles above the sea level at the poles. the waters of the ocean move freely and take the same form as if the whole globe were fluid, and the solid parts of the earth have the same degree of convexity, which shows that it took its form from its own rotation upon its axis while in a fluid state. this would also show that in the primal ages, when the earth was in a plastic or fluid state, it had the same rate of rotation as at present. "the lifting up of the mountain ranges also is best explained by supposing that the earth was once molten. the earth cooled, a crust was formed, and by farther cooling and contraction of the molten mass within the crust wrinkled and formed mountain chains. thus the higher temperature of the geologic ages, the form of the earth as if it were a revolving fluid mass, and the corrugation of its surface--these, joined with its present internal heat, point to the fact that it was once molten and fluid to its surface. the benefits of this heat laid up in store on the day of creation we still enjoy." "before the class is dismissed," said mr. hume, "i should like to say a few words." "i have nothing farther to say to-day," answered mr. wilton, "and we should be glad to hear you now. say on." "i wish only to say that these lessons have led me to such thoughts of god's wisdom and goodness as i never had before. of course it is not strange that this should be the case with me. i now look at everything with new eyes. it is not merely this one element of heat in nature that moves my admiration, but i have been led to consider a thousand things in which the goodness of god is shown. my thoughts of the divine goodness are as fresh and interesting to me as my impressions of his righteousness and holiness are startling. for years i have tried with might and main to look upon the dark side of the world and to exaggerate its physical evils. i have searched for disorder and want of adaptation. as long as i misunderstood the purpose of the creation, i thought i was successful in impugning the wisdom of the arrangements of this physical world. while i supposed that the earth must needs be the creator's masterpiece in beauty and pleasantness and all manner of perfections, designed just to give sensual pleasure to its inhabitants, i could find, or thought i found, many faults in the creator's work. now i withdraw all my former charges. my eyes are opened. the rougher elements of man's life will henceforth have a new meaning to me. i see that god seeks not so much present pleasure for men as their holiness. he lays a solid foundation for their happiness. he seeks to render men blessed by bringing them into likeness and union with himself. these are new views to me, and i thank my heavenly father that this new light has dawned upon me. i feel now that i can bear the ills of this life cheerfully, understanding that the lord is using them as a means of spiritual discipline. it seems to me as if this lower world and man's lowly life were already glorified by a beam of light falling from heaven. i hope that my young friends have been as much profited as i have been." "i rejoice with you, mr. hume. 'we know that all things work together for good to them that love god.' this light has shone upon me for many years." chapter xv. economy of heat. "in this final lesson i wish," said mr. wilton, "to bring before you some general views of the whole subject of the agency and management of heat. "when jesus had fed the five thousand men upon the mountain side by the sea of galilee, he said to his disciples, 'gather up the fragments that remain, that nothing be lost.' the christ who spoke these words was the same christ by whom 'all things were created that are in heaven and that are in the earth, visible and invisible.' these words inculcate the propriety of saving, the very opposite of extravagance and wastefulness. the same prudent economy we find in all god's works. nothing is wasted. god provides bountifully; he is not stinted in his works; we find nothing narrow or mean; his resources are ample for all his undertakings. perhaps a careless observer might charge him with prodigality and wastefulness. the wilderness rejoices in beauty and fertility upon which no human eye gazes, and which supplies no human want. 'full many a gem of purest ray serene the dark unfathomed caves of ocean bear; full many a flower is born to blush unseen, and waste its sweetness on the desert air.' rich fruit grows ruddy and golden in the autumnal sun only to fall and decay. how small a part of the seeds which might germinate and reproduce the parent plant ever fulfill this their legitimate object! but this is not waste. as for the beauty with which the unpeopled wastes are smiling, we know not what other beings besides man 'grow glad at the sight.' fruits and grains and seeds were appointed as much to nourish the animal kingdom as to reproduce plants and trees. and that which decays is not wasted. the oak lifts high its leafy arms and does battle with the tempests for a century, and then having served its purpose in nature, if man does not call it to the higher mission of serving his purposes, nature begins to pull down the structure she has reared and rebuild the elements in other forms--such forms as man perchance may need. the fruit that falls and decays is not wasted; it shall blush with golden tints in other forms and in other years. god pulls down the old that he may build the new. the same elements appear and reappear in a thousand shapes. there is endless change, but no waste. this sentiment, 'gather up the fragments, that nothing be lost,' which is proclaimed throughout all nature, is uttered most emphatically in the management of heat. god has provided most bountiful stores of heat, but has left no heat to go to waste. will you, mr. hume, suggest one of the general arrangements for the economical use of heat?" "i think that the arrangement for economizing heat which ought to be mentioned first is the confinement of heat to the locality where it is needed." "will you explain that a little farther, mr. hume?" "all living creatures are confined near the surface of the earth. they penetrate only a few feet into the earth and soar a few hundred feet above it. heat is therefore confined to the region of the earth's surface. it penetrates but a little way below the surface, and when warm air rises into the higher regions, heat becomes latent. the higher parts of the atmosphere are cold, and in the empty spaces of the heavens the temperature is we know not how low. god has provided for heating only that part of the world which needs to be heated. i think you spoke of this in some one of the earlier lessons." "perhaps i did. but i refer to it again to call especial attention to the idea of the economical use of heat. who will mention another method by which heat is economized?" no one answered. "i asked the question, but did not expect an answer. god shows economy in the use of heat by accomplishing many different results by its agency. i do not mean that the same identical heat accomplishes different results at the same time. the same force cannot accomplish two works. as man cannot spend his money and at the same time keep it, no more can heat be used and not used up in that form. the heat which raises the temperature can do nothing else at the same time, and when it is employed as force it ceases to affect temperature. but by this one agency of heat the creator brings very various works to pass. heat expands bodies, relaxes cohesive attraction, and brings the chemical affinities into activity. by this means the elements of nature are subdued to human uses, seeds germinate, all the processes of vegetable life go on, and digestion and nutrition are carried forward in the bodies of animals. by the agency of heat the winds blow, the deep waters of the ocean circulate, clouds are formed, dew and rain refresh the earth, rivers flow, and all the activities of life fill the world. the employment of one agency for the accomplishment of so many works indicates economy in the expenditure of force and means. moreover, the same heat appears and reappears again and again, passing from the sensible to the latent form and back again, asserting itself alternately in raising the temperature and as active force. a beam of heat falls upon our world: it is partly absorbed by the earth, and warms it. a part of that warmth is used in setting the chemical affinities in action in the sprouting of seeds; a part warms the air by conduction; a part is radiated, and being stopped by the vapor in the air, warms it; the heat of the air is partly used in the evaporation of water: the vapor formed is condensed and waters the earth, and gives out the heat by which it was formed; that raises the temperature of the air; a part of it is used in deoxidizing carbonic acid and building up the forests; the forest tree falls by the woodman's axe, is burned for fuel, and gives out its heat again, or if it falls and decays, the result is the same; the heat given out by combustion cooks the laborer's dinner and warms his room, or it goes out again, and is used in preparing food for the growing wheat; that wheat is used for food, and by slow combustion in the blood the heat is again evolved, the body is warmed, and the chemical operations of digestion and nutrition are maintained; the heat is radiated or conducted from the body into the atmosphere, and again raises the temperature and goes to do other work. at last, so far as our earth is concerned, it escapes into the stellar spaces, and goes to bless other worlds. in all these operations no heat-force is frittered away and wasted and lost. this is one of the accepted doctrines of physical science. heat is used bountifully, but economically and without waste. "even the inequalities and variations of temperature must be counted economy in the use of heat. the heat of midday is not needed at all hours, and therefore it is not always provided; the heat of summer is not always useful, and is therefore not given; a higher temperature for a part of the year and a part of the day is necessary, and is bestowed. the smallest amount of heat is so disposed as to accomplish the largest result. keep in mind, then, the economical aspect of god's management of heat. "i would also have you remember how few are the principles involved in all the ways and means for transporting heat and equalizing temperature. all the various phenomena which we have examined can be brought under two general principles. the first principle or method is the heating and cooling of bodies. bodies absorb heat; they part with their heat by conduction or radiation. if they are heated and cooled without change of place, heat is transported in time, but not in place. if the body be removed from one place to another between the heating and the cooling or between the cooling and the heating, heat is transported in both time and space. this applies alike to solids, liquids, and gases; each one is a carrier of heat in proportion to its specific heat. "the second principle or method is the transportation of heat by the change of sensible to latent heat and its restoration to a sensible state. under this principle there are four cases: " . heat is employed in the evaporation of liquids, and is restored again to use as affecting temperature by the condensation of the vapor. " . heat is employed in liquifying solids, and becomes latent thereby, and returns to the sensible state when the liquid solidifies. these two principles find their grandest application in the changes of water: of this application i have chiefly spoken; but they apply also to other bodies--to metals as well as to liquids. " . heat is rendered latent in the expansion of gases from removal of pressure, and latent heat becomes sensible by the compression of gases. " . heat is employed in the deoxidation of carbonic acid or other combinations of oxygen, and is evolved in combustion. while in the latent condition, heat may be kept without loss for an unlimited period of time or transported from equator to pole. by the various applications of these two general principles, all the different methods of equalizing temperature are determined. "i would have you remember also that these processes for transporting heat and modifying temperature are not confined to the regular changes of days and seasons and the permanent differences of zones, but apply to every possible difference of temperature. one minute the sun shines out in full splendor; the next, a cloud hides his face and cuts off his fervent beams; the methods employed to soften the heat of the one minute and the chill of the next are the same which equalize the temperature of the seasons. evaporation carries off the heat from the seething tropics, evaporation carries off the excess of heat from the bodies of animals and men. the same methods are equally efficient upon the grandest and upon the smallest scale. "in this connection let me give one or two illustrations of the delicacy with which general principles adapt themselves to the minutest circumstances. when the earth is wet, it is fitting that evaporation should go on rapidly and remove the excess of water, but when the ground is drier, it is fitting that evaporation should be checked and the remaining moisture spared. this result is secured not merely by the lack of moisture at the surface, but also by the decreased capacity of the earth for absorbing heat. a dark color absorbs heat more readily than a lighter color, and the earth becomes, as a general rule, darker when wet; and lighter when dry. moist earth, therefore, receives heat more readily than dry earth, and the excessive moisture is the more rapidly carried off by evaporation. "another more interesting illustration is presented by the odor of flowers. in its place i told you that watery vapor hinders the radiation of heat from the earth. dark heat is absorbed by it. the same is true of other gases, and also of the odors of fragrant substances. a bed of flowers fills the air around with odors. by these odors much of the heat radiated by the earth is stopped. by this means the air around the blooming flowers is warmed. the invisible fragrance raises the temperature and secures for the blooming plants a more genial atmosphere. the lord provides for the flowers when most of all they need to be cherished by a congenial warmth. "this completes what i have to say to you upon the subject of heat. i might have gone far more into particulars, and extended these lessons over six months instead of three. we started with the design of finding out whether the works of nature have anything to say about a wise and good creator. we could not examine the whole circle of god's works, and therefore chose a single department--that of heat. i will leave yourselves to decide whether we have found marks of divine wisdom and goodness, whether nature has had anything to say to _us_ about a creator." "it seems to me," said samuel, "that if the works of nature do not show god's goodness and wisdom, it would be hard to tell what works would show them. i think i shall always, after this, look upon the earth and sky with more interest than i have ever felt in them before; i shall always look upon them as having something to do with god." "we certainly ought," said mr. wilton, "to study nature in such a manner and with such a spirit that we shall be led to reverence and worship the creator. some very good men are afraid of scientific study, as if there were something in it to draw men from belief in the scriptures and the jehovah revealed in them; and it cannot be denied that not a few unbelievers have tried to find a foundation and a defence for their infidelity in scientific studies; but such men are not made skeptics by earnest and reverent study of god's works: they were unbelievers before and aside from physical studies, and they only try to glorify their rejection of the bible and christ by deifying science and the creation and holding them up in opposition to inspired revelations. if ever you find the works of god separating you from god, you may know at once that you misunderstand those works or come to them with a wrong spirit. 'the undevout astronomer,' it has been said, 'is mad,' and the same might, with good reason, be said of every undevout student of physical science. "in selecting heat for our examination, i did not take the only rich department of nature's works. the practical chemist would find a richer and broader field of research, and so would the anatomist and animal physiologist, the geologist, or the physical geographer. i purposely chose a comparatively narrow field, in order that our course of study might not become wearisome by its length. you will find ample scope in the fields of natural science for your largest powers, and enough to carry your thoughts reverently to the great creator and governor. "in one respect the study of nature resembles the study of the sacred scriptures. it is a revelation; it is an embodiment of god's thoughts; in it god has expressed himself; and nature, by most suggestive symbols and types, teaches much more moral truth and spiritual sentiment than some men think. in the brute creation it gives us, in pantomime, all the virtues and graces and all repulsive vices and cruel passions. to this book of nature we ought to come without prejudice, reverently inquiring what is written therein. we must study it thoroughly and interpret it as we interpret the written word, comparing scripture with scripture. it is a great attainment to be able to read and understand the thoughts of god embodied in his works. "in another respect, the book of nature and the sacred scriptures have very little in common. the bible is occupied pre-eminently with moral duties and spiritual relationship. its great themes are sin and salvation. christ is the great central truth. one might compare the scriptures to a picture in which one central figure seizes every eye, and by whose radiance the whole picture is filled with light, and that central figure is christ; or we might compare the bible to a sublime oratorio, the glorious symphony of the ages; through it all is heard one strain, sweetly exultant as angel voices, faintly heard at first amid the sadness of the fall, but rising still above the terrific bass of sinai and its ever-repeating echoes, growing more clear and strong upon the harps of the prophets, till its rapturous beauty pours itself triumphant along the plains of bethlehem. in this revelation of salvation from the guilt and ruin of sin the bible stands alone. upon this subject nature is silent. salvation by christ is the gem enshrined in the scriptures. but what is the setting for this gem? the works of god on the earth and in the heavens. the prophets were men in sympathy with nature. how david sung the praises of the divine handiwork!--'o lord, how manifold are thy works; in wisdom hast thou made them all.' 'the heavens declare the glory of god and the firmament showeth his handiwork. day unto day uttereth speech, and night unto night showeth knowledge. there is no speech nor language where their voice is not heard.' how christ unfolded the deepest spiritual truths by the symbols of nature! but if the casket be so worthy, what shall be said of the gem which is enshrined within? that is the pearl of great price. to that book which speaks in no doubtful voice of deliverance from sin let us turn with increasing reverence; and above all, let us come to him who came to reveal our god, who came to be as well as to make a revelation of god, being himself 'the brightness of his glory and the express image of his person.' i am glad that you all now feel that you know him whom to know is everlasting life." from these words of mr. wilton you will conclude that ansel has at length found rest in christ. in another brief chapter i will tell you of his experience, and then bid you adieu. chapter xvi. a day of joy and gladness. the reader has already learned that after ansel had confessed himself an anxious inquirer and professed himself willing to obey christ, he remained three or four weeks still in darkness. others found peace in believing, but he felt no joyful confidence that christ had received him and forgiven his sins. he sometimes felt almost discouraged, and sometimes was tempted to complain of god for not treating him as favorably as others, or to feel chagrined because others were rejoicing, while he found no light. but he fought against these evil thoughts and insinuations of satan, and did not flag in his private devotions or cease to confess himself, always and everywhere, an anxious inquirer, still in darkness, but desiring to find the grace of god. if ever he was tempted to push away all concern about salvation and return by force to his former careless state, the words of christ would come to his mind: "will ye also go away?" and peter's answer, "lord, to whom shall we go? for thou hast the words of eternal life." the alternative, salvation by christ or the loss of his soul, stared him in the face. "i can but perish if i go; i am resolved to try; for if i stay away, i know, i must for ever die." great interest was felt for him and much prayer was offered in his behalf, but he seemed to make no progress toward a better state. mr. wilton had talked with him, but had failed to discover what it was that hindered his humble acceptance of the grace of christ. after long and anxious musing upon ansel's character and surroundings and the previous conversations which he had had with him, mr. wilton determined to probe him more fully. for this reason he invited ansel to his study, where the following conversation transpired: "good-morning, my young friend; how do you find yourself to-day?" "i am feeling, i think, very much as when i was here a week ago." "are you becoming discouraged and almost ready to give up all effort to follow christ?" "i do sometimes feel very much discouraged, but i am not ready to give up my interest in religion." "have you no more enjoyment in reading the scriptures and in your prayer in secret than you had a week ago?" "i think that i am trying to do right in doing these things, and i enjoy them better than i should if i felt that i was doing something wrong, but i do not feel as i think a christian ought to feel." "are your thoughts and feelings and opinions about christ and salvation the same as they were six weeks ago?" "i think they are very different." "i am glad to hear that; but can you tell how they are different?" "at that time i felt that i was a sinner, but was fighting against that feeling. i wished that christ would let me alone, and that the holy spirit would not trouble me. but now i very much wish that i may feel my sins, and that christ may come to me and save me. i wish to follow the spirit." "did you expect a month ago that at this time you would be feeling and acting as you now feel and act?" "no, sir; i meant then to fight it through, and not let anybody know how i felt." "do you wish now that you had fought it through, as you proposed, and kept all your feelings to yourself?" "i am very thankful that i did not keep on hiding my feelings. i almost tremble to think what the result would have been." "you have said that you wish to spend your life in serving christ. does it seem to you a hard and painful work--a work that you would get rid of if you could--or does working for christ and confessing christ before men seem attractive?" "i think his service seems pleasant; there is no other life that seems half as pleasant." "do you believe that christ is able to save you?" "i suppose he is. if he cannot save me, there is no hope for me, for i cannot save myself." "do you believe that he is willing to save you?" "i think he is, if i come to him and trust in him. i suppose he is willing to save all who come to him." "are you unwilling to come to him--to trust him and submit to him?" "i don't know; i have tried to come to christ, but i have met with no such change as i have always supposed that a christian ought to have." "what do you think it is that hinders your coming into light and joy as others have done?" "i cannot tell. i suppose it must be something or other in myself, but i cannot guess what it is." "i would like to ask you a few questions which you may think rather close and personal, and which you may find it hard to answer frankly. you know the spiritual adviser, as well as the physician, must first of all find out the condition of the patient." "i am willing to have you ask any questions you please, and i will try to answer them as well as i can." "did you ever think, ansel, that you were very ambitious?" "i knew that, like many others, i was a little ambitious, but i never thought that i was very much so." "perhaps you were more ambitious than you thought. you know that you would work day and night rather than not stand at the head of every class you were in. on the play-ground you asserted your position as leader in every game. did you not carry the same idea of being chief into your plans and expectations for the future? you were ambitious of standing the very first whatever course of life you might follow. was not this so?" "i don't know: i can't deny it; i think it was." "it is possible, ansel, that you are trying to carry the same ambition into the kingdom of christ. perhaps you have wished in conversion some brilliant experience which would draw attention to you. tell me how this is. would you be satisfied to have a commonplace experience, such as thousands of others have, which would attract no special notice? have you not formed an idea of the great and brilliant change you must pass through, and are you not refusing to take anything else from the lord's hands?" tears gathered in ansel's eyes, and his face worked painfully. at length he answered: "your question is a hard one to answer, but i cannot deny it; i am afraid it is so. i have heard persons tell of the great load of sin like a pack on their shoulders, and of the earth seeming as if it would open and swallow them up, of sleepless nights and unspeakable anguish, and then of light and joy, so that they could never doubt that they were converted. i have been expecting that i was to have such an experience, but i have not seen it. is it wrong to wish for such an experience?" "it is certainly wrong to _insist_ upon such an experience. god leads each one to himself in his own chosen way. there was but one saul, whom christ met and blinded with the dazzling light. as a general rule, when a sinner makes up his mind in what way he will be converted, the lord will disappoint him. if he fixes in his mind that he will not come to an anxious-seat, or will not confess his feelings till he can say that his sins are pardoned, or will not do anything else, the lord will very likely bring him to do the very thing he resolved that he would not do. if he attempts to bring his ambitious aspirations into christ's kingdom, he will be disappointed. 'the first shall be last and the last first.' men become great in christian service by counting themselves the least of all, and humbling themselves to become the servants of all. you need to examine yourself in this matter. if you have looked for something great and startling, be contented with something small and commonplace. it is an unspeakable privilege to be brought into christ's kingdom in any manner. it is sometimes a great blessing to have a very unmarked and plain style of conversion. such a convert is compelled to look to the truly scriptural evidences of a change of heart instead of resting upon the evidence, often deceptive, of a great and sudden illumination or a fancied voice from heaven. some of the greatest and best of men have been unable to tell at all the time of their conversion. richard baxter could not tell even the year of his change. the best experiences i have known have been those where the converts could tell very little about themselves; they had been doing something else besides looking into themselves to watch the motions of their own thoughts." "i will try to do as you say. but what kind of evidence am i to look for?" "the same kind of evidence which you now look for in me or any other christian. it is not one thing to come to christ and another thing to follow christ. the best evidence that a sinner has come to christ is that he actually follows christ and serves him. 'by their fruits ye shall know them.' 'bring forth fruits meet for repentance,' said john the baptist. bring forth fruits that show that your thoughts about sin, and about christ, and about the service of christ have been changed. look for the same kind of evidence in yourself that you would look for in any stranger whom you should meet. but above all things take the words of jesus as true and rest on them; consecrate yourself to jesus with all the heart; with lowliness of mind hold yourself ready for any work or any sacrifice; you will find that evidences will take care of themselves. when men come into sympathy with christ, when they believe his words, walk with him, and talk with him, and bear the cross with him, when they enter into a partnership of service and suffering with christ,--the spirit bears witness with their spirits that they are born of god." "i will try to follow your advice, and am very thankful that you have spoken about my ambitious spirit." "another caution i wish to give you. do not think that you, by any methods or by cherishing any spirit, are to make yourself fit to be saved. if you are saved at all, christ must take you as a sinner, and a great sinner. if you get rid of your spirit of pride, it will be by christ's saving you from it. let me also suggest to you that which a consideration of your associations suggested to me, that you may have stumbled at the idea of baptism. you must have heard baptism spoken of very disrespectfully, and it is possible that you may have learned to look upon it as a humiliation and a reproach. you may have recoiled from the thought of submitting to it." "that was my feeling once, but since i have been willing to have my feelings known i have ceased to be afraid of what those who despise religion may say." "be careful now, since you feel that your sympathies are with the christian band, that your love of greatness does not lead you to resist the spirit. be willing to be small. be thankful for small gifts. i trust that your present feelings will before long give place to a humble trust, a childlike confidence, and a holy boldness in christ, and that your usefulness in the kingdom of god will be all the greater because he now requires you in the beginning to trample under foot your budding pride and die to all human ambitions." when ansel gave up the idea of a wonderful conversion, a sudden illumination which should bring with it something of éclat, he found that he could understand the scriptures better and have more enjoyment in his religious duties. while he humbled himself, hoping for little, he found his soul soon filled with a deep, quiet joy. the next saturday afternoon was the regular time for the covenant-meeting, and also, according to custom, for hearing the experiences of any who wished to unite with the church by baptism. ansel, peter, and mr. hume came, along with others, to present themselves to the church. in regard to mr. hume there had been much speculation among his former comrades as to what course he would take. some said: "mr. hume will never wet the sole of his foot in that river. don't you remember how he used to laugh at the idea of being plunged in the river in honor of a dead man? he may talk in meeting, but it is a very different thing to go down into the river with the whole hillside covered with people." others said: "we can't tell what has come over him, but he will not go back now. he has gone too far to retreat." some even ventured to approach mr. hume himself with their raillery: "what do you think now of being dipped in the river in honor of a dead man?" "i think that i would be willing to be baptized a thousand times if i could recall by that means what i have spoken against baptism." "and what, mr. hume, about the ice water?" "you know and i know," he answered, "that we always respected those who did not shrink from cold water for christ's sake. what effeminacy, what more than effeminacy, for a resolute man to hesitate and tremble at baptism! we should be ashamed of such weakness in any worldly matter. i have given you occasion for all your raillery, but as i once was a leader in evil, so i wish that i might lead you to better things." ansel, peter, and the rest gave an account of their religious experiences, and last of all mr. hume. "what leads you," asked mr. wilton, "to present yourself to the church, asking for baptism?" "i think that the love of christ leads me. i have done a great deal against christ, and now i wish, if possible, to do something to show my love for him. i come to obey the word and example of christ by being buried with him in baptism." they were received for baptism, and the time of administration fixed at half-past twelve o'clock the next day. the lord's day was cold and blustering. many were disappointed, for they hoped that the day would prove warm and sunny. but the blustering day did not prevent the gathering of a great company by the riverside. as the congregations left the churches they turned their steps toward the place of baptism. ungodly men turned out, and those who never came to hear the preaching of the gospel flocked together to see the gospel preached by this symbolic service. the word had gone out that mr. hume was to be baptized, and this drew together his former associates. at the place chosen the river swept around in a gentle curve and the bank rose up like a magnificent amphitheatre; while just above, the land put out into the water and threw the current upon the opposite side. here gathered almost the entire population of the village to witness that simple and solemn service which from the days of john the baptist has thrilled so many hearts. the candidates came warmly clad, brought from their own homes in a close carriage. gathered there, the little band of christians, surrounded by so great a cloud of witnesses, first sang the hymn commencing: "thou hast said, exalted jesus, take thy cross and follow me; shall the word with terror seize us? shall we from the burden flee? lord, i'll take it, and rejoicing follow thee." then mr. wilton read with a voice that reached all the company a few passages from the new testament which authorized and commanded that service. after that he prayed that the joyful presence of christ might attend those about to follow him in baptism, that believers might be encouraged, and careless sinners awakened. one by one the converts were buried with christ, and one by one they came up out of the water, forgetting all else in the joy of obedience. they sang the words consecrated by use at so many riversides: "oh how happy are they who their saviour obey, and have laid up their treasure above! tongue can never express the sweet comfort and peace of a soul in its earliest love." these words found a response in many hearts. high up upon the river bank were gathered a little knot of mocking unbelievers. one among them, seven years before, had publicly professed his faith in christ. for a little time he seemed to be treading in the lord's ways, but falling among evil associates, he not only neglected christian duties, but became a professed unbeliever. he read infidel books and loaned them to others. he sought to sow the seeds of unbelief wherever he went. upon this lord's day he stood with others profanely mocking at the sacred service. with shivering, tremulous accents he exclaimed, "poor harry gill is very cold; i would not go into the water to please any christ for five hundred dollars." that young man went home with deep conviction of sin upon him. two days after, mr. wilton was called at ten o'clock at night to visit him. he was trembling like an aspen leaf with his deep anguish of conscience, and for two days and nights his body shook under his fear. then little by little faith took the place of fear, and hope smiled upon him. he was the next person whom mr. wilton baptized. look in upon the christian band assembled that lord's day evening. upon the faces of those who had been baptized there was no sign that the service of that day had been painful; if they had done the duty as a cross, the cross must have been quickly followed by a crown of joy, for every face was radiant with light. among them was one little girl twelve years of age whose face, as she rose from the water, shone like the face of an angel, and the transfiguration of that moment had hardly begun to fade away. ansel was peacefully happy, and from the face of mr. hume the old look of dissatisfaction was all gone; his soul had entered into rest, and he felt at home. every one of them testified that it had been the happiest day of his life. they declared themselves willing for christ's sake to be baptized a hundred times if he commanded. they had already found that "in keeping his commandments there is great reward." i should be glad, kind reader, to trace with you the christian course of these disciples through the years that follow. but we must leave them. i am sure, however, that their course will be upward. their experience was not the mere effervescence of fickle feeling. the word of god germinated in their hearts; they had root in themselves. they believed, they believed the truths of the gospel, and therefore they felt, and therefore they acted. "whatsoever is born of god overcometh the world," and believing that they were truly born of the spirit, we are confident that "he which hath begun a good work in them will perform it until the day of jesus christ." none trinity site: - . a national historic landmark white sands missile range, new mexico contents: radiation at trinity site. how to get to trinity site. trinity site national historic landmark. the manhattan project. the theory. building a test site. jumbo. bomb assembly. the test. after the explosion. it's the schmidt house. afterwards. white sands missile range. reading list. "the effects could well be called unprecedented, magnificent, beautiful, stupendous, and terrifying. no man-made phenomenon of such tremendous power had ever occurred before. the lighting effects beggared description. the whole country was lighted by a searing light with the intensity many times that of the midday sun." brig. gen. thomas farrell radiation at trinity site in deciding whether to visit ground zero at trinity site, the following information may prove helpful to you. radiation levels in the fenced, ground zero area are low. on an average the levels are only times greater than the region's natural background radiation. a one-hour visit to the inner fenced area will result in a whole body exposure of one-half to one milliroentgen. to put this in perspective, a u.s. adult receives an average exposure of milliroentgens every year from natural and medical sources. for instance, the department of energy says we receive between and milliroentgens every year from the sun and from to milliroentgens every year from our food. living in a brick house adds milliroentgens of exposure every year compared to living in a frame house. finally, flying coast to coast in a jet airliner gives an exposure of between three and five milliroentgens on each trip. although radiation levels are low, some feel any extra exposure should be avoided. the decision is yours. it should be noted that small children and pregnant women are potentially more at risk than the rest of the population and are generally considered groups who should only receive exposure in conjunction with medical diagnosis and treatment. again, the choice is yours. at ground zero, trinitite, the green, glassy substance found in the area, is still radioactive and must not be picked up. typical radiation exposures for americans per the national council on radiation protection on hour at ground zero = / mrem cosmic rays from space = mrem at sea level per year radioactive minerals in rocks and soil = mrems per year radioactivity from air, water, and food = anywhere from to mrem per year about mrem per chest x-ray and mrem for whole-mouth dental x- rays smoking one pack of cigarettes a day for one year = mrem miscellaneous such as watch dials and smoke detectors = mrem per year how to get to trinity site trinity site, where the world's first atomic bomb was exploded in , is normally open to the public twice a year--on the first saturday in april and october. trinity is located on the northern end of the , -square-mile white sands missile range, n.m., between the towns of carrizozo and socorro, n.m. there are two ways of entering the restricted missile range on tour days. visitors can enter through the range's stallion range center which is five miles south of highway . the turnoff is miles east of san antonio, n.m., and miles west of carrizozo, n.m. the stallion gate will be open a.m. to p.m. visitors arriving at the gate between those hours will receive handouts and will be allowed to drive unescorted the miles to trinity site. the road is paved and marked. the other way of entering the missile range is by travelling with a caravan sponsored by the alamogordo (n.m.) chamber of commerce. the caravan forms at the otero county fairgrounds in alamogordo and leaves at a.m. visitors entering this way will travel as an escorted group with military police to and from trinity site. the drive is miles round trip. there are no service station facilities on the missile range. the caravan is scheduled to leave trinity site at : p.m. for the return to alamogordo. the caravan may leave later if there is a large number of vehicles in the returning caravan. in , an additional open house will be conducted on july , the th anniversary of the trinity test. visitors may enter the missile range through the stallion range center gate from to a.m. there will be no caravan leaving from alamogordo, n.m., for this event. the early hours will allow visitors to be on-site at : : a.m., the time the trinity site detonation occurred, and should help visitors avoid the -plus degree afternoon temperatures common here in july. included on the trinity site tour is ground zero where the atomic bomb was placed on a -foot steel tower and exploded on july , . a small monument now marks the spot. visitors also see the mcdonald ranch house where the world's first plutonium core for a bomb was assembled. the missile range provides historical photographs and a fat man bomb casing for display. there are no ceremonies or speakers. portable toilet facilities are available on site. hot dogs and sodas are sold at the parking lot. cameras are allowed at trinity site, but their use is strictly prohibited anywhere else on white sands missile range. for more information, contact the white sands missile range public affairs office at ( ) - / . trinity site national historic landmark trinity site is where the first atomic bomb was tested at : : a.m. mountain war time on july , . the kiloton explosion not only led to a quick end to the war in the pacific but also ushered the world into the atomic age. all life on earth has been touched by the event which took place here. the , -acre area was declared a national historic landmark in . the landmark includes base camp, where the scientists and support group lived; ground zero, where the bomb was placed for the explosion; and the mcdonald ranch house, where the plutonium core to the bomb was assembled. on your visit to trinity site you will be able to see ground zero and the mcdonald ranch house. in addition, on your drive into the trinity site area you will pass one of the old instrumentation bunkers which is beside the road just west of ground zero. the manhattan project the story of trinity site begins with the formation of the manhattan project in june . the project was given overall responsibility of designing and building an atomic bomb. at the time it was a race to beat the germans who, according to intelligence reports, were building their own atomic bomb. under the manhattan project three large facilities were constructed. at oak ridge, tenn., huge gas diffusion and electromagnetic process plants were built to separate uranium from its more common form, uranium . hanford, wash. became the home for nuclear reactors which produced a new element called plutonium. both uranium and plutonium are fissionable and can be used to produce an atomic explosion. los alamos was established in northern new mexico to design and build the bomb. at los alamos many of the greatest scientific minds of the day labored over the theory and actual construction of the device. the group was led by dr. j. robert oppenheimer who is credited with being the driving force behind building a workable bomb by the end of the war. the theory los alamos scientists devised two designs for an atomic bomb--one using the uranium and another using the plutonium. the uranium bomb was a simple design and scientists were confident it would work without testing. the plutonium bomb worked by compressing the plutonium into a critical mass which sustains a chain reaction. the compression of the plutonium ball was to be accomplished by surrounding it with lens-shaped charges of conventional explosives. they were designed to all explode at the same instant. the force is directed inward, thus smashing the plutonium from all sides. in an atomic explosion, a chain reaction picks up speed as atoms split, releasing neutrons plus great amounts of energy. the escaping neutrons strike and split more atoms, thus releasing still more neutrons and energy. in a nuclear explosion this all occurs in a millionth of a second with billions of atoms being split. project leaders decided a test of the plutonium bomb was essential before it could be used as a weapon of war. from a list of eight sites in california, texas, new mexico and colorado, trinity site was chosen as the test site. the area already was controlled by the government because it was part of the alamogordo bombing and gunnery range which was established in . the secluded jornado del muerto was perfect as it provided isolation for secrecy and safety, but was still close to los alamos. building a test site in the fall of soldiers started arriving at trinity site to prepare for the test. marvin davis and his military police unit arrived from los alamos at the site on dec. , . the unit set up security checkpoints around the area and had plans to use horses to ride patrol. according to davis the distances were too great and they resorted to jeeps and trucks for transportation. the horses were sometimes used for polo, however. davis said that capt. bush, base camp commander, somehow got the soldiers real polo equipment to play with but they preferred brooms and a soccer ball. other recreation at the site included volleyball and hunting. davis said capt. bush allowed the soldiers with experience to use the army rifles to hunt deer and pronghorn. the meat was then cooked up in the mess hall. leftovers went into soups which davis said were excellent. of course, some of the soldiers were from cities and unfamiliar with being outdoors a lot. davis said he went to relieve a guard at the mockingbird gap post and the soldier told davis he was surprised by the number of "crawdads" in the area considering it was so dry. davis gave the young man a quick lesson on scorpions and warned him not to touch. throughout other personnel arrived at trinity site to help prepare for the test. carl rudder was inducted into the army on jan. , . he said he passed through four camps, took basic for two days and arrived at trinity site on feb. . on arriving he was put in charge of what he called the "east jesus and socorro light and water company." it was a one-man operation--himself. he was responsible for maintaining generators, wells, pumps and doing the power line work. a friend of rudder's, loren bourg, had a similar experience. he was a fireman in civil life and ended up trained as a fireman for the army. he worked as the station sergeant at los alamos before being sent to trinity site in april . in a letter bourg said, "i was sent down here to take over the fire prevention and fire department. upon arrival i found i was the fire department, period." as the soldiers at trinity site settled in they became familiar with socorro. they tried to use the water out of the ranch wells but found it so alkaline they couldn't drink it. in fact, they used navy salt-water soap for bathing. they hauled drinking water from the fire house in socorro. gasoline and diesel was purchased from the standard bulk plant in socorro. according to davis, they established a post office box, number , in socorro so getting their mail was more convenient. the trips into town also offered them the chance to get their hair cut in a real barbershop. if they didn't use the shop, sgt. greyshock used horse clippers to trim their hair. jumbo the bomb design to be used at trinity site actually involved two explosions. first there would be a conventional explosion involving the tnt and then, a fraction of a second later, the nuclear explosion, if a chain reaction was maintained. the scientists were sure the tnt would explode, but were initially unsure of the plutonium. if the chain reaction failed to occur, the tnt would blow the very rare and dangerous plutonium all over the countryside. because of this possibility, jumbo was designed and built. originally it was feet long, feet in diameter and weighed tons. scientists were planning to put the bomb in this huge steel jug because it could contain the tnt explosion if the chain reaction failed to materialize. this would prevent the plutonium from being lost. if the explosion occurred as planned, jumbo would be vaporized. jumbo was brought to pope, n.m., by rail and unloaded. a specially built trailer with wheels was used to move jumbo the miles to trinity site. as confidence in the plutonium bomb design grew it was decided not to use jumbo. instead, it was placed in a steel tower about yards from ground zero. the blast destroyed the tower, but jumbo survived intact. today jumbo rests at the entrance to ground zero so all can see it. the ends are missing because, in , the army detonated eight -pound bombs inside it. because jumbo was standing on end, the bombs were stacked in the bottom and the asymmetry of the explosion blew the ends off. to calibrate the instruments which would be measuring the atomic explosion and to practice a countdown, the manhattan scientists ran a simulated blast on may . they stacked tons of tnt onto a -foot wooden platform just southeast of ground zero. louis hemplemann inserted a small amount of radioactive material from hanford into tubes running through the stack of crates. the scientists hoped to get a feel for how the radiation might spread in the real test by analyzing this test. the explosion destroyed the platform, leaving a small crater with trace amounts of radiation in it. bomb assembly on july the two hemispheres of plutonium were carried to the george mcdonald ranch house just two miles from ground zero. at the house, brig. gen. thomas farrell, deputy to maj. gen. leslie groves, was asked to sign a receipt for the plutonium. farrell later said, "i recall that i asked them if i was going to sign for it shouldn't i take it and handle it. so i took this heavy ball in my hand and i felt it growing warm, i got a certain sense of its hidden power. it wasn't a cold piece of metal, but it was really a piece of metal that seemed to be working inside. then maybe for the first time i began to believe some of the fantastic tales the scientists had told about this nuclear power." at the mcdonald ranch house the master bedroom had been turned into a clean room for the assembly of the bomb core. according to robert bacher, a member of the assembly team, they tried to use only tools and materials from a special kit. several of these kits existed and some were already on their way to tinian, the island in the pacific which was the base for the bombers. the idea was to test the procedures and tools at trinity as well as the bomb itself. at one minute past midnight on friday, july , the explosive assembly left los alamos for trinity site. later in the morning, assembly of the plutonium core began. according to raemer schreiber, robert bacher was the advisor and marshall holloway and philip morrison had overall responsibility. louis slotin, boyce mcdaniel and cyril smith were responsible for the mechanical assembly in the ranch house. later holloway was responsible for the mechanical assembly at the tower. in the afternoon of the th the core was taken to ground zero for insertion into the bomb mechanism. the bomb was assembled under the tower on july . the plutonium core was inserted into the device with some difficulty. on the first try it stuck. after letting the temperatures of the plutonium and casing equalize the core slid smoothly into place. once the assembly was complete many of the men took a welcome relief and went swimming in the water tank east of the mcdonald ranch house. the next morning the entire bomb was raised to the top of the foot steel tower and placed in a small shelter. a crew then attached all the detonators and by p.m. it was complete. the test three observation points were established at , yards from ground zero. these were wooden shelters protected by concrete and earth. the south bunker served as the control center for the test. the automatic firing device was triggered from there as key men such as dr. robert oppenheimer, head of los alamos, watched. none of the manned bunkers are left. many scientists and support personnel, including gen. leslie groves, head of the manhattan project, watched the explosion from base camp which was ten miles southwest of ground zero. all the buildings at base camp were removed after the test. most visiting vips watched from compania hill, miles northwest of ground zero. the test was scheduled for a.m. july , but rain and lightning early that morning caused it to be postponed. the device could not be exploded under rainy conditions because rain and winds would increase the danger from radioactive fallout and interfere with observation of the test. at : a.m. the crucial weather report came through announcing calm to light winds with broken clouds for the following two hours. at : the countdown started and at : : the device exploded successfully. to most observers the brilliance of the light from the explosion--watched through dark glasses--overshadowed the shock wave and sound that arrived later. hans bethe, one of the contributing scientists, wrote "it looked like a giant magnesium flare which kept on for what seemed a whole minute but was actually one or two seconds. the white ball grew and after a few seconds became clouded with dust whipped up by the explosion from the ground and rose and left behind a black trail of dust particles." joe mckibben, another scientist, said, "we had a lot of flood lights on for taking movies of the control panel. when the bomb went off, the lights were drowned out by the big light coming in through the open door in the back." others were impressed by the heat they immediately felt. military policeman davis said, "the heat was like opening up an oven door, even at miles." dr. phillip morrison said, "suddenly, not only was there a bright light but where we were, miles away, there was the heat of the sun on our faces....then, only minutes later, the real sun rose and again you felt the same heat to the face from the sunrise. so we saw two sunrises." after the explosion although no information on the test was released until after the atomic bomb was used as a weapon against japan, people in new mexico knew something had happened. the shock broke windows miles away and was felt by many at least miles away. army officials simply stated that a munitions storage area had accidentally exploded at the alamogordo bombing range. the explosion did not make much of a crater. most eyewitnesses describe the area as more of a small depression instead of a crater. the heat of the blast did melt the desert sand and turn it into a green glassy substance. it was called trinitite and can still be seen in the area. at one time trinitite completely covered the depression made by the explosion. afterwards the depression was filled and much of the trinitite was taken away by the nuclear energy commission. to the west of the monument is a low structure which is protecting an original portion of the crater area. trinitite is visible through openings in the roof. it's the schmidt house the george mcdonald ranch house sits within an 'x ' low stone wall. the house was built in by franz schmidt, a german immigrant, and an addition was constructed on the north side in the 's by the mcdonalds. there is a display about the schmidt family in the house during each open house. the ranch house is a one-story, , square-foot building. it is built of adobe which was plastered and painted. an ice house is located on the west side along with an underground cistern which stored rain water running off the roof. at one time the north addition contained a toilet and bathtub which drained into a septic tank northwest of the house. there is a large, divided water storage tank and a chicago aeromotor windmill east of the house. the scientists and support people used the north tank as a swimming pool during the long hot summer of . south of the windmill are the remains of a bunkhouse and a barn which was part garage. further to the east are corrals and holding pens. the buildings and fixtures east of the house have been stabilized to prevent further deterioration. the ranch was abandoned in when the alamogordo bombing and gunnery range took over the land to use in training world war ii bombing crews. the house stood empty until the manhattan project support personnel arrived in early . inside the house the northeast room (the master bedroom) was designated the assembly room. work benches and tables were installed. to keep dust and sand out of instruments and tools, the windows were covered with plastic. tape was used to fasten the edges of the plastic and to seal doors and cracks in the walls. the explosion, only two miles away, did not significantly damage the house. most of the windows were blown out, but the main structure was intact. years of rain water dripping through holes in the roof did much more damage. the barn did not do as well. during the trinity test the roof was bowed inward and some of the roofing was blown away. the roof has since collapsed. the house stood empty and deteriorating until when the u.s. army stabilized the house to prevent any further damage. shortly after, the department of energy and u.s. army provided the funds for the national park service to completely restore the house. the work was done in . all efforts were directed at making the house appear as it did on july , . afterwards the story of what happened at trinity site did not come to light until after the second atomic bomb was exploded over hiroshima, japan, on august . president truman made the announcement that day. three days later, august , the third atomic bomb devastated the city of nagasaki, and on august the japanese surrendered. trinity site became part of what was then white sands proving ground. the proving ground was established on july , , as a test facility to investigate the new rocket technology emerging from world war ii. the land, including trinity site and the old alamogordo bombing range, came under the control of the new rocket and missile testing facility. interest in trinity site was immediate. in september press tours to the site started. one of the famous photos of ground zero shows robert oppenheimer and general leslie groves surrounded by a small group of reporters as they examine one of the footings to the foot tower on which the bomb was placed. that picture was taken sept. . the exposed footing is still visible at ground zero. on sept. - , george cremeens, a young radio reporter from krnt in des moines, visited the site with soundman frank lagouri. they flew over the crater and interviewed dr. kenneth bainbridge, trinity test director, and capt. howard bush, base camp commander. back in iowa, cremeens created four -minute reports on his visit which aired sept. , , and . a -minute composite was made and aired on the abc radio network. for his work cremeens received a local peabody award for "outstanding reporting and interpretation of the news." at first trinity site was encircled with a fence and radiation warning signs were posted. the site remained off-limits to military and civilian personnel of the proving ground and closed to the public. in the atomic energy commission let a contract to clean up the site. much of the trinitite was scraped up and buried. in september about people attended the first trinity site open house. a few years later a small group from tularosa visited the site on an anniversary of the explosion to conduct a religious service and prayers for peace. similar visits have been made annually in recent years on the first saturday in october. in the inner oblong fence was added. in the corridor barbed wire fence which connects the outer fence to the inner one was completed. jumbo was moved to the parking lot in . visits to the site are now made in april and october because it is generally so hot in july on the jornada del muerto. white sands missile range white sands missile range has developed from a simple desert testing site for the v- into one of the most sophisticated test facilities in the world. the mission of white sands missile range begins with a customer--a service developer, or another federal agency, which is ready to find out if engineers and scientists have built something which will perform according to job specifications. it ends when an exhaustive series of tests has been completed and a data report has been delivered to the customer. between the beginning and the end of the test program, be it the army tactical missile system or newly designed automobiles, range employees are involved in every operation connected with the customer and his product. the range can and does provide everything from rat traps to telephones, from equipment hoists and flight safety to microsecond timing. we shake, rattle and roll the product, roast it, freeze it, subject it to nuclear radiation, dip it in salt water and roll it in the mud. we test its paint, bend its frame and find out what effect its propulsion material has on flora and fauna. in the end, if it's a missile, we fire it, record its performance and bring back the pieces for post mortem examination. all test data is reduced and the customer receives a full report. for more information on trinity site or white sands missile range contact: public affairs office (stews-pa) white sands missile range white sands missile range, n.m. - reading list the day the sun rose twice, by ferenc szasz, university of new mexico press, . manhattan: the army and the atomic bomb, by vincent jones, center of military history, u. s. army. trinity, by kenneth bainbridge, los alamos publication (la- -h). the making of the atomic bomb, by richard rhodes, simon and schuster, . now it can be told, by general leslie groves, da capo press, . day one, by peter wyden, simon and schuster, . city of fire: los alamos and the atomic age, - , by james kunetka, university of new mexico press, . los alamos - : the beginning of an era, los alamos publication (lasl- - ). day of trinity, by lansing lamont, atheneum. radiological survey and evaluation of the fallout area from the trinity test: chupadera mesa and white sands missile range, n. m., los alamos publication (la- -ms). life magazine, august and september , . time magazine, august and , . project trinity - by carl maag and steve rohrer united states atmospheric nuclear weapons tests nuclear test personnel review prepared by the defense nuclear agency as executive agency for the department of defense destroy this report when it is no longer needed. do not return to sender. please noitify the defense nuclear agency, attn: stti, wasington d.c. , if your address is incorrect, if you wish to be deleted from the distribution list, or if the addressee is no longer employed by your organization. since declassified contents: list of figures list of abbreviations and acronyms report documentation page fact sheet preface chapters: introduction . historical background of project trinity . the project trinity site . the project trinity organization . military and civilian participants in project trinity the activities at project trinity . preshot activities . detonation and postshot activities . activities after july radiation protection at project trinity . organization . site monitoring group . offsite monitoring group dosimetry analysis of participants in project trinity . film badge records . gamma radiation exposure reference list list of figures - location of alamogordo bombing range - trinity site and major installations - tent used as guard post at project trinity - truck used as guard post at project trinity - organization of project trinity - the trinity shot-tower - the trinity detonation, hours, july - the south shelter (control point) - inside one of the shelters - the base camp, headquarters for project trinity - the base camp, headquarters for project trinity - project trinity personnel wearing protective clothing - "jumbo" after the trinity detonation list of abbreviations and acronyms the following abbreviations and acronyms are used in this volume: aec atomic energy commission dod department of defense lasl los alamos scientific laboratory maud [committee for the] military application of uranium detonation med manhattan engineer district r/h roentgens per hour utm universal transverse mercator report documentation page security classification of this page (when data entered): unclassified . report number: dna f . govt accession no.: . recipient's catalog number: . title (and subtitle): project trinity - . type of report & period covered: final report . performing org. report number: jrb - - - - . author(s): carl maag, steve rorer . contract or grant number(s): dna - -c- . performing organization name and address: jrb associates westpark drive mclean, virginia . program element. project, task area & work unit numbers: subtask u qaxmk - . controlling office name and address: director defense nuclear agency washington, d.c. . report date: december . number of pages: . monitoring agency name & address(if different from controlling office): . security class. (of this report): unclassified a. declassification/downgrading schedule: n/a since unclassified . distribution statement (of this report): approved for public release; distribution unlimited. . distribution statement (of the abstract entered in block , if different from report): . supplementary notes: this work was sponsored by the defense nuclear agency under rdt&e rmss code b u qaxmk h d. for sale by national technical information service, springfield, va . the defense nuclear agency action officer, lt. col. h. l. reese, usaf, under whom this work was done, wishes to acknowledge the research and editing contribution of numerous reviewers in the military services and other organizations in addition to those writers listed in block . . key words (continue on reverse side if necessary and identify by block number): trinity los alamos scientific laboratory alamogordo bombing range manhattan engineer district manhattan project personnel dosimetry radiation exposure nuclear weapons testing . abstract: this report describes the activities of an estimated , personnel, both military and civilian, in project trinity, which culminated in detonation of the first nuclear device, in new mexico in . scientific and diagnostic experiments to evaluate the effects of the nuclear device were the primary activities engaging military personnel. fact sheet defense nuclear agency public affairs office washington, d c. subject: project trinity project trinity, conducted by the manhattan engineer district (med), was designed to test and assess the effects of a nuclear weapon. the trinity nuclear device was detonated on a -foot tower on the alamogordo bombing range in south-central new mexico at hours on july . the nuclear yield of the detonation was equivalent to the energy released by detonating kilotons of tnt. at shot-time, the temperature was . degrees celsius, and surface air pressure was millibars. the winds were nearly calm at the surface; at , feet above mean sea level, they were from the southwest at knots. the winds blew the cloud resulting from the detonation to the northeast. from july through , about , military and civilian personnel took part in project trinity or visited the test site. the location of the test site and its major installations are shown in the accompanying figures. military and scientific activities all participants in project trinity, both military and civilian, were under the authority of the med. no military exercises were conducted. the los alamos scientific laboratory (lasl), which was staffed and administered by the university of california (under contract to the med), conducted diagnostic experiments. civilian and military scientists and technicians, with assistance from other military personnel, placed gauges, detectors, and other instruments around ground zero before the detonation. four offsite monitoring posts were established in the towns of nogal, roswell, socorro, and fort sumner, new mexico. an evacuation detachment consisting of to enlisted men and officers was established in case protective measures or evacuation of civilians living offsite became necessary. at least of these personnel were from the provisional detachment number , company "b," of the th technical service unit, army corps of engineers. military police cleared the test area and recorded the locations of all personnel before the detonation. a radiological monitor was assigned to each of the three shelters, which were located to the north, west, and south of ground zero. soon after the detonation, the monitors surveyed the area immediately around the shelters and then proceeded out the access road to its intersection with the main road, broadway. personnel not essential to postshot activities were transferred from the west and south shelters to the base camp, about kilometers southwest of ground zero. personnel at the north shelter were evacuated when a sudden rise in radiation levels was detected; it was later learned that the instrument had not been accurately calibrated and levels had not increased as much as the instrument indicated. specially designated groups conducted onsite and offsite radiological surveys. safety standards and procedures the safety criteria established for project trinity were based on calculations of the anticipated dangers from blast pressure, thermal radiation, and ionizing radiation. the tr- group, also known as the medical group, was responsible for radiological safety. a limit of roentgens of exposure during a two-month period was established. the site and offsite monitoring groups were both part of the medical group. the site monitoring group was responsible for equipping personnel with protective clothing and instruments to measure radiation exposure, monitoring and recording personnel exposure according to film badge readings and time spent in the test area, and providing for personnel decontamination. the offsite monitoring group surveyed areas surrounding the test site for radioactive fallout. in addition to these two monitoring groups, a small group of medical technicians provided radiation detection instruments and monitoring. radiation exposures at project trinity dosimetry information is available for about individuals who either participated in project trinity activities or visited the test site between july and january . the listing does not indicate the precise military or unit affiliation of all personnel. less than six percent of the project trinity participants received exposures greater than roentgens. twenty-three of these individuals received exposures greater than but less than roentgens; another individuals received between and roentgens. preface from to , the u.s. government, through the manhattan engineer district (med) and its successor agency, the atomic energy commission (aec), conducted tests of nuclear devices at sites in the united states and in the atlantic and pacific oceans. in all, an estimated , department of defense (dod)* participants, both military and civilian, were present at the tests. project trinity, the war-time effort to test-fire a nuclear explosive device, was the first atmospheric nuclear weapons test. * the med, which was part of the army corps of engineers, administered the u.s. nuclear testing program until the aec came into existence in . before dod was established in , the army corps of engineers was under the war department. in , years after the last above-ground nuclear weapons test, the centers for disease control** noted a possible leukemia cluster among a small group of soldiers present at shot smoky, a test of operation plumbbob, the series of atmospheric nuclear weapons tests conducted in . since that initial report by the centers for disease control, the veterans administration has received a number of claims for medical benefits from former military personnel who believe their health may have been affected by their participation in the weapons testing program. ** the centers for disease control are part of the u.s. department of health and human services (formerly the u.s. department of health, education, and welfare). in late , dod began a study to provide data to both the centers for disease control and the veterans administration on potential exposures to ionizing radiation among the military and civilian participants in atmospheric nuclear weapons testing. dod organized an effort to: o identify dod personnel who had taken part in the atmospheric nuclear weapons tests o determine the extent of the participants' exposure to ionizing radiation o provide public disclosure of information concerning participation by military personnel in project trinity. methods and sources used to prepare this volume this report on project trinity is based on historical and technical documents associated with the detonation of the first nuclear device on july . the department of defense compiled information for this volume from documents that record the scientific activities during project trinity. these records, most of which were developed by participants in trinity, are kept in several document repositories throughout the united states. in compiling information for this report, historians, health physicists, radiation specialists, and information analysts canvassed document repositories known to contain materials on atmospheric nuclear weapons tests conducted in the southwestern united states. these repositories included armed services libraries, government agency archives and libraries, federal repositories, and libraries of scientific and technical laboratories. researchers examined classified and unclassified documents containing information on the participation of personnel from the med, which supervised project trinity, and from the los alamos scientific laboratory (lasl), which developed the trinity device. after this initial effort, researchers recorded relevant information concerning the activities of med and lasl personnel and catalogued the data sources. many of the documents pertaining specifically to med and lasl participation were found in the defense nuclear agency technical library and the lasl records center. information on the fallout pattern, meteorological conditions, and nuclear cloud dimensions is taken from volume of the general electric company-tempo's "compilation of local fallout data from test detonations - , extracted from dasa ," unless more specific information is available elsewhere. organization of this volume the following chapters detail med and lasl participation in project trinity. chapter provides background information, including a description of the trinity test site. chapter describes the activities of med and lasl participants before, during, and after the detonation. chapter discusses the radiological safety criteria and procedures in effect for project trinity, and chapter presents the results of the radiation monitoring program, including information on film badge readings for participants in the project. the information in this report is supplemented by the reference manual: background materials for the conus volumes." the manual summarizes information on radiation physics, radiation health concepts, exposure criteria, and measurement techniques. it also lists acronyms and includes a glossary of terms used in the dod reports addressing test events in the continental united states. chapter introduction project trinity was the name given to the war-time effort to produce the first nuclear detonation. a plutonium-fueled implosion device was detonated on july at the alamogordo bombing range in south-central new mexico. three weeks later, on august, the first uranium-fueled nuclear bomb, a gun-type weapon code-named little boy, was detonated over the japanese city of hiroshima. on august, the fat man nuclear bomb, a plutonium-fueled implosion weapon identical to the trinity device, was detonated over another japanese city, nagasaki. two days later, the japanese government informed the united states of its decision to end the war. on september , the japanese empire officially surrendered to the allied governments, bringing world war ii to an end. in the years devoted to the development and construction of a nuclear weapon, scientists and technicians expanded their knowledge of nuclear fission and developed both the gun-type and the implosion mechanisms to release the energy of a nuclear chain reaction. their knowledge, however, was only theoretical. they could be certain neither of the extent and effects of such a nuclear chain reaction, nor of the hazards of the resulting blast and radiation. protective measures could be based only on estimates and calculations. furthermore, scientists were reasonably confident that the gun-type uranium-fueled device could be successfully detonated, but they did not know if the more complex firing technology required in an implosion device would work. successful detonation of the trinity device showed that implosion would work, that a nuclear chain reaction would result in a powerful detonation, and that effective means exist to guard against the blast and radiation produced. . historical background of project trinity the development of a nuclear weapon was a low priority for the united states before the outbreak of world war ii. however, scientists exiled from germany had expressed concern that the germans were developing a nuclear weapon. confirming these fears, in the germans stopped all sales of uranium ore from the mines of occupied czechoslovakia. in a letter sponsored by group of concerned scientists, albert einstein informed president roosevelt that german experiments had shown that an induced nuclear chain reaction was possible and could be used to construct extremely powerful bombs ( ; )*. * all sources cited in the text are listed alphabetically in the reference list at the end of this volume. the number given in the text corresponds to the number of the source document in the reference list. in response to the potential threat of a german nuclear weapon, the united states sought a source of uranium to use in determining the feasibility of a nuclear chain reaction. after germany occupied belgium in may , the belgians turned over uranium ore from their holdings in the belgian congo to the united states. then, in march , the element plutonium was isolated, and the plutonium- isotope was found to fission as readily as the scarce uranium isotope, uranium- . the plutonium, produced in a uranium-fueled nuclear reactor, provided the united states with an additional source of material for nuclear weapons ( ; ). in the summer of , the british government published a report written by the committee for military application of uranium detonation (maud). this report stated that a nuclear weapon was possible and concluded that its construction should begin immediately. the maud report, and to a lesser degree the discovery of plutonium, encouraged american leaders to think more seriously about developing a nuclear weapon. on december , president roosevelt appointed the s- committee to determine if the united states could construct a nuclear weapon. six months later, the s- committee gave the president its report, recommending a fast-paced program that would cost up to $ million and that might produce the weapon by july ( ). the president accepted the s- committee's recommendations. the effort to construct the weapon was turned over to the war department, which assigned the task to the army corps of engineers. in september , the corps of engineers established the manhattan engineer district to oversee the development of a nuclear weapon. this effort was code-named the "manhattan project" ( ). within the next two years, the med built laboratories and production plants throughout the united states. the three principal centers of the manhattan project were the hanford, washington, plutonium production plant; the oak ridge, tennessee, u- production plant; and the los alamos scientific laboratory in northern new mexico. at lasl, manhattan project scientists and technicians, directed by dr. j. robert oppenheimer,* investigated the theoretical problems that had to be solved before a nuclear weapon could be developed ( ). * this report identifies by name only those lasl and med personnel who are well-known historical figures. during the first two years of the manhattan project, work proceeded at a slow but steady pace. significant technical problems had to be solved, and difficulties in the production of plutonium, particularly the inability to process large amounts, often frustrated the scientists. nonetheless, by sufficient progress had been made to persuade the scientists that their efforts might succeed. a test of the plutonium implosion device was necessary to determine if it would work and what its effects would be. in addition, the scientists were concerned about the possible effects if the conventional explosives in a nuclear device, particularly the more complex implosion-type device, failed to trigger the nuclear reaction when detonated over enemy territory. not only would the psychological impact of the weapon be lost, but the enemy might recover large amounts of fissionable material. in march , planning began to test-fire a plutonium-fueled implosion device. at lasl, an organization designated the x- group was formed within the explosives division. its duties were "to make preparations for a field test in which blast, earth shock, neutron and gamma radiation would be studied and complete photographic records made of the explosion and any atmospheric phenomena connected with the explosion" ( ). dr. oppenheimer chose the name trinity for the project in september ( ). . the project trinity site the trinity site was chosen by manhattan project scientists after thorough study of eight different sites. the site selected was an area measuring by kilometers* in the northwest corner of the alamogordo bombing range. the alamogordo bombing range was located in a desert in south-central new mexico called the jornada del muerto ("journey of death"). figure - shows the location of the bombing range. the site was chosen for its remote location and good weather and because it was already owned by the government. med obtained permission to use the site from the commanding general of the second air force (army air forces) on september ( ). figure - shows the trinity site with its major installations. * throughout this report, surface distances are given in metric units. the metric conversion factors include: meter = . feet; meter = . yards; and kilometer = . miles. vertical distances are given in feet; altitudes are measured from mean sea level, while heights are measured from surface level, unless otherwise noted. ground zero for the trinity detonation was at utm coordinates .** three shelters, located approximately , meters ( , yards) north, west, and south of ground zero, were built for the protection of test personnel and instruments. the shelters had walls of reinforced concrete and were buried under a few feet of earth. the south shelter was the control point for the test ( ). the base camp, which was the headquarters for project trinity, was located approximately kilometers southwest of ground zero. the principal buildings of the abandoned mcdonald ranch, where the active parts of the trinity device were assembled, stood , meters southeast of ground zero. seven guard posts, which were simply small tents or parked trucks like the ones shown in figures - and - , dotted the test site ( ). ** universal transverse mercator (utm) coordinates are used in this report. the first three digits refer to a point on an east- west axis, and the second three digits refer to a point on a north-south axis. the point so designated is the southwest corner of an area meters square. . the project trinity organization the organization that planned and conducted project trinity grew out of the x- group. lasl, though administered by the university of california, was part of the manhattan project, supervised by the army corps of engineers manhattan engineer district. the chief of med was maj. gen. leslie groves of the army corps of engineers. major general groves reported to both the chief of engineers and the army chief of staff. the army chief of staff reported to the secretary of war, a cabinet officer directly responsible to the president. figure - outlines the organization of project trinity. the director of the project trinity organization was dr. kenneth bainbridge. dr. bainbridge reported to dr. j. robert oppenheimer, the director of lasl. a team of nine research consultants advised dr. bainbridge on scientific and technical matters ( ). the project trinity organization was divided into the following groups ( ): o the trinity assembly group, responsible for assembling and arming the nuclear device o the tr- (services) group, responsible for construction, utilities, procurement, transportation, and communications o the tr- group, responsible for air-blast and earth-shock measurements o the tr- (physics) group, responsible for experiments concerning measurements of ionizing radiation o the tr- group, responsible for meteorology o the tr- group, responsible for spectrographic and photographic measurements o the tr- group, responsible for the airblast-airborne condenser gauges o the tr- (medical) group, responsible for the radiological safety and general health of the project trinity participants. each of these groups was divided into several units. individuals were also assigned special tasks outside their groups, such as communications and tracking the trinity cloud with a searchlight ( ). . military and civilian participants in project trinity from march until the beginning of , several thousand people participated in project trinity. these included not only the lasl scientists, but also scientists, technicians, and workmen employed at med installations throughout the united states. according to entrance logs, film badge data, and other records, about , people either worked at or visited the trinity site from july through ( ; ; ; ; ). although supervised by major general groves and the army corps of engineers, many manhattan project personnel were civilians. military personnel were assigned principally to support services, such as security and logistics, although soldiers with special skills worked with the civilians ( ; ). most of the military personnel were part of the army corps of engineers, although navy and other army personnel were also assigned to the project ( ; ). chapter the activities at project trinity the trinity nuclear device was detonated on a -foot tower (shown in figure - ) at utm coordinates on the alamogordo bombing range, new mexico, at mountain war time, on july . the detonation had a yield of kilotons and left an impression . meters deep and meters wide. the cloud resulting from the detonation rose to an altitude of , feet ( ). the trinity detonation is shown in figure - . at shot-time, the temperature was . degrees celsius, and the surface air pressure was millibars. winds at shot-time were nearly calm at the surface but attained a speed of knots from the southwest at , feet. at , feet, the wind speed was knots from the southwest. the winds blew the cloud to the northeast ( ). . preshot activities construction of test site facilities on the alamogordo bombing range began in december . the first contingent of personnel, military policemen, arrived just before christmas. the number of personnel at the test site gradually increased until the peak level of about was reached the week before the detonation ( ; ). on may at hours, lasl scientists and technicians exploded tons of conventional high explosives at the test site. the explosives were stacked on top of a -foot tower and contained tubes of radioactive solution to simulate, at a low level of activity, the radioactive products expected from a nuclear explosion. the test produced a bright sphere which spread out in an oval form. a column of smoke and debris rose as high as , feet before drifting eastward. the explosion left a shallow crater . meters deep and meters wide. monitoring in the area revealed a level of radioactivity low enough to allow workers to spend several hours in the area ( ; ). the planned firing date for the trinity device was july . on june , dr. oppenheimer changed the test date to no earlier than july and no later than july. on june, the earliest firing date was moved to july, even though better weather was forecast for and july. because the allied conference in potsdam, germany, was about to begin and the president needed the results of the test as soon as possible, the trinity test organization adjusted its schedules accordingly and set shot-time at hours on july ( ; ; ). the final preparations for the detonation started at on july. to prevent unnecessary danger, all personnel not essential to the firing activities were ordered to leave the test site. during the night of july, these people left for viewing positions on compania hill,* kilometers northwest of ground zero. they were joined by several spectators from lasl ( ; ). * "compania" also appears as "compana," "campagne," or "compagna" in various sources. project personnel not required to check instruments within the ground zero area stationed themselves in the three shelters or at other assigned locations. the military police at guard posts , , and blocked off all roads leading into the test site, and the men at guard post , the only access to the ground zero area from the base camp, ensured that no unauthorized individuals entered the area ( ; ). at hours on july, military policemen from guard posts , , , and met to compare their logs of personnel authorized to be in the ground zero area. the guards then traveled along the access roads to clear out all project personnel. as individuals left for their assigned shelters or stations, their departures from the test area were recorded in the military police logs. by the area sweep was completed, and the military police went to their shelters and stations. a final check of personnel was made in each shelter ( ; ; ). at the time of detonation, project personnel were in the three shelters: in the north shelter, in the west shelter, and in the south shelter. dr. oppenheimer, dr. bainbridge, and other key personnel awaited the firing at the south shelter, which served as the control point. figure - shows the exterior of the south shelter; figure - gives an interior view of one of the shelters, most likely the south. although most of the shelter occupants were civilians, at least military participants were spread among the three shelters ( ; ). the remainder of the test site personnel were positioned at the base camp kilometers south-southwest of ground zero, or on compania hill, or at the guard posts. important government officials, such as general groves and dr. vannevar bush, director of the u.s. office of scientific research and development, viewed the detonation from a trench at the base camp. the base camp is depicted in figure - . the military police of guard posts and were instructed to be in foxholes approximately five kilometers west and north, respectively, from their posts. the military police of guard posts and were instructed to be in foxholes south of mockingbird gap. a radiological safety monitor was assigned to the group from guard post . guard post personnel were to be in the south shelter, guard post personnel in the west shelter, and guard post personnel in the north shelter. the military police of guard post remained at that post, meters east of the base camp ( ). an evacuation detachment of between and officers and enlisted men was stationed near guard post , about kilometers northwest of ground zero. these men were on standby in case ranches and towns beyond the test site had to be evacuated. five radiological safety monitors were assigned to this detachment. ninety-four men of the evacuation detachment belonged to provisional detachment number , company "b," of the th technical service unit, army corps of engineers, from lasl. the identity of the remaining evacuation personnel has not been documented ( ; ; ; ; ). with the exception of the shelter occupants ( personnel) and evacuation detachment (between and men), the number of personnel at the test site at the time of detonation has not been documented. film badge records show that approximately people were at the test site at some time during july. the shelter occupants and men of the evacuation detachment are on this list. it has not been possible to pinpoint the location of many of the remaining personnel. some were at the base camp or on compania hill. since many of these people returned to the test site after shot-time to work on experiments, their film badges registered exposures from residual radioactivity on july. based on the documented personnel totals, at least the following individuals were at the test site when the device was detonated ( ; ; - ; ; ): o shelter occupants at shelters , meters north, south, and west of ground zero o to officers and enlisted men of the evacuation detachment, located kilometers northwest of ground zero near guard post o five radiological safety monitors assigned to the evacuation detachment to perform offsite monitoring of nearby towns and residences o one radiological safety monitor assigned to guard post o two military policemen at each of the seven guard posts (indicated by photographs such as figures - and - ). . detonation and postshot activities because of bad weather, the project trinity director (dr. bainbridge) delayed the detonation, which had been scheduled for hours. by , however, the forecast was better, and shot-time was set for . this gave the scientists minutes to arm the device and prepare the instruments in the shelters. the final countdown began at , and the device was detonated at : mountain war time from the control point in the south shelter ( ; ). no one was closer than , meters to ground zero at the time of the detonation. with the exception of a few men holding the ropes of barrage balloons or guiding cameras to follow the fireball as it ascended, all shelter personnel were in or behind the shelters. some left the shelters after the initial flash to view the fireball. as a precautionary measure, they had been advised to lie on the ground before the blast wave arrived. project personnel located beyond the shelters, such as at the base camp and on compania hill, were also instructed to lie on the ground or in a depression until the blast wave had passed ( ). however, the blast wave at these locations was not as strong as had been expected. in order to prevent eye damage, dr. bainbridge ordered the distribution of welder's filter glass. because it was not known exactly how the flash might affect eyesight, it was suggested that direct viewing of the fireball not be attempted even with this protection. the recommended procedure was to face away from ground zero and watch the hills or sky until the fireball illuminated the area. then, after the initial flash had passed, one could turn around and view the fireball through the filter glass. despite these well-publicized instructions, two participants did not take precautions. they were temporarily blinded by the intense flash but experienced no permanent vision impairment ( ; ). people as far away as santa fe and el paso saw the brilliant light of the detonation. windows rattled in the areas immediately surrounding the test site, waking sleeping ranchers and townspeople. to dispel any rumors that might compromise the security of project trinity, the government announced that an army munitions dump had exploded. however, immediately after the destruction of hiroshima, the government revealed to the public what had actually occurred in the new mexico desert ( ; ). immediately after the shot, medical group personnel began the radiological monitoring activities described in section . . . at , when most of the monitoring activities were completed, preparations began for entrance into the ground zero area. to regulate entry into the area, a "going-in board" was established, consisting of dr. bainbridge, the chief of the medical group, and a special scientific consultant. its purpose was to determine whether a party had a valid reason for entering the ground zero area. the board functioned for three days. military police at guard post and at three roadblocks set up along broadway controlled entry into the area. guard posts , , , and were within , meters of ground zero and thus remained unmanned. at the south shelter, the medical group set up a "going-in" station where personnel were required to stop to put on protective clothing (coveralls, booties, caps, and cotton gloves) and pick up monitoring equipment before entering the ground zero area. since it was not known how much radioactive material might be suspended in the air, all personnel entering the ground zero area wore complete protective covering and respirators for the first three days after the detonation. figure - shows two project trinity personnel wearing protective clothing ( ). on the day of the shot, five parties entered the ground zero area. one party consisted of eight members of the earth-sampling group. they obtained samples by driving to within meters of ground zero in a tank specially fitted with rockets to which retrievable collectors were fastened in order to gather soil samples from a distance. this group made several sampling excursions on and july. the tank carried two personnel (a driver and a passenger) each trip. no member of this party received a radiation exposure of more than roentgen ( ). five other men from the earth-sampling group entered the ground zero area in a second tank, lined with lead for radiation protection. the tank, carrying the driver and one passenger, made five trips into the ground zero area to retrieve soil samples on and july. on two trips, the tank passed over ground zero; on the others, it approached to within about meters of ground zero. the men scooped up soil samples through a trap door in the bottom of the tank. one driver who made three trips into the ground zero area received the highest exposure, roentgens ( ). this lead-lined tank was also used by ten men to observe the radiation area. these men, traveling two at a time, made five trips into the area on shot-day but never approached closer than , meters to ground zero. the highest exposure among these ten men was . roentgens ( ). the next party to approach ground zero consisted of a photographer and a radiological safety monitor. wearing protective clothing and respirators, the two men were about meters northwest of ground zero photographing "jumbo" from to hours. "jumbo," shown in figure - , was a massive container built to contain the high-explosive detonation of the trinity device and to allow recovery of the fissionable material if the device failed to produce a nuclear detonation. the plan to use "jumbo," however, was abandoned when the scientists concluded that a fairly large nuclear explosion was certain. the container remained on the ground near the shot-tower during the detonation. both the photographer and the monitor received an estimated radiation exposure between . and roentgen ( ; ). the last party to "go in" on shot-day consisted of six men retrieving neutron detectors. they entered the test area at hours. three of the men went to a point meters south of ground zero to pull out cables carrying neutron detectors located meters south of ground zero. the group wore protective clothing and respirators and spent about minutes in the area. the remaining three men drove as close as meters southwest of ground zero to retrieve neutron detectors. they got out of their vehicle only once, at about meters from ground zero, and spent a total of about ten minutes making this trip through the area. each man's radiation exposure measured less than roentgen ( ). most of the soldiers of the evacuation detachment remained in their bivouac area near guard post . according to a report written by the detachment commander, a reinforced platoon was sent to the town of bingham, about kilometers northeast of the test site, while offsite radiological safety monitors surveyed the area. the evacuation detachment was dismissed at hours on shot-day when it became evident from offsite monitoring that evacuations would not be undertaken. the detachment returned to lasl at on july ( ). two b- aircraft from kirtland field, albuquerque, new mexico, participated in post-shot events. their planned mission was to pass over the test area shortly before the explosion to simulate a bomb drop. after the trinity device had been detonated, the aircraft would circle near ground zero, while the men onboard would measure the atmospheric effects of the nuclear explosion. this would enable them to determine whether a delivery aircraft would be endangered. however, because of bad weather on shot-day, dr. oppenheimer canceled the aircraft's flight in the ground zero area. instead, the two b- s, each with men onboard, flew along the perimeter of the bombing range and observed the shot from a distance of to kilometers. among those observers was a navy captain who was also the med chief of ordnance ( ; ; ). . activities after july on , , and july, all personnel and visitors had to receive permission to approach ground zero from the "going-in board." on these three days, groups were authorized to go beyond the broadway roadblocks. most of those who sought this authorization were scientists and military support personnel whose job required that they work near ground zero. except for a group of two military men and three civilians who went to ground zero on and july and a group of two civilians who approached as close as meters on july, the reentry personnel came no closer than meters to ground zero. of these personnel, the individual who received the highest exposure during the three days was an army sergeant who received roentgens. during the same period, two civilians received roentgens and . roentgens, respectively. all other personnel received exposures of roentgens or less ( ; ). after the "going-in board" was disbanded on july, permission to enter the ground zero area had to be obtained from dr. bainbridge or one of his deputies. many scientists entered the ground zero area after july to retrieve instruments or to perform experiments. the population of the trinity test site was diminishing, however, as the emphasis shifted to preparing the devices that were to be dropped on japan ( ). on july, a week after the shot, chain barricades with prominent signs warning against trespassing were placed meters north, south, and west of ground zero. these barricades were supplemented with two concentric circles of red flags , and , meters from ground zero. except during bad weather, the entire ground zero area was visible from the roadblocks. no unauthorized person was ever detected entering the ground zero area ( ). on august, the broadway roadblocks were removed, and mounted military policemen began patrolling around ground zero at a distance of meters. each guard was assigned to a daily six-hour shift for a period of two weeks; in the third week, the guard was assigned tasks away from the ground zero area. the mounted guards and their horses wore film badges. no exposure greater than . roentgen was registered. on september, the mounted patrol moved to a distance of meters from ground zero, just outside a fence installed a week earlier to seal off the area. the same rotating patrol schedule was used. the guards' film badge readings showed an average daily exposure of . roentgens. the mounted patrol at the fence continued until early ( ). between july and november , groups entered the ground zero area. most of these parties entered in the month after shot-day. these were the scientists and technicians conducting experiments or retrieving data. by the beginning of september, most of those who entered the ground zero area were invited guests ( ). also during the period july through november, at least soldiers were at the trinity test site. twenty-five of these men were support personnel who never went within meters of ground zero. the remaining men were technical personnel, laborers who erected the -meter fence, or military policemen who served as guides. eleven of these men, probably members of the fence detail, spent several days at about meters from ground zero. working three to five hours per day between august and august, they would have been the only group to stay longer than one hour in the ground zero area. of the remaining personnel who approached within meters from ground zero, spent minutes and ten spent between minutes and one hour in the ground zero area. only people received exposures of to roentgens between july and november. most received less than roentgen. after november , no one approached closer than the fence which was meters from ground zero, although about civilian and military personnel worked at or visited the trinity site through ( ; ). according to dosimetry data, entrance logs, and other records, about , individuals were at the test site at some time between july and the end of . this number includes not only the scientists, technicians, and military personnel who were part of project trinity but also many visitors. some of the scientists took their wives and children on a tour of the area near ground zero, particularly to view the green glass called "trinitite," which covered the crater floor. trinitite was the product of the detonation's extreme beat, which melted and mixed desert sand, tower steel, and other debris ( ; ; ; ). chapter radiation protection at project trinity the tr- or medical group, shown in the figure - organizational chart, was responsible for radiological safety at project trinity. many of the physicians and scientists in the medical group had worked with radioactive materials before and were trained in radiological safety procedures. the chief of the medical group supervised the radiological safety operations and reported to the trinity director. in addition to providing medical care to trinity personnel, this group established radiological safety programs to: o minimize radiation exposure of personnel on the test site and in offsite areas o provide monitors to conduct radiological surveys onsite and offsite o provide and maintain radiation detection instruments o provide protective clothing and equipment. an exposure limit of roentgens during a two-month period was established. personnel were provided with radiation detection instruments to determine their exposures ( ). . organization the medical group consisted of physicians, scientists, and administrators, as well as radiological monitors. many of these personnel were nonmilitary, but all worked on the manhattan project under the administration of the army corps of engineers manhattan engineer district. the medical group was divided into two monitoring groups, the site monitoring group, which was responsible for onsite monitoring, and the offsite monitoring group. each reported to the chief of the medical group, and each communicated with the other during the monitoring activities. in addition to these two groups, a small group of medical technicians provided radiation detection instruments to medical group personnel ( ; ). . site monitoring group the site monitoring group consisted of a chief monitor, three other monitors, and several medical doctors. this group had the following functions ( ; ): o conduct ground surveys of the test area and mark areas of radioactivity o conduct surveys of the base camp and roads leading into the test area o provide protective clothing and equipment, including film badges and pocket dosimeters, to personnel o monitor all personnel for radioactive contamination and provide for their decontamination o maintain a record of radiation exposures received by personnel. the site monitoring group monitored the radiation exposures of personnel in the test area. the time spent by personnel in radiation areas was limited, and radiation detection instruments were provided to permit continuous monitoring of exposure rates. in many cases, a monitor from the site monitoring group accompanied project personnel into the test area to monitor exposure rates ( ; ). two members of the site monitoring group, a monitor and a physician with radiological safety training, were assigned to each shelter. the supervising monitor was stationed at the base camp and was in radio and telephone communication with all three shelters and the offsite ground and aerial survey teams. before any personnel were allowed to leave the shelter areas, a radiological safety monitor and a military policeman from each shelter advanced along the roads to broadway to check radiation levels. they wore respirators to prevent them from inhaling radioactive material ( ; ). since it was expected that any dust from the cloud would fall on one of the shelter areas within minutes of the shot, plans had been made to evacuate personnel as soon as the monitors completed their initial survey. because the cloud moved to the northeast, the south shelter (the control point) was not completely evacuated, although nonessential personnel were sent to the base camp. the west shelter was emptied of all personnel except a searchlight crew spotlighting the cloud as it moved away ( ; ). only at the north shelter did an emergency evacuation occur. about minutes after the shot, a detection instrument indicated a rapid rise in the radiation levels within the shelter. at the same time, a remote ionization monitoring device detected a rapid increase in radiation. because of these two readings, all north shelter personnel were immediately evacuated to the base camp, kilometers to the south. film badges worn by personnel stationed at the north shelter, however, showed no radiation exposure above the detectable level. it was later discovered that the meter of the detector in the north shelter had not retained its zero calibration setting, and radiation at the north shelter had not reached levels high enough to result in measurable exposures of the personnel who had been positioned there. however, fallout activity was later detected in the north shelter area, proof that part of the cloud did head in that direction. this also explains why the monitoring device detected rising radiation levels ( ; ). after ascertaining that radiation levels along the roads leading from the shelters to broadway were within acceptable limits, the radiological safety monitors and military police established roadblocks at important intersections leading to ground zero. the north shelter monitor and military police set up a post where the north shelter road ran into broadway. the west shelter monitor and a military policeman blocked vatican road where it intersected broadway. the south shelter monitor and military police set up a roadblock where broadway intersected pennsylvania avenue ( ). the monitor assigned to guard post surveyed the mockingbird gap area to ensure that it was safe for the guards to return to their post. this position controlled access to the mcdonald ranch road, which led directly to ground zero ( ). at hours, the chief monitor departed from the base camp with a military policeman to monitor the entire length of broadway. they first checked the roadblock at pennsylvania avenue and broadway. next they drove to the roadblock at vatican road and broadway. upon the chief monitor's arrival, the west shelter monitor traveled about nine kilometers west on vatican road to monitor guard post so that the military police could reoccupy the post. the monitoring excursion to guard post continued until the chief monitor had returned from guard post , located kilometers northwest of the vatican road roadblock on broadway ( ; ). the chief monitor arrived at guard post at about hours and found the post empty. he then continued five kilometers north along broadway to the foxholes from which the military police had watched the detonation. there he found the guards, the five radiological safety monitors assigned to the evacuation detachment, and the commanding officer of the evacuation detachment ( ; ). the military policemen refused to return to guard post , insisting that they had received orders over their two-way radio from the base commander to evacuate their post and head for san antonio, new mexico, a town kilometers northwest of the guard post. the base commander had noted that portions of the cloud were heading northwestward and, fearing that fallout from the cloud would contaminate guard post , had ordered the military police to evacuate. the chief monitor, however, had found no significant radiation levels anywhere along the northern part of broadway nor around guard post . the base commander, after being contacted by the chief monitor, drove to the foxholes and ordered the guards to return to their post. this was the only unplanned incident during the onsite monitoring ( ). after guard post was reoccupied, the chief monitor returned to the roadblock at the intersection of broadway and the north shelter road. the north shelter monitor informed the chief monitor of the sudden evacuation of the north shelter, whereupon the chief monitor surveyed the north shelter area and found intensities of only . and . roentgens per hour (r/h). the chief monitor then contacted the south shelter and informed dr. bainbridge that the north shelter region was safe for those who needed to return, that broadway was safe from the base camp to guard post , and that guard post was now manned so that personnel leaving for lasl could be checked out ( ). the chief monitor then returned to the south shelter and assembled the monitors from the three roadblocks and guard post to prepare for entrance into the ground zero area. the time was about hours. the military police at the roadblocks were given radiation meters to survey the adjoining area. broadway from the south shelter to guard post was remonitored occasionally to reassure the military police that there was no radiation problem. monitors also surveyed the base camp for hours after the detonation. no radiation above background levels was detected there ( ). the following brief description of the radiological environment in the trinity test area is based primarily on the results of the remote gamma recorders situated in the test area and on results of the road surveys conducted after the detonation ( ). within about , meters of ground zero (except to the north), radiation intensities between . and . r/h were detected during the first few minutes after the detonation. these readings decreased to less than . r/h within a few hours. at greater distances to the east, south, and west, radiation levels above background were not detected ( ). the cloud drifted to the northeast, and higher gamma readings due to fallout were encountered in this direction. about five minutes after the detonation, a reading of r/h was recorded , meters north of ground zero. several minutes later, the intensity there had increased to greater than r/h, and it continued to increase for several more minutes. gamma detectors , meters north of ground zero, however, recorded no radiation above background levels. this indicated that the cloud had passed over or near the , -meter area and only partially over the , -meter area where the north shelter was located. subsequent ground surveys of this area found no gamma intensities higher than . r/h ( ). gamma radiation levels at and around ground zero were much higher than in other onsite areas because of induced activity in the soil. twenty-four hours after the detonation, the gamma intensity at ground zero was estimated to be to r/h. this estimate was based on data provided by the tank crew that drove to ground zero to obtain soil samples. the intensity decreased to about r/h at meters from ground zero. gamma intensities of . r/h or more were confined within a circular area extending about , meters from ground zero (except in areas of fallout). one week after the shot, the gamma intensity at ground zero was about r/h. after days, intensities at ground zero had decreased to r/h, and intensities of . r/h or more were not encountered beyond about meters from ground zero. gamma intensities of to r/h were found at ground zero three months after the detonation ( ; ). . offsite monitoring group four two-man teams and one five-man team supervised by the chief offsite monitor constituted the offsite monitoring group. before the detonation, the four two-man teams established monitoring posts in towns outside the test area. these towns were nogal, roswell, fort sumner, and socorro, all in new mexico. the five-man team remained at guard post to assist in evacuation of nearby residences if the trinity cloud drifted in that direction. these residences, the fite house and the homes in the town of tokay, were and kilometers northwest of ground zero, respectively. since the cloud drifted to the northeast, evacuation was not required. all offsite monitoring teams were in radio or telephone contact with personnel at the base camp ( ). offsite monitoring teams in areas northeast of ground zero encountered gamma readings ranging from . to r/h two to four hours after the detonation. three hours after the detonation, surveys taken in bingham, new mexico (located kilometers northeast of ground zero) found gamma intensities of about . r/h. radiation readings at the town of white, nine kilometers southeast of bingham, were . r/h three hours after the detonation and . r/h two hours later. another team monitoring in a canyon kilometers east of bingham found a gamma intensity of about r/h. five hours later, the intensity had decreased to . r/h. it was estimated that peak intensities of gamma radiation from fallout on shot-day were about r/h at an occupied ranch house in this canyon area ( ; ; ). monitoring teams resurveyed these towns about one month after the trinity detonation. at bingham, gamma readings of . r/h and . r/h were found at ground level outdoors and at waist level inside a building, respectively. at the town of white, the highest outdoor gamma reading was . r/h. inside a building, the highest reading was . r/h ( ). surveys taken in the canyon area one month after the detonation indicated that gamma intensities at ground level had decreased to . r/h. the occupied ranch house was also surveyed, both inside and outside. the highest reading outdoors was . r/h, and the highest reading indoors was . r/h ( ; ). monitoring was also conducted in offsite areas other than those to the north and northeast of ground zero. monitors found no radiation readings above background levels ( ). significant fallout from the trinity cloud did not reach the ground within about kilometers northeast of ground zero. from this point, the fallout pattern extended out kilometers and was kilometers wide. gamma intensities up to r/h were measured in this region several hours after the detonation. one month later, intensities had declined to . r/h or less ( ). chapter dosimetry analysis of participants in project trinity this chapter summarizes the radiation doses received by participants in various activities during project trinity. the sources of this dosimetry information are the safety and monitoring report for personnel at trinity, which includes a compilation of film badge readings for all participants up to january , and film badge data from the records of the reynolds electrical and engineering company, which contain readings through ( ; ). these sources list individual participants with their cumulative gamma radiation exposures. . film badge records during trinity, the film badge was the primary device used to measure the radiation dose received by individual participants. the site monitoring plan indicates that film badges were to be issued to participants. the film badge was normally worn at chest level on the outside of clothing and was designed to measure the wearer's exposure to gamma radiation from external sources. these film badges were insensitive to neutron radiation and did not measure the amount of radioactive material that might have been inhaled or ingested ( ). personnel from the medical group had responsibility for issuing, receiving, processing, and interpreting film badges for project trinity. the site monitoring group compiled the film badge records for both onsite and offsite personnel. radiological safety personnel and military police recorded the names and identification numbers of individuals as they entered the test area. this information was recorded in an entry logbook and on a personal exposure data card. upon leaving the test area, individuals returned their film badges to the check station. when the film badges were processed and interpreted, the reading was entered on the individuals exposure data card. in this manner, the number of times an individual entered the test area and his cumulative exposure history were recorded and maintained ( ). . gamma radiation exposure the safety and monitoring report lists film badge readings for about individuals who participated in project trinity from july to january ( ). this list includes both military and nonmilitary personnel who were involved with the trinity operation and postshot activities. however, records are available for only of the to members of the evacuation detachment ( ). in addition, some of these film badge listings may be for personnel who were only peripherally involved with trinity activities, such as family members and official guests who visited the site. according to the safety and monitoring report, by january , individuals had received cumulative gamma exposures greater than but less than roentgens. an additional individuals received gamma exposures between and roentgens. personnel who received gamma exposures exceeding roentgens represent less than six percent of the project trinity participants with recorded exposures. as described below, these exposures generally resulted when personnel approached ground zero several times ( ). information is available regarding the activities of some of these personnel. one of the drivers of the earth-sampling group's lead-lined tank, an army sergeant who traveled three times to ground zero, received an exposure of roentgens. a second tank driver, also an army sergeant, received an exposure of . roentgens. three members of the earth-sampling group, all of whom traveled in the tank to ground zero, received exposures of , . , and roentgens. an army photographer who entered the test area six times between july and october received . roentgens ( ). four individuals involved with excavating the buried supports of the trinity tower from october to october received gamma exposures ranging from . to . roentgens. film badge readings for this three-day period indicate that the two individuals who operated mechanical shovels received . and . roentgens, while the two who supervised and monitored the excavation received exposures of . and . roentgens. the individual receiving . roentgens during the excavation operation had received . roentgens from a previous exposure, making his total exposure roentgens ( ). an army captain who accompanied all test and observer parties into the ground zero area between september and october received a total gamma exposure of . roentgens ( ). the activities and times of exposure are not known for other personnel with exposures over roentgens. according to the dosimetry records for , about people visited the test site that year. no one ventured inside the fence surrounding ground zero, and no one received an exposure greater than roentgen ( ; ). reference list the following list of references represents the documents consulted in preparation of the project trinity volume. availability information an availability statement has been included at the end of the reference citation for those readers who wish to read or obtain copies of source documents. availability statements were correct at the time the bibliography was prepared. it is anticipated that many of the documents marked unavailable may become available during the declassification review process. the coordination and information center (cic) and the national technical information service (ntis) will be provided future dna-wt documents bearing an ex after the report number. source documents bearing an availability statement of cic may be reviewed at the following address: department of energy coordination and information center (operated by reynolds electrical & engineering co., inc.) attn: mr. richard v. nutley s. highland p.o. box las vegas, nevada phone: ( ) - fts: - source documents bearing an availability statement of ntis may be purchased from the national technical information service. when ordering by mail or phone, please include both the price code and the ntis number. the price code appears in parentheses before the ntis order number. national technical information service port royal road springfield, virginia phone: ( ) - (sales office) additional ordering information or assistance may be obtained by writing to the ntis, attention: customer service, or by calling ( ) - . project trinity references *available from ntis; order number appears before the asterisk. **available at cic. ***not available, see availability information page. ****requests subject to privacy act restrictions. . aebersold, paul. july th nuclear explosion-safety and monitoring of personnel (u). los alamos scientific laboratory, atomic energy commission. los alamos, nm.: lasl. la- . january , . pages.*** . bainbridge, k. t. memorandum to all concerned, subject: tr circular no. --total personnel at tr. [base camp, trinity site: nm.] july , . page.** . bainbridge, k. t. trinity. los alamos scientific laboratory. los alamos, nm.: lasl, la- -h and washington, d. c.: gpo. may . pages.** . bramlet, walt. memorandum for thomas j. hirons, subject: dod participants in atmospheric tests, wo/encl. los alamos scientific laboratory. los alamos, nm. isd- . february , . pages.** . general electric company--tempo. compilation of local fallout data from test detonations - . vol. : "continental us tests." washington, d. c.: defense nuclear agency. dna - (ex.). . pages. (a ) ad/ao .* . groves, leslie r., ltg, usa. memorandum for secretary of war, [subject: trinity]. [washington, d.c.] july . pages.** . groves, leslie r., ltg, usa (ret.). now it can be told: the story of the manhattan project. new york, ny.: harper and row. . pages. . headquarters, th technical service unit, provisional detachment no. i (company "b"). [extract from: daily diary, provisional detachment no. (company "b"), th technical service unit.] army corps of engineers, department of war. [santa fe, nm.] july . pages.** . headquarters, special service detachment. supplemental special guard orders, with appendix. los alamos scientific laboratory, manhattan engineer district. [alamogordo, nm.] july . pages.** . hempelmann, l. h., m.d. [extracts from: "preparation and operational plan of medical group (tr- ) for nuclear explosion july ."] los alamos scientific laboratory, atomic energy commission. los alamos, nm.: lasl. la- (deleted). june , . pages.*** . hoffman, j. g. [extracts from "health physics report on radioactive contamination throughout new mexico following the nuclear explosion, part a--physics."] los alamos scientific laboratory, manhattan engineer district. [los alamos, nm.] [ .] pages.** . lamont, lansing. day of trinity. new york, ny.: atheneum. . pages. . los alamos scientific laboratory, public relations office. "los alamos: beginning of an era, - ." atomic energy commission. los alamos, nm.: lasl. . pages.** . oppenheimer, j. r. memorandum for group leaders, subject: trinity test. los alamos scientific laboratory. los alamos, nm. june , . pages.** . palmer, t. o., maj., usa. evacuation detachment at trinity. [manhattan engineer district, army corps of engineers.] [los alamos, nm.] [ july .] pages.** . reynolds electrical & engineering company, inc. [personnel radiation exposures, , ] las vegas, nv. microfilm.**** . warren, s. l., col., usa. directions for personnel at base camp at time of shot. los alamos scientific laboratory, manhattan engineer district. [alamogordo, nm.] july . page.** . warren, s. l., col, usa; hempelmann, l. h., m.d. extracts from: personal notes, subject: events in camp immediately following shot--july , . . pages.** . weisskopf, v.; hoffman, j.; aebersold, paul; hempelmann, l. h. memorandum for george kistiakowsky, subject: measurement of blast, radiation, heat and light and radioactivity at trinity. [los alamos, nm.] september . pages.** photos and maps of trinity (atomic test) site the picture files are courtesy of u.s. army white sands missile range public affairs office: basecamp.gif -- base camp for trinity site workers. blast.gif -- trinity test blast at seconds. crater.gif -- oppenheimer and groves examine tower piling in crater. gadget .gif -- lifting the "gadget" into the -foot tower. gadget .gif -- norris bradbury with the "gadget". jumbo.gif -- unloading jumbo. mcdonald.gif -- mcdonald-schmidt ranch house, where plutonium core was assembled. patch.gif -- patch issued to manhattan project military participants. tr_map .gif -- map of roads to trinity site and visitors' site map. whitsand.gif -- emblem of the u.s. army white sands missile range. generously made available by the internet archive.) curiosities of light and sight. curiosities of light and sight by shelford bidwell, m.a., ll.b., f.r.s. _with fifty illustrations_ london: swan sonnenschein & co., limited paternoster square preface. the following chapters are based upon notes of several unconnected lectures addressed to audiences of very different classes in the theatres of the royal institution, the london institution, the leeds philosophical and literary society, and caius house, battersea. in preparing the notes for publication the matter has been re-arranged with the object of presenting it, as far as might be, in methodical order; additions and omissions have been freely made, and numerous diagrams, illustrative of the apparatus and experiments described, have been provided. i do not know that any apology is needed for offering the collection as thus re-modelled to a larger public. though the essays are, for the most part, of a popular and informal character, they touch upon a number of curious matters of which no readily accessible account has yet appeared, while, even in the most elementary parts, an attempt has been made to handle the subject with some degree of freshness. the interesting subjective phenomena which are associated with the sense of vision do not appear to have received in this country the attention they deserve. this little book may perhaps be of some slight service in suggesting to experimentalists, both professional and amateur, an attractive field of research which has hitherto been only partially explored. contents. page. chapter i. light and the eye chapter ii. colour and its perception chapter iii. some optical defects of the eye chapter iv. some optical illusions chapter v. curiosities of vision list of diagrams. fig. page. . image of slit and spectrum . diagram of the eye . abney's colour-patch apparatus . partially intercepted spectrum . stencil cards . helmholtz's curves of colour sensations . könig's curves . stencil card for complementary colours . another form . slide for mixing any two spectral colours . refraction of monochromatic light by lens . refraction of dichromatic light . narrow spectrum as seen from a distance . spectrum formed with v-shaped slit . bezold's device for demonstrating non-achromatism of the eye . crossed lines showing the effect of astigmatism . another design showing the same . star-like images of luminous points . sutures of the crystalline lens . multiple images of a luminous point . the same, showing an increased number of images . the same when a slit is held before the eye . multiple images of an electric lamp filament . the same seen through a slit - . illusion of length . another form . another form . another form . another form . illusion of inclination . zöllner's lines . slide for showing illusions of motions . illusion of motion . illusion of luminosity . illusion of colour . recurrent vision demonstrated with a vacuum tube . the same with a rotating disk . apparatus for showing recurrent vision with spectral colours . charpentier's "dark band" . charpentier's effect shown with the hand . multiple dark bands . temporary insensitiveness of the eye after illumination . visual sensations attending a period of illumination . benham's artificial spectrum top . demonstration of red colour-borders . black and white screens for the same . rotating disk for the same . demonstration of blue colour-borders . disk for experiments on the origin of the colour-borders . disk for the subjective transformation of colours chapter i. light and the eye. in the present scientific age every one knows that light is transmitted across space through the medium of the luminiferous ether. this ether fills the whole of the known universe, as far at least as the remotest star visible in the most powerful telescopes, and is often said to be possessed of properties of so paradoxical a character that their unreserved acceptance has always been a matter of considerable difficulty. the ether is a thing of immeasurable tenuity, being many millions of times rarer than the most perfect vacuum of which we have any experience: it offers no sensible obstruction to the movements of the celestial bodies, and even the flimsiest of material substances can pass through it as if it were nothing. yet we have been taught that this same ether is an elastic solid with a great degree of rigidity, its resistance to distortion being, in comparison with the density, nearly ten thousand million times greater than that of steel: thus was explained the prodigious speed with which it propagates transverse vibrations. a few years ago, a distinguished leader in science endeavoured in the course of a lecture to illustrate these apparently incompatible properties with the aid of a large slab of burgundy pitch. he showed that the pitch was hard and brittle, yet, as he said, a bullet laid upon the slab would, in the course of a few months, sink into and penetrate through it, the hard brittle mass being really a very viscous fluid. the ether, it was suggested, resembled the pitch in having the rigidity of a solid and yet gradually yielding; it was, in fact, a rigid solid for luminiferous vibrations executed in about a hundred-billionth part of a second, and at the same time highly mobile to bodies like the earth going through it at the rate of twenty miles in a second. this illustration, felicitous as it is, would, however, scarcely avail to force conviction upon an unwilling mind, even if it were admitted that the period of an ether wave is necessarily no more than a hundred-billionth of a second or thereabouts, which is probably very far from the truth. but, indeed, the elastic solid theory of the ether has failed to give a consistent explanation of some of the most important points in observational optics; and, in spite of the exalted position which it has held, it can now hardly be regarded as representing a physical reality. the famous researches of hertz have established upon a secure experimental basis the hypothesis of maxwell that light is an electro-magnetic phenomenon. such electrical radiations as can be produced by suitable instruments are found to behave in exactly the same manner as those to which light is due. they travel through space with the same speed; they can be reflected, refracted, polarised, and made to exhibit interference effects. no fact in physics can be much more firmly established than that of the essential identity of light and electricity. it follows then that the displacements of the ether which constitute light-waves are not necessarily of the same gross mechanical nature as those which we see on the surface of water, or which occur in the air when sound is transmitted through it. the displacements which the ether undergoes are not mechanical--primarily at all events--but electrical. every one knows what a simple mechanical displacement is. if we push aside the bob of a suspended pendulum, that is a mechanical displacement. but if we electrify a stick of sealing wax by rubbing it with flannel, the surrounding ether undergoes electric displacement, and no one understands what electric displacement really is. ultimately, no doubt, it will turn out to be of a mechanical nature, but it is almost certainly not a simple bodily distortion such as is caused, for example, when one presses a jelly with the finger. since, then, it is no longer necessary to assume that the exceedingly rare and subtile ether is a jelly-like solid in order to account for the manner in which it transmits light, one of the most serious difficulties in the way of its acceptance is removed. it is true that nothing is definitely known concerning the mechanism which takes the place of the simple transverse vibrations formerly postulated, but every one will admit that it is far easier to believe in what we know nothing about than in what we know to be impossible. all scientific men are in fact agreed in recognising the real and genuine existence throughout space of an ether capable, among other things, of transmitting at the speed of , miles per second disturbances which, whatever their precise nature, are of the kind which mathematicians are accustomed to call waves. how an ether wave is constituted will probably be known when we have found out exactly what electricity is: and that may be never. the sensation of light results from the action of ether waves upon the organism of the eye, but the old belief that the sensation was primarily due to a series of mere mechanical impulses or beats, just as that of sound results from the mechanical impact of air-waves upon the drum of the ear, cannot any longer be upheld. the essential nature of the action exerted by ether waves is still undetermined, though many guesses at the truth have been hazarded. it may be electrical or it may be chemical; possibly it is both. ether-waves, we know, are competent to bring about chemical changes, as in the familiar instance of the photographic processes; they can also produce electric phenomena, as, for example, when they fall upon a suitably prepared piece of selenium; but there is no evidence that they can exert any direct mechanical action of a vibratory character, and indeed it is barely conceivable that any portion of our organism should be adapted to take up vibrations of such enormous rapidity as those which characterise light-waves. of the multitude of ether-waves which traverse space it is only comparatively few that have the power of exciting the sensation of light. as regards limited range of sensibility there is a very close analogy between hearing and seeing. no sensation of sound (at least of continuous sound) is produced when air-waves beat upon our ears unless the rate of the successive impulses lies within certain definite limits. it is just so with vision. if ether-waves fall upon our eyes at a less rate than about billions per second, or at a greater rate than billions per second, no sensation of light is perceived. there is another and more generally convenient way of stating this fact. since all waves found in the ether travel through space at exactly the same speed-- , miles a second--it follows that the length[ ] of each of a series of homogeneous waves must be inversely proportional to their frequency, that is, to the rate at which they strike a fixed object, such as the eye. instead, therefore, of specifying waves by their frequency we may equally well specify them by their length. waves whose frequency is billions per second have a length of about / inch, this being the one four hundred billionth part of , miles; and those whose frequency is billions have a wave-length of / inch. waves, then, of a length greater than / inch or less than / inch have no effect upon our organs of vision.[ ] in relation to this important fact it will be convenient to refer to a familiar but very beautiful experiment--the formation of a spectrum. an electric lamp is enclosed in an iron lantern, having in its front an upright slit; from this slit there issues a narrow beam of white light, which is made up of rays of many different wave-lengths, all mixed up together. by causing the light to pass through a prism the mixed rays are sorted out side by side according to their several wave-lengths, forming a broad, many-hued band or "spectrum" upon a white screen placed to receive it. (see fig. .) to the visible rays of the longest wave-length is due the red colour on the extreme left. waves of somewhat shorter length produce the adjoining stripe of orange, and the succeeding colours--yellow, green, and blue--correspond respectively to waves of shorter and shorter lengths. lastly there comes a patch of violet due to those of the visible rays whose wave-length is the shortest of all. the wave-length of the light at the extreme edge of the red is about / inch, and as we pass along the spectrum the wave-length gradually diminishes, until at the extreme outer edge of the violet it is about / inch, or not much more than half that at the other end. [illustration: _fig. .--image of slit and of spectrum._] the two ends of the spectrum gradually fade away into darkness, and the point that i wish to insist upon and make perfectly clear is this:--the position of the boundaries terminating the visible spectrum does not depend upon anything whatever in the nature of light regarded as a physical phenomenon. ether waves which are much longer and much shorter than those which illuminate the spectrum certainly exist, and evidence of their existence is easily obtainable. but we cannot see them; they fall upon our eyes without exciting the faintest sensation of light. the visible spectrum is limited solely by the physiological constitution of our organs of vision, and the fact that it begins and ends where it does is, from a physical point of view, a mere accident. the spectrum actually projected upon the screen is in truth much longer than that portion of it which any one can see: it extends for a considerable distance beyond the violet at the one end and beyond the red at the other, these invisible portions being known as the ultra-violet and infra-red regions. people's eyes differ in regard to range of sensibility just as their ears do. i believe the sensibility of my own eyes to be normal, but if i were to indicate the two points where the spectrum appears to me to begin and to end, a great many persons would certainly be inclined to disagree with me and place the boundaries somewhere else. some, indeed, could see nothing whatever in what appears to most of us to be a brilliant portion of the red. again, it is by no means probable that in all animals and insects the limits of vision are the same as they are in man. we might naturally expect that larger and perhaps more coarsely constructed eyes than our own would respond to waves of greater average length, while the visual organs of small insects might on the other hand be more sensitive to shorter waves. the point is not one that can be easily settled, because we are unable to cross-examine an animal as to what it sees under different conditions. but sir john lubbock, taking advantage of the dislike which ants when in their nests have for light, has proved by a series of very exhaustive and conclusive experiments that these insects are most sensitive to rays which our own eyes cannot perceive at all. that region of the spectrum which appears brightest to the eye of an ant is what we should call a perfectly dark one, lying outside the violet, where the incident waves have a length of less than / inch. as lord salisbury said at oxford, the function of the ether is to undulate, and, in fact, it transports energy from one place to another by wave-motion. some of its waves, such as those which proceed from an electric-light dynamo, may be thousands of miles in length, others may be shorter than a millionth of an inch, as is perhaps the case with those associated with professor röntgen's x-rays; but all, so far as is known, are of essentially the same character, differing from one another only as the billows of the atlantic differ from the ripples on the surface of a pond. no matter how the disturbance is first set up, whether by the sun, or by a dynamo, or by a warm flat-iron, in every case the ether conveys nothing at all but the energy of wave-motion, and when the waves, encountering some material obstacle which does not reflect them, become quenched, their energy takes another form, and some kind of work is done, or heat is generated in the obstacle. the whole, or at least the greater part, of the energy given up by the waves is in most cases transformed into heat, but under special circumstances, as, for instance, when the waves fall upon a green leaf or a living eye, a few of them may perform work of an electrical or chemical nature. the process of the transmission of energy from one body to another by propagation through an intervening medium has long been spoken of as "radiation," and in recent years the same term has been largely employed to denote the energy itself while in the stage of transmission. "radiation" in the latter sense--meaning ether wave-energy--includes what is often improperly called light. light, people say, takes about eight minutes in travelling from the sun to the earth. but while it is on its journey it is not light in the true sense of the word; neither does anything of the nature of light ever start from the sun. light has no more existence in nature outside a living body than the flavour of onions has; both are merely sensations. if a boy throws a stone which hits you in the face, you feel a pain; but you do not say that it was a pain which left the boy's hand and travelled through space from him to you. the stone, instead of causing pain in a sentient being, might have broken a window, or knocked down an apple. just so, the same radiation which, when it chances to encounter an eye, produces a certain sensation, will produce a chemical decomposition if it falls upon a cabbage, an electrical effect in a selenium cell, or a heating effect in almost anything. why, then, should it be specially identified with the sensation? "radiation" also includes, and is nearly synonymous with, what is often miscalled radiant heat. after what has been already indicated, i need hardly say that there is no such thing as radiant heat. the truth is that the sun or other hot body generates wave-energy in the ether at the expense of some of its own heat, and any distant substance which absorbs a portion of this energy generally (but not necessarily) acquires an equivalent quantity of heat. the _result_ may be exactly the same as if heat left the hot body and travelled across space to the substance; but the _process_ is different. it is like sending a sovereign to a friend by a postal order. you part with a sovereign and he receives one, but the piece of paper which goes through the post is not a sovereign. it is strictly correct to say that the sun loses heat by radiation, just as you lose a sovereign by investing it in the purchase of a postal order. but that is not the same thing as saying that the sun radiates heat. the term "radiation" has the advantage of avoiding any suggestion of the fallacy that there is some essential difference in the nature of the ether-waves which may happen to terminate their respective careers in the production of light or heat or chemical action or something else; but it is, unfortunately, impossible in the present condition of things to use it as freely as one could wish without pedantry, and we must still often speak of light or of heat when radiation would express our meaning with greater accuracy. light, then--to use the term unblushingly in its objectionable but well understood sense--has the property of stimulating certain nerves which exist in many living beings, with the result that, in some unknown and probably unknowable manner, a special sensation is called into play--the sensation of luminosity. and in order that the creature may be able not only to perceive light but also to see things, that is, to appreciate the forms of external objects, it is generally provided with an optical apparatus by means of which the incident light is suitably distributed over a large number of independent sensitive elements. in man and the higher animals the optical apparatus, or eye, consists of a stiff globular shell, having in front an opening provided with a system of lenses, and, at the back of the interior, a delicate perceptive membrane, upon which the transmitted light is received. so much of the light emitted or reflected from an external object as passes through the lenses, is distributed by them in such a manner as to form what is called an "image" upon the membrane, every elementary point of the image receiving the light which issues from a corresponding point of the object, and no other. the contrivance evidently bears a close resemblance to a photographic camera, the sensitive plate or film, upon which the picture is projected, being analogous to the perceptive membrane. i am not going to attempt a detailed description of the human eye. it will be sufficient to point out briefly some of its principal features as indicated in the annexed diagrammatic section, fig. . [illustration: _fig. .--diagram of the eye._] the opening in front of the globe is covered by a slightly protuberant transparent medium c, which is shaped like a small watch-glass, and on account of its horn-like structure has been named the _cornea_. the space between the cornea c and the body marked l is filled with a watery liquid a, known as the aqueous humour: this liquid with its curved surfaces constitutes a meniscus lens, convex on the outer side and concave on the inner. then comes the biconvex _crystalline lens_ l, an elastic gelatinous-looking solid, which is easily distorted by pressure. the convexity of this lens can be varied by the action of a surrounding muscle m m, and in this way the focus is adjusted for objects at different distances from the eye. when the muscle is relaxed and the lens in its natural condition, the curvature of its surfaces is such that a sharp image is formed of objects distant about forty feet and upwards. when by an effort of will, the muscle is contracted, the lens becomes more convex, and distinct pictures can thus be focussed of things which are only a few inches away. this process of adjustment by muscular effort is technically known as "accommodation." the remainder of the globe is filled with the so-called _vitreous body_ v, which derives its name from its fancied resemblance to liquid glass: it might perhaps be more properly likened to a thin colourless jelly. the vitreous body plays a part in the refraction of the light. the perceptive membrane, or _retina_ r r, which lines rather more than half the interior of the eye-ball, is an exceedingly complex structure. though its average thickness is less than / inch it is known to consist of nine distinct layers, most of which are marvels of minute intricacy. of these layers i shall notice only two, the so-called _bacillary layer_, which is in immediate contact with the inner coating of the eye-ball, and the _fibrous layer_, or layer of optic nerve fibres, which is only separated from the vitreous body by a thin protective film. the bacillary layer (from _bacillum_, a wand) consists of a vast assemblage of little elongated bodies called _rods_ and _cones_, which are placed side by side and set perpendicularly to the surfaces of the retina, or in other words, radially to the eye-ball. let us try to make the arrangement clear by an illustration. imagine a small portion of the inner surface of the eye-ball, one-tenth of an inch square, to be magnified diameters (four million times), and let the enlarged area be represented by the floor of a room feet square. procure a quantity of cedar pencils, and set them on the floor in an upright position and very close to one another. it will be found that the number of pencils required to fill the space will be about half-a-million. to make the analogy more complete, let some of the pencils be sharpened to a long tapering point at their lower ends, the greater number remaining uncut, just as received from the manufacturers. neglecting details which are immaterial for our present purpose, we may regard the uncut pencils as representing upon an enormously magnified scale the rods of the retina, and the pointed ones the cones. the flat upper ends of the pencils may be painted in different uniform colours, and arranged so as to form a large picture in mosaic, and if this is looked at from such a distance that its image on the retina is a tenth of an inch square (which will be the case when the picture is about forty yards away) all possibility of distinguishing the separate elements which compose it will be lost, and the picture will seem to be a perfectly continuous one. although the light which enters the eye cannot reach the rods and cones until it has traversed all the other layers of the retina, yet these intervening layers, being transparent, offer little obstruction to its passage, and it can hardly be doubted that the rods and cones are the special organs upon which light exerts its action, the picture focussed upon their ends being in truth an exceedingly fine mosaic. from every separate element of the mosaic--from every single rod and cone--there proceeds a slender transparent filament: all these make their way through the intermediate layers of the retina, without, as is believed, any break of functional continuity, and emerge near its internal surface; here they bend over at right angles, and the thousands of filaments form a tangle which lines the inside of the eye like a fine network, and constitute the layer of optic nerve-fibres already referred to. the filaments, or nerve-fibres, do not however terminate within the eye; they all pass through the hole marked n in the figure, and thence, in the form of a many-stranded cable, constituting the _optic nerve_, they are led to the brain, to which each individual fibre is separately attached. if, therefore, what i have said is true--and, though it has not, i believe, been all rigorously proved, yet the evidence in its support is exceedingly cogent--it follows that every one of the multitude of rods and cones has its own independent line of communication with the brain. the mind, which is mysteriously connected with the brain, is thus afforded the means of localising all the points of luminous excitation relatively to one another, and furnished with data for estimating the form of the object from which the light proceeds. there are two small regions of the retina which are of special interest. one of them lies just over the opening n where the optic nerve enters. here it is evident that there can be no rods and cones, their place being wholly occupied by strands of nerve-fibre. now it is remarkable that this spot is totally insensitive to light. the other interesting portion is situated opposite the middle of the front opening, and is marked by a small yellow patch, in the centre of which is a depression or pit, which is shown in an exaggerated form at f, and is called the _fovea_. it has been ascertained that the depression is due partly to the absence of the layer of nerve-fibres, which are here bent aside out of their natural course, and partly to a local reduction in the thickness of some of the intermediate retinal layers. this spot, being at the centre of the field of vision, occupies a position of great importance, and the evident purpose of the superficial depression is to allow the light to reach the underlying bacillary layer with as little obstruction as possible. it is noteworthy that the bacillary layer beneath the yellow spot is composed entirely of cones, the rods, which elsewhere are in excess, being altogether wanting. the only other accessory of the visual apparatus to which i shall refer is the _iris_ (i i, fig. ), a coloured disk having a central perforation. this can be seen through the cornea and is consequently a very familiar object. the iris serves the same purpose as the stop, or diaphragm, of a photographic lens, its function being to limit and regulate the quantity of light which is admitted into the eye. the size of the central opening, or _pupil_, varies automatically with the intensity of the illumination: in a strong light the opening becomes small; in a feeble light or in darkness it is enlarged. the pupil also contracts when the eye is focussed upon a near object and dilates when the vision is directed to a distance. this brief sketch may serve to give some slight idea of the complexity and delicacy of the visual apparatus. only a few of its more salient features have been touched upon; when our scrutiny is carried into details the complexity becomes bewildering. even such simple-looking things as the cornea and the vitreous body turn out on close examination to be most elaborately constituted. much, no doubt, remains to be discovered, and of what has already been investigated much is at present only partially understood. and yet, though it is true that man is "fearfully and wonderfully made," it is equally true that he is far from perfect; and while there is no structure in the whole human anatomy which exhibits so abundant a profusion of marvels as the eye, there is perhaps none which is marked with imperfections so striking. many of its defects are the more striking because they are so obvious, being such as would never be tolerated in optical instruments of human manufacture. in any fairly good camera or telescope or microscope we should expect to find that the lenses were symmetrically figured, free from striæ and properly centred; also that they were achromatic and efficiently corrected for spherical aberration. in the eye not one of these elementary requirements is fulfilled. the external surface of the lens formed by the aqueous humour and the cornea is not a surface of revolution, such as would be fashioned by a turning lathe or a lens-grinding machine; its curvature is greater in a vertical than in a horizontal direction, and the distinctness of the focussed image is consequently impaired. again, the crystalline lens is constructed of a number of separate portions which are imperfectly joined together. striæ occur along the junctions, and the light which traverses them, instead of being uniformly refracted, is scattered irregularly. moreover the system of lenses is not centred upon a common axis; neither is it achromatic, while the means employed for correcting spherical aberration are inadequate. the purchaser of an optical instrument which turned out to have such faults as these would certainly, as the late professor helmholtz remarked, be justified in returning it to the maker and blaming him severely for his carelessness. i would not, of course, have it believed that scientific men are conceited enough to imagine themselves capable of designing a better eye than is to be found in nature. that would be an absurdity. they are quite ready to admit that there may exist sufficiently good reasons for the undoubted blemishes which have been indicated, as well as for others which will be referred to later. it is indeed well known that the general efficiency of a machine as a whole may often be best secured by the sacrifice of ideal perfection in some of its parts. with all its anomalies the eye fulfils its proper function very perfectly, and is regarded by those who have studied it most closely with feelings of wonder and humble admiration.[ ] chapter ii. colour and its perception. it was explained in the last chapter that we see things through the agency of the light--emitted or reflected--which proceeds from them to the eye, and is suitably distributed over the retina by the action of a system of lenses. now the "image" thus formed is not generally perceived as a simple monochromatic one, darker in some parts, lighter in others, like a black and white engraving. it is, in most cases at least, characterised by a variety of colours, the light which comes from different objects, or from different parts of the same object, having the power of exciting different colour sensations. light which has the property of exciting the sensation of any colour is commonly spoken of as coloured light. the light reflected by a soldier's coat, for example, may be called red light, because when it falls upon the eye it gives rise to a sensation of redness. but it must be understood that this mode of expression is only a convenient abbreviation, for there can, of course, be no objective colour in the light or "radiation" itself. wherein, then, does coloured light differ from white? why do things appear to be variously coloured when illuminated by light which is colourless? and how do coloured lights affect the visual organs so as to evoke appropriate sensations? these are questions--the first two of a physical character, the last partly physiological and partly psychological--which it is now proposed to discuss. the matter has already been touched upon, though very slightly, in connection with the spectrum. let us again turn to the spectrum and consider it a little more fully. it is easily seen that the luminous band contains six principal hues or tones of colour--red, orange, yellow, green, blue, and violet. (see fig. , page .) these however merge into one another so gradually that it is impossible to say exactly where any one colour begins and ends. look, for instance, at the somewhat narrow but very conspicuous stripe of yellow. towards the right of this stripe the colour gradually becomes greenish-yellow; a little further on it is yellowish-green, and at length, by insensible gradations, a full, pure green is reached. the six most prominent hues of the spectrum are, in fact, supplemented by an immense multitude of subordinate ones, the total number which the eye can recognise as distinct being not less than a thousand. all the colours that we see in nature, with the exception of the purples (about which i shall say more presently), are here represented, and every single variety of tone in the prismatic scale corresponds with one, and only one, definite wave-length of light. the source of all these colours is, as we know, a beam of white or colourless light, the constituents of which have been sorted out and arranged so that they fall side by side upon the screen in the order of their several wave-lengths. if, then, these coloured constituents were all mixed together again, it would be reasonable to expect that pure white light would be reproduced. the experiment has been performed in a great many different ways, several of which were devised by newton himself, and the result admits of no doubt whatever. the method which i intend to describe is not quite so simple as some others, but it has great advantages in the way of convenient manipulation, and affords the means of demonstrating a number of interesting colour effects in an easily intelligible manner. by the simple operation of moving aside a lens out of the track of the light, we can gather up and thoroughly mix together all the variously coloured rays of the spectrum and cause them to form upon the screen a bright circular patch, which, though due to a mixture of a thousand different hues, is absolutely white. when the lens is replaced, which is done in an instant, the mixture is again analysed into its component parts, and the spectrum reappears. the arrangement of the apparatus, which is essentially the same as that devised by captain abney, and called by him the "colour-patch apparatus," is shown in the annexed diagram (fig. ). [illustration: _fig. .--abney's colour-patch apparatus._] the light of an electric lamp a placed inside the lantern is concentrated by the condensing lenses b upon a narrow adjustable slit c. the framework of this slit is attached to one end of a telescope tube, which carries at the other end an achromatic lens d of about inches focus. the rays having been rendered parallel by d are refracted by the prism e; they then pass through a circular opening in the brass plate f to the lens g, the focal length of which is inches, and form a little bright spectrum upon a white card held in a grooved support at h. the card being removed, we place at k a lens having a diameter of - / inches and a focal length of inches or more, and adjust it so that a sharply defined image of the hole in the brass plate f is formed upon the distant white screen l. if all the lenses are correctly placed, this image, though formed entirely by the rays which constituted the little spectrum at h, will be perfectly free from colour even around the edge. if we wish to project upon the screen l an enlarged image of the little spectrum, we have only to use another suitable lens i in conjunction with k: the diameter of that used by myself is - / inches, and its focal length - / inches. when we have once found by trial the position in which this supplementary lens gives the clearest image[ ] it is easy to arrange a contrivance for removing and replacing it correctly without need of any further adjustment. this apparatus shows then that ordinary white light may be regarded as a mixture of all the variously coloured lights which occur in the spectrum, the sensation produced when it falls upon the eye being consequently a compound one. from these and similar experiments the scientific neophyte is not unlikely to draw an erroneous conclusion. white light, he is apt to think, is _always_ due to the combined action of rays of every possible wave-length, while coloured light consists of rays of one definite wave-length only. neither of these inferences would be correct. it is not true that white light necessarily contains rays of all possible wave-lengths: the sensation of whiteness may, as will be shown by and bye, be produced quite as effectively by the combination of only two or three different wave-lengths. nor is it true that such colours as we see in nature are always due to light of a single wave-length; light of this kind is indeed rarely met with outside laboratories and lecture rooms. far more commonly coloured light consists of mixed rays, and like ordinary white light, it may, and generally does, contain all the colours of the spectrum, but in different proportions. this last assertion is easily proved. by means of a slip of card we may intercept a portion of the little spectrum formed at h (fig. ). the dark shadow of the card in the enlarged spectrum on the screen is shown in fig. . it will be noticed that the shadow cuts off a part only of the red, orange, and yellow light, allowing the remainder to pass through the projection lenses. there are still rays of every possible wave-length from extreme red to extreme violet, but the proportion of those towards the red end is less than it was before the card was interposed. [illustration: _fig. .--partially intercepted spectrum._] if now we remove the lens i (fig. ) and so mix the colours of this mutilated spectrum, the bright round patch where the mixed rays fall upon the screen will no longer appear white but greenish-blue. if we transfer the card to the other end of the little spectrum, so as to cause a partial eclipse of the violet, blue, and green rays, the colour of the patch will be changed to orange. if we remove the card altogether, the patch will once more become white. it follows _a fortiori_ that when any portion of the little spectrum is eclipsed totally, instead of only partially, the light from the remainder will appear, when combined, to be coloured. very beautiful changes of hue are exhibited by the bright patch when a narrow opaque strip, such as the small blade of a pocket knife, is slowly moved along the little spectrum at h, eclipsing different portions of it in succession. the patch first becomes green, then by imperceptible gradations it changes successively to blue, purple, scarlet, orange, yellow, and finally, when the knife has completed its course, all colour disappears and the patch is again white. we may improve upon this crude experiment, and, after captain abney's plan, prepare a number of small cardboard stencils, with openings corresponding to any selected parts of the little spectrum. when a card so prepared is placed at h (fig. ) the bright patch upon the screen is formed by the combination of the selected rays, all the others being quenched. we shall find that under these conditions the bright patch is generally, but not always, coloured. [illustration: _fig. .--stencil cards._] the first diagram in fig. represents a blackened card, which allows only the red and a little of the orange to pass through. when this is inserted in the grooved holder at h, the bright patch immediately turns red. the second diagram shows another, which transmits the middle portion of the spectrum, but blocks the red and the violet at its two ends: with this card the colour of the patch becomes green. the third card has openings for the violet and the red rays: this turns the patch a beautiful purple, a hue which, as already mentioned, is not produced by light of any single wave-length. the purples are mixtures of red and violet or of red and blue. now i have in my possession three pieces of glass (or, to be strictly accurate, two pieces of glass and one glass-mounted gelatine film) which, when placed transversely in the beam of light, either at h (fig. ) or anywhere else, behave exactly like these three cardboard stencils. the first glass cuts off all the spectrum except the red and part of the orange, just as the first stencil does, though the line of demarcation is not quite so sharp. this is in fact a piece of red glass, or in other words the light that it transmits produces the sensation of red. the second glass, like the second stencil, allows the whole of the spectral rays to pass freely except the red and the violet, which disappear as if they were obstructed by an opaque body. this is a green glass. and the third (which is really a film of gelatine) cuts out the middle of the spectrum but transmits the red and violet ends. the colour of the gelatine is purple.[ ] the glasses and the gelatine in question act like the cardboard stencils in completely cutting off some of the spectral rays and transmitting others, and they owe their apparent colours to the combined influence which the transmitted rays exert upon the eye. many other coloured glasses merely weaken some of the rays, without entirely quenching any. a piece of pale yellow glass, for example, when placed in the path of the beam of light from which the spectrum on the screen is formed, simply diminishes the brightness of the blue region and does not wholly quench any of the rays; and again, a common kind of violet-coloured glass enfeebles, but does not quite obliterate, the middle portion of the spectrum. from such observations as these we infer that the glasses derive their respective colours from the light which falls upon them. the first glass would not appear red if seen in a light which contained no red rays. this is easily proved by an experiment with the colour-patch apparatus. the spectrum being once more combined into a bright white patch (which turns red if the glass is for a moment interposed), let all the red rays and part of the orange be cut off with a suitable stencil. the re-combined light is no longer white but greenish-blue, as is evidenced by the colour of the patch; and nothing that is illuminated by this light can possibly appear red. the piece of red glass, if placed in the beam, will now cast a perfectly black shadow, and a square of bright red paper held in the middle of the patch will look as black as ink. it will be shown later how we may obtain light which, although it appears to the eye to differ in no respect from ordinary white daylight, yet contains no red component, and is consequently as powerless as this greenish-blue light to reveal any red colour in the objects which it illuminates. if we substitute a stencil which admits only red rays, we shall obtain a beam of light in which no colour but red can be seen. green and blue glasses when exposed to this light will cast black shadows, while pieces of green and blue paper will become either black or dark grey. we see then that the colours of transparent objects, like the glasses used in these experiments, are brought out by a process of filtration. certain of the coloured ingredients of white light are filtered out and quenched inside the glass, and it is to the remaining ingredients which pass through unimpeded that the observed colour is due. the energy of the absorbed rays is not lost of course, for energy, like matter, is indestructible. it is transformed into heat. a coloured glass held in a strong beam of light will in a short time become sensibly warmer than one that is clear and colourless. in studying colour effects as produced by coloured glasses, we have at the same time been learning how the great majority of natural objects--not only those which are transparent but also those called opaque--become possessed of their colours. for the truth is that few things are perfectly opaque. when white light falls upon a coloured body, it generally penetrates to a small depth below the surface, and in so doing loses by absorption some of its coloured components, just as it does in passing through the pieces of glass. but before it has gone very far--generally much less than a thousandth part of an inch--it has encountered a number of little reflecting surfaces due to optical irregularities, which turn the light back again and compel it to pass a second time through the same thickness of the substance: it thus becomes still more effectively sifted, and on emerging is imbued with a colour due to such of the components as have not been quenched in the course of their double journey through a superficial layer of the substance. any coloured rays reflected by an object must necessarily be contained in the light by which the object is seen. the following is a curious experiment illustrating this. a large bright spectrum is projected upon a screen and in the green or blue portion of it is held a wall poster. the letters and figures upon the paper are seen to stand out boldly as if printed with the blackest ink. but if the poster is moved into the red part of the spectrum, the printing at once disappears as if by magic, and the paper appears perfectly blank. the explanation is that the letters are printed in red ink--they can reflect no light but red. green or blue light falling upon them is absorbed and quenched, and the letters consequently appear black. on the other hand when the poster is illuminated by the red rays of the spectrum, the letters reflect just as much light as the paper itself, and are therefore indistinguishable from it. anything which, when illuminated by a source of white light, reflects all its various components equally and without absorbing a larger proportion of some than of others, appears white or grey. between white and grey there is no essential difference except in luminosity, or brightness, that is to say, in the quantity of light reflected to the eye, or--to go a step further back--in the amplitude of the ether waves. under different conditions of illumination any substance which reflects all the rays of the spectrum equally may appear either white or grey, or even black. a snowball can easily be made to look blacker than pitch, and a block of pitch whiter than snow. it must have struck many of those who have thought about the matter at all as a most remarkable coincidence that sunlight should be white. white light, as we have seen, consists of a mixture of variously-coloured rays in very different and apparently arbitrary proportions, and if these proportions were a little changed the light would no longer be quite colourless. no ordinary artificial light is so exactly white as that of the sun. the light of candles, gas, oil, and electric glow-lamps is yellow; that of the electric arc (when unaffected by atmospheric absorption) is blue, and that of the incandescent gas burner green. it is exceedingly convenient that the light which serves us for the greater part of our waking lives should happen to be just so constituted that it is colourless. but on a little further reflection it will, i think, appear that this is not the right way to look at the matter. it is precisely because the hue called white is the one which is associated with the light of our sun that we regard whiteness as synonymous with absence of colour. we take sunlight as our standard of neutrality, and anything that reflects it without altering the proportions of its constituents we consider as being colourless. there can be little doubt that if the sun were purple instead of white, our sentiments as regards these two hues would be interchanged; we should talk quite naturally of "a pure purple, entirely free from any trace of colour," or perhaps describe a lady's costume as being of a "gaudy white." even as things are, the standard of neutrality is not quite a hard and fast one. we have a tendency to regard any artificial light which we may happen to be using, as more free from colour than it would turn out to be if compared directly with sunlight. if in the middle of the day we go suddenly into a gas-lit room, we cannot fail to observe how intensely yellow the illumination at first appears; in a few minutes, however, the colour loses its obtrusiveness and we cease to take much notice of it. the effect may be partly a physiological one, depending upon unequal fatigue of the various perceptive nerves of the retina; but i believe that it is to a large extent due to mental judgment. the standard of whiteness, or colour-zero, can apparently be changed within certain limits in a very short time, and, as we shall see later, this is only one of many instances in which our organs of vision seem to be incapable of recognising a constant standard of reference. and now let us consider how it comes about that each elementary portion of the retina--at least in its central region--has the power of distinguishing so many hundreds of different hues. it is incredible that every little area of microscopic dimensions should be furnished with such a multitude of independent organs as would be necessary if each of the many colours met with in nature required a separate organ for its perception; and it is not necessary to suppose anything of the kind. experiment shows that all the various hues of the spectrum, as well as all (including white) that can be formed from their mixture, may be derived from no more than three distinct colours. there are, in fact, an indefinite number of triads of colours which, in suitable combinations, are capable of producing the sensation of every tone, tint, and shade of colour which the eye of man has ever beheld. old-fashioned books, such as an early edition of ganot's "physics," tell us that the three "primary" colours are red, yellow, and blue, and that all others are produced by mixtures of these. this was the basis of sir david brewster's theory, which attained a very wide popularity, and even at the present time is held as an article of faith among the great majority of intelligent persons who have not paid any special attention to science. but it is not true. a fatal objection to it is the well-ascertained fact that no combination of red, yellow, and blue, or of any two of them, such as blue and yellow, for example, will produce green. yet every painter knows that if he mixes blue and yellow pigments together he gets green. that is one of the first things that a child learns when he is allowed to play with a box of water-colours, and no doubt brewster was misled by the fact. the truth is, that the colours of all, or almost all, known blue and yellow pigments happen to be composite. an ordinary blue paint reflects not only blue light, but a large quantity of green as well; while an ordinary yellow paint reflects a large quantity of green light in addition to yellow. when such paints are mixed together, the blue and yellow hues neutralise one another, and only the green, which is common to both, remains. the spectrum apparatus will make this clearer. hold a piece of bright blue glass before the slit; the light passing through the glass will be analysed by the prism, and you will see that it really contains almost as much green as blue. if a yellow glass is substituted, not only will yellow light be transmitted, but, as before, a considerable quantity of green. if now both glasses be placed together before the slit, what will happen? the yellow glass will stop the blue light transmitted by the blue glass, the blue glass will stop the yellow light transmitted by the yellow glass, and only the green light which both glasses have the power of transmitting will pass through unimpeded, forming a band of pure green colour upon the screen. the combination of simple blue and yellow lights of suitable relative luminosities results in the formation of white or neutral light. if the blue is a little in excess, the combined light will be of a bluish tint; if the yellow is in excess, the combination will have a yellowish tint. it will never contain any trace of green. the combination of simple spectral blue and yellow is easily effected by the colour-patch apparatus, and the result will be found to bear out what has been said. since, then, no mixture of red, yellow, and blue, or of any two of them, will produce green, we cannot regard these colours as being, in brewster's sense of the term, primary ones. but it is quite possible to find a group of three different hues--and indeed many such groups--which when made to act upon the eye simultaneously and in the right proportions can give rise to the sensation of any colour whatever. now this experimental fact is obviously suggestive of a possible converse, namely, that almost every colour sensation may in reality be a compound one, the resultant of not more than three simple sensations. assuming this to be so, it is evident that if each elementary area of the retina were provided with only three suitable colour organs, nothing more would be requisite for the perception of an indefinite number of distinct colours. such a hypothesis was first proposed by thomas young at the beginning of the present century; but it came before its time and met with no attention until fifty years later, when it was unearthed by the distinguished physicist and physiologist, helmholtz, who accorded to it his powerful support and modified it in one or two important details. [illustration: _fig. .--helmholtz's curves of colour perception._] according to the young-helmholtz theory, as it is now called, there are three different kinds of nerve-fibres distributed over the retina. the first, when separately stimulated, produce the sensation of red, the second that of green, and the third that of violet. light having the same wave-length as the extreme red rays of the spectrum stimulates the red nerve-fibres only; that having the same wave-length as the extreme violet rays stimulates the violet nerve-fibres only. light of all intermediate wave-lengths, corresponding to the orange, yellow, green, and blue of the spectrum, stimulates all three sets of nerve-fibres at once, but in different degrees. the proportionate stimulation of the red, green, and violet nerves throughout the spectrum is indicated in fig. , which is derived from the rough sketch first given by helmholtz. the yellow rays of the spectrum, it will be seen, excite the red and green nerves strongly, and the violet feebly; green light excites the green nerves strongly, and the red and violet moderately; while blue light excites the green and violet nerves strongly, and the red feebly. [illustration: _fig. .--könig's curves._] fig. shows another set of curves given more recently by dr. könig as the result of many thousands of experiments made, not only upon persons whose vision was normal, but also upon some who were colour-blind. könig found that the equations he obtained were best satisfied by assuming as the normal fundamental sensations a purplish red (not to be found in the spectrum), a green like that of wave-length , and a blue like that of wave-length approximately, the two latter, however, being purer or more saturated than any actual spectrum colour. but könig's curves are not consistent with every class of vision which he examined, and the question as to what are the true fundamental colour-sensations, if such really exist at all, cannot yet be regarded as finally settled.[ ] the young-helmholtz theory of colour-vision, whether or not it is destined in the future to be superseded by some other, has at all events proved an invaluable guide in experimental work, and there are very few colour phenomena of which it is not competent to offer a satisfactory explanation. it has at present only one serious rival--the theory of hering, which, although it seems to be curiously attractive to many physiologists, can hardly be said to present less serious difficulties than that which it seeks to displace. neither of these competing theories has yet had its fundamental assumptions confirmed by any direct evidence, and the advantage must rest with the one which best accords with the facts of colour vision. in my judgment the older of the two is to be greatly preferred as a useful working hypothesis. certain curiosities of vision with which i propose to deal in a future chapter depend upon the properties of what are known as complementary colours. two colours are said to be complementary to each other when their combination in proper proportions results in the formation of white. [illustration: _fig. .--stencil card for complementary colours._] if we produce a compound hue by mixing together the colours of any portion of the spectrum, and a second compound hue by mixing the remainder of the spectrum, it must be evident that these two hues are necessarily complementary, for when they are united they contain together all the elements of the entire spectrum, and therefore appear as white. this may be illustrated with the aid of the colour-patch apparatus. place at h (fig. ) a cardboard stencil of the form shown in fig. , and focus upon it a little spectrum, the principal hues of which are indicated by the letters r o y g b v (red, orange, yellow, green, blue, violet). the two oblong apertures in the card should be of exactly the same height, and the card so placed that one aperture may admit rays extending from the red end of the spectrum to about the middle of the green, while the other admits rays from the remainder of the spectrum. if now the lower aperture be covered, only the red, orange, yellow, and part of the green rays will pass through the stencil, and these being combined by the lens k (fig. ) will form upon the screen a bright patch, the colour of which will be yellow. if the upper aperture be covered, and the rest of the green, together with the blue and violet rays, allowed to pass through the other, the colour of the patch will become blue; and if both apertures be uncovered at the same time, rays from the whole length of the spectrum will pass through the stencil, and the patch will, of course, turn white. the yellow and the blue which were compounded from the two portions of the spectrum are, therefore, in accordance with the definition, complementary colours. in a similar manner by dividing the spectrum into any two portions whatever--as, for example, by the complicated stencil shown in fig. --we can obtain an indefinite number of pairs of complementary colours. [illustration: _fig. .--stencil card for complementary colours._] but it is by no means indispensable that both or either of a pair of complementary colours should be compound. to prove this, two strips of card with narrow vertical openings a and b are prepared as shown in fig. . the cards are placed one above the other and can be slipped in a horizontal direction, so that the narrow openings can be brought into any desired part of the spectrum which is indicated in outline by the dotted oblong. [illustration: _fig. .--slide for mixing any two spectral colours._] bring the opening a of the upper card into the yellow of the spectrum and the opening b of the lower card into the blue. the bright patch formed upon the screen will then be illuminated by simple blue and yellow rays; yet it will be white--not green, as it would be if brewster's theory were correct. if upon the first trial the white should not be absolutely pure, it can easily be made so by partially covering either a or b--the first if the white is yellowish, the second if it is bluish. simple spectral blue and yellow are therefore no less truly complementary colours than are the compound hues formed when the spectrum is divided into two parts. it is noticeable, however, that the white light resulting from the combination of blue and yellow, though it cannot be distinguished by the eye from ordinary white light, is yet possessed of very different properties. most coloured objects when illuminated by it have their hues greatly altered; a piece of ribbon, for example, which in common light is bright red, will appear when held in the blue-yellow light to be of a dark slate colour, almost black. if the opening a is placed in any part whatever of the spectrum except the green, it will always be possible, by moving b backwards or forwards, to find some other part where the colour is complementary to that at a. to green there is no simple complementary; a purple is required, which is not found in the spectrum, but may be formed by combining small portions of spectral blue and red. for studying mixtures of three simple colours, a third slide may be added to the two shown in fig. . the following little table gives the principal pairs of complementary colours. table of complementary colours. red greenish-blue orange sky-blue yellow blue greenish-yellow violet green purple chapter iii. some optical defects of the eye. more than one reference has been made to the fact that the sense of sight, even in its best normal condition, is characterised by certain defects and anomalies. some of these arise directly from causes inherent in the design or structure of the eye itself, and may be broadly classified as physical; others are of psychological origin, and result from the erroneous interpretations placed by the mind upon the phenomena presented to it through the medium of the optic nerve and the brain. among the numerous physical defects of the eye none is more remarkable than the absence of means for properly correcting chromatic aberration. this defect is remarkable because it appears--at least to those who are without actual experience in the manufacture of eyes--to be one which might very easily have been avoided. so far as a mere theorist can judge, an achromatic arrangement of lenses would have been just as simple and just as cheap (if i may use the term) as the arrangement with which we find ourselves provided. it is true that we manage to go through life very well with our uncorrected lenses, and indeed it is hardly possible by ordinary observation to detect any evidence of the imperfection. yet its existence in a glaring degree is undoubted, and can be readily demonstrated by a great variety of methods. the conclusion is inevitable that with achromatic eyes our vision would be improved, but whether there may not possibly exist reasons why such an improvement could only be achieved at a disproportionately high cost is a question which cannot at present be answered. without going into matters which are dealt with in every elementary text book of optics or general physics, it may be desirable to explain shortly what is meant by the terms chromatic aberration, and achromatism. [illustration: _fig. .--refraction of monochromatic light by a lens._] let l l, fig. , represent in section a circular convex lens, and p a luminous point, which is most conveniently supposed to be situated on the axis of the lens. imagine p to be surrounded in the first instance by a glass shade which transmits only monochromatic red light. so much of the light from p as falls upon the lens will be refracted to a point at the conjugate focus f, and after passing this point will diverge again; the refracted light rays will, in fact, form a double cone, of which f is the apex. if a white screen be held at f, there will be focussed upon it a small clearly-defined image of the luminous point. if, however, the screen be moved nearer to or further from the lens, it will cut the cone of light, and the image will then no longer appear as a point, but as a circular red disk, which will be larger the greater the distance of the screen from f. such a disk is known as a "diffusion circle." suppose now that we substitute for the red glass, surrounding the source of light, a purple one capable of transmitting not only red rays but violet as well. the lens will cause both the red and the violet rays which pass through it to converge; but since the violet rays are more refrangible--more easily refracted or bent aside out of their straight course--than the red, there will now be two double cones, as shown in fig. , where the contours of the red cones are represented by solid lines and those of the violet by dots. [illustration: _fig. .--refraction of dichromatic light._] the focus of the red rays will as before be at f, but that of the violet will be nearer to the lens, as at h, and this being so, it is evident that a well defined image of the purple source of light cannot possibly be formed upon a screen placed anywhere behind the lens. held in the position indicated by the line c c, where it passes through the focus of the red rays, the screen cuts one of the cones of violet light, and the image at f will appear to be surrounded by a violet halo. held at a a, the screen evidently receives an image with a red halo round it. only at b b, in the plane where the surfaces of the red and violet cones cut one another, will it be possible to obtain an image without a coloured border; but here good definition is unattainable, for neither the red nor the violet rays are in focus, and the luminous point is represented by a purple disk or diffusion circle of sensible diameter. if rays of every possible refrangibility are allowed to fall upon the lens, as is the case when the source of light is not shielded by any coloured glass, there will be formed an indefinite number of pairs of cones, the apices of which will lie along the straight line joining h and f. it is clear that all these cones cannot possibly intersect in a single plane, and consequently no position can be found where the edge of the projected image is perfectly free from colour, though at a certain distance from the lens, where the brightest constituents of the light--namely, the yellow and green--are approximately focussed, the coloured border is least conspicuous, and is of a purple tint, due to the mixture of the red and violet rays. for these reasons a single glass lens cannot, except with homogeneous light, be made to give a perfectly distinct image of a luminous point, nor of an illuminated object, the surface of which may be regarded as an assemblage of points. such a lens, therefore, is never employed when good definition is required. the confusion resulting from the unequal refrangibility of the differently coloured rays is said to be due to the chromatic aberration of the lens. in connection with this matter, the history of physical optics contains an interesting little episode. it occurred to sir isaac newton that although a single lens could never be free from chromatic aberration, yet it might be possible to arrange a so-called achromatic combination of lenses in such a manner as to overcome the defect and bring all the rays issuing from a point, whatever their refrangibility, to one focus. experiments which he undertook for the purpose of testing the matter led him to form the conclusion that such a result could never be attained, the amount of colour dispersion in all substances being, as he stated, always exactly proportional to that of refraction. for this reason he confidently announced that it was useless to attempt the construction of a really good refracting telescope, and so great was the authority attaching to his name that for many years all efforts in that direction were abandoned. nevertheless from time to time certain philosophers ventured to surmise that newton might perhaps have been mistaken, and the curious thing is that they all based their scepticism upon what they considered the self-evident fact of the achromatism of the eye. the system of lenses in the eye, they argued, being unquestionably achromatic, why should not an equally effective combination be constructed artificially? at length, more than eighty years after newton had made and published his fundamental experiments, it occurred to a working optician, john dollond, that it might be worth while to repeat them, and upon doing so he at once found that newton was wrong in his facts, the results as recorded by him being in direct opposition to the truth. with proper respect for the memory of a great man it is usual to speak of newton's observation as a "hasty" one, but if in these days a junior science student were to be guilty of a similar lapse, his conduct would not impossibly be stigmatised as grossly careless. having established newton's error, dollond found little difficulty in constructing achromatic lenses of very satisfactory quality; telescopes of his manufacture long enjoyed the highest reputation, and the best optical instruments of the present day are the direct offspring of his invention. those who entertained the opinion that newton's conclusion was erroneous were therefore in the right, but it is remarkable that the reason upon which that opinion rested was altogether invalid, for, as i have said, the lenses of the eye are by no means achromatic. of the many ways in which this can be demonstrated, the following is one of the most impressive. let a long and narrow spectrum of the electric light be projected upon a white screen, the prisms and lenses being carefully arranged in such a manner as to ensure that the upper and lower edges of the spectrum are clearly defined and strictly parallel. to an observer standing close to the screen, the spectrum will present the appearance of a bright parti-coloured rectangle. but viewed from a distance of a few feet the spectrum will not seem to be rectangular, its upper and lower edges no longer appearing to be parallel, but to diverge, fan-like, towards the blue and violet, as shown in fig. . this is because the violet and some of the blue rays proceeding from an object at a little distance cannot by any effort be focussed upon the retina. they are too much refracted, and the mechanism by which the eye is adjusted is incompetent to diminish the convexity of the lenses sufficiently to enable them to project a clear image. every point is expanded into a luminous circle, which is the larger the more refrangible the rays, and it is the extension of these diffusion circles beyond the proper boundaries of the image that gives the appearance of increased breadth. it is a simple matter to counteract the effects of undue convexity by means of a concave lens. if a normal-eyed person, to whom the violet end of the spectrum when seen from a distance appears blurred and widened, will look at it through suitable glasses adapted for short sight, he will at once see it clearly defined and of its proper width. [illustration: _fig. .--narrow spectrum as seen from a distance._] let a rectangular patch of white light having about the same dimensions as the rectangular spectrum be now thrown upon the screen. the light reflected from the patch will contain, as before, all the various spectral colours, but they will be mixed or superposed, instead of being spread out side by side. the patch will send forth, among others, can yellow and green rays, which the eye easily focus; it will also send out violet rays, which, as we have shown, cannot be focussed by the unassisted eye. owing to the existence of diffusion circles there must necessarily be formed upon the retina a violet image larger than the approximately superposed images due to rays of brighter colours. viewed from a distance therefore the white patch might be expected to exhibit a violet border. yet it may be confidently asserted that the observer will not be conscious of seeing any such border, for though one actually exists, it is possessed of such comparatively feeble luminosity that it is lost in the glare produced by the brighter rays. it is, however, possible to cut off these brighter rays by interposing between the projection lantern and the screen a combination of glasses which has been found by trial with a spectroscope to transmit only dark blue and violet light. the rectangle will then be of a blue-violet colour, and when looked at closely, will still be quite clear and sharply defined, but viewed from a little distance it will appear blurred and of an exaggerated size. another and perhaps even better way of demonstrating this last effect is to enclose the source of light (which should be a powerful one, such as an arc lamp or limelight) inside a box having a ground-glass window in one side. when the window is covered by the coloured glasses its outline cannot be clearly distinguished unless the observer is near, but if he uses suitable concave spectacles, he will be able to see it quite distinctly, even from a considerable distance. it is well known that ideas of distance are associated with certain colours. a room gives one the impression of being larger when it is papered or painted a blue-violet colour than when its colouring is red. in the former case the walls seem to retire from the spectator, in the latter to approach him. so too a red spot upon a violet ground appears to be distinctly raised above the surface, while a violet spot upon a red ground appears to be depressed. these phenomena are fully explained by the imperfect achromatism of the eye. when we look at a red object, we have to adjust the crystalline lens by means of the ciliary muscle in exactly the same way as when we look at a near object; in both cases it is necessary to increase the convexity of the lens, and so diminish its focal length, in order to obtain a clear image upon the retina. and again, when we wish to see a blue or violet thing distinctly, the ciliary muscle must be relaxed and the convexity of the lens as far as possible diminished, just as if the gaze were directed to the horizon. we are accustomed to estimate the distances of things largely by the muscular effort required to focus their images, and thus it happens that the colour red comes to be associated in our minds with nearness, and violet with remoteness. these psychological effects are perfectly well marked even with the impure colours met with in ordinary life, but they are naturally more evident when the colours observed are pure, like those of the spectrum. a beautiful example is that presented by the pair of short bright spectra formed upon the screen when a double slit is used shaped like the letter v. the gorgeously coloured v seems to stand out in strong relief like a pair of inclined boards, the nearer edges being red, the farther ones violet. (see fig. .) [illustration: _fig. .--spectrum formed with v-shaped slit._] in many other ways, and with little or no apparatus, any one may easily convince himself that the different constituents of white light are not equally refracted by the lenses of the eye. look, for instance, at the incandescent filament of an electric lamp through a piece[ ] of common dark blue cobalt glass, which has the property of obstructing the coloured rays corresponding to the middle of the spectrum, while transmitting the red and the blue. seen from a distance of only a few inches, the filament appears to be pale blue with a bright red border, the blue rays being perfectly focussed, while the red form diffusion circles. move some six or eight feet away and look again; the colours will now be reversed, the filament appearing red and the border blue-violet. from a still greater distance--about fifteen or twenty feet--the whole lamp-bulb will seem to be filled with a blue-violet glow, due to large diffusion circles, while the red image of the filament may be even more clearly defined than before. no doubt it is partly owing to the non-achromatism of the eye that distant arc lights always appear to have a yellowish hue, even when the air is quite clear; a considerable proportion of their blue and violet components must necessarily be lost by extensive diffusion.[ ] again, look at a sunlit landscape or a printed wall poster through a combination of coloured glasses which will transmit only the violet end of the spectrum. you will find yourself for the time terribly short-sighted, everything appearing blurred and indistinct. but if you resort to the usual corrective for myopia, and put on a pair of concave spectacles, your normal vision will be restored; trees and houses will be seen as clearly as the feebleness of the light transmitted by the coloured glasses will permit, and the letters of the poster will become easily legible. now, of course, the interposition of coloured glasses does not actually give rise to these blurred images; it merely enables one to detect their existence. under ordinary conditions they always accompany the clearer images produced by the more luminous rays, and their presence cannot fail to exert a detrimental effect upon the general definition. such blurs must at least tend to fog the darker portions of the focussed picture, and though we are not distinctly conscious of their existence, it is certain that if they were annulled the acuteness of our vision would be improved. the diffusion circles produced by the red rays, when the eye is accommodated (as it commonly is) for the yellow and green, are less conspicuous than those due to the most refrangible rays. yet i find it impossible to focus a red object, such as the filament of an electric lamp screened by a properly selected deep red glass, when placed at the ordinary distance of distinct vision--some nine or ten inches from the eye--without the aid of a convex lens. in this case one is not too short-sighted but too long-sighted to see the object distinctly; in other words, the lenses of the eye cannot refract the red rays sufficiently to produce well-defined images upon the retina, and the refraction has to be increased by artificial means. though, as i have said, it is difficult, or even impossible to detect any trace of a coloured border when looking at a bright object for which the eye is accommodated, it is quite easy to bring such borders into prominence if the object is at a distance a little too great or too small for distinct vision. a very remarkable device for the purpose is one due to von bezold. this may be illustrated by using a non-achromatic glass lens, such as a common magnifying glass, to project a transparency or lantern-slide upon which is painted a target-like design, consisting of a series of circular black bands surrounding a circular black spot.[ ] (see fig. .) [illustration: _fig. .--bezold's diagram._] suppose the glass lens to represent the lenses of a gigantic eye (in a definite condition of accommodation) and the screen the retina. the imaginary eye is looking at the design on the lantern-slide, and when this is at the distance of most distinct vision a fairly well defined image of the target is formed upon the retinal screen. now gradually move the lantern slide towards the lens (or the lens towards the slide), thus bringing it too near for distinct vision. this has the effect of enlarging the diffusion circles formed by the less refrangible rays corresponding to the red end of the spectrum, and at the same time of diminishing those formed by the more refrangible rays corresponding to the violet end. the first result is that the circular dark bands become reddish brown, and the spaces between them bluish. as the distance between the lens and the slide is still further diminished, the tints become more varied and brilliant, until at last there appears a beautiful series of coloured rings around a bright red central spot. these effects are not produced when the lens employed is an achromatic one; with such a lens the diffusion circles are all enlarged or diminished together, and a to-and-fro movement of the lantern slide (or of the lens) merely affects the definition of the image without causing any perceptible dispersion of colour. now it is noteworthy that the chromatic phenomena exhibited with the uncorrected glass lens are quite well shown by the lenses of the eye. it is only necessary to hold the lantern-slide before a bright background and gradually bring it so close to the eye that the design cannot be seen distinctly. the black bands will then appear to turn brown, the white ones blue, and the central spot bright red. the printed diagram (fig. ) will itself show the colours if it is held at a distance of four to five inches from one eye in a good light. one more experiment may be referred to. look with one eye at a well-lighted page of print, and with a strip of brown paper, held quite near the eye, cover about half the pupil. the black letters will now appear to be bordered with colour--blue towards the apparent edge of the brown paper, orange on the opposite side. if the letters are white on a black ground, as sometimes happens in the case of advertisements, the colours will be interchanged. the cause of the coloured borders will be readily understood from an inspection of the diagram fig. ; but it must be remembered that the images on the retina are inverted. thus it is proved beyond all question that the lenses of the eye do not form an achromatic combination. another peculiarity by which the eye is affected, and which does not occur in optical instruments, is that known as _astigmatism_. the surface of the cornea, which, with the aqueous humour, forms the outer lens, is not often perfectly spherical; generally it is shaped something like the bowl of a spoon, the curvature being greater vertically than horizontally. rays issuing from a luminous point do not, after refraction by such a lens, cross at a single focus, but along two short straight lines, the one horizontal the other vertical, which are at different distances from the lens; thus a distinct image of a small point cannot anywhere be produced. [illustration: _fig. .--effect of astigmatism._] a very curious result follows from this deformity. if two straight lines are drawn at right angles to each other, as in fig. , it is impossible to see both of them quite clearly at the same time. when the paper is held at a certain short distance from the eye--about eight or nine inches--the horizontal line appears black and well defined, while the other is rather grey and indistinct; at a greater distance the upright line seems to be the blacker. the effect is very well shown by the diagram, fig. . to most persons the lines occupying the middle portion will appear either much blacker or much lighter than those at the two ends, though in fact they are exactly alike. when this form of astigmatism is excessive, it may be corrected by the use of spectacles fitted with cylindrical lenses. [illustration: _fig. .--effect of astigmatism._] but there is a different kind of astigmatism--irregular astigmatism it is called--to which every one is more or less a victim, and which cannot be relieved by any artificial appliances. fortunately it does not often cause much practical inconvenience. irregular astigmatism is commonly demonstrated in the following manner. with the point of a fine needle, prick a very small hole in a sheet of tinfoil. hold up the tinfoil to the light and look at the hole with one eye, the other being closed. even at the distance of most distinct vision--ten inches or thereabouts,--there will probably be a ragged appearance about the hole, as if it were not perfectly round. but if you bring the tinfoil an inch or two nearer to the eye, the hole will not seem to be even approximately circular; it will assume the form of a little star with five or more distinct rays. the configuration of the star is not generally the same for the right eye as for the left; the rays may differ in number and in relative magnitude, and may be inclined at different angles to the vertical. fig. shows the stars as they appear to my two eyes, when the illumination is rather strong. [illustration: _fig. .--star-like images of luminous point._] if several holes are pricked in the tinfoil, each will of course originate a separate star, and all the stars as seen by the same eye will appear to be figured upon the same model, though some may be larger or brighter than others. [illustration: _fig. .--sutures of crystalline lens._] there can be no doubt that the stellate form observed in these experiments, as well as that of the stars of heaven themselves (which with perfect vision would be seen simply as luminous points), is a consequence of the singular structure of the crystalline lens of the eye. this does not consist of one uniform homogeneous mass like a glass lens, but of a number of separate portions pieced together radially, as indicated diagrammatically in fig. . in the eye of a newly-born child there are three such portions, and the radial junctions on one side of the lens are not opposite to those on the other, but are intermediate. in the figure the junctions at the front of the lens are represented by continuous lines and those at the back by dots. the number of sutures found in the adult lens is generally greater than six. but while it is certain that these radial sutures are in some way closely connected with the luminous rays which appear to proceed from a bright point, it must be confessed that no adequate explanation has yet been given of the precise manner in which the phenomenon is brought about. ophthalmologists seem to have been contented with vague statements about irregular refraction, but what kind of irregularity would sufficiently account for all the facts of observation has never, so far as i know, been exactly determined. the problem can hardly be very difficult of solution, and would, no doubt, readily yield to the joint efforts of a physicist and a physiologist. the phenomena of irregular astigmatism as exhibited by a normal eye are exceedingly curious, and perhaps i may be allowed to refer briefly to one or two experiments which i have myself made on the subject.[ ] [illustration: _fig. .--multiple images of a luminous point._] light from an enclosed electric lamp of twenty-five candle power was admitted through a circular aperture about / -inch ( mm.) in diameter perforated in a brass plate; a sheet of ground glass and another of ruby-red glass were placed behind the aperture. when the little disk of monochromatic light thus formed was looked at through a concave lens of eleven inches focal length from a suitable distance--nearly two feet in my own case--it appeared as seven bright round spots upon a less luminous ground. the appearance is represented in a somewhat idealised form in fig. ; but the spots were not quite so distinct nor so regularly disposed as there shown, neither was their configuration exactly the same for the right eye as for the left. on gradually increasing the distance each circumferential spot became at first elongated radially and afterwards split up into two circular ones; at the same time new spots were developed upon the luminous ground, the approximate symmetry of the figure being still retained. fig. represents a certain stage in this process of expansion. the appearance was happily likened by an observer who repeated the experiment to that of a large unripe blackberry. as the distance was still further increased, the spots continued to multiply, ultimately becoming very numerous; their arrangement however soon became much less regular, and the definition of most of them less distinct. at about twenty feet there was seen a luminous patch, roughly circular in outline, and covered with irregular speckles; superposed upon this were strings of bright, partially overlapping spots, corresponding apparently to the sutures of the crystalline lens. [illustration: _fig. .--increased number of images._] when the hole was looked at from a moderate distance through a narrow slit (about / inch wide) interposed between the eye and the lens, there was seen only a single row of circular spots, which were arranged sinuously, as shown in fig. . a slight movement of the slit in the direction perpendicular to its length produced a wave-like motion of the circles, suggestive, as pointed out by the excellent observer before referred to of the wriggling of a caterpillar. [illustration: _fig. .--multiple images seen through a slit._] by sufficiently increasing the distance between the source of light and the eye, as many as twenty-four or twenty-five bright spots might be made to appear in the row, but they could not be counted with any great certainty. at a still longer distance or with a lens of shorter focus (convex or concave) they became less distinct, and finally seemed to be resolved into a multitude of small blurred images--probably several hundreds--which were separated from one another by hazy dark lines. [illustration: _fig. .--images of an electric lamp filament._] i thought that the observations might be rendered easier if the source of light had a more distinctive and conspicuous form than that of a simple circle. some experiments were therefore made with semi-circular and triangular holes, and these were in some respects preferable; but far better results were afterwards obtained by using as a source of light the horse-shoe shaped filament of an electric lamp, screened by a coloured glass. when such a lamp was looked at through a lens, concave or convex, of about six inches focus, from a distance of a few feet, the roughly oval patch of luminosity formed upon the retina, instead of being a mere ill-defined blur, such as would be produced if the transparent media of the eye were composed of homogeneous substances like glass or water, appeared to be made up of a crowd of separate images of the filament, some being brighter than others, as is shown in the diagram fig. . [illustration: _fig. a.--images with horizontal slit._] [illustration: _fig. b.--images with vertical slit._] if a spectroscope slit was interposed between the eye and the lens, and its width suitably adjusted, only a single row of filaments was observed, the appearances with the slit in horizontal, vertical, and intermediate positions being as represented in fig. , a, b, c. as before, it was found possible by gradually retiring from the lamp to bring the number of images up to about twenty-five, but attentive examination showed that most of these really consisted of clusters, each composed of perhaps fifteen or twenty confused images of the filament. a stronger lens still further separated the constituents of the clusters, exhibiting a total number of indistinctly seen images which was estimated to amount to nearly five hundred. assuming the diameter of the pupil of the eye to be one-fifth of an inch, these observations seem to indicate as a cause of the phenomenon some fairly regular anatomical structure, situated in or near the crystalline lens and composed of elements measuring about / inch in length or breadth. whether the structure which gives rise to these multiple images is to be found in the fibres of the crystalline lens itself, or in the membranes which cover it, is a question upon which i will not venture an opinion. [illustration: _fig. c.--images with oblique slit._] it is indeed wonderful that an organ affected by peculiarities of which those that have been referred to are merely specimens, should give such well-defined pictures as it does when accommodated for the objects looked at. chapter iv. some optical illusions. optical illusions generally result from the mind's faulty interpretation of phenomena presented to it through the medium of the visual organs. they are of many different kinds, but a large class, which at first sight may seem to have little or nothing in common, arise, i believe, from a single cause, namely, the inability of the mind to form and adhere to a definite scale or standard of measurement. in specifying quantities and qualities by physical methods, the standards of reference that we employ are invariable. we may, for example, measure a length by reference to a rule, an interval of time by a clock, a mass or weight by comparison with standardised lumps of metal, and in all such cases--provided that our instruments are good ones and skilfully used--we have every confidence in the constancy and uniformity of our results. but two lengths, which when tested with the same foot rule are found to be exactly equal, are not necessarily equal in the estimate formed of them by the mind. look, for instance, at the two lines in fig. . according to the foot rule each of them is just one inch in length, but the mind unhesitatingly pronounces the upright one to be considerably longer than the other; the standard which it applies is not, like a physical one, identical in the two cases. many other examples might be cited illustrative of the general uncertainty of mental estimates. [illustration: _fig. .--illusion of length._] the variation of the vague mental standard which we unconsciously employ seems to be governed by a law of very wide if not universal application. though this law is in itself simple and intelligible enough, it cannot easily be formulated in terms of adequate generality. the best result of my efforts is the following unwieldy statement:--the mental standard which is applied in the estimation of a quality or a condition tends to assimilate itself, as regards the quality or condition in question, to the object or other entity under comparison of which the same (quality or condition) is an attribute. in plainer but less precise language, there is a disposition to minimise extremes of whatever kind; to underestimate any deviation from a mean or average state of things, and consequently to vary our conception of the mean or standard condition in such a manner that the deviation from it which is presented to our notice in any particular instance may seem to be small rather than large. thus, when we look at a thing which impresses us as being long or tall, the mental standard of length is at once increased. it is as if, in making a physical measurement, our foot rule were automatically to add some inches to its length, while still supposed to represent a standard foot: clearly anything measured by means of the augmented rule would seem to contain a fewer number of feet, and, therefore, to be shorter than if the rule had not undergone a change. it is not an uncommon thing for people visiting switzerland for the first time to express disappointment at the apparently small height of the mountains. a mountain of , feet certainly does not seem to be twenty times as lofty as a hill of . the fact is that a different scale of measurement is applied in the two cases; though the observer is unaware of it, the mountain is estimated in terms of a larger unit than the hill. [illustration: _fig. .--illusion of length._] if we mentally compare two adjacent things of unequal length, such as the two straight lines in fig. , there is a tendency to regard the shorter one as longer than it would appear if seen alone, and the longer one as shorter. the lower of the two lines in the figure is just twice as long as the other, but it does not look so; each is regarded as differing less than it really does from an imaginary line of intermediate length. [illustration: _fig. .--illusion of length._] two divergently oblique lines attached to the ends of a straight line as at a, fig. , suggest to the mind the idea of lengths greater than that of the straight line itself; the latter, being thought of as comparatively small, is therefore estimated in terms of a smaller unit than would be employed if the attachments were absent, and consequently appears longer. if, on the other hand, the attachments are made convergent, as at b, shorter lengths are suggested; the length of the given line is regarded as exceeding an average or mean; the standard applied in estimating it is accordingly increased, and the line is made to seem unduly short. in spite of appearances to the contrary, the two lines a and b are actually of the same length. by duplicating the attached lines, as shown in fig. , their misleading effect becomes intensified. here we have a well-known illusion of which several explanations have been proposed. the fallacy is, i think, sufficiently accounted for by variation of the mental standard, in accordance with the law to which i have called attention. [illustration: _fig. .--illusion of length._] a number of other paradoxical effects may be referred to the operation of the same law. fig. shows a curious specimen. at each end of the diagram is a short upright line; exactly in the middle is another; between the middle and the left hand end are inserted several more lines, the space to the right of the middle being left blank. any one looking casually at the diagram would be inclined to suppose that it was not equally divided by what purports to be the middle line, the left hand portion appearing sensibly longer than the other. [illustration: _fig. .--illusion of distance._] it is not difficult to indicate the source of the illusion. when we look at the left hand portion we attend to the small subdivisions, and the mental unit becomes correspondingly small; while in the estimation of the portion which is not subdivided a larger unit is applied. as one more example i may refer to a familiar trap for the unwary. ask a person to mark upon the wall of a room the height above the floor which he thinks will correspond to that of a gentleman's tall hat. unless he has been beguiled on a former occasion, he will certainly place the mark several inches too high. obviously the height of a hat is unconsciously estimated in terms of a smaller standard than that of a room. the illusion presented by the horizontal and vertical lines in fig. (p. ) depends, though a little less directly, upon a similar cause. we habitually apply a larger standard in the estimation of horizontal than of vertical distances, because the horizontal magnitudes to which we are accustomed are upon the whole very much greater than the vertical ones. the heights of houses, towers, spires, trees, or even mountains are insignificant in comparison with the horizontal extension of the earth's surface, and of many things upon it, to which our notice is constantly directed. for this reason, we have come to associate horizontality with greater extension and verticality with less, and, in conformity with our law, a given distance appears longer when reckoned vertically than when reckoned horizontally. hence the illusion in fig. . but it is not only in regard to lengths and distances that the law in question holds good; in most, if not all cases in which a psycho-optical estimate is possible, the mental standard is unstable and tends to assimilate itself, as regards the quality or condition to be estimated, to the entity in which the same is manifested. this is true, for example, in judging of an angle of inclination or slope; of a motion in space; of luminous intensity, or of the purity of a colour. every cyclist knows how difficult it is to form a correct judgment of the steepness of a hill by merely looking at it. not only may a slope seem to be greater or less than it really is, but under certain circumstances a dead level sometimes appears as an upward or downward inclination, while a gentle ascent may even be mistaken for a descent, and _vice versa_. we usually specify a slope by its inclination to a level plane which is parallel to the plane of the horizon, or at right angles to the direction of gravity. at any given spot the level is, physically considered, definite and unalterable. in forming a mental judgment of an inclination, we employ as our standard of reference an imaginary plane which is intended to be identical with the physical level. but our mental plane is not absolutely stable; when we refer a slope to it, we unconsciously give the mental plane a slight tilt, tending to make it parallel with the slope. hence the inclination of a simple slope, when misleading complications are absent, is always underestimated. [illustration: _fig. .--illusion of inclination._] this may be illustrated by the diagram fig. . if a b represents a truly horizontal line, the slope of the oblique line c d is correctly specified by the angle c o a. but if we have no instrument at hand to fix the level for us, we shall infallibly imagine it to be in some such position as that indicated (in an exaggerated degree) by the dotted line e f, while the true level a b will appear to slope oppositely to c d. this class of illusion is remarkably well demonstrated by zöllner's lines, fig. ; the two thick lines which appear to diverge from left to right, are in truth strictly parallel. [illustration: _fig. .--zöllner's lines._] i need not discuss in further detail the various illusions to which a cyclist is subjected when slopes of different inclinations succeed one another: they all follow simply from the same general principle. a thing is said to be in motion when it is changing its position relatively to the earth, which for all practical purposes may be regarded as motionless. the state, as regards motion, of the earth and anything rigidly attached to it, therefore constitutes the physical zero or standard to which the motion of everything terrestrial is referred. but the corresponding mental standard, especially when it cannot easily be checked by comparison with some stationary object, is liable to deviate from the physical one; it tends in fact to move in the same direction as the moving body which is under observation, and the apparent speed of the body is consequently rather less than it should be. the influence exerted upon the judgment sometimes even persists for an appreciable period after the exciting cause has ceased to be operative, as when the moving body is lost sight of or has suddenly come to rest; in such cases fixed objects, being compared with the delusive mental standard, appear for a few seconds to be moving in the opposite direction. i have devised a lantern slide (fig. ) by the aid of which this phenomenon may be rendered very evident. in a square plate of metal is cut a vertical slot, which is shaded in the figure; behind the plate is an opaque disk, which, by means of suitable mechanism, can be made to rotate about its centre. the disk has a spiral opening cut in it of the same width as the slot, as indicated by the dotted line. the slide is placed in an optical lantern, and the light passing through the aperture formed where the slot is crossed by the spiral opening, produces a small bright patch upon a white screen hung at a suitable distance from the lantern. [illustration: _fig. .--slide for showing illusions of motion._] when the disk is turned in the direction indicated by the arrow, the bright patch moves upwards and ultimately disappears; but at the moment of its disappearance a fresh patch starts from below, which also moves in the upward direction; thus there is formed upon the screen a continuous succession of ascending bright patches. after these have been observed for about a quarter of a minute, the disk is suddenly stopped, and the persistence of the fallacious mental standard is at once demonstrated. for the bright patch does not appear to be at rest, as it actually is, but to creep steadily downwards, continuing to do so more and more slowly for perhaps as long as ten seconds. the upward motion of the bright patches had led the observer to assume a slower upward motion as the zero, or standard of no motion, and reference of the really stationary patch to this physically false standard induces the illusion that the patch is descending. this experiment is most successful when the bright patches are projected upon the middle of a large screen. the disk should turn about three times in a second, and the room should be feebly illuminated, but not quite dark. [illustration: _fig. .--illusions of motion._] a very remarkable illusion which no doubt depends upon the same principle as the last, though its form is entirely different, is that to which the diagram fig. relates. so far as i am aware, it has not before been noticed. two intersecting straight lines, the one upright and the other sloping, as shown in the figure, are drawn upon a card. the card is to be held vertically before the eyes at the distance of most distinct vision, and waved up and down through a distance of a few inches. the oblique line will then appear to oscillate transversely, as if it were not rigidly attached to the card. this is the result of underestimating the speed at which the card is moved. rather than recognise the true state of things, the mind prefers to accept the suggestion that the upward or downward movement of the point of intersection is in part due to oppositely directed horizontal movements of the lines themselves upon the surface of the card. when the card is descending the vertical line is supposed to slide a little to the right and the oblique line to the left, which would have the effect of lowering their point of intersection independently of the downward movement of the card itself. when the card ascends, these horizontal movements are supposed to be reversed, and the point of intersection consequently raised. the assumption is exactly analogous to that made when an angle of slope is unwittingly minimised. another example of the instability of a mental standard occurs in the estimation of luminosity. the luminosity of a bright object, if reckoned in terms of the same unit as that applied in judging of a less bright one, would appear to be greater than it actually does appear, and this quite independently of any effects of fatigue. [illustration: _fig. .--illusion of luminosity._] the fact is well illustrated by a familiar experiment. fig. is photographed from a transparency made by superposing several different lengths of gelatine film so as to form a series of steps. at the right-hand end of the image the light has passed through only one layer of the film; in the next division it has traversed two layers, in the next, three, and in the last, four. the luminosity of each of the four squares into which the oblong is divided is, in a physical sense, quite uniform, but the mental standard of luminosity varies for different parts of the image, increasing or decreasing, as the case may be, not _per saltum_, but smoothly and continuously, with the result that each square looks brighter towards the left than towards the right. the appearance, which is often likened to that presented by a fragment of a fluted column, is equally well shown when the diagram is illuminated instantaneously by an electric spark, and cannot, therefore, be accounted for by retinal fatigue. if the squares are separated from one another by distinct lines of demarcation, however fine, the standard of luminosity becomes uniform for each square, and the illusion vanishes. this fact sufficiently disposes of the hypothesis which has been advanced to the effect that the phenomenon is due to physiological causes. i now propose to discuss a curious consequence of the fluctuation of unaided judgment as regards the purity of a colour. when any colour occupies a predominant place in the field of vision, we are apt to consider it as being less pure, or paler, than we should if it were less conspicuous, our standard of whiteness tending to approximate itself to the colour in question. for the sake of clearness let us first confine our attention to a definite colour--say red. an absolutely pure red is one that is entirely free from any admixture of white; in proportion as it contains more and more white, the more impure, or in other words, the more pale does it become, until at last all trace of perceptible redness is lost and the colour is indistinguishable from white. [illustration: _fig. .--illusion of colour._] a convenient way of picturing the scale of purity is shown in fig . the shaded oblong may be supposed to represent a painted strip of cardboard or paper. at the extreme right hand end the colour is supposed to be absolutely pure red; towards the left the red gradually becomes paler or more dilute, and at the middle of the diagram it has merged into perfect whiteness. the figures to from left to right denote the percentage of free red contained in the mixture at different parts of the scale; the luminosity is supposed to be uniform throughout. now the white light with which the red is diluted may be regarded as consisting of two parts, one of which is of exactly the same hue as the pure red itself, and the other an equivalent proportion of the complementary colour, which in the present case will be greenish-blue. the fact therefore really is that, as we pass along the scale from to , the _total_ quantity of red in the mixture is not reduced to nothing, but only to one half, while at the same time greenish-blue is added in proportions increasing from nought at the extreme right to per cent. of the whole at the middle of the card. the ordinates of the quadrilateral figure e d b f show the proportion of red, and those of the triangle e f b the proportion of greenish-blue, at different parts of the scale. regarding the portion of the strip which lies above the point marked , as representing the zero of colour--that is, whiteness or greyness, which is essentially the same as whiteness--let us continue the diagram in the negative direction, gradually reducing the quantity of red until it falls from per cent. of the whole at f to nothing at a, and at the same time increasing that of the greenish-blue from per cent. at f to per cent. at a. the resultant hue in the portion of the card between f and a will be greenish-blue, which begins to be perceptible as a very pale tint just to the left of f, and increases in purity as a is approached, at which point the colour will be entirely free from any admixture with white. we have in the scale thus presented to our imagination a pair of colours, each occupying one-half of the scale, and gradually diminishing in purity towards the middle line; here only, just at the stage where one colour merges into the other, is there no colour at all, and this region represents the fixed physical zero or standard from which is reckoned the purity of a colour corresponding to any other portion of the scale. the completed scale, it will be observed, though originally intended only for the case of red, turns out to be equally serviceable for greenish-blue: if we consider greenish-blue as positive, then the red, being on the other side of zero, must be regarded as negative. any other possible pairs of complementary colours may be similarly treated. this device enables us at once to understand the consequence of mentally displacing the zero, while physically the scale remains unchanged. when red is the prevailing colour in the field of vision, we are inclined to consider it unduly pale; in other words we imagine it to be nearer the zero of the scale than is actually the case, and so are led to shift our standard of whiteness from the middle slightly towards the red end of the scale. the new position assigned to white, being a little to the right of the point marked in fig. , is one where, under customary circumstances, the colour would be called pale red. at the same time, an object which is normally white, and is exactly matched at the middle of the scale, would be a little to the left of the imaginary zero, and would consequently appear to be of a greenish-blue tint. this apparent transformation of white or grey into a decided colour is most striking when the inducing colour is considerably diluted with white or is of feeble luminosity. a small fragment of neutral grey paper, placed upon a much larger piece of a bright red hue, generally appears at the first glance[ ] to be greenish-blue, but if the light is at all strong, only slightly so. if, however, a sheet of white tissue paper is laid over the whole, the greenish-blue tint immediately becomes startlingly distinct, and may even appear more decided than the red itself as seen through the tissue. the same piece of grey paper, when placed upon a green ground, appears rose-coloured, and upon a blue ground, yellow, the effect being always greatly increased by the diluent action of superposed tissue paper. there seem to be several reasons, partly physical and partly psychological, why these contrast colours, as they are called, are more pronounced when the colour that calls them into existence either has a somewhat pale tint or is feebly illuminated. probably the most important is of a purely physical character. the refracting media of the eye are much less perfectly transparent than a good glass lens is; they are sensibly turbid or opalescent, and in consequence of this defect some of the light which falls upon them is irregularly scattered over the retina. if we look at a bright red object with a small white patch upon it, the image of the patch as formed upon the retina is not, physically speaking, perfectly white, but slightly coloured by diffused red light; owing however to the psychological influence to which our attention has been directed, the faint red coloration is not consciously perceived; the same mental displacement of the zero which, when the exciting colour was feeble, led us to regard white (or grey) as bluish-green, now causes what is actually pale red to appear white. there is no need whatever to assume that the contrast colours with which we have been dealing are of physiological origin and due to an inductive action excited in portions of the retina adjacent to those upon which coloured light falls. on the contrary, it would be a matter for surprise if the case in question presented an exception to the comprehensive law which governs the fluctuation of the mental judgment. of the operation of this law i have quoted several very diverse instances, and the number might easily have been increased. nor is it only in relation to optical phenomena that the law holds good; in its most general form, supplemented it may be in some instances by obvious corollaries, it is applicable to almost every case in which physical attributes of whatever kind are the subject of unassisted mental judgment. chapter v. curiosities of vision. the function of the eye, regarded as an optical instrument, is limited to the formation of luminous images upon the retina. from a purely physical point of view it is a simple enough piece of apparatus, and, as was forcibly pointed out by helmholtz, it is subject to a number of defects which can be demonstrated by the simplest tests, and which, if they occurred in a shop-bought instrument, would be considered intolerable. what takes place in the retina itself under luminous excitation, and how the sensation of sight is produced, are questions which belong to the sciences of physiology and psychology; and in the physiological and psychological departments of the visual machinery we meet with an additional host of objectionable peculiarities from which any humanly-constructed apparatus is by the nature of the case free. yet in spite of all these drawbacks our eyes do us excellent service, and provided that they are free from actual malformation and have not suffered from injury or disease, we do not often find fault with them. this, however, is not because they are as good as they might be, but because with incessant practice we have acquired a very high degree of skill in their use. if anything is more remarkable than the ease and certainty with which we have learnt to interpret ocular indications, when they are in some sort of conformity with external objects, it is the pertinacity with which we refuse to be misled when our eyes are doing their best to deceive us. in our earliest years we began to find out that we must not believe all we saw; experience gradually taught us that on certain points and under certain circumstances the indications of our organs of vision were uniformly meaningless or fallacious, and we soon discovered that it would save us trouble and add to the comfort of life if we cultivated a habit of completely ignoring all such visual sensations as were of no practical value. in this most of us have been remarkably successful; so much so, that if, from motives of curiosity, or for the sake of scientific experiment, we wish to direct our attention to the sensations in question, and to see things as they actually appear, we can only do so with the greatest difficulty; sometimes, indeed, not at all, unless with the assistance of some specially contrived artifice. in the present chapter it is proposed to discuss a few of the less familiar vagaries of the visual organs, and to show how they may be demonstrated. some of the experiments may, it is to be feared, be found rather difficult; success will depend mainly upon the experimentalist's ability to lay aside habit and prejudice, and give close attention to his visual sensations; but it is hardly to be expected that an unskilled person will at the first attempt observe all the phenomena which will be referred to. among the most annoying of the eccentricities which characterise the sense of vision is that known as the persistence of impressions. the sensation of sight which is produced by an illuminated object does not cease at the moment when the exciting cause is removed or changed in position; it continues for a period which is generally said to be about a tenth of a second, but may sometimes be much more or less. it is for this reason that we cannot see the details of anything which is in rapid motion, but only an indistinct blur, resulting from the confusion of successive impressions. if a cardboard disk, which is painted in conspicuous black and white sectors is caused to rotate at a sufficiently high speed, the divisions are completely lost sight of, and the whole surface appears to be of a uniformly grey hue. but if the rapidly rotating disk is illuminated by a properly timed series of electric flashes, it looks as if it were at rest, and in spite of the intermittent nature of the light, the black and white sectors can be seen quite continuously, though as a matter of fact the intervals of darkness are very much longer than those of illumination. persistent impressions of this kind are often spoken of as positive after-images. there is a very remarkable phenomenon accompanying the formation of positive after-images, especially those following brief illumination, which seems, until comparatively recent times, to have entirely escaped the notice of the most acute observers. it was first observed accidentally by professor c. a. young, when he was experimenting with a large electrical machine which had been newly acquired for his laboratory. he noticed that when a powerful leyden jar discharge took place in a darkened room, any conspicuous object was seen twice at least, with an interval of a trifle less than a quarter of a second, the first time vividly, the second time faintly. often it was seen a third time, and sometimes, but only with great difficulty, even a fourth time. he gave to this phenomenon the name of recurrent vision; it may perhaps be more appropriately denominated the young effect. by means of the powerful machine presented to the royal institution by mr. wimshurst, used in conjunction with a battery of leyden jars, the young effect has been successfully shown to a large assembly. but it is quite easy to demonstrate it on a small scale with any influence machine which will give a spark about an inch long. one of the terminals of the machine should be connected by a wire with the inner coating of a half-pint leyden jar, the other with the outer coating, and the discharging balls should be set a quarter of an inch apart. the observer's eyes must be shielded from the direct light of the spark by any convenient screen, such as a large book set on end. the best object for the experiment is a sheet of white paper, placed in an upright position a few inches away from the terminals of the machine and exposed to the full light of the discharge. the room being darkened, let the machine be worked slowly, while the eyes are turned towards the white paper. this will be seen for a moment when the spark passes, and, after a dark interval of about one-fifth of a second, it will make another brief appearance. after a further short interval of darkness, a second recurrent image will often be seen. it may be remarked that the effect is most striking when the eyes are not directed exactly upon the white paper, but above or on one side of it; the proper distance of the paper from the spark-gap should be found by trial. under favourable conditions i have observed as many as six or seven reappearances of an object which was illuminated by a single discharge. these followed one another at the usual rate--about five in a second--and produced a twinkling or quivering effect, closely resembling that attending a flash of lightning which is not directly seen. there can indeed be little doubt that the proverbial quiver of the lightning-flash is in many cases merely an effect of recurrent vision, though sometimes, of course, as has been shown by photographs, the discharge is really multiple. some years ago i called attention to a very different method of exhibiting a recurrent image. the apparatus used for the purpose consists of a vacuum tube mounted in the usual way upon a horizontal axis capable of rotation. when the tube is illuminated by a rapid succession of discharges from an induction coil, and is made to rotate very slowly by clockwork (turning once in every two or three seconds), a very curious phenomenon may be noticed. at a distance of a few degrees behind the tube and separated from it by an interval of perfect darkness, comes a ghost. this ghost is in form an exact reproduction of the tube; it is very clearly defined, and though its apparent luminosity is somewhat feeble, it can in most cases be seen without difficulty. the varied colours of the original are, however, absent, the whole of the phantom tube being of a uniform bluish or violet tint. if the rotation is suddenly stopped the ghost still moves steadily on until it reaches the luminous tube, with which it coalesces and so disappears. (see fig. , where the recurrent image is represented by dotted lines.) [illustration: _fig. .--recurrent vision demonstrated with a vacuum tube._] more recently a fresh series of experiments were undertaken in connection with the young effect and certain allied matters, the results being embodied in a communication to the royal society (proc. roy. soc., , vol. , p. ). among other things an attempt was made to ascertain how far a recurrent image was affected by the colour of the exciting light. with this object two methods of experimenting were employed. in the first, coloured light was obtained by passing white light through coloured glasses; in the second and more perfect series of experiments, the pure coloured light of the spectrum was used. among other results it was found that, _cæteris paribus_, the recurrent image was much stronger with green light than with any other, and that when the excitation was produced by pure red light, however intense, there was no recurrent image at all. [illustration: _fig. .--recurrent vision with rotating disk._] for a repetition of my first experiment a mechanical lantern slide is required containing a metal disk about three inches in diameter which can be caused to rotate slowly and steadily about its centre. near the edge of the disk is a small circular aperture. the slide is placed in a limelight lantern, and a bright image of the hole is focussed upon a distant screen, all other light being carefully shut off. when the disk is turned slowly, the spot of light upon the screen goes round and round, and it is generally possible to see at once that the bright primary spot appears to be followed at a short distance by a much feebler spot of a violet colour, which is the recurrent image of the first. (see fig. .) it is essential to keep the direction of the eyes perfectly steady, which is not a very easy thing to do without practice. if a green glass is placed before the lens, the ghost will be at its best, and should be seen quite clearly and easily, provided that no attempt is made to follow it with the eyes. with an orange glass the ghost becomes less distinctly visible, and its colour generally appears to be greenish-blue, instead of violet as before. when a red glass is substituted, the ghost completely disappears. if the speed of rotation is sufficiently high, the red spot is considerably elongated during its revolution, and its colour ceases to be uniform, the tail assuming a light bluish-pink tint. but however great the speed, no complete separation of the spot into red and pink portions can be effected, and no recurrent image is ever found. the spectrum method of observation can only be carried out on a small scale, and is not suited for exhibition to an audience. it, however, affords the best means of ascertaining how far the apparent colour of the recurrent image depends upon that of the primary, a matter of some theoretical interest. [illustration: _fig. .--recurrent vision with spectrum._] the arrangement adopted is shown in the annexed diagram (fig. ). l is a lantern containing an oxyhydrogen light or an electric arc lamp, s is an adjustable slit, m a projection lens, p a bisulphide of carbon prism, d a metal plate in the middle of which is a circular aperture millimetres ( / inch) in diameter. a bright spectrum, or centimetres in length (about inches), is projected upon this metal plate, and a small selected portion of it passes through the round hole; thence the coloured light goes through the lens n to the little mirror q, which reflects it upon the white screen r. by properly adjusting the position of the lens n a sharp monochromatic image of the round hole in the plate d is focussed upon the screen r. to the back of the mirror q is attached a horizontal arm which is not quite perpendicular to the mirror, its inclination being capable of adjustment. the arm is turned slowly by clock-work, thus causing the coloured spot on the screen to revolve in a circular orbit about centimetres ( foot) in diameter, its recurrent image following at a short distance behind it. when the mirror turns once in - / seconds, this image appears about ° behind the coloured spot, the corresponding time-interval being about one-fifth of a second. using this apparatus, it was found that white light was followed by a violet recurrent image; after blue and green, when the image was brightest, its colour was also violet; after yellow and orange it appeared blue or greenish blue. on the other hand, when a complete spectrum was caused to revolve upon the screen, the whole of its recurrent image from end to end appeared violet; there was no suspicion of blue or greenish-blue at the less refrangible end. for this and other reasons given in the paper it was concluded that the true colour was in all cases really violet, the blue and greenish-blue apparently seen in conjunction with the much brighter yellow and orange of the primary being merely an illusory effect of contrast. it seems likely, then, that the phenomenon which has been spoken of as recurrent vision, is due principally, if not entirely, to an action of the violet nerve-fibres. recurrent vision is, no doubt, generally most conspicuous after a very brief period of retinal illumination, such as was employed in the experiments which we have been discussing; this is evidently due to the fact that the effect is most easily perceived when the sensibility of the retina has not been impaired by fatigue. but by a little effort it may be detected even after very prolonged illumination, and a practised observer can hardly avoid noticing a short flash of bluish light which manifests itself about a quarter of a second after the lights in a room have been suddenly extinguished; the phenomenon forces itself upon my attention almost every night when i turn off the electric lights. it need hardly be pointed out that it represents only a transient phase of the well known positive after-image, and it had even been observed in a vague and uncertain sort of way long before the date of professor young's experiment. helmholtz, for example, mentions the case of a positive after-image which seemed to disappear and then to brighten up again, but he goes on to explain--erroneously, as it turns out--that the seeming disappearance was illusory. m. charpentier, of nancy, whose work in physiological optics is well known, was the first to notice and record a remarkable phenomenon which, in some form or other, must present itself many times daily to every person who is not blind, but which until about seven years ago had been absolutely and universally ignored. the law which is associated with charpentier's name is this:--when darkness is succeeded by light, the stimulus which the retina at first receives, and which causes the sensation of luminosity, is followed by a brief period of insensibility, resulting in the sensation of momentary darkness. it appears that the dark period begins about one sixtieth of a second after the light has first been admitted to the eye, and lasts for about an equal time. the whole alternation from light to darkness and back again to light is performed so rapidly, that except under certain conditions, which, however, occur frequently enough, it cannot be detected. [illustration: _fig. .--charpentier's dark band._] the apparatus which charpentier employed for demonstrating and measuring the duration of this effect is very simple. it consists of a blackened disk with a white sector, mounted upon an axis. when the disk is illuminated by sunlight and turned rather slowly, the direction of the gaze being fixed upon the centre, there appears upon the white sector, close behind its leading edge, a narrow but quite conspicuous dark band. (see fig. .) the portion of the retina which at any moment is apparently occupied by the dark band, is that upon which the light reflected by the leading edge of the white sector impinged one sixtieth of a second previously. but no special apparatus is required to show the dark reaction. in fig. an attempt has been made to illustrate what any one may see if he simply moves his hand between his eyes and the sky or any strongly illuminated white surface. the hand appears to be followed by a dark outline separated from it by a bright interval. the same kind of thing happens, in a more or less marked degree, whenever a dark object moves across a bright background, or a bright object across a dark background. [illustration: _fig. .--charpentier's effect shown with the hand._] in order to see the effect distinctly by charpentier's original method, the illumination must be strong. if, howover, the arrangement is slightly varied, so that transmitted instead of reflected light is made use of, comparatively feeble illumination is sufficient. a very effective way is to turn a small metal disk, having an open sector of about °, in front of a sheet of ground or opal glass behind which is a lamp. by an arrangement of this kind upon a larger scale, the effect may easily be rendered visible to an audience. the eyes should not be allowed to follow the disk in its rotation, but should be directed steadily upon the centre. the acute and educated vision of charpentier enabled him, even when working with his black and white disk, to detect the existence, under favourable conditions, of a second, and sometimes a third, band of greatly diminished intensity, though he remarks that the observation is a very difficult one. what is probably the same effect can, however, as pointed out in my paper of , be shown quite easily in a different manner. if a disk with a narrow radial slit, about half a millimetre ( / inch) wide, is caused to rotate at the rate of about one turn per second in front of a bright background, such as a sheet of ground glass with a lamp behind it, the moving slit assumes the appearance of a fan-shaped luminous patch, the brightness of which diminishes with the distance from the leading edge. and if the eyes are steadily fixed upon the centre of the disk, it will be noticed that this bright image is streaked with a number of dark radial bands, suggestive of the ribs or sticks of a fan. near the circumference as many as four or five such dark streaks can be distinguished without difficulty; towards the centre they are less conspicuous, owing to the overlapping of the successive images of the slit. the effect is roughly indicated in fig. . [illustration: _fig. .--multiple dark bands._] the dark reaction known as the charpentier effect occurs at the beginning of a period of illumination. there is also a dark reaction of very short duration at the end of a period of illumination. it should be explained that, owing to what is called the proper light of the retina, ordinary darkness does not appear absolutely black: even in a dark room on a dark night with the eyes carefully covered, there is always some sensation of luminosity which would be sufficient to show up a really black image if one could be produced. now the darkness which is experienced after the extinction of a light is for a small fraction of a second more intense than common darkness. the first mention of this dark reaction perhaps occurs in an article contributed to _nature_ in , in which it was stated that when the current was cut off from an illuminated vacuum tube "the luminous image was almost instantly replaced by a corresponding image which seemed to be intensely black upon a less dark background," and which was estimated to last from a-quarter to a-half second. "abnormal darkness," it was added, "follows as a reaction after luminosity." [illustration: _fig. .--temporary insensitiveness of the eye._] in the royal society paper before referred to the point is further discussed, and a method is described by which the stage of reaction may be easily exhibited and its duration approximately measured. if a translucent disk, made of stout drawing-paper and having an open sector, is caused to rotate slowly in front of a luminous background, a narrow radial dark band, like a streak of black paint, appears upon the paper very near the edge which follows the open sector. from the space covered by this band when the disk was rotating at a known speed, the duration of the dark reaction was calculated to be about one-fiftieth of a second; my original estimate was therefore an excessive one. the experiment is illustrated in fig. . one more interesting point should be noticed in the train of visual phenomena which attend a period of illumination. the sensation of luminosity which is excited when light first strikes the eye is for about a sixtieth of a second much more intense than it subsequently becomes. this is shown by the fact, which is obvious enough when once attention has been directed to it, that the bright band, which in the charpentier disk intervenes between the dark band and the leading edge of the white sector, appears to be much more strongly illuminated than any other portion of the sector. the complete order of visual phenomena observed when the retina is exposed to the action of light for a limited time may therefore be summed up as follows:-- ( ) immediately upon the impact of the light there is experienced a sensation of luminosity, the intensity of which increases for about one-sixtieth of a second: more rapidly towards the end of that period than at first. ( ) then ensues a sudden re-action, lasting also for about one-sixtieth of a second, in virtue of which the retina becomes partially insensible to renewed or continued luminous impressions. these two effects may be repeated in a diminished degree, as often as three or four times. ( ) the stage of fluctuation is succeeded by a sensation of steady luminosity, the intensity of which is, however, considerably below the mean of that experienced during the first one-sixtieth of a second. ( ) after the external light has been shut off, a sensation of diminishing luminosity continues for a short time, and is succeeded by a brief interval of darkness. ( ) then follows a sudden and clearly-defined sensation of what may be called abnormal darkness--darker than common darkness--which lasts for about one-sixtieth of a second, and is followed by another interval of ordinary darkness. ( ) finally, in about a fifth of a second after the extinction of the external light, there occurs another transient impression of luminosity, generally violet coloured, after which the uniformity of the darkness remains undisturbed. fig. , which is copied from my paper, gives a rough diagrammatic representation of the above described chain of sensations. no account is here taken of the comparatively feeble after-images which succeed the recurrent image, and may last for several seconds. i propose now to say a few words about a curious phenomenon of vision which a short time ago excited considerable interest. [illustration: _fig. .--visual sensations attending a period of illumination._] [illustration: _fig. .--benham's top._] in the year mr. c. e. benham brought out a pretty little toy which he called the artificial spectrum top. it consists of a cardboard disk, one half of which is painted black, while on the other half are drawn four successive groups of curved black lines at different distances from the centre, as shown in fig. . when the disk rotates rather slowly, each group of black lines generally appears to assume a different colour, the nature of which depends upon the speed of the rotation and the intensity and quality of the light. under the best conditions the inner and outer groups of lines become bright red and dark blue; at the same time the intermediate groups also appear tinted, but the hues which they assume are rather uncertain and difficult to specify. by far the most striking of the colours exhibited by the top is the red, and next to that the blue; this latter is, however, sometimes described as bluish-green. some experiments carried out by myself in (proc. roy. soc., vol. , p. ) seem to indicate pretty clearly the cause of the remarkable bright red colour, and also that of the blue. the more feeble tints of the two intermediate groups of lines perhaps result from similar causes in a modified form, but these have not yet been investigated. in the red colour we have another striking example of an exceedingly common phenomenon which is habitually disregarded; indeed i can find no record of its ever having been noticed at all. the fact is that whenever a bright image is suddenly formed upon the retina after a period of comparative darkness, this image appears for a short time to be surrounded by a narrow coloured border, the colour, under ordinary conditions of illumination, being red. if the light is very strong, the transient border is greenish-blue, but this colour, as will be explained later, turned out to be merely an after-effect of red. sometimes, when the object is in motion, both red and blue are seen together. the observations were first made in the following manner. a blackened zinc plate, in which is a small round hole covered with a piece of thin writing-paper, is fixed over a larger opening in a wooden board; thus we are furnished with a sharply-defined translucent disk, which is surrounded by a perfectly opaque substance. an arrangement is provided for covering the translucent disk with a shutter, which can be opened very rapidly by releasing a strong spring. if this apparatus is held between the eyes and a lamp, and the translucent disk is suddenly disclosed by working the shutter, the disk appears for a short time to be surrounded by a narrow red border. the width of the border is perhaps a millimetre ( / inch), and the appearance lasts for something like a tenth of a second. most people are at first quite unable to recognise this effect, the difficulty being, not to see it, but to know that one sees it. those who have been accustomed to visual observations generally perceive it without any difficulty when they know what to look for, and no doubt it would be very evident to a baby which had not advanced very far in the education of its eyes. the observation is made rather less difficult by a further device. if the disk is divided into two parts by an opaque strip across the middle, it is clear that each half disk will have its red border, and if the strip is made sufficiently narrow, the red borders along its edges will meet or perhaps overlap, and the whole strip will, for a moment after the shutter is opened, appear red. a disk was thus prepared by gumming across the paper a very narrow strip of tinfoil. the effect produced when such a disk is suddenly exposed is indicated in fig. , the red colour being represented by shading. [illustration: _fig. .--demonstration of red borders._] a simpler apparatus is, however, quite sufficient for showing the phenomenon,[ ] and with practice one can even acquire the power of seeing it without any artificial aid at all. i have many times noticed flashes of red upon the black letters of a book that i was reading, or upon the edges of the page: bright metallic, or polished objects often show it when they pass across the field of vision in consequence of a movement of the eyes, and it was an accidental observation of this kind which suggested the following easy way of exhibiting the effect experimentally. an incandescent electric lamp was fixed behind a round hole in a sheet of metal which was attached to a board. the hole was covered with two or three thicknesses of writing paper, making a bright disk of nearly uniform luminosity. when this arrangement was moved rather quickly either backwards and forwards or round and round in a small circle, the edge of the streak of light thus formed appeared to be bordered with red. if this experiment is performed with a strong light behind the paper, the streak becomes bordered with greenish-blue instead of red. with an intermediate degree of illumination, both blue and red may be seen together. most of the effects that have so far been described were produced by transmitted light, but reflected light will show them equally well. if you place a printed book in front of you near a good lamp and interpose a dark screen before your eyes, then, when the screen is suddenly withdrawn, the printed letters will for a moment appear red, quickly changing to black. some practice is required before this observation can be made satisfactorily, but by a simple device it is possible to obliterate the image of the letters before the redness has had time to disappear; the colour then becomes quite easily perceptible. hold two screens together side by side, a black one and a white one, in such a manner that an open space is left between them. (see fig. .) in the first place let the black screen cover the printing; then quickly move the screens sideways so that the printed letters may be for a moment exposed to view through the gap, stopping the movement as soon as the page is covered by the white screen. during the brief glimpse that will be had of the black letters while the gap is passing over them, they will, if the illumination is suitable, appear to be bright red. [illustration: _fig. .--black and white screens._] [illustration: _fig. .--disk for red borders._] we may go a step further. cut out a disk of white cardboard, divide it into two equal parts by a straight line through the centre, and paint one half black.[ ] at the junction of the black and white portions cut out a gap, which may conveniently be of the form of a sector of °. (see fig. .) stick a long pin through the centre and hold the arrangement by the pointed end of the pin a few inches above a printed page near a good light. make the disk spin at the rate of about five or six turns a second by striking the edge with the finger. as before, the letters when seen through the gap will appear red, and persistence will render the repeated impressions almost continuous so long as the rotation is kept up; any one seeing the printing for the first time through the rotating disk would believe that it was done with red ink. care must be taken that the disk does not cast a shadow upon the page, and that the intensity of the illumination is properly adjusted. i have devised several rather more elaborate contrivances for making the disks rotate at a uniform speed; one of these is shown in fig. . in none of these experiments does an extended black surface ever appear red, but only black dots or lines. and the lines must not be too thick; if their thickness is much more than a millimetre ( / inch), the lines, as seen by an observer from the usual distance for reading, do not become red throughout, but only along their edges. the red appearance does not in fact originate in the black lines themselves: these serve merely as a background for showing up the red border which fringes externally the white portions of the paper, and the width of this border does not exceed about one-fifth of a degree. but by employing a sufficiently large disk and selecting designs or letters composed of lines of suitable thickness, the colour effect has been shown to a large audience. when the disk is turned in the opposite direction, so that the gap is preceded by white and followed by black, the lines of the design appear at first sight to become dark blue instead of red. attentive observation, however, shows that the apparently blue tint is not formed upon the lines themselves, as the red tint was, but upon the white ground just outside them. this introduces to our notice another border phenomenon, which seems to present itself when a dark patch is suddenly formed on a bright ground, for that is essentially what takes place when the disk is turned the reverse way. i made some attempts to obtain more direct evidence that such a dark patch appeared for a moment to have a blue border, and after some trouble succeeded in doing so. a circular aperture was cut in a wooden board and covered with white paper; a lamp was placed behind the board, and thus a bright disk was obtained, as in the former experiment. an arrangement was prepared by means of which one half of this bright disk could be suddenly covered by a metal shutter, and it was found that when this was done a narrow blue band appeared on the bright ground just beyond and adjoining the edge of the shutter when it had come to rest. the blue band lasted for about a tenth of a second, and it seemed to disappear by retreating into the black edge of the shutter. the phenomenon is illustrated in fig. , where the shaded band indicates the blue border. [illustration: _fig. .--demonstration of blue border._] we have then to account, if possible, for the two facts that, in the formation of these transient colour-borders, the red sensation occurs in a portion of the retina which has not itself been exposed to the direct action of light, while the blue occurs in a portion which is steadily illuminated, both colour sensations being referred to localities adjacent to those in which a change of illumination has suddenly taken place. accepting the young-helmholtz theory of colour vision, the effects must, i think, be attributed to a sympathetic affection of the red nerve fibres. when the various nerve fibres occupying a limited portion of the retina are suddenly stimulated by white light (or by any kind of light which contains a red constituent) the immediately surrounding red nerve fibres are for a short period excited sympathetically, while the violet and green fibres are not so excited, or in a much less degree. and again, when light is suddenly cut off from a patch in a bright field, there occurs a sympathetic insensitive reaction in the red fibres just outside the darkened patch, in virtue of which they cease for a moment to respond to the luminous stimulus; the green and violet fibres, by continuing to respond uninterruptedly, give rise to the sensation of a blue border. it is perhaps desirable to refer briefly to another proposed explanation of the phenomenon, which occurred to myself at an early stage of the investigation, and has since been suggested by many different persons. the explanation in question is of a purely physical character, and depends upon the non-achromatism of the eye. [illustration: _fig. .--disk for experiments on the origin of colour-borders._] without going into details, it will suffice to quote a single experiment which is of itself fatal to any such theory. prepare a disk like that shown in fig. , and spin it above a page of printing. the letters beneath the zone which is partly black and partly white will, under the usual conditions, turn red, but those beneath the remainder of the disk will retain their blackness. the demarcation is quite definite, and a single printed word may be made to appear red in the middle and black at its two ends. now it is, of course, impossible that the lenses of the eye should be perfectly accommodated for the letters which appear black, and at the same time seriously out of focus for the others. this explanation, therefore, simple and obvious as it may seem, is altogether untenable. whether or not the hypothesis which i have suggested is correct in all its details, it is, i think, sufficiently obvious that the red and blue colours of benham's top are due to exactly the same causes as the colours observed in my own experiments, for the essential conditions are the same in both cases. the last curiosity which i will notice is connected with the fact already mentioned, that when the illumination is strong, the transient border-colours are nearly reversed, greenish-blue appearing in place of red, and brick-red in place of blue. i was for a long time quite unable to imagine any reasonably probable explanation of this circumstance, but a clue was finally obtained from consideration of the fact that greenish-blue is the complementary colour to red, and in a subsequent memoir (proc. roy. soc., vol. , p. ) some experiments were described which show, as i believe conclusively, that the greenish-blue borders seen in a strong light are simply negative after-images of the usual red one. these negative after-images are of the familiar kind that are observed after one has gazed for some time at a bright coloured object. if a red "wafer" lying upon a sheet of white or grey paper is looked at steadily for about half a minute, and the gaze is then suddenly transferred to some other part of the paper, a greenish-blue ghost of the wafer will be seen. the portion of the retina upon which the red image at first falls becomes fatigued and partially insensible to red light; it is therefore unable to appreciate the red component of the white light afterwards reflected to it by the paper, and the sensation of the complementary colour consequently predominates; hence the greenish-blue ghost, which is called the negative after-image of the wafer. the new experiments show that, if a certain condition is fulfilled, the usual prolonged stare becomes unnecessary, a momentary glance sufficing to produce a strong but fugitive after-image. the condition is that the part of the retina where the image is to be formed, shall have been darkened immediately before excitation by the bright object. the retinal nerves, when in darkness, rapidly acquire a state of sensitiveness far exceeding the normal average in the light, but quickly diminishing again under the influence of illumination. this peculiar sensitiveness may, indeed, be both gained and lost in a small fraction of a second, and is therefore very favourable for the rapid generation of negative after-images. once more making use of the black and white screens depicted in fig. , let the black screen first cover the paper upon which the wafer is lying; this will darken a portion of the retina, and render it sensitive. then let the screens be quickly moved sideways, so that the wafer, after having been seen for a moment through the opening, may be immediately covered by the white screen. a bright but evanescent greenish-blue ghost will succeed the red impression. but the most curious thing is that if the illumination is strong, and the screens are moved at the proper speed, no trace of red will be seen at all; it will appear exactly as if the actual colour of the wafer seen through the gap were greenish-blue. i am informed that analogous phenomena have been observed in other branches of physiology; a well-defined reaction sometimes occurs when no direct evidence can be detected of the existence of the excitation to which the reaction must be due. as in the former experiments, the effect may be shown continuously by means of a rotating disk with an open sector. the annexed diagram (fig. ) indicates a convenient apparatus for the purpose. the disk is made of thin metal, and properly balanced; the dark portion of the surface is covered with black velvet, and the light portion with unglazed grey or buff paper. it should turn some six or eight times in a second, while its front is well illuminated either by bright diffused daylight or by a powerful lamp. a red card placed behind the rotating disk is made to appear green, a green card pink, and a blue one yellow, while a black patch painted upon a white ground appears lighter than the ground itself. i have prepared some designs which demonstrate the phenomenon in a very striking manner. one of these is a picture of a lady with indigo-blue hair, an emerald-green face, and a scarlet gown, who is represented as admiring a violet sunflower with purple leaves. seen through the disk, the lady's tresses appear flaxen, her complexion a delicate pink, and her dress a light peacock-blue; the petals of the sunflower also become yellow, and its foliage green. other designs show equally remarkable transformations of colour. [illustration: _fig. .--disk for transforming colours._] i have mentioned only a few among many curious phenomena which have presented themselves in the course of these investigations. it is not improbable that a careful study of the subjective effects produced by intermittent illumination would lead to results tending to clear up several doubtful points in the theory of colour vision. william byles & sons, printers, , fleet street, london, and bradford. footnotes: [ ] it should be clearly understood that the length of each wave of a series is measured by the distance between the crests of two successive waves. the length of water-waves which break upon a sea shore is not the length along the crest of a single wave measured in a direction parallel to the shore, as the uninitiated are apt to suppose. the true wave-length, or distance from crest to crest of successive waves, can be well observed from the top of a cliff. [ ] in practice, wave-lengths are expressed in ten-millionths of a millimetre. the wave-lengths of the lines a and h of the solar spectrum, which approximately coincide with the limits of visibility, are and ten-millionths of a millimetre. [ ] possibly the human eye is at present in process of transformation from an inferior type to a different and more perfect one. [ ] it is sometimes necessary to place the lens i on the other side of k. [ ] it is easy to find specimens of red and green glass suitable for this experiment. the proper kind of purple is not so commonly met with. [ ] some recent experiments on artificial colour-blindness (proc. roy. soc., feb., ) have led mr. burch to the conclusion that there are really _four_ fundamental colour-sensations--a red, a green, a blue, and a violet. his results are, however, thought to be capable of a different interpretation. [ ] or through several pieces superposed. [ ] a violet-coloured haze may sometimes be actually seen around the opal globes of the electric lamps in the streets. [ ] a "focus" electric lamp was used in the lantern. [ ] proc. roy. soc., jan., . [ ] after a few seconds' observation the greenish-blue colour often becomes much more intense, but this is an effect of fatigue, with which we are not at present concerned. [ ] see _nature_, vol. , p. (feb. th, ). [ ] or, for best results, use a balanced metal disk covered with black velvet and white paper. makers of electricity by brother potamian, f.s.c., d.sc., lond. professor of physics in manhattan college, n. y. and james j. walsh, m. d., ph.d., ll.d. dean and professor of nervous diseases and of the history of medicine at fordham university school of medicine; professor of physiological psychology at the cathedral college, new york [illustration] fordham university press new york copyright, , fordham university press, new york. preface this volume represents an effort in the direction of what may be called the biographical history of electricity. the controlling idea in its preparation was to provide brief yet reasonably complete sketches of the lives of the great pioneer workers in electricity, the ground-breaking investigators who went distinctly beyond the bounds of what was known before their time, not merely to add a fringe of information to previous knowledge, but to make it easy for succeeding generations to reach conclusions in electrical science that would have been quite impossible until their revealing work was done. the lives of these men are not only interesting as scientific history, but especially as human documents, showing the sort of men who are likely to make great advances in science and, above all, demonstrating what the outlook of such original thinkers was on all the great problems of the world around us. in recent times, many people have come to accept the impression that modern science leads to such an exclusive occupation with things material, that scientists almost inevitably lose sight of the deeper significance of the world of mystery in which humanity finds itself placed on this planet. the lives of these great pioneers in electricity, however, do not lend the slightest evidence in confirmation of any such impression. they were all of them firm believers in the existence of providence, of a creator, of man's responsibility for his acts to that creator, and of a hereafter of reward and punishment where the sanction of responsibility shall be fulfilled. besides, they were men characterized by some of the best qualities in human nature. their fellows liked them for their unselfishness, for their readiness to help others, for their devotedness to their work and to their duties as teachers, citizens and patriots. almost without exception, they were as far above the average of mankind in their personal ethics as they were in their intellectual qualities. the lives of such men, who were inspiring forces in their day, are as illuminating as they are instructive and encouraging. perhaps never more than now do we need such inspiration and illumination to lift life to a higher plane of purpose and accomplishment, than that to which it is so prone to sink when material interests attract almost exclusive attention. contents page peregrinus and columbus norman and gilbert franklin and some contemporaries galvani, discoverer of animal electricity volta, the founder of electrical science coulomb hans christian oersted andré marie ampère ohm, the founder of mathematical electricity faraday clerk maxwell lord kelvin illustrations page the double pivoted needle of petrus peregrinus first pivoted compass, peregrinus, magnetic declination at new york " " " san francisco " " in london, in and first dip-circle, invented by norman in norman's illustration of magnetic dip gilbert's orb of virtue, behavior of compass-needle on a terrella or spherical lodestone gilbert's "versorium" or electroscope gordon's electric chimes, modern form of leyden jar, with movable coatings three coated panes in series " panes in parallel " jars in parallel " jars in cascade discharge by alternate contacts tassel of long threads or light strips of paper procopius divisch ( - ) the divisch lightning conductor ( ) set of pointed rods galvani (portrait) opposite page volta " " " oersted " " " ampère " " " faraday " " " clerk maxwell (portrait) opposite page lord kelvin " " " makers of electricity. chapter i. peregrinus and columbus. the ancients laid down the laws of literary form in prose as well as in verse, and bequeathed to posterity works which still serve as models of excellence. their poets and historians continue to be read for the sake of the narrative and beauty of the style; their philosophers for breadth and depth of thought; and their orators for judicious analysis and impassioned eloquence. in the exact sciences, too, the ancients were conspicuous leaders by reason of the number and magnitude of the discoveries which they made. you have only to think of euclid and his "elements," of apollonius and his conics, of eratosthenes and his determination of the earth's circumference, of archimedes and his mensuration of the sphere, and of the inscription on plato's academy, _let none ignorant of geometry enter my door_, to realize the fondness of the greek mind for abstract truth and its suppleness and ingenuity in mathematical investigation. but the sciences of observation did not advance with equal pace; nor was this to be expected, as time is an essential element in experimentation and in the collection of data, both of which are necessary for the framing of theories in explanation of natural phenomena. the slowness of advance is well seen in the development of the twin subjects of electricity and magnetism. as to the lodestone, with which we are concerned at present, the attractive property was the only one known to ancient philosophy for a period of six hundred years, from the time of thales to the age of the cæsars, when lucretius wrote on the nature of things in latin verse. lucretius records the scant magnetic knowledge of his predecessors and then proceeds to unfold a theory of his own to account for the phenomena of the wonder-working stone. book vi. of "de natura rerum" contains his speculations anent the magnet, together with certain observations which show that the poet was not only a thinker, but somewhat of an experimenter as well. thus he recognizes magnetic _repulsion_ when he says: "it happens, too, at times that the substance of the iron recedes from the stone as if accustomed to start back from it, and by turns to follow it." this recognition of the repelling property of the lodestone is immediately followed by the description of an experiment which is frequently referred to in works on magnetic philosophy. it reads: "thus have i seen raspings of iron, lying in brazen vessels, thrown into agitation and start up when the magnet was moved beneath"; or metrically, and oft in brazen vessels may we mark ringlets of samothrace, or fragments fine struck from the valid iron bounding high when close below, the magnet points its powers. this experiment, seen and recorded by lucretius, is of special interest to the student of magnetic history because of the use which is made of iron filings and also because it has led certain writers to credit the poet with a knowledge of what is known to-day by the various names of magnetic figures, magnetic curves, magnetic spectrum. we do not, however, share this view, because we see no adequate resemblance between the positions assumed by the bristling particles of iron in the one case, as described by the roman poet, and the continuous symmetrical curves of our laboratories in the other. if lucretius noticed such curves in his brazen vessels, he does not say so; nor does the meagre description of magnetic phenomena given in book vi. warrant us in assuming that he did. the use of iron filings to map out the entire field of force that surrounds a magnet was unknown to classical antiquity; it was not known to peregrinus or roger bacon in the thirteenth century or even to gilbert in the sixteenth. the credit for reviving the use of filings and employing them to show the direction of the resultant force at any point in the neighborhood of a magnet, belongs to cabeo, an italian jesuit, who described and illustrated it in his "philosophia magnetica," published at ferrara in the year . on page of that celebrated work will be found a figure, the first of the kind, showing the position taken by the filings when plentifully sifted over a lodestone: thick tufts at the polar ends with curved lines in the other parts of the field. the samothracian rings mentioned in the passage quoted above were light, hollow rings of iron which, for the amusement of the crowd, the jugglers of the times held suspended one from the other by the power of a lodestone. writing of the lodestone, lucretius says: its viewless, potent virtues men surprise, its strange effects, they view with wond'ring eyes, when without aid of hinges, links or springs a pendent chain we hold of steely rings. dropt from the stone--the stone the binding source-- ring cleaves to ring and owns magnetic force; those held above, the ones below maintain; circle 'neath circle downward draws in vain whilst free in air disports the oscillating chain. though the roman poet was acquainted with two of the leading properties of the lodestone, viz., attraction and repulsion, there is nothing in the lines quoted above or in any other lines of his great didactic poem to indicate that he was aware of the remarkable difference which there is between one end of a lodestone and the other. the polarity of the magnet, as we term it, was unknown to him and remained unknown for a period of years. during that long period nothing of importance was added to the magnetic lore of the world. true, a few fables were dug out of the tomes of ancient writers which gained credence and popularity, partly by reason of the fondness of the human mind for the marvelous, and partly also by reason of the reputation of the authors who stood sponsors for them. pliny ( - a. d.) devotes several pages of his "natural history" to the nature and geographical distribution of various kinds of lodestones, one of which was said to repel iron just as the normal lodestone attracts it. needless to say that the mineral kingdom does not hold such a stone, although pliny calls it _theamedes_ and says that it was found in ethiopia. pliny is responsible for another myth which found favor with subsequent writers for a long time, when he says that a certain architect intended to place a mass of magnetite in the vault of an alexandrian temple for the purpose of holding an iron statue of queen arsinoe suspended in mid-air. of like fabulous character is the oft-repeated story about mahomet, that an iron sarcophagus containing his remains was suspended by means of the lodestone between the roof of the temple at mecca and the ground. as a matter of fact, mahomet died at medina and was buried there in the ordinary manner, so that the story as currently told of the suspension of his coffin in the "holy city" of mecca, contains a twofold error, one of place and the other of position. by a recent ( ) imperial irade of the sultan of turkey, the tomb is lit up by electric light in a manner that is considered worthy of the "prophet of islam." four centuries after pliny, claudian, the last of the latin poets as he is styled, wrote an idyl of fifty-seven lines on the magnet, which contains nothing but poetic generalities. st. ambrose ( - ) and palladius ( - ), writing on the brahmans of india, tell how certain magnetic mountains were said to draw iron nails from passing ships and how wooden pegs were substituted for nails in vessels going to taprobane, the modern ceylon. st. augustine ( - ) records in his "de civitate dei" the wonder which he felt in seeing scraps of iron contained in a silver dish follow every movement of a lodestone held underneath. with time, the legendary literature of the magnet became abundant and in some respects amusing. thus we read of the "flesh" magnet endowed with the extraordinary power of adhering to the skin and even of drawing the heart out of a man; the "gold" magnet which would attract particles of the precious metal from an admixture of sand; the "white" magnet used as a philter; magnetic unguents of various kinds, one of which, when smeared over a bald head, would make the hair grow; magnetic plasters for the relief of headache; magnetic applications to ease toothaches and dispel melancholy; magnetic nostrums to cure the dropsy, to quell disputes and even reconcile husband and wife. no less fictitious was the pernicious effect on the lodestone attributed in the early days of the mariner's compass to onions and garlic; and yet, so deeply rooted was the belief in this figment that sailors, while steering by the compass, were forbidden the use of these vegetables lest by their breath they might intoxicate the "index of the pole" and turn it away from its true pointing. more reasonable than this prohibition was the maritime legislation of certain northern countries for the protection of the lodestone on shipboard. according to this penal code, a sailor found guilty of tampering with the lodestone used for stroking the needles, was to have the guilty hand held to a mast of the ship by a dagger thrust through it until, by tearing the flesh away, he wrenched himself free. it was only at the time of the crusades that people in europe began to recognize the _directive_ property of the magnet, in virtue of which a freely suspended compass-needle takes up a definite position relatively to the north-and-south line, property which is serviceable to the traveler on land and supremely useful to the navigator on sea. it is commonly said that the compass was introduced into europe by the returning crusaders, who heard of it from their mussulman foes. these, in turn, derived their knowledge from the chinese, who are credited with its use on sea as far back as the third century of our era.[ ] among the earliest references to the sailing compass is that of the trouvère guyot de provins,[ ] who wrote, about the year , a satirical poem of three thousand lines, in which the following passage occurs: the mariners employ an art which cannot deceive. an ugly stone and brown, to which iron joins itself willingly they have; after applying a needle to it, they lay the latter on a straw and put it simply in the water where the straw makes it float. then the point turns direct. to the star with such certainty that no man will ever doubt it, nor will it ever go wrong. when the sea is dark and hazy, that one sees neither star nor moon, then they put a light by the needle and have no fear of losing their way. the point turns towards the star; and the mariners are taught to follow the right way. it is an art which cannot fail. the author was a caustic and fearless critic, who lashed with equal freedom the clergy and laity, nobles and princes, and even the reigning pontiff himself, all of whom should be for their subjects, according to the satirist, what the pole-star is for mariners--a beacon to guide them over the stormy sea of life. guyot traveled extensively in his early years, but later in life retired from a world which he despised, and ended his days in the peaceful seclusion of the benedictine abbey of cluny. an interesting reference, of a similar nature to that of the minstrel guyot, is found in the spanish code of laws known as las siete partidas of alfonso el sabio, begun in and completed in . it says: "and even as mariners guide themselves in the dark night by the needle, which is their connecting medium between the lodestone and the star, and thus shows them where they go alike in bad seasons as in good; so those who are to give counsel to the king ought always to guide themselves by justice, which is the connecting medium between god and the world, at all times to give their guerdon to the good and their punishment to the wicked, to each according to his deserts."[ ] it will be necessary to give a few more extracts from writers of the first half of the thirteenth century in order to show how little was known about the magnet and how crude were the early appliances used in navigation when peregrinus appeared on the scene. cardinal jacques de vitry, who lived in the east for some years, wrote his "history of the orient" between the years and , in which he says: "an iron needle after touching the lodestone, turns towards the north star, so that such a needle is necessary for those who navigate the seas." this passage of the celebrated cardinal seems to indicate that even then the compass was widely known and commonly used in navigation. neckam ( - ), the augustinian abbot of cirencester, wrote in his "utensilibus": "among the stores of a ship, there must be a needle _mounted on a dart_ which will oscillate and turn until the point looks to the north; the sailors will thus know how to direct their course when the pole-star is concealed through the troubled state of the atmosphere." this passage is of historical value, as it contains what is probably the earliest known reference to a mounted or pivoted compass. prior to the introduction of this mode of suspension, the needle was floated on a straw, in a reed, on a piece of cork or a strip of wood, all of which modes of flotation, when taken in conjunction with the unsteadiness of the vessel in troubled waters, must have made observation difficult and unsatisfactory. brunetto latini ( - ) makes a passing reference to the new magnetic knowledge in his "livres dou tresor," which he wrote in , during his exile in paris. "the sailors navigate the seas," he says, "guided by the two stars called tramontanes; and each of the two parts of the lodestone directs the end of the needle that has touched it to the particular star to which that part of the stone itself turns." though a statesman, orator and philosopher of ability, the preceptor of dante in florence and guest of friar bacon in oxford, brunetto has not got the philosophy of the needle quite right in this passage; for the part that has been touched by the north end of a lodestone will acquire south polarity and will not, therefore, turn towards the same "tramontane" as the end of the stone by which it was touched. dante himself admitted the occult influence on the compass-needle that emanates from the pole-star when he wrote: "out of the heart of one of the new lights there came a voice that, needle to the star, made me appear in turning thitherward. _paradise_, xii., - . the next writer on the compass is raymond lully ( - ), who was noted for his versatility, voluminous writings and extensive travels as well as for the zeal which he displayed in converting the african moors. lully writes in his "de contemplatione": "as the needle after touching the lodestone, turns to the north, so the mariners' needle directs them over the sea." this brings us to the last of our ante-peregrinian writers who make definite allusions to the use of the compass for navigation purposes, viz., roger bacon, one of the glories of the thirteenth century as he would be of the twentieth. it was at the request of his patron, pope clement iv., that bacon wrote his "opus majus," a work in which he treats of all the sciences and in which he advocates the experimental method as the right one for the study of natural phenomena and the only one that will serve to extend the boundaries of human knowledge. in a section on the magnet, a clear distinction is drawn between the physical properties of the two ends of a lodestone; for "iron which has been touched by a lodestone," he says, "follows the end by which it has been touched and turns away from the other." besides being a recognition of magnetic polarity, this is equivalent to saying that unlike poles attract while like poles repel each other. bacon further remarks, by way of corroboration, that if a strip of iron be floated in a basin, the end that was touched by the lodestone will follow the stone, while the other end will flee from it as a lamb from the wolf. there is, however, an earlier recognition known of the polarity of the lodestone; for abbot neckam, fifty years before, called attention to the dual nature of the physical action of the lodestone, attracting in one part (say) by sympathy and repelling at the other by antipathy. it was the common belief in bacon's time and for centuries after, that the compass-needle was directed by the pole-star, often called the sailor's star; but bacon himself did not think so, preferring to believe with peregrinus, that it was controlled not by any one star or by any one constellation, but by the entire celestial sphere. other contemporaries of his sought the cause of the directive property not in the heavens at all, but in the earth itself, attributing it to hypothetical mines of iron which, naturally enough, they located in regions situated near the pole. peregrinus records this opinion, which he criticises and rejects, saying in chapter x. that persons who hold such a doctrine "are ignorant of the fact that in many different parts of the globe the lodestone is found; from which it would follow that the needle should turn in different directions, according to the locality, which is contrary to experience." a little further on he gives his own view, saying: "it is evident from the foregoing chapters that we must conclude that not only from the north pole (of the world), but also from the south pole rather than from the veins of mines, virtue flows into the poles of the lodestone." observations had to accumulate and much experimentation had to be done before it was finally established that the cause of the directive property of the magnet is not to be sought in the remote star depths at all, but in the earth itself, the whole terrestrial globe acting as a colossal magnet, partly in virtue of magnetic ore lying near the surface and partly also in virtue of electrical currents, due to solar heat, circulating in the crust of the earth. of the early years of pierre le pélérin (petrus peregrinus), nothing is known save that he was born of wealthy parents in maricourt, a village of picardy in northern france. from his academic title of magister, we infer that he received the best instruction available at the time, probably in the university of paris, which was then in the height of its fame. his reputation for mathematical learning and mechanical skill crossed the channel and reached friar bacon in the university of oxford. in his "opus tertium," the franciscan friar records the esteem in which he held his picard friend, saying: "i know of only one person who deserves praise for his work in experimental philosophy, because he does not care for the discourses of men or their wordy warfare, but quietly and diligently pursues the works of wisdom. therefore it is that what others grope after blindly, as bats in the evening twilight, this man contemplates in all their brilliancy because he is master of experiment." continuing the appraisal of his gallic friend's achievements, he says: "he knows all natural sciences, whether pertaining to medicine and alchemy or to matters celestial and terrestrial. he has worked diligently in the smelting of ores and also in the working of minerals; he is thoroughly acquainted with all sorts of arms and implements used in military service and in hunting, besides which he is skilled in agriculture and also in the measurement of lands. it is impossible to write a useful or correct treatise on experimental philosophy without mentioning this man's name. moreover, he pursues knowledge for its own sake; for if he wished to obtain royal favor, he could easily find sovereigns to honor and enrich him." this is at once a beautiful tribute to the work and character of peregrinus and an emphatic recognition of the paramount importance of laboratory methods for the advancement of learning. it is evident from such testimony, coming as it does from an eminent member of the brotherhood of science, that the world had not to wait for the advent of chancellor bacon or for the publication of his _novum organum_ in , to learn how to undertake and carry out a scientific research to a reliable issue. call the method what you will, inductive, deductive or both, the method advocated by the franciscan friar of the thirteenth century was the one followed at all times from archimedes to peregrinus and from peregrinus to gilbert, none of whom knew anything of lord bacon's pompous phrases and lofty commendation of the inductive method of inquiry for the advancement of physical knowledge. be it said in passing, that bacon, eminent as he undoubtedly was in the realm of the higher philosophy, was, nevertheless, neither a mathematician nor a man of science; he never put to a practical test the rules which he laid down with such certitude and expectancy for the guidance of physical inquiry. moreover, there is not a single discovery in science made during the three centuries that have elapsed since the promulgation of the baconian doctrine that can be ascribed to it; it has been steadily ignored by men renowned in the world for their scientific achievements and has been absolutely barren of results. peregrinus, on the other hand, does not stop to enumerate opinions, he does not even quote aristotle; but he experiments, observes, reasons and draws conclusions which he puts to the further test of experiment before finally accepting them. then and then only does he rise from the order of the physicist to that of the philosopher, from correlating facts and phenomena to the discovery of the laws which govern them and the causes that produce them. furthermore, he was in no hurry to let the world know that he was grinding lodestones one day and pivoting compass-needles the next; what he cared for supremely was to discover facts, new phenomena, new methods. peregrinus was not an essayist, nor was he a man of mere book-learning. he was a clear-headed thinker, a close and resourceful worker, a man who preferred facts to phrases and observation to speculation. at one period of his life, master peter applied his ingenuity to the solution of a problem in practical optics, involving the construction of a burning-mirror of large dimensions somewhat after the manner of archimedes; but though he spent three years on the enterprise and a correspondingly large sum of money, we are not told by friar bacon, who mentions the fact, what measure of success was achieved. bacon, however, avails himself of the occasion to insinuate a possible cause of failure, for he says that nothing is difficult of accomplishment to his friend _unless it be for want of means_. centuries later, the french naturalist buffon took up the same optical problem, with a view to showing that the feat attributed to archimedes during the siege of syracuse by the romans was not impossible of accomplishment. for this purpose, he used small mirrors in the construction of a large concave reflector, with which he ignited wood at a distance of feet and succeeded in melting lead at a distance of feet. as this was done in the winter time in paris, it was concluded that it would have been quite possible to set a roman trireme on fire from a safe distance by the concentrated energy of a sicilian sun. if peregrinus was alert in mind, he appears to have been very active in body. prompted, no doubt, by the higher motives of christian faith and perhaps a little, too, by his fondness for travel and adventure, he took the cross in early life and joined one of the crusading expeditions of the time. that he went to the land of the paynim, we have no direct evidence; but we infer the fact from the title of peregrinus or pilgrim, by which he is known, his full name being pierre le pélérin de maricourt, or, in the latinized form, petrus peregrinus de maricourt. in , we find him engaged in a military expedition undertaken by charles duke of anjou, for the purpose of bringing back to his allegiance as king of the two sicilies the revolted city of lucera in southern italy. he served in what might be called the engineering corps of the army, and was engaged in fortifying the camp and constructing engines of defense and attack. unlike his companions in arms, peregrinus does not allow himself to be wholly absorbed with military duties, nor does he waste his leisure hours in frivolous amusements; his mind is on higher things; he is engrossed with a problem in practical mechanics which required him to devise a piece of mechanism that would keep an armillary sphere in motion for a time. in outlining the necessary mechanism, as he conceived it, he was gradually led to consider the general and more fascinating problem of perpetual motion itself, with the result that he waxed somewhat enthusiastic when he thought that he saw the possibility of constructing an ever-turning wheel in which the motive power would be magnetic attraction, the attraction of a lodestone for a number of iron teeth arranged at equal distances on the periphery of a wheel. the device looked well on paper, beyond which stage it was not carried, perhaps for want of leisure, or more probably for want of the necessary material and tools. had peregrinus been able to test his theoretical views on the _magnetic motor_ by actual experiment, the delusive character of perpetual motion would have been recognized at an early epoch in the world's history, and much time and money spared for more profitable investment. this very wheel, which was designed in the trenches before lucera in , was probably the cause of the withering rebuke which justin huntly mccarthy administers in his "history of the french revolution," vol. i., p. , where he says: "in the long record of rascaldom from _peregrinus_ to bamfylde moore carew, no single rascal stands forward with such magnificent effrontery, such majestic impudence, such astonishing success as cagliostro." to say the least, this is a very serious slip of the pen on the part of the irish historian of the french revolution, in which a scientific pioneer of the first rank and a patriot of exalted type is mistaken for a charlatan of the deepest dye. although peregrinus puts the burden of constructing his wheel on others, he does not appear to have considered it a vain conceit; for, in the beginning of the last chapter of the "epistola" he says: "in this chapter, i will make known to you the construction of a wheel which, in a remarkable manner, moves continuously." he is writing from southern italy to his friend siger (syger, sygerus), at home in picardy; and that this friend may the better comprehend the mechanism of the wheel, he proceeds to describe in a systematic manner the various properties of the lodestone, all of which he had investigated and many of which he had discovered. the "epistola" of peregrinus is, therefore, the first treatise on the magnet ever written; it stands as the first great landmark in magnetic philosophy. the work is divided into two parts--the first contains ten chapters and the latter three. "at your request," he says to his friend, "i will make known to you in an unpolished narrative the undoubted though hidden virtue of the lodestone, concerning which philosophers, up to the present time, give us no information. out of affection for you, i will write in simple style about things entirely unknown to the ordinary individual." [illustration: fig. the double pivoted needle of petrus peregrinus, a. d., ] after this declaration as to the original character of his work peregrinus proceeds: "you must know that whoever wishes to experiment should be acquainted with the nature of things; he must also be skilled in manipulation, in order that by means of this stone, he may produce those marvelous results." the titles of the chapters will give an idea of the comprehensive character of the magnetic work accomplished by the author and, at the same time, will serve to show how much was known about the lodestone in the thirteenth century. part i. chap. i. purpose of this work. ii. qualifications of the experimenter. iii. characteristics of a good lodestone. iv. how to distinguish the poles of a lodestone. v. how to tell which pole is north and which south. vi. how one lodestone attracts another. vii. how iron touched by a lodestone turns towards the poles of the world. viii. how a lodestone attracts iron. ix. why the north pole of one lodestone attracts the south pole of another, and _vice versa_. x. an inquiry into the natural virtue of the lodestone. part ii. chap. i. construction of an instrument for measuring the azimuth of the sun, the moon or any star when in the horizon. ii. construction of a better instrument for the same purpose. iii. the art of making a wheel of perpetual motion. an attentive reading of the thirteen chapters of this treatise of , words will show that: ( ) peregrinus assigns a definite position to what he calls the _poles_ of a lodestone and gives practical directions for determining which is north and which south. ( ) he establishes the two fundamental laws of magnetism, that like poles repel and unlike poles attract each other. ( ) he demonstrates by experiment that every fragment of a lodestone is a complete magnet, and shows how the fragments should be put together in order to reproduce the polarity of the unbroken stone. ( ) he shows how a pole of a lodestone may neutralize a weaker one of the same name and even reverse its polarity. ( ) he pivots a magnetized needle and surrounds it with a circle divided into degrees. this brief summary shows the great advance made by the author on what was known about the lodestone before his time. most of the salient facts in magnetism are clearly described and some of their applications pointed out. so thorough and complete was this apprehension and explanation of magnetic phenomena that nothing of importance was added to it for the next three hundred years. [illustration: fig. first pivoted compass, peregrinus, ] in the compass which peregrinus devised for use in navigation, a light magnetic needle was thrust through a slender vertical axis made of wood, which axis also carried a pointer of brass or silver at right angles to the needle. according to the belief of the time, the magnetic needle gave the north and south points of the horizon, while the brass pointer determined the east and west points. this compass, double pivoted be it noticed, was provided with a graduated circle and a movable arm, having a pair of upright pins at its extremities, which movable arm enabled the navigator to determine the magnetic bearing of the sun, moon or any star at the time of rising or setting. "by means of this instrument," the author says in chap. ii., "you can direct your course towards cities and islands and any other place wherever you may wish to go, by land or by sea, provided you know the latitude and longitude of the place which you want to reach." the invention of the compass has been attributed to one flavio gioja, a seafaring man of amalfi, a flourishing maritime town in southern italy. if we admit that gioja was a real and not a fictitious person, we cannot, however, admit the claim which is made by his countrymen, when they say that he gave to the mariner the use of the compass in the year ; for we have seen that peregrinus distinctly states that his compass, described in , could be relied upon for guidance by the traveler on land as well as by the voyager on sea. to gioja may belong the merit of having simplified and improved the compass. it is likely that he suspended the needle on one pivot instead of the two used by peregrinus, and that he added the compass-card with its thirty-two divisions, attaching it to the needle itself, thereby adding materially to the practical character of the compass as a nautical instrument. on the other hand, a claim has been made for peregrinus which cannot be admitted. it was put forward by his itinerant countryman thévenot, in the seventeenth century, to the effect that the author of the "epistola" was acquainted with magnetic declination, in virtue of which a freely suspended magnet does not point north and south, but cuts the geographical meridian at a definite angle. writing in , thévenot says in his "recueil de voyages" that: "it was a matter of general belief down to the present day, that the declination of the magnetic needle was first observed sometime in the beginning of the last ( th) century. i have found, however, that there was a declination of five degrees in the year , having found it recorded in a manuscript with the title "epistola petri adsigerii," etc. the title of the manuscript seen by thévenot is not, however, as he gives it above, but "epistola petri ad sygerium," etc., which is quite a different reading. there are twenty-eight manuscript copies of the "epistola" known to exist; and only one of them, that of the university of leyden, contains the passage alluded to by thévenot. this manuscript was the object of careful study and critical examination by wenckebach ( ) and other competent scholars, who pronounced it a spurious addition made some time in the early part of the th century.[ ] in the time of peregrinus, it is probable that the declination did not exceed three degrees in paris or on the shores of the mediterranean, a quantity so small that it would have been difficult of detection; and, if detected, would have been attributed either to errors in the construction of the instrument used or to inaccuracy on the part of the observer. this is what happened to columbus when, on his return to spain, having reported the many and definite observations on the variation of the compass which he had made on his outward voyage, he was told by the learned ones of the day that _he_ was in error and not the needle, because the latter was everywhere true to the pole. [illustration: fig. ] this oft-stated and widely-believed fidelity of the needle to the pole is not, however, founded on fact; it is the exception, the rare exception, not the rule, despite the couplet of the poet: th' obedient steel with living instinct moves and veers for ever to the pole it loves; or this other, so turns the faithful needle to the pole, though mountains rise between and oceans roll. that the magnet does not turn to the pole of the world is common knowledge to-day, when the high school tyro will tell you that in new york it points ° _west_ of north, while in san francisco it points ° _east_ of north. if he happens to be well up, he may refer to the position of the agonic line on the globe along which the needle stands true to the pole, while all places to the east of that line in our hemisphere have westerly declination and those to the west have easterly declination. indeed, magnetic charts show places where the needle points east and west instead of north and south, and others where the north-seeking end points directly south. such varying and conflicting behavior of the compass-needle serves to show the irregular manner in which the earth's magnetism is distributed and also the intensity of distributing forces which exist at certain places. it is one of the gems in the crown of columbus, that he observed, measured and recorded this strange behavior of the magnetic needle in his narrative of the voyage. true, he did not notice it until he was far out on the trackless ocean. a week had elapsed since he left the lordly teneriffe, and a few days since the mountainous outline of gomera had disappeared from sight. the memorable night was that of september th, . there was no mistaking it; the needle of the _santa maria_ pointed a little west of north instead of due north. some days later, on september th, the pilots, having taken the sun's amplitude, reported that the variation had reached a whole point of the compass, the alarming amount of degrees. the surprise and anxiety which columbus manifested on those occasions may be taken as indications that the phenomenon was new to him. as a matter of fact, however, his needles were not true even at the outset of the voyage from the port of palos, where, though no one was aware of it, they pointed about ° _east_ of north. this angle diminished from day to day as the admiral kept the prow of his caravel directed to the west, until it vanished altogether, after which the needles veered to the _west_, and kept moving westward for a time as the flag-ship proceeded on her voyage. [illustration: fig. magnetic declination in london in and in ] columbus thus determined a place on the atlantic in which the magnetic meridian coincided with the geographical and in which the needle stood true to the pole. six years later, in , sebastian cabot found another place on the same ocean, a little further north, in which the compass lay exactly in the north-and-south line. these two observations, one by columbus and the other by cabot, sufficed to determine the position of the _agonic line_, or line of no variation, for that locality and epoch. the _columbian_ line acquired at once considerable importance, in the geographical and the political world, because of the proposal that was made to discard the island of ferro and take it for the prime meridian from which longitude would be reckoned east and west, and also because it was selected by pope alexander vi. to serve as a line of reference in settling the rival claims of the kingdoms of portugal and castile with regard to their respective discoveries. it was decided that all recently discovered lands lying to the east of that line should belong to portugal; and those to the west, to castile. the line of no variation, like all other isomagnetic lines, has shifted its position with time, so that it runs to-day considerably to the west of the place assigned to it by columbus in and by the papal bull of the following year. columbus did not speak of the disquieting observation which he made on the night of the th of september; he thought of it, and wondered greatly what might be the cause of such an unexpected and untoward phenomenon. his silence on the matter did not avail, for the keen-eyed sailors noticed the westerly deflection of the needle when, after a few days, it became quite apparent. they grew alarmed, believing that the laws of nature were changing as they advanced farther and farther into the unknown. it was a trying moment for the admiral, but his ingenuity and tactfulness rose to the occasion. he told his seamen that the needle did not point to the _cynosure_ or last star in the tail of the little bear, as commonly supposed, but to a fixed point in the celestial sphere at which there was no star, adding that the "cynosure" itself, the polaris of our days, was not stationary, but had a rotational movement of its own like all other heavenly bodies. we do not know what columbus thought of his explanation, born of the stress of the moment, but the esteem in which he was held by pilots and sailors alike for his knowledge of astronomy and cosmography led them to accept it. their fears were allayed, a mutiny was averted and a successful termination to their voyage rendered possible. captains of ocean-liners would give to-day a different answer to a passenger who might consult them about the splinter of steel which serves to guide their fleet vessels in darkest nights, through howling tempests and over billowy seas. the mysterious influence that controls it, they would say, comes neither from polaris nor the pole of the world, nor from the heavens above, but from the earth beneath. such an explanation was not thought of until it was clearly shown a hundred years later that this globe of ours acts like a colossal lodestone, controlling every magnet in our laboratories and observatories, and every needle on board the merchantmen and fighting-monsters that plough our seas and oceans. without any intuition of modern theory, columbus made two discoveries in terrestrial magnetism, as we have seen, each of fundamental importance, whether considered from the view-point of pure science or that of practical navigation, viz., (a) that the needle is not true to the pole and (b) that the angular displacement of the needle from true orientation, the _variation of the compass_, as it is called in nautical parlance, differs with the place of the observer. these two discoveries as well as the location of a place of no variation on the atlantic ocean entitle columbus to a prominent place among the founders of the _science_ of terrestrial magnetism. later observers discovered that even for a given place this element of magnetic declination has not a constant value, but undergoes changes which complete their cycle, some in a day, others in a year, and others again in centuries. the last or _secular_ change in the direction of the magnetic needle was discovered by gellibrand, of london, in (published in ); the _annual_, by cassini, at paris, - ; and the _diurnal_, by graham, of london, in . the first observation of magnetic declination on _land_ appears to have been made about the year by george hartmann ( - ), vicar of the church of st. sebald in nuremberg, who found it to be ° east in rome, where he was living at the time. hartmann's observation of the declination in rome and also in nuremberg, where the needle pointed ° east of north, will be found in a letter which he wrote in to duke albert of prussia and which remained unpublished until the year . returning to the treatise of peregrinus on the magnet, it should be said that for several centuries the twenty-eight manuscript copies lay undisturbed on the dusty shelves of city and university libraries. in , four years after the appearance of the first printed edition (augsburg, ), taisnier, a belgian writer on magnetics, who is also described as poet-laureate and doctor "utriusque juris," was among the earliest to discover the "epistola," from which he copied extensively in his little quarto on the magnet and its effects, thus showing that there were literary pirates in those days. it was also well known to gilbert, to cabeo and kircher; but despite the references of these writers, the "epistola" remained practically unknown until cavallo, of london, called attention to the leyden manuscript in the third edition of his "treatise on magnetism,"[ ] , by giving part of the text and accompanying it with a translation. later, in , libri, historian of the mathematical sciences in italy, gave excerpts from the paris codex with translation; but the scholar who contributed most of all to make the work of peregrinus known is the italian barnabite, timoteo bertelli, who published in a critical study of the various manuscripts of the letter, principally those which he found in rome and in florence, adding copious notes of historic, bibliographic and scientific value. father bertelli was professor of physics in the collegio della quercia, in florence, where he took an active interest in italian seismology besides carrying on investigations in meteorology, telegraphy and electricity. born in bologna in , he died in florence in march, . the following list of manuscript copies of the "epistola" is taken from a scholarly paper by professor silvanus p. thompson, of london, which appeared in the "proceedings of the british academy" for :-- the bodleian library seven vatican four british museum one bibliothèque nationale, paris two biblioteca riccardiana, florence one trinity college, dublin one gonville and caius, cambridge one the university of leyden one geneva one turin one erfurt three vienna three s. p. thompson two the first printed edition of the "epistola" was prepared for the press in by achilles gasser, a man well versed in the science and philosophy of his day; another edition, which will probably be considered the _textus receptus_, is that which was prepared and published by bertelli in . no complete translation in any language of this historical work on magnetism was made until , when prof. silvanus p. thompson, of london, published his "epistle of peter peregrinus of maricourt to sygerus of foncaucourt, soldier, concerning the magnet." unfortunately, this translation was printed for private circulation and limited to copies. two years later, , brother arnold, f. s. c., presented a memoir on peregrinus, including a translation of the "epistola," for the m. sc. degree of manhattan college, new york city, which translation was published some months later by the mcgraw publishing company, new york. these are the only complete translations of the "letter" of peregrinus on the magnet which have yet appeared. brother potamian. footnotes: [ ] see klaproth, "lettre à m. le baron a. de humbolt sur l'invention de la boussole." ; also encyc. brit., article _compass_. [ ] provins, town miles southeast of paris. [ ] southey, "omniana," vol. i., p. , ed. . [ ] annali di matematica pura ed applicata. rome, . [ ] also in _rees_ encyclopedia, article _compass_. chapter ii. norman and gilbert. we have seen that in the thirteenth century the directive property of the lodestone was recognized by peregrinus and used by him in his pivoted compass; and that in the fifteenth, columbus discovered magnetic declination on sea as well as its variation with place. the next cardinal fact in terrestrial magnetism, magnetic dip, was discovered in by robert norman, a compass-maker of limehouse, london. norman possessed many of the fine qualities of mind, hand and disposition that are indispensable in the make-up of the original investigator. in pivoting his compass-needles, he soon noticed that, however carefully they were balanced before being magnetized, they did not remain horizontal after magnetization, the north-seeking end always going down through a small angle. he next had the happy idea of swinging a needle on a horizontal axis, so that it might be free to move up and down in a vertical plane, with the result that the north-seeking end again went down through a constant but much greater angle. [illustration: fig. the first dip-circle, invented by norman in ] like declination, the first discovered of the three magnetic elements, the dip was found to vary with place on the earth's surface, being ° at the magnetic equator and ° at either pole. it was with a norman dip-circle, greatly improved, that ross in found the north magnetic pole of the earth to be in boothia felix in latitude ° '. n., and longitude ° '. w.; and it was with a similar instrument that amundsen recently studied the magnetic conditions of that arctic region, the exact location of the pole itself being finally determined by an earth-inductor or spinning coil of the latest make. though the results of his observations have not yet been made public, it is generally known that they indicate a spot for the magnetic pole close to that found by sir james ross. it is not expected, however, that the location of the pole by the norwegian commander shall exactly coincide with that of the english captain, because the magnetic pole is believed to have nomadic tendencies of its own like our geographical pole, only much more pronounced in magnitude. after moving westward for some time at the rate of a mile per year, it retraced its steps and is now back again in the vicinity of its starting place. besides his dip-circle, norman also devised a simple and very apt illustration of magnetic inclination. thrusting a steel needle through a round piece of cork, he pared the latter down until the system, consisting of the needle and the cork, sank to a certain depth in a glass vessel containing water, and there took up a horizontal position. the needle was next removed from the water and magnetized with great care, so as not to disturb its position in the cork. when placed again in the water, the needle sank to its former depth and settled down at an angle of ° to the horizon. the same illustration shows another experiment which norman made in order to determine whether the earth exerts a force of translation on a magnet, in virtue of which the magnet would tend to move bodily toward the pole. for this purpose, he floated a magnetized piece of steel wire on the surface of the water and noticed that, wherever placed, it merely swung round into the magnetic meridian without showing any tendency to move northward or southward toward the rim of the vessel. hartmann, who observed the declination of the needle on land as stated on p. , appears also to have been the first to notice magnetic inclination. having balanced a steel needle with great precision, he found that, after magnetization, it did not remain horizontal, the north-seeking end invariably dipping through an angle of °. the smallness of the angle in this experiment was due to the fact that the needle used by the nuremberg vicar could move only in a horizontal plane, whereas norman's was free to move in a vertical circle. had hartmann used such a device, he would have obtained more than ° for the dip instead of the ° which he records. [illustration: fig. norman's illustration of magnetic dip] as already remarked, the letter in which hartmann consigns these capital observations was written in , but was not published until the third decade of the nineteenth century, so that norman has clearly the full merit of independent discovery. in the directions which norman gives for making observations of dip, he states explicitly that the instrument must be adjusted "duley according to the variation of the place," which means that the plane of the circle must be turned into what was called after his time "the magnetic meridian." the discovery of magnetic dip led norman to discard the view generally held in his time, which placed the controlling influence of the compass-needle in far-off celestial space; for he says that the _poynt respective_ which the magnet indicates, but to which it is not bodily drawn, is not in the heavens above, but in the earth itself. his words are: "and by the declining of the needle is also proved that the poynt respective is rather in the earth than in the heavens, as some have imagined; and the greatest reason why they so thought, as i judge, was because they were never acquainted with this declining in the needle." here we have a radical departure from the scientific creed of the time, a notable advance in scientific theory, an entirely new philosophy founded by norman, the compass-maker, and greatly developed twenty-four years later by his fellow-citizen, gilbert, the physician. [illustration: fig. gilbert's _orb of virtue_, ] norman made another remark of great importance in the new philosophy, the justness of which was appreciated by gilbert, his contemporary, but more so by faraday and clerk maxwell, two centuries later. it refers to the space surrounding a magnet, natural or artificial, which cubical space gilbert, following norman, called an _orb of virtue_. that the influence or "effluvium" of the magnet extends throughout the entire space may readily be seen by carrying a compass-needle round a magnet from point to point, far away as well as close by. the phrase "orb of virtue," or sphere of magnetic influence, appears to describe the actual magnetic condition of the space in question more pertinently than our modern equivalent of "magnetic field." the words of norman are very remarkable: "i am of opinion that if this vertue could by anie means be made visible to the eie of man, it would be found in a sphericall forme, extending round about the stone in great compasse and the dead bodie of the stone in the middle thereof." the lines which immediately follow this statement, pregnant with significance, show the deep religious feeling of the author. they read: "and this i have partly proved and made visible to be seene in some manner, and god sparing mee life, i will herein make further experience and that not curiouslie but in the feare of god as neere as he shall give me grace and meane to annexe the same unto a booke of navigation which i have had long in hand."--chap. viii. it is evident from the pages of the _newe attractive_ ( ) that norman was animated with the right spirit of inquiry, which is calm, deliberate and judicious, which leads to the discovery of facts, to their coordination and experimental illustration before explanations are thought of and long before new theories are propounded. the style in which this little treatise is written has a charm of its own, mainly by reason of its quaintness. at the end of his address to the candid reader, which, after the manner of the times, was somewhat belabored and rhetorical in character, norman breaks away from common inadequate prose; and, giving wings to his imagination, writes a lyric on the magnet which is the first metrical composition in english that we have on such a subject. it reads:-- the magnes or loadstone's challenge. give place ye glittering sparks, ye glimmering diamonds bright, ye rubies red, and saphires brave wherein ye most delight. in breefe, yee stones inricht, and burnisht all with golde, set forth in lapidaries shops, for jewells to be sold. give place, give place i say, your beautie, gleame and glee, is all the vertue for the which, accepted so you bee. magnes, the loadstone i, your painted sheath defie, without my help in indian seas, the best of you might lie. i guide the pilot's course, his helping hand i am, the mariner delights in me, so doth the marchant man. my vertue lies unknowne, my secrets hidden are, by me, the court and commonweale, are pleasured very farre. no ship could sail on seas, her course to run aright, nor compass shew the ready way were magnes not of might. blush then, and blemish all, bequeath to mee thats due, your seats in golde, your price in plate, which jewellers do renue. its i, its i alone, whom you usurp upon, magnes my name, the loadstone cal'd, the prince of stones alone. if this you can deny, then seem to make reply, and let the painfull sea-man judge, the which of us doth lie. the mariner's judgement. the loadstone is the stone, the onely stone alone, deserving praise above the rest whose vertues are unknown. the marchant's verdict. the diamonds bright, the saphires brave, are stones that bear the name, but flatter not, and tell the troath, magnes deserves the same. (edition of .) norman's _newe attractive_ was well known to gilbert, as were also the _epistola_ of peregrinus, the _magiae naturalis_ of porta, and indeed all books treating of the lodestone, the magnet, or the compass-needle. his own work _de magnete_, published in the year , is a compendium of the world's knowledge of magnetism and electricity at the time. in its pages, he not only discusses the opinions of others, but describes discoveries of his own made during the twenty years which he ardently devoted to the pursuit of experimental science, crowning his investigations with theories in electricity and magnetism as became a true philosopher. impressed by the originality of gilbert's treatise, the practical ingenuity and philosophic acumen displayed throughout, hallam wrote in his _introduction to the literature of europe_: "gilbert not only collected all the knowledge which others had possessed on the subject, but became at once the father of experimental philosophy in this island; and, by a singular felicity and acuteness of genius, the founder of theories which have been received after the lapse of ages and are almost universally received into the creed of science." at a period when natural science was taught in the schools of europe mainly from text-books, we find gilbert proclaiming by example and advocacy the paramount value of experiment for the advancement of learning. he was unsparing in his denunciation of the superficiality and verbosity of mere bookmen, and had no patience with writers who treated their subjects "esoterically, reconditely and mystically." for him, the laboratory method was the only one that could secure fruitful results and contribute effectively to the advancement of learning. it is true that men of unusual ability and strong character strove before his time to adjust the claims of authority in matters scientific. while respectful of the teachings of recognized leaders, they were not, however, awed into acquiescence by an academical "magister dixit." on the contrary, they wanted to test with their eyes in order to judge with reason; believing in the importance of experiment, they sought to acquire a knowledge of nature from nature herself. such were albert the great and friar bacon. albert did not bow obsequiously to the authority of aristotle or any of his arabian commentators; he investigated for himself and became, for his age, a distinguished botanist, physiologist and mineralogist. the franciscan monk of ilchester has left us in his _opus majus_ a lasting memorial of his practical genius. in the section entitled "scientia experimentalis," he affirms that "without experiment, nothing can be adequately known. an argument proves theoretically, but does not give the certitude necessary to remove all doubt, nor will the mind repose in the clear view of truth, unless it find it by way of experiment." and in his _opus tertium_: "the strongest arguments prove nothing, so long as the conclusions are not verified by experience. experimental science is the queen of sciences and the goal of all speculation." no one, even in our own times, wrote more strongly in favor of the practical method than did this follower of st. francis in the thirteenth century. being convinced that there can be no conflict between scientific and revealed truths, he became an irrepressible advocate for observation and experiment in the study of the phenomena and forces of nature. the example of peregrinus, of albert and friar bacon, not to mention others like vincent of beauvais, the dominican encyclopedist, was, however, not sufficient to wean students from the easy-going routine of book-learning. a few centuries had to elapse before the weaning was effectively begun; and the man who contributed in a marked degree to this result was gilbert the philosopher of colchester ( - ). having received the elements of his education in the grammar school of colchester, his native town, gilbert entered st. john's college, cambridge, from which university he took his b. a. degree in , m. a. in and m. d. in . in all, he appears to have been connected with the university for a period of eleven or twelve years, as student, fellow, and examiner. on leaving cambridge, gilbert traveled for four years on the continent, principally in italy, visiting medical schools and studying methods of treatment under the leading physicians and surgeons of the day as well as discussing scientific theory with the leaders of thought. on his return to england in , he practised medicine in london "with great applause and success." he was elected president of the royal college of physicians in , and appointed physician to queen elizabeth in and to her successor, james i., in . on one occasion, he hears that baptista porta, whom he calls "a philosopher of no ordinary note," said that a piece of iron rubbed with a diamond turns to the north. he suspects this to be heresy. so, forthwith he proceeds to test the statement by experiment. he was not dazzled by the reputation of baptista porta; he respected porta, but respected truth even more. he tells us that he experimented with seventy diamonds in presence of many witnesses, employing a number of iron bars and pieces of wire, manipulating them with the greatest care while they floated on corks; and concludes his long and exhaustive research by plaintively saying: "yet never was it granted me to see the effect mentioned by porta." though it led to a negative result, this probing inquiry was a masterpiece of experimental work. gilbert incidentally regrets that the men of his time "are deplorably ignorant with respect to natural things," and the only way he sees to remedy this is to make them "quit the sort of learning that comes only from books and that rests only on vain arguments and conjectures," for he shrewdly remarks that "even men of acute intelligence without actual knowledge of facts and in the absence of experiment easily fall into error." acting on this intimate conviction, he labored for twenty years over the theories and experiments which he sets forth in his great work on the magnet. "there is naught in these books," he tells us, "that has not been investigated, and again and again done and repeated under our eyes." he begs any one that should feel disposed to challenge his results to repeat the experiments for himself "carefully, skilfully and deftly, but not heedlessly and bunglingly." it has been said that we are indebted to sir francis bacon, queen elizabeth's chancellor, for the inductive method of studying the phenomena of nature. bacon's merit lies in the fact that he not only minutely analyzed the method, pointing out its uses and abuses, but also that he showed it to be the only one by which we can attain an accurate knowledge of the physical world around us. his sententious eulogy went forth to the world of scholars invested with all the importance, authority and dignity which the high position and worldwide fame of the philosophic chancellor could give it. but while bacon thought and wrote in his study, gilbert labored and toiled in his workshop. by his pen, bacon made a profound impression on the philosophic mind of his age; by his researches, gilbert explored two provinces of nature and added them to the domain of science. bacon was a theorist, gilbert an investigator. for twenty years he shunned the glare of society and the throbbing excitement of public life; he wrenched himself away from all but the strictest exigencies of his profession, in order to devote himself undistractedly to the pursuit of science. and all this forty years before the appearance of bacon's _novum organum_, the very work which contains the philosopher's "large thoughts and lofty phrases" on the value of experiment as a means for the advancement of learning. during that long period gilbert haunted colchester, where he delved into the secrets of nature and prepared the materials for his great work on the magnet. the publication of this latin treatise made him known in the universities at home and especially abroad: he was appreciated by all the great physicists and mathematicians of his age; by such men as sir kenelm digby; by william barlowe, a great "magneticall" man; by kepler, the astronomer, who adopted and defended his views; by galileo himself, who said: "i extremely admire and envy the author of _de magnete_." the science of magnetism owes more to gilbert than to any other man, peregrinus ( ) excepted. he repeated for himself the numerous and ingenious experiments of the medieval philosopher, and added much of his own which he discovered during the long period of a life devoted to the diligent exploration of this domain in the world of natural knowledge. the ancients spoke of the lodestone as the magnesian stone, from its being found in abundance in the vicinity of magnesia, a city of asia minor. in his latin treatise of (small) folio pages, gilbert uses the adjective form of the term, but never the noun "magnetismus" itself. our english term _magnetism_ appears for the first time on page of archdeacon barlowe's "magneticall advertisements," published in ; while the surprising compound, "electro-magnetismos," is the title of a chapter in father kircher's "magnes, sive de arte magnetica," printed in the year . gilbert showed that a great number of bodies could be electrified; but maintained that those only could exhibit magnetic properties which contain iron. he satisfies himself of this by rubbing with a lodestone such substances as wood, gold, silver, copper, zinc, lead, glass, etc., and then floating them on corks, quaintly adding that they show "no poles, because the energy of the lodestone has no entrance into their interior." to-day we know that nickel and cobalt behave like iron, whilst antimony, bismuth, copper, silver and gold are susceptible of being influenced by powerful electro-magnets, showing what has been termed diamagnetic phenomena. even liquids and gases, in faraday's classical experiments, yielded to the influence of his great magnet; and professor dewar, in the same royal institution, exposed some of his liquid air and liquid oxygen to the influence of faraday's electromagnet and found them to be strongly attracted, thus behaving like the paramagnetic bodies, iron, nickel and cobalt. gilbert observes in all his magnets two points, one near each end, in which the force, or, as he terms it, "the supreme attractional power," is concentrated. like peregrinus, he calls these points the _poles_ of the magnet, and the line joining them its magnetic axis. with the aid of his steel versorium, he recognizes that similar poles are mutually hostile, whilst opposite poles seize and hold each other in friendly embrace. he also satisfies himself that the energy of magnets resides not only in their extremities, but that it permeates "their inmost parts, being entire in the whole and entire in each part." this is exactly what peregrinus said in and what we say to-day; it is nothing else than the molecular theory proposed by weber, extended by ewing and universally accepted. at any rate, gilbert is quite certain that whatever magnetism may be, it is not, like electricity, a material, ponderable substance. he ascertained this by weighing in the most accurate scales of a goldsmith a rod of iron before and after it had been rubbed with the lodestone, and then observing that the weight is precisely the same in both cases, being "neither less nor more." without referring to the prior discovery of norman, whom he calls "a skilled navigator and ingenious artificer," gilbert satisfies himself that not only the magnet, but all the space surrounding it, possesses magnetic properties; for the magnet "sends its force abroad in all directions, according to its energy and quality." this region of influence norman called a sphere of "vertue," and gilbert an "orbis virtutis," which is the latin equivalent; we call it a "magnetic field," or field of force, which is less expressive and less appropriate. with wonderful intuition, gilbert sees this space filled with lines of magnetic virtue passing out radially from his spherical lodestone, which lines he calls "rays of magnetic force." clerk maxwell was so fascinated with this beautiful concept that he made it the work of his life to study the field of force due to electrified bodies, to magnets and to conductors conveying currents; his powerful intellect visualized those lines and gave them accurate mathematical expression in the great treatise on electricity and magnetism which he gave to the world in . gilbert observes that the lodestone may be spherical or oblong; "whatever the shape, imperfect or irregular, verticity is present; there are poles," and the lodestones "have the selfsame way of turning to the poles of the world." he knows that a compass-needle is not drawn bodily towards the pole, and does not hesitate in this instance to give credit to his countryman, robert norman, for having clearly stated this fact and aptly demonstrated it. following norman, he floats a needle in a vessel by means of a piece of cork, and notices that on whatever part of the surface of the water it may be placed, the needle settles down after a few swings invariably in the same direction. his words are: "it revolves on its iron center and is not borne towards the rim of the vessel." gilbert knew nothing about the mechanical couple that came into play, but he knew the fact; and, with the instinct of the philosopher, tested it in a variety of ways. we explain the orientation of the compass-needle by saying that it is acted upon by a pair of equal and opposite forces due to the influence of the terrestrial magnetic poles on each end of the needle and by showing that such a couple can produce rotation, but not translation. we find gilbert working not only with steel needles and iron bars, but also with rings of iron. he strokes them with a natural magnet and feels certain that he has magnetized them. he assures us that "one of the poles will be at the point rubbed and the other will be at the opposite side." to show that the ring is really magnetized, he cuts it across, opens it out, and finds that the ends exhibit polar properties. a favorite piece of apparatus with gilbert, as with peregrinus, was a lodestone ground down into globular form. he called it a terrella, a miniature earth, and used it extensively for reproducing the phenomena described by magnetizers, travelers and navigators. he breaks up terrellas, in order to examine the magnetic condition of their inner parts. there is not a doubtful utterance in his description of what he finds; he speaks clearly and emphatically. "if magnetic bodies be divided, or in any way broken up, each several part hath a north and a south end"; _i.e._, each part will be a complete magnet. [illustration: fig. behavior of compass-needle on a _terrella_ or spherical lodestone] we find him also comparing magnets by what is known to us as the "magnetometer method." he brings the magnetized bars in turn near a compass-needle and concludes that the magnet or the lodestone which is able to make the needle go round is the best and strongest. he also seeks to compare magnets by a process of weighing, similar to what is called, in laboratory parlance, the "test-nail" method. he also inquires into the effect of heat upon his magnets, and finds that 'a lodestone subjected to any great heat loses some of its energy.' he applies a red-hot iron to a compass-needle and notices that it 'stands still, not turning to the iron.' he thrusts a magnetized bar into the fire until it is red-hot and shows that it has lost all magnetic power. he does not stop at this remarkable discovery, for he proceeds to let his red-hot bars cool while lying in various positions, and finds: ( ) that the bar will acquire magnetic properties if it lie in the magnetic meridian; and ( ) that it will acquire none if it lie east and west. these effects he rightly attributes to the inductive action of the earth. gilbert marks these and other experiments with marginal asterisks; small stars denoting minor and large ones important discoveries of his. there are in all large and small asterisks, as well as illustrations in _de magnete_. this implies a vast amount of original work, and forms no small contribution to the foundations of electric and magnetic science. gilbert clearly realized the phenomena and laws of magnetic induction. he tells us that "as soon as a bar of iron comes within the lodestone's sphere of influence, though it be at some distance from the lodestone itself, the iron changes instantly and has its form renewed; it was before dormant and inert; but now is quick and active." he hangs a nail from a lodestone; a second nail from the first, a third from the second and so on--a well-known experiment, made every day for elementary classes. nor is this all, for he interposes between the lodestone and his iron nail, thick boards, walls of pottery and marble, and even metals, and he finds that there is naught so solid as to do away with its force or to check it, save a plate of iron. all that can be added to this pregnant observation is that the plate of iron must be very thick in order to carry all the lines of force due to the magnet, and thus completely screen the space beyond. but gilbert is astonishing when he goes on to make thick boxes of gold, glass and marble; and, suspending his needle within them, declares with excusable enthusiasm that, regardless of the box which imprisons the magnet, it turns to its predestined points of north and south. he even constructs a box of iron, places his magnet within, observes its behavior, and concludes that it turns north and south, and would do so were "it shut up in iron vaults sufficiently roomy." in this, he was in error, for experiments show that if the sides of the box are thin, the needle will experience the directive force of the earth; but if they are sufficiently thick--thick as the walls of an ordinary safe--the inside of such a box will be completely screened; none of the earth's magnetic lines will get into it so that the needle will remain indifferently in any position in which it is placed. some years ago, the physical laboratory of st. john's college, oxford, was screened from the obtrusive lines of neighboring dynamos by building two brick walls parallel to each other and eight inches apart and filling in the space with scrap iron. a delicate magnetometer showed that such a structure allowed no leakage of lines of force through it, but offered an impenetrable barrier to the magnetic influence of the working dynamos. gilbert's greatest discovery is that the earth itself acts as a vast globular magnet having its magnetic poles, axis and equator. the pole which is in our hemisphere, he variously calls north, boreal or arctic. whilst that in the other hemisphere he calls south, austral or antarctic. he sought to explain the magnetic condition of our globe by the presence, especially in its innermost parts, of what he calls true, terrene matter, homogeneous in structure and endowed with magnetic properties, so that every separate fragment exhibits the whole force of magnetic matter. he is quite aware that his theory is a grand generalization; and admits that it is "a new and till now unheard-of view," and so confident is he in its worth that he is not afraid to say that "it will stand as firm as aught that ever was produced in philosophy, backed by ingenious argumentation or buttressed by mathematical demonstration." in developing his theory of terrestrial magnetism, gilbert fell into certain errors, chiefly for want of data, but partly also by reason of his adherence to the view that the earth exactly resembled his terrella in its magnetic action. accordingly, he believed that the magnetic poles of the earth were diametrically opposite each other and that they coincided with the poles of rotation, whence it followed that the magnetic meridian everywhere coincided with the geographical, and that the magnet, unless influenced by local disturbances, stood true to the pole. it was, however, well known from the thrilling experience of columbus and the constant report of travelers that this was not the case. gilbert himself says that at the time of writing, in the year , the needle pointed - / ° east of north in london; but what he did not know and could not have known was that this easterly deviation was decreasing from year to year, to vanish altogether in , after which the needle began to decline to the west. this magnetic declination sorely perplexed gilbert, as it did not fit in with his theory. yet an explanation was needed; and as the earth must be considered a normal and well-behaved magnet, though of cosmical size, gilbert turns the difficulty by saying that this variation is nothing else than "a sort of perturbation of the directive force" caused by inequalities in the earth's surface by continents and mountain masses: "since the earth's surface is diversified by elevations of land and depths of seas, great continental lands, oceans and seas differing in every way while the power that produces all magnetic movements comes from the constant magnetic earth-substance which is strongest in the most massive continent and not where the surface is water or fluid or unsettled, it follows that toward a massive body of land or continent rising to some height in any meridian, there is a measurable magnetic leaning from the true pole toward the east or the west." so convinced is gilbert of the true and satisfactory character of his explanation that he goes on to say that, "in northern regions, the compass varies because of the northern eminences; in southern regions, because of southern eminences. on the equator, if the eminences on both sides were equal, there would be no variation." in a later chapter of book iv., he adds that, "in the heart of great continents there is no variation; so, too, in the midst of great seas." as continents and mountain-chains are among the permanent features of our planet, gilbert concluded that the misdirection of the needle was likewise permanent or constant at any given place, a conclusion which observations made after gilbert's time showed to be incorrect. gilbert writes: "as the needle hath ever inclined toward the east or toward the west, so even now does the arc of variation continue to be the same in whatever place or region, be it sea or continent; so, too, will it be forever unchanging." this we know to be untrue, and gilbert, too, could have known as much had he brought the experimental method, which he used with such consummate skill and fruitful results in other departments of his favorite studies, to bear on this particular element of terrestrial magnetism. he labored with incredible ardor and persistence for twenty years in his workshops at colchester over the experiments in electricity, magnetism and terrestrial magnetism which he embodies and discusses in his original and epoch-making book, _de magnete_, published in the year ; a period of twenty years was long enough for such a careful observer as he was to detect the slow change in magnetic declination discovered by his friend gellibrand in , published by him in , and known to-day as the "secular variation." it is true the quantity to be measured was small; but what is surprising is that such an industrious and resourceful experimenter as gilbert was does not record in his pages any observations of his own on declination or dip, elements of primary importance in magnetic theory. shortly after the voyage of columbus it was thought that the _longitude_ of a place could be found from its magnetic declination. gilbert, however, did not think so, and accordingly scores those who championed that view. "porta," he says, "is deluded by a vain hope and a baseless theory"; livius sanutus "sorely tortures himself and his readers with like vanities"; and even the researches of stevin, the great flemish mathematician, on the cause of variation in the southern regions of the earth are "utterly vain and absurd." with regard to dip, gilbert erroneously held that for any given latitude it had a constant value. he was so charmed with this constancy that he proposed it as a means of determining _latitude_. there is no diffidence in his mind about the matter; he is sure that with his "inclinatorium" or dip-circle, together with accompanying tables, calculated for him by briggs, of logarithmic fame, an observer can find his latitude "in any part of the world without the aid of the sun, planets or fixed stars in foggy weather as well as in darkness." after such a statement, it is no wonder that he waxes warm over the capabilities of his instrument and allows himself to exclaim: "we can see how far from idle is magnetic philosophy; on the contrary, how delightful it is, how beneficial, how divine! seamen tossed by the waves and vexed with incessant storms while they cannot learn even from the heavenly luminaries aught as to where on earth they are, may with the greatest ease gain comfort from an insignificant instrument and ascertain the latitude of the place where they happen to be." gilbert dwells at length on the inductive action of the earth. he hammers heated bars of iron on his anvil and then allows them to cool while lying in the magnetic meridian. he notes that they become magnetized, and does not fail to point out the polarity of each end. he likewise attributes to the influence of the earth the magnetic condition acquired by iron bars that have for a long time lain fixed in the north-and-south position and ingenuously adds: "for great is the effect of long-continued direction of a body towards the poles." to the same cause, he attributes the magnetization of iron crosses attached to steeples, towers, etc., and does not hesitate to say that the foot of the cross always acquires north-seeking polarity. in a similar manner, every vertical piece of iron, like railings, lamp-posts, and fire-irons, becomes a magnet under the inductive action of the earth. in the case of our modern ships, the magnetization of every plate and vertical post, intensified by the hammering during construction, converts the whole vessel into a magnetic magazine, the resulting complex "field" rendering the adjustment of the compasses somewhat difficult and unreliable. the unreliable character of the adjustment arises mainly from the changing magnetism of the ship with change of place in the earth's magnetic field, the effect increasing slightly from the magnetic equator to the poles. with luminous insight into the phenomena of terrestrial magnetism, gilbert observes that in the neighborhood of the poles, a compass-needle, tending as it does to dip greatly, must in consequence experience only a feeble directive power. to which he adds that "at the poles there is no direction," meaning, no doubt, that a compass-needle would remain in any horizontal position in which it might be placed when in the vicinity of the magnetic pole. this is precisely the experience of all arctic explorers, who find that their compasses become less and less active as they sail northward, the reason being that the horizontal component of the earth's magnetic force, which alone controls the movements of the compass-needle, decreases as the ship advances and vanishes altogether at the magnetic pole. when once a high latitude is reached, captains do not depend upon their compasses for their bearings, but have recourse to astronomical observations. in his account of magnetic work carried on in the neighborhood of the magnetic pole, amundsen says: "at prescott island the compass, which for some time had been somewhat sluggish, refused entirely to act, and we could as well have used a stick to steer by." as a physician, gilbert valued iron for its medicinal properties, but denounced quacks and wandering mountebanks who practised "the vilest imposture for lucre's sake," using powdered lodestone for the cure of wounds and disorders. "headaches," he said, "are no more cured by application of a lodestone than by putting on an iron helmet or a steel hat"; and again: "to give it in a draught to dropsical persons is either an error of the ancients or an impudent tale of their copyists." elsewhere he condemns prescriptions of lodestone as "an evil and deadly advice" and as "an abominable imposture." in the sixth and last book of _de magnete_, gilbert sets forth his views on such astronomical subjects as the figure of the earth, its suspension in space, rotation on its axis and revolution around the sun. as to the figure of our planet, the primitive view widely credited in early times was that the earth is a flat, uneven mass floating in a boundless ocean. the hindoos, however, did not accept the flatland doctrine, but taught that the earth was a convex mass which rested on the back of a triad of elephants having for their support the carapace of a gigantic tortoise. of course, they did not say how the complaisant chelonian contrived to maintain his wonderful state of equilibrium under the superincumbent mass. aristotle ( - , b. c.) taught that the earth, fixed in the center of the universe, is not flat as a disk, but round as an orange, giving as proofs ( ) the gradual disappearance of a ship standing out to sea and ( ) the form of the shadow cast by the earth in lunar eclipses, to which others added ( ) the change in the altitude of circumpolar stars readily noticeable in traveling north or south. aristarchus of samos ( - ), one of the great astronomers of antiquity, went further, not fearing to teach that the earth is spherical in form, that it turns on its axis daily and revolves annually around the sun. such orthodox teaching did not, however, commend itself to people generally, as they did not exactly like the idea of being whisked round with their houses and cities at a dangerous speed, preferring to explain celestial phenomena by the rotation of the vast celestial sphere, with all the starry host, round a flat, immovable earth. for them such a system of cosmography recommended itself by its simplicity and reasonableness as well as by the sense of stability, rest and comfort which it brought along with it. ptolemy, who flourished at alexandria about a. d. and whose name is associated with a system of the world, also held that the earth is spherical in form, giving at the same time some very ingenious proofs of his belief. st. augustine, in the fourth century, was not opposed to the doctrine of a round earth, though he felt the religious difficulty arising from the existence of the antipodes, which difficulty reached its acute stage four hundred years later. it is well to remember that the church did not condemn the existence of an antipodean world; what it did condemn was the teaching of virgilius, bishop of salzburg, to the effect that this world, lying under the equator, was inhabited by a race of men not descended from adam. virgilius also taught that the antipodes had a sun and moon different from ours, an astronomical opinion for which he was never molested by ecclesiastical authority. boethius, the worthy representative of the natural and the higher philosophy of the sixth century, wrote of the earth as globe-like in form, but small in comparison with the heavens. isidore of seville, in the seventh century, "the most learned man of his age," and the encyclopedic bede, in the eighth, rejected the theory of a flat, discoidal earth and returned to the spherical form of the early greek astronomers. but again, centuries had to elapse before people could be brought to tolerate views of the world that seemed so directly opposed to the daily testimony of their senses. the strong, conclusive arguments which alone establish this theory on a firm basis were, however, not known to copernicus and could not have been known in an age that preceded the invention of the telescope and in which the astronomer had to be the constructor of his own crude wooden instruments. the wonder is that copernicus did such excellent observational work on the banks of the vistula with the rough appliances at his disposal. the arguments which he put forward and urged with consummate skill for the acceptance of his revolutionary theory were its general simplicity and probability. of proofs clear and decisive, he gave none; yet, while he was working on his epoch-making treatise, begun in and published in with dedication to pope paul iii., a direct proof of the earth's spherical form was given by the return ( ) from the philippines along an eastern route of one of magellan's ships, which had reached those distant isles after crossing the western ocean, which the portuguese navigator called the "pacific," from the tranquility of its waters. for a direct proof of the earth's _annual_ motion, the world had to wait two hundred years more, until bradley discovered the "aberration of light" in ; and for a direct demonstration of its _diurnal_ motion until foucault made his pendulum experiment in the panthéon, in . we cannot let gilbert's reference to a "weightless earth" pass without a few remarks to justify our approval of the statement. the idea connoted by the term weight is the pull which the earth exerts on the mass of a body; thus, when we say that an iron ball weighs six pounds, we mean that the earth pulls it downwards with a force equal to the weight of six pounds. that the weight of a given lump of matter is not a constant but a dependent quantity may be seen from a number of considerations. its weight in vacuo, for instance, is different from its weight in air, and this latter differs considerably from its weight in water or in oil. again, if we take our experimental ball down the shaft of a mine, the spring-balance used to measure the pull of the earth on it will not record six pounds but something less; and the further we descend, the less will the spring-balance be found to register. at a depth of two thousand miles below the surface, the ball would be found to have lost half its weight; and at a depth of four thousand, all its weight. at the earth's center a "box of weights" would still be called a "box of weights," though neither the box itself nor its enclosed standards singly or collectively would have any weight whatever. it has been shown experimentally that two masses weigh slightly less when placed one above the other than when placed side by side, because in the latter case their common mass-center is measurably nearer to the center of the earth. every mother knows that when a boy is sent to buy a pound of candy, it is the mass of the sweet stuff that makes him happy, and not its weight, for this acts more like an incumbrance while he is bringing it home. of course, weight is every day used, and correctly, as a measure of mass, for every student of mechanics writes without the least hesitation, w=mg. by which he simply means that the weight of a body is directly proportional to its mass (m), which is constant wherever the body may be taken, and to the intensity of gravity (g), which varies slightly with geographical position. as both scale-pans of an ordinary balance are equally affected by the local value of _g_, it follows that equilibrium is established only when the two _masses_--that of the body and that of the standards--are themselves equal: hence weighing is in reality only a process of comparing masses, _i.e._, a process of "massing." if we bring our experimental ball to the top of a hill or to the summit of a mountain or aloft in a balloon, we find the pull on the registering spring growing less and less as we go higher and higher, from which we naturally conclude that if we could go far enough out into circumterrestrial space, say, towards the moon, the ball would lose its weight entirely; it would cease to stretch the spring of the measuring balance, its weight vanishing at a definite, calculable distance from the earth's center. if carried beyond that point the ball would come under the moon's preponderating attraction and would begin to depress anew the index of the balance until at the surface of our satellite it would be found to weigh exactly _one_ pound. if transferred to the planet mars the ball would weigh _two_ pounds, and if to the surface of the giant planet jupiter, _sixteen_ pounds. but while its weight thus changes continually, its mass or quantity of matter, the stuff of which it is made, remains constant all the while, being equally unaffected by such variables as motion, position or even temperature. returning from celestial space to our more congenial terrestrial surroundings, we find a similar inconstancy in the weight of the ball as we travel from the equator toward either pole, the weight being least at the equator and slightly greater at either end of our axis of rotation. this change is fully accounted for by the spheroidal figure of the earth and its motion of rotation, in virtue of which, while going from the equator toward the pole, our distance from the center of attraction undergoes a slight diminution, as does also the component of the local centrifugal force, which is in opposition to gravity. from all this, it will be seen that the weight of a body is more of the nature of an accidental rather than an essential property of matter, whereas its mass is a necessary and unvarying property. hence we speak with propriety of the conservation of mass just as we speak with equal propriety of the conservation of energy; but we may never speak or write of the conservation of weight. the mass of our iron ball is precisely the same away from the surface of the earth as it is anywhere on the surface, whether a thousand miles below the surface or a thousand miles above it; and the same it would be found in any part of the solar system or of the starry universe to which it might be taken. since weight is nothing else than the pull which the earth exerts on a body, it follows that, big and massive as our planet is, it must, nevertheless, be weightless; for it cannot with any degree of propriety be said to pull itself. it is incapable of producing even an infinitesimal change in the position of its mass-center, or center of gravity, as this centroid is sometimes called. the earth attracts _itself_ with no force whatever; but is attracted and governed in its annual movement by the sun, the central controlling body of our system, while the moon and planets play only the part of petty disturbers. it would, however, be right to speak of the _weight_ of the earth _relatively_ to the sun; for the sun attracts the mass of our planet with a certain definite force, readily calculable from the familiar formula for central force, viz., mv^ /r., in which _m_ is the mass of the earth, _v_ its orbital velocity and _r_ its distance from the sun. supplying the numbers, the weight of the earth relatively to the sun, comes out to be , , , or × ^{ } tons weight, or, in words, three million million million tons weight. it may here be noted that the velocity _v_ of the earth in its orbit is a varying quantity, depending on distance from the sun. as this distance is least in december and greatest in june, it follows that the earth is heavier relatively to the sun in winter than it is in summer. the _mass_ of the earth, on the other hand, is not a relative and variable quantity, but a constant and independent one, which would not be affected either by the sudden annihilation of all the other members of the solar system or by the instantaneous or successive addition of a thousand orbs. mass being the product of volume by density, that of the earth is , , , or × ^{ } tons mass, which reads six thousand million million million tons mass. the number which expresses the mass of the earth is thus very different from that which represents its weight relatively to the sun. it is obvious that the latter would be a much greater quantity if our planet were transferred to the orbit of venus and very much less if transferred to that of far-off jupiter, but the number which expresses its _mass_ would remain precisely the same in both cases, viz., the value given above. in elaborating his theory of magnetism, and especially his magnetic theory of the earth, gilbert made extensive use of lodestone-globes, which he called "terrellas," _i.e._, miniature models of the earth. in pursuing his searching inquiry, he was gradually led from these "terrellas" to his great induction that the earth itself is a colossal, globe-like magnet. following norman, "the ingenious artificer," of limehouse, london, he also showed that the entire cubical space which surrounds a lodestone is an "orb of virtue," or region of influence, from which he inferred that the earth itself must have its "orb of virtue," or magnetic field, extending outward to a very great distance. gilbert does not, for a moment, think that this theory of terrestrial magnetism, the first ever given to the world, is a wild speculation. far from it; he is convinced that "it will stand as firm as aught that ever was produced in philosophy, backed by ingenious argumentation or buttressed by mathematical demonstration." if the earth has a magnetic field, he argued, why not the moon, the planets and the sun itself, "the mover and inciter of the universe"? given these planetary magnetic fields, gilbert seems to have no difficulty in finding out the forces necessary to account for the crucial difficulties of the copernican doctrine. nor is the medium absent that is needed for the mutual action of magnetic globes, for we are assured that it is none other than the universal _ether_, which, he says "is without resistance." gilbert disposes of the cosmographic puzzle of the "suspension" of the earth in space by saying, and saying justly, that the earth "has no heaviness of its own," and, therefore, "does not stray away into every region of the sky." to emphasize the statement, he continues: "the earth, in its own place, is in no wise heavy, nor does it need any balancing"; and again, "the whole earth itself has no weight." "by the wonderful wisdom of the creator," he elsewhere says, "forces were implanted in the earth that the globe itself might with steadfastness take direction." gilbert holds that the daily rotation of the earth on its axis is also caused, and maintained with strict uniformity, by the same prevalent system of magnetic forces, for "lest the earth should in divers ways perish and be destroyed, she rotates in virtue of her _magnetic energy_, and such also are the movements of the rest of the planets." just how this magnetic energy acts to produce the rotatory motion of a massive globe gilbert does not say. nor was he able to solve such a magnetic riddle, for there was nothing in his philosophy to explain how a lodestone-globe in free space should ever become a perpetual magnetic motor. oddly enough he disagrees with peregrinus, who maintained in his _epistola_, , that a terrella, or spherical lodestone, poised in the meridian, would turn on its axis regularly every hours. he naively says: "we have never chanced to see this; nay, we doubt if there is such a movement." continuing, he brings out his clinching argument: "this daily rotation seems to some philosophers wonderful and incredible because of the ingrained belief that the mighty mass of earth makes an orbital movement in hours; it were more incredible that the moon should in the space of hours traverse her orbit or complete her course; more incredible that the sun and mars should do so; still more that jupiter and saturn; more than wonderful would be the velocity of the fixed stars and firmament." here he finds himself obliged to berate ptolemy for being "over-timid and scrupulous in apprehending a break up of this nether world were the earth to move in a circle. why does he not apprehend universal ruin, dissolution, confusion, conflagration and stupendous celestial and super-celestial calamities from a motion (that of the starry sphere) which surpasses all imagination, all dreams and fables and poetic license, a motion ineffable and inconceivable?" gilbert is not clear and emphatic on the other doctrine of copernicus, the revolution of the earth and planets around the sun. he does, however, say that each of the moving globes "has circular motion either in a great circular orbit or on its own axis, or in both ways." again: "the earth by some great necessity, even by a virtue innate, evident and conspicuous, is turned circularly about the sun." elsewhere he affirms that the moon circles round the earth "by a magnetic compact of both." he returns to this point in his _de mundo nostro_, saying, "the force which emanates from the moon reaches to the earth; and, in like manner, the _magnetic virtue_ of the earth pervades the region of the moon." we have here an implied interaction between two magnetic fields, rather a clever idea for a magnetician of the sixteenth century. in one case, the reaction is between the field of the earth and that of the moon, compelling the latter to rotate round its primary once every month; and the second, between the field of the earth and that of the sun, compelling our planet to revolve round the center of our system once every year. though an inefficient cause of the annual motion of our planet, this interaction of two magnetic fields had, nevertheless, something in common with the idea of the mutual action of material particles postulated in the newtonian theory of universal gravitation. this magnetic assumption by which gilbert sought to defend the theory of the universe propounded by copernicus was a very vulnerable point in his astronomical armor which was promptly detected and fiercely assailed by a galaxy of continental writers; all of them churchmen, physicists and astronomers of note. they accepted gilbert's electric and magnetic discoveries and warmed up to his experimental method; they did not discard his theory of terrestrial magnetism, but rejected and scoffed at the use which he made of it to justify the heliocentric theory. they poked fun at the english philosopher for his magnetic hypothesis of planetary rotation and revolution, and succeeded in discrediting the copernican doctrine. error prevailed for a time, but newton's _principia_, published in , gave the ptolemaic system the _coup de grâce_. gilbert's hypothesis of the interaction of planetary magnetic fields gave way to universal gravitation, and copernicanism was finally triumphant. throughout the pages of gilbert's treatise, he shows himself remarkably chary in bestowing praise, but surprisingly vigorous in denunciation. st. thomas is an instance of the former, for it is said that he gets at the nature of the lodestone fairly well; and it is admitted that "with his godlike and perspicacious mind, he would have developed many a point had he been acquainted with magnetic experiments." taisnier, the belgian, is an example of the latter, whose plagiarism from peregrinus wrings from our indignant author such withering words as "may the gods damn all such sham, pilfered, distorted works, which so muddle the minds of students!" besides his treatise on the magnet, gilbert is the author of an extensive work entitled, "de mundo nostro sublunari," in which he defends the modern system of the universe propounded by copernicus and gives his views on important cosmical problems. this work was published after the author's death, first at stettin in , and again at amsterdam in . chancellor bacon was well acquainted with this treatise of our philosopher; indeed he had in his collection the only two manuscript copies ever made, one in latin and the other in english, a very singular and significant fact in view of the chancellor's attitude toward gilbert. putting it crudely, one would like to know how he obtained possession of the manuscripts and what was his motive in keeping them hidden away from the philosophers of the day. "it is considered surprising," writes prof. silvanus p. thompson, "that bacon, who had the manuscripts in his possession and held them for years unpublished, should have written severe strictures upon their dead author and his methods, while at the very same time posing as the discoverer of the inductive method in science, a method which gilberd (gilbert) had practised for years before."[ ] that bacon was no admirer of gilbert's physical and cosmical theories the following passages will show. in the "novum organum" the chancellor wrote: "his philosophy is an instance of extravagant speculation founded on insufficient data"; again, "as the alchemists made a philosophy out of a few experiments of the furnace, gilbert, our countryman, hath made a philosophy out of the lodestone" ("the advancement of learning"); lastly, "gilbert hath attempted a general system on the magnet, endeavoring to build a ship out of materials not sufficient to make the rowing-pins of a boat" ("de augmentis scientiarum"). one is tempted to ask how this strange disregard which bacon entertained for the scientific views of the greatest natural philosopher of his age and country came to exist? was it due to a feeling of jealousy that could not brook a rival in the domain of the higher philosophy, or was it because bacon, the anti-copernican, wanted to write down gilbert, the defender of the heliocentric theory, in the british isles? when reading bacon's depreciatory remarks we have to remember that his mathematical and physical outfit was very limited even for the age in which he lived; from which it is safe to infer that he was but little qualified to pass judgment on the value of the electric and magnetic work accomplished in the workshops at colchester or on the theories to which they gave rise. bacon deserves praise for denouncing the prevalent system of natural philosophy which was mainly authoritative, speculative and syllogistic instead of experimental, deductive and inductive, but he was inconsistent and forgetful of his own principles when he belittled the greatest living enemy of mere book-learning, and the most earnest advocate, by word and example, of the laboratory methods for the advancement of learning. to avoid misapprehension, it should be here stated that bacon was not always censorious in his treatment of his illustrious fellow-citizen, for in several places he writes approvingly of the electric and magnetic experiments contained in _de magnete_, which he calls in his _advancement of learning_, "a painfull (_i.e._, painstaking) experimentall booke." in other places he draws so freely on gilbert without acknowledgment as to come dangerously near the suspicion of plagiarism. gilbert died, probably of the plague, in the sixtieth year of his age, on december th, , and was buried in the chancel of holy trinity church, colchester, where a mural tablet records in latin the chief facts of his life. dr. fuller in his "worthies of england" ( ) describes gilbert as tall of stature and cheerful of "complexion," a happiness, he quaintly remarks, not ordinarily found in so hard a student and retired a person." concluding his appreciation of the philosopher, fuller writes: "mahomet's tomb at mecha[ ] is said strangely to hang up, attracted by some invisible loadstone; but the memory of this doctor will never fall to the ground, which his incomparable book _de magnete_ will support to eternity." animated by a similar spirit of national pride, dryden wrote gilbert shall live till loadstones cease to draw, or british fleets the boundless ocean awe. we shall close these remarks by hallam's estimate of gilbert as a scientific pioneer, contained in his _introduction to the literature of europe_. "the year ," he says, "was the first in which england produced a remarkable work in physical science; but this was one sufficient to raise a lasting reputation for its author. gilbert, a physician, in his latin treatise on the magnet, not only collected all the knowledge which others had possessed on the subject, but became at once the father of experimental philosophy in this island; and, by a singular felicity and acuteness of genius, the founder of theories which have been revived after a lapse of ages and are almost universally received into the creed of science." for well-nigh three hundred years, _de magnete_ remained untranslated, being read only by the scholarly few. the first translation was made by p. fleury mottelay, of new york, and published by messrs. wiley and sons in the year . mr. mottelay has given much attention to the bibliography of the twin sciences of electricity and magnetism, as the foot-notes which he has added to the translation abundantly prove. a second translation appeared in the tercentenary year, , and was the work of the members of the gilbert club, london, among whom were dr. joseph larmor and prof. silvanus p. thompson. it is a page-for-page translation with facsimile illustrations, initial letters and tail-pieces. as one would infer from the numerous references contained in _de magnete_, gilbert had a considerable collection of valuable books, classical and modern, bearing on the subject of his life-work; but these, as well as his terrellas, globes, minerals and instruments, perished in the great fire of london, , with the buildings of the college of physicians, in which they were located. a portrait of gilbert was preserved in the bodleian library, oxford, for many years; but has long since disappeared from its walls. on the occasion of the three hundredth anniversary ( ) of gilbert's death, a fine painting representing the doctor in the act of showing some of his electrical experiments to queen elizabeth and her court (including sir walter raleigh, sir francis drake and cecil, lord burleigh, famous secretary of state), was presented to the mayor of colchester by the london institute of electrical engineers. a replica of the painting was sent to the st. louis exposition, , where it formed one of the attractions of the electricity building. the house in which gilbert was born ( ) still stands in holy trinity street, colchester, where it is frequently visited by persons interested in the history of electric and magnetic science. brother potamian. footnotes: [ ] "souvenir of gilberd's tercentenary," p. . [ ] see magnetic myths, page . chapter iii. franklin and some contemporaries. as already seen, the writers of greece and rome knew little about the lodestone; we have now to add that the knowledge of electricity which they possessed was of the same elementary character. they knew that certain resinous substances, such as amber and jet had, when rubbed, the property of attracting straws, feathers, dry leaves and other light bodies; beyond this, their philosophy did not go. the middle ages added little to the subject, as the schoolmen were occupied with questions of a higher order. the saxon heptarchy came and went, alcuin taught in the schools of charlemagne, cardinal langton compelled a landless and worthless king to sign magna charta, universities were founded with papal sanction in italy, france, germany, england and scotland, copernicus wrote his treatise on the revolution of heavenly bodies and dedicated it to pope paul iii., tycho brahé made his famous astronomical observations at uranienborg and befriended at prague the penniless kepler, and columbus gave a new world to castile and leon--all this before the man appeared who, using amber as guide, discovered a new world of phenomena, of thought and philosophy. this man was no other that gilbert, whose discoveries in magnetism were described in an earlier chapter. the trunk line of his work was magnetism; electricity was only a siding. one was the main subject of a life-long quest while the other was only a digression. it was a digression in which the qualities of the native-born investigator are seen at their very best: alertness and earnestness, resourcefulness and perseverance, all rewarded by a rich harvest of valuable results. it is refreshing and inspiring to read the second book of gilbert's treatise, _de magnete_, in which are recorded in quick succession the twenty important discoveries which he made in his new field of labor. [illustration: fig. gilbert's "versorium" or electroscope] at the very outset, he found it necessary to invent a recording instrument to test the electrification produced by rubbing a great variety of substances. this he appropriately called a _versorium_; we would call it an electroscope. "make to yourself," he says, "a rotating needle of any sort of metal three or four fingers long and pretty light and poised on a sharp point." he then briskly rubs and brings near his versorium glass, sulphur, opal, diamond, sapphire, carbuncle, rock-crystal, sealing-wax, alum, resin, etc., and finds that all these attract his suspended needle, and not only the needle, but everything else. his words are remarkable: "all things are drawn to electrics." here is a great advance on the amber and jet, the only two bodies previously known as having the power to attract "straws, chaff and twigs," the usual test-substances of the ancients. pursuing his investigations, he finds numerous bodies which perplex him, because when rubbed they do not affect his electroscope. among these, he enumerates: bone, ivory, marble, flint, silver, copper, gold, iron, even the lodestone itself. the former class he called _electrica_, electrics; the latter was termed _anelectrica_, non-electrics. to gilbert we, therefore, are indebted for the terms electric and electrical, which he took from the greek name for amber instead of succinic and succinical, their latin equivalents. the noun electricity was a coinage of a later period, due probably to sir thomas browne, in whose _pseudodoxia epidemica_, , it occurs in the singular number on page and in the plural on page . it may interest the reader to be here retold that we owe the chemical term _affinity_ to albertus magnus, _barometer_ to boyle, _gas_ to van helmont, _magnetism_ to barlowe, magnetic _inclination_ to bond, electric _circuit_ to watson, electric _potential_ to green, _galvanometer_ to cumming, _electro-magnetism_ to kircher, _electromagnet_ to sturgeon, and _telephone_ to wheatstone. gilbert was perplexed by the anomalous behavior of his non-electrics. he toiled and labored hard to find out the cause. he undertook a long, abstract, philosophical discussion on the nature of bodies which, from its very subtlety, failed to reveal the cause of his perplexing anomaly. gilbert failed to discover the distinction between conductors and insulators; and, as a consequence, never found out that similarly electrified bodies repel each other. had he but suspended an excited stick of sealing-wax, what a promised land of electrical wonders would have unfolded itself to his vision and what a harvest of results such a reaper would have gathered in! from solids, gilbert proceeds to examine the behavior of liquids, and finds that they, too, are susceptible of electrical influence. he notices that a piece of rubbed amber when brought near a drop of water deforms it, drawing it out into a conical shape. he even experiments with smoke, concluding that the small carbon particles are attracted by an electrified body. some years ago, sir oliver lodge, extending this observation, proposed to lay the poisonous dust floating about in the atmosphere of lead works by means of large electrostatic machines. he even hinted in his royal institution lecture that they might be useful in dissipating mists and fogs, and recommended that a trial be made on some of our ocean-steamers. gilbert next tries heat as an agent to produce electrification. he takes a red-hot coal and finds that it has no effect on his electroscope; he heats a mass of iron up to whiteness and finds that it, too, exerts no electrical effect. he tries a flame, a candle, a burning torch, and concludes that all bodies are attracted by electrics save those that are afire or flaming, or extremely rarefied. he then reverses the experiment, bringing near an excited body the flame of a lamp, and ingenuously states that the body no longer attracts the pivoted needle. he thus discovered the neutralizing effect of flames, and supplied us with the readiest means that we have to-day for discharging non-conductors. he goes a step further; for we find him exposing some of his electrics to the action of the sun's rays in order to see whether they acquired a charge; but all his results were negative. he then concentrates the rays of the sun by means of lenses, evidently expecting some electrical effect; but finding none, concludes with a vein of pathos that the sun imparts no power, but dissipates and spoils the electric effluvium. professor righi has shown that a clean metallic plate acquires a positive charge when exposed to the ultraviolet radiation from any artificial source of light, but that it does not when exposed to solar rays. the absence of electrical effects in the latter case is attributed to the absorptive action of the atmosphere on the shorter waves of the solar beam. of course gilbert permits himself some speculation as to the nature of the agent with which he was dealing. he thought of it, reasoned about it, pursued it in every way; and came to the conclusion that it must be something extremely tenuous indeed, but yet substantial, ponderable, material. "as air is the effluvium of the earth," he says, "so electrified bodies have an effluvium of their own, which they emit when stimulated or excited"; and again: "it is probable that amber exhales something peculiar that attracts the bodies themselves." these views are quite in line with the electronic theory of electricity in vogue to-day, which invests that elusive entity with an atomic structure. it is held that the tiny particles or electrons that are shot out from the cathode terminal of a vacuum tube with astounding velocity are none other than particles of negative electricity, pure and simple. they have mass and inertia, both of which properties are held to be entirely electrical, though quite analogous to the mass and inertia of ordinary, ponderable matter. history shows that scientific theories have their periods of infancy, maturity and decay. when they have served their purpose, like the scaffolding of a building, they are removed from sight and stored away, say, in a limbo of discarded philosophy, for use of the historian of science or of the metaphysician writing on the nature of human knowledge. such was the fate of gilbert's "effluvium" theory of electricity, of the fluid theories of dufay and franklin, and the ether-strain theory of recent years. "each physical hypothesis," says prof. fleming, "serves as a lamp to conduct us a certain stage in the journey. it illumines a limited portion of the path, throwing light before and behind for some distance; but it has to be discarded and exchanged at intervals because it has become exhausted and because its work is done." it is a little surprising that the phenomenon of electrical repulsion should have escaped the attention of one so skilled in experimentation as gilbert. yet such was the case; and gilbert even went so far as to deny its very existence, saying, "electrics attract objects of every kind; they never repel." this error reminds one of gilbert's own saying that "men of acute intelligence, without actual knowledge of facts, and in the absence of experiment, easily slip and err." just twenty-nine years after gilbert had penned this aphorism, there appeared in ferrara an extensive work on electric and magnetic philosophy, by the jesuit cabeo, in which this electrical repulsion was recognized and described. having rubbed one of his electrics, cabeo noticed that it attracted grains of dust at first and afterward repelled them suddenly and violently. in the case of threads, hairs or filaments of any kind, he observed that they quivered a little before being flung away like sawdust. this self-repelling property of electricity, described in the year , opened up a new field of inquiry, which was actively explored by a number of brilliant electricians in england and on the continent. this was especially the case after the building of the first frictional machine by otto von guericke in . the burgomaster of magdeburg had already acquired european fame by the original and sensational experiments on atmospheric pressure which he made in presence of the emperor and his nobles in solemn diet assembled ( ). von guericke seems to have been of a mind with gilbert concerning writers on natural science who treat their subjects "esoterically, miracle-mongeringly, abstrusely, reconditely, mystically"; for he affirms that "oratory, elegance of diction or skill in disputation avails nothing in the field of natural science." von guericke's machine consisted of a ball of sulphur, with the hand of the operator or assistant as rubber. some years later, the sulphur ball was replaced by newton (some say hauksbee) by a glass globe, which, in turn, was exchanged for a glass cylinder by gordon, a scotch benedictine, who was professor of natural philosophy in the university of erfurt. in , martin de planta, of sus, in switzerland, constructed a plate-machine which was subsequently improved by ramsden of london. the frictional machine, as it was rightly called, has been superseded by the influence machine, a type of static generator which is at once efficient, reliable and easy of operation. the best known form for laboratory use is that of wimshurst ( - ), of london. andrew gordon, the scotch benedictine to whom reference has just been made, was a man of an inventive turn of mind. besides, the cylindrical electric machine which he constructed, he devised several ingenious pieces of electrical apparatus, among which are the _electric chimes_ usually ascribed to franklin. they are fully described in his _versuch einer erklärung der electricität_, published in . on page , he says that he was led to try an electrical method of ringing bells; and then adds: "for this purpose i placed two small wine-glasses near each other, one of which stood on an electrified board, while the other, placed at a distance of an inch from it, was connected with the ground. between the two, i suspended a little clapper by a silk thread, which clapper was attracted by the electrified glass and then repelled to the grounded one, giving rise to a sound as it struck each glass. as the clapper adhered somewhat to the glasses, the effect on the whole was not agreeable. i, therefore, substituted two small metallic gongs suspended one from an electrified conductor and the other from a grounded rod, the gongs being on the same level and one inch apart. when the clapper was lowered and adjusted, it moved at once to the electrified bell, from which it was driven over to the other, and kept on moving to and fro, striking the bell each time with pleasing effect until the electrified bell lost its charge." in the illustration, _a_ is connected with the electrified conductor; _b_ is the insulated clapper; _c_ the grounded gong. [illustration: fig. gordon's electric chimes, ] gordon's book was published in erfurt in , while the year is that in which franklin applied the chimes to his experimental rod to apprise him of the approach of an electric storm, an application which was original and quite in keeping with the practical turn of mind that characterized our journeyman-printer, philosopher and statesman. unquestionably, franklin had all the ingenuity and constructive ability needed to make such an appliance; but there is no evidence that he actually invented it. though franklin neither claimed nor disclaimed the chimes as his own, all his admirers would have preferred less reticence on his part when the discoveries and inventions of contemporary workers in the electrical field were concerned. he had attained sufficient eminence to permit him to look appreciatingly and encouragingly on the efforts of others. gordon also invented a toy electric motor in which rotation was effected by the reaction of electrified air-particles escaping from a number of sharp points. one of these motors consisted of a star of light rays cut from a sheet of tin and pivoted at the center, with the ends of the rays slightly bent aside and all in the same direction. when electrified, gordon noticed that the star required no extraneous help to set it in motion. it was a self-starting electric-motor. in the dark, the points were tipped with light, and as they revolved traced out a luminous circle which "could neither be blown out nor decreased." the reader will recognize in this description taken from gordon's _versuch_, page , the _electric whirl_ of the lecture-table; gordon's name is never associated with it, but that of hamilton (hamilton's "fly" or hamilton's "mill") sometimes is! this irrepressible monk seems to have been one of the earliest electrocutors, for it is said that many an innocent chaffinch fell victim to discharges from his machine; and we would be disposed to think of him as a wizard on learning that he ignited spirits by using an electrified stream of water, to the astonishment and mystification of the spectators. abbé menon was kinder to the feathered tribe than his black-cowled brother of erfurt; he did not subject them to a powerful discharge, but rather to a gentle electrification for the purpose of determining what physical or physiological effect the agent would have on the animal system. the abbé found that cats, pigeons, sparrows and chaffinches lost weight by being electrified for five or six hours at a time, from which he concluded that electricity augments the slow, continuous perspiration of animals. the same was found to take place with the human body itself. the reader will remember that stephen gray in suspended a boy by means of silken cords for the purpose of electrification; abbé nollet did the same, and doubtless his friend abbé menon adopted a similar mode of insulation for complacent electrical subjects. an easier mode of operating would have been to make the child stand on a cake of resin, the insulating property of which had been discovered by stephen gray. about this time, , franklin appears on the scene, and though he devoted but nine years ( - ) of his life to the study of electricity, he made discoveries in that fascinating branch of human knowledge that will hand his name down the centuries. franklin's life is interesting and instructive on account of the difficulties which he met and overcame, for his strength of will, tenacity of purpose, the philosophy which he followed, his devotedness to science, and the success which he achieved. our philosopher's moral code comprised the thirteen virtues of temperance, silence, order, resolution, frugality, industry, sincerity, justice, moderation, cleanliness, tranquility, chastity and humility. to each of these virtues franklin attached a precept which makes edifying reading even at the present day: _temperance_, eat not to dullness, drink not to elation; _silence_, speak not but what may benefit others or yourself, avoid trifling conversation; _order_, let all your things have their places, let each part of your business have its time; _resolution_, resolve to perform what you ought, perform without fail what you resolve; _frugality_, make no expense, but do good to others or yourself, _i.e._, waste nothing; _industry_, lose no time, be always employed in something useful, cut off all unnecessary actions; _sincerity_, use no hurtful deceit, think innocently and justly, and if you speak, speak accordingly; _justice_, wrong no one by doing injury or omitting the benefits that are your duty; _moderation_, avoid extremes, forbear resenting injuries so much as you think they deserve; _cleanliness_, tolerate no uncleanliness in body, clothes or habitation; _tranquility_, be not disturbed by trifles or accidents common or unavoidable; _chastity_ (no remark); _humility_, imitate jesus. this last virtue seems to have given franklin very much concern; for he admits that he had the appearance of humility, and immediately adds that in reality there is no passion of the human breast so hard to subdue as pride. he is shrewd enough to say that "even if i could conceive that i had completely overcome it, i should probably be proud of my humility." like many another, the virtue which gave him the most trouble was _order_, and this never became conspicuously apparent at any time of his long life. in his endeavors after the higher life, he seems to have been animated with the earnest spirit of the ascetic who binds himself to strive after perfection as laid down in the maxims and counsels of the gospel. it is not without surprise and perhaps a feeling too of self-condemnation, that we read the means which he adopted to reach a high moral standard. taking for granted that he had a true appreciation of right and wrong, he did not see why he should not always act according to the dictates of conscience. to improve himself morally and advance in the higher life, he adopted a means that should have proved effective. taking the first of the thirteen fundamental virtues, he applied himself to its acquisition for a whole week together, after which he took the second, then the third, and so on with the rest. he thought that by making daily acts of the virtue, it would become habitual with him at the end of the week. when the last of the thirteen virtues had received its share of attention, he returned to the first one on the list and proceeded round the cycle again. being a man of purpose and tenacity, he completed the circle of his chosen virtues four times a year; subsequently he extended the time of individual practise so as to take a whole year for the course; and later on, he devoted several years to the completion of his list. as an aid in this work of self-betterment, franklin examined himself daily, registering his failures in a little book which was ruled for the purpose, a column being allowed for each day and a line for each of the thirteen virtues. he naively tells us the result of this exercise of daily introspection in these words: "i am surprised to find myself so much fuller of faults than i had imagined; but i had the satisfaction of seeing them diminish." the evening examination of conscience was always concluded by the following prayer written by franklin himself: "o powerful goodness! bountiful father! merciful guide! increase in me that wisdom which discovers my truest interest. strengthen my resolutions to perform what that wisdom dictates. accept my kind offices to thy other children as the only return in my power for thy continual favors to me." an extensive reader, franklin found in thomson's poems some lines that appealed to him very strongly by the beauty of the sentiment expressed. he called them "a little prayer," which he recited from time to time: "father of light and life, thou lord supreme, oh, teach me what is good; teach me thyself. save me from folly, vanity and vice; from every low pursuit; and fill my soul with knowledge, conscious peace and virtue pure; sacred, substantial, never-failing bliss!" his was a praiseworthy attempt at emancipating himself from the thraldom of passion and raising himself to the high plane of perfection required by the master when he said "follow me." doubtless, as time wore on, he must have felt as many before and since, that the spirit is willing but the flesh is weak. in his autobiography, franklin attributes his success in business not only to his self-control, uniformity of conduct, philosophical indifference to slight or pique, but also to his habits of frugality, the result in part of his early training. "my original habits of frugality continuing," he says, "and my father having frequently repeated a proverb of solomon, 'seest thou a man diligent in his business? he shall stand before kings,' i from thence considered industry as a means of obtaining wealth and distinction, which encouraged me, tho' i did not think that i should ever literally _stand before kings_, which, however, has since happened." our aged philosopher proceeds to tell us of his good fortune with a little bit of pardonable vanity, to which, by the way, he was never a great stranger, despite his philosophy, acquired virtue, and staid character. referring to the kings of the earth, he informs us that he "_stood_ before five, and even had the honor of _sitting down_ with one to dinner." an important event in franklin's life was the founding by him of the first public library in the country in the year . though but twenty-six years of age, he seems to have been as well aware as any of the millionaire philanthropists of to-day, of the good that may be accomplished among common people by providing them with suitable reading matter. he watched with eagerness the progress of his experiment and was pleased with the success that crowned it. he observes that such libraries "tend to improve the conversation of americans and to make common tradesmen and farmers as intelligent (well-informed?) as most gentlemen from other countries." peter collinson, fellow of the royal society of london, who had dealings with some philadelphia merchants, was led to take an active interest in the library. this he did by sending over a number of books and papers relating to electricity together with an "electrical tube" with instructions for its use. these literary and scientific contributions sent from london from time to time, excited much interest among the charter members of the library company, and principally that of franklin himself. he had heard something of the new order of phenomena which was just then engaging the attention of european physicists. in the summer of , while on a visit to boston, his native place, he assisted at a lecture on electricity by a certain dr. spence, a scotchman, who sought to illustrate the properties of electrified bodies by such experiments as could be made with glass tubes and suitable rubbers, the rudimentary apparatus available at the time. franklin was impressed by what he saw and heard, even though he indulged in a little destructive criticism when he said that the experiments were "imperfectly made," because the lecturer was "not very expert." when franklin wrote those words, he knew by repeated and painful experience the difficulty of getting satisfactory results from rubbing glass tubes or rotating glass globes, owing to the provoking attraction which plain, untreated glass has for moisture. knowing this, he might have been less severe in his strictures on his friend, the peripatetic electrician. it is evident, however, that the experiments which he witnessed surprised and pleased him, for, having shortly afterward received some electrical tubes together with a paper of instructions, from his london friend, peter collinson, he set to work for himself without delay. we may well say of him that what his right hand found to do, he did calmly, but with all his might. a twelve-month had not elapsed before he wrote: "i never was engaged in any study that so totally engrossed my attention and time as this has lately done; for, what with making experiments when i can be alone and repeating them to my friends and acquaintance who, from the novelty of the thing, come continually in crowds to see them, i have had little leisure for anything else." ( .) here we see the calm, persistent character of the philosopher united with the affability and communicativeness of the gentleman. for the sake of encouraging others as well, perhaps, as through a sense of personal relief, franklin had a number of long tubes of large bore blown at the local glass-house, which tubes he distributed to his friends that they, too, might engage in research work. in this way, rubbing and rubbing of an energetic kind became quite an occupation in the franklin circle. kinnersley, whose name still survives in works on static electricity in connection with an electric "thermometer" which he devised, was among the band of ardent workers who ungrudgingly acknowledged franklin's superior acumen, comprehensive grasp of detail and wondrous insight into the mechanism of the new phenomena. if we say that franklin was not a genius, it is only for the purpose of adding that even in those early electrical studies he displayed an uncommon amount of the unlimited capacity for taking pains which is said to be associated with that brilliant gift. he tested all his results with great care and in a variety of ways before accepting any of them as final; and considered his explanations of them provisional, being ever ready to modify them or give them up altogether if shown to conflict with the simple workings of nature. as early as , the refined and tactful dufay, in france, showed by numerous experiments on woods, stones, books, oranges and metals that all solid bodies were susceptible of electrification. this was a notable advance which swept away gilbert's classification of bodies into electrics and non-electrics. the french physicist soon drew from his observations the conclusion that electrification produced by friction is of two kinds, to which he applied the terms vitreous and resinous, the former being developed when glass is rubbed with silk and the latter when amber or common sealing-wax is rubbed with flannel. he noticed, too, that silk strings repelled each other when both were touched either with excited glass or sealing-wax; but that they attracted each other when touched one with glass and the other with sealing-wax. from these observations, he deduced the electrostatic laws, that similarly electrified bodies attract while dissimilarly electrified bodies repel each other. the law of distance was discovered later by coulomb, who, in , showed that the law of repulsion as well as of attraction between two electrified particles varies inversely as the square of the distance. in the year , the law of the inverse square for magnets was stated by john michell, who expressed it by saying that the "attraction and repulsion decrease as the square of the distance from the respective poles increases." michell was fourth wrangler of his year ( - ), fellow of queen's college, cambridge, and inventor of the _torsion balance_, which, however, he did not live to use; but which, in the hands of cavendish, yielded important results on the mean density of the earth. coulomb probably re-invented the "balance" and applied the practical, laboratory instrument which he made it, to the study of the quantitative laws of electricity and magnetism. to observe and correlate phenomena is the special work of the physicist; to speculate on ultimate causes is the privilege of the philosopher. dufay was both. the theory which he offered was a simple one, even if untrue to nature. it was a good working hypothesis for the time being. according to this theory, there are two distinct, independent electrical fluids mutually attractive but self-repelling. with that postulate, dufay was able to offer a plausible explanation of a great many phenomena that puzzled the electricians of the time. franklin, however, held a different view; rejecting the dual nature of electricity, he propounded his one-fluid theory, which was found equally capable of explaining electrical phenomena. a body having an excess of the fluid was said to be _positively_ charged, while one with a deficit was said to be _negatively_ charged. the sign plus was used in one case and the sign minus in the other; and just as two algebraical quantities of equal magnitude but opposite sign give zero when added together, so a conductor to which equal quantities of positive and negative electricity would be given would be in the neutral state. the franklinian theory was welcomed in england, germany and italy, but it met with opposition in france from the brilliant abbé nollet and the followers of dufay. each of the rival theories affords a mental conception of the forces in play and also a consistent explanation of the resulting phenomena. their simplicity, and, at the same time, the comprehensiveness of explanation which they afford, will continue to give them a place in our text-books for many years to come. efforts are being made to apply the _electronic_ theory to the various phenomena of electrostatics, the electron being the smallest particle of electricity that can have separate, individual existence. it is many times smaller than the hydrogen atom, the smallest of chemical atoms, and it possesses all the properties of negative electricity. by the loss of one or more electrons, a body becomes positively electrified, whereas by the acquisition of one or more electrons it becomes negatively electrified. the electron at rest gives rise to the phenomena of electrostatics; in motion, it gives rise to electrical currents, electromagnetism and electric radiation. we do not know what led franklin to call positive the electrification of glass when rubbed with silk, and negative that of sealing-wax when rubbed with flannel. if he meant to imply that positive is the more important of the two, he erred, for many reasons can be given to show the preponderating influence of negative electricity; but it is too late now to change the terminology. if asked to point out differences between the physical effects of positive and negative electrification, we would refer to the positive brush, which is finer and much more developed than the negative; to the wimshurst machine, with its positive brushes on one side and negative "beads" on the other; to the positive charge acquired by a clean plate of zinc when exposed to ultraviolet light; to the ordinary vacuum tube in which there is a violet glow at the cathode end or negative terminal; to crookes's tubes, x-ray tubes and other high vacuum tubes, in which electrified particles, kelvin's _molecular torrent_, are shot out from the negative electrode with great velocity; and to arc-lamps using a direct current in which the plus carbon is hollowed out crater-like, has the higher temperature and wastes away twice as fast as the negative. the year is an _annus mirabilis_ in the history of electricity, for it was in the january of that year that an attempt to electrify water by musschenbroek, of leyden, led to the discovery of the principle of the electrostatic condenser. whatever may be thought of the claim for priority put forward in favor of dean von kleist, of cammin in pomerania, or of cunæus, of leyden, it is certain that the discovery became known throughout europe by the startling announcement and sensational description given of it by musschenbroek, a renowned professor of a renowned university. he was not only surprised but terror-stricken by the effect of the electric energy which he had unconsciously stored up in his little phial; for after telling his french friend réaumur, the physicist, that he felt the commotion in his arms, shoulders and chest, he added that he would not take another shock for the whole kingdom of france! a resolution destined to be broken, like so many others before and since. [illustration: fig. modern form of leyden jar with movable coatings] very different was the sentiment of bose, professor of physics in the university of wittenberg, who is credited with saying that he would like to die by the electric shock, that he might live in the memoirs of the french academy of sciences. the leyden jar became at once the scientific curiosity and universal topic of discussion of the time; and not only was it the curiosity, but also the _crux_ of the day, puzzling investigators, perplexing philosophers and giving rise to animated controversies. the mystery was soon dispelled, however, when franklin began in his searching inquiry into the electric conditions of each element of the jar. nothing escaped his subtle mind and nothing was left undone by his deft hand. the evidence of experiment and the logic of facts carried at last conviction even with londoners and parisians, who were wont to look upon americans as mere colonists, who had neither time nor opportunity for scientific pursuits, being obliged to hew their way through virgin forests or drive the roving indian back from their frontiers into the wilds of the west. the theory of the leyden jar given by franklin years ago has stood the test of time. it has met with universal acceptance; and, despite our manifold advances, but little of permanent value has been added to it. it is very interesting to follow the main lines of this magnificent research. franklin electrifies, in the usual way, water contained in a small flask, complaisantly taking the shock on completing the circuit. to find where the charge resides, whether in the hand of the operator, as some said, or in the water, as others maintained, he again electrifies the water and pours it into another flask, which fails, however, to give a shock, thus showing that the charge had not been carried over with the water. convinced that the charge was still somewhere in the first phial, he carefully poured water into it again; and found, to his intense satisfaction, that it was capable of giving an excellent shock. it was now clear to him that the energy of the charge was either in the hand of the experimenter or in the glass itself, or in both. to determine this nice point, he proceeds to construct a "jar" which could easily be taken to pieces. for this purpose, he selected a pane of glass; and, laying it on the extended hand, placed a sheet of lead on its upper surface. the leaden plate was then electrified; and when touched with the finger, a spark was seen and a shock felt. by the addition of another plate to the lower surface, the shocking power of this simple condenser was increased. in this efficient form he had a readily dissectible condenser, which allowed him to throw off and replace the coatings at will, and thereby to prove beyond cavil that the seat of the stored-up electric energy is not in the conductors, but in the glass itself. this was a discovery of the first magnitude and one destined to associate the name of franklin with those of the most eminent electricians down the ages. fig. shows the modern form of the jar with movable coatings. [illustration: fig. three coated panes in _series_] [illustration: fig. three panes in _parallel_] in the "fulminating" pane, as it came to be called, we have one of the eleven elements of franklin's historic battery of . it is interesting to notice that he was accustomed to connect his "panes" in series while charging (fig. ), but that he preferred to join similar coatings together, that is, to couple them in "parallel" (fig. ), for powerful discharges. fig. shows three jars in "parallel." later on, he arranged leyden jars so that the inside coating of one could be hooked to the outside coating of another, the first of the series hanging down from the prime conductor of the machine, while the last one was grounded. "what is driven out of the tail of the first," he quaintly says, "serves to charge the second; what is driven out of the second serves to charge the third, and so on." this has become known as the "cascade" method of charging a battery, owing to the flow of electricity from one jar to the next (fig. ). electricians, however, have discarded the picturesque "cascade" for the prosaic term of "series" or "tandem" arrangement. [illustration: fig. three jars in _parallel_] [illustration: fig. three jars in _cascade_] franklin also noticed that a phial cannot be charged while standing on wax or on glass, or even while hanging from the prime conductor, unless communication be formed between its outer coating and the floor, the reason given being that "the jar will not suffer a charging unless as much fire can go out of it one way as is thrown in by the other." ( .) following his very ingenious philadelphia friend and co-worker, kinnersley, he varies the mode of charging by electrifying the outside of the jar and grounding the inner coating; for "the phial will be electrified as strongly if held by the hook and the coating applied to the globe as when held by the coating and the hook applied to the globe." ( .) the globe here referred to is the glass globe of franklin's frictional machine of american make, which, when rotated, was electrified positively by contact with the hand or with a leather rubber. franklin also used a sulphur ball or "brimstone" globe, and observed that the electrification produced on it differed in kind from that developed on the glass globe. ( .) it may here be stated that the first to use a _leather cushion_ as a substitute for the hand in the frictional machine, was winkler, of leipzig ( ); the efficiency of the rubber was increased by canton, of london, who covered it with an _amalgam_ of tin and mercury ( ). bose, of wittenberg, had previously added the _prime-conductor_, which greatly augmented the electrical capacity and output of the machine. in franklin imitated the effect of lightning on the compasses of a ship by the action of a jar discharge on an unmagnetized steel needle. "by electricity," he says, "we have frequently given polarity to needles and reversed it at pleasure." similar experiments are made to-day in every lecture-course on static electricity; but the experimenter, when wise, does not announce beforehand which end of the needle will be north and which south, as he is just as likely to be wrong as right, the uncertainty being due to the fact that the discharge of a leyden jar is not a current of electricity in one direction, but rather a few sudden rushes or rapid surgings of electricity to and fro; in other words, it is oscillatory in character instead of being continuous in one direction. franklin did not know this; although he made a very pertinent remark in when he likened the mechanical condition of the glass of a charged jar to that of a bent rod or a stretched spring. "so, a straight spring," he says, "when forcibly bent must, to restore itself, contract that side which in the bending was extended, and extend that side which was contracted." franklin knew, of course, that the bent rod, when released, would swing to and fro a few times before settling down to its state of rest; but he failed to see the analogy between it and the strained glass of the charged leyden jar. it is to joseph henry ( - ), the faraday of america, that we owe the recognition and statement of the oscillatory character of the discharge from leyden jars and condensers generally. he discovered and published this cardinal fact in . his words deserve recording. "the discharge, whatever may be its nature, is not correctly represented (employing for simplicity the theory of franklin) by the single transfer of an imponderable fluid from one side of the jar to the other; the phenomenon requires us to admit _the existence of a principal discharge in one direction and then several reflex actions backward and forward, each more feeble than the preceding, until equilibrium is attained._"[ ] the italics are prof. henry's. it is precisely this oscillatory character of the spark-discharge that enables us to send out trains of electric waves into the all-pervading ether, and thus to communicate, by "wireless," with remote stations. having conclusively proved that the energy of a charged condenser resides in the dielectric, franklin next tries to find whether "the electric matter" in the case of conductors is limited to the surface or whether it penetrates to an appreciable depth. to ascertain this, he insulates a silver fruit-can and brings a charged ball, held by a silk thread, into contact with the outer surface. on testing after removal, he found that the ball retained some of its charge, whilst it lost all if allowed to touch the bottom of the vessel. surprised at this unexpected difference, he repeated the experiment again and again, only to find the ball every time without a trace of charge after contact with the interior of the vessel. this perplexed and puzzled him. "the fact is singular," he says, "and you require the reason? i do not know it. i find a frank acknowledgment of one's ignorance is not only the easiest way to get rid of a difficulty, but the likeliest way to obtain information, and therefore i practice it. i think it an honest policy. those who affect to be thought to know everything, often remain long ignorant of many things that others could and would instruct them in, if they appeared less conceited." this was in . cavendish in and coulomb in independently attacked the same problem; and having proved by their classic experiments that a static charge is limited to the surface of conductors, it was but a step to infer that such a distribution of electricity implies that the law of force between two elements of charge, or between two point-charges, is the law of the inverse square of the distance. it will also be remembered that faraday, not knowing what had been accomplished eighty years before in philadelphia, used for one of his best-known experiments an ice-pail, into which he lowered an electrified ball for the purpose of showing the exact equality of the induced and the inducing charge. the similarity of apparatus and mode of procedure are remarkable. in pursuing his work, franklin placed a charged jar on a cake of wax and other insulating materials, and drew sparks from it by touching successively the knob and the outer coating, repeating the process a great number of times to his infinite delight. he next attached a brass rod to the outside, bending it and bringing the other end close to the knob (fig. ) connected with the inner coating. between these two he suspended a leaden ball by a silk thread and found, as he expected, that it played to and fro between the terminals for a considerable time. observe that we have here a definite mass maintained in a state of reciprocating motion by a series of electric attractions and repulsions. we have in fact an electro-motor, closely resembling the star and the chimes of gordon, the benedictine, ; a mere toy, if you will, but still a remarkable invention. we repeat the same experiment to-day only with a little more harmony, by substituting for the knobs two little bells, which emit a soft, musical note when struck by the interhanging clapper. [illustration: fig. discharge by alternate contacts] this experiment has further significance, for, like gordon's chimes, it is an instance of the conveyance of electricity from one point of space to another by means of a material carrier, a mode of transfer which has since been called "electric convection," the full meaning of which was not revealed until rowland ( - ), made his famous experiment of in the laboratory of the university of berlin with a highly-charged, rapidly-revolving, ebonite disc. it was apropos of this experiment that the illustrious clerk maxwell, of the university of cambridge, wrote to his friend, professor tait, of edinburgh, saying that: "the mounted disc of ebonite had whirled before, but whirled in vain; rowland of troy, that doughty knight, convection currents did obtain, in such a disc, of power to wheedle from its loved north, the needle." we may here say that franklin was no stranger to the work done by the electrical pioneers of the old world, his diligent london friend, peter collinson, keeping him advised by means of letters, books and pamphlets, in which inspiration and practical hints must have been found. he certainly was well acquainted with the achievements of dr. watson and dr. bevis, of london, as well as with the theories and experiments of dufay and abbé nollet in paris. it is germane to the subject to say that dr. bevis used mercury and iron filings for the inner coating of his jars, as well as sheet lead for both. he also experimented with coated panes of glass instead of jars. about this, franklin wrote to collinson: "i perceive by the ingenious mr. watson's last book, lately received, that dr. bevis had used, before we had, panes of glass to give a shock; though till that book came to hand, i thought to have communicated it to you as a novelty." ( .) franklin gave way to a little pleasant humor when, in , he proposed to wind up the "electrical season" by a banquet à la lucullus, to be given to a few of his friends and fellow-workers, not in a sumptuously decorated hall, but _al fresco_, on the banks of the schuylkill. "a turkey is to be killed for our dinner by the electrical shock," he wrote, "and roasted by the electrical jack before a fire kindled by the electrical bottle, when the healths of all the famous electricians in england, holland, france and germany are to be drunk in electrified bumpers under the discharge of guns fired from the electrical battery." it is hardly to be supposed that such an elaborate program was carried out. indeed the difficulty of preparing the apparatus and getting it ready for action on the banks of a river were formidable enough to say the least. franklin, however, had a leyden battery capable of doing considerable electrocution, for with two jars of six gallons capacity each, he knocked six men to the ground; the same two jars sufficed to kill a hen outright, whereas it required five, he tells us, to kill a turkey weighing ten pounds. the "electrical bumper" was a wine-glass containing an allowance, let us say, of some favorite brand and charged in the usual way. on approaching the lips the two coatings would be brought within striking-distance and a spark would take place, if not to the delight of the performer, at least to the amusement of the on-lookers. it was subsequently remarked that guests whose upper lip was adorned with a moustache could quaff the nectar with impunity, as every bristle would play the part of a filiform lightning-rod and prevent the apprehended, disruptive discharge! not quite so humorous was his suggestion of a hammock to be used by timid people during an electric storm: "a hammock or swinging-bed, suspended by silk cords equally distant from the walls on every side, and from the ceiling and floor above and below, affords the safest situation a person can have in any room whatever; and which, indeed, may be deemed quite free from danger of any stroke of lightning." ( .) in his experiments on puncturing bodies by the spark-discharge, franklin does not fail to notice the double burr produced when paper is used.[ ] his words are: "when a hole is struck through pasteboard by the electrified jar, if the surfaces of the pasteboard are not confined or compressed, there will be a bur raised all round the hole on both sides the pasteboard, for the bur round the outside of the hole is the effect of the explosion every way from the centre of the stream and not an effect of direction." ( .) the spelling is franklin's _unreformed_. the to-and-fro nature of the discharge was thought, at a time, to account satisfactorily for the burr raised on each side of the pasteboard; but trowbridge, of harvard, has shown that even a unidirectional discharge, such as can be obtained by inserting a wet string or any high resistance in the circuit, would produce a double burr, from which we infer, confirming franklin, that this effect of the discharge is caused by the sudden expansion of air within the paper itself. by the year , franklin had reached the conclusion that the lightning of the skies is identical with that of our laboratories, basing his belief on the following analogies which he enumerates in the notes or "minutes" which he kept of his experiments: "the electric fluid agrees with lightning in these particulars: ( ) giving light; ( ) color of the light; ( ) crooked direction; ( ) swift motion; ( ) being conducted by metals; ( ) crack or noise in exploding; ( ) rending bodies it passes through; ( ) destroying animals; ( ) melting metals; ( ) firing inflammable substances; and ( ) sulphurous smell." but although he felt the full force of the analogical argument, franklin knew that the matter could not be finally settled without an appeal to experiment; and accordingly he adds: "the electric fluid is attracted by points; we do not know whether this property is in lightning. but since they agree in all the particulars wherein we can already compare them, is it not probable that they agree likewise in this? let the experiment be made." ( .) in writing to collinson in july, , he tells his london friend how the experiment may be made: "on the top of some high tower or steeple, place a kind of sentry-box--big enough to contain a man--and an electrical stand. from the middle of the stand let an iron rod rise and pass, bending out of the door, and then upright or feet, pointed very sharp at the end. if the electrical stand be kept clean and dry, a man standing on it, when such clouds are passing low, might be electrified and afford sparks, the rod drawing fire to him from the cloud." collinson brought some of franklin's letters to the notice of fellow-members of the royal society with a view to their insertion in the _philosophical transactions_ of that learned body; but even his epoch-making letter to dr. mitchell, of london, on the identity of lightning and electricity, was dismissed with derisive laughter. the royal society made amends in due time for their contemptuous treatment of the american philosopher by electing him member of the society and by awarding him the copley medal in . disappointed as he was, collinson collected franklin's letters and published them under the title of _new experiments and observations on electricity made at philadelphia in america_. the pamphlet appeared in , and was immediately translated into french by m. d'alibard at the request of the great naturalist count de buffon. the experiments described in the pamphlet, and especially that of the pointed conductor, were taken up in paris with great enthusiasm by de buffon himself, by d'alibard, a botanist of distinction, and by de lor, a professor of physics. following out the instructions given by franklin, they were all able to report success: d'alibard on may th, de lor on may th, and de buffon on may th, . de buffon erected his rod on the tower of his château at montbar; de lor, over his house in paris, and d'alibard, at his country seat at marly, a little town eighteen miles from paris. d'alibard was not at home on the eventful afternoon of may th; but before leaving marly, he had drilled a certain coiffier in what he should do in case an electric storm came on during his absence. though a hardy and resolute old soldier and proud of the confidence placed in him, coiffier grew alarmed at the long and noisy discharges which he drew from the _insulated_ rod on the afternoon of may th. while the storm was still at its height he sent for the prior of the place, raulet by name, who hastened to the spot, followed by many of his parishioners. after witnessing a number of brilliant and stunning discharges, the priest drew up an account of the incident and sent it, at once, by coiffier himself to d'alibard, who was then in paris. without delay d'alibard prepared a memoir on the subject which he communicated to the académie des sciences three days later, viz.: on may th. in the concluding paragraph, the polished academician pays a graceful tribute to the philosopher of the western world: "it follows from all the experiments and observations contained in the present paper, and more especially from the recent experiment at marly-la-ville, that the matter of lightning is, beyond doubt, the same as that of electricity; it has become a reality, and i believe that the more we realize what he (franklin) has published on electricity, the more will we acknowledge the great debt which physical science owes him." we may, in passing, correct the error of those who credit french physicists with having originated the idea of the pointed conductor. such writers should read the words of d'alibard in the beginning of his memoir, where he says: "en suivant la route que m. franklin nous a tracée, j'ai obtenu une satisfaction complète"; that is, "in following the way traced out by franklin, i have met with complete success." to franklin, therefore, belongs the idea of the pointed rod of , which became the lightning conductor of subsequent years; to the parisian savants belongs the great distinction of having been the first to make the experiment and verify the franklinian view of the identity of the lightning of our skies with the electricity of our laboratories. franklin had precise ideas on the action of his pointed conductors, clearly recognizing their twofold function: ( ) that of preventing a dangerous rise of potential by disarming the cloud; and ( ) that of conveying the discharge to earth, if struck. in some of his letters, he complains of people who concentrate their attention on the preventive function, forgetting the other entirely. "wherever my opinion is examined in europe," he wrote in , "nothing is considered but the probability of these rods preventing a stroke, which is only a part of the use i proposed for them; and the other part, their conducting a stroke which they may happen not to prevent, seems to be totally forgotten, though of equal importance and advantage." a favorite illustration of franklin's showing the discharging power of points, consisted in insulating a cannon ball against which rested a pellet of cork, hung by a silk thread. on electrifying the ball, the cork flies off and remains suspended at a distance, falling back at once, as soon as a needle is brought near the ball. ( .) he also used tassels consisting of fifteen or twenty long threads (fig. ), and even cotton-fleece, the filaments of which stand out when electrified, but come together when a pointed rod is held underneath. he also noticed that the filaments do not collapse when the point of the rod is covered with a small ball. ( .) [illustration: fig. tassel of long threads or light strips of paper] franklin's views on lightning-rods met with some opposition in france from the brilliant abbé nollet, and in england from dr. benjamin wilson. the latter was mainly instrumental in bringing about the famous controversy of "points _vs._ knobs." in , a committee was appointed by the royal society to consider the best means of protecting the powder-magazines at purfleet from lightning. on the committee with dr. wilson were henry cavendish, the distinguished chemist and physicist, and sir john pringle, president of the royal society. the report favored sharp conductors against blunt ones advocated by dr. wilson. five years later, in , the question was again brought up, and again the new committee decided in favor of pointed terminals, convinced "that the experiments and reasons made and alleged to the contrary by mr. wilson were inconclusive." dr. wilson, being a man of influence, succeeded in having his views taken up by the board of ordnance. it has been remarked that this controversy would never have attracted attention but for the fact that the discoverer of the effect of points was franklin. he was an american and the dispute with the colonies was then at its height. the war of the revolution had begun, and the british forces had already met with serious reverses. no patriot could, therefore, admit any good in points. george iii. took sides, decreed that the points on the royal conductors at kew should be covered with balls, and ordered sir john pringle to support dr. wilson. sir john gave the dignified answer: "sire, i cannot reverse the laws and operations of nature"; to which the king, incensed that so incompetent a man should hold such an important office, replied: "then, sir john, perhaps you had better resign," which sir john did. a wit of the time put the matter epigrammatically when he wrote: "while you, great george, for knowledge hunt and sharp conductors change to blunt, the nation's out of joint; franklin a wiser course pursues, and all your thunder useless views by keeping to the point." it was in connection with this heated controversy that franklin wrote the following admirable words: "i have never entered into any controversy in defence of my philosophical opinions. i leave them to take their chance in the world. if they are _right_, truth and experience will support them; if _wrong_, they ought to be refuted and rejected. the king's changing his _pointed_ conductors for _blunt_ ones is, therefore, a matter of small importance to me." it was not until september, , that franklin raised a rod over his own house. this experimental conductor was made of iron fitted with a sharp steel point and rising seven or eight feet above the roof, the other end being buried five feet in the ground. in order to avoid useless personal displacement, franklin, the economist of time, made an automatic annunciator similar to that devised by gordon in , and described by watson in his _sequel_, , to apprize him of the advent of a good thunder-gust. instead of making the rod of one continuous length, it was divided on the staircase, opposite his chamber door, the ends being drawn apart to a horizontal distance of a few inches. screwing a pair of tiny gongs to the ends, he suspended between them a brass ball, held by a silk thread, to act as clapper. whenever a thundercloud came hovering by, the bells began to ring, thereby summoning the philosopher to his "laboratory" on the staircase. franklin's rod, erected over his house in the summer of , was evidently intended by him for experimental rather than protective purposes. there is no doubt whatever in his mind about the use of such pointed conductors for the protection of buildings and ships against the destructive effects of lightning. he expressly says, in an article printed in _poor richard's almanack_ for , that "it has pleased god in his infinite goodness to mankind, to discover to them the means of securing their habitations and other buildings from mischief by thunder and lightning. the method is this: provide a small iron rod (it may be made of the rod-iron used by the nailers), but of such a length, that one end being ft. or ft. in the moist ground, the other may be ft. or ft. above the highest part of the building. to the upper end of the rod fasten about a foot of brass-wire, the size of a common knitting needle, sharpened to a fine point; the rod may be secured to the house by a few small staples. if the house or barn be long, there may be a rod and point at each end, and a middling wire along the ridge from one to the other. a house thus furnished will not be damaged by lightning, it being attracted by the points and passing through the metal into the ground without hurting anything. vessels also, having a sharp-pointed rod fixed on the top of their masts, with a wire from the foot of the rod reaching down round one of the shrouds to the water, will not be hurt by lightning." it is well known, as dr. rotch, director of the blue hill observatory, recently pointed out, that the matter for these almanacs was prepared by franklin himself under the pen-name of richard saunders. as the above passage appeared in the almanac for , it is obvious that it must have been ready sometime toward the end of . furthermore, we know that it was actually in the hands of the printer in the middle of october of that year, for the _pennsylvania gazette_ of oct. th says that the almanac was then in press and that it would be on sale shortly. whence it follows that the year is the year of the invention of the lightning rod, and not or as often stated. the instructions given by franklin include all the essentials necessary for the erection of a lightning conductor. it may be made of iron or copper, flat or round, but must make good "sky" and good "earth." the former condition is secured by screwing to the top of the rod either copper or platinum terminals ending in sharp points; and the latter, by burying the lower end deep in moist soil. between "sky" and "earth" the rod must be continuous. the function of the rod is twofold, as franklin well recognized, preventive and preservative. it prevents the stroke, under ordinary conditions, by the action of the points, which send off copious streams of air and dust particles electrified oppositely to that of the cloud. even at a distance, the dangerous potential of the cloud is reduced by these convection currents and the stroke ordinarily averted. it is clear that ten points are more efficacious than one, and fifty more than five. hence the number of points which we see distributed over the higher and more conspicuous parts of a building, all of which are carefully connected with the lightning conductor. however well a building may theoretically be protected, conditions will occasionally arise when the rod will inevitably be struck; its preservative function then comes into play, by which it carries the energy of the disruptive discharge safely to earth. the experience of more than a century shows that the lightning-rod affords protection in the great majority of cases; but it would be at least a mild exaggeration to say that it never failed, even when properly constructed. at first, the erection of lightning-rods was opposed in the new world as well as in the old: some based their opposition to the novelty on religious grounds, saying that, as lightning and thunder are tokens of divine wrath, it would be impious to interfere in any way with their manifestations. this objection was met by saying that for a parity of reason we should avoid protecting ourselves against the inclemencies of the weather. others opposed the use of the rods on the score that they invited or attracted the flash, which was answered by saying that they attract lightning as much as a rain-pipe attracts a shower, and no more. the death of professor richmann, of the university of st. petersburg, also tended to retard the adoption of the rod for the protection of buildings; but the invalidity of that objection became apparent when the circumstances of the accident became known. richmann's conductor was like d'alibard's ( ), an experimental rod, and as such was insulated at the lower end. it was, therefore, not a lightning-rod at all, inasmuch as it was not grounded. on august th, , during a violent electric storm, richmann happened to be close to his exploring rod observing the indications of a roughly-made electrometer, when a sharp thunder-clap was heard, and at the same instant a ball of fire was seen by richmann's assistant to dart from the apparatus and strike the head of the unfortunate professor, who fell over on a near-by chest and expired instantly. his assistant was stunned for a while. on regaining consciousness, he ran to the aid of the professor; but it was too late, the body was lifeless. in recording this tragic event, priestley, the historian of electricity, says that, "it is not given to every electrician to die in so glorious a manner as the justly envied richmann." for one, we do not "envy" professor richmann's fate, and we think that the phrase "tragic manner" would better suit the circumstances of his death than the "glorious manner" of dr. priestley. risks of a similar character were taken by franklin in philadelphia, de romas in bordeaux, and d'alibard's representative at marly, when experimenting with kites and insulated rods; they took their lives in their hands, though they may not have thought so. a few years ago, sir william preece said that a man might with impunity "clasp a copper rod an inch in diameter, the bottom of which is well connected with moist earth, while the top of it receives a violent flash of lightning; the conductor might even be surrounded by gunpowder in the heaviest storm without risk or danger." it is not on record that the english electrician ever clasped a lightning conductor or even stood in close proximity to one during an electric storm. the above statement was as sensational as it was unwise and foolhardy. the neighborhood of a rod during a storm is a zone of danger, owing to the electrical surgings which are set up in it, and, as such, is to be avoided. the death of richmann caused quite a sensation throughout europe, and naturally the lightning-rod came in for severe condemnation. among the memoirs to which the fatality gave rise was one written in the heart of moravia and addressed to the celebrated euler, director of the academy of sciences at berlin. the writer was a monk of the premonstratensian order, whose field of labor was at prenditz. in the year , this country priest made experiments with lightning conductors on a scale that transcended anything done in paris, london or philadelphia. the accompanying illustrations show the conductor which divisch (also diwisch) raised at prenditz (also brenditz) in the summer of that year to demonstrate publicly the efficacy of such apparatus in breaking up thunder-clouds and neutralizing the destructive energy pent up in their electric charges. prenditz, it would appear, suffered severely from electric storms; and it was mainly for the safety of the locality that the good priest devoted himself with earnestness to the study of electrical phenomena. as such a man deserves to live in the memory of posterity, we have sought out the leading facts of his career mainly from father alphons zák, of pernegg, in lower austria, a distinguished writer of the order to which divisch belonged, and have woven such details as we obtained from him and others into the simple narrative that follows. [illustration: fig. procopius divisch ( - )] procopius divisch (prokop diwisch) was born on aug. st, , at helkowitz-senftenberg in bohemia. he spent his youth at znaim, where he studied the humanities and philosophy at the college conducted by the jesuit fathers in that moravian city. in , when in his twenty-third year, he decided to quit the common ways of the world in order to lead the higher life in the premonstratensian order at kloster-bruck. at the ripe age of , divisch was ordained priest, in , after which he taught philosophy and theology to classes of young aspirants to the ecclesiastical state. in he went to the university of salzburg and won his double doctorate in theology and philosophy. three years later, in , he was appointed parish priest of prenditz, a small moravian town on the road to austerlitz, since of napoleonic fame. here he remained for five years, returning in to bruck as prior of the kloster or monastery situated there. at the end of the seven years' war of the austrian succession, he quitted bruck, in , for his parish at prenditz, where he spent the last twenty years of his life in the pastoral ministrations of his sacred office and in electrical experimentation, of which he was very fond. the curative property of the new agent was heralded throughout europe about this time in terms of unmeasured praise. some of divisch's ailing parishioners, believing him to be an expert in electrical manipulation, applied to him for a little alleviation of their woes. the good-hearted priest did not turn them away, but thought it desirable to treat them to the therapeutic effect of such sparks as he could get from his homemade frictional machine. the results were various, depending probably on the confidence and imagination of the patient. several remarkable cures seem to have been effected either by the electric spark or by the persuasive powers of the operator, or by both combined, with the result that people far and wide were divided in their opinion of the pastor of prenditz. some physicians said that he was interfering with their practice, and even clergymen found fault with him for indulging in work which they thought unsuited to the cloth. a general impression, too, seems to have prevailed that his electrical experiments, especially those with his lightning conductor, were likely to prove harmful in more ways than one. on the other hand, divisch had admirers in high places, among whom were the emperor francis i. of germany and his imperial consort, maria theresa. having been invited to vienna, divisch repaired to the austrian capital, where, with the aid of father franz, another electrical devotee, he gave a demonstration of the wonderful capability of the new form of energy before the grandees of the empire. when he came to the electrical property of points, he showed their discharging power in a very original way, one which must have made his assistant uneasy for a while. at times, the machine worked by father franz gave excellent results; at others, it failed to generate. it was noticed by the critical few that when the machine failed, divisch was close by; while when it worked normally, he was at some distance away. after a number of such alternations of success and failure which sorely perplexed the assistant, himself a man of renown in vienna, divisch explained the occurrence by saying, with a merry twinkle in his eye, that the failure of the machine to generate when he was close to it, apparently seeking out the cause of the breakdown, was due to a number of pin-like conductors which he had concealed for the purpose in his peruke and which neutralized the charge on the rotating generator! the identity of the lightning of our skies with the artificial electricity of our laboratories was suspected by many before the middle of the eighteenth century. englishmen like hauksbee, hall, gray, freke, martin and watson; germans like bose and winkler, and frenchmen like abbé nollet, had already published their suspicions and conjectures anent the matter. franklin, too, had indicated twelve points of analogy between the two, in , in his letter to collinson, of london. though he felt the force of the analogical agreement, he also felt that the matter could not be definitely settled without an appeal to experiment. accordingly, he added: "the electric fluid is attracted by points; we do not know whether this property is in lightning. but since they agree in all the particulars wherein we can already compare them, is it not probable that they agree likewise in this? let the experiment be made." [illustration: fig. the divisch lightning conductor ( )] [illustration: fig. ] the experiment was made by franklin himself by means of his kite two years later, in the summer of , and also by the lightning-rod which he erected over his own house in the autumn of the same year. doubtless divisch heard of the marvelous effects obtained from d'alibard's insulated conductor at marly; at any rate, he erected in an open space at some little distance from his rectory at prenditz, a lightning conductor feet in height. as will be seen from the illustration, it bristled with points, for the bohemian wizard argued rightly that five points would be more efficient than one, and more efficacious than five. the weird-looking structure destined to ward off the lightning of heaven had no less than well-distributed points. lodge says in his _lightning conductors_: "points to the sky are recognized as correct; only i wish to advocate more of them, any number of them, like barbed wire along ridges and eaves. if you want to neutralize a thunder-bolt, three points are not as effective as ." this was written in ; nearly years before that date, we find a simple parish priest of an obscure village in moravia using precisely such a multiple system of short, pointed conductors for the protection of life and property. this lightning conductor or _meteorological machine_, as divisch called it, was erected by him at prenditz on june th, . on the top of the rod will be seen three light vanes, which were added in the interest of the feathered race in order to prevent incautious members from incurring the risk of electrocution by alighting on the apparatus during a storm. the wind whirled the vanes round like the cups of an anemometer, and thus kept the birds away from the zone of danger. [illustration: fig. set of pointed rods] several trials came to the electrical pastor, and from quarters least expected. it happened in the second year after the erection of the apparatus that the summer was unusually dry, in consequence of which the crops failed almost completely. the farmers of the neighborhood were always suspicious of the strange-looking mast of prenditz; and, be it said, that they were more than diffident about the propriety of interfering with the forces of nature even under the plea of protection, forgetting that they took great care to protect themselves against heat and cold, rain, snow and hail. the country ladies, no doubt, used parasols for one kind of protection; and the gentry, umbrellas for another. anyhow, the people of prenditz and the good folk around did not like the lofty mast, with its outstretched arms and bristling rows of suspicious-looking iron points connected to the ground by means of four long, heavy chains. for the nonce, they deemed their pastor a queer fellow, who thought that he could avert the anger of heaven by the oddest kind of a machine which they ever laid their eyes on. it was argued in the councils of the hamlets that, whatever advantages divisch claimed for his "machine," they were all of a negative character. it _prevented_ the lightning stroke, he said; that might be, but they did not _see_ the prevention. what they did see and keenly realize was the failure of their crops. that affected them very closely; and if, as they supposed, the apparatus of prenditz had anything to do with it, the sooner they got rid of the machine the better. divisch, it must be said, was liked by his people; but despite his popularity, the men of violence carried the day and the machine was doomed. popular passion, excited by personal interest, got the better of the consideration due to the pastor. on an appointed day, a band of bellicose farmers came down on the village and wrecked the apparatus which had cost the priest so much thought and manual labor and on which, knowingly and justly, he relied for the protection of the homesteads of his rustic flock. this recalls a similar incident of mob violence which occurred at st. omer in the north of france, where a manufacturer of that quaint old town, who had been in america and seen the usefulness of lightning conductors, proceeded to erect one over his own house. hardly was it completed before the populace gathered together; and, when passion was sufficiently aroused by inflammatory remarks of the demagogues, the house was attacked and the conductor torn down. the manufacturer complained of the inaction of the "gardiens de la paix" and appealed to the courts to uphold his right to protect his home against lightning. he entrusted his case to a young, brilliant lawyer, as yet unknown to fame, but one destined to achieve unenviable notoriety during the revolutionary period. this, the first defender of the lightning-rod in a court of justice, was robespierre. the news of the untoward event soon reached the ears of the premonstratensian's superiors at kloster-bruck; and, as they very wisely considered that the duty of a country priest is primarily to attend to the spiritual welfare of his people, rather than to invent machines for their protection against the bolts of heaven, they advised him to yield to the prejudice of his people and not reconstruct the objectionable apparatus. father divisch accepted the friendly advice of his superiors and obeyed like a good premonstratensian monk. the remains of the shattered "meteorological machine" were sent to the abbey at bruck, where they could be seen for many years afterward. as a consequence of this act of vandalism, divisch gave up experimenting with lightning-rods and with electricity itself. the villagers were satisfied, but the world at large lost the benefit that might accrue from the researches on atmospheric electricity which divisch would have carried on during the remaining nineteen years of his life. in giving up electricity, the disappointed priest turned his attention, first, to acoustics and then, practical man as he was, to the construction of musical instruments. it was not long before his genius brought out an orchestrion of wind and stringed instruments which was played like an organ with hands and feet, and which was capable of different combinations. prince henry of prussia offered a considerable sum of money for the invention, but divisch died while the preliminaries of sale were arranging, and negotiations were broken off. the instrument remained for many years in the abbey at bruck, where it was in daily use for the canonical office. it is a curious coincidence that franklin was also interested in musical instruments. he is credited with having devised an improved form of glass harmonica, one of which he presented to queen marie antoinette. despite the bitter experience of divisch, the introduction of lightning conductors into italy was warmly advocated some years later by padre toaldo ( - ), an admirer and correspondent of franklin. it was through his influence and personal activity that the magnificent thirteenth-century cathedral of siena was protected with lightning conductors after having been repeatedly struck during the centuries and seriously damaged. toaldo published in his celebrated work on the protection of public edifices and private buildings against lightning; it contributed greatly to reassure public opinion on the value of "franklinian rods," as the conductors were commonly called. it is a matter of regret that franklin used the words "the electric fluid is attracted by the points" in the passage quoted above, inasmuch as in the popular mind such "attraction" courts rather than averts danger. as already said, the rod no more "attracts" lightning than a rain-pipe attracts a downpour. franklin knew very well the twofold function of his rods, the _preventive_, by which they tend to ward off the stroke by gradually and silently neutralizing the excessive energy of the cloud; and the other, the _preservative_, by which they convey the discharge safely to earth when struck. he even complains of people who concentrate their attention on the preventive function, forgetting the other entirely, adding that, "wherever my opinion is examined in europe, nothing is considered but the probability of these rods preventing a stroke, which is only a part of the use which i proposed for them; and the other part, their conducting a stroke which they may happen not to prevent, seems to be totally forgotten, though of equal importance and advantage." ( .) at a time, it was customary to make the rods rise to a considerable height above the building, in the belief that the diameter of the circle of protection was four times the height of the rod. such a rule was an arbitrary one which facts soon showed to be unreliable and unsafe. it is now recognized that there is no such thing as a definite area of protection. were this a literary chapter, we would point out that either of the expressions "electric" storm or "lightning" storm is preferable to _thunder-storm_, because electricity or lightning is the active agent or principal feature of the impressive phenomenon. no one thinks of calling a hailstorm by the descriptive term of _patter-storm_; yet that would be just as logical and appropriate an appellative in one case as thunder-storm is in the other. _thunder-tube_ is certainly a startling misnomer applied to the long, narrow, glazed tubes formed in siliceous materials by the fervid heat of the flash, but not in any way by the sound-waves produced by the crash. _thunder-bolt_ does not mean, despite the common opinion, a white-hot mass that accompanies the discharge; it is purely and simply the flash itself. a glowing mass that happens to come down in the track of the discharge is a _meteorite_, a body of cosmic not terrestrial origin, a visitor from space that chose the rarefied path of the flash for its descent to earth. again, there are no _thunder-clouds_ in nature, only electric clouds or lightning clouds; nor is there ever _thunder in the air_ save when the lightning breaks from cloud to cloud, or leaps from cloud to earth, or strikes from earth to cloud. but though thunder is only occasionally in the air, electricity always is. we have a normal electrical field in all seasons, times and places. though it is the lightning that kills and not the thunder, we would not, however, object to the following inscription which we found on a tombstone: "here lies (so and so), oh! what a wonder, she was killed outright by a peal of thunder," because the suddenness of the peal may have given the aged lady a shock from which her failing heart was unable to recover. we are well aware that such criticism of technical terms in popular use will have no reform effect whatever; because as long as people will say "the sun rises" and "the stars set," they will continue to speak of thunder-clouds and thunder-storms, thunder-tubes and thunder-bolts. though containing an element of error, these expressions have the sanction of the centuries; and so, they have come to stay. returning to divisch, that worthy priest and pioneer electrician died at prenditz in his sixty-ninth year, on dec. st, , and was buried in the little churchyard where he had blessed many a grave during the twenty-five years of his ministration. a simple inscription marks the place of his interment, but a monument will soon be erected to his memory which will tell the passerby where sleeps the premonstratensian pioneer of the lightning-rod. about three months before the erection of his rod, _i.e._, in june, , the idea occurred to franklin that he could approach the region of clouds just as well by means of a common kite. here are his words anent the novel and famous experiment with the "lightning kite": "make a small cross of two light strips of cedar, the arms so long as to reach to the four corners of a large thin silk handkerchief when extended; tie the corners of the handkerchief to the extremities of the cross, so you have the body of a kite, which, being properly accommodated with a tail, loop and string, will rise in the air, like those made of paper; but this, being of silk, is fitter to bear the wet and wind of a thunder-gust without tearing. to the top of the upright stick is to be fixed a very sharp-pointed wire, rising a foot or two above the wood. in the end of the twine, next the hand, is to be held a silk ribbon, and where the silk and cord join a key may be fastened. this kite is to be raised when a thunder-gust appears to be coming on, and the person who holds the string must stand within a door or window, or under some cover, so that the silk ribbon may not be wet; and care must be taken that the twine does not touch the frame of the door or window. as soon as any of the thunder-clouds come over the kite, the pointed wire will draw the electric fire from them, and the kite with all the twine will be electrified, and the loose filaments of the twine will stand out every way and be attracted by an approaching finger. and when the rain has wetted the kite, so that it can conduct the electric fire freely, you will find it stream out plentifully from the key on the approach of your knuckle. at this key the phial may be charged, and from electric fire thus obtained spirits may be kindled and all the other electric experiments be performed which are usually done by the help of a rubbed glass globe or tube, and thereby the sameness of the electric matter with that of lightning completely demonstrated."[ ] here we have the electric kite and manner of using it fully described without, however, any direct statement that the author himself actually experimented with it, although he does say that the experiment was successfully carried out. this is strictly true, but it may be safely contended that the precautions enumerated, the observation about the fibres of the cord, its improved conductivity when wetted by the rain and the like, all bespeak a knowledge of practical conditions that could be obtained only by way of experiment. but if franklin is not outspoken on the matter, some of his contemporaries are. here is the kite incident as related in the _continuation of the life of dr. franklin_, by dr. stuber, a philadelphian and intimate friend of the franklins: "while franklin was waiting for the erection of a spire, it occurred to him that he might have more ready access to the region of clouds by means of a common kite. he prepared one by fastening two cross-sticks to a silk handkerchief, which would not suffer so much from the rain as paper. to the upright stick was affixed an iron point. the string was, as usual, of hemp, except the lower end, which was silk. where the hempen string terminated, a key was fastened. with this apparatus, on the appearance of a thunder-gust approaching, he went out into the commons, accompanied by his son, to whom alone he communicated his intentions, well knowing the ridicule which, too generally for the interest of science, awaits unsuccessful experiments in philosophy. he placed himself under a shed to avoid the rain. his kite was raised. a thunder-cloud passed over it. no sign of electricity appeared. he almost despaired of success, when suddenly he observed the loose fibres of his string move toward an erect position. he now presented his knuckle to the key and received a strong spark. repeated sparks were drawn from the key, the phial was charged, a shock given, and all the experiments made which are usually performed with electricity." this testimony of a man who enjoyed the unlimited confidence of franklin has a very matter-of-fact ring about it; there is not a note of uncertainty, not a word indicating doubt that his friend and neighbor went out to the fields accompanied by his robust son, carrying along with them a queer assortment of electrical impedimenta. this son, william by name, was twenty-two years of age at the time; and as he died in , eleven years after the publication of dr. stuber's biographical sketch, he had ample time to contradict the kite story if instead of being a fact it were a mere romance. nor is this all, for dr. stuber's narrative, given above, appears textually in the "memoirs of the life and writings of benjamin franklin," edited by his grandson william temple franklin. the doctor, be it remarked, was very fond of his grandson, whose "faithful service and filial attachment" he warmly commends in several of his letters, and whose regard for the memory of the statesman led him to undertake the task of preparing his works for publication. on page , vol. i., he tells us that "as dr. franklin mentioned his electrical discoveries only in a very transient way, and as they are of a most important and interesting nature, it has been thought that a short disgression on the subject would be excusable and not void of entertainment. for this purpose the following account of the same, including the first experiment of the lightning kite, as given by dr. stuber, is here given." in these concluding lines we have the testimony of franklin's grandson to the authenticity of the "lightning kite" story. moreover, the account as given by stuber evidently meets with his cordial approval, since he transcribes it verbatim; and, as if to invest the quotations with unimpeachable authority, he tells us in the preface, p. viii., that "they deserve entire dependence because of the accuracy of the information imparted." a word now from priestley, also one of franklin's intimate friends. in his _history of electricity_, fourth edition, p. , he says that "dr. franklin, astonishing as it must have appeared, continued actually to bring lightning from the heavens by means of an electrical kite which he raised when a storm of thunder was perceived to be coming on." then follows a description taken almost word for word from dr. stuber, whom he styles "the best authority on the subject." if, perchance, the above testimony should not be deemed conclusive and final, all lingering doubt must be removed by franklin's own words, for in his _autobiography_, after briefly referring to the experiments made in france with pointed conductors, he adds: "i will not swell this narrative with an account of that capital experiment (the pointed conductor), nor of the infinite pleasure which i received on the success of a similar one i made soon after with a kite at philadelphia, as both are to be found in histories of electricity." here, at last, we have franklin's own word for it, that he made the kite experiment, and that he made it "soon after" the demonstration of his electrical discoveries which m. de lor gave, by request, before louis xv. and his court. the "lightning kite" is, therefore, not a myth, as some have ventured to think, having been fully described by franklin in his letter to peter collinson, dated october th, , and having been made by him some time in june of the same year. we have now to see whether franklin was anticipated in the idea of the kite or in its use for electrical purposes. there are some who hold that he was anticipated by m. de romas as to the idea, but not the actual experiment; while others credit the french magistrate with both. let us examine the evidence which there is for these opinions. m. de romas lived in nérac, a small town some seventy-five miles south of bordeaux. he was a member of the bar; and at the time of the franklinian furor in europe was a judge of the district court. he took an interest in scientific matters quite unusual for men of his profession, proceeding, as soon as he had read of the efficiency of pointed conductors, to study their behavior for himself. his experiments met with surprising success, and were as much admired by the local savants as they were dreaded by the common folk. letters containing his observations were regularly sent to the academy of bordeaux, where they were read with lively interest on account of their character and novelty. from the published _actes_ of that body we learn that the first kite used by de romas was raised by him on may th, . disappointment, however, attended this attempt, no electrical manifestation being observed, although rain fell and wetted the hempen cord. the magistrate of nérac attributed his failure to the resistance of the string; and, like a good electrician, surprisingly good for the time, determined to improve its conductivity by wrapping a fine copper wire round its entire length. when this long and tedious operation was completed, he went out again to the fields on a stormy day, when, assisted by two of his friends, he raised the kite and soon got torrents of sparks from the wire-wound cord. this was on june th, . the experiment was repeated from time to time, both for his own satisfaction and that of his assistants as well as for the entertainment of his ever-growing class of admiring spectators. kites - / ft. long and ft. wide were raised and even ft. above ground when flashes nine feet long and an inch thick were drawn, so the account says, with the report of a pistol. the effect must have been truly spectacular. the kite was held by a silk ribbon fastened to the end of the hempen cord. it is then a matter of history vouched for by the _actes_ of the academy of bordeaux that may th, , is the day on which the first use of a kite for electrical purposes was made in france; on the other hand, it is to be remembered that franklin flew his "lightning kite" in june, , almost a year earlier. as far, then, as the _fact_ is concerned, the philadelphia philosopher was not anticipated by the justice of nérac. from facts let us pass to writings. franklin's letter to collinson, in which he describes the electric kite, is dated october th, , while that of m. de romas, on which the claim for priority is founded, was addressed by him to the academy of bordeaux on july th, , three months earlier. after a lengthy and interesting account of his experiments with pointed conductors, he concludes his communication as follows: "c'est là, monsieur, ce qu'il y a de plus important, car j'aurais bien d'autres particularités à vous communiquer; mais ma lettre, devenue d'une excessive longueur, m'avertit de finir. je me réserve de mettre au jour la dernière (quoiquelle ne soit qu'un jeu d'enfant) lorsque je me serai assuré de la réussite par l'expérience que je me propose d'en faire et que je ne negligerai pas." in english this would read: "such, sir, are the more important points which i have to communicate, and to which many others might be added, were it not for the excessive length of this letter, which warns me that it is time to bring it to a close. i will, however, give publicity to the last one of all (though it is only a child's plaything) as soon as i shall have assured myself of its success by an experiment which i have devised and which i shall not fail to make." the words in brackets--"though it is only a child's plaything"--are all important, for it is on them and on them alone that the claim for priority has been put forth and maintained. it will be seen that the word kite (_cerf-volant_), does not occur in the letter, so that there can be no absolute certainty as to the nature of the _jeu d'enfant_ which the author had in mind, though it is very likely that the kite was meant. in his _mémoire sur les moyens de se garantir de la foudre dans les maisons_, he says, after describing some experiments that he had made with pointed rods: "néanmoins toujours plein du désir d'augmenter le volume du feu électricque, il fallut chercher le moyen pour y parvenir. en conséquence, je me plongeai dans de nouvelles méditations. enfin une demi-heure après, tout au plus, le cerf-volant des enfants se présenta tout à coup à mon esprit, et il me tardait de la mettre à l'épreuve. par malheur, je n'en avais pas le temps." in english: "being anxious to augment the quantity of electric fire, i began to think of some means to effect my purpose, and soon became quite absorbed with the subject. not more than half an hour elapsed before the idea of the kite suddenly occurred to me, and i longed for an opportunity to try it; but unfortunately i had not sufficient leisure at the time." the work in which this passage occurs was published at bordeaux in , shortly after the death of the author. de romas always maintained that he did not borrow the idea of the kite from any one, but that it occurred to him while pursuing his experiments with pointed conductors. it must be admitted that de romas could not have been acquainted with franklin's performance of june, , when he sent to the bordeaux academy his letter of july th, of the same year, for we cannot suppose that in an age of sailing vessels such news would cross the atlantic and reach an obscure provincial town in the southwest of france in the space of a month. on the other hand, it is equally improbable that a vague allusion to the electrical use of a kite made at nérac on july th, by a man entirely unknown to fame as was de romas, should be talked of on the banks of the schuylkill before october th, the date of franklin's memorable letter to collinson. moreover, the "_jeu d'enfant_" allusion as well as the very use of the kite by de romas failed so completely to attract the attention of scientific men of his own country that he frequently and bitterly complained down to the end of his life, in , of their persistent neglect of his claims to recognition. from all this, we conclude: (_a_) that franklin's "lightning kite" is not a myth, the experiment having been made by him in june, , and fully described by him in a memorable letter written to peter collinson, of london, dated october th of the same year: (_b_) that de romas independently had the idea of using a kite for electrical purposes as early as july th, ; but that he did not carry out his idea until may th, ; and, furthermore, that he did not succeed in getting any electrical manifestations until june th, , his success then being due, at least in part, to the clever idea which he had of entwining the cord with a fine copper wire. therefore, _suum cuique_. in conclusion, we would say that the cardinal and enduring achievements of franklin are: ( ) his rejection of the two-fluid theory of electricity and substitution of the one-fluid theory; ( ) his coinage of the appropriate terms _positive_ and _negative_, to denote an excess or a deficit of the common electric fluid; ( ) his explanation of the leyden jar, and, notably, his recognition of the paramount role played by the glass or dielectric; ( ) his experimental demonstration of the identity of lightning and electricity; and ( ) his invention of the lightning conductor for the protection of life and property, together with his clear statement of its preventive and protective functions. if franklin was well acquainted with electrical phenomena, it is safe to say that his knowledge of human nature was wider and deeper still. this appears continually in his _autobiography_, in his political writings, in business transactions and diplomatic relations. on one occasion, while his re-election as clerk of the general assembly was pending, a certain member made a long speech against him. franklin listened with calm, dignified composure; and after his election, instead of resenting the opposition of the offending member, he determined that it would be better to disarm his antagonism and win his friendship. for this purpose he sent the assemblyman a courteously-worded request for the loan of a very scarce book which was in his library. the book was sent to franklin, who returned it within a week with a note of thanks, which had the desired effect. commenting on the event, our philosopher says that "it is more profitable to remove than to resent inimical proceedings." some of franklin's views on general political economy are tersely set forth in the following passage: "there seem, in fine, to be but three ways for a nation to acquire wealth. the first is by _war_, as the romans did in plundering their conquered neighbor; this is _robbery_. the second is by _commerce_, which is generally _cheating_. the third is by _agriculture_, the only _honest way_ wherein man receives a real increase of the seed thrown into the ground, in a kind of continual miracle wrought by the hand of god in his favour, as a reward for his innocent life and virtuous industry." franklin asserts his religious convictions in many passages of his "autobiography" as well as on many occasions of his public life. shocked by "tom" paine's views of fundamental religious truths, he says: "i have read your manuscript with some attention. by the argument which it contains against a particular providence, though you allow a general providence, you strike at the foundation of all religion. for, without the belief of a providence that takes cognizance of, guards and guides, and may favour particular persons, there is no motive to worship a deity, to fear his displeasure, or to pray for his protection. i will not enter into any discussion of your principles, though you seem to desire it. at present, i shall only give you my opinion that, though your reasonings are very subtile and may prevail with some readers, you will not succeed so as to change the general sentiments of mankind on that subject; and the consequence of printing this piece will be a great deal of odium drawn upon yourself, mischief to you, and no benefit to others. he that spits against the wind, spits in his own face." this aphorism recalls the ripe wisdom contained in many of the sayings of "poor richard," for franklin was a deep thinker, shrewd observer and quaint expositor of his own philosophy. continuing, he fleeces paine in the following noble words: "but were you to succeed, do you imagine any good would be done by it? you yourself may find it easy to live a virtuous life without the assistance afforded by religion; you having a clear perception of the advantages of virtue and the disadvantages of vice, and possessing strength of resolution sufficient to enable you to resist common temptations. but think how great a portion of mankind consists of weak and ignorant men and women, and of inexperienced, inconsiderate youth of both sexes, who have need of the motives of religion to restrain them from vice, to support them to virtue, and retain them in the practice of it till it becomes _habitual_, which is the great point for its security. and perhaps you are indebted to her originally, that is, to your religious education for the habits of virtue upon which you now justly value yourself. you might easily display your excellent talents of reasoning upon a less hazardous subject, and thereby obtain a rank with our most distinguished authors. for among us, it is not necessary, as among the hottentots, that a youth, to be raised into the company of men, should prove his manhood by beating his mother." franklin concludes this magnificent expression of his religious faith by the solemn warning: "i would advise you, therefore, not to attempt unchaining the tiger, but to burn this piece before it is seen by any other person; whereby you will save yourself a great deal of mortification by the enemies it may raise against you, and perhaps a good deal of regret and repentance. if men are so wicked _with_ religion, what would they be _without_ it?" franklin's belief in the cardinal doctrine of the resurrection of the body is well expressed in the epitaph which he wrote for himself in , when in his twenty-second year. it reads the body of benjamin franklin printer, (like the cover of an old book its contents torn out and stript of its lettering and gilding) lies here, food for worms. but the work shall not be lost for it will (as he believed) appear once more in a new and more elegant edition revised and corrected by the author. however, when the statesman and philosopher was laid at rest beside his wife in the cemetery of christ church, philadelphia, in , the marble slab which marked the grave bore no other inscription than franklin's name and the date of his death. appreciating the great loss which the country sustained by the death of franklin, congress ordered a general mourning for one month throughout the fourteen states of the union; and the french national assembly decreed three days of public mourning at the instance of mirabeau, who said in his address that "the genius that gave freedom to america and scattered torrents of light upon europe, has returned to the bosom of the divinity. antiquity would have erected altars to that mortal who for the advantage of the human race, embracing both heaven and earth in his vast mind, knew how to subdue both thunder and tyranny." the fugitive apprentice boy of turned out to be one of the most esteemed and eminent americans of his day. of an even temper and well-balanced mind, he was plain in dress, simple in manner, easy of approach and friendly to all. the success which he achieved during his long career of eighty-five years, shows what may be done by seizing the opportunities which come to every one, by concentration of mind, application to duty and tenacity of purpose. he attained distinction in science, in letters, in diplomacy; he stood for good government and true liberty. his name is a household one in his own country, where monuments, institutions and cities will bear it down to posterity. addenda. _the lightning kite._ fully described by franklin in a letter to peter collinson, of london, dated october th, . stuber in his "continuation of the life of dr. franklin," and priestley in his "history of electricity," affirm that franklin made the experiment in june, . franklin's son, william, never denied the story, although he figured in it as an active character. william temple franklin, who prepared for publication his grandfather's works, gives the kite story almost verbatim from stuber. finally, franklin himself states that he made the experiment: memoirs, vol. i., p. . _franklin and de romas._ june, : franklin raises his kite in a field near philadelphia. july , : letter of de romas to the academy of bordeaux, in which a probable reference is made to the kite as _un jeu d'enfant_. october th, : franklin describes the "lightning kite" in a letter to peter collinson, of london. may th, : first use by de romas of the electric kite in the fields around nérac; no result. june th, : first success by de romas with his electric kite. _pointed conductor._ suggested by franklin in letter to peter collinson, of london, dated july th, . d'alibard, following franklin's instructions, gets torrents of discharges from his iron rod feet high at marly, may th, . de lor gets good results from his conductor feet high, erected over his house in paris, may th, . de buffon succeeds with his rod on may th, . franklin erected the first rod over his house in philadelphia in september, . it was made of iron with a sharp steel point rising seven or eight feet above the roof, the other end being sunk five feet in the ground. franklin charged a leyden jar from his rod in april, . professor richmann, of st. petersburg, was killed by a flash from his apparatus on august th, . brother potamian. [illustration: aloisio galvani] footnotes: [ ] scientific writings of joseph henry, vol. i., p. . [ ] frequently referred to as lullin's experiment. [ ] every schoolboy knows that the electricity which passed down the kite-string was not drawn from the clouds, but was due to their inductive action on the pointed conductor attached to the kite. kant calls franklin the "modern prometheus." chapter iv. galvani, discoverer of animal electricity. it is a well-known fact, often commented on in the history of medicine, that harvey, the discoverer of the circulation of the blood, did not give the details of his discovery to the public for some twenty years after he had first reached it. the reason for his delay was twofold. with the characteristic patience of a real investigator in science, harvey wanted to work out the details of his discovery for himself before giving it to the public, and wished to be sure of all he would have to say about it before committing it to print. he had not, as had indeed none of the really great discoverers in science, that intense desire for publicity which causes smaller men to rush into print with their embryonic discoveries, or oftener, their supposed discoveries, the moment they get their first distant glimpse of a new truth or see some mirage of a distant scientific principle, perhaps already well known, in their heated imaginations. small men squabble about priority in small discoveries, and rush headlong into print, lest some one should anticipate their wonderful observation. the example of harvey can scarcely be commended too highly, for if followed, it would save the world of science a lot of bother and obviate the necessity of taking back many things that have been proclaimed in the name of science. fortunately, it has been the rule among genuine students of science, not because of any deliberate imitation of their great predecessors, but because of modest assurance of the worth of their work and honest desire to perfect it before giving it to the world. luigi, or, as he preferred to be known himself, aloysio galvani, for the young prince of the house of gonzaga whose canonization made him st. aloysius was his patron in baptism and a favorite in life, presents an interesting exemplification of this characteristic trait of the really great discoverer in science, to wait calmly and work faithfully for thorough confirmation of his views before publishing them. his admirable patience in reaching the real significance of his discovery before proclaiming the results of his investigations is only a typical illustration of the modest thorough scientist that he was. it used to be said that galvani's discovery of the twitchings of the frog's legs, which led him to give himself to serious investigations into animal electricity, was made more or less by accident in . his views on the subject of animal electricity were not formally published until the appearance of his treatise, de viribus electricitatis in motu musculari commentarius, in the eighth volume of the memoirs of the institute of science of bologna, published in . this would seem to indicate that only five years elapsed between his original observation and the publication of his views. even this interval may seem long enough to our modern notions of at least supposed rapidity of scientific progress, but we know now, from documents in the possession of the institute of science at bologna, that, twenty years previous to the publication of this commentary, galvani was deeply interested in the action of electricity upon the muscles of frogs, and was diligently and fruitfully occupied during his spare time with investigations upon this subject. when, in makers of modern medicine,[ ] i called special attention to the fact that practically all of the greatest discoverers in medicine had made their cardinal discovery, or at least the far-reaching observation that opened up for them the special career in investigation that was to make them famous, before they were thirty-five, one of my critics doubted the assertion and suggested the case of galvani as a distinct exception. ordinarily, it is presumed that his discovery of the twitchings of frogs' legs under the influence of electricity was made in , when he was in his forty-ninth year. as a matter of fact, however, his first observations were made and his attention attracted to the importance of the subject when he was scarcely more than thirty. his career is indeed a striking example of the earliness in life at which a great man's work is likely to come to him, and yet illustrates very aptly the patience with which he devotes himself to it, without seeking the idle reputation to be derived from immediate announcement, if he really has the true spirit of the scientific investigator. galvani began original work of a high order very early in his medical career. his graduation thesis on the human skeleton treated especially of the formation and development of bone, and attracted no little attention. it is noteworthy because of the breadth of view in it, for it touches on the various questions relative to osteology, from the standpoint of physics and chemistry, as well as medicine and surgery. it was sufficient to obtain for its author the place of lecturer in anatomy in the university of bologna, besides the post of director of the teaching of anatomy in the institute of sciences, a subsidiary institution. here, from the very beginning, galvani's course was popular. he was not, as we note elsewhere, a fluent talker, but he was one of the first who introduced experimental demonstrations of his subject into his lectures, and this made his teaching very attractive and drew crowds to his university courses. galvani's work as an anatomist, however, was done much more in comparative anatomy than in the study of the human being. he selected birds for the special subject of his first investigations in the field, and his monograph on the kidneys of birds attracted widespread attention among the scientists of europe. as the farthest removed from man of the beings that are warm-blooded, these creatures have always attracted particular attention, and, quite apart from any interest in evolution, were the subject of special investigation. owing to the facility with which they can be studied in embryonic stages in the hatching egg, most of the peculiarities of their structure and development are very well known now. the kidneys of the bird are especially interesting, because they represent a different phase of development from that of human beings. galvani had selected, then, one of the cardinal or turning-point subjects in comparative anatomy. as he pointed out very clearly, the kidneys of birds differ very much among themselves, and the intense muscular action of this creature makes a large amount of excretory material, that must be disposed of, and consequently demands much more active kidney function than occurs in most other classes of animals. galvani studied every feature--the vessels, the nerves, the canals--and almost necessarily pointed out many new points or added hitherto unknown details. he next devoted himself to the study of the ear of the bird. this might seem to be of little special interest, since hearing is not one of the most characteristic qualities of the winged species. it so happens, however, that the semi-circular canals which are closely connected with the auditory apparatus in all animals are extremely large in birds. as a consequence of this, the avian auditory structures assume an importance in comparative anatomy quite like that of the kidneys in the same species. after galvani had completed his studies, he found that he had been anticipated by another great italian anatomist of the time, antonio scarpa (of scarpa's triangle in human anatomy), who afterwards became the chief surgeon to napoleon. galvani abandoned the idea of publishing his book then, but published a short article, in which he added much to scarpa's details and conclusions. his additions were particularly with regard to the semi-circular canals, which are probably the organ of direction, the necessity for which, in this species, for the purpose of flying, is so easy to understand. he also described with great care the single ossicle or small bone, which replaces the chain of little bones that exist in mammal ears, and pointed out that the shape of this bone and its appendages enabled it to fulfil, though single, all the functions of the hammer, the anvil and the stirrup bones in human beings. galvani's careful study of the semi-circular canals of various species of birds can perhaps be better appreciated from the fact that he made it a point to measure their size exactly, as compared to the semi-circular canals of most other creatures. he found that the semi-circular canals of the hawk, for instance, were larger than the corresponding structures in man or even in the cow or the horse. as these latter animals are many hundred times larger than the largest birds, the special significance of the canals in birds becomes manifest. in certain of the birds, as he pointed out, these structures are not semi-circles, nor indeed of circular form at all, but take on much more the shape of an ellipse, and, indeed, sometimes the arc of curvature of the ellipse is quite acute. he seems to have had no hint, however, of the function that we have in modern times assigned to these structures, that of presiding over direction and equilibrium, and discusses in his rather vigorous latin what the physiological significance of them may be as regards hearing. he thinks that they add something to the acuity of hearing, and would seem to imply that in birds flying rapidly through the air, there was the necessity for a more perfect hearing apparatus than among other creatures, and that this was the reason for the huge development of their semi-circular canals. at this time the science of comparative anatomy was just beginning to attract widespread attention. john hunter, in london, was doing a great work in this line, which placed him in the front rank of contributors to biology and collectors of important facts in all the sciences allied to anatomy and physiology. galvani's work on birds, then, made him a pioneer in the biological sciences that were to attract so much attention during the nineteenth century. his experimental work in comparative anatomy, strange as it might seem, and apparently not to be expected, led him into the domain of electricity, through the observation of certain phenomena of animal electricity and the effects of electrical currents on animals. like so many other great discoveries in science, galvani's first attraction to his subject of animal electricity is often said to have been the result of a happy accident. of course it is easy to talk of accidents in these cases. archimedes and his bath; the fall of the apple for newton; laennec's observation of the boys tapping on a log in the courtyard of the louvre and the ready conduction of sound, from which he got his idea for the invention of the stethoscope; lord kelvin's eye-glass falling and showing him how a weightless arm for his electrometer might be obtained in a beam of light,--may all be called happy accidents if you will. without the inventive scientific genius ready to take advantage of them, however, these accidents would not have been raised to the higher plane of important incidents in the history of science. these phenomena had probably occurred under men's eyes hundreds of times before, but there was no great mind ready to receive the seeds of thought suggested, nor to follow out the conclusions so obviously indicated. galvani's observation of the twitching of the muscles of the frog under the influence of electricity, may be called one of the happy accidents of scientific development, but it was galvani's own genius that made the accident happy. there are two stories told as to the method of the first observation in this matter. both of them make his wife an important factor in the discovery. according to a popular but less authentic form of the history, galvani was engaged in preparing some frogs' legs as a special dainty for his wife, who was ill and liked this delicacy very much. he thought so much of her that he was doing this himself, in the hope that she would be thus more readily tempted to eat them. while so engaged, he exposed the large nerve of the animal's hind legs, and at the same time split the skin covering the muscles. in doing this he touched the nerve muscle preparation, as this has come to be called, with the scalpel and the forceps simultaneously, with the result that twitchings occurred. while seeking the cause of these twitchings, the idea of animal electricity came to him. the other form of the story is told a little later in galvani's own words in the analysis of his monograph on animal electricity. he does not mention his wife in it, but there is a tradition that she was present in the laboratory when the phenomenon of the twitching of the frog's legs was first noticed, and indeed that it was she who called his attention to the curious occurrence. she was a woman of well-developed intellect, and her association with her father and also with her husband made her well acquainted with the anatomy and physiology of the day. she realized that what had occurred was quite out of the ordinary. she is even said to have suggested their possible connection with the presence and action of the electric apparatus. husband and wife, then, together, by means of a series of observations determined that, whenever the apparatus was not in use the phenomenon of the convulsive movements of the frog's legs did not take place, notwithstanding irritation by the scalpel. whenever the electric apparatus was working, however, then the phenomenon in question always took place. according to either form of the story, if we accept the traditions in the matter, madame galvani had an important part in the discovery. galvani's most important contribution to science is undoubtedly his de viribus electricitatis in motu musculari commentarius--commentary on the forces of electricity in their relation to muscular motion. like many another epoch-making contribution to science, it is not a large work, but in his collected works in the edition of , occupies altogether sixty-four pages, of scarcely more than two hundred and fifty words to the page. there are probably not more than fifteen thousand words in it altogether. it was published originally in the eighth volume of the memoirs of the institute of science at bologna, in , but a reprint of it, with some modifications, was issued at modena in the following year. this modenese edition, published by the societa typographica, was annotated by professor giovanni aldini, who also wrote an accompanying dissertation, de animalis electricae theoriae ortu atque incrementis, on the rise and development of the theory of animal electricity. in this volume was also published a letter from galvani to professor carminati, in italian, on the seat of animal electricity. these two editions are the sources to which we must turn for whatever galvani tried to make known with regard to animal electricity. this little volume consists of four parts: the first of which is devoted to a consideration of the effects of artificial electricity on muscular motion; the second is on the effect of atmospheric electricity on muscular motion; the third is on the effect of animal electricity on muscular motion; and the fourth consists of a series of conjectures and some conclusions from his observations. the arrangement of the work, as can readily be understood from this, is thoroughly scientific. galvani proceeds from what was best known and most evident to what he knew less about, trying to enlarge the bounds of knowledge and then suggesting the conclusions that might be drawn from his work and offering a number of hints as to the possible significance of many of the phenomena that might form suggestive material for further experimentation along this same line. in spite of the forbiddingness of the latin to a modern scientist, as a rule, the little work is well worthy of study because of its eminently scientific method and the excellent evidence it affords of the way serious students of science approached a scientific thesis before the beginning of the nineteenth century. the first paragraph of this dissertation is of such fundamental significance, because it represents the primal work done in animal electricity, that it has seemed to me worth while presenting entire. the original latin from which the translation is made, and from which a good idea of galvani's latin style may be obtained, is given in a note.[ ] "i had dissected a frog and had prepared it, as in figure of the fifth plate (in which is shown a nerve muscle preparation), and had placed it upon a table on which there was an electric machine, while i set about doing certain other things. the frog was entirely separated from the conductor of the machine, and indeed was at no small distance away from it. while one of those who were assisting me touched lightly and by chance the point of his scalpel to the internal crural nerves of the frog, suddenly all the muscles of its limbs were seen to be so contracted that they seemed to have fallen into tonic convulsions. another of my assistants, who was making ready to take up certain experiments in electricity with me, seemed to notice that this happened only at the moment when a spark came from the conductor of the machine. he was struck with the novelty of the phenomenon, and immediately spoke to me about it, for i was at the moment occupied with other things and mentally preoccupied. i was at once tempted to repeat the experiment, so as to make clear whatever might be obscure in it. for this purpose i took up the scalpel and moved its point close to one or the other of the crural nerves of the frog, while at the same time one of my assistants elicited sparks from the electric machine. the phenomenon happened exactly as before. strong contractions took place in every muscle of the limb, and at the very moment when the sparks appeared, the animal was seized as it were with tetanus." galvani then explains in detail how he made observations on control frogs at moments when there were no electric sparks, and decided that the contact with the scalpel was only effective in producing twitchings when there was a simultaneous electric spark. he noted, also, that occasionally the contractions did not occur, in spite of the fulfilment of the conditions mentioned. he traced this to fatigue. he then proceeded to vary the experiment in many ways, decreasing the size of the scalpel, increasing and decreasing the size of the electric machine and varying the method of preparation of the frog, so as to decide just what the significance of the phenomenon was. in a general way, it may be said that this study shows galvani as one of the most careful of experimentalists, though he has often been declared to be a theorizer, rather than an observer. a very interesting anticipation of galvani's original experiment, made long before his time by a great naturalist, the story of which serves to show that discoveries made before their time, that is, before people are ready to follow them up, fail to attract attention, has been called to my attention by brother potamian. in the second volume of the dutch naturalist swammerdam's works, page , is to be found the following passage:[ ] "another experiment that is at once very curious and suggestive can be made if one separates the largest of the muscles of the thigh of the frog and so prepares it with its adherent nerve as to leave it unhurt. if after this has been done you take the tendons of this muscle, one in each hand, and irritate the hanging nerve by a little forceps or other instrument, the muscle will recover the former motion which it had lost. you will see at once that it contracts and that there will be an effort as it were to bring together the two hands which hold its tendons. this i demonstrated, in the year , to the illustrious duke of tuscany then reigning, when he was at the moment in a state of mind that prompted him not to favor me. this same experiment can be repeated with the same muscle as often and for as long a time as any portion of the nerve remains uninjured, so that we may, therefore, irritate the muscle to its former contraction as often as we wish." as a foundation classic in electricity, galvani's de viribus electricitatis deserves more detailed analysis. the first part of the monograph is taken up with experiments of many kinds, with what may be called artificial sources of electricity--the electric machine, the leyden jar, and other modes of electrical development. the second part treats of the effects of atmospheric electricity upon muscular motion, by which expression galvani means lightning, though he also observed various electrical manifestations in the muscles of his frogs when there was no actual lightning but only darkening of the heavens, without actual passage of the current flash from one cloud to another or from the clouds to the earth. in this matter, galvani displayed quite as much courage as patient observation. he knew the fate of richmann, the russian scientist, who had been struck dead by a lightning-bolt while making experiments not very different, yet he dared to place a lightning conductor on the highest point of his house, and to this conductor he attached a wire, which ran down to his laboratory. during a storm, he suspended on this metallic circuit, by means of their sciatic nerves, frogs' legs and the legs of other animals prepared for the purpose. to the feet of the animals he attached another wire sufficiently long to reach down to the bottom of a well, thus grounding the circuit. not satisfied with this study of the influence of lightning and large electrical disturbances in the air on the preparation of the frog as he had made it, galvani set about discovering whether even the slight differences in electrical potential which occur during the day in atmospheric electricity might not give rise, even in fair weather, to certain contractions of the frog's muscles. he made his observations for many days at many different hours and under varying conditions of light and shade, of heat and cold, without finding anything. there were occasional contractions, but they bore no definite relation to variations in the atmosphere, or the electric state of the atmosphere. galvani satisfied himself of this very thoroughly, and with a patience and diligence worthy of emulation by a fellow at a modern university working on a foundation for the determination of a particular question. the third part of the work is the most important as well as the longest, and contains the ideas which are original with galvani, but which met most opposition in his time and have only been properly appreciated in recent years. galvani came to the conclusion that there is such a thing as animal electricity. this led to a famous controversy with volta, in which their contemporaries judged that galvani had the worst of it; but, as so often happens, their successors a century later would judge that galvani's views were more in accord with what we know at the present time. criticism is always easier than scientific advance, and in a controversy it is usually the man who writes most forcibly, rather than the one who thinks most deeply, who secures the assent of readers. this makes controversy in matters of science always unfortunate, for it does much more to retard than to help scientific progress. galvani insists, at the end of this chapter on animal electricity, that what he writes is entirely the result of experiment, and that he has tried in every way to make his experiments from a thoroughly critical standpoint. those who repeat his observations will find this to be true, though he confesses that there are times when conditions not well understood seem to hinder the results that he usually obtained. the fourth part of his commentary is taken up with certain conjectures, as he calls them, and some conclusions from his work. in this he suggests the use of electricity for the cure of certain nervous diseases, and especially for the treatment of the various forms of paralysis. the use of electricity for these cases had been previously suggested, and bertholinus had told the story of patients who were utterly unable to move and who had recovered after having been in the neighborhood where a lightning-bolt had struck. to the minds of physicians of that time, this must have seemed proof positive of the curative value of lightning, and, therefore, of electricity, for paralytic conditions. the remedy was heroic, if not indeed positively risky, but its good effect could not be doubted. unfortunately, as is always true in medical matters, the real question at issue in these cases is not so much the value of the remedy as the propriety of the diagnosis. paralysis, in the sense of inability to use one or more limbs, may be due to many causes. there are a number of forms of functional or hysterical palsy, that is, of incapacity to use certain groups of muscles not dependent on any organic lesion, but upon some curious state of the nervous system which may pass away entirely, and which, indeed, seem to be dependent on the patient's state of mind. a number of so-called paralytic patients were cured by the earthquake in san francisco; some are made to do the apparently impossible every year; they get up and walk because of the shock due to a fire or burglars. we know now that the electrical status of the individual is very carefully protected from disturbance by external electrical forces. what galvani began has borne fruit in diagnosis more than treatment, so that his prophecy has been amply fulfilled. "the application of this method may throw light on the subject and experience may help us to understand it." among his conclusions, galvani hints that electricity may not only proceed from the clouds during electrical disturbances, but also may proceed from the earth itself, and that living beings may be affected by this. he suggests, therefore, that plants and animals may be influenced in their growth and in their health by such electrical changes. he adds the suggestion that there may be some intimate connection between electrical phenomena and earthquakes, and suggests that, in countries where earthquakes are frequent, observations should be made by means of frogs' limbs in order to see whether there may not be some definite change in the electrical conditions of the atmosphere before and during the earthquake. he seems to have had some idea that the curious feelings which at times come before an earthquake to human beings, though they seem even more noticeable in animals, may be due to this change in atmospheric electricity.[ ] we are rather prone to think that news of scientific discoveries traveled slowly in europe in the eighteenth century. there is abundant evidence of the contrary in these sketches of electricians, and galvani's case is one of the most striking. how much attention galvani's discovery attracted and how soon definite details of it spread to the other end of europe may be judged from the fact that, in , mr. richard fowler published a small book at edinburgh bearing the title, experiments and observations relative to the influence lately discovered by m. galvani, and commonly called animal electricity.[ ] this little book, which may be seen at the surgeons general library, washington, and in the library of the american institute of electrical engineers, new york, details a large number of experiments that fowler had made during the preceding year or more, so that galvani's work must have reached him within a few months after its publication. fowler mentions the fact that galvani had been occupied many years before this in the study of electric fishes, especially the _torpedo_, the _gymnotus electricus_ and _silurus electricus_. he also mentions a curious observation of cotugno, who, a few years before, had received a shock from a mouse while dissecting the little animal, which makes it clear that imagination played a role in helping to the introduction of the newer ideas with regard to animal electricity.[ ] but before his discovery was to attract so much attention, galvani had to work it out, and this is the merit of the man. it is almost needless to say, these experiments upon frogs were not accomplished in a few days or a few weeks. galvani had his duties as professor of anatomy to attend to besides the obligations imposed upon him as a busy practitioner of medicine and surgery. at that time, it was not nearly so much the custom as it is at the present, to use frogs for experiments, with the idea that conclusions might be obtained of value for the biological sciences generally, and especially for medicine. there has always been such an undercurrent of feeling, that such experiments have been more or less a beating of the air. galvani found this opposition not only to his views with regard to animal electricity as enunciated after experimental demonstration, but also met with no little ridicule because of the supposed waste of time at occupations that could not be expected to lead to any practical results. it was the custom of scientific men to laugh somewhat scornfully at his patient persistence in studying out every detail of electrical action on the frog, and one of the supposedly prominent scientists of the time even dubbed him "the frog dancing master." this did not, however, deter galvani from his work, though some of the bitter things must have proved cutting enough, and might have discouraged a smaller man, less confident of the scientific value of the work that he was doing. his relations with his patients--for during all of his career he continued to practice, especially surgery and obstetrics--were of the friendliest character. while his distinction as a professor at the university gave him many opportunities for practice among the rich, he was always ready and willing to help the poor, and, indeed, seemed to feel more at home among poor patients than in the society of the wealthy and the noble. even toward the end of his life, when the loss of many friends, and especially his wife, made him retire within himself much more than before, he continued to exercise his professional skill for the benefit of the poor, though he often refused to take cases that might have proved sources of considerable gain to him. early in life, when he was very busy between his professorial work and his practice, he remarked more than once, on refusing to take the cases of wealthy patients, that they had the money with which to obtain other physicians, while the poor did not, and he would prefer to keep some time for his services to them. when ailing and miserable toward the end of his life, he still continued his practice, and was especially ready to spend his time with the poor. he was dying himself, as one of his biographers says, when he got up from a sick bed to see a dying woman who sent for him. he was one of the most popular professors that the university of bologna ever had. he was not, in the ordinary sense of the word, an orator, but he was a born teacher. the source of the enthusiasm which he aroused in his hearers was undoubtedly his own love for teaching and the power it gave him to express even intricate problems in simple, straightforward language. more than any of his colleagues, he understood that experiments and demonstrations must be the real groundwork of the teaching of science. accordingly, very few of his lectures were given without the aid of these material helps to attract attention. besides, he was known to be one who delighted to answer questions, and was perfectly frank about the limitations of his knowledge whenever there was no real answer to be given to a question that had been proposed. though an original discoverer of the first rank, he was extremely modest, particularly when talking about the details of his discoveries or subjects relating to them. galvani was not a good talker, though he seems to have been a good teacher. he had little of that facility which wins friends easily and enables a man to shine with a borrowed lustre of knowledge, often enough quite superficial. what he said was almost sure to have a very serious meaning. while there is no doubt that galvani was a genius, in the sense that he was one of the precious few who take the step across the boundary of the unknown and make a path along which it is easy for others to follow in reaching hitherto trackless regions in human speculation, he also had what is undoubtedly the main element in talent, for he was possessed to a high degree of the faculty for hard work. for this he regulated the hours of his labor very carefully. only thus could he have accomplished what he did. it must not be forgotten that he was teaching anatomy and obstetrics at the university of bologna, and, surprising as it may seem, doing both these tasks well. he was besides accomplishing good work in comparative anatomy and physiology by original investigations of a high order. in spite of all this, which would seem occupation enough and more for any one man, he was able to keep up a rather demanding practice. he did not have many friends, but those whom he admitted to his intimacy were bound to him with the proverbial hoops of steel. with two men in bologna he spent most of his leisure. they were dr. julio cæsare cingari, a distinguished physician of the city, and the well-known astronomer who held the chair of astronomy at the university, francisco sacchetti. with these he passed many a pleasant hour, and week after week they met at one another's houses to discuss scientific questions and the lighter topics of the day. galvani was thoroughly respected by all the members of the faculty at bologna, though he did not seek many friendships, and indeed probably would have more or less resented the intrusions of acquaintances, because of the time that it would take from him. he was a very retiring man, caring not at all for social things, and least of all for that personal fame which has been so well defined as the being known by those whom one does not know. his happiness in life came to him from his work and from his domestic relations. his wife was one of those marvelous women, rarer than they should be, one is tempted to say, who are enough interested in their husband's intellectual work to add to the zest of discovery in the discussion of it with them, and who yet realize that it is by minimizing the little worries of life that they can best help their husbands. a very interesting phase of the italian university life of that time is revealed in two important incidents of galvani's university career. one of his professors--one, by the way, for whom he seems to have had a great deal of respect, and to whose lectures he devoted much attention, was laura caterina maria bassi, the distinguished woman professor of philosophy at the university of bologna, about the middle of the eighteenth century. it is doubtless to her teaching that galvani owes some of his thorough-going conservatism in philosophic speculation, a conservatism that was of great service to him later on in life, in the midst of the ultra-radical principles which became fashionable just before and during the french revolution. madame bassi seems to have had her influence on him for good not only during his student career, but also later in life, for she was the wife of a prominent physician in bologna, and galvani was often in social contact with her during her years of connection with the university. as might, perhaps, be expected, seeing that his own happy domestic life showed him that an educated woman might be the center of intellectual influence, galvani seems to have had no spirit of opposition to even the highest education for women. this is very well illustrated by the first formal lecture in his course on anatomy at the university, which had for its subject the models for the teaching of anatomy that had been made by madame manzolini.[ ] in the early part of the eighteenth century, madame manzolini had been the professor of anatomy at the university of bologna, and in order to make the teaching of this difficult subject easier and more definite, she modeled with great care and delicate attention to every detail, so that they imitated actual dissections of the human body very closely, a set of wax figures, which replaced the human body for demonstration purposes, at least at the beginning of the anatomical course. galvani, in taking up the work of lecturer in anatomy, appreciated how much such a set of models would serve to make the introduction to anatomical study easy, yet at the same time without diminishing its exactness, and accordingly introduced his students to madame manzolini's set of models in his very first lecture. at the time, not a few of the teachers of anatomy at the italian universities were inclined to consider the use of these models as rather an effeminate proceeding. galvani's lack of prejudice in the matter shows the readiness of the man to accept the best, wherever he found it, without regard to persons or feelings. galvani's personal character was very pleasant, yet rather grave and serious. his panegyrist, professor giuseppe venturoli, in the eulogium of galvani, delivered in the public academy of the institute of bologna ( ) within five years after galvani's death, says that galvani was far from that coldness or lack of interest which sometimes characterizes scientists in their social relations, and which, as he naïvely says, is sometimes praised and sometimes blamed by those who write about them. another side of galvani's character is more interesting. he was ready to do all in his power for the poor. he conducted his obstetrical clinic particularly with a liberal benevolence and charity that deserve to be mentioned. when it is considered how much time his teaching and his charity took from him, it is rather surprising to find that he had enough left to enable him to devote himself with so much success to the difficult tasks he set himself in research and to the time-taking labors of controversy, which occupied many years after the announcement of his discoveries. the most striking proof of the thorough conscientiousness with which he faced the duties of life is to be found in his conduct after the establishment of the so-called cis-alpine republic in italy. this was a government established merely by force of arms, maintained through french influence, without the consent of the people, and a plain usurpation of the rights of the previous government. galvani considered himself bound in duty to the authority under which he had lived all his previous life and to which he had sworn fealty. when the university of bologna was reorganized under the new government, the first requirement of all those who were made professors was that they should take the oath of allegiance to the new government. this he refused to do. his motives can be readily understood, and though practically all the other professors of the university had taken the oath, he did not consider that this freed him from his conscientious obligations in the matter. accordingly he was dropped from the roll of professors and deprived of the never very large salary which he had obtained from this chair. on this sum he had practically depended for his existence, and he began to suffer from want. while he had been a successful practitioner of medicine, especially of surgery, he had always been very liberal, and had spent large sums of money in demonstrations for his lectures and personal experimentation and in materials for the museums of the university. he began to suffer from actual want, and friends had to come to his assistance. he refused, however, to give up his scruples in the matter and accept the professorship which was still open to him. finally, at the end of two years, influence was brought to bear on the new government, and galvani was allowed to accept his chair in the university without taking the oath of allegiance. this tribute came too late, however, and within a short time after his restoration to his professorship he died. galvani's conduct in this affair is the key-note to his character and conduct through life. for him duty was the paramount word, and success meant the accomplishment of duty. for getting on in the world and material rewards he had no use unless they came as the consequence of duty fulfilled. his action in the matter of the university professorship has of course been much discussed by his biographers. his eulogist, professor venturoli, whom we have already quoted, and whose eulogium is to be found in the complete edition of galvani's works issued at bologna in ,[ ] has much to say with regard to galvani's religious sentiments. he says: "the great founder in electricity was deeply religious, and his piety clothed a heart that was not less affectionate and sensitive to affection than it was intrepid and courageous. when called upon to take the civic oath in a formula involved in ambiguous words, he did not believe that he ought, on so serious an occasion, to permit himself anything but the clear and precise expression of his sentiments, full as they were of honesty and rectitude. refusing to take advantage of the suggestion that he should modify the oath by some declaration apart from the prescribed formula, though it might still be generally understood that he had taken the oath, he refused constantly to commit himself to any such subterfuge. it is not our duty here to ask whether his conclusion was correct or not. he followed the voice of his conscience, which ever must be the standard of duty, and it certainly would have been a fault to have deviated from it. it is sad to think that this great man, deprived of his position, saw himself, for an instant at least, exposed to the danger of ending his career, deprived of the recompense which he so richly deserved and to which his past services to the state and the university had given him so just a title. this is all the more sad when we realize that the vicissitudes of his delicate health, much more than his age, now rendered such recompense doubly necessary. it is a gracious thing to recall, however, the noble firmness with which he maintained himself against so serious a blow. his courage is all the more admirable as one can see how absolutely without affectation it is. he was not ostentatious in his goodness, and did not permit himself to be cast down by the unfortunate conditions, but constantly preserved in the midst of adverse fortune that modest, imperturbable and dignified conduct which had always characterized him in the midst of his prosperity and his glory." that his action in this matter was very properly appreciated by his contemporaries, and that the moral influence of his example was not lost, can be realized from the expressions used by alibert, the secretary-general of the medical society of emulation, in the historical address on galvani which he delivered before that society in paris in : "galvani constantly refused to take the civil oath demanded by the decrees of the cis-alpine republic. who can blame him for having followed the voice of his conscience--that sacred, interior voice which alone prescribes the duties of man and which has preceded all human laws? who could not praise him for having sacrificed all such exemplary resignation, all the emoluments of his professorship, rather than violate the solemn engagements made under religious sanction?" in the same panegyric there is a very curiously interesting passage with regard to galvani's habit of frequently closing his lectures by calling attention to the complexity yet the purposefulness of natural things, and the inevitable conclusion that they must have been created with a definite purpose by a supreme being possessed of intelligence. at the time that alibert wrote his memoir, it was the fashion to consider, at least in france, that christianity was a thing of the past, and that while theism might remain, that would be all that could be expected to survive the crumbling effect of the emancipation of man. he says: "we have seen already what was galvani's zeal and his love for the religion which he professed. we may add that, in his public demonstrations, he never finished his lectures without exhorting his pupils to a renewal of their faith, by leading them always back to the idea of the eternal providence which develops, preserves and causes life to flow among so many different kinds of things. i write now," he continues, "in the age of reason, of tolerance and of light. must i then defend galvani in the eyes of posterity for one of the most beautiful sentiments that can spring from the nature of man? no; and they are but little initiated in the saner mechanism of philosophy who refuse to recognize the truths established on evidence so strong and so authentic. _breves haustus in philosophia ad atheismum ducunt, longiores autem reducunt ad deum_--small draughts of philosophy lead to atheism, but longer draughts bring one back to god"--(which may be better translated, perhaps, for english readers by pope's well known lines, "a little learning [in philosophy] is a dangerous thing; drink deep or touch not the pierian spring"). galvani has been honored by his fellow-citizens of bologna as one of their greatest townsmen, and by the university as one of her worthiest sons. in , a medal was struck in his honor, on the reverse of which, surrounding a figure of the genius of science, were the two legends: "mors mihi vita," "death is life for me," and "spiritus intus alit," "the spirit works within," which were favorite expressions of the great scientist while living, and are lively symbols of the spirit which animated him. in , a monument was erected to him in the courtyard of the university of bologna. it is surmounted by his bust, made by the most distinguished bolognian sculptor of the time, de maria. on the pedestal there are two figures in bas-relief, executed by the same sculptor, which represent religion and philosophy, the inspiring genius of galvani's life. before he died, he asked, as had his favorite poet dante, whose divina commedia had been one of the pleasures of life and above all one of the consolations of his times of adversity, to be buried in the humble habit of a member of the third order of st. francis. he is said to have valued his fellowship with the sons of the "poor little man of assisi" more than the many honorary fellowships of various kinds which had been conferred upon him by scientific societies all over europe. with him passed away one of the great pioneers of modern science and one of the most lovable men in all the history of science. his death took place just before the close of the eighteen century, dec. , , but his work was destined to be one of the harbingers of a great period of electrical development. footnotes: [ ] fordham university press, . [ ] ranam dissecui, atque praeparavi ut in fig. tab. v. eamque in tabula, omnia mihi alia proponens, in qua erat mechina electrica fig. , collocavi ab ejus conductore penitus sejunctam, atque haud brevi intervallo dissitam; dum scalpelli cuspidem unus ex iis, qui mihi operam dabant, cruralibus hujus ranae internis nervis dd casu vel leviter admoveret, continuo omnes artuum musculi ita contrahi visi sunt, ut in vehementiores incidisse tonicas convulsiones viderentur. eorum vero alter, qui nobis electricitatem tentantibus praesto erat, animadvertere sibi visus est, rem contingere dum ex conductere machinae scintilla extorqueretur fig. b. rei novitatem ille admiratus de eadem statim me alia omnino molientem ac mecum ipso cogitantem admonuit. his ego incredibili cum studio, et cupiditate incensus idem experiundi, et quod occultum in re esset in lucem pro ferendi admovi propterea et ipse scalpelli cuspidem uni vel alteri crurali nervo, quo tempore unus aliquis ex iis, qui aderant, scintillam eliceret. phoenomenon eadem omnino ratione contigit; vehementes nimirum contractiones in singulos artum musculos, perinde ac si tetano praeparatum animal esset correptum, eodem ipso temporis momento inducebantur, quo scintillae extorquerentur. [ ] for the sake of those who might care to see how the great dutch naturalist expressed these curious scientific notions in latin, the original text seems worth while giving. "jucundissimum porro juxta ac utilissimum experimentum aliud institui potest, si quidam e maximis musculis de ranae femore separetur, atque una cum adhaerente suo nervo ita praeparetur, ut hic illaesus permaneat. quodsi enim, hoc peracto, utrumque musculi hujus tendinem a, a manibus prehenderis. nervumque ejus propendentem forsicula aliove quodam instrumento de in irritaveris b; pristinum, quem amiserat, motum suum mox recuperabit musculus. videbis hinc ilico eum contrahi, binasque manus, quae tendines ejus adtinent, ad se mutuo veluti adducere: prout olim jam, anno , illustrissimo duci hetrusco, cummaxime regnanti, demonstravi; quum is immerito sane favore ad me invisere non dedignaretur. hoc ipsum veto experimentum eodem in musculo tam crebro & diu reiterari potest, donec ulla nervi pars illaesa fuerit: ut ideo toties sic ad pristinam contractionem suam lacessere musculum valeamus, quoties nobis libuerit." [ ] with galvani's attention to medical electricity, it is not surprising that for several years, beginning with , an italian medical journal called il galvani, with the sub-title giornale di elettro-idro-ed aero terapia, was published at milan. its directors were the brothers themistocles and ulysses santopadre. those who think that an exaggeration of claims for electrical influence on various diseases is of comparatively recent date, will do well to consult that journal. the prophylaxis of yellow-fever is suggested by means of static electricity. the cause of yellow-fever is declared to be a disturbance of the electro-magnetic conditions of the body. everything, from skin diseases to uterine inertia, chloroform asphyxia, aphasia, and the various forms of paralysis, and basedow's disease, are described as cured by electrical treatment. so does science become the nursing mother of quackery. [ ] edinburgh, . [ ] in , one of the theses presented for the fellowship of the royal college of surgeons of edinburgh was on the subject of galvanism, or at least on galvani's work, by francis barker, who signs himself hibernicus, an evidence of the fact that irishmen often went to edinburgh for their scientific training. this thesis serves to show that galvani's work was already attracting the attention even of the most distant of western universities. [ ] it is interesting to note that the two successful inventions for lessening the necessity for deterrent dissecting work are due to women--professor manzolini and her wax models, and alessandra giliani, the assistant of mondino, father of dissection, (d. ), who knew how "to fill the veins with various colored fluids which would harden, and paint these same vessels and color them so naturally that they brought mondino great fame and credit." (old chronicler.) [ ] opere edite ed inedite del professore luigi galvani raccolte e pubblicate per cura dell'accademia delle scienze dell'instituto di bologna, bologna tipografia di emilio dall'olmo. mdcccxli. chapter v. volta the founder of electrical science. up to the end of the eighteenth century, discoverers in electrical science had usually been students of science in other departments, whose attention to electricity had been attracted in passing as it were. occasionally, indeed, they had been only interested amateurs, inquisitive as to the curious phenomena of magnetism. it is surprising how many of these pioneers in electricity were clergymen, though that fact is seldom realized. it can be seen very readily in my chapter on clergymen pioneers in electricity, in catholic churchmen in science (second series, dolphin press, phila., ). with volta's career, however, was initiated the story of the electrical scientists who devoted themselves almost exclusively to this department of physics, though more or less necessarily paying some attention to related subjects. volta's discovery of a practical instrument for measuring electricity, as well as of comparatively simple apparatus producing a continuous current, changed the whole face of the science of electricity. after these inventions, regular work could be readily done in the investigation of problems in the science of electricity without discouragement or inadequate instruments, discontinuous electrical phenomena, disturbances of experiments by the weather, and other conditions which had been hitherto so unfavorable to electrical experimentation. volta's invention of the pile, or battery, so deservedly called after him, caused electrical science to take on an entirely new aspect, and the modern development of electricity was assured. it has been well said that no other invention, not even the steam-engine, meant so much for the transformation of modern life as this new apparatus for the production of a continuous electric current. [illustration: alessandro volta] the man who worked this revolution in electrical science was no mere inventor who, by a happy chance, brought together practical factors that had been well known before but had never been combined. he was one of the greatest scientists of a period particularly rich in examples of original scientific genius of a high order. before his death, he came to be acknowledged by the scientific world of his time as one of the greatest leaders of thought, not alone in electricity, but in all departments of the physical sciences. his life forms for this reason an important chapter in the history of science and scientific development. like most of the distinguished scientific discoverers of the last two centuries, alessandro volta was born in very humble circumstances. his father was a member of the italian nobility, but had wasted his patrimony so completely that the family was in extreme poverty when the distinguished son was born, on the eighteenth of february, . this poverty was so complete that volta said of it, later in life: "my father owned nothing except a small dwelling worth about fourteen thousand lire; and as he left behind him seventeen thousand lire of debt, i was actually poorer than poor." a good idea of the circumstances in which volta's childhood was passed may be gathered from the fact that he could not even secure copy-books for his first school exercises except through the kindness of friends. volta had shown signs of genius from early boyhood, and yet had been discouragingly slow in his intellectual development as a child. in fact, it was feared that he was congenitally lacking in intelligence to a great degree. it is said that he was more than four years old before he ever uttered a word. this does not mean before he learned to talk connectedly, but before he could utter even such familiar expressions as "father," "mother," and the like. he was considered to be dumb; and, as is not infrequently the mistaken notion with regard to children dumb for any reason, he was thought to be almost an idiot. the first word he ever uttered is said to have been a vigorous "no!" which was heard when one of his relatives insisted on his doing something that he did not wish to do. at the age of seven, however, he had so far overcome all difficulties of speech as to be looked upon as a very bright child. owing to this late, unexpected development, his parents seem to have regarded him as a sort of living miracle, and felt certain that he was destined to accomplish great things. his father said of him later, "we had a jewel in the house and did not know it." fortunately for volta, one of his uncles was archdeacon of the cathedral, and another was one of the canons. these relatives helped him to obtain an education, the way being made especially easy by the fact that at this time all the jesuit colleges subsisted on foundations and collected no fees from any of their students; so that all that was necessary for his uncles to do for him was to contribute to his expenses outside of college. according to tradition, the jesuits not only helped volta in his education, but assisted him in obtaining his books and even in his living expenses while at their college. at the age of about sixteen, his education was complete, even including a year of philosophy. this is probably an indication of his talent as a student; though it was not an unusual thing in the southern countries for students to graduate at sixteen, or even younger, after a course equivalent to that now required for the bachelor's degree in arts. we have gotten far away from this early graduation, although it is still sometimes possible in italian universities; and one of the brightest men i ever knew was an italian who had graduated with a degree equivalent to our a. b. before he was sixteen. when volta graduated, however, such early completion of the undergraduate course was not at all unusual in italy, and boys of thirteen and fourteen, almost as a rule, entered the undergraduate department to complete their course for a degree at seventeen or eighteen. one of our greatest physicians in this country, benjamin rush, was only seventeen when he completed his college course, and such examples were not at all rare. indeed, the possibility for these men to devote themselves much earlier than is possible now to their serious life-work, yet with the development of mind which comes from a university course in the arts, was probably a distinct help to the success of their scientific careers. one is tempted to think that possibly such justification of earlier graduation, as we find among the distinguished scientists of a century ago, might make us reflect deeply before lending ourselves to what herbert spencer thought a phase of evolution, the lengthening of childhood, for it is just possible that the earlier recognition of manhood may mean more for individual development. of course, geniuses are exceptions to rule, and an argument founded on their careers may mean very little for the generality of students. like many another of the great scientists, volta was not that constant source of satisfaction to his teachers while at school that might possibly be expected. he had little interest in the conventional elementary education of the time, he was frequently distracted during school hours, and even as a mere boy often asked questions with regard to natural phenomena that were puzzlers to his masters, and sometimes complained of their lack of knowledge. he fortunately outgrew this priggishness, for in later childhood he seems to have been one of those talented children who learn rapidly and who are impatient at being kept back while their slower fellow-pupils are having drilled into them what came so easy to readier talents. in his classical studies, however, volta was deeply interested. he was especially enthusiastic over poetry, and at school devoted the spare time that his readiness of acquisition left him to the reading of virgil and tasso. these favorite authors became so familiar to him that he could repeat much of them by heart, and even in old age could cap verses from them better than any of his friends, even those all of whose lives had been devoted exclusively to literary occupations. during his walks, when an old man, he often entertained himself by repeating long passages from the classic latin and italian poets. even at this time, volta's interest in the physical sciences was very marked. there is still extant a latin poem of about five hundred verses, in which he sets forth the observations of priestley, the discoverer of oxygen, whom it used to be the custom to call the father of modern chemistry. this poem shows his thorough familiarity with the work of the great english investigator. volta's model was lucretius. lest it should be a source of surprise that an italian scientist had recourse to latin for even a poetic account of scientific discoveries, it may be well to recall that latin was still the universal language of science at that time, and volta's great contemporary in electricity, galvani, wrote his original monograph on animal electricity in that language, and even the father of pathology wrote his first great treatise, de causis et sedibus morborum, in that tongue. as to his adoption of verse as a vehicle for scientific writing, it must not be forgotten that, at the time when volta was writing his poem, another distinguished writer on scientific subjects, erasmus darwin, the grandfather of charles darwin of the last generation, was composing his "zoonomia; or, animal biography," in english verse. didactic verse was quite the fashion of the time, and some of it, even when it came from acknowledged poets, had not more poetry than volta's effusion. as if to make up for his lack of linguistic faculty when young, volta seems to have had a special gift for languages when he grew older. before the age of twenty, he knew french as well as his mother tongue, read german and english fluently, and low dutch and spanish were not beyond his comprehension. besides his verses in latin he wrote poetry also in french and italian, always with cleverness at least, and at times with true poetic feeling. while attending the jesuit school, he expressed, it is said, a desire to enter the order. as his father, however, had been with the jesuits for eleven years and had then given up his studies, his family feared a repetition of such an experience; and so his clergymen uncles took him away from the school and sent him for a while to the seminary at benzi. after a time volta abandoned the idea of becoming a priest, but would not consent to follow the wishes of the family council further, at least not to the extent of becoming a lawyer. though he studied law for a time, he constantly wandered away to the reading of books on the natural sciences and to the study of natural objects. finally he was allowed to give up law to devote himself exclusively to science. fortunately, one of the canons of the cathedral of como, a former fellow-student of his and a man of considerable means, was also interested in the natural sciences, and obtained the books and instruments necessary to enable volta and himself to continue their studies. father gattoni seems to have realized at once the possibilities for great advances in science that lay in volta's wonderful powers of observation, and encouraged him in every way. as a consequence, some of the important experiments that laid the foundation of the modern science of electricity and proved the beginning of volta's world-wide reputation were carried on in gattoni's rooms. as a young man, volta was so completely devoted to scientific investigations that there could be no doubt of the bent of his genius for original work of a high order. his power of concentration of attention on a subject was supreme. biographers emphasize that there was no time, much less inclination, for the levities that so often appeal to the growing youth. he was almost too staid and preoccupied with his work for his own health and the comfort of his friends. when he became interested in a series of experiments, he often forgot the flight of time, and was known to miss meals, and inadvertently to put off going to bed--apparently quite unconscious of his physical necessities. this intense concentration of mind had its disadvantages. one of his friends complained playfully that he made a rather disagreeable traveling companion on account of his tendency to become abstracted; and on occasions this friend was deeply mortified to see volta, when in company, take out a pocket-handkerchief that had been used for some purpose in the laboratory--which showed unmistakable signs of its previous employment as a cleansing agent for dirty instruments or hands, though its possessor was evidently unconscious of its appearance. more than once, too, his handkerchief proved, when taken out for its natural uses, to be as preoccupied as its owner: specimens of rocks or natural curiosities that he had gathered and inadvertently allowed to remain in his pocket came with it. all during his life he retained an unusual faculty for concentrating his attention, which at times amounted to complete abstraction from his surroundings. it is related that, one cold morning his students at the university of pavia found him in his shirt sleeves, so intent on arranging the experiments that were to illustrate his morning lecture that he was unconscious of the time, and even did not notice their coming into the room until they had been for some time in their seats and he had finally completed the arrangement for the demonstrations. he was constantly occupied with problems in natural science, looking for the explanation of phenomena that he did not understand as well as gathering new data by observation and experiment. he was gifted with the supremely inquisitive spirit, in the scientific sense of the epithet, and could not be satisfied with accepting things as he found them without knowing the reasons for them. volta furnishes another excellent illustration of how soon genius gets at its life-work. we have his own authority for the fact that he had come to certain conclusions with regard to the explanation of electrical phenomena, which, when he was only nineteen years of age, he set forth in a letter to the abbé nollet, who was then one of the best known experimenters and writers on electrical phenomena in europe. though so young, volta had tried to simplify franklin's theory of electricity by assuming that there was an action only between a (supposed) electrical substance and matter. it is curious to see how much he anticipated what was to be the thinking for more than a century after his time and practically down to the present day. he considers that all bodies, in their normal state, contain electricity in such proportion that electrical equilibrium is established within them. electrical phenomena, then, are due to disturbances of this equilibrium. such disturbances may be produced by physical means, as by friction or by chemical means, and even atmospheric electricity may be explained in the former way. volta's first formal paper on electricity, bearing the title _de vi attractiva ignis electrici_, was published in , when he was twenty-four years of age. his second paper, _novus ac simplicissimus, electricorum tentaminum apparatus_--new and very simple. apparatus for electrical tests, shows that volta was getting beyond the stage of theorizing about electricity into the experimental work, which was to form the foundation of his contributions to electrical science. it is not surprising, then, that when he was just past thirty, in , he was able to announce to priestley his invention of the electrophorus. priestley is usually thought of as one of the founders of modern chemistry, but he was known to his own generation, especially at this time, as the writer of a very interesting and complete history of electricity. it is characteristic of volta's careful ways, that the reason for his letter to priestley was in order to obtain information from him as to what extent this invention, which volta knew, as far as he was concerned, to be original with himself, was novel in the domain of electrical advance.[ ] with the intense interest in his work that we have noted, it is not surprising to find volta's investigations proving fruitful. his active inventive genius stood him in good stead in enabling him to demonstrate principles by working instruments. the electrophorus is but one of the instruments that show the very practical character of the man. he was especially taken with the idea of securing some method of measuring electricity. among other things, he invented the condensing electroscope, in which, instead of the ribbons of gold leaf now employed, he used straws. with this instrument he was able to demonstrate the presence of minute quantities of electricity developed under circumstances in which ordinarily the occurrence of any such phenomena would be unsuspected. these two instruments, the electroscope and the electrophorus, lifted the department of electricity out of the realm of theory into that of accurate scientific demonstration, and made the electrical departments of the physical laboratories of the time much more interesting and important than they had been before. though so early occupied with electricity, volta did not confine himself to this subject, nor even to the wider field of physics, and that he did not hesitate, in his scientific inquisitiveness, to follow clues even in chemistry, is well illustrated by his first step in the investigation of gases. his attention being called to bubbles breaking on the surface of lake maggiore while on a fishing excursion, he set about finding their source, and noted that whenever the bottom of the lake near the shore was stirred somewhat a number of bubbles arose, and that the gas thus set free was inflammable. he constructed an electrical pistol in which gases thus set free were exploded by a spark from the electrophorus. about the same time, on the principle of the electrical pistol, he invented the eudiometer, an apparatus by means of which the oxygen content of air could be determined. with regard to these inventions, arago calls attention to a special quality that is peculiar to all of volta's work. "there is not a single one of the discoveries of professor volta," says the distinguished french scientist, "which can be said to be the result of chance. every instrument with which he has enriched science existed in principle in his imagination before an artisan began to put it into a material shape." after these inventions and his previous work, it is not surprising that in volta was offered the professorship of experimental physics in the college of como. here he labored for five years, until he received a call, in , to the professorship of physics at the university of pavia, where he was destined to remain in an active teaching capacity for a period of forty years. volta began his life-work as professor of physics at pavia by extending his observations on gases. he was the first to demonstrate the expansion of gases under heat, especially as regards their increased expansibility at higher temperatures. many observers had been at work on this problem before his time, but there were serious discrepancies in the results reported. volta was the first to point out the reasons for the apparent inconsistencies of previous investigators' findings; and from his observations alone some valuable data might have been obtained for the establishment of what has since become known as the "law of charles." at this time, his knowledge of english enabled him to follow english discoveries closely, and he seems to have paid particular attention to the work of cavendish and priestley. not long after cavendish's description of the method of obtaining pure hydrogen, volta made a series of observations on the relations of spongy platinum to this gas, and pointed out the spontaneous ignition that takes place when the two substances are brought together. this experiment is the basis of what has since been known as the hydrogen lamp, called, from the german observer who first made it a practical instrument, dobereiner's lamp. after seven years of teaching, volta was given the opportunity to visit various parts of europe, and took advantage of the occasion to meet most of the celebrated men of science. his linguistic faculty stood him in good stead during this sabbatical year, and his travel aided him in completing a thorough acquaintanceship with european languages as well as with scientists. his practical character led him, during his trip, to note the growth of the potato and its uses in various european countries, and he brought the plant home with him to italy in order to introduce it among the farmers. he succeeded in making his countrymen realize its value, and the introduction of the potato is one of the reasons for which italians have always looked up to him as a benefactor of his native land. how modern this makes a vegetable we are inclined to think of as having been always an important food resource of the race! about the middle of the third quarter of the eighteenth century, by one of the fortunate accidents that happen, however, only to genius, galvani, at the time professor of anatomy in bologna, had been led to make the observation that if a frog, so prepared that its hind leg is attached to the trunk only by means of the sciatic nerve, happens to be touched by a metal instrument in such a way as to put nerve and muscle in connection with each other through the metal instrument, a very curious phenomenon is observed, the muscles of the almost severed leg becoming spasmodically contracted and then relaxed whenever the contacts were made and broken. galvani noted the phenomenon first in connection with an electric machine, and looked for an explanation of it in electricity, thinking that there was an analogy between it and the discharge of the leyden jar. after several years of careful observation, he published a monograph on the subject, which at once attracted attention all over europe. volta was very much interested in galvani's work, and took up the development of it from the physical side. at first he agreed with the explanation offered by galvani, who considered that his experiment demonstrated the presence of electricity in animal bodies, and who proposed to introduce the term "animal electricity." after careful investigation, however, galvani's assertion that animal electricity existed in a form entirely independent of any external electricity, though it had been accepted by most of the distinguished men of science of the time, seemed to volta without experimental verification. for many years his most determined efforts were used to demonstrate that the muscle twitchings observed were not due to the presence of animal electricity (galvanism as it had come to be called), but to the fact that the metals touching the different portions of the moist nerve muscle preparation really set up minute currents of ordinary electricity. some of the experiments which he devised for this purpose were extremely ingenious, and show how thoroughly empirical were his methods and how modern his scientific spirit. in the course of his experiments he found that a difference in the metals of which the arc was composed, when used for the purpose of eliciting the so-called animal electricity, made a great difference in the electrical phenomena observed and in the amount of muscle twitchings obtained. in one brilliant series of experiments, moreover, he showed that, even when the metallic portions touching nerve and muscle were identical, there might still be distinct electrical phenomena, if only an artificial difference in temperature of the end of the metallic arc were produced. volta was even able to demonstrate that such minute physical differences as the filing of one end of the metallic arc used might give rise to small currents of electricity. in the midst of these experiments, he came to the realization that two portions of metal of different kinds, separated by a moist non-conducting material, might be made to produce a constant current of electricity for some time. more than this, however, he found that discs of metal of different kinds might be piled on top of one another with intervening discs of moist cloth, and so produce proportionately stronger currents as more and more of the metal plates were employed. this was the origin of the voltaic pile, as it has been called--the first battery for the production at will of regular currents of electricity of definite strength.[ ] while engaged at this he succeeded in demonstrating what has come to be known as volta's basic experiment; namely, that two plates of metal of different kinds become electrically excited merely by contact. this was practically the beginning of the great advance in applied electricity which ushered in our modern electrical era. it seems a simple matter now, looking back over the century that has elapsed since then, to have taken the successive steps that volta did for the construction of his electrical pile and for the demonstration of the principle of contact electricity. groping, as he was, in the dark, however, it took him three years to make the progress that we have described in a few words. how great his discoveries appeared, even to the most distinguished of his scientific contemporaries, can best be judged from an expression of one of the greatest of french electrical scientists, arago, who declared "volta's pile the most wonderful instrument that has ever come from the hand of man, not excluding even the telescope or the steam-engine." an excellent description of just how volta made his electric pile and what he was able to accomplish with it experimentally in the laboratory, is to be found in the numbers for january and february, , of the stimmen aus maria-laach--a literary and scientific periodical published by the german jesuits. this article on alessandro volta, by father kneller, s. j., was written shortly after the celebration of the hundredth anniversary of volta's invention of the electric pile, when there had just been a fresh sorting over of volta's documents, and contains a very full set of references to the biographical material for volta's life. father kneller says: "before this, no one thought for a moment of any possibility of the practical application of electricity. but all at once the whole situation changed. after eight years of observation and experiment, volta accomplished one day, at the beginning of , in his laboratory at como, the construction of an instrument which was to revolutionize the study and the practical applications of electricity. he made a pile composed of a large number of equal-sized copper and zinc discs. on each copper disc he placed one of zinc, and then on this a moistened piece of cloth, and continued the series of alternate discs and cloths in this order until he had a rather high column. this was an apparatus as simple as possible and from which no one but volta could possibly have promised any results. the inventor, however, knew what he was about. "as soon as he had connected the upper and lower metal plates by means of a wire, there began to flow from the zinc to the copper a secret something, which by the application of the ends of the wire to muscles caused them to twitch; which appeared before the eye as light; applied to the tongue, gave a sensation of taste; caused a thin wire to glow and even to burn between carbon points; produced a blinding light; decomposed water into its constituents; dissolved hitherto unknown metals out of salts and earth; made iron magnetic; directed the magnetic needle out of its path; inclosed wire coils caused new electric currents to be set up; to say nothing of the awful spectacle which occurred when, under the influence of the electric current, the bodies of executed criminals again gave movements of the limbs, their thoraxes really heaved and sank as if they really breathed, and even a dead grasshopper was caused to spring and apparently to sing again. "only now, after the discovery of this new kind of electricity--which did not work merely by jerks, but flowed in a constant stream from pole to pole--only now was this mighty natural agent won to the service of man. volta is, therefore, above all others, the one who broke ground not only for an immense amount of new knowledge in physics, chemistry and physiology, but who also made possible rapid progress in practical electricity, in telegraphy, in electric motors and power machines, in electroplating and the marvelous results in electro-galvanism which constitute our most wonderful mechanical effects at the present time." soon after volta's discovery of the electric pile, or voltaic pile, as it was called in his honor, his reputation spread throughout europe. at the beginning of , he sent a detailed description of the voltaic pile to the royal society of london. during the year the scientific journals all over europe were filled with discussions of his discovery. the french academy of sciences invited him to paris in order to demonstrate his discoveries to the members of that body. volta was now looking forward to some peaceful years of study, and, so far as he was personally concerned, would surely have refused the invitation. circumstances were such, however, that it became a civic duty for him to proceed to paris. at this time napoleon was first consul, and the italian cities wished to propitiate his favor as far as possible. it was considered a wise thing by the city to send a special delegation to paris, and, as they knew napoleon was deeply interested in scientific discoveries that promised practical results, the name of volta was suggested as one of the official delegates. as an associate, professor brugnatelli, who had made some important investigations in chemistry, and who was later to be an extender of the practical application of volta's discoveries by the invention of the first method of electroplating, was the other member of the delegation. it is a curious reflection on the facilities for travel at the time, that it took twenty-six days for the delegates to reach paris from pavia. shortly after their arrival in paris, the travelers were formally introduced to the members of the french institute, and a number of sessions of the academy were held, at which volta's discoveries were discussed. volta read a communication on the identity of electricity and galvanism. napoleon, as first consul, was present at these sessions in the robe of an academician, and was not only an interested listener, but occasionally, by pertinent questions, drew out significant details of former experiments and volta's own theories with regard to the nature of the phenomena observed. at the end of the first meeting, at which volta took a prominent part, napoleon spent several hours with him talking about the prospects of electricity. in his letters to his brothers and to his wife at this time, volta expressed his pleasure at finding how much attention his discoveries were attracting all over europe. as he said himself, germany, france and england were full of them, and all the distinguished scientists were eager to do him honor. in france, he was chosen one of the eight foreign members of the institute, and was made knight commander of the legion of honor and of the order of the iron crown. napoleon selected him as one of the first members of the italian academy, which he was then in course of establishing, and conferred on him the honor of senator and count of the kingdom of italy. the french academy, after having heard volta's own description of his experiments and discoveries, contrary to its usual custom, voted to him by acclamation its gold medal. more important still, bonaparte made him a present of lire, and conferred upon him an annual income of lire from the public purse. it is an index of volta's feeling as a faithful son of the church, that as this income was allotted to him from the revenues of the bishopric of adria, he would consent to receive it only after napoleon's decree had been confirmed by the pope. volta had been for nearly twenty years in the university of pavia before he finally found for himself a wife. he was then past forty-nine years of age. his wife was the youngest daughter of count ludovico peregrini. she had six sisters, one of whom became a nun, and all the others were married before volta sought the hand of the youngest. writing to a friend, he says, "that her sisters had distinguished themselves so much by piety, prudence, good sense and practical economy in their households as well as by the most admirable qualities of heart and mind, that he considered himself very fortunate in obtaining a branch from the family tree; and he took her in preference to others that had been offered to him, even though they were possessed of greater physical beauty, more exalted piety and a larger dowry." the marriage seems to have been a very happy one, notwithstanding the considerable disparity of ages and the very matter-of-fact spirit with which it was entered into by one of the parties at least. the charming intimacy of his domestic life may be judged from some of his letters to his wife when he was traveling. she was always his confidante with regard to new things in science that he saw, and especially as regards the kindly reception which he met with from scientists and the readiness with which they accepted his views. at first, so many of his ideas were new, that it is not surprising that they were looked at somewhat askance by contemporary scientists. when, on his journeys through france, he noticed the trend of opinion setting in favor of his views in electricity, he took pains to tell his wife, and apparently found his greatest pleasure in having her share the joy of his triumph. one of the severest blows that he suffered was the untimely death of his eldest son, flaminio, in . "this loss," he wrote to one of his nephews not long after, "strikes me so much to heart that i do not think i shall ever have another happy day." the relations between himself and his children were all of the kindliest nature; and the character of the man comes out perhaps even more clearly in the traditions that are still extant with regard to the devotion of his servants to him, and especially his body-servant, polonio. volta was always a simple and unpretentious person, notwithstanding the fact that scientific and even political honors had been heaped upon him toward the end of his life. it was rather difficult, for instance, to get him to change his old clothes for new ones. this feat was usually accomplished by polonio, who, when he thought the time had arrived for his master to put on the newer clothes, would engage him in some scientific explanation of a morning; then handing him the new garments, volta would put them on, and would be wearing them for some time before he noticed it. the old servant was then generally able to persuade him that it was time to make the change. toward the end of his career, volta led a retired life in a country house not far from his native city of como. foreigners often came to see or even have the privilege of a few words with the distinguished scientist who was regarded as the patriarch of electrical science. to volta, the being on exhibition was always an unpleasant function. he did not care to be lionized, and frequently refused to allow himself even to be seen unless his visitors had a scientific motive. on such occasions, the only chance of the visitors was to secure the good will of polonio. he would engage his unsuspecting master in a discussion of clouds or wind, or some appearance in the heavens, or something in the leaves of the neighboring trees, and would then bring him to the portico, that he might see the supposed phenomenon. this would give occasion for the visitors to get at least a glimpse of the scientist, who usually failed to suspect the real purpose for which he had been tempted out of doors. while thus living in the country, volta's piety became a sort of proverb among the country people. every morning at an early hour, in company with his servant, he could be seen with bowed head making his way to the church. here he heard mass, and usually the office of the day, in which all the canons of the cathedral took part. he had a special place on the epistle side of the altar, not far from the organ. his favorite method of prayer was the rosary. he was not infrequently held up to the people by the parish priest as a model of devotion. whenever he was in the country, every evening saw him taking his walk towards the church. on these occasions, he was usually accompanied by members of his family, and they entered the church for an evening visit to the blessed sacrament. his behavior toward those who lived in the vicinity of his country place endeared him to all the peasantry. he was not only liberal in giving alms, but made it a point to visit frequently the houses of the poor and help them as much as possible by counsel and suggestion. his scientific knowledge was at command for their benefit, and he was often able to tell them how to avoid many dangers. he gave them definite ideas with regard to the importance of cleanliness and the necessity of cooking their food very carefully so as to prevent diseases occasioned by badly cooked material. he also taught them to distinguish between the wholesome and the spurred rye, from which their polenta was prepared, in order to escape the dreaded pellagra, the disease so common in italy, which comes from the use of diseased grain. he endeared himself so much to the people of the countryside that they invented a special name for him, which proclaimed the tenderness of their liking for the man. they knew how much he was honored for his wonderful discoveries in electricity, and many of them had even seen some of the (to them at least) inexplicable phenomena that he could produce at will by means of various electrical contrivances. at first they called him a "magician"; but as this word has, particularly for the italian peasantry, a suspicion of evil in it, they added the adjective "beneficent," and he was generally known as _il mago benefico_. his interest in these gentle, kindly people may be appreciated from the fact that he knew practically all of his country neighbors by name, and, as a rule, he was familiar also with the conditions of their families and their household affairs. not infrequently he would stop and talk to them about such things, and this favor was always considered as a precious mark of his neighborly courtesy by the peasantry. such was the simplicity of the man whose name is undoubtedly one of the greatest in the history of science. the great beginnings of the chapter on applied electricity are all his. there was nothing he touched in his work that he did not illuminate. his was typically the mind of the genius, ever reaching out beyond the boundaries of the known--an abundant source of leading and light for others. far from being a doubter in matters religious, his scientific greatness seemed only to make him readier to submit to what are sometimes spoken of as the shackles of faith, though to him belief appealed as a completion of knowledge of things beyond the domain of sense or the ordinary powers of intellectual acquisition. like pasteur, a century later, the more he knew, the more ready was he to believe and the more satisfying he found his faith. this is a very different picture of the great scientific mind from that ordinarily presented as characteristic of scientific thinkers. but volta is not an exception; rather does he represent the rule, so far as the very great scientists are concerned; for it is only the second-rate minds, those destined to follow but not to lead, in science, who have so constantly proclaimed the opposition of science to faith. volta's well-known confession of faith declares his state of mind with regard to religion better than any words of a biographer, and it is a striking commentary on the impression that has in some inexplicable way gained wide acceptance, that a man cannot be a great scientist and a firm believer in religion. a distinguished professor of psychology at one of the large american universities said not long since, that a scientist must keep his science and religion apart, or there will be serious consequences for his religion. volta's opinion in this matter is worth remembering. having heard it said that, though he continued to practice his religion, this was more because he did not want to offend friends, that he did not care to scandalize his neighbors, and did not want the poor folk around him to be led by his example into giving up what he knew to be their most fruitful source of consolation in the trials of life, while in the full exercise of his intellectual faculties, volta deliberately wrote out his confession of faith so that all the world of his own and the after time might know it. "if some of my faults and negligences may have by chance given occasion to some one to suspect me of infidelity, i am ready, as some reparation for this and for any other good purpose, to declare to such a one and to every other person and on every occasion and under all circumstances that i have always held, and hold now, the holy catholic religion as the only true and infallible one, thanking without end the good god for having gifted me with such a faith, in which i firmly propose to live and die, in the lively hope of attaining eternal life. i recognize my faith as a gift of god, a supernatural faith. i have not, on this account, however, neglected to use all human means that could confirm me more and more in it and that might drive away any doubt which could arise to tempt me in matters of faith. i have studied my faith with attention as to its foundations, reading for this purpose books of apologetics as well as those written with a contrary purpose, and trying to appreciate the arguments pro and contra. i have tried to realize from what sources spring the strongest arguments which render faith most credible to natural reason and such as cannot fail to make every well-balanced mind which has not been perverted by vice or passion embrace it and love it. may this protest of mine, which i have deliberately drawn up and which i leave to posterity, subscribed with my own hand and which shows to all and everyone that i do not blush at the gospel--may it, as i have said, produce some good fruit.--signed at milan, jan. th, , alessandro volta." when volta wrote this, he was just approaching his sixtieth year and was in the full maturity of his powers. he lived for twelve years after this, looked up to as one of the great thinkers of europe and as one of the most important men of italy of this time. far from being in his dotage, then, he was at the moment surely, if ever, in the best position to know his own mind with regard to his faith and his relations to the creator. there is a famous picture of volta, by magaud, in marseilles. it chronicles the fact that volta had become a count, a senator and a member of the french institute, so appointed by napoleon, and that he is in some sense therefore a frenchman. magaud has painted him standing, with his electric apparatus on one side and the scriptures on the other. near him is placed his friend sylvio pellico, whose little book, "my ten years' imprisonment," has endeared him to thousands of readers all over the world. pellico had doubted the presence of providence in the world and the existence of a hereafter. in the midst of his doubts, he turned to volta. "in thy old age, o volta!" said pellico, "the hand of providence placed in thy pathway a young man gone astray. oh! thou, said i to the ancient seer, who hast plunged deeper than others into the secrets of the creator, teach me the road that will lead me to the light." and the old man made answer: "i too have doubted, but i have sought. the great scandal of my youth was to behold the teachers of those days lay hold of science to combat religion. for me to-day i see only god everywhere." footnotes: [ ] wilcke, a swedish investigator of electric phenomena, constructed in two machines involving the principle of the electrophorus.--(brother potamian.) [ ] brother potamian has called my attention to the fact that volta's work on the origin of electricity from two different metals when, though connected, they were yet separated by some moist medium, was curiously anticipated by an observation described by sulzer, in a book called nouvelle théorie des plaisirs, . in this he states that, if a silver and a lead coin, placed one above and the other under the tongue, be brought in contact a sour taste develops, which he considers to be due to vibrations set up by the contact of the two metals. he seems also to have had a dash of light before the eyes, so that all the elements necessary for the discovery of the voltaic pile were in his hands, and indeed he was making what has since become one of the classical experiments, by which certain physiological effects of the electric current are demonstrated. chapter vi. coulomb. great discoverers in science must usually be satisfied with having their names attached to some one phase of scientific development, be it an instrument, a law, a unit of measurement, a process of investigation or some phenomenon which they first observed. the originality of coulomb's genius will be better appreciated, since besides having a unit of electricity named after him, there is also a law in electro-magnetics and a torsion-balance that will always be associated with his name. few men have been more ingenious in their ability to put complex ideas into practical shape and give them simple mechanical expression by instrumental methods. while his name is to be forever associated with the science of electrostatics, he was profoundly interested in other departments of physics, and for him to be interested always meant that he would illuminate previous knowledge by practical hints and suggestions and carry the conclusions of his predecessors a little farther into science than they had ever gone before. his was typically an experimental genius, and he must be considered one of the men of whom not more than half a dozen are born in a century, who are, in kipling's strong term, "masterless"; who do not need to be taught, but who find for themselves a path into the domain of the unknown. coulomb investigated the fundamental law in electricity and magnetism, that attractions and repulsions are inversely as the square of the distances, and showed that it held accurately for point-charges and point-poles. he demonstrated that these interesting phenomena were not chance manifestations of irregular forces, but that they represented a definite mode of action of force, thus setting this department of knowledge on a scientific basis. while in practical significance ohm's law, discovered nearly a half century later, is of much more import, coulomb's discoveries are fundamental in character and, coming in the very beginnings of modern electrical science, did much to guide the infant science in the ways it should follow. the establishing of this law contributed very largely to the rapid development of the twin sciences of electricity and magnetism. it is experimental observation that means most for a rising science; and, in fact, that coulomb should have been the pioneer in it stamps him as possessed not only of great originality, but also of the power of independent thinking, which is perhaps the most precious quality for the man of science. the french investigator succeeded in demonstrating his law by two distinct methods which are still used for illustration purposes in our physical laboratories. in the first, he employed the torsion-balance devised by michell, and re-invented by himself, an instrument of exact measurement which, in his hands, yielded as invaluable results as it did in those of faraday half a century later. the instrument depends on the principle first established by coulomb himself, that when a wire is twisted, the angle of torsion is directly proportional to the force of torsion. in the application of this principle, a fine wire is suspended in a glass case, on the sides of which there is a graduated scale to measure the degree of repulsion between two like poles of a magnet or between similarly electrified bodies. in his second research on the law of the inverse square, coulomb used what is known as the method of oscillations. a magnetic needle swinging under the influence of the earth's magnetism is known to act like a pendulum, and as such obeys the laws of pendular motion. in applying this method, coulomb caused the magnetic needle to oscillate, first, under the influence of the earth's magnetism alone and then under the combined influence of the earth and the magnet placed at varying distances from the needle. the most interesting feature of this work is the manner in which coulomb succeeded in eliminating the important factor of the earth's magnetism from the problem. it is so simple and ingenious that it commands the admiration of investigators, who employ it in their laboratory work even to the present day. it is clear, then, that the international committee which selected the term coulomb for the electromagnetic unit of electrical quantity gave honor where it was eminently due. coulomb stands out as a man of precision and accuracy, whose methods of exact measurement revolutionized the rising science, and whose researches and discoveries in physics and mechanics furnish ample justification for giving him a place among the makers of electricity. he was one of the gifted men whose original works ushered in so gloriously the nineteenth century, and who laid the deep and firm foundations on which the last three generations have built up the magnificent temple of electrical science. charles augustin de coulomb was born at angoulême, june th, . his ancestors for several generations had been magistrates, and were looked upon as representatives of the country nobility. he made his university studies in paris, and while still young, entered the army. from the very beginning, however, his genius for mathematics was recognized, and he was employed in the capacity of military engineer. to americans, it will be interesting to know that his first engineering project was undertaken at martinique, where he constructed fort bourbon. his sterling character and remarkable ability secured him rapid advancement in the service. in spite of the fact that the climate did not agree with him, he remained for three years on the island, because he would not employ the political influence that might have secured his recall, since he thought it his duty to serve his country in an important colonial post. nearly all his comrades perished by fever. it is the irony of fate that after his return to france a change in the ministry deprived him of the just recompense of his devotion to country, and he did not receive the special extraordinary promotion which he had earned in this special detail. during a short stay that he made at paris after his return, he sought the society of men of science as far as possible, and succeeded in getting in touch with all that was most promising in scientific progress at the time. he was already known rather favorably by many of the scientific men of the capital because of the paper on the statics of vaults, a monograph on static problems in architecture, which he presented to the academy of sciences in . his next military assignment was to rochefort. here he composed his monograph on "the theory of simple machines," which carried off the double prize that had been offered by the academy of sciences for the solution of problems connected with this important question. this attracted the attention not only of the scientific world, but also of his military superiors. as a result, he was sent successively to cherburg and to the isle of aix, to direct engineering works, and accomplished the tasks involved with success. two years later, when he was about forty-five years of age, he was elected member of the academy of sciences by a unanimous vote. he was a man of great personal magnetism, and all those who came in contact with him learned to like him for his straightforward character and for the absolute righteousness of his life. few men have made firmer friends than coulomb, as few have ever shown more unselfish devotion to duty and to conscience than he, though under circumstances that were neither spectacular nor theatrical. it was harder to face the deadly climate of martinique than it would have been to take one's place at the head of a forlorn hope in an outburst of enthusiastic courage; and coulomb was to have other trials of quite as deterrent a nature, and was to meet them with the same imperturbed sense of duty. graft is sometimes supposed to be temptation peculiar only to our own times, but the opportunities for it have always been present in such work as coulomb had to oversee, and the army engineer of all ages has had to stand or fall before it. it was proposed, about this time, to build a system of government canals in brittany. such a canal-system would, as is easy to understand, cost an enormous sum of money and give magnificent opportunities for speculation of various kinds. no small objection had been made to the project, on the score that it would not confer all the benefits on the region that were claimed for it, and coulomb was commissioned by the minister of marine to determine the question of the advisability of constructing the canals, and of the probable effect which they would have on the commerce of the country. after careful investigation, he came to the conclusion that the advantages which were expected to accrue from the project would not compensate for the enormous expense that would be entailed. this decision aroused the angry protest of a strong political faction, who expected to reap wealth and personal advantages of many kinds from the scheme, and who protested bitterly against coulomb's report. he was able to support his conclusions in the matter, however, with such unanswerable mathematical and engineering arguments, that his opinion prevailed and the project was given up. as a consequence, instead of the opportunity to serve a political party with every avenue to preferment and, above all, to wealth open for him, he found himself, for the time being, deprived even of the opportunity to devote himself further to his favorite occupations in military engineering. the excuse given for this interruption in his career, for there has always been an excuse for such action, was that proper representations for permission to make the report had not been made to the minister of marine; and instead of commendation, coulomb received what was practically a reprimand. wounded by this injustice, which was manifestly due to the fact that his honest report had displeased those who expected to reap personal benefit from the canal project, and disgusted with a service in which such things were possible, coulomb sent in his resignation. the minister of marine realized that the acceptance of the proffered resignation would surely expose the ministry at least to suspicion as to the reasons why coulomb's report was not accepted with good grace. permission to retire from the service was refused, as this would insure his silence. he was ordered back to brittany to continue his work there, possibly with the idea that this unfavorable experience would be sufficient of itself to make him understand what was expected of him and render him a little more complacent to the wishes of those in authority. if any such ideas were entertained, they were destined to grievous disappointment. coulomb was not of those who, seeing duty plainly, refuse to follow it because some personal advantage or disadvantage intervenes. selfish reasons did not appeal to his character nor obscure the issues. he went back to brittany, ready to express his firm opinion in the matter and with integrity of soul untouched. the consequence was that the provincial authorities, recognizing their true interests, acknowledged the error they had come near falling into, and now wished to reward the engineer handsomely for his unswerving devotion to duty. coulomb as promptly refused a reward for doing his duty as he had ignored even the appearance of a bribe to avoid it. only after considerable pressure was he prevailed upon to accept the best timepiece they could procure, on which the arms of the province were engraved. it had what was quite rare in those days, a second's hand, and he constantly made use of this in all his experimental work thereafter. a french biographer says that, never was a souvenir better chosen nor more suitably employed. coulomb's merits were recognized by the government authorities not long after, and he was made superintendent of the fountains of france. a few years later, he was promoted to the position of curator of plans and relief maps of the military staff of france, and was chosen as one of the commission of the french academy of sciences who went to england in order to study hospital conditions there. at this time, he was at the acme of his career. his grade was that of lieutenant colonel of engineers, a position much higher in the foreign armies at that time than would be the post with the corresponding title in our army. he had been made a chevalier of st. louis, and it looked as though a brilliant future were opening out before him. each year, for a decade, had seen the publication of one or more memoirs on important subjects, nearly every one of which contained some original material of the highest value, destined not only to add to coulomb's reputation, but to furnish basic information for the further development of science. in , however, the revolution broke out, and there was an end to all coulomb's opportunities for work. he was utterly out of sympathy with the movement, the worst consequences of which he foresaw from the beginning, and he at once handed in his resignation of the various positions that he occupied under the government. he went into almost absolute retirement, devoting himself to the education of his children. during this time, however, he did not cease to cultivate science, inasmuch as he gave the finishing touch to various papers which he had previously outlined. unfortunately, however, his departure from paris made it impossible for him to continue his investigations in electricity for want of apparatus, and so there is a ten years' interruption in his life of scientific activity and of original work. besides, it cannot be surprising that he should not have had the heart to go on with his work under the awful social conditions that prevailed. many of his friends lost their lives during the stormy period of the revolution; most of the others were banished or were in hiding. his beloved country had gone into an unfortunate eclipse, as he could not help but consider it; most of the nations of the earth were indeed in league against her, and the end was not yet in sight. it would be too much to expect of human nature that it should devote itself to abstruse problems in science at moments of such disturbance as this, and so some of the possibilities of coulomb's original genius were lost to science during that calamitous period. like many of the great discoveries of science, coulomb's most important work was done in the course of other investigations, and came by what might be called a happy accident. he had been investigating the qualities of wire of various kinds, especially with regard to their elasticity, so as to be able to determine the limits of their use in various engineering projects. when he discovered that the elasticity of torsion of a wire was a constant property, he proceeded to utilize it in the calculation of such delicate phenomena as those of electric and magnetic forces. the first instrument for this purpose that he constructed consisted simply of a long magnetized needle suspended horizontally by a fine wire. supposing this needle to be at rest, if one moves it away from the magnetic meridian by a certain number of degrees, the twisted wire will have a definite tendency to untwist and to bring back the needle to its original position by a series of oscillations whose frequency can be readily observed. for such observations, it is possible to obtain the value of the force acting on the needle and causing it to move to and fro at a given rate. this was the underlying idea which received very simple expression in the ingenious instrument which coulomb devised and called a torsion-balance. with it, he set about determining the law which governs the mutual action of magnets and of electrified bodies with regard to distance, and found it to be the same as that which newton found to hold for bodies distributed throughout the universe, that is, that attraction and repulsion vary inversely as the square of the distance. he also proved, with the aid of his torsion-balance, that the forces of attraction and repulsion vary as the product of the strength of the poles in one case and as the product of the electric charges in the other. these were the important discoveries of coulomb's life; they served to earn for him the right to have his name given to the unit of electrical quantity, the _coulomb_. coulomb did not stop here, however, but proceeded to apply his laws to various other phenomena. he proved that electricity distributes itself entirely over the surface of a body without penetrating the mass of the conductor, and he showed by calculation that this result was a necessary consequence of the law of repulsion. a list of the papers which he published on electricity and magnetism, the titles of which, with french accuracy of expression, furnish an excellent idea of their contents, shows the thoroughly progressive and scientific spirit of the man, and how well he proceeded from the known to the less known, always widening the bounds of knowledge. suffice it to say here that the observations of coulomb were not only original, but that they concerned some of the most difficult questions in electricity, and that he was clearing the ground for others in such a way as to make future work and quantitative measurements in electricity reliable and comparatively easy. it is because of this pioneer work that coulomb deserves so much praise. it was not long before coulomb's observations were confirmed by others, and then the beginnings of the modern development of electricity became manifest, owing not a little to the researches and inventions, the genius and ingenuity of this french military engineer. some phases of electrical development attributed to others really belong to coulomb. a typical example of this detraction from his merit is the attribution to biot of the solution of the problem of the complete discharge of an electrified sphere by means of two hollow hemispheres. this experiment is fully described by coulomb, and he even emphasizes the fact that the external discharging bodies need not necessarily be of the same shape as the charged sphere. some of what coulomb accepted as principles in electricity have proved in the course of time, not to be the realities that he thought them; but the progress that has led to such contradictions of his opinions has been mainly rendered possible by his own discoveries. the fable of the eagle stricken by the arrow containing some of its own feathers, is so old that one might think that, when the progress of a science due to a scientist brings men beyond the position he occupied, they would not blame him for backwardness. this is, however, one of the curious critical methods in the history of science that has most frequently to be deprecated by the historian who is tracing origins and developments. coulomb's papers, with the exception of his memoir on "problems in statics applied to architecture," his "researches on the methods of executing works under water without the necessity of pumping," his "theory of simple machines," and his researches "on windmills," which form separate monographs, were all published together in a single volume by the french physical society in .[ ] this volume contains, besides his investigations on the best way of making magnetic needles, his theoretic and experimental investigations on the force of torsion and on the elasticity of metallic threads, which were undertaken in order to enable him to make his electric torsion-balance something more than mere guess-work. all the other papers are concerned directly with electricity or magnetism, and show how actively, nearly a hundred and twenty-five years ago, a great mind was engaged with problems in electricity which we are apt to consider as belonging more properly to our own time. the list of papers published in these memoirs, arranged in chronological order, gives a good idea of the development of electrical science in coulomb's own mind. there is a logical as well as a chronological order to be observed in them. in , when he was just approaching his fiftieth year, there were three subjects with regard to which coulomb's experimental observations enabled him to set down some definite principles. the first of these was the construction and use of an electric balance, founded on the property which wires have of exhibiting a torque proportional to the angle of torsion. the second was the determination of the laws, according to which the magnetic and electric "fluids," as coulomb and investigators in electricity called them at that time, act both as regards repulsion and attraction. the third was the determination of the quantity of electricity which an insulated body loses in a given time from contact with air more or less moist. in , he published a paper in which he demonstrated what he considered the principal properties of the electric fluid. these are, that this fluid does not spread itself on a substance by any chemical affinity or any elective attraction, but that it distributes itself over various bodies that are placed in contact, entirely in accordance with their shape; and also that in electrical conductors, the charge is limited to the surface of the conductor and does not penetrate to any appreciable depth. in , his only paper was on the manner in which the electrical fluid divides itself between two conducting bodies placed in contact, and on the distribution of this fluid over the different parts of the surface of these bodies. he continued his investigations into this subject in , and also succeeded in determining the density of the electricity at different points on the surface of conducting bodies. in , he began to work more particularly on magnetism. his first paper on the subject was published that year. unfortunately, as we have said, the revolution interrupted his scientific investigations at this point, and for the next eleven years we have nothing from his pen. as a nobleman, he was compelled to leave paris, and this not only put him out of touch with scientific work generally, but deprived him of the opportunities of using such apparatus as was necessary to carry on his experiments. that he acted prudently in leaving paris, the careers of other scientists amply prove. lavoisier continued to carry on his chemical investigations during the stormy times of the revolution, but his stay in the capital eventually cost him his life. abbé haüy, the father of crystallography,[ ] who, because of his contributions to the science of pyro-electricity, is of special interest to us, continued to work at his crystals throughout even the reign of terror. when thrown into prison, he asked and obtained permission to have his crystals with him. his friends saved him from lavoisier's fate, but not without an effort, as his life was seriously endangered. it is easy to understand, however, that a member of the nobility like coulomb, whose life had been spent in military affairs, should not be able to devote himself seriously to scientific matters while his country was in such a turmoil. in , he resumed his investigations once more, but now they are concerned more particularly with magnetism. the first was a theoretical and practical determination of the forces which hold different magnetic needles, magnetized to saturation, in the magnetic meridian. this was followed, in the same year, by a paper which, like its predecessor, was published among the memoirs of the institute of france, which had replaced the royal academy of sciences, to which body many of coulomb's papers of the former time had been presented, and in whose publications they originally appeared. this second paper detailed his experiments on the determination of the force of cohesion of fluids and the law of resistance in them, when the movements were very slow. when the french institute was organized under napoleon in , coulomb was named among its first members. it is believed that he was even chosen to occupy a place in the first government of the state, but a man more interested in politics obtained the place, a fortunate circumstance for science. coulomb was named, however, one of the inspectors of public instruction, then the highest place in the education department, and he did much to restore to france the educational system that had been destroyed during the revolution. in this rather trying work he was noted for the kindliness yet firmness of his character, while his absolute fairness and sense of justice were recognized on all sides. unfortunately coulomb was not long spared to continue his work. he took up his experimental and mathematical investigations, on his return to the capital, with great enthusiasm, but his health had been undermined and his work had been rudely interrupted. after , no further paper by him appears to have been published until . this gave the result of different methods employed in order to produce in blades and bars of steel the greatest degree of magnetism. for some time preceding this, in spite of increasing ill-health, he had continued his experiments on the influence of temperature on the magnetism of steel. his work on this subject was not destined to be completed, for not long after passing his seventieth year, in june of this year, his health gave way completely, and he died august d, . his final observations were gathered by biot, carefully preserved, and assigned a place in the volume of coulomb's memoirs, issued by the french physical society. personally, coulomb was noted for great seriousness of character, though with this was mingled a gentleness of disposition that made for him some cordial friendships among his scientific contemporaries. he had but few friends, but those who were admitted to his intimacy made up by the depth of their affection for the smallness of their number. even those who had occasion to meet him but once or twice, carried away from their meeting an affectionate remembrance of his kindliness and courtesy and readiness to help wherever he could be of service. he was extremely happy in his family relations, and this proved to be a great source of consolation to him during the years when the progress of the french revolution took him away from science and made him almost despair of his country. it is not surprising that biot, the great french physicist, in writing of coulomb in his mélanges scientifiques et littéraires, vol. iii. (paris, ), should have held coulomb up as a model of the simple, earnest, helpful life and as a man of the most exemplary character. he says: "coulomb lived among the men of his time in patience and charity. he was distinguished among them mainly by his separation from their passions and their errors, and he always maintained himself calm, firm and dignified _in se totus teres atque rotundus_, as horace says, a complete, perfect and well-rounded character." few men have deserved so noble a eulogy as this, written nearly fifty years after his death, by one who had known coulomb himself and his contemporaries well; it has none of the exaggeration of a funeral panegyric, and is evidently founded on details of knowledge with regard to the great electrician which had become a tradition among french scientists, and which biot has forever crystallized into the history of science by his emphatic expression. one could scarcely wish for a better epitaph than biot's summing up of coulomb's personal character: "all those who knew coulomb know how the gravity of his character was tempered by the sweetness of his disposition, and those who had the happiness to meet him at their entrance into a scientific career have kept the most tender remembrance of his gentle good-heartedness." [illustration: hans christian oersted] footnotes: [ ] collection de memoires relatifs à la physique publiés par la société française de physique. tome i., mémoires de coulomb. paris. gauthier-villars, imprimeur-libraire du bureau des longitudes, de l'École polytechnique, quai des augustins, , . [ ] catholic churchmen in science, the dolphin press, philadelphia, . chapter vii. hans christian oersted. whatever may be thought of the value of controversy in other departments of knowledge, it has certainly proved useful in the progress of experimental science. witness the animated and prolonged discussion which took place between volta and galvani, and which led to enduring results for the welfare of mankind. wishing to prove the correctness of his theory of electrification by contact against galvani's animal electricity, volta devoted himself unremittingly to experimentation until, in the century year , his brilliant work culminated in the invention of the "pile" or electric battery which bears his name. a suspicion had been growing for many years in the minds of physicists, that there must be some degree of relationship, probably an intimate one, between magnetism and electricity, between magnetic and electric forces. in the year , van swinden, a celebrated dutch physicist, published a work on electricity in which he described and commented upon a number of analogies which he had observed between the two orders of phenomena; but, voluminous as was the work, it threw no light on the nature of the suspected relationship. it was well known, in the case of houses and ships struck by lightning, that knives, forks and other articles made of steel were often found to be permanently magnetized. following up this pregnant observation, experimenters often sought to impart magnetic properties to steel needles by leyden-jar discharges, but with indifferent success. sometimes there would be a trace of magnetism left and sometimes none. in no case was it possible to say beforehand which end of the knitting-needle would have north polarity and which south. though we are better equipped to-day for research work than were our predecessors in the electrical field fifty years ago, we are still unable to predict the polarity that will result in a bar of iron from a given condenser discharge. the uncertainty arises from the fact disclosed by joseph henry in and well known to-day that, under ordinary circumstances, all such discharges consist of a rush of electricity to and fro, that is, they give rise to an oscillatory current of exceedingly short duration. were it otherwise, that is, were the discharge unidirectional, the needle would always be magnetized to a degree of intensity proportional to the energy released; and it would be possible in every case to foretell with certainty the resulting polarity which the needle would acquire. with the advent of the voltaic battery, a generator which supplies a steady flow of current in one direction, the interesting problem of relationship between electric and magnetic forces was again attacked; and this time with considerable success. probably the earliest investigator afield was romagnosi, an italian physician residing in trent (tyrol), who, in the year , published in the "gazetta" of his town an account of an experiment which he had made, and which showed that he was working on promising lines. what he did was this: having connected one end of a silver chain to a voltaic pile, and having carried the chain through a glass tube for the purpose of insulation, he presented the free end, terminating in a knob, to a compass-needle, also insulated. at first, the needle was attracted; and, after contact, repelled. whatever romagnosi thought of his experiment and its theoretical bearing, the attraction and subsequent repulsion of the compass-needle which he said he observed were electrostatic and not electromagnetic effects. the italian physician was indeed on the verge of a great discovery; but he halted in his course and lost his opportunity. mojon, professor of chemistry in genoa, was a little more fortunate, though he, too, failed to improve his opportunities. in , he sought to magnetize steel needles by placing them for a period of twenty days in circuit with a battery of one hundred elements of the crown-of-cups type, and had the satisfaction of finding them permanently magnetized when withdrawn from the circuit. unlike the electrostatic effect of his fellow-countryman romagnosi, this was unquestionably an electromagnetic effect, the first link in the long chain connecting electricity with magnetism. that this result attracted wide attention at the time, as it well deserved, is evident from the notice given by izarn in his "manuel du galvanisme," and by aldini in his "essai théorique et expérimental sur le galvanisme," both of which were published in paris in the same year, . though the manuals of izarn and aldini served to give a fresh impetus to the quest of the relationship between electricity and magnetism, it was not, however, until the year that the cardinal discovery was made by one philosopher and the intimate relationship revealed by another. then all europe rang with the names of oersted, the fortunate discoverer of the "magnetic effect" of the electric current, and ampère, whose masterly analysis disclosed the nature of the long-sought-for connection. in the delight of the hour, men called oersted the columbus, and ampère the newton, of electricity. though a philosopher of a high order and lecturer of interest and brilliancy, oersted was, nevertheless, a poor experimentalist. he was fine in the abstract, awkward in the concrete. often did he call for the assistance of a student to perform an experiment for the class under his direction. hansteen, who is celebrated for his very fine work in terrestrial magnetism, often had this privilege, for he was clear of mind and deft of hand. writing to faraday, he said: "oersted was a man of genious, but very unsuccessful as a demonstrator, for he could not manipulate instruments." in seeking for some evidence of a physical interaction between electricity and magnetism, oersted on one occasion, placed a wire conveying a current vertically across a compass-needle; and, on obtaining no result, seemed greatly disappointed. he evidently expected the needle to respond in some way to the energy of the current; and so it would have responded had he placed the wire in any other position than the particular one which he selected. the danish philosopher now hesitates; and for lack of coolness, patience and resourcefulness, runs the risk of losing the crowning glory of his life. he is disappointed at his failure; and for the nonce, contents himself with brooding over it. [illustration: fig. the magnetic effect of an electric current. oersted, ] on another occasion, having a stronger battery at his disposal, he determined to try the experiment again, in the hope that the greater energy at his command would provoke the magnet to respond. this time, he stretched the wire over and parallel to the compass needle, when, to his intense delight, the magnet turned aside as soon as the circuit was closed. the result was pronounced and instantaneous. the professor, an enthusiast by nature, waxed warm over his good fortune, and well might he do so, as the discovery which he had just made was destined to revolutionize existing modes of transmitting intelligence to distant parts and bring remotest countries into direct, and immediate relation with one another. that oersted fell into ecstasy over his success was but natural, though it is not stated that he exhibited his enthusiasm by the performance of any unusual feat. when lavoisier made a discovery, he was wont to take hold of his assistant and go dancing around with him for sheer joy. after making a certain successful experiment in his laboratory, gay-lussac gave vent to his feelings by dancing round the room, and clapping his hands the while. it is related that, when davy saw the first globules of potassium burst through the crust of potash and take fire, his delight knew no bounds. he also took to dancing, and some time had to elapse before he was sufficiently composed to continue his work. even the cool and self-possessed faraday occasionally waxed warm on seeing his efforts crowned with success. it is said that, when he got a wire conveying a current to revolve round the pole of a magnet, he rubbed his hands vigorously and danced around the table, his face beaming with delight: "there they go, there they go; we have succeeded at last," he said. he then gleefully proposed to cease work for the day and spend the evening at astley's seeing the feats of well-trained horses! having realized that his experiment was one of fundamental importance in physical theory, our philosopher proceeds to repeat it under varying conditions. he places the wire conveying the current in front of the needle, behind it, under it, across it; he reverses the current in each case, and notices the direction in which the needle turns. though he states results very clearly, he gives no general rule whereby the direction of the deflection may be foretold from that of the current. a _memoria technica_ to meet all cases that may occur was needed, and was promptly supplied by ampère, who, with a flash of genius, devised the rule of the little swimmer. others have been added since, such as the cork-screw rule and the rule involving the outspread right hand; but the swimmer appeals in a manner quite its own to the fancy of the youthful student. it pleases while it instructs; it is ingenious while yet remarkably simple. it has been said that the philosopher of copenhagen was led by mere accident to the experiment which will hand his name down the ages; but inasmuch as he was looking, during thirteen years, for a result analogous to the one which he obtained, it is only right to give him full credit for the success which he achieved. it has been well remarked, that the seeds of great discoveries are constantly floating around us, but take root only in minds well prepared to receive them. accidents of the oersted type happen only to men who deserve them, as was the case with musschenbroek and galvani in the eighteenth century, and with roentgen in the nineteenth. the electrification of a flask of water, the twitching of frogs' legs in response to electric sparks, and the blackening of a sensitive screen by a distant, shielded crookes's tube, led to the electrostatic condenser in the first case, to "galvanism" in the second, and to the photography of the invisible in the third. writing of oersted's discovery, faraday said that "it burst open the gates of a domain in science, dark till then, and filled it with a flood of light." the discovery of was hailed throughout europe by an extraordinary outburst of enthusiasm. oersted was complimented and congratulated on all sides. honors were showered upon him: the royal society of london awarded him the copley medal; the french academy of sciences gave him its gold medal for the physico-mathematical sciences; prussia conferred upon him the ordre pour le mérite, and his own country made him a knight of the daneborg. oersted lost no time in preparing a memoir on the subject of his work, a copy of which was sent to the learned societies and most renowned philosophers of europe. the memoir, which was written in latin and dated july st, , consisted of four quarto pages with the title "experiments on the effect of the electric conflict on the magnetic needle." a perusal of this paper brings home the conviction that oersted realized fairly well the forces which came into play in his experiment; for in one place, he speaks of the effect as due to a transverse force emanating from the conductor conveying the current, and again as a conflict acting in a revolving manner around the wire. a complete statement of the nature of the mechanical force exerted by a conductor conveying a current on a magnetic needle was given almost immediately by ampère, a master analyst and accomplished experimentalist. [illustration: fig. magnetic field surrounding a conductor carrying a current] it will stand for all time in the history of science, that in less than two months after the publication of oersted's memoir, ampère succeeded in showing the mechanical effect in magnitude and direction of an element of current not only on the magnetic needle itself, but also on a similar element of an adjacent conductor conveying a current, thereby founding a new science in the department of physics, the science of electro-dynamics. oersted does not appear to have given thought to the practical possibilities of his discovery. while appreciating the utilitarian in science, he evidently preferred the pursuit of knowledge for its own sake. in a discourse which he delivered in before the university of copenhagen, he put himself on record when he said that "the real laborer in the scientific field chooses knowledge as his highest aim." so said plato ages before, and so said archimedes, who held that it was undesirable for a philosopher to seek to apply the discoveries of science to any practical end. the screw which he invented, his catapults and burning mirrors, show, however, that when necessary the syracusan mathematician could come down from the serene heights of investigation to the prosaic arena of application. before oersted spoke of "the real laborer," thomas young had affirmed that "those who possess the genuine spirit of scientific investigation are content to proceed in their researches without inquiring at every step what they gain by their newly discovered lights, and to what practical purposes they are applicable." [illustration: fig. magnetic whirl surrounding a wire through which a current is passing] young's most illustrious successor in the royal institution, michael faraday, devoted himself calmly but unflinchingly to research work, in the conviction that no discovery, however remote in its nature, from the subject of daily observation, could with reason be declared wholly inapplicable to the benefit of mankind. after discovering in that electric currents could be produced by the relative motion of magnets and coils of wire, a discovery which is the basis of all the electric engineering of our day, faraday constructed several experimental machines embodying this principle, and then turned away abruptly from the work, saying, "i had rather been desirous of discovering new facts and new relations dependent on magneto-electric induction than of exalting the force of those already obtained, being assured that the latter would find their full development hereafter." our own joseph henry, whose sterling merit is universally recognized, beautifully said in this connection: "he who loves truth for its own sake feels that its highest claims are lowered by being continually summoned to the bar of immediate and palpable utility." oersted seems to have shared the opinion largely held by the scientific men of his day, that electricity is mainly a magnetic phenomenon. ampère, for one, did not think so, as is evident from the beautiful theory which he devised to explain the magnetism of a bar by minute electric currents flowing round each individual molecule of the iron. to the french physicist, magnetism was purely an electrical phenomenon. [illustration: fig. ampère's molecular currents] though propounded more than eighty years ago, this theory is still in harmony with all facts and phenomena in the domain of magnetism known to-day. it is important to remember, when thinking of this physical theory, that the amperian currents in question are confined to the molecule, and that they do not flow from one molecule to another. critics have urged against the theory that the molecules must be heated by the circulation of these elementary currents, to which objection it has been replied that, as we know nothing of the nature of the molecule, we cannot say that it offers any resistance to the current; and, therefore, we cannot affirm that there is any development of heat due to the circulation of these elementary currents. it is to ampère's credit that he was also the first to propose a practical application of oersted's discovery, an application that was nothing less than the electric telegraph itself. he suggested that the deflection of the magnetic needle could be used for the transmission of signals from one place to another by means of as many needles and circuits as there are letters in the alphabet. if ampère had only recalled the optical and mechanical telegraphs in use in his day, such as the swinging of lanterns by night and wigwagging of flags and the movements of semaphores by day, he might have reduced his twenty-four circuits to one, using the two elements, viz., motion of the needle to the right and motion to the left, to make up the entire alphabet. morse substituted the dot and the dash for these deflections, and thus rendered the reception of messages automatic and permanent. in connection with this proposal to use a magnetic needle for the transmission of intelligence, the reader will no doubt recall the lover's telegraph, so beautifully described by addison in the "spectator" for december th, ; but ingeniously conceived as it was, this magnetic telegraph was purely and simply a creation of the imagination. this canny conceit has been attributed to cardinal bembo, the elegant scholar and private secretary to pope leo x.; but it was his friend porta, the versatile philosopher, who made it widely known by the vivid description which he gave of it in his celebrated work on "natural magic," published at naples in . this sympathetic telegraph consisted, we are told, of a magnetic needle poised in the center of a dial-plate, with the letters of the alphabet written around it. the two fortunate individuals privileged to hold _wireless_ correspondence with each other having agreed as to the day and the hour, proceed to the room in which the wonderful instrument is kept, where, as soon as one of them turns the needle of his transmitter to a letter, the distant needle turns at once in sympathy to the same letter on its dial! such is the power of magnetic sympathy, that the instruments will work successfully though hills, forests, lakes or mountains intervene! porta has it: "to a friend at a distance shut up in prison, we may relate our minds; which, i do not doubt, may be done by means of compasses having the alphabet written around them." [illustration: fig. the "sympathetic telegraph" from cabeo's _philosophia magnetica_, ] this sympathetic magnetic telegraph figures extensively in the scientific literature of the sixteenth and seventeenth centuries: some believed in the figment, others condemned it. addison described it in elegant prose, and akenside in beautiful verse. perhaps the most famous composition on the subject is a short latin poem, written, after the style and vein of lucretius, in by famianus strada, an italian jesuit. a few years after its publication in the author's "prolusiones," a metrical translation was made by hakewill and inserted on page of his "apologie, or declaration of the power and providence of god," . owing to the interest that attaches to this celebrated composition and the difficulty of getting hakewill's "apologie," we append his version of the poem. the loade above all other stones hath this strange property if sundry steels thereto or needles you apply, such force and motion thence they draw that they incline to turn them to the bear, which near the pole doth shine. nay, more, as many steels as touch that virtuous stone in strange and wondrous sort conspiring all in one together move themselves and situate together: as if one of those steels at rome be stirred, the other the self-same way will stir though they far distant be, and all through nature's force and secret sympathy; well then if you of aught would fain advise your friend that dwells far off, to whom no letter you can send; a large smooth round table make, write down the crisscross row in order on the verge thereof, and then bestow the needle in the midst which touch'd the loade that so what note soe'er you list, it straight may turn unto. then frame another orb in all respects like this describe the edge and lay the steel thereon likewise, the steel which from the self-same magnes motion drew; this orb send with thy friend what time he bids adieu. but on the days agree at first, when you do mean to prove if the steel stir, and to what letter it doth move. this done, if with thy friend thou closely wouldst advise, who in a country off far distant from thee lies, take thou the orb and steel which on the orb was set the crisscross on the edge thou seest in order writ. what notes will frame thy words, to them direct thy steel and it sometimes to this, sometimes to that note wheel turning it round about so often till you find you have compounded all the meaning of your mind. thy friend that dwells far off, o strange! doth plainly see the steel so stir though it by no man stirréd be, running now here, now there: he conscious of the plot as the steel-guide pursues, and reads from note to note. then gathering into words those notes, he clearly sees what's needful to be done, the needle truchman is. now, when the steel doth cease its motion; if thy friend think it convenient answer back to send, the same course he may take; and, with his needle write touching the several notes which so he list indite. would god, men would be pleased to put this course in use, their letters would arrive more speedy and more sure, no rivers would them stop nor thieves them intercept; princes with their own hands, their business might effect. we scribes, from black sea 'scaped, at length with hearty wills at th' altar of the loade would consecrate our quills. another translation of the poem was made by dr. samuel ward and published at the end of his "wonders of the loadstone," . [illustration: fig. the "sympathetic telegraph" from turner's _ars notoria_, ] ampère's suggestion, made, as we have seen, in the year , was not the first proposal to use electricity for telegraphic purposes. already, in , a writer in _the scots magazine_, signing himself c. m. (charles morrison, of greenock, according to sir david brewster, and charles marshall, of paisley, according to latimer clark), outlined a method involving the use of frictional electricity; and lesage, of geneva, constructed a short experimental line, in , consisting of twenty-four wires and a pith-ball electroscope. but the man who attained the greatest success in the employment of static electricity for this purpose was ronalds, of london, who, in , erected a single-wire line eight miles long in his gardens at hammersmith, with a pair of pith-balls and a rotating disc for receiving instrument. when well satisfied that his system was practicable and reliable, ronalds wrote to the head of the intelligence department in london urging the adoption of his invention for the public service; but he was promptly brought to realize the scant encouragement so often extended to inventors by persons in high places, that responsible official politely informing him "that telegraphs of all kinds are wholly unnecessary," and that no other than the mechanical one in daily use would be adopted. when penning these words, the representative of the british government must have forgotten the experience of , when the result of the battle of salamanca was semaphored from plymouth to london, on which occasion a fog cut off the message after the transmission of the first two words, "wellington defeated," the remainder of the despatch, "the french at salamanca," reaching the capital only on the following morning! a rapid sketch of the life of our philosopher, whose discovery of the magnetic effect of the voltaic current in led to the invention of the electric telegraph, cannot be without interest. hans christian oersted was born on august th, , in the little town of rudkjöbing, in the island of langeland, denmark. being the son of poor parents, his early years were spent in very narrow circumstances. he and his younger brother were mainly indebted to their own efforts for whatever instruction they received in the rudiments of learning. the town in which they lived being small, offered few opportunities for education, even if the family exchequer had been such as to permit the boys to take advantage of them. there was a german wigmaker in the place, however, who was a little more advanced in knowledge than the generality of the townspeople. he and his wife liked the oersted boys, who were very frequently to be found in the wigmaker's shop. the good housewife taught them to read, while the artist himself taught them a little german. hans christian advanced so rapidly in his studies that he acquired a reputation for precociousness, which, with the usual prejudice against bright children, made the neighbors shake their heads prophetically and say: "the child will not live; he is too bright to last long." hans christian learned the elements of arithmetic from an old school-book which he picked up by chance; and no sooner had he advanced a little, than he set about instructing his brother. very probably, the teacher benefited quite as much by this process of instruction as the pupil. adversity is a good school for the formation of character as well as for the acquisition of knowledge. it is evident, from the lives of such men as oersted, faraday, kepler, ohm, and others who were brought up in the lap of poverty, that it is not so much educational opportunity that is needed for the development of mind which we call education, as the earnest determination and the abiding desire to have it. even boyhood creates its own opportunities for education despite intervening obstacles, if it has only a decided eagerness, a pronounced thirst for knowledge. about the time that the young oersteds entered their teens, their father secured the services of a private teacher to give them some instruction in the rudiments of latin and greek. this accidental preceptor was only a wandering student who happened to be in the place at the time; but the boys, in their eagerness to learn, derived more benefit from his lessons than many boys of their age often do nowadays from the help and encouragement of a carefully selected and academically equipped tutor. at the age of twelve, oersted senior was taken into his father's apothecary-shop in quality of assistant, a position which seemed destined to put an end to all opportunities for further advancement in the path of learning. when a boy goes into a drug-store in an official capacity, his future career is usually settled; he is a druggist to the end. his new avocation, however, proved to be the beginning of new intellectual activities for oersted. the chemical side of his work became a source of new information to him, and also a stimulus to learn all that he could of chemistry and kindred subjects. science became a hobby with the young apothecary, and everything relating to it appealed to him. what hans learned, he as usual imparted to his brother, who was already becoming interested in other departments of learning, especially the law. the desire of the boys to advance grew with their stock of knowledge. accordingly, when, in , hans was only seventeen years of age and his brother sixteen, they both matriculated at the university of copenhagen. their father was able to help them but little, so that they were obliged to live quietly and sparingly, a condition distinctly favorable to consecutive and efficient study. they became so successful in their pursuits that they soon began to attract attention. having passed creditable examinations, they were recommended for pecuniary assistance from an educational fund established by the government for the purpose. even then, as receipts were hardly equal to expenses, they sought to increase their little revenue by giving private lessons in their leisure hours. here we have a striking example of what may be accomplished by men who work their way through college in the teeth of adverse circumstances; in these two brothers, we have proof of the truth that it is the student's mind, his willingness and determination to work, that count in education more than the golden opportunities that may fall to his lot. in the year , oersted prepared a thesis on "the architectonics of natural metaphysics," which won for him his doctorate in philosophy. though the young doctor did not hesitate to discuss metaphysical problems and even to disagree with kant at a time when most teutonic minds were deeply under the influence of the philosopher of königsberg, his chief interests, however, centered in the experimental sciences, in physics and chemistry. in spite of his devotedness to science, oersted allowed himself, by way of distraction, an occasional excursion into the field of literature. a great literary and artistic movement was making itself felt in the northern part of europe at the time. the æsthetic awakening of the teutonic nations had come after three centuries of religious and political unrest, ill adapted to intellectual development. lessing and winkelmann, goethe and schiller, the two schlegels and klopstock as well as the young poets, uhland and koerner, were either already at work or were about to enter on their distinguished careers, and the neighboring scandinavian nations were beginning to be seriously affected by the movement which was going on among their brethren. in the third year of his university course, oersted entered the lists as a competitor for literary honors on the question, "what are the limits of prose and poetry?" and had the satisfaction of winning the gold medal offered for the contest. in spite of this episode, indicative of devotedness to the muses, oersted passed a brilliant pharmaceutical examination; and in the following year succeeded in capturing another prize, this time for a medical essay. after such a period of preparation, it might be expected that a brilliant career would open up for oersted; but, unfortunately, he could not afford to wait for slow academic rewards, as it was absolutely necessary for him to set about earning his livelihood. for this purpose, shortly after graduation, he accepted the position of manager of a drug-store. as the salary attached to the office was rather slender, he increased his resources by giving lectures in the evening on the familiar subjects of chemistry, natural philosophy and metaphysics. about this time, the _wanderlust_, or passion for travel, took possession of our young philosopher; and under its influence, he resolved to see for himself what men of scientific avocations were doing in france and in germany. his own pinched circumstances would not allow him to undertake such a journey; but he was fortunate enough to win a _stipendium cappelianum_ which allowed him to travel at the expense of the government for a period of five years, though he used it only for three. if ever pecuniary aid was productive of enduring results, it was so in this case. in , at the age of twenty-four, oersted set out from copenhagen on his grand tour, determined to make it a scientific as well as sentimental journey. in germany, which he first visited, he met klaproth, the orientalist; werner, the mineralogist; olbers the astronomer; the philosophers fichte, schelling and the two schlegels; and above all, the young and brilliant physicist johann wilhelm ritter, who discussed with him the theory of the wonderful "pile" invented by volta in the previous year, . in paris, oersted spent about fifteen months, during which time he was in habitual relations with many of the savants who were just then reflecting great lustre on french science. to mention but a few: there was cuvier, the leading naturalist of his age; abbé haüy, crystallographer of world-wide reputation; biot, the brilliant expounder of physics; charles, the discoverer of the law which bears his name; berthollet, the associate of monge the mathematician, and lavoisier, the chemist. on his return to the danish capital in , oersted delivered courses of lectures on electricity and magnetism, light and heat, before numerous and cultured audiences; and such was the success which he achieved that he was appointed, at the age of twenty-nine, to the chair of physics in the university of copenhagen. for nearly forty-five years he was destined to occupy this academical position, so that his connection with that seat of learning rounded out the full period of half a century. while sedulously occupied with the duties of his chair and the pursuit of his favorite scientific subjects, oersted was not unmindful of his civic and altruistic obligations. he frequently gave popular scientific lectures, which were open to women as well as to men. he helped in the organization of a bureau through which lectures would be given in various parts of the country, and thus became a pioneer in what we call to-day the university extension movement. when democratic ideas began to be discussed in denmark after the french revolution of , oersted was one of those who took part in the onward movement for the betterment of the people. in , he coöperated in the foundation of the society for the freedom of the press; and when christian viii. ascended the throne, he addressed the new monarch in a speech of liberal tendency, hailing him because of the interest which he took in the advancement of science and in the uplift of the masses. an idea of the position accorded to oersted by his colleagues in the world of science may be gathered from an address made by sir john herschel at the closing session of the southampton meeting of the british association in , in which the distinguished astronomer said: "in science, there is but one direction which the needle will take when pointed towards the european continent, and that is towards my esteemed friend, professor oersted. to look at his cool manner, who would think that he wielded such an intense power, capable of altering the whole state of science, and almost the knowledge of the world? he has at this meeting developed some of those recondite and remarkable forces of nature which he was the first to discover, and which went almost to the extent of obliging us to alter our views on the most ordinary laws of energy and motion. he elaborated his ideas with slowness and certainty, bringing them forward only after a long lapse of time. how often did i wish to heaven that we could trample down, and strike forever to earth, the hasty generalizations which mark the present age, and bring up another and safer system of investigation, such as that which marked the inquiries of our friend? it was in deep recesses, as it were, of a cell, that a faint idea first occurred to oersted. he waited long and calmly for the dawn which at length broke upon him, altering the whole relations of science and life. the electric telegraph and other wonders of modern science were but mere effervescences from the surface of this deep, recondite discovery of his. if we were to characterize, by any figure, the usefulness of oersted to science, we would regard him as a fertilizing shower descending from heaven, which brought forth a new crop, delightful to the eye and pleasing to the heart." it may be noticed that in oersted's day early specialization was fortunately unknown. his education was broad and his intellectual activities broader still. quite as interesting as many of his scientific researches are some of his contributions to philosophy and some of his views on the significance of the material universe. oersted, a man of the world with a wide range of interests and a philosopher who lived at high intellectual altitudes, was one of the all-round men in the history of thought who took active part in science, in literature, in politics and in social problems. he had the opportunity of meeting many of the renowned scientists and philosophers of the century, and had been very closely in touch with some of them. he was a regular attendant at scientific congresses, in which he distinguished himself by the leading part which he took in their deliberations. his opinions, therefore, on the great problems of life, religious, moral, social and political, challenge our respect even where they do not compel our approval. our danish philosopher deserves, then, to stand as the spokesman of his generation of savants on the great questions that concern man's relations to his fellow-men, to an all-wise providence and to an enduring hereafter. his opinions on these matters are all the more interesting because they are in open contradiction with what is sometimes thought to be the views of scientists on such subjects. one of the passages of his paper on "all existence, a dominion of reason," contains some surprising anticipations of ideas that created a great stir in the intellectual world some fifty years ago. in , that is, thirteen years before the publication of darwin's "origin of species," oersted discussed evolution and suggested explanations that are generally considered to have been forced from apologists when compelled to take up the work of reconciling christian doctrines with scientific conclusions. writing in the middle 'forties, he said: "if we are now thoroughly convinced that everything in the material world is produced from similar particles of matter, by the same forces and in obedience to the same laws, we must allow that the planets have been formed according to the same laws as our own earth. they have been in process of development during immeasurable periods of time, and have undergone numerous transformations which have also influenced the vegetable and animal kingdoms of those remote periods. the lower forms of life advanced by gradual stages to higher and more complex states of organization, till at length (in a comparatively recent period) a self-conscious being was evolved, the crowning work of this long-continued process of development. accordingly, we must allow a similar order of organic development to take place on the other planets of our solar family. there may be some which have not as yet attained the same degree of development that we have reached; but everywhere throughout the universe, creatures endowed with reason appear in due time, just as man appeared on our own globe. their understanding is intimately connected with the organs of sense which they possess; therefore, the nature of their mental faculties cannot be essentially different from our own. that i may avoid even the appearance of materialism, i must direct attention to the conciliatory principle, that the natural environment from which man springs must be recognized as the work of the eternal, creative spirit. in other words, our conception of the universe is incomplete, if not comprehended as a constant and continuous work of the eternally creating spirit." thus far oersted; let us here recall what lord kelvin, the representative scientist of his day, quoted with approval on a memorable occasion from the danish scientist with regard to the basic truths of science, philosophy and religion. "it will not be foreign to our purpose if, called upon by the solemnities of this day, we endeavor to establish our conviction of the harmony that subsists between religion and science, by showing how the man of science must look upon his pursuits, if he understands them rightly, as an exercise of religion. "if my purpose here was merely to show that science necessarily engenders piety, i should appeal to the great truth everywhere recognized, that the essence of all religion consists in love toward god. the conclusion would then be easy, that love of him from whom all truth proceeds must create the desire to acknowledge truth in all her paths; but as we desire here to recognize science herself as a religious duty, it will be requisite for us to penetrate deeper into its nature. it is obvious, therefore, that the searching eye of man, whether he regards his own inward being or the creation surrounding him, is always led to the eternal source of all things. in all inquiry, the ultimate aim is to discover that which really exists and to contemplate it in its pure light apart from all that deceives the careless observer by only a seeming existence. the philosopher will then comprehend what, amidst ceaseless change, is the constant and uncreated, which is hidden behind unnumbered creations, the bond of union which keeps things together in spite of their manifold divisions and separations. he must soon acknowledge that the independent can only be the constant and the constant the independent, and that true unity is inseparable from either of these. and thus it is in the nature of thought that it finds no quiet resting place, no pause, except in the invariable, eternal, uncaused, all-causing, all-comprehensive omniscience. "but, if this one-sided view does not satisfy him, if he seeks to examine the world with the eye of experience, he perceives that all those things of whose reality the multitude feels most assured never have an enduring existence, but are always on the road between birth and death. if he now properly comprehends the whole array of nature, he perceives that it is not merely an idea or an abstract notion, as it is called; but that reason and the power to which everything is indebted for its essential nature are only the revelation of a self-sustained being. how can he, when he sees this, be otherwise animated than by the deepest feeling of humility, of devotion and of love? if anyone has learned a different lesson from his observation of nature, it could only be because he lost his way amidst the dispersion and variety of creation and had not looked upwards to the eternal unity of truth." as already said, oersted lived to celebrate the fiftieth year of his connection with his university. this was in november, , on which occasion his friends, pupils and the public generally united together in honoring him as a professor whose warm and animated lectures enraptured audiences; as a leader in the scientific advance of the times; and as a christian to whom nature was but a manifestation of the deity's combined wisdom and creative power. the aged scientist, much touched by this popular demonstration as well as by the tokens of esteem given him by the king, spoke of this jubilee celebration as the happiest day of his life. the reader will recall another great man, great in the world of politics and great on the field of battle, who said that the happiest day of _his_ life was that of his first communion. a few months after celebrating his golden jubilee, oersted passed away, after a short illness, on march th, , deeply mourned by all. oersted was eminent as a scholar and equally eminent as a man; lenient in his judgment of others, he was strict with regard to himself; simple in his ways and frugal in living, he was benevolent to others, being always ready to give a helping hand wherever needed. to such a man may well be applied these beautiful words with which priestley begins his "history of electricity": "a life spent in the contemplation of the productions of divine power, wisdom and goodness, would be a life of devotion. the more we see of the wonderful structure of the world and of the laws of nature, the more clearly do we comprehend their admirable uses to make all percipient creation happy, a sentiment which cannot but fill the heart with unbounded love, gratitude and joy." a statue to the memory of oersted was unveiled in copenhagen on september th, , in presence of the king of denmark, the king of greece, the danish crown prince and members of the royal family, as well as numerous high officials, representatives of learned societies and a vast body of students and people assembled together to do honor to a man who was distinguished alike by his scientific attainments and philosophical acumen, and who, during his long life, never faltered in his devotedness to the welfare of his country as he never weakened in his defense of the great truths of religion. brother potamian. chapter viii. andrÉ marie ampÈre. few men of the nineteenth century are so interesting as andré marie ampère, who is, as we have seen, deservedly spoken of as the founder of the science of electro-dynamics. extremely precocious as a boy, so that, like his immediate predecessor in discovery, oersted the dane, his rapid intellectual development drew down upon him ominous expressions from those who knew him, he more than fulfilled the highest promise of his early years. his was no one-sided genius. he was interested in everything, and his memory was as retentive as his intellect was comprehensive. he grew up, indeed, to be a young man of the widest possible interests. literature never failed to have its attraction for him, though science was his favorite study and mathematics his hobby. the mathematical mind is commonly supposed to run in very precise grooves, yet ampère was always a speculator, and his speculations were most suggestive for his contemporaries and subsequent generations. indeed, his mathematics, far from being a hindrance to his penetrating outlook upon the hazier confines of science, rather seemed to help the penetrations it gave. while he was so great a scientist that arago, so little likely to exaggerate his french contemporary's merit, has said of ampère's discovery identifying magnetism and electricity, that "the vast field of physical science perhaps never presented so brilliant a discovery, conceived, verified, and completed with such rapidity," his friends knew this great scientist as one of the kindliest and most genial of men, noted for his simplicity, his persuasive sympathy and his tender regard for all those with whom he was brought into intimate relations. [illustration: andrÉ marie ampÈre] the commonly accepted formula for a great scientist, that he is a man wrapt up in himself and his work, enmeshed so completely in the scientific speculations that occupy him that he has little or no time for great humanitarian interests, so that his human sympathies are likely to atrophy, is entirely contradicted by the life of ampère. he was no narrow specialist, and, indeed, it may be said that not a single one of these great discoverers in electricity whom we are considering in this volume was of the type that is sometimes accepted as indicative of scientific genius and originality. after reading their lives, one is prone to have the feeling that men who lack that wider sympathy which, in the famous words of the old latin poet, makes everything human of interest to them, are not of the mental calibre to make supreme discoveries, even though they may succeed in creating a large amount of interest in their scientific speculations in their own generation. it is the all-round man who does supreme original work of enduring quality. andré marie ampère was born at lyons, january d, . his father, jean jacques ampère, was a small merchant who made a comfortable living for his family, but no more. his father and mother were both well informed for their class and time, and were well esteemed by their neighbors. his mother especially was known for an unalterable sweetness of character and charitable beneficence which sought out every possible occasion for its exercise. she was universally beloved by those who knew her, and the charm of ampère's manner, which made for him a friend of every acquaintance, was undoubtedly a manifestation of the same family strain. shortly after the birth of their son, the parents gave up business and retired on a little property situated in the country not far from lyons. it was in this little village, without any school-teacher and with only home instruction, that the genius of the future savant, who was to be one of the distinguished scientific men of the nineteenth century, began to show itself. for ampère was not only a genius, but, what is so often thought to be an almost absolute preclusion of any serious achievement later in life, a precocious genius. the first marvelous faculty that began to develop in him was an uncontrollable tendency to arithmetical expression. before he knew how to make figures, he had invented for himself a method of doing even rather complicated problems in arithmetic by the aid of a number of pebbles or peas. during an illness that overtook him as a child, his mother, anxious because of the possible evil effects upon his health of mental work, took his pebbles away from him. he supplied their place, however, during the leisure hours of his convalescence, when time hung heavy on his child hands, by bread crumbs. he craved food, but, according to the "starving" medical _régime_ of the time, he was allowed only a single biscuit in three days. it required no little self-sacrifice on his part, then, to supply himself with counters from this scanty supply, and his persistence, in spite of hunger, evidently indicates that this mathematical tendency was stronger than his appetite for food. this is all the more surprising, since children are usually scarcely more than little animals in the matter of eating, and commonly satisfy their physical cravings without an after-thought of any kind. ampère learned to read when but very young, and then began to devour all the books which came to hand. usually, the precocious taste for reading specializes on some particular subject; but everything was grist that came to the child ampère's mental mill, and it was all ground up; and, strangest of all, much of it was assimilated. travel, history, poetry, occupied him quite as much as romance; and, amazing as it may appear, even philosophy was not disdained while he was still under ten years of age. it seems amusing to read the declaration of the french biographer, that if this boy of ten had any special predilection in literature, it was for homer, lucan, tasso, fénelon, corneille and voltaire, yet it must be taken seriously. when he was about fifteen, this omnivorous intellectual genius came across a french encyclopedia in twenty folio volumes. this seemed to him a veritable golconda of endless riches of information. each of the volumes had its turn. the second was begun as soon as the first was finished, and the reading of the third followed, and so on, until every one of the volumes had been completely read. references to other volumes might be looked up occasionally, but this did not distract him into taking other portions of the works out of alphabetical order. surprising as it must seem, most of this heterogeneous mass of information, far from being forgotten at once, was deeply engraved on his wonderful memory. more than once in after-life, when many years had passed, it was a surprise to his friends to find how much information ampère had amassed on some abstruse and unfamiliar subject, and how readily he was able to pour forth details of information that seemed quite out of his line. he would then confess that the encyclopedia article on the subject, read so many years before, was still fresh in his mind, or at least that its information was so stored away as to be readily available. we have heard much of gladstone's memory in more recent years; but that seems to have been nothing compared to this wonderful faculty which recalled for ampère, even as an old man, the unrelated details of every encyclopedia article that had passed under his eyes half a century before, when he was a boy of ten to fourteen. the modest family library soon proved utterly insufficient to occupy the mind of this young, enthusiastic student; and his father, sympathetic to his ardent curiosity, took him to lyons from time to time, where he might have the opportunity to consult volumes of various kinds that might catch his fancy. at this time, his old mathematical tendency reasserted itself. he wished to learn something about the higher mathematics. he found in a library in lyons the works of bernoulli and of euler. when the delicate-looking boy, whom the librarian considered little more than a child, put in his request to the town library for these serious mathematical works, the old gentleman said to him: "the works of bernoulli and euler! what are you thinking of, my little friend? these works figure among the most difficult writings that ever came from the mind of man." "i hope to be able to understand them," replied the boy. "i suppose you know," said the librarian, "that they are written in latin." this was a disagreeable surprise for young ampère. as yet he had not studied latin. he went home, resolved, however, to remove this hindrance to his study of the higher mathematics. at the end of the month, owing to his assiduity, the obstacle had entirely disappeared; and though he could read only mathematical latin and had later to study the language from another standpoint, in order to understand the classics, he was now able to pursue the study of mathematics in latin to his heart's content. the even tenor of the boy's life, deeply engaged as he was in studies of every description, was destined to be very seriously disturbed. when he was but fourteen, in , the revolution came, with its glorious promise and then its awful consummation. ampère's father was seriously alarmed at the revolutionary course things were taking in france, and had the fatal inspiration to leave his country home and betake himself to the city of lyons. for a time, he occupied a position as magistrate. after the siege of lyons, the revolutionary tribunal established there took up the project of making the lyonnese patriotic, as they called it, by properly punishing the citizens for their failure to sympathize at first with the revolutionary government, and soon a series of horrible massacres began. new victims were claimed every day, and ampère's father was one of those who had to suffer. the real reason for his condemnation was that he had accepted a position under the old government, though the pretext stated on the warrant for his arrest was that he was an aristocrat. this is the only evidence we have that the ampère family was in any way connected with the nobility. the day on which he was sentenced to die, jean jacques ampère wrote to his wife a letter of sublime simplicity, in which his christian resignation of spirit, his lofty courage, yet thoroughly practical commonsense, are manifest. he warned his wife to say nothing about his fate to their daughter josephine, though he hoped that his son would be better able to stand the blow, and perhaps prove a consolation to his mother. the news proved almost too much for the young ampère, and for a time his reason was despaired of. all his faculties seemed to be shocked for the moment into insensibility. biographers tell us that he wandered around, building little piles of sand, gazing idly at the stars or vacantly into space, wearing scarcely any of the expression of a rational being. his friends could harbor only the worst possible expectations for him, and even his physical health suffered so much that it seemed he would not long survive. one day, by chance, rousseau's "letters on botany" fell into his hands. they caught his attention, and he became interested in their charming narrative style, and as a result, his reason awoke once more. he began to study botany in the field, and soon acquired a taste for the reading of linnæus. at the same time, classic poetry, especially such as contained descriptions of nature, once more appealed to him, and so he took up his classical studies. he varied the reading of the poets with dissections of flowers, and yet succeeded in following both sets of studies so attentively that, forty years afterward, he was still perfectly capable of taking up the technical description of the plants that he had then studied, and while acting as a university inspector, he composed latin verses during his horseback rides from one inspection district to another, without ever having to consult a gradus or a dictionary for the quantities, yet without making a single mistake. his memory for subjects once learned, was almost literally infallible. something of his love for nature can be appreciated from an incident of his early manhood, which is not without its amusing side. ampère was very near-sighted, and had been able to read books all his life only by holding them very close to his eyes. this makes it all the more difficult to understand how he succeeded in reading so much. his near-sightedness was so marked that he had no idea of beauties of scenery beyond him, and was often rather put out at the enthusiastic description of scenes through which he passed _en diligence_, when his fellow-travelers spoke of the beauties of the scenes around them. ampère, like most people who do not share, or at least appreciate, the enthusiasm of others for beautiful things around them, was in this mood, mainly because he was not able to see them in the way that others did, and, therefore, could not have the same pleasure in them. this lack in himself was unconscious, of course, as in all other cases, and, far from lessening, rather emphasized the tendency to be impatient with others, and rather made him more ready to think how foolish they were to go into ecstasies over something that to him was so insignificant. one day, while ampère was making the journey along the saone into lyons, it happened that there sat beside him on the stage-coach a young man who suffered from near-sightedness very nearly in the same degree as ampère himself, but whose myopia had been corrected by means of properly fitting glasses. these glasses were just exactly what ampère needed in order to correct his vision completely. the young fellows became interested in each other, and, during the course of their conversation, his companion suggested to ampère, seeing how near-sighted he was, that he should try his glasses. he put them on, and at once nature presented herself to him under an entirely different aspect. the vision was so unexpected, that the description which he had so often heard from his fellow-travelers, but could not appreciate, now recurred to him, and he could not help exclaiming in raptures, "oh! what a smiling country! what picturesque, graceful hills! how the rich, warm tones are harmoniously blended in the wonderful union of sky and mountain vista!" all of these now spoke emphatically to his delicate sensibility, and a new world was literally revealed to him. ampère was so overcome by this unexpected sight, which gave him so much pleasure, that he burst into tears from depth of emotion, and could not satisfy himself with looking at all the beauties of nature that had been hidden from him for so long. ever after, natural scenery was one of the greatest pleasures that he had in life, and the beauties of nature, near or distant, meant more to him than any other gratification of the senses. in spite of the fact that ampère had devoted considerable attention to acoustics as a young man, and had studied the ways in which the waves of air by which sounds are formed and propagated, he had absolutely no ear for music, and was as tone-deaf as he had been blind before his discovery with regard to the glasses. musical notes constituted a mathematical problem for ampère, but nothing more. this continued to be the case until about thirty years of age. then, one day, he attended a musical soirée, at which the principal portions of the program were taken from glück. it is easy to understand that this master of harmony possessed no charms for a tone-deaf young man. he became uneasy during the course of the musical program, and his uneasiness became manifest to others. after the selections of the german composer were finished, however, some simple but charming melodies were unexpectedly introduced, and ampère suddenly found himself transported into a new world. if we are to believe his biographers, once more his emotion was expressed by an abundance of tears, which ampère seems to have had at command and to have been quite as ready to give way to in public as any of homer's heroes of the olden time. blind until he was nearly twenty, he used to say of himself, he had been deaf until he was thirty. in spite of his failure to respond in youth, once it had been awakened to appreciation, his soul vibrated profoundly to all the beauties of color and sound, and, later in life, they gave rise in him to depths of emotion which calmer individuals of less delicate sensibilities could scarcely understand, much less sympathize with. between his two supreme experiences in vision and sound, there had come to ampère another and even profounder emotion. he tells the story himself, in words that probably express his feelings better than any possible description of his biographer could do, and that show us how wonderfully sensitive his soul was to emotion of all kinds. he had just completed his twenty-first year when he fell head over heels in love. though he wrote very little, as a rule, he has left us a rather detailed description in diaries, evidently kept for the purpose, of the state of his feelings at this time. these bear the title, "_amorum_," the story of his love. on the first page these words occur: "one day as i was taking an evening walk, just after the setting of the sun, making my way along a little brook," then there is a hiatus, and he was evidently quite unable to express all that he felt. it seems that he was gathering botanical specimens, wearing an excellent set of spectacles ever since his adventure on the stage-coach had shown him the need of them, when he suddenly perceived at some distance two young and charming girls who were gathering flowers in the field. he looked at one of them, and he knew that his fate was sealed. up to that time, as he says, the idea of marriage had never occurred to him. one might think that the idea would occur very gently at first, then grow little by little; but that was not ampère's way. he wanted to marry her that very day. he did not know her name; he did not know her family; he had never even heard her voice, but he knew that she was the destined one. fortunately for the young lady and himself, she had very sensible parents. they demanded how he would be able to support a wife. ampère was quite willing to do anything that they should suggest. his father had left enough to support the family, but not enough to enable him to support a wife in an independent home; and until he had some occupation, the parents of his bride-to-be refused to listen to his representations. for a time, he consented to be a salesman in a silk store in lyons, in order to have some occupation which might eventually give him enough money to enable him to marry. fortunately, however, he was diverted from a commercial vocation which might thus have absorbed a great scientist, and arrangements were made which permitted him to continue his intellectual life, yet have the woman of his choice. she was destined to make life happier far for him than is the usual lot of man, and he was ever ready to acknowledge how much she meant for his happiness. with literature, poetry, love and settling down in life to occupy him, it is hard to think of ampère as a young man doing great work in science, but he did; and his work deservedly attracted attention even from his very early years. it was in pure mathematics, perhaps, above all other branches, that ampère attracted the attention of his generation. ordinary questions he did not care for. problems which the fruitless efforts of twenty centuries had pronounced insoluble attracted him at once. even the squaring of the circle claimed his attention for a while, though he got well beyond it even before his boyhood passed away. there is a manuscript note from the secretary of the academy of lyons, which shows that on july th, , ampère, then not quite thirteen years of age, addressed to that learned body a paper on the "squaring of the circle." later, during the same year, he submitted an analogous memoir, entitled, "the rectification of an arc of a circle, less than a semi-circumference." arago says that he was tempted to suppress this story of ampère's coquetting with so dangerous a problem, for ampère rather flattered himself that he had almost solved it. it was only after arago recalled how many geniuses in mathematics had occupied themselves with this same problem, that he saw his way clearly not to share the scruples of those who might think this incident a reflection on ampère's mathematical genius. after all, anaxagoras, hippocrates, archimedes and apollonius, among the ancients, and among the moderns, willebrod snell, huyghens, gregory, wallis, and finally newton, the mathematician of the heavens, occupied themselves seriously with this very problem. arago even notes that some men, by their speculations on the squaring of the circle, were led to distinguished discoveries, and mentions the name of father grégoire de saint-vincent, the distinguished flemish mathematician of the society of jesus, to whom, as a direct result of his studies in attempted circle-squaring, we owe the discovery of the properties of hyperbolic space, limited by the curve and its asymptotes, as well as the expansion of log ( + _x_) in ascending powers of _x_. montucla, the historian of mathematics, writing of père saint-vincent, said that, "no one ever squared the circle with so much ability or with so much success." there was, however, a fallacy in his magnificent work which was pointed out by the celebrated huyghens. shortly after the beginning of the nineteenth century, ampère, as one of his french biographers rather characteristically declares, redeemed whatever of mathematical sinning there might have been, in indulging in fond dalliance with the squaring of the circle, by a series of mathematical papers, each of which was in itself a distinct advance on previous knowledge, and at the same time, definite evidence of his mathematical ability. the first paper, published in , was a contribution to solid geometry, bearing the title, "on oblique polyhedrons." his next paper, written in , though not published until , was a treatise on the advantages to be derived in the theory of curves from due consideration of the osculating parabola. another treatise, written about the same time, had for title, "investigations on the application of the general formulæ of the calculus of variations to problems in mechanics." this concerned problems which had interested and, in most cases, proved too hard of solution even for such men as galileo, jacques bernoulli, leibnitz, huyghens and jean bernoulli. arago's expression with regard to this work is: "the treatise of ampère contains, in fact, new and very remarkable properties of the _catenary_ (la chainette) and its development." he adds: "there is no small merit in discovering hiatuses in subjects explored by such men as leibnitz, huyghens and the two bernoullis. i must not forget to add that the analysis of our associate unites elegance with simplicity." it is not surprising, after such marks of mathematical genius, that ampère was appointed to the chair of mathematics at the École polytechnique, where he came to be looked upon as one of the most distinguished of french mathematicians. in , he became a candidate for the position left vacant by the death of the famous lagrange; and at this time, presented to the academy general considerations on the integration of partial differential equations of the first and the second order. after his election to the academy, ampère continued to present important papers at its various sessions. among these, three are especially noteworthy: one was a demonstration of père mariotte's law (known to english students as boyle's law); another bore the title, "demonstration of a new theory from which can be deduced all the laws of refraction, ordinary and extraordinary"; a third was a memoir on the "determination of the curved surfaces of luminous waves in a medium whose elasticity differs in each of the three dimensions." in his eulogy of ampère, which, together with his article in the "dictionnaire universelle de biographie," we have followed rather closely, arago calls particular attention to the fact that in paris, ampère moved in two intellectual circles quite widely separated in their interests and sympathies. among the first group, were the members of the old "institute" and professors and examiners of the École polytechnique and professors of the collège de france. in the other, were the men whose names have since become widely known as students of psychology, of whom cabanis may be taken as the representative. ampère had as great a passion for psychology, and was as ready to devote himself to fathoming and analyzing the mysteries of the mind, as he was to work out a problem in advanced mathematics, or throw light on difficult questions in the physical sciences. these two sets of interests are seldom united in the same man, though occasionally they are found. at the end of the nineteenth century, we had the spectacle of very distinguished men of science in physics, and even in biology--sir william crookes, sir oliver lodge, professor charles richet, professor lombroso and even mr. alfred russell wallace--interested in psychic and spiritualistic manifestations of many kinds as well as in natural science; and, inasmuch as they did so, they would have found ampère a brother spirit. ampère indeed dived rather deeply into what would be called, somewhat slightingly, perhaps, in our generation, metaphysical speculation. at one time, he contemplated the publication of a book which was to be called "an introduction to philosophy." he had made elaborate theories with regard to many metaphysical questions, and had written articles on "the theory of relations," "the history of existence," "subjective and objective knowledge" and "absolute morality." arago calls attention to the fact that napoleon's famous anathema against ideology, far from discouraging ampère, rather seemed to stimulate him in his studies, and he declared that it would surely contribute to the propagation of this kind of speculation, rather than to its suppression. it was simply another case of napoleon overreaching himself, though this was in the domain of ideas and not in the realm of politics, where his fate was to reach him some time later. how deeply interested ampère became in metaphysics will perhaps be best appreciated from the fact that, for progress in metaphysics, exercise in disputation is needed, and had been the custom in the old medieval universities. ampère once made an arrangement to travel from paris to lyons and stay there for some time, provided a definite promise was made that at least four afternoons a week should be devoted to discussions on ideology. the journey to lyons, a distance of two hundred and fifty miles, was no easy undertaking in those days. the paris, lyons and mediterranean express now whirls one down to the capital of the silk district in a night; but in ampère's time, it took many days, and the journey was by no means without inconveniences, which were likely to be so troublesome that a prolonged rest was needed after it was over. ampère seems quite to have exhausted the interest of his friends in lyons, who found his metaphysical speculations too high for them, though they themselves were specializing in the subject and would be glad to tempt him into discussions of the exact sciences; but in lyrical strain he apostrophizes psychological studies: "how can i abandon the country, the flowers and running waters for the arid streets of the city! how give up streams and groves for deserts scorched by the rays of a mathematical sun, which, diffusing over all surrounding objects the most brilliant light, withers and dries them down to the very roots! how much more agreeable to wander under flitting shades, where truth seems to flee before us to incite us to pursue, than walk in straight paths where the eye embraces all at a glance!" had ampère been less successful as a mathematician or an investigator of physical science, these expressions would seem little short of ridiculous. as it is, they provide food for thought. ampère seemed to realize that, for the intellectual man, the only satisfaction was not in successful research so much as in application of mind to what promised results. as in everything else, it was the chase, and not the capture, that counted. seldom has this idea been applied to intellectual things with so much force as it seems to have appealed to ampère, and one is reminded of malebranche's famous expression, "if i had truth in my hand, i would be tempted to let it go for the pleasure of recapturing it." the principal source of ampère's fame, however, for future generations, was to be in his researches in the science of electro-dynamics. the name of this science will ever be inseparably linked with that of ampère, its founder. it was for that reason, of course, that the international congress of electricians decided to give his name to the unit of current strength, so that it has now become a household word, and will continue so for ages to come. in spite of the resemblances, much more than superficial, between magnetism and electricity, the identification of these two with each other seemed as yet very distant. it is curiously interesting, however, to note that ampère himself, in a program of his course, printed in , announced that the "professor will demonstrate that electrical and magnetic phenomena must be attributed to two different fluids which act independently of each other." ampère's fame was to be founded on the direct contradiction of this proposition, which he proposed and triumphantly defended by a marvelous series of experimental illustrations eighteen years later. in the meantime, the discovery of another distinguished scientist, doing his work many hundreds of miles away, was to prove the stimulus to ampère's constructive imagination, so as to enable him to fill out many obscure points of knowledge with regard to magnetism and electricity. this suggestive discovery was that of oersted, the sketch of whose life and work immediately precedes this. oersted demonstrated that a current of electricity will affect a magnetic needle. this epoch-making discovery reached paris by way of switzerland. the experiment was repeated before the french academy of sciences by a member of the academy of geneva, on september th, . the date has some importance in the history of science, for just seven days later, on the th of september, ampère presented, at the session of the academy of sciences, a still more important fact, to which he had been led by the consideration of oersted's discovery while testing it by way of control experiment. this brilliant discovery of ampère, arago summed up in these words: "two parallel conducting wires attract each other when the current traverses them in the same direction. on the contrary, they repel each other when the current flows in opposite directions. the phenomenon described by oersted was called, very appropriately, electromagnetic, whilst the phenomena described by ampère, in which the magnet played no part, received at his suggestion the general name of electro-dynamics, which has since been applied to them." at first it was said that these phenomena were nothing more than manifestations of the ordinary attractive and repelling power of the two forms of electricity which had been so carefully studied, especially in france, during the eighteenth century. ampère at once disposed of any such idea as this, however, by pointing out that bodies similarly electrified repel each other, whilst those that are in opposite electrical states attract each other. in the case of conductors conveying currents, there is attraction when these are in the same direction, and repulsion when they flow in the opposite direction. this reasoning absolutely precluded all possibility of further doubt in the matter, and this particular form of objection to ampère's discoveries was dropped at once. having satisfactorily disposed of other objections, ampère was content neither to rest quietly in his discovery nor merely to develop various experimental phases of it which would be extremely interesting and popularly attractive, but which at the same time might mean very little for science. with his mathematical mind, ampère resolved to work out a mathematical theory which would embrace not only all the phenomena of magnetism then known, but also the complete theory of the science of electro-dynamics. needless to say, such a problem was extremely difficult. arago has compared it to newton's solution of the problem of gravitation by mathematics. considering the comparatively small amount of data that ampère had at his command, this problem might very well be compared to that which leverrier took up with so much success, when he set about discovering by calculation only the planet neptune, as yet unknown, which was disturbing the movements of uranus. it might be thought that these discoveries of ampère would be welcomed with great enthusiasm. as a matter of fact, however, new discoveries that are really novel always have, as almost their surest index, the fact that contemporaries refuse to accept them. the more versed a man is in the science in which the discovery comes, the more likely is he to delay his acceptance of the novelty. this is not so surprising, since, as a rule, new discoveries are nearly always very simple expressions of great truths that seem obvious once they are accepted, yet have never been thought of. they mean, therefore, that men who consider themselves distinguished in a particular science have missed some easily discoverable phenomenon or its full significance, and so, to accept a new discovery in their department of learning men must confess their own lack of foresight. it may be pointed out that the same thing happened with regard to ohm, only it was much more serious. years of ohm's life were wasted because of the refusal of his contemporaries to accept his "law" at his valuation. arago, in his life of ampère, recalls that when fresnel discovered the transverse character of waves of light, his observations created the same doubts and uncertainty in the same individuals who a few years later refused to accept ampère's conclusions. arago puts it, that as he was ambitious of a high place in the world of ideas, he should have expected to find his adversaries precisely those already occupying the highest places. ampère never looked on himself as a mere specialist in physical science, however, and it is extremely interesting to know that he dared to take sides in a discussion between cuvier and geoffroy-saint-hilaire, with regard to the unity of structure in organized beings. while the purely physical scientists mostly sat mute during the discussion, ampère took an active share in it, and ventured to subject himself to what perhaps, above all things, a frenchman dreads, the ridicule of his colleagues. arago thought that he held his own very well in this discussion, which involved some of the ideas that were afterwards to be the subject of profound study and prolonged investigation later in the nineteenth century, because of the announcement of the theory of evolution. after his discoveries in electricity ampère came to be acknowledged as one of the greatest of living scientists, and was honored as such by most of the distinguished scientific societies of europe. his work was not confined to electricity alone, however, and late in life he prepared what has been well called a remarkable work on the classification of the sciences. this showed that, far from being a mere electrical specialist or even a profound thinker in physics, he understood better probably than any man of his time the interrelations of the sciences to one another. he was a broad-minded, profound thinker in the highest sense of the words, and in many things seems to have had almost an intuition of the intimate processes of nature; a sharer in secrets as yet unrevealed, though he was at the same time an untiring experimenter, eminently successful, as is so evident in his electrical researches, in arranging experiments so as to compel answers to the questions which he put to nature. in the midst of all this preoccupation of mind with science and all the scientific problems that were working in men's minds in his time, from the constitution of matter to the nature of life, above all engaged in experimental work, he was a deeply religious man in his opinions and practices. he had indeed the simple piety of a child. during the awful period of the french revolution, he had some doubts with regard to religious truths; but once these were dispelled, he became one of the most faithful practical catholics of his generation. he seldom passed a day without finding his way into a church, and his favorite form of prayer was the rosary. frederick ozanam tells the story of how he himself, overtaken by misgivings with regard to faith, and roaming almost aimlessly through the streets of paris trying to think out solutions for his doubts, and the problems that would so insistently present themselves respecting the intellectual foundations of christianity, finally wandered one day into a church, and found ampère there in an obscure corner, telling his beads. ozanam himself was moved to do the same thing, for ampère was then looked upon as one of the greatest living scientists of france. under the magic touch of an example like this and the quiet influence of prayer, ozanam's doubts vanished, never to return. saint-beuve, whose testimony in a matter like this would surely be unsuspected of any tendency to make ampère more catholic than he was, in his introduction to ampère's essay on the philosophy of the sciences (paris, ), says: "the religious struggles and doubts of his earlier life had ceased. what disturbed him now lay in less exalted regions. years ago, his interior conflicts, his instinctive yearning for the eternal, and a lively correspondence with his old friend, father barrett, combined with the general tendency of the time of the restoration, had led him back to that faith and devotion which he expressed so strikingly in .... during the years which followed, up to the time of his death, we were filled with wonder and admiration at the way in which, without effort, he united religion and science; faith and confidence in the intellectual possibilities of man with adoring submission to the revealed word of god." ozanam, to whose thoroughly practical christianity while he was professor of foreign literatures at the university of paris we owe the foundation of the conferences of st. vincent de paul, which so long anticipated the "settlement work" of the modern time and have done so much for the poor in large cities ever since, was very close to ampère, lived with him indeed for a while, said that, no matter where conversations with him began, they always led up to god. the great french scientist and philosopher used to take his broad forehead between his hands after he had been discussing some specially deep question of science or philosophy and say: "how great is god, ozanam! how great is god and how little is our knowledge!" of course this has been the expression of most profound thinkers at all times. st. augustine's famous vision of the angel standing by the sea emptying it out with a teaspoon, which has been rendered so living for most of us by botticelli's great picture, is but an earlier example of the same thing. one of ampère's greatest contemporaries, laplace, re-echoed the same sentiment, perhaps in less striking terms, when he declared that what we know is but little, while what we do not know is infinite. for anyone who desires to study the beautiful christian simplicity of a truly great soul, there is no better human document than the "journal and correspondence of ampère," published some years after his death. he himself wrote out the love story of his life; and it is perhaps one of the most charming of narratives, certainly the most delightful autobiographic story of this kind that has ever been told. it is human to the very core, and it shows a wonderfully sympathetic character in a great man, whose work was destined a few years later to revolutionize physics and to found the practical science of electro-dynamics. when ampère's death was impending, it was suggested that a chapter of the "imitation of christ" should be read to him; but he said, no! declaring that he preferred to be left alone for a while, as he knew the "imitation" by heart and would repeat those chapters in which he found most consolation. with the profoundest sentiments of piety and confidence in providence, he passed away june th, , at marseilles. with all his solid piety, this man was not so distant from ordinary worldly affairs as not to take a lively interest in all that was happening around him and, above all, all that concerned the welfare of men. he was especially enthusiastic for the freedom of the south american republics, eagerly following the course of bolivar and canaris, and rejoicing at the success of their efforts. south american patriots visiting paris found a warm welcome at his hands, and also introductions that made life pleasant for them at the french capital. his house was always open to them, and no service that he performed for them seemed too much. ampère was beloved by his family and his friends; he was perhaps the best liked man among his circle of acquaintances in paris because of the charming geniality of his character and his manifold interests. he was kind, above all, to rising young men in the intellectual world around him, and was looked up to by many of them as almost a second father. his charity towards the poor was proverbial, and this side of his personality and career deserves to be studied quite as much as what he was able to accomplish for science. the beauty of his character was rooted deeply in the religion that he professed, and in our day, when it has come to be the custom for so many to think that science and faith are inalterably opposed, the lesson of this life, so deeply imbued with both of these great human interests, deserves to be studied. ozanam, who knew him best, has brought out this extremely interesting union of intellectual qualities, in a passage that serves very well to sum up the meaning of ampère's life. "in addition to his scientific achievements," says ozanam, "this brilliant genius has other claims upon our admiration and affection. he was our brother in the faith. it was religion which guided the labors of his mind and illuminated his contemplations; he judged all things, science itself, by the exalted standard of religion.... this venerable head which was crowned by achievements and honors, bowed without reserve before the mysteries of faith, down even below the line which the church has marked for us. he prayed before the same altars before which descartes and pascal had knelt; beside the poor widow and the small child who may have been less humble in mind than he was. nobody observed the regulations of the church more conscientiously, regulations which are so hard on nature and yet so sweet in the habit. above all things, however, it is beautiful to see what sublime things christianity wrought in his great soul; this admirable simplicity, the unassumingness of a mind that recognized everything except its own genius; this high rectitude in matters of science, now so rare, seeking nothing but the truth and never rewards and distinction; the pleasant and ungrudging amiability; and lastly, the kindness with which he met everyone, especially young people. i can say that those who know only the intelligence of the man, know only the less perfect part. if he thought much, he loved more." chapter ix. ohm, the founder of mathematical electricity. lord kelvin, himself one of the greatest of the electrical scientists of the nineteenth century, in commenting some years ago on ohm's law, said that it was such an extremely simple expression of a great truth in electricity, that its significance is probably not confined to that department of physical phenomena, but that it is a law of nature in some much broader way. re-echoing this expression of his colleague, professor george chrystal, of edinburgh, in his article on electricity in the encyclopedia britannica (ix. edition), says that ohm's law "must now be allowed to rank with the law of gravitation and the elementary laws of statical electricity as a _law of nature_ in the strictest sense." in a word, to these leaders and teachers in physical science of the generation after his, though within a comparatively short time after ohm's death, there has come the complete realization of the absolutely fundamental character of the discovery made by george simon ohm, when he promulgated the principle that a current of electricity is to be measured by the electromotive force, divided by the resistance in the circuit. the very simplicity of this expression is its supreme title to represent a great discovery in natural science. it is the men who reach such absolutely simple formulæ for great fundamental truths that humanity has come, and rightly, to consider as representing its greatest men in science. like most of the distinguished discoverers in science who have displayed marked originality, ohm came from what is usually called the lower classes, his ancestors having had to work for their living for as long as the history of the family can be traced. his father was a locksmith, and succeeded his father at the trade. the head of the family for many generations had been engaged at this handicraft. the first of them of whom there is any definite record was ohm's great-grandfather, wilhelm ohm, who was a locksmith at westerholt, not far from münster, in westphalia. wilhelm ohm's son, johann vincent, the grandfather of the great electrician, during his years as a journeyman locksmith had spent some time in france, and subsequently settled down in kadolzburg, a small suburb of erlangen, in bavaria. in , he obtained the position of locksmith to the university of erlangen, and became a citizen of that municipality. both of his sons followed the trade of their father. the elder of these, johann wolfgang, worked at his trade as a journeyman in a number of the small cities of germany, and only after ten years of absence in what, because of the independent condition of the states now known as the german empire, were then considered foreign parts, did he wander back to his native place. on his return he received the mastership in his craft, and shortly after, about , married a young woman named beck. george simon ohm, the electrical scientist, was the first child of this marriage, and was born march th, . a second son, born three years later, also became distinguished in after-life for his mathematical ability. this younger brother, after having filled a number of teaching positions in various german educational institutions, was called as professor of mathematics to berlin, where he died in . while their father, johann wolfgang ohm, followed his trade of locksmith for a living, like many another handicraftsman, he had many mental interests which he cultivated in leisure hours, and doubtless dwelt on while his hands were occupied with the mere routine work of his trade. it is curiously interesting to find that he devoted himself, during the hours he could spare from his occupation, to two such diverse intellectual occupations as mathematics and kant's philosophy; but they had no newspapers in those days, and a man, even of the artisan class, had some time for serious mental occupation. it might be thought, under these circumstances, that he would be but the most passing of amateurs in either of these subjects, and have a very superficial knowledge of them. this probably was true for his philosophy fad, for there are not many who have ever thought themselves more than amateurs in kantism, and even kant himself, i believe, thought that only one scholar ever really understood his system, and subsequently said he had some doubts even about that one; but in mathematics, the elder ohm seems to have attained noteworthy success. hofrath langsdorff, who was the professor of mathematics at erlangen during the last decade of the eighteenth century, and who was called to heidelberg in , a fact that would seem quite enough to set beyond all question that his opinion in this matter may be taken as that of a competent judge, declared that the elder ohm's mathematical knowledge was far above the ordinary, and that he knew much more than the elements even of the higher mathematics. under these circumstances, it is not surprising that the father should have tried to encourage in both his boys a taste for mathematics, nor that he should have taken their mathematical instruction into his own hands and succeeded in making excellent mathematicians of them, even in their early years. he was so successful in this, indeed, that langsdorff, after a five-hour examination of the brothers when they were respectively and , did not hesitate to declare that the erlangen locksmith's family was likely to be remembered as containing a pair of brothers who, for success in mathematics, might rival the famous bernoulli brothers, so well known at that time. this might be thought only a bit of neighborly praise, meant to warm a father's heart, yet it seems indeed to have been given quite seriously. certainly the event justified the prophecy. it is not surprising that, with such a forecast to encourage him, the father should have been ready to make every sacrifice to enable both his sons to prepare for the university. he continued his instruction of them, then, in mathematics, though he insisted at the same time that they should continue to keep up their occupation of locksmiths. in spite of his enthusiasm for mathematics, the old gentleman seems to have cherished no illusions with regard to the likelihood of pure mathematics ever serving them as a lucrative means of livelihood. it was a very satisfying intellectual interest, but a good trade was much more apt to prove their constant and substantial standby, unless, of course, the boys should actually prove to be the geniuses foretold. he seems to have realized to the full, coleridge's idea that, like the literary man, the mathematician should have some other occupation, though he might not go to the extent of following oliver wendell holmes' well-known addition to coleridge's formula, that he should, as far as possible, confine himself to the other occupation. the boys were given the opportunity to attend the gymnasium of erlangen, and seem to have had excellent success in their general studies besides mathematics.[ ] in , when george, the subject of our sketch, was sixteen years of age, he was graduated from the gymnasium and was ready for the university. on may d, , he took his matriculation examination before the faculty of erlangen, electing the course of mathematics, physics and philosophy. later in life he told his friends that it was his deep love for the mathematics of these studies, and his persuasion that in them the student was brought in contact with the most important factors for absolute intellectual cultivation, that tempted him to take them up. to this he did not hesitate to add that there seemed to him to be some call of a higher voice, as if he had a vocation to dedicate himself to the cultivation and extension of these important subjects. he had been but some two years at the university, when for a time his studies had to be interrupted, partly for lack of means to pursue them, but partly because to his father, at least, the university course was not the source of such satisfaction as he had anticipated from his son's ability in mathematics. while ohm took his studies seriously, he was not by any means a mere "grind," and, indeed, the reputation which he acquired at the university for many of the qualities which make for a student's popularity among his fellows, was not such as would be likely to appeal to a very serious-minded father. ohm had acquired the fame of being one of the best dancers in the university; he was a brilliant billiard player and an unrivalled skater; all of which indicates that as a young man he had the physical development and acuteness of sense so necessary to enable him to gain prestige in all these sports. his father, in spite of his desire for his son's university career, was quite willing, then, at the end of september, , to have him take up a position as teacher of mathematics in the school kept by pastor zehnder, in the canton berne, in switzerland. his very youthful appearance (he was only years of age at the time, quite boyish looking and not even large for his years) caused the head of this institution no little surprise when he came with letters of introduction showing that he was to be the new teacher in mathematics. he could scarcely believe his eyes for a time. within a few months, however, he was convinced of the ability and the capacity for work of his new addition to the faculty, who seems to have given, from the very beginning, excellent satisfaction in his rather important position. ohm remained there some three years and a half and then moved to neunberg, where, independent of any educational institution, he set himself up as a private tutor in mathematics. his reason for so doing, as he himself tells, was that he wished to devote himself to the study of pure mathematics more than was possible in a regular teaching position. for this same reason also he refused a number of offers of positions as teacher of mathematics, which would ordinarily be considered quite flattering to a young man of only . another reason for refusing these offers was that he wished to perfect himself in french, and he had an excellent opportunity afforded him for conversation in this language in the conditions in which he was placed in neunberg. this last may seem an unusual reason, but it is characteristic of ohm's determination always to add to his power of understanding and expression. most young men in ohm's circumstances are so occupied with the thought of immediate success in life, that every possible abbreviation of their studies which will bring them nearer the opportunity to make their own living is likely to be heartily welcomed. ohm, however, realized that his own intellectual development was more important, especially at this time, even than getting on in the world; and for this reason his life has an added interest, not only for students themselves, but especially for those who have the best interests of students at heart and wish to be able to cite examples of how a little delay in getting at one's actual life-work, or, still more, at a remunerative occupation, may serve the very useful purpose of preparing a man so much the better to bring out his best intellectual possibilities when he does settle down to his work. at easter, , ohm returned to erlangen, after having spent nearly two years perfecting himself in mathematics. he then finished his studies at the university, which seems not to have had the rule of requiring attendance for a definite period before coming up for its degree, but permitted him to take the examinations for the doctorate of philosophy on the strength of the work he had done, and gave him his degree on the th of october of the same year. with the drawing tighter of the bands of red tape in educational institutions in more recent years, ohm would have found it difficult to get his degree thus readily, though it was the university rather than the graduate who was eventually to be honored by it. after this, he became _privatdocent_ in mathematics at the university, and taught for three semesters. he met with marked success and became very popular with the students. after a year and a half, however, he gave up his university position to accept the professorship of mathematics at the realschule of bamberg. while ohm was here, the spirit of young germany awoke at the news of napoleon's unfortunate moscow campaign, in which his good fortune seemed to have definitely abandoned the great emperor of the french. most of the students of the universities of germany were deeply aroused by it, and those who know körner's and uhland's songs will have some idea of the depth of patriotic feeling that was stirred in thousands of young german hearts, who thought that now the opportunity for the fatherland to throw off the hated foreign yoke forever, had come at last. ohm debated with himself whether he should volunteer with the crowds of young men who were so bravely giving up everything, that the fatherland might be free. two things deterred him. if he went as a soldier, the material assistance he was able to give his father, and which, as the old man was now advancing in years and had spent most of his little savings upon his sons, was needed, would have to be given up. the other motive that kept him at home was, according to his german biographer in the allgemeine deutsche biographie, which we have been following for most of these details, because he felt that what he might be able to accomplish in other fields besides those of battle would eventually prove more beneficial for his fatherland, and indeed for the whole of humanity, than anything he could do as a soldier, even with the patriotic motive to help his country to throw off the yoke of the foreign usurper, which had proven so hard to bear. as we have already seen, it was a characteristic trait of ohm all through life, that he cherished the idea, which acquired almost the force of a premonition, that he was destined for great things. ohm continued his work as a teacher, then, instead of volunteering for the army; but, as might be expected, found the monotonous work of drilling young students in mathematics extremely unsatisfactory after a time. at the end of a year and a half of service at bamberg, he asked for a change in the conditions of his teaching position. instead of this, he received a transfer to the bamberg pro-gymnasium, where he was to teach latin until a regular teacher was appointed. in spite of his representations that the teaching position offered him was utterly at variance with his talents and his inclinations, he was compelled to accept this occupation for a time, though after some delay there came the assurance that, just as soon as possible, he would be assigned to a position as teacher of mathematics. in spite of his unfortunate circumstances, which would ordinarily be thought quite enough to keep him from serious work until he was settled in a position more suited to his tastes, he devoted himself to the writing of his first book during this time, and it was published by enke, in erlangen, in the spring of . its title was, "outlines of the study of geometry as a means of intellectual culture." it comprised nearly two hundred pages, and gives the best possible insight into the ability and intelligence of the author, then a young man of only twenty-eight. as a sort of appendix, he gives a short sketch of his father, evidently introduced, not quite so much for the purpose of filially confessing his obligations to the old locksmith mathematician, nor with the idea of repaying some of his immeasurable debt for all the opportunities which the sacrifices of paternal affection had brought into the life of his sons, as to emphasize the excellent educational influence which his father's mathematical training had had upon his boys, and thus prove his thesis as to the value of mathematical studies in education. few filial tributes were ever more deserved or given more convincingly or with less suggestion of the conventional attitude of son to father. now that mathematics has come to occupy probably even a less prominent place in education than it did in ohm's time, though the burden of his complaint with regard to educational methods was that geometry was not used as a daily developmental subject as much as it should be, it may be interesting to recall some of the reasons which he advanced for urging its greater employment as an instrument for mental training. he thought that rational geometry should occupy a place of honor among our means of education. its quality as a mode of pure reasoning, though so closely related to the senses, made easy the transition from sensation to thought, which is such an important element in education; while its eminently simple character, though combined with definite demands upon the constructive faculties, made it appropriate in a high degree for the education of the young out of the field of merely imitative use of the intellect, into that of independent thinking and following out of ideas. "geometry," says ohm, "when properly taught, not with the fruitless drilling usually employed in teaching it, but in such ways as to secure deep personal attention, must take rank above all other branches of education, in enabling the student to break down the barrier which separates mere understanding from personal investigation. it forces a man whose thoughts were, up to this time, only the repetition of others' thoughts, to think for himself and to light for himself in his own mind the torches which enable him to see things clearly for himself, and not merely in the dimness of the half light that is thrown on them by the explanations of others." geometrical methods always had a special fascination for ohm, and practically all of his books and writings bear the impress of that close dependence of all parts on one another, that absolutely logical connection so characteristic of geometric accuracy of thought. his was the sort of mind likely to be benefited by mathematical training. such minds are, however, comparatively few, for most men are not rational in any sense of the word, that would make them dependent on logical reasoning. perhaps it is as well that they are not, for many of those lacking in logic or mathematical accuracy of thought and absoluteness of conclusion, still continue to accomplish much in the world of thought and do much valuable planning for the complexities of human affairs, where strict logic will not always solve the intricate yet incomplete problems that present themselves in human relations, where, indeed, individual unknown factors often make any but an approximate solution impossible. the opinions of the critics as to ohm's "outlines of geometry" were, as might be easily anticipated, not all flattering, since only a few of the critics were able to place themselves on the ideal standpoint of mathematical subjectivity from which he had written his book. king frederick william iii., of prussia, is said to have read it with much interest, however, and the royal pleasure doubtless drew attention to ohm's work, and may have contributed to the fact that, shortly after its publication, in september, , ohm was invited by the royal consistory of cologne to take the position of head professor of mathematics and physics in the gymnasium of that city. this post was not only honorable, it was also highly remunerative, at least from the standpoint of teachers' wages as they were at that time, and ohm eagerly accepted the position. lamont, who was the director of the royal observatory at munich, has written a memorial of ohm which contains much valuable information. the body of it is an address delivered at a meeting of the faculty of the university of munich in honor of thaddeus siber and george simon ohm, but its value has been much enhanced by notes added before publication. siber was a benedictine who was professor in the philosophical department at munich, and died the same year as ohm. lamont says that he received his information as to intimate details of ohm's life from his brother, prof. martin ohm, of berlin. his sketch is, therefore, absolutely authoritative. lamont says with regard to this period of teaching at cologne: "ohm's first position of importance, in any way worthy of his talents, was the professorship of mathematics at the large jesuit gymnasium in cologne, in , where the special gift that he possessed, of making the study of mathematics not only comprehensible but attractive to boys, brought him success and recognition." for nearly ten years ohm had the opportunity to put into practice in this jesuit gymnasium of the rhineland, the principles which he had so much at heart, for he was apparently given the full freedom of his department of teaching. he succeeded so well that he received wide and hearty recognition for his work. the mathematical studies of the cologne gymnasium stood higher than had ever been the case before, and this was all ohm's work. in the years before his teaching in the rhenish city, those who were distinguished in mathematics at the university of bonn had not come, as a rule, from cologne, but from other places; but now nearly all the mathematical prize-takers of bonn came from among ohm's students, and the best of the candidates for teaching positions in physics and mathematics had also, as a rule, had the advantages of his training. among the best of his scholars at this time was the afterwards well-known mathematician, lejeune-dirichlet, who taught in berlin with jacobi and steiner and succeeded gauss in göttingen. another of his most distinguished pupils was the astronomer heis, who occupied a modest position at the munster academy, but whose merits were above the post which he occupied, and who was distinguished for the excellency of his original work and his ability as a mathematician. one very interesting fact with regard to ohm's teaching, was that he was successful in catching and holding the interest not only of those of his students who were later to specialize in mathematics, but also of those who took up mathematics only as a subject for mental development, that was to be applied to other purposes later in life, and who found ohm's teaching of the greatest possible service. among these, the well-known german literary man, jacob venedey, of cologne, has expressed his affection and gratitude for his old teacher in a very striking way in his sketch of the cathedral at cologne, written in the banishment that came to so many vigorous german thinkers after the failure of the revolution of ' . in sending a copy of this to ohm, venedey says: "honored sir:--it will perhaps be a source of wonder to you that a student who apparently learned so little from you and your colleagues that he must now earn his bread by writing, should continue to cherish for you the liveliest gratitude. it is not the fault of mathematics that only the dimmest recollection of them remains with me. i shall never forget the personality of my professor, however, nor his ways and methods of teaching. i frequently recount your way with us boys, and i have the liveliest remembrance of your influence as a teacher. there are seldom weeks, there never is a month, when i fail to recall you. this is no mere compliment that i am paying to you, since i know you too well to think that flattery would mean anything to you, as it would be unworthy of you, and i for my part am not one of those who like to bandy compliments. i have often wished to meet you again, and a hundred times i thought that i saw you because some one at a distance had something that recalled you. i may say to you that you accomplished something for me in those days of teaching that i would not have been able to accomplish for myself. i can only think of you, then, with the highest feelings of reverence approaching what might well be called love. it will be a happy day, indeed, for me if i am ever in a position to make an hour of existence happier for you in any way." while ohm so zealously continued his instruction in both the upper classes of the gymnasium, he never lost from sight that higher aim of original research and investigation to which his genius disposed him. his choice of a subject for original investigation wavered for a long time between mathematics and physics, but, as he himself declared, his experience having shown him that authority was prone to play a large role in mathematics, while the field was more open for personal research and observation in physics, he resolved to take up that department for his special studies, consoled by the idea that physics cannot be properly pursued without mathematics. looking around to select a subject that would serve as a striking preface to his work in this department, though resolved at the same time to avoid one where he would be without rivalry, he found it all ready to his hand in what one of his contemporaries called the enigmatic phenomena of the galvanic current. this was to prove a fortunate selection, indeed, both for himself and the opportunity afforded his genius as well as for the science of electricity itself. he then began a series of investigations, always experimental in character, and with the mathematical explanations of the phenomena observed carefully worked out. accounts of these studies appeared from time to time in the year-book for chemistry and physics, issued by schweigger. after some ten years, these were collected together, or at least the principal portions of them, and published in the second half of the year-book for the year . the apparatus for his experiments was fortunately at command in the gymnasium at cologne, but without his mechanical skill, obtained from his experience as a locksmith when a boy, it would have been impossible so to vary his experiments and modify his instruments as to bring out many of the phenomena that he succeeded in demonstrating. nearly all of the great discoverers in science have been handy men possessed of mechanical skill, and this is as true for medicine, as i have shown in "makers of modern medicine,"[ ] though it might perhaps not be expected, as it is here in electricity, where it seems very natural. ohm felt, in , that he had succeeded in exhausting nearly all that he could learn for himself, and as he wished to have opportunities for further study, and especially for further reading, he asked for an academic furlough that would carry him over the next year. the work that he had already accomplished was beginning to be appreciated, and after discussion of the papers that he had published up to that time, the requested furlough was promptly granted; and in a letter in which the school authorities praised his school work as well as his original investigations, they allowed him to take the sabbatic year for the furtherance of science on one-half the usual salary, though with the condition also that more would be allowed to him in case this seemed necessary and the conditions justified it. this furlough was perhaps the most important event in ohm's life. he employed it in bringing to a focus the ideas with regard to electricity which had been gradually worked out in his mind during the past ten years. in may, , within six months after the beginning of his exclusive devotion to the subject, ohm's article on the mathematics of the galvanic current appeared. it proved a scientific achievement of the first rank, that was to be epoch-making in the domain of electricity. it settled the conditions under which electrical tension exists in various bodies, and made it clear that there is a fundamental law of electrical conduction which could be expressed by an easy, simple formula. ohm's preface to his little book, that was to work such a revolution in electricity and was to remain for all time one of the classics in this department of science, is typical of the man in many ways. its modesty could not very well be exceeded. its simplicity constitutes in itself an appeal to the reader's interest. i know nothing in the literature of the history of science quite like it in these regards, unless it be the preface of auenbrugger's little book on percussion, in which he laid the foundation of modern clinical diagnosis.[ ] the two men have many more qualities in common than the authorship of modest prefaces to their books. both of them were geniuses whose names the aftertime will not willingly let die, and both of them accomplished their work apart from the stream of university life in their time, and met with a like fate in the neglect, for some time at least, by their distinguished colleagues of the important discoveries that they had made. ohm's preface deserves to be quoted because of its classic quality: "i herewith present to the public a theory of galvanic electricity as a special part of electrical science in general, and shall successively, as time, inclination and means permit, arrange more such portions together into a whole, if this first essay shall in some degree repay the sacrifice it has cost me. the circumstances in which i have hitherto been placed have not been suitable either to encourage me in the pursuit of novelties or to enable me to become acquainted with works relating to the same department of literature throughout its whole extent. i have, therefore, chosen for my first attempt a department of science in which i have the least to apprehend competition. "may the well-disposed reader accept whatever i have accomplished with the same love for science as that with which it is sent forth!--the author, berlin, may st, ." in his preface to the american edition of the "galvanic circuit investigated mathematically,"[ ] mr. thomas d. lockwood, vice-president of the american institute of electrical engineers, said of this masterpiece of ohm's: "a sufficient reason for republishing an english translation of the wonderful book of professor g. s. ohm is the difficulty with which the only previous translation (that of taylor's scientific memoirs) is procurable. "besides this, however, the intrinsic value of the book is so great that it should be read by all electricians who care for more than superficial knowledge. "it is most remarkable to note, at this time, how completely ohm stated his famous law that the electromotive force divided by the resistance is equal to the strength of the current." with regard to the book as a whole, mr. lockwood says, after suggesting certain anticipations of ohm's ideas which had been made in the preceding century: "ohm's work stands alone, and, reading it at the present time, one is filled with wonder at the prescience, respect for his patience and prophetic soul, and admiration of the immensity and variety of ground covered by his little book, which is indeed his best monument." like many another great discovery in physical science, ohm's work failed to receive the immediate appreciation which it deserved. it cannot be said, however, that it failed to attract attention. it would be easier, indeed, to forgive the scientists of the day if this were true. not long after its appearance, abstracts from it were made by fechner in leipzig, by pfaff in erlangen, and poggendorff in berlin, which showed that these scientists understood very clearly the significance and comprehended the wide application of ohm's law as claimed by its author. from these men there was no question of hostile criticism. professor pohl, of the university of berlin, however, in the berlin "year-book of scientific criticism," did not hesitate to express his utter disagreement, and declared that ohm's work was fallacious and should be rejected. other writers of the time treated ohm's article more or less indifferently, as a merely conventional contribution to science. professor pohl's opinion was taken to represent the conclusions of the faculty of the university of berlin, especially noted for mathematical ability. this was to prove a serious hindrance to ohm in the university career which he had planned for himself. at berlin they had the ear of the minister of education, and it was not long before ohm felt that the criticisms of his work were making themselves felt in a direction unfavorable to him. not long after the appearance of his book, there came a disagreement between ohm and the educational authorities. ohm felt that this was due to failure to recognize the significance of his work, and that under the circumstances he could not hope for the appreciation that would provide him with the opportunities he deserved. he insisted on sending in his resignation as a teacher. nothing could change his determination in the matter, not even the pleas of his former scholars, and his resignation had to be accepted. ohm had hoped for a teaching position in a university. the minister of education declared that, while his work as a teacher had been accomplished with careful industry and diligence and conscientious attention to duty, the ministry regretted that, in spite of thorough appreciation of him and admiration for his excellent work as a scientist, they could not find for him a position outside of the gymnasium. how utterly trivial the conventional expressions sound, now that we know that they brought about for the time being the interruption of one of the most brilliant scientific careers in europe. of course, the geese cannot be expected to appreciate the swans, and it was not the minister's fault, but that of some of ohm's own colleagues. the next six years of his life, the precious years between and , ohm had to give up the idea of teaching in a university, and devote himself to some private tutoring in berlin, with a stipend of about three hundred dollars a year, miserable enough, yet sufficient, as would appear, for ohm's simple mode of life. this he owed to the kindness of gen. radowitz, who employed him to teach mathematics in a military school in berlin. at the end of this time, when he was nearly years of age, his unfortunate situation attracted the attention of king ludwig i., of bavaria, who offered him the chair of professor of physics at the polytechnic school in nuremberg, which had recently by royal rescript been raised to the status of a royal institute, with the same rank in educational circles as a lyceum for the study of humanities. here ohm's duties were shortly to be multiplied. he became the inspector of scientific instruction, after having occupied for some time the professorship of mathematics, and later became the rector of the polytechnic school, a position which he held for some ten years, fulfilling its duties with the greatest conscientiousness and fidelity. ohm continued his work at nuremberg for more than fifteen years. during this time, he succeeded in making his mark in every one of the departments of physics. he is usually considered as owing his reputation as an experimental and mathematical scientist to his researches in electricity. as a matter of fact, every branch of physics was illuminated by his work, and perhaps nothing shows the original genius of the man better than the fact that everything which he took up revealed new scientific aspects in his hands. the only wonder is that he should have remained so long in a subordinate position in the educational world at nuremberg, and received his appointment as university professor of physics at munich only in . in the midst of the administrative educational work that came to him at nuremberg, ohm did not neglect original investigation, but somehow succeeded in finding time for experiment and study. having made a cardinal discovery in electricity, of the value of which surely no one was more aware than himself, ohm might have been expected, as soon as his new post gave him the opportunity, to devote himself quite exclusively to this department of science. instead, he turned for a time to the related subjects of sound, heat and light, devoting himself especially to their mathematics. he did this, as he said himself, to complete for his own satisfaction his knowledge of the scientific foundations of the imponderables, as heat, light and electricity were then called, but also because he wished, for the sake of his students, to get closely in touch with what had been accomplished by recent investigators in physics. it is almost a universal rule in science, that no matter how distinguished an investigator may be, he makes but one cardinal discovery. ohm, however, was destined, after having brilliantly illuminated electricity by the discovery of a great law, to throw nearly as bright a light on the domain of acoustics; and there is a law in this department of physics which is deservedly called by his name, though it is often associated with that of helmholtz. helmholtz himself was always most emphatic in his insistence on ohm's priority in the matter, and constantly speaks of the law in question by ohm's name. perhaps no better evidence of the breadth of ohm's interest in science, his supreme faculty for experimentation, or the originality of his investigating genius, can be found than the fact that he thus discovered, by experimental and mathematical methods, the solution to important problems in two such distinct departments of physical science as electricity and acoustics. before his time, the question of electrical resistance was absolutely insoluble. the problem in acoustics was not less obscure, as may be judged from the fact that, though some of the best physicists and mathematicians of europe during the eighteenth century--and there were giants in those days, among others, brook taylor in england, d'alembert in france, johann bernoulli and euler in germany, and finally, daniel bernoulli--had devoted themselves to its solution, it remained nevertheless unsolved. here, as in electricity, the simplicity of the solution which ohm found shows how direct were his methods of thinking and how thorough his modes of investigation. perhaps the most striking feature of ohm's work in acoustics, and, above all, his solution of an important problem in music, is the fact that he himself, unlike most of his german compatriots, had no ear for music and no liking for it. in his address delivered at the public meeting of the royal bavarian academy of sciences at munich, in march, , the hundredth anniversary of the birth of ohm, eugene lommel, in discussing the scientific work of ohm, said: "inasmuch as his law in acoustics furnished the clearest insight into the hitherto incomprehensible nature of musical tones, it dominates the acoustics of to-day no less completely than ohm's law of the electric current dominates the science of electricity."[ ] this law concerns the resolution of tones into their constituents. the ideas laid down by ohm were almost absolutely novel. they were so new that none of the workers in acoustics could think that ohm had made a great discovery. his law states that the human ear perceives only pendulum-like vibration as a simple tone. every other periodic motion it resolves into a collection of pendulum-like vibrations, which it then hears in the sound as a series of single tones, fundamentals and overtones. ohm arrived at this law from mathematical considerations, making use of fourier's series; for its experimental verification he was compelled to use the well-cultivated ear of a friend, inasmuch as he was himself, as we have said, quite devoid of musical appreciation. ohm's results were too distant from the accustomed ideas of investigators of sound at that time to be accepted by them. seebeck, who was one of the most prominent scientists of the time in acoustics, did not hesitate to criticise severely, just as pohl had made little of ohm's law of the electric current. while, however, foreigners were to teach german scientists the value of the advance that their great colleague in electricity had made, the privilege of pointing out the significance of his work in sound was to be a compatriot's good fortune. it was nearly a score of years, however, before this vindication was to take place. then helmholtz, a decade after ohm's death, furnished the experimental means which enabled even the unskilled ear to resolve a sound into its simple partial tones, and revolutionized the theory of music by his classic work, "the science of the perception of sound," which is based entirely on ohm's law of acoustics. ohm, in the appendix to his work, "the galvanic circuit treated mathematically," dared to suggest certain speculations with regard to the ultimate structure of matter. he said: "there are properties of space-filling matter which we are accustomed to look upon as belonging to it. there are other properties which heretofore we have been inclined to look upon as accidents or guests of matter, which abide with it from time to time. for these properties man has thought out causes, if not foreign, at least extrinsic, and they pass as immaterial independent phases of nature under the names light, heat, electricity, etc. it must be possible so to conceive the structure of physical bodies that, along with the properties of the first class, at the same time and necessarily those of the second shall be given." it is all the more interesting to come upon ohm's speculations on this subject of the ultimate constitution of matter, because within a few years of his time, pasteur, then only a comparatively young man, had also been taken with the idea of getting at the constitution of matter by his observations upon dissymmetry, which he abandoned after a time, however, because he found other and more practical subjects to devote himself to, though he never gave up the thought that he might some time return to them and perhaps discover the underlying principles of matter from observations in this subject. it was not until the last five years of his life, when ohm was already past sixty, that he was to enjoy the satisfaction of an ambition which he had cherished from his earliest years as a teacher, and which, in spite of untoward circumstances, had been a precious stimulus in his work. for some twenty years he had hoped some time to be able to devote himself to the investigation of the physical constitution of matter. unfortunately, when the opportunity came, the manifold duties of his teaching position prevented the completion of his great work, and doubtless robbed his generation and ours of a precious heritage in the mathematics of the structure of matter, which would doubtless have been of the greatest possible value. it is of course idle to speculate as to what he might have accomplished if left to his original investigation. the problem which he now took up was much more difficult than any of his preceding tasks. it would have seemed, however, quite as hopeless to those who lived before ohm's laws, to look for a single complete law of the resistance of the electrical current in the circuit or of the overtones in music, as it is to us to think of a simple mathematical formula for atomic relations. what ohm accomplished in these other cases by his wonderful power of eliminating all the unnecessary factors in the problem, would surely have helped him here. the main power of genius, after all, is its faculty of eliminating the superfluous, which always obscures the real question at issue to such a degree for ordinary minds, that they are utterly unable to see even the possibility of a simple solution of it. art has been defined as the elimination of the superfluous; discovery in science might well be defined in the same terms. under the circumstances, we cannot help regretting that ohm was not allowed the time and the opportunity to work out the thoughts with which he was engaged. it would have been even more satisfactory if the precious years of his ripe middle age had not been wasted in trivial, conventional tasks, so that he might have been permitted to devote his academic leisure, sooner than was actually the case, to the problem which had been so constantly in mind since he made his great generalization in the laws of electricity. unfortunately, most of ohm's time had now to be taken up with his teaching duties. only for his self-sacrifice in the matter, his success as a teacher would doubtless have been less marked. science itself must have suffered, however, from this pre-occupation of mind with a round of conventional duties, since ohm could no longer devote his time to original research. in the meantime, his great discovery was coming to its own. during these ten years since the publication of his book, a number of distinguished physicists in every country--poggendorff, and especially fechner, in germany, jacobi and lenz in russia, henry in america, rosenkoeld in sweden, and de heer in holland--took up the problems of the current strength of electricity as set forth in ohm's law, and confirmed his conclusion by their investigations along similar lines. the french physicist and member of the academy of sciences, pouillet, applied ohm's ideas to thermo-electricity and pyro-electricity, employing his terms and bringing his work to the notice of foreigners generally, so that a translation of ohm's work was made into english. ohm's work at once attracted the attention that it deserved in england. the royal society conferred on him the copley medal, which had been founded as a reward for important discoveries in the domain of natural knowledge. before ohm's time only one other german scientist, carl friedrich gauss, of göttingen, had ever been thus honored. the words employed by the royal society in conferring this distinction showed how thoroughly the representatives of english science appreciated ohm's work. they said that he had set forth the laws of the electric current very clearly, and thus accomplished the solution of a problem which was as important in the realm of applied science as it had hitherto been in the schools. recognition now became the rule, and ohm had the satisfaction of having all his colleagues in the physical sciences acknowledge the significance of his work. ohm's recognition, then, came from foreigners first, and only afterwards from his fellow-countrymen. immediate appreciation might have meant much for him, and even this tardy recognition gave him renewed courage and new strength to go on with his work. he gave effective expression at once to his gratitude and to the stimulus that had been afforded him by the dedication to the royal society of london of the great work, "contributions to molecular physics," which he planned. the year after he received the copley medal, he was made a foreign associate of the royal society of england, and from this time on his discoveries began to find their way into text-books as fundamental doctrines in the science of electricity. german and foreign scientific bodies followed the english example so happily set for them, and began to give him their recognition as a physicist of the first rank. ohm's further observations were, for a time, not accepted so readily as his first law. the reason for this was that ohm was so far ahead of his times that there was not as yet in existence a suitable electroscope to test their truth. finally, the invention of an exact electrometer by dellman, and its application by professor kohlrausch, of marburg, made the experimental confirmation of all his work quite as significant as for his law. it is a striking reflection on ohm's career, though not very encouraging for the discoverer in science, to realize that some important discoveries, which thus proved eventually quite as epoch-making as his law, had lain for practically ten years neglected, and their magnificently endowed author had been allowed to eke out a rather difficult existence in teaching, not in the important department of science in which he was so great a master, but in certain conventional phases of mathematics which might very well have been taught by almost anyone who knew the elements of higher mathematics. ohm's case is not a solitary phenomenon in the history of science, however, but rather follows the rule, that a genuine novelty is seldom welcomed by the leaders of science at any given moment; but, on the contrary, rather decried, and its discoverer always frigidly put in his proper place by those who resent his audacity in presuming to teach _them_ something new in their _own_ science. having thus illuminated electricity and acoustics, ohm turned his attention to the department of optics. his power to simplify difficulties and get at the heart of obscure problems is illustrated by his contribution to this subject, made while he was professor of physics in the university of munich. optics had early engaged his attention, and in he published a paper in poggendorff's annalen, bearing the title, "a description of some simple and easily managed arrangements for making the experiment of the interference of light." with his usual faculty for simplifying things, he showed that the interference prisms which were made so carefully by the french could be constructed from common plate-glass. he was indeed able to demonstrate that a simple strip from the edge of a piece of such glass could be used for this purpose. he pursued this absorbing subject until - , and then set himself the difficult task of developing a general theory of these phenomena of interference which are so rich in form and color. the problem was indeed alluring, but some of the best minds in nineteenth century science in europe had been engaged at it, without bringing much order out of the chaos, and it would have looked quite unpromising to anyone but ohm, to whom, the greater the difficulty of a subject, the more the attraction it possessed. with his wonderful power of synthesis and his capacity to discover a clue to the way through a maze of difficulties, ohm succeeded in finding a formula of great simplicity and beauty and which covered all the individual colors. it was only after he had reached his conclusions and was actually publishing his results, that the german scientist found that he had been anticipated by professor langberg, of christiania, in norway, with regard to the principal points of his investigation, though not as to all its details. professor langberg[ ] had published his article in the norwegian magazine for natural sciences in , and an abstract of it had appeared the following year in the first complementary volume (erganzungsband) of poggendorff's annalen. of this publication by professor langberg, ohm had known absolutely nothing. he had even gone to some pains to find out, before undertaking his own investigation, whether anything had been published on the matter. at the sessions of the german naturalists' association, held in , he had called the attention of many prominent physicists and mineralogists who were present at that meeting to the colored concentric ellipses which occur in connection with certain crystals used in the investigation of polarization. he asked whether these had ever been seen before, or whether anything had been written about them. all of those whom he consulted declared that they had not observed them, and that, so far as they knew, nothing had been published with regard to them. accordingly, ohm proceeded with his work, only to find, after its formal publication, that he had been almost entirely anticipated and that the merit of original discovery belonged to his norwegian colleague. when his attention was called to the publication, ohm was perfectly ready to acknowledge the priority of professor langberg's claim and to give him all the credit that belonged to his discovery. at the beginning of the second part of his article, he said: "i know not whether i should consider it lucky or unlucky that the extremely meritorious work of langberg should have entirely escaped me and should have been lost to general recollection. certain it is that, if i had had any knowledge of it before, my present investigations, which were occasioned by this elliptical system, would not have been made and i would have been spared a deal of work. in that case, however, a number of other and scarcely less important scientific principles would have remained hidden for the time being at least. under the circumstances, the profound truth of the old proverb, 'man proposes, but god disposes,' has been brought home to me again. what originally set me investigating this subject now proves to be without interest for science, since the problem has been solved before. on the other hand, a number of things of which i had no hint at all at the beginning of my researches, have come to take its place and compensate for it." perhaps nothing will show better than this, ohm's disposition toward that providence which overrules everything, and somehow, out of the mixture of good and evil in life, accomplishes things that make for the great purpose of creation. his eminently inquiring attitude towards science, which had on three occasions led him to tackle problems that had puzzled the greatest of experimental scientists, has been shown. he must have been, above all things, a man of a scientific turn of mind, in the sense that he was not ready to accept what had previously been accepted even by distinguished authorities in science, but was ready to look for new clews that would lead him to simpler explanations than any that had been offered before. in spite of this inquiring disposition, so eminently appropriate to the scientist, and constituting the basis of his success as an experimenter and scientific synthesist, he seems to have no doubts about the old explanation of the creation nor the all-wise directing power of a divine providence. this is all the more interesting, because already the materialistic view of things, which claims to know nothing except what can be learned from the matter around us, had begun to make its way in europe, especially in scientific circles, but ohm remained untouched by it. another example of this same state of mind in ohm is to be found in the preface to his last great work, his contribution to molecular physics, in which he hoped to sum up all that he could discover and demonstrate mathematically with regard to the constitution of matter. he knew that he was taking up a work that would require many years and much laborious occupation of mind. he realized, too, that his duties as professor of physics and mathematics as well as the directorship of the museum and the consultancy to the department of telegraphs, left him comparatively little time for the work. he foresaw that he might not be able to finish it, yet hoped against hope that he would. in the preface to the first volume, he declared that he would devote himself to it at every possible opportunity, and that he hoped that _god would spare him to complete it_. this simplicity of confidence in the almighty is indeed a striking characteristic of the man. the work which ohm began thus with such humble trust in god, was to contain his conclusions concerning the nature, size, form and mode of action of the atom, with the idea of being able to deduce, by the aid of analytical mechanics, all the phenomena of matter. unfortunately, he was spared only to write the first, an introductory volume which bears the title, "elements of the analytical geometry of space on a system of oblique co-ordinates." this did not touch, as he confesses, the ultimate problem he had in mind. the second volume was to have contained the dynamics of the structures of bodies, and a third and fourth were to be devoted to the physical investigation of the atom and its relation to other atoms and matter in general. ohm devoted himself, however, with too much ardor to his duties as teacher, to allow himself to give the time to his own work that would have enabled him to finish it. among other things that he did for his students was to complete a text-book of physics. he confesses that he had always felt an aversion to working at a text-book, and yet was impelled to take up the task because he felt that in electricity, in sound and in optics, the only way in which his students would get his ideas, many of which were the result of his own work, was to have a text-book by himself, and he felt bound in duty to do this for them, as he had accepted the position of instructor. he succeeded in completing the book very rapidly by lithographing his lectures immediately after delivery and distributing copies to his classes. it is almost needless to say that the work was, in its way, thoroughly original. it was accomplished with the ease with which he was always able to do things; but, unfortunately, the strain of the work told on him at his years much more than when, as a younger man, he was able to work without fatigue. he acknowledges, at the close of the preface, that the task has been too great, and that he should not have undertaken its accomplishment, and especially not in the hasty way in which it was done. this preface was dated easter, . within a few months, ohm's strength began to fail, and the end was not long in coming. according to the translation of the address of lommel, as it appeared in the annual report of the smithsonian institute for , ohm died as the result of repeated attacks of epilepsy, on july th, . the date is correct; the mode of death, however, is surely reported under a misunderstanding. the physician who hears of epilepsy is prone at once to inquire as to its origin, and to wonder how long the patient had been suffering from it. there are no reports of previous attacks of epilepsy, and the sudden development of genuine epilepsy in fatal form at the age of is quite unlikely. his german biographer, bauernfeind, who is quoted by lommel as one of the authorities for the details of ohm's life, and who was a pupil and intimate friend, gives quite a different account. up to the very last day of his life, ohm continued his lectures. his duties as professor appealed to his conscience as no others. on thursday, july th, , he delivered his last lecture. that night at ten o'clock he died. the cause of his death was given as a repeated apopleptic stroke. it is evidently because of the occurrence of more apopleptic seizures than one, that the assertion of epilepsy was introduced unto the account of his death. for some days before his death, ohm had been very weak, but had continued to fulfil every duty. to us in the modern time, it may seem surprising that there should be lectures in a university in july; but the second semester of the university year in germany is not supposed to come to a close until the first of august, when the summer vacation begins, and lectures are continued until well on into july. the manner of ohm's death, as told by his biographer friend, at once corrects the idea of epilepsy, and also shows that his passing came without any of the preliminary suffering that makes death a real misfortune. a half hour before his death, he had been entertaining some friends with lively recollections of the events of his early life in cologne and treves. he had been quite gay in the stories that he told, and almost boyishly happy in the recollections of those early days. for one for whom duty had meant so much in life, and who had always tried so faithfully to fulfil it, no happier call to higher things could possibly be imagined than that which came to ohm. on the following sunday he was followed to the grave by numbers of friends, by all his colleagues and by most of the students of the munich university. the university felt that it had suffered a great loss, and no signs of its grief were felt to be too much. ohm was buried in the old munich graveyard, where his bones still rest, beneath the simple memorial not unworthy of the modest scientist who did his work patiently and quietly, yet with never-failing persistency; who cared not for the applause of the multitude, and accomplished so much quite independently of any of the ordinary helps from others and from great educational institutions that are often supposed to be almost indispensably necessary for the accomplishment of original scientific work. ohm's personal appearance will be of interest to many of those to whom his discoveries have made him appeal as one of the great original thinkers in modern science. he was almost small in stature, even below middle height; and those who remember virchow, may get something of an idea of his appearance when told that those who saw ohm and knew virchow, considered that there was a certain reminder of each other in the two men. according to his intimate friend and biographer, he had a very expressive face, with a high, somewhat doubled forehead. his eyes were deep and full of intelligence. his mouth, very sharply defined, betrayed, at the first glance, at once the earnest thinker and the pleasant man of friendly disposition. he was always restful and never seemed to be distracted. he talked but little, but his conversation was always interesting, and, except when he was in some particularly serious mood, was always likely to have a vein of light humor in it. he did not hesitate to introduce a sparkle of wit now and then into his lectures, and especially knew how gently to make fun of mistakes made by his pupils, yet in such a way as not to hurt their feelings, but to make them realize the necessity for more careful thought before giving answers, and for appreciating principles before speculating on them. he was particularly careful not to do anything that would offend his students in any way, and it is to this care that the success of his method of teaching has been especially attributed. his habits of life were from the beginning of his career simple, and they continued to be so until the end. he was never married, and he himself attributed this to the unfavorable condition of his material resources at the beginning of his career as a teacher, and the fact that the improvement in these did not really come until he was well past fifty years of age. he once confessed to a friend that he missed those modest pleasures of family life which do so much to give courage and strength for the greater as well as the lesser sufferings of life. most of his years of teaching he spent in boarding houses. only after his appointment to the professorship at munich was he able to have a dwelling for himself, which was presided over by a near relative. ohm is remembered as a teacher rather than as an educational administrator. his pupils recall him as one who was able to be eminently suggestive, while at the same time he succeeded in making it easy to acquire the details of information. the didactic lecture, as a method of teaching, did not appeal to him, and his success was due to the application of quite other methods. he realized how much personal influence meant, and the peculiarity of his system of teaching was an almost uninterrupted lively personal intercourse with his pupils. demonstrations and exercises at the board always occupied the first half of his two-hour lesson, and only the other half was devoted to the setting forth of new matter. in this way, ohm succeeded not only in influencing each student according to his personal endowments, but he also began the training of future teachers by giving them a living example of what their work should be. the success of ohm as a teacher was recognized on all sides. his attitude towards his scholars was very different from that which was assumed by many teachers. instead of being a mere conveyer of scientific information, he was himself "a high priest of science," as one of his pupils declared, supplying precious inspiration, and not merely pointing out the limits of lessons and finding out whether they were known, but making work productively interesting, while neglecting none of the details. his pupils became distinguished engineers, and as this is the period in which the state railroads were being built, there was plenty of opportunity for them to apply the instruction they had received. not only were the reports of the royal commission of inspection repeated evidence of ohm's success as a teacher, but the technical schools which were under the care of ohm's disciples soon came to be recognized as far above the average, and as representing not only the successful teaching of technics on his part, but also the influence that his example as a teacher had in forming others to carry on the work. how much ohm was beloved by those who knew him best can be properly appreciated from the following passage from the panegyric delivered in munich in , not long after his death, by professor lamont, who had known him intimately: "nature," he said, "conferred upon ohm goodness of heart and unselfishness to an unusual degree. these precious qualities formed the groundwork of all his intercourse with his fellows. despite the underlying strength of his character, which kept him faithfully at work during all his career, whenever there was question of merely personal advantage to himself, he preferred to yield to pressure from without, rather than rouse himself to resistance, and he thus avoided all bitterness in life. the unfortunate events which forced him, during the early part of his career, from an advantageous position back into private life, did not produce any misanthropic feelings in him, and when later a brilliant recognition gave him that rank in the world of science which by right belonged to him, his simplicity of conduct was not in any way modified, nor was the modesty of his disposition at all altered." in a word, ohm was one of those rare geniuses whose magnanimity placed him above the vicissitudes of fortune. his power to do original work was not disturbed by the opposition which a really new discoverer invariably meets, but his unfailing equanimity was just as little exalted into conceit and pretentiousness by the praise which so justly came to him once the real significance of his scientific work dawned upon the world. with the realization of all that ohm's work meant in the department of electricity, it is easy to understand how his name deserves a place in the science for all time. in order permanently to honor his memory, the international congress of electricians, which met at paris in , confirmed the action of the british association of , by giving the name _ohm_ to the unit of electrical resistance. this is an ideal monument to the great worker. it is as simple and modest a reward as even he would have wished, expressing as it does, the gratitude of succeeding generations of scientists for all time. footnotes: [ ] ohm's brother, martin ohm, deserves a passing word, because his life is characteristically different in certain ways and because, above all, it represents academic success, while ohm's was almost an academic failure. he finally received the professorship in mathematics at berlin, and came to be considered as one of the greatest professors of the subject in europe. their careers form typical examples of the fact, often notable in history, that talent finds a ready welcome in the academic world, while genius is often neglected, and indeed may be, and often is, the target for bitter opposition. the younger ohm's writings are mainly with regard to mathematics, but nearly always from some general rather than special standpoint, and very often with regard to the educational side of the subject. his first book was on analytic and higher geometry in their elements. he then wrote class text-books of mathematics and mechanics. one of his works, the spirit of mathematical analysis and its relation to a logical system, because of its value as an educational document attracted widespread attention. this book, translated by ellis into english, was published in london in . one of martin ohm's earlier books should be of special interest to educators because of its subject. its rather lengthy title is, "an attempt to formulate a short, fundamental, clear method to enable those without a taste for mathematics to learn the mathematics necessary for the higher and technical schools." [ ] fordham university press, . [ ] makers of modern medicine, fordham university press, new york, . [ ] new york, van nostrand company, . [ ] published in the annual report of the smithsonian institute for the year , washington, . [ ] in the address on the scientific work of george simon ohm, published by the smithsonian institute in , this name is translated sangberg. in the article by baurenfeind, in the allegmeine deutsche biographie, the name is spelled langberg. the form of the old german l may have suggested the letter s, or it may have slipped in as a typographical error. chapter x. faraday. the maxim current among european scientists, that it is well to wait before accepting any scientific discovery to see what will be said about it on the other side of the rhine, throws a rather curious sidelight on the supposed absoluteness of scientific knowledge. gallic enthusiasm or german subtlety may evolve plausible theories that look like scientific discoveries, but the destructive criticism of the neighbor nation usually saves the scientific world from deception. not infrequently, the english-speaking scientists held the balance between these rivals in the intellectual world, and their adhesion to either party or side of a question secured its dominance. when all three, germans and french and english, are agreed as to the value of a scientific discovery, then it may be looked upon as having some of the absoluteness, or at least possesses for the moment the finality of scientific truth. if this triple agreement be taken as the criterion of the significance of a great scientist's work, then must michael faraday be considered as without doubt one of the greatest scientists of our time, and probably the greatest experimental scientist that the world has known. [illustration: michael faraday] dubois reymond, in berlin, declared faraday "the greatest experimentalist of all times, and the greatest physical discoverer that ever lived." professor martius said before the academy of sciences at munich, "deservedly has faraday been called the greatest experimenter of his epoch, and that the greatest epoch of scientific experimentation down to our time." dumas, the french chemist, in the panegyric delivered before the french academy of sciences, declared that faraday was "the greatest scientific scholar that the academy ever possessed." in order to give a picture of what he had accomplished in electricity, added dumas, one would have to write a complete treatise on that subject. "there is nothing in this department of science that faraday has not investigated completely or very materially modified. much of this chapter of our modern science is his creation and belongs undeniably to him." beside these testimonies from french and german scientific contemporaries must be placed tyndall's appreciation, which sets forth his brother scientist's merits. "take him all in all," he said, "it must be admitted, i think, that michael faraday was the greatest experimental scientist that the world has ever seen." nor did these magnificent appreciations of faraday cease when the enthusiasm for his memory, immediately after his death, had faded somewhat into sober realization of his merits. when dumas summed up faraday in the first faraday lecture of the english chemical society, he said: "faraday was a type of the most fortunate and the most accomplished of the learned men of our age. his hand, in the execution of his conceptions, kept pace with his mind in designing them; he never wanted boldness when he undertook an experiment, never lacked resources to insure success, and was full of discretion when interpreting results. his hardihood, which never halted once he had undertaken a task, and his wariness, which felt its way carefully in adopting a received conclusion, will ever serve as models for the experimentalist." it is evident that the life of faraday should be of supreme interest for a generation that is mainly interested in experimental science, and it so happens that his career contains many other sources of interest; for faraday was a self-made man, who owed very little to anyone but himself and his own genius. besides, he was a deep thinker with regard to all the problems of human life as well as those of science, and while he was a genial, kindly friend to those near him, the charming associate whom scientific intimates always welcomed, he had no illusions with regard to life being the end of all things, but looked confidently to the hereafter, and shaped his life here from that point of view. michael faraday was born at newington butts, now called stoke newington, an outskirt of london, in surrey, september d, . his father was a journeyman blacksmith whose health was not very good, and as a consequence, the family suffered not a little from poverty. both his parents were noted for their good habits, industrious lives and deep religious feelings. in spite of their poverty, as is much oftener the case than is sometimes thought, their children were brought up very carefully and had a precious training in high principles. like most of his great colleagues in scientific discovery, faraday had to begin to earn his livelihood early in life. of educational opportunities he had practically none. he learned to read and write, and probably had a certain slight training in doing simple sums in arithmetic, but that was the extent of his formal teaching, and much of that he got at home. he had to help in the support of his family, and so it seemed fortunate that not far away from his home there was a bookstore and bindery, the owner of which became interested in the faradays and took michael as an errand boy when he was scarcely thirteen years of age. it was here that the future scientist began his education for himself and, strange as it may seem, laid the deep foundation of his knowledge of science. for the first year he carried newspapers around to the customers, and did his work so faithfully that at the end of this time the book-binder offered to take him as an apprentice to the trade, without the usual premium which used to be rather strictly required for teaching boys their trades at that time. faraday accepted this offer, but proved to be interested much more than in the outsides of the books he bound. whatever of leisure there was he took advantage of to read a number of works on experimental science that happened to be in the shop. luckily for him, some of these were classics. as an introduction to chemistry, he had mrs. marcet's "conversations on chemistry" and robert boyle's "notes about the producibleness of chimicall principles." he was even more interested in electricity than in chemistry, however, and lyons' "experiments on electricity" and the article on electricity in the encyclopedia britannica, whetted his interest and made the boy wish for more of such information. there probably could not be a better proof of the fact that, a man who really has intellectual interests will find the material with which to satisfy them, in spite of untoward circumstances, than this boyish experience of faraday. it is a curious anticipation of faraday's after-career that he at once began to demonstrate by personal experiment some of the statements that he found in the books. he procured a stock of chemicals as far as his meagre salary would allow, and constructed a practical electrical machine, though he had nothing better than a large glass bottle to serve as a cylinder for it. when not yet fourteen, he noticed an advertisement of a set of lectures on natural philosophy. he was at once taken with the idea of going to them, but the price of admission, one shilling, seemed to place them entirely beyond him. his elder brother, who followed his father's trade of blacksmith, had more money than he, and, when properly cajoled, was persuaded to provide the necessary shillings, and so faraday got to the lectures. elder brothers do not often have to lend shillings to their juniors for admission to scientific lectures now any more than in faraday's time, so that the incident seems worth noting. in attendance at these lectures, faraday not only learned much that was new to him in science, but met a number of earnest fellow-students and formed some life-long friendships. he took copious notes, and afterwards wrote them out in a fine, legible hand, making excellent drawings in perspective of the apparatus employed in the experiments. his notes were so extensive that faraday bound them himself, in four volumes, with an index. these volumes are still preserved in the library of the royal institution as one of the precious treasures among its faraday relics.[ ] the whole story of these early years of faraday's life is a series of illustrations of how a young man without the necessary opportunities for his favorite studies can make them for himself. everything seemed to be against his acquiring a thorough knowledge of science, yet he succeeded in creating for himself the equivalent of a good scientific course out of his meagre chances to hear lectures and read books on his favorite subject in the intervals of a busy life as book-seller and book-binder. things did not always continue to run along as pleasantly in life for young faraday as while he was working for his book-binder friend as an apprentice. with the conclusion of his apprenticeship he became a journeyman book-binder, and his first employer proved to be a hard task-master. it did not matter how much work faraday did or how well, it never quite satisfied this french émigré, until it is no wonder that faraday looked for another occupation. for a time, he had the congenial occupation of acting as amanuensis for sir humphry davy, who, while working on a new violent explosive, probably chloride of hydrogen, met with an accident which prevented him from using his eyes for some time. this occupation, pleasant and even alluring as it was, lasted only for a few days, however. it had the fortunate result of suggesting to faraday to apply to sir humphry davy in person for a position not long after, and it eventually brought him the position of assistant at the royal institution. his anxiety to secure this post had been increased by the growing realization that a business life was not to his liking. it seemed to him a waste of time, or worse, for a man to give himself up to the making of money. even thus young he had the ambition to add to the knowledge possessed by mankind, and the insatiable desire to increase the opportunities of others to learn whatever they were interested in. accordingly, he set about finding the chance to devote himself entirely to science. in writing years after to dr. paris, he says: "my desire to escape from trade, which i thought vicious and selfish, and to enter into the service of science, which i imagined made its pursuers amiable and liberal, induced me at last to take the bold and simple step of writing to sir humphry davy, expressing my wishes, and a hope that, if an opportunity came in his way, he should favor my views; and at the same time i sent the notes i had taken of his lectures." davy called, not long after, on one of his friends, who was at the time honorary inspector of the models and apparatus at the royal institution, and with the letter before him asked: "here is a letter from a young man named faraday; he has been attending my lectures and wants me to give him employment at the royal institution. what can i do?" "do?" replied the inspector; "put him to wash bottles. if he is good for anything, he will do it directly; if he refuses, he is good for nothing." "no, no," replied davy, "we must try him with something better than that." davy wrote a kind reply, and arranged for an interview with young faraday. in this, however, he candidly advised him to stick to his business, telling him very plainly that "science was a harsh mistress, and, from a pecuniary point of view, but poorly rewarded those who devoted themselves to her service." he apparently put an end to all further consideration of the subject by promising faraday the book-binding work of the institution, and his own besides. faraday was not satisfied to go back to the book-shop, even with all this kindly patronage, but there was nothing else for it, and so for a time he continued at his duties and spent his spare moments reading science and his evenings at scientific lectures, or in remaking the experiments he had seen and others suggested by them, and above all in rewriting the notes that he had taken. there is no livelier picture in all the history of science, of how a man will, in spite of all obstacles, get the things he cares for, if he really cares for them, than that of faraday thus teaching himself science in the face of what seems almost insurmountable discouragement. fortunately, not long after he had been thus forcibly called to the attention of sir humphry davy, the former assistant in the laboratory of the royal institution not only neglected his duties, but became a source of considerable annoyance. his misfortune proved faraday's opportunity. he was offered the post. the salary was only twenty-five shillings a week, but he accepted it very willingly. one might think that at last his scientific career was opened for him, but his new post was no sinecure. the labors required from him, indeed, were so manifold that it is somewhat surprising that he found any time for his own improvement. his duties as set forth in writing were: "to attend and assist the lecturers and professors preparing for and during lectures. where any instruments or apparatus may be required, to attend to their careful removal from the model room and laboratory to the lecture room, and to clean and replace them after being used, reporting to the managers such accidents as shall require repair, a constant diary being kept by him for that purpose. that in one day in each week he be employed in keeping clean the models in the repository, and that all the instruments in the glass cases be cleaned and dusted at least once within a month." the previous assistant had complained of the amount of work that was required of him. it is easy to see that his duties were rather exacting and time-taking. faraday did not confine himself to them, though he did perform them with great assiduity. his interest in experimental chemistry was soon noted, and he was allowed to take his share in the experiments going on in the laboratory. some of his first work was the extraction of sugar from beet-root; but he was soon to have abundant experience of the deterring side of chemistry. not long after he began his work in the laboratory, he had to manufacture some bisulphide of carbon, one of the most nauseating of compounds. he found it disgusting enough as an experience, but the study of it brought its compensation. it was much more than foul odors that faraday had to encounter, for davy was still occupying himself with the study of the explosives, in the investigation of which he had been injured the previous year. faraday suffered from four or five explosions during the course of the first month or two of his employment. indeed, the substance with which they were experimenting proved so unreliable in this regard that, after a second rather serious injury to davy, further study of it was given up. once faraday had secured his post at the royal institution, his life-work was before him, and he became deeply engaged in scientific speculations, investigations and experiments of all kinds. the young man who had found and made opportunities when they were so distant and difficult, now made use of all that were so ready at hand. he did not confine himself to his laboratory work, however, but seems always to have felt that the contact of minds engaged along the same lines was the best possible way to be stimulated to knowledge. he applied and was admitted as member of the philosophical society of london, an association of some two score of men occupied with many things during the day, but interested in science, so far as they could get the books and the opportunities for its study. they met every wednesday evening and discussed various subjects in science or, as they called it then, in philosophy, and they seem to have occupied themselves with many questions in the social as well as the natural sciences. these men, most of whom were older than faraday, soon came to look up to him because of the depth and increasing breadth of his knowledge, and we have some emphatic expressions of their admiration for him. faraday's earliest successful scientific investigation was accomplished in chemistry. this might have been expected, from the fact that he began his work with sir humphry davy, whose principal scientific investigations had been concerned with chemistry. his own great scientific work was to be done in electricity. even in the brief time that he devoted to chemistry, however, he succeeded in making some discoveries of deep significance. for instance, in his special study of chlorine, he demonstrated the existence of the two chlorides of carbon which had not hitherto been obtained. above all, he impressed his personality upon methods in chemistry. he was the first to realize how much technics were to mean in the modern advancement of science, and he made methodic chemistry, in distinction from practical chemistry, the object of very special study. his work on _chemical manipulation_ did more to train successful students of chemistry and to make good investigators in this department of science than any other single work in his generation. it has continued to be of interest down even to our own time, and is well worthy of consultation by all those who are interested in chemistry as a science, and especially in original research in that subject. it was with regard to gases, however, that faraday's most striking chemical work was done. he succeeded in liquefying several gases, and was the first to make clear that all matter could probably exist in each of the three different states--solid, liquid and gaseous--according as the proper conditions for each particular state were present. one might almost have expected that the serious dangers incurred in his early days in the royal institution, when his chief, sir humphry davy, suffered so severely and he himself was more than once involved, might have deterred him from further investigation along similar lines; but faraday's ardor for scientific investigation overcame any hesitancy there might have been. the effect of gases upon human beings proved as attractive to faraday as it had been to davy. his experiments upon chlorine threatened to prove seriously injurious to his throat, and he was warned of the danger that he was running in the effort to determine whether such gases were respirable and what their effects upon human beings were. the warning was disregarded, however, though he exercised somewhat more care in subsequent observations. his experiments in the respiration of gases finally led him to a discovery of cardinal importance in the very practical field of anæsthesia. sir humphry davy, just at the beginning of the nineteenth century, had made a series of interesting experiments on nitrous oxide gas, the so-called "laughing gas," and had pointed out very definitely its anæsthetic properties. while suffering from toothache he had inhaled the gas, and had experienced prompt alleviation of the pain. he described in detail these curious effects, and suggested that there might be a place for nitrous oxide in surgery, at least for minor operations. the words he employed with regard to this subject show that the idea of anæsthesia, as we now understand it, had come to him very definitely. not quite a score of years later, faraday, recalling the experiments of davy with nitrous oxide, studied sulphuric ether, and showed that the inhalation of the vapor of this substance produced anæsthetic effects very similar to those of nitrous oxide gas, but with the possibility of prolonging them much more easily and apparently with less danger than would be the case with the latter. in every history of anæsthesia, these two sets of experiments at the royal institution must be set down as foundation-stones, and faraday's name particularly must be hailed as one of the initiators of a supremely beneficent advance in modern surgery. faraday had given up business to devote himself to science, and he was not to be seduced from the purpose of making his life unselfish and doing things, not for money, but for the good of science and his own satisfaction. as a practical chemist, he soon had many opportunities to increase his salary by making analyses for industrial purposes. during one year, the amount of work thus offered him was paid for so well that it formed an addition of some £ sterling to his salary. it took away precious time, however, that he might otherwise devote to original work. as soon as faraday realized this possibility of interference with his scientific investigations, he cut it off, quite content to live on the modest salary of his position at the royal institution. his action in the matter would remind one very much of pasteur, in the latter half of the century, when asked by the empress eugénie, to whom he had been just exhibiting his discoveries in fermentation, whether he would not apply these to actual manufacture and so make a fortune for himself in brewing. pasteur replied that he thought it unworthy of a french scientist to devote his time to money-making, with all the world of science open before him.[ ] with a conscientious patriotism, however, that was typical of the man and his ways, there was one exception to this rule of not taking outside work that faraday made. in a letter to lord auckland, long afterward, he says: "i have given up for the last ten years or more, all professional occupation and voluntarily resigned a large income, that i might pursue in some degree my own objects of research. but in doing this i have always, as a good subject, held myself ready to assist the government if still in my power, but not for pay; for, except in one instance (and then only for the sake of the person joined with me), i refused to take it. i have had the honor and pleasure of application, and that very recently, from the admiralty, the ordnance, the home office, the woods and forests and other departments, all of which i have replied to and will reply to as long as strength is left me." as we have said, faraday's principal work was accomplished in the domain of electricity. his supreme discovery, and, indeed, the most important practical discovery in the whole realm of electricity, was that of the induction effect of a current of electricity on a neighboring circuit. this was accomplished by experimental work of the highest order. toward the end of , when he was about thirty-three, he came to the definite conclusion that an electric current might be obtained by the motion of a magnet. his mind had been prepared for such a conclusion by oersted's significant discovery in july, , that an electric current acts somewhat like a magnet when the wire through which it flows is free to move. this discovery, definitely connecting electricity and magnetism, had been elaborated to an important degree by ampère, and its sphere of application broadened by wollaston. the curious though not unusual result in such cases, that it is not those who are in immediate touch with a great discoverer who develop or even apply his work, was illustrated by the fact that ampère, the frenchman, took up oersted's discovery first, while wollaston, working in england, had been the next one to follow successfully in the path thus opened up. it takes genius to go even a slight step farther into the unknown; the trained talent of disciples does not suffice. it was now faraday, though not under wollaston's influence, who was to continue successfully these labors. in spite of his persuasion that a magnet would produce by induction an electric current, and the further step that a current in one wire could induce a current in another, experiments during seven years had brought him very little nearer the actual demonstration of this important principle. those who think that great discoveries are made by accident and almost fall into the laps of their makers, as the apple upon newton, should recall these seven years of unsuccessful labor on the part of faraday. finally, in , he obtained the first definite evidence that an electric current can induce another in a different circuit. the discovery meant so much for him, that he hesitated to believe in his own success. nearly a month after this first demonstration for himself, he wrote to his friend phillips: "i am busy just now again on electro-magnetism, and think i have got hold of a good thing, but can't say. it may be a weed instead of a fish that, after all my labor, i may at last pull up." he had long suspected, as we have said, that induction should occur, and he had tried currents of different strength, but without result. one day he noticed that, though he could not produce a permanent induced current, whenever the primary current started or stopped, there was a movement of the galvanometer connected with the secondary circuit, though the galvanometer remained at zero so long as the primary current flowed steadily. from this he proceeded to the demonstration that a bar magnet suddenly thrust into a helix of copper wire produced the same effect on the galvanometer, and evidently induced a transient current. when the magnet was withdrawn, the galvanometer needle swung in the opposite direction, showing another current, so that electrical currents were evidently induced by the relative motions of a magnet and a conductor. he continued his experiments in many different forms, and in the short space of a little more than a week, once the first definite hint was obtained, succeeded in so completely finding out the phenomena of electro-magnetic induction that scarcely more than practical applications in this subject were left for his successors. faraday's explanation of the induction of currents in the secondary circuit was probably quite as important a contribution to science as the series of experiments by which he demonstrated the occurrence of induced currents. his mind was not of the order that would accept action at a distance; that is, without some conducting medium through which the action took place. the old aphorism of the scholastics, "_actio in distans repugnat_"--action at a distance, that is, without a medium intervening, is absurd--would have appealed to him as a basic truth. the explanation that he outlined for induced currents was based on the lines of magnetic force, which he had so often delineated by means of iron filings. it was a favorite occupation of his, at moments of comparative leisure, to make varied pictures in iron filings of magnetic fields as they were exhibited under the influence of different combinations of magnets. he strewed iron filings over "gum paper," and then when the filings had arranged themselves in certain definite lines, he threw a jet of steam on the paper, which melted the gum and fixed the filings in position. he explained electrical action as the transmission of force along such lines as these, and he thought the whole electric field was filled with them. probably the best summary of faraday's work on induction and its significance has been given us by clerk maxwell, in his article on faraday, in the ninth edition of the encyclopedia britannica. there is no doubt but that maxwell, above all men of the nineteenth century, was in a position to judge of the meaning of faraday's work. he was not the sort of a man to say things in a panegyric mood, and his article on faraday is indeed a model of well-considered judgment and critical illumination. summing up the significance not only of faraday's great discovery of induction, but also his theory in explanation of that discovery, he does not hesitate to say that his (faraday's) opinion is the nearest approach to truth that has been advanced in this much-discussed subject. "after nearly half a century of labor of this kind, we may say that, though the practical applications of faraday's great discovery have increased and are increasing in number and value every year, no exception to the statement of these laws as given by faraday has been discovered; no new law has been added to them; and faraday's original statement remains to this day the only one which asserts no more than can be verified by experiment, and the only one by which the theory of the phenomena can be expressed in a manner which is actually and numerically accurate, and at the same time within the range of elementary methods of exposition." with what eminent care and absolute truth faraday's conclusions were reached may be judged from some further expressions of clerk maxwell's in the article just quoted, with regard to the attitude of certain mathematicians toward faraday's work. in this matter, clerk maxwell, in talking on a theme that he had made especially his own, and in which his opinion must carry the greatest possible weight, said: "up to the present time, the mathematicians who have rejected faraday's method of stating his law as unworthy of the precision of their science, have never succeeded in devising any essentially different formula which shall fully express the phenomena, without introducing the hypotheses about the mutual action of things which have no physical existence, such as elements of currents, which flow out of nothing, then along the wire, and finally sink into nothing again." faraday's results were described in papers afterwards incorporated in his first series of "experimental researches," which were read before the royal society, november th, . these papers probably contain the best possible proof of faraday's genius as an experimentalist and a leader in scientific observation. within a few months after his first successful experiment, he had succeeded in bringing to perfection the whole doctrine of induction by currents and magnets, had laid down the fundamental ideas which were to constitute the formal basis of electro-magnetism for all time. perhaps no better idea of the importance of the discovery thus made by faraday can be given than will be found in clerk maxwell's compendious paragraph on this subject, in his sketch of faraday, in the encyclopedia britannica. it may be said that no one in all the nineteenth century was more capable of appreciating properly the value of faraday's work than this great electrical mathematician, who laid the firm foundation of mathematical electricity during the latter part of the nineteenth century. clerk maxwell says: "this was of course a great triumph, and nobody appreciated this fact better than faraday himself, who had been working at its problems for many years. one of the first problems that he had set himself in his note-book as a young man, was 'to convert magnetism into electricity,' and this he had now done. within a month of the time that his first successful experiment was formed, he succeeded in obtaining induction currents by means of the earth's magnetism. within a year he took the further immense step of obtaining a spark from the induced current. this would ordinarily have seemed quite impossible, since sparks occur only if the electromotive force is very high, and it was very low in his induced currents. he found, however, that if the circuit of wire in which a current was flowing is broken while the current is passing, a little bridge of metallic vapor is formed, across which the spark leaps. the difficulty with the experiment was to break the circuit during the extremely short period while the current is flowing. faraday succeeded in doing this, and as a result obtained the first germ of the electric light. when he demonstrated this experiment by a very ingenious apparatus at the meeting of the british association at oxford, all were deeply interested, yet probably no one, even the most sanguine of the scientists present, thought for a moment that they saw the beginning of a far-reaching revolution of all the lighting of the world." perhaps the most interesting of faraday's discoveries, from the scientific standpoint, because they throw so much light on the problems of all the related phenomena of magnetism, heat, light, even electricity, were those in which a ray of polarized light was used as a means of investigating the condition of transparent bodies when acted on by electric and magnetic forces. faraday himself, when he was just thirty years of age, made a note in his commonplace laboratory book, in which all his observations were carefully detailed, that serves to show how much this subject had begun to interest him thus early in his career. he mentions that he had polarized a ray of lamp-light by reflection, and had made various experiments to ascertain whether any depolarizing action was exerted on it by water placed between the poles of a voltaic battery in a glass cistern, or by various fluids which were decomposed by the voltaic action during the course of the experiment. besides water, the fluids used were weak solutions of sulphate of soda and strong sulphuric acid. none of them had any effect on the polarized light, either during the passage of the voltaic current or when this was shut off. no particular arrangement of particles in reference to polarized light could be found from these observations. such a note, with utter failure for conclusion, is common enough in faraday's note-book. he was never discouraged, however, by failure at the beginning. once a subject has been taken up seriously, it is almost inevitable that further observations with regard to it will be found during the course of the year. because he had asked one question of nature and had not obtained a satisfactory answer, was never a reason why he should not ask further questions along the same line; and, above all, why he should not ask the same question in another way. after having tried a continuous current, faraday next experimented on the effect of making and breaking the circuit. he did not expect very much from this, but he hoped that under circumstances when no decomposition would ensue as the effect of the current, he might find some indication of the polarization. it was nearly twenty-five years before faraday succeeded in solving the problem that he had thus set himself as a young man, and nearly twenty years more were to pass before he made the relation between magnetism and light the subject of his very last experimental work. nothing discouraged him. when he had resolved to investigate something, he continued to make his experiments over and over again in different ways, until finally he got an answer to his question and a solution to the problem. indeed, his perseverance in anything that he undertook was a striking characteristic of the man and one of the most important elements in his success in life. his tenacity of purpose showed itself equally in little as in great things. arranging some apparatus one day with a philosophical instrument-maker, he let fall on the floor a small piece of glass. he made several ineffectual attempts to pick it up. "never mind," said his companion, "it is not worth the trouble." "well, but, murray, i don't like to be beaten by something that i have once tried to do." faraday was sure that there was some very definite relation between electricity and light. his experiments, however, did not enable him to demonstrate this until nearly fifteen years after his successful experiment on induction. in september, , he placed a piece of heavy glass made of silico-borate of lead in the field of a magnet, and found that, when a beam of polarized light was transmitted through the glass in the direction of the lines of force, there was a rotation of the plane of polarization. later experiments showed him that all transparent solids and liquids were capable of producing this rotation in greater or less degree. when no magnet was used and the transparent substance was placed within a coil of wire through which an electric current was flowing, similar effects were produced. this was the demonstration of a definite relation between light and electricity. later, faraday found that magnets had a directive action upon the glass. he then made experiments upon gases, and found that they too exhibited magnetic phenomena, and that, indeed, the diurnal variations of the compass-needle were due to the sun's heat diminishing the magnetic permeability of the oxygen of the air. further experiments with gases showed him that nitrogen was absolutely neutral in its reaction. it might have been expected, from faraday's early interest in chemistry, that when he turned to electricity and made discoveries in that field of research, he would naturally take up the problem of tracing the laws and demonstrating the relationships of the points of contact of the two great sciences. after his completion, then, of the subject of induction, faraday devoted himself to the experimental proof of the identity of frictional and voltaic electricity, and to showing that chemistry and physics have a common ground. his inductive electrical machine could deflect a magnet and decompose iodide of potash. with his tendency to measure things, he determined that the amount of electricity required to decompose a grain of water was equal to , charges of his large battery of leyden jars. on the other hand, the current from a frictional machine deflected the needle of his galvanometer in the same way as the induced current of electricity, so that all the elements of the proof of the identity of the two forms of phenomena were now in his hands. that he should have proceeded to the demonstration of the laws of electrolysis, was the next most natural result. he showed that the amount of any compound decomposed by the electric current is exactly proportional to the whole quantity of electricity which has passed through the electrolyte. different substances are variously refractory to dissolution under the influence of the electric current, but each one always acts in the same way and requires the same amount of current. substances that are closely related to one another chemically, are also related to one another in the amount of electricity required to bring about decomposition of their various compounds. he showed, of course, that there are differences of electrical relationship that make the results produced in the decomposition of various compounds very different. polarization, for instance, sets in to a much greater degree in the decomposition of some substances than of others. one consequence is that the resistance to the passage of the electric current differs markedly, and the opposing electromotive force will stop the current or hamper its effects in many cases, so that, until after actual experiment, the quantitative effect of the passage of the electric current through a solution cannot be determined. faraday's opinions as to the significance of electricity in the animal economy are very interesting because of his profound knowledge of electrical phenomena and their place in nature. it is all the more interesting because it is so simple, and most scientists would be apt to say that its very simplicity is a very taking argument for its truth. "as living creatures produce heat, and a heat certainly identical with that of our hearths, why should they not produce electricity also, and an electricity in like manner identical with that of our machines? like heat, like chemical action, electricity is an implement of life, and nothing more." while faraday often occupied himself with subjects connected with matter and force that are likely to remain mysteries for long after his time, and often had thoughts to express with regard to the nature of atoms and of imponderable agents, whatever he had to say about these subjects was not vague and speculative, but, on the contrary, was concrete and usually of such a practical character as to add something new to our knowledge of them. few men have ever succeeded in getting closer to the mysteries that underlie natural phenomena than faraday; yet no one was ever less carried away into vague theoretic speculations with regard to them, nor tempted to think that because he knew much more than most other men with regard to complex natural problems, that therefore he knew enough to be able to solve the mysteries that existed all around him. he had none at all of what would ordinarily be called pride of intellect, but, on the contrary, had the humility of the true scientist. knowing so much only made him realize more poignantly how much he was ignorant of. with regard to his speculations on matter and force and the imponderables, helmholtz, the great german physicist, once summed up faraday's contributions very succinctly in a way to show the practical nature of faraday's intellect. he said: "it is these things that faraday in his mature works ever seeks to purify more and more from everything that is theoretical and is not the direct and simple expression of the fact. for instance, he contended against the action of forces at a distance, and the adoption of two electrical and two magnetic fluids, as well as all hypotheses contrary to the law of the conservation of force, which he early foresaw, though he misunderstood it in its scientific expression. and it is just in this direction that he exercised the most unmistakable influence, first of all, on the english physicist, and then on the physicists of all the world." inventors and promoters of useful inventions, frequently benefited by the advice of faraday or by his general help. a remarkable instance of this was told by mr. cyrus w. field. at the commencement of his great enterprise, when he wished to unite the old and the new world by the telegraphic cable, he sought the advice of the great electrician, and faraday told him that he doubted the possibility of getting a message across the atlantic. mr. field saw that this fatal objection must be settled at once, and begged faraday to make the necessary experiments, offering to pay him properly for his services. the philosopher, however, declined all remuneration, but worked away at the question, and presently reported to mr. field: "it can be done; but you will not get an instantaneous message." "how long will it take?" was the inquiry. "oh! perhaps a second." "well, that's quick enough for me," was the conclusion of the american; and the enterprise was proceeded with. faraday was far from being a mere laboratory student; he was much more even than a great teacher of physics. he was a magnificent popular lecturer, and did an incalculable amount to bring physics to the attention and the serious interest of his generation. a contemporary has described one of his lectures at the royal institution in such a way as to give us some idea, even at this distant date, of faraday's power over his audience, of his own wonderful interest in the subject and his marvelous ability to communicate that interest to others. it was of the very nature of the man that he should not be cold and formal, for he was not a man of the head alone, but, above all, a man whose heart and affections were greatly developed, and he had powers of enthusiasm that placed him high among the artistic spirits of mankind. our american poet, stedman, once declared that the intellectual quality of the poet, the creator in the realm of thought, and of the scientist, the original worker in the domain of science, differed but little from one another, and must be considered as collateral expressions of the same form of intellectual genius. with this in mind, his contemporary's enthusiastic description of his lectures will not seem overdrawn. "it was an irresistible eloquence, which compelled attention and insisted upon sympathy. it waked the young from their visions, and the old from their dreams. there was a gleaming in his eyes which no painter could copy, and which no poet could describe. their radiance seemed to send a strange light into the very heart of his congregation; and when he spoke, it was felt that the stir of his voice and the fervor of his words could belong only to the owner of those kindling eyes. his thought was rapid, and made itself a way in new phrases, if it found none ready made, as the mountaineer cuts steps in the most hazardous ascent with his own axe. his enthusiasm sometimes carried him to the point of ecstasy." faraday's habit of testing opinions by experiment, and the frequent disillusions which he encountered with regard to things of which he thought he knew something definite, served to make him extremely careful as regards expressions of opinion. some of his thoughts on this subject are worth while recalling because they remain perennially true, and anyone in any generation will find that, as his experience grows, he gets more and more into this faraday mood of doubting his own opinion and listening with more readiness to that of others. as a rule, this is said not to be true of those who are in advancing years, but the greater minds among the older men do not get set in their ways. flourens might have said that because of constant exercise the connective tissue in the brains of such men does not form to the same extent as in others, and does not make them case-hardened. as a consequence, they retain far on in years their sympathy for others' opinions and their openness of mind. comparatively, they are so few, however, that this expression of faraday's becomes a striking commentary on his large-mindedness. "for proper self-education, it is necessary that a man examine himself, and that not carelessly either.... a first result of this habit of mind will be an internal conviction of ignorance in many things respecting which his neighbors are taught, and that his opinions and conclusions on such matters ought to be advanced with reservation. a mind so disciplined will be open to correction upon good grounds in all things, even in those it is best acquainted with, and should familiarize itself with the idea of such being the case." perhaps it is even more interesting, because more humanly sympathetic, to find that faraday distrusted his opinions of people even more than his opinions of things, and that he himself tried to be very slow to take offence at what was said to him, and counselled greatest discretion to others in judging of the significance of supposed slights. "let me, as an old man who ought by this time to have profited by experience, say that when i was younger, i found i often misinterpreted the intentions of people, and found that they did not mean what at the time i supposed they meant; and further, that, as a general rule, it was better to be a little dull of apprehension when phrases seemed to imply pique and quick in perception, when, on the contrary, they seemed to imply kindly feeling. the real truth never fails ultimately to appear, and opposing parties, if wrong, are sooner convinced when replied to forbearingly than when overwhelmed." few lives have been happier than that of faraday. he gave up the ordinary ambition of men to make what is called a successful career of money-making, and constantly guarded himself from slipping back, as so many do, to the ruin of their original purpose. he lived a long life in peace, occupied with work that he liked above all things, and surely serves as the best illustration of the maxim: "blessed is the man who has found his work." work is said to be one of the primal curses laid upon man; but if, when the creator would ban it turns to blessing in the way that work has done, then may one well ask what will his blessings prove. faraday even had what is rarer in life than happiness, the consciousness of his happiness. usually it is so elusive that it escapes reflection. at the close of his career, when he wrote, in , to the managers of the royal institution resigning most of his duties, he expressed this feeling very beautifully, and at the same time so simply and clearly as to make his letter of resignation a precious bit of literature. "i entered the royal institution in march, , nearly forty-nine years ago, and, with the exception of a comparatively short period, during which i was abroad on the continent with sir h. davy, i have been with you ever since. during that time i have been most happy in your kindness, and in the fostering care which the royal institution has bestowed upon me. thank god, first, for all his gifts! i have next to thank you and your predecessors for the unswerving encouragement and support which you have given me during that period. my life has been a happy one, and all i desired. during its progress, i have tried to make a fitting return for it to the royal institution, and through it to science. but the progress of years (now amounting in number to three-score and ten) having brought forth, first, the period of development, and then that of maturity, has ultimately produced for me that of gentle decay. this has taken place in such a manner as to make the evening of life a blessing; for, while increasing physical weakness occurs, a full share of health, free from pain, is granted with it; and while memory and certain other faculties of the mind diminish, my good spirits and cheerfulness do not diminish with them." for nearly five years after he had given up to a great degree his work at the royal institution, he faced death, not with the equanimity of the stoic, but with the peaceful happiness of the believer in providence and a hereafter. even the loss of his memory, dear as it must have been to a man who had spent all his life in storing it with the great facts of science, does not seem seriously to have disturbed him. he realized the necessity for patience, and took the lesson of its necessity to heart, so that there was no difficulty in it. once when calling on his friend, the distinguished scientist, barlow, who had for a lifetime almost worked beside him at the royal institution, but who was now suffering from paralysis, he said: "barlow, you and i are waiting; that is what we have to do now; and we must try to do it patiently." when the full realization that his powers were leaving him first came to him, he wrote to his niece what he thought ought to be the feelings of the believer in providence toward death, and his letter shows how thoroughly he had imbibed the great lessons of christianity, and how much of consolation his faith was to him in this darkest hour before the dawn of that other life, in which he had as implicit confidence as in any of the great scientific principles that he had demonstrated by experiment. he wrote: "i cannot think that death has, to the christian, anything in it that should make it a rare, or other than a constant thought. out of the thought of death comes the view of the life beyond the grave, as out of the view of sin (that true and real view which the holy spirit alone can give to man) comes the glorious hope.... my worldly faculties are slipping away day by day. happy is it for all of us, that the true good lies not in them. as they ebb, may they leave us as little children, trusting in the father of mercies and accepting his unspeakable gift." and when the dark shadow was creeping over him, he wrote to the comte de paris: "i bow before him who is the lord of all, and hope to be kept waiting patiently for his time and mode of releasing me, according to his divine word and the great and precious promises whereby his people are made partakers of the divine nature." probably the feature of the careers of darwin and spencer which are saddest for their adherents, and which made those who refused to be recognized as among their followers appreciate their one-sidedness, is the confession by both of them, that they had lost their interest in poetry and even in literature of all kinds, and toward the end of their lives particularly lost entirely their appreciation of things artistic. as might be expected from what we know of faraday, this was not at all the case with him; but, on the contrary, down to the end of his life, he retained all his youthful admiration for the poets. his niece tells the story of hearing him often read poetry, and of how much he used to be affected by his favorite poems. in one of her letters she says: "but of all things, i used to like to hear him read 'childe harold'; and never shall i forget the way in which he read the description of the storm on lake leman. he took great pleasure in bryon, and coleridge's 'hymn to mont blanc' delighted him. when anything touched his feelings as he read--and it happened not infrequently--he would show it not only in his voice, but by tears in his eyes also." as a young man, he was so completely taken up with the scientific studies that he could not think that he would ever find time for the ordinary interests of life. especially was this true with regard to the question of marriage. he felt that he would never marry, and he seems rather to have pitied those, the weakness of whose nature pushed them on to assume many duties in life and look for merely selfish happiness. it was as a very young man that he wrote: "what is't that comes in false, deceitful guise, making dull fools of those that 'fore were wise? 'tis love." when the time came, however, he altered this opinion. among the elders of the church which he attended in london was a mr. barnard, a silversmith. faraday occasionally spent an evening at his house, and incidentally met his daughter sarah. he had not met her many times before his ideas as to what love might mean in life were completely changed, and not long after making her acquaintance he wrote her a letter, in which he recants and asks her to be more than a friend. his letter is rather interesting as love letters go. "you know me as well or better than i do myself. you know my former prejudices and my present thoughts; you know my weaknesses, my vanity, my whole mind; you have converted me from one erroneous way; let me hope that you will attempt to correct what others are wrong.... again and again i attempt to say what i feel, but i cannot. let me, however, claim not to be the selfish being that wishes to bend his affections for his own sake only. in whatever way i can best minister to your happiness, either by assiduity or by absence, it shall be done. do not injure me by withdrawing your friendship, or punish me for aiming to be more than a friend by making me less; and if you cannot grant me more, leave me what i possess but hear me." in spite of the sincere feeling of this letter, the lady hesitated. for a time she left london, apparently in order to give herself a breathing spell from the ardor of his suit. in spite of his deep interest in science, faraday followed her to the seacoast, and after they had wandered together for several days at margate and dover, where shakespeare's cliff was an especial haunt of theirs, the lady relented. faraday returned to london bubbling over with happiness. he was not quite thirty when they were married, and at the time his salary did not amount to more than a thousand dollars a year. it was distinctly not a marriage of reason. most of the happiness of his life came to him from his marriage. many years afterward, he called it "an event which, more than any other, contributed to my happiness and healthful state of mind." with years, this feeling only deepened and strengthened. in the midst of his scientific triumphs, his first thought was always of her. when his attendance at scientific congresses took him away from her, his letters were frequent, and always expressive of his longing to be with her. one of his biographers has said "that doubtless at any time between their marriage and his final illness, he might have written to her as he did from birmingham, at the time of the meeting of the british association there." "after all, there is no pleasure like the tranquil pleasure of home; and here, the moment i leave the table, i wish i were with you _in quiet_. oh! what happiness is ours! my runs into the world in this way only serve to make me esteem that happiness the more." faraday had probably lost more illusions than most men, and came to the true appreciation of things as they are. in spite of his life-long study, he had no illusions with regard to the education of the intellect merely, or the possession of superior intellectual faculties as moral factors. his keen observation of men had made any such mistake as that impossible. on the other hand, he had often noted that the ignorant, or at least those lacking education, were very admirable in conduct and in principle, and so we have his suggestive testimony: "i should be glad to think that high mental powers insured something like a high moral sense, but have often been grieved to see the contrary; as also, on the other hand, my spirit has been cheered by observing in some lowly and uninstructed creature such a healthful and honorable and dignified mind as made one in love with human nature. when that which is good mentally and morally meet in one being, that that being is more fitted to work out and manifest the glory of god in the creation, i fully admit." faraday's very definite expression of what he considers must be the position of the man of science with regard to a hereafter and the existence of god, is worth while recalling here, because it was such a modest yet forceful presentation of the attitude of mind that every thinking modern scientist must occupy in this matter, the attitude which all of faraday's great fellow-workers in the domain of electricity also occupy. it is indeed the position that has been assumed by all the great scientists who bowed humbly to faith, though so many lesser lights have found this apparently impossible. at a lecture given in at the royal institution, faraday said: "high as man is placed above the creatures around him, there is a higher and far more exalted position within his view; and the ways are infinite in which he occupies his thoughts about the fears, or hopes, or expectations of a future life. i believe that the truth of that future cannot be brought to his knowledge by any exertion of his mental powers, however exalted they may be; that it is made known to him by other teaching than his own, and is received through simple belief of the testimony given.... yet even in earthly matters, i believe that 'the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and godhead'; and i have never seen anything incompatible between those things of man which can be known by the spirit of man which is within him, and those higher things concerning his future which he cannot know by that spirit." elsewhere he had said: "when i consider the multitude of associate forces which are diffused through nature; when i think of that calm and tranquil balancing of their energies which enables elements, most powerful in themselves, most destructive to the world's creatures and economy, to dwell associated together and be made subservient to the wants of creation, i rise from the contemplation more than ever impressed with the wisdom, the beneficence, and grandeur beyond our language to express, of the great disposer of all!" dr. gladstone, in his life of faraday, which we have so often put into requisition, has given in one striking paragraph a description of the passing of faraday, that in its simplicity is worthy of the great man whom it so well represents. it is so different from what is ordinarily supposed to be the attitude of the scientist towards death, that when by contrast we recall that faraday is acknowledged to be the greatest experimental scientist of the nineteenth century, the man of his generation most honored by scientific societies at home and abroad--his honorary memberships numbered nearly one hundred--it must be considered as a very curious contradiction of what is the usual impression in this matter: "when his faculties were fading fast, he would sit long at the western window, watching the glories of the sunset; and one day, when his wife drew his attention to a beautiful rainbow that then spanned the sky, he looked beyond the falling shower and the many-colored arch and observed, 'he hath set his testimony in the heavens.' on august th, , quietly, almost imperceptibly, came the release. there was a philosopher less on earth, and a saint more in heaven." when we come to the end of the life of this greatest of experimentalists, the most striking remembrance is that of the supreme original genius of this great discoverer in electricity, whose work was such a stimulus to others, whose conclusions were to prove the basis for so much of the work of his contemporaries and his successors in electrical investigation, and whose place in the world of science is assured beside such men as newton and kepler and harvey and the other great pioneers in science. there is no doubt at all, however, that our heartiest feelings are aroused by the picture of the wonderfully rounded existence of the great scientist, his pervasive humanity, his largeness of soul and sympathy, his understanding of men in their ways through his own complete knowledge of himself, that is so strikingly displayed. we feel sure that faraday himself would have cared less for his fame as a great scientist than for the summary of his life which has been given us by his friend, bence jones, who said: "his was a life-long strife, to seek and say that which he thought was true and to do that which he thought was kind." footnotes: [ ] some of the books bound by faraday at this time are still preserved in the library of the royal institution, together with his notes on various courses of lectures, some of which are mentioned more particularly later on in this sketch, as they were also bound by him. among the manuscripts in the collection are letters from many of the important scientific scientists of europe. [ ] makers of modern medicine, fordham university press, n. y., . chapter xi. clerk maxwell. natural science in every department developed very wonderfully from its experimental side during the first half of the nineteenth century. facts and observations accumulated to such an amount that, shortly after the middle of the century, there was felt the need of a great mathematical genius to bring the results of experiment into their proper places in the great body of applied and theoretic science. nearly always such a demand meets with adequate response in its own due time. clerk maxwell came at this most opportune moment for science. no mathematical problem was too abstruse or difficult for him, and whatever he took up seriously he always illuminated, and usually solved its problems as completely as can be hoped for in the present state of scientific knowledge. it was particularly in electricity that his mathematical faculty proved of the greatest value, and that he found the abundant opportunities of which he knew so well how to take advantage. [illustration: james clerk maxwell] clerk maxwell's theory of electricity, as developed in his classic treatise on "electricity and magnetism," is well called by prof. peter guthrie tait, "one of the most splendid monuments ever raised by the genius of a single individual." this book became the guide and companion of more physical scientists during the nineteenth century than perhaps any other written in that period. it was not alone in england or in english-speaking countries that it was accepted as an authority and constantly referred to, but everywhere throughout the world of science. not to know it, was to argue that a man knew nothing of the profounder truths of electrical science and was only a seeker after superficial information. clerk maxwell was known and esteemed by all the great physical scientists of the world. his name is less widely known than that of most of the great discoverers in electricity, because mathematical achievement always has less popular attraction; but he deserves to be known by all who are interested in science, not only because of his magnificent contributions to mathematical electricity, but quite as much for qualities of heart and mind that stamp him as one of the very great men of the century so rapidly receding from us. clerk maxwell, as he is usually called, because he was the representative of a younger branch of the well-known scottish family of clerk of penicuik, was born in edinburgh, june th, . as with nearly every other person who reaches distinction in after-life, there are stories told of his precociousness which probably have more meaning in this case than in most others, since they exhibit real traits that were characteristic of the man. as a child, it is said that he was never satisfied until he had found out for himself everything that he could about anything that attracted his attention. he wanted to know where the streams of water came from, where and whence all the pipes ran, and the course of bell-wires and the like. his frequently repeated question was, "what's the go o' that." if an attempt were made to put him off with some indefinite answer, then he would insist, "but what's the particular go of it." this was probably the most prominent trait in his after-life. general explanations of phenomena that satisfied other men never satisfied him. he was a nature student from the beginning, and even as a boy he devised all sorts of ingenious mechanical contrivances. pet animals were his special delight, but for experimental purposes always, and his selection of pets would probably have startled some people. he received his early education at the edinburgh academy, and his university education at the university of edinburgh, where he graduated in . his liking for mathematics, which had already been very strongly exhibited, led him, at the age of nineteen, to go to cambridge. here, for a term or two, he was a student at peterhouse, but afterwards found a more sympathetic place for his mathematical tastes at trinity. he took his degree at cambridge in , though only with the rank of second wrangler, routh being senior. in the more serious and more exacting examination for the smith's prize, he was declared equal with the senior wrangler. his mathematical talents had developed very early, and it is not surprising that the rest of his life should have been devoted mainly to the teaching of mathematics and in investigations connected with applied mathematics. it was not success at the university that determined his career, for he had shown his marvelous mathematical ability much earlier than that, and had given some astonishing examples of his power to treat complex scientific problems in mathematical journals. indeed, his original contributions to the higher mathematics began before he was fifteen years of age. he was a striking example of the fact that a great genius usually finds his work very early in life, and usually accomplishes something significant in it, at once the harbinger and the token of the future, before he is twenty-five. while clerk maxwell was at the edinburgh academy, professor j. d. b. forbes, in , communicated to the royal society of edinburgh a short paper by his youthful student on "a mechanical method of tracing oval curves" (cartesian ovals). in spite of the prejudice that exists with regard to precocious genius and the distinct feeling that it is not likely to prove an enduring quality, clerk maxwell continued to do excellent original work all through his teens. when he was but eighteen, he contributed two important papers to the transactions of the royal society of edinburgh. one of these was on "the theory of rolling curves," and the other on "the equilibrium of elastic solids." these are now remembered, not only because of clerk maxwell's subsequent distinguished career, but because of their distinct value as contributions to science. both of them demonstrate not only his ability to work out subtle mathematical problems at this very early age, but show the possession by him of a power of investigation for original work that stamps them as well worthy of consideration in themselves, quite apart from the repute of their author or the successful accomplishments of his subsequent life. with regard to one of those edinburgh papers of clerk maxwell's eighteenth year, prof. guthrie tait said "that in it he laid the foundation of one of the singular discoveries of his later life, the temporary double refraction produced in viscous liquid by sheering stress." after his magnificent mathematical training at cambridge, it is not surprising that this academic career of great original work should be continued by contributions to science of ever-increasing importance. immediately after his graduation, he read to the cambridge philosophical society one of the few purely mathematical papers that he ever published. this had for its title, "on the transformation of surfaces by bending." expert mathematicians who read the paper, realized at once that there was a new genius in the field of mathematics. during the same year, the young scotch mathematician took the first step in that series of electrical investigations which was to occupy so much of his attention in after-life, and which was to prove the source of his greatest inspirations. this consisted of the publication of an elaborate paper on faraday's "lines of force." while we think of maxwell as a mathematical physicist, it must not be forgotten that he was also one of the leading experimental scientists of that great epoch, the nineteenth century. only a man who was himself a great experimenter could have properly appreciated and developed, from the mathematical standpoint, the works of such men as cavendish and faraday. from his early years, maxwell displayed a distinct fondness for experimentation, and this even extended to experiments upon himself. in many ways this trait of his would remind us of johann müller, the great father of modern german medicine.[ ] like müller, there was danger also of maxwell's experiments on himself getting him into trouble. for instance, at one time his love of experiment led him to try sleeping in the evening and getting up to work at midnight, so as to have the long, silent hours of the night to himself. in the sketch of his life by dr. garnett,[ ] a letter from one of his friends is quoted with regard to this nocturnal habit, which is amusing as well as interesting. the friend wrote: "from to : a. m. he took exercise by running along the upper corridor, down the stairs, along the lower corridor, then up the stairs, and so on until the inhabitants of the rooms along his track got up and laid _perdus_ behind their sporting doors, to have shots at him with boots, hair-brushes, etc., as he passed." his love of fun, his sharp wit, his extensive knowledge, and, above all, his complete unselfishness, rendered him a universal favorite, in spite of the temporary inconveniences which his experiments may have occasionally caused to his fellow-students. in , clerk maxwell received the adams prize for his essay on "the stability of the motion of saturn's rings." he shows very clearly that these annular appendages consist of a large number of small masses. this work would seem to be very distant from anything that maxwell had attempted before, and would indeed seem to the superficial observer, at least, to be quite out of his sphere. it was the mathematics of it that attracted him, and the fact that the problem was difficult, indeed, one of the most difficult at that time before astronomers, only added zest to his resolve to fathom it. all his life, mathematics continued to be his favorite form of work, and his power to express the most complex physical phenomena in mathematical formulæ gave him a reputation throughout europe unsurpassed by anyone of his generation. the more a problem seemed incapable of direct statement in mathematical terms, provided it represented a great occurrence in nature, the more maxwell was attracted to it; and the training of these early years in thus setting mathematics to the solution of physical relations, was to serve him in good stead when he came to try his hand at demonstrating the meaning of electricity in mathematical terms. just before this, in , maxwell, though only twenty-five years of age, was offered the chair of natural history, which included most of the physical sciences, at marischal college, aberdeen. with the attention that his mathematical papers attracted, it is not surprising that after four years of teaching experience he was invited to king's college, london. he held his new position for eight years, and then his health required him to retire to his estate in kirkcudbrightshire. after three years of retirement, his english alma mater demanded his services, and the temptation to get back to an academic career was so great that he could not resist it. he became, in , professor of experimental physics at cambridge. to him, more than to anyone else, is due the magnificent development of the physical sciences which took place at cambridge during the last quarter of the nineteenth century. unfortunately, he was not destined to live to enjoy the fruits of his labor in organizing the scientific side of the university, but it was under his direction that the plans of the cavendish laboratory were prepared, and he superintended every step of the progress of the building. it was under his careful management, too, that the purchase of the very valuable collection of apparatus, with which it was equipped by the duke of devonshire, was made, and maxwell's work here counts for much in the history of english science. he died in , when only forty-eight years of age, but he had deeply impressed himself upon the science of the nineteenth century. for quite one-half of his scant half-century span of life he had occupied a prominent place in england, and after the age of thirty-five had come to be generally recognized as one of the leading physical scientists of the world. his career is, as we have said, a striking illustration of how early in life a man's real work is likely to come to him, and how little success in original investigation is dependent on that development of mind which is supposed to be due only to long years of application to a particular branch of study. manifestly it is the original genius that counts for most, and not any training that it receives, except such as comes from its own maturing powers. environment, if unfavorable, does not hamper it much, nor keep it from reaching the proper terminus of its destiny; and poor health only serves to prevent the exercise of its full powers, but does not eclipse the manifestation of its capacity. clerk maxwell's important contribution to science was the demonstration that electro-magnetic effects travel through space in the form of transverse waves similar to those of light and having the same velocity. we have become so familiar with the ideas contained in this explanation, that they seem almost obvious now. they came, however, as a great surprise to clerk maxwell's generation, and at first seemed to be merely a theoretic expression of a mathematical formula. not long afterwards, however, maxwell's explanation was corroborated by hertz, who showed that these waves were propagated just as waves of light are, and that they exhibit the phenomena of reflection, refraction and polarization. hertz went on from his demonstration of the actuality of maxwell's mathematical theory to the demonstration of further electrical waves. these hertzian waves, as they were called, were a startling discovery, but remained only a scientific curiosity until they were taken advantage of for wireless telegraphy, when a new era of applied electrical science began. how his success in this was accomplished will be best understood from prof. guthrie tait's account of maxwell's devotion to electricity as a life-work. he says: "but the great work of his life was devoted to electricity. he began by reading with the most profound admiration and attention the whole of faraday's extraordinary self-revelations, and proceeded to translate the ideas of that master into the succinct and expressive notation of the mathematicians. a considerable part of this translation was accomplished during his career as an undergraduate in cambridge. the writer had the opportunity of perusing the ms. on faraday's lines of force, in a form little different from the final one, a year before maxwell took his degree. his great object, as it was also the great object of faraday, was to over-turn the idea of action at a distance. the splendid researches of poisson and gauss had shown how to reduce all the phenomena of statical electricity to mere attractions and repulsions exerted at a distance by particles of an imponderable on one another. sir w. thomson had, in , shown that a totally different assumption, based upon other analogies, led (by its own special mathematical methods) to precisely the same results. he treated the resultant electric force at any point as an analogous flux of heat from the sources distributed, in the same manner as the supposed electric particles. this paper of thomson's, whose ideas maxwell afterwards developed in an extraordinary manner, seems to have given the first hint that there are at least two perfectly distinct methods of arriving at the known formulæ of statical electricity. the step to magnetic phenomena was comparatively simple; but it was otherwise as regards electromagnetic phenomena, where current electricity is essentially involved. an exceedingly ingenious, but highly artificial, theory had been devised by weber, which was found capable of explaining all the phenomena investigated by ampère as well as the induction currents of faraday. but this was based upon the assumption of a distance-action between electric particles, whose intensity depended upon their relative motion as well as on their position. this was, of course, more repugnant to maxwell's mind than the statical distance-action developed by poisson. the first paper of maxwell's in which an attempt at an admissible physical theory of electromagnetism was made, was communicated to the royal society in . but the theory in a fully developed form, first appeared in his great treatise on electricity and magnetism ( ). availing himself of the admirable generalized coördinate system of lagrange, maxwell has shown how to reduce all electric and magnetic phenomena to stresses and motions of a material medium, and as one preliminary, but excessively severe, test of the truth of this theory has shown that, if the electromagnetic medium be that which is required for the explanation of the phenomena of light, the velocity of light in vacuo should be numerically the same as the ratio of the electromagnetic and electrostatic units. we do not as yet certainly know either of these quantities very exactly, but the mean values of the best determination of each separately agree with one another more closely than do the various values of either. there seems to be no longer any possibility of doubt that maxwell has taken the first grand step towards the discovery of the true nature of electrical phenomena. had he done nothing but this, his fame would have been secure for all time. but, striking as it is, this forms only one small part of the contents of his truly marvelous work." maxwell's prediction as to the propagation of electric waves has received its full confirmation, as we have said, in the brilliant experiments of hertz, and in the subsequent application of the hertzian waves to wireless telegraphy in our own time. it was not by mere chance that this development of maxwell's thinking came. hertz himself declared, in the introduction to his collected papers, that he owed the suggestion of his work to faraday and maxwell, and above all to maxwell's speculations as to the nature of electricity and its relations to light. hertz said: "the hypothesis that light is an electric phenomenon is thus made highly probable. to give a strict proof of this hypothesis would logically require experiments upon light itself. there is an obvious comparison between the experiments and the theory, in connection with which they were really undertaken. since , science has been in possession of a theory which maxwell constructed upon faraday's views, and which we therefore call the faraday-maxwell theory. this theory affirms the occurrence of the class of phenomena here discovered, just as positively as the remaining electric theories are compelled to deny it. from the outset, maxwell's theory excelled all others in its elaboration and in the abundance of relations between the various phenomena which it included." how much maxwell's work was appreciated across the channel, may be realized from what poincaré said: "so sure did the results of his (maxwell's) theory appear as worked out for the deepest problems, that a feeling of distrust and suspicion is likely to be mingled with our admiration for his magnificent work. it is only after prolonged study and at the cost of many efforts that this feeling is dissipated." maxwell's explanation of electricity is that it is a strain or stress in the ether, that it is a condition or mode, and not a substance. one distinguished foreign contemporary who had read maxwell's books with the greatest interest, declared that he could not be quite satisfied, since nowhere did he find what a charge of electricity is, though he seemed to find satisfactory information with regard to everything else. maxwell realized, however, the limitations of his speculation very well, and hesitated, above all, to bind his mathematical conclusions to statements that might prove eventually only surmises founded on insufficient information from the standpoint of observation. even when he gave his explanation, he did not insist on it as absolute, but, as pointed out by poincaré, discussed it only as a possibility. the french scientist said: "maxwell does not give a mechanical explanation of electricity and magnetism; he is only concerned to show that such an explanation is possible." maxwell thoroughly believed in having a hobby as well as his regular work, and during the time while he was devoting himself to the mathematical explanation of electricity he turned for recreation to certain problems in physics, in physiology and psychology, relating to color. he worked almost as great a revolution in our knowledge of color-vision as in any other subject that he took up. principal garnett has condensed so well what clerk maxwell accomplished in the matter of color-vision, in his sketch of him in "the heroes of science,"[ ] that i prefer to quote his explanation. he says: "it has been stated that thomas young propounded a theory of color-vision which assumes that there exists three separate color sensations, corresponding to red, green and violet, each having its own special organs, the excitement of which causes the perception of the corresponding color, other colors being due to the excitement of two or more of these simple sensations in different proportions. maxwell adopted blue instead of violet for the third sensation, and showed that, if a particular red, green, and blue were selected and placed at the angular points of an equilateral triangle, the colors formed by mixing them being arranged as in young's diagram, all the shades of the spectrum would be ranged along the sides of this triangle, the center being neutral grey. for the mixing of colored lights, he at first employed the color top; but instead of painting circles with colored sectors, the angles of which could not be changed, he used circular discs of colored paper slit along one radius. any number of such discs can be combined so that each shows a sector at the top, and the angle of each sector can be varied at will by sliding the corresponding disc between the others. maxwell used discs of two different sizes, the small discs being placed above the larger on the same pivot, so that one set forked a central circle and the other set a ring surrounding it. he found that, with discs of five different colors, of which one might be white and another black, it was always possible to combine them so that the inner circle and the outer ring exactly matched. from this he showed that there could be only three conditions to be satisfied in the eye, for two conditions were necessitated by the nature of the top, since the smaller sectors must exactly fill the circle and so must the larger. maxwell's experiments, therefore, confirmed, in general, young's theory. they showed, however, that the relative delicacy of the several color sensations is different in different eyes, for the arrangement which produced an exact match in the case of one observer, had to be modified for another; but this difference of delicacy proved to be very conspicuous in color-blind persons, for in most of the cases of color-blindness examined by maxwell the red sensation was completely absent, so that only two conditions were required by color-blind eyes, and a match could therefore always be made in such cases with four discs only. holmgren has since discovered cases of color-blindness in which the violet sensation is absent. he agrees with young in making the third sensation correspond to violet rather than blue. maxwell explained the fact that persons color-blind to the red divide colors into blues and yellows, by the consideration that, although yellow is a complex sensation corresponding to a mixture of red and green, yet in nature, yellow tints are so much brighter than greens, that they excite the green sensation more than green objects themselves can do; and hence greens and yellows are called yellow by such color-blind persons, though their perception of yellow is really the same as perception of green by normal eyes. later on, by a combination of adjustable slits, prisms, and lenses arranged in a 'color box,' maxwell succeeded in mixing, in any desired proportions, the light from any three portions of the spectrum, so that he could deal with pure spectral colors instead of the complex combinations of differently colored lights afforded by colored papers. from these experiments, it appears that no ray of the solar spectrum can affect one color sensation alone, so that there are no colors in nature so pure as to correspond to the pure simple sensations, and the colors occupying the angular points of maxwell's diagram affect all three color sensations, though they influence two of them to a much smaller extent than the third. a particular color in the spectrum corresponds to light which, according to the undulatory theory, physically consists of waves, all of the same period; but it may affect all three of the color sensations of a normal eye, though in different proportions. thus yellow-light of a given wave-length affects the red and green sensations considerably and the blue (or violet) slightly, and the same effect may be produced by various mixtures of red or orange and green." for his researches on the perception of color, the royal society awarded clerk maxwell the rumford medal in . besides this more or less theoretic work, however, maxwell made some interesting and important discoveries and inventions in optics. for instance, he noted the great differences that exist in the eyes of dark and fair complexions to different colors when the light falls upon the center of the yellow spot, the so-called fovea centralis, or central pit of the retina. his researches with regard to this led him to the discovery that this portion of the retina is largely lacking in sensibility to blue light. he was able to demonstrate this by his experiment of looking through a glass vessel containing a solution of chrome alum, when the central portion of the field of vision appears of a light red color for the first second or two. he was also the inventor of an ingenious optical apparatus, a real image stereoscope. a still more important discovery was that of the double refraction which is produced for the time in viscous liquids when they are stirred and their motion is not as yet stopped. maxwell showed that canada balsam, for instance, when stirred, acquired a distinct power of double refraction, which it retained so long as the stress in the fluid produced by stirring remained. other departments of physics were not neglected. for instance, one of his greatest investigations was that on the kinetic theory of gases. geniuses had been working before him on this line, for, as pointed out by professor tait, this theory owed its origin to daniel bernoulli, the greatest mathematician of the eighteenth century, and had been developed by the successful labors of herapath, joule and, above all, of clausius. the work of these men put the general accuracy of the theory beyond all doubt and led to its very general acceptance, yet the details of it needed to be elaborated before it could become definitely scientific. its greatest developments are due to maxwell, and in this field maxwell appeared as an experimenter on the laws of gaseous friction as well as a mathematician. his work with regard to color had showed his ingenuity as an experimentalist, and this is still further illustrated by his carefully arranged experiments on gases. indeed, his work in this line makes it very clear that nothing was too difficult for him, and that anything that he turned his hand to in the field of science he was sure to accomplish with eminent success. it was not only his scientific monographs, however, that indicate how great a scientist clerk maxwell was, but his text-books, even those of more or less elementary character, which he wrote bring out this same idea. he wrote, for instance, an admirable text-book on the theory of heat, which went through many editions. students of the subject, even those who were not far advanced, found it clear and easier of study than many a less exhaustive work. he also wrote an elementary treatise on matter and motion, which has gone through several editions. one might think that so small a work would scarcely interest him enough to tempt him to put forth his powers at their best, and that at most it would be a conventional condensation of previous knowledge. prof. tait, who surely must be taken as a good judge in the matter, says that "even this, like his other and larger works, is full of valuable material worthy of the most attentive perusal not of students alone, but of the very foremost scientific men." one of the characteristic traits of maxwell was his desire to impart information to others. this extended not only to his academic relations, but, above all, to the working classes, who might have few opportunities for the obtaining of the information that was so interesting with regard to natural subjects. everywhere that he held an academic post in his life, he gave lectures to the workmen. he was an extremely interesting talker, and one of his friends said of him: "i do believe there is not a single subject on which he cannot talk, and talk well, too, displaying always the most curious and out-of-the-way information." one of his private tutors said of him: "it is not possible for maxwell to think incorrectly on physical subjects." it is easy to understand, then, how much his lectures to the working people at aberdeen, at edinburgh, and at kings college, london, as well as at cambridge, meant for them. if men like maxwell would take up the popularization of science generally, then there would be much less opprobrium attached to the expression popular science than there has been only too often in the past, and is even at present. just as maxwell set himself to the solution of the most difficult problems in physics, so he did not hesitate to give himself also to the discussion of problems in ethics. here his power of penetration, the rigid logic of his mind, and his power to follow out conclusions to their ultimate significance, were quite as manifest as any scientific writing. it is almost the rule to find that scientists either ignore the great problems of man's place in nature and his destiny, or treat them very superficially. agnosticism had become the fad of the moment, and was just beginning to make itself felt as a fashion in thinking when clerk maxwell was doing his great work. maxwell was not an agnostic in science, and because he could not solve all the problems that came to him with regard to electricity and the constitution of matter, this did not keep him from setting himself to the task of seeing what should be his thoughts with regard to these subjects. he had none of the agnostic's feelings with regard to them, that since we cannot know all about them definitely and absolutely, therefore it is not worth while studying them at all. had maxwell been tempted to any such line of thought, we would have missed some of the most helpful scientific speculations and suggestions that have ever been made. no one knew better than maxwell, that his speculations on matter and electricity were theories, and that what he was offering to science were not definite explanations, but possible hypotheses. he has emphasized this himself over and over again. this inability of the human intellect at the present moment to solve all the questions that its inquiring spirit can evoke, did not keep him from investigating and following up his investigations by mathematical deductions and mechanical suggestions just as far as possible. he had the same attitude of mind toward the great problems of man's relation to his fellow-man, to the universe, and to a hereafter. while he felt that he could not solve the problems entirely, he felt also that his reasoning was quite sufficient to enable him to get a little nearer to the heart mystery of them and to understand something of their significance. in his later years, the question of the existence of pain and suffering in the world had, because of darwin's attitude towards them and his declaration that since he was unable to understand them they carried him away from the thought of a beneficent creator, attracted much attention. we have an essay of clerk maxwell's, then, on "aspects of pain," in which he discusses particularly pain as discipline. it is, of course, the old story, that men rise on stepping-stones of their dead selves, and that the successive deaths of self represent a triumphant progress, but it comes with a new vigor from this great scientist. we all know that it is the man who has suffered who is able to do things, and we are all well aware that the man who has lived in comfort all his life is almost sure to be lacking in character when a great crisis comes upon him. indeed, as clerk maxwell re-states it, this is such a commonplace that one wonders why the problem of pain should have seemed so hard to understand. there is an essay of his, also, on "science and free will," which seems to deserve special notice. he has no illusions with regard to determinism. he is perfectly sure that he is free and that the great majority of men around him do or do not things as they choose. he points out that science makes for determinism only if one takes a very narrow view of it. free will is not only compatible with scientific thinking, but it represents what would be expected as a culmination of the significance of life. in a word, clerk maxwell wrote as suggestively with regard to the great problems of human life as with regard to the physical nature around him that claimed so much of his interest. he was a true natural philosopher, and his interests were not limited merely to the lower orders of beings. because of the supreme power of clerk maxwell's mind to seek out the very heart of difficulties, the conclusions which he reached with regard to the existence of matter and the causes for the ultimate qualities which it exhibits, have an enduring interest. mathematics is sometimes said to lead minds into scepticism. cardinal newman even thought that the mathematical cast of mind was the farthest removed from that which might be expected to accept things confidently on faith. clerk maxwell's intellect was eminently mathematical; yet, far from sending him over into the camp of the agnostics, his tendency to get at the ultimate reasons for things seemed almost to push him to conclusions with regard to the origin of matter, and especially its ultimate constituents, not ordinarily supposed to be scientific. a passage like the following, for instance, which may be found in his book on "the theory of heat," london, , page , brings out this tendency very well: "but if we suppose the molecules to be made at all, or if we suppose them to consist of something previously made, why should we expect any irregularity to exist among them? if they are, as we believe, the only material things which still remain in the precise condition in which they first began to exist, why should we not rather look for some indication of that spirit of order, our scientific confidence in which is never shaken by the difficulty which we experience in tracing it in the complex arrangements of visible things, and of which our moral estimation is shown in all our attempts to think and speak the truth, and to ascertain the exact principles of distributive justice?" the argument from design for creation is often said in our day to have lost its weight. for clerk maxwell, however, this was evidently not the case. on the contrary, he seemed to find in the detailed knowledge of the ultimate constituents of matter which had come in recent years, additional proofs of the great design which permeates nature. he had come to the conclusion that not only were the groups of atoms which make up living things so ordered as to produce definite results, because there was a great purpose and, above all, a great designer behind nature, but he also reached the position that the separate atoms of matter were so ordered with regard to one another, and in that ordering were so closely related to corresponding qualities in higher beings, that only the presence of a great design in nature could possibly account for all these wonderful attributes, which were to be found even in the smallest portions of matter. he said in his article on the atom, in the ninth edition of the encyclopedia britannica: "what i thought of was not so much that uniformity of result which is due to uniformity in the process of formation, as a uniformity intended and accomplished by the same wisdom and power of which uniformity, accuracy, symmetry, consistency, and continuity of plan are as important attributes as the contrivance of the special utility of each individual thing." here is the old argument for the existence of god, from the design exhibited in the universe, rehabilitated by its application to the minutest portions of matter, whose qualities demand such an explanation quite as much as the highest adaptations of nature. perhaps the most striking expression of all with regard to the atoms that clerk maxwell permitted himself, is that in which he finds the type of what is best in man, in every minute portion of the universe, planted there by the creator just as surely as they are in his highest beings, because they represent the most precious qualities of his own nature as they are reflected in the creation that he called into existence. "they (the atoms) continue this day as they were created, perfect in number and measure and weight, and from the ineffaceable characters impressed on them we may learn that those aspirations, after accuracy in measurement, truth in statement, and justice in action, which we reckon among our noblest attributes as men, are ours because they are essential constituents of the image of him who in the beginning created not only the heaven and the earth, but the materials of which heaven and earth consist." a very interesting side of maxwell's life is that which shows his continued interest in literature, and even his occasional dippings into poetry. though he reached distinction in mathematics and physics so early in his career, he yet found time to indulge a liking for the classics, and we even find some rather good translations of horace's odes from his pen. the translation of a part of the ajax of sophocles from the greek is a striking testimony to the breadth of maxwell's intellectual interests. all during life, however, he permitted himself occasionally the luxury of fitting words into verse forms, and sometimes with a success that deserves much more than passing interest. it is very probable that the following verses, for instance, which are the first and last stanzas of a poem on the formula for being happy in life and were meant to be sung (or at least so he would hint) to the tune of "il segreto per esser felice," will strike many a sympathetic chord in the modern time. there are some folks that say they have found out a way to be healthy and wealthy and wise:-- "let your thoughts be but few, do as other folks do, and never be caught by surprise. let your motto be follow the fashion, but let other people alone; do not love them nor hate them nor care for their fate, but keep a lookout for your own. then what though the world may run riot, still playing at catch who catch can, you may just eat your dinner in quiet and live like a sensible man." in nature i read quite a different creed, there everything lives in the rest; each feels the same force as it moves in its course, and all by one blessing are blest. the end that we live for is single, but we labor not therefor alone; for together we feel how by wheel within wheel we are helped by a force not our own. so we flee not the world and its dangers, for he that has made it is wise; he knows we are pilgrims and strangers, and he will enlighten our eyes. there probably was not a more nicely logical or more accurately reasoning intellect among all our nineteenth century scientists than that of the great mathematical electrician. he had none of the one-sidedness of the merely experimental scientist, nor, on the other hand, the narrowness of the exclusively speculative philosopher. with a power of analysis that was seldom equaled during the century, he had a power of synthesis that probably surpassed any of his contemporaries in any part of europe. his ideas with regard to matter and its ultimate constitution are most suggestive. his suggestion of a strain in the ether as an explanation of electricity, thus enabling scientists to get away from the curious theories of the foretime which had required them to accept "action at a distance," that is, without any connecting medium, shows his power of following out abstruse ideas to definite practical conclusions. his religious life, then, will be a surprise to those who think that science leads men away from religion. in the life of clerk maxwell, written by campbell and garnett,[ ] there is a passage from his friend and sometime pastor, guillemard, in which the details of his religious life are given so fully as scarcely to require any further gleaning of information in this regard. "he was a constant, regular attendant at church, and seldom, if ever, failed to join in our monthly late celebration of holy communion, and he was a generous contributor to all our parish charitable institutions. but his illness drew out the whole heart and soul and spirit of the man; his firm and undoubting faith in the incarnation and all its results; in the full sufficing of atonement; in the works of the holy spirit. he had gauged and fathomed all the schemes and systems of philosophy, and had found them utterly empty and unsatisfying--'unworkable' was his own word about them--and he turned with simple faith to the gospel of the saviour." his faith was not disturbed at the near approach of death, but, on the contrary, seemed strengthened. his biographers tell the story of some of the expressions used to his friends during these last days, which furnish manifest proof of this. some of these passages are so characteristic and so striking that they deserve to be in the note-book of those to whom the modern idea that science is opposed to religion or faith may sometimes have been a source of worry, or at least an occasion for argument. here is a typical one of these passages: "mr. colin mackenzie has repeated to us two sayings of his during those last days, which may be repeated here: 'old chap, i have read up many queer religions; there is nothing like the old thing, after all; and i have looked into most philosophical systems, and i have seen that none will work without a god.'" it must not be imagined, because clerk maxwell was a deeply religious man, that, therefore, he was frigid or formal or extremely serious, or inclined to be puritanic with regard to the pleasures of life, or a fanatic in the matter of taking all the good-natured fun there might be in anything that turned up. he was far from over-serious, or what has been called, though not quite properly, ascetic; but, on the contrary, was often, indeed usually, the soul of the party with which he was at the moment. he had none at all of the self-centered interest of the narrow-minded, but had many friends, and was liked by all his acquaintances. his friends were enthusiastic about his kindness of heart and the thorough congeniality of his disposition. on this point, the sketch of him in the national dictionary of biography gives a charming picture: "as a man, maxwell was loved and honored by all who knew him; to his pupils, he was the kindest and most sympathetic of teachers; to his friends, he was the most charming of companions, brimful of fun, the life and soul of a red lion dinner at the british association meetings; but in due season brave and thoughtful, with keen interest in problems that lay outside the domain of his own work, and throughout his life a stern foe to all that was superficial or untrue. on religious questions, his beliefs were strong and deeply rooted." it may be added to this, that his religion had nothing of the merely formal about it, nor was it perfunctory. it entered into most of the details of his life, and the fact that, every day as the head of the house he led evening prayers for the family, was only a token of the deep hold which religion had upon his life. when his last illness came, though he knew that his end was not far off, and at his age sometimes the approach of death hampers religious faith because it does seem that longer life might be afforded to one who has been so faithful in his realization of the obligations of life, clerk maxwell's piety increased rather than diminished. a favorite expression of his during his last days was the verselet from richard baxter, which one would be apt to think of as frequently repeated by some feminine devotee rather than by the greatest mathematical scientist of the nineteenth century: "lord, it belongs not to my care, whether i die or live; to love and serve thee is my share, and that thy grace must give." a friend who knew him intimately says: "in private life, clerk maxwell was one of the most lovable of men, a sincere and unostentatious christian. though perfectly free from any trace of envy or ill-will, he yet showed on fit occasions his contempt for that pseudo-science which seeks for the applause of the ignorant by professing to reduce the whole system of the universe to a fortuitous sequence of uncaused events." in these phases of his intellectual life, the greatest of the mathematical electricians of the nineteenth century deserves to be taken as the type of the man of science, rather than the many mediocre intelligences whose minds were not large enough apparently for the two sets of truths--those of the moral as well as of the physical order. [illustration: lord kelvin] footnotes: [ ] see life of johann müller, in makers of modern medicine, fordham university press, n. y., . [ ] heroes of science physicists, n.y., young & co., . [ ] heroes of science physicists, by wm. garnett, m. a., d. c. l. london society for promoting christian knowledge, northumberland ave., charing cross, w. c. new york, e. and j. b. young. [ ] the life of james clerk maxwell, with a selection from his correspondence and occasional writings, and a sketch of his contributions to science. lewis campbell and william garnett. london, . chapter xii. lord kelvin. few men lived to witness so many remarkable discoveries in science and so many applications of the same to the welfare of the race as did the man whose name stands at the head of this chapter. when william thomson, the future lord kelvin, first saw the light of day, the voltaic pile was in a rudimentary and inefficient form. it is true that water had been decomposed by the current from a pile in ,[ ] that the magnetic effect of the current had been discovered in , and the possibility of a practical form of an electric telegraph suggested in the same year; but ohm's law was still one of nature's secrets, electromagnetic induction was undiscovered, and the doctrine of energy but ill understood. light, electricity and magnetism were regarded as distinct forces, and heat was thought to be a material substance, to which the name caloric was assigned. what young, fresnel and ampère were in the early years of the nineteenth century; what faraday, regnault and joseph henry were some time later, kelvin became in the 'fifties, a leader in the intellectual and scientific life of the time, a leader destined to extend the frontiers of knowledge, to establish an accurate system of electrical measurement, and to enrich the world with instruments of marvelous ingenuity and precision. william thomson, born in belfast in , received his early training in the royal academic institute of that city. when eight years of age, he left his native land, exchanging the shores of antrim for the banks of the clyde. his father, james thomson, a mathematician of note, having been appointed to the chair of mathematics in the university of glasgow (founded in ), proceeded early in the summer of to the commercial metropolis of scotland, accompanied by his two sons william and james, both of whom were destined to add lustre to the family name. after a period of preparatory study, the two brothers, who were ten and eleven years of age, respectively, matriculated at the university. with the iron-clad regulations that govern admission to american colleges and universities, these boys would at best have been admitted to one of our high schools, and kept there until they reached the maturity required by the age limit. by the time young william attained that limit, he had already finished his work at the university, and captured the first prizes in mathematics, astronomy and natural philosophy. he was then only sixteen years of age, small of stature, but a giant in intellect; brilliant, versatile, and with a passion for work. it was his good fortune, also, to come under the influence of a great teacher, in the person of prof. nichol. "i have to thank what i heard in the natural philosophy class," he said in , "for all i did in connection with submarine cables. the knowledge of fourier was my start in the theory of signaling through submarine cables, which occupied a large part of my after-life. the inspiring character of dr. nichol's personality and his bright enthusiasm live still in my mental picture of those old days." having heard fourier's treatise on the mathematical theory of heat spoken of one day as a remarkable and inspiring work, young thomson astonished the professor when, at the end of the lecture, he addressed dr. nichol with the query, "do you think that i could read it?" to which the professor smilingly replied: "well, the mathematical part is very difficult." many a student would have left fourier alone for the nonce, after listening to a statement so little calculated to excite courage or awaken interest: but thomson was not an ordinary student; and, however forbidding the answer which he received, he was determined all the same to handle the volume and seek its inspiration. without delay, he got the book from the university library, and grew so delighted with the new ideas of the french mathematician about sine-expansions and cosine-expansions, that in the space of two weeks he had "turned over all the pages" of the book, as he modestly put it. in the summer of , he accompanied his father and his brother on a tour through germany, partly to see the country and partly also, to acquire a practical knowledge of the language. in both these objects, he was somewhat hindered by his fondness for mathematical studies, which led him to include in his impedimenta for the trip a copy of fourier's _théorie analytique de la chaleur_. most students out on a summer's vacation, especially in foreign parts, would doubtless have preferred to give their minds rest and congenial distraction rather than keep on reading and pondering over abstract mathematical concepts. our young tourist, on the other hand, seems to have thought of little else than of fourier's "mathematical poem," as clerk maxwell called the work, a "poem" that continued to have a charm for him all through life. it is a noteworthy fact that thomson continually returned to the ideas and methods of this suggestive treatise on the flow of heat, and that he applied them with great success to problems in thermal conductivity, in electricity and in submarine telegraphy. shortly after returning home, thomson was sent to the university of cambridge, where he entered st. peter's college, commonly called peterhouse, one of the oldest colleges of the university, its foundation dating back to the year . though he, no doubt, followed in a general way the directions given him by william hopkins, "the best of private tutors," and kept in view the requirements of the honors examination, called the "mathematical tripos," for which he intended to present himself at the end of his course, he found his studies somewhat routinal and uninspiring. original work was more to his taste than conventional subjects; his tutor, however, thought mainly of placing this brilliant pupil at the head of the wranglers, and hailing him the senior wrangler of the year, for which purpose, the beaten track must be followed, the standard works read, favorite problems worked out, short-cuts conned and rapidity of output exercised. stokes, of pembroke, had been senior wrangler in ; cayley, of trinity, in ; and adams, of john's, in ; why not thomson, of peterhouse, in , argued hopkins, who had the distinction of being second wrangler of the previous year? but when the ordeal was over and the work of all candidates appraised, thomson's name was second on the list, with parkinson, of john's, at the top. hopkins was disappointed, as he had a right to be, for it was thought by many and said by some that parkinson was not fit to sharpen thomson's pencils. at the examination for the smith's prizes, which immediately followed, and which was generally regarded as a higher honor and a better test of original ability, the order was reversed, and thomson's star blazed out with the brilliancy of the first magnitude. we have here an instructive instance of the failure of an examination to place rightly the most gifted man; that of sylvester, in , and clerk maxwell, in , both of whom were second wranglers, are equally so. examinations, however, seldom fail in justly rating candidates when originality is not a necessary qualification, but only a sound knowledge and liberal interpretation of the subjects laid down in the syllabus; a good memory and rapidity of writing will do the rest. thomson committed the fatal mistake in the tripos examination of devoting too much time to a particular question in which he was deeply interested. it was a curious coincidence that the solution which parkinson sent in to the same question was almost identical with that of his rival for mathematical honors. on being questioned about the matter by the moderators, parkinson said that he had read the solution some time before in the _cambridge mathematical journal_; thomson's explanation was that the solution given in the journal was his! as he had not memorized the details, he was obliged of course to work the problem out _de novo_. parkinson in later years wrote a treatise on elementary mechanics that has long since made way for others; thomson, on the other hand, published in collaboration with tait a _treatise on natural philosophy_ for advanced students, which became at once the accepted standard. throughout this treatise, the view is emphasized that physics deals with realities more than with theories, with mutual relations more than with their mathematical expression. helmholtz thought so highly of this work that he translated it into german, saying in his preface: "william thomson, one of the most penetrating and ingenious thinkers, deserves the thanks of the scientific world, in that he takes us into the workshop of his thoughts and unravels the guiding threads which have helped him to master and set in order the most resisting and confused material." and again: "following the example given by faraday, he avoids as far as possible hypotheses about unknown subjects, and endeavors to express by his mathematical treatment of problems simply the law of observable phenomena." we are not to think of thomson, the undergraduate, as of one who gave himself up, mind and body, to his favorite studies; he knew how to combine, in some measure, the _dulce_ with the _utile_, for he was fond of music, and so proficient in the art that he was elected president of the musical society. he also took a practical interest in aquatic sports, and on the cam he could ply his sculls with the best of the men. indeed, he was fond of the water all through life, his _lalla rookh_ being well known on the clyde and in the solent. expert in the navigation of his yacht, he liked to be out on the deep, caressed by wind and buffeted by wave, on which occasions he usually studied, pencil in hand, problems connected with navigation and hydrodynamics. thomson was never without his note-book. even in his journeys to london, when he usually took the night train to save time, his mind was active, and the green-book was in frequent requisition to receive thoughts that occurred relative to problems that engaged his attention. unlike many mortals, he was able to sleep soundly on those night trips, although in the early days he had none of the luxuries of traveling which we consider indispensable to our comfort. helmholtz records that, being on the _lalla rookh_ on one occasion, thomson "carried the freedom of intercourse so far that he always had a mathematical note-book with him; and as soon as an idea occurred to him, he began to reckon right in the midst of company." this reminds us of the answer which newton gave to a friend who asked him how he accomplished so much. "by constantly thinking of it," was the brief reply. concentration of the faculties is necessary for all good work; a distracted mind never achieved anything of value in philosophy, in science, in religious worship. concentration is like a convex lens, which brings rays to a focus; whereas distraction is like a concave lens, which breaks them up into a number of divergent and scattered elements. on leaving cambridge in , thomson proceeded to london, and was warmly received by faraday, then of world-wide reputation. he next went to paris, where, in the laboratory of regnault, he devoted himself to original research, under the direction of that great and accurate physicist who was then carrying out his classic work on the thermal constants of bodies. the year marks an epoch in thomson's life; for, in that year, he was chosen to succeed nichol, his friend and master, in the chair of natural philosophy in the university of glasgow. though only in his twenty-second year, he chose for the subject of his inaugural address the age of the earth, a subject which continued to have a life-long interest for him because of its very fascination, and perhaps, too, because of the opposition which his views aroused on the part of biologists and geologists. these demanded untold æons for the original fire-mist to cool down and form a spinning globe fit to be the abode of organic life, whereas thomson endeavored to show the weakness of the arguments which they advanced to uphold their claim for unlimited time. basing his estimate on the rate of increase of temperature as we go below the earth's surface, he concluded that the earth required from to million years, and probably less, to cool from its molten state to its present condition. impressed by the value of the experimental work which he did under regnault in paris, prof. thomson gave himself no rest until he secured a place in which the demonstrations of the lecture-room could be supplemented by qualitative and quantitative work in the laboratory. this was the first "physical laboratory" open to students in great britain, a fact that makes the year a memorable one in the history of university development. two apartments were allotted him for experimental purposes, _viz._, an abandoned wine-cellar and a disused examination-room, to which, as time went on, were added a corridor, some spare attics, and even the university tower itself, so great was the power of annexation possessed by the young professor. in those dark and cheerless rooms, a few old instruments were installed, after which students were invited and work begun. a band of men, whose ardor was enkindled by the glowing enthusiasm of the presiding genius, gathered around him, and helped him to carry out investigations on the properties of metals, on moduli of elasticity, elastic fatigue and atmospheric electricity. among this band of earnest students it will suffice to mention the names of the late prof. ayrton, an eminent electrician; prof. john perry, known for his homeric battles in favor of reform in the teaching of mathematics; sir william ramsay, the discoverer of the "newer" gases of the atmosphere; and prof. andrew gray, who succeeded his master in the university of glasgow. writing of his laboratory experiences, prof. ramsay says: "i remember that my first exercise, which occupied over a week, was to take the kinks out of a bundle of copper wire. having achieved this with some success, i was placed opposite a quadrant electrometer and made to study its construction and use." "although this method," he adds, "is not without its disadvantages--for systematic instruction is of much value--there is something to be said for it. on the one hand, too long a course of experimenting on old and well known lines is likely to imbue the young student with the idea that all physics consists in learning the use of apparatus and repeating measurements which have already been made. on the other hand, too early attempts to investigate the unknown are likely to prove fruitless for want of manipulative skill and for want of knowledge of what has already been done." prof. gray wrote: "in the physical laboratory, prof. thomson was both inspiring and distracting. he continually thought of new things to be tried, and interrupted the course of work with interpolated experiments which often robbed the previous sequence of operations of their final result." it may bring a grain of consolation to teachers who meet with troublesome elements in the discharge of their duties, to know that thomson, great and brilliant as he was, had similar experiences now and again. at one time a book of mathematical data would be removed from the place assigned to it, upon which he would give orders that it should be chained to the table; at others, there would be no chalk near the blackboard, and then the assistant would be solemnly instructed to have one hundred pieces available next time. on one occasion, he settled in a very novel manner the case of a student who insisted on disturbing the class by moving his foot back and forth on the floor. calling his assistant, thomson told him in a whisper to go down into the room under the tiers of seats, to listen attentively, and locate the wandering foot by its distance from two adjacent walls of the building. on his return to the lecture-room, the triumphant assistant gave the desired coordinates to the professor, who took out his tape at once and measured off the distances, by which the outwitted offender was mathematically located. in obedience to orders, the latter rose and left the room, muttering a few graceful epithets as he went, in honor of descartes, the founder of a system of geometry that could serve so well the twofold purpose of the detective and the mathematician. it was the custom in glasgow to open the daily sessions, morning and afternoon, with prayer, the selection of which was left to the discretion of the professor. thomson usually recited from memory the third collect from the morning service of the church of england, to which he sometimes added reflections of his own for the spiritual benefit of his hearers. in his teaching, prof. thomson was particularly insistent that his students should not bow their intellects in mute admiration before an array of mathematical symbols; but that, on all occasions, they should seek the physical meaning behind them. writing on his blackboard one day _dx/dt_, he was not satisfied when told that it represented the ratio of the increment of _x_ to the increment of the independent variable _t_ (time); he wanted the student to say it represents velocity. he himself was so wont to look for the physical meaning of symbols that, like the prophets of old, he saw many things that were hidden from the eyes of ordinary mortals. he had the rare gift of translating mathematical equations into real facts; and he strove all throughout his life, by word and writing, to purify mathematical theory from mere assumptions. he often said that he could not understand a thing until he was able to make, or at least conceive, a model of it. he had a "keen mathematical instinct," as prof. silvanus p. thomson puts it in a letter to the writer, an insight that "grew to see things." he often left matters in the dark for years, then returned to see them in the clear light of truth. at the age of sixteen, he wrote a mathematical essay on the figure of the earth; and at eighty-three, took it up again in order to add a note to the argument! thomson was discursive in his lectures, and was never able to boil the matter down to suit the taste and digestive powers of the ordinary student. the activity of his mind and its fecundity were such that new ideas, new problems, new modes of treatment were continually occurring, and with such fascination that he would leave the main subject to indulge in what often proved prolonged digressions. one of his bugbears was our system of weights and measures, which he denounced in season and out of season as "insane," "brain-wasting" and "dangerous." occasionally epithets of a more caloric nature would escape the lips of the indignant professor, who, as a consequence of his denunciation, had always to be indulgent to students who chanced to be shaky in the matter of troy weight, avoirdupois weight or even apothecaries weight. in later years, i heard lord kelvin at the royal institution, london, on some of his favorite dynamical subjects, such as the gyrostat, vortex rings and the like. however impressed by his keen eye, intellectual forehead, his mastery of the subject and wealth of illustration, i was no less impressed by his vivacity, his enthusiasm and the rapidity with which he could leave a train of thought and return to it again. at meetings of the british association, he always had something illuminating to say; but not infrequently, carried away by a torrent of ideas, he would indulge in a superfluity of detail, forgetting that other speakers had to be heard and other papers read. the idea of connecting the old world with the new by means of an electric cable laid on the bed of the ocean, seemed to most people in the 'fifties quixotic and utopian. manufacturers said such a cable could not be made; engineers, that it could not be laid; electricians, that it could not be worked; and financiers, that if laid and worked, it would never pay. but with a field to look after the financial interests of the scheme, and a thomson to attend to electrical quantities, there was no tilting at windmills, and the utopian scheme became in due time the cable whose core pulsated with the news of the world. as early as , bishop mullock, of st. john's, n. f., addressed to an american newspaper, called the _courier_, a letter in which he advocated a telegraph line from newfoundland to new york, so that the news of mail steamers could be intercepted and wired to that city. in , the "newfoundland electric telegraph company" was formed for the purpose of carrying out a similar plan. this was to be accomplished by means of a telegraph line from cape race, at the eastern extremity of newfoundland to cape ray, on the western, as well as by short cables over to cape breton island, to prince edward island and the mainland, and thence by ordinary telegraph lines to canada and the united states. but owing to the want of money, nothing was done. the first attempt at laying a cable under the atlantic was made by the atlantic telegraph company in , after a careful survey of the ocean had revealed the existence of a submarine plain, or extended table-land, on which the cable could rest undisturbed by passing keels, monsters of the deep or angry billows. the result was the first of a series of failures, which caused great perplexity and depression at the time; for, after miles had been paid out from valentia on the irish coast, the cable suddenly parted, burying in fathoms of water an electrical conductor which had cost $ , for its manufacture. a second attempt was made in , when the u. s. frigate _niagara_ and h. m. s. _agamemnon_, each carrying half of the cable, met in mid-ocean; and, after splicing the two ends together, steamed away in opposite directions, the _niagara_ toward newfoundland and the _agamemnon_ toward valentia. fortunately for the enterprise, prof. thomson was on board the english ship as chief electrician. no doubt, his mind turned many a time during those anxious days to fourier's differential equation for the flow of heat along a conductor, and his own application of it to the conduction of the electric current through the copper core of the cable as it came up from the tanks, trailed out behind the ship, dipped silently into the blue water and slowly settled down to its bed of ooze on the ocean floor. after a series of disheartening mishaps, necessitating as many returns of the ships to the rendezvous in mid-ocean, the _agamemnon_ landed the shore-end safely in valentia; and the _niagara_, after rolling and pitching for days and nights in tempestuous seas, landed hers in trinity bay on the morning of august th, , on which historic date the telegraphic union of the two worlds was finally consummated and the great feat of the century accomplished. though not fully realized at the time by the capitalists who financed his scheme, by the engineers and electricians who carried it out, or even by statesmen, economists and social reformers, the slender copper cord, buried away from human ken amidst the _débris_ of minute organisms, was destined to effect a revolution in the affairs of men greater than any achieved by the wisdom of sages or the policy of legislators. owing to the electrostatic capacity of the cable, signaling would have been difficult and unsatisfactory had it not been for the resourcefulness of prof. thomson, who devised his reflecting galvanometer to serve as receiving instrument. the principle of the mirror applied in this way was not new, for it had been suggested by poggendorff and even used by gauss in connection with very heavy magnets. the magnets used by thomson, on the other hand, were strips of watch-spring weighing about a grain each, so that even a very weak current coming through the cable would be sufficient to produce strong displacements of the spot of light on the scale. thomson was clearly the first to insist on small dimensions in magnetic instruments, and to show that reduction in size would be attended with corresponding increase in sensitiveness. the mirror galvanometer, surrounded with a thick iron case to screen it from the magnetic field due to the iron of the ship, the "iron-clad galvanometer" as it was called, was used for the first time on the telegraphic expedition of . the instrument itself, which was fitted up on board the _niagara_ and which was connected with so many episodes of thrilling interest, was placed by prof. thomson in the collection of historical apparatus in the university of glasgow, where it is at the present day. beautiful as was the invention of the mirror galvanometer, it gave neither warning of the beginning of a message nor a permanent record of it. sitting in his dark room, the operator had to be always on the alert for the first swing of the spot of light over the scale. to obviate these drawbacks, thomson, after some thinking and more talking with his friend white, of glasgow, finally patented the _siphon-recorder_, in which a glass siphon of capillary dimensions is pulled to the right or left by the action of the current flowing through a light movable coil, and is thus made to register signals in ink on a vertical strip of paper which is kept in uniform motion by a train of clockwork. it is by this simple but very ingenious instrument that messages are received and recorded to-day at all the cable-stations of the world. the inaugural message through the cable came from the directors of the atlantic telegraph company in great britain to the directors in america, saying: "europe and america are united by telegraph; glory to god in the highest, on earth peace and good will toward men." the message from queen victoria to president buchanan, consisting of words, took minutes in transmission; it read: "the queen desires to congratulate the president upon the successful completion of this great international work, in which the queen has taken the deepest interest. "the queen is convinced that the president will join with her in fervently hoping that the electric cable which now connects great britain with the united states will prove an additional link between the nations whose friendship is founded upon their common interests and reciprocal esteem. "the queen has much pleasure in thus communicating with the president, and renewing to him her wishes for the prosperity of the united states." the reply of president buchanan was as follows: "the president cordially reciprocates the congratulations of her majesty, the queen, on the success of the great international enterprise accomplished by the science, skill and indomitable energy of the two countries. it is a triumph more glorious, because far more useful to mankind, than was ever won by conqueror on the field of battle. "may the atlantic telegraph, under the blessing of heaven, prove to be a bond of perpetual peace and friendship between the kindred nations, and an instrument destined by divine providence to diffuse religion, civilization, liberty and law throughout the world. in this view will not all nations of christendom spontaneously unite in the declaration that it shall be forever neutral, and that its communications shall be held sacred in passing to their places of destination, even in the midst of hostilities?" the historian of the enterprise was mr. john mullaly, of new york, who was on the _niagara_ as secretary to prof. morse and subsequently to mr. cyrus w. field and correspondent of the _new york herald_. he has published three interesting works on the subject: a _trip to newfoundland, with an account of the laying of the submarine cable_ (between port au basque and north sydney), ; _the ocean telegraph_, ; and _the first atlantic telegraph cable_, a pamphlet of pages, reprinted from the "journal of the franklin institute," . from it, we learn that archbishop hughes was one of the principal american subscribers to the capital of the atlantic cable company. when, in , the subject of laying a cable under the atlantic ocean began to be seriously considered, thomson, who was then only years of age, discussed in a series of masterly papers the theory of signaling through such conductors, showing _inter alia_ that the instruments used on land-lines would be inoperative on cables, and also that the same speed of transmission could not be attained on cables as on ordinary telegraph lines. it was shown at the same time, that these differences are due to the fact that, unlike an air-line, the cable is an electrical _condenser_ in which the copper core is separated from the waters of the ocean by a layer of gutta percha, a nonconducting material. as a submerged cable is, therefore, a long leyden jar of great electrical capacity, it follows that a signal sent in at the american end will not reach the other instantly; for while the current flows along the conductor, it has also to charge up the cable as it progresses, which operation retards the signals, and also deprives them of the clearness and sharpness with which they were sent. the phenomenon is analogous to the diffusion of heat along a bar, the temperature of the various cross-sections rising in gradual succession until the distant end is reached. the mathematical investigations of thomson showed the necessity of working slowly, and of using weak currents as well as very delicate receiving instruments. the interval of time required for the transmission of a signal from newfoundland to valentia is about one second. some years later, in , thomson had the opportunity of putting his theoretical views to the test of experiment on a grand, commercial scale, and had the satisfaction of finding that all his conclusions were confirmed. electricians of the early period distrusted the inexperienced young man who had never erected a mile of telegraph line or even served for a month in a telegraph office; but their distrust was followed by admiration when they saw the efficient manner in which he handled every problem and dealt with every difficulty that occurred while laying the cable of . it was generally admitted that, had it not been for the brilliant work of the young glasgow professor, many years would have passed away before the old world and the new would have been brought into telegraphic communication. like all interested in the enterprise, thomson was greatly shocked when the news reached him that signals could no longer be transmitted through the cable, which, after costing so much money, so much thought and labor, now lay a useless thing in two and a half miles of water. attempts were made to raise it, but without success. during its short life of less than a month, messages were flashed through the cable, aggregating words of , letters. the failure of the pioneer cable has been attributed to a variety of causes, chief of which were defective construction and imperfect paying-out machinery, which produced unequal strains in the cable. defective as the cable was at the moment of immersion, the various troubles became intensified with time, until at last, when provoked by the feebleness of the signals, the injudicious electrician at valentia had recourse to the great penetrative power of the induction coil, and gave the dying cable the _coup de grâce_. an experiment made by mr. latimer clark is not only germane to the subject, but is also of very great interest. writing from valentia on sept. th, , mr. latimer clark says: "with a single galvanic cell, composed of a few drops of acid in a _silver thimble_[ ] and a fragment of zinc, weighing a grain or two, conversation may easily, though slowly, be carried on through one of the cables ( , ) or through the two joined together at newfoundland; and although in the latter case, the spark, twice traversing the breadth of the atlantic, has to pass through miles of cable, its effects at the receiving end are visible in the galvanometer in a little more than a second after contact is made with the battery. the deflections are not of a dubious character, but full and long, the spot of light traversing freely a space of in. or in. on the scale; and it is manifest that a battery many times smaller would suffice to produce similar effects." not to be outdone by the english electrician, mr. william dickerson devised the gun-cap cell, which he used in with success in transmitting signals from heart's content, newfoundland, to valentia on the irish coast. a piece of no. bare copper wire was procured, one end of which was firmly twisted around the head of an empty _percussion-cap_. to one end of another similar length of wire was bound, with fine copper wire, a short strip of zinc bent at a right angle to form the anode element of the diminutive cell. after charging the cell with a drop of acidulated water of the size of an ordinary well-formed tear, and properly connecting the terminals with earth and cable, signals were transmitted over the cable by the infinitesimal current generated by this novel cell. the receiving operator reported that the signals were "awfully small"; but they were intelligible, and messages were successfully transmitted under the ocean by this tiny element. contrast with this lilliputian cell the enormous power that was used on the cable of toward the end of its short existence, when batteries of and daniell cells were employed to force signals across. when, in , it was decided to make another attempt at laying a cable under the atlantic, prof. thomson, whose reputation was enhanced during the seven intervening years by a number of communications on the theory and practice of submarine telegraphy, was again retained as scientific expert in a consultative sense, with mr. cromwell f. varley as chief electrician. in accordance with the costly experience that had been gained, a new cable was made and coiled on board the _great eastern_,[ ] a leviathan which was well fitted for the work by the great man[oe]uvring power afforded by its screw and paddles combined. leaving valentia, the big ship steamed with her prow to the west at a slow rate of speed, in order to give the cable time to sink beneath the waves and adapt itself to the configuration of the ocean floor. eleven hundred miles had been successfully paid out when, to the consternation of all, the cable suddenly snapped and disappeared in more than two miles of water. attempts were made during the next nine days to recover it from those abysmal depths; and, though grappled many times during those trying hours, it gave way each time under the strain to which it was subjected. like its predecessors of and , the cable of was finally abandoned to its fate, and the _great eastern_ returned home with three greatly disappointed men on board, _viz._, prof. thomson, mr. c. f. varley and captain (later sir james) anderson. in the following year, a sum of three-quarters of a million sterling, nearly $ , , , was offered to the directors of the "telegraph construction company" if they would complete the cable of and lay a new one. after consultation and careful consideration, the offer was accepted and the cable constructed according to the best engineering knowledge available. in , prof. thomson was again on board the _great eastern_ with captain anderson; and this time the big ship had snugly coiled up in her deep, cavernous tanks _the_ cable that was destined to put europe and america in permanent telegraphic communication. with a well-manufactured cable, improved paying-out machinery and an experienced staff of mechanical engineers, not to mention the foremost electricians of the day, the immersion of the cable was successfully effected, after which the american end of the cable of was raised, a new length spliced on, and the shore-end safely landed in trinity bay. europe and america were thus united together by two electric bonds. it may here be mentioned that ocean cables are usually made in three sections, called, respectively, the shore-end, the intermediate section and the deep-sea section. it is clear that the submerged conductor needs the greatest protection in the shallow water that surrounds the coast, where it lies on a pebbly or rocky bottom, exposed to the drifting action of currents and tides, as well as to the haling flukes of the anchors of storm-tossed ships. in deep water, on the other hand, there is neither shingly bottom nor violent movement to displace and abrade the cable; for all is quiet and peaceful in the profound depths where the god of the trident holds his court; and hence few coverings and a light armor afford sufficient protection. the wear and tear in the ocean depths is a vanishing quantity when compared with the abrasive effects near coast-lines. looking at the sections of an ocean cable, the biggest and heaviest is the shore-end, while the thinnest and lightest is that which goes down into the depths of the sea. the lengths of the various sections are determined by the survey of the route, which is always carefully made before completing the specification of the cable. moreover, as the position of the cable-ship at noon every day is known from its longitude and latitude, it follows that the location of the cable on the bed of the ocean is also exactly known. when a cable is broken either by an upheaval or by a subsidence of the ocean floor, the distance of the rupture from the shore end is determined by an electrical test, after which a repair-ship is dispatched to the spot, when the cable is lifted, the "fault" cut away, a new length spliced on, and the amended cable allowed to settle down into its watery depths. at the present time (july, ), there are sixteen cables carrying the work of the north atlantic, at an average speed of words a minute duplex, or words a minute, counting both directions. this cable narrative affords as striking an illustration of the _triumph of failure_ as any recorded in the history of human enterprise. it was a victory of mind over matter; of character and tactfulness, energy and endurance over difficulties of every kind, moral and financial, mechanical and meteorological. the four expeditions of , , and represent years of hard work, anxiety and distressing failures; but, sustained by the patience of hope and by an unshaken confidence in the soundness of the enterprise as well as in the ability of their staff, the directors of the atlantic company were well rewarded for the disappointment occasioned and the monetary losses incurred. "it has been a long struggle," said the initial promoter of the enterprise, mr. cyrus w. field, speaking at a banquet given in his honor on november th, , at the metropolitan hotel, new york, "a long struggle of nearly thirteen years of anxious watching and ceaseless toil. often my heart was ready to sink. many times, when wandering in the forests of newfoundland in pelting rain, or on the decks of ships in dark, stormy nights, i almost accused myself of madness and folly to sacrifice the peace of my family for what might have proved but a dream. i have seen my companions, one after another, fall by my side, and i feared that i, too, might not live to see the end. and yet one hope has led me on; i prayed that i might not taste of death till the work was accomplished. that prayer has been answered; and now, beyond all acknowledgments to men, is the feeling of gratitude to almighty god." it was men like field and thomson that the poet had in mind when he wrote: the wise and active conquer difficulties by daring to attempt them. sloth and folly shiver and shrink at sight of toil and labor, and make the impossibility they fear. shortly after his return home, prof. thomson was knighted for his splendid services in connection with sub-oceanic cables, and was also honored with the freedom of the city of glasgow. if while journeying over land or sea, sir william's mind was always active, his eyes were also open and observant. in the numerous voyages which he undertook in the interest of cable companies, he seems to have been struck by the unreliable character of the ordinary apparatus used in taking soundings, consisting of a heavy weight suspended by a thick hempen cord unwound from a reel. owing to the massiveness of the cord, the motion of the ship and currents in the water would necessarily deflect it from the vertical, so that the soundings recorded would be in excess of the true depth. to remedy this defect, thomson replaced the rope, at first by a steel wire, and later by a thin strand of steel wires, on which the speed of the ship has but little effect; the sinker descends vertically with considerable velocity, and is raised with equal rapidity by suitable winding-up machinery placed in the stern of the ship. the sinker carries a gauge consisting of a quill-tube open at the lower end and closed at the top. the inside, which is coated with silver chromate, shows by the discoloration produced by the action of the sea water how far the water has compressed the air in the tube. by comparison with a graduated ruler, the depth is then read off. when the sinker reaches bottom, the heavy weight is detached automatically, so that there is but little strain on the wire as it ascends with its thermometer and battery of tubes containing samples of the depths reached. a story is told in connection with this sounding-machine which shows the vivacity and wit of the inventor. having brought his friend joule into white's one day, he pointed to a number of coils of steel wire lying on the floor, informing his english friend of "mechanical-equivalent" fame at the same time that he intended the wire for sounding purposes. upon joule's innocently asking what note it would sound, he received the prompt answer, "the deep sea"! another subject to which sir william gave some attention after his experiences on the ocean is the navigating compass. his observations led him to distrust the long, heavy needles then in general use on shipboard. besides the friction to which the pressure on the pivot gives rise and which necessarily diminishes the sensitiveness of the needle, there was another objection, due to the difficulty experienced in successfully applying steel magnets and soft-iron masses to compensate for the magnetism of the ship and for the changes induced in it by change of place in the earth's magnetic field. as a result, prof. thomson devised a compass-card which is remarkable for its lightness and sensitiveness. it is made of two sets of magnets, containing four needles each, arranged symmetrically on the right and left of the pivot. the four needles, forming a set, are of unequal length, ranging from - / to inches, with the shortest outermost. such a card, with its associated correctors of steel magnets and soft-iron balls, has added greatly to the safety and certainty of navigation; and as such, it is used to-day in the merchant service and in the navies of most countries of the world. as we have seen, thomson had the keen, racy wit of his race. lecturing before the members of the birmingham and midland institute in , he placed himself and his nationality on record in a very humorous way. his subject was "the six gateways of knowledge." as will be remembered by the readers of _the pilgrim's progress_, old bunyan likened the soul to a citadel on a hill having no means of communication with the outer world save by live gates, _viz._, the eye gate, the ear gate, the mouth gate, the nose gate and the feel gate. these are the five senses by which we obtain our knowledge of the material world which surrounds us. but prof. thomson took issue with bunyan, with reid, and the metaphysicians of all time in maintaining in this lecture that we have six gateways of knowledge instead of five, justifying the position which he took by affirming that the sense of touch is really twofold, one of heat and the other of force. it does not appear, however, that he made any marked impression on the philosophic thought of the day, for psychologists continued to write with undisturbed equanimity of the five senses and not the six. it was on this occasion that prof. thomson said: "the only census of the senses, so far as i am aware, that ever before made them more than five was the irishman's reckoning of seven senses. i presume the irishman's seventh sense was common sense; and i believe that the possession of that virtue by my countrymen, _i speak as an irishman_, i say the large possession of the seventh sense which i believe irishmen have, will do more to alleviate the woes of ireland than the removal of 'the melancholy ocean' which surrounds its shores." for the successful operation of cables, telegraph lines and scientific investigations of all sorts, a system of practical electrical units, accepted by all companies and countries of the world, was soon found to be indispensable. the pioneer in the movement for establishing an international system of electrical standards was mr. j. latimer clark, who, assisted by his distinguished partner, (sir) charles bright, prepared a paper on "the formation of standards of electrical quantity and resistance," which was read at the manchester meeting of the british association in . prof. thomson was present; and, at his instance, a committee was appointed to report on the general question of electrical units. this was the first meeting of a committee that was destined to accomplish much in the electric and electromagnetic field; it was the initial impulse of a movement that brought renown to the entire body of english electricians. such units as the ohm, the volt and the farad met with immediate acceptance, while later on the ampere, the coulomb, the watt and the joule were introduced. among the members of this body besides prof. thomson, were such able men as clerk maxwell, joule, lord rayleigh, sir william siemens, johnstone stoney, balfour stewart, and carey foster. the world is then indebted to the insistence and advocacy of prof. thomson for the general acceptance of the "c.g.s." system of measurement, which involves the centimeter (length), the gram (mass), and the second (time) as the fundamental units from which all others are derived. prof. thomson has claims in the "wireless" field also; for as far back as , he studied the nature of the discharge of a condenser and proved mathematically that, under certain conditions easily realized in practice, such discharges are of an oscillatory character, consisting of a forward and a backward rush of electricity between the two coatings of the condenser. as pointed out on page , prof. henry had reached the same conclusion in , and helmholtz in ; but thomson's insight into the phenomenon is keen and his mathematical analysis of it very remarkable. just as the to-and-fro motions of the prongs of a tuning-fork give rise to sound-waves in the air, so the electric oscillation due to a condenser discharge sets up in the universal ether electric waves which flash the news of the world over continents and oceans with unthinkable velocity. by special request, sir william thomson gave, in , a course of lectures at the johns hopkins university, baltimore, to an audience of "professional fellow-students in physical science," as he called the _élite_ of american men of science, twenty-one in number, assembled to hear him. these accomplished physicists he also affectionately called his "twenty-one coefficients." the subject was the wave-theory of light, and the object of the lecturer was to show how far the phenomena of light, such as its transmission, refraction and dispersion, could be explained within the limits of the elastic solid theory of the ether, which makes that hypothetical medium rigid, highly elastic and non-gravitational. from the very first lecture, sir william assumed a cold and diffident attitude toward the rival theory of clerk maxwell, which makes light an electromagnetic phenomenon; and though his own presented formidable difficulties, and its rival was universally accepted, the veteran professor assured his hearers that the elastic solid theory is the "only tenable foundation for the wave-theory of light in the present ( ) state of our knowledge." despite the energy which he displayed, his luminous argumentation and close logic, kelvin made no converts among his "twenty-one coefficients"; and it soon became evident that he was championing a lost cause. newton did the same when he held tenaciously to the corpuscular theory of light; and in doing so, let it be said, that he retarded the acceptance of the wave-theory and the advance of science by a hundred years. a few years after the baltimore lectures, official recognition of his distinguished services and of his eminence in science came to sir william thomson when, in , he was raised to the peerage, with the title of baron kelvin of netherhall, kelvin being the name of a stream which passes near the buildings of the university of glasgow and flows into the clyde, while netherhall is that of his country-seat at largs, in ayrshire, miles from glasgow. as to the structure of matter, kelvin lived to see the "atom" of his youth and mature years shattered into fragments, and the atomic theory of matter rapidly yielding to the electronic. though he maintained an open mind toward the new school of physics, he was reserved and conservative toward the revolutionary doctrine of extreme radio-activists. he did not believe in the transformation of one elementary form of matter into another; and he strenuously combated the theory of the spontaneous disintegration of the atom. notwithstanding a long life devoted to the study of mathematical and experimental physics, during which kelvin unraveled many a difficult problem in electricity and magnetism and added many a beautiful skein to the texture of our knowledge in electrostatics and electrokinetics, that illustrious man, the acknowledged leader in physical science, made a public admission in which caused a great stir throughout the scientific world. it was on the occasion of the celebration of the golden jubilee of his professorship of natural philosophy in the university of glasgow. delegates had come from all parts of the world; kings and princes had sent their representatives; universities and learned societies of every country of the old world and the new vied with one another in doing honor to the scientist who had figured so long and so conspicuously in the advances of the age. it was on that solemn occasion and in presence of such a notable assembly that kelvin made the astonishing admission that, although he had been a diligent student of electricity and magnetism for a period exceeding fifty years, and although he had pondered every day for forty years over the nature of the ether and the constitution of matter, he knew no more about their essence, about what they really are, than he knew at the beginning of his professional work. this confession, remarkable by reason of the man who made it and the circumstances in which it was made, has always appeared to the writer of these lines as having more of the ring of disappointment in it than of blank failure. kelvin's great analytical mind early and persistently strove to penetrate the closely guarded secrets of nature; and because dame nature did not yield to his open sesame, but persisted in her reticence, the philosopher grew pessimistic and disappointed; and, under the sway of such feelings, he summed up the result of his life-quest after the ultimate problems in science and pronounced it a "failure." a "failure" it was not, if science is the discovery and registration of the laws of god as revealed in the universe of mind and matter; for few men of his generation, if any, made more contributions of the first order to the theory of electrostatics, to the doctrine of energy, to hydrodynamics and the thermo-electric properties of matter. this note of disappointment, or wail of despondency, had been sounded before by faraday, who said that, the more he studied electrical phenomena, the less he seemed to know about electricity itself. was not laplace animated by a kindred feeling when he spoke about the infinitude of our ignorance? lastly, was not this intense feeling of our limited powers precisely that which, after all his discoveries in mathematics, in optics and in celestial mechanics, made newton compare himself to a child standing on the beach with the vast ocean of truth before him, unfathomed and unexplored? kelvin gave a beautiful example to the world when, after resigning the chair which he had occupied for fifty-five years in the university of glasgow, he immediately proceeded to enter his name on the undergraduate list, intimating by such an act that, whether a man is a professor-in-ordinary of natural philosophy or a professor emeritus, he must ever be a student, in close touch with nature. lord kelvin had the happiness of enjoying good health throughout all the years of his long career, a happiness due in part to nature, and in part also to the simplicity, frugality and regularity of his life. as already said, he was fond of cruising in european waters in his yacht _lalla rookh_ during the summer months, and even venturing out on the atlantic as far as madeira, for, he loved the sea, and what is more, he loved it best when far from shore. in later years, however, owing to facial neuralgia, he was accustomed to spend a month or so every summer with lady kelvin at aix-les-bains, from which visits he always derived much benefit. while making some experiments in a corridor of his beautiful home at netherhall, he caught a chill on november d, , from which he never rallied, despite the cares and attentions that were fondly lavished upon him. the bulletins that were issued concerning his condition were read all the world over with more concern than if they referred to a reigning sovereign or an heir apparent. every teacher of physics, mathematical or experimental; every man interested in the advance of science and the spread of knowledge, anxiously awaited news from the sick-room of the illustrious patient--news that was transmitted to the ends of the earth by the siphon-recorder invented by the dying scientist in the heyday of his life; and when the word came that kelvin had breathed his last, that cablegram brought universal sorrow for the quenching of the brightest light of the age and the loss of the leading scientist, the model man and faithful christian. it was in the fitness of things that the man who was considered the greatest since newton should be buried in westminster abbey, and that the mortal remains of lord kelvin should find a resting-place next to the grave of the genius who thought out the _principia_ and discovered the gravitational law which governs the planetary as well as the stellar universe. if asked to say what impressed me most in lord kelvin, i would mention the cordial manner in which he welcomed those who sought advice; the encouragement which he held out to students; his absolute devotion to truth; his fair-mindedness and candor; his reverence in dealing with the problems of the soul and the destiny of man; and the uniform, tranquil happiness of his life, due, under god, to his profound religious belief and noble christian life. a man of strong convictions, kelvin did not, however, wear his religion on his sleeve, but treasured it in the depths of his heart, where it was never disturbed by the tossing and ever-changing wave-forms of individual opinion. he quietly but uniformly maintained that physical science demands the existence and action of creative power; and he did not shrink from affirming this conviction whenever circumstances seemed to require it, as was the case on the memorable occasion of his presidential address to the members of the british association in . in concluding that brilliant discourse, he said: "but strong, overpowering proofs of intelligent and benevolent design lie all around us; and if ever perplexities, whether metaphysical or scientific, turn us away from them for a time, they come back upon us with irresistible force, showing to us, through nature, the influence of free will, and teaching us that all living beings depend on one ever-acting creator and ruler." once when particularly disgusted with the materialistic views of those who, while denying the existence of a creator, attributed the wonders of nature, animate and inanimate, to the potency of a fortuitous concourse of atoms, he wrote to liebig, asking him if a leaf or a flower could be formed or even made grow by chemical forces, to which he received the significant reply from the famous chemist of giessen: "i would more readily believe that a book on chemistry or on botany could grow out of dead matter by chemical processes." we have already referred to the custom which obtained in the university of glasgow, of beginning the daily sessions by invoking the blessing of heaven on the work about to be undertaken. having liberty in the matter of choice, prof. thomson selected for this purpose a prayer from the morning service of the church of england, which reads: "o lord, our heavenly father, almighty and everlasting god, who hast safely brought us to the beginning of this day; defend us in the same with thy mighty power; and grant that this day we fall into no sin, neither run into any kind of danger; but that all our doings may be ordered by thy governance, to do always what is righteous in thy sight; through jesus christ, our lord, amen." academical honors were showered upon lord kelvin by seats of learning, ancient and modern; he was a d. c. l. oxford, ll. d. cambridge, and a d. sc. london; he was president of the royal society from to ; president of the british association in ; knight of the prussian order _pour le mérite_, and foreign associate of the _institut de france_. his published works include a "treatise on natural philosophy," vols., written in collaboration with prof. tait, of edinburgh (the two authors were often referred to as t and t'); "contributions to electrostatics and magnetism"; "collected mathematical and physical papers," vols.; "popular lectures and addresses," vols.; and the "baltimore lectures." these, as well as the instruments which he devised for navigation, for the finest work of the laboratory, as well as for the commercial measurement of current, potential, and energy, form a monument to lord kelvin that will be _aere perennius_. brother potamian. footnotes: [ ] water was decomposed in by van troostwijk and cuthberson, by means of sparks from an electrical machine. prof. ostwald considers this the first instance of the decomposition of a chemical compound by electricity. [ ] the thimble was borrowed from miss fitzgerald, daughter of the knight of kerry, who was living at valentia. [ ] broken up a few years ago for scrap iron. index. a abbey, westminster, academical honors, academy of science, royal, action at a distance, adams prize, addison, , advancement of learning, affinity, agonic line, , akenside, albert the great, albertus magnus, alibert, aldin, , alfonso el sabio, almanack, poor richard's, ampère, jean jacques, , , , amperean currents, amundsen, , anaesthesia, anaxagoras, anatomy, comparative, ancients in the exact sciences, anderson, anelectrica, animal electricity, , , , , annus mirabilis, apollonius, arago, , , archimedes, , , , , ; burning mirror, architecture, architectonics of metaphysics, aristarchus of samos, aristotle, arsinoe, queen, aspects of pain, assisi, poor little man of, atheism, atlantic telegraph co., atoms, attraction and repulsion, auenbrugger, autobiography of franklin, ayrton, b bacon, chancellor, bacon, roger, , , balance, electric, balancing of energies, baltimore lecture, barlowe, wm., , , barometer, barrett, father, bassi, laura, battery, voltaic, bauernfeind, baxter, richard, bear, little, bede, beet sugar, bembo, cardinal, bence jones, bernoulli, , , bernoulli, daniel, bernoulli, johann, bertelli, bertholinus, berthollet, beuve, saint, bevis, biot, , , birds' ears; kidneys; semi-circular canals, boethius, bolivar, bond, bose, boyle, , brewster, sir david, briggs, bright, sir charles, brook taylor, browne, sir thomas, brugnatelli, prof., brunetto latini, buffon, , , bunyan, burning mirror, byron, c cabanis, cabeo, , , cable, submarine, ; telegraph, cabot, sebastian, calculus of variations, canada balsam, canals, semi-circular, canton, carthesian ovals, carminate, prof., cascade, cassini, cavallo, cavendish, , , , ; laboratory, cayley, cell, gun-cap, charles, law of, , chelonian, complaisant, chemical manipulation, childe harold, christianity, chrystal, churchmen in science, cingari, circle, graduated, circuit, clark, latimer, , clausius, clergymen pioneers in electricity, clerk maxwell, , , clerk of penicuik, cluny, coffin of mahomet, coleridge, , collinson, peter, color vision, columbian line, columbus, , ; on electricity, como, college of, compass-card, ; variation of the, concentration, concourse of atoms, conference of st. vincent de paul, contributions to molecular physics, copernicus, copley medal, coulomb, , , ; character, ; memoirs, creator and ruler, creatures, crookes, , cumming, cunatus, current, oscillatory, curves, rolling, cuthberson, cuvier, , cynosure, d dante, darwin, , , davy, sir humphry, , davy, , d'alembert, d'alibard, , de causis et sedibus morborum, declination, de civitate dei, degrees and residence, de heer, dellman, de magnete, de mundo nostro, de mundo nostro sublunari, de natura rerum, de romas, de vi attractiva, de viribus electricitatis, development, process of, devotion, life of, dewar, dickerson, william, didactic lecture, digby, sir kenelen, dip-circle, discoveries by accident, ; in science, ; new, ; practical, disposer, great, divinia commedia, divisch, dobereiner's lamp, dryden, dubois, reymond, dufay, , dumas, dynamics of bodies, e earth's magnetism, earthquakes and electricity, earthquakes and magnetism, ear of the bird, elastic solids, electrica, electrical bumper, ; jack, ; pistol, ; treatment, ; tube, electricitatis, electric light, ; matter, ; motor, electricity, electro-dynamics, electromagnet, electro-magnetics, electro-magnetism, electro-magnetismos, electron, electronic theory, electrophorus, electroscope, epilepsy, epitaph of franklin, eratosthenes, ether, ; universal, euclid, eudiometer, eugénie, empress, euler, , , ewing, examination of conscience, existence, history of, ; of god, f failure, triumph of, , faith, confession of, faraday, , , , , , , ; eloquence, ; marriage, ; money making, ; notebooks, , ; parents, ; passing of, ; perseverance, ; poverty, ; statement of law, faraday-maxwell theory, father of mercies, father of pathology, fechner, , fénelon, fichte, field, cyrus w., , , field of force, filial tributes, flavio gioja, foster, carey, foucault, fourier, fowler, francis i., emperor, franklin, , ; and paine, franklinian rods, franz, father, freedom of the press, free will, , fresnel, , frog dancing master, fuller, fulminating pane, future, truth of, g galileo, , gateway of knowledge, galtoni, father, , galvani, , , ; anticipation of original experiment, ; madame, ; the physician, the teacher, ; wife, galvanometer, , garnett, gasser, gauss, , gay-lussac, gellibrand, , genius, precocious, geometry, ; and intellectual culture, , gilbert, , , , giliani, alessandra, gioja, gladstone, , glass harmonica, god disposes, goethe, graduation, early, graft, graham, gray, stephen, ; prof. andrew, great eastern, green, gregory, grind, guericke, otto von, guyot de provins, gymnotus electricus, gyrostat, h hakewill, hallam, hamilton, handy-men, hansteen, happy in life, hartmann, , harvey, , hauksbee, haüy, abbé, headaches, helmholtz, , , heis, henry, , ; joseph, , , , herapath, herschel, sir john, hertzian waves, hippocrates, hobby, holmes, oliver wendell, homer, horace, , hottentots, hunter, john, huyghens, hymn to mont blanc, i identity of lightning and electricity, il mago benefico, imitation of christ, inclination, induction, ; theory of, ; sparks, institute of france, interference, phenomena of, iron filings, ; raspings of, isidore of seville, isomagnetic lines, invisible things of god, izarn, j jacobi, jesuit gymnasium, johns hopkins university, joule, , k kant, , kelvin, lord, , , kepler, , kidneys of the bird, kinnersley, kircher, , , kite incident, ; lightning, klaproth, , kleist, dean von, klopstock, kneller, father, knowledge, subjective and objective, koerner, , kohlrausch, l laboratory, first physical, laennec, lagrange, , lamont, larmor, dr. joseph, langberg, langsdorff, languages, special gift for, laplace, , learning, a little, lectures to the working people, leibnitz, lejeune dirichlet, lenz, lesage, lessing, leverrier, libri, light an electric phenomenon, ; 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principia, , nollet, abbé, , , , norman, , novum organum, o oersted, , , ; discusses evolution, ohm, martin, , ohm's law, , , ; of acoustics, ; goodness of heart, ohm's personal appearance, ; preface, olbers, opus majus, opus tertiam, orb of virtue, orchestrion, origin of species, ostwald, oval curves, ozanam, p paine, palladius, paralysis, paris, dr., parkinson, pascal, pasteur, , , pavia, university of, pellagra, pellico, sylvio, peregrinus, , , perry, prof. john, pfaff, philosopher of copenhagen, philosophia magnetica, philosophical society, philosophy, small draughts of, physics text-book, pierre le pélérin, pile, pivoted compass, plagiarism, planta, martin de, plato, , pliny, poet and scientist, poem, mathematical, poggendorff, , , pohl, poincaré, polaric, polarity, , polarization, polyhedrons, pope alexander vi., ; clement iv., ; paul iii., ; leo x., popularization of science, porta, positive, potential, potato, pouillet, power, feeble directive, preece, sir william, premonstratensian order, premonition, priestley, , , , , pringle, sir john, priority in discoveries, prometheus, modern, providence, ; particular, general, pseudodoxia epidemica, psychology, ptolemy, q quacks, quackery, r radowitz, general, rainbow, ramsay, sir wm., ramsden, rayleigh, lord, raymond lully, reid, religion, republic, cis-alpine, repulsion, magnetic, resurrection, retina, richet, richmann, righi, ritter, robespierre, roentgen, romagnosi, ronaldo, ross, sir james, rotch, rousseau, rowland, rush, benjamin, s sacchetti, samothracian rings, saturn's rings, scarpa, schelling, schiller, schlegel, schweigger, science and free will, ; and religion, ; classification of, ; experimental, ; high priest of, sebec, secular variation, semi-circular canals, senses, seven, series, seventh sense, shakespeare's cliff, siena, cathedral, siger, silurus electricus, siphon-recorder, skill, mechanical, smith's prize, , snell, sophocles, soundings of deep sea, sound, perception of, southey, spectator, spence, dr., sphere, electrified, spirit of mathematical analysis, squaring of the circle, saint aloysius, ; augustine, , ; francis, third order of, ; thomas, saint-hilaire, geoffroy, statics, stereoscope, real image, stethoscope, stevin, stewart, balfour, stimmen aus maria-laach, stokes, stoney, strada, strain in the ether, structure of physical bodies, stuber, dr., sturgeon, sugar from beet-root, sulzer, superfluous, elimination of, suspension of the earth, swammerdam, t taisnier, , tait, , , tampering with the lodestone, tandem, taprobane, tasso, , taylor's scientific memoirs, telephone, terrella, terrestrial magnetism, terror, reign of, test-nail method, text-books, maxwell's, thales, theory of induction, ; of the leyden-jar, ; two-fluid, thévenot, thimble-cell, thompson, james, ; silvanus p., , , , ; wm., thunderbolt, toaldo, padre, torpedo, torsion balance, , torque, tripos, truth of the future, twitchings of frogs, tycho brahé, tyndall, u uhland, , understanding and personal investigation, university degrees, unworkable, ; extension, uranus, v van helmont, van troostwijk, variation of the compass, vaults, the statics of, venedey, venturoli, verses, latin, virchow, virgil, virgilius, vitry, cardinal jacques de, volta, ; anticipation of, ; faith, ; honored, ; piety; ; pile, voltaic pile, voltaire, vortex, w wallace, , watson, , waves, hertzian, wealth, three ways to, weber, weight, accidental, ; and mass of the earth, , wenckebach, werner, wheatstone, wilson, dr. benjamin, wimshurst, windmills, winkelmann, winkler, works, sham, pilfered, distorted, ; under-water, worthies of england, y young, z zák, father alphons, fordham university press series makers of modern medicine--a series of biographies of the men to whom we owe the important advances in the development of modern medicine. by james j. walsh, m. d., ph. d., ll.d., dean and professor of the history of medicine at fordham university school of medicine, n. y. second edition, . pp. price, $ . net. _the london lancet_ said: "the list is well chosen, and we have to express gratitude for so convenient and agreeable a collection of biographies, for which we might otherwise have to search through many scattered books. the sketches are pleasantly written, interesting, and well adapted to convey the thoughtful members of our profession just the amount of historical knowledge that they would wish to obtain. we hope that the book will find many readers." _the new york times_: "the book is intended primarily for students of medicine, but laymen will find it not a little interesting." _il morgagni_ (italy): "professor walsh narrates important lives in modern medicine with an easy style that makes his book delightful reading. it certainly will give the young physician an excellent idea of who made our modern medicine." _the lamp_: "this exceptionally interesting book is from the practiced hand of dr. james j. walsh. it is a suggestive thought that each of the great specialists portrayed were god-fearing men, men of faith, far removed from the shallow materialism that frequently flaunts itself as inherently worthy of extra consideration for its own sake." _the church standard_ (_protestant episcopal_): "there is perhaps no profession in which the lives of its leaders would make more fascinating reading than that of medicine, and dr. walsh by his clever style and sympathetic treatment by no means mars the interest which we might thus expect." _the new york medical journal_: "we welcome works of this kind; they are evidence of the growth of culture within the medical profession, which betokens that the time has come when our teachers have the leisure to look backward to what has been accomplished." _science_: "the sketches are extremely entertaining and useful. perhaps the most striking thing is that everyone of the men described was of the catholic faith, and the dominant idea is that great scientific work is not incompatible with devout adherence to the tenets of the catholic religion." the popes and science--the story of the papal relations to science from the middle ages down to the nineteenth century. by james j. walsh, m. d., ph. d., ll.d. pp. price, $ . net. prof. pagel, professor of history at the university of berlin: "this book represents the most serious contribution to the history of medicine that has ever come out of america." sir clifford allbutt, regius professor of physic at the university of cambridge (england): "the book as a whole is a fair as well as a scholarly argument." _the evening post_ (new york) says: "however strong the reader's prejudice...he cannot lay down prof. walsh's volume without at least conceding that the author has driven his pen hard and deep into the 'academic superstition' about papal opposition to science." in a previous issue it had said: "we venture to prophesy that all who swear by dr. andrew d. white's history of the warfare of science with theology in christendom will find their hands full, if they attempt to answer dr. james j. walsh's the popes and science." _the literary digest_ said: "the book is well worth reading for its extensive learning and the vigor of its style." _the southern messenger_ says: "books like this make it clear that it is ignorance alone that makes people, even supposedly educated people, still cling to the old calumnies." _the nation_ (new york) says: "the learned fordham physician has at command an enormous mass of facts, and he orders them with logic, force and literary ease. prof. walsh convicts his opponents of hasty generalizing if not anti-clerical zeal." _the pittsburg post_ says: "with the fair attitude of mind and influenced only by the student's desire to procure knowledge, this book becomes at once something to fascinate. on every page authoritative facts confute the stereotyped statement of the purely theological publications." prof. welch, of johns hopkins, quoting martial, said: "it is pleasant indeed to drink at the living fountain-heads of knowledge after previously having had only the stagnant pools of second-hand authority." prof. piersol, professor of anatomy at the university of pennsylvania, said: "i have been reading the book with the keenest interest, for it indeed presents many subjects in what to me at least is a new light. every man of science looks to the beacon--truth--as his guiding mark, and every opportunity to replace even time-honored misconceptions by what is really the truth must be welcomed." _the independent_ (new york) said: "dr. walsh's books should be read in connection with attacks upon the popes in the matter of science by those who want to get both sides." other books by the same author fordham university press series makers of modern medicine (second thousand). lives of the dozen men to whom nineteenth century medical science owes most. cloth, octavo, pp., with portrait of pasteur. new york, : second edition, . $ . , net. the popes and science (second thousand). the history of the papal relations to science during the middle ages and down to our own time. new york, . $ . , net. old-time makers of medicine, in preparation. to be issued winter, . makers of astronomy, in preparation. the dolphin press series catholic churchmen in science (first series). lives of seven catholic ecclesiastics who were among the great founders of science. the dolphin press, philadelphia, . price, $ . , net. catholic churchmen in science (second series). lives of four great clerical founders in science and clerical pioneers in electricity and jesuit astronomers. the dolphin press, philadelphia, . price, $ . , net. * * * * * the thirteenth greatest of centuries (second edition, third thousand). the story of the rise of the universities, and of the origin of modern art, letters, science, liberty and democracy in a single century. catholic summer school press, new york, . $ . , net. in collaboration essays in pastoral medicine. o'malley and walsh. medical information for pastors, superiors and nurses, and applications of ethical principles for physicians, judges, lawyers, etc. longmans, green & co. (fourth thousand), new york, . $ . , net. transcriber's notes: page , "passings" changed to "passing" (...nails from passing ships and how wooden pegs were substituted for nails in vessels...) page , "conville" changed to "gonville" (gonville and caius, cambridge) page , added word "of" (...contribute effectively to the advancement of learning.) page , changed "philosphical" to "philosophical" (philosophical transactions) page , changed "formal" to "former" (...the muscle will recover the former motion...) page , changed "inadventently" to "inadvertently" (...miss meals, and inadvertently to put off...) page , changed "cicumstances" to "circumstances" (...that, under ordinary circumstances, all...) page , changed two cases of "pyschology" to "psychology" (...widely known as students of psychology, of whom...); (...great a passion for psychology, and...) page , changed "allegmeine" to "allgemeine" (...german biographer in the allgemeine deutsche...) page , changed "know" to "known" (...who had known him intimately:...) page , changed "galvonometer" to "galvanometer" (...machine deflected the needle of his galvanometer in the...) page , changed "abderdeen" to "aberdeen" (...physical sciences, at marischal college, aberdeen.) page , changed "realtive" to "relative" (...thoughts that occurred relative to problems...) page , changed "suface" to "surface" (...as we go below the earth's surface, he...) page , changed numerical order of index entry "henry; joseph" page , changed "keppler" to "kepler" page , added missing page reference " " to "mahomet; sarcophagus" page , changed "poggendorf" to "poggendorff" page , changed alphabetical order of "winkelmann" advertising material originally located at beginning of text moved to end. learn one thing every day july serial no. the mentor the weather by c. f. talman of the united states weather bureau department of science $ . per year fifteen cents a copy old probabilities shall tomorrow's weather be fair or foul? blow wind--blow moistly from the south, for i go afishing. "nay, good friend," exclaims the golfer, "the day must be dry and the wind in the west." the farmer moistens his finger and points it toward the sky. "rain, come, quickly, for my crops," is his prayer. but the maiden's voice is full of pleading: "let the sun shine tomorrow that my heart may be light on my wedding day." * * * * * and so, through the days and seasons, humanity with all its varied needs, turns anxiously, entreatingly to old probabilities. and how is it possible for him to satisfy the conflicting demand? he may, on the same day, please the farmer in the west, the fisherman in the south, the golfer in the northern hills, and the bride in the eastern town. but how can he suit them all in one locality on a single day? old probabilities is willing and he loves humanity, but his powers and privileges are limited. there are those who say that it is due to the kind endeavors of old probabilities to satisfy everybody that our weather has at times become so strangely mixed. * * * * * old probabilities is a gentle family name and came out of the affection of the people. the name was a matter of pleasantry. it was given to the chief of the united states weather bureau when the department was first established by congress, and its source lay in the phrase, "it is probable," with which all the weather predictions began. but old probabilities, genial prophet and lover of his fellow men, is passing away, for the officer who organized the weather bureau became in time displeased with the name and changed the form of the daily prediction so as to read, "the indications are." the phrase is formal and severe. there is naught but cold comfort in it. our hearts turn back fondly to old probabilities and his friendly assurance: "it is _probable_ that tomorrow will be fair." [illustration: chickamauga park, tenn., in an ice storm] the weather by charles fitzhugh talman _librarian of the u. s. weather bureau_ the mentor · department of science · july , _mentor gravures_ central office of the u. s. weather bureau, washington, d. c. a simple weather station a majestic cumulus cloud the observatory on monte rosa launching a meteorological kite the effects of snow and ice--the campus, princeton university [entered as second-class matter, march , , at the postoffice at new york, n. y., under the act of march , . copyright, , by the mentor association, inc.] it is easy to lay too much stress upon the unimportant aspects of weather. it furnishes a bit of conversation over the teacups; it accentuates the twinges of rheumatism; it spoils a holiday. all this, however, is mere byplay. the real work of the weather--the work that explains the existence of costly weather bureaus, such as the one upon which our government spends more than a million and a half dollars annually--is momentous beyond calculation. consider such facts and figures as these: the head of the british meteorological office recently declared that bad weather costs the farmers of the british isles about one hundred million dollars a year. in our own country it has been estimated that a difference of one inch in the rainfall occurring during july in six states means a difference of two hundred and fifty million dollars in the value of the corn (maize) crop. the world over, the damage wrought by hail-storms is said to average about two hundred million dollars a year. in the city of galveston a single hurricane once destroyed twenty million dollars' worth of property and six thousand human lives. thus we might proceed indefinitely. the fact is that man's welfare is conditioned to an enormous extent and in an endless variety of ways by the vicissitudes of the atmosphere; hence the study of weather--meteorology--is one of the most important of sciences. it is also one of the most strikingly neglected! at the office of the weather bureau in washington there is a meteorological library of some thirty-five thousand volumes. but meteorological libraries are rare; meteorological books are scarce in other libraries; and meteorologists are so uncommon that whoever declares himself one is likely to be asked, "what _is_ a meteorologist?" [illustration: stations of the united states weather bureau showing two extreme types: one, an office on the twenty-ninth floor of the whitehall building, new york city, with instruments installed on the roof; the other, an independent observatory building, with free exposure on all sides, at st. joseph, mo.] the "meteors" studied by the meteorologist are not shooting stars, but the phenomena of the atmosphere,--rain and snow, cloud and fog, wind and sunshine, and whatever else enters into the composition of weather and climate. the atmosphere the ocean of air in which human beings live, even as deep-sea fishes live at the bottom of the liquid ocean, is called the _atmosphere_. unlike the liquid ocean, it diminishes rapidly in density from the bottom upward. at an altitude of three and one-half miles it is only half as dense as at sea-level. this is higher than the highest permanent habitations of man. mountain-climbers and balloonists have attained greater altitudes; but above a level of about five miles the air is too greatly rarefied to support life. balloonists who ascend still higher must carry a supply of oxygen with them. a little above the ten-mile level the air is only one-eighth as dense as at sea-level. the atmosphere extends at least miles above the earth, at which height its density is computed to be only one two-millionth as great as at sea-level. the weather with which human beings are concerned may be said to extend upward seven or eight miles; _i.e._, to the level of the higher clouds. the layer of the atmosphere lying between sea-level and the upper cloud level has certain characteristics that distinguish it from the air above it, and is known as the _troposphere_. the heating of the atmosphere by the sun is the beginning of all weather, and the temperature of the air is the most important weather element. as soon as we begin to study atmospheric temperature, we encounter a paradox. the heat of the air is all derived from the sun (except a minute quantity from the interior of the earth, and an infinitesimal quantity from other heavenly bodies), and it would therefore seem at first glance that the upper layers of the atmosphere should be warmer than the lower. experience proves the reverse to be the case. a mountain overgrown with tropical vegetation on its lower slopes is, if high enough, crowned with eternal snows. a thermometer carried upward in the air shows under average conditions a fall of temperature of one degree (fahrenheit) for every feet of ascent. this fall of temperature with ascent continues to the upper limit of the troposphere, where the average temperature is something like degrees below zero. [illustration: the new idea in weather observatories the observatory of the ebro (spain), founded by spanish jesuits, is devoted to studying the interrelations of sun, earth and air. its admirable equipment includes apparatus for the direct and spectroscopic study of the sun, for measuring solar radiation, atmospheric electricity, earth currents, terrestrial magnetism, and earthquakes; besides the ordinary routine of a meteorological observatory. the results of all these observations are published side by side, to facilitate comparison.] above the troposphere is a region called the _stratosphere_, or _isothermal layer_, in which an ascending thermometer shows irregular and generally small changes of temperature--not infrequently a rise of temperature with ascent. the exploration of the stratosphere is one of the most fascinating fields of meteorological research, but lies somewhat beyond the scope of an essay on weather. it is carried out chiefly with the aid of small free balloons, some of which (sounding balloons) bear self-registering thermometers and other instruments, while others (pilot balloons) bear no instruments, but show by their movements the drift of the air currents. the greatest altitude ever attained by a sounding-balloon was . miles; by a pilot-balloon, . miles. the branch of meteorology dealing with the study of the upper air is called _aërology_. [illustration: a lonely outpost on the verge of the antarctic the argentine meteorological station in the south orkneys. once a year an expedition is sent from buenos aires to relieve the staff of four observers. this is the southernmost permanently inhabited spot on the globe; and it has not even wireless communication with the rest of the world.] reverting to the temperature of man's environment, the reason why the atmosphere is warmest at the bottom is this: the sun's rays come to us from outer space in the form of vibrations in the ether, and warm the air to only a slight extent in passing through it. they are absorbed by the ground, and converted into heat waves. the air is then warmed by contact with the warm ground. lastly, the warming of the lower air gives rise to air-currents, which distribute the heat through the atmosphere. barometric pressure if our weather were uniform, it would furnish little matter for conversation; in fact, would hardly be weather at all. changeableness is the salient feature of weather, and to understand weather changes one must know something about barometric pressure. like all other forms of matter, the invisible air has weight. at sea-level it exerts a downward pressure averaging . pounds to the square inch. atmospheric pressure is measured by means of an instrument called the _barometer_, in which the weight of the air is balanced against a column of mercury. as the height of the mercurial column varies with the pressure of the air, and is taken as the measure of the latter, we follow the practice of expressing pressure (a force) in linear units (inches or millimeters). this practice is retained even in the use of the aneroid barometer, which contains no mercurial column. hence, when we say that the average barometric pressure at sea-level is . "inches," we are really expressing in a roundabout way the weight of the air at that level. [illustration: how the camera analyzes lightning the same flashes photographed with (_a_) a stationary camera, and (_b_) a camera revolving on a vertical axis. one of the flashes is seen to have consisted of several successive discharges along an identical path courtesy of u. s. bureau of standards and popular science monthly.] barometric pressure not only varies somewhat regularly with altitude--diminishing as we ascend--but also less regularly from place to place in a horizontal direction, and from time to time at a given place. in studying the weather meteorologists frequently wish to compare the barometric pressures prevailing at a certain time at a number of places lying in the same horizontal plane. given a system of meteorological stations scattered over a certain territory, the first step is to secure simultaneous readings of the barometers at these stations. then, if the stations are at various altitudes, as they commonly are, corrections must be applied to the readings to reduce all to a common plane; the plane adopted for this purpose is sea-level. since most stations are _above_ sea-level, and since atmospheric pressure diminishes with altitude, reduction to sea-level generally involves applying an _additive_ correction. the weather map now please attend carefully to what follows; because i am going to attempt to put into a minimum number of words the essential facts concerning the _weather map_, the best clue to weather mysteries yet devised by man. at about stations of the weather bureau, distributed over the united states, the barometer and other meteorological instruments are read twice a day; viz., at a. m. and p. m., eastern standard time. the readings are promptly telegraphed in cipher to washington, where they are entered on a map. the barometer readings at the different stations, reduced to sea-level as just explained, will vary, say, from to inches. lines, called _isobars_, are now drawn through places having the same pressure; the intervals between the lines corresponding to differences in pressure of one-tenth of an inch. lines (_isotherms_) are also drawn to connect places having the same temperature, a little arrow at each station shows the direction of the wind at that point, and various other symbols are used to facilitate the interpretation of the map; but the isobars are more important than anything else. [illustration: cirro-stratus the appearance of this cloud precedes by a day or so the arrival of rainy and stormy weather] [illustration: alto-cumulus] [illustration: fair weather cumulus this cloud marks the summit of an ascending air current, and appears toward midday or early afternoon in the warm season. when the air rises powerfully to great heights, cumulus is built up in mountainous masses and may become cumulo-nimbus, the thundercloud.] here is the weather map for the morning of january , . the solid curved lines are isobars, representing barometric pressures ranging all the way from . to . inches. it will be seen at a glance that these lines tend to assume roughly circular forms, inclosing regions where the pressure is lower or higher than the average. moreover, the little arrows (which "fly with the wind") show that the winds round a center of low pressure tend to blow in a direction contrary to that followed by the hands of a clock (in the southern hemisphere the reverse is true), but instead of blowing in circles are inclined somewhat inward toward the center. round a center of high pressure (in the northern hemisphere) the typical circulation of the winds is exactly opposite ("clockwise," and inclined outward), though the accompanying map does not show this particularly well. [illustration: weather map january , ] an area of low pressure, with its system of winds, is called a _cyclone_, or _low_. an area of high pressure, with its system of winds, is called an _anticyclone_, or _high_. note that a cyclone is not necessarily a storm, though the one shown on this map, with its center not far from new york city, was a very violent storm, which, when this map was drawn, was sweeping up the atlantic coast. (popular usage applies the term "cyclone" to the tornado.) the strength of the winds in a cyclone depends upon the contrast in barometric pressure between its center and its outer border. a cyclone with crowded isobars always has strong winds; when the isobars are widely spaced the winds are gentle. [illustration: ascent of a sounding balloon the first made in the united states; at st. louis, mo., in ] these areas of low and high pressure, in addition to their movements about their centers, move bodily across the country, in a general west-to-east direction, at an average speed of over miles a day. this double movement may be compared to that of a carriage-wheel, rotating and advancing at the same time. most of our cyclones enter the country from the canadian north-west--though many come from other regions--and nearly all of them pass off to sea in the neighborhood of the gulf of st. lawrence. their route across the country varies greatly, depending in part upon the season. [illustration: the kite house at an aerological observatory some of the kites are much the worse for wear after flying in a storm] the weather in cyclones and anticyclones barometric pressure is not an element of weather, in the ordinary sense of the term, since the fluctuations of pressure that occur in the human environment are entirely inappreciable to the senses. we have seen, however, that pressure is intimately related to wind, which is a weather element of much importance. in noting that systems of high and low pressure are constantly traveling across the country, and that they are accompanied by winds having fairly definite characteristics in relation to each, we have taken an important step toward bringing order out of the (to the uninitiated) chaotic sequence of weather. obviously, a system of telegraphic weather reports makes it possible to keep close watch of these wind systems, and, from their locations on today's weather map, to form some idea where they will be tomorrow. thus the weather forecaster is enabled to give notice of the imminence of those violent winds that destroy life and property at sea, and, to a less extent, on land. there is an element of uncertainty in such predictions--since storms, unlike railway trains, are not confined to fixed routes and regular schedules--but the practised forecaster acquires an instinct that helps him to forestall their vagaries. [illustration: sending up a meteorological balloon on lake constance between switzerland and germany.] now what is true of wind is also true to a certain extent of the other elements of weather,--they bear typical relations to the distribution of atmospheric pressure. cyclones are usually preceded by rising temperature and accompanied by cloudiness and rain or snow; anticyclones are usually preceded by falling temperature and attended by fair weather. referring again to the map of january , , and following the course of the isotherms, or temperature lines, we see that abnormally cold weather prevailed over the middle western and southern states. the isotherm of zero dips far south across northern texas, arkansas, mississippi, alabama, and tennessee; while in the upper mississippi and missouri valleys the temperatures were from to degrees below zero. these regions were, in fact, in the grip of a severe "cold wave," which had entered the country a day or two before, preceding the anticyclone here seen central north of dakota. cold northwesterly winds were sweeping over the great plains, and as far south as the gulf. [illustration: hoarfrost minute crystals of ice deposited from the air. under a magnifying-glass they show a variety of beautiful forms] the same map shows typical weather accompanying the cyclone central on the atlantic coast. from the seaboard west to the mississippi valley rain or snow had fallen within the previous twenty-four hours (indicated by shading), and snow (indicated by s) was falling at the moment of observation at a majority of stations within this area. elsewhere in the same region the weather was cloudy. the foregoing remarks indicate in a general way the significance of the weather map and the principles upon which scientific weather predictions are based. the endless procession of highs and lows brings to any place on the map constant alternations of heat and cold, storm and sunshine. the forecaster watches the procession, and draws his inferences as to what will happen in this or that part of the country within the next day or two (forty-eight hours is about the limit of his outlook). "long-range" forecasting is still a thing of the remote future. forecasts for a week in advance, are, indeed made by the weather bureau with the aid of reports from a chain of stations extending round the globe, but these are in very general terms. [illustration: marvin rain and snow gage with trumpet-shaped wind-shield at top. in the middle is seen the cylindrical collector. this is removed and weighed with its contents to ascertain the amount of rain or snow that has fallen] in january, , the bureau began publishing a "daily weather map of the northern hemisphere." this publication is, at present, suspended on account of the war. some weather miscellanies it would require a book, rather than a brief essay, to describe all the vicissitudes of weather, and many books that attempt to do this have been written.[a] we have space here only to mention a few important features of the weather met with in our own country. [a] see "brief list of meteorological textbooks and reference books," d ed., by c. fitzhugh talman. for sale by the superintendent of documents, washington, d. c. price cents. the southern and southeastern part of a cyclone, some hundreds of miles from the center, is a favorite breeding-ground for _thunderstorms_ and _tornadoes_. thunderstorms of the type known as "heat thunderstorms" also occur with no special relation to cyclonic centers in regions where the ground has been intensely heated. in either case the storm is built up by rapidly ascending air, which cools and condenses its water vapor, first into enormous clouds (_cumulo-nimbus_, or "_thunderheads_"), and then into rain, frequently accompanied by hail. it would be necessary to go to some length to explain the familiar electrical manifestations of the thunderstorm--some points, indeed, are not perfectly clear to meteorologists--but it should be stated that these are always the result, not the cause, of the storm. _lightning_ is an electrical discharge between cloud and earth, or cloud and cloud, and _thunder_ is simply the violent soundwave set up by the sudden expansion of the heated air along the path of the discharge,--the same acoustic phenomenon that accompanies an ordinary explosion. [illustration: the effects of an ice storm at canton, n. y. march - , ] [illustration: summit hotel at summit, cal. on march , . a three-story building whose first story is buried under twenty-six feet of snow] a _tornado_ (popularly miscalled a "cyclone") is an extremely violent vortex in the air, usually less than , feet in diameter. besides its very rapid rotary motion, it has a progressive motion at a speed averaging forty or fifty miles an hour. its position at any moment is marked by a black funnel-shaped cloud, which grows downward from the sky and does not at all times reach the earth. a waterspout at sea is an identical phenomenon, though usually less violent. along its narrow path the tornado demolishes everything,--wooden houses are blown to splinters, trees uprooted or stripped of their branches, structures of heavy masonry laid in ruins. something like a hundred lives are lost each year in these storms, on an average, and one of them (st. louis, may , ) destroyed thirteen million dollars' worth of property. [illustration: courtesy of the scientific american a waterspout near beaufort, n. c., in august, ] [illustration: turpain's thunderstorm recorder or ceraunograph. this is one of several instruments designed to register the natural electric waves, or "strays," which sometimes interfere seriously with the transmission of wireless telegrams. strays are often generated by lightning discharges, near or distant, and this instrument therefore serves to give notice of an approaching thunderstorm] a _blizzard_ is a high, cold wind, accompanied by blinding snow, which in winter sometimes blows out of the front of an advancing anticyclone, especially in our north-central states. a similar wind, with or without snow, is called in texas a _norther_. a _chinook_ is a warm, dry wind that descends the eastern slope of the rocky mountains in montana, wyoming and colorado, and flows north-eastward over the plains. its effects are most pronounced in winter, when it brings about a very sudden rise in the temperature--in extreme cases as much as forty degrees in fifteen minutes! it causes snow to vanish as if by magic, and is appropriately nicknamed the "snow-eater." "_cloudburst_" is merely a picturesque name for a very heavy shower; usually a thunder-shower. [illustration: in the wake of a tornado the tornado destroyed a house and barn, but left a path in the center with practically no harm done] _west india hurricanes_ occasionally visit the united states, especially in the late summer and early autumn. these storms begin as violent cyclones of small extent ( to miles in diameter), usually somewhere east of the west indies, sweep in a long curve across the caribbean sea, and then turn north, either passing up along the atlantic coast or crossing the gulf of mexico into the southern united states. soon after entering the temperate zone they increase in size and diminish in violence, but are still vigorous enough on reaching the gulf or south atlantic coast to cause great devastation. low-lying shores are often inundated by the immense waves they generate. _cold waves_ are the rapid and severe falls in temperature that sometimes occur in winter, especially at the front of an anticyclone. warnings of these occurrences, issued by the weather bureau twenty-four to thirty-six hours in advance, often result in the saving of millions of dollars' worth of merchandise susceptible to damage by freezing. _frosts_ in the spring and autumn are also predicted with great success, to the immense advantage of farmers, market-gardeners, and horticulturists. the practice of smudging or heating orchards, now so widespread, is usually carried on under the advice of the weather bureau, which gives prompt notice to the orchardist when such precautions are in order. the bureau publishes charts showing the average and extreme dates of the last frost in spring and the first frost in autumn for all parts of the country. [illustration: looking down on a sea of fog from mt. tamalpais, california] a _fog_ is a cloud resting on the surface of the earth. in the united states fog is commonest along the northern and middle parts of the atlantic and pacific coasts. in the interior of the country, especially the western part, it is of rare occurrence, the average number of days a year with fog being less than ten. lastly--weather fallacies are rife. _indian summer_ is merely a type of mild, hazy, heavenly weather that prevails intermittently during our long american autumns. the _equinoctial storm_ is a myth; the climate has not "changed" anywhere within the span of a human lifetime (one year differs from another, but there is no progressive or permanent change); and the _moon_ has nothing whatever to do with the weather. supplementary reading climate and weather _by h. n. dickson_ american weather _by a. w. greely_ weather science _by r. g. k. lempfert_ some facts about the weather _by w. marriott_ second edition. meteorology _by w. i. milham_ the latest general textbook on the subject in english. forecasting weather _by w. n. shaw_ elementary meteorology _by f. waldo_ consult also the numerous publications of the united states weather bureau, which will be found in most public libraries. *** information concerning the above books and articles may be had on application to the editor of the mentor. the open letter "what is lightning and what causes it?" the question came to us a few days after we had made announcement of a "weather" number of the mentor. it was a natural question, for lightning is the most sensational of all weather phenomena. it has always had a fearful sort of fascination for humanity. to the ancients it came as a bolt of wrath from the hand of jove. to the fire-worshipers it was a warning message. to parched travelers it was a bright promise, for it heralded the coming of rain. to the superstitious it was a signal flash from the spirit world. and to those of nervous temperament it was a highly disturbing phenomenon producing emotions varying from uneasiness and alarm to hysteria. the question then, "what is lightning and what causes it?" has an interest for all. i referred it to mr. talman, the author of the mentor article on "the weather." his reply follows. * * * * * "not so many generations ago 'natural philosophers' thought that inflammable gases, exhaled from the earth, took fire spontaneously in the air, and that this was lightning. the idea also prevailed--and it is not yet quite extinct--that a stroke of lightning involved the hurling down from the sky of a mass of rock, called a 'thunderbolt.' in the eighteenth century people became quite familiar with the process of generating, by friction, a mysterious something called 'electricity,' which, when it passed from one body to another through a small layer of intervening air, produced sparks. several philosophers noticed the resemblance between these sparks and lightning. it remained, however, for benjamin franklin to prove that lightning was really an electrical discharge on a large scale. the experiments by which he proposed to demonstrate this were successfully performed, first by others, in france, and then, by franklin himself, at philadelphia. with the aid of his famous kite he drew down from a thundercloud a little of the 'electrical fluid' (as it was then called), and produced tiny sparks from an iron key at the lower end of the wet kite-string. "we do not even yet know what electricity is, but we know a great deal about the way it behaves and the effects it produces. there are two kinds of electricity, which we call _positive_ and _negative_. a body is said to be _charged_ when it has an excess of either kind, and the two kinds have a tendency to unite and neutralize each other's effects. thunderclouds become heavily charged with electricity. we are not quite sure how this happens, but it is now commonly believed that the strong uprising currents of air that occur in the storm, in the process of breaking up the water-drops in the cloud also separate positive from negative electricity; leaving the former in excess in the part of the cloud next to the earth, and carrying the latter far aloft. "by a process called 'induction' the positive charge in the cloud draws an excess of negative electricity to the surface of the ground underneath. the stronger the contrast between these opposite charges, the harder they try to break through the interposing barrier of the air (which is a poor conductor of electricity) and to neutralize each other. at length they succeed in doing so. a powerful stream of electricity flows for an instant between cloud and earth. its passage heats the air and makes it luminous--just as the passage of an electric current heats the filament of an electric lamp and makes it luminous. this is lightning. "these discharges occur not only between the clouds and the earth, but also, and probably more often, between clouds charged with opposite kinds of electricity. "the sudden expansion of the heated air along the path of the discharge affects our ears just as does the sudden expansion of the air at the mouth of a gun when it is fired. in each case a wave is sent through the air in all directions from the place of disturbance, and our ear-drums are set in vibration. that is thunder." * * * * * take courage then, you timid ones, who wince in the lightning's flash and tremble under the thunder's roll. thunder is simply a vibration of your ear drums--and, when you hear the thunder, be assured, all danger is over. [signature: w. d. moffat] editor the mentor association established for the development of a popular interest in art, literature, science, history, nature, and travel the plan of the association the purpose of the mentor association is to give its members, in an interesting and attractive way the information in various fields of knowledge which everybody wants and ought to have. the information is imparted by interesting reading matter, prepared under the direction of leading authorities, and by beautiful pictures, reproduced by the most highly perfected modern processes. the object of the mentor association is to enable people to acquire useful knowledge without effort, so that they may come easily and agreeably to know the world's great men and women, the great achievements and the permanently interesting things in art, literature, science, history, nature, and travel. the purpose of the association is carried out by means of simple readable text and beautiful illustrations in the 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john milton . joan of arc . furniture of the revolutionary period . the ring of the nibelung . the golden age of greece . chinese rugs . the war of . great galleries of the world: the national gallery, london . masters of the violin . american pioneer prose writers . old silver . shakespeare's country . historic gardens of new england numbers to follow july . american poets of the soil. _by burges johnson, associate professor of literature, vassar college._ august . argentina. _by e. m. newman, lecturer and traveler._ * * * * * [illustration: central office of the united states weather bureau, washington, d. c.] weather services at home and abroad monograph number one in the mentor reading course posted up in public offices, in hotel corridors, and other conspicuous places in our cities, the official weather map is a familiar sight. even more familiar is the official weather forecast, displayed, as a rule, on the first page of the daily newspaper, and sent broadcast over the country on the little brown cards which one may see in the village postoffice as well as in the city drug-store. when a great storm sweeps over land or sea, detailed official reports concerning its progress and characteristics are published in the daily press. when a lawsuit involves a dispute as to the temperature or the state of the sky on a certain day, the official weather records are consulted. how much do you know about the branch of the national government that is charged with the duty of keeping watch of the weather--recording its vagaries as they occur, and also predicting them, as far as is humanly possible? besides its office in washington, where more than two hundred persons are constantly employed, the weather bureau has about two hundred stations, manned by professional meteorologists and observers. one of these will be found in almost every large city, while some are in towns of very modest importance. a regular weather bureau station is well worth a visit. the instrumental equipment of these stations is almost superhuman in the accuracy with which it sets down on paper the chronicle of weather happenings from day to day and from moment to moment. little less marvelous is the system by means of which weather information--past, present and future--is disseminated from these official foci. the postoffice, the telephone, the telegraph (wire and wireless) are all pressed into service to the fullest extent--especially in giving timely notice of approaching storms and other destructive forms of weather. these agencies are supplemented by visible and audible signals, in the shape of flags, lanterns, railway whistles and so forth. contrary to popular belief, the weather bureau does not exist primarily for the purpose of telling the public (with a considerable margin of uncertainty) whether it will be advisable, on the morrow, to carry an umbrella or wear an overcoat. the important work of the bureau is twofold. it consists, first, in the prediction of those atmospheric visitations, such as storms, floods, and cold waves, which endanger life and property on a large scale; and, second, in the maintenance of the records that form the basis of climatic statistics. in both these directions the bureau splendidly justifies its existence. our national weather service was founded in , and for twenty years was maintained by the signal corps of the army. in it was established on the present basis, as the weather bureau of the department of agriculture. most civilized countries possess official services for the observation and prediction of weather, though no other is organized on quite so grandiose a scale as ours. the british meteorological office, the prussian meteorological institute, the central meteorological bureau of france, and the central physical observatory of petrograd are among the leading institutions of this character in the old world. admirable weather services also exist in india, japan, australia, canada, argentina and elsewhere. [illustration: a simple weather station] meteorological instruments monograph number two in the mentor reading course the history of meteorological instruments dates back at least as far as the fourth century before the christian era, when the depth of rainfall was measured in india by some form of gauge. we again hear of rain-gauges being used in palestine in the first century of the present era. thermometers with fixed scales were used in italy in the seventeenth century, and the great galileo, born in pisa in , took part in perfecting these instruments. wind-vanes were known to the ancients. the earliest one of which we have any record surmounted the famous tower of the winds at athens. in the middle ages the weathercock became the usual adornment of church steeples. the barometer was invented by torricelli in . most meteorological instruments, however, are of quite recent origin, and this is true especially of these types of apparatus that make automatic records, thus replacing, to a large extent, the human observer. our picture on the other side of this sheet shows the instruments used by the "co-operative" observers of the weather bureau. these observers, of whom there are about , , well distributed over the country, serve the government without pay, and their painstaking observations have alone made possible a detailed survey of our climate. in the picture we see, on the right, an ordinary rain-gauge, and, on the left, a thermometer-screen containing two thermometers; viz., a maximum thermometer, for recording the highest temperature of the day, and a minimum thermometer, for recording the lowest. the screen, which is of wood, painted white, serves to shield the instruments from the rays of the sun, while permitting free ventilation. under these conditions the thermometers show the temperature of the _air_; whereas when exposed to direct sunlight a thermometer shows the temperature acquired by the instrument itself, and this may differ materially from the air temperature. in contrast to this simple equipment, we find at a regular meteorological station, or observatory, an impressive collection of apparatus for observing and recording nearly all the elements of weather. the pressure of the air is measured by the mercurial barometer, and registered continuously by the barograph; the temperature of the air is automatically recorded by the thermograph. other self-registering instruments maintain continuous records of the force and direction of the wind, the amount and duration of rainfall, the duration of sunshine, the humidity of the air, etc. there are also instruments for measuring evaporation, the height and movement of clouds, the intensity of solar radiation, the elements of atmospheric electricity, and various other phenomena of the atmosphere. [illustration: a majestic cumulus cloud] clouds and rainfall monograph number three in the mentor reading course the international cloud classification, now generally used by meteorologists, is an amplification of one introduced by an ingenious english quaker, luke howard, in the year . howard distinguished seven types of cloud, to which he gave the latin names _cirrus_, _cumulus_, _stratus_, _cirro-cumulus_, _cirro-stratus_, _cumulo-stratus_, and _nimbus_. in passing, it may be of interest to note that, a few years after howard's classification was published, an attempt was made by one thomas forster to introduce "popular" equivalents of these terms. forster proposed to call cirrus "curlcloud," cumulus "stackencloud," stratus "fallcloud," etc. in other words, he assumed that because howard's names were latin in form they were unsuitable for use by the layman, and therefore needed to be supplemented by english names--although the proposed substitutes were, on the whole, somewhat longer and more difficult to pronounce than the originals! a parallel undertaking would be an attempt to discourage the public from calling the wind-flower "anemone," or virgin's bower "clematis." forster's superfluous names have never taken root in our language. the highest clouds--cirrus and cirro-stratus--are feathery in appearance, and consist of minute crystals of ice. their altitude above sea-level averages about five miles, but is frequently much greater than this. all other clouds are composed of little drops of water--not hollow vesicles of water, as was once supposed. neither crystals nor drops actually "float" in the air. they are constantly falling with respect to the air around them, though, as the air itself often has an upward movement, the cloud particles are not always falling with reference to the earth. in any case, their rate of fall depends upon their size, and in the case of the smaller particles is very slow. under some conditions the particles evaporate before reaching the earth, while under others they maintain a solid or liquid form and constitute rain or snow. a fog is a cloud lying at the earth's surface. rainfall is one of the most important elements of climate, chiefly because of its effects upon vegetation. it is measured in terms of the depth of water that would lie on the ground if none of it ran off, soaked in, or evaporated; and this is, in practice, determined by collecting the rain, as it falls, in a suitable receiver, or rain-gauge. usually the gauge is so shaped as to magnify the actual depth of rainfall, in order to facilitate measurement. snow is measured in two ways; first, as snow, and, second, in terms of its "water equivalent." the latter measurement is commonly effected by melting the snow and pouring it into the rain-gauge, where it is measured as rain. by this expedient we are enabled to combine measurements of rain and snow, in order to get the total "precipitation" of a place during a given period. nature is notoriously partial in her distribution of this valuable element over the earth. a region having an average annual rainfall of less than ten inches is normally a desert, though irrigation or "dry-farming" methods may enable its inhabitants to practice agriculture. the heaviest average annual rainfall in the united states (not including alaska) is about inches, in tillamook county, oregon. the rainiest meteorological station in the world is cherrapunji, india, with an average of about inches per annum.[b] [b] this is the latest official record. there are several rain-gauges at cherrapunji, and the average amount of rain collected by any one of them varies considerably with the length of the record. hence the widely divergent values of the rainfall at this famous station published in encyclopædias and other reference books. [illustration: the observatory on monte rosa] the outposts of meteorology monograph number four in the mentor reading course the expression used in our title seems a fitting one to apply to a number of meteorological observatories and stations maintained for the benefit of science in regions remote from the comforts and conveniences of civilization. some are on the summits of lofty mountains, the ascent of which is laborious and even perilous. others are situated in the bleak wildernesses of the circumpolar zones. public attention has all too rarely been called to the heroism and self-sacrifice of the men who constitute the staffs of these lonely outposts. the institution shown in our gravure--officially known, in honor of the dowager queen of italy, as the regina margherita observatory--crowns the summit of monte rosa, on the northern italian frontier, and is , feet above sea-level. it is devoted not only to meteorological investigations, but to studies of the physiological effects of great altitudes and various other researches, and is open to the _savants_ of all nationalities who are courageous enough to scale the second highest summit of the alps. it is habitable for only about two months; viz., from the middle of july to the middle of september. each year a temporary telephone line is constructed connecting the observatory with the plains of italy. this is the highest telephone line in the world, and its installation is an arduous undertaking. a permanent line is impossible, on account of the shifting of the glaciers and snowfields on which the poles must be erected. there is also a meteorological observatory on mont blanc, but it is not at the summit and is not quite so high as that on monte rosa. the solar observatory which once stood at the very top of mont blanc no longer exists. the united states signal service (now the weather bureau) formerly maintained observatories on pike's peak ( , feet) and mount washington ( , feet). the loftiest of meteorological stations was, however, that formerly operated by harvard college observatory on the summit of el misti, peru ( , feet). for a number of years the united states weather bureau maintained a large and important observatory at mount weather, at the crest of the blue ridge, near bluemont, virginia. in the old world one of the most famous of mountain meteorological observatories was that which stood on ben nevis ( , ), the highest summit in the british isles. this was closed in . if the conditions of life at these high-level stations are such as to repel any but the ardent lover of science, the same is true in even greater measure of those endured by the little band of meteorologists who man the observatory maintained by the government of argentina at laurie island, in the south orkneys, on the verge of the antarctic. every year a party of four is sent out from buenos aires to spend a year of exile in this inhospitable spot, which is generally ice-bound, and has not even wireless communication with the rest of the world. this station has been in operation since . the staff, which is changed each year, has embraced men of several nationalities--scotch, american and others. far within the arctic circle two meteorological observatories are maintained in spitsbergen; but these are, at least, connected with the world by radiotelegraphy. if the hopes of explorer peary are accomplished, an observatory will, one of these days, be established at the south pole. [illustration: launching a meteorological kite] the air above us monograph number five in the mentor reading course meteorologists are not content to limit their investigations to the stratum of air lying close to the earth's surface. even before the demands of the aeronaut for information concerning the structure and phenomena of the atmosphere far overhead became pressing, many efforts had been made to secure such information, in view of its important bearing upon many scientific problems. as long ago as the year a balloonist, equipped with various meteorological instruments, made an ascent from london and brought back an interesting series of observations, which were communicated to the royal society. for more than a century the manned balloon was the principal means of sounding the upper atmosphere. nowadays, as a rule, the meteorologist, instead of going aloft in person, sends up a kite or a balloon to which are attached automatically registering instruments. when the aerial vehicle returns to earth its record shows in detail the conditions encountered during the journey. everybody remembers how franklin brought lightning from the clouds; but it is a far cry from the simple apparatus that served franklin's purpose to the "box kite" of modern meteorology. science has perfected the kite almost beyond recognition. it has been shorn of that crucial feature of the schoolboy article, the tail. even the kite "string" has become several miles of steel piano wire, wound around the drum of a power-driven winch, with elaborate apparatus for recording the force of the pull, and the angles of azimuth and altitude. captive balloons are sometimes used for similar investigations. when, however, it is desired to attain great altitudes the meteorologist has recourse to the so-called "sounding-balloon," which is not tethered to the earth. this is usually made of india-rubber, and when launched is inflated to less than its full capacity. as it rises to regions of diminished air pressure it gradually expands, and finally bursts at an elevation approximately determined in advance. a linen cap, serving as a parachute, or sometimes an auxiliary balloon which does not burst, serves to waft the apparatus, with its delicate self-registering instruments, gently to the ground. this commonly happens many miles--sometimes two hundred or more--from the place of ascent. attached to the apparatus is a ticket offering the finder a reward for its return, and giving instructions as to packing and shipping. sooner or later it usually comes back; though often months after it falls. indeed, the large percentage of records recovered, even in sparsely settled countries, is not the least remarkable feature of this novel method of research. the instruments attached to sounding-balloons register the temperature of the air, the barometric pressure, and sometimes the humidity. by means of the sounding-balloon the air is explored to heights of twenty miles and more! the records obtained by means of these balloons have, within the past fifteen years, completely revolutionized our ideas concerning the upper atmosphere. still another device employed by meteorologists is the pilot-balloon. this is also a free balloon, but carries no meteorological instruments. its motion in the air is followed by means of a theodolite, and it serves to show the speed and direction of the wind at different levels. during the winter of - a pilot-balloon sent up from godhavn, greenland, by a danish exploring expedition reached the unprecedented altitude of more than miles. [illustration: the effects of snow and ice--the campus, princeton university] our winters monograph number six in the mentor reading course in the year thomas jefferson wrote in his "notes on virginia": "a change of climate is taking place very sensibly. *** snows are less frequent and less deep. they do not often lie below the mountains more than one, two or three days, and very rarely a week. the snows are remembered to have been formerly frequent, deep, and of long continuance. the elderly inform me that the earth used to be covered with snow about three months in every year." probably long before the white man came to america the patriarchs of the indian tribes regaled the young men and maidens gathered about the campfire with reminiscences of the deep snows that prevailed in a previous generation. in short the "old-fashioned winter" is a _perennial myth_, perpetuated by a familiar process of self-delusion! the occasional periods of abundant snow make a more lasting impression upon our minds than the long intervals in which this element was scarce or lacking. the resulting misconception is promptly dissipated when we consult the weather records, which, in some parts of the country, extend back more than a century, and prove that there has been no actual change in the climate within the period they embrace. of course the erroneous idea is, in some cases, due to the fact that one's childhood was spent in a part of the country in which the snowfall is normally heavier than in that where one has recently lived. the average yearly snowfall over the new england states, new york, and the borders of the great lakes is from to inches, and upward. over the north central states it is much less. in the southern tier of states and along almost the whole of our pacific coast snow is a rarity. the heaviest snowfall in this country probably occurs in the high sierra nevada of california, near the border of nevada. in some places in these mountains more than feet of snow falls in an average winter, while more than feet has been recorded in extreme cases. here it is a common occurrence for one-story houses to be buried, to the eaves, or above. the southern pacific railway, which intersects this region, has built miles of snowsheds, at a cost of $ , a mile over single track and $ , a mile over double track. in an average year $ , is spent on these sheds in upkeep and renewals. flat-roofed houses are unknown in this vicinity; all roofs are gabled at a sharp angle to shed the snow. a picturesque feature of our american winters is the "ice storm," so enthusiastically described by mark twain: "... when a leafless tree is clothed with ice from the bottom to the top--ice that is as bright and clear as crystal; when every bough and twig is strung with ice-beads, frozen dew-drops, and the whole tree sparkles cold and white, like the shah of persia's diamond plume." such is the artist's view of the phenomenon; but, alas! these same ice storms cause endless inconvenience and heavy expense every winter to the electrical industries, by breaking wires. prepared by the editorial staff of the mentor association illustration for the mentor, vol. , no. , serial no. copyright, , by the mentor association, inc. note: project gutenberg also has an html version of this file which includes the original illustrations. see -h.htm or -h.zip: (http://www.gutenberg.org/files/ / -h/ -h.htm) or (http://www.gutenberg.org/files/ / -h.zip) conversations on natural philosophy, in which the elements of that science are familiarly explained. _illustrated with plates._ by the author of conversations on chemistry, &c. with corrections, improvements, and considerable additions in the body of the work; _appropriate questions, and a glossary:_ by dr. thomas p. jones, professor of mechanics, in the franklin institute of the state of pennsylvania. philadelphia: published and sold by john grigg, no. north fourth street. stereotyped by l. johnson. . _eastern district of pennsylvania, to wit:_ be it remembered, that, on the twenty-fourth day of april, in the fiftieth year of the independence of the united states of america, a. d. , john grigg, of the said district, hath deposited in this office the title of a book, the right whereof he claims as proprietor, in the words following, to wit: "conversations on natural philosophy, in which the elements of that science are familiarly explained. illustrated with plates. by the author of conversations on chemistry, &c. with corrections, improvements, and considerable additions, in the body of the work; appropriate questions, and a glossary: by dr. thomas p. jones, professor of mechanics, in the franklin institute, of the state of pennsylvania." in conformity to the act of the congress of the united states, entitled "an act for the encouragement of learning, by securing the copies of maps, charts, and books, to the authors and proprietors of such copies, during the times therein mentioned;"--and also to the act, entitled, "an act supplementary to an act, entitled, 'an act for the encouragement of learning, by securing the copies of maps, charts, and books, to the authors and proprietors of such copies during the times therein mentioned,' and extending the benefits thereof to the arts of designing, engraving, and etching, historical and other prints." d. caldwell, clerk of the eastern district of pennsylvania. preface. notwithstanding the great number of books which are written, expressly for the use of schools, and which embrace every subject on which instruction is given, it is a lamentable fact, that the catalogue of those which are well adapted to the intended purpose, is a very short one. almost all of them have been written, either by those who are without experience as teachers, or by teachers, deficient in a competent knowledge of the subjects, on which they treat. every intelligent person, who has devoted himself to the instruction of youth, must have felt and deplored, the truth of these observations. in most instances, the improvement of a work already in use, will be more acceptable, than one of equal merit would be, which is entirely new; the introduction of a book into schools, being always attended with some difficulty. the "conversations on chemistry," written by mrs. marcet, had obtained a well-merited celebrity, and was very extensively adopted as a school-book, before the publication of her "conversations on natural philosophy." this, also, has been much used for the same purpose; but, the observation has been very general, among intelligent teachers, that, in its execution, it is very inferior to the former work. the editor of the edition now presented to the public, had undertaken to add to the work, questions, for the examination of learners; and notes, where he deemed them necessary. he soon found, however, that the latter undertaking would be a very unpleasant one, as he must have pointed out at the bottom of many of the pages, the defects and mistakes in the text; whilst numerous modes of illustration, or forms of expression, which his experience as a teacher, had convinced him would not be clear to the learner, must, of necessity, have remained unaltered. he therefore determined to revise the whole work, and with the most perfect freedom, to make such alterations in the body of it, as should, in his opinion, best adapt it to the purpose for which it was designed. were the book, as it now stands, carefully compared with the original, it would be found, that, in conformity with this determination, scarcely a page of the latter, remains unchanged. verbal alterations have been made, errors, in points of fact, have been corrected; and new modes of illustration have been introduced, whenever it was thought that those already employed, could be improved; or when it was known, that, from local causes, they are not familiar, in this country. the editor feels assured, that, in performing this task, he has rendered the book more valuable to the teacher, and more useful to the pupil; and he doubts not that the intelligent author of it, would prefer the mode which has been adopted, to that which was at first proposed. the judicious teacher will, of course, vary the questions according to circumstances; and those who may not employ them at all, as questions, will still find them useful, in directing the pupil to the most important points, in every page. the glossary has been confined to such terms of science as occur in the work; and is believed to include all those, of which a clear definition cannot be found in our common dictionaries. contents. conversation i. on general properties of bodies. introduction. general properties of bodies. impenetrability. extension. figure. divisibility. inertia. attraction. attraction of cohesion. density. rarity. heat. attraction of gravitation. conversation ii. on the attraction of gravity. attraction of gravitation, continued. of weight. of the fall of bodies. of the resistance of the air. of the ascent of light bodies. conversation iii. on the laws of motion. of motion. of the inertia of bodies. of force to produce motion. direction of motion. velocity, absolute and relative. uniform motion. retarded motion. accelerated motion. velocity of falling bodies. momentum. action and reaction equal. elasticity of bodies. porosity of bodies. reflected motion. angles of incidence and reflection. conversation iv. on compound motion. compound motion, the result of two opposite forces. of curvilinear motion, the result of two forces. centre of motion, the point at rest, while the other parts of the body move round it. centre of magnitude, the middle of a body. centripetal force, that which impels a body towards a fixed central point. centrifugal force, that which impels a body to fly from the centre. fall of bodies in a parabola. centre of gravity, the point about which the parts balance each other. conversation v. on the mechanical powers. of the power of machines. of the lever in general. of the lever of the first kind, having the fulcrum between the power and the weight. of the lever of the second kind, having the weight between the power and the fulcrum. of the lever of the third kind, having the power between the fulcrum and the weight. of the pulley. of the wheel and axle. of the inclined plane. of the wedge. of the screw. conversation vi. astronomy. causes of the motion of the heavenly bodies. of the earth's annual motion. of the planets, and their motion. of the diurnal motion of the earth and planets. conversation vii. on the planets. of the satellites and moons. gravity diminishes as the square of the distance. of the solar system. of comets. constellations, signs of the zodiac. of copernicus, newton, &c. conversation viii. on the earth. of the terrestrial globe. of the figure of the earth. of the pendulum. of the variation of the seasons, and of the length of days and nights. of the causes of the heat of summer. of solar, siderial, and equal or mean time. conversation ix. on the moon. of the moon's motion. phases of the moon. eclipses of the moon. eclipses of jupiter's moons. of latitude and longitude. of the transits of the inferior planets. of the tides. conversation x. hydrostatics. on the mechanical properties of fluids. definition of a fluid. distinction between fluids and liquids. of non-elastic fluids, scarcely susceptible of compression. of the cohesion of fluids. of their gravitation. of their equilibrium. of their pressure. of specific gravity. of the specific gravity of bodies heavier than water. of those of the same weight as water. of those lighter than water. of the specific gravity of fluids. conversation xi. of springs, fountains, &c. of the ascent of vapour and the formation of clouds. of the formation and fall of rain, &c. of the formation of springs. of rivers and lakes. of fountains. conversation xii. pneumatics. on the mechanical properties of air. of the spring or elasticity of the air. of the weight of the air. experiments with the air pump. of the barometer. mode of weighing air. specific gravity of air. of pumps. description of the sucking pump. description of the forcing pump. conversation xiii. on wind and sound. of wind in general. of the trade wind. of the periodical trade winds. of the aerial tides. of sound in general. of sonorous bodies. of musical sounds. of concord or harmony, and melody. conversation xiv. on optics. of luminous, transparent, and opaque bodies. of the radiation of light. of shadows. of the reflection of light. opaque bodies seen only by reflected light. vision explained. camera obscura. image of objects on the retina. conversation xv. optics--_continued._ of the angle of vision, and reflection of mirrors. angle of vision. reflection of plain mirrors. reflection of convex mirrors. reflection of concave mirrors. conversation xvi. on refraction and colours. transmission of light by transparent bodies. refraction. refraction by the atmosphere. refraction by a lens. refraction by the prism. of colour from the rays of light. of the colours of bodies. conversation xvii. on the structure of the eye, and optical instruments. description of the eye. of the image on the retina. refraction by the humours of the eye. of the use of spectacles. of the single microscope. of the double microscope. of the solar microscope. magic lanthorn. refracting telescope. reflecting telescope. glossary, conversation i. on general properties of bodies. introduction. general properties of bodies. impenetrability. extension. figure. divisibility. inertia. attraction. attraction of cohesion. density. rarity. heat. attraction of gravitation. emily. i must request your assistance, my dear mrs. b., in a charge which i have lately undertaken: it is that of instructing my youngest sister, a task, which i find proves more difficult than i had at first imagined. i can teach her the common routine of children's lessons tolerably well; but she is such an inquisitive little creature, that she is not satisfied without an explanation of every difficulty that occurs to her, and frequently asks me questions which i am at a loss to answer. this morning, for instance, when i had explained to her that the world was round like a ball, instead of being flat as she had supposed, and that it was surrounded by the air, she asked me what supported it. i told her that it required no support; she then inquired why it did not fall as every thing else did? this i confess perplexed me; for i had myself been satisfied with learning that the world floated in the air, without considering how unnatural it was that so heavy a body, bearing the weight of all other things, should be able to support itself. _mrs. b._ i make no doubt, my dear, but that i shall be able to explain this difficulty to you; but i believe that it would be almost impossible to render it intelligible to the comprehension of so young a child as your sister sophia. you, who are now in your thirteenth year, may, i think, with great propriety, learn not only the cause of this particular fact, but acquire a general knowledge of the laws by which the natural world is governed. _emily._ of all things, it is what i should most like to learn; but i was afraid it was too difficult a study even at my age. _mrs. b._ not when familiarly explained: if you have patience to attend, i will most willingly give you all the information in my power. you may perhaps find the subject rather dry at first; but if i succeed in explaining the laws of nature, so as to make you understand them, i am sure that you will derive not only instruction, but great amusement from that study. _emily._ i make no doubt of it, mrs. b.; and pray begin by explaining why the earth requires no support; for that is the point which just now most strongly excites my curiosity. _mrs. b._ my dear emily, if i am to attempt to give you a general idea of the laws of nature, which is no less than to introduce you to a knowledge of the science of natural philosophy, it will be necessary for us to proceed with some degree of regularity. i do not wish to confine you to the systematic order of a scientific treatise, but if we were merely to examine every vague question that may chance to occur, our progress would be but very slow. let us, therefore, begin by taking a short survey of the general properties of bodies, some of which must necessarily be explained before i can attempt to make you understand why the earth requires no support. when i speak of _bodies_, i mean substances, of whatever nature, whether solid or fluid; and _matter_ is the general term used to denote the substance, whatever its nature be, of which the different bodies are composed. thus, the wood of which this table is made, the water with which this glass is filled, and the air which we continually breathe, are each of them _matter_. _emily._ i am very glad you have explained the meaning of the word matter, as it has corrected an erroneous conception i had formed of it: i thought that it was applicable to solid bodies only. _mrs. b._ there are certain properties which appear to be common to all bodies, and are hence called the _essential or inherent properties_ of bodies; these are _impenetrability_, _extension_, _figure_, _divisibility_, _inertia_ and _attraction_. these are also called the general properties of bodies, as we do not suppose any body to exist without them. by _impenetrability_ is meant the property which bodies have of occupying a certain space, so that where one body is, another can not be, without displacing the former; for two bodies can not exist in the same place at the same time. a liquid may be more easily removed than a solid body; yet it is not the less substantial, since it is as impossible for a liquid and a solid to occupy the same space at the same time, as for two solid bodies to do so. for instance, if you put a spoon into a glass full of water, the water will flow over to make room for the spoon. _emily._ i understand this perfectly. liquids are in reality as substantial or as impenetrable as solid bodies, and they appear less so, only because they are more easily displaced. _mrs. b._ the air is a fluid differing in its nature from liquids, but no less impenetrable. if i endeavour to fill this phial by plunging it into this bason of water, the air, you see, rushes out of the phial in bubbles, in order to make way for the water, for the air and the water can not exist together in the same space, any more than two hard bodies; and if i reverse this goblet, and plunge it perpendicularly into the water, so that the air will not be able to escape, the water will no longer be able to fill the goblet. _emily._ but it rises some way into the glass. _mrs. b._ because the water compresses or squeezes the air into a smaller space in the upper part of the glass; but, as long as it remains there, no other body can occupy the same place. _emily._ a difficulty has just occurred to me, with regard to the impenetrability of solid bodies; if a nail is driven into a piece of wood, it penetrates it, and both the wood and the nail occupy the same space that the wood alone did before? _mrs. b._ the nail penetrates between the particles of the wood, by forcing them to make way for it; for you know that not a single atom of wood can remain in the space which the nail occupies; and if the wood is not increased in size by the addition of the nail, it is because wood is a porous substance, like sponge, the particles of which may be compressed or squeezed closer together; and it is thus that they make way for the nail. we may now proceed to the next general property of bodies, _extension_. a body which occupies a certain space must necessarily have extension; that is to say, _length_, _breadth_ and _depth_ or thickness; these are called the dimensions of extension: can you form an idea of any body without them? _emily._ no; certainly i can not; though these dimensions must, of course vary extremely in different bodies. the length, breadth and depth of a box, or of a thimble, are very different from those of a walking stick, or of a hair. but is not height also a dimension of extension? _mrs b._ height and depth are the same dimension, considered in different points of view; if you measure a body, or a space, from the top to the bottom, you call it depth; if from the bottom upwards, you call it height; thus the depth and height of a box are, in fact, the same thing. _emily._ very true; a moment's consideration would have enabled me to discover that; and breadth and width are also the same dimension. _mrs. b._ yes; the limits of extension constitute _figure_ or shape. you conceive that a body having length, breadth and depth, can not be without form, either symmetrical or irregular? _emily._ undoubtedly; and this property admits of almost an infinite variety. _mrs. b._ nature has assigned regular forms to many of her productions. the natural form of various mineral substances is that of crystals, of which there is a great variety. many of them are very beautiful, and no less remarkable by their transparency or colour, than by the perfect regularity of their forms, as may be seen in the various museums and collections of natural history. the vegetable and animal creation appears less symmetrical, but is still more diversified in figure than the mineral kingdom. manufactured substances assume the various arbitrary forms which the art of man designs for them; and an infinite number of irregular forms are produced by fractures and by the dismemberment of the parts of bodies. _emily._ such as a piece of broken china, or glass? _mrs. b._ or the masses and fragments of stone, and other mineral substances, which are dug out of the earth, or found upon its surface; many of which, although composed of minute crystals, are in the lump of an irregular form. we may now proceed to _divisibility_; that is to say, a susceptibility of being divided into an indefinite number of parts. take any small quantity of matter, a grain of sand for instance, and cut it into two parts; these two parts might be again divided, had we instruments sufficiently fine for the purpose; and if by means of pounding, grinding, and other similar methods, we carry this division to the greatest possible extent, and reduce the body to its finest imaginable particles, yet not one of the particles will be destroyed, but will each contain as many halves and quarters, as did the whole grain. the dissolving of a solid body in a liquid, affords a very striking example of the extreme divisibility of matter; when you sweeten a cup of tea, for instance, with what minuteness the sugar must be divided to be diffused throughout the whole of the liquid. _emily._ and if you pour a few drops of red wine into a glass of water, they immediately tinge the whole of the water, and must therefore be diffused throughout it. _mrs. b._ exactly so; and the perfume of this lavender water will be almost as instantaneously diffused throughout the room, if i take out the stopper. _emily._ but in this case it is only the perfume of the lavender, and not the water itself that is diffused in the room. _mrs. b._ the odour or smell of a body is part of the body itself, and is produced by very minute particles or exhalations which escape from the odoriferous bodies. it would be impossible that you should smell the lavender water, if particles of it did not come in actual contact with your nose. _emily._ but when i smell a flower, i see no vapour rise from it; and yet i perceive the smell at a considerable distance. _mrs. b._ you could, i assure you, no more smell a flower, the odoriferous particles of which did not touch your nose, than you could taste a fruit, the flavoured particles of which did not come in contact with your tongue. _emily._ that is wonderful indeed; the particles then, which exhale from the flower and from the lavender water, are, i suppose, too small to be visible? _mrs. b._ certainly: you may form some idea of their extreme minuteness, from the immense number which must have escaped in order to perfume the whole room; and yet there is no sensible diminution of the liquid in the phial. _emily._ but the quantity must really be diminished? _mrs. b._ undoubtedly; and were you to leave the bottle open a sufficient length of time, the whole of the water would evaporate and disappear. but though so minutely subdivided as to be imperceptible to any of our senses, each particle would continue to exist; for it is not within the power of man to destroy a single particle of matter: nor is there any reason to suppose that in nature an atom is ever annihilated. _emily._ yet, when a body is burnt to ashes, part of it, at least, appears to be effectually destroyed: look how small is the residue of ashes in the fire place, from all the fuel which has been consumed within it. _mrs. b._ that part of the fuel, which you suppose to be destroyed, evaporates in the form of smoke, and vapour, and air, whilst the remainder is reduced to ashes. a body, in burning, undergoes no doubt very remarkable changes; it is generally subdivided; its form and colour altered; its extension increased: but the various parts, into which it has been separated by combustion, continue in existence, and retain all the essential properties of bodies. _emily._ but that part of a burnt body which evaporates in smoke has no figure; smoke, it is true, ascends in columns into the air, but it is soon so much diffused as to lose all form; it becomes indeed invisible. _mrs. b._ invisible, i allow; but we must not imagine that what we no longer see no longer exists. were every particle of matter that becomes invisible annihilated, the world itself would in the course of time be destroyed. the particles of smoke, when diffused in the air, continue still to be particles of matter as well as when more closely united in the form of coals: they are really as substantial in the one state as in the other, and equally so when by their extreme subdivision they become invisible. no particle of matter is ever destroyed: this is a principle you must constantly remember. every thing in nature decays and corrupts in the lapse of time. we die, and our bodies moulder to dust; but not a single atom of them is lost; they serve to nourish the earth, whence, while living, they drew their support. the next essential property of matter is called _inertia_ or inactivity; this word expresses the resistance which matter makes to a change from a state of rest, to that of motion, or from a state of motion to that of rest. bodies are equally incapable of changing their actual state, whether it be of motion or of rest. you know that it requires force to put a body which is at rest in motion; an exertion of strength is also requisite to stop a body which is already in motion. the resistance of the body to a change of state, in either case, arises from its _inertia_. _emily._ in playing at base-ball i am obliged to use all my strength to give a rapid motion to the ball; and when i have to catch it, i am sure i feel the resistance it makes to being stopped. but if i did not catch it, it would soon fall to the ground and stop of itself. _mrs. b._ matter being inert it is as incapable of stopping of itself as it is of putting itself into motion: when the ball ceases to move, therefore, it must be stopped by some other cause or power; but as it is one with which you are yet unacquainted, we can not at present investigate its effects. the last property which appears to be common to all bodies is _attraction_. all bodies consist of infinitely small particles of matter, each of which possesses the power of attracting or drawing towards it, and uniting with any other particle sufficiently near to be within the influence of its attraction; but in minute particles this power extends to so very small a distance around them, that its effect is not sensible, unless they are (or at least appear to be) in contact; it then makes them stick or adhere together, and is hence called the _attraction of cohesion_. without this power, solid bodies would fall in pieces, or rather crumble to atoms. _emily._ i am so much accustomed to see bodies firm and solid, that it never occurred to me that any power was requisite to unite the particles of which they are composed. but the attraction of cohesion does not, i suppose, exist in liquids; for the particles of liquids do not remain together so as to form a body, unless confined in a vessel? _mrs. b._ i beg your pardon; it is the attraction of cohesion which holds this drop of water suspended at the end of my finger, and keeps the minute watery particles of which it is composed united. but as this power is stronger in proportion as the particles of bodies are more closely united, the cohesive attraction of solid bodies is much greater than that of fluids. the thinner and lighter a fluid is, the less is the cohesive attraction of its particles, because they are further apart; and in elastic fluids, such as air, there is no cohesive attraction among the particles. _emily._ that is very fortunate; for it would be impossible to breathe the air in a solid mass; or even in a liquid state. but is the air a body of the same nature as other bodies? _mrs. b._ undoubtedly, in all essential properties. _emily._ yet you say that it does not possess one of the general properties of bodies--attraction. _mrs. b._ the particles of air are not destitute of the power of attraction, but they are too far distant from each other to be influenced by it so as to produce cohesion: and the utmost efforts of human art have proved ineffectual in the attempt to compress them, so as to bring them within the sphere of each other's attraction, and make them cohere. _emily._ if so, how is it possible to prove that they are endowed with this power? _mrs. b._ the air is formed of particles precisely of the same nature as those which enter into the composition of liquid and solid bodies, in each of which we have a proof of their attraction. _emily._ it is then, i suppose, owing to the different degrees of cohesive attraction in different substances, that they are hard or soft, and that liquids are thick or thin. _mrs. b._ yes; but you would express your meaning better by the term _density_, which denotes the degree of closeness and compactness of the particles of a body. in philosophical language, density is said to be that property of bodies by which they contain a certain quantity of matter, under a certain bulk or magnitude. _rarity_ is the contrary of density; it denotes the thinness and subtilty of bodies: thus you would say that mercury or quicksilver was a very dense fluid; ether, a very rare one. those bodies which are the most dense, do not always cohere the most strongly; lead is more dense than iron, yet its particles are more easily separated. _caroline._ but how are we to judge of the quantity of matter contained in a certain bulk? _mrs. b._ by the weight: under the same bulk bodies are said to be dense in proportion as they are heavy. _emily._ then we may say that metals are dense bodies, wood comparatively a rare one, &c. but, mrs. b., when the particles of a body are so near as to attract each other, the effect of this power must increase as they are brought by it closer together; so that one would suppose that the body would gradually augment in density, till it was impossible for its particles to be more closely united. now, we know that this is not the case; for soft bodies, such as cork, sponge, or butter, never become, in consequence of the increasing attraction of their particles, as hard as iron? _mrs. b._ in such bodies as cork and sponge, the particles which come in contact are so few as to produce but a slight degree of cohesion: they are porous bodies, which, owing to the peculiar arrangement of their particles, abound with interstices, or pores, which separate the particles. but there is also a fluid much more subtile than air, which pervades all bodies, this is _heat_. heat insinuates itself more or less between the particles of all bodies, and forces them asunder; you may therefore consider heat, and the attraction of cohesion, as constantly acting in opposition to each other. _emily._ the one endeavouring to rend a body to pieces, the other to keep its parts firmly united. _mrs. b._ and it is this struggle between the contending forces of heat and attraction, which prevents the extreme degree of density which would result from the sole influence of the attraction of cohesion. _emily._ the more a body is heated then, the more its particles will be separated. _mrs. b._ certainly: we find that bodies not only swell or dilate, but lose their cohesion, by heat: this effect is very sensible in butter, for instance, which expands by the application of heat, till at length the attraction of cohesion is so far diminished that the particles separate, and the butter becomes liquid. a similar effect is produced by heat on metals, and all bodies susceptible of being melted. liquids, you know, are made to boil by the application of heat; the attraction of cohesion then yields entirely to the repulsive power; the particles are totally separated and converted into steam or vapour. but the agency of heat is in no body more sensible than in air, which dilates and contracts by its increase or diminution in a very remarkable degree. _emily._ the effects of heat appear to be one of the most interesting parts of natural philosophy. _mrs. b._ that is true; but heat is so intimately connected with chemistry, that you must allow me to defer the investigation of its properties till you become acquainted with that science. to return to its antagonist, the attraction of cohesion; it is this power which restores to vapour its liquid form, which unites it into drops when it falls to earth in a shower of rain, which gathers the dew into brilliant gems on the blades of grass. _emily._ and i have often observed that after a shower, the water collects into large drops on the leaves of plants; but i cannot say that i perfectly understand how the attraction of cohesion produces this effect. _mrs. b._ rain, when it first leaves the clouds, is not in the form of drops, but in that of mist or vapour, which is composed of very small watery particles; these in their descent mutually attract each other, and those that are sufficiently near in consequence unite and form a drop, and thus the mist is transformed into a shower. the dew also was originally in a state of vapour, but is, by the mutual attraction of the particles, formed into small globules on the blades of grass: in a similar manner the rain upon the leaf collects into large drops, which when they become too heavy for the leaf to support, fall to the ground. _emily._ all this is wonderfully curious! i am almost bewildered with surprise and admiration at the number of new ideas i have already acquired. _mrs. b._ every step that you advance in the pursuit of natural science, will fill your mind with admiration and gratitude towards its divine author. in the study of natural philosophy, we must consider ourselves as reading the book of nature, in which the bountiful goodness and wisdom of god are revealed to all mankind; no study can tend more to purify the heart, and raise it to a religious contemplation of the divine perfections. there is another curious effect of the attraction of cohesion which i must point out to you; this is called capillary attraction. it enables liquids to rise above their ordinary level in capillary tubes: these are tubes, the bores of which are so extremely small that liquids ascend within them, from the cohesive attraction between the particles of the liquid and the interior surface of the tube. do you perceive the water rising in this small glass tube, above its level in the goblet of water, into which i have put one end of it? _emily._ oh yes; i see it slowly creeping up the tube, but now it is stationary: will it rise no higher? _mrs. b._ no; because the cohesive attraction between the water and the internal surface of the tube is now balanced by the weight of the water within it; if the bore of the tube were narrower the water would rise higher; and if you immerse several tubes of bores of different sizes, you will see it rise to different heights in each of them. in making this experiment, you should colour the water with a little red wine, in order to render the effect more obvious. all porous substances, such as sponge, bread, linen, &c. may be considered as collections of capillary tubes: if you dip one end of a lump of sugar into water, the fluid will rise in it, and wet it considerably above the surface of the water into which you dip it. _emily._ in making tea i have often observed that effect, without being able to account for it. _mrs. b._ now that you are acquainted with the attraction of cohesion, i must endeavour to explain to you that of _gravitation_, which is probably a modification of the same power; the first is perceptible only in very minute particles, and at very small distances; the other acts on the largest bodies, and extends to immense distances. _emily._ you astonish me: surely you do not mean to say that large bodies attract each other? _mrs. b._ indeed i do: let us take, for example, one of the largest bodies in nature, and observe whether it does not attract other bodies. what is it that occasions the fall of this book, when i no longer support it? _emily._ can it be the attraction of the earth? i thought that all bodies had a natural tendency to fall. _mrs. b._ they have a natural tendency to fall, it is true; but that tendency is produced entirely by the attraction of the earth: the earth being so much larger than any body on its surface, forces every body, which is not supported, to fall upon it. _emily._ if the tendency which bodies have to fall results from the earth's attractive power, the earth itself can have no such tendency, since it cannot attract itself, and therefore it requires no support to prevent it from falling. yet the idea that bodies do not fall of their own accord, but that they are drawn towards the earth by its attraction, is so new and strange to me, that i know not how to reconcile myself to it. _mrs. b._ when you are accustomed to consider the fall of bodies as depending on this cause, it will appear to you as natural, and surely much more satisfactory, than if the cause of their tendency to fall were totally unknown. thus you understand that all matter is attractive, from the smallest particle to the largest mass; and that bodies attract each other with a force proportional to the quantity of matter they contain. _emily._ i do not perceive any difference between the attraction of cohesion and that of gravitation; is it not because every particle of matter is endowed with an attractive power, that large bodies consisting of a great number of particles, are so strongly attractive? _mrs. b._ true. there is, however, this difference between the attraction of particles and that of masses, that the former takes place only when the particles are contiguous, whilst the latter is exerted when the masses are far from each other. the attraction of particles frequently counteracts the attraction of gravitation. of this you have an instance in the attraction of capillary tubes, in which liquids ascend by the attraction of cohesion, in opposition to that of gravity. it is on this account that it is necessary that the bore of the tube should be extremely small; for if the column of water within the tube is not very minute, the attraction of cohesion would not be able either to raise or support it in opposition to its gravity; because the increase of weight, in a column of water of a given height, is much greater than the increase in the attracting surface of the tube, when its size is increased. you may observe also, that all solid bodies are enabled by the force of the cohesive attraction of their particles to resist that of gravity, which would otherwise disunite them, and bring them to a level with the ground, as it does in the case of a liquid, the cohesive attraction of which is not sufficient to enable it to resist the power of gravity. _emily._ and some solid bodies appear to be of this nature, as sand, and powder for instance: there is no attraction of cohesion between their particles? _mrs. b._ every grain of powder, or sand, is composed of a great number of other more minute particles, firmly united by the attraction of cohesion; but amongst the separate grains there is no sensible attraction, because they are not in sufficiently close contact. _emily._ yet they actually touch each other? _mrs. b._ the surface of bodies is in general so rough and uneven, that when in apparent contact, they touch each other only by a few points. thus, when i lay this book upon the table, the binding of which appears perfectly smooth, so few of the particles of its under surface come in contact with the table, that no sensible degree of cohesive attraction takes place; for you see that it does not stick or cohere to the table, and i find no difficulty in lifting it off. it is only when surfaces, perfectly flat and well polished, are placed in contact, that the particles approach in sufficient number, and closely enough, to produce a sensible degree of cohesive attraction. here are two plates of polished metal, i press their flat surfaces together, having previously interposed a few drops of oil, to fill up every little porous vacancy. now try to separate them. _emily._ it requires an effort beyond my strength, though there are handles for the purpose of pulling them asunder. is the firm adhesion of the two plates merely owing to the attraction of cohesion? _mrs. b._ there is no force more powerful, since it is by this that the particles of the hardest bodies are held together. it would require a weight of several pounds to separate these plates. in the present example, however, much of the cohesive force is due to the attraction subsisting between the metal and the oil which is interposed; as without this, or some other fluid, the points of contact would still be comparatively few, although we may have employed our utmost art, in giving flat surfaces to the plates. _emily._ in making a kaleidoscope, i recollect that the two plates of glass, which were to serve as mirrors, stuck so fast together, that i imagined some of the gum i had been using had by chance been interposed between them; but i am now convinced that it was their own natural cohesive attraction which produced this effect. _mrs. b._ very probably it was so; for plate-glass has an extremely smooth, flat surface, admitting of the contact of a great number of particles, when two plates are laid upon each other. _emily._ but, mrs. b., the cohesive attraction of some substances is much greater than that of others; thus glue, gum and paste, cohere with singular tenacity. _mrs. b._ bodies which differ in their natures in other respects, differ also in their cohesive attraction; it is probable that there are no two bodies, the particles of which attract each other with precisely the same force. there are some other modifications of attraction peculiar to certain bodies; namely, that of magnetism, of electricity, and of affinity, or chemical attraction; but we shall confine our attention merely to the attraction of cohesion and of gravity; the examination of the latter we shall resume at our next meeting. questions . (pg. ) what is intended by the term _bodies_? . (pg. ) is the term _matter_, restricted to substances of a particular kind? . (pg. ) name those properties of bodies, which are called inherent. . (pg. ) what is meant by impenetrability? . (pg. ) can a liquid be said to be impenetrable? . (pg. ) how can you prove that air is impenetrable? . (pg. ) if air is impenetrable, what causes the water to rise some way into a goblet, if i plunge it into water with its mouth downward? . (pg. ) when i drive a nail into wood, do not both the iron and the wood occupy the same space? . (pg. ) in how many directions, is a body said to have extension? . (pg. ) how do we distinguish the terms height and depth? . (pg. ) what constitutes the _figure_, or _form_ of a body? . (pg. ) what is said respecting the form of minerals? . (pg. ) what of the vegetable and animal creation? . (pg. ) what of artificial, and accidental forms? . (pg. ) what is meant by divisibility? . (pg. ) what examples can you give, to prove that the particles of a body are minute in the extreme? . (pg. ) what produces the odour of bodies? . (pg. ) how do odours exemplify the minuteness of the particles of matter? . (pg. ) can matter be in any way annihilated? . (pg. ) what becomes of the fuel, which disappears in our fires? . (pg. ) how can that part which evaporates, be still said to possess a substantial form? . (pg. ) what do we mean by _inertia_? . (pg. ) give an example to prove that force is necessary, either to give or to stop motion. . (pg. ) what general power do the particles of matter exert upon other particles? . (pg. ) what is that species of attraction called, which keeps bodies in a solid state? . (pg. ) does the attraction of cohesion exist in liquids, and how is its existence proved? . (pg. ) if the particles of air attract each other, why do they not cohere? . (pg. ) from what then do you infer that they possess attraction? . (pg. ) how do you account for some bodies being hard and others soft? . (pg. ) what is meant by the term _density_? . (pg. ) do the most dense bodies always cohere the most strongly? . (pg. ) how do we know that one body is more dense than another? . (pg. ) what is there which acts in opposition to cohesive attraction, tending to separate the particles of bodies? . (pg. ) what would be the consequence if the repulsive power of heat were not exerted? . (pg. ) if we continue to increase the heat, what effects will it produce on bodies? . (pg. ) what body has its dimensions most sensibly affected by change of temperature? . (pg. ) what power restores vapours to the liquid form? . (pg. ) what examples can you give? . (pg. ) how are drops of rain and of dew said to be formed? . (pg. ) what is meant by a capillary tube? . (pg. ) what effect does attraction produce when these are immersed in water? . (pg. ) what is the reason that the water rises to a certain height only? . (pg. ) give some familiar examples of capillary attraction. . (pg. ) in what does _gravitation_ differ from cohesive attraction? . (pg. ) what causes bodies near the earth's surface, to have a tendency to fall towards it? . (pg. ) what remarkable difference is there between the attraction of gravitation, and that of cohesion? . (pg. ) in what instances does the power of cohesion counteract that of gravitation? . (pg. ) why will water rise to a less height, if the size of the tube is increased? . (pg. ) why do not two bodies cohere, when laid upon each other? . (pg. ) can two bodies be made sufficiently flat to cohere with considerable force? . (pg. ) what is the reason that the adhesion is greater when oil is interposed? . (pg. ) what other modifications of attraction are there, besides those of cohesion and of gravitation? conversation ii. on the attraction of gravity. attraction of gravitation, continued. of weight. of the fall of bodies. of the resistance of the air. of the ascent of light bodies. emily. i have related to my sister caroline all that you have taught me of natural philosophy, and she has been so much delighted by it, that she hopes you will have the goodness to admit her to your lessons. _mrs. b._ very willingly; but i did not think you had any taste for studies of this nature, caroline. _caroline._ i confess, mrs. b., that hitherto i had formed no very agreeable idea either of philosophy, or philosophers; but what emily has told me has excited my curiosity so much, that i shall be highly pleased if you will allow me to become one of your pupils. _mrs. b._ i fear that i shall not find you so tractable a scholar as emily; i know that you are much biased in favour of your own opinions. _caroline._ then you will have the greater merit in reforming them, mrs. b.; and after all the wonders that emily has related to me, i think i stand but little chance against you and your attractions. _mrs. b._ you will, i doubt not, advance a number of objections; but these i shall willingly admit, as they will afford an opportunity of elucidating the subject. emily, do you recollect the names of the general properties of bodies? _emily._ impenetrability, extension, figure, divisibility, inertia and attraction. _mrs. b._ very well. you must remember that these are properties common to all bodies, and of which they cannot be deprived; all other properties of bodies are called accidental, because they depend on the relation or connexion of one body to another. _caroline._ yet surely, mrs. b., there are other properties which are essential to bodies, besides those you have enumerated. colour and weight, for instance, are common to all bodies, and do not arise from their connexion with each other, but exist in the bodies themselves; these, therefore, cannot be accidental qualities? _mrs. b._ i beg your pardon; these properties do not exist in bodies independently of their connexion with other bodies. _caroline._ what! have bodies no weight? does not this table weigh heavier than this book; and, if one thing weighs heavier than another, must there not be such a thing as weight? _mrs. b._ no doubt: but this property does not appear to be essential to bodies; it depends upon their connexion with each other. weight is an effect of the power of attraction, without which the table and the book would have no weight whatever. _emily._ i think i understand you; it is the attraction of gravity which makes bodies heavy. _mrs. b._ you are right. i told you that the attraction of gravity was proportioned to the quantity of matter which bodies contain: now the earth consisting of a much greater quantity of matter than any body upon its surface, the force of its attraction must necessarily be greatest, and must draw every thing so situated towards it; in consequence of which, bodies that are unsupported fall to the ground, whilst those that are supported, press upon the object which prevents their fall, with a weight equal to the force with which they gravitate towards the earth. _caroline._ the same cause then which occasions the fall of bodies, produces their weight also. it was very dull in me not to understand this before, as it is the natural and necessary consequence of attraction; but the idea that bodies were not really heavy of themselves, appeared to me quite incomprehensible. but, mrs. b., if attraction is a property essential to matter, weight must be so likewise; for how can one exist without the other? _mrs. b._ suppose there were but one body existing in universal space, what would its weight be? _caroline._ that would depend upon its size; or more accurately speaking, upon the quantity of matter it contained. _emily._ no, no; the body would have no weight, whatever were its size; because nothing would attract it. am i not right, mrs. b.? _mrs. b._ you are: you must allow, therefore, that it would be possible for attraction to exist without weight; for each of the particles of which the body was composed, would possess the power of attraction; but they could exert it only amongst themselves; the whole mass having nothing to attract, or to be attracted by, would have no weight. _caroline._ i am now well satisfied that weight is not essential to the existence of bodies; but what have you to object to colours, mrs. b.; you will not, i think, deny that they really exist in the bodies themselves. _mrs. b._ when we come to treat of the subject of colours, i trust that i shall be able to convince you, that colours are likewise accidental qualities, quite distinct from the bodies to which they appear to belong. _caroline._ oh do pray explain it to us now, i am so very curious to know how that is possible. _mrs. b._ unless we proceed with some degree of order and method, you will in the end find yourself but little the wiser for all you learn. let us therefore go on regularly, and make ourselves well acquainted with the general properties of bodies before we proceed further. _emily._ to return, then, to attraction, (which appears to me by far the most interesting of them, since it belongs equally to all kinds of matter,) it must be mutual between two bodies; and if so, when a stone falls to the earth, the earth should rise part of the way to meet the stone? _mrs. b._ certainly; but you must recollect that the force of attraction is proportioned to the quantity of matter which bodies contain, and if you consider the difference there is in that respect, between a stone and the earth, you will not be surprised that you do not perceive the earth rise to meet the stone; for though it is true that a mutual attraction takes place between the earth and the stone, that of the latter is so very small in comparison to that of the former, as to render its effect insensible. _emily._ but since attraction is proportioned to the quantity of matter which bodies contain, why do not the hills attract the houses and churches towards them? _caroline._ what an idea, emily! how can the houses and churches be moved, when they are so firmly fixed in the ground! _mrs. b._ emily's question is not absurd, and your answer, caroline, is perfectly just; but can you tell us why the houses and churches are so firmly fixed in the ground? _caroline._ i am afraid i have answered right by mere chance; for i begin to suspect that bricklayers and carpenters could give but little stability to their buildings, without the aid of attraction. _mrs. b._ it is certainly the cohesive attraction between the bricks and the mortar, which enables them to build walls, and these are so strongly attracted by the earth, as to resist every other impulse; otherwise they would necessarily move towards the hills and the mountains; but the lesser force must yield to the greater. there are, however, some circumstances in which the attraction of a large body has sensibly counteracted that of the earth. if whilst standing on the declivity of a mountain, you hold a plumb-line in your hand, the weight will not fall perpendicular to the earth, but incline a little towards the mountain; and this is owing to the lateral, or sideways attraction of the mountain, interfering with the perpendicular attraction of the earth. _emily._ but the size of a mountain is very trifling, compared to the whole earth. _mrs. b._ attraction, you must recollect, is in proportion to the quantity of matter, and although that of the mountain, is much less than that of the earth, it may yet be sufficient to act sensibly upon the plumb-line which is so near to it. _caroline._ pray, mrs. b., do the two scales of a balance hang parallel to each other? _mrs. b._ you mean, i suppose, in other words to inquire whether two lines which are perpendicular to the earth, are parallel to each other? i believe i guess the reason of your question; but i wish you would endeavour to answer it without my assistance. _caroline._ i was thinking that such lines must both tend by gravity to the same point, the centre of the earth; now lines tending to the same point cannot be parallel, as parallel lines are always at an equal distance from each other, and would never meet. _mrs. b._ very well explained; you see now the use of your knowledge of parallel lines: had you been ignorant of their properties, you could not have drawn such a conclusion. this may enable you to form an idea of the great advantage to be derived even from a slight knowledge of geometry, in the study of natural philosophy; and if after i have made you acquainted with the first elements, you should be tempted to pursue the study, i would advise you to prepare yourselves by acquiring some knowledge of geometry. this science would teach you that lines which fall perpendicular to the surface of a sphere cannot be parallel, because they would all meet, if prolonged to the centre of the sphere; while lines that fall perpendicular to a plane or flat surface, are always parallel, because if prolonged, they would never meet. _emily._ and yet a pair of scales, hanging perpendicular to the earth, appear parallel? _mrs. b._ because the sphere is so large, and the scales consequently converge so little, that their inclination is not perceptible to our senses; if we could construct a pair of scales whose beam would extend several degrees, their convergence would be very obvious; but as this cannot be accomplished, let us draw a small figure of the earth, and then we may make a pair of scales of the proportion we please. (fig. . pl. i.) _caroline._ this figure renders it very clear: then two bodies cannot fall to the earth in parallel lines? _mrs. b._ never. _caroline._ the reason that a heavy body falls quicker than a light one, is, i suppose, because the earth attracts it more strongly. _mrs. b._ the earth, it is true, attracts a heavy body more than a light one; but that would not make the one fall quicker than the other. _caroline._ yet, since it is attraction that occasions the fall of bodies, surely the more a body is attracted, the more rapidly it will fall. besides, experience proves it to be so. do we not every day see heavy bodies fall quickly, and light bodies slowly? _emily._ it strikes me, as it does caroline, that as attraction is proportioned to the quantity of matter, the earth must necessarily attract a body which contains a great quantity more strongly, and therefore bring it to the ground sooner than one consisting of a smaller quantity. _mrs. b._ you must consider, that if heavy bodies are attracted more strongly than light ones, they require more attraction to make them fall. remember that bodies have no natural tendency to fall, any more than to rise, or to move laterally, and that they will not fall unless impelled by some force; now this force must be proportioned to the quantity of matter it has to move: a body consisting of particles of matter, for instance, requires ten times as much attraction to bring it to the ground in the same space of time as a body consisting of only particles. [illustration: plate i.] _caroline._ i do not understand that; for it seems to me, that the heavier a body is, the move easily and readily it falls. _emily._ i think i now comprehend it; let me try if i can explain it to caroline. suppose that i draw towards me two weighty bodies, the one of lbs. the other of lbs. must i not exert ten times as much strength to draw the larger one to me, in the same space of time, as is required for the smaller one? and if the earth draws a body of lbs. weight to it in the same space of time that it draws a body of lbs. does it not follow that it attracts the body of lbs. weight with ten times the force that it does that of lbs.? _caroline._ i comprehend your reasoning perfectly; but if it were so, the body of lbs. weight, and that of lbs. would fall with the same rapidity; and the consequence would be, that all bodies, whether light or heavy, being at an equal distance from the ground, would fall to it in the same space of time: now it is very evident that this conclusion is absurd; experience every instant contradicts it; observe how much sooner this book reaches the floor than this sheet of paper, when i let them drop together. _emily._ that is an objection i cannot answer. i must refer it to you, mrs. b. _mrs. b._ i trust that we shall not find it insurmountable. it is true that, according to the laws of attraction, all bodies at an equal distance from the earth, should fall to it in the same space of time; and this would actually take place if no obstacle intervened to impede their fall. but bodies fall through the air, and it is the resistance of the air which makes bodies of different density fall with different degrees of velocity. they must all force their way through the air, but dense heavy bodies overcome this obstacle more easily than rarer or lighter ones; because in the same space they contain more gravitating particles. the resistance which the air opposes to the fall of bodies is proportioned to their surface, not to their weight; the air being inert, cannot exert a greater force to support the weight of a cannon ball, than it does to support the weight of a ball (of the same size) made of leather; but the cannon ball will overcome this resistance more easily, and fall to the ground, consequently, quicker than the leather ball. _caroline._ this is very clear and solves the difficulty perfectly. the air offers the same resistance to a bit of lead and a bit of feather of the same size; yet the one seems to meet with no obstruction in its fall, whilst the other is evidently resisted and supported for some time by the air. _emily._ the larger the surface of a body, then, the more air it covers, and the greater is the resistance it meets with from it. _mrs. b._ certainly: observe the manner in which this sheet of paper falls; it floats awhile in the air, and then gently descends to the ground. i will roll the same piece of paper up into a ball: it offers now but a small surface to the air, and encounters therefore but little resistance: see how much more rapidly it falls. the heaviest bodies may be made to float awhile in the air, by making the extent of their surface counterbalance their weight. here is some gold, which is one of the most dense bodies we are acquainted with; but it has been beaten into a very thin leaf, and offers so great an extent of surface in proportion to its weight, that its fall, you see, is still more retarded by the resistance of the air, than that of the sheet of paper. _caroline._ that is very curious: and it is, i suppose, upon the same principle that a thin slate sinks in water more slowly than a round stone. but, mrs. b., if the air is a real body, is it not also subjected to the laws of gravity? _mrs. b._ undoubtedly. _caroline._ then why does it not, like all other bodies, fall to the ground? _mrs. b._ on account of its spring or elasticity. the air is an _elastic fluid_; and the peculiar property of elastic bodies is to resume, after compression, their original dimensions; and you must consider the air of which the atmosphere is composed as existing in a state of compression, for its particles being drawn towards the earth by gravity, are brought closer together than they would otherwise be, but the spring or elasticity of the air by which it endeavours to resist compression, gives it a constant tendency to expand itself, so as to resume the dimensions it would naturally have, if not under the influence of gravity. the air may therefore be said constantly to struggle with the power of gravity without being able to overcome it. gravity thus confines the air to the regions of our globe, whilst its elasticity prevents it from falling, like other bodies, to the ground. _emily._ the air then is, i suppose, thicker, or i should rather say more dense, near the surface of the earth, than in the higher regions of the atmosphere; for that part of the air which is nearer the surface of the earth must be most strongly attracted. _mrs. b._ the diminution of the force of gravity, at so small a distance as that to which the atmosphere extends (compared with the size of the earth) is so inconsiderable as to be scarcely sensible; but the pressure of the upper parts of the atmosphere on those beneath, renders the air near the surface of the earth much more dense than in the upper regions. the pressure of the atmosphere has been compared to that of a pile of fleeces of wool, in which the lower fleeces are pressed together by the weight of those above; these lie light and loose, in proportion as they approach the uppermost fleece, which receives no external pressure, and is confined merely by the force of its own gravity. _emily._ i do not understand how it is that the air can be springy or elastic, as the particles of which it is composed must, according to the general law, attract each other; yet their elasticity, must arise from a tendency to recede from each other. _mrs. b._ have you forgotten what i told you respecting the effects of heat, a fluid so subtile that it readily pervades all substances, and even in solid bodies, counteracts the attraction of cohesion? in air the quantity of heat interposed is so great, as to cause its particles actually to repel each other, and it is to this that we must ascribe its elasticity; this, however, does not prevent the earth from exerting its attraction upon the individual particles of which it consists. _caroline._ it has just occurred to me that there are some bodies which do not gravitate towards the earth. smoke and steam, for instance, rise instead of falling. _mrs. b._ it is still gravity which produces their ascent; at least, were that power destroyed, these bodies would not rise. _caroline._ i shall be out of conceit with gravity, if it is so inconsistent in its operations. _mrs. b._ there is no difficulty in reconciling this apparent inconsistency of effect. the air near the earth is heavier than smoke, steam, or other vapours; it consequently not only supports these light bodies, but forces them to rise, till they reach a part of the atmosphere, the weight of which is not greater than their own, and then they remain stationary. look at this bason of water; why does the piece of paper which i throw into it float on the surface? _emily._ because, being lighter than the water, it is supported by it. _mrs. b._ and now that i pour more water into the bason, why does the paper rise? _emily._ the water being heavier than the paper, gets beneath it, and obliges it to rise. _mrs. b._ in a similar manner are smoke and vapour forced upwards by the air; but these bodies do not, like the paper, ascend to the surface of the fluid, because, as we observed before, the air being less dense, and consequently lighter as it is more distant from the earth, vapours rise only till they attain a region of air of their own density. smoke, indeed ascends but a very little way; it consists of minute particles of fuel, carried up by a current of heated air, from the fire below: heat, you recollect, expands all bodies; it consequently rarefies air, and renders it lighter than the colder air of the atmosphere; the heated air from the fire carries up with it vapour and small particles of the combustible materials which are burning in the fire. when this current of hot air is cooled by mixing with the atmosphere, the minute particles of coal, or other combustible, fall; it is this which produces the small black flakes which render the air, and every thing in contact with it, in london, so dirty. _caroline._ you must, however, allow me to make one more objection to the universal gravity of bodies; which is the ascent of air balloons, the materials of which are undoubtedly heavier than air: how, therefore, can they be supported by it? _mrs. b._ i admit that the materials of which balloons are made are heavier than the air; but the air with which they are filled is an elastic fluid, of a different nature from atmospheric air, and considerably lighter; so that on the whole the balloon is lighter than the air which it displaces, and consequently will rise, on the same principle as smoke and vapour. now, emily, let me hear if you can explain how the gravity of bodies is modified by the effect of the air? _emily._ the air forces bodies which are lighter than itself to ascend; those that are of an equal weight will remain stationary in it; and those that are heavier will descend through it: but the air will have some effect on these last; for if they are not much heavier, they will with difficulty overcome the resistance they meet with in passing through it, they will be borne up by it, and their fall will be more or less retarded. _mrs. b._ very well. observe how slowly this light feather falls to the ground, while a heavier body, like this marble, overcomes the resistance which the air makes to its descent much more easily, and its fall is proportionally more rapid. i now throw a pebble into this tub of water; it does not reach the bottom near so soon as if there were no water in the tub, because it meets with resistance from the water. suppose that we could empty the tub, not only of water, but of air also, the pebble would then fall quicker still, as it would in that case meet with no resistance at all to counteract its gravity. thus you see that it is not the different degrees of gravity, but the resistance of the air, which prevents bodies of different weight from falling with equal velocities; if the air did not bear up the feather, it would reach the ground as soon as the marble. _caroline._ i make no doubt that it is so; and yet i do not feel quite satisfied. i wish there was any place void of air, in which the experiment could be made. _mrs. b._ if that proof will satisfy your doubts, i can give it you. here is a machine called an _air pump_, (fig. . pl. .) by means of which the air may be expelled from any close vessel which is placed over this opening, through which the air is pumped out. glasses of various shapes, usually called receivers, are employed for this purpose. we shall now exhaust the air from this tall receiver which is placed over the opening, and we shall find that bodies within it, whatever their weight or size, will fall from the top to the bottom in the same space of time. _caroline._ oh, i shall be delighted with this experiment; what a curious machine! how can you put the two bodies of different weight within the glass, without admitting the air? _mrs. b._ a guinea and a feather are already placed there for the purpose of the experiment: here is, you see, a contrivance to fasten them in the upper part of the glass; as soon as the air is pumped out, i shall turn this little screw, by which means the brass plates which support them will be removed, and the two bodies will fall.--now i believe i have pretty well exhausted the air. _caroline._ pray let me turn the screw.--i declare, they both reached the bottom at the same instant! did you see, emily, the feather appeared as heavy as the guinea? _emily._ exactly; and fell just as quickly. how wonderful this is! what a number of entertaining experiments might be made with this machine! _mrs. b._ no doubt there are a great many; but we shall reserve them to elucidate the subjects to which they relate: if i had not explained to you why the guinea, and the feather fell with equal velocity, you would not have been so well pleased with the experiment. _emily._ i should have been as much surprised, but not so much interested; besides, experiments help to imprint on the memory the facts they are intended to illustrate; it will be better therefore for us to restrain our curiosity, and wait for other experiments in their proper places. _caroline._ pray by what means is this receiver exhausted of its air? _mrs. b._ you must learn something of mechanics in order to understand the construction of a pump. at our next meeting, therefore, i shall endeavour to make you acquainted with the laws of motion, as an introduction to that subject. questions . (pg. ) what are those properties of bodies called, which are not common to all? . (pg. ) why are they so called? . (pg. ) what is the cause of weight in bodies? . (pg. ) what is the reason that all bodies near to the surface of the earth, are drawn towards it? . (pg. ) if attraction is the cause of weight, could you suppose it possible for a body to possess the former and not the latter property? . (pg. ) when a stone falls to the ground, in which of the two bodies does the power of attraction exist? . (pg. ) if the attraction be mutual, why does not the earth approach the stone, as much as the stone approaches the earth? . (pg. ) if attraction be in proportion to the mass, why does not a hill, draw towards itself, a house placed near it? . (pg. ) how can the attraction of a mountain be rendered sensible? . (pg. ) why cannot two lines which are perpendicular to the surface of the earth be parallel to each other? . (pg. ) draw a small figure of the earth to exemplify this, as in fig. . plate . . (pg. ) if bodies were not resisted by the air, those which are light, would fall as quickly as those which are heavy, how can you account for this? . (pg. ) what then is the reason that a book, and a sheet of paper, let fall from the same height, will not reach the ground in the same time? . (pg. ) what then will be the effect of increasing the surface of a body? . (pg. ) what could you do to a sheet of paper, to make it fall quickly, and why? . (pg. ) inform me how a very dense body may be made to float in the air? . (pg. ) the air is a real body, why does it not fall to the ground? . (pg. ) the air is more dense near the surface of the earth, and decreases in density as you ascend, how is this accounted for, and to what is it compared? . (pg. ) what is it which causes the particles of air to recede from each other, and seems to destroy their mutual attraction? . (pg. ) smoke and vapour ascend in the atmosphere, how can you reconcile this with gravitation? . (pg. ) how would you illustrate this by the floating of a piece of paper on water? . (pg. ) does smoke rise to a great height in the air, and if not, what prevents its so doing? . (pg. ) what limits the height to which vapours rise? . (pg. ) of what does smoke consist? . (pg. ) air balloons are formed of heavy materials, how will you account for their rising in the air? . (pg. ) what influence does the air exert, on bodies less dense than itself, on those of equal, and on those of greater density? . (pg. ) if the air could be entirely removed, what influence would this have upon the falling of heavy and light bodies? . (pg. ) how could this be exemplified by means of the air pump? conversation iii. on the laws of motion. of motion. of the inertia of bodies. of force to produce motion. direction of motion. velocity, absolute and relative. uniform motion. retarded motion. accelerated motion. velocity of falling bodies. momentum. action and reaction equal. elasticity of bodies. porosity of bodies. reflected motion. angles of incidence and reflection. mrs. b. the science of mechanics is founded on the laws of motion; it will therefore be necessary to make you acquainted with these laws before we examine the mechanical powers. tell me, caroline, what do you understand by the word motion? _caroline._ i think i understand it perfectly, though i am at a loss to describe it. motion is the act of moving about, of going from one place to another, it is the contrary of remaining at rest. _mrs. b._ very well. motion then consists in a change of place; a body is in motion whenever it is changing its situation with regard to a fixed point. now since we have observed that one of the general properties of bodies is inertia, that is, an entire passiveness, either with regard to motion or rest, it follows that a body cannot move without being put into motion; the power which puts a body into motion is called _force_; thus the stroke of the hammer is the force which drives the nail; the pulling of the horse that which draws the carriage, &c. force then is the cause which produces motion. _emily._ and may we not say that gravity is the force which occasions the fall of bodies? _mrs. b._ undoubtedly. i have given you the most familiar illustrations in order to render the explanation clear; but since you seek for more scientific examples, you may say that cohesion is the force which binds the particles of bodies together, and heat that which drives them asunder. the motion of a body acted upon by a single force, is always in a straight line, and in the direction in which it received the impulse. _caroline._ that is very natural; for as the body is inert, and can move only because it is impelled, it will move only in the direction in which it is impelled. the degree of quickness with which it moves, must, i suppose, also depend upon the degree of force with which it is impelled. _mrs. b._ yes; the rate at which a body moves, or the shortness of the time which it takes to move from one place to another, is called its velocity; and it is one of the laws of motion, that the velocity of the moving body is proportional to the force by which it is put in motion. we must distinguish between absolute and relative velocity. the velocity of a body is called _absolute_, if we consider the motion of the body in space, without any reference to that of other bodies. when, for instance, a horse goes fifty miles in ten hours, his velocity is five miles an hour. the velocity of a body is termed _relative_, when compared with that of another body which is itself in motion. for instance, if one man walks at the rate of a mile an hour, and another at the rate of two miles an hour, the relative velocity of the latter is double that of the former; but the absolute velocity of the one is one mile, and that of the other two miles an hour. _emily._ let me see if i understand it--the relative velocity of a body is the degree of rapidity of its motion compared with that of another body; thus if one ship sail three times as far as another ship in the same space of time, the velocity of the former is equal to three times that of the latter. _mrs. b._ the general rule may be expressed thus: the velocity of a body is measured by the space over which it moves, divided by the time which it employs in that motion: thus if you travel one hundred miles in twenty hours, what is your velocity in each hour? _emily._ i must divide the space, which is one hundred miles, by the time, which is twenty hours, and the answer will be five miles an hour. then, mrs. b., may we not reverse this rule, and say that the time is equal to the space divided by the velocity; since the space, one hundred miles, divided by the velocity, five miles per hour, gives twenty hours for the time? _mrs. b._ certainly; and we may say also that the space is equal to the velocity multiplied by the time. can you tell me, caroline, how many miles you will have travelled, if your velocity is three miles an hour, and you travel six hours? _caroline._ eighteen miles; for the product of multiplied by , is . _mrs. b._ i suppose that you understand what is meant by the terms _uniform_, _accelerated_ and _retarded_ motion. _emily._ i conceive uniform motion to be that of a body whose motion is regular, and at an equal rate throughout; for instance a horse that goes an equal number of miles every hour. but the hand of a watch is a much better example, as its motion is so regular as to indicate the time. _mrs. b._ you have a right idea of uniform motion; but it would be more correctly expressed by saying, that the motion of a body is uniform when it passes over equal spaces in equal times. uniform motion is produced by a force having acted on a body once and having ceased to act; as, for instance, the stroke of a bat on a ball. _caroline._ but the motion of a ball is not uniform; its velocity gradually diminishes till it falls to the ground. _mrs. b._ recollect that the ball is inert, and has no more power to stop, than to put itself in motion; if it falls, therefore, it must be stopped by some force superior to that by which it was projected, and which destroys its motion. _caroline._ and it is no doubt the force of gravity which counteracts and destroys that of projection; but if there were no such power as gravity, would the ball never stop? _mrs. b._ if neither gravity nor any other force, such as the resistance of the air, opposed its motion, the ball, or even a stone thrown by the hand, would proceed onwards in a right line, and with a uniform velocity for ever. _caroline._ you astonish me! i thought that it was impossible to produce perpetual motion? _mrs. b._ perpetual motion cannot be produced by art, because gravity ultimately destroys all motion that human power can produce. _emily._ but independently of the force of gravity, i cannot conceive that the little motion i am capable of giving to a stone would put it in motion for ever. _mrs. b._ the quantity of motion you communicate to the stone would not influence its duration; if you threw it with little force it would move slowly, for its velocity you must remember, will be proportional to the force with which it is projected; but if there is nothing to obstruct its passage, it will continue to move with the same velocity, and in the same direction as when you first projected it. _caroline._ this appears to me quite incomprehensible; we do not meet with a single instance of it in nature. _mrs. b._ i beg your pardon. when you come to study the motion of the celestial bodies, you will find that _nature_ abounds with examples of perpetual motion; and that it conduces as much to the harmony of the system of the universe, as the prevalence of it on the surface of the earth, would to the destruction of all our comforts. the wisdom of providence has therefore ordained insurmountable obstacles to perpetual motion here below; and though these obstacles often compel us to contend with great difficulties, yet these appear necessary to that order, regularity and repose, so essential to the preservation of all the various beings of which this world is composed. now can you tell me what is _retarded motion_? _caroline._ retarded motion is that of a body which moves every moment slower and slower: thus when i am tired with walking fast, i slacken my pace; or when a stone is thrown upwards, its velocity is gradually diminished by the power of gravity. _mrs. b._ retarded motion is produced by some force acting upon the body in a direction opposite to that which first put it in motion: you who are an animated being, endowed with power and will, may slacken your pace, or stop to rest when you are tired; but inert matter is incapable of any feeling of fatigue, can never slacken its pace, and never stop, unless retarded or arrested in its course by some opposing force; and as it is the laws of inert bodies of which mechanical philosophy treats, i prefer your illustration of the stone retarded in its ascent. now emily, it is your turn; what is _accelerated motion_? _emily._ accelerated motion, i suppose, takes place when the velocity of a body is increased; if you had not objected to our giving such active bodies as ourselves as examples, i should say that my motion is accelerated if i change my pace from walking to running. i cannot think of any instance of accelerated motion in inanimate bodies; all motion of inert matter seems to be retarded by gravity. _mrs. b._ not in all cases; for the power of gravitation sometimes produces accelerated motion; for instance, a stone falling from a height, moves with a regularly accelerated motion. _emily._ true; because the nearer it approaches the earth, the more it is attracted by it. _mrs. b._ you have mistaken the cause of its accelerated motion; for though it is true that the force of gravity increases as a body approaches the earth, the difference is so trifling at any small distance from its surface, as not to be perceptible. accelerated motion is produced when the force which put a body in motion, continues to act upon it during its motion, so that its velocity is continually increased. when a stone falls from a height, the impulse which it receives from gravitation in the first instant of its fall, would be sufficient to bring it to the ground with a uniform velocity: for, as we have observed, a body having been once acted upon by a force, will continue to move with a uniform velocity; but the stone is not acted upon by gravity merely at the first instant of its fall; this power continues to impel it during the whole time of its descent, and it is this continued impulse which accelerates its motion. _emily._ i do not quite understand that. _mrs. b._ let us suppose that the instant after you have let a stone fall from a high tower, the force of gravity were annihilated; the body would nevertheless continue to move downwards, for it would have received a first impulse from gravity; and a body once put in motion will not stop unless it meets with some obstacle to impede its course; in this case its velocity would be uniform, for though there would be no obstacle to obstruct its descent, there would be no force to accelerate it. _emily._ that is very clear. _mrs. b._ then you have only to add the power of gravity constantly acting on the stone during its descent, and it will not be difficult to understand that its motion will become accelerated, since the gravity which acts on the stone at the very first instant of its descent, will continue in force every instant, till it reaches the ground. let us suppose that the impulse given by gravity to the stone during the first instant of its descent, be equal to one; the next instant we shall find that an additional impulse gives the stone an additional velocity, equal to one; so that the accumulated velocity is now equal to two; the following instant another impulse increases the velocity to three, and so on till the stone reaches the ground. _caroline._ now i understand it; the effects of preceding impulses continue, whilst gravity constantly adds new ones, and thus the velocity is perpetually increased. _mrs. b._ yes; it has been ascertained, both by experiment, and calculations which it would be too difficult for us to enter into, that heavy bodies near the surface of the earth, descending from a height by the force of gravity, fall sixteen feet the first second of time, three times that distance in the next, five times in the third second, seven times in the fourth, and so on, regularly increasing their velocities in the proportion of the odd numbers , , , , , &c. according to the number of seconds during which the body has been falling. _emily._ if you throw a stone perpendicularly upwards, is it not the same length of time in ascending, that it is in descending? _mrs. b._ exactly; in ascending, the velocity is diminished by the force of gravity; in descending, it is accelerated by it. _caroline._ i should then imagine that it would fall, quicker than it rose? _mrs. b._ you must recollect that the force with which it is projected, must be taken into the account; and that this force is overcome and destroyed by gravity, before the body begins to fall. _caroline._ but the force of projection given to a stone in throwing it upwards, cannot always be equal to the force of gravity in bringing it down again; for the force of gravity is always the same, whilst the degree of impulse given to the stone is optional; i may throw it up gently, or with violence. _mrs. b._ if you throw it gently, it will not rise high; perhaps only sixteen feet, in which case it will fall in one second of time. now it is proved by experiment, that an impulse requisite to project a body sixteen feet upwards, will make it ascend that height in one second; here then the times of the ascent and descent are equal. but supposing it be required to throw a stone twice that height, the force must be proportionally greater. you see then, that the impulse of projection in throwing a body upwards, is always equal to the action of the force of gravity during its descent; and that whether the body rises to a greater or less distance, these two forces balance each other. i must now explain to you what is meant by the _momentum_ of bodies. it is the force, or power, with which a body in motion, strikes against another body. the momentum of a body is the product of its _quantity of matter_, multiplied by its _quantity of motion_; in other words, its weight multiplied by its velocity. _caroline._ the quicker a body moves, the greater, no doubt, must be the force which it would strike against another body. _emily._ therefore a light body may have a greater momentum than a heavier one, provided its velocity be sufficiently increased; for instance, the momentum of an arrow shot from a bow, must be greater than that of a stone thrown by the hand. _caroline._ we know also by experience, that the heavier a body is, the greater is its force; it is not therefore difficult to understand, that the whole power, or momentum of a body, must be composed of these two properties, its weight and its velocity: but i do not understand why they should be _multiplied_, the one by the other; i should have supposed that the quantity of matter, should have been _added_ to the quantity of motion? _mrs. b._ it is found by experiment, that if the weight of a body is represented by the number , and its velocity also by , its momentum will be represented by , not by , as would be the case, were these figures added, instead of being multiplied together. _emily._ i think that i now understand the reason of this; if the quantity of matter is increased three-fold, it must require three times the force to move it with the same velocity; and then if we wish to give it three times the velocity, it will again require three times the force to produce that effect, which is three times three, or nine; which number therefore, would represent the momentum. _caroline._ i am not quite sure that i fully comprehend what is intended, when weight, and velocity, are represented by numbers alone; i am so used to measure space by yards and miles, and weight by pounds and ounces, that i still want to associate them together in my mind. _mrs. b._ this difficulty will be of very short duration: you have only to be careful, that when you represent weights and velocities by numbers, the denominations or values of the weights and spaces, must not be changed. thus, if we estimate the weight of one body in ounces, the weight of others with which it is compared, must be estimated in ounces, and not in pounds; and in like manner, in comparing velocities, we must throughout, preserve the same standards both of space and of time; as for instance, the number of feet in one second, or of miles in one hour. _caroline._ i now understand it perfectly, and think that i shall never forget a thing which you have rendered so clear. _mrs. b._ i recommend it to you to be very careful to remember the definition of the momentum of bodies, as it is one of the most important points in mechanics: you will find that it is from opposing velocity, to quantity of matter, that machines derive their powers. the _reaction_ of bodies, is the next law of motion which i must explain to you. when a body in motion strikes against another body, it meets with resistance from it; the resistance of the body at rest will be equal to the blow struck by the body in motion; or to express myself in philosophical language, _action_ and _reaction_ will be equal, and in opposite directions. _caroline._ do you mean to say, that the action of the body which strikes, is returned with equal force by the body which receives the blow? _mrs. b._ exactly. _caroline._ but if a man strike another on the face with his fist, he surely does not receive as much pain by the reaction, as he inflicts by the blow? _mrs. b._ no; but this is simply owing to the knuckles, having much less feeling than the face. here are two ivory balls suspended by threads, (plate . fig. .) draw one of them, a, a little on one side,--now let it go;--it strikes, you see, against the other ball b, and drives it off, to a distance equal to that through which the first ball fell; but the motion of a is stopped; because when it struck b, it received in return a blow equal to that it gave, and its motion was consequently destroyed. _emily._ i should have supposed, that the motion of the ball a was destroyed, because it had communicated all its motion to b. _mrs. b._ it is perfectly true, that when one body strikes against another, the quantity of motion communicated to the second body, is lost by the first; but this loss proceeds from the reaction of the body which is struck. here are six ivory balls hanging in a row, (fig. .) draw the first out of the perpendicular, and let it fall against the second. you see none of the balls except the last, appear to move, this flies off as far as the first ball fell; can you explain this? _caroline._ i believe so. when the first ball struck the second, it received a blow in return, which destroyed its motion; the second ball, though it did not appear to move, must have struck against the third; the reaction of which set it at rest; the action of the third ball must have been destroyed by the reaction of the fourth, and so on till motion was communicated to the last ball, which, not being reacted upon, flies off. _mrs. b._ very well explained. observe, that it is only when bodies are elastic, as these ivory balls are, and when their masses are equal, that the stroke returned is equal to the stroke given, and that the striking body loses all its motion. i will show you the difference with these two balls of clay, (fig. .) which are not elastic; when you raise one of these, d, out of the perpendicular, and let it fall against the other, e, the reaction of the latter, on account of its not being elastic, is not sufficient to destroy the motion of the former; only part of the motion of d will be communicated to e, and the two balls will move on together to _d_ and _e_, which is not so great a distance as that through which d fell. observe how useful reaction is in nature. birds in flying strike the air with their wings, and it is the reaction of the air, which enables them to rise, or advance forwards; reaction being always in a contrary direction to action. _caroline._ i thought that birds might be lighter than the air, when their wings were expanded, and were by that means enabled to fly. _mrs. b._ when their wings are spread, this does not alter their weight, but they are better supported by the air, as they cover a greater extent of surface; yet they are still much too heavy to remain in that situation, without continually flapping their wings, as you may have noticed when birds hover over their nests: the force with which their wings strike against the air, must equal the weight of their bodies, in order that the reaction of the air, may be able to support that weight; the bird will then remain stationary. if the stroke of the wings is greater than is required merely to support the bird, the reaction of the air will make it rise; if it be less, it will gently descend; and you may have observed the lark, sometimes remaining with its wings extended, but motionless; in this state it drops quietly into its nest. _caroline._ this is indeed a beautiful effect of the law of reaction! but if flying is merely a mechanical operation, mrs. b., why should we not construct wings, adapted to the size of our bodies, fasten them to our shoulders, move them with our arms, and soar into the air? _mrs. b._ such an experiment has been repeatedly attempted, but never with success; and it is now considered as totally impracticable. the muscular power of birds, is incomparably greater in proportion to their weight, than that of man; were we therefore furnished with wings sufficiently large to enable us to fly, we should not have strength to put them in motion. in swimming, a similar action is produced on the water, to that on the air, in flying; in rowing, also, you strike the water with the oars, in a direction opposite to that in which the boat is required to move, and it is the reaction of the water on the oars which drives the boat along. _emily._ you said, that it was in elastic bodies only, that the whole motion of one body, would be communicated to another; pray what bodies are elastic, besides the air? _mrs. b._ in speaking of the air, i think we defined elasticity to be a property, by means of which bodies that are compressed, return to their former state. if i bend this cane, as soon as i leave it at liberty, it recovers its former position; if i press my finger upon your arm, as soon as i remove it, the flesh, by virtue of its elasticity, rises and destroys the impression i made. of all bodies, the air is the most eminent for this property, and it has thence obtained the name of an elastic fluid. hard bodies are in the next degree elastic; if two ivory, or hardened steel balls are struck together, the parts at which they touch, will be flattened; but their elasticity will make them instantaneously resume their former shape. _caroline._ but when two ivory balls strike against each other, as they constantly do on a billiard table, no mark or impression is made by the stroke. _mrs. b._ i beg your pardon; you cannot, it is true, perceive any mark, because their elasticity instantly destroys all trace of it. soft bodies, which easily retain impressions, such as clay, wax, tallow, butter, &c. have very little elasticity; but of all descriptions of bodies, liquids are the least elastic. _emily._ if sealing-wax were elastic, instead of retaining the impression of a seal, it would resume a smooth surface, as soon as the weight of the seal was removed. but pray what is it that produces the elasticity of bodies? _mrs. b._ there is great diversity of opinion upon that point, and i cannot pretend to decide which approaches nearest to the truth. elasticity implies susceptibility of compression, and the susceptibility of compression depends upon the porosity of bodies; for were there no pores or spaces between the particles of matter of which a body is composed, it could not be compressed. _caroline._ that is to say, that if the particles of bodies were as close together as possible, they could not be squeezed closer. _emily._ bodies then, whose particles are most distant from each other, must be most susceptible of compression, and consequently most elastic; and this you say is the case with air, which is perhaps the least dense of all bodies? _mrs. b._ you will not in general find this rule hold good; for liquids have scarcely any elasticity, whilst hard bodies are eminent for this property, though the latter are certainly of much greater density than the former; elasticity implies, therefore, not only a susceptibility of compression, but depends upon the power possessed by the body, of resuming its former state after compression, in consequence of the peculiar arrangement of its particles. _caroline._ but surely there can be no pores in ivory and metals, mrs. b.; how then can they be susceptible of compression? _mrs. b._ the pores of such bodies are invisible to the naked eye, but you must not thence conclude that they have none; it is, on the contrary, well ascertained that gold, one of the most dense of all bodies, is extremely porous; and that these pores are sufficiently large to admit water when strongly compressed, to pass through them. this was shown by a celebrated experiment made many years ago at florence. _emily._ if water can pass through gold, there must certainly be pores or interstices which afford it a passage; and if gold is so porous, what must other bodies be, which are so much less dense than gold! _mrs. b._ the chief difference in this respect, is i believe, that the pores in some bodies are larger than in others; in cork, sponge and bread, they form considerable cavities; in wood and stone, when not polished, they are generally perceptible to the naked eye; whilst in ivory, metals, and all varnished and polished bodies, they cannot be discerned. to give you an idea of the extreme porosity of bodies, sir isaac newton conjectured that if the earth were so compressed as to be absolutely without pores, its dimensions might possibly not be more than a cubic inch. _caroline._ what an idea! were we not indebted to sir isaac newton for the theory of attraction, i should be tempted to laugh at him for such a supposition. what insignificant little creatures we should be! _mrs. b._ if our consequence arose from the size of our bodies, we should indeed be but pigmies, but remember that the mind of newton was not circumscribed by the dimensions of its envelope. _emily._ it is, however, fortunate that heat keeps the pores of matter open and distended, and prevents the attraction of cohesion from squeezing us into a nut-shell. _mrs. b._ let us now return to the subject of reaction, on which we have some further observations to make. it is because reaction is in its direction opposite to action, that _reflected motion_ is produced. if you throw a ball against the wall, it rebounds; this return of the ball is owing to the reaction of the wall against which it struck, and is called _reflected motion_. _emily._ and i now understand why balls filled with air rebound better than those stuffed with bran or wool; air being most susceptible of compression and most elastic, the reaction is more complete. _caroline._ i have observed that when i throw a ball straight against the wall, it returns straight to my hand; but if i throw it obliquely upwards, it rebounds still higher, and i catch it when it falls. _mrs. b._ you should not say straight, but perpendicularly against the wall; for straight is a general term for lines in all directions which are neither curved nor bent, and is therefore equally applicable to oblique or perpendicular lines. _caroline._ i thought that perpendicularly meant either directly upwards or downwards? _mrs. b._ in those directions lines are perpendicular to the earth. a perpendicular line has always a reference to something towards which it is perpendicular; that is to say, that it inclines neither to the one side or the other, but makes an equal angle on every side. do you understand what an angle is? _caroline._ yes, i believe so: it is the space contained between two lines meeting in a point. _mrs. b._ well then, let the line a b (plate . fig. .) represent the floor of the room, and the line c d that in which you throw a ball against it; the line c d, you will observe, forms two angles with the line a b, and those two angles are equal. _emily._ how can the angles be equal, while the lines which compose them are of unequal length? _mrs. b._ an angle is not measured by the length of the lines, but by their opening, or the space between them. _emily._ yet the longer the lines are, the greater is the opening between them. _mrs. b._ take a pair of compasses and draw a circle over these spaces, making the angular point the centre. _emily._ to what extent must i open the compasses? _mrs. b._ you may draw the circle what size you please, provided that it cuts the lines of the angles we are to measure. all circles, of whatever dimensions, are supposed to be divided into equal parts, called degrees; the opening of an angle, being therefore a portion of a circle, must contain a certain number of degrees: the larger the angle the greater is the number of degrees, and two angles are said to be equal, when they contain an equal number of degrees. _emily._ now i understand it. as the dimension of an angle depends upon the number of degrees contained between its lines, it is the opening, and not the length of its lines, which determines the size of the angle. _mrs. b._ very well: now that you have a clear idea of the dimensions of angles, can you tell me how many degrees are contained in the two angles formed by one line falling perpendicularly on another, as in the figure i have just drawn? _emily._ you must allow me to put one foot of the compasses at the point of the angles, and draw a circle round them, and then i think i shall be able to answer your question: the two angles are together just equal to half a circle, they contain therefore degrees each; degrees being a quarter of . _mrs. b._ an angle of degrees or one-fourth of a circle is called a right angle, and when one line is perpendicular to another, and distant from its ends, it forms, you see, (fig. .) a right angle on either side. angles containing more than degrees are called obtuse angles, (fig. .) and those containing less than degrees are called acute angles, (fig. .) _caroline._ the angles of this square table are right angles, but those of the octagon table are obtuse angles; and the angles of sharp pointed instruments are acute angles. [illustration: plate ii.] _mrs. b._ very well. to return now to your observation, that if a ball is thrown obliquely against the wall, it will not rebound in the same direction; tell me, have you ever played at billiards? _caroline._ yes, frequently; and i have observed that when i push the ball perpendicularly against the cushion, it returns in the same direction; but when i send it obliquely to the cushion, it rebounds obliquely, but on an opposite side; the ball in this latter case describes an angle, the point of which is at the cushion. i have observed too, that the more obliquely the ball is struck against the cushion, the more obliquely it rebounds on the opposite side, so that a billiard player can calculate with great accuracy in what direction it will return. _mrs. b._ very well. this figure (fig. . plate .) represents a billiard table; now if you draw a line a b from the point where the ball a strikes perpendicular to the cushion, you will find that it will divide the angle which the ball describes into two parts, or two angles; the one will show the obliquity of the direction of the ball in its passage towards the cushion, the other its obliquity in its passage back from the cushion. the first is called _the angle of incidence_, the other _the angle of reflection_; and these angles are always equal, if the bodies are perfectly elastic. _caroline._ this then is the reason why, when i throw a ball obliquely against the wall, it rebounds in an opposite oblique direction, forming equal angles of incidence and of reflection. _mrs. b._ certainly; and you will find that the more obliquely you throw the ball, the more obliquely it will rebound. we must now conclude; but i shall have some further observations to make upon the laws of motion, at our next meeting. questions . (pg. ) on what is the science of mechanics founded? . (pg. ) in what does motion consist? . (pg. ) what is the consequence of inertia, on a body at rest? . (pg. ) what do we call that which produces motion? . (pg. ) give some examples. . (pg. ) what may we say of gravity, of cohesion, and of heat, as forces? . (pg. ) how will a body move, if acted on by a single force? . (pg. ) what is the reason of this? . (pg. ) what do we intend by the term velocity, and to what is it proportional? . (pg. ) velocity is divided into absolute and relative; what is meant by absolute velocity? . (pg. ) how is relative velocity distinguished? . (pg. ) how do we measure the velocity of a body? . (pg. ) the time? . (pg. ) the space? . (pg. ) what is uniform motion? and give an example. . (pg. ) how is uniform motion produced? . (pg. ) a ball struck by a bat gradually loses its motion; what causes produce this effect? . (pg. ) if gravity did not draw a projected body towards the earth, and the resistance of the air were removed, what would be the consequence? . (pg. ) in this case would not a great degree of force be required to produce a continued motion? . (pg. ) what is retarded motion? . (pg. ) give some examples. . (pg. ) what is accelerated motion? . (pg. ) give an example. . (pg. ) explain the mode in which gravity operates in producing this effect. . (pg. ) what number of feet will a heavy body descend in the first second of its fall, and at what rate will its velocity increase? . (pg. ) what is the difference in the time of the ascent and descent, of a stone, or other body thrown upwards? . (pg. ) by what reasoning is it proved that there is no difference? . (pg. ) what is meant by the momentum of a body? . (pg. ) how do we ascertain the momentum? . (pg. ) how may a light body have a greater momentum than one which is heavier? . (pg. ) why must we _multiply_ the weight and velocity together in order to find the momentum? . (pg. ) when we represent weight and velocity by numbers, what must we carefully observe? . (pg. ) why is it particularly important, to understand the nature of momentum? . (pg. ) what is meant by reaction, and what is the rule respecting it? . (pg. ) how is this exemplified by the ivory balls represented in plate . fig. ? . (pg. ) explain the manner in which the six balls represented in fig. , illustrate this fact. . (pg. ) what must be the nature of bodies, in which the whole motion is communicated from one to the other? . (pg. ) what is the result if the balls are not elastic, and how is this explained by fig. ? . (pg. ) how will reaction assist us in explaining the flight of a bird? . (pg. ) how must their wings operate in enabling them to remain stationary, to rise, and to descend? . (pg. ) why cannot a man fly by the aid of wings? . (pg. ) how does reaction operate in enabling us to swim, or to row a boat? . (pg. ) what constitutes elasticity? . (pg. ) give some examples. . (pg. ) what name is given to air, and for what reason? . (pg. ) what hard bodies are mentioned as elastic? . (pg. ) do elastic bodies exhibit any indentation after a blow? and why not? . (pg. ) what do we conclude from elasticity respecting the contact of the particles of a body? . (pg. ) are those bodies always the most elastic, which are the least dense? . (pg. ) give examples to prove that this is not the case. . (pg. ) all bodies are believed to be porous, what is said on this subject respecting gold? . (pg. ) what conjecture was made by sir isaac newton, respecting the porosity of bodies in general? . (pg. ) if you throw an elastic body against a wall, it will rebound; what is this occasioned by, and what is this return motion called? . (pg. ) what do we mean by a perpendicular line? . (pg. ) what is an angle? . (pg. ) what is represented by fig. . plate ? . (pg. ) have the length of the lines which meet in a point, any thing to do with the measurement of an angle? . (pg. ) what use can we make of compasses in measuring an angle? . (pg. ) into what number of parts do we suppose a whole circle divided, and what are these parts called? . (pg. ) when are two angles said to be equal? . (pg. ) upon what does the dimension of an angle depend? . (pg. ) what number of degrees, and what portion of a circle is there in a right angle? . (pg. ) how must one line be situated on another to form two right angles? (fig. .) . (pg. ) figure represents an angle of more than degrees, what is that called? . (pg. ) what are those of less than degrees called as in fig. ? . (pg. ) if you make an elastic ball strike a body at right angles, how will it return? . (pg. ) how if it strikes obliquely? . (pg. ) explain by fig. what is meant by the angles of incidence and of reflection. conversation iv. on compound motion. compound motion, the result of two opposite forces. of curvilinear motion, the result of two forces. centre of motion, the point at rest while the other parts of the body move round it. centre of magnitude, the middle of a body. centripetal force, that which impels a body towards a fixed central point. centrifugal force, that which impels a body to fly from the centre. fall of bodies in a parabola. centre of gravity, the point about which the parts balance each other. mrs. b. i must now explain to you the nature of compound motion. let us suppose a body to be struck by two equal forces in opposite directions, how will it move? _emily._ if the forces are equal, and their directions are in exact opposition to each other, i suppose the body would not move at all. _mrs. b._ you are perfectly right; but suppose the forces instead of acting upon the body in direct opposition to each other, were to move in lines forming an angle of ninety degrees, as the lines y a, x a, (fig. . plate .) and were to strike the ball a, at the same instant; would it not move? _emily._ the force x alone, would send it towards b, and the force y towards c; and since these forces are equal, i do not know how the body can obey one impulse rather than the other; and yet i think the ball would move, because as the two forces do not act in direct opposition, they cannot entirely destroy the effect of each other. _mrs. b._ very true; the ball therefore will not follow the direction of either of the forces, but will move in a line between them, and will reach d in the same space of time, that the force x would have sent it to b, and the force y would have sent it to c. now if you draw two lines, one from b, parallel to a c, and the other from c, parallel to a b, they will meet in d, and you will form a square; the oblique line which the body describes, is called the diagonal of the square. _caroline._ that is very clear, but supposing the two forces to be unequal, that the force x, for instance, be twice as great as the force y? _mrs. b._ then the force x, would drive the ball twice as far as the force y, consequently you must draw the line a b (fig. .) twice as long as the line a c, the body will in this case move to d; and if you draw lines from the points b and c, exactly as directed in the last example, they will meet in d, and you will find that the ball has moved in the diagonal of a rectangle. _emily._ allow me to put another case. suppose the two forces are unequal, but do not act on the ball in the direction of a right angle, but in that of an acute angle, what will result? _mrs. b._ prolong the lines in the directions of the two forces, and you will soon discover which way the ball will be impelled; it will move from a to d, in the diagonal of a parallelogram, (fig. .) forces acting in the direction of lines forming an obtuse angle, will also produce motion in the diagonal of a parallelogram. for instance, if the body set out from b, instead of a, and was impelled by the forces x and y, it would move in the dotted diagonal b c. we may now proceed to curvilinear motion: this is the result of two forces acting on a body; by one of which, it is projected forward in a right line; whilst by the other, it is drawn or impelled towards a fixed point. for instance, when i whirl this ball, which is fastened to my hand with a string, the ball moves in a circular direction, because it is acted on by two forces; that which i give it, which represents the force of projection, and that of the string which confines it to my hand. if, during its motion you were suddenly to cut the string, the ball would fly off in a straight line; being released from that confinement which caused it to move round a fixed point, it would be acted on by one force only; and motion produced by one force, you know, is always in a right line. _caroline._ this circular motion, is a little more difficult to comprehend than compound motion in straight lines. _mrs. b._ you have seen how the water is thrown off from a grindstone, when turned rapidly round; the particles of the stone itself have the same tendency, and would also fly off, was not their attraction of cohesion, greater than that of water. and indeed it sometimes happens, that large grindstones fly to pieces from the rapidity of their motion. _emily._ in the same way, the rim and spokes of a wheel, when in rapid motion, would be driven straight forwards in a right line, were they not confined to a fixed point, round which they are compelled to move. _mrs. b._ very well. you must now learn to distinguish between what is called the _centre_ of motion, and the _axis_ of motion; the former being considered as a point, the latter as a line. when a body, like the ball at the end of the string, revolves in a circle, the centre of the circle is called the centre of its motion, and the body is said to revolve in a plane; because a line extended from the revolving body, to the centre of motion, would describe a plane, or flat surface. when a body revolves round itself, as a ball suspended by a string, and made to spin round, or a top spinning on the floor, whilst it remains on the same spot; this revolution is round an imaginary line passing through the body, and this line is called its axis of motion. _caroline._ the axle of a grindstone, is then the axis of its motion; but is the centre of motion always in the middle of a body? _mrs. b._ no, not always. the middle point of a body, is called its centre of magnitude, or position, that is, the centre of its mass or bulk. bodies have also another centre, called the centre of gravity, which i shall explain to you; but at present we must confine ourselves to the axis of motion. this line you must observe remains at rest, whilst all the other parts of the body move around it; when you spin a top, the axis is stationary, whilst every other part is in motion round it. _caroline._ but a top generally has a motion forwards besides its spinning motion; and then no point within it can be at rest? _mrs. b._ what i say of the axis of motion, relates only to circular motion; that is to say, motion round a line, and not to that which a body may have at the same time in any other direction. there is one circumstance to which you must carefully attend; namely, that the further any part of a body is from the axis of motion, the greater is its velocity: as you approach that line, the velocity of the parts gradually diminish till you reach the axis of motion, which is perfectly at rest. _caroline._ but, if every part of the same body did not move with the same velocity, that part which moved quickest, must be separated from the rest of the body, and leave it behind? _mrs. b._ you perplex yourself by confounding the idea of circular motion, with that of motion in a right line; you must think only of the motion of a body round a fixed line, and you will find, that if the parts farthest from the centre had not the greatest velocity, those parts would not be able to keep up with the rest of the body, and would be left behind. do not the extremities of the vanes of a windmill move over a much greater space, than the parts nearest the axis of motion? (plate . fig. .) the three dotted circles represent the paths in which three different parts of the vanes move, and though the circles are of different dimensions, each of them is described in the same space of time. _caroline._ certainly they are; and i now only wonder, that we neither of us ever made the observation before: and the same effect must take place in a solid body, like the top in spinning; the most bulging part of the surface must move with the greatest rapidity. _mrs. b._ the force which draws a body towards a centre, round which it moves, is called the _centripetal_ force; and that force, which impels a body to fly from the centre, is called the _centrifugal_ force; when a body revolves round a centre, these two forces constantly balance each other; otherwise the revolving body would either approach the centre or recede from it, according as the one or the other prevailed. _caroline._ when i see any body moving in a circle, i shall remember, that it is acted on by two forces. _mrs. b._ motion, either in a circle, an ellipsis, or any other curve-line, must be the result of the action of two forces; for you know, that the impulse of one single force, always produces motion in a right line. _emily._ and if any cause should destroy the centripetal force, the centrifugal force would alone impel the body, and it would, i suppose, fly off in a straight line from the centre to which it had been confined. _mrs. b._ it would not fly off in a right line from the centre; but in a right line in the direction in which it was moving, at the instant of its release; if a stone, whirled round in a sling, gets loose at the point a, (plate . fig. .) it flies off in the direction a b; this line is called a _tangent_, it touches the circumference of the circle, and forms a right angle with a line drawn from that point of the circumference to the centre of the circle c. _emily._ you say, that motion in a curve-line, is owing to two forces acting upon a body; but when i throw this ball in a horizontal direction, it describes a curve-line in falling; and yet it is only acted upon by the force of projection; there is no centripetal force to confine it, or produce compound motion. _mrs. b._ a ball thus thrown, is acted upon by no less than three forces; the force of projection, which you communicate to it; the resistance of the air through which it passes, which diminishes its velocity, without changing its direction; and the force of gravity, which finally brings it to the ground. the power of gravity, and the resistance of the air, being always greater than any force of projection we can give a body, the latter is gradually overcome, and the body brought to the ground; but the stronger the projectile force, the longer will these powers be in subduing it, and the further the body will go before it falls. _caroline._ a shot fired from a cannon, for instance, will go much further, than a stone projected by the hand. _mrs. b._ bodies thus projected, you observe, describe a curve-line in their descent; can you account for that? _caroline._ no; i do not understand why it should not fall in the diagonal of a square. _mrs. b._ you must consider that the force of projection is strongest when the ball is first thrown; this force, as it proceeds, being weakened by the continued resistance of the air, the stone, therefore, begins by moving in a horizontal direction; but as the stronger powers prevail, the direction of the ball will gradually change from a horizontal, to a perpendicular line. _projection_ alone, would drive the ball a, to b, (fig. .) _gravity_ would bring it to c; therefore, when acted on in different directions, by these two forces, it moves between, gradually inclining more and more to the force of gravity, in proportion as this accumulates; instead therefore of reaching the ground at d, as you suppose it would, it falls somewhere about e. _caroline._ it is precisely so; look emily, as i throw this ball directly upwards, how gravity and the resistance of the air conquer projection. now i will throw it upwards obliquely: see, the force of projection enables it, for an instant, to act in opposition to that of gravity; but it is soon brought down again. _mrs. b._ the curve-line which the ball has described, is called in geometry a _parabola_; but when the ball is thrown perpendicularly upwards, it will descend perpendicularly; because the force of projection, and that of gravity, are in the same line of direction. [illustration: plate iii.] we have noticed the centres of magnitude, and of motion; but i have not yet explained to you, what is meant by the _centre of gravity_; it is that point in a body, about which all the parts exactly balance each other; if therefore that point be supported, the body will not fall. do you understand this? _emily._ i think so; if the parts round about this point have an equal tendency to fall, they will be in equilibrium, and as long as this point is supported, the body cannot fall. _mrs. b._ caroline, what would be the effect, were the body supported in any other single point? _caroline._ the surrounding parts no longer balancing each other, the body, i suppose, would fall on the side at which the parts are heaviest. _mrs. b._ infallibly; whenever the centre of gravity is unsupported, the body must fall. this sometimes happens with an overloaded wagon winding up a steep hill, one side of the road being more elevated than the other; let us suppose it to slope as is described in this figure, (plate . fig. .) we will say, that the centre of gravity of this loaded wagon is at the point a. now your eye will tell you, that a wagon thus situated, will overset; and the reason is, that the centre of gravity a, is not supported; for if you draw a perpendicular line from it to the ground at c, it does not fall under the wagon within the wheels, and is therefore not supported by them. _caroline._ i understand that perfectly; but what is the meaning of the other point b? _mrs. b._ let us, in imagination take off the upper part of the load; the centre of gravity will then change its situation, and descend to b, as that will now be the point about which the parts of the less heavily laden wagon will balance each other. will the wagon now be upset? _caroline._ no, because a perpendicular line from that point falls within the wheels at d, and is supported by them; and when the centre of gravity is supported, the body will not fall. _emily._ yet i should not much like to pass a wagon in that situation, for, as you see, the point d is but just within the left wheel; if the right wheel was raised, by merely passing over a stone, the point d would be thrown on the outside of the left wheel, and the wagon would upset. _caroline._ a wagon, or any carriage whatever, will then be most firmly supported, when the centre of gravity falls exactly between the wheels; and that is the case in a level road. _mrs. b._ the centre of gravity of the human body, is a point somewhere in a line extending perpendicularly through the middle of it, and as long as we stand upright, this point is supported by the feet; if you lean on one side, you will find that you no longer stand firm. a rope-dancer performs all his feats of agility, by dexterously supporting his centre of gravity; whenever he finds that he is in danger of losing his balance, he shifts the heavy pole which he holds in his hands, in order to throw the weight towards the side that is deficient; and thus by changing the situation of the centre of gravity, he restores his equilibrium. _caroline._ when a stick is poised on the tip of the finger, is it not by supporting its centre of gravity? _mrs. b._ yes; and it is because the centre of gravity is not supported, that spherical bodies roll down a slope. a sphere being perfectly round, can touch the slope but by a single point, and that point cannot be perpendicularly under the centre of gravity, and therefore cannot be supported, as you will perceive by examining this figure. (fig. . plate .) _emily._ so it appears: yet i have seen a cylinder of wood roll up a slope; how is that contrived? _mrs. b._ it is done by plugging or loading one side of the cylinder with lead, as at b, (fig. . plate .) the body being no longer of a uniform density, the centre of gravity is removed from the middle of the body to some point in or near the lead, as that substance is much heavier than wood; now you may observe that should this cylinder roll down the plane, as it is here situated, the centre of gravity must rise, which is impossible; the centre of gravity must always descend in moving, and will descend by the nearest and readiest means, which will be by forcing the cylinder up the slope, until the centre of gravity is supported, and then it stops. _caroline._ the centre of gravity, therefore, is not always in the middle of a body. _mrs. b._ no, that point we have called the centre of magnitude; when the body is of an uniform density, and of a regular form, as a cube, or sphere, the centres of gravity and of magnitude are in the same point; but when one part of the body is composed of heavier materials than another, the centre of gravity can no longer correspond with the centre of magnitude. thus you see the centre of gravity of this cylinder plugged with lead, cannot be in the same spot as the centre of magnitude. _emily._ bodies, therefore, consisting but of one kind of substance, as wood, stone, or lead, and whose densities are consequently uniform, must stand more firmly, and be more difficult to overset, than bodies composed of a variety of substances, of different densities, which may throw the centre of gravity on one side. _mrs. b._ that depends upon the situation of the materials; if those which are most dense, occupy the lower part, the stability will be increased, as the centre of gravity will be near the base. but there is another circumstance which more materially affects the firmness of their position, and that is their form. bodies that have a narrow base are easily upset, for if they are a little inclined, their centre of gravity is no longer supported, as you may perceive in fig. . _caroline._ i have often observed with what difficulty a person carries a single pail of water; it is owing, i suppose, to the centre of gravity being thrown on one side; and the opposite arm is stretched out to endeavour to bring it back to its original situation; but a pail hanging to each arm is carried with less difficulty, because they balance each other, and the centre of gravity remains supported by the feet. _mrs. b._ very well; i have but one more remark to make on the centre of gravity, which is, that when two bodies are fastened together by an inflexible rod, they are to be considered as forming but one body; if the two bodies be of equal weight, the centre of gravity will be in the middle of the line which unites them, (fig. .) but if one be heavier than the other, the centre of gravity will be proportionally nearer the heavy body than the light one. (fig. .) if you were to carry a rod or pole with an equal weight fastened at each end of it, you would hold it in the middle of the rod, in order that the weights should balance each other; whilst if the weights were unequal, you would hold it nearest the greater weight, to make them balance each other. _emily._ and in both cases we should support the centre of gravity; and if one weight be very considerably larger than the other, the centre of gravity will be thrown out of the rod into the heaviest weight. (fig. .) _mrs. b._ undoubtedly. questions . (pg. ) if a body be struck by two equal forces in opposite directions, what will be the result? . (pg. ) what is fig. . plate . intended to represent? . (pg. ) how would the ball move, and how would you represent the direction of its motion? . (pg. ) what is supposed respecting the forces represented in fig. ? . (pg. ) how would the body move if so impelled? . (pg. ) if the forces are unequal and not at right angles, how would the body move, as illustrated by fig. ? . (pg. ) how must a body be acted on, to produce motion in a curve, and what example is given? . (pg. ) when is a body said to revolve in a plane, and what is meant by the centre of motion? . (pg. ) what is intended by the axis of motion, and what are examples? . (pg. ) what is the middle point of a body called? . (pg. ) what is said of the axis of motion, whilst the body is revolving? . (pg. ) when a body revolves on an axis, do all its parts move with equal velocity? . (pg. ) how is this explained by fig. . plate ? . (pg. ) what are the two forces called which cause a body to move in a curve; and what proportion do these two forces bear to each other when a body revolves round a centre? . (pg. ) if the centripetal force were destroyed, how would a body be carried by the centrifugal? . (pg. ) explain what is meant by a _tangent_, as shown in fig. . plate . . (pg. ) what forces impede a body thrown horizontally? . (pg. ) give the reason why a body so projected, falls in a curve. (fig. . plate .) . (pg. ) the curve in which it falls, is not a part of a true circle: what is it denominated? . (pg. ) what is the _centre of gravity_ defined to be? . (pg. ) what results from supporting, or not supporting the centre of gravity? . (pg. ) what is intended to be explained by fig. . plate ? . (pg. ) what would be the effect of taking off the upper portion of the load? . (pg. ) when will a carriage stand most firmly? . (pg. ) what is said of the centre of gravity of the human body, and how does a rope dancer preserve his equilibrium? . (pg. ) why cannot a sphere remain at rest on an inclined plane? (fig. . plate .) . (pg. ) a cylinder of wood, may be made to rise to a small distance up an inclined plane. how may this be effected? (fig. . plate .) . (pg. ) when do we find the centres of gravity, and of magnitude in different points? . (pg. ) what influence will the density of the parts of a body exert upon its stability? . (pg. ) what other circumstance materially affects the firmness of position? (fig. . plate .) . (pg. ) why is it more easy to carry a weight in each hand, than in one only? . (pg. ) what is said respecting two bodies united by an inflexible rod? . (pg. ) what is fig. , plate , intended to illustrate? what fig. ; what fig. ? conversation v. on the mechanical powers. of the power of machines. of the lever in general. of the lever of the first kind, having the fulcrum between the power and the weight. of the lever of the second kind, having the weight between the power and the fulcrum. of the lever of the third kind, having the power between the fulcrum and the weight. mrs. b. we may now proceed to examine the mechanical powers; they are six in number: the _lever_, the _pulley_, the _wheel_ and _axle_, the _inclined plane_, the _wedge_ and the _screw_; one or more of which enters into the composition of every machine. a mechanical power is an instrument by which the effect of a given force is increased, whilst the force remains the same. in order to understand the power of a machine, there are four things to be considered. st. the power that acts: this consists in the effort of men or horses, of weights, springs, steam, &c. dly. the resistance which is to be overcome by the power: this is generally a weight to be moved. the power must always be superior to the resistance, otherwise the machine could not be put in motion. _caroline._ if for instance the resistance of a carriage was greater than the strength of the horses employed to draw it, they would not be able to make it move. _mrs. b._ dly. we are to consider the support or prop, or as it is termed in mechanics, the _fulcrum_; this you may recollect is the point upon which the body turns when in motion; and lastly, the respective velocities of the power, and of the resistance. _emily._ that must in general depend upon their respective distances from the fulcrum, or from the axis of motion; as we observed in the motion of the vanes of the windmill. _mrs. b._ we shall now examine the power of the lever. the _lever is an inflexible rod or bar, moveable about a fulcrum, and having forces applied to two or more points on it_. for instance, the steel rod to which these scales are suspended is a lever, and the point in which it is supported, the fulcrum, or centre of motion; now, can you tell me why the two scales are in equilibrium? _caroline._ being both empty, and of the same weight, they balance each other. _emily._ or, more correctly speaking, because the centre of gravity common to both, is supported. _mrs. b._ very well; and where is the centre of gravity of this pair of scales? (fig. . plate .) _emily._ you have told us that when two bodies of equal weight were fastened together, the centre of gravity was in the middle of the line that connected them; the centre of gravity of the scales must therefore be supported by the fulcrum f of the lever which unites the two scales, and which is the centre of motion. _caroline._ but if the scales contained different weights, the centre of gravity would no longer be in the fulcrum of the lever, but remove towards that scale which contained the heaviest weight; and since that point would no longer be supported, the heavy scale would descend, and out-weigh the other. _mrs. b._ true; but tell me, can you imagine any mode by which bodies of different weights can be made to balance each other, either in a pair of scales, or simply suspended to the extremities of the lever? for the scales are not an essential part of the machine; they have no mechanical power, and are used merely for the convenience of containing the substance to be weighed. _caroline._ what! make a light body balance a heavy one? i cannot conceive that possible. _mrs. b._ the fulcrum of this pair of scales (fig. .) is moveable, you see; i can take it off the beam, and fasten it on again in another part; this part is now become the fulcrum, but it is no longer in the centre of the lever. _caroline._ and the scales are no longer true; for that which hangs on the longest side of the lever descends. _mrs. b._ the two parts of the lever divided by the fulcrum, are called its arms; you should therefore say the longest arm, not the longest side of the lever. your observation is true that the balance is now destroyed; but it will answer the purpose of enabling you to comprehend the power of a lever, when the fulcrum is not in the centre. _emily._ this would be an excellent contrivance for those who cheat in the weight of their goods; by making the fulcrum a little on one side, and placing the goods in the scale which is suspended to the longest arm of the lever, they would appear to weigh more than they do in reality. _mrs. b._ you do not consider how easily the fraud would be detected; for on the scales being emptied they would not hang in equilibrium. if indeed the scale on the shorter arm was made heavier, so as to balance that on the longer, they would appear to be true, whilst they were really false. _emily._ true; i did not think of that circumstance. but i do not understand why the longest arm of the lever should not be in equilibrium with the other? _caroline._ it is because the momentum in the longest, is greater than in the shortest arm; the centre of gravity, therefore, is no longer supported. _mrs. b._ you are right, the fulcrum is no longer in the centre of gravity; but if we can contrive to make the fulcrum in its present situation become the centre of gravity, the scales will again balance each other; for you recollect that the centre of gravity is that point about which every part of the body is in equilibrium. _emily._ it has just occurred to me how this may be accomplished; put a great weight into the scale suspended to the shortest arm of the lever, and a smaller one into that suspended to the longest arm. yes, i have discovered it--look mrs. b., the scale on the shortest arm will carry lbs., and that on the longest arm only one, to restore the balance. (fig. .) _mrs. b._ you see, therefore, that it is not so impracticable as you imagined, to make a heavy body balance a light one; and this is in fact the means by which you observed that an imposition in the weight of goods might be effected, as a weight of ten or twelve ounces, might thus be made to balance a pound of goods. if you measure both arms of the lever, you will find that the length of the longer arm, is three times that of the shorter; and that to produce an equilibrium, the weights must bear the same proportion to each other, and that the greater weight, must be on the shorter arm. let us now take off the scales, that we may consider the lever simply; and in this state you see that the fulcrum is no longer the centre of gravity, because it has been removed from the middle of the lever; but it is, and must ever be, the centre of motion, as it is the only point which remains at rest, while the other parts move about it. [illustration: plate iv.] _caroline._ the arms of the lever being different in length, it now exactly resembles the steelyards, with which articles are so frequently weighed. _mrs. b._ it may in fact be considered as a pair of steelyards, by which the same power enables us to ascertain the weight of different articles, by simply increasing the distance of the power from the fulcrum; you know that the farther a body is from the axis of motion, the greater is its velocity. _caroline._ that i remember, and understand perfectly. _mrs. b._ you comprehend then, that the extremity of the longest arm of a lever, must move with greater velocity than that of the shortest arm, and that its momentum is greater in proportion. _emily._ no doubt, because it is farthest from the centre of motion. and pray, mrs. b., when my brothers play at _see-saw_, is not the plank on which they ride, a kind of lever? _mrs. b._ certainly; the log of wood which supports it from the ground is the fulcrum, and those who ride, represent the power and the resistance at the ends of the lever. and have you not observed that when those who ride are of equal weight, the plank must be supported in the middle, to make the two arms equal; whilst if the persons differ in weight, the plank must be drawn a little farther over the prop, to make the arms unequal, and the lightest person, who may be supposed to represent the power, must be placed at the extremity of the longest arm. _caroline._ that is always the case when i ride on a plank with my youngest brother; i have observed also that the lightest person has the best ride, as he moves both further and quicker; and i now understand that it is because he is more distant from the centre of motion. _mrs. b._ the greater velocity with which your little brother moves, renders his momentum equal to yours. _caroline._ yes; i have the most weight, he the greatest velocity; so that upon the whole our momentums are equal. but you said, mrs. b., that the power should be greater than the resistance, to put the machine in motion; how then can the plank move if the momentums of the persons who ride are equal? _mrs. b._ because each person at his descent touches and pushes against the ground with his feet; the reaction of which gives him an impulse which produces the motion; this spring is requisite to destroy the equilibrium of the power and the resistance, otherwise the plank would not move. did you ever observe that a lever describes the arc of a circle in its motion? _emily._ no; it appears to me to rise and descend perpendicularly; at least i always thought so. _mrs. b._ i believe i must make a sketch of you and your brother riding on a plank, in order to convince you of your error. (fig. . plate .) you may now observe that a lever can move only round the fulcrum, since that is the centre of motion; it would be impossible for you to rise perpendicularly, to the point a; or for your brother to descend in a straight line, to the point b; you must in rising, and he in descending, describe arcs of your respective circles. this drawing shows you also how much superior his velocity must be to yours; for if you could swing quite round, you would each complete your respective circles, in the same time. _caroline._ my brother's circle being much the largest, he must undoubtedly move the quickest. _mrs. b._ now tell me, do you think that your brother could raise you as easily without the aid of a lever? _caroline._ oh no, he could not lift me off the ground. _mrs. b._ then i think you require no further proof of the power of a lever, since you see what it enables your brother to perform. _caroline._ i now understand what you meant by saying, that in mechanics, velocity is opposed to weight, for it is my brother's velocity which overcomes my weight. _mrs. b._ you may easily imagine, what enormous weights may be raised by levers of this description, for the longer, when compared with the other, that arm is to which the power is applied, the greater will be the effect produced by it; because the greater is the velocity of the power compared to that of the weight. levers are of three kinds; in the first the fulcrum is between the power and the weight. _caroline._ this kind then comprehends the several levers you have described. _mrs. b._ yes, when in levers of the first kind, the fulcrum is equally distant from the power and the weight, as in the balance, there will be an equilibrium, when the power and the weight are equal to each other; it is not then a mechanical power, for nothing can in this case be gained by velocity; the two arms of the lever being equal, the velocity of their extremities must be so likewise. the balance is therefore of no assistance as a mechanical power, although it is extremely useful in estimating the respective weights of bodies. but when (fig. .) the fulcrum f of a lever is not equally distant from the power and the weight, and the power p acts at the extremity of the longest arm, it may be less than the weight w; its deficiency being compensated by its superior velocity, as we observed in the _see-saw_. _emily._ then when we want to lift a great weight, we must fasten it to the shortest arm of a lever, and apply our strength to the longest arm? _mrs. b._ if the case will admit of your putting the end of the lever under the resisting body, no fastening will be required; as you will perceive, when a nail is drawn by means of a hammer, which, though bent, is a lever of the first kind; the handle being the longest arm, the point on which it rests, the fulcrum, and the distance from that to the part which holds the nail, the short arm. but let me hear, caroline, whether you can explain the action of this instrument, which is composed of two levers united in one common fulcrum. _caroline._ a pair of scissors! _mrs. b._ you are surprised; but if you examine their construction, you will discover that it is the power of the lever, that assists us in cutting with scissors. _caroline._ yes; i now perceive that the point at which the two levers are screwed together, is the fulcrum; the power of the fingers is applied to the handles, and the article to be cut, is the resistance; therefore, the longer the handles, and the shorter the points of the scissors, the more easily you cut with them. _emily._ that i have often observed, for when i cut paste-board or any hard substance, i always make use of that part of the scissors nearest the screw or rivet, and i now understand why it increases the power of cutting; but i confess that i never should have discovered scissors to have been double levers; and pray are not snuffers levers of a similar description? _mrs. b._ yes, and most kinds of pincers; the great power of which consists in the great relative length of the handles. did you ever notice the swingle-tree of a carriage to which the horses are attached when drawing? _emily._ o yes; this is a lever of the first kind, but the fulcrum being in the middle, the horses should draw with equal power, whatever may be their strength. _mrs. b._ that is generally the case, but it is evident that by making one arm longer than the other, it might be adapted to horses of unequal strength. _caroline._ and of what nature are the other two kinds of levers? _mrs. b._ in levers of the second kind, the weight, instead of being at one end, is situated between the power and the fulcrum, (fig. .) _caroline._ the weight and the fulcrum have here changed places; and what advantage is gained by this kind of lever? _mrs. b._ in moving it, the velocity of the power must necessarily be greater than that of the weight, as it is more distant from the centre of the motion. have you ever seen your brother move a snow-ball by means of a strong stick, when it became too heavy for him to move without assistance? _caroline._ oh yes; and this was a lever of the second kind, (fig. .) the end of the stick, which he thrusts under the ball, and which rests on the ground, becomes the fulcrum; the ball is the weight to be moved, and the power his hands, applied to the other end of the lever. in this instance there is a great difference in the length of the arms of the lever; for the weight is almost close to the fulcrum. _mrs. b._ and the advantage gained is proportional to this difference. the most common example that we have of levers of the second kind, is in the doors of our apartments. _emily._ the hinges represent the fulcrum, our hands the power applied to the other end of the lever; but where is the weight to be moved? _mrs. b._ the door is the weight, which in this example occupies the whole of the space between the power and the fulcrum. nut crackers are double levers of this kind: the hinge is the fulcrum, the nut the resistance, and the hands the power. in levers of the third kind (fig. .) the fulcrum is again at one extremity, the weight or resistance at the other, and the power is applied between the fulcrum and the resistance. _emily._ the fulcrum, the weight, or the power, then, each in its turn, occupies some part of the lever between its extremities. but in this third kind of lever, the weight being farther than the power from the centre of motion, the difficulty of raising it seems increased rather than diminished. _mrs. b._ that is very true; a lever of this kind is therefore never used, unless absolutely necessary, as is the case in raising a ladder in order to place it against a wall; the man who raises it cannot place his hands on the upper part of the ladder, the power, therefore, is necessarily placed much nearer to the fulcrum than to the weight. _caroline._ yes, the hands are the power, the ground the fulcrum, and the upper part of the ladder the weight. _mrs. b._ nature employs this kind of lever in the structure of the human frame. in lifting a weight with the hand, the lower part of the arm becomes a lever of the third kind; the elbow is the fulcrum, the muscles of the fleshy part of the arm, the power; and as these are nearer to the elbow than to the hand, it is necessary that their power should exceed the weight to be raised. _emily._ is it not surprising that nature should have furnished us with such disadvantageous levers? _mrs. b._ the disadvantage, in respect to power, is more than counterbalanced by the convenience resulting from this structure of the arm; and it is that no doubt which is best adapted to enable it to perform its various functions. there is one rule which applies to every lever, which is this: in order to produce an equilibrium, the power must bear the same proportion to the weight, as the length of the shorter arm does to that of the longer; as was shown by emily with the weights of _lb._ and of _lb._ fig. . plate . we have dwelt so long on the lever, that we must reserve the examination of the other mechanical powers, to our next interview. questions . (pg. ) how many mechanical powers are there, and what are they named? . (pg. ) what is a mechanical power defined to be? . (pg. ) what four particulars must be observed? . (pg. ) upon what will the velocities depend? . (pg. ) what is a lever? . (pg. ) give a familiar example. . (pg. ) when and why do the scales balance each other, and where is their centre of gravity? (fig. . plate .) . (pg. ) why would they not balance with unequal weights? . (pg. ) were the fulcrum removed from the middle of the beam what would result? . (pg. ) what do we mean by the arms of a lever? . (pg. ) how may a pair of scales be false, and yet appear to be true? . (pg. ) if the fulcrum be removed from the centre of gravity, how may the equilibrium be restored? . (pg. ) how is this exemplified by fig. . plate ? . (pg. ) what proportion must the weights bear to the lengths of the arms? . (pg. ) on what principle do we weigh with a pair of steelyards, and what will be the difference in the motion of the extremities of such a lever? . (pg. ) how is this exemplified by fig. . plate ? . (pg. ) what line is described by the ends of a lever? fig. . plate . . (pg. ) how many kinds are there; and in the first how is the fulcrum situated? . (pg. ) when may the fulcrum be so situated that this lever is not a mechanical power, and why? . (pg. ) what is represented by fig. . plate ? . (pg. ) give a familiar example of the use of a lever of the first kind. . (pg. ) in what instruments are two such levers combined? . (pg. ) how may two horses of unequal strength, be advantageously coupled in a carriage? . (pg. ) describe a lever of the second kind. (fig. . plate .) . (pg. ) what is represented in fig. . plate , and in what proportion does this lever gain power? . (pg. ) what is said respecting a door? . (pg. ) describe a lever of the third kind. . (pg. ) in what instance do we use this? . (pg. ) what remarks are made on its employment in the limbs of animals? . (pg. ) what are the conditions of equilibrium in every lever? conversation v. continued. on the mechanical powers. of the pulley. of the wheel and axle. of the inclined plane. of the wedge. of the screw. mrs. b. the pulley is the second mechanical power we are to examine. you both, i suppose, have seen a pulley? _caroline._ yes, frequently: it is a circular, and flat piece of wood or metal, with a string which runs in a groove round it: by means of which, a weight may be pulled up; thus pulleys are used for drawing up curtains. _mrs. b._ yes; but in that instance the pulleys are fixed; that is, they retain their places, and merely turn round on their axis; these do not increase the power to raise the weights, as you will perceive by this figure. (plate . fig. .) observe that the fixed pulley is on the same principle as the lever of a pair of scales, in which the fulcrum f being in the centre of gravity, the power p and the weight w, are equally distant from it, and no advantage is gained. _emily._ certainly; if p represents the power employed to raise the weight w, the power must be greater than the weight in order to move it. but of what use then is a fixed pulley in mechanics? _mrs. b._ although it does not increase the power, it is frequently useful for altering its direction. a single fixed pulley enables us to draw a curtain up, by pulling the string connected with it downwards; and we should be at a loss to accomplish this simple operation without its assistance. _caroline._ there would certainly be some difficulty in ascending to the head of the curtain, in order to draw it up. indeed i now recollect having seen workmen raise weights to a considerable height by means of a fixed pulley, which saved them the trouble of going up themselves. _mrs. b._ the next figure represents a pulley which is not fixed; (fig. .) and thus situated, you will perceive that it affords us mechanical assistance. a is a moveable pulley; that is, one which is attached to the weight to be raised, and which consequently moves up or down with it. there is also a fixed pulley d, which is only of use to change the direction of the power p. now it is evident that the velocity of the power, will be double that of the weight w; for if the rope be pulled at p, until the pulley a ascends with the weight to the fixed pulley d, then both parts of the rope, c and b, must pass over the fixed pulley, and consequently the hand at p, will have descended through a space equal to those two parts; but the weight will have ascended only one half of that distance. _caroline._ that i understand: if p drew the string but one inch, the weight would be raised only half an inch, because it would shorten the strings b and c half an inch each, and consequently the pulley with the weight attached to it, can be raised only half an inch. _emily._ but i do not yet understand the advantage of moveable pulleys; they seem to me to increase rather than diminish the difficulty of raising weights, since you must draw the string double the length that you raise the weight; whilst with a single pulley, or without any pulley, the weight is raised as much as the string is shortened. _mrs. b._ the advantage of a moveable pulley consists in dividing the difficulty; we must, it is true, draw twice the length of the string, but then only half the strength is required that would be necessary to raise the weight without the assistance of a moveable pulley. _emily._ so that the difficulty is overcome in the same manner as it would be, by dividing the weight into two equal parts, and raising them successively. _mrs. b._ exactly. you must observe, that with a moveable pulley the velocity of the power, is double that of the weight; since the power p (fig. .) moves two inches whilst the weight w moves one inch; therefore the power need not be more than half the weight, to make their momentums equal. _caroline._ pulleys act then on the same principle as the lever; the deficiency of weight in the power, being compensated by its superior velocity, so as to make their momentums equal. _mrs. b._ you will find, that all gain of power in mechanics is founded on the same principle. _emily._ but may it not be objected to pulleys, that a longer time is required to raise a weight by their aid, than without it? for what you gain in power, you lose in time. _mrs. b._ that, my dear, is the fundamental law in mechanics: it is the case with the lever, as well as the pulley; and you will find it to be so with all the other mechanical powers. _caroline._ i do not see any advantage in the mechanical powers then, if what we gain by them in one way, is lost in another. _mrs. b._ since we are not able to increase our natural strength is not any instrument of obvious utility, by means of which we may reduce the resistance or weight of any body, to the level of that strength? this the mechanical powers enable us to accomplish. it is true, as you observe, that it requires a sacrifice of time to attain this end, but you must be sensible how very advantageously it is exchanged for power. if one man by his natural strength could raise one hundred pounds only, it would require five such men to raise five hundred pounds; and if one man performs this by the help of a suitable engine, there is then no actual loss of time; as he does the work of five men, although he is five times as long in its accomplishment. you can now understand, that the greater the number of moveable pulleys connected by a string, the more easily the weight is raised; as the difficulty is divided amongst the number of strings, or rather of parts into which the string is divided, by the pulleys. two, or more pulleys thus connected, form what is called a tackle, or system of pulleys. (fig. .) you may have seen them suspended from cranes to raise goods into warehouses. _emily._ when there are two moveable pulleys, as in the figure you have shown to us, (fig. .) there must also be two fixed pulleys, for the purpose of changing the direction of the string, and then the weight is supported by four strings, and of course, each must bear only one fourth part of the weight. _mrs. b._ you are perfectly correct, and the rule for estimating the power gained by a system of pulleys, is to count the number of strings by which the weight is supported; or, which amounts to the same thing, to multiply the number of moveable pulleys by two. in shipping, the advantages of both an increase of power, and a change of direction, by means of pulleys, are of essential importance: for the sails are raised up the masts by the sailors on deck, from the change of direction which the pulley effects, and the labour is facilitated by the mechanical power of a combination of pulleys. [illustration: plate v.] _emily._ but the pulleys on ship-board do not appear to me to be united in the manner you have shown us. _mrs. b._ they are, i believe, generally connected as described in figure , both for nautical, and a variety of other purposes; but in whatever manner pulleys are connected by a single string, the mechanical power is the same. the third mechanical power, is the wheel and axle. let us suppose (plate . fig. ) the weight w, to be a bucket of water in a well, which we raise by winding round the axle the rope, to which it is attached; if this be done without a wheel to turn the axle, no mechanical assistance is received. the axle without a wheel is as impotent as a single fixed pulley, or a lever, whose fulcrum is in the centre: but add the wheel to the axle, and you will immediately find the bucket is raised with much less difficulty. the velocity of the circumference of the wheel is as much greater than that of the axle, as it is further from the centre of motion; for the wheel describes a great circle in the same space of time that the axle describes a small one, therefore the power is increased in the same proportion as the circumference of the wheel is greater than that of the axle. if the velocity of the wheel is twelve times greater than that of the axle, a power twelve times less than the weight of the bucket, would balance it; and a small increase would raise it. _emily._ the axle acts the part of the shorter arm of the lever, the wheel that of the longer arm. _caroline._ in raising water, there is commonly, i believe, instead of a wheel attached to the axle, only a crooked handle, which answers the purpose of winding the rope round the axle, and thus raising the bucket. _mrs. b._ in this manner (fig. ;) now if you observe the dotted circle which the handle describes in winding up the rope, you will perceive that the branch of the handle a, which is united to the axle, represents the spoke of a wheel, and answers the purpose of an entire wheel; the other branch b affords no mechanical aid, merely serving as a handle to turn the wheel. wheels are a very essential part of most machines; they are employed in various ways; but, when fixed to the axle, their mechanical power is always the same: that is, as the circumference of the wheel exceeds that of the axle, so much will the energy of the power be increased. _caroline._ then the larger the wheel, in proportion to the axle, the greater must be its effect? _mrs. b._ certainly. if you have ever seen any considerable mills or manufactures, you must have admired the immense wheel, the revolution of which puts the whole of the machinery into motion; and though so great an effect is produced by it, a horse or two has sufficient power to turn it; sometimes a stream of water is used for that purpose, but of late years, a steam-engine has been found both the most powerful and the most convenient mode of turning the wheel. _caroline._ do not the vanes of a windmill represent a wheel, mrs. b.? _mrs. b._ yes; and in this instance we have the advantage of a gratuitous force, the wind, to turn the wheel. one of the great benefits resulting from the use of machinery is, that it gives us a sort of empire over the powers of nature, and enables us to make them perform the labour which would otherwise fall to the lot of man. when a current of wind, a stream of water, or the expansive force of steam, performs our task, we have only to superintend and regulate their operations. the fourth mechanical power is the inclined plane; this is generally nothing more than a plank placed in a sloping direction, which is frequently used to facilitate the raising of weights, to a small height, such as the rolling of hogsheads or barrels into a warehouse. it is not difficult to understand, that a weight may much more easily be rolled up a slope than it can be raised the same height perpendicularly. but in this, as well as the other mechanical powers, the facility is purchased by a loss of time (fig. ;) for the weight, instead of moving directly from a to c, must move from b to c, and as the length of the plane is to its height, so much is the resistance of the weight diminished. _emily._ yes; for the resistance, instead of being confined to the short line a c, is spread over the long line b c. _mrs. b._ the wedge, which is the next mechanical power, is usually viewed as composed of two inclined planes (fig. :) you may have seen wood-cutters use it to cleave wood. the resistance consists in the cohesive attraction of the wood, or any other body which the wedge is employed to separate; the advantage gained by this power is differently estimated by philosophers; but one thing is certain, its power is increased, in proportion to the decrease of its thickness, compared with its length. the wedge is a very powerful instrument, but it is always driven forward by blows from a hammer, or some other body having considerable momentum. _emily._ the wedge, then, is rather a compound than a distinct mechanical power, since it is not propelled by simple pressure, or weight, like the other powers. _mrs. b._ it is so. all cutting instruments are constructed upon the principle of the inclined plane, or the wedge: those that have but one edge sloped, like the chisel, may be referred to the inclined plane; whilst the axe, the hatchet, and the knife, (when used to split asunder) are used as wedges. _caroline._ but a knife cuts best when it is drawn across the substance it is to divide. we use it thus in cutting meat, we do not chop it to pieces. _mrs. b._ the reason of this is, that the edge of a knife is really a very fine saw, and therefore acts best when used like that instrument. the screw, which is the last mechanical power, is more complicated than the others. you will see by this figure, (fig. .) that it is composed of two parts, the screw and the nut. the screw s is a cylinder, with a spiral protuberance coiled round it, called the thread; the nut n is perforated to receive the screw, and the inside of the nut has a spiral groove, made to fit the spiral thread of the screw. _caroline._ it is just like this little box, the lid of which screws on the box as you have described; but what is this handle l which projects from the nut? _mrs. b._ it is a lever, which is attached to the nut, without which the screw is never used as a mechanical power. the power of the screw, complicated as it appears, is referable to one of the most simple of the mechanical powers; which of them do you think it is? _caroline._ in appearance, it most resembles the wheel and axle. _mrs. b._ the lever, it is true, has the effect of a wheel, as it is the means by which you turn the nut, or sometimes the screw, round; but the lever is not considered as composing a part of the screw, though it is true, that it is necessarily attached to it. _emily._ the spiral thread of the screw resembles, i think, an inclined plane: it is a sort of slope, by means of which the nut ascends more easily than it would do if raised perpendicularly; and it serves to support it when at rest. _mrs. b._ very well: if you cut a slip of paper in the form of an inclined plane, and wind it round your pencil, which will represent the cylinder, you will find that it makes a spiral line, corresponding to the spiral protuberance of the screw. (fig. .) _emily._ very true; the nut then ascends an inclined plane, but ascends it in a spiral, instead of a straight line: the closer the threads of the screw, the more easy the ascent: it is like having shallow, instead of steep steps to ascend. _mrs. b._ yes; excepting that the nut takes no steps, as it gradually winds up or down; then observe, that the closer the threads of the screw, the less is its ascent in turning round, and the greater is its power; so that we return to the old principle,--what is saved in power is lost in time. _emily._ cannot the power of the screw be increased also, by lengthening the lever attached to the nut? _mrs. b._ certainly. the screw, with the addition of the lever, forms a very powerful machine, employed either for compression or to raise heavy weights. it is used by book-binders, to press the leaves of books together; it is used also in cider and wine presses, in coining, and for a variety of other purposes. _emily._ pray, mrs. b., by what rule do you estimate the power of the screw? _mrs. b._ by measuring the circumference of the circle, which the end of the lever would form in one whole revolution, and comparing this with the distance from the centre of one thread of the screw, to that of its next contiguous turn; for whilst the lever travels that whole distance, the screw rises or falls only through the distance from one coil to another. _caroline._ i think that i have sometimes seen the lever attached to the screw, and not to the nut, as it is represented in the figure. _mrs. b._ this is frequently done, but it does not in any degree affect the power of the instrument. all machines are composed of one or more of these six mechanical powers we have examined; i have but one more remark to make to you relative to them, which is, that friction in a considerable degree diminishes their force: allowance must therefore always be made for it, in the construction of machinery. _caroline._ by friction, do you mean one part of the machine rubbing against another part contiguous to it? _mrs. b._ yes; friction is the resistance which bodies meet with in rubbing against each other; there is no such thing as perfect smoothness or evenness in nature; polished metals, though they wear that appearance more than most other bodies, are far from really possessing it; and their inequalities may frequently be perceived through a good magnifying glass. when, therefore, the surfaces of the two bodies come in contact, the prominent parts of the one, will often fall into the hollow parts of the other, and occasion more or less resistance to motion. _caroline._ but if a machine is made of polished metal, as a watch for instance, the friction must be very trifling? _mrs. b._ in proportion as the surfaces of bodies are well polished, the friction is doubtless diminished; but it is always considerable, and it is usually computed to destroy one-third of the power of a machine. oil or grease is used to lessen friction: it acts as a polish, by filling up the cavities of the rubbing surfaces, and thus making them slide more easily over each other. _caroline._ is it for this reason that wheels are greased, and the locks and hinges of doors oiled? _mrs. b._ yes; in these instances the contact of the rubbing surfaces is so close, and they are so constantly in use, that they require to be frequently oiled, or a considerable degree of friction is produced. there are two kinds of friction; the first is occasioned by the rubbing of the surfaces of bodies against each other, the second, by the rolling of a circular body; as that of a carriage wheel upon the ground: the friction resulting from the first is much the most considerable, for great force is required to enable the sliding body to overcome the resistance which the asperities of the surfaces in contact oppose to its motion, and it must be either lifted over, or break through them; whilst, in the second kind of friction, the rough parts roll over each other with comparative facility; hence it is, that wheels are often used for the sole purpose of diminishing the resistance from friction. _emily._ this is one of the advantages of carriage wheels, is it not? _mrs. b._ yes; and the larger the circumference of the wheel the more readily it can overcome any considerable obstacles, such as stones, or inequalities in the road. when, in descending a steep hill, we fasten one of the wheels, we decrease the velocity of the carriage, by increasing the friction. _caroline._ that is to say, by converting the rolling friction into the rubbing friction. and when you had casters put to the legs of the table, in order to move it more easily, you changed the rubbing into the rolling friction. _mrs. b._ there is another circumstance which we have already noticed, as diminishing the motion of bodies, and which greatly affects the power of machines. this is the resistance of the medium, in which a machine is worked. all fluids, whether elastic like air, or non-elastic like water and other liquids, are called mediums; and their resistance is proportioned to their density; for the more matter a body contains, the greater the resistance it will oppose to the motion of another body striking against it. _emily._ it would then be much more difficult to work a machine under water than in the air? _mrs. b._ certainly, if a machine could be worked in _vacuo_, and without friction, it would not be impeded, but this is unattainable; a considerable reduction of power must therefore be allowed for, from friction and the resistance of the medium. we shall here conclude our observations on the mechanical powers. at our next meeting i shall endeavour to give you an explanation of the motion of the heavenly bodies. questions . (pg. ) describe a pulley, and its use. . (pg. ) what is meant by a fixed pulley and why is not power gained by its employment? (fig. . plate .) . (pg. ) of what use is the fixed pulley? . (pg. ) how is the power gained by a moveable pulley, explained by means of fig. . plate ? . (pg. ) what proportion must the power bear to the weight in fig. , that their momentums may be equal? . (pg. ) what is a fundamental law as respects power and time? . (pg. ) if to gain power we must lose time, what advantage do we derive from the mechanical powers? . (pg. ) what name is given to two or more pulleys connected by one string? . (pg. ) how do we estimate the power gained by a system of pulleys? . (pg. ) what is represented by fig. . plate ? . (pg. ) how does the wheel operate in increasing power? . (pg. ) how is this compared with the lever? . (pg. ) how does a handle fixed to an axle, represent a wheel, fig. ? . (pg. ) how could we increase the power in this instrument? . (pg. ) what other forces besides the power of men, do we employ to move machines? . (pg. ) what will serve as an example of an inclined plane? . (pg. ) in what proportion does it gain power? (fig. .) . (pg. ) to what is the wedge compared? (fig. .) . (pg. ) how does its power increase? . (pg. ) why is it rather a compound than a simple power? . (pg. ) what common instruments act upon the principle of the inclined plane, or the wedge? . (pg. ) why does a knife cut best when drawn across? . (pg. ) the screw has two essential parts; what are they? . (pg. ) what other instrument is used to turn the screw? . (pg. ) how can you compare the screw with an inclined plane? fig. . . (pg. ) by what two means may the power of the screw be increased? . (pg. ) how do we estimate the power gained by the screw? . (pg. ) is the lever always attached to the nut, as in the figure? . (pg. ) what is said respecting the composition of all machines, and for what must allowance always be made in estimating their power? . (pg. ) what is meant by friction, and what causes it? . (pg. ) how may friction be diminished? . (pg. ) friction is of two kinds, what are they? . (pg. ) for what purpose are wheels often used? . (pg. ) when is the friction of a carriage wheel changed from the rolling to the rubbing friction? . (pg. ) what is a medium, and in what proportion does it diminish motion? . (pg. ) under what circumstances must a body be placed, in order to move without impediment? conversation vi. causes of the motion of the heavenly bodies. of the earth's annual motion. of the planets and their motion. of the diurnal motion of the earth and planets. caroline. i am come to you to-day quite elated with the spirit of opposition, mrs. b.; for i have discovered such a powerful objection to your theory of attraction, that i doubt whether even your conjuror newton, with his magic wand of gravitation, will be able to dispel it. _mrs. b._ well, my dear, pray what is this weighty objection? [illustration: plate vi.] _caroline._ you say that the earth revolves in its orbit round the sun once in a year, and that bodies attract in proportion to the quantity of matter they contain; now we all know the sun to be much larger than the earth: why, therefore does it not draw the earth into itself; you will not, i suppose, pretend to say that we are falling towards the sun? _emily._ however plausible your objection appears, caroline, i think you place too much reliance upon it: when any one has given such convincing proofs of sagacity and wisdom as sir isaac newton, when we find that his opinions are universally received and adopted, is it to be expected that any objection we can advance should overturn them? _caroline._ yet i confess that i am not inclined to yield implicit faith even to opinions of the great newton: for what purpose are we endowed with reason, if we are denied the privilege of making use of it, by judging for ourselves. _mrs. b._ it is reason itself which teaches us, that when we, novices in science, start objections to theories established by men of knowledge and wisdom, we should be diffident rather of our own than of their opinion. i am far from wishing to lay the least restraint on your questions; you cannot be better convinced of the truth of a system, than by finding that it resists all your attacks, but i would advise you not to advance your objections with so much confidence, in order that the discovery of their fallacy may be attended with less mortification. in answer to that you have just proposed, i can only say, that the earth really is attracted by the sun. _caroline._ take care, at least, that we are not consumed by him, mrs. b. _mrs. b._ we are in no danger; but newton, our magician, as you are pleased to call him, cannot extricate himself from this difficulty without the aid of some cabalistical figures, which i must draw for him. let us suppose the earth, at its creation, to have been projected forwards into universal space: we know that if no obstacle impeded its course it would proceed in the same direction, and with a uniform velocity for ever. in fig. . plate , a represents the earth, and s the sun. we shall suppose the earth to be arrived at the point in which it is represented in the figure, having a velocity which would carry it on to b in the space of one month; whilst the sun's attraction would bring it to c in the same space of time. observe that the two forces of projection and attraction do not act in opposition, but perpendicularly, or at a right angle to each other. can you tell me now, how the earth will move? _emily._ i recollect your teaching us that a body acted upon by two forces perpendicular to each other, would move in the diagonal of a parallelogram; if, therefore, i complete the parallelogram, by drawing the lines c d, b d, the earth will move in the diagonal a d. _mrs. b._ a ball struck by two forces acting perpendicularly to each other, it is true, moves in the diagonal of a parallelogram; but you must observe that the force of attraction is continually acting upon our terrestrial ball, and producing an incessant deviation from its course in a right line, which converts it into that of a curve-line; every point of which may be considered as constituting the diagonal of an infinitely small parallelogram. let us retain the earth a moment at the point d, and consider how it will be affected by the combined action of the two forces in its new situation. it still retains its tendency to fly off in a straight line; but a straight line would now carry it away to f, whilst the sun would attract it in the direction d s; how then will it proceed? _emily._ it will go on in a curve-line, in a direction between that of the two forces. _mrs. b._ in order to know exactly what course the earth will follow, draw another parallelogram similar to the first, in which the line d f describes the force of projection, and the line d s that of attraction; and you will find that the earth will proceed in the curve-line d g. _caroline._ you must now allow me to draw a parallelogram, mrs. b. let me consider in what direction will the force of projection now impel the earth. _mrs. b._ first draw a line from the earth to the sun representing the force of attraction; then describe the force of projection at a right angle to it. _caroline._ the earth will then move in the curve g i, of the parallelogram g h i k. _mrs. b._ you recollect that a body acted upon by two forces, moves through a diagonal, in the same time that it would have moved through one of the sides of the parallelogram, were it acted upon by one force only. the earth has passed through the diagonals of these three parallelograms, in the space of three months, and has performed one quarter of a circle; and on the same principle it will go on till it has completed the whole of the circle. it will then recommence a course, which it has pursued ever since it first issued from the hand of its creator, and which there is every reason to suppose it will continue to follow, as long as it remains in existence. _emily._ what a grand and beautiful effect resulting from so simple a cause! _caroline._ it affords an example, on a magnificent scale, of the curvilinear motion, which you taught us in mechanics. the attraction of the sun is the centripetal force, which confines the earth to a centre; and the impulse of projection, the centrifugal force, which impels the earth to quit the sun, and fly off in a tangent. _mrs. b._ exactly so. a simple mode of illustrating the effect of these combined forces on the earth, is to cut a slip of card in the form of a carpenter's square, as a, b, c; (fig. . plate .) the point b will be a right angle, the sides of the square being perpendicular to each other; after having done this you are to describe a small circle at the angular point b, representing the earth, and to fasten the extremity of one of the legs of the square to a fixed point a, which we shall consider as the sun. thus situated, the two sides of the square will represent both the centrifugal and centripetal forces; a b, representing the centripetal, and b c, the centrifugal force; if you now draw it round the fixed point, you will see how the direction of the centrifugal force varies, constantly forming a tangent to the circle in which the earth moves, as it is constantly at a right angle with the centripetal force. _emily._ the earth then, gravitates towards the sun, without the slightest danger either of approaching nearer, or receding further from it. how admirably this is contrived! if the two forces which produce this curved motion, had not been so accurately adjusted, one would ultimately have prevailed over the other, and we should either have approached so near the sun as to have been burnt, or have receded so far from it as to have been frozen. _mrs. b._ what will you say, my dear, when i tell you, that these two forces are not, in fact, so proportioned as to produce circular motion in the earth? we actually revolve round the sun in an elliptical or oval orbit, the sun being situated in one of the foci or centres of the oval, so that the sun is at some periods much nearer to the earth, than at others. _caroline._ you must explain to us, at least, in what manner we avoid the threatened destruction. _mrs. b._ let us suppose that when the earth is at a, (fig. .) its projectile force should not have given it a velocity sufficient to counterbalance that of gravity, so as to enable these powers conjointly to carry it round the sun in a circle; the earth, instead of describing the line a c, as in the former figure, will approach nearer the sun in the line a b. _caroline._ under these circumstances, i see not what is to prevent our approaching nearer and nearer the sun, till we fall into it: for its attraction increases as we advance towards it, and produces an accelerated velocity in the earth, which increases the danger. _mrs. b._ there is another seeming danger, of which you are not aware. observe, that as the earth approaches the sun, the direction of its projectile force is no longer perpendicular to that of its attraction, but inclines more nearly to it. when the earth reaches that part of its orbit at b, the force of projection would carry it to d, which brings it nearer the sun instead of bearing it away from it. _emily._ if, then, we are driven by one power, and drawn by the other to this centre of destruction, how is it possible for us to escape? _mrs. b._ a little patience, and you will find that we are not without resource. the earth continues approaching the sun with a uniformly increasing accelerated motion, till it reaches the point e; in what direction will the projectile force now impel it? _emily._ in the direction e f. here then the two forces act perpendicularly to each other, the lines representing them forming a right angle, and the earth is situated just as it was in the preceding figure; therefore, from this point, it should revolve round the sun in a circle. _mrs. b._ no, all the circumstances do not agree. in motion round a centre, you recollect that the centrifugal force increases with the velocity of the body, or in other words, the quicker it moves the stronger is its tendency to fly off in a right line. when the earth, therefore, arrives at e, its accelerated motion will have so far increased its velocity, and consequently its centrifugal force, that the latter will prevail over the force of attraction, and force the earth away from the sun till it reaches g. _caroline._ it is thus then that we escape from the dangerous vicinity of the sun; and in proportion as we recede from it, the force of its attraction, and, consequently, the velocity of the earth's motion, are diminished. _mrs. b._ yes. from g the direction of projection is towards h, that of attraction towards s, and the earth proceeds between them with a uniformly retarded motion, till it has completed its revolution. thus you see that the earth travels round the sun, not in a circle, but an ellipsis, of which the sun occupies one of the _foci_; and that in its course, the earth alternately approaches and recedes from it, without any danger of being either swallowed up, or being entirely carried away from it. _caroline._ and i observe, that what i apprehended to be a dangerous irregularity, is the means by which the most perfect order and harmony are produced. _emily._ the earth travels then at a very unequal rate, its velocity being accelerated as it approaches the sun, and retarded as it recedes from it. _mrs. b._ it is mathematically demonstrable, that, in moving round a point towards which it is attracted, a body passes over equal areas, in equal times. the whole of the space contained within the earth's orbit, is in fig. , divided into a number of areas or surfaces; , , , , &c. all of which are of equal dimensions, though of very different forms; some of them, you see, are long and narrow, others broad and short: but they each of them contain an equal quantity of space. an imaginary line drawn from the centre of the earth to that of the sun, and keeping pace with the earth in its revolution, passes over equal areas in equal times; that is to say, if it is a month going from a to b, it will be a month going from b to c, and another from c to e, and so on; and the areas a b s, b c s, c e s, will be equal to each other, although the lines a b, b c, c e, are unequal. _caroline._ what long journeys the earth has to perform in the course of a month, in one part of her orbit, and how short they are in the other part! _mrs. b._ the inequality is not so considerable as appears in this figure; for the earth's orbit is not so eccentric as it is there described; and in reality, differs but little from a circle: that part of the earth's orbit nearest the sun is called its _perihelion_, that part most distant from the sun, its _aphelion_; and the earth is above three millions of miles nearer the sun at its perihelion than at its aphelion. _emily._ i think i can trace a consequence from these different situations of the earth; are not they the cause of summer and winter? _mrs. b._ on the contrary, during the height of summer, the earth is in that part of its orbit which is most distant from the sun, and it is during the severity of winter, that it approaches nearest to it. _emily._ that is very extraordinary; and how then do you account for the heat being greatest, when we are most distant from the sun? _mrs. b._ the difference of the earth's distance from the sun in summer and winter, when compared with its total distance from the sun, is but inconsiderable. the earth, it is true, is above three millions of miles nearer the sun in winter than in summer; but that distance, however great it at first appears, sinks into insignificance in comparison with millions of miles, which is our mean distance from the sun. the change of temperature, arising from this difference, would scarcely be sensible, even were it not completely overpowered by other causes which produce the variations of the seasons; but these i shall defer explaining, till we have made some further observations on the heavenly bodies. _caroline._ and should not the sun appear smaller in summer, when it is so much further from us? _mrs. b._ it actually does, when accurately measured; but the apparent difference in size, is, i believe, not perceptible to the naked eye. _emily._ then, since the earth moves with the greatest velocity in that part of its orbit in which it is nearest the sun, it must have completed its journey through that half of its orbit, in a shorter time than through the other? _mrs. b._ yes, it is about seven days longer performing the summer-half of its orbit, than the winter-half; and the summers are consequently seven days longer in the northern, than they are in the southern hemisphere. the revolution of all the planets round the sun, is the result of the same causes, and is performed in the same manner, as that of the earth. _caroline._ pray what are the planets? _mrs. b._ they are those celestial bodies, which revolve like our earth, about the sun; they are supposed to resemble the earth also in many other respects; and we are led by analogy, to suppose them to be inhabited worlds. _caroline._ i have heard so, but do you not think such an opinion too great a stretch of the imagination? _mrs. b._ some of the planets are proved to be larger than the earth; it is only their immense distance from us, which renders their apparent dimensions so small. now, if we consider them as enormous globes, instead of small twinkling spots, we shall be led to suppose that the almighty would not have created them merely for the purpose of giving us a little light in the night, as it was formerly imagined; and we should find it more consistent with our ideas of the divine wisdom and beneficence, to suppose that these celestial bodies should be created for the habitation of beings, who are, like us, blessed by his providence. both in a moral, as well as a physical point of view, it appears to me more rational to consider the planets as worlds revolving round the sun; and the fixed stars as other suns, each of them attended by their respective system of planets, to which they impart their influence. we have brought our telescopes to such a degree of perfection, that from the appearances which the moon exhibits when seen through them, we have very good reason to conclude that it is a habitable globe: for though it is true that we cannot discern its towns and people, we can plainly perceive its mountains and valleys: and some astronomers have gone so far as to imagine that they discovered volcanos. _emily._ if the fixed stars are suns, with planets revolving round them, why should we not see those planets as well as their suns? _mrs. b._ in the first place, we conclude that the planets of other systems (like those of our own) are much smaller than the suns which give them light; therefore at a distance so great as to make the suns appear like fixed stars, the planets would be quite invisible. secondly, the light of the planets being only reflected light, is much more feeble than that of the fixed stars. there is exactly the same difference as between the light of the sun and that of the moon; the first being a fixed star, the second a planet. _emily._ but the planets appear to us as bright as the fixed stars, and these you tell us are suns like our own; why then do we not see them by daylight, when they must be just as luminous as they are in the night? _mrs. b._ both are invisible from the same cause: their light is so faint, compared to that of the sun, that it is entirely effaced by it: the light emitted by the fixed stars may probably be as great as that of our sun, at an equal distance; but they being so much more remote, it is diffused over a greater space, and is in consequence proportionally lessened. _caroline._ true; i can see much better by the light of a candle that is near me, than by that of one at a great distance. but i do not understand what makes the planets shine? _mrs. b._ what is that which makes the gilt buttons on your brothers coat shine? _caroline._ the sun. but if it was the sun which made the planets shine, we should see them in the day-time, when the sun shone upon them; or if the faintness of their light prevented our seeing them in the day, we should not see them at all, for the sun cannot shine upon them in the night. _mrs. b._ there you are in error. but in order to explain this to you, i must first make you acquainted with the various motions of the planets. you know, that according to the laws of attraction, the planets belonging to our system all gravitate towards the sun; and that this force, combined with that of projection, will occasion their revolution round the sun, in orbits more or less elliptical, according to the proportion which these two forces bear to each other. but the planets have also another motion: they revolve upon their axis. the axis of a planet is an imaginary line which passes through its centre, and on which it turns; and it is this motion which produces day and night. it is day on that side of the planet which faces the sun; and on the opposite side, which remains in darkness, it is night. our earth, which we consider as a planet, is hours in performing one revolution on its axis; in that period of time, therefore, we have a day and a night; hence this revolution is called the earth's diurnal or daily motion; and it is this revolution of the earth from west to east which produces an apparent motion of the sun, moon and stars, in a contrary direction. let us now suppose ourselves to be beings independent of any planet, travelling in the skies, and looking upon the earth from a point as distant from it as from other planets. _caroline._ it would not be flattering to us, its inhabitants, to see it make so insignificant an appearance. _mrs. b._ to those accustomed to contemplate it in this light, it could never appear more glorious. we are taught by science to distrust appearances; and instead of considering the fixed stars and planets as little points, we look upon them either as brilliant suns, or habitable worlds; and we consider the whole together as forming one vast and magnificent system, worthy of the divine hand by which it was created. _emily._ i can scarcely conceive the idea of this immensity of creation; it seems too sublime for our imagination;--and to think that the goodness of providence extends over millions of worlds throughout a boundless universe--ah! mrs. b., it is we only who become trifling and insignificant beings in so magnificent a creation! _mrs. b._ this idea should teach us humility, but without producing despondency. the same almighty hand which guides these countless worlds in their undeviating course, conducts with equal perfection, the blood as it circulates through the veins of a fly, and opens the eye of the insect to behold his wonders. notwithstanding this immense scale of creation, therefore, we need not fear that we shall be disregarded or forgotten. but to return to our station in the skies. we were, if you recollect, viewing the earth at a great distance, in appearance a little star, one side illumined by the sun, the other in obscurity. but would you believe it, caroline, many of the inhabitants of this little star imagine that when that part which they inhabit is turned from the sun, darkness prevails throughout the universe, merely because it is night with them; whilst, in reality, the sun never ceases to shine upon every planet. when, therefore, these little ignorant beings look around them during their night, and behold all the stars shining, they cannot imagine why the planets, which are dark bodies, should shine; concluding, that since the sun does not illumine themselves, the whole universe must be in darkness. _caroline._ i confess that i was one of these ignorant people; but i am now very sensible of the absurdity of such an idea. to the inhabitants of the other planets, then, we must appear as a little star? _mrs. b._ yes, to those which revolve round our sun; for since those which may belong to other systems, (and whose existence is only hypothetical) are invisible to us, it is probable that we also are invisible to them. _emily._ but they may see our sun as we do theirs, in appearance a fixed star? _mrs. b._ no doubt; if the beings who inhabit those planets are endowed with senses similar to ours. by the same rule we must appear as a moon to the inhabitants of our moon; but on a larger scale, as the surface of the earth is about thirteen times as large as that of the moon. _emily._ the moon, mrs. b., appears to move in a different direction, and in a different manner from the stars? _mrs. b._ i shall defer the explanation of the motion of the moon till our next interview, as it would prolong our present lesson too much. questions . (pg. ) what revolution does the earth perform in a year? . (pg. ) had the earth received a projectile force only, at the time of its creation, how would it have moved? . (pg. ) what do the lines a b, and a c, represent in fig. . plate ? . (pg. ) what have you been taught respecting a body acted upon by two forces at right angles with each other? . (pg. ) how does the force of gravity change the diagonal into a curved line? . (pg. ) describe the operation of the forces of projection and of gravity as illustrated by the parallelograms in the figure? . (pg. ) what is the law respecting the time required for motion in the diagonal? . (pg. ) what portion of a year is represented by the three diagonals in the figure? . (pg. ) how will what you have learned respecting motion in a curve, apply to the earth's motion? . (pg. ) in what form are you directed to cut a piece of card to aid in illustrating the two forces acting upon the earth? . (pg. ) how must you apply it to this purpose? (fig. . plate .) . (pg. ) if these two forces did not exactly balance each other, what would result? . (pg. ) does the earth revolve in a circular orbit? . (pg. ) what results from its motion in an ellipsis? . (pg. ) what is represented by the lines a c, a b, in fig. . plate ? . (pg. ) were the projectile force to carry the earth from b to d, (fig. .) what would result? . (pg. ) when it has arrived at e, what angle will be formed by the lines representing the two forces? . (pg. ) what effect will the accelerated motion then produce? . (pg. ) what is the form of the earth's orbit, and what circumstances produce this form? . (pg. ) what is the consequence as regards the regularity of the earth's motion? . (pg. ) what law governs as regards the spaces passed over, and how is this explained by fig. . plate ? . (pg. ) what is meant by _perihelion_, and by _aphelion_? . (pg. ) what is the difference of the distance of the earth from the sun, in these two points? . (pg. ) at what season of the year is it nearest to, and at what furthest from the sun? . (pg. ) what is the mean distance of the earth from the sun? . (pg. ) why is but little effect produced, as regards temperature, by the change of distance? . (pg. ) has it any influence on the sun's apparent size? . (pg. ) are the summer and winter, half years, of the same length; what is their difference, and what is the cause? . (pg. ) what are the planets? . (pg. ) what circumstances render it probable that they are habitable globes? . (pg. ) what is believed respecting the fixed stars? . (pg. ) what discoveries have been made in the moon? . (pg. ) what prevents our seeing the planets, if there are any, which revolve round the fixed stars? . (pg. ) what prevents our seeing the stars and planets in the day-time? . (pg. ) what other motions have the earth and planets, besides that in their orbits? . (pg. ) what is the imaginary line called, round which they revolve? . (pg. ) how does this occasion night and day? . (pg. ) in what direction does the earth turn upon its axis, and what apparent motion of the sun, moon, and stars is thereby produced? . (pg. ) what must be the appearance of the earth to an inhabitant of one of the planets? . (pg. ) what the appearance of the sun to the inhabitants of planets in other systems? . (pg. ) what the appearance of the earth to an inhabitant of the moon? conversation vii. of the planets. of the satellites or moons. gravity diminishes as the square of the distance. of the solar system. of comets. constellations, signs of the zodiac. of copernicus, newton, &c. mrs. b. the planets are distinguished into primary and secondary. those which revolve immediately about the sun are called primary. many of these are attended in their course by smaller planets, which, revolve round them: these are called secondary planets, satellites, or moons. such is our moon which accompanies the earth, and is carried with it round the sun. _emily._ how then can you reconcile the motion of the secondary planets to the laws of gravitation; for the sun is much larger than any of the primary planets; and is not the power of gravity proportional to the quantity of matter? _caroline._ perhaps the sun, though much larger, may be less dense than the planets. fire you know, is very light, and it may contain but little matter, though of great magnitude. _mrs. b._ we do not know of what kind of matter the sun is made; but we may be certain, that since it is the general centre of attraction of our system of planets, it must be the body which contains the greatest quantity of matter in that system. you must recollect, that the force of attraction is not only proportional to the quantity of matter, but to the degree of proximity of the attractive body: this power is weakened by being diffused, and diminishes as the distance increases. _emily._ then if a planet was to lose one-half of its quantity of matter, it would lose one half of its attractive power; and the same effect would be produced by removing it to twice its former distance from the sun; that i understand. _mrs. b._ not so perfectly as you imagine. you are correct as respects the diminution in size, because the attractive force is in the same proportion as the quantity of matter; but were you to remove a planet to double its former distance, it would retain but one-fourth part of its gravitating force; for attraction decreases not in proportion to the simple increase of the distance, but as the squares of the distances increase. _caroline._ i do not exactly comprehend what is meant by the squares, in this case, although i know very well what is in general intended by a square. _mrs. b._ by the square of a number we mean the product of a number, multiplied by itself; thus two, multiplied by two, is four, which is therefore the square of two; in like manner the square of three, is nine, because three multiplied by three, gives that product. _emily._ then if one planet is three times more distant from the sun than another, it will be attracted with but one-ninth part of the force; and if at four times the distance, with but one-sixteenth, sixteen being the square of four? _mrs. b._ you are correct; the rule is, that _the attractive force is in the inverse proportion of the square of the distance_. and it is easily demonstrated by the mathematics, that the same is the case with every power that emanates from a centre; as for example, the light from the sun, or from any other luminous body, decreases in its intensity at the same rate. _caroline._ then the more distant planets, move much slower in their orbits; for their projectile force must be proportioned to that of attraction? but i do not see how this accounts for the motion of the secondary, round the primary planets, in preference to moving round the sun? _emily._ is it not because the vicinity of the primary planets, renders their attraction stronger than that of the sun? _mrs. b._ exactly so. but since the attraction between bodies is mutual, the primary planets are also attracted by the satellites which revolve round them. the moon attracts the earth, as well as the earth the moon; but as the latter is the smaller body, her attraction is proportionally less; therefore, neither the earth revolves round the moon, nor the moon round the earth; but they both revolve round a point, which is their common centre of gravity, and which is as much nearer to the earth than to the moon, as the gravity of the former exceeds that of the latter. _emily._ yes, i recollect your saying, that if two bodies were fastened together by a wire or bar, their common centre of gravity would be in the middle of the bar, provided the bodies were of equal weight; and if they differed in weight, it would be nearer the larger body. if then, the earth and moon had no projectile force which prevented their mutual attraction from bringing them together, they would meet at their common centre of gravity. _caroline._ the earth then has a great variety of motion, it revolves round the sun, round its own axis, and round the point towards which the moon attracts it. _mrs. b._ just so; and this is the case with every planet which is attended by satellites. the complicated effect of this variety of motions, produces certain irregularities, which, however, it is not necessary to notice at present, excepting to observe that they eventually correct each other, so that no permanent derangement exists. the planets act on the sun, in the same manner as they are themselves acted on by their satellites; for attraction, you must remember, is always mutual; but the gravity of the planets (even when taken collectively) is so trifling compared with that of the sun, that were they all placed on the same side of that luminary, they would not cause him to move so much as one-half of his diameter towards them, and the common centre of gravity, would still remain within the body of the sun. the planets do not, therefore, revolve round the centre of the sun, but round a point at a small distance from its centre, about which the sun also revolves. _emily._ i thought the sun had no motion? _mrs. b._ you were mistaken; for besides that round the common centre of gravity, which i have just mentioned, which is indeed very inconsiderable, he revolves on his axis in about days; this motion is ascertained by observing certain spots which disappear, and reappear regularly at stated times. [illustration: plate vii.] _caroline._ a planet has frequently been pointed out to me in the heavens; but i could not perceive that its motion differed from that of the fixed stars, which only appear to move. _mrs. b._ the great distance of the planets, renders their apparent motion so slow, that the eye is not sensible of their progress in their orbits, unless we watch them for some considerable length of time: but if you notice the nearness of a planet to any particular fixed star, you may in a few nights perceive that it has changed its distance from it, whilst the stars themselves always retain their relative situations. the most accurate idea i can give you of the situation and motion of the planets in their orbits, will be by the examination of this diagram, (plate . fig. .) representing the solar system, in which you will find every planet, with its orbit delineated. _emily._ but the orbits here are all circular, and you said that they were elliptical. the planets appear too, to be moving round the centre of the sun; whilst you told us that they moved round a point at a little distance from thence. _mrs. b._ the orbits of the planets are so nearly circular, and the common centre of gravity of the solar system, so near the centre of the sun, that these deviations are too small to be represented. the dimensions of the planets, in their proportion to each other, you will find delineated in fig. . mercury is the planet nearest the sun; his orbit is consequently contained within ours; his vicinity to the sun, prevents our frequently seeing him, so that very accurate observations cannot be made upon mercury. he performs his revolution round the sun in about days, which is consequently the length of his year. the time of his rotation on his axis is not known; his distance from the sun is computed to be millions of miles, and his diameter miles. the heat of this planet is supposed to be so great, that water cannot exist there but in a state of vapour, and that even quicksilver would be made to boil. _caroline._ oh, what a dreadful climate! _mrs. b._ though we could not live there, it may be perfectly adapted to other beings, destined to inhabit it; or he who created it may have so modified the heat, by provisions of which we are ignorant, as to make it habitable even by ourselves. venus, the next in the order of planets, is millions of miles from the sun: she revolves about her axis in hours and minutes, and goes round the sun in days, hours. the orbit of venus is also within ours; during nearly one-half of her course in it, we see her before sun-rise, and she is then called the morning star; in the other part of her orbit she rises later than the sun. _caroline._ in that case we cannot see her, for she must rise in the day time? _mrs. b._ true; but when she rises later than the sun, she also sets later; so that we perceive her approaching the horizon after sun-set: she is then called hesperus, or the evening star. do you recollect those beautiful lines of milton? now came still evening on, and twilight gray had in her sober livery all things clad; silence accompanied; for beast and bird, they to their grassy couch, these to their nests were slunk, all but the wakeful nightingale; she all night long her amorous descant sung; silence was pleas'd; now glowed the firmament with living sapphires. hesperus that led the starry host, rode brightest, till the moon rising in clouded majesty, at length apparent queen unveil'd her peerless light, and o'er the dark her silver mantle threw. the planet next to venus is the earth, of which we shall soon speak at full length. at present i shall only observe that we are millions of miles distant from the sun, that we perform our annual revolution in days hours and minutes; and are attended in our course by a single moon. next follows mars. he can never come between us and the sun, like mercury and venus; his motion is, however, very perceptible, as he may be traced to different situations in the heavens; his distance from the sun is millions of miles; he turns round his axis in hours and minutes; and he performs his annual revolution, in about of our days: his diameter is miles. then follow four very small planets, juno, ceres, pallas and vesta, which have been recently discovered, but whose dimensions, and distances from the sun, have not been very accurately ascertained. they are generally called asteroids. jupiter is next in order: this is the largest of all the planets. he is about millions of miles from the sun, and completes his annual period in nearly of our years. he turns round his axis in about ten hours. he is above times as big as our earth; his diameter is , miles. the respective proportions of the planets cannot, therefore, you see, be conveniently delineated in a diagram. he is attended by four moons. the next planet is saturn, whose distance from the sun, is about millions of miles; his diurnal rotation is performed in hours and a quarter: his annual revolution is nearly of our years. his diameter is , miles. this planet is surrounded by a luminous ring, the nature of which, astronomers are much at a loss to conjecture: he has seven moons. lastly, we observe the planet herschel, discovered by dr. herschel, by whom it was named the georgium sidus, and which is attended by six moons. _caroline._ how charming it must be in the distant planets, to see several moons shining at the same time; i think i should like to be an inhabitant of jupiter or saturn. _mrs. b._ not long i believe. consider what extreme cold must prevail in a planet, situated as saturn is, at nearly ten times the distance at which we are from the sun. then his numerous moons are far from making so splendid an appearance as ours; for they can reflect only the light which they receive from the sun; and both light, and heat, decrease in the same ratio or proportion to the distances, as gravity. can you tell me now how much more light we enjoy than saturn? _caroline._ the square of ten is a hundred; therefore, saturn has a hundred times less--or to answer your question exactly, we have a hundred times more light and heat, than saturn--this certainly does not increase my wish to become one of the poor wretches who inhabit that planet. _mrs. b._ may not the inhabitants of mercury, with equal plausibility, pity us for the insupportable coldness of our situation; and those of jupiter and saturn for our intolerable heat? the almighty power which created these planets, and placed them in their several orbits, has no doubt peopled them with beings, whose bodies are adapted to the various temperatures and elements, in which they are situated. if we judge from the analogy of our own earth, or from that of the great and universal beneficence of providence, we must conclude this to be the case. _caroline._ are not comets, in some respects similar to planets? _mrs. b._ yes, they are; for by the reappearance of some of them, at stated times, they are known to revolve round the sun; but in orbits so extremely eccentric, that they disappear for a great number of years. if they are inhabited, it must be by a species of beings very different, not only from the inhabitants of this, but from those of any of the other planets, as they must experience the greatest vicissitudes of heat and cold; one part of their orbit being so near the sun, that their heat, when there, is computed to be greater than that of red-hot iron; in this part of its orbit, the comet emits a luminous vapour, called the tail, which it gradually loses as it recedes from the sun; and the comet itself totally disappears from our sight, in the more distant parts of its orbit, which extends considerably beyond that of the furthest planet. the number of comets belonging to our system cannot be ascertained, as some of them are several centuries before they make their reappearance. the number that are known by their regular reappearance is, i believe, only three, although their whole number is very considerable. _emily._ pray, mrs. b., what are the constellations? _mrs. b._ they are the fixed stars; which the ancients, in order to recognise them, formed into groups, and gave the names of the figures, which you find delineated on the celestial globe. in order to show their proper situations in the heavens, they should be painted on the internal surface of a hollow sphere, from the centre of which you should view them; you would then behold them as they appear to be situated in the heavens. the twelve constellations, called the signs of the zodiac, are those which are so situated, that the earth, in its annual revolution, passes directly between them, and the sun. their names are aries, taurus, gemini, cancer, leo, virgo, libra, scorpio, sagittarius, capricornus, aquarius, pisces; the whole occupying a complete circle, or broad belt, in the heavens, called the zodiac. (plate . fig. .) hence, a right line drawn from the earth, and passing through the sun, would reach one of these constellations, and the sun is said to be in that constellation at which the line terminates: thus, when the earth is at a, the sun would appear to be in the constellation or sign aries; when the earth is at b, the sun would appear in cancer; when the earth was at c, the sun would be in libra; and when the earth was at d, the sun would be in capricorn. you are aware that it is the real motion of the earth in its orbit, which gives to the sun this apparent motion through the signs. this circle, in which the sun thus appears to move, and which passes through the middle of the zodiac, is called the ecliptic. _caroline._ but many of the stars in these constellations appear beyond the zodiac. [illustration: plate viii.] _mrs. b._ we have no means of ascertaining the distance of the fixed stars. when, therefore, they are said to be in the zodiac, it is merely implied that they are situated in that direction, and that they shine upon us through that portion of the heavens, which we call the zodiac. _emily._ but are not those large bright stars, which are called stars of the first magnitude, nearer to us, than those small ones which we can scarcely discern? _mrs. b._ it may be so; or the difference of size and brilliancy of the stars may proceed from their difference of dimensions; this is a point which astronomers are not enabled to determine. considering them as suns, i see no reason why different suns should not vary in dimensions, as well as the planets belonging to them. _emily._ what a wonderful and beautiful system this is, and how astonishing to think that every fixed star may probably be attended by a similar train of planets! _caroline._ you will accuse me of being very incredulous, but i cannot help still entertaining some doubts, and fearing that there is more beauty than truth in this system. it certainly may be so; but there does not appear to me to be sufficient evidence to prove it. it seems so plain and obvious that the earth is motionless, and that the sun and stars revolve round it;--your solar system, you must allow, is directly in opposition to the evidence of our senses. _mrs. b._ our senses so often mislead us, that we should not place implicit reliance upon them. _caroline._ on what then can we rely, for do we not receive all our ideas through the medium of our senses? _mrs. b._ it is true that they are our primary source of knowledge; but the mind has the power of reflecting, judging, and deciding upon the ideas received by the organs of sense. this faculty, which we call reason, has frequently proved to us, that our senses are liable to err. if you have ever sailed on the water, with a very steady breeze, you must have seen the houses, trees, and every object on the shore move, while you were sailing. _caroline._ i remember thinking so, when i was very young; but i now know that their motion is only apparent. it is true that my reason, in this case, corrects the error of my sight. _mrs. b._ it teaches you, that the apparent motion of the objects on shore, proceeds from your being yourself moving, and that you are not sensible of your own motion, because you meet with no resistance. it is only when some obstacle impedes our motion, that we are conscious of moving; and if you were to close your eyes when you were sailing on calm water, with a steady wind, you would not perceive that you moved, for you could not feel it, and you could see it only by observing the change of place of the objects on shore. so it is with the motion of the earth: every thing on its surface, and the air that surrounds it, accompanies it in its revolution; it meets with no resistance: therefore, like the crew of a vessel sailing with a fair wind, in a calm sea, we are insensible of our motion. _caroline._ but the principal reason why the crew of a vessel in a calm sea do not perceive their motion, is, because they move exceedingly slow, while the earth, you say, revolves with great velocity. _mrs. b._ it is not because they move slowly, but because they move steadily, and meet with no irregular resistances, that the crew of a vessel do not perceive their motion; for they would be equally insensible to it, with the strongest wind, provided it were steady, that they sailed with it, and that it did not agitate the water; but this last condition, you know, is not possible, for the wind will always produce waves which offer more or less resistance to the vessel, and then the motion becomes sensible, because it is unequal. _caroline._ but, granting this, the crew of a vessel have a proof of their motion, which the inhabitants of the earth cannot have,--the apparent motion of the objects on shore, or their having passed from one place to another. _mrs. b._ have we not a similar proof of the earth's motion, in the apparent motion of the sun and stars? imagine the earth to be sailing round its axis, and successively passing by every star, which, like the objects on land, we suppose to be moving instead of ourselves. i have heard it observed by an ærial traveller in a balloon, that the earth appears to sink beneath the balloon, instead of the balloon rising above the earth. it is a law which we discover throughout nature, and worthy of its great author, that all its purposes are accomplished by the most simple means; and what reason have we to suppose this law infringed, in order that we may remain at rest, while the sun and stars move round us; their regular motions, which are explained by the laws of attraction, on the first supposition, would be unintelligible on the last, and the order and harmony of the universe be destroyed. think what an immense circuit the sun and stars would make daily, were their apparent motions, real. we know many of them, to be bodies more considerable than our earth; for our eyes vainly endeavour to persuade us, that they are little brilliants sparkling in the heavens; while science teaches us that they are immense spheres, whose apparent dimensions are diminished by distance. why then should these enormous globes daily traverse such a prodigious space, merely to prevent the necessity of our earth's revolving on its axis? _caroline._ i think i must now be convinced. but you will, i hope, allow me a little time to familiarise to myself, an idea so different from that which i have been accustomed to entertain. and pray, at what rate do we move? _mrs. b._ the motion produced by the revolution of the earth on its axis, is about seventeen miles a minute, to an inhabitant on the equator. _emily._ but does not every part of the earth move with the same velocity? _mrs. b._ a moment's reflection would convince you of the contrary: a person at the equator must move quicker than one situated near the poles, since they both perform a revolution in hours. _emily._ true, the equator is farthest from the axis of motion. but in the earth's revolution round the sun, every part must move with equal velocity? _mrs. b._ yes, about a thousand miles a minute. _caroline._ how astonishing!--and that it should be possible for us to be insensible of such a rapid motion. you would not tell me this sooner, mrs. b., for fear of increasing my incredulity. before the time of newton, was not the earth supposed to be in the centre of the system, and the sun, moon, and stars to revolve round it? _mrs. b._ this was the system of ptolemy, in ancient times; but as long ago as the beginning of the sixteenth century it was generally discarded, and the solar system, such as i have shown you, was established by the celebrated astronomer copernicus, and is hence called the copernican system. but the theory of gravitation, the source from which this beautiful and harmonious arrangement flows, we owe to the powerful genius of newton, who lived at a much later period, and who demonstrated its truth. _emily._ it appears, indeed, far less difficult to trace by observation the motion of the planets, than to divine by what power they are impelled and guided. i wonder how the idea of gravitation could first have occurred to sir isaac newton? _mrs. b._ it is said to have been occasioned by a circumstance from which one should little have expected so grand a theory to have arisen. during the prevalence of the plague in the year , newton retired into the country to avoid the contagion: when sitting one day in an orchard, he observed an apple fall from a tree, and was led to consider what could be the cause which brought it to the ground. _caroline._ if i dared to confess it, mrs. b., i should say that such an inquiry indicated rather a deficiency than a superiority of intellect. i do not understand how any one can wonder at what is so natural and so common. _mrs. b._ it is the mark of superior genius to find matter for wonder, observation, and research, in circumstances which, to the ordinary mind, appear trivial, because they are common; and with which they are satisfied, because they are natural; without reflecting that nature is our grand field of observation, that within it, is contained our whole store of knowledge; in a word, that to study the works of nature, is to learn to appreciate and admire the wisdom of god. thus, it was the simple circumstance of the fall of an apple, which led to the discovery of the laws upon which the copernican system is founded; and whatever credit this system had obtained before, it now rests upon a basis from which it cannot be shaken. _emily._ this was a most fortunate apple, and more worthy to be commemorated than all those that have been sung by the poets. the apple of discord for which the goddesses contended; the golden apples by which atalanta won the race; nay, even the apple which william tell shot from the head of his son, cannot be compared to this! questions . (pg. ) into what two classes are the planets divided, and how are they distinguished? . (pg. ) by what reasoning do you prove that the sun contains a greater quantity of matter than any other body in the system? . (pg. ) what two circumstances govern the force with which bodies attract each other? . (pg. ) were a planet removed to double its former distance from the sun, what would be the effect upon its attractive force? . (pg. ) why would it be reduced to one-fourth? . (pg. ) what is meant by the square of a number, and what examples can you give? . (pg. ) what then would be the effect of removing it to three, or four times its former distance? . (pg. ) how is the rule upon this subject expressed? . (pg. ) does this apply to any power excepting gravitation? . (pg. ) how is it that a secondary planet revolves round its primary, and is not drawn off by the sun? . (pg. ) what is said respecting the revolution of the moon, and of the earth, round a common centre of gravity? . (pg. ) by what law in mechanics is this explained? . (pg. ) what motions then has the earth, and are these remarks confined to it alone? . (pg. ) what effect have the planets upon the sun, and what is said of the common centre of gravity of the system? . (pg. ) what other motion has the sun, and how is it proved? . (pg. ) how may you observe the motion of a planet, by means of a fixed star? . (pg. ) what is represented by fig. . plate ? . (pg. ) why are the orbits represented as circular? . (pg. ) in what order do the planets increase in size as represented, fig. . plate ? . (pg. ) what are we told respecting mercury? . (pg. ) what respecting venus? . (pg. ) when does venus become a morning, and when an evening star? . (pg. ) what is said of the earth? . (pg. ) what of mars? . (pg. ) what four small planets follow next? . (pg. ) what is said of jupiter? . (pg. ) what of saturn? . (pg. ) what of herschel? . (pg. ) why do we conclude that the moons of saturn afford less light than ours? . (pg. ) in what proportion will the light and heat at saturn be diminished, and why? . (pg. ) what do the comets resemble, and what is remarkable in their orbits? . (pg. ) what is said of the number of comets? . (pg. ) what is a constellation? . (pg. ) how are the twelve constellations, or signs, called the zodiac, situated? . (pg. ) name them. . (pg. ) what is meant by the sun being in a sign? . (pg. ) what causes the apparent change of the sun's place? . (pg. ) the stars appear of different magnitudes, by what may this be caused? . (pg. ) we are not sensible of the motion of the earth; what fact is mentioned to illustrate this point? . (pg. ) what does this teach us? . (pg. ) would the slowness, or the rapidity of the motion, if steady, produce any sensible difference? . (pg. ) if we do not feel the motion of the earth, how may we be convinced of its reality? . (pg. ) were we to deny the motion of the earth upon its axis, what must we admit respecting the heavenly bodies? . (pg. ) what distance is an inhabitant on the equator carried in a minute by the diurnal motion of the earth? . (pg. ) why is not the velocity every where equally great? . (pg. ) what distance does the earth travel in a minute, in its revolution round the sun? . (pg. ) what was formerly supposed respecting the motion of all the heavenly bodies? . (pg. ) what do we mean by the copernican system, and what is said respecting copernicus and newton? . (pg. ) what circumstance is said to have given rise to the speculations of newton, on the subject of gravitation? conversation viii. on the earth. of the terrestrial globe. of the figure of the earth. of the pendulum. of the variation of the seasons, and of the length of days and nights. of the causes of the heat of summer. of solar, siderial, and equal or mean time. mrs. b. as the earth is the planet in which we are the most particularly interested, it is my intention this morning, to explain to you the effects resulting from its annual, and diurnal motions; but for this purpose, it will be necessary to make you acquainted with the terrestrial globe: you have not either of you, i conclude, learnt the use of the globes? _caroline._ no; i once indeed, learnt by heart, the names of the lines marked on the globe, but as i was informed they were only imaginary divisions, they did not appear to me worthy of much attention, and were soon forgotten. _mrs. b._ you supposed, then, that astronomers had been at the trouble of inventing a number of lines, to little purpose. it will be impossible for me to explain to you the particular effects of the earth's motion, without your having acquired a knowledge of these lines: in plate . fig. . you will find them all delineated: and you must learn them perfectly, if you wish to make any proficiency in astronomy. _caroline._ i was taught them at so early an age, that i could not understand their meaning; and i have often heard you say, that the only use of words, was to convey ideas. _mrs. b._ a knowledge of these lines, would have conveyed some idea of the manner in which they were designed to divide the globe into parts; although the use of these divisions, might at that time, have been too difficult for you to understand. childhood is the season, when impressions on the memory are most strongly and most easily made: it is the period at which a large stock of terms should be treasured up, the precise application of which we may learn when the understanding is more developed. it is, i think, a very mistaken notion, that children should be taught such things only, as they can perfectly understand. had you been early made acquainted with the terms which relate to figure and motion, how much it would have facilitated your progress in natural philosophy. i have been obliged to confine myself to the most common and familiar expressions, in explaining the laws of nature; although i am convinced that appropriate and scientific terms, might have conveyed more precise and accurate ideas, had you been prepared to understand them. _emily._ you may depend upon our carefully learning the names of these lines, mrs. b.; but before we commit them to memory, will you have the goodness to explain them to us? _mrs. b._ most willingly. this figure of a globe, or sphere, represents the earth; the line which passes through its centre, and on which it turns, is called its axis, and the two extremities of the axis a and b, are the poles, distinguished by the names of the north and the south pole. the circle c d, which divides the globe into two equal parts between the poles, and equally distant from them, is called the equator, or equinoctial line; that part of the globe to the north of the equator, is the northern hemisphere; that part to the south of the equator, the southern hemisphere. the small circle e f, which surrounds the north pole, is called the arctic circle; that g h, which surrounds the south pole, the antarctic circle; these are also called polar circles. there are two circles, intermediate between the polar circles and the equator; that to the north i k, called the tropic of cancer; that to the south, l m, called the tropic of capricorn. lastly, this circle, l k, which divides the globe into two equal parts, crossing the equator and extending northward as far as the tropic of cancer, and southward as far as the tropic of capricorn, is called the ecliptic. the delineation of the ecliptic on the terrestrial globe is not without danger of conveying false ideas; for the ecliptic (as i have before said) is an imaginary circle in the heavens, passing through the middle of the zodiac, and situated in the plane of the earth's orbit. _caroline._ i do not understand the meaning of the plane of the earth's orbit. _mrs. b._ a plane, is an even flat surface. were you to bend a piece of wire, so as to form a hoop, you might then stretch a piece of cloth, or paper over it, like the head of a drum; this would form a flat surface, which might be called the plane of the hoop. now the orbit of the earth, is an imaginary circle, surrounding the sun, and you can readily imagine a plane extending from one side of this circle to the other, filling up its whole area: such a plane would pass through the centre of the sun, dividing it into hemispheres. you may then imagine this plane extended beyond the limits of the earth's orbit, on every side, until it reached those fixed stars which form the signs of the zodiac; passing through the middle of these signs, it would give you the place of that imaginary circle in the heavens, call the ecliptic; which is the sun's apparent path. let fig. . plate , represent such a plane, s the sun, e the earth with its orbit, and a b c d the ecliptic passing through the middle of the zodiac. [illustration: plate ix.] _emily._ if the ecliptic relates only to the heavens, why is it described upon the terrestrial globe? _mrs. b._ it is convenient for the demonstration of a variety of problems in the use of the globes; and besides, the obliquity of this circle to the equator is rendered more conspicuous by its being described on the same globe; and the obliquity of the ecliptic shows how much the earth's axis is inclined to the plane of its orbit. but to return to fig. . plate . the spaces between the several parallel circles on the terrestrial globe are called zones: that which is comprehended between the tropics is distinguished by the name of the torrid zone; the spaces which extend from the tropics to the polar circles, the north and south temperate zones; and the spaces contained within the polar circles, the frigid zones. by the term zone is meant a belt, or girdle, the frigid zones, however, are not belts, but circles, extending - / degrees from their centres, the poles. the several lines which, you observe to be drawn from one pole to the other, cutting the equator at right angles, are called meridians; the number of these is unlimited, as a line passing through any place, directly to the poles, is called the meridian of that place. when any one of these meridians is exactly opposite to the sun, it is mid-day, or twelve o'clock in the day, at all the places situated any where on that meridian; and, at the places situated on the opposite meridian, it is consequently midnight. _emily._ to places situated equally distant from these two meridians, it must then be six o'clock. _mrs. b._ yes; if they are to the east of the sun's meridian it is six o'clock in the afternoon, because they will have previously passed the sun; if to the west, it is six o'clock in the morning, and that meridian will be proceeding towards the sun. those circles which divide the globe into two equal parts, such as the equator and the ecliptic, are called greater circles; to distinguish them from those which divide it into two unequal parts, as the tropics, and polar circles, which are called lesser circles. all circles, you know, are imagined to be divided into equal parts, called degrees, and degrees are again divided into equal parts, called minutes. the diameter of a circle is a right line drawn across it, and passing through its centre; were you, for instance, to measure across this round table, that would give you its diameter; but were you to measure all round the edge of it, you would then obtain its circumference. now emily, you may tell me exactly how many degrees are contained in a meridian? _emily._ a meridian, reaching from one pole to the other, is half a circle, and must therefore contain degrees. _mrs. b._ very well; and what number of degrees are there from the equator to one of the poles? _caroline._ the equator being equally distant from either pole, that distance must be half of a meridian, or a quarter of the circumference of a circle, and contain degrees. _mrs. b._ besides the usual division of circles into degrees, the ecliptic is divided into twelve equal parts, called signs, which bear the name of the constellations through which this circle passes in the heavens. the degrees measured on the meridians from the equator, either towards the north, or towards the south, are called degrees of latitude, of which there may be ; those measured from east to west, either on the equator, or any of the lesser circles, are called degrees of longitude, of which there may be ; these lesser circles are also called parallels of latitude. of these parallels there may be any number; a circle drawn from east to west, at any distance from the equator, will always be parallel to it, and is therefore called a parallel of latitude. _emily._ the degrees of longitude must then vary in length, according to the dimensions of the circle on which they are reckoned; those, for instance, at the polar circles, will be considerably smaller than those at the equator? _mrs. b._ certainly; since the degrees of circles of different dimensions do not vary in number, they must necessarily vary in length. the degrees of latitude, you may observe, never vary in length; for the meridians on which they are reckoned are all of the same dimensions. _emily._ and of what length is a degree of latitude? _mrs. b._ sixty geographical miles, which is equal to - / english statute miles; or about one-sixth more than a common mile. _emily._ the degrees of longitude at the equator, must then be of the same dimensions, with a degree of latitude. _mrs. b._ they would, were the earth a perfect sphere; but it is not exactly such, being somewhat protuberant about the equator, and flattened towards the poles. this form proceeds from the superior action of the centrifugal power at the equator, and as this enlarges the circle, it must, in the same proportion, increase the length of the degrees of longitude measured on it. _caroline._ i thought i had understood the centrifugal force perfectly, but i do not comprehend its effects in this instance. _mrs. b._ you know that the revolution of the earth on its axis, must give to every particle a tendency to fly off from the centre, that this tendency is stronger, or weaker, in proportion to the velocity with which the particle moves; now a particle situated near to one of the poles, makes one rotation in the same space of time as a particle at the equator; the latter, therefore, having a much larger circle to describe, travels proportionally faster, consequently the centrifugal force is much stronger at the equator than in the polar regions: it gradually decreases as you leave the equator and approach the poles, at which points the centrifugal force, entirely ceases. supposing, therefore, the earth to have been originally in a fluid state, the particles in the torrid zone would recede much farther from the centre than those in the frigid zones; thus the polar regions would become flattened, and those about the equator elevated. as a large portion of the earth is covered with water, the creator gave to it the form, denominated an _oblate spheroid_, otherwise the polar regions would have been without water, and those about the equator, would have been buried several miles below the surface of the ocean. _caroline._ i did not consider that the particles in the neighbourhood of the equator, move with greater velocity than those about the poles; this was the reason i could not understand you. _mrs. b._ you must be careful to remember, that those parts of a body which are farthest from the centre of motion, must move with the greatest velocity: the axis of the earth is the centre of its diurnal motion, and the equatorial regions the parts most distant from the axis. _caroline._ my head then moves faster than my feet; and upon the summit of a mountain, we are carried round quicker than in a valley? _mrs. b._ certainly; your head is more distant from the centre of motion than your feet; the mountain-top than the valley; and the more distant any part of a body is from the centre of motion, the larger is the circle it will describe, and the greater therefore must be its velocity. _emily._ i have been reflecting, that if the earth is not a perfect circle---- _mrs. b._ a sphere you mean, my dear: a circle is a round line, every part of which is equally distant from the centre; a sphere or globe is a round body, the surface of which is every where equally distant from the centre. _emily._ if, then, the earth is not a perfect sphere, but prominent at the equator, and depressed at the poles, would not a body weigh heavier at the equator than at the poles? for the earth being thicker at the equator, the attraction of gravity perpendicularly downwards must be stronger. _mrs. b._ your reasoning has some plausibility, but i am sorry to be obliged to add, that it is quite erroneous; for the nearer any part of the surface of a body is to the centre of attraction, the more strongly it is attracted; because it is then nearest to the whole mass of attracting matter. in regard to its effects, you might consider the whole power of gravity, as placed at the centre of attraction. _emily._ but were you to penetrate deep into the earth, would gravity increase as you approached the centre? _mrs. b._ certainly not; i am referring only to any situation on the surface of the earth. were you to penetrate into the interior, the attraction of the parts above you, would counteract that of the parts beneath you, and consequently diminish the power of gravity in proportion as you approach the centre; and if you reached that point, being equally attracted by the parts all around you, the effects of gravity would cease, and you would be without weight. _emily._ bodies, then, should weigh less at the equator than at the poles, since they are more distant from the centre of gravity in the former than in the latter situation? _mrs. b._ and this is really the case; but the difference of weight would be scarcely sensible, were it not augmented by another circumstance. _caroline._ and what is this singular circumstance, which seems to disturb the laws of nature? _mrs. b._ one that you are well acquainted with, as conducing more to the preservation than the destruction of order,--the centrifugal force. this we have just observed to be strongest at the equator; and as it tends to drive bodies from the centre, it is necessarily opposed to, and must lessen the power of gravity, which attracts them towards the centre. we accordingly find that bodies weigh lightest at the equator, where the centrifugal force is greatest; and heaviest at the poles, where this power is least: the weight being diminished at the equator, by both the causes mentioned. _caroline._ has the experiment been made in these different situations? _mrs. b._ louis xiv. of france, sent philosophers both to the equator, and to lapland, for this purpose: the severity of the climate, and obstruction from the ice, have hitherto rendered every attempt to reach the pole abortive; but the difference of gravity at the equator, and in lapland is very perceptible. _caroline._ yet i do not comprehend how the difference of weight could be ascertained, for if the body under trial decreased in weight, the weight which was opposed to it in the opposite scale must have diminished in the same proportion. for instance, if a pound of sugar did not weigh so heavy at the equator as at the poles, the leaden pound which served to weigh it, would not be so heavy either; therefore they would still balance each other, and the different force of gravity could not be ascertained by this means. _mrs. b._ your observation is perfectly just: the difference of gravity in bodies situated at the poles, and at the equator, cannot be ascertained by weighing them; a pendulum was therefore used for that purpose. _caroline._ what, the pendulum of a clock? how could that answer the purpose? _mrs. b._ a pendulum consists of a line, or rod, to one end of which a weight is attached, and by the other end it is suspended to a fixed point, about which it is made to vibrate. when not in motion, a pendulum, obeying the general law of attraction, hangs like a plumb line, perpendicular to the surface of the earth, but if you raise the pendulum, gravity will bring it back to its perpendicular position. it will, however, not remain stationary there, for the momentum it has acquired during its descent, will impel it onwards, and if unobstructed, it will rise on the opposite side to an equal height; from thence it is brought back by gravity, and is again forced upwards, by the impulse of its momentum. _caroline._ if so, the motion of a pendulum would be perpetual, and i thought you said, that there was no perpetual motion on the earth. _mrs. b._ the motion of a pendulum is opposed by the resistance of the air in which it vibrates, and by the friction of the part by which it is suspended: were it possible to remove these obstacles, the motion of a pendulum would be perpetual, and its vibrations perfectly regular; each being of equal distance, and performed in equal times. _emily._ that is the natural result of the uniformity of the power which produces these vibrations, for the force of gravity being always the same, the velocity of the pendulum must consequently be uniform. _caroline._ no, emily, you are mistaken; the force is not every where the same, and therefore the effect will not be so either. i have discovered it, mrs. b.; since the force of gravity is less at the equator than at the poles, the vibrations of the pendulum will be slower at the former place than at the latter. _mrs. b._ you are perfectly right, caroline; it was by this means that the difference of gravity was discovered, and the true figure of the earth ascertained. _emily._ but how do they contrive to regulate their time in the equatorial and polar regions? for, since in our part of the earth the pendulum of a clock vibrates exactly once in a second, if it vibrates faster at the poles, and slower at the equator, the inhabitants must regulate their clocks in a manner different from us. _mrs. b._ the only alteration required is to lengthen the pendulum in one case, and to shorten it in the other; for the velocity of the vibrations of a pendulum depends on its length; and when it is said that a pendulum vibrates quicker at the pole than at the equator, it is supposed to be of the same length. a pendulum which vibrates seconds in this latitude is about - / inches long. in order to vibrate at the equator in the same space of time, it must be somewhat shorter; and at the poles, it must be proportionally lengthened. the vibrations of a pendulum, resemble the descent of a body on an inclined plane, and are produced by the same cause; now you must recollect, that the greater the perpendicular height of such a plane, in proportion to its length, the more rapid will be the descent of the body; a short pendulum ascends to a greater height than a larger one, in vibrating a given distance, and of course its descent must be more rapid. i shall now, i think, be able to explain to you the cause of the variation of the seasons, and the difference in the length of the days and nights in those seasons; both effects resulting from the same cause. in moving round the sun, the axis of the earth is not perpendicular to the plane of its orbit. supposing this round table to represent the plane of the earth's orbit, and this little globe, the earth; through this i have passed a wire, representing its axis and poles. in moving round the table, i do not hold the wire perpendicular to it, but obliquely. _emily._ yes, i understand, the earth does not go round the sun in an upright position, its axis is slanting or oblique; and, it of course, forms an angle with a line drawn perpendicular to the plane of the earth's orbit. _mrs. b._ all the lines, which you learnt in your last lesson, are delineated on this little globe; you must consider the ecliptic as representing the plane of the earth's orbit; and the equator, which crosses the ecliptic in two places, then shows the degree of obliquity of the axis of the earth; which amounts to - / degrees, very nearly. the points in which the ecliptic intersects the equator, are called the equinoctial points. but i believe i shall render the effects of the obliquity of the earth's axis clearer to you, by the revolution of the little globe round a candle, which shall represent the sun. (plate ix. fig. .) as i now hold it, at a, you see it in the situation in which it is in the midst of summer, or what is called the summer solstice, which is on the st of june. _emily._ you hold the wire awry, i suppose, in order to show that the axis of the earth is not upright? _mrs. b._ yes; in summer, the north pole is inclined towards the sun. in this season, therefore, the northern hemisphere enjoys much more of his rays than the southern. the sun, you see, now shines over the whole of the north frigid zone, and notwithstanding the earth's diurnal revolution, which i imitate by twirling the ball on the wire, it will continue to shine upon it as long as it remains in this situation, whilst the south frigid zone is at the same time completely in darkness. _caroline._ that is very strange; i never before heard that there was constant day or night in any part of the world! how much happier the inhabitants of the north frigid zone must be than those of the southern; the first enjoy uninterrupted day, while the last are involved in perpetual darkness. _mrs. b._ you judge with too much precipitation; examine a little further, and you will find, that the two frigid zones share an equal fate. we shall now make the earth set off from its position in the summer solstice, and carry it round the sun; observe that the pole is always inclined in the same direction, and points to the same spot in the heavens. there is a fixed star situated near that spot, which is hence called the north polar star. now let us stop the earth at b, and examine it in its present situation; it has gone through one quarter of its orbit, and is arrived at that point at which the ecliptic cuts, or crosses, the equator, and which is called the autumnal equinox. _emily._ the sun now shines from one pole to the other, just as it would constantly do, if the axis of the earth were perpendicular to its orbit. _mrs. b._ because the inclination of the axis is now neither towards the sun, nor in the contrary direction; at this period of the year, the days and nights are equal in every part of the earth. but the next step she takes in her orbit, you see, involves the north pole in darkness, whilst it illumines that of the south; this change was gradually preparing as i moved the earth from summer to autumn; the arctic circle, which was at first entirely illumined, began to have short nights, which increased as the earth approached the autumnal equinox; and the instant it passed that point, the long night of the north pole commences, and the south pole begins to enjoy the light of the sun. we shall now make the earth proceed in its orbit, and you may observe that as it advances, the days shorten and the nights lengthen, throughout the northern hemisphere, until it arrives at the winter solstice, on the st of december, when the north frigid zone is entirely in darkness, and the southern has uninterrupted daylight. [illustration: plate x.] _caroline._ then, after all, the sun which i thought so partial, confers his favours equally on all. _mrs. b._ not so either: the inhabitants of the torrid zone have much more heat than we have, as the sun's rays fall perpendicularly twice in the course of a year, on every place within the tropics, while they shine more or less obliquely on the rest of the world, and almost horizontally at the poles; for during their long day of six months, the sun moves round their horizon without either rising or setting; the only observable difference, is that it is more elevated by a few degrees at mid-day, than at midnight. _emily._ to a person placed in the temperate zone, in the situation in which we are in england, the sun will shine neither so obliquely as it does on the poles, nor vertically as at the equator; but its rays will fall upon him more obliquely in autumn, and winter, than in summer. _caroline._ and therefore, the inhabitants of the temperate zones, will not have merely one day, and one night, in the year, as happens at the poles, nor will they have equal days, and equal nights, as at the equator; but their days and nights will vary in length, at different times of the year, according as their respective poles incline towards, or from the sun, and the difference will be greater in proportion to their distance from the equator. _mrs. b._ we shall now follow the earth through the other half of her orbit, and you will observe, that now exactly the same changes take place in the southern hemisphere, as those we have just remarked in the northern. day commences at the south pole, when night sets in at the north pole; and in every other part of the southern hemisphere the days are longer than the nights, while, on the contrary, our nights are longer than our days. when the earth arrives at the vernal equinox, d, where the ecliptic again cuts the equator, on the st of march, she is situated, with respect to the sun, exactly in the same position, as in the autumnal equinox; and the only difference with respect to the earth, is, that it is now autumn in the southern hemisphere, whilst it is spring with us. _caroline._ then the days and nights are again every where equal. _mrs. b._ yes, for the half of the globe which is enlightened, extends exactly from one pole to the other, the sun has just risen to the north pole, and is just setting to the south pole; but in every other part of the globe, the day and night is of twelve hours length; hence the word equinox, which is derived from the latin, meaning equal night. as our summer advances, the days lengthen in the northern hemisphere, and shorten in the southern, till the earth reaches the summer solstice, when the north frigid zone is entirely illumined, and the southern is in complete darkness; and we have now brought the earth again to the spot from whence we first accompanied her. _emily._ this is indeed a most satisfactory explanation of the cause of the different lengths of our days and nights, and of the variation of the seasons; and the more i learn, the more i admire the simplicity of means by which such wonderful effects are produced. _mrs. b._ i know not which is most worthy of our admiration, the causes, or the effects of the earth's revolution round the sun. the mind can find no object of contemplation more sublime, than the course of this magnificent globe, impelled by the combined powers of projection and attraction, to roll in one invariable course, around the source of light and heat: and what can be more delightful than the beneficent effects of this vivifying power on its attendant planet. it is at once the grand principle which animates and fecundates nature. _emily._ there is one circumstance in which this little ivory globe appears to me to differ from the earth; it is not quite dark on that side of it which is turned from the candle, as is the case with the earth when neither moon nor stars are visible. _mrs. b._ this is owing to the light of the candle, being reflected by the walls of the room, on every part of the globe, consequently that side of the globe, on which the candle does not directly shine, is not in total darkness. now the skies have no walls to reflect the sun's light on that side of our earth which is in darkness. _caroline._ i beg your pardon, mrs. b., i think that the moon, and stars, answer the purpose of walls in reflecting the sun's light to us in the night. _mrs. b._ very well, caroline; that is to say, the moon and planets; for the fixed stars, you know, shine by their own light. _emily._ you say, that the superior heat of the equatorial parts of the earth, arises from the rays falling perpendicularly on those regions, whilst they fall obliquely on these more northern regions; now i do not understand why perpendicular rays should afford more heat than oblique rays. _caroline._ you need only hold your hand perpendicularly over the candle, and then hold it sideways obliquely, to be sensible of the difference. _emily._ i do not doubt the fact, but i wish to have it explained. _mrs. b._ you are quite right; if caroline had not been satisfied with ascertaining the fact, without understanding it, she would not have brought forward the candle as an illustration; the reason why you feel so much more heat if you hold your hand perpendicularly over the candle, than if you hold it sideways, is because a stream of heated vapour constantly ascends from the candle, or any other burning body, which being lighter than the air of the room, does not spread laterally but rises perpendicularly, and this led you to suppose that the rays were hotter in the latter direction. had you reflected, you would have discovered that rays issuing from the candle sideways, are no less perpendicular to your hand when held opposite to them, than the rays which ascend when your hand is held over them. the reason why the sun's rays afford less heat when in an oblique direction, than when perpendicular, is because fewer of them fall upon an equal portion of the earth; this will be understood better by referring to plate . fig. , which represents two equal portions of the sun's rays, shining upon different parts of the earth. here it is evident, that the same quantity of rays fall on the space a b, as fall on the space b c; and as a b is less than b c, the heat and light will be much stronger in the former than in the latter; a b, you see, represents the equatorial regions, where the sun shines perpendicularly; and b c, the temperate and frozen climates, where his rays fall more obliquely. _emily._ this accounts not only for the greater heat of the equatorial regions, but for the greater heat of our summers, as the sun shines less obliquely in summer than in winter. _mrs. b._ this you will see exemplified in figure , in which the earth is represented, as it is situated on the st of june, and england receives less oblique, and consequently a greater number of rays, than at any other season; and figure , shows the situation of england on the st of december, when the rays of the sun fall most obliquely upon her. but there is also another reason why oblique rays give less heat, than perpendicular rays; which is, that they have a greater portion of the atmosphere to traverse; and though it is true, that the atmosphere is itself a transparent body, freely admitting the passage of the sun's rays, yet it is always loaded more or less with dense and foggy vapour, which the rays of the sun cannot easily penetrate; therefore, the greater the quantity of atmosphere the sun's rays have to pass through in their way to the earth, the less heat they will retain when they reach it. this will be better understood, by referring to fig. . the dotted line round the earth, describes the extent of the atmosphere, and the lines which proceed from the sun to the earth, the passage of two equal portions of the sun's rays, to the equatorial and polar regions; the latter you see, from its greater obliquity, passes through a greater extent of atmosphere. _caroline._ and this, no doubt, is the reason why the sun, in the morning and in the evening, gives so much less heat, than at mid-day. _mrs. b._ the diminution of heat, morning and evening, is certainly owing to the greater obliquity of the sun's rays; and they are also affected by the other, both the cause, which i have just explained to you; the difficulty of passing through a foggy atmosphere is perhaps more particularly applicable to them, as mist and vapours are prevalent about the time of sunrise and sunset. but the diminished obliquity of the sun's rays, is not the sole cause of the heat of summer; the length of the days greatly conduces to it; for the longer the sun is above the horizon, the more heat he will communicate to the earth. _caroline._ both the longest days, and the most perpendicular rays, are on the st of june; and yet the greatest heat prevails in july and august. _mrs. b._ those parts of the earth which are once heated, retain the heat for some length of time, and the additional heat they receive, occasions an elevation of temperature, although the days begin to shorten, and the sun's rays to fall more obliquely. for the same reason, we have generally more heat at three o'clock in the afternoon, than at twelve, when the sun is on the meridian. _emily._ and pray, have the other planets the same vicissitudes of seasons, as the earth? _mrs. b._ some of them more, some less, according as their axes deviate more or less from the perpendicular, to the plane of their orbits. the axis of jupiter, is nearly perpendicular to the plane of his orbit; the axes of mars, and of saturn, are each, inclined at angles of about sixty degrees; whilst the axis of venus is believed to be elevated only fifteen or twenty degrees above her orbit; the vicissitudes of her seasons must therefore be considerably greater than ours. for further particulars respecting the planets, i shall refer you to bonnycastle's introduction to astronomy. i have but one more observation to make to you, relative to the earth's motion; which is, that although we have but days and nights in the year, she performs complete revolutions on her axis, during that time. _caroline._ how is that possible? for every complete revolution must bring the same place back to the sun. it is now just twelve o'clock, the sun is, therefore, on our meridian; in twenty-four hours will it not have returned to our meridian again, and will not the earth have made a complete rotation on its axis? _mrs. b._ if the earth had no progressive motion in its orbit whilst it revolves on its axis, this would be the case; but as it advances almost a degree westward in its orbit, in the same time that it completes a revolution eastward on its axis, it must revolve nearly one degree more in order to bring the same meridian back to the sun. _caroline._ oh, yes! it will require as much more of a second revolution to bring the same meridian back to the sun, as is equal to the space the earth has advanced in her orbit; that is, nearly a degree; this difference is, however, very little. _mrs. b._ these small daily portions of rotation, are each equal to the three hundred and sixty-fifth part of a circle, which at the end of the year amounts to one complete rotation. _emily._ that is extremely curious. if the earth then, had no other than its diurnal motion, we should have days in the year. _mrs. b._ we should have days in the same period of time that we now have ; but if we did not revolve round the sun, we should have no natural means of computing years. you will be surprised to hear, that if time is calculated by the stars instead of the sun, the irregularity which we have just noticed does not occur, and that one complete rotation of the earth on its axis, brings the same meridian back to any fixed star. _emily._ that seems quite unaccountable; for the earth advances in her orbit with regard to the fixed stars, the same as with regard to the sun. _mrs. b._ true, but then the distance of the fixed stars is so immense, that our solar system is in comparison to it but a spot, and the whole extent of the earth's orbit but a point; therefore, whether the earth remain stationary, or whether it revolved in its orbit during its rotation on its axis, no sensible difference would be produced with regard to the fixed stars. one complete revolution brings the same meridian back to the same fixed star; hence the fixed stars appear to go round the earth in a shorter time than the sun by three minutes fifty-six seconds of time. _caroline._ these three minutes fifty-six seconds is the time which the earth takes to perform the additional three hundred and sixty-fifth part of the circle, in order to bring the same meridian back to the sun. _mrs. b._ precisely. hence the stars gain every day three minutes fifty-six seconds on the sun, which makes them rise that portion of time earlier every day. when time is calculated by the stars it is called sidereal time; when by the sun, solar, or apparent time. _caroline._ then a sidereal day is three minutes fifty-six seconds shorter, than a solar day of twenty-four hours. _mrs. b._ i must also explain to you what is meant by a sidereal year. the common year, called the solar or tropical year, containing days, five hours, forty-eight minutes and fifty-two seconds, is measured from the time the sun sets out from one of the equinoxes, or solstices, till it returns to the same again; but this year is completed, before the earth has finished one entire revolution in its orbit. _emily._ i thought that the earth performed one complete revolution in its orbit, every year; what is the reason of this variation? _mrs. b._ it is owing to the spheroidal figure of the earth. the elevation about the equator produces much the same effect as if a similar mass of matter, collected in the form of a moon, revolved round the equator. when this moon acted on the earth, in conjunction with, or in opposition to the sun, variations in the earth's motion would be occasioned, and these variations produce what is called the precession of the equinoxes. [illustration: plate xi.] _emily._ what does that mean? i thought the equinoctial points, were fixed points in the heavens, in which the equator cuts the ecliptic. _mrs. b._ these points are not quite fixed, but have an apparently retrograde motion, among the signs of the zodiac; that is to say, instead of being at every revolution in the same place, they move backwards. thus if the vernal equinox is at a, (fig. . plate xi.) the autumnal one, will be at b, instead of c, and the following vernal equinox, at d, instead of at a, as would be the case if the equinoxes were stationary, at opposite points of the earth's orbit. _caroline._ so that when the earth moves from one equinox to the other, though it takes half a year to perform the journey, it has not travelled through half its orbit. _mrs. b._ and, consequently, when it returns again to the first equinox, it has not completed the whole of its orbit. in order to ascertain when the earth has performed an entire revolution in its orbit, we must observe when the sun returns in conjunction with any fixed star; and this is called a sidereal year. supposing a fixed star situated at e, (fig. . plate xi.) the sun would not appear in conjunction with it, till the earth had returned to a, when it would have completed its orbit. _emily._ and how much longer is the sidereal, than the solar year? _mrs. b._ only twenty minutes; so that the variation of the equinoctial points is very inconsiderable. i have given them a greater extent in the figure, in order to render them sensible. in regard to time, i must further add, that the earth's diurnal motion on an inclined axis, together with its annual revolution in an elliptic orbit, occasions so much complication in its motion, as to produce many irregularities; therefore the true time cannot be measured by the apparent place of the sun. a perfectly correct clock, would in some parts of the year be before the sun, and in other parts after it. there are but four periods in which the sun and a perfect clock would agree, which is the th of april, the th of june, the d of august, and the th of december. _emily._ and is there any considerable difference between solar time, and true time? _mrs. b._ the greatest difference amounts to between fifteen and sixteen minutes. tables of equation are constructed for the purpose of pointing out, and correcting these differences between solar time and equal or mean time, which is the denomination given by astronomers, to true time. questions . (pg. ) what does the line a b, (fig. plate .) represent, and what are its extremities called? . (pg. ) what is meant by the equator, and how is it situated? . (pg. ) there are two hemispheres; how are they named and distinguished? . (pg. ) what are the circles near the poles called? . (pg. ) what do the lines i k, and l m, represent? . (pg. ) what circle is in part represented by the line l k? . (pg. ) against what mistake must you guard respecting this line? . (pg. ) what is meant by a plane, and how could one be represented? . (pg. ) describe what is intended by the plane of the earth's orbit. . (pg. ) extending this plane to the fixed stars, what circle would it form, and among what particular stars would it be found? . (pg. ) what is fig. . plate , designed to represent? . (pg. ) the ecliptic does not properly belong to the earth, for what purpose then is it described on the terrestrial globe? . (pg. ) what does the obliquity of the ecliptic to the equator serve to show? . (pg. ) within what limits do you find the torrid zone? . (pg. ) what two zones are there between the torrid, and the two frigid zones? . (pg. ) where are the frigid zones situated? . (pg. ) what is meant by the term zone; and are the frigid zones properly so called? . (pg. ) how do meridian lines extend, and what is meant by the meridian of a place? . (pg. ) what is said of the meridian to which the sun is opposite, and where is it then midnight? . (pg. ) what hour is it then, at places exactly half way between these meridians? . (pg. ) how are greater and lesser circles distinguished? . (pg. ) what part of a circle is a degree, and how are these further divided? . (pg. ) what is the diameter, and what the circumference of a circle, and what proportion do they bear to each other? . (pg. ) what part of a circle is a meridian? . (pg. ) how many degrees are there between the equator and the poles? . (pg. ) into what parts, besides degrees, is the ecliptic divided? . (pg. ) how are degrees of latitude measured, and to what number do they extend? . (pg. ) on what circles are degrees of longitude measured, and to what number do they extend? . (pg. ) what is a parallel of latitude? . (pg. ) degrees of longitude vary in length; what is the cause of this? . (pg. ) what is the length of a degree of latitude, and why do not these vary? . (pg. ) what causes the equator to be somewhat larger than a great circle passing through the poles, and what effect has this on degrees of longitude measured on the equator? . (pg. ) what is the cause of this form being given to the earth? . (pg. ) what would have been a consequence of the centrifugal force, had the earth been a perfect sphere? . (pg. ) a body situated at the poles, is attracted more forcibly than if placed at the equator, what is the reason? . (pg. ) what effect would be produced upon the gravity of a body, were it placed beneath the surface of the earth, and what supposing it at its centre? . (pg. ) what two circumstances combine, to lessen the weight of a body on the equator? . (pg. ) why could not this be proved by weighing a body at the poles, and at the equator? . (pg. ) what is a pendulum? . (pg. ) what causes it to vibrate? . (pg. ) why are not its vibrations perpetual? . (pg. ) two pendulums of the same length, will not, in different latitudes, perform their vibrations in equal times, what is the cause of this? . (pg. ) to what use has this property of the pendulum been applied? . (pg. ) what change must be made in pendulums situated at the equator and at the poles, to render their vibrations equal? . (pg. ) what do the vibrations of a pendulum resemble, and why will it vibrate more rapidly if shortened? . (pg. ) in the revolution of the earth round the sun, what is the position of its axis? . (pg. ) how much is the axis of the earth inclined, and with what line does it form this angle? . (pg. ) what is represented by fig. , plate ? . (pg. ) how is the north pole inclined in the middle of our summer, and what effect has this on the north frigid zone? . (pg. ) in what direction does the north pole always point? . (pg. ) what is shown by the position of the earth at b, in the figure? . (pg. ) how does the sun then shine at the poles, and what is the effect on the days and nights? . (pg. ) when the earth has passed the autumnal equinox, what changes take place at the poles, and also in the whole northern and southern hemispheres? . (pg. ) why is the heat greatest within the torrid zone? . (pg. ) how does the sun appear at the poles, during the period of day there? . (pg. ) in what will the days and nights differ in the temperate zone, from those at the poles, and at the equator? . (pg. ) trace the earth from the winter solstice to the vernal equinox, and inform me what changes take place. . (pg. ) what takes place at the time of the vernal equinox, and what is meant by the term? . (pg. ) in proceeding from the vernal equinox to the summer solstice, what changes take place? . (pg. ) from what cause arises the superior heat of the equatorial regions? . (pg. ) why should oblique rays afford less heat than those which are perpendicular? . (pg. ) how is this explained by fig. . plate ? . (pg. ) how do you account for the superior heat of summer, and how is this exemplified in fig. and , plate ? . (pg. ) what other cause lessens the intensity of oblique rays? . (pg. ) how is this explained by fig. ? . (pg. ) what causes conspire to lessen the solar heat in the morning and evening? . (pg. ) the greatest heat of summer is after the solstice, and the greatest heat of the day, after o'clock, although the sun's rays are then most direct, how is this accounted for? . (pg. ) is there any change of seasons in the other planets? . (pg. ) what is said respecting the axes of jupiter, of mars, and of saturn? . (pg. ) in days, how many times does the earth revolve on its axis? . (pg. ) how is this accounted for? . (pg. ) do the fixed stars require the same time as the sun, to return to the same meridian? . (pg. ) how is this accounted for? . (pg. ) what is meant by the solar and the sidereal day? . (pg. ) what is the difference in time between them? . (pg. ) what is the length of the tropical year? . (pg. ) the solar year is completed before the earth has made a complete revolution in its orbit, by what is this caused? . (pg. ) what is this called, and what is represented respecting it by fig. , plate ? . (pg. ) by what means can we ascertain the period of a complete revolution of the earth in its orbit, as illustrated by the fixed star e, in fig. ? . (pg. ) what difference is there in the length of the solar and sidereal year? . (pg. ) why can we not always ascertain the true time by the apparent place of the sun? . (pg. ) what would be the greatest difference between solar, and true time, as indicated by a perfect clock? conversation ix. on the moon. of the moon's motion. phases of the moon. eclipses of the moon. eclipses of jupiter's moons. of latitude and longitude. of the transits of the inferior planets. of the tides. mrs. b. we shall, to-day, confine our attention to the moon, which offers many interesting phenomena. the moon revolves round the earth in the space of about twenty-nine days and a half; in an orbit, the plane of which is inclined upwards of five degrees to that of the earth; she accompanies us in our revolution round the sun. _emily._ her motion then must be of a complicated nature; for as the earth is not stationary, but advances in her orbit, whilst the moon goes round her, the moon, in passing round the sun, must proceed in a sort of scolloped circle. _mrs. b._ that is true; and there are also other circumstances which interfere with the simplicity, and regularity of the moon's motion, but which are too intricate for you to understand at present. the moon always presents the same face to us, by which it is evident that she turns but once upon her axis, while she performs a revolution round the earth; so that the inhabitants of the moon have but one day, and one night, in the course of a lunar month. _caroline._ we afford them, however, the advantage of a magnificent moon to enlighten their long nights. _mrs. b._ that advantage is put partial; for since we always see the same hemisphere of the moon, the inhabitants of that hemisphere alone, can perceive us. _caroline._ one half of the moon then enjoys our light, while the other half has constantly nights of darkness. if there are any astronomers in those regions, they would doubtless be tempted to visit the other hemisphere, in order to behold so grand a luminary as we must appear to them. but, pray, do they see the earth under all the changes, which the moon exhibits to us? _mrs. b._ exactly so. these changes are called the phases of the moon, and require some explanation. in fig. , plate , let us say, that s represents the sun, e the earth, and a b c d e f g h, the moon, in different parts of her orbit. when the moon is at a, her dark side being turned towards the earth, we shall not see her as at _a_; but her disappearance is of very short duration, and as she advances in her orbit, we perceive her under the form of a new moon: when she has gone through one eighth of her orbit at b, one quarter of her enlightened hemisphere will be turned towards the earth, and she will then appear horned as at _b_; when she has performed one quarter of her orbit, she shows us one half of her enlightened side, as at _c_, and this is called her first quarter; at _d_ she is said to be gibbous, and at _e_ the whole of the enlightened side appears to us, and the moon is at full. as she proceeds in her orbit, she becomes again gibbous, and her enlightened hemisphere turns gradually away from us, until she arrives at g, which is her third quarter; proceeding thence she completes her orbit and disappears, and then again resumes her form of a new moon, and passes successively, through the same changes. when the moon is new, she is said to be in conjunction with the sun, as they are then both in the same direction from the earth; at the time of full moon, she is said to be in opposition, because she and the sun, are at opposite sides of the earth; at the time of her first and third quarters, she is said to be in her quadratures, because she is then one-fourth of a circle, or °, from her conjunction, or the period of new moon. _emily._ are not the eclipses of the sun produced by the moon passing between the sun and the earth? _mrs. b._ yes; when the moon passes between the sun and the earth, she intercepts his rays, or, in other words, casts a shadow on the earth, then the sun is eclipsed, and daylight gives place to darkness, while the moon's shadow is passing over us. when, on the contrary, the earth is between the sun and the moon, it is we who intercept the sun's rays, and cast a shadow on the moon; she is then said to be eclipsed, and disappears from our view. _emily._ but as the moon goes round the earth every month, she must be, once during that time, between the earth and the sun; and the earth must likewise be once between the sun and the moon, and yet we have not a solar and a lunar eclipse every month? _mrs. b._ i have already informed you, that the orbits of the earth and moon are not in the same plane, but cross or intersect each other; and the moon generally passes either above or below that of the earth, when she is in conjunction with the sun, and does not therefore intercept its rays, and produce an eclipse; for this can take place only when the moon is in, or near her nodes, which is the name given to those two points in which her orbit crosses that of the earth; eclipses cannot happen at any other time, because it is then only, that they are both in a right line with the sun. _emily._ and a partial eclipse of the moon takes place, i suppose, when, in passing by the earth, she is not sufficiently above or below the shadow, to escape it entirely? _mrs. b._ yes, one edge of her disk then dips into the shadow, and is eclipsed; but as the earth is larger than the moon, when eclipses happen precisely at the nodes, they are not only total, but last for upwards of three hours. [illustration: plate xii.] a total eclipse of the sun rarely occurs, and when it happens, the total darkness is confined to one particular part of the earth, the diameter of the shadow not exceeding miles; evidently showing that the moon is smaller than the sun, since she cannot entirely hide it from the earth. in fig. , plate , you will find a solar eclipse described; s is the sun, m the moon, and e the earth; and the moon's shadow, you see, is not large enough to cover the earth. the lunar eclipses, on the contrary, are visible from every part of the earth, where the moon is above the horizon; and we discover, by the length of time which the moon is passing through the earth's shadow, that it would be sufficient to eclipse her totally, were she many times her actual size; it follows, therefore, that the earth is much larger than the moon. in fig. , s represents the sun, which pours forth rays of light in straight lines, in every direction. e is the earth, and m the moon. now a ray of light coming from one extremity of the sun's disk, in the direction a b, will meet another, coming from the opposite extremity, in the direction c b; the shadow of the earth cannot therefore extend beyond b; as the sun is larger than the earth, the shadow of the latter is conical, or in the figure of a sugar loaf; it gradually diminishes, and is much smaller than the earth where the moon passes through it, and yet we find the moon to be, not only totally eclipsed, but to remain for a considerable length of time in darkness, and hence we are enabled to ascertain its real dimensions. _emily._ when the moon eclipses the sun to us, we must be eclipsed to the moon? _mrs. b._ certainly; for if the moon intercepts the sun's rays, and casts a shadow on us, we must necessarily disappear to the moon, but only partially, as in fig. . _caroline._ there must be a great number of eclipses in the distant planets, which have so many moons? _mrs. b._ yes, few days pass without an eclipse taking place; for among the number of satellites, one or the other of them are continually passing either between their primary and the sun; or between the planet, and each other. astronomers are so well acquainted with the motion of the planets, and their satellites, that they have calculated not only the eclipses of our moon, but those of jupiter, with such perfect accuracy, that it has afforded a means of ascertaining the longitude. _caroline._ but is it not very easy to find both the latitude and longitude of any place by a map or globe? _mrs. b._ if you know where you are situated, there is no difficulty in ascertaining the latitude or longitude of the place, by referring to a map; but supposing that you had been a length of time at sea, interrupted in your course by storms, a map would afford you very little assistance in discovering where you were. _caroline._ under such circumstances, i confess i should be equally at a loss to discover either latitude, or longitude. _mrs. b._ the latitude is usually found by taking the altitude of the sun at mid-day; that is to say, the number of degrees that it is elevated above the horizon, for the sun appears more elevated as we approach the equator, and less as we recede from it. _caroline._ but unless you can see the sun, how can you take its altitude? _mrs. b._ when it is too cloudy to see the sun, the latitude is sometimes found at night, by the polar star; the north pole of the earth, points constantly towards one particular part of the heavens, in which a star is situated, called the polar star: this star is visible on clear nights, from every part of the northern hemisphere; the altitude of the polar star, is therefore the same number of degrees, as that of the pole; the latitude may also be determined by observations made on any of the fixed stars: the situation therefore of a vessel at sea, with regard to north and south, is easily ascertained. the difficulty is, respecting east and west, that is to say, its longitude. as we have no eastern poles from which we can reckon our distance, some particular spot, or line, must be fixed upon for that purpose. the english, reckon from the meridian of greenwich, where the royal observatory is situated; in french maps, you will find that the longitude is reckoned from the meridian of paris. the rotation of the earth on its axis in hours from west to east, occasions, you know, an apparent motion of the sun and stars in a contrary direction, and the sun appears to go round the earth in the space of hours, passing over fifteen degrees, or a twenty-fourth part of the earth's circumference every hour; therefore, when it is twelve o'clock in london, it is one o'clock in any place situated fifteen degrees to the east of london, as the sun must have passed the meridian of that place, an hour before he reaches that of london. for the same reason it is eleven o'clock in any place situated fifteen degrees to the west of london, as the sun will not come to that meridian till an hour later. if then the captain of a vessel at sea, could know precisely what was the hour at london, he could, by looking at his watch, and comparing it with the hour at the spot in which he was, ascertain the longitude. _emily._ but if he had not altered his watch, since he sailed from london, it would indicate the hour it then was in london. _mrs. b._ true; but in order to know the hour of the day at the spot in which he is, the captain of a vessel regulates his watch by the sun when it reaches the meridian. _emily._ then if he had two watches, he might keep one regulated daily, and leave the other unaltered; the former would indicate the hour of the place in which he was situated, and the latter the hour at london; and by comparing them together, he would be able to calculate his longitude. _mrs. b._ you have discovered, emily, a mode of finding the longitude, which i have the pleasure to tell you, is universally adopted: watches of a superior construction, called chronometers, or time-keepers, are used for this purpose, and are now made with such accuracy, as not to vary more than four or five seconds in a whole year; but the best watches are liable to imperfections, and should the time-keeper go too fast or too slow, there would be no means of ascertaining the error; implicit reliance, cannot consequently be placed upon them. recourse, therefore, is sometimes had to the eclipses of jupiter's satellites. a table is made, of the precise time at which the several moons are eclipsed to a spectator at london; when they appear eclipsed to a spectator in any other spot, he may, by consulting the table, know what is the hour at london; for the eclipse is visible at the same moment, from whatever place on the earth it is seen. he has then only to look at his watch, which he regulates by the sun, and which therefore points out the hour of the place in which he is, and by observing the difference of time there, and at london, he may immediately determine his longitude. let us suppose, that a certain moon of jupiter is always eclipsed at six o'clock in the evening; and that a man at sea consults his watch, and finds that it is ten o'clock at night, where he is situated, at the moment the eclipse takes place, what will be his longitude? _emily._ that is four hours later than in london: four times fifteen degrees, make ; he would, therefore, be sixty degrees east of london, for the sun must have passed his meridian before it reaches that of london. _mrs. b._ for this reason the hour is always later than in london, when the place is east longitude, and earlier when it is west longitude. thus the longitude can be ascertained whenever the eclipses of jupiter's moons are visible. _caroline._ but do not the primary planets, sometimes eclipse the sun from each other, as they pass round in their orbits? _mrs. b._ they must of course sometimes pass between each other and the sun, but as their shadows never reach each other, they hide so little of his light, that the term eclipse is not in this case used; this phenomenon is called a transit. the primary planets do not any of them revolve in the same plane, and the times of their revolution round the sun is considerable, it therefore but rarely happens that they are at the same time, in conjunction with the sun, and in their nodes. it is evident also, that a planet must be inferior (that is within the orbit of another) in order to its apparently passing over the disk of the sun. mercury, and venus, have sometimes passed in a right line between us, and the sun, but being at so great a distance from us, their shadows did not extend so far as the earth; no darkness was therefore produced on any part of our globe; but the planet appeared like a small black spot, passing across the sun's disk. it was by the last transit of venus, that astronomers were enabled to calculate, with some degree of accuracy, the distance of the earth from the sun, and the dimensions of the latter. _emily._ i have heard that the tides are affected by the moon, but i cannot conceive what influence it can have on them. _mrs. b._ they are produced by the moon's attraction, which draws up the waters of that part of the ocean over which the moon passes, so as to cause it to stand considerably higher than the surrounding parts. _caroline._ does attraction act on water more powerfully than on land? i should have thought it would have been just the contrary, for land is certainly a more dense body than water? _mrs b._ tides do not arise from water being more strongly attracted than land, for this certainly is not the case; but the cohesion of fluids, being much less than that of solid bodies, they more easily yield to the power of gravity; in consequence of which, the waters immediately below the moon, are drawn up by it, producing a full tide, or what is commonly called, high water, at the spot where it happens. so far, the theory of the tides is not difficult to understand. _caroline._ on the contrary, nothing can be more simple; the waters, in order to rise up under the moon, must draw the waters from the opposite side of the globe, and occasion ebb-tide, or low water, in those parts. _mrs. b._ you draw your conclusion rather too hastily, my dear; for according to your theory, we should have full tide only once in about twenty-four hours, that is, every time that we were below the moon, while we find that in this time we have two tides, and that it is high water with us, and with our antipodes, at the same time. _caroline._ yet it must be impossible for the moon to attract the sea in opposite parts of the globe, and in opposite directions, at the same time. _mrs. b._ this opposite tide, is rather more difficult to explain, than that which is immediately beneath the moon; with a little attention, however, i hope i shall be able to make you understand the explanation which has been given of it, by astronomers. it must be confessed, however, that the theory upon this subject, is attended with some difficulties. you recollect that the earth and the moon mutually attract each other, but do you suppose that every part of the earth is equally attracted by the moon? _emily._ certainly not; you have taught us that the force of attraction decreases, with the increase of distance, and therefore that part of the earth which is farthest from the moon, must be attracted less powerfully, than that to which she is nearest. _mrs. b._ this fact will aid us in the explanation which i am about to give to you. in order to render the question more simple, let us suppose the earth to be every where covered by the ocean, as represented in (fig. . pl. .) m is the moon, a b c d the earth. now the waters on the surface of the earth, about a, being more strongly attracted than any other part, will be elevated: the attraction of the moon at b and c being less, and at d least of all. the high tide at a, is accounted for from the direct attraction of the moon; to produce this the waters are drawn from b and c, where it will consequently be low water. at d, the attraction of the moon being considerably decreased, the waters are left relatively high, which height is increased, by the centrifugal force of the earth being greater at d than at a, in consequence of its greater distance from the common centre of gravity x, between the earth and the moon. _emily._ the tide a, then, is produced by the moon's attraction, and the tide d, is produced by the centrifugal force, and increased by the feebleness of the moon's attraction, in those parts. _caroline._ and when it is high water at a and d, it is low water at b and c: now i think i comprehend the nature of the tides, though i confess it is not quite so easy as i at first thought. but, mrs. b., why does not the sun produce tides, as well as the moon; for its attraction is greater than that of the moon? _mrs. b._ it would be at an equal distance, but our vicinity to the moon, makes her influence more powerful. the sun has, however, a considerable effect on the tides, and increases or diminishes them as it acts in conjunction with, or in opposition to the moon. _emily._ i do not quite understand that. _mrs. b._ the moon is a month in going round the earth; twice during that time, therefore, at full and at change, she is in the same direction as the sun; both, then act in conjunction on the earth, and produce very great tides, called spring tides, as represented in fig. , at a and b; but when the moon is at the intermediate parts of her orbit, that is in her quadratures, the sun, instead of affording assistance, weakens her power, by acting in opposition to it; and smaller tides are produced, called neap tides, as represented at m, in fig. . _emily._ i have often observed the difference of these tides, when i have been at the sea side. but since attraction is mutual between the moon and the earth, we must produce tides in the moon; and these must be more considerable in proportion as our planet is larger. and yet the moon does not appear of an oval form. _mrs. b._ you must recollect, that in order to render the explanation of the tides clearer, we suppose the whole surface of the earth to be covered with the ocean; but that is not really the case, either with the earth or the moon, and the land which intersects the water, destroys the regularity of the effect. thus, in flowing up rivers, in passing round points of land, and into bays and inlets, the water is obstructed, and high water must happen much later, than would otherwise be the case. _caroline._ true; we may, however, be certain that whenever it is high water, the moon is immediately over our heads. _mrs. b._ not so either; for as a similar effect is produced on that part of the globe immediately beneath the moon, and on that part most distant from it, it cannot be over the heads of the inhabitants of both those situations, at the same time. besides, as the orbit of the moon is very nearly parallel to that of the earth, she is never vertical, but to the inhabitants of the torrid zone. _caroline._ in the torrid zone, then, i hope you will grant that the moon is immediately over, or opposite the spots where it is high water? _mrs. b._ i cannot even admit that; for the ocean naturally partaking of the earth's motion, in its rotation from west to east, the moon, in forming a tide, has to contend against the eastern motion of the waves. all matter, you know, by its inertia, makes some resistance to a change of state; the waters, therefore, do not readily yield to the attraction of the moon, and the effect of her influence is not complete, till three hours after she has passed the meridian, where it is full tide. when a body is impelled by any force, its motion may continue, after the impelling force ceases to act: this is the case with all projectiles. a stone thrown from the hand, continues its motion for a length of time, proportioned to the force given to it: there is a perfect analogy between this effect, and the continued rise of the water, after the moon has passed the meridian at any particular place. _emily._ pray what is the reason that the tide is three-quarters of an hour later every day? _mrs. b._ because it is twenty-four hours and three-quarters before the same meridian, on our globe, returns beneath the moon. the earth revolves on its axis in about twenty-four hours; if the moon were stationary, therefore, the same part of our globe would, every twenty-four hours, return beneath the moon; but as during our daily revolution, the moon advances in her orbit, the earth must make more than a complete rotation, in order to bring the same meridian opposite the moon: we are three-quarters of an hour in overtaking her. the tides, therefore, are retarded, for the same reason that the moon rises later by three-quarters of an hour, every day. we have now, i think, concluded the observations i had to make to you on the subject of astronomy; at our next interview, i shall attempt to explain to you the elements of hydrostatics. questions . (pg. ) in what time does the moon revolve round the earth? what is the inclination of her orbit? and how does she accompany the earth? . (pg. ) as the moon revolves round the earth, and also accompanies it in its annual revolution, in what form would you draw the moon's orbit? . (pg. ) what causes the moon always to present the same face to the earth, and what must be the length of a day and night to its inhabitants? . (pg. ) can the earth be seen from every part of the moon, and will it always exhibit the same appearance? . (pg. ) what are the changes of the moon called? . (pg. ) how are these changes explained by fig. . plate ? . (pg. ) what is meant by her first quarter? . (pg. ) what by her being horned, and her being gibbous? . (pg. ) what by her being full? . (pg. ) what by her third quarter? . (pg. ) what is meant by her conjunction?--what by her being in opposition?--what by her quadratures? . (pg. ) by what are eclipses of the sun caused? . (pg. ) what causes eclipses of the moon? . (pg. ) what is meant by the moon's nodes? . (pg. ) why do not eclipses happen at every new and full moon? . (pg. ) what causes partial eclipses of the moon? . (pg. ) when the moon is exactly in one of her nodes, what length of time will she be eclipsed? . (pg. ) are total eclipses of the sun frequent, and when they happen what is their extent? . (pg. ) what does this prove respecting the size of the moon? . (pg. ) what is shown in fig. , plate ? . (pg. ) how are lunar eclipses visible, and what is proved by their duration? . (pg. ) what is illustrated by fig. , plate ? . (pg. ) what remark is made respecting those planets which have several moons? . (pg. ) what use is made of the eclipses of the satellites of jupiter? . (pg. ) how is the latitude of a place usually found? . (pg. ) by what other means may latitude be found? . (pg. ) from what is longitude reckoned? . (pg. ) how does the rotation of the earth upon its axis, govern the time at different places? . (pg. ) what two circumstances, if known, will enable you to find your longitude from a given place? . (pg. ) by what means may a captain find the time at london, and in the place where his ship may be? . (pg. ) how may the eclipses of jupiter's satellites be used to find the longitude? . (pg. ) give an example. . (pg. ) how will you know whether the longitude is east or west? . (pg. ) what is meant by the transit of a planet? . (pg. ) why can we see transits of venus and mercury only? . (pg. ) by what are tides caused? . (pg. ) why is not a similar effect produced on the land? . (pg. ) in what two parts of the world is it high water at the same time? . (pg. ) what circumstances respecting the decrease of attraction are taken into account, in explaining the tides? . (pg. ) how are the high tides at a and d, and the low ones at b and c, in fig. . pl. , accounted for? . (pg. ) has the sun any influence on the tides, and why is it less than that of the moon? . (pg. ) what is meant by spring tides, and how are they produced? . (pg. ) what by neap tides, and how are they caused? . (pg. ) what circumstances affect the time of the tide in rivers, bays, &c.? . (pg. ) why in the open ocean, is it high water, some hours after the moon has passed the meridian? . (pg. ) why are the tides three-quarters of an hour later every day? conversation x. on the mechanical properties of fluids. definition of a fluid. distinction between fluids and liquids. of non-elastic fluids. scarcely susceptible of compression. of the cohesion of fluids. of their gravitation. of their equilibrium. of their pressure. of specific gravity. of the specific gravity of bodies heavier than water. of those of the same weight as water. of those lighter than water. of the specific gravity of fluids. mrs. b. we have hitherto confined our attention to the mechanical properties of solid bodies, which have been illustrated, and, i hope, thoroughly impressed upon your memory, by the conversations we have subsequently had, on astronomy. it will now be necessary for me to give you some account of the mechanical properties of fluids--a science which, when applied to liquids, is divided into two parts, hydrostatics and hydraulics. hydrostatics, treats of the weight and pressure of fluids; and hydraulics, of the motion of fluids, and the effects produced by this motion. a fluid is a substance which yields to the slightest pressure. if you dip your hand into a basin of water, you are scarcely sensible of meeting with any resistance. _emily._ the attraction of cohesion is then, i suppose, less powerful in fluids, than in solids? _mrs. b._ yes; fluids, generally speaking, are bodies of less density than solids. from the slight cohesion, of the particles of fluids, and the facility with which they slide over each other, it is inferred, that they have but a slight attraction for each other, and that this attraction is equal, in every position of their particles, and therefore produces no resistance to a perfect freedom of motion among themselves. _caroline._ pray what is the distinction between a fluid and a liquid? _mrs. b._ liquids comprehend only one class of fluids. there is another class, distinguished by the name of elastic fluids, or gases, which comprehends the air of the atmosphere, and all the various kinds of air with which you will become acquainted, when you study chemistry. their mechanical properties we shall examine hereafter, and confine our attention this morning, to those of liquids, or non-elastic fluids. water, and liquids in general, are scarcely susceptible of being compressed, or squeezed into a smaller space, than that which they naturally occupy. such, however, is the extreme minuteness of their particles, that by strong compression, they sometimes force their way through the pores of the substance which confines them. this was shown by a celebrated experiment, made at florence many years ago. a hollow globe of gold was filled with water, and on its being submitted to great pressure, the water was seen to exude through the pores of the gold, which it covered with a fine dew. many philosophers, however, think that this experiment is too much relied upon, as it does not appear that it has ever been repeated; it is possible, therefore, that there may have been some source of error, which was not discovered by the experimenters. fluids, appear to gravitate more freely, than solid bodies; for the strong cohesive attraction of the particles of the latter, in some measure counteracts the effect of gravity. in this table, for instance, the cohesion of the particles of wood, enables four slender legs to support a considerable weight. were the cohesion destroyed, or, in other words, the wood converted into a fluid, no support could be afforded by the legs, for the particles no longer cohering together, each would press separately and independently, and would be brought to a level with the surface of the earth. _emily._ this want of cohesion is then the reason why fluids can never be formed into figures, or maintained in heaps; for though it is true the wind raises water into waves, they are immediately afterwards destroyed by gravity, and water always finds its level. _mrs. b._ do you understand what is meant by the level, or equilibrium of fluids? _emily._ i believe i do, though i feel rather at a loss to explain it. is not a fluid level when its surface is smooth and flat, as is the case with all fluids, when in a state of rest? _mrs. b._ smooth, if you please, but not flat; for the definition of the equilibrium of a fluid is, that every part of the surface is equally distant from the point to which they gravitate, that is to say, from the centre of the earth; hence the surface of all fluids must be spherical, not flat, since they will partake of the spherical form of the globe. this is very evident in large bodies of water, such as the ocean, but the sphericity of small bodies of water, is so trifling, that their surfaces appear flat. this level, or equilibrium of fluids, is the natural result of their particles gravitating independently of each other; for when any particle of a fluid, accidentally finds itself elevated above the rest, it is attracted down to the level of the surface of the fluid, and the readiness with which fluids yield to the slightest impression, will enable the particle by its weight, to penetrate the surface of the fluid, and mix with it. _caroline._ but i have seen a drop of oil, float on the surface of water, without mixing with it. _mrs. b._ they do not mix, because their particles repel each other, and the oil rises to the surface, because oil is a lighter liquid than water. if you were to pour water over it, the oil would still rise, being forced up by the superior gravity of the water. here is an instrument called a spirit-level, (fig. , plate .) which is constructed upon the principle of the equilibrium of fluids. it consists of a short tube a b, closed at both ends, and containing a little water, or more commonly some spirits: it is so nearly filled, as to leave only a small bubble of air; when the tube is perfectly horizontal, this bubble will occupy the middle of it, but when not perfectly horizontal, the water runs to the lower, and the bubble of air or spirit rises to the upper end; by this instrument, the level of any situation, to which we apply it, may be ascertained. from the strong cohesion of their particles, you may therefore consider solid bodies as gravitating in masses, while every particle of a fluid may be considered as separate, and gravitating independently of each other. hence the resistance of a fluid, is considerably less, than that of a solid body; for the resistance of the particles, acting separately, is more easily overcome. _emily._ a body of water, in falling, does certainly less injury than a solid body of the same weight. _mrs. b._ the particles of fluids, acting thus independently, press against each other in every direction, not only downwards, but upwards, and laterally or sideways; and in consequence of this equality of pressure, every particle remains at rest, in the fluid. if you agitate the fluid, you disturb this equality of pressure, and the fluid will not rest, till its equilibrium is restored. [illustration: plate xiii.] _caroline._ the pressure downwards is very natural; it is the effect of gravity; one particle, weighing upon another, presses on it; but the pressure sideways, and particularly the pressure upwards, i cannot understand. _mrs. b._ if there were no lateral pressure, water would not run out of an opening on the side of a vessel. if you fill a vessel with sand, it will not continue to run out of such an opening, because there is scarcely any lateral pressure among its particles. _emily._ when water runs out of the side of a vessel, is it not owing to the weight of the water, above the opening? _mrs. b._ if the particles of fluids were arranged in regular columns, thus, (fig. .) there would be no lateral pressure, for when one particle is perpendicularly above the other, it can only press downwards; but as it must continually happen, that a particle presses between two particles beneath, (fig. .) these last, must suffer a lateral pressure. _emily._ the same as when a wedge is driven into a piece of wood, and separates the parts, laterally. _mrs. b._ yes. the lateral pressure proceeds, therefore, entirely from the pressure downwards, or the weight of the liquid above; and consequently, the lower the orifice is made in the vessel, the greater will be the velocity of the water rushing out of it. here is a vessel of water (fig. .), with three stop cocks at different heights; we shall open them, and you will see with what different degrees of velocity, the water issues from them. do you understand this, caroline? _caroline._ oh yes. the water from the upper spout, receiving but a slight pressure, on account of its vicinity to the surface, flows but gently; the second cock, having a greater weight above it, the water is forced out with greater velocity, whilst the lowest cock, being near the bottom of the vessel, receives the pressure of almost the whole body of water, and rushes out with the greatest impetuosity. _mrs. b._ very well; and you must observe, that as the lateral pressure, is entirely owing to the pressure downwards, it is not affected by the horizontal dimensions of the vessel, which contains the water, but merely by its depth; for as every particle acts independently of the rest, it is only the column of particles immediately above the orifice, that can weigh upon, and press out the water. _emily._ the breadth and width of the vessel then, can be of no consequence in this respect. the lateral pressure on one side, in a cubical vessel, is, i suppose, not so great as the pressure downwards upon the bottom. _mrs. b._ no; in a cubical vessel, the pressure downwards will be double the lateral pressure on one side; for every particle at the bottom of the vessel is pressed upon, by a column of the whole depth of the fluid, whilst the lateral pressure diminishes from the bottom upwards to the surface, where the particles have no pressure. _caroline._ and from whence proceeds the pressure of fluids upwards? that seems to me the most unaccountable, as it is in direct opposition to gravity. _mrs. b._ and yet it is in consequence of their pressure downwards. when, for example, you pour water into a tea-pot, the water rises in the spout, to a level with the water in the pot. the particles of water at the bottom of the pot, are pressed upon by the particles above them; to this pressure they will yield, if there is any mode of making way for the superior particles, and as they cannot descend, they will change their direction, and rise in the spout. suppose the tea-pot to be filled with columns of particles of water, similar to that described in fig. ., the particle , at the bottom, will be pressed laterally by the particle , and by this pressure be forced into the spout, where, meeting with the particle , it presses it upwards, and this pressure will be continued from to , from to , and so on, till the water in the spout, has risen to a level with that in the pot. _emily._ if it were not for this pressure upwards, forcing the water to rise in the spout, the equilibrium of the fluid would be destroyed. _caroline._ true; but then a tea-pot is wide and large, and the weight of so great a body of water as the pot will contain, may easily force up and support so small a quantity, as will fill the spout. but would the same effect be produced, if the spout and the pot, were of equal dimensions? _mrs. b._ undoubtedly it would. you may even reverse the experiment, by pouring water into the spout, and you will find that the water will rise in the pot, to a level with that in the spout; for the pressure of the small quantity of water in the spout, will force up and support, the larger quantity in the pot. in the pressure upwards, as well as that laterally, you see that the force of pressure, depends entirely on the height, and is quite independent of the horizontal dimensions of the fluid. as a tea-pot is not transparent, let us try the experiment by filling this large glass goblet, by means of this narrow tube, (fig. .) _caroline._ look, emily, as mrs. b. fills it, how the water rises in the goblet, to maintain an equilibrium with that in the tube. now, mrs. b., will you let me fill the tube, by pouring water into the goblet? _mrs. b._ that is impossible. however, you may try the experiment, and i doubt not that you will be able to account for its failure. _caroline._ it is very singular, that if so small a column of water as is contained in the tube, can force up and support the whole contents of the goblet; that the weight of all the water in the goblet, should not be able to force up the small quantity required to fill the tube:--oh, i see now the reason, the water in the goblet, cannot force that in the tube above its level, and as the end of the tube, is considerably higher than the goblet, it can never be filled by pouring water into the goblet. _mrs. b._ and if you continue to pour water into the goblet when it is full, the water will run over, instead of rising above its level in the tube. i shall now explain to you the meaning of the _specific gravity_ of bodies. _caroline._ what! is there another species of gravity, with which we are not yet acquainted? _mrs. b._ no: the specific gravity of a body, means simply its weight, compared with that of another body, of the same size. when we say, that substances, such as lead, and stones, are heavy, and that others, such as paper and feathers, are light, we speak comparatively; that is to say, that the first are heavy, and the latter light, in comparison with the generality of substances in nature. would you call wood, and chalk, light or heavy bodies? _caroline._ some kinds of wood are heavy, certainly, as oak and mahogany; others are light, as cedar and poplar. _emily._ i think i should call wood in general, a heavy body; for cedar and poplar, are light, only in comparison to wood of a heavier description. i am at a loss to determine whether chalk should be ranked as a heavy, or a light body; i should be inclined to say the former, if it was not that it is lighter than most other minerals. i perceive that we have but vague notions of light and heavy. i wish there was some standard of comparison, to which we could refer the weight of all other bodies. _mrs. b._ the necessity of such a standard, has been so much felt, that a body has been fixed upon for this purpose. what substance do you think would be best calculated to answer this end? _caroline._ it must be one generally known, and easily obtained; lead or iron, for instance. _mrs. b._ the metals, would not answer the purpose well, for several reasons; they are not always equally compact, and they are rarely quite pure; two pieces of iron, for instance, although of the same size, might not, from the causes mentioned, weigh exactly alike. _caroline._ but, mrs. b., if you compare the weight, of equal quantities of different bodies, they will all be alike. you know the old saying, that a pound of feathers, is as heavy as a pound of lead? _mrs. b._ when therefore we compare the weight of different kinds of bodies, it would be absurd to take quantities of equal _weight_, we must take quantities of equal _bulk_; pints or quarts, not ounces or pounds. _caroline._ very true; i perplexed myself by thinking that quantity referred to weight, rather than to measure. it is true, it would be as absurd to compare bodies of the same size, in order to ascertain which was largest, as to compare bodies of the same weight, in order to discover which was heaviest. _mrs. b._ in estimating the specific gravity of bodies, therefore, we must compare equal bulks, and we shall find that their specific gravity, will be proportional to their weights. the body which has been adopted as a standard of reference, is distilled, or rain water. _emily._ i am surprised that a fluid should have been chosen for this purpose, as it must necessarily be contained in some vessel, and the weight of the vessel, will require to be deducted. _mrs. b._ you will find that the comparison will be more easily made with a fluid, than with a solid; and water you know can be every where obtained. in order to learn the specific gravity of a solid body, it is not necessary to put a certain measure of it in one scale, and an equal measure of water into the other scale: but simply to weigh the body under trial, first in air, and then in water. if you weigh a piece of gold, in a glass of water, will not the gold displace just as much water, as is equal to its own bulk? _caroline._ certainly, where one body is, another cannot be at the same time; so that a sufficient quantity of water must be removed, in order to make way for the gold. _mrs. b._ yes, a cubic inch of water, to make room for a cubic inch of gold; remember that the bulk, alone, is to be considered; the weight, has nothing to do with the quantity of water displaced, for an inch of gold, does not occupy more space, and therefore will not displace more water, than an inch of ivory, or any other substance, that will sink in water. well, you will perhaps be surprised to hear that the gold will weigh less in water, than it did out of it? _emily._ and for what reason? _mrs. b._ on account of the upward pressure of the particles of water, which in some measure supports the gold, and by so doing, diminishes its weight. if the body immersed in water, was of the same weight as that fluid, it would be wholly supported by it, just as the water which it displaces, was supported, previous to its making way for the solid body. if the body is heavier than the water, it cannot be wholly supported by it; but the water will offer some resistance to its descent. _caroline._ and the resistance which water offers to the descent of heavy bodies immersed in it, (since it proceeds from the upward pressure of the particles of the fluid,) must in all cases, i suppose, be the same? _mrs. b._ yes: the resistance of the fluid, is proportioned to the bulk, and not to the weight, of the body immersed in it; all bodies of the same size, therefore, lose the same quantity of their weight in water. can you form any idea what this loss will be? _emily._ i should think it would be equal to the weight of the water displaced; for, since that portion of the water was supported before the immersion of the solid body, an equal weight of the solid body, will be supported. _mrs. b._ you are perfectly right; a body weighed in water, loses just as much of its weight, as is equal to that of the water it displaces; so that if you were to put the water displaced, into the scale to which the body is suspended, it would restore the balance. you must observe, that when you weigh a body in water, in order to ascertain its specific gravity, you must not sink the dish of the balance in the water; but either suspend the body to a hook at the bottom of the dish, or else take off the dish, and suspend to the arm of the balance a weight to counterbalance the other dish, and to this attach the solid to be weighed, (fig. .) now suppose that a cubic inch of gold, weighed ounces out of water, and lost one ounce of its weight by being weighed in water, what would be its specific gravity? _caroline._ the cubic inch of water it displaced, must weigh that one ounce; and as a cubic inch of gold, weighs ounces, gold is times, as heavy as water. _emily._ i recollect having seen a table of the comparative weights of bodies, in which gold appeared to me to be estimated at thousand times, the weight of water. _mrs. b._ you misunderstood the meaning of the table. in the estimation you allude to, the weight of water was reckoned at . you must observe, that the weight of a substance when not compared to that of any other, is perfectly arbitrary; and when water is adopted as a standard, we may denominate its weight by any number we please; but then the weight of all bodies tried by this standard, must be signified by proportional numbers. _caroline._ we may call the weight of water, for example, one, and then that of gold, would be nineteen; or if we choose to call the weight of water , that of gold would be , . in short, specific gravity, means how many times more a body weighs, than an equal bulk of water. _mrs. b._ it is rather the weight of a body compared with a portion of water equal to it in bulk; for the specific gravity of many substances, is less than that of water. _caroline._ then you cannot ascertain the specific gravity of such substances, in the same manner as that of gold; for a body that is lighter than water, will float on its surface, without displacing any of it. _mrs. b._ if a body were absolutely without weight, it is true that it would not displace a drop of water, but the bodies we are treating of, have all some weight, however small; and will, therefore, displace some quantity. if the body be lighter than water, it will not sink to a level with its surface, and therefore it will not displace so much water as is equal to its bulk; but only so much, as is equal to its weight. a ship, you must have observed, sinks to some depth in water, and the heavier it is laden, the deeper it sinks, as it always displaces a quantity of water, equal to its own weight. _caroline._ but you said just now, that in the immersion of gold, the bulk, and not the weight of body, was to be considered. _mrs. b._ that is the case with all substances which are heavier than water; but since those which are lighter, do not displace so much as their own bulk, the quantity they displace is not a test of their specific gravity. in order to obtain the specific gravity of a body which is lighter than water, you must attach to it a heavy one, whose specific gravity is known, and immerse them together; the specific gravity of the lighter body, may then be easily calculated from observing the loss of weight it produces, in the heavy body. _emily._ but are there not some bodies which have exactly the same specific gravity as water? _mrs. b._ undoubtedly; and such bodies will remain at rest in whatever situation they are placed in water. here is a piece of wood which i have procured, because it is of a kind which is precisely the weight of an equal bulk of water; in whatever part of this vessel of water you place it, you will find that it will remain stationary. _caroline._ i shall first put it at the bottom; from thence, of course, it cannot rise, because it is not lighter than water. now i shall place it in the middle of the vessel; it neither rises nor sinks, because it is neither lighter nor heavier than the water. now i will lay it on the surface of the water; but there it sinks a little--what is the reason of that, mrs. b.? _mrs. b._ since it is not lighter than the water, it cannot float upon its surface; since it is not heavier than water, it cannot sink below its surface: it will sink therefore, only till the upper surface of both bodies are on a level, so that the piece of wood is just covered with water. if you poured a few drops of water into the vessel, (so gently as not to give them momentum) they would mix with the water at the surface, and not sink lower. _caroline._ i now understand the reason, why, in drawing up a bucket of water out of a well, the bucket feels so much heavier when it rises above the surface of the water in the well; for whilst you raise it in the water, the water within the bucket being of the same specific gravity as the water on the outside, will be wholly supported by the upward pressure of the water beneath the bucket, and consequently very little force will be required to raise it; but as soon as the bucket rises to the surface of the well, you immediately perceive the increase of weight. _emily._ and how do you ascertain the specific gravity of fluids? _mrs. b._ by means of an hydrometer; this instrument is made of various materials, and in different forms, one of which i will show you. it consists of a thin brass ball a, (fig. , plate .) with a graduated tube b, and the specific gravity of the liquid, is estimated by the depth to which the instrument sinks in it, or by the weight required to sink it to a given depth. there is a small bucket c, suspended at the lower end, and also a little dish on the graduated tube; into either of these, small weights may be put, until the instrument sinks in the fluid, to a mark on the tube b; the amount of weight necessary for this, will enable you to discover the specific gravity of the fluid. i must now take leave of you; but there remain yet many observations to be made on fluids: we shall, therefore, resume this subject at our next interview. questions . (pg. ) what are the two divisions of the science which treats of the mechanical properties of liquids? . (pg. ) of what do hydrostatics and hydraulics treat? . (pg. ) what is a fluid defined to be? . (pg. ) from what is fluidity supposed to arise? . (pg. ) into what two classes are fluids divided? . (pg. ) what is said of the incompressibility of liquids, and what experiment is related? . (pg. ) ought this experiment to be considered as conclusive? . (pg. ) why do fluids appear to gravitate more freely than solids? . (pg. ) when is a fluid said to be in equilibrium? . (pg. ) what is there in the nature of a fluid, which causes it to seek this level? . (pg. ) what circumstances occasion oil to float upon water? . (pg. ) what is the nature and use of the instrument represented in fig. , plate ? . (pg. ) what difference is there in the gravitation of solid masses, and of fluids? . (pg. ) what results as regards the pressure of fluids? . (pg. ) how is this illustrated by fig. , , plate ? . (pg. ) from what does the lateral pressure proceed? and to what is it proportioned, as exemplified in fig. , plate ? . (pg. ) has the extent of the surface of a fluid, any effect upon its pressure downwards? . (pg. ) what will be the difference between the pressure upon the bottom, and upon one side of a cubical vessel? . (pg. ) what occasions the upward pressure, and how is it explained by fig. , plate ? . (pg. ) how could the equilibrium of fluids be exemplified by pouring water in at the spout of a tea-pot? . (pg. ) how by the apparatus represented at fig. , plate ? . (pg. ) what is meant by the specific gravity of a body? . (pg. ) what do we in common mean by calling a body heavy, or light? . (pg. ) why would not the metals answer to compare other bodies with? . (pg. ) what must be supposed equal in estimating the specific gravity of a body? . (pg. ) what has been adopted as a standard for comparison? . (pg. ) what is the first step in ascertaining the specific gravity of a solid? . (pg. ) what quantity of water will the solid displace? . (pg. ) why will a solid weigh less in water than in air, and to what will the loss of weight be equal? . (pg. ) what is the arrangement represented by fig. , plate ? . (pg. ) what is stated of gold as an example? . (pg. ) in comparing a body with water, this is sometimes called , what must be observed? . (pg. ) what quantity of water is displaced, by a body floating upon its surface? . (pg. ) how can you find the specific gravity of a solid which is lighter than water? . (pg. ) what is observed of a body whose specific gravity is the same as that of water? . (pg. ) what is the reason that in drawing a bucket of water from a well, its weight is not perceived until it rises above the surface? . (pg. ) describe the instrument represented by fig. , plate , and also how, and for what it is used? conversation xi. of springs, fountains, &c. of the ascent of vapour and the formation of clouds. of the formation and fall of rain, &c. of the formation of springs. of rivers and lakes. of fountains. caroline. there is a question i am very desirous of asking you, respecting fluids, mrs. b., which has often perplexed me. what is the reason that the great quantity of rain which falls upon the earth and sinks into it, does not, in the course of time, injure its solidity? the sun and the wind, i know, dry the surface, but they have no effect on the interior parts, where there must be a prodigious accumulation of moisture. _mrs. b._ do you not know, that, in the course of time, all the water which sinks into the ground, rises out of it again? it is the same water which successively forms seas, rivers, springs, clouds, rain, and sometimes hail, snow and ice. if you will take the trouble of following it through these various changes, you will understand why the earth is not yet drowned, by the quantity of water which has fallen upon it, since its creation; and you will even be convinced, that it does not contain a single drop more water now, than it did at that period. let us consider how the clouds were originally formed. when the first rays of the sun warmed the surface of the earth, the heat, by separating the particles of water, rendered them lighter than the air. this, you know, is the case with steam or vapour. what then ensues? _caroline._ when lighter than the air, it will naturally rise; and now i recollect your telling us in a preceding lesson, that the heat of the sun transformed the particles of water into vapour; in consequence of which, it ascended into the atmosphere, where it formed clouds. _mrs. b._ we have then already followed water through two of its transformations; from water it becomes vapour, and from vapour clouds. _emily._ but since this watery vapour is lighter than the air, why does it not continue to rise; and why does it unite again, to form clouds? _mrs. b._ because the atmosphere diminishes in density, as it is more distant from the earth. the vapour, therefore, which the sun causes to exhale, not only from seas, rivers, and lakes, but likewise from the moisture on the land, rises till it reaches a region of air of its own specific gravity; and there, you know, it will remain stationary. by the frequent accession of fresh vapour, it gradually accumulates, so as to form those large bodies of vapour, which we call clouds: and the particles, at length uniting, become too heavy for the air to support, and fall to the ground. _caroline._ they do fall to the ground, certainly, when it rains; but, according to your theory, i should have imagined, that when the clouds became too heavy, for the region of air in which they were situated, to support them, they would descend, till they reached a stratum of air of their own weight, and not fall to the earth; for as clouds are formed of vapour, they cannot be so heavy as the lowest regions of the atmosphere, otherwise the vapour would not have risen. _mrs. b._ if you examine the manner in which the clouds descend, it will obviate this objection. in falling, several of the watery particles come within the sphere of each other's attraction, and unite in the form of a drop of water. the vapour thus transformed into a shower, is heavier than any part of the atmosphere, and consequently descends to the earth. _caroline._ how wonderfully curious! _mrs. b._ it is impossible to consider any part of nature attentively, without being struck with admiration at the wisdom it displays; and i hope you will never contemplate these wonders, without feeling your heart glow with admiration and gratitude, towards their bounteous author. observe, that if the waters were never drawn out of the earth, all vegetation would be destroyed by the excess of moisture; if, on the other hand, the plants were not nourished and refreshed by occasional showers, the drought would be equally fatal to them. if the clouds constantly remained in a state of vapour, they might, as you remarked, descend into a heavier stratum of the atmosphere, but could never fall to the ground; or were the power of attraction more than sufficient to convert the vapour into drops, it would transform the cloud into a mass of water, which, instead of nourishing, would destroy the produce of the earth. water then ascends in the form of vapour, and descends in that of rain, snow, or hail, all of which ultimately become water. some of this falls into the various bodies of water on the surface of the globe, the remainder upon the land. of the latter, part reascends in the form of vapour, part is absorbed by the roots of vegetables, and part descends into the earth, where it forms springs. _emily._ is there then no difference between rain water, and spring water? _mrs. b._ they are originally the same; but that portion of rain water which goes to supply springs, dissolves a number of foreign particles, which it meets with in its passage through the various soils it traverses. _caroline._ yet spring water is more pleasant to the taste, appears more transparent, and, i should have supposed, would have been more pure than rain water. _mrs. b._ no; excepting distilled water, rain water is the most pure we can obtain; it is its purity which renders it insipid; whilst the various salts and different ingredients, dissolved in spring water, give it a species of flavour, which habit renders agreeable; these salts do not, in any degree, affect its transparency; and the filtration it undergoes, through gravel and sand, cleanses it from all foreign matter, which it has not the power of dissolving. _emily._ how is it that the rain water does not continue to descend by its gravity, instead of collecting together, and forming springs? _mrs. b._ when rain falls on the surface of the earth, it continues making its way downwards through the pores and crevices in the ground. when several drops meet in their subterraneous passage, they unite and form a little rivulet; this, in its progress, meets with other rivulets of a similar description, and they pursue their course together within the earth, till they are stopped by some substance, such as rock, or clay, which they cannot penetrate. _caroline._ but you say that there is some reason to believe that water can penetrate even the pores of gold, and it cannot meet with a substance more dense? _mrs. b._ but if water penetrate the pores of gold, it is only when under a strong compressive force, as in the florentine experiment; now in its passage towards the centre of the earth, it is acted upon by no other power than gravity, which is not sufficient to make it force its way, even through a stratum of clay. this species of earth, though not remarkably dense, being of great tenacity, will not admit the particles of water to pass. when water encounters any substance of this nature, therefore, its progress is stopped, and it is diffused through the porous earth, and sometimes the pressure of the accumulating waters, forms a bed, or reservoir. this will be more clearly explained by fig. , plate , which represents a section, of the interior of a hill or mountain. a, is a body of water, such as i have described, which, when filled up as high as b, (by the continual accession of water it receives from the ducts or rivulets _a_, _a_, _a_, _a_,) finds a passage out of the cavity, and, impelled by gravity, it runs on, till it makes its way out of the ground at the side of the hill, and there forms a spring, c. _caroline._ gravity impels downwards towards the centre of the earth; and the spring in this figure runs in an horizontal direction. _mrs. b._ not entirely. there is some declivity from the reservoir, to the spot where the water issues out of the ground; and gravity, you know, will bring bodies down an inclined plane, as well as in a perpendicular direction. _caroline._ but though the spring may descend, on first issuing, it must afterwards rise to reach the surface of the earth; and that is in direct opposition to gravity. _mrs. b._ a spring can never rise above the level of the reservoir whence it issues; it must, therefore, find a passage to some part of the surface of the earth, that is lower, or nearer the centre, than the reservoir. it is true that, in this figure, the spring rises in its passage from b to c; but this, i think, with a little reflection, you will be able to account for. _emily._ oh, yes; it is owing to the pressure of fluids upwards; and the water rises in the duct, upon the same principle as it rises in the spout of a tea-pot; that is to say, in order to preserve an equilibrium with the water in the reservoir. now i think i understand the nature of springs: the water will flow through a duct, whether ascending or descending, provided it never rises higher than the reservoir. _mrs. b._ water may thus be conveyed to every part of a town, and to the upper part of the houses, if it is originally brought from a height, superior to any to which it is conveyed. have you never observed, when the pavements of the streets have been mending, the pipes which serve as ducts for the conveyance of the water through the town? _emily._ yes, frequently; and i have remarked that when any of these pipes have been opened, the water rushes upwards from them, with great velocity; which, i suppose, proceeds from the pressure of the water in the reservoir, which forces it out. _caroline._ i recollect having once seen a very curious glass, called tantalus's cup; it consists of a goblet, containing a small figure of a man, and whatever quantity of water you pour into the goblet, it never rises higher than the breast of the figure. do you know how that is contrived? _mrs. b._ it is by means of a syphon, or bent tube, which is concealed in the body of the figure. this tube rises through one of the legs, as high as the breast, and there turning, descends through the other leg, and from thence through the foot of the goblet, where the water runs out. (fig. , plate .) when you pour water into the glass a, it must rise in the syphon b, in proportion as it rises in the glass; and when the glass is filled to a level with the upper part of the syphon, the water will run out through the other leg of the figure, and will continue running out, as fast as you pour it in; therefore the glass can never fill any higher. _emily._ i think the new well that has been made at our country-house, must be of that nature. we had a great scarcity of water, and my father has been at considerable expense to dig a well; after penetrating to a great depth, before water could be found, a spring was at length discovered, but the water rose only a few feet above the bottom of the well; and sometimes it is quite dry. [illustration: plate xiv.] _mrs. b._ this has, however, no analogy to tantalus's cup; but is owing to the very elevated situation of your country-house. _emily._ i believe i guess the reason. there cannot be a reservoir of water near the summit of a hill; as in such a situation, there will not be a sufficient number of rivulets formed, to supply one; and without a reservoir, there can be no spring. in such situations, therefore, it is necessary to dig very deep, in order to meet with a spring; and when we give it vent, it can rise only as high as the reservoir from whence it flows, which will be but little, as the reservoir must be situated at some considerable depth below the summit of the hill. _caroline._ your explanation appears very clear and satisfactory; but i can contradict it from experience. at the very top of a hill, near our country-house, there is a large pond, and, according to your theory, it would be impossible there should be springs in such a situation to supply it with water. then you know that i have crossed the alps, and i can assure you, that there is a fine lake on the summit of mount cenis, the highest mountain we passed over. _mrs. b._ were there a lake on the summit of mount blanc, which is the highest of the alps, it would indeed be wonderful. but that on mount cenis, is not at all contradictory to our theory of springs; for this mountain is surrounded by others, much more elevated, and the springs which feed the lake must descend from reservoirs of water, formed in those mountains. this must also be the case with the pond on the top of the hill; there is doubtless some more considerable hill in the neighbourhood, which supplies it with water. _emily._ i comprehend perfectly, why the water in our well never rises high: but i do not understand why it should occasionally be dry. _mrs. b._ because the reservoir from which it flows, being in an elevated situation, is but scantily supplied with water; after a long drought, therefore, it may be drained, and the spring dry, till the reservoir be replenished by fresh rains. it is not uncommon to see springs flow with great violence in wet seasons, which at other times, are perfectly dry. _caroline._ but there is a spring in our grounds, which more frequently flows in dry, than in wet weather; how is that to be accounted for? _mrs. b._ the spring, probably, comes from a reservoir at a great distance, and situated very deep in the ground: it is, therefore, some length of time before the rain reaches the reservoir; and another considerable portion must elapse, whilst the water is making its way, from the reservoir, to the surface of the earth; so that the dry weather may probably have succeeded the rains, before the spring begins to flow; and the reservoir may be exhausted, by the time the wet weather sets in again. _caroline._ i doubt not but this is the case, as the spring is in a very low situation, therefore, the reservoir may be at a great distance from it. _mrs. b._ springs which do not constantly flow, are called intermitting, and are occasioned by the reservoir being imperfectly supplied. independently of the situation, this is always the case, when the duct, or ducts, which convey the water into the reservoir, are smaller than those which carry it off. _caroline._ if it runs out, faster than it runs in, it will of course sometimes be empty. do not rivers also, derive their source from springs? _mrs. b._ yes, they generally take their source in mountainous countries, where springs are most abundant. _caroline._ i understood you that springs were more rare, in elevated situations. _mrs. b._ you do not consider that mountainous countries, abound equally with high, and low situations. reservoirs of water, which are formed in the bosoms of mountains, generally find a vent, either on their declivity, or in the valley beneath; while subterraneous reservoirs, formed in a plain, can seldom find a passage to the surface of the earth, but remain concealed, unless discovered by digging a well. when a spring once issues at the surface of the earth, it continues its course externally, seeking always a lower ground, for it can no longer rise. _emily._ then what is the consequence, if the spring, or, as i should now rather call it, the rivulet, runs into a situation, which is surrounded by higher ground? _mrs. b._ its course is stopped; the water accumulates, and it forms a pool, pond, or lake, according to the dimensions of the body of water. the lake of geneva, in all probability, owes its origin to the rhone, which passes through it: if, when the river first entered the valley, which now forms the bed of the lake, it found itself surrounded by higher grounds, its waters would there accumulate, till they rose to a level with that part of the valley, where the rhone now continues its course beyond the lake, and from whence it flows through valleys, occasionally forming other small lakes, till it reaches the sea. _emily._ and are not fountains, of the nature of springs? _mrs. b._ exactly. a fountain is conducted perpendicularly upwards, by the spout or adjutage a, through which it flows; and it will rise nearly as high as the reservoir b, from whence it proceeds. (plate . fig. .) _caroline._ why not quite as high? _mrs. b._ because it meets with resistance from the air, in its ascent; and its motion is impeded by friction against the spout, where it rushes out. _emily._ but if the tube through which the water rises be smooth, can there be any friction? especially with a fluid, whose particles yield to the slightest impression. _mrs. b._ friction, (as we observed in a former lesson,) may be diminished by polishing, but can never be entirely destroyed; and though fluids, are less susceptible of friction, than solid bodies, they are still affected by it. another reason why a fountain will not rise so high as its reservoir, is, that as all the water which spouts up, has to descend again, it in doing so, presses, or strikes against the under parts, and forces them sideways, spreading the column into a head, and rendering it both wider, and shorter, than it otherwise would be. at our next meeting, we shall examine the mechanical properties of the air, which being an elastic fluid, differs in many respects, from liquids. questions . (pg. ) why do not the frequent rains, fill the earth with water? . (pg. ) why will vapour rise? to what height will it ascend, and what will it form? . (pg. ) how may drops of rain be formed? . (pg. ) what becomes of the water after it has fallen to the earth? . (pg. ) what is the difference between rain water, and that from springs? . (pg. ) why is rain more pure than spring water? . (pg. ) why is spring water more agreeable to the palate? . (pg. ) what causes the water to collect and form springs? . (pg. ) why cannot water penetrate through clay? . (pg. ) what is represented by fig. , plate ? . (pg. ) how can you account for its rising upwards, as represented at c? . (pg. ) in conveying water by means of pipes, how must the reservoir be situated? . (pg. ) what is the instrument called, which is represented in plate , fig. ,--and how does it operate? . (pg. ) why are wells rarely well supplied with water, in elevated situations? . (pg. ) when water is found in elevated situations, whence is it supplied? . (pg. ) wells and springs, at some periods well supplied, fail at others; how is this accounted for? . (pg. ) some springs flow abundantly in dry weather, which occasionally fail in wet weather, how may this be explained? . (pg. ) what is meant by intermitting springs? . (pg. ) whence do rivers, in general, derive their water? . (pg. ) why do springs abound more in mountainous, than in level countries? . (pg. ) how are lakes formed? . (pg. ) what causes water to rise in fountains, and how is this explained by figure , plate ? . (pg. ) why will not the fountain rise to the height of the water in the reservoir? conversation xii. on the mechanical properties of air. of the spring or elasticity of the air. of the weight of the air. experiments with the air pump. of the barometer. mode of weighing air. specific gravity of air. of pumps. description of the sucking pump. description of the forcing pump. mrs. b. at our last meeting we examined the properties of fluids in general, and more particularly of such as are called non-elastic fluids, or liquids. there is another class of fluids, distinguished by the name of æriform, or elastic fluids, the principal of which is the air we breathe, which surrounds the earth, and is called the atmosphere. _emily._ there are then other kinds of air, besides the atmosphere? _mrs. b._ yes; a great variety; but they differ only in their chemical, and not in their mechanical properties; and as it is the latter we are to examine, we shall not at present inquire into their composition, but confine our attention to the mechanical properties of elastic fluids in general. _caroline._ and from whence arises this difference, between elastic, and non-elastic fluids? _mrs. b._ there is no attraction of cohesion, between the particles of elastic fluids; so that the expansive power of heat, has no adversary to contend with, but gravity; any increase of temperature, therefore, expands elastic fluids considerably, and a diminution, proportionally condenses them. the most essential point, in which air, differs from other fluids is in its spring or elasticity; that is to say, its power of increasing, or diminishing in bulk, accordingly as it is more, or less, compressed: a power of which i have informed you, liquids are almost wholly deprived. _emily._ i think i understand the elasticity of the air very well from what you formerly said of it; but what perplexes me is, its having gravity; if it is heavy, and we are surrounded by it, why do we not feel its weight? _caroline._ it must be impossible to be sensible of the weight of such infinitely small particles, as those of which the air is composed: particles which are too small to be seen, must be too light to be felt. _mrs. b._ you are mistaken, my dear; the air is much heavier than you imagine; it is true, that the particles which compose it, are small; but then, reflect on their quantity: the atmosphere extends in height, a great number of miles from the earth, and its gravity is such, that a man of middling stature, is computed (when the air is heaviest) to sustain the weight of about tons. _caroline._ is it possible! i should have thought such a weight would have crushed any one to atoms. _mrs. b._ that would, indeed, be the case, if it were not for the equality of the pressure, on every part of the body; but when thus diffused, we can bear even a much greater weight, without any considerable inconvenience. in bathing we support the weight and pressure of the water, in addition to that of the atmosphere; but because this pressure is equally distributed over the body, we are scarcely sensible of it; whilst if your shoulders, your head, or any particular part of your frame, were loaded with the additional weight of a hundred pounds, you would soon sink under the fatigue. besides this, our bodies contain air, the spring of which, counterbalances the weight of the external air, and renders us insensible of its pressure. _caroline._ but if it were possible to relieve me from the weight of the atmosphere, should i not feel more light and agile? _mrs. b._ on the contrary, the air within you, meeting with no external pressure to restrain its elasticity, would distend your body, and at length bursting some of the parts which confined it, put a period to your existence. _caroline._ this weight of the atmosphere, then, which i was so apprehensive would crush me, is, in reality, essential to my preservation. _emily._ i once saw a person cupped, and was told that the swelling of the part under the cup, was produced by taking away from that part, the pressure of the atmosphere; but i could not understand how this pressure produced such an effect. _mrs. b._ the air pump affords us the means of making a great variety of interesting experiments, on the weight, and pressure of the air: some of them you have already seen. do you not recollect, that in a vacuum produced within the air pump, substances of various weights, fell to the bottom in the same time; why does not this happen in the atmosphere? _caroline._ i remember you told us it was owing to the resistance which light bodies meet with, from the air, during their fall. _mrs. b._ or, in other words, to the support which they received from the air, and which prolonged the time of their fall. now, if the air were destitute of weight, how could it support other bodies, or retard their fall? i shall now show you some other experiments, which illustrate, in a striking manner, both the weight, and elasticity of air. i shall tie a piece of bladder over this glass receiver, which, you will observe, is open at the top as well as below. _caroline._ why do you wet the bladder first? _mrs. b._ it expands by wetting, and contracts in drying; it is also more soft and pliable when wet, so that i can make it fit better, and when dry, it will be tighter. we must hold it to the fire in order to dry it; but not too near, lest it should burst by sudden contraction. let us now fix it on the air pump, and exhaust the air from underneath it--you will not be alarmed if you hear a noise? _emily._ it was as loud as the report of a gun, and the bladder is burst! pray explain how the air is concerned in this experiment. _mrs. b._ it is the effect of the weight of the atmosphere, on the upper surface of the bladder, when i had taken away the air from the under surface, so that there was no longer any reaction to counterbalance the pressure of the atmosphere, on the receiver. you observed how the bladder was pressed inwards, by the weight of the external air, in proportion as i exhausted the receiver: and before a complete vacuum was formed, the bladder, unable to sustain the violence of the pressure, burst with the explosion you have just heard. i shall now show you an experiment, which proves the expansion of the air, contained within a body, when it is relieved from the pressure of the external air. you would not imagine that there was any air contained within this shrivelled apple, by its appearance; but take notice of it when placed within a receiver, from which i shall exhaust the air. _caroline._ how strange! it grows quite plump, and looks like a fresh-gathered apple. _mrs. b._ but as soon as i let the air again into the receiver, the apple, you see, returns to its shrivelled state. when i took away the pressure of the atmosphere, the air within the apple, expanded, and swelled it out; but the instant the atmospheric air was restored, the expansion of the internal air, was checked and repressed, and the apple shrunk to its former dimensions. you may make a similar experiment with this little bladder, which you see is perfectly flaccid, and appears to contain no air: in this state i shall tie up the neck of the bladder, so that whatever air remains within it, may not escape, and then place it under the receiver. now observe, as i exhaust the receiver, how the bladder distends; this proceeds from the great dilatation of the small quantity of air, which was enclosed within the bladder, when i tied it up; but as soon as i let the air into the receiver, that which the bladder contains, condenses and shrinks into its small compass, within the folds of the bladder. _emily._ these experiments are extremely amusing, and they afford clear proofs, both of the weight, and elasticity of the air; but i should like to know, exactly, how much the air weighs. _mrs. b._ a column of air reaching to the top of the atmosphere, and whose base is a square inch, weighs about lbs. therefore, every square inch of our bodies, sustains a weight of lbs.: and if you wish to know the weight of the whole of the atmosphere, you must reckon how many square inches there are on the surface of the globe, and multiply them by . _emily._ but can we not ascertain the weight of a small quantity of air? _mrs. b._ with perfect ease. i shall exhaust the air from this little bottle, by means of the air pump: and having emptied the bottle of air, or, in other words, produced a vacuum within it, i secure it by turning this screw adapted to its neck: we may now find the exact weight of this bottle, by putting it into one of the scales of a balance. it weighs, you see, just two ounces; but when i turn the screw, so as to admit the air into the bottle, the scale which contains it, preponderates. _caroline._ no doubt the bottle filled with air, is heavier than the bottle void of air; and the additional weight required to bring the scales again to a balance, must be exactly that of the air which the bottle now contains. _mrs. b._ that weight, you see, is almost two grains. the dimensions of this bottle, are six cubic inches. six cubic inches of air, therefore, at the temperature of this room, weighs nearly grains. _caroline._ why do you observe the temperature of the room, in estimating the weight of the air? _mrs. b._ because heat rarefies air, and renders it lighter; therefore the warmer the air is, which you weigh, the lighter it will be. if you should now be desirous of knowing the specific gravity of this air, we need only fill the same bottle, with water, and thus obtain the weight of an equal quantity of water--which you see is grs.; now by comparing the weight of water, to that of air, we find it to be in the proportion of about to . as you are acquainted with decimal arithmetic, you will understand what i mean, when i tell you, that water being called , the specific gravity of air, will be . . i will show you another instance, of the weight of the atmosphere, which i think will please you: you know what a barometer is? _caroline._ it is an instrument which indicates the state of the weather, by means of a tube of quicksilver; but how, i cannot exactly say. _mrs. b._ it is by showing the weight of the atmosphere, which has great influence on the weather. the barometer, is an instrument extremely simple in its construction. in order that you may understand it, i will show you how it is made. i first fill with mercury, a glass tube a b, (fig. , plate .) about three feet in length, and open only at one end; then stopping the open end, with my finger, i immerse it in a cup c, containing a little mercury. _emily._ part of the mercury which was in the tube, i observe, runs down into the cup; but why does not the whole of it subside, for it is contrary to the law of the equilibrium of fluids, that the mercury in the tube, should not descend to a level with that in the cup? _mrs. b._ the mercury that has fallen from the tube, into the cup, has left a vacant space in the upper part of the tube, to which the air cannot gain access; this space is therefore a perfect vacuum; the mercury in the tube, is relieved from the pressure of the atmosphere, whilst that in the cup, remains exposed to it. _caroline._ oh, now i understand it; the pressure of the air on the mercury in the cup, forces it to rise in the tube, where there is not any air to counteract the external pressure. _emily._ or rather supports the mercury in the tube, and prevents it from falling. _mrs. b._ that comes to the same thing; for the power that can support mercury in a vacuum, would also make it ascend, when it met with a vacuum. thus you see, that the equilibrium of the mercury is destroyed, only to preserve the general equilibrium of fluids. _caroline._ but this simple apparatus is, in appearance, very unlike a barometer. _mrs. b._ it is all that is essential to a barometer. the tube and the cup, or a cistern of mercury, are fixed on a board, for the convenience of suspending it; the brass plate on the upper part of the board, is graduated into inches, and tenths of inches, for the purpose of ascertaining the height at which the mercury stands in the tube; and the small moveable metal plate, serves to show that height, with greater accuracy. _emily._ and at what height, will the weight of the atmosphere sustain the mercury? _mrs. b._ about or inches, as you will see by this barometer; but it depends upon the weight of the atmosphere, which varies much, in different states of the weather. the greater the pressure of the air on the mercury in the cup, the higher it will ascend in the tube. now can you tell me whether the air is heavier, in wet, or in dry weather? _caroline._ without a moment's reflection, the air must be heaviest in wet weather. it is so depressing, and makes one feel so heavy, while in fine weather, i feel as light as a feather, and as brisk as a bee. _mrs. b._ would it not have been better to have answered with a moment's reflection, caroline? it would have convinced you, that the air must be heaviest in dry weather; for it is then, that the mercury is found to rise in the tube, and consequently, the mercury in the cup, must be most pressed by the air. _caroline._ why then does the air feel so heavy, in bad weather? _mrs. b._ because it is less salubrious, when impregnated with damp. the lungs, under these circumstances, do not play so freely, nor does the blood circulate so well; thus obstructions are frequently occasioned in the smaller vessels, from which arise colds, asthmas, agues, fevers, &c. _emily._ since the atmosphere diminishes in density, in the upper regions, is not the air more rare, upon a hill, than in a plain; and does the barometer indicate this difference? _mrs. b._ certainly. this instrument, is so exact in its indications, that it is used for the purpose of measuring the height of mountains, and of estimating the elevation of balloons; the mercury descending in the tube, as you ascend to a greater height. _emily._ and is no inconvenience experienced, from the thinness of the air, in such elevated situations? _mrs. b._ oh, yes; frequently. it is sometimes oppressive, from being insufficient for respiration; and the expansion which takes place, in the more dense air contained within the body, is often painful: it occasions distention, and sometimes causes the bursting of the smaller blood-vessels, in the nose, and ears. besides in such situations, you are more exposed, both to heat, and cold; for though the atmosphere is itself transparent, its lower regions, abound with vapours, and exhalations, from the earth, which float in it, and act in some degree as a covering, which preserves us equally from the intensity of the sun's rays, and from the severity of the cold. _caroline._ pray, mrs. b., is not the thermometer constructed on the same principles as the barometer? _mrs. b._ not at all. the rise and fall of the fluid in the thermometer, is occasioned by the expansive power of heat, and the condensation produced by cold: the air has no access to it. an explanation of it would, therefore, be irrelevant to our present subject. _emily._ i have been reflecting, that since it is the weight of the atmosphere, which supports the mercury, in the tube of a barometer, it would support a column of any other fluid, in the same manner. _mrs. b._ certainly; but as mercury, is heavier than all other fluids, it will support a higher column, of any other fluid; for two fluids are in equilibrium, when their height varies, inversely as their densities. we find the weight of the atmosphere, is equal to sustaining a column of water, for instance, of no less than feet above its level. _caroline._ the weight of the atmosphere, is then, as great as that of a body of water of feet in height. _mrs. b._ precisely; for a column of air, of the height of the atmosphere, is equal to a column of water of about feet, or one of mercury, of from to inches. the common pump, is dependent on this principle. by the act of pumping, the pressure of the atmosphere is taken off the water, which, in consequence, rises. the body of a pump, consists of a large tube or pipe, whose lower end is immersed in the water which it is designed to raise. a kind of stopper, called a piston, is fitted to this tube, and is made to slide up and down it, by means of a metallic rod, fastened to the centre of the piston. _emily._ is it not similar to the syringe, or squirt, with which you first draw in, and then force out water? _mrs. b._ it is; but you know that we do not wish to force the water out of the pump, at the same end of the pipe, at which we draw it in. the intention of a pump, is to raise water from a spring, or well; the pipe is, therefore, placed perpendicularly over the water, which enters it at the lower extremity, and it issues at a horizontal spout, towards the upper part of the pump; to effect this, there are, besides the piston, two contrivances called valves. the pump, therefore, is rather a more complicated piece of machinery, than the syringe. _caroline._ pray, mrs. b., is not the leather, which covers the opening, in the lower board of a pair of bellows, a kind of valve? _mrs. b._ it is, valves are made in various forms; any contrivance, which allows a fluid to pass in one direction, and prevents its return, is called a valve; that of the bellows, and of the common pump, resemble each other, exactly. you can now, i think, understand the structure of the pump. its various parts, are delineated in this figure: (fig. . plate .) a b is the pipe, or body of the pump, p the piston, v a valve, or little door in the piston, which, opening upwards, admits the water to rise through it, but prevents its returning, and y, is a similar valve, placed lower down in the body of the pump; h is the handle, which in this model, serves to work the piston. when the pump is in a state of inaction, the two valves are closed by their own weight; but when, by working the handle of the pump, the piston ascends; it raises a column of air which rested upon it, and produces a vacuum, between the piston, and the lower valve y; the air beneath this valve, which is immediately over the surface of the water, consequently expands, and forces its way through it; the water, then, relieved from the pressure of the air, ascends into the pump. a few strokes of the handle, totally excludes the air from the body of the pump, and fills it with water, which, having passed through both the valves, runs out at the spout. _caroline._ i understand this perfectly. when the piston is elevated, the air, and the water, successively rise in the pump, for the same reason as the mercury, rises in the barometer. _emily._ i thought that water was drawn up into a pump, by suction, in the same manner as water may be sucked through a straw. _mrs. b._ it is so, into the body of the pump; for the power of suction, is no other than that of producing a vacuum over one part of the liquid, into which vacuum the liquid is forced, by the pressure of the atmosphere, on another part. the action of sucking through a straw, consists in drawing in, and confining the breath, so as to produce a vacuum in the mouth; in consequence of which, the air within the straw, rushes into the mouth, and is followed by the liquid, into which, the lower end of the straw, is immersed. the principle, you see, is the same, and the only difference consists in the mode of producing a vacuum. in suction, the muscular powers answer the purpose of the piston and valve. _emily._ water cannot, then, be raised by a pump, above feet; for the pressure of the atmosphere will not sustain a column of water, above that height. _mrs. b._ i beg your pardon. it is true that there must never be so great a distance as feet, from the level of the water in the well, to the valve in the piston, otherwise the water would not rise through that valve; but when once the water has passed that opening, it is no longer the pressure of air on the reservoir, which makes it ascend; it is raised by lifting it up, as you would raise it in a bucket, of which the piston formed the bottom. this common pump is, therefore, called the sucking, or lifting pump, as it is constructed on both these principles. the rod to which the piston is attached, must be made sufficiently long, to allow the piston to be within feet of the surface of the water in the well, however deep it may be. there is another sort of pump, called the forcing pump: it consists of a forcing power, added to the sucking part of the pump. this additional power, is exactly on the principle of the syringe: by raising the piston, you draw the water into the pump, and by causing it to descend, you force the water out. _caroline._ but the water must be forced out at the upper part of the pump; and i cannot conceive how that can be done by the descent of the piston. _mrs. b._ figure , plate , will explain the difficulty. the large pipe, a b, represents the sucking part of the pump, which differs from the lifting pump, only in its piston p, being unfurnished with a valve, in consequence of which the water cannot rise above it. when, therefore, the piston descends, it shuts the valve y, and forces the water (which has no other vent) into the pipe d: this is likewise furnished with a valve v, which, opening upwards, admits the water to pass, but prevents its return. the water, is thus first raised in the pump, and then forced into the pipe, by the alternate ascending, and descending motion of the piston, after a few strokes of the handle to fill the pipe, from whence the water issues at the spout. _emily._ does not the air pump, which you used in the experiments, on pneumatics, operate upon the same principles as the sucking pump? _mrs. b._ exactly. the air pump which i used (plate , fig. ,) has two hollow, brass cylinders, called barrels, which are made perfectly true. in each of those barrels, there is a piston; these are worked up, and down, by the same handle; the pistons, are furnished with valves, opening upwards, like those of the common pump: there are valves also, placed at the lower part of each barrel, which open upwards; there are therefore two pumps, united to produce the same effect: two tubes, connect these barrels with the plate, upon which i placed the receivers, which were to be exhausted. _emily._ i now understand how the air pump acts; the receiver contains air, which is exhausted, just as it is by the common pump, before the water begins to rise. _mrs. b._ having explained the mechanical properties of air, i think it is now time to conclude our lesson. when next we meet, i shall give you some account of wind, and of sound, which will terminate our observations on elastic fluids. _caroline._ and i shall run into the garden, to have the pleasure of pumping, now that i understand the construction of a pump. _mrs. b._ and, to-morrow, i hope you will be able to tell me, whether it is a forcing, or a common lifting pump. questions . (pg. ) into what two kinds are fluids divided? . (pg. ) there are different kinds of elastic fluids, in what properties are they alike, and in what do they differ? . (pg. ) in what particular do elastic, differ from non-elastic, fluids? . (pg. ) what is meant by the elasticity of air? . (pg. ) what is said respecting the weight of the atmosphere? . (pg. ) why do we not feel the pressure of the air? . (pg. ) what would be the effect of relieving us from atmospheric pressure? . (pg. ) how may the weight of the air be shown by the aid of the air pump, and a piece of bladder? . (pg. ) how is this explained? . (pg. ) how may its elasticity be exhibited, by an apple, and by a bladder? . (pg. ) what is the absolute weight of a given column of atmospheric air, and how could its whole pressure upon the earth be ascertained? . (pg. ) how can the weight of a small bulk of air be found? . (pg. ) in ascertaining the weight of air, we take account of its temperature--why? . (pg. ) how could you ascertain the specific gravity of air, and what would it be? . (pg. ) what are the essential parts of a barometer, as represented plate , fig. ? . (pg. ) what sustains the mercury in the tube? . (pg. ) of what use are the divisions in the upper part of the instrument? . (pg. ) to what height will the mercury rise, and what occasions this height to vary? . (pg. ) when is the mercury highest, in wet, or in dry weather? . (pg. ) what occasions the sensation of oppression, in damp weather? . (pg. ) why will the barometer indicate the height of mountains, or of balloons? . (pg. ) is any inconvenience experienced by persons ascending to great heights, and from what cause? . (pg. ) what occasions the rise and fall of the mercury, in a thermometer? . (pg. ) to what height will the pressure of the atmosphere raise a column of water? . (pg. ) what governs the difference between the height of the mercury, and of the water? . (pg. ) how does the common pump, raise water from a well? . (pg. ) what is meant by a piston? . (pg. ) describe the construction, and use, of a valve. . (pg. ) what are the parts of the pump, as represented, fig. , plate .? . (pg. ) how do these parts act, in raising the water? . (pg. ) in what does that which is commonly called suction, consist? . (pg. ) how must the piston be situated in the pump? . (pg. ) what other kind of pump is described? . (pg. ) how is the forcing pump constructed, as shown in plate , fig. ? . (pg. ) describe the construction and operation of the air pump, (fig. , plate .) conversation xiii. on wind and sound. of wind in general. of the trade-wind. of the periodical trade-winds. of the aerial tides. of sounds in general. of sonorous bodies. of musical sounds. of concord or harmony, and melody. mrs. b. well, caroline, have you ascertained what kind of pump you have in your garden? _caroline._ i think it must be merely a lifting pump, because no more force is required to raise the handle than is necessary to lift its weight; and as in a forcing pump, by raising the handle, you force the water into the smaller pipe, the resistance the water offers, must require an exertion of strength, to overcome it. _mrs. b._ i make no doubt you are right; for lifting pumps, being simple in their construction, are by far the most common. i have promised to-day to give you some account of the nature of wind. wind is nothing more than the motion of a stream, or current of air, generally produced by a partial change of temperature in the atmosphere; for when any one part is more heated than the rest, that part is rarefied, the air in consequence rises, and the equilibrium is destroyed. when this happens, there necessarily follows a motion of the surrounding air towards that part, in order to restore it; this spot, therefore, receives winds from every quarter. those who live to the north of it, experience a north wind; those to the south, a south wind:--do you comprehend this? _caroline._ perfectly. but what sort of weather must those people have, who live on the spot, where these winds meet and interfere? _mrs. b._ they have most commonly turbulent and boisterous weather, whirlwinds, hurricanes, rain, lightning, thunder, &c. this stormy weather occurs most frequently in the torrid zone, where the heat is greatest: the air being more rarefied there, than in any other part of the globe, is lighter, and consequently, ascends; whilst the air from the north and south, is continually flowing in, to restore the equilibrium. _caroline._ this motion of the air, would produce a regular and constant north wind, to the inhabitants of the northern hemisphere; and a south wind, to those of the southern hemisphere, and continual storms at the equator, where these two adverse winds would meet. _mrs. b._ these winds do not meet, for they each change their direction before they reach the equator. the sun, in moving over the equatorial regions from east to west, rarefies the air as it passes, and causes the denser eastern air to flow westwards, in order to restore the equilibrium, thus producing a regular east wind, about the equator. _caroline._ the air from the west, then, constantly goes to meet the sun, and repair the disturbance which his beams have produced in the equilibrium of the atmosphere. but i wonder how you will reconcile these various winds, mrs. b.; you first led me to suppose there was a constant struggle between opposite winds at the equator, producing storm and tempest; but now i hear of one regular invariable wind, which must naturally be attended by calm weather. _emily._ i think i comprehend it: do not these winds from the north and south, combine with the easterly wind about the equator, and form, what are called, the trade-winds? _mrs. b._ just so, my dear. the composition of the two winds, north and east, produces a constant north-east wind; and that of the two winds, south and east, produces a regular south-east wind; these winds extend to about thirty degrees on each side of the equator, the regions further distant from it, experiencing only their respective northerly and southerly winds. _caroline._ but, mrs. b., if the air is constantly flowing from the poles, to the torrid zone, there must be a deficiency of air, in the polar regions? _mrs. b._ the light air about the equator, which expands, and rises into the upper regions of the atmosphere, ultimately flows from thence, back to the poles, to restore the equilibrium: if it were not for this resource, the polar, atmospheric regions, would soon be exhausted by the stream of air, which, in the lower strata of the atmosphere, they are constantly sending towards the equator. _caroline._ there is then a sort of circulation of air in the atmosphere; the air in the lower strata, flowing from the poles towards the equator, and in the upper strata, flowing back from the equator, towards the poles. _mrs. b._ exactly; i can show you an example of this circulation, on a smaller scale. the air of this room, being more rarefied, than the external air, a wind or current of air is pouring in from the crevices of the windows and doors, to restore the equilibrium; but the light air, with which the room is filled, must find some vent, in order to make way for the heavy air that enters. if you set the door a-jar, and hold a candle near the upper part of it, you will find that the flame will be blown outwards, showing that there is a current of air flowing out from the upper part of the room.--now place the candle on the floor, close by the door, and you will perceive, by the inclination of the flame, that there is also a current of air, setting into the room. _caroline._ it is just so; the upper current is the warm light air, which is driven out to make way for the stream of cold dense air, which enters the room lower down. _mrs. b._ besides the general, or trade-winds, there are others, which are called periodical, because they blow in contrary directions, at particular periods. _emily._ i have heard, mrs. b., that the periodical winds, called, in the torrid zone, the sea and land breezes, blow towards the land, in the day time, and towards the sea, at night: what is the reason of that? _mrs. b._ the land reflects into the atmosphere, a much greater quantity of the sun's rays, than the water; therefore, that part of the atmosphere which is over the land, is more heated and rarefied, than that which is over the sea: this occasions the wind to set in upon the land, as we find that it regularly does on the coast of guinea, and other countries in the torrid zone. there, they have only the sea breeze, but on the islands, they have, in general, both a land and sea breeze, the latter being produced in the way described; whilst at night, during the absence of the sun, the earth cools, and the air is consequently condensed, and flows from the land, towards the sea, occasioning the land breeze. _emily._ i have heard much of the violent tempests, occasioned by the breaking up of the monsoons; are not they also regular trade-winds? _mrs. b._ they are called periodical trade-winds, as they change their course every half year. this variation is produced by the earth's annual course round the sun; the north pole being inclined towards that luminary one half of the year, the south pole, the other half. during the summer of the northern hemisphere, the countries of arabia, persia, india, and china, are much heated, and reflect great quantities of the sun's rays into the atmosphere, by which it becomes extremely rarefied, and the equilibrium consequently destroyed. in order to restore it, the air from the equatorial southern regions, where it is colder, (as well as from the colder northern parts,) must necessarily have a motion towards those parts. the current of air from the equatorial regions, produces the trade-winds for the first six months, in all the seas between the heated continent of asia, and the equator. the other six months, when it is summer in the southern hemisphere, the ocean and countries towards the southern tropic are most heated, and the air over those parts, more rarefied: then the air about the equator alters its course, and flows exactly in an opposite direction. _caroline._ this explanation of the monsoons is very curious; but what does their breaking up mean? _mrs. b._ it is the name given by sailors to the shifting of the periodical winds; they do not change their course suddenly, but by degrees, as the sun moves from one hemisphere, to the other: this change is usually attended by storms and hurricanes, very dangerous for shipping; so that those seas are seldom navigated at the season of the equinoxes. _emily._ i think i understand the winds in the torrid zone perfectly well; but what is it that occasions the great variety of winds, which occur in the temperate zones? for, according to your theory, there should be only north and south winds, in those climates. _mrs. b._ since so large a portion of the atmosphere, as is over the torrid zone, is in continued agitation, these agitations in an elastic fluid, which yields to the slightest impression, must extend every way, to a great distance; the air, therefore, in all climates, will suffer more or less perturbation, according to the situation of the country, the position of mountains, valleys, and a variety of other causes: hence it is easy to conceive, that almost every climate, must be liable to variable winds; this is particularly the case in high latitudes, where the earth is less powerfully affected by the sun's rays, than near the equator. _caroline._ i have observed, that the wind, whichever way it blows, almost always falls about sun-set. _mrs. b._ because the rarefaction of air in the particular spot which produces the wind, diminishes as the sun declines, and consequently the velocity of the wind, abates. _emily._ since the air is a gravitating fluid, is it not affected by the attraction of the moon and the sun, in the same manner as the waters? _mrs. b._ undoubtedly; but the ærial tides are as much greater than those of water, as the density of water exceeds that of air, which, as you may recollect, we found to be about to . _caroline._ what a prodigious protuberance that must occasion! how much the weight of such a column of air, must raise the mercury in the barometer! _emily._ as this enormous tide of air is drawn up and supported, as it were, by the moon, its weight and pressure, i should suppose, would be rather diminished than increased? _mrs. b._ the weight of the atmosphere is neither increased nor diminished by the ærial tides. the moon's attraction augments the bulk, as much as it diminishes the weight, of the column of air; these effects, therefore, counterbalancing each other, the ærial tides do not affect the barometer. _caroline._ i do not quite understand that. _mrs. b._ let us suppose that the additional bulk of air at high tide, raises the barometer one inch; and on the other hand, that the support which the moon's attraction affords the air, diminishes its weight or pressure, so as to occasion the mercury to fall one inch; under these circumstances the mercury must remain stationary. thus, you see, that we can never be sensible of ærial tides by the barometer, on account of the equality of pressure of the atmosphere, whatever be its height. the existence of ærial tides is not, however, hypothetical; it is proved by the effect they produce on the apparent position of the heavenly bodies; but this i cannot explain to you, till you understand the properties of light. _emily._ and when shall we learn them? _mrs. b._ i shall first explain to you the nature of sound, which is intimately connected with that of air; and i think at our next meeting, we may enter upon the subject of optics. we have now considered the effects produced by the wide, and extended agitation, of the air; but there is another kind of agitation, of which the air is susceptible--a vibratory trembling motion, which, striking on the drum of the ear, produces _sound_. _caroline._ is not sound produced by solid bodies? the voice of animals, the ringing of bells, the music of instruments, all proceed from solid bodies. i know of no sound but that of the wind, which is produced by the air. _mrs. b._ sound, i assure you, results from a tremulous motion of the air; and the sonorous bodies you enumerate, are merely the instruments by which that peculiar species of motion, is communicated to the air. _caroline._ what! when i ring this little bell, is it the air that sounds, and not the bell? _mrs. b._ both the bell, and the air, are concerned in the production of sound. but sound, strictly speaking, is a perception excited in the mind, by the motion of the air, on the nerves of the ear; the air, therefore, as well as the sonorous bodies which put it in motion, is only the cause of sound, the immediate effect is produced by the sense of hearing: for without this sense, there would be no sound. _emily._ i can with difficulty conceive that. a person born deaf, it is true, has no idea of sound, because he hears none; yet that does not prevent the real existence of sound, as all those who are not deaf, can testify. _mrs. b._ i do not doubt the existence of sound, to all those who possess the sense of hearing; but it exists neither in the sonorous body, nor in the air, but in the mind of the person whose ear is struck, by the vibratory motion of the air, produced by a sonorous body. sound, therefore, is a sensation, produced in a living body; life, is as necessary to its existence, as it is to that of feeling or seeing. to convince you that sound does not exist in sonorous bodies, but that air or some other vehicle, is necessary to its production, endeavour to ring the little bell, after i have suspended it under a receiver in the air pump, from which i shall exhaust the air.... _caroline._ this is indeed very strange: though i agitate it so violently, it produces but little sound. _mrs. b._ by exhausting the receiver, i have cut off the communication between the air and the bell; the latter, therefore, cannot impart its motion, to the air. _caroline._ are you sure that it is not the glass, which covers the bell, that prevents our hearing it? _mrs. b._ that you may easily ascertain, by letting the air into the receiver, and then ringing the bell. _caroline._ very true; i can hear it now, almost as loud, as if the glass did not cover it; and i can no longer doubt but that air is necessary to the production of sound. _mrs. b._ not absolutely necessary, though by far the most common vehicle of sound. liquids, as well as air, are capable of conveying the vibratory motion of a sonorous body, to the organ of hearing; as sound can be heard under water. solid bodies also, convey sound, as i can soon convince you by a very simple experiment. i shall fasten this string by the middle, round the poker; now raise the poker from the ground, by the two ends of the string, and hold one to each of your ears:--i shall now strike the poker, with a key, and you will find that the sound is conveyed to the ear by means of the strings, in a much more perfect manner, than if it had no other vehicle than the air. _caroline._ that it is, certainly, for i am almost stunned by the noise. but what is a sonorous body, mrs. b.? for all bodies are capable of producing some kind of sound, by the motion they communicate to the air. _mrs. b._ those bodies are called sonorous, which produce clear, distinct, regular, and durable sounds, such as a bell, a drum, musical strings, wind instruments, &c. they owe this property to their elasticity; for an elastic body, after having been struck, not only returns to its former situation, but having acquired momentum by its velocity, like the pendulum, it springs out on the opposite side. if i draw the string a b, (fig. , plate ,) which is made fast at both ends, to c, it will not only return to its original position, but proceed onwards, to d. this is its first vibration; at the end of which, it will retain sufficient velocity to bring it to e, and back again to f, which constitutes its second vibration; the third vibration, will carry it only to g and h, and so on, till the resistance of the air destroys its motion. the vibration of a sonorous body, gives a tremulous motion to the air around it, very similar to the motion communicated to smooth water, when a stone is thrown into it. this, first produces a small circular wave, around the spot in which the stone falls; the wave spreads, and gradually communicates its motion to the adjacent waters, producing similar waves to a considerable extent. the same kind of waves are produced in the air, by the motion of a sonorous body, but with this difference, that as air, is an elastic fluid, the motion does not consist of regularly extending waves, but of vibrations; and are composed of a motion, forwards and backwards, similar to those of the sonorous body. they differ also, in the one taking place in a plane, the other, in all directions: the ærial undulations, being spherical. _emily._ but if the air moves backwards, as well as forwards, how can its motion extend so as to convey sound to a distance? _mrs. b._ the first sphere of undulations, which are produced immediately around the sonorous body, by pressing against the contiguous air, condenses it. the condensed air, though impelled forward by the pressure, reacts on the first set of undulations, driving them back again. the second set of undulations which have been put in motion, in their turn, communicate their motion, and are themselves driven back, by reaction. thus, there is a succession of waves in the air, corresponding with the succession of waves in the water. _caroline._ the vibrations of sound, must extend much further than the circular waves in water, since sound is conveyed to a great distance. _mrs. b._ the air is a fluid so much less dense than water, that motion is more easily communicated to it. the report of a cannon produces vibrations of the air, which extend to several miles around. _emily._ distant sound takes some time to reach us, since it is produced at the moment the cannon is fired; and we see the light of the flash, long before we hear the report. _mrs. b._ the air is immediately put in motion, by the firing of a cannon; but it requires time for the vibrations to extend to any distant spot. the velocity of sound, is computed to be at the rate of feet in a second. _caroline._ with what astonishing rapidity the vibrations must be communicated! but the velocity of sound varies, i suppose, with that of the air which conveys it. if the wind sets towards us from the cannon, we must hear the report sooner than if it set the other way. _mrs. b._ the direction of the wind makes less difference in the velocity of sound, than you would imagine. if the wind sets from us, it bears most of the ærial waves away, and renders the sound fainter; but it is not very considerably longer in reaching the ear, than if the wind blew towards us. this uniform velocity of sound, enables us to determine the distance of the object, from which it proceeds; as that of a vessel at sea, firing a cannon, or that of a thunder cloud. if we do not hear the thunder, till half a minute after we see the lightning, we conclude the cloud to be at the distance of six miles and a half. _emily._ pray, how is the sound of an echo produced? _mrs. b._ when the ærial vibrations meet with an obstacle, having a hard and regular surface, such as a wall, or rock, they are reflected back to the ear, and produce the same sound a second time; but the sound will then appear to proceed, from the object by which it is reflected. if the vibrations fall perpendicularly on the obstacle, they are reflected back in the same line; if obliquely, the sound returns obliquely, in the opposite direction, the angle of reflection being equal to the angle of incidence. _caroline._ oh, then, emily, i now understand why the echo of my voice behind our house is heard so much plainer by you than it is by me, when we stand at the opposite ends of the gravel walk. my voice, or rather, i should say, the vibrations of air it occasions, fall obliquely on the wall of the house, and are reflected by it, to the opposite end of the gravel walk. _emily._ very true; and we have observed, that when we stand in the middle of the walk, opposite the house, the echo returns to the person who spoke. _mrs. b._ speaking-trumpets, are constructed on the principle, that sound is reflected. the voice, instead of being diffused in the open air, is confined within the trumpet; and the vibrations which would otherwise spread laterally, fall against the sides of the instrument, and are reflected from the different points of incidence, so as to combine with those vibrations which proceed straight forwards. the vibrations are thus forced onwards, in the direction of the trumpet, so as greatly to increase the sound, to a person situated in that direction. figure , plate , will give you a clearer idea, of the speaking-trumpet; in this, lines are drawn to represent the manner, in which we may imagine the sound to be reflected. there is a point in front of the trumpet, f, which is denominated its focus, because the sound is there more intense, than at any other spot. the trumpet used by deaf persons, acts on the same principle; although it does not equally increase the sound. _emily._ are the trumpets used as musical instruments, also constructed on this principle? _mrs. b._ so far as their form tends to increase the sound, they are; but, as a musical instrument, the trumpet becomes itself the sonorous body, which is made to vibrate by blowing into it, and communicates its vibrations to the air. i will attempt to give you, in a few words, some notion of the nature of musical sounds, which, as you are fond of music, must be interesting to you. if a sonorous body be struck in such a manner, that its vibrations, are all performed in regular times, the vibrations of the air, will correspond with them; and striking in the same regular manner on the drum of the ear, will produce the same uniform sensation, on the auditory nerve, and excite the same uniform idea, in the mind; or, in other words, we shall hear one musical tone. but if the vibrations of the sonorous body, are irregular, there will necessarily follow a confusion of ærial vibrations; for a second vibration may commence, before the first is finished, meet it half way on its return, interrupt it in its course, and produce harsh jarring sounds, which are called _discords_. _emily._ but each set of these irregular vibrations, if repeated alone, and at equal intervals, would, i suppose, produce a musical tone? it is only their irregular interference, which occasions discord. _mrs. b._ certainly. the quicker a sonorous body vibrates, the more acute, or sharp, is the sound produced; and the slower the vibrations, the more grave will be the note. _caroline._ but if i strike any one note of the piano-forte, repeatedly, whether quickly or slowly, it always gives the same tone. _mrs. b._ because the vibrations of the same string, at the same degree of tension, are always of a similar duration. the quickness, or slowness of the vibrations, relate to the single tones, not to the various sounds which they may compose, by succeeding each other. striking the note in quick succession, produces a more frequent repetition of the tone, but does not increase the velocity of the vibrations of the string. the duration of the vibrations of strings, or wires, depends upon their length, their thickness, or weight, and their degree of tension: thus, you find, the low bass notes are produced by long, thick, loose strings; and the high treble notes by short, small, and tight strings. _caroline._ then, the different length, and size, of the strings of musical instruments, serve to vary the duration of the vibrations, and consequently, the acuteness or gravity of the notes? _mrs. b._ yes. among the variety of tones, there are some which, sounded together, please the ear, producing what we call harmony, or concord. this arises from the agreement of the vibrations of the two sonorous bodies; so that some of the vibrations of each, strike upon the ear at the same time. thus, if the vibrations of two strings are performed in equal times, the same tone is produced by both, and they are said to be in unison. _emily._ now, then, i understand why, when i tune my harp, in unison with the piano-forte, i draw the strings tighter, if it is too low, or loosen them, if it is too high a pitch: it is in order to bring them to vibrate, in equal times, with the strings of the piano-forte. _mrs. b._ but concord, you know, is not confined to unison; for two different tones, harmonize in a variety of cases. when the vibrations of one string (or other sonorous body) vibrate in double the time of another, the second vibration of the latter, will strike upon the ear, at the same instant, as the first vibration of the former; and this is the concord of an octave. if the vibrations of two strings are as two to three, the second vibration of the first, corresponds with the third vibration of the latter, producing the harmony called, a fifth. _caroline._ so, then, when i strike the key-note with its fifth, i hear every second vibration of one, and every third of the other, at the same time? _mrs. b._ yes; and the key-note, struck with the fourth, is likewise a concord, because the vibrations, are as three to four. the vibrations of a major third, with the key-note, are as four to five; and those of a minor third, as five to six. there are other tones, which, though they cannot be struck together without producing discord, if struck successively, give us that succession of pleasing sounds, which is called melody. harmony, you perceive, arises from the combined effect of two, or more concordant sounds, while melody, is the result of certain simple sounds, which succeed each other. upon these general principles, the science of music is founded; but, i am not sufficiently acquainted with it, to enter into it any further. we shall now, therefore, take leave of the subject of sound; and, at our next interview, enter upon that of optics, in which we shall consider the nature of light, vision, and colours. questions . (pg. ) what is wind, and how is it generally produced? . (pg. ) how do the winds blow, around the place where the air becomes rarefied? . (pg. ) what effect is likely to be produced where the winds meet? . (pg. ) in what part of the globe is the air most rarefied, and what is the consequence? . (pg. ) how do these winds change their direction as they approach the equator? . (pg. ) how are the trade-winds produced, and how far do they extend? . (pg. ) how is the equilibrium in the air restored? . (pg. ) how can contrary currents of air be shown in a room? . (pg. ) what causes this? . (pg. ) what is meant by a periodical wind? . (pg. ) what occasions the land and sea breezes, and where do they prevail? . (pg. ) what are monsoons? . (pg. ) how do they change, and what is the cause? . (pg. ) what is meant by their breaking up, and what effect is in general produced? . (pg. ) why is the wind most variable in high latitudes? . (pg. ) why is the wind apt to lessen about sunset? . (pg. ) what effect must the sun and moon produce upon the atmosphere, from their attraction? . (pg. ) why do not the ærial tides affect the barometer? . (pg. ) how is sound produced? . (pg. ) does sound exist in the sonorous body, if not, what is it? . (pg. ) by what experiment might we prove that air is the principal vehicle of sound? . (pg. ) what other bodies convey sound, and how can it be shown that they do so? . (pg. ) what is meant by a sonorous body? . (pg. ) to what do they owe this property? . (pg. ) how is this explained by fig. , plate ? . (pg. ) how is it illustrated by a stone thrown into water, and how far does this illustration apply? . (pg. ) how are the vibrations propagated? . (pg. ) how can we prove that sound, does not travel as rapidly as light? . (pg. ) at what rate is sound said to travel? . (pg. ) is the velocity much influenced by the direction of the wind? . (pg. ) how will sound enable us to judge of the distance of objects? . (pg. ) how are echoes produced? . (pg. ) what is the operation and effect of the speaking-trumpet (fig. , plate )? . (pg. ) how is a musical tone produced? . (pg. ) what occasions discords? . (pg. ) upon what does the acuteness or gravity of a sound depend? . (pg. ) does the force, with which a string is struck, affect the rapidity of its vibrations? . (pg. ) how are the strings made to produce the high and low notes? . (pg. ) what is meant by harmony, or concord, and how is it produced? . (pg. ) when are strings said to be in unison? . (pg. ) how are octaves produced? . (pg. ) how are fifths produced? . (pg. ) how major and minor thirds? . (pg. ) what is meant by melody, and in what particular does it differ from harmony? [illustration: plate xv.] conversation xiv. on optics. of luminous, transparent, and opaque bodies. of the radiation of light. of shadows. of the reflection of light. opaque bodies seen only by reflected light. vision explained. camera obscura. image of objects on the retina. caroline. i long to begin our lesson to-day, mrs. b., for i expect that it will be very entertaining. _mrs. b._ _optics is that branch of philosophy, which treats of the nature and properties of light._ it is certainly one of the most interesting branches of natural philosophy, but not one of the easiest to understand; i must, therefore, beg that you will give me your undivided attention. i shall first inquire, whether you comprehend the meaning of a _luminous body_, an _opaque body_, and a _transparent body_. _caroline._ a luminous body is one that shines; an opaque.... _mrs. b._ do not proceed to the second, until we have agreed upon the definition of the first. all bodies that shine, are not luminous; for a luminous body is one that shines by its own light; as the sun, the fire, a candle, &c. _emily._ polished metal then, when it shines with so much brilliancy, is not a luminous body? _mrs. b._ no, for it would be dark, if it did not receive light from a luminous body; it belongs, therefore, to the class of dark, as well as of opaque bodies, which comprehends all such as are neither luminous, nor will admit the light to pass through them. _emily._ and transparent bodies, are those which admit the light to pass through them, such as glass and water. _mrs. b._ you are right. transparent, or pellucid bodies, are frequently called mediums, because they allow the rays of light to pass through them; and the rays which pass through, are said to be transmitted by them. light, when emanated from the sun, or any other luminous body, is projected forward in straight lines, in every possible direction; so that the luminous body, is not only the general centre, from whence all the rays proceed; but every point of it, may be considered as a centre, which radiates light in every direction. (fig. , plate .) _emily._ but do not the rays which are projected in different directions, and cross each other, interfere, and impede each other's course? _mrs. b._ not at all. the particles of light, are so extremely minute, that they are never known to interfere with each other. a ray of light, is a single line of light, projected from a luminous body; and a pencil of rays, is a collection of rays, proceeding from any one point of a luminous body, as fig. . _caroline._ is light then a substance composed of particles, like other bodies? _mrs. b._ that is a disputed point, upon which i cannot pretend to decide. in some respects, light is obedient to the laws which govern bodies; in others, it appears to be independent of them: thus, though its course is guided by the laws of motion, it does not seem to be influenced by those of gravity. it has never been discovered to have weight, though a variety of interesting experiments have been made with a view of ascertaining that point; but we are so ignorant of the intimate nature of light, that an attempt to investigate it, would lead us into a labyrinth of perplexity, if not of error; we shall, therefore, confine our attention to those properties of light, which are well ascertained. let us return to the examination of the effects of the radiation of light, from a luminous body. since the rays of light are projected in straight lines, when they meet with an opaque body through which they are unable to pass, they are stopped short in their course; for they cannot move in a curve line round the body. _caroline._ no, certainly; for it would require some other force besides that of projection, to produce motion in a curve line. _mrs. b._ the interruption of the rays of light, by the opaque body, produces, therefore, darkness on the opposite side of it: and if this darkness fall upon a wall, a sheet of paper, or any object whatever, it forms a shadow. _emily._ a shadow, then, is nothing more than darkness produced by the intervention of an opaque body, which prevents the rays of light from reaching an object behind it. _caroline._ why then are shadows of different degrees of darkness; for i should have supposed, from your definition of a shadow, that it would have been perfectly black? _mrs. b._ it frequently happens that a shadow is produced by an opaque body, interrupting the course of the rays from one luminous body, while light from another, reaches the space where the shadow is formed; in which case, the shadow is proportionally fainter. this happens when the opaque body is lighted by two candles: if you extinguish one of them, the shadow will be both deeper, and more distinct. _caroline._ but yet it will not be perfectly dark. _mrs. b._ because it is still slightly illuminated by light reflected from the walls of the room, and other surrounding objects. you must observe, also, that when a shadow is produced by the interruption of rays from a single luminous body, the darkness is proportioned to the intensity of the light. _emily._ i should have supposed the contrary; for as the light reflected from surrounding objects on the shadow, must be in proportion to the intensity of the light, the stronger the light, the more the shadow will be illumined. _mrs. b._ your remark is perfectly just; but as we have no means of estimating the degrees of light, and of darkness, but by comparison, the strongest light will appear to produce the deepest shadow. hence a total eclipse of the sun, occasions a more sensible darkness than midnight, as it is immediately contrasted with the strong light of noonday. _caroline._ the reappearance of the sun, after an eclipse, must, by the same contrast, appear remarkably brilliant. _mrs. b._ certainly. there are several things to be observed, in regard to the form, and extent, of shadows. if the luminous body a (fig. .) is larger than the opaque body b, the shadow will gradually diminish in size, till it terminates in a point. _caroline._ this is the case with the shadows of the earth, and the moon; as the sun, which illumines them, is larger than either of those bodies. and why is it not the case with the shadows of terrestrial objects? their shadows, far from diminishing, are always larger than the object, and increase with the distance from it. _mrs. b._ in estimating the effect of shadows, we must consider the dimensions of the luminous body; when the luminous body is less, than the opaque body, the shadow will increase with the distance. this will be best exemplified, by observing the shadow of an object lighted by a candle. _emily._ i have often noticed, that the shadow of my figure, against the wall, grows larger, as it is more distant from me, which is owing, no doubt, to the candle that shines on me, being much smaller than myself. _mrs. b._ yes. the shadow of a figure as a, (fig. .) varies in size, according to the distance of the several surfaces b c d e, on which it is described. _caroline._ i have observed, that two candles, produce two shadows from the same object; whilst it would appear, from what you said, that they should rather produce only half a shadow, that is to say, a very faint one. _mrs. b._ the number of lights (in different directions) while it decreases the intensity of the shadows, increases their number, which always corresponds with that of the lights; for each light, makes the opaque body cast a different shadow, as illustrated by fig. . which represents a ball a, lighted by three candles, b, c, d; and you observe the light b, produces the shadow _b_, the light c, the shadow _c_, and the light d, the shadow _d_; but neither of these shadows will be very dark, because the light of one candle only, is intercepted by the ball; and the spot is still illuminated by the other two. _emily._ i think we now understand the nature of shadows very well; but pray, what becomes of the rays of light, which opaque bodies arrest in their course, and the interruption of which, is the occasion of shadows? _mrs. b._ your question leads to a very important property of light, _reflection_. when rays of light encounter an opaque body, they cannot pass through it, and part of them are absorbed by it, and part are reflected, and rebound; just as an elastic ball rebounds, when struck against a wall. by reflection, we mean that the light is turned back again, through the same medium which it had traversed in its first course. _emily._ and is light, in its reflection, governed by the same laws, as solid, elastic bodies? _mrs. b._ exactly. if a ray of light fall perpendicularly on an opaque body, it is reflected back in the same line, towards the point whence it proceeded. if it fall obliquely, it is reflected obliquely, but in the opposite direction; the ray which falls upon the reflecting surface, is called the incident ray, and that which leaves it, the reflected ray; the angle of incidence, is always equal to the angle of reflection. you recollect that law in mechanics? _emily._ oh yes, perfectly. _mrs. b._ if you will shut the shutters, we will admit a ray of the sun's light, through a very small aperture, and i can show you how it is reflected. i now hold this mirror, so that the ray shall fall perpendicularly upon it. _caroline._ i see the ray which falls upon the mirror, but not that which is reflected by it. _mrs. b._ because it is turned directly back again; and the ray of incidence, and that of reflection, are confounded together, both being in the same line, though in opposite directions. _emily._ the ray then, which appears to us single, is really double, and is composed of the incident ray, proceeding to the mirror, and of the reflected ray, returning from the mirror. _mrs. b._ exactly so. we will now separate them, by holding the mirror m, (fig. ,) in such a manner, that the incident ray, a b, shall fall obliquely upon it--you see the reflected ray, b c, is marching off in another direction. if we draw a line from the point of incidence b, perpendicularly, to the mirror, it will divide the angle of incidence, from the angle of reflection, and you will see that they are equal. _emily._ exactly; and now, that you hold the mirror, so that the ray falls more obliquely upon it, it is also reflected more obliquely, preserving the equality of the angles of incidence, and of reflection. _mrs. b._ it is by reflected rays only, that we see opaque objects. luminous bodies, send rays of light immediately to our eyes, but the rays which they send to other bodies, are invisible to us, and are seen, only when they are reflected by those bodies, to our eyes. _emily._ but have we not just seen the ray of light, in its passage from the sun to the mirror, and its reflections? yet, in neither case, were those rays in a direction to enter our eyes. _mrs. b._ what you saw, was the light reflected to your eyes, by small particles of dust floating in the air, and on which the ray shone, in its passage to, and from, the mirror. _caroline._ yet i see the sun, shining on that house yonder, as clearly as possible. _mrs. b._ indeed you cannot see a single ray, which passes from the sun to the house; you see, by the aid of those rays, which enter your eyes; therefore, it is the rays which are reflected by the house, to you, and not those which proceed directly from the sun, to the house, that render the building visible to you. _caroline._ why then does one side of the house appear to be in sunshine, and the other in shade? for, if i cannot see the sun shine upon it, the whole of the house should appear in the shade. _mrs. b._ that side of the house, which the sun shines upon, receives, and reflects more light, and therefore, appears more luminous and vivid, than the side which is in shadow; for the latter is illumined only, by rays reflected upon it by other objects; these rays are, therefore, twice reflected before they reach your sight; and as light is more, or less, absorbed by the bodies it strikes upon, every time a ray is reflected, its intensity is diminished. _caroline._ still i cannot reconcile to myself, the idea that we do not see the sun's rays shining on objects, but only those which such objects reflect to us. _mrs. b._ i do not, however, despair of convincing you of it. look at that large sheet of water; can you tell why the sun appears to shine on one part of it only? _caroline._ no, indeed; for the whole of it is equally exposed to the sun. this partial brilliancy of water, has often excited my wonder; but it has struck me more particularly by moonlight. i have frequently observed a vivid streak of moonshine on the sea, while the rest of the water remained in deep obscurity, and yet there was no apparent obstacle to prevent the moon from shining equally on every part of the water. _mrs. b._ by moonlight the effect is more remarkable, on account of the deep obscurity of the other parts of the water; while by the sun's light, the effect is too strong for the eye to be able to observe it so distinctly. _caroline._ but, if the sun really shines on every part of that sheet of water, why does not every part of it, reflect rays to my eyes? _mrs. b._ the reflected rays, are not attracted out of their natural course, by your eyes. the direction of a reflected ray, you know, depends on that of the incident ray; the sun's rays, therefore, which fall with various degrees of obliquity upon the water, are reflected in directions equally various; some of these will meet your eyes, and you will see them, but those which fall elsewhere, are invisible to you. _caroline._ the streak of sunshine, then, which we now see upon the water, is composed of those rays which by their reflection, happen to fall upon my eyes? _mrs. b._ precisely. _emily._ but is that side of the house yonder, which appears to be in shadow, really illuminated by the sun, and its rays reflected another way? _mrs. b._ no; that is a different case, from the sheet of water. that side of the house, is really in shadow; it is the west side, which the sun cannot shine upon, till the afternoon. _emily._ those objects, then, which are illumined by reflected rays, and those which receive direct rays from the sun, but which do not reflect those rays towards us, appear equally in shadow? _mrs. b._ certainly; for we see them both illumined by reflected rays. that part of the sheet of water, over which the trees cast a shadow, by what light do you see it? _emily._ since it is not by the sun's direct rays, it must be by those reflected on it from other objects, and which it again reflects to us. _caroline._ but if we see all terrestrial objects by reflected light, (as we do the moon,) why do they appear so bright and luminous? i should have supposed that reflected rays, would have been dull and faint, like those of the moon. _mrs. b._ the moon reflects the sun's light, with as much vividness as any terrestrial object. if you look at it on a clear night, it will appear as bright as a sheet of water, the walls of a house, or any object seen by daylight, and on which the sun shines. the rays of the moon are doubtless feeble, when compared with those of the sun; but that would not be a fair comparison, for the former are incident, the latter, reflected rays. _caroline._ true; and when we see terrestrial objects by moonlight, the light has been twice reflected, and is consequently, proportionally fainter. _mrs. b._ in traversing the atmosphere, the rays, both of the sun, and moon, lose some of their light. for though the pure air, is a transparent medium, which transmits the rays of light freely, we have observed, that near the surface of the earth, it is loaded with vapours and exhalations, by which some portion of them are absorbed. _caroline._ i have often noticed, that an object on the summit of a hill, appears more distinct, than one at an equal distance in a valley, or a plain; which is owing, i suppose, to the air being more free from vapours in an elevated situation, and the reflected rays, being consequently brighter. _mrs. b._ that may have some sensible effect; but, when an object on the summit of a hill, has a back ground of light sky, the contrast with the object, makes its outline more distinct. _caroline._ i now feel well satisfied, that we see opaque objects, only by reflected rays; but i do not understand, how these rays, show us the objects from which they proceed. _mrs. b._ i shall hereafter describe the structure of the eye, very particularly, but will now observe, that the small round spot, which is generally called the sight of the eye, is properly denominated the _pupil_; and that the _retina_, is an expansion of the optic nerve on the back part of the ball of the eye, upon which, as upon a screen, the rays fall, which enter at the pupil. the rays of light, enter at the pupil of the eye, and proceed to the retina; and there they describe the figure, colour, and (excepting size) form a perfect representation of the object, from which they proceed. we shall again close the shutters, and admit the light, through the small hole made for that purpose, and you will see a picture, on the wall, opposite the aperture, similar to that which is delineated on the retina of the eye. the picture is somewhat confused, but by using a lens, to bring the rays to a focus, it will be rendered very distinct. _caroline._ oh, how wonderful! there is an exact picture in miniature of the garden, the gardener at work, the trees blown about by the wind. the landscape, would be perfect, if it were not reversed; the ground, being above, and the sky beneath. _mrs. b._ it is not enough to admire, you must understand, this phenomenon, which is called a _camera obscura_, or dark chamber; from the necessity of darkening the room, in order to exhibit it. the camera obscura, sometimes consists of a small box, properly fitted up, to represent external objects. this picture, you now see, is produced by the rays of light, reflected from the various objects in the garden, and which are admitted through the hole in the window shutter. [illustration: plate xvi.] the rays from the glittering weathercock, at the top of the alcove, a, (plate .) represent it in this spot, _a_; for the weathercock, being much higher than the aperture in the shutter, only a few of the rays, which are reflected by it, in an obliquely descending direction, can find entrance there. the rays of light, you know, always move in straight lines; those, therefore, which enter the room, in a descending direction, will continue their course in the same direction, and will consequently fall upon the lower part of the wall opposite the aperture, and represent the weathercock, reversed in that spot, instead of erect, in the uppermost part of the landscape. _emily._ and the rays of light, from the steps, (b) of the alcove, in entering the aperture, ascend, and will describe those steps in the highest, instead of the lowest, part of the landscape. _mrs. b._ observe, too, that the rays coming from the alcove, which is to our left, describe it on the wall, to the right; while those, which are reflected by the walnut tree, c d, to our right, delineate its figure in the picture, to the left, _c d_. thus the rays, coming in different directions, and proceeding always in right lines, cross each other at their entrance through the aperture; those which come from above, proceed below, those from the right, go to the left, those from the left, towards the right; thus every object is represented in the picture, as occupying a situation, the very reverse of that which it does in nature. _caroline._ excepting the flower-pot, e f, which, though its position is reversed, has not changed its situation in the landscape. _mrs. b._ the flower-pot, is directly in front of the aperture; so that its rays, fall perpendicularly upon it, and consequently proceed perpendicularly to the wall, where they delineate the object, directly behind the aperture. _emily._ and is it thus, that the picture of objects, is painted on the retina of the eye? _mrs. b._ precisely. the pupil of the eye, through which the rays of light enter, represents the aperture in the window-shutter; and the image, delineated on the retina, is exactly similar to the picture on the wall. _caroline._ you do not mean to say, that we see only the representation of the object, which is painted on the retina, and not the object itself? _mrs. b._ if, by sight, you understand that sense, by which the presence of objects is perceived by the mind, through the means of the eyes, we certainly see only the image of those objects, painted on the retina. _caroline._ this appears to me quite incredible. _mrs. b._ the nerves, are the only part of our frame, capable of sensation: they appear, therefore, to be the instruments, which the mind employs in its perceptions; for a sensation, always conveys an idea, to the mind. now it is known, that our nerves can be affected only by contact; and for this reason, the organs of sense, cannot act at a distance: for instance, we are capable of smelling only particles which are actually in contact with the nerves of the nose. we have already observed, that the odour of a flower consists in effluvia, composed of very minute particles, which penetrate the nostrils, and strike upon the olfactory nerves, which instantly convey the idea of odour to the mind. _emily._ and sound, though it is said to be heard at a distance, is, in fact, heard only when the vibrations of the air, which convey it to our ears, strike upon the auditory nerve. _caroline._ there is no explanation required, to prove that the senses of feeling and of tasting, are excited only by contact. _mrs. b._ and i hope to convince you, that the sense of sight, is so likewise. the nerves, which constitute the sense of sight, are not different in their nature from those of the other organs; they are merely instruments which convey ideas to the mind, and can be affected only on contact. now, since real objects cannot be brought to touch the optic nerve, the image of them is conveyed thither by the rays of light, proceeding from real objects, which actually strike upon the optic nerve, and form that image which the mind perceives. _caroline._ while i listen to your reasoning, i feel convinced; but when i look upon the objects around, and think that i do not see them, but merely their image painted in my eyes, my belief is again staggered. i cannot reconcile to myself, the idea, that i do not really see this book which i hold in my hand, nor the words which i read in it. _mrs. b._ did it ever occur to you as extraordinary, that you never beheld your own face? _caroline._ no; because i so frequently see an exact representation of it in the looking-glass. _mrs. b._ you see a far more exact representation of objects on the retina of your eye: it is a much more perfect mirror, than any made by art. _emily._ but is it possible, that the extensive landscape, which i now behold from the window, should be represented on so small a space, as the retina of the eye? _mrs. b._ it would be impossible for art to paint so small and distinct a miniature; but nature works with a surer hand, and a more delicate pencil. that power alone, which forms the feathers of the butterfly, and the organs of the minutest insect, can pourtray so admirable and perfect a miniature, as that which is represented on the retina of the eye. _caroline._ but, mrs. b., if we see only the image of objects, why do we not see them reversed, as you showed us they were, in the camera obscura? is not that a strong argument against your theory? _mrs. b._ not an unanswerable one, i hope. the image on the retina, it is true, is reversed, like that in the camera obscura; as the rays, from the different parts of the landscape, intersect each other on entering the pupil, in the same manner as they do, on entering the camera obscura. the scene, however, does not excite the idea of being inverted, because we always see an object in the direction of the rays which it sends to us. _emily._ i confess i do not understand that. _mrs. b._ it is, i think, a difficult point to explain clearly. a ray which comes from the upper part of an object, describes the image on the lower part of the retina; but, experience having taught us, that the direction of that ray is from above, we consider that part of the object it represents as uppermost. the rays proceeding from the lower part of an object, fall upon the upper part of the retina; but as we know their direction to be from below, we see that part of the object they describe as the lowest. _caroline._ when i want to see an object above me, i look up; when an object below me, i look down. does not this prove that i see the objects themselves? for if i beheld only the image, there would be no necessity for looking up or down, according as the object was higher or lower, than myself. _mrs. b._ i beg your pardon. when you look up, to an elevated object, it is in order that the rays reflected from it, should fall upon the retina of your eyes; but the very circumstance of directing your eyes upwards, convinces you that the object is elevated, and teaches you to consider as uppermost, the image it forms on the retina, though it is, in fact, represented in the lowest part of it. when you look down upon an object, you draw your conclusion from a similar reasoning; it is thus that we see all objects in the direction of the rays which reach our eyes. but i have a further proof in favour of what i have advanced, which, i hope, will remove your remaining doubts: i shall, however, defer it till our next meeting, as the lesson has been sufficiently long to-day. questions . (pg. ) what is optics? . (pg. ) what is meant by a luminous body? . (pg. ) what is meant by a dark body, and what by an opaque body? . (pg. ) what are transparent bodies? . (pg. ) what is a medium? . (pg. ) how is light projected from luminous bodies, and how, from every point of such bodies, (fig. , plate ?) . (pg. ) why do not the rays of light from different points, stop each other's progress? . (pg. ) what is a ray, and what a pencil of rays? fig. , plate . . (pg. ) do we know whether light is a substance, similar to bodies in general? . (pg. ) when a ray of light falls upon an opaque body, what is the result? . (pg. ) in what does shadow consist? . (pg. ) why are they, in general, but partially dark? . (pg. ) upon what does the intensity of a shadow depend? . (pg. ) how are shadows affected by the size of the luminous body, as represented in plate , fig. ? . (pg. ) when is the shadow larger than the intercepting body? . (pg. ) what is explained by fig. , plate ? . (pg. ) what will be the effect of several lights, as in fig. , plate ? . (pg. ) why will neither of these shadows be very dark? . (pg. ) what becomes of the light which falls upon an opaque body? . (pg. ) what is meant by reflection? . (pg. ) what is meant by the incident, and reflected rays? . (pg. ) what is the result, when the incident ray falls perpendicularly, and what, when it falls obliquely? . (pg. ) what two angles are always equal in this case? . (pg. ) to what law in mechanics, is this analogous, as represented in fig. , plate ? . (pg. ) what is represented by fig. , plate ? . (pg. ) by what light are we enabled to see opaque, and by what, luminous bodies? . (pg. ) what enables us to see a ray of light in its passage, through a darkened room? . (pg. ) by what reasoning would you prove that an object, such, for example, as a house, is seen by reflected light? . (pg. ) why may one side of such object appear more bright than another side? . (pg. ) how is the fact exemplified by the sun, or moon, shining upon water? . (pg. ) why is this best evinced by moonlight? . (pg. ) by what light do we see the moon, and why is it comparatively feeble? . (pg. ) what circumstance, renders objects seen by moonlight, still less vivid? . (pg. ) what is meant by the pupil of the eye? . (pg. ) what by the retina? . (pg. ) how do the rays of light operate on the eye in producing vision? . (pg. ) how may this be exemplified, in a darkened room? . (pg. ) what is meant by a _camera obscura_? . (pg. ) how is it explained in plate ? . (pg. ) why are the objects inverted and reversed? . (pg. ) what analogy is there between the camera obscura, and the eye? . (pg. ) is it the object, or its picture on the retina, which presents to the mind an idea of the object seen? . (pg. ) by what organs is sensation produced, and how must these organs be affected? . (pg. ) how will the idea of contact, apply to objects not touching the eye? . (pg. ) why do not objects appear reversed to the eye, as in the camera obscura? conversation xv. optics--_continued_. on the angle of vision, and the reflection of mirrors. angle of vision. reflection of plain mirrors. reflection of convex mirrors. reflection of concave mirrors. caroline. well, mrs. b., i am very impatient to hear what further proofs you have to offer, in support of your theory. you must allow, that it was rather provoking to dismiss us as you did at our last meeting. _mrs. b._ you press so hard upon me with your objections, that you must give me time to recruit my forces. can you tell me, caroline, why objects at a distance, appear smaller than they really are? _caroline._ i know no other reason than their distance. _mrs. b._ it is a fact, that distance causes objects to appear smaller, but to state the fact, is not to give the reason. we must refer again to the camera obscura, to account for this circumstance; and you will find, that the different apparent dimensions of objects at different distances, proceed from our seeing, not the objects themselves, but merely their image on the retina. fig. , plate , represents a row of trees, as viewed in the camera obscura. i have expressed the direction of the rays, from the objects to the image, by lines. now, observe, the ray which comes from the top of the nearest tree, and that which comes from the foot of the same tree, meet at the aperture, forming an angle of about twenty-five degrees; the angle under which we see any object, is called, the visual angle, or, angle of vision. these rays cross each other at the aperture, forming equal angles on each side of it, and represent the tree inverted in the camera obscura. the degrees of the image, are considerably smaller than those of the object, but the proportions are perfectly preserved. [illustration: plate xvii.] now, let us notice the upper and lower ray, from the most distant tree; they form an angle of not more than twelve or fifteen degrees, and an image of proportional dimensions. thus, two objects of the same size, as the two trees of the avenue, form figures of different sizes in the camera obscura, according to their distance; or, in other words, according to the angle of vision under which they are seen. do you understand this? _caroline._ perfectly. _mrs. b._ then you have only to suppose, that the representation in the camera obscura, is similar to that on the retina. now, since objects of the same magnitudes, appear to be of different dimensions, when at different distances from us, let me ask you which it is, that you see; the real objects, which, we know, do not vary in size, or the images, which, we know, do vary, according to the angle of vision under which we see them? _caroline._ i must confess, that reason is in favour of the latter. but does that chair, at the further end of the room, form an image on my retina, much smaller than this which is close to me? they appear exactly of the same size. _mrs. b._ our senses are imperfect, but the experience we acquire by the sense of touch, corrects the illusions of our sight, with regard to objects within our reach. you are so perfectly convinced, of the real size of objects, which you can handle, that you do not attend to the apparent difference. does that house appear to you much smaller, than when you are close to it? _caroline._ no, because it is very near us. _mrs. b._ and yet you can see the whole of it, through one of the windows of this room. the image of the house on your retina must, therefore, be smaller than that of the window through which you see it. it is your knowledge of the real magnitude of the house which prevents your attending to its apparent size. if you were accustomed to draw from nature, you would be fully aware of this difference. _emily._ and pray, what is the reason that, when we look up an avenue, the trees not only appear smaller as they are more distant, but seem gradually to approach each other, till they meet in a point? _mrs. b._ not only the trees, but the road which separates the two rows, forms a smaller visual angle, in proportion as it is more distant from us; therefore, the width of the road gradually diminishes, as well as the size of the trees, till at length the road apparently terminates in a point, at which the trees seem to meet. _emily._ i am very glad to understand this, for i have lately begun to learn perspective, which appeared to me a very dry study; but now that i am acquainted with some of the principles on which it is founded, i shall find it much more interesting. _caroline._ in drawing a view from nature, it seems that we do not copy the real objects, but the image they form on the retina of our eyes? _mrs. b._ certainly. in sculpture, we copy nature as she really exists; in painting, we represent her, as she appears to us. we must now conclude the observations that remain to be made, on the angle of vision. if the rays, proceeding from the extremities of an object, with an ordinary degree of illumination, do not enter the eye under an angle of more than two seconds, which is the - th part of a degree, it is invisible. there are, consequently, two cases in which objects may be invisible; if they are either so small, or so distant, as to form an angle of less than two seconds of a degree. in like manner, if the velocity of a body does not exceed degrees in an hour, its motion is imperceptible. _caroline._ a very rapid motion may then be imperceptible, provided the distance of the moving body, is sufficiently great. _mrs. b._ undoubtedly; for the greater its distance, the smaller will be the angle, under which its motion will appear to the eye. it is for this reason, that the motion of the celestial bodies is invisible, although inconceivably rapid. _emily._ i am surprised, that so great a velocity as degrees an hour, should be invisible. _mrs. b._ the real velocity depends upon the space comprehended in each degree, and upon the time, in which the moving body, passes over that space. but we can only know the extent of this space, by knowing the distance of the moving body, from its centre of motion; for supposing two men to set off at the same moment from a and b, (fig. .) to walk each to the end of their respective lines, c and d; if they perform their walk in the same space of time, they must have proceeded at a very different rate; and yet to an eye situated at e, they will appear to have moved with equal velocity, because they will both have gone through an equal number of degrees, though over a very unequal length of ground. the number of degrees over which a body moves in a given time, is called its angular velocity; two bodies, you see, may have the same angular, or apparent velocity, whilst their real velocities may differ almost infinitely. sight is an extremely useful sense, no doubt, but it cannot always be relied on, it deceives us both in regard to the size and the distance of objects; indeed, our senses would be very liable to lead us into error, if experience did not set us right. _emily._ between the two, i think that we contrive to acquire a tolerably accurate idea of objects. _mrs. b._ at least sufficiently so, for the general purposes of life. to convince you how requisite experience is, to correct the errors of sight, i shall relate to you, the case of a young man, who was blind from his infancy, and who recovered his sight at the age of fourteen, by the operation of couching. at first, he had no idea, either of the size, or distance of objects, but imagined that every thing he saw touched his eyes; and it was not, till after having repeatedly felt them, and walked from one object to another, that he acquired an idea of their respective dimensions, their relative situations, and their distances. _caroline._ the idea that objects touched his eyes, is, however, not so absurd, as it at first appears; for if we consider that we see only the image of objects, this image actually touches our eyes. _mrs. b._ that is, doubtless, the reason of the opinion he formed, before the sense of touch had corrected his judgment. _caroline._ but since an image must be formed on the retina of each of our eyes, why do we not see objects double? _mrs. b._ the action of the rays, on the optic nerve of each eye, is so perfectly similar, that they produce but a single sensation; the mind, therefore, receives the same idea, from the retina of both eyes, and conceives the object to be single. _caroline._ this is difficult to comprehend, and i should think, can be but conjectural. _mrs. b._ i can easily convince you, that you have a distinct image of an object formed on the retina of each eye. look through the window, with both eyes open, at some object exactly opposite to one of the upright bars of the sash. _caroline._ i now see a tree, the body of which, appears to be in a line exactly opposite to one of the bars. _mrs. b._ if you now shut your right eye, and look with the left, it will appear to the left of the bar; then by closing the left eye, and looking with the other, it will appear to the right of the bar. _caroline._ that is true, indeed! _mrs. b._ there are, evidently, two representations of the tree in different situations, which must be owing to an image of it being formed on each eye; if the action of the rays, therefore, on each retina, were not so perfectly similar as to produce but one sensation, we should see double; and we find that to be the case with some persons, who are afflicted with a disease in one eye, which prevents the rays of light from affecting it in the same manner as the other. _emily._ pray, mrs. b., when we see the image of an object in a looking-glass, why is it not inverted, as in the camera obscura, and on the retina of the eye? _mrs. b._ because the rays do not enter the mirror by a small aperture, and cross each other, as they do at the orifice of a camera obscura, or the pupil of the eye. when you view yourself in a mirror, the rays from your eyes fall perpendicularly upon it, and are reflected in the same line; the image is, therefore, described behind the glass, and is situated in the same manner as the object before it. _emily._ yes, i see that it is; but the looking-glass is not nearly so tall as i am, how is it, therefore, that i can see the whole of my figure in it? _mrs. b._ it is not necessary that the mirror should be more than half your height, in order that you may see the whole of your person in it, (fig. .) the ray of light a b, from your eye, which falls perpendicularly on the mirror b d, will be reflected back, in the same line; but the ray from your feet, will fall obliquely on the mirror, for it must ascend in order to reach it; it will, therefore, be reflected in the line a d: and since we view objects in the direction of the reflected rays, which reach the eye, and since the image appears at the same distance, behind the mirror, that the object is before it, we must continue the line a d to e, and the line c d to f, at the termination of which, the image will be represented. [illustration: plate xviii.] _emily._ then i do not understand why i should not see the whole of my person in a much smaller mirror, for a ray of light from my feet would always reach it, though more obliquely. _mrs. b._ true; but the more obliquely the ray falls on the mirror, the more obliquely it will be reflected; the ray would, therefore, be reflected above your head, and you could not see it. this is shown by the dotted line (fig. .) now stand a little to the right of the mirror, so that the rays of light from your figure may fall obliquely on it---- _emily._ there is no image formed of me in the glass now. _mrs. b._ i beg your pardon, there is; but you cannot see it, because the incident rays, falling obliquely on the mirror, will be reflected obliquely, in the opposite direction; the angles of incidence, and reflection, being equal. caroline, place yourself in the direction of the reflected rays, and tell me whether you do not see emily's image in the glass? _caroline._ let me consider.--in order to look in the direction of the reflected rays, i must place myself as much to the left of the glass, as emily stands to the right of it.--now i see her image, not straight before me, however, but before her; and it appears at the same distance behind the glass, that she is in front of it. _mrs. b._ you must recollect, that we always see objects in the direction of the last rays, which reach our eyes. figure represents an eye, looking at the image of a vase, reflected by a mirror; it must see it in the direction of the ray a b, as that is the ray which brings the image to the eye; prolong the ray to c, and in that spot will the image appear. _caroline._ i do not understand why a looking-glass reflects the rays of light; for glass is a transparent body, which should transmit them! _mrs. b._ it is not the glass that reflects the rays which form the image you behold, but the silvering behind it; this silvering is a compound of mercury and tin, which forms a brilliant metallic coating. the glass acts chiefly as a transparent case, through which the rays find an easy passage, to, and from, the quicksilver. _caroline._ why then should not mirrors be made simply of mercury? _mrs. b._ because mercury is a fluid. by amalgamating it with tinfoil, it becomes of the consistence of paste, attaches itself to the glass, and forms, in fact, a metallic mirror, which would be much more perfect without its glass cover, for the purest glass is never perfectly transparent; some of the rays, therefore, are lost during their passage through it, by being either absorbed, or irregularly reflected. this imperfection of glass mirrors, has introduced the use of metallic mirrors, for optical purposes. _emily._ but since all opaque bodies reflect the rays of light, i do not understand why they are not all mirrors. _caroline._ a curious idea indeed, sister; it would be very gratifying to see oneself in every object at which one looked. _mrs. b._ it is very true that all opaque objects reflect light; but the surface of bodies, in general, is so rough and uneven, that the reflection from them is extremely irregular, and prevents the rays from forming an image on the retina. this, you will be able to understand better, when i shall explain to you the nature of vision, and the structure of the eye. you may easily conceive the variety of directions in which rays would be reflected by a nutmeg-grater, on account of the inequality of its surface, and the number of holes with which it is pierced. all solid bodies more or less resemble the nutmeg-grater, in these respects; and it is only those which are susceptible of receiving a polish, that can be made to reflect the rays with regularity. as hard bodies are of the closest texture, the least porous, and capable of taking the highest polish, they make the best mirrors; none, therefore, are so well calculated for this purpose, as metals. _caroline._ but the property of regular reflection, is not confined to this class of bodies; for i have often seen myself, in a highly polished mahogany table. _mrs. b._ certainly; but as that substance is less durable, and its reflection less perfect, than that of metals, i believe it would seldom be chosen, for the purpose of a mirror. there are three kinds of mirrors used in optics; the _plain_, or _flat_, which are the common mirrors we have just mentioned; _convex_ mirrors, and _concave_ mirrors. the reflection of the two latter, is very different from that of the former. the plain mirror, we have seen, does not alter the direction of the reflected rays, and forms an image behind the glass, exactly similar to the object before it. a convex mirror has the peculiar property of making the reflected rays diverge, by which means it diminishes the image; and a concave mirror makes the rays converge, and under certain circumstances, magnifies the image. _emily._ we have a convex mirror in the drawing-room, which forms a beautiful miniature picture of the objects in the room; and i have often amused myself with looking at my magnified face in a concave mirror. but i hope you will explain to us, why the one enlarges, while the other diminishes the objects it reflects. _mrs. b._ let us begin by examining the reflection of a convex mirror. this is formed of a portion of the exterior surface of a sphere. when several parallel rays fall upon it, that ray only which, if prolonged, would pass through the centre or axis of the mirror, is perpendicular to it. in order to avoid confusion, i have, in fig. , plate , drawn only three parallel lines, a b, c d, e f, to represent rays falling on the convex mirror, m n; the middle ray, you will observe, is perpendicular to the mirror, the others fall on it, obliquely. _caroline._ as the three rays are parallel, why are they not all perpendicular to the mirror? _mrs. b._ they would be so to a flat mirror; but as this is spherical, no ray can fall perpendicularly upon it which is not directed towards the centre of the sphere. _emily._ just as a weight falls perpendicularly to the earth, when gravity attracts it towards the centre. _mrs. b._ in order, therefore, that rays may fall perpendicularly to the mirror at b and f, the rays must be in the direction of the dotted lines, which, you may observe, meet at the centre o of the sphere, of which the mirror forms a portion. now, can you tell me in what direction the three rays, a b, c d, e f, will be reflected? _emily._ yes, i think so: the middle ray, falling perpendicularly on the mirror, will be reflected in the same line: the two outer rays falling obliquely, will be reflected obliquely to g and h; for the dotted lines you have drawn are perpendiculars, which divide the angles of incidence and reflection, of those two rays. _mrs. b._ extremely well, emily: and since we see objects in the direction of the reflected ray, we shall see the image l, which is the point at which the reflected rays, if continued through the mirror, would unite and form an image. this point is equally distant, from the surface and centre of the sphere, and is called the imaginary focus of the mirror. _caroline._ pray, what is the meaning of focus? _mrs. b._ a point at which converging rays, unite. and it is in this case, called an imaginary focus; because the rays do not really unite at that point, but only appear to do so: for the rays do not pass through the mirror, since they are reflected by it. _emily._ i do not yet understand why an object appears smaller, when viewed in a convex mirror. _mrs. b._ it is owing to the divergence of the reflected rays. you have seen that a convex mirror, by reflection, converts parallel rays into divergent rays; rays that fall upon the mirror divergent, are rendered still more so by reflection, and convergent rays are reflected either parallel, or less convergent. if then, an object be placed before any part of a convex mirror, as the vase a b, fig. , for instance, the two rays from its extremities, falling convergent on the mirror, will be reflected less convergent, and will not come to a focus, till they arrive at c; then an eye placed in the direction of the reflected rays, will see the image formed in (or rather behind) the mirror, at _a b_. _caroline._ but the reflected rays, do not appear to me to converge less than the incident rays. i should have supposed that, on the contrary, they converged more, since they meet in a point. _mrs. b._ they would unite sooner than they actually do, if they were not less convergent than the incident rays: for observe, that if the incident rays, instead of being reflected by the mirror, continued their course in their original direction, they would come to a focus at d, which is considerably nearer to the mirror than at c; the image, is, therefore, seen under a smaller angle than the object; and the more distant the latter is from the mirror, the smaller is the image reflected by it. you will now easily understand the nature of the reflection of concave mirrors. these are formed of a portion of the internal surface of a hollow sphere, and their peculiar property is to converge the rays of light. can you discover, caroline, in what direction the three parallel rays, a b, c d, e f, are reflected, which fall on the concave mirror, m n, (fig. .)? _caroline._ i believe i can. the middle ray is sent back in the same line, in which it arrives, that being the direction of the axis of the mirror; and the two others will be reflected obliquely, as they fall obliquely on the mirror. i must now draw two dotted lines perpendicular to their points of incidence, which will divide their angles of incidence and reflection; and in order that those angles may be equal, the two oblique rays must be reflected to l, where they will unite with the middle ray. _mrs. b._ very well explained. thus you see, that when any number of parallel rays fall on a concave mirror, they are all reflected to a focus: for in proportion as the rays are more distant from the axis of the mirror, they fall more obliquely upon it, and are more obliquely reflected; in consequence of which they come to a focus in the direction of the axis of the mirror, at a point equally distant from the centre, and the surface, of the sphere; and this point is not an imaginary focus, as happens with the convex mirror, but is the true focus at which the rays unite. _emily._ can a mirror form more than one focus, by reflecting rays? _mrs. b._ yes. if rays fall convergent on a concave mirror, (fig. ,) they are sooner brought to a focus, l, than parallel rays; their focus is, therefore, nearer to the mirror m n. divergent rays are brought to a more distant focus than parallel rays, as in figure , where the focus is at l; but what is called the true focus of mirrors, either convex or concave, is that of parallel rays, and is equally distant from the centre, and the surface of the spherical mirror. i shall now show you the real reflection of rays of light, by a metallic concave mirror. this is one made of polished tin, which i expose to the sun, and as it shines bright, we shall be able to collect the rays into a very brilliant focus. i hold a piece of paper where i imagine the focus to be situated; you may see by the vivid spot of light on the paper, how much the rays converge: but it is not yet exactly in the focus; as i approach the paper to that point, observe how the brightness of the spot of light increases, while its size diminishes. _caroline._ that must be occasioned by the rays approaching closer together. i think you hold the paper just in the focus now, the light is so small and dazzling--oh, mrs. b., the paper has taken fire! _mrs. b._ the rays of light cannot be concentrated, without, at the same time, accumulating a proportional quantity of heat: hence concave mirrors have obtained the name of burning mirrors. _emily._ i have often heard of the surprising effects of burning mirrors, and i am quite delighted to understand their nature. _caroline._ it cannot be the true focus of the mirror, at which the rays of the sun unite, for as they proceed from so large a body, they cannot fall upon the mirror parallel to each other. _mrs. b._ strictly speaking, they certainly do not. but when rays, come from such an immense distance as the sun, they may be considered as parallel: their point of union is, therefore, the true focus of the mirror, and there the image of the object is represented. now that i have removed the mirror out of the influence of the sun's rays, if i place a burning taper in the focus, how will its light be reflected? (fig. .) _caroline._ that, i confess, i cannot say. _mrs. b._ the ray which falls in the direction of the axis of the mirror, is reflected back in the same line; but let us draw two other rays from the focus, falling on the mirror at b and f; the dotted lines are perpendicular to those points, and the two rays will, therefore, be reflected to a and e. _caroline._ oh, now i understand it clearly. the rays which proceed from a light placed in the focus of a concave mirror fall divergent upon it, and are reflected, parallel. it is exactly the reverse of the former experiment, in which the sun's rays fell parallel on the mirror, and were reflected to a focus. _mrs. b._ yes: when the incident rays are parallel, the reflected rays converge to a focus; when, on the contrary, the incident rays proceed from the focus, they are reflected parallel. this is an important law of optics, and since you are now acquainted with the principles on which it is founded, i hope that you will not forget it. _caroline._ i am sure that we shall not. but, mrs. b., you said that the image was formed in the focus of a concave mirror; yet i have frequently seen glass concave mirrors, where the object has been represented within the mirror, in the same manner as in a convex mirror. _mrs. b._ that is the case only, when the object is placed between the mirror and its focus; the image then appears magnified behind the mirror, or, as you would say, within it. _caroline._ i do not understand why the image should be larger than the object. _mrs. b._ this results from the convergent property of the concave mirror. if an object, a b, (fig. .) be placed between the mirror and its focus, the rays from its extremities fall divergent on the mirror, and on being reflected, become less divergent, as if they proceeded from c: to an eye placed in that situation, the image will appear magnified behind the mirror at _a b_, since it is seen under a larger angle than the object. you now, i hope, understand the reflection of light by opaque bodies. at our next meeting, we shall enter upon another property of light, no less interesting, and which is called _refraction_. questions . (pg. ) what is meant by the angle of vision, or the visual angle? . (pg. ) why do objects of the same size appear smaller when distant, than when near? . (pg. ) why do not two objects, known to be equal in size, appear to differ, when at different distances from the eye? . (pg. ) how is this exemplified, by a house seen through a window? . (pg. ) why do rows of trees, forming an avenue, appear to approach as they recede from the eye, until they eventually seem to meet? . (pg. ) in drawing a view from nature, what do we copy? . (pg. ) what is the difference in sculpture, in this respect? . (pg. ) excepting the rays from an object enter the eye, under a certain angle, they cannot be seen; what must this angle exceed? . (pg. ) what two circumstances may cause the angle to be so small, as not to produce vision? . (pg. ) motion may be so slow as to become imperceptible, what is said on this point? . (pg. ) under what circumstances may a body, moving with great rapidity, appear to be at rest? . (pg. ) upon what does the real velocity of a body, depend? . (pg. ) what must be known, to enable us to ascertain the real space contained in a degree? . (pg. ) what is explained by fig. , plate ? . (pg. ) what is said respecting the evidence afforded by our senses, and how do we correct the errors into which they would lead us? . (pg. ) an image of a visible object is formed upon the retina of each eye, why, therefore, are not objects seen double? . (pg. ) by what experiment can you prove that a separate image of an object is formed in each eye? . (pg. ) under what circumstances are objects seen double? . (pg. ) why is not the image of an object inverted in the common mirror? . (pg. ) your whole figure may be seen in a looking-glass, which is not more than half your height; how is this shown in fig. . plate ? . (pg. ) why is the image invisible to the person, when not standing directly before the glass? . (pg. ) in what situation may a second person see the image reflected? . (pg. ) in what direction will an object always appear to the eye? . (pg. ) how is this explained by fig. , plate ? . (pg. ) what is it that reflects the rays in a looking-glass? . (pg. ) all opaque bodies reflect some light, why do they not all act as mirrors? . (pg. ) what substances form the most perfect mirrors, and for what reason? . (pg. ) what are the three kinds of mirrors usually employed for optical purposes? . (pg. ) how are the rays of light affected by them? . (pg. ) what is the form of a convex mirror, and how do parallel rays fall upon it, as represented in fig. , plate ? . (pg. ) what is represented by the dotted line in the same figure? . (pg. ) explain by the figure, how the parallel rays will be reflected. . (pg. ) at what distance behind such a mirror, would an image, produced by parallel rays, be formed? . (pg. ) what is that point denominated? . (pg. ) what is meant by a focus? . (pg. ) why is the point behind the mirror, called the _imaginary focus_? . (pg. ) why does an object appear to be lessened by a convex mirror, (fig. .)? . (pg. ) what is a concave mirror, and what its peculiar property? . (pg. ) how are parallel rays reflected by a concave mirror, as explained by fig. , plate ? . (pg. ) where is the focus of parallel rays, in a concave mirror? . (pg. ) if rays fall on it convergent, how are they reflected? . (pg. ) how if divergent? . (pg. ) how, and why, may concave, become burning mirrors? . (pg. ) why may rays of light coming from the sun, be viewed as parallel to each other? . (pg. ) if a luminous body, as a burning taper, be placed in the focus of a concave mirror, how will the rays from it, be reflected? (fig. .) . (pg. ) what fact is explained by fig. , plate ? conversation xvi. on refraction and colours. transmission of light by transparent bodies. refraction. refraction by the atmosphere. refraction by a lens. refraction by the prism. of colour from the rays of light. of the colours of bodies. mrs. b. the refraction of light will furnish the subject of to-day's lesson. _caroline._ that is a property of which i have not the faintest idea. _mrs. b._ it is the effect which transparent mediums produce on light in its passage through them. opaque bodies, you know, reflect the rays, and transparent bodies transmit them; but it is found, that _if a ray, in passing from one medium, into another of different density, fall obliquely, it is turned out of its course. the ray of light is then said to be refracted._ _caroline._ it must then be acted on by some new power, otherwise it would not deviate from its first direction. _mrs. b._ the power which causes the deviation of the ray, appears to be the attraction of the denser medium. let us suppose the two mediums to be air, and water; if a ray of light passes from air, into water, it is more strongly attracted by the latter, on account of its superior density. _emily._ in what direction does the water attract the ray? _mrs. b._ the ray is attracted perpendicularly towards the water, in the same manner in which bodies are acted upon by gravity. if then a ray, a b, (fig. , plate .) fall perpendicularly on water, the attraction of the water acts in the same direction as the course of the ray: it will not, therefore, cause a deviation, and the ray will proceed straight on, to e. but if it fall obliquely, as the ray c b, the water will attract it out of its course. let us suppose the ray to have approached the surface of a denser medium, and that it there begins to be affected by its attraction; this attraction, if not counteracted by some other power, would draw it perpendicularly to the water, at b; but it is also impelled by its projectile force, which the attraction of the denser medium cannot overcome; the ray, therefore, acted on by both these powers, moves in a direction between them, and instead of pursuing its original course to d, or being implicitly guided by the water to e, proceeds towards f, so that the ray appears bent or broken. _caroline._ i understand that very well; and is not this the reason that oars appear bent in the water? _mrs. b._ it is owing to the refraction of the rays, reflected by the oar; but this is in passing from a dense, to a rare medium, for you know that the rays, by means of which you see the oar, pass from water into air. _emily._ but i do not understand why refraction takes place, when a ray passes from a dense into a rare medium; i should suppose that it would be less, attracted by the latter, than by the former. _mrs. b._ and it is precisely on that account that the ray is refracted. let the upper half of fig. , represent glass, and the lower half water, let c b represent a ray, passing obliquely from the glass, into water: glass, being the denser medium, the ray will be more strongly attracted by that which it leaves than by that which it enters. the attraction of the glass acts in the direction a b, while the impulse of projection would carry the ray to f; it moves, therefore, between these directions towards d. _emily._ so that a contrary refraction takes place, when a ray passes from a dense, into a rare medium. [illustration: plate xix.] _mrs. b._ the rule upon this subject is this; _when a ray of light passes from a rare into a dense medium, it is refracted towards the perpendicular; when from a dense into a rare medium, it is refracted from the perpendicular_. by the perpendicular is meant a line, at right angle with the refracting surface. this may be seen in fig. , and fig. , where the lines a e, are the perpendiculars. _caroline._ but does not the attraction of the denser medium affect the ray before it touches it? _mrs. b._ the distance at which the attraction of the denser medium acts upon a ray, is so small, as to be insensible; it appears, therefore, to be refracted only at the point at which it passes from one medium into the other. now that you understand the principle of refraction, i will show you the real refraction of a ray of light. do you see the flower painted at the bottom of the inside of this tea-cup? (fig. .) _emily._ yes.--but now you have moved it just out of sight; the rim of the cup hides it. _mrs. b._ do not stir. i will fill the cup with water, and you will see the flower again. _emily._ i do, indeed! let me try to explain this: when you drew the cup from me, so as to conceal the flower, the rays reflected by it, no longer met my eyes, but were directed above them; but now that you have filled the cup with water, they are refracted, and bent downwards when passing out of the water, into the air, so as again to enter my eyes. _mrs. b._ you have explained it perfectly: fig. . will help to imprint it on your memory. you must observe that when the flower becomes visible by the refraction of the ray, you do not see it in the situation which it really occupies, but the image of the flower appears higher in the cup; for as objects always appear to be situated in the direction of the rays which enter the eye, the flower will be seen at b, in the direction of the refracted ray. _emily._ then, when we see the bottom of a clear stream of water, the rays which it reflects, being refracted in their passage from the water into the air, will make the bottom appear higher than it really is. _mrs. b._ and the water will consequently appear more shallow. accidents have frequently been occasioned by this circumstance; and boys, who are in the habit of bathing, should be cautioned not to trust to the apparent shallowness of water, as it will always prove deeper than it appears. the refraction of light prevents our seeing the heavenly bodies in their real situation: the light they send to us being refracted in passing into the atmosphere, we see the sun and stars in the direction of the refracted ray; as described in fig. , plate ., the dotted line represents the extent of the atmosphere, above a portion of the earth, e b e: a ray of light coming from the sun s, falls obliquely on it, at a, and is refracted to b; then, since we see the object in the direction of the refracted ray, a spectator at b, will see an image of the sun at c, instead of its real situation, at s. _emily._ but if the sun were immediately over our heads, its rays, falling perpendicularly on the atmosphere, would not be refracted, and we should then see the real sun, in its true situation. _mrs. b._ you must recollect that the sun, is vertical only to the inhabitants of the torrid zone; its rays, therefore, are always refracted, in this latitude. there is also another obstacle to our seeing the heavenly bodies in their real situations: light, though it moves with extreme velocity, is about eight minutes and a quarter, in its passage from the sun to the earth; therefore, when the rays reach us, the sun must have quitted the spot he occupied on their departure; yet we see him in the direction of those rays, and consequently in a situation which he had abandoned eight minutes and a quarter, before. _emily._ when you speak of the sun's motion, you mean, i suppose, his apparent motion, produced by the diurnal motion of the earth? _mrs. b._ certainly; the effect being the same, whether it is our earth, or the heavenly bodies, which move: it is more easy to represent things as they appear to be, than as they really are. _caroline._ during the morning, then, when the sun is rising towards the meridian, we must (from the length of time the light is in reaching us) see an image of the sun below that spot which it really occupies. _emily._ but the refraction of the atmosphere, counteracting this effect, we may, perhaps, between the two, see the sun in its real situation. _caroline._ and in the afternoon, when the sun is sinking in the west, refraction, and the length of time which the light is in reaching the earth, will conspire to render the image of the sun, higher than it really is. _mrs. b._ the refraction of the sun's rays, by the atmosphere, prolongs our days, as it occasions our seeing an image of the sun, both before he rises, and after he sets; when below our horizon, he still shines upon the atmosphere, and his rays are thence refracted to the earth: so likewise we see an image of the sun, previously to his rising, the rays that fall upon the atmosphere being refracted to the earth. _caroline._ on the other hand, we must recollect that light is eight minutes and a quarter on its journey; so that, by the time it reaches the earth, the sun may, perhaps, have risen above the horizon. _emily._ pray, do not glass windows, refract the light? _mrs. b._ they do; but this refraction would not be perceptible, were the surfaces of the glass, perfectly flat and parallel, because, in passing through a pane of glass, the rays suffer two refractions, which, being in contrary directions, produce nearly the same effect as if no refraction had taken place. _emily._ i do not understand that. _mrs. b:_ fig. , plate , will make it clear to you: a a represents a thick pane of glass, seen edgeways. when the ray b approaches the glass, at c, it is refracted by it; and instead of continuing its course in the same direction, as the dotted line describes, it passes through the pane, to d; at that point returning into the air, it is again refracted by the glass, but in a contrary direction to the first refraction, and in consequence proceeds to e. now you must observe that the ray b c and the ray d e being parallel, the light does not appear to have suffered any refraction: the apparent, differing so little from the true place of any object, when seen through glass of ordinary thickness. _emily._ so that the effect which takes place on the ray entering the glass, is undone on its quitting it. or, to express myself more scientifically, when a ray of light passes from one medium into another, and through that into the first again, the two refractions being equal, and in opposite directions, no sensible effect is produced. _caroline._ i think the effect is very sensible, for, in looking through the glass of the window, i see objects very much distorted; articles which i know to be straight, appear bent and broken, and sometimes the parts seem to be separated to a distance from each other. _mrs. b._ that is because common window glass is not flat, its whole surface being uneven. rays from any object, falling upon it under different angles, are, consequently, refracted in various ways, and thus produce the distortion you have observed. _emily._ is it not in consequence of refraction, that the glasses in common spectacles, magnify objects seen through them? _mrs. b._ yes. glasses of this description are called _lenses_; of these, there are several kinds, the names of which it will be necessary for you to learn. every lens is formed of glass, ground so as to form a segment of a sphere, on one, or both sides. they are all represented at fig. , plate . the most common, is the _double convex_ lens, d. this is thick in the middle, and thin at the edges, like common spectacles, or reading glasses. a b, is a _plano-convex_ lens, being flat on one side, and convex on the other. e is a _double concave_, being, in all respects, the reverse of d. c is a _plano-concave_, flat on one side, and concave on the other. f is called a _meniscus_, or _concavo-convex_, being concave on one, and convex on the other side. a line passing through the centre of a lens, is called its _axis_. _caroline._ i should like to understand how the rays of light are refracted, by means of a lens. _mrs. b._ when parallel rays (fig. ) fall on a double convex _lens_, that only, which falls in the direction of the axis of the lens, is perpendicular to the surface; the other rays, falling obliquely, are refracted towards the axis, and will meet at a point beyond the lens, called its _focus_. of the three rays, a b c, which fall on the lens d e, the rays a and c are refracted in their passage through it, to _a_, and _c_; and on quitting the lens, they undergo a second refraction in the same direction, which unites them with the ray b, at the focus f. _emily._ and what is the distance of the focus, from the surface of the lens? _mrs. b._ the focal distance depends both upon the form of the lens, and on the refracting power of the substance of which it is made: in a glass lens, both sides of which are equally convex, the focus is situated nearly at the centre of the sphere, of which the surface of the lens forms a portion; it is at the distance, therefore, of the radius of the sphere. the property of those lenses which have a convex surface, is to collect the rays of light to a focus; and of those which have a concave surface, on the contrary, to disperse them. for the rays a and c, falling on the concave lens x y, (fig. , plate .) instead of converging towards the ray b, in the axis of the lens, will each be attracted towards the thick edges of the lens, both on entering and quitting it, and will, therefore, by the first refraction, be made to diverge to _a_, _c_, and by the seconds, to _d_, _e_. [illustration: plate xx.] _caroline._ and lenses which have one side flat, and the other convex, or concave, as a and b, (fig. , plate .) are, i suppose, less powerful in their refractions? _mrs. b._ yes; the focus of the plano-convex, is at the distance of the diameter of a sphere, of which the convex surface of the lens, forms a portion; as represented in figure , plate . the three parallel rays, a b c, are brought to a focus by the plano-convex lens, x y, at f. _emily._ you have not explained to us, mrs. b., how the lens serves to magnify objects. _mrs. b._ by turning again to fig. , plate . you will readily understand this. let a c, be an object placed before the lens, and suppose it to be seen by an eye at f; the ray from the point a, will be seen in the direction f g, that from c, in the direction f h; the visual angle, therefore, will be greatly increased, and the object must appear larger, in proportion. i must now explain to you the refraction of a ray of light, by a triangular piece of glass, called a prism. (fig. .) _emily._ the three sides of this glass are flat; it cannot, therefore, bring the rays to a focus; nor do i suppose that its refraction will be similar to that of a flat pane of glass, because it has not two sides parallel; i cannot, therefore, conjecture what effect the refraction by a prism, can produce. _mrs. b._ the refractions of the ray, both on entering and on quitting the prism, are in the same direction, (fig. .) on entering the prism p, the ray a is refracted from b to c, and on quitting it from c to d. in the first instance it is refracted towards, and in the last, from the perpendicular; each causing it to deviate in the same way, from its original course, a b. i will show you this by experiment; but for this purpose it will be advisable to close the window-shutters, and admit, through the small aperture, a ray of light, which i shall refract, by means of this prism. _caroline._ oh, what beautiful colours are represented on the opposite wall! there are all the colours of the rainbow, and with a brightness, i never saw equalled. (fig. , plate .) _emily._ i have seen an effect, in some respects similar to this, produced by the rays of the sun shining upon glass lustres; but how is it possible that a piece of white glass can produce such a variety of brilliant colours? _mrs. b._ the colours are not formed by the prism, but existed in the ray previously to its refraction. _caroline._ yet, before its refraction, it appeared perfectly white. _mrs. b._ the white rays of the sun, are composed of rays, which, when separated, produce all these colours, although when blended together, they appear colourless or white. sir isaac newton, to whom we are indebted for the most important discoveries respecting light and colours, was the first who divided a white ray of light, and found it to consist of an assemblage of coloured rays, which formed an image upon the wall, such as you now see exhibited, (fig. .) in which are displayed the following series of colours: red, orange, yellow, green, blue, indigo, and violet. _emily._ but how does a prism separate these coloured rays? _mrs. b._ by refraction. it appears that the coloured rays have different degrees of refrangibility; in passing through the prism, therefore, they take different directions according to their susceptibility of refraction. the violet rays deviate most from their original course; they appear at one of the ends of the spectrum, a b: contiguous to the violet, are the blue rays, being those which have somewhat less refrangibility; then follow, in succession, the green, yellow, orange, and lastly, the red, which are the least refrangible of the coloured rays. _caroline._ i cannot conceive how these colours, mixed together, can become white? _mrs. b._ that i cannot pretend to explain: but it is a fact that the union of these colours, in the proportions in which they appear in the spectrum, produce in us the idea of whiteness. if you paint a circular piece of card, in compartments, with these seven colours, as nearly as possible in the proportion, and of the shade exhibited in the spectrum, and whirl it rapidly on a pin, it will appear white; as the velocity of the motion, will have the effect of blending the colours, in the impression which they make upon the eye. but a more decisive proof of the composition of a white ray is afforded, by reuniting these coloured rays, and forming with them, a ray of white light. _caroline._ if you can take a ray of white light to pieces, and put it together again, i shall be quite satisfied. _mrs. b._ this can be done by letting the coloured rays, which have been separated by a prism, fall upon a lens, which will converge them to a focus; and if, when thus reunited, we find that they appear white as they did before refraction, i hope you will be convinced that the white rays, are a compound of the several coloured rays. the prism p, you see, (fig. .) separates a ray of white light, into seven coloured rays, and the lens l l brings them to a focus at f, where they again appear white. _caroline._ you succeed to perfection: this is indeed a most interesting and conclusive experiment. _emily._ yet, mrs. b., i cannot help thinking, that there may, perhaps, be but three distinct colours in the spectrum, red, yellow, and blue; and that the four others may consist of two of these colours blended together; for, in painting, we find, that by mixing red and yellow, we produce orange; with different proportions of red and blue, we make violet or any shade of purple; and yellow, and blue, form green. now, it is very natural to suppose, that the refraction of a prism, may not be so perfect as to separate the coloured rays of light completely, and that those which are contiguous, in order of refrangibility, may encroach on each other, and by mixing, produce the intermediate colours, orange, green, violet, and indigo. _mrs. b._ your observation is, i believe, neither quite wrong, nor quite right. dr. wollaston, who has performed many experiments on the refraction of light, in a more accurate manner than had been previously done, by receiving a very narrow line of light on a prism, found that it formed a spectrum, consisting of rays of four colours only; but they were not exactly those you have named as primitive colours, for they consisted of red, green, blue, and violet. a very narrow line of yellow was visible, at the limit of the red and green, which dr. wollaston attributed to the overlapping of the edges of the red and green light. _caroline._ but red and green mixed together, do not produce yellow? _mrs. b._ not in painting; but it may be so in the primitive rays of the spectrum. dr. wollaston observed, that, by increasing the breadth of the aperture, by which the line of light was admitted, the space occupied by each coloured ray in the spectrum, was augmented, in proportion as each portion encroached on the neighbouring colour, and mixed with it; so that the intervention of orange and yellow, between the red and green, is owing, he supposes, to the mixture of these two colours; and the blue is blended on the one side with the green, and on the other with the violet, forming the spectrum, as it was originally observed by sir isaac newton, and which i have just shown you. the rainbow, which exhibits a series of colours, so analogous to those of the spectrum, is formed by the refraction of the sun's rays, in their passage through a shower of rain; every drop of which acts as a prism, in separating the coloured rays as they pass through it; the combined effect of innumerable drops, produces the bow, which you know can be seen, only when there are both rain, and sunshine. _emily._ pray, mrs. b., cannot the sun's rays be collected to a focus by a lens, in the same manner as they are by a concave mirror? _mrs. b._ the same effect in concentrating the rays, is produced by the refraction with a lens, as by the reflection from a concave mirror: in the first, the rays pass through the glass and converge to a focus, behind it, in the latter, they are reflected from the mirror, and brought to a focus, before it. a lens, when used for the purpose of collecting the sun's rays, is called a burning glass. i have before explained to you, the manner in which a convex lens, refracts the rays, and brings them to a focus; (fig. , plate .) as these rays contain both light and heat, the latter, as well as the former, is refracted; and intense heat, as well as light, will be found in the focal point. the sun now shines very bright; if we let the rays fall on this lens, you will perceive the focus. _emily._ oh yes: the point of union of the rays, is very luminous. i will hold a piece of paper in the focus, and see if it will take fire. the spot of light is extremely brilliant, but the paper does not burn? _mrs. b._ try a piece of brown paper;--that, you see, takes fire almost immediately. _caroline._ this is surprising; for the light appeared to shine more intensely, on the white, than on the brown paper. _mrs. b._ the lens collects an equal number of rays to a focus, whether you hold the white or the brown paper, there; but the white paper appears more luminous in the focus, because most of the rays, instead of entering into the paper, are reflected by it; and this is the reason that the paper does not readily take fire: whilst, on the contrary, the brown paper, which absorbs more light and heat than it reflects, soon becomes heated and takes fire. _caroline._ this is extremely curious; but why should brown paper, absorb more rays, than white paper? _mrs. b._ i am far from being able to give a satisfactory answer to that question. we can form but mere conjecture on this point; it is supposed that the tendency to absorb, or reflect rays, depends on the arrangement of the minute particles of the body, and that this diversity of arrangement renders some bodies susceptible of reflecting one coloured ray, and absorbing the others; whilst other bodies, have a tendency to reflect all the colours, and others again, to absorb them all. _emily._ and how do you know which colours bodies have a tendency to reflect, or which to absorb? _mrs. b._ because a body always appears to be of the colour which it reflects; for, as we see only by reflected rays, it can appear of the colour of those rays, only. _caroline._ but we see all bodies of their own natural colour, mrs. b.; the grass and trees, green; the sky, blue; the flowers of various hues. _mrs. b._ true; but why is the grass green?--because it absorbs all, except the green rays; it is, therefore, these only which the grass and trees reflect to our eyes, and this makes them appear green. the flowers, in the same manner, reflect the various colours of which they appear to us; the rose, the red rays; the violet, the blue; the jonquil, the yellow, &c. _caroline._ but these are the permanent colours of the grass and flowers, whether the sun's rays shine on them or not. _mrs. b._ whenever you see those colours, the flowers must be illumined by some light; and light, from whatever source it proceeds, is of the same nature; composed of the various coloured rays which paint the grass, the flowers, and every coloured object in nature. _caroline._ but, mrs. b., the grass is green, and the flowers are coloured, whether in the dark, or exposed to the light? _mrs. b._ why should you think so? _caroline._ it cannot be otherwise. _mrs. b._ a most philosophical reason indeed! but, as i never saw them in the dark, you will allow me to dissent from your opinion. _caroline._ what colour do you suppose them to be, then, in the dark? _mrs. b._ none at all; or black, which is the same thing. you can never see objects, without light. white light is compounded of rays, from which all the colours in nature are produced; there, therefore, can be no colour without light; and though a substance is black, or without colour, in the dark, it may become coloured, as soon as it becomes visible. it is visible, indeed, only by the coloured rays which it reflects; therefore, we can see it only when coloured. _caroline._ all you say seems very true, and i know not what to object to it; yet it appears at the same time incredible! what, mrs. b., are we all as black as negroes in the dark? you make me shudder at the thought. _mrs. b._ your vanity need not be alarmed at the idea, as you are certain of never being seen, in that state. _caroline._ that is some consolation, undoubtedly; but what a melancholy reflection it is, that all nature which appears so beautifully diversified with colours, is really one uniform mass of blackness! _mrs. b._ is nature less pleasing for being coloured, as well as illumined, by the rays of light? and are colours less beautiful, for being accidental, rather than essential properties of bodies? providence seems to have decorated nature with the enchanting diversity of colours, which we so much admire, for the sole purpose of beautifying the scene, and rendering it a source of sensible gratification: it is an ornament which embellishes nature, whenever we behold her. what reason is there to regret, that she does not wear it when she is invisible? _emily._ i confess, mrs. b., that i have had my doubts, as well as caroline, though she has spared me the pains of expressing them: but i have just thought of an experiment, which, if it succeed, will, i am sure, satisfy us both. it is certain, that we cannot see bodies in the dark, to know whether they have then any colour. but we may place a coloured body in a ray of light, which has been refracted by a prism; and if your theory is true, the body, of whatever colour it naturally is, must appear of the colour of the ray in which it is placed; for since it receives no other coloured rays, it can reflect no others. _caroline._ oh! that is an excellent thought, emily; will you stand the test, mrs. b.? _mrs. b._ i consent: but we must darken the room, and admit only the ray which is to be refracted; otherwise, the white rays will be reflected on the body under trial, from various parts of the room. with what do you choose to make the experiment? _caroline._ this rose: look at it, mrs. b., and tell me whether it is possible to deprive it of its beautiful colour? _mrs. b._ we shall see.--i expose it first to the red rays, and the flower appears of a more brilliant hue; but observe the green leaves---- _caroline._ they appear neither red nor green; but of a dingy brown with a reddish glow? _mrs. b._ they cannot appear green, because they have no green rays to reflect; neither are they red, because green bodies absorb most of the red rays. but though bodies, from the arrangement of their particles, have a tendency to absorb some rays, and reflect others, yet it is not natural to suppose, that bodies are so perfectly uniform in their arrangement, as to reflect only pure rays of one colour, and perfectly to absorb the others; it is found, on the contrary, that a body reflects, in great abundance, the rays which determine its colour, and the others in a greater or less degree, in proportion as they are nearer to or further from its own colour, in the order of refrangibility. the green leaves of the rose, therefore, will reflect a few of the red rays, which, blended with their natural blackness, give them that brown tinge: if they reflected none of the red rays, they would appear perfectly black. now i shall hold the rose in the blue rays---- _caroline._ oh, emily, mrs. b. is right! look at the rose: it is no longer red, but of a dingy blue colour. _emily._ this is the most wonderful, of any thing we have yet learnt. but, mrs. b., what is the reason that the green leaves, are of a brighter blue than the rose? _mrs. b._ the green leaves reflect both blue and yellow rays, which produce a green colour. they are now in a coloured ray, which they have a tendency to reflect; they, therefore, reflect more of the blue rays than the rose, (which naturally absorbs that colour,) and will, of course, appear of a brighter blue. _emily._ yet, in passing the rose through the different colours of the spectrum, the flower takes them more readily than the leaves. _mrs. b._ because the flower is of a paler hue. bodies which reflect all the rays, are white; those which absorb them all, are black: between these extremes, bodies appear lighter or darker, in proportion to the quantity of rays they reflect or absorb. this rose is of a pale red; it approaches nearer to white than to black, and therefore, reflects rays, more abundantly than it absorbs them. _emily._ but if a rose has so strong a tendency to reflect rays, i should imagine that it would be of a deep red colour. _mrs. b._ i mean to say, that it has a general tendency to reflect rays. pale coloured bodies, reflect all the coloured rays to a certain degree, their paleness, being an approach towards whiteness: but they reflect one colour more than the rest: this predominates over the white, and determines the colour of the body. since, then, bodies of a pale colour, in some degree reflect all the rays of light, in passing through the various colours of the spectrum, they will reflect them all, with tolerable brilliancy; but will appear most vivid, in the ray of their natural colour. the green leaves, on the contrary, are of a dark colour, bearing a stronger resemblance to black, than to white; they have, therefore, a greater tendency to absorb, than to reflect rays; and reflecting very few of any, but the blue, and yellow rays, they will appear dingy, in passing through the other colours of the spectrum. _caroline._ they must, however, reflect great quantities of the green rays, to produce so deep a colour. _mrs. b._ deepness or darkness of colour, proceeds rather from a deficiency, than an abundance of reflected rays. remember, that if bodies reflected none of the rays, they would be black; and if a body reflects only a few green rays, it will appear of a dark green; it is the brightness, and intensity of the colour, which show that a great quantity of rays are reflected. _emily._ a white body, then, which reflects all the rays, will appear equally bright in all the colours of the spectrum. _mrs. b._ certainly. and this is easily proved by passing a sheet of white paper, through the rays of the spectrum. white, you perceive, results from a body reflecting all the rays which fall upon it; black, is produced, when they are all absorbed; and colour, arises from a body possessing the power to decompose the solar ray, by absorbing some parts, and reflecting others. _caroline._ what is the reason that articles which are blue, often appear green, by candle-light? _mrs. b._ the light of a candle, is not of so pure a white as that of the sun: it has a yellowish tinge, and when refracted by the prism, the yellow rays predominate; and blue bodies reflect some of the yellow rays, from their being next to the blue, in the order of refrangibility; the superabundance of yellow rays, which is supplied by the candle, gives to blue bodies, a greenish hue. _caroline._ candle-light must then give to all bodies, a yellowish tinge, from the excess of yellow rays; and yet it is a common remark, that people of a sallow complexion, appear fairer, or whiter, by candle-light. _mrs. b._ the yellow cast of their complexion is not so striking, when every surrounding object has a yellow tinge. _emily._ pray, why does the sun appear red, through a fog? [illustration: plate xxi.] _mrs. b._ it is supposed to be owing to the rays, which are most refrangible, being also the most easily reflected: in passing through an atmosphere, loaded with moisture, as in foggy weather, and also in the morning and evening, when mists prevail, the _violet_, _indigo_, _blue_, and _green_ rays, are reflected back by the particles which load the air; whilst the _yellow_, _orange_, and _red_ rays, being less susceptible of reflection, pass on, and reach the eye. _caroline._ and, pray, why is the sky of a blue colour? _mrs. b._ you should rather say, the atmosphere; for the sky is a very vague term, the meaning of which, it would be difficult to define, philosophically. _caroline._ but the colour of the atmosphere should be white, since all the rays traverse it, in their passage to the earth. _mrs. b._ do not forget that the direct rays of light which pass from the sun to the earth, do not meet our eyes, excepting when we are looking at that luminary, and thus intercept them; in which case, you know, that the sun appears white. the atmosphere is a transparent medium, through which the sun's rays pass freely to the earth; but the particles of which it is composed, also reflect the rays of light, and it appears that they possess the property of reflecting the blue rays, the most copiously: the light, therefore, which is reflected back into the atmosphere, from the surface of the earth, falls upon these particles of air, and the blue rays are returned by reflection: this reflection is performed in every possible direction; so that whenever we look at the atmosphere, some of these rays fall upon our eyes; hence we see the air of a blue colour. if the atmosphere did not reflect any rays, though the objects, on the surface of the earth, would be illuminated, the sky would appear perfectly black. _caroline._ oh, how melancholy would that be; and how pernicious to the sight, to be constantly viewing bright objects against a black sky. but what is the reason that bodies often change their colour; as leaves, which wither in autumn, or a spot of ink, which produces an iron-mould on linen? _mrs. b._ it arises from some chemical change, which takes place in the arrangement of the component parts; by which they lose their tendency to reflect certain colours, and acquire the power of reflecting others. a withered leaf thus no linger reflects the blue rays; it appears, therefore, yellow, or has a slight tendency to reflect several rays, which produce a dingy brown colour. an ink spot on linen, at first absorbs all the rays; but, from the action of soap, or of some other agent, it undergoes a chemical change, and the spot partially regains its tendency to reflect colours, but with a preference to reflect the yellow rays, and such is the colour of the iron-mould. _emily._ bodies, then, far from being of the colour which they appear to possess, are of that colour to which they have the greatest aversion, with which they will not incorporate, but reject, and drive from them. _mrs. b._ it certainly is so; though i scarcely dare venture to advance such an opinion, whilst caroline is contemplating her beautiful rose. _caroline._ my poor rose! you are not satisfied with depriving it of colour, but even make it have an aversion to it; and i am unable to contradict you. _emily._ since dark bodies, absorb more solar rays than light ones, the former should sooner be heated if exposed to the sun? _mrs. b._ and they are found, by experience, to be so. have you never observed a black dress, to be warmer than a white one? _emily._ yes, and a white one more dazzling: the black is heated by absorbing the rays, the white is dazzling, by reflecting them. _caroline._ and this was the reason that the brown paper was burnt in the focus of the lens, whilst the white paper exhibited the most luminous spot, but did not take fire. _mrs. b._ it was so. it is now full time to conclude our lesson. at our next meeting, i shall give you a description of the eye. questions . (pg. ) what is meant by the refraction of light? . (pg. ) what is believed to be the cause of refraction? . (pg. ) how is a ray refracted in passing obliquely from air into water? . (pg. ) how is this refraction explained in fig. , plate ? . (pg. ) what is fig. intended to explain? . (pg. ) what is the rule respecting refraction, by different mediums? . (pg. ) what is meant by the perpendicular? . (pg. ) how does fig. , plate , elucidate the law of refraction? . (pg. ) what will be the effect on the apparent situation of the flower? . (pg. ) what effect has refraction upon the apparent depth of a stream of water? . (pg. ) how does the atmosphere refract the rays of the sun, as represented, fig. ? . (pg. ) why have we the rays of the sun always refracted? . (pg. ) what length of time is required for light to travel from the sun, to the earth? . (pg. ) what effect has this upon his apparent place? . (pg. ) how is the length of the day affected by refraction? . (pg. ) how are rays refracted, which fall obliquely upon a flat pane of glass, (fig. , plate ?) . (pg. ) what is the reason that objects are distorted, when seen through common window glass? . (pg. ) what is meant by a lens? . (pg. ) what are the five kinds called, represented at fig. , plate ? . (pg. ) what is meant by the axis of a lens? . (pg. ) how are parallel rays, refracted by the double convex lens, fig. , plate ? . (pg. ) what is meant by the focus of a lens? . (pg. ) what is the focal distance of parallel rays, from a double convex lens? . (pg. ) how are the rays refracted by a concave lens, fig. , plate ? . (pg. ) what is the effect of one plane side in a lens? . (pg. ) how is the focus of the plano-convex lens situated, fig. , plate ? . (pg. ) how does a convex lens magnify objects, fig. , plate ? . (pg. ) what is the article denominated which is represented at fig. , plate ? . (pg. ) how will a ray be refracted, which enters on one side of the prism, in the direction a b? . (pg. ) what effect is produced by this refraction, as represented in fig. , plate ? . (pg. ) of what are the rays of white light said to be composed? . (pg. ) what colours are produced? . (pg. ) by what property, in light, does refraction enable us to separate these different rays? . (pg. ) what experiment may be performed with a piece of card, so as to exemplify the compound nature of light? . (pg. ) how can the same be shown by a lens, fig. . plate ? . (pg. ) is it certain that there are seven primitive colours in the spectrum? . (pg. ) how is the rainbow produced, and what is necessary to its production? . (pg. ) how are the solar rays affected by a convex lens? . (pg. ) why is such a lens, called a burning glass? . (pg. ) why are bodies of a dark colour, more readily inflamed, than those which are white? . (pg. ) what is believed to be the reason, why some bodies absorb more rays than others? . (pg. ) what determines the colour of any particular body? . (pg. ) what exemplifications are given? . (pg. ) by what reasoning is it proved, that bodies do not retain their colours in the dark? . (pg. ) what proof of the truth of this theory of colours, may be afforded by the prism? . (pg. ) why will green leaves, when exposed to the red ray, appear of a dingy brown? . (pg. ) bodies, in general, when placed in a ray differing in colour from their own, appear of a mixed hue, what causes this? . (pg. ) why will bodies of a pale, or light hue, most perfectly, assume the different colours of the spectrum? . (pg. ) upon what property in a body, does the darkness of its colour depend? . (pg. ) why do some bodies appear white, others black, and others of different colours? . (pg. ) from what cause do blue articles appear green, by candle-light? . (pg. ) what is believed to be the cause, of the red appearance of the sun, through a fog, or misty atmosphere? . (pg. ) from what is the blue colour of the sky, thought to arise? . (pg. ) what would be the colour of the sky, did not the atmosphere reflect light? . (pg. ) from what cause do some bodies change their colour, as leaves formerly green, become brown, and ink, yellow? . (pg. ) why is a black dress, warmer in the sunshine, than a white one of the same texture? conversation xvii. on the structure of the eye, and optical instruments. description of the eye. of the image on the retina. refraction by the humours of the eye. of the use of spectacles. of the single microscope. of the double microscope. of the solar microscope. magic lanthorn. refracting telescope. reflecting telescope. mrs. b. the body of the eye, is of a spherical form: (fig. . plate .) it has two membranous coats, or coverings; the external one, _a a a_, is called the sclerotica, this is commonly known under the name of the white of the eye; it has a projection in that part of the eye which is exposed to view, _b b_, which is called the transparent cornea, because, when dried, it has nearly the consistence of very fine horn, and is sufficiently transparent for the light to obtain free passage through it. the second membrane which lines the cornea, and envelops the eye, is called the choroid, _c c c_; this has an opening in front, just beneath the cornea, which forms the pupil, or sight of the eye, _d d_, through which the rays of light pass into the eye. the pupil is surrounded by a coloured border called the iris, _e e_, which, by its muscular motion, always preserves the pupil of a circular form, whether it is expanded in the dark, or contracted by a strong light. this you will understand better by examining fig. . _emily._ i did not know that the pupil was susceptible of varying its dimensions. _mrs. b._ the construction of the eye is so admirable, that it is capable of adapting itself, more or less, to the circumstances in which it is placed. in a faint light, the pupil dilates so as to receive an additional quantity of rays, and in a strong light, it contracts, in order to prevent the intensity of the light from injuring the optic nerve. observe emily's eyes, as she sits looking towards the windows: the pupils appear very small, and the iris, large. now, emily, turn from the light, and cover your eyes with your hand, so as entirely to exclude it, for a few moments. _caroline._ how very much the pupils of her eyes are now enlarged, and the iris diminished! this is, no doubt, the reason why the eyes suffer pain, when from darkness, they suddenly come into a strong light; for the pupil being dilated, a quantity of rays must rush in, before it has time to contract. _emily._ and when we go from a strong light, into obscurity, we at first imagine ourselves in total darkness; for a sufficient number of rays cannot gain admittance into the contracted pupil, to enable us to distinguish objects: but in a few minutes it dilates, and we clearly perceive objects which were before invisible. _mrs. b._ it is just so. the choroid _c c_, is embued with a black liquor, which serves to absorb all the rays that are irregularly reflected, and to convert the body of the eye, into a more perfect camera obscura. when the pupil is expanded to its utmost extent, it is capable of admitting ten times the quantity of light, that it does when most contracted. in cats, and animals which are said to see in the dark, the power of dilatation and contraction of the pupil, is still greater; it is computed that the pupils of their eyes may admit one hundred times more light at one time than at another. within these coverings of the eye-ball, are contained, three transparent substances, called humours. the first occupies the space immediately behind the cornea, and is called the aqueous humour, _f f_, from its liquidity and its resemblance to water. beyond this, is situated the crystalline humour, _g g_, so called from its clearness and transparency: it has the form of a lens, and refracts the rays of light in a greater degree of perfection, than any that have been constructed by art: it is attached by two muscles, _m m_, to each side of the choroid. the back part of the eye, between the crystalline humour and the retina, is filled by the vitreous humour, _h h_, which derives its name from a resemblance it is supposed to bear, to glass, or vitrified substances. [illustration: plate xxii.] the membranous coverings of the eye are intended chiefly for the preservation of the retina, _i i_, which is by far the most important part of the eye, as it is that which receives the impression of the objects of sight, and conveys it to the mind. the retina is formed by the expansion of the optic nerve, and is of a most perfect whiteness: this nerve proceeds from the brain, enters the eye, at _n_, on the side next the nose, and is finely spread over the interior surface of the choroid. the rays of light which enter the eye, by the pupil, are refracted by the several humours in their passage through them, and unite in a focus on the retina. _caroline._ i do not understand the use of these refracting humours: the image of objects was represented in the camera obscura, without any such assistance. _mrs. b._ that is true; but the representation became much more strong and distinct, when we enlarged the opening of the camera obscura, and received the rays into it, through a lens. i have told you, that rays proceed from bodies in all possible directions. we must, therefore, consider every part of an object which sends rays to our eyes, as points from which the rays diverge, as from a centre. _emily._ these divergent rays, issuing from a single point, i believe you told us, were called a pencil of rays? _mrs. b._ yes. now, divergent rays, on entering the pupil, do not cross each other; the pupil, however, is sufficiently large to admit a small pencil of them; and these, if not refracted to a focus, by the humours, would continue diverging after they had passed the pupil, would fall dispersed upon the retina, and thus the image of a single point, would be expanded over a large portion of the retina. the divergent rays from every other point of the object, would be spread over a similar extent of space, and would interfere and be confounded with the first; so that no distinct image could be formed, and the representation on the retina would be confused, both in figure and colour. fig. . represents two pencils of rays, issuing from two points of the tree, a b, and entering the pupil c, refracted by the crystalline humour d, and forming on the retina, at _a b_, distinct images of the spot they proceed from. fig. . differs from the preceding, merely from not being supplied with a lens; in consequence of which, the pencils of rays are not refracted to a focus, and no distinct image is formed on the retina. i have delineated only the rays issuing from two points of an object, and distinguished the two pencils in fig. . by describing one of them with dotted lines: the interference of these two pencils of rays on the retina, will enable you to form an idea of the confusion which would arise, from thousands and millions of points, at the same instant pouring their divergent rays upon the retina. _emily._ true; but i do not yet well understand, how the refracting humours, remedy this imperfection. _mrs. b._ the refraction of these several humours, unites the whole of a pencil of rays, proceeding from any one point of an object, to a corresponding point on the retina, and the image is thus rendered distinct and strong. if you conceive, in fig. ., every point of the tree to send forth a pencil of rays, similar to those from a b, every part of the tree will be as accurately represented on the retina, as the points _a b_. _emily._ how admirably, how wonderfully, is this contrived! _caroline._ but since the eye absolutely requires refracting humours, in order to have a distinct representation formed on the retina, why is not the same refraction equally necessary, for the images formed in the camera obscura? _mrs. b._ it is; excepting the aperture through which we receive the rays into the camera obscura, is extremely small; so that but very few of the rays diverging from a point, gain admittance; but when we enlarged the aperture, and furnished it with a lens, you found the landscape more perfectly represented. _caroline._ i remember how obscure and confused the image was, when you enlarged the opening, without putting in the lens. _mrs. b._ such, or very similar, would be the representation on the retina, unassisted by the refracting humours. you will now be able to understand the nature of that imperfection of sight, which arises from the eyes being too prominent. in such cases, the crystalline humour, d, (fig. .) being extremely convex, refracts the rays too much, and collects a pencil, proceeding from the object a b, into a focus, f, before they reach the retina. from this focus, the rays proceed, diverging, and consequently form a very confused image on the retina, at _a b_. this is the defect in short-sighted people. _emily._ i understand it perfectly. but why is this defect remedied by bringing the object nearer to the eye, as we find to be the case with short-sighted people? _mrs. b._ the nearer you bring an object to your eye, the more divergent the rays fall upon the crystalline humour, and consequently they are not so soon converged to a focus: this focus, therefore, either falls upon the retina, or at least approaches nearer to it, and the object is proportionally distinct, as in fig. . _emily._ the nearer, then, you bring an object to a lens, the further the image recedes behind it. _mrs. b._ certainly. but short-sighted persons have another resource, for objects which they can not bring near to their eyes; this is, to place a concave lens, c d, (fig. , plate .) before the eye, in order to increase the divergence of the rays. the effect of a concave lens, is, you know, exactly the reverse of a convex one: it renders parallel rays divergent, and those which are already divergent, still more so. by the assistance of such glasses, therefore, the rays from a distant object, fall on the pupil, as divergent as those from a less distant object; and, with short-sighted people, they throw the image of a distant object, back, as far as the retina. _caroline._ this is an excellent contrivance, indeed. _mrs. b._ and tell me, what remedy would you devise for such persons as have a contrary defect in their sight; that is to say, who are long-sighted, in whom the crystalline humour, being too flat, does not refract the rays sufficiently, so that they reach the retina before they are converged to a point? _caroline._ i suppose that a contrary remedy must be applied to this defect; that is to say, a convex lens, l m, fig. , to make up for the deficiency of convexity of the crystalline humour, o p. for the convex lens would bring the rays nearer together, so that they would fall, either less divergent, or parallel, on the crystalline humour; and, by being sooner converged to a focus, would fall on the retina. _mrs. b._ very well, caroline. this is the reason why elderly people, the humours of whose eyes are decayed by age, are under the necessity of using convex spectacles. and when deprived of that resource, they hold the object at a distance from their eyes, as in fig. , in order to bring the focus more forward. _caroline._ i have often been surprised, when my grandfather reads without his spectacles, to see him hold the book at a considerable distance from his eyes. but i now understand the cause; the more distant the object is from the crystalline lens, the nearer to it, will the image be formed. _emily._ i comprehend the nature of these two opposite defects very well; but i cannot now conceive, how any sight can be perfect: for, if the crystalline humour is of a proper degree of convexity, to bring the image of distant objects to a focus on the retina, it will not represent near objects distinctly; and if, on the contrary, it is adapted to give a clear image of near objects, it will produce a very imperfect one, of distant objects. _mrs. b._ your observation is very good, emily; and it is true, that every person would be subject to one of these two defects, if we had it not in our power to adapt the eye, to the distance of the object; it is believed that this is accomplished, by our having a command over the crystalline lens, so as to project it towards, or draw it back from the object, as circumstances require, by means of the two muscles, to which the crystalline humour is attached; so that the focus of the rays, constantly falls on the retina, and an image is formed equally distinct, either of distant objects, or of those which are near. _caroline._ in the eyes of fishes, which are the only eyes i have ever seen separate from the head, the cornea does not protrude, in that part of the eye which is exposed to view. _mrs. b._ the cornea of the eye of a fish is not more convex than the rest of the ball of the eye; but to supply this deficiency, their crystalline humour is spherical, and refracts the rays so much, that it does not require the assistance of the cornea to bring them to a focus on the retina. _emily._ pray, what is the reason that we cannot see an object distinctly, if we place it very near to the eye? _mrs. b._ because the rays fall on the crystalline humour, too divergent to be refracted to a focus on the retina; the confusion, therefore, arising from viewing an object too near the eye, is similar to that which proceeds from a flattened crystalline humour; the rays reach the retina before they are collected to a focus, (fig. .) if it were not for this imperfection, we should be able to see and distinguish the parts of objects, which, from their minuteness, are now invisible to us; for, could we place them very near the eye, the image on the retina would be so much magnified, as to render them visible. _emily._ and could there be no contrivance, to convey the rays of objects viewed, close to the eye, so that they should be refracted to a focus on the retina? _mrs. b._ the microscope is constructed for this purpose. the single microscope (fig. .) consists simply of a convex lens, commonly called a magnifying glass; in the focus of which the object is placed, and through which it is viewed: by this means, you are enabled to place your eye very near to the object, for the lens a b, by diminishing the divergence of the rays, before they enter the pupil c, makes them fall parallel on the crystalline humour d, by which they are refracted to a focus on the retina, at r r. _emily._ this is a most admirable invention, and nothing can be more simple; for the lens magnifies the object, merely by allowing us to bring it nearer to the eye. [illustration: plate xxiii.] _mrs. b._ those lenses, therefore, which have the shortest focus will magnify the object most, because they enable us to place it nearest to the eye. _emily._ but a lens, that has the shortest focus, is most bulging or convex; and the protuberance of the lens will prevent the eye from approaching very near to the object. _mrs. b._ this is remedied by making the lens extremely small: it may then be spherical without occupying much space, and thus unite the advantages of a short focus, and of allowing the eye to approach the object. there is a mode of magnifying objects, without the use of a lens: if you look through a hole, not larger than a small pin, you may place a minute object near to the eye, and it will be distinct, and greatly enlarged. this piece of tin has been perforated for the purpose; place it close to your eye, and this small print before it. _caroline._ astonishing! the letters appear ten times as large as they do without it: i cannot conceive how this effect is produced. _mrs. b._ the smallness of the hole, prevents the entrance into the eye, of those parts of every pencil of rays which diverge much; so that, notwithstanding the nearness of the object, those rays from it, which enter the eye, are nearly parallel, and are, therefore, brought to a focus by the humours of the eye. _caroline._ we have a microscope at home, which is a much more complicated instrument than that you have described. _mrs. b._ it is a double microscope, (fig. .) in which you see, not the object a b, but a magnified image of it, _a b_. in this microscope, two lenses are employed; the one, l m, for the purpose of magnifying the object, is called the object-glass, the other, n o, acts on the principle of the single microscope, and is called the eye-glass. there is another kind of microscope, called the solar microscope, which is the most wonderful from its great magnifying power: in this we also view an image formed by a lens, not the object itself. as the sun shines, i can show you the effect of this microscope; but for this purpose, we must close the shutters, and admit only a small portion of light, through the hole in the window-shutter, which we used for the camera obscura. we shall now place the object a b, (plate , fig. .) which is a small insect, before the lens c d, and nearly at its focus: the image e f, will then be represented on the opposite wall, in the same manner, as the landscape was in the camera obscura; with this difference, that it will be magnified, instead of being diminished. i shall leave you to account for this, by examining the figure. _emily._ i see it at once. the image e f is magnified, because it is farther from the lens, than the object a b; while the representation of the landscape was diminished, because it was nearer the lens, than the landscape was. a lens, then, answers the purpose equally well, either for magnifying or diminishing objects? _mrs. b._ yes: if you wish to magnify the image, you place the object near the focus of the lens; if you wish to produce a diminished image, you place the object at a distance from the lens, in order that the image may be formed in, or near the focus. _caroline._ the magnifying power of this microscope is prodigious: but the indistinctness of the image, for want of light, is a great imperfection. would it not be clearer, if the opening in the shutter were enlarged, so as to admit more light? _mrs. b._ if the whole of the light admitted, does not fall upon the object, the effect will only be to make the room lighter, and the image consequently less distinct. _emily._ but could you not by means of another lens, bring a large pencil of rays to a focus on the object, and thus concentrate upon it the whole of the light admitted? _mrs. b._ very well. we shall enlarge the opening, and place the lens x y (fig. .) in it, to converge the rays to a focus on the object a b. there is but one thing more wanting to complete the solar microscope, which i shall leave to caroline's sagacity to discover. _caroline._ our microscope has a small mirror attached to it, upon a moveable joint, which can be so adjusted as to receive the sun's rays, and reflect them upon the object: if a similar mirror were placed to reflect light upon the lens, would it not be a means of illuminating the object more perfectly? _mrs. b._ you are quite right. p q (fig. .) is a small mirror, placed on the outside of the window-shutter, which receives the incident rays s s, and reflects them on the lens x y. now that we have completed the apparatus, let us examine the mites on this piece of cheese, which i place near the focus of the lens. _caroline._ oh, how much more distinct the image now is, and how wonderfully magnified! the mites on the cheese look like a drove of pigs scrambling over rocks. _emily._ i never saw any thing so curious. now, an immense piece of cheese has fallen: one might imagine it an earthquake: some of the poor mites must have been crushed; how fast they run--they absolutely seem to gallop. but this microscope can be used only for transparent objects; as the light must pass through them, to form the image on the wall? _mrs. b._ very minute objects, such as are viewed in a microscope, are generally transparent, but when opaque objects are to be exhibited, a mirror m n (fig. .) is used to reflect the light on the side of the object next the wall: the image is then formed by light reflected from the object, instead of being transmitted through it. _emily._ pray, is not a magic lanthorn constructed on the same principles? _mrs. b._ yes, with this difference; the objects to be magnified, are painted upon pieces of glass, and the light is supplied by a lamp, instead of the sun. the microscope is an excellent invention to enable us to see and distinguish objects, which are too small to be visible to the naked eye. but there are objects, which, though not really small, appear so to us, from their distance; to these, we cannot apply the same remedy; for when a house is so far distant, as to be seen under the same angle as a mite which is close to us, the effect produced on the retina is the same: the angle it subtends is not large enough for it to form a distinct image on the retina. _emily._ since it is impossible, in this case, to make the object approach the eye, cannot we by means of a lens bring an image of it, nearer to us? _mrs. b._ yes; but then the object being very distant from the focus of the lens, the image would be too small to be visible to the naked eye. _emily._ then, why not look at the image through another lens, which will act as a microscope, enable us to bring the image close to the eye, and thus render it visible? _mrs. b._ very well, emily; i congratulate you on having invented a telescope. in figure , the lens c d, forms an image e f, of the object a b; and the lens x y, serves the purpose of magnifying that image; and this is all that is required in a common refracting telescope. _emily._ but in fig. , the image is not inverted on the retina, as objects usually are: it should therefore appear to us inverted; and that is not the case in the telescopes i have looked through. _mrs. b._ when it is necessary to represent the image erect, two other lenses are required; by which means a second image is formed, the reverse of the first, and consequently upright. these additional glasses are used to view terrestrial objects; for no inconvenience arises from seeing the celestial bodies inverted. _emily._ the difference between a microscope and a telescope, seems to be this:--a microscope produces a magnified image, because the object is nearest the lens; and a telescope produces a diminished image, because the object is furthest from the lens. _mrs. b._ your observation applies only to the lens c d, or object-glass, which serves to bring an image of the object nearer the eye; for the lens x y, or eye-glass, is, in fact, a microscope, as its purpose is to magnify the image. when a very great magnifying power is required, telescopes are constructed with concave mirrors, instead of lenses. these are called reflecting telescopes, because the image is reflected by metallic mirrors. concave mirrors, you know, produce by reflection, an effect similar to that of convex lenses, by refraction. in reflecting telescopes, therefore, mirrors are used in order to bring the image nearer the eye; and a lens, or eye-glass, the same as in the refracting telescope, to magnify the image. the advantage of the reflecting telescope is, that mirrors whose focus is six feet, will magnify as much as lenses of a hundred feet: an instrument of this kind may, therefore, possess a high magnifying power, and yet be so short, as to be readily managed. _caroline._ but i thought it was the eye-glass only which magnified the image; and that the other lens, served to bring a diminished image nearer to the eye. _mrs. b._ the image is diminished in comparison with the object, it is true; but it is magnified, if you compare it to the dimensions of which it would appear without the intervention of any optical instrument; and this magnifying power is greater in reflecting, than in refracting telescopes. we must now bring our observations to a conclusion, for i have communicated to you the whole of my very limited stock of knowledge of natural philosophy. if it enable you to make further progress in that science, my wishes will be satisfied; but remember, in order that the study of nature may be productive of happiness, it must lead to an entire confidence in the wisdom and goodness of its bounteous author. questions . (pg. ) what is the form of the body of the eye? fig. , plate . . (pg. ) what is its external coat called? . (pg. ) what is the transparent part of this coat denominated? . (pg. ) what is the second coat named? . (pg. ) what opening is there in this? . (pg. ) what is the coloured part which surrounds the pupil? . (pg. ) the pupils dilate and contract, what purpose does this answer? . (pg. ) how could you observe the dilatation and contraction of the pupils? . (pg. ) what purpose is the choroid said to answer? . (pg. ) in what animals is the change in the iris greatest? . (pg. ) what are the three humours denominated, and how are they situated? . (pg. ) what is the part represented at _i i_, and of what does it consist? . (pg. ) what are the respective uses of the humours, and of the retina? . (pg. ) why is it necessary the rays should be refracted? . (pg. ) how is this illustrated by fig. and , plate ? . (pg. ) what causes a person to be short-sighted? fig. , plate . . (pg. ) why does placing an object near the eye, enable such, to see distinctly? fig. . . (pg. ) a concave lens remedies this defect; how? fig. , plate . . (pg. ) what is the remedy, when a person is long-sighted? fig. . . (pg. ) why does holding an object far from the eye, help such persons? fig. . . (pg. ) how is the eye said to adapt itself to distant, and to near objects? . (pg. ) why are objects rendered indistinct, when placed very near to the eye? fig. , plate . . (pg. ) what is the single microscope, fig. , and how does it magnify objects? . (pg. ) how may objects be magnified without the aid of a lens? . (pg. ) why can an object, very near to the eye, be distinctly seen, when viewed through a small hole? . (pg. ) describe the double microscope, as represented in fig. , plate . . (pg. ) how does the solar microscope, (fig. plate .) operate? . (pg. ) why may minute objects be greatly magnified by this instrument? . (pg. ) in its more perfect form it has other appendages, as seen in fig. , what are they? and what their uses? . (pg. ) what is added when opaque objects are to be viewed? fig. . . (pg. ) in what does the magic lanthorn differ from the solar microscope? . (pg. ) what are the use and structure of the telescope, as shown in fig. ? . (pg. ) when terrestrial objects are to be viewed, why are two additional lenses employed? . (pg. ) what part of the telescope performs the part of a microscope? . (pg. ) in what does the reflecting, differ from the refracting telescope? . (pg. ) what advantages, do reflecting, possess over refracting telescopes? glossary. accelerated motion. motion is said to be accelerated, when the velocity is continually increasing. accidental properties. those properties of bodies which are liable to change, as colour, form, &c. acute.--see angle. air. an elastic fluid. the atmosphere which surrounds the earth, is generally understood by this term, but there are many kinds of air. the term is synonymous with _gas_. air pump. an instrument by which vessels may be exhausted of air. altitude. the height in degrees of the sun, or any heavenly body, above the horizon. angle. the space contained between two lines inclined to each other, and which meet in a point. angles are measured in degrees, upon a segment of a circle described by placing one leg of a pair of compasses on the angular point, and with the other, describing the segment between the two lines. if the segment be exactly - th of a circle, it is called a _right_ angle, and contains deg. if more than - th of a circle, it is an _obtuse_ angle. if less, an _acute_ angle. see plate . angle of incidence, is the space contained between a ray which falls obliquely upon a body, and a line perpendicular to the surface of the body, at the point where the ray falls. angle of reflection. the space contained between a reflected ray, and a line perpendicular to the reflecting point. angle of vision, or visual angle. the space contained between lines drawn from the extreme parts of any object, and meeting in the eye. antarctic circle. a circle extending round the south pole, at the distance of - degrees from it. the same as the south frigid zone. aphelion. that part of the orbit of a planet, in which its distance from the sun is the greatest. area. the surface enclosed between the lines which form the boundary of any figure, whether regular or irregular. aries. see sign. asteroids. the name given to the four small planets, ceres, juno, pallas, and vesta. astronomy. the science which treats of the motion and other phenomena of the sun, the planets, the stars, and the other heavenly bodies. atmosphere. the air which surrounds the earth, extending to an unknown height. wind is this air in motion. attraction. a tendency in bodies to approach each other, and to exist in contact. attraction of cohesion. that attraction which causes matter to remain in masses, preventing them from falling into powder. for this attraction to exist, the particles must be contiguous. attraction of gravitation. by this attraction, masses of matter, placed at a distance, have a tendency to approach each other. attraction is mutual between the sun and the planets. axis of the earth, or of any of the planets. an imaginary line passing through their centres, and terminating at their poles; round this their diurnal revolutions are performed. axis of motion. the imaginary line, around which all the parts of a body revolve, when it has a spinning motion. axis of a lens, or mirror. a line passing through the centre of a lens, or mirror, in a direction perpendicular to its surface. balloon. any hollow globe. the term is generally applied to those which are made to ascend in the air. barometer. commonly called a weather-glass. it has a glass tube, containing quicksilver, which by rising and falling, indicates any change in the pressure of the atmosphere, and thus frequently warns us of changes in the weather. body. the same as _matter_. it may exist in the solid, liquid, or æriform state; and includes every thing with which we become acquainted by the aid of the senses. burning-glass, or mirror. a lens, or a mirror, by which the rays of light, and heat, are brought to a focus, so as to set bodies on fire. camera obscura, a darkened room; or more frequently a box, admitting light by one opening, where a lens is placed; which, bringing the rays of light, from external objects, to a focus, presents a perfect picture of them, in miniature. capillary tubes. tubes, the bore of which is very small. glass tubes are usually employed, to show the phenomenon of _capillary attraction_. fluids in which they are immersed, rise in such tubes above the level of that in the containing vessel. centre of a circle. a point, equally distant from every part of its circumference. centre of gravity. that point within a body, to which all its particles tend, and around which they exactly balance each other. a system of bodies, as the planets, may have a common centre of gravity, around which they revolve in their orbits; whilst each, like the earth, has its particular centre of gravity within itself. centre of motion. that point about which the parts of a revolving body move, which point is, itself, considered as in a state of rest. centre of magnitude. the middle point of any body. suppose a globe, one side of which is formed of lead, and the other of wood, the centres of magnitude and of gravity, would not be in the same points. central forces. those which either impel a body towards, or from, a centre of motion. centrifugal. that which gives a tendency to fly from a centre. centripetal. that which impels a body, towards a centre. circle. a figure; the periphery, or circumference of which, is every where equally distant, from the point, called its centre. circle, great. on the globe, or earth, is one that divides it into two equal parts, or hemispheres. the equator, and meridian lines, are great circles. circle, lesser. those which divide the globe into unequal parts. the tropical, arctic and antarctic circles, and all parallels of latitude, are lesser circles. circumference. the boundary line of any surface, as that which surrounds the centre of a circle; the four sides of a square, &c. comets. bodies which revolve round the sun, in very long ovals, approaching him very nearly in their perihelion, but in their aphelion, passing to a distance immeasurably great. cohesion. see attraction. compressible. capable of being forced into a smaller space. concave. hollowed out; the inner surface of a watch-glass is concave, and may represent the form of a _concave mirror_, or _lens_. convex. projecting, or bulging out, as the exterior surface of a watch-glass, which may represent the form of a _convex mirror_, or _lens_. cone. a body somewhat resembling a sugar-loaf; that is, having a round base, and sloping at the sides, until it terminates in a point. conjunction. when three of the heavenly bodies are in a straight or right line, if you take either of the extreme bodies, the other two are in conjunction with it; because a straight line drawn from it, might pass through the centres of both, and join them together. at the time of new moon, the moon and sun are in conjunction with the earth; and the moon and earth, are in conjunction with the sun. constellation, or sign. a collection of stars. astronomers have imagined pictures drawn in the heavens, so as to embrace a number of contiguous stars, and have named the group after the animal, or other article supposed to be drawn; an individual star is generally designated by its fancied location; as upon the ear of _leo_, the lion, &c. convergent rays, are those which approach each other, so as eventually to meet in the same point. crystals. bodies of a regular form, having flat surfaces, and well defined angles. nitre, and other salts, are familiar examples. many masses of matter, are composed of crystals too minute to be discerned without glasses. curvilinear, consisting of a line which is not straight, as a portion of a circle, of an oval, or any curved line. cylinder. a body in the form of a roller, having flat circular ends, and being of equal diameter throughout. degree. if a circle of any size be divided into equal parts, each of these parts is called a degree. one quarter of a circle contains ninety degrees; one twelfth of a circle, thirty degrees. the actual length of a degree, must depend upon the size of the circle. a degree upon the equator, upon a meridian, or any great circle of the earth, is equal to - / miles. straight lines are sometimes divided into equal parts, called degrees; but these divisions are arbitrary, bearing no relationship to the degrees upon a circle. density. closeness of texture. when two bodies are equal in bulk, that which weighs the most, has the greatest density. diagonal. a line drawn so as to connect two remote angles of a square, or other four-sided figure. dilatation. the act of increasing in size. bodies in general, dilate when heated, and contract by cooling. discord. when the vibrations of the air, produced by two musical tones, do not bear a certain ratio to each other, a jarring sound is produced, which is called discord. divergent rays. those which proceed from the same point, but are continually receding from each other. divisibility. capability of being divided, or of having the parts separated from each other. this is called one of the _essential properties_ of matter; because, however minute the particles may be, they must still contain as many halves, quarters, &c. as the largest mass of matter. echo. a sound reflected back, by some substance, so situated as to produce this effect. eclipse. the interruption of the light of the sun, or of some other heavenly body, by the intervention of an opaque body. the moon passing between the earth and the sun, causes an eclipse of the latter. ecliptic. a circle in the heavens. the apparent path of the sun, through the twelve signs of the zodiac. this is caused by the actual revolution of the earth, round the sun. it is called the ecliptic, because eclipses always happen in the direction of that line, from the earth. elasticity. that property of bodies, by which they resume their dimensions and form, when the force which changed them is removed. air is eminently elastic. two ivory balls, struck together, become flattened at the point of contact; but immediately resuming their form, they react upon each other. ellipsis. an oval. this figure differs from a circle, in being unequal in its diameters, and in having two centres, or points, called its _foci_. the orbits of the planets are all elliptical. equator. that imaginary line which divides the earth into northern and southern hemispheres, and which is equally distant from each pole. equilibrium. when two articles exactly balance each other, they are in equilibrium. they may, notwithstanding, be very unequal in weight, but they must be so situated, that, if set in motion, their momentums would be equal. equinox. the two periods of time at which the nights and days are every where of equal length. the _vernal_ equinox is in march, when the sun enters the sign _aries_; the _autumnal_ equinox in september, when the sun enters _libra_. at these periods, the sun is vertical at the equator. exhalations. all those articles which arise from the earth, and mixing with the atmosphere, form vapour. expansion. the same as dilatation, which see. extension. one of the essential properties of matter; that by which it occupies some space, to the exclusion of all other matter. figure. all matter must exist in some form, or shape; hence figure is deemed an essential property of matter. fluid. a form of matter, in which its particles readily flow, or slide, over each other. airs, or gases, are called elastic fluids, because they are readily reduced to a smaller bulk by pressure. liquids, are denominated non-elastic fluids, because they suffer but little diminution of bulk, by any mechanical force. focus. that point in which converging rays unite. force. that power which acts upon a body, either tending to create, or to stop motion. fountain. a jet, or stream of water, forced upwards by the weight of other water, by the elasticity of air, or some other mechanical pressure. friction. the rubbing of bodies together, by which their motion is retarded. friction may be lessened, but cannot be destroyed. frigid zones. the spaces or areas, contained within the arctic and antarctic circles. fulcrum. a prop. the point or axis, by which a body is supported, and about which it is susceptible of motion. gas. any kind of air; of these there are several. the atmosphere consists of two kinds, mixed, or combined with each other. geometry. that branch of the mathematics, which treats of lines, of surfaces, and of solids; and investigates their properties, and proportions. globe. a sphere, or ball. it has a point in its centre of magnitude, from which its surface is every where equally distant. gravity. that species of attraction which appears to be common to matter, existing in its particles, and giving to them, and of course to the masses which they compose, a tendency to approach each other. by gravity a stone falls to the earth, and by it the heavenly bodies tend towards each other. harmony. a combination of musical sounds, produced by vibrations which bear a certain ratio to each other; and which thence affect the mind agreeably, when heard at the same time. sounds not so related, produce discord. hemisphere. half a sphere or globe. a plane passing through the centre of a globe, will divide it into hemispheres. horizon. this is generally divided into _sensible_, and _rational_. the sensible horizon is that portion of the surface of the earth, to which our vision extends. our rational horizon is that circle in the heavens which bounds our vision, when on the ocean, an extended plane, or any elevated situation. in the heavens our sensible, and our rational horizon are the same; its plane would divide the earth into hemispheres at degrees from us; and a person standing on that part of the earth which is directly opposite to us, would, at the same moment, see in his horizon, the same heavenly bodies, which would be seen in ours. horizontal. level; not inclined, or sloping. a perfectly round ball, placed upon a flat surface, which is placed horizontally, will remain at rest. hydraulics. that science which treats of water in motion, and the means of raising, conducting, and using it for moving machinery, or other purposes. hydrostatics. treats of the weight, pressure, and equilibrium of fluids, when in a state of rest. hydrometer. an instrument used to ascertain the specific gravity of different fluids, which it does, by the depth to which it sinks when floating on them. image. the picture of any object which we perceive either by reflected or refracted light. all objects which are visible, become so by forming images on the retina. impenetrability. that property of matter, by which it excludes all other matter from occupying the same space with itself at the same time. if two particles could exist in the same space, so also might any greater number, and indeed all the matter in the universe, might be collected in a single point. incidence. the direction in which a body, or a ray of light, moves in its approach towards any substance, upon which it strikes. inclined plane. one of the six mechanical powers. any plane surface inclined to the horizon, may be so denominated. inertia. one of the inherent properties of matter. want of power, or of any active principle within itself, by which it can change its own state, whether of motion, or of rest. inherent properties. those properties which are absolutely necessary to the existence of a body; called also essential properties. all others are denominated accidental. colour is an accidental--extension, an essential property of matter. latitude. distance from the equator, in a direct line towards either pole. this distance is measured in degrees and minutes. the degree of latitude cannot exceed ninety, or one quarter of a circle. places to the south of the equator, are in south latitude, and those to the north, in north latitude. latitude, parallels of. lines drawn upon the globe, parallel to the equator, are so called; every place situated on such a line, has the same latitude, because equally distant from the equator. lens. a glass, ground so that one or both surfaces form segments of a sphere, serving either to magnify, or diminish objects seen through them. glasses used in spectacles are lenses. lever. one of the mechanical powers. an inflexible bar of wood or metal, supported by a fulcrum, or prop; and employed to increase the effect of a given power. libra. one of the twelve signs of the zodiac. that into which the sun enters, at the autumnal equinox. light. that principle, by the aid of which we are able to discern all visible objects. it is generally believed to be a substance emitted by luminous bodies, and, exciting vision by passing into the eye. longitude. distance measured in degrees and minutes, either in an eastern, or a western direction, from any given point either on the equator, or on a parallel of latitude. degrees of longitude may amount to , or half a circle. a degree of longitude measured upon the equator, is of the same length with a degree of latitude; but as the poles are approached, the degrees of longitude diminish in length, because the circles upon which they are measured, become less. lunar. relating to _luna_, the moon. lunation. the time in which the moon completes its circuit. a lunar month. luminous bodies. those which emit light from their own substance; not shining by borrowed, or reflected light. machine. any instrument, either simple or compound, by which any mechanical effect is produced. a needle, and a clock, are both machines. magic lanthorn, or lantern. an optical instrument, by which transparent pictures, painted upon glass, are magnified and exhibited on a white wall or screen, in a darkened room. the phantasmagoria, is a species of magic lanthorn. mathematics. the science of numbers and of extension. common arithmetic, is a lower branch of the mathematics. in its higher departments, it extends to every thing which is capable of being either numbered or measured. matter. substance. every thing with which we become acquainted by the aid of the senses; every thing however large, or however minute, which has length, breadth, and thickness. mechanics. that science which investigates the principles, upon which the action of every machine depends; and teaches their proper application in overcoming resistance, and in producing motion, in all the useful purposes to which they are applied. medium. in optics, is any body which transmits light. air, water, glass, and all other transparent bodies, are media. medium also denotes that in which any body moves. air is the medium which conveys sound, and which enables birds to fly. melody. a succession of such single musical sounds, as form a simple air or tune. mercury. that planet which is nearest to the sun. quicksilver, a metal, which remains fluid at the common temperature of the atmosphere. it is capable of being rendered solid, by intense cold. meridian. midday. a meridian line, is one which extends directly from one pole of the earth to the other; crossing the equator at right angles. it is therefore half of a great circle. the hour of the day is the same at every place situated on the same meridian. longitude is measured from any given meridian, to the opposite meridian. places at the same distance in degrees, to the east or west of any meridian, have the same longitude. microscope. an optical instrument, by which minute objects, are magnified, so as to enable us to perceive and examine such as could not be seen by the naked eye. mineral. earths, stones, metals, salts, and in general all substances dug out of the earth, are denominated minerals. minute. in time, the sixtieth part of an hour. in length, the sixtieth part of a degree. a minute of time, is an unvarying period; but a minute in length varies in extent, with the degree of which it forms a part. the degrees and minutes are equal in number, upon a common ring, upon the equator of the earth, or, on any circle of the heavens. mirrors. polished surfaces of metal, or of glass coated with metal, for the purpose of reflecting the rays of light, and the images of objects. common looking-glasses, are mirrors. those used in reflecting telescopes, are made of metal. mobility. capable of being moved from one place to another. this is accounted one of the essential properties of matter, because we cannot conceive of its existence without this capacity. momentum. the force, or power, with which a body in motion acts upon any other body, or tends to preserve its own quantity of motion. the momentum of a body, is compounded of its quantity of matter, and its velocity. a body weighing one pound, moving with a velocity of two miles in a minute, will possess the same momentum with one weighing two pounds, moving with a velocity of one mile in a minute. motion. a continued and successive change of place, either of a whole body, or of the particles of which a body is composed; the earth in revolving upon its axis only, would not change its place as a body, but all the particles of which it is composed, would revolve round a common axis of motion. in revolving in its orbit, its whole mass is constantly occupying a new portion of space. natural philosophy. that science which enquires into the laws which govern all the natural bodies in the universe, in all their changes of place, or of state. neap tides. those tides which occur when the moon is in her quadratures, or half way between new, and full moon; at these periods the tides are the lowest. nodes. those points in the orbit of the moon, or of a planet, where it crosses the ecliptic or plane of the earth's orbit. when passing to the north of the ecliptic, it is called the ascending node; when to the south of it, the descending node. oblate. see spheroid. octagon. a figure with eight sides, and consequently with eight angles. opaque. not transparent; refusing a passage to the rays of light. optics. that branch of science which treats of light, and vision. it is generally divided into two parts. _catoptrics_, which treats of the reflection of light, and _dioptrics_, which treats of its refraction. orbit. the line in which a primary planet moves in its revolution round the sun; or a secondary planet, in its revolution round its primary. these orbits are all elliptical, or oval. parabola. a particular kind of curve; that which a body describes in rising and in falling, when thrown upwards, in any direction not perpendicular to the horizon. parallelogram. a figure with four sides, having those which are opposite, parallel to each other. a square, an oblong square, and the figure usually called a diamond, are parallelograms. parallel lines. all lines, whether straight or curved, which are every where at an equal distance from each other, are parallel lines. parallel of latitude. see latitude. perihelion. that part of the orbit of a planet, in which it approaches the sun most nearly. pendulum. a body suspended by a rod, or line, so that it may vibrate, or oscillate, backwards and forwards. pendulums of the same length, perform their vibrations in the same time, whatever may be their weight, and whether the arc of vibration, be long or short. percussion. the striking of bodies against each other. the force of this, depends upon the momentum of the striking body. period. the time required for the revolution of one of the heavenly bodies in its orbit. perpendicular. making an angle of degrees with the horizon. when two lines which meet, make an angle of degrees, they are perpendicular to each other. phases. the various appearances of the disc, or face of the moon, and of the planets; that portion of them which we see illuminated by the rays of the sun. phenomenon. any natural appearance is properly so called; the term, however, is usually applied to extraordinary appearances, as eclipses, transits, &c. piston. that part of a pump, or other engine which is made to fit into a hollow cylinder, or barrel; and to move up and down in it, in order to raise water, or for any other purpose. plane. a perfectly flat surface. the plane of the orbit of a planet, is an imaginary flat surface, extending to every part of the orbit. planet. those bodies which revolve round the sun, in orbits nearly circular. they are divided into _primary_, and _secondary_; these latter are also called satellites, or moons; they revolve round the primary planets, and accompany them in their courses round the sun. plumb-line. a string, or cord, by which a weight is suspended; it is used for the purpose of finding a line perpendicular to the horizon; the weight being always attracted towards the centre of the earth. pneumatics. that branch of natural philosophy, which treats of the mechanical properties of the atmosphere, or of air in general. poles. the extremities of the axis of motion either of our earth, or of any other revolving sphere. the poles of the earth have never been visited; the regions by which they are surrounded, being obstructed by impassable barriers of ice. power. that force which we apply to any mechanical instrument, to effect a given purpose, is denominated power, from whatever source it may be derived. we have the power of weights, of springs, of horses, of men, of steam, &c. prism. the instrument usually so called, is employed in optics to decompose the solar ray: it consists of a piece of solid glass, several inches in length, and having three flat sides; the ends are equal in size, and are of course triangular. precession of the equinoxes. every equinox takes place a few seconds of a degree, before the earth arrives at that part of the ecliptic in which the preceding equinox occurred. this phenomenon is called the precession of the equinoxes. there is consequently a gradual change of the places of the signs of the zodiac: a fact, the discovery of which has thrown much light on ancient chronology. projection. that force by which motion is given to a body, by some power acting upon it, independently of gravity. pulley. one of the six mechanical powers. a wheel turning upon an axis, with a line passing over it. it is the moveable pulley only, which gives any mechanical advantage. pump. an hydraulic, or pneumatic instrument, for the purpose of raising water, or exhausting air. quadrant. a quarter of a circle. an instrument used to measure the elevation of a body in degrees above the horizon. quadratures of the moon. that period in which she appears in the form of a semicircle. she is then either in her first, or her last quarter; and exactly half way, between the places of new, and of full moon. radiation. the passage of light or heat in rays, or straight lines; these being projected from every luminous, or heated point, in all directions. radius. the distance from the centre of a circle, to its circumference; or one half of its diameter. in the plural denominated radii. rainbow. an appearance in the atmosphere, occasioned by the decomposition of solar light, in its refraction, and reflection, in passing through drops of rain. the bow can be seen, only when the sun is near the horizon, when the back is turned towards it, and there is a shower in the opposite direction. ray. a single line of light, emitted in one direction, from any luminous point. reaction. every body, whether in a state of motion, or at rest, tends to remain in such state, and resists the action of any other body upon it, with a force equal to that action. this resistance, is called its _reaction_. receiver. this name is applied to glass vessels of various kinds, appertaining to the air pump, and from which the air may be exhausted. they are made to contain, or receive, any article upon which an effect is to be produced, by taking off the pressure of the atmosphere. refraction, of the rays of light, is the bending of those rays, when they pass obliquely from one medium into another of different density. a stick held obliquely in water, appears bent or broken at the surface of the fluid. refrangibility. capacity of being refracted. light is decomposed by the prism, because its component parts are refrangible in different degrees, by the same refracting medium. repulsion. the reverse of attraction. a tendency in particles, or in masses of matter, to recede from each other. the matter of heat within a body, appears to counteract the attraction of its particles, so as to prevent absolute contact. retina. that part of the ball of the eye, upon which the images of visible objects are formed; and from which, the idea of such forms, is conveyed to the mind. revolution, of a planet; is either diurnal, or annual; the former, is its turning upon its own axis; the latter, is its passage in its orbit. satellites. moons, secondary planets. segment of a circle. a portion, or part of a circle; called also, an arc of a circle. semi-diameter. half the diameter. the semi-diameter of the earth, is the distance from its surface, to its centre. siderial. belonging to the stars. a siderial day, is the time required for a star to reappear on a given meridian. a siderial year, the period in which the sun appears to have travelled round the ecliptic, so as to have arrived opposite to any particular star, from which his course was calculated. signs, or constellations. collections, or groups, of stars. those of the zodiac are twelve, corresponding with the twelve months in the year. in the centre of these the ecliptic is situated. the sun appears to pass in succession through these signs; entering the first degree of aries, which is accounted the first sign, about the st of march. sky. that vast expanse, or space, in which the heavenly bodies are situated. its blue appearance is supposed to arise from the particles of which the atmosphere is composed, possessing the property of reflecting the blue rays, in greatest abundance. solar. appertaining to, or governed by, the sun: as the solar system, the solar year, solar eclipses. solid. not fluid. having its parts connected so as to form a mass. solid bodies, are not absolutely so, all undoubtedly containing pores, or spaces void of matter. solstices. the middle of summer and the middle of winter; those two points in the orbit of the earth, in which its poles point most directly towards the sun. sonorous bodies. those bodies which are capable of being put into a state of vibration, so as to emit sounds. specific gravity. the relative weight of bodies of different species, when the same bulk of each is taken. water has been chosen as the standard for comparison. if we say that the specific gravity of a body is , we mean, that its weight is six times as great as that of a portion of water, exactly equal to it in bulk. spectrum. that appearance of differently coloured rays, which is produced by the refraction of the solar ray, by means of a prism, is called the prismatic spectrum; it exhibits most distinctly, and beautifully, all the colours seen in the rainbow. sphere. a globe, or ball. spheroid. spherical; a body approaching nearly to a sphere in its figure. the earth, is denominated an _oblate spheroid_; it not being an exact sphere, but flattened at the poles, so as to cause the polar diameter to be upwards of thirty miles less than the equatorial. oblate, is the reverse of oblong, and means shorter in one direction, than in another. spring tides. those tides which occur at the time of new, or of full moon. the tides then rise to a greater height than at any other period. square. a figure having four sides of equal length, and its angles all right angles. in numbers; the product of a number multiplied into itself; thus, the square of is , and the square of is . star. the _fixed_ stars are so called, because they retain their relative situations; while the planets, by revolving in their orbits, appear to wander amongst the fixed stars. subtend. this term is applied to the measurement of an angle; when the lines by which it is bounded recede but little from each other, they are said to subtend; that is, to be contained under, a small angle. superficies. the surface of any figure. space extended in length and width. system. the mutual connexion, and dependance of things, upon each other. the solar, or copernican system, includes the sun, the planets, with their moons, and the comets. tangent. a straight line touching the circumference of a circle; but which would not cut off any portion of it, were it extended beyond the touching point, in both directions. telescope. an instrument by which distant objects may be distinctly seen; the images of objects being brought near to the eye, and greatly magnified. temperate zones. those portions of the surface of the earth situated between - / and - / degrees of latitude. within these boundaries, the sun is never vertical; nor does he ever remain, during a whole day, below the horizon. thermometer. an instrument for measuring the temperature of the atmosphere, or of other bodies. torrid zone. that portion of the earth which extends - / degrees on each side of the equator, to the tropical circles; within this limit, the sun is vertical, twice in the year. transit. mercury or venus, are said to transit the sun, when they pass between the earth and that luminary. they then appear like dark spots, upon the face of the sun. transparent. allowing the rays of light to pass freely through. the reverse of opaque. glass, water, air, &c. are transparent bodies. tropics. two circles on the globe on either hemisphere, at the distance of - / degrees from the equator. beyond these circles, the sun is never vertical: and the countries within them, are denominated tropical. twilight. that portion of the morning or evening, in which the light of the sun is perceptible, although he is below the horizon. vacuum. space void of matter. such is supposed to be the space in which the planets revolve. we are said to produce a vacuum, when we exhaust the air from a receiver. valve. a part of a pump, and of some other instruments, which opens to admit the passage of a fluid in one direction, but closes when pressed in the opposite direction, so as to prevent the return of the fluid; a pair of bellows is furnished with a valve. vapour. exhalations from fluid or solid substances, generally mixing with the atmosphere. the most abundant, is that from water. vertical. exactly over our heads: ninety degrees above our horizon. vibration. the alternate motion of a body, forwards and backwards; swinging, as a pendulum. visual. belonging to vision; as the visual angle, or that angle formed by the rays of light which enter the eye, from the extremities of any object. undulation. a vibratory, or wave-like motion communicated to fluids. sound, is said to be propagated by the undulatory, or vibratory motion of the air. wedge. one of the mechanical powers; the form of the wedge is well known. it is of extensive use; serving to rend bodies of great strength, and to raise enormous weights. wheel and axle. one of the mechanical powers, used under various modifications. cranes for raising weights, the wheels and pinions of clocks and watches, windlasses, &c. are all applications of this power. zodiac. a broad belt in the heavens, extending nearly eight degrees on each side of the ecliptic; the planes of the orbits of all the planets are included within this space. this belt is divided into twelve parts or signs, each containing degrees. these signs are: _aries_; the ram. _taurus_; the bull. _gemini_; the twins. _cancer_; the crab. _leo_; the lion. _virgo_; the virgin. _libra_; the scales. _scorpio_; the scorpion. _sagittarius_; the archer. _capricornus_; the goat. _aquarius_; the waterer. _pisces_; the fishes. the first six are called northern signs; because the sun is in them, during that half of the year, in which he is vertical to the north of the equator; the last six, are called southern signs; because, during his journey among them, he is vertical to the south of the equator. the sun enters _aries_, at the time of the _vernal equinox_; _cancer_, at the _summer solstice_; _libra_, at the _autumnal equinox_; and _capricornus_, at the _winter solstice_. the sun is said to enter a sign, when the earth in going round in its orbit, enters the opposite sign. thus, when the sun appears in the first degree of _libra_, it is in consequence of the earth having arrived opposite to the first degree of aries. a line then drawn from the earth, and passing through the centre of the sun, would, if extended to the fixed stars, touch the first degree of libra. zone. the earth is divided into zones, or belts. see frigid, temperate, and torrid zones. index. a. air, , , , , . air-pump, , . angle, . acute, . obtuse, . right, . of incidence, , , , . of reflection, , , , . visual, , , . angular velocity, . antarctic circle, . aphelion, . arctic circle, . atmosphere, , , , , , , . colour of, . reflection of, . refraction of, . attraction, , , , , . of cohesion, , , . capillary, . of gravitation, , , , , , , , . avenue, . auditory nerve, . axis, . of motion, . of the earth, , . of mirrors, . of a lens, . b. balloon, . barometer, . bass, . bladder, . bodies, . elastic, . fall of, , , , . luminous, . opaque, . sonorous, , . transparent, . bulk, . c. camera obscura, , , . capillary tubes, . centre, . of gravity, , , , . of magnitude, , . of motion, , , . centrifugal force, , , , . centripetal force, , . ceres, . circle, , . circumference, . clouds, . colours, , . comets, . compression, . concord, . constellation, . convergent rays, , . crystals, . curvilinear motion, , . cylinder, . d. day, , , . degrees, , , , , . of latitude, , . of longitude, , . density, . diagonal, . diameter, . discords, . diurnal, . divergent rays, , . divisibility, , . e. earth, , , , , . echo, . eclipse, , . ecliptic, , , . elasticity, . elastic bodies, , . fluids, , , , . ellipsis, . equinox, , . precession of, . equator, , . essential properties, . exhalations, . extension, , . eye, , . f. fall of bodies, , , . figure, , . fluids, , . elastic, , , , . equilibrium of, , , . non-elastic, . pressure of, . flying, . focus, . of concave mirrors, . of convex mirrors, , . of a lens, . imaginary, . virtual, . force, . centrifugal, , , , . centripetal, , . projectile, , . of gravity, , . fountains, . friction, , , . frigid zone, . fulcrum, . g. general properties of bodies, . georgium sidus, . glass, . burning, . refraction of, . gold, , . gravity, , , , . h. harmony, . heat, , , . hemisphere, , . herschel, . hydraulics, . hydrometer, . hydrostatics, . i. image on the retina, , . reversed, . in plain mirror, . in concave do. . in convex do. . impenetrability, . inclined plane, , . inertia, , , . inherent properties, . juno, . jupiter, . l. lake, , . latitude, , . lens, . concave, . convex, . meniscus, . plano-concave, . plano-convex, . lever, , . first kind, . second kind, . third kind, . light, . pencil of, . of the moon, , . absorption of, . reflected, . refraction of, . liquids, . longitude, , . luminous bodies, . lunar month, . eclipse, . m. machine, , . magic lanthorn, . mars, . matter, , . mechanics, . mediums, , . melody, . mercury, (planet) , , . mercury, or quicksilver, , , . meridians, . microscope, . single, . double, . solar, , . minerals, . minutes, . momentum, , . monsoons, . month, lunar, . moon, , , , , . moonlight, , . motion, , , . accelerated, . axis of, . centre of, , . compound, . curvilinear, , . diurnal, . perpetual, . retarded, . reflected, . uniform, . mirrors, . axis of, . burning, . concave, , , . convex, , . plane or flat, . reflection of, . n. neap tides, . nerves, . auditory, , . olfactory, . optic, , . night, . nodes, . o. octave, . odour, . opaque bodies, , . optics, . orbit, . p. pallas, . parabola, . parallel lines, . parallel of latitude, . pellucid bodies, . pencil of rays, . pendulum, . perihelion, . perpendicular lines, . phases, . piston, , . plane, , . planets, , , . poles, , , . polar star, , . porosity, , . powers, mechanical, . projection, , , . precession of equinoxes, . pulley, , . pump, . sucking and lifting, . forcing, , . air, , . pupil of the eye, . r. rain, , . rainbow, . rarity, . ray of light, , . reflected, , . incident, . rays, intersecting, . reaction, . receiver, . reflection of light, , . angle of, , , . of mirrors, . of plane mirrors, . of concave do. . of convex do. . reflected motion, . refraction, , . of the atmosphere, . of glass, . of a lens, . of a prism, . resistance, . retina, . image on, . rivers, . rivulets, . s. satellites, , , . saturn, . scales, or balance, . screw, , . shadow, , . siderial time, . sight, . signs of the zodiac, , . smoke, , . solar microscope, . solstice, , . sound, . acute, . musical, . space, . specific gravity, . of air, . spectrum, . speaking-trumpet, . sphere, . springs, . spring tides, . square, , . stars, , , . storms, . substance, . summer, , . sun, , , , , . swimming, . syphon, . t. tangent, , . telescope, , . reflecting, . refracting, . temperate zone, , . thermometer, . tides, , . neap, . spring, . ærial, . time, , . siderial, . equal, . solar, . tone, . torrid zone, , , . transit, . transparent bodies, . treble and bass, . tropics, . v. valve, . vapour, , , , . velocity, , . venus, . vesta, . vibration, , . vision, , . vision, angle of, , . double, . u. undulation, . unison, . w. water, , . spring, . rain, . level of, . wedge, , . weight, . wheel and axle, , . wind, . trade, . periodical, . winter, , . y. year, . siderial, . solar, . z. zodiac, . zone, . torrid, , , . temperate, , . frigid, , . the end. to all teachers. school books. smiley's geography and atlas, and sacred and ancient geography for schools. the above works will be found useful and very valuable as works of reference, as well as for schools. the maps, composing the atlases, will be found equal in execution and correctness to those on the most extensive scale. the author has received numerous recommendations, among which are the following: dear sir--i have looked over your "_easy introduction to the study of geography_," together with your "_improved atlas_." i have no hesitation in declaring, that i consider them works of peculiar merit. they do honour to your industry, research, and talent, and i am satisfied, will facilitate the improvement of the student in geographical science. with sentiments of sincere consideration, i am yours truly, wm. staughton, d. d. _president of columbia college, district of columbia._ mr. thomas smiley. _philadelphia, sept. , ._ * * * * * _extract from the minutes of the philadelphia academy of teachers._ _november , ._ resolved unanimously, that the academy of teachers highly approve the superior merits of mr. smiley's "_easy introduction to the study of geography_," and the accompanying atlas, and cordially recommend them to the patronage of the public. b. mayo, _president._ i. i. hitchcock, _secretary._ the new federal calculator, or scholar's assistant. containing the most concise and accurate rules for performing the operations in common arithmetic; together with numerous examples under each of the rules, varied so as to make them conformable to almost every kind of business. for the use of schools and counting houses. by thomas t. smiley, teacher: author of an easy introduction to the study of geography. also, of sacred geography for the use of schools. among the numerous recommendations received to the work, are the following: mr. john grigg. _phila. march , ._ sir--i have examined with as much care as my time would admit, "the new federal calculator," by thomas t. smiley. it appears to me to be a treatise on arithmetic of considerable merit. there are parts in mr. smiley's work which are very valuable; the rules given by him in barter, loss and gain, and exchange, are a great desideratum in a new system or treatise on arithmetic, and renders his book superior to any on the subject now in use; and when it is considered that the calculations in the work are made in federal money, the only currency now known in the united states, and that appropriate questions follow the different rules, by which the learner can be exercised as to his understanding of each part as he progresses; i hesitate not to say, that, in my opinion, it is eminently calculated to promote instruction in the science on which it treats. mr. smiley deserves the thanks of the public and the encouragement of teachers, for his attempt to simplify and improve the method of teaching arithmetic. i am yours respectfully, wm. p. smith, _preceptor of mathematics and natural philosophy, no. , south tenth street._ * * * * * sir--i have carefully examined "the new federal calculator, or scholar's assistant," by thomas t. smiley, on which you politely requested my opinion; and freely acknowledge that i think it better calculated for the use of the united states schools and counting-houses than any book on the subject that i have seen. the author's arrangement of the four primary rules is, in my opinion, a judicious and laudable innovation, claiming the merit of improvement; as it brings together the rules nearest related in their nature and uses. his questions upon the rules throughout, appear to me to be admirably calculated to elicit the exertions of the learner. but above all, the preference he has given to the currency of his own country, in its numerous examples, has stamped a value upon this little work, which i believe has not fallen to the lot of any other book of the kind, as yet offered to the american public. i am, sir, yours respectfully, john mackay. _charleston, (s. c.) march , ._ * * * * * _from the united states gazette._ among the numerous publications of the present day, devoted to the improvement of youth, we have noticed a new edition of smiley's arithmetic, just published by j. grigg. the general arrangement of this book is an improvement upon the arithmetics in present use, being more systematic, and according to the affinities of different rules. the chief advantage of the present over the first edition, is a correction of several typographical errors, a circumstance which will render it peculiarly acceptable to teachers. in referring to the merits of this little work, it is proper to mention that a greater portion of its pages are devoted to federal calculation, than is generally allowed in primary works in this branch of study. the heavy tax of time and patience which our youth are now compelled to pay to the errors of their ancestors, by performing the various operations of pounds, shillings, and pence, should be remitted, and we are glad to notice that the federal computation is becoming the prominent practice of school arithmetic. in recommending mr. smiley's book to the notice of parents and teachers, we believe that we invite their attention to a work that will really prove an "assistant" to them, and a "_guide_" to their interesting charge. * * * * * the editors of the new york telegraph, speaking of smiley's arithmetic, observe that they have within a few days attentively examined the above arithmetic, and say, "we do not hesitate to pronounce it an improvement upon every work of the kind previously before the public; and as such, recommend its adoption in all our schools and academies." a key to the above arithmetic, in which all the examples necessary for a learner are wrought at large, and also solutions given of all the various rules. designed principally to facilitate the labour of teachers, and assist such as have not the opportunity of a tutor's aid. by t. t. smiley, author of the new federal calculator, &c. &c. torrey's spelling book, or first book for children. i have examined mr. jesse torrey's "familiar spelling book." i think it a great improvement in the primitive, and not least important branches of education, and shall introduce it into the seminaries under my care, as one superior to any which has yet appeared. ira hill, a. m. _boonsborough, feb. , ._ the increasing demand for this work is the best evidence of its merits. a pleasing companion for little girls and boys, blending instruction with amusement; being a selection of interesting stories, dialogues, fables, and poetry. designed for the use of primary schools and domestic nurseries. by jesse torrey, jr. to secure the perpetuation of our republican form of government to future generations, let divines and philosophers, statesmen and patriots, unite their endeavours to renovate the age, by impressing the minds of the people with the importance of educating their _little boys and girls_. s. adams. _report of the committee of the philadelphia academy of teachers: adopted nov. , ._ the committee, to whom was referred mr. jesse torrey's "pleasing companion for little girls and boys," beg leave to report, that they have perused the "pleasing companion," and have much pleasure in pronouncing as their opinion, that it is a compilation much better calculated for the exercise and improvement of small children in the art of reading, and especially in the more rare art of understanding what they read, than the books in general use. all which is respectfully submitted. i. irvine hitchcock, pardon davis, charles mead, _committee_. a true copy from the minutes of the academy. c. b. trego, _secretary_. _nov. , ._ the moral instructor and guide to virtue, by jesse torrey, jr. among the numerous recommendations to this valuable school book, are the following:-- _extract of a note from the hon. thomas jefferson, late president of the united states._ "i thank you, sir, for the copy of your '_moral instructor_.' i have read the first edition with great satisfaction, and encouraged its reading in my family." * * * * * _extracts of a letter from the hon. james madison, late president of the united states._ "sir--i have received your letter of the th, with a copy of the _moral instructor_. "i have looked enough into your little volume to be satisfied, that both the original and selected parts contain information and instruction which may be useful, not only to juvenile but most other readers. "with friendly respects, james madison." dr. torrey. * * * * * _from roberts vaux, president of the controllers of the public schools in philadelphia._ "the moral instructor" is a valuable compilation. it appears to be well adapted for elementary schools, and it will give me pleasure to learn that the lessons which it contains are furnished for the improvement of our youth generally. respectfully, roberts vaux. _philadelphia, th month, ._ history of england, from the first invasion by julius cæsar, to the accession of george the fourth, in eighteen hundred and twenty: comprising every political event worthy of remembrance; a progressive view of religion, language, and manners; of men eminent for their virtue or their learning; their patriotism, eloquence, or philosophical research; of the introduction of manufactures, and of colonial establishments. with an interrogative index, for the use of schools. by william grimshaw, author of a history of the united states, &c. history of the united states, from their first settlement as colonies, to the cession of florida, in : comprising every important political event; with a progressive view of the aborigines; population, religion, agriculture, and commerce; of the arts, sciences, and literature; occasional biographies of the most remarkable colonists, writers, and philosophers, warriors, and statesmen; and a copious alphabetical index. by william grimshaw, author of a history of england, &c. also, questions adapted to the above history, and a key, adapted to the questions, for the use of teachers. "_university of georgia, athens, june , ._ "dear sir, "with grateful pleasure, i have read the two small volumes of mr. grimshaw, (a history of england, and a history of the united states) which you some time since placed in my hands. on a careful perusal of them, i feel no difficulty in giving my opinion, that they are both, as to style and sentiment, works of uncommon merit in their kind; and admirably adapted to excite, in youthful minds, the love of historical research. "with sincere wishes for the success of his literary labours, "i am very respectfully, your friend, "m. waddel, _president_. "e. jackson, esq." * * * * * "d. jaudon presents his respectful compliments to mr. grimshaw, and is much obliged by his polite attention, and the handsome compliment of his history of the united states with the questions and key. "mr. j. has been in the use of this book for some time; but anticipates still more pleasure to himself, and profit to his pupils, in future, from the help and facility which the questions and key will afford in the study of these interesting pages. "_october th, ._" * * * * * _golgotha, p. edwd. va. sep. , ._ "dear sir, "mr. grimshaw's 'history of the united states,' &c. was some time ago put into my hands by mr. b----, who requested me to give you my opinion as to the merits of the work. the history of the late war is well managed by your author: it has more of detail and interest than the former part; and i consider it much superior to any of the many compilations on that subject, with which the public has been favoured. it may be said of the entire performance, that it is decidedly the best chronological series, and the chastest historical narrative, suited to the capacity of the juvenile mind, that has yet appeared. its arrangement is judicious; its style neat, always perspicuous, and often elegant; and its principles sound. "american writings on men and things connected with america, have been long needed for the young; and i am happy to find, that mr. grimshaw has not only undertaken to supply this want, but also to _americanise_ foreign history for the use of our schools. in a word, sir, i am so fond of american fabrics, and so anxious to show myself humbly instrumental in giving our youth american feeling and character whilst at school; that i shall without hesitation recommend mr. grimshaw's works to my young pupils, as introductory to more extensive historical reading. in fine, the work is so unobjectionable, and puts so great a mass of necessary information within the reach of school-boys, at so cheap a rate, that i feel the highest pleasure in recommending it to the public, and wish you extensive sales. "yours respectfully, "william branch, jr. "mr. benjamin warner, "_philadelphia._" * * * * * "_history of the united states, from their first settlement as colonies, to the peace of ghent, &c._ by william grimshaw, pp. , mo. "this is the third time, within the space of two years, that we have had occasion to review a volume from the hand of mr. grimshaw. he writes with great rapidity; and improves as he advances. this is the most correctly written of all his productions. we could wish that a person so well formed for close, and persevering study, as he must be, might find encouragement to devote himself to the interests of literature." "mr. g. has our thanks for the best concise and comprehensive history of the united states which we have seen." _theological review, october, ._ * * * * * "_history of england, from the first invasion by julius cæsar, to the peace of ghent, &c._ _for the use of schools._ by william grimshaw. philadelphia, . benjamin warner. mo. pp. . "we have copied so much of the title of this work, barely to express our decided approbation of the book, and to recommend its general introduction into schools. it is one of the best books of the kind to be found, and is instructive even to an adult reader. we should be pleased that teachers would rank it among their class-books; for it is well calculated to give correct impressions, to its readers, of the gradual progress of science, religion, government, and many other institutions, a knowledge of which is beneficial in the present age. among the many striking merits of this book, are, the perspicuity of the narrative, and chasteness of the style. it is with no little pleasure we have learned, that the author has prepared a similar history _of the united states_; a work long wanted, to fill up a deplorable chasm in the education of american youth." _analectic magazine, october, ._ * * * * * "_philadelphia, june, ._ "sir--i have read with pleasure and profit your history of england. i think it is written with perspicuity, chasteness, and impartiality. well written history is the best political instructor, and under a government in which it is the blessing of the country that the people govern, its pages should be constantly in the hands of our youth, and lie open to the humblest citizen in our wide-spread territories. your book is eminently calculated thus to diffuse this important knowledge, and therefore entitled to extensive circulation; which i most cordially wish. with much respect, "your obedient servant, "langdon cheves. "william grimshaw, esq." grimshaw's improved edition of goldsmith's greece.--among the numerous recommendations to this valuable school book, are the following:-- although there are many worthless school books, there are but few which are equally impure and inaccurate with the original editions of goldsmith's histories, for the use of schools. i congratulate both teachers and pupils upon the appearance of mr. grimshaw's edition of the "history of greece," which has been so completely expurgated, and otherwise corrected, as to give it the character of a new work, admirably adapted to the purpose for which it is intended. thos. p. jones, _professor of mechanics in the franklin institute of the state of pennsylvania, and late principal of the north carolina female academy._ _philadelphia, sept. , ._ * * * * * mr. john grigg. dear sir--agreeably to your request i have examined, with attention, "goldsmith's greece, revised and corrected, and a vocabulary of proper names appended, with prosodial marks, to assist in their pronunciation, by william grimshaw;" and i feel a perfect freedom to say, that the correction of numerous grammatical and other errors, by mr. grimshaw, together with the rejection of many obscene and indelicate passages improper for the perusal of youth, gives this edition, in my opinion, a decided preference over the editions of that work heretofore in use. the questions and key, likewise supplied by mr. grimshaw to accompany this edition, afford a facility for communicating instruction, which will be duly appreciated by every judicious teacher. i am, sir, yours truly, thos. t. smiley. _philadelphia, sept. , ._ * * * * * the editor of the united states gazette, in speaking of this work, says--"goldsmith's greece, without a revision, is not calculated for schools; it abounds in errors, in indelicate description, improper phrases, and is, indeed, a proof how very badly a good author can write, if indeed there is not much room to doubt goldsmith ever composed the histories to which his name is attached. mr. grimshaw has adopted the easy descriptive style of that writer, retained his facts, connected his dates, and entirely and handsomely adapted his work to the school desk. the book of questions and the accompanying key, are valuable additions to the work, and will be found most serviceable to teacher and pupil. "from a knowledge of the book, and some acquaintance with the wants of those for whom it was especially prepared, we unhesitatingly recommend grimshaw's greece as one of the best (in our opinion, the very best of) works of the kind that has been offered to the public." the united states speaker, compiled by t. t. smiley--preferred generally to the columbian orator and scott's lessons, and works of that kind, by teachers who have examined it. goldsmith's history of greece, improved by grimshaw, with a vocabulary of the proper names contained in the work, and the prosodial accents, in conformity with the pronunciation of lempriere--with questions and a key, as above. grimshaw's etymological dictionary and expositor of the english language. * * * * * transcriber's note: spelling variations where there is no obviously preferred choice have been preserved, except as noted below. irregularities include: "bason" and "basin;" derivatives of "enquire" and "inquire;" "learned" and "learnt;" "sidereal" and "siderial;" "sun-rise" and "sunrise;" "sun-set" and "sunset." the original use of commas was preserved, except where explicitly noted below. the original spelling of "pourtray" was preserved. both roman and arabic numerals are used to number the plates; the text was left as is. preserved the non-standard order in the index, where u comes after v. removed extra comma after "which" on page v: "about which the parts." changed "sideral" to "siderial" on page vi: "solar, siderial, and equal." added comma after "mrs. b." on page : "your assistance, my dear mrs. b., in a charge." changed "errroneous" to "erroneous" on page : "an erroneous conception." added comma after "mrs. b." twice on page : "yet surely, mrs. b., there;" and "but, mrs. b., if attraction." added commas before and after "mrs. b." on page : "pray, mrs. b., do." changed "pullies" to "pulleys" on page : "a system of pulleys." changed "plate . fig. " to "plate . fig. " on page in the body of the text and in the associated question, to designate the correct figure. changed "twelves" to "twelve" on page : "twelve times less." changed "stream" to "steam" on page : "expansive force of steam." changed "pray mrs. b," to "pray, mrs. b.," on page . changed "nonelastic" to "non-elastic" on page : "non-elastic like water." removed extra comma after "one" on page : "one would ultimately have prevailed." changed "eliptical" to "elliptical" on page : "elliptical or oval orbit." changed "eclipse" to "ellipsis" on page : "motion in an ellipsis." changed "elipsis" to "ellipsis" on page : "but an ellipsis." changed "fig. plate " in the question on page to "fig. . plate " to designate the correct figure. changed "day-light" to "daylight" on page : "see them by daylight." changed the second question numbered to " " from page . changed "eliptical" to "elliptical" on page : "they were elliptical." capitalised "mercury" on page : "made upon mercury." added question mark on page after "those beautiful lines of milton." removed repeated word, "it", on page : "provided it were steady." changed "aeriform" to "æriform" on page (in versions supporting full latin- character set). changed "atmospherical" to "atmospheric" on page : "the atmospheric air." changed "rarifies" to "rarefies" on page : "heat rarefies air." changed "to day" to "to-day" on page : "our lesson to-day." changed "re-appearance" to "reappearance" on page : "reappearance of the sun." changed question to " " on page to maintain proper sequence. changed "proportionably" to "proportionally" on page : "proportionally distinct." inserted comma after "circle" on page in the glossary entry for "circle, lesser." inserted period on page at the end of the glossary entry for "cylinder." changed "musisical" to "musical" on page in the glossary entry for "harmony." changed "perpendidicular" to "perpendicular" on page : "perpendicular to each other." changed "oppoite" to "opposite" on page : "the opposite direction." capitalised "aries" on page : "the first degree of aries." change "jr." to "jr." in the advertisement for "a pleasing companion ...": "by jesse torrey, jr." _lrl accelerators_ the -inch synchrocyclotron lawrence radiation laboratory university of california, berkeley, california pub. no. d m june [illustration: synchrocyclotron building] _contents_ page the -inch synchrocyclotron principle of operation of a conventional cyclotron the principle of phase stability design and construction of the -inch synchrocyclotron magnet vacuum system ion source radiofrequency system internal targets and beam extractor cyclotron experiments nuclear physics biophysics nuclear chemistry bibliography appendix the -inch synchrocyclotron his success with the -inch cyclotron in led dr. e. o. lawrence to propose a much more powerful accelerator, one which could produce new types of nuclear rearrangements and even create particles. grants totaling $ , , permitted work to start on the -inch cyclotron in august .[ ] it was designed to accelerate atomic particles to an energy of million electron volts (mev), five times that possible with the -inch machine. [illustration: fig. . the electromagnet under construction during the period to .] before the new cyclotron could be finished world war ii began. construction on the cyclotron was therefore halted. however, because of interest in separating the isotopes of uranium by the electromagnetic method, work on the giant magnet continued at an even faster pace. this magnet would contain tons of steel in its yoke and pole pieces, and tons of copper in its exciting coils (fig. ). by may the magnet was completed. during that summer it was used in a pilot plant to separate the first significant amounts of u^{ } ever obtained. the -inch magnet remained in use in a research and development program at berkeley until the end of the war, supplying information to oak ridge, tennessee, where a large separation plant had been erected. construction on the rest of the cyclotron was resumed in . by that time a new principle had been discovered which made it possible to obtain ion beams of much higher energy than originally hoped for. yet a considerably lower accelerating voltage could be used. this important discovery was made independently by dr. v. veksler in russia and by dr. edwin m. mcmillan, present director of the lawrence radiation laboratory. before attempting to discuss this principle, we should first review the operation of a conventional cyclotron. principle of operation of a conventional cyclotron [illustration: fig. . basic parts of a cyclotron.] the main parts of a cyclotron are represented in fig. . charged particles (ions) are accelerated inside an evacuated tank. this is to prevent the beam from colliding with air molecules and being scattered. the vacuum tank is placed between the poles of an electromagnet, whose field bends the ion beam into a circular orbit. the operation begins when the ions are introduced into the region between two accelerating electrodes, or "dees."[ ] because the ions carry a positive electric charge, they are attracted toward that dee which is electrically negative at the moment. were it not for the magnetic field, the ions would be accelerated in a straight line; instead they are deflected into a circular path back toward the dee gap. by the time the ions again reach the dee gap, the sign of the electric potential on the dees is reversed, so that now the ions are attracted toward the opposite dee. as this process of alternating the electric potential is repeated, the ions gain speed and energy with each revolution. this causes them to spiral outward. finally they strike a target inserted into their path or are extracted from the cyclotron for use as an external beam. the time required for an ion to complete one loop remains constant as it spirals outward. this is because its velocity increases sufficiently to make up for the increased distance it travels during each turn. this means that the electric potential applied to the dees must alternate at a constant frequency, called the "resonant frequency." the resonant frequency f is given by the relationship he f = --------- , ( ) [pi]mc where h, e, [pi], c, and m are constants. h is the strength of the magnetic field of the cyclotron, e is the electric charge carried by the ion, [pi] equals . , c is a conversion factor, and m is the mass of the ion. for example, the resonant frequency for protons accelerated in a , -gauss magnetic field is . megacycles (mc).[ ] we call such a rapidly alternating potential a "radiofrequency voltage" and the electronic circuit for producing it a "radiofrequency oscillator." the energy e of an ion emerging from the cyclotron is given by h^ r^ e^ e = ------- ---- , ( ) mc^ where h, e, and m are as defined above, and r is the radius at which the beam is extracted. from this equation we see that for a given type of ion (where e and m are constant), the energy depends on the diameter and strength of the magnet, but not directly upon the voltage applied to the dees. the number of revolutions that an ion can make in a conventional cyclotron is limited to about to . this is due to a very curious effect: as an ion is accelerated, its mass increases! [this phenomenon is explained by einstein's special theory of relativity (see fig. ).] referring back to eq. ( ), we see that if the ion mass (m) does not remain constant, but rather increases, then the resonant frequency (f) decreases. but since the dee potential continues alternating at a constant frequency, an ion soon begins to arrive "late" at the dee gap. by the time it has made about to turns an ion is so badly out of phase that it is no longer accelerated. suppose now that we want to obtain an energy of mev. because an ion can make a maximum of about turns, the accelerating potential would have to be about , volts. however, professor lawrence hoped to reach mev with the new -inch cyclotron. this meant that the accelerating voltage would have to be about , , volts. preventing such a high voltage from sparking promised to be one of many formidable engineering problems. [illustration: fig. . graph showing how the mass of an object increases as its velocity approaches that of light.] footnotes: [ ] the grants were as follows: rockefeller foundation--$ , , ; john and mary markle foundation--$ , ; the research corporation--$ , . the university of california added a guarantee of $ , to bring the total building fund to $ , , . [ ] in the first cyclotrons the electrodes were shaped like the letter "d." [ ] we have the values h = , gauss, e = . × ^{- } electrostatic units, and m = . × ^{- } gram. to find f, we write , ( . ) ^{- } f = ---------------------------------- , ( . )( . ) ^{- } ( ) ^{ } f = . mc. the principle of phase stability fortunately, drs. veksler and mcmillan showed that relatively low dee voltages can be used to accelerate ions to very high energies. this is possible if the oscillator frequency is continuously decreased to keep it in synchronism with the decreasing rotational frequency of the ions. this would allow an ion to make many revolutions without becoming out of phase. this principle of phase stability was experimentally verified with the -inch cyclotron before being incorporated into the design of the -inch machine. because it utilizes this principle, this machine has usually been referred to as a "synchrocyclotron" or "frequency-modulated cyclotron." however, it is sometimes called simply a "cyclotron." the -inch synchrocyclotron was first operated in november . with a maximum dee voltage of only , volts, it accelerated deuterons to mev and alpha particles to mev.[ ] in it was modified to permit production of -mev protons also. between and the synchrocyclotron was rebuilt so that now the following energies can be obtained: protons deuterons alpha particles helium- nuclei[ ] ------- --------- --------------- --------------- mev mev mev mev in reaching an energy of mev a proton, for example, makes , revolutions in just milliseconds (msec). it travels a distance of miles and attains a velocity of , miles per second, or % of the speed of light! during this brief journey its mass increases %, giving very convincing evidence for the validity of einstein's theory. similar data for other ions may be found in the appendix. footnotes: [ ] a deuteron is the nucleus of an atom of heavy hydrogen and contains one proton and one neutron; it carries a single positive electric charge. an alpha particle is the nucleus of a helium atom and is made up of two protons and two neutrons; it carries two positive charges. [ ] the machine is equipped for helium- operation, but to date it has not been used for that purpose. design and construction of the -inch synchrocyclotron _magnet_ during the rebuilding of the cyclotron, the diameter of the magnet pole pieces was increased from to - / inches. also, the pole gap at the center was reduced from to inches. these changes increased the weight of steel in the magnet from to tons. the main exciting coils, which contain turns of copper-bar conductor each, were not altered. two auxiliary coils containing turns each were added. this brought the total weight of copper from to tons. the coils are layer-wound around the pole pieces close to the pole gap. other data about the coils are given in the appendix. the effect of these modifications was to increase the field strength at the center of the pole gap from , to , gauss. this increase made it possible to obtain the higher-energy ions. power is supplied to the coils by two motor generator sets, which produce the direct current required for a steady magnetic field. the direct current from the motor generators is regulated so that the magnetic-field fluctuation is less than one part in , . this is necessary if one wants an external beam of nearly uniform energy. in order to prevent the beam from becoming unstable and striking the dee, the magnetic field must be strongest at the center and decrease radially (fig. a). with flat pole faces the field does not decrease uniformly. to give the desired rate of decrease, the pole faces are shimmed with concentric steel rings of varying thickness, as shown in fig. b. in a radially decreasing magnetic field, the lines of magnetic flux bow outward, as represented in fig. b. ions moving in a magnetic field are deflected at right angles to these flux lines. ions above the midplane of the cyclotron are directed downward; those below the midplane are directed upward. in this way an ion oscillates about the midplane and vertical focusing is achieved. [illustration: fig. . (a) plot of magnetic-field strength vs radius. the field strength decreases gradually out to a radius of about -in., after which it falls off sharply. this point marks the maximum usable radius for particle orbits. further out they are unstable. (b) magnetic flux lines are represented as broken arrows, and focusing forces as solid arrows. an ion above the midplane is directed downward, while an ion below the midplane is directed upward.] radial focusing is accomplished in a somewhat analogous manner. if the magnetic field decreases with radius, radial restoring forces are established. an ion at too large a radius is directed inward, and an ion at too small a radius is directed outward. in this fashion, the ion oscillates about the synchronous orbit. thus, radial focusing is achieved. _vacuum system_ the vacuum tank (acceleration chamber) is a steel box × ft and ft high. it is evacuated to a pressure of ^{- } millimeter of mercury (about one -millionth of atmospheric pressure). the pumping equipment consists of six oil-diffusion pumps and four mechanical vacuum pumps. the pumping speed of the six -in. oil-diffusion pumps is a total of , liters/sec. _ion source_ the ion source is a simple arc-type. hydrogen gas is allowed to leak into the ion-source enclosure near a tungsten filament, which is heated to incandescence. electrons emitted by the filament knock off electrons from hydrogen atoms, leaving free protons. the protons then escape into the acceleration chamber through a hole in the ion-source housing. once inside, the protons are accelerated by the dee potential. deuterons or alpha particles are obtained in a similar fashion using deuterium or helium gas in place of hydrogen. _radiofrequency system_ the -inch synchrocyclotron has a single dee instead of the double-dee arrangement described above for illustrative purposes. the accelerating electric field is developed between the dee and a dummy dee which is grounded to the vacuum tank. using a single dee does not change the principle of operation, yet it offers the advantage of allowing more space for auxiliary equipment inside the vacuum tank. also, the construction is much simpler. the dummy dee is not essential for operation, but it does improve performance. [illustration: fig. . radiofrequency cycle for accelerating protons. sixty-four such cycles are repeated each second.] radiofrequency power is supplied to the dee by a vacuum-tube oscillator. the frequency of oscillation must decrease during the acceleration cycle, as indicated above. for protons, the frequency at the start of acceleration is megacycles (mc). at the end of acceleration the frequency is only mc (see fig. ). this change in frequency is achieved by varying the electrical capacitance in the tuned circuit of the oscillator. (this is what you do when you dial a different station on a radio.) this tuned circuit, which is called the cyclotron resonator, is shown in fig. . [illustration: fig. . cyclotron resonator.] because the frequency must change over such a wide range (from to mc), the electrical capacitance must be varied by a factor of to . this is done by a variable capacitor of unique design. it resembles two giant tuning forks. as the blades of the tuning forks vibrate, the capacitance is alternately increased and decreased by the required amount. these two tuning forks must be kept in step with great precision. this is to prevent the oscillator from exciting lateral rf resonances. with a cyclotron of this size, this is a problem. these resonances, if excited, would cause loss of beam. the method for keeping the blades moving together is as follows: the blades are made to vibrate at their resonant frequency, which is approximately cycles per second. one set of blades operates at its natural frequency as a tuning-fork oscillator. the second set of blades is driven from an amplified sample of the signal from the first; its natural period is adjusted automatically to equal that of the first. the amplitude of each set is regulated to within . in.; the phase angle between the blades is regulated to within deg. ions are accelerated only when the radiofrequency is decreasing (fig. ). the remaining portion of the cycle is "dead time." thus, pulses, each of about microseconds' duration, are obtained every second. the average ion current of a pulsed beam is much less than for a continuous beam, such as that obtained from a conventional cyclotron (see table i). this is part of the price paid for higher energies. _internal targets and beam extractor_ the simplest target is one placed inside the vacuum tank where the circulating beam will strike it. the target may be any substance that the physicist or chemist wants to irradiate. the target material is attached to a movable probe. if the experimenter wants to use the full-energy beam, he places the target at the maximum usable radius of the circulating beam ( inches). however, if he desires to use ions having less than the maximum energy, he inserts the target further into the cyclotron so that it is intercepted sooner. table i ============================================================= comparison of external-beam energy and current for a synchrocyclotron and a conventional cyclotron ------------------------------------------------------------- -inch synchrocyclotron ------------------------- protons deuterons alpha particles ------- --------- --------------- beam energy -- maximum (mev) beam intensity -- peak current ([mu]a)[ ] beam intensity -- average current ([mu]a) . . . -inch cyclotron ----------------- beam energy -- maximum (mev) beam intensity -- peak current ([mu]a) beam intensity -- average current ([mu]a) ------------------------------------------------------------- [ ] [mu]a = microampere ============================================================= [illustration: fig. . plan view of the cyclotron, showing the method for obtaining an external beam of protons, deuterons, or alpha particles.] some experiments require an external beam of protons, deuterons, or alpha particles. a beam of this type can be brought out of the machine by means of a lecouteur regenerator (fig. ). the construction of the regenerator is very simple. it is made of a number of steel laminations of various sizes. what the regenerator does is perturb the magnetic field of the cyclotron at one radial position. each time the beam passes through the regenerator it receives a kick. with each kick the beam builds up its radial amplitude, until finally it enters a magnetic channel. this channel focuses the beam and steers it outside the main magnetic field. once outside, the beam travels through an evacuated tube, which is integral with the main vacuum tank. by means of a steering magnet, the beam can be sent into either the physics cave or the medical cave. (these experimental areas are called "caves" because they are rooms inside the massive concrete shielding wall.) other experiments may require an external beam of mesons.[ ] a meson beam is obtained in the following way (fig. ): a movable target such as a block of carbon is placed inside the cyclotron near the end of the outward-spiraling proton beam. when the proton beam hits this target, a shower of mesons is produced. these mesons are bent in various directions by the main magnetic field. some of them pass through a thin metal window in the vacuum-tank wall and are focused by a magnetic lens into a beam. this meson beam then travels through a hole in the concrete shielding wall into the meson cave. the maximum intensity of this extracted meson beam depends on both the charge and energy desired. beams of more than , mesons per second have been obtained through an aperture × in. in the shielding wall. cyclotron experiments _nuclear physics_ about % of the operating time of the -inch synchrocyclotron is devoted to experiments in nuclear physics. most of the experiments study the production and interaction of [pi] mesons. these particles are considered to be essential factors in the intense but short-range forces that bind the nucleus together. the three types of [pi] mesons are designated according to their electric charge as [pi]^+, [pi]^ , and [pi]^-.[ ] these mesons materialize only in high-energy nuclear collisions. [illustration: fig. . method for obtaining external meson beam.] of great importance are those experiments that determine the probability of producing each of the three types of mesons in a nuclear collision. this type of experiment is repeated for different beam energies and target elements. other experiments measure the energy and direction of emission of [pi] mesons from a target. [illustration: fig. . a typical experiment. scintillation counters at a, b, c, d, and e record the passage of charged particles.] a typical [pi]-meson experiment is represented in fig. . the purpose of this experiment was to detect the spin directions of protons as they are knocked out of a liquid hydrogen target by a [pi]-meson beam. (like the earth, a proton spins on its axis.) an extracted proton beam from the cyclotron enters the physics cave from the left, striking a polyethylene target and producing [pi] mesons. a beam of these mesons is formed by a series of two bending magnets and three focusing magnets. this beam passes through a carbon absorber to remove unwanted particles. the meson beam then strikes the liquid hydrogen target. a few of the incoming mesons scatter, knocking protons out of the liquid hydrogen. scintillation counters at a and b record the passage of a proton, thus defining its direction. the scattered mesons are counted by a scintillation counter at c. a few of the protons scatter off the carbon target and are detected by counters at e and d. from the detection of such events, the spin directions (polarization) of the recoil protons can be analyzed. in this way, more is learned about the fundamental [pi]-proton interaction. further studies of the interactions of [pi] mesons are made in the meson cave. other experiments performed there are concerned with [mu] mesons. the [mu] meson (muon) is a particle created in the decay of a [pi] meson and is the principal constituent of cosmic rays striking the surface of the earth. the muon is unstable, eventually undergoing a radioactive decay into an electron. although the muon does not experience nuclear forces, it can interact weakly with nuclei. the behavior of the muon is well understood, but its role as one of the elementary particles is unknown. that is, if the muon did not exist, what effect would this have on the structure of matter? the answer to this question, among others, is being sought by physicists using the -inch cyclotron. _biophysics_ experiments in biophysics are conducted in the medical cave. in these the interest lies not in nuclear interactions but in the effect of ionizing radiation on living tissue. high-energy beams of particles can be used for selective destruction of specific areas of the brain. this permits physiological mapping of the functions of the brain in experimental animals. it further offers a therapeutic approach to the treatment of brain tumors. one of the important investigational programs is concerned with the relationship of the pituitary gland to the growth rate of certain cancers and to some endocrine disorders. _nuclear chemistry_ for techniques of radiochemistry to be employed successfully, high interaction rates (and therefore high beam intensities) are needed. for this reason, chemistry targets are usually inserted right into the cyclotron so that they can be bombarded directly by the circulating beam. after the bombardment is completed the target is removed from the cyclotron. it is then taken to a chemistry laboratory and subjected to detailed chemical procedures. individual elements are removed, and the radioactive isotopes of each element are identified by quantitative counting techniques. in some cases a mass spectrometer is used to analyze the products. many deductions about the nature of the breakup of the target nucleus can be drawn from the pattern of the observed radioactive products. sometimes the nucleus splits almost in half. this is called fission. more frequently smaller parts of the nucleus are split off. two general types of reactions, known as spallation and fragmentation, are distinguished. one of the goals of this research is to learn more about the constitution of the nucleus and of the forces which bind the particles in the interior of the nucleus. footnotes: [ ] mesons are elementary particles intermediate in mass between the electron and proton. [ ] it may be interesting to note that the [pi]^ meson was discovered with this cyclotron in . this was the first particle to be discovered with an accelerator. all particles that had been previously discovered were observed first in cosmic rays or some other form of natural radiation. bibliography . gerald a. behman, particle accelerators: i. bibliography, ii. list of accelerator installations, ucrl- , january , . . samuel glasstone, the acceleration of charged particles, in _sourcebook on atomic energy_, second edition (van nostrand, princeton, ), ch. ix. . m. s. livingston, _high-energy accelerators_ (interscience publishers, new york, ). . m. stanley livingston and edwin m. mcmillan, history of the cyclotron, physics today _ _, - (october ). . e. m. mcmillan, particle accelerators, in _experimental nuclear physics_, emilio segrè, editor, vol. iii (wiley, new york, ), part xiii. . bob h. smith _et al._, the electrical aspects of the ucrl -mev synchrocyclotron, ucrl- rev., october , . . robert l. thornton, frequency-modulation and radiofrequency system for the modified berkeley cyclotron, ucrl- , april , . . robert r. wilson, particle accelerators, scientific american _ _, - (march ). appendix summary of specifications present fields of research % of time -------------------------- --------- nuclear physics nuclear chemistry biophysics _scheduled operation_ hours/week _performance_ _internal beams_ alpha helium- protons deuterons particles ions ------- --------- --------- -------- maximum energy (mev) energy spread (mev) beam intensity average current ([mu]a) . . . peak current ([mu]a) beam radius, maximum (in.) time required for acceleration (msec) . . number of revolutions during acceleration , , , distance traveled during acceleration (miles) velocity at maximum energy (% of speed of light) mass increase at maximum energy (% of rest mass) range of full-energy particles (in. of aluminum) _external beams_ physics cave meson cave ----------------------- ------------------- protons neutrons [pi]^+ [pi]^+ [pi]^- ------- -------- ------ -------- --------- energy (mev) | | energy spread (mev) | | beam area (cm^ ) | | flux (particles/cm^ -sec) × ^ × ^ × ^ | | _acceleration chamber (vacuum tank)_ size length (ft) width (ft) height (ft) material: mild steel operating pressure (mm hg) ^- vacuum pumps: six -in. oil-diffusion pumps with -in. boosters: one beach-russ -cfm; one kinney -cfm; two kinney -cfm. pumping speed of oil-diffusion pumps (liters/sec) , _magnet_ core diameter (in.) pole-tip diameter (in.) . pole gap at center (in.) magnetic field strength (gauss) at center , at radius of . in., where n = . , weight of steel (tons) , magnet coils main coils auxiliary coils ---------- --------------- material solid copper hollow copper ( / × in.) ( - / × - / in.) weight of copper (tons) number of turns (total) , ampere turns . × ^ . × ^ current (amp) voltage (v) power (kw) coolant oil water _radiofrequency system_ dee system number of dees size length (in.) width (in.) height (in.) material: / -in.-thick copper, stretched over a stainless steel frame dee aperture (in.) - / oscillator type: self-excited grounded-grid tube: one machlett ml dc input, operating condition (kw) dee bias, maximum dc (v) protons deuterons alpha particles ------- --------- --------------- rf duty cycle (%) dee-to-ground voltage, peak (kv) frequency-modulation system type: vibrating-reed (tuning-fork) capacitor number of units: two (two blades each) blades size width (in.) length (in.) thickness: tapered from . to . in. weight (lb) vibrational frequency (cps) electrical capacitance ([mu][mu]f) to , peak-to-peak excursion (in.) minimum separation of blade and stator (mils) protons deuterons alpha particles ------- --------- --------------- frequency sweep (mc) - - . - . ion source: conventional open-arc type _beam extractor_ lecouteur-type regenerator combined with magnetic channel _building and facilities_ room dimensions diameter (ft) height (ft) crane type: radial capacity (tons) overhead span (ft) concrete shielding: ft thick on sides, ft on top _history_ design started: january . construction started: august . first operation for deuterons and alpha particles: november . for protons: december . rebuilt: - . [illustration: synchrocyclotron building] [transcriber's note: the following changes have been made to the printed text: page , added closing quote (are called "caves" because) page , "iostopes" corrected to "isotopes" ] vol. iii, pp. - , may , the national geographic magazine geography of the air annual report by vice-president a. w. greely washington published by the national geographic society price cents. { } vol. iii, pp. - , may , the national geographic magazine geography of the air. annual report by vice-president a. w. greely. (_presented to the society january , ._) in fulfilling the duties growing out of his official position in connection with this society, your vice-president of the geography of the air has been so closely occupied with executive and other official duties devolving upon him as to preclude his giving that amount of time and labor to this annual report that the subject merits. indeed, no report would be submitted this year had it not seemed better to insure a continuity of these annual addresses, even if one of them might not be up to the high standard which should be maintained for them. it must have impressed every general reader of scientific journals that the past year has been marked by the publication of an unusual number of controversial articles relating entirely or in part to meteorology. some of the discussions of this subject appear to be in the nature of speculation, which, by good authority, is defined to be "chiefly the work of the imagination, and has little to do with realities." the status of the meteorological discussion which has been going on for some time seems to be this: a number of men, applying themselves to investigation in separate branches or stages of the same science, are attempting to reconcile their views, which, based as they are upon entirely different processes of investigation, are not entirely accordant. some, at least, of these writers are still apparently groping in the preliminary, the "natural history" stage of the { } science of meteorology, while one alone stands as the exponent of the "natural philosophy" of meteorology. to me it seems that it could not have failed to impress any interested reader who has followed the late publications on the convectional theory that, in order to clear the ground for definite meteorological discussion, it is necessary to determine the exact meaning of the various technical terms employed by the various writers. whether from looseness of verbiage originally or from the not infrequent habit of disputants when worsted to change their ground by claiming to be misunderstood, we find that some writers are unwilling either to stand by their first criticisms or to openly abandon them; they prefer to explain away their defective statements and gradually shift around to positions almost diametrically opposed to those originally assumed. the generally accepted theory as to cyclones attributes their initiatory formation to an unequal distribution of temperature with resulting mean diminution of pressure, and the movement of the air from places of high to places of low pressure, the lower air ascending with a gyratory motion, while air particles moving from opposite directions form couples which produce rotation. when energetic motions raise the ascending air to such a height that the temperature, cooled dynamically in ascending, goes below the dew-point, then the great store of latent heat thereby set free becomes, it is assumed, the main source of energy in maintaining the upward convectional movement. the subsidiary causes are attributed to the diminution of pressure on the collapse of the vapor, and also to the direct absorption of the sun's heat at the upper cloud surface. in anticyclones a slow gyratory descending motion of the air is assumed. ferrel considers the cyclone and anticyclone one system, and believes that air flowing into the cyclone from a "high" at the ground passes out into the higher atmospheric strata. dr. hann has put forth the hypothesis that the genesis of cyclones and anticyclones may be sought in the general atmospheric circulation through a difference of temperature of the air from the equator to the poles. he speaks of a congestion in the upper or anti-trade winds, where the air heaps up to a great height, this being the cause of the anticyclones; and he maintains that the low temperature of the "high" is due to ground radiation, and that no part of the high pressure is the result of low temperature. { } to this hypothesis of dr. hann, ascribing the genesis of storms to the general circulation of the atmosphere, no application of the laws of dynamics has yet been made with a view of developing it into an acceptable "theory." if it should be established it does not follow that it will in any way affect the truth of the commonly accepted "convectional system," which, founded as it is on the well-known laws of thermo-dynamics, is not likely to be successfully assailed. there may be an improved nomenclature for the laws of statics and dynamics that will express to the mind more clearly the relation of cause and effect; but until the advance of scientific research modifies the present formulation of these laws the convectional theory will be generally accepted as giving the true interpretation of all the phenomena to which it could be applied. professor russell, in commenting on this subject, expresses the opinion that the low temperature is due to the convective interchange of air at a low temperature in the upper strata with air of a high temperature in lower strata, such convective interchange tending to make the whole body of air of a temperature coinciding throughout with the adiabatic rate of upward diminution, with the consequent result of rendering the air at the surface of the earth cooler than previously and the upper air warmer. when the upward diminution of temperature is less than the adiabatic rate, in the forced circulation of air crossing a mountain ridge, there occurs the dynamic heating which is observed in the case of the foehn winds. the low temperature near the earth he does not believe could ever be entirely produced by nocturnal radiation from the ground. the high pressure, in his opinion, is largely the result of greater density due to low temperature, as is very clearly indicated by the fact that the temperature is almost inversely proportional to the pressure, and that the places of lower temperature substantially coincide with the places of greatest pressure. in advancing hypotheses and inviting discussion the real object is, or at least should be, to discover the essential cause or causes which determine the initial formation and subsequent maintenance and progress of the cyclone. some real progress in charting lines of equal density seems to have been made by m. nils ekholm following professor abbe's system of "isostaths," one using the term density, the other buoyancy. professor abbe also introduces the factor of the orographic gradient, but the { } latter is simply the measure of a resistance. the objection to this form of determination is this, that it is a measure of mass only. the density of two masses of air is determined to be the same; but as the density may result from two entirely different causes, their physical relations cannot be fully expressed in units of gravity. the methods of professor abbe and of m. nils ekholm undoubtedly give good results, partly from the coincidence that humidity usually varies directly as the temperature. the method proposed by captain james allen in , which is briefly described in appendix to the annual report of the chief signal officer for , appears to afford the means of more clearly expressing the relations that exist between the mass of the atmosphere and the forces available for the generation and movement of storms. its tentative application at the signal office has anticipated and explained storm movements not indicated or accounted for by the usual methods. as pertinent to this matter, there is instanced a study of the progress of thunder-storms made by berg, who observes that the line of storm front in every case investigated made a decidedly conspicuous bend into the densest part of the lines representing the absolute humidity. * * * * * scientific conditions have so changed that in these later years it becomes more and more difficult for investigators to publish any work which may be characterized as _magnum opus_. under this head, however, must be classed buchan's important memoir on the distribution of atmospheric pressure, temperature, and wind direction over the whole world; a large quarto volume, which contains much new material. it has been incorporated with the results of observations during the challenger expedition, in which series this work appears. the isobars and isotherms for each month in the year for the whole earth are charted on mercator's projection, and for the northern hemisphere on a chart constructed on a polar projection. in connection with an abstruse subject, to which buchan has paid so much attention, the diurnal variation of pressure, he opines from the challenger observations that the oscillations are due to the heat taken from the solar rays directly in passing through the air and instantaneously communicated through the whole mass from top to bottom by heating and evaporation of water on innumerable dust particles. the afternoon minimum, he thinks, is caused by upward currents removing a portion of the lower air. marked { } differences exist between the continental and insular types, since on islands the morning minimum is unusually large and the afternoon minimum so small as to disappear, while in continental types the reverse conditions obtain. * * * * * werner von siemens, in answering sprung's criticism on his general air currents, after repelling certain statements of sprung, describes his own theories, which are worthy of restating: . all winds are caused by the disturbances of indifferent equilibrium, and the motion of the air is to restore equilibrium. . these disturbances are caused through overheating of the layers of air near the surface of the earth by insolation, through unsymmetrical cooling of the higher layers by radiation, and through the heaping up of air masses caused by obstructions. . the disturbances are adjusted by ascending currents, wherein the particular species of acceleration occurs in which the increase of velocity is proportioned to the diminution of pressure. . the upward currents correspond to equally great descending currents in which there is a decrease of velocity corresponding to the acceleration in the upward velocity. . if the region of overheating of the air is limited locally, a local upward current reaching to the highest layers of air arises, and whirlwinds appear with interior spirally ascending currents and outside similar spiral descending currents. the result of this is dispersion of the superfluous heat of the lower air by which the adiabatic equilibrium is disturbed throughout the whole column of air taking part in the whirling motion. . in case the region of disturbance of the indifferent (or adiabatic) equilibrium is very extensive, as, for example, the whole of the tropical zone, the temperature adjustment can no longer be accomplished by locally ascending whirls, and a whirling current must then arise involving the whole atmosphere. the same conditions apply to these as to the local whirls of accelerated upward motion and retarded descent in such a manner that the velocity at different altitudes arising from heat converted to work is approximately proportional to the prevailing pressure at the place. . in consequence of the meridional motion produced and maintained by conversion of heat into work, the whole atmosphere in every latitude must rotate with approximately the same absolute velocity. thus the meridional currents produced by overheating combine with the currents embracing the whole { } wind system of the earth, with the result of disseminating the excess of temperature and humidity of the torrid zone over the temperate and arctic zones, thereby producing the prevailing winds. . this is accomplished by the production of alternating local depressions and elevations of barometric pressure by the disturbance of indifferent equilibrium in the upper layers of the air. . "highs" and "lows" are a consequence of the temperatures and velocities of the upper currents. whence it follows that the most important problem of meteorology is the investigation of the causes and consequences of the disturbance of indifferent equilibrium of the atmosphere, and the weightiest problem in weather prediction is the investigation of the geographical origin or extraction of air currents pursuing their course above us toward the pole. * * * * * in pomortsew's treatise on synoptic meteorology, published in russia, there are full chapters on prediction of weather, whether from synoptic charts, from observations at a single place, or from prognostics of great length based on researches on the succession of warm and cold months. it also contains pomortsew's investigations on the types of pressure distribution in eastern europe, as well as the average path of cyclones. * * * * * the favorable opportunities afforded by the eiffel tower have been utilized by french meteorologists. m. angot states that during the anti-cyclone of november, , the temperature on the tower was several degrees higher than below. the change of weather set in earlier, with a strong and warm wind, on the tower, while the air at the ground was cold and calm. wind observations on the tower show a ratio of . at that height ( meters) to the velocity at a height of meters, as determined from days' observations, which, remarkable at such a small height, discloses the peculiarity of high mountain stations. * * * * * partsch, writing on evidence of climatic changes within historical times in the mediterranean region, remarks that too much attention has been given to changes in crops, the introduction of plants, and the limits of domestic animals. he states that existing information as to the harvest time of ancient days indicates an unchanged climate, while the land-locked lakes in tunis, which afford the best evidence on rainfall variation, show absolutely no climatic change. * * * * * { } van bebber, in writing on weather types, claims that a line drawn from the center of a cyclone perpendicularly in the direction of the heaviest gradients will in general be perpendicular to the subsequent path of the "low," and that these lows leave high temperature on the right hand. * * * * * hill, in describing hail-stones and tornadoes in india, explains them on the principle of the great diminution of temperature upwards in the air, but a critic, in combating this theory, objects to the high and low stations selected to show temperatures. * * * * * the so-called "weather plant" of the tropics has passed through the process of investigation with the usual result. it appears surprising that in these days it should be believed that any plant or animal can foretell weather hours in advance, particularly after considering the vast amount of proof as to the enormous rapidity with which weather-changes progress from day to day. * * * * * hugo meyer, in treating the precipitation of central germany for the ten years ending in , pertinently remarks that the same significance does not attach to the same rainfall for all places and different times of the year, for this average value is not the amount most likely to fall in any particular interval of time, since there is a limit to the extent of the negative deviations on one side--that is, or no rainfall, while on the positive side there is no limit. the most probable depth of rainfall, therefore, is less than the mean value, the preponderance of negative over positive deviations being about per cent. and sometimes as great as per cent. * * * * * professor w. m. davis wrote an interesting review of professor ferrel's popular treatise on the winds, published a year ago. commenting on the review, the editor of _meteorologische zeltschrift_, vienna, remarks on a very important omission in the treatise, namely, the absence of all reference to the diurnal variation of the wind and the many interesting relations it bears to other phenomena, a notable omission in a treatise specially devoted to winds. the treatment of the monsoon wind and its relation to the general circulation is highly commended by the editor, and indicated as being all new. * * * * * your vice-president has elsewhere expressed his opinion that monsoon winds, applying the term by liberal construction to signify winds which recur with returning seasons, cannot with { } any degree of correctness be asserted to prevail in the united states. it is true that the prevailing surface winds of the greater part of the united states come from the western quadrants--that is, between southwest and northwest--and so are in substantial harmony with the general atmospheric circulation as shown by the upper-wind currents of mount washington (from the northwest) and pike's peak (from the southwest). but, apart from the easterly and northeasterly trades on the florida coast, it appears from the records that in no case for any considerable section of the country do per cent. of the winds blow, for any consecutive number of months, either from any single point or from two neighboring points of the compass. occasionally, however, the local configuration of the country is such that winds are drawn up or down valleys, and, being diverted from their free and proper direction, the wind in such cases follows the trend of the valley or depression. * * * * * in general your vice-president would feel inclined to refer only casually to the work proceeding from the bureau over which he has the honor to preside, but this year has been marked by special researches and investigations of general interest. as the work of investigation has been entrusted to the professors of the signal service, due credit should not be refused them from their own official chief. special reference should be made to the work of professor charles f. marvin, whose successful experiments on wind pressures and velocities have attracted the attention of experts both in europe and in this country. unfortunately there was available only a small sum (about one hundred dollars) for the expense of experiments, but with this petty sum, supplemented by his ingenuity, professor marvin has very satisfactorily determined the coëfficients of the various forms of the robinson anemometer, with which instrument the velocity of the wind is very generally determined. following these investigations, the royal meteorological society of england reopened the question, which, after a costly set of experiments with results widely differing from those of professor marvin, had been considered closed. the general results of these researches, which are believed to be sufficiently definite for general questions, are not only prized by the scientist, but they are of value to the engineer and the builder. indeed, to all interested in costly structures or extended works liable to harm from wind pressures, the factor of safety is { } a matter of no small pecuniary importance. these experiments show that, as was formerly believed to be the case, the wind pressure varies as the square of the velocity of the wind, expressed in miles per hour; but a most important fact has developed, namely, that the pressure in pounds per square foot is equal to the miles of hourly velocity multiplied by . instead of . , as was formerly assumed. professor marvin was not content with one system of experiments, but he further attacked the problem in a direct manner by a method which checked and verified his experiments with the whirling machine. on the summit of mount washington, at an elevation of , feet, he obtained simultaneously and under the same conditions, by automatic and electrical apparatus, continuous registration of the pressure of the wind in pounds per square foot and of the velocity in miles per hour. the results thus verified can be considered as conclusive from a general standpoint. the corrections for the robinson anemometer thus determined from these experiments are comparatively unimportant at low velocities, say from to miles per hour, being only a fraction of a mile per hour. the uncorrected velocities, however, are in all cases too large, and by greater and greater amounts the higher the velocity. at miles per hour the observed velocities are about miles per hour too high, and for an indicated velocity of miles the experiments show that the actual velocity is but a fraction over miles per hour. the anemometer formula found to satisfy most closely the entire range of experiments has the following form for velocities in miles per hour: log. _v_ = . + . log. _v_. this difference indicated by the formula may seem small and insignificant, as it is in the case of light winds, but at very high velocities the differences are very great. for instance, an actual velocity of miles per hour may occur at some time in almost any locality of the united states for a few minutes, and even greater velocities are occasionally reported, apart from severe tornadoes. under the old coëfficients for the robinson anemometer an actual velocity of miles per hour would have been reported as miles per hour, which under the old factor of . would mean a pressure of . pounds per square foot; but when considered with reference to the true velocity of miles, under { } the new factor of . , the pressure would only be . pounds per square foot--a reduction of over per cent. from the pressure-values formerly accepted. professor marvin has undertaken to verify, and also to extend to even lower temperatures, the observations of regnault as to the pressure of aqueous vapor at low temperatures, especial attention being given to temperature conditions from ° centigrade to - ° centigrade. these observations disclose, below ° centigrade, small but constant differences from the values assigned by regnault. in all this work professor marvin has shown such ingenuity of resource, such skill in adapting means to the end, and such deftness in improvising and manufacturing the requisite instruments as have elicited commendation from all who have seen his work and followed his methods. your vice-president alludes to this not only to give that credit rightfully due to professor marvin, but to illustrate this as a type of the highly important work which is being done in all branches of science here in washington by young men sometimes illy equipped as to means, and still more illy paid. men engaged in work of original investigation should receive higher pay than clerks in charge of routine duties; but unfortunately the majority of them do not. * * * * * the work of professor hazen in charting tornadoes and in determining their relative frequency and severity is directly in the line of the geography of the air. great attention had previously been given to this subject by lieutenant john p. finley, who, with indefatigable industry, had accumulated an enormous mass of data relative to these violent outbursts of nature's forces. the figures and deductions previously put forth under the authority of the signal service having been questioned, the chief signal officer felt obliged, in view of the growing practical importance of the question, as indicated by the great sums annually paid out in the ohio valley and in the trans-mississippi region for protection against tornadoes, to reöpen the subject. instructions of the most conservative character were given to professor hazen to determine carefully the prevalence and number of tornadoes in the united states, the areas devastated by them, and the number of lives lost annually. this work was carefully scrutinized during its progress to see that it should be devoid of theory and rest on the solid basis of fact. the results are most assuring to every { } one, and must serve to allay the unreasonable fears of the inhabitants of the so-called "tornado districts." it appears that there is no part of the united states in which annually more than one square mile of devastation or severe destruction can be expected for each , square miles, although cases of _limited destruction_ may occur annually for about every , square miles of area. in no state may destructive tornadoes be expected, on an average, more than once in two years; and the area over which total destruction can be expected is, as shown by the foregoing figures, exceedingly small, even in localities most liable to these violent storms. the annual death casualties from tornadoes have averaged, in the last years, annually; but it is believed that the death rate from lightning is greater than that from tornadoes, since during march to august, , the names of are on record who have lost their lives by lightning, although the data are incomplete, especially as regards the southern states. these statistics cannot be passed by lightly, however, and it is doubtful if in the main they are much in error. by them it appears from five years' record that the average annual death rate by lightning in the united states is . per million of inhabitants, or . above the average. in sweden, for sixty years, the average has been . ; in france, for forty-nine years, . ; in baden, for seventeen years, . ; and in prussia, for fifteen years, . per million. other figures, given by a life-insurance agent in st. louis, which the author claims to have compiled with great care, place the average annual rate of death from lightning in the united states at , being more than double the deaths from tornadoes. it must be understood that these figures are not vouched for, and must be very cautiously received, as originating with companies interested pecuniarily in the statistics. on the whole, therefore, it may be safely assumed that tornadoes are not so destructive to life as thunder-storms. * * * * * professor thomas russell has formulated a method for prediction of cold waves. they always occur after "lows" and before "highs," and different cold waves vary in extent from three "units" to sixty. a "unit" of temperature-fall is taken as a fall of twenty degrees over an area of , square miles. the temperature-fall curves in the united states are approximately elliptical in shape. perfect ellipses represent actual temperature falls with an error not exceeding six degrees in { } most cases. these fall lines are intersections of planes with a cone which graphically represents the totality of temperature-fall, the contents of the cone being equal to the area of its base multiplied by its altitude, which is the greatest fall in temperature at the center of the cold wave. a formula has been devised, based on special cases, representing the amount of fall in terms of the amount of barometric depression in a "low," and the amount of excess if a "high," and the density of the isothermal lines in the region. from proper consideration of the type of low area, shape of isobars, and position of the long axis, definite conclusions can be drawn as to the subsequent shape of the elliptical twenty-degree temperature-fall area and its position. a method has been devised, also by professor russell, for determining the maximum fall of temperature at the center of the cold wave. the maximum fall and extent of fall being known, from suitably prepared tables, the area of twenty-degree fall can be derived. previously prepared pieces of card-board are laid in the proper position on a map of suitable scale, and lines drawn around them. between the line representing the twenty-degree fall and the center, the other falls of thirty degrees, forty degrees, etc., are sketched in. * * * * * the foregoing sketch of the geography of the air may appear too superficial and limited for the purposes of this society, but its further elaboration was impracticable. indeed, the subject of meteorology could hardly have been touched upon this year had it not been for the courtesy of professor russell in placing at my disposal notes upon translations from foreign publications, especially from the german; which publications i have been unable to examine save in a casual way. the address, as it is, is submitted only in the hope that it may serve, if no other purpose, at least to indicate the great interest which now obtains in the geography of the air, and which manifests itself in the production of meteorological pamphlets and publications too numerous to permit any one charged with important executive duties to examine them all, even in a non-critical way. none _on-line data-acquisition systems in nuclear physics, _ ad hoc panel on on-line computers in nuclear research committee on nuclear science national research council _national academy of sciences washington, d.c. _ this is a report of work under contract nsf-c , t.o. between the national science foundation and the national academy of sciences and under contract at( - ) between the u.s. atomic energy commission and the national academy of sciences. _available from_ committee on nuclear science constitution avenue washington, d.c. preface the first digital electronic device employed to collect nuclear data was the binary electronic counter (scaler) of the 's. in the next decade single and multichannel pulse-height analyzers appeared, still using vacuum tubes. in the 's the development of multichannel analyzers continued vigorously, with vast improvement of the analog-to-digital converter sections and with the introduction of computer-type memories, based first on acoustic delay lines and a short time later on ferrite cores. the replacement of vacuum tubes by transistors beginning in the latter half of the 's accelerated the pace of development and application of all types of electronic laboratory instruments. the 's was the decade of the computer. before the 's almost no on-line computers were used in nuclear research, but since about the computer has moved into the nuclear laboratory. it provides the research worker with an immensely flexible, powerful, and accurate tool capable of raising the research output of a laboratory while eliminating the most tedious part of the experimental work. the phenomenal speed of development of computer hardware, software, and methodology contributes to the difficulty experienced by everybody involved in decision-making processes regarding data-acquisition systems. since the cost of a computer system is often a sizable fraction of the total cost of a new laboratory, there is urgent need for a set of guiding rules or principles for use by a laboratory director planning a system, a reviewer going over a proposal for support, or a potential funding agency considering proposals and reviews. the purpose of this report is to assist in filling this need. the material presented is current through . although we deal with a field that is developing rapidly, we hope that a substantial portion of the material covered will have long-lasting value. the report was prepared by the ad hoc panel on on-line computers in nuclear research of the committee on nuclear science, national research council. appointed in march , the panel first met in washington, d.c., on april , . the original members of the panel were h. w. fulbright, h. l. gelernter, l. j. lidofsky, d. ophir (through late ), l. b. robinson, and m. w. sachs. in june , this group prepared an interim report. l. j. lidofsky was on sabbatical leave in europe and therefore could not participate during the academic year - . early in j. f. mollenauer and j. hahn joined the panel. the panel has reviewed the present state of the field and has attempted to anticipate future needs. we have agreed on many important matters, including especially useful design features for computers employed in data acquisition, as well as types of organization of data-acquisition systems suitable for various purposes, types of software that manufacturers should supply, and approximate costs of systems, and we present a number of recommendations in these areas. however, the panel makes no recommendation on standards for computer hardware, such as logic levels and polarities, because of a conviction that these are now rapidly being established as a result of sound engineering progress and the pressure of economic competition in the fast-moving computer business. throughout this report we have expressed opinions based on our own experience and on the best information at our disposal. the nature of the report seemed to demand some discussion of properties of specific computers by name. we have tried to be neither misleading nor unjust in our evaluations. we wish to thank everyone who has aided us, especially p. w. mcdaniel, c. v. smith, and g. rogosa of the u.s. atomic energy commission and the many scientists in aec-and nsf-sponsored laboratories who supplied the basic data on which the economic survey chapter is based. we are indebted to several members of the staff of the department of physics and astronomy of the university of rochester for assistance in the preparation of the manuscript, especially mrs. brignall and mrs. hughes. we also received initial directions and many helpful suggestions from d. a. bromley, chairman of the committee on nuclear science, f. s. goulding, chairman of the subcommittee on instrumentation and methods, w. s. rodney and p. donovan of the national science foundation, and charles k. reed, executive secretary of the committee on nuclear science. h. w. fulbright, _chairman_ h. l. gelernter j. f. mollenauer j. hahn l. b. robinson l. j. lidofsky m. w. sachs contents . the tasks and the computer a. introduction b. the tasks c. the computers d. matching computers to tasks e. on characteristic features of computers and related equipment . data-acquisition systems a. introduction b. a small time-shared data-acquisition system based on a pdp- computer c. a small system based on a pdp- computer d. a medium-sized on-line computer system e. a large system based on a single computer (the yale-ibm nuclear-data-acquisition system) f. multiple-computer systems g. a process-control system: the brookhaven multiple spectrometer control system h. relationship to a remote computing center . a review and analysis of expenditures a. the nature of the data b. breakdown of data for analysis c. types of computers d. some total costs e. breakdown of costs by systems f. rotating memory devices g. systems on-line with computing centers h. anticipated future expenditures i. investment in accelerators, computer systems, and laboratory budgets j. process-control application . summary and recommendations on system planning a. the need for on-line computer systems b. where should large-scale calculations be done? c. exercising economic judgment in planning d. on the utility of small and medium-sized computers e. growth considerations f. short summary of conclusions regarding system planning appendix a: tables of properties of small and medium-sized computers appendix b: background information for chapter , a review and analysis of expenditures chapter the tasks and the computers a. introduction on-line data-acquisition computer systems are made in a wide range of types and sizes. in all cases at least one electronic computer is involved--a stored-program machine--because wired-program devices such as pulse-height analyzers are not considered to be computers. the rest of the system typically consists of input/output (i/o) devices such as analog-to-digital converters (adc's), printers, cathode-ray oscilloscopes, plotters, and control devices, which may include, in addition to the console typewriter, switch boxes to simplify the control of special types of operations and perhaps a set of logic circuits associated with the input system, used to provide preliminary selection of incoming data. in a small but increasing number of cases a computer is seen dedicated entirely to a "process-control" application such as the automatic adjustment of the shim coils of a variable-energy cyclotron or the control of data acquisition in a nuclear-scattering experiment, adjustments such as changing the angle of observation being made essentially under direct automatic control of the computer. the smallest on-line systems use the smallest commercially available computers; the largest use computers bigger than those which until recently served most computing centers. large systems sometimes include one or more satellite computers. the cost of individual systems ranges from $ , to $ , , , approximately. the total cost of computer systems in low-energy nuclear laboratories is estimated by now to have reached about $ , , . (there has been a larger expenditure in the high-energy nuclear field, where computer systems have been employed extensively for some years longer and where experiments are so expensive that the economic advantages of computer use were quickly recognized.) b. the tasks we first list the main uses to which on-line computer systems have been put. we start with the simple operations, which we call class . _class operations_: a. accepting digital data from external devices and storing it in computer memory. b. preliminary processing of incoming data, on-line, before storage. this usually involves only operations of logic and simple arithmetic. c. controlling the presentation of data via cathode-ray oscilloscope or typewriter, often for the purpose of monitoring the progress of an experiment. d. controlling the recording of digital data on magnetic tape, paper tape, or other storage medium. e. controlling an incremental plotter. f. controlling the output of large quantities of data via a line printer. g. transmission of quantities of data between two computers or between a computer and a pulse-height analyzer or other device having a magnetic core memory. * * * * * several operations of intermediate complexity we will label class . _class operations_: a. processing of data already accumulated and stored either in memory or on tape or other medium (off-line processing). this data reduction is often more complicated and lengthy than the preliminary on-line processing referred to in (class b). b. calculation of information required by the experimenter during the experiment, for example, kinematics tables and particle energies corresponding to field strengths in analyzer magnets. c. process-control operations, in which the computer directs or regulates a sequence of events in an experiment. under program control the computer monitors the course of the experiment and supplies signals that cause automatic changes in experimental conditions, such as starting and stopping times of event counting, angles of observation of scattered particles, and accelerator energies. such applications are designed to relieve the experimenter of unnecessary labor and to reduce the probability of error in routine operations. * * * * * our final class involves even more complex calculations. _class operations_: a. complicated treatment of reduced data, including least squares and curve fitting. b. large-scale calculations such as those required for the evaluation of theoretical nuclear scattering and reaction cross sections, e.g., dwba calculations, which may each require running times of the order of minutes, even at a modern computing center. * * * * * apparently class operations do not always have to be done during the course of the experiment; in fact, they can in most cases be carried out later, leisurely, at the local computing center. nonetheless, calculations of the first type, and to a lesser extent the second, are currently being done at laboratories having large, powerful computers in their on-line data-acquisition systems. c. the computers . introduction because computers have proved useful in so many fields, many varieties are now on the market, quite a few of them having properties highly suitable for nuclear-data acquisition. the properties particularly useful are, first, the ease with which a great variety of external input and output devices can be attached (interfaced to the computer); second, provisions for rapid, efficient response to interrupt signals from external devices; and third, usually a means of transferring data from external devices directly into blocks of memory without use of the central processor, the transfer possibly requiring only a single memory cycle per word. (this is referred to as direct memory access through a direct data channel.) several types of small computers have appeared on the market during the past year, some having -bit words, but they are too small for general data-acquisition use, although valuables for special applications. for present purposes, the smallest useful machines have a minimum memory size of ( k) -bit words, which can usually be enlarged to k words by the addition of memory modules, while the larger machines have minimum memories of at least k, with provision for expansion to several hundred k. regardless of their size, the machines of the present generation all have memory cycle times around or µsec. . rough classification of computers before proceeding with the discussion it is convenient to find a simple scheme for classifying computers. the scheme adopted here is to divide them into three loosely defined classes--small, medium, and large--essentially on the basis of the properties of the basic central processors: small word length to bits useful memory size k number of bits in instruction or floating-point hardware orally offered approximate cost range $ to $ , medium word length to bits useful memory size to k number of bits in instruction to floating-point hardware option sometimes offered approximate cost range $ , to $ , large word length to bits useful memory size at least k number of bits in instruction or more floating-point hardware approximate cost range $ , or more computers do not fall neatly into these three classifications, especially since manufacturers offer many optional features; therefore, some argument about the assignment of a particular machine to one or the other class is possible. this is especially true with respect to the small and medium types. the properties of a large number of small and medium-sized computers are given in appendix a. information on larger machines can be found in the adams associates _computer characteristics quarterly_. d. matching computers to tasks having classified both the computers and the jobs that they may be called on to do, we now ask this question: how suitable is each of the three types of computers for each of the three classes of jobs, given that in every case the acquisition system consists of a single computer coupled to all necessary input and output equipment? . large computers we start with the large computer system. all classes of jobs can be handled by this powerful system. however, we should question the wisdom of assembling a system based on a large machine unless a substantial amount of numerical calculating is anticipated, because the essential advantage of the large computer--the advantage that costs so much--is its capacity for rapidly executing highly accurate floating-point arithmetical operations. . small computers the small computer system can handle the jobs of data acceptance, data manipulation, and output characteristic of the simple class operations, but they are suitable for very few jobs involving floating-point arithmetic. in fact, we must usually be skeptical about the use of small machines for any of the class operations except those of the process-control type, which in many cases would involve little if any arithmetic. (process-control applications have been rather few to date, but a rapid increase can be expected in this field, especially because of the convenience and low cost of small modern computers.) it is apparent that these machines have been designed as economical instruments specifically intended to handle class jobs. the smallest word length of a machine in this group, bits, is sufficient for storing in one word the output of a -channel adc unit, but it is not quite so convenient for handling the output of a typical scaler, which would likely require the use of two words. the capability of even a small computer system to convert experimental information into digital form, to transfer it into memory, to manipulate it, and to present it for inspection in a digested, convenient form, all at a high rate and essentially without error, is of immense value to an experimenter who has to cope with the abundant outflow of data from a modern nuclear experiment. . medium-sized computers the capabilities of medium-sized computers are less clear. these machines are superior to the small ones mainly in two respects: they have a more flexible command structure (i.e., they have a larger set of wired-in operations), and, usually, they have a longer word length. these features make them easier to program and give them a limited, but important, capability to execute floating-point operations sufficiently quickly and accurately for many purposes, even though these operations must in most cases be programmed, in the absence of floating-point hardware. we can reasonably conclude that the medium-sized machines will serve for any use listed in classes and . certain simpler calculations of class a are also expected to prove feasible, but few, if any, of those of class b. e. on characteristic features of computers and related equipment the value of any feature depends on its need in the application involved; therefore detailed, absolute statements regarding each characteristic usually cannot be made. however, the panel has discussed various features at some length, and we present here some general comments on the pros and cons of these features. among the items discussed are some, such as word length and cycle time, that represent basic, inherent properties of the computer; while a great many others, such as priority interrupts, are customarily offered as options. . word length the shorter the word length the cheaper the hardware, generally speaking, but the less the accuracy in calculations unless multiple precision is used. for example, although the -bit words of the pdp- match the accuracy of data from most adc's, they are too small not to match the output data from most counters; furthermore, indirect addressing is often required because a single word is too short to include both the operation code and the absolute address of a memory location. apart from addressing considerations, a -bit word is too small for many uses, e.g., in general-purpose pulse-height analyzer applications where bits or, better, bits should be considered a minimum. fortran programs for numerical calculations are in general best run on machines having at least -bit words, although -bit words are usually acceptable here when double precision can be used. . number of memory words in general the more words that a system can retain the better; but the greater the memory, the greater the expense. the cost must be weighed against the need. for simple handling of data, a k memory may be adequate, but in a large shared-time general-purpose machine a k or greater memory is essential. in the latter case, the resident shared-time monitor will probably occupy at least k of the memory, so with a k memory only k would be left accessible to users, and experience has shown that this much can be taken up completely by one user compiling a fortran iv program. a k memory is adequate for many process-control applications, but it is too small for many other applications such as general-purpose pulse-height analyzer use, where an k memory is highly desirable. adding a supplemental rotating memory device (disk or drum), at a cost per word about percent that of core storage, is often preferable to adding core memory. see below. . cycle time for most purposes the typical memory cycle time of to µsec is quite adequate. some of the modern computers have cycle times under µsec. . direct data channels these allow sequential depositing of digital data from external devices directly into blocks of computer memory without intervention of the central processor (direct memory access, dma). such input may require only one computer cycle per word, that being the next cycle after the one during which the interrupt signal arrives. this is the fastest means of getting data into memory, but it requires more external hardware and more complex interfacing than input through an accumulator of the central processor. most data-acquisition machines provide both possibilities. direct data channels can be valuable for interfacing to magnetic disks, drums, and tapes. . priority interrupts (nested) these can be very useful. they may cost as little as $ each, depending on the machine, and can be used to reduce greatly the overhead running time losses of the computer. in complicated data-taking applications many interrupt lines are desirable; to priority levels are generally adequate. the usual fortran compiler cannot compile programs that respond properly to interrupts, although a relocatable object code generated by the compiler can always be assembled with a machine-language subroutine designed to handle interrupts. enlargement of fortran compilers for data-acquisition use to include statements designed to handle interrupts is desirable. (see, for example, the discussion of the yale-ibm system, chapter , section e.) . mass storage magnetic media--drums, disks, and standard magnetic tapes--are employed here. dec tapes are useful and reliable, but they have only a small capacity. the use of such microtapes is also limited by their incompatibility with typical computer-center equipment. reliable, inexpensive incremental magnetic tape units are now available which can be operated asynchronously at about hz, too slow for many purposes. some of them can also be run much faster in a synchronous mode. drums and disks are highly desirable because they provide program-controlled rapid access to great volumes of data. typically, access times are of the order of µsec. in the past few years, good and inexpensive disks have been developed which are now on the market. some suppliers are ibm, cdc, datadisk, burroughs, dec, and sds. disk storage is cheaper per word than core storage by two orders of magnitude; therefore, it is preferable for applications where data can be organized serially and where access and transfer time requirements can be relaxed somewhat. for example, a small dec disk system for the pdp- holds up to k -bit words and has an average access time of µsec and a transfer rate of , -bit words per sec. it costs $ for the first k of capacity, plus $ for each additional k, including interfacing through the direct data channel. larger and faster versions are available. disks (or drums) should be important in future systems. magnetic tapes of the ibm-compatible type are valuable, especially for communication with machines at computing centers, but tape drives and interfacing are usually expensive. it often costs $ , or more to get a single tape drive in service, although the next few are usually less expensive. the cheapest tape drives available cost about $ . the cost of interfacing depends greatly on the particular computer. it may be as little as $ , but it is often in the neighborhood of $ , or $ , . . program input method because they provide immediate access, the most satisfactory program storage media are magnetic disks and drums, followed by the ibm tape. the most satisfactory cheap device for input of programs is the high-speed, punched-tape reader, but the advantages of using small "cartridge-type" magnetic tapes have recently been emphasized. recently, card readers have appeared which are much cheaper than the older ibm models. they can read - cards per minute. they cost about $ plus interfacing. examples: soroban, general devices, uptime. a simple means of restoring the basic loader program (other than toggling!) is desirable. many computers have this feature, e.g., the ibm series; the sds sigma , sigma , and pdp- . . memory protection hardware memory protection is necessary in multiprogram systems. it is very helpful in any machine with a batch-processing resident monitor and in other special situations. . parity check this feature is useful for purposes as detecting memory failures, but it is usually not worth its cost in computer speed and in capital investment in the case of a small system. . ease and cost of interfacing this is a big subject, partly because the organization of computers for input and output of data varies with the manufacturer. some computers such as the hewlett-packard and the dec models are especially easy to interface, whereas the automatic channels of the sds sigma computers and the ordinary ibm machines (e.g., the series) are very difficult. the ibm machines require an expensive control unit. it is said that before a competent engineer could order plug boards for sigma interfacing he would have to study the system for a month or two. however, once interfaced, these machines permit rapid input of data. interfacing a $ calcomp plotter to the automatic channel of an ibm or sigma series machine may cost much more than the cost of the plotter. . typewriters many small computers use teletype machines as console typewriters. the asr- teletype has not performed well, but it has recently been improved. the asr- and ksr- have excellent records, and the newer asr- and ksr- ( characters/sec) are very good. the ibm selectric has had a mixed reliability record which is, however, improving. in every case, expert routine maintenance is required. . index registers these are a valuable asset to efficient programming. at least one, and preferably more, is desirable, especially in the medium and large computers. . line printers these are of great use for obtaining a permanent ("hard copy") record, especially when large volumes of output are produced; however, they are expensive, usually costing $ , or more (including interfacing). in order to avoid tying up a large central processor during typewriter output of masses of data, a line printer is not only very useful, it is essential for efficient operation (and to spare the typewriter). a line printer can be _immensely helpful_ and can save much time in the process of developing and debugging programs. the cost, however, will often preclude its addition to a modest system. if the system has an ibm-compatible tape drive, the computer output can be written on tape and later carried to a computing center for printing. several industrial concerns are known to be working on new types of printers, some being dry-copy, nonpercussive types. one type which has already been marketed, the inktronic printer, operates by spraying ink at the paper from small tubes. the characters are well formed. it operates at about characters per second and costs $ . conveniently, it requires standard teletype interfacing, and it can be ordered with an optional keyboard. although it has exhibited a few new-product ailments in its first months or so of use, it shows promise of becoming a very useful device. another printer operating on a similar principle has just appeared--the a.b. dick company's videojet printer, priced at about $ . . plotters the overwhelming favorite is still the incremental machine called the calcomp plotter. it costs about $ and is easily interfaced to many computers. it is very accurate (about . in. in each direction) and provides valuable output to the experimenter. it can be programmed to plot experimental points and theoretical curves together on white paper in india ink, relieving draftsmen of considerable work and doing a more precise job. other incremental plotters are now on the market, e.g., the houston instruments version. varian has developed an electrostatic plotter to sell for about $ , . . cathode-ray tube displays at least four types are in use. the standard scheme involves the displaying of bright spots under control of the computer, which has generated appropriate words to cause _x_ and _y_ deflections of the spot when those words have been transformed by adc's in the crt unit. the pattern is rewritten continuously. a light pen held against a particular part of the display pattern can be used to signal the computer. this scheme works well but may produce a flickering image if the computer is interrupted frequently to handle higher priority jobs or if the display is so complicated that the rewriting period exceeds / sec. the expensive hardware option called a character generator is considered not worthwhile unless large amounts of text are to be displayed. on a in. x in. raster a matrix of dots x is sensible. a second scheme involves a disk or drum on which the computer writes the words to generate the pattern. separate reading heads send the words to the crt unit. thus the display, automatically rewritten over and over, is updated from time to time by the computer. the light-spot cursor and joy-stick method replace the light pen in this case. (in passing, it is worth remarking that a light pen is only as effective as the computer program allows it to be, that the effort of programming for light-pen control is usually not trivial, and that a substantial amount of core storage may be required. a means of display control perhaps not so popular as it should be is sense-switch control.) a third scheme makes use of a modern storage crt. the computer sends the pattern to the crt only once, and the display can persist until erased. this method is flicker-free and inexpensive, but the pattern is not so distinct and sometimes not so bright as in the above schemes. however, it is cheap. furthermore, the storage tube can be used alternately as an ordinary crt with quite satisfactory resolution. a storage version is thus possible which reverts to the standard scheme, for high-resolution inspection, when a button is pushed. the storage-tube scheme is probably the best buy for use in a typical small system. the tektronix company has recently announced a storage-tube device, type , which is said to generate a continuous video signal suitable for driving large-screen television monitors. a fourth scheme involves the generation of a video (analog) signal corresponding to the display, written on a disk or drum by the computer. reading heads then send the video information to a crt having a tv raster synchronized with the rotation of the medium. this is a good scheme where many displays are needed, but it is too expensive for many applications, costing upwards of $ , for the first unit. (for example, the data disc system display costs about $ , .) one display feature considered desirable by many nuclear physicists is rotation of isometric data plots. this can be accomplished in one of two ways: recomputing every displayed dot or using an appropriate analog device (potentiometer). because the latter is so cheap, clearly its use is more desirable than the recomputation of the rotated view. also, using a light pen on a recomputed display is especially difficult because the inverse computation has to be performed in order to maintain proper correlation with the original data. however, it should be noted that the tv raster technique is limited in this respect: rotating potentiometers cannot be used, and the image must be recomputed. the technology of displays is developing rapidly. . the role of external devices in many cases, especially where typical standard operations are involved, it is preferable to use external devices to handle preliminary selection and sorting of events, rather than to ask the computer to do the entire job. for example, particle identification by use of signals from two counters involves one or two multiplications and additions, which can be carried out almost instantly by a fairly simple external analog device, whereas a small computer would likely require at least µsec for the job, assuming calculation, and perhaps µsec, assuming table look-up. . time sharing computers as small as a pdp- have been successfully time-shared by several users in special applications. the justification given is that all the peripheral hardware can be shared also, so that the added constraints and programming difficulties are balanced by savings in hardware costs. computers have also been shared for simultaneous on-line data-taking in low-data-rate experiments. in working out the economics of time-sharing, the added hardware (such as crt's and remote consoles and memory protection) needed to allow simultaneous access by more than one user, as well as the extra memory space needed by the time-sharing monitor, should be considered. the greatest costs, however, lie in the added constraints placed on each of the users and in the greatly increased cost of programming. in many cases the use of two or more identical computers is preferable. however, in large, expensive systems time-sharing can be very useful. . software that should be supplied by manufacturer complete documentation should be provided, including listings, step-by-step user instructions, and some fully worked out examples. a. hardware diagnostic routines: to test memory addressing, instruction set and to test correct operation of every peripheral and special hardware feature. b. systems to edit, assemble, and debug programs in symbolic machine language: these should efficiently use any special i/o device such as magnetic tape, disk, or line printer. c. efficient subroutines should be provided for operation of any special peripheral device purchased from the computer manufacturer. symbolic language source tapes or card decks, listings with comments, and examples of use should be included. d. conversational fortran-type programs provided by some manufactures are useful for supplemental calculations. note: the following points apply particularly to the medium and large machines and become increasingly important as the computer becomes larger and more complex. e. fortran compiler and operating system, with convenient method to insert machine language instructions and subroutines. good compile and run-time diagnostics are essential. f. mathematical subroutines should be provided in binary and source language. g. complete specifications and documentation for the programming system should be supplied, so that programs prepared by users can be made compatible. it may be objected that this will cost too much, but not to do so will be very costly and frustrating to many users. . note on the cost of programming experience at brookhaven and berkeley has shown that a programmer can produce between and debugged and documented lines of program per day, depending on such factors as experience, when he is working on reasonably straightforward programming. when working on a complicated monitor system he would be considerably less productive. system programming is obviously very expensive, therefore the average person exploring the computer market would be well advised to consider the software support along with the hardware offered in each case. manufacturers vary greatly in this respect. a major contributing factor to the persistent popularity of the pdp- is that the software support is so extensive. _in general, the newer a computer, the less software is likely to be available._ chapter data-acquisition systems a. introduction . history the movement toward computer systems began in earnest about . much of the early work depended on the use of magnetic tape for storage of data, either raw or partially digested, the analysis of data being carried out later, off-line. more recently, computers have been used increasingly for on-line processing. the early work is well known and will not be described here. some of the more recent systems are basically very close descendants of one or another of the early systems. many varieties are now in service. most incorporate small or medium-sized computers, however, extensive new experience has been gained during the past two or three years of operation of a few large time-shared systems, in particular those in the tandem van de graaff accelerator laboratories at yale and at rochester, perhaps the first large systems in operation which were planned systematically for nuclear research. both operate with multiprogramming monitor control, background calculations being possible, on a low-priority basis, simultaneously with data acquisition. . possible systems simple rules for the design of various types of data-acquisition systems cannot be stated, but some examples of possible systems can be given. (see figure .) a. a simple system for pulse-height analysis work can be assembled from a small computer, a -in. tektronix cro, an adc unit, and a teletype with paper-tape attachment for a cost of about $ , , providing that a competent engineer is available, not counting programming and engineering costs. a calcomp plotter could be added for about $ . to maintain and operate the system at least a half-time technician-programmer would be required. [illustration: figure basic data-acquisition system.] b. a general-purpose system for use in an accelerator laboratory can be assembled from a medium-sized computer, two typewriters, four -bit ( - ) adc's, six -bit ( - , ) counters, a in. x in. cro display unit with light pen, two tape drives (for ibm tape), a calcomp plotter, and a fast paper-tape reader for about $ , plus the cost of engineering service and programming. at least one full-time technician-programmer would be needed for maintenance and programming. c. a large shared-time system of the smallest configuration which makes much sense consists of a large computer with a k memory, two typewriters, a fast punched-tape reader, four dec tapes (or the equivalent), one ibm-compatible magnetic tape, one cro with light pen, one incremental plotter, input devices for experimental data (adc's, counters, etc.), plus an interfacing system to link the external input-output devices to the computer. the interfacing system may include a fixed-wired "front end," such as that used at yale, a small computer, such as that used at rochester, or both. the hardware would probably cost over $ , exclusive of engineering, and to this must be added a large expense for programming, even if the manufacturer supplies a satisfactory shared-time monitor plus all the usual software. three men would be needed to assemble, maintain, and operate the system: an engineer, technician-programmer, and a full-time programmer, or some equivalent combination, assuming use of the system in a large laboratory with an active and continually developing research program. thus the cost of this "stripped-down" system must be expected to reach $ , before it is in full operation, and the cost of keeping it going, including salaries, overhead, and replacement parts will likely exceed $ , per year, although this could perhaps be trimmed somewhat once the system is running. furthermore, to run efficiently, the system would need additional components: another k (at least) of core memory, another ibm-compatible magnetic-tape drive, and a line printer. a rotating memory device would also be helpful. these would raise the cost by well over $ , . it is apparent that large time-shared systems are so expensive that they can ordinarily be justified only in the largest, most lively research establishments. . small computers as satellites in medium and large systems the use of small computers for coupling input and output devices to the main computer offers a number of attractive advantages, especially now that mass production and competition have brought the prices down so low that a large amount of hardware nearly ideal for the purpose is available at a bargain. some advantages: ( ) the small machine can control data acquisition, accumulating blocks of data while the large machine is doing background calculations, interrupting those calculations only occasionally to transfer raw or partially processed data. ( ) the small machine can continuously control the monitor cro. ( ) it can control output devices such as a plotter, line printer, rotating memory, or tape drive. ( ) it can carry out many logic operations on the incoming data. experience has shown that such operations are numerous, and from the economic point of view they should not be allowed to tie up the larger machine, which, at the same time, can better be engaged in complicated calculations. in some cases the use of two small satellite computers can easily be justified. the chief disadvantage: programming can be complicated. however, if the larger machine already has a time-shared monitor which recognizes the small machine as a typical input-output device (as is the case with the pdp- plus pdp- system at rochester) the programming problem is not bad. in the following five sections descriptions of a number of data-acquisition systems of various types and sizes will be given in order to illustrate concretely some practical system configurations. in each case a breakdown of costs and a discussion of the lessons learned in connection with planning, construction, and operation will be included. the systems are of the following types: two small, one medium, one large, two multiple-cpu, and one process control. b. a small time-shared data-acquisition system based on a pdp- computer . introduction in , two identical computer systems based on pdp- computers were set up in two different locations at the lawrence radiation laboratory (lrl), to be used by several groups of experimenters (see figure ). assembly of the hardware for the first system was completed months after delivery of the computer. assembly of the second system required only months. two years after operation commenced, the first satisfactory time-sharing monitor was completed and put into service. the basic use of these systems is pulse-height analysis. in principle many other types of operation are possible. [illustration: figure pdp- data-acquisition system at lawrence radiation laboratory.] [illustration: figure a switch panel used for data taking and control of crt display in conjunction with the pdp- computer. the switch-setting codes can be read into the pdp- accumulator under program control and are used to select branch points in the program. as many as eight of these units can be connected to the system. the lights are used to indicate program status.] . operational features data-reduction jobs currently possible in the shared-time operating mode include spectrum stripping, normalization, smoothing, storage and retrieval of data from magnetic tapes, graph plotting, printout, energy calibration, background fitting, peak integration, and transfer of data from a remote analyzer. remote control of the computer from up to eight experimental locations is possible using inexpensive switch panels (figure ). remote slave crt display is also provided. multiparameter pulse-height acquisition and analysis can be done on a time-shared basis but often requires all the computer's time and memory. . hardware the hardware configuration is shown in figure . the pdp- computer was supplied by the digital equipment corporation with an k memory ( bits) extended arithmetic hardware, microtape (dectape), paper tape, teletype, and cathode-ray tube (crt). the other items were built or interfaced at lrl. automatic memory increment and memory-protection hardware, together with suitable programming, allow a user to carry out simple data-reduction jobs with a live crt display while two other users are independently acquiring separate, -channel, pulse-height spectra in part of the computer memory, with computer-controlled-gain stabilization. adc dead time per pulse is less that µsec. up to words of the memory can be used for data (one pha channel per word), while machine language programs fill the remaining words of memory. . lessons learned from operating experience the system works well for pulse-height analysis, but for new applications, e.g., nuclear magnetic resonance magnet control, it needs additional hardware and programs. two groups of experimenters, doing chiefly pulse-height analysis experiments are very satisfied with the system. another group, with a wider range of interests, has been dissatisfied because of the time lag to implement new experiments. one programmer is now engaged full time preparing more programs. lack of free computer time has become a limitation for both users and programmers. provision for programming at the outset was inadequate. one full-time systems programmer should have been assigned to these systems for months. experimenters need fortran or similar language capability. a disk, or more core memory, would make this practical. the memory size is totally inadequate for multiple users because of the large amount of data space needed for the high-resolution spectra now obtainable with ge(li) detectors. an external k memory is being acquired for data acquisition in each system so that more of the computer memory can be used for computing. memory crt's are needed to provide independent displays for each user. a separate teletype for each user would be invaluable. a disk memory is needed for rapid overlay of programs and for sorting of multiparameter data. a "czar" should have been appointed for day-to-day assignment of facilities, consultations with users, and routine maintenance and upkeep of the hardware and programs. the "czar" could be a good electronics technician interested both in programming and in physics. . costs system costs for the pdp- with time-sharing are given in table . fabrication time is included as a dollar cost. engineering and programming times shown are one half those charged against two identical systems. additional special-purpose experimental equipment commonly used with the system includes gain stabilizers, analog pulse derandomizers, amplifiers, pulse pileup rejection, low-noise preamplifiers, and ge(li) detectors. table system costs--pdp- with time-sharing[a] --------------------------------------------------------- items costs man-months --------------------------------------------------------- cpu ( k, eae) pdp- , $ , dual microtape , calcomp plotter , calcomp interface / crt controller , large screen crt , small crt's ( in.) , / mag tape ( bpi, ips) , (with erase head) mag tape interface memory protection , direct memory multiplexer , adc's ( -channel, µsec , - per count) adc multiplex interface (automatic , memory increment) -parameter input to adc , (analog multiplexer) remote memory switch panels , cabling to experiments , k external, -bit, -µsec memory , ---------- ------ programming ---------- ------ $ , --------------------------------------------------------- [a] the pdp- is no longer made. its modern equivalent is the pdp- . c. a small system based on a pdp- computer . history and hardware this second example of a small computer system is also taken from experience at lrl. it was planned in february and first put into operation in the summer of . data were first taken with the aid of the system in the spring of , and the system programming was completed in may . the system is used extensively in experiments with the bevatron. [illustration: figure pdp- data-acquisition system at lawrence radiation laboratory.] the computer-system hardware consists of the items shown in figure . the pdp- has k of memory. the disk is a data disc unit with a removable disk ( tracks and a movable head) on the same shaft as a smaller disk and three fixed data heads. two of the fixed-head tracks are devoted to the display: they drive a hardware-translator continuously. a single display track is used when the number of points does not exceed . for larger displays two tracks are used alternately. the display is controlled from the switch panel. on-line operating functions may be controlled both from the switch panel and from the teletype. . programming three classes of programming have been completed: _system programs_: symbolic text editor, assembler, a general-purpose library system--all disk oriented. _data-taking programs_: these cause the adc to be read, control elementary sorting, update histograms resident on the disk, write raw data on tape, and monitor the beam. the bevatron has approximately sec of beam every sec. during a beam pulse the computer is devoted entirely to acquiring data, saving raw data in core, on the disk, and on tape. after a beam pulse, the -disk histograms are updated, then the display programs are read into core memory and the display is updated. the system is designed to be capable of accepting over events per beam burst, and it has met this requirement. _simple data-analysis programs_: these compute displays (linear, log, isometric, and contour--all double precision), read out the sealers, monitor the real-time clock, allow resorting of raw data from tape, and generate tapes for remote plotting. . lessons from operating experience the system now functions as originally intended and does its job very satisfactorily. the experimenter relies heavily on the main computer center for data processing. in assembling this system now, one would buy the disk already interfaced by the computer manufacturer; furthermore the manufacturer now offers programs that would greatly reduce the programming costs. a memory scope would eliminate the need for a disk-to-crt display interface. less-expensive magnetic tapes are now available. however, it would be better to buy the tape already interfaced by the manufacturer of the computer. the added cost of buying a -or -bit computer would have been almost completely offset by savings in the cost of programming. the addition of a fast printer (e.g., inktronic $ ) would have paid for itself in time saved during programming but would not be of much use in experiment. . costs the costs of the pdp- are given in table . table system costs pdp- -- --------------------------------------------------- costs man-months --------------------------------------------------- cpu, k, -bit $ , - data disk , - disk interface , crt display control , crt - mag tape ( bpi, ips) , mag tape interface , misc. interfaces , scalers (on loan) - - adc ( -channel, -µsec , - dead time) -parameter input to adc , - (analog multiplexer) remote console (switch panel) - time-of-day clock , - -------- $ , systems programs data-handling programs engineering diagnosis, debugging ---- --------------------------------------------------- [illustration: figure block diagram of emr data-acquisition system at columbia university.] d. a medium-sized on-line computer system . introduction an emr computer system has been installed and is being prepared for use with columbia university's neutron velocity spectrometer data-acquisition and analysis system. the spectrometer is characterized by high data rates and many events per burst. at present, peak arrival rates are approximately ^ events per second, with - events per burst and a burst rate of hz. the arrival distribution is random; therefore, percent of the interarrival intervals are nsec long, and percent are nsec long. in the future, peak arrival rates of ^ events/sec and - events per burst are possible, with a burst rate of hz. with an appropriate time-of-flight "front end," the will be able to handle the anticipated faster rates. the emr is a -bit, -nsec computer. the memory has a multibus structure which permits each bus to communicate simultaneously with a separate memory module. up to four memory buses may be purchased. the columbia system has two memory buses. if a high-speed buffered data channel is used, block transfer may occur at memory cycle speeds. with two buses, data may be stored in two memory modules at rates up to twice memory speed. alternatively, one bus, channel, and one or more memory modules may be dedicated to data acquisition, while the central processor and standard peripheral devices, using the second bus, simultaneously operate in the remaining memory modules. . description of the system a block diagram of the columbia system is given in figure . the system has three k core modules. memory bus is dedicated to a high-speed channel serving the time-of-flight acquisition system. memory bus serves both the central processor and a second high-speed channel. low-speed input-output devices, such as the operator's console, teletype, card reader, and plotter communicate directly through the processor. the high-speed input-output devices, namely, a magnetic tape unit, line printer, fixed head disk, and interactive crt display, communicate through the channel. the box designated as "time-of-flight system" represents special-purpose electronics, including a -mhz clock, time-quantizing circuits which "clock" an input event from one of the detectors to the nearest clock pulse following its arrival, a -mhz counter, and a -word derandomizing buffer capable of storing a new word of data (i.e., arrival time) every nsec. the number of channels, nominally , , is limited not by the front end but by the amount of core available for histogram storage in the system. (for the high data rates anticipated in the future, the time-of-flight clock speed and derandomizing buffer data acceptance rate will be increased to mhz. at the same time, an accumulating buffer of several hundred words capacity, with a -mhz data acceptance rate, will be added to empty the derandomizing buffer and store temporarily the time-of-arrival data prior to its transmission to the system.) [illustration: figure diagram illustrating mode of utilization of core memory in the columbia system shown in figure .] . how the system is used during the time-of-flight experiment, memory is utilized as follows (see figure ). the channel dedicated to data acquisition writes on alternate bursts, into two buffer regions, of approximately words each, in the top of memory module . the remaining parts of memory module and all module will be devoted to histogram storage (i.e., time-of-flight channels). module will contain a stripped-down monitor program and all data-handling programs, including buffer regions for the external devices other than the time-of-flight front end. programs will be capable of referring to all module or in full concurrency with data acquisition. reference to module will also overlap data acquisition, except for a period of high input data rate of -to -µsec duration per burst. with the type of memory allocation described, the system will permit the use of all standard i/o devices, concurrent with the essential operations of input data buffering and histogram generation. thus, new data may be stored on, or old data retrieved from, the disk or magnetic tape; either new or old data may be displayed on the crt; and the same or other data may be output with the plotter or line printer. control information will be input from the teletype, the operator's console, or from special-purpose switches. the importance to the physicist is that hard copy output is immediately available during data acquisition and may be used to monitor, or modify, the experiment. subsequent to the input data increase, a high-speed memory incrementing channel will be used to input time-of-flight data directly to the histogram area. with this channel the buffer area in module will no longer be required. histogram data will be stored in all modules and , and no program intervention will be required for histogram generation. between data-acquisition runs, the system will be used for data analysis. . present status the computer, with two memory modules and one channel and bus, was delivered in july . the remaining memory module channel and bus were delivered in the fall of , the crt arrived in june , and the line printer (which was not purchased from emr) came shortly afterward. the first time-of-flight run with this system was scheduled for december . during the period from delivery to the first run, one full-time programmer and approximately half the time of one physicist were devoted to the debugging of manufacturer-supplied programs and the writing of the on-line programs required for the run. it has been hoped that the system would be used extensively for the analysis of previously acquired data, beginning shortly after delivery; however, very little such use has proved possible, essentially because of the unreliability of the -cpm card reader supplied by emr. the lack of a line printer was also a factor. a more reliable reader has been purchased. the delivery of a line printer should rectify the second need. the development of high-speed, buffered, time-of-flight front ends has been a continuing interest at columbia. it is therefore difficult to estimate the precise costs of the time-of-flight system developed for use with the . a rough estimate of the design and development time is approximately engineer man years. . lessons from development and testing experience columbia chose to order the emr , even though at that time ( ) it was not in production, because it seemed a very powerful machine which matched the needs of the system planners. the alternate possibility open was to order a larger, much more expensive machine of proven capability. as it turned out, difficulties in the development of the caused a delay of over a year in the delivery of the main frame and of over two years in the delivery of the crt display. (when these delays became apparent, emr loaned columbia a -bit computer and also a small display for use during the interim period.) the emr is perhaps the most powerful -bit computer available today, in spite of one or two changes in the original specifications, but in order to get it columbia apparently traded time for money. . cost the costs of the columbia university emr system are given in table . table cost of columbia university emr system (prices from emr except where indicated) ---------------------------------------------------------- central processor with k core memory $ , additional k core memory , teletype, model , word/byte buffered channels , additional memory bus and control , additional cabinet assembly , card reader, cpm , magnetic disk and control , tape transport and control , levels, priority interrupt , crt display, including vector generator and light pen , -lpm line printer[b] , -------- $ , ---------------------------------------------------------- [b] purchased from printer manufacturer with interface. e. a large system based on a single computer (the yale-ibm nuclear-data-acquisition system) . introduction since early , yale and ibm research have been engaged in a joint study in the application of computers to nuclear-data acquisition. the main goal was the production of an integrated hardware-software system which is fully under the control of the experimenter in the sense that he can define his entire data acquisition and analysis process with a fortran program. the joint study may be divided into four areas: ( ) development of a suitable general and powerful data-acquisition interface and control unit (front end) with a set of compatible nuclear instrumentation modules (scalers, adc's, and general-purpose input registers). ( ) development of a suitable display system. ( ) development of a data-acquisition language (as an extension to fortran) and the necessary library routines to support this language. ( ) development of a general-purpose multiprogramming system for the selected computer (the ibm system/ , model ) into which the data-acquisition system could be incorporated. the first three areas became operational in july , within three months after delivery of the computer, using the standard batch programming system for the / as a basis. development continues on the multiprogramming system, which has now reached a state where users inexperienced in using the system can compile and execute fortran programs, but the data-acquisition components are not yet operational. . description of the system nuclear data are input by means of a general-purpose nuclear-data-acquisition interface and control unit, organized around the concept of an event, an occurrence in the real world which causes the outputs of a group of instruments selected by the user to be read into computer memory. sixteen independent events are provided for, with each of which may be associated any or all of different instruments (scalers, adc's, or general-purpose monitor registers) by means of a diode plug board matrix. the instruments themselves, also designed and built by ibm, are modular and completely interchangeable and enable the experimenter to configure his experiment in any way desired, i.e., to determine not only which instruments are to be read but also in what order. exclusion logic is provided to prevent processing of certain events if and when other defined events occur simultaneously (figure ). the cathode-ray-tube display unit provides a x point plotting oscilloscope with seven levels of intensity, character-generation hardware, a light pen, and a programmed function keyboard, by means of which the user can call in programs by pushing buttons. such programs can perform any function from changing displays gains to curve-fitting. they may be system-supplied or user-written and may be (and usually are) written fortran. a parallel, high-resolution photographic system permits computer assembly of publication quality illustrations. [illustration: figure block diagram of the ibm / system at yale.] the data-acquisition and display-programming system is composed of a group of subroutines which may be called from fortran programs for performing the various processes in data acquisition and display. for this purpose, a considerable number of additional statements have been added to the fortran language. these statements perform such functions as defining multidimensional pulse-height analyzers in the computer memory, performing pulse-height analysis using incoming data as channel numbers, and defining separate programs to process each of the classes of input events. all the special statements that make up the new data-acquisition language are implemented by means of a preprocessor which converts them into fortran coding, which the standard / fortran compiler then processes. . software system the general-purpose multiprogramming operating system is a multilevel priority system designed to provide access to the system simultaneously by an, in principle, unlimited number of users, each with unique priority. unlimited means that there is no arbitrary restriction on the number of users; the nth user can always get access if the facilities his particular job requires are not already in use. two types of user are recognized by the system: the basic unit of execution is the logical user, or task. each logical user has a unique priority level. switching between users is carried out as a response to i/o, timer, or external interrupts, at which time the highest priority user in a position to execute gains control of the central processor. the basic unit of memory protection is the physical user, composed of one or more logical users engaged in a common cause. physical users correspond to real people doing independent work simultaneously. by dividing his work up among a group of logical users, a person may take advantage of the parallel processing capabilities of the system in a natural way. since logical users within a physical user are not memory-protected against each other, they may communicate rapidly, at full machine speed. communication between different physical users is also possible, via real or simulated i/o devices. while this system by no means guarantees execution time to any but the highest priority user, it is adequate in a single-experiment environment. the assumption is that the experimenter, who has actual control of the computer at all times, loads his logical users in the order in which he requires their priorities. following this, other users load their jobs, getting whatever memory and i/o facilities remain. the amount of processor time available to the other users varies inversely with the experimenter's counting rate and the amount of processing he does on his data. in most experiments, the experimenter uses significantly less than percent of the processor time simply because those experiments requiring the sophistication of the computer also have rather low counting rates. _a priori_, it is estimated that the simple priority algorithm described above is not only adequate but pays a dividend in terms of reduced system overhead time as compared to a more elaborate algorithm. it also guarantees that no data will be lost due to the lower priority users being in the machine. in general the new system will provide all the facilities of ps within the multiprogramming framework, including execution of the fortran compiler simultaneously with data acquisition. table -------------------------------------------------------------- the basic system -------------------------------------------------------------- cpu: with k-byte memory, -µsec registers, external interrupts, floating point, one high-speed multiplex channel, one low-speed multiplex channel, and one single-disk storage drive $ , . ----------- standard i/o gear: one -v tape drive and control, data adapter for front end, card reader/punch, printer, data control unit, four disk cartridges , . ---------- data-acquisition and display subsystem[c]: display system (rpq on ) with function keyboard and light pen , . -vii scientific interface and control unit (front end) , . lecroy model m general-purpose registers and adc interface to , as designed for maryland , . lecroy model b scaler banks (each contains eight -bit scalers with separate inhibit, strobe, and reset, as modified for maryland) , . lecroy interfaces to connect model b to , . northern scientific -channel adc's , . ---------- data-acquisition and display subtotal , . =========== total $ , . -------------------------------------------------------------- [c] as previously stated, the data-acquisition and display subsystem intalled at yale is the laboratory prototype of the ibm equipment, for which yale paid only a nominal sum. table ------------------------------------------------------------- additional items needed to make a system identical to the yale system ------------------------------------------------------------- cpu and peripherals: additional high-speed multiplex channel with extra subchannel, high-speed ( / µsec) general registers, additional single-disk storage drive, memory protect, additional tape drive ( iv),[d] (calcomp) plotter and adapter, keypunch, six additional disk cartridges $ , . data-acquisition subsystem additional northern scientific k adc's , . additional lecroy m registers , . ---------- costs of extras , . grand total for basic system , . ---------- cost to copy the yale system total $ , . ------------------------------------------------------------- [d] about $ , can be saved on tape drives by using the slowest ones ( k bytes/sec) rather than the k bytes/sec units shown here. . costs to the laboratory interested in developing a system of the magnitude of the yale system, but not a copy, it must be reiterated that neither yale's out-of-pocket costs nor the cost of copying the system represents the total cost of development. ibm's development costs are not known, but they may be assumed to be very large. from ibm's viewpoint, the adc and scaler project is the least successful part of the whole project. although those instruments are technically excellent, ibm is either unwilling or unable to sell them at a price competitive with the costs of front-end and interface equipment available from the traditional nuclear instrument manufacturers. however, adc's and scalers available from the traditional sources can easily be interfaced to the front end (whose price is in keeping with its power and versatility). the university of maryland has followed this procedure. we therefore present the cost of copying the yale system by some other laboratory. in tables and following the example of maryland, we have not selected ibm adc's and scalers but rather less expensive components from traditional manufacturers, together with suitable adapters available commercially. the prices shown are to be considered strictly reference numbers and in no way constitute price quotations. . general comments on experience with the system starting by producing an operational data-acquisition software system running within the standard batch programming system for the / enabled the system to become operational within three months of the delivery of the computer. this not only enabled it to do useful work almost immediately but also enabled important experience to be gained which is being applied to the development of the multiprogrammed version. one of the main lessons so far is that a batch-oriented system barely begins to tap the real-time potentials of a computer such as the / . in a batch system, whatever analysis is needed during data acquisition must be somehow tied to the processing of events. if this is not possible, it is necessary to stop data acquisition in order to do analysis even though, on a millisecond time scale, plenty of cpu time is available during acquisition. multiprogramming software is necessary in order to utilize this available time. this means that multiprogramming not only makes the machine available to several people at a time, but, more important, it makes large amounts of parallel processing power available to the experimenter. it has also been shown quite conclusively that the ability of the physicist to program his own experiment (in fortran) gives him enormous power, power which simply would not be available on a suitable time scale if he has to queue up for the services of a system programmer. while the generalized event structure gives the experimenter considerable ability to deal with complex experimental situations, it has an overhead associated with it which limits it to about events per second. this is, of course, adequate for all experiments that demand such an event structure. for simple pulse-height analysis, it is unnecessary overhead, but it can be "turned off" in a trivial way, by simply defining the completion of filling of the buffer as an event and calling a special pulse-height-analysis program to process the entire buffer, bypassing the event sorting. this allows for close to , pulse-height analyses per second. there are, however, few situations that justify using a computer as powerful as the / in a manner just described (i.e., doing nothing but simple addition , times per second). therefore, such simple experiments will shortly be handled by means of a link between an existing multichannel analyzer and the computer. the system does not suffer from having the front end directly connected to the / . the data channel on the / is sufficiently sophisticated so that it performs all the functions that one might relegate to a small cpu placed between the front end and the / , without any interference with the program currently running in the cpu. the one application described above, which does warrant a separate processor, is handled best by attaching the processor to a separate input port rather than by placing it between the front end and the computer. this enables it to do its intended job without acting as a bottleneck in jobs requiring the power of the front end. it also, incidentally, will function as a completely separate data input terminal if two simultaneous terminals should ever be required. the particular display system employed has worked very well. because the display list is in the main memory of the computer, programming of light-pen and other manipulative actions is extremely easy, but at the price of large amounts of memory being tied up. it is clear that the system cannot support two such terminals if they are to be truly independent of each other. it is equally clear that the display is as useful in data analysis as it is in data acquisition. a second display terminal is therefore being added. the selected unit (built by computer displays, inc.) is oriented around the tektronix storage oscilloscope. it provides both alphameric and graphic display, as well as an interactive device (a cursor moved by means of a joystick) for a price of $ to $ , plus the cost of interfacing to the computer. f. multiple-computer systems . introduction at the rutgers-bell (rb) nuclear physics laboratory, work has been done with two different two-cpu systems. the first of these represented essentially two duplicate processors (figure ), and the second, now in the process of implementation, two processors of different size and capability (figure ). while full data are not yet available on the actual performance of the second system, an outline of the experience to date will be given. [illustration: figure the two-central-processor system of rutgers-bell.] . two equivalent processors the initial success of the original rb sds data-acquisition system was soon tempered by a result of its popularity: during most experiments the computer was unavailable for program development or data analysis. since most experiments required the use of displays and light pens in at least one stage of data analysis, the computer center could not handle the work. [illustration: figure the new rutgers-bell sigma -sigma system.] the solution adopted was to acquire another computer with the same instruction set (an sds ) and to provide switches such that the line printer, card reader, and plotter could be run from either computer. no provision was made for direct transfer of data from one computer to the other. . lessons from operating experience in practice this system worked out quite well. there was complete interchangeability of programs from the to the , which differed only in being five times faster. normally the switchable peripherals were run from the ; when the group taking data wished to print or plot current spectra, they consulted with the users, then used the peripherals with little more difficulty than permanently attached units would have involved. a further advantage of the switchable peripherals, in addition to the cost saving, was that the experiments associated with the could proceed while the peripherals were being serviced. the is exceedingly reliable, averaging less that one main frame failure per year, and the is nearly as reliable. the vast majority of service calls have been occasioned by the peripherals and have competed with data analysis but not with accelerator utilization. in addition to the switched peripherals, both computers were equipped with two magnetic tape transports, electric typewriter, and high-speed paper-tape reader and punch. while these units were also subject to downtime, the paper-tape system and the typewriter could be exchanged between the and . only the magnetic-tape transports required the use of the cpu during servicing, and the presence of two transports has usually meant that the second one could carry the load until the weekly accelerator maintenance period. while the reliability record of the central processors has been excellent, that of many of the peripherals has not. here is an excellent justification for renting computing equipment: if units do not work well, they can be returned. for a time, a low-cost card reader ( cards per minute) built by ncr for sds was used. it was unacceptable in reliability and was replaced by the univac reader which came with the . another unit returned was a cartridge magnetic-tape system built by sds. the ampex tm- magnetic-tape transports on both the and have been consistently poor in reliability, but no other unit has been available to replace them. a manufacturer's name does not seem to be a guarantee of good or bad quality--the line printer, also made by ncr, has been excellent both in reliability and print quality. . limitations on a twin-computer system while the two-computer system generally rated high in user satisfaction, considerations of performance have led to the design of a larger and more powerful system with totally new components. the , without wired multiplication or floating-point operations, was too slow for theoretical computation or for many types of data analysis such as those using monte carlo methods. interactive methods of analysis, using a display and light pen, have been found very effective in the cases where the could accommodate them but have not been available through either the bell laboratories or rutgers computer centers. a further limitation on the earlier system was that only one person could use the at a time. the generation of a display involved the full time of the cpu, and while multiprogramming might have been able to divert some cpu time, the k memory size did not permit it. data acquisition on the was limited in array size to the capacity of the core memory. for multiparameter experiments, three, six, or even twelve -channel arrays have been stored in core, but the advantages of live display available with core storage have discouraged anyone from handling large arrays by logging raw data on magnetic tape for analysis later. memory expansion would have been desirable, but the necessity of making the expansion on both the and the effectively doubled the cost. limited flexibility, then, is a major drawback of this type of system. as long as only two users needed to be accommodated, and each could adapt to exactly half of the total core storage, it was satisfactory and provided redundant facilities to guard against experiment downtime due to computer failures. . new directions in ordering a new computer powerful enough to handle most of the nuclear physics laboratory's data analysis and theoretical computing tasks, cost ruled out the acquisition of a pair of program-compatible computers. it was recognized that desirable features of the original system would have to be obtained in new ways. accessibility of the system for programming could be improved by running a simple time-sharing monitor on it. reliability could be enhanced by avoiding bargain peripherals and using only items of demonstrated high quality and by the capability of running the peripherals on either computer. the use of a separate cpu for data collection still seemed particularly desirable, however. a combination of a large (by present standards) computer with a powerful small computer as a front end was designed. it includes a display disk for refreshing displays without cpu attention, as well as for storing data arrays too large to be kept in core. the computers selected were a k, -bit sds sigma and a k, -bit sigma . the new system, with separate and nonequivalent computers, will have advantages over the old system in data analysis and general computation, because these will be done on the larger computer, either in time sharing or batch mode. time sharing should enhance the flexibility of the system by making it easier to generate and debug new programs, in addition to improving the accessibility. for the data-collection computer, rb will lose the advantage of a separate computer on which complete debugging of programs may be done. this loss can be tolerated since the fraction of the load carried by the sigma will be less than that carried by the in the old system. in the old system, very few distinct data input or display programs were written. a few subroutines and their calling parameters sufficed for all needs for six years; the logic and i/o operations unique to each experiment were written in fortran by the experimenters. in the new system, the sigma will be concerned with the operations used in the data acquisition and formating of displays; most of the rest can be left in the sigma , with routines sent over to the sigma under the time-sharing system. if the user should prefer, he can operate the sigma directly and make use of the sigma only for data storage. until very recently, program development on the sigma has been slow because it lacked means of getting program listings quickly. we have now developed an assembler for the sigma which runs on the sigma . the availability of card reader input and line printer output has greatly speeded sigma software development. the loading of sigma programs is also much more convenient, since they can be stored on the sigma disk and loaded exactly as sigma programs. it seems highly desirable to have an assembler for any small data-acquisition computer capable of running on another machine; the means of transporting the object code to the small computer is of less importance. the reliability of the new equipment has been excellent. only the card reader has had any downtime of consequence, and modifications seem to have resolved its problems. the sigma main frame has had no failures in months, and the sigma has had only one in the past year. if this record continues, the loss of the redundancy inherent in the old / system will not have any serious effects. . computer-independent data bus system one component of the new system is taking on an increasingly important role, although it had not been a part of the original planning. that is the computer-independent data bus consisting of system controller, bin controller, and register units. only the system controller is specific to a particular computer; moreover the same system controller design could be used on both the sigma and sigma by restricting the data path to bits. the register units are used to interface external devices to the computer quite cheaply; a typical register used here to interface an existing calcomp plotter to the new computers costs about $ in parts and labor. similar units are used to interface the sigma to the sigma and to the , to drive a temporary core-resident display on the sigma , to read pushbutton inputs, and to read adc's. the display disk controller now under construction uses these registers to furnish control information, although the data go directly to and from the core. at the present time, the registers are read and written under program interrupt control, but the design is not limited to program-controlled operation. by substituting a controller designed to operate automatically (directly to core or to the i/o processor) speeds approaching or µsec per word transferred could be obtained. such interfaces have been built for various computers using the european camac bus system, which is conceptually similar. the system is highly modular and is built into nim bins with modified back connectors. exchange of modular units has been very helpful in debugging the system, and presumably it will also be helpful in case of failures in operation. this is a much more satisfactory situation than that which was obtained with the adc interface on which rb collaborated with brookhaven. the latter unit was built with computer-type construction: commercial logic cards and wire-wrapped back panel. debugging of that unit was exceedingly laborious because of the lack of modularity in its components. the computer-independent bus system has not been expensive in manpower. it has required about man-months in design and debugging and somewhat less time in construction. the registers cost about $ , as mentioned, and the controllers $ to $ depending on the need for cable drivers. . costs the costs of the rb multiple-computer system are given in table . the figures are approximate and not the result of detailed accounting. table systems costs of rutgers-bell multiple-computer system --------------------------------------------------------------------- a. original / system systems/programming man-year interface design man-months man-months interface construction $ , including display $ , computer costs $ , /month; bought with rental allowance for $ , $ , /month; including line printer and card reader maintenance and updating man-year, over years b. new system (including some components not yet acquired) planning and expediting - / man-years systems programming man-years to date, more expected adc interface engineering design $ , construction and test , spent outside parts , time spent locally man-months data bus system parts $ , design and debug man-months construction man-months display disk system disk with heads and amplifiers $ , interface to sigma , sigma , and displays $ , (estimated, since design is not complete) -in. displays with analog rotators and light pens $ , three teletypes $ , miscellaneous technical work man-year computer costs sigma $ , sigma $ , purchase equivalent, but part is leased totals for new system cash costs $ , time man-years professional man-years technician --------------------------------------------------------------------- g. a process-control system: the brookhaven multiple spectrometer control system (mscs) . introduction in , a system based on an sds computer was put into operation at the brookhaven national laboratory to control data-acquisition processes involving eight neutron spectrometers and one x-ray spectrometer. the neutron spectrometers are located on the floor surrounding the high flux beam reactor (hfbr); the x-ray spectrometer was placed in the same building in order to facilitate linking it to the computer. the system can control the execution of experiments on all nine sets of apparatus simultaneously, yet each experimenter feels that he is working essentially independently of all other users. the system controls all angular rotations of crystals and counters, all detector counting, the data displays, the input and output operations, and automatic error responses. it can also perform most of the calculations necessary for real-time guidance of the course of the experiments. for example, the experimenter can mount a crystal on a goniometer at approximately the correct angular orientation, then he can specify to the computer where several peaks should be found, whereupon the computer will direct the execution of a trial experiment to find where the peaks do, in fact, occur, executing least-squares calculations in the process, after which the error in crystal orientation is known and the angular scales are automatically corrected. in another example, the computer is given as input information the crystal constants (unit cell) and the zone orientation of the crystal on the goniometer and is asked to produce a scanning of a given part of reciprocal space. the computer then calculates where to look, turns to a correct angle to check the intensity of a central peak, and performs the other necessary steps, making many decisions as it controls the execution of the entire experiment. . description of system when it was first assembled, the system included only two teletypes, both located near the computer. early in , a communications network was added to permit the installation of a local, assigned typewriter at each of the nine spectrometer stations, as well as three assignable remote teletypes located in the chemistry and physics buildings. this network incorporates a varian i computer. it permits any ordinary operation to be carried out from any of the remote stations, except program loading, which still must be done via the high-speed paper-tape reader at the computer. [illustration: figure the multiple-spectrometer control system at brookhaven national laboratory.] [illustration: figure block diagram of a single-spectrometer control station of the mscs shown in figure . [from d. r. beaucage, m. a. kelley, d. ophir, s. rankowitz, r. j. spinrad, and r. van norton, nucl. instrum. methods _ _, ( ).]] the major parts of the system (figure ) are the sds computer with a k, -bit memory, a bulk storage memory section comprising two magnetic tapes units and one , -word drum, the communication network, and the nine local control stations (scs) at the spectrometers. each scs (figure ) contains the stepping motors required for computer control of angular rotations of crystals and counters, together with shaft rotation encoders (optional, incremental type) to feed information back to the computer. each scs also includes manual controls, the electronic counters associated with the radiation detectors, counter displays, a decoding and control section, and other related equipment. . lessons from operating experience a. the system now does "all things imagined to be necessary." b. the computer has proved to be remarkably reliable, with a record of about , hours of use without a breakdown. c. a reasonable amount of preventive maintenance is done, mostly during the one week of four that the reactor is shut down. d. one person serves as operator and programmer (for simple jobs). he also transports magnetic tapes to the computing center for off-line data processing and performs smaller tasks. the average user does not need to do any programming. e. fortunately, the people who have written most of the programs have remained in attendance and have updated the programs frequently. machine-language programming has not proved to be a bad chore because the system is a fixed-hardware setup. f. modes of data collection can easily be changed. g. the overall performance is excellent. the only problem is an occasional wiping out of a program due to the fact that there is no hardware memory-protection feature in the computer. these accidents are estimated to cost at most a loss of a few percent of the running time. . costs the costs in manpower and dollars of the mscs are given in table . table cost in manpower and dollars of mscs ------------------------------------------------------------- a. engineering design and costs (professional only) over calendar months man-months cost ---------- -------- electronic equipment development, design, construction, and startup $ , mechanical development and liaison , system coordination, development, design, coding, etc. , parameter generating, data analysis, and programming , ---- -------- total $ , b. construction time and costs (technical) over calendar months man-months cost ---------- -------- system construction and interconnection $ , debugging and startup , documentation and drafting , ---- -------- total $ , c. major components cost ---------- original cost sds ( k mem., interrupts) $ , -kc mag. tape and controller , add'l. -kc tape , magnetic drum memory , spectrometer control stations , off-line paper-tape pre. unit , ---------- total $ , replacement: the major components of the mscs cannot be replaced by new line units as they are no longer in production. d. operating costs normal use per year --------- computer operator/programmer $ , computer maintenance , materials , misc. (minor system improvements) , overhead , --------- total $ , manpower required computer operator/programmer systems programmer (as required) maintenance personnel (part-time) note: all manpower with the exception of the on-call systems programmer has been costed in a above. e. mscs communications network the communications network adjunct to the mscs was started october , and it became operational early in . man- calendar cost months months ------- ------ -------- . engineering design and programming $ , . components (commercially available) , - - . construction , . ------------------------ totals $ , . ------------------------------------------------------------- h. relationship to a remote computing center . the small computer with a fast data link to a remote general computing facility although the use of a small data-acquisition and experiment-control computer on-line to a remote computing center machine is not uncommon in high-energy particle physics applications, we know of few such systems presently operating in low-energy nuclear physics. for the purposes of this discussion, we define "general computing facility" to be a relatively large-scale centralized installation charged with the responsibility of servicing a wide range of computing needs. the typical university computing center is our model for such a facility. in light of the fact that only a few years back the remote computer on-line to a general computing facility was considered to be the wave of the future, with plans for such systems under vigorous discussion at many low-energy physics installations, it is at first sight surprising that there is so little progress to report at this time. the van de graaff accelerator laboratory at the state university of new york at stony brook was one such facility planning to couple a pdp- on hand to an ibm system / available at the university computing center. it is instructive to examine what happened there. in , with the completion of the new accelerator scheduled within a year, it was decided that the best way to acquire the desired power and flexibility in computing support was through a coupled system of the kind under discussion. plans were formulated for a high-speed transmission line to a control unit on a selector channel at the computer center. since true time-sharing of the system / was not in the offing, a k-byte partition of high-speed core storage was to be permanently dedicated to the needs of experimental physics (including the particle-physics group), and a high-speed program-swapping drum and at least one tape drive were to be assigned to the physics users as well. what actually happened was that as funds became available to the low-energy physics group to implement its share of the remote link to the computer center, sentiment shifted to the point of view that the funds could more usefully be invested in a second pdp- installed at the accelerator, and the second small-to-intermediate class computer was in fact purchased. the two pdp- 's are coupled only by a switchable tape drive, with no plans at present for direct channel-to-channel communication. plans for a remote link to the computing center have been completely dropped; any further funds for computing will be invested in larger high-speed core stores for the pdp- 's, at least in the foreseeable future. conversations with the principals involved in the operation of the stony brook low-energy physics facility fail to yield a clear and uniform explanation of the change in computing outlook. one cannot escape the impression that the group was not wildly enthusiastic about the proposed remote linkup in the first place, and that the evident immediate benefits to the group of a second pdp- on hand for program debugging and experiment setup while the second machine was running an experiment were irresistible when compared to the future promise of a remote link to the ibm / . the physicists were not anxious to undertake what was expected to be a substantial systems program development task for the coupled system, being unconvinced that the result would be worth the effort. while they still wish to increase the computing power available to them on-site, they have elected to achieve that end by expanding high-speed core storage on their machines, at least until true time-sharing becomes available at the central computing facility. the coupled system at the university of manitoba cyclotron is representative of what was intended at stony brook. at that installation, the pdp- is linked to the computing center's ibm / by a control unit commercially available from dec for about $ , . the unit connects the pdp- (or its successor, the pdp- ) directly to a system selector channel, without requiring an additional control unit. the maximum data-transfer rate at manitoba over a -foot twisted pair cable is k bytes/sec. a relatively unsophisticated set of system programs has been written to control communication and transfer of data between the two computers. the only experiment to which the coupled system (as distinct from the stand-alone use of the pdp- ) has been applied is a p-p bremsstrahlung measurement, where the data are developed in wire spark chambers and plastic scintillation counters. information from the wire chambers defines proton trajectories, and pulse heights from the counters determine their energies. the pdp- first tries to reconstruct a vertex from the proton trajectories. if a point of origin can be determined for the protons to the required accuracy, the relevant coordinates for the proton trajectories and the pulse heights are sent to the ibm / for full kinematic and statistical analysis of the individual event; otherwise, the event is rejected. the large computer also prepares displays and plots of physical interest that are returned to the pdp- for display on the local crt or output on the local _x-y_ plotter. the remote computer operates in a multiprogramming rather than in a time-shared environment, with an assigned partition of k bytes. because of the well-designed program overlay feature of the / operating system, the manitoba group does not find itself restricted by this relatively small partition. because of other demands on the computing center, however, they are restricted in the use of this partition to hours/day and days/week. the operation of the coupled system is controlled almost entirely from the pdp- teletype, with / operator intervention required only for initial loading of the partition, off-line printout, and, of course, mounting magnetic tapes at the computing center. users of the manitoba system are pleased with the cooperation and service they have received from the computing center thus far, and they are anticipating no difficulties developing as their demands on the central computing facility increase. but while use of the coupled system for experiments other than that described is clearly possible and desirable, no information was available on plans for the future. the brookhaven on-line remote network (brooknet), where a pair of cdc machines sharing a common one million _word_ extended core storage unit may be interfaced over a high-speed channel to as many as remote data-acquisition computers, can be considered an extreme example of a coupled system. although the software for brooknet is reported to be complete and debugged, the system has not yet begun routine operation, and the first remote computer intended for low-energy physics application (a pdp- ) has not yet been delivered. (the only brooknet user at present is the chemistry department, which has a remote batch terminal: teletype, card reader, and printer.) . reasons for lack of popularity why has linking data-acquisition computers directly to computing centers not proved as popular as the obvious advantage of having access to an extremely powerful computer would lead one to expect? there are a number of contributing factors: . since the remote computer can be used only if it is in operating condition and if the necessary personnel are present, the physicist stands to lose some of his independence and flexibility of operation (often not four-shift operation). . most remote computers operate on a multiprogramming basis, hence prompt interrupts are not available. the waiting time for attention might typically be several tenths of a second, therefore the computer in the physics laboratory should be fairly powerful in order to handle the preliminary processing and buffering. with such a computer at work the necessity for fairly rapid access to the large remote machine may entirely disappear, or else the experimenter may be able to store partly processed data on magnetic tape for subsequent further reduction off-line at the computing center. . the total amount of time available to one user of a shared-time system per day is always limited. the amount of access time guaranteed by the computing center may not be sufficient. . in some cases there is a question of charges, and the total expense of involvement with the computing center may be comparable over a period of several years with the extra cost of buying a sufficiently large local computer for the laboratory to be able to handle all the essential on-line calculations. even though the calculations may take longer in terms of machine time, they may not require as much lapsed real time if there are stringent limitations on computer center access time. chapter a review and analysis of expenditures in this chapter we present a review and an analysis of total expenditures for on-line computing in a large number of laboratories supported by the atomic energy commission and the national science foundation through . (appendix b gives the background for this economic survey.) a. the nature of the data laboratory directors were requested to supply a separate report covering each data-acquisition system currently in use or under construction and, in addition, to supply an estimate of anticipated future requirements for the period - . the high-energy field was excluded. information was also requested on process-control applications, e.g., systems to control accelerator operation or to monitor progress and to execute control functions during the course of an experiment. in every case details were to be supplied regarding the nature and capability of the system and its cost in dollars and manpower during the design, construction, and operation phases. in all, different systems were reported by different institutions (listed in appendix b). berkeley, brookhaven, and oak ridge together reported . the various systems range in total cost (including manpower) from about $ , to about $ , , . most are in operation, but a few are under construction, and a few others are in the advanced proposal or design stage. plans for substantial expansions and proposed expansions of existing systems were also reported. there was a wide range of thoroughness of compliance with the request; for example, cost estimates ranged from the most meticulous analyses down to one case where no cost information whatever was supplied. in assessing the reliability and completeness of the data the reviewer concluded that in general the costs of manufactured hardware items such as central processors (cpu's), line printers, card readers, rotating memory devices, etc. should be regarded as reasonably accurate, while estimates of the amount of manpower used, and its cost, seemed much less reliable; in fact, the manpower item was frequently not covered, especially in connection with the preparation of systems software. whenever a report was more or less complete, and there seemed to be a reasonable good basis for doing so, the reviewer estimated appropriate values for missing items by making use of figures given in more complete reports on similar systems constructed or operated under similar circumstances. [illustration: figure breakdown of system for analysis.] with regard to labor costs, government laboratory people seem to be in a much better position to supply figures than are university people. the reviewer got the impression that the university respondents have, on the average, a much less clear idea of the dollar value of people's time and a much less clear idea of how to estimate realistically the man-hours consumed by various projects. b. breakdown of data for analysis because of the nature of the data the reviewer separated each system into three parts for the purpose of analysis: ( ) the data-acquisition central processor (cpu); ( ) the standard computer input-output (i/o) devices such as magnetic tapes, disks, card readers, printers; ( ) the complete data-acquisition subsystem (das). (see figure .) this breakdown has the advantage that the costs of the first two parts of the system are usually fairly accurately known. the cost of the das includes the price of all manufactured units closely involved in its assembly, including scalers, adc's, pulse-height analyzers, and the like (but not detection equipment), together with the expenses associated with all special construction, including engineering, fabrication, and parts. all engineering and fabrication costs associated with the entire system can logically be charged against the das, because the cpu and i/o parts, being assembled from standard manufactured items, generally are installed by the manufacturer without much effort or expense on the part of the laboratory personnel. questions occasionally arose in connection with the assignment of the cost of interfacing the das to the cpu. such costs were assigned to the das when the units involved were of a custom-built nature and to the cpu when they were manufacturer's items incorporated in the computer frame. the very wide range of types of data-acquisition equipment in use necessarily contributes to the spread in das costs. although a number of items of uncertain costs are lumped together in this definition of the das, the procedure adopted is believed to have led to a valuable overall picture of the pattern of expenditures. table types of computers used in the systems reported ------------------------------------------------------- type number ------------------------------------------------------- asi asi ( ) cdc a cdc ( ) ddp ddp ( ) emr emr ( ) ibm ibm / ibm ( ) pdp- pdp- pdp- pdp- , a pdp- , i ( ) pdp- scc ( ) sds sigma sds sigma sds sigma sds ( ) sds sds sds sel b sel a ( ) varian i ( ) --- total ------------------------------------------------------- a fourth item of importance in the analysis is the cost of system software programming. this is almost entirely a manpower item, assuming that program testing and debugging can be carried out without charge for the computer time involved. here there is considerable uncertainty in the estimates, especially with respect to university installations as well as systems which have been in operation for a long time, e.g., the large system at argonne. the total cost of a system is taken to be the sum of the four items listed above, namely, the cpu, the standard i/o system, the das, and the system software expenditures. in all likelihood the total costs tend to be too small rather than too large because of incomplete assignments of charges of various sorts, especially manpower. in many cases the totals seem reliable to or percent, while in a few others an error of or even percent would not be surprising. c. types of computers table gives a listing of the different types of computers incorporated in the systems reported, together with the number of units of each type mentioned. of the types, are machines designed with this general sort of application in mind; the exceptional three are the cdc a, the cdc , and the ibm . evidently, the pdp machines are the most popular ( units), followed by sds types ( units), and ibm types ( units). d. some total costs of the system reports, were sufficiently complete to be useful in a detailed analysis. a histogram showing the distribution of these in total cost is given in figure . one immediately sees that few systems cost less than $ , ; in fact only four were reported in this range. however, it must be pointed out that information was solicited regarding only those systems which had cost approximately $ , or more. the most common range is $ , to $ , , with examples. the total cost of the system at the yale van de graaff laboratory was not known when the histogram was prepared, but the hardware is reported to cost about $ , to duplicate and about $ , to copy, so if allowance is made for the cost of developing the software and for other manpower uses the cost would rise substantially. (this system is not one of the . the conditions under which the yale-ibm development are being carried out are so special that manpower costs cannot be assigned on the basis used in other cases. see chapter , section e.) [illustration: figure histogram showing the distribution of data-acquisition systems in total cost.] a breakdown of total costs for the systems is given in table , showing separately the total amounts involved in each of the four categories defined above. evidently, about percent of the cost goes for standard computer hardware, while about percent goes for special hardware and software required for data acquisition. table shows separately the hardware and labor costs in the das item. evidently, hardware is twice as expensive as labor in this case, on the average. table summary from "complete" reports ---------------------------------------------------------- percentage subsystem cost of total ---------------------------------------------------------- cpu's with memory and tty $ , , . standard peripherals , , . data-acquisition subsystem , , . systems software , . ----------- ----- total $ , , . ---------------------------------------------------------- table data-acquisition subsystem ---------------------------------------------------------- hardware $ , , labor , , ---------- total $ , , ---------------------------------------------------------- [illustration: figure cost of standard peripheral equipment plotted against central processor costs for systems.] [illustration: figure cost of data-acquisition subsystem plotted against central processor costs for systems.] e. breakdown of costs by systems in figure the cost of the standard i/o equipment is shown plotted against the cost of the cpu for different systems. the high point labeled "t" represents a system having many high-speed magnetic tape drives. the low point labeled "r" represents the rochester system, which must be considered unbalanced, because its only "standard" i/o equipment is four dectapes, which should, perhaps, have been defined as cpu items, since they cannot be used for communication with most computing centers. if a line printer and two ibm-compatible tape units were added, the rochester point would have to be raised at least as high as the position r'. the straight line shown in figure was drawn with a slope of one half. it may perhaps be taken to represent a rough statistical reflection of the collective experience accumulated over the past six years or so regarding the relative costs of i/o and cpu equipment. in figure das costs are plotted against cpu costs for the same systems. here the spread of the points is worse than in the previous case, as expected for the reasons mentioned earlier. the exceptionally high point labeled "pha" represents a system with three large pulse-height analyzers, two of them , -channel units, in the das. the straight line shown has the equation _y_ = . + . _x_. the overall das cost is percent of the total cpu cost. f. rotating memory devices one magnetic drum unit and disks were reported to be in service (in eight different laboratories). plans were reported for the installation of six more disk units and one drum (in five different laboratories). recognition of the importance of rotating memory devices in display applications is evident in the reports. g. systems on-line with computing centers two systems were clearly stated to be in successful on-line operation with external computing centers. (at least one more example, at the university of manitoba, is known: there a pdp- system is linked to an ibm / .) there are plans in various stages of development to connect nine different data-acquisition systems on-line with computing center machines, in most cases to operate on a delayed-access basis. h. anticipated future expenditures in cases where updating or enlarging of existing systems was said to be in progress, the costs reported were usually assigned by the reviewer to the present system, especially when money for the expansion seemed already available or very likely to become available. in many cases plans were in a less advanced state, but a fairly definite idea of the amount of money to be requested for expansion or for completely new systems was expressed. table summarizes these anticipated costs. table anticipated future expenditures ------------------------------------------ for expansion of systems $ , , for additional systems , , ---------- total $ , , ------------------------------------------ i. investment in accelerators, computer systems, and laboratory budgets c. v. smith and george rogosa have kindly made available approximate aec budget figures for nine typical university laboratories chosen from those which had returned information in response to dr. mcdaniels' request. (the laboratories are colorado, kansas, maryland, minnesota, texas, wisconsin, washington, yale linac, and yale van de graaff.) after adding similar information for rochester, it was possible to get a rough idea of the relative capital investments in accelerators and in computer systems and to compare those figures with the annual operating budgets (for ). total annual budget ------------------------ = . cost of bare accelerator total computer cost . ± . by averaging ------------------------ = . -> separate ratios for each cost of bare accelerator system total computer cost ------------------------ = . total annual budget if the ratio of the total computer cost to the annual budget is calculated for each of the ten cases, and then the results are averaged, one gets . ± . . if one quite unusual set of data (from a laboratory with a small aec budget) is eliminated the last result becomes . ± . , while the earlier results remain essentially unaltered. for the same nine examples we find that the average of the ratios of total computer system costs to bare accelerator costs is . ± . , thus this ratio is significantly more consistent. it is emphasized that the results given in this paragraph refer only to experience at universities. j. process-control application tables and give a summary of present and anticipated process-control applications disclosed by the survey. table current process-control applications ----------------------------------------------------------------- laboratory systems ----------------------------------------------------------------- anl van de graaff accelerator; large scattering chamber setup; x-ray and neutron diffractometers; automatic plate scanner bnl neutron spectrometers; x-ray and neutron spectrometers, nine in all michigan state cyclotron shim coils ornl slow neutron time-of-flight to measure capture and fission cross sections yale electron linac and beam optics; experiments with the linac ----------------------------------------------------------------- table future process-control applications ---------------------------------------------------------- laboratory systems ---------------------------------------------------------- michigan state control of entire accelerator system minnesota tandem van de graaff accelerator and beam transport system stanford nuclear reaction experiments ucla limited control of cyclotron ---------------------------------------------------------- chapter summary and recommendations on system planning a. the need for on-line computer systems the ultimate justification for assembling and using on-line data-acquisition systems must be made in terms of research output. the same considerations underlying judgments on the support of experimental research in other ways must therefore apply to computer systems. some reasons often given for the use of on-line computer systems are these: . modern experiments produce vast quantities of data which can be handled efficiently only by automatic calculating machinery. the experimenter gains greatly in effectiveness when the data are immediately converted into machine language, reduced by the computer, and presented to the experimenter in a convenient form. _comment_: undoubtedly true. fortunately a small system can satisfy this requirement in many cases. . some experiments "cannot" be done by other means. _comment_: more likely true in practice than in principle. . investment in a computer system is sometimes sound because it leads to a net reduction in the overall cost of performing experiments, either by eliminating some of the labor cost, by reducing the consumption of accelerator time, or in some other way. _comment_: true in many cases. making estimates of projected savings is easier in _ad hoc_ cases than in general. . having facilities immediately accessible for calculating nuclear-reaction kinematics, magnetic analyzer field strengths, and other phenomena during the course of experiments saves time and promotes efficiency. _comment_: true, however, much of this work can be done ahead of time, and much of it requires only a relatively short, simple calculation which can be executed on a medium-sized computer, sometimes on a small one. . given a sufficiently large computer system in the laboratory, its use for complicated data reduction and for theoretical calculations may produce an important saving of funds which might otherwise have been spent at the computing center. _comment_: this point may sometimes be valid, depending on a number of conditions, but the installation of a large computer as part of the data-acquisition system essentially on the basis of this argument is questionable, in view of the excellent facilities offered by modern computing centers. . some expense for the _development_ of computer systems and computer systems methods is justifiable as an investment in methodology. _comment_: true, although there is some question about the choice of places where such work should be done and about the correct source of funds to support it. b. where should large-scale calculations be done? at the very outset of planning one should examine very closely the question of the large-scale calculating required in the overall execution of the research program of the laboratory; then, if, as usual, it turns out that a substantial amount of complex calculating is anticipated, one should consider carefully the feasibility of planning to do that part of the work at the most readily accessible computer center in the vicinity, so as to be able to concentrate one's own energies and resources, especially capital investment, on the data-acquisition system. the use of a modern computer center offers enormous advantages, and most computing centers would welcome support. if this course of action is chosen, provisions must be planned from the start for computer-language communication between the computer center and the nuclear research laboratory via a medium such as magnetic tape. (direct wire transmission will often not prove feasible.) some key questions are: . how much large-scale computing is anticipated? . how much waiting time for results is tolerable? . can the local computing center handle the needs, and at what cost? . if the local computing center can handle all the needs, but only after acquiring certain additional support for equipment or manpower, might not the better course of action be to provide that support rather than to set up separate facilities? . can setting up a large system truly be justified? have all the extra costs and complexities of the large system been taken into account, including those associated with input and output devices, operation, maintenance, programming, management, and space? c. exercising economic judgment in planning since the ultimate criterion is research output, the role assigned to a computer system must depend on the nature of the work being planned. in some cases where a very specific use is intended, for example, in the case of a process-control application such as the argonne plate scanner or an accelerator controller, the conditions are simple enough to make economic judgment relatively easy to apply. in the case encountered in setting up an accelerator laboratory where a wide variety of experiments is to be performed, conditions are much more complex. it is now widely accepted that any such laboratory should have a computer system, but what is not so clear is how extensive and expensive it should be. in other words, points - in b are accepted, and point is conceded possibly to be applicable. if sufficient funds are available, one sensible way to proceed is to use the accumulated collective experience outlined above. for example, one can say that experience has shown that the total investment in the computer system will be in reasonable balance with the capital investment in the bare accelerator if the ratio of costs is about one to five. departures from the rule may then be made to adjust to special circumstances. following this procedure means extrapolating from past experience, which may not prove a good guide, but this approach is similar to that often used in other matters bearing on the support of research. probability is involved. it should be noted that the actual expenditures for on-line equipment for nuclear research have far exceeded those projected at the "grossinger conference on the utilization of multiparameter analyzers in nuclear physics" in . in times of economic stringency it may be necessary to take a hard look at points - in b above before deciding how large a computer can be justified. a medium-sized computer is sufficient for most data-acquisition demands but not for large-scale calculations of a theoretical nature or for an occasional complicated piece of data reduction. often it will be advisable to plan on carrying out all large calculations at the computing center, in which case a medium-sized computer will probably suffice for data acquisition, and a saving of about half in capital investment and operating expenses can be achieved. d. on the utility of small and medium-sized computers if economic realities and good judgment should dictate the choice of a smaller system, the laboratory will still be well off. there is a tendency not to recognize the full capabilities of modern medium-sized and small computers, which, given intelligent programming, are very powerful. although programming is in general expensive, the return for a modest amount of it in terms of data-acquisition performance may be very impressive. for example, the use of tables calculated ahead of time, stored on magnetic tape at the computing center, and read into the data-acquisition machine along with its control program offers a way to bypass the need for various sorts of calculations which might have been done on-line on a larger system. increased efficiency of data acquisition often comes from the use of such methods, reflected in increased data-handling rates. e. growth considerations the system planner should try to anticipate a possible future expansion. in the case of a cut-and-dried process-control application it will often be safe to assume that the system will not have to grow, but recent history shows that in the case of general-purpose systems growth is the rule. in fact, systems have sometimes had to be replaced by entirely new ones. the system planner must beware of pitfalls. if, in anticipation of a greater future need, a much larger cpu is ordered than current use demands, the anticipated need may not develop. or, if it happens that the money initially available for capital investment is so limited that it is all exhausted in buying the cpu, leaving the system badly short of conventional i/o equipment, then the system will remain painfully unbalanced until substantial additional funds appear. if those funds do not appear, the capability of the system will remain far less than the presence of the large cpu would suggest. (this is what happened at rochester, where three years after the system was installed there is still no card reader, line printer, or conventional magnetic tape drive system; in fact, there is no computer-language medium for communication with the university of rochester computing center.) [illustration: figure a data-acquisition system based on a medium-sized computer. prices are actual costs for equipment supplied by a well-known manufacturer. this system is powerful enough to satisfy most data-acquisition needs at a typical low-energy accelerator laboratory.] the correct strategy to employ in every case should be consistent with the size of the laboratory and with the capabilities of its staff. a laboratory with a small engineering staff and with modest computing needs for the immediate future should certainly not plan to set up a large system. instead it could sensibly begin with a manufacturer-assembled, trimmed-down version of the comparison system (figure ), which could be enlarged later as occasion demanded and funds permitted. f. short summary of conclusions regarding system planning . planning and procuring a data-acquisition system today it is no longer necessary to develop one's own system. times have changed greatly. many systems now exist which work well and are worth copying. manufacturers and suppliers are prepared to deliver entire systems assembled and ready to operate, complete with all the necessary system software and varying amounts of utility software. although it may at first sight seem more economical to assemble a system within the laboratory, by use of laboratory personnel, in most cases it is now better to buy the system from a single supplier, completely installed and operable, saving one's own resources for matters more directly concerned with research. the costs in time and effort to develop a new computer system have been much larger than predicted, in almost every case known to the authors. large laboratories having strong engineering staffs are an exception; outside of industrial plants they are the places where new system development and assembly makes the most sense. . large-scale computations and computing centers in general it is best to plan to do all very large-scale computing jobs (e.g., shell model and scattering theory calculations) at a large computing center and to set up in the laboratory a system which is just large enough to handle comfortably the data-acquisition jobs. usually a medium-sized or small system will suffice. however, in some circumstances this will not be true. . remote large computing center on-line for data acquisition direct transmission-line coupling to a large, remote computing center may prove practical for handling occasional low-priority bursts of data processing, for example, when one can be satisfied with guaranteed access within about µsec, say, and a maximum guaranteed total access duration of no more than a few percent of any day. such a hookup may also be valuable for the handling of data input and output in the remote batch mode of operation, especially if a card reader (or high-speed paper tape or storage device) and a line printer are available for this use, in the laboratory. however, there are few if any examples of successful high-priority prompt-interrupt operation. one should be extremely skeptical about the feasibility of relying on this last mode of operation. . buying versus renting rental rates have typically been set so that if the anticipated use period exceeds about three years, economic prudence suggests purchasing a computer rather than renting, providing that the necessary funds for capital investment are available. this can only be true, of course, because the life expectancy of modern computers is quite long, certainly over five years. (also, one hesitates to trade in an old computer for which an excellent software collection exists!) the argument against renting standard peripherals is weaker, because they are electromechanical in nature and therefore have shorter lifetimes; furthermore, they tend to become outmoded. renting can be especially attractive in special circumstances. for example, a line printer can be rented for the early period of operation of a system, while extensive program development work is in progress, and returned later, when the work has been finished. . new computer or current model? computers are rapidly getting better and cheaper. this month's machine is much more powerful than last month's, dollar for dollar. new machines will always be appealing, but the prospective purchaser must balance their appeal against considerations of probable delivery date, software availability, completeness of documentation for both software and hardware, and in general the manufacturer's support capability. unfortunately, these factors usually weigh against a new machine. as a rule, even a medium-sized system based on a new model machine will not be in full operation for approximately one year after delivery, unless both the hardware and the software have been tried and proven in a previous installation. on the other hand, in the case of an older model the same factors may all be favorable, but now the machine probably gives less computing per dollar, and the advantage of an early return on the investment must be weighed carefully against the likelihood of somewhat earlier obsolescence. at some time during the life of a computer the manufacturer will very likely cease to support its software and, usually later, its hardware. . importance of software software is all-important, and it is very expensive to develop, both in time and money; hence a system planner should favor a central processor for which a large amount of software is supplied by the manufacturer, especially system software. in general, when a particular type of machine has already been delivered to many customers the manufacturer may be relied upon to supply the essential software needed to run a system: an assembler, i/o routines for standard devices, and usually a fortran compiler. the larger machines will be supplied with some sort of operating system (monitor), either for batch or time-shared operation. however, the specialized software needed for data acquisition will usually not be available unless it has already been developed by another user. a laboratory with limited programming resources should therefore give great weight to obtaining a system already provided with all essential software and should direct its own programming efforts to specific data-acquisition problems. contracting with an outside company for development of the specialized software is also possible, although the cost will probably exceed the salaries of in-house personnel hired to do the same job, and communication with an outside group is inconvenient. . utility of modern small computers many small, powerful computers are now on the market. they are inexpensive but very reliable. for many data-taking purposes they are quite sufficient, when equipped with appropriate peripheral devices and an adequate program library. . utility of disks and drums magnetic disk and drum bulk storage devices have also undergone much development recently. many good, small versions are now on the market at rather low prices. the capabilities of these units must not be overlooked. attaching a modern disk unit to a modern, small or medium-sized computer produces a powerful but economical combination. . need for adequate peripheral devices unless an appropriate set of standard input-output devices is provided, the computer will not be used efficiently. a balanced system with a small computer is likely to prove much more useful than an unbalanced system with a medium-sized computer. what is necessary will, of course, depend upon the uses of the system. for example, if a large amount of program development is anticipated, the inclusion of a line printer should certainly be considered, because universal experience has shown that line printers are immensely valuable during program development; on the other hand, as a rule they are not so important in most data-taking operations. . peripherals (brand x) it is often cheaper initially to use peripheral devices from a separate manufacturer, with interfacing provided either by the user or by an outside commercial firm. in this case difficulties lie in guessing the reliability of the devices and in achieving software compatibility. software developed by a computer manufacturer usually takes advantage of the peculiarities of his own peripherals. if an outside device is purchased, the additional cost for programming during the lifetime of the system should be considered. if competent engineering effort is available, an interface compatible with the computer manufacturer's software may be built, with a possible saving in programming cost. . input-output bus structures standardized input-output bus structures designed to simplify interfacing to computers have recently been developed. conspicuous among them is the camac system already accepted as standard in many european laboratories. it is now being introduced into a few american laboratories. before it can be accepted as a standard system here, a number of questions must be answered. for example, what types of external devices should be interfaced in this way, just adc's data registers, counters, and the like, or should line printers, card readers, and related devices be included? also, how much trouble will be encountered with manufacturers' i/o software, and how much will any necessary rewriting cost? also will all computer i/o structures lend themselves to such a system; specifically, are multiport systems suitable? a national committee is now studying the camac system to see if it, or something similar, should be recommended as standard in the united states. even after being recommended as standard, however, any such system cannot be considered successful unless manufacturers accept it and market a wide variety of compatible devices. from the manufacturer's point of view the risks here may seem considerably greater than they were in the case of the nim bins. it seems wise to keep watching for the outcome of this interesting development. . necessity for competence in machine-language programming whenever a new type of device is interfaced to a system, some form of machine-language programming must almost always be done in order to permit the handling of input-output operations involving the new device. this is true even in places such as yale, where the design emphasizes a maximum use of fortran. for this and other reasons, there should be at least one person on call who is skilled in machine-language programming and who understands the system. . manpower for programming and maintenance the manpower required to maintain the hardware and software of any system naturally depends on the size of the installation and the uses to which it is put. typically, a continuing effort must be expended on the improvement of system software and the writing of new data-acquisition programs. the existing hardware must be given preventive maintenance and repairs. furthermore from time to time a hardware change must be made. also, there are administrative matters; even the smallest system should have within the laboratory at least one person who will devote a large part of his time to administration, to the education of users, and to related matters. in many cases the laboratory has a contract with an outside firm, often the computer manufacturer, for maintenance of the computer, and sometimes the rest of the system as well. in other cases all or part of this work is done by laboratory personnel. sometimes several laboratory people are competent both in machine-language programming and in diagnosing and repairing hardware ills. such people are very valuable, especially if they are also competent to do interfacing of new devices. in some cases the experimenters do much of their own data-acquisition programming, in others essentially all programming is done by professionals. in some university laboratories much use is made of part-time student programmers, of whom there is now a considerable supply because of the growth of education in programming, both in high schools and at colleges. students are sometimes remarkably good at this work and stand to profit later from the experience, but they are transients, and effort expended in training them is lost when they leave. very roughly speaking, a small system will require a good fraction of the time of a technician-programmer, a medium system will require at least one full-time technician-programmer, and a full-time programmer, or some equivalent combination, assuming an active research program. appendix a tables of properties of small and medium-sized computers the comprehensive tables of properties of small and medium-sized computers appearing on the next pages are from d. j. theis and l. c. hobbs, "mini-computers for real-time applications," _datamation_, vol. , no. , p. (march ) and are reprinted here by permission of the publisher, f. d. thompson publications, inc., mason street, greenwich, conn. . ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- digital digital data mate decade electronic equipment equipment computer control data data computer computer assoc. corp. corp. automation corporation general systems, inc. corp. inc. emr hewlett-packard hewlett-packard hewlett-packard honeywell manufacturer/model number pdp- pdp- /l pdc- nova data mate- / a a b ddp- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- memory memory cycle time (µs) . . . . . . . . . . . . ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- memory word length (bits) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- minimum memory size (words) k k k k k k k k k k k k k ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- memory increment size (words) k k k k k, k, k k k k k k k k k ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- maximum memory size (words) k k k k k k k k k k k k k ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- parity check (std., opt., no) opt. opt. no std. no opt. std. no std. opt. opt. opt. opt. ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- memory protect (std., opt., no) opt. opt. no std. no std. std. std. std. no opt. opt. opt. ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- cpu features instruction word length (s) / / / / / ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- number of accumulators (or std. std. general purpose registers that opt. opt. can be used as accumulators) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- number of hardware registers std. std. (not including index registers) opt. opt. ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- number of index registers (auto. (auto. hardware hardware hardware hardware memory hardware hardware none none none none (indicate whether they are index mem. index mem. memory memory hardware, memory or other reg.) reg.) techniques) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- how many bits for operation code ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- how many bits for address modes ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- number of addressing modes - ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- how many bits for address / / / / / ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- in this machine one can directly address _________ words in , , , , , , . , , _________ µs and indirectly . . . . . . . . . . . . . address _________ words in k k k k k k k k k k k k k _________ µs . . . . . . . . . . . . . ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- indirect addressing single- single- multi- multi- multi- multi- single- multi- multi- multi- multi- multi- multi- (multi-level, single-level, no) level level level level level level level level level level level level level ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- arithmetic operations store time for full word (µs) . . . . . . . . . . . . . ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- add time for full word (µs) . . . . . . . . . . . . . ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- fixed-point hardware mult/divide opt. opt. no std. no std. opt. std. std. no opt. opt. no (std., opt., no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- multiply time hardware (µs) . to . . to . -- -- . . . . to . . . ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- divide time hardware (µs) . to . . to . -- -- . . . . to . . . ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- multiply time software (µs) max. max. -- . to . n/a . ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- divide time software (µs) max. max. -- . to . n/a . ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- i/o capability data path width (bits) / ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- direct memory access (dma) std. no std. opt. std. opt. opt. opt. std. no opt. opt. opt. channel (std., opt., no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- maximum dma word transfer rate mhz -- khz khz khz mhz . mhz khz . mhz -- khz khz mhz ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- number of external priority none interrupt levels provided in basic system ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- maximum number of external interrupts ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- response time (µs) including . . . . . . . . . . . . . time to save registers of interrupted program and initiate new program execution ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- other features power failure protect opt. opt. opt. std. std. std. opt. std. std. opt. opt. std. std. (std., opt., no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- automatic restart after power opt. opt. opt. opt. opt. std. opt. no opt. opt. opt. opt. opt. failure (std., opt., no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- real-time clock or internal timer std. opt. opt. opt. opt. opt. opt. opt. opt. opt. opt. opt. opt. (std., opt., no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- software assembler ( pass, pass, both) pass pass pass pass pass pass pass pass both pass pass pass both ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- relocatable assembler (yes, no) yes yes yes yes no yes yes yes yes yes yes yes no ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- minimum core size necessary to use k k k k -- k k k k k k k -- this relocatable assembler ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- macro assembler capability yes yes no yes no yes no no yes no no no no ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- compilers available (specify fortran fortran none asa basic none none fortran fortran asa basic algol, asa algol, asa algol, asa none explicitly, e.g., fortran ii, iv iv fortran iv iv fortran basic fortran basic fortran basic fortran iv, asa basic fortran, etc.) fortran iv ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- conversational compilers (e.g., focal none none none none none chat doi none basic basic basic none focal, basic, cal, etc.) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- real-time executive monitor yes yes no yes no no no no yes no no yes no available (yes, no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- disc operating system available yes yes no yes no no no yes yes no yes yes yes (yes, no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- basic mainframe costs basic system price with k words n/a $ , $ , $ , $ , $ , $ , $ , n/a $ , $ , n/a $ , including power supplies ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- price of asr- teletype (if not -- $ $ , $ , $ , $ , $ , $ , -- $ , $ , -- $ , already included in basic system (asr- ) price) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- total system price, including -- $ , $ , $ , $ , $ , $ , $ , -- $ , $ , -- $ , asr- teletype and cpu (asr- ) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- basic system price with k words $ , $ , $ , $ , $ , $ , $ , $ , $ , $ , $ , $ , $ , including adequate power supplies, enclosure, control panel ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- price of asr- teletype (if not included $ $ , $ , $ , $ , $ , $ , $ , $ , $ , $ , $ , already included in basic system (asr- ) price) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- total system price including $ , $ , $ , $ , $ , $ , $ , $ , $ , $ , $ , $ , $ , asr- teletype and cpu (asr- ) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- peripherals available magnetic tape available (yes, no) yes yes yes yes yes yes yes yes yes yes yes yes yes ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- approximate price for $ , $ , $ , $ , $ , $ , $ , $ , $ , $ , $ , $ , $ , operational unit (including to to to to to to to to to controller, computer options $ , $ , $ , $ , $ , $ , $ , $ , $ , necessary, etc.) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- mass storage device available yes yes yes yes yes yes yes yes yes no yes yes yes (yes, no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- approximate price of operational $ , $ , $ , $ , $ , $ , $ , $ , $ , -- $ , $ , $ , unit (including controller, to to to to to to to computer options necessary, etc.) $ , $ , $ , $ , $ , $ , $ , ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- high speed paper tape reader yes yes yes yes yes yes yes yes yes yes yes yes yes (yes, no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- speed (char/sec) / ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- included combination $ , $ , $ , /$ , $ , $ , combination combination $ , $ , $ , $ , approximate price of operational unit $ , $ , $ , ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- high speed paper tape punch yes yes yes yes yes yes yes yes yes yes yes yes yes (yes, no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- speed (char/sec) . ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- included combination $ , $ , $ , $ , $ , combination combination $ , $ , $ , $ , approximate price of operational unit $ , $ , $ , ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- note: n/a = not announced--or not available ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- information scientific scientific systems systems technology, inc. lockheed control data engineering engineering honeywell iti- interdata interdata ibm ibm electronics raytheon raytheon corp. systems laboratories laboratories manufacturer/model number ddp- (model ) model model mac- sigma a b ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- memory memory cycle time (µs) . . / . . / . . / . . / . / . . . . . . ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- memory word length (bits) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- minimum memory size (words) k k k k k k k k k k k k k ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- memory increment size (words) k k k, k k, k k k k k k k k k k ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- maximum memory size (words) k k k k k k k k k k k k k ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- parity check (std., opt., no) opt. opt. opt. opt. std. std. opt. no opt. opt. std. opt. std. ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- memory protect (std., opt., no) opt. opt. opt. opt. no std. opt. no opt. opt. opt. opt. opt. ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- cpu features instruction word length(s) / / / / / / / ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- number of accumulators (or general purpose registers that can be used as accumulators) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- number of hardware registers (not including index registers) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- number of index registers (indicate whether they are hardware hardware memory hardware memory hardware hardware hardware hardware hardware hardware hardware, memory or other techniques) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- how many bits for operation code / ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- how many bits for address modes ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- number of addressing modes ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- how many bits for address / / / / / / / ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- in this machine one can directly address _________ words in , , , , , , , , , , , , _________ µs and indirectly . . / . . / . . / . . . . . . . . . . address ________ words in k k -- -- k k k -- -- k k k k _________ µs . . / . -- -- . . . -- -- . . . . ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- indirect addressing multi- multi-level no no single- single- multi-level no no single- single- multi-level multi-level (multi-level, single-level, no) level level level level level ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- arithmetic operations store time for full word (µs) . . / . . . . . . . . . . . . ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- add time for full word (µs) . . / . . . . . . . . . . . . ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- fixed-point hardware mult/divide opt. opt. opt. opt. std. std. opt. opt. opt. opt. opt. std. std. (std., opt., no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- multiply time--hardware (µs) . . . . - . . to . . . . ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- divide time--hardware (µs) . . . . . . . . . ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- multiply time--software (µs) . -- -- -- -- -- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- divide time--software (µs) . , , -- -- . -- -- -- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- i/o capability data path width (bits) / ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- direct memory access (dma) opt. opt. opt. opt. std. std. opt. opt. opt. opt. std. opt. opt. channel (std., opt., no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- maximum dma word transfer rate mhz mhz khz khz khz khz khz khz . mhz . mhz khz khz . mhz ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- number of external priority interrupt levels provided in basic system ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- maximum number of external interrupts ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- response time (µs) including . . . - . . - . . . . . . . . . . time to save registers of interrupted program and initiate new program execution ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- other features power failure protect std. opt. opt. opt. no opt. opt. opt. opt. std. opt. std. std. (std., opt., no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- automatic restart after power opt. opt. opt. opt. no opt. opt. std. std. opt. opt. opt. opt. failure (std., opt., no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- real time clock or internal timer opt. opt. opt. opt. no std. opt. opt. opt. opt. opt. opt. opt. (std., opt., no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- software assembler ( pass, pass, both) both pass both both pass pass pass both both pass pass pass pass ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- relocatable assembler (yes, no) yes yes yes yes yes yes yes yes yes yes yes yes yes ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- minimum core size necessary to use n/a k k k k k k k k k k k k this relocatable assembler ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- macro assembler capability no yes no no yes yes yes yes yes yes yes yes yes ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- compilers available (specify fortran iv fortran iv none none asa basic asa basic asa fortran iv fortran iv asa basic fortran iv fortran iv fortran iv explicitly e.g., fortran ii, iv, extended extended standard fortran asa basic fortran asa basic asa basic fortran, etc.) fortran fortran fortran iv asa basic fortran fortran iv fortran ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- conversational compilers (e.g., fortran iv none fortran fortran apl none none none none none none none none focal, basic, cal, etc.) basic ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- real-time executive monitor yes yes no no no yes no yes yes yes yes no yes available (yes, no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- disc operating system available yes no no no yes yes no yes yes yes yes yes yes (yes, no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- basic mainframe costs basic system price with k words $ , $ , $ , $ , $ , $ , $ , $ , $ , $ , n/a $ , n/a including power supplies ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- price of asr- teletype (if not $ , $ , $ , $ , included $ , included included included $ , -- included -- already included in basic system price) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- total system price, including $ , $ , $ , $ , $ , $ , $ , $ , $ , $ , -- $ , -- asr- teletype and cpu ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- basic system price with k words $ , $ , $ , $ , $ , $ , $ , $ , $ , $ , $ , $ , $ , including adequate power supplies, enclosure, control panel ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- price of asr- teletype (if not $ , $ , $ , $ , included $ , included included included $ , $ , included included already included in basic system (asr- ) price) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- total system price including $ , $ , $ , $ , $ , $ , $ , $ , $ , $ , $ , $ , $ , asr- teletype and cpu (asr- ) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- peripherals available magnetic tape available (yes, no) yes yes yes yes no yes yes yes yes yes yes yes yes ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- approximate price for $ , $ , $ , $ , -- $ , n/a $ , $ , $ , $ , $ , $ , operational unit (including to to to controller, computer options $ , $ , $ , necessary, etc.) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- mass storage device available yes n/a yes yes yes yes no yes yes yes yes yes yes (yes, no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- approximate price of operational $ , -- $ , $ , included $ , -- $ , $ , $ , $ , $ , $ , unit (including controller, to computer options necessary, etc.) $ , ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- high speed paper tape reader yes yes yes yes yes no yes yes yes yes yes yes yes (yes, no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- speed (char/sec) -- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- $ , $ , $ , $ , $ , -- n/a $ , $ , $ , combination $ , $ , approximate price of operational unit $ , ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- high speed paper tape punch yes yes yes yes no no yes yes yes yes yes yes yes (yes, no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- speed (char/sec) -- -- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- $ , $ , $ , $ , -- -- n/a $ , $ , $ , combination $ , $ , approximate price of operational unit $ , ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- note: n/a = not announced--or not available ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- tempo digital digital digital spear business computers, equipment equipment equipment general computers, information computer data general inc. varian corp. corp. corp. automation motorola inc. technology automation technology automation varian manufacturer/model number tempo i linc- pdp / pdp /l spc- mdp- micro linc / pdc- dt- spc- i ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- memory memory cycle time (µs) . . . . . . . . . . . . ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- memory word length (bits) / ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- minimum memory size (words) k k k k k k k k k k k k k ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- memory increment size (words) k k k k k k k k k, k, k k k k k ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- maximum memory size (words) k k k k k k k k k k k k k ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- parity check (std., opt., no) opt. opt. opt. opt. opt. opt. no opt. opt. no no opt. opt. ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- memory protect (std., opt., no) opt. opt. no std. std. no no no no no opt. no std. ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- cpu features instruction word length(s) / / / / , , / / / , , / ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- number of accumulators (or general purpose registers that can be used as accumulators) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- number of hardware registers (not including index registers) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- number of index registers hardware hardware memory memory memory hardware hardware memory none none none hardware hardware (indicate whether they are hardware, memory or other techniques) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- how many bits for operation code , , , ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- how many bits for address modes none ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- number of addressing modes ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- how many bits for address / / , , , / / / ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- in this machine one can directly address _________ words in , , , , , , , _________ µs and indirectly . . . . . . . . . . . . . address _________ words in k k k k k k k k k k k k k _________ µs . . . . . . . . . . . . . ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- indirect addressing multi-level multi-level single-level single-level single-level single-level single-level single-level single-level multi-level multi-level single-level multi-level (multi-level, single-level, no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- arithmetic operations store time for full word (µs) . . . . . . . . . . . . . ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- add time for full word (µs) . . . . . . . . . . . . . ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- fixed-point hardware mult/divide opt. opt. mult.-std. opt. no no no mult.-std. opt. no no no no (std., opt., no) div.-opt. div.-no ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- multiply time--hardware (µs) n/a -- -- -- n/a -- -- -- -- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- divide time--hardware (µs) - n/a -- -- -- -- n/a -- -- -- -- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- multiply time--software (µs) -- -- n/a n/a n/a , , n/a ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- divide time--software (µs) -- n/a n/a n/a , , n/a ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- i/o capability data path width (bits) / / / / / ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- direct memory access (dma) opt. opt. std. opt. opt. opt. opt. std. std. no no opt. opt. channel (std., opt., no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- maximum dma word transfer rate khz khz khz khz khz khz khz mhz khz -- -- khz khz ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- number of external priority none interrupt levels provided in basic system ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- maximum number of external interrupts ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- response time (µs) including . . . . . n/a . . . . . . time to save registers of interrupted program and initiate new program execution ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- other features power failure protect std. opt. opt. opt. opt. opt. opt. std. opt. opt. opt. opt. opt. (std., opt., no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- automatic restart after power opt. opt. opt. opt. opt. opt. opt. no opt. opt. opt. opt. opt. failure (std., opt., no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- real-time clock or internal timer opt. opt. opt. opt. opt. std. std. opt. opt. opt. opt. std. opt. (std., opt., no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- software assembler ( pass, pass, both) both pass both both both pass pass pass pass pass pass pass pass ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- relocatable assembler (yes, no) yes no yes yes yes yes yes no no no yes yes yes ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- minimum core size necessary to use k -- k k k k k -- -- -- k k k this relocatable assembler ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- macro assembler capability yes no yes yes yes no yes no no no no no no ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- compilers available (specify asa fortran ii fortran ii fortran ii fortran ii none none none asa none none none none explicitly, e.g., fortran ii, basic algol algol algol basic iv, asa basic fortran, etc.) fortran fortran ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- conversational compilers (e.g., none none basic focal focal no no no no no no no no focal, basic, cal, etc.) focal basic basic lap- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- real-time executive monitor no no no no no yes yes yes yes no no yes no available (yes, no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- disc operating system available no no yes yes yes no no no no no yes no no (yes, no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- basic mainframe costs basic system price with k words $ , $ , $ , $ , $ , $ , $ , $ , [a] $ , $ , $ , $ , $ , including power supplies ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- price of asr- teletype (if not included $ , included included included $ , $ , included included $ , $ , $ , $ , already included in basic system price) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- total system price, including $ , $ , $ , $ , $ , $ , $ , $ , [a] $ , $ , $ , $ , $ , asr- teletype and cpu ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- basic system price with k words $ , $ , $ , $ , $ , $ , $ , $ , [a] $ , $ , $ , $ , $ , including adequate power supplies, enclosure, control panel ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- price of asr- teletype (if not included $ , included included included $ , $ , included included $ , $ , $ , $ , already included in basic system price) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- total system price including $ , $ , $ , $ , $ , $ , $ , $ , [e] $ , $ , $ , $ , $ , asr- teletype and cpu ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- peripherals available magnetic tape available (yes, no) yes yes yes yes yes yes yes yes yes yes yes yes yes ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- approximate price for $ , n/a $ , $ , $ , $ , n/a n/a $ , $ , $ , $ , $ , operational unit (including to to controller, computer options $ , $ , necessary, etc.) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- mass storage device available yes yes yes yes yes yes yes yes yes yes yes yes yes (yes, no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- approximate price of operational n/a n/a $ , $ , $ , $ , n/a n/a $ , $ , $ , $ , n/a unit (including controller, to to to to to computer options necessary, etc.) $ , $ , $ , $ , $ , ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- high speed paper tape reader yes yes yes yes yes yes yes yes yes yes yes yes yes (yes, no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- speed (char/sec) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- approximate price of operational unit n/a n/a $ , $ , $ , $ , n/a n/a $ , $ , $ , $ , $ , ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- high speed paper tape punch yes yes yes yes yes yes yes yes yes yes yes yes yes (yes, no) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- speed (char/sec) / / / ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- n/a n/a $ , $ , $ , $ , n/a n/a $ , $ , $ , $ , - $ , approximate price of operational unit $ , ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- note: n/a = not announced--or not available [e] price includes mag tapes and crt with keyboard appendix b background information for chapter , a review and analysis of expenditures at the november "grossinger conference on the utilization of multiparameter analyzers in nuclear physics" a paper by w. f. miller and h. w. fulbright was presented in which data-analysis systems then in use in aec-sponsored laboratories in the fields of high-and low-energy nuclear physics were reviewed. by that time many applications of computers had already been made in the high-energy field, while there were only a few examples of computer systems to be found in low-energy laboratories, and those were rather simple. chapter gives a similar review, but in this case the high-energy field is excluded; the emphasis is concentrated on the economic aspects of data-acquisition systems used in low-and medium-energy physics. in the earlier paper, only aec-sponsored laboratories were covered, but in the present case some nsf-supported laboratories are also included. chapter is a condensed version of a paper presented by h. w. fulbright at the skytop "conference on computer systems in experimental nuclear physics" in march . the first part of chapter presents a review and a simple analysis of the expenditures for on-line computing in a total of different laboratories supported by the aec and nsf. the second part presents a discussion of trends visible in, or suggested by the analysis, along with some other remarks about the support of on-line computing facilities in nuclear-physics research laboratories. most of the information was supplied by the aec. it was requested by paul w. mcdaniel in letters sent in december . information was received from about percent of those from whom it was requested. it was then forwarded to the author c. v. smith, arriving in the first two weeks of february . the nsf found that certain administrative regulations made the sending out of a questionnaire a complicated procedure, so a different approach had to be adopted in their case. letters requesting the information were sent by the reviewer himself directly to laboratory directors, the appropriate names and addresses having been kindly supplied by william rodney of the nsf. here the response was less complete. most of the returns arrived by february , . a large amount of information was available for analysis. in many cases the laboratory involved had done a thorough job, and the numbers presented in those cases were especially valuable in providing a basis for estimating expenses for various items omitted in less complete reports from other laboratories, particularly in the case of manpower. in some ways, the information necessarily remained incomplete because no practical means of obtaining it occurred to the reviewer; the organization of the material in the analysis reflects this fact. institutions reporting systems ------------------------------------------------------------ place number of systems --------------------------------------- ----------------- brookhaven national laboratory university of california at los angeles university of kansas lawrence radiation laboratory university of maryland university of minnesota oak ridge national laboratory university of texas texas a & m yale university of wisconsin university of colorado argonne national laboratory columbia university university of washington university of pennsylvania university of iowa ohio state university university of rochester michigan state university stanford university rutgers-bell labs. transcriber's note on page , item . was written as . in the original. in this text version, the caret character has been used to represent exponents, e.g. ^ . _italic text_ has been enclosed in underscores. radiation by p. phillips d.sc. (b'ham), b.sc. (london), b.a. (cantab.) london: t. c. & e. c. jack long acre, w.c., and edinburgh new york: dodge publishing co. contents chap. introduction i. the nature of radiant heat and light ii. graphic representation of waves iii. the meaning of the spectrum iv. the laws of radiation v. full radiation vi. the transformation of absorbed radiation vii. pressure of radiation viii. the relation between radiant heat and electric waves index {vii} introduction we are so familiar with the restlessness of the sea, and with the havoc which it works on our shipping and our coasts, that we need no demonstration to convince us that waves can carry energy from one place to another. few of us, however, realise that the energy in the sea is as nothing compared with that in the space around us, yet such is the conclusion to which we are led by an enormous amount of experimental evidence. the sea waves are only near the surface and the effect of the wildest storm penetrates but a few yards below the surface, while the waves which carry light and heat to us from the sun fill the whole space about us and bring to the earth a continuous stream of energy year in year out equal to more than million million horsepower. the most important part of the study of radiation of energy is the investigation of the characters of the waves which constitute heat and light, but there is another method of transference of energy included in the term radiation; the source of the energy behaves like a battery of guns pointing in all directions and pouring out a continuous hail of bullets, which strike against obstacles and so give up the energy due to their motion. this method is relatively unimportant, and is usually treated of separately when considering the subject of radioactivity. we shall therefore not consider it in this book. { } radiation chapter i the nature of radiant heat and light +similarity of heat and light.+--that light and heat have essentially the same characters is very soon made evident. both light and heat travel to us from the sun across the ninety odd millions of miles of space unoccupied by any material. [illustration: figure ] both are reflected in the same way from reflecting surfaces. thus if two parabolic mirrors be placed facing each other as in the diagram (fig. ), with a source of light l at the focus of one of them, an inverted image of the light will be formed at the focus i of the other one, and may be received on a small screen placed there. the paths of two of the rays are shown by the dotted lines. if l be now replaced by a heated ball and a[ ] blackened thermometer bulb be placed at i, the thermometer will indicate a sharp rise of temperature, showing that the rays of heat are focussed there as well as the rays of light. [ ] see page . { } both heat and light behave in the same way in passing from one transparent substance to another, _e.g._ from air into glass. this can be readily shown by forming images of sources of heat and of light by means of a convex lens, as in the diagram (fig. ). [illustration: fig. .] the source of light is represented as an electric light bulb, and two of the rays going to form the image of the point of the bulb are represented by the dotted lines. the image is also dotted and can be received on a screen placed in that position. if now the electric light bulb be replaced by a heated ball or some other source of heat, we find by using a blackened thermometer bulb again that the rays of heat are brought to a focus at almost the same position as the rays of light. the points of similarity between radiant heat and light might be multiplied indefinitely, but as a number of them will appear in the course of the book these few fundamental ones will suffice at this point. +the corpuscular theory.+--a little over a century ago everyone believed light to consist of almost inconceivably small particles or corpuscles shooting out at enormous speed from every luminous surface and causing the sensation of sight when impinging { } on the retina. this was the corpuscular theory. it readily explains why light travels in straight lines in a homogeneous medium, and it can be made to explain reflection and refraction. +reflection.+--to explain reflection, it is supposed that the reflector repels the particles as they approach it, and so the path of one particle would be like that indicated by the dotted line in the diagram (fig. ). [illustration: fig. .] until reaching the point a we suppose that the particle does not feel appreciably the repulsion of the surface. after a the repulsion bends the path of the particle round until b is reached, and after b the repulsion becomes inappreciable again. the effect is the same as a perfectly elastic ball bouncing on a perfectly smooth surface, and consequently the angle to the surface at which the corpuscle comes up is equal to the angle at which it departs. +refraction.+--to explain refraction, it is supposed that when the corpuscle comes very close to the surface of the transparent substance it is attracted by the denser substance, e.g. glass, more than by the lighter substance, e.g. air. thus a particle moving along the dotted line in air (fig. ) would reach the { } point a before the attraction becomes appreciable, and therefore would be moving in a straight line. between a and b the attraction of the glass will be felt and will therefore pull the particle round in the path indicated. beyond b, the attraction again becomes inappreciable, because the glass will attract the particle equally in all directions, and therefore the path will again become a straight line. we notice that by this process the direction of the path has become more nearly normal to the surface, and this is as it should be. further, by treating the angles between the two paths and the normal mathematically we may deduce the laws of refraction which have been obtained experimentally. one other important point should be noticed. since the surface has been attracting the particle between a and b the speed of the particle will be greater in the glass than in the air. [illustration: fig. ] +ejection and refraction at the same surface.+--a difficulty very soon arises from the fact that at nearly all transparent surfaces some light is reflected and some refracted. how can the same surface sometimes repel and sometimes attract a corpuscle? newton surmounted this difficulty by attributing a polarity to each particle, so that one end was repelled and the other attracted by the reflecting and refracting { } surface. thus, whether a particle was reflected or refracted depended simply upon which end happened to be foremost at the time. by attributing suitable characteristics to the corpuscles, newton with his superhuman ingenuity was able to account for all the known facts, and as the corpuscles were so small that direct observation was impossible, and as newton's authority was so great, there was no one to say him nay. +wave theory. rectilinear propagation.+--true, huyghens in had propounded the theory that light consists of waves of some sort starting out from the luminous body, and he had shown how readily it expressed a number of the observed facts; but light travels in straight lines, or appears to do so, and waves bend round corners and no one at that time was able to explain the discrepancy. thus for nearly a century the theory which was to be universally accepted remained lifeless and discredited. the answer of the wave theory to the objection now is, that light does bend round corners though only slightly and that the smallness of the bend is quite simply due to the extreme shortness of the light waves. the longer waves are, the more they bend round corners. this can be noticed in any harbour with a tortuous entrance, for the small choppy waves are practically all cut off whereas a considerable amount of the long swell manages to get into the harbour. +interference of light. illustration by ripples+.--the revival of the wave theory dates from the discovery by dr. young of the phenomenon of interference of light. in order to understand this we will { } consider the same effect in the ripples on the surface of mercury. a tuning-fork, t (fig. ), has two small styles, s s, placed a little distance apart and dipping into the mercury contained in a large shallow trough. when the tuning-fork is set into vibration, the two styles will move up and down in the mercury at exactly the same time and each will start a system of ripples exactly similar to the other. at any instant each system will be a series of concentric circles with its centre at the style, and the crests of the ripples will be at equal distance from each other with the troughs half-way between the crests. [illustration: fig. .] the ripples from one style will cross those from the other, and a curious pattern, something like that in fig. , will be formed on the mercury. s s represents the position of the two styles, while the plain circles denote the positions of the crests and the dotted circles the positions of the troughs at any instant. where two plain circles cross it is evident that both systems of ripples are producing a crest, and so the two produce an exaggerated crest. similarly where two dotted circles cross an exaggerated trough is produced. thus in the shaded portions of the diagram we get more violent ripples than those due to a single style. where a plain circle cuts a dotted one, however, one system of ripples produces a { } crest and the other a trough, and between them the mercury is neither depressed below nor raised above its normal level. at these points, therefore, the effect of one series of ripples is just neutralised by the effect of the other and no ripples are produced at all. this occurs in the unshaded regions of the diagram. the mutual destruction of the effects of the two sets of waves is "interference." [illustration: fig. .] now imagine a row of little floats placed along the line edcbabcde. at the lettered points the floats will be violently agitated, but at the points midway between the letters they will be unmoved. this exactly represents the effect of two interfering sources of light s, s, sending light which is received by a screen at the dotted line edcbabcde. the lettered points will be brightly illuminated while the intermediate points will be dark. in practice it is found impossible to make two { } sources of light whose vibrations start at exactly the same time and are exactly similar, but this difficulty is surmounted by using one source of light and splitting the waves from it into two portions which interfere. +young's experiment.+--dr. young's arrangement is diagrammatically represented in fig. . light of a certain wave length is admitted at a narrow slit s, and is intercepted by a screen in which there are two narrow slits a and b parallel to the first one. [illustration: fig. .] a screen receives the light emerging from the two slits. if the old corpuscular theory were true there would be two bright bands of light, the one at p and the other at q, but instead dr. young observed a whole series of parallel bright bands with dark spaces in between them. evidently the two small fractions of the original waves which pass through a and b spread out from a and b and interfere just as if they were independent sources like the two styles in the mercury ripples experiment. { } +speed of light in rare and dense media.+--the discovery of interference again brought the wave theory into prominence, and in the death-blow was given to the corpuscular theory by foucault, who showed that light travels more slowly in a dense medium such as glass or water than in a light medium such as air. this is what the wave theory anticipates, while the reverse is anticipated by the corpuscular theory. but if light and heat consist of waves, what kind of waves are they and how are they produced? +elastic solid theory.+--in the earlier days of the wave theory it was supposed that the whole of space was filled with something which acted like an elastic solid material in which the vibrations of the atoms of a luminous body started waves in all directions, just as the vibrations of a marble embedded in a jelly would send out waves through the jelly. these waves are quite easily imagined in the following way. if one end of an elastic string be made to oscillate to and fro a series of waves travels along the string. if a large number of these strings are attached to an oscillating point and stretch out in all directions, the waves will travel along each string, and if the strings are all exactly alike will travel at the same speed along all of them. any particular crest of a wave will thus at any instant lie on the surface of a sphere whose centre is the oscillating point. if now we imagine that the strings are so numerous that they fill the whole of the space we have a conception of the transmission of waves by an elastic solid. +electromagnetic waves.+--since maxwell published { } his electromagnetic theory in it has been universally held that heat and light consist of electro-magnetic waves. these are by no means so easy to imagine as the elastic waves, as there is no actual movement of the medium; an alternating condition of the medium is carried onward, not an oscillation of position. when a stick of sealing-wax or ebonite is rubbed with flannel it becomes possessed of certain properties which it did not have before. it will attract light pieces of paper or pith that are brought near to it, it will repel a similar rubbed piece of sealing-wax or ebonite and will attract a rod of quartz which has been rubbed with silk. the quartz rod which has been rubbed with silk has the same property of attracting light bodies which the ebonite and sealing-wax rod has, but it repels another rubbed quartz rod and attracts a rubbed ebonite or sealing-wax rod. +positive and negative electrification.+--the ebonite is said to be negatively electrified and the quartz positively electrified. when the two rods, one positively and the other negatively electrified, are placed near to one another, we may imagine the attraction to be due to their being joined by stretched strings filling up all the space around them. if a very small positively electrified body be placed between the two it will tend to move from the quartz to the ebonite, _i.e._ in the direction of the arrows. [illustration: fig. .] +the electric field. lines of force.+--the space { } surrounding the electrified sticks in which the forces due to them are appreciable is called the electric field, and the direction in which a small positively electrified particle tends to move is called the direction of the field. the lines along which the small positive charge would move are called lines of force. the conception of the electric field as made up of stretched elastic strings is, of course, a very crude one, but there is evidently some change in the medium in the electric field which is somewhat analogous to it. [illustration: fig. .] +electric oscillations.+--if the position of the two rods is reversed, then of course the direction of the field at a point between them is reversed, and if this reversal is repeated rapidly, we shall have the direction of the field alternating rapidly. if these alternations become sufficiently rapid they are conveyed outwards in much the same way as the oscillations of position are conveyed in an ordinary ripple. thus suppose the two rods are suddenly placed in the position in the diagram. the field is not established instantaneously, the lines of force taking a short time to establish themselves in their ultimate positions. during this time the lines of force will be travelling outwards to a in the direction of the dotted arrow. { } before they reach a let us suppose that the position of the rods is reversed. then the direction of the lines is reversed and these reversed lines will travel outwards towards a, following in the track of the original lines. thus a continuous procession of lines of force, first in one direction and then in the opposite direction, will be moving out perpendicular to themselves in the direction of the dotted arrow. this constitutes an electric wave. +magnetic oscillation, lines of force, and field+.--almost exactly the same kind of description applies to a magnetic wave. the space near to the north and south poles of a magnet is modified in somewhat the same way as that between the electrified rods, and the magnetic lines of force are the lines along which a small north magnetic pole would move. we may imagine a rapid alternation of the magnetic field by the rapid reversal of the positions of the north and south poles, and we may imagine the transmission of the alternations by means of the procession of magnetic lines of force. +changes in magnetic field.+--but experiment shows that whenever the magnetic field at any place is changing an electric field is produced during the alteration, and _vice-versa_. electric and magnetic waves must therefore always accompany one another, and the two sets of waves together constitute electro-magnetic waves. these are the waves which a huge amount of experimental evidence leads us to believe constitute heat, light, the electric waves used in wireless telegraphy, { } and the invisible ultraviolet waves which are so active in inducing chemical action. +oscillation of electric charges within the atom.+--we have seen how these waves might be produced by the oscillation of two electrified rods, and it is supposed that the light coming from luminous bodies is produced in a similar way. there are many reasons for believing that there exist in the atoms of all substances, minute negatively electrified particles which may rotate in small orbits or oscillate to and fro within the atom. there also exists an equal positive charge within the atom. as the negative particles rotate or oscillate in the atom, it is evident that the field between them and the positively electrified part of the atom alternates, and so electro-magnetic waves are sent out. { } chapter ii graphic representation of waves a system of ripples on the surface of water appears in vertical section at any instant somewhat as in fig. . the dotted line ab represents the undisturbed surface of the wafer, and the solid line the actual surface. if the disturbance which is causing the ripples is an oscillation of perfectly regular period the individual ripples will be all alike, except they will get shallower as they become more remote from the disturbance. [illustration: fig. .] +wave-length.+--the distance between two successive crests will be the same everywhere, and this distance or the distance between any two corresponding points on two successive ripples is called the wave-length. evidently, the wave-length is the distance in which the whole wave repeats itself. +phase.+--the position of a point in the wave is called the phase of the point. thus the difference of phase between the two points a and c is a quarter { } of a wave-length. as the waves move on along the surface it is evident that each drop of water executes an up and down oscillation, and at the points c, c the drop has reached its highest position and at the points t, t its lowest. +amplitude.+--the largest displacement of the drop, _i.e._ the distance from the dotted line to c or to t, is called the amplitude of the wave. the time taken for a drop to complete one whole oscillation, _i.e._ the time taken for a wave to travel one whole wave-length forward, is called the period of the wave. the number of oscillations in one second, _i.e._ the number of wave-lengths travelled in one second, is called the frequency. [illustration: fig. .] although there is no visible displacement in the waves of light and heat, yet we may represent them in much the same way. thus if ab, fig. , represents the line along which a ray of light is travelling, the length np is drawn to scale to represent the value of the electric field at the point n, and is drawn upwards from the line ab when the field is in one direction and downwards when it is in the opposite direction. thus the direction of the field at different points in the wave xy, fig. , is shown by the dotted arrows as if due to electrified rods of quartz and ebonite placed above and below xy. in the case of the electromagnetic wave, the { } amplitude will be the maximum value to which the electric field attains in either direction, and the other terms--wave-length, phase, period and frequency--will have the same meaning as for water ripples. +wave form.+--waves not only differ in amplitude, wave-length, and frequency, but also in wave form. waves may have any form, _e.g._ fig . or we may have a solitary irregular disturbance such as is caused by the splash of a stone in water. [illustration: fig. .] but there is one form of motion of a particle in a wave which is looked upon as the simplest and fundamental form. it is that form which is executed by the bob of a pendulum, the balance wheel of a watch, the prong of a tuning-fork, and most other vibrations where the controlling force is provided by a spring or by some other elastic solid. it is called "simple harmonic motion" or "simple periodic motion," and the essential feature of it is that the force restoring the displaced particle to its undisturbed position is proportional to its displacement from the undisturbed position. a wave in which all the particles execute simple harmonic motion has the form in fig. or fig. , which is therefore looked upon as the fundamental wave form or simple wave form. simple waves will vary only in amplitude, wave-length, and frequency, and the energy in the wave will depend upon these quantities. { } +energy in a simple wave.+--if the velocity is the same for all wave-lengths, then the frequency will evidently be inversely proportional to the wave-length and the energy will depend upon the amplitude and the wave-length. the kinetic energy of any moving body, _i.e._ the energy due to its motion, is proportional to the square of its velocity, and we may apply this to the motion of the particles in a wave and to show how the energy depends upon the amplitude and wave-length. since the distance travelled by a particle in a single period of the wave will be equal to four times the amplitude, the velocity at any point in the wave must be proportional to the amplitude and therefore the kinetic energy is proportional to the square of the amplitude. with the same amplitude but with different wave-lengths, we see that the time in which the oscillation is completed is proportional to the wave-length and that the velocity is therefore inversely proportional to the wave-length. the kinetic energy is therefore inversely proportional to the square of the wave-length. +addition of waves.+--the superposition of two waves so as to obtain the effect of both waves at the same place is carried out very simply. the displacements at any point due to the two waves separately are algebraically added together, and this sum is the actual displacement. in fig. the dotted lines represent two simple waves, one of which has double the wave-length of the other. at any point p on the solid line, the displacement pn is equal to { } the algebraic sum of the displacement nq due to one of the waves and nr due to the other. the solid line, therefore, represents the resulting wave. we may repeat this process for any number of simple waves, and by suitably choosing the wave-length and amplitude of the simple waves we may build up any desired form of wave. the mathematician fourier has shown that any form of wave, even the single irregular disturbance, can thus be expressed as the sum of a series of simple waves and that the wave-lengths of these simple waves are equal to the original wave-length, one-half of it, one-third, one-quarter, one-fifth, and so on in an infinite series. fourier has also shown that only one such series is possible for any particular form of wave. [illustration: fig. .] the importance of this mathematical expression lies in the fact that in a number of ways fourier's series of simple waves is manufactured from the original wave and the different members of the series become separated. thus the most useful way in which we can represent any wave is, not to draw the actual form of a wave, but to represent what simple waves go to form it and to show how much energy there is in each particular simple wave. { } +energy--wave-length curve.+--this can be done quite simply as in fig. . the distance pn from the line oa being drawn to scale to represent the energy in the simple wave whose length is represented by on. [illustration: fig. .] thus the simple wave of length ox has the greatest amount of energy in it. [illustration: fig. .] fig. wall represent a simple wave of wave-length ox, the energy in all the other waves being zero. { } the three curves given in fig. give a comparison of the waves from the sun, an arc lamp, and an ordinary gas-burner. [illustration: fig. .] { } chapter iii the meaning of the spectrum +the spectrum. dispersion.+--when a narrow beam of white light is transmitted through a prism of glass or of any other transparent substance, it is deflected from its original direction and is at the same time spread out into a small fan of rays instead of remaining a single ray. if a screen is placed in the path of these rays a coloured band is formed on it, the least deflected part of the band being red and the colours ranging from red through orange, yellow, green, blue, and indigo, to violet at the most deflected end of the band. this band of colours is called the spectrum of the white light used, and the spreading out of the rays is called dispersion. +newton's experiment.+--newton first discovered this fact with an arrangement like that in fig. . [illustration: fig. .] if by any means the fan of coloured rays be combined again into a single beam, white light is reformed, and newton therefore came to the conclusion that white light was a mixture of the various colours in the spectrum, and that the only function of the prism was to separate the constituents. of the nature of the constituents newton had little knowledge, since he had rejected the wave theory, which could alone give the clue. { } we now believe that white light is an irregular wave, and that the prism manufactures from it the fourier's series of waves to which it is equivalent. it is supposed that the manufacture is effected by means of the principle of resonance. as an example of resonance let a small tap be given to a pendulum just as it commences each swing. then because the taps are so timed that each of them increases the swing of the pendulum by a small amount, they will very soon cause the pendulum to swing very violently even though the effect of a single tap can scarcely be detected at all. thus when any body which has a free period of vibration is subject to periodic impulses of the same period as its own, it will vibrate very vigorously and absorb nearly all the energy of the impulses. +electrons and their vibrations.+--there is conclusive evidence to show that in the atoms of all substances, and therefore of the glass of which the prism is composed, there are a number of minute negatively electrified particles which are called electrons. these are held in position by a positive charge on the rest of the { } atom, and if they are displaced from their usual positions by any means they will vibrate about these positions. the time of vibration of the electron will depend upon its position in the atom and upon the position of neighbouring atoms. in solid or liquid bodies the neighbouring atoms are so near that they have a considerable influence in modifying the period of an electron or a system of electrons, and consequently we may find almost any period of vibration in one or other of these electrons or systems. as the wave of light with its alternating electric fields comes up to the prism, the field will first displace the electrons in one direction and then in the other, and so on. if the period of one particular type of electron happens to coincide with the period of the wave, that electron will vibrate violently and will in its turn send out a series of waves in the glass. if the wave is an irregular one it will start all the electrons vibrating, but those electrons will vibrate most violently whose periods are equal to the periods of the fourier's constituents which have the greatest energy. thus we shall actually have the fourier's constituent waves separated into the vibrations of different electrons. but the speed with which any simple wave travels in glass or in any transparent medium, other than a vacuum, is dependent upon its period. the shorter the period, _i.e._ the shorter the wave-length, the slower is the speed in most transparent substances. but the slower the speed in the prism the more is the ray deviated, and therefore we conclude that the violet end of the spectrum consists of the shortest waves while the red end consists of the { } longest waves, and that the different parts of the spectrum are simple waves of different period. +the whole spectrum.+--the visible spectrum is by no means the whole of the series of fourier's waves, however. the eye is sensitive only to a very small range of period, while there exists in sunlight a range many times as great. those waves of shorter period than the violet end of the visible spectrum will be deviated even more than the violet, and will therefore be beyond the violet. they are called the ultra violet rays, and can easily be detected by means of their chemical activity. they cause a number of substances to glow, and therefore by coating the screen on which the spectrum is received with one of these substances, the violet end of the spectrum is extended by this glow. the waves of longer period than the red rays will be deviated less than the red, and will therefore lie beyond the red end of the visible spectrum. they are called the infra-red rays, and are chiefly remarkable for their heating effect. all the rays are absorbed when they fall on to a perfectly dull, black surface, and their energy is converted into heat. this heating effect provides the best way of measuring the energy in the different parts of the spectrum, and of thus constructing curves similar to those given in fig. . the instrument moat commonly used is called langley's bolometer. it consists of a fine strip of blackened platinum, which can be placed in any part of the spectrum at will and thus absorb the waves over a very small range of wave-length. it is heated by { } them, and the rise in temperature is found by measuring the electrical resistance of the strip. the electrical resistance of all conductors varies with the temperature, and since resistance can be measured with extreme accuracy this forms a very sensitive and accurate method. +spectrum of an incandescent solid or liquid.+--the spectra given by different sources of light show certain marked differences. an incandescent solid or liquid gives a continuous spectrum, _i.e._ all the different wave-lengths are represented, but the part of the spectrum which has the greatest energy is different for different substances and for different temperatures: cf. arc and gas flame in fig. . this is quite in keeping with the idea already suggested that in solids and liquids there are electrons of almost every period of vibration. when they are agitated by being heated, a mixture of simple waves of all periods will be sent out giving a very irregular wave. gases may also become incandescent. thus when any compound of sodium is put into a colourless flame the flame becomes coloured an intense yellow. this is due to the vapour of sodium, and the agitation of the electrons in it is probably due to the chemical action in which the compound is split up into sodium and some other parts. we may also make the gas incandescent by enclosing it at low pressure in a vacuum tube and passing an electrical discharge through it. the glow in the tube gives the spectrum of the gas. incandescent gases give a very characteristic kind of spectrum. { } it consists usually of a limited number of narrow lines, the rest of the spectrum being almost perfectly dark. the light therefore consists of a few simple waves of perfectly definite period. this would suggest that in the atom of a gas there are only a few electrons which are concerned in the emission of the light waves. thus the spectra of gases and of incandescent solids are represented in character by the curves in fig. . [illustration: fig. .] +spectrum analysis.+--the lines in a gas spectrum are so sharply defined and are so definitely characteristic of the particular gas that they serve as a delicate method of detecting the presence of some elements. these spectra which are emitted by incandescent bodies are called emission spectra. but not only do different materials emit different kinds of light when raised to incandescence, but they also absorb light differently when it passes through them. when white light is passed through some transparent solids or liquids and then through a prism, it is found that whole regions of the spectrum are absent. thus a potassium permanganate solution { } which is not too concentrated absorbs the whole of the middle part of the spectrum, allowing the red and blue rays to pass through. since with solids and liquids the absorbed regions are large and somewhat ill-defined, the absorption spectra are not of any great use in the detection of substances. the absorption spectra of gases show the same sharply defined characteristics as the emission spectra. thus if white light from an arc lamp passes through a flame coloured yellow with sodium vapour, the spectrum of the issuing light has two sharply defined narrow dark lines close together in the yellow part of the spectrum in exactly the same position as the two bright yellow lines which incandescent sodium vapour itself gives out. the flame has therefore absorbed just those waves which it gives out. this is perfectly general, and applies to solids and liquids as well as to gases. it is perfectly in keeping with our view of the refraction of light by the resonance of electrons to the fourier's constituents which have the same period. for if the electrons have a certain period of vibration they will resound to waves of that period and therefore absorb their energy. +spectrum of the sun.+--one of the most interesting examples of the absorption by incandescent gases of their own characteristic lines is provided by the sun. the spectrum of the sun is crossed by a large number of fine dark lines which were mapped out by fraunhöfer and are therefore called fraunhöfer lines. these lines are found to be in the position of the characteristic lines of a number of known elements, { } and therefore we assume that these elements are present in the sun. the interior of the sun is liquid or solid owing to the pressure of the mass round it. it therefore emits a continuous spectrum. but the light has to pass through the outer layers of incandescent vapour, and these layers absorb from the light their characteristic waves and so produce the dark lines in the spectrum. the spectra of stars show similar characters to those of the sun, and therefore we assume them to be in the same condition as the sun. the spectra of nebulæ consist only of bright lines, and we therefore assume that nebulæ consist of incandescent masses of gas which have not yet cooled enough to have liquid or solid nuclei. { } chapter iv the laws of radiation +absorbing power.+--a perfectly dull black surface is simply one which absorbs all the light which is falling on it and reflects or diffuses none of it back. if the surface absorbs the heat as well as the light completely, it is called a perfect or full absorber. other surfaces merely absorb a fraction of the heat and light falling on them, and this fraction, expressed usually as a percentage, is called the absorbing power of the surface. the absorbing powers of different kinds of surfaces can be measured in a great many ways, but the following may be taken as fairly typical. a perfectly steady beam of heat and light is made to fall on a small metallic disc, and the amount of heat which is absorbed per second is calculated from the mass of the metal and the rate at which its temperature rises. the disc is first coated with lamp-black, and the rate at which it then receives heat is taken as the rate at which a full absorber absorbs heat under these conditions. the disc is then coated with the surface whose absorbing power is to be measured, and the experiment is repeated. then the rate at which heat is received in the second case divided by the rate at which it is received in the first is the absorbing power of the second surface. { } experiments with a large number of surfaces show that the lighter in colour and the more polished is the surface, the smaller is its absorbing power. +radiating power.+--but the character of the surface affects not only the rate at which heat and light are absorbed, but also the rate at which they are emitted. for example, if we heat a fragment of a willow pattern china plate in a blowpipe flame until it is bright red hot, we shall notice that the dark pattern now stands out brighter than the rest. thus the dark pattern, which absorbs more of the light which falls on it when it is cold, emits more light than the rest of the plate when it is hot. this is one example of a general rule, for it is found that the most perfect absorbers are the greatest radiators, and _vice-versa_. the perfectly black surface is therefore taken as a standard in measuring the heat and light emitted by surfaces, in exactly the same way as for heat and light absorbed. thus the emissive or radiating power of a surface is defined as the quantity of heat radiated per second by the surface divided by the amount radiated per second by a perfectly black surface under the same conditions. as it is somewhat paradoxical to call a surface a perfectly black surface when it may even be white hot, the term "a full radiator" has been suggested as an alternative and will be used in this book. [illustration: fig. ] +relation between absorbing and radiating powers.+--the exact relation between the absorbing and radiating powers of a surface was first determined by ritchie by means of an ingenious experiment. two equal air-tight metal chambers a and b were connected by a glass tube bent twice at right angles as { } in fig. . a drop of mercury in the horizontal part of this tube acted as an indicator. when one of the vessels became hotter than the other, the air in it expanded and the mercury index moved towards the colder side. between the two metal chambers a third equal one was mounted which could be heated up by pouring boiling water into it and could thus act as a radiator to the other two. one surface of this radiator was coated with lamp-black and the opposite one with the surface under investigation, _e.g._ cinnabar. the inner surfaces of the other two vessels were coated in the same way, the one with lamp-black, the other with cinnabar. the middle vessel was first placed so that the lamp-blacked surface was opposite to a cinnabar one, and _vice-versa_. in this position, when hot water was poured into it no movement of the mercury drop was detected, and therefore the amounts of heat received by the two outer vessels must have been exactly equal. on the one side the heat given out by the cinnabar surface of the middle vessel is only a fraction, equal to its radiating power, of the heat given out by the black surface. all the heat given out by the cinnabar surface to the black surface opposite to it is absorbed, however, while of the heat given out by the black surface to the cinnabar surface opposite it only a fraction is absorbed equal to the absorbing power of the cinnabar surface. thus on the one side only a fraction is sent out but all of it is absorbed, and on the other side all is sent out and only a fraction absorbed. since { } the quantities absorbed are exactly equal, it is obvious that the two fractions must be exactly equal, or the absorbing and radiating powers of any surface are exactly equal. this result is known as kirchoff's law, and it applies solely to radiation which is caused by temperature. later experiments have shown that it applies to each individual wave-length, _i.e._ to any portion of the spectrum which we isolate, as well as to the whole radiation. thus at any particular temperature let the dotted line in fig. represent the wave-length--energy curve for a full radiator, and let the solid line represent it for the surface under investigation. then for any wave-length, on, the radiating power of the surface would be equal to qn divided by pn. [illustration: fig. .] now a wave-length--energy curve may be as easily constructed for absorbed as for emitted radiation by means of a langley's bolometer. the strip of the bolometer is first coated with lamp-black and the spectrum of the incident radiation is explored in exactly the same way as is described in chapter iii. { } the strip is then coated with the surface under investigation and the spectrum is again explored. since the incident radiation is exactly the same in the two experiments, the differences in the quantities of heat absorbed must be due solely to the difference in the absorbing powers of the two surfaces. in fig. the dotted line represents the wave-length--energy curve for the radiation absorbed by the blackened bolometer strip, and the solid line the curve for the strip coated with the surface under investigation. [illustration: fig. .] the actual form of the curves may and probably will be quite different from the form in fig. , but it will be found for the same wave-length on that pn/qn is exactly the same in the two figures. it has already been mentioned that dull, dark-coloured surfaces radiate the most heat, and that polished surfaces radiate the least. a radiator for heating a room should therefore have a dull, dark surface, while a vessel which is designed to keep its contents from losing heat should have a highly polished exterior. a perfectly transparent substance would radiate no energy, whatever the temperature to which it is { } raised, for its absorbing power is zero and therefore its radiating power is also zero. no perfectly transparent substances exist, but some substances are a very near approach to it. a fused bead of microcosmic salt heated in a small loop of platinum wire in a blowpipe flame may be raised to such a temperature that it is quite painful to look at the platinum wire, yet the bead itself is scarcely visible at all. any speck of metallic dust on the surface of the bead will at the same time shine out like a bright star. +gases as radiators.+--most gases are an even nearer approach to the perfectly transparent substance, and consequently, with one or two exceptions, the simple heating of gases causes no appreciable radiation from them. of course, gases do radiate heat and light under some circumstances, but the radiation seems to be produced either by chemical action, as in the flames coloured by metallic vapours, or by electric discharge, as in vacuum tubes, the arc or the electric spark. the agitation of the electrons is thus produced in a different way in gases, and we must not apply kirchoff's law to them, although at first sight they appear to conform to it. we have seen that the particular waves which an incandescent gas radiates are also absorbed by it. this we should expect, because the particular electron which has such a period of vibration that it sends out a certain wave-length will naturally be in tune to exactly similar waves which fall on it, and will so resound to them, and absorb their energy. the quantitative law, however, that the absorbing power is exactly equal to the radiating power, is not true for gases. { } +emission of polarised light.+--one very interesting result of kirchoff's law is the emission of polarized light by glowing tourmaline and by one or two other crystal when they are heated to incandescence. in ordinary light the vibrations are in all directions perpendicular to the line along winch the light travels, that is, the vibrations at any point are in a plane perpendicular to this line. now any vibration in a plane may be expressed as the sum of two component vibrations, one component in one direction and the other in a perpendicular direction. if we divide up the vibrations all along the wave in this way we shall have two waves, one of which has its vibrations all in one direction and the other in a perpendicular direction. such waves, in which the vibrations all lie in one plane, are said to be plane polarised. tourmaline is possessed of the curious property of absorbing vibrations in one direction of the crystal much more rapidly than it does those vibrations perpendicular to this direction, and therefore light which passes through it emerges partially, or in some cases wholly, plane polarised. since the absorbing power of tourmaline is different for the two components, the emissive power should also be different, and that component which was most absorbed should be radiated most strongly. this was found to be true by kirchoff himself, who detected and roughly measured the polarised light emitted. subsequently in , pflüger carried out exact experiments which gave a beautiful confirmation of the law. { } chapter v full radiation +the full radiator.+--we have assumed that a lamp-blacked surface is a perfect absorber, and consequently a full radiator, but although it is a very near approach to the ideal it is not absolutely perfect. no actual surface is a perfectly full radiator, but the exact equivalent of one has been obtained by an ingenious device. a hollow vessel which is blackened on the inside has a small aperture through which the radiation from the interior of the vessel can escape. if the vessel is heated up, therefore, the small aperture may act as a radiator. the radiation which emerges through the aperture from any small area on the interior of the vessel is made up of two parts, one part which it radiates itself, and the other part which it scatters back from the radiation which it receives from the other parts of the interior of the vessel. these two together are equal to the energy sent out by a full radiator, and therefore the small aperture acts as a full radiator: _e.g._ suppose the inner surface has an absorbing power of per cent., then it radiates per cent. of the full radiation and absorbs per cent. of the radiation coming up to it therefore scattering back per cent. we have therefore coming from the inner surface per cent. { } radiated and per cent. scattered, and the radiated and scattered together make per cent. [illustration: fig. .] one form in which such radiators have been used is shown in section in fig. . a double walled cylindrical vessel of brass has a small hole, _a_, in one end. steam can be passed through the space between the double walls, thus keeping the temperature of the inner surface at ° c. a screen with a hole in it just opposite to the hole in the vessel, or rather several such screens, are placed in front of the vessel in order to shield any measuring instrument from any radiation except that emerging through the hole. +the full absorber.+--in an exactly similar way an aperture in a hollow vessel will act as a full absorber, for the fraction of the incident radiation which is scattered on the inner surface again impinges on another portion of the surface and so all is ultimately absorbed except a minute fraction which is scattered out again through the aperture. the variation in the heat radiated by a full radiator at different temperatures forms a very important part of the study of radiation, and a very large number of experiments and theoretical investigations have been devoted to it. these investigations may be divided into two sections: those concerned with the total quantity of heat radiated at different temperatures and those concerned with the variation in the character of the spectrum with varying temperatures. { } the experiments in the first section have been carried out mainly in two ways. in the first, the rate of cooling of the full radiator has been determined, and from the rate of cooling at any temperature the rate at which heat was lost by radiation was immediately calculated. newton was the first to investigate in this way by observing the rate at which a thermometer bulb cooled down when it was surrounded by an enclosure which was kept at a uniform temperature. he found that the rate of cooling, and therefore the rate at which heat was lost by the thermometer, was proportional to the difference of temperature between the thermometer and its surroundings. this rule is known as newton's law of cooling, and is still used when it is desired to correct for the heat lost during an experiment where the temperature differences are small. it is only true, however, for very small differences of temperature between the thermometer and its surroundings, and as early as martine had found that it was only true for a very limited range of temperature. +prévost's theory of exchanges.+--in , prévost of geneva, when endeavouring to explain the supposed radiation of cold, introduced the line of thought, that any body is not to be regarded as radiating heat only when its temperature is falling, or absorbing heat only when its temperature is rising, but that both processes are continually and simultaneously going on. the amount of heat radiated will depend on the temperature and character of the body itself, while the amount absorbed will depend upon the condition of the surroundings as well as upon the nature { } of the body. if the amount of heat radiated is greater than the amount absorbed the body will fall in temperature, and _vice-versa_. this view of prévost's is called the theory of exchanges, and we can see that it is a necessary consequence of our ideas as to the production of heat and light waves by the agitation of electrons in the radiating body. if the rate of cooling of a body at a certain temperature is measured when it is placed in an enclosure at a lower temperature, it must be borne in mind that the rate of loss of heat is equal to the rate at which heat is radiated minus the rate at which it is absorbed from the enclosure. a second way in which the heat lost by a body has been measured at different temperatures is by heating a conductor such as a thin platinum strip by means of an electric current, and measuring the temperature to which the conductor has attained. when its temperature is steady, all the energy given to it by the current must be lost as heat, and therefore the electrical energy, which can very easily be calculated, must be equal to the heat radiated by the body minus the heat received from the enclosure. so many attempts have been made to establish, by one or other of these two methods, the relation between the quantity of heat radiated and the temperature, that it is impossible to give even a passing reference to most of them. unfortunately, the results do not show the agreement with one another which we would like, but probably the most correct result is that stated by stefan in , after a close inspection of the experimental results of dulong and { } petit. he stated that the quantity of heat radiated per second by a full radiator is proportional to the fourth power of its absolute temperature.[ ] thus the quantity of heat radiated by one square centimetre of the surface of a full radiator whose absolute temperature is t, is equal to et(sup) , where e is some constant multiplier which must be determined by experiment and which is called the radiation constant. if the absolute temperature of the enclosure in which the surface is placed is t, then the rate at which the surface is losing heat will be e(t(sup) -t(sub) (sup) ), for it will receive heat at the rate et(sub) (sup) and will radiate it at the rate et(sup) . [ ] see page . stefan's fourth power law has been verified by a number of good experiments, notably those of lummer and pringsheim (_congrés international de physique_, vol. ii. p. ), so that although some experiments do not agree with it, we are probably justified in taking it as correct. in boltzmann added still further evidence in support of this law by deriving it theoretically. he applied to a space containing the waves of full radiation the two known laws which govern the transformation of energy, by imagining the space to be taken through a cycle of compressions and expansions in just the same way as a gas is compressed and expanded in what is known as carnot's cycle. +variation of spectrum with temperature.+--the variation of the character of the spectrum of a full radiator has been determined mainly by the use of langley's bolometer, but the general nature of the change may be readily observed by the eye. { } as the temperature of a full radiator rises it first gives out only invisible heat waves; as soon as its temperature exceeds about ° c. it begins to emit some of the longest visible rays; and as the temperature rises further, more and more of the visible rays in the spectrum are emitted until, when the radiator is white hot, the whole of the visible spectrum. is produced. thus the higher the temperature of the radiator the more of the shorter waves are produced. [illustration: fig. .] by means of langley's bolometer the distribution of energy in the spectrum has been measured accurately, with the results of confirming and amplifying the general results just stated. the energy in the spectrum of even the hottest of terrestrial radiators is mostly in the longer waves of the infra-red, but the position of the maximum of energy moves to shorter and shorter wave-lengths as the temperature rises, and so more of the shorter waves make their appearance. the sun is not a full radiator, but is nearly so, and its temperature is so high that the maximum of energy in its spectrum is in the visible part near to the red end. { } fig. shows the results obtained by lummer and pringsheim, and brings out clearly the shift of the maximum with rising temperature and also the position of the greatest part of the energy in the infrared region. +wien's laws.+--examination of the results also shows that the wave-length at which the maximum energy occurs is inversely proportional to the absolute temperature and that the actual energy at the maximum point is proportional to the fifth power of the absolute temperature. these two results have both been derived theoretically by wien[ ] in a similar way to that in which boltzmann derived stefan's fourth power law, _i.e._ by imagining a space filled with the radiation to be taken through a cycle of compressions and rarefactions. [ ] _wied. ann._, , p. ; , p. . wien derived an amplification of the last result by showing that if a wave-length in the spectrum of a full radiator at one temperature and another wave-length in the spectrum at another temperature are so related as to be inversely proportional to the two absolute temperatures, they may be said to correspond to each other, and the energy in corresponding wave-lengths at different temperatures is proportional to the fifth power of the absolute temperature. we see therefore that if the distribution of energy in the spectrum of the full radiator be known at any one temperature it may be calculated for any other temperature by applying these two laws of corresponding wave-lengths and the energy in them. neither of them give us any information, however, { } about the actual distribution of energy at any one temperature from which we may calculate that at any other temperature. for that, some relation must be found between the energy and the wave-length. planck, by reasoning founded on the electromagnetic character of the waves, derived such a relation, but both his reasoning and his results are a little too complicated to be introduced here. his results have been confirmed in the most striking manner by experiments carried out by rubens and kurlbaum (_ann. der physik_, , p. , ). they measured the energy in a particular wave-length (. cms., _i.e._ nearly times the wave-length of red light) in the radiation of a full radiator from a temperature of ° up to ° absolute, and their results are given in the following table: absolute temperature. observed energy. energy calculated from planck's formula. - . - . - . - + + . . . . . we have therefore the means of calculating both the total quantity and the kind of radiation given out by any full radiator at any temperature, and a number of very interesting problems may be solved by means of the results. { } +efficiency in lighting.+--one very simple problem is concerned with efficiency in lighting. we see by reference to fig. , that in the radiation from the electric arc very little of the energy is in the visible part of the spectrum even though the temperature in the arc is the highest yet obtained on the earth, whereas the energy in the visible part of the spectrum from a gas flame is almost wholly negligible. the problem of efficient lighting is to get as big a proportion as possible of the energy into the visible part of the spectrum, and therefore the higher the temperature the greater the efficiency. this is the reason of the greater efficiency of the incandescent gas mantle over the ordinary gas burner, for the introduction of the air into the gas allows the combustion to be much more complete, and therefore the temperature of the mantle becomes very much higher than that of the carbon particles in the ordinary flame. the modern metallic filament electric lamps have filaments made of metals whose melting point is extremely high, and they may therefore be raised to a much higher temperature than the older carbon filaments. the arc is even more efficient than the metallic filament lamps, because its temperature is higher still; and we must assume that the temperature of the sun is very much higher even than the arc, since its maximum of energy lies in the visible spectrum. +temperature of the sun.+--the actual temperature of the sun may be calculated approximately by means of stefan's fourth power law. we will first assume that the earth and the sun are both full radiators, and { } that the earth is a good conductor, so that its temperature is the same all over. the first assumption is very nearly true, and we will make a correction for the small error it introduces; and the second, although far from true, makes very little difference to the final result, for it is found that the values obtained on the opposite assumption that the earth is an absolute non-conductor differ by less than per cent. from those calculated on the first assumption. we will further assume that the heat radiated out by the earth is exactly equal to the heat which it receives from the sun. this is scarcely an assumption, but rather an experimental fact, for experiment shows that heat is conducted from the interior of the earth to the exterior, and so is radiated, but at such a small rate that it is perfectly negligible compared with the rate at which the earth is receiving heat from the sun. the sun occupies just about one , th part of the hemisphere of the heavens or one , th part of the whole sphere. if the whole sphere surrounding the earth were of sun brightness, the earth would be in an enclosure at the temperature of the sun, and would therefore be at that temperature itself. the sphere would be sending heat at , times the rate at which the sun is sending it, and the earth would be radiating it at , times its present rate. but the rate at which it radiates is proportional to the fourth power of its absolute temperature, and therefore its temperature would be the fourth root of , times its present temperature, _i.e._ . times. if the radiating or absorbing power of the earth's surface be taken as / , which is somewhere near the mark, { } the calculation gives the number . instead of . . the average temperature of the earth's surface is probably about ° c. or ° absolute, and therefore the temperature of the sun is x . , _i.e._ about ° absolute. it is easy to see that if we had known the temperature of the sun and not of the earth, we could have calculated that of the earth by reversing the process. by this means we can estimate the temperatures of the other planets, at any rate of those for which we may make the same assumptions as for the earth. probably those planets which are very much larger than the earth are still radiating a considerable amount of heat of their own, and therefore to them the calculation will not apply; but the smaller planets mercury, venus and mars have probably already radiated nearly all their own heat and are now radiating only such heat as they receive from the sun. the temperatures calculated in this way are-- average absolute temperature mercury . . . . . . . . . ° venus . . . . . . . . . . ° earth . . . . . . . . . . ° mars . . . . . . . . . . ° since the freezing point of water is ° absolute, we see that the average temperature of mars is ° c. below freezing, and it is almost certain that no part of mars ever gets above freezing point. in a very similar way we may find the temperature to which a non-conducting surface reaches when it is exposed to full sunlight by equating the heat absorbed to the heat radiated, and the result comes { } to ° absolute, _i.e._ ° c., or considerably above boiling point. this would be the upper limit to the temperature of the surface of the moon at a point where the sun is at its zenith. on the surface of the earth the sunlight has had to pass through the atmosphere, and in perfectly bright sunshine it is estimated that only three-fifths of the heat is transmitted. any surface is also radiating out into surroundings which are at about ° absolute. taking into account these two facts, we find that the upper limit to a non-conducting surface in full sunshine on the earth is about ° absolute, or only a few degrees less than the boiling point of water. +effective temperature of space.+--the last problem we will attack by means of the fourth power law is the estimation of the effective temperature of space, _i.e._ the temperature of a full absorber shielded from the sun and far away from any planet. it is estimated by experiment that zenith sun radiation is five million times the radiation from the stars. this estimate is only very rough, as the radiation from the stars is so minute. as the sun only occupies one , th part of the heavens, the radiation from a sunbright hemisphere would be five million times , times starlight, _i.e._ , , , times. the temperature of the sun is therefore the fourth root of this quantity times the effective temperature of space, _i.e._ about times. since the temperature of the sun is about °, the temperature of space is a little under ° absolute; _i.e._ lower than - ° c. { } +note on absolute temperature.+--it is found that, if a gas such as air has its temperature raised or lowered while its pressure is kept uniform, for every one degree centigrade rise or fall its volume is increased or decreased by one two hundred and seventy-third of its volume at freezing point, _i.e._ at ° centigrade. if therefore it continued in the same way right down to - ° centigrade, its volume would be reduced to zero at this temperature. this temperature is therefore called the absolute zero of temperature, and temperatures reckoned from it are called absolute temperatures. to get absolute temperatures from centigrade temperatures we evidently need to add °. { } chapter vi the transformation of absorbed radiation no account of radiation would be complete without mentioning what becomes of the radiation which bodies absorb, but a good deal of the subject is in so uncertain a state that very little space will be devoted to it. +absorbed radiation converted into heat.+--the most common effect of absorbed radiation is to raise the temperature of the absorbing body, and so cause it to re-emit long heat-waves. as the usual arrangement is for the absorbing body to be at a lower temperature than the radiating one, the waves given out by the absorber are longer than those given out by the radiator, and so the net result is the transformation of shorter waves into longer ones. but we have seen by prévost's theory of exchanges that radiator and absorber are interchangeable, and therefore we see that those waves which are emitted by the absorber and absorbed by the radiator are re-emitted by the latter as shorter waves. the mechanism by means of which the waves are converted into heat in the body is still a mystery. that the waves should cause the electrons to vibrate is perfectly clear, but how the vibrations of the electrons are converted into those vibrations of the atoms { } and molecules which constitute heat is still unsolved, and the reverse process is, of course, equally puzzling. the heating of the body and the consequent re-emission of heat-waves is not, however, the only process which goes on. in a large number of substances, waves are given out under the stimulus of other waves without any heating of the body at all. in most of these cases the emission stops as soon as the stimulating waves are withdrawn, and in these cases the phenomenon has been called fluorescence. the name has been derived from fluor spar, the substance which was first observed to exhibit this peculiar emission of waves. a familiar example of fluorescence is provided by paraffin-oil, which glows with a blue light when it is illuminated with ordinary sunlight or daylight. perhaps the easiest way to view it is to project a narrow beam of light through the paraffin-oil contained in a glass vessel and view the oil in a direction perpendicular to the beam. the latter will then show up a brilliant blue. a water solution of sulphate of quinine, made acid by a few drops of sulphuric acid, also exhibits a blue fluorescence, while a water solution of æsculin (made by pouring hot water over some scraps of horse-chestnut bark) shines with a brilliant blue light. some lubricating oils fluoresce with a green light, as does also a solution in water of fluorescene, named thus because of its marked fluorescence. a solution of chlorophyll in alcohol, which can be readily prepared by soaking green leaves in alcohol, shows a red fluorescence; uranium glass--the canary glass of which small vases are very frequently { } made--exhibits a brilliant green fluorescence, as does also crystal uranium nitrate. it is found, on observing the spectrum of the fluorescent light, that a fairly small range of waves is emitted showing a well-marked maximum of intensity at a wave-length which is characteristic of the particular fluorescing substance. there also seems to be a limited range of waves which can induce this fluorescence, and this range also depends upon the fluorescing substance. as a rule, the inducing waves are shorter in length than the induced fluorescence, but this rule has some very marked exceptions. the fact that only a limited range of waves produces fluorescence explains a noticeable characteristic of the phenomenon. if the fluorescing solutions are at all strong the fluorescence is confined to the region close to where the light enters the solution, thus showing that the rays which are responsible for inducing the glow become rapidly absorbed, whereas the remainder of the light goes on practically unabsorbed. +phosphorescence.+--sometimes the emission of the induced light continues for some time after the inducing waves are withdrawn, and then the phenomenon is termed phosphorescence, since phosphorus emits a continuous glow without rise of temperature. sometimes the glow will continue for several hours after the exciting rays have been cut off, a good example of this being provided by balmain's luminous paint, which is a sulphide of calcium. with other substances the glow will only continue for a very small fraction of a second, so that it is impossible to { } say where fluorescence ends and where phosphorescence begins. in order to determine the duration of the glow in the case of these small times, an arrangement consisting of two rotating discs, each of which have slits in them, is set up. through the slits in one of them the substance is illuminated, and through the slits in the other the substance is observed while the light is cut off. by adjusting the position of the discs with regard to each other the slits may be made to follow one another after greater or shorter intervals, and so the time of observation can be made greater or smaller after the illumination is cut off. all the bodies which have been observed to exhibit phosphorescence are solid. +theory of fluorescence.+--it is fairly simple to imagine a mechanism by which fluorescence might be brought about, as we might assume a relation between the periods of oscillation of certain types of electron in the substance and the period of the stimulating waves. thus resonance might occur, and the consequent vibrations of the electrons would start a series of secondary waves. if, however, we assume resonance, it is difficult to see why there is a range of wave-lengths produced and another range of wave-lengths which may produce them. we should have expected one definite wave-length or a few definite ones producing one or a few definite wave-lengths in the glow, while if a whole range of waves will produce the effect it is difficult to see why all bodies do not exhibit the phenomenon. but the phenomenon of phosphorescence finally { } disposes of any such description, for the two phenomena have no sharp distinction between them. some substances are known in which the phosphorescence lasts for such an extremely small fraction of a second after the stimulating waves are withdrawn that it is difficult to know whether to call the effect fluorescence or phosphorescence. it is probable, therefore, that both are due to the same action. now a wave of orange light completes about five hundred million million vibrations in one second, and therefore if an orange-coloured phosphoresence were to last for only one five-hundredth of a second it would mean that the electrons responsible for it vibrate one million million times after the stimulus is removed. this is hardly credible, and becomes more credible when we remember that in some phosphorescent substances the effect lasts for many hours. +chemical theory of phosphoresence.+--it is more probable that the stimulating rays produce an actual chemical change in the phosphorescent substance. for instance, it is possible that the vibrations of a certain type of electron in one kind of atom become so violent as to detach it from the atom and the temporarily free electron attaches itself immediately to another kind of atom. the new arrangement may be quite stable; it is so in the action of light on a photographic plate, but it may only be stable when the electrons are being driven out of their original atoms, and in this case the electrons will begin to return to their old allegiance as soon as the stimulus is withdrawn. in the return { } process the electrons will naturally be agitated, and will therefore emit waves having their characteristic period. the rate at which the return process takes place will evidently depend upon the stability of the new arrangement. if it is extremely unstable, the whole return may only occupy a fraction of a second, but if it is nearly as stable as the original arrangement the return may be extremely slow. on this view, then, those substances will phosphoresce which have an electron which is fairly easily detached from its atom and which will attach itself to another atom, forming an arrangement which is less stable than the original. +temperature and phosphorescence.+--a confirmation of this chemical view is provided by the effect of temperature on phosphorescence. the rate of a chemical change is usually very largely increased by rise of temperature, and further, at very low temperatures a large number of chemical changes which take place quite readily at ordinary temperatures do not take place at all. similarly at very low temperatures the action of the light may be more or less stable. for example, dewar cooled a fragment of ammonium-platino-cyanide by means of liquid hydrogen, and exposed it to a strong light. after removing the light no phosphorescence was observed, though at ordinary temperatures a brilliant green phosphorescence is exhibited, but on allowing the fragment to warm up it presently glows very brightly. a partial stability is shown by balmain's luminous paint, for if it be kept in the dark until it becomes quite non-luminous it will begin to glow again for a { } short time if warmed up in any way. by means of this property the infra-red region of the spectrum may be made visible. for this purpose a screen is coated with the paint, exposed to strong sunlight, and then placed so as to receive the spectrum. the first effect of the invisible heat rays is to make the portions of the screen on which they fall brighter than their surroundings; but this causes the phosphorescence to be emitted more rapidly, and soon it is all emitted, leaving a dark region where the heat has destroyed the phosphorescence. on the whole, then, those substances which phosphoresce at ordinary temperatures do so more rapidly as the temperature rises. but dewar has found a number of substances which phosphoresce only at low temperatures, _e.g._ gelatine, celluloid, paraffin, ivory and horn. this is not a fatal objection to the idea of chemical change, as some chemical actions will only take place at low temperatures, but it is an objection as quite a large number of substances only phosphoresce at low temperatures, whereas there are not many chemical reactions which will only take place there. as a matter of fact, even if the idea of a chemical change be the true one, it is not a very satisfactory one, as chemical changes are undoubtedly very complicated ones, and it would be too difficult to trace the change from the vibration of an electron to the chemical change, and _vice-versa_. no satisfactory theory therefore exists to account for the absorption and the remission of the waves, whether accompanied or unaccompanied by a rise in temperature of the absorbing body. { } chapter vii pressure of radiation +prediction of pressure by maxwell.+--had the fact that light exerts a pressure been known in newton's time there is no doubt that it would have been hailed as conclusive proof of the superiority of the corpuscular theory over the wave theory. yet, ironically enough, it was reserved for james clerk maxwell to predict its existence and calculate its value on the assumption of his electromagnetic wave theory; and further, the measurement of its value has given decisive evidence in favour of the wave theory, for the value predicted by the latter is only one-half that predicted by the corpuscular theory, and the measurements by nicholls and hull agree to within per cent. with the wave theory value. maxwell showed that all waves which come up to and are absorbed by a surface exert a pressure on every square centimetre of the surface equal to the amount of energy contained in one cubic centimetre of the beam. if the surface is a perfect reflector, the reflected waves produce an equal back pressure, and therefore the pressure is doubled. as the waves are reflected back along their original direction, the energy in the beam will also be doubled, and so { } the pressure will still be equal to the energy per cubic centimetre of the beam. as the energy which is received in one second from the sun on any area can be measured by measuring the heat absorbed, and since the speed of light is known, we can calculate the energy contained in one cubic centimetre of full sunlight, and hence the pressure on one square centimetre of surface. for the energy received on one square centimetre of surface in one second must have been spread originally over a length of beam equal to the distance which the light has travelled in one second, _i.e._ over a length equal to the speed of light. if we divide that energy, therefore, by the speed of light, we shall get the energy in a one-centimetre length of the beam, and therefore in one cubic centimetre. this turns out to be an extremely small pressure indeed, being only a little more than the weight of half a milligram, on a square metre of surface. maxwell suggested that a much greater energy of radiation might be obtained by means of the concentrated rays of an electric lamp. such rays falling on a thin, metallic disc delicately suspended in a vacuum might perhaps produce an observable mechanical effect. nearly thirty years after maxwell's suggestion it was successfully carried out by prof. lebedew of moscow, who used precisely the arrangement which maxwell had suggested. +measurement of the pressure.+--a beam of light from an arc lamp was concentrated on to a disc suspended very delicately in an exhausted glass { } globe about inches across. actually four discs were suspended, as in fig. , and arrangements were made to concentrate the beam on to either side of any of the four discs. [illustration: fig. .] the suspension was a very fine quartz fibre _q_. the discs _d_, _d_, _d_, _d_, were half a centimetre in diameter and were fixed on two light arms, so that their centres were one centimetre from the glass rod, _g_, which carried them. a mirror, _m_, served to measure the angle through which the whole system was twisted owing to the pressure of the beam on one of the discs. in order to measure the angle a telescope viewed the reflection of a scale in _m_, and as _m_ turned different divisions of the scale came into view. the two discs on the left were polished and therefore the pressure on them should be about twice that on the blackened discs on the right. having measured the angle through which a beam of light has turned the system, it is a simple matter to measure the force which would cause this twist in the fibre q. in order to test whether the pressure agrees with the calculated value, we must find the energy in the beam of light. this was done by receiving the beam on a blackened block of copper and measuring the rate at which its temperature rose. from this rate and the weight of copper it is easy to calculate the amount of heat received per second, and therefore the amount of energy received per second on one square { } centimetre of the area. knowing the speed of the light we can, as suggested above, calculate the energy in one cubic centimetre of the beam. lebedew's result was in very fair accord with the calculated value. the chief difficulty in the experiment is to eliminate the effects due to the small amount of gas which remains in the globe. each disc is heated by the beam of light, and the gas in contact with it becomes heated and causes convection currents in the gas. at very low pressures a slightly different action of the gas becomes a disturbing factor. this effect is due to the molecules which come up to the disc becoming heated and rebounding from the disc with a greater velocity than that with which they approached it. the rebound of each molecule causes a backward kick on to the disc, and the continual stream of molecules causes a steady pressure. this would be the same on both sides of the disc if both sides were at the same temperature, but since the beam of light comes up to one side, that side becomes hotter than the other and there will be an excess of pressure on that side. this action is called "radiometer" action, because it was first made use of by crookes in detecting radiation. between the scylla of convection currents at higher pressures and the charybdis of radiometer action at lower pressures, there seems to be a channel at a pressure of about two or three centimetres of mercury. for here the convection currents are small and the radiometer action has scarcely begun to be appreciable. by working at this pressure and using one or two { } other devices for eliminating and allowing for the gas action, professors nicholls and hull also measured the pressure of light in an exceedingly careful and masterly way. their results were extremely consistent among themselves, and agreed with the calculated value to within one per cent. those who know the difficulty of measuring such minute forces, and the greatness of the disturbing factors, must recognise in this result one of the finest experimental achievements of our time. +effect of light pressure in astronomy.+--forces due to light pressure are so small that we should not expect to be able to detect their effects on astronomical bodies, and certainly we cannot hope to observe them in the large bodies of our system. the pressure of the sunlight on the whole surface of the earth is about , tons weight. this does not sound small until we compare it with the pull of the sun for the earth, which is two hundred million million times as great. when we consider very small bodies, however, we find that the pressure of the light may even exceed the gravitational pull, and therefore these small particles will be driven right away from our system. in order to show that the light pressure becomes more and more important, let us imagine two spheres of the same material, one of which has four times the radius of the other. then the weight of the larger one, that is its gravitational pull, will be sixty-four times as great as that of the smaller one, while the area, and therefore the light pressure, will be sixteen times as great. { } the light pressure is therefore four times as important in the sphere of one-quarter the radius. for a sphere whose radius is one two hundred million millionth of the radius of the earth and of the same density, the pressure of the light would equal the pull of the sun, and therefore such a sphere would not be attracted to the sun at all. this is an extremely small particle, much smaller than the finest visible dust, but even for much larger things the light pressure has an appreciable effect. thus for a sphere of one centimetre radius and of the same density as the earth, the pressure due to the sunlight is one seventy-four thousandth of the pull due to gravitation. it therefore need not move in its orbit with quite such a high speed in order that it may not fall into the sun, and its year is therefore lengthened by about three minutes. the lengthening out of comets' tails as they approach the sun, and the apparent repulsion of the tail by the sun, has sometimes been attributed to pressure of sunlight, but it is pretty certain that the forces called into play are very much greater than can be accounted for by the light. +doppler effect.+--the doppler effect also has some influence on the motion of astronomical bodies. when a body which is receiving waves moves towards the source of the waves, it receives the waves more rapidly than if it were still, and therefore the pressure is greater. when the body is moving away from the source it receives the waves less rapidly, and hence the pressure of light on it is less than for a stationary body. if a body is moving in an elliptical orbit, it is moving towards the sun in one part of its orbit and { } away in another part; it will therefore be retarded in both parts, and the ultimate result will be that the orbit will be circular. the doppler effect can act in another way. a body which is receiving waves from the sun on one side is thereby heated and emits waves in all directions. as it is moving in its orbit it will crowd up the waves which it sends out in front of it and lengthen out those which it sends out behind it. but the energy per cubic centimetre will be greater where the waves are crowded up than where they are drawn out, and therefore the body will experience a retarding force in its orbit. as the body tends to move more slowly it falls in a little towards the sun, and so approaches the sun in a spiral path. +three effects of light pressure.+--we thus have three effects of light pressure on bodies describing an orbit round the sun. the first effect is to lengthen their period of revolution, the second is to make their orbits more circular, and the third is to make them gradually approach the sun in a spiral path. these effects are quite inappreciable for bodies anything like the size of the earth, but for small bodies of the order of one centimetre diameter or less the effects would be quite large. our system is full of such bodies, as is evidenced by the number of them which penetrate our atmosphere and form shooting stars. the existence of such bodies is somewhat of a problem, as whatever estimate of the sun's age we accept as correct, he is certainly of such an age that if these bodies had existed at his beginning they would all have been drawn in to him long ago. we must therefore { } suppose that they are continually renewed in some way, and since we can see no sufficient source inside the solar system, we must come to the conclusion that they are renewed from outside. there is every reason to believe that some of them originate in comets which have become disintegrated and spread out along their orbits. these form the meteoric showers. thus the very finest dust is driven by the sun right out of our system, and all the rest he is gradually drawing in to himself. { } chapter viii the relation between radiant heat and electric waves in this concluding chapter it is proposed to show how the wave-lengths of radiant heat have been determined and to state what range of wave-lengths has been experimentally observed. it is then proposed to show how electromagnetic waves have been produced by straightforward electrical means and how their wave-lengths have been measured. the similarity in properties of the radiant heat and of the electric waves will be noted, leading to the conclusion that the difference between the two sets of waves is merely one of wave-length. +diffraction grating.+--the best method of measuring the wave-lengths of heat and light is by means of the "diffraction grating." this consists essentially of a large number of fine parallel equidistant slits placed very close to one another. for the measurement of the wave-lengths of light and of the shorter heat waves, it is usually produced by ruling a large number of very fine close equidistant lines on a piece of glass or on a polished mirror by means of a diamond point. the ruled lines are opaque on the glass and do not reflect on the mirror, and consequently the spaces in between act as slits. { } +rowland's gratings.+--the ruling of these gratings is a very difficult and tedious business, but the difficulties have been surmounted in a very remarkable manner by rowland, so that the gratings ruled on his machine have become standard instruments throughout the world. he succeeded in ruling gratings inches in diameter with , lines to the inch, truly a remarkable performance when we remember that if the diamond point develops the slightest chip in the process, the whole grating is spoilt. [illustration: fig. .] the action of the grating can be made clear by means of fig. . let a, b, c, d represent the { } equidistant slits in a grating, and let the straight lines to the left of the grating represent at any instant the crests of some simple plane waves coming up to the grating. the small fractions of the original waves emerging from the slits a, b, c, d will spread out from the slits so that the crests of the small wavelets may at any instant be represented by a series of concentric circles, starting from each slit as centre. the series of crests from each slit are represented in the figure. now notice that a line pq parallel to the original waves lies on one of the crests from each slit, and therefore the wavelets will make up a plane wave parallel to the original wave. this may therefore be brought to a focus by means of a convex lens just as if the grating were removed, except that the intensity of the wave is less. but a line, lm, also lies on a series of crests, the crest from a being one wave-length behind that from b, the one from b a wave-length behind that from c, and so on. the wavelets will therefore form a plane wave lm, which will move in the direction perpendicular to itself (_i.e._ the direction dk) and may be brought to a focus in that direction by means of a lens. draw ch and dk perpendicular to lm, and draw ce perpendicular to dk, _i.e._ parallel to lm. the difference between ch and dk is evidently one wave-length, _i.e._ de is one wave-length. if [greek: alpha] is the angle between the direction of pq and lm, de is evidently equal to cd sin [greek: alpha] and therefore one wave-length=cd sin [greek: alpha]. from the ruling of the grating we know the value { } of cd, and therefore by measuring [greek: alpha] we can calculate the wave-length. we find that a third line rs also lies on a series of crests, and therefore a plane wave sets out in the direction perpendicular to rs. we notice here that the crest from a is two wave-lengths behind that from b, and so on, and therefore if [greek: beta] is the angle between rs and pq, cd sin [greek: beta] is equal to two wave-lengths. similarly we get another plane wave for a three wave-lengths difference, and so on. the intensity of the wavelets falls off fairly rapidly as they become more oblique to their original direction, and therefore the intensity of these plane waves also falls off rather rapidly as they become more oblique to the direction in which pq goes. we see that the essential condition for the plane wave to set out in any direction, is that the difference in the distances of the plane wave from two successive slits shall be exactly a whole number of wave-lengths. should it depart ever so little from this condition we should see, on drawing the line, that there lie on the line an equal number of crests and troughs, and therefore, if a lens focus waves in this direction, the resulting effect is zero. the directions of the waves pq, lm, rs, &c., will therefore be very sharply defined and will admit of very accurate determination. +dispersion by grating.+--evidently the deviations [greek: alpha], [greek: beta] will be greater the greater is de, _i.e._ the greater the wave-length, and therefore the light or heat will be "dispersed" into its different wave-lengths as in the prism; but in this case the dispersion { } is opposite to that in the normal prism, the long waves being dispersed most and the short waves least. evidently, too, the smaller the distance cd the greater the angle, and therefore for the extremely short wave-lengths of light and of ultraviolet rays we require the distance between successive slits to be extremely small. [illustration: fig. .] +the spectrometer.+--the grating is usually used with a spectrometer, as shown in plan diagrammatically in fig. . the slit s from which the waves radiate is placed at the principal focus of the lens l, and therefore the waves emerge from l as plane waves which come up to the grating g. the telescope t is first turned until it views the slit directly, _i.e._ until the plane waves like pq in fig. are brought to a focus at the principal focus f of the objective of the telescope. the eyepiece e views the image of the slit s which is formed at f. the telescope is then turned through an angle, [greek: alpha], until it views the second image of the slit which will be formed by the plane waves similar to lm in fig. . the angle [greek: alpha] is carefully measured by the graduated circle on the spectrometer, { } and hence the wave-length of a particular kind of light, or of a particular part of the spectrum, is measured. this spectrometer method is exactly the method used for measuring the wave-lengths in the visible part of the spectrum. for the ultraviolet rays, instead of viewing the image of the slit by means of the eyepiece of the telescope, a photographic plate is placed at the principal focus f of the objective of the telescope, and serves to detect the existence and position of these shorter waves. for the heat rays a langley's bolometer strip is placed at f, in fact the bolometer strip might be used throughout, but it is not quite so sensitive for the visible and ultraviolet rays as the eye and the photographic plate. +absorption by glass and quartz.+--two main difficulties arise in these experiments. the first one is that although glass, or better still quartz, is extremely transparent to ultraviolet, visible, and the shorter infra-red waves, yet it absorbs some of the longer heat waves almost completely. for these waves, therefore, some arrangement must be devised in which they are not transmitted through a glass diffraction grating or through glass or quartz lenses. to effect this, the convex lenses are replaced by concave mirrors and the ruled grating is replaced by one which is made of very fine wires, which are stretched on a frame parallel to and equidistant from each other. the wire grating cannot be constructed with such fine or close slits as the ruled grating, but for the longer waves this is unnecessary. { } +reflecting spectrometer.+--an arrangement used by rubens is represented roughly in plan in fig. . l represents the source of heat, the rays from which are reflected at the concave mirror m, and brought to a focus on the slit s. emerging from s the rays are reflected at m(sub) and are thereby rendered parallel before passing through the wire grating g. after passing through the grating, the rays are reflected at m(sub) and are thereby focussed on to a bolometer strip placed at b. turning the mirror m(sub) in this arrangement is evidently equivalent to turning the telescope in the ordinary spectrometer arrangement. [illustration: fig. .] +absorption of waves by air.+--by using a spectrometer in an exhausted vessel schumann discovered that waves existed in the ultraviolet region of much smaller wave-length than any previously found, and that these waves were almost completely absorbed on passing through a few centimetres of air. to all longer waves, however, air seems to be extremely transparent. the second difficulty arises from the fact, already explained, that a diffraction grating produces not one, but a number of spectra. if only a small range of waves exists, this will lead to no confusion, but if a large range is being investigated, we may get two or more of these spectra overlapping. suppose, for example, we have some waves of wave-length de (in fig. ), some of wave-length one-half { } de and some of one-third de. then in the direction dk we shall get plane waves of each of these wave-lengths setting out and being brought to a focus in the same place. this difficulty can be fairly simply surmounted where the measurement of wave-length alone is required, by placing in the path of the rays from the source of light, suitable absorbing screens, which will only allow a very small range of wave-lengths to pass through them. there will then be no overlapping and no confusion. where the actual distribution of energy in the spectrum of any source of heat is to be determined the difficulty becomes more serious, and probably there is some error in the determinations, especially in the longest waves, which are masked almost completely by the overlapping shorter waves. +rest-strahlen or residual rays.+--a very beautiful method of isolating very long heat waves, and so freeing them from the masking effect of the shorter waves, was devised by rubens and nichols. it is found that when a substance very strongly absorbs any waves that pass through it, it also strongly reflects at its surface the same waves. for example, a sheet of glass used as a fire-screen will cut off most of the heat coming from the fire, although it is perfectly transparent to the light. if, now, it is placed so as to reflect the light and heat from the fire, it is found to reflect very little light but a very large proportion of the heat. some substances have a well-defined absorption band, _i.e._ they absorb a particular wave-length very strongly, and these substances will therefore reflect { } this same wave-length strongly. if instead of a single reflection a number of successive reflections be arranged, at each reflection the proportion of the strongly reflected wave-length is increased until ultimately there is practically only this one wave-length present. it can therefore be very easily measured. these waves resulting from a number of successive reflections, rest-strahlen or residual rays as they have been named, have been very largely used for investigating long waves. quartz gives rest-strahlen of length . centimetres and very feeble ones of . centimetres long. sylvite gives the longest rays yet isolated, the wave-length being . centimetres. +range of the waves.+--the lengths of the waves thus far measured are:-- schumann waves . . . . . . . . . to . cms. ultraviolet . . . . . . . . . . to . " violet . . . . . . . . . . . . . " green . . . . . . . . . . . . . " red . . . . . . . . . . . . . . to . " infra-red . . . . . . . . . . . to about . " rest-strahlen from quartz . . . and . " rest-strahlen from sylvite . . . " thus the longest waves are six hundred times the length of the shortest. the corresponding range of wave-lengths of sound would be a little more than eight octaves, of which the visible part of the spectrum is less than one. electromagnetic induction.--in the attempt to explain the nature of an electromagnetic wave (pp. - ) it was stated that an electric wave must always be accompanied by a magnetic wave. in order to { } understand the production of these waves, the relation between electric and magnetic lines of force must be stated in more detail. a large number of quite simple experiments show that whenever the electric field at any point is changing, _i.e._ whenever the lines of force are moving perpendicular to themselves, a magnetic field is produced at the point, and this magnetic field lasts while the change is taking place. an exactly similar result is observed when the magnetic field at a point is changing--an electric field is produced which lasts while the magnetic field is changing. when the electric field changes, therefore, there is both an action and a reaction--a magnetic field is produced and this change in magnetic field produces a corresponding electric field. this induced electric field is always of such a kind as to delay the change in the original electric field; if the original field is becoming weaker the induced field is in the same direction, thus delaying the weakening, and if the original field is becoming stronger the induced field is in the opposite direction, thus delaying the increase. +momentum of moving electric field.+--imagine now a small portion of an electric field moving at a steady speed; it will produce, owing to its motion, a steady magnetic field. if now the motion be stopped, the magnetic field will be destroyed, and the change in the magnetic field will produce an electric field so as to delay the change, _i.e._ so as to continue the original motion. the moving electric field thus has momentum in exactly the same way as a moving mass has. the parallel between the two { } is strictly accurate. the mass has energy due to its motion, and in order to stop the mass this energy must be converted into some other form of energy and work must therefore be done. the electric field has energy due to its motion--the energy of the magnetic field--and therefore to stop the motion of the electric field, the energy of the magnetic field must be converted into some other form, and work must therefore be done. one consequence of the momentum of a moving mass is well illustrated by the pendulum. the bob of the pendulum is in equilibrium when it is at its lowest point, but when it is displaced from that point and allowed to swing, it does not swing to its lowest point and stay there, but is carried beyond that point by its momentum. the work done in displacing the bob soon brings it to rest on the other side, and it swings back again only to overshoot the mark again. the friction in the support of the pendulum and the resistance of { } the air to the motion makes each swing a little smaller than the one before it, so that ultimately the swing will die down to zero and the pendulum will come to rest at its lowest point. the graph of the displacement of the bob at different times will therefore be something like fig. . should the pendulum be put to swing, not in air, but in some viscous medium like oil, its vibrations would be damped down very much more rapidly, and if the medium be viscous enough the vibrations may be suppressed, altogether, the pendulum merely sinking to its lowest position. [illustration: fig. .] +electric oscillation.+--these conditions have their exact counterpart in the electric field. to understand them, three properties of lines of force must be borne in mind: (i.) lines of force act as if in tension and therefore always tend to shorten as much as possible; (ii.) the ends of lines of force can move freely on a conductor; (iii.) lines of force in motion possess momentum. now imagine two conducting plates a and b, fig. , charged positively and negatively, and therefore connected by lines of force as indicated. let the two plates be suddenly connected by the wire _w_, so that the ends of the lines of force may freely slide from a to b or _vice-versa_, and therefore all the lines will slide upwards along a and b, and then towards each other along _w_, until they shrink to zero { } somewhere in _w_. the condition of equilibrium will evidently be reached when all the lines have thus shrunk to zero, but the lines which are travelling from a towards b will have momentum and will therefore overshoot the equilibrium condition and pass right on to b. that is, the positive ends of the lines will travel on to b, and similarly the negative ends will pass on to a. the lines of force between a and b will therefore be reversed. the tension in the lines will soon bring them to rest, and they will slide back again, overshoot the mark again, reach a limit in the original direction and still again slide back. the field between a and b will therefore be continually reversed, but each time its value will be a little less, until ultimately the vibrations will die down to zero. thus if we were to replace the displacement in fig. by the value of the field between a and b we should have an exactly similar graph. [illustration: fig. .] the amount by which the oscillations are damped down will depend upon the character of the wire _w_. if it is a very poor conductor it will offer a large resistance to the sliding of the lines along it, and the vibrations will be quickly damped down or, if the resistance is great enough, be suppressed altogether. this rapid alternation of the electric field will send out electromagnetic waves which die down as the oscillations decrease. +the spark discharge.+--in practice the wire _w_ is not actually used, but the air itself suddenly becomes a conductor and makes the connection. when the electric field at a point in the air exceeds a certain limiting strength, the air seems to break down and { } suddenly become a conductor and remains one for a short time. this breaking down is accompanied by light and heat, and is known as the spark discharge or electric spark. +experiments of hertz.+--in the brilliant experiments carried out by hertz at karlsruhe between and , he not only demonstrated the existence of the waves produced in this way, but he showed that they are reflected and refracted like ordinary light, he measured their wave-length and roughly measured their speed, this latter being equal to the speed of light within the errors of experiment. [illustration: fig. .] one arrangement used by hertz is shown in plan in fig. . a ruhmkorff coil r serves to charge the two conductors a and b until the air breaks down at the gap g, and a spark passes. before the spark is { } produced, the lines of force on the lower side of ab will in form be something like the dotted lines in the figure, but as soon as the air becomes a conductor, the positive ends of the lines will surge from a towards b and on to b, and the negative ends will surge on to a. these to and fro surgings will continue for a little while, but will gradually die out. as the surgings are all up and down ab, the electric vibrations in the electromagnetic waves sent out { } will all be parallel to ab, and therefore they will be polarised. [illustration: fig. .] this is characteristic of all electric waves, as no single sparking apparatus will produce anything but waves parallel to the spark gap. the electric vibrations coming up to a conductor placed in the position of the wire rectangle, m, will cause surging of the lines along it, and, if these surgings are powerful enough, will cause a spark to pass across the small gap s. such a rectangle was therefore used by hertz as a detector of the waves, but since that time many detectors of very much greater sensitiveness have been devised. +reflection.+--in order to show that these waves are reflected in the same way as light waves, hertz placed the sparking knobs, g, at the focus of a large parabolic metallic reflector, and his detector, d, at the focus of a similar reflector placed as in fig. , but much farther away (cf. fig. ). in this position sparking at g produced strong sparking in the detector, although the distance was such that no sparking was produced without the reflectors. +refraction.+--the refraction of the waves was { } shown by means of a large prism made of pitch. this had an angle of ° and was about . metres high and . metres broad. [illustration: fig. .] setting it up as shown in plan in fig. , strong sparking was produced in the detector, thus showing that the rays of electric waves were deflected by ° on passing through the prism. moving the mirror and detector in either direction from the line lm, made the sparks decrease rapidly in intensity, so that the exact position of lm can be determined with considerable definiteness. +wave-length, by stationary waves.+--the wave-lengths of the oscillations were found by means of what are known as stationary waves. when two exactly similar sets of waves are travelling in opposite directions over the same space, they produce no effects at certain points called nodes. these nodes are just half a wave-length apart. their production can be understood by reference to fig. . the dotted lines represent the two waves which are travelling in the direction indicated by the arrows. in a the time is chosen when the waves are exactly superposed, and the resultant displacement will be represented by the solid line. the points marked with a cross will be points at which the displacement is zero. [illustration: fig. .] in b each wave has travelled a distance equal to a quarter of a wave-length, and it will be seen that the two sets of waves cause equal and opposite displacements. the resulting displacement is therefore zero, as indicated by the solid line. in c the waves have travelled another quarter of a wave-length and { } are superposed again, but in this case the displacements will be in the opposite directions from those in a. in d, still another quarter wave-length has been traversed by each wave, and another quarter wave-length would bring back the position a. in e, we have the successive positions of the wave drawn in one diagram, and we notice that the points indicated by a cross are always undisplaced and their distance apart is one-half a wave-length. hertz produced these conditions by setting up his coil and sparking knobs at some distance from a reflecting wall, fig. . then the waves which are coming up to the wall and those which are reflected { } from the wall will be travelling in opposite directions over the same space. true, the reflected waves will be rather weaker than the original ones, so that there will be a little displacement even at the nodes, but there will be a well-marked minimum. thus when the detector is placed at a, b, c or d no sparking or very feeble sparking occurs, while midway between these points the sparking is very vigorous, and the distance between two successive minima is one-half a wave-length. [illustration: fig. .] the wave-length will depend upon the size, form, &c., of the conductors between which the sparking occurs, for the time which the lines of force take to surge backwards and forwards in the conductors will depend upon these things. other things being equal, the smaller the conductors the smaller the time and therefore the shorter the wave-length. the shortest wave which hertz succeeded in producing was centimetres long, but since then waves as little as millimetres long have been produced. { } the waves which are produced in a modern wireless telegraphy apparatus are miles in length. we thus see that there is rather a large gap between the longest heat waves which have been isolated, . cms., and the shortest electric waves, . cms. the surprising fact, however, is that this gap is so small, for the heat waves are produced by vibrations within a molecule, or at most within a small group of molecules, whereas the electric surgings, even in the smallest conductors, take place over many many millions of molecules. in conclusion, therefore, we see that from the schumann waves up to the longest heat waves a little over eight octaves of electromagnetic waves have been detected, then after a gap of between five and six octaves the ordinary electrically produced electromagnetic waves begin and extend on through an almost indefinite number of octaves. { } books for further reading j. h. poynting, _the pressure of light_. e. edser, _heat for advanced students_: the chapters on radiation. e. edser, _light for advanced students_: the chapters on the spectrum. b. w. wood, _physical optics_: the chapters on fluorescence and phosphorescence, laws of radiation, nature of white light, and absorption of light. { } index absorbing power, -- and radiating power, absorption, spectra, -- by glass and quartz, -- by air, addition of waves, amplitude, balmain, luminous paint, boltzmann, laws of radiation, convection currents, corpuscular theory, -- reflection and refraction by, crookes' radiometer, dewar, temperature and phosphorescence, diffraction grating, -- dispersion by, -- wire grating, dispersion, , doppler effect, efficiency in lighting, elastic solid theory, electric field, electric charges within the atom, electric oscillations, , electrification, positive and negative, electromagnetic induction, electromagnetic waves, , electrons, energy in simple wave, energy--wave-length curve, fluorescence, -- theory of, foucault, speed of light in different media, fourier's series of waves, , fraunhöfer lines, full radiator and absorber, , gases as radiators, huyghens' wave theory, hertz, experiments on electric waves, -- reflection, -- refraction, -- wave-length by stationary waves, infra-red rays, interference, kirchoff's law, langley, bolometer, , , , lebedew, pressure of light, lummer and pringsheim, law of radiation, , magnetic oscillations, maxwell, electromagnetic theory, -- pressure of light, momentum of moving electric field, newton, dispersion, -- corpuscular theory, -- law of cooling, nichols, rubens and, rest-strahlen, nicholls and hull, pressure of light, , pflÃ�ger, emission from tourmaline, phase, phosphorescence, -- chemical theory of, -- temperature and phosphorescence, planck, energy and wave-length, polarised light, emission from tourmaline, pressure of light, prediction of by maxwell, -- measurement by lebedew, -- measurement by nicholls and hull, , -- on the earth, -- on fine dust, -- on comets' tails, -- three effects of in astronomy, prévost, theory of exchanges, radiating power, radiometer action, reflection, corpuscular theory, -- of electric waves, refraction, corpuscular theory, -- of electric waves, resonance, rest-strahlen or residual rays, ripples on mercury, ritchie, radiating and absorbing powers, rowland, gratings, rubens and kurlbaum, proof of planck's law, rubens and nichols, rest-strahlen, schumann waves, simple harmonic motion, simple periodic motion, spark discharge, spectrometer, -- reflecting, spectrum, -- the whole, -- incandescent solid or liquid, -- incandescent gas, -- analysis, -- emission and absorption, -- sun, -- stars and nebulæ, -- and temperature, stationary waves, stefan, law of radiation, temperature, absolute, -- of planets, -- of space, -- of sun, ultraviolet rays, , wave form, wave-length, -- range of, -- of electric waves, wave theory, rectilinear propagation, wien, law of radiation, young, interference, printed by ballantyne, hanson & co. edinburgh & london /tb the people's books "a wonderful enterprise, admirably planned, and deserving the highest success."--_the nation_. the first dozen volumes . +botany: the modern study of plants+. by m. c. stopes, d.sc., ph.d., f.l.s. "a wonderful 'multum in parvo,' and cannot fail, by its lucidity and pleasant method of exposition, to give the reader not only a clear conception of the science of botany as a whole, but also a desire for fuller knowledge of plant life."--_notes and queries_. . +heredity+. by j. a. s. watson, b.sc. "accurate, and written in a simple manner which will stimulate those who are interested to wider reading."--_athenæum_. . +organic chemistry+. by professor j. b. cohen, b.sc., f.r.s. "an excellently clear and efficient treatise on a subject not easily confined within a short or untechnical discourse."--_the manchester guardian_. . +the principles of electricity+. by norman r. campbell, m.a. "as for mr. norman campbell's treatise 'in petto' i cannot but think it a model of its kind. he takes next to nothing for granted."--_sunday times_. . +the science of the stars+. by e. w. maunder, f.r.a.s., of the royal observatory, greenwich. "will convey to the attentive reader an enormous amount of information in a small space, being clear and abreast of current knowledge."--_the athenæum_. . +henri bergson: the philosophy of change+. by h. wildon carr. "the fact that m. bergson has read the proof-sheets of mr. carr's admirable survey will give it a certain authoritativeness for the general reader."--_daily news_. . +roman catholicism+. by h. b. coxon. preface, mgr. r. h. benson. "this small book is one which cannot fail to be of use to those who desire to know what catholics do, and do not, believe."--_the catholic chronicle_. . +mary queen of scots+. by e. o'neill, m.a. "mrs. o'neill, on 'mary queen of scots,' is splendid; it is an attempt to give the very truth about a subject on which all feel interest and most lie freely."--_daily express_. . +women's suffrage+. by m. g. fawcett, ll.d. "mrs. fawcett's admirably concise and fair-minded historical sketch of the women's suffrage movement. we could hardly ask for a better summary of events and prospects."--_daily news_. . +shakespeare+. by professor c. h. herford, litt.d. "well worth a place alongside professor raleigh's book in the 'english men of letters.' ... sets a high note and retains it without effort."--_observer_. . +pure gold--a choice of lyrics and sonnets+. by h. c. o'neill. "an anthology of good poetry such as we might expect from a man of taste."--_daily news_. . _dante_. by a. g. ferrers howell. "it is a fine piece of scholarship, and should be read by any one who is beginning the study of dante, or indeed any one who is interested generally in the early process of european literature, for the process is here admirably analysed."--_the manchester guardian_. the second dozen volumes (ready) . +the foundations of science+. by w. c. d. whetham, m.a., f.r.s. . +inorganic chemistry+. by professor k. c. c. baly, f.r.s. . +radiation+. by p. phillips, d.sc. . +lord kelvin+. by a. russell, m.a., d.sc., m.i.e.e. . +huxley+. by professor g. leighton, m.d. . +the growth of freedom+. by h. w. nevinson. . +julius caesar: soldier, statesman, emperor+. by hilary hardinge. . +england in the middle ages+. by mrs. e. o'neill, m.a. . +francis bacon+. by professor a. r. skemp, m.a. . +the brontÃ�s+. by miss flora masson. . +a dictionary of synonyms+. by austin k. gray, b.a. . +home rule+. by l. g. redmond howard. _list of other volumes in preparation may be had_. london and edinburgh: t. c. & e. c. jack new york: dodge publishing co. a medley of weather lore a medley of weather lore collected by m. e. s. wright "an almanack is out at twelve months day, my legacy it doth endure for aye, but take you notice, though 'tis but a hint, it far exceeds some books of greater print." the shepherd's legacy. (_john claridge, _) horace g. commin, bournemouth the anchor press, ltd., tiptree, essex. contents page preface january february march april may june july august september october november december index preface in this collection of weather lore and poetry i beg to acknowledge with gratitude permission from messrs. macmillan to quote lines from tennyson, charles turner, alfred austin, matthew arnold, christina rossetti, t. e. brown, and francis doyle. from messrs. longman and green from jean ingelow, from "four bridges," and "an afternoon at a parsonage." andrew lang, from "a ballade of summer." william morris' from "the earthly paradise," and "love is enough," and edwin arnold, from "bloom of an almond tree." from messrs. kegan paul and trench from lewis morris. from messrs. chatto and windus (by the courtesy of mr. theodore watts-dunton) for the inclusion of verses by a. swinburne, and from the walter scott publishing company for the use of selections of r. w. emerson and owen meredith. i have endeavoured to avoid infringing copyrights, but if i should have done so inadvertently i beg that my sincere apologies maybe accepted. m. e. s. wright. a medley of weather lore a medley of weather lore january ancient cornish name for the month: mis-jenver, cold air month. * * * * * jewel for the month: garnet. constancy. * * * * * if janiveer calends be summerly gay, 'twill be wintry weather till the calends of may. * * * * * the wind of the south will be productive of heat and fertility; the wind of the west, of milk and fish; the wind from the north, of cold and storm; the wind from the east, of fruit on the trees. _scotland._ * * * * * at new year's tide the days lengthen a cock's stride. _proverb in the north._ * * * * * a cold january, a feverish february, a dusty march, a weeping april, a windy may, presage a good year and gay. _france._ * * * * * warwickshire countrymen to ensure good luck bow nine times to the first new moon of the year. * * * * * a snow year, a rich year. * * * * * the blackest month of all the year is the month of janiveer. * * * * * through all the sad and weary hours which cold and dark and storms will bring, we scarce believe in what we know-- that time drags on at last to spring. * * * * * the empty pastures blind with rain. * * * * * if the grass grow in janiveer 'twill be the worse for 't all the year. * * * * * a fair day in winter is the mother of a storm. * * * * * under water famine, under snow bread. * * * * * march in janiveer, janiveer in march i fear. * * * * * a year of snow a year of plenty. _spain._ * * * * * winter time for shoeing; peascod time for wooing. _devon._ * * * * * on twelve-eve in christmas, they used to set up as high as they can a sieve of oats, and in it a dozen candles set round, and in the centre one larger, all lighted. this in memory of our saviour and his apostles, lights of the world. _westmeath custom._ * * * * * in the south-hams of devonshire, on the eve of the epiphany, the farmer attended by his workmen, with a large pitcher of cyder, goes to the orchard, and there, encircling one of the best bearing trees, they drink the following toast three several times: "here's to thee, old apple-tree, whence thou mayst bud, and whence thou mayst blow! and whence thou mayst bear apples enow! hats-full, caps-full! bushel-bushel-sacks-full! and my pockets full too! huzza!" * * * * * old custom of blessing apple trees on twelfth day. apple-tree, apple-tree, bear apples for me: hats full, laps full, sacks full, caps full: apple-tree, apple-tree, bear apples for me. * * * * * "twelfth-day--came in a tiffany suit, white and gold, like a queen on a frost-cake, all royal, glittering, and epiphanous." _elia._ * * * * * january the fourteenth will be either the coldest or wettest day of the year. _huntingdon._ * * * * * st. anthony. (_january th._) it is affirmed of him that all the world bemoaned his death, for afterwards there fell no rain from heaven for three years. st. vincent. (_january nd. old style. february rd. new style._) remember in st. vincent's day if the sun his beams display, 'tis a token bright and clear, that you will have a prosperous year. * * * * * winter's thunder's summer's wonder. * * * * * st. paul's eve. (_january th._) winter's white shrowd doth cover all the grounde, and caecias blows his bitter blaste of woe; the ponds and pooles, and streams in ice are bounde, and famished birds are shivering in the snowe. * * * * * still round about the house they flitting goe, and at the windows seek for scraps of foode which charity with hand profuse doth throwe, right weeting that in neede of it they stoode, for charity is shown by working creatures goode. the sparrowe pert, the chaffinche gay and cleane, the redbreast welcome to the cotter's house, the livelie blue tomtit, the oxeye greene, the dingie dunnock, and the swart colemouse; the titmouse of the marsh, the nimble wrenne, the bullfinch and the goldspink, with the king of birds the goldcrest. the thrush, now and then, the blackbird, wont to whistle in the spring, like christians seeke the heavenlie food saint paul doth bring. _dr. forster._ * * * * * st. paul's day. if saint paul's day be fair and clear, it promises then a happy year; but if it chance to snow or rain, then will be dear all sorts of grain; or if the wind do blow aloft, great stirs will vex the world full oft; and if dark clouds do muff the sky, then foul and cattle oft will die. _t. passenger._ * * * * * of gardens. for the latter part of january and february, the mezerion tree, which then blossoms; crocus vernus, both the yellow and the gray; primroses, anemones, the early tulippa, hyacinthus orientalis, chamairis, frettellaria. _bacon._ * * * * * a january spring is worth no thing. * * * * * pluck broom, broom still, cut broom, broom kill. _tusser._ * * * * * good gardener mine, make garden fine, set garden pease, and beans if ye please. set respis and rose, young roots of those. who now sows oats gets gold and groats. who sows in may, gets little that way. _tusser._ * * * * * a kindly good january freezeth pot by the fire. * * * * * o winter! wilt thou never--never go! o summer! but i weary for thy coming! _david gray._ * * * * * if the robin sings in the bush, then the weather will be coarse; but if the robin sings on the barn, then the weather will be warm. _norfolk._ february ancient cornish name: hu-evral, whirling month. * * * * * jewel: amethyst. sincerity. * * * * * one month is past, another is begun, since merry bells rang out the dying year, and buds of rarest green began to peer, as if impatient for a warmer sun; and though the distant hills are bleak and dun, the virgin snowdrop, like a lambent fire, pierces the cold earth, with its green-streaked spire; and in dark woods the wandering little one may find a primrose. _hartley coleridge._ * * * * * fair rising from her icy couch, wan herald of the floral year, the snowdrop marks the spring's approach, ere yet the primrose groups appear, or peers the arum from its spotted veil, or violets scent the cold capricious gale. _charlotte smith._ * * * * * candlemas shined, and the winter's behind. * * * * * if candlemas day be fair and bright the winter will take another flight; but if it should be dark and drear then winter is gone for another year. * * * * * when on the purification sun hath shined, the greater part of winter comes behind. * * * * * the badger peeps out of his hole on candlemas day, and if he finds snows, walks abroad; but if he sees the sun shining, he draws back into his hole. _german saying._ * * * * * on candlemas day if the thorns hang-a-drop, then you are sure of a good pea crop. * * * * * when the wind's in the east on candlemas day, there it will stick till the second of may. * * * * * february fill the ditch, black or white we don't care which. _hants._ * * * * * all the months of the year fear a fair februeer. * * * * * the dim droop of a sombre february day. * * * * * there is an old proverb, that birds of a feather on saint valentine's day will meet together. * * * * * . why, valentine's a day to choose a mistress, and our freedom lose? may i my reason interpose, the question with an answer close? to imitate we have a mind, and couple like the winged kind. _john dunton._ * * * * * i early rose, just at the break of day, before the sun had chased the stars away; afield i went, amid the morning dew, to milk my kine (for so should housewives do), thee first i spied, and the first swain we see. in spite of fortune, shall our true-love be. _gay._ * * * * * shrove-tide. beef and bacon's out of season, i want a pan to parch my peason. _berks._ * * * * * knick-knock, the pan's hot, and we are come a-shroving, for a piece of pancake, or a piece of bacon, or a piece of truckle cheese of your own making. _hants._ * * * * * on shrove tuesday night, though the supper be fat, before easter day thou mayst fast for that. _isle of man._ * * * * * pancake bell. (_congleton._) the housekeeper goes to the huxter's shop, and the eggs are brought home, and there's flop! flop! flop! and there's batter, and butter, and savoury smell, while merrily rings the pancake bell. * * * * * so much sun as shineth on pancake tuesday, the like will shine every day in lent. * * * * * a hoar frost, third day crost, the fourth lost. _lancs._ * * * * * bean sowing. one for the mouse, one for the crow, one to rot, one to grow. * * * * * sow peason and beans in the wane of the moone, who soweth them sooner, he soweth too soon; that they with the planet may rest and rise, and flourish with bearing, most plentiful wise. _tusser._ * * * * * if february gives much snow a fine summer it doth foreshow. * * * * * now set for thy pot best herbs to be got, for flowers go set, all sorts ye can get. _tusser._ * * * * * in oxfordshire the first bee seen in february is saluted, as this is said to bring good luck. * * * * * thrush's song. "did he do it? did he do it? come and see, come and see; knee deep, knee deep; cherry sweet, cherry sweet, to me! to me! to me!" * * * * * the pretty lark, climbing the welkin clear, chaunts with a "cheer, here, peer, i near my dear!" when stooping thence, seeming her fall to rue, "adieu," she cries, "adieu! dear love, adieu!" * * * * * when after a rough and stormy day there is a lull in the wind at the going down of the sun, old men say: "us shall have better weather now, for the wind's gone to sleep with the sun." _devon._ * * * * * when a moorland shepherd meets his sheep on a winter's night coming down from the hilltops (where they prefer to sleep) he knows that a storm is brewing. march ancient cornish name: miz-merp, horse month. * * * * * jewel: bloodstone. courage and wisdom. * * * * * upon st. david's day put oats and barley in the clay. * * * * * the leeke is white and green, whereby is ment that britaines are both stout and eminent; next to the lion and the unicorne, the leek's the fairest emblyn that is worne. _harleian ms._ * * * * * on the first of march the crows begin to search, by the first of april they are sitting still, by the first of may they are a' flown away; croupin' greedy back again, wi' october's wind and rain. * * * * * he who freely lops in march will get his lap full of fruit. _portuguese saying._ * * * * * tossing his mane of snows in wildest eddies and tangles, warlike march cometh in, hoarse, with tempestuous breath. through all the moaning chimneys, and 'thwart all the hollows and angles, round the shuddering house, breathing of winter and death. _w. d. howells._ * * * * * welcome, o march! whose kindly days and dry make april ready for the throstle's song, thou first redresser of the winter's wrong. _w. morris._ * * * * * of gardens. for march there come violets, especially the single blue, which are the earliest; the early daffodil, the daisy, the almond tree in blossom, the peach tree in blossom, the cornelian (dogwood) tree in blossom, sweetbrier. _bacon._ * * * * * a frosty winter, and a dusty march, and a rain about aperill, and another about the lammas time when the corn begins to fill, is worth a ploughy of gold and all her pins theretill. * * * * * come gather the crocus-cups with me, and dream of the summer coming; saffron, and purple, and snowy white, all awake to the first bees humming. the white is there for the maiden-heart, and the purple is there for sorrow; the saffron is there for the true true love, and they'll all be dead to-morrow. _sebastian evans._ * * * * * beside the garden path the crocus now puts forth its head to woo the genial breeze, and finds the snowdrop, hardier visitant, already basking in the solar ray. upon the brooke the water cresses float more greenly, and the bordering reeds exalt higher their speary summits. joyously, from stone to stone, the ouzel flits along, startling the linnet from the hawthorn bough; while on the elm-tree, overshadowing deep the low-roofed cottage white, the blackbird sits cheerily hymning the awakened year. * * * * * blank earth-baldness clothes itself afresh, and breaks into the crocus-purple hour. _tennyson._ * * * * * spring's an expansive time: yet i don't trust march with its peck of dust, nor april with its rainbow-crowned brief showers, nor even may, whose flowers one frost may wither through the sunless hours. _c. rossetti._ * * * * * if it does not freeze on the tenth of march a fertile year may be expected. * * * * * in march is good graffing, the skilful do know, so long as the wind in the east do not blow: from moon being changed, till past be the prime, for graffing and cropping is very good time. _tusser._ * * * * * in march and in april, from morning to night, in sowing and setting good huswives delight: to have in a garden or other like plot, to trim up their house, and to furnish their pot. _tusser._ * * * * * to the daffodil. o love-star of the unbeloved march, when cold and shrill, forth flows beneath a low dim-lighted arch the wind that beats sharp crag and barren hill, and keeps unfilmed the lately torpid rill! herald and harbinger! with thee begins the year's great jubilee! of her solemnities sublime (a sacristan whose gusty taper flashes through earliest morning vapour) thou ring'st dark nocturns and dim prime. birds that have yet no heart for song gain strength with thee to twitter, and, warm at last, where hollies throng, the mirror'd sunbeams glitter. _a. de vere._ * * * * * the softest turf of english green, with sloping walks and trees between, and then a bed of flowers half-seen. here daffodils in early spring and violets their off'rings bring, and sweetest birds their hymns outsing. * * * * * when country roads begin to thaw in mottled spots of damp and dust, and fences by the margin draw along the frozen crust their graphic silhouettes, i say, the spring is coming round this way. when suddenly some shadow bird goes wavering beneath the gaze, and through the hedge the moan is heard of kine that fain would graze in grasses new, i smile and say, the spring is coming round this way. _whitcomb riley._ * * * * * oh, what a dawn of day! how the march sun feels like may! all is blue again after last night's rain. _browning._ * * * * * no summer flowers are half so sweet as those of early spring. * * * * * under the furze is hunger and cold, under the broom is silver and gold. * * * * * the spring. when wintry weather's all a-done, an' brooks do sparkle in the zun, an' naisy-builden rooks do vlee wi' sticks toward their elem tree; when birds do zing, an' we can zee upon the bough the buds o' spring-- then i'm as happy as a king, a'vield wi' health an' sunshine. vor then the cowslips hangin' flow'r a-wetted in the zunny show'r, do grow wi' vi'lets, sweet o' smell, beside the wood-screen'd graegle's bell; where drushes aggs, wi' sky-blue shell, do lie in mossy nest among thorns, while they do zing their zong at evenin' in the zunsheen. _w. barnes._ * * * * * a camomile bed,-- the more it is trodden, the more it will spread. * * * * * thunder in spring cold will bring. * * * * * march search, april try, may will prove if you live or die. * * * * * march wind and may sun makes clothes white and maids dun. * * * * * march does from april gain three days, and they're in rain, returned by april in's bad kind, three days, and they're in wind. * * * * * sun set in a clear, easterly wind's near; sun set in a bank, westerly will not lack. _scotland._ * * * * * in the morning look toward the south east; in the evening look toward the north west. _china._ * * * * * pale moon doth rain, red moon doth blow, white moon doth neither rain nor snow. _latin proverb._ * * * * * any person neglecting to kill the first butterfly he may see for the season will have ill luck throughout the year. _devon and hants._ * * * * * st. patrick's day. (_march th._) gervase of tilbury gives a legend that on st. patrick's day, to do homage to him, the fish rise from the sea, pass in procession before his altar, and then disappear. * * * * * divination by a daffodil. when a daffodil i see hanging down his head t'wards me, guesse i may what i must be: first, i shall decline my head; secondly, i shall be dead; lastly, safely buryed. _herrick._ * * * * * hail! once again, that sweet strong note! loud on my loftiest larch, thou quaverest with thy mottled throat, brave minstrel of bleak march! _a. austin._ * * * * * march twenty-first, spring begins. * * * * * where the wind is at twelve o'clock on the twenty-first of march, there she'll bide for three months afterwards. _surrey and hants._ * * * * * when the wind blows from n.e.--a uniformly dry quarter during the week of the vernal equinox--it is an all but unfailing guide to the general character of the ensuing season. * * * * * our vernal signs the ram begins, then comes the bull, in may the twins; the crab in june, next leo shines, and virgo ends the northern signs. the balance brings autumnal fruits, the scorpion stings, the archer shoots; december's goat brings wintry blast, aquarius rain, the fish come last. _e. c. brewer._ * * * * * spring is here when you can tread on nine daisies at once on the village green. * * * * * there is a saying that if boys be beaten with an elder stick it hinders their growth. * * * * * when our lord falls in our lady's lap england will meet with a great mishap. * * * * * there is a tradition amongst new forest gipsies that you must not soap your face on good friday, as it is said that soapsuds were thrown in our lord's face on the day of his crucifixion. * * * * * thou wilt remember one warm morn when winter crept aged from the earth, and spring's first breath blew soft from the moist hills; the blackthorn boughs, so dark in the bare wood, when glistening in the sunshine were white with coming buds, like the bright side of a sorrow, and the banks had violets opening from sleep like eyes. _browning._ * * * * * if apples bloom in march, in vain for 'um you'll sarch; if apples bloom in april, why then they'll be plentiful; if apples bloom in may, you may eat 'um night and day. * * * * * from whatever quarter the wind blows on palm sunday, it will continue to blow for the greater part of the coming summer. _hants._ * * * * * as many days of fog in march, so many days of frost in may, on corresponding days. _hants._ in spring a tub of rain makes a spoonful of mud. in autumn a spoonful of rain makes a tub of mud. * * * * * there is a tradition that twin lambs are scarce in leap year. * * * * * sleep with your head to the north--you will have sickness; to the south--long life; to the east--health and riches; to the west--fame. april ancient cornish name: miz-ebrall primrose month. * * * * * jewel for the month: sapphire. frees from enchantment. * * * * * if it thunders on all fool's day it brings good crops of grain and hay. * * * * * the first thunder of the year awakes all the frogs and all the snakes. * * * * * ms. years old. the first monday in april cain was born, and abel was slain. the second monday in august sodom and gomorrah were destroyed. the thirty-first of december judas was born, who betrayed christ. these are dangerous days to begin any business, fall sick, or undertake any journey. * * * * * a wet good friday and easter day brings plenty of grass, but little good hay. _leicester._ * * * * * parsley sown on good friday bears a heavier crop than that sown on any other day. parsley seed goes nine times to the devil before coming up. it only comes up partially because the devil takes his tithe of it. _old country sayings._ oh! faint, delicious, spring-tide violet, thin odour, like a key. turns noiselessly in memory's wards to let a thought of sorrow free. _w. story._ * * * * * what affections the violet wakes! what loved little islands, twice seen in their lakes, can the wild water-lily restore! what landscapes i read in the primroses looks, and what pictures of pebbled and minnowy brooks, in the vetches that tangled their shore. _campbell._ * * * * * descend sweet april from yon watery bow, and, liberal, strew the ground with budding flowers, with leafless crocus, leaf-veiled violet, auricula with powdered cup, primrose that loves to lurk below the hawthorn shade. _graham._ * * * * * spring is strong and virtuous, broad--sowing, cheerful, plenteous, quickening underneath the mould grains beyond the price of gold. so deep and large her bounties are, that one broad, long midsummer day shall to the planet overpay the ravage of a year of war. _emerson._ * * * * * in wild moor or sterile heath, bright with the golden furze, beneath o'erhanging bush or shelving stone, the little stonechat dwells alone, or near his brother of the whin; among the foremost to begin his pretty love-songs tinkling sound, and rest low seated on the ground; not heedless of the winding pass, that leads him through the secret grass. _bishop chant._ * * * * * the lark sung loud; the music at his heart had called him early; upward straight he went, and bore in nature's quire the merriest part. _c. turner._ * * * * * how violets came blew. love on a day (wise poets tell) some time in wrangling spent, whether the violets sho'd excell, or she, in sweetest scent. but venus having lost the day, poore girles, she fell on you, and beat ye so (as some dare say), her blowes did make ye blew. _herrick._ * * * * * april fourteenth, first cuckoo day. _sussex._ * * * * * in former times shropshire labourers used to give up work for the rest of the day when they heard the first note of the cuckoo. * * * * * there is an old superstition that where one hears the cuckoo first there one will spend most of the year. * * * * * use maketh maistry, this hath been said alway; but all is not alway as all men do say. in april, the koocoo can sing her song by rote, in june of tune she cannot sing a note: at first koocoo, koocoo, sing still can she do; at last kooke, kooke, kooke, six kookes to one coo. _john heywood, ._ * * * * * ode to the cuckoo. hail, beauteous stranger of the grove! thou messenger of spring! now heaven repairs thy rural seat and woods thy welcome sing. what time the daisy decks the green, thy certain voice we hear; hast thou a star to guide thy path, or mark the rolling year? _michael bruce._ * * * * * "cuckoo! cuckoo!" the first we've heard! "cuckoo! cuckoo!" god bless the bird scarce time to take his breath, and now "cuckoo!" he saith. cuckoo! cuckoo! three cheers! and let the welkin ring! he has not folded wing since last he saw algiers. _t. e. brown._ * * * * * april fifteenth, first swallow day. _sussex._ * * * * * he comes! he comes! who loves to bear soft sunny hours and seasons fair; the swallow hither comes to rest his sable wing and snowy breast. * * * * * april and may, the keys of the year. _spanish._ * * * * * the first sunday after easter settles the weather for the whole summer. _sweden._ * * * * * "the rippling smile of the april rain." _a. austin._ * * * * * a cold april the barn will fill. * * * * * although it rains, throw not away thy watering-pot. * * * * * plant your 'taturs when you will, they won't come up before april. _wilts._ * * * * * when there are many more swifts than swallows in the spring, expect a hot and dry summer. * * * * * april cold with dropping rain willows and lilacs brings again, the whistle of returning birds, and, trumpet-lowing of the herds. * * * * * i met queen spring in the hanger that slopes to the river gray; yestreen the thrushes sang her, but she came herself to-day. _bourdillon._ * * * * * when the sloe tree is as white as a sheet, sow your barley, whether it be dry or wet. * * * * * as yet but single, the bluebells with the grasses mingle; but soon their azure will be scrolled upon the primrose cloth of gold. _a. austin._ * * * * * april, pride of murmuring winds of spring, that beneath the winnowed air, trap with subtle nets and sweet flora's feet, flora's feet, the fleet and fair. _belleau._ * * * * * hark! the hours are softly calling, bidding spring arise, to listen to the raindrops falling from the cloudy skies, to listen to earth's weary voices, louder every day, bidding her no longer linger on her charmed way; but hasten to her task of beauty scarcely yet begun; by the first bright day of summer it should all be done. _a. a. procter._ * * * * * to the blackbird golden bill! golden bill! lo! the peep of day; all the air is cool and still, from the elm tree on the hill, chant away: while the moon drops down the west, like thy mate upon her nest, and the stars before the sun melt, like snow-flakes, one by one, let thy loud and welcome lay pour along few notes, but strong. _montgomery._ * * * * * fled are the frosts, and now the fields appear re-clothed in fresh and verdant diaper. thaw'd are the snows, and now the lusty spring gives to each mead a neat enamelling. the palms put forth their gemmes, and every tree now swaggers in her leavy gallantry. _herrick._ * * * * * ye who have felt and seen spring's morning smiles and soul enlivening green, say, did you give the thrilling transport way? did your eye brighten, when young lambs at play leap'd o'er your path with animated pride, or graz'd in merry clusters by your side? _bloomfield._ * * * * * when in the spring the gay south-west awakes, and rapid gusts now hide, now clear, the sun, round each green branch a fitful glimmering shakes, and through the lawns and flowery thickets run (tossed out of shadow into splendour brief) the silver shivers of the under-leaf. _f. doyle._ april. winter is so quite forced hence and locked up underground, that ev'ry sense hath several objects: trees have got their heads, the fields their coats; that now the shining meads do boast the paunse, lily, and the rose; and every flower doth laugh as zephyr blows, the seas are now more even than the land; the rivers run as smoothed by his hand; only their heads are crisped by his stroke. _ben jonson._ * * * * * of gardens. in april, follow the double white violet, the wallflower, the stock-gilliflower, the cowslip, flower de liece, and lilies of all natures, rosemary flowers, the tulippa, the double peony, the pale daffodil, the french honeysuckle, the cherry-tree in blossom, the damascene, the plum trees in blossom, the whitethorn in leaf, the lilac tree. _bacon._ * * * * * the primrose. lady of the springe, the lovely flower that first doth show her face; whose worthy prayse the pretty byrds do syng, whose presence sweet the wynter's cold doth chase. * * * * * almond blossom. blossom of the almond trees, april's gift to april's bees, birthday ornament of spring, flora's fairest daughterling; coming when no flowerets dare trust the cruel outer air; when the royal kingcup bold dares not don his throat of gold; and the sturdy blackthorn spray keeps his silver for the may; coming when no flowerets would save thy lowly sisterhood; early violets, blue and white, dying for their love of light. _edwin arnold._ * * * * * there is a rapturous movement, a green growing, among the hills and valleys once again, and silent rivers of delight are flowing into the hearts of men. there is a purple weaving on the heather, night drops down starry gold upon the furze, wild rivers and wild birds sing songs together, dead nature breathes and stirs. _trench._ * * * * * april! the hawthorn and the eglantine, purple woodbine, streak'd pink, and lily cup and rose, and thyme and marjorum are spreading, where thou art treading, and their sweet eyes for thee unclose. the little nightingale sits singing aye on leafy spray, and in her fitful strain doth run a thousand and a thousand changes, with voice that ranges through every sweet division. _belleau._ * * * * * the ballad-singers and the troubadours, the street-musicians of the heavenly city, the birds, who make sweet music for us all, in our dark hours, as david did for saul. the thrush that carols at the dawn of day, from the green steeples of the piny woods, linnet and meadow-lark, and all the throng that dwell in nests and have the gift of song. _longfellow._ * * * * * the lark, that shuns on lofty boughs to build her humble nest, lies silent in the field; but if (the promise of a cloudless day) aurora, smiling, bids her rise and play, then straight she shows 'twas not for want of voice, or power to climb, she made so low a choice; singing she mounts; her airy wings are stretched towards heaven, as if from heaven her voice she fetched. _waller._ * * * * * lark's song. (_wessex._) "twighee, twighee! there's not a shoemaker in all the world can make a shoe for me." "why so? why so?" "because my heel's as long as my toe." * * * * * sweet april, smiling through her tears, shakes raindrops from her hair and disappears. may ancient cornish name: miz-me, flowery month. * * * * * jewel for the month: emerald. discovers false friends. * * * * * lo, the young month comes, all smiling, up this way. * * * * * the irish say that fire and salt are the two most sacred things given to man, and if you give them away on may day you give away your luck for the year. * * * * * the fair maid, who, the first of may, goes to the fields at break of day, and washes in dew from the hawthorn tree, will ever after handsome be. * * * * * it is unlucky to go on the water the first monday in may. _irish saying._ * * * * * whoever is ill in the month of may, for the rest of the year will be healthy and gay. * * * * * leave cropping from may to michaelmas day. * * * * * the last year's leaf, its time is brief upon the beechen spray; the green bud springs, the young bird sings, old leaf, make room for may: begone, fly away, make room for may. oh, green bud, smile on me awhile; oh, young bird, let me stay: what joy have we, old leaf, in thee? make room, make room for may: begone, fly away, make room for may. _henry taylor._ * * * * * there are twelve months in all the year, as i hear many say, but the merriest month in all the year is the merry month of may. * * * * * they who bathe in may will soon be laid in clay; they who bathe in june will sing another tune. _yorkshire._ * * * * * come listen awhile to what we shall say, concerning the season, the month we call may; for the flowers they are springing, and the birds they do sing, and the baziers (auriculas) are sweet in the morning of may. when the trees are in bloom, and the meadows are green, the sweet smiling cowslips are plain to be seen; the sweet ties of nature, which we plainly do see, for the baziers are sweet in the morning of may. _lancashire._ * * * * * summer is near, and buttercups blow, and sunshine glimmers aloft; and winds play tunes which merrily flow, though in melody mellow and soft; then sing the song of the green spring-time, the season of promise and bloom, when buds have birth, and the gladdened earth awakes from her wintry tomb. _hogg._ * * * * * flowery may, who from her green lap throws the yellow cowslip and the pale primrose. _milton._ * * * * * of gardens. in may and june come pinks of all sorts, especially the blush pink; roses of all kinds, except the musk which comes later; honeysuckles, strawberries, bugloss, columbine, the french marigold, flos africanus, cherry tree in fruit, ribes, figs in fruit, rasps, vine flowers, lavender in flowers, the sweet satyrian (orchis) with the white flower, herba muscaria (grape hyacinth), lilium convallium, the apple tree in blossom. _bacon._ * * * * * a lovely morn, so still, so very still, it hardly seems a growing day of spring, though all the odorous buds are blossoming, and the small matin birds were glad and shrill some hours ago; but now the woodland rill murmurs along, the only vocal thing, save when the wee wren flits with stealthy wing, and cons by fits and bits her evening trill. _hartley coleridge._ * * * * * if you sweep the house with blossomed broom in may, you're sure to sweep the head of the house away. * * * * * come out of doors! 'tis spring! 'tis may! the trees be green, the fields be gay, the weather warm, the winter blast with all his train of clouds is past. mother of blossoms! and of all that's fair afield from spring to fall, the cuckoo, over white-waved seas, do come to sing in thy green trees, and butterflies, in giddy flight, do gleam the most by thy gay light. _w. barnes._ * * * * * all the land in flowery squares, beneath a broad and equal blowing wind, smelt of the coming summer, as one large cloud drew downward: but all else of heaven was pure up to the sun, and may from verge to verge. _tennyson._ * * * * * hush! hush! the nightingale begins to sing, and stops, as ill-contented with her note; then breaks from out the bush with hurried wing, restless and passionate. she tunes her throat, laments awhile in wavering trills, and then floods with a stream of sweetness all the glen. _jean ingelow._ * * * * * dark winter is waning, bright summer is reigning, the world is regaining, its beauty in may. the wild woods are ringing with birds sweetly singing, where dewdrops are clinging to flowret and spray. the sunshine entrances my heart when it dances, and glimmers and glances, through greenwood so gay. _from celtic lyre._ * * * * * old may day. (_may th._) on! what a may-day--what a dear may-day! feel what a breeze, love, undulates o'er us; meadow and trees, love, glisten before us; light, in all showers, falls from the flowers, hear how they ask us; "come and sit down." _from venetian._ (_burrati._) * * * * * old may day is the usual time for turning out cattle into the pastures, though frequently then very bare of grass. _hone._ * * * * * the three most unpopular saints in the calender are pancratius, servatius, and bonifacius, known both in germany and austria as the "three icemen"; and during may , , and many gardeners keep nightly watch and light outdoor fires. * * * * * who shears his sheep before st. gervatius' (or servatius') day loves more his wool than his sheep. * * * * * when the corn is over the crow's back the frost is over. _cheshire._ * * * * * go and look at oats in may, you will see them blown away; go and look again in june, you will sing another tune. * * * * * the oak before the ash, prepare your summer sash; the ash before the oak, prepare your summer cloak. _dorset._ * * * * * a windy may makes a fair year. * * * * * cut thistles in may, they grow in a day; cut thistles in june, that is too soon; cut thistles in july, then they will die. * * * * * in the middle of may comes the tail of the winter. _france._ * * * * * when passing o'er this streamlet, one fragrant morn in may, the meadows, wet with dewdrops, shone bright at dawn of day; the crimson-breasted robin was pouring forth his lay; the cuckoo's note of gladness arose from scented spray. the mavis warbles loudly from yonder leafy tree; the wren now joins the chorus, and chirps aloud with glee; the linnet is preparing her cheerfulness to show, while black-cocks greet their partners with cooing soft and low. _from celtic lyre._ * * * * * may's warm, slow, yellow moonlit summer nights. * * * * * among east coast folk there is a pretty belief, very widely held, that in may, when the sea-fowl are hatching out on the saltings, providence checks the spring tides so that they do not rise high enough to interfere with the birds. these they call by the appropriate name of "bird tides." * * * * * the linnet's warble, sinking towards a close, hints to the thrush 'tis time for their repose; the shrill-voiced thrush is heedless, and again the monitor revives his own sweet strain; but both will soon be mastered, and the copse be left as silent as the mountain-tops, ere some commanding star dismiss to rest the throng of rooks, that now from twig or nest, (after a steady flight on home-bound wings, and a last game of mazy hoverings around their ancient grove) with cawing noise, disturb the liquid music's equipoise. _wordsworth._ * * * * * the starlings are come! and merry may, and june, and the whitethorn and the hay, and the violet, and then the rose, and all sweet things are coming. * * * * * he that would live for aye must eat sage in may. * * * * * a dry may and a dripping june brings all things into tune. _bedford._ * * * * * hawthorn bloom and elder flowers will fill a house with evil powers. _warwick._ * * * * * the simplers. (_xviith. century._) here's pennyroyal and marigolds! come, buy my nettle-tops. here's water-cress and scurvy-grass! come buy my sage of virtue, ho! come buy my wormwood and mugwort! here's all fine herbs of every sort: here's southernwood that's very good, dandelion and house-leek; here's dragon's tongue and wood-sorrel, with bear's-foot and horehound. * * * * * lazy cattle wading in the water where the ripples dimple round the buttercups of gold. _whitcomb riley._ * * * * * when the dimpled water slippeth, full of laughter on its way, and her wing the wagtail dippeth, running by the brink at play; when the poplar leaves atremble turn their edges to the light, and the far-off clouds resemble veils of gauze most clear and white; and the sunbeams fall and flatter woodland moss and branches brown, and the glossy finches chatter up and down, up and down: though the heart be not attending, having music of her own, on the grass, through meadows wending, it is sweet to walk alone. _jean ingelow._ * * * * * moonwort. there is a herb, some say, whose virtue's such it in the pasture, only with a touch, unshods the new-shod steed. _withers._ * * * * * wood-pigeon. "coo-pe-coo, me and my poor two; two sticks across, and a little bit of moss, and it will do, do, do." _notts._ * * * * * the pigeon never knoweth woe, until abenting it doth go. _old couplet._ * * * * * if you scare the flycatcher away, no good luck with you will stay. _somerset._ * * * * * may th, yack-bob day. _westmorland._ * * * * * may, thou month of rosy beauty, month when pleasure is a duty; month of maids that milk the kine, bosom rich, and breath divine; month of bees, and month of flowers month of blossom-laden bowers; month of little hands with daisies, lover's love, and poets' praises. oh, thou merry month complete! may, thy very name is sweet. _leigh hunt._ * * * * * when clamour that doves in the lindens keep mingles with musical flash of the weir, where drowned green tresses of crowsfoot creep, then comes in the sweet o' the year! when big trout late in the twilight leap, when the cuckoo clamoureth far and near, when glittering scythes in the hayfield reap, then comes in the sweet o' the year! _andrew lang._ * * * * * oh! come quickly, show thee soon; come at once with all thy noon, manly, joyous, gipsy june. _leigh hunt._ june ancient cornish name: miz-epham, summer month, or head of summer. * * * * * jewel for the month: agate. long life, health, and prosperity. * * * * * when the white pinks begin to appear, then is the time your sheep to shear. _old rhyme._ * * * * * over the meadow, in sunshine and shadow, the meadow-larks trill and the bumble-bees drone. _whitcomb riley._ * * * * * if it raineth on the eighth of june a wet harvest men will see. * * * * * the broom having plenty of blossoms, or the walnut tree, is a sign of a fruitful year of corn. * * * * * a calm june puts the farmer in tune. * * * * * a dripping june puts all things in tune. * * * * * come away! the sunny hours woo thee far to founts and bowers! o'er the very waters now, in their play, flowers are shedding beauty's glow-- come away! where the lily's tender gleam quivers on the glancing stream, come away! all the air is filled with sound, soft and sultry, and profound; murmurs through the shadowy grass lightly stray; faint winds whisper as they pass: come away! where the bee's deep music swells from the trembling foxglove bells. come away! _mrs. hemans._ * * * * * pansies! pansies! how i love you, pansies! jaunty-faced, laughing-lipped, and dewy-eyed with glee. _whitcomb riley._ * * * * * the flower beds all were liberal of delight; roses in heaps were there, both red and white, lilies angelical, and gorgeous glooms of wall-flowers, and blue hyacinths, and blooms, hanging thick clusters from light boughs; in short, all the sweet cups to which the bees resort. _leigh hunt._ * * * * * oh! the rosy month of june i hail as summer's queen; the hills and valleys sing in joy, and all the woods are green; and streamlets flow in gladsome song, the birds are all in tune; and nature smiles in summer's pride, in the rosy month of june. the sixth month of the year is the month of june, when the weather's too hot to be borne, the master doth say, as he goes on his way, "to-morrow my sheep shall be shorn." _somerset._ * * * * * here the rosebuds in june and the violets are blowing, the small birds they warble from every green bough; here the pink and the lily, and the daffadowndilly, to adorn and perfume the sweet meadows in june; 'tis all before the plough the fat oxen go slow; but the lads and the lasses to the sheep-shearing go. _sussex song._ * * * * * below the hill's an ash; below the ash, white elder-flow'rs do blow: below the elder is a bed o' robinhoods a' blushin' red; and there, wi' nunch es all a-spread, the hay-meakers, wi' each a cup o' drink, do smile to zee hold up the rain, an' sky a-clearin'. _w. barnes._ * * * * * by fragrant gales in frolic play the floating corn's green waves are fann'd, and all above, broad summer day! and all below, bright summer land. _owen meredith._ * * * * * the sweet west wind is flying over the purple sea, and the amber daylight dying on roadway, hill, and tree; the cattle bells are ringing among the slanting downs, and children's voices flinging glad echoes through the towns: "oh, summer day! so soon away!" the happy-hearted sigh and say: "sweet is thy light, and sad thy flight, and sad the words--good-night, good-night." the wan white clouds are trailing low o'er the level plain, and the wind brings with its wailing the chill of the coming rain; fringed by the faded heather, wide pools of water lie, and birds and leaves together whirl through the evening sky. "haste thee away, oh, winter day!" the weary-hearted weep and say: "sad is thy light, and slow thy flight, and sweet the words--good-night, good-night." * * * * * 'twas one of the charmed days when the genius of god doth flow, the wind may alter twenty ways, a tempest cannot blow; it may blow north, it still is warm; or south, it still is clear; or east, it smells like a clover farm; or west, no thunder fear. _emerson._ * * * * * where woodbines wander, and the wallflower pushes its way alone; and where in wafts of fragrance, sweetbrier bushes make themselves known, with banks of violets for southern breezes to seek and find, and trellis'd jessamine that trembles in the summer wind. where clove carnations overgrow the places where they were set, and, mist-like, in the intervening spaces creeps mignonette. * * * * * st. barnabas day. (_old style. june st._) barnaby bright, barnaby bright, the longest day and the shortest night. * * * * * the ignorant believe that any person fasting on midsummer eve, and sitting in the church porch, will, at midnight, see the spirits of the persons of that parish who will die that year, come and knock at the church door, in the order and succession in which they will die. _hone._ * * * * * when mack'rel ceaseth from the seas, john baptist brings grass-beef and pease. _tusser._ * * * * * (?) then doth the joyfull feast of john the baptist take his turne, when bonfires great with loftie flame, in every town doe burne; and yong men round about with maides doe daunce in every streete, with garlands wroughte of motherworth, or else with vervain sweete. _barnaby googe._ * * * * * 'twas midsummer; the warm earth teemed with flowers; the kingcups gold, the perfumed clover, 'mid the crested grass; the plantains rearing high their flowery crowns above the daisied coverts; overhead, the hawthorns, white and rosy, bent with bloom, the broad-fanned chestnuts spiked with frequent flowers, and white gold-hearted lilies on the stream. _lewis morris._ * * * * * old kentish song. my one man, my two men, will mow me down the medda'; my three men, my four men, will carry away togedda'; my five men, my six men, and there ain't no more, will mow my hay, and carry away, and mow me down the medda'. * * * * * soon will high midsummer pomps come on, soon will the musk carnation break and swell, soon shall we have gold-dusted snapdragon, sweet william with his homely cottage-smell, and stocks in fragrant blow. _matthew arnold._ * * * * * signs of rain. the hollow winds begin to blow, the clouds look black, the glass is low, the soot falls down, the spaniels sleep, the spiders from their cobwebs creep, last night the sun went pale to bed, the moon in halo hid her head, the boding shepherd heaves a sigh, for see! a rainbow spans the sky. the walls are damp, the ditches smell, clos'd is the pink ey'd pimpernel. hark! how the chairs and tables crack; old betty's joints are on the rack. loud quack the ducks, the peacocks cry, the distant hills are looking nigh. how restless are the snorting swine! the busy flies disturb the kine. low o'er the grass the swallow wings; the cricket, too, how loud it sings. puss on the hearth, with velvet paws, sits smoothing o'er her whiskered jaws. through the clear stream the fishes rise, and nimbly catch the incautious flies. the sheep are seen at early light cropping the meads with eager bite. tho' june, the air is cold and chill; the mellow blackbird's voice is still. the glow-worms, numerous and bright, illumed the dewy dell last night. at dusk the squalid toad was seen hopping, crawling, o'er the green. the frog has lost his yellow vest, and in a dingy suit is dress'd. the leech disturb'd is newly risen quite to the summit of his prison. the whirling winds the dust obeys, and in the rapid eddy plays. my dog, so altered in his taste, quits mutton bones on grass to feast; and see yon rooks, how odd their flight, they imitate the gliding kite, or seem precipitate to fall, as if they felt the piercing ball. 'twill surely rain--i see with sorrow, our jaunt must be put off to-morrow. an excuse for not accepting the invitation of a friend to make a country excursion. _edward jenner._ * * * * * pondweed sinks before rain. * * * * * fir cones close for wet, open for fine weather. * * * * * cows and sheep lie down before rain to keep a dry place to lie on. * * * * * when the clouds go up the hill, they'll send down water to turn a mill. _hants._ * * * * * if nights three dewless there be, 'twill rain you're sure to see. * * * * * if bees stay at home rain will soon come. if they fly away fine will be the day. * * * * * . if the down flyeth off colt's foot, dandelyon and thistles, when there is no winde, it is a sign of rain. * * * * * when a cock drinks in summer it will rain a little after. _italy._ * * * * * when sheep begin to go up the mountains, shepherds say it will be fine weather. * * * * * sea gull, sea gull, sit on the sand; it's never good weather when you're on the land. * * * * * pimpernel, pimpernel, tell me true, whether the weather be fine or no; no heart can think, no tongue can tell, the virtues of the pimpernel. * * * * * when rain causes bubbles to rise in water it falls upon, the shower will last long. _essex._ * * * * * a saturday's rainbow, a week's rotten weather. _south ireland._ * * * * * a sunshiny shower never lasts half an hour. _bedford._ * * * * * when oxen do lick themselves against the hair, it betokeneth rain to follow shortly after. * * * * * beast do take comfort in a moist air: and it maketh them eat their meat better, and therefore sheep will get up betimes in the morning to feed against rain, and cattle, and deer, and coneys will feed hard before rain, and a heifer will put up his nose and snuff in the air against rain. worms, vermin, etc., likewise do foreshew rain: for earth-worms will come forth, and moles will cast up more, and fleas bite more against rain. _bacon._ * * * * * to talk of the weather is nothing but folly, for when it rains on the hill, the sun shines in the valley. * * * * * maayres taails an' mackerel sky, not long wet, nor not long dry. _berkshire._ * * * * * when the wind veers against the sun, trust it not, for back 'twill run. * * * * * rainbow to windward, foul falls the day; rainbow to leeward, damp runs away. * * * * * when sheep do huddle by tree and bush, bad weather is coming with wind and slush. * * * * * a rainbow at morn, put your hook in the corn; a rainbow at eve, put your head in the sheave. _cornwall._ * * * * * clouds without rain in summer indicate wind. * * * * * saturday's moon, sunday seen the foulest weather there ever hath been. * * * * * when the new moon comes in at midnight, or within thirty minutes before or after, the following month will be fine. * * * * * saturday change, and sunday full, is always wet, and always wull. _northants._ * * * * * if mist's in the new moon, rain in the old; if mist's in the old moon, rain in the new. * * * * * a fog and a small moon bring an easterly wind soon. _cornwall._ * * * * * if saturday's moon comes once in seven years, it comes too soon. * * * * * full moon. the nearer to twelve in the afternoon, the drier the moon. the nearer to twelve in the forenoon, the wetter the moon. _hereford._ * * * * * when the moon is at the full, mushrooms you may freely pull; but when the moon is on the wane, wait, ere you think to pluck again. * * * * * the moon and the weather may change together; but change of the moon does not change the weather; if we'd no moon at all, and that may seem strange, we still should have weather that's subject to change. * * * * * midsummer fairies. the pastoral cowslips are our little pets, and daisy stars, whose firmament is green; pansies, and those veiled nuns, meek violets, sighing to that warm world from which they screen; and golden daffodils, plucked for may's queen; and lovely harebells, quaking on the heath; and hyacinth, long since a fair youth seen, whose tuneful voice, turned fragrance in his breath, kissed by sad zephyr, guilty of his death. _hood._ * * * * * the sun has long been set, the stars are out by twos and threes, the little birds are piping yet among the bushes and the trees; there's a cuckoo, and one or two thrushes, and a far-off wind that rushes, and a sound of water that gushes, and the cuckoo's sovereign cry fills all the hollow of the sky, who would "go parading," in london "and masquerading," on such a night in june, with the beautiful soft half-moon, and all these innocent blisses? on such a night as this is! _wordsworth._ * * * * * when the wind's in the south the rain's in its mouth. * * * * * no weather is ill if the wind be still. _old saying._ * * * * * all through the sultry hours of june, from morning blithe to golden noon, and till the star of evening climbs the gray-blue east, a world too soon, there sings a thrush within the limes. god's poet, hid in foliage green, sings endless songs, himself unseen; right seldom come his silent times. linger, ye summer hours serene! sing on, dear thrush, amid the limes! _mortimer collins._ * * * * * a wet june makes a dry september. _cornwall._ july ancient cornish name: miz-gorepham, head of the summer month. * * * * * jewel for the month: ruby. discovers poison. * * * * * if the first of july be rainy weather, 'twill rain more or less for four weeks together. * * * * * in my nostrils the summer wind blows the exquisite scent of the rose: oh! for the golden, golden wind, breaking the buds as it goes! breaking the buds and bending the grass, and spilling the scent of the rose. _aldrich._ * * * * * i sometimes think that never blows so red the rose as where some buried cæsar bled; that every hyacinth the garden wears dropt in its lap from some once lovely head. _omar khayyam._ * * * * * of gardens. in july come gilliflowers of all varieties, musk roses, the lime tree in blossom, early pears, and plums in fruit, ginnetings, quadlins. _bacon._ * * * * * a tuft of evening primroses, o'er which the mind may hover till it dozes; o'er which it well might take a pleasant sleep, but that 'tis ever startled by the leap of buds into ripe flowers. _keats._ * * * * * now the glories of the year may be viewed at the best, and the earth doth now appear in her fairest garments dress'd: sweetly smelling plants and flowers do perfume the garden bowers; hill and valley, wood and field, mixed with pleasure profits yield. _george withers._ * * * * * blue flags, yellow flags, flags all freckled, which will you take? yellow, blue, speckled! take which you will, speckled, blue, yellow, each in its way has not a fellow. _c. rossetti._ swelling downs, where sweet air stirs blue hair-bells lightly, and where prickly furze buds lavish gold. _keats._ * * * * * mouse-ear, or scorpion grass, any manner of way ministered to horses brings this help unto them, that they cannot be hurt, while the smith is shoeing of them, therefore it is called of many, _herba clavorum_, the herb of nails. _old saying, before ._ * * * * * sweet is the rose, but growes upon a brere; sweet is the junipere, but sharp his bough; sweet is the eglantine, but pricketh nere; sweet is the firbloome, but his braunche is rough; sweet is the cypresse, but his rynd is tough; sweet is the nut, but bitter is his pill; sweet is the broome-flowre, but yet sowre enough; and sweet is moly, but his root is ill. so every sweet with sowre is tempered still, that maketh it the coveted be more: for easie things, that may be got at will, most sorts of men doe set but little store. _spenser._ * * * * * where the copse-wood is the greenest, where the fountain glistens sheenest, where the morning dew lies longest, there the lady fern grows strongest. _walter scott._ * * * * * faire dayes: or, dawnes deceitful. faire was the dawne; and but e'ene now the skies shew'd like to creame, enspir'd with strawberries: but on a sudden, all was chang'd and gone that smil'd in that first sweet complexion. then thunder-claps and lightning did conspire to teare the world, or set it all on fire. what trust to things below, whenas we see, as men, the heavens have their hypocrisie? _herrick._ * * * * * summer in the penniless can stir the frozen prayer, summer sends a golden glow through needy bones a-while; bright and breezy is the dawn, and soft the balmy air; summer, 'tis the breath of heaven, 'tis god's own gracious smile. _from victor hugo._ * * * * * the nightingale and the cuckow both grow hoarse at the rising of sirius the dogge star. * * * * * not rend off, but cut off ripe bean with a knife, for hindering stalk of her vegetive life. so gather the lowest, and leaving the top, shall teach thee a trick for to double thy crop. _tusser._ * * * * * a shower of rain in july, when the corn begins to fill, is worth a plough of oxen, and all belongs theretill. * * * * * st. swithun. (_july th_.) saint swithun's day, if thou dost rain, for forty days it will remain; saint swithun's day, if thou be fair, for forty days 'twill rain na mair. _scotland._ * * * * * st. swithun christens the apples. * * * * * no tempest good july, lest the corn look ruely. * * * * * while wormwood hath seed, get a handful or twain, to save against march, to make flea to refrain: where chamber is sweepid, and wormwood is strown, no flea for his life, dare abide to be known. * * * * * the flower girl. . come buy, come buy my mystic flowers, all ranged with due consideration, and culled in fancy's fairy bowers, to suit each age and every station. . for those who late in life would tarry, i've snowdrops, winter's children cold; and those who seek for wealth to marry, may buy the flaunting marigold. . i've ragwort, ragged robins, too, cheap flowers for those of low condition; for bachelors i've buttons blue; and crown imperials for ambition. . for sportsmen keen, who range the lea, i've pheasant's eye and sprigs of heather; for courtiers with the supple knee, i've parasites and prince's feather. . for thin tall fops i keep the rush, for peasants still am nightshade weeding; for rakes i've devil-in-the-bush, for sighing strephons, loves-lies-bleeding. but fairest blooms affection's hand for constancy and worth disposes, and gladly weaves at your command a wreath of amaranths and roses. _mrs. corbold._ * * * * * london street-call. (_about years old._) will you buy, lady, buy my sweet blooming lavender? there are sixteen blue branches a penny. you will buy it once, you will buy it twice, it makes your clothes smell so very nice. it will scent your pocket-handkerchief, and it will scent your clothes as well. now is your time, and do not delay: come and buy your lavender, all fresh cut from mitcham every day. * * * * * i do not want change: i want the same old and loved things, the same wild flowers, the same trees and soft ash-green; the turtle doves, the blackbirds, the coloured yellow-hammer sing, sing, singing so long as there is light to cast a shadow on the dial, for such is the measure of his song, and i want them in the same place. _richard jefferies._ * * * * * st. james's day. (_new style. july th._) 'till saint james's day be past and gone, there may be hops, or there may be none. _hereford._ * * * * * july, to whom, the dog-star in her train, st. james gives oisters, and st. swithin rain. _churchill._ * * * * * oh! golden, golden summer, what is it thou hast done? thou hast chased each vernal roamer with thy fiercely burning sun. glad was the cuckoo's hail. where may we hear it now? thou hast driven the nightingale from the waving hawthorn bough. thou hast shrunk the mighty river; thou hast made the small brook flee; and the light gales faintly quiver through the dark and shadowy tree. _w. howitt._ august ancient cornish name: miz-east, harvest month. * * * * * jewel for the month: sardonyx. insures happiness in marriage. august first. (_loaf-mass day._) day of offering first fruits, when a loaf was given to the priests in place of the first fruits. * * * * * at latter lammas, i.e. never. * * * * * the august gold of earth. * * * * * all things rejoiced beneath the sun, the weeds, the river, and the cornfields, and the reeds; the willow-leaves that glanced in the light breeze, and the firm foliage of the larger trees. _shelley._ of gardens. in august come plums of all sorts in fruit, pears, apricots, berberies, filberds, musk melons, monkshoods of all colour. _bacon._ * * * * * august st. (_snipe shooting may begin._) snipe's song: "don't take" local name for snipe. nipcake, don't take, don't take, don't take; gie the lasses milk and bread, and gie the laddies don't take, don't take, don't take. _scottish midlands._ * * * * * august th. (_old style._) st. james's day. oyster day. who eats oysters on st. james's day will never want. * * * * * wheat sways heavy, oats are airy, barley bows a graceful head, short and small shoots up canary; each of these is some one's bread-- bread for man or bread for beast, or at very least a bird's savoury feast. _c. rossetti._ * * * * * it is always windy in barley harvests; it blows off the heads for the poor. * * * * * on thursday at three look out and you'll see what friday will be. * * * * * no weather is ill if the wind be still. * * * * * for morning rain leave not your journey. * * * * * never a fisherman med there be, if fishes could hear as well as see. _kent._ * * * * * if the sage tree thrives and grows, the master's _not_ master, and that he knows. _warwick._ * * * * * a garden must be looked into, and dressed as a body. * * * * * to smell wild thyme will renew spirits and energy in long walks under an august sun. * * * * * friday's a day as'll have his trick, the fairest or foulest day o' the wick. * * * * * dry august and warm doth harvest no harm. * * * * * put in the sickles and reap, for the morning of harvest is red, and the long, large ranks of the corn, coloured and clothed as the morn, stand thick in the fields and deep, for them that faint to be fed. _swinburne._ * * * * * summer is purple, and drowsed with repletion. * * * * * now yellow harvests wave on every field, now bending boughs the hoary chestnut yield, now loaded trees resign their annual store, and on the ground the mellow fruitage pour. _beattie._ (_from_ "_virgil_.") * * * * * august th. (_st. roche's day._) formerly celebrated in england as a general harvest home. * * * * * good huswives in summer will save their own seeds against the next year, as occasion needs; one seed for another to make an exchange, with fellowly neighbourhood seemeth not strange. _tusser._ * * * * * on one side is a field of drooping oats, through which the poppies show their scarlet coats. _keats._ * * * * * august th. (_st. bartholomew's day._) if st. bartholomew's day be misty, the morning beginning with a hoar frost, then cold weather will soon ensue, and a sharp winter attended with many biting frosts. _thomas passenger._ * * * * * st. bartlemy's mantle wipes dry all the tears that st. swithun can cry. _portugal._ * * * * * ...happy britannia!... rich is thy soil, and merciful thy clime; thy streams unfailing in the summer's drought; unmatch'd thy guardian oaks; thy vallies float with golden waves; and on thy mountains flocks bleat numberless; while roving round their sides, bellow the blackening herds in lusty droves. beneath thy meadows glow, and rise unquell'd against the mower's scythe. _thomson._ september ancient cornish name: miz-guerda gala, white straw month. * * * * * jewel for the month: chrysolite. antidote to madness. * * * * * if the woodcock had but the partridge's thigh, he'd be the best bird that ever did fly. if the partridge had but the woodcock's breast, he'd be the best bird that ever was dress'd. * * * * * harvest hwome. the ground is clear. there's nar a ear o' stannen corn a-left out now, vor win' to blow or rain to drow; 'tis all up seafe in barn or mow. here's health to them that plough'd an' zow'd; here's health to them that reap'd an' mow'd, an' them that had to pitch an' lwoad, or tip the rick at harvest hwome. the happy zight,--the merry night; the men's delight,--the harvest hwome. _w. barnes._ * * * * * we have ploughed, we have sowed, we have reaped, we have mowed, we have brought home every load, hip, hip, hip, harvest home. _gloucester._ * * * * * harvest toast. here's a health to the barley mow, here's a health to the man who very well can both harrow and plough and sow. when it is well sown, see it is well mown, both raked and gravell'd clean, and a barn to lay it in, here's a health to the man who very well can both thrash and fan it clean. _suffolk._ * * * * * tramping after grouse or partridge through the soft september air, both my pockets stuffed with cartridge, and my heart devoid of care. * * * * * september blow soft. till the fruit's in the loft. * * * * * of gardens. in september come grapes, apples, poppies of all colours, peaches, melocotones (yellow peaches), nectarines, cornelians, wardens, quinces. _bacon._ * * * * * spring was o'er happy and knew not the reason, and summer dreamed sadly, for she thought all was ended in her fulness of wealth that might not be amended; but this is the harvest and the garnering season, and the leaf and the blossom in the ripe fruit are blended. _w. morris._ * * * * * a bloom upon the apple tree when the apples are ripe is a sure termination to somebody's life. * * * * * september dries up wells or breaks down bridges. _portugal._ * * * * * many haws, many sloes, many cold toes. * * * * * when september thirteenth falls on a friday, the autumn will be dry and sunny. _france._ * * * * * september fifteenth is said to be fine in six years out of seven. * * * * * onion skin very thin, mild winter coming in; onion skin thick and tough, coming winter cold and rough. * * * * * set strawberries, wife, i love them for life. _tusser._ * * * * * the barberry, respis, and gooseberry too, look now to be planted as other things do: the gooseberry, respis, and roses all three, with strawberries under them trimly agree. _tusser._ * * * * * wild with the winds of september wrestled the trees of the forest, as jacob of old with the angel. _longfellow._ that mellow season of the year when the hot sun singes the yellow leaves till they be gold, and with a broader sphere the moon looks down on ceres and her sheaves. _hood._ * * * * * when the falling waters utter something mournful on their way, and departing swallows flutter, taking leave of bank and brae; when the chaffinch idly sitteth with her mate upon the sheaves, and the wistful robin flitteth over beds of yellow leaves; when the clouds like ghosts that ponder evil fate, float by and frown, and the listless wind doth wander up and down, up and down: through the fields and fallows wending, it is sad to walk alone. _jean ingelow._ * * * * * st. matthew. (_september st._) st. matthee shut up the bee. * * * * * the flush of the landscape is o'er, the brown leaves are shed on the way, the dye of the lone mountain-flower grows wan and betokens decay. all silent the song of the thrush, bewilder'd she cowers in the dale; the blackbird sits lone on the bush-- the fall of the leaf they bewail. _hogg._ * * * * * summer is gone on swallow's wings, and earth has buried all her flowers; no more the lark, the linnet sings, but silence sits in faded bowers. there is a shadow on the plain of winter, ere he comes again. _hood._ * * * * * the feathers of the willow are half of them grown yellow above the swelling stream; and ragged are the bushes, and rusty now the rushes, and wild the clouded gleam. the thistle now is older, his stalk begins to moulder, his head is white as snow; the branches all are barer, the linnet's song is rarer, the robin pipeth now. _dixon._ * * * * * nothing stirs the sunny silence, save the drowsy humming of the bees round the rich, ripe peaches on the wall, and the south wind sighing in the trees, and the dead leaves rustling as they fall: while the swallows, one by one, are gathering, all impatient to be on the wing, and to wander from us, seeking their beloved spring. _adelaide procter._ * * * * * the garden. what wondrous life is this i lead! ripe apples drop about my head. the luscious clusters of the vine upon my mouth do crush their wine. the nectarine, and curious peach into my hands themselves do reach. stumbling on melons, as i pass, insnared with flowers, i fall on grass. _andrew marvell._ * * * * * st. michael's day. (_september th._) in the sarum missal st. michael is invoked as a "most glorious and warlike prince," "chief officer of paradise," "captain of god's hosts," "the receiver of souls," "the vanquisher of evil spirits," and "the admirable general." _from hone._ * * * * * if michaelmas day be fair, the sun will shine much in the winter; though the wind at northeast will frequently reign long, and be very sharp and nipping. _thomas passenger._ * * * * * fresh herring plenty michael brings, with fatted crones (old ewes) and such old things. _tusser._ * * * * * when the tenants come to pay their quarter's rent, they bring some fowl at midsummer, a dish of fish in lent, at christmas a capon, at michaelmas a goose, and somewhat else at new year's tide, for fear their lease fly loose. _g. gascoigne._ * * * * * geese now in their prime season are, which if well roasted are good fare: yet, however, friends take heed how too much on them you feed, lest, when as your tongues run loose, your discourse do smell of goose. _"poor robin," ._ * * * * * if you eat goose on michaelmas day you will never want money all the year round. * * * * * old saying. the michaelmas moon rises nine nights alike soon. * * * * * the moon in the wane, gather fruit for to last; but winter fruit gather, when michael is past; though michers (thieves) that love not to buy nor to crave, make some gather sooner, else few for to have. _tusser._ october ancient cornish name: miz-hedra, watery month. * * * * * jewel: opal. hope. * * * * * october fourth. st. francis and st. benedight died . * * * * * st. francis and st. benedight, blesse this house from wicked wight from the night-mare, and the goblin that is night good-fellow-robin; keep it from all evil spirits, fairies, weezils, rats, and ferrets: from curfew time, to the next prime. _william cartwright._ * * * * * who soweth in rain hath weed to his pain; but worse shall he speed that soweth ill seed. _tusser._ * * * * * when autumn, sad but sunlit, doth appear, with his gold hand gilding the falling leaf, bringing up winter to fulfil the year, bearing upon his back the ripened sheaf; when all the hills with woolly seed are white, when lightning fires and gleams do meet from far the sight; when the fair apple, flushed as even sky, doth bend the tree unto the fertile ground, when juicy pears and berries of black dye do dance in air and call the eye around: then, be the even foul or be it fair, methinks my heart's delight is stained with some care. _chatterton._ * * * * * there is strange music in the stirring wind, when lowers the autumnal eve, and all alone to the dark wood's cold covert thou art gone, whose ancient trees on the rough slope reclined rock, and at times scatter their tresses sere. _w. l. bowles._ * * * * * of gardens. in october and beginning of november come services, medlars, bullaces, roses cut or removed to come late, hollyoaks, and such like. _bacon._ * * * * * seed-time. october's gold is dim--the forests rot, the weary rain falls ceaseless, while the day is wrapt in damp. in mire of village way the hedgerow leaves are stampt, and, all forgot, the broodless nest sits visible in the thorn. autumn, among her drooping marigolds, weeps all her garnered fields and empty folds and dripping orchards, plundered and forlorn. _david gray._ * * * * * autumn days. yellow, mellow, ripened days, sheltered in a golden coating; o'er the dreamy, listless haze, white and dainty cloudlets floating winking at the blushing trees, and the sombre, furrowed fallow; smiling at the airy ease of the southward flying swallow. sweet and smiling are thy ways, beauteous, golden, autumn days! shivering, quivering, tearful days, fretfully and sadly weeping; dreading still, with anxious gaze, icy fetters round thee creeping; o'er the cheerless, withered plain, woefully and hoarsely calling; pelting hail and drenching rain, on thy scanty vestments falling. sad and mournful are thy ways, grieving, wailing, autumn days! _will. carleton._ * * * * * moan, oh ye autumn winds! summer has fled, the flowers have closed their tender leaves and die; the lily's gracious head all low must lie, because the gentle summer now is dead. mourn, mourn, oh autumn winds, lament and mourn; how many half-blown buds must close and die; hopes with the summer born all faded lie, and leave us desolate and earth forlorn! _a. a. procter._ * * * * * st. simon and st. jude's day. (_october th._) it is a bedford custom for boys to cry baked pears about the town, with the following words:-- who knows what i have got? in a hot pot? baked wardens--all hot! who knows what i have got? * * * * * october brings the cold weather down, when the wind and the rain continue; he nerves the limbs that are lazy grown, and braces the languid sinew; so while we have voices and lungs to cheer, and the winter frost before us, come chant to the king of the mortal year, and thunder him out in chorus. _e. e. bowen._ * * * * * "decay, decay," the wildering west winds cry; "decay, decay," the moaning woods reply; the whole dead autumn landscape, drear and chill, strikes the same chord of desolate sadness still. * * * * * full moon in october without frost, no frost till full moon in november. * * * * * hoar frost and gipsies never stay nine days in a place. * * * * * there are always nineteen fine days in october. _kentish saying._ * * * * * an april frost is sharp, but kills not; sad october's storm strikes when the juices and the vital sap are ebbing from the leaf. _henry taylor._ november ancient cornish name: miz-dui, black month. * * * * * jewel for the month: topaz. fidelity. * * * * * november st. (_all saints' day._) on all saints' day hard is the grain. the leaves are dropping, the puddle is full, at setting off in the morning woe to him that will trust a stranger. on all saints' day blustering is the weather, unlike the beginning of the past fair season: besides god there is none that knows the future. _from the welsh. ._ * * * * * apples, peares, hawthorns, quicksetts, oakes. sett them at all hallow-tyde, and command them to grow; sett them at candlemas-tide and entreat them to grow. _wilts._ * * * * * who sets an apple tree may live to see it end, who sets a pear tree may set it for a friend. _hereford._ * * * * * their loveliness of life and leaf at last the waving trees have shed; the garden ground is sown with grief, the gay chrysanthemum is dead. but oh! remember this: there must be birth and blossoming; nature will waken with a kiss next spring! _clement scott._ * * * * * thorny balls, each three in one, the chestnuts throw in our path in showers! for the drop of the woodland fruit's begun, these early november hours. _browning._ * * * * * there never was a juster debt than what the dry do pay for wet; never a debt was paid more nigh as what the wet do pay for dry! * * * * * a wet sunday, a fine monday, wet the rest of the week. _winchester._ * * * * * an early winter, a surly winter. * * * * * st. martin's day. (_november th._) if martinmas ice can bear a duck, the winter will be all mire and muck. * * * * * 'tween martinmas and yule, water's wine in every pool. * * * * * if it is cold, fair, and dry at martinmas, the cold in winter will not last long. _old saying._ * * * * * young and old must go warm at martinmas. _italy._ * * * * * weary the cloud falleth out of the sky. dreary the leaf lieth low, all things must come to the earth by-and-by, out of which all things grow. _owen meredith._ * * * * * the year's on the wane, there is nothing adorning, the night has no eve, and the day has no morning; cold winter gives warning. _hood._ * * * * * the melancholy days are come, the saddest of the year, of wailing winds, and naked woods, and meadows brown and sere; heaped in the hollows of the grove, the withered leaves lie dead, they rustle to the eddying gust, and to the rabbit's tread. the robin and the wren are flown, and from the shrub the jay, and from the wood-tops calls the crow, through all the gloomy day. _w. cullen bryant._ * * * * * november th. (_st. edmund's day._) set garlike and pease st. edmund to please. _tusser._ * * * * * if on friday it rain, 'twill on sunday again; if friday be clear, have for sunday no fear. * * * * * from twelve to two see what the day will do. * * * * * november rd. (_st. clement's day._) catherine and clement, be here, be here; some of your apples, and some of your beer; some for peter, and some for paul, and some for him that made us all. clement was a good old man, for his sake give us some; not of the worst, but some of the best, and god will send your soul to rest. _worcestershire._ * * * * * november th. (_st. andrew's day._) on st. andrew's the night is twice as long as the day. _portugal._ december ancient cornish name: miz-kavardine, following black month. * * * * * jewel for the month: turquoise. prosperity. * * * * * though now no more the musing ear delights to listen to the breeze, that lingers o'er the green-wood shade, i love thee, winter! well. sweet are the harmonies of spring, sweet is the summer's evening gale, and sweet the autumnal winds that shake the many-colour'd grove. and pleasant to the sober'd soul the silence of the wintry scene, when nature shrouds herself, entranced in deep tranquillity. _southey._ * * * * * december frost and january flood never boded husbandman good. * * * * * when there are three days cold, expect three days colder. * * * * * of gardens. i do hold it, in the royal ordering of gardens, there ought to be gardens for all the months in the year, in which, severally, things of beauty may be then in season. for december and january, and the latter part of november, you must take such things as are green all winter, holly, ivy, bays, juniper, cypress trees, yew, pines, fir trees, rosemary, lavender, periwinkle, the white, the purple, and the blue; germander, flags, orange trees, lemon trees, and myrtles, if they be stoved; and sweet marjoram, warm set. _bacon._ * * * * * if frost do continue, take this for a law, the strawberries look to be covered with straw, laid overly trim upon crotches and bows, and after uncovered, as weather allows. the gilliflower also, the skilful do know, doth look to be covered in frost and in snow: the knot and the border, and rosemary gay, do crave the like succour, for dying away. _tusser._ * * * * * december th. (_st. nicholas's eve._) st. nicholas, besides being the patron of children, was supposed to have provided marriage portions for portionless maidens. saint nicholas money used to give to maidens secretlie, who, that he still may use his wonted liberalitie, the mothers all their children on the eve do cause to fast, and, when they every one at night in senseless sleepe are cast, both apples, nuttes, and peares they bring, and other things beside, as caps, and shooes and petticotes, which secretlie they hide, and in the morning found, they say, that this saint nicholas brought: thus tender mindes to worship saints, and wicked things are taught. _from "the popish kingdom," ._ _barnaby googe._ st. nicholas, archbishop of myra, patron saint of virgins, boys, sailors, and the worshipful company of parish clerks of the city of london. _hone._ * * * * * the drooping year is in the wane, no longer floats the thistle-down; the crimson heath is wan and sere; the sedge hangs withering by the mere, and the broad fern is rent and brown. the owl sits huddling by himself, the cold has pierced his body through; the patient cattle hang their head; the deer are 'neath their winter shed; the ruddy squirrel's in his bed, and each small thing within its burrow. _mary howitt._ * * * * * december st. (_st. thomas's day._) st. thomas grey st. thomas grey, the longest night and the shortest day. * * * * * look at the weathercock on st. thomas's day at twelve o'clock, and see which way the wind is, and there it will stick for the next three months. _warwickshire._ * * * * * there is never a saturday in the year but what the sun it doth appear. * * * * * if birds pipe afore christmas they'll greet after. _scotland._ * * * * * mystic mistletoe flaunted, such as the druids cut down with golden hatchets at yuletide. _longfellow._ * * * * * william stukeley, arch druid ( - ), says: "the druids cut mistletoe off the trees with their upright hatchets of brass, called celts, put upon the ends of their staffs, which they carried in their hands." * * * * * mistletoe is said to be the forbidden tree in the middle of the trees of eden. * * * * * if christmas day on monday be, a great winter that year you'll see. * * * * * what chyld on that day boorn be, of gret worscheyp schall he be. _ms. in bodleian._ * * * * * if that christmas day should fall upon friday, know well all that winter season shall be easy, save great winds aloft shall fly. * * * * * easter in snow, christmas in mud; christmas in snow, easter in mud. _germany._ * * * * * so now is come our joyful feast; let every man be jolly; each room with ivy leaves is drest and every post with holly. though some churls at our mirth repine, round your foreheads garlands twine; drown sorrow in a cup of wine, and let us all be merry. _george wither._ * * * * * carol of queen anne's time, . thrice welcome christmas, which makes us good cheer, mince pies and plum porridge, good ale and strong beer, with pig, goose and capon, the best that may be, so well doth the weather and our stomachs agree. observe how the chimneys do smoke all about-- the cooks are providing for dinner, no doubt! * * * * * kindle the christmas brand, and then till sunrise let it burn; which quenched, then lay it up agen till christmas next return. part must be kept, wherewith to tend the christmas log next year; and when 'tis safely kept, the fiend can do no mischief there. _warwickshire._ * * * * * december th. (_st. stephen's day._) blessed be st. stephen, there's no fast upon his even. _old saying._ * * * * * bishop hall says: "on st. stephen's day blessings are implored upon pastures." * * * * * december th. (_innocents' day, or childermas day._) according to the monks, it was very unlucky to begin any work on childermas day, and what soever day that falls on, whether on the monday, or tuesday, or any other, nothing must be begun on that day through the year. _henry bourne, ._ * * * * * days lengthen a cock's stride each day after christmas. * * * * * know the best season to laugh and to sing, is winter, is summer, is autumn, is spring. _old song._ * * * * * hagman heigh old yorkshire custom for hag- or wood-man to go round to ask for money on new year's eve. * * * * * new year's eve. hark, the cock crows, and yon bright star tells us the day himself's not far; and see where, breaking from the night, he gilds the western hills with light. with him old janus doth appear, peeping into the future year, with such a look as seems to say, the prospect is not good that way. _charles cotton._ * * * * * if new year's eve night wind blows south it betokeneth warmth and growth; if west, much milk, and fish in the sea; if north, much cold and storms there will be; if east, the trees will bear much fruit; if north-east, flee it man and brute. * * * * * the new year, with its yet unacted history, claims the homage of our last departing chime; then we hush ourselves in awe before the mystery, of the youngest and the freshest birth of time. * * * * * the good old year is with the past; oh, be the new as kind! _pope._ index a bloom upon, a calm june, a camomile bed, a cold january, a dripping june, a dry may, a fair day, a fog and a small moon, a frosty winter, a garden must be looked, a hoar frost, a january spring, a kindly good january, all the land, all the months, all things rejoiced, all through the sultry, a lovely morning, although it rains, among the east coast, an april frost, an early winter, any individual, apples, peares, apple-tree, april and may, april cold, april pride of, april! the hawthorn, a rainbow, a saturday's rainbow, a shower of rain, as many days, a snow year, a sunshiny shower, as yet but single, at latter lammas, at new year's tide, a tuft of evening primroses, a wet good friday, a wet june, a wet sunday, a windy may, a year of snow, barnaby bright, beasts do take comfort, beef and bacon, below the hill, beside the garden path, bishop hall, blank earth-baldness, blessed be st. stephen, blossom of the almond, blue flags, by fragrant gales, candlemas shined, catherine and clement, childermas day, clouds without rain, come away, come buy, come gather, come listen, come out of doors, coo-pi-coo, cows and sheep, cuckoo, cut thistles in may, dark winter is waning, days lengthen, decay, decay, december frost, descend sweet april, did he do it?, dry august, easter in snow, faire was the dawn, fair rising, february fill the ditch, fir cones, first cuckoo day, first swallow day, fled are the frosts, flowery may, for march, for morning rain, for the latter part, fresh herring, friday's a day, from twelve to two, from whatever quarter, full moon in october, geese now, gervase of tilbury, go and look at oats, golden bill, good gardener mine, good huswives, hagman heigh, hail beauteous stranger, hail once again, happy britannia!, hark! the cock crows, hark! the hours, hawthorn bloom, he comes, he comes!, here's a health, here's a penny-royal, here the rosebuds, he that would live, he that freely lops, hoar frost and gipsies, hush! hush!, i do hold it, i do not want change, i early rose, if apples bloom, if bees stay at home, if birds pipe, if candlemas day, if christmas day, if february, if frosts do continue, if it does not freeze, if it is cold, if it raineth, if it thunders, if janiveer calends, if martinmas, if michaelmas day, if mist's in the new moon, if new year's eve, if nights three, if on friday, if st. bartholomew, if st. paul's day, if saturday's moon, if that christmas, if the down, if the first of july, if the grass grow, if the robin, if the sage tree, if the woodcock, if you eat, if you scare, if you sweep, i met queen spring, in april, in august, in former times, in july, in march and in april, in march is good graffing, in may and june, in my nostrils, in october and beginning, in oxfordshire, in september, i sometimes think, in spring, in the morning, in the sarum, in the south, in wild moor, it is affirmed, it is always, it is unlucky, january the fourteenth, july, to whom, kindle the christmas, knick, knock, know the best season, lady of the springe, lazy cattle, leave cropping from may, loaf-mass day, look at the weathercock, lo, the young month, love on a day, maaryes taailes, many haws, march does from april gain, march in janiveer, march search, march wind, may's warm, may, thou month, mistletoe is said, moan, oh ye autumn, moonwort, mouse ear, my one man, mystic mistletoe, never a fisherman, nipcake, nipcake, no summer flower, nothing stirs, no tempest, no weather is ill, not rend-off, now set for thy pot, now the glories, now yellow harvests, october brings the cold, october's gold, oh! come quickly, oh! faint delicious, oh! golden, golden, oh! the rosy month, oh! what a dawn, oh! what a may day, oh! winter, oh! love-star, old may day, old yorkshire custom, on all saint's day, one for the mouse, one month is past, onion skin, on one side is a field, on st. andrew's, on shrove tuesday, on the first of march, on thursday at three, on twelve-eve, our vernal signs, over the meadow, pale moon, pansies! pansies, parsley sown, pimpernel, plant your 'taturs, pluck broom, pondweed sinks, put in the sickle, rainbow to windward, remember in st. vincent's, st. barnabas, st. francis, st. matthew, st. nicholas, archbishop, st. nicholas, st. roche, st. swithun is christening, st. swithun's day, st. thomas, saturday change, saturday's moon, sea-gull, september blow soft, september dries, september the fifteenth, set garlike, set strawberries, shivering, quivering, sleep with your head, so now is come, soon will high, sow peason, spring is here, spring is strong, spring's an expansive, spring was o'er happy, summer in the penniless, summer is gone, summer is near, summer is purple, sunset in a clear, sweet april, sweet is the rose, swelling downs, that mellow season, the august gold, the badger peeps, the ballad singers, the barberry, the blackest month, the broom, the dim droop, the drooping year, the empty pastures, the fair maid, the feathers of the willow, the first monday, the first sunday, the first thunder, the flower beds, the flush of the landscape, the good old year, the ground is clear, the hollow winds, the housekeeper, the ignorant, the irish, their loveliness, the lark, the last year's leaf, the leeke is white, the linnet, the melancholy days, the michaelmas moon, the moon and the weather, the moon in the wane, the nearer to twelve, the new year, the nightingale, the oak before the ash, the pastoral cowslips, the pigeon never, the pretty lark, the rippling smile, the sixth month, the softest turf, the starlings are come, the sun has long, the sweet west wind, the wind of the south, the year's on the wane, then doth the joyful, there are always, there are twelve, there is an old proverb, there is an old, there is a rapturous, there is a saying, there is a tradition, there is never a saturday, there is strange music, there never was, they who bathe in may, thorny balls, though now no more, thou wilt remember, three icemen, thrice welcome christmas, through all the sad, thunder in spring, till st. james's, to smell wild thyme, tossing his mane, to talk of the weather, tramping after grouse, 'twas midsummer, 'twas one of the charmed, 'tween martinmas, twelfth day, twighee, twighee, under the furze, under water, upon st. david's day, use maketh maistry, warwickshire countrymen, weary the cloud, we have ploughed, welcome, o march, what affections, what child, what wondrous life, wheat sways heavy, when a cock drinks, when a daffodil, when after a rough, when a moorland, when autumn sad, when clamour, when country roads, when in spring, when mack'rel, when on the purification, when our lord, when oxen, when passing o'er, when rain causes bubbles, when september, when sheep begin, when sheep do huddle, when the clouds, when the corn, when the dimpled, when the falling, when the moon, when the new moon, when the sloe tree, when the tenants, when the white pinks, when the wind blows, when the wind's in the east, when the wind's in the south, when the wind veers, when there are many, when there are three, when wintry weather, where the copse, where the wind is, where woodbines, while wormwood, who eats oysters, whoever is ill, who knows what, who sets an apple, who shears his sheep, who soweth in rain, why valentine, wild with the winds, will you buy, winter is so, winter's thunder, winter's white shroud, winter time, yack-bob day, yellow, mellow, ye who have felt, young and old, transcriber's notes -obvious print and punctuation errors fixed. -a table of contents was not in the original work; one has been produced and added by transcriber. [transcriber's note: spelling maintained as closely as possible to the original document, while obvious typos have been corrected. emdashes in original text for negative temperatures changed to minus signs to standardize temperatures.] climatic changes their nature and causes published on the foundation established in memory of theodore l. glasgow other books by the same authors ellsworth huntington a. _four books showing the development of knowledge as to historical pulsations of climate._ the pulse of asia. boston, . explorations in turkestan. expedition of . washington, . palestine and its transformation. boston, . the climatic factor, as illustrated in arid america. washington, . b. _two books illustrating the effect of climate on man._ civilization and climate. new haven, . world power and evolution. new haven, . c. _four books illustrating the general principles of geography._ asia: a geography reader. chicago, . the red man's continent. new haven, . principles of human geography (with s. w. cushing). new york, . business geography (with f. e. williams). new york, . d. _a companion to the present volume._ earth and sun: an hypothesis of weather and sunspots. new haven. in press. stephen sargent visher geography, geology and biology of southern dakota. vermilion, . the biology of northwestern south dakota. vermilion, . the geography of south dakota. vermilion, . handbook of the geology of indiana (with others). indianapolis, . hurricanes of australia and the south pacific. melbourne, . climatic changes their nature and causes by ellsworth huntington research associate in geography in yale university and stephen sargent visher associate professor of geology in indiana university [illustration] new haven yale university press london: humphrey milford: oxford university press mdccccxxii copyright by yale university press published . the theodore l. glasgow memorial publication fund the present volume is the fifth work published by the yale university press on the theodore l. glasgow memorial publication fund. this foundation was established september , , by an anonymous gift to yale university in memory of flight sub-lieutenant theodore l. glasgow, r.n. he was born in montreal, canada, and was educated at the university of toronto schools and at the royal military college, kingston. in august, , he entered the royal naval air service and in july, , went to france with the tenth squadron attached to the twenty-second wing of the royal flying corps. a month later, august , , he was killed in action on the ypres front. to thomas chrowder chamberlin of the university of chicago whose clear and masterly discussion of the great problems of terrestrial evolution has been one of the most inspiring factors in the writing of this book _there is a toy, which i have heard, and i would not have it given over, but waited upon a little. they say it is observed in the low countries (i know not in what part), that every five and thirty years the same kind and suit of years and weathers comes about again; as great frosts, great wet, great droughts, warm winters, summers with little heat, and the like, and they call it the prime; it is a thing i do the rather mention, because, computing backwards, i have found some concurrence._ francis bacon preface unity is perhaps the keynote of modern science. this means unity in time, for the present is but the outgrowth of the past, and the future of the present. it means unity of process, for there seems to be no sharp dividing line between organic and inorganic, physical and mental, mental and spiritual. and the unity of modern science means also a growing tendency toward coöperation, so that by working together scientists discover much that would else have remained hid. this book illustrates the modern trend toward unity in all of these ways. first, it is a companion volume to _earth and sun_. that volume is a discussion of the causes of weather, but a consideration of the weather of the present almost inevitably leads to a study of the climate of the past. hence the two books were written originally as one, and were only separated from considerations of convenience. second, the unity of nature is so great that when a subject such as climatic changes is considered, it is almost impossible to avoid other subjects, such as the movements of the earth's crust. hence this book not only discusses climatic changes, but considers the causes of earthquakes and attempts to show how climatic changes may be related to great geological revolutions in the form, location, and altitude of the lands. thus the book has a direct bearing on all the main physical factors which have molded the evolution of organic life, including man. in the third place, this volume illustrates the unity of modern science because it is preëminently a coöperative product. not only have the two authors shared in its production, but several of the yale faculty have also coöperated. from the geological standpoint, professor charles schuchert has read the entire manuscript in its final form as well as parts at various stages. he has helped not only by criticisms, suggestions, and facts, but by paragraphs ready for the printer. in the same way in the domain of physics, professor leigh page has repeatedly taken time to assist, and either in writing or by word of mouth has contributed many pages. in astronomy, the same cordial coöperation has come with equal readiness from professor frank schlesinger. professors schuchert, schlesinger, and page have contributed so materially that they are almost co-authors of the volume. in mathematics, professor ernest w. brown has been similarly helpful, having read and criticised the entire book. in certain chemical problems, professor harry w. foote has been our main reliance. the advice and suggestions of these men have frequently prevented errors, and have again and again started new and profitable lines of thought. if we have made mistakes, it has been because we have not profited sufficiently by their coöperation. if the main hypothesis of this book proves sound, it is largely because it has been built up in constant consultation with men who look at the problem from different points of vision. our appreciation of their generous and unstinted coöperation is much deeper than would appear from this brief paragraph. outside the yale faculty we have received equally cordial assistance. professor t. c. chamberlin of the university of chicago, to whom, with his permission, we take great pleasure in dedicating this volume, has read the entire proof and has made many helpful suggestions. we cannot speak too warmly of our appreciation not only of this, but of the way his work has served for years as an inspiration in the preliminary work of gathering data for this volume. professor harlow shapley of harvard university has contributed materially to the chapter on the sun and its journey through space; professor andrew e. douglass of the university of arizona has put at our disposal some of his unpublished results; professors s. b. woodworth and reginald a. daly, and mr. robert w. sayles of harvard, and professor henry f. reid of johns hopkins have suggested new facts and sources of information; professor e. r. cumings of indiana university has critically read the entire proof; conversations with professor john p. buwalda of the university of california while he was teaching at yale make him another real contributor; and mr. wayland williams has contributed the interesting quotation from bacon on page x of this book. miss edith s. russell has taken great pains in preparing the manuscript and in suggesting many changes that make for clearness. many others have also helped, but it is impossible to make due acknowledgment because such contributions have become so thoroughly a part of the mental background of the book that their source is no longer distinct in the minds of the authors. the division of labor between the two authors has not followed any set rules. both have had a hand in all parts of the book. the main draft of chapters vii, viii, ix, xi, and xiii was written by the junior author; his contributions are also especially numerous in chapters x and xv; the rest of the book was written originally by the senior author. contents page i. the uniformity of climate ii. the variability of climate iii. hypotheses of climatic change iv. the solar cyclonic hypothesis v. the climate of history vi. the climatic stress of the fourteenth century vii. glaciation according to the solar cyclonic hypothesis viii. some problems of glacial periods ix. the origin of loess x. causes of mild geological climates xi. terrestrial causes of climatic changes xii. post-glacial crustal movements and climatic changes xiii. the changing composition of oceans and atmosphere xiv. the effect of other bodies on the sun xv. the sun's journey through space xvi. the earth's crust and the sun list of illustrations page fig. . climatic changes and mountain building fig. . storminess at sunspot maxima vs. minima fig. . relative rainfall at times of increasing and decreasing sunspots , fig. . changes of climate in california and in western and central asia fig. . changes in california climate for years, as measured by growth of sequoia trees fig. . distribution of pleistocene ice sheets fig. . permian geography and glaciation fig. . effect of diminution of storms on movement of water fig. . cretaceous paleogeography fig. . climatic changes of , years as inferred from the stars fig. . sunspot curve showing cycles, to fig. . seasonal distribution of earthquakes fig. . wandering of the pole from to tables page . the geological time table . types of climatic sequence . correlation coefficients between rainfall and growth of sequoias in california . correlation coefficients between rainfall records in california and jerusalem . theoretical probability of stellar approaches . thirty-eight stars having largest known parallaxes , . destructive earthquakes from to compared with sunspots . seasonal march of earthquakes . deflection of path of pole compared with earthquakes . earthquakes in to compared with departures of the projected curve of the earth's axis from the eulerian position chapter i the uniformity of climate the rôle of climate in the life of today suggests its importance in the past and in the future. no human being can escape from the fact that his food, clothing, shelter, recreation, occupation, health, and energy are all profoundly influenced by his climatic surroundings. a change of season brings in its train some alteration in practically every phase of human activity. animals are influenced by climate even more than man, for they have not developed artificial means of protecting themselves. even so hardy a creature as the dog becomes notably different with a change of climate. the thick-haired "husky" of the eskimos has outwardly little in common with the small and almost hairless canines that grovel under foot in mexico. plants are even more sensitive than animals and men. scarcely a single species can flourish permanently in regions which differ more than °c. in average yearly temperature, and for most the limit of successful growth is °.[ ] so far as we yet know every living species of plant and animal, including man, thrives best under definite and limited conditions of temperature, humidity, and sunshine, and of the composition and movement of the atmosphere or water in which it lives. any departure beyond the limits means lessened efficiency, and in the long run a lower rate of reproduction and a tendency toward changes in specific characteristics. any great departure means suffering or death for the individual and destruction for the species. since climate has so profound an influence on life today, it has presumably been equally potent at other times. therefore few scientific questions are more important than how and why the earth's climate has varied in the past, and what changes it is likely to undergo in the future. this book sets forth what appear to be the chief reasons for climatic variations during historic and geologic times. it assumes that causes which can now be observed in operation, as explained in a companion volume entitled _earth and sun_, and in such books as humphreys' _physics of the air_, should be carefully studied before less obvious causes are appealed to. it also assumes that these same causes will continue to operate, and are the basis of all valid predictions as to the weather or climate of the future. in our analysis of climatic variations, we may well begin by inquiring how the earth's climate has varied during geological history. such an inquiry discloses three great tendencies, which to the superficial view seem contradictory. all, however, have a similar effect in providing conditions under which organic evolution is able to make progress. the first tendency is toward uniformity, a uniformity so pronounced and of such vast duration as to stagger the imagination. superposed upon this there seems to be a tendency toward complexity. during the greater part of geological history the earth's climate appears to have been relatively monotonous, both from place to place and from season to season; but since the miocene the rule has been diversity and complexity, a condition highly favorable to organic evolution. finally, the uniformity of the vast eons of the past and the tendency toward complexity are broken by pulsatory changes, first in one direction and then in another. to our limited human vision some of the changes, such as glacial periods, seem to be waves of enormous proportions, but compared with the possibilities of the universe they are merely as the ripples made by a summer zephyr. the uniformity of the earth's climate throughout the vast stretches of geological time can best be realized by comparing the range of temperature on the earth during that period with the possible range as shown in the entire solar system. as may be seen in table , the geological record opens with the archeozoic era, or "age of unicellular life," as it is sometimes called, for the preceding cosmic time has left no record that can yet be read. practically no geologists now believe that the beginning of the archeozoic was less than one hundred million years ago; and since the discovery of the peculiar properties of radium many of the best students do not hesitate to say a billion or a billion and a half.[ ] even in the archeozoic the rocks testify to a climate seemingly not greatly different from that of the average of geologic time. the earth's surface was then apparently cool enough so that it was covered with oceans and warm enough so that the water teemed with microscopic life. the air must have been charged with water vapor and with carbon dioxide, for otherwise there seems to be no possible way of explaining the formation of mudstones and sandstones, limestones of vast thickness, carbonaceous shales, graphites, and iron ores.[ ] although the archeozoic has yielded no generally admitted fossils, yet what seem to be massive algæ and sponges have been found in canada. on the other hand, abundant life is believed to have been present in the oceans, for by no other known means would it be possible to take from the air the vast quantities of carbon that now form carbonaceous shales and graphite. in the next geologic era, the proterozoic, the researches of walcott have shown that besides the marine algæ there must have been many other kinds of life. the proterozoic fossils thus far discovered include not only microscopic radiolarians such as still form the red ooze of the deepest ocean floors, but the much more significant tubes of annelids or worms. the presence of the annelids, which are relatively high in the scale of organization, is generally taken to mean that more lowly forms of animals such as coelenterates and probably even the mollusca and primitive arthropods must already have been evolved. that there were many kinds of marine invertebrates living in the later proterozoic is indicated by the highly varied life and more especially the trilobites found in the oldest cambrian strata of the next succeeding period. in fact the cambrian has sponges, primitive corals, a great variety of brachiopods, the beginnings of gastropods, a wonderful array of trilobites, and other lowly forms of arthropods. since, under the postulate of evolution, the life of that time forms an unbroken sequence with that of the present, and since many of the early forms differ only in minor details from those of today, we infer that the climate then was not very different from that of today. the same line of reasoning leads to the conclusion that even in the middle of the proterozoic, when multicellular marine animals must already have been common, the climate of the earth had already for an enormous period been such that all the lower types of oceanic invertebrates had already evolved. table the geological time table[ ] cosmic time formative era. birth and growth of the earth. beginnings of the atmosphere, hydrosphere, continental platforms, oceanic basins, and possibly of life. no known geological record. geologic time archeozoic era. origin of simplest life. proterozoic era. age of invertebrate origins. an early and a late ice age, with one or more additional ones indicated. paleozoic era. age of primitive vertebrate dominance. _cambrian period._ first abundance of marine animals and dominance of trilobites. _ordovician period._ first known fresh-water fishes. _silurian period._ first known land plants. _devonian period._ first known amphibians. "table mountain" ice age. _mississippian period._ rise of marine fishes (sharks). _pennsylvanian period._ rise of insects and first period of marked coal accumulation. _permian period._ rise of reptiles. another great ice age. mesozoic era. age of reptile dominance. _triassic period._ rise of dinosaurs. the period closes with a cool climate. _jurassic period._ rise of birds and flying reptiles. _comanchean period._ rise of flowering plants and higher insects. _cretaceous period._ rise of archaic or primitive mammalia. cenozoic era. age of mammal dominance. _early cenozoic or eocene and oligocene time._ rise of higher mammals. glaciers in early eocene of the laramide mountains. _late cenozoic or miocene and pliocene time._ transformation of ape like animals into man. _glacial or pleistocene time._ last great ice age. present time psychozoic era. age of man or age of reason. includes the present or "recent time," estimated to be probably less than , years. moreover, they could live in most latitudes, for the indirect evidences of life in the archeozoic and proterozoic rocks are widely distributed. thus it appears that at an almost incredibly early period, perhaps many hundred million years ago, the earth's climate differed only a little from that of the present. the extreme limits of temperature beyond which the climate of geological times cannot have departed can be approximately determined. today the warmest parts of the ocean have an average temperature of about °c. on the surface. only a few forms of life live where the average temperature is much higher than this. in deserts, to be sure, some highly organized plants and animals can for a short time endure a temperature as high as °c. ( °f.). in certain hot springs, some of the lowest unicellular plant forms exist in water which is only a little below the boiling point. more complex forms, however, such as sponges, worms, and all the higher plants and animals, seem to be unable to live either in water or air where the temperature averages above °c. ( °f.) for any great length of time and it is doubtful whether they can thrive permanently even at that temperature. the obvious unity of life for hundreds of millions of years and its presence at all times in middle latitudes so far as we can tell seem to indicate that since the beginning of marine life the temperature of the oceans cannot have averaged much above °c. even in the warmest portions. this is putting the limit too high rather than too low, but even so the warmest parts of the earth can scarcely have averaged much more than ° warmer than at present. turning to the other extreme, we may inquire how much colder than now the earth's surface may have been since life first appeared. proterozoic fossils have been found in places where the present average temperature approaches °c. if those places should be colder than now by °c., or more, the drop in temperature at the equator would almost certainly be still greater, and the seas everywhere would be permanently frozen. thus life would be impossible. since the contrasts between summer and winter, and between the poles and the equator seem generally to have been less in the past than at present, the range through which the mean temperature of the earth as a whole could vary without utterly destroying life was apparently less than would now be the case. these considerations make it fairly certain that for at least several hundred million years the average temperature of the earth's surface has never varied more than perhaps °c. above or below the present level. even this range of °c. ( °f.) may be double or triple the range that has actually occurred. that the temperature has not passed beyond certain narrow limits, whatever their exact degree, is clear from the fact that if it had done so, all the higher forms of life would have been destroyed. certain of the lowest unicellular forms might indeed have persisted, for when dormant they can stand great extremes of dry heat and of cold for a long time. even so, evolution would have had to begin almost anew. the supposition that such a thing has happened is untenable, for there is no hint of any complete break in the record of life during geological times,--no sudden disappearance of the higher organisms followed by a long period with no signs of life other than indirect evidence such as occurs in the archeozoic. a change of °c. or even of ° in the average temperature of the earth's surface may seem large when viewed from the limited standpoint of terrestrial experience. viewed, however, from the standpoint of cosmic evolution, or even of the solar system, it seems a mere trifle. consider the possibilities. the temperature of empty space is the absolute zero, or - °c. to this temperature all matter must fall, provided it exists long enough and is not appreciably heated by collisions or by radiation. at the other extreme lies the temperature of the stars. as stars go, our sun is only moderately hot, but the temperature of its surface is calculated to be nearly °c., while thousands of miles in the interior it may rise to , ° or , ° or some other equally unknowable and incomprehensible figure. between the limits of the absolute zero on the one hand, and the interior of a sun or star on the other, there is almost every conceivable possibility of temperature. today the earth's surface averages not far from °c., or ° above the absolute zero. toward the interior, the temperature in mines and deep wells rises about °c. for every meters. at this rate it would be over °c. at a depth of ten miles, and over ° at miles. let us confine ourselves to surface temperatures, which are all that concern us in discussing climate. it has been calculated by poynting[ ] that if a small sphere absorbed and re-radiated all the heat that fell upon it, its temperature at the distance of mercury from the sun would average about °c.; at the distance of venus, °; the earth °; mars - °; neptune - °. a planet much nearer the sun than is mercury might be heated to a temperature of a thousand, or even several thousand, degrees, while one beyond neptune would remain almost at absolute zero. it is well within the range of possibility that the temperature of a planet's surface should be anywhere from near - °c. up to perhaps °c. or more, although the probability of low temperature is much greater than of high. thus throughout the whole vast range of possibilities extending to perhaps , °, the earth claims only ° at most, or less than per cent. this may be remarkable, but what is far more remarkable is that the earth's range of ° includes what seem to be the two most critical of all possible temperatures, namely, the freezing point of water, °c., and the temperature where water can dissolve an amount of carbon dioxide equal to its own volume. the most remarkable fact of all is that the earth has preserved its temperature within these narrow limits for a hundred million years, or perchance a thousand million. to appreciate the extraordinary significance of this last fact, it is necessary to realize how extremely critical are the temperatures from about ° to °c., and how difficult it is to find any good reason for a relatively uniform temperature through hundreds of millions of years. since the dawn of geological time the earth's temperature has apparently always included the range from about the freezing point of water up to about the point where protoplasm begins to disintegrate. henderson, in _the fitness of the environment_, rightly says that water is "the most familiar and the most important of all things." in many respects water and carbon dioxide form the most unique pair of substances in the whole realm of chemistry. water has a greater tendency than any other known substance to remain within certain narrowly defined limits of temperature. not only does it have a high specific heat, so that much heat is needed to raise its temperature, but on freezing it gives up more heat than any substance except ammonia, while none of the common liquids approach it in the amount of additional heat required for conversion into vapor after the temperature of vaporization has been reached. again, water substance, as the physicists call all forms of h_{ }o, is unique in that it not only contracts on melting, but continues to contract until a temperature several degrees above its melting point is reached. that fact has a vast importance in helping to keep the earth's surface at a uniform temperature. if water were like most liquids, the bottoms of all the oceans and even the entire body of water in most cases would be permanently frozen. again, as a solvent there is literally nothing to compare with water. as henderson[ ] puts it: "nearly the whole science of chemistry has been built up around water and aqueous solution." one of the most significant evidences of this is the variety of elements whose presence can be detected in sea water. according to henderson they include hydrogen, oxygen, nitrogen, carbon, chlorine, sodium, magnesium, sulphur, phosphorus, which are easily detected; and also arsenic, cæsium, gold, lithium, rubidium, barium, lead, boron, fluorine, iron, iodine, bromine, potassium, cobalt, copper, manganese, nickel, silver, silicon, zinc, aluminium, calcium, and strontium. yet in spite of its marvelous power of solution, water is chemically rather inert and relatively stable. it dissolves all these elements and thousands of their compounds, but still remains water and can easily be separated and purified. another unique property of water is its power of ionizing dissolved substances, a property which makes it possible to produce electric currents in batteries. this leads to an almost infinite array of electro-chemical reactions which play an almost dominant rôle in the processes of life. finally, no common liquid except mercury equals water in its power of capillarity. this fact is of enormous moment in biology, most obviously in respect to the soil. although carbon dioxide is far less familiar than water, it is almost as important. "these two simple substances," says henderson, "are the common source of every one of the complicated substances which are produced by living beings, and they are the common end products of the wearing away of all the constituents of protoplasm, and of the destruction of those materials which yield energy to the body." one of the remarkable physical properties of carbon dioxide is its degree of solubility in water. this quality varies enormously in different substances. for example, at ordinary pressures and temperatures, water can absorb only about per cent of its own volume of oxygen, while it can take up about times its own volume of ammonia. now for carbon dioxide, unlike most gases, the volume that can be absorbed by water is nearly the same as the volume of the water. the volumes vary, however, according to temperature, being absolutely the same at a temperature of about °c. or °f., which is close to the ideal temperature for man's physical health and practically the same as the mean temperature of the earth's surface when all seasons are averaged together. "hence, when water is in contact with air, and equilibrium has been established, the amount of free carbonic acid in a given volume of water is almost exactly equal to the amount in the adjacent air. unlike oxygen, hydrogen, and nitrogen, carbonic acid enters water freely; unlike sulphurous oxide and ammonia, it escapes freely from water. thus the waters can never wash carbonic acid completely out of the air, nor can the air keep it from the waters. it is the one substance which thus, in considerable quantities relative to its total amount, everywhere accompanies water. in earth, air, fire, and water alike these two substances are always associated. "accordingly, if water be the first primary constituent of the environment, carbonic acid is inevitably the second,--because of its solubility possessing an equal mobility with water, because of the reservoir of the atmosphere never to be depleted by chemical action in the oceans, lakes, and streams. in truth, so close is the association between these two substances that it is scarcely correct logically to separate them at all; together they make up the real environment and they never part company."[ ] the complementary qualities of carbon dioxide and water are of supreme importance because these two are the only known substances which are able to form a vast series of complex compounds with highly varying chemical formulæ. no other known compounds can give off or take on atoms without being resolved back into their elements. no others can thus change their form freely without losing their identity. this power of change without destruction is the fundamental chemical characteristic of life, for life demands complexity, change, and growth. in order that water and carbon dioxide may combine to form the compounds on which life is based, the water must be in the liquid form, it must be able to dissolve carbon dioxide freely, and the temperature must not be high enough to break up the highly complex and delicate compounds as soon as they are formed. in other words, the temperature must be above freezing, while it must not rise higher than some rather indefinite point between °c. and the boiling point, where all water finally turns into vapor. in the whole range of temperature, so far as we know, there is no other interval where any such complex reactions take place. the temperature of the earth for hundreds of millions of years has remained firmly fixed within these limits. the astonishing quality of the earth's uniformity of temperature becomes still more apparent when we consider the origin of the sun's heat. what that origin is still remains a question of dispute. the old ideas of a burning sun, or of one that is simply losing an original supply of heat derived from some accident, such as collision with another body, were long ago abandoned. the impact of a constant supply of meteors affords an almost equally unsatisfactory explanation. moulton[ ] states that if the sun were struck by enough meteorites to keep up its heat, the earth would almost certainly be struck by enough so that it would receive about half of per cent as much heat from them as from the sun. this is millions of times more heat than is now received from meteors. if the sun owes its heat to the impact of larger bodies at longer intervals, the geological record should show a series of interruptions far more drastic than is actually the case. it has also been supposed that the sun owes its heat to contraction. if a gaseous body contracts it becomes warmer. finally, however, it must become so dense that its rate of contraction diminishes and the process ceases. under the sun's present condition of size and density a radial contraction of feet per year would be enough to supply all the energy now radiated by that body. this seems like a hopeful source of energy, but kelvin calculated that twenty million years ago it was ineffective and ten million years hence it will be equally so. moreover, if this is the source of heat, the amount of radiation from the sun would have to vary enormously. twenty million years ago the sun would have extended nearly to the earth's orbit and would have been so tenuous that it would have emitted no more heat than some of the nebulæ in space. some millions of years later, when the sun's radius was twice as great as at present, that body would have emitted only one-fourth as much heat as now, which would mean that on the earth's surface the theoretical temperature would have been ° below the present level. this is utterly out of accord with the uniformity of climate shown by the geological record. in the future, if the sun's contraction is the only source of heat, the sun can supply the present amount for only ten million years, which would mean a change utterly unlike anything of which the geological record holds even the faintest hint.[ ] altogether the problem of how the sun can have remained so uniform and how the earth's atmosphere and other conditions can also have remained so uniform throughout hundreds of millions of years is one of the most puzzling in the whole realm of nature. if appeal is taken to radioactivity and the breaking up of uranium into radium and helium, conditions can be postulated which will give the required amount of energy. such is also the case if it be supposed that there is some unknown process which may induce an atomic change like radioactivity in bodies which are now supposed to be stable elements. in either case, however, there is as yet no satisfactory explanation of the _uniformity_ of the earth's climate. a hundred million or a thousand million years ago the temperature of the earth's surface was very much the same as now. the earth had then presumably ceased to emit any great amount of heat, if we may judge from the fact that its surface was cool enough so that great ice sheets could accumulate on low lands within ° of the equator. the atmosphere was apparently almost like that of today, and was almost certainly not different enough to make up for any great divergence of the sun from its present condition. we cannot escape the stupendous fact that in those remote times the sun must have been essentially the same as now, or else that some utterly unknown factor is at work. footnotes: [footnote : w. a. setchell: the temperature interval in the geographical distribution of marine algæ; science, vol. , , p. .] [footnote : j. barrell: rhythms and the measurements of geologic time; bull. geol. soc. am., vol. , dec., , pp. - .] [footnote : pirsson and schuchert: textbook of geology, , pp. - .] [footnote : from charles schuchert in the evolution of the earth and its inhabitants: edited by r. s. lull, new haven, , but with revisions by professor schuchert.] [footnote : j. h. poynting: radiation in the solar system; phil. trans. a, , , p. .] [footnote : l. j. henderson: the fitness of the environment, .] [footnote : henderson: _loc. cit._, p. .] [footnote : f. r. moulton: introduction to astronomy, .] [footnote : moulton: _loc. cit._] chapter ii the variability of climate the variability of the earth's climate is almost as extraordinary as its uniformity. this variability is made up partly of a long, slow tendency in one direction and partly of innumerable cycles of every conceivable duration from days, or even hours, up to millions of years. perhaps the easiest way to grasp the full complexity of the matter is to put the chief types of climatic sequence in the form of a table. +-----------------------------------------------------------+ | | | table | | | | types of climatic sequence | | | | . cosmic uniformity. . brückner periods. | | . secular progression. . sunspot cycles. | | . geologic oscillations. . seasonal alternations. | | . glacial fluctuations. . pleionian migrations. | | . orbital precessions. . cyclonic vacillations. | | . historical pulsations. . daily vibrations. | | | +-----------------------------------------------------------+ in assigning names to the various types an attempt has been made to indicate something of the nature of the sequence so far as duration, periodicity, and general tendencies are concerned. not even the rich english language of the twentieth century, however, furnishes words with enough shades of meaning to express all that is desired. moreover, except in degree, there is no sharp distinction between some of the related types, such as glacial fluctuations and historic pulsations. yet, taken as a whole, the table brings out the great contrast between two absolutely diverse extremes. at the one end lies well-nigh eternal uniformity, or an extremely slow progress in one direction throughout countless ages; at the other, rapid and regular vibrations from day to day, or else irregular and seemingly unsystematic vacillations due to cyclonic storms, both of which types are repeated millions of times during even a single glacial fluctuation. the meaning of cosmic uniformity has been explained in the preceding chapter. its relation to the other types of climatic sequences seems to be that it sets sharply defined limits beyond which no changes of any kind have ever gone since life, as we know it, first began. secular progression, on the other hand, means that in spite of all manner of variations, now this way and then the other, the normal climate of the earth, if there is such a thing, has on the whole probably changed a little, perhaps becoming more complex. after each period of continental uplift and glaciation--for such are preëminently the times of complexity--it is doubtful whether the earth has ever returned to quite its former degree of monotony. today the earth has swung away from the great diversity of the glacial period. yet we still have contrasts of what seem to us great magnitude. in low depressions, such as turfan in the central deserts of eurasia, the thermometer sometimes ranges from °f. in the morning to ° in the shade at noon. on a cloudy day in the amazon forest close to the seashore, on the contrary, the temperature for months may rise to ° by day and sink no lower than ° at night. the reasons for the secular progression of the earth's climate appear to be intimately connected with those which have caused the next, and, in many respects, more important type of climatic sequence, which consists of geological oscillations. both the progression and the oscillations seem to depend largely on three purely terrestrial factors: first, the condition of the earth's interior, including both internal heat and contraction; second, the salinity and movement of the ocean; and third, the composition and amount of the atmosphere. to begin with the earth's interior--its loss of heat appears to be an almost negligible factor in explaining either secular progression or geologic oscillation. according to both the nebular and the planetesimal hypotheses, the earth's crust appears to be colder now than it was hundreds or thousands of millions of years ago. the emission of internal heat, however, had probably ceased to be of much climatic significance near the beginning of the geological record, for in southern canada glaciation occurred very early in the proterozoic era. on the other hand, the contraction of the earth has produced remarkable effects throughout the whole of geological time. it has lessened the earth's circumference by a thousand miles or more, as appears from the way in which the rocks have been folded and thrust bodily over one another. according to the laws of dynamics this must have increased the speed of the earth's rotation, thus shortening the day, and also having the more important effect of increasing the bulge at the equator. on the other hand, recent investigations indicate that tidal retardation has probably diminished the earth's rate of rotation more than seemed probable a few years ago, thus lengthening the day and diminishing the bulge at the equator. thus two opposing forces have been at work, one causing acceleration and one retardation. their combined effect may have been a factor in causing secular progression of climate. it almost certainly was of much importance in causing pronounced oscillations first one way and then the other. this matter, together with most of those touched in these first chapters, will be expanded in later parts of the book. on the whole the tendency appears to have been to create climatic diversity in place of uniformity. the increasing salinity of the oceans may have been another factor in producing secular progression, although of slight importance in respect to oscillations. while the oceans were still growing in volume, it is generally assumed that they must have been almost fresh for a vast period, although chamberlin thinks that the change in salinity has been much less than is usually supposed. so far as the early oceans were fresher than those of today, their deep-sea circulation must have been less hampered than now by the heavy saline water which is produced by evaporation in warm regions. although this saline water is warm, its weight causes it to descend, instead of moving poleward in a surface current; this descent slows up the rise of the cold water which has moved along in the depths of the ocean from high latitudes, and thus checks the general oceanic circulation. if the ancient oceans were fresher and hence had a freer circulation than now, a more rapid interchange of polar and equatorial water presumably tended to equalize the climate of all latitudes. again, although the earth's atmosphere has probably changed far less during geological times than was formerly supposed, its composition has doubtless varied. the total volume of nitrogen has probably increased, for that gas is so inert that when it once becomes a part of the air it is almost sure to stay there. on the other hand, the proportions of oxygen, carbon dioxide, and water vapor must have fluctuated. oxygen is taken out constantly by animals and by all the processes of rock weathering, but on the other hand the supply is increased when plants break up new carbon dioxide derived from volcanoes. as for the carbon dioxide, it appears probable that in spite of the increased supply furnished by volcanoes the great amounts of carbon which have gradually been locked up in coal and limestone have appreciably depleted the atmosphere. water vapor also may be less abundant now than in the past, for the presence of carbon dioxide raises the temperature a little and thereby enables the air to hold more moisture. when the area of the oceans has diminished, and this has recurred very often, this likewise would tend to reduce the water vapor. moreover, even a very slight diminution in the amount of heat given off by the earth, or a decrease in evaporation because of higher salinity in the oceans, would tend in the same direction. now carbon dioxide and water vapor both have a strong blanketing effect whereby heat is prevented from leaving the earth. therefore, the probable reduction in the carbon dioxide and water vapor of the earth's atmosphere has apparently tended to reduce the climatic monotony and create diversity and complexity. hence, in spite of many reversals, the general tendency of changes, not only in the earth's interior and in the oceans, but also in the atmosphere, appears to be a secular progression from a relatively monotonous climate in which the evolution of higher organic forms would scarcely be rapid to an extremely diverse and complex climate highly favorable to progressive evolution. the importance of these purely terrestrial agencies must not be lost sight of when we come to discuss other agencies outside the earth. in table the next type of climatic sequence is geologic oscillation. this means slow swings that last millions of years. at one extreme of such an oscillation the climate all over the world is relatively monotonous; it returns, as it were, toward the primeval conditions at the beginning of the secular progression. at such times magnolias, sequoias, figs, tree ferns, and many other types of subtropical plants grew far north in places like greenland, as is well known from their fossil remains of middle cenozoic time, for example. at these same times, and also at many others before such high types of plants had evolved, reef-making corals throve in great abundance in seas which covered what is now wisconsin, michigan, ontario, and other equally cool regions. today these regions have an average temperature of only about °f. in the warmest month, and average well below freezing in winter. no reef-making corals can now live where the temperature averages below °f. the resemblance of the ancient corals to those of today makes it highly probable that they were equally sensitive to low temperature. thus, in the mild portions of a geologic oscillation the climate seems to have been so equable and uniform that many plants and animals could live and at other times even miles farther from the equator than now. at such times the lands in middle and high latitudes were low and small, and the oceans extended widely over the continental platforms. thus unhampered ocean currents had an opportunity to carry the heat of low latitudes far toward the poles. under such conditions, especially if the conception of the great subequatorial continent of gondwana land is correct, the trade winds and the westerlies must have been stronger and steadier than now. this would not only enable the westerlies, which are really southwesterlies, to carry more heat than now to high latitudes, but would still further strengthen the ocean currents. at the same time, the air presumably contained an abundance of water vapor derived from the broad oceans, and an abundance of atmospheric carbon dioxide inherited from a preceding time when volcanoes contributed much carbon dioxide to the air. these two constituents of the atmosphere may have exercised a pronounced blanketing effect whereby the heat of the earth with its long wave lengths was kept in, although the energy of the sun with its shorter wave lengths was not markedly kept out. thus everything may have combined to produce mild conditions in high latitudes, and to diminish the contrast between equator and pole, and between summer and winter. such conditions perhaps carry in themselves the seeds of decay. at any rate while the lands lie quiet during a period of mild climate great strains must accumulate in the crust because of the earth's contraction and tidal retardation. at the same time the great abundance of plants upon the lowlying plains with their mild climates, and the marine creatures upon the broad continental platforms, deplete the atmospheric carbon dioxide. part of this is locked up as coal and part as limestone derived from marine plants as well as animals. then something happens so that the strains and stresses of the crust are released. the sea floors sink; the continents become relatively high and large; mountain ranges are formed; and the former plains and emergent portions of the continental platforms are eroded into hills and valleys. the large size of the continents tends to create deserts and other types of climatic diversity; the presence of mountain ranges checks the free flow of winds and also creates diversity; the ocean currents are likewise checked, altered, and diverted so that the flow of heat from low to high latitudes is diminished. at the same time evaporation from the ocean diminishes so that a decrease in water vapor combines with the previous depletion of carbon dioxide to reduce the blanketing effect of the atmosphere. thus upon periods of mild monotony there supervene periods of complexity, diversity, and severity. turn to table and see how a glacial climate again and again succeeds a time when relative mildness prevailed almost everywhere. or examine fig. and notice how the lines representing temperatures go up and down. in the figure schuchert makes it clear that when the lands have been large and mountain-making has been important, as shown by the high parts of the lower shaded area, the climate has been severe, as shown by the descent of the snow line, the upper shaded area. in the diagram the climatic oscillations appear short, but this is merely because they have been crowded together, especially in the left hand or early part. there an inch in length may represent a hundred million years. even at the right-hand end an inch is equivalent to several million years. the severe part of a climatic oscillation, as well as the mild part, will be shown in later chapters to bear in itself certain probable seeds of decay. while the lands are being uplifted, volcanic activity is likely to be vigorous and to add carbon dioxide to the air. later, as the mountains are worn down by the many agencies of water, wind, ice, and chemical decay, although much carbon dioxide is locked up by the carbonation of the rocks, the carbon locked up in the coal is set free and increases the carbon dioxide of the air. at the same time the continents settle slowly downward, for the earth's crust though rigid as steel is nevertheless slightly viscous and will flow if subjected to sufficiently great and enduring pressure. the area from which evaporation can take place is thereby increased because of the spread of the oceans over the continents, and water vapor joins with the carbon dioxide to blanket the earth and thus tends to keep it uniformly warm. moreover, the diminution of the lands frees the ocean currents from restraint and permits them to flow more freely from low latitudes to high. thus in the course of millions of years there is a return toward monotony. ultimately, however, new stresses accumulate in the earth's crust, and the way is prepared for another great oscillation. perhaps the setting free of the stresses takes place simply because the strain at last becomes irresistible. it is also possible, as we shall see, that an external agency sometimes adds to the strain and thereby determines the time at which a new oscillation shall begin. in table the types of climatic sequences which follow "geologic oscillations" are "glacial fluctuations," "orbital precessions" and "historical pulsations." glacial fluctuations and historical pulsations appear to be of the same type, except as to severity and duration, and hence may be considered together. they will be treated briefly here because the theories as to their causes are outlined in the next two chapters. oddly enough, although the historic pulsations lie much closer to us than do the glacial fluctuations, they were not discovered until two or three generations later, and are still much less known. the most important feature of both sequences is the swing from a glacial to an inter-glacial epoch or from the arsis or accentuated part of an historical pulsation to the thesis or unaccented part. in a glacial epoch or in the arsis of an historic pulsation, storms are usually abundant and severe, the mean temperature is lower than usual, snow accumulates in high latitudes or upon lofty mountains. for example, in the last such period during the fourteenth century, great floods and droughts occurred alternately around the north sea; it was several times possible to cross the baltic sea from germany to sweden on the ice, and the ice of greenland advanced so much that shore ice caused the norsemen to change their sailing route between iceland and the norse colonies in southern greenland. at the same time in low latitudes and in parts of the continental interior there is a tendency toward diminished rainfall and even toward aridity and the formation of deserts. in yucatan, for example, a diminution in tropical rainfall in the fourteenth century seems to have given the mayas a last opportunity for a revival of their decaying civilization. [illustration: _fig. . climatic changes and mountain building._ (_after schuchert, in the evolution of the earth and its inhabitants, edited by r. s. lull._) diagram showing the times and probable extent of the more or less marked climate changes in the geologic history of north america, and of its elevation into chains of mountains.] among the climatic sequences, glacial fluctuations are perhaps of the most vital import from the standpoint of organic evolution; from the standpoint of human history the same is true of climatic pulsations. glacial epochs have repeatedly wiped out thousands upon thousands of species and played a part in the origin of entirely new types of plants and animals. this is best seen when the life of the pennsylvanian is contrasted with that of the permian. an historic pulsation may wipe out an entire civilization and permit a new one to grow up with a radically different character. hence it is not strange that the causes of such climatic phenomena have been discussed with extraordinary vigor. in few realms of science has there been a more imposing or more interesting array of theories. in this book we shall consider the more important of these theories. a new solar or cyclonic hypothesis and the hypothesis of changes in the form and altitude of the land will receive the most attention, but the other chief hypotheses are outlined in the next chapter, and are frequently referred to throughout the volume. between glacial fluctuations and historical pulsations in duration, but probably less severe than either, come orbital precessions. these stand in a group by themselves and are more akin to seasonal alternations than to any other type of climatic sequence. they must have occurred with absolute regularity ever since the earth began to revolve around the sun in its present elliptical orbit. since the orbit is elliptical and since the sun is in one of the two foci of the ellipse, the earth's distance from the sun varies. at present the earth is nearest the sun in the northern winter. hence the rigor of winter in the northern hemisphere is mitigated, while that of the southern hemisphere is increased. in about ten thousand years this condition will be reversed, and in another ten thousand the present conditions will return once more. such climatic precessions, as we may here call them, must have occurred unnumbered times in the past, but they do not appear to have been large enough to leave in the fossils of the rocks any traces that can be distinguished from those of other climatic sequences. we come now to brückner periods and sunspot cycles. the brückner periods have a length of about thirty-three years. their existence was suggested at least as long ago as the days of sir francis bacon, whose statement about them is quoted on the flyleaf of this book. they have since been detected by a careful study of the records of the time of harvest, vintage, the opening of rivers to navigation, and the rise or fall of lakes like the caspian sea. in his book on _klimaschwankungen seit _, brückner has collected an uncommonly interesting assortment of facts as to the climate of europe for more than two centuries. more recently, by a study of the rate of growth of trees, douglass, in his book on _climatic cycles and tree growth_, has carried the subject still further. in general the nature of the -year periods seems to be identical with that of the - or - year sunspot cycle, on the one hand, and of historic pulsations on the other. for a century observers have noted that the variations in the weather which everyone notices from year to year seem to have some relation to sunspots. for generations, however, the relationship was discussed without leading to any definite conclusion. the trouble was that the same change was supposed to take place in all parts of the world. hence, when every sort of change was found somewhere at any given sunspot stage, it seemed as though there could not be a relationship. of late years, however, the matter has become fairly clear. the chief conclusions are, first, that when sunspots are numerous the average temperature of the earth's surface is lower than normal. this does not mean that all parts are cooler, for while certain large areas grow cool, others of less extent become warm at times of many sunspots. second, at times of many sunspots storms are more abundant than usual, but are also confined somewhat closely to certain limited tracks so that elsewhere a diminution of storminess may be noted. this whole question is discussed so fully in _earth and sun_ that it need not detain us further in this preliminary view of the whole problem of climate. suffice it to say that a study of the sunspot cycle leads to the conclusion that it furnishes a clue to many of the unsolved problems of the climate of the past, as well as a key to prediction of the future. passing by the seasonal alternations which are fully explained as the result of the revolution of the earth around the sun, we may merely point out that, like the daily vibrations which bring table to a close, they emphasize the outstanding fact that the main control of terrestrial climate is the amount of energy received from the sun. this same principle is illustrated by pleionian migrations. the term "pleion" comes from a greek word meaning "more." it was taken by arctowski to designate areas or periods where there is an excess of some climatic element, such as atmospheric pressure, rainfall, or temperature. even if the effect of the seasons is eliminated, it appears that the course of these various elements does not run smoothly. as everyone knows, a period like the autumn of in the eastern united states may be unusually warm, while a succeeding period may be unseasonably cool. these departures from the normal show a certain rough periodicity. for example, there is evidence of a period of about twenty-seven days, corresponding to the sun's rotation and formerly supposed to be due to the moon's revolution which occupies almost the same length of time. still other periods appear to have an average duration of about three months and of between two and three years. two remarkable discoveries have recently been made in respect to such pleions. one is that a given type of change usually occurs simultaneously in a number of well-defined but widely separated centers, while a change of an opposite character arises in another equally well-defined, but quite different, set of centers. in general, areas of high pressure have one type of change and areas of low pressure the other type. so systematic are these relationships and so completely do they harmonize in widely separated parts of the earth, that it seems certain that they must be due to some outside cause, which in all probability can be only the sun. the second discovery is that pleions, when once formed, travel irregularly along the earth's surface. their paths have not yet been worked out in detail, but a general migration seems well established. because of this, it is probable that if unusually warm weather prevails in one part of a continent at a given time, the "thermo-pleion," or excess of heat, will not vanish but will gradually move away in some particular direction. if we knew the path that it would follow we might predict the general temperature along its course for some months in advance. the paths are often irregular, and the pleions frequently show a tendency to break up or suddenly revive. probably this tendency is due to variations in the sun. when the sun is highly variable, the pleions are numerous and strong, and extremes of weather are frequent. taken as a whole the pleions offer one of the most interesting and hopeful fields not only for the student of the causes of climatic variations, but for the man who is interested in the practical question of long-range weather forecasts. like many other climatic phenomena they seem to represent the combined effect of conditions in the sun and upon the earth itself. the last of the climatic sequences which require explanation is the cyclonic vacillations. these are familiar to everyone, for they are the changes of weather which occur at intervals of a few days, or a week or two, at all seasons, in large parts of the united states, europe, japan, and some of the other progressive parts of the earth. they do not, however, occur with great frequency in equatorial regions, deserts, and many other regions. up to the end of the last century, it was generally supposed that cyclonic storms were purely terrestrial in origin. without any adequate investigation it was assumed that all irregularities in the planetary circulation of the winds arise from an irregular distribution of heat due to conditions within or upon the earth itself. these irregularities were supposed to produce cyclonic storms in certain limited belts, but not in most parts of the world. today this view is being rapidly modified. undoubtedly, the irregularities due to purely terrestrial conditions are one of the chief contributory causes of storms, but it begins to appear that solar variations also play a part. it has been found, for example, that not only the mean temperature of the earth's surface varies in harmony with the sunspot cycle, but that the frequency and severity of storms vary in the same way. moreover, it has been demonstrated that the sun's radiation is not constant, but is subject to innumerable variations. this does not mean that the sun's general temperature varies, but merely that at some times heated gases are ejected rapidly to high levels so that a sudden wave of energy strikes the earth. thus, the present tendency is to believe that the cyclonic variations, the changes of weather which come and go in such a haphazard, irresponsible way, are partly due to causes pertaining to the earth itself and partly to the sun. from this rapid survey of the types of climatic sequences, it is evident that they may be divided into four great groups. first comes cosmic uniformity, one of the most marvelous and incomprehensible of all known facts. we simply have no explanation which is in any respect adequate. next come secular progression and geologic oscillations, two types of change which seem to be due mainly to purely terrestrial causes, that is, to changes in the lands, the oceans, and the air. the general tendency of these changes is toward complexity and diversity, thus producing progression, but they are subject to frequent reversals which give rise to oscillations lasting millions of years. the processes by which the oscillations take place are fully discussed in this book. nevertheless, because they are fairly well understood, they are deferred until after the third group of sequences has been discussed. this group includes glacial fluctuations, historic pulsations, brückner periods, sunspot cycles, pleionian migrations, and cyclonic vacillations. the outstanding fact in regard to all of these is that while they are greatly modified by purely terrestrial conditions, they seem to owe their origin to variations in the sun. they form the chief subject of _earth and sun_ and in their larger phases are the most important topic of this book also. the last group of sequences includes orbital precessions, seasonal alternations, and daily variations. these may be regarded as purely solar in origin. yet their influence, like that of each of the other groups, is much modified by the earth's own conditions. our main problem is to separate and explain the two great elements in climatic changes,--the effects of the sun, on the one hand, and of the earth on the other. chapter iii hypotheses of climatic change the next step in our study of climate is to review the main hypotheses as to the causes of glaciation. these hypotheses apply also to other types of climatic changes. we shall concentrate on glacial periods, however, not only because they are the most dramatic and well-known types of change, but because they have been more discussed than any other and have also had great influence on evolution. moreover, they stand near the middle of the types of climatic sequences, and an understanding of them does much to explain the others. in reviewing the various theories we shall not attempt to cover all the ground, but shall merely state the main ideas of the few theories which have had an important influence upon scientific thought. the conditions which any satisfactory climatic hypothesis must satisfy are briefly as follows: ( ) due weight must be given to the fact that changes of climate are almost certainly due to the combined effect of a variety of causes, both terrestrial and solar or cosmic. ( ) attention must also be paid to both sides in the long controversy as to whether glaciation is due primarily to a diminution in the earth's supply of heat or to a _redistribution_ of the heat through changes in atmospheric and oceanic circulation. at present the great majority of authorities are on the side of a diminution of heat, but the other view also deserves study. ( ) a satisfactory hypothesis must explain the frequent synchronism between two great types of phenomena; first, movements of the earth's crust whereby continents are uplifted and mountains upheaved; and, second, great changes of climate which are usually marked by relatively rapid oscillations from one extreme to another. ( ) no hypothesis can find acceptance unless it satisfies the somewhat exacting requirements of the geological record, with its frequent but irregular repetition of long, mild periods, relatively cool or intermediate periods like the present, and glacial periods of more or less severity and perhaps accompanying the more or less widespread uplifting of continents. at least during the later glacial periods the hypothesis must explain numerous climatic epochs and stages superposed upon a single general period of continental upheaval. moreover, although historical geology demands cycles of varied duration and magnitude, it does not furnish evidence of any rigid periodicity causing the cycles to be uniform in length or intensity. ( ) most important of all, a satisfactory explanation of climatic changes and crustal deformation must take account of all the agencies which are now causing similar phenomena. whether any other agencies should be considered is open to question, although the relative importance of existing agencies may have varied. i. _croll's eccentricity theory._ one of the most ingenious and most carefully elaborated scientific hypotheses is croll's[ ] precessional hypothesis as to the effect of the earth's own motions. so well was this worked out that it was widely accepted for a time and still finds a place in popular but unscientific books, such as wells' _outline of history_, and even in scientific works like wright's _quaternary ice age_. the gist of the hypothesis has already been given in connection with the type of climatic sequence known as orbital precessions. the earth is million miles away from the sun in january and million in july. the earth's axis "precesses," however, just as does that of a spinning top. hence arises what is known as the precession of the equinoxes, that is, a steady change in the season at which the earth is in perihelion, or nearest to the sun. in the course of , years the time of perihelion varies from early in january through the entire twelve months and back to january. moreover, the earth's orbit is slightly more elliptical at certain periods than at others, for the planets sometimes become bunched so that they all pull the earth in one direction. hence, once in about one hundred thousand years the effect of the elliptical shape of the earth's orbit is at a maximum. croll argued that these astronomical changes must alter the earth's climate, especially by their effect on winds and ocean currents. his elaborate argument contains a vast amount of valuable material. later investigation, however, seems to have proven the inadequacy of his hypothesis. in the first place, the supposed cause does not seem nearly sufficient to produce the observed results. second, croll's hypothesis demands that glaciation in the northern and southern hemisphere take place alternately. a constantly growing collection of facts, however, indicates that glaciation does not occur in the two hemispheres alternately, but at the same time. third, the hypothesis calls for the constant and frequent repetition of glaciation at absolutely regular intervals. the geological record shows no such regularity, for sometimes several glacial epochs follow in relatively close succession at irregular intervals of perhaps fifty to two hundred thousand years, and thus form a glacial period; and then for millions of years there are none. fourth, the eccentricity hypothesis provides no adequate explanation for the glacial stages or subepochs, the historic pulsations, and the other smaller climatic variations which are superposed upon glacial epochs and upon one another in bewildering confusion. in spite of these objections, there can be little question that the eccentricity of the earth's orbit and the precession of the equinoxes with the resulting change in the season of perihelion must have some climatic effect. hence croll's theory deserves a permanent though minor place in any full discussion of the causes of climatic changes. ii. _the carbon dioxide theory._ at about the time that the eccentricity theory was being relegated to a minor niche, a new theory was being developed which soon exerted a profound influence upon geological thought. chamberlin,[ ] adopting an idea suggested by tyndall, fired the imagination of geologists by his skillful exposition of the part played by carbon dioxide in causing climatic changes. today this theory is probably more widely accepted than any other. we have already seen that the amount of carbon dioxide gas in the atmosphere has a decided climatic importance. moreover, there can be little doubt that the amount of that gas in the atmosphere varies from age to age in response to the extent to which it is set free by volcanoes, consumed by plants, combined with rocks in the process of weathering, dissolved in the ocean or locked up in the form of coal and limestone. the main question is whether such variations can produce changes so rapid as glacial epochs and historical pulsations. abundant evidence seems to show that the degree to which the air can be warmed by carbon dioxide is sharply limited. humphreys, in his excellent book on the _physics of the air_, calculates that a layer of carbon dioxide forty centimeters thick has practically as much blanketing effect as a layer indefinitely thicker. in other words, forty centimeters of carbon dioxide, while having no appreciable effect on sunlight coming toward the earth, would filter out and thus retain in the atmosphere all the outgoing terrestrial heat that carbon dioxide is capable of absorbing. adding more would be like adding another filter when the one in operation has already done all that that particular kind of filter is capable of doing. according to humphreys' calculations, a doubling of the carbon dioxide in the air would in itself raise the average temperature about . °c. and further carbon dioxide would have practically no effect. reducing the present supply by half would reduce the temperature by essentially the same amount. the effect must be greater, however, than would appear from the figures given above, for any change in temperature has an effect on the amount of water vapor, which in turn causes further changes of temperature. moreover, as chamberlin points out, it is not clear whether humphreys allows for the fact that when the centimeters of co_{ } nearest the earth has been heated by terrestrial radiation, it in turn radiates half its heat outward and half inward. the outward half is all absorbed in the next layer of carbon dioxide, and so on. the process is much more complex than this, but the end result is that even the last increment of co_{ }, that is, the outermost portions in the upper atmosphere, must apparently absorb an infinitesimally small amount of heat. this fact, plus the effect of water vapor, would seem to indicate that a doubling or halving of the amount of co_{ }, would have an effect of more than . °c. a change of even °c. above or below the present level of the earth's mean temperature would be of very appreciable climatic significance, for it is commonly believed that during the height of the glacial period the mean temperature was only ° to °c. lower than now. nevertheless, variations in atmospheric carbon dioxide do not necessarily seem competent to produce the relatively rapid climatic fluctuations of glacial epochs and historic pulsations as distinguished from the longer swings of glacial periods and geological eras. in chamberlin's view, as in ours, the elevation of the land, the modification of the currents of the air and of the ocean, and all that goes with elevation as a topographic agency constitute a primary cause of climatic changes. a special effect of this is the removal of carbon dioxide from the air by the enhanced processes of weathering. this, as he carefully states, is a very slow process, and cannot of itself lead to anything so sudden as the oncoming of glaciation. but here comes chamberlin's most distinctive contribution to the subject, namely, the hypothesis that changes in atmospheric temperature arising from variations in atmospheric carbon dioxide are able to cause a reversal of the deep-sea oceanic circulation. according to chamberlin's view, the ordinary oceanic circulation of the greater part of geological time was the reverse of the present circulation. warm water descended to the ocean depths in low latitudes, kept its heat while creeping slowly poleward, and rose in high latitudes producing the warm climate which enabled corals, for example, to grow in high latitudes. chamberlin holds this opinion largely because there seems to him to be no other reasonable way to account for the enormously long warm periods when heat-loving forms of life lived in what are now polar regions of ice and snow. he explains this reversed circulation by supposing that an abundance of atmospheric carbon dioxide, together with a broad distribution of the oceans, made the atmosphere so warm that the evaporation in low latitudes was far more rapid than now. hence the surface water of the ocean became a relatively concentrated brine. such a brine is heavy and tends to sink, thereby setting up an oceanic circulation the reverse of that which now prevails. at present the polar waters sink because they are cold and hence contract. moreover, when they freeze a certain amount of salt leaves the ice and thereby increases the salinity of the surrounding water. thus the polar water sinks to the depths of the ocean, its place is taken by warmer and lighter water from low latitudes which moves poleward along the surface, and at the same time the cold water of the ocean depths is forced equatorward below the surface. but if the equatorial waters were so concentrated that a steady supply of highly saline water kept descending to low levels, the direction of the circulation would have to be reversed. the time when this would occur would depend upon the delicate balance between the downward tendencies of the cold polar water and of the warm saline equatorial water. suppose that while such a reversed circulation prevailed, the atmospheric co_{ } should be depleted, and the air cooled so much that the concentration of the equatorial waters by evaporation was no longer sufficient to cause them to sink. a reversal would take place, the present type of circulation would be inaugurated, and the whole earth would suffer a chill because the surface of the ocean would become cool. the cool surface-water would absorb carbon dioxide faster than the previous warm water had done, for heat drives off gases from water. this would hasten the cooling of the atmosphere still more, not only directly but by diminishing the supply of atmospheric moisture. the result would be glaciation. but ultimately the cold waters of the higher latitudes would absorb all the carbon dioxide they could hold, the slow equatorward creep would at length permit the cold water to rise to the surface in low latitudes. there the warmth of the equatorial sun and the depleted supply of carbon dioxide in the air would combine to cause the water to give up its carbon dioxide once more. if the atmosphere had been sufficiently depleted by that time, the rising waters in low latitudes might give up more carbon dioxide than the cold polar waters absorbed. thus the atmospheric supply would increase, the air would again grow warm, and a tendency toward deglaciation, or toward an inter-glacial condition would arise. at such times the oceanic circulation is not supposed to have been reversed, but merely to have been checked and made slower by the increasing warmth. thus inter-glacial conditions like those of today, or even considerably warmer, are supposed to have been produced with the present type of circulation. the emission of carbon dioxide in low latitudes could not permanently exceed the absorption in high latitudes. after the present type of circulation was finally established, which might take tens of thousands of years, the two would gradually become equal. then the conditions which originally caused the oceanic circulation to be reversed would again destroy the balance; the atmospheric carbon dioxide would be depleted; the air would grow cooler; and the cycle of glaciation would be repeated. each cycle would be shorter than the last, for not only would the swings diminish like those of a pendulum, but the agencies that were causing the main depletion of the atmospheric carbon dioxide would diminish in intensity. finally as the lands became lower through erosion and submergence, and as the processes of weathering became correspondingly slow, the air would gradually be able to accumulate carbon dioxide; the temperature would increase; and at length the oceanic circulation would be reversed again. when the warm saline waters of low latitudes finally began to sink and to set up a flow of warm water poleward in the depths of the ocean, a glacial period would definitely come to an end. this hypothesis has been so skillfully elaborated, and contains so many important elements that one can scarcely study it without profound admiration. we believe that it is of the utmost value as a step toward the truth, and especially because it emphasizes the great function of oceanic circulation. nevertheless, we are unable to accept it in full for several reasons, which may here be stated very briefly. most of them will be discussed fully in later pages. ( ) while a reversal of the deep-sea circulation would undoubtedly be of great climatic importance and would produce a warm climate in high latitudes, we see no direct evidence of such a reversal. it is equally true that there is no conclusive evidence against it, and the possibility of a reversal must not be overlooked. there seem, however, to be other modifications of atmospheric and oceanic circulation which are able to produce the observed results. ( ) there is much, and we believe conclusive, evidence that a mere lowering of temperature would not produce glaciation. what seems to be needed is changes in atmospheric circulation and in precipitation. the carbon dioxide hypothesis has not been nearly so fully developed on the meteorological side as in other respects. ( ) the carbon dioxide hypothesis seems to demand that the oceans should have been almost as saline as now in the proterozoic era at the time of the first known glaciation. chamberlin holds that such was the case, but the constant supply of saline material brought to the ocean by rivers and the relatively small deposition of such material on the sea floor seem to indicate that the early oceans must have been much fresher than those of today. ( ) the carbon dioxide hypothesis does not attempt to explain minor climatic fluctuations such as post-glacial stages and historic pulsations, but these appear to be of the same nature as glacial epochs, differing only in degree. ( ) another reason for hesitation in accepting the carbon dioxide hypothesis as a full explanation of glacial fluctuations is the highly complex and non-observational character of the explanation of the alternation of glacial and inter-glacial epochs and of their constantly decreasing length. ( ) most important of all, a study of the variations of weather and of climate as they are disclosed by present records and by the historic past suggests that there are now in action certain other causes which are competent to explain glaciation without recourse to a process whose action is beyond the realm of observation. these considerations lead to the conclusion that the carbon dioxide hypothesis and the reversal of the oceanic circulation should be regarded as a tentative rather than a final explanation of glaciation. nevertheless, the action of carbon dioxide seems to be an important factor in producing the longer oscillations of climate from one geological era to another. it probably plays a considerable part in preparing the way for glacial periods and in making it possible for other factors to produce the more rapid changes which have so deeply influenced organic evolution. iii. _the form of the land._ another great cause of climatic change consists of a group of connected phenomena dependent upon movements of the earth's crust. as to the climatic potency of changes in the lands there is practical agreement among students of climatology and glaciation. that the height and extent of the continents, the location, size, and orientation of mountain ranges, and the opening and closing of oceanic gateways at places like panama, and the consequent diversion of oceanic currents, exert a profound effect upon climate can scarcely be questioned. such changes may be introduced rapidly, but their disappearance is usually slow compared with the rapid pulsations to which climate has been subject during historic times and during stages of glacial retreat and advance, or even in comparison with the epochs into which the pleistocene, permian, and perhaps earlier glacial periods have been divided. hence, while crustal movements appear to be more important than the eccentricity of the earth's orbit or the amount of carbon dioxide in the air, they do not satisfactorily explain glacial fluctuations, historic pulsations, and especially the present little cycles of climatic change. all these changes involve a relatively rapid swing from one extreme to another, while an upheaval of a continent, which is at best a slow geologic process, apparently cannot be undone for a long, long time. hence such an upheaval, if acting alone, would lead to a relatively long-lived climate of a somewhat extreme type. it would help to explain the long swings, or geologic oscillations between a mild and uniform climate at one extreme, and a complex and varied climate at the other, but it would not explain the rapid climatic pulsations which are closely associated with great movements of the earth's crust. it might prepare the way for them, but could not cause them. that this conclusion is true is borne out by the fact that vast mountain ranges, like those at the close of the jurassic and cretaceous, are upheaved without bringing on glacial climates. moreover, the marked permian ice age follows long after the birth of the hercynian mountains and before the rise of others of later permian origin. iv. _the volcanic hypothesis._ in the search for some cause of climatic change which is highly efficient and yet able to vary rapidly and independently, abbot, fowle, humphreys, and others,[ ] have concluded that volcanic eruptions are the missing agency. in _physics of the air_, humphreys gives a careful study of the effect of volcanic dust upon terrestrial temperature. he begins with a mathematical investigation of the size of dust particles, and their quantity after certain eruptions. he demonstrates that the power of such particles to deflect light of short wave-lengths coming from the sun is perhaps thirty times more than their power to retain the heat radiated in long waves from the earth. hence it is estimated that if a krakatoa were to belch forth dust every year or two, the dust veil might cause a reduction of about °c. in the earth's surface temperature. as in every such complicated problem, some of the author's assumptions are open to question, but this touches their quantitative and not their qualitative value. it seems certain that if volcanic explosions were frequent enough and violent enough, the temperature of the earth's surface would be considerably lowered. actual observation supports this theoretical conclusion. humphreys gathers together and amplifies all that he and abbot and fowle have previously said as to observations of the sun's thermal radiation by means of the pyrheliometer. this summing up of the relations between the heat received from the sun, and the occurrence of explosive volcanic eruptions leaves little room for doubt that at frequent intervals during the last century and a half a slight lowering of terrestrial temperature has actually occurred after great eruptions. nevertheless, it does not justify humphreys' final conclusion that "phenomena within the earth itself suffice to modify its own climate, ... that these and these alone have actually caused great changes time and again in the geologic past." humphreys sees so clearly the importance of the purely terrestrial point of view that he unconsciously slights the cosmic standpoint and ignores the important solar facts which he himself adduces elsewhere at considerable length. in addition to this the _degree_ to which the temperature of the earth as a whole is influenced by volcanic eruptions is by no means so clear as is the fact that there is some influence. arctowski,[ ] for example, has prepared numerous curves showing the march of temperature month after month for many years. during the period from to , which includes the great eruption of katmai in alaska, low temperature is found to have prevailed at the time of the eruption, but, as arctowski puts it, on the basis of the curves for stations in all parts of the world: "the supposition that these abnormally low temperatures were due to the veil of volcanic dust produced by the katmai eruption of june , , is completely out of the question. if that had been the case, temperature would have decreased from that date on, whereas it was decreasing for more than a year before that date." köppen,[ ] in his comprehensive study of temperature for a hundred years, also presents a strong argument against the idea that volcanic eruptions have an important place in determining the present temperature of the earth. a volcanic eruption is a sudden occurrence. whatever effect is produced by dust thrown into the air must occur within a few months, or as soon as the dust has had an opportunity to be wafted to the region in question. when the dust arrives, there will be a rapid drop through the few degrees of temperature which the dust is supposed to be able to account for, and thereafter a slow rise of temperature. if volcanic eruptions actually caused a frequent lowering of terrestrial temperature in the hundred years studied by köppen, there should be more cases where the annual temperature is decidedly below the normal than where it shows a large departure in the opposite direction. the contrary is actually the case. a still more important argument is the fact that the earth is now in an intermediate condition of climate. throughout most of geologic time, as we shall see again and again, the climate of the earth has been milder than now. regions like greenland have not been the seat of glaciers, but have been the home of types of plants which now thrive in relatively low latitudes. in other words, the earth is today only part way from a glacial epoch to what may be called the normal, mild climate of the earth--a climate in which the contrast from zone to zone was much less than now, and the lower air averaged warmer. hence it seems impossible to avoid the conclusion that the cause of glaciation is still operating with considerable although diminished efficiency. but volcanic dust is obviously not operating to any appreciable extent at present, for the upper air is almost free from dust a large part of the time. again, as chamberlin suggests, let it be supposed that a krakatoan eruption every two years would produce a glacial period. unless the most experienced field workers on the glacial formations are quite in error, the various glacial epochs of the pleistocene glacial period had a joint duration of at least , years and perhaps twice as much. that would require , krakatoan eruptions. but where are the pits and cones of such eruptions? there has not been time to erode them away since the pleistocene glaciation. their beds of volcanic ash would presumably be as voluminous as the glacial beds, but there do not seem to be accumulations of any such size. even though the same volcano suffered repeated explosions, it seems impossible to find sufficient fresh volcanic debris. moreover, the volcanic hypothesis has not yet offered any mechanism for systematic glacial variations. hence, while the hypothesis is important, we must search further for the full explanation of glacial fluctuations, historic pulsations, and the earth's present quasi-glacial climate. v. _the hypothesis of polar wandering._ another hypothesis, which has some adherents, especially among geologists, holds that the position of the earth's axis has shifted repeatedly during geological times, thus causing glaciation in regions which are not now polar. astrophysicists, however, are quite sure that no agency could radically change the relation between the earth and its axis without likewise altering the orbits of the planets to a degree that would be easily recognized. moreover, the distribution of the centers of glaciation both in the permian and pleistocene periods does not seem to conform to this hypothesis. vi. _the thermal solar hypothesis._ the only other explanations of the climatic changes of glacial and historic times which now seem to have much standing are two distinct and almost antagonistic solar hypotheses. one is the idea that changes in the earth's climate are due to variations in the heat emitted by the sun and hence in the temperature of the earth. the other is the entirely different idea that climatic changes arise from solar conditions which cause a _redistribution of the earth's atmospheric pressure_ and hence produce changes in winds, ocean currents, and especially storms. this second, or "cyclonic," hypothesis is the subject of a book entitled _earth and sun_, which is to be published as a companion to the present volume. it will be outlined in the next chapter. the other, or thermal, hypothesis may be dismissed briefly. unquestionably a permanent change in the amount of heat emitted by the sun would permanently alter the earth's climate. there is absolutely no evidence, however, of any such change during geologic time. the evidence as to the earth's cosmic uniformity and as to secular progression is all against it. suppose that for thirty or forty thousand years the sun cooled off enough so that the earth was as cool as during a glacial epoch. as glaciation is soon succeeded by a mild climate, some agency would then be needed to raise the sun's temperature. the impact of a shower of meteorites might accomplish this, but that would mean a very sudden heating, such as there is no evidence of in geological history. in fact, there is far more evidence of sudden cooling than of sudden heating. moreover, it is far beyond the bounds of probability that such an impact should be repeated again and again with just such force as to bring the climate back almost to where it started and yet to allow for the slight changes which cause secular progression. another and equally cogent objection to the thermal form of solar hypothesis is stated by humphreys as follows: "a change of the solar constant obviously alters all surface temperatures by a roughly constant percentage. hence a decrease of the heat from the sun would in general cause a decrease of the interzonal temperature gradients; and this in turn a less vigorous atmospheric circulation, and a less copious rain or snowfall--exactly the reverse of the condition, namely, abundant precipitation, most favorable to extensive glaciation." this brings us to the end of the main hypotheses as to climatic changes, aside from the solar cyclonic hypothesis which will be discussed in the next chapter. it appears that variations in the position of the earth at perihelion have a real though slight influence in causing cycles with a length of about , years. changes in the carbon dioxide of the air probably have a more important but extremely slow influence upon geologic oscillations. variations in the size, shape, and height of the continents are constantly causing all manner of climatic complications, but do not cause rapid fluctuations and pulsations. the eruption of volcanic dust appears occasionally to lower the temperature, but its potency to explain the complex climatic changes recorded in the rocks has probably been exaggerated. finally, although minor changes in the amount of heat given out by the sun occur constantly and have been demonstrated to have a climatic effect, there is no evidence that such changes are the main cause of the climatic phenomena which we are trying to explain. nevertheless, in connection with other solar changes they may be of high importance. footnotes: [footnote : james croll: climate and time, .] [footnote : t. c. chamberlin: an attempt to frame a working hypothesis of the cause of glacial periods on an atmospheric basis; jour. geol., vol. vii, , pp. - , - , - . t. c. chamberlin and r. d. salisbury: geology, vol. ii, , pp. - , - , and vol. iii, pp. - . s. arrhenius (kosmische physik, vol. ii, , p. ) carried out some investigations on carbon dioxide which have had a pronounced effect on later conclusions. f. frech adopted arrhenius' idea and developed it in a paper entitled ueber die klima-aenderungen der geologischen vergangenheit. compte rendu, tenth (mexico) congr. geol. intern., (= ), pp. - . the exact origin of the carbon dioxide theory has been stated so variously that it seems worth while to give the exact facts. prompted by the suggestion, of tyndall that glaciation might be due to depletion of atmospheric carbon dioxide, chamberlin worked up the essentials of his early views before he saw any publication from arrhenius, to whom the idea has often been attributed. in or earlier chamberlin began to give the carbon dioxide hypothesis to his students and to discuss it before local scientific bodies. in he prepared a paper on "a group of hypotheses bearing on climatic changes," jour. geol., vol. v ( ), to be read at the meeting of the british association at toronto, basing his conclusions on tyndall's determination of the competency of carbon dioxide as an absorber of heat radiated from the earth. he had essentially completed this when a paper by arrhenius, "on the influence of carbonic acid in the air upon the temperature of the ground," phil. mag., , pp. - , first came to his attention. chamberlin then changed his conservative, tentative statement of the functions of carbon dioxide to a more sweeping one based on arrhenius' very definite quantitative deductions from langley's experiments. both langley and arrhenius were then in the ascendancy of their reputations and seemingly higher authorities could scarcely have been chosen, nor a finer combination than experiment and physico-mathematical development. arrhenius' deductions were later proved to have been overstrained, while langley's interpretation and even his observations were challenged. chamberlin's latest views are more like his earlier and more conservative statement.] [footnote : c. g. abbot and f. e. fowle: volcanoes and climate; smiths. misc. coll., vol. , , pp. w. j. humphreys: volcanic dust and other factors in the production of climatic and their possible relation to ice ages; bull. mount weather observatory, vol. , part , , pp. also, physics of the air, .] [footnote : h. arctowski: the pleonian cycle of climatic fluctuations; am. jour. sci., vol. , , pp. - . see also annals of the new york academy of sciences, vol. , .] [footnote : w. köppen: Über mehrjährige perioden der witterung ins besondere üzer die ii-jährige periode der temperatur. also, lufttemperaturen sonnenflecke und vulcanausbrüche; meteorologische zeitschrift, vol. , , pp. - .] chapter iv the solar cyclonic hypothesis the progress of science is made up of a vast succession of hypotheses. the majority die in early infancy. a few live and are for a time widely accepted. then some new hypothesis either destroys them completely or shows that, while they contain elements of truth, they are not the whole truth. in the previous chapter we have discussed a group of hypotheses of this kind, and have tried to point out fairly their degree of truth so far as it can yet be determined. in this chapter we shall outline still another hypothesis, the relation of which to present climatic conditions has been fully developed in _earth and sun_; while its relation to the past will be explained in the present volume. this hypothesis is not supposed to supersede the others, for so far as they are true they cannot be superseded. it merely seems to explain some of the many conditions which the other hypotheses apparently fail to explain. to suppose that it will suffer a fate more glorious than its predecessors would be presumptuous. the best that can be hoped is that after it has been pruned, enriched, and modified, it may take its place among the steps which finally lead to the goal of truth. in this chapter the new hypothesis will be sketched in broad outline in order that in the rest of this book the reader may appreciate the bearing of all that is said. details of proof and methods of work will be omitted, since they are given in _earth and sun_. for the sake of brevity and clearness the main conclusions will be stated without the qualifications and exceptions which are fully explained in that volume. here it will be necessary to pass quickly over points which depart radically from accepted ideas, and which therefore must arouse serious question in the minds of thoughtful readers. that, however, is a necessary consequence of the attempt which this book makes to put the problem of climate in such form that the argument can be followed by thoughtful students in any branch of knowledge and not merely by specialists. therefore, the specialist can merely be asked to withhold judgment until he has read all the evidence as given in _earth and sun_, and then to condemn only those parts that are wrong and not the whole argument. without further explanation let us turn to our main problem. in the realm of climatology the most important discovery of the last generation is that variations in the weather depend on variations in the activity of the sun's atmosphere. the work of the great astronomer, newcomb, and that of the great climatologist, köppen, have shown beyond question that the temperature of the earth's surface varies in harmony with variations in the number and area of sunspots.[ ] the work of abbot has shown that the amount of heat radiated from the sun also varies, and that in general the variations correspond with those of the sunspots, although there are exceptions, especially when the spots are fewest. here, however, there at once arises a puzzling paradox. the earth certainly owes its warmth to the sun. yet when the sun emits the most energy, that is, when sunspots are most numerous, the earth's surface is coolest. doubtless the earth receives more heat than usual at such times, and the upper air may be warmer than usual. here we refer only to the air at the earth's surface. another large group of investigators have shown that atmospheric pressure also varies in harmony with the number of sunspots. some parts of the earth's surface have one kind of variation at times of many sunspots and other parts the reverse. these differences are systematic and depend largely on whether the region in question happens to have high atmospheric pressure or low. the net result is that when sunspots are numerous the earth's storminess increases, and the atmosphere is thrown into commotion. this interferes with the stable planetary winds, such as the trades of low latitudes and the prevailing westerlies of higher latitudes. instead of these regular winds and the fair weather which they bring, there is a tendency toward frequent tropical hurricanes in the lower latitudes and toward more frequent and severe storms of the ordinary type in the latitudes where the world's most progressive nations now live. with the change in storminess there naturally goes a change in rainfall. not all parts of the world, however, have increased storminess and more abundant rainfall when sunspots are numerous. some parts change in the opposite way. thus when the sun's atmosphere is particularly disturbed, the contrasts between different parts of the earth's surface are increased. for example, the northern united states and southern canada become more stormy and rainy, as appears in fig. , and the same is true of the southwest and along the south atlantic coast. in a crescent-shaped central area, however, extending from wyoming through missouri to nova scotia, the number of storms and the amount of rainfall decrease. [illustration: _fig. . storminess at sunspot maxima vs. minima._ (_after kullmer._) based on nine years' nearest sunspot minima and nine years' nearest sunspot maxima in the three sunspot cycles from to . heavy shading indicates excess of storminess when sunspots are numerous. figures indicate average yearly number of storms by which years of maximum sunspots exceed those of minimum sunspots.] the two controlling factors of any climate are the temperature and the atmospheric pressure, for they determine the winds, the storms, and thus the rainfall. a study of the temperature seems to show that the peculiar paradox of a hot sun and a cool earth is due largely to the increased storminess during times of many sunspots. the earth's surface is heated by the rays of the sun, but most of the rays do not in themselves heat the air as they pass through it. the air gets its heat largely from the heat absorbed by the water vapor which is intimately mingled with its lower portions, or from the long heat waves sent out by the earth after it has been warmed by the sun. the faster the air moves along the earth's surface the less it becomes heated, and the more heat it takes away. this sounds like a contradiction, but not to anyone who has tried to heat a stove in the open air. if the air is still, the stove rapidly becomes warm and so does the air around it. if the wind is blowing, the cool air delays the heating of the stove and prevents the surface from ever becoming as hot as it would otherwise. that seems to be what happens on a large scale when sunspots are numerous. the sun actually sends to the earth more energy than usual, but the air moves with such unusual rapidity that it actually cools the earth's surface a trifle by carrying the extra heat to high levels where it is lost into space. there has been much discussion as to why storms are numerous when the sun's atmosphere is disturbed. many investigators have supposed it was due entirely and directly to the heating of the earth's surface by the sun. this, however, needs modification for several reasons. in the first place, recent investigations show that in a great many cases changes in barometric pressure precede changes in temperature and apparently cause them by altering the winds and producing storms. this is the opposite of what would happen if the effect of solar heat upon the earth's surface were the only agency. in the second place, if storms were due exclusively to variations in the ordinary solar radiation which comes to the earth as light and is converted into heat, the solar effect ought to be most pronounced when the center of the sun's visible disk is most disturbed. as a matter of fact the storminess is notably greatest when the edges of the solar disk are most disturbed. these facts and others lead to the conclusion that some agency other than heat must also play some part in producing storminess. the search for this auxiliary agency raises many difficult questions which cannot yet be answered. on the whole the weight of evidence suggests that electrical phenomena of some kind are involved, although variations in the amount of ultra-violet light may also be important. many investigators have shown that the sun emits electrons. hale has proved that the sun, like the earth, is magnetized. sunspots also have magnetic fields the strength of which is often fifty times as great as that of the sun as a whole. if electrons are sent to the earth, they must move in curved paths, for they are deflected by the sun's magnetic field and again by the earth's magnetic field. the solar deflection may cause their effects to be greatest when the spots are near the sun's margin; the terrestrial deflection may cause concentration in bands roughly concentric with the magnetic poles of the earth. these conditions correspond with the known facts. farther than this we cannot yet go. the calculations of humphreys seem to indicate that the direct electrical effect of the sun's electrons upon atmospheric pressure is too small to be of appreciable significance in intensifying storms. on the other hand the peculiar way in which activity upon the margins of the sun appears to be correlated not only with atmospheric electricity, but with barometric pressure, seems to be equally strong evidence in the other direction. possibly the sun's electrons and its electrical waves produce indirect effects by being converted into heat, or by causing the formation of ozone and the condensation of water vapor in the upper air. any one of these processes would raise the temperature of the upper air, for the ozone and the water vapor would be formed there and would tend to act as a blanket to hold in the earth's heat. but any such change in the temperature of the upper air would influence the lower air through changes in barometric pressure. these considerations are given here because the thoughtful reader is likely to inquire how solar activity can influence storminess. moreover, at the end of this book we shall take up certain speculative questions in which an electrical hypothesis will be employed. for the main portions of this book it makes no difference how the sun's variations influence the earth's atmosphere. the only essential point is that when the solar atmosphere is active the storminess of the earth increases, and that is a matter of direct observation. let us now inquire into the relation between the small cyclonic vacillations of the weather and the types of climatic changes known as historic pulsations and glacial fluctuations. one of the most interesting results of recent investigations is the evidence that sunspot cycles on a small scale present almost the same phenomena as do historic pulsations and glacial fluctuations. for instance, when sunspots are numerous, storminess increases markedly in a belt near the northern border of the area of greatest storminess, that is, in southern canada and thence across the atlantic to the north sea and scandinavia. (see figs. and .) corresponding with this is the fact that the evidence as to climatic pulsations in historic times indicates that regions along this path, for instance greenland, the north sea region, and southern scandinavia, were visited by especially frequent and severe storms at the climax of each pulsation. moreover, the greatest accumulations of ice in the glacial period were on the poleward border of the general regions where now the storms appear to increase most at times of solar activity. [illustration: _fig. a. relative rainfall at times of increasing and decreasing sunspots._ heavy shading, more rain with increasing spots. light shading, more rain with decreasing spots. no data for unshaded areas. figures indicate percentages of the average rainfall by which the rainfall during periods of increasing spots exceeds or falls short of rainfall during periods of decreasing spots. the excess or deficiency is stated in percentages of the average. rainfall data from walker: sunspots and rainfall.] [illustration: _fig. b. relative rainfall at times of increasing and decreasing sunspots._ heavy shading, more rain with increasing spots. light shading, more rain with decreasing spots. no data for unshaded areas. figures indicate percentages of the average rainfall by which the rainfall during periods of increasing spots exceeds or falls short of rainfall during periods of decreasing spots. the excess or deficiency is stated in percentages of the average. rainfall data from walker: sunspots and rainfall.] even more clear is the evidence from other regions where storms increase at times of many sunspots. one such region includes the southwestern united states, while another is the mediterranean region and the semi-arid or desert parts of asia farther east. in these regions innumerable ruins and other lines of evidence show that at the climax of each climatic pulsation there was more storminess and rainfall than at present, just as there now is when the sun is most active. in still earlier times, while ice was accumulating farther north, the basins of these semi-arid regions were filled with lakes whose strands still remain to tell the tale of much-increased rainfall and presumable storminess. if we go back still further in geological times to the permian glaciation, the areas where ice accumulated most abundantly appear to be the regions where tropical hurricanes produce the greatest rainfall and the greatest lowering of temperature at times of many sunspots. from these and many other lines of evidence it seems probable that historic pulsations and glacial fluctuations are nothing more than sunspot cycles on a large scale. it is one of the fundamental rules of science to reason from the known to the unknown, from the near to the far, from the present to the past. hence it seems advisable to investigate whether any of the climatic phenomena of the past may have arisen from an intensification of the solar conditions which now appear to give rise to similar phenomena on a small scale. the rest of this chapter will be devoted to a _résumé_ of certain tentative conclusions which have no bearing on the main part of this book, but which apply to the closing chapters. there we shall inquire into the periodicity of the climatic phenomena of geological times, and shall ask whether there is any reason to suppose that the sun's activity has exhibited similar periodicity. this leads to an investigation of the possible causes of disturbances in the sun's atmosphere. it is generally assumed that sunspots, solar prominences, the bright clouds known as faculæ, and other phenomena denoting a perturbed state of the solar atmosphere, are due to some cause within the sun. yet the limitation of these phenomena, especially the sunspots, to restricted latitudes, as has been shown in _earth and sun_, does not seem to be in harmony with an internal solar origin, even though a banded arrangement may be normal for a rotating globe. the fairly regular periodicity of the sunspots seems equally out of harmony with an internal origin. again, the solar atmosphere has two kinds of circulation, one the so-called "rice grains," and the other the spots and their attendant phenomena. now the rice grains present the appearance that would be expected in an atmospheric circulation arising from the loss of heat by the outer part of a gaseous body like the sun. for these reasons and others numerous good thinkers from wolf to schuster have held that sunspots owe their periodicity to causes outside the sun. the only possible cause seems to be the planets, acting either through gravitation, through forces of an electrical origin, or through some other agency. various new investigations which are described in _earth and sun_ support this conclusion. the chief difficulty in accepting it hitherto has been that although jupiter, because of its size, would be expected to dominate the sunspot cycle, its period of . years has not been detected. the sunspot cycle has appeared to average . years in length, and has been called the -year cycle. nevertheless, a new analysis of the sunspot data shows that when attention is concentrated upon the major maxima, which are least subject to retardation or acceleration by other causes, a periodicity closely approaching that of jupiter is evident. moreover, when the effects of jupiter, saturn, and the other planets are combined, they produce a highly variable curve which has an extraordinary resemblance to the sunspot curve. the method by which the planets influence the sun's atmosphere is still open to question. it may be through tides, through the direct effect of gravitation, through electro-magnetic forces, or in some other way. whichever it may be, the result may perhaps be slight differences of atmospheric pressure upon the sun. such differences may set in motion slight whirling movements analogous to terrestrial storms, and these presumably gather momentum from the sun's own energy. since the planetary influences vary in strength because of the continuous change in the relative distances and positions of the planets, the sun's atmosphere appears to be swayed by cyclonic disturbances of varying degrees of severity. the cyclonic disturbances known as sunspots have been proved by hale to become more highly electrified as they increase in intensity. at the same time hot gases presumably well up from the lower parts of the solar atmosphere and thereby cause the sun to emit more heat. thus by one means or another, the earth's atmosphere appears to be set in commotion and cycles of climate are inaugurated. if the preceding reasoning is correct, any disturbance of the solar atmosphere must have an effect upon the earth's climate. if the disturbance were great enough and of the right nature it might produce a glacial epoch. the planets are by no means the only bodies which act upon the sun, for that body sustains a constantly changing relation to millions of other celestial bodies of all sizes up to vast universes, and at all sorts of distances. if the sun and another star should approach near enough to one another, it is certain that the solar atmosphere would be disturbed much more than at present. here we must leave the cyclonic hypothesis of climate and must refer the reader once more to _earth and sun_ for fuller details. in the rest of this book we shall discuss the nature of the climatic changes of past times and shall inquire into their relation to the various climatic hypotheses mentioned in the last two chapters. then we shall inquire into the possibility that the solar system has ever been near enough to any of the stars to cause appreciable disturbances of the solar atmosphere. we shall complete our study by investigating the vexed question of why movements of the earth's crust, such as the uplifting of continents and mountain chains, have generally occurred at the same time as great climatic fluctuations. this would not be so surprising were it not that the climatic phenomena appear to have consisted of highly complex cycles while the uplift has been a relatively steady movement in one direction. we shall find some evidence that the solar disturbances which seem to cause climatic changes also have a relation to movements of the crust. footnotes: [footnote : the so-called sunspot numbers to which reference is made again and again in this book are based on a system devised by wolf and revised by a. wolfer. the number and size of the spots are both taken into account. the numbers from to may be found in the monthly weather review for april, , and from to in the same journal for .] chapter v the climate of history[ ] we are now prepared to consider the climate of the past. the first period to claim attention is the few thousand years covered by written history. strangely enough, the conditions during this time are known with less accuracy than are those of geological periods hundreds of times more remote. yet if pronounced changes have occurred since the days of the ancient babylonians and since the last of the post-glacial stages, they are of great importance not only because of their possible historic effects, but because they bridge the gap between the little variations of climate which are observable during a single lifetime and the great changes known as glacial epochs. only by bridging the gap can we determine whether there is any genetic relation between the great changes and the small. a full discussion of the climate of historic times is not here advisable, for it has been considered in detail in numerous other publications.[ ] our most profitable course would seem to be to consider first the general trend of opinion and then to take up the chief objections to each of the main hypotheses. in the hot debate over this problem during recent decades the ideas of geographers seem to have gone through much the same metamorphosis as have those of geologists in regard to the climate of far earlier times. as every geologist well knows, at the dawn of geology people believed in climatic uniformity--that is, it was supposed that since the completion of an original creative act there had been no important changes. this view quickly disappeared and was superseded by the hypothesis of progressive cooling and drying, an hypothesis which had much to do with the development of the nebular hypothesis, and which has in turn been greatly strengthened by that hypothesis. the discovery of evidence of widespread continental glaciation, however, necessitated a modification of this view, and succeeding years have brought to light a constantly increasing number of glacial, or at least cool, periods distributed throughout almost the whole of geological time. moreover, each year, almost, brings new evidence of the great complexity of glacial periods, epochs, and stages. thus, for many decades, geologists have more and more been led to believe that in spite of surprising uniformity, when viewed in comparison with the cosmic possibilities, the climate of the past has been highly unstable from the viewpoint of organic evolution, and its changes have been of all degrees of intensity. geographers have lately been debating the reality of historic changes of climate in the same way in which geologists debated the reality of glacial epochs and stages. several hypotheses present themselves but these may all be grouped under three headings; namely, the hypotheses of ( ) progressive desiccation, ( ) climatic uniformity, and ( ) pulsations. the hypothesis of progressive desiccation has been widely advocated. in many of the drier portions of the world, especially between ° and ° from the equator, and preëminently in western and central asia and in the southwestern united states, almost innumerable facts seem to indicate that two or three thousand years ago the climate was distinctly moister than at present. the evidence includes old lake strands, the traces of desiccated springs, roads in places now too dry for caravans, other roads which make detours around dry lake beds where no lakes now exist, and fragments of dead forests extending over hundreds of square miles where trees cannot now grow for lack of water. still stronger evidence is furnished by ancient ruins, hundreds of which are located in places which are now so dry that only the merest fraction of the former inhabitants could find water. the ruins of palmyra, in the syrian desert, show that it must once have been a city like modern damascus, with one or two hundred thousand inhabitants, but its water supply now suffices for only one or two thousand. all attempts to increase the water supply have had only a slight effect and the water is notoriously sulphurous, whereas in the former days, when it was abundant, it was renowned for its excellence. hundreds of pages might be devoted to describing similar ruins. some of them are even more remarkable for their dryness than is niya, a site in the tarim desert of chinese turkestan. yet there the evidence of desiccation within years is so strong that even so careful and conservative a man as hann,[ ] pronounces it "überzeugend." a single quotation from scores that might be used will illustrate the conclusions of some of the most careful archæologists.[ ] among the regions which were once populous and highly civilized, but which are now desert and deserted, there are few which were more closely connected with the beginnings of our own civilization than the desert parts of syria and northern arabia. it is only of recent years that the vast extent and great importance of this lost civilization has been fully recognized and that attempts have been made to reduce the extent of the unexplored area and to discover how much of the territory which has long been known as desert was formerly habitable and inhabited. the results of the explorations of the last twenty years have been most astonishing in this regard. it has been found that practically all of the wide area lying between the coast range of the eastern mediterranean and the euphrates, appearing upon the maps as the syrian desert, an area embracing somewhat more than , square miles, was more thickly populated than any area of similar dimensions in england or in the united states is today if one excludes the immediate vicinity of the large modern cities. it has also been discovered that an enormous desert tract lying to the east of palestine, stretching eastward and southward into the country which we know as arabia, was also a densely populated country. how far these settled regions extended in antiquity is still unknown, but the most distant explorations in these directions have failed to reach the end of ruins and other signs of former occupation. the traveler who has crossed the settled, and more or less populous, coast range of northern syria and descended into the narrow fertile valley of the orontes, encounters in any farther journey toward the east an irregular range of limestone hills lying north and south and stretching to the northeast almost halfway to the euphrates. these hills are about , feet high, rising in occasional peaks from , to , feet above sea level. they are gray and unrelieved by any visible vegetation. on ascending into the hills the traveler is astonished to find at every turn remnants of the work of men's hands, paved roads, walls which divided fields, terrace walls of massive structure. presently he comes upon a small deserted and partly ruined town composed of buildings large and small constructed of beautifully wrought blocks of limestone, all rising out of the barren rock which forms the ribs of the hills. if he mounts an eminence in the vicinity, he will be still further astonished to behold similar ruins lying in all directions. he may count ten or fifteen or twenty, according to the commanding position of his lookout. from a distance it is often difficult to believe that these are not inhabited places; but closer inspection reveals that the gentle hand of time or the rude touch of earthquake has been laid upon every building. some of the towns are better preserved than others; some buildings are quite perfect but for their wooden roofs which time has removed, others stand in picturesque ruins, while others still are level with the ground. on a far-off hilltop stands the ruin of a pagan temple, and crowning some lofty ridge lie the ruins of a great christian monastery. mile after mile of this barren gray country may be traversed without encountering a single human being. day after day may be spent in traveling from one ruined town to another without seeing any green thing save a terebinth tree or two standing among the ruins, which have sent their roots down into earth still preserved in the foundations of some ancient building. no soil is visible anywhere except in a few pockets in the rock from which it could not be washed by the torrential rains of the wet season; yet every ruin is surrounded with the remains of presses for the making of oil and wine. only one oasis has been discovered in these high plateaus. passing eastward from this range of hills, one descends into a gently rolling country that stretches miles away toward the euphrates. at the eastern foot of the hills one finds oneself in a totally different country, at first quite fertile and dotted with frequent villages of flat-roofed houses. here practically all the remains of ancient times have been destroyed through ages of building and rebuilding. beyond this narrow fertile strip the soil grows drier and more barren, until presently another kind of desert is reached, an undulating waste of dead soil. few walls or towers or arches rise to break the monotony of the unbroken landscape; but the careful explorer will find on closer examination that this region was more thickly populated in antiquity even than the hill country to the west. every unevenness of the surface marks the site of a town, some of them cities of considerable extent. we may draw certain very definite conclusions as to the former conditions of the country itself. there was soil upon the northern hills where none now exists, for the buildings now show unfinished foundation courses which were not intended to be seen; the soil in depressions without outlets is deeper than it formerly was; there are hundreds of olive and wine presses in localities where no tree or vine could now find footing; and there are hillsides with ruined terrace walls rising one above the other with no sign of earth near them. there was also a large natural water supply. in the north as well as in the south we find the dry beds of rivers, streams, and brooks with sand and pebbles and well-worn rocks but no water in them from one year's end to the other. we find bridges over these dry streams and crudely made washing boards along their banks directly below deserted towns. many of the bridges span the beds of streams that seldom or never have water in them and give clear evidence of the great climatic changes that have taken place. there are well heads and well houses, and inscriptions referring to springs; but neither wells nor springs exist today except in the rarest instances. many of the houses had their rock-hewn cisterns, never large enough to have supplied water for more than a brief period, and corresponding to the cisterns which most of our recent forefathers had which were for convenience rather than for dependence. some of the towns in southern syria were provided with large public reservoirs, but these are not large enough to have supplied water to their original populations. the high plateaus were of course without irrigation; but there are no signs, even in the lower flatter country, that irrigation was ever practiced; and canals for this purpose could not have completely disappeared. there were forests in the immediate vicinity, forests producing timbers of great length and thickness; for in the north and northeast practically all the buildings had wooden roofs, wooden intermediate floors, and other features of wood. costly buildings, such as temples and churches, employed large wooden beams; but wood was used in much larger quantities in private dwellings, shops, stables, and barns. if wood had not been plentiful and cheap--which means grown near by--the builders would have adopted the building methods of their neighbors in the south, who used very little wood and developed the most perfect type of lithic architecture the world has ever seen. and here there exists a strange anomaly: northern syria, where so much wood was employed in antiquity, is absolutely treeless now; while in the mountains of southern syria, where wood must have been scarce in antiquity to have forced upon the inhabitants an almost exclusive use of stone, there are still groves of scrub oak and pine, and travelers of half a century ago reported large forests of chestnut trees.[ ] it is perfectly apparent that large parts of syria once had soil and forests and springs and rivers, while it has none of these now, and that it had a much larger and better distributed rainfall in ancient times than it has now. professor butler's careful work is especially interesting because of its contrast to the loose statements of those who believe in climatic uniformity. so far as i am aware, no opponent of the hypothesis of climatic changes has ever even attempted to show by careful statistical analysis that the ancient water supply of such ruins was no greater than that of the present. the most that has been done is to suggest that there may have been sources of water which are now unknown. of course, this might be true in a single instance, but it could scarcely be the case in many hundreds or thousands of ruins. although the arguments in favor of a change of climate during the last two thousand years seem too strong to be ignored, their very strength seems to have been a source of error. a large number of people have jumped to the conclusion that the change which appears to have occurred in certain regions occurred everywhere, and that it consisted of a gradual desiccation. many observers, quite as careful as those who believe in progressive desiccation, point to evidences of aridity in past times in the very regions where the others find proof of moisture. lakes such as the caspian sea fell to such a low level that parts of their present floors were exposed and were used as sites for buildings whose ruins are still extant. elsewhere, for instance in the tian-shan mountains, irrigation ditches are found in places where irrigation never seems to be necessary at present. in syria and north africa during the early centuries of the christian era the romans showed unparalleled activity in building great aqueducts and in watering land which then apparently needed water almost as much as it does today. evidence of this sort is abundant and is as convincing as is the evidence of moister conditions in the past. it is admirably set forth, for example, in the comprehensive and ably written monograph of leiter on the climate of north africa.[ ] the evidence cited there and elsewhere has led many authors strongly to advocate the hypothesis of climatic uniformity. they have done exactly as have the advocates of progressive change, and have extended their conclusions over the whole world and over the whole of historic times. the hypotheses of climatic uniformity and of progressive change both seem to be based on reliable evidence. they may seem to be diametrically opposed to one another, but this is only when there is a failure to group the various lines of evidence according to their dates, and according to the types of climate in which they happen to be located. when the facts are properly grouped in both time and space, it appears that evidence of moist conditions in the historic mediterranean lands is found during certain periods; for instance, four or five hundred years before christ, at the time of christ, and a. d. the other kind of evidence, on the contrary, culminates at other epochs, such as about b. c. and in the seventh and thirteenth centuries after christ. it is also found during the interval from the culmination of a moist epoch to the culmination of a dry one, for at such times the climate was growing drier and the people were under stress. this was seemingly the case during the period from the second to the fourth centuries of our era. north africa and syria must then have been distinctly better watered than at present, as appears from butler's vivid description; but they were gradually becoming drier, and the natural effect on a vigorous, competent people like the romans was to cause them to construct numerous engineering works to provide the necessary water. the considerations which have just been set forth have led to a third hypothesis, that of pulsatory climatic changes. according to this, the earth's climate is not stable, nor does it change uniformly in one direction. it appears to fluctuate back and forth not only in the little waves which we see from year to year or decade to decade, but in much larger waves, which take hundreds of years or even a thousand. these in turn seem to merge into and be imposed on the greater waves which form glacial stages, glacial epochs, and glacial periods. at the present time there seems to be no way of determining whether the general tendency is toward aridity or toward glaciation. the seventh century of our era was apparently the driest time during the historic period--distinctly drier than the present--but the thirteenth century was almost equally dry, and the twelfth or thirteenth before christ may have been very dry. the best test of an hypothesis is actual measurements. in the case of the pulsatory hypothesis we are fortunately able to apply this test by means of trees. the growth of vegetation depends on many factors--soil, exposure, wind, sun, temperature, rain, and so forth. in a dry region the most critical factor in determining how a tree's growth shall vary from year to year is the supply of moisture during the few months of most rapid growth.[ ] the work of douglass[ ] and others has shown that in arizona and california the thickness of the annual rings affords a reliable indication of the amount of moisture available during the period of growth. this is especially true when the growth of several years is taken as the unit and is compared with the growth of a similar number of years before or after. where a long series of years is used, it is necessary to make corrections to eliminate the effects of age, but this can be done by mathematical methods of considerable accuracy. it is difficult to determine whether the climate at the beginning and end of a tree's life was the same, but it is easily possible to determine whether there have been pulsations while the tree was making its growth. if a large number of trees from various parts of a given district all formed thick rings at a certain period and then formed thin ones for a hundred years, after which the rings again become thick, we seem to be safe in concluding that the trees have lived through a long, dry period. the full reasons for this belief and details as to the methods of estimating climate from tree growth are given in _the climatic factor_. the results set forth in that volume may be summarized as follows: during the years and , under the auspices of the carnegie institution of washington, measurements were made of the thickness of the rings of growth on the stumps of about sequoia trees in california. these trees varied in age from to nearly years. the great majority were over years of age, seventy-nine were over years, and three over . even where only a few trees are available the record is surprisingly reliable, except where occasional accidents occur. where the number approximates , accidental variations are largely eliminated and we may accept the record with considerable confidence. accordingly, we may say that in california we have a fairly accurate record of the climate for years and an approximate record for years more. the final results of the measurements of the california trees are shown in fig. , where the climatic variations for years in california are indicated by the solid line. the high parts of the line indicate rainy conditions, the low parts, dry. an examination of this curve shows that during years there have apparently been climatic variations more important than any which have taken place during the past century. in order to bring out the details more clearly, the more reliable part of the california curve, from b. c. to the present time, has been reproduced in fig. . this is identical with the corresponding part of fig. , except that the vertical scale is three times as great. [illustration: _fig. . changes of climate in california (solid line) and in western and central asia (dotted line)._ note. the curves of figs. and are reproduced as published in _the solar hypothesis_ in . later work, however, has indicated that in the asiatic curve the dash lines, which were tentatively inserted in , are probably more nearly correct than the dotted lines. still further evidence indicates that the asiatic curve is nearly like that of california in its main features.] the curve of tree growth in california seems to be a true representation of the general features of climatic pulsations in the mediterranean region. this conclusion was originally based on the resemblance between the solid line of fig. , representing tree growth, and the dotted line representing changes of climate in the eastern mediterranean region as inferred from the study of ruins and of history before any work on this subject had been done in america.[ ] the dotted line is here reproduced for its historical significance as a stage in the study of climatic changes. if it were to be redrawn today on the basis of the knowledge acquired in the last twelve years, it would be much more like the tree curve. for example, the period of aridity suggested by the dip of the dotted line about a. d. was based largely on professor butler's data as to the paucity of inscriptions and ruins dating from that period in syria. in the recent article, from which a long quotation has been given, he shows that later work proves that there is no such paucity. on the other hand, it has accentuated the marked and sudden decay in civilization and population which occurred shortly after a. d. he reached the same conclusion to which the present authors had come on wholly different grounds, namely, that the dip in the dotted line about a. d. is not warranted, whereas the dip about a. d. is extremely important. in similar fashion the work of stein[ ] in central asia makes it clear that the contrast between the water supply about b.c. and in the preceding and following centuries was greater than was supposed on the basis of the scanty evidence available when the dotted line of fig. was drawn in . [illustration: _fig. . changes in california climate for years, as measured by growth of sequoia trees._ fig. is the same as the later portion of fig. , except that the vertical scale has been magnified threefold. it seems probable that the dotted line at the right is more nearly correct than the solid line. during the thirty years since the end of the curve the general tendency appears in general to have been somewhat upward.] since the curve of the california trees is the only continuous and detailed record yet available for the climate of the last three thousand years, it deserves most careful study. it is especially necessary to determine the degree of accuracy with which the growth of the trees represents ( ) the local rainfall and ( ) the rainfall of remote regions such as palestine. perhaps the best way to determine these matters is the standard mathematical method of correlation coefficients. if two phenomena vary in perfect unison, as in the case of the turning of the wheels and the progress of an automobile when the brakes are not applied, the correlation coefficient is . , being positive when the automobile goes forward and negative when it goes backward. if there is no relation between two phenomena, as in the case of the number of miles run by a given automobile each year and the number of chickens hatched in the same period, the coefficient is zero. a partial relationship where other factors enter into the matter is represented by a coefficient between zero and one, as in the case of the movement of the automobile and the consumption of gasoline. in this case the relation is very obvious, but is modified by other factors, including the roughness and grade of the road, the amount of traffic, the number of stops, the skill of the driver, the condition and load of the automobile, and the state of the weather. such partial relationships are the kind for which correlation coefficients are most useful, for the size of the coefficients shows the relative importance of the various factors. a correlation coefficient four times the probable error, which can always be determined by a formula well known to mathematicians, is generally considered to afford evidence of some kind of relation between two phenomena. when the ratio between coefficient and error rises to six, the relationship is regarded as strong. few people would question that there is a connection between tree growth and rainfall, especially in a climate with a long summer dry season like that of california. but the growth of the trees also depends on their position, the amount of shading, the temperature, insect pests, blights, the wind with its tendency to break the branches, and a number of other factors. moreover, while rain commonly favors growth, great extremes are relatively less helpful than more moderate amounts. again, the roots of a tree may tap such deep sources of water that neither drought nor excessive rain produces much effect for several years. hence in comparing the growth of the huge sequoias with the rainfall we should expect a correlation coefficient high enough to be convincing, but decidedly below . . unfortunately there is no record of the rainfall where the sequoias grow, the nearest long record being that of sacramento, nearly miles to the northwest and close to sea level instead of at an altitude of about feet. applying the method of correlation coefficients to the annual rainfall of sacramento and the growth of the sequoias from to , we obtain the results shown in table . the trees of section a of the table grew in moderately dry locations although the soil was fairly deep, a condition which seems to be essential to sequoias. in this case, as in all the others, the rainfall is reckoned from july to june, which practically means from october to may, since there is almost no summer rain. thus the tree growth in is compared with the rainfall of the preceding rainy season, - , or of several preceding rainy seasons as the table indicates. +-------------------------------------------------------------------+ | table | | | | correlation coefficients between rainfall and | | growth of sequoias in california[ ] | | | | (_r_) = _correlation coefficient_ | | (_e_) = _probable error_ | | (_r_/_e_) = _ratio of coefficient to probable error_ | | | | a. sacramento rainfall and growth of sequoias in dry | | locations, - | | | | (_r_) (_e_) (_r_/_e_) | | ------ ------ ----- | | year of rainfall - . ± . . | | years of rainfall + . ± . . | | years of rainfall + . ± . . | | years of rainfall + . ± . . | | | | b. sacramento rainfall and growth of sequoias mostly in | | moist locations, - | | | | years of rainfall + . ± . . | | years of rainfall + . ± . . | | years of rainfall + . ± . . | | years of rainfall + . ± . . | | years of rainfall + . ± . . | | years of rainfall (+ . ) ± . . | | years of rainfall + . ± . . | | years of rainfall + . ± . . | | | | c. sacramento rainfall and growth of sequoias in moist | | locations, - | | | | years of rainfall + . ± . . | | | | d. annual sequoia growth and rainfall of preceding years | | at stations on southern pacific railroad | | | | = _years_ | | = _altitude_ (_feet_) | | = _rainfall_ (_inches_) | | = _approximate distance from sequoias_ (_miles_) | | | | (_r_) (_e_) (_r_/_e_) | | --------- ---- ----- --- ------ ------ --------- | | sacramento, - . + . ± . . | | colfax, - . + . ± . . | | summit, - . + . ± . . | | truckee, - . + . ± . . | | boca, - . + . ± . . | | winnemucca, - . + . ± . . | | | +-------------------------------------------------------------------+ in the first line of section a a correlation coefficient of only - . , which is scarcely six-tenths of the probable error, means that there is no appreciable relation between the rainfall of a given season and the growth during the following spring and summer. the roots of the sequoias probably penetrate so deeply that the rain and melted snow of the spring months do not sink down rapidly enough to influence the trees before the growing season comes to an end. the precipitation of two preceding seasons, however, has some effect on the trees, as appears in the second line of section a, where the correlation coefficient is + . , or . times the probable error. when the rainfall of three seasons is taken into account the coefficient rises to + . , or . times the probable error, while with four years of rainfall the coefficient begins to fall off. thus the growth of these eighteen sequoias on relatively dry slopes appears to have depended chiefly on the rainfall of the second and third preceding rainy seasons. the growth in , for example, depended largely on the rainfall in the rainy seasons of - and - . section b of the table shows that with trees, growing chiefly in moist depressions where the water supply is at a maximum, the correlation between growth and rainfall, + . for ten years' rainfall, is even higher than with the dry trees. the seepage of the underground water is so slow that not until four years' rainfall is taken into account is the correlation coefficient more than four times the probable error. when only the trees growing in moist locations are employed, the coefficient between tree growth and the rainfall for ten years rises to the high figure of + . , or . times the probable error, as appears in section c. these figures, as well as many others not here published, make it clear that the curve of sequoia growth from to affords a fairly close indication of the rainfall at sacramento, provided allowance be made for a delay of three to ten years due to the fact that the moisture in the soil gradually seeps down the mountain-sides and only reaches the sequoias after a considerable interval. if a rainfall record were available for the place where the trees actually grow, the relationship would probably be still closer. the record at fresno, for example, bears out this conclusion so far as it goes. but as fresno lies at a low altitude and its rainfall is of essentially the sacramento type, its short record is of less value than that of sacramento. the only rainfall records among the sierras at high levels, where the rainfall and temperature are approximately like those of the sequoia region, are found along the main line of the southern pacific railroad. this runs from oakland northeastward seventy miles across the open plain to sacramento, then another seventy miles, as the crow flies, through colfax and over a high pass in the sierras at summit, next twenty miles or so down through truckee to boca, on the edge of the inland basin of nevada, and on northeastward another miles to winnemucca, where it turns east toward ogden and salt lake city. section d of table shows the correlation coefficients between the rainfall along the railroad and the growth of the sequoias. at sacramento, which lies fairly open to winds from the pacific and thus represents the general climate of central california, the coefficient is nearly five times the probable error, thus indicating a real relation to sequoia growth. then among the foothills of the sierras at colfax, the coefficient drops till it is scarcely larger than the probable error. it rises rapidly, however, as one advances among the mountains, until at boca it attains the high figure of + . or eight times the probable error, and continues high in the dry area farther east. in other words the growth of the sequoias is a good indication of the rainfall where the trees grow and in the dry region farther east. in order to determine the degree to which the sequoia record represents the rainfall of other regions, let us select jerusalem for comparison. the reasons for this selection are that jerusalem furnishes the only available record that satisfies the following necessary conditions: ( ) its record is long enough to be important; ( ) it is located fairly near the latitude of the sequoias, °n versus °n; ( ) it is located in a similar type of climate with winter rains and a long dry summer; ( ) it lies well above sea level ( feet) and somewhat back from the seacoast, thus approximating although by no means duplicating the condition of the sequoias; and ( ) it lies in a region where the evidence of climatic changes during historic times is strongest. the ideal place for comparison would be the valley in which grow the cedars of lebanon. those trees resemble the sequoias to an extraordinary degree, not only in their location, but in their great age. some day it will be most interesting to compare the growth of these two famous groups of old trees. +-------------------------------------------------------------------+ | table | | | | correlation coefficients between | | rainfall records in california | | and jerusalem | | | | (_r_) = _correlation coefficient_ | | (_e_) = _probable error_ | | (_r_/_e_) = _ratio of coefficient to probable error_ | | | | a. jerusalem rainfall for years and various groups of | | sequoias[ ] | | | | (_r_) (_e_) (_r_/_e_)| | ------ ----- ---------| | trees measured by douglass + . ± . . | | trees, moist locations, groups ia, | | iia, iiia, va + . ± . . | | trees, in moist locations, in | | dry, i, ii, iii + . ± . . | | trees, in moist locations, in | | dry, i, ii, iii, v + . ± . . | | | | b. rainfall at jerusalem and at stations in california and nevada | | | | = _altitude_ (_feet_) | | = _years_ | | | | -- years -- -- years -- | | (_r_) (_r_/_e_) (_r_) (_r_/_e_) | | ---- --------- ------ ------- ------ ------- | | sacramento, - + . . + . . | | colfax, - + . . + . . | | summit, - + . . + . . | | truckee, - + . . + . . | |[a]boca, - + . . + . . | | winnemucca, - + . . + . . | | san bernardino, - + . . + . . | | | | c. rainfall for years at california and nevada stations, | | - | | | | (_r_) (_r_/_e_) | | ------ ------- | | sacramento and san bernardino + . . | | san bernardino and winnemucca + . . | | | +-------------------------------------------------------------------+ the correlation coefficients for the sequoia growth and the rainfall at jerusalem are given in section a, table . they are so high and so consistent that they scarcely leave room for doubt that where a hundred or more sequoias are employed, as in fig. , their curve of growth affords a good indication of the fluctuations of climate in western asia. the high coefficient for the eleven trees measured by douglass suggests that where the number of trees falls as low as ten, as in the part of fig. from to b. c., the relation between tree growth and rainfall is still close even when only one year's growth is considered. where the unit is ten years of growth, as in figs. and , the accuracy of the tree curve as a measure of rainfall is much greater than when a single year is used as in table . when the unit is raised to thirty years, as in the smoothed part of fig. previous to b. c., even four trees, as from to , probably give a fair approximation to the general changes in rainfall, while a single tree prior to b. c. gives a rough indication. table shows a peculiar feature in the fact that the correlations of section a between tree growth and the rainfall of jerusalem are decidedly higher than those between the rainfall in the two regions. only at sacramento and boca are the rainfall coefficients high enough to be conclusive. this, however, is not surprising, for even between sacramento and san bernardino, only miles apart, the correlation coefficient for the rainfall by three-year periods is only . times the probable error, as appears in section c of table , while between san bernardino and winnemucca miles away, the corresponding figure drops to . . it must be remembered that in some respects the growth of the sequoias is a much better record of rainfall than are the records kept by man. the human record is based on the amount of water caught by a little gauge a few inches in diameter. every gust of wind detracts from the accuracy of the record; a mile away the rainfall may be double what it is at the gauge. each sequoia, on the other hand, draws its moisture from an area thousands of times as large as a rain gauge. moreover, the trees on which figs. and are based were scattered over an area fifty miles long and several hundred square miles in extent. hence they represent the summation of the rainfall over an area millions of times as large as that of a rain gauge. this fact and the large correlation coefficients between sequoia growth and jerusalem rainfall should be considered in connection with the fact that all the coefficients between the rainfall of california and nevada and that of jerusalem are positive. if full records of the complete rainfall of california and nevada on the one hand and of the eastern mediterranean region on the other were available for a long period, they would probably agree closely. just how widely the sequoias can be used as a measure of the climate of the past is not yet certain. in some regions, as will shortly be explained, the climatic changes seem to have been of an opposite character from those of california. in others the californian or eastern mediterranean type of change seems sometimes to prevail but is not always evident. for example, at malta the rainfall today shows a distinct relation to that of jerusalem and to the growth of the sequoias. but the correlation coefficient between the rainfall of eight-year periods at naples, a little farther north, and the growth of the sequoias at the end of the periods is - . , or only . times the probable error and much too small to be significant. this is in harmony with the fact that although naples has summer droughts, they are not so pronounced as in california and palestine, and the prevalence of storms is much greater. jerusalem receives only per cent of its rain in the seven months from april to october, and sacramento , while malta receives per cent and naples . nevertheless, there is some evidence that in the past the climatic fluctuations of southern italy followed nearly the same course as those of california and palestine. this apparent discrepancy seems to be explained by our previous conclusion that changes of climate are due largely to a shifting of storm tracks. when sunspots are numerous the storms which now prevail in northern italy seem to be shifted southward and traverse the mediterranean to palestine just as similar storms are shifted southward in the united states. this perhaps accounts for the agreement between the sequoia curve and the agricultural and social history of rome from about b. c. to a. d., as explained in _world power and evolution_. for our present purposes, however, the main point is that since rainfall records have been kept the fluctuations of climate indicated by the growth of the sequoias have agreed closely with fluctuations in the rainfall of the eastern mediterranean region. presumably the same was true in the past. in that case, the sequoia curve not only is a good indication of climatic changes or pulsations in regions of similar climate, but may serve as a guide to coincident but different changes in regions of other types. an enormous body of other evidence points to the same conclusion. it indicates that while the average climate of the present is drier than that of the past in regions having the mediterranean type of winter rains and summer droughts, there have been pronounced pulsations during historic times so that at certain times there has actually been greater aridity than at present. this conclusion is so important that it seems advisable to examine the only important arguments that have been raised against it, especially against the idea that the general rainfall of the eastern mediterranean was greater in the historic past than at present. the first objection is the unquestionable fact that droughts and famines have occurred at periods which seem on other evidence to have been moister than the present. this argument has been much used, but it seems to have little force. if the rainfall of a given region averages thirty inches and varies from fifteen to forty-five, a famine will ensue if the rainfall drops for a few years to the lower limit and does not rise much above twenty for a few years. if the climate of the place changes during the course of centuries, so that the rainfall averages only twenty inches, and ranges from seven to thirty-five, famine will again ensue if the rainfall remains near ten inches for a few years. the ravages of the first famine might be as bad as those of the second. they might even be worse, because when the rainfall is larger the population is likely to be greater and the distress due to scarcity of food would affect a larger number of people. hence historic records of famines and droughts do not indicate that the climate was either drier or moister than at present. they merely show that at the time in question the climate was drier than the normal for that particular period. the second objection is that deserts existed in the past much as at present. this is not a real objection, however, for, as we shall see more fully, some parts of the world suffer one kind of change and others quite the opposite. moreover, deserts have always existed, and when we talk of a change in their climate we merely mean that their boundaries have shifted. a concrete example of the mistaken use of ancient dryness as proof of climatic uniformity is illustrated by the march of alexander from india to mesopotamia. hedin gives an excellent presentation of the case in the second volume of his _overland to india_. he shows conclusively that alexander's army suffered terribly from lack of water and provisions. this certainly proves that the climate was dry, but it by no means indicates that there has been no change from the past to the present. we do not know whether alexander's march took place during an especially dry or an especially wet year. in a desert region like makran, in southern persia and beluchistan, where the chief difficulties occurred, the rainfall varies greatly from year to year. we have no records from makran, but the conditions there are closely similar to those of southern arizona and new mexico. in and the rainfall for five stations in that region was as follows: +------------------------------------------------------------+ | | | _mean rainfall | | during period | | _ _ _ _ since | | observations | | began_ | | yuma, arizona, . . . | | phoenix, arizona, . . . | | tucson, arizona, . . . | | lordsburg, new mexico, . . . | | el paso, texas (on new | | mexico border), . . . | | ---- ----- ----- | | average, . . . | | | +------------------------------------------------------------+ these stations are distributed over an area nearly miles east and west. manifestly a traveler who spent the year in that region would have had much more difficulty in finding water and forage than one who traveled in the same places in . during the rainfall was per cent less than the average, and during it was per cent more than the average. let us suppose, for the sake of argument, that the average rainfall of southeastern persia is six inches today and was ten inches in the days of alexander. if the rainfall from year to year varied as much in the past in persia as it does now in new mexico and arizona, the rainfall during an ancient dry year, corresponding in character to , would have been about . inches. on the other hand, if we suppose that the rainfall then averaged less than at present,--let us say four inches,--a wet year corresponding to in the american deserts might have had a rainfall of about ten inches. this being the case, it is clear that our estimate of what alexander's march shows as to climate must depend largely on whether b. c. was a wet year or a dry year. inasmuch as we know nothing about this, we must fall back on the fact that a large army accomplished a journey in a place where today even a small caravan usually finds great difficulty in procuring forage and water. moreover, elephants were taken miles across what is now an almost waterless desert, and yet the old historians make no comment on such a feat which today would be practically impossible. these things seem more in harmony with a change of climate than with uniformity. nevertheless, it is not safe to place much reliance on them except when they are taken in conjunction with other evidence, such as the numerous ruins, which show that makran was once far more densely populated than now seems possible. taken by itself, such incidents as alexander's march cannot safely be used either as an argument for or against changes of climate. the third and strongest objection to any hypothesis of climatic changes during historic times is based on vegetation. the whole question is admirably set forth by j. w. gregory,[ ] who gives not only his own results, but those of the ablest scholars who have preceded him. his conclusions are important because they represent one of the few cases where a definite statistical attempt has been made to prove the exact condition of the climate of the past. after stating various less important reasons for believing that the climate of palestine has not changed, he discusses vegetation. the following quotation indicates his line of thought. a sentence near the beginning is italicized in order to call attention to the importance which gregory and others lay on this particular kind of evidence: some more certain test is necessary than the general conclusions which can be based upon the historical and geographical evidence of the bible. in the absence of rain gauge and thermometric records, _the most precise test of climate is given by the vegetation; and fortunately the palm affords a very delicate test of the past climate of palestine and the eastern mediterranean_.... the date palm has three limits of growth which are determined by temperature; thus it does not reach full maturity or produce ripe fruit of good quality below the mean annual temperature of °f. the isothermal of ° crosses southern algeria near biskra; it touches the northern coasts of cyrenaica near derna and passes egypt near the mouth of the nile, and then bends northward along the coast lands of palestine. to the north of this line the date palm grows and produces fruit, which only ripens occasionally, and its quality deteriorates as the temperature falls below °. between the isotherms of ° and °, limits which include northern algeria, most of sicily, malta, the southern parts of greece and northern syria, the dates produced are so unripe that they are not edible. in the next cooler zone, north of the isotherm of °, which enters europe in southwestern portugal, passes through sardinia, enters italy near naples, crosses northern greece and asia minor to the east of smyrna, the date palm is grown only for its foliage, since it does not fruit. hence at benghazi, on the north african coast, the date palm is fertile, but produces fruit of poor quality. in sicily and at algiers the fruit ripens occasionally and at rome and nice the palm is grown only as an ornamental tree. the date palm therefore affords a test of variations in mean annual temperature of three grades between ° and °. this test shows that the mean annual temperature of palestine has not altered since old testament times. the palm tree now grows dates on the coast of palestine and in the deep depression around the dead sea, but it does not produce fruit on the highlands of judea. its distribution in ancient times, as far as we can judge from the bible, was exactly the same. it grew at "jericho, the city of palm trees" (deut. xxxiv: and chron. xxviii: ), and at engedi, on the western shore of the dead sea ( chron. xx: ; sirach xxiv: ); and though the palm does not still live at jericho--the last apparently died in --its disappearance must be due to neglect, for the only climatic change that would explain it would be an increase in cold or moisture. in olden times the date palm certainly grew on the highlands of palestine; but apparently it never produced fruit there, for the bible references to the palm are to its beauty and erect growth: "the righteous shall flourish like the palm" (ps. xcii: ); "they are upright as the palm tree" (jer. x: ); "thy stature is like to a palm tree" (cant. vii: ). it is used as a symbol of victory (rev. vii: ), but never praised as a source of food. dates are not once referred to in the text of the bible, but according to the marginal notes the word translated "honey" in chron. xxxi: may mean dates.... it appears, therefore, that the date palm had essentially the same distribution in palestine in old testament times as it has now; and hence we may infer that the mean temperature was then the same as now. if the climate had been moister and cooler, the date could not have flourished at jericho. if it had been warmer, the palms would have grown freely at higher levels and jericho would not have held its distinction as _the_ city of palm trees.[ ] in the main gregory's conclusions seem to be well grounded, although even according to his data a change of ° or ° in mean temperature would be perfectly feasible. it will be noticed, however, that they apply to temperature and not to rainfall. they merely prove that two thousand years ago the mean temperature of palestine and the neighboring regions was not appreciably different from what it is today. this, however, is in no sense out of harmony with the hypothesis of climatic pulsations. students of glaciation believe that during the last glacial epoch the mean temperature of the earth as a whole was only ° or °c. lower than at present. if the difference between the climate of today and of the time of christ is a tenth as great as the difference between the climate of today and that which prevailed at the culmination of the last glacial epoch, the change in two thousand years has been of large dimensions. yet this would require a rise of only half a degree centigrade in the mean temperature of palestine. manifestly, so slight a change would scarcely be detectable in the vegetation. the slightness of changes in mean temperature as compared with changes in rainfall may be judged from a comparison of wet and dry years in various regions. for example, at berlin between and the ten most rainy years had an average precipitation of mm. and a mean temperature of . °c. on the other hand, the ten years of least rainfall had an average of mm. and a mean temperature of . °. in other words, a difference of mm., or per cent, in rainfall was accompanied by a difference of only . °c. in temperature. such contrasts between the variability of mean rainfall and mean temperature are observable not only when individual years are selected, but when much longer periods are taken. for instance, in the western gulf region of the united states the two inland stations of vicksburg, mississippi, and shreveport, louisiana, and the two maritime stations of new orleans, louisiana, and galveston, texas, lie at the margins of an area about miles long. during the ten years from to their rainfall averaged . inches,[ ] while during the ten years from to it averaged only . inches. even in a region so well watered as the gulf states, such a change-- per cent more in the first decade than in the second--is important, and in drier regions it would have a great effect on habitability. yet in spite of the magnitude of the change the mean temperature was not appreciably different, the average for the four stations being . °f. during the more rainy decade and . °f. during the less rainy decade--a difference of only . °f. it is worth noticing that in this case the wetter period was also the warmer, whereas in berlin it was the cooler. this is probably because a large part of the moisture of the gulf states is brought by winds having a southerly component. similar relationships are apparent in other places. we select jerusalem because we have been discussing palestine. at the time of writing, the data available in the _quarterly journal of the palestine exploration fund_ cover the years from - and - . among these twenty-five years the thirteen which had most rain had an average of . inches and a temperature of . °f. the twelve with least rain had . inches and a temperature of . °. a difference of per cent in rainfall was accompanied by a difference of only . °f. in temperature. the facts set forth in the preceding paragraphs seem to show that extensive changes in precipitation and storminess can take place without appreciable changes of mean temperature. if such changed conditions can persist for ten years, as in one of our examples, there is no logical reason why they cannot persist for a hundred or a thousand. the evidence of changes in climate during the historic period seems to suggest changes in precipitation much more than in temperature. hence the strongest of all the arguments against historic changes of climate seems to be of relatively little weight, and the pulsatory hypothesis seems to be in accord with all the known facts. before the true nature of climatic changes, whether historic or geologic, can be rightly understood, another point needs emphasis. when the pulsatory hypothesis was first framed, it fell into the same error as the hypotheses of uniformity and of progressive change--that is, the assumption was made that the whole world is either growing drier or moister with each pulsation. a study of the ruins of yucatan, in , and of guatemala, in , as is explained in _the climatic factor_, has led to the conclusion that the climate of those regions has changed in the opposite way from the changes which appear to have taken place in the desert regions farther south. these maya ruins in central america are in many cases located in regions of such heavy rainfall, such dense forests, and such malignant fevers that habitation is now practically impossible. the land cannot be cultivated except in especially favorable places. the people are terribly weakened by disease and are among the lowest in central america. only a hundred miles from the unhealthful forests we find healthful areas, such as the coasts of yucatan and the plateau of guatemala. here the vast majority of the population is gathered, the large towns are located, and the only progressive people are found. nevertheless, in the past the region of the forests was the home of by far the most progressive people who are ever known to have lived in america previous to the days of columbus. they alone brought to high perfection the art of sculpture; they were the only american people who invented the art of writing. it seems scarcely credible that such a people would have lived in the worst possible habitat when far more favored regions were close at hand. therefore it seems as if the climate of eastern guatemala and yucatan must have been relatively dry at some past time. the maya chronology and traditions indicate that this was probably at the same time when moister conditions apparently prevailed in the subarid or desert portions of the united states and asia. fig. shows that today at times of many sunspots there is a similar opposition between a tendency toward storminess and rain in subtropical regions and toward aridity in low latitudes near the heat equator. thus our final conclusion is that during historic times there have been pulsatory changes of climate. these changes have been of the same type in regions having similar kinds of climate, but of different and sometimes opposite types in places having diverse climates. as to the cause of the pulsations, they cannot have been due to the precession of the equinoxes nor apparently to any allied astronomical cause, for the time intervals are too short and too irregular. they cannot have been due to changes in the percentage of carbon dioxide in the atmosphere, for not even the strongest believers in the climatic efficacy of that gas hold that its amount could fluctuate in any such violent way as would be necessary to explain the pulsations shown in the california curve of tree growth. volcanic activity seems more probable as at least a partial cause, and it would be worth while to investigate the matter more fully. nevertheless, it can apparently be only a minor cause. in the first place, the main effect of a cloud of dust is to alter the temperature, but gregory's summary of the palm and the vine shows that variations in temperature are apparently of very slight importance during historic times. again, ruins on the bottoms of enclosed salt lakes, old beaches now under the water, and signs of irrigation ditches where none are now needed indicate a climate drier than the present. volcanic dust, however, cannot account for such a condition, for at present the air seems to be practically free from such dust for long periods. thus we now experience the greatest extreme which the volcanic hypothesis permits in one direction, but there have been greater extremes in the same direction. the thermal solar hypothesis is likewise unable to explain the observed phenomena, for neither it nor the volcanic hypothesis offers any explanation of why the climate varies in one way in mediterranean climates and in an opposite way in regions near the heat equator. this leaves the cyclonic hypothesis. it seems to fit the facts, for variations in cyclonic storms cause some regions to be moister and others drier than usual. at the same time the variations in temperature are slight, and are apparently different in different regions, some places growing warm when others grow cool. in the next chapter we shall study this matter more fully, for it can best be appreciated by examining the course of events in a specific century. footnotes: [footnote : much of this chapter is taken from the solar hypothesis of climatic changes; bull. geol. soc. am., vol. , .] [footnote : ellsworth huntington: explorations in turkestan, ; the pulse of asia, ; palestine and its transformation, ; the climatic factor, ; world power and evolution, .] [footnote : j. hann: klimatologie, vol. , , p. .] [footnote : h. c. butler: desert syria, the land of a lost civilization; geographical review, feb., , pp. - .] [footnote : this is due to the fact that where these forests occur, in gilead for example, the mountains to the west break down, so that the west winds with water from the mediterranean are able to reach the inner range without having lost all their water. it is one of the misfortunes of syria that its mountains generally rise so close to the sea that they shut off rainfall from the interior and cause the rain to fall on slopes too steep for easy cultivation.] [footnote : h. leiter: die frage der klimaanderung waherend geschichtlicher zeit in nordafrika. abhandl. k. k. geographischen gesellschaft, wien, , p. .] [footnote : a most careful and convincing study of this problem is embodied in an article by j. w. smith: the effects of weather upon the yield of corn; monthly weather review, vol. , , pp. - . on the basis of the yield of corn in ohio for years and in other states for shorter periods, he shows that the rainfall of july has almost as much influence on the crop as has the rainfall of all other months combined. see his agricultural meteorology, new york, .] [footnote : see chapter by a. e. douglass in the climatic factor; and his book on climatic cycles and tree-growth; carnegie inst., . also article by m. n. stewart: the relation of precipitation to tree growth, in the monthly weather review, vol. , .] [footnote : the dotted line is taken from palestine and its transformation, pp. and .] [footnote : m. a. stein: ruins of desert cathay, london, .] [footnote : in the preparation and interpretation of this table the help of mr. g. b. cressey is gratefully acknowledged.] [footnote : for the tree data used in these comparisons, see the climatic factor p. , and a. e. douglass: climatic cycles and tree growth, p. .] [footnote a: one year interpolated.] [footnote : j. w. gregory: is the earth drying up? geog. jour., vol. , , pp. - and - .] [footnote : geog. jour., vol. , pp. - .] [footnote : see a. j. henry: secular variation of precipitation in the united states; bull. am. geog. soc., vol. , , pp. - .] chapter vi the climatic stress of the fourteenth century in order to give concreteness to our picture of the climatic pulsations of historic times let us take a specific period and see how its changes of climate were distributed over the globe and how they are related to the little changes which now take place in the sunspot cycle. we will take the fourteenth century of the christian era, especially the first half. this period is chosen because it is the last and hence the best known of the times when the climate of the earth seems to have taken a considerable swing toward the conditions which now prevail when the sun is most active, and which, if intensified, would apparently lead to glaciation. it has already been discussed in _world power and evolution_, but its importance and the fact that new evidence is constantly coming to light warrant a fuller discussion. to begin with europe; according to the careful account of pettersson[ ] the fourteenth century shows a record of extreme climatic variations. in the cold winters the rivers rhine, danube, thames, and po were frozen for weeks and months. on these cold winters there followed violent floods, so that the rivers mentioned inundated their valleys. such floods are recorded in summers in the th century. there is, of course, nothing astonishing in the fact that the inundations of the great rivers of europe were more devastating to years ago than in our days, when the flow of the rivers has been regulated by canals, locks, etc.; but still the inundations in the th and th centuries must have surpassed everything of that kind which has occurred since then. in the waters of the rhine rose so high that they inundated the city of mayence and the cathedral "usque ad cingulum hominis." the walls of cologne were flooded so that they could be passed by boats in july. this occurred also in in the midst of the month of february, which is of course an unusual season for disasters of the kind. again in other years the drought was so intense that the same rivers, the danube, rhine, and others, nearly dried up, and the rhine could be forded at cologne. this happened at least twice in the same century. there is one exceptional summer of such evil record that centuries afterwards it was spoken of as "the old hot summer of ." pettersson goes on to speak of two oceanic phenomena on which the old chronicles lay greater stress than on all others: the first [is] the great storm-floods on the coast of the north sea and the baltic, which occurred so frequently that not less than nineteen floods of a destructiveness unparalleled in later times are recorded from the th century. the coastline of the north sea was completely altered by these floods. thus on january , , half of the island heligoland and many other islands were engulfed by the sea. the same fate overtook the island of borkum, torn into several islands by the storm-flood of january , which remoulded the frisian islands into their present shape, when also wendingstadt, on the island of sylt, and thiryu parishes were engulfed. this flood is known under the name of "the great man-drowning." the coasts of the baltic also were exposed to storm-floods of unparalleled violence. on november , , the island of ruden was torn asunder from rugen by the force of the waves. time does not allow me to dwell upon individual disasters of this kind, but it will be well to note that of the nineteen great floods on record eighteen occurred in the cold season between the autumnal and vernal equinoxes. the second remarkable phenomenon mentioned by the chronicles is the freezing of the entire baltic, which occurred many times during the cold winters of these centuries. on such occasions it was possible to travel with carriages over the ice from sweden to bornholm and from denmark to the german coast (lubeck), and in some cases even from gotland to the coast of estland. norlind[ ] says that "the only authentic accounts" of the complete freezing of the baltic in the neighborhood of the kattegat are in the years , , , and . of these is "much the most uncertain," while was the coldest year ever recorded, as appears from the fact that horses and sleighs crossed regularly from sweden to germany on the ice. not only central europe and the shores of the north sea were marked by climatic stress during the fourteenth century, but scandinavia also suffered. as pettersson puts it: on examining the historic (data) from the last centuries of the middle ages, dr. bull of christiania has come to the conclusion that the decay of the norwegian kingdom was not so much a consequence of the political conditions at that time, as of the frequent failures of the harvest so that corn [wheat] for bread had to be imported from lübeck, rostock, wismar and so forth. the hansa union undertook the importation and obtained political power by its economic influence. the norwegian land-owners were forced to lower their rents. the population decreased and became impoverished. the revenue sank to per cent. even the income from church property decreased. in corn was imported from lübeck to a value of one-half million kroner. the trade balance inclined to the disadvantage of norway whose sole article of export at that time was dried fish. (the production of fish increased enormously in the baltic regions off south sweden because of the same changes which were influencing the lands, but this did not benefit norway.) dr. bull draws a comparison with the conditions described in the sagas when nordland [at the arctic circle] produced enough corn to feed the inhabitants of the country. at the time of asbjörn selsbane the chieftains in trondhenäs [still farther north in latitude °] grew so much corn that they did not need to go southward to buy corn unless three successive years of dearth had occurred. the province of trondheim exported wheat to iceland and so forth. probably the turbulent political state of scandinavia at the end of the middle ages was in a great measure due to unfavorable climatic conditions, which lowered the standard of life, and not entirely to misgovernment and political strife as has hitherto been taken for granted. during this same unfortunate first half of the fourteenth century england also suffered from conditions which, if sufficiently intensified, might be those of a glacial period. according to thorwald rogers[ ] the severest famine ever experienced in england was that of - , and the next worst was in . in fact, from to great scarcity of food prevailed most of the time. other famines of less severity occurred in and . "the same cause was at work in all these cases," says rogers, "incessant rain, and cold, stormy summers. it is said that the inclemency of the seasons affected the cattle, and that numbers perished from disease and want." after the bad harvest of the price of wheat, which was already high, rose rapidly, and in may, , was about five times the average. for a year or more thereafter it remained at three or four times the ordinary level. the severity of the famine may be judged from the fact that previous to the great war the most notable scarcity of wheat in modern england and the highest relative price was in december, . at that time wheat cost nearly three times the usual amount, instead of five as in . during the famine of the early fourteenth century "it is said that people were reduced to subsist upon roots, upon horses and dogs, and stories are told of even more terrible acts by reason of the extreme famine." the number of deaths was so great that the price of labor suffered a permanent rise of at least per cent. there simply were not people enough left among the peasants to do the work demanded by the more prosperous class who had not suffered so much. after the famine came drought. the year appears to have been peculiarly dry, and , , , , and were also dry. in general these conditions do little harm in england. they are of interest chiefly as showing how excessive rain and drought are apt to succeed one another. these facts regarding northern and central europe during the fourteenth century are particularly significant when compared with the conclusions which we have drawn in _earth and sun_ from the growth of trees in germany and from the distribution of storms. a careful study of all the facts shows that we are dealing with two distinct types of phenomena. in the first place, the climate of central europe seems to have been peculiarly continental during the fourteenth century. the winters were so cold that the rivers froze, and the summers were so wet that there were floods every other year or oftener. this seems to be merely an intensification of the conditions which prevail at the present time during periods of many sunspots, as indicated by the growth of trees at eberswalde in germany and by the number of storms in winter as compared with summer. the prevalence of droughts, especially in the spring, is also not inconsistent with the existence of floods at other seasons, for one of the chief characteristics of a continental climate is that the variations from one season to another are more marked than in oceanic climates. even the summer droughts are typically continental, for when continental conditions prevail, the difference between the same season in different years is extreme, as is well illustrated in kansas. it must always be remembered that what causes famine is not so much absolute dryness as a temporary diminution of the rainfall. the second type of phenomena is peculiarly oceanic in character. it consists of two parts, both of which are precisely what would be expected if a highly continental climate prevailed over the land. in the first place, at certain times the cold area of high pressure, which is the predominating characteristic of a continent during the winter, apparently spread out over the neighboring oceans. under such conditions an inland sea, such as the baltic, would be frozen, so that horses could cross the ice even in the far west. in the second place, because of the unusually high pressure over the continent, the barometric gradients apparently became intensified. hence at the margin of the continental high-pressure area the winds were unusually strong and the storms of corresponding severity. some of these storms may have passed entirely along oceanic tracks, while others invaded the borders of the land, and gave rise to the floods and to the wearing away of the coast described by pettersson. turning now to the east of europe, brückner's[ ] study of the caspian sea shows that that region as well as western europe was subject to great climatic vicissitudes in the first half of the fourteenth century. in - the caspian sea, after rising rapidly for several years, stood thirty-seven feet above the present level and it probably rose still higher during the succeeding decades. at least it remained at a high level, for hamdulla, the persian, tells us that in a place called aboskun was under water.[ ] still further east the inland lake of lop nor also rose at about this time. according to a chinese account the dragon town on the shore of lop nor was destroyed by a flood. from himley's translation it appears that the level of the lake rose so as to overwhelm the city completely. this would necessitate the expansion of the lake to a point eighty miles east of lulan, and fully fifty from the present eastern end of the kara koshun marsh. the water would have to rise nearly, or quite, to a strand which is now clearly visible at a height of twelve feet above the modern lake or marsh. in india the fourteenth century was characterized by what appears to have been the most disastrous drought in all history. apparently the decrease in rainfall here was as striking as the increase in other parts of the world. no statistics are available but we are told that in the great famine which began in even the mogul emperor was unable to obtain the necessaries of life for his household. no rain worth mentioning fell for years. in some places the famine lasted three or four years, and in some twelve, and entire cities were left without an inhabitant. in a later famine, - , which occurred in bengal shortly after the foundation of british rule in india, but while the native officials were still in power, a third of the population, or ten out of thirty millions, perished. the famine in the first half of the fourteenth century seems to have been far worse. these indian famines were apparently due to weak summer monsoons caused presumably by the failure of central asia to warm up as much as usual. the heavier snowfall, and the greater cloudiness of the summer there, which probably accompanied increased storminess, may have been the reason. the new world as well as the old appears to have been in a state of climatic stress during the first half of the fourteenth century. according to pettersson, greenland furnishes an example of this. at first the inhabitants of that northland were fairly prosperous and were able to approach from iceland without much hindrance from the ice. today the north atlantic ocean northeast of iceland is full of drift ice much of the time. the border of the ice varies from season to season, but in general it extends westward from iceland not far from the arctic circle and then follows the coast of greenland southward to cape farewell at the southern tip and around to the western side for fifty miles or more. except under exceptional circumstances a ship cannot approach the coast until well northward on the comparatively ice-free west coast. in the old sagas, however, nothing is said of ice in this region. the route from iceland to greenland is carefully described. in the earliest times it went from iceland a trifle north of west so as to approach the coast of greenland after as short an ocean passage as possible. then it went down the coast in a region where approach is now practically impossible because of the ice. at that time this coast was icy close to the shore, but there is no sign that navigation was rendered difficult as is now the case. today no navigator would think of keeping close inland. the old route also went _north_ of the island on which cape farewell is located, although the narrow channel between the island and the mainland is now so blocked with ice that no modern vessel has ever penetrated it. by the thirteenth century, however, there appears to have been a change. in the kungaspegel or _kings' mirror_, written at that time, navigators are warned not to make the east coast too soon on account of ice, but no new route is recommended in the neighborhood of cape farewell or elsewhere. finally, however, at the end of the fourteenth century, nearly years after the kungaspegel, the old sailing route was abandoned, and ships from iceland sailed directly southwest to avoid the ice. as pettersson says: ... at the end of the thirteenth and the beginning of the fourteenth century the european civilization in greenland was wiped out by an invasion of the aboriginal population. the colonists in the vesterbygd were driven from their homes and probably migrated to america leaving behind their cattle in the fields. so they were found by ivar bardsson, steward to the bishop of gardar, in his official journey thither in . the eskimo invasion must not be regarded as a common raid. it was the transmigration of a people, and like other big movements of this kind [was] impelled by altered conditions of nature, in this case the alterations of climate caused by [or which caused?] the advance of the ice. for their hunting and fishing the eskimos require an at least partially open arctic sea. the seal, their principal prey, cannot live where the surface of the sea is entirely frozen over. the cause of the favorable conditions in the viking-age was, according to my hypothesis, that the ice then melted at a higher latitude in the arctic seas. the eskimos then lived further north in greenland and north america. when the climate deteriorated and the sea which gave them their living was closed by ice the eskimos had to find a more suitable neighborhood. this they found in the land colonized by the norsemen whom they attacked and finally annihilated. finally, far to the south in yucatan the ancient maya civilization made its last flickering effort at about this time. not much is known of this but in earlier periods the history of the mayas seems to have agreed quite closely with the fluctuations in climate.[ ] among the mayas, as we have seen, relatively dry periods were the times of greatest progress. let us turn now to fig. once more and compare the climatic conditions of the fourteenth century with those of periods of increasing rainfall. southern england, ireland, and scandinavia, where the crops were ruined by extensive rain and storms in summer, are places where storminess and rainfall now increase when sunspots are numerous. central europe and the coasts of the north sea, where flood and drought alternated, are regions which now have relatively less rain when sunspots increase than when they diminish. however, as appears from the trees measured by douglass, the winters become more continental and hence cooler, thus corresponding to the cold winters of the fourteenth century when people walked on the ice from scandinavia to denmark. when such high pressure prevails in the winter, the total rainfall is diminished, but nevertheless the storms are more severe than usual, especially in the spring. in southeastern europe, the part of the area whence the caspian derives its water, appears to have less rainfall during times of increasing sunspots than when sunspots are few, but in an equally large area to the south, where the mountains are higher and the run-off of the rain is more rapid, the reverse is the case. this seems to mean that a slight diminution in the water poured in by the volga would be more than compensated by the water derived from persia and from the oxus and jaxartes rivers, which in the fourteenth century appear to have filled the sea of aral and overflowed in a large stream to the caspian. still farther east in central asia, so far as the records go, most of the country receives more rain when sunspots are many than when they are few, which would agree with what happened when the dragon town was inundated. in india, on the contrary, there is a large area where the rainfall diminishes at times of many sunspots, thus agreeing with the terrible famine from which the moguls suffered so severely. in the western hemisphere, greenland, arizona, and california are all parts of the area where the rain increases with many sunspots, while yucatan seems to lie in an area of the opposite type. thus all the evidence seems to show that at times of climatic stress, such as the fourteenth century, the conditions are essentially the same as those which now prevail at times of increasing sunspots. as to the number of sunspots, there is little evidence previous to about . yet that little is both interesting and important. although sunspots have been observed with care in europe only a little more than three centuries, the chinese have records which go back nearly to the beginning of the christian era. of course the records are far from perfect, for the work was done by individuals and not by any great organization which continued the same methods from generation to generation. the mere fact that a good observer happened to use his smoked glass to advantage may cause a particular period to appear to have an unusual number of spots. on the other hand, the fact that such an observer finds spots at some times and not at others tends to give a valuable check on his results, as does the comparison of one observer's work with that of another. hence, in spite of many and obvious defects, most students of the problem agree that the chinese record possesses much value, and that for a thousand years or more it gives a fairly true idea of the general aspect of the sun. in the chinese records the years with many spots fall in groups, as would be expected, and are sometimes separated by long intervals. certain centuries appear to have been marked by unusual spottedness. the most conspicuous of these is the fourteenth, when the years to were particularly noteworthy, for spots large enough to be visible to the naked eye covered the sun much of the time. hence wolf,[ ] who has made an exhaustive study of the matter, concludes that there was an absolute maximum of spots about . while this date is avowedly open to question, the great abundance of sunspots at that time makes it probable that it cannot be far wrong. if this is so, it seems that the great climatic disturbances of which we have seen evidence in the fourteenth century occurred at a time when sunspots were increasing, or at least when solar activity was under some profoundly disturbing influence. thus the evidence seems to show not merely that the climate of historic times has been subject to important pulsations, but that those pulsations were magnifications of the little climatic changes which now take place in sunspot cycles. the past and the present are apparently a unit except as to the intensity of the changes. footnotes: [footnote : o. pettersson: the connection between hydrographical and meteorological phenomena; quarterly journal of the royal meteorological society, vol. , pp. - .] [footnote : a. norlind: einige bemerkungen über das klima der historischen zeit nebst einem verzeichnis mittelaltlicher witterungs erscheinungen; lunds univ. arsskrift, n. f., vol. , , pp.] [footnote : thorwald rogers: a history of agriculture and prices in england.] [footnote : e. brückner: klimaschwankungen seit , vienna, .] [footnote : for a full discussion of the changes in the caspian sea, see the pulse of asia, pp. - .] [footnote : s. q. morley: the inscriptions at copán; carnegie inst. of wash., no. , . ellsworth huntington: the red man's continent, .] [footnote : see summary of wolf's work with additional information by h. fritz; zürich vierteljahrschrift, vol. , , pp. - .] chapter vii glaciation according to the solar-cyclonic hypothesis[ ] the remarkable phenomena of glacial periods afford perhaps the best available test to which any climatic hypothesis can be subjected. in this chapter and the two that follow, we shall apply this test. since much more is known about the recent great ice age, or pleistocene glaciation, than about the more ancient glaciations, the problems of the pleistocene will receive especial attention. in the present chapter the oncoming of glaciation and the subsequent disappearance of the ice will be outlined in the light of what would be expected according to the solar-cyclonic hypothesis. then in the next chapter several problems of especial climatic significance will be considered, such as the localization of ice sheets, the succession of severe glacial and mild inter-glacial epochs, the sudden commencement of glaciation and the peculiar variations in the height of the snow line. other topics to be considered are the occurrence of pluvial or rainy climates in non-glaciated regions, and glaciation near sea level in subtropical latitudes during the permian and proterozoic. then in chapter ix we shall consider the development and distribution of the remarkable deposits of wind-blown material known as loess. facts not considered at the time of framing an hypothesis are especially significant in testing it. in this particular case, the cyclonic hypothesis was framed to explain the historic changes of climate revealed by a study of ruins, tree rings, and the terraces of streams and lakes, without special thought of glaciation or other geologic changes. indeed, the hypothesis had reached nearly its present form before much attention was given to geological phases of the problem. nevertheless, it appears to meet even this severe test. according to the solar-cyclonic hypothesis, the pleistocene glacial period was inaugurated at a time when certain terrestrial conditions tended to make the earth especially favorable for glaciation. how these conditions arose will be considered later. here it is enough to state what they were. chief among them was the fact that the continents stood unusually high and were unusually large. this, however, was not the primary cause of glaciation, for many of the areas which were soon to be glaciated were little above sea level. for example, it seems clear that new england stood less than a thousand feet higher than now. indeed, salisbury[ ] estimates that eastern north america in general stood not more than a few hundred feet higher than now, and w. b. wright[ ] reaches the same conclusion in respect to the british isles. nevertheless, widespread lands, even if they are not all high, lead to climatic conditions which favor glaciation. for example, enlarged continents cause low temperature in high latitudes because they interfere with the ocean currents that carry heat polewards. such continents also cause relatively cold winters, for lands cool much sooner than does the ocean. another result is a diminution of water vapor, not only because cold air cannot hold much vapor, but also because the oceanic area from which evaporation takes place is reduced by the emergence of the continents. again, when the continents are extensive the amount of carbonic acid gas in the atmosphere probably decreases, for the augmented erosion due to uplift exposes much igneous rock to the air, and weathering consumes the atmospheric carbon dioxide. when the supply of water vapor and of atmospheric carbon dioxide is small, an extreme type of climate usually prevails. the combined result of all these conditions is that continental emergence causes the climate to be somewhat cool and to be marked by relatively great contrasts from season to season and from latitude to latitude. when the terrestrial conditions thus permitted glaciation, unusual solar activity is supposed to have greatly increased the number and severity of storms and to have altered their location, just as now happens at times of many sunspots. if such a change in storminess had occurred when terrestrial conditions were unfavorable for glaciation, as, for example, when the lands were low and there were widespread epicontinental seas in middle and high latitudes, glaciation might not have resulted. in the pleistocene, however, terrestrial conditions permitted glaciation, and therefore the supposed increase in storminess caused great ice sheets. the conditions which prevail at times of increased storminess have been discussed in detail in _earth and sun_. those which apparently brought on glaciation seem to have acted as follows: in the first place the storminess lowered the temperature of the earth's surface in several ways. the most important of these was the rapid upward convection in the centers of cyclonic storms whereby abundant heat was carried to high levels where most of it was radiated away into space. the marked increase in the number of tropical cyclones which accompanies increased solar activity was probably important in this respect. such cyclones carry vast quantities of heat and moisture out of the tropics. the moisture, to be sure, liberates heat upon condensing, but as condensation occurs above the earth's surface, much of the heat escapes into space. another reason for low temperature was that under the influence of the supposedly numerous storms of pleistocene times evaporation over the oceans must have increased. this is largely because the velocity of the winds is relatively great when storms are strong and such winds are powerful agents of evaporation. but evaporation requires heat, and hence the strong winds lower the temperature.[b] the second great condition which enabled increased storminess to bring on glaciation was the location of the storm tracks. kullmer's maps, as illustrated in fig. , suggest that a great increase in solar activity, such as is postulated in the pleistocene, might shift the main storm track poleward even more than it is shifted by the milder solar changes during the twelve-year sunspot cycle. if this is so, the main track would tend to cross north america through the middle of canada instead of near the southern border. thus there would be an increase in precipitation in about the latitude of the keewatin and labradorean centers of glaciation. from what is known of storm tracks in europe, the main increase in the intensity of storms would probably center in scandinavia. fig. in chapter v bears this out. that figure, it will be recalled, shows what happens to precipitation when solar activity is increasing. a high rate of precipitation is especially marked in the boreal storm track, that is, in the northern united states, southern canada, and northwestern europe. another important condition in bringing on glaciation would be the fact that when storms are numerous the total precipitation appears to increase in spite of the slightly lower temperature. this is largely because of the greater evaporation. the excessive evaporation arises partly from the rapidity of the winds, as already stated, and partly from the fact that in areas where the air is clear the sun would presumably be able to act more effectively than now. it would do so because at times of abundant sunspots the sun in our own day has a higher solar constant than at times of milder activity. our whole hypothesis is based on the supposition that what now happens at times of many sunspots was intensified in glacial periods. a fourth condition which would cause glaciation to result from great solar activity would be the fact that the portion of the yearly precipitation falling as snow would increase, while the proportion of rain would diminish in the main storm track. this would arise partly because the storms would be located farther north than now, and partly because of the diminution in temperature due to the increased convection. the snow in itself would still further lower the temperature, for snow is an excellent reflector of sunlight. the increased cloudiness which would accompany the more abundant storms would also cause an unusually great reflection of the sunlight and still further lower the temperature. thus at times of many sunspots a strong tendency toward the accumulation of snow would arise from the rapid convection and consequent low temperature, from the northern location of storms, from the increased evaporation and precipitation, from the larger percentage of snowy rather than rainy precipitation, and from the great loss of heat due to reflection from clouds and snow. if events at the beginning of the last glacial period took place in accordance with the cyclonic hypothesis, as outlined above, one of the inevitable results would be the production of snowfields. the places where snow would accumulate in special quantities would be central canada, the labrador plateau, and scandinavia, as well as certain mountain regions. as soon as a snowfield became somewhat extensive, it would begin to produce striking climatic alterations in addition to those to which it owed its origin.[ ] for example, within a snowfield the summers remain relatively cold. hence such a field is likely to be an area of high pressure at all seasons. the fact that the snowfield is always a place of relatively high pressure results in outblowing surface winds except when these are temporarily overcome by the passage of strong cyclonic storms. the storms, however, tend to be concentrated near the margins of the ice throughout the year instead of following different paths in each of the four seasons. this is partly because cyclonic lows always avoid places of high pressure and are thus pushed out of the areas where permanent snow has accumulated. on the other hand, at times of many sunspots, as kullmer has shown, the main storm track tends to be drawn poleward, perhaps by electrical conditions. hence when a snowfield is present in the north, the lows, instead of migrating much farther north in summer than in winter, as they now do, would merely crowd on to the snowfield a little farther in summer than in winter. thus the heavy precipitation which is usual in humid climates near the centers of lows would take place near the advancing margin of the snowfield and cause the field to expand still farther southward. the tendency toward the accumulation of snow on the margins of the snowfields would be intensified not only by the actual storms themselves, but by other conditions. for example, the coldness of the snow would tend to cause prompt condensation of the moisture brought by the winds that blow toward the storm centers from low latitudes. again, in spite of the general dryness of the air over a snowfield, the lower air contains some moisture due to evaporation from the snow by day during the clear sunny weather of anti-cyclones or highs. where this is sufficient, the cold surface of the snowfields tends to produce a frozen fog whenever the snowfield is cooled by radiation, as happens at night and during the passage of highs. such a frozen fog is an effective reflector of solar radiation. moreover, because ice has only half the specific heat of water, and is much more transparent to heat, such a "radiation fog" composed of ice crystals is a much less effective retainer of heat than clouds or fog made of unfrozen water particles. shallow fogs of this type are described by several polar expeditions. they clearly retard the melting of the snow and thus help the icefield to grow. for all these reasons, so long as storminess remained great, the pleistocene snowfields, according to the solar hypothesis, must have deepened and expanded. in due time some of the snow was converted into glacial ice. when that occurred, the growth of the snowfield as well as of the ice cap must have been accelerated by glacial movement. under such circumstances, as the ice crowded southward toward the source of the moisture by which it grew, the area of high pressure produced by its low temperature would expand. this would force the storm track southward in spite of the contrary tendency due to the sun. when the ice sheet had become very extensive, the track would be crowded relatively near to the northern margin of the trade-wind belt. indeed, the pleistocene ice sheets, at the time of their maximum extension, reached almost as far south as the latitude now marking the northern limit of the trade-wind belt in summer. as the storm track with its frequent low pressure and the subtropical belt with its high pressure were forced nearer and nearer together, the barometric gradient between the two presumably became greater, winds became stronger, and the storms more intense. this zonal crowding would be of special importance in summer, at which time it would also be most pronounced. in the first place, the storms would be crowded far upon the ice cap which would then be protected from the sun by a cover of fog and cloud more fully than at any other season. furthermore, the close approach of the trade-wind belt to the storm belt would result in a great increase in the amount of moisture drawn from the belt of evaporation which the trade winds dominate. in the trade-wind belt, clear skies and high temperature make evaporation especially rapid. indeed, in spite of the vast deserts it is probable that more than three-fourths of the total evaporation now taking place on the earth occurs in the belt of trades, an area which includes about one-half of the earth's surface. the agency which could produce this increased drawing northward of moisture from the trade-wind belt would be the winds blowing into the lows. according to the cyclonic hypothesis, many of these lows would be so strong that they would temporarily break down the subtropical belt of high pressure which now usually prevails between the trades and the zone of westerly winds. this belt is even now often broken by tropical cyclones. if the storms of more northerly regions temporarily destroyed the subtropical high-pressure belt, even though they still remained on its northern side, they would divert part of the trade winds. hence the air which now is carried obliquely equatorward by those winds would be carried spirally northward into the cyclonic lows. precipitation in the storm track on the margin of the relatively cold ice sheet would thus be much increased, for most winds from low latitudes carry abundant moisture. such a diversion of moisture from low latitudes probably explains the deficiency of precipitation along the heat equator at times of solar activity, as shown in fig. . taken as a whole, the summer conditions, according to the cyclonic hypothesis, would be such that increased evaporation in low latitudes would coöperate with increased storminess, cloudiness, and fog in higher latitudes to preserve and increase the accumulation of ice upon the borders of the ice sheet. the greater the storminess, the more this would be true and the more the ice sheet would be able to hold its own against melting in summer. such a combination of precipitation and of protection from the sun is especially important if an ice sheet is to grow. the meteorologist needs no geologic evidence that the storm track was shoved equatorward by the growth of the ice sheet, for he observes a similar shifting whenever a winter's snow cap occupies part of the normal storm tract. the geologist, however, may welcome geologic evidence that such an extreme shift of the storm track actually occurred during the pleistocene. harmer, in , first pointed out the evidence which was repeated with approval by wright of the ireland geological survey in .[ ] according to these authorities, numerous boulders of a distinctive chalk were deposited by pleistocene icebergs along the coast of ireland. their distribution shows that at the time of maximum glaciation the strong winds along the south coast of ireland were from the northeast while today they are from the southwest. such a reversal could apparently be produced only by a southward shift of the center of the main storm track from its present position in northern ireland, scotland, and norway to a position across northern france, central germany, and middle russia. this would mean that while now the centers of the lows commonly move northeastward a short distance north of southern ireland, they formerly moved eastward a short distance south of ireland. it will be recalled that in the northern hemisphere the winds spiral into a low counter-clockwise and that they are strongest near the center. when the centers pass not far north of a given point, the strong winds therefore blow from the west or southwest, while when the centers pass just south of that point, the strong winds come from the east or northeast. in addition to the consequences of the crowding of the storm track toward the trade-wind belt, several other conditions presumably operated to favor the growth of the ice sheet. for example, the lowering of the sea level by the removal of water to form the snowfields and glaciers interfered with warm currents. it also increased the rate of erosion, for it was equivalent to an uplift of all the land. one consequence of erosion and weathering was presumably a diminution of the carbon dioxide in the atmosphere, for although the ice covered perhaps a tenth of the lands and interfered with carbonation to that extent, the removal of large quantities of soil by accelerated erosion on the other nine-tenths perhaps more than counterbalanced the protective effect of the ice. at the same time, the general lowering of the temperature of the ocean as well as the lands increased the ocean's capacity for carbon dioxide and thus facilitated absorption. at a temperature of °f. water absorbs per cent more carbon dioxide than at °. the high waves produced by the severe storms must have had a similar effect on a small scale. thus the percentage of carbon dioxide in the atmosphere was presumably diminished. of less significance than these changes in the lands and the air, but perhaps not negligible, was the increased salinity of the ocean which accompanied the removal of water to form snow, and the increase of the dissolved mineral load of the rejuvenated streams. increased salinity slows up the deep-sea circulation, as we shall see in a later chapter. this increases the contrasts from zone to zone. at times of great solar activity the agencies mentioned above would apparently coöperate to cause an advance of ice sheets into lower latitudes. the degree of solar activity would have much to do with the final extent of the ice sheets. nevertheless, certain terrestrial conditions would tend to set limits beyond which the ice would not greatly advance unless the storminess were extraordinarily severe. the most obvious of these conditions is the location of oceans and of deserts or semi-arid regions. the southwestward advance of the european ice sheet and the southeastward advance of the labradorean sheet in america were stopped by the atlantic. the semi-aridity of the great plains, produced by their position in the lee of the rocky mountains, stopped the advance of the keewatin ice sheet toward the southwest. the advance of the european ice sheet southeast seems to have been stopped for similar reasons. the cessation of the advance would be brought about in such an area not alone by the light precipitation and abundant sunshine, but by the dryness of the air, and also by the power of dust to absorb the sun's heat. much dust would presumably be drawn in from the dry regions by passing cyclonic storms and would be scattered over the ice. the advance of the ice is also slowed up by a rugged topography, as among the appalachians in northern pennsylvania. such a topography besides opposing a physical obstruction to the movement of the ice provides bare south-facing slopes which the sun warms effectively. such warm slopes are unfavorable to glacial advance. the rugged topography was perhaps quite as effective as the altitude of the appalachians in causing the conspicuous northward dent in the glacial margin in pennsylvania. where glaciers lie in mountain valleys the advance beyond a certain point is often interfered with by the deployment of the ice at the mouths of gorges. evaporation and melting are more rapid where a glacier is broad and thin than where it is narrow and thick, as in a gorge. again, where the topography or the location of oceans or dry areas causes the glacial lobes to be long and narrow, the elongation of the lobe is apparently checked in several ways. toward the end of the lobe, melting and evaporation increase rapidly because the planetary westerly winds are more likely to overcome the glacial winds and sweep across a long, narrow lobe than across a broad one. as they cross the lobe, they accelerate evaporation, and probably lessen cloudiness, with a consequent augmentation of melting. moreover, although lows rarely cross a broad ice sheet, they do cross a narrow lobe. for example, nansen records that strong lows occasionally cross the narrow southern part of the greenland ice sheet. the longer the lobe, the more likely it is that lows will cross it, instead of following its margin. lows which cross a lobe do not yield so much snow to the tip as do those which follow the margin. hence elongation is retarded and finally stopped even without a change in the earth's general climate. because of these various reasons the advances of the ice during the several epochs of a glacial period might be approximately equal, even if the durations of the periods of storminess and low temperature were different. indeed, they might be sub-equal, even if the periods differed in intensity as well as length. differences in the periods would apparently be manifested less in the extent of the ice than in the depth of glacial erosion and in the thickness of the terminal moraines, outwash plains, and other glacial or glacio-fluvial formations. having completed the consideration of the conditions leading to the advance of the ice, let us now consider the condition of north america at the time of maximum glaciation.[ ] over an area of nearly four million square miles, occupying practically all the northern half of the continent and part of the southern half, as appears in fig. , the surface was a monotonous and almost level plain of ice covered with snow. when viewed from a high altitude, all parts except the margins must have presented a uniformly white and sparkling appearance. along the margins, however, except to the north, the whiteness was irregular, for the view must have included not only fresh snow, but moving clouds and dirty snow or ice. along the borders where melting was in progress there was presumably more or less spottedness due to morainal material or glacial débris brought to the surface by ice shearage and wastage. along the dry southwestern border it is also possible that there were numerous dark spots due to dust blown onto the ice by the wind. [illustration: _fig. . distribution of pleistocene ice sheets._ (_after schuchert._)] the great white sheet with its ragged border was roughly circular in form, with its center in central canada. yet there were many departures from a perfectly circular form. some were due to the oceans, for, except in northern alaska, the ice extended into the ocean all the way from new jersey around by the north to washington. on the south, topographic conditions made the margin depart from a simple arc. from new jersey to ohio it swung northward. in the mississippi valley it reached far south; indeed most of the broad wedge between the ohio and the missouri rivers was occupied by ice. from latitude ° near the junction of the missouri and the mississippi, however, the ice margin extended almost due north along the missouri to central north dakota. it then stretched westward to the rockies. farther west lowland glaciation was abundant as far south as western washington. in the rockies, the cascades, and the sierra nevadas glaciation was common as far south as colorado and southern california, respectively, and snowfields were doubtless extensive enough to make these ranges ribbons of white. between these lofty ranges lay a great unglaciated region, but even in the great basin itself, in spite of its present aridity, certain ranges carried glaciers, while great lakes expanded widely. in this vast field of snow the glacial ice slowly crept outward, possibly at an average speed of half a foot a day, but varying from almost nothing in winter at the north, to several feet a day in summer at the south.[ ] the force which caused the movement was the presence of the ice piled up not far from the margins. almost certainly, however, there was no great dome from the center in canada outward, as some early writers assumed. such a dome would require that the ice be many thousands of feet thick near its center. this is impossible because of the fact that ice is more voluminous than water (about per cent near the freezing point). hence when subjected to sufficient pressure it changes to the liquid form. as friction and internal heat tend to keep the bottom of a glacier warm, even in cold regions, the probabilities are that only under very special conditions was a continental ice sheet much thicker than about feet. in antarctica, where the temperature is much lower than was probably attained in the united states, the ice sheet is nearly level, several expeditions having traveled hundreds of miles with practically no change in altitude. in shackleton's trip almost to the south pole, he encountered a general rise of feet in miles. mountains, however, projected through the ice even near the pole and the geologists conclude that the ice is not very thick even at the world's coldest point, the south pole. along the margin of the ice there were two sorts of movement, much more rapid than the slow creep of the ice. one was produced by the outward drift of snow carried by the outblowing dry winds and the other and more important was due to the passage of cyclonic storms. along the border of the ice sheet, except at the north, storm presumably closely followed storm. their movement, we judge, was relatively slow until near the southern end of the mississippi lobe, but when this point was passed they moved much more rapidly, for then they could go toward instead of away from the far northern path which the sun prescribes when solar activity is great. the storms brought much snow to the icefield, perhaps sometimes in favored places as much as the hundred feet a year which is recorded for some winters in the sierras at present. even the unglaciated intermontane great basin presumably received considerable precipitation, perhaps twice as much as its present scanty supply. the rainfall was enough to support many lakes, one of which was ten times as large as great salt lake; and grass was doubtless abundant upon many slopes which are now dry and barren. the relatively heavy precipitation in the great basin was probably due primarily to the increased number of storms, but may also have been much influenced by their slow eastward movement. the lows presumably moved slowly in that general region not only because they were retarded and turned from their normal path by the cold ice to the east, but because during the summer the area between the sierra snowfields on the west and the rocky mountain and mississippi valley snowfields on the east was relatively warm. hence it was normally a place of low pressure and therefore of inblowing winds. slow-moving lows are much more effective than fast-moving ones in drawing moisture northwestward from the gulf of mexico, for they give the moisture more time to move spirally first northeast, under the influence of the normal southwesterly winds, then northwest and finally southwest as it approaches the storm center. in the case of the present lows, before much moisture-laden air can describe such a circuit, first eastward and then westward, the storm center has nearly always moved eastward across the rockies and even across the great plains. a result of this is the regular decrease in precipitation northward, northwestward, and westward from the gulf of mexico. along the part of the glacial margins where for more than miles the north american ice entered the atlantic and the pacific oceans, myriads of great blocks broke off and floated away as stately icebergs, to scatter boulders far over the ocean floor and to melt in warmer climes. where the margin lay upon the lands numerous streams issued from beneath the ice, milk-white with rock flour, and built up great outwash plains and valley trains of gravel and sand. here and there, just beyond the ice, marginal lakes of strange shapes occupied valleys which had been dammed by the advancing ice. in many of them the water level rose until it reached some low point in the divide and then overflowed, forming rapids and waterfalls. indeed, many of the waterfalls of the eastern united states and canada were formed in just this way and not a few streams now occupy courses through ridges instead of parallel to them, as in pre-glacial times. in the zone to the south of the continental ice sheet, the plant and animal life of boreal, cool temperate, and warm temperate regions commingled curiously. heather and arctic willow crowded out elm and oak; musk ox, hairy mammoth, and marmot contested with deer, chipmunk, and skunk for a chance to live. near the ice on slopes exposed to the cold glacial gales, the immigrant boreal species were dominant, but not far away in more protected areas the species that had formerly lived there held their own. in europe during the last two advances of the great ice sheet the caveman also struggled with fierce animals and a fiercer climate to maintain life in an area whose habitability had long been decreasing. the next step in our history of glaciation is to outline the disappearance of the ice sheets. when a decrease in solar activity produced a corresponding decrease in storminess, several influences presumably combined to cause the disappearance of the ice. most of their results are the reverse of those which brought on glaciation. a few special aspects, however, some of which have been discussed in _earth and sun_, ought to be brought to mind. a diminution in storminess lessens upward convection, wind velocity, and evaporation, and these changes, if they occurred, must have united to raise the temperature of the lower air by reducing the escape of heat. again a decrease in the number and intensity of tropical cyclones presumably lessened the amount of moisture carried into mid-latitudes, and thus diminished the precipitation. the diminution of snowfall on the ice sheets when storminess diminished was probably highly important. the amount of precipitation on the sheets was presumably lessened still further by changes in the storminess of middle latitudes. when storminess diminishes, the lows follow a less definite path, as kullmer's maps show, and on the average a more southerly path. thus, instead of all the lows contributing snow to the ice sheet, a large fraction of the relatively few remaining lows would bring rain to areas south of the ice sheet. as storminess decreased, the trades and westerlies probably became steadier, and thus carried to high latitudes more warm water than when often interrupted by storms. steadier southwesterly winds must have produced a greater movement of atmospheric as well as oceanic heat to high latitudes. the warming due to these two causes was probably the chief reason for the disappearance of the european ice sheet and of those on the pacific coast of north america. the two greater american ice sheets, however, and the glaciers elsewhere in the lee of high mountain ranges, probably disappeared chiefly because of lessened precipitation. if there were no cyclonic storms to draw moisture northward from the gulf of mexico, most of north america east of the rocky mountain barrier would be arid. therefore a diminution of storminess would be particularly effective in causing the disappearance of ice sheets in these regions. that evaporation was an especially important factor in causing the ice from the keewatin center to disappear, is suggested by the relatively small amount of water-sorted material in its drift. in south dakota, for example, less than per cent of the drift is stratified.[ ] on the other hand, salisbury estimates that perhaps a third of the labradorean drift in eastern wisconsin is crudely stratified, about half of that in new jersey, and more than half of the drift in western europe. when the sun's activity began to diminish, all these conditions, as well as several others, would coöperate to cause the ice sheets to disappear. step by step with their disappearance, the amelioration of the climate would progress so long as the period of solar inactivity continued and storms were rare. if the inactivity continued long enough, it would result in a fairly mild climate in high latitudes, though so long as the continents were emergent this mildness would not be of the extreme type. the inauguration of another cycle of increased disturbance of the sun, with a marked increase in storminess, would inaugurate another glacial epoch. thus a succession of glacial and inter-glacial epochs might continue so long as the sun was repeatedly disturbed. footnotes: [footnote : this chapter is an amplification and revision of the sketch of the glacial period contained in the solar hypothesis of climatic changes; bull. geol. soc. am., vol. , .] [footnote : r. d. salisbury: physical geography of the pleistocene, in outlines of geologic history, by willis, salisbury, and others, , p. .] [footnote : the quaternary ice age, , p. .] [footnote b: for fuller discussion of climatic controls see s. s. visher: seventy laws of climate, annals assoc. am. geographers, .] [footnote : many of these alterations are implied or discussed in the following papers: . f. w. harmer: influence of winds upon the climate of the pleistocene; quart. jour. geol. soc., vol. , , p. . . c. e. p. brooks: meteorological conditions of an ice sheet; quart. jour. royal meteorol. soc., vol. , , pp. - , and the evolution of climate in northwest europe; _op. cit._, vol. , , pp. - . . w. h. hobbs: the rôle of the glacial anticyclone in the air circulation of the globe; proc. am. phil. soc., vol. , , pp. - .] [footnote : w. b. wright: the quaternary ice age, , p. .] [footnote : the description of the distribution of the ice sheet is based on t. c. chamberlin's wall map of north america at the maximum of glaciation, .] [footnote : chamberlin and salisbury: geology, , vol. , and w. h. hobbs: characteristics of existing glaciers, .] [footnote : s. s. visher: the geography of south dakota; s. d. geol. surv., .] chapter viii some problems of glacial periods having outlined in general terms the coming of the ice sheets and their disappearance, we are now ready to discuss certain problems of compelling climatic interest. the discussion will be grouped under five heads: (i) the localization of glaciation; (ii) the sudden coming of glaciation; (iii) peculiar variations in the height of the snow line and of glaciation; (iv) lakes and other evidences of humidity in unglaciated regions during the glacial epochs; (v) glaciation at sea level and in low latitudes in the permian and proterozoic eras. the discussion of perhaps the most difficult of all climatic problems of glaciation, that of the succession of cold glacial and mild inter-glacial epochs, has been postponed to the next to the final chapter of this book. it cannot be properly considered until we take up the history of solar disturbances. i. the first problem, the localization of the ice sheets, arises from the fact that in both the pleistocene and the permian periods glaciation was remarkably limited. in neither period were all parts of high latitudes glaciated; yet in both cases glaciation occurred in large regions in lower latitudes. many explanations of this localization have been offered, but most are entirely inadequate. even hypotheses with something of proven worth, such as those of variations in volcanic dust and in atmospheric carbon dioxide, fail to account for localization. the cyclonic form of the solar hypothesis, however, seems to afford a satisfactory explanation. the distribution of the ice in the last glacial period is well known, and is shown in fig. . four-fifths of the ice-covered area, which was eight million square miles, more or less, was near the borders of the north atlantic in eastern north america and northwestern europe. the ice spread out from two great centers in north america, the labradorean east of hudson bay, and the keewatin west of the bay. there were also many glaciers in the western mountains, especially in canada, while subordinate centers occurred in newfoundland, the adirondacks, and the white mountains. the main ice sheet at its maximum extension reached as far south as latitude ° in kansas and kentucky, and ° in illinois. huge boulders were transferred more than one thousand miles from their source in canada. the northward extension was somewhat less. indeed, the northern margin of the continent was apparently relatively little glaciated and much of alaska unglaciated. why should northern kentucky be glaciated when northern alaska was not? in europe the chief center from which the continental glacier moved was the scandinavian highlands. it pushed across the depression now occupied by the baltic to southern russia and across the north sea depression to england and belgium. the alps formed a center of considerable importance, and there were minor centers in scotland, ireland, the pyrenees, apennines, caucasus, and urals. in asia numerous ranges also contained large glaciers, but practically all the glaciation was of the alpine type and very little of the vast northern lowland was covered with ice. in the southern hemisphere glaciation at low latitudes was less striking than in the northern hemisphere. most of the increase in the areas of ice was confined to mountains which today receive heavy precipitation and still contain small glaciers. indeed, except for relatively slight glaciation in the australian alps and in tasmania, most of the pleistocene glaciation in the southern hemisphere was merely an extension of existing glaciers, such as those of south chile, new zealand, and the andes. nevertheless, fairly extensive glaciation existed much nearer the equator than is now the case. in considering the localization of pleistocene glaciation, three main factors must be taken into account, namely, temperature, topography, and precipitation. the absence of glaciation in large parts of the arctic regions of north america and of asia makes it certain that low temperature was not the controlling factor. aside from antarctica, the coldest place in the world is northeastern siberia. there for seven months the average temperature is below °c., while the mean for the whole year is below - °c. if the temperature during a glacial period averaged °c. lower than now, as is commonly supposed, this part of siberia would have had a temperature below freezing for at least nine months out of the twelve even if there were no snowfield to keep the summers cold. yet even under such conditions no glaciation occurred, although in other places, such as parts of canada and northwestern europe, intense glaciation occurred where the mean temperature is much higher. the topography of the lands apparently had much more influence upon the localization of glaciation than did temperature. its effect, however, was always to cause glaciation exactly where it would be expected and not in unexpected places as actually occurred. for example, in north america the western side of the canadian rockies suffered intense glaciation, for there precipitation was heavy because the westerly winds from the pacific are forced to give up their moisture as they rise. in the same way the western side of the sierra nevadas was much more heavily glaciated than the eastern side. in similar fashion the windward slopes of the alps, the caucasus, the himalayas, and many other mountain ranges suffered extensive glaciation. low temperature does not seem to have been the cause of this glaciation, for in that case it is hard to see why both sides of the various ranges did not show an equal percentage of increase in the size of their icefields. from what has been said as to temperature and topography, it is evident that variations in precipitation have had much more to do with glaciation than have variations in temperature. in the arctic lowlands and on the leeward side of mountains, the slight development of glaciation appears to have been due to scarcity of precipitation. on the windward side of mountains, on the other hand, a notable increase in precipitation seems to have led to abundant glaciation. such an increase in precipitation must be dependent on increased evaporation and this could arise either from relatively high temperature or strong winds. since the temperature in the glacial period was lower than now, we seem forced to attribute the increased precipitation to a strengthening of the winds. if the westerly winds from the pacific should increase in strength and waft more moisture to the western side of the canadian rockies, or if similar winds increased the snowfall on the upper slopes of the alps or the tian-shan mountains, the glaciers would extend lower than now without any change in temperature. although the incompetence of low temperature to cause glaciation, and the relative unimportance of the mountains in northeastern canada and northwestern europe throw most glacial hypotheses out of court, they are in harmony with the cyclonic hypothesis. the answer of that hypothesis to the problem of the localization of ice sheets seems to be found in certain maps of storminess and rainfall in relation to solar activity. in fig. a marked belt of increased storminess at times of many sunspots is seen in southern canada. a comparison of this with a series of maps given in _earth and sun_ shows that the stormy belt tends to migrate northward in harmony with an increase in the activity of the sun's atmosphere. if the sun were sufficiently active the belt of maximum storminess would apparently pass through the keewatin and labradorean centers of glaciation instead of well to the south of them, as at present. it would presumably cross another center in greenland, and then would traverse the fourth of the great centers of pleistocene glaciation in scandinavia. it would not succeed in traversing northern asia, however, any more than it does now, because of the great high-pressure area which develops there in winter. when the ice sheets expanded from the main centers of glaciation, the belt of storms would be pushed southward and outward. thus it might give rise to minor centers of glaciers such as the patrician between hudson bay and lake superior, or the centers in ireland, cornwall, wales, and the northern ural mountains. as the main ice sheets advanced, however, the minor centers would be overridden and the entire mass of ice would be merged into one vast expanse in the atlantic portion of each of the two continents. in this connection it may be well to consider briefly the most recent hypothesis as to the growth and hence the localization of glaciation. in and more fully in , hobbs,[ ] advanced the anti-cyclonic hypothesis of the origin of ice sheets. this hypothesis has the great merit of focusing attention upon the fact that ice sheets are pronounced anti-cyclonic regions of high pressure. this is proved by the strong outblowing winds which prevail along their margins. such winds must, of course, be balanced by inward-moving winds at high levels. abundant observations prove that such is the case. for example, balloons sent up by barkow near the margin of the antarctic ice sheet reveal the occurrence of inblowing winds, although they rarely occur below a height of meters. the abundant data gathered by guervain on the coast of greenland indicate that outblowing winds prevail up to a height of about meters. at that height inblowing winds commence and increase in frequency until at an altitude of over meters they become more common than outblowing winds. it should be noted, however, that in both antarctica and greenland, although the winds at an elevation of less than a thousand meters generally blow outward, there are frequent and decided departures from this rule, so that "variable winds" are quite commonly mentioned in the reports of expeditions and balloon soundings. the undoubted anti-cyclonic conditions which hobbs thus calls to the attention of scientists seem to him to necessitate a peculiar mechanism in order to produce the snow which feeds the glaciers. he assumes that the winds which blow toward the centers of the ice sheets at high levels carry the necessary moisture by which the glaciers grow. when the air descends in the centers of the highs, it is supposed to be chilled on reaching the surface of the ice, and hence to give up its moisture in the form of minute crystals. this conclusion is doubtful for several reasons. in the first place, hobbs does not seem to appreciate the importance of the variable winds which he quotes arctic and antarctic explorers as describing quite frequently on the edges of the ice sheets. they are one of many signs that cyclonic storms are fairly frequent on the borders of the ice though not in its interior. thus there is a distinct and sufficient form of precipitation actually at work near the margin of the ice, or exactly where the thickness of the ice sheet would lead us to expect. another consideration which throws grave doubt on the anti-cyclonic hypothesis of ice sheets is the small amount of moisture possible in the highs because of their low temperature. suppose, for the sake of argument, that the temperature in the middle of an ice sheet averages °f. this is probably much higher than the actual fact and therefore unduly favorable to the anti-cyclonic hypothesis. suppose also that the decrease in temperature from the earth's surface upward proceeds at the rate of °f. for each feet, which is per cent less than the actual rate for air with only a slight amount of moisture, such as is found in cold regions. then at a height of , feet, where the inblowing winds begin to be felt, the temperature would be - °f. at that temperature the air is able to hold approximately . grain of moisture per cubic foot when fully saturated. this is an exceedingly small amount of moisture and even if it were all precipitated could scarcely build a glacier. however, it apparently would not be precipitated because when such air descends in the center of the anti-cyclone it is warmed adiabatically, that is, by compression. on reaching the surface it would have a temperature of ° and would be able to hold . grain of water vapor per cubic foot; in other words, it would have a relative humidity of about per cent. under no reasonable assumption does the upper air at the center of an ice sheet appear to reach the surface with a relative humidity of more than or per cent. such air cannot give up moisture. on the contrary, it absorbs it and tends to diminish rather than increase the thickness of the sheet of ice and snow. but after the surplus heat gained by descent has been lost by radiation, conduction, and evaporation, the air may become super-saturated with the moisture picked up while warm. hobbs reports that explorers in antarctica and greenland have frequently observed condensation on their clothing. if such moisture is not derived directly from the men's own bodies, it is apparently picked up from the ice sheet by the descending air, and not added to the ice sheet by air from aloft. the relation of all this to the localization of ice sheets is this. if hobbs' anti-cyclonic hypothesis of glacial growth is correct, it would appear that ice sheets should grow up where the temperature is lowest and the high-pressure areas most persistent; for instance, in northern siberia. it would also appear that so far as the topography permitted, the ice sheets ought to move out uniformly in all directions; hence the ice sheet ought to be as prominent to the north of the keewatin and labradorean centers as to the south, which is by no means the case. again, in mountainous regions, such as the glacial areas of alaska and chile, the glaciation ought not to be confined to the windward slope of the mountains so closely as is actually the fact. in each of these cases the glaciated region was large enough so that there was probably a true anti-cyclonic area comparable with that now prevailing over southern greenland. in both places the correlation between glaciation and mountain ranges seems much too close to support the anti-cyclonic hypothesis, for the inblowing winds which on that hypothesis bring the moisture are shown by observation to occur at heights far greater than that of all but the loftiest ranges. ii. the sudden coming of glaciation is another problem which has been a stumbling-block in the way of every glacial hypothesis. in his _climates of geologic times_, schuchert states that the fossils give almost no warning of an approaching catastrophe. if glaciation were solely due to uplift, or other terrestrial changes aside from vulcanism, schuchert holds that it would have come slowly and the stages preceding glaciation would have affected life sufficiently to be recorded in the rocks. he considers that the suddenness of the coming of glaciation is one of the strongest arguments against the carbon dioxide hypothesis of glaciation. according to the cyclonic hypothesis, however, the suddenness of the oncoming of glaciation is merely what would be expected on the basis of what happens today. changes in the sun occur suddenly. the sunspot cycle is only eleven or twelve years long, and even this short period of activity is inaugurated more suddenly than it declines. again the climatic record derived from the growth of trees, as given in figs. and , also shows that marked changes in climate are initiated more rapidly than they disappear. in this connection, however, it must be remembered that solar activity may arise in various ways, as will appear more fully later. under certain conditions storminess may increase and decrease slowly. iii. the height of the snow line and of glaciation furnishes another means of testing glacial hypotheses. it is well established that in times of glaciation the snow line was depressed everywhere, but least near the equator. for example, according to penck, permanent snow extended feet lower than now in the alps, whereas it stood only feet below the present level near the equator in venezuela. this unequal depression is not readily accounted for by any hypothesis depending solely upon the lowering of temperature. by the carbon dioxide and the volcanic dust hypotheses, the temperature presumably was lowered almost equally in all latitudes, but a little more at the equator than elsewhere. if glaciation were due to a temporary lessening of the radiation received from the sun, such as is demanded by the thermal solar hypothesis, and by the longer periods of croll's hypothesis, the lowering would be distinctly greatest at the equator. thus, according to all these hypotheses, the snow line should have been depressed most at the equator, instead of least. the cyclonic hypothesis explains the lesser depression of the snow line at the equator as due to a diminution of precipitation. the effectiveness of precipitation in this respect is illustrated by the present great difference in the height of the snow line on the humid and dry sides of mountains. on the wet eastern side of the andes near the equator, the snow line lies at , feet; on the dry western side, at , feet. again, although the humid side of the himalayas lies toward the south, the snow line has a level of , feet, while farther north, on the dry side, it is , feet.[ ] the fact that the snow line is lower near the margin of the alps than toward the center points in the same direction. the bearing of all this on the glacial period may be judged by looking again at fig. in chapter v. this shows that at times of sunspot activity and hence of augmented storminess, the precipitation diminishes near the heat equator, that is, where the average temperature for the whole year is highest. at present the great size of the northern continents and their consequent high temperature in summer, cause the heat equator to lie north of the "real" equator, except where australia draws it to the southward.[ ] when large parts of the northern continents were covered with ice, however, the heat equator and the true equator were probably much closer than now, for the continents could not become so hot. if so, the diminution in equatorial precipitation, which accompanies increased storminess throughout the world as a whole, would take place more nearly along the true equator than appears in fig. . hence so far as precipitation alone is concerned, we should actually expect that the snow line near the equator would rise a little during glacial periods. another factor, however, must be considered. köppen's data, it will be remembered, show that at times of solar activity the earth's temperature falls more at the equator than in higher latitudes. if this effect were magnified it would lower the snow line. the actual position of the snow line at the equator during glacial periods thus appears to be the combined effect of diminished precipitation, which would raise the line, and of lower temperature, which would bring it down. before leaving this subject it may be well to recall that the relative lessening of precipitation in equatorial latitudes during the glacial epochs was probably caused by the diversion of moisture from the trade-wind belt. this diversion was presumably due to the great number of tropical cyclones and to the fact that the cyclonic storms of middle latitudes also drew much moisture from the trade-wind belt in summer when the northern position of the sun drew that belt near the storm track which was forced to remain south of the ice sheet. such diversion of moisture out of the trade-wind belt must diminish the amount of water vapor that is carried by the trades to equatorial regions; hence it would lessen precipitation in the belt of so-called equatorial calms, which lies along the heat equator rather than along the geographical equator. another phase of the vertical distribution of glaciation has been the subject of considerable discussion. in the alps and in many other mountains the glaciation of the pleistocene period appears to have had its upper limit no higher than today. this has been variously interpreted. it seems, however, to be adequately explained as due to decreased precipitation at high altitudes during the cold periods. this is in spite of the fact that precipitation in general increased with increased storminess. the low temperature of glacial times presumably induced condensation at lower altitudes than now, and most of the precipitation occurred upon the lower slopes of the mountains, contributing to the lower glaciers, while little of it fell upon the highest glaciers. above a moderate altitude in all lofty mountains the decrease in the amount of precipitation is rapid. in most cases the decrease begins at a height of less than feet above the base of the main slope, provided the slope is steep. the colder the air, the lower the altitude at which this occurs. for example, it is much lower in winter than in summer. indeed, the higher altitudes in the alps are sunny in winter even where there are abundant clouds lower down. iv. the presence of extensive lakes and other evidences of a pluvial climate during glacial periods in non-glaciated regions which are normally dry is another of the facts which most glacial hypotheses fail to explain satisfactorily. beyond the ice sheets many regions appear to have enjoyed an unusually heavy precipitation during the glacial epochs. the evidence of this is abundant, including numerous abandoned strand lines of salt lakes and an abundance of coarse material in deltas and flood plains. j. d. whitney,[ ] in an interesting but neglected volume, was one of the first to marshal the evidence of this sort. more recently free[ ] has amplified this. according to him in the great basin region of the united states sixty-two basins either contain unmistakable evidence of lakes, or belong to one of the three great lake groups named below. two of these, the lake lahontan and the lake bonneville groups, comprise twenty-nine present basins, while the third, the owens-searles chain, contained at least five large lakes, the lowest being in death valley. in western and central asia a far greater series of salt lakes is found and most of these are surrounded by strands at high levels. many of these are described in _explorations in turkestan_, _the pulse of asia_, and _palestine and its transformation_. there has been a good deal of debate as to whether these lakes actually date from the glacial period, as is claimed by c. e. p. brooks, for example, or from some other period. the evidence, however, seems to be convincing that the lakes expanded when the ice also expanded. according to the older glacial hypotheses the lower temperature which is postulated as the cause of glaciation would almost certainly mean less evaporation over the oceans and hence less precipitation during glacial periods. to counteract this the only way in which the level of the lakes could be raised would be because the lower temperature would cause less evaporation from their surfaces. it seems quite impossible, however, that the lowering of temperature, which is commonly taken to have been not more than °c., could counteract the lessened precipitation and also cause an enormous expansion of most of the lakes. for example, ancient lake bonneville was more than ten times as large as its modern remnant, great salt lake, and its average depth more than forty times as great.[ ] many small lakes in the old world expanded still more.[ ] for example, in eastern persia many basins which now contain no lake whatever are floored with vast deposits of lacustrine salt and are surrounded by old lake bluffs and beaches. in northern africa similar conditions prevail.[ ] other, but less obvious, evidence of more abundant rainfall in regions that are now dry is found in thick strata of gravel, sand, and fine silt in the alluvial deposits of flood plains and deltas.[ ] the cyclonic hypothesis supposes that increased storminess accounts for pluvial climates in regions that are now dry just as it accounts for glaciation in the regions of the ice sheets. figs. and , it will be remembered, illustrate what happens when the sun is active. solar activity is accompanied by an increase in storminess in the southwestern united states in exactly the region where elevated strands of diminished salt lakes are most numerous. in fig. , the same condition is seen in the region of salt lakes in the old world. judging by these maps, which illustrate what has happened since careful meteorological records were kept, an increase in solar activity is accompanied by increased rainfall in large parts of what are now semi-arid and desert regions. such precipitation would at once cause the level of the lakes to rise. later, when ice sheets had developed in europe and america, the high-pressure areas thus caused might force the main storm belt so far south that it would lie over these same arid regions. the increase in tropical hurricanes at times of abundant sunspots may also have a bearing on the climate of regions that are now arid. during the glacial period some of the hurricanes probably swept far over the lands. the numerous tropical cyclones of australia, for example, are the chief source of precipitation for that continent.[ ] some of the stronger cyclones locally yield more rain in a day or two than other sources yield in a year. v. the occurrence of widespread glaciation near the tropics during the permian, as shown in fig. , has given rise to much discussion. the recent discovery of glaciation in latitudes as low as ° in the proterozoic is correspondingly significant. in all cases the occurrence of glaciation in low and middle latitudes is probably due to the same general causes. doubtless the position and altitude of the mountains had something to do with the matter. yet taken by itself this seems insufficient. today the loftiest range in the world, the himalayas, is almost unglaciated, although its southern slope may seem at first thought to be almost ideally located in this respect. some parts rise over , feet and certain lower slopes receive inches of rain per year. the small size of the himalayan glaciers in spite of these favorable conditions is apparently due largely to the seasonal character of the monsoon winds. the strong outblowing monsoons of winter cause about half the year to be very dry with clear skies and dry winds from the interior of asia. in all low latitudes the sun rides high in the heavens at midday, even in winter, and thus melts snow fairly effectively in clear weather. this is highly unfavorable to glaciation. the inblowing southern monsoons bring all their moisture in midsummer at just the time when it is least effective in producing snow. conditions similar to those now prevailing in the himalayas must accompany any great uplift of the lands which produces high mountains and large continents in subtropical and middle latitudes. hence, uplift alone cannot account for extensive glaciation in subtropical latitudes during the permian and proterozoic. [illustration: _fig. . permian geography and glaciation._ (_after schuchert._)] the assumption of a great general lowering of temperature is also not adequate to explain glaciation in subtropical latitudes. in the first place this would require a lowering of many degrees,--far more than in the pleistocene glacial period. the marine fossils of the permian, however, do not indicate any such condition. in the second place, if the lands were widespread as they appear to have been in the permian, a general lowering of temperature would diminish rather than increase the present slight efficiency of the monsoons in producing glaciation. monsoons depend upon the difference between the temperatures of land and water. if the general temperature were lowered, the reduction would be much less pronounced on the oceans than on the lands, for water tends to preserve a uniform temperature, not only because of its mobility, but because of the large amount of heat given out when freezing takes place, or consumed in evaporation. hence the general lowering of temperature would make the contrast between continents and oceans less than at present in summer, for the land temperature would be brought toward that of the ocean. this would diminish the strength of the inblowing summer monsoons and thus cut off part of the supply of moisture. evidence that this actually happened in the cold fourteenth century has already been given in chapter vi. on the other hand, in winter the lands would be much colder than now and the oceans only a little colder, so that the dry outblowing monsoons of the cold season would increase in strength and would also last longer than at present. in addition to all this, the mere fact of low temperature would mean a general reduction in the amount of water vapor in the air. thus, from almost every point of view a mere lowering of temperature seems to be ruled out as a cause of permian glaciation. moreover, if the permian or proterozoic glacial periods were so cold that the lands above latitude ° were snow-covered most of the time, the normal surface winds in subtropical latitudes would be largely equatorward, just as the winter monsoons now are. hence little or no moisture would be available to feed the snowfields which give rise to the glaciers. it has been assumed by marsden manson and others that increased general cloudiness would account for the subtropical glaciation of the permian and proterozoic. granting for the moment that there could be universal persistent cloudiness, this would not prevent or counteract the outblowing anti-cyclonic winds so characteristic of great snowfields. therefore, under the hypothesis of general cloudiness there would be no supply of moisture to cause glaciation in low latitudes. indeed, persistent cloudiness in all higher latitudes would apparently deprive the himalayas of most of their present moisture, for the interior of asia would not become hot in summer and no inblowing monsoons would develop. in fact, winds of all kinds would seemingly be scarce, for they arise almost wholly from contrasts of temperature and hence of atmospheric pressure. the only way to get winds and hence precipitation would be to invoke some other agency, such as cyclonic storms, but that would be a departure from the supposition that glaciation arose from cloudiness. let us now inquire how the cyclonic hypothesis accounts for glaciation in low latitudes. we will first consider the terrestrial conditions in the early permian, the last period of glaciation in such latitudes. geologists are almost universally agreed that the lands were exceptionally extensive and also high, especially in low latitudes. one evidence of this is the presence of abundant conglomerates composed of great boulders. it is also probable that the carbon dioxide in the air during the early permian had been reduced to a minimum by the extraordinary amount of coal formed during the preceding period. this would tend to produce low temperature and thus make the conditions favorable for glaciation as soon as an accentuation of solar activity caused unusual storminess. if the storminess became extreme when terrestrial conditions were thus universally favorable to glaciation, it would presumably produce glaciation in low latitudes. numerous and intense tropical cyclones would carry a vast amount of moisture out of the tropics, just as now happens when the sun is active, but on a far larger scale. the moisture would be precipitated on the equatorward slopes of the subtropical mountain ranges. at high elevations this precipitation would be in the form of snow even in summer. tropical cyclones, however, as is shown in _earth and sun_, occur in the autumn and winter as well as in summer. for example, in the bay of bengal the number recorded in october is fifty, the largest for any month; while in november it is thirty-four, and december fourteen as compared with an average of forty-two for the months of july to september. from january to march, when sunspot numbers averaged more than forty, the number of tropical hurricanes was per cent greater than when the sunspot numbers averaged below forty. during the months from april to june, which also would be times of considerable snowy precipitation, tropical hurricanes averaged per cent more numerous with sunspot numbers above forty than with numbers below forty, while from july to september the difference amounted to per cent. even at this season some snow falls on the higher slopes, while the increased cloudiness due to numerous storms also tends to preserve the snow. thus a great increase in the frequency of sunspots is accompanied by increased intensity of tropical hurricanes, especially in the cooler autumn and spring months, and results not only in a greater accumulation of snow but in a decrease in the melting of the snow because of more abundant clouds. at such times as the permian, the general low temperature due to rapid convection and to the scarcity of carbon dioxide presumably joined with the extension of the lands in producing great high-pressure areas over the lands in middle latitudes during the winters, and thus caused the more northern, or mid-latitude type of cyclonic storms to be shifted to the equatorward side of the continents at that season. this would cause an increase of precipitation in winter as well as during the months when tropical hurricanes abound. many other circumstances would coöperate to produce a similar result. for example, the general low temperature would cause the sea to be covered with ice in lower latitudes than now, and would help to create high-pressure areas in middle latitudes, thus driving the storms far south. if the sea water were fresher than now, as it probably was to a notable extent in the proterozoic and perhaps to some slight extent in the permian, the higher freezing point would also further the extension of the ice and help to keep the storms away from high latitudes. if to this there is added a distribution of land and sea such that the volume of the warm ocean currents flowing from low to high latitudes was diminished, as appears to have been the case, there seems to be no difficulty in explaining the subtropical location of the main glaciation in both the permian and the proterozoic. an increase of storminess seems to be the key to the whole situation. one other possibility may be mentioned, although little stress should be laid on it. in _earth and sun_ it has been shown that the main storm track in both the northern and southern hemispheres is not concentric with the geographical poles. both tracks are roughly concentric with the corresponding magnetic poles, a fact which may be important in connection with the hypothesis of an electrical effect of the sun upon terrestrial storminess. the magnetic poles are known to wander considerably. such wandering gives rise to variations in the direction of the magnetic needle from year to year. in the compass in england pointed - / ° w. of n. and in ° ' w. such a variation seems to mean a change of many miles in the location of the north magnetic pole. certain changes in the daily march of electromagnetic phenomena over the oceans have led bauer and his associates to suggest that the magnetic poles may even be subject to a slight daily movement in response to the changes in the relative positions of the earth and sun. thus there seems to be a possibility that a pronounced change in the location of the magnetic pole in permian times, for example, may have had some connection with a shifting in the location of the belt of storms. it must be clearly understood that there is as yet no evidence of any such change, and the matter is introduced merely to call attention to a possible line of investigation. any hypothesis of permian and proterozoic glaciation must explain not only the glaciation of low latitudes but the lack of glaciation and the accumulation of red desert beds in high latitudes. the facts already presented seem to explain this. glaciation could not occur extensively in high latitudes partly because during most of the year the air was too cold to hold much moisture, but still more because the winds for the most part must have blown outward from the cold northern areas and the cyclonic storm belt was pushed out of high latitudes. because of these conditions precipitation was apparently limited to a relatively small number of storms during the summer. hence great desert areas must have prevailed at high latitudes. great aridity now prevails north of the himalayas and related ranges, and red beds are accumulating in the centers of the great deserts, such as those of the tarim basin and the transcaspian. the redness is not due to the original character of the rock, but to intense oxidation, as appears from the fact that along the edges of the desert and wherever occasional floods carry sediment far out into the midst of the sand, the material has the ordinary brownish shades. as soon as one goes out into the places where the sand has been exposed to the air for a long time, however, it becomes pink, and then red. such conditions may have given rise to the high degree of oxidation in the famous permian red beds. if the air of the early permian contained an unusual percentage of oxygen because of the release of that gas by the great plant beds which formed coal in the preceding era, as chamberlin has thought probable, the tendency to produce red beds would be still further increased. it must not be supposed, however, that these conditions would absolutely limit glaciation to subtropical latitudes. the presence of early permian glaciation in north america at boston and in alaska and in the falkland islands of the south atlantic ocean proves that at least locally there was sufficient moisture to form glaciers near the coast in relatively high latitudes. the possibility of this would depend entirely upon the form of the lands and the consequent course of ocean currents. even in those high latitudes cyclonic storms would occur unless they were kept out by conditions of pressure such as have been described above. the marine faunas of permian age in high latitudes have been interpreted as indicating mild oceanic temperatures. this is a point which requires further investigation. warm oceans during times of slight solar activity are a necessary consequence of the cyclonic hypothesis, as will appear later. the present cold oceans seem to be the expectable result of the pleistocene glaciation and of the present relatively disturbed condition of the sun. if a sudden disturbance threw the solar atmosphere into violent commotion within a few thousand years during permian times, glaciation might occur as described above, while the oceans were still warm. in fact their warmth would increase evaporation while the violent cyclonic storms and high winds would cause heavy rain and keep the air cool by constantly raising it to high levels where it would rapidly radiate its heat into space. nevertheless it is not yet possible to determine how warm the oceans were at the actual time of the permian glaciation. some faunas formerly reported as permian are now known to be considerably older. moreover, others of undoubted permian age are probably not strictly contemporaneous with the glaciation. so far back in the geological record it is very doubtful whether we can date fossils within the limits of say , years. yet a difference of , years would be more than enough to allow the fossils to have lived either before or after the glaciation, or in an inter-glacial epoch. one such epoch is known to have occurred and nine others are suggested by the inter-stratification of glacial till and marine sediments in eastern australia. the warm currents which would flow poleward in inter-glacial epochs must have favored a prompt reintroduction of marine faunas driven out during times of glaciation. taken all and all, the permian glaciation seems to be accounted for by the cyclonic hypothesis quite as well as does the pleistocene. in both these cases, as well as in the various pulsations of historic times, it seems to be necessary merely to magnify what is happening today in order to reproduce the conditions which prevailed in the past. if the conditions which now prevail at times of sunspot minima were magnified, they would give the mild conditions of inter-glacial epochs and similar periods. if the conditions which now prevail at times of sunspot maxima are magnified a little they seem to produce periods of climatic stress such as those of the fourteenth century. if they are magnified still more the result is apparently glacial epochs like those of the pleistocene, and if they are magnified to a still greater extent, the result is permian or proterozoic glaciation. other factors must indeed be favorable, for climatic changes are highly complex and are unquestionably due to a combination of circumstances. the point which is chiefly emphasized in this book is that among those several circumstances, changes in cyclonic storms due apparently to activity of the sun's atmosphere must always be reckoned. footnotes: [footnote : w. h. hobbs: characteristics of existing glaciers, . the rôle of the glacial anticyclones in the air circulation of the globe; proc. am. phil. soc., vol. , , pp. - .] [footnote : r. d. salisbury: physiography, .] [footnote : griffith taylor: australian meteorology, , p. .] [footnote : j. d. whitney: climatic changes of the later geological times, .] [footnote : e. e. free: u. s. dept. of agriculture, bull. , . mr. free has prepared a summary of this bulletin which appears in the solar hypothesis, bull. geol. sec. of am., vol. , pp. - .] [footnote : g. k. gilbert: lake bonneville; monograph , u. s. geol. surv.] [footnote : c. e. p. brooks: quart. jour. royal meteorol. soc., , pp. - .] [footnote : h. j. l. beadnell: a. egyptian oasis, london, . ellsworth huntington: the libyan oasis of kharga; bull. am. geog. soc., vol. , sept., , pp. - .] [footnote : s. s. visher: the bajada of the tucson bolson of southern arizona; science, n. s., mar. , . ellsworth huntington: the basins of eastern persia and seistan, in explorations in turkestan.] [footnote : griffith taylor: australian meteorology, , p. .] chapter ix the origin of loess one of the most remarkable formations associated with glacial deposits consists of vast sheets of the fine-grained, yellowish, wind-blown material called loess. somewhat peculiar climatic conditions evidently prevailed when it was formed. at present similar deposits are being laid down only near the leeward margin of great deserts. the famous loess deposits of china in the lee of the desert of gobi are examples. during the pleistocene period, however, loess accumulated in a broad zone along the margin of the ice sheet at its maximum extent. in the old world it extended from france across germany and through the black earth region of russia into siberia. in the new world a still larger area is loess-covered. in the mississippi valley, tens of thousands of square miles are mantled by a layer exceeding twenty feet in thickness and in many places approaching a hundred feet. neither the north american nor the european deposits are associated with a desert. indeed, loess is lacking in the western and drier parts of the great plains and is best developed in the well-watered states of iowa, illinois, and missouri. part of the loess overlies the non-glacial materials of the great central plain, but the northern portions overlie the drift deposits of the first three glaciations. a few traces of loess are associated with the kansan and illinoian, the second and third glaciations, but most of the america loess appears to have been formed at approximately the time of the iowan or fourth glaciation, while only a little overlies the drift sheets of the wisconsin age. the loess is thickest near the margin of the iowan till sheet and thins progressively both north and south. the thinning southward is abrupt along the stream divides, but very gradual along the larger valleys. indeed, loess is abundant along the bluffs of the mississippi, especially the east bluff, almost to the gulf of mexico.[ ] it is now generally agreed that all typical loess is wind blown. there is still much question, however, as to its time of origin, and thus indirectly as to its climatic implications. several american and european students have thought that the loess dates from inter-glacial times. on the other hand, penck has concluded that the loess was formed shortly before the commencement of the glacial epochs; while many american geologists hold that the loess accumulated while the ice sheets were at approximately their maximum size. w. j. mcgee, chamberlin and salisbury, keyes, and others lean toward this view. in this chapter the hypothesis is advanced that it was formed at the one other possible time, namely, immediately following the retreat of the ice. these four hypotheses as to the time of origin of loess imply the following differences in its climatic relations. if loess was formed during typical inter-glacial epochs, or toward the close of such epochs, profound general aridity must seemingly have prevailed in order to kill off the vegetation and thus enable the wind to pick up sufficient dust. if the loess was formed during times of extreme glaciation when the glaciers were supplying large quantities of fine material to outflowing streams, less aridity would be required, but there must have been sharp contrasts between wet seasons in summer when the snow was melting and dry seasons in winter when the storms were forced far south by the glacial high pressure. alternate floods and droughts would thus affect broad areas along the streams. hence arises the hypothesis that the wind obtained the loess from the flood plains of streams at times of maximum glaciation. if the loess was formed during the rapid retreat of the ice, alternate summer floods and winter droughts would still prevail, but much material could also be obtained by the winds not only from flood plains, but also from the deposits exposed by the melting of the ice and not yet covered by vegetation. the evidence for and against the several hypotheses may be stated briefly. in support of the hypothesis of the inter-glacial origin of loess, shimek and others state that the glacial drift which lies beneath the loess commonly gives evidence that some time elapsed between the disappearance of the ice and the deposition of the loess. for example, abundant shells of land snails in the loess are not of the sort now found in colder regions, but resemble those found in the drier regions. it is probable that if they represented a glacial epoch they would be depauperated by the cold as are the snails of far northern regions. the gravel pavement discussed below seems to be strong evidence of erosion between the retreat of the ice and the deposition of the loess. turning to the second hypothesis, namely, that the loess accumulated near the close of the inter-glacial epoch rather than in the midst of it, we may follow penck. the mammalian fossils seem to him to prove that the loess was formed while boreal animals occupied the region, for they include remains of the hairy mammoth, woolly rhinoceros, and reindeer. on the other hand, the typical inter-glacial beds not far away yield remains of species characteristic of milder climates, such as the elephant, the smaller rhinoceros, and the deer. in connection with these facts it should be noted that occasional remains of tundra vegetation and of trees are found beneath the loess, while in the loess itself certain steppe animals, such as the common gopher or spermaphyl, are found. penck interprets this as indicating a progressive desiccation culminating just before the oncoming of the next ice sheet. the evidence advanced in favor of the hypothesis that the loess was formed when glaciation was near its maximum includes the fact that if the loess does not represent the outwash from the iowan ice, there is little else that does, and presumably there must have been outwash. also the distribution of loess along the margins of streams suggests that much of the material came from the flood plains of overloaded streams flowing from the melting ice. although there are some points in favor of the hypothesis that the loess originated ( ) in strictly inter-glacial times, ( ) at the end of inter-glacial epochs, and ( ) at times of full glaciation, each hypothesis is much weakened by evidence that supports the others. the evidence of boreal animals seems to disprove the hypothesis that the loess was formed in the middle of a mild inter-glacial epoch. on the other hand, penck's hypothesis as to loess at the end of inter-glacial times fails to account for certain characteristics of the lowest part of the loess deposits and of the underlying topography. instead of normal valleys and consequent prompt drainage such as ought to have developed before the end of a long inter-glacial epoch, the surface on which the loess lies shows many undrained depressions. some of these can be seen in exposed banks, while many more are inferred from the presence of shells of pond snails here and there in the overlying loess. the pond snails presumably lived in shallow pools occupying depressions in the uneven surface left by the ice. another reason for questioning whether the loess was formed at the end of an inter-glacial epoch is that this hypothesis does not provide a reasonable origin for the material which composes the loess. near the alps where the loess deposits are small and where glaciers probably persisted in the inter-glacial epochs and thus supplied flood plain material in large quantities, this does not appear important. in the broad upper mississippi basin, however, and also in the black earth region of russia there seems to be no way to get the large body of material composing the loess except by assuming the existence of great deserts to windward. but there seems to be little or no evidence of such deserts where they could be effective. the mineralogical character of the loess of iowan age proves that the material came from granitic rocks, such as formed a large part of the drift. the nearest extensive outcrops of granite are in the southwestern part of the united states, nearly a thousand miles from iowa and illinois. but the loess is thickest near the ice margin and thins toward the southwest and in other directions, whereas if its source were the southwestern desert, its maximum thickness would probably be near the margin of the desert. the evidence cited above seems inconsistent not only with the hypothesis that the loess was formed at the end of an inter-glacial epoch, but also with the idea that it originated at times of maximum glaciation either from river-borne sediments or from any other source. a further and more convincing reason for this last conclusion is the probability and almost the certainty that when the ice advanced, its front lay close to areas where the vegetation was not much thinner than that which today prevails under similar climatic conditions. if the average temperature of glacial maxima was only °c. lower than that of today, the conditions just beyond the ice front when it was in the loess region from southern illinois to minnesota would have been like those now prevailing in canada from new brunswick to winnipeg. the vegetation there is quite different from the grassy, semi-arid vegetation of which evidence is found in the loess. the roots and stalks of such grassy vegetation are generally agreed to have helped produce the columnar structure which enables the loess to stand with almost vertical surfaces. we are now ready to consider the probability that loess accumulated mainly during the retreat of the ice. such a retreat exposed a zone of drift to the outflowing glacial winds. most glacial hypotheses, such as that of uplift, or depleted carbon dioxide, call for a gradual retreat of the ice scarcely faster than the vegetation could advance into the abandoned area. under the solar-cyclonic hypothesis, on the other hand, the climatic changes may have been sudden and hence the retreat of the ice may have been much more rapid than the advance of vegetation. now wind-blown materials are derived from places where vegetation is scanty. scanty vegetation on good soil, it is true, is usually due to aridity, but may also result because the time since the soil was exposed to the air has not been long enough for the soil to be sufficiently weathered to support vegetation. even when weathering has had full opportunity, as when sand bars, mud flats, and flood plains are exposed, vegetation takes root only slowly. moreover, storms and violent winds may prevent the spread of vegetation, as is seen on sandy beaches even in distinctly humid regions like new jersey and denmark. thus it appears that unless the retreat of the ice were as slow as the advance of vegetation, a barren area of more or less width must have bordered the retreating ice and formed an ideal source of loess. several other lines of evidence seemingly support the conclusion that the loess was formed during the retreat of the ice. for example, shimek, who has made almost a lifelong study of the iowan loess, emphasizes the fact that there is often an accumulation of stones and pebbles at its base. this suggests that the underlying till was eroded before the loess was deposited upon it. the first reaction of most students is to assume that of course this was due to running water. that is possible in many cases, but by no means in all. so widespread a sheet of gravel could not be deposited by streams without destroying the irregular basins and hollows of which we have seen evidence where the loess lies on glacial deposits. on the other hand, the wind is competent to produce a similar gravel pavement without disturbing the old topography. "desert pavements" are a notable feature in most deserts. on the edges of an ice sheet, as hobbs has made us realize, the commonest winds are outward. they often attain a velocity of eighty miles an hour in antarctica and greenland. such winds, however, usually decline rapidly in velocity only a few score miles from the ice. thus their effect would be to produce rapid erosion of the freshly bared surface near the retreating ice. the pebbles would be left behind as a pavement, while sand and then loess would be deposited farther from the ice where the winds were weaker and where vegetation was beginning to take root. such a decrease in wind velocity may explain the occasional vertical gradation from gravel through sand to coarse loess and then to normal fine loess. as the ice sheet retreated the wind in any given place would gradually become less violent. as the ice continued to retreat the area where loess was deposited would follow at a distance, and thus each part of the gravel pavement would in turn be covered with the loess. the hypothesis that loess is deposited while the ice is retreating is in accord with many other lines of evidence. for example, it accords with the boreal character of the mammal remains as described above. again, the advance of vegetation into the barren zone along the front of the ice would be delayed by the strong outblowing winds. the common pioneer plants depend largely on the wind for the distribution of their seeds, but the glacial winds would carry them away from the ice rather than toward it. the glacial winds discourage the advance of vegetation in another way, for they are drying winds, as are almost all winds blowing from a colder to a warmer region. the fact that remains of trees sometimes occur at the bottom of the loess probably means that the deposition of loess extended into the forests which almost certainly persisted not far from the ice. this seems more likely than that a period of severe aridity before the advance of the ice killed the trees and made a steppe or desert. penck's chief argument in favor of the formation of loess before the advance of the ice rather than after, is that since loess is lacking upon the youngest drift sheet in europe it must have been formed before rather than after the last or würm advance of the ice. this breaks down on two counts. first, on the corresponding (wisconsin) drift sheet in america, loess is present,--in small quantities to be sure, but unmistakably present. second, there is no reason to assume that conditions were identical at each advance and retreat of the ice. indeed, the fact that in europe, as in the united states, nearly all the loess was formed at one time, and only a little is associated with the other ice advances, points clearly against penck's fundamental assumption that the accumulation of loess was due to the approach of a cold climate. having seen that the loess was probably formed during the retreat of the ice, we are now ready to inquire what conditions the cyclonic hypothesis would postulate in the loess areas during the various stages of a glacial cycle. fig. , in chapter iv, gives the best idea of what would apparently happen in north america, and events in europe would presumably be similar. during the nine maximum years on which fig. is based the sunspot numbers averaged seventy, while during the nine minimum years they averaged less than five. it seems fair to suppose that the maximum years represent the average conditions which prevailed in the past at times when the sun was in a median stage between the full activity which led to glaciation and the mild activity of the minimum years which appear to represent inter-glacial conditions. this would mean that when a glacial period was approaching, but before an ice sheet had accumulated to any great extent, a crescent-shaped strip from montana through illinois to maine would suffer a diminution in storminess ranging up to per cent as compared with inter-glacial conditions. this is in strong contrast with an increase in storminess amounting to or even per cent both in the boreal storm belt in canada and in the subtropical belt in the southwest. such a decrease in storminess in the central united states would apparently be most noticeable in summer, as is shown in _earth and sun_. hence it would have a maximum effect in producing aridity. this would favor the formation of loess, but it is doubtful whether the aridity would become extreme enough to explain such vast deposits as are found throughout large parts of the mississippi basin. that would demand that hundreds of thousands of square miles should become almost absolute desert, and it is not probable that any such thing occurred. nevertheless, according to the cyclonic hypothesis the period immediately before the advent of the ice would be relatively dry in the central united states, and to that extent favorable to the work of the wind. as the climatic conditions became more severe and the ice sheet expanded, the dryness and lack of storms would apparently diminish. the reason, as has been explained, would be the gradual pushing of the storms southward by the high-pressure area which would develop over the ice sheet. thus at the height of a glacial epoch there would apparently be great storminess in the area where the loess is found, especially in summer. hence the cyclonic hypothesis does not accord with the idea of great deposition of loess at the time of maximum glaciation. finally we come to the time when the ice was retreating. we have already seen that not only the river flood plains, but also vast areas of fresh glacial deposits would be exposed to the winds, and would remain without vegetation for a long time. at that very time the retreat of the ice sheet would tend to permit the storms to follow paths determined by the degree of solar activity, in place of the far southerly paths to which the high atmospheric pressure over the expanded ice sheet had previously forced them. in other words, the conditions shown in fig. would tend to reappear when the sun's activity was diminishing and the ice sheet was retreating, just as they had appeared when the sun was becoming more active and the ice sheet was advancing. this time, however, the semi-arid conditions arising from the scarcity of storms would prevail in a region of glacial deposits and widely spreading river deposits, few or none of which would be covered with vegetation. the conditions would be almost ideal for eolian erosion and for the transportation of loess by the wind to areas a little more remote from the ice where grassy vegetation had made a start. the cyclonic hypothesis also seems to offer a satisfactory explanation of variations in the amount of loess associated with the several glacial epochs. it attributes these to differences in the rate of disappearance of the ice, which in turn varied with the rate of decline of solar activity and storminess. this is supposed to be the reason why the iowan loess deposits are much more extensive than those of the other epochs, for the iowan ice sheet presumably accomplished part of its retreat much more suddenly than the other ice sheets.[ ] the more sudden the retreat, the greater the barren area where the winds could gather fine bits of dust. temporary readvances may also have been so distributed and of such intensity that they frequently accentuated the condition shown in fig. , thus making the central united states dry soon after the exposure of great amounts of glacial débris. the closeness with which the cyclonic hypothesis accords with the facts as to the loess is one of the pleasant surprises of the hypothesis. the first draft of fig. and the first outlines of the hypothesis were framed without thought of the loess. yet so far as can now be seen, both agree closely with the conditions of loess formation. footnotes: [footnote : chamberlin and salisbury: geology, , vol. iii, pp. - .] [footnote : it may have retreated soon after reaching its maximum. if so, the general lack of thick terminal moraines would be explained. see page .] chapter x causes of mild geological climates in discussions of climate, as of most subjects, a peculiar psychological phenomenon is observable. everyone sees the necessity of explaining conditions different from those that now exist, but few realize that present conditions may be abnormal, and that they need explanation just as much as do others. because of this tendency glaciation has been discussed with the greatest fullness, while there has been much neglect not only of the periods when the climate of the earth resembled that of the present, but also of the vastly longer periods when it was even milder than now. how important the periods of mild climate have been in geological times may be judged from the relative length of glacial compared with inter-glacial epochs, and still more from the far greater relative length of the mild parts of periods and eras when compared with the severe parts. recent estimates by r. t. chamberlin[ ] indicate that according to the consensus of opinion among geologists the average inter-glacial epoch during the pleistocene was about five times as long as the average glacial epoch, while the whole of a given glacial epoch averaged five times as long as the period when the ice was at a maximum. climatic periods far milder, longer, and more monotonous than any inter-glacial epoch appear repeatedly during the course of geological history. our task in this chapter is to explain them. knowlton[ ] has done geology a great service by collecting the evidence as to the mild type of climate which has again and again prevailed in the past. he lays special stress on botanical evidence since that pertains to the variable atmosphere of the lands, and hence furnishes a better guide than does the evidence of animals that lived in the relatively unchanging water of the oceans. the nature of the evidence has already been indicated in various parts of this book. it includes palms, tree ferns, and a host of other plants which once grew in regions which are now much too cold to support them. with this must be placed the abundant reef-building corals and other warmth-loving marine creatures in latitudes now much too cold for them. of a piece with this are the conditions of inter-glacial epochs in europe, for example, when elephants and hippopotamuses, as well as many species of plants from low latitudes, were abundant. these conditions indicate not only that the climate was warmer than now, but that the contrast from season to season was much less. indeed, knowlton goes so far as to say that "relative uniformity, mildness, and comparative equability of climate, accompanied by high humidity, have prevailed over the greater part of the earth, extending to, or into, polar circles, during the greater part of geologic time--since, at least, the middle paleozoic. this is the regular, the ordinary, the normal condition." ... "by many it is thought that one of the strongest arguments against a gradually cooling globe and a humid, non-zonally disposed climate in the ages before the pleistocene is the discovery of evidences of glacial action practically throughout the entire geologic column. hardly less than a dozen of these are now known, ranging in age from huronian to eocene. it seems to be a very general assumption by those who hold this view that these evidences of glacial activities are to be classed as ice ages, largely comparable in effect and extent to the pleistocene refrigeration, but as a matter of fact only three are apparently of a magnitude to warrant such designation. these are the huronian glaciation, that of the 'permo-carboniferous,' and that of the pleistocene. the others, so far as available data go, appear to be explainable as more or less local manifestations that had no widespread effect on, for instance, ocean temperatures, distribution of life, et cetera. they might well have been of the type of ordinary mountain glaciers, due entirely to local elevation and precipitation." ... "if the sun had been the principal source of heat in pre-pleistocene time, terrestrial temperatures would of necessity have been disposed in zones, whereas the whole trend of this paper has been the presentation of proof that these temperatures were distinctly non-zonal. therefore it seems to follow that the sun--at least the present small-angle sun--could not have been the sole or even the principal source of heat that warmed the early oceans." knowlton is so strongly impressed by the widespread fossil floras that usually occur in the middle parts of the geological periods, that as schuchert[ ] puts it, he neglects the evidence of other kinds. in the middle of the periods and eras the expansion of the warm oceans over the continents was greatest, while the lands were small and hence had more or less insular climates of the oceanic type. at such times, the marine fauna agrees with the flora in indicating a mild climate. large colony-forming foraminifera, stony corals, shelled cephalopods, gastropods and thick-shelled bivalves, generally the cemented forms, were common in the far north and even in the arctic. this occurred in the silurian, devonian, pennsylvanian, and jurassic periods, yet at other times, such as the cretaceous and eocene, such forms were very greatly reduced in variety in the northern regions or else wholly absent. these things, as schuchert[ ] says, can only mean that knowlton is right when he states that "climatic zoning such as we have had since the beginning of the pleistocene did not obtain in the geologic ages prior to the pleistocene." it does not mean, however, that there was a "non-zonal arrangement" and that the temperature of the oceans was everywhere the same and "without widespread effect on the distribution of life." students of paleontology hold that as far back as we can go in the study of plants, there are evidences of seasons and of relatively cool climates in high latitudes. the cycads, for instance, are one of the types most often used as evidence of a warm climate. yet wieland,[ ] who has made a lifelong study of these plants, says that many of them "might well grow in temperate to cool climates. until far more is learned about them they should at least be held as valueless as indices of tropic climates." the inference is "that either they or their close relatives had the capacity to live in every clime. there is also a suspicion that study of the associated ferns may compel revision of the long-accepted view of the universality of tropic climates throughout the mesozoic." nathorst is quoted by wieland as saying, "i think ... that during the time when the gingkophytes and cycadophytes dominated, many of them must have adapted themselves for living in cold climates also. of this i have not the least doubt." another important line of evidence which knowlton and others have cited as a proof of the non-zonal arrangement of climate in the past, is the vast red beds which are found in the proterozoic, late silurian, devonian, permian, and triassic, and in some tertiary formations. these are believed to resemble laterite, a red and highly oxidized soil which is found in great abundance in equatorial regions. knowlton does not attempt to show that the red beds present equatorial characteristics in other respects, but bases his conclusion on the statement that "red beds are not being formed at the present time in any desert region." this is certainly an error. as has already been said, in both the transcaspian and takla makan deserts, the color of the sand regularly changes from brown on the borders to pale red far out in the desert. kuzzil kum, or red sand, is the native name. the sands in the center of the desert apparently were originally washed down from the same mountains as those on the borders, and time has turned them red. since the same condition is reported from the arabian desert, it seems that redness is characteristic of some of the world's greatest deserts. moreover, beds of salt and gypsum are regularly found in red beds, and they can scarcely originate except in deserts, or in shallow almost landlocked bays on the coasts of deserts, as appears to have happened in the silurian where marine fossils are found interbedded with gypsum. again, knowlton says that red beds cannot indicate deserts because the plants found in them are not "pinched or depauperate, nor do they indicate xerophytic adaptations. moreover, very considerable deposits of coal are found in red beds in many parts of the world, which implies the presence of swamps but little above sea-level." students of desert botany are likely to doubt the force of these considerations. as macdougal[ ] has shown, the variety of plants in deserts is greater than in moist regions. not only do xerophytic desert species prevail, but halophytes are present in the salty areas, and hygrophytes in the wet swampy areas, while ordinary mesophytes prevail along the water courses and are washed down from the mountains. the ordinary plants, not the xerophytes, are the ones that are chiefly preserved since they occur in most abundance near streams where deposition is taking place. so far as swamps are concerned, few are of larger size than those of seistan in persia, lop nor in chinese turkestan, and certain others in the midst of the asiatic deserts. streams flowing from the mountains into deserts are almost sure to form large swamps, such as those along the tarim river in central asia. lake chad in africa is another example. in it, too, reeds are very numerous. putting together the evidence on both sides in this disputed question, it appears that throughout most of geological time there is some evidence of a zonal arrangement of climate. the evidence takes the form of traces of cool climates, of seasons, and of deserts. nevertheless, there is also strong evidence that these conditions were in general less intense than at present and that times of relatively warm, moist climate without great seasonal extremes have prevailed very widely during periods much longer than those when a zonal arrangement as marked as that of today prevailed. as schuchert[ ] puts it: "today the variation on land between the tropics and the poles is roughly between ° and - °f., in the oceans between ° and °f. in the geologic past the temperature of the oceans for the greater parts of the periods probably was most often between ° and °f., while on land it may have varied between ° and °f. at rare intervals the extremes were undoubtedly as great as they are today. the conclusion is therefore that at all times the earth had temperature zones, varying between the present-day intensity and times which were almost without such belts, and at these latter times the greater part of the earth had an almost uniformly mild climate, without winters." it is these mild climates which we must now attempt to explain. this leads us to inquire what would happen to the climate of the earth as a whole if the conditions which now prevail at times of few sunspots were to become intensified. that they could become greatly intensified seems highly probable, for there is good reason to think that aside from the sunspot cycle the sun's atmosphere is in a disturbed condition. the prominences which sometimes shoot out hundreds of thousands of miles seem to be good evidence of this. suppose that the sun's atmosphere should become very quiet. this would apparently mean that cyclonic storms would be much less numerous and less severe than during the present times of sunspot minima. the storms would also apparently follow paths in middle latitudes somewhat as they do now when sunspots are fewest. the first effect of such a condition, if we can judge from what happens at present, would be a rise in the general temperature of the earth, because less heat would be carried aloft by storms. today, as is shown in _earth and sun_, a difference of perhaps per cent in the average storminess during periods of sunspot maxima and minima is correlated with a difference of °c. in the temperature at the earth's surface. this includes not only an actual lowering of . °c. at times of sunspot maxima, but the overcoming of the effect of increased insolation at such times, an effect which abbot calculates as about . °c. if the storminess were to be reduced to one-half or one-quarter its present amount at sunspot minima, not only would the loss of heat by upward convection in storms be diminished, but the area covered by clouds would diminish so that the sun would have more chance to warm the lower air. hence the average rise of temperature might amount to as much at ° or °c. another effect of the decrease in storminess would be to make the so-called westerly winds, which are chiefly southwesterly in the northern hemisphere and northwesterly in the southern hemisphere, more strong and steady than at present. they would not continually suffer interruption by cyclonic winds from other directions, as is now the case, and would have a regularity like that of the trades. this conclusion is strongly reënforced in a paper by clayton[ ] which came to hand after this chapter had been completed. from his studies of the solar constant and the temperature of the earth which are described in _earth and sun_, he reaches the following conclusion: "the results of these researches have led me to believe: . that if there were no variation in solar radiation the atmospheric motions would establish a stable system with exchanges of air between equator and pole and between ocean and land, in which the only variations would be daily and annual changes set in operation by the relative motions of the earth and sun. . the existing abnormal changes, which we call weather, have their origins chiefly, if not entirely, in the variations of solar radiation." if cyclonic storms and "weather" were largely eliminated and if the planetary system of winds with its steady trades and southwesterlies became everywhere dominant, the regularity and volume of the poleward-flowing currents, such as the gulf stream and the atlantic drift in one ocean, and the japanese current in another, would be greatly increased. how important this is may be judged from the work of helland-hansen and nansen.[ ] these authors find that with the passage of each cyclonic storm there is a change in the temperature of the surface water of the atlantic ocean. winds at right angles to the course of the drift drive the water first in one direction and then in the other but do not advance it in its course. winds with an easterly component, on the other hand, not only check the drift but reverse it, driving the warm water back toward the southwest and allowing cold water to well up in its stead. the driving force in the atlantic drift is merely the excess of the winds with a westerly component over those with an easterly component. suppose that the numbers in fig. represent the strength of the winds in a certain part of the north atlantic or north pacific, that is, the total number of miles moved by the air per year. in quadrant a of the left-hand part all the winds move from a more or less southwesterly direction and produce a total movement of the air amounting to thirty units per year. those coming from points between north and west move twenty-five units; those between north and east, twenty units; and those between east and south, twenty-five units. since the movement of the winds in quadrants b and d is the same, these winds have no effect in producing currents. they merely move the water back and forth, and thus give it time to lose whatever heat it has brought from more southerly latitudes. on the other hand, since the easterly winds in quadrant c do not wholly check the currents caused by the westerly winds of quadrant a, the effective force of the westerly winds amounts to ten, or the difference between a force of thirty in quadrant a and of twenty in quadrant c. hence the water is moved forward toward the northeast, as shown by the thick part of arrow a. [illustration: _fig. . effect of diminution of storms on movement of water._] now suppose that cyclonic storms should be greatly reduced in number so that in the zone of prevailing westerlies they were scarcely more numerous than tropical hurricanes now are in the trade-wind belt. then the more or less southwesterly winds in quadrant a´ in the right-hand part of fig. would not only become more frequent but would be stronger than at present. the total movement from that quarter might rise to sixty units, as indicated in the figure. in quadrants b´ and d´ the movement would fall to fifteen and in quadrant c´ to ten. b´ and d´ would balance one another as before. the movement in a´, however, would exceed that in c´ by fifty instead of ten. in other words, the current-making force would become five times as great as now. the actual effect would be increased still more, for the winds from the southwest would be stronger as well as steadier if there were no storms. a strong wind which causes whitecaps has much more power to drive the water forward than a weaker wind which does not cause whitecaps. in a wave without a whitecap the water returns to practically the original point after completing a circle beneath the surface. in a wave with a whitecap, however, the cap moves forward. any increase in velocity beyond the rate at which whitecaps are formed has a great influence upon the amount of water which is blown forward. several times as much water is drifted forward by a persistent wind of twenty miles an hour as by a ten-mile wind.[ ] in this connection a suggestion which is elaborated in chapter xiii may be mentioned. at present the salinity of the oceans checks the general deep-sea circulation and thereby increases the contrasts from zone to zone. in the past, however, the ocean must have been fresher than now. hence the circulation was presumably less impeded, and the transfer of heat from low latitudes to high was facilitated. consider now the magnitude of the probable effect of a diminution in storms. today off the coast of norway in latitude °n. and longitude °e., the mean temperature in january is °c. and in july °c. this represents a plus anomaly of about ° in january and ° in july; that is, the norwegian coast is warmer than the normal for its latitude by these amounts. suppose that in some past time the present distribution of lands and seas prevailed, but norway was a lowland where extensive deposits could accumulate in great flood plains. suppose, also, that the sun's atmosphere was so inactive that few cyclonic storms occurred, steady winds from the west-southwest prevailed, and strong, uninterrupted ocean currents brought from the caribbean sea and gulf of mexico much greater supplies of warm water than at present. the norwegian winters would then be warmer than now not only because of the general increase in temperature which the earth regularly experiences at sunspot minima, but because the currents would accentuate this condition. in summer similar conditions would prevail except that the warming effect of the winds and currents would presumably be less than in winter, but this might be more than balanced by the increased heat of the sun during the long summer days, for storms and clouds would be rare. if such conditions raised the winter temperature only °c. and the summer temperature °c., the climate would be as warm as that of the northern island of new zealand (latitude °- °s.). the flora of that part of new zealand is subtropical and includes not only pines and beeches, but palms and tree ferns. a climate scarcely warmer than that of new zealand would foster a flora like that which existed in far northern latitudes during some of the milder geological periods. if, however, the general temperature of the earth's surface were raised ° because of the scarcity of storms, if the currents were strong enough so that they increased the present anomaly by per cent, and if more persistent sunshine in summer raised the temperature at that season about °c., the january temperature would be °c. and the july temperature °c. these figures perhaps make summer and winter more nearly alike than was ever really the case in such latitudes. nevertheless, they show that a diminution of storms and a consequent strengthening and steadying of the southwesterlies might easily raise the temperature of the norwegian coast so high that corals could flourish within the arctic circle. another factor would coöperate in producing mild temperatures in high latitudes during the winter, namely, the fogs which would presumably accumulate. it is well known that when saturated air from a warm ocean is blown over the lands in winter, as happens so often in the british islands and around the north sea, fog is formed. the effect of such a fog is indeed to shut out the sun's radiation, but in high latitudes during the winter when the sun is low, this is of little importance. another effect is to retain the heat of the earth itself. when a constant supply of warm water is being brought from low latitudes this blanketing of the heat by the fog becomes of great importance. in the past, whenever cyclonic storms were weak and westerly winds were correspondingly strong, winter fogs in high latitudes must have been much more widespread and persistent than now. the bearing of fogs on vegetation is another interesting point. if a region in high latitudes is constantly protected by fog in winter, it can support types of vegetation characteristic of fairly low latitudes, for plants are oftener killed by dry cold than by moist cold. indeed, excessive evaporation from the plant induced by dry cold when the evaporated water cannot be rapidly replaced by the movement of sap is a chief reason why large plants are winterkilled. the growing of transplanted palms on the coast of southwestern ireland, in spite of its location in latitude °n., is possible only because of the great fogginess in winter due to the marine climate. the fogs prevent the escape of heat and ward off killing frosts. the tree ferns in latitude °s. in new zealand, already referred to, are often similarly protected in winter. therefore, the relative frequency of fogs in high latitudes when storms were at a minimum would apparently tend not merely to produce mild winters but to promote tropical vegetation. the strong steady trades and southwesterlies which would prevail at times of slight solar activity, according to our hypothesis, would have a pronounced effect on the water of the deep seas as well as upon that of the surface. in the first place, the deep-sea circulation would be hastened. for convenience let us speak of the northern hemisphere. in the past, whenever the southwesterly winds were steadier than now, as was probably the case when cyclonic storms were relatively rare, more surface water than at present was presumably driven from low latitudes and carried to high latitudes. this, of course, means that a greater volume of water had to flow back toward the equator in the lower parts of the ocean, or else as a cool surface current. the steady southwesterly winds, however, would interfere with south-flowing surface currents, thus compelling the polar waters to find their way equatorward beneath the surface. in low latitudes the polar waters would rise and their tendency would be to lower the temperature. hence steadier westerlies would make for lessened latitudinal contrasts in climate not only by driving more warm water poleward but by causing more polar water to reach low latitudes. at this point a second important consideration must be faced. not only would the deep-sea circulation be hastened, but the ocean depths might be warmed. the deep parts of the ocean are today cold because they receive their water from high latitudes where it sinks because of low temperature. suppose, however, that a diminution in storminess combined with other conditions should permit corals to grow in latitude °n. the ocean temperature would then have to average scarcely lower than °c. and even in the coldest month the water could scarcely fall below about °c. under such conditions, if the polar ocean were freely connected with the rest of the oceans, no part of it would probably have a temperature much below °c., for there would be no such thing as ice caps and snowfields to reflect the scanty sunlight and radiate into space what little heat there was. on the contrary, during the winter an almost constant state of dense fogginess would prevail. so great would be the blanketing effect of this that a minimum monthly temperature of °c. for the coldest part of the ocean may perhaps be too low for a time when corals thrived in latitude °. the temperature of the ocean depths cannot permanently remain lower than that of the coldest parts of the surface. temporarily this might indeed happen when a solar change first reduced the storminess and strengthened the westerlies and the surface currents. gradually, however, the persistent deep-sea circulation would bring up the colder water in low latitudes and carry downward the water of medium temperature at the coldest part of the surface. thus in time the whole body of the ocean would become warm. the heat which at present is carried away from the earth's surface in storms would slowly accumulate in the oceans. as the process went on, all parts of the ocean's surface would become warmer, for equatorial latitudes would be less and less cooled by cold water from below, while the water blown from low latitudes to high would be correspondingly warmer. the warming of the ocean would come to an end only with the attainment of a state of equilibrium in which the loss of heat by radiation and evaporation from the ocean's surface equaled the loss which under other circumstances would arise from the rise of warm air in cyclonic storms. when once the oceans were warmed, they would form an extremely strong conservative force tending to preserve an equable climate in all latitudes and at all seasons. according to the solar cyclonic hypothesis such conditions ought to have prevailed throughout most of geological time. only after a strong and prolonged solar disturbance with its consequent storminess would conditions like those of today be expected. in this connection another possibility may be mentioned. it is commonly assumed that the earth's axis is held steadily in one direction by the fact that the rotating earth is a great gyroscope. having been tilted to a certain position, perhaps by some extraneous force, the axis is supposed to maintain that position until some other force intervenes. cordeiro,[ ] however, maintains that this is true only of an absolutely rigid gyroscope. he believes that it is mathematically demonstrable that if an elastic gyroscope be gradually tilted by some extraneous force, and if that force then ceases to act, the gyroscope as a whole will oscillate back and forth. the earth appears to be slightly elastic. cordeiro therefore applies his formulæ to it, on the following assumptions: ( ) that the original position of the axis was nearly vertical to the plane of the ecliptic in which the earth revolves around the sun; ( ) that at certain times the inclination has been even greater than now; and ( ) that the position of the axis with reference to the earth has not changed to any great extent, that is, the earth's poles have remained essentially stationary with reference to the earth, although the whole earth has been gyroscopically tilted back and forth repeatedly. with a vertical axis the daylight and darkness in all parts of the earth would be of equal duration, being always twelve hours. there would be no seasons, and the climate would approach the average condition now experienced at the two equinoxes. on the whole the climate of high latitudes would give the impression of being milder than now, for there would be less opportunity for the accumulation of snow and ice with their strong cooling effect. on the other hand, if the axis were tilted more than now, the winter nights would be longer and the winters more severe than at present, and there would be a tendency toward glaciation. thus cordeiro accounts for alternating mild and glacial epochs. the entire swing from the vertical position to the maximum inclination and back to the vertical may last millions of years depending on the earth's degree of elasticity. the swing beyond the vertical position in the other direction would be equally prolonged. since the axis is now supposed to be much nearer its maximum than its minimum degree of tilting, the duration of epochs having a climate more severe than that of the present would be relatively short, while the mild epochs would be long. cordeiro's hypothesis has been almost completely ignored. one reason is that his treatment of geological facts, and especially his method of riding rough-shod over widely accepted conclusions, has not commended his work to geologists. therefore they have not deemed it worth while to urge mathematicians to test the assumptions and methods by which he reached his results. it is perhaps unfair to test cordeiro by geology, for he lays no claim to being a geologist. in mathematics he labors under the disadvantage of having worked outside the usual professional channels, so that his work does not seem to have been subjected to sufficiently critical analysis. without expressing any opinion as to the value of cordeiro's results we feel that the subject of the earth's gyroscopic motion and of a possible secular change in the direction of the axis deserves investigation for two chief reasons. in the first place, evidences of seasonal changes and of seasonal uniformity seem to occur more or less alternately in the geological record. second, the remarkable discoveries of garner and allard[ ] show that the duration of daylight has a pronounced effect upon the reproduction of plants. we have referred repeatedly to the tree ferns, corals, and other forms of life which now live in relatively low latitudes and which cannot endure strong seasonal contrasts, but which once lived far to the north. on the other hand, sayles,[ ] for example, finds that microscopical examination of the banding of ancient shales and slates indicates distinct seasonal banding like that of recent pleistocene clays or of the squantum slate formed during or near the permian glacial period. such seasonal banding is found in rocks of various ages: (a) huronian, in cobalt shales previously reported by coleman; (b) late proterozoic or early cambrian in hiwassee slate; (c) lower cambrian, in georgian slates of vermont; (d) lower ordovician, in georgia (rockmart slate), tennessee (athens shale), vermont (slates), and quebec (beekmantown formation); and (e) permian in massachusetts (squantum slate). how far the periods during which such evidence of seasons was recorded really alternated with mild periods, when tropical species lived in high latitudes and the contrast of seasons was almost or wholly lacking, we have as yet no means of knowing. if periods characterized by marked seasonal changes should be found to have alternated with those when the seasons were of little importance, the fact would be of great geological significance. the discoveries of garner and allard as to the effect of light on reproduction began with a peculiar tobacco plant which appeared in some experiments at washington. the plant grew to unusual size, and seemed to promise a valuable new variety. it formed no seeds, however, before the approach of cold weather. it was therefore removed to a greenhouse where it flowered and produced seed. in succeeding years the flowering was likewise delayed till early winter, but finally it was discovered that if small plants were started in the greenhouse in the early fall they flowered at the same time as the large ones. experiments soon demonstrated that the time of flowering depends largely upon the length of the daily period when the plants are exposed to light. the same is true of many other plants, and there is great variety in the conditions which lead to flowering. some plants, such as witch hazel, appear to be stimulated to bloom by very short days, while others, such as evening primrose, appear to require relatively long days. so sensitive are plants in this respect that garner and allard, by changing the length of the period of light, have caused a flowerbud in its early stages not only to stop developing but to return once more to a vegetative shoot. common iris, which flowers in may and june, will not blossom under ordinary conditions when grown in the greenhouse in winter, even under the same temperature conditions that prevail in early summer. again, one variety of soy beans will regularly begin to flower in june of each year, a second variety in july, and a third in august, when all are planted on the same date. there are no temperature differences during the summer months which could explain these differences in time of flowering; and, since "internal causes" alone cannot be accepted as furnishing a satisfactory explanation, some external factor other than temperature must be responsible. the ordinary varieties of cosmos regularly flower in the fall in northern latitudes if they are planted in the spring or summer. if grown in a warm greenhouse during the winter months the plants also flower readily, so that the cooler weather of fall is not a necessary condition. if successive plantings of cosmos are made in the greenhouse during the late winter and early spring months, maintaining a uniform temperature throughout, the plantings made after a certain date will fail to blossom promptly, but, on the contrary, will continue to grow till the following fall, thus flowering at the usual season for this species. this curious reversal of behavior with advance of the season cannot be attributed to change in temperature. some other factor is responsible for the failure of cosmos to blossom during the summer months. in this respect the behavior of cosmos is just the opposite of that observed in iris. certain varieties of soy beans change their behavior in a peculiar manner with advance of the summer season. the variety known as biloxi, for example, when planted early in the spring in the latitude of washington, d. c., continues to grow throughout the summer, flowering in september. the plants maintain growth without flowering for fifteen to eighteen weeks, attaining a height of five feet or more. as the dates of successive plantings are moved forward through the months of june and july, however there is a marked tendency for the plants to cut short the period of growth which precedes flowering. this means, of course, that there is a tendency to flower at approximately the same time of year regardless of the date of planting. as a necessary consequence, the size of the plants at the time of flowering is reduced in proportion to the delay in planting. the bearing of this on geological problems lies in a query which it raises as to the ability of a genus or family of plants to adapt itself to days of very different length from those to which it is wonted. could tree ferns, ginkgos, cycads, and other plants whose usual range of location never subjects them to daylight for more than perhaps fourteen hours or less than ten, thrive and reproduce themselves if subjected to periods of daylight ranging all the way from nothing up to about twenty-four hours? no answer to this is yet possible, but the question raises most interesting opportunities of investigation. if cordeiro is right as to the earth's elastic gyroscopic motion, there may have been certain periods when a vertical or almost vertical axis permitted the days to be of almost equal length at all seasons in all latitudes. if such an absence of seasons occurred when the lands were low, when the oceans were extensive and widely open toward the poles, and when storms were relatively inactive, the result might be great mildness of climate such as appears sometimes to have prevailed in the middle of geological eras. suppose on the other hand that the axis should be tilted more than now, and that the lands should be widely emergent and the storm belt highly active in low latitudes, perhaps because of the activity of the sun. the conditions might be favorable for glaciation at latitudes as low as those where the permo-carboniferous ice sheets appear to have centered. the possibilities thus suggested by cordeiro's hypothesis are so interesting that the gyroscopic motion of the earth ought to be investigated more thoroughly. even if no such gyroscopic motion takes place, however, the other causes of mild climate discussed in this chapter may be enough to explain all the observed phenomena. many important biological consequences might be drawn from this study of mild geological climates, but this book is not the place for them. in the first chapter we saw that one of the most remarkable features of the climate of the earth is its wonderful uniformity through hundreds of millions of years. as we come down through the vista of years the mild geological periods appear to represent a return as nearly as possible to this standard condition of uniformity. certain changes of the earth itself, as we shall see in the next chapter, may in the long run tend slightly to change the exact conditions of this climatic standard, as we might perhaps call it. yet they act so slowly that their effect during hundreds of millions of years is still open to question. at most they seem merely to have produced a slight increase in diversity from season to season and from zone to zone. the normal climate appears still to be of a milder type than that which happens to prevail at present. some solar condition, whose possible nature will be discussed later, seems even now to cause the number of cyclonic storms to be greater than normal. hence the earth's climate still shows something of the great diversity of seasons and of zones which is so marked a characteristic of glacial epochs. footnotes: [footnote : rollin t. chamberlin: personal communication.] [footnote : f. h. knowlton: evolution of geologic climates; bull. geol. soc. am., vol. , , pp. - .] [footnote : chas. schuchert: review of knowlton's evolution of geological climates, in am. jour. sci., .] [footnote : g. r. wieland: distribution and relationships of the cycadeoids; am. jour. bot., vol. , , pp. - .] [footnote : d. t. macdougal: botanical features of north american deserts; carnegie instit. of wash., no. , .] [footnote : _loc. cit._] [footnote : h. h. clayton: variation in solar radiation and the weather; smiths. misc. coll., vol. , no. , washington, .] [footnote : b. helland hansen and f. nansen: temperature variations in the north atlantic ocean and in the atmosphere; misc. coll., smiths. inst., vol. , no. , washington, .] [footnote : the climatic significance of ocean currents is well discussed in croll's climate and time, , and his climate and cosmogony, .] [footnote : f. j. b. cordeiro: the gyroscope, .] [footnote : w. w. garner and h. a. allard: flowering and fruition of plants as controlled by length of day; yearbook dept. agri., , pp. - .] [footnote : report of committee on sedimentation, national research council, april, .] chapter xi terrestrial causes of climatic changes the major portion of this book has been concerned with the explanation of the more abrupt and extreme changes of climate. this chapter and the next consider two other sorts of climatic changes, the slight secular progression during the hundreds of millions of years of recorded earth history, and especially the long slow geologic oscillations of millions or tens of millions of years. it is generally agreed among geologists that the progressive change has tended toward greater extremes of climate; that is, greater seasonal contrasts, and greater contrasts from place to place and from zone to zone.[ ] the slow cyclic changes have been those that favored widespread glaciation at one extreme near the ends of geologic periods and eras, and mild temperatures even in subpolar regions at the other extreme during the medial portions of the periods. as has been pointed out in an earlier chapter, it has often been assumed that all climatic changes are due to terrestrial causes. we have seen, however, that there is strong evidence that solar variations play a large part in modifying the earth's climate. we have also seen that no known terrestrial agency appears to be able to produce the abrupt changes noted in recent years, the longer cycles of historical times, or geological changes of the shorter type, such as glaciation. nevertheless, terrestrial changes doubtless have assisted in producing both the progressive change and the slow cyclic changes recorded in the rocks, and it is the purpose of this chapter and the two that follow to consider what terrestrial changes have taken place and the probable effect of such changes. the terrestrial changes that have a climatic significance are numerous. some, such as variations in the amount of volcanic dust in the higher air, have been considered in an earlier chapter. others are too imperfectly known to warrant discussion, and in addition there are presumably others which are entirely unknown. doubtless some of these little known or unknown changes have been of importance in modifying climate. for example, the climatic influence of vegetation, animals, and man may be appreciable. here, however, we shall confine ourselves to purely physical causes, which will be treated in the following order: first, those concerned with the solid parts of the earth, namely: (i) amount of land; (ii) distribution of land; (iii) height of land; (iv) lava flows; and (v) internal heat. second, those which arise from the salinity of oceans, and third, those depending on the composition and amount of atmosphere. the terrestrial change which appears indirectly to have caused the greatest change in climate is the contraction of the earth. the problem of contraction is highly complex and is as yet only imperfectly understood. since only its results and not its processes influence climate, the following section as far as page is not necessary to the general reader. it is inserted in order to explain why we assume that there have been oscillations between certain types of distribution of the lands. the extent of the earth's contraction may be judged from the shrinkage indicated by the shortening of the rock formations in folded mountains such as the alps, juras, appalachians, and caucasus. geologists are continually discovering new evidence of thrust faults of great magnitude where masses of rock are thrust bodily over other rocks, sometimes for many miles. therefore, the estimates of the amount of shrinkage based on the measurements of folds and faults need constant revision upward. nevertheless, they have already reached a considerable figure. for example, in , professor a. heim estimated the shortening of the meridian passing through the modern alps and the ancient hercynian and caledonian mountains as fully a thousand miles in europe, and over five hundred miles for the rest of this meridian.[ ] this is a radial shortening of about miles. possibly the shrinkage has been even greater than this. chamberlin[ ] has compared the density of the earth, moon, mars, and venus with one another, and found it probable that the radial shrinkage of the earth may be as much as miles. this result is not so different from heim's as appears at first sight, for heim made no allowance for unrecognized thrust faults and for the contraction incident to metamorphism. moreover, heim did not include shrinkage during the first half of geological time before the above-mentioned mountain systems were upheaved. according to a well-established law of physics, contraction of a rotating body results in more rapid rotation and greater centrifugal force. these conditions must increase the earth's equatorial bulge and thereby cause changes in the distribution of land and water. opposed to the rearrangement of the land due to increased rotation caused by contraction, there has presumably been another rearrangement due to tidal retardation of the earth's rotation and a consequent lessening of the equatorial bulge. g. h. darwin long ago deduced a relatively large retardation due to lunar tides. a few years ago w. d. macmillan, on other assumptions, deduced only a negligible retardation. still more recently taylor[ ] has studied the tides of the irish sea, and his work has led jeffreys[ ] and brown[ ] to conclude that there has been considerable retardation, perhaps enough, according to brown, to equal the acceleration due to the earth's contraction. from a prolonged and exhaustive study of the motions of the moon brown concludes that tidal friction or some other cause is now lengthening the day at the rate of one second per thousand years, or an hour in almost four million years if the present rate continues. he makes it clear that the retardation due to tides would not correspond in point of time with the acceleration due to contraction. the retardation would occur slowly, and would take place chiefly during the long quiet periods of geologic history, while the acceleration would occur rapidly at times of diastrophic deformation. as a consequence, the equatorial bulge would alternately be reduced at a slow rate, and then somewhat suddenly augmented. the less rigid any part of the earth is, the more quickly it responds to the forces which lead to bulging or which tend to lessen the bulge. since water is more fluid than land, the contraction of the earth and the tidal retardation presumably tend alternately to increase and decrease the amount of water near the equator more than the amount of land. thus, throughout geological history we should look for cyclic changes in the relative area of the lands within the tropics and similar changes of opposite phase in higher latitudes. the extent of the change would depend upon (a) the amount of alteration in the speed of rotation, and (b) the extent of low land in low latitudes and of shallow sea in high latitudes. according to slichter's tables, if the earth should rotate in twenty-three hours instead of twenty-four, the great amazon lowland would be submerged by the inflow of oceanic water, while wide areas in hudson bay, the north sea, and other northern regions, would become land because the ocean water would flow away from them.[ ] following the prompt equatorward movement of water which would occur as the speed of rotation increased, there must also be a gradual movement or creepage of the solid rocks toward the equator, that is, a bulging of the ocean floor and of the lands in low latitudes, with a consequent emergence of the lands there and a relative rise of sea level in higher latitudes. tidal retardation would have a similar effect. suess[ ] has described widespread elevated strand lines in the tropics which he interprets as indicating a relatively sudden change in sea level, though he does not suggest a cause of the change. however, in speaking of recent geological times, suess reports that a movement more recent than the old strands "was an accumulation of water toward the equator, a diminution toward the poles, and (it appears) as though this last movement were only one of the many oscillations which succeed each other with the same tendency, i.e., with a positive excess at the equator, a negative excess at the poles." (vol. ii, p. .) this creepage of the rocks equatorward seemingly might favor the growth of mountains in tropical and subtropical regions, because it is highly improbable that the increase in the bulge would go on in all longitudes with perfect uniformity. where it went on most rapidly mountains would arise. that such irregularity of movement has actually occurred is suggested not only by the fact that many cenozoic and older mountain ranges extend east and west, but by the further fact that these include some of our greatest ranges, many of which are in fairly low latitudes. the himalayas, the javanese ranges, and the half-submerged caribbean chains are examples. such mountains suggest a thrust in a north and south direction which is just what would happen if the solid mass of the earth were creeping first equatorward and then poleward. a fact which is in accord with the idea of a periodic increase in the oceans in low latitudes because of renewed bulging at the equator is the exposure in moderately high latitudes of the greatest extent of ancient rocks. this seems to mean that in low latitudes the frequent deepening of the oceans has caused the old rocks to be largely covered by sediments, while the old lands in higher latitudes have been left more fully exposed to erosion. another suggestion of such periodic equatorward movements of the ocean water is found in the reported contrast between the relative stability with which the northern part of north america has remained slightly above sea level except at times of widespread submergence, while the southern parts have suffered repeated submergence alternating with great emergence.[ ] furthermore, although the northern part of north america has been generally exposed to erosion since the proterozoic, it has supplied much less sediment than have the more southern land areas.[ ] this apparently means that much of canada has stood relatively low, while repeated and profound uplift alternating with depression has occurred in subtropical latitudes, apparently in adjustment to changes in the earth's speed of rotation. the uplifts generally followed the times of submergence due to equatorward movement of the water, though the buckling of the crust which accompanies shrinkage doubtless caused some of the submergence. the evidence that northern north america stood relatively low throughout much of geological time depends not only on the fact that little sediment came to the south from the north, but also on the fact that at times of especially widespread epicontinental seas, the submergence was initiated at the north.[ ] this is especially true for ordovician, silurian, devonian, and jurassic times in north america. general submergence of this kind is supposed to be due chiefly to the overflowing of the ocean when its level is slowly raised by the deposition of sediment derived from the erosion of what once were continental highlands but later are peneplains. the fact that such submergence began in high latitudes, however, seems to need a further explanation. the bulging of the rock sphere at the equator and the consequent displacement of some of the water in low latitudes would furnish such an explanation, as would also a decrease in the speed of rotation induced by tidal retardation, if that retardation were great enough and rapid enough to be geologically effective. the climatic effects of the earth's contraction, which we shall shortly discuss, are greatly complicated by the fact that contraction has taken place irregularly. such irregularity has occurred in spite of the fact that the processes which cause contraction have probably gone on quite steadily throughout geological history. these processes include the chemical reorganization of the minerals of the crust, a process which is illustrated by the metamorphism of sedimentary rocks into crystalline forms. the escape of gases through volcanic action or otherwise has been another important process. although the processes which cause contraction probably go on steadily, their effect, as chamberlin[ ] and others have pointed out, is probably delayed by inertia. thus the settling of the crust or its movement on a large scale is delayed. perhaps the delay continues until the stresses become so great that of themselves they overcome the inertia, or possibly some outside agency, whose nature we shall consider later, reënforces the stresses and gives the slight impulse which is enough to release them and allow the earth's crust to settle into a new state of equilibrium. when contraction proceeds actively, the ocean segments, being largest and heaviest, are likely to settle most, resulting in a deepening of the oceans and an emergence of the lands. following each considerable contraction there would be an increase in the speed of rotation. the repeated contractions with consequent growth of the equatorial bulge would alternate with long quiet periods during which tidal retardation would again decrease the speed of rotation and hence lessen the bulge. the result would be repeated changes of distribution of land and water, with consequent changes in climate. i. we shall now consider the climatic effect of the repeated changes in the relative amounts of land and water which appear to have resulted from the earth's contraction and from changes in its speed of rotation. during many geologic epochs a larger portion of the earth was covered with water than at present. for example, during at least twelve out of about twenty epochs, north america has suffered extensive inundations,[ ] and in general the extensive submergence of europe, the other area well known geologically, has coincided with that of north america. at other times, the ocean has been less extensive than now, as for example during the recent glacial period, and probably during several of the glacial periods of earlier date. each of the numerous changes in the relative extent of the lands must have resulted in a modification of climate.[ ] this modification would occur chiefly because water becomes warm far more slowly than land, and cools off far more slowly. an increase in the lands would cause changes in several climatic conditions. (a) the range of temperature between day and night and between summer and winter would increase, for lands become warmer by day and in summer than do oceans, and cooler at night and in winter. the higher summer temperature when the lands are widespread is due chiefly to the fact that the land, if not snow-covered, absorbs more of the sun's radiant energy than does the ocean, for its reflecting power is low. the lower winter temperature when lands are widespread occurs not only because they cool off rapidly but because the reduced oceans cannot give them so much heat. moreover, the larger the land, the more generally do the winds blow outward from it in winter and thus prevent the ocean heat from being carried inland. so long as the ocean is not frozen in high latitudes, it is generally the chief source of heat in winter, for the nights are several months long near the poles, and even when the sun does shine its angle is so low that reflection from the snow is very great. furthermore, although on the average there is more reflection from water than from land, the opposite is true in high latitudes in winter when the land is snow-covered while the ocean is relatively dark and is roughened by the waves. another factor in causing large lands to have extremely low temperature in winter is the fact that in proportion to their size they are less protected by fog and cloud than are smaller areas. the belt of cloud and fog which is usually formed when the wind blows from the ocean to the relatively cold land is restricted to the coastal zone. thus the larger the land, the smaller the fraction in which loss of heat by radiation is reduced by clouds and fogs. hence an increase in the land area is accompanied by an increase in the contrasts in temperature between land and water. (b) the contrasts in temperature thus produced must cause similar contrasts in atmospheric pressure, and hence stronger barometric gradients. (c) the strong gradients would mean strong winds, flowing from land to sea or from sea to land. (d) local convection would also be strengthened in harmony with the expansion of the lands, for the more rapid heating of land than of water favors active convection. (e) as the extent of the ocean diminished, there would normally be a decrease in the amount of water vapor for three reasons: ( ) evaporation from the ocean is the great source of water vapor. other conditions being equal, the smaller the ocean becomes, the less the evaporation. ( ) the amount of water vapor in the air diminishes as convection increases, since upward convection is a chief method by which condensation and precipitation are produced, and water vapor removed from the atmosphere. ( ) nocturnal cooling sufficient to produce dew and frost is very much more common upon land than upon the ocean. the formation of dew and frost diminishes the amount of water vapor at least temporarily. (f) any diminution in water vapor produced in these ways, or otherwise, is significant because water vapor is the most essential part of the atmosphere so far as regulation of temperature is concerned. it tends to keep the days from becoming hot or the nights cold. therefore any decrease in water vapor would increase the diurnal and seasonal range of temperature, making the climate more extreme and severe. thus a periodic increase in the area of the continents would clearly make for periodic increased climatic contrasts, with great extremes, a type of climatic change which has recurred again and again. indeed, each great glaciation accompanied or followed extensive emergence of the lands.[ ] whether or not there has been a _progressive_ increase from era to era in the area of the lands is uncertain. good authorities disagree widely. there is no doubt, however, that at present the lands are more extensive than at most times in the past, though smaller, perhaps, than at certain periods. the wide expanse of lands helps explain the prominence of seasons at present as compared with the past. ii. the contraction of the earth, as we have seen, has produced great changes in the distribution as well as in the extent of land and water. large parts of the present continents have been covered repeatedly by the sea, and extensive areas now covered with water have been land. in recent geological times, that is, during the pliocene and pleistocene, much of the present continental shelf, the zone less than feet below sea level, was land. if the whole shelf had been exposed, the lands would have been greater than at present by an area larger than north america. when the lands were most elevated, or a little earlier, north america was probably connected with asia and almost with europe. asia in turn was apparently connected with the larger east indian islands. in much earlier times land occupied regions where now the ocean is fairly deep. groups of islands, such as the east indies and malaysia and perhaps the west indies, were united into widespreading land masses. figs. and , illustrating the paleography of the permian and the cretaceous periods, respectively, indicate a land distribution radically different from that of today. so far as appears from the scattered facts of geological history, the changes in the distribution of land seem to have been marked by the following characteristics: ( ) accompanying the differentiation of continental and oceanic segments of the earth's crust, the oceans have become somewhat deeper, and their basins perhaps larger, while the continents, on the average, have been more elevated and less subject to submergence. hence there have been less radical departures from the present distribution during the relatively recent cenozoic era than in the ancient paleozoic because the submergence of continental areas has become less general and less frequent. for example, the last extensive epeiric or interior sea in north america was in the cretaceous, at least ten million years ago, and according to barrell perhaps fifty million, while in europe, according to de lapparent,[ ] a smaller share of the present continent has been submerged since the cretaceous than before. indeed, as in north america, the submergence has decreased on the average since the paleozoic era. ( ) the changes in distribution of land which have taken place during earth history have been cyclic. repeatedly, at the close of each of the score or so of geologic periods, the continents emerged more or less, while at the close of the groups of periods known as eras, the lands were especially large and emergent. after each emergence, a gradual encroachment of the sea took place, and toward the close of several of the earlier periods, the sea appears to have covered a large fraction of the present land areas. ( ) on the whole, the amount of land in the middle and high latitudes of the northern hemisphere appears to have increased during geologic time. such an increase does not require a growth of the continents, however, in the broader sense of the term, but merely that a smaller fraction of the continent and its shelf should be submerged. ( ) in tropical latitudes, on the other hand, the extent of the lands seems to have decreased, apparently by the growth of the ocean basins. south america and africa are thought by many students to have been connected, and africa was united with india via madagascar, as is suggested in fig. . the most radical cyclic as well as the most radical progressive changes in land distribution also seem to have taken place in tropical regions.[ ] [illustration: _fig. . cretaceous paleogeography._ (_after schuchert._)] although there is much evidence of periodic increase of the sea in equatorial latitudes and of land in high latitudes, it has remained for the zoölogist metcalf to present a very pretty bit of evidence that at certain times submergence along the equator coincided with emergence in high latitudes, and vice versa. certain fresh water frogs which carry the same internal parasite are confined to two widely separated areas in tropical and south temperate america and in australia. the extreme improbability that both the frogs and the parasites could have originated independently in two unconnected areas and could have developed by convergent evolution so that they are almost identical in the two continents makes it almost certain that there must have been a land connection between south america and australia, presumably by way of antarctica. the facts as to the parasites seem also to prove that while the land connection existed there was a sea across south america in equatorial latitudes. the parasite infests not only the frogs but the american toads known as bufo. now bufo originated north of the equator in america and differs from the frogs which originated in southern south america in not being found in australia. this raises the question of how the frogs could go to australia via antarctica carrying the parasite with them, while the toads could not go. metcalf's answer is that the toads were cut off from the southern part of south america by an equatorial sea until after the antarctic connection between the old world and the new was severed. as patagonia let go of antarctica by subsidence of the intervening land area, there was a probable concomitant rise of land through what is now middle south america and the northern and southern portions of this continent came together.[ ] these various changes in the earth's crust have given rise to certain specific types of distribution of the lands, which will now be considered. we shall inquire what climatic conditions would arise from changes in (a) the continuity of the lands from north to south, (b) the amount of land in tropical latitudes, and (c) the amount of land in middle and high latitudes. (a) at present the westward drift of warm waters, set in motion by the trade winds, is interrupted by land masses and turned poleward, producing the important gulf stream drift and japan current in the northern hemisphere, and corresponding, though less important, currents in the southern hemisphere. during the past, quite different sets of ocean currents doubtless have existed in response to a different distribution of land. repeatedly, in the mid-cretaceous (fig. ) and several other periods, the present american barrier to the westward moving tropical current was broken in central america. even if the supposed continent of "gondwana land" extended from africa to south america in equatorial latitudes, strong currents must still have flowed westward along its northern shore under the impulse of the peculiarly strong trade winds which the equatorial land would create. nevertheless at such times relatively little warm tropical water presumably entered the north atlantic, for it escaped into the pacific. at several other times, such as the late ordovician and mid-devonian, when the isthmian barrier existed, it probably turned an important current northward into what is now the mississippi basin instead of into the atlantic. there it traversed an epeiric, or mid-continental sea open to both north and south. hence its effectiveness in warming arctic regions must have been quite different from that of the present gulf stream. (b) we will next consider the influences of changes in the amount of equatorial and tropical land. as such lands are much hotter than the corresponding seas, the intensity and width of the equatorial belt of low pressure must be great when they are extensive. hence the trade winds must have been stronger than now whenever tropical lands were more extensive than at present. this is because the trades are produced by the convection due to excessive heat along the heat equator. there the air expands upward and flows poleward at high altitudes. the trade wind consists of air moving toward the heat equator to take the place of the air which there rises. when the lands in low latitudes were wide the trade winds must also have dominated a wide belt. the greater width of the trade-wind belt today over africa than over the atlantic illustrates the matter. the belt must have been still wider when gondwana land was large, as it is believed to have been during the paleozoic era and the early mesozoic. an increase in the width of the equatorial belt of low pressure under the influence of broad tropical lands would be accompanied not only by stronger and more widespread trade winds, but by a corresponding strengthening of the subtropical belts of high pressure. the chief reason would be the greater expansion of the air in the equatorial low pressure belt and the consequent more abundant outflow of air at high altitudes in the form of anti-trades or winds returning poleward above the trades. such winds would pile up the air in the region of the high-pressure belt. moreover, since the meridians converge as one proceeds away from the equator, the air of the poleward-moving anti-trades tends to be crowded as it reaches higher latitudes, thus increasing the pressure. unless there were a corresponding increase in tropical cyclones, one of the most prominent results of the strengthened trades and the intensified subtropical high-pressure belt at times of broad lands in low latitudes would be great deserts. it will be recalled that the trade-wind lowlands and the extra-tropical belt of highs are the great desert belts at present. the trade-wind lowlands are desert because air moving into warmer latitudes takes up water except where it is cooled by rising on mountain-sides. the belt of highs is arid because there, too, air is being warmed, but in this case by descending from aloft. again, if the atmospheric pressure in the subtropical belt should be intensified, the winds flowing poleward from this belt would necessarily become stronger. these would begin as southwesterlies in the northern hemisphere and northwesterlies in the southern. in the preceding chapter we have seen that such winds, especially when cyclonic storms are few and mild, are a powerful agent in transferring subtropical heat poleward. if the strength of the westerlies were increased because of broad lands in low latitudes, their efficacy in transferring heat would be correspondingly augmented. it is thus evident that any change in the extent of tropical lands during the geologic past must have had important climatic consequences in changing the velocity of the atmospheric circulation and in altering the transfer of heat from low latitudes to high. when the equatorial and tropical lands were broad the winds and currents must have been strong, much heat must have been carried away from low latitudes, and the contrast between low and high latitudes must have been relatively slight. as we have already remarked, leading paleogeographers believe that changes in the extent of the lands have been especially marked in low latitudes, and that on the average there has been a decrease in the extent of land within the tropics. gondwana land is the greatest illustration of this. in the same way, on the numerous paleogeographic maps of north america, most paleogeographers have shown fairly extensive lands south of the latitude of the united states during most of the geologic epochs.[ ] (c) there is evidence that during geologic history the area of the lands in middle and high latitudes, as well as in low latitudes, has changed radically. an increase in such lands would cause the winters to grow colder. this would be partly because of the loss of heat by radiation into the cold dry air over the continents in winter, and partly because of increased reflection from snow and frost, which gather much more widely upon the land than upon the ocean. furthermore, in winter when the continents are relatively cold, there is a strong tendency for winds to blow out from the continent toward the ocean. the larger the land the stronger this tendency. in asia it gives rise to strong winter monsoons. the effect of such winds is illustrated by the way in which the westerlies prevent the gulf stream from warming the eastern united states in winter. the gulf stream warms northwestern europe much more than the united states because, in europe, the prevailing winds are onshore. another effect of an increase in the area of the lands in middle and high latitudes would be to interpose barriers to oceanic circulation and thus lower the temperature of polar regions. this would not mean glaciation in high latitudes, however, even when the lands were widespread as in the mesozoic and early tertiary. students of glaciology are more and more thoroughly convinced that glaciation depends on the availability of moisture even more than upon low temperature. in conclusion it may be noted that each of the several climatic influences of increased land area in the high latitudes would tend to increase the contrasts between land and sea, between winter and summer, and between low latitudes and high. in other words, so far as the effect upon high latitudes themselves is concerned, an expansion of the lands there would tend in the same direction as a diminution in low latitudes. in so far as the general trend of geological evolution has been toward more land in high latitudes and less in low, it would help to produce a progressive increase in climatic diversity such as is faintly indicated in the rock strata. on the other hand, the oscillations in the distribution of the lands, of which geology affords so much evidence, must certainly have played an important part in producing the periodic changes of climate which the earth has undergone. iii. throughout geological history there is abundant evidence that the process of contraction has led to marked differences not only in the distribution and area of the lands, but in their height. on the whole the lands have presumably increased in height since the proterozoic, somewhat in proportion to the increased differentiation of continents and oceans.[ ] if there has been such an increase, the contrast between the climate of ocean and land must have been accentuated, for highlands have a greater diurnal and seasonal range of temperature than do lowlands. the ocean has very little range of either sort. the large range at high altitudes is due chiefly to the small quantity of water vapor, for this declines steadily with increased altitude. a diminution in the density of the other constituents of the air also decreases the blanketing effect of the atmosphere. in conformity with the great seasonal range in temperature at times when the lands stand high, the direction of the wind would be altered. when the lands are notably warmer than the oceans, the winds commonly flow from land to sea, and when the continents are much colder than the oceans, the direction is reversed. the monsoons of asia are examples. strong seasonal winds disturb the normal planetary circulation of the trade winds in low latitudes and of the westerlies in middle latitudes. they also interfere with the ocean currents set in motion by the planetary winds. the net result is to hinder the transfer of heat from low latitudes to high, and thus to increase the contrasts between the zones. local as well as zonal contrasts are also intensified. the higher the land, the greater, relatively speaking, are the cloudiness and precipitation on seaward slopes, and the drier the interior. indeed, most highlands are arid. henry's[ ] recent study of the vertical distribution of rainfall on mountain-sides indicates that a decrease sets in at about feet in the tropics and only a little higher in mid-latitudes. in addition to the main effects upon atmospheric circulation and precipitation, each of the many upheavals of the lands must have been accompanied by many minor conditions which tended toward diversity. for example, the streams were rejuvenated, and instead of meandering perhaps over vast flood plains they intrenched their channels and in many cases dug deep gorges. the water table was lowered, soil was removed from considerable areas, the bare rock was exposed, and the type of dominant vegetation altered in many places. an almost barren ridge may represent all that remains of what was once a vast forested flood plain. thus, increased elevation of the land produces contrasted conditions of slope, vegetation, availability of ground water, exposure to wind and so forth, and these unite in diversifying climate. where mountains are formed, strong contrasts are sure to occur. the windward slopes may be very rainy, while neighboring leeward slopes are parched by a dry foehn wind. at the same time the tops may be snow-covered. increased local contrasts in climatic conditions are known to influence the intensity of cyclonic storms,[ ] and these affect the climatic conditions of all middle and high latitudes, if not of the entire earth. the paths followed by cyclonic storms are also altered by increased contrast between land and water. when the continents are notably colder than the neighboring oceans, high atmospheric pressure develops on the lands and interferes with the passage of lows, which are therefore either deflected around the continent or forced to move slowly. the distribution of lofty mountains has an even more striking climatic effect than the general uplift of a region. in proterozoic times there was a great range in the lake superior region; in the late devonian the acadian mountains of new england and the maritime provinces of canada possibly attained a height equal to the present rockies. subsequently, in the late paleozoic a significant range stood where the ouachitas now are. accompanying the uplift of each of these ranges, and all others, the climate of the surrounding area, especially to leeward, must have been altered greatly. many extensive salt deposits found now in fairly humid regions, for example, the pennsylvanian and permian deposits of kansas and oklahoma, were probably laid down in times of local aridity due to the cutting off of moisture-bearing winds by the mountains of llanoria in louisiana and texas. hence such deposits do not necessarily indicate periods of widespread and profound aridity. when the causes of ancient glaciation were first considered by geologists, about the middle of the nineteenth century, it was usually assumed that the glaciated areas had been elevated to great heights, and thus rendered cold enough to permit the accumulation of glaciers. the many glaciers occurring in the alps of central europe where glaciology arose doubtless suggested this explanation. however, it is now known that most of the ancient glaciation was not of the alpine type, and there is adequate proof that the glacial periods cannot be explained as due directly and solely to uplift. nevertheless, upheavals of the lands are among the most important factors in controlling climate, and variations in the height of the lands have doubtless assisted in producing climate oscillations, especially those of long duration. moreover, the progressive increase in the height of the lands has presumably played a part in fostering local and zonal diversity in contrast with the relative uniformity of earlier geological times. iv. the contraction of the earth has been accompanied by volcanic activity as well as by changes in the extent, distribution, and altitude of the lands. the probable part played by volcanic dust as a contributory factor in producing short sudden climatic variations has already been discussed. there is, however, another though probably less important respect in which volcanic activity may have had at least a slight climatic significance. the oldest known rocks, those of the archean era, contain so much igneous matter that many students have assumed that they show that the entire earth was once liquid. it is now considered that they merely indicate igneous activity of great magnitude. in the later part of proterozoic time, during the second quarter of the earth's history according to schuchert's estimate, there were again vast outflowings of lava. in the lake superior district, for example, a thickness of more than a mile accumulated over a large area, and lavas are common in many areas where rocks of this age are known. the next quarter of the earth's history elapsed without any correspondingly great outflows so far as is known, though several lesser ones occurred. toward the end of the last quarter, and hence quite recently from the geological standpoint, another period of outflows, perhaps as noteworthy as that of the proterozoic, occurred in the cretaceous and tertiary. the climatic effects of such extensive lava flows would be essentially as follows: in the first place so long as the lavas were hot they would set up a local system of convection with inflowing winds. this would interfere at least a little with the general winds of the area. again, where the lava flowed out into water, or where rain fell upon hot lava, there would be rapid evaporation which would increase the rainfall. then after the lava had cooled, it would still influence climate a trifle in so far as its color was notably darker or lighter than that of the average surface. dark surfaces absorb solar heat and become relatively warm when the sun shines upon them. dark objects likewise radiate heat more rapidly than light-colored objects. hence they cool more rapidly at night, and in the winter. as most lavas are relatively dark they increase the average diurnal range of temperature. hence even after they are cool they increase the climatic diversity of the land. the amount of heat given to the atmosphere by an extensive lava flow, though large according to human standards, is small compared with the amount received from the sun by a like area, except during the first few weeks or months before the lava has formed a thick crust. furthermore, probably only a small fraction of any large series of flows occurred in a given century or millennium. moreover, even the largest lava flows covered an area of only a few hundredths of one per cent of the earth's surface. nevertheless, the conditions which modify climate are so complicated that it would be rash to state that this amount of additional heat has been of no climatic significance. like the proverbial "straw that broke the camel's back," the changes it would surely produce in local convection, atmospheric pressure, and the direction of the wind may have helped to shift the paths of storms and to produce other complications which were of appreciable climatic significance. v. the last point which we shall consider in connection with the effect of the earth's interior upon climate is internal heat. the heat given off by lavas is merely a small part of that which is emitted by the earth as a whole. in the earliest part of geological history enough heat may have escaped from the interior of the earth to exert a profound influence on the climate. knowlton,[ ] as we have seen, has recently built up an elaborate theory on this assumption. at present, however, accurate measurements show that the escape of heat is so slight that it has no appreciable influence except in a few volcanic areas. it is estimated to raise the average temperature of the earth's surface less than . °c.[ ] in order to contribute enough heat to raise the surface temperature °c., the temperature gradient from the interior of the earth to the surface would need to be ten times as great as now, for the rate of conduction varies directly with the gradient. if the gradient were ten times as great as now, the rocks at a depth of two and one-half miles would be so hot as to be almost liquid according to barrell's[ ] estimates. the thick strata of unmetamorphosed paleozoic rocks indicate that such high temperatures have not prevailed at such slight depths since the proterozoic. furthermore, the fact that the climate was cold enough to permit glaciation early in the proterozoic era and at from one to three other times before the opening of the paleozoic suggests that the rate of escape of heat was not rapid even in the first half of the earth's recorded history. yet even if the general escape of heat has never been large since the beginning of the better-known part of geological history, it was presumably greater in early times than at present. if there actually has been an appreciable decrease in the amount of heat given out by the earth's interior, its effects would agree with the observed conditions of the geological record. it would help to explain the relative mildness of zonal, seasonal, and local contrasts of climate in early geological times, but it would not help to explain the long oscillations from era to era which appear to have been of much greater importance. those oscillations, so far as we can yet judge, may have been due in part to solar changes, but in large measure they seem to be explained by variations in the extent, distribution, and altitude of the lands. such variations appear to be the inevitable result of the earth's contraction. footnotes: [footnote : chas. schuchert: the earth's changing surface and climate during geologic time; in lull: the evolution of the earth and its inhabitants, , p. .] [footnote : quoted by j. cornet: cours de géologie, , p. .] [footnote : t. c. chamberlin: the order of magnitude of the shrinkage of the earth; jour. geol., vol. , , pp. - , - .] [footnote : g. i. taylor: philosophical transactions, a. , , pp. - ; monthly notices royal astron. soc., jan., , vol. , p. .] [footnote : j. jeffreys: monthly notices royal astron. soc., jan., , vol. , p. .] [footnote : e. w. brown: personal communication.] [footnote : c. s. slichter: the rotational period of a heterogeneous spheroid; in contributions to the fundamental problems of geology, by t. c. chamberlin, _et al._, carnegie inst. of wash., no. , .] [footnote : e. suess: the face of the earth, vol. ii, p. , .] [footnote : chas. schuchert: the earth's changing surface and climate; in lull: the evolution of the earth and its inhabitants, , p. .] [footnote : j. barren: rhythms and the measurement of geologic time; bull. geol. soc. am., vol. , , p. .] [footnote : chas. schuchert: _loc. cit._, p. .] [footnote : t. c. chamberlin: diastrophism, the ultimate basis of correlation; jour. geol., vol. , ; chas. schuchert: _loc. cit._] [footnote : pirsson-schuchert: textbook of geology, , vol. ii, p. ; chas. schuchert: paleogeography of north america; bull. geol. soc. am., vol. , pp. - ; reference on p. .] [footnote : the general subject of the climatic significance of continentality is discussed by c. e. p. brooks: continentality and temperature; quart. jour. royal meteorol. soc., april, , and oct., .] [footnote : chas. schuchert: climates of geologic time; in the climatic factor; carnegie institution, , p. .] [footnote : a. de lapparent: traité de géologie, .] [footnote : chas. schuchert: historical geology, , p. .] [footnote : m. m. metcalf: upon an important method of studying problems of relationship and of geographical distribution; proceedings national academy of sciences, vol. , july, , pp. - .] [footnote : chas. schuchert: paleogeography of north america; bull. geol. soc. am., vol. , ; and willis, salisbury, and others: outlines of geologic history, .] [footnote : chas. schuchert: the earth's changing surface and climate; in lull: the evolution of the earth and its inhabitants, , p. .] [footnote : a. j. henry: the decrease of precipitation with altitude; monthly weather review, vol. , , pp. - .] [footnote : chas. f. brooks: monthly weather review, vol. , , p. ; and also a. j. henry and others: weather forecasting in the united states, .] [footnote : f. h. knowlton: evolution of geologic climates; bull. geol. soc. am., vol. , dec., , pp. - .] [footnote : talbert, quoted by i. bowman: forest physiography, , p. .] [footnote : j. barrell: rhythms and the measurement of geologic time; bull. geol. soc. am., vol. , , pp. - .] chapter xii post-glacial crustal movements and climatic changes an interesting practical application of some of the preceding generalizations is found in an attempt by c. e. p. brooks[ ] to interpret post-glacial climatic changes almost entirely in terms of crustal movement. we believe that he carries the matter much too far, but his discussion is worthy of rather full recapitulation, not only for its theoretical value but because it gives a good summary of post-glacial changes. his climatic table for northwest europe as reprinted from the annual report of the smithsonian institution for , p. , is as follows: _phase_ _climate_ _date_ . the last great arctic climate. , - , b. c. glaciation. . the retreat of the severe continental , - b. c. glaciers. climate. . the continental phase. continental climate. - b. c. . the maritime phase. warm and moist. - b. c. . the later forest phase. warm and dry. - b. c. . the peat-bog phase. cooler and moister. b. c.- a. d. . the recent phase. becoming drier. a. d.- brooks bases his chronology largely on de geer's measurements of the annual layers of clay in lake bottoms but makes much use of other evidence. according to brooks the last glacial epoch lasted roughly from , to , b. c., but this includes a slight amelioration of climate followed by a readvance of the ice, known as the buhl stage. during the time of maximum glaciation the british isles stood twenty or thirty feet higher than now and scandinavia was "considerably" more elevated. the author believes that this caused a fall of °c. in the temperature of the british isles and of °c. in scandinavia. by an ingenious though not wholly convincing method of calculation he concludes that this lowering of temperature, aided by an increase in the area of the lands, sufficed to start an ice sheet in scandinavia. the relatively small area of ice cooled the air and gave rise to an area of high barometric pressure. this in turn is supposed to have caused further expansion of the ice and to have led to full-fledged glaciation. about , b. c. the retreat of the ice began in good earnest. even though no evidence has yet been found, brooks believes there must have been a change in the distribution of land and sea to account for the diminution of the ice. the ensuing millenniums formed the magdalenian period in human history, the last stage of the paleolithic, when man lived in caves and reindeer were abundant in central europe.[ ] at first the ice retreated very slowly and there were periods when for scores of years the ice edge remained stationary or even readvanced. about , b. c. the edge of the ice lay along the southern coast of sweden. during the next years it withdrew more rapidly to about °n. then came the fennoscandian pause, or gschnitz stage, when for about years the ice edge remained in one position, forming a great moraine. brooks suggests that this pause about b. c. was due to the closing of the connection between the atlantic ocean and the baltic sea and the synchronous opening of a connection between the baltic and the white seas, whereby cold arctic waters replaced the warmer atlantic waters. he notes, however, that about b. c. the obliquity of the ecliptic was probably nearly ° greater than at present. this he calculates to have caused the climate of germany and sweden to be °f. colder than at present in winter and °f. warmer in summer. the next climatic stage was marked by a rise of temperature till about b. c. during this period the ice at first retreated, presumably because the climate was ameliorating, although no cause of such amelioration is assigned. at length the ice lay far enough north to allow a connection between the baltic and the atlantic by way of lakes wener and wetter in southern sweden. this is supposed to have warmed the baltic sea and to have caused the climate to become distinctly milder. next the land rose once more so that the baltic was separated from the atlantic and was converted into the ancylus lake of fresh water. the southwest baltic region then stood feet higher than now. the result was the daun stage, about b. c., when the ice halted or perhaps readvanced a little, its front being then near ragunda in about latitude °. why such an elevation did not cause renewed glaciation instead of merely the slight daun pause, brooks does not explain, although his calculations as to the effect of a slight elevation of the land during the main period of glaciation from , to , b. c. would seem to demand a marked readvance. after b. c. there ensued a period when the climate, although still distinctly continental, was relatively mild. the winters, to be sure, were still cold but the summers were increasingly warm. in sweden, for example, the types of vegetation indicate that the summer temperature was °f. higher than now. storms, brooks assumes, were comparatively rare except on the outer fringe of great britain. there they were sufficiently abundant so that in the northwest they gave rise to the first peat-bog period, during which swamps replaced forests of birch and pine. southern and eastern england, however, probably had a dry continental climate. even in northwest norway storms were rare as is indicated by remains of forests on islands now barren because of the strong winds and fierce storms. farther east most parts of central and northern europe were relatively dry. this was the early neolithic period when man advanced from the use of unpolished to polished stone implements. not far from b. c. the period of continental climate was replaced by a comparatively moist maritime climate. brooks believes that this was because submergence opened the mouth of the baltic and caused the fresh ancylus lake to give place to the so-called litorina sea. the temperature in sweden averaged about °f. higher than at present and in southwestern norway °. more important than this was the small annual range of temperature due to the fact that the summers were cool while the winters were mild. because of the presence of a large expanse of water in the baltic region, storms, as our author states, then crossed great britain and followed the baltic depression, carrying the moisture far inland. in spite of the additional moisture thus available the snow line in southern norway was higher than now. at this point brooks turns to other parts of the world. he states that not far from b. c., a submergence of the lands, rarely amounting to more than twenty-five feet, took place not only in the baltic region but in ireland, iceland, spitzbergen, and other parts of the arctic ocean, as well as in the white sea, greenland, and the eastern part of north america. evidences of a mild climate are found in all those places. similar evidence of a mild warm climate is found in east africa, east australia, tierra del fuego, and antarctica. the dates are not established with certainty but they at least fall in the period immediately preceding the present epoch. in explanation of these conditions brooks assumes a universal change of sea level. he suggests with some hesitation that this may have been due to one of pettersson's periods of maximum "tide-generating force." according to pettersson the varying positions of the moon, earth, and sun cause the tides to vary in cycles of about , , and years, though the length of the periods is not constant. when tides are high there is great movement of ocean waters and hence a great mixture of the water at different latitudes. this is supposed to cause an amelioration of climate. the periods of maximum and minimum tide-generating force are as follows: maxima b. c.-------- b. c.-------- b. c.-------a. d. minima --------- b. c.-------- b. c.-------a. d. --------- brooks thinks that the big trees in california and the norse sagas and germanic myths indicate a rough agreement of climatic phenomena with pettersson's last three dates, while the mild climate of b. c. may really belong to b. c. he gives no evidence confirming pettersson's view at the other three dates. to return to brooks' sketch of the relation of climatic pulsations to the altitude of the lands, by b. c., that is, toward the close of the neolithic period, further elevation is supposed to have taken place over the central latitudes of western europe. southern britain, which had remained constantly above its present level ever since , b. c., was perhaps ninety feet higher than now. ireland was somewhat enlarged by elevation, the straits of dover were almost closed, and parts of the present north sea were land. to these conditions brooks ascribes the prevalence of a dry continental climate. the storms shifted northward once more, the winds were mild, as seems to be proved by remains of trees in exposed places; and forests replaced fields of peat and heath in britain and germany. the summers were perhaps warmer than now but the winters were severe. the relatively dry climate prevailed as far west as ireland. for example, in drumkelin bog in donegal county a corded oak road and a two-story log cabin appear to belong to this time. fourteen feet of bog lie below the floor and twenty-six above. this period, perhaps - b. c., was the legendary heroic age of ireland when "the vigour of the irish reached a level not since attained." this, as brooks points out, may have been a result of the relatively dry climate, for today the extreme moisture of ireland seems to be a distinct handicap. in scandinavia, civilization, or at least the stage of relative progress, was also high at this time. by b. c. the land had assumed nearly its present level in the british isles and the southern baltic region, while northern scandinavia still stood lower than now. the climate of britain and germany was so humid that there was an extensive formation of peat even on high ground not before covered. this moist stage seems to have lasted almost to the time of christ, and may have been the reason why the romans described britain as peculiarly wet and damp. at this point brooks again departs from northwest europe to a wider field: it is possible that we have to attribute this damp period in northwest europe to some more general cause, for ellsworth huntington's curves of tree-growth in california and climate in western asia both show moister conditions from about b. c. to a. d. , and the same author believes that the mediterranean lands had a heavier rainfall about b. c. to a. d. . it seems that the phase was marked by a general increase of the storminess of the temperate regions of the northern hemisphere at least, with a maximum between ireland and north germany, indicating probably that the baltic again became the favourite track of depressions from the atlantic. brooks ends his paper with a brief résumé of glacial changes in north america, but as the means of dating events are unreliable the degree of synchronism with europe is not clear. he sums up his conclusions as follows: on the whole it appears that though there is a general similarity in the climatic history of the two sides of the north atlantic, the changes are not really contemporaneous, and such relationship as appears is due mainly to the natural similarity in the geographical history of two regions both recovering from an ice age, and only very partially to world-wide pulsations of climate. additional evidence on this head will be available when baron de geer publishes the results of his recent investigations of the seasonal glacial clays of north america, especially if, as he hopes, he is able to correlate the banding of these clays with the growth-rings of the big trees. when we turn to the northwest of north america, this is brought out very markedly. for in yukon and alaska the ice age was a very mild affair compared with its severity in eastern america and scandinavia. as the land had not a heavy ice-load to recover from, there were no complicated geographical changes. also, there were no fluctuations of climate, but simply a gradual passage to present conditions. the latter circumstance especially seems to show that the emphasis laid on geographical rather than astronomical factors of _great_ climatic changes is not misplaced. brooks' painstaking discussion of post-glacial climatic changes is of great value because of the large body of material which he has so carefully wrought together. his strong belief in the importance of changes in the level of the lands deserves serious consideration. it is difficult, however, to accept his final conclusion that such changes are the main factors in recent climatic changes. it is almost impossible, for example, to believe that movements of the land could produce almost the same series of climatic changes in europe, central asia, the western and eastern parts of north america, and the southern hemisphere. yet such changes appear to have occurred during and since the glacial period. again there is no evidence whatever that movements of the land have anything to do with the historic cycles of climate or with the cycles of weather in our own day, which seem to be the same as glacial cycles on a small scale. also, as dr. simpson points out in discussing brooks' paper, there appears "no solution along these lines of the problem connected with rich vegetation in both polar circles and the ice-age which produced the ice-sheet at sea-level in northern india." nevertheless, we may well believe that brooks is right in holding that changes in the relative level and relative area of land and sea have had important local effects. while they are only one of the factors involved in climatic changes, they are certainly one that must constantly be kept in mind. footnotes: [footnote : c. e. p. brooks: the evolution of climate in northwest europe. quart. jour. royal meteorol. soc., vol. , , pp. - .] [footnote : h. f. osborn: men of the old stone age, n. y., ; j. m. tyler: the new stone age in northwestern europe, n. y., .] chapter xiii the changing composition of oceans and atmosphere having discussed the climatic effect of movements of the earth's crust during the course of geological time, we are now ready to consider the corresponding effects due to changes in the movable envelopes--the oceans and the atmosphere. variations in the composition of sea water and of air and in the amount of air must almost certainly have occurred, and must have produced at least slight climatic consequences. it should be pointed out at once that such variations appear to be far less important climatically than do movements of the earth's crust and changes in the activity of the sun. moreover, in most cases, they are not reversible as are the crustal and solar phenomena. hence, while most of them appear to have been unimportant so far as climatic oscillations and fluctuations are concerned, they seemingly have aided in producing the slight secular progression to which we have so often referred. there is general agreement among geologists that the ocean has become increasingly saline throughout the ages. indeed, calculations of the rate of accumulation of salt have been a favorite method of arriving at estimates of the age of the ocean, and hence of the earliest marine sediments. so far as known, however, no geologist or climatologist has discussed the probable climatic effects of increased salinity. yet it seems clear that an increase in salinity must have a slight effect upon climate. salinity affects climate in four ways: ( ) it appreciably influences the rate of evaporation; ( ) it alters the freezing point; ( ) it produces certain indirect effects through changes in the absorption of carbon dioxide; and ( ) it has an effect on oceanic circulation. ( ) according to the experiments of mazelle and okada, as reported by krümmel,[ ] evaporation from ordinary sea water is from to per cent less rapid than from fresh water under similar conditions. the variation from to per cent found in the experiments depends, perhaps, upon the wind velocity. when salt water is stagnant, rapid evaporation tends to result in the development of a film of salt on the top of the water, especially where it is sheltered from the wind. such a film necessarily reduces evaporation. hence the relatively low salinity of the oceans in the past probably had a tendency to increase the amount of water vapor in the air. even a little water vapor augments slightly the blanketing effect of the air and to that extent diminishes the diurnal and seasonal range of temperature and the contrast from zone to zone. ( ) increased salinity means a lower freezing temperature of the oceans and hence would have an effect during cold periods such as the present and the pleistocene ice age. it would not, however, be of importance during the long warm periods which form most of geologic time. a salinity of about . per cent at present lowers the freezing point of the ocean roughly °c. below that of fresh water. if the ocean were fresh and our winters as cold as now, all the harbors of new england and the middle atlantic states would be icebound. the baltic sea would also be frozen each winter, and even the eastern harbors of the british isles would be frequently locked in ice. at high latitudes the area of permanently frozen oceans would be much enlarged. the effect of such a condition upon marine life in high latitudes would be like that of a change to a warmer climate. it would protect the life on the continental shelf from the severe battering of winter storms. it would also lessen the severity of the winter temperature in the water for when water freezes it gives up much latent heat,--eighty calories per cubic centimeter. part of this raises the temperature of the underlying water. the expansion of the ice near northern shores would influence the life of the lands quite differently from that of the oceans. it would act like an addition of land to the continents and would, therefore, increase the atmospheric contrasts from zone to zone and from continental interior to ocean. in summer the ice upon the sea would tend to keep the coastal lands cool, very much as happens now near the arctic ocean, where the ice floes have a great effect through their reflection of light and their absorption of heat in melting. in winter the virtual enlargement of the continents by the addition of an ice fringe would decrease the snowfall upon the lands. still more important would be the effect in intensifying the anti-cyclonic conditions which normally prevail in winter not only over continents but over ice-covered oceans. hence the outblowing cold winds would he strengthened.[ ] the net effect of all these conditions would apparently be a diminution of snowfall in high latitudes upon the lands even though the summer snowfall upon the ocean and the coasts may have increased. this condition may have been one reason why widespread glaciation does not appear to have prevailed in high latitudes during the proterozoic and permian glaciations, even though it occurred farther south. if the ocean during those early glacial epochs were ice-covered down to middle latitudes, a lack of extensive glaciation in high latitudes would be no more surprising than is the lack of pleistocene glaciation in the northern parts of alaska and asia. great ice sheets are impossible without a large supply of moisture. ( ) among the indirect effects of salinity one of the chief appears to be that the low salinity of the water in the past and the greater ease with which it froze presumably allowed the temperature of the entire ocean to be slightly higher than now. this is because ice serves as a blanket and hinders the radiation of heat from the underlying water. the temperature of the ocean has a climatic significance not only directly, but indirectly through its influence on the amount of carbon dioxide held by the oceans. a change of even °c. from the present mean temperature of °c. would alter the ability of the entire ocean to absorb carbon dioxide by about per cent. this, according to f. w. clarke,[ ] is because the oceans contain from eighteen to twenty-seven times as much carbon dioxide as the air when only the free carbon dioxide is considered, and about seventy times as much according to johnson and williamson[ ] when the partially combined carbon dioxide is also considered. moreover, the capacity of water for carbon dioxide varies sharply with the temperature.[ ] hence a rise in temperature of only °c. would theoretically cause the oceans to give up from to times as much carbon dioxide as the air now holds. this, however, is on the unfounded assumption that the oceans are completely saturated. the important point is merely that a slight change in ocean temperature would cause a disproportionately large change in the amount of carbon dioxide in the air with all that this implies in respect to blanketing the earth, and thus altering temperature. ( ) another and perhaps the most important effect of salinity upon climate depends upon the rapidity of the deep-sea circulation. the circulation is induced by differences of temperature, but its speed is affected at least slightly by salinity. the vertical circulation is now dominated by cold water from subpolar latitudes. except in closed seas like the mediterranean the lower portions of the ocean are near the freezing point. this is because cold water sinks in high latitudes by reason of its superior density, and then "creeps" to low latitudes. there it finally rises and replaces either the water driven poleward by the winds, or that which has evaporated from the surface.[ ] during past ages, when the sea water was less salty, the circulation was presumably more rapid than now. this was because, in tropical regions, the rise of cold water is hindered by the sinking of warm surface water which is relatively dense because evaporation has removed part of the water and caused an accumulation of salt. according to krümmel and mill,[ ] the surface salinity of the subtropical belt of the north atlantic commonly exceeds . per cent and sometimes reaches . per cent, whereas the underlying waters have a salinity of less than . per cent and locally as little as . per cent. the other oceans are slightly less saline than the north atlantic at all depths, but the vertical salinity gradients along the tropics are similar. according to the smithsonian physical tables, the difference in salinity between the surface water and that lying below is equivalent to a difference of . in density, where the density of fresh water is taken as . . since the decrease in density produced by warming water from the temperature of its greatest density ( °c.) to the highest temperatures which ever prevail in the ocean ( °c. or °f.) is only . , the more saline surface waters of the dry tropics are at most times almost as dense as the less saline but colder waters beneath the surface, which have come from higher latitudes. during days of especially great evaporation, however, the most saline portions of the surface waters in the dry tropics are denser than the underlying waters and therefore sink, and produce a temporary local stagnation in the general circulation. such a sinking of the warm surface waters is reported by krümmel, who detected it by means of the rise in temperature which it produces at considerable depths. if such a hindrance to the circulation did not exist, the velocity of the deep-sea movements would be greater. if in earlier times a more rapid circulation occurred, low latitudes must have been cooled more than now by the rise of cold waters. at the same time higher latitudes were presumably warmed by a greater flow of warm water from tropical regions because less of the surface heat sank in low latitudes. such conditions would tend to lessen the climatic contrast between the different latitudes. hence, in so far as the rate of deep-sea circulation depends upon salinity, the slowly increasing amount of salt in the oceans must have tended to increase the contrasts between low and high latitudes. thus for several reasons, the increase of salinity during geologic history seems to deserve a place among the minor agencies which help to explain the apparent tendency toward a secular progression of climate in the direction of greater contrasts between tropical and subpolar latitudes. changes in the composition and amount of the atmosphere have presumably had a climatic importance greater than that of changes in the salinity of the oceans. the atmospheric changes may have been either progressive or cyclic, or both. in early times, according to the nebular hypothesis, the atmosphere was much more dense than now and contained a larger percentage of certain constituents, notably carbon dioxide and water. the planetesimal hypothesis, on the other hand, postulates an increase in the density of the atmosphere, for according to this hypothesis the density of the atmosphere depends upon the power of the earth to hold gases, and this power increases as the earth grows bigger with the infall of material from without.[ ] whichever hypothesis may be correct, it seems probable that when life first appeared on the land the atmosphere resembled that of today in certain fundamental respects. it contained the elements essential to life, and its blanketing effect was such as to maintain temperatures not greatly different from those of the present. the evidence of this depends largely upon the narrow limits of temperature within which the activities of modern life are possible, and upon the cumulative evidence that ancient life was essentially similar to the types now living. the resemblance between some of the oldest forms and those of today is striking. for example, according to professor schuchert:[ ] "many of the living genera of forest trees had their origin in the cretaceous, and the giant sequoias of california go back to the triassic, while ginkgo is known in the permian. some of the fresh-water molluscs certainly were living in the early periods of the mesozoic, and the lung-fish of today (ceratodus) is known as far back as the triassic and is not very unlike other lung-fishes of the devonian. the higher vertebrates and insects, on the other hand, are very sensitive to their environment, and therefore do not extend back generically beyond the cenozoic, and only in a few instances even as far as the oligocene. of marine invertebrates the story is very different, for it is well known that the horseshoe crab (limulus) lived in the upper jurassic, and nautilus in the triassic, with forms in the devonian not far removed from this genus. still longer-ranging genera occur among the brachiopods, for living lingula and crania have specific representatives as far back as the early ordovician. among living foraminifers, lagena, globigerina, and nodosaria are known in the later cambrian or early ordovician. in the middle cambrian near field, british columbia, walcott has found a most varied array of invertebrates among which are crustaceans not far removed from living forms. zoölogists who see these wonderful fossils are at once struck with their modernity and the little change that has taken place in certain stocks since that far remote time. back of the paleozoic, little can be said of life from the generic standpoint, since so few fossils have been recovered, but what is at hand suggests that the marine environment was similar to that of today." at present, as we have repeatedly seen, little growth takes place either among animals or plants at temperatures below °c. or above °c., and for most species the limiting temperatures are about ° and °. the maintenance of so narrow a scale of temperature is a function of the atmosphere, as well as of the sun. without an atmosphere, the temperature by day would mount fatally wherever the sun rides high in the sky. by night it would fall everywhere to a temperature approaching absolute zero, that is - °c. some such temperature prevails a few miles above the earth's surface, beyond the effective atmosphere. indeed, even if the atmosphere were almost as it is now, but only lacked one of the minor constituents, a constituent which is often actually ignored in statements of the composition of the air, life would be impossible. tyndall concludes that if water vapor were entirely removed from the atmosphere for a single day and night, all life--except that which is dormant in the form of seeds, eggs, or spores--would be exterminated. part would be killed by the high temperature developed by day when the sun was high, and part, by the cold night. the testimony of ancient glaciation as to the slight difference in the climate and therefore in the atmosphere of early and late geological times is almost as clear as that of life. just as life proves that the earth can never have been extremely cold during hundreds of millions of years, so glaciation in moderately low latitudes near the dawn of earth history and at several later times, proves that the earth was not particularly hot even in those early days. the gentle progressive change of climate which is recorded in the rocks appears to have been only in slight measure a change in the mean temperature of the earth as a whole, and almost entirely a change in the distribution of temperature from place to place and season to season. hence it seems probable that neither the earth's own emission of heat, nor the supply of solar heat, nor the power of the atmosphere to retain heat can have been much greater a few hundred million years ago than now. it is indeed possible that these three factors may have varied in such a way that any variation in one has been offset by variations of the others in the opposite direction. this, however, is so highly improbable that it seems advisable to assume that all three have remained relatively constant. this conclusion together with a realization of the climatic significance of carbon dioxide has forced most of the adherents of the nebular hypothesis to abandon their assumption that carbon dioxide, the heaviest gas in the air, was very abundant until taken out by coal-forming plants or combined with the calcium oxide of igneous rocks to form the limestone secreted by animals. in the same way the presence of sun cracks in sedimentary rocks of all ages suggests that the air cannot have contained vast quantities of water vapor such as have been assumed by knowlton and others in order to account for the former lack of sharp climatic contrast between the zones. such a large amount of water vapor would almost certainly be accompanied by well-nigh universal and continual cloudiness so that there would be little chance for the pools on the earth's water-soaked surface to dry up. furthermore, there is only one way in which such cloudiness could be maintained and that is by keeping the air at an almost constant temperature night and day. this would require that the chief source of warmth be the interior of the earth, a condition which the proterozoic, permian, and other widespread glaciations seem to disprove. thus there appears to be strong evidence against the radical changes in the atmosphere which are sometimes postulated. yet some changes must have taken place, and even minor changes would be accompanied by some sort of climatic effect. the changes would take the form of either an increase or a decrease in the atmosphere as a whole, or in its constituent elements. the chief means by which the atmosphere has increased appear to be as follows: (a) by contributions from the interior of the earth via volcanoes and springs and by the weathering of igneous rocks with the consequent release of their enclosed gases;[ ] (b) by the escape of some of the abundant gases which the ocean holds in solution; (c) by the arrival on the earth of gases from space, either enclosed in meteors or as free-flying molecules; (d) by the release of gases from organic compounds by oxidation, or by exhalation from animals and plants. on the other hand, one or another of the constituents of the atmosphere has presumably decreased (a) by being locked up in newly formed rocks or organic compounds; (b) by being dissolved in the ocean; (c) by the escape of molecules into space; and (d) by the condensation of water vapor. the combined effect of the various means of increase and decrease depends partly on the amount of each constituent received from the earth's interior or from space, and partly on the fact that the agencies which tend to deplete the atmosphere are highly selective in their action. our knowledge of how large a quantity of new gases the air has received is very scanty, but judging by present conditions the general tendency is toward a slow increase chiefly because of meteorites, volcanic action, and the work of deep-seated springs. as to decrease, the case is clearer. this is because the chemically active gases, oxygen, co_{ }, and water vapor, tend to be locked up in the rocks, while the chemically inert gases, nitrogen and argon, show almost no such tendency. though oxygen is by far the most abundant element in the earth's crust, making up more than per cent of the total, it forms only about one-fifth of the air. nitrogen, on the other hand, is very rare in the rocks, but makes up nearly four-fifths of the air. it would, therefore, seem probable that throughout the earth's history, there has been a progressive increase in the amount of atmospheric nitrogen, and presumably a somewhat corresponding increase in the mass of the air. on the other hand, it is not clear what changes have occurred in the amount of atmospheric oxygen. it may have increased somewhat or perhaps even notably. nevertheless, because of the greater increase in nitrogen, it may form no greater percentage of the air now than in the distant past. as to the absolute amounts of oxygen, barrell[ ] thought that atmospheric oxygen began to be present only after plants had appeared. it will be recalled that plants absorb carbon dioxide and separate the carbon from the oxygen, using the carbon in their tissues and setting free the oxygen. as evidence of a paucity of oxygen in the air in early proterozoic times, barrell cites the fact that the sedimentary rocks of that remote time commonly are somewhat greyish or greenish-grey wackes, or other types, indicating incomplete oxidation. he admits, however, that the stupendous thicknesses of red sandstones, quartzite, and hematitic iron ores of the later proterozoic prove that by that date there was an abundance of atmospheric oxygen. if so, the change from paucity to abundance must have occurred before fossils were numerous enough to give much clue to climate. however, barrell's evidence as to a former paucity of atmospheric oxygen is not altogether convincing. in the first place, it does not seem justifiable to assume that there could be no oxygen until plants appeared to break down the carbon dioxide, for some oxygen is contributed by volcanoes,[ ] and lightning decomposes water into its elements. part of the hydrogen thus set free escapes into space, for the earth's gravitative force does not appear great enough to hold this lightest of gases, but the oxygen remains. thus electrolysis of water results in the accumulation of oxygen. in the second place, there is no proof that the ancient greywackes are not deoxidized sediments. light colored rock formations do not necessarily indicate a paucity of atmospheric oxygen, for such rocks are abundant even in recent times. for example, the tertiary formations are characteristically light colored, a result, however, of deoxidation. finally, the fact that sedimentary rocks, irrespective of their age, contain an average of about . per cent more oxygen than do igneous rocks,[ ] suggests that oxygen was present in the air in quantity even when the earliest shales and sandstones were formed, for atmospheric oxygen seems to be the probable source of the extra oxygen they contain. the formation of these particular sedimentary rocks by weathering of igneous rocks involves only a little carbon dioxide and water. although it seems probable that oxygen was present in the atmosphere even at the beginning of the geological record, it may have been far less abundant then than now. it may have been removed from the atmosphere by animals or by the oxidation of the rocks almost as rapidly as it was added by volcanoes, plants, and other agencies. after this chapter was in type, st. john[c] announced his interesting discovery that oxygen is apparently lacking in the atmosphere of venus. he considers that this proves that venus has no life. furthermore he concludes that so active an element as oxygen cannot be abundant in the atmosphere of a planet unless plants continually supply large quantities by breaking down carbon dioxide. but even if the earth has experienced a notable increase in atmospheric oxygen since the appearance of life, this does not necessarily involve important climatic changes except those due to increased atmospheric density. this is because oxygen has very little effect upon the passage of light or heat, being transparent to all but a few wave lengths. those absorbed are chiefly in the ultra violet. the distinct possibility that oxygen has increased in amount, makes it the more likely that there has been an increase in the total atmosphere, for the oxygen would supplement the increase in the relatively inert nitrogen and argon, which has presumably taken place. the climatic effects of an increase in the atmosphere include, in the first place, an increased scattering of light as it approaches the earth. nitrogen, argon, and oxygen all scatter the short waves of light and thus interfere with their reaching the earth. abbot and fowle,[ ] who have carefully studied the matter, believe that at present the scattering is quantitatively important in lessening insolation. hence our supposed general increase in the volume of the air during part of geological times would tend to reduce the amount of solar energy reaching the earth's surface. on the other hand, nitrogen and argon do not appear to absorb the long wave lengths known as heat, and oxygen absorbs so little as to be almost a non-absorber. therefore the reduced penetration of the air by solar radiation due to the scattering of light would apparently not be neutralized by any direct increase in the blanketing effect of the atmosphere, and the temperature near the earth's surface would be slightly lowered by a thicker atmosphere. this would diminish the amount of water vapor which would be held in the air, and thereby lower the temperature a trifle more. in the second place, the higher atmospheric pressure which would result from the addition of gases to the air would cause a lessening of the rate of evaporation, for that rate declines as pressure increases. decreased evaporation would presumably still further diminish the vapor content of the atmosphere. this would mean a greater daily and seasonal range of temperature, as is very obvious when we compare clear weather with cloudy. cloudy nights are relatively warm while clear nights are cool, because water vapor is an almost perfect absorber of radiant heat, and there is enough of it in the air on moist nights to interfere greatly with the escape of the heat accumulated during the day. therefore, if atmospheric moisture were formerly much more abundant than now, the temperature must have been much more uniform. the tendency toward climatic severity as time went on would be still further increased by the cooling which would result from the increased wind velocity discussed below; for cooling by convection increases with the velocity of the wind, as does cooling by conduction. any persistent lowering of the general temperature of the air would affect not only its ability to hold water vapor, but would produce a lessening in the amount of atmospheric carbon dioxide, for the colder the ocean becomes the more carbon dioxide it can hold in solution. when the oceanic temperature falls, part of the atmospheric carbon dioxide is dissolved in the ocean. this minor constituent of the air is important because although it forms only . per cent of the earth's atmosphere, abbot and fowle's[ ] calculations indicate that it absorbs over per cent of the heat radiated outward from the earth. hence variations in the amount of carbon dioxide may have caused an appreciable variation in temperature and thus in other climatic conditions. humphreys, as we have seen, has calculated that a doubling of the carbon dioxide in the air would directly raise the earth's temperature to the extent of . °c., and a halving would lower it a like amount. the indirect results of such an increase or decrease might be greater than the direct results, for the change in temperature due to variations in carbon dioxide would alter the capacity of the air to hold moisture. two conditions would especially help in this respect; first, changes in nocturnal cooling, and second, changes in local convection. the presence of carbon dioxide diminishes nocturnal cooling because it absorbs the heat radiated by the earth, and re-radiates part of it back again. hence with increased carbon dioxide and with the consequent warmer nights there would be less nocturnal condensation of water vapor to form dew and frost. local convection is influenced by carbon dioxide because this gas lessens the temperature gradient. in general, the less the gradient, that is, the less the contrast between the temperature at the surface and higher up, the less convection takes place. this is illustrated by the seasonal variation in convection. in summer, when the gradient is steepest, convection reaches its maximum. it will be recalled that when air rises it is cooled by expansion, and if it ascends far the moisture is soon condensed and precipitated. indeed, local convection is considered by c. p. day to be the chief agency which keeps the lower air from being continually saturated with moisture. the presence of carbon dioxide lessens convection because it increases the absorption of heat in the zone above the level in which water vapor is abundant, thus warming these higher layers. the lower air may not be warmed correspondingly by an increase in carbon dioxide if abbot and fowle are right in stating that near the earth's surface there is enough water vapor to absorb practically all the wave lengths which carbon dioxide is capable of absorbing. hence carbon dioxide is chiefly effective at heights to which the low temperature prevents water vapor from ascending. carbon dioxide is also effective in cold winters and in high latitudes when even the lower air is too cold to contain much water vapor. moreover, carbon dioxide, by altering the amount of atmospheric water vapor, exerts an indirect as well as a direct effect upon temperature. other effects of the increase in air pressure which we are here assuming during at least the early part of geological times are corresponding changes in barometric contrasts, in the strength of winds, and in the mass of air carried by the winds along the earth's surface. the increase in the mass of the air would reënforce the greater velocity of the winds in their action as eroding and transporting agencies. because of the greater weight of the air, the winds would be capable of picking up more dust and of carrying it farther and higher; while the increased atmospheric friction would keep it aloft a longer time. the significance of dust at high levels and its relation to solar radiation have already been discussed in connection with volcanoes. it will be recalled that on the average it lowers the surface temperature. at lower levels, since dust absorbs heat quickly and gives it out quickly, its presence raises the temperature of the air by day and lowers it by night. hence an increase in dustiness tends toward greater extremes. from all these considerations it appears that if the atmosphere has actually evolved according to the supposition which is here tentatively entertained, the general tendency of the resultant climatic changes must have been partly toward long geological oscillations and partly toward a general though very slight increase in climatic severity and in the contrasts between the zones. this seems to agree with the geological record, although the fact that we are living in an age of relative climatic severity may lead us astray. the significant fact about the whole matter is that the three great types of terrestrial agencies, namely, those of the earth's interior, those of the oceans, and those of the air, all seem to have suffered changes which lead to slow variations of climate. many reversals have doubtless taken place, and the geologic oscillations thus induced are presumably of much greater importance than the progressive change, yet so far as we can tell the purely terrestrial changes throughout the hundreds of millions of years of geological time have tended toward complexity and toward increased contrasts from continent to ocean, from latitude to latitude, from season to season, and from day to night. throughout geological history the slow and almost imperceptible differentiation of the earth's surface has been one of the most noteworthy of all changes. it has been opposed by the extraordinary conservatism of the universe which causes the average temperature today to be so like that of hundreds of millions of years ago that many types of life are almost identical. nevertheless, the differentiation has gone on. often, to be sure, it has presumably been completely masked by the disturbances of the solar atmosphere which appear to have been the cause of the sharper, shorter climatic pulsations. but regardless of cosmic conservatism and of solar impulses toward change, the slow differentiation of the earth's surface has apparently given to the world of today much of the geographical complexity which is so stimulating a factor in organic evolution. such complexity--such diversity from place to place--appears to be largely accounted for by purely terrestrial causes. it may be regarded as the great terrestrial contribution to the climatic environment which guides the development of life. footnotes: [footnote : encyclopædia britannica, th edition: article "ocean."] [footnote : c. e. p. brooks: the meteorological conditions of an ice sheet and their bearing on the desiccation of the globe; quart. jour. royal meteorol. soc., vol. , , pp. - .] [footnote : data of geochemistry, fourth ed., ; bull. no. , u. s. geol. survey.] [footnote : quoted by schuchert in the evolution of the earth.] [footnote : smithsonian physical tables, sixth revision, , p. .] [footnote : chamberlin, in a very suggestive article "on a possible reversal of oceanic circulation" (jour. of geol., vol. , pp. - , ), discusses the probable climatic consequences of a reversal in the direction of deep-sea circulation. it is not wholly beyond the bounds of possibility that, in the course of ages the increasing drainage of salt from the lands not only by nature but by man's activities in agriculture and drainage, may ultimately cause such a reversal by increasing the ocean's salinity until the more saline tropical portion is heavier than the cooler but fresher subpolar waters. if that should happen, greenland, antarctica, and the northern shores of america and asia would be warmed by the tropical heat which had been transferred poleward beneath the surface of the ocean, without loss _en route_. subpolar regions, under such a condition of reversed deep-sea circulation, might have a mild climate. indeed, they might be among the world's most favorable regions climatically.] [footnote : encyclopædia britannica: article "ocean."] [footnote : chamberlin and salisbury: geology, vol. ii, pp. - , ; and t. c. chamberlin: the origin of the earth, .] [footnote : personal communication.] [footnote : r. t. chamberlin: gases in rocks, carnegie inst. of wash., no. , .] [footnote : j. barrell: the origin of the earth, in evolution of the earth and its inhabitants, , p. , and more fully in an unpublished manuscript.] [footnote : f. w. clarke: data of geochemistry, fourth ed., , bull. no. , u. s. geol. survey, p. .] [footnote : f. w. clarke: _loc. cit._, pp. - et al.] [footnote c: chas. e. st. john: science service press reports from the mt. wilson observatory, may, .] [footnote : abbot and fowle: annals astrophysical observatory; smiths. inst., vol. ii, , p. . f. e. fowle: atmospheric scattering of light; misc. coll. smiths. inst., vol. , .] [footnote : abbot and fowle: _loc. cit._, p. .] chapter xiv the effect of other bodies on the sun if solar activity is really an important factor in causing climatic changes, it behooves us to subject the sun to the same kind of inquiry to which we have subjected the earth. we have inquired into the nature of the changes through which the earth's crust, the oceans, and the atmosphere have influenced the climate of geological times. it has not been necessary, however, to study the origin of the earth, nor to trace its earlier stages. our study of the geological record begins only when the earth had attained practically its present mass, essentially its present shape, and a climate so similar to that of today that life as we know it was possible. in other words, the earth had passed the stages of infancy, childhood, youth, and early maturity, and had reached full maturity. as it still seems to be indefinitely far from old age, we infer that during geological times its relative changes have been no greater than those which a man experiences between the ages of perhaps twenty-five and forty. similar reasoning applies with equal or greater force to the sun. because of its vast size it presumably passes through its stages of development much more slowly than the earth. in the first chapter of this book we saw that the earth's relative uniformity of climate for hundreds of millions of years seems to imply a similar uniformity in solar activity. this accords with a recent tendency among astronomers who are more and more recognizing that the stars and the solar system possess an extraordinary degree of conservatism. changes that once were supposed to take place in thousands of years are now thought to have required millions. hence in this chapter we shall assume that throughout geological times the condition of the sun has been almost as at present. it may have been somewhat larger, or different in other ways, but it was essentially a hot, gaseous body such as we see today and it gave out essentially the same amount of energy. this assumption will affect the general validity of what follows only if it departs widely from the truth. with this assumption, then, let us inquire into the degree to which the sun's atmosphere has probably been disturbed throughout geological times. in _earth and sun_, as already explained, a detailed study has led to the conclusion that cyclonic storms are influenced by the electrical action of the sun. such action appears to be most intense in sunspots, but apparently pertains also to other disturbed areas in the sun's atmosphere. a study of sunspots suggests that their true periodicity is almost if not exactly identical with that of the orbital revolution of jupiter, . years. other investigations show numerous remarkable coincidences between sunspots and the orbital revolution of the other planets, including especially saturn and mercury. this seems to indicate that there is some truth in the hypothesis that sunspots and other related disturbances of the solar atmosphere owe their periodicity to the varying effects of the planets as they approach and recede from the sun in their eccentric orbits and as they combine or oppose their effects according to their relative positions. this does not mean that the energy of the solar disturbances is supposed to come from the planets, but merely that their variations act like the turning of a switch to determine when and how violently the internal forces of the sun shall throw the solar atmosphere into commotion. this hypothesis is by no means new, for in one form or another it has been advocated by wolfer, birkeland, e. w. brown, schuster, arctowski, and others. the agency through which the planets influence the solar atmosphere is not yet clear. the suggested agencies are the direct pull of gravitation, the tidal effect of the planets, and an electro-magnetic effect. in _earth and sun_ the conclusion is reached that the first two are out of the question, a conclusion in which e. w. brown acquiesces. unless some unknown cause is appealed to, this leaves an electro-magnetic hypothesis as the only one which has a reasonable foundation. schuster inclines to this view. the conclusions set forth in _earth and sun_ as to the electrical nature of the sun's influence on the earth point somewhat in the same direction. hence in this chapter we shall inquire what would happen to the sun, and hence to the earth, on their journey through space, if the solar atmosphere is actually subject to disturbance by the electrical or other effects of other heavenly bodies. it need hardly be pointed out that we are here venturing into highly speculative ground, and that the verity or falsity of the conclusions reached in this chapter has nothing to do with the validity of the reasoning in previous chapters. those chapters are based on the assumption that terrestrial causes of climatic changes are supplemented by solar disturbances which produce their effect partly through variations in temperature but also through variations in the intensity and paths of cyclonic storms. the present chapter seeks to shed some light on the possible causes and sequence of solar disturbances. let us begin by scanning the available evidence as to solar disturbances previous to the time when accurate sunspot records are available. two rather slender bits of evidence point to cycles of solar activity lasting hundreds of years. one of these has already been discussed in chapter vi, where the climatic stress of the fourteenth century was described. at that time sunspots are known to have been unusually numerous, and there were great climatic extremes. lakes overflowed in central asia; storms, droughts, floods, and cold winters were unusually severe in europe; the caspian sea rose with great rapidity; the trees of california grew with a vigor unknown for centuries; the most terrible of recorded famines occurred in england and india; the eskimos were probably driven south by increasing snowiness in greenland; and the mayas of yucatan appear to have made their last weak attempt at a revival of civilization under the stimulus of greater storminess and less constant rainfall. the second bit of evidence is found in recent exhaustive studies of periodicities by turner[ ] and other astronomers. they have sought every possible natural occurrence for which a numerical record is available for a long period. the most valuable records appear to be those of tree growth, nile floods, chinese earthquakes, and sunspots. turner reaches the conclusion that all four types of phenomena show the same periodicity, namely, cycles with an average length of about to years. he suggests that if this is true, the cycles in tree growth and in floods, both of which are climatic, are probably due to a non-terrestrial cause. the fact that the sunspots show similar cycles suggests that the sun's variations are the cause. these two bits of evidence are far too slight to form the foundation of any theory as to changes in solar activity in the geological past. nevertheless it may be helpful to set forth certain possibilities as a stimulus to further research. for example, it has been suggested that meteoric bodies may have fallen into the sun and caused it suddenly to flare up, as it were. this is not impossible, although it does not appear to have taken place since men became advanced enough to make careful observations. moreover, the meteorites which now fall on the earth are extremely small, the average size being computed as no larger than a grain of wheat. the largest ever found on the earth's surface, at bacubirito in mexico, weighs only about fifty tons, while within the rocks the evidences of meteorites are extremely scanty and insignificant. if meteorites had fallen into the sun often enough and of sufficient size to cause glacial fluctuations and historic pulsations of climate, it seems highly probable that the earth would show much more evidence of having been similarly disturbed. and even if the sun should be bombarded by large meteors the result would probably not be sudden cold periods, which are the most notable phenomena of the earth's climatic history, but sudden warm periods followed by slow cooling. nevertheless, the disturbance of the sun by collision with meteoric matter can by no means be excluded as a possible cause of climatic variations. allied to the preceding hypothesis is shapley's[ ] nebular hypothesis. at frequent intervals, averaging about once a year during the last thirty years, astronomers have discovered what are known as novæ. these are stars which were previously faint or even invisible, but which flash suddenly into brilliancy. often their light-giving power rises seven or eight magnitudes--a thousand-fold. in addition to the spectacular novæ there are numerous irregular variables whose brilliancy changes in every ratio from a few per cent up to several magnitudes. most of them are located in the vicinity of nebulæ, as is also the case with novæ. this, as well as other facts, makes it probable that all these stars are "friction variables," as shapley calls them. apparently as they pass through the nebulæ they come in contact with its highly diffuse matter and thereby become bright much as the earth would become bright if its atmosphere were filled with millions of almost infinitesimally small meteorites. a star may also lose brilliancy if nebulous matter intervenes between it and the observer. if our sun has been subjected to any of these changes some sort of climatic effect must have been produced. in a personal communication shapley amplifies the nebular climatic hypothesis as follows: within light years of the sun in many directions (taurus, cygnus, ophiuchus, scorpio) are great diffuse clouds of nebulosity, some bright, most of them dark. the probability that stars moving in the general region of such clouds will encounter this material is very high, for the clouds fill enormous volumes of space,--e.g., probably more than a hundred thousand cubic light years in the orion region, and are presumably composed of rarefied gases or of dust particles. probably throughout all our part of space such nebulosity exists (it is all around us, we are sure), but only in certain regions is it dense enough to affect conspicuously the stars involved in it. if a star moving at high velocity should collide with a dense part of such a nebulous cloud, we should probably have a typical nova. if the relative velocity of nebulous material and star were low or moderate, or if the material were rare, we should not expect a conspicuous effect on the star's light. in the nebulous region of orion, which is probably of unusually high density, there are about known stars, varying between % and % of their total light--all of them irregularly--some slowly, some suddenly. apparently they are "friction variables." some of the variables suddenly lose % of their light as if blanketed by nebulous matter. in the trifid nebula there are variables like those of orion, in messier also, and probably many of the or so around the rho ophiuchi region belong to this kind. i believe that our sun could not have been a typical nova, at least not since the archeozoic, that is for perhaps a billion years. i believe we have in geological climates final proof of this, because an increase in the amount of solar radiation by times as in the typical nova, would certainly punctuate emphatically the life cycle on the earth, even if the cause of the nova would not at the same time eliminate the smaller planets. but the sun may have been one of these miniature novæ or friction variables; and i believe it very probable that its wanderings through this part of space could not long leave its mean temperature unaffected to the amount of a few per cent. one reason we have not had this proposal insisted upon before is that the data back of it are mostly new--the orion variables have been only recently discovered and studied, the distribution and content of the dark nebulæ are hardly as yet generally known. this interesting hypothesis cannot be hastily dismissed. if the sun should pass through a nebula it seems inevitable that there would be at least slight climatic effects and perhaps catastrophic effects through the action of the gaseous matter not only on the sun but on the earth's own atmosphere. as an explanation of the general climatic conditions of the past, however, shapley points out that the hypothesis has the objection of being vague, and that nebulosity should not be regarded as more than "a possible factor." one of the chief difficulties seems to be the enormously wide distribution of as yet undiscovered nebulous matter which must be assumed if any large share of the earth's repeated climatic changes is to be ascribed to such matter. if such matter is actually abundant in space, it is hard to see how any but the nearest stars would be visible. another objection is that there is no known nebulosity near at hand with which to connect the climatic vicissitudes of the last glacial period. moreover, the known nebulæ are so much less numerous than stars that the chances that the sun will encounter one of them are extremely slight. this, however, is not an objection, for shapley points out that during geological times the sun can never have varied as much as do the novæ, or even as most of the friction variables. thus the hypothesis stands as one that is worth investigating, but that cannot be finally rejected or accepted until it is made more definite and until more information is available. another suggested cause of solar variations is the relatively sudden contraction of the sun such as that which sometimes occurs on the earth when continents are uplifted and mountains upheaved. it seems improbable that this could have occurred in a gaseous body like the sun. lacking, as it does, any solid crust which resists a change of form, the sun probably shrinks steadily. hence any climatic effects thus produced must be extremely gradual and must tend steadily in one direction for millions of years. still another suggestion is that the tidal action of the stars and other bodies which may chance to approach the sun's path may cause disturbances of the solar atmosphere. the vast kaleidoscope of space is never quiet. the sun, the stars, and all the other heavenly bodies are moving, often with enormous speed. hence the effect of gravitation upon the sun must vary constantly and irregularly, as befits the geological requirements. in the case of the planets, however, the tidal effect does not seem competent to produce the movements of the solar atmosphere which appear to be concerned in the inception of sunspots. moreover, there is only the most remote probability that a star and the sun will approach near enough to one another to produce a pronounced gravitational disturbance in the solar atmosphere. for instance, if it be assumed that changes in jupiter's tidal effect on the sun are the main factor in regulating the present difference between sunspot maxima and sunspot minima, the chances that a star or some non-luminous body of similar mass will approach near enough to stimulate solar activity and thereby bring on glaciation are only one in twelve billion years, as will be explained below. this seems to make a gravitational hypothesis impossible. another possible cause of solar disturbances is that the stars in their flight through space may exert an electrical influence which upsets the equilibrium of the solar atmosphere. at first thought this seems even more impossible than a gravitational effect. electrostatic effects, however, differ greatly from those of tides. they vary as the diameter of a body instead of as its mass; their differentials also vary inversely as the square of the distance instead of as the cube. electrostatic effects also increase as the fourth power of the temperature or at least would do so if they followed the law of black bodies; they are stimulated by the approach of one body to another; and they are cumulative, for if ions arrive from space they must accumulate until the body to which they have come begins to discharge them. hence, on the basis of assumptions such as those used in the preceding paragraph, the chances of an electrical disturbance of the solar atmosphere sufficient to cause glaciation on the earth may be as high as one in twenty or thirty million years. this seems to put an electrical hypothesis within the bounds of possibility. further than that we cannot now go. there may be other hypotheses which fit the facts much better, but none seems yet to have been suggested. in the rest of this chapter the tidal and electrical hypotheses of stellar action on the sun will be taken up in detail. the tidal hypothesis is considered because in discussions of the effect of the planets it has hitherto held almost the entire field. the electrical hypothesis will be considered because it appears to be the best yet suggested, although it still seems doubtful whether electrical effects can be of appreciable importance over such vast distances as are inevitably involved. the discussion of both hypotheses will necessarily be somewhat technical, and will appeal to the astronomer more than to the layman. it does not form a necessary part of this book, for it has no bearing on our main thesis of the effect of the sun on the earth. it is given here because ultimately the question of changes in solar activity during geological times must be faced. in the astronomical portion of the following discussion we shall follow jeans[ ] in his admirable attempt at a mathematical analysis of the motions of the universe. jeans divides the heavenly bodies into five main types. ( ) spiral nebulæ, which are thought by some astronomers to be systems like our own in the making, and by others to be independent universes lying at vast distances beyond the limits of our galactic universe, as it is called from the galaxy or milky way. ( ) nebulæ of a smaller type, called planetary. these lie within the galactic portion of the universe and seem to be early stages of what may some day be stars or solar systems. ( ) binary or multiple stars, which are extraordinarily numerous. in some parts of the heavens they form or even per cent of the stars and in the galaxy as a whole they seem to form "fully one third." ( ) star clusters. these consist of about a hundred groups of stars in each of which the stars move together in the same direction with approximately the same velocity. these, like the spiral nebulæ, are thought by some astronomers to lie outside the limits of the galaxy, but this is far from certain. ( ) the solar system. according to jeans this seems to be unique. it does not fit into the general mathematical theory by which he explains spiral nebulæ, planetary nebulæ, binary stars, and star clusters. it seems to demand a special explanation, such as is furnished by tidal disruption due to the passage of the sun close to another star. the part of jeans' work which specially concerns us is his study of the probability that some other star will approach the sun closely enough to have an appreciable gravitative or electrical effect, and thus cause disturbances in the solar atmosphere. of course both the star and the sun are moving, but to avoid circumlocution we shall speak of such mutual approaches simply as approaches of the sun. for our present purpose the most fundamental fact may be summed up in a quotation from jeans in which he says that most stars "show evidence of having experienced considerable disturbance by other systems; there is no reason why our solar system should be expected to have escaped the common fate." jeans gives a careful calculation from which it is possible to derive some idea of the probability of any given degree of approach of the sun and some other star. of course all such calculations must be based on certain assumptions. the assumptions made by jeans are such as to make the probability of close approaches as great as possible. for example, he allows only million years for the entire evolution of the sun, whereas some astronomers and geologists would put the figure ten or more times as high. nevertheless, jeans' assumptions at least show the order of magnitude which we may expect on the basis of reasonable astronomical conclusions. according to the planetary hypothesis of sunspots, the difference in the effect of jupiter when it is nearest and farthest from the sun is the main factor in starting the sunspot cycle and hence the corresponding terrestrial cycle. the climatic difference between sunspot maxima and minima, as measured by temperature, apparently amounts to at least a twentieth and perhaps a tenth of the difference between the climate of the last glacial epoch and the present. we may suppose, then, that a body which introduced a gravitative or electrical factor twenty times as great as the difference in jupiter's effect at its maximum and minimum distances from the sun would cause a glacial epoch if the effect lasted long enough. of course the other planets combine their effects with that of jupiter, but for the sake of simplicity we will leave the others out of account. the difference between jupiter's maximum and minimum tidal effect on the sun amounts to per cent of the planet's average effect. the corresponding difference, according to the electrical hypothesis, is about per cent, for electrostatic action varies as the square of the distance instead of as the cube. let us assume that a body exerting four times jupiter's present tidal effect and placed at the average distance of jupiter from the sun would disturb the sun's atmosphere twenty times as much as the present difference between sunspot maxima and minima, and thus, perhaps, cause a glacial period on the earth. on the basis of this assumption our first problem is to estimate the frequency with which a star, visible or dark, is likely to approach near enough to the sun to produce a _tidal_ effect four times that of jupiter. the number of visible stars is known or at least well estimated. as to dark stars, which have grown cool, arrhenius believed that they are a hundred times as numerous as bright stars; few astronomers believe that there are less than three or four times as many. dr. shapley of the harvard observatory states that a new investigation of the matter suggests that eight or ten is probably a maximum figure. let us assume that nine is correct. the average visible star, so far as measured, has a mass about twice that of the sun, or about times that of jupiter. the distances of the stars have been measured in hundreds of cases and thus we can estimate how many stars, both visible and invisible, are on an average contained in a given volume of space. on this basis jeans estimates that there is only one chance in thirty billion years that a visible star will approach within . times the distance of neptune from the sun, that is, within about eight billion miles. if we include the invisible stars the chances become one in three billion years. in order to produce four times the tidal effect of jupiter, however, the average star would have to approach within about four billion miles of the sun, and the chances of that are only one in twelve billion years. the disturbing star would be only per cent farther from the sun than neptune, and would almost pass within the solar system. even though jeans holds that the frequency of the mutual approach of the sun and a star was probably much greater in the distant past than at present, the figures just given lend little support to the tidal hypothesis. in fact, they apparently throw it out of court. it will be remembered that jeans has made assumptions which give as high a frequency of stellar encounters as is consistent with the astronomical facts. we have assumed nine dark stars for every bright one, which may be a liberal estimate. also, although we have assumed that a disturbance of the sun's atmosphere sufficient to cause a glacial period would arise from a tidal effect only twenty times as great as the difference in jupiter's effect when nearest the sun and farthest away, in our computations this has actually been reduced to thirteen. with all these favorable assumptions the chances of a stellar approach of the sort here described are now only one in twelve billion years. yet within a hundred million years, according to many estimates of geological time, and almost certainly within a billion, there have been at least half a dozen glaciations. our use of jeans' data interposes another and equally insuperable difficulty to any tidal hypothesis. four billion miles is a very short distance in the eyes of an astronomer. at that distance a star twice the size of the sun would attract the outer planets more strongly than the sun itself, and might capture them. if a star should come within four billion miles of the sun, its effect in distorting the orbits of all the planets would be great. if this had happened often enough to cause all the glaciations known to geologists, the planetary orbits would be strongly elliptical instead of almost circular. the consideration here advanced militate so strongly against the tidal hypothesis of solar disturbances that it seems scarcely worth while to consider it further. let us turn now to the electrical hypothesis. here the conditions are fundamentally different from those of the tidal hypothesis. in the first place the electrostatic effect of a body has nothing to do with its mass, but depends on the area of its surface; that is, it varies as the square of the radius. second, the emission of electrons varies exponentially. if hot glowing stars follow the same law as black bodies at lower temperatures, the emission of electrons, like the emission of other kinds of energy, varies as the fourth power of the absolute temperature. in other words, suppose there are two black bodies, otherwise alike, but one with a temperature of ° c. or ° on the absolute scale, and the other with ° on the absolute scale. the temperature of one is twice as high as that of the other, but the electrostatic effect will be sixteen times as great.[ ] third, the number of electrons that reach a given body varies inversely as the square of the distance, instead of as the cube which is the case with tide-making forces. in order to use these three principles in calculating the effect of the stars we must know the diameters, distances, temperature, and number of the stars. the distances and number may safely be taken as given by jeans in the calculations already cited. as to the diameters, the measurements of the stars thus far made indicate that the average mass is about twice that of the sun. the average density, as deduced by shapley[ ] from the movements of double stars, is about one-eighth the solar density. this would give an average diameter about two and a half times that of the sun. for the dark stars, we shall assume for convenience that they are ten times as numerous as the bright ones. we shall also assume that their diameter is half that of the sun, for being cool they must be relatively dense, and that their temperature is the same as that which we shall assume for jupiter. as to jupiter we shall continue our former assumption that a body with four times the effectiveness of that planet, which here means with twice as great a radius, would disturb the sun enough to cause glaciation. it would produce about twenty times the electrostatic effect which now appears to be associated with the difference in jupiter's effect at maximum and minimum. the temperature of jupiter must also be taken into account. the planet is supposed to be hot because its density is low, being only about . that of water. nevertheless, it is probably not luminous, for as moulton[ ] puts it, shadows upon it are black and its moons show no sign of illumination except from the sun. hence a temperature of about °c., or approximately ° on the absolute scale, seems to be the highest that can reasonably be assigned to the cold outer layer whence electrons are emitted. as to the temperature of the sun, we shall adopt the common estimate of about °c. on the absolute scale. the other stars will be taken as averaging the same, although of course they vary greatly. when jeans' method of calculating the probability of a mutual approach of the sun and a star is applied to the assumptions given above, the results are as shown in table . on that basis the dark stars seem to be of negligible importance so far as the electrical hypothesis is concerned. even though they may be ten times as numerous as the bright ones there appears to be only one chance in billion years that one of them will approach the sun closely enough to cause the assumed disturbance of the solar atmosphere. on the other hand, if all the visible stars were the size of the sun, and as hot as that body, their electrical effect would be fourfold that of our assumed dark star because of their size, and times as great because of their temperature, or approximately , times as great. under such conditions the theoretical chance of an approach that would cause glaciation is one in million years. if the average visible star is somewhat cooler than the sun and has a radius about two and one-half times as great, as appears to be the fact, the chances rise to one in thirty-eight million years. a slight and wholly reasonable change in our assumptions would reduce this last figure to only five or ten million. for instance, the earth's mean temperature during the glacial period has been assumed as °c. lower than now, but the difference may have been only °. again, the temperature of the outer atmosphere of jupiter where the electrons are shot out may be only ° or ° absolute, instead of °. or the diameter of the average star may be five or ten times that of the sun, instead of only two and one-half times as great. all this, however, may for the present be disregarded. the essential point is that even when the assumptions err on the side of conservatism, the results are of an order of magnitude which puts the electrical hypothesis within the bounds of possibility, whereas similar assumptions put the tidal hypothesis, with its single approach in twelve billion years, far beyond those limits. the figures for betelgeuse in table are interesting. at a meeting of the american association for the advancement of science in december, , michelson reported that by measurements of the interference of light coming from the two sides of that bright star in orion, the observers at mount wilson had confirmed the recent estimates of three other authorities that the star's diameter is about million miles, or times that of the sun. if other stars so much surpass the estimates of only a decade or two ago, the average diameter of all the visible stars must be many times that of the sun. the low figure for betelgeuse in section d of the table means that if all the stars were as large as betelgeuse, several might often be near enough to cause profound disturbances of the solar atmosphere. nevertheless, because of the low temperature of the giant red stars of the betelgeuse type, the distance at which one of them would produce a given electrical effect is only about five times the distance at which our assumed average star would produce the same effect. this, to be sure, is on the assumption that the radiation of energy from incandescent bodies varies according to temperature in the same ratio as the radiation from black bodies. even if this assumption departs somewhat from the truth, it still seems almost certain that the lower temperature of the red compared with the high temperature of the white stars must to a considerable degree reduce the difference in electrical effect which would otherwise arise from their size. table theoretical probability of stellar approaches --------------------------------------------------------------------- | | | | | | | |_average | | | _dark stars_ | _sun_ | star_ |_betelgeuse_| --------------------------------------------------------------------- a. approximate | | | | | radius in miles | , | , | , , | , , | | | | | | b. assumed | | | | | temperature above| | | | | absolute zero. | ° c. | ° c.| ° c.| ° c. | | | | | | c. approximate | | | | | theoretical | | | | | distance at which| | | | | star would cause | | | | | solar disturbance| | | | | great enough to | | | | | cause glaciation | | | | | (billions[ ] | | | | | of miles). | . | | | | | | | | | d. average | | | | | interval between | | | | | approaches | | | | | close enough to | | | | | cause glaciation | | | | | if all stars | , , , | | | | were of given |[ ] | | | | type. years. | | , , | , , | , | --------------------------------------------------------------------- thus far in our attempt to estimate the distance at which a star might disturb the sun enough to cause glaciation on the earth, we have considered only the star's size and temperature. no account has been taken of the degree to which its atmosphere is disturbed. yet in the case of the sun this seems to be one of the most important factors. the magnetic field of sunspots is sometimes or times as strong as that of the sun in general. the strength of the magnetic field appears to depend on the strength of the electrical currents in the solar atmosphere. but the intensity of the sunspots and, by inference, of the electrical currents, may depend on the electrical action of jupiter and the other planets. if we apply a similar line of reasoning to the stars, we are at once led to question whether the electrical activity of double stars may not be enormously greater than that of isolated stars like the sun. if this line of reasoning is correct, the atmosphere of every double star must be in a state of commotion vastly greater than that of the sun's atmosphere even when it is most disturbed. for example, suppose the sun were accompanied by a companion of equal size at a distance of one million miles, which would make it much like many known double stars. suppose also that in accordance with the general laws of physics the electrical effect of the two suns upon one another is proportional to the fourth power of the temperature, the square of the radius, and the inverse square of the distance. then the effect of each sun upon the other would be sixty billion ( × ^{ }) times as great as the present electrical effect of jupiter upon the sun. just what this would mean as to the net effect of a pair of such suns upon the electrical potential of other bodies at a distance we can only conjecture. the outstanding fact is that the electrical conditions of a double star must be radically different and vastly more intense than those of a single star like the sun. this conclusion carries weighty consequences. at present twenty or more stars are known to be located within about trillion miles of the sun (five parsecs, as the astronomers say), or . light years. according to the assumptions employed in table an average single star would influence the sun enough to cause glaciation if it came within approximately billion miles. if the star were double, however, it might have an electrical capacity enormously greater than that of the sun. then it would be able to cause glaciation at a correspondingly great distance. today alpha centauri, the nearest known star about twenty-five trillion miles, or . light years from the sun, and sirius, the brightest star in the heavens, is about fifty trillion miles away, or . light years. if these stars were single and had a diameter three times that of the sun, and if they were of the same temperature as has been assumed for betelgeuse, which is about fifty times as far away as alpha centauri, the relative effects of the three stars upon the sun would be, approximately, betelgeuse , alpha centauri , sirius . but alpha centauri is triple and sirius double, and both are much hotter than betelgeuse. hence alpha centauri and even sirius may be far more effective than betelgeuse. the two main components of alpha centauri are separated by an average distance of about , , , miles, or somewhat less than that of neptune from the sun. a third and far fainter star, one of the faintest yet measured, revolves around them at a great distance. in mass and brightness the two main components are about like the sun, and we will assume that the same is true of their radius. then, according to the assumptions made above, their effect in disturbing one another electrically would be about , times the total effect of jupiter upon the sun, or times the effect that we have assumed to be necessary to produce a glacial period. we have already seen in table that, according to our assumptions, a single star like the sun would have to approach within billion miles of the solar system, or within per cent of a light year, in order to cause glaciation. by a similar process of reasoning it appears that if the mutual electrical excitation of the two main parts of alpha centauri, regardless of the third part, is proportional to the apparent excitation of the sun by jupiter, alpha centauri would be times as effective as the sun. in other words, if it came within , , , , miles of the sun, or . light years, it would so change the electrical conditions as to produce a glacial epoch. in that case alpha centauri is now so near that it introduces a disturbing effect equal to about one-sixth of the effect needed to cause glaciation on the earth. sirius and perhaps others of the nearer and brighter or larger stars may also create appreciable disturbances in the electrical condition of the sun's atmosphere, and may have done so to a much greater degree in the past, or be destined to do so in the future. thus an electrical hypothesis of solar disturbances seems to indicate that the position of the sun in respect to other stars may be a factor of great importance in determining the earth's climate. footnotes: [footnote : h. h. turner: on a long period in chinese earthquake records; mon. not. royal astron. soc., vol. , , pp. - ; vol. , , pp. - ; long period terms in the growth of trees; _idem_, pp. - .] [footnote : harlow shapley: note on a possible factor in geologic climates; jour. geol., vol. , no. , may, ; novæ and variable stars, pub. astron. soc. pac., no. , aug., .] [footnote : j. h. jeans: problems of cosmogony and stellar dynamics, cambridge, .] [footnote : this fact is so important and at the same time so surprising to the layman, that a quotation from the electron theory of matter by o. w. richardson, , pp. and is here added. "it is a very familiar fact that when material bodies are heated they emit electromagnetic radiations, in the form of thermal, luminous, and actinic rays, in appreciable quantities. such an effect is a natural consequence of the electron and kinetic theories of matter. on the kinetic theory, temperature is a measure of the violence of the motion of the ultimate particles; and we have seen that on the electron theory, electromagnetic radiation is a consequence of their acceleration. the calculation of this emission from the standpoint of the electron theory alone is a very complex problem which takes us deeply into the structure of matter and which has probably not yet been satisfactorily resolved. fortunately, we can find out a great deal about these phenomena by the application of general principles like the conservation of energy and the second law of thermodynamics without considering special assumptions about the ultimate constitution of matter. it is to be borne in mind that the emission under consideration occurs at all temperatures although it is more marked the higher the temperature.... the energy per unit volume, _in vacuo_, of the radiation in equilibrium in an enclosure at the absolute temperature, t, is equal to a universal constant, a, multiplied by the fourth power of the absolute temperature. since the intensity of the radiation is equal to the energy per unit volume multiplied by the velocity of light, it follows that the former must also be proportional to the fourth power of the absolute temperature. moreover, if e is the total emission from unit area of a perfectly black body, we see from p. that e=a´t^{ }, where a´ is a new universal constant. this result is usually known as stefan's law. it was suggested by stefan in the inaccurate form that the total radiant energy of emission from bodies varies as the fourth power of the absolute temperature, as a generalization from the results of experiments. the credit for showing that it is a consequence of the existence of radiation pressure combined with the principles of thermodynamics is due to bartoli and boltzmann."] [footnote : quoted by moulton in his introduction to astronomy.] [footnote : introduction to astronomy.] [footnote : the term billions, here and elsewhere, is used in the american sense, ^{ }.] [footnote : the assumed number of stars here is ten times as great as in the other parts of this line.] chapter xv the sun's journey through space having gained some idea of the nature of the electrical hypothesis of solar disturbances and of the possible effect of other bodies upon the sun's atmosphere, let us now compare the astronomical data with those of geology. let us take up five chief points for which the geologist demands an explanation, and which any hypothesis must meet if it is to be permanently accepted. these are ( ) the irregular intervals at which glacial periods occur; ( ) the division of glacial periods into epochs separated sometimes by hundreds of thousands of years; ( ) the length of glacial periods and epochs; ( ) the occurrence of glacial stages and historic pulsations in the form of small climatic waves superposed upon the larger waves of glacial epochs; ( ) the occurrence of climatic conditions much milder than those of today, not only in the middle portion of the great geological eras, but even in some of the recent inter-glacial epochs. . the irregular duration of the interval from one glacial epoch to another corresponds with the irregular distribution of the stars. if glaciation is indirectly due to stellar influences, the epochs might fall close together, or might be far apart. if the average interval were ten million years, one interval might be thirty million or more and the next only one or two hundred thousand. according to schuchert, the known periods of glacial or semi-glacial climate have been approximately as follows: list of glacial periods . archeozoic. ( / of geological time or perhaps much more) no known glacial periods. . proterozoic. ( / of geological time) a. oldest known glacial period near base of proterozoic in canada. evidence widely distributed. b. indian glacial period; time unknown. c. african glacial period; time unknown. d. glaciation near end of proterozoic in australia, norway, and china. . paleozoic. ( / of geological time) a. late ordovician(?). local in arctic norway. b. silurian. local in alaska. c. early devonian. local in south africa. d. early permian. world-wide and very severe. . mesozoic and cenozoic. ( / of geological time) a-b. none definitely determined during mesozoic, although there appears to have been periods of cooling (a) in the late triassic, and (b) in the late cretacic, with at least local glaciation in early eocene. c. severe glacial period during pleistocene. this table suggests an interesting inquiry. during the last few decades there has been great interest in ancient glaciation and geologists have carefully examined rocks of all ages for signs of glacial deposits. in spite of the large parts of the earth which are covered with deposits belonging to the mesozoic and cenozoic, which form the last quarter of geological time, the only signs of actual glaciation are those of the great pleistocene period and a few local occurrences at the end of the mesozoic or beginning of the cenozoic. late in the triassic and early in the jurassic, the climate appears to have been rigorous, although no tillites have been found to demonstrate glaciation. in the preceding quarter, that is, the paleozoic, the permian glaciation was more severe than that of the pleistocene, and the devonian than that of the eocene, while the ordovician evidences of low temperature are stronger than those at the end of the triassic. in view of the fact that rocks of paleozoic age cover much smaller areas than do those of later age, the three paleozoic glaciations seem to indicate a relative frequency of glaciation. going back to the proterozoic, it is astonishing to find that evidence of two highly developed glacial periods, and possibly four, has been discovered. since the indian and the african glaciations of proterozoic times are as yet undated, we cannot be sure that they are not of the same date as the others. nevertheless, even two is a surprising number, for not only are most proterozoic rocks so metamorphosed that possible evidences of glacial origin are destroyed, but rocks of that age occupy far smaller areas than either those of paleozoic or, still more, mesozoic and cenozoic age. thus the record of the last three-quarters of geological time suggests that if rocks of all ages were as abundant and as easily studied as those of the later periods, the frequency of glacial periods would be found to increase as one goes backward toward the beginnings of the earth's history. this is interesting, for jeans holds that the chances that the stars would approach one another were probably greater in the past than at present. this conclusion is based on the assumption that our universe is like the spiral nebulæ in which the orbits of the various members are nearly circular during the younger stages. jeans considers it certain that in such cases the orbits will gradually become larger and more elliptical because of the attraction of one body for another. thus as time goes on the stars will be more widely distributed and the chances of approach will diminish. if this is correct, the agreement between astronomical theory and geological conclusions suggests that the two are at least not in opposition. the first quarter of geological time as well as the last three must be considered in this connection. during the archeozoic, no evidence of glaciation has yet been discovered. this suggests that the geological facts disprove the astronomical theory. but our knowledge of early geological times is extremely limited, so limited that lack of evidence of glaciation in the archeozoic may have no significance. archeozoic rocks have been studied minutely over a very small percentage of the earth's land surface. moreover, they are highly metamorphosed so that, even if glacial tills existed, it would be hard to recognize them. third, according to both the nebular and the planetesimal hypotheses, it seems possible that during the earliest stages of geological history the earth's interior was somewhat warmer than now, and the surface may have been warmed more than at present by conduction, by lava flows, and by the fall of meteorites. if the earth during the archeozoic period emitted enough heat to raise its surface temperature a few degrees, the heat would not prevent the development of low forms of life but might effectively prevent all glaciation. this does not mean that it would prevent changes of climate, but merely changes so extreme that their record would be preserved by means of ice. it will be most interesting to see whether future investigations in geology and astronomy indicate either a semi-uniform distribution of glacial periods throughout the past, or a more or less regular decrease in frequency from early times down to the present. . the pleistocene glacial period was divided into at least four epochs, while in the permian at least one inter-glacial epoch seems certain, and in some places the alternation between glacial and non-glacial beds suggests no less than nine. in the other glaciations the evidence is not yet clear. the question of periodicity is so important that it overthrows most glacial hypotheses. indeed, had their authors known the facts as established in recent years, most of the hypotheses would never have been advanced. the carbon dioxide hypothesis is the only one which was framed with geologically rapid climatic alternations in mind. it certainly explains the facts of periodicity better than does any of its predecessors, but even so it does not account for the intimate way in which variations of all degrees from those of the weather up to glacial epochs seem to grade into one another. according to our stellar hypothesis, occasional groups of glacial epochs would be expected to occur close together and to form long glacial periods. this is because many of the stars belong to groups or clusters in which the stars move in parallel paths. a good example is the cluster in the hyades, where boss has studied thirty-nine stars with special care.[ ] the stars are grouped about a center about light years from the sun. the stars themselves are scattered over an area about thirty light years in diameter. they average about the same distance apart as do those near the sun, but toward the center of the group they are somewhat closer together. the whole thirty-nine sweep forward in essentially parallel paths. boss estimates that , years ago the cluster was only half as far from the sun as at present, but probably that was as near as it has been during recent geological times. all of the thirty-nine stars of this cluster, as moulton[ ] puts it, "are much greater in light-giving power than the sun. the luminosities of even the five smallest are from five to ten times that of the sun, while the largest are one hundred times greater in light-giving power than our own luminary. their masses are probably much greater than that of the sun." if the sun were to pass through such a cluster, first one star and then another might come so near as to cause a profound disturbance in the sun's atmosphere. . another important point upon which a glacial hypothesis may come to grief is the length of the periods or rather of the epochs which compose the periods. during the last or pleistocene glacial period the evidence in america and europe indicates that the inter-glacial epochs varied in length and that the later ones were shorter than the earlier. chamberlin and salisbury, from a comparison of various authorities, estimate that the intervals from one glacial epoch to another form a declining series, which may be roughly expressed as follows: - - - - , where unity is the interval from the climax of the late wisconsin, or last glacial epoch, to the present. most authorities estimate the culmination of the late wisconsin glaciation as twenty or thirty thousand years ago. penck estimates the length of the last inter-glacial period as , years and the preceding one as , .[ ] r. t. chamberlin, as already stated, finds that the consensus of opinion is that inter-glacial epochs have averaged five times as long as glacial epochs. the actual duration of the various glaciations probably did not vary in so great a ratio as did the intervals from one glaciation to another. the main point, however, is the irregularity of the various periods. the relation of the stellar electrical hypothesis to the length of glacial epochs may be estimated from column c, in table . there we see that the distances at which a star might possibly disturb the sun enough to cause glaciation range all the way from billion miles in the case of a small star like the sun, to billion in the case of betelgeuse, while for double stars the figure may rise a hundred times higher. from this we can calculate how long it would take a star to pass from a point where its influence would first amount to a quarter of the assumed maximum to a similar point on the other side of the sun. in making these calculations we will assume that the relative rate at which the star and the sun approach each other is about twenty-two miles per second, or million miles per year, which is the average rate of motion of all the known stars. according to the distances in table this gives a range from about years up to about , , which might rise to a million in the case of double stars. of course the time might be relatively short if the sun and a rapidly moving star were approaching one another almost directly, or extremely long if the sun and the star were moving in almost the same direction and at somewhat similar rates,--a condition more common than the other. here, as in so many other cases, the essential point is that the figures which we thus obtain seem to be of the right order of magnitude. . post-glacial climatic stages are so well known that in europe they have definite names. their sequence has already been discussed in chapter xii. fossils found in the peat bogs of denmark and scandinavia, for example, prove that since the final disappearance of the continental ice cap at the close of the wisconsin there has been at least one period when the climate of europe was distinctly milder than now. directly overlying the sheets of glacial drift laid down by the ice there is a flora corresponding to that of the present tundras. next come remains of a forest vegetation dominated by birches and poplars, showing that the climate was growing a little warmer. third, there follow evidences of a still more favorable climate in the form of a forest dominated by pines; fourth, one where oak predominates; and fifth, a flora similar to that of the black forest of germany, indicating that in scandinavia the temperature was then decidedly higher than today. this fifth flora has retreated southward once more, having been driven back to its present latitude by a slight recurrence of a cool stormy climate.[ ] in central asia evidence of post-glacial stages is found not only in five distinct moraines but in a corresponding series of elevated strands surrounding salt lakes and of river terraces in non-glaciated arid regions.[ ] in historic as well as prehistoric times, as we have already seen, there have been climatic fluctuations. for instance, the twelfth or thirteenth century b. c. appears to have been almost as mild as now, as does the seventh century b. c. on the other hand about b. c., at the time of christ, and in the fourteenth century there were times of relative severity. thus it appears that both on a large and on a small scale pulsations of climate are the rule. any hypothesis of climatic changes must satisfy the periods of these pulsations. these conditions furnish a problem which makes difficulty for almost all hypotheses of climatic change. according to the present hypothesis, earth movements such as are discussed in chapter xii may coöperate with two astronomical factors. one is the constant change in the positions of the stars, a change which we have already called kaleidoscopic, and the other is the fact that a large proportion of the stars are double or multiple. when one star in a group approaches the sun closely enough to cause a great solar disturbance, numerous others may approach or recede and have a minor effect. thus, whenever the sun is near groups of stars we should expect that the earth would show many minor climatic pulsations and stages which might or might not be connected with glaciation. the historic pulsations shown in the curve of tree growth in california, fig. , are the sort of changes that would be expected if movements of the stars have an effect on the solar atmosphere. not only are fully a third of all the visible stars double, as we have already seen, but at least a tenth of these are known to be triple or multiple. in many of the double stars the two bodies are close together and revolve so rapidly that whatever periodicity they might create in the sun's atmosphere would be very short. in the triplets, however, the third star is ordinarily at least ten times as far from the other two as they are from each other, and its period of rotation sometimes runs into hundreds or thousands of years. an actual multiple star in the constellation polaris will serve as an example. the main star is believed by jeans to consist of two parts which are almost in contact and whirl around each other with extraordinary speed in four days. if this is true they must keep each other's atmospheres in a state of intense commotion. much farther away a third star revolves around this pair in twelve years. at a much greater distance a fourth star revolves around the common center of gravity of itself and the other three in a period which may be , years. still more complicated cases probably exist. suppose such a system were to traverse a path where it would exert a perceptible influence on the sun for thirty or forty thousand years. the varying movements of its members would produce an intricate series of cycles which might show all sorts of major and minor variations in length and intensity. thus the varied and irregular stages of glaciation and the pulsations of historic times might be accounted for on the hypothesis of the proximity of the sun to a multiple star, as well as on that of the less pronounced approach and recession of a number of stars. in addition to all this, an almost infinitely complex series of climatic changes of long and short duration might arise if the sun passed through a nebula. . we have seen in chapter viii that the contrast between the somewhat severe climate of the present and the generally mild climate of the past is one of the great geological problems. the glacial period is not a thing of the distant past. geologists generally recognize that it is still with us. greenland and antarctica are both shrouded in ice sheets in latitudes where fossil floras prove that at other periods the climate was as mild as in england or even new zealand. the present glaciated regions, be it noted, are on the polar borders of the world's two most stormy oceanic areas, just where ice would be expected to last longest according to the solar cyclonic hypothesis. in contrast with the semi-glacial conditions of the present, the last inter-glacial epoch was so mild that not only men but elephants and hippopotamuses flourished in central europe, while at earlier times in the middle of long eras, such as the paleozoic and mesozoic, corals, cycads, and tree ferns flourished within the arctic circle. if the electro-stellar hypothesis of solar disturbances proves well founded, it may explain these peculiarities. periods of mild climate would represent a return of the sun and the earth to their normal conditions of quiet. at such times the atmosphere of the sun is assumed to be little disturbed by sunspots, faculæ, prominences, and other allied evidences of movements; and the rice-grain structure is perhaps the most prominent of the solar markings. the earth at such times is supposed to be correspondingly free from cyclonic storms. its winds are then largely of the purely planetary type, such as trade winds and westerlies. its rainfall also is largely planetary rather than cyclonic. it falls in places such as the heat equator where the air rises under the influence of heat, or on the windward slopes of mountains, or in regions where warm winds blow from the ocean over cold lands. according to the electro-stellar hypothesis, the conditions which prevailed during hundreds of millions of years of mild climate mean merely that the solar system was then in parts of the heavens where stars--especially double stars--were rare or small, and electrical disturbances correspondingly weak. today, on the other hand, the sun is fairly near a number of stars, many of which are large doubles. hence it is supposed to be disturbed, although not so much as at the height of the last glacial epoch. after the preceding parts of this book had been written, the assistance of dr. schlesinger made it possible to test the electro-stellar hypothesis by comparing actual astronomical dates with the dates of climatic or solar phenomena. in order to make this possible, dr. schlesinger and his assistants have prepared table , giving the position, magnitude, and motions of the thirty-eight nearest stars, and especially the date at which each was nearest the sun. in column where the dates are given, a minus sign indicates the past and a plus sign the future. dr. shapley has kindly added column , giving the absolute magnitudes of the stars, that of the sun being . , and column , showing their luminosity or absolute radiation, that of the sun being unity. finally, column shows the effective radiation received by the sun from each star when the star is at a minimum distance. unity in this case is the effect of a star like the sun at a distance of one light year. it is well known that radiation of all kinds, including light, heat, and electrical emissions, varies in direct proportion to the exposed surface, that is, as the square of the radius of a sphere, and inversely as the square of the distance. from black bodies, as we have seen, the total radiation varies as the fourth power of the absolute temperature. it is not certain that either light or electrical emissions from incandescent bodies vary in quite this same proportion, nor is it yet certain whether luminous and electrical emissions vary exactly together. nevertheless they are closely related. since the light coming from each star is accurately measured, while no information is available as to electrical emissions, we have followed dr. shapley's suggestion and used the luminosity of the stars as the best available measure of total radiation. this is presumably an approximate measure of electrical activity, provided some allowance be made for disturbances by outside bodies such as companion stars. hence the inclusion of column . table thirty-eight stars having largest known parallaxes star code groombr. ++[greek: ê] cassiop. ++[greek: k] tucanæ [greek: t] ceti [greek: d]_ eridani ++[greek: e] eridani ++ ( )^ eridani cordoba z. weisse ++[greek: a] can. maj. (sirius) ++[greek: a] can. min. (procyon) ++fedorenko - groombr. weisse lalande lalande lalande ++[greek: a] centauri ++[greek: x] bootes ++lalande weisse lacaille ++[greek: b] argel - . - barnard's star ++ p ophiuchi ++[greek: s] [greek: s] draconis ++[greek: a] aquilæ (altair) ++ cygni lacaille [greek: e] indi ++krüger lacaille lalande c. g. a. (++ double star.) ( ) ( ) ( ) ( ) ( ) ( ) right declination visual spectrum proper radial star ascension [greek: d] mag. m motion velocity code [greek: a] km. per sec. ------------------------------------------------------------------ ^h ^m. + ° ' . ma ". + . + . f . + . + . f . ..... . - . f . + . - . k . - ------------------------------------------------------------------ . - . g . + . - . k . + . - . g . - . - . k . + . - . k . ..... ------------------------------------------------------------------ . - - . a . - . + . f . - . + . ma . + . + . k p . - . + . ... . ..... ------------------------------------------------------------------ . + . mb . - . + . k . + . - . ... . ..... . + . k . ..... . - . g . + ------------------------------------------------------------------ . + . k p . + . - . kp . + . + . ... . ..... . - . k . ..... . - . k . - ------------------------------------------------------------------ . + . k . ..... . + . mb . - . + . k . ..... . + . k . ..... . + . g . + ------------------------------------------------------------------ . + . a . - . + . k . - . - . g . + . - . k . - . + . ... . ..... ------------------------------------------------------------------ . - . k . + . + . ma . ..... . - . g . + ( ) ( ) ( ) ( ) ( ) present minimum magnitude luminosity effective parallax distance at min. dist. | radiation [greek: p] light yrs. | | at | | | | minimum | ( ) | ( ) | ( ) | distance star | maximum | time of | absolute | from sun code | parallax | minimum | magnitude | | | | | distance | | | | ----------------------------------------------------------------------- ". ". . - . . . . . . . - . . . . . .... .... ...... .... . . ........ . . . - . . . . . . . + . . . . ----------------------------------------------------------------------- . . . - . . . . . . . - . . . . . . . + . . . . . . . - . . . . . .... .... ...... .... . . ........ ----------------------------------------------------------------------- . . . + - . . . . . . . + . . . . . . . - . . . . . . . + . . . . . .... .... ...... .... . . ........ ----------------------------------------------------------------------- . . . + . . . . . . . - . . . . . .... .... ...... .... . . ........ . .... .... ...... .... . . ........ . . . - - . . . . ----------------------------------------------------------------------- . . . - . . . . . . . - . . . . . .... .... ...... .... . . ........ . .... .... ...... .... . . ........ . . . + . . . . ----------------------------------------------------------------------- . .... .... ...... .... . . ........ . . . + . . . . . .... .... ...... .... . . ........ . .... .... ...... .... . . ........ . . . - . . . . ----------------------------------------------------------------------- . . . + - . . . . . . . + . . . . . . . - . . . . . . . + . . . . . .... .... ....... .... . . ........ ----------------------------------------------------------------------- . . . - . . . . . .... .... ....... .... . . ........ . . . - . . . . ----------------------------------------------------------------------- on the basis of column and of the movements and distances of the stars as given in the other columns fig. has been prepared. this gives an estimate of the approximate electrical energy received by the sun from the nearest stars for , years before and after the present. it is based on the twenty-six stars for which complete data are available in table . the inclusion of the other twelve would not alter the form of the curve, for even the largest of them would not change any part by more than about half of per cent, if as much. nor would the curve be visibly altered by the omission of all except four of the twenty-six stars actually used. the four that are important, and their relative luminosity when nearest the sun, are sirius , , altair , , alpha centauri , , and procyon , . the figure for the next star is only , while for this star combined with the other twenty-one that are unimportant it is only , . figure is not carried more than , years into the past or into the future because the stars near the sun at more remote times are not included among the thirty-eight having the largest known parallaxes. that is, they have either moved away or are not yet near enough to be included. indeed, as dr. schlesinger strongly emphasizes, there may be swiftly moving, bright or gigantic stars which are now quite far away, but whose inclusion would alter fig. even within the limits of the , years there shown. it is almost certain, however, that the most that these would do would be to raise, but not obliterate, the minima on either side of the main maximum. [illustration: _fig. . climatic changes of , years as inferred from the stars._] in preparing fig. it has been necessary to make allowance for double stars. passing by the twenty-two unimportant stars, it appears that the companion of sirius is eight or ten magnitudes smaller than that star, while the companions of procyon and altair are five or more magnitudes smaller than their bright comrades. this means that the luminosity of the faint components is at most only per cent of that of their bright companions and in the case of sirius not a hundredth of per cent. hence their inclusion would have no visible effect on fig. . in alpha centauri, on the other hand, the two components are of almost the same magnitude. for this reason the effective radiation of that star as given in column is doubled in fig. , while for another reason it is raised still more. the other reason is that if our inferences as to the electrical effect of the sun on the earth and of the planets on the sun are correct, double stars, as we have seen, must be much more effective electrically than single stars. by the same reasoning two bright stars close together must excite one another much more than a bright star and a very faint one, even if the distances in both cases are the same. so, too, other things being equal, a triple star must be more excited electrically than a double star. hence in preparing fig. all double stars receive double weight and each part of alpha centauri receives an additional per cent because both parts are bright and because they have a third companion to help in exciting them. according to the electro-stellar hypothesis, alpha centauri is more important climatically than any other star in the heavens not only because it is triple and bright, but because it is the nearest of all stars, and moves fairly rapidly. sirius and procyon move slowly in respect to the sun, only about eleven and eight kilometers per second respectively, and their distances at minimum are fairly large, that is, and . light years. hence their effect on the sun changes slowly. altair moves faster, about twenty-six kilometers per second, and its minimum distance is . light years, so that its effect changes fairly rapidly. alpha centauri moves about twenty-four kilometers per second, and its minimum distance is only . light years. hence its effect changes very rapidly, the change in its apparent luminosity as seen from the sun amounting at maximum to about per cent in , years against per cent for altair, for sirius, and for procyon. the vast majority of the stars change so much more slowly than even procyon that their effect is almost uniform. all the stars at a distance of more than perhaps twenty or thirty light years may be regarded as sending to the sun a practically unchanging amount of radiation. it is the bright stars within this limit which are important, and their importance increases with their proximity, their speed of motion, and the brightness and number of their companions. hence alpha centauri causes the main maximum in fig. , while sirius, altair, and procyon combine to cause a general rise of the curve from the past to the future. let us now interpret fig. geologically. the low position of the curve fifty to seventy thousand years ago suggests a mild inter-glacial climate distinctly less severe than that of the present. geologists say that such was the case. the curve suggests a glacial epoch culminating about , years ago. the best authorities put the climax of the last glacial epoch between twenty-five and thirty thousand years ago. the curve shows an amelioration of climate since that time, although it suggests that there is still considerable severity. the retreat of the ice from north america and europe, and its persistence in greenland and antarctica agree with this. and the curve indicates that the change of climate is still persisting, a conclusion in harmony with the evidence as to historic changes. if alpha centauri is really so important, the effect of its variations, provided it has any, ought perhaps to be evident in the sun. the activity of the star's atmosphere presumably varies, for the orbits of the two components have an eccentricity of . . hence during their period of revolution, . years, the distance between them ranges from , , , to , , , miles. they were at a minimum distance in , , , , , , , and will be again in . in fig. , showing sunspot variations, it is noticeable that the years and come just at the ends of periods of unusual solar activity, as indicated by the heavy horizontal line. a similar period of great activity seems to have begun about . if its duration equals the average of its two predecessors, it will end about . back in the fourteenth century a period of excessive solar activity, which has already been described, culminated from to , or just before the two parts of alpha centauri were at a minimum distance. thus in three and perhaps four cases the sun has been unusually active during a time when the two parts of the star were most rapidly approaching each other and when their atmospheres were presumably most disturbed and their electrical emanations strongest. [illustration: _fig. . sunspot curve showing cycles, to ._ _note._ the asterisks indicate two absolute minima of sunspots in and , and the middle years ( and ) of two periods when the sunspot maxima never fell below . if alpha centauri has an effect on the sun's atmosphere, the end of another such period would be expected not far from .] the fact that alpha centauri, the star which would be expected most strongly to influence the sun, and hence the earth, was nearest the sun at the climax of the last glacial epoch, and that today the solar atmosphere is most active when the star is presumably most disturbed may be of no significance. it is given for what it is worth. its importance lies not in the fact that it proves anything but that no contradiction is found when we test the electro-stellar hypothesis by facts which were not thought of when the hypothesis was framed. a vast amount of astronomical work is still needed before the matter can be brought to any definite conclusion. in case the hypothesis stands firm, it may be possible to use the stars as a help in determining the exact chronology of the later part of geological times. if the hypothesis is disproved, it will merely leave the question of solar variations where it is today. it will not influence the main conclusions of this book as to the causes and nature of climatic changes. its value lies in the fact that it calls attention to new lines of research. footnotes: [footnote : lewis boss: convergent of a moving cluster in taurus; astronom. jour., vol. , no. , , pp. - .] [footnote : f. r. moulton: in introduction to astronomy, .] [footnote : a. penck: die alpen im eiszeitalter, leipzig, .] [footnote : r. d. salisbury: physical geography of the pleistocene, in outlines of geologic history, by willis and salisbury, , pp. - .] [footnote : davis, pumpelly, and huntington: explorations in turkestan, carnegie inst. of wash., no. , . in north america the stages have been the subject of intensive studies on the part of taylor, leverett, goldthwait, and many others.] chapter xvi the earth's crust and the sun although the problems of this book may lead far afield, they ultimately bring us back to the earth and to the present. several times in the preceding pages there has been mention of the fact that periods of extreme climatic fluctuations are closely associated with great movements of the earth's crust whereby mountains are uplifted and continents upheaved. in attempting to explain this association the general tendency has been to look largely at the past instead of the present. hence it has been almost impossible to choose among three possibilities, all beset with difficulties. first, the movements of the crust may have caused the climatic fluctuations; second, climatic changes may cause crustal movements; and third, variations in solar activity or in some other outside agency may give rise to both types of terrestrial phenomena. the idea that movements of the earth's crust are the main cause of geological changes of climate is becoming increasingly untenable as the complexity and rapidity of climatic changes become more clear, especially during post-glacial times. it implies that the earth's surface moves up and down with a speed and facility which appear to be out of the question. if volcanic activity be invoked the problem becomes no clearer. even if volcanic dust should fill the air frequently and completely, neither its presence nor absence would produce such peculiar features as the localization of glaciers, the distribution of loess, and the mild climate of most parts of geological time. nevertheless, because of the great difficulties presented by the other two possibilities many geologists still hold that directly or indirectly the greater climatic changes have been mainly due to movements of the earth's crust and to the reaction of the crustal movements on the atmosphere. the possibility that climatic changes are in themselves a cause of movements of the earth's crust seems so improbable that no one appears to have investigated it with any seriousness. nevertheless, it is worth while to raise the question whether climatic extremes may coöperate with other agencies in setting the time when the earth's crust shall be deformed. as to the third possibility, it is perfectly logical to ascribe both climatic changes and crustal deformation to some outside agency, solar or otherwise, but hitherto there has been so little evidence on this point that such an ascription has merely begged the question. if heavenly bodies should approach the earth closely enough so that their gravitational stresses caused crustal deformation, all life would presumably be destroyed. as to the sun, there has hitherto been no conclusive evidence that it is related to crustal movements, although various writers have made suggestions along this line. in this chapter we shall carry these suggestions further and shall see that they are at least worthy of study. as a preliminary to this study it may be well to note that the coincidence between movements of the earth's crust and climatic changes is not so absolute as is sometimes supposed. for example, the profound crustal changes at the end of the mesozoic were not accompanied by widespread glaciation so far as is yet known, although the temperature appears to have been lowered. nor was the violent volcanic and diastrophic activity in the miocene associated with extreme climates. indeed, there appears to have been little contrast from zone to zone, for figs, bread fruit trees, tree ferns, and other plants of low latitudes grew in greenland. nevertheless, both at the end of the mesozoic and in the miocene the climate may possibly have been severe for a time, although the record is lost. on the other hand, kirk's recent discovery of glacial till in alaska between beds carrying an undoubted middle silurian fauna indicates glaciation at a time when there was little movement of the crust so far as yet appears.[ ] thus we conclude that while climatic changes and crustal movements usually occur together, they may occur separately. according to the solar-cyclonic hypothesis such a condition is to be expected. if the sun were especially active when the terrestrial conditions prohibited glaciation, changes of climate would still occur, but they would be milder than under other circumstances, and would leave little record in the rocks. or there might be glaciation in high latitudes, such as that of southern alaska in the middle silurian, and none elsewhere. on the other hand, when the sun was so inactive that no great storminess occurred, the upheaval of continents and the building of mountains might go on without the formation of ice sheets, as apparently happened at the end of the mesozoic. the lack of absolute coincidence between glaciation and periods of widespread emergence of the lands is evident even today, for there is no reason to suppose that the lands are notably lower or less extensive now than they were during the pleistocene glaciation. in fact, there is much evidence that many areas have risen since that time. yet glaciation is now far less extensive than in the pleistocene. any attempt to explain this difference on the basis of terrestrial changes is extremely difficult, for the shape and altitude of continents and mountains have not changed much in twenty or thirty thousand years. yet the present moderately mild epoch, like the puzzling inter-glacial epochs of earlier times, is easily explicable on the assumption that the sun's atmosphere may sometimes vary in harmony with crustal activity, but does not necessarily do so at all times. turning now to the main problem of how climatic changes may be connected with movements of the earth's crust, let us follow our usual method and examine what is happening today. let us first inquire whether earthquakes, which are one of the chief evidences that crustal movements are actually taking place in our own times, show any connection with sunspots. in order to test this, we have compared _milne's catalogue of destructive earthquakes_ from to , with wolf's sunspot numbers for the same period month by month. the earthquake catalogue, as its compiler describes it, "is an attempt to give a list of earthquakes which have announced changes of geological importance in the earth's crust; movements which have probably resulted in the creation or the extension of a line of fault, the vibrations accompanying which could, with proper instruments, have been recorded over a continent or the whole surface of our world. small earthquakes have been excluded, while the number of large earthquakes both for ancient and modern times has been extended. as an illustration of exclusion, i may mention that between and , which are years taken at random, i find in mallet's catalogue entries. only thirty-seven of these, which were accompanied by structural damage, have been retained. other catalogues such as those of perry and fuchs have been treated similarly."[ ] if the earthquakes in such a carefully selected list bear a distinct relation to sunspots, it is at least possible and perhaps probable that a similar relation may exist between solar activity and geological changes in the earth's crust. the result of the comparison of earthquakes and sunspots is shown in table . the first column gives the sunspot numbers; the second, the number of months that had the respective spot numbers during the century from to . column c shows the total number of earthquakes during the months having any particular degree of spottedness; while d, which is the significant column, gives the average number of destructive earthquakes per month under each of the six conditions of solar spottedness. the regularity of column d is so great as to make it almost certain that we are here dealing with a real relationship. column f, which shows the average number of earthquakes in the month succeeding any given condition of the sun, is still more regular except for the last entry. table destructive earthquakes from to compared with sunspots a: _sunspot numbers_ b: _number of months per wolf's table_ c: _number of earthquakes_ d: _average number of earthquakes per month_ e: _number of earthquakes in succeeding month_ f: _average number of earthquakes in succeeding month_ a b c d e f - . . - . . - . . - . . - . . over . . the chance that six numbers taken at random will arrange themselves in any given order is one in . in other words, there is one chance in that the regularity of column d is accidental. but column f is as regular as column d except for the last entry. if columns d and e were independent there would be one chance in about , that the six numbers in both columns would fall in the same order, and one chance in , that five numbers in each would fall in the same order. but the two columns are somewhat related, for although the after-shocks of a great earthquake are never included in milne's table, a world-shaking earthquake in one region during a given month probably creates conditions that favor similar earthquakes elsewhere during the next month. hence the probability that we are dealing with a purely accidental arrangement in table is less than one in , and greater than one in , . it may be one in , or , . in any event it is so slight that there is high probability that directly or indirectly sunspots and earthquakes are somehow connected. in ascertaining the relation between sunspots and earthquakes it would be well if we could employ the strict method of correlation coefficients. this, however, is impossible for the entire century, for the record is by no means homogeneous. the earlier decades are represented by only about one-fourth as many earthquakes as the later ones, a condition which is presumably due to lack of information. this makes no difference with the method employed in table , since years with many and few sunspots are distributed almost equally throughout the entire nineteenth century, but it renders the method of correlation coefficients inapplicable. during the period from onward the record is much more nearly homogeneous, though not completely so. even in these later decades, however, allowance must be made for the fact that there are more earthquakes in winter than in summer, the average number per month for the fifty years being as follows: jan. . may . sept. . feb. . june . oct. . mar. . july . nov. . apr. . aug. . dec. . the correlation coefficient between the departures from these monthly averages and the corresponding departures from the monthly averages of the sunspots for the same period, - , are as follows: sunspots and earthquakes of same month: + . , or . times the probable error. sunspots of a given month and earthquakes of that month and the next: + . , or . times the probable error. sunspots of three consecutive months and earthquakes of three consecutive months allowing a lag of one month, i.e., sunspots of january, february, and march compared with earthquakes of february, march, and april; sunspots of february, march, and april with earthquakes of march, april, and may, etc.; + . , or . times the probable error. these coefficients are all small, but the number of individual cases, months, is so large that the probable error is greatly reduced, being only ± . or ± . . moreover, the nature of our data is such that even if there is a strong connection between solar changes and earth movements, we should not expect a large correlation coefficient. in the first place, as already mentioned, the earthquake data are not strictly homogeneous. second, an average of about two and one-half strong earthquakes per month is at best only a most imperfect indication of the actual movement of the earth's crust. third, the sunspots are only a partial and imperfect measure of the activity of the sun's atmosphere. fourth, the relation between solar activity and earthquakes is almost certainly indirect. in view of all these conditions, the regularity of table and the fact that the most important correlation coefficient rises to more than four times the probable error makes it almost certain that the solar and terrestrial phenomena are really connected. we are now confronted by the perplexing question of how this connection can take place. thus far only three possibilities present themselves, and each is open to objections. the chief agencies concerned in these three possibilities are heat, electricity, and atmospheric pressure. heat may be dismissed very briefly. we have seen that the earth's surface becomes relatively cool when the sun is active. theoretically even the slightest change in the temperature of the earth's surface must influence the thermal gradient far into the interior and hence cause a change of volume which might cause movements of the crust. practically the heat of the surface ceases to be of appreciable importance at a depth of perhaps twenty feet, and even at that depth it does not act quickly enough to cause the relatively prompt response which seems to be characteristic of earthquakes in respect to the sun. the second possibility is based on the relationship between solar and terrestrial electricity. when the sun is active the earth's atmospheric electrical potential is subject to slight variations. it is well known that when two opposing points of an ionized solution are oppositely charged electrically, a current passes through the liquid and sets up electrolysis whereby there is a segregation of materials, and a consequent change in the volume of the parts near the respective electrical poles. the same process takes place, although less freely, in a hot mass such as forms the interior of the earth. the question arises whether internal electrical currents may not pass between the two oppositely charged poles of the earth, or even between the great continental masses and the regions of heavier rock which underlie the oceans. could this lead to electrolysis, hence to differentiation in volume, and thus to movements of the earth's crust? could the results vary in harmony with the sun? bowie[ ] has shown that numerous measurements of the strength and direction of the earth's gravitative pull are explicable only on the assumption that the upheaval of a continent or a mountain range is due in part not merely to pressure, or even to flowage of the rocks beneath the crust, but also to an actual change in volume whereby the rocks beneath the continent attain relatively great volume and those under the oceans a small volume in proportion to their weight. the query arises whether this change of volume may be related to electrical currents at some depth below the earth's surface. the objections to this hypothesis are numerous. first, there is little evidence of electrolytic differentiation in the rocks. second, the outer part of the earth's crust is a very poor conductor so that it is doubtful whether even a high degree of electrification of the surface would have much effect on the interior. third, electrolysis due to any such mild causes as we have here postulated must be an extremely slow process, too slow, presumably, to have any appreciable result within a month or two. other objections join with these three in making it seem improbable that the sun's electrical activity has any direct effect upon movements of the earth's crust. the third, or meteorological hypothesis, which makes barometric pressure the main intermediary between solar activity and earthquakes, seems at first sight almost as improbable as the thermal and electrical hypotheses. nevertheless, it has a certain degree of observational support of a kind which is wholly lacking in the other two cases. among the extensive writings on the periodicity of earthquakes one main fact stands out with great distinctness: earthquakes vary in number according to the season. this fact has already been shown incidentally in the table of earthquake frequency by months. if allowance is made for the fact that february is a short month, there is a regular decrease in the frequency of severe earthquakes from december and january to june. since most of milne's earthquakes occurred in the northern hemisphere, this means that severe earthquakes occur in winter about per cent oftener than in summer. the most thorough investigation of this subject seems to have been that of davisson.[ ] his results have been worked over and amplified by knott,[ ] who has tested them by schuster's exact mathematical methods. his results are given in table .[ ] here the northern hemisphere is placed first; then come the east indies and the malay archipelago lying close to the equator; and finally the southern hemisphere. in the northern hemisphere practically all the maxima come in the winter, for the month of december appears in fifteen cases out of the twenty-five in column d, while january, february, or november appears in six others. it is also noticeable that in sixteen cases out of twenty-five the ratio of the actual to the expected amplitude in column g is four or more, so that a real relationship is indicated, while the ratio falls below three only in japan and zante. the equatorial data, unlike those of the northern hemisphere, are indefinite, for in the east indies no month shows a marked maximum and the expected amplitude exceeds the actual amplitude. even in the malay archipelago, which shows a maximum in may, the ratio of actual to expected amplitude is only . . turning to the southern hemisphere, the winter months of that hemisphere are as strongly marked by a maximum as are the winter months of the northern hemisphere. july or august appears in five out of six cases. here the ratio between the actual and expected amplitudes is not so great as in the northern hemisphere. nevertheless, it is practically four in chile, and exceeds five in peru and bolivia, and in the data for the entire southern hemisphere. table seasonal march of earthquakes after davisson and knott a: _region_ b: _limiting dates_ c: _number of shocks_ d: _maximum month_ e: _amplitude_ f: _expected amplitude_ g: _ratio of actual to expected amplitude_ a b c d e f g northern hemisphere - dec. . . . northern hemisphere - dec. . . . europe - dec. . . . europe - dec. . . . southeast europe - dec. . . . vesuvius district - dec. . . . italy: old tromometre - dec. . . . old tromometre - dec. . . . normal tromometre - dec. . . . balkan, etc. - dec. . . . hungary, etc. - dec. . . . italy - dec.(sept.) . . . grecian archip. - dec.-jan. . . . austria - jan. . . . switzerland, etc. - jan. . . . asia - feb. . . . north america - nov. . . . california - oct. . . . japan - dec. . . . japan - dec.-jan. . . . japan - feb. . . . japan - oct. . . . zante - aug. . . . italy, north - sept.(nov.) . . . of naples east indies - aug., oct., . ? . . or dec.? malay archip. - may . . . new zealand - aug.-sept. . . . chile - july . . . southern hemisphere - july . . . new zealand - march, may . . . chile - ? july, dec. . . . peru, bolivia - july . . . the whole relationship between earthquakes and the seasons in the northern and southern hemispheres is summed up in fig. taken from knott. the northern hemisphere shows a regular diminution in earthquake frequency from december until june, and an increase the rest of the year. in the southern hemisphere the course of events is the same so far as summer and winter are concerned, for august with its maximum comes in winter, while february with its minimum comes in summer. in the southern hemisphere the winter month of greatest seismic activity has over per cent more earthquakes than the summer month of least activity. in the northern hemisphere this difference is about per cent, but this smaller figure occurs partly because the northern data include certain interesting and significant regions like japan and china where the usual conditions are reversed.[ ] if equatorial regions were included in fig. , they would give an almost straight line. the connection between earthquakes and the seasons is so strong that almost no students of seismology question it, although they do not agree as to its cause. a meteorological hypothesis seems to be the only logical explanation.[ ] wherever sufficient data are available, earthquakes appear to be most numerous when climatic conditions cause the earth's surface to be most heavily loaded or to change its load most rapidly. the main factor in the loading is apparently atmospheric pressure. this acts in two ways. first, when the continents become cold in winter the pressure increases. on an average the air at sea level presses upon the earth's surface at the rate of . pounds per square inch, or over a ton per square foot, and only a little short of thirty million tons per square mile. an average difference of one inch between the atmospheric pressure of summer and winter over ten million square miles of the continent of asia, for example, means that the continent's load in winter is about ten million million tons heavier than in summer. second, the changes in atmospheric pressure due to the passage of storms are relatively sharp and sudden. hence they are probably more effective than the variations in the load from season to season. this is suggested by the rapidity with which the terrestrial response seems to follow the supposed solar cause of earthquakes. it is also suggested by the fact that violent storms are frequently followed by violent earthquakes. "earthquake weather," as dr. schlesinger suggests, is a common phrase in the typhoon region of japan, china, and the east indies. during tropical hurricanes a change of pressure amounting to half an inch in two hours is common. on september , , at false point lighthouse on the bay of bengal, the barometer fell about an inch in six hours, then nearly an inch and a half in not much over two hours, and finally rose fully two inches inside of two hours. a drop of two inches in barometric pressure means that a load of about two million tons is removed from each square mile of land; the corresponding rise of pressure means the addition of a similar load. such a storm, and to a less degree every other storm, strikes a blow upon the earth's surface, first by removing millions of tons of pressure and then by putting them on again.[ ] such storms, as we have seen, are much more frequent and severe when sunspots are numerous than at other times. moreover, as veeder[ ] long ago showed, one of the most noteworthy evidences of a connection between sunspots and the weather is a sudden increase of pressure in certain widely separated high pressure areas. in most parts of the world winter is not only the season of highest pressure and of most frequent changes of veeder's type, but also of severest storms. hence a meteorological hypothesis would lead to the expectation that earthquakes would occur more frequently in winter than in summer. on the chinese coast, however, and also on the oceanic side of japan, as well as in some more tropical regions, the chief storms come in summer in the form of typhoons. these are the places where earthquakes also are most abundant in summer. thus, wherever we turn, storms and the related barometric changes seem to be most frequent and severe at the very times when earthquakes are also most frequent. [illustration: _fig. . seasonal distribution of earthquakes. (after davisson and knott.)_ solid line ---- northern hemisphere. dashed line .... southern hemisphere.] other meteorological factors, such as rain, snow, winds, and currents, probably have some effect on earthquakes through their ability to load the earth's crust. the coming of vegetation may also help. these agencies, however, appear to be of small importance compared with the storms. in high latitudes and in regions of abundant storminess most of these factors generally combine with barometric pressure to produce frequent changes in the load of the earth's crust, especially in winter. in low latitudes, on the other hand, there are few severe storms, and relatively little contrast in pressure and vegetation from season to season; there is no snow; and the amount of ground water changes little. with this goes the twofold fact that there is no marked seasonal distribution of earthquakes, and that except in certain local volcanic areas, earthquakes appear to be rare. in proportion to the areas concerned, for example, there is little evidence of earthquakes in equatorial africa and south america. the question of the reality of the connection between meteorological conditions and crustal movements is so important that every possible test should be applied. at the suggestion of professor schlesinger we have looked up a very ingenious line of inquiry. during the last decades of the nineteenth century, a long series of extremely accurate observations of latitude disclosed a fact which had previously been suspected but not demonstrated, namely, that the earth wabbles a little about its axis. the axis itself always points in the same direction, and since the earth slides irregularly around it the latitude of all parts of the earth keeps changing. chandler has shown that the wabbling thus induced consists of two parts. the first is a movement in a circle with a radius of about fifteen feet which is described in approximately days. this so-called eulerian movement is a normal gyroscopic motion like the slow gyration of a spinning top. this depends on purely astronomical causes, and no terrestrial cause can stop it or eliminate it. the period appears to be constant, but there are certain puzzling irregularities. the usual amplitude of this movement, as schlesinger[ ] puts it, "is about ". , but twice in recent years it has jumped to ". . such a change could be accounted for by supposing that the earth had received a severe blow or a series of milder blows tending in the same direction." these blows, which were originally suggested by helmert are most interesting in view of our suggestion as to the blows struck by storms. the second movement of the pole has a period of a year, and is roughly an ellipse whose longest radius is fourteen feet and the shortest, four feet; or, to put it technically, there is an annual term with a maximum amplitude of about ". . this, however, varies irregularly. the result is that the pole seems to wander over the earth's surface in the spiral fashion illustrated in fig. . it was early suggested that this peculiar wandering of the pole in an annual period must be due to meteorological causes. jeffreys[ ] has investigated the matter exhaustively. he assumes certain reasonable values for the weight of air added or subtracted from different parts of the earth's surface according to the seasons. he also considers the effect of precipitation, vegetation, and polar ice, and of variations of temperature and atmospheric pressure in their relation to movements of the ocean. then he proceeds to compare all these with the actual wandering of the pole from to . while it is as yet too early to say that any special movement of the pole was due to the specific meteorological conditions of any particular year, jeffreys' work makes it clear that meteorological causes, especially atmospheric pressure, are sufficient to cause the observed irregular wanderings. slight wanderings may arise from various other sources such as movements of the rocks when geological faults occur or the rush of a great wave due to a submarine earthquake. so far as known, however, all these other agencies cause insignificant displacements compared with those arising from movements of the air. this fact coupled with the mathematical certainty that meteorological phenomena must produce some wandering of the pole, has caused most astronomers to accept jeffreys' conclusion. if we follow their example we are led to conclude that changes in atmospheric pressure and in the other meteorological conditions strike blows which sometimes shift the earth several feet from its normal position in respect to the axis. [illustration: _fig. . wandering of the pole from to ._ (_after moulton._)] if the foregoing reasoning is correct, the great and especially the sudden departures from the smooth gyroscopic circle described by the pole in the eulerian motion would be expected to occur at about the same time as unusual earthquake activity. this brings us to an interesting inquiry carried out by milne[ ] and amplified by knott.[ ] taking albrecht's representation of the irregular spiral-like motion of the pole, as given in fig. , they show that there is a preponderance of severe earthquakes at times when the direction of motion of the earth in reference to its axis departs from the smooth eulerian curve. a summary of their results is given in table . the table indicates that during the period from to there were nine different times when the curve of fig. changed its direction or was deflected by less than ° during a tenth of a year. in other words, during those periods it did not curve as much as it ought according to the eulerian movement. at such times there were world-shaking earthquakes, or an average of about . per tenth of a year. according to the other lines of table , in thirty-two cases the deflection during a tenth of a year was between ° and °, while in fifty-six cases it was from ° to °. during these periods the curve remained close to the eulerian path and the world-shaking earthquakes averaged only . and . . then, when the deflection was high, that is, when meteorological conditions threw the earth far out of its eulerian course, the earthquakes were again numerous, the number rising to . when the deflection amounted to more than °. table deflection of path of pole compared with earthquakes _no. of _no. of _average no. _deflection_ deflections_ earthquakes_ of earthquakes_ - ° . - ° . - ° . - ° . over ° . in order to test this conclusion in another way we have followed a suggestion of professor schlesinger. under his advice the eulerian motion has been eliminated and a new series of earthquake records has been compared with the remaining motions of the poles which presumably arise largely from meteorological causes. for this purpose use has been made of the very full records of earthquakes published under the auspices of the international seismological commission for the years to , the only years for which they are available. these include every known shock of every description which was either recorded by seismographs or by direct observation in any part of the world. each shock is given the same weight, no matter what its violence or how closely it follows another. the angle of deflection has been measured as milne measured it, but since the eulerian motion is eliminated, our zero is approximately the normal condition which would prevail if there were no meteorological complications. dividing the deflections into six equal groups according to the size of the angle, we get the result shown in table . table earthquakes in - compared with departures of the projected curve of the earth's axis from the eulerian position _average angle of deflection_ _average daily number (_ periods of / year each_) of earthquakes_ - . ° . . ° . . ° . . ° . . ° . . ° . here where some twenty thousand earthquakes are employed the result agrees closely with that of milne for a different series of years and for a much smaller number of earthquakes. so long as the path of the pole departs less than about ° from the smooth gyroscopic eulerian path, the number of earthquakes is almost constant, about eight and a quarter per day. when the angle becomes large, however, the number increases by nearly per cent. thus the work of milne, knott, and jeffreys is confirmed by a new investigation. apparently earthquakes and crustal movements are somehow related to sudden changes in the load imposed on the earth's crust by meteorological conditions. this conclusion is quite as surprising to the authors as to the reader--perhaps more so. at the beginning of this investigation we had no faith whatever in any important relation between climate and earthquakes. at its end we are inclined to believe that the relation is close and important. it must not be supposed, however, that meteorological conditions are the _cause_ of earthquakes and of movements of the earth's crust. even though the load that the climatic agencies can impose upon the earth's crust runs into millions of tons per square mile, it is a trifle compared with what the crust is able to support. there is, however, a great difference between the cause and the occasion of a phenomenon. suppose that a thick sheet of glass is placed under an increasing strain. if the strain is applied slowly enough, even so rigid a material as glass will ultimately bend rather than break. but suppose that while the tension is high the glass is tapped. a gentle tap may be followed by a tiny crack. a series of little taps may be the signal for small cracks to spread in every direction. a few slightly harder taps may cause the whole sheet to break suddenly into many pieces. yet even the hardest tap may be the merest trifle compared with the strong force which is keeping the glass in a state of strain and which would ultimately bend it if given time. the earth as a whole appears to stand between steel and glass in rigidity. it is a matter of common observation that rocks stand high in this respect and in the consequent difficulty with which they can be bent without breaking. because of the earth's contraction the crust endures a constant strain, which must gradually become enormous. this strain is increased by the fact that sediment is transferred from the lands to the borders of the sea and there forms areas of thick accumulation. from this has arisen the doctrine of isostasy, or of the equalization of crustal pressure. an important illustration of this is the oceanward and equatorial creep which has been described in chapter xi. there we saw that when the lands have once been raised to high levels or when a shortening of the earth's axis by contraction has increased the oceanic bulge at the equator, or when the reverse has happened because of tidal retardation, the outer part of the earth appears to creep slowly back toward a position of perfect isostatic adjustment. if the sun had no influence upon the earth, either direct or indirect, isostasy and other terrestrial processes might flex the earth's crust so gradually that changes in the form and height of the lands would always take place slowly, even from the geological point of view. thus erosion would usually be able to remove the rocks as rapidly as they were domed above the general level. if this happened, mountains would be rare or unknown, and hence climatic contrasts would be far less marked than is actually the case on our earth where crustal movements have repeatedly been rapid enough to produce mountains. nature's methods rarely allow so gradual an adjustment to the forces of isostasy. while the crust is under a strain, not only because of contraction, but because of changes in its load through the transference of sediments and the slow increase or decrease in the bulge at the equator, the atmosphere more or less persistently carries on the tapping process. the violence of that process varies greatly, and the variations depend largely on the severity of the climatic contrasts. if the main outlines of the cyclonic hypothesis are reliable, one of the first effects of a disturbance of the sun's atmosphere is increased storminess upon the earth. this is accompanied by increased intensity in almost every meteorological process. the most important effect, however, so far as the earth's crust is concerned would apparently be the rapid and intense changes of atmospheric pressure which would arise from the swift passage of one severe storm after another. each storm would be a little tap on the tensely strained crust. any single tap might be of little consequence, even though it involved a change of a billion tons in the pressure on an area no larger than the state of rhode island. yet a rapid and irregular succession of such taps might possibly cause the crust to crack, and finally to collapse in response to stresses arising from the shrinkage of the earth. another and perhaps more important effect of variations in storminess and especially in the location of the stormy areas would be an acceleration of erosion in some places and a retardation elsewhere. a great increase in rainfall may almost denude the slopes of soil, while a diminution to the point where much of the vegetation dies off has a similar effect. if such changes should take place rapidly, great thicknesses of sediment might be concentrated in certain areas in a short time, thus disturbing the isostatic adjustment of the earth's crust. this might set up a state of strain which would ultimately have to be relieved, thus perhaps initiating profound crustal movements. changes in the load of the earth's crust due to erosion and the deposition of sediment, no matter how rapid they may be from the geological standpoint, are slow compared with those due to changes in barometric pressure. a drop of an inch in barometric pressure is equivalent to the removal of about five inches of solid rock. even under the most favorable circumstances, the removal of an average depth of five inches of rock or its equivalent in soil over millions of square miles would probably take several hundred years, while the removal of a similar load of air might occur in half a day or even a few hours. thus the erosion and deposition due to climatic variations presumably play their part in crustal deformation chiefly by producing crustal stresses, while the storms, as it were, strike sharp, sudden blows. suppose now that a prolonged period of world-wide mild climate, such as is described in chapter x, should permit an enormous accumulation of stresses due to contraction and tidal retardation. suppose that then a sudden change of climate should produce a rapid shifting of the deep soil that had accumulated on the lands, with a corresponding localization and increase in strains. suppose also that frequent and severe storms play their part, whether great or small, by producing an intensive tapping of the crust. in such a case the ultimate collapse would be correspondingly great, as would be evident in the succeeding geological epoch. the sea floor might sink lower, the continents might be elevated, and mountain ranges might be shoved up along lines of special weakness. this is the story of the geological period as known to historical geology. the force that causes such movements would be the pull of gravity upon the crust surrounding the earth's shrinking interior. nevertheless climatic changes might occasionally set the date when the gravitative pull would finally overcome inertia, and thus usher in the crustal movements that close old geologic periods and inaugurate new ones. this, however, could occur only if the crust were under sufficient strain. as lawson[ ] says in his discussion of the "elastic rebound theory," the sudden shifts of the crust which seem to be the underlying cause of earthquakes "can occur only after the accumulation of strain to a limit and ... this accumulation involves a slow creep of the region affected. in the long periods between great earthquakes the energy necessary for such shocks is being stored up in the rocks as elastic compression." if a period of intense storminess should occur when the earth as a whole was in such a state of strain, the sudden release of the strains might lead to terrestrial changes which would alter the climate still further, making it more extreme, and perhaps permitting the storminess due to the solar disturbances to bring about glaciation. at the same time if volcanic activity should increase it would add its quota to the tendency toward glaciation. nevertheless, it might easily happen that a very considerable amount of crustal movement would take place without causing a continental ice sheet or even a marked alpine ice sheet. or again, if the strains in the earth's crust had already been largely released through other agencies before the stormy period began, the climate might become severe enough to cause glaciation in high latitudes without leading to any very marked movements of the earth's crust, as apparently happened in the mid-silurian period. footnotes: [footnote : e. kirk: paleozoic glaciation in alaska; am. jour. sci., , p. .] [footnote : j. milne: catalogue of destructive earthquakes; rep. brit. asso. adv. sci., .] [footnote : wm. bowie: lecture before the geological club of yale university. see am. jour. sci., .] [footnote : chas. davisson: on the annual and semi-annual seismic periods; roy. soc. of london, philosophical transactions, vol. , , _ff._] [footnote : c. g. knott: the physics of earthquake phenomena, oxford, .] [footnote : in table the first column indicates the region; the second, the dates; and the third, the number of shocks. the fourth column gives the month in which the annual maximum occurs when the crude figures are smoothed by the use of overlapping six-monthly means. in other words, the average for each successive six months has been placed in the middle of the period. thus the average of january to june, inclusive, is placed between march and april, that for february to july between april and may, and so on. this method eliminates the minor fluctuations and also all periodicities having a duration of less than a year. if there were no annual periodicity the smoothing would result in practically the same figure for each month. the column marked "amplitude" gives the range from the highest month to the lowest divided by the number of earthquakes and then corrected according to schuster's method which is well known to mathematicians, but which is so confusing to the layman that it will not be described. next, in the column marked "expected amplitude," we have the amplitude that would be expected if a series of numbers corresponding to the earthquake numbers and having a similar range were arranged in accidental order throughout the year. this also is calculated by schuster's method in which the expected amplitude is equal to the square root of "pi" divided by the number of shocks. when the actual amplitude is four or more times the expected amplitude, the probability that there is a real periodicity in the observed phenomena becomes so great that we may regard it as practically certain. if there is no periodicity the two are equal. the last column gives the number of times by which the actual exceeds the expected amplitude, and thus is a measure of the probability that earthquakes vary systematically in a period of a year.] [footnote : n. f. drake: destructive earthquakes in china; bull. seism. soc. am., vol. , , pp. - , - .] [footnote : the only other explanation that seems to have any standing is the psychological hypothesis of montessus de ballore as given in les tremblements de terre. he attributes the apparent seasonal variation in earthquakes to the fact that in winter people are within doors, and hence notice movements of the earth much more than in summer when they are out of doors. there is a similar difference between people's habits in high latitudes and low. undoubtedly this does have a marked effect upon the degree to which minor earthquake shocks are noticed. nevertheless, de ballore's contention, as well as any other psychological explanation, is completely upset by two facts: first, instrumental records show the same seasonal distribution as do records based on direct observation, and instruments certainly are not influenced by the seasons. second, in some places, notably china, as drake has shown, the summer rather than the winter is very decidedly the time when earthquakes are most frequent.] [footnote : a comparison of tropical hurricanes with earthquakes is interesting. taking all the hurricanes recorded in august, september, and october, from to , and the corresponding earthquakes in milne's catalogue, the correlation coefficient between hurricanes and earthquakes is + . , with a probable error of ± . , the month being used as the unit. this is not a large correlation, yet when it is remembered that the hurricanes represent only a small part of the atmospheric disturbances in any given month, it suggests that with fuller data the correlation might be large.] [footnote : ellsworth huntington: the geographic work of dr. m. a. veeder; geog. rev., vol. , march and april, , nos. and .] [footnote : frank schlesinger: variations of latitude; their bearing upon our knowledge of the interior of the earth; proc. am. phil. soc., vol. , , pp. - . also smithsonian report for , pp. - .] [footnote : harold jeffreys: causes contributory to the annual variations of latitude; monthly notices, royal astronomical soc., vol. , , pp. - .] [footnote : john milne: british association reports for and .] [footnote : c. g. knott: the physics of earthquake phenomena, oxford, .] [footnote : a. c. lawson: the mobility of the coast ranges of california; univ. of calif. pub., geology, vol. , no. , pp. - .] conclusion here we must bring this study of the earth's evolution to a close. its fundamental principle has been that the present, if rightly understood, affords a full key to the past. with this as a guide we have touched on many hypotheses, some essential and some unessential to the general line of thought. the first main hypothesis is that the earth's present climatic variations are correlated with changes in the solar atmosphere. this is the keynote of the whole book. it is so well established, however, that it ranks as a theory rather than as an hypothesis. next comes the hypothesis that variations in the solar atmosphere influence the earth's climate chiefly by causing variations not only in temperature but also in atmospheric pressure and thus in storminess, wind, and rainfall. this, too, is one of the essential foundations on which the rest of the book is built, but though this cyclonic hypothesis is still a matter of discussion, it seems to be based on strong evidence. these two hypotheses might lead us astray were they not balanced by another. this other is that many climatic conditions are due to purely terrestrial causes, such as the form and altitude of the lands, the degree to which the continents are united, the movement of ocean currents, the activity of volcanoes, and the composition of the atmosphere and the ocean. only by combining the solar and the terrestrial can the truth be perceived. finally, the last main hypothesis of this book holds that if the climatic conditions which now prevail at times of solar activity were magnified sufficiently and if they occurred in conjunction with certain important terrestrial conditions of which there is good evidence, they would produce most of the notable phenomena of glacial periods. for example, they would explain such puzzling conditions as the localization and periodicity of glaciation, the formation of loess, and the occurrence of glaciation in low latitudes during permian and proterozoic times. the converse of this is that if the conditions which now prevail at times when the sun is relatively inactive should be intensified, that is, if the sun's atmosphere should become calmer than now, and if the proper terrestrial conditions of topographic form and atmospheric composition should prevail, there would arise the mild climatic conditions which appear to have prevailed during the greater part of geological time. in short, there seems thus far to be no phase of the climate of the past which is not in harmony with an hypothesis which combines into a single unit the three main hypotheses of this book, solar, cyclonic, and terrestrial. outside the main line of thought lie several other hypotheses. several of these, as well as some of the main hypotheses, are discussed chiefly in _earth and sun_, but as they are given a practical application in this book they deserve a place in this final summary. each of these secondary hypotheses is in its way important. yet any or all may prove untrue without altering our main conclusions. this point cannot be too strongly emphasized, for there is always danger that differences of opinion as to minor hypotheses and even as to details may divert attention from the main point. among the non-essential hypotheses is the idea that the sun's atmosphere influences that of the earth electrically as well as thermally. this idea is still so new that it has only just entered the stage of active discussion, and naturally the weight of opinion is against it. although not necessary to the main purpose of this book, it plays a minor rôle in the chapter dealing with the relation of the sun to other astronomical bodies. it also has a vital bearing on the further advance of the science of meteorology and the art of weather forecasting. another secondary hypothesis holds that sunspots are set in motion by the planets. whether the effect is gravitational or more probably electrical, or perhaps of some other sort, does not concern us at present, although the weight of evidence seems to point toward electronic emissions. this question, like that of the relative parts played by heat and electricity in terrestrial climatic changes, can be set aside for the moment. what does concern us is a third hypothesis, namely, that if the planets really determine the periodicity of sunspots, even though not supplying the energy, the sun in its flight through space must have been repeatedly and more strongly influenced in the same way by many other heavenly bodies. in that case, climatic changes like those of the present, but sometimes greatly magnified, have presumably arisen because of the constantly changing position of the solar system in respect to other parts of the universe. finally, the fourth of our secondary hypotheses postulates that at present the date of movements of the earth's crust is often determined by the fact that storms and other meteorological conditions keep changing the load upon first one part of the earth's surface and then upon another. thus stresses that have accumulated in the earth's isostatic shell during the preceding months are released. in somewhat the same way epochs of extreme storminess and rapid erosion in the past may possibly have set the date for great movements of the earth's crust. this hypothesis, like the other three in our secondary or non-essential group, is still so new that only the first steps have been taken in testing it. yet it seems to deserve careful study. in testing all the hypotheses here discussed, primary and secondary alike, the first necessity is a far greater amount of quantitative work. in this book there has been a constant attempt to subject every hypothesis to the test of statistical facts of observation. nevertheless, we have been breaking so much new ground that in many cases exact facts are not yet available, while in others they can be properly investigated only by specialists in physics, astronomy, or mathematics. in most cases the next great step is to ascertain whether the forces here called upon are actually great enough to produce the observed results. even though they act only as a means of releasing the far greater forces due to the contraction of the earth and the sun, they need to be rigidly tested as to their ability to play even this minor rôle. still another line of study that cries aloud for research is a fuller comparison between earthquakes on the one hand and meteorological conditions and the wandering of the poles on the other. finally, an extremely interesting and hopeful quest is the determination of the positions and movements of additional stars and other celestial bodies, the faint and invisible as well as the bright, in order to ascertain the probable magnitude of their influence upon the sun and thus upon the earth at various times in the past and in the future. perhaps we are even now approaching some star that will some day give rise to a period of climatic stress like that of the fourteenth century, or possibly to a glacial epoch. or perhaps the variations in others of the nearer stars as well as alpha centauri may show a close relation to changes in the sun. throughout this volume we have endeavored to discover new truth concerning the physical environment that has molded the evolution of all life. we have seen how delicate is the balance among the forces of nature, even though they be of the most stupendous magnitude. we have seen that a disturbance of this balance in one of the heavenly bodies may lead to profound changes in another far away. yet during the billion years, more or less, of which we have knowledge, there appears never to have been a complete cataclysm involving the destruction of all life. one star after another, if our hypothesis is correct, has approached the solar system closely enough to set the atmosphere of the sun in such commotion that great changes of climate have occurred upon the earth. yet never has the solar system passed so close to any other body or changed in any other way sufficiently to blot out all living things. the effect of climatic changes has always been to alter the environment and therefore to destroy part of the life of a given time, but with this there has invariably gone a stimulus to other organic types. new adaptations have occurred, new lines of evolutionary progress have been initiated, and the net result has been greater organic diversity and richness. temporarily a great change of climate may seem to retard evolution, but only for a moment as the geologist counts time. then it becomes evident that the march of progress has actually been more rapid than usual. thus the main periods of climatic stress are the most conspicuous milestones upon the upward path toward more varied adaptation. the end of each such period of stress has found the life of the world nearer to the high mentality which reaches out to the utmost limits of space, of time, and of thought in the search for some explanation of the meaning of the universe. each approach of the sun to other bodies, if such be the cause of the major climatic changes, has brought the organic world one step nearer to the solution of the greatest of all problems,--the problem of whether there is a psychic goal beyond the mental goal toward which we are moving with ever accelerating speed. throughout the vast eons of geological time the adjustment of force to force, of one body of matter to another, and of the physical environment to the organic response has been so delicate, and has tended so steadily toward the one main line of mental progress that there seems to be a purpose in it all. if the cosmic uniformity of climate continues to prevail and if the uniformity is varied by changes as stimulating as those of the past, the imagination can scarcely picture the wonders of the future. in the course of millions or even billions of years the development of mind, and perhaps of soul, may excel that of today as far as the highest known type of mentality excels the primitive plasma from which all life appears to have arisen. index * indicates illustrations. abbot, c. g., cited, , , , , . aboskun, . africa, earthquakes, ; east, _see_ east africa; lakes, ; north, _see_ north africa. african glaciation, . air, _see_ atmosphere. alaska, glacial till in, ; ice age in, . albrecht, cited, . alexander, march of, f. allard, h. a., cited, , . alpha centauri, companion of, ; distance from sun, ; luminosity, ; speed of, ; variations, . alps, loess in, ; precipitation in, ; snow level in, . altair, companion of, ; luminosity, ; speed of, . amazon forest, temperature, . ancylus lake, . andes, snow line, . animals, climate and, . antarctica, mild climate, ; thickness of ice in, ; winds, , . anti-cyclonic hypothesis, ff. appalachians, effect on ice sheet, . arabia, civilization in, . aral, sea of, . archean rocks, . archeozoic, f.; climate of, . arctic ocean, submergence, . arctowski, h., cited, , , . argon, increase of, . arizona, rainfall, , ; trees measured in, . arrhenius, s., cited, , . arsis, of pulsation, . asbjörn selsbane, corn of, . asia, atmospheric pressure, ; central, changes of climate, * ; central, post-glacial climate, ; climate, ; glaciation in, ; storminess in, ; western, climate in, f. atlantic ocean, storminess, . atmosphere, changes, f., ; composition of, - ; effect on temperature, . atmospheric circulation, glaciation and, . atmospheric electricity, solar relations of, . atmospheric pressure, earthquakes and, ; evaporation and, ; increase in, ; redistribution of, ; variation, . australia, east, mild climate, ; precipitation, . axis, earth's, ; wabbling of, . bacon, sir francis, cited, . bacubirito, meteor at, . baltic sea, as lake, ; freezing of, ; ice, ; storm-floods, ; submergence, . bardsson, ivar, . barkow, cited, . barometric pressure, solar relations of, . barrell, j., cited, , , , . bartoli, a. g., cited, . bauer, l. a., cited, . beaches, under water, . beadnell, h. j. l., cited, . beluchistan, rainfall, . bengal, bay of, cyclones in, . bengal, famine in, f. berlin, rainfall and temperature, . betelgeuse, f.; distance from sun, . bible, climatic evidence in, f.; palms in, . binary stars, . birkeland, k., cited, . black earth region, loess in, . boca, cal., correlation coefficients, , . boltzmann, l., cited, . bonneville, lake, , . borkum, storm-flood in, . boss, l. cited, , . botanical evidence of mild climates, ff. boulders, on irish coast, . bowie, w., cited, . bowman, i., cited, . britain, forests, ; level of land, . british isles, height of land, ; temperature, . brooks, c. e. p., cited, , , , , . brooks, c. f., cited, . brown, e. w., cited, , . brückner, e., cited, . brückner periods, f. bufo, habitat of, . buhl stage, . bull, dr., cited, , . butler, h. c., cited, , ff., , . california, changes of climate, * ; correlations of rainfall, ; measurements of sequoias in, , ff.; rainfall, . cambrian period, f. canada, storminess, f., ; storm tracks in, . cape farewell, shore ice at, . carbon dioxide, erosion and, f.; from volcanoes, ; hypothesis, ; importance of, , f.; in permian, ; in atmosphere, , , ; in ocean, ; nebular hypothesis and, ; theory of glaciation, ff. caribbean mountains, origin of, . carnegie institution of washington, . caspian sea, climatic stress, ; rainfall, f.; rise and fall, ; ruins in, . cenozoic, climate, ; fossils, . central america, maya ruins, . chad, lake, swamps of, . chamberlin, r. t., cited, , , . chamberlin, t. c., cited, , , , , f., , , , , , , , , . chandler, s. c., cited, . chinese earthquakes, periodicity of, . chinese, sunspot observations, f. chinese turkestan, desiccation in, . chronology, glacial, . clarke, f. w., cited, , . clayton, h. h., cited, f. climate, effect of contraction, ff.; affect of salinity, ; in history, - ; uniformity, - ; variability, - . climates, mild, causes of, - ; mild, periods of, . climatic changes, and crustal movements, ff.; hypotheses of, - ; mountain-building and, * ; post-glacial crustal movements and, - ; terrestrial causes of, - . climatic sequence, f. climatic stages, post-glacial, . climatic stress, in fourteenth century, - . climatic uniformity, hypothesis of, , f. climatic zoning, . cloudiness, glaciation and, , . clouds, as protection, . colfax, cal., correlation coefficients, . cologne, flood at, . compass, variations, . continental climate, variations, . continents, effect on climate, f. contraction, effect on climate, ff., , ; effect on lands, ; heat of sun and, f.; irregular, ; of the earth, ; of the sun, ; stresses caused by, . convection, carbon dioxide and, . corals, in high latitudes, , , , . cordeiro, f. j. b., cited, , , . correlation coefficients, earthquakes and sunspots, ; jerusalem rainfall and sequoia growth, ff.; rainfall and tree growth, ff. cosmos, effect of light, . cressey, g. b., cited, . cretaceous, lava, ; mountain ranges, ; paleogeography, * ; submergence of north america, . croll, j., cited, ff., . croll's hypothesis, snow line, . crust, climate and movements of, , , ; movements of, ; strains in, . currents and planetary winds, . cycads, . cyclonic hypothesis, ; loess and, ; permian glaciation and, ; snow line, . cyclonic storms, in glacial epochs, f.; solar electricity and, (_see_ storms, storminess). cyclonic vacillations, f.; nature of, ff. daily vibrations, f. danube, frozen, . darwin, g. h., cited, . daun stage, . davis, w. m., cited, . davisson, c., cited, , , . day, c. p., cited, . day, length of, , . dead sea, palms near, . death valley, . de ballore, m., cited, , . deep-sea circulation, rapidity, ; salinity and, ; solar activity and, . de geer, s., cited, , . de lapparent, a., cited, . denmark, fossils, . "desert pavements," . deserts, abundant flora of, ; and pulsations theory, ff.; red beds of, . devonian, climate, ; mountains, . dog, climate and, . donegal county, ireland, . double stars, , ; electrical effect of, . douglass, a. e., cited, , , f., , , . dragon town, destruction of, , . drake, n. f., cited, , . droughts, and pulsations theory, f.; in england, ; in india, f. drumkelin bog, ireland, log cabin in, . dust, at high levels, . earth, crust of and the sun, - ; internal heat, ; nature of mild climate, ; position of axis, ; rigidity of, ; temperature gradient, ; temperature of surface, . earthquakes, and seasons, , ; and sunspots, f.; and tropical hurricanes, ; and wandering of pole, f.; cause of, ; compared with departures from eulerian position, ; seasonal distribution of, ; seasonal march, . "earthquake weather," . east africa, mild climate, . east indies, earthquakes of, . eberswalde, tree growth at, f. ecliptic, obliquity of, . electrical currents, in solar atmosphere, . electrical emissions, variation of, . electrical hypothesis, , f., ff. electrical phenomena, storminess and, . electricity, and earthquakes, ; solar, . electro-magnetic hypothesis, . electrons, solar, ; variation of, . electro-stellar hypothesis, . elevation, climatic changes and, . engedi, palms in, . england, climatic stress, f.; storminess and rainfall, . eocene, climate, . equinoxes, precession of, . erosion, storminess and, . eskimo, in greenland, . eulerian movement, , . euphrates, . europe, climatic stress, ff., f.; climatic table, ; glaciation in, ; ice sheet, ; inundations of rivers, ; post-glacial climate, ; rainfall, ; submergence, , . evaporation, and glaciation, , ; atmospheric pressure and, ; from plants, ; importance, ; in trade-wind belt, ; rapidity of, . evening primrose, effect of light, . evolution, climate and, ; geographical complexity and, ; glaciation and, ; of the earth, . faculæ, cause of, . false point lighthouse, barometric pressure at, . famine, cause of, ; in england, f.; in india, f.; pulsations theory and, f. faunas, and mild climates, f.; in permian, f. fennoscandian pause, . flowering, light and, . fog, and glaciation, ; as protection, ; temperature and, . forests, climate and, . form of the land, ff. fossil floras, and mild climates, ; in antarctica, ; in greenland, . fossils, , ; and loess, ; archeozoic, f.; cenozoic, ; dating of, ; glaciation and, ; in peat bogs, ; mild climate, ; proterozoic, , f. fourteenth century, climatic stress in, - . fowle, f. e., cited, , , , . frech, f., cited, . free, e. e., cited, . freezing, salinity and, . fresno, rainfall record, . "friction variables," . frisian islands, storm-flood, . fritz, h., cited, . frogs, distribution of, . fuchs, cited, . galaxy, . galveston, tex., rainfall and temperature, . garner, w. w., cited, , . gasses, in air, . geographers, and climatic changes, ff. geological time table, * . geologic oscillations, f., ff., , . geologists, changes in ideas of, f. germanic myths, . germany, forests, ; growth of trees in, ; storms in, . gilbert, g. k., cited, . glacial epochs, causes of, ; dates of, ; intervals between, f.; length of, f. glacial fluctuations, ff.; nature of, ff. glacial period, at present, ; ice in, f.; length of, ; list, ; temperature, . glaciation, and loess, f.; and movement of crust, ; conditions favorable for, ; extent of, ; hypotheses of, ff.; in southern canada, ; localization of, ff.; permian, * ; solar-cyclonic hypothesis of, - ; suddenness of, ; upper limit of, . goldthwait, j. w., cited, . gondwana land, , . gravitation, effect on sun, ; pull of, . great basin, in glacial period, ; salt lakes in, . great ice age, see pleistocene. great plains, effect on ice sheet, . greenland, climatic stress, ff.; ice, ; rainfall, ; storminess, ; submergence, ; vegetation, , , ; winds, , . gregory, j. w., cited, ff., . gschnitz stage, . guatemala, ruins in, . guervain, cited, . gyroscope, earth as, . hale, g. e., cited, , . hamdulla, cited, . hann, j., cited, . hansa union, operations of, . harmer, f. w., cited, , . heat, and earthquakes, ; earth's internal, . hedin, s., cited, . heim, a., cited, . heligoland, flood in, . helland-hansen, b., cited, . helmert, f. r., cited, . henderson, l. j., cited, , , , . henry, a. j., cited, , . hercynian mountains, . high pressure and glaciation, , . himalayas, glaciation, ; origin of, ; snow line, . himley, cited, . historic pulsations, f.; nature of, ff. history, climate of, - ; climatic pulsations and, . hobbs, w. h., cited, , , , . hot springs, temperature of, . humphreys, w. j., cited, , f., , , , , . hurricanes, in arid regions, ; sunspots and, . hyades, cluster in, . ice, accumulations, f.; advances of, ; distribution of, ; drift, . ice sheets, disappearance, ; limits, ; localization, ff.; rate of retreat, ; thickness, . iceland, submergence, . iowan ice sheet, rapid retreat, . iowan loess, . india, drought, f.; famine, f.; rainfall, . indian glaciation, . inter-glacial epoch, permian, . internal heat of earth, . ireland, drumkelin bog, ; in glacial period, ; level of land, ; storminess and rainfall, ; submergence, . irish sea, tides, . irrigation ditches, abandoned, . isostasy, ff. italy, southern, climate of, f. japan, earthquakes of, . javanese mountains, origin of, . jaxartes, . jeans, j. h., cited, , , , , . jeffreys, h., cited, , , . jeffreys, j., cited, . jericho, palms in, . jerusalem, rainfall, ; rainfall and temperature, ; rainfall in, and sequoia growth, ff. johnson, cited, . judea, palms in, . jupiter, and sunspots, ; effect of, ; periodicity of, f.; temperature of, ; tidal effect of, . jurassic, climate, ; mountain ranges, . kansas, variations of seasons, . kara koshun marsh, lop nor, . keewatin center, ; evaporation in, . keewatin ice sheet, . kelvin, lord, cited, f. keyes, c. r., cited, . kirk, e., cited, . knott, c. g., cited, , , , , , . knowlton, f. h., cited , , , , . köppen, w., , , . krakatoa, glaciation and, ; volcanic hypothesis and, . krümmel, o., cited, , . kullmer, c. j., cited, , , ; map of storminess, * . _kungaspegel_, sea routes described, . labor, price in england, . labradorean center of glaciation, . lahontan, lake, . lake strands, _see_ strands. lake superior, lava, . lakes, during glacial periods, f.; in semi-arid regions, ; of great basin, ; ruins in, . land, and water, climatic effect of, ff.; distribution of, , form of, ff.; range of temperature and, . lavas, climatic effect of, . lawson, a. c., cited, . lebanon, cedars of, . leiter, h., cited, . leverett, f., cited, . life, atmosphere and, f.; chemical characteristic of, ; effect of salinity, ; of glacial period, ; persistence of forms, . light, effect of atmosphere on, ; effect on plants, ff.; ultra-violet, storminess and, ; variation of, . litorina sea, . loess, date of, ff.; origin of, , . lop nor, rise of, ; swamps, . lows, and glacial lobes, ; movements of, ; see storms and cyclones. lulan, . lull, r. s., cited, , . macdougal, d. t., cited, . mcgee, w. j., cited, . macmillan, w. d., cited, . magdalenian period, . magnetic fields of sunspots, . magnetic poles, relation to storm tracks, . makran, climate, ; rainfall, . malay archipelago, earthquakes of, . mallet, r., cited, . malta, rainfall, . manson, m., cited, . mayas, civilization, ; ruins, . mayence, flood at, . mazelle, e., cited, . mediterranean, climate of, ; rainfall records, ; storminess in, . mercury, and sunspots, . mesozoic, climate, ; crustal changes, ; emergence of lands, . messier, ; variables, . metcalf, m. m., cited, . meteorological factors and earthquakes, f. meteorological hypothesis of crustal movements, . meteors, and sun's heat, , . michelson, a. a., cited, . middle silurian, fauna in alaska, . mild climates, _see_ climates, mild. milky way, . mill, h. r., cited, . milne, j., cited, , , , , . miocene, crustal changes, . mississippi basin, loess in, . mogul emperor, and famine, . monsoons, character of, ; direction of, ; indian famines and, . moulton, f. r., cited, , , . mountain building, climatic changes and, * . mountains, folding of, ; rainfall, on, . multiple stars, . nansen, f., cited, , . naples, rainfall, . nathorst, cited, . nebulæ, . nebular hypothesis, , . neolithic period, . nevada, correlations of rainfall, . new england, height of land, . new mexico, rainfall, . new orleans, la., rainfall and temperature, . new zealand, climate, ; tree ferns, . newcomb, s., cited, . nile floods, periodicity in, . nitrogen, in atmosphere, . niya, chinese turkestan, desiccation at, . nocturnal cooling, changes in, f. norlind, a., cited, . norsemen, route to greenland, . norse sagas, . north africa, climate of, ; roman aqueducts in, . north america, at maximum glaciation, ff.; emergence of lands, ; glaciation in, ; height of land, ; interior sea in, ; inundations, ; loess in, ; submergence of lands, , . north atlantic ocean, salinity, . north sea, climatic stress, ff.; floods around, , ; rainfall, ; storminess, . northern hemisphere, earthquakes of, . norway, decay, ; temperature, . novæ, . oceanic circulation, carbon dioxide and, ff. oceanic climate, characteristics, . oceanic currents, diversion, ; influence of land distribution, . oceans, age of, ; composition of, - ; deepening of, ; salinity, , ; temperature, , , , . okada, t., cited, . old testament, temperature, . orbital precessions, . ordovician, climate, . organic evolution, glacial fluctuations and, . orion, nebulosity near, ; stars near, . orontes, . osborn, h. f., cited, . owens-searles, lakes, . oxus, . oxygen, in atmosphere, , ; in permian, . ozone, cause of, . paleolithic, . paleozoic, climate, ; mountains in, . palestine, change of climate, f. palms, climatic change and, f.; in ireland, . palmyra, ruins of, . parallaxes of stars, f. patrician center, . peat-bog period, first, . penck, a., cited, , , , , . pennsylvanian, life of, . periodicities, f. periodicity, of climatic phenomena, f.; of glaciation, ; of sunspots, . permian, climate, ; distribution of glaciation, ; glaciation, , , * , ; glaciation and mountains, ; life of, ; red beds, ; temperature, f. perry, cited, . persia, lakes, ; rainfall, . pettersson, o., cited, ff., f., , , . pirsson, l. v., cited, , . planetary hypothesis, , . planetary nebulæ, . planets, and sunspots, ; effect of star on, ; sunspot cycle and, ; temperatures, f. plants, climate and, f.; effect of light, ff. pleion, defined, . pleionian migrations, f. pleistocene, climate, ; duration of, ; glaciation, ff.; ice sheets, * . pluvial climate, causes of, ; during glacial periods, . po, frozen, . polaris, . polar wandering, hypothesis of, f. pole and earthquakes, . post-glacial crustal movements and climatic changes, - . poynting, j. h., cited, . precessional hypothesis, f. precipitation, and glaciation, , ; during glacial period, ; snow line and, ; temperature and, . procyon, companion of, ; luminosity, ; speed of, . progressive change, . progressive desiccation, hypothesis of, ff. proterozoic, f.; fossils, f.; glaciation, , , , ; lava, ; mountains in, ; oceanic salinity, f.; oxygen in air, ; red beds, ; temperature, f. pulsations, hypothesis of, , ff. pulsatory climatic changes, ff. pulsatory hypothesis, . pumpelly, r., cited, . radiation, variation of, . radioactivity, heat of sun and, f. rainfall, changes in, f.; glaciation and, ; sunspots and, , * , ; tree growth and, . red beds, , . rhine, flood, ; frozen, . rho ophiuchi, variables, . "rice grains," . richardson, o. w., cited, . rigidity, of earth, . roads, climate and, . rogers, thorwald, cited, . romans, aqueduct of, . rome, history of, . rotation, of earth, f. ruden, storm-flood, . rugen, storm-flood, . ruins, as climatic evidence, ; rainfall and, . sacramento, correlation coefficients, f., ; rainfall, ; rainfall record, . sagas, cited, f. st. john, c. e., cited, . salinity, deep-sea circulation and, ; effect on climate, ; in north atlantic, ; ocean temperature and, ; of ocean, , . salisbury, r. d., cited, , , , , , , , . salt, in ocean, . san bernardino, correlation of rainfall, . saturn, and sunspots, ; sunspot cycle and, . sayles, r. w., cited, . scandinavia, climatic stress, f.; fossils, ; post-glacial climate, ; rainfall, ; storminess, , ; temperature, . scandinavian center of glaciation, . schlesinger, f., cited, , , , , . schuchert, c., cited, , , , * , * , , * , , , , , , , , , * , , , , . schuster, a., cited, , , , . sculpture, maya, . sea level and glaciation, . seasonal alternations, f. seasonal banding, f. seasonal changes, geological, . seasons, and earthquakes, , , , ; evidences of, . secular progression, ff., . seistan, swamps, . sequoias, measurements of, ff.; rainfall record, . setchell, w. a., cited, . shackleton, e., cited, . shapley, h., cited, , , , , . shimek, e., cited, , . shreveport, la., rainfall and temperature, f. shrinkage of the earth, . siberia, and glaciation, . sierras, rainfall records, . simpson, g. c., cited, . sirius, companion of, ; distance from sun, ; luminosity, ; speed of, . slichter, c. s., cited, . smith, j. w., cited, . snowfall, glaciation and, , . snowfield, climatic effects of, . snow line, height of, ; in andes, ; in himalayas, . solar activity, cycles of, ; deep-sea circulation and, ; ice and, . solar constant, . solar-cyclonic hypothesis, - , ; glaciation and, - . solar prominences, cause of, . solar system, ; conservation of, ; proximity to stars, . solar variations, storms and, . south america, earthquakes, . south pole, thickness of ice at, . southern hemisphere, earthquakes, ; glaciation in, f. southern pacific railroad, rainfall records along, . soy beans, effect of light, f. space, sun's journey through, - . spiral nebulæ, f.; universe of, . spitzbergen, submergence, . springs, climate and, . stars, approach to sun, ; binary, ; clusters, , ; effect on solar atmosphere, ; dark, ; parallaxes of, f.; tidal action of, . stefan's law, . stein, m. a., cited, . stellar approaches, probability of, . storm belt in arid regions, . storm-floods, in fourteenth century, . storminess, and erosion, ; and ice, ; effect on glaciation, ; sunspots and, ; temperature and, , . storms, blows of, , ; increase, ; movement of, f.; movement of water and, * ; origin of, f.; sunspots and, , ; _see_ cyclones and lows. storm tracks, during glacial period, ; location, ; relation to magnetic poles, ; shifting of, . strands, climate and, ; in semi-arid regions, ; of salt lakes, . suess, e., cited, . sun, and the earth's crust, - ; approach to star, ; atmosphere of, , ; atmosphere of, and weather, ; cooling of, ; contraction of, ; disturbances of, ; effect of other bodies on, - ; heat, ; journey through space, - ; knowlton's hypothesis of, . suncracks, . sunspot cycles, f. sunspots, and earthquakes, ; causes of, ; magnetic field of, ; maximum of, ; mild climates and, ; number, f.; periodicity, ; planetary hypothesis of, ; records, ; storminess and, ; storms and, ; temperature of earth and, , . sunspot variations, . swamps, as desert phenomena, . sylt, storm-flood, . syria, civilization in, ; inscriptions in, ; roman aqueducts in, . syrian desert, ruins in, . talbert, cited, . tarim basin, red beds, . tarim desert, desiccation, . tarim river, swamps, . taylor, g., cited, , , , . temperature, change of in atlantic, ; changes in, ; climatic change and, ; critical, ; geological time and, ; glacial period, ; glaciation and, , , ; gradient of earth, ; of ocean, ; in norway, ; in permian, f.; in proterozoic, f.; limits, ff.; precipitation and, ; range of, , ; solar activity and, ; storminess and, , , ; sunspots and, , ; volcanic eruptions and, ; zones, . terrestial causes of climatic changes, - . tertiary, lava, . thames, frozen, . thermal solar hypothesis, f., . thermo-pleion, movements of, . thesis, of pulsations, . thiryu, storm-flood, . tian-shan mountains, irrigation in, . tidal action of stars, . tidal effect, of jupiter, ; of planets, . tidal hypothesis, . tidal retardation, effect on land and sea, ; rotation of earth and, f.; stress caused by, . tides, cycles of, . time, geological, _see_ geological time. toads, distribution of, . tobacco plant, effect of light, . topography, and glaciation, . transcaspian basin, red beds, . tree ferns, in new zealand, . tree growth, periodicity in, ; rainfall and, . trees, in california, ; measurement of, ff. triassic, climate, . trifid nebula, variables, . trondheim, wheat in, . trondhenäs, corn in, . tropical cyclones, in glacial epochs, f.; occurrence, ; solar activity and, . tropical hurricanes, earthquakes and, ; sunspots and, . turfan, temperature, . turner, h. h., cited, . tyler, j. m., cited, . tyndall, j., cited, , . typhoon region, "earthquake weather," . typhoons, occurrence, . united states, rainfall and temperature in gulf region, f.; salt lakes in, ; southwestern, climate, ; storminess, f., . variables, . veeder, m. a., cited, . vegetation, theory of pulsations and, . venus, atmosphere of, . vesterbygd, invasion of, . vicksburg, miss., rainfall and temperature, f. volcanic activity, climate and, ; movement of the earth's crust and, ; times of uplifting lands and, . volcanic dust, climatic changes and, . volcanic hypothesis, climatic change and, ff.; snow line, . volcanoes, activity of, . volga, . walcott, c. d., cited, , . wandering of the pole, . water, importance, . water vapor, condensation of, ; effect on life, ; in atmosphere, . wave, effect on movement of water, . weather, changes of, f.; origin of, ; variations, . wells, h. g., cited, . wendingstadt, storm-flood, . westerlies, f. wheat, price in england, . white sea, submergence, . whitney, j. d., cited, . wieland, g. r., cited, . williamson, e. d., cited, . willis, b., cited, . winds, at ice front, ; effect on currents, ; glaciation and, ; in antarctica, ; in glacial period, ; in greenland, ; planetary system of, ; velocity, . witch hazel, effect of light, . wolf, j. r., cited, , , . wolfer, cited, . wright, w. b., cited, , , . writing, among mayas, . yucatan, maya civilization, , ; rainfall, ; ruins, . yukon, ice age in, . zante, earthquakes of, . zonal crowding, . printed in the united states of america proofreading team at https://www.pgdp.net/ for project gutenberg (this book was produced from scanned images of public domain material from the google books project.) transcribers note: the plain text version of this ebook includes mathematical formulas in tex notation. these formulas are enclosed between dollar signs. the html version includes the same formulas rendered in their original form. einstein's theories of relativity and gravitation a selection of material from the essays submitted in the competition for the eugene higgins prize of $ , compiled and edited, and introductory matter supplied by j. malcolm bird, associate editor, scientific american new york scientific american publishing co., munn & co. preface the obstacles which the layman finds to understanding einstein's relativity theories lie not so much in the inherent difficulty of these theories themselves as in the difficulty of preparing the mind for their reception. the theory is no more difficult than any scientific development of comparable depth; it is not so difficult as some of these. but it is a fact that for a decent understanding of it, a large background of scientific knowledge and scientific habit of thought is essential. the bulk of the writers who have attempted to explain einstein to the general reader have not realized the great gulf which lies between the mental processes of the trained mathematician and those of the man in the street. they have not perceived that the lay reader must be personally conducted for a long distance from the vestibule of the temple of science before he comes to einstein, and that he cannot by any possibility make this journey unaided. the result has been to pitchfork the reader into the intricacies of the subject without adequate preparation. the present volume avoids this mistake with the utmost care. it avoids it, in fact, with such deliberation as to make it in order to say a word in explanation of what will at first glance seem an extraordinary arrangement of material. it was to be expected, doubtless, that this book would open with a brief statement of the genesis and the outcome of the einstein prize essay contest for the $ , prize offered by mr. eugene higgins. it was doubtless to be expected that, after this had been dismissed, the winning essay would be given the post of honor in advance of all other material bearing actually on the einstein theories. when the reader observes that this has not been done, he will by all means expect a word of explanation; and it is mainly for the purpose of giving this that we make these introductory remarks. the essays submitted in the contest, and in particular the comments of a few disappointed readers upon mr. bolton's prize essay, make quite plain what might have been anticipated--that in the small compass of , words it is not possible both to prepare the reader's mind for a discussion of relativity and to give a discussion that shall be adequate. mr. bolton himself, in replying to a protest that he had not done all this, has used the word "miracle"--we think it a well-advised one. no miracle was expected as a result of the contest, and none has been achieved. but in awarding the prize, the judges had to decide whether it was the best preliminary exposition or the best discussion that was wanted. they decided, and rightly we believe, that the award should go to an actual statement of what the einstein theories are and what they do, rather than to a mere introduction, however well conceived and well executed the latter might be. nevertheless, we should be closing our eyes to a very obvious fact if we did not recognize that, without something in the way of preparation, the general reader is not going to pursue mr. bolton's essay, or any other essay on this subject, with profit. it is in order the more forcefully to hold out inducements to him to subject himself to this preparation that we place at the head of the book the chapters designed to give it to him. chapter ii. is intended so to bring the mind of the reader into contact with certain philosophical problems presented to us by our experiences with the external world and our efforts to learn the facts about it, that he may approach the subject of relativity with an appreciation of the place it occupies as a phase of human thought and a pillar of the scientific structure. until the reader is aware of the existence of these problems and the directions taken by the efforts, successful and unsuccessful, to unravel them, he is not equipped to comprehend the doctrine of relativity at all; he is in much the same case as a child whose education had reached only the primer stage, if asked to read the masterpieces of literature. he lacks not alone the vocabulary, but equally the mental background on which the vocabulary is based. it will be noted that in this and the chapters immediately following it, the editor has supplied material freely. the obvious interpretation is that satisfactory material covering the desired ground was not found in any of the essays; for we are sure the scope and number of the credited excerpts will make it clear that all contributions were adequately scrutinized in search of available passages. this "inadequacy" of the competing essays has been severely commented upon by several correspondents, and the inference drawn that as a whole the offerings were not up to the mark. such a viewpoint is wholly unjust to the contestants. the essays which paid serious attention to the business of paving the way to relativity necessarily did so at the expense of completeness in the later paragraphs where specific explanation of the einstein theories was in order. mr. law, whose essay was by all means the best of those that gave much space to introductory remarks, found himself left with only words in which to tell what it was that he had been introducing. the majority of the contestants appear to have faced the same question as to subject matter which the judges faced, and to have reached the same decision. they accordingly devoted their attention toward the prize, rather than toward the production of an essay that would best supplement that of the winner. it is for this very reason that, in these preliminary chapters, so large a proportion of the material has had to be supplied by the editor; and this very circumstance is a tribute to the good judgment of the competitors, rather than ground for criticism of their work. the general introduction of chapter ii. out of the way, chapters iii. and iv. take up the business of leading the reader into the actual subject of relativity. the subject is here developed in what may be called the historical order--the order in which it took form in einstein's own mind. both in and outside the contest of which this book is the outcome, a majority of those who have written on relativity have followed this order, which is indeed a very natural one and one well calculated to give to the rather surprising assumptions of relativity a reasonableness which they might well appear to the lay mind to lack if laid down more arbitrarily. in these two chapters no effort is made to carry the argument beyond the formulation of the special principle of the relativity of uniform motion, but this principle is developed in considerably more detail than would be the case if it were left entirely to the competing essayists. the reason for this is again that we are dealing with a phase of the subject which is of subordinate importance so far as a complete statement of the general theory of relativity is concerned, but which is of the greatest significance in connection with the effort of the layman to acquire the proper preliminary orientation toward the larger subject. chapter v. goes back again to general ground. among the ideas which the competing essayists were forced to introduce into their text on a liberal scale is that of non-euclidean geometry. the entire formulation of the general theory of relativity is in fact an exercise in this. the essayists--good, bad and indifferent alike--were quite unanimous in their decision that this was one thing which the reader would have to assume the responsibility of acquiring for himself. certainly they were justified in this; for the editor has been able to explain what non-euclidean geometry is only by using up considerably more space than the contestants had for an entire essay. no effort has been made to set forth any of the details of any of the various non-euclidean geometries; it has simply been the aim to draw the dividing line between euclidean and non-euclidean, and to make the existence of the latter appear reasonable, so that when the essayists come to talk about it the reader will not feel hopelessly at sea. in other words, this is another case of providing the mental background, but on such a scale that it has seemed necessary to give a separate chapter to it. chapter vi. completes the preliminary course in the fundamentals of relativity by tying up together the findings of chapter v. and those of chapters iii. and iv. it represents more or less of a last-minute change of plan; for while it had been the editor's intent from the beginning to place the material of chapters ii.-v. in its present position, his preliminary impression would have been that the work of the present chapter vi. would be adequately done by the essayists themselves. his reading of the essays, however, convinced him that it had not so been done--that with the possible exception of mr. francis, the essayists did not make either a serious or a successful effort to show the organic connection between the special theory of relativity and the minkowski space-time structure, or the utter futility of trying to reconcile ourselves to the results of the former without employing the ideas of the latter. so chapter vi. was supplied to make good this deficiency, and to complete the mental equipment which the reader requires for his battle with the general theory. in laying down a set of general principles to govern the award of the prize, one of the first things considered by the judges was the relative importance of the special and the general theories. it was their opinion that no essay could possibly qualify for the prize which did not very distinctly give to the general theory the center of the stage; and that in fact discussion of the special theory was pertinent only so long as it contributed, in proportion to the space assigned it, to the attack upon the main subject. the same principle has been employed in selecting essays for complete or substantially complete reproduction in this volume. writers who dealt with the special theory in any other sense than as a preliminary step toward the general theory have been relegated to the introductory chapters, where such excerpts from their work have been used as were found usable. the distinction of publication under name and title is reserved for those who wrote consistently and specifically upon the larger subject--with the one exception of dr. russell, whose exposition of the special theory is so far the best of those submitted and at the same time so distinctive that we have concluded it will appear to better advantage by itself than as a part of chapters iii. and iv. following after mr. bolton's essay we have tried to arrange the various contributions, not at all in any order of merit, but in the order that will make connected reading of the book most nearly possible and profitable. each essay should be made easier of reading by the examination of those preceding it; at the same time each, by the choice of ground covered and by the emphasis on points not brought out sharply by its predecessors, should throw new light upon these predecessors. the reader will find that no two of the essays given thus in full duplicate or even come close to duplicating one another. they have of course been selected with this in view; each represents the best of several essays of substantially the same character. not all of them require comment here, but concerning some of them a word may well be said. mr. francis, we believe, has succeeded in packing more substance into his , words than any other competitor. mr. elliot has come closer than anybody else to really explaining relativity in terms familiar to everybody, without asking the reader to enlarge his vocabulary and with a minimum demand so far as enlarging his mental outlook is concerned. were it not for certain conspicuous defects, his essay would probably have taken the prize. in justice to the judges, we should state that we have taken the liberty of eliminating mr. elliot's concluding paragraph, which was the most objectionable feature of his essay. dr. dushman chose for his title the one which we adopted for this book. it became necessary, therefore, for us to find a new title for his essay; aside from this instance, the main titles appearing at the heads of the various complete essays are those of the authors. the subtitles have in practically every instance been supplied editorially. dr. pickering submitted two essays, one written from the viewpoint of the physicist, the other from that of the astronomer. to make each complete, he naturally found it necessary to duplicate between them certain introductory and general material. we have run the two essays together into a single narrative, with the elimination of this duplicated material; aside from this blue-penciling no alteration has been made in dr. pickering's text. this text however served as the basis of blue-penciling that of several other contestants, as indicated in the foot notes. for the reader who is qualified or who can qualify to understand it, dr. murnaghan's essay is perhaps the most illuminating of all. even the reader who does not understand it all will realize that its author brings to the subject a freshness of viewpoint and an originality of treatment which are rather lacking in some of the published essays, and which it will readily be understood were conspicuously lacking in a good many of the unpublished ones. dr. murnaghan of all the competitors has come closest to making a contribution to science as well as to the semi-popular literature of science. in the composite chapters, the brackets followed by reference numbers have been used as the most practicable means of identifying the various individual contributions. we believe that this part of the text can be read without allowing the frequent occurrence of these symbols to distract the eye. as to the references themselves, the asterisk marks the contributions of the editor. the numbers are those attached to the essays in order of and at the time of their receipt; it has been more convenient to use these than to assign consecutive numbers to the quoted essays. the several numbers identify passages from the essays of the following contestants: : frederick w. shurlock, derby, england. : l. l. whyte, cambridge, england. : prof. moritz schlick, university of rostock, germany. : c. e. rose, m.e., little rock, ark. : h. gartelmann, bremen, germany. : prof. joseph s. ames, johns hopkins university, baltimore. : james o. g. gibbons, east orange. n. j. : charles h. burr, philadelphia. : l. f. h. de miffonis. b.a., c.e., ottawa, canada. : charles a. brunn, kansas city. : j. elias fries, fellow a.i.e.e., birmingham, ala. : dean w. p. graham, syracuse university, syracuse, n. y. : rev. george thomas manley, london. : prof. j. a. schouten, delft, netherlands. : elwyn f. burrill, berkeley, cal. : dorothy burr, bryn mawr, pa. : c. w. kanolt, bureau of standards, washington. : robert stevenson, new york. : leopold schorsch, new york. : dr. m. c. mott-smith, los angeles, calif. : edward a. clarke, columbus, o. : edward a. partridge, philadelphia. : col. john millis, u. s. a., chicago. : george f. marsteller, detroit. : d. b. hall, cincinnati. : francis farquhar, york, pa. : dr. george de bothezat, dayton, o. : professor a. e. caswell, university of oregon, eugene, ore. : c. e. dimick, new london, conn. : earl r. evans, washington, d. c. : norman e. gilbert, dartmouth college, hanover, n. h. : a. d'abro. new york. : l. m. alexander, cincinnati. : kenneth w. reed, east cleveland, o. : prof. e. n. da c. andrade, ordnance college, woolwich, england. : professor andrew h. patterson, university of north carolina, chapel hill, n. c. : prof. arthur gordon webster, clark college, worcester, mass. : walter van b. roberts, princeton university, n. j. : paul m. batchelder, austin, tex. : prof. r. w. wood, johns hopkins university, baltimore. : e. p. fairbairn, m.c., b.sc., glasgow. : r. f. deimel, hoboken, n. j. : lieut. w. mark angus, u. s. n., philadelphia. : edward adams richardson, kansas city. : prof. william benjamin smith, tulane university, new orleans. : james rice, university of london, london. : william hemmenway pratt, lynn, mass. : r. bruce lindsay, new bedford, mass. : frank e. law, montclair, n. j. in addition to the specific credit given by these references for specifically quoted passages, the editor feels that he ought to acknowledge his general indebtedness to the competing essayists, collectively, for the many ideas which he has taken away from their text to clothe in his own words. this does not mean that the editor has undertaken generally to improve upon the language of the competitors, but merely that the reading of all their essays has given him many ideas of such complex origin that he could not assign credit if he would. table of contents i.--the einstein $ , prize: how the contest came to be held, and some of the details of its conduct. by the editor ii.--the world--and us: an introductory discussion of the philosophy of relativity, and of the mechanism of our contact with time and space. by various contributors and the editor iii.--the relativity of uniform motion: classical ideas on the subject; the ether and the apparent possibility of absolute motion; the michelson-morley experiment and the final negation of this possibility. by various contributors and the editor iv.--the special theory of relativity: what einstein's study of uniform motion tells us about time and space and the nature of the external reality. by various contributors and the editor v.--that parallel postulate: modern geometric methods; the dividing line between euclidean and non-euclidean; and the significance of the latter. by the editor vi.--the space-time continuum: minkowski's world of events, and the way in which it fits into einstein's structure. by the editor and a few contributors vii.--relativity: the winning essay in the contest for the eugene higgins $ , prize. by lyndon bolton, british patent office, london viii.--the new concepts of time and space: the essay in behalf of which the greatest number of dissenting opinions have been recorded. by montgomery francis, new york ix.--the principle of relativity: a statement of what it is all about, in ideas of one syllable. by hugh elliot, chislehurst, kent, england x.--space, time and gravitation: an outline of einstein's theory of general relativity. by w. de sitter, university of leyden xi.--the principle of general relativity: how einstein, to a degree never before equalled, isolates the external reality from the observer's contribution. by e. t. bell, university of seattle xii.--force vs. geometry: how einstein has substituted the second for the first in connection with the cause of gravitation. by saul dushman, schenectady xiii.--an introduction to relativity: a treatment in which the mathematical connections of einstein's work are brought out more strongly and more successfully than usual in a popular explanation. by harold t. davis, university of wisconsin xiv.--new concepts for old: what the world looks like after einstein has had his way with it. by john g. mchardy, commander r. n., london xv.--the new world: a universe in which geometry takes the place of physics, and curvature that of force. by george frederick hemens, m.c., b.sc., london xvi.--the quest of the absolute: modern developments in theoretical physics, and the climax supplied by einstein. by dr. francis d. murnaghan, johns hopkins university, baltimore xvii.--the physical side of relativity: the immediate contacts between einstein's theories and current physics and astronomy. by professor william h. pickering, harvard college observatory, mandeville, jamaica xviii.--the practical significance of relativity: the best discussion of the special theory among all the competing essays. by prof. henry norris russell, princeton university xix.--einstein's theory of relativity: a simple explanation of his postulates and their consequences. by t. royds, kodaikanal observatory, india xx.--einstein's theory of gravitation: the discussion of the general theory and its most important application, from the essay by prof. w. f. g. swann, university of minnesota, minneapolis xxi.--the equivalence hypothesis: the discussion of this, with its difficulties and the manner in which einstein has resolved them, from the essay by prof. e. n. da c. andrade, ordnance college, woolwich, england xxii.--the general theory: fragments of particular merit on this phase of the subject. by various contributors i. the einstein $ , prize how the contest came to be held, and some of the details of its conduct by the editor in january, , an anonymous donor who was interested in the spread of correct scientific ideas offered through the scientific american a prize of $ for the best essay explaining, in simple non-technical language, that paradise of mathematicians and bugaboo of plain ordinary folk--the fourth dimension. many essays were submitted in this competition, and in addition to that of the winner some twenty were adjudged worthy of ultimate publication. it was felt that the competition had added distinctly to the popular understanding of this significant subject; that it had done much to clear up popular misconception of just what the mathematician means when he talks of four or even more dimensions; and that it had therefore been as successful as it was unusual in character. in november, , the world was startled by the announcement from london that examination of the photographs taken during the total solar eclipse of may th had been concluded, and that predictions based upon the einstein theories of relativity had been verified. in the reaction from the long surfeit of war news an item of this sort was a thoroughly journalistic one. long cable dispatches were carried in the news columns all over the world; einstein and his theories were given a prominent place on the front pages day after day; leading scientists in great number were called upon to tell the public through the reportorial medium just what the excitement was all about, just in what way the classical scientific structure had been overthrown. instead of being a mere nine days' wonder, the einstein theories held their place in the public mind. the more serious periodicals devoted space to them. first and last, a very notable group of scientific men attempted to explain to the general reader the scope and content of einstein's system. these efforts, well considered as they were, could be no more than partially successful on account of the very radical character of the revisions which the relativity doctrine demands in our fundamental concepts. such revisions cannot be made in a day; the average person has not the viewpoint of the mathematician which permits a sudden and complete exchange of one set of fundamentals for another. but the whole subject had caught the popular attention so strongly, that even complete initial failure to discover what it was all about did not discourage the general reader from pursuing the matter with determination to come to some understanding of what had happened to newton and newtonian mechanics. the donor and the prize in may, , mr. eugene higgins, an american citizen long resident in paris, a liberal patron of the arts and sciences, and a lifelong friend of the scientific american and its proprietors, suggested that the success of the fourth dimension prize contest of had been so great that it might be desirable to offer another prize in similar fashion for the best popular essay on the einstein theories. he stated that if in the opinion of the scientific american these theories were of sufficient importance, and the probability of getting a good number of meritorious essays were sufficiently great, and the public need and desire for enlightenment were sufficiently present, he would feel inclined to offer such a prize, leaving the conduct of the contest to the scientific american as in the former event. it was the judgment of the editors of the scientific american that all these provisos should be met with an affirmative, and that mr. higgins accordingly could with propriety be encouraged to offer the prize. in his preliminary letter mr. higgins had suggested that in view of the apparent greater importance of the subject to be proposed for discussion by the contestants of , the prize offered should probably be more liberal than in the former instance. this view met with the approval of the editors as well; but they were totally unprepared for the receipt, late in june, of a cablegram from mr. higgins stating that he had decided to go ahead with the matter, and that he was forwarding a draft for $ , to represent the amount of the prize. such a sum, exceeding any award open to a professional man with the single exception of the nobel prize, for which he cannot specifically compete, fairly took the breath of the editors, and made it immediately clear that the contest would attract the widest attention, and that it should score the most conspicuous success. it also made it clear that the handling of the contest would be a more serious matter than had been anticipated. in spite of the fact that it would not for some time be possible to announce the identity of the judges, it was felt that the prospective contestants should have every opportunity for extensive preparation; so the contest was announced, and the rules governing it printed as far as they could be determined on such short shrift, in the scientific american for july , . several points of ambiguity had to be cleared up after this initial publication. in particular, it had been mr. higgins' suggestion that in the very probable event of the judges' inability to agree upon the winning essay, the prize might, at their discretion, be divided between the contributors of the best two essays. this condition was actually printed in the first announcement, but the post office department insisted upon its withdrawal, on the ground that with it in force the contestant would not know whether he were competing for $ , or for $ , , and that this would introduce the "element of chance" which alone was necessary, under the federal statutes, to make the contest a lottery. so this provision was replaced by one to the effect that in the event the judges were not able to agree, the einstein editor should cast the deciding vote between the essays respectively favored by them. the announcement attracted the widest attention, and was copied in newspapers and magazines all over the world. inquiries poured in from all quarters, and the einstein editor found it almost impossible to keep himself supplied with proofs of the conditions and rules to mail in response to these inquiries. it was immediately clear that there was going to be a large number of essays submitted, and that many distinguished names would be listed among the competitors. the judges in the scientific american for september , announcement was carried in the following words: "we are assured with complete certainty that the competition for the five-thousand-dollar prize will be very keen, and that many essays will be submitted which, if they bore the names of their authors, would pass anywhere as authoritative statements. the judges will confront a task of extraordinary difficulty in the effort to determine which of these efforts is the best; and we believe the difficulties are such that multiplication of judges would merely multiply the obstacles to an agreement. it is altogether likely that the initial impressions of two or three or five judges would incline toward two or three or five essays, and that any final decision would be attainable only after much consultation and discussion. it seems to us that by making the committee as small as possible while still preserving the necessary feature that its decision represent a consensus, we shall simplify both the mental and the physical problem of coming to an agreement. we believe that the award should if possible represent a unanimous decision, without any minority report, and that such a requirement is far more likely to be met among two men than among three or five. at the same time, the bringing together of two men and the details of general administration of their work together are far simpler than if there were three or five. so we have finally decided to have but two judges, and in this we have the endorsement of all the competent opinion that we have consulted. "the gentlemen who have consented to act as judges are professors leigh page and edwin plimpton adams, of the departments of physics of yale and princeton universities, respectively. both are of the younger generation of physicists that has paid special attention to those phases of mathematics and physics involved in the einstein theories, and both have paid special attention to these theories themselves. we are gratified to be able to put forward as judges two men so eminently qualified to act. we feel that we may here appropriately quote professor page, who says in his acceptance: 'as the large prize offers a great inducement, i had thought of entering the contest. however i realize that not many people in this country have made a considerable study of einstein's theory, and if all who have should enter the contest, it would be difficult to secure suitable judges.' without any desire to put the gentleman in the position of pleading for himself, we think this suggests very well the extent to which the scientific american, the contestants, and the public at large, are indebted to professors page and adams for their willingness to serve in the difficult capacity of judges." it might appropriately have been added to this announcement that it was altogether to the credit of science and the scientific spirit that the first two gentlemen approached with the invitation to act as judges were willing to forego their prospects as contestants in order thus to contribute to the success of the contest. three thousand words of the conditions, the one which evoked most comment was that stating the word limit. this limit was decided upon after the most careful discussion of the possibilities of the situation. it was not imagined for a moment that any contestant would succeed in getting within , words a complete discussion of all aspects of the special and the general theories of relativity. it was however felt that for popular reading a single essay should not be much if any longer than this. moreover, i will say quite frankly that we should never have encouraged mr. higgins to offer such a prize if we had supposed that the winning essay was the only thing of value that would come from the contest, or if we had not expected to find in many of the other essays material which would be altogether deserving of the light. from the beginning we had in view the present volume, and the severe restriction in length was deliberately imposed for the purpose of forcing every contestant to stick to what he considered the most significant viewpoints, and to give his best skill to displaying the theories of einstein to the utmost advantage from these viewpoints. we felt that divergent viewpoints would be more advantageously treated in this manner than if we gave each contestant enough space to discuss the subject from all sides; and that the award of the prize to the essay which, among other requirements, seemed to the judges to embody the best choice of material, would greatly simplify the working of the contest without effecting any injustice against those contestants who displayed with equal skill less happily chosen material. perhaps on this point i may again quote with profit the editorial page of the scientific american: "an essay of three thousand words is not long enough to lose a reader more than once; if it does lose him it is a failure, and if it doesn't it is a competitor that will go into the final elimination trials for the prize. if we can present, as a result of the contest, six or a dozen essays of this length that will not lose the lay reader at all, we shall have produced something amply worth the expenditure of mr. higgins' money and our time. for such a number of essays of such character will of necessity present many different aspects of the einstein theories, and in many different ways, and in doing so will contribute greatly to the popular enlightenment. "really the significant part of what has already appeared is not the part that is intelligible, but rather the part that, being unintelligible, casts the shadow of doubt and suspicion on the whole. the successful competitor for the prize and his close contestants will have written essays that, without any claim to completeness, will emphasize what seems to each author the big outstanding feature; and every one of them will be intelligible. together they will in all probability be reasonably complete, and will retain the individual characteristic of intelligibility. they will approach the various parts of the field from various directions--we could fill this page with suggestions as to how the one item of the four-dimensional character of einstein's time-space might be set forth for the general reader. and when a man must say in three thousand words as much as he can of what eminent scientists have said in whole volumes--well, the result in some cases will be sheer failure, and in others a product of the first water. the best of the essays will shine through intelligent selection of what is to be said, and brilliant success in saying it. it is to get a group of essays of this character, not to get the single essay which will earn the palm, that the prize is offered." the competing essays at all times after the first announcement the einstein editor had a heavy correspondence; but the first real evidence that the contest was under way came with the arrival of the first essay, which wandered into our office in the middle of september. about a week later they began to filter in at the rate of one or two per day--mostly from foreign contestants who were taking no chances on the mails. heavy returns did not commence until about ten days before the closing date. the great avalanche, however, was reserved for the morning of monday, november st. here we had the benefit of three days' mail; there were about essays. among those which were thrown out on the ground of lateness the honors should no doubt go to the man who mailed his offering in the hague on october st. essays were received in greater quantity from germany than from any other foreign country, doubtless because of the staggering value of $ , when converted into marks at late rates. england stood next on the list; and one or more essays were received from austria, czechoslovakia, jugoslavia, france, switzerland, the netherlands, denmark, italy, chile, cuba, mexico, india, jamaica, south africa and the fiji islands. canada, of course, contributed her fair share; and few of our own states were missing on the roll-call. the general level of english composition among the essays from non-english-speaking sources was about what might have been expected. a man may have a thorough utilitarian knowledge of a foreign tongue, but when he attempts intensive literary competition with a man who was brought up in that tongue he is at a disadvantage. we read french and german with ease and spanish and italian without too much difficulty, ourselves; we should never undertake serious writing in any of these languages. not many of the foreign contributions, of course, were as ludicrous as the one we quote to some extent in our concluding chapter, but most of them were distinctly below par as literary compositions. drs. de sitter and schlick were the notable exceptions to this; both showed the ability to compete on a footing of absolute equality with the best of the native product. we dare say it was a foregone conclusion that many essays should have been over the limit, and that a few should have been over it to the point of absurdity. the winning essay contains , words, plus or minus a reasonable allowance for error in counting; that it should come so far from being on the ragged edge should be sufficient answer to those who protested against the severity of the limitation. one inquirer, by the way, wanted to know if , words was not a misprint for , . another contestant suggested that instead of disqualifying any essay that was over the line, we amputate the superfluous words at the end. this was a plausible enough suggestion, since any essay able to compete after such amputation must necessarily have been one of extreme worth; but fortunately we did not have to decide whether we should follow the scheme. perhaps twenty of the essays submitted were so seriously in excess of the limit that it was not even necessary to count their words in detail; most of these offenders ran to , words or thereabouts, and one--a good one, too, from which we use a good deal of material in this volume--actually had , . on the other extreme were a few competitors who seemed to think that the shortest essay was necessarily the best, and who tried to dismiss the subject with or , words. by a curious trick of chance there were submitted in competition for the prize exactly essays. of course a few of these did not require serious consideration--this is inevitable in a contest of such magnitude. but after excluding all the essays that were admittedly not about the einstein theories at all, and all those whose english was so execrable as to make them quite out of the question, and all those which took the subject so lightly as not to write reasonably close to the limit of , words, and all those which were given over to explanation of the manner in which einstein's theories verify those of the writer, and all those in which the writer attempted to substitute his own cosmic scheme for einstein's--after all this, there remained some essays which were serious efforts to explain in simple terms the nature and content and consequences of special and general relativity. looking for the winner the einstein editor was in sufficiently close touch with the details of the adjudication of the essays to have every realization of the difficulty of this work. the caliber of the essays submitted was on the whole high. there were many which would have been well worthy of the prize in the absence of others that were distinctly better--many which it was not possible to eliminate on the ground of specific faults, and which could only be adjudged "not the best" by detailed comparison with specific other essays. it was this detailed comparison which took time, and which so delayed the award that we were not able to publish the winning essay any sooner than february th. especially difficult was this process of elimination after the number of surviving essays had been reduced to twenty or less. the advantages of plan possessed by one essay had to be weighed against those of execution exhibited in another. a certain essay had to be critically compared with another so like it in plan that the two might have been written from a common outline, and at the same time with a third as unlike it in scope and content as day and night. and all the time there was present in the background the consciousness that a prize of $ , hung upon the decision to be reached. for anyone who regards this as an easy task we have no worse wish than that he may some day have to attack a similar one. we had anticipated that the bulk of the superior essays would be among those received during the last day or two of the contest; for we felt that the men best equipped to attack the subject would be the most impressed with its seriousness. here we were quite off the track. the seventeen essays which withstood most stubbornly the judges' efforts at elimination were, in order of receipt, numbers , , , , , , , , , , , , , , , , : a fairly even distribution. the winner was the nd essay received. the judges held their final meeting in the editorial office on january , . the four essays which were before the committee at the start of the session were speedily cut to three, and then to two; and after an all-day session the judges found themselves conscientiously able to agree on one of these as the best. this unanimity was especially gratifying, the more so since it by no means was to be confidently expected, on a priori grounds, that it would be possible of attainment. even the einstein editor, who might have been called upon for a final decision but wasn't, can hardly be classed as a dissenter; for with some slight mental reservations in favor of the essay by mr. francis which did not enter the judges' final discussion at all, and which he rather suspects appeals more to his personal taste than to his soundest judgment, he is entirely in accord with the verdict rendered. the fact that the prize went to england was no surprise to those acquainted with the history of einstein's theories. the special theory, promulgated fifteen years ago, received its fair share of attention from mathematicians all over the world, and is doubtless as well known and as fully appreciated here as elsewhere. but it has never been elevated to a position of any great importance in mathematical theory, simply because of itself, in the absence of its extension to the general case, it deserves little importance. it is merely an interesting bit of abstract speculation. the general theory was put out by einstein in finished form during the war. owing to the scientific moratorium, his paper, and hence a clear understanding of the new methods and results and of the sweeping consequences if the general theory should prevail, did not attain general circulation outside germany until some time in or even later. had it not been for eddington it is doubtful that the british astronomers would have realized that the eclipse expeditions were of particular consequence. therefore at the time of these expeditions, and even as late as the november announcement of the findings, the general body of scientific men in america had not adequately realized the immense distinction between the special and the general theories, had not adequately appreciated that the latter led to distinctive consequences of any import, and we fear in many cases had not even realized explicitly that the deflection of light and the behavior of mercury were matters strictly of the general and in no sense of the special theory. certainly when the american newspapers were searching frantically for somebody to interpret to their public the great stir made by the british announcement that einstein's predictions had been verified, they found no one to do this decently; nor were our magazines much more successful in spite of the greater time they had to devote to the search. in a word, there is not the slightest room for doubt that american science was in large measure caught asleep at the switch--perhaps for no reason within its control; and that american writers were in no such favorable case to write convincingly on the subject as were their british and continental contemporaries. so it was quite in accord with what might have been expected to find, on opening the identifying envelopes, that not alone the winning essay, but its two most immediate rivals, come from members of that school of british thought which had been in contact with the einstein theories in their entirety for two years longer than the average american of equal competence. this riper familiarity with the subject was bound to yield riper fruit. indeed, had it not been for the handicap of writing in a strange language, it is reasonable to assume that the scientists of germany would have made a showing superior to that of either americans or british--and for the same reason that britain showed to better advantage than america. the winner of the prize mr. bolton, the winner of the big prize, we suppose may fairly be referred to as unknown in a strict scientific sense. indeed, at the time of the publication of his essay in the scientific american nothing could be learned about him on the american side of the water beyond the bare facts that he was not a young man, and that he had for a good many years occupied a position of rank in the british patent office. (it will be recalled that einstein himself was in the swiss patent office for some time.) in response to the request of the scientific american for a brief biographical sketch that would serve to introduce him better to our readers, mr. bolton supplied such a concise and apparently such a characteristic statement that we can do no better than quote it verbatim. "i was born in dublin in , but i have lived in england since . my family belonged to the landed gentry class, but i owe nothing to wealth or position. i was in fact put through school and college on an income which a workman would despise nowadays. after attending sundry small schools, i entered clifton college in . my career there was checkered, but it ended well. i was always fairly good at natural science and very fond of all sorts of mechanical things. i was an honest worker but no use at classics, and as i did practically nothing else for the first four years at clifton, i came to consider myself something of a dunce. but a big public school is a little world. everyone gets an opportunity, often seemingly by accident, and it is up to him to take it. mine did not come till i was nearly . as i was intended for the engineering profession, i was sent to the military side of the school in order to learn some mathematics, at which subject i was then considered very weak. this was certainly true, as at that time i barely knew how to solve a quadratic, i was only about halfway through the third book of euclid, and i knew no trigonometry. but the teaching was inspiring, and i took readily to mathematics. one day it came out that i had been making quite a good start with the differential calculus on my own without telling anybody. after that all was well. i left clifton in with a school exhibition and a mathematical scholarship at clare college, cambridge. "after taking my degree in as a wrangler, i taught science and mathematics at wellington college, but i was attracted by what i had heard of the patent office and i entered it through open competition in . during my official career i have been one of the comptroller's private secretaries and i am now a senior examiner. during the war i was attached to the inventions department of the ministry of munitions, where my work related mainly to anti-aircraft gunnery. i have contributed, and am still contributing to official publications on the subject. "i have written a fair number of essays on various subjects, even on literature, but my only extra-official publications relate to stereoscopic photography. i read a paper on this subject before the royal photographic society in which was favorably noticed by dr. von rohr of messrs. zeiss of jena. i have also written in the amateur photographer. "i have been fairly successful at athletics, and i am a member of the leander club." that mr. bolton did not take the prize through default of serious competition should be plain to any reader who examines the text from competing essays which is to be found in this volume. the reference list of these competitors, too, supplemented by the names that appear at the heads of complete essays, shows a notable array of distinguished personalities, and i could mention perhaps a dozen more very well known men of science whose excellent essays have seemed a trifle too advanced for our immediate use, but to whom i am under a good deal of obligation for some of the ideas which i have attempted to clothe in my own language. before leaving the subject, we wish to say here a word of appreciation for the manner in which the judges have discharged their duties. the reader will have difficulty in realizing what it means to read such a number of essays on such a subject. we were fortunate beyond all expectation in finding judges who combined a thorough scientific grasp of the mathematical and physical and philosophical aspects of the matter with an extremely human viewpoint which precluded any possibility of an award to an essay that was not properly a popular discussion, and with a willingness to go to meet each other's opinions that is rare, even among those with less ground for confidence in their own views than is possessed by drs. page and adams. ii. the world--and us an introductory discussion of the philosophy of relativity, and of the mechanism of our contact with time and space by various contributors and the editor from a time beyond the dawn of history, mankind has been seeking to explain the universe. at first the effort did not concern itself further probably than to make a supposition as to what were the causes of the various phenomena presented to the senses. as knowledge increased, first by observation and later by experiment also, the ideas as to these causes passed progressively through three stages--the theological (the causes were thought to be spirits or gods); the metaphysical (the causes were thought in this secondary or intermediate stage to be some inherent, animating, energizing principles); and the scientific (the causes were finally thought of as simply mechanical, chemical, and magneto-electrical attractions and repulsions, qualities or characteristics of matter itself, or of the thing of which matter is itself composed.) with increase of knowledge, and along with the inquiry as to the nature of causes, there arose an inquiry also as to what reality was. what was the essential nature of the stuff of which the universe was made, what was matter, what were things in themselves, what were the noumena (the realities), lying back of the phenomena (the appearances)? gradually ideas explaining motion, force, and energy were developed. at the same time inquiry was made as to the nature of man, the working of his mind, the nature of thought, the relation of his concepts (ideas) to his perceptions (knowledge gained through the sense) and the relations of both to the noumena (realities).] [the general direction taken by this inquiry has been that of a conflict between two schools of thought which we may characterize as those of absolutism and of relativism.]* [the ancient greek philosophers believed that they could tap a source of knowledge pure and absolute by sitting down in a chair and reasoning about the nature of time and space, and the mechanism of the physical world.] [they maintained that the mind holds in its own right certain concepts than which nothing is more fundamental. they considered it proper to conceive of time and space and matter and the other things presented to their senses by the world as having a real existence in the mind, regardless of whether any external reality could be identified with the concept as ultimately put forth. they could even dispute with significance the qualities which were to be ascribed to this abstract conceptual time and space and matter. all this was done without reference to the external reality, often in defiance of that reality. the mind could picture the world as it ought to be; if the recalcitrant facts refused to fit into the picture, so much the worse for them. we all have heard the tale of how generation after generation of greek philosophers disputed learnedly why and how it was that a live fish could be added to a brimming pail of water without raising the level of the fluid or increasing the weight; until one day some common person conceived the troublesome idea of trying it out experimentally to learn whether it were so--and found that it was not. true or false, the anecdote admirably illustrates the subordinate place which the externals held in the absolutist system of greek thought.]* [under this system a single observer is competent to examine a single phenomenon, and to write down the absolute law of nature by referring the results to his innate ideas of absolute qualities and states. the root of the word absolute signifies "taking away," and in its philosophical sense the word implies the ability of the mind to subtract away the properties or qualities from things, and to consider these abstract qualities detached from the things; for example, to take away the coldness from ice, and to consider pure or abstract coldness apart from anything that is cold; or to take away motion from a moving body, and to consider pure motion apart from anything that moves. this assumed power is based upon the socratic theory of innate ideas. according to this theory the mind is endowed by nature with the absolute ideas of hardness, coldness, roundness, equality, motion, and all other absolute qualities and states, and so does not have to learn them. thus a socratic philosopher could discuss pure or absolute being, absolute space and absolute time.] getting away from the greek ideas [this greek mode of thought persisted into the late middle ages, at which time it was still altogether in order to dispose of a troublesome fact of the external world by quoting aristotle against it. during the renaissance, which intellectually at least marks the transition from ancient to modern, there came into being another type of absolutism, equally extreme, equally arbitrary, equally unjustified. the revolt against the mental slavery to greek ideas carried the pendulum too far to the other side, and early modern science as a consequence is disfigured by what we must now recognize as gross materialism. the human mind was relegated to the position of a mere innocent bystander. the external reality was everything, and aside from his function as a recorder the observer did not in the least matter. the whole aim of science was to isolate and classify the elusive external fact. the rôle of the observer was in every possible way minimized. it was of course his duty to get the facts right, but so far as any contribution to these was concerned he did not count--he was definitely disqualified. he really played the part of an intruder; from his position outside the phenomena he was searching for the absolute truth about these phenomena. the only difference between his viewpoint and that of aristotle was that the latter looked entirely inside himself for the elusive "truth," while the "classical" scientist, as we call him now, looked for it entirely outside himself. let me illustrate the difference between the two viewpoints which i have discussed, and the third one which i am about to outline, by another concrete instance. the greeks, and the medievals as well, were fond of discussing a question which embodies the whole of what i have been saying. this question involved, on the part of one who attempted to answer it, a choice between the observer and the external world as the seat of reality. it was put in many forms; a familiar one is the following: "if the wind blew down a great tree at a time and place where there was no conscious being to hear, would there be any noise?" the greek had to answer this question in the negative because to him the noise was entirely a phenomenon of the listener. the classical scientist had to answer it in the affirmative because to him the noise was entirely a phenomenon of the tree and the air and the ground. today we answer it in the negative, but for a very different reason from that which swayed the greek. we believe that the noise is a joint phenomenon of the observer and the externals, so that in the absence of either it must fail to take existence. we believe there are sound waves produced, and all that; but what of it? there is no noise in the presence of the falling tree and the absence of the observer, any more than there would be in the presence of the observer and the absence of the tree and the wind; the noise, a joint phenomenon of the observer and the externals, exists only in their joint presence. relativism and reality this is the viewpoint of relativism. the statue is golden for one observer and silver to the other. the sun is rising here and setting in another part of the world. it is raining here and clear in chicago. the observer in delft hears the bombardment of antwerp and the observer in london does not. if they were to be consistent, both the greek and the medieval-modern absolutist would have to dispute whether the statue were "really" golden or silver, whether the sun were "really" rising or setting, whether the weather were "really" fair or foul, whether the bombardment were "really" accompanied by loud noises or not; and on each of these questions they would have to come to an agreement or confess their methods inadequate. but to the relativist the answer is simple--whether this or that be true depends upon the observer. in simple cases we understand this full well, as we have always realized it. in less simple cases we recognize it less easily or not at all, so that some of our thought is absolutist in its tendencies while the rest is relativistic. einstein is the first ever to realize this fully--or if not this, then the first ever to realize it so fully as to be moved toward a studied effort to free human thought from the mixture of relativism and absolutism and make it consistently the one or the other. this brings it about that the observed fact occupies a position of unexpected significance. for when we discuss matters of physical science under a strictly relativistic philosophy, we must put away as metaphysical everything that smacks of a "reality" partly concealed behind our observations. we must focus attention upon the reports of our senses and of the instruments that supplement them. these observations, which join our perceptions to their external objects, afford us our only objective manifestations; them we must accept as final--subject always to such correction as more refined observations may suggest. the question whether a "true" length or area or mass or velocity or duration or temperature exists back of the numerical determination, or in the presence of a determination that is subject to correction, or in the absence of any determination at all, is a metaphysical one and one that the physicist must not ask. length, area, mass, velocity, duration, temperature--none of these has any meaning other than the number obtained by measurement.]* [if several different determinations are checked over and no error can be found in any of them, the fault must lie not with the observers but with the object, which we must conclude presents different values to different observers.] [we are after all accustomed to this viewpoint; we do not demand that pittsburgh shall present the same distance from new york and from philadelphia, or that the new yorker and the philadelphian come to any agreement as to the "real" distance of pittsburgh. the distance of pittsburgh depends upon the position of the observer. nor do we demand that the man who locates the magnetic pole in one spot in and in another in come to a decision as to where it "really" is; we accept his statement that its position depends upon the time of the observation. what this really means is that the distance to pittsburgh and the position of the magnetic pole are joint properties of the observer and the observed--relations between them, as we might put it. this is obvious enough in the case of the distance of pittsburgh; it is hardly so obvious in the case of the position of the magnetic pole, varying with the lapse of time. but if we reflect that the observation of and that of were both valid, and both represented the true position of the pole for the observer of the date in question, we must see that this is the only explanation that shows us the way out. i do not wish to speak too definitely of the einstein theories in these introductory remarks, and so shall refrain from mentioning explicitly in this place the situation which they bring up and upon which what i have just said has direct bearing. it will be recognized when it arises. what must be pointed out here, however, is that we are putting the thing which the scientist calls the "observed value" on a footing of vastly greater consequence than we should have been willing offhand to concede to it. so far as any single observer is concerned, his own best observed values are themselves the external world; he cannot properly go behind the conditions surrounding his observations and speak of a real external world beyond these observations. any world which he may think of as so existing is purely a conceptual world, one which for some reason he infers to exist behind the deceptive observations. provided he makes this reservation he is quite privileged to speculate about this concealed world, to bestow upon it any characteristics that he pleases; but it can have no real existence for him until he becomes able to observe it. the only reality he knows is the one he can directly observe. laws of nature the observations which we have been discussing, and which we have been trying to endow with characteristics of "reality" which they are frequently not realized to possess, are the raw material of physical science. the finished product is the result of bringing together a large number of these observations.]* [the whole underlying thought behind the making of observations, in fact, is to correlate as many as possible of them, to obtain some generalization, and finally to express this in some simple mathematical form. this formulation is then called a "law of nature."] [much confusion exists because of a misunderstanding in the lay mind of what is meant by a "law of nature." it is perhaps not a well chosen term. one is accustomed to associate the word law with the idea of necessity or compulsion. in the realm of nature the term carries no such meaning. the laws of nature are man's imperfect attempts to explain natural phenomena; they are not inherent in matter and the universe, not an iron bar of necessity running through worlds, systems and suns. laws of nature are little more than working hypotheses, subject to change or alteration or enlargement or even abandonment, as man's vision widens and deepens. no sanctity attaches to them, and if any one, or all, of them fail to account for any part, or all, of the phenomena of the universe, then it or they must be supplemented or abandoned.] [the test of one of these laws is that it can be shown to include all the related phenomena hitherto known and that it enables us to predict new phenomena which can then be verified. if new facts are discovered that are not in agreement with one of these generalized statements, the assumptions on which the latter is based are examined, those which are not in accordance with the new facts are given up, and the statement is modified so as to include the new facts.] [and if one remembers that the laws of physics were formerly based on a range of observations much narrower than at present available, it seems natural that in the light of this widening knowledge one law or another may be seen to be narrow and insufficient. new theories and laws do not necessarily disprove old ones, but explain certain discrepancies in them and penetrate more deeply into their underlying principles, thereby broadening our ideas of the universe. to follow the new reasoning we must rid ourselves of the prejudice behind the old, not because it is wrong but because it is insufficient. the universe will not be distorted to fit our rules, but will teach us the rules of existence.] [always, however, we must guard against the too easy error of attributing to these rules anything like absolute truth.]* [the modern scientist has attained a very business-like point of view toward his "laws of nature." to him a law is fundamentally nothing but a short-hand way of expressing the results of a large number of experiments in a single statement. and it is important to remember that this mere shortening of the description of a lot of diverse occurrences is by no means any real explanation of how and why they happened. in other words, the aim of science is not ultimately to explain but only to discover the relations that hold good among physical quantities and to embody all these relations in as few and as simple physical laws as possible.] [this is inherently the method of relativism.]* [under it a set of phenomena is observed. there are two or many observers, and they write down their several findings. these are reviewed by a final observer or judge, who strains out the bias due to the different viewpoints of the original observers. he then writes down, not any absolute law of nature governing the observed phenomena, but a law as general as possible expressing their interrelations.] [and through this procedure modern science and philosophy reveal with increasing emphasis that we superimpose our human qualities on external nature to such an extent that] [we have at once the strongest practical justification, in addition to the arguments of reason, for our insistence that the contact between objective and subjective represented by the observation is the only thing which we shall ever be able to recognize as real. we may indulge in abstract metaphysical speculation to our heart's content, if we be metaphysically inclined; we may not attempt to impose the dicta of metaphysics upon the physical scientist.]* concepts and realities [from the inquiry and criticism which have gone on for centuries has emerged the following present-day attitude of mind toward the sum total of our knowledge. the conceptual universe in our minds in some mysterious way parallels the real universe, but is totally unlike it. our conceptions (ideas) of matter, molecules, atoms, corpuscles, electrons, the ether, motion, force, energy, space, and time stand in the same or similar relation to reality as the x's and y's of the mathematician do to the entities of his problem. matter, molecules, atoms, corpuscles, electrons, the ether, motion, force, energy, space, and time do not exist actually and really as we conceive them, nor do they have actually and really the qualities and characteristics with which we endow them. the concepts are simply representations of things outside ourselves; things which, while real, have an essential nature not known to us. matter, molecules, atoms, corpuscles, electrons, the ether, motion, force, energy, space and time are merely devices, symbols, which enable us to reason about reality. they are parts of a conceptual mechanism in our minds which operates, or enables our minds to operate, in the same sequence of events as the sequence of phenomena in the external universe, so that when we perceive by our senses a group of phenomena in the external universe, we can reason out what result will flow from the interaction of the realities involved, and thus predict what the situation will be at a given stage in the sequence. but while our conceptual universe has thus a mechanical aspect, we do not regard the real universe as mechanical in its nature.] [this may be illustrated by a little story. entering his friend's house, a gentleman is seized unawares from behind. he turns his head but sees nothing. his hat and coat are removed and deposited in their proper places by some invisible agent, seats and tables and refreshments appear in due time where they are required, all without any apparent cause. the visitor shivers with horror and asks his host for an explanation. he is then told that the ideas "order" and "regularity" are at work, and that it is they who acquit themselves so well of their tasks. these ideas cannot be seen nor felt nor seized nor weighed; they reveal their existence only by their thoughtful care for the welfare of mankind. i think the guest, coming home, will relate that his friend's house is haunted. the ghosts may be kind, benevolent, even useful; yet ghosts they are. now in newtonian mechanics, absolute space and absolute time and force and inertia and all the other apparatus, altogether imperceptible, appearing only at the proper time to make possible a proper building up of the theory, play the same mysterious part as the ideas "order" and "regularity" in my story. classical mechanics is haunted.] [as a matter of fact, we realize this and do not allow ourselves to be imposed upon with regard to the true nature of these agencies.]* [we use a mechanistic terminology and a mechanistic mode of reasoning only because we have found by experience that they facilitate our reasoning. they are the tools which we find produce results. they are adapted to our minds, but perhaps it would be better to say that our minds are so constructed as to render our conceptual universe necessarily mechanical in its aspect in order that our minds may reason at all. two things antithetic are involved--subject (our perceiving mind which builds up concepts) and object (the external reality); and having neither complete nor absolute knowledge of either, we cannot affirm which is more truly to be said to be mechanistic in its nature, though we may suspect that really neither is. we no longer think of cause and effect as dictated by inherent necessity, we simply regard them as sequences in the routine of our sense-impressions of phenomena. in a word, we have at length grasped the idea that our notions of reality, at present at least, whatever they may become ultimately, are not absolute, but simply relative. we see, too, that we do not explain the universe, but only describe our perceptions of its contents. the so-called laws of nature are simply statements of formulæ which resume or sum up the relationships and sequences of phenomena. our effort is constantly to find formulæ which will describe the widest possible range of phenomena. as our knowledge increases, that is, as we perceive new phenomena, our laws or formulæ break down, that is, they fail to afford a description in brief terms of all of our perceptions. it is not that the old laws are untrue, but simply that they are not comprehensive enough to include all of our perceptions. the old laws are often particular or limiting instances of the new laws.] [from what we have said of the reality of observations it follows that we must support that school of psychology, and the parallel school of philosophy, which hold that concepts originate in perceptions. but this does not impose so strong a restriction upon conceptions as might appear. the elements of all our concepts do come to us from outside; we manufacture nothing out of whole cloth. but when perception has supplied a sufficient volume of raw material, we may group its elements in ways foreign to actual occurrence in the perceptual world, and in so doing get conceptual results so entirely different from what we have consciously perceived that we are strongly tempted to look upon them as having certainly been manufactured in our minds without reference to the externals. of even more significance is our ability to abstract from concrete objects and concrete incidents the essential features which make them alike and different. but unlike the greeks, we see that our concept of coldness is not something with which we were endowed from the beginning, but merely an abstraction from concrete experiences with concrete objects that have been cold. the concepts of space and time when we have formed the abstract ideas of coldness and warmth, and have had experience indicating that the occurrence of these properties varies in degree, we are in a position to form the secondary abstract notion covered by the word "temperature." when we have formed the abstract ideas of size and position and separation, we are similarly in a position to form a secondary abstraction to which we give the name "space." not quite so easy to trace to its definite source but none the less clearly an abstraction based on experience, is our idea of what we call "time." none of us are deceived as to the reality of these abstractions.]* [we do not regard space as real in the sense that we regard a chair as real; it is merely an abstract idea convenient for the location of material objects like the chair.] [nor do we regard time as real in this sense. things occupy space, events occupy time; space and time themselves we realize are immaterial and unreal; space does not exist and time does not happen in the same sense that material objects exist and events occur. but we find it absolutely necessary to have, among the mental machinery mentioned above as the apparatus by aid of which we keep track of the external world, these vessels for that world to exist in and move in. space and time, then, are concepts.]* [it is not strange, however, that when confronted with the vast and bewildering complexity of the universe and the difficulty of keeping separate and distinct in our minds our perceptions and conceptions, we should at times and as respects certain things project our conceptions illegitimately into the perpetual universe and mistake them for perceptions. the most notable example perhaps of this projection has occurred in the very case of space and time, most fundamental of all of our concepts. we got to think of these as absolute, as independent of each other and of all other things, and as always existing and continuing to exist whether or not we or anything else existed--space as a three-dimensional, uniform continuum, having the same properties in all directions; time as a one-dimensional, irreversible continuum, flowing in one direction. it is difficult to get back to the idea that space and time so described and defined are concepts merely, for the idea of their absolute existence is ingrained in us as the result probably of long ancestral experience.] [newton's definitions of course represent the classical idea of time and space. he tells us that "absolute, true and mathematical time flows in virtue of its own nature, uniformly and without reference to any external object;" and that "absolute space, by virtue of its own nature and without reference to any external object, always remains the same and is immovable." of course from modern standpoints it is absurd to call either of these pronouncements a definition; but they represent about as well as any words can the ideas which newton had about time and space, and they make it clear enough that he regarded both as having real existence in the external world. if space and time are to be the vessels of our universe, and if the only thing that really matters is measured results, it is plain enough that we must have, from the very beginning, means of measuring space and time. whether we believe space and time to have real existence or not, it is obvious that we can measure neither directly. we shall have to measure space by measuring from one material object to another; we shall have to measure time by some similar convention based on events. we shall later have something further to say about the measurement of time; for the present we need only point out that]* [newtonian time is measured independently of space; and the existence is presupposed of a suitable timekeeper.] [the space of galileo and newton was conceived of as empty, except in so far as certain parts of it were occupied by matter. positions of bodies in this space were in general determined by reference to] [a "coordinate system" of some kind. this is again something that demands a certain amount of discussion. the reference frame for space the mathematician, following the lead of the great french all-around genius, descartes, shows us very clearly how to set up, for the measurement of space, the framework known as the cartesian coordinate system. the person of most ordinary mathematical attainments will realize that to locate a point in a plane we must have two measurements; and we could probably show this person, without too serious difficulty, that we can locate a point in any surface by two measurements. an example of this is the location of points on the earth's surface by means of their latitude and longitude. it is equally clear that if we add a third dimension and attempt to locate points in space, we must add a third measurement. in the case of points on the earth's surface, this might be the elevation above sea level, which would define the point not as part of the spherical surface of the earth but as part of the solid sphere. or we may fall back on dr. slosson's suggestion that in order to define completely the position of his laboratory, we must make a statement about broadway, and one about th street, and one telling how many flights of stairs there are to climb. in any event, it should be clear enough that the complete definition of a point in space calls for three measurements. the mathematician formulates all this with the utmost precision. he asks us to]* [pick out any point whatever in space and call it $o$. we then draw or conceive to be drawn through this point three mutually perpendicular lines called coordinate axes, which we may designate $ox$, $oy$ and $oz$, respectively. finally, we consider the three planes also mutually perpendicular like the two walls and the floor of a room that meet in one common corner, which are formed by the lines $ox$ and $oy$, $oy$ and $oz$, and $oz$ and $ox$, respectively. these three planes are called coordinate planes. and then any other point $p$ in space can be represented with respect to $o$ by its perpendicular distances from each of the three coordinate planes--the distances $x$, $y$, $z$ in the figure. these quantities are called the coordinates of the point.] [to the layman there seems something altogether naive in this notion of the scientist's setting up the three sides of a box in space and using them as the basis of all his work. the layman somehow feels that while it is perfectly all right for him to tell us that he lives at (one coordinate) th street (two coordinates) on the third floor (three coordinates), it is rather trivial business for the serious-minded scientist to consider the up-and-down, the forward-and-back, the right-and-left of every point with which he has occasion to deal. there seems to the layman something particularly inane and foolish and altogether puerile about a set of coordinate axes, and you simply can't make him believe that the serious-minded scientist has to monkey with any such funny business. he can't be induced to take this coordinate-axis business seriously. nevertheless, the fact is that the scientist takes it with the utmost seriousness. it is necessary for him to define the positions of points; and he does do it by means of a set of coordinate axes. the scientist, however, is not interested in points of empty space. the point is to him merely part again of the conceptual machinery which he uses in his effort to run along with the external world. he knows there are no real points, but it suits his convenience to keep track of certain things that are real by representing them as points. but these things are in practically every instance material bodies; and in practically every instance, instead of staying put in one spot, they insist upon moving about through space. the scientist has to use his coordinate system, not merely to define a single position of such a "point," but to keep track of the path over which it moves and to define its position in that path at given moments. time and the coordinate system this introduces the concept of time into intimate relationship with the spatial coordinate system. and at once we feel the lack of a concrete, visualized fourth dimension.]* [if we want to fix objects in the floor alone, the edge of the room running toward the ceiling would become unnecessary and could be dropped from our coordinate system. that is, we need only two coordinates to fix the position of a point in a plane. suppose instead of discarding the third coordinate, we use it to represent units of time. it then enables us to record the time it took a moving point in the floor to pass from position to position. certain points in the room would be vertically above the corresponding points occupied by the moving point in its path across the floor; and the vertical height above the floor of such points corresponds to a value of the time-coordinate which indicates the time it took the point to move from position to position.] [just as the path of the point across the floor is a continuous curve (for the mathematician, it should be understood, this term "curve" includes the straight line, as a special case in which the curvature happens to be zero); so the series of points above these in the room forms a continuous curve which records for us, not merely the path of the point across the floor, but in addition the time of its arrival at each of its successive positions. in the algebraic work connected with such a problem, the third coordinate behaves exactly the same, regardless of whether we consider it to represent time or a third spatial dimension; we cannot even tell from the algebra what it does represent. when we come to the more general case of a point moving freely through space, we have but three coordinates at our disposal; there is not a fourth one by aid of which we can actually diagram its time-space record. nevertheless, we can write down the numerical and algebraic relations between its three space-coordinates and the time which it takes to pass from one position to another; and by this means we can make all necessary calculations. its motion is completely defined with regard both to space and to time. we are very apt to call attention to the fact that if we did have at our disposal a fourth, space-coordinate, we could use it to represent the time graphically, as before, and actually construct a geometric picture of the path of our moving point with regard to space and time. and on this account we are very apt to speak as though the time measurements constituted a fourth coordinate, regardless of any question of our ability to construct a picture of this coordinate. the arrival of a point in a given position constitutes an event; and this event is completely defined by means of four coordinates--three in space, which we can picture on our coordinate axes, and one in time which we cannot. the set of coordinate axes in space, together with the zero point from which we measure time, constitute what we call a frame of reference. if we are not going to pay any attention to time, we can think of the space coordinate system alone as constituting our reference frame. this expression appears freely throughout the subsequent text, and always with one or the other of these interpretations. we see, then, how we can keep track of a moving point by keeping track of the successive positions which it occupies in our reference frame.]* [now we have implied that these coordinate axes are fixed in space; but there is nothing to prevent us from supposing that they move.] [if they do, they carry with them all their points; and any motion of these points which we may speak about will be merely motion with reference to the coordinate system. if we find something outside our coordinate system that is not moving, the motion of points in our system with regard to those outside it will be a combination of their motion with regard to our coordinate axes and that of these axes with regard to the external points. this will be a great nuisance; and it represents a state of affairs which we shall try to avoid. we shall avoid it, if at all, by selecting a coordinate system with reference to which we, ourselves, are not moving; one which partakes of any motion which we may have. or perhaps we shall sometimes wish to reverse the process, in studying the behavior of some group of bodies, and seek a set of axes which is at rest with respect to these bodies; one which partakes of any motion they may have. the choice of a coordinate frame all this emphasizes the fact that our coordinate axes are not picked out for us in advance by nature, and set down in some one particular spot. we select them for ourselves, and we select them in the most convenient way. but different observers, or perhaps the same observer studying different problems, will find it advantageous to utilize different coordinate systems.]* [the astronomer has found it possible, and highly convenient, to select a coordinate frame such that the great majority of the stars have, on the whole, no motion with respect to it.] [such a system would be most unsuited for investigations confined to the earth; for these we naturally select a framework attached to the earth, with its origin o at the earth's center if our investigation covers the entire globe and at some more convenient point if it does not, and in either event accompanying the earth in its rotation and revolution. but such a framework, as well as the one attached to the fixed stars, would be highly inconvenient for an investigator of the motions of the planets; he would doubtless attach his reference frame to the sun.] [in this connection a vital question suggests itself. is the expression of natural law independent of or dependent upon the choice of a system of coordinates? and to what extent shall we be able to reconcile the results of one observer using one reference frame, and a second observer using a different one? the answer to the second question is obvious.]* [true, if any series of events is described using two different sets of axes, the descriptions will be different, depending upon the time system adopted and the relative motion of the axes. but if the connection between the reference systems is known, it is possible by mathematical processes to deduce the quantities observed in one system if those observed in the other are known.] [this process of translating the results of one observer into those of another is known as a transformation; and the mathematical statement of the rule governing the transformation is called the equation or the equations (there are usually several of them) of the transformation.]* [transformations of this character constitute a well-developed branch of mathematics.] [when we inquire about the invariance of natural law it is necessary to be rather sure of just what we mean by this expression. the statement that a given body is moving with a velocity of miles per hour is of course not a natural law; it is a mere numerical observation. but aside from such numerical results, we have a large number of mathematical relations which give us a more or less general statement of the relations that exist between velocities, accelerations, masses, forces, times, lengths, temperatures, pressures, etc., etc. there are some of these which we would be prepared to state at once as universally valid--distance travelled equals velocity multiplied by time, for instance. we do not believe that any conceivable change of reference systems could bring about a condition in which the product of velocity and time, as measured from a certain framework, would fail to equal distance as measured from this same framework. there are other relations more or less of the same sort which we probably believe to be in the same invariant category; there are others, perhaps, of which we might be doubtful; and presumably there are still others which we should suspect of restricted validity, holding in certain reference systems only and not in others. the question of invariance of natural law, then, may turn out to be one which may be answered in the large by a single statement; it may equally turn out to be one that has to be answered in the small, by considering particular laws in connection with particular transformations between particular reference systems. or, perhaps, we may find ourselves justified in taking the stand that an alleged "law of nature" is truly such a law only in the event that it is independent of the change from one reference system to another. in any event, the question may be formulated as follows: observer a, using the reference system r, measures certain quantities $t$, $w$, $x$, $y$, $z$. observer b, using the reference system s, measures the same items and gets the values $t'$, $w'$, $x'$, $y'$, $z'$. the appropriate transformation equations for calculating the one set of values from the other is found. if a mathematical relation of any sort is found to exist between the values $t$, $w$, $x$, $y$, $z$, will the same relation exist between the values $t'$, $w'$, $x'$, $y'$, $z'$? if it does not, are we justified in still calling it a law of nature? and if it does not, and we refrain from calling it such a law, may we expect in every case to find some relation that will be invariant under the transformation, and that may therefore be recognized as the natural law connecting $t$, $w$, $x$, $y$ and $z$? i have found it advisable to discuss this point in such detail because here more than in any other single place the competing essayists betray uncertainty of thought and sloppiness of expression. it doesn't amount to much to talk about the invariance of natural laws and their persistence as we pass from one coordinate system to another, unless we are fairly well fortified with respect to just what we mean by invariance and by natural law. we don't expect the velocity of a train to be miles per hour alike when we measure it with respect to a signal tower along the line and with respect to a moving train on the other track. we don't expect the angular displacement of mars to change as rapidly when he is on the other side of the sun as when he is on our side. but we do, i think, rather expect that in any phenomenon which we may observe, we shall find a natural law of some sort which is dependent for its validity neither upon the units we employ, nor the place from which we make our measurements, nor anything else external to the phenomenon itself. we shall see, later, whether this expectation is justified, or whether it will have to be discarded in the final unravelling of the absolutist from the relativistic philosophy which, with einstein, we are to undertake.]* iii the relativity of uniform motion classical ideas on the subject; the ether and the apparent possibility of absolute motion; the michelson-morley experiment and the final negation of this possibility by various contributors and the editor when we speak of a body as being "in motion," we mean that this body is changing its position "in space." now it is clear that the position of an object can only be determined with reference to other objects: in order to describe the place of a material thing we must, for example, state its distances from other things. if there were no such bodies of reference, the words "position in space" would have no definite meaning for us.] [the number of such external bodies of reference which it is necessary to cite in order to define completely the position of a given body in space depends upon the character of the space dealt with. we have seen that when we visualize the space of our experience as a surface of any character, two citations are sufficient; and that when we conceive of it as surrounding us in three dimensions we require three. it will be realized that the mathematician is merely meeting this requirement when he sets up his system of coordinate axes to serve as a reference frame.]* [what is true of "place" must be true also of "motion," since the latter is nothing but change of place. in fact, it would be impossible to ascribe a state of motion or of rest to a body poised all alone in empty space. whether a body is to be regarded as resting or as moving, and if the latter at what speed, depends entirely upon the objects to which we refer its positions in space.] [as einstein sits at his desk he appears to us to be at rest; but we know that he is moving with the rotation of the earth on its axis, with the earth in its orbit about the sun, and with the solar system in its path through space--a complex motion of which the parts or the whole can be detected only by reference to appropriately chosen ones of the heavenly bodies. no mechanical test has ever been devised which will detect this motion,] [if we reserve for discussion in its proper place the foucault pendulum experiment which will reveal the axial rotation of our globe.]* [no savage, if he were to "stand still," could be convinced that he was moving with a very high velocity or in fact that he was moving at all.] [you drop a coin straight down a ship's side: from the land its path appears parabolic; to a polar onlooker it whirls circle-wise; to dwellers on mars it darts spirally about the sun; to a stellar observer it gyrates through the sky] [in a path of many complications. to you it drops in a straight line from the deck to the sea.]* [yet its various tracks in ship-space, sea-space, earth-space, sun-space, star-space, are all equally real,] [and the one which will be singled out for attention depends entirely upon the observer, and the objects to which he refers the motion.]* [the earth moves in the solar system, which is itself approaching a distant star-cluster. but we cannot say whether we are moving toward the cluster, or the cluster toward us,] [or both, or whether we are conducting a successful stern chase of it, or it of us,]* [unless we have in mind some third body with reference to which the motions of earth and star-cluster are measured.] [and if we have this, the measurements made with reference to it are of significance with regard to it, rather than with regard to the earth and the star-cluster alone.]* [we can express all this by saying "all motions are relative; there is no such thing as absolute motion." this line of argument has in fact been followed by many natural philosophers. but is its result in agreement with actual experience? is it really impossible to distinguish between rest and motion of a body if we do not take into consideration its relations to other objects? in fact it can easily be seen that, at least in many cases, no such distinction is possible. who is moving? imagine yourself sitting in a railroad car with veiled windows and running on a perfectly straight track with unchanging velocity: you would find it absolutely impossible to ascertain by any mechanical means whether the car were moving or not. all mechanical instruments behave exactly the same, whether the car be standing still or in motion.] [if you drop a ball you will see it fall to the floor in a straight line, just as though you had dropped it while standing on the station platform. furthermore, if you drop the ball from the same height in the two cases, and measure the velocities with which it strikes the car floor and the station platform, or the times which it requires for the descent, you will find these identical in the two cases.] [any changes of speed or of direction (as when the car speeds up or slows down or rounds a curve) can be detected by observing the behavior of bodies in the car, without apparent reference to any outside objects. this becomes particularly obvious with sudden irregularities of motion, which manifest themselves by shaking everything in the car. but a uniform motion in a straight line does not reveal itself by any phenomenon within the vehicle.] [moreover, if we remove the veil from our window to the extent that we may observe the train on the adjoining track, we shall be able to make no decision as to whether we or it be moving. this is indeed an experience which we have all had.]* [often when seated in a train about to leave the station, we have thought ourselves under way, only to perceive as the motion becomes no longer uniform that another train has been backing into the station on the adjoining track. again, as we were hurried on our journey, we have, raising suddenly our eyes, been puzzled to say whether the passing train were moving with us or against us or indeed standing still; or more rarely we have had the impression that both it and we seemed to be at rest, when in truth both were moving rapidly with the same speed.] [even this phrase "in truth" is a relative one, for it arises through using the earth as an absolute reference body. we are indeed naive if we cannot appreciate that there is no reason for doing this beyond convenience, and that to an observer detached from the earth it were just as reasonable to say that the rails are sliding under the train as that the train is advancing along the rails. one of my own most vivid childhood recollections is of the terror with which, riding on a train that passed through a narrow cut, i hid my head in the maternal lap to shut out the horrid sight of the earth rushing past my window. the absence of a background in relatively slow retrograde motion was sufficient to prevent my consciousness from drawing the accustomed conclusion that after all it was really the train that was moving.]* mechanical relativity [so we can enunciate the following principle: when a body is in uniform rectilinear motion relatively to a second body, then all phenomena take place on the first in exactly the same manner as on the second; the physical laws for the happenings on both bodies are identical.] [and between a system of bodies, nothing but relative motion may be detected by any mechanical means whatever; any attempt to discuss absolute motion presupposes a super-observer on some body external to the system. even then, the "absolute" motion is nothing but motion relative to this super-observer. by no mechanical means is uniform straight-line motion of any other than relative character to be detected. this is the principle of mechanical relativity. there is nothing new in this. it was known to galileo, it was known to newton, it has been known ever since. but the curious persistence of the human mind in habits of thought which confuse relativity with absolutism brought about a state of affairs where we attempted to know this and to ignore it at the same time. we shall have to return to the mathematical mode of reasoning to see how this happened. the mathematician has a way all his own of putting the statement of relativity which we have made. he recalls, what we have already seen, that the observer on the earth who is measuring his "absolute" motion with respect to the earth has merely attached his reference framework to the earth; that the passenger in the train who measures all motion naively with respect to his train is merely carrying his coordinate axes along with his baggage, instead of leaving them on the solid ground; that the astronomer who deals with the motion of the earth about the sun, or with that of the "fixed" stars against one another, does so simply by the artifice of hitching his frame of reference to the sun or to one of the fixed stars. so the mathematician points out that dispute as to which of two bodies is in motion comes right down to dispute as to which of two sets of coordinate axes is the better one, the more nearly "natural" or "absolute." he therefore phrases the mechanical principle of relativity as follows: among all coordinate systems that are merely in uniform straight-line motion to one another, no one occupies any position of unique natural advantage; all such systems are equivalent for the investigation of natural laws; all systems lead to the same laws and the same results. the mathematician has thus removed the statement of relativity from its intimate association with the external observed phenomena, and transferred it to the observer and his reference frame. we must either accept the principle of relativity, or seek a set of coordinate axes that have been singled out by nature as an absolute reference frame. these axes must be in some way unique, so that when we refer phenomena to them, the laws of nature take a form of exceptional simplicity not attained through reference to ordinary axes. where shall we look for such a preferred coordinate system?]* the search for the absolute [older theory clung to the belief that there was such a thing as absolute motion in space.] [as the body of scientific law developed from the sixteenth century onward, the not unnatural hypothesis crept in, that these laws (that is to say, their mathematical formulations rather than their verbal statements) would reveal themselves in especially simple forms, were it possible for experimenters to make their observations from some absolute standpoint; from an absolutely fixed position in space rather than from the moving earth.] [somewhere a set of coordinate axes incapable of motion was to be found,] [a fixed set of axes for measuring absolute motion; and for two hundred years the world of science strove to find it,] [in spite of what should have been assurance that it did not exist. but the search failed, and gradually the universal applicability of the principle of relativity, so far as it concerned mechanical phenomena, grew into general acceptance.]* [and after the development, by the great mathematicians of the eighteenth century, of newton's laws of motion into their most complete mathematical form, it was seen that so far as these laws are concerned the absolutist hypothesis mentioned is quite unsupported. no complication is introduced into newton's laws if the observer has to make his measurements in a frame of reference moving uniformly through space; and for measurements in a frame like the earth, which moves with changing speed and direction about the sun and rotates on its axis at the same time, the complication is not of so decisive a nature as to give us any clue to the earth's absolute motion in space. but mechanics, albeit the oldest, is yet only one of the physical sciences. the great advance made in the mathematical formulation of optical and electromagnetic theory during the nineteenth century revived the hope of discovering absolute motion in space by means of the laws derived from this theory.] [newton had supposed light to be a material emanation, and if it were so, its passage across "empty space" from sun and stars to the earth raised no problem. but against newton's theory huyghens, the dutch astronomer, advanced the idea that light was a wave motion of some sort. during the newtonian period and for many years after, the corpuscular theory prevailed; but eventually the tables were turned.]* [men made rays of light interfere, producing darkness (see page ). from this, and from other phenomena like polarization, they had deduced that light was a form of wave motion similar to water ripples; for these interfere, producing level surfaces, or reinforce each other, producing waves of abnormal height. but if light were to be regarded as a form of wave motion--and the phenomena could apparently be explained on no other basis--then there must be some medium capable of undergoing this form of motion.] [transmission of waves across empty space without the aid of an intermediary material medium would be "action at a distance," an idea repugnant to us. trammeled by our tactual, wire-pulling conceptions of a material universe, we could not accustom ourselves to the idea of something--even so immaterial a something as a wave--being transmitted by nothing. we needed a word--ether--to carry light if not to shed it; just as we need a word--inertia--to carry a projectile in its flight.] [it was necessary to invest this medium with properties to account for the observed facts. on the whole it was regarded as the perfect fluid.] [the ether was imagined as an all-pervading, imponderable substance filling the vast emptiness through which light reaches us, and as well the intermolecular spaces of all matter. nothing more was known definitely, yet this much served as a good working hypothesis on the basis of which maxwell was enabled to predict the possibility of radio communication. by its fruits the ether hypothesis justified itself; but does the ether exist?] the ether and absolute motion [if it does exist, it seems quite necessary, on mere philosophical grounds, that it shall be eligible to serve as the long-sought reference frame for absolute motion. surely it does not make sense to speak of a homogeneous medium filling all space, sufficiently material to serve as a means of communication between remote worlds, and in the next breath to deny that motion with respect to this medium is a concept of significance.]* [such a system of reference as was offered by the ether, coextensive with the entire known region of the universe, must necessarily serve for all motions within our perceptions.] [the conclusion seems inescapable that motion with respect to the ether ought to be of a sufficiently unique character to stand out above all other motion. in particular, we ought to be able to use the ether to define, somewhere, a system of axes fixed with respect to the ether, the use of which would lead to natural laws of a uniquely simply description. maxwell's work added fuel to this hope.]* [during the last century, after the units of electricity had been defined, one set for static electrical calculations and one for electromagnetic calculations, it was found that the ratio of the metric units of capacity for the two systems was numerically equal to what had already been found as the velocity with which light is transmitted through the hypothetical ether. one definition refers to electricity at rest, the other to electricity in motion. maxwell, with little more working basis than this, undertook to prove that electrical and optical phenomena were merely two aspects of a common cause,] [to which the general designation of "electromagnetic waves" was applied. maxwell treated this topic in great fullness and with complete success. in particular, he derived certain equations giving the relations between the various electrical quantities involved in a given phenomenon. but it was found, extraordinarily enough, that these relations were of such character that, when we subject the quantities involved to a change of coordinate axes, the transformed quantities did not preserve these relations if the new axes happened to be in motion with respect to the original ones. this, of course, was taken to indicate that motion really is absolute when we come to deal with electromagnetic phenomena, and that the ether which carries the electromagnetic waves really may be looked to to display the properties of an absolute reference frame. reference to the phenomenon of aberration, which dr. pickering has discussed adequately in his essay and which i need therefore mention here only by name, indicated that the ether was not dragged along by material bodies over and through which it might pass. it seemed that it must filter through such bodies, presumably via the molecular interstices, without appreciable opposition. were this not the case, we should be in some doubt as to the possibility of observing the velocity through the ether of material bodies; if the ether adjacent to such bodies is not dragged along or thrown into eddies, but "stands still" while the bodies pass, there seems no imaginable reason for anything other than the complete success of such observations. and of course these are of the utmost importance, the moment we assign to the ether the rôle of absolute reference frame. the earth and the ether one body in motion with respect to the ether is our earth itself. we do not know in advance in what direction to expect this motion or what magnitude to anticipate that it will have. but one thing is clear.]* [in its motion around the sun, the earth has, at opposite points on its orbit, a difference in velocity with respect to the surrounding medium which is double its orbital velocity with respect to the sun. this difference comes to miles per second. the earth should therefore, at some time in the year, show a velocity equal to or greater than / miles per second, with reference to the universal medium. the famous michelson-morley experiment of was carried out with the expectation of observing this velocity.] [the ether, of course, and hence velocities through it, cannot be observed directly. but it acts as the medium for the transmission of light.]* [if the velocity of light through the ether is $c$ and that of the earth through the ether is $v$, then the velocity of light past the earth, so the argument runs, must vary from $c-v$ to $c+v$, according as the light is moving exactly in the same direction as the earth, or in the opposite direction,] [or diagonally across the earth's path so as to get the influence only of a part of the earth's motion. this of course assumes that $c$ has always the same value; an assumption that impresses one as inherently probable, and one that is at the same time in accord with ordinary astronomical observation. it is not possible to measure directly the velocity of light ( , miles per second, more or less) with sufficient accuracy to give any meaning to the variation in this velocity which might be effected by adding or subtracting that of the earth in its orbit (a mere / miles per second). it is, however, possible to play a trick on the light by sending it back and forth over several paths, and comparing (not measuring absolutely, but merely comparing) with great minuteness the times consumed in these several round trips. a journey upstream and back the number of letters the scientific american has received questioning the michelson-morley experiment indicates that many people are not acquainted with the fundamental principle on which it is based. so let us look at a simple analogous case. suppose a swimmer or a rower make a return trip upstream and down, contending with the current as he goes up and getting its benefit when he comes down. obviously, says snap judgment, since the two legs of the journey are equal, he derives exactly as much benefit from the current when he goes with it as he suffers handicap from it when he goes against it; so the round trip must take exactly the same time as a journey of the same length in still water, the argument applying equally in the case where the "swimmer" is a wave of light in the ether stream. but let us look now at a numerical case. a man can row in still water at four miles per hour. he rows twelve miles upstream and back, in a current of two miles per hour. at a net speed of two miles per hour he arrives at his turning point in six hours. at a net speed of six miles per hour he makes the down-stream leg in two hours. the elapsed time for the journey is eight hours; in still water he would row the twenty-four miles in six hours. if we were to attempt an explanation of this result in words we should say that by virtue of the very fact that it does delay him, the adverse current prolongs the time during which it operates; while by virtue of the very fact that it accelerates his progress, the favoring current shortens its venue. the careless observer realizes that distances are equal between the two legs of the journey, and unconsciously assumes that times are equal. if the journey be made directly with and directly against the stream of water or ether or what not, retardation is effected to its fullest extent. if the course be a diagonal one, retardation is felt to an extent measurable as a component, and depending for its exact value upon the exact angle of the path. felt, however, it must always be. here is where we begin to get a grip on the problem of the earth and the ether. in any problem involving the return-trip principle, there will enter two velocities--that of the swimmer and that of the medium; and the time of retardation. if we know any two of these items we can calculate the third. when the swimmer is a ray of light and the velocity of the medium is that of the ether as it flows past the earth, we know the first of these two; we hope to observe the retardation so that we may calculate the second velocity. the apparatus for the experiment is ingenious and demands description. the michelson-morley experiment the machine is of structural steel, weighing , pounds. it has two arms which form a greek cross. each arm is feet in length. the whole apparatus is floated in a trough containing pounds of mercury. four mirrors are arranged on the end of each arm, sixteen in all, with a seventeenth mirror, m, set at one of the inside corners of the cross, as diagrammed. a source of light (in this case a calcium flame) is provided, and its rays directed by a lens toward the mirror m. part of the light is allowed to pass straight through m to the opposite arm of the cross, where it strikes mirror . it is reflected back across the arm to mirror , thence to , and so on until it reaches mirror . thence it is reflected back to mirror , to , and so on, retracing its former path, and finally is caught by the reverse side of the mirror m and is sent to an observer at o. in retracing its path the light sets up an interference phenomenon (see below) and the interference bands are visible to the observer, who is provided with a telescope to magnify the results. a second part of the original light-beam is reflected off at right angles by the mirror m, and is passed to and fro on the adjacent arms of the machine, in exactly the same manner and over a similar path, by means of the mirrors i, ii, iii, ... viii. this light finally reaches the observer at the telescope, setting up a second set of interference bands, parallel to the first. a word now about this business of light interference. light is a wave motion. the length of a wave is but a few millionths of an inch, and the amplitude is correspondingly minute; but none the less, these waves behave in a thoroughly wave-like manner. in particular, if the crests of two waves are superposed, there is a double effect; while if a crest of one wave falls with a trough of another, there is a killing-off or "interference". under ordinary circumstances interference of light waves does not occur. this is simply because under ordinary circumstances light waves are not piled up on one another. but sometimes this piling up occurs; and then, just so sure as the piled-up waves are in the same phase they reinforce one another, while if they are in opposite phase they interfere. and the conditions which we have outlined above, with the telescope and the mirrors and the ray of light retracing the path over which it went out, are conditions under which interference does occur. if the returning wave is in exact phase with the outgoing one, the effect is that of uniform double illumination; if it is in exactly opposite phase the effect is that of complete extinguishing of the light, the reversed wave exactly cancelling out the original one. if the two rays are partly in phase, there is partial reinforcement or partial cancelling out, according to whether they are nearly in phase or nearly out of phase. finally, if the mirrors are not set absolutely parallel--as must in practice be the case when we attempt to measure their parallelism in terms of the wave-length of light--adjacent parts of the light ray will vary in the extent to which they are out of phase, since they will have travelled a fraction of a wave-length further to get to and from this, that or the other mirror. there will then appear in the telescope alternate bands of illumination and darkness, whose width and spacing depend upon all the factors entering into the problem. if it were possible for us to make the apparatus with such a degree of refinement that the path from mirror m via mirrors , , , etc., back through m and into the telescope, were exactly the same length as that from flame to telescope by way of the mirrors i, ii, iii, etc.--exactly the same to a margin of error materially less than a single wave-length of light--why, then, the two sets of interference fringes would come out exactly superposed provided the motion of the earth through the "ether" turn out to have no influence upon the velocity of light; or, if such influence exist, these fringes would be displaced from one another to an extent measuring the influence in question. but our ability to set up this complicated pattern of mirrors at predetermined distances falls far short of the wave-length as a measure of error. so in practice all that we can say is that having once set the instrument up, and passed a beam of light through it, there will be produced two sets of parallel interference fringes. these sets will fail of superposition--each fringe of one set will be removed from the corresponding fringe of the other set--by some definite distance. then, any subsequent variation in the speed of light along the two arms will at once be detected by a shifting of the interference bands through a distance which we shall be able to measure. the verdict under the theories and assumptions governing at the time of the original performance of this experiment, it will be readily seen that if this machine be set up in an "ether stream" with one arm parallel to the direction of the stream and the other at right angles thereto, there will be a difference in the speed of the light along the two arms. then if the apparatus be shifted to a position oblique to the ether stream, the excess velocity of the light in the one arm would be diminished, and gradually come to zero at the -degree angle, after which the light traveling along the other arm would assume the greater speed. in making observations, therefore, the entire apparatus was slowly rotated, the observers walking with it, so that changes of the sort anticipated would be observed. the investigators were, however, ignorant of the position in which the apparatus ought to be set to insure that one of the arms lie across the ether drift; and they were ignorant of the time of year at which the earth's maximum velocity through the ether was to be looked for. in particular, it is plain that if the solar system as a whole is moving through the ether at a rate less than the earth's orbital velocity, there is a point in our orbit where our velocity through the ether and that around the sun just cancel out and leave us temporarily in a state of "absolute rest." so it was anticipated that the experiment might have to be repeated in many orientations of the machine and at many seasons of the year in order to give a series of readings from which the true motion of the earth through the ether might be deduced. for those who have a little algebra the demonstration which dr. russell gives on a subsequent page will be interesting as showing the situation in perfectly general terms. it will be realized that the more complicated arrangement of mirrors in the experiment as just described is simply an eightfold repetition of the simple experiment as outlined by dr. russell, and that it was done so for the mere sake of multiplying by eight the distances travelled and hence the difference in time and in phase. and now for the grand climax. the experiment was repeated many times, with the original and with other apparatus, indoors and outdoors, at all seasons of the year, with variation of every condition that could imaginably affect the result. the apparatus was ordinarily such that a shift in the fringes of anywhere from one-tenth to one one-hundredth of that which would have followed from any reasonable value for the earth's motion through the ether would have been systematically apparent. the result was uniformly negative. at all times and in all directions the velocity of light past the earth-bound observer was the same. the earth has no motion with reference to the ether! [the amazing character of this result is not by any possibility to be exaggerated.]* [according to one experiment the ether was carried along by a rapidly moving body and according to another equally well-planned and well-executed experiment a rapidly moving body did not disturb the ether at all. this was the blind alley into which science had been led.] the "contraction" hypothesis [numerous efforts were made to explain the contradiction.]* [it is indeed a very puzzling one, and it gave physicists no end of trouble. however lorentz and fitzgerald finally put forward an ingenious explanation, to the effect that the actual motion of the earth through the ether is balanced, as far as the ability of our measuring instruments is concerned, by a contraction of these same instruments in the direction of their motion. this contraction obviously cannot be observed directly because all bodies, including the measuring instruments themselves (which after all are only arbitrary guides), will suffer the contraction equally. according to this theory, called the lorentz-fitzgerald contraction theory,] [all bodies in motion suffer such contraction of their length in the direction of their motion;] [the contraction being made evident by our inability to observe the absolute motion of the earth, which it is assumed must exist.] [this would suffice to show why the michelson-morley experiment gave a negative result, and would preserve the concept of absolute motion with reference to the ether.] [this proposal of lorentz and fitzgerald loses its startling aspect when we consider that all matter appears to be an electrical structure, and that the dimensions of the electric and magnetic fields which accompany the electrons of which it is constituted change with the velocity of motion.] [the forces of cohesion which determine the form of a rigid body are held to be electromagnetic in nature; the contraction may be regarded as due to a change in the electromagnetic forces between the molecules.] [as one writer has put it, the orientation, in the electromagnetic medium, of a body depending for its very existence upon electromagnetic forces is not necessarily a matter of indifference.]* [granting the plausibility of all this, on the basis of an electromagnetic theory of matter, it leaves us in an unsatisfactory position. we are left with a fixed ether with reference to which absolute motion has a meaning, but that motion remains undetected and apparently undetectable. further, if we on shore measure the length of a moving ship, using a yard-stick which is stationary on shore, we shall obtain one result. if we take our stick aboard it contracts, and so we obtain a greater length for the ship. not knowing our "real" motion through the ether, we cannot say which is the "true" length. is it not, then, more satisfactory to discard all notion of true length as an inherent quality of bodies, and, by regarding length as the measure of a relation between a particular object and a particular observer, to make one length as true as the other?] [the opponents of such a viewpoint contend that michelson's result was due to a fluke; some mysterious counterbalancing influence was for some reason at work, concealing the result which should normally have been expected. einstein refuses to accept this explanation;] [he refuses to believe that all nature is in a contemptible conspiracy to delude us.]* [the fitzgerald suggestion is further unsatisfactory because it assumes all substances, of whatever density, to undergo the same contraction; and above all for the reason that it sheds no light upon other phenomena.] [it is indeed a very special explanation; that is, it applies only to the particular experiment in question. and indeed it is only one of many possible explanations. einstein conceived the notion that it might be infinitely more valuable to take the most general explanation possible, and then try to find from this its logical consequences. this "most general explanation" is, of course, simply that it is impossible in any way whatever to measure the absolute motion of a body in space.] [accordingly einstein enunciated, first the special theory of relativity, and later the general theory of relativity. the special theory was so called because it was, limited to uniform rectilinear and non-rotary motions. the general theory, on the other hand, dealt not only with uniform rectilinear motions, but with any arbitrary motion whatever. taking the bull by the horns the hypothesis of relativity asserts that there can be no such concept as absolute position, absolute motion, absolute time; that space and time are inter-dependent, not independent; that everything is relative to something else. it thus accords with the philosophical notion of the relativity of all knowledge.] [knowledge is based, ultimately, upon measurement; and clearly all measurement is relative, consisting merely in the application of a standard to the magnitude measured. all metric numbers are relative; dividing the unit multiplies the metric number. moreover, if measure and measured change proportionately, the measuring number is unchanged. should space with all its contents swell in fixed ratio throughout, no measurement could detect this; nor even should it pulse uniformly throughout. furthermore, were space and space-contents in any way systematically transformed (as by reflection in curved mirrors) point for point, continuously, without rending, no measurement could reveal this distortion; experience would proceed undisturbed.] [mark twain said that the street in damascus "which is called straight," is so called because while it is not as straight as a rainbow it is straighter than a corkscrew. this expresses the basic idea of relativity--the idea of comparison. all our knowledge is relative, not absolute. things are big or little, long or short, light or heavy, fast or slow, only by comparison. an atom may be as large, compared to an electron, as is a cathedral compared to a fly. the relativity theory of einstein emphasizes two cases of relative knowledge; our knowledge of time and space, and our knowledge of motion.] [and in each case, instead of allowing the notions of relativity to guide us only so far as it pleases us to follow them, there abandoning them for ideas more in accord with what we find it easy to take for granted, einstein builds his structure on the thesis that relativity must be admitted, must be followed out to the bitter end, in spite of anything that it may do to our preconceived notions. if relativity is to be admitted at all, it must be admitted in toto; no matter what else it contradicts, we have no appeal from its conclusions so long as it refrains from contradicting itself.]* [the hypothesis of relativity was developed by einstein through a priori methods, not the more usual a posteriori ones. that is, certain principles were enunciated as probably true, the consequences of these were developed, and these deductions tested by comparison of the predicted and the observed phenomena. it was in no sense attained by the more usual procedure of observing groups of phenomena and formulating a law or formula which would embrace them and correctly describe the routine or sequence of phenomena. the first principle thus enunciated is that it is impossible to measure or detect absolute translatory motion through space, under any circumstances or by any means. the second is that the velocity of light in free space appears the same to all observers regardless of the relative motion of the source of light and the observer. this velocity is not affected by motion of the source toward or away from the observer,] [if we may for the moment use this expression with its implication of absolute motion.]* [but universal relativity insists that motion of the source toward the observer is identical with motion of the observer toward the source.] [it will be seen that we are at once on the horns of a dilemma. either we must give up relativity before we get fairly started on it, or we must overturn the foundations of common sense by admitting that time and space are so constituted that when we go to meet an advancing light-impulse, or when we retreat from it, it still reaches us with the same velocity as though we stood still waiting for it. we shall find when we are through with our investigation that common sense is at fault; that our fixed impression of the absurdity of the state of affairs just outlined springs from a confusion between relativism and absolutism which has heretofore dominated our thought and gone unquestioned. the impression of absurdity will vanish when we have resolved this confusion.]* questions of common sense [but it is obvious from what has just been said that if we are to adopt einstein's theory, we must make very radical changes in some of our fundamental notions, changes that seem in violent conflict with common sense. it is unfortunate that many popularizers of relativity have been more concerned to astonish their readers with incredible paradoxes than to give an account such as would appeal to sound judgment. many of these paradoxes do not belong essentially to the theory at all. there is nothing in the latter that an enlarged and enlightened common sense would not readily endorse. but common sense must be educated up to the necessary level.] [there was a time when it was believed, as a result of centuries of experience, that the world was flat. this belief checked up with the known facts, and it could be used as the basis for a system of science which would account for things that had happened and that were to happen. it was entirely sufficient for the time in which it prevailed. then one day a man arose to point out that all the known facts were equally accounted for on the theory that the earth was a sphere. it was in order for his contemporaries to admit this, to say that so far as the facts in hand were concerned they could not tell whether the earth was flat or round--that new facts would have to be sought that would contradict one or the other hypothesis. instead of this the world laughed and insisted that the earth could not be round because it was flat; that it could not be round because then the people would fall off the other side. but the field of experimentation widened, and men were able to observe facts that had been hidden from them. presently a man sailed west and arrived east; and it became clear that in spite of previously accepted "facts" to the contrary, the earth was really round. the previously accepted "facts" were then revised to fit the newly discovered truth; and finally a new system of science came into being, which accounted for all the old facts and all the new ones. at intervals this sort of thing has been repeated. a galileo shows that preconceived ideas with regard to the heavens are wrong, and must be revised to accord with his newly promulgated principles. a newton does the same for physics--and people unlearn the "fact" that motion has to be supported by continued application of force, substituting the new idea that it actually requires force to stop a moving body. a harvey shows that the things which have been "known" for generations about the human body are not so. a lyell and a darwin force men to throw overboard the things they have always believed about the way in which the earth and its creatures came into being. every science we possess has passed through one or more of these periods of readjustment to new facts. shifting the mental gears now we are apt to lose sight of the true significance of this. it is not alone our opinions that are altered; it is our fundamental concepts. we get concepts wholly from our perceptions, making them to fit those perceptions. whenever a new vista is opened to our perceptions, we find facts that we never could have suspected from the restricted viewpoint. we must then actually alter our concepts to make the new facts fit in with the greatest degree of harmony. and we must not hesitate to undertake this alteration, through any feeling that fundamental concepts are more sacred and less freely to be tampered with than derived facts.]* [we do, to be sure, want fundamental concepts that are easy for a human mind to conceive; but we also want our laws of nature to be simple. if the laws begin to become, intricate, why not reshape, somewhat, the fundamental concepts, in order to simplify the scientific laws? ultimately it is the simplicity of the scientific system as a whole that is our principal aim.] [as a fair example, see what the acceptance of the earth's sphericity did to the idea represented by the word "down." with a flat earth, "down" is a single direction, the same throughout the universe; with a round earth, "down" becomes merely the direction leading toward the center of the particular heavenly body on which we happen to be located. it is so with every concept we have. no matter how intrinsic a part of nature and of our being a certain notion may seem, we can never know that new facts will not develop which will show it to be a mistaken one. today we are merely confronted by a gigantic example of this sort of thing. einstein tells us that when velocities are attained which have just now come within the range of our close investigation, extraordinary things happen--things quite irreconcilable with our present concepts of time and space and mass and dimension. we are tempted to laugh at him, to tell him that the phenomena he suggests are absurd because they contradict these concepts. nothing could be more rash than this. when we consider the results which follow from physical velocities comparable with that of light, we must confess that here are conditions which have never before been carefully investigated. we must be quite as well prepared to have these conditions reveal some epoch-making fact as was galileo when he turned the first telescope upon the skies. and if this fact requires that we discard present ideas of time and space and mass and dimension, we must be prepared to do so quite as thoroughly as our medieval fathers had to discard their notions of celestial "perfection" which demanded that there be but seven major heavenly bodies and that everything center about the earth as a common universal hub. we must be prepared to revise our concepts of these or any other fundamentals quite as severely as did the first philosopher who realized that "down" in london was not parallel to "down" in bagdad or on mars.]* [in all ordinary terrestrial matters we take the earth as a fixed body, light as instantaneous. this is perfectly proper, for such matters. but we carry our earth-acquired habits with us into the celestial regions. though we have no longer the earth to stand on, yet we assume, as on the earth, that all measurements and movements must be referred to some fixed body, and are only then valid. we cling to our earth-bound notion that there is an absolute up-and-down, back-and-forth, right-and-left, in space. we may admit that we can never find it, but we still think it is there, and seek to approach it as nearly as possible. and similarly from our earth experiences, which are sufficiently in a single place to make possible this simplifying assumption, we get the idea that there is one universal time, applicable at once to the entire universe.] [the difficulty in accepting einstein is entirely the difficulty in getting away from these earth-bound habits of thought.]* iv the special theory of relativity what einstein's study of uniform motion tells us about time and space and the nature of the external reality by various contributors and the editor whatever the explanation adopted for the negative result of the michelson-morley experiment, one thing stands out clearly: the attempt to isolate absolute motion has again failed.]* [einstein generalizes this with all the other and older negative results of similar sort into a negative deduction to the effect that no experiment is possible upon two systems which will determine that one of them is in motion and the other at rest.] [he elevates the repeated failure to detect absolute motion through space into the principle that experiment will never reveal anything in the nature of absolute velocities. he postulates that all laws of nature can and should be enunciated in such forms that they are as true in these forms for one observer as for another, even though these observers with their frames of reference be in motion relative to one another.] [there are various ways of stating the principle of the relativity of uniform motion which has been thus arrived at, and which forms the basis of the special theory of einstein. if we care to emphasize the rôle of mathematics and the reference frame we may say that]* [any coordinate system having a uniform rectilinear motion with respect to the bodies under observation may be interchangeably used with any other such system in describing their motions;] [or that the unaccelerated motion of a system of reference cannot be detected by observations made on this system alone.] [or we can let this aspect of the matter go, and state the relativity postulate in a form more intelligible to the non-mathematician by simply insisting that it is impossible by any means whatever to distinguish any other than the relative motion between two systems that are moving uniformly. as dr. russell puts it on a later page, we can assume boldly that the universe is so constituted that uniform straight-ahead motion of an observer and all his apparatus will not produce any difference whatever in the result of any physical process or experiment of any kind. as we have seen, this is entirely reasonable, on philosophical grounds, until we come to consider the assumptions of the past century with regard to light and its propagation. on the basis of these assumptions we had expected the michelson-morley experiment to produce a result negativing the notion of universal relativity. it refused to do this, and we agree with einstein that the best explanation is to return to the notion of relativity, rather than to invent a forced and special hypothesis to account for the experiment's failure. but we must now investigate the assumptions underlying the theory of light, and remove the one that requires the ether to serve as a universal standard of absolute motion. light and the ether it is among the possibilities that the wave theory of light itself will in the end be more or less seriously modified. it is even more definitely among the possibilities that the ether will be discarded.]* [certainly when lord kelvin estimates that its mass per cubic centimeter is . , , , , , gram, while sir oliver lodge insists that the correct figure is , , , , , grams, it is quite evident that we know so little about it that it is better to get along without it if we can.] [but to avoid confusion we must emphasize that einstein makes no mention whatsoever of the ether; his theory is absolutely independent of any theory of the ether.] [save as he forbids us to employ the ether as a standard of absolute motion, einstein does not in the least care what qualities we assign to it, or whether we retain it at all. his demands are going to be made upon light itself, not upon the alleged medium of light transmission. when two observers in relative motion to one another measure their velocities with respect to a third material object, they expect to get different results. their velocities with regard to this object properly differ, for it is no more to be taken as a universal super-observer than either of them. but if they get different results when they come to measure the velocity with which light passes their respective systems, relativity is challenged. light is with some propriety to be regarded as a universal observer; and if it will measure our velocities against each other we cannot deny it rank as an absolute standard. if we are not prepared to abandon universal relativity, and adopt one of the "fluke" explanations for the michelson-morley result, we must boldly postulate that in free space light presents the same velocity $c$ to all observers--whatever the source of the light, whatever the relative motion between source and observer, whatever the relative motion between the several observers. the departure here from the old assumption lies in the circumstance that the old physics with its ether assigned to light a velocity universally constant in this ether; we have stopped talking about the medium and have made the constant $c$ refer to the observer's measured value of the velocity of light with regard to himself. we are fortified in this assumption by the michelson-morley result and by all other observations bearing directly upon the matter. nevertheless, as mr. francis says in his essay, we feel instinctively that space and time are not so constituted as to make it possible, if i pass you at miles per hour, for the same light-impulse to pass us both at the same speed $c$.]* [the implicit assumptions underlying this feeling, be they true or false, are now so interwoven with the commonly received notions of space and time that any theory which questions them has all the appearance of a fantastic and unthinkable thing.] [we cannot, however, go back on our relativity; so when]* [einstein shows us that an entirely new set of time and space concepts is necessary to reconcile universe relativity with this fundamental fact of the absolute constancy of the observed velocity of light in vacuo,] [all that is left for us to do is to inquire what revisions are necessary, and submit to them.]* [the conceptual difficulties of the theory arise principally from attributing to space and time the properties of things. no portion of space can be compared with another, save by convention; it is things which we compare. no interval of time can be compared with another, save by convention. the first has gone when the second becomes "now".] [it is events that we compare, through the intervention of things. our measurements are never of space or of time, but only of the things and the events that occupy space and time. and since the measurements which we deal with as though they were of space and of time lie at the foundation of all physical science, while at the same time themselves constituting, as we have seen, the only reality of which we are entitled to speak, it is in order to examine with the utmost care the assumptions underlying them. that there are such assumptions is clear--the very possibility of making measurements is itself an assumption, and every technique for carrying them out rests on an assumption. let us inquire which of these it is that relativity asks us to revise.]* the measurement of time and space [time is generally conceived as perfectly uniform. how do we judge about it? what tells us that the second just elapsed is equal to the one following? by the very nature of time the superposition of its successive intervals is impossible. how then can we talk about the relative duration of these intervals? it is clear that any relationship between them can only be conventional.] [as a matter of fact, we habitually measure time in terms of moving bodies. the simplest method is to agree that some entity moves with uniform velocity. it will be considered as travelling equal distances in equal intervals of time, the distances to be measured as may be specified by our assumptions governing this department of investigation.] [the motions of the earth through which we ultimately define the length of day and year, the division of the former into , "equal" intervals as defined by the motions of pendulum or balance wheel through equal distances, are examples of this convention of time measurement. even when we correct the motions of the earth, on the basis of what our clocks tell us of these motions, we are following this lead; the earth and the clocks fall out, it is plain that one of them does not satisfy our assumption of equal lengths in equal times, and we decide to believe the clock.]* [the foregoing concerning time may be accepted as inherent in time itself. but concerning lengths it may be thought that we are able to verify absolutely their equality and especially their invariability. let us have the audacity to verify this statement. we have two lengths, in the shape of two rods, which coincide perfectly when brought together. what may we conclude from this coincidence? only that the two rods so considered have equal lengths at the same place in space and at the same moment. it may very well be that each rod has a different length at different locations in space and at different times; that their equality is purely a local matter. such changes could never be detected if they affected all objects in the universe. we cannot even ascertain that both rods remain straight when we transport them to another location, for both can very well take the same curvature and we shall have no means of detecting it. euclidean geometry assumes that geometrical objects have sizes and shapes independent of position and of orientation in space, and equally invariable in time. but the properties thus presupposed are only conventional and in no way subject to direct verification. we cannot even ascertain space to be independent of time, because when comparing geometrical objects we have to conceive them as brought to the same place in space and in time.] [even the statement that when they are made to coincide their lengths are equal is, after all, itself an assumption inherent in our ideas of what constitutes length. and certainly the notion that we can shift them from place to place and from moment to moment, for purposes of comparison, is an assumption; even euclid, loose as he was from modern standards in this business of "axioms," knew this and included a superposition axiom among his assumptions. as a matter of fact, this procedure for determining equality of lengths is not always available. it assumes, it will be noted, that we have free access to the object which is to be measured--which is to say, it assumes that this object is at rest with respect to us. if it is not so at rest, we must employ at least a modification of this method; a modification that will in some manner involve the sending of signals. even when we employ the euclidean method of superposition directly, we must be assured that the respective ends of the lengths under comparison coincide at the same time. the observer cannot be present at both ends simultaneously; at best he can only be present at one end and receive a signal from the other end. the problem of communication accordingly, in making the necessary assumptions to cover the matter of measuring lengths, we must make one with regard to the character of the signals which are to be employed for this purpose. if we could assume a system of signalling that would consume no time in transmission all would be simple enough. but we have no experience with such a system. even if we believe that it ought to be possible thus to transmit signals at infinite velocity, we may not, in the absence of our present ability to do this, assume that it is possible. so we may only assume, with einstein, that for our signals we shall employ the speediest messenger with which we are at present acquainted. this of course is light, the term including any of the electromagnetic impulses that travel at the speed $c$. of course in the vast majority of cases the distance that any light signal in which we are interested must go to reach us is so small that the time taken by its transmission can by no means be measured. we are then, to all intents and purposes, at both places--the point of origin of the signal and the point of receipt--simultaneously. but this is not the question at all. waiving the fact that in astronomical investigations this approximation no longer holds, the fact remains that it is, in every case, merely an approximation. approximations are all right in observations, where we know that they are approximations and act accordingly. but in the conceptual universe that parallels the external reality, computation is as good an agent of observation as visual or auditory or tactile sensation; if we can compute the error involved in a wrong procedure the error is there, regardless of whether we can see it or not. we must have methods which are conceptually free from error; and if we attempt to ignore the velocity of our light signals we do not meet this condition. the measurement of lengths demands that we have a criterion of simultaneity between two remote points--remote in inches or remote in light-years, it does not matter which. there is no difficulty in defining simultaneity of two events that fall in the same point--or rather, in agreeing that we know what we mean by such simultaneity. but with regard to two events that occur in remote places there may be a question. a scientific definition differs from a mere description in that it must afford us a means of testing whether a given item comes under the definition or not. there is some difficulty in setting up a definition of simultaneity between distant events that satisfies this requirement. if we try simply to fall back upon our inherent ideas of what we mean by "the same instant" we see that this is not adequate. we must lay down a procedure for determining whether two events at remote points occur at "the same instant," and check up alleged simultaneity by means of this procedure. einstein says, and we must agree with him, that he can find but one reasonable definition to cover this ground. an observer can tell whether he is located half way between two points of his observation; he can have mirrors set up at these points, send out light-signals, and note the time at which he gets back the reflection. he knows that the velocity of both signals, going and coming, is the same; if he observes that they return to him together so that their time of transit for the round trip is the same, he must accept the distances as equal. he is then at the mid-point of the line joining the two points under observation; and he may define simultaneity as follows, without introducing anything new or indeterminate: two events are simultaneous if an observer midway between them sees them at the same instant, by means, of course, of light originating at the points of occurrence.]* [it is this definition of simultaneity, coupled with the assumption that all observers, on whatever uniformly moving systems, would obtain the same experimental value for the velocity of light, that leads to the apparent paradoxes of the special theory of relativity. if it be asked why we adopt it, we must in turn ask the inquirer to propose a better system for defining simultaneous events on different moving bodies.] [there is nothing in this definition to indicate, directly, whether simultaneity persists for all observers, or whether it is relative, so that events simultaneous to one observer are not so to another. the question must then be investigated; and the answer, of course, will hinge upon the possibility of making proper allowances for the time of transit of the light signals that may be involved. it seems as though this ought to be possible; but a simple experiment will indicate that it is not, unless the observers involved are at rest with respect to one another. an einsteinian experiment let us imagine an indefinitely long, straight railroad track, with an observer located somewhere along it at the point $m$. according to the convention suggested above, he has determined points $a$ and $b$ in opposite directions from him along the track, and equally distant from him. we shall imagine, further, than a beneficent providence supplies two lightning flashes, one striking at $a$ and one at $b$, in such a way that observer $m$ finds them to be simultaneous. while all this is going on, a train is passing--a very long train, amply long enough to overlap the section $amb$ of the track. among the passengers there is one, whom we may call $m'$, who is directly opposite $m$ at the instant when, according to $m$, the lightning strikes. observe he is not opposite $m$ when $m$ sees the flashes, but a brief time earlier--at the instant when, according to $m$'s computation, the simultaneous flashes occurred. at this instant there are definitely determined the points $a'$ and $b'$, on the train; and since we may quite well think of the two systems--train-system and track-system--as in coincidence at this instant, $m'$ is midway between $a'$ and $b'$, and likewise is midway between $a$ and $b$. now if we think of the train as moving over the track in the direction of the arrow, we see very easily that $m'$ is running away from the light from $a$ and toward that from $b$, and that, despite--or if you prefer because of--the uniform velocity of these light signals, the one from $b$ reaches him, over a slightly shorter course, sooner than the one from $a$, over the slightly longer course. when the light signals reach $m$, $m'$ is no longer abreast of him but has moved along a wee bit, so that at this instant when $m$ has the two signals, one of these has passed $m'$ and the other has yet to reach him. the upshot is that the events which were simultaneous to $m$ are not so to $m'$. it will probably be felt that this result is due to our having, somewhat unjustifiably and inconsistently, localized on the train the relative motion between train and track. but if we think of the track as sliding back under the train in the direction opposite to the arrow, and carrying with it the points $a$ and $b$; and if we remember that this in no way affects $m$'s observed velocity of light or the distances $am$ and $bm$ as he observes them: we can still accept his claim that the flashes were simultaneous. then we have again the same situation: when the flashes from $a$ and from $b$ reach $m$ at the same moment, in his new position a trifle to the left of his initial position of the diagram, the flash from $a$ has not yet reached $m'$ in his original position while that from $b$ has passed him. regardless of what assumption we make concerning the motion between train-system and track-system, or more elegantly regardless of what coordinate system we use to define that motion, the event at $b$ precedes that at $a$ in the observation of $m'$. if we introduce a second train moving on the other track in the opposite direction, the observer on it will of course find that the flash at $a$ precedes that at $b$--a disagreement not merely as to simultaneity but actually as to the order of two events! if we conceive the lightning as striking at the points $a'$ and $b'$ on the train, these points travel with $m'$ instead of with $m$; they are fixed to his coordinate system instead of to the other. if you carry out the argument now, you will find that when the flashes are simultaneous to $m'$, the one at $a$ precedes that at $b$ in $m$'s observation. a large number of experiments more or less similar in outline to this one can be set up to demonstrate the consequences, with regard to measured values of time and space, of relative motion between two observers. i do not believe that a multiplicity of such demonstrations contributes to the intelligibility of the subject, and it is for this reason that i have cut loose from immediate dependence upon the essayists in this part of the discussion, concentrating upon the single experiment to which einstein himself gives the place of importance. who is right? we may permit mr. francis to remind us here that neither $m$ nor $m'$ may correct his observation to make it accord with the other fellow's. the one who does this is admitting that the other is at absolute rest and that he is himself in absolute motion; and this cannot be. they are simply in disagreement as to the simultaneity of two events, just as two observers might be in disagreement about the distance or the direction of a single event. this can mean nothing else than that, under the assumptions we have made, simultaneity is not an absolute characteristic as we had supposed it to be, but, like distance and direction, is in fact merely a relation between observer and objective, and therefore depends upon the particular observer who happens to be operating and upon the reference frame he is using. but this is serious. my time measurements depend ultimately upon my space measurements; the latter, and hence both, depend closely upon my ideas of simultaneity. yours depend upon your reading of simultaneity in precisely the same way.]* [suppose the observer on the track, in the above experiment, wants to measure the length of something on the car, or the observer on the car something on the track. the observer, or his assistant, must be at both ends of the length to be measured at the same time, or get simultaneous reports in some way from these ends; else they will obtain false results. it is plain, then, that with different criteria of what the "same time" is, the observers in the two systems may get different values for the measured lengths in question.] [who is right? according to the principle of relativity a decision on this question is absolutely impossible. both parties are right from their own points of view; and we must admit that two events in two different places may be simultaneous for certain observers, and yet not simultaneous for other observers who move with respect to the first ones. there is no contradiction in this statement, although it is not in accordance with common opinion, which believes simultaneousness to be something absolute. but this common opinion lacks foundation. it cannot be proved by direct perception, for simultaneity of events can be perceived directly,] [and in a manner involving none of our arbitrary assumptions,]* [only if they happen at the same place; if the events are distant from each other, their simultaneity or succession can be stated only through some method of communicating by signals. there is no logical reason why such a method should not lead to different results for observers who move with regard to one another. from what we have said, it follows immediately that in the new theory not only the concept of simultaneousness but also that of duration is revealed as dependent on the motion of the observer.] [demonstration of this should be superfluous; it ought to be plain without argument that if two observers cannot agree whether two instants are the same instant or not, they cannot agree on the interval of time between instants. in the very example which we have already examined, one observer says that a certain time-interval is zero, and another gives it a value different from zero. the same thing happens whenever the observers are in relative motion.]* [two physicists who measure the duration of a physical process will not obtain the same result if they are in relative motion with regard to one another. they will also find different results for the length of a body. an observer who wants to measure the length of a body which is moving past him must in one way or another hold a measuring rod parallel to its motion and mark those points on his rod with which the ends of the body come into simultaneous coincidence. the distance between the two marks will then indicate the length of the body. but if the two markings are simultaneous for one observer, they will not be so for another one who moves with a different velocity, or who is at rest, with regard to the body under observation. he will have to ascribe a different length to it. and there will be no sense in asking which of them is right: length is a purely relative concept, just as well as duration.] the relativity of time and space [the degree to which distance and time become relative instead of absolute quantities under the special theory of relativity can be stated very definitely. in the first place, we must point out that the relativity of lengths applies with full force only to lengths that lie parallel to the direction of relative motion. those that lie exactly perpendicular to that direction come out the same for both observers; those that lie obliquely to it show an effect, depending upon the angle, which of course becomes greater and greater as the direction of parallelism is approached. the magnitude of the effect is easily demonstrated, but with this demonstration we do not need to be concerned here. it turns out that if an observer moving with a system finds that a certain time interval in the system is $t$ seconds and that a certain length in the system is $l$ inches, then an observer moving parallel with $l$ and with a velocity $v$ relative to the system will find for these the respective values $t \div k$ and $l \times k$, where $$k = \sqrt{ - v^ /c^ }.$$ $c$ in this expression of course represents the velocity of light. it will be noted that the fraction $v^ /c^ $ is ordinarily very small; that the expression under the radical is therefore less than but by a very slight margin; and that the entire expression $k$ is itself therefore less than but by an even slighter margin. this means, then, that the observer outside the system finds the lengths in the system to be a wee bit shorter and the time intervals a wee bit longer than does the observer in the system. another way of putting the matter is based, ultimately, upon the fact that in order for the observer in the system to get the larger value for distance and the smaller value for time, his measuring rod must go into the distance under measurement more times than that of the moving observer, while his clock must beat a longer second in order that less of them shall be recorded in a given interval between two events. so it is often said that the measuring rod as observed from without is contracted and the clock runs slow. this does not impress me as a happy statement, either in form or in content.]* [the argument that these formulae are contradicted by human experience can be refuted by examining a concrete instance. if a train is , feet long at rest, how long will it be when running a mile a minute?] [i have quoted this question exactly as it appears in the essay from which it is taken, because it is such a capital example of the objectionable way in which this business is customarily put. for the statement that lengths decrease and time-intervals increase "with velocity" is not true in just this form. the velocity, to have meaning, must be relative to some external system; and it is the observations from that external system that are affected. so long as we confine ourselves to the system in which the alleged modifications of size are stated as having taken place, there is nothing to observe that is any different from what is usual; there is no way to establish that we are enjoying a velocity, and in fact within the intent of the relativity theory we are not enjoying a velocity, for we are moving with the objects which we are observing. it is inter-systemic observations, and these alone, that show the effect. when we travel with the system under observation, we get the same results as any other observer on this system; when we do not so travel, we must conduct our observations from our own system, in relative motion to the other, and refer our results to our system. now when no particular observer is specified, we must of course assume an observer connected with the train, or with whatever the body mentioned. to that observer it doesn't make the slightest difference what the train does; it may stand at rest with respect to some external system or it may move at any velocity whatsoever; its length remains always , feet. in order for this question to have the significance which its propounder means it to have, i must restate it as follows: a train is , feet long as measured by an observer travelling with it. if it passes a second observer at miles per hour, what is its length as observed by him? the answer is now easy.]* [according to the formula the length of the moving train as seen from the ground will be $$ , \times \sqrt{ - ( )^ / ( , \times , )^ } = . , , , $$ feet, a change entirely too small for detection by the most delicate instruments. examination of the expression k shows that in so far as terrestrial movements of material objects are concerned it is equal to ] [within a far smaller margin than we can ever hope to make our observations. even the diameter of the earth, as many of the essayists point out, will be shortened only / inches for an outside observer past whom it rushes with its orbital speed of . miles per second. but slight as the difference may be in these familiar cases, its scientific importance remains the same.]* relativity and reality [a simple computation shows that this effect is exactly the amount suggested by lorentz and fitzgerald to explain the michelson-morley experiment.] [this ought not to surprise us, since both that explanation and the present one are got up with the same purpose. if they both achieve that purpose they must, numerically, come to the same thing in any numerical case. it is, however, most emphatically to be insisted that the present "shortening" of lengths]* [no longer appears as a "physical" shortening caused by absolute motion through the ether but is simply a result of our methods of measuring space and time.] [where fitzgerald and lorentz had assumed that a body in motion has its dimensions shortened in the direction of its motion,] [this very form of statement ceases to possess significance under the relativity assumption.]* [for if we cannot tell which of two bodies is moving, which one is shortened? the answer is, both--for the other fellow. for each frame of reference there is a scale of length and a scale of time, and these scales for different frames are related in a manner involving both the length and the time.] [but we must not yield to the temptation to say that all this is not real; the confinement of a certain scale of length and of time to a single observation system does not in the least make it unreal.]* [the situation is real--as real as any other physical event.] [the word physical is used in two senses in the above paragraph. it is denied that the observed variability in lengths indicates any "physical" contraction or shrinkage; and on the heels of this it is asserted that this observed variability is of itself an actual "physical" event. it is difficult to express in words the distinction between the two senses in which the term physical is employed in these two statements, but i think this distinction ought to be clear once its existence is emphasized. there is no material contraction; it is not right to say that objects in motion contract or are shorter; they are not shorter to an observer in motion with them. the whole thing is a phenomenon of observation. the definitions which we are obliged to lay down and the assumptions which we are obliged to make in order, first, that we shall be able to measure at all, and second, that we shall be able to escape the inadmissible concept of absolute motion, are such that certain realities which we had supposed ought to be the same for all observers turn out not to be the same for observers who are in relative motion with respect to one another. we have found this out, and we have found out the numerical relation which holds between the reality of the one observer and that of the other. we have found that this relation depends upon nothing save the relative velocity of the two observers. as good a way of emphasizing this as any is to point out that two observers who have the same velocity with respect to the system under examination (and whose mutual relative velocity is therefore zero) will always get the same results when measuring lengths and times on that system. the object does not go through any process of contraction; it is simply shorter because it is observed from a station with respect to which it is moving. similar remarks might be made about the time effect; but the time-interval is not so easily visualized as a concrete thing and hence does not offer such temptation for loose statement. the purely relative aspect of the matter is further brought out if we consider a single example both backwards and forwards. systems $s$ and $s'$ are in relative motion. an object in $s$ which to an observer in $s$ is $l$ units long, is shorter for an observer in $s'$--shorter by an amount indicated through the "correction factor" $k$. now if we have, in the first instance, made the objectionable statement that objects are shorter in system $s'$ than they are in $s$, it will be quite natural for us to infer from this that objects in $s$ must be longer than those in $s'$; and from this to assert that when the observer in $s$ measures objects lying in $s'$, he gets for them greater lengths than does the home observer in $s'$. but if we have, in the first instance, avoided the objectionable statement referred to, we shall be much better able to realize that the whole business is quite reciprocal; that the phenomena are symmetric with respect to the two systems, to the extent that we can interchange the systems in any of our statements without modifying the statements in any other way. objects in $s$ appear shorter and times in $s$ appear longer to the external "moving" observer in $s'$ than they do to the domestic observer in $s$. exactly in the same way, objects in $s'$ appear shorter to observers in the foreign system $s$ than to the home observer in $s'$, who remains at rest with respect to them. i think that when we get the right angle upon this situation, it loses the alleged startling character which has been imposed upon it by many writers. the "apparent size" of the astronomer is an analogy in point. objects on the moon, by virtue of their great distance, look smaller to observers on the earth than to observers on the moon. do objects on the earth, on this account, look larger to a moon observer than they do to us? they do not; any suggestion that they do we should receive with appropriate scorn. the variation in size introduced by distance is reciprocal, and this reciprocity does not in the least puzzle us. why, then, should that introduced by relative motion puzzle us? time and space in a single package our old, accustomed concepts of time and space, which have grown up through countless generations of our ancestors, and been handed down to us in the form in which we are familiar with them, leave no room for a condition where time intervals and space intervals are not universally fixed and invariant. they leave no room for us to say that]* [one cannot know the time until he knows where he is, nor where he is until he knows the time,] [nor either time or place until he knows something about velocity. but in this concise formulation of the difference between what we have always believed and what we have seen to be among the consequences of einstein's postulates of the universal relativity of uniform motion, we may at once locate the assumption which, underlying all the old ideas, is the root of all the trouble. the fact is we have always supposed time and space to be absolutely distinct and independent entities.]* [the concept of time has ever been one of the most absolute of all the categories. it is true that there is much of the mysterious about time; and philosophers have spent much effort trying to clear up the mystery--with unsatisfactory results. however, to most persons it has seemed possible to adopt an arbitrary measure or unit of duration and to say that this is absolute, independent of the state of the body or bodies on which it is used for practical purposes.] [time has thus been regarded as something which of itself flows on regularly and continuously, regardless of physical events concerning matter.] [in other words, according to this view, time is not affected by conditions or motions in space.] [we have deliberately chosen to ignore the obvious fact that time can never appear to us, be measured by us, or have the least significance for us, save as a measure of something that is closely tied up with space and with material space-dimensions. not merely have we supposed that time and space are separated in nature as in our easiest perceptions, but we have supposed that they are of such fundamentally distinct character that they can never be tied up together. in no way whatever, assumes the euclidean and newtonian intellect, may space ever depend upon time or time upon space. this is the assumption which we must remove in order to attain universal relativity; and while it may come hard, it will not come so hard as the alternative. for this alternative is nothing other than to abandon universal relativity. this course would leave us with logical contradictions and discrepancies that could not be resolved by any revision of fundamental concepts or by any cleaning out of the augean stables of old assumptions; whereas the relativity doctrine as built up by einstein requires only such a cleaning out in order to leave us with a strictly logical and consistent whole. the rôle of hercules is a very difficult one for us to play. einstein has played it for the race at large, but each of us must follow him in playing it for himself. some further consequences i need not trespass upon the subject matter of those essays which appear in full by going here into any details with regard to the manner in which time and space are finally found to depend upon one another and to form the parts of a single universal whole. but i may appropriately point out that if time and space are found to be relative, we may surely expect some of the less fundamental concepts that depend upon them to be relative also. in this expectation we are not disappointed. for one thing,]* [mass has always been assumed to be a constant, independent of any motion or energy which it might possess. just as lengths and times depend upon relative motion, however, it is found that mass, which is the remaining factor in the expression for energy due to motion, also depends upon relative velocities. the dependence is such that if a body takes up an amount of energy $e$ with respect to a certain system, the body behaves, to measurements made from that system, as though its mass had been increased by an amount $e/c^ $, where $c$ is as usual the velocity of light.] [this should not startle us. the key to the situation lies in the italicized words above, which indicate that the answer to the query whether a body has taken up energy or not depends upon the seat of observation. if i take up my location on the system $s$, and you on the system $s'$, and if we find that we are in relative motion, we must make some assumption about the energy which was necessary, initially, to get us into this condition. suppose we are on two passing trains.]* [the chances are that either of us will assume that he is at rest and that it is the other train which moves, although if sufficiently sophisticated one of us may assume that he is moving and that the other train is at rest.] [whatever our assumption, whatever the system, the localization of the energy that is carried in latent form by our systems depends upon this assumption. indeed, if our systems are of differing mass, our assumptions will even govern our ideas of the amount of energy which is represented by our relative motion; if your system be the more massive, more energy would have to be localized in it than in mine to produce our relative motion. if we did not have the universal principle of relativity to forbid, we might make an arbitrary assumption about our motions and hence about our respective latent energies; in the presence of this veto, the only chance of adjustment lies in our masses, which must differ according to whether you or i observe them.]* [for most of the velocities with which we are familiar $e/c^ $ is, like the difference between $k$ and unity, such an extremely small quantity that the most delicate measurements fail to detect it. but the electrons in a highly evacuated tube and the particles shot out from radioactive materials attain in some cases velocities as high as eight-tenths that of light. when we measure the mass of such particles at different velocities we find that it actually increases with the velocity, and in accordance with the foregoing law.] [this observation, in fact, antedates einstein's explanation, which is far more satisfactory than the earlier differentiation between "normal mass" and "electrical mass" which was called upon to account for the increase.]* [but if the quantity $e/c^ $ is to be considered as an actual increase in mass, may it not be possible that all mass is energy? this would lead to the conclusion that the energy stored up in any mass is $mc^ $. the value is very great, since $c$ is so large; but it is in good agreement with the internal energy of the atom as calculated from other considerations. it is obvious that conservation of mass and of momentum cannot both hold good under a theory that translates the one into the other. mass is then not considered by einstein as conservative in the ordinary sense, but it is the total quantity of mass plus energy in any closed system that remains constant. small amounts of energy may be transformed into mass, and vice versa.] [other features of the theory which are often displayed as consequences are really more in the nature of assumptions. it will be recalled that when we had agreed upon the necessity of employing signals of some sort, we selected as the means of signalling the speediest messenger with which we happened to be acquainted. our subsequent difficulties were largely due to the impossibility of making a proper allowance for this messenger's speed, even though we knew its numerical value; and as a consequence, this speed enters into our formulae. now we have not said in so many words that $c$ is the greatest speed attainable, but we have tacitly assumed that it is. we need not, therefore, be surprised if our formulae give us absurd results for speeds higher than $c$, and indicate the impossibility of ever attaining these. whatever we put into a problem the algebra is bound to give us back. if we look at our formula for $k$, we see that in the event of $v$ equalling $c$, lengths become zero and times infinite. the light messenger itself, then, has no dimension; and for it time stands still. if we suppose $v$ to be greater than $c$, we get even more bizarre results, for then the factor $k$ is the square root of a negative number, or as the mathematician calls it an "imaginary" quantity; and with it, lengths and times become imaginary too. the fact that time stops for it, and the fact that it is the limiting velocity, give to $c$ certain of the attributes of the mathematician's infinity. certainly if it can never be exceeded, we must have a new formula for the composition of velocities. otherwise when my system passes yours at a speed of , miles per second, while yours passes a third in the same direction at the same velocity, i shall be passing this third framework at the forbidden velocity of , miles per second--greater than $c$. in fact einstein is able to show that an old formula, which had already been found to connect the speed of light in a material medium with the speed of that medium, will now serve universally for the composition of velocities. when we combine the velocities $v$ and $u$, instead of getting the resultant $v+u$ as we would have supposed, we get the resultant $$(v + u) / ( + (uv / c^ ))\,,$$ or $$c^ (v + u) / (c^ + uv)$$ this need not surprise us either, if we will but reflect that the second velocity effects a second revision of length and time measurements between the systems involved. and now, if we let either $v$, or $u$, or even both of them, take the value $c$, the resultant still is $c$. in another way we have found $c$ to behave like the mathematician's infinity, to which, in the words of the blind poet, if we add untold thousands, we effect no real increment. assumption and consequence a good many correspondents who have given the subject sufficient thought to realize that the limiting character of the velocity $c$ is really read into einstein's system by assumption have written, in more or less perturbed inquiry, to know whether this does not invalidate the whole structure. the answer, of course, is yes--provided you can show this assumption to be invalid. the same answer may be made of any scientific doctrine whatever, and in reference to any one of the multitudinous assumptions underlying it. if we were to discover, tomorrow, a way of sending signals absolutely instantaneously, einstein's whole structure would collapse as soon as we had agreed to use this new method. if we were to discover a signalling agent with finite velocity greater than that of light, relativity would persist with this velocity written in its formulae in the place of $c$. it is a mistake to quote einstein's theory in support of the statement that such a velocity can never be. an assumption proves its consequences, but never can prove itself; it must remain always an assumption. but in the presence of long human experience supporting einstein's assumption that no velocity in excess of $c$ can be found, it is fair to demand that it be disputed not with argument but with demonstration. the one line of argument that would hold out a priori hope of reducing the assumption to an absurdity would be one based on the familiar idea of adding velocities; but einstein has spiked this argument before it is started by replacing the direct addition of velocities with another method of combining them that fits in with his assumption and as well with the observed facts. the burden of proof is then on the prosecution; anyone who would contradict our assertion that $c$ is the greatest velocity attainable may do so only by showing us a greater one. until this has been done, the admission that it may properly be attempted can in no way be construed as a confession of weakness on the part of einstein. it may be well to point out that in no event may analogy be drawn with sound, as many have tried to do. in the first place sound requires a material medium and its velocity with regard to this rather than relative to the observer we know to be fixed; in the second place, requiring a material medium, sound is not a universal signalling agent; in the third place, we know definitely that its velocity can be exceeded, and are therefore barred from making the assumption necessary to establish the analogy. the very extraordinary behavior of light in presenting a velocity that is the same for all observers, and in refusing to betray the least material evidence of any medium for its transmission, rather fortifies us in believing that einstein's assumption regarding the ultimate character of this velocity is in accord with the nature of things. relativity and the layman a great deal can be said in the direction of general comment making the special theory and its surprising accompaniments easier of acceptance, and we shall conclude the present discussion by saying some of these things.]* [it has been objected that the various effects catalogued above are only apparent, due to the finite velocity of light--that the real shape and size of a body or the real time of an event cannot be affected by the point of view or the motion of an observer. this argument would be perfectly valid, if there were real times and distances; but there are not. these are earth-bound notions, due to our experience on an apparently motionless platform, with slow-moving bodies. under these circumstances different observations of the same thing or of the same event agree. but when we no longer have the solid earth to stand on, and are dealing with velocities so high that the relativity effects become appreciable, there is no standard by which to resolve the disagreements. no one of the observations can claim to be nearer reality than any other. to demand the real size of a thing is to demand a stationary observer or an instantaneous means of information. both are impossible. when relativity asks us to give up our earth-bound notions of absolute space and absolute time the sensation, at first, is that we have nothing left to stand on. so must the contemporaries of columbus have felt when told that the earth rested on--nothing. the remedy too is similar. just as they had to be taught that falling is a local affair, that the earth is self-contained, and needs no external support--so we must be taught that space and time standards are local affairs. each moving body carries its own space and time standards with it; it is self-contained. it does not need to reach out for eternal support, for an absolute space and time that can never quite be attained. all we ever need to know is the relation of the other fellow's space and time standards to our own. this is the first thing relativity teaches us.] [the consequences of einstein's assumptions have led many to reject the theory of relativity, on the ground that its conclusions are contrary to common sense--as they undoubtedly are. but to the contemporaries of copernicus and galileo the theory that the earth rotates on its axis and revolves around the sun was contrary to common sense; yet this theory prevailed. there is nothing sacred about common sense; in the last analysis its judgments are based on the accumulated experience of the human race. from the beginning of the world up to the present generation, no bodies were known whose velocities were not extremely small compared with that of light. the development of modern physics has led to discovery of very much larger velocities, some as high as , miles per second. it is not to be wondered at that such an enlargement of our experience requires a corresponding enlargement or generalization of the concepts of space and time. just as the presupposition of primitive man that the earth was flat had to be given up in the light of advancing knowledge, so we are now called upon to give up our presupposition that space and time are absolute and independent in their nature. the reader must not expect to understand the theory of relativity in the sense of making it fit in with his previous ideas. if the theory be right these ideas are wrong and must be modified, a process apt to be painful.] [all the reader can do is to become familiar with the new concepts, just as a child gets used to the simple relations and quantities he meets until he "understands" them.] [mr. francis has said something of the utmost significance when he points out that "understanding" really means nothing in the world except familiarity and accustomedness.]* [the one thing about the relativity doctrine that we can hope thus to understand at once and without pain is the logical process used in arriving at our results.] [particularly is it hard to give a satisfactory explanation of the theory in popular language, because the language itself is based on the old concepts; the only language which is really adequate is that of mathematics.] [unless we have, in addition to the terms of our ordinary knowledge, a set of definitions that comes with a wide knowledge of mathematics and a lively sense of the reality of mathematical constructions, we are likely to view the theory of relativity through a fog of familiar terms suddenly become self-contradictory and deceptive. not that we are unfamiliar with the idea that some of our habitual notions may be wrong; but knowledge of their illusory nature arises and becomes convincing only with time. we may now be ready to grant that the earth, seemingly so solid, is really a whirling globe rushing through space; but we are no more ready immediately to accept the bald assertion that this space is not what it seems than our ancestors were to accept the idea that the earth was round or that it moved.] [what we must have, if we are to comprehend relativity with any degree of thoroughness, is the mathematician's attitude toward his assumptions, and his complete readiness to swap one set of assumptions for another as a mere part of the day's work, the spirit of which i have endeavored to convey in the chapter on non-euclidean geometry.]* physics vs. metaphysics [the ideas of relativity may seem, at first sight, to be giving us a new and metaphysical theory of time and space. new, doubtless; but certainly the theory was meant by its author to be quite the opposite of metaphysical. our actual perception of space is by measurement, real and imagined, of distances between objects, just as our actual perception of time is by measurement. is it not less metaphysical to accept space and time as our measurements present them to us, than to invent hypotheses to force our perceptual space into an absolute space that is forever hidden from us?] [in order not to be metaphysical, we must eliminate our preconceived notions of space and time and motion, and focus our attention upon the indications of our instruments of observation, as affording the only objective manifestations of these qualities and therefore the only attributes which we can consider as functions of observed phenomena.] [einstein has consistently followed the teachings of experience, and completely freed himself from metaphysics.] [that this is not always easy to do is clear, i think, if we will recall the highly metaphysical character often taken by the objections to action-at-a-distance theories and concepts; and if we will remind ourselves that it was on purely metaphysical grounds that newton refused to countenance huyghens' wave theory of light. whether, as in the one case, it leads us to valid conclusions, or, as in the other, to false ones, metaphysical reasoning is something to avoid. einstein, i think, has avoided it about as thoroughly as anyone ever did.]* v that parallel postulate modern geometric methods; the dividing line between euclidean and non-euclidean; and the significance of the latter by the editor the science of geometry has undergone a revolution of which the outsider is not informed, but which it is necessary to understand if we are to attain any comprehension of the geometric formulation of einstein's results; and especially if we are to appreciate why it is proper and desirable to formulate these results geometrically at all. the classical geometer regarded his science from a narrow viewpoint, as the study of a certain set of observed phenomena--those of the space about us, considered as an entity in itself and divorced from everything in it. it is clear that some things about that space are not as they appear (optical illusions), and that other things about it are true but by no means apparent (the sum-of-squares property of a right triangle, the formulæ for surface and volume of a sphere, etc.). while many things about space are "obvious," these need in the one case disproof and in the other discovery and proof. with all their love of mental processes for their own sake, it is then not surprising that the greeks should have set themselves the task of proving by logical process the properties of space, which a less thoughtful folk would have regarded as a subject only for observational and experimental determination. but, abstract or concrete, the logical structure must have a starting point; and it is fair to demand that this consist in a statement of the terms we are going to use and the meanings we are going to attach to them. in other words, the first thing on the program will be a definition, or more probably, several definitions. now the modern scientist has a somewhat iconoclastic viewpoint toward definitions, and especially toward the definition of his very most fundamental ideas. we do not speak here in terms of dictionary definitions. these have for object the eminent necessity of explaining the meaning and use of a word to some one who has just met it for the first time. it is easy enough to do this, if the doer possesses a good command of the language. it is not even a matter of grave concern that the words used in the definition be themselves known to the reader; if they are not, he must make their acquaintance too. dr. johnson's celebrated definition of a needle stands as perpetual evidence that when he cannot define a simple thing in terms of things still simpler, the lexicographer is forced to define it in terms of things more complex. or we might demonstrate this by noting that the best dictionaries are driven to define such words as "and" and "but" by using such complex notions as are embodied in "connective," "continuative," "adversative," and "particle." it is otherwise with the scientist who undertakes to lay down a definition as the basis of further procedure in building up the tissue of his science. here a degree of rigorous logic is called for which would be as superfluous in the dictionary as the effort there to attain it would be out of place. the scientist, in building up a logical structure that will withstand every assault, must define everything, not in terms of something which he is more or less warranted in supposing his audience to know about, but actually in terms of things that have already been defined. this really means that he must explain what he is talking about in terms of simpler ideas and simpler things, which is precisely what the lexicographer does not have to worry about. this is why it is quite trivial to quote a dictionary definition of time or space or matter or force or motion in settlement of a controversy of scientific or semi-scientific nature. terms we cannot define but the scientist who attempts to carry out this ideal system of defining everything in terms of what precedes meets one obstacle which he cannot surmount directly. even a layman can construct a passable definition of a complex thing like a parallelopiped, in terms of simpler concepts like point, line, plane and parallel. but who shall define point in terms of something simpler and something which precedes point in the formulation of geometry? the scientist is embarrassed, not in handling the complicated later parts of his work, but in the very beginning, in dealing with the simplest concepts with which he has to deal. suppose a dictionary were to be compiled with the definitions arranged in logical rather than alphabetical order: every word to be defined by the use only of words that have already been defined. the further back toward the beginning we push this project, the harder it gets. obviously we can never define the first word, or the second, save as synonymous with the first. in fact we should need a dozen words, more or less, to start with--god-given words which we cannot define and shall not try to define, but of which we must agree that we know the significance. then we have tools for further procedure; we can start with, say, the thirteenth word and define all the rest of the words in the language, in strictly logical fashion. what we have said about definitions applies equally to statements of fact, of the sort which are going to constitute the body of our science. in the absence of simpler facts to cite as authority, we shall never be able to prove anything, however simple this may itself be; and in fact the simpler it be, the harder it is to find something simpler to underlie it. if we are to have a logical structure of any sort, we must begin by laying down certain terms which we shall not attempt to define, and certain statements which we shall not try to prove. mathematics, physics, chemistry--in the large and in all their many minor fields--all these must start somewhere. instead of deceiving ourselves as to the circumstances surrounding their start, we prefer to be quite frank in recognizing that they start where we decide to start them. if we don't like one set of undefined terms as the foundation, by all means let us try another. but always we must have such a set. the classical geometer sensed the difficulty of defining his first terms. but he supposed that he had met it when he defined these in words free of technical significance. "a point is that which has position without size" seemed to him an adequate definition, because "position" and "size" are words of the ordinary language with which we may all be assumed familiar. but today we feel that "position" and "size" represent ideas that are not necessarily more fundamental than those of "line" and "point," and that such a definition begs the question. we get nowhere by replacing the undefined terms "point" and "line" and "plane," which really everybody understands, by other undefined terms which nobody understands any better. in handling the facts that it was inconvenient to prove, the classical geometer came closer to modern practice. he laid down at the beginning a few statements which he called "axioms," and which he considered to be so self-evident that demonstration was superfluous. that the term "self-evident" left room for a vast amount of ambiguity appears to have escaped him altogether. his axioms were axioms solely because they were obviously true. laying the foundation the modern geometer falls in with euclid when he writes an elementary text, satisfying the beginner's demand for apparent rigor by defining point and line in some fashion. but when he addresses to his peers an effort to clarify the foundations of geometry to a further degree of rigor and lucidity than has ever before been attained, he meets these difficulties from another quarter. in the first place he is always in search of the utmost possible generality, for he has found this to be his most effective tool, enabling him as it does to make a single general statement take the place and do the work of many particular statements. the classical geometer attained generality of a sort, for all his statements were of any point or line or plane. but the modern geometer, confronted with a relation that holds among points or between points and lines, at once goes to speculating whether there are not other elements among or between which it holds. the classical geometer isn't interested in this question at all, because he is seeking the absolute truth about the points and lines and planes which he sees as the elements of space; to him it is actually an object so to circumscribe his statements that they may by no possibility refer to anything other than these elements. whereas the modern geometer feels that his primary concern is with the fabric of logical propositions that he is building up, and not at all with the elements about which those propositions revolve. it is of obvious value if the mathematician can lay down a proposition true of points, lines and planes. but he would much rather lay down a proposition true at once of these and of numerous other things; for such a proposition will group more phenomena under a single principle. he feels that on pure scientific grounds there is quite as much interest in any one set of elements to which his proposition applies as there is in any other; that if any person is to confine his attention to the set that stands for the physicist's space, that person ought to be the physicist, not the geometer. if he has produced a tool which the physicist can use, the physicist is welcome to use it; but the geometer cannot understand why, on that ground, he should be asked to confine his attention to the materials on which the physicist employs that tool. it will be alleged that points and lines and planes lie in the mathematician's domain, and that the other things to which his propositions may apply may not so lie--and especially that if he will not name them in advance he cannot expect that they will so lie. but the mathematician will not admit this. if mathematics is defined on narrow grounds as the science of number, even the point and line and plane may be excluded from its field. if any wider definition be sought--and of course one must be--there is just one definition that the mathematician will accept: dr. keyser's statement that "mathematics is the art or science of rigorous thinking." the immediate concern of this science is the means of rigorous thinking--undefined terms and definitions, axioms and propositions. its collateral concern is the things to which these may apply, the things which may be thought about rigorously--everything. but now the mathematician's domain is so vastly extended that it becomes more than ever important for him to attain the utmost generality in all his pronouncements. one barrier to such generalization is the very name "geometry," with the restricted significance which its derivation and long usage carry. the geometer therefore must have it distinctly understood that for him "geometry" means simply the process of deducing a set of propositions from a set of undefined primitive terms and axioms; and that when he speaks of "a geometry" he means some particular set of propositions so deduced, together with the axioms, etc., on which they are based. if you take a new set of axioms you get a new geometry. the geometer will, if you insist, go on calling his undefined terms by the familiar names "point," "line," "plane." but you must distinctly understand that this is a concession to usage, and that you are not for a moment to restrict the application of his statements in any way. he would much prefer, however, to be allowed new names for his elements, to say "we start with three elements of different sorts, which we assume to exist, and to which we attach the names a, b and c--or if you prefer, primary, secondary and tertiary elements--or yet again, names possessing no intrinsic significance at all, such as ching, chang and chung." he will then lay down whatever statements he requires to serve the purposes of the ancient axioms, all of these referring to some one or more of his elements. then he is ready for the serious business of proving that, all his hypotheses being granted, his elements a, b and c, or i, ii and iii, or ching, chang and chung, are subject to this and that and the other propositions. the objection will be urged that the mathematician who does all this usurps the place of the logician. a little reflection will show this not to be the case. the logician in fact occupies the same position with reference to the geometer that the geometer occupies with reference to the physicist, the chemist, the arithmetician, the engineer, or anybody else whose primary interest lies with some particular set of elements to which the geometer's system applies. the mathematician is the tool-maker of all science, but he does not make his own tools--these the logician supplies. the logician in turn never descends to the actual practice of rigorous thinking, save as he must necessarily do this in laying down the general procedures which govern rigorous thinking. he is interested in processes, not in their application. he tells us that if a proposition is true its converse may be true or false or ambiguous, but its contrapositive is always true, while its negative is always false. but he never, from a particular proposition "if a is b then c is d," draws the particular contrapositive inference "if c is not d then a is not b." that is the mathematician's business. the rÃ�le of geometry the mathematician is the quantity-production man of science. in his absence, the worker in each narrower field where the elements under discussion take particular concrete forms could work out, for himself, the propositions of the logical structure that applies to those elements. but it would then be found that the engineer had duplicated the work of the physicist, and so for many other cases; for the whole trend of modern science is toward showing that the same background of principles lies at the root of all things. so the mathematician develops the fabric of propositions that follows from this, that and the other group of assumptions, and does this without in the least concerning himself as to the nature of the elements of which these propositions may be true. he knows only that they are true for any elements of which his assumptions are true, and that is all he needs to know. whenever the worker in some particular field finds that a certain group of the geometer's assumptions is true for his elements, the geometry of those elements is ready at hand for him to use. now it is all right purposely to avoid knowing what it is that we are talking about, so that the names of these things shall constitute mere blank forms which may be filled in, when and if we wish, by the names of any things in the universe of which our "axioms" turn out to be true. but what about these axioms themselves? when we lay them down in ignorance of the identity of the elements to which they may eventually apply, they cannot by any possibility be "self-evident." we may, at pleasure, accept as self-evident a statement about points and lines and planes; or one about electrons, centimeters and seconds; or one about integers, fractions, and irrational numbers; or one about any other concrete thing or things whatever. but we cannot accept as self-evident a statement about chings, changs and chungs. so we must base our "axioms" on some other ground than this; and our modern geometer has his ground ready and waiting. he accepts his axioms on the ground that it pleases him to do so. to avoid all suggestion that they are supposed to be self-evident, or even necessarily true, he drops the term "axiom" and substitutes for it the more color-less word "postulate." a postulate is merely something that we agreed to accept, for the time being, as a basis of further argument. if it turns out to be true, or if we can find circumstances under which and elements to which it applies, any conclusions which we deduce from it by trustworthy processes are valid within the same limitations. and the propositions which tell us that, if our postulates are true, such and such conclusions are true--they, too, are valid, but without any reservation at all! perhaps an illustration of just what this means will not be out of place. let it be admitted, as a postulate, that $ + $ is greater, by , than $ + $. let us then consider the statement: "if $ + = $, then $ + = $." we know--at least we are quite certain--that $ + $ is not equal to , if by " " and " " and " " we mean what you think we mean. we are equally sure, on the same grounds, that $ + $ is not equal to . but, under the one assumption that we have permitted ourselves, it is unquestionable that if $ + $ were equal to , then $ + $ certainly would be equal to . so, while the conclusion of the proposition which i have put in quotation marks is altogether false, the proposition itself, under our assumption, is entirely true. i have taken an illustration designed to be striking rather than to possess scientific interest; i could just as easily have shown a true proposition leading to a false conclusion, but of such sort that it would be of decided scientific interest as telling us one of the consequences of a certain assumption. what may we take for granted? this is all very fine; but how does the geometer know what postulates to lay down? one is tempted to say that he is at liberty to postulate anything he pleases, and investigate the results; and that whether or not his postulate ever be realized, the propositions that he deduces from it, being true, are of scientific interest. actually, however, it is not quite as simple as all that. if it were sufficient to make a single postulate it would be as simple as all that; but it turns out that this is not sufficient any more than it is sufficient to have a single undefined term. we must have several postulates; and they must be such, as a whole, that a geometry flows out of them. the requirements are three. in the first place, the system of postulates must be "categorical" or complete--there must be enough of them, and they must cover enough ground, for the support of a complete system of geometry. in practice the test for this is direct. if we got to a point in the building up of a geometry where we could not prove whether a certain thing was one way always, or always the other way, or sometimes one way and sometimes the other, we should conclude that we needed an additional postulate covering this ground directly or indirectly. and we should make that postulate--because it is precisely the things that we can't prove which, in practical work, we agree to assume. even euclid had to adopt this philosophy. in the second place, the system of postulates must be consistent--no one or more of them may lead, individually or collectively, to consequences that contradict the results or any other or others. if in the course of building up a geometry we find we have proved two propositions that deny one another, we search out the implied contradiction in our postulates and remedy it. finally, the postulates ought to be independent. it should not be possible to prove any one of them as a consequence of the others. if this property fails, the geometry does not fail with it; but it is seriously disfigured by the superfluity of assumptions, and one of them should be eliminated. if we are to assume anything unnecessarily, we may as well assume the whole geometry and be done with it. the geometer's business then is to draw up a set of postulates. this he may do on any basis whatever. they may be suggested to him by the behavior of points, lines and planes, or by some other concrete phenomena; they may with equal propriety be the product of an inventive imagination. on proceeding to deduce their consequences, he will discover and remedy any lack of categoricity or consistence or independence which his original system of postulates may have lacked. in the end he will have so large a body of propositions without contradiction or failure that he will conclude the propriety of his postulates to have been established, and the geometry based on them to be a valid one. and what is it all about? is this geometry ever realized? strictly it is not the geometer's business to ask or answer this question. but research develops two viewpoints. there is always the man who indulges in the pursuit of facts for their sake alone, and equally the man who wants to see his new facts lead to something else. one great mathematician is quoted as enunciating a new theory of surpassing mathematical beauty with the climacteric remark "and, thank god, no one will ever be able to find any use for it." an equally distinguished contemporary, on being interrogated concerning possible applications for one of his most abstruse theorems, replied that he knew no present use for it; but that long experience had made him confident that the mathematician would never develop any tool, however remote from immediate utility, for which the delvers in other fields would not presently find some use. if we wish, however, we may inquire with perfect propriety, from the side lines, whether a given geometry is ever realized. we may learn that so far as has yet been discovered there are no elements for which all its postulates are verified, and that there is therefore no realization known. on the other hand, we may more likely find that many different sets of elements are such that the postulates can be interpreted as applying to them, and that we therefore have numerous realizations of the geometry. as a human being the geometer may be interested in all this, but as a geometer it really makes little difference to him. when we look at space about us, we see it, for some reason grounded in the psychological history of the human race, as made up in the small of points, which go to make up lines, which in turn constitute planes. or we can start at the other end and break space down first into planes, then into lines, finally into points. our perceptions and conceptions of these points, lines and planes are very definite indeed; it seems indeed, as the greeks thought, that certain things about them are self-evident. if we wish to take these self-evident properties of point, line and plane, and combine with them enough additional hair-splitting specifications to assure the modern geometer that we have really a categorical system of assumptions, we shall have the basis of a perfectly good system of geometry. this will be what we unavoidably think of as the absolute truth with regard to the space about us; but you mustn't say so in the presence of the geometer. it will also be what we call the euclidean geometry. it has been satisfactory in the last degree, because not only space, but pretty much every other system of two or three elements bearing any relations to one another can be made, by employing as a means of interpretation the cartesian scheme of plotting, to fit into the framework of euclidean geometry. but it is not the only thing in the world of conceptual possibilities, and it begins to appear that it may not even be the only thing in the world of cold hard fact that surrounds us. to see just how this is so we must return to euclid, and survey the historical development of geometry from his day to the present time. euclid's geometry point, line and plane euclid attempts to define. modern objection to these efforts was made clear above. against euclid's specific performance we urge the further specific fault that his "definitions" are really assumptions bestowing certain properties upon points, lines and planes. these assumptions euclid supplements in his axioms; and in the process of proving propositions he unconsciously supplements them still further. this is to be expected from one whose justification for laying down an axiom was the alleged obvious character of the statement made. if some things are too obvious to require demonstration, others may be admitted as too obvious to demand explicit statement at all. thus, if euclid has two points a and b in a plane, on opposite sides of a line m, he will draw the line ab and without further formality speak of the point c in which it intersects m. that it does so intercept m, rather than in some way dodges it, is really an assumption as to the nature of lines and planes. or again, euclid will speak of a point d on the line ab, between or outside the points a and b, without making the formal assumption necessary to insure that the line is "full" of points so that such a point as d must exist. that such assumptions as these are necessary follows from our previous remarks. if we think of our geometry as dealing with "chings," "changs," and "chungs," or with elements i, ii and iii, it is no longer in the least degree obvious that the simplest property in the world applies to these elements. if we wish any property to prevail we must state it explicitly. with the postulates embodied in his definitions, those stated in his axioms, and those which he reads into his structure by his methods of proof, euclid has a categorical set--enough to serve as foundation for a geometry. we may then climb into euclid's shoes and take the next step with him. we follow him while he proves a number of things about intersecting lines and about triangles. to be sure, when he proves that two triangles are identically constituted by moving one of them over on top of the other, we may protest on the ground that the admission of motion, especially of motion thus imposed from without, into a geometry of things is not beyond dispute. if euclid has caught our modern viewpoint, he will rejoin that if we have any doubts as to the admissibility of motion he will lay down a postulate admitting it, and we shall be silenced. having exhausted for the present the interest of intersecting lines, our guide now passes to a consideration of lines in the same plane that never meet. he defines such lines as parallel. if we object that he should show the existence of a derived concept like this before laying down a definition that calls for it to exist, he can show that two lines drawn perpendicular to the same line never meet. he will execute this proof by a special sort of superposition, which requires that the plane be folded over on itself, through the third dimension of surrounding space, rather than merely slid along upon itself. we remain quiet while euclid demonstrates that if two lines are cut by any transversal in such a way as to make corresponding angles at the two intersections equal, the lines are parallel. it is then in order to investigate the converse: if the lines are parallel to begin with, are the angles equal? axioms made to order this sounds innocent enough; but in no way was euclid able to devise a proof--or, for that matter, a disproof. so he took the only way out, and said that if the lines were parallel, obviously they extended in the same direction and made the angles equal. the thing was so obvious, he argued, that it was really an axiom and he didn't have to prove it; so he stated it as an axiom and proceeded. he didn't state it in precisely the form i have used; he apparently cast about for the form in which it would appear most obvious, and found a statement that suited him better than this one, and that comes to the same thing. this statement tells us that if the transversal makes two corresponding angles unequal, the lines that it cuts are not parallel and do meet if sufficiently prolonged. but wisely enough, he did not transplant this axiom, once he had arrived at it, to the beginning of the book where the other axioms were grouped; he left it right where it was, following the proposition that if the angles were equal the lines were parallel. this of course was so that it might appeal back, for its claim to obviousness, to its demonstrated converse of the proposition. euclid must have been dissatisfied with this cutting of the gordian knot; his successors were acutely so. for twenty centuries the parallel axiom was regarded as the one blemish in an otherwise perfect work; every respectable mathematician had his shot at removing the defect by "proving" the objectionable axiom. the procedure was always the same: expunge the parallel axiom, in its place write another more or less "obvious" assumption, and from this derive the parallel statement more or less directly. thus if we may assume that the sum of the angles of a triangle is always exactly degrees, or that there can be drawn only one line through a given point parallel to a given line, we can prove euclid's axiom. sometimes the substitute assumption was openly made and stated, as in the two instances cited; as often it was admitted into the demonstration implicitly, as when it is quietly assumed that we can draw a triangle similar to any given triangle and with area as great as we please, or when parallels are "defined" as everywhere equidistant. but such "proofs" never satisfied anyone other than the man who made them; the search went merrily on for a valid "proof" that should not in substance assume the thing to be proved. locating the discrepancy saccheri, an italian jesuit, would have struck bottom if he had had a little more imagination. he gave an exhaustive reductio ad absurdum, on the basis of the angle-sum theorem. this sum must be (a) greater than or (b) equal to or (c) less than degrees. saccheri showed that if one of these alternatives occurs in a single triangle, it must occur in every triangle. the first case gave little trouble; admitting the possibility of superposing in the special manner mentioned above, which he did implicitly, he showed that this "obtuse-angled hypothesis" contradicted itself. he pursued the "acute-angled hypothesis" for a long time before he satisfied himself that he had caught it, too, in an inconsistency. this left only the "right-angled hypothesis," proving the euclidean angle-sum theory and through it the parallel postulate. but saccheri was wrong: he had found no actual contradiction in the acute-angled hypothesis--for none exists therein. the full facts were probably first known to gauss, who had a finger in every mathematical pie that had to do with the transition to modern times. they were first published by lobatchewsky, the russian, who anticipated the hungarian john bolyai by a narrow margin. all three worked independently of saccheri, whose book, though theoretically available in italian libraries, was actually lost to sight and had to be rediscovered in recent years. like saccheri, lobatchewsky investigated alternative possibilities. but he chose another point of attack: through a given point it must be possible to draw, in the same plane with a given line (a) no lines or (b) one line or (c) a plurality of lines, which shall not meet the given line. the word parallel is defined only in terms of the second of these hypotheses, so we avoid it here. these three cases correspond, respectively, to those of saccheri. the first case lobatchewsky ruled out just as did saccheri, but accepting consciously the proviso attached to its elimination; the third he could not rule out. he developed the consequences of this hypothesis as far as euclid develops those of the second one, sketching in a full outline for a system of geometry and trigonometry based on a plurality of "non-cutters." this geometry constitutes a coherent whole, without a logical flaw. this made it plain what was the matter with euclid's parallel axiom. nobody could prove it from his other assumptions because it is not a consequence of these. true or false, it is independent of them. trinity church is in new york, faneuil hall is in boston, but faneuil hall is not in boston because trinity is in new york; and we could not prove that faneuil hall was in boston if we knew nothing about america save that trinity is in new york. the mathematicians of , years had been pursuing, on a gigantic scale, a delusion of post hoc, ergo propter hoc. what the postulate really does moreover, in the absence of an assumption covering the ground, we shall not know which of the alternatives (a), (b), (c) holds. but when one holds in a single case it holds permanently, as saccheri and lobatchewsky both showed. so we cannot proceed on this indefinite basis; we must know which one is to hold. without the parallel postulate or a substitute therefor that shall tell us the same thing or tell us something different, we have not got a categorical set of assumptions--we cannot build a geometry at all. that is why euclid had to have his parallel postulate before he could proceed. that is why his successors had to have an assumption equivalent to his. the reason why it took so long for this to percolate into the understanding of the mathematicians was that they were thinking, not in terms of the modern geometry and about undefined elements; but in terms of the old geometry and about strictly defined and circumscribed elements. if we understand what is meant by euclidean line and plane, of course the parallel postulate, to use the old geometer's word, is true--of course, to adopt the modern viewpoint, if we agree to employ an element to which that assumption applies, the assumption is realized. the very fact of accepting the "straight" line and the "flat" plane of euclid constitutes acceptance of his parallel postulate--the only thing that can separate his geometry from other geometries. but of course we can't prove it; the prior postulates which we would have to use in such an attempt apply where it does not apply, and hence it cannot possibly be consequences of. to all this the classical euclidean rejoins that we seem to have in mind elements of some sort to which, with one reservation, his postulates apply. he wants to know what these elements look like. we can, and must, produce them--else our talk about generality is mere drivel. but we must take care that the euclidean geometer does not try to apply to our elements the notions of straightness and flatness which inhere in the parallel postulate. we cannot satisfy and defy that postulate at the same time. if we do not insist on this point, we shall find that we are reading non-euclidean properties into euclidean geometry, and interpreting the elements of the latter as straight lines that are not straight, flat planes that are not flat. it is not the mission of non-euclidean geometry thus to deny the possibility of euclidean geometry; it merely demands a place of equal honor. the geometry of surfaces let us ask the euclidean geometer whether he can recognize his plane after we have crumpled it up like a piece of paper en route to the waste basket. he will hesitate only long enough to recall that in the special case of superposition he has reserved for himself the privilege of deforming his own plane, and to realize that he can always iron his plane out smooth again after we are through with it. this emphasizes the true nature of the two-dimensionality which is the fundamental characteristic of the plane (and of other things, as we shall directly see). the plane is two-dimensional in points not because two sets of mutually perpendicular euclidean straight lines can be drawn in it defining directions of north-south and east-west, but because a point in it can be located by means of two measures. the same statement may be made of anything whatever to which the term "surface" is applicable; anything, however crumpled or irregular it be, that possesses length and breadth without thickness. the surface of a sphere, of a cylinder, of an ellipsoid, of a cone, of a doughnut (mathematically known as a torus), of a gear wheel, of a french horn, all these possess two-dimensionality in points; on all of them we can draw lines and curves and derive a geometry of these figures. if we get away from the notion that geometry of two dimensions must deal with planes, and adopt in place of this idea the broader restriction that it shall deal with surfaces, we shall have the generalization which the euclidean has demanded that we produce, and the one which in the hands of the modern geometer has shown results. in this two-dimensional geometry of surfaces in general, that of the plane is merely one special case. certain of the features met in that case are general. if we agree that we know what we mean by distance, we find that on every surface there is a shortest distance between two points, together with a series of lines or curves along which such distances are taken. these lines or curves we call geodesics. on the plane the geodesic is the straight line. on surfaces in general the geodesic, whatever its particular and peculiar shape, plays the same rôle that is played by the straight line in the plane; it is the secondary element of the geometry, the surface itself and all other surfaces of its type are the tertiary elements. and it is a fact that we can take all the possible spheres, or all the possible french-horn surfaces, and conceive of space as we know it being broken down by analysis into these surfaces instead of into planes. the only reason we habitually decompose space into planes is because it comes natural to us to think that way. but geometric points, lines and surfaces must be recognized as abstractions without actual existence, for all of them lack one or more of the three dimensions which such existence implies. these figures exist in our minds but not in the external world about us. so any decomposition of space into geometric elements is a phenomenon of the mind only; it has no parallel and no significance in the external world, and is made in one way or in another purely at our pleasure. there isn't a true, honest-to-goodness geometrical plane in existence any more than there is an honest-to-goodness spherical surface: so on intrinsic grounds one decomposition is as reasonable as another. certain of the most fundamental postulates are obeyed by all surfaces. as we attempt to discriminate between surfaces of different types, and get, for instance, a geometry that shall be valid for spheres and ellipsoids but not for conicoids in general, we must do so by bringing in additional postulates that embody the necessary restrictions. a characteristic shared by planes, spheres, and various other surfaces is that the geodesics can be freely slid along upon themselves and will coincide with themselves in all positions when thus slid; with a similar arrangement for the surface itself. but the plane stands almost unique among surfaces in that it does not force us to distinguish between its two sides; we can turn it over and still it will coincide with itself; and this property belongs also to the straight line. it does not belong to the sphere, or to the great circles which are the geodesics of spherical geometry; when we turn one of these over, through the three-dimensional space that surrounds it, we find that the curvature lies in the wrong way to make superposition possible. if we postulate that superposition be possible under such treatment, we throw out the sphere and spherical geometry; if we postulate that superposition be only by sliding the surface upon itself we admit that geometry--as saccheri failed to see, as lobatchewsky realized, and as riemann showed at great length in rehabilitating the "obtuse-angled hypothesis." lobatchewsky's acute-angled geometry is realized on a surface of the proper sort, which admits of unrestricted superposition; but it is not the sort of a surface that i care to discuss in an article of this scope. euclidean geometry is the natural and easy one, i suppose, because it makes it easy to stop with three dimensions. if we take a secondary element, a geodesic, which is "curved" in the euclidean sense, we get a tertiary element, a surface, which is likewise curved. then unless we are to make an altogether abrupt and unreasonable break, we shall find that just as the curved geodesic generated a curved surface, the curved surface must give rise to a "curved space"; and just as the curved geodesic needed a second dimension to curve into, and the curved surface a third, so the curved three-space requires a fourth. once started on this sort of thing, there doesn't really seem to be any end. euclidean or non-euclidean nevertheless, we must face the possibility that the space we live in, or any other manifold of any sort whatever with which we deal on geometric principles, may turn out to be non-euclidean. how shall we finally determine this? by measures--the euclidean measures the angles of an actual triangle and finds the sum to be exactly degrees; or he draws parallel lines of indefinite extent and finds them to be everywhere equally distant; and from these data he concludes that our space is really euclidean. but he is not necessarily right. we ask him to level off a plot of ground by means of a plumb line. since the line always points to the earth's center, the "level" plot is actually a very small piece of a spherical surface. any test conducted on this plot will exhibit the numerical characteristics of the euclidean geometry; yet we know the geometry of this surface is riemannian. the angle-sum is really greater than degrees; lines that are everywhere equidistant are not both geodesics. the trouble, of course, is that on this plot we deal with so minute a fraction of the whole sphere that we cannot make measurements sufficiently refined to detect the departure from euclidean standards. so it is altogether sensible for us to ask: "is the universe of space about us really euclidean in whatever of realized geometry it presents to us? or is it really non-euclidean, but so vast in size that we have never yet been able to extend our measures to a sufficiently large portion of it to make the divergence from the euclidean standard discernible to us?" this discussion is necessarily fragmentary, leaving out much that the writer would prefer to include. but it is hoped that it will nevertheless make it clear that when the contestants in the einstein competition speak of a non-euclidean universe as apparently having been revealed by einstein, they mean simply that to einstein has occurred a happy expedient for testing euclideanism on a smaller scale than has heretofore been supposed possible. he has devised a new and ingenious sort of measure which, if his results be valid, enables us to operate in a smaller region while yet anticipating that any non-euclidean characteristics of the manifold with which we deal will rise above the threshold of measurement. this does not mean that euclidean lines and planes, as we picture them in our mind, are no longer non-euclidean, but merely that these concepts do not quite so closely correspond with the external reality as we had supposed. as to the precise character of the non-euclideanism which is revealed, we may leave this to later chapters and to the competing essayists. we need only point out here that it will not necessarily be restricted to the matter of parallelism. the parallel postulate is of extreme interest to us for two reasons; first because historically it was the means by which the possibilities and the importance of non-euclidean geometry were forced upon our attention; and second because it happens to be the immediate ground of distinction between euclidean geometry and two of the most interesting alternatives. but euclidean geometry is characterized, not by a single postulate, but by a considerable number of postulates. we may attempt to omit any one of these so that its ground is not specifically covered at all, or to replace any one of them by a direct alternative. we might conceivably do away with the superposition postulate entirely, and demand that figures be proved equivalent, if at all, by some more drastic test. we might do away with the postulate, first properly formulated by hilbert, on which our ideas of the property represented in the word "between" depend. we might do away with any single one of the euclidean postulates, or with any combination of two or more of them. in some cases this would lead to a lack of categoricity and we should get no geometry at all; in most cases, provided we brought a proper degree of astuteness to the formulation of alternatives for the rejected postulates, we should get a perfectly good system of non-euclidean geometry: one realized, if at all, by other elements than the euclidean point, line and plane, and one whose elements behave toward one another differently from the euclidean point, line and plane. merely to add definiteness to this chapter, i annex here the statement that in the geometry which einstein builds up as more nearly representing the true external world than does euclid's, we shall dispense with euclid's (implicit) assumption, underlying his (explicitly stated) superposition postulate, to the effect that the act of moving things about does not affect their lengths. we shall at the same time dispense with his parallel postulate. and we shall add a fourth dimension to his three--not, of course, anything in the nature of a fourth euclidean straight line perpendicular, in euclidean space, to three lines that are already perpendicular to each other, but something quite distinct from this, whose nature we shall see more exactly in the next chapter. if the present chapter has made it clear that it is proper for us to do this, and has prevented anyone from supposing that the results of doing it must be visualized in a euclidean space of three dimensions or of any number of dimensions, it will have served its purpose. vi the space-time continuum minkowski's world of events, and the way it fits into einstein's structure by the editor, except as noted seeking a basis for the secure formulation of his results, and especially a means for expressing mathematically the facts of the dependence which he had found to exist between time and space, einstein fell back upon the prior work of minkowski. it may be stated right here that the idea of time as a fourth dimension is not particularly a new one. it has been a topic of abstract speculation for the best part of a century, even on the part of those whose notions of the fourth dimension were pretty closely tied down to the idea of a fourth dimension of euclidean point-space, which would be marked by a fourth real line, perpendicular to the other three, and visible to us if we were only able to see it. moreover, every mathematician, whether or not he be inclined to this sort of mental exercise, knows well that whenever time enters his equations at all, it does so on an absolutely equal footing with each of his space coordinates, so that as far as his algebra is concerned he could never distinguish between them. when the variables $x$, $y$, $z$, $t$ come to the mathematician in connection with some physical investigation, he knows before he starts that the first three represent the dimensions of euclidean three-space and that the last stands for time. but if the algebraic expressions of such a problem were handed to him independently of all physical tie-up, he would never be able to tell, from them alone, whether one of the four variables represented time, or if so, which one to pick out for this distinction. it was minkowski who first formulated all this in a form susceptible of use in connection with the theory of relativity. his starting point lies in the distinction between the point and the event. mr. francis has brought this out rather well in his essay, being the only competitor to present the euclidean geometry as a real predecessor of newtonian science, rather than as a mere part of the newtonian system. i think his point here is very well taken. as he says, euclid looked into the world about him and saw it composed of points. ignoring all dynamic considerations, he built up in his mind a static world of points, and constructed his geometry as a scientific machine for dealing with this world in which motion played no part. it could to be sure be introduced by the observer for his own purposes, but when so introduced it was specifically postulated to be a matter of no moment at all to the points or lines or figures that were moved. it was purely an observational device, intended for the observer's convenience, and in the bargain a mental device, calling for no physical action and the play of no force. so far as euclid in his daily life was obliged to take cognizance of the fact that in the world of work-a-day realities motion existed, he must, as a true greek, have looked upon this as a most unfortunate deviation of the reality from his beautiful world of intellectual abstraction, and as something to be deplored and ignored. even in their statuary the greeks clung to this idea. a group of marvelous action, like the laocoon, they held to be distinctly a second rate production, a prostitution of the noble art; their ideal was a figure like the majestic zeus--not necessarily a mere bust, be it understood, but always a figure in repose without action. their statuary stood for things, not for action, just as their geometry stood for points, not for events. galileo and newton took a different viewpoint. they were interested in the world as it is, not as it ought to be; and if motion appears to be a fundamental part of that world, they were bound to include it in their scheme. this made it necessary for them to pay much more attention to the concept of time and its place in the world than did the greeks. in the superposition process, and even when he allowed a curve to be generated by a moving point, the sole interest which euclid had in the motion was the effect which was to be observed upon his static figures after its completion. in this effect the rate of the motion did not enter. so all questions of velocity and time are completely ignored, and we have in fact the curious spectacle of motion without time. to galileo and newton, on the other hand, the time which it took a body to pass from one point of its path to another was of paramount importance. the motion itself was the object of their study, and they recognized the part played by velocity. but galileo and newton were still sufficiently under the influence of euclid to fit the observed phenomena of motion, so far as they could, upon euclid's static world of points. this they effected by falling in with the age-old procedure of regarding time and space as something entirely disassociated and distinct. the motion of an object--in theory, of a point--was to be recorded by observing its successive positions. with each of these positions a time was to be associated, marking the instant at which the point attained that position. but in the face of this association, space and time were to be maintained as entirely separate entities. the four-dimensional world of events this severe separation of time and space minkowski has now questioned, with the statement that the elements of which the external world is composed, and which we observe, are not points at all, but are events. this calls for a revision of our whole habit of thought. it means that the perceptual world is four-dimensioned, not three-dimensioned as we have always supposed; and it means, at the very least, that the distinction between time and space is not so fundamental as we had supposed. [this should not impress us as strange or incomprehensible. what do we mean when we say that a plane is two-dimensional? simply that two coordinates, two numbers, must be given to specify the position of any point of the plane. similarly for a point in the space of our accustomed concepts we must give three numbers to fix the position--as by giving the latitude and longitude of a point on the earth and its height above sea-level. so we say this space is three-dimensional. but a material body is not merely somewhere; it is somewhere now,] or was somewhere yesterday, or will be somewhere tomorrow. the statement of position for a material object is meaningless unless we at the same time specify the time at which it held that position. [if i am considering the life-history of an object on a moving train, i must give three space-coordinates and one time-coordinate to fix each of its positions.] and each of its positions, with the time pertaining to that position, constitutes an event. the dynamic, ever-changing world about us, that shows the same aspect at no two different moments, is a world of events; and since four measures or coordinates are required to fix an event, we say this world of events is four-dimensional. if we wish to test out the soundness of this viewpoint, we may well do so by asking whether the naming of values for the four coordinates fixes the event uniquely, as the naming of three under the old system fixes the point uniquely. suppose we take some particular event as the one from which to measure, and agree upon the directions to be taken by our space axes, and make any convention about our time-axis which subsequent investigation may show to be necessary. certainly then the act of measuring so many miles north, and so many west, and so many down, and so many seconds backward, brings us to a definite time and place--which is to say, to a definite event. perhaps nothing "happened" there, in the sense in which we usually employ the word; but that is no more serious than if we were to locate a point with reference to our familiar space coordinate system, and find it to lie in the empty void of interstellar space, with no material body occupying it. in this second case we still have a point, which requires, to insure its existence and location, three coordinates and nothing more; in the first case we still have an event, which requires for its existence and definition four coordinates and nothing more. it is not an event about which the headline writers are likely to get greatly excited; but what of that? it is there, ready and waiting to define any physical happening that falls upon it, just as the geometer's point is ready and waiting to define any physical body that chances to fall upon it. a continuum of points it is now in order to introduce a word, which i shall have to confess the great majority of the essayists introduce, somewhat improperly, without explanation. but when i attempt to explain it, i realize quite well why they did this. they had to have it; and they didn't have space in their three thousand words to talk adequately about it and about anything else besides. the mathematician knows very well indeed what he means by a continuum; but it is far from easy to explain it in ordinary language. i think i may do best by talking first at some length about a straight line, and the points on it. if the line contains only the points corresponding to the integral distances , , , etc., from the starting point, it is obviously not continuous--there are gaps in it vastly more inclusive than the few (comparatively speaking) points that are present. if we extend the limitations so that the line includes all points corresponding to ordinary proper and improper fractions like / and / and / --what the mathematician calls the rational numbers--we shall apparently fill in these gaps; and i think the layman's first impulse would be to say that the line is now continuous. certainly we cannot stand now at one point on the line and name the "next" point, as we could a moment ago. there is no "next" rational number to / , for instance; / comes before it and / comes after it, but between it and either, or between it and any other rational number we might name, lie many others of the same sort. yet in spite of the fact that the line containing all these rational points is now "dense" (the technical term for the property i have just indicated), it is still not continuous; for i can easily define numbers that are not contained in it--irrational numbers in infinite variety like $\sqrt{ }$; or, even worse, the number pi = . ... which defines the ratio of the circumference of a circle to the diameter, and many other numbers of similar sort. if the line is to be continuous, there may be no holes in it at all; it must have a point corresponding to every number i can possibly name. similarly for the plane, and for our three-space; if they are to be continuous, the one must contain a point for every possible pair of numbers $x$ and $y$, and the other for every possible set of three numbers $x$, $y$ and $z$, that i can name. there may be no holes in them at all. a line is a continuum of points. a plane is a continuum of points. a three-space is a continuum of points. these three cases differ only in their dimensionality; it requires but one number to determine a point of the first continuum, two and three respectively in the second and third cases. but the essential feature is not that a continuum shall consist of points, or that we shall be able to visualize a pseudo-real existence for it of just the sort that we can visualize in the case of line, plane and point. the essential thing is merely that it shall be an aggregate of elements numerically determined in such a way as to leave no holes, but to be just as continuous as the real number system itself. examples, however, aside from the three which i have used, are difficult to construct of such sort that the layman shall grasp them readily; so perhaps, fortified with the background of example already presented, i may venture first upon a general statement. the continuum in general suppose we have a set of "elements" of some sort--any sort. suppose that these elements possess one or more fundamental identifying characteristics, analogous to the coordinates of a point, and which, like these coordinates, are capable of being given numerical values. suppose we find that no two elements of the set possess identically the same set of defining values. suppose finally--and this is the critical test--that the elements of the set are such that, no matter what numerical values we may specify, it we do specify the proper number of defining magnitudes we define by these an actual element of the set, that corresponds to this particular collection of values. our elements then share with the real number system the property of leaving no holes, of constituting a continuous succession in every dimension which they possess. we have then a continuum. whatever its elements, whatever the character of their numerical identifiers, whatever the number n of these which stands for its dimension, there may be no holes or we have no continuum. there must be an element for every possible combination of n numbers we can name, and no two of these combinations may give the same element. granted this condition, our elements constitute a continuum. as i have remarked, it is not easy to cite examples of continua which shall mean anything to the person unaccustomed to the term. the totality of carbon-oxygen-nitrogen-hydrogen compounds suggested by one essayist as an example is not a continuum at all, for the set contains elements corresponding only to integer values of the numbers which tell us how many atoms of each substance occur in the molecule. we cannot have a compound containing $\sqrt $ carbon atoms, or $ \pi$ oxygen atoms. perhaps the most satisfactory of the continua, outside the three euclidean space-continua already cited, [is the manifold of music notes. this is four-dimensional; each note has four distinctions--length, pitch, intensity, timbre--to distinguish it perfectly, to tell how long, how high, how loud, how rich.] we might have a little difficulty in reducing the characteristic of richness to numerical expression, but presumably it could be done; and we should then be satisfied that every possible combination of four values, $l$, $p$, $i$, $t$ for these four identifying characteristics would give us a musical effect, and one to be confused with no other. there is in the physical world a vast quantity of continua of one sort or another. the music-note continuum brings attention to the fact that not all of these are such that their elements make their appeal to the visual sense. this remark is a pertinent one; for we are by every right of heritage an eye-minded race, and it is frequently necessary for us to be reminded that so far as the external world is concerned, the verdict of every other sense is entirely on a par with that of sight. the things which we really see, like matter, and the things which we abstract from these visual impressions, like space, are by no means all there is to the world. euclidean and non-euclidean continua if we are dealing with a continuum of any sort whatever having one or two or three dimensions, we are able to represent it graphically by means of the line, the plane, or the three-space. the same set of numbers that defines an element of the given continuum likewise defines an element of the euclidean continuum of the same dimensionality; so the one continuum corresponds to the other, element for element, and either may stand for the other. but if we have a continuum of four or more dimensions, this representation breaks down in the absence of a real, four-dimensional euclidean point-space to serve as a picture. this does not in the least detract from the reality of the continuum which we are thus prevented from representing graphically in the accustomed fashion. the euclidean representation, in fact, may in some cases be unfortunate--it may be so entirely without significance as to be actually misleading. for in the euclidean continuum of points, be it line, plane or three-space, there are certain things which we ordinarily regard as secondary derived properties, but which possess a great deal of significance none the less. in particular, in the euclidean plane and in euclidean three-space, there is the distance between two points. i have indicated, in the chapter on non-euclidean geometry, that the parallel postulate of euclid, which distinguishes his geometry from others, could be replaced by any one of numerous other postulates. grant euclid's postulate and you can prove any of these substitutes; grant any of the substitutes and you can prove euclid's postulate. now it happens that there is one of these substitutes to which modern analysis has given a position of considerable importance. it is merely our good old friend the pythagorean theorem, that the square on the hypotenuse equals the sum of the squares on the sides; but it is dressed in new clothes for the present occasion. mr. francis' discussion of this part of the subject, and especially his figure, ought to make it clear that this theorem can be considered as dealing with the distance between any two points. when we so consider it, and take it as the fundamental, defining postulate of euclidean geometry which distinguishes this geometry from others, we have a statement of considerable content. we have, first, that the characteristic property of euclidean space is that the distance between two points is given by the square root of the sum of the squares of the coordinate-differences for these points--by the expression $$d = \sqrt{(x - x)^ + (y - y)^ + (z - z)^ }\,,$$ where the large letters represent the coordinates of the one point and the small ones those of the other. we have more than this, however; we have that this distance is the same for all observers, no matter how different their values for the individual coordinates of the individual points. and we have, finally, as a direct result of looking upon the thing from this viewpoint, that the expression for $d$ is an "invariant"; which simply means that every observer may use the same expression in calculating the value of $d$ in terms of his own values for the coordinates involved. the distance between two points in our space is given numerically by the square root of the sum of the squares of my coordinate-differences for the two points involved; it is given equally by the square root of the sum of the squares of your coordinate-differences, or those of any other observer whatsoever. we have then a natural law--the fundamental natural law characterizing euclidean space. if we wish to apply it to the euclidean two-space (the plane) we have only to drop out the superfluous coordinate-difference; if we wish to see by analogy what would be the fundamental natural law for a four-dimensional euclidean space, we have only to introduce under the radical a fourth coordinate-difference for the fourth dimension. if we were not able to attach any concrete meaning to the expression for $d$ the value of all this would be materially lessened. consider, for instance, the continuum of music notes. there is no distance between different notes. there is of course significance in talking about the difference in pitch, in intensity, in duration, in timbre, between two notes; but there is none in a mode of speech that implies a composite expression indicating how far one note escapes being identical with another in all four respects at once. the trouble, of course, is that the four dimensions of the music-note continuum are not measurable in terms of a common unit. if they were, we should expect to measure their combination more or less absolutely in terms of this same unit. we can make measurements in all three dimensions of euclidean space with the same unit, with the same measuring rod in fact. [this presents a peculiarity of our three-space which is not possessed by all three-dimensional manifolds. riemann has given another illustration in the system of all possible colors, composed of arbitrary proportions of the three primaries, red, green and violet. this system forms a three-dimensional continuum; but we cannot measure the "distance" or difference between two colors in terms of the difference between two others.] accordingly, in spite of the fact that the euclidean three-space gives us a formal representation of the color continuum, and in spite of the fact that the hypothetical four-dimensional euclidean space would perform a like office for the music-note continuum, this representation would be without significance. we should not say that the geometry of these two manifolds is euclidean. we should realize that any set of numerical elements can be plotted in a euclidean space of the appropriate dimensionality; and that accordingly, before allowing such a plot to influence us to classify the geometry of the given manifold as euclidean, we must pause long enough to ask whether the rest of the euclidean system fits into the picture. if the square root of the sum of the squares of the coordinate-differences between two elements possesses significance in the given continuum, and if it is invariant between observers of that continuum who employ different bases of reference, then and only then may we allege the euclidean character of the given continuum. if under this test the given continuum fails of euclideanism, it is in order to ask what type of geometry it does present. if it is of such character that the "distance" between two elements possesses significance, we should answer this question by investigating that distance in the hope of discovering a non-euclidean expression for it which will be invariant. if it is not of such character, we should seek some other characteristic of single elements or groups of elements, of real physical significance and of such sort that the numerical expression for it would be invariant. if the continuum with which we have to do is one in which the "distance" between two elements possesses significance, and if it turns out that the invariant expression for this distance is not the pythagorean one, but one indicating the non-euclideanism of our continuum, we say that this continuum has a "curvature." this means that, if we interpret the elements of our continuum as points in space (which of course we may properly do) and if we then try to superpose this point-continuum upon a euclidean continuum, it will not "go"; we shall be caught in some such absurdity as trying to force a sphere into coincidence with a plane. and of course if it won't go, the only possible reason is that it is curved or distorted, like the sphere, in such a way as to prevent its going. it is unfortunate that the visualizing of such curvature requires the visualizing of an additional dimension for the curved continuum to curve into; so that while we can picture a curved surface easily enough, we can't picture a curved three-space or four-space. but that is a barrier to visualization alone, and in no sense to understanding. our world of four dimensions it will be observed that we have now a much broader definition of non-euclideanism than the one which served us for the investigation of euclid's parallel postulate. if we may at pleasure accept this postulate or replace it by another and different one, we may presumably do the same for any other or any others of euclid's postulates. the very statement that the distance between elements of the continuum shall possess significance, and shall be measurable by considering a path in the continuum which involves other elements, is an assumption. if we discard it altogether, or replace it by one postulating that some other joint property of the elements than their distance be the center of interest, we get a non-euclidean geometry. so for any other of euclid's postulates; they are all necessary for a euclidean system, and in the absence of any one of them we get a non-euclidean system. now the four-dimensional time-space continuum of minkowski is plainly of a sort which ought to make susceptible of measurement the separation between two of its events. we can pass from one element to another in this continuum--from one event to another--by traversing a path involving "successive" events. our very lives consist in doing just this: we pass from the initial event of our career to the final event by traversing a path leading us from event to event, changing our time and space coordinates continuously and simultaneously in the process. and while we have not been in the habit of measuring anything except the space interval between two events and the time interval between two events, separately, i think it is clear enough that, considered as events, as elements in the world of four dimensions, there is a less separation between two events that occur in my office on the same day than between two which occur in my office a year apart; or between two events occurring minutes apart when both take place in my office than when one takes place there and one in london or on betelgeuse. it is not at all unreasonable, a priori, then, to seek a numerical measure for the separation, in space-time of four-dimensions, of two events. if we find it, we shall doubtless be asked just what its subjective significance to us is. this must be answered with some circumspection. it will presumably be something which we cannot observe with the visual sense alone, or it would have forced itself upon our attention thousands of years ago. it ought, i should think, to be something that we would sense by employing at the same time the visual sense and the sense of time-passage. in fact, i might very plausibly insist that, by my very remarks about it in the above paragraph, i have sensed it. minkowski, however, was not worried about this phase of the matter. he had only to identify the invariant expression for distance; sensing it could wait. he found, of course, that this expression was not the euclidean expression for a four-dimensional interval. he had discarded several of the euclidean assumptions and could not expect that the postulate governing the metric properties of euclid's space would persist. especially had he violated the euclidean canons in discarding, with einstein, the notion that nothing which may happen to a measuring rod in the way of uniform translation at high velocity can affect its measures. so he had to be prepared to find that his geometry was non-euclidean; yet it is surprising to learn how slightly it deviates from that of euclid. without any extended discussion to support the statement, we may say that he found that when two observers measure the time- and the space-coordinates of two events, using the assumptions and therefore the methods of einstein and hence subjecting themselves to the condition that their measures of the pure time-interval and of the pure space-interval between these events will not necessarily be the same, they will discover that they both get the same value for the expression $$s = \sqrt{(x - x)^ + (y - y)^ + (z - z)^ - (ct - ct)^ }\,.$$ if our acceptance of this as the numerical measure of the separation in space-time between the two events should lead to contradiction we could not so accept it. no contradiction arises however and we may therefore accept it. and at once the mathematician is ready with some interpretative remarks. the curvature of space-time the invariant expression for separation, it will be seen, is in the same form as that of the euclidean four-dimensional invariant save for the minus sign before the time-difference (the appearance of the constant $c$ in connection with the time coordinate $t$ is merely an adjustment of units; see page ). this tells us that not alone is the geometry of the time-space continuum non-euclidean in its methods of measurement, but also in its results, to the extent that it possesses a curvature. it compares with the euclidean four-dimensional continuum in much the same way that a spherical surface compares with a plane. as a matter of fact, a more illuminating analogy here would be that between the cylindrical surface and the plane, though neither is quite exact. to make this clear requires a little discussion of an elementary notion which we have not yet had to consider. our three-dimensional existence often reduces, for all practical purposes, to a two-dimensional one. the objects and the events of a certain room may quite satisfactorily be defined by thinking of them, not as located in space, but as lying in the floor of the room. mathematically the justification for this viewpoint is got by saying that we have elected to consider a slice of our three-dimensional world of the sort which we know as a plane. when we consider this plane and the points in it, we find that we have taken a cross-section of the three-dimensional world. a line in that world is now reduced, for us, to a single point--the point where it cuts our plane; a plane is reduced to a line--the line where it cuts our plane; the three-dimensional world itself is reduced to our plane itself. everything three-dimensional falls down into its shadow in our plane, losing in the process that one of the three dimensions which is not present in our plane. for simplicity's sake it is usual to take a cross-section of space parallel to one of our coordinate axes. we think of our three dimensions as extending in the directions of those axes; and it is easier to take a horizontal or vertical section which shall simply wipe out one of these dimensions than to take an oblique section which shall wipe out a dimension that consists partly of our original length, and partly of our original width, and partly of our original height. if we have a four-dimensional manifold to begin with, we may equally shake out one of the four dimensions, one of the four coordinates, and consider the three-dimensional result of this process as a cross-section of the original four-dimensional continuum. and where, in cross-sectioning a three-dimensioned world, we have but three choices of a coordinate to eliminate, in cross-sectioning a world of four dimensions we have four choices. by dropping out either the $x$, or the $y$, or the $z$, or the $t$, we get a three-dimensioned cross-section. now our accustomed three-dimensional space is strictly euclidean. when we cross-section it, we get a euclidean plane no matter what the direction in which we make the cut. likewise the euclidean plane is wholly euclidean, because when we cross-section it in any direction whatever we get a euclidean line. a cylindrical surface, on the other hand, is neither wholly euclidean nor wholly non-euclidean in this matter of cross-sectioning. if we take a section in one direction we get a euclidean line and if we take a section in the other direction we get a circle (if the cylindrical surface be a circular one). and of course if we take an oblique section of any sort, it is neither line nor circle, but a compromise between the two--the significant thing being that it is still not a euclidean line. the space-time continuum presents an analogous situation. when we cross-section it by dropping out any one of the three space dimensions, we get a three-dimensional complex in which the distance formula is still non-euclidean, retaining the minus sign before the time-difference and therefore retaining the geometric character of its parent. but if we take our cross-section in such a way as to eliminate the time coordinate, this peculiarity disappears. the signs in the invariant expression are then all plus, and the cross-section is in fact our familiar euclidean three-space. if we set up a surface geometry on a sphere, we find that the elimination of one dimension leaves us with a line-geometry that is still non-euclidean since it pertains to the great circles of the sphere rather than to euclidean straight lines. in shaking minkowski's continuum down into a three-dimensional one by eliminating any one of his coordinates, if we eliminate either the $x$, the $y$ or the $z$, we have left a three-dimensional geometry in which the disturbing minus sign still occurs in the distance-formula, and which is therefore still non-euclidean. if we omit the $t$, this does not occur. we see, then, that the time dimension is the disturbing factor, the one which gives to space-time its non-euclidean character so far as the possession of curvature is concerned. and we see that this curvature is not the same in all directions, and in one direction is actually zero--whence the attempted analogy with a cylinder instead of with a sphere. many writers on relativity try to give the space-time continuum an appeal to our reason and a character of inevitableness by insisting on the lack of any fundamental distinction between space and time. the very expression for the space-time invariant denies this. time is distinguishable from space. the three dimensions of space are quite indistinguishable--we can interchange them without affecting the formula, we can drop one out and never know which is gone. but the very formula singles out time as distinct from space, as inherently different in some way. it is not so inherently different as we have always supposed; it is not sufficiently different to offer any obstacle to our thinking in terms of the four-dimensional continuum. but while we can group space and time together in this way, [this does not mean at all that space and time cease to differ. a cook may combine meat with potatoes and call the product hash, but meat and potatoes do not thereby become identical.] the question of visualization to the layman there is a great temptation to say that while, mathematically speaking, the space-time continuum may be a great simplification, it does not really represent the external world. to be sure, you can't see the space-time continuum in precisely the same way that you can the three-dimensional space continuum, but this is only because einstein finds the time dimension to be not quite freely interchangeable with the space dimension. yet you do perceive this space-time continuum, in the manner appropriate for its perception; and it would be just as sensible to throw out the space continuum itself on the ground that perception of the two is not of exactly the same sort, as to throw out the space-time continuum on this ground. with appropriate conventions, either may stand as the mental picture of the external world; it is for us to choose which is the more convenient and useful image. einstein tells us that his image is the better, and tells us why. before we look into this, we must let him tell us something more about the geometry of his continuum. what he tells us is, in its essentials, just this. the observer in a pure space continuum of three dimensions finds that as he changes his position, his right-and-left, his backward-and-forward, and his up-and-down are not fixed directions inherent in nature, but are fully interchangeable. the observers, in the adjoined sketch, whose verticals are as indicated by the arrows, find very different vertical and horizontal components for the distance between the points $o$ and $p$; a similar situation would prevail if we used all three space directions. the statement analogous to this for einstein's four-dimensional continuum of space and time combined is that, as observers change their relative motion, their time axes take slightly different directions, so that what is purely space or purely time for the one becomes space with a small component in the time direction, or time with a small component in the space direction, for the other. this it will be seen explains fully why observers in relative motion can differ about space and time measurements. we should not be at all surprised if the two observers of the figure reported different values for horizontals and verticals; we should realize that what was vertical for one had become partly horizontal for the other. it is just so, says einstein, with his observers of time and space who are in relative motion to one another; what one sees as space the other sees as partly time, because their time axes do not run quite parallel. the natural question here, of course, is "well, where are their time axes?" if you know what to look for, of course, you ought to be able to perceive them in just the way you perceive ordinary time intervals--with the reservation that they are imaginary, after all, just like your space axes, and that you must only expect to see them in imagination. if you look for a fourth axis in euclidean three-space to represent your time axis, you will of course not find it. but you will by all means agree with me that your time runs in a definite direction; and this it is that defines your time axis. einstein adds that if you and i are in relative motion, my time does not run in quite the same direction as yours. how shall we prove it? well, how would we prove it if he told us that our space axes did not run in precisely the same direction? of course we could not proceed through direct measures upon the axes themselves; we know these are imaginary. what we should do would be to strike out, each of us, a very long line indeed in what seemed the true horizontal direction; and we should hope that if we made them long enough, and measured them accurately enough, we should be able to detect any divergence that might exist. this is precisely what we must do with our time axes if we wish to verify einstein's statement that they are not precisely parallel; and what better evidence could we demand of the truth of this statement than the evidence already presented--that when we measure our respective time components between two events, we get different results? what it all leads to the preceding chapters have been compiled and written with a view to putting the reader in a state of mind and in a state of informedness which shall enable him to derive profit from the reading of the actual competing essays which make up the balance of the book. for this purpose it has been profitable to take up in detail the preliminaries of the special theory of relativity, and to allow the general theory to go by default, in spite of the fact that it is the latter which constitutes einstein's contribution of importance to science. the reason for this is precisely the same as that for taking up euclidean geometry and mastering it before proceeding to the study of newtonian mechanics. the fundamental ideas of the two theories, while by no means identical, are in general terms the same; and the conditions surrounding their application to the special theory are so very much simpler than those which confront us when we apply them to the more general case, that this may be taken as the controlling factor in a popular presentation. we cannot omit the general theory from consideration, of course; but we can omit it from our preliminary discussion, and leave its development to the complete essays which follow, and which in almost every case give it the larger half of their space which its larger content demands. in the process of the slow and difficult preparation of the lay mind for the assimilation of an altogether new set of fundamental ideas, it is altogether desirable to give the special theory, with its simpler applications of these ideas, a place out of proportion to its importance in einstein's completed structure; and this we have therefore done. the special theory, postulating the relativity of uniform motion and deducing the consequences of that relativity, is often referred to as a "special case" of the general theory, in which this restriction of uniformity is removed. this is not strictly speaking correct. the general theory, when we have formulated it, will call our attention to something which we really knew all the time, but to which we chose not to give heed--that in the regions of space to which we have access, uniform motion does not exist. all bodies in these regions are under the gravitational influence of the other bodies therein, and this influence leads to accelerated motion. nothing in our universe can possibly travel at uniform velocity; the interference of the rest of the bodies in the universe prevents this. obviously, we ought not to apply the term "special case" to a case that never occurs. nevertheless, this case is of extreme value to us in our mental processes. many of the motions with which we are concerned are so nearly at constant velocity that we find it convenient to treat them as though they were uniform, either ignoring the resulting error or correcting for it at the end of our work. in many other cases we are able to learn what actually occurs under accelerated motion by considering what would have occurred under uniform motion were such a thing possible. science is full of complications which we unravel in this fashion. the physicist deals with gas pressures by assuming temperatures to be constant, though he knows temperature never is constant; and in turn he deals with temperatures by assuming pressures to be constant. after this, he is able to predict what will happen when, as in nature, pressures and temperatures are varying simultaneously. by using as a channel of attack the artificially simple case that never occurs, we get a grip on the complex case that gives us a true picture of the phenomenon. and because in actual nature we can come as close as we please to this artificial case, by supposing the variable factor to approach constancy, so when we assume it to be absolutely constant we speak of the result as the limiting case. this situation does not occur, but is the limiting case for those that do occur. when, in the matter of motion, we abandon the artificial, limiting case of uniform velocity and look into the general, natural one of unrestricted motion, we find that the structure which we have built up to deal with the limiting case provides us with many of the necessary ideas and viewpoints. this is what we expect--in it lies the value of the limiting case. we shall see that the relativity of time and space, established for the limiting case, holds good in the general one. we shall see that the idea of the four-dimensional space-time continuum as representing the external world persists, forming the whole background of the general theory much more definitely than in the special theory. incidentally we shall see that the greater generality of the case under consideration will demand a greater degree of generality in the geometry of this continuum, a non-euclideanism of a much more whole-hearted type than that of the special theory. but all the revisions of fundamental concepts which we have been at such pains to make for the sake of the special theory will remain with us in the general. with this we may consider our preliminary background as established, and give our attention to the essayists, who will try to take us more deeply into the subject than we have yet gone, without losing us in its intricacies. vii relativity the winning essay in the contest for the eugene higgins $ , prize by lyndon bolton senior examiner, british patent office london the reader is probably acquainted with the method of specifying positions of points in a plane by their distances from two mutually perpendicular lines, or if the points are in space by their distances from three mutually perpendicular planes like adjacent sides of a flat-sided box. the method is in fact in common use for exhibiting relations between quantities by graphs or diagrams. these sets of axes, as they are called, together with any scales used for measuring, must be supposed rigid, otherwise the events or points which they are used to specify are indefinite. the lengths which locate any point with reference to a set of axes are called its coordinates. when such systems are used for physical purposes, they must be supplemented by clocks to enable the times at which events occur to be determined. the clocks must be synchronized, and must go at the same rate, but it must suffice here to state that this is possible without indicating how these conditions can be attained. a system of axes with its clocks will hereinafter be called a frame of reference, and every observer will be supposed to be provided with such a frame partaking of his motion. all the objects which partake of an observer's motion will be called his system. it is a question whether among all possible frames of reference any one frame or class of frames is more suited than another for the mathematical statement of physical laws. this is for experience to decide, and a principle of relativity is a statement embodying the answer. the mechanical principle of relativity it has been ascertained that all such frames are equally suitable for the mathematical statement of general mechanical laws, provided that their motion is rectilinear and uniform and without rotation. this fact is comprehended in the general statement that all unaccelerated frames of reference are equivalent for the statement of the general laws of mechanics. this is the mechanical principle of relativity. it is well recognized however that the laws of dynamics as hitherto stated involve the assumptions that the lengths of rigid bodies are unaffected by the motion of the frame of reference, and that measured times are likewise unaffected; that is to say that any length measured on his own system by either of two relatively moving observers appears the same to both observers, or that lengths of objects and rates of clocks do not alter whatever the motion relative to an observer. these assumptions seem so obvious that it is scarcely perceived that they are assumptions at all. yet this is the case, and as a matter of fact they are both untrue. the special principle of relativity although all unaccelerated frames of reference are equivalent for the purposes of mechanical laws, this is not the case for physical laws generally as long as the above suppositions are adhered to. electromagnetic laws do alter their form according to the motion of the frame of reference; that is to say, if these suppositions are true, electromagnetic agencies act in different ways according to the motion of the system in which they occur. there is nothing a priori impossible in this, but it does not agree with experiment. the motion of each locality on the earth is continually changing from hour to hour but no corresponding changes occur in electromagnetic actions. it has however been ascertained that on discarding these suppositions the difficulty disappears, and electromagnetic laws retain their form under all circumstances of unaccelerated motion. according to the theory of relativity, the correct view which replaces these suppositions is deducible from the following postulates: ( ) by no experiment conducted on his own system can an observer detect the unaccelerated motion of his system. ( ) the measure of the velocity of light in vacuo is unaffected by relative motion between the observer and the source of light. both these postulates are well established by experiment. the first may be illustrated by the familiar difficulty of determining whether a slowly moving train one happens to be sitting in, or an adjacent one, is in motion. the passenger has either to wait for bumps (that is, accelerations) or else he has to look out at some adjacent object which he knows to be fixed, such as a building (that is, he has to perform an experiment on something outside his system), before he can decide. the second postulate is an obvious consequence of the wave theory of light. just as waves in water, once started by a ship, travel through the water with a velocity independent of the ship, so waves in space travel onward with a speed bearing no relation to that of the body which originated them. the statement however is based on experiment, and can be proved independently of any theory of light. it is not difficult to deduce from these postulates certain remarkable conclusions relating to the systems of two observers, a and b, in relative motion, among them the following: ( ) objects on b's system appear to a to be shorter in the direction of relative motion than they appear to b. ( ) this opinion is reciprocal. b thinks that a's measurements on a's system are too great. ( ) similarly for times: each observer thinks that the other's clocks have a slower rate than his own, so that b's durations of time appear shorter to b than to a, and conversely. ( ) events which appear simultaneous to a do not in general appear so to b, and conversely. ( ) lengths at right angles to the direction of motion are unaffected. ( ) these effects vary with the ratio of the relative velocity to that of light. the greater the relative velocity, the greater the effects. they vanish if there is no relative velocity. ( ) for ordinary velocities the effects are so small as to escape notice. the remarkable point however is their occurrence rather than their magnitude. ( ) the observers similarly form different estimates of the velocities of bodies on each other's systems. the velocity of light however appears the same to all observers. taking into account these revised views of lengths and times the mechanical principle of relativity may be extended to physical laws generally as follows: all unaccelerated frames of reference are equivalent for the statement of the general laws of physics. in this form the statement is called the special, or restricted, principle of relativity, because it is restricted to unaccelerated frames of reference. naturally the laws of classical mechanics now require some modification, since the suppositions of unalterable lengths and times no longer apply. the four dimensional continuum lengths and times therefore have not the absolute character formerly attributed to them. as they present themselves to us they are relations between the object and the observer which change as their motion relative to him changes. time can no longer be regarded as something independent of position and motion, and the question is what is the reality? the only possible answer is that objects must be regarded as existing in four dimensions, three of these being the ordinary ones of length, breadth and thickness, and the fourth, time. the term "space" is applicable only by analogy to such a region; it has been called a "continuum," and the analogue of a point in ordinary three-dimensional space has been appropriately called an "event." by "dimension" must be understood merely one of four independent quantities which locate an event in this continuum. in the nature of the case any clear mental picture of such a continuum is impossible; mankind does not possess the requisite faculties. in this respect the mathematician enjoys a great advantage. not that he can picture the thing mentally any better than other people, but his symbols enable him to abstract the relevant properties from it and to express them in a form suitable for exact treatment without the necessity of picturing anything, or troubling whether or not the properties are those on which others rely for their conceptions. gravitation and acceleration the limitation of statements of general law to uniformly moving systems is hardly satisfactory. the very concept of general law is opposed to the notion of limitation. but the difficulties of formulating a law so that the statement of it shall hold good for all observers, whose systems may be moving with different and possibly variable accelerations, are very great. accelerations imply forces which might be expected to upset the formulation of any general dynamical principles, and besides, the behavior of measuring rods and clocks would be so erratic as to render unmeaning such terms as rigidity and measured time, and therefore to preclude the use of rigid scales, or of a rigid frame of reference which is the basis of the foregoing investigation. the following example taken from einstein will make this clear, and also indicate a way out of the difficulty. a rotating system is chosen, but since rotation is only a particular case of acceleration it will serve as an example of the method of treating accelerated systems generally. moreover, as it will be seen, the attribution of acceleration to the system is simply a piece of scaffolding which can be discarded when the general theory has been further developed. let us note the experiences of an observer on a rotating disk which is isolated so that the observer has no direct means of perceiving the rotation. he will therefore refer all the occurrences on the disk to a frame of reference fixed with respect to it, and partaking of its motion. he will notice as he walks about on the disk that he himself and all the objects on it, whatever their constitution or state, are acted upon by a force directed away from a certain point upon it and increasing with the distance from that point. this point is actually the center of rotation, though the observer does not recognize it as such. the space on the disk in fact presents the characteristic properties of a gravitational field. the force differs from gravity as we know it by the fact that it is directed away from instead of toward a center, and it obeys a different law of distance; but this does not affect the characteristic properties that it acts on all bodies alike, and cannot be screened from one body by the interposition of another. an observer aware of the rotation of the disk would say that the force was centrifugal force; that is, the force due to inertia which a body always exerts when it is accelerated. next suppose the observer to stand at the point of the disk where he feels no force, and to watch someone else comparing, by repeated applications of a small measuring rod, the circumference of a circle having its center at that point, with its diameter. the measuring rod when laid along the circumference is moving lengthwise relatively to the observer, and is therefore subject to contraction by his reckoning. when laid radially to measure the diameter this contraction does not occur. the rod will therefore require a greater proportional number of applications to the circumference than to the diameter, and the number representing the ratio of the circumference of the circle to the diameter thus measured will therefore be greater than . +, which is its normal value. moreover the relative velocity decreases as the center is approached, so that the contraction of the measuring rod is less when applied to a smaller circle; and the ratio of the circumference to the diameter, while still greater than the normal, will be nearer to it than before, and the smaller the circle the less the difference from the normal. for circles whose centers are not at the point of zero force the confusion is still greater, since the velocities relative to the observer of points on them now change from point to point. the whole scheme of geometry as we know it is thus disorganized. rigidity becomes an unmeaning term since the standards by which alone rigidity can be tested are themselves subject to alteration. these facts are expressed by the statement that the observer's measured space is non-euclidean; that is to say, in the region under consideration measurements do not conform to the system of euclid. the same confusion arises in regard to clocks. no two clocks will in general go at the same rate, and the same clock will alter its rate when moved about. the general principle of relativity the region therefore requires a space-time geometry of its own, and be it noted that with this special geometry is associated a definite gravitational field, and if the gravitational field ceases to exist, for example if the disk were brought to rest, all the irregularities of measurement disappear, and the geometry of the region becomes euclidean. this particular case illustrates the following propositions which form the basis of this part of the theory of relativity: ( ) associated with every gravitational field is a system of geometry, that is, a structure of measured space peculiar to that field. ( ) inertial mass and gravitational mass are one and the same. ( ) since in such regions ordinary methods of measurement fail, owing to the indefiniteness of the standards, the systems of geometry must be independent of any particular measurements. ( ) the geometry of space in which no gravitational field exists in euclidean. [ ] the connection between a gravitational field and its appropriate geometry suggested by a case in which acceleration was their common cause is thus assumed to exist from whatever cause the gravitational field arises. this of course is pure hypothesis, to be tested by experimental trial of the results derived therefrom. gravitational fields arise in the presence of matter. matter is therefore presumed to be accompanied by a special geometry, as though it imparted some peculiar kink or twist to space which renders the methods of euclid inapplicable, or rather we should say that the geometry of euclid is the particular form which the more general geometry assumes when matter is either absent or so remote as to have no influence. the dropping of the notion of acceleration is after all not a very violent change in point of view, since under any circumstances the observer is supposed to be unaware of the acceleration. all that he is aware of is that a gravitational field and his geometry coexist. the prospect of constructing a system of geometry which does not depend upon measurement may not at first sight seem hopeful. nevertheless this has been done. the system consists in defining points not by their distances from lines or planes (for this would involve measurement) but by assigning to them arbitrary numbers which serve as labels bearing no relation to measured distances, very much as a house is located in a town by its number and street. if this labeling be done systematically, regard being had to the condition that the label-numbers of points which are close together should differ from one another by infinitesimal amounts only, it has been found that a system of geometry can actually be worked out. perhaps this will appear less artificial when the fact is called to mind that even when standards of length are available no more can be done to render lengths of objects amenable to calculation than to assign numbers to them, and this is precisely what is done in the present case. this system of labeling goes by the name of "gaussian coordinates" after the mathematician gauss who proposed it. it is in terms of gaussian coordinates that physical laws must be formulated if they are to have their widest generality, and the general principle of relativity is that all gaussian systems are equivalent for the statement of general physical laws. for this purpose the labeling process is applied not to ordinary space but to the four dimensional space-time continuum. the concept is somewhat difficult and it may easily be aggravated into impossibility by anyone who thinks that he is expected to visualize it. fortunately this is not necessary; it is merely one of these irrelevancies to which those who are unaccustomed to think in symbols are liable. it will now be seen that among physical laws the law of gravitation stands pre-eminent, for it is gravitating matter which determines the geometry, and the geometry determines the form of every other law. the connection between the geometry and gravitation is the law of gravitation. this law has been worked out, with the result that newton's law of the inverse square is found to be approximate only, but so closely approximate as to account for nearly all the motions of the heavenly bodies within the limits of observation. it has already been seen that departure from the euclidean system is intensified by rapidity of motion, and the movements of these bodies are usually too slow for this departure to be manifest. in the case of the planet mercury the motion is sufficiently rapid, and an irregularity in its motion which long puzzled astronomers has been explained by the more general law. another deduction is that light is subject to gravitation. this has given rise to two predictions, one of which has been verified. the verification of the other is as yet uncertain, though the extreme difficulty of the necessary observations may account for this. since light is subject to gravitation it follows that the constancy of the velocity of light assumed in the earlier part of this paper does not obtain in a gravitational field. there is really no inconsistency. the velocity of light is constant in the absence of gravitation, a condition which unaccelerated motion implies. the special principle of relativity is therefore a limiting case of the general principle. viii the new concepts of time and space the essay in behalf of which the greatest number of dissenting opinions have been recorded by montgomery francis new york we have all had experiences, on trains and boats, illustrating our inability to tell, without looking off to some external body, whether we are at rest or moving uniformly; and when we do so look, to tell, without reference to the ground or some other point external to both systems, whether ours or the other be the seat of motion. uniform motion must be relative, because we find nowhere in the universe a body in the unique state of absolute rest from which alone absolute motion might be measured. true, the wave theory of light with its homogeneous space-filling ether seemed to provide a reference standard for the concept of absolute motion, and for its measurement by experiment with light rays. but when michelson and morley looked for this absolute motion they found no trace of it. to the physicist, observational student of the external world, nothing exists save observationally; what he can never observe is not there. so: i. by no means whatever may we regard uniform straight-line motion as other than relative. as a further direct consequence of the michelson-morley experiment we have: ii. light in a vacuum presents the same velocity, $c = , $ miles per second, to all observers whatever their velocity of relative motion. in addition to being experimentally established, this is necessary to support i, for if light will distinguish between our velocities, its medium is necessarily a universal standard for absolute motion. but it is contrary to common sense to suppose that if i pass you at miles per hour, the same light impulse can pass us both at the same speed, $c$. we feel, instinctively, that space and time are not so constituted as to make this possible. but the fact has been repeatedly demonstrated. and when common sense and fundamental concepts clash with facts, it is not the facts that must yield. we have survived such crises, notably one where we had to change the fundamental concept of up-and-down; if another one is here, says einstein, let us meet it. this the special theory of relativity does. it accepts postulates i and ii above; their consequences it deduces and interprets. for extensive demonstration of these i lack space, and this has been satisfactorily done by others so it is not my chief duty; but clearly they will be startling. for the very ray of light which refuses to recognize our relative motion is the medium through which i must observe your system and you mine. it turns out that i get different values for lengths and time intervals in your system than you get, and vice versa. and we are both right! for me to accept your "correction" were for me to admit that you are at absolute rest and i in absolute motion, that your measure of light velocity is right and mine wrong: admissions barred by the postulates. we have nothing to correct; we can only recognize the reason for the discrepancy; and knowing our relative velocity, each can calculate from his own results what the other's will be. we find, of course, that at ordinary velocities the discrepancy is many times too small for detection; but at relative velocities at all comparable with that of light it rises above the observational horizon. to inquire the "true" length is meaningless. chicago is east of denver, west of pittsburgh, south of milwaukee; we do not consider this contradictory, or demand the "true" direction of chicago. einstein finds that the concept of length, between points in space or events in time, does not as we had supposed represent an intrinsic property of the points or the events. like direction, it is merely a relation between these and the observer--a relation whose value changes with the observer's velocity relative to the object. if our ideas of the part played in the world by time and space do not permit us to believe this, we must alter these ideas. let us see how we may do this. a world of points to deal with points in a plane the mathematician draws two perpendicular lines, and locates any point, as $p$, by measuring its distances, $x$ and $y$, from these "coordinate axes." the directions of his axes acquire for him a peculiar significance, standing out above other directions; he is apt to measure the distances $x-x$ and $y-y$ between the points $p$ and $q$ in these directions, instead of measuring the single distance $pq$. we do the same thing when we say that the railroad station is five blocks north and two east. the mathematician visualizes himself as an observer, located on his coordinate framework. for another observer on another framework, the horizontal and vertical distances $x'-x'$ and $y'-y'$ between $p$ and $q$ are different. but for both, the distance from $p$ direct to $q$ is the same. in each case the right triangle tells us that: $$pq = \sqrt{(x-x)^ + (y-y)^ } = \sqrt{(x'-x')^ + (y'-y')^ }$$ imagine an observer so dominated by his coordinate system that he knows no way of relating $p$ with $q$ save by their horizontal and vertical separation. his whole scheme of things would be shattered by the suggestion that other observers on other reference frames find different horizontal and vertical components. we have to show him the line $pq$. we have to convince him that this length is the absolute property enjoyed by his pair of points; that horizontals and verticals are merely relations between the points and the observer, result of the observer's having analyzed the distance $pq$ into two components; that different observers effect this decomposition differently; that this seems not to make sense to him only because of his erroneous concept of a fundamental difference between verticals and horizontals. the four-dimensional world of events we too have created a distinction in our minds corresponding to no sufficient reality. our minds seize on time as inherently separable from space. we see the world made up of things in a continuum of three space dimensions; to make this dead world live there runs through it a one-dimensional time continuum, imposed from without, unrelated. but did you ever observe anything suggesting the presence of time in the absence of space, or vice versa? no; these vessels of the universe always occur together. association of the space dimensions into a manifold from which time is excluded is purely a phenomenon of the mind. the space continuum cannot begin to exist until the time dimension is supplied, nor can time exist without a place to exist in. the external world that we observe is composed, not of points, but of events. if a point lacks position in time it does not exist; give it this position and it becomes an event. this world of events is four-dimensional--which means nothing more terrifying than that you must make four measures to locate an event. it does not mean, at all, that you must visualize four mutually perpendicular lines in your accustomed three-space or in a four-space analogous to it. if this world of four dimensions seems to lack reality you will be able to exhibit no better reality for your old ideas. time belongs, without question; and not as an afterthought, but as part of the world of events. to locate an event we use four measures: $x$, $y$ and $z$ for space, $t$ for time. using the same reference frame for time and space, we locate a second event by the measures $x$, $y$, $z$, $t$. minkowski showed that the quantity $$\sqrt{(x-x)^ + (y-y)^ + (z-z)^ - (ct-ct)^ }$$ is the same for all observers, no matter how different their $x$'s, $y$'s, $z$'s and $t$'s; just as in the plane the quantity $$\sqrt{(x-x)^ + (y-y)^ }$$ is the same for all observers, no matter how different their $x$'s and $y$'s. such a quantity, having the same value for all observers, is absolute. in the plane it represents the true, absolute distance between the points--their intrinsic property. in dealing with events it represents the true, absolute "interval," in time and space together between the events. it is not space, nor time, but a combination of the two. we have always broken it down into separate space and time components. in this we are as naive as the plane observer who could not visualize the distance $pq$ until it was split into separate horizontals and verticals. he understood with difficulty that another observer, employing a different reference frame because in different position, would make the decomposition differently. we understand with difficulty that another observer, employing a different reference frame because in uniform motion relative to us, will decompose the "interval" between events into time and space components different from ours. time and space are relative to the observer; only the interval representing space-time is absolute. so common sense stands reconciled to the special theory of relativity. successive steps toward generality is then our laboriously acquired geometry of points in a three-dimensional space to go into the discard? by no means. jeans, investigating the equilibrium of gaseous masses, found the general case too difficult for direct attack. so he considered the case where the masses involved are homogeneous and incompressible. this never occurs; but it throws such light on the general case as to point the way toward attack on it. euclidean geometry excludes motion, save that engineered by the observer; and then the time is immaterial. time does not enter at all; the three space dimensions suffice. this simple case never occurs where matter exists; but its conclusions are of value in dealing with more general cases. when we look into a world alleged to be that of euclid and find motion, we may retain the euclidean concept of what constitutes the world and invent a machinery to account for the motion; or we may abandon the euclidean world, as inadequate, in favor of a more general one. we have adopted the second alternative. newton's laws tells us that a body free to move will do so, proceeding in a straight line at uniform velocity until interfered with. we do not ask, nor does the theory tell us, whence comes the initial motion. there is no machinery to produce it; it is an inherent property of newton's world--assured by the superposition of the time continuum upon euclid's world to make newton's, accepted without question along with that world itself. but newton saw that his world of uniform motion, like euclid's, was never realized. in the neighborhood of one particle a second is interfered with, forced to give up its uniform motion and acquire a constant acceleration. this newton explained by employing the first of the alternatives mentioned above. he tells us that in connection with all matter there exists a force which acts on other matter in a certain way. he does not display the actual machinery through which this "force" works, because he could not discover any machinery; he had to stop with his brilliant generalization of the observed facts. and all his successors have failed to detect the slightest trace of a machinery of gravitation. einstein asks whether this is not because the machinery is absent--because gravitation, like position in euclid's world and motion in newton's, is a fundamental property of the world in which it occurs. his point of attack here lay in precise formulation of certain familiar facts that had never been adequately appreciated. these facts indicate that even accelerated motion is relative, in spite of its apparently real and absolute effects. gravitation and acceleration an observer in a closed compartment, moving with constant acceleration through empty space, finds that the "bottom" of his cage catches up with objects that he releases; that it presses on his feet to give him the sensation of weight, etc. it displays all the effects that he would expect if it were at rest in a gravitational field. on the other hand, if it were falling freely under gravitational influence, its occupant would sense no weight, objects released would not leave his hand, the reaction from his every motion would change his every position in his cage, and he could equally well assume himself at rest in a region of space free from gravitational action. accelerated motion may always be interpreted, by the observer on the system, as ordinary force effects on his moving system, or as gravitational effects on his system at rest. an alternative statement of the special theory is that the observed phenomena of uniform motion may equally be accounted for by supposing the object in motion and the observer with his reference frame at rest, or vice versa. we may similarly state the general theory: the observed phenomena of uniformly accelerated motion may in every case be explained on a basis of stationary observer and accelerated objective, or of stationary objective with the observer and his reference system in accelerated motion. gravitation is one of these phenomena. it follows that if the observer enjoy properly accelerated axes (in time-space, of course), the absolute character of the world about him must be such as to present to him the phenomenon of gravitation. it remains only to identify the sort of world, of which gravitation as it is observed would be a fundamental characteristic. euclid's and newton's systems stand as first and second approximations to that world. the special relativity theory constitutes a correction of newton, presumably because it is a third approximation. we must seek in it those features which we may most hopefully carry along, into the still more general case. newton's system retained the geometry of euclid. but minkowski's invariant expression tells us that einstein has had to abandon this; for in euclidean geometry of four dimensions the invariant takes the form: $$\sqrt{(x-x)^ + (y-y)^ + (z-z)^ + (t-t)^ }\,,$$ analogous to that of two and three dimensions. it is not the presence of the constant $c$ in minkowski's formula that counts; this is merely an adjustment so that we may measure space in miles and time in the unit that corresponds to a mile. it is the minus sign where euclidean geometry demands a plus that makes minkowski's continuum non-euclidean. the editor has told us what this statement means. i think he has made it clear that when we speak of the geometry of the four-dimensional world, we must not read into this term the restrictions surrounding the kind of geometry we are best acquainted with--that of the three-dimensional euclidean continuum. so i need only point out that if we are to make a fourth (and we hope, final) approximation to the reality, its geometry must preserve the generality attained by that of the third step, if it goes no further. einstein's time-space world einstein accordingly examined the possible non-euclidean geometries of four dimensions, in search of one displaying fundamental characteristics which, interpreted in terms of space-time, would lead to the observed facts of gravitation. the mathematics of this investigation is that part of his work which, we are told, but twelve men can follow; so we may only outline his conclusions. if we assume that in the neighborhood of matter the world of space-time is non-euclidean, and that its curvature or distortion or non-euclideanism is of a certain type already known to mathematicians; that the curvature of this world in the neighborhood of matter increases with the mass, and decreases as the distance from the matter increases; and that every particle of matter that is not interfered with travels through space-time in the most direct path possible in that continuum; then the observed facts of gravitation are accounted for as an inherent geometric property of this space-time world. we usually say that the presence of matter distorts this world, and that this distortion gives the track of particles through the region affected its non-uniform character. gravitation then is not a force at all; it is the fundamental nature of things. a body free to move through the world must follow some definite path. euclid says it will stand still; newton that it will traverse a straight line in three-space at uniform time-rate; einstein that it will move in a "geodesic" through time-space--in every-day language, that it will fall. the numerical consequences of einstein's theory are, within the limits of observation, the same as those of newton's for all bodies save one--mercury. this planet shows a small deviation from the path predicted by newton's law; einstein's theory gives its motion exactly. again, when modern research showed that light must be affected by gravitation, einstein's theory, because of the extreme velocity of light, deviates from newton's, where the speed is less a determining factor; and observations of starlight deflected by the sun during the eclipse were in much better accord with einstein's theory than newton's. moreover, the special theory predicts that mass is an observational variable like length and duration. radioactive emanations have a velocity high enough to give appreciable results here, and the prediction is verified, tending to support the general theory by supporting its limiting case. we like always to unify our science; and seldom, after effecting a unification, are we forced to give it up. einstein for the first time brings mechanical, electromagnetic and gravitational phenomena within one structure. this is one reason why physicists are so open minded toward his theory--they want it to be true. the layman's last doubt the final answer to any series of questions is inevitably "because the world is so constructed." the things we are content to leave on that basis are those to which we are accustomed, and which we therefore think we understand; those for which this explanation leaves us unsatisfied are those which are new and unfamiliar. newton told us that the world of three-dimensional space with one-dimensional time superposed was so constructed that bodies left to themselves would go on forever in a straight line at constant speed. we think we understand this, but our understanding consists merely of the unspoken query, "why, of course; what is there to prevent?" the greeks, an intelligent people, looked at this differently; they would have met newton with the unanimous demand "why so; what is there to keep them going?" so if, in seeking an explanation of anything, we come sooner than we had expected to the finality "because the world is so constructed," let us not feel that we have been cheated. ix the principle of relativity a statement of what it is all about, in ideas of one syllable by hugh elliot chislehurst, kent, england the invariance of the laws of nature was one of the most popular themes of nineteenth century philosophy. for it was not till last century that general acceptance was accorded to the doctrine of the "uniformity of law," adumbrated in ancient times by epicurus and lucretius. it is now a cardinal axiom of science that the same cause in the same conditions is always followed by the same effect. there exists in nature no indeterminate element; all things are governed by fixed laws, and the discovery of these laws is the main business of science. it is necessary to guard against reading into this statement an erroneous idea of the content of a "law of nature." such a law is of course not an enactment of any sort; and it is not even to be thought of as an actual explanation of the how and why of the phenomena with which it has to do. it really is nothing but an expression of our belief in the pronouncement of the preceding paragraph, that like conditions do produce like results. it is a prediction based on past experience, and is of value merely in that past experience leads us to credit its accuracy. the composite essay beginning on page discusses this question of the reality of natural laws, and should be consulted in connection with the present contribution.--editor. this great philosophic principle was derived of course from the study of natural science; i.e., from observations and experiments conducted upon the earth. their comprehensiveness is therefore limited by the fact that the observer is always in a state of rest, or nearly so, as compared with the earth. all observers upon the earth are moving through space at the same velocity; and it was possible to argue that the uniformity of law might only hold good, when experiments were conducted at this velocity. an observer moving at very different velocity might discover that the laws of nature under these new conditions were somewhat different. such a view could indeed never be very plausible, for motion is only a relative conception. imagine a universe consisting of infinite "empty" space, in which there is poised a single material body. how shall we determine whether this body is at rest, or whether it is moving at high or low velocity through space? it is never getting nearer to anything or farther from anything, since there is no other body for it to get nearer to or farther from. if we say it is moving at a uniform velocity of a thousand miles a second, our statement really has no significance. we have no more reason for affirming that it is in motion than we have for affirming that it is at rest. in short, there is no such thing as absolute motion; the conception of motion only arises when there are two or more bodies changing their position relatively to one another. this is what is meant by the relativity of motion. it seemed therefore improbable that the laws of nature would be different if the observer were moving at high velocity; for the movement of the observer is not an absolute quantity, but merely a statement of his relation to other bodies, and if there were no other bodies, the statement itself would be meaningless. the behavior of light now among the established laws of nature is that which specifies the velocity of light moving through a vacuum. if the laws of nature are invariable, this velocity will always be the same. but consider what would happen under the following circumstances: suppose that we are at rest, and that an observer on another body flies past us at , miles a second. suppose that at the moment he passes, a piece of flint projecting from him grazes a piece of steel projecting from us, giving rise to a spark; and that we both thereupon set about to measure the velocity of the light so produced. after one second, we should find that the light had traveled about , miles away, and since during this second the other observer had traveled , miles, we should infer that the light traveling in his direction was only about , miles ahead of him. we should also infer that he would find this out by his experiment, and that he would estimate the velocity of light as only , miles a second in his own direction, and , miles a second in the opposite direction. but if this is so, then that law of nature which specifies the velocity of light is quite different for him and for us: the laws of nature must be dependent upon the observer's motion--a conclusion which appears incompatible with the idea of the relativity of motion. and it so happens that it is also contradictory to experimental conclusions. experiments undertaken to settle the point show that each observer finds the same velocity for the light of the spark; and after one second, each observer finds that the light has traveled , miles from himself. but how is it possible that when it has traveled , miles in the same direction as the other observer who himself has moved , miles meanwhile, he should still think it , miles ahead of him? that is the initial paradox; and since there has been no room for error in the experiments, we are forced to conclude that there was something wrong in the assumptions and preconceptions with which we started. space and time there can in fact be only one interpretation. if we each find that the light has moved the same number of miles in the same number of seconds, then we must be meaning something different when we speak of miles and seconds. we are speaking in different languages. some subsidence has occurred in the foundations of our systems of measurement. we are each referring to one and the same objective fact; but since we describe it quite differently, and at first sight incompatibly, some profound alteration must have occurred in our perceptions--all unsuspected by ourselves. it has been shown precisely what this alteration is. a body moving at high velocity must become flattened in the direction of its motion; all its measuring apparatus, when turned in that direction, is shortened, so that no hint of the flattening can be obtained from it. furthermore, the standards of time are lengthened out, and clocks go slower. the extent of this alteration in standards of space and time is stated in the equations of the so-called lorentz transformation. objection might be urged to the above paragraph on the ground that the connection of the observer with the variability of measured lengths and times is not sufficiently indicated, and that this variability therefore might be taken as an intrinsic property of the observed body--which of course it is not.--editor. we are accustomed to describe space as being of three dimensions, and time as being of one dimension. as a matter of fact, both space and time are "ideas," and not immediate sense-perceptions. we perceive matter; we then infer a universal continuum filled by it, which we call space. if we had no knowledge of matter, we should have no conception of space. similarly in the case of time: we perceive one event following another, and we then invent a continuum which we call time, as an abstraction based on the sequence of events. we do not see space, and we do not see time. they are not real things, in the sense that matter is real, and that events are real. they are products of imagination: useful enough in common life, but misleading when we try to look on the universe as a whole, free from the artificial divisions and landmarks which we introduce into it for practical convenience. hence it is perhaps not so surprising after all that in certain highly transcendental investigations, these artificial divisions should cease to be a convenience, and become a hindrance. take for instance our conception of time. it differs from our conception of space in that it has only one dimension. in space, there is a right and left, an up and down, a before and after. but in time there is only before and after. why should there be this limitation of the time-factor? merely because that is the verdict of all our human experience. but is our human experience based on a sufficiently broad foundation to enable us to say that, under all conditions and in all parts of the universe, there can be only one time-direction? may not our belief in the uniformity of time be due to the uniformity of the motion of all observers on the earth? such in fact is the postulate of relativity. we now believe that, at velocities very different from our own, the standard of time would also be different from ours. from our point of view, that different standard of time would not be confined to the single direction fore and aft, as we know it, but would also have in it an element of what we might call right and left. true, it would still be of only one dimension, but its direction would differ from the direction of our time. it would still run like a thread through the universe, but not in the direction which we call straight forward. it would have a slant in it, and the angle of the slant depends upon the velocity of motion. it does not follow that because we are all traveling in the same direction down the stream of time, therefore that stream can only flow in the direction which we know. "before" and "after" are expressions which, like right and left, depend upon our personal situation. if we were differently situated, if to be precise we were moving at very high velocity, we should, so to speak, be facing in a new direction and "before" and "after" would imply a different direction of progress from that with which we are now familiar. the world of reality but, after all, the objective universe is the same old universe however fast we are moving about in it, and whatever way we are facing. these details merely determine the way we divide it up into space and time. the universe is not affected by any arbitrary lines which we draw through it for our personal convenience. for practical purposes, we ascribe to it four dimensions, three in space and one in time. clearly if the time direction is altered, all dimensions both of space and time must have different readings. if, for instance, the time direction slopes away to the left, as compared with ours, then space measurements to right and left must be correspondingly altered. an analogy will simplify the matter. suppose we desire to reach a point ten miles off in a roughly northeasterly direction. we might do so by walking six miles due east and then eight miles due north. we should then be precisely ten miles from where we started. but suppose our compass were out of order, so that its north pole pointed somewhat to the west of north. then in order to get to our destination, we might have to walk seven miles in the direction which we thought was east, and a little more than seven miles in the direction which we thought was north. we should then reach the same point as before. both observers have walked according to their lights, first due east and then due north, and both have reached the same point: the one observer is certain that the finishing point is six miles east of the starting-point, while the other is sure it is seven miles. now we on the earth are all using a compass which points in the same direction as regards time. but other observers, on bodies moving with very different velocity, have a compass in which the time-direction is displaced as compared with ours. hence our judgments of distances will not be alike. in our analogy, the northerly direction corresponds to time, and the easterly direction to space; and so long as we use the same compass we do not differ in our measurements of distances. but for any one who has a different notion of the time-direction, not only time intervals but space distances will be judged differently. in short, the universe is regarded as a space-time continuum of four dimensions. a "point" in space-time is called an "event"--that which occurs at a specified moment and at a specified place. the distance between two points in space-time is called their "interval." all observers will agree as to the magnitude of any interval, since it is a property of the objective universe; but they will disagree as to its composition in space and time separately. in short, space and time are relative conceptions; their relativity is a necessary consequence of the relativity of motion. the paradox named at the outset is overcome; for the two observers measuring the velocity of the light produced as they passed one another, were using different units of space and time. and hence emerges triumphant the special principle of relativity, which states that the laws of nature are the same for all observers, whether they are in a state of rest or of uniform motion in a straight line. accelerated motion uniform motion in a straight line is however a very special kind of motion. our experience in ordinary life is of motions that are neither uniform nor in a straight line; both speed and direction of motion are altering. the moving body is then said to undergo "acceleration": which means either that its speed is increasing or diminishing, or that its direction of motion is changing, or both. if we revert to our former supposition of a universe in which there is only a single body in "empty" space, we clearly cannot say whether it has acceleration any more than whether it is moving, there being no outside standard of comparison; and the general principle of relativity asserts the invariance of the laws of nature for all states of motion of the observer. in this case, however, a difference might be detected by an observer on the moving body itself. it would be manifested to him as the action of a force; such for instance as we feel when a train in which we are traveling is increasing or reducing speed, or when, without changing speed, it is rounding a corner. the force dies away as soon as the velocity becomes uniform. thus acceleration reveals itself to us under the guise of action by a force. force and acceleration go together, and we may either say that the acceleration is due to the force, or the impression of force to the acceleration. now when we are traveling with accelerated motion, we have quite a different idea of what constitutes a straight line from that which we had when at rest or in uniform motion. if we are moving at uniform velocity in an airplane and drop a stone to the earth it will appear to us in the airplane to fall in a straight line downward, while to an observer on the earth it will appear to describe a parabola. this is due to the fact that the stone gathers speed as it falls; it is subject to the acceleration associated with gravity. acceleration obliterates the fundamental difference between a straight and curved line. unless we know what is the absolute motion of the stone, and the two observers, we cannot say whether the line is "really" a straight or a curved line. since absolute motion is an illegitimate conception, it follows that there is no such thing as "really" straight or "really" curved. these are only appearances set up as a consequence of our relative motions with respect to the bodies concerned. if there were no such thing as acceleration--if the stone fell to the earth at uniform velocity--then an observer on the earth or anywhere else would agree that it fell in a straight line; and straight lines would always be straight lines. under these circumstances, euclidean geometry would be absolutely true. but if we are in a state of acceleration, then what we think are straight lines are "really" curved lines, and euclidean geometry, based on the assumption that its lines are straight, must founder when tested by more accurate measurements. and in point of fact we are in a state of acceleration: for we are being acted upon by a force--namely, the force of gravitation. wherever there is matter, there is gravitation; wherever there is gravitation there is acceleration; wherever there is acceleration euclidean geometry is inaccurate. hence in the space surrounding matter a different geometry holds the field; and bodies in general move through such space in curved lines. different parts of space are thus characterized by different geometrical properties. all bodies in the universe proceed on their established courses through space and time. but when they come to distorted geometrical areas, their paths naturally seem to us different from when they were moving through less disturbed regions. they exhibit the difference by acquiring an acceleration; and we explain the acceleration by alleging the existence of a force, which we call the force of gravitation. but their motions can in fact be perfectly predicted if we know the geometry of the space through which they are traveling. the predictions so based have in fact proved more accurate than those based on the law of gravitation. x space, time and gravitation an outline of einstein's theory of general relativity by w. de sitter professor of astronomy in the university of leyden "henceforth space by itself and time by itself shall sink to mere shadows, and only a union of the two shall preserve reality." the prophecy contained in the above-quoted words, spoken by minkowski at the meeting of german "naturforscher und aerzte" at cologne in , has, however, only been completely fulfilled by einstein's "allgemeine relativitäts-theorie" of , which incorporated gravitation into the union. in the following pages an attempt is made to set forth, without using any technical language, the leading ideas of that theory: i will confine myself to the theory as published by einstein in november, , which forms a consistent whole, complete in itself; and i will not refer to later developments, which are still more or less tentative, and not necessary for the understanding of the theory. the mathematics used by einstein is the so-called absolute differential calculus. it is not more difficult or recondite than that used in other branches of theoretical physics, but it is somewhat unfamiliar to most of us, because it is not generally taught in the regular university courses. i will, however, in this essay abstain from using any mathematics at all, at least, i will not be using it openly. it is of course unavoidable to use at least the results of the mathematical reasoning, if not the reasoning itself; but so long as they are not put into formulas they will, it is hoped, not look so formidable to the reader. referring to the quoted words of minkowski, we may ask what is meant by "reality." physical science, like common sense, takes for granted that there is a reality behind the phenomena, which is independent of the person by whom, and the particular methods by which it is observed, and which is also there when it is not observed. strictly speaking, all talk about what is not observed is metaphysics. nevertheless the physicist unhesitatingly believes that his laws are general, and that the phenomena continue to happen according to them when nobody is looking. and since it would be impossible to prove that they did not, he is fully entitled to his belief. the observed phenomena are the effects of the action of this reality, of which we assume the existence, on the observer's senses--or apparatus, which are extended and refined sense-organs. the laws governing the phenomena therefore must convey some information regarding this reality. we shall never by any means be able to know anything else about it but just these laws. to all intents and purposes the laws are the reality, if we eliminate from them all that refers to the observer alone. what refers to the reality is called "absolute," and what involves reference to the observer "relative." the elimination of the relative is one of the things the theory of relativity has set out to do. the external world and its geometry to describe the phenomena and derive laws from them, we locate them in space and time. to do this we use geometry. here it is that the part contributed by the observer comes in. there are an infinite number of geometries, and a priori there seems to be no reason to choose one rather than the other. taking geometry of two dimensions as an example, we can draw figures on a piece of paper, and discuss their properties, and we can also do so on the shell of an egg. but we cannot draw the same figures on the egg as on the paper. the ones will be distorted as compared with the others: the two surfaces have a different geometry. similarly it is not possible to draw an accurate map of the earth on a sheet of paper, because the earth is spherical and its representation on the flat paper is always more or less distorted. the earth requires spherical geometry, which differs from the flat, or euclidean, geometry of the paper. up to a few years ago euclidean (i.e. flat) geometry of three dimensions had been exclusively used in physical theories. why? because it is the true one, is the one answer generally given. now a statement about facts can be true or false, but a mathematical discipline is neither true nor false; it can only be correct--i.e. consistent in itself--or incorrect, and of course it always is correct. the assertion that a certain geometry is the "true" one can thus only mean, that it is the geometry of "true" space, and this again, if it is to have any meaning at all, can only mean that it corresponds to the physical "reality." leaving aside the question whether this reality has any geometry at all, we are confronted with the more immediately practical consideration how we shall verify the asserted correspondence. there is no other way than by comparing the conclusions derived from the laws based upon our geometry, with observations. it thus appears that the only justification for the use of the euclidean geometry is its success in enabling us to "draw an accurate map" of the world. as soon as any other geometry is found to be more successful, that other must be used in physical theories, and we may, if we like, call it the "true" one. accurate observations always consist of measures, determining the position of material bodies in space. but the positions change, and for a complete description we also require measures of time. an important remark must be made here. nobody has ever measured a pure space-distance, nor a pure lapse of time. the only thing that can be measured is the distance from a body at a certain point of space and a certain moment of time, to a body (either the same or another) at another point and another time. we can even go further and say that time cannot be measured at all. we profess to measure it by clocks. but a clock really measures space, and we derive the time from its space-measures by a fixed rule. this rule depends on the laws of motion of the mechanism of the clock. thus finally time is defined by these laws. this is so, whether as a "clock" we use an ordinary chronometer, or the rotating earth, or an atom emitting light-waves, or anything else that may be suggested. the physical laws, of course, must be so adjusted that all these devices give the same time. about the reality of time, if it has any, we know nothing. all we know about time is that we want it. we cannot adequately describe nature with the three space-coordinates alone, we require a fourth one, which we call time. we might thus say with some reason that the physical world has four dimensions. but so long as it was found possible adequately to describe all known phenomena by a space of three dimensions and an independent time, the statement did not convey any very important information. only after it had been found out that the space-coordinates and the time are not independent, did it acquire a real meaning. as is well known the observation by which this was found out is the famous experiment of michelson and morley. it led to the "special" theory of relativity, which is the one referred to by minkowski in . in it a geometry of four dimensions is used, not a mere combination of a three-dimensional space and a one-dimensional time, but a continuum of truly fourfold order. this time-space is not euclidean, since the time-component and the three space-components are not on the same footing, but its fundamental formula has a great resemblance to that of euclidean geometry. we may call it "pseudo-euclidean." this theory, which we need not explain here, was very satisfactory so far as the laws of electromagnetism, and especially the propagation of light, were concerned, but it did not include gravitation, and mechanics generally. we then had this curious state of affairs, that physicists actually believed in two different "realities." when they were thinking of light they believed in minkowski's time-space; when they were thinking of gravitation they believed in the old euclidean space and independent time. this, of course, could not last. attempts were made so to alter newton's law of gravitation that it would fit into the four-dimensional world of the special relativity-theory, but these only succeeded in making the law, which had been a model of simplicity, extremely complicated, and, what was worse, it became ambiguous. it is einstein's great merit to have perceived that gravitation is of such fundamental importance, that it must not be fitted into a ready-made theory, but must be woven into the space-time geometry from the beginning. and that he not only saw the necessity of doing this, but actually did it. gravitation and its place in the universe to see the necessity we must go back to newton's system of mechanics. newton did two things (amongst others). he canonised galileo's system of mechanics into his famous "laws of motion," the most important of which is the law of inertia, which says that: a body, that is not interfered with, moves in a straight line with constant velocity. the velocity, of course, can be nil, and the body at rest. this is a perfectly general law, the same for all material bodies, whatever their physical or chemical status. newton took good care exactly to define what he meant by uniform motion in a straight line, and for this purpose he introduced the absolute euclidean space and absolute time as an essential part of his system of laws at the very beginning of his great work. the other thing newton did was to formulate the law of gravitation. gravitation was in his system considered as an interference with the free, or inertial, motion of bodies, and accordingly required a law of its own. but gravitation has this in common with inertia, and in this it differs from all other interferences, that it is perfectly general. all material bodies are equally subjected to it, whatever their physical or chemical status may be. but there is more. gravitation and inertia are actually indistinguishable from each other, and are measured by the same number: the "mass". this was already remarked by newton himself, and from his point of view it was a most wonderful accidental coincidence. if an apple falls from the tree, that which makes it fall is its weight, which is the gravitational attraction by the earth, diminished by the centrifugal force due to the earth's rotation and the apple's inertia. in newton's system the gravitational attraction is a "real" force, whereas the centrifugal force is only "fictitious". but the one is as real as the other. the most refined experiments, already begun by newton himself, have not succeeded in distinguishing between them. their identity is actually one of the best established facts in experimental physics. from this identity of "fictitious," or inertial, and "real," or gravitational, forces it follows that locally a gravitational field can be artificially created or destroyed. thus inside a closed room which is falling freely, say a lift of which the cable has been broken, bodies have no weight: a balance could be in equilibrium with different weights in the two scales. having thus come to the conclusion that gravitation is not an interference, but is identical with inertia, we are tempted to restate the law of motion, so as to include both, thus: bodies which are not interfered with--do not move in straight lines, but--fall. now this is exactly what einstein did. only the "falling" of course requires a precise mathematical definition (like the uniform motion in a straight line), and the whole gist of his theory is the finding of that definition. in our earthly experience the falling never lasts long, very soon something--the floor of the room, or the earth itself--interferes. but in free space bodies go on falling forever. the motion of the planets is, in fact, adequately described as falling, since it consists in nothing else but obeying newton's law of gravitation together with his law of inertia. a body very far removed from all other matter is not subjected to gravitation, consequently it falls with constant velocity in a straight line according to the law of inertia. the problem was thus to find a mathematical definition of "falling," which would embrace the uniform straight-line motion very far from all matter as well as the complex paths of the planets around the sun, and of an apple or a cannon-ball on earth. gravitation and space-time for the definition of the uniform rectilinear motion of pure inertia newton's euclidean space and independent time were sufficient. for the much more complicated falling under the influence of gravitation and inertia together, evidently a more complicated geometry would be needed. minkowski's pseudo-euclidean time-space also was insufficient. einstein accordingly introduced a general non-euclidean four-dimensional time-space, and enunciated his law of motion thus: bodies which are not interfered with move in geodesics. a geodesic in curved space is exactly the same thing as a straight line in flat space. we only call it by its technical name, because the name "straight line" would remind us too much of the old euclidean space. if the curvature gets very small, or zero, the geodesic becomes very nearly, or exactly, a straight line. the problem has now become to assign to time-space such curvatures that the geodesics will exactly represent the tracks of falling bodies. space of two dimensions can just be flat, like a sheet of paper, or curved, like an egg. but in geometry of four dimensions there are several steps from perfect flatness, or "pseudo-flatness," to complete curvature. now the law governing the curvature of einstein's time-space, i.e., the law of gravitation, is simply that it can never, outside matter, be curved more than just one step beyond perfect (pseudo-)flatness. since i have promised not to use any mathematics i can hardly convey to the reader an adequate idea of the difficulty of the problem, nor do justice to the elegance and beauty of the solution. it is, in fact, little short of miraculous that this solution, which was only adopted by einstein because it was the simplest he could find, does so exactly coincide in all its effects with newton's law. thus the remarkably accurate experimental verification of this law can at once be transferred to the new law. in only one instance do the two laws differ so much that the difference can be observed, and in this case the observations confirm the new law exactly. this is the well known case of the motion of the perihelion of mercury, whose disagreement with newton's law had puzzled astronomers for more than half a century. since einstein's time-space includes minkowski's as a particular case, it can do all that the other was designed to do for electro-magnetism and light. but it does more. the track of a pulse of light is also a geodesic, and time-space being curved in the neighborhood of matter, rays of light are no longer straight lines. a ray of light from a star, passing near the sun, will be bent round, and the star consequently will be seen in a different direction from where it would be seen if the sun had not been so nearly in the way. this has been verified by the observations of the eclipse of the sun of of may . there is one other new phenomenon predicted by the theory, which falls within the reach of observation with our present means. gravitation chiefly affects the time-component of the four-dimensional continuum, in such a way that natural clocks appear to run slower in a strong gravitational field than in a weak one. thus, if we make the hypothesis--which, though extremely probable, is still a hypothesis--that an atom emitting or absorbing light-waves is a natural clock, and the further hypothesis--still very probable, though less so than the former--that there is nothing to interfere with its perfect running, then an atom on the sun will give off light-waves of smaller frequency than a similar atom in a terrestrial laboratory emits. opinions as yet differ as to whether this is confirmed or contradicted by observations. the great strength and the charm of einstein's theory do however not lie in verified predictions, nor in the explanation of small outstanding discrepancies, but in the complete attainment of its original aim: the identification of gravitation and inertia, and in the wide range of formerly apparently unconnected subjects which it embraces, and the broad view of nature which it affords. outside matter, as has been explained, the law of gravitation restricts the curvature of time-space. inside continuous matter the curvature can be of any arbitrary kind or amount; the law of gravitation then connects this curvature with measurable properties of the matter, such as density, velocity, stress, etc. thus these properties define the curvature, or, if preferred, the curvature defines the properties of matter, i.e. matter itself. from these definitions the laws of conservation of energy, and of conservation of momentum, can be deduced by a purely mathematical process. thus these laws, which at one time used to be considered as the most fundamental ones of mechanics, now appear as simple corollaries from the law of gravitation. it must be pointed out that such things as length, velocity, energy, momentum, are not absolute, but relative, i.e. they are not attributes of the physical reality, but relations between this reality and the observer. consequently the laws of conservation are not laws of the real world, like the law of gravitation, but of the observed phenomena. there is, however one law which, already before the days of relativity, had come to be considered as the most fundamental of all, viz: the principle of least action. now action is absolute. accordingly this principle retains its central position in einstein's theory. it is even more fundamental than the law of gravitation, since both this law, and the law of motion, can be derived from it. the principle of least action, so far as we can see at present, appears to be the law of the real world. xi the principle of general relativity how einstein, to a degree never before equalled, isolates the external reality from the observer's contribution by e. t. bell university of washington seattle einstein's general relativity is of such vast compass, being coextensive with the realm of physical events, that in any brief account a strict selection from its numerous aspects is prescribed. the old, restricted principle being contained in the general, we shall treat the latter, its close relations with gravitation, and the significance of both for our knowledge of space and time. the essence of einstein's generalization is its final disentanglement of that part of any physical event which is contributed by the observer from that which is inherent in the nature of things and independent of all observers. the argument turns upon the fact that an observer must describe any event with reference to some framework from which he makes measurements of time and distance. thus, suppose that at nine o'clock a ball is tossed across the room. at one second past nine the ball occupies a definite position which we can specify by giving the three distances from the centre of the ball to the north and west walls and the floor. in this way, refining our measurements, we can give a precise description of the entire motion of the ball. our final description will consist of innumerable separate statements, each of which contains four numbers corresponding to four measurements, and of these one will be for time and three for distances at the time indicated. imagine now that a man in an automobile looks in and observes the moving ball. suppose he records the motion. to do so, he must refer to a timepiece and some body of reference. say he selects his wrist-watch, the floor of his auto and two sides meeting in a corner. fancy that just as he begins his series of observations his auto starts bucking and the main-spring of his watch breaks, so that he must measure "seconds" by the crazy running-down of his watch, and distances with reference to the sides of his erratic auto. despite these handicaps he completes a set of observations, each of which consists of a time measured by his mad watch and three distances reckoned from the sides of his bucking machine. let us assume him to have been so absorbed in his experiment that he noticed neither the disorders of his watch nor the motion of his auto. he gives us his sets of measurements. we remark that his seconds are only small fractions of ours, also his norths and wests are badly mixed. if we interpret his sets in terms of our stationary walls and sober clock we find the curious paradox that the ball zigzagged across the room like an intoxicated bee. he obstinately argues that we know no more than he about how the ball actually moved. for we got a smooth description, he asserts, by choosing an artificially simple reference framework, having no necessary relations whatever to the ball. the crooked path plotted from his observations proves, he declares, that the ball was subject to varying forces of which we in the room suspected nothing. he contends that our room was being jarred by a system of forces which exactly compensated and smoothed out the real jaggedness of path observed by himself. but if we know all about his watch and auto we can easily apply necessary corrections to his measurements, and, fitting the corrected set to our reference-framework of walls and clock, recover our own smooth description. for consistency we must carry our readjustments farther. the path mapped from our measurements is a curve. perhaps the curvature was introduced by some peculiarity of our reference framework? possibly our own room is being accelerated upward, so that it makes the ball's true path--whatever that may be--appear curved downward, just as the autoist's zigzags made the path he mapped appear jagged. tradition attributes the downward curving to the tug of gravity. this force we say accelerates the ball downward, producing the curved path. is this the only possible explanation? let us see. gravitation and acceleration imagine a man in a room out of which he cannot see. he notices that when he releases anything it falls to the floor with a constant acceleration. further he observes that all his objects, independently of their chemical and physical properties, are affected in precisely the same way. now, he previously has experimented with magnets, and has remarked that they attract certain bodies in essentially the same way that the things which he drops are "attracted" to whatever is beneath the floor. having explained magnetic attraction in terms of "forces," he makes his first hypothesis: (a) he and his room are in a strong "field of force," which he designates gravitational. this force pulls all things downward with a constant acceleration. here he notes a singular distinction between magnetic and gravitational "forces": magnets attract only a few kinds of matter, notably iron; the novel "force," if indeed a force at all, acts similarly upon all kinds of matter. he makes another hypothesis: (b) his room and he are being accelerated upward. either (a) or (b) describes the facts perfectly. by no experiment can he discriminate between them. so he takes the great step, and formulates the equivalence hypothesis: a gravitational field of force is precisely equivalent in its effects to an artificial field of force introduced by accelerating the framework of reference, so that in any small region it is impossible to distinguish between them by any experiment whatever. next reconsidering his magnetic "forces," he extends the equivalence hypothesis to cover all manifestations of force: the effects attributed to forces of any kind whatever can be described equally well by saying that our reference frameworks are accelerated; and moreover there is possible no experiment which will discriminate between the descriptions. if the accelerations are null, the frameworks are at rest or in uniform motion relatively to one another. this special case is the "restricted" principle of relativity, which asserts that it is impossible experimentally to detect a uniform motion through the ether. being thus superfluous for descriptions of natural phenomena, the ether may be abandoned, at least temporarily. the older physics sought this absolute ether framework to which all motions could be unambiguously referred, and failed to find it. the most exacting experiments, notably that of michelson-morley, revealed no trace of the earth's supposed motion through the ether. fitzgerald accounted for the failure by assuming that such motion would remain undetected if every moving body contracted by an amount depending upon its velocity in the direction of motion. the contraction for ordinary velocities is imperceptible. only when as in the case of the beta particles, the velocity is an appreciable fraction of the velocity of light, is the contraction revealed. this contraction follows immediately from einstein's generalization constructed upon the equivalence hypothesis and the restricted relativity principle. we shall see that the contraction inevitably follows from the actual geometry of the universe. [ ] let us return for a moment to the moving ball. four measures, three of distances and one of time, are required in specifying its position with reference to some framework at each point and at each instant. all of these measures can be summed up in one compendious statement--the equations of motion showed how in changing from our room to his accelerated auto we found a new summary, "transformed equations," which seemed to indicate that the ball had traversed a strong, variable field of force. is there then in the chaos of observational disagreements anything which is independent of all observers? there is, but it is hidden at the very heart of nature. paths through the world of four dimensions to exhibit this, we must recall a familiar proposition of geometry: the square on the longest side of a right-angled triangle is equal to the sum of the squares on the other two sides. it has long been known that from this alone all the metrical properties of euclidean space--the space in which for , years we have imagined we were living--can be deduced. metrical properties are those depending upon measurement. now, in the geometry of any space, euclidean or not, there is a single proposition of a similar sort which tells us how to find the most direct distance between any two points that are very close together. this small distance is expressed in terms of the two sets of distance measurements by which the end-points are located, just as two neighboring positions of our ball were located by two sets of four measurements each. we say by analogy that two consecutive positions of the ball are separated by a small interval of time-space. from the formula for the very small interval of time-space we can calculate mathematically all the metrical properties of the time and space in which measurements for the ball's motion must be made. so in any geometry mathematical analysis predicts infallibly the truth about all facts depending upon measurements from the simple formula of the interval between neighboring points. thus, on a sphere the sum of the angles of any triangle formed by arcs of great circles exceeds °, and this follows from the formula for the shortest ("geodesic") distance between neighboring points on the spherical surface. we saw that it takes four measurements, one for time and three for distances, to fix an elementary event, viz., the position of the centre of our ball at any instant. a system of all possible such sets of four measurements each, constitutes what mathematicians call a four-dimensional space. the study of the four-dimensional time-space geometry, once its shortest-distance proposition is known, reveals all those relations in nature which can be ascertained by measurements, that is, experimentally. we have then to find this indispensable proposition. imagine the path taken by a particle moving solely under the influence of gravitation. this being the simplest possible motion of an actual particle in the real world, it is natural to guess that its path will be such that the particle moves from one point of time-space to another by the most direct route. this in fact is verified by forming the equations of the free particle's motion, which turn out to be precisely those that specify a geodesic (most direct line) joining the two points. on the (two-dimensional) surface of a sphere such a line is the position taken by a string stretched between two points on the surface, and this is the shortest distance on the surface between them. but in the time-space geometry we find a remarkable distinction: the interval between any two points of the path taken is the longest possible, and between any two points there is only one longest path. translated into ordinary space and time this merely asserts that the time taken between any two points on the natural path is the longest possible. recall now that when the line-formula for any kind of space is known all the metrical properties of that space are completely determined, and combine with this what we have just found, namely, the equations of motion of a particle subject only to gravitation are the same equations as those which fix the line-formula for the four-dimensional time-space. since gravitation alone determines the motion of the particle, and since this motion is completely described by the very equations which fix all the metrical properties of time-space, it follows that the metrical (experimentally determinable) properties of time-space are equivalent to those of gravitation, in the sense that each set of properties implies the other. the universe of space-time we have found the thing in nature which is independent of all observers, and it turns out to be the very structure of time-space itself. the motion of the free particle obviously is a thing unconditioned by accidents of observation; the particle under the influence of gravitation alone must go a way of its own. and if some observer in an artificial field of force produced by the acceleration of his reference framework describes the path as knotted, he merely is foisting eccentricities of his own motion upon the direct path of the particle. the conclusion is rational, for we believe that time-space exists independently of any man's way of perceiving it. incidentally note that this space is that of the physical world. for only by measurements of distances and times can we become aware of our extension in time and space. if beyond this time-space geometry of measurements there is some "absolute geometry," science can have no concern with it, for never can it be revealed by the one exploring device we possess--measurement. we have followed a single particle. let us now form a picture of several. any event can be analyzed into a multitude of coincidences in time-space. for consider two moving particles--say electrons. if they collide they both are in very approximately one place at the same time. we imagine the path of an electron through time-space plotted by a line (in four-dimensional space), which will deviate from a "most direct" (geodesic) path if the electron is subjected to forces. this is the "world-line" of the electron. if the world lines of several electrons intersect at one point in time-space, the intersection pictures the fact of their coincidence somewhere and somewhen; for all their world-lines having a time-space point in common, at some instant they must have been in collision. each point of a world-line pictures the position at a certain place at a certain time; and it is the intersections of world-lines which correspond to physical events. of what lies between the intersections we have no experimental knowledge. imagine the world-lines of all the electrons in the universe threading time-space like threads in a jelly. the intersections of the tangle are a complete history of all physical events. now distort the jelly. clearly the mutual order of the intersections will be unchanged, but the distances between them will be shortened or lengthened. to a distortion of the jelly corresponds a special choice (by some observer) of a reference framework for describing the order of events. he cannot change the natural sequence of events. again we have found something which is independent of all observers. we can now recapitulate our conclusions and state the principle of relativity in its most general form. ( ) observers describe events by measures of times and distances made with regard to their frameworks of reference. ( ) the complete history of any event is summarized in a set of equations giving the positions of all the particles involved at every instant. ( ) two possibilities arise. (a) either these equations are the same in form for all space-time reference frameworks, persisting formally unchanged for all shifts of the reference scheme; or (b), they subsist only when some special framework is used, altering their form as they are referred to different frameworks. if (b) holds, we naturally assume that the equations, and the phenomena which they profess to represent, owe their existence to some peculiarity of the reference framework. they do not, therefore, describe anything which is inherent in the nature of things, but merely some idiosyncrasy of the observer's way of regarding nature. if (a) holds, then obviously the equations describe some real relation in nature which is independent of all possible ways of observing and recording it. ( ) in its most general form the principle of relativity states that those relations, and those alone, which persist unchanged in form for all possible space-time reference frameworks are the inherent laws of nature. to find such relations einstein has applied a mathematical method of great power--the calculus of tensors--with extraordinary success. this calculus threshes out the laws of nature, separating the observer's eccentricities from what is independent of him, with the superb efficiency of a modern harvester. the residue is a physical geometry--or geometrical physics--of time-space, in which it appears that the times and spaces contributed by the several observers' reference frameworks are shadows of their own contrivings; while the real, enduring universe is a fourfold order of time and space indissolubly bound together. one observer separates this time-space into his own "time" and "space" in one way, determined by his path through the world of events; another, moving relatively to the first, separates it differently, and what for one is time shades into space for another. this time-space geometry is non-euclidean. it is "warped" (curved), the amount of warping at any place being determined by the intensity of the gravitational field there. thus again gravitation is rooted in the nature of things. in this sense it is not a force, but a property of space. wherever there is matter there is a gravitational field, and hence a warping of space. conversely, as long ago imagined by clifford, wherever there is a warping of space, there is matter; and matter is resolved ultimately into wrinkles in time-space. to visualize a warped space, consider a simple analogy. a man walks away from a polished globe; his image recedes into the mirror-space, shortening and thinning as it goes, and thinning (in the direction of motion) faster than it shortens. everything around him experiences a like effect. if he tries to discover this by a footrule it automatically shortens faster as he turns it into the horizontal position, so his purpose eludes him. the mirror-space is warped in the direction of the image's motion. so is our own. for all bodies, as evidenced by the fitzgerald contraction, shorten in the direction of motion. and just as the image can never penetrate the mirror-space a greater distance than half its radius, so probably time-space is curved in such a way that our universe, like the surface of a sphere, is finite in extent, but unbounded. xii force vs. geometry how einstein has substituted the second for the first in connection with the cause of gravitation by saul dushman general electric laboratories schenectady, n. y. the theory of relativity represents a most strikingly original conception of time and space, which was suggested by einstein in order to correlate with all our past experience certain observations made in recent years. it is therefore extremely comprehensive in its scope; it demands from us a radical revision in our notions of time and space; it throws new light on the nature of mass and energy, and finally, it furnishes a totally new conception of the old problem of gravitation. the starting point of the theory is the familiar observation that motion is always relative: that is, to define the motion of any object we must always use some point of reference. thus we speak of the velocity of a train as miles per hour with respect to the earth's surface, but would find it impossible to determine its absolute speed, or motion in space, since we know of no star whose position can be spoken of as absolutely fixed. these and similar considerations have led to the conclusion, pointed out by newton and others, that it is impossible by any mechanical experiments on the earth to measure its velocity in space. however, the results of observations on the phenomena of light and electricity led to the revival of the same problem under another form. as well known, there was evolved from these discoveries, the theory that light and electrical energy are of the same nature, and are in each case manifestations of wave-disturbances propagated through a hypothetical medium, the ether, with a velocity of , miles per second. the problem therefore arose as to whether the earth and all stellar bodies move through this ether. in that case it ought to be possible to measure the velocity of the earth with respect to this medium, and under these conditions we could speak, in a sense, of absolute motion. a large number of experiments has been tried with this end in view. the most famous of these, and the one which stimulated the subsequent development of the theory of relativity, was that carried out by michelson and morley in . to understand the significance of this experiment we shall refer briefly to an analogous observation which is quite familiar. does it take longer to swim to a point mile up a stream and back or to a point mile across stream and back? the experienced swimmer will answer that the up-and-down journey takes longer. if we assume that the swimmer has a speed of miles an hour in still water and that the current is miles an hour, we find that, while it requires five-eighths hour to make the up-and-down journey, it takes only one-half hour for the trip across stream and back. the ratio between the times required for the two journeys is thus five-fourths, and if this is written in the form $$\frac{ }{\sqrt{ - (\frac{ }{ })^ }}$$ it shows how the result depends upon the square of the ratio of the speeds of the swimmer and the current. now the earth is moving in its orbit about the sun with a velocity of miles per second. if the earth moves through the ether and a light-beam passes from one mirror to another and back again, the time taken for this journey ought to be longer when the light-path is in the direction of the earth's motion than when it is at right angles to this direction. for we can consider the light as a swimmer having a speed of , miles per second and travelling in a stream whose current is miles per second. when michelson and morley tried the experiment they could not observe any difference in the velocity of light in the two directions. the experiment has since been repeated under various conditions, but always with negative results. einstein's contribution to science consists in interpreting this result as being in accord with newton's ideas on mechanical relativity in that it demonstrates the impossibility of measuring absolute motion, not only by mechanical, but also by optical or electrical experiments. consequently the velocity of light must be regarded as constant and independent of the motion of either source or observer. the relativity of uniform motion let us consider some of the consequences which follow from this principle. an observer travelling with say one-half the velocity of light in the same direction as a ray of light would find that the latter has the usual velocity of , miles per second. similarly an observer travelling in the opposite direction to that of the light-ray, with one-half the velocity of light, would obtain the same result. einstein has shown that these conclusions can be valid only if the units of time and space used by the two observers depend upon their relative motions. a careful calculation shows that the unit of length used by either observer appears to the other observer contracted when placed in the direction of their relative motion (but not, when placed at right angles to this direction), and the unit of time used by either observer appears to the other too great. moreover, the ratio of the units of length or of time varies with the square of the relative speed of the two observers, according to a relation which is similar to that mentioned above for the swimmer in the current. this relation shows that as the relative speed approaches that of light the discrepancy between the units increases. thus, for an observer moving past our earth with a velocity which is nine-tenths that of light, a meter stick on the earth would be centimeters as measured by him, while a second on our clocks would be about two and a half seconds as marked by his clock. similarly, what he calls a meter length would, for us, be only centimeters and he would appear to us to be living about two and a half times slower than we are. each observer is perfectly consistent in his measurements of time and space as long as he confines his observations to his own system, but when he tries to make observations on another system moving past his, he finds that the results which he obtains do not agree with those obtained by the other observer. it is not surprising that in accordance with this conclusion it also follows that the mass of a body must increase with its velocity. for low velocities the increase is so small that we cannot ever hope to measure it, but as the velocity of light is approached the difference becomes more and more appreciable and a body having the velocity of light would possess infinite mass, which simply means that such a velocity cannot be attained by any material object. this conclusion has been experimentally confirmed by observations on the mass of the extremely small negatively charged particles which are emitted by radioactive elements. some of these particles are ejected with velocities which are over nine-tenths that of light, and measurements show that the increase in mass is in accord with this theory. the relativity theory also throws new light on the nature of mass itself. according to this view, mass and energy are equivalent. the absolute destruction of gram of any substance, if possible, would yield an amount of energy which is one hundred million times as much as that obtained by burning the same mass of coal. conversely, energy changes are accompanied by changes in mass. the latter are ordinarily so inappreciably small as to escape our most refined methods of measurements, but in the case of the radioactive elements we actually observe this phenomenon. from this standpoint, also, the laws of conservation of energy and of mass are shown to be intimately related. universal relativity so far we have dealt with what has been designated as the special theory of relativity. this, as we have seen, applies to uniform motion only. in extending the theory to include non-uniform or accelerated motion, einstein has at the same time deduced a law of gravitation which is much more general than that of newton. a body falling towards the earth increases in velocity as it falls. the motion is said to be accelerated. we ascribe this increase in velocity to a gravitational force exerted by the earth on all objects. as shown by newton, this force acts between all particles of matter in the universe, and varies inversely as the square of the distance, and directly as the product of the masses. of course, we have had a number of theories of gravitation, and none of them have proven successful. einstein, however, was the first one to suggest a conception of gravitation which has proven extremely significant. he points out that a gravitational force is non-existent for a person falling freely with the acceleration due to gravity. for this person there is no sensation of weight, and if he were in a closed box which is also falling with the same acceleration, he would be unable to decide as to whether his system were falling or situated in interplanetary space where there is no gravitational field. furthermore, if he were to carry out any optical or electrical experiments in this box he would observe the same results as an experimenter on the earth. a ray of light would travel in a straight line so far as this observer can perceive, while an external observer would, of course, judge differently. einstein shows that this is equally true for all kinds of acceleration including that due to rotation. in the case of a rotating body there exists a centrifugal force which tends to make objects on the surface fly outwards, but for an external observer this force does not exist any more than gravity exists for the observer falling freely. thus we can draw the general conclusion that a gravitational field or any other field of force may be eliminated by choosing an observer moving with the proper acceleration. for this observer, however, the laws of optics and electricity must be just as valid as for an observer on the earth. in postulating this equivalence hypothesis einstein merely makes use of the very familiar observation that, independently of the nature of the material, all bodies possess the same acceleration in a given field of force. the problem which einstein now sets out to solve is that of determining the law which shall describe the motion of any system in a field of force in such a general manner as to leave unaltered the fundamental relations of electricity and optics. in connection with the solution of this problem he finds it necessary to discard the limitations placed on us by ordinary or euclidean geometry. in this manner geometrical concepts as well as those of force are completely robbed of all notions of absoluteness, and the goal of a general theory of relativity is attained. the geometry of gravitation let us consider a circular disc rotating with a uniform peripheral speed. according to the deductions from the "special theory" of relativity, an observer situated near the edge of this disc, but not rotating with it, will observe that units of length measured along the circumference of the disc are contracted. on the other hand, measurements along the diameter, which is at right angles to the direction of motion of the circumference, will show no contraction whatever, and, consequently the observer will find that the ratio of circumference to diameter has not the well known value . ... but exceeds this value, the difference being greater and greater as the peripheral speed approaches that of light. that is, the laws of ordinary geometry no longer hold true. however, we know other cases in which the ordinary or euclidean geometry is not applicable. thus suppose that on the surface of a sphere we describe a series of concentric circles. since the surface is curved, we are not surprised at finding that the circumference of any one of these circles is less than . ... times the distance across the circle as measured on the surface of the sphere. what this means, therefore, is that we cannot use euclidean geometry to describe measurements on the surface of a sphere, and every schoolboy knows this from comparing mercator's projection of the earth's surface with the actual representation on a globe. when we come to think of it, the reason we realize all this is because our sense of three dimensions enables us to differentiate flat surfaces from those that are curved. let us, however, imagine a two-dimensional being living on the surface of a large sphere. so long as his measurements are confined to relatively small areas he will find it possible to describe all his measurements in terms of euclidean geometry. as, however, his area of operation increases he will begin to observe greater and greater discrepancies. being unfamiliar with the existence of such a three-dimensional object as a sphere, and therefore not realizing that he is on the surface of one, our intelligent two-dimensional being will conclude that the disturbance in his geometry is due to the action of a force, and by means of plausible assumptions on the "law" of this force he will reconcile his observations with the laws of plane geometry. now since an acceleration in a gravitational field is identical with that due to centrifugal force produced by rotation, we concluded that the geometry in a gravitational field must also be non-euclidean. that is, space in the neighborhood of matter is distorted or curved. the curvature of space bears the same relation to three dimensions that the curvature of a spherical surface bears to two dimensions, and that is why we do not perceive it, any more than the intelligent two-dimensional being would be aware of the distortion of his space (or surface). furthermore, like this being, we have assumed the existence of a gravitational force to account for discrepancies in our geometrical measurements. the identification in this manner of gravitational effects with geometrical curvature of space enables einstein to derive a general law for the path of any particle in a gravitational field, with respect both to space and to time. furthermore, the law expresses this motion in terms which are independent of the relative motion and position of the observer, and satisfies the condition that the fundamental laws of physics be equally valid for all observers. the solution of the problem involved the use of a new kind of higher calculus, elaborated by two italian mathematicians, ricci and levi-civita. the result is a law of motion which is extremely general in its validity. for low velocities it approximates to newton's solution, and in the absence of a gravitational field it leads to the same conclusions as the special theory of relativity. there are three deductions from this law which have aroused a great deal of interest, and the confirmation of two of these by actual observation must be regarded as striking proof of einstein's theory. xiii an introduction to relativity a treatment in which the mathematical connections of einstein's work are brought out more strongly and more successfully than usual in a popular explanation by harold t. davis, university of wisconsin, madison, wis. one of the first questions which appears in philosophy is this: what is the great reality that underlies space and time and the phenomena of the physical universe? kant, the philosopher, dismissed it as a subjective problem, affirming that space and time are "a priori" concepts beyond which we can say no more. then the world came upon some startling facts. in a paper appeared by professor albert einstein which asserted that the explanation of certain remarkable discoveries in physics gave us a new conception of this strange four-dimensional manifold in which we live. thus, the great difference between the space and time of philosophy and the new knowledge is the objective reality of the latter. it rests upon an amazing sequence of physical facts, and the generalized theory, which appeared several years later, founded as it is upon the abstruse differential calculus of riemann, christoffel, ricci and levi-civita, emerges from its maze of formulas with the prediction of real phenomena to be sought for the in the world of facts. we shall, therefore, approach the subject from this objective point of view. let us go to the realm of actual physical events and see how the ideas of relativity gradually unfolded themselves from the first crude wonderings of science to the stately researches that first discovered the great ocean of ether and then penetrated in such a marvelous manner into some of its most mysterious properties. the electromagnetic theory of light suppose that we go out on a summer night and look into the dark depths of the sky. a thousand bright specks are flashing there, blue, red, yellow against the dark velvet of space. and as we look we must all be impressed by the fact that such remote objects as the stars can be known to us at all. how is it that light, that curious thing which falls upon the optic nerve and transmits its pictures to the brain, can ever reach us through the black regions of interstellar space? that is the question which has for its answer the electromagnetic theory of light. the first theory to be advanced was newton's "corpuscular" theory which supposed that the stars are sending off into space little pellets of matter so infinitesimally small that they can move at the rate of , miles a second without injuring even so delicate a thing as the eye when they strike against it. but in , when thomas young made the very important discovery of interference, this had to give way to the wave theory, first proposed by huyghens in the th century. the first great deduction from this, of course, was the "luminiferous ether," because a wave without some medium for its propagation was quite unthinkable. certain peculiar properties of the ether were at once evident, since we deduce that it must fill all space and at the same time be so extremely tenuous that it will not retard to any noticeable degree the motion through it of material bodies like the planets. but how light was propagated through the ether still remained a perplexing problem and various theories were proposed, most prominent among them being the "elastic solid" theory which tried to ascribe to ether the properties of an elastic body. this theory, however, laid itself open to serious objection on the ground that no longitudinal waves had been detected in the ether, so that it began to appear that further insight into the nature of light had to be sought for in another direction. this was soon forthcoming for in a new theory was proposed by james clerk maxwell which seemed to solve all of the difficulties. maxwell had been working with the facts derived from a study of electrical and magnetic phenomena and had shown that electromagnetic disturbances were propagated through the ether at a velocity identical with that of light. this, of course, might have been merely a strange coincidence, but maxwell went further and demonstrated the interesting fact that an oscillating electric charge should give rise to a wave that would behave in a manner identical with all of the known properties of a light wave. one particularly impressive assertion was that these waves, consisting of an alternating electric field accompanied by an alternating magnetic field at right angles to it, and hence called electromagnetic waves, would advance in a direction perpendicular to the alternating fields. this satisfied the first essential property of light rays, i.e., that they must be transverse waves, and the ease with which it explained all of the fundamental phenomena of optics and predicted a most striking interrelation between the electrical and optical properties of material bodies, gave it at once a prominent place among the various theories. the electromagnetic theory, however, had to wait until for verification when heinrich hertz, in a series of brilliant experiments, succeeded in producing electromagnetic waves in the laboratory and in showing that they possessed all of the properties predicted by maxwell. these waves moved with the velocity of light: they could be reflected, refracted, and polarized: they exhibited the phenomenon of interference and, in short, could not be distinguished from light waves except for their difference in wave length. the michelson-morley experiment with the final establishment of the electromagnetic theory of light as a fact of physics, we have at last endowed the ether with an actual substantiality. the "empty void" is no longer empty, but a great ocean of ether through which the planets and the suns turn without ever being aware that it is there. in a. a. michelson undertook an experiment, originally suggested by maxwell, to determine the relative motion of our earth to the ether ocean and six years later he repeated it with the assistance of e. w. morley. the experiment is now known as the michelson-morley experiment and since it is the great physical fact upon which the theory of relativity rests, it will be well for us to examine it in detail. since we can scarcely think that our earth is privileged in the universe and that it is at rest with respect to this great ether ocean that fills space, we propose to discover how fast we are actually moving. but the startling fact is that the experiment devised for this purpose failed to detect any motion whatever of the earth relative to the ether. [ ] the explanation of this very curious fact was given by both h. a. lorentz and g. f. fitzgerald in what is now widely known under the name of the "contraction hypothesis." it is nothing more nor less than this: every solid body undergoes a slight change in dimensions, of the order of ($v^ /c^ $), when it moves with a velocity $v$ through the ether. the reason why the experiment failed, then, was not because the earth was not moving through the ether, but because the instruments with which the experiment was being conducted had shrunk just enough to negative the effect that was being looked for. [ ] the lorentz transformation we can not at this point forebear introducing a little mathematics to further emphasize the theory and the very logical nature of this contraction hypothesis. let us suppose that we were on a world that was absolutely motionless with respect to the ether and were looking at a ray of light. the magnetic and electric fields which form the ray can be described by means of four mathematical expressions which have come to bear the name of "maxwell's field equations." now suppose that we ask ourselves the question: how must these equations be changed so that they will apply to a ray of light which is being observed by people on a world that is moving with a velocity v through the ether? the answer is immediate. from the michelson-morley experiment we know that we can not tell how fast or how slowly we are moving with respect to the ether. this means that no matter what world we may be upon, the form of the maxwell field equations will always be the same, even though the second set of axes (or frame of reference) may be moving with high velocity with respect to the first. starting from this hypothesis (called in technical language the covariance of the equations with respect to a transformation of coordinates), lorentz found that the transformation which leaves the field equations unchanged in form was the following: $$x' = k(x - vt), y' = y, z' = z, t' = k(t - vx/c)$$ where $k$ is as on page . and what, now, can be deduced from these very simple looking equations? in the first place we see that the space of $x'$, $y'$, $z'$, $t'$ is not our ordinary concept of space at all, but a space in which time is all tangled up with length. to put it more concretely, we may deduce from them the interesting fact that whenever an aviator moves with respect to our earth, his shape changes, and if he were to compare his watch with one on the earth, he would find that his time had changed also. a sphere would flatten into an ellipse, a meter stick would shorten up, a watch would slow down and all because, as h. minkowski has shown us from these very equations, we are really living in a physical world quite different from the world of euclid's geometry in which we are accustomed to think we live. a variety of objections has very naturally been made to this rather radical hypothesis in an attempt to discredit the entire theory, but it is easily seen that any result obtained through the field equations must necessarily be in conformity with the theory of contraction, since this theory is only the physical interpretation of that transformation which leaves the field equations unaltered. indeed, it is even possible to postulate the lorentz transformation together with the assumption that each element of charge is a center of uniformly diverging tubes of strain and derive the maxwell field equations from this, which shows from another point of view the truly fundamental nature of the transformation. the first theory of relativity the whole question of the ether had arrived at this very interesting point when professor einstein in stated the theory of relativity. he had noticed that the equations of dynamics as formulated by newton did not admit the lorentz transformation, but only the simple galilean transformation: $$x' = x - vt, y' = y, z' = z, t' = t\,.$$ here, indeed, was a curious situation. two physical principles, that of dynamics and that of electromagnetism, were coexistent and yet each one admitted a different transformation when the system of reference was transferred to axes moving with constant velocity with respect to the ether. now the electromagnetic equations and their transformation had been shown to be in accord with experimental fact, whereas it had long been felt that newton's equations were only a first approximation to the truth. for example, the elliptic orbit of a planet had been observed by leverrier to exhibit a disquieting tendency to rotate in the direction of motion. this precession, which in the case of mercury was as large as '' per century, could not be accounted for in any way by the ordinary newtonian laws and was, consequently, a very celebrated case of discordance in gravitational astronomy. with this example clearly before him, einstein took the great step and said that the laws of dynamics and all other physical laws had to be remade so that they, also, admit the lorentz transformation. that is to say, the laws of physical phenomena, or rather the mathematical expressions for these laws, are covariant (unchanged in form) when we apply the lorentz transformation to them. the deductions from the michelson-morley experiment now seem to have reached their ultimate conclusion. one discordant fact in this new theory remained, however. that same precession of the perihelion of mercury which had first lead einstein to his theory remained unsettled. when the new approximations were applied to the formula of orbital motion, a precession was, indeed, obtained, but the computed value fell considerably below that of the observed '' per century. the inclusion of gravitation with the idea of investigating the problem from the very bottom, einstein now undertook a broader and more daring point of view. in the first place he said that there is no apparent reason in the great scheme of world events why any one special system of coordinates should be fundamental to the description of phenomena, just as in the special theory a ray of light would appear the same whether viewed from a fixed system or a system moving with constant velocity with respect to the ether. this makes the very broad assumption that no matter what system of coordinates we may use, the mathematical expressions for the laws of nature must be the same. in einstein's own words, then, the first principle of this more general theory of relativity must be the following: "the general laws of nature are expressed through equations which hold for all systems of coordinates, that is, they are covariant with respect to arbitrary substitutions." [ ] but this was not enough to include gravitation so einstein next formulated what he was pleased to call his "equivalence hypothesis." this is best illustrated by an example. suppose that we are mounting in an elevator and wish to investigate the world of events from our moving platform. we mount more and more rapidly, that is with constant acceleration, and we appear to be in a strong gravitational field due to our own inertia. suppose, on the other hand, that the elevator descends with an acceleration equal to that of gravity. we would now feel certain that we were in empty space because our own relative acceleration has entirely destroyed that of the earth's gravitational field and all objects placed upon scales in an elevator would apparently be without weight. applying this idea, then, einstein decided to do away with gravitation entirely by referring all events in a gravitational field to a new set of axes which should move with constant acceleration with respect to the first. in other words we are going to deal with a system moving with uniform acceleration with respect to the ether, just as we considered a system moving with uniform velocity in the special theory. the next step in the construction of this complicated theory is to reduce these two hypotheses to the language of mathematics and this was accomplished by einstein with the help of m. grossmann by means of the theory of tensors. on account of the very great intricacy of the details, we must content ourselves with the mere statement that this really involved the generalization of the famous expressions known as laplace's and poisson's equations, on the explicit assumption that these two equations would still describe the gravitational field when we are content to use a first approximation to the truth. the set of ten differential equations which einstein got as a result of his generalization he called his field equations of gravitation. [ ] xiv new concepts for old what the world looks like after einstein has had his way with it by john g. mchardy, commander r.n., london "the new-created world, which fame in heaven long had foretold, a fabric wonderful, of absolute perfection." einstein's theory of relativity has led to determining a key law of nature--the law of gravitation--which is also the basic law of mechanics. thus it embraces a whole realm of physics, and promises, through the researches of professor weyl, to embrace another realm--electro-dynamics. its limitations are not yet reached, for einstein has already postulated therefrom a theory of a finite, yet unbounded, universe. this essay, however, is mainly concerned with mechanics, and electrical forces are not considered. to have synthesised newton's two great principles--his law of motion and law of gravitation--interpreting in the process the empirical law of equality of gravitational and inertial mass, is alone an immense achievement; but einstein's researches have opened up a new world to the physicist and philosopher which is of greater importance. he has given us a vision of the immaterial world, a geometrical or mathematical vision, which is more satisfying than the "ether" conceptions hitherto presented. the fabric of his vision is not baseless. it is this fabric we shall consider, touching on certain aspects of the einstein theory in the endeavor to present an image in miniature of his edifice of thought and to show the firmness of its foundations. that they are well and truly laid was demonstrated by the verification, from observations made during the solar eclipse in , of einstein's prediction of the displacement of a wave of light in a gravitational field, showing light to have the property of weight. the physical world is shown by einstein to be a world of "relations." underlying it there is an absolute world of which physical phenomena are the manifestation. "give me matter and motion," says descartes, "and i will construct the world." "give me a world in which there are ordered relations," says the relativist, "and i will show you the behavior of matter therein" (mechanics). we first view this underlying world as an abstraction, abstracting energy ("bound" as in matter and electrons, "free" as in light), and its attribute force. this abstraction we will call the "world-frame." later, we will study the underlying world in connection with energy, and will call this absolute world the "world-fabric." the connection between the geometrical character of the world-frame and the geometrical characters of the world-fabric is the key to the law of gravitation. the world-frame this is our conception of a world, if such were possible, entirely free from the influence of energy. we may conceive of it as an amorphous immaterial something containing "point-events" (a point-event being an instant of time at a point in space--a conception, not a definition). these point-events have a fourfold order and definite relation in this frame, i.e. they can be specified by four variables or coordinates in reference to some base called a reference system, with respect to which they are forward or backward, right or left, above or below, sooner or later. this shows the world-frame to be four-dimensional. thus an aggregate of point-events (or an "event," which implies limited extension in space and limited duration in time) [ ] would have what we familiarly describe as length, breadth, height and time. to express these metrical properties most simply we must choose a four-dimensional reference system having a particular form--rectilinear axes (cartesian coordinates), and a particular motion--uniform and rectilinear, i.e. unaccelerated, and non-rotating with respect to the path of a light ray. we call this an inertial system because newton's law of inertia holds for such a system alone. this system indicates how observers partition the world-frame into space and time. it restricts observers to uniform rectilinear motion, and observations to bodies and light-pulses in such motion. thus gravitational and other forces are discounted, and we obtain world-frame conditions notwithstanding the fact that observers are in the presence of energy. now the separation between point-events which have a definite relation to each other must be absolute. the separation between two points in a plane is defined by the unique distance between them (the straight line joining them). between point-events the analogue of this unique distance, which we call the "separation-interval" (to indicate its time-like and space-like nature), is also unique. its unique and absolute character give it great importance as thereby it is the same for all observers regardless of their reference system. if, in place of the rather cumbersome expression $x-x$ to indicate the difference between the $x$-coordinates of two points, we employ the more compact expression $dx$; if for the benefit of readers who have a little algebra but no analysis we state explicitly that this expression is a single symbol for a single quantity, and has nothing to do with any product of two quantities $d$ and $x$; and if we extend this notation to all our coordinates: then it is clear from previous essays that the distance $s$ between two points in a plane referred to a rectilinear system $ox$, $oy$, is given by the simple equation $s^ = (dx)^ + (dy)^ $. einstein and minkowski show that the value for the separation interval $\omega$, the analogue of $s$, referred to an inertial system is given by the equation $$\omega^ = (dx)^ + (dy)^ + (dz)^ - (dt)^ \,,$$ which is seen to be a modified extension to four dimensions of the equation for $s$. we must measure $t$ in the same units as $x$, $y$, $z$. by taking the constant velocity of light ( , kilometres per second) as unit velocity, we can measure in length or time indiscriminately. [ ] we will analyse briefly this equation as it epitomizes the special theory of relativity. if the world-frame had been euclidean the equation would have been $$\omega^ = (dx)^ + (dy)^ + (dz)^ + (dt)^ $$ but this would not satisfy the "transformation equations" which resulted from the special theory. these transformation equations arose directly from a reconciliation between two observed facts; (a) the observed agreement of all natural phenomena with the "restricted principle of relativity"--a principle which shows that absolute rectilinear motion cannot be established--(as regards mechanics this was recognized by newton; the michelson-morley and other experiments showed this principle also applied to optical and electro-dynamical phenomena); and (b) the observed disagreement of optical and electro-dynamical phenomena (notably the constancy of light velocity) with the laws of dynamics as given by classical mechanics, e.g., in regard to the compounding of relative velocities. einstein effected this reconciliation by detecting a flaw in classical mechanics. he showed that by regarding space and time measurements as relative to the observer--not absolute as newton defined them--there was nothing incompatible between the principle of relativity and the laws of dynamics so modified. newton's definitions were founded on conception. einstein's recognition of the relativity of space and time is based on observation. equation ( ) shows that the geometry of the world-frame referred to an inertial system is semi-euclidean (hyperbolic), and that space and time measurements are relative to the observer's inertial reference system. the equation shows that the world-frame has a certain geometrical character which we distinguish as four-dimensional "flatness." it is everywhere alike (homaloidal). its flat character is shown by the straight line nature of the separation-interval and of the system to which it is most simply referred. thus we have found two absolute features in the world-frame--( ) its geometrical character--"flatness"; ( ) the separation-interval--which can be expressed in terms of measurable variables called space and time partitions, this partitioning being dependent on the observer's motion. we are now in a position to explore the world-fabric. already we see that, studied under inertial conditions (free of force), it agrees with the world-frame. the world-fabric the general theory of relativity is largely concerned with the investigation of the world-fabric. consider the world-frame to be disturbed. we may regard this disturbance, which manifests itself in physical phenomena, as energy, or more correctly "action." when energy is thwarted in its natural flow, force is manifested, with which are associated non-uniform motions such as accelerations and rotations. this disturbed world-frame we distinguish as the world-fabric. it is found to have various non-euclidean characters differing from the simple "flat" character of the world-frame according to the degree of disturbance (action) in the region. disturbance gives the fabric a geometrical character of "curvature"; the more considerable the disturbance, the greater the curvature. thus an empty region (not containing energy, but under its influence) has less curvature than a region in which free energy abounds. our problem, after showing the relativity of force (especially gravitational force), is to determine the law underlying the fabric's geometrical character; to ascertain how the degree of curvature is related to the energy influencing a region, and how the curvature of one region is linked by differential equations to that of neighboring regions. such a law will be seen to be the law of gravitation. we study the world-fabric by considering tracks on which material particles and light-pulses progress; we find such tracks regulated and defined by the fabric's curvature, and not, as hitherto supposed, by attractive force inherent in matter. as a track is measurable by summing the separation-intervals between near-by point-events on it, all observers will agree which is the unique track between two distant point-events. einstein postulates that freely progressing bodies will follow unique tracks, which are therefore called natural tracks (geodesics). if material bodies are prevented from following natural tracks by contact with matter or other causes, the phenomenon of gravitational force is manifested relative to them. whenever the natural flow of energy is interrupted force is born. for example, when the piston interrupts the flow of steam, or golf ball flow of club, force results--the interruption is mutual, and the force relative to both. likewise when the earth interrupts the natural track of a particle (or observer) gravitational force is manifested relative to both. so long as a body moves freely no force is appreciated by it. a falling aviator (neglecting air resistance) will not appreciate any gravitational force. he follows a natural track, thereby freeing himself from the force experienced in contact with matter. he acquires an accelerating motion with respect to an inertial system. by acquiring a particular accelerating motion an observer can annul any force experienced in any small region where the field of force can be considered constant. thus einstein, interpreting the equality of gravitational and inertial mass, showed that the same quality manifests itself according to circumstances as "weight" or as inertia, and that all force is purely relative and may be treated as one phenomenon (an interruption in energy flow). this "principle of equivalence" shows that small portions of the world-fabric, observed from a freely moving particle (free of force), could be treated as small portions of the world-frame. [ ] if such observations were practicable, we could determine the fabric curvature by referring point-event measurements to equation ( ). we cannot observe from unique tracks but we can observe them from our restrained situation. their importance is now apparent, because, by tracing them over a region, we are tracing something absolute in the fabric--its geometrical character. we study this curvature by exploring separation-intervals on the tracks of freely moving bodies, relating these separation-intervals to actual measurements in terms of space and time components depending on the observer's reference system. the law of curvature must be the law of gravitation. to illustrate the lines on which einstein proceeded to survey the world-fabric from the earth we will consider a similar but more simple problem--the survey of the sea-surface curvature from an airship. we study this curvature by exploring small distances on the tracks of ships (which we must suppose can only move uniformly on unique tracks--arcs of great circles), relating such distances to actual measurements in terms of length and breadth components depending on the observer's reference system. this two-dimensional surface problem can be extended to the four-dimensional fabric one. we consider the surface to be covered by two arbitrarily drawn intersecting series of curves: curves in one series not intersecting each other, vide figure. this gaussian system of coordinates is appropriate only when the smaller the surface considered, the more nearly it approximates to euclidean conditions. it admits of defining any point on the surface by two numbers indicating the curves intersecting at that point. $p$ is defined by $x_ $, $x_ $. $p_ $ (very near $p$) is defined by $x_ + dx_ $, $x_ + dx_ $. the equation for the minute distance $s$ between two adjacent points in such a system is given by the general formula $$s^ = g_{ }d{x_ }^ + g_{ }dx_ dx_ + g_{ }d{x_ }^ \,.$$ the $g$'s may be constants or functions of $x_ $, $x_ $. their value is dependent on the observer's reference system and on the geometrical character of the surface observed. the curves being arbitrary, the formula is appropriate for any reference system, or even if the observer does not know exactly what his reference system is. (the fabric observer does not know what his space and time partitioning actually is because he is in a gravitational field). it is the $g$'s which disclose the geometry of an observer's partitions, and their values also contain a reflection of the character of the region observed. we find $s$ by direct exploration with a moving ship ($\omega$ is found by direct exploration with a freely moving particle); $dx_ $, $dx_ $ are the observed length and breadth measurement differences which we have to relate to $s$. by making sufficient observations in a small area and referring them to the general formula we can find the values of the $g$'s for the observer's particular reference system. different values for $g$'s will be found if the observer changes his reference system, but there is a limitation to the values so obtainable owing to the part played by the surface itself, which is diffidently expressing its intrinsic geometrical character in the $g$'s in each observation. einstein's results thus we approach the absolute character of the surface through the relative nature of the observer's reference system. there is a relationship common to all values of the $g$'s that belong to the same curvature. this relationship is expressed by a differential equation. it is this equation of curvature that the airship's observer must find. einstein's problem was similar, but he was concerned with four dimensions, which entailed a general formula with ten $g$'s, and he had to find a set of differential equations of the second order to determine the law of fabric curvature. he divided the fabric into regions: i. world-frame--beyond influence of energy. ii. empty region--free of energy, but under its influence. iii. region containing free energy only. each region has a characteristic curvature. by means of an absolute differential calculus--a wonderful mathematical scaffolding erected by riemann, christoffel and others--involving the theory of tensors, he succeeded in finding such a set of equations. he kept the following points in view: ( ) the equations must not only give the character of region ii, but must satisfy the special case of region i; ( ) they must be independent of any partitioning system, because the general theory of relativity demands that a law of nature be in a form appropriate for all observers whatever their position and motion; ( ) they must be concerned with energy which is conserved, not mass which the special theory showed dependent on velocity. this set of differential equations which shows how the curvature of the fabric at any point links to the curvature at neighboring points is the law of gravitation, a law which has been severely tested by the practical observation of the solar eclipse already referred to. at a first approximation these equations degenerate into newton's law. at a second approximation they account for the motion of the perihelion of mercury, which had hitherto baffled astronomers. all the laws of mechanics are deducible from this law of world-fabric curvature, i.e. conservation of energy (which includes conservation of mass since we re-define mass as energy) and conservation of momentum (re-defined by a relativist). it must be noted that this law and the general theory show that the velocity of light is not absolutely constant, but, like everything else, a light-pulse is affected by the fabric curvature in a gravitational field. in conclusion we will contrast some conspicuous differences in the old world view of classical mechanics and the new view presented by einstein. . a three-dimensional ether medium with variously conceived properties which communicated the supposed inherent attractive force in matter in some unexplained way, and transmitted electromagnetic waves, has been replaced by a four-dimensional external world-fabric, the geometrical character of which controls the motion of matter (energy) and accounts for all mechanical laws. . after separating the observer's subjective share in definitions from nature's share in the things defined, space, time, and force, hitherto regarded as absolute, have been shown to be purely relative and dependent on the observer's track. mass has also proved to be relative to velocity unless re-defined as energy. as classical mechanics bases all definitions on space, time, and mass units, the relativity of such defined quantities is now apparent. . newton's laws of motion, his law of gravitation, and the laws of conservation, hitherto regarded as unrelated, are now synthesised in a basic law of mechanics. einstein has not disturbed the electric theory of matter, and both the old and new physics have in common the "principle of least action." we obtain a glimpse of this principle in the unique tracks pursued by freely moving bodies, which may be regarded as tracks of least effort, force only being manifested as an expression of the fabric's resentment when bodies depart from these natural tracks. einstein has approached nearer to the truth in regard to the laws underlying nature, and, as always, this means a simplification. his theory, which entails a readjustment of such fundamental conceptions as space and time, opens up fresh fields to scientific investigation and to philosophic thought. it reveals a bridge uniting the domains of physics and philosophy, and it heralds a new era in the history of science. xv the new world a universe in which geometry takes the place of physics, and curvature that of force by george frederick hemens, m.c., b.sc., london it is familiar knowledge that the line, the surface and ordinary euclidean space are to be regarded as spaces of one, two and three dimensions respectively and readers of this journal are aware that a hypothetical space of four dimensions has been closely investigated. the most convenient space to study is the surface or two-space, since we can regard it as embedded in a three-space. if a surface is curved it is generally impossible to draw a straight line on it, for as we see clearly, the "straightest" line is changing its direction at every point. to describe this property accurately it is necessary to ascribe to each point a magnitude which expresses what happens to the direction of a short line in the region when displaced a short distance parallel to itself. this is called the direction-defining magnitude. different sets of values of this magnitude relate to surfaces of different curvatures. a second fundamental property has recently been pointed out. there is inherent in every part of a space a measure of length peculiar to that particular region and which in general varies from region to region. to describe this variation accurately it is necessary to ascribe to each point another magnitude called the length-defining magnitude, which expresses the change from each point to the next of the unit of length. these two magnitudes define the surface completely. similarly, a space of any number of dimensions is defined completely by a similar pair of magnitudes. a space is the "field" of such a magnitude-pair and the nature of these magnitudes defines the dimensions of the space. the four-space usually described is the euclidean member of an infinity of four-spaces. when we look into a mirror we see a space differing from ordinary space in that right and left are interchanged and this is described mathematically by saying that if we locate points as usual by specifying three distances $x_ $, $x_ $, $x_ $ of the point from three mutually perpendicular planes, then a point $x_ $, $x_ $, $x_ $, in actual space corresponds with a point $x_ $, $x_ $, $ -x_ $ in the mirrored space: in other words the mirrored space is derived from the real space by multiplying the $x_ $ coordinates by $- $. if we were to multiply by $\sqrt{- }$ instead of $- $ we should derive a different space; in this case, however, we have no mirror to show us what it looks like. such a space is said to have one negative dimension and it has the peculiar property that in the figure derived from the right triangle of ordinary space the square of the "hypotenuse" equals the difference and not the sum of the squares of the other two sides, so that the length of a line may sometimes have to be represented by the square-root of a negative number, a "complex" number. in considering what at first sight may appear to be fantastic statements made by this theory, it must be borne in mind that all our knowledge of the external universe comes through our sense-impressions, and our most confident statements about external things are really of the nature of inferences from these sense-impressions and, being inferences, liable to be wrong. so that if the theory says that a stone lying on the ground is not a simple three-dimensional object, and that its substance is not the same as its substance a moment before, the matter is one for due consideration and not immediate disbelief. the idea that the universe extends in time as well as in space is not new, and fiction-writers have familiarized us with wonderful machines in which travellers journey in time and are present at various stages of the world's history. this conception of the universe, to which the name "space-time" is usually applied, is adopted by the new theory and assigned the status of a physical reality. the world geometry the fundamental creed of the new theory is that the space-time universe constitutes a true four-dimensional space of one negative dimension, this dimension being time. the variations from point to point of the direction-defining and length-defining magnitudes generate the geometrical properties of curvature, etc., and these are cognised by the human mind as physical phenomena: our sense-impressions are nothing more nor less than perceptions of the geometry of a fourspace. so instead of inferring from our sense-impressions the existence of matter, motion and the like as we are accustomed to do, we should with equal justice infer the existence of a geometrical fourspace. thus it becomes necessary to prepare a dictionary in which the familiar things of our world are identified with those geometrical properties of the four-space which really constitute them, and in so doing parts of our geometrical knowledge assume the guise of new physical knowledge. through the fourspace our consciousness travels, cognising a changing three-dimensional section of it as it goes and thus giving rise to time. it becomes aware that the fourspace is pleated or folded along lines all running roughly in the same direction, and possibly because this is the easiest direction to follow, it travels along the lines. the direction of this motion is the negative dimension. thus consciousness is always aware of the nearly constant forms of the cross-sections of the pleats along which it travels. these unvarying forms constitute matter: matter is the form of a section through a uniform pleat of the fourspace--a three-dimensional aspect of a four-dimensional curvature; so that in strict accuracy we should say that a stone is the shape or form of a changing section of a four-dimensional object, the complete object being a long fold in the fourspace. the physical interpretation of this conservation of form of the cross-section is that matter is conserved. it is thus seen that the conscious mind, by following these pleats, has so determined time that the law of the conservation of matter must hold. the mathematical treatment of the subject makes it clear that practically all other physical laws similarly follow as a direct result of this choice of time. the type of order prevailing in the physical universe, the laws of gravitation, heat, motion and the rest are not directly imposed by some external power, but are apparently chosen by mind itself. in the neighborhood of these pleats the fourspace is still curved, but to a smaller degree. this we cognise as energy or as a field of force. thus energy is seen to be the same kind of thing as matter and would therefore be expected to have weight. this was experimentally demonstrated in when light was in effect actually weighed. conversely, matter consists of energy; and it is calculated that one liter of water contains sufficient energy to develop a million horsepower for about four years. it is now believed that the sun's energy is derived from the disintegration of the matter of which it is made. the method of establishing these identifications will be clear from the following: we already knew that matter is made up of electrons and that radiant energy is electromagnetic and before the advent of this theory it was regarded as certain that practically all observed physical phenomena except gravitation were manifestations of the electromagnetic field. the new theory has confirmed this belief. it is found that the gravitational and electromagnetic conditions of the universe are completely defined if to each point of space-time a gravitational and an electric potential are ascribed. these are magnitudes of the same nature as the direction-defining and length-defining magnitudes which must necessarily be associated with every point of space-time if it is a true "space," and they are therefore identified with these. by performing ordinary mathematical operations on these magnitudes statements of fact clothed in mathematical form are obtained, which are to be interpreted on the one hand as physical laws and on the other as geometrical properties of the fourspace. nearly all our physical laws are derivable mathematically in this way, so that an extensive identification is effected which has been fruitful of results. it has been mentioned that a slight curvature is sometimes cognised as force and as this identification appeared originally as a postulate its history is interesting. the genesis of the theory an experiment by michelson and morley ( ), on which the whole theory is based, made it appear that if a man measures the velocity at which light passes him he will get the same result whether he is stationary, rushing to meet the light, or moving in the same direction as the light. the solution was provided by einstein in . he suggested that since we know the results of these determinations ought not to agree, something must have happened to the clocks and measuring-rods used in measuring the velocity so that the standards of length and time were not the same in the three cases, the alterations being exactly such as to make the velocity of light constant. this solution is universally accepted as true and is the fundamental postulate. thus the length of a stick and the rate at which time passes will change as the velocity of the person observing these things changes. if a man measured the length of an aeroplane going past him at , miles per second it would measure only half the length observed when stationary. if the aeroplane were going with the velocity of light, its length would vanish though its breadth and height would be unaltered. similarly, if of two twin brothers one were continually moving with reference to the other their ages would gradually diverge, for time would go at different rates for the two. if one moved with the velocity of light, time would stand still for him while for the other it would go on as usual. to get actually younger it would be necessary to move quicker than light which is believed to be impossible. the velocity of light is assumed to be the greatest velocity occurring in nature. evidently then if the distance in space and the interval in time separating two given events, such as the firing of a gun and the bursting of the shell, are measured by two observers in uniform relative motion, their estimates will not agree. consider now the simple problem of measuring the distance between two points on an ordinary drawing-board. if we draw two perpendicular axes, we can define this distance by specifying the lengths of the projections on the two axes of the line joining the points. if we choose two different axes the projections will not be the same but will define the same length. similarly, in a euclidean four-space the distance between two points will be defined by the projections on the four axes, but if these axes be rotated slightly, the projections will be different, but will define the same length. now, returning to the two observers just mentioned, it was noticed by minkowski in that if the space measurements between the two events are split into the usual three components, and if the time measurements are multiplied by $\sqrt{- }$, the difference between the two sets of measurements is exactly the same as would have occurred had these two events been points in a euclidean fourspace, and two different observations made of their distance apart using two sets of axes inclined to each other. the velocity of light is made equal to in this calculation by a suitable choice of units. this discovery threw a vivid light on the problem of space-time, showing that it is probably a true four-space of one negative dimension, a simple derivative of the much-discussed and now familiar euclidean four-space. although this discovery gave a tremendous impetus to the progress of the theory, it is probable that it holds a deeper significance not yet revealed. it is probably a statement of the "stuff" of which the four-space is made, and perhaps also of how it is made; but the problem remains unsolved. it thus becomes plain that our two observers are merely looking at the same thing from different viewpoints. each has just as much right as the other to regard himself as being at rest in ordinary space (this is the postulate of the relativity of uniform motion) and to regard his time direction as a straight line in the four-space. the difference is merely that the two time axes are inclined to each other. if, however, one were moving with an acceleration with reference to the other his path in the four-space will appear curved to the other, though he himself, since he regards it as his time axis, will still assume it to be straight. if there is a body moving in what one observer sees to be a straight line, the other will, of course, in general see it as curved, and following the usual custom, since this body, without apparent reason, deviates from the straight path, will say there must be some force acting on it. thus the curvature of his time axis, due to his accelerated motion, makes it appear that there is round him a field of force, which causes freely moving bodies to deviate from the straight path. now if space-time is itself inherently curved it is not generally possible for any line in it to be straight any more than it is possible for any line on the surface of a sphere to be straight. hence, all axes must be curved, and all observers, whatever their states of motion, must experience fields of force which are of the same nature as those due to motion only. the extra force experienced when a lift begins to rise is an example of force due to pure motion: gravitation is the similar force due to an inherent curvature of the four-space, and it was the postulate that these forces were similar that made possible einstein's solution of the general problem of gravitation. the time diagram the correlation of time with its geometrical analogue is of absorbing interest. representing velocity by the common method of plotting a curve showing positions at various times and marking distances horizontally and times vertically, the velocity of light being , $mm'$ and $nn'$ will both represent this velocity. since this is assumed to be the greatest velocity occurring in nature, all other possible velocities are represented by lines falling within the upper and lower v's. now this diagram correctly represents two dimensions of minkowski's euclidean four-space so, transmuting to real but flat four-space by multiplying times by $\sqrt{- }$, it is seen that there is a region outside which no effect can be propagated from $o$ since that would involve the existence of a velocity greater than that of light. this region represents the future of $o$. similarly, $o$ can only be affected by events within the region derived from the downward-opening v, which therefore represents the past of $o$. the region between the two represents events which may be either simultaneous with $o$ or not, according to the velocity of the observer at $o$. thus in this theory an event dictated by free-will, could affect points in its "future" region, but not in any other, which agrees with experience and shows that the theory is not essentially "determinist." if "free-will" is really free, the future is not yet determined, and the fourspace must be in some way formed by the will as time progresses. the trains of thought inspired by einstein's postulates have already carried us to a pinnacle of knowledge unprecedented in the history of man. on every hand, as we look out upon the universe from our new and lofty standpoint, unexpected and enthralling vistas open up before us, and we find ourselves confronting nature with an insight such as no man has ever before dared aspire to. it is completely unthinkable that this theory can ever be swept aside. apart from experimental verifications which, in point of fact, lend it the strongest support, no one could work through the theory without feeling that here, in truth, the inner workings of the universe were laid bare before him. the harmony with nature is far too complete for any doubt to arise of its truth. xvi the quest of the absolute modern developments in theoretical physics, and the climax supplied by einstein by dr. francis d. murnaghan, johns hopkins university baltimore we shall discuss the more important aspects of the theory popularly known as the "einstein theory of gravitation" and shall try to show clearly that this theory is a natural outcome of ideas long held by physicists in general. these ideas are: (a) the impossibility of "action at a distance;" in other words we find an instinctive repugnance to admit that one body can affect another, remote from it, instantaneously and without the existence of an intervening medium. (b) the independence of natural, i.e., physical, laws of their mathematical mode of expression. thus, when an equation is written down as the expression of a physical law it must be satisfied, no matter what units we choose in order to measure the quantities occurring in the equation. as our physics teacher used to say "the expression of the law must have in every term the same dimensions." more than this the choice of the quantities used to express the law--if there be a choice open--must have no effect on its correctness. as we were told--"all physical laws are capable of expression as relations between vectors or else as relations between magnitudes of the same dimensions." we shall hope to make this clearer in its proper place in the essay, as its obvious generalization is einstein's cardinal principle of relativity. the measurements which an experimental physicist makes are always the expression of a coincidence of two points in space at the same time. if we ask such an experimenter what he means by a point in space he tells us that, for him, the term has no meaning until he has a material body with reference to which he can locate the point by measurements; in general it requires three measurements and he expresses this by saying that space has three dimensions. he measures his distance, as a rule, parallel to three mutually perpendicular lines fixed in the material body--a cartesian reference-frame so-called. so that a "point in space" is equivalent to a given material reference-frame and three numbers or coordinates. if, for any reason, we prefer to use a new material reference-frame the coordinates or measurements will change and, if we know the relative positions of the two material reference-frames, there is a definite relation between the two sets of three coordinates which is termed a transformation of coordinates. but which particular material reference-frame shall we use? the first choice would, we think, be that attached to the earth. but, even yet, we are in doubt as there are numberless cartesian frameworks attached to the earth (as to any material body) and it is here that our idea (b) begins to function. we say it must be immaterial which of these cartesian frames we use. in each frame a vector has three components and when we change from one frame to another the components change in such a way that if two vectors have their three components equal in one framework they will be equal in any other attached to the same material system. so our idea (b), which says that our physical equations must be vector equations, is equivalent to saying that the choice of the framework attached to any given material body can have no effect on the mode of expression of a natural law. shall we carry over our idea (b) to answer the next question: "to which material body shall we attach our framework?" to this question newton gave one answer and einstein another. we shall first consider newton's position and then we may hope to see clearly where the new theory diverges from the classical or newtonian mechanics. newton's answer was that there is a particular material frame with reference to which the laws of mechanics have a remarkably simple form commonly known as "newton's laws of motion" and so it is preferable to use this framework which is called an absolute frame. what is the essential peculiarity of an absolute frame? newton was essentially an empiricist of bacon's school and he observed the following facts. let us suppose we have a framework of reference attached to the earth. then a small particle of matter under the gravitational influence of surrounding bodies, including the earth, takes on a certain acceleration $a_ $. now suppose the surrounding bodies removed (since we cannot remove the earth we shall have to view the experiment as an abstraction), and another set introduced; the particle, being again at its original position, will begin to move with an acceleration $a_ $. if both sets of surrounding bodies are present simultaneously the particle begins to move with an acceleration which is approximately but not quite the sum of $a_ $ and $a_ $. newton postulated there there is a certain absolute reference frame in which the approximation would be an equality; and so the acceleration, relative to the material frame, furnishes a convenient measure of the effect of the surrounding bodies--which effect we call their gravitational force. notice that if the effect of the surrounding bodies is small the acceleration is small and so we obtain as a limiting case, newton's law of inertia which says that a body subject to no forces has no acceleration; a law which, as poincaré justly observed, can never be subjected to experimental justification. the natural questions then arise: which is the absolute and privileged reference-frame and how must the simple laws be modified when we use a frame more convenient for us--one attached to the earth let us say? the absolute frame is one attached to the fixed stars; and to the absolute or real force defined as above, we must add certain terms, usually called centrifugal forces. these are referred to as fictitious forces because, as it is explained, they are due to the motion of the reference-frame with respect to the absolute frame and in no way depend on the distribution of the surrounding bodies. gravitational force and centrifugal forces have in common the remarkable property that they depend in no way on the material of the attracted body nor on its chemical state; they act on all matter and are in this way different from other forces met with in nature, such as magnetic or electric forces. further newton found that he could predict the facts of observation accurately on the hypothesis that two small particles of matter attracted each other, in the direction of the line joining them, with a force varying inversely as the square of the distance between them. this law is an "action at a distance" law and so is opposed to the idea (a). we have tacitly supposed that the space in which we make our measurements is that made familiar to us by the study of euclid's elements. the characteristic property of this space is that stated by the theorem of pythagoras that the distance between two points is found by extracting the square root of the sum of the squares of the differences of the cartesian coordinates of the two points. mathematicians have long recognized the possibility of other types of space and einstein has followed their lead. he abandons the empiricist method and when asked what he means by a point in space replies that to him a point in space is equivalent to four numbers how obtained it is unnecessary to know a priori; in certain special cases they may be the three cartesian coordinates of the experimenter (measured with reference to a definite material framework) together with the time. accordingly he says his space is of four dimensions. between any two "points" we may insert a sequence of sets of four numbers, varying continuously from the first set to the second, thus forming what we call a curve joining the two points. now we define the "length" of this curve in a manner which involves all the points on it and stipulate that this length has a physical reality, i.e., according to our idea (b) its value is independent of the particular choice of coordinates we make in describing the space. among all the joining curves there will be one with the property of having the smallest length; this is called a geodesic and corresponds to the straight line in euclidean space. we must now, for lack of an a priori description of the actual significance of our coordinates, extend the idea of vector so that we may speak of the components of a vector no matter what our coordinates may actually signify. in this way are introduced what are known as tensors; if two tensors are equal, i.e., have all their components equal, in any one set of coordinates they are equal in any other and the fundamental demand of the new physics is that all physical equations which are not merely the expression of equality of magnitudes must state the equality of tensors. in this way no one system of coordinates is privileged above any other and the laws of physics are expressed in a form independent of the actual coordinates chosen; they are written, as we may say, in an absolute form. the gravitational hypothesis einstein flatly denies newton's hypothesis that there is an absolute system (and, indeed, many others before him had found it difficult to admit that so insignificant a part of the universe as our fixed star system should have such a privileged position as that accorded to it in the newtonian mechanics). in any system, he says, we have no reason to distinguish between the so-called real gravitational force and the so-called fictitious centrifugal forces--if we wish so to express it gravitational force is fictitious force. [ ] a particle moving in the neighborhood of material bodies moves according to a law of inertia--a physical law expressible, therefore, in a manner quite independent of the choice of coordinates. the law of inertia is that a particle left to itself moves along the geodesics or shortest lines in the space. if the particle is remote from other bodies the space has the euclidean character and we have newton's law of inertia; otherwise the particle is in a space of a non-euclidean character (the space being always the four-dimensional space) and the path of the particle is along a geodesic in that space. einstein, in order to make the theory more concrete, makes a certain stipulation as to the nature of the gravitational space which stipulation is expressed, as are all physical laws, by means of a tensor equation--and this is sometimes called his law of gravitation. perhaps it will be well, in exemplification, to explain why light rays, which pass close to the sun, should be bent according to the new theory. it is assumed that light rays travel along certain geodesics known as minimal geodesics. the sun has an intense gravitational field near it--or, as we now say, the departure of the four-dimensional space from the euclidean is very marked for points near the sun--but for points so remote as the earth this departure is so small as to be negligible. hence the form of the geodesics near the sun is different from that near the earth. if the space surrounding the sun were euclidean the actual paths of the light rays would appear different from geodesics or straight-lines. hence einstein speaks of the curvature of the light rays due to the gravitational field of the sun; but we must not be misled by a phrase. light always travels along geodesics (or straight lines--the only definition we have of a straight line is that it is a geodesic); but, owing to the "distortion" of the space they traverse, due to the sun, these geodesics reach us with a direction different from that they would have if they did not pass through the markedly non-euclidean space near the sun. the consideration of the fundamental four-dimensional space as being non-euclidean where matter is present gives a possibility of an answer to the world old question: is space finite or infinite? is time eternal or finite? the fascinating possibility arises that the space may be like the two-dimensional surface of a sphere which to a limited experience seems infinite in extent and flat or euclidean in character. a new columbus now asks us to consider other possibilities in which we should have a finite universe--finite not only as to space measurement but as to time (for the space may be such that all of the four coordinates of its points are bounded in magnitude). however, although einstein speaks of the possibility of a finite universe, we do not, personally, think his argument convincing. points on a sphere may be located by the cartesian coordinates of their stereographic projections on the equatorial plane and these coordinates, which might well be those actually measured, are not bounded. the special relativity theory in our account of the einstein theory we have not followed its historical order of development for two reasons. firstly, the earlier special relativity theory properly belongs to a school of thought diametrically opposed to that furnishing the "general theory of relativity" and, secondly, the latter cannot be obtained from the former by the process of generalization as commonly understood. einstein, when proposing the earlier theory, adopted the position of the empiricist so that to him the phrase, a point in space, had no meaning without a material framework of reference in which to measure space distances. when he came to investigate what is meant by time and when he asked the question "what is meant by the statement that two remote events are simultaneous?" it became evident that some mode of communication between the two places is necessary; the mode adopted was that by means of light-signals. the fundamental hypothesis was then made that the velocity of such signals is independent of the velocity of their source (some hypothesis is necessary if we wish to compare the time associated with events, when one material reference-system is used, and the corresponding time when another in motion relative to the first is adopted). it develops that time and space measurements are inextricably interwoven; there is no such thing as the length of a body or the duration of an event but rather these are relative to the reference-system. [ ] minkowski introduced the idea of the space of events--of four dimensions--but this space was supposed euclidean like the three-dimensional space of his predecessors. to einstein belongs the credit of taking from this representation a purely formal mathematical character and of insisting that the "real" space--whose distances have a physical significance--is the four-dimensional space. but we cannot insist too strongly on the fact that in the gravitational space of the general theory there is no postulate of the constancy of velocity of a light-signal and accordingly no method of assigning a time to events corresponding to that adopted in the special theory. in this latter theory attention was confined to material systems moving with uniform velocity with respect to each other and it developed that the velocity of light was the ultimate velocity faster than which no system could move--a result surprising and a priori rather repugnant. it is merely a consequence of our mode of comparing times of events; if some other method--thought transference, let us say--were possible the velocity of this would be the "limiting velocity." in conclusion we should remark that the postulated equivalence of "gravitational" and "centrifugal" forces demands that anything possessed of inertia will be acted upon by a gravitational field and this leads to a possible identification of matter and energy. further our guiding idea (a) will prompt us to say, following the example of faraday in his electrical researches, that the geodesics of a gravitational space have a physical existence as distinct from a mere mathematical one. the four-dimensional space we may call the ether, and so restore this bearer of physical forces to the position it lost when, as a three-dimensional idea in the special relativity theory, it had to bear an identical relation to a multitude of relatively moving material systems. the reason for our seemingly paradoxical title for an essay on relativity will be clear when it is remembered that in the new theory we consider those space-time properties which are absolute or devoid of reference to any particular material reference-frame. nevertheless, although the general characteristics of the theory are thus described, without reference to experiment, when the theory is to be tested it is necessary to state what the four coordinates discussed actually are--how they are determined by measurement. it is our opinion that much remains to be done to place this portion of the subject on a satisfactory basis. for example, in the derivation of the nature of the gravitational space, surrounding a single attracting body, most of the accounts use cartesian coordinates as if the space were euclidean and step from these to polar coordinates by the formulæ familiar in euclidean geometry. but these details are, perhaps, like matters of elegance, if we shall be allowed to give einstein's quotation from boltzmann, to be left to the "tailor and the cobbler." xvii the physical side of relativity the immediate contacts between einstein's theories and current physics and astronomy by professor william h. pickering harvard college observatory, mandeville, jamaica the theory of relativity will be treated first from the physical side, leaving the three astronomical tests to which it has been put to be discussed later. there is one astronomical fact however that must be mentioned in this connection, and this is the discovery of the aberration of light by bradley in . it is found that every star in the heavens apparently describes a small annual ellipse, whose major axis is '' in length. this bradley showed to be due to a combination of the velocity of the earth in its orbit, and the velocity of light; and it is so explained in all the elementary text-books on astronomy. it implies a stationary ether through which the earth is moving. the importance of this statement will appear presently. the subject is usually illustrated by supposing a man to go out in a rainstorm carrying a vertical tube. if the rain is falling vertically, and the man stands still, the sides of the tube will not be wet, save by an occasional drop, but if the tube is moved, it must then be inclined forward in order to keep it dry. the angle of inclination, which corresponds to aberration, will depend on the relative velocity of the tube, corresponding to the earth, and the rain drops which correspond to the waves of light. if three lines are dropped upon a point in space, each line being perpendicular to the plane containing the other two, we have what is known as a system of coordinates. einstein's original theory of relativity, which he now designates as the "special theory," depends on two principles. the first is that "every law of nature which holds good with respect to a coordinate system $k$ must also hold good for any other system $k'$, provided that $k$ and $k'$ are in uniform movement of translation." the second principle is that "light in a vacuum has a definite and constant velocity, independent of the velocity of its source." these two sentences may be considered as authoritative, being quoted in einstein's own words. the first of these principles need not greatly surprise us. the second is not well expressed, because it is ambiguous. he does not say how the first "velocity" is measured, whether relatively to the ether or relatively to the observer. in fact this is the very gist of the whole matter, as we shall presently see. in the case of sound the velocity is constant with regard to the medium, the air, in the case of light it is supposed to be constant with regard to the observer. it reaches him with a constant velocity, no matter how he moves. in order to understand this statement clearly let us consider the appended tabular diagram. on a calm day imagine a source of sound at $s$ in line a. this may be either a gun or a bell. imagine an observer , feet distant, located at $o$. the velocity of sound in air is , feet per second. this velocity we will take as unity, as indicated in the third column, and the velocity with which the sound reaches the observer is also , as shown in the fourth. it will reach him in a unit interval of second, as shown in the fifth. if the bell is struck, it will give its normal pitch or frequency, which we will also call unity, in the sixth column. now imagine case b where the observer is on a train advancing toward $s$. when he is , feet distant, the gun is fired, but as he is advancing toward it, he hears it at $o$ in rather less than a second, as shown in the fifth column. the velocity of the sound with regard to him is rather more than unity, as shown in the fourth column. if the bell is sounded, the pitch, that is the frequency, is raised, because he receives more sound waves per second than before. in case c the observer is stationary, but the source of sound is receding. at a distance of , feet the gun is fired, and the observer hears it after an interval of just one second, as in case a. the velocities with regard to the observer and through the medium are also unity. if the bell is struck the pitch is lowered, since he receives fewer sound waves per second, the reverse of case b. velocity source in to interval frequency observer medium observer air a s o b s + - + o c s - o d s + - o ether a s o b s - + o c s - o d s - o in case d imagine the source and the observer , feet apart, and advancing on the same train. when the gun is fired, the velocity of the sound waves will be greater with regard to the observer, and he will hear the sound in less than a second, as in case b. when the bell is struck it will have the normal pitch, the same as in case a. we find therefore that for sound the velocity with regard to the medium is always unity, while the velocity with regard to the observer, and the interval elapsed, depend only on the motion of the observer himself, and are independent of the motion of the source. the frequency of the vibrations, on the other hand, depends only on the relative motion of the observer and the source, but is independent of their common motion in any direction. further, it makes no difference whether the source and the observer are moving on a train, or whether they are stationary, and a uniform wind is blowing past them. in the case of light waves we shall find a very different state of affairs, although the rules for frequency are the same as they are for sound. in case a we have the normal conditions, where both the source and observers are stationary. in case b we have a representation of the michelson-morley experiment as supplemented by that of majorana, where the source is stationary and the observer advances. unlike the case of sound, the interval elapsed, as shown by the experiment, is now the same as in case a, and since the distance to the observer is less, the velocity of light with respect to the ether must also be less than unity. since the observer is advancing against the light, this will permit the velocity of light with regard to the observer to remain unity, in conformity with the second principle of relativity. compare with case b for sound. as jeans expresses it, "the velocity of light in all directions is the same, whatever the motion of the observer." that is to say it appears to be the same to him, however he moves. case c represents einstein's statement, as confirmed by majorana's experiment. it does not differ from case c for sound. case d is more complex, but accepting the statement above that the velocity is constant with regard to the observer, we see that the velocity through the medium must be less, and that the interval elapsed will be constant, as in case b. could we use the brighter stars and planets as sources of light, several of these cases could be further tested. this brings us at once to statements that contradict our common sense. for instance, jeans says "no matter what the velocity of the observer is, the light surface, as observed by that observer, is invariably a sphere having that observer as center." that is to say the light surface, or wave front, is a contracting, not an expanding, sphere. this, if confirmed, would go a long way toward making our universe a subjective rather than an objective phenomenon. again imagine a flash of light, such as an explosion, to occur when an observer is in a given position. it makes no difference how the observer may move while the light is approaching him, whether several miles forward or backward, the light will reach him in exactly the same time, as is shown by michelson's experiment. or if two observers are at the same spot when the explosion occurs, and one moves forward, and the other backward, they will both see the explosion at exactly the same instant. this sounds ridiculous, but not only is it what jeans says, but it is the logical interpretation of einstein's second principle, if einstein means by velocity, velocity with regard to the observer. if he means velocity with regard to the medium, then the case is exactly the same as that of sound in air, and michelson's experiment as well as the maxwell-lorentz theory of light are contradicted. this theory is now universally accepted, and michelson's experiment has been carefully repeated by other observers, and fully confirmed. this is the very heart of the relativity question. if we state the matter objectively it comes to this. the velocity of light with regard to the ether is a variable quantity, depending merely on where the observer chooses to go. as eddington well says, "these relations to the ether have no effect on the phenomena and can be disregarded--a step which appears to divest the ether of the last remnants of substantiality." the only way of avoiding this apparent absurdity seems to be to consider that the ether moves with the earth. michelson's result would then be fully explained. of course this can only be true for a few miles above the earth's surface. beyond that the ether must either be stationary or move with the sun. the velocity of light with regard to the ether would then be a constant, just as the velocity of sound is constant with regard to the air. this would contradict einstein's second principle as it is generally understood. the trouble with this suggestion is that it fails to account for aberration, which, as already explained, appears to require that the earth should be moving through the ether. to meet this emergency would involve some modification of the undulatory theory of light, which apparently would not be impossible, but has not yet been made. in einstein brought out an extension of his first principle. this he calls the "general theory of relativity." it states that in our choice of coordinate systems we "should not be limited in any way so far as their state of motion is concerned." this leads to the three astronomical consequences mentioned later in this paper, two of which have been more or less confirmed, and the third practically contradicted as far as quantitative measures are concerned. as is well known the kinetic energy of a moving body may be expressed as $e = / mv^ $, but if the body is charged electrically, the fraction becomes $ / (m + m')v^ $, where $m'$ is a quantity dependent on the square of the electrical charge. that is to say, we have the normal mass of the body, and also what we may call its electrical mass. if when in this condition a portion of the mass is electrical, the question at once occurs to us, why may not the whole mass be electrical, in other words, a form of energy? although this has not been satisfactorily proved hitherto, yet such is the general belief among physicists. as einstein puts it "inert mass is nothing else than latent energy." the same idea is sometimes expressed as "the mass of ordinary matter is due to the electromagnetic energy of its ultimate particles, and electromagnetic energy wherever found must possess mass, i.e., inertia." if that is so, since a ray of light on the undulatory theory is a form of electromagnetic energy, it too must possess mass. since all mass with which we are familiar is subject to the attraction of gravitation, it seemed likely that a ray of light would be bent out of its course in passing near the sun, and this as we have seen was proved to be true at the recent solar eclipse. that portion of the mass of a body due to its electrical charge can be readily shown experimentally to vary with the velocity of the body. einstein has shown the same to be true of the normal mass, as is illustrated in the advance of the perihelion of the orbit of mercury. he has also pointed out that gravitation, inertia and centrifugal force are all closely related, and obey similar laws. thus if we rise from the earth with accelerated velocity, we apparently increase our weight. again if the velocity of rotation of the earth on its axis should be increased, our weight would be diminished. these facts are suggestive when we come to consider the ultimate cause of gravitation. another fact which must be rather startling to the older school of scientists is that momentum is no longer simply $mv$, mass times velocity, but that the velocity of light $c$, comes into the question, and the formula for momentum now assumes the form of $$\frac{m v}{\sqrt{ - \frac{v^ }{c^ }}}$$ for ordinary velocities this correction is extremely small, but it has been shown to be necessary, both theoretically and experimentally, when dealing with the high velocities with which we are now familiar. the theory of relativity is so widespread in its application that several other theories have become more or less intimately combined with it, for which einstein is in no way responsible. one of these is known as the fitzgerald-lorentz theory, that all bodies are subject to a contraction in the direction of their motions through space. this was first suggested in order to explain the michelson-morley experiment, but has proved inadequate to do so, particularly when the observer is receding from the source. this contraction is expressed by the same factor used in the denominator of the revised expression for momentum, given above. again the quantity $c$ is so enormous, that even for large bodies at planetary velocities the contraction amounts to very little. thus the earth moving at a speed of eighteen miles per second in its orbit, is flattened only / , , , or . inches. on the other hand for high velocities of many thousand miles per second, such as we have become familiar with in the case of the radioactive substances, the flattening is a very considerable fraction of the diameter of the moving body, one-half or more, and in the case of the corpuscles of light, if that theory were adopted, this flattening becomes equal to the diameter, and their thickness is reduced to zero. when we view einstein's theories from the astronomical standpoint, the earliest fact bearing on relativity that we need consider was the discovery of aberration, by bradley, in , as seen above. in airy observed the star g draconis through a telescope filled with water. since the velocity of light is less in water than in air, we should naturally expect to find the aberration appreciably increased. it was found, on the other hand, however, to be unaffected. in the results of the famous michelson-morley experiment were published. in this experiment the velocity of light was measured in various directions with regard to the motion of the earth in its orbit. if the ether were stationary, and the earth moving through it, different velocities should be obtained in different directions. such was not the case however, and the experiment indicated that the ether moved with the earth. it thus flatly contradicted the conclusions founded on aberration. einstein's special theory of relativity, of , as we have seen, resolves this contradiction. but as we shall presently see, it is the general theory, of , that leads to astronomical applications of broad scope. it indicates, for instance, that there is no essential difference between gravitation and inertia. this idea may be crudely illustrated by our feelings of increased weight when an elevator starts rapidly upwards. a man while falling freely in space ceases to feel the pull of gravitation. but we must not as yet conceive of the theory of relativity as a universally accepted and unquestioned truth of science. eddington is its leading english exponent, and he is supported by such men as jeans, larmor, and jeffreys. on the other hand, the theory has been severely criticised by lodge, fowler, silberstein, and sampson. few american scientists have expressed any opinions in print on the subject, and the recent eclipse observations, to which we shall refer later, are to be repeated with more suitable instruments for verification in , in the hope of obtaining more accurate and accordant results. an appurtenance of the einstein theories which bears much the same relation to them as does the lorentz-fitzgerald contraction, mentioned above, is the idea, first clearly stated by minkowski, that time is a kind of space--a fourth dimension. this the reader will doubtless find to be the most difficult portion of the theory to picture in his own mind. it is entirely unsupported by experiment or observation, necessarily so, and is based wholly on mathematical and philosophical conceptions. our distinction between space and time seems to be that the direction in which we progress without effort is time; the other directions, in which we have to make an exertion to move ourselves, or in which we are carried, are space. how many dimensions empty space may have, we really have no means of knowing, because we can neither see nor feel it. matter we know has three, length, breadth, and thickness, also that it lies remote from us in three corresponding directions. these facts may have given us the erroneous impression that space too has only three dimensions. now it is claimed that time is a fourth, and that there are also others. in order to illustrate this, eddington asks us to imagine a movie film taken of a man or of any moving object. let the separate pictures be cut apart and piled on one another. this would form a sort of pictorial history of the individual for a brief interval in his life, in the form of a cube. if we attempt to pick it up, it falls apart, thus clearly showing the difference between time and space. but suppose it now all glued together in one solid cube, so that it is no easier to cut a section in one direction than in another. that is minkowski's idea of space and time, and further, that the direction in which we should cut it depends merely on the velocity with which we are moving through space. i should cut it parallel to the films, but a man on a rapidly moving star, in order to separate it into space and time, would cut it in an inclined direction. that is a thing which may be true, but it is one which we believe no mortal man can clearly picture to himself. on the other hand turner has recently made a very interesting point, namely, that the fourth dimension as actually treated by the mathematicians is not time itself, but time multiplied by a constant--the velocity of light. without affecting the astronomical proofs of relativity at all, this simplifies our conceptions enormously. in ordinary everyday life time and space cannot be identical, any more than a yard can be identical with a quart. on what is known to physicists as the centimeter-gram-second system, distance is represented by $l$, mass by $m$, and time by $t$. velocity is then distance divided by time, $l/t$, or as we say in english units, so many feet per second, and the fourth dimension may be expressed as time multiplied by velocity, $t \times l/t = l$. that is to say, it is simply distance, just like the other three dimensions. to say that time is the fourth dimension from this point of view, appears to us just as ridiculous as it would be to attempt to measure the velocity of a train in quarts. it is quite correct, however, although unusual, to speak of a given train as moving at a speed of quarts per square inch per second, $l^ /l^ t = l/t$. this would be equivalent to a velocity of miles per hour. if i wish to give a complete dimensional description of myself in my four dimensions, i must give my length, my breadth, and my thickness, ever since i came into being, and also the course i have traversed through space since that time. this latter distance will be expressed in terms of a unit whose length is , miles, the distance traversed by light in one second. the distance which i travel through space annually is enormous, and very complex as to direction. it involves not merely my own motions as i cross the room, or take a train or steamer, but also those due to the rotation of the earth on its axis, its revolution round the sun, and the motion of the latter through the heavens. in general i travel, or in other words increase my length in the fourth dimension, by over , units a year. the fourth dimension accordingly, if this view is accepted, is simply a distance like the other three, and perfectly easy to understand. we now come to the three actual tests by which the theory has been tried. the planets as is well known revolve about the sun in ellipses, with the sun in one of the foci. that is to say, the sun is not in the center, but a little on one side of it. the end of the ellipse where the planet comes nearest to the sun is called the perihelion, and here the planet is moving most rapidly. the other end is called the aphelion, and here the motion is slowest. according to newton's theory of gravitation, if a spherical sun possesses a single planet or companion, its orbit will be permanently fixed in space unless perturbed by some other body. if a second planet exist, it will cause the perihelion of the first slowly to advance. according to einstein the mass of a planet depends in part on its velocity. it will therefore be less at aphelion where it is moving slowly than at perihelion where it is moving rapidly, consequently in addition to the newtonian attraction we have another one which increases as we approach the sun. the effect of this will be to cause the perihelion of the orbit to advance, whether there is a second planet or not. among the larger planets mercury has the most eccentric orbit, and it also moves most rapidly, so that it is particularly well adapted to test the relativity theory. the observed advance of its perihelion is '' per century, instead of the theoretical figure '', due to the other planets--a difference of ''. this has long been a puzzling discrepancy between observation and the law of gravitation. prior to einstein, attempts were made to eliminate it by assuming a certain oblateness of the solar disk. if the equatorial diameter exceeded the polar by only ''. the whole advance would be accounted for, but not only has this ellipticity failed of detection, but if it existed, it should produce a very noticeable and inadmissible change in the inclination of mercury's orbit, amounting to about '' per century, as has been demonstrated by both herzer and newcomb. einstein from computations alone, without introducing any new constants or hypotheses whatever, showed, if the theory of relativity be accepted, that the sun should produce an acceleration of '' per century, thus entirely accounting for the observed discrepancy, far within the limits of accuracy of the observations. the only other planet whose orbit has a large eccentricity, and that is suitable for investigation, is the planet mars. here the discrepancy between observation and theory is very slight, only '', and a portion of that may be due to the attraction of the asteroids. this deviation is so slight that it may well be due entirely to accidental errors of observation, but however that may be, einstein's theory reduces it to ''. . this all seems very satisfactory and complete, but the trouble with it is that the coincidence for mercury is rather too good. it is based on the assumption that the sun is a perfect sphere, and that the density of its surface is uniform from the equator to the poles. this would doubtless be true if the sun did not revolve on its axis. in point of fact it does revolve, in a period in general of about days. consequently an object on its equator must experience a certain amount of centrifugal force. therefore if its surface were of uniform density the shape of the sun would be an oblate spheroid. it can be readily shown that the theoretical excess of the equatorial over the polar diameter, due to the centrifugal force, should amount to only ''. , an amount which could hardly be detected by observation, and might readily be concealed by a slight excess of equatorial over polar density. any reasonable excess of density at the center would diminish this result but slightly. the molecular weight of the central material is probably about . this computed equatorial excess is one-twelfth of the amount necessary to cause the observed advance, and should therefore cause an advance of the perihelion of about ''. per century, reducing the difference between the observed advance and that caused by gravitation to ''. . according to einstein the advance due to relativity should be, as we saw, '', a discrepancy of ''. per century, or per cent. jeffreys has remarked that any discrepancy such as '' "would be fatal to a theory such as einstein's, which contains no arbitrary constituent capable of adjustment to suit empirical facts." it must be pointed out here however, that so far as known, this small correction to the motion of mercury's perihelion has not previously been suggested, so that there has been no opportunity hitherto for its criticism by others. it was due largely to the success with mercury that it was decided to put the relativity theory to another test. according to the newtonian theory, as stated by newton himself, corpuscles as well as planets have mass, and must therefore be attracted by the sun. according to einstein, owing to their high velocity, this attraction must be twice as great as it would be according to the theory of gravitation. if the ray of light proceeding from a star were to pass nearly tangent to the sun's limb it should be deflected ''. according to newton. according to the theory of relativity it should be deflected ''. . stars of course cannot usually be observed near the sun. it is therefore necessary to take advantage of a total solar eclipse, when the sun is completely hidden by the moon, in order to secure these observations. two expeditions, one to africa, and one to south america, observed successfully the total eclipse of may , . the former was located on the island of principe in the gulf of guinea. the latter was located at sobral, brazil. their equipment and results are shown in the following table, where the successive columns give the location, the aperture in inches of the telescopes employed, their focus in feet, the number of plates secured, the number of stars measured, their mean deduced deflection from their true positions by the attraction of the sun, and the deviations from the theoretical results. in the first and last line of the table shown herewith, this location aperture focus plates stars defl. dev. principe ''. - ''. sobral . (+ . )   ,,   . + . deviation is taken from einstein's computed value of ''. . in the second line the difference shown is from the value required by the newtonian theory, ''. . the results obtained with this telescope were rejected however, although they were much the most numerous, because it was found that for some reason, supposed to be the heating of the mirror by the sun before the eclipse, the star images were slightly out of focus, and were therefore considered unreliable. the results with the two other telescopes were not very accordant, but the -inch had the longer focus, secured the greater number of plates, and showed the greater number of stars. the results obtained with it therefore appear to have been the more reliable. they differ from einstein's prediction by per cent. in future expeditions to test this question, the mirror in front of the telescope will be eliminated. we now come to the final test which has been applied to einstein's theory. einstein showed that in the intense gravitational field of the sun, the theory of relativity required that all of the spectrum lines should be shifted slightly toward the red end. the shift however is exceedingly small, and can only be detected and measured with the most powerful modern instruments. moreover only certain lines can be used, because owing to varying pressure in the solar atmosphere, which affects many lines, as well as to rapid motion in the line of sight, which may affect all of them, still larger displacements are liable to occur. according to the theory of relativity the displacement of the lines should be $+ . a$. st. john at mt. wilson found a displacement for the cyanogen lines of only $+ . a$. evershed at kodaikanal found + . at the north pole of the sun, and + . at the south pole. these latter values however were only for the stronger lines. the weaker lines give much smaller shifts, as do those of calcium and magnesium. according to einstein all lines should give nearly the same shift, an amount proportional to the wave length. it therefore appears that we must conclude by saying that einstein's theory of relativity has been partially, but not completely, verified. the reference numbers in the above text have nothing to do with the numbers used in other parts of this volume to acknowledge the work of the various contestants; they refer to dr. pickering's sources, as follows: journ. brit. astron. assoc., , , . comptes rendus, , , and , . monthly notices r. a. s., , , . monthly notices r. a. s., , , . astro-physical journal, , , . journ. brit. astron. assoc., , , . monthly notices, r. a. s., , , . amer. journ. sci., , . monthly notices, r. a. s., , , . the observatory , april. from an oxford note book. monthly notices, r. a. s., . , de sitter, , , , jeans, , jeffreys. "gravitation and the principle of relativity," eddington. royal institution of great britain, . journ. brit. astron. assoc., , , . "the interior of a star," eddington. scientia, , , . monthly notices, r. a. s., , , . monthly notices. r. a. s., , , . journ. brit. astron. assoc., , , . astro-physical journ., , , . journ. brit. astron. assoc., , , . xviii the practical significance of relativity the best discussion of the special theory among all the competing essays by professor henry norris russell, princeton university can a small child catch a baseball moving sixty miles an hour without getting hurt? we should probably answer "no"--but suppose that the boy and his father were sitting side by side in an express train, and the ball was tossed lightly from one to the other. then there would be no trouble about it, whether the train was standing still, or going at full speed. only the relative motion of ball and boy would count. this every-day experience is a good illustration of the much discussed principle of relativity, in its simplest form. if there were no jolting, the motion of the train, straight ahead at a uniform speed, would have no effect at all upon the relative motions of objects inside it, nor on the forces required to produce or change these motions. indeed, the motion of the earth in its orbit, which is free from all jar, but a thousand times faster, does not influence even the most delicate apparatus. we are quite unconscious of it, and would not know that the earth was moving, if we could not see other bodies outside it. this sort of relativity has been recognized for more than two centuries and lies at the bottom of all our ordinary dynamical reasoning, upon which both science and engineering are based. but there are other things in nature besides moving bodies,--above all, light, which is intimately related to electricity and magnetism, and can travel through empty space, between the stars. it moves at the enormous speed of , miles per second, and behaves exactly like a series of vibrations or "waves." we naturally think of it as travelling through some medium, and call this thing, which carries the light, the "ether." can we tell whether we are moving through this ether, even though all parts of our apparatus move together, and at the same rate? suppose that we have two mirrors, $m$ and $n$, at equal distances, $d$, from a point $o$, but in directions at right angles to one another, and send out a flash of light from $o$. if everything is at rest, the reflected flashes will evidently come back to $o$ at the same instant, and the elapsed time will be $ d/c$ seconds if $c$ is the velocity of light. but suppose that $o$, $m$, and $n$ are fastened to a rigid frame work, and all moving in the direction $o m$, with velocity $v$. the light which goes from $o$ toward $m$, at the speed $c$, will overtake it with the difference of their speeds, $c-v$, taking $d/(c-v)$ seconds to reach $m$. on the way back, $o$ will be advancing to meet it, and the return trip will occupy $d/(c+v)$ seconds. the elapsed time for the round trip comes out $ cd/(c^ -v^ )$ seconds, which is longer than when the system was at rest--the loss of time in the "stern chase" exceeding the saving on the return. the light which is reflected from n has a different history. when it starts, $o$ and $n$ have certain positions in the ether, $o_ $ and $n_ $. by the time it reaches the mirror, this is at $n_ $, and $o$ is at $o_ $, and when it returns, it finds $o$ at $o_ $. the distances for the outward and inward journeys are now equal, but (as is obvious from the figure), each of them is greater than $d$, or $o_ n_ $, and the time for the round trip will be correspondingly increased. a simple calculation shows that it is $ d/\sqrt{c^ -v^ }$. the increase above the time required when the system was at rest is less in this case than the preceding. hence, if the apparatus is moving through the ether, the flashes reflected from $m$ and $n$ will not return at the same instant. for such velocities as are attainable--even the miles per second of the earth in its orbit--the difference is less than a hundred-millionth of the elapsed time. nevertheless, michelson and morley tried to detect it in their famous experiment. a beam of light was allowed to fall obliquely upon a clear glass mirror (placed at $o$ in the diagram) which reflected part of it toward the mirror, $m$, and let the rest pass through to the mirror $n$. by reuniting the beams after their round trips, it was possible to tell whether one had gained upon the other by even a small fraction of the time of vibration of a single light wave. the apparatus was so sensitive that the predicted difference, though amounting to less than a millionth part of a billionth of a second, could easily have been measured; but they actually found no difference at all--though the earth is certainly in motion. other optical experiments, more intricate, and even more delicate, were attempted, with the same object of detecting the motion of the earth through the ether; and they all failed. the special theory and its surprising consequences it was upon these facts that einstein based his original, or "special" theory of relativity. he assumed boldly that the universe is so constituted that uniform straight-ahead motion of an observer and all his apparatus will not produce any difference whatever in the result of any physical process or experiment of any kind. granting this, it follows that if all objects in the visible universe were moving uniformly together in any direction, no matter how fast, we could not find this out at all. we cannot determine whether the universe, as a whole, is at rest or in motion, and may as well make one guess as another. only the relative motions of its parts can be detected or studied. this seems simple and easy enough to understand. but the consequences which follow from it are extraordinary, and at first acquaintance seem almost absurd. in the first place, if an observer measures the velocity of light, he must always get the same result, no matter how fast he and his apparatus are moving, or in what direction (so long as the motion is uniform and rectilinear). this sounds harmless; but let us go back to the michelson-morley experiment where the light came back in exactly the same time from the two mirrors. if the observer supposes himself to be at rest, he will say that the distances $o m$ and $o n$ were equal. but if he fancies that the whole universe is moving in the direction $o m$, he will conclude that $m$ is nearer to $o$ than $n$ is--for if they were equidistant, the round-trip would take longer in the first case, as we have proved. if once more he fancies that the universe is moving in the direction $o n$, he will conclude that n is nearer to $o$ than $m$ is. his answer to the question which of the two distances, $o m$ or $o n$, is the greater will therefore depend on his assumption about the motion of the universe as a whole. similar complications arise in the measurement of time. suppose that we have two observers, $a$ and $b$, provided with clocks which run with perfect uniformity, and mirrors to reflect light signals to one another. at noon exactly by his clock, $a$ sends a flash of light towards $b$. $b$ sees it come in at : by his clock. the flash reflected from $b$'s mirror reaches $a$ at : by $a$'s clock. they communicate these observations to one another. if $a$ and $b$ regard themselves as being at rest, they will agree that the light took as long to go out as it did to come back, and therefore that it reached $b$ at just : by $a$'s clock, and that the two clocks are synchronized. but they may, if they please, suppose that they (and the whole universe) are moving in the direction from $a$ towards $b$, with half the speed of light. they will then say that the light had a "stern chase" to reach $b$, and took three times as long to go out as to come back. this means that it got to $b$ at / minutes past noon by $a$'s clock, and that $b$'s clock is slow compared with $a$'s. if they should assume that they were moving with the same speed in the opposite direction, they would conclude that $b$'s clock is half a minute fast. hence their answer to the question whether two events at different places happen at the same time, or at different times, will depend on their assumption about the motion of the universe as a whole. once more, let us suppose that $a$ and $b$, with their clocks and mirrors, are in relative motion, with half the speed of light, and pass one another at noon by both clocks. at : by $a$'s clock, he sends a flash of light, which reaches $b$ at : by his clock, is reflected, and gets back to $a$'s clock at : . they signal these results to each other, and sit down to work them out. $a$ thinks that he is at rest, and $b$ moving. he therefore concludes that the light had the same distance to go out as to return to him and took two seconds each way, reaching $b$ at : by $a$'s clock, and that the two clocks, which agreed then, as well as at noon, are running at the same rate. $b$, on the contrary, thinks that he is at rest and $a$ in motion. he then concludes that $a$ was much nearer when he sent out the flash than when he got it back, and that the light had three times as far to travel on the return journey. this means that it was : by $a$'s clock at the instant when the light reached $b$ and $b$'s clock read : . hence $a$'s clock is running slow, compared with $b$'s. hence the answer to the question whether two intervals of time, measured by observers who are in motion relative to one another, are of the same or of different durations, depends upon their assumptions about the motion of the universe as a whole. now we must remember that one assumption about the motion of the universe as a whole is exactly as good--or bad--as another. no possible experiment can distinguish between them. hence on the principle of relativity, we have left no absolute measurement of time or space. whether two distances in different directions are to be called equal or not--whether two events in different places are to be called simultaneous or not--depends on our arbitrary choice of such an assumption, or "frame of reference." all the various schemes of measurement corresponding to these assumptions will, when applied to any imaginable experiment, predict exactly the same phenomena. but, in certain important cases, these predictions differ from those of the old familiar theory, and, every time that such experiments have been tried, the result has agreed with the new theory, and not with the old. we are therefore driven to accept the theory of relativity, strange as it is, as being more nearly "true to nature" than our older ideas. fortunately, the difference between the results of the two become important only when we assume that the whole visible universe is moving together much faster than any of its parts are moving relatively to one another. unless we make such an unwarranted assumption, the differences are so small that it takes the most ingenious and precise experiments to reveal them. the generalization not content with all this, einstein proceeded, a few years ago, to develop a "general" theory of relativity, which includes the effects of gravitation. to make this idea clear, let us imagine two observers, each, with his measuring instruments, in a large and perfectly impervious box, which forms his "closed system." the first observer, with his box and its contents, alone in space, is entirely at rest. the second observer, with his box and its contents, is, it may be imagined, near the earth or the sun or some star, and falling freely under the influence of its gravitation. this second box and its contents, including the observer, will then fall under the gravitational force, that is, get up an ever-increasing speed, but at exactly the same rate, so that there will be no tendency for their relative positions to be altered. according to newton's principles, this will make not the slightest difference in the motions of the physical objects comprising the system or their attractions on one another, so that no dynamical experiment can distinguish between the condition of the freely falling observer in the second box and the observer at rest in the first. but once more the question arises: what could be done by an optical experiment? einstein assumed that the principle of relativity still applied in this case, so that it would be impossible to distinguish between the conditions of the observers in the two boxes by any optical experiment. it can easily be seen that it follows from this new generalized relativity that light cannot travel in a straight line in a gravitational field. imagine that the first observer sets up three slits, all in a straight line. a ray of light which passes through the first and second will obviously pass exactly through the third. suppose the observer in the freely falling system attempts the same experiment, having his slits $p$, $q$, $r$, equally spaced, and placing them at right angles to the direction in which he is falling. when the light passes through $p$, the slits will be in certain position $p_ q_ r_ $ (figure). by the time it reaches $q$, they will have fallen to a lower level, $p_ q_ r_ $, and when it reaches $r$, they will be still lower, $p_ q_ r_ $. the times which the light takes to move from $p$ to $q$ and $q$ to $r$ will be the same: but, since the system is falling ever faster and faster the distance $r_ r_ $ will be greater than $q_ q_ $. hence, if the light which has passed through $p$ and $q$ moves in a straight line, it will strike above $r$, as is illustrated by the straight line in the figure. but, on einstein's assumption, the light must go through the third slit, as it would do in the system at rest, and must therefore move in a curved line, like the curved line in the figure, and bend downward in the direction of the gravitational force. the tests calculation shows that the deviation of light by the moon or planets would be too small to detect. but for a ray which had passed near the sun, the deflection comes out . '', which the modern astronomer regards as a large quantity, easy to measure. observations to test this can be made only at a total eclipse, when we can photograph stars near the sun, on a nearly dark sky. a very fine chance came in may, , and two english expeditions were sent to brazil and the african coast. these photographs were measured with extreme care, and they show that the stars actually appear to be shifted, in almost exactly the way predicted by einstein's theory. another consequence of "general relativity" is that newton's law of gravitation needs a minute correction. this is so small that there is but a single case in which it can be tested. on newton's theory, the line joining the sun to the nearest point upon a planet's orbit (its perihelion) should remain fixed in direction, (barring certain effects of the attraction of the other planets, which can be allowed for). on einstein's theory it should move slowly forward. it has been known for years that the perihelion of mercury was actually moving forward, and all explanations had failed. but einstein's theory not only predicts the direction of the motion, but exactly the observed amount. einstein also predicts that the lines of any element in the solar spectrum should be slightly shifted towards the red, as compared with those produced in our laboratories. different observers have investigated this, and so far they disagree. the trouble is that there are several other influences which may shift the lines, such as pressure in the sun's atmosphere, motion of currents on the sun's surface, etc., and it is very hard to disentangle this gordian knot. at present, the results of these observations can neither be counted for or against the theory, while those in the other two cases are decisively favorable. the mathematical expression of this general relativity is intricate and difficult. mathematicians--who are used to conceptions which are unfamiliar, if not incomprehensible, to most of us--find that these expressions may be described (to the trained student) in terms of space of four dimensions and of the non-euclidean geometry. we therefore hear such phrases as "time as a sort of fourth dimension," "curvature of space" and others. but these are simply attempts--not altogether successful--to put mathematical relationships into ordinary language, instead of algebraic equations. more important to the general reader are the physical bearings of the new theory, and these are far easier to understand. various assumptions which we may make about the motion of the universe as a whole, though they do not influence the observed facts of nature, will lead us to different ways of interpreting our observations as measurements of space and time. theoretically, one of these assumptions is as good as any other. hence we no longer believe in absolute space and time. this is of great interest philosophically. practically, it is unimportant, for, unless our choice of an assumption is very wild, our conclusions and measurements will agree substantially with those which are already familiar. finally, the "general" relativity shows that gravitation and electro-magnetic phenomena--(including light) do not form two independent sides of nature, as we once supposed, but influence one another (though slightly) and are parts of one greater whole. xix einstein's theory of relativity a simple explanation of his postulates and their consequences by t. royds, kodaikanal observatory, india einstein's theory of relativity seeks to represent to us the world as it really is instead of the world of appearances which may be deceiving us. when i was in town last week to buy yards of calico i watched the draper very carefully as he measured the cloth to make sure i was not cheated. yet experiment can demonstrate, and einstein's theory can explain, that the draper's yardstick became longer or shorter according to the direction in which it was held. the length of the yardstick did not appear to me to change simply because everything else in the same direction, the store, the draper, the cloth, the retina of my eye, changed length in the same ratio. einstein's theory points out not only this, but every case where appearances are deceptive, and tries to show us the world of reality. einstein's theory is based on the principle of relativity and before we try to follow his reasoning we must spend a little time in understanding what he means by "relativity" and in grasping how the idea arises. suppose i wish to define my motion as i travel along in an automobile. i may be moving at the rate of miles an hour relative to objects fixed on the roadside, but relative to a fellow-passenger i am not moving at all; relative to the sun i am moving with a speed of / miles per second in an elliptical orbit, and again relative to the stars i am moving in the direction of the star vega at a speed of miles per second. thus motion can only be defined relative to some object or point of reference. now this is not satisfactory to the exact scientist. scientists are not content with knowing, for example, that the temperature of boiling water is + ° c. relative to the temperature of freezing; they have set out to determine absolute temperatures and have found that water boils at ° c. above absolute zero. why should i not, therefore, determine the absolute motion of the automobile, not its motion relative to the road, earth, sun or stars, but relative to absolute rest? michelson and morley set out in their famous experiment to measure the absolute velocity of their laboratory, which was, of course, fixed on the earth. the experiment consisted of timing two rays of light over two equal tracks at right angles to each other. when one track was situated in the direction of the earth's motion they expected to get the same result as when two scullers of equal prowess are racing in a river, one up and down the stream and the other across and back; the winner will be the sculler rowing across the stream, as working out an example will convince. even if the earth had been stationary at the time of one experiment, the earth's motion round the sun would have been reversed months later and would then have given double the effect. they found, however, that the two rays of light arrived always an exact dead heat. all experimenters who have tried since have arrived at the same result and found it impossible to detect absolute motion. the principle of relativity has its foundation in fact on these failures to detect absolute motion. this principle states that the only motion we can ever know about is relative motion. if we devise an experiment which ought to reveal absolute motion, nature will enter into a conspiracy to defeat us. in the michelson and morley experiment the conspiracy was that the track in the direction of the earth's absolute motion should contract its length by just so much as would allow the ray of light along it to arrive up to time. we see, therefore, that according to the principle of relativity motion must always remain a relative term, in much the same way as vertical and horizontal, right and left, are relative terms having only meaning when referred to some observer. we do not expect to find an absolute vertical and are wise enough not to attempt it; in seeking to find absolute motion physicists were not so wise and only found themselves baffled. the principle that all motion is relative now requires to be worked out to all its consequences, as has been done by einstein, and we have his theory of relativity. einstein conceives a world of four dimensions built up of the three dimensions of space, namely up and down, backwards and forwards, right and left, with time as the fourth dimension. this is an unusual conception to most of us, so let us simplify it into something which we can more easily picture but which will still allow us to grasp einstein's ideas. let us confine ourselves for the present to events which happen on this sheet of paper, i.e., to space of two dimensions only and take time as our third dimension at right angles to the plane of the paper. we have thus built up a three dimensional world of space-time which is every bit as useful to us as a four dimensional representation so long as we only need study objects moving over the sheet of paper. suppose a fly is crawling over this sheet of paper and let us make a movie record of it. if we cut up the strip of movie film into the individual pictures and cement them together one above another in their proper order, we shall build up a solid block of film which will be a model of our simplified world of space-time and in which there will be a series of dots representing the motion of the fly over the paper. just as i can state the exact position of an object in my room by defining its height above the floor, its distance from the north wall and its distance from the east wall, so we can reduce the positions of the dots to figures for use in calculations by measuring their distances from the three faces intersecting in the lines $ox$, $oy$, and $ot$, where $oxaytbcd$ represents the block of film. the mathematician would call the three lines $ox$, $oy$, $ot$ the coordinate axes. measuring all the dots in this way we shall obtain the motion of the fly relative to the coordinate axes $ox$, $oy$, $ot$. if we add a block $otdyefgh$ of plain film we can use $ex$, $eh$, $ef$ as coordinate axes and again obtain the motion of the fly relative to these new axes; or we can add block after block so as to keep the axes moving. we can conceive of other changes of axes. the operator making the movie record might have taken the fly for the hero of the piece and moved the camera about so as to keep the fly more or less central in the picture; or he might, by turning the handle first fast and then slow and by moving the camera, have made the fly appear to be doing stunts. moving the camera would change the axes of $x$ and $y$, and turning the handle at different speeds would change the axis of time. again, we might change the axes by pushing the block out of shape or by distorting it into a state of strain. whatever change of axes we make, any dot in the block of film will signify a coincidence of the fly with a certain point of the paper at a certain time, and the series of dots will, in every case, be a representation of the motion of the fly. maybe the representation will be a distorted one, but who is to say which is the absolutely undistorted representation? the principle of relativity which we laid down before says that no one set of coordinates will give the absolute motion of the fly, so that one set is as good as another. the principle that all motion is relative means, therefore, that no matter how we change our coordinates of space-time, the laws of motion which we deduce must be the same for all changes. to use an analogy, the sculptured head of shakespeare on my table may appear to have hollow cheeks when i admit light from the east window only, or to have sunken eyes with light from the skylight in the roof, but the true shape of the head remains the same in all lights. hence, if with reference to two consecutive dots in our block of film a mathematical quantity can be found which will not change no matter how we changes our axes of coordinates, that quantity must be an expression of the true law of motion of the fly between the two points of the paper and the two times represented by these two dots. einstein has worked out such a quantity remaining constant for all changes of coordinates of the four dimensional world of space-time. in passing we may notice a feature of einstein's world of space-time which we shall doubtless find it difficult to conceive, namely, that there is no essential difference between a time and a distance in space. since one set of coordinates is as good as another, we can transform time into space and space into time according as we choose our axes. for example if we change $ox$, $ot$, the axes of $x$ and time in fig. , into $ox'$, $ot'$ by a simple rotation, the new time represented by $ot'$ consists partly of $oa$ in the old time and partly of $ob in the old $x$ direction. referring to our block of movie film again, it means that although i might separate the block into space and time by slicing it into the original pictures, i can just as readily slice it in any direction i choose and still get individual pictures representing the motion of the fly but with, of course, new time and space. so whilst i may be believe that a liner has travelled , miles in days, an observer on a star who knows nothing of my particular axes in space-time may say, with equal truth, that it went , miles in days. thus, time and space are not two separate identities in einstein's view; there only exists a world of four dimensions which we can split up into time and space as we choose. let us see now how einstein explains gravitation. when a body is not acted on by any forces (except gravitation) the quantity which remains constant for all changes of coordinates implies that the body will follow that path in the space of an outside observer which takes the least time. it is an observed fact that one body attracts another by gravitation; that is, the path of one body is bent from its course by the presence of another. now we can bend the path of the fly in our block of film by straining the block in some way. suppose, therefore, that i strain the world so as to bend the path of a body exactly as the gravitation due to some other body bends it; i.e., by a change of coordinates i have obtained the same effect as that produced by gravitation. einstein's theory, therefore, explains gravitation as a distortion of the world of space-time due to the presence of matter. suppose first that a body is moving with no other bodies near; according to einstein it will take the path in space which requires the least time, i.e., a straight line as agrees with our experience. if now the world be strained by the presence of another body or by a change of coordinates it will still pursue the path of least time, but this path is now distorted from the straight line, just as in a similar way the path on a globe requiring the least time to travel follows a great circle. so, on einstein's view of gravitation, the earth moves in an elliptical path around the sun not because a force is acting on it, but because the world of space-time is so distorted by the presence of the sun that the path of least time through space is the elliptical path observed. there is, therefore, no need to introduce any idea of "force" of gravitation. einstein's theory explains gravitation only in the sense that he has explained it away as a force of nature and makes it a property of space-time, namely, a distortion not different from an appropriate change of coordinates. he does not, however, explain how or why a body can distort space-time. it is noteworthy that whilst the law of gravitation and the law of uniform motion in a straight line when no force is acting were separate and independent laws under newton, einstein finds one explanation for both under the principle of relativity. [ ] xx einstein's theory of gravitation the discussion of the general theory and its most important application, from the essay by prof. w. f. g. swann, university of minnesota, minneapolis newton's great discovery regarding the motion of the planets consisted in his showing that these could all be summed up in the following statement: consider any planet in its relation to all particles in the universe. write down, for the planet, in the line joining it to any particle, an acceleration proportional to the mass of the particle and to the inverse square of its distance from the planet. then calculate the planet's resultant acceleration by combining all the accelerations thus obtained. we have here purposely avoided the use of the word "force," for newton's law is complete as a practical statement of fact without it; and this word adds nothing to the law by way of enhancing its power in actual use. nevertheless, the fact that the acceleration is made up as it were of non-interfering contributions from each particle in the line joining it to the planet strongly suggests to the mind something of the nature of an elastic pull for which the particle is responsible, and to which the planet's departure from a straight-line motion is due. the mind likes to think of the elastic; ever since the time of newton people have sought to devise some mechanism by which these pulls might be visualized as responsible for the phenomena in the same way as one pictures an elastic thread as controlling the motion of a stone which swings around at its end. this search has been always without success; and now einstein has found a rather different law which fits the facts better than newton's law. it is of such a type that it does not lend itself conveniently to expression in terms of force; the mind would gain nothing by trying to picture such forces as are necessary. it compensates for this, however, in being capable of visualization in terms of what is ultimately a much simpler concept. in order to appreciate the fundamental ideas involved, suppose for a moment that gravitation could be annihilated, completely, and suppose i find myself upon this earth in empty space. you shall be seated at some point in space and shall watch my doings. if i am in the condition of mind of the people of the reign of king henry viii, i shall believe that the earth does not rotate. if i let go a stone, there being no gravity, i shall find that it flies away from me with an acceleration. you will know, however, that the stone really moves in a straight line with constant velocity, and that the apparent acceleration which i perceive is due to the earth's rotation. if i have argued that acceleration is due to force, i shall say that the earth repels the stone, and shall try to find the law governing the variation of this force with distance. i may go farther, and try to imagine some reason for the force, some pushing action transmitted from the earth to the stone through a surrounding medium; and, you will pity me for all this wasted labor, and particularly for my attempt to find a mechanism to account for the force, since you know that if i would only accept your measurements all would appear so simple. let us probe this matter a little farther, however, from the stand-point of myself. i must believe in the reality of the force, since i have to be tied to my chair to prevent my departure from the earth. i might wonder how this field of force would affect the propagation of light, chemical action and so forth. for, even though i had discovered that, by using your measures, i could transform away the apparent effects of my field of force as far as concerned its power to hurl stones about, i could still regard this as a mathematical accident, and believe that the force was really there. although i might suspect that the same transformation of view-point that would annul the field's effect as regards the stones would also annul its effect as regards light, etc., i should not be sure of this, as you would be; and my conscience would hardly allow me to do more than look upon the assumption of complete equivalence between the apparent field and a change in the system of measurement as a hypothesis. i should be strongly tempted to make the hypothesis, however. now the question raised by einstein is whether the force of gravity, which we experience as a very real thing, may be put upon a footing which is in some way analogous to that of the obviously fictitious centrifugal force cited above: whether gravitation may be regarded as a figment of our imagination engendered by the way in which we measure things. he found that it could be so regarded. he went still farther, and in his principle of equivalence, he postulated that the apparent effects of gravitation in all phenomena could be attributed to the same change in the system of our measurements that would account for the ordinary phenomena of gravitation. on the basis of this hypothesis he was able to deduce for subsequent experimental verification, the effects of gravitation on light. he did not limit himself to such simple changes in our measurements as were sufficient to serve the purpose of the problem of centrifugal force cited above; but, emboldened by the assumptions, in the older theory of relativity, of change in standards of length and time on account of motion, he went even farther than this, and considered the possibility of change of our measures due to mere proximity to matter. his problem amounted to an attempt to find some way in which it was possible to conceive our scales and clocks as altered, relatively to some more fundamental set, so as to allow of the planetary motions being uniform and rectilinear with respect to these fundamental measures, although they appear as they do to us. if we allow our imaginations perfect freedom as to how the scales may be altered, we shall not balk at assuming alterations varying in any way we please, einstein does, however, introduce restrictions for reasons which we will now discern. if we imagine our whole universe, with its observers, planetary orbits, instruments, and everything else, embedded in a jelly, and then distort the jelly and contents in any way, the numbers at which our planetary orbits (or rather their telescopic images) intersect our scales will be unaltered. moreover, we could vary, in any manner, the times at which all objects (including the clock hands) occupied their distorted positions, and the hand of some clock near the point where the planetary image crossed the scale would record for this occurrence the same dial reading as before. an inhabitant of this distorted universe would be absolutely unconscious of the change. now the general theory of relativity which expresses itself in slightly varied forms, amounts to satisfying a certain philosophical craving of the mind, by asserting that the laws of nature which control our universe ought to be such that another universe like the above, whose inhabitants would be unconscious of their change, would also satisfy these laws, not merely from the standpoint of its own inhabitants, but also from the standpoint of our measurements. in other words, this second universe ought to appear possible to us as well as to its inhabitants. einstein decides to make his theory conform to this philosophical desire, and this greatly limits the modifications of clocks and scales which he permits himself for the purpose of representing gravitation. further, if we express the alterations of the measures as functions of proximity to matter, velocity and so forth, our expressions for these alterations will include, as a particular case, that where matter is absent, although the scales and observer may still remain. our alteration of the scales and clocks with velocity must thus revert, for this case, to that corresponding to the older theory of relativity, in order to avoid predicting that two observers, in uniform motion relative to each other in empty space, will measure different values for the velocity of light. in this way, the velocity of light comes to play a part in expressing the alterations of the measures. even with these restrictions, einstein was able to do the equivalent of finding an alteration of scales and clocks in the presence of matter which would account for our finding that the planetary motions take place very nearly in accordance with newton's law. the new law has accounted with surprising accuracy for certain astronomical irregularities for which newton's law failed to account, and has predicted at least one previously unknown phenomenon which was immediately verified. in conclusion, it may be of interest to state how the new law describes the motion of a particle in the vicinity of a body like the earth. the law amounts to stating that, if we measure a short distance, radially as regards the earth's center, we must allow for the peculiarity of our units by dividing by $$\sqrt{ -\frac{ mg}{c^ r}}$$ where $r$ is the distance from the earth's center, $m$ the mass of the earth, $c$ the velocity of light, and $g$ the newtonian gravitational constant. tangential measurements require no correction, but intervals of time as measured by our clocks must be multiplied, for each particular place, by the above factor. then, in terms of the corrected measures so obtained, the particle will be found to describe a straight line with constant velocity although, in terms of our actual measures, it appears to fall with an acceleration. xxi the equivalence hypothesis the discussion of this, with its difficulties and the manner in which einstein has resolved them, from the essay by prof. e. n. da c. andrade, ordnance college, woolwich, england having shown that, of several systems all moving with reference to one another with uniform motion, no one is entitled to any preference over the others, and having deduced the laws for such systems, einstein was confronted with a difficulty which had long been felt. a body rotating, which is a special case of an accelerated body, can be distinguished from one at rest, without looking outside it, by the existence of the so-called centrifugal forces. this circumstance, which gives certain bodies an absolute or preferential motion, is unpalatable to the relativist; he would like there to be no difference as regards forces [ ] between the case when the earth rotates with reference to outside bodies (the stars) considered as fixed, and the case when the earth is considered fixed and all outside bodies rotate around it. this point cannot be investigated by direct experiment; we can spin a top but we cannot keep a top at rest and spin the world round it, to see if the forces are same. in considering the problem of how to devise laws which should make all rotations relative, einstein conceived the brilliant yet simple idea that gravitation could be brought into the scheme as an acceleration effect, since both ordinary accelerational forces and gravitational forces are proportional to the same thing, the mass of a body. the impossibility of separating the two kinds of effect can be easily seen by considering the starting of an elevator. when the elevator is quickly accelerated upwards we feel a downward pull, just as if the gravitational pull had been increased, and if the acceleration continued to be uniform, bodies tested with a spring balance would all weigh more in the elevator than they did on firm ground. in a similar way the whole of the gravitational pull may be considered to be an accelerational effect, the difficulty being to devise laws of motion which will give the effects that we find by actual observation. but it is obvious that we cannot, by ordinary mechanics, consider the earth as being accelerated in all directions, which we should have to do, apparently, to account for the fact that the gravitational pull is always toward the center. [it is obvious that we cannot explain gravitation by assuming that the earth's surface is continually moving outward with an accelerated velocity.] so einstein found that, as long as we treat the problem by euclid's geometry, we cannot reach a satisfactory solution. but he found that to the four-dimensional space made up of the three ordinary dimensions of space, together with the time-dimension which we have already mentioned in discussing the special theory, may be attributed a peculiar geometry, the nature of which departs more and more from euclidean geometry as we approach a gravitational body, and the net result of which is to make possible the universal correspondence of gravitation and acceleration. this modification of the geometry of space is often spoken of as the "curvature of space," an expression which is puzzling, especially as the space which is "curved" is four-dimensional time-space. but we can get an idea of what is meant by considering figures, triangles say, drawn on the surface, of a sphere. these triangles, although drawn on a surface, will not have the same properties as triangles drawn on flat paper--their three angles will not together equal right angles. they will be non-euclidean. this is only a rough analogy, but we can see that the curvature of the surface causes a departure from euclidean geometry for plane figures, and consequently the departure from euclidean laws extended to four dimensions may be referred to as caused by "curvature of space." it is difficult to imagine a lump of matter affecting the geometry of the space round it. once more we must use a rough illustration. imagine a very hot body, and that, knowing nothing of its properties, we have to measure up the space round it with metal measuring-rods. the nearer we are to the body, the longer the rods will become, owing to the expansion of the metal. when we measure out a square, one side of which is nearer the body than the opposite side, its angles will not be right angles. if we knew nothing of the laws of heat we should say that the body had made the space round it non-euclidean. einstein found, then, that by taking the properties of space, as given by measurement, to be modified in the neighborhood of masses of matter, he could devise general laws according to which gravitational effects would be produced, and there would be no absolute rotation. all forces will be the same whether a body rotates with everything outside it fixed, or the body is fixed, and everything rotates round it. all motion is then relative, and the theory is one of "general relativity." the velocity of light is, however, no longer constant, and its path is not a straight line, if it is passing near gravitating matter. this does not contradict the special theory, which did not allow for gravitation. rather, the special theory is a particular case to which the generalised theory reduces when there is no matter about, just as the newtonian dynamics is a special case of the special theory, which we obtain when all velocities are small compared to that of light. xxii the general theory fragments of particular merit on this phase of the subject by various contributors when dorothy was carried by the cyclone from her home in kansas to the land of oz, together with her uncle's house and her little dog toto, she neglected to lower the trap door over the hole in the floor which formerly led to the cyclone cellar and toto stepped through. dorothy rushed to the opening expecting to see him dashed onto the rocks below but found him floating just below the floor. she drew him back into the room and closed the trap. the author of the chronicle of dorothy's adventures explains that the same force which held up the house held up toto but this explanation is not necessary. dorothy was now floating through space and house and dog were subject to the same forces of gravitation which gave them identical motions. dorothy must have pushed the dog down onto the floor and in doing so must herself have floated to the ceiling whence she might have pushed herself back to the floor. in fact gravitation was apparently suspended and dorothy was in a position to have tried certain experiments which einstein has never tried because he was never in dorothy's unique position.] the principle of equivalence, of which einstein's suspended cage experiment is the usual illustration, and upon which the generalized theory of relativity is built, is thus stated by prof. eddington: "a gravitational field of force is precisely equivalent to an artificial field of force, so that in any small region it is impossible, by any conceivable experiment, to distinguish between them. in other words, force is purely relative." this may be otherwise stated by going back to our idea of a four-dimensional world, the points of which represent the positions and times of events. if we mark in such a space-time the successive positions of an object we get a line, or curve, which represents the whole history of the object, inasmuch as it shows us the position of the object at every time. the reader may imagine that all events happen in one plane, so that only two perpendicular dimensions are needed to fix positions in space, with a third perpendicular dimension for time. he may then conceive, if he may not picture, an analogous process for four-dimensional space-time. these lines, "tracks of objects through space-time," were called by minkowski "world-lines." we may now say that all the events we observe are the intersections of world-lines. the temperature at noon was °. this means that if i plot the world-line of the top of the mercury column and the world-line of a certain mark on the glass they intersect in a certain point of space-time. all that we know are intersections of these world-lines. suppose now we have a large number of them drawn in our four-dimensional world, satisfying all known intersections, and let us suppose the whole embedded in a jelly. we may distort this jelly in any way, changing our coordinates as we please, but we shall neither destroy nor create intersections of world-lines. it may be proved that a change from one system of reference, to which observations are referred, to any other system, moving in any way with respect to the first system, may be pictured as a distortion of the four-dimensional jelly. the laws of nature, therefore, being laws that describe intersections, must be expressible in a form independent of the reference system chosen. from these postulates, einstein was able to show such a formulation possible. his law may be stated very simply:--all bodies move through space-time in the straightest possible tracks. the fact that an easy non-mathematical explanation can not be given, of how this law is reached, or of just why the straightest track of mercury through space-time will give us an ellipse in space after we have split space-time up into space and time, is no valid objection to the theory. newton's law that bodies attract with a force proportional to their masses and inversely proportional to the square of the distance is simple, but no one has ever given an easy non-mathematical proof of how that law requires the path of mercury to be an ellipse, with the sun at a focus, instead of some other curve.] one of the grave difficulties we have in gaining a satisfactory comprehension of einstein's conceptions, is that they do not readily relate themselves to our modes of geometrical thought. within limits we may choose our own geometry, but it may be at the cost of unwieldy complication. if we think with newton in terms of euclidean geometry and consider the earth as revolving around the sun, the motions of our solar system can be stated in comparatively simple terms. if, on the other hand, we should persist in stating them, as ptolemy would have done, from the earth as a relatively stationary center, our formulas will become complicated beyond ready comprehension. for this reason it is much simpler in applying the theory of relativity, and in considering and describing what actually happens in the physical universe, to use geometrical conceptions to which the actual conditions can be easily related. we find such an instrument in non-euclidean geometry, wherein space will appear as though it were projected from a slightly concave mirror. it is in this sense that some speak of space as curved. the analogy is so suggestive it tempts one to linger over it. unless there were material objects within the range of the mirror, its conformation would be immaterial; the thought of the space which the mirror, as it were, circumscribes, is dependent upon the presence of such material objects. the lines of light and of all other movement will not be quite "straight" from the view-point of euclidean geometry. a line drawn in a universe of such a nature must inevitably return upon itself. nothing therefore, can ever pass out of this unlimitedly great but yet finite cosmos. but even now, since our imaginary mirror is only very slightly concave, it follows that for limited regions like the earth or even the solar system, our conception of geometry may well be rectilinear and euclidean. newton's law of gravitation will be quite accurate with only a theoretical modification drawn from the theory of relativity.] the way in which a curvature of space might appear to us as a force is made plainer by an example. suppose that in a certain room a marble dropped anywhere on the floor always rolled to the center of the room; suppose the same thing happened to a baseball, a billiard ball, and a tennis ball. these results could be explained in two ways; we might assume that a mysterious force of attraction existed at the center of the floor, which affected all kinds of balls alike; or we might assume that the floor was curved. we naturally prefer the latter explanation. but when we find that in the neighborhood of a large material body all other bodies move toward it in exactly the same manner, regardless of their nature or their condition, we are accustomed to postulate a mysterious attractive force (gravitation); einstein, on the contrary, adopts the other alternative, that the space around the body is curved.] in the ordinary "analytical geometry," the position and motion of all the points considered is referred to a rigid "body" or "frame of reference." this usually consists of an imaginary room of suitable size. the position of any point is then given by three numbers, i.e., its distances from one side wall and from the back wall and its height above the floor. these three numbers can only give one point, every other point having at least one number different. in four-dimensional geometry a fourth wall may be vaguely imagined as perpendicular to all three walls, and a fourth number added, giving the distance of the "point" from this wall also. since "rigid" bodies do not exist in gravitational fields the "frame of reference" must be "non-rigid." the frame of reference in the gaussian system need not be rigid, it can be of any shape and moving in any manner, in fact a kind of jelly. a "point" or "event" in the four dimensioned world is still given by four numbers but these numbers do not represent distances from anywhere; all that is necessary is that no two events shall have exactly the same four numbers to represent them, and that two events which are very close together shall be represented by numbers which differ only slightly from one another. this system assumes so little that it will be seen to be very wide in its scope; although to the ordinary mind, what is gained in scope seems more than that lost in concreteness. this does not concern the mathematician, however, and by using this system he gains his object, proving that the general laws of nature remain the same when expressed in any gaussian coordinate system whatever.] einstein enunciates a general principle that it is possible to find a transformation of coordinate axes which is exactly equivalent to any force, and in particular one which is equivalent to the force of gravitation. that is by concentrating our attention on the transformation which is a purely mathematical operation we can afford to neglect the force completely. to get a better idea of this principle of equivalence as it is called, let us consider a relatively simple example (which actually has nothing to do with gravitation, but which will serve to make our notions clearer.) a person on the earth unconsciously refers all his experiences, i.e., the motions of the objects around him to a set of axes fixed in the earth on which he stands. however, we know that the earth is rotating about its axis, and his axes of reference are also rotating with respect to the space about him. from the point of view of general relativity it is exactly because we do refer motions on the surface of the earth to axes rotating with the earth that we experience the so-called centrifugal force of the earth's rotation, with which everyone is familiar. if we could find it convenient to transform from moving axes to fixed axes, the force would vanish, since it is exactly equivalent to the transformation from one set of axes to the other. however, we find it unnatural to refer daily experiences to axes that are not placed where we happen to be, and so we prefer to take the force and rotating axes instead of no force and fixed axes.] we seem to have a direct experience of force in our muscular sensations. by pushing or pulling we can set bodies in motion. it is natural to assume, that something similar occurs, when nature set bodies in motion. but is this not a relic of animism? the savage and the ancients peopled all the woods and skies with gods and demons, who carries on the activities of nature by their own bodily efforts. today we have dispossessed the demons, but the ghost of a muscular pull still holds the planets in place.] the general theory is an extension of the special theory which enables the law of gravitation to be deduced. not in newton's form, it is true, but in a better form, that is, one that accounts for two important facts otherwise not explained. but it is a far more general theory that indicated above. it is a complete study of the relations between laws expressed by means of any four coordinates (of which three space and one time is a special case), and the same laws expressed in the four coordinates of a system having any motion whatever with respect to the first system. by restricting this general study in accordance with certain postulates about the nature of the universe we live in, we arrive at a number of conclusions which fit more closely with observed facts that the conclusions drawn from newton's theory.] notes [ ] it will be noted that mr. bolton pronounces the geometry of space to be euclidean in the absence of gravitational fields, not that of space-time. this is in accord with what was pointed out on page .--editor. [ ] the author here comes perilously close to ascribing to this "contraction" the sort of physical reality which it does not possess. see page .--editor. [ ] dr. davis went rather fully into the algebra of the michelson-morley experiment. but dr. russell has covered the same ground in a form somewhat more advantageous from the typographical viewpoint, and the point is not one which it is profitable to discuss twice; so we eliminate this part of dr. davis' text.--editor. [ ] this statement is objectionable, as explained in chapter iv.--editor. [ ] a. einstein: die grundlage der allgemeinen relativitätstheorie. ann. d. physik. , vol. , page . [ ] at this point we have again used the blue pencil on dr. davis' text, his discussion of the three observational tests of the general theory adding nothing to dr. pickering's.--the editor. [ ] commander mchardy uses the term "event" in a sense somewhat different from that seen in a majority of the essays. he reserves for the four-dimensional element--the instant of time at a point in space--the name "point-event"; and the term "event" he applies to a collection of these forming, together, an observable whole. an actual physical happening, like a railroad wreck or a laboratory experiment, it will be realized is of the latter sort, occupying an appreciable region of space rather than a single point, and an appreciable interval of time rather than a single second. to the element, the "point-event" of commander mchardy's essay, this bears the same relation that the geometer's solid bears to his point. this comment is in no sense to be taken as criticism of commander mchardy's terminology, which rather appeals to us; we make it merely to guard against confusion in the reader's mind.--editor. [ ] this paragraph is the result of an editorial revision of the author's text, designed to retain the substance of his presentation, while tying up what he has to say more definitely with the preceding essays, and eliminating the distinction between finite and infinitesimal intervals, which we believe to be out of place in an essay of this character. we will not apologize to our mathematical readers for having used finite and differential notation in the same equation, in violation of mathematical convention.--editor. [ ] although gravitational force in a small region can be imitated or annulled by accelerating motion, there remains the disturbing influence of gravitational matter already referred to and expressed in the fabric curvature. it is this that defines how unique tracks run, or rather, how bodies progress.--author. [ ] not all gravitational fields may be transformed away by a proper choice of coordinates. if this were so, the space, whose nature is independent of any choice of coordinates, would always be euclidean.--author. [ ] thus when it it said that a body contracts or that a clock runs slow when it is put in motion no actual physical change is implied. the judgment of different observers--one at rest with respect to the body and one not--are different.--author. [ ] the balance of dr. royds' essay is given to a discussion of the phenomena of mercury's perihelial advance, the deflection of light under the gravitational field of the sun, and the shift in spectral lines, in connection with which alone einstein's theory makes predictions which are sufficiently at variance with those of newtonian science to be of value in checking up the theory observationally. in the interest of space conservation and in the presence of dr. pickering's very complete discussion of these matters we omit dr. royds' statement.--editor. [ ] there is, on any view, no difference as regards observation of position only.--author. _nature's miracles, vol. iii._ electricity and magnetism by elisha gray, ph.d., ll.d. william briggs - richmond st. west, toronto c. w. coates, montreal, que. s. f. huestis, halifax, n.s. contents. chapter page introduction v i. the author's design ii. history of electrical science iii. history of magnetism iv. theory and nature of magnetism v. theory of electricity vi. electrical currents vii. electric generators viii. atmospheric electricity ix. electrical measurement x. the electric telegraph xi. receiving messages xii. miscellaneous methods xiii. multiple transmission xiv. way duplex system xv. the telephone xvi. how the telephone talks xvii. submarine telegraphy xviii. short-line telegraphs xix. the telautograph xx. some curiosities xxi. wireless telegraphy xxii. niagara falls power--introduction xxiii. niagara falls power--appliances xxiv. niagara falls power--appliances xxv. electrical products--carborundum xxvi. electrical products--bleaching-powder xxvii. electrical products--aluminum xxviii. electrical products--calcium carbide xxix. the new era introduction. for the benefit of the readers of vol. iii, who have not read the general introduction found in vol. i, a word as to the scope and object of this volume will not be amiss. it will be plain to any one on seeing the size of the little book that it cannot be an exhaustive treatise on a subject so large as that of electricity. this volume, like the others, is intended for popular reading, and technical terms are avoided as far as possible, or when used clearly explained. the subject is treated historically, theoretically, and practically. as the author has lived through the period during which the science of electricity has had most of its growth, he naturally and necessarily deals somewhat in reminiscence. all he hopes to do is to plant a few seed-thoughts in the minds of his readers that will awaken an interest in the study of natural science; and especially in its most fascinating branch--electricity. if vol. i is at hand, please read the introduction. it will bring you into closer sympathy with the author and his mode of treatment. again, if the reader is especially interested in the theory of electricity it will help him very much if he first reads vols. i and ii, as a preparation for a better understanding of vol. iii. all the natural sciences are so closely related that it is difficult to get a clear insight into any one of them without at least a general idea of all the others. nature's miracles. electricity and magnetism. chapter i. the author's design. the writer has spent much of his time for thirty-five years in the study of electricity and in inventing appliances for purposes of transmitting intelligence electrically between distant points, and is perhaps more familiar with the phenomena of electricity than with those of any other branch of physics; yet he finds it still the most difficult of all the natural sciences to explain. to give any satisfactory theory as to its place with and relation to other forms of energy is a perplexing problem. it is said that lord kelvin lately made the statement that no advance had been made in explaining the real nature of electricity for fifty years. while this statement--if he really made it--is rather broad, it must be acknowledged that all the theories so far advanced are little better than guesses. but there is value in guessing, for one man's guess may lead to another that is better, and, as it is rarely the case that each one does not give us a little different view of the matter, it may be that out of the multiplicity of guesses there may some time be a suggestion given to some investigator that will solve the problem, or at least carry the theme farther back and establish its true relationship to the other forms of energy. i cannot but think that there is yet a simple statement to be made of energy in its relation to matter that will establish a closer relationship between the different branches of physical science. and this, most likely, will be brought about by a better understanding of the nature of the interstellar substance called ether, and its relation to all forms and conditions of sensible matter and energy. in the talks that will follow it will be the endeavor of the writer to give such a simple and popular exposition of the phenomena and applications of electricity, in a general way only, that the popular reader may get, at least, an elementary understanding of the subject so far as it is known. as we have said, the descriptions will have to be elementary, for nothing else can be done without such elaborate technical drawings and specifications as would be impossible in our limited space, and would not be clear to the ordinary reader who knows nothing of the science. thousands who are employed in various ways with enterprises, the foundations of which are electrical, know nothing of electricity as a science. a friend of mine, who is a professor of physics in one of our colleges, was traveling a few years ago, and in his wanderings he came across some sort of a factory where an electric motor was employed. being on the alert for information, he stepped in and introduced himself to the engineer, and began asking him questions about the electric motor of which he had charge. the professor could talk ohm, ampères, and volts smoothly, and he "fired" some of these electrotechnical names at the engineer. the engineer looked at him blankly and said: "you can't prove it by me. i don't know what you're talking about. all i know is to turn on the juice and let her buzz." how much "juice" is wasted in this cut-and-dry world of ours and how much could be saved if only all were even fairly intelligent regarding the laws of nature! a great deal of the business of this world is run on the "let her buzz" theory, and the public pays for the waste. it will continue to be so until a higher order of intelligence is more generally diffused among the people. a fountain can rise no higher than its source. a business will never exceed the intelligence that is put into it, nor will a government ever be greater than its people. let us begin the subject of electricity by going somewhat into its past history. it is always well to know the history of any subject we are studying, for we often profit as much by the mistakes of others as by their successes. i shall also give the theories advanced by different investigators, and if i should have any thoughts of my own on the subject i shall be free to give them, for i have just as good a right to make a guess as any one. it must be confessed, however, that the older i grow the less i feel that i know about the subject of electricity, or anything else, in comparison with what i see there is yet to be known. i once met a young man who had just graduated from college, and in his conversation he stated that he had taken a course in electricity. i asked him how long he had studied the subject. he said "three months." i asked him if he understood it--and he said that he did. i told him that he was the man that the world was looking for; that i had studied it for thirty years and did not understand it yet. "a little learning is a dangerous thing"--for it puffs us up, and we feel that we know it all and have the world in our grasp; but after we have tried our "little learning" on the world for a while and have received the many hard knocks that are sure to come, we are sooner or later brought up in front of the mirror of experience, and we "see ourselves as others see us," and are not satisfied with the view. whatever the theories may be regarding electricity, and however unsatisfactory they may be, there are certain well-defined facts and phenomena that are of the greatest importance to the world. these we may understand: and to this end let us especially direct our efforts. chapter ii. history of electrical science. electricity as a well-developed science is not old. those of us who have lived fifty years have seen nearly all its development so far as it has been applied to useful purposes, and those who have lived over twenty-five years have seen the major portion of its development. thales of miletus, b.c., discovered, or at least described, the properties of amber when rubbed, showing that it had the power to attract and repel light substances, such as straws, dry leaves, etc. and from the greek word for amber--elektron--came the name electricity, denoting this peculiar property. theophrastus and pliny made the same observations; the former about b.c., and the latter about a.d. it is also said that the ancients had observed the effects of animal electricity, such as that of the fish called the torpedo. pliny and aristotle both speak of its power to paralyze the feet of men and animals, and to first benumb the fish which it then preyed upon. it is also recorded that a freed-man of tiberius was cured of the gout by the shocks of the torpedo. it is further recorded that wolimer, the king of the goths, was able to emit sparks from his body. coming down to more modern times--a.d. --we find dr. gilbert, an englishman, taking up the investigation of the electrical properties of various substances when submitted to friction, and formulating them in the order of their importance. in these experiments we have the beginnings of what has since developed into a great science. he made the discovery that when the air was dry he could soon electrify the substances rubbed, but when it was damp it took much longer and sometimes he failed altogether. in francis hawksbee, an experimental philosopher, discovered that mercury could be rendered luminous by agitating it in an exhausted receiver. (it is a question whether this phenomenon should not be classed with that of phosphorescence rather than electricity.) the number of investigators was so great that all of them cannot be mentioned. it often happens that those who do really most for a science are never known to fame. a number of people will make small contributions till the structure has by degrees assumed large proportions, when finally some one comes along and puts a gilded dome on it and the whole structure takes his name. this is eminently true of many of the more important developments in the science and applications of electricity during the last twenty-five or thirty years. following hawksbee may be mentioned stephen gray, sir isaac newton, dr. wall, m. dupay and others. dupay discovered the two conditions of electrical excitation known now as positive and negative conditions. in the leyden jar was invented. it takes its name from the city of leyden, where its use was first discovered. it is a glass jar, coated inside and out with tin-foil. the inside coating is connected with a brass knob at the top, through which it can be charged with electricity. the inner and outer coatings must not be continuous but insulated from each other. the author's name is not known, but it is said that three different persons invented it independently, to wit, a monk by the name of kleist, a man by the name of cuneus, and professor muschenbroeck of leyden. this was an important invention, as it was the forerunner of our own franklin's discoveries and a necessary part of his outfit with which he established the identity of lightning and electricity. every american schoolboy has heard, from fourth of july orations, how "franklin caught the forked lightning from the clouds and tamed it and made it subservient to the will of man." how my boyish soul used to be stirred to its depths by this oratorical display of electrical fireworks! franklin had long entertained the idea that the lightning of the clouds was identical with what is called frictional electricity, and he waited long for a church spire to be erected in his adopted home, the quaker city, in order that he might make the test and settle the question. but the quakers did not believe in spires, and franklin's patience had a limit. franklin had the theory that most investigators had at that time, that electricity was a fluid and that certain substances had the power to hold it. there were two theories prevalent in those days--both fluid theories. one theory was that there were two fluids, a positive and a negative. franklin held to the theory of a single fluid, and that the phenomenon of electricity was present only when the balance or natural amount of electricity was disturbed. according to this theory, a body charged with positive electricity had an excessive amount, and, of course, some other body somewhere else had less than nature had allotted to it; hence it was charged with negative electricity. a leyden jar, for instance, having one of its coatings (say the inside) charged with positive or + electricity, the other coating will be charged with negative or - electricity. the former was only a name for an amount above normal and the latter a name for a shortage or lack of the normal amount. as we have said, franklin believed in the identity of lightning and electricity, and he waited long for an opportunity to demonstrate his theory. he had the leyden jar, and now all he needed was to establish some suitable connection between a thunder-cloud and the earth. previous to franklin had written a paper in which he showed the likeness between the lightning spark and that of frictional electricity. he showed that both sparks move in crooked lines--as we see it in a storm-cloud, that both strike the highest or nearest points, that both inflame combustibles, fuse metals, render needles magnetic and destroy animal life. all this did not definitely establish their identity in the mind of franklin, and he waited long for an opportunity, and finally, finding that no one presented itself, he did what many men have had to do in other matters; he made one. in the month of june, , tired of waiting for a steeple to be erected, franklin devised a plan that was much better and probably saved the experiment from failure; for the steeple would probably not have been high enough. he constructed a kite by making a cross of light cedar rods, fastening the four ends to the four corners of a large silk handkerchief. he fixed a loop to tie the kite string to and balanced it with a tail, as boys do nowadays. he fixed a pointed wire to the upper end of one of the cross sticks for a lightning-rod, and then waited for a thunder-storm. when it came, with the help of his boy, he sent up the kite. he tied a loop of silk ribbon on the end of the string next his hand--as silk was known to be an insulator or non-conductor--and having tied a key to the string he waited the result, standing within a door to prevent the silk loop from getting wet and thus destroying its insulating qualities. the cloud had nearly passed and he feared his long waited for experiment had failed, when he noticed the loose fibers of the string standing out in every direction, and saw that they were attracted by the approach of his finger. the rain now wet the string and made a better conductor of it. soon he could draw sparks with his knuckle from the key. he charged a leyden jar with this electrical current from the thunder-cloud, and performed all the experiments with it that he had done with ordinary electricity, thus establishing the identity of the two and confirming beyond a doubt what he had long before believed was true. in after experiments franklin found that sometimes the electricity of the clouds was positive and at other times negative. from this experiment franklin conceived the idea of erecting lightning-rods to protect buildings, which are used to this day. the news spread all over europe, not through the medium of electricity, however, but as soon as sailing vessels and stage-coaches could carry it. many philosophers repeated the experiments and at least one man sacrificed his life through his interest in the new discovery. in professor richman of st. petersburg erected on his house a metal rod which terminated in a leyden jar in one of the rooms. on the st of may he was attending a meeting of the academy of sciences. he heard a roll of thunder and hurried home to watch his apparatus. he and one of the assistants were watching the apparatus when a stroke of lightning came down the rod and leaped to the professor's head. he was standing too near it and was instantly killed. passing over many names of men who followed in the wake of franklin we come to the next era-making discovery, namely, that of galvanic electricity. in the year an incident occurred in the household of one luigi galvani, an italian physician and anatomist, that led to a new and important branch of electrical science. galvani's wife was preparing some frogs for soup, and having skinned them placed them on a table near a newly charged electric machine. a scalpel was on the table and had been in contact with the machine. she accidentally touched one of the frogs to the point of the scalpel, when, lo! the frog kicked, and the kick of that dead frog changed the whole face of electrical science. she called her husband and he repeated the experiment, and also appropriated the discovery as well, and he has had the credit of it ever since, when really his wife made the discovery. galvani supposed it to be animal electricity and clung to that theory the rest of his life, making many experiments and publishing their results; but the discovery led others to solve the problem. alessandro volta, a professor of natural philosophy at pavia, italy, was, it must be said, the founder of the science of galvanic or voltaic electricity. stimulated by the discovery of galvani he attributed the action of the frog's muscles, not to animal electricity, but to some chemical action between the metals that touched it. to prove his theory, he constructed a pile made of alternate layers of zinc, copper, and a cloth or pasteboard saturated in some saline solution. by repeating these trios--copper, zinc, and the saturated cloth--he attained a pile that would give a powerful shock. it is called the voltaic pile. i have a clear idea of the construction of this form of pile, founded on experience. it was my habit when a boy to make everything that i found described, if it were possible. the bottom of my mother's wash-boiler was copper, and just the thing to make the square plates of copper to match the zinc ones, made from another piece of domestic furniture used under the stove. i shocked my mother twice--first with the voltaic pile that i had constructed, and again when she found out where the metal plates came from. the sequel to all this was--but why dwell upon a painful subject! galvanism and voltaic electricity are the same. volta was the first to construct what is termed the galvanic battery. the unit of electrical pressure or electromotive force is called the volt, and takes its name from volta, the great founder of the science of galvanic or voltaic electricity. from this pile constructed by volta innumerable forms of batteries have been devised. the evolution of the galvanic battery in all its forms, from volta to the present day, would fill a large volume if all were described. the discoveries of michael faraday ( - ), the distinguished english chemist and physicist, led to another phase of the science that has revolutionized modern life. faraday made an experiment that contains the germ of all forms of the modern dynamo, which is a machine of comparatively recent development. he found that by winding a piece of insulated wire around a piece of soft iron and bringing the two ends (of the wire) very close together, and then placing the iron across the poles of a permanent magnet and suddenly jerking it away, a spark would pass between the two ends of the wire that was wound around the piece of soft iron. here was an incipient dynamo-electric machine--the germ of that which plays such an important part in our modern civilization. having brought our history down to the present day, it would seem scarcely necessary to recite that which everybody knows. it is well, however, to call a halt once in a while and compare our present conditions of civilization with those of the past. our world is filled with croakers who are always sighing for the good old days. but we can easily imagine that if they could go back to those days their croaking would be still louder than it is. before the advent of electricity many things were impossible that are easy now. in the old days the world was very, very large; now, thanks to electricity, it is knocking at the door of every man's house. the lumbering stage-coach that was formerly our limited express--limited to thirty or forty miles a day--has been supplanted by one that covers miles in the same time, and this high rate of speed is made possible only by the use of the electric telegraph. in the old days all europe could be involved in a great war and the news of it would be weeks in reaching our shores, but now the firing of the first gun is heard at every fireside the world over, almost before the smoke has cleared away. our planet is threaded with iron nerves that run over mountains and under seas, whose trembling atoms, thrilled with the electric fire, speak to us daily and hourly of the great throbbing life of the whole civilized world. electricity has given us a voice that can be heard a thousand miles, and not only heard, but recognized. it has given us a pen that will write our autograph in new york, although we are still in chicago. it has given us the best light, both from an optical and a sanitary standpoint, that the world has ever seen. the old-fashioned, jogging horse-car has been supplanted by the electric "trolley," and we no longer have our feelings harrowed with pity for the poor old steeds that pulled those lumbering coaches through the streets, with men and women crowded in and hanging on to straps, while everybody trod on every other body's toes. "in olden times we took a car drawn by a horse, if going far, and felt that we were blest; now the conductor takes the fare and puts a broomstick in the air-- and lightning does the rest. "in other days, along the street, a glimmering lantern led the feet, when on a midnight stroll; but now we catch, when night is nigh, a piece of lightning from the sky and stick it on a pole. "time was when one must hold his ear close to a whispering voice to hear, like deaf men--nigh and nigher; but now from town to town he talks and puts his nose into a box and whispers through a wire." so jogs the old world along. we sometimes think it is slow, but when we look back a few years and see what has been accomplished it seems to have had a marvelously rapid development. something like fifty years ago a professor of physics in one of our colleges was giving his class a course in electricity. the electric telegraph was too little known at that time to cut much of a figure in the classroom, so the stock experiments were those made with the frictional electric machine and the leyden jar. one day the professor had, in one hour's time, taken his class through a course of electricity, and at the end he said: "gentlemen, you were born too late to witness the development of this great science." i often wonder if the good professor is ever allowed to part the veil that separates us from the great beyond and to look down upon this busy world of ours in which electricity plays such an important part in our every-day life; and if so, what he thinks of that little speech he made to the boys fifty years or more ago. if we make an analysis of the history of the science of electricity we shall see that it has progressed in successive eras, shortening as they approach our time. for a period of years, from thales to franklin, but little or no progress was made beyond the further development of the phenomena of frictional electricity--the most important invention being that of the leyden jar. from franklin to volta was forty-eight years, and from volta to faraday about thirty-two years. from this time on the development was very rapid as compared with the old days. soon after faraday, morse, henry, wheatstone, and others began experiments that have grown, during fifty or sixty years, into a most colossal system of electric telegraphs, telephones, electric lights and electric railroads. in the latter days marvel has succeeded marvel with such rapid strides that the ink is scarcely dry from the description of one before another crowds itself upon our attention. where it will all end no one knows, but that it has ended no one believes. the human mind has become so accustomed to these periodic revelations of the marvelous that it must have the stimulus once in a while or it suffers as the toper does when deprived of his cups. the commercial instinct of the news-vender is not slow to see the situation, and if the development is too slow to suit the public demand his fertile brain supplies the lack. so that every few days we hear of some great discovery made by some one it may be unknown to fame. it has served its purpose. the public mind has had its mental toddy and has been saved from a fit of intellectual delirium tremens that it was in danger of from lack of its accustomed stimulus. having given you a very limited outline of the history of electricity, from ancient times down to the present, we will endeavor now to give you an elementary notion of the science as it stands to-day. to the common mind the science is a blank page. so little is known of it by the ordinary reader, who is fairly intelligent in other matters, that to account for anything that we do not understand it is only necessary to say that it is an electrical phenomenon and he accepts it. electricity is a synonym for all that we cannot understand. inasmuch as magnetism is so closely related to electricity in its uses as related to every-day life, we will carry the two subjects along together, as the one will to a large extent help to explain the other. in our next chapter we will look at the history of magnetism. chapter iii. history of magnetism. it is said that the word magnetism is derived from the name of a greek shepherd, called magnes, who once observed on mount ida the attractive properties of loadstone when applied to his iron shepherd's crook. it is more likely that the name came from magnesia, a country in lydia, where it was first discovered. it was also called lapis heracleus. heraclea was the capital of magnesia. loadstone is a magnetic ore or oxide of iron found in the natural state, and has at some time by natural processes been rendered magnetic--that is, given the power of attracting iron, and, when suspended, of pointing to the north and south poles. the power of the natural magnet was known at a very early age in the history of man. it was referred to by homer, pythagoras, and aristotle. pliny also speaks of it, and refers to one dinocares, who recommended to ptolemy philadelphus to build a temple at alexandria and suspend in its vault a statue of the queen by the attractive power of "loadstones." there is also mention of a statue being suspended in like manner in the temple of serapis, alexandria. it is claimed that the chinese knew of and used the magnetic needle in the earliest times and that travelers by land employed this needle suspended by a string to guide them in their journeys across the country a thousand years before christ. notwithstanding the claims of the chinese and arabians to the discovery of the use of the magnetic needle, modern authors question whether the ancients were familiar with any artificial construction of a magnetic needle, however much they may have studied and used the loadstones. no doubt the loadstone in its natural state was used by mariners to steer their ships by, long before its artificial counterpart was invented. in a history of the discovery of iceland, by are frode, who was born in , it is stated that a mariner by name of folke gadenhalen sailed from norway in search of iceland in the year , and that he carried with him three ravens as guides, for he says, "in those times seamen had no loadstones in the northern countries." the magnetic needle as applied to the mariner's compass was known in the eleventh century, as proved by various authors. in an old french poem, the manuscript of which still exists, the mariner's compass is clearly mentioned. the author was guyot, of provence, who was alive in . like electricity, magnetism has had a long history, but little use was made of it till modern times beyond that of the mariner's compass. it can readily be seen what an important factor it was in the science of navigation. long after the discovery of the compass needle there were many perplexing problems arising, and all sorts of theories were advanced to account for the various phenomena. the variation of the needle was one of these problems. it is said that columbus was the first to discover the variation of the needle, as well as america. this is disputed, however, as every man's pretensions usually are. however this may be, columbus had to invent some plausible theory to account for this variation to prevent a mutiny among his crew. they were very superstitious and thought that they were sailing into a new world where the laws of nature were different from those of spain. one phenomenon that disturbed columbus was the dip of the needle. as we move in a northerly direction a magnetic needle dips, and it was the observation of this phenomenon in different latitudes that finally resulted in the invention of the dipping needle. it is well known that one pole of a magnetic needle points to the north and the other to the south. in other words, what is called the north pole of a needle points to one of the magnetic poles of the earth which is in the direction of the north pole, though not the same as the geographical pole. a dipping needle revolves on an axis so that it can point to any declination. if we should construct one that is perfectly balanced, so as to lie in a perfectly horizontal direction before it is magnetized, it will dip--in this latitude--downward toward the north after magnetization. if we keep moving northward it will continue to dip downward till we come to the true magnetic pole, when what is called the north pole of the needle will point directly downward. if we go back to the equator the needle will lie horizontally again. we call the end of the needle that points to the north the north pole. it is really the south pole, because unlike poles attract each other. if the magnetic poles of the earth are at the north and south geographical poles, the south pole of the needle will point north. but it is less confusing to call the end of the needle that points north the north pole. the nomenclature is purely arbitrary. it was not until it was learned that magnets could be made by electricity that they became commercially important outside of their use in navigation. the advent of electricity has brought magnetism to the front as one of the great factors in our modern civilization. and we might say with equal force that the discovery of magnetism has brought electricity to the front. the truth is that they depend upon each other. electricity would be robbed of a large part of its importance as a factor in modern life if it were not for its relation to magnetism. even electric lighting would be impossible, commercially, if it were not for the part magnetism plays in the production of electricity for this purpose. it could not be successfully carried on with any battery but the storage-battery, and the storage-battery is dependent upon the dynamo, and the dynamo is a magneto-electric machine. when we come to analyze the relation between magnetism and electricity we cannot separate them without robbing each of a large part of its usefulness. they are interdependent forces. as in the case of electricity there have been many theories regarding magnetism. one philosopher in the old days accounts for the variation of the compass-needle on the theory that there are two globes, one revolving within the other, and that any derangement of their normal movements in relation to each other affects the needle. evidently there were cranks in those days as well as now. another theory of magnetism was that there were two fluids--a boreal and an austral--one developing north polarity and the other south polarity. in the next chapter the nature of magnetism in the light of modern investigation will be discussed. chapter iv. theory and nature of magnetism. iron and steel have a peculiar property called magnetism. it is an attraction in many ways unlike the attraction of cohesion or the attraction of gravitation. it is very certain that magnetism is an inherent property of the molecules of iron and steel, and, to a small degree, other forms of matter. that is to say, the molecules are little natural magnets of themselves. it is as unnecessary to inquire why they are magnets as it is to inquire why the molecules of all ordinary substances possess the attraction of cohesion. the one is as easy to explain as the other. people of all ages have insisted upon making a greater mystery of all electrical and magnetic phenomena than they do of other natural forces. ampère's theory is that electric currents are flowing around the molecules which render them magnetic; but it is just as easy to suppose that magnetism is an inherent quality of the molecule. (the word molecule is here used as referring to the smallest particle of iron.) these little molecular magnets, so small that , million million million of them can be put into a cubic inch of space, have their attractions satisfied by forming into little molecular rings, with their unlike poles together, so that when the iron is in a natural or unmagnetized condition it does not attract other iron. if i should take a ring of hardened steel and cut it into two or more pieces and magnetize them, each one of the pieces would be an independent magnet. if now i put them together in the form of a ring they will cling together by their mutual attraction for each other. before i put them together into a ring each piece would attract and adhere to other pieces of iron or steel. but as soon as they are put together in the ring they are satisfied with their own mutual attraction, and the ring as a whole will not attract other pieces of iron. suppose the pieces forming the ring--it may be only two, if you choose--are as small as the molecules we have described, the same thing would be true of them. each molecular ring would have its magnetic attractions satisfied and would not attract other molecules outside of its own little circle. when the iron is in the neutral state it will not as a mass attract another piece of iron, because the millions of little natural magnets of which it is made up have their attractive force all turned in upon themselves. now, if we make a helix, or coil, of insulated wire and put a piece of iron into it, and pass a current of electricity through the helix, the iron becomes a magnet. why? because the electric current has the power to break up these molecular magnetic rings and turn all their like poles in one direction, so that their attractions are no longer satisfied among themselves, and with a combined effort they reach outside and attract any piece of iron that is within reach. in this state we say it is magnetized. most people think that we have put something into the iron, but we have not; we have only developed and made active its inherent power. it must be kept in mind that it takes power to develop this magnetic power from its state of neutrality and that something is never made from nothing. when this power is developed it will do work in falling back to its natural state. the power is natural to the molecules of the metal. it is only being exerted in a new direction. the millions of little natural magnets have been forced to combine their attractions into one whole and exert it on something outside of themselves. they are under a strain in this condition, like a bent bow, and there is a tendency to fly back to the natural position, and if it is soft iron and not steel, they will fly back as soon as the power that wrenched them apart and is holding them apart is taken away. this power is the electric current. now break the current, and the little natural magnets, that have been so ruthlessly torn from their home circle attachments, fly back to them again with the speed of lightning, and the iron rod as a whole is no longer a magnet. the power to become so under the electrical strain is in it still--only latent. the kind of magnet that we have been describing is called an electromagnet. it is a magnet only so long as the electric current is passing around it. there is another kind of magnet called a permanent magnet that will remain a magnet after the current is taken away. the permanent magnet is made of steel and hardened; then its poles are placed, to the poles of a powerful magnet, either electro or permanent, when its molecular rings are wrenched apart and arranged in a polarized position as heretofore described. now take it away from the magnet and it will be found to retain its magnetism. the molecules tend to fly back the same as those of the soft iron, but they cannot because hardened steel is so much finer grained than soft iron, and the molecules are so close together that they are held in position by a friction that is called its coercive force. the soft iron is comparatively free from this coercive force, because its molecules are free to move on each other, so that when they are wrenched out of their natural position they fly back by their own attractions as soon as the force holding them apart is taken away. the molecules of hardened steel are unable to fly back, although they tend to do it just as much as in the iron, and so it is called a permanent magnet. its molecules also are under a strain, like a bent bow. (the form of such a magnet is usually that of a horse-shoe, or u.) let us use a homely illustration that may help us to understand. let ten boys represent the molecules in a piece of iron. let them pair off into five pairs and each one clasp his mate in his arms; each one, say, is exerting a force of ten pounds, and it would require a force of twenty pounds to pull any one of the pairs apart. the five pairs are exerting a force of one hundred pounds, but this force is not felt outside of themselves. now let them unclasp themselves and take hold of a rope that is tied to a post, and all pull with the same force that they were using, to wit, ten pounds each, and all pull in the same direction, and they would put a strain of one hundred pounds upon the post, the same power that they were exerting upon themselves before they combined their efforts on something outside of themselves. so with the magnet. so long as the force of each molecule is wholly spent upon its neighbor there is nothing left for exterior use. but as soon as they all line up and pull conjointly in the same direction their combined force is felt outside. the analogy may not be perfect, but it will help you to get a mental picture of what takes place in iron when it is magnetized. we have now described the magnet and the inherent power residing in the molecular structure of iron. it is this magic power slumbering in its molecules and the ability of the electric current to arouse them to action at will and to hold them in action and at will let them fly back to their normal position, that gives to electricity and magnetism--twin sisters in nature's household--their great value as the servants of man. there would be no virtue in winding up a weight if it could not run down and do work in its fall. simply bending a bow would never send the arrow flying over its course; it must be released as well. the magnet could not accomplish the great work it does if we could only charge it and not have the ability to discharge it. without this ability the electric motor would not revolve, the electric light would not burn, the click of the telegraph would not be heard, the telephone would not talk, nor would the telautograph write. i have said that the permanent magnet would hold its charge after once having been magnetized. this is true only in a sense and under favorable conditions. if made of the best of steel for the purpose and hardened and tempered in just the right way, it will hold its charge if it is given something to do. if a piece of iron is placed across its poles it also becomes a magnet and its molecules turn and work in harmony with those of the mother magnet. these magnetic lines of force reach around in a circuit. even before the iron, or "keeper," as it is called, is put across its poles there are lines of force reaching around through the air or ether from one pole to another. (for a description of ether see chap. v.) this is called the "field" of the magnet, and when the iron is placed in this field the lines of force pass through it in a closed circuit, and if the "keeper" is large enough to take care of all the lines of force in the field the magnet will not attract other bodies, because its attraction is satisfied, like its prototype in the molecular ring described above. we speak of lines of force, not that force is necessarily exerted in a bundle of lines but as a convenient way of telling the strength of a magnetic field. the practical limit of the magnetization of soft iron (called saturation) is , lines to the square centimeter. as long as we give our magnet something to do, up to the measure of its capacity, it will keep up its power. we may make other magnets with it, thousands, yea, millions of them, and it not only does not lose its power but may be even stronger for having done this work. if, however, we hang it up without its "keeper," and give it nothing to do, it gradually returns to its natural condition in the home circle of molecular rings. little by little the coercive force is overcome by the constant tendency of the molecule to go back to its natural position among its fellows. the magnet furnishes many beautiful lessons, as indeed do all the natural phenomena. every man has within him a latent power that needs only to be aroused and directed in the right way to make his influence felt upon his fellows. like the magnet, the man who uses his power to help his fellows up to the measure of his limitations not only has been a benefactor to his race, but is himself a stronger and better man for having done so. but, again, like the magnet, if he allows these god-given powers to lie still and rust for want of legitimate use he gradually loses the power he had and becomes simply a moving thing without influence or use in a world in which he vegetates. but let us leave philosophy and go back to science. one of the striking exhibitions of magnetism is found in the earth. the earth itself is a great magnet; and there is good reason for believing that it is an electromagnet of great power. the magnetic poles of the earth are not exactly coincident with the geographical poles, and they are not constant. there is a gradual deviation going on, but as it follows a certain law mariners are able to tell just what the deviation should be at a certain time. the magnetic pole revolves around the polar axis of the earth once in about years. a thermal current (one produced by heat) of electricity seems to flow around the earth caused by the irregularities of temperature at the earth's surface, as the sun makes his daily round. these earth currents vary at times, and other phenomena are the occasion. this will be discussed when we come to electric storms. the value of the earth's magnetism is seen most in the science of navigation. a magnetic needle is only a slender permanent magnet suspended very delicately, and when not under local influence it points north and south on the magnetic axis. the law of its action may be explained as follows: take a straight bar magnet of fairly good power and suspend a magnetic needle over it. the needle will arrange itself parallel to the bar magnet. the north pole of the needle will point toward the south pole of the bar magnet. in the presence of the magnet the needle is not affected by the earth, but yields to a superior force. if, however, the bar magnet is taken out of the way of the needle it will immediately arrange itself north and south. of course if the earth's magnetic axis changes the needle will vary with it. this variation is uniform and in navigation is reduced to a science, so that the mariner knows how much to allow for the variation. columbus, as heretofore mentioned, was supposed to have first noticed this variation and it made him trouble. he did not know how to account for it, and as his crew thought the laws of nature were changing because they were so far from home he saw the necessity for some sort of explanation. so, like the brave man that he was, he hatched up a theory that satisfied the crew, and although in the light of the closing years of the nineteenth century it was a questionable one, it worked well enough in practice to serve his purpose. we have already stated that the earth was a great magnet, and that probably it was an electromagnet, caused by earth currents circulating around the globe. you want to know how the earth can be a magnet unless it has an iron core like an electromagnet. magnetism or magnetic lines of force may be developed without the presence of iron. when we pass a current of electricity through a wire, magnetic lines of force are thrown out at right angles with the direction of the current. this will be fully explained further on. if we wind the wire into a coil, or helix, these magnetic lines are concentrated. if now we suspend this helix, or, better, float it on water so that it can move freely, and pass a current of electricity through it, the helix will arrange itself north and south the same as a magnetic needle. its attractive properties are feeble in comparison with that of the iron, but it obeys the laws of a magnet. the earth is probably a magnet of this kind, consisting mostly of lines of force. however, the iron in the earth is affected magnetically, as we have evidence in the loadstone. the earth has the power also to magnetize iron through the medium of its magnetic field, that reaches out in lines of force from pole to pole like those of the artificial magnet. if we hold a bar of iron in line with the magnetic axis of the earth and dip it in line with the dipping needle and then strike it a few blows on the end, it will be found to be feebly magnetic. the blows have partly loosened the molecules and during the moment that they unclasped themselves the earth's magnetism has through its lines of force caught them for a time and held them a little out of their natural position--as they are in a state of rest. the peculiar changing light that we sometimes see in the northern sky, that is called the aurora borealis (northern light), is indirectly due to intense magnetic lines of force that radiate from the north magnetic pole of the earth. those lines of force are able to cause the rarified air molecules to become feebly incandescent, giving them the appearance that we see in a tube that is a partial vacuum when electricity is passed through it. while these auroral displays may be seen almost any night in the far north, they vary greatly in their intensity, so it is only once in a while that they are visible in the temperate latitudes. what are called magnetic storms occur occasionally, and at such times the telegraph service will sometimes be paralyzed on all the east and west lines for many hours. strong earth-currents will flow east and west, and be so powerful and so erratic that it is sometimes impossible to use the telegraph. it sometimes happens that the operators can throw off their batteries and work on the earth-current alone. sometimes it is necessary to make a complete metallic circuit to get away from the influence of the earth in order to use the telegraph. currents equal to the force of , cells of ordinary battery have been developed sometimes in telegraph wires. this of course is a mere fraction of what is passing through the earth under the wire through which the current flowed. on the th and th of november, , a magnetic storm occurred that extended around the globe, as it was felt wherever there were telegraph wires. these magnetic storms are attended by brilliant displays of the aurora, and this fact strengthens the theory that the earth is a great electromagnet; for the stronger the electrical current the more powerful we should expect the magnetism to be, and this is shown by the action of the magnetic needle at such times. the stronger the magnet the more intense will be the lines of force, and naturally the more intense the light, if indeed these lines of force are the cause of the light. there is evidently some close relation between the two. another coincidence is that at the times of these storms there is an unusual display of sun-spots. these sun-spots seem to be great holes that have been blown through the photosphere of the sun. the photosphere is a great luminous body of gaseous matter that is believed to envelop the sun, so that we do not see the core of the sun unless it is when we look into one of these spots. in some way, evidently, the sun affects the earth by radiating magnetic lines of force which are cut by the earth's revolution, and so creating currents of electricity. the sun is the field-magnet, and the earth is the revolving armature of nature's great dynamo-electric machine. it would seem that the radiant energy that comes out through these spots or these holes in the sun's envelope, are more potent to develop earth-currents than the ordinary rays; and so, when for a brief while in the revolution of the earth about the sun, these extra potent rays strike the earth, an unusual energy is developed, and these unusual phenomena are the consequence. these phenomena seem to occur periodically; some years (about eleven) intervening. all the forces and phenomena of nature are thus seen to be in a state of unrest. and it is to this unrest, which does not stop with visible things, but pervades even the atoms of matter throughout the universe, that we are indebted for the ability to carry on all the activities of life, and for life itself. for universal quiet would mean universal death. the cyclone and tornado that devastate and strike terror to a whole region are only eccentricities of nature when she is setting her house to rights. the play of natural forces has disturbed her equilibrium, and she is but making an effort to restore it. chapter v. theory of electricity. in the series of chapters on heat (vol. ii) and in the chapter on magnetism the word molecule was frequently used synonymously with atom. in chemistry a distinction is made, and as we can better explain the theory, at least, of electricity by keeping this distinction in mind we will refer to it here. it has been stated that there are between sixty and seventy elementary substances. an elementary substance cannot be destroyed as such. it can be united with other elements and form chemical compounds of almost endless variety. the smallest particle of an elementary substance is called in chemistry an atom. the smallest particle of a compound substance is called a molecule. the atom is the unit of the element, and the molecule is the unit of the compound as such. it follows, then, that there are as many different kinds of atoms as there are elements, and as many different kinds of molecules as there are compounds. if the elements have a molecular structure then two or more atoms of the same kind must combine to make a molecule of an elementary substance. two atoms of hydrogen combine with one of oxygen to form one molecule of water. it cannot exist as water in any smaller quantity. if we subdivide it, it no longer exists as water, but as the original gases from which it was compounded. we have shown in the series on sound, heat and light that they are all modes of motion. sound is transmitted in longitudinal waves through air and other material substance as vibration. heat is a motion of the ultimate particles or atoms of matter, and light is a motion of the luminiferous ether transmitted in waves that are transverse. electricity is also undoubtedly a mode of motion related in a peculiar way to the atoms of the conductor. notice that there is a difference between conduction and radiation. the former transmits energy by a transference of motion from atom to atom or molecule to molecule within the body, while the latter does it by a vibration of the ether outside--as light, radiant heat, and electromagnetic lines of force. for the benefit of those persons who have not read vol. ii, where the nature of ether is discussed somewhat, let us refer to it here, as it plays an important part in the explanation of electrical phenomena. ether is a tenuous and highly elastic substance that fills all interstellar and interatomic space. it has few of the qualities of ordinary matter. it is continuous and has no molecular structure. it offers no perceptible resistance, and the closest-grained substances of ordinary matter are more open to the ether than a coarse sieve is to the finest flour. it fills all space, and, like eternity, it has no limits. some physicists suppose--and there is much plausibility in the supposition--that the ether is the one substance out of which all forms of matter come. that the atoms of matter are vortices or little whirlpools in the ether; and that rigidity and other qualities of matter all arise in the ether from different degrees or kinds of motion. electricity is not a fluid, or any form of material substance, but a form of energy. energy is expressed in different ways, and, while as energy it is one and the same, we call it by different names--as heat energy, chemical energy, electrical energy, and so on. they will all do work, and in that respect are alike. one difficulty in explaining electrical phenomena is the nomenclature that the science is loaded down with. all the old names were adopted when electricity was regarded as a fluid, hence the word "current." it is spoken of as "flowing" when it does not flow any more than light flows. if a man wants to write a treatise on electricity--outside of the mere phenomena and applications--and wants to make a large book of it, he would better tell what he does not know about it, for in that way he can make a volume of almost any size. but if he wants to tell what it really is, and what he really knows it is, a primer will be large enough. this much we know--that it is one of many expressions of energy. chemistry teaches that heat is directly related to the atoms of matter. atoms of different substances differ greatly in weight. for instance, the hydrogen atom is the unit of atomic weight, because it is the lightest of all of them. taking the hydrogen atom as the unit, in round numbers the iron atom weighs as much as atoms of hydrogen, copper a little over , silver , gold . heat acts upon matter according to the number of atoms in a given space, and not as its weight. knowing the relative weights of the atoms of the different metals named, it would be possible to determine by weight the dimensions of different pieces of metal so that they will contain an equal number of atoms. if we take pieces of iron, copper, silver and gold, each of such weight as that all the pieces will contain the same number of atoms, and subject them to heat till all are raised to the same temperature, it will be found that they have all absorbed practically the same quantity of heat without regard to the different weights of matter. it will be observed that the piece of silver, for instance, will have to weigh nearly twice as much as the iron in order to contain the same number of atoms, but it will absorb the same amount of heat as the piece of iron containing the same number of atoms, if both are raised to the same temperature. in view of the above fact it seems that heat acts especially upon the atoms of matter and is a peculiar form of atomic motion. heat is one kind of motion of the atoms, while electricity may be another form of motion of the same. the two motions may be carried on together. the earth has a compound motion. it revolves upon its axis once in twenty-four hours, and it also revolves around the sun once each year. so you see that there are different kinds of motion that may be communicated to the same body--all producing different results. the motion of the individual atom as heat may be, and is, as rapid as light itself when the temperature is sufficiently high, but it does not travel along a conductor rapidly as the electro-atomic motion will. if we apply heat to the end of a metal rod it will travel slowly along the rod. but if we make the rod a conductor of electricity it travels from atom to atom with a speed nearer that of the light ray through the ether. some modern writers have attempted to explain all the phenomena of electricity as having their origin in a certain play of forces upon the ether, and there is no doubt but that the ether plays an important part in all electrical phenomena as a medium through which energy is transferred; but ether-waves that are set in motion by the electrical excitation of ordinary matter are no more electricity than the ether-waves set up by the sun in the cold regions of space are heat. they become heat only when they strike matter. heat, _as such_, begins and ends in matter;--so (i believe) does electricity. do not be discouraged with these feeble attempts to explain the theory of electricity. all i even hope to do is to establish in your minds this fundamental thought, to wit, that there is really but one energy, and that it is always expressed by some form of motion or the ability to create motion. motions differ, and hence are called by different names. if i should set an emery-wheel to revolving and hold a piece of steel against it the piece of steel would become heated and incandescent particles would fly off, making a brilliant display of fireworks. the heat that has been developed is the measure of the mechanical energy that i have used against the emery-wheel. now, let us substitute for the emery-wheel another wheel of the same size made of vulcanized rubber, glass or resin. i set it to revolving at the same speed, and instead of the piece of steel, i now hold a silk handkerchief or a catskin against the wheel with the same force that i did the steel. if now i provide a leyden jar and some points to gather up the electricity that will be produced (instead of the heat generated in the other case), it would be found that the energy developed in the one case would exactly balance that of the other, if it were all gathered up and put into work. the electricity stored in the jar is in a state of strain, like a bent bow, and will recoil, when it has a chance, with a power commensurate with the time it has been storing and the amount of energy used in pressing against the wheel. if now i connect my two hands, one with the inside and the other with the outside of the jar, this stored energy will strike me with a force equal to all the energy i have previously expended in pressing against the wheel, minus the loss in heat. if i did it for a long enough time this electrical spring would be wound up to such a tension that the recoil would destroy life if one put himself in the path of its discharge. if all the heat in the first case were gathered up and made to bend a stiff spring, and one should put himself in its way when released, this mechanical spring would strike with the same power that the electrical spring did when the leyden jar was discharged. this statement assumes that all the energy in the second experiment was stored as electricity in the jar. you will be able to see from the above illustration that heat, electrical energy, and mechanical energy are really the same. then you ask, how do they differ? simply in their phenomena--their outward manifestations. while there is much that we cannot know about any of the phenomena of nature, it is a great step in advance if we can establish a close relationship between them. it helps to free electricity from many vagaries that exist in the minds of most people regarding it; vagaries that in ignorant minds amount to superstition. while it possesses wonderful powers, they give it attributes that it does not possess. not long ago a favorite headline of the medical electrician's advertisement was "electricity is life," and it was a common thing to see street-venders dealing out this "life" in shocking quantities to the innocent multitudes--ten cents' worth in as many seconds. science divides electricity into two kinds--static and dynamic. static comes from a greek word, meaning to stand, and refers to electricity as a stationary charge. dynamic is from the greek word meaning power, and refers to electricity in motion. when franklin made his celebrated kite experiment, the electricity came down the string, and from the key on the end of the string he stored it in a leyden jar. while the electricity was moving down the string it was dynamic, but as soon as it was stored in the leyden jar it became static. current electricity is dynamic. a closed telegraphic circuit is charged dynamically, while the prime conductor of a frictional electric machine is charged statically. the distinction is arbitrary and in a sense a misnomer. when we rub a piece of hard rubber with a catskin it is statically charged because the substances are what are called non-conductors, and the charge cannot be conducted readily away. all substances are divided into two classes, to wit, conductors or non-electrics, and non-conductors or electrics, more commonly called dielectrics. these, however, are relative terms, as no substance is either a perfect conductor or a perfect non-conductor. the metals, beginning with silver as the best, are conductors. ebonite, paraffine, shellac, etc., are insulators, or very poor conductors. the best conductors offer some resistance to the passage of the current and the best insulators conduct to some extent. if we make a comparison of electric conductors we find that the metals that conduct heat best also conduct electricity best. this, it seems to me, is a confirmation of the atomic theory of electricity so far as it means anything. if a good conductor, as silver, is subjected to intense cold by putting it into liquid air, its conductivity is greatly increased. it is well known that heating a conductor ordinarily diminishes its power to conduct electricity. this shows that, in order that electrical motion of the atom may have free play, the heat motion must be suppressed. chapter vi. electric currents. the simplest form of an electric machine is one in which the operator is a prominent part of the operation. electricity, like magnetism, operates in a closed circuit, even when it is static--so-called. take a stick of sealing-wax, say, in your left hand, and rub it with a piece of fur or silk with your right hand, and you have the simplest form of electric machine--the one that was known to the ancients, and the one from which the science, great as it is to-day, had its beginnings. the stick of sealing-wax is one element of the battery, and the piece of fur or silk is the other, while your hands, arm and body form the conductor that connects the two poles, and the friction is the exciting agent and may be said to take the place of the fluid of a battery. the electrical conditions are not wholly static, as a slow current is passing around through your arms and body from one pole to the other. even if the conditions were wholly static there would be polarized lines of force, in a state of strain, reaching around in a closed circuit. if we rub the wax with the fur and then take it away the wax has a charge of electricity and will attract light objects. if we had rubbed a piece of metal or some good conductor it would have been warmed instead of electrified. in both cases the particles of the substances have been affected, and if the atomic theory is correct--and it seems plausible--in the former case the atoms are partly put into electrical motion and partly into a state of electrical strain that we call static (standing) electricity; while in the latter case the atoms are put into the peculiar motion that belongs to heat. the former we call electricity, and the latter we call heat. the electro-atomic motion under some circumstances readily turns to heat, which seems to be the tendency of all forms of energy. the electric light is a result of this tendency. all non-conductors, or electrics, have a complex molecular structure, and, while their atoms when subjected to friction are put into a state of electrostatic strain, they are not able readily to respond as a conductor of dynamic electricity. the electric-light filament in the incandescent lamp is a much poorer conductor than the copper wire that leads up to it. the copper wire is readily responsive to the electrical influence, but the carbon filament is not. so electrical action that freely passes along the wire, is resisted and becomes heat action in the filament, and light is the attendant of intense heat. but, to go back to the sources of electricity. frictional electric machines have been constructed in great variety. all, however, embrace the essentials set forth in the sealing-wax experiment, and would be difficult to describe without cuts. let us, therefore, consider another source of electricity, which was the outgrowth of the discovery of galvani (or rather his wife), and reduced to concrete form by volta. we refer to the galvanic or voltaic battery. if we put a bar of zinc into a glass vessel and pour sulphuric acid and water into it, there will be a boiling, and an evolution of hydrogen gas, and energy is released in the form of heat, so that the fluid and the glass vessel become heated. now let us put a bar of copper or a stick of carbon into the glass, but not in contact with the zinc; connect the ends (that are not immersed) of the two elements--copper and zinc--with a metal wire or any conductor, and a new condition is set up. heat is no longer evolved to the same extent, but most of the energy becomes electrical in character, and an electrical chain of action takes place in the circuit that has now been formed. taking the zinc as the starting point, the so-called current flows from the zinc through the fluid to the copper and from the copper through the wire to the zinc. a chain of polarized atomic activity is established in the circuit, similar to the closed circuit of magnetic lines of force, only the latter is static, while the former is dynamic. you ask what is the difference? well, it is much easier to ask a question than it is to answer it. you will remember that in the chapter on magnetism it was stated that the molecules of a magnet were little natural magnets, and that their attractions were satisfied within themselves; that when their local attachments were broken up and all their like poles turned in one direction they could act upon other pieces of iron outside of the magnet. outside and between the poles there are magnetic lines of force reaching out from one pole to the other. if we put a piece of iron across the poles these lines of force are gathered up and pass through the iron. this is purely a static condition. let us go back to the cell of battery. when the elements are in position (the copper, the acidulated water and the zinc), and the two wires attached to the two metals which are the two poles of the battery not yet connected, there is a condition induced in these two wires that did not exist before the acidulated water was poured in, although the circuit is not yet established. if we test the two wires we find a difference of potential--a state of strain, so to speak--that did not exist before the acid acted on the zinc and liberated what was stored energy. it is in a static condition, like the magnet, and electrical lines of force are reaching out from both wires so that the ether is in a state of strain between the two poles. the air molecules may partake of it, but we have to bring in the ether as a substance, because the same conditions would practically exist if the two wires were in a vacuum. if now we connect the two wires, we have established a metallic circuit between the two poles of the battery, the static conditions are relieved, the lines of force are gathered up into the wire, and the phenomenon that we call a current is established and we have dynamic or moving electricity. having established the so-called electric current we will now try to show you that there really is no current. the idea of a current involves the idea of a fluid substance flowing from one point to another. when you were a boy did you never set up a row of bricks on their ends, just far enough apart so that if you pushed one over they all fell one after another? now, imagine rows of molecules or atoms, and in your imagination they may be arranged like the bricks, so that they are affected one by the other successively with a rapidity that is akin to that of light-waves, and you can conceive how a motion may be communicated from end to end of a wire hundreds of miles in length in a small fraction of a second, and no material substance has been carried through the wire--only energy. we do not mean to say that the row of bricks illustrates the exact mode of molecular or atomic motion that takes place in a conductor. what we mean is, that in some way motion is passed along from atom to atom. to give you a better conception of an electric current, let us go back of the galvanic cell to the electric machine. if both poles of the machine are attached to rods terminating in round knobs we can set the machine in action and keep up a steady stream of disruptive discharges that will, if their frequency is great enough, perform the function of a current, and we have dynamic electricity from a statical machine; when the acid of the galvanic battery breaks down a molecule of zinc, energy is set free, and in the battery we have what corresponds to a disruptive discharge of infinitesimal proportions. this discharge would have been immediately converted into heat energy if the copper element had been left out of the battery, but as it is, it impresses itself on the atomic "brick" next to it, which establishes a chain of atomic movement throughout the circuit. this may constitute, if you please, a line of electrical force. but as thousands of these disruptive discharges are taking place simultaneously as many different lines of force are established. you must not conceive of these chains of atoms as simply thrown down like the bricks and left lying there, but that the atom is active; that it has the power to pick itself up again in an infinitesimally short time and is again knocked down (following the illustration of the bricks) by the next discharge along its line or chain of atoms. if you could get a mental picture of this action you would see that the whole conductor is in a most violent state of atomic motion of a peculiar kind. at the same time a part of this electrical motion is being converted into a heat motion of the atoms, and finally it all returns to heat unless some of it is stored up somewhere as potential energy. if the current has driven a motor that has wound up a weight, a part is stored up in the weight, which has the ability to do work if it is allowed to run down. if it drives machinery as it runs down, the mechanical motion is the expression of the stored energy. when the weight has run down the energy will be represented by the heat created by friction of the journals of the wheels and pulleys and the heating of the air. if the weight is allowed to fall suddenly it will heat the air to some extent, but mostly the earth and the weight itself will be heated. if the source of energy (the battery) is great and the pressure high and the conductor is too small to carry the energy developed in the battery as electricity, heat is developed, and if the heat is sufficiently intense, light also. we have seen (vol. ii) that heat motion when it reaches a sufficiently high rate throws the ether into a vibratory motion that we call light. however, this vibratory motion of the ether is set up long before it reaches the luminous stage; in other words, there are dark rays of the ether. we find that the electro-atomic motions of a conductor have the power to impress themselves upon the ether. [illustration: fig. . a is the primary line; _a_, the battery: _b_, the key. b is the secondary line in which is placed the galvanometer _c_.] let us try another experiment to show that this is the case, not only, but that the impressed ether can transfer these impressions to still another conductor. suppose we stretch two parallel wires for, say, half a mile, or any distance, only a few feet apart, and make of each a complete circuit by rounding the end of the course and returning the wire to the starting point (as shown in fig. ). put in one of these circuits a battery, and a circuit-breaker (a common telegraph-key), and in the other circuit a galvanometer (an instrument for detecting the presence and measuring the intensity of a galvanic current, by means of a dial and a deflecting needle or pointer). now if we touch the key and close the circuit in a, the needle of the galvanometer in b will swing in one direction from zero on the dial; and if we release the key, breaking the circuit in a, the needle will swing back in the opposite direction. in neither case will the needle stay deflected, but will at once return to zero. this shows that when the battery current was allowed to complete its circuit through wire a by closing its key, an electrical action was instantly felt in wire b, although there was no material connection between them other than the air, which is a non-conductor. the current in the second circuit is called an induced current. why this current? according to one theory, when we close the primary circuit the surrounding ether is thrown into a peculiar state of strain that we will call magnetic or electrical lines of force. when the ether wave strikes the second wire there is a molecular movement from a state of rest to a state of static strain. during the time that the molecules are moving from the normal to the strained position in sympathy with the ether we have the condition of a dynamic current, which lasts only a moment. this state of strain continues till the circuit is opened (breaking the wire-line), when all the electrical lines of force vanish and the molecular strain of the second wire is relieved, and we again have the conditions, momentarily, for a current of the opposite polarity, and the needle will swing in the opposite direction because the molecules or atoms have, in their recoil to the natural state, moved in an opposite direction. going back to fig. , let us further study the phenomena under other conditions. in our first circuit (a) there is a battery and a circuit-breaker, which is a common telegraph-key. now close the key so that a current will be established. (remember that "current" is only a name for a condition of dynamic charge.) place a piece of soft iron across the wire at right angles with the direction of the wire, when of course it will be at right angles with the direction of the current, and you will find now that the iron is more or less magnetic, depending upon the amount of current passing through the wire. if we wind a number of turns of insulated wire through which the current is passing around the iron the magnetism will be increased. in practice there are a certain number of turns and a certain sized wire that will give the best results with a given number of cells of battery (or a given voltage or pressure), operating in a closed circuit of a given resistance. all these questions are worked out mathematically in many standard books on the subject. it is not the intention in these talks to develop the science mathematically but to set out the fundamental physical facts and applications of electricity. under the conditions above named magnetism is developed in the soft iron bar. if we open the key the current will cease and the magnetism will vanish--that is to say, the molecules will turn back to their neutral position by their own attractions, as has been described in a previous chapter. magnetism developed in this way is called electromagnetism. (see chap. iv.) if we use a piece of hardened steel instead of the soft iron it will become magnetic and remain so when the circuit is opened, because the natural tendency of the molecules to turn back to the neutral position is not great enough to overcome the coercive force, or molecular friction, of hardened steel, as has been also described in a previous chapter. to make the best electromagnet we need qualities of iron just the opposite from those of the permanent magnet. for the former we need the purest of soft iron, well annealed (heated to redness and slowly cooled, making it less brittle), so that its molecules are free to turn; while for the latter we need hardened steel, so that when the molecules are once wrenched into the magnetic condition they cannot, of themselves, turn back to the neutral state. the great value of the electromagnet lies in its ability to readily discharge, or go back to the neutral state, when the current is broken. let us now go back to the beginning of our experiment. when we closed the key and established the current through the wire we found that a piece of iron held at right angles to the wire, although not touching it, became magnetic. we have already said that when the circuit was open, the battery being in circuit, there were electrical lines of force established in the ether, between the two poles of the battery, and that they were gathered up into the conducting wire when the circuit was closed. we now find that there are other lines of force of a different nature established in the ether when the circuit is closed. these we call magnetic lines of force, or the magnetic field of the charged wire, and they are established at right angles to the direction of the current. these magnetic lines of force acting through the ether from an electrically charged conductor are able to break up the natural molecular magnetic rings, referred to in chapter iv, and turn all their like poles in the same direction--thus making one compound magnet of the iron which in the neutral state consisted of millions of little natural magnets whose attractions were satisfied by a joining of their unlike poles. most writers account for all of the phenomena of induced currents in a second wire as coming directly from these magnetic lines of force developed upon closing the circuit. so much for theory based upon a set of facts that make the theory seem probable. if you don't like it give us a better one. if it is correct the writer claims no credit; it is merely a compilation of suggestions from many sources, including his own experience. we are simply seeking after truth. the man who is an earnest seeker after scientific truth cannot afford to pursue his investigations with any prejudice in favor of one theory more than another, unless the facts sustain him, and then he is not acting from prejudice, but is led by the facts. many people make pets of their theories; and they become attached to them as they do their children; and they look upon a man who destroys them by a presentation of the facts as an enemy. i once knew a lady who became so attached to her family doctor that, she said, she would rather die under his treatment, if necessary, than to be cured by any other doctor. there are many people who are imbued with this kind of spirit not only in matters scientific, but in matters religious as well. such people are not the kind who contribute to the world's progress, but are the hindrances that have to be overcome. chapter vii. electric generators. of the sources of electricity we have mentioned two: friction, and galvanism or chemical action. there are hundreds of forms of the latter species of apparatus for generating electrical energy, so we will mention only a few of the more prominent ones. it is not our intention to go into the chemistry of batteries. there are too many exhaustive works on this subject lying on the shelves of libraries that are accessible to all. all galvanic batteries act on one general principle--the generation of electricity by the chemical action of acid on metal plates; but the chemistry of their action is very different. in all batteries the potential energy of one element is greater than the other. the acid of the battery dissolves the element of greater potentiality, and its energy is freed and under right conditions takes on the form of electricity. the potential of zinc, for instance, is greater than that of copper, and the measure of the difference is called the "electromotive force," the unit of which is the "volt." electromotive force is another name for pressure; the symbol for which is _e.m.f._ if we were to put two zinc plates in the battery fluid and connect them in the ordinary way there would be no electricity evolved (assuming that they were perfectly homogeneous), because they are both of the same potential, or have the same possible amount of stored electrical energy measured by its working power. if one of the zinc plates were softer than the other, a feeble current would be developed, for one would be more readily acted upon by the acids than the other. the battery that has been most used in america for telegraphic purposes is called the gravity-battery. it is constructed by putting a copper plate in some form at the bottom of a jar, usually of glass, and filling it partly full of the crystals of sulphate of copper, commonly called "bluestone." zinc, usually cast in some open form, so as to expose a large surface to the solution, is suspended in the upper part of the jar, which is then filled with water till it covers the zinc. the zinc is the positive metal, but it is called the negative pole. the energy developed by the zinc passes from zinc to copper and out on the circuit from the copper pole. hence the copper came to be called the positive pole, although in relation to zinc it is negative. copper would, however, be positive to some other metal whose potential was less. so you see that metals are relative, not absolute, in their character as positive and negative elements. the galvanic battery has been almost entirely superseded in this country for telegraphic purposes by the dynamo, a machine developing electrical currents by mechanical power. another form of battery that is extensively used for some kinds of heavy current work is called the storage-battery. the man who did the most, perhaps, to bring the storage-battery to its present state of perfection was planté, a frenchman, who died only a short time ago. although very many types of battery have been developed, it is found that, after all, the lines on which he developed it make the most efficient battery. there is a common notion that electricity is stored in the storage-battery. energy is stored, that will produce electricity when it is set free, just the same as energy is stored in zinc. the storage-battery, when ready for action, is one form of acid or primary battery. it has been made by passing a current of electricity through it until the chemical relations of the two lead plates have been changed so that the potential of one is greater than that of the other. a simple storage-battery element is made up of two plates of lead held out of contact with each other by some insulating substance the same as the elements of an ordinary battery. the cell is filled with dilute sulphuric acid, and there will be no electrical action till the cell has been charged by running a current of electricity through it and forming a lead oxide on one plate. now, take off the charging battery and connect the two poles, and electricity will flow until the oxide has partly changed back into spongy metallic lead, when it must be renewed by recharging. i remember perfectly well the first galvanic battery i ever saw, for it was of my own construction. it is now nearly fifty years ago, and yet it seems but yesterday--such is the flight of time. i related to you in another chapter how i made a voltaic battery--or pile, as it was called--by cutting up my mother's boiler and her stove-zinc, and the domestic incident that followed. well, a little later i made a real galvanic battery as follows: i lived in the country and far from town or city, and my facilities were extremely limited, so that i pursued my scientific investigations under great difficulties. my only text-book was an old comstock's philosophy. in the book was a crude cut of a morse register and a short description of its construction, including the battery. i determined to make a register, and i did. it was all constructed of wood except the magnet and its armature and the embossing-point, which latter was made of the end of a nail. the thing that seemed out of reach was the electromagnet. i had no money; and there was no one that believed i could do it, and if i could "what good would come of it?" i made friends with a blacksmith by keeping flies off a horse while he nailed the shoes on, and "blowing the bellows" and occasionally using the "sledge" for him. when i thought the obligation had accumulated a sufficient "voltage" (to express it electrically) i communicated to the blacksmith the situation and what i wanted. the good-natured old fellow was not long in bending up a u magnet of soft iron and forging out an armature. the next step was to wind the u with insulated wire. the only thing that i had ever seen of the kind was an iron wire called "bonnet" wire that was wrapped with cotton thread. this, however, was not available, so i captured a piece of brass bell-wire and wound strips of cotton cloth around it for insulation--and in that way completed the magnet. now everything was ready but the battery. i went at its construction with a feeling almost akin to awe, for i could not believe that it would do as described in the book. i procured a candy-jar from the grocer and found some pieces of sheet zinc and copper. these i rolled together into loose spirals and placed one inside the other so that they would not touch, when i was ready for the solution. the druggist trusted me for a half pound of "blue vitriol," and i put it into my battery and filled it with water. i waited awhile for it to dissolve, and then connected my magnet in circuit, when--to my astonishment and delight--it would lift a pound or more. it was a great triumph. i never have had one since that gave me the same satisfaction. but i had my triumph all to myself. i was still the same "tinker" (a name i had long carried), and a nuisance to be endured but not encouraged. the dynamo is the form of generator now in general use where heavy currents of electricity are needed. it is aptly described by a writer in modern machinery, mr. john a. grier, as a thing that when "at rest is a lifeless piece of mechanism; in action it has a living spirit as full of mystery as the soul of man." this is a poetic way of describing it that conveys to the mind a sense of the power and beauty of natural law in action, that would not come from a mere recital of the cold scientific facts. the facts, however, are necessary: but let us draw from them all the poetry and all the practical lessons that we can as we go along; for it is this blending of the poetic with the practical that lends a charm to our every-day "grind," and lightens the load of many a weary hour. the dynamo is a machine that converts mechanical into electrical energy, and the great practical value of energy in this form is that it can be distributed through a conductor economically for many miles. we can transmit mechanical power by means of a rope or cable for a limited distance, but at tremendous loss through friction. we can transmit power through pipes by compressed air or steam, but there is a great loss, especially in the case of steam, by condensation from cold. none of these methods are available for long distances. another advantage electricity has over other forms of energy is the speed with which it can be transmitted from one place to another. in this respect it has no rival except light. but we have not been able to harness light and make it available to carry either freight or news, except in the latter case for a short distance by flashing it in agreed signals. the heliostat can be used when the sun shines to transmit news by flashes of sunlight chopped up into the morse code and thrown from point to point by a moving mirror. but this is limited as to distance; besides, the sun does not always shine. it has the disadvantage in that respect that the old semaphore-telegraph did that was in use in wellington's day. these semaphores were constructed in various ways, but a common form was that of moving arms that could be seen from hill to hill or point to point. by a code of moving signals news was repeated from point to point and it can be easily imagined that many mistakes occurred, to say nothing of the time it required for repetition. when the battle of waterloo was fought--so the story goes--news was sent to england by means of the semaphore-telegraph. the dispatch read, "wellington defeated--" at that point in the message a thick fog came up and lasted for three days, so that no further news could be sent or received. in the telegraphic parlance of to-day the line was "busted." for three long days all london was in deep mourning, when finally the fog lifted, which repaired the telegraphic line, and the balance of the dispatch was received--"the french at waterloo." mourning changed to rejoicing and the english have rejoiced ever since when they think of either wellington or waterloo. but to return to the dynamo. the name dynamo is an abbreviation for dynamo-electric machine. a machine for producing dynamic electricity. there are many forms of the dynamo, just as there are in the evolution of every important machine, and there will be many more. but the fundamental, underlying principle of them all is contained in an experiment made by faraday. faraday took the soft iron "keeper" of a permanent magnet and wound insulated wire around it and brought the two ends of the wire close together. he now placed the keeper, with the wire wound around it, across the poles of the permanent magnet, and wrenched it away suddenly, when he observed a spark pass between the ends of the wires. this would occur when he approached the poles as well as when he took it away. he discovered that the currents were momentary and occurred at the moment of approach or recession, and that the currents developed by the approach were of opposite polarity to those occurring at the recession. when the "keeper" was put on the poles of the magnet it was magnetized by having its molecular rings broken up and the poles of the little natural magnets all turned in one direction. during the time that the molecules of the keeper are changing they are in a dynamic or moving condition. by some mysterious action of the ether between the iron and the wire wrapped around it there is a corresponding molecular action in the wire that is dynamic for a moment only, and during that moment we have the phenomenon of an electric current. when the magnet and soft iron are separated this molecular state of strain is relieved and the molecules of both the iron and the wire wound about it return to normal, and in the act of returning we have a dynamic or moving condition, resulting in a current, only in the opposite direction. (see chap. vi.) now mount the permanent magnet in a frame and mount the soft iron with the wire on it (which in this shape is an electromagnet) on a revolving arm and so set it on the arm that its ends will come close to, but not touch, the poles of the permanent magnet. now revolve the arm, and every time the electromagnet or keeper approaches the permanent magnet a current of one polarity will be momentarily developed in the wire of the electromagnet, which is moving. when it is opposite the poles, it has reached the maximum charge and, now, as it passes on it discharges and a current of the opposite polarity is developed in the wire. the more rapidly we revolve the arm the more voltage (electrical pressure) the current it develops will have. it will be plain to all that we might make the electromagnet stationary and revolve the permanent magnet and get the same result. if the permanent magnet were strong enough and the electromagnet the right size as to iron, windings, etc., and we revolve the arm with sufficient rapidity, we could get an alternating current of electricity that would produce an electric light. i have not and cannot here give you the construction of a modern alternating-current dynamo. i have simply described the simplest form of dynamo, and all of them operate upon the fundamental principle of a permanent magnetic field and an electromagnet, moving in a certain relation to each other. the field may revolve or the electromagnet may revolve, whichever is the most convenient to construct. the field-magnet may be a permanent magnet or an electromagnet, made permanent during the operation of the dynamo by a part of the current generated by the machine being directed through a coil surrounding soft iron; or the field-current may come from an outside source. this is the kind of field-magnet universally used for dynamo work, as a much stronger magnetism is developed in this way than it is possible to obtain from any system of permanent steel magnets. the usual construction is to have a stationary field-magnet and then a series of electromagnets mounted and revolving upon a shaft in the center of the magnetic field. the rotating part is called the armature, and is so wound with insulated wire that successive induced currents are created in the armature windings and discharged through brushes which rest on revolving segments that connect with the armature windings. these induced currents succeed each other with such rapidity as to amount in practice to a steady current. however, the separate pulsations are easily heard in any telephone when the circuit is near to that of a dynamo circuit. the dynamo current is not nearly so steady as the battery current, although both are probably made up of separate discharges. in the dynamo there is a discharge every time the electromagnet of the armature cuts through the lines of force of the magnetic field, and in the galvanic battery every time a molecule is broken up and its little measure of energy is set free. in the dynamo the pulsations are so far apart as to make a musical tone of not very high pitch, but in the galvanic battery the pitch of the tone, if there is one, would require a special ear to hear it--one tuned, it may be, up near the rate of light vibration. there are two types of dynamo, one generating a direct and the other an alternating current. (by alternating we mean first a positive and then a negative current impulse.) we cannot enter into a technical description of the dynamo in a popular treatise such as this. the dynamo has evolved from the germ discovered by faraday, till to-day it is a machine, the construction of which requires the highest class of engineering skill. when in action it seems like a great living presence, scattering its energy in every direction in a way that is at once a marvel and a blessing to mankind. but we must not give all the credit to the dynamo. as the moon shines with a reflected light, so the dynamo gives off energy by a power delegated to it by the steam-engine that rotates it, and the steam-engine owes its life to the burning coal, and the burning coal is only giving up an energy that was stored ages ago by the magic of the sunbeam; and the sun--? well, we are getting close on to the borders of theology, and being only scientists we had better stop with the sun. there is still another way of generating electricity besides those that we have named; which are friction, chemical action, and the magneto-electric mode of generating a current. electricity may be generated by heat. if we connect antimony and bismuth bars together and apply heat at the junction of the metals and then connect the free ends of the two bars to a galvanometer, it will indicate a current. these pairs can be multiplied, and in this way increase the voltage or pressure, and, of course, increase the current, if we assume that there is resistance in the circuit to be overcome. if there were absolutely no resistance in the circuit--a condition we never find--there would be no advantage in adding on elements in series. substances differ in their resistance to the passage of electricity--the less the resistance the better the conductor. the german electrician, g. s. ohm ( - ), investigated this and propounded a law upon which the unit for resistances is based, and this unit takes his name and is called the "ohm." any two metals having a difference of potential will give the phenomena of thermo-electricity. antimony and bismuth having a great difference of potential are commonly used. the use made of thermal currents is chiefly for determining slight differences of temperature. an apparatus called the thermo-electric pile has been constructed out of a great number of pairs of antimony and bismuth bars. this instrument in connection with a galvanometer makes a most delicate means of determining slight changes of temperature. if one face of a thermopile is exposed to a temperature greater than its own, the needle will move in one direction; if to a temperature lower than its own, the needle will be deflected in the opposite direction. if both faces of the pile are exposed to the same changes of temperature simultaneously, of course no electrical manifestations will occur. the earth is undoubtedly a great thermal battery that is kept in action by the constant changes of temperature going on at the earth's surface, caused by its rotation every twenty-four hours on its axis. the sun, of course, is at some point heating the earth, which at other points is cooling, making a constant change of potential between different points. if we heat a metal ring at one point a current of electricity will flow around it--especially if it is made of two dissimilar metals--until the heat is equally distributed throughout the ring. some years ago, when the postal telegraph company first began operations between new york and chicago, the writer made observations twice a day for some time of the temperature and direction of the earth-current. the first two wires constructed gave only two ohms resistance to the mile, which facilitated the experiments. i found that in almost every instance the current flowed from the point of higher temperature to the lower. if the temperature in new york were higher at the time of observations than in chicago the current would flow westward, and if the conditions were reversed the current would be reversed also. chapter viii. atmospheric electricity. nature has another mode of generating electricity, called atmospheric. the normal conditions of potential between the earth and the upper atmosphere seem to be that the atmosphere is positively electrified and the earth negatively. these conditions change, apparently from local causes, for short periods during storms. in some way the sun's rays have the power directly or indirectly to give the globules of moisture in the air a potential different from that of the earth. in clear weather we find the air near to the earth in a neutral condition, but gradually assuming the condition of a positive charge as we ascend; so that the upper air and the earth are oppositely charged like the two sides of a leyden jar or two leaves of a condenser. this condition is intensified and localized when a thunder-cloud passes over the earth. the moisture globules have been charged with potential energy by the power of the sun's rays when evaporation took place; but in this state the energy is neither heat nor electricity, but a state of strain like a bent bow or a wound-up spring. when these moisture globules condense into drops of water the potential energy is set free and becomes active either as heat or electricity. the cloud gathers up the energy into a condensed form, and when the tension gets too great a discharge takes place between the cloud and the earth or from one cloud to another, which to an extent equalizes the energy. in most cases of thunder and lightning it is only a discharge from cloud to cloud unequally charged. this does not relieve the tension between the earth and the cloud, but distributes it over a larger area. the reason for this constant electrical difference between the earth and the upper regions of atmosphere is not well understood, except that primarily it is an effect of the sun's rays. evaporation may and probably does play a part, and the same causes that give rise to the auroral display may contribute in some way to the same result. evaporation does not always take place at the earth's surface. cloud formations may be evaporated in the upper air into invisible moisture spherules, and charged at the time with potential energy. if we go up into a high mountain when the conditions are right, we can witness the effect of this condition of electrical charge or strain between the upper regions of the atmosphere and the earth, and the tendency to equalize the potentials between the clouds and the earth. often one's hair will stand on end, not from fright, but from electricity passing down from the upper regions to the earth. when the tension is very great a loud hissing sound as of many musical tones of not very good quality may be heard, and a brush or fine-pointed radiation of electricity may be seen from every point, even from your finger-ends. the thunder is not usually so loud on high mountains for two reasons--one because the air is rare, but the chief reason is that the mountain acts as a great lightning-rod and gradually discharges the cloud or atmosphere, for often the phenomena may occur when the sky is clear. i remember being on top of what is called the mosquito range, between alma and leadville in colorado, during the passage of a thunder shower. there was no heavy thunder, but a constant fusillade of snapping sounds, accompanied by flashes not very intense. i could feel the shocks, but not painfully. a part of the time i was in the cloud and became for the time being a veritable lightning-rod. after the cloud passed it crawled down the mountainside as if clinging to it, all the time bombarding it with little electric missiles. after the cloud left the mountain and passed over the valley i could hear loud thunder, because the charge would have to accumulate quite a quantity, so to speak, before it could discharge. these heavy discharges when the cloud is some distance from the earth would be dangerous to life, while the light ones, when the cloud is in contact with the earth, are not. many wonderful and destructive effects come from these lightning discharges and many lives are lost every year from this cause. i do not suppose it is possible to be on one's guard continually, but many lives are needlessly lost either from ignorance or carelessness. although there is a just prejudice against lightning-rods as ordinarily constructed, it is still just as possible to protect your house and its inmates from the destroying effects of lightning as from rain. if, for instance, we lived in metal houses that had perfect contact all round them with moist earth, or better, with a water-pipe that has a large surface contact with the earth, the lightning would never hurt the house or the inmates. in such a case you simply carry the surface of the earth to the top of your house, electrically speaking, and neutralization takes place there in case the lightning strikes the house. a house that is heated with hot water can easily be made lightning-proof by a little work at the top and bottom of the heating system. all the heavy metal of the house should be a part of the lightning-rod. points should be erected at the chimneys, and if there is a metal roof they should be connected with it. then connect the roof with rods from several points with the ground. here is where most rods fail. the ground connection is not sufficient. the earth is a poor conductor, and we have to make up by having a large metal surface in contact with it. it is best to have the rod connected with the water pipe, if there is one, and have it connected with metal running all around the house as low down as the bottom of the cellar, for sometimes there is an upward stroke, and you never can tell where it is coming up. if you have a heating system it should be thoroughly grounded and the top pipe connected with the rods at the chimneys. these rods need not be insulated as is the usual practice. if you are outdoors during a thunder-storm never get under a tree, but if you are twenty or thirty feet away it may save your life, because, if it comes near enough to strike you, it will probably take the tree in preference. it seeks the earth by the easiest passage. an oil-tank and a barn are dangerous places, if the one has oil in it and the other is filled with hay and grain. a column of gas is rising that acts as a conductor for lightning. of course if the barn is properly protected with rods it will be safe. sometimes a cloud is so heavily charged that the lightning comes down like an avalanche, and in such a case the rods must have great capacity and be close together to fully protect a building. there is a popular notion that rods draw the lightning and increase the damage rather than otherwise. this is a mistake. points will draw off electricity from a charged body silently. it would be possible to so protect a district of any size in such a way that thunder would never be heard within its boundaries if we should erect rods enough and run them high enough into the upper air. the points--if they were close enough together--would silently draw off the electricity from a cloud as fast as it formed, and thus effectually prevent any disruptive discharge from taking place. chapter ix. electrical measurement. having given a short account of some of the sources of electricity, let us now proceed to describe some of the practical uses to which it is put, and at the same time describe the operation of the appliances used. before proceeding further, however, we ought to tell how electricity is measured. we have pounds for weight, feet and inches for lineal measure, and pints, quarts, gallons, pecks and bushels for liquid and dry measure, and we also have ohms, volts, ampères and ampère-hours for electricity. when a current of electricity flows through a conductor the conductor resists its flow more or less according to the quality and size of the conductor. silver and copper are good conductors. silver is better than copper. calling silver , copper will be only . if we have a mile of silver wire and a mile of iron wire and want the iron wire to carry as much electricity as the silver and have the same battery for both, we will have to make the iron wire over seven times as large. that is, the area of a cross-section of the iron wire must be over seven times that of the silver wire. but if we want to keep both wires the same size and still force the same amount of current through each we must increase the pressure of the battery connected with the iron wire. we measure this pressure by a unit called the "volt," named for volta, the inventor or discoverer of the voltaic battery. the volt is the unit of pressure or electromotive force. (in all these cases a "unit" is a certain amount or quantity--as of resistance, electromotive force, etc.--fixed upon as a standard for measuring other amounts of the same kind.) the iron wire offers a resistance that is about seven times greater than silver to the passage of the current. to illustrate by water pressure: if we should have two columns of water, and a hole at the bottom of each column, one of them seven times larger than the other, the water would run out much faster from the larger hole if the columns were the same height. now, if we keep the column with the larger hole at a fixed height a certain amount of water will flow through per second. if we raise the height of the column having the small hole we shall reach a point after a time when there will be as much water flow through the small hole per second as there is flowing through the large hole. this result has been accomplished by increasing the pressure. so, we can accomplish a similar result in passing electricity through an iron wire at the same rate it flows through a silver wire of the same size, by increasing the pressure, or electromotive power; and this is called increasing the voltage. the quality of the iron wire that prevents the same amount of current from flowing through it as the silver is called its resistance. the unit of resistance, as mentioned in the last chapter, is called the ohm, and the more ohms there are in a wire as compared with another, the more volts we have to put into the battery to get the same current. the unit for measuring the current is called the "ampère," named after the french electrician, a. m. ampère ( - ). now, to make practical application of these units. the volt is the potential or pressure of one cell of battery called a standard cell, made in a certain way. the electromotive force of one cell of a daniell battery is about one volt. one ohm is the resistance offered to the passage of a current having one volt pressure by a column of mercury one millimeter in cross-section and . centimeters in length. ordinary iron telegraph-wire measures about thirteen ohms to the mile. now connect our standard cell--one volt--through one ohm resistance and we have a current of one ampère. unit electromotive force (volt) through unit resistance (ohm) gives unit of current (ampère). it is not the intention to treat the subject mathematically, but i will give you a simple formula for finding the amount of current if you know the resistance and the voltage. the electromotive force divided by the resistance gives the current. c = e/r or current (ampères) equals electromotive force (volts) divided by the resistance (ohms). but still further: one ampère of current having one volt pressure will develop one watt of power, which is equal to / of a horse-power. (the watt is named in honor of james watt, the scottish inventor of the steam-engine-- - ). in other words, watts equal one horse-power. by multiplying volts and ampères together we get watts. if we want to carry only a small current for a long distance we do not need to use large cells, but many of them. we increase the pressure or voltage by increasing the number of cells set up in series. if we have a wire of given length and resistance and find we need volts to force the right amount or strength of current through it, and the electromotive force of the cells we are using is one volt each, it will require cells. if we have a battery that has an e. m. f. of two volts to the cell, as the storage-battery has, fifty cells would answer. if we want a very strong current of great volume, so to speak, for electric light or power, and use a galvanic battery, we should have to have cells of large surface and lower resistance both inside and outside the cells. when dynamos are used they are so constructed that a given number of revolutions per minute will give the right voltage. in fact, the dynamo has to be built for the amount of current that must be delivered through a given resistance. the same holds good for a dynamo as for a galvanic battery. if any one factor is fixed, we must adapt the others to that one in order to get the result we want. there are many other units, but to introduce them here would only confuse the reader. the advanced student is referred to the text-books. with this much as a preliminary we are prepared to take up the applications of electricity, which to most people will be more interesting than what has gone before. chapter x. the electric telegraph. in the year strada, an italian jesuit, proposed to telegraph news without wires by means of two sympathetic needles made of loadstone so balanced that when one was turned the other would turn with it. each needle was to have a dial with the letters on it. this would have been very nice if it had only worked, but it was not based on any known law of nature. many attempts at telegraphing with electricity were made by different people during the eighteenth century. about franklin succeeded in firing spirits by means of a wire across the schuylkill river, using, as all the other experimenters had done, frictional electricity. in an anonymous letter was written to scott's magazine describing a method by which it was possible to communicate at a distance by electricity. the writer proposed the use of a wire for each letter of the alphabet, that should terminate in pith balls at the receiving end, and under the balls were to be strips of paper corresponding to the letters of the alphabet. the message was to be sent by discharging static electricity through the wire corresponding to the first letter of a word when the paper would be attracted to the pith ball and read by the observer. then the wire corresponding to the second letter of the word was to be charged in like manner, and so on till the whole message was spelled out. this was the first practical (i.e., possible) suggestion for a telegraph. the writer also proposed to have the wires strung on insulators, which was a great advance over the other attempts. the communication was anonymous, as no doubt, like many others, the author feared the ridicule of his neighbors. it requires a vast amount of moral courage to stand up before the world and openly advocate some new theory that has never come within the experience of any one before. it requires much now, but it required more then; for a man in those days would have been roasted for what in these days he would be toasted. the rank and file of humanity have been opposed to innovations in all ages, but no progress could have been made without innovations. there always has to be a first time. galileo is said to have been forced to retract, on his knees, some theory he advanced about the motion of the earth, and its relation to the sun and other heavenly bodies. notwithstanding this retraction the seed-thought sown by galileo took root in other minds, which led to the triumph of scientific truth over religious fanaticism. the writer in scott's magazine did not have the opportunity to put his ideas into practice, so the glory of the invention fell to others. such men as this unknown writer are made of finer stuff, and they stand alone on the frontier of progress. they do not fear the bullets of an enemy half so much as the gibes of a friend. much of their work is done quietly and without notice, and when something of real importance is worked out theoretically and experimentally, some one seizes upon it and proclaims it from the housetops and attaches to it his name; but perhaps years after the real inventor (the man who taught the so-called inventor how to do it) is dead, some one writes a book that reveals the truth, and then the hero-loving people erect a monument to his memory. such a man was our own professor joseph henry, so long the presiding genius at the smithsonian institution at washington. he worked out all the problems of the present american telegraphic system and demonstrated it practically. everything that made the so-called morse telegraph a success had long before been described and demonstrated by henry. yet with the modest grace that was ingrained in the man he yielded all to the one who was instrumental in constructing the first telegraph line between baltimore and washington. great credit is due to such men as morse and cyrus w. field--neither of them inventors, but promoters of great systems of communication that are of unspeakable benefit to mankind. henry pointed out the way, and morse carried it into effect. morse has had no more credit than was due him, but has henry had as much as is due him? no great invention was ever yet the work, wholly, of one man. we americans are too apt to forget this. i shall always remember henry as a most unassuming, kindly, genial man, and i shall never forget his kindness to me. in i began my researches in telephony, having applied for a patent for an apparatus for transmitting musical tones telegraphically. this consisted of a means of transmitting musical tones through a wire and reproducing them on a metal plate (stretched on the body of a violin to give it resonance) by rubbing the plate with the hand--the latter being a part of the circuit. the examiner refused the application at first on the ground that the inventor or operator could not be a part of his machine. i took my apparatus and went to washington, first calling upon professor henry, never having met him before. he received me most kindly, and allowed me to string wires from room to room in the institute, and when he had witnessed the experiments he seemed as delighted as a child. i now brought the patent office official over to the smithsonian and soon convinced him that the inventor could be a part of his own machine. the same year i went abroad, and henry gave me a letter to tyndall. it was very fortunate for me that he did, for tyndall was very shy at first, and it was only henry's letter that gave me a hearing for a moment. the history of the few days that followed this first interview with tyndall at the royal institution would make very interesting reading, if i felt at liberty to publish it. suffice it to say that he was convinced in a few minutes after he had reached the experimental stage that not all my work had been anticipated by wheatstone, as he asserted before seeing the experiments. wheatstone had transmitted the tones of a piano, mechanically, from one room to another by a wooden rod placed upon the sound-board and terminating in another room in contact with another sound-board. but this was very different from transmitting musical tones and melodies from one city to another through a wire, as i could do with my electrotelephonic apparatus. it is a curious fact that the world is divided into two great classes, leaders and followers. or we might say, originators and copyists; the former division being very small, while the latter is very large. as late as the european philosophers were trying to construct a telegraphic system based upon two ideas, announced a long time before, to wit, the use of static or frictional electricity, and a wire for every letter. it does not seem to have occurred to any one to devise a code consisting of motions differently related as to time, and to use simply one wire. in oersted discovered the effect of a galvanic current on a magnetic needle, and published a pamphlet concerning his discovery. this stimulated others, and ampère applied it to the galvanometer the same year. arago applied it to soft iron, and here was the germ of the electromagnet. we see that as far back as we had the galvanic battery and the electromagnet, the two great essentials of the modern telegraph. however, there remained another great discovery to be made before these elements could be utilized for telegraphic purposes. one cell of battery was used, and the magnet was made by winding one layer of wire spirally around the iron, so that each spiral was out of touch with its neighbor. barlow of england, a fellow of the royal society, tried the effect of a current through a wire feet long, and found that the power was so diminished that he was discouraged, and in a paper gave it as his opinion that galvanism was of no use for telegraphing at a distance. this paper stimulated others, and it was reserved for our own joseph henry, already referred to, to show not only how to construct a magnet for long-distance telegraphy, but also how to adapt the battery to the distance. he showed us that by insulating the wire and using several layers of whirls, instead of one, and by using enough cells of battery coupled up in series to get more voltage, as we now express it, it was possible to transmit signals to a distance. he not only set forth the theory, but he constructed a line of bell-wire feet long and worked his magnet by making the armature strike a bell for the signals, which is the basis of the modern "sounder." nothing was needed but to construct a line and devise a code to be read by sound, to have practically our modern morse telegraph. this line was constructed in . in henry, who was then at princeton, constructed a line and worked it as it is to-day worked, with a relay and local circuit, so that at that period all the problems had been worked out. but, like the speaking-telephone in its early inception, no one appreciated its real importance. henry himself did not think it worth while to take out a patent. two years later the secretary of the treasury sent out a circular letter of inquiry to know if some system of telegraphic communication could not be devised. the learned heads of the franklin institute of philadelphia, the oldest scientific society in america, advised that a semaphore system be established between new york and washington, consisting of forty towers with swinging arms, the same as used in the days of wellington. among other replies to the circular letter of the secretary was one from samuel f. b. morse. morse was not a scientist or even an inventor, at least not at that time. he was an artist of some note. in , while crossing the ocean, morse, in connection with one dr. jackson of boston, devised a code of telegraphic signs intended to be used in a chemical telegraph system. some years later morse adapted henry's signal-instrument to a recorder, called the morse register, and this was the instrument used in the early days of the morse telegraph. what morse seems to have really invented was the register, which made embossed marks on a strip of paper, and the code of dots and dashes representing letters, now known as the morse alphabet, although this latter is questioned. morse took his apparatus to washington and exhibited it to the members of congress in the year , but it was four years before a bill was passed that enabled him to try the experiment between baltimore and washington. we will let him describe in his own words the closing day of congress. he says: "my bill had indeed passed the house of representatives and it was on the calendar of the senate, but the evening of the last day had commenced with more than bills to be considered and passed upon before mine could be reached. wearied out with the anxiety of suspense, i consulted one of my senatorial friends. he thought the chance of reaching it to be so small that he advised me to consider it as lost. in a state of mind which i must leave you to imagine, i returned to my lodgings to make preparations for returning home the next day. my funds were reduced to the fraction of a dollar. in the morning, as i was about to sit down to breakfast, the servant announced that a young lady desired to see me in the parlor. it was the daughter of my excellent friend and college classmate, the commissioner of patents, henry l. ellsworth. she had called, she said, by her father's permission, and in the exuberance of her own joy, to announce to me the passage of my telegraph bill at midnight, but a moment before the senate adjourned. this was the turning-point of the telegraph invention in america. as an appropriate acknowledgment of the young lady's sympathy and kindness--a sympathy which only a woman can feel and express--i promised that the first dispatch by the first line of telegraph from washington to baltimore should be indited by her; to which she replied: 'remember, now, i shall hold you to your word.' about a year from that time the line was completed, and, everything being prepared, i apprised my young friend of the fact. a note from her inclosed this dispatch: 'what hath god wrought?' these were the first words that passed on the first completed line in america." the first telegraph-line in america was put into operation in the spring of at the beginning of polk's administration. i remember as a boy having the two cities, baltimore and washington, pointed out to me on the map, and how the story of the telegraph impressed me. congress appropriated $ , for the construction of the line, and $ to keep it running the first year. it was placed under the control of the postmaster-general, and the line was thrown open to the public. the tariff was fixed at one cent for every four words. it was open for business on april , , and for the first few days the revenue was exceedingly small. on the morning of the first day a gentleman came in and wanted to "see it work." the operator told him that he would be glad to show it at the regular tariff of one cent for four words. the gentleman grew angry and said that he was influential with the administration, and that if he did not show him the working free of charge he would see to it that he lost his job. his bluff did not succeed. the operator referred him to the postmaster-general, and thus the stormy interview ended. no patrons came in for the next three days, but a great number stood around hoping to see the instrument start up, but no one was willing to invest a cent--probably from fear of being laughed at. on the fourth day the same gentleman who had threatened the young man with dismissal came back and invested a cent, and this was the first and only revenue for four days. the message that was sent only came to one-half cent, but as the operator could not make change the stranger laid down the cent and departed. his name ought to be known to fame as the first man patron of the telegraph. [illustration: fig. . a gives a diagram view of a morse telegraph-line with three stations. b is the battery; c c c, the transmitting keys in the three offices; d d d, the relay magnets; e e e, the armatures that are actuated by the magnets.] the operation of the morse telegraph is very simple if we grant all that has gone before. all that is needed is the wire, the battery, and the key, as shown in fig. (page ), and a relay--an extra electromagnet which receives the electric current and by its means puts into or out of action a small local battery on a short circuit in which is placed the receiving or recording apparatus. thus we have a wire starting from the earth in new york and passing through a battery, a key and a relay, and thence to boston on poles, with insulators on which the wire is strung, and through another instrument, key and battery in boston, the same as at the new york end, and into the ground, leaving the earth to complete one-half of the circuit. when the keys at both ends are closed the batteries are active and the armatures or "keepers" are attracted so that the armature levers rest on the forward stops. (see diagram fig. .) if either one of the keys is opened the current stops flowing and the magnetism vanishes from all the electromagnets on the line, and a spring or retractile of some kind pulls the armatures away from the magnets and the levers rest on their back stops. in this way all the levers of all the magnets are made to follow the motions of any key. if there are more than two magnets in circuit (and there may be twenty or more) they all respond in unison to the working of one key, so that when any one station is sending a dispatch all the other stations get it. but there is a "call" for each office, so that the operator only heeds the instrument when he hears his own call. operators become so expert in reading by sound that they may lie down and sleep in the room, and, although the instrument is rattling away all the time, he does not hear it till his own call is made, when he immediately awakes. in the old days messages were received on slips of paper by the morse register by means of dots and dashes. gradually the operator learned to read by sound, till now this mode of receiving is almost universal the world over. reading by sound was of american origin. it is a spoken language, and when one becomes accustomed to it it is like any other language. this code language has some advantages over articulate speech, as well as many disadvantages. a gentleman who was connected with a louisville telegraph office told me that one of the best operators he ever knew was as deaf as a post. he would receive the message by holding his knee against the leg of the table upon which the sounder was mounted, and through the sense of feeling receive the long and short vibrations of the table, and by this means read as well or better than through the ear, because he was not distracted by other sounds. a story is told of the late general stager that at one time he was on a train that was wrecked at some distance from any station. he climbed a telegraph pole, cut the wire and by alternately joining and separating the ends sent a message, detailing the story of the wreck, to headquarters, and asked for assistance. he then held the two ends of the wire on each side of his tongue and tasted out the reply--that help was coming. any one who has ever tasted a current knows that it is very pronounced. a story similar to this is told of the early days when the bain chemical system was used between washington city and some other point. this system made marks on chemically-prepared paper; as the current passed through it left marks on the paper from the decomposition of the chemicals. some of the preparations emitted an odor during the time that the current passed. the occurrence to which we refer took place at presidential election time. at some station out of washington an operator was employed who had a blind sister, and this sister knew the morse alphabet well before she became blind. one evening a signal came to get ready for a message containing the returns from the election. in the hurry, and just as the message had started, the lamp was upset and they were in total darkness--at least, the brother was. the sister, poor girl, had been in darkness a long time. the blind sister leaned over the stylus through which the current flowed to the paper and smelled out as well as spelled out the message, and repeated it to her astonished brother. by the old semaphore system the motions were sensed through the eye as well as the early method of cable signaling. it will be seen from the above that the morse code may be communicated through any one of the five senses. chapter xi. receiving messages. with but few exceptions the morse code is the one almost universally used the world over. as it is used in europe, it is slightly changed from our american code, but they all depend upon dots, dashes, and spaces, related in different combinations, for the different letters. notwithstanding its universal use it is not free from serious difficulties in transmission unless it is repeated back to the sender for correction; and then in some cases it is impossible to be sure, owing to difficulties of punctuation and capitalizing, and the further difficulty of running the signals together, caused, it may be, by faulty transmission, induced currents from other wires, "swinging crosses" or atmospheric electricity. sometimes it is a psychological difficulty in the mind of the receiving-operator. the telegraph companies have to suffer damages from all these and many other unforeseen causes. prescott tells some curious things that happened in the early days, growing out of the peculiarities of the receiving-operator. at one time he was reporting by telegraph one of webster's speeches made at albany in in which there were many pithy interrogative sentences, and he was desirous of having the interrogation-points appear. so to make sure, whenever he wished an interrogation-point he said "question" at the end of almost every sentence. next day he was horrified on reading the speech to see the ends of the sentences bristling with the word "question." some time back in the fifties a gentleman in boston telegraphed to a house in new york to "forward sample forks by express." the message when received by the new york merchant read: "forward sample for k. s. by express." the new york merchant did not know who k. s. was, nor did he gather from the dispatch what kind of sample he wanted. so he went to the telegraph office to have the matter cleared up. the boston operator repeated the message, saying "sample forks." "that's the way i received it and so delivered it--sample for k. s.," said new york. "but," says boston, "i did not say for k. s.; i said f-o-r-k-s." new york had read it wrong in the start and could not get it any other way. "what a fool that boston fellow is. he says he did not say for k. s., but for k. s." boston had to resort to the united states mail before the mystery was solved. curiously enough, the old method of recording the dots and dashes on the paper strip was not so reliable as the present mode of reading by sound. a man can put his individuality to some extent into a sounder, and when one becomes used to his style it is much easier to read him accurately by sound than by the paper impressions. some people never could learn to read either by paper or sound. an instance of this kind is given of a middle-aged man who was employed by a railroad company as depot master and telegraph operator, in the old days of the paper strip. one day he rushed out and hailed the conductor of a train that had just pulled into the station, and told him that ---- train had broken both driving-wheels and was badly smashed up. the conductor could read the mystic symbols, so he took the tape and deciphered the dispatch as follows: "ask the conductor of the boston train to examine carefully the connecting-rods of both driving-wheels, and if not in good condition to await orders." it is further related of this same operator that when he got into real difficulty with his "tape" he used to run over to the regular commercial office to have his messages translated. one day he rushed into his neighbor's office trailing the tape behind him and saying: "i am sure an awful accident has happened by the way the message was rattled off." a playful dog had torn off a large part of the strip as it trailed along, so only a part was left. it read, "good morning, uncle ben. when are you----" the dog had swallowed the balance of the dispatch. sometimes the morse code is not only funny but disastrous. a gentleman wanted to borrow money of some capitalists who, not knowing his financial standing, telegraphed to a banker who they knew could post them. they received an answer, "note good for large amount." the gentleman borrowed a "large amount," but afterward when it came to be investigated it was found that the dispatch was originally written "not," instead of "note," which made "all the difference in the world." it has been stated that any one of the five senses may be called into service to interpret the morse code into words and ideas. a story is told by mr. prescott that he says is true, as he knew the party. a friend of his, by name langenzunge, who knew the morse code, had served under general taylor (who at this time was president) at palo alto, in mexico. the general had just promised him an office; soon after he left washington for the west over the baltimore and ohio on a freight train; the president was taken seriously ill, and his friend hearing of it was troubled not only because he loved the old general, but on account of the change in his own prospects. the train stopped somewhere on the potomac at midnight and remained there for four hours. uneasy and sad, he wandered down the track and climbed a pole, cut the wire and placed the ends each side of his tongue and tasted out the fatal message--"died at half-past ten." the shock (not the electric) was so great that he almost fell from the pole. what a situation! a man climbs a pole at midnight miles from the sick friend he loves, puts his tongue to inanimate wire, and is told in concrete language--through the sense of taste--that his friend is dead. this is only one of the many, many wonderful episodes of the telegraph. chapter xii. miscellaneous methods. "it never rains but it pours." almost simultaneously with the demonstration of the morse telegraph other types were devised. there were the needle systems of cooke and wheatstone, the chemical telegraph of alexander bain, and soon the printing telegraph of house, and later that of hughes. the latter is in use on the continent of europe, and a modification of it has a very limited use on some american lines. the bain telegraph used a key and battery the same as the morse system, but it did not depend upon electromagnetism as the morse system does. when in operation a strip of paper was made to move under an iron stylus at the receiving-end of the line. the paper was saturated with some chemical that would discolor by the electrolytic action of the current. when a message was sent the paper was set to moving by a clock mechanism or otherwise, under the stylus that was pressing on the paper as it passed over a metal roller or bed-plate. the transmitting-operator worked his key precisely as in sending an ordinary message by the morse system. the effect was to send currents through the receiving-stylus chopped into long or short marks, or the dots and dashes of the morse code, and recorded on the tape in marks that were blue or brown, according to the chemical used. a few lines were established in this country on the bain system, but it never came into general use. a number of systems, called "automatic," grew out of the bain system. bain himself devised, perhaps, the first automatic telegraph. the fundamental principle of all automatic telegraphs depends upon the preparation of the message before sending, and is usually punched in a strip of paper and then run through between rollers that allow the stylus to ride on the paper and drop through the holes that represent the dots and lines of the morse alphabet. every time the stylus drops through a hole in the paper it makes electrical contact and sends a current, long or short, according to the length of the hole. the object of the automatic system was to send a large amount of business through a single wire in a short time. it does not save operators, as the messages have to be prepared for transmission, and then translated at the receiving-end and put into ordinary writing for delivery. the automatic system is not used except for special purposes, and the one that seems to be the most favored is that of wheatstone. the system is in use in england and in america to a limited degree. early in the history of the telegraph a printing system was devised. wheatstone and others had proposed systems of printing telegraphs in europe, but these never passed the experimental stage. the first printing telegraph introduced in america was invented by royal e. house of vermont, and first introduced in on a line between cincinnati and jeffersonville, a distance of miles. in a line for commercial use was established between new york and philadelphia, and for some years following many lines were equipped with the house printing telegraph instrument. the late general anson stager was a house operator at one time. all printing telegraph instruments, while differing greatly in detail, have certain things in common, to wit: a means for bringing the type into position, an inking device, a printing mechanism, a paper feed, and a means for bringing the type-wheels into unison. there are two general types of printing instruments, the step-by-step, and the synchronously moving type-wheels. the house printer was a step-by-step instrument and consisted of two parts, a transmitter and a receiver. the transmitter consists of a keyboard like a piano, with twenty-eight keys. these keys are held in position by springs. under the keys is a cylinder having twenty-eight pins on it corresponding to the twenty-six letters of the alphabet and a dot and a space. this cylinder was driven by some power. in those days it was by man-power. it was carried by a friction, so that it could be easily stopped by the depression of any one of the keys that interfered with one of the pins. one revolution of the cylinder would break and close the current twenty-eight times, making twenty-eight steps. the receiving-instrument consisted of a type-wheel and means for driving it. it was somewhat complicated, and can only be described in a general way. if the cylinder of the transmitter was set to rotating it would break and close twenty-eight times each revolution. (there were fourteen closes and fourteen breaks, each break and each close of the current representing a step.) the type-wheel of the receiver was divided into twenty-eight parts, having twenty-six letters and a dot and space, each break moved it one step and each close a step; so that if the cylinder, with its twenty-eight pins, started in unison with the type-wheel, with its twenty-eight letters and spaces, they would revolve in unison. the keys were lettered, and if any one was depressed the pin corresponding to it on the cylinder would strike it and stop the rotation of the cylinder, which stopped the breaking and closing of the circuit, which in turn stopped the rotation of the type-wheel--and not only stopped it, but also put it in a position so that the letter on the type-wheel corresponding to the letter on the key that was depressed was opposite the printing mechanism. the printing was done on a strip of paper, which was carried forward one space each time it printed. the printing mechanism was so arranged that so long as the wheel continued to rotate it was held from printing, but the moment the type-wheel stopped it printed automatically. the messages were delivered on strips of paper as they came from the machine. in david e. hughes of kentucky patented a type-printing telegraph that employed a different principle for rotating the type-wheel. the electric current was used for printing the letters and unifying the type-wheels with the transmitting-apparatus. the transmitter, cylinder, and the type-wheel revolved synchronously, or as nearly so as possible, and the printing was done without stopping the type-wheel. whenever a letter was printed the type-wheel was corrected if there was any lack of unison. this type of machine in a greatly improved form is still used on some of the western union lines, especially between new york, boston, philadelphia, and washington. it is also in use in one of its forms in most of the european countries. chapter xiii. multiple transmission. although the printing and automatic systems of telegraphing are used in america to some extent, the larger part is done by the morse system of sound-reading and copying from it, either by pen or the typewriter. in the early days only one message could be sent over one wire at the same time, but now from four to six or even more messages may be sent over the same wire simultaneously without one message interfering with the other. like most other inventions, many inventors have contributed to the development of multiple transmission, till finally some one did the last thing needed to make it a success. the first attempts were in the line of double transmission, and many inventors abroad have worked on this problem. moses g. farmer of salem, mass., proposed it as early as , and patented it in . gintl, preece, siemens and halske and others abroad had from time to time proposed different methods of double transmission, but no one of them was a perfect success. when the line was very long there was a difficulty that seemed insurmountable. in the common parlance of telegraphy, there was a "kick" in the instrument that came in and mutilated the signals. about joseph b. stearns of boston made a certain application of what is called a "condenser" to duplex telegraphy that cured the "kick," and from that time to this it has been a success. farther along i will tell you what occasioned this "kick" and how it was cured. if this or some other method could be applied as successfully to cure the many chronic "kickers" in the world it would be a great blessing to mankind. it has always been a mystery to the uninitiated how two messages could go in opposite directions and not run into one another and get wrecked by the way. if you will follow me closely for a few minutes i will try to tell you. we have already stated that an electromagnet is made by winding an insulated wire around a soft iron core. if we pass a current of electricity through this wire the core becomes magnetic, and remains so as long as the current passes around it. in duplex telegraphy we use what is called a differential magnet. a differential electromagnet is wound with two insulated wires and so connected to the battery that the current divides and passes around the iron core in opposite directions. now if an equal current is simultaneously passed through each of the wires of the coil in opposite directions the effect on the iron will be nothing, because one current is trying to develop a certain kind of polarity at each pole of the magnet, while the current in the other wire is trying to develop an opposite kind in each pole. there is an equal struggle between the two opposing forces, and the result is no magnetism. this assumes that the two currents are exactly the same strength. if we break the current in one of the coils we immediately have magnetism in the iron; or if we destroy the balance of the two currents by making one stronger than the other we shall have magnetism of a strength that measures the difference between the two. without specifically describing here the entire mechanism--since this is not a text-book or a treatise--we may say that a duplex telegraph-line is fitted with these differentially wound electromagnets at every station. when station a (fig. ) is connected to the line by the positive pole of its battery, station b will have its negative pole to line and its positive to earth. when a depresses his key to send a message, half the current passes by one set of coils around his differential magnet through a short resistance-coil to the earth, and the other half by the contrary coil around the magnet to the line, and so to station b. the divided current does not affect a's own station, being neutralized by the differential magnet, but it does affect b, whose instrument responds and gives him the message. now b may at the same time send a message to a by half of his own divided current from his own end of the line. [illustration: fig. . represents a duplex -mile telegraph-line. a and b are the two terminal stations; b b´, the batteries; k k´, the keys; d d´, the small resistance-coils, equal to the battery-resistance when the latter is not in circuit; r r´, resistances each equal to the -mile line; and c c´, condensers giving the artificial lines r r´ the same capacity as the -mile line.] the puzzle to most people is: how can the signals pass each other in different directions on the same wire? but the signals do not have to pass each other. in effect, they pass; but in fact, it is like going round a circle--the earth forming half. a sends his message over the line to b. b sends his message to a through the earth and up a's ground-wire. the operative who is sending with positive pole to line _pushes_ his current through--so to speak--while the operative who is sending with the negative pole to line _pulls_ more current in the same direction through the line whenever he closes his key. this may not be a strictly scientific statement; but, as long as we speak of a "current" flowing from positive to negative poles (which is the invariable course electricity takes), it is the way to look at the matter understandingly. the short "resistance-coil" at each end, fortified by a "condenser" made of many leaves of isolated tin-foil, to give it capacity, offers precisely the same resistance to the current as the miles of wire line; so that the twin currents that run around the differential magnet exactly neutralize each other and make no effect in the office the message starts from; while one of them takes to the earth, and the other to the line to carry the message. this condenser is necessary, because the short resistance-coil affects the current immediately, while the long line with its greater amount of metal does not give the same amount of resistance till it is filled from end to end, which requires a fraction of a second. during this time, however, more current is passing through the differential coil connected with the line than through the short resistance-coil; and the unequal flow causes the relay armature to jump, or "kick." the condenser, with the many leaves of tin-foil, supplies the greater metal surface to be traversed by the short line current, causes the flow to be equal in both circuits at all times, and thus cures the "kick." it is this quality of a condenser that enables us to give to an artificial line of any resistance all the qualities, including capacity, and exhibit all the phenomena of a real line of any length, and it was this quality that enabled mr. stearns to take the "kick" out of duplex transmission and thus change the whole system, which created a new era in telegraphy. we have just spoken of the "capacity" of a circuit, and stated that it was determined by the mass of metal used. this capacity is measured by a standard of capacity that is arbitrary and consists of a condenser, constructed so that a given amount of surface of tin-foil may be plugged in or out. the practical unit of capacity is called the micro-farad, the real unit is the farad, and takes its name from faraday. but let us go back to multiple systems of transmission. there are many other systems of simultaneous transmission aside from the duplex, and all of them are classed under the general head of multiple telegraphy. first there is the quadruplex, that sends two messages each way simultaneously, making one wire do the work of four single wires--as they were used at first. the quadruplex is very extensively used by the western union telegraph company and others. it would be difficult to explain it in a popular article, so we will not attempt it. there is another form of multiple telegraph that was used on the postal telegraph line when it first started--which was invented and perfected by the writer--that can be more easily explained. in i discovered a method of transmitting musical tones telegraphically, and the thing that set my mind in that direction was a domestic incident. it is a curious fact that most inventions have their beginnings in some incident or observation that comes within the experience of some one who is able to see and interpret the meaning of such incidents or observations. i do not mean to say that inventions are usually the result of a happy thought, or accident; the germ may be, but the germ has to have the right kind of soil to take root in and the right kind of culture afterward. it is a rare thing that an invention, either of commercial or scientific importance, ever comes to perfection without hard work--midnight oil and daylight toil; and it is rarely, if ever, that a discovery or an invention based upon a discovery does not have, sooner or later, a practical use, although we sometimes have to wait centuries to find it put. we had to wait forty-four years after the galvanic battery was discovered before it became a useful servant of man. it was fifty years or more after the discovery by faraday of magneto-electricity before it found a useful application beyond that of a mere toy, but now it is one of the most useful servants we have, as shown in its wonderful development in electric lighting and electric railroads, to say nothing of its heating qualities and the useful purpose it serves in driving machinery. the interesting discoveries of professor crookes in passing a current of electricity through tubes of high vacua waited many years before they found a practical use in the x-ray, that promises to be of great service in medicine and surgery. the transmission of musical harmonies telegraphically, while in itself of great scientific interest, was of no practical use, but it led to other inventions, of which it is the base, that are transcendently useful in every-day life. the transmission of harmonic sounds by electricity underlies the principle of the telephone. there is a vast difference, in principle, between the transmission of simple melody, which is a combination of musical tones transmitted successively--one tone following another--and the transmission of harmony, which involves the transmission of two or more tones simultaneously. the former can be transmitted by a make-and-break current. in the latter case one tone has to be superposed upon another and must be transmitted with a varying but a continuously closed current. i make a distinction between a closed circuit and a closed current. in the case of the arc-light the circuit is open (that is, broken), technically speaking, but the current is still flowing. the reason why the reiss and other metallic contact telephone transmitters cannot successfully be used for telephone purposes is that metal points will not allow of sufficient separation of the transmitting points without breaking the current as well as the circuit. carbon contacts admit of a much wider separation without actually stopping the flow of the current, which latter is a necessity for perfect telephonic transmission, and it was the use of carbon that made that form of transmitter a success. there are other forms, or at least one other form that does not depend upon the length of the voltaic arc formed when the electrodes are separated. of this we will speak another time. now let us go back to the domestic incident referred to above. one evening in the winter of - i came home from my laboratory work and went into the bathroom to make my toilet for dinner. i found my nephew, mr. charles s. sheppard, together with some of his playmates, taking electrical "shocks" from a little medical induction-coil that i heard humming in the closet. he had one terminal of the coil connected to the zinc lining of the bathtub--which was dry at that time--while he held the other in his left hand, and with his right was taking shocks from the lining of the tub by rubbing his hand against the zinc. i noticed that each time he made contact with the tub, as he rubbed it for a short distance, a peculiar sound was emitted from under his hand, not unlike the sound made by the electrotome that was vibrating in the closet. my interest was immediately aroused, and i took the electrode out of his hand and for some time experimented with it, going to the cupboard from time to time to change the rate of vibration of the electrotome, and thus change the quality of the sound. i noticed that the sound or tone under my hand, if it could be so called, changed with each change of the rate of vibration. the thing that most interested me was that the peculiar characteristics of the noise were reproduced. in those few minutes i laid out work enough for years of experiment, and as a result i was late to dinner. this discovery opened up to my mind the possibility of three things--the transmission of music and of speech or articulate words through a telegraph-wire, and the transmission of a number of messages over a single wire. i constructed a keyboard consisting of one octave and made a set of reeds tuned to the notes of the scale, and then when some one would play a melody i could reproduce it in two ways: one by placing my body in the circuit and rubbing a metal plate--it might be the bottom of a tin pan, a joint of stovepipe or otherwise--anything that was metal and would vibrate would give the effect. another way was to connect an electromagnet (having a diaphragm or reed across its poles) in the circuit at the receiving-end and mount it on some kind of a soundboard. i made a great number of different kinds of receivers that were capable of receiving either musical or articulate sounds, as has many times been proven by experiment. i carried two sets of experiments along together; the one looking toward a system of multiple telegraphy and the other the transmission of articulate speech. let us first look into the multiple telegraph and take the other up under the head of the telephone. when the electrical keyboard was completed i found that i could transmit not only a melody but a harmony; that more than one tone could be transmitted simultaneously. this discovery opened up a long series of experiments with the view of sending a number of messages simultaneously by means of musical tones differing in pitch. i had already demonstrated that several tones could be transmitted at once, but they would speak all alike (with the same loudness) on the receiving-instrument. i now went to work on an instrument that responded for one note only and succeeded beyond my expectations. i made three different kinds of receiving-instruments. the first was a steel strap about eight inches long by three-eighths wide. this strap was mounted in an iron frame in front of an electromagnet. a thumbscrew enabled me to stretch the strap till it would vibrate at the required pitch. if, for instance, the sending-reed vibrated at the rate of times per second and the strap of the receiver was stretched to a tension that would give vibrations per second when plucked, it would then respond to the vibrations of the sending-reed but not to those of another reed of a different rate of vibration. if we take mounted tuning-forks tuned in pairs of different pitches, say four pairs, so that each fork has a mate that is in exact accord with it, and place them all in the same room, and sound one of them for a few seconds and then stop it, upon examining the other forks you will find all of them quiet except the mate of the one that was sounded. this one will be sounding. if we now sound four of the forks and then stop them the other four will be sounding from sympathy because the mate of each one of them has been sounded. if only two forks differing in pitch are sounded only two of the others will sound in sympathy. in the first case only one set of sound-waves were set up in the air, and the fork that found itself in accord with this set responded. when four forks differing in pitch were sounded there were four sets of tone-waves superposed upon each other existing in the air, so that each of the remaining forks found a set of waves in sympathy with its own natural rate of vibration and so responded. now apply this principle to the harmonic telegraph and you can understand its operation. at the transmitting-end of a line of wire there are a certain number of forks or reeds kept vibrating continuously. these reeds each have a fixed rate of vibration and bear a harmonic relation to each other so as not to have sound-interference or "beats." at the receiving-end of the line there are as many electromagnets as there are transmitting-reeds, and each magnet has a reed or strap in front of it tuned to some one of the transmitting-reeds, so that each transmitting-reed has a mate in exact harmony with it at the receiving-end of the line. keys are so arranged at the transmitting-end as to throw the tones corresponding to them to line when depressed. in other words, when the key belonging to battery b and vibrator is depressed (see fig. ) the effect is to send electrical pulsations through the line corresponding in rate per second to that of the vibrator. the same is true of battery b´ and vibrator . during the time any key is depressed--we will say of tone no. --this tone will be transmitted through the line and be reproduced by its mate--the one tuned in accord with it--at the receiving-station. by a succession of long and short tones representing the morse code a message can be sent. numbers two, three and four might be sending at the same time, but they would not interfere with number one or with each other. in - the writer succeeded in sending eight simultaneous messages between new york and philadelphia by the harmonic method. [illustration: fig. . in this diagram, and are tuned reeds; a a are receivers tuned to the reeds and respectively; and a are in unison, also and a, but the two groups (the s and the s) differ from each other in pitch.] there were two ways of reading by the harmonic method. one was by the long and short tone-sounds and the other by the ordinary sounder. the vibration of the receiving-reed was made to open and close a local circuit like a common morse relay and thus operate the sounder. it is useless to try to send a message if the sender and receiver are out of tune with each other in this system. what is true in science is true in life. if we are out of tune with our surroundings we only beat the air, and our efforts are in vain. we get no sympathetic response. chapter xiv. way duplex system. a novel form of double transmission was invented by the writer soon after the completion of the harmonic system, and was an outgrowth of it. it is still in use on some of the railroad-lines. an ordinary railroad telegraph-line has an instrument in circuit in every office along the road, chiefly for purposes of train-dispatching. as we have heretofore explained, whenever any one office is sending, the dispatch is heard in all of the offices. the "way duplex" system permits of the use of the line for through business simultaneously with the operation of the local offices. that is to say, any station along the line may be telegraphing with any other station by the ordinary morse method, and at the same time messages may be passing back and forth between the two end offices. this is accomplished by the following method: at each end of the line there is a tuned reed, such as we have described in our last chapter, that is kept constantly in vibration by a local battery during working hours. this vibrator is so arranged in relation to the battery that whenever the key belonging to it is depressed the current all through the line is rendered vibratory. there is also in circuit at each end of the line a harmonic relay, that is tuned in accord with the vibrating reed of the sender. if either key belonging to this part of the system is opened, as in the act of sending a message, these harmonic relays, being tuned in sympathy with the sending-vibrator, will respond, thus sending morse characters made up of a tone broken into dots and dashes. this tone can be read directly from the relay, or, as is usually the case, it causes the sounder to operate in the common way. you will at once inquire why the ordinary morse instruments in the local offices are not affected by these vibratory signals, and also why the harmonic instruments at the end office are not affected by the working of the local offices. the local office does not open the circuit entirely, but simply cuts out a resistance by the operation of the special harmonic key. when a resistance is thrown into an electric circuit it weakens the current in proportion to the amount of resistance interposed. you will see that there is some current still left in the line when the key is open, but the spring of the relay at the local office is so adjusted as to pull the armatures away from the magnets whenever the current is weakened by throwing in the resistance, so that by this means an ordinary morse telegraphic relay may be worked without ever entirely opening the circuit. in the way duplex system there is a resistance at each station that is cut in and out by the operation of its key, which causes all the instruments in the line to work simultaneously except the two harmonic relays located one at each end of the line. these will not respond to anything but the vibratory signal. in order to prevent the morse relays at the local offices from responding to the vibratory current a condenser is connected around them. this condenser serves two purposes: it enables the short impulses of the vibrating current to pass around the relays without having to be resisted by the coils of the magnets, and between the pulsations each condenser will discharge through the relay at the local offices, and thus fill in the gap between the pulsations, producing the effect on the relay of a steady current. when a line is thus equipped it may be treated in every respect as two separate wires, one of them doing way business and the other through business. it is a curious blending of science and mechanism. another interesting application was made of the system of transmission by musical tones--by edison, some years ago. we refer to the transmission of messages to and from a moving railroad-train with the head office at the end of the line. in this case the message was transmitted a part of the distance through the air;--another instance of wireless telegraphy. the operation was as follows: one of the wires strung on the poles nearest to the track was fitted up with a vibrator and key at the end of the line similar to that of the way duplex just described. in one of the cars was another battery, key and vibrator, and as only one tone was used, no tone-selecting device or harmonic relay was needed, but instead an ordinary receiving-telephone was used to read the long and short sounds sent over the lines. one end of the battery in the car was connected through the wheels to the earth, while the other end was connected to the metal roof of the car. being thus equipped, we will suppose our train to be out on the road forty or fifty miles from either end of the line, moving at the rate of forty miles an hour. the operator at chicago, say, wishes to send a message to the moving train; he operates his key in the ordinary manner, which makes the current on the line vibratory during the time the key is depressed. these electrical vibrations cause magnetic vibrations, or ether-waves, to radiate in every direction from the wire, at right angles to the direction of the current, like rays of light. when they strike the roof of the car they create electrical impulses in the metal by induction (described in chap. vi). these impulses pass through a telephone located in the car to the ground. a morse operator listening, with the telephone to his ear, will hear the message through the medium of a musical tone chopped up into the morse code. in like manner the operator in the car may transmit a message to the roof of the car and thence through the air to the wire, which will be heard, by any one listening, in a telephone which is connected in that circuit,--and, as a matter of fact, it will be heard from any wire that may be strung on any of the poles on either side of the road. some years ago an experiment of this kind was made on one of the roads between milwaukee and chicago. what wonderful things can be done with electricity! as a servant of man it is reliable and accurate--seeming almost to have the qualities of docility--when under intelligent direction, that is in accord with the laws of nature; but under other conditions it changes from the willing servant to a hard master, hesitating not to destroy life or property without regard to persons or things. chapter xv. telephony. in the foregoing chapters i have described the method of transmitting musical tones telegraphically and its applications to multiple telegraphy, as well as to a mode of communicating with a moving railroad-train. as i stated in a former chapter, after discovering a method of transmitting harmony as well as melody, i had in mind two lines of development, one in the direction of multiple telegraphy, and the other that of the transmission of articulate speech. i will not attempt to give the names of all the people who have contributed to the development of the telephone (as this alone would fill a volume) but only describe my own share in the work--leaving history to give each one due credit for his part. while i do not intend, here, to enter into any controversy regarding the priority of the invention of the telephone, i wish to say that from the time i began my researches, in the winter of - , until some time after i had filed my specification for a speaking or articulating telephone, in the winter of - , i had no idea that any one else had done or was doing anything in this direction. i wish to say further that if i had filed my description of a telephone as an application for a patent instead of as a caveat, and had prosecuted it to a patent, without changing a word in the specification as it stands to-day, i should have been awarded the priority of invention by the courts. i am borne out in this assertion by the highest legal authority. in law, a _caveat_ (latin word, meaning "let him beware") is a warning to other inventors, to protect an incomplete invention; whereas in fact the invention to be protected may be complete. an _application_ for a patent is presumed by the law to be for a completed invention; but it may be, and very often is, incomplete. it would often make a very great difference if decisions were rendered according to the facts in the case rather than according to rules of law and practice, that sometimes work great injustice to individuals. as has been said in another chapter, in the summer of i went to europe in the interest of the telephone, taking my apparatus, as then developed, with me. i came home early in the fall and resumed my experimental work. many interesting as well as amusing things occurred during these experiments. i remember that in the fall or early winter of i was in milwaukee with my apparatus carrying on some experiments on a wire between milwaukee and chicago. i had my musical transmitter along, and one evening, for the entertainment of some friends at the newhall house, a wire was stretched across the street from the telegraph office into one of the rooms of the hotel. a great number of tunes were played at the telegraph-office by mr. goodridge, who was my assistant at that time, which were transmitted across the street, as before stated. in those days it was a common practice in telegraphy to use one battery for a great number of lines. for instance, starting with one ground-wire which connected with, say, the negative pole of the battery, from the positive pole two, three or a half-dozen lines might be connected, running in various directions, connecting with the ground at the further end, thus completing their circuits. for use in transmitting tones across the street that evening we connected our line-wire on to the telegraph company's battery, which consisted of or more cells, and which had four or five more lines radiating from the end of the battery to different parts of wisconsin. our line was tapped on to the battery (without changing any of its connections) twenty cells from the ground-wire. in transmitting, each vibration would momentarily shut off these twenty cells from the lines that were connected with the whole battery. the effect of this (an effect that we did not anticipate at the time) was to send a vibratory current out on all the lines that were connected with that single battery as well as across the street. a great many familiar tunes were played during the course of an hour or two which, unconsciously for us, were creating great consternation throughout the state of wisconsin, in many of the offices through which these various lines passed. next morning reports and inquiries began to come in from various towns and cities west, northwest and north, giving details of the phenomena that were noticed on the instruments located in the various offices along the lines. they reported their relays as singing tunes; one party said he thought the instruments were holding a prayer-meeting from the fact that they seemed to be singing hymn-tunes for quite a while, but this notion was finally dissipated, because they grew hilarious and sang "yankee doodle." one operator, up in the pine woods of northern wisconsin, did not seem to take the cheerful view of it that some of the others did. he was sitting alone in the telegraph-office that evening when he thought he heard the notes of a bugle in the distance; he got up and went to the door to listen, but could hear nothing; but on coming back into the room he heard the same bugle notes very faintly. he was inclined to be somewhat superstitious and grew very nervous; finally, on looking around, he located the sound in his relay, but this did not help matters with him. with superstitious awe he listened to the instrument for a few moments, while it gave out the solemn tones of "old hundred," then it suddenly jumped into a hilarious rendering of "yankee doodle." this was too much for our nervous friend, and hastily putting on his overcoat, he left the office for the night. on another occasion, when i was giving a lecture in one of the cities outside of chicago, where exhibitions of music transmitted from chicago were given, one of the operators along the line was very much astonished by his switchboard suddenly becoming musical. orders had been given for the instruments in all the local offices to be cut out of the particular line that i was using. hence the instrument in this particular office was not in the circuit through which the tunes were being transmitted. the wire, however, ran through his switchboard, and owing probably to a loose connection, or an induced effect, there was a spark that leaped across a short space at each electrical pulsation that passed through the line, thus reproducing the notes of the various tunes played. you will remember in one of the chapters on sound (volume ii.), it is stated that a musical tone is made up of a succession of sounds repeated at equal intervals, and that the pitch of the tone is determined by the number of sound-impulses per second. applying this law to the sparks, you will be able to see how the switchboard played tunes for the operator. in the foregoing experiments in transmitting musical tones telegraphically, i used a great many different varieties of receivers. some of them were designed with metal diaphragms mounted over single electromagnets, not unlike the receiver of an ordinary telephone. these instruments would both transmit and receive articulate speech when placed in circuit with the right amount of battery to furnish the necessary magnetism. however, they were not used in that way at the time they were first made--in . these i called common receivers, as they were designed to reproduce all tones equally well. i designed and constructed another form of receiver, based somewhat upon the theory of the harmonic telegraph. this consisted of an electromagnet of considerable size, mounted upon a wooden rod about ten feet long. mounted upon this rod were also resonating boxes or tubes made of wood of the right size to have their air-cavities correspond with the various pitches of the transmitting-reeds, so that each tone would be re-enforced by some one of these air-cavities, thus giving a louder and more resonant effect to the musical notes. here were two types of receiver, one that would receive one sound as well as another, but none of them so loud, while the other was constructed on the principle of selection and re-enforcement, so that a particular note would be sounded by the box having a cavity corresponding to the pitch of the tone, and was much louder and of much better quality than i could get from the diaphragm receiver. one of these receivers pointed to the harmonic telegraph and the other to the speaking telephone. i knew that i had a receiver that would reproduce articulate speech or anything else that could be transmitted. my first conceptions of an articulate speech-transmitter were somewhat complicated. i conceived of a funnel made of thin metal having a great number of little riders, insulated from the funnel at one end and resting lightly in contact with the funnel at the other end. these riders were to be made of all sizes and weights so as to be responsive to all rates of vibration. in the light of the present day we know that such an arrangement would have transmitted articulate speech, but perhaps not so well as a single point would do when properly adjusted. my mind clung to this idea till in the fall of , when an observation i made upon the street changed the whole course of my thinking and solved the problem. the incident i refer to took place in milwaukee, where i was then experimenting. one day while out on an errand i noticed two boys with fruit-cans in their hands having a thread attached to the center of the bottom of each can and stretched across the street, perhaps feet apart. they were talking to each other, the one holding his mouth to his can and the other his ear. at that time i had not heard of this "lovers' telegraph," although it was old. it is said to have been used in china years ago. the two boys seemed to be conversing in a low tone with each other and my interest was immediately aroused. i took the can out of one of the boy's hands (rather rudely as i remember it now), and putting my ear to the mouth of it i could hear the voice of the boy across the street. i conversed with him a moment, then noticed how the cord was connected at the bottom of the two cans, when, suddenly, the problem of electrical speech-transmission was solved in my mind. i did not have an opportunity immediately to construct an instrument, as i had a partner who was furnishing money for the development of the harmonic telegraph and would not listen to any collateral experiments. i remember sitting down by this partner one day and telling him what i could do in the way of transmitting speech through a wire. i told him i thought it would be very valuable if worked out. he gave me a look that i shall never forget, but he did not say a word. the look conveyed more meaning than all the words he could have said, and i did not dare broach the subject again. however, as soon as i found opportunity, without saying a word to anybody except my patent lawyer, i filed a description, accompanied by drawings, of a speaking telephone which stands in history to-day as the first complete description on record of the operation of the speaking telephone. it described an apparatus which, when constructed, worked as described, and it is a matter of history that the first articulate speech electrically transmitted in this country was by a transmitter constructed on the principle described, and almost identically after the drawings in my caveat. while the transmitter described in this caveat was not the best form, it would transmit speech, and it contained the foundation principle of all the telephone transmitters in use to-day. there are two methods of transmitting speech. one is known as the magneto method and the other that of varying the resistance of the circuit. my first transmitter was devised on the latter principle. i append to this extracts from my specification filed feb. , : _to all whom it may concern:_--be it known that i, elisha gray of chicago, in the county of cook and state of illinois, have invented a new art of transmitting vocal sounds telegraphically, of which the following is a specification: it is the object of my invention to transmit the tones of the human voice through a telegraphic circuit, and reproduce them at the receiving-end of the line, so that actual conversations can be carried on by persons at long distances apart. i have invented and patented methods of transmitting musical impressions or sounds telegraphically, and my present invention is based upon a modification of the principle of said invention, which is set forth and described in letters patent of the united states, granted to me july , , respectively numbered , and , , and also in an application for letters patent of the united states, filed by me, feb. , . * * * my present belief is that the most effective method of providing an apparatus capable of responding to the various tones of the human voice is a tympanum, drum, or diaphragm, stretched across one end of the chamber, carrying an apparatus for producing fluctuations in the potential of the electric circuit and consequently varying in its power. * * * the vibrations thus imparted are transmitted through an electric circuit to the receiving-station, in which circuit is included an electromagnet of ordinary construction, acting upon a diaphragm to which is attached a piece of soft iron, and which diaphragm is stretched across a receiving vocalizing chamber _c_, somewhat similar to the corresponding vocalizing chamber _a_. the diaphragm at the receiving-end of the line is thus thrown into vibrations corresponding with those at the transmitting-end, and audible sounds or words are produced. the obvious practical application of my improvement will be to enable persons at a distance to converse with each other through a telegraphic circuit, just as they now do in each other's presence, or through a speaking-tube. i claim as my invention the art of transmitting vocal sounds or conversations telegraphically through an electric circuit. this specification was accompanied by cuts of the transmitter and receiver connected by a line-wire and showing one person talking to the transmitter and another listening at the receiver. these cuts may be seen in various books on the subject of telephony. chapter xvi. how the telephone talks. everybody knows what the telephone is because it is in almost every man's house. but while everybody knows what it is, there are very few (comparatively speaking) that know how it works. if you remember what has been said about sound and electromagnetism it will not be hard to understand. when any one utters a spoken word the air is thrown into shivers or vibrations of a peculiar form, and every different sound has a different form. therefore, every articulate word differs from every other word, not only as a shape in the air, but as a sensation in the brain, where the air-vibrations have been conducted through the organ of hearing; otherwise we could not distinguish between one word and another. every different word produces a different sensation because there is a physical difference, as a shape or motion, in the air where it is uttered. if one word contains simultaneous air-motions and another you can see that there is a physical or mechanical difference in the air. the construction of the simplest form of telephone is as follows: take a piece of iron rod one-half or three-quarters of an inch long and one-quarter inch thick, and after putting a spool-head on each end to hold the wire in place wind it full of fine insulated copper wire; fasten the end of this spool to the end of a straight-bar permanent magnet. then put the whole into a suitable frame, and mount a thin circular diaphragm (membrane or plate) of iron or steel, held by its edges, so that the free end of the spool will come near to but not touch the center of the diaphragm. this diaphragm must be held rigidly at the edges. now if the two ends of the insulated copper wires are brought out to suitable binding-screws the instrument is done. the permanent steel magnet serves a double purpose. when the telephone was first used commercially, the instrument now used as a receiver was also used as a transmitter. as a transmitter it is a dynamo-electric machine. every time the iron diaphragm is moved in the magnetic field of the pole of the permanent magnet, which in this case is the free end of the spool (the iron of the spool being magnetic by contact with the permanent magnet), there is a current set up in the wire wound on the spool; a short impulse, lasting only as long as the movement lasts. the intensity of the impulse will depend upon the amplitude and quickness of the movement of the diaphragm. if there is a long movement there will be a strong current and vice versa. if a sound is uttered, and even if the multitude of sounds that are required to form a word, be spoken to the diaphragm, the latter partakes in kind of the air-motions that strike it. it swings or vibrates in the air, and if it is a perfect diaphragm it moves exactly as the air does, both as to amplitude and complexity of movement. you will remember that in the chapter on sound-quality (vol. ii) it was said that there were hundreds and sometimes thousands of superposed motions in the tones of some voices that gave them the element we call quality. all these complex motions are communicated by the air to the diaphragm, and the diaphragm sets up electric currents in the wire wound on the spool, corresponding exactly in number and form, so that the current is molded exactly as the air-waves are. now, if we connect another telephone in the circuit, and talk to one of them, the diaphragm of the other will be vibrated by the electric current sent, and caused to move in sympathy with it and make exactly the same motions relatively, both as to number and amplitude. it will be plain that if the receiving diaphragm is making the same motions as the transmitting diaphragm, it will put the air in the same kind of motion that the air is in at the transmitting end, and will produce the same sensation when sensed by the brain through the ear. if the air-motion is that of any spoken word it will be the same at both ends of the line, except that it will not be so intense at the receiving-end; it is the same relatively. and this is how the telephone talks. i have said that the permanent magnet had two functions. in the case of the transmitter it is the medium through which mechanical is converted into electrical energy. it corresponds to the field-magnet of the dynamo, while the diaphragm corresponds to the revolving armature, and the voice is the steam-engine that drives it. in the second place, it puts a tension on the diaphragm and also puts the molecules of the iron core of the magnet in a state of tension or magnetic strain, and in that condition both the molecules and the diaphragm are much more sensitive to the electric impulses sent over the wire from the transmitter. this fact was experimented upon by the writer as far back as and published in the journal of the american electrical society. at the present day this form of telephone is used only as a receiver. transmitters have been made in a variety of forms, but there are only two generic methods of transmission. one is the magneto method--the one we have described--and the other is effected by varying the resistance of a battery current. the former will work without a battery, as the voice acting on the wire around the magnet through the diaphragm creates the current; in the latter the current is created by the battery but molded by the voice. in the latter method the current passes through carbon contacts that are moved by the diaphragm. carbon is the best substance, because it will bear a wider separation of contact without actually breaking the current. when carbon points are separated that have an electric current passing through them, there is an arc formed on the same principle as the electric arc-light. great improvements in details have been made in the telephone since its first use, but no new principles have been discovered as applied to transmission. we have spoken in another place regarding the various claimants to the invention of the telephone, but here is one that has been overlooked. a young man from the country was in a telegraph-office at one time and was left alone while the operator went to dinner. suddenly the sounder started up and rattled away at such a rate that the countryman thought something should be done. he leaned down close to the instrument and shouted as loudly as possible these words: "the operator has gone to dinner." from what we know now of the operation of the telephone i have no doubt but that he transmitted his voice to some extent over the wire. this young man's claims have never been put forward before, and we are doing him tardy justice. but his claim is quite as good as many others set forth by people who think they invent, whenever it occurs to them that something new might possibly be done, if only somebody would do it. and when that somebody does do it they lay claim to it. in the early days of the telephone it was not supposed that a vocal message could be transmitted to a very great distance. however, as time went on and experiments were multiplied the distance to which one could converse with another through a wire kept on increasing. in these days, as every one knows, it is a daily occurrence that business men converse with each other, telephonically, for a distance of miles or more; in fact, it is possible to transmit the voice through a single circuit about as great a distance as it is possible to practically telegraph. this leads us to speak of another telegraphic apparatus which we have not heretofore mentioned, and that is the telegraphic repeater. it is a common notion that messages are sent through a single circuit across the continent, but this is not the case, although the circuits are very much longer than they were some years ago. the repeater is an instrument that repeats a message automatically from one circuit to another. for instance, if chicago is sending a message to new york through two circuits, the division being in buffalo, the repeater will be located at buffalo and under the control of both the operator at chicago and the operator in new york. when chicago is sending, one part of the repeater works in unison with the chicago key and is the key to the new york circuit, which begins at buffalo. when new york is sending the other part of the repeater operates, which becomes a key which repeats the message to the chicago line. in this way the practical result is the same as though the circuit were complete from new york to chicago. at the present day some of the copper wires and perhaps some of the larger iron wires are used direct from chicago to new york without repetition, but all messages between new york and san francisco are automatically repeated at least twice and under certain conditions of weather oftener. i can remember that in wet weather in the old days, with such wires as they had then (being no. iron with bad joints, which gave the circuit a high resistance) that these repeaters would be inserted at toledo, cleveland, buffalo and albany in order to work from chicago to new york. under such conditions the transmission would necessarily be slow, because an armature time will be lost at each repeater. regarding each repeater as a key, when chicago depresses his key the armature of the next repeater must act, and then the next successively, and all of this takes time, although only a small fraction of a second. the repeater was a very delicate instrument and had to be handled by a skilled operator. every wire must be in its place or the instrument would fail to operate. i remember on one occasion in cleveland that along in the middle of the night the repeater failed to work. the operator knew nothing of the principle of its operation, so that when it failed he had to appeal to some of his superiors. at this time there was no one in the office who knew how to adjust it, so they had to send up to the house of the superintendent and arouse him from his sleep and bring him down to the office. he looked under the table and found that one of the wires had loosened from its binding-post and was hanging down. he said immediately, "here's the trouble; i should think you could have seen it yourself." the operator replied, "i did see that, but i didn't think one wire would make any difference." he learned the lesson that all electricians have had to learn--that even one wire makes all the difference in the world. but this operator was no worse in that respect than some of his superiors. one of the heads of the cleveland office at one time in the early days wanted to give some directions to the office at buffalo. he told the operator at the key to tell buffalo so and so, when the operator replied: "i can't do it; buffalo has his key open." the official immediately said with severity: "tell him to close it." he forgot that it would be as difficult for him to tell him to close it, as it would have been to have sent the original message. but let us go back to the telephone. while it is possible to send a message from new york to san francisco by telegraph, it is not possible to telephone that distance, because as yet no one has been able to devise a repeater that will transfer spoken words from one line to another satisfactorily. but unless the printer and publisher bestir themselves some one may accomplish the feat before this little book reaches the reader. if this proves to be true, let the writer be the first to congratulate the successful inventor. chapter xvii. submarine cables. the first attempts at transmitting messages through wires laid in water were made about . these early experiments were not very successful, because the art of wire-insulation had not attained any degree of perfection at that time. it was not until gutta-percha began to be used as an insulator for submarine lines that any substantial progress was made. the first line, so history states, that was successfully laid and operated was across the hudson river in . this line was constructed for the use of the magnetic telegraph company. in the following year experiments with gutta-percha insulation were successfully made, and about a cable was laid across the english channel between dover and calais (twenty-seven miles), consisting of a single strand of wire having a covering of gutta-percha. the insulation was destroyed in a day or two, which demonstrated the fact that all submarine cables must be protected by some kind of armor. in another cable was laid between these two points, containing four conductors insulated with gutta-percha, and over all was an armor of iron wire. twenty-one years later this cable was still working, and for all we know is working now. after this successful demonstration other cables were laid for longer distances. these short-line cables served to demonstrate the relative value of different material for insulating purposes under water, and it has been found that gutta-percha possesses qualities superior to almost every other material as an insulator for submarine cables, although there are many better materials for air-line insulation. gutta-percha when exposed to air becomes hardened and will crack, but under water it seems to be practically indestructible. ocean telegraphy really dates from the laying of the first successful atlantic cable. there were many problems that had to be solved, which could be done only by the very expensive experiment of laying a cable across the atlantic ocean. in the first place a survey had to be made of the bottom of the ocean between the shores of america and great britain. the most available route was discovered by lieutenant maury of the united states navy, who made a series of deep-sea soundings, and discovered that, from newfoundland to the west coast of ireland the bottom of the ocean was comparatively even, but gradually deepening toward the coast of ireland until it reached a depth of fathoms. it was not so deep but that the cable could be laid on the bottom, nor so shallow as to be in danger of the waves, icebergs or large sea-animals. the water below a certain depth is always still and not affected by winds or ocean currents. at many other points in crossing the ocean, high mountains and deep valleys are encountered, possessing all the topographical features of dry land--as the ocean bed is only a great submerged continent. the beginning of the laying of the first atlantic cable was on aug. , . on the morning of aug. , , a year later, after a series of mishaps and adverse circumstances that would have discouraged most men, the country was electrified by a dispatch from cyrus w. field of new york (to whom the final success of the atlantic cable is mainly due), that the cable had been successfully laid and worked. but this cable worked only from the th of august to the st of september, having sent in that time messages. the insulation became impaired at some point, when an attempt to force the current through by means of a large battery only increased the difficulty. the failure of this first cable served to teach manufacturers and engineers how to construct cables with reference to the conditions under which they are to be used. it was found that in the deep sea a much smaller and less expensive cable could be used than would answer at the shore ends, where the water is shallow. the shore ends of an ocean cable are made very large, as compared to the deep-sea portions, so as to resist the effect of the waves and other interfering obstacles. it was further learned that the most successful mode of transmitting signals through the cable was with a small battery of low voltage, and by the use of very delicate instruments for receiving the messages. it is not possible to employ such instruments on cables as are used on land-lines, while it would not be a difficult feat to transmit even twice the distance over land-lines strung on poles, using the ordinary morse telegraph. the water of the ocean is a conductor, as well as the heavy armor that surrounds the insulation of the cable. when a current is transmitted through the conducting wires, in the center of the cable, they set up a countercharge in the armor and the water above it, somewhat as an electrified cloud will induce a charge in the earth under it, of an opposite nature. this countercharge, being so close to the conducting wire, has a retarding effect upon the current transmitted through it. an ordinary land-line that is strung on poles that are high up from the ground has this effect reduced to a minimum, but the greater the number of wires clustered together on the same poles the more difficult it becomes to send rapid signals through any one of them. the instrument used for receiving cable messages was devised by sir william thompson, now lord kelvin. one form consists of a very short and delicate galvanometer-needle carrying a tiny mirror. this mirror is so related to a beam of light thrown upon it that it reflects it upon a graduated screen at some distance away, so that its motions are magnified many hundred times as it appears upon the screen. an operator sits in a dark room with his eye on the screen and his hand upon the key of an ordinary morse instrument. he reads the signal at sight, and with his key transmits it to a sounder, which may be in another room, where it is read and copied by another operator. another form of receiving-instrument carries, instead of the mirror, a delicate capillary glass tube that feeds ink from a reservoir, and by this means the movements of the needle are recorded on a moving strip of paper. the symbols (representing letters) are formed by combinations of zigzag lines. this instrument is the syphon-recorder. chapter xviii. short-line telegraphs. early in the history of the telegraph short lines began to be used for private purposes, and as the morse code was familiar only to those who had studied it and were expert operators on commercial lines, some system had to be devised that any one with an ordinary english education could use; as the expense of employing two morse operators would be too great for all ordinary business enterprises. these short lines are called private lines, and the instruments used upon them were called private-line telegraph-instruments. of course they are now nearly all superseded by the telephone, but they are a part of history. one of the earliest forms of short-line instruments was called the dial-telegraph. one of the first inventors, if not the first, of this form of instrument was professor wheatstone of england, who perfected a dial-telegraph-instrument about the year . the receiving-end of this instrument consisted of a lettered dial-face, under which was clockwork mechanism and an escape-wheel controlled by an electromagnet. each time the circuit was opened or closed the wheel would move forward one step, and each step represented one of the letters of the alphabet, so that the wheel, like the type-wheel of a printing telegraph, had fourteen teeth, each tooth representing two steps. as the reciprocating movement of the escapement had a pallet or check-piece on each side of the wheel, its movement was arrested twenty-eight times in each revolution. these twenty-eight steps correspond to the twenty-six letters of the alphabet, a dot and a space. on the shaft of the escape-wheel is fastened a hand or pointer, which revolves over a dial-face having the twenty-six letters of the alphabet, also a dot and space. the pointer was so adjusted that when the escape-wheel was arrested by one of the pallets it would stop over a letter, showing thus, letter by letter, the message which the sender was spelling out. the transmitter consisted of a crank with a knob and a pointer on it, which was mounted over a dial that was lettered in the same way as the face of the receiving-instrument. a revolution of this crank would break and close the circuit twenty-eight times; that is to say, there were fourteen breaks and fourteen closes of the circuit. if now the transmitting-pointer and the receiving-pointer are unified so that they both start from the same point on the dial, and the transmitting-crank is rotated from left to right, the receiving-pointer will follow it up to the limit of its speed. in transmitting a message the sender would turn his crank, or pointer, to the first letter of the word he wished to transmit, making a short pause, and then move on to the next letter, and so on to the end of the message, making a short pause on each letter. the end of a word was indicated by turning the pointer to the space-mark on the dial. the receiving-operator would read by the pauses of the needle on the various letters. this was a system of reading by sight. there have been many forms of this dial-telegraph worked out by different inventors at different times, and quite a number of them were used in the old days. it was a slow process of telegraphing, but it was suited to the age in which it flourished. one of the difficulties of a dial-telegraph consisted in the readiness with which the transmitter and receiver would get out of unison with each other; and when this happened of course a message is unintelligible, and you have to stop and unify again. about the writer invented a dial-telegraph to obviate this difficulty. in this system a transmitter and receiver were combined in one instrument, and instead of a crank there were buttons arranged around the dial in a circle, one opposite each letter. when not in operation the pointers of both instruments at both stations stood at zero. in the act of transmitting the operator would depress the button opposite the letter he wished to indicate, when immediately the pointers of both instruments would start up and move automatically, step by step, until the pointer came in contact with the stem of the depressed button, when it would be arrested, and at the same time cut out the automatic transmitting-mechanism and cause both needles to remain stationary during the time the button was depressed. upon releasing the button the pointers both fall back to zero at one leap. the first private line equipped by this instrument was for rockefeller, andrews & flagler, which was the firm name of the parties who afterward organized the standard oil company. this line was built between their office on the public square in cleveland and their works over on the cuyahoga flats. it seemed, however, to be the fate of the writer to make new inventions that would supersede the old ones before they were fairly brought into use. very soon after the dial-telegraph began to be used, printing telegraph instruments for private-line purposes superseded them. about a printing instrument was devised for stock reporting, which in one of its forms is still in use. soon after the invention of this form of printer a company was organized to operate not only these stock-reporting lines, but short lines for all sorts of private purposes. following the invention of the stock-reporting instrument there were several adaptations made of the printing telegraph for private-line purposes. among others the writer invented one known as "gray's automatic printer," a cut and a description of which may be found on page in "electricity and electric telegraph," by george b. prescott, published in . this instrument was adopted by the gold and stock telegraph company as their standard private-line printer. it was first introduced in the year , and at the time the telephone began to be used there were large numbers of these printers in operation in all of the leading cities and towns in the united states. while this has been superseded to a large extent by the telephone, there are still a few isolated cases where it is used. short lines have multiplied for all sorts of purposes, until to-day the money invested in them largely exceeds the amount invested in the regular commercial telegraphic enterprises. the invention of the telephone created such a demand for short-line service that some scheme had to be devised not only to make room for the necessary wires, but to so cheapen the instruments as to bring them within reach of the ability of the ordinary man of business. this problem has been solved (but not without many difficulties) by the inauguration of what is known as the "central station." by this system one party simply controls a single wire from his office or residence to the central station; here he can have his line connected with any other wire running into this same station, by calling the central operator and asking for the required number. it is useless to tell the public that very often this number is "busy," and here is the great drawback to the central-station system. this is especially true in large cities, where there are a great number of lines. the switchboards in large cities are necessarily very complicated affairs, and it requires a number of operators to answer the many calls that are constantly coming in. each central-station operator presides over a certain section of the board, and as this section has to be related in a certain way to every other section, it is easy to see wherein arises the complication. in large cities the central stations themselves have to be divided and located in different districts, being connected by a system of trunk lines. chapter xix. the telautograph. so far we have described several methods of electrical communication at a distance, including the reading of letters and symbols at sight (as by the dial-telegraph and the morse code embossed on a strip of paper); printed messages and messages received by means of arbitrary sounds, and culminating in the most wonderful of all, the electrical transmission of articulate speech. none of these systems, however, are able to transmit a message that completely identifies the sender without confirmation in the form of an autograph letter by mail. in there was exhibited in the electrical building at the world's fair an instrument invented by the writer called the telautograph. as the word implies, it is a system by which a man's own handwriting may be transmitted to a distance through a wire and reproduced in facsimile at the receiving-end. this instrument has been so often described in the public prints that we will not attempt to do it here, for the reason that it would be impossible without elaborate drawings and specifications. it is unnecessary to state that it differs in a fundamental way from other facsimile systems of telegraphy. suffice it to say that as one writes his message in one city another pen in another city follows the transmitting-pen with perfect synchronism; it is as though a man were writing with a pen with two points widely separated, both moving at the same time and both making exactly the same motions. by this system a man may transact business with the same accuracy as by the united states mail, and with the same celerity as by the electric telegraph. a broker may buy or sell with his own signature attached to the order, and do it as quickly as he could by any other method of telegraphing, and with absolute accuracy, secrecy and perfect identification. in , when this apparatus was first publicly exhibited, it operated by means of four wires between stations, and while the work it did was faultless, the use of four wires made it too expensive and too cumbersome for commercial purposes; so during all the years since then the endeavor has been to reduce the number of wires to two, when it would stand on an equality with the telephone in this respect. it is only lately that this improvement has been satisfactorily accomplished, and, for reasons above stated, no serious attempt has been made to introduce it as yet; but it has been used for a long enough time to demonstrate its practicability and commercial value. companies have been organized both in europe and america for the purpose of putting the telautograph into commercial use. by means of a switch located in each subscriber's office the wires may be switched from a telephone to a telautograph, or vice versa, in a moment of time. by this arrangement a man may do all the preliminary work of a business transaction through the telephone, and when he is ready to put it into black and white switch in the telautograph and write it down. for ordinary exchange work this is undoubtedly the true way to use the telautograph, because one system of wires and one central-station system will answer for both modes of communication, and in this way an enormous saving can be made to the public. there is no question in the mind of any one who is familiar with the operation of both the telephone and telautograph but that some day they will both be used, either in the same or separate systems, as they each have distinctly separate fields of usefulness,--the telephone for desultory conversation, the telautograph for accurate business transactions. the question may arise in the minds of experts how the two systems can be worked in the same set of cables, and this leads us to discuss the phenomena of induction. every one who has listened at a telephone has heard a jumble of noises more or less pronounced, which is the effect of the working of other wires in proximity to those of the telephone. if, when a morse telegraph instrument is in operation on one of a number of wires strung on the same poles, we should insert a telephone in any one of the wires that were strung on the same poles or on another set of poles even across the street, we could hear the working of this morse wire in the telephone, more or less pronounced, according to the distance the wire is from the morse circuit. this phenomenon is the result of induction, caused by magnetic ether-waves that are set up whenever a circuit is broken and closed, as explained in chapter vi. the telephone is perhaps the most sensitive of all instruments, and will detect electrical disturbances that are too feeble to be felt on almost any other instrument, hence the telephone is preyed upon by every other system of electrical transmission, and for this reason has to adopt means of self-protection. it has been found that the surest way to prevent interference in the telephone from neighboring wires is to use what is called a metallic circuit--that is to say, instead of running a single wire from point to point and grounding at each end, as in ordinary telegraph systems, the telephone circuit is completed by using a second wire instead of the earth. as a complete defense against the effects of induced currents the wires should be exactly alike as to cross-section (or size) and resistance. they should be insulated and laid together with a slight twist. this latter is to cause the two wires so twisted to average always the same distance from any contiguous wire. one factor in determining the intensity of an induced current is the distance the wire in which it flows is from the source of induction. a telephone put in circuit at the end of the two wires that are thus laid together will be practically free from the effects of induced currents that are set up by the working of contiguous wires--for this reason: whenever a current is induced in one of the slack-twisted wires it is induced in both alike; the two impulses being of the same polarity meet in the telephone, where they kill each other. in order to have a perfect result we must have perfect conditions, which are never attained absolutely, but nearly enough for all practical purposes. in the early days of telephony great difficulty was experienced in using a single wire grounded at each end in the ordinary way, if it ran near other wires that were in active use. as time passed on and the electric light and electric railroad came into operation these difficulties were immensely increased, till now in large cities the telephone companies are fast being driven to the double-wire system, which will soon become universal for telephonic purposes the world over, except perhaps in a few country places where there is freedom from other systems of electrical transmission. to successfully work the telephone and telautograph through the same cables, these protective devices against induction must be very carefully provided and maintained. chapter xx. some curiosities. until within recent years it was never supposed that a sunbeam would ever laugh except in poetry. but the modern scientist has taken it out of the realm of poetry and put it into the prosy play of every-day life. the radiophone, invented by a. g. bell, is an instrument by which articulate or other sounds are transmitted through the medium of a ray of light. it has as yet no practical application and has never gone beyond the experimental stage, but as a bit of scientific information it is very interesting. if we introduce into an electric circuit a piece of selenium, prepared in a certain way, its resistance as an electric conductor undergoes a radical change when a beam of sunlight is thrown upon it. for instance, a selenium cell, so called, that in the dark would measure ohms resistance, would have only about ohms when exposed to sunlight. this amount of variation in a short circuit of low resistance would produce a considerable change in the strength of a current passing through it from a battery of a given voltage. if now we connect a selenium cell to one pole of a battery, and thence through a telephone and back to the other pole, we have completed an electric circuit, of which the selenium cell is a part, and any variation of resistance in this cell, if made suddenly, will be heard in the telephone. let the diaphragm of a telephone transmitter have a very light, thin mirror on one side of it, and a beam of sunlight be thrown upon it and reflected from that on to the selenium cell, which may be some distance away. then, if the diaphragm is thrown into vibration by an articulate word or other sound, the light-ray is also thrown into vibration, which causes a vibratory change of resistance in the selenium cell in sympathy with the light-vibrations; and this in turn throws the electric current into a sympathetic vibratory state which is heard in the telephone. so that if a person laughs or talks or sings to the diaphragm, the sunbeam laughs, talks and sings and tells its story to the electric current, which impresses itself upon the telephone as audible sounds--articulate or otherwise. instead of the telephone, battery and selenium cell, a block of vulcanite or certain other substances may be used as a receiver; as a light-ray thrown into vibration has the power to produce sound or sympathetic vibration in certain substances. another curious application of the selenium cell has been attempted, but has scarcely gone beyond the domain of theory. this apparatus, if perfected, might be called a telephote. it is an apparatus by which an illuminated picture at one end of a line of many wires is reproduced upon a screen at the other end. the light is not actually transmitted, but only its effects. suppose a picture is laid off into small squares and there is a selenium cell corresponding to each square and for each selenium cell there is a wire that runs to a distant station in which circuit there is a battery. at the distant station there are little shutters, one for each wire, that are controlled by the electric current and so adjusted that when the cell at the transmitting-end is in the dark the shutter will be closed. now if a strong light be thrown upon the picture at the transmitting-end, and each square of the picture reflects the light upon its corresponding selenium cell, the high lights of the picture will reflect stronger light than the shadows, and therefore the wires corresponding to the high-light squares will have a stronger current of electricity flowing through them, because the resistance of the circuit is less than the ones connected with the darker shadows. so that the degree of current-strength in the various wires will correspond to the intensity of light reflected by the different sections of the picture. the shutters are so adjusted that the amount of opening depends upon the strength of current. the shutters corresponding to the high lights of the picture will open the widest and throw the strongest light upon the screen, from a source of light that is placed behind the shutters. the shutters that open the least will be those that are operated upon by the shadows of the picture. inasmuch as a picture thrown on a screen from a source of light is wholly made up of lights and shadows, the theory is that this apparatus perfectly constructed would transmit any picture to a distance, through telegraph-wires. it must not be understood that the rays of light are transmitted through the wires as sound-vibrations are. light, per se, can be transmitted only through the luminiferous ether, as we have seen in the chapter on light in volume ii. while we are talking about these curious methods of telegraphic transmission, i wish to refer to an apparatus constructed by the writer in - , for the purpose of receiving musical tones or compositions transmitted from a distance through a wire by electricity. (a cut of this apparatus is shown on page of "electricity and electric telegraph," by prescott, issued in .) it consists of a disk of metal rotated by a crank mounted on a suitable stand. the electric circuit passes through the disk to the hand of the operator in contact with it, thence running through the line-wire to the distant station. now, if a tune is played at that station, upon an electrical key-board, as described in a previous chapter, and the disk rotated with the fingers in contact with it, the tune or other sounds will be reproduced at the ends of the fingers. after the telephone was invented and put into use i used this revolving disk as a receiver for speech as well as music, and by this means persons may carry on an oral conversation through the ends of their fingers. this apparatus has been confounded in the minds of some people with edison's electromotograph. the phenomena of the electromotograph were produced by chemical effects, while that of the apparatus just described is electrostatic in its action. the electrostatic disk was made in the winter of - , while edison's electrochemical discovery was made some time later. chapter xxi. wireless telegraphy. broadly speaking, "wireless telegraphy" is any method of transmitting intelligible signals to a distance without wires; and this includes the old semaphore systems of visual signals, such as flags and long arms of wood by day, and lights by night; also the heliograph (an apparatus for flashing sunlight), and sound signals, made either through the air or water. electrical conduction, either through rarefied air or the earth, also comes under this heading. the name "wireless telegraphy," however, is specifically applied to a system of signaling by means of ether-waves induced by electrical discharges of very high voltage. ether-waves of a greater or less degree are always set up whenever there are sudden electrical disturbances, however slight. ether-waves, electrically induced, are probably as old as the universe. when "there were thunders and lightnings" from the cloud that hovered over mount sinai in the time of moses, ether-waves of great power were sent out through the camp of israel. but the people of those days had no "coherer" or telephone or any other means of converting these waves into visual or audible signals. thousands of years had to elapse before the intellect of man could grasp the meaning of these natural phenomena sufficiently to harness them and make them subservient to his will. many people have been powerfully "shocked"--some even killed--by the impact of ether-waves set up by powerful discharges of lightning between the clouds and the earth--when they were not in the direct path of the lightning-stroke. the history of electro-wireless telegraphy, like that of all inventions, is one of successive stages, and all the work was not done by one man. the one who gets the most credit is usually the one who puts on the finishing touches and brings it out before the public. he may have done much toward its development or he may have done but little. in the year morse transmitted a battery current through the water of a canal eighty feet wide so as to affect a galvanometer on the opposite side from the battery. this was wireless telegraphy by _conduction_ through water. in joseph henry produced an effect on a galvanometer by ether-waves through a distance of twenty feet by an arrangement of batteries and circuits like that shown in fig. , chapter vi. this was called _induction_, and is still so called when electrical effects are produced from one wire to another through the ether for short distances. all induction-coils and transformers (see chapter xxiv) are operated by effects produced through the ether from the primary to the secondary coil--but through very short distances. in professor trowbridge transmitted an electrical current through the earth for one mile so as to produce signals in a telephone. in - professor dolbear used for a short distance (fifty feet) substantially the same arrangement as marconi now uses, except that the former used a telephone as a receiver. he used an induction-coil having one end of the secondary wire connected with the earth, while the other was attached to a wire running up into the air. at the receiving-end a wire starting from the earth extended into the air, passing through a telephone, which acted as a receiver. in he used a kite to elevate the wire, through which electrical discharges of high voltage were made into the air to produce ether-waves--the receiver being feet away. dolbear's experiments were public fourteen years ago, but at that time there was no interest in such matters, so that his work received little or no attention. in dr. hertz of germany made some experiments in producing and detecting ether-waves, and he did a great deal to awaken an interest in the subject, so that others began investigations that have led to its present use as a means of telegraphing to a distance of many miles. in professor branly of paris invented the coherer. in it was improved by lodge and by him used as a detector of ether-waves. in , ten years after dolbear had used it with the kite at the transmitting-end and telephone at the receiving-end, marconi, an italian, substituted the coherer of branly for the telephone of dolbear. this coherer is constructed and operated as follows: it consists of a glass tube, of comparatively small diameter, loosely filled with metal filings of a certain grade. this body of metal-dust is made a part of a local battery circuit in which is placed an ordinary electric bell or telegraphic sounder. the resistance of this body of filings is so great that current enough will not pass through it to ring the bell or actuate the sounder until an ether-wave strikes it and the wire attached to it, when the metal particles are made to cohere to such an extent that the conductivity of the mass is greatly increased; so that a current of sufficient volume will now pass through the bell-magnet to ring it. before the next signal comes the filings must be made to de-cohere; and to accomplish this a little "tapper," that works automatically between the signals, strikes the glass tube with a succession of light blows. briefly stated, the wireless system of marconi, in its essentials, consists of a powerful induction-coil with one end of the secondary wire connected with the earth, while the other extends into the air a greater or less distance according to the distance it is desired to send signals. the greater the distance the higher the wire should extend into the air. at the receiving-end a wire of corresponding height is erected, also connected with the earth. in this wire--as a part of its circuit--is placed the coherer. in a local circuit that is connected to the upright wire in parallel with the coherer is placed a battery, a sounder, or a bell, that is rung when the filings cohere. when an ether-wave is set up by a discharge of electricity into the air it strikes the perpendicular wire of the receiver, and that portion of the wave that strikes is converted into electricity, which is called an induced current. it is this current, as it discharges through the coherer to the earth, that causes the filings to unite so as to close the local circuit and operate the sounder. to send a message it is only necessary to make the discharges into the air, at the sending-end, correspond to the morse alphabet. while marconi has done more than any other man to improve and popularize wireless telegraphy, history shows that he invented none of the essential elements so far as the system has been made public. what he seems to have really done was to substitute the coherer of branly and lodge, with its adjuncts, for the telephone of dolbear. there is no doubt but that marconi has done much to improve and enlarge the capacity of the apparatus and to demonstrate to the world some of its possibilities. he has been an indefatigable worker and deserves great credit; but without the work of those who preceded him he could not have succeeded: the honors should be divided. this system has been used at various times for reporting yacht-races, and between ships. it is said also to have been used to some extent in the south african war. there is much to be done yet, however, before it can be made entirely reliable for defensive work in time of war. as it is now, all an enemy would have to do to destroy its usefulness would be to set an ether-wave-producer to work automatically anywhere within the "sphere of influence" of the system--to speak diplomatically--when it would render unintelligible any message that should be sent. to make the system of the greatest value some sort of selective receiver must be invented that will select signals sent from a transmitter that is designed to work with it. there is no doubt but that wireless telegraphy will some time play an important part in many spheres of usefulness. there is another mode (already referred to) for transmitting signals electrically without wires through the earth instead of through the air, but in this case it is not through the medium of induction, but conduction. it has been explained in former chapters that earth-currents are constantly flowing from one point to another where the potentials are unequal. sometimes these inequalities of potential are caused by heat and sometimes by electricity, as in the case of a thunder-storm. if a cloud is heavily charged with positive electricity, say, the earth underneath will have an equal charge of negative electricity. let us illustrate it by the tides. as the moon passes over the ocean it attracts the water toward it and tends to pile up, as it were, at the nearest point between the earth and the moon. suppose that (while the water is thus piled up at a point under the moon) we could suddenly suspend the attraction between the earth and the moon--the water would begin immediately to flow off by the force of gravitation until it had found a common level. suppose in the place of the moon we have a cloud containing a static charge of positive electricity--it attracts a negative charge to a point on the earth nearest the cloud. if now a discharge takes place between the earth and cloud the potential between the two will suddenly become equalized and the static charge that was accumulated in the earth is released and it dissipates in every direction, seeking an equilibrium, following the analogy of the water; the difference being that in one case the movement is very slow, while in the other it is as "quick as lightning." about eighteen years ago i had a short telephone-line between my house and that of one of my neighbors. this line was equipped with what was known in those days as magneto-transmitters, such as we have described in a previous chapter on the subject of telephony. when a line is equipped in this way no batteries are needed, as the voice generates the current, on the principle employed in the dynamo-electric machine. often on summer evenings, when the sky appears to be cloudless, we can see faint flashes of lightning on the horizon, an appearance which is commonly called "heat-lightning." as a matter of fact, i do not suppose there is any such thing as heat-lightning, but what we see is the effect of very distant storm-clouds. often at such times i have held the telephone receiver to my ear and could hear simultaneously with each flash a slight sound in the telephone. this effect could be produced in the earth by a simple discharge between two or more clouds, which would distribute the electrical discharge over a greater area. and because my line had connection with the earth it could have been disturbed electrically by conduction instead of induction; or it may have been the effect of ether-waves set up by the lightning discharges. there is no doubt in my mind but that both of these effects (ether-waves and conduction through earth) may be felt when a discharge takes place between a cloud and the earth. if we could, by operating an ordinary telegraphic key, cause the lightning to discharge from cloud to earth, and some one was listening at a telephone in a circuit that was grounded at both ends miles or more distant from the cloud, the man who controlled the discharges by the key could transmit the morse code through the earth to the man who was listening at the telephone. thousands of people might be listening at telephones in every direction from the transmitting-station, and they would all get the same message. if the receiving-station is near to the point where there is a heavy discharge from the clouds to the earth the earth-current is very strong--flowing out in every direction. for some years i had an underground line between my house and laboratory, and no part of the line between the two stations was above ground. many and many times during the prevalence of a thunder-storm have the telephone-bells been made to ring at both ends of the line by a discharge from the cloud to the earth, and in some cases the discharge was several miles away. the wires could not have been affected so powerfully in any other way than through the earth. it will be seen by the foregoing statements that it is possible to transmit messages through the earth for long distances, but the difficulty in the way of its becoming a general system is twofold. first, we cannot always have a thunder-cloud at hand from which to transmit our signals, and, secondly, the signals would be received alike at every station simultaneously. chapter xxii. niagara falls power--introduction. as our readers know, niagara falls is situated upon the niagara river, which is the connecting-link between lake erie and lake ontario. the surface of lake erie lies feet above that of lake ontario. the high level upon which lake erie is situated abruptly terminates at queenstown, which is near the point where the niagara river empties into lake ontario. from lake erie to the falls the level of the river is gradually lowered a little less than feet, and most of this (making "the rapids") occurs in the last mile above the point where it takes a perpendicular plunge of feet into a narrow gorge extending for seven miles, through which the river runs, gradually falling also feet in that distance. the river above the falls is broad, varying from one to three miles in width, but below that point it is suddenly narrowed up to a distance of from to yards. it is supposed that at one time the fall was situated at the bluff overlooking queenstown, near lake ontario, and at that time was very much higher than it is at present. through long ages of time the water has gradually eaten away the rock, thus forming the gorge. it is estimated by different geologists that the time required to wear away the rock back to the present position of the fall has required from , to , years. some authorities place the rate of wear at three feet per annum and others not more than one. it is well known, however, that this erosion is constantly going on, and if nothing is done to check it the time will come when the gorge will extend up to lake erie and drain it, practically, to the bottom. this is a matter, however, that the people of this and those of several succeeding generations need not worry about. in the early days, before the country was settled and the banks of the river were lined with trees, and no houses, hotels or horse-cars were to be seen; when the puffing of the locomotive was not heard echoing from shore to shore; when no bridges spanned the river to mar its beauty, and when nature was the only architect and beautifier, niagara falls must have been one of the most attractive spots on the earth; at least it is the place of all places where the mighty energies of nature are gathered together in one grand exhibition of sublime power. here for ages this same grand exhibition had been going on, and although there was no human eye to see it, those of us who believe that nature is not a thing of chance, but that it was planned by an intelligence infinitely superior to that of any man, can easily imagine that the great architect and beautifier of this same nature, not only plans but enjoys the work of his own hand. why not? for ages the same sun, in his daily round, has reflected that beautifully colored rainbow, here the product of sunshine and mist. the same water, through these successive ages, has been lifted to the clouds by the power of the sun's rays, and has been carried back to the fountain-heads on the wings of the wind, and there has been condensed into raindrops, that have fallen on land, lake and river, and in turn has been carried over this same waterfall in its onward course toward the sea, only again to be caught up into the clouds; and thus through an eternal round it has been kept moving by that mighty engine of nature, the sun. it is said that "the mill will never grind with the water that has passed." this is true only in poetry. as a matter of fact, "the water that has passed" may often return to help the mill to grind again. water-powers have been utilized in a small way for many years for the purpose of generating electricity through the medium of the dynamo, but nowhere in the world has the application of the force been made for this purpose on such a grand scale as at niagara falls. when one stands on the bank of the river and sees the great waterfall as it plunges over the precipice, exerting a force of from five to ten million horse-power, one is overwhelmed in contemplation of its possibilities as a source of energy that may be converted into work, mechanical and chemical, through the medium of electricity. the genius of man has devised a way by which some of this constantly wasting energy may be converted into electricity and distributed to different points to perform various kinds of work. but the amount utilized as yet is scarcely a drop when compared with that which might be if the whole torrent could be set to work in the same manner as a very small portion of it now is. chapter xxiii. niagara falls power--appliances. some years ago a company was formed for the purpose of utilizing, to some extent, this greatest of all water-powers. a tunnel of large capacity was run from a point a short distance below the falls on a level a little above the river at that point. the general direction of this tunnel is up the river; it is about a mile and one-half in length, terminating at a point near the bank of the river a mile or more above the falls. above the end of this tunnel an upright pit comes to the surface, where a power-house of large dimensions has been constructed of solid masonry. it is long enough at present to contain ten dynamos of mammoth size. along the side of this power-house a deep broad canal is cut, which communicates with the river at that point, and through which flows the water that is to furnish the power. of course the water level of this canal is the same as that of the river. the foundations of the power-house extend to the bottom of the tunnel, which at that point is feet below the surface of the ground. to put it in other words, the cellar or pit under the power-house is feet deep and communicates with the great tunnel, which has its outlet below the falls. each of the ten dynamos is driven by a turbine water-wheel situated near the bottom of the pit heretofore described. the turbine-wheel is on the lower end of a continuous shaft, which reaches from a point near the bottom of the tunnel to a point ten or fifteen feet above the floor of the power-house (which is about on a level with the surface of the ground). this shaft is incased in a water-tight cylinder of such diameter as will admit a sufficient amount of water, and connects with the turbine wheel at the bottom in the ordinary way. the water is admitted into the top of this cylinder from the canal, so that the wheel is under the pressure of a falling column of water over feet high. the water, forcing its way out at the bottom through the turbine, revolves it and its long, upward-reaching shaft with great power, and enables it to work the dynamos in the power-house above, as will be described. the water discharges through the wheel in such a manner as to lift the whole shaft, thus taking away the tremendous end-thrust downward that would otherwise interfere greatly with the running of the machine through friction. after the water has done its work it flows off through the tunnel into the river below the falls. to the upper end of the power-shaft is attached a great revolving umbrella-shaped hood; to the periphery (circumference) of this hood is attached a forged steel ring, inches in thickness, about feet in diameter and from to feet in width. the whole of the revolving portion--including the ring upon which are mounted the field-magnets, the hood, and the shaft running to the bottom of the pit, where the turbine wheel is attached--weighs about thirty-five tons. the dynamos belong to the alternating type, and are comparatively simple in construction. in a previous chapter upon the dynamo it was stated that the fundamental feature was the relation that the field-magnet and the armature sustained to each other, and that in some cases the field-magnet revolves while the part that is technically called the armature remains stationary. in other cases the armature revolves and the field-magnets are stationary. in the latter case brushes and commutators are used, to catch and transfer the generated electricity, while in the former these are not needed, which simplifies the construction of the machine. as we have stated, the dynamos used at niagara are constructed with revolving field-magnets that are bolted on to the inner surface of the steel ring that is carried by the hood, so that there are no brushes connected with the machine except the small ones used to carry the current to the field-magnets. the current for power purposes is generated in a large stationary armature about ten feet in diameter and of the same depth as the revolving ring. the revolutions of the ring send out currents of alternating polarity, and each of the ten machines will furnish electrical energy equal to horse-power, so that when the work that is now under way is completed , horse-power can be furnished in the form of electricity. about , horse-power is now actually delivered to the various industrial enterprises. the dynamos are set horizontally, since the shaft which connects them with the turbine wheel stands in a perpendicular position. not all of the energy that is developed by the water-wheel is converted into electricity, but some of it appears as heat. in order to prevent the heat from becoming so great as to be dangerous to the machine it must be constructed in such a way as to admit of sufficient ventilation for cooling purposes. the armature is so constructed that there are air-passages running all through it, and on top of the revolving hood are two bonnet-shaped air-tubes set in such a way as to force the air down through the armature, which carries off the heat and warms the power-house, on the principle of a hot-air furnace. this great machine--which, in a way, is so simple in its construction--when in action conveys to the mind of the beholder a sense of wonderful power. it is only when we stand in the presence of such exhibitions as may be seen in this power-house, devised and executed by the genius of man, and in that greater presence, the mighty falls of niagara, that we get something of a conception of the power of the silent yet potent energy of the great king of daylight, the sun. there are very many interesting details that work in connection with this great power-plant, some of which we will describe, in a general way. standing within a few feet of each one of the great dynamos is a very beautifully constructed piece of machinery called the governor. the governor regulates the speed of the dynamos by partially opening and closing the water-gates that regulate the flow of water into the turbines. the question may be asked, why is there any regulation needed, if there is always an even head of water? there are two reasons--one because the load on the dynamo is constantly changing, and another that the head of water changes, although this latter fluctuation is in long periods. if the circuit leading out from the dynamo is broken, the rotating part of the dynamo will move with great ease and little power, as compared with what is required when the circuit is closed, and the current is going out and doing work. the increased amount of energy that will be required to keep the dynamo moving at a certain rate of speed when the load is on--in other words, when the circuit is closed--will depend upon the amount of current that is going out from the dynamo to perform work at other points. as the amount of current used outside for the various purposes is constantly changing, it follows that the load on the dynamo is constantly changing also. as the load changes, the speed will change, unless the amount of water that is flowing into the turbine is changed in a like proportion; hence the necessity for a governor that will perform this work. you can easily imagine that it will require a great amount of power to move the gate up or down with such a pressure of water behind it. it is not possible here to explain the operation of the governor in detail, as that could not be done without elaborate drawings; suffice it to say that the whole thing is controlled by a small ball governor such as we see used in ordinary steam-engines for regulating steam-pressure. the rising or falling of the balls of this governor to only a very slight extent will bring into action a power that is driven by the turbine itself, which is able to move the water-gate in either direction according as the balls rise or fall. for instance, if the balls rise beyond their normal position, it shows that the dynamo is increasing in speed, and immediately machinery is brought into action that shuts the water off in a small degree, just enough to bring the speed back to normal. if the balls drop to any extent, it shows that the load is too great for the amount of water, and that the dynamo is decreasing in speed; immediately the power is brought into action, now in the opposite direction, and the water-gate is opened wider. these slight variations of speed are constantly going on, and the constant opening and closing of the gate follows with them. it is a beautiful piece of machinery, and is beautifully adapted to the work it has to perform. it is continually standing guard over this greater piece of machinery that is exerting an energy of horse-power and prevents it from going wrong, both in doing "that which it should not do and leaving undone that which it should do." it is a machine that, when in action, points a moral to every thinking person who beholds it. every man has such a governor if he only has the inclination to use it. i have said further back that the water-head varies, but usually at long periods. this variation is chiefly caused by changes of wind, and it is very much greater than one would suppose without studying the causes. lake erie lies in an easterly and westerly direction, and when the wind blows constantly for a time from the west, with considerable force, the water piles up at the eastern end of the lake, which causes the level of the niagara river to rise to a very sensible extent. it is not so noticeable above the falls as below, because of the great difference in the width of the river at these two points. sometimes the river below the falls, as it flows through the narrow gorge, will vary in height from twenty to forty feet. when the wind stops blowing from the west and suddenly changes and blows from the east, it carries the water of the lake away from the east toward the west end, which will produce a corresponding depression in the niagara river. no doubt there is an effect produced by the difference of annual rainfall, but the effect from this cause is not so marked as that from the changing winds. another appliance used in the power-house, chiefly for handling heavy loads and transferring them from one point to another, is called the electric crane. it is mounted upon tracks located on each side of the power-house. the crane spans the whole distance, and runs on this track by means of trucks from one end of the power-house to the other. running across this crane is another track which carries the lifting-machinery, consisting of block and tackle, able to sustain a weight of fifty tons. situated at one end of the crane are one or more electric motors, which are able, under the control of the engineer, to produce a motion in any direction, which is the resultant of a compound motion of the two cars acting crosswise to each other together with the perpendicular motion of the lifting-rope connected with the block and tackle. it seems like a thing endowed with human reason, when we see it move off to a distant part of the building, reach down and pick up a piece of metal weighing several tons, carry it to some other portion of the building and lower it into place, to the fraction of an inch. while the machine itself does not reason, there is a reasoning being at the helm, who controls it and makes it subservient to his will. the machine is to the engineer who manipulates it what a man's brain is to the man himself. the brain is the instrument through which the unseen man expresses his will and impresses his work upon men and things in the visible world. chapter xxiv. niagara falls power--appliances. in the last chapter i described some of the appliances used in connection with the power-house. there are many things that are commonplace as electrical appliances when used with currents of low voltage and small quantity, that become extremely interesting when constructed for the purpose of handling such currents as are developed by the dynamos used at niagara. for instance, it is a very commonplace and simple thing to break and close a circuit carrying such a current as is used for ordinary telegraphic purposes, but it requires quite a complicated and scientifically constructed device to handle currents of large volume and great pressure. if such a current as is generated by a dynamo giving out horse-power under a pressure of volts should be broken at a single point in a conductor, there would be a flash and a report, attended with such a degree of heat and such power for disintegration that it would destroy the instrument. the circuit-breakers used at niagara are constructed with a very large number of contacts made of metal sleeves, or tubes, say one inch in diameter, so constructed that one will slide within the other; the sleeves being slotted so as to give them a little spring that secures a firm contact. these are all connected together electrically, on each half of the switch, as one conductor, so that when the switch is closed the current is divided into as many parts as there are points of contact in the switch. suppose there are of these contact-points, a one-hundredth part of the current would be flowing through each one of them. if, now, these points are so arranged that they can be all simultaneously separated, the spark that will occur at each break will be very small as compared with what it would be if the whole current were flowing through a single point, and it would be so small that there would be no danger attending the opening of the switch. these switches are carefully guarded, being boxed in and under the control of a single individual. there is another apparatus that is a necessary part of every manufacturing or other kind of plant that uses electricity from this power-house, and this is called the transformer. many of you are familiar with the box-shaped apparatus that is used in connection with electric lighting when the alternating current is used. where simply heating effects are required, such as in electric lighting, for instance, the alternating current can be used to greater advantage than the direct current when it has to be carried to some distance, owing to the fact that it may be a current of high voltage. a greater amount can be carried through a small conductor; thus greatly reducing the cost of an electrical plant that distributes power to a distance. a transformer is an apparatus that changes the current from one voltage to another. in the ordinary electric-light plant, such as is used in a small town or village, the current that is sent out from the power-station has a pressure of from to volts, according to the distance to which it is sent. it would not do, however, for the current to enter a dwelling at this high pressure, because it is dangerous to handle, and the liability to fires originating from the current would be greatly increased. at some point, therefore, outside of the building, and not a great distance from it, a transformer is inserted which changes the voltage, say, from down to or , according to the kind of lamps used. some lamps are constructed to be used with a current of fifty volts and others for or more. the lamp must always be adapted to the current or the current to the lamp, as you choose. the human body may be placed in a circuit where such low voltage is used without danger, but it would be exceedingly dangerous to be put in contact with a pressure of or more volts, such as is used for lighting purposes. in principle the transformer is nothing more or less than an induction-coil on a very large scale. the ordinary induction-coil, such as is used for medical purposes, is ordinarily constructed by winding a coarse wire around an iron core. this core is usually made of a bundle of soft iron wires, because the wires more readily magnetize and demagnetize than a solid iron core would. around this coil of coarse wire, which we call the primary coil, is wound a secondary coil of finer wire. if now a battery is connected with the primary coil, which is made of the coarse wire, and the circuit is interrupted by some sort of mechanical circuit-breaker, each time the primary or battery circuit is opened there will be a momentary impulse in the secondary circuit of a much higher voltage; and at the moment the primary circuit is closed there will be another impulse in this secondary circuit in the opposite direction. the latter impulse is called the initial and the former the terminal impulse. a current created in this manner is called an _induced_ current. the initial current is not so strong as the terminal in this particular arrangement. if we should take hold of the two wires connected with the two poles of the battery and bring them together so as to close the circuit, and then separate them so as to break it we should scarcely feel any sensation--if there were only one or two cells, such as are ordinarily used with such coils. but if we connect these wires to the coils of the induction apparatus and then take hold of the two ends of the secondary coil and break and close the primary circuit we should feel a painful shock at each break and close, although the actual amount of current flowing through the secondary wire is not as great as that which flows through the primary; but the voltage (or electromotive force) is higher, and thus is able to drive what current there is through a conductor of higher resistance, such as the human body. for this reason there is more current forced through the body, which is a poor conductor, than can be by a direct battery current which has a lower voltage. if now we should take a battery of a number of cells, so as to get a voltage equal to that given off by the secondary coil, and connect it with the fine-wire coil instead of the coarse-wire coil--thus making what was before the secondary coil the primary--by breaking and closing the battery circuit as before we shall get a secondary or induced current in the coarse-wire coil, but it will be a current of low voltage, and will not produce the painful sensation that the secondary coil did. we have now described the principle of a transformer as it is worked out in an ordinary induction-coil. as has been stated, at niagara falls the current comes from the dynamos with an electromotive force or pressure of volts. for some purposes this voltage is not high enough, and for other purposes it is too high; therefore it has to be transformed before it is used! for some purposes this transformation takes place in the power-house, and for others it takes place at the establishment where it is used. for instance, take the current that is sent to buffalo, a distance of from twenty to thirty miles. the current first runs to a transformer connected with the power-house, where it is "stepped-up" (to use the parlance of the craft) from a voltage of to , . it is carried to buffalo through wire conductors that are strung on poles, and is there "stepped-down" again through another transformer to the voltage required for use at that place. the object of raising the voltage from to , in this case is to save money in the construction of the line of conductors between the two points. if the voltage were left at --the conductors remaining the same as they are now--the loss in transmission would be very great, owing to the resistance which these wires would offer to a current of such comparatively low voltage as . to overcome this difficulty--if the voltage is not increased--it would be necessary to use conductors that are very much larger in cross-section (thicker) than the present ones are. and as these conductors are made of copper the expense would be too great to admit of any profit to the company. if we go back to an illustration we used in one of the early chapters on electricity we can better explain what takes place by increasing the voltage. if we have a column of water kept at a level say of ten feet above a hole where it discharges, that is one inch in diameter, a certain definite amount of water will discharge there each minute. if now we substitute for the hole that is one inch in diameter one that is only one-half inch in diameter a very much smaller amount of water will discharge each minute, if the head is kept at the same point--namely, ten feet. but if now we raise the column of water we shall in time reach a height which will produce a pressure that will cause as much water to discharge per minute through the one-half-inch hole as before discharged through the one-inch hole with only the pressure of a ten-foot column. this is exactly what takes place when the voltage is "stepped-up," which is equivalent to an increase of pressure. it will be seen from the foregoing that these transformers have to be made with reference to the use the current is to be put to. in general shape they are alike in appearance, the difference being chiefly in the relation the primary sustains to the secondary coils. there is another kind of transformer that is used when it is necessary to have the current always running in the same direction. this transformer, as heretofore explained, does not change the voltage of the current, but simply transforms what was an alternating into a direct current. by alternating current we mean one that is made up of impulses of alternating polarity--first a positive and then a negative. the direct current is one whose impulses are all of one polarity. the direct current is required for all purposes where electrolysis (chemical decomposition by electricity, as of silver for silver-plating, etc.) is a part of the process. the alternating current may be used without transformation in all processes where heat is the chief factor. for motive power either current may be used, only the electromotors have to be constructed with reference to the kind of current that is used. the rotary transformer, which may be driven by any power, consists of a wheel carrying a rotating commutator so arranged with reference to brushes that deliver the current to the commutator and carry it away from the same, that the brushes leading out from the transformer will always have impulses of the same polarity delivered to them. in the parlance of the craft, the transformers that are used to change the voltage from high to low, or vice versa, are called "static transformers," simply because they are stationary, we suppose. the others are called rotary, or moving transformers, to distinguish them from the other forms. the operation of the latter is purely mechanical, while the former is electrical. in some instances where the static transformers are very large they develop a great amount of heat, so much that it is necessary to devise means for dissipating it as fast as created. in some instances this is done by air-currents forced through them, but in others, where they are very large, oil is kept circulating through the transformer from a tank that is elevated above it, the oil being pumped back by a rotary pump into the tank where it is cooled by a coil of pipe located in the oil, through which cold water is continually circulating. by this means cold oil is constantly flowing down through the transformer, where it absorbs the heat, which in turn is pumped back into the tank, where it is cooled. having now traced the energy from the water-wheel through the various transformations and having described in a very general way the apparatus both for generating electricity and for transforming it to the right voltage necessary for the various uses to which it is put, we will proceed in our next chapter to follow it out to the points where it is delivered, and trace it through its processes, and the part it plays in creating the products of these various commercial establishments. chapter xxv. electrical products--carborundum. the production of electricity in such enormous quantities as are generated at niagara falls has led to many discoveries and will lead to many more. products that at one time existed only in the chemical laboratory for experimental purposes, have been so cheapened by utilizing electrical energy in their manufacture, as to bring them into the play of every-day life. still other products have only been discovered since the advent of heavy electrical currents. a substance called carborundum, which was discovered as late as , has now become the basis of an industry of no small importance. it is a substance not unlike a diamond in hardness, and not very unlike it in its composition. the chief use to which it is put is for grinding metals and all sorts of abrasive work. it is manufactured into wheels, in structure like the emery-wheel, and serves the same purpose. it is much more expensive than the emery-wheel, but it is claimed that it will do enough more and better work to make it fully as economical. it was my pleasure and privilege to visit the factory at niagara falls, and through the courtesy of mr. fitzgerald, the chemist in charge of the works, i learned much of the manufacture and use of carborundum. the crude materials used in the manufacture of carborundum are, sand, coke, sawdust and salt; the compound is a combination of coke and sand. it combines at a very high heat, such as can be had only from electricity. when cooled down the product forms into beautiful crystals with iridescent colors. the predominating colors are blue and green, and yet when subjected to sunlight it shows all the colors of the solar spectrum to a greater or less degree. the crystals form into hexagonal shapes, and sometimes they are quite large, from a quarter to a half inch on a side. the salt does not enter into the product as a part of the compound, neither does the sawdust. the salt acts as a flux to facilitate the union of the silica and carbon. the sawdust is put into the mixture to render it porous so that the gases that are formed by the enormous heat can readily pass off, thus preventing a dangerous explosion that might otherwise occur. in fact, these explosions have occurred, which led to the necessity of devising some means for the ready escape of the gases. the process of manufacture as it is carried on at niagara is interesting. the visitor is first taken into the rooms where are stored the crude material, the sand, coke, sawdust and salt. the sand is of the finest quality and very white. the coke is first crushed and screened, the part which is reduced to sufficient fineness is mixed by machinery with the right proportion of sand, salt and sawdust. the coarser pieces of coke are used for what is called the core of the furnace, which will be described later on. this mixture is carried to the furnace-room, which has a capacity for ten furnaces, but not all of these will be found in operation at one time. here the workmen will be taking the manufactured material from a furnace that has been completed, and there another furnace is in process of construction, while a third is under full heat, so that one sees the whole process at a glance. these furnaces are built of brick, about sixteen feet in length and about five feet in width and depth. the ends and bed of the furnace are built of brick, and might be called stationary structures. the sides are also built of brick laid up loosely without mortar; each time the material is placed in the furnace, and each time the furnace is emptied, the side-walls are taken down. a furnace is made ready for firing by placing a mass of the mixture on the bottom, and building the sides up about four feet high (or half the height when it shall be completed). a trough, about twenty or twenty-one inches wide and half as deep, is scooped out the whole length of the pulverized stuff, and in this is placed what has before been referred to as the core of the furnace, namely, pure coke broken into small pieces, but not pulverized, as in the case of the other mixture. the amount used is carefully weighed, so as to have the core the proper size that experiment has proved to give the best results. the core is filled in and rounded over till it is in circular form, being about twenty-one inches in diameter. at each end of the furnace the core connects with a number of carbon rods--about sixty in all--that are thirty inches long and three inches in diameter. these carbon rods are connected with a solid iron frame that stands flush with the outer end of the furnace. on the inside the spaces between the rods are packed full of graphite, which is simply carbon or coke with all the impurities driven out, so as to make good electrical connections with the core. this core corresponds, electrically speaking, to the filament in an ordinary incandescent lamp, only it is fourteen feet long and twenty-one inches in diameter. the mixed material is now piled up over this core, and the walls at the sides are built up until the whole structure stands about eight feet from the floor--a mass of the fine pulverized mixture, with a core of broken coke electrically connected at the ends. it is now ready for the application of electricity, which completes the work. let us go back to the transformer-room and examine the electrical appliances that bring the current down to a proper voltage to produce the heat necessary to cause a union between the silica of the sand and the carbon of the coke, which results in the beautiful carborundum crystals that we have heretofore described. the current is delivered from the niagara power company under a pressure of volts. the conductors run first into the transformer-room, which adjoins the furnace-room, and is there transformed down from volts to an average of about volts. the transformers at these works have a capacity of about horse-power. about per cent of this power is converted into heat in the process of transformation, making a loss in electrical energy of a little over horse-power. this heat would be sufficient to destroy the transformer if some arrangement were not provided to carry it off. we have already described how this is done through the medium of a circulation of oil. because of the low voltage and enormous quantity of the current passing from the transformer to the furnace very large conductors are required. the two conductors running to the furnace have a cross-section of eight square inches, and this enormous current, representing over horse-power, is passed through the core of the furnace, and is kept running through it constantly for a period of twenty-four to thirty-six hours. let us consider for a moment what horse-power means; as this will give us some conception of the enormous energy expended in producing carborundum. a horse-power is supposed to be the force that one horse can exert in pulling a load, and this is the unit of power. however, a horse-power as arbitrarily fixed is about one-quarter greater than the average real horse-power. if horses were hitched up in series, one in front of the other, and each horse should occupy the space of twelve feet, say, it would make a line of horses , feet long, which would be something over two miles. imagine the load that a string of horses two miles long could draw, if all were pulling together, and you will get something of an idea of the energy expended during the burning of one of these carborundum furnaces. within a half hour after the current is turned on a gas begins to be emitted from the sides and top of the furnace, and when a match is applied to it, it lights and burns with a bluish flame during the whole process. it is estimated that over five and one-half tons of this gas is thrown off during the burning of a single furnace. this gas is called carbon monoxide, and is caused by the carbon of the coke uniting with the oxygen of the sand. when we consider the vast amount of material that comes away from the furnace in the form of gas it is easy to see why it is necessary to introduce sawdust or some equivalent material into the mixture, in order to give the whole bulk porosity, so that the gas can readily escape. we should also expect that after five and one-half tons had been taken away from the whole bulk that it would shrink in size. this is found to be the case. the top of the mass of material sinks down to a considerable extent by the end of the time it has been exposed to this intense heat. gradually, after the current has been turned on, the core becomes heated, first to a red, and afterwards to an intense white heat. this heat is communicated to the material surrounding the core, producing various effects in the different strata, owing to the fact that it is not possible to keep a uniform heat throughout the whole bulk of material. some of it will be "overdone" and some of it "underdone." the material which lies immediately in contact with the core will be overheated, and that, which at one stage was carborundum, has become disintegrated by overheating. the silica of the compound has been driven off, leaving a shell of graphitic substance formed from the coke. after the current is shut off and the furnace has cooled down, a cross-section through the whole mass becomes a very interesting study. the core itself, owing to the intense heat it has been subjected to, has had the impurities driven out of the coke, leaving a substance like black lead, that will make a mark like a lead-pencil, and is really the same substance, known as plumbago, in one of its forms. it is the carbon left after the impurities have been driven out of the coke. surrounding the core for a distance of ten or twelve inches, radiating in every direction, beautifully colored crystals of carborundum are found, so that a single furnace will yield over pounds of this material. beyond this point the heat has not been great enough to cause the union between the carbon and silica, which leaves a stratum of partly-formed carborundum; outside of that the mixture is found to be unchanged. these carborundum crystals are next crushed under rollers of enormous weight, after which the crushed material is separated into various grades for use in making grinding-wheels of different degrees of fineness. this crushed material is now mixed with certain kinds of clay, to hold it together, and then pressed into wheels of various sizes in a hydraulic press, and afterward carried into kilns and burned the same as ordinary pottery or porcelain. these wheels vary in size from one to sixteen inches. the substances used as a bond in manufacturing wheels are kaolin, a kind of clay, and feldspar. while carborundum has already a large place as a commercial product, there is no doubt but that the uses to which it will be put will vastly increase as time goes on. this product may be called an artificial one, and never would have been known had it not been for the intense heating effects that are obtained from the use of electricity. it certainly never could have been brought into play as one of the useful agencies in manufacturing and the arts. it is not known to exist as a natural product, which at first thought would seem a little strange in view of the evidences of intense heat that at one time existed in the earth. its absence in nature is explained by mr. fitzgerald by the fact that "the temperatures of formation and of decomposition lie very close together." chapter xxvi. electrical products--bleaching-powder. another industry that has assumed large proportions at niagara falls, owing to the vast quantity of electricity produced there, is the manufacture of a commercial product called bleaching-powder, or chloride of lime. every one knows that chloride of sodium is simply common salt, so extensively used wherever people and animals exist. simple and harmless as it is, while it exists as a compound of the original elements, when separated into those elements they are each very unpleasant and even dangerous substances to handle. salt is one of the most common substances in nature. it is found in many parts of the world in solid beds, and is one of the prominent constituents of sea-water. salt is a compound of chlorine and a metal called sodium. sodium in its pure state has a strong affinity for oxygen, so much so that when a lump of it is thrown into water it takes fire and burns violently with a yellow flame. chlorine, the substance with which it unites to form common salt, is a greenish-colored gas, the fumes of which are very offensive and very dangerous even to breathe, if the quantity is very considerable. it is a curious fact in nature that two such substances as chlorine and sodium, both of them so difficult and dangerous to handle, should unite together to form such a useful and harmless compound as common salt. the important element in bleaching-powder is the chlorine which it contains. it is extensively used in the manufacture of paper and in all other materials where bleaching is required. the object of combining it with lime, forming a chloride of lime, is simply to have a convenient method of holding the chlorine in a safe and convenient manner until it is needed for use. the chemical works at niagara falls manufacture bleaching-powder on a very large scale. the part that electricity plays is to separate the chlorine from the sodium as it exists in common salt. at the works i was first taken into a room where a large quantity of salt was stored. a belt with little carrier-buckets on it picked up this salt and carried it into another room, where it was thrown into a vast mixing-vat containing water. the salt was mixed with water until a saturated solution was obtained. in a large room, covering one-half acre or more of ground, were assembled a great number of shallow vessels, about by feet square and foot deep. these vessels were sealed up so that they were gas-tight. communicating with all of these vessels were pipes connecting with the great tank containing the saturated solution of salt. from the top or cover of each vessel is a pipe running to a main pipe that carries off the chlorine gas into another room as fast as it is formed. through each one of these vessels a current of electricity passes; the whole system consuming about horse-power. the electric current, as it passes through the brine, separates the chlorine from the sodium, the chlorine passing in the form of gas up through the pipes, before mentioned, into the main pipe, where it is carried into another large room and discharged into a system of gas-tight chambers. upon the floor of these chambers is spread a coating of unslacked lime ground into a fine powder. the lime has a strong affinity for the chlorine gas and rapidly absorbs it, forming chloride of lime. when the lime is fully saturated with the chlorine the gas is turned off from that chamber, which is then opened up and the chloride taken out for shipment. a new coating of lime is now spread in the chamber and the gas is turned on and the process repeated. there are a number of these chambers, so that the operation in all of its phases is going on continuously. the room where the chlorine gas is formed is thoroughly ventilated, a precaution which is very necessary in case any one of the vats should spring a leak, as they sometimes do. in each one of these vats where the electrolytic process is going on there are two products constantly passing off; one, as before mentioned, is chlorine gas, and the other caustic soda in solution. the solution in the vat is constantly being renewed by the saturated solution of salt from the reservoir before mentioned. there is one stream continuously coming into the vat and two going out, caused by the decomposing power of the electric current. the solution of caustic soda is carried to large evaporating-pans, where the water is driven out of it, leaving the caustic soda in dry, white sticks of crystalline formation. in this process the electric current, which comes from the power-house with an energy of horse-power, has to be transformed twice; first, to bring it to the proper voltage for the work of decomposition, and, secondly, to change it from an alternating to a direct current, by which all electrolytic processes are carried on. you will notice that the electrical energy expended in this establishment is double that used in the manufacture of carborundum. the caustic soda, which is one of the products from the decomposition of salt, is taken to another establishment, where, by still another electrical process, metallic sodium is manufactured. the process here being a secret one, the writer did not have the privilege of examining the details. chapter xxvii electrical products--aluminum. another comparatively new article of manufacture now produced in large quantities at niagara falls is aluminum. until within the last few years this metal was not used to any extent by manufacturers, because of the great expense attending its production. now, however, it is produced in such quantities as to make it about as cheap as brass, bulk for bulk. aluminum is a very light metal, with a color somewhat lighter than silver; its specific gravity being about one-third that of iron. aluminum is found in one of its compounds in great quantities in nature, especially in certain kinds of clay and in a state of silicate, as in feldspar and its associated minerals. it is found in great quantities in southern georgia, where it is mixed with the red oxide of iron that abounds in that region. here, it exists as alumina, which is an oxide of aluminum. before it is taken to the reduction-works the alumina is separated from all other substances. it is a white powder, tasteless, and not easily acted upon by acids. electricity is the chief agent in the production of metallic aluminum. the reduction company buys this alumina, which has been separated from the clay or ores where it is mined. in a large room there are located a great number of iron vats or crucibles, lined with carbon, about two or two and one-half feet deep, five or six feet long and four feet wide. immediately over each vat is constructed a metal framework, through which are inserted a large number of carbon rods about eighteen or twenty inches long and from two to two and one-half inches in diameter. this framework is electrically insulated from the iron crucibles. the framework and the carbons are connected with the positive conductor of the electric current, and the vat or crucible with the negative. these conductors are very large, something like a foot in width and an inch in thickness, and made of some good conductor of electricity. they have to be very large because they carry a current equal to horse-power. the current is one of great volume, but very low voltage; the electromotive force at each vat or crucible being only about seven volts. as the process is electrolytic, and not simply a heating process, the direct current must be used, and therefore the current coming from the power-house must be transformed twice; first to bring it to a proper voltage and secondly to change it from an alternating to a direct current. these iron vats or crucibles are connected up in series, electrically, and then they are filled with the alumina and certain other materials, which act either as a flux or as a means of increasing the conductivity of the mixture; just what this substance is, is probably one of the secrets of the process. when all of the crucibles are filled with the mixture the current is turned on and is kept on continuously night and day seven days in the week. all of the material in the different crucibles is heated to redness, when the process of separation takes place. the oxygen of the alumina is thrown off as a gas, and other residuum floats to the top of the crucible and is skimmed off. metallic aluminum in a melted state sinks to the bottom of the crucible, where it is dipped out from time to time with large iron ladles and poured into sand and molded into blocks similar to that of pig iron. from time to time, as the metal is dipped out, fresh alumina with the other substances are thrown in on top of the crucible, so that the process is continually going on, day and night, week in and week out. the heat in the process of reducing alumina, as we have before seen, is not the chief factor; it simply serves to reduce the compound to a fluid state so that the electrolytic action can readily take place. therefore it is not necessary to be brought to a white heat, as it is in the case of the production of carborundum, described elsewhere. it was extremely interesting to observe the wonderful magnetic effects that were produced in iron when brought into proximity with these enormous electrical conductors. the voltage was so low that one could handle them with impunity. the iron crucibles became so magnetic that a heavy bar of iron seven or eight feet long would cling to their sides, so that it would be held in an upright position. bars of iron would cling to the conductor at any point along its length, and, although these conductors were carrying an energy of over horse-power, they produced no perceptible effect upon the human body. the reason for this lies in the fact, first, that the body is not made of magnetic material, and, secondly, the pressure is so low that the body--being a poor conductor--would not easily allow the low-pressure current to pass through it. aluminum is fast becoming an important article of commerce, and it is destined to become more and more so on account of its extreme lightness as compared to other metals. it is found to be valuable also when used as an alloy with many of the other metals. one of the great drawbacks to its more extensive use lies in the fact that as yet no satisfactory method has been devised for soldering it. undoubtedly in time this difficulty will be solved, when its use will be greatly increased. it is estimated that in its various compounds aluminum forms about one-twelfth of the crust of the earth. chapter xxviii. electrical products--calcium carbide. another important use to which electricity is put at niagara falls is the manufacture of a new product, called calcium carbide. like carborundum and aluminum, this product could not have been produced in commercial quantities in advance of a means for producing electricity in enormous volume. calcium carbide is a compound of calcium and carbon. calcium is a white metal not found in the natural state, but exists chiefly as a carbonate of lime, which is ordinary limestone, including the various forms of marble. as a pure metal it is hard to obtain and very hard to maintain, as it readily oxidizes when in contact with the air. the symbol for calcium carbide is cac_{ }, which means that a molecule of this carbide is compounded of one atom of calcium and two atoms of carbon. ca stands for calcium and c for carbon. when the symbol has no figure following it, it means that one atom only enters into the compound; but if a figure follows, it means that as many atoms enter in as the figure represents. the process of manufacturing calcium carbide is as follows: ordinary lime before it is slacked is ground to a fine powder; then it is mixed with powdered coke or carbon in the proper quantities, so that when a chemical union takes place the proportion will be as before stated, one atom of calcium to two of carbon. as is well known, lime is procured by exposing ordinary limestone to a red heat for some hours together. the heat disengages the carbon dioxide, leaving only a combination of calcium and oxygen, which is common lime. the mixture of ground lime and coke is put into a crucible that surrounds the arc of an electric light of enormous dimensions; the carbon conductors amounting to an area of one square foot or more. in order to cause the carbon to unite with the calcium a very intense heat is required, such a heat as can be obtained only in the arc of an electric light. when the enormous current is turned on (amounting to over horse-power) the mixture is melted, and after an exposure to this intense heat for a given length of time the oxygen of the unslacked lime is thrown off and the carbon unites with the calcium, which remains in the proportions of one atom of calcium to two of carbon, as before stated. this, it will be noted, is purely a heat process, and an intense one at that. no electrolytic action being required, the alternating current is used without transformation to the direct current, as is necessary in the manufacture of bleaching-powder and aluminum, both of which are electrolytic processes. when the operation is completed the current is turned off and the compound allowed to cool. in cooling it assumes a slate color, which is slightly iridescent when exposed to light. it also crystallizes to a certain extent. the value of this new product consists in its ability to evolve acetylene gas in large quantities. a molecule of acetylene gas is composed of two atoms of carbon to two of hydrogen. to evolve the gas it is necessary only to pour water upon the calcium carbide, when a union takes place between the carbon of the carbide and the hydrogen of the water in the proportions above stated. if there is water enough the whole of the carbon will pass off with the gas, leaving a residuum of slacked lime. the value of acetylene gas lies in its very intense illuminating power. this is due to the fact that the gas is very rich in carbon as compared with other illuminating gases. it burns with a pure white light when properly mixed with air or oxygen, but if there is a lack of air it burns with a smoky flame. in this case the carbon is not all consumed and escapes into the air in the form of soot or smoke, but when burned with the proper mixture of oxygen or common air it becomes one of the most brilliant of illuminants. acetylene, like most other gases, becomes explosive when mixed with air in certain proportions. whether it is more dangerous to handle than ordinary illuminating gases the writer is not prepared to say, as he has not had the opportunity to make a thorough comparison between it and other gases from an experimental standpoint. experiment, after all, is the only sure road to absolute knowledge. theories are beautiful in books and lectures, but they often fail in the laboratory. acetylene is now being introduced as an illuminating gas for domestic and other purposes. several methods of handling it have been proposed. one is to condense it into strong metal cylinders and deliver it in that form; another is to erect generators at convenient places and generate the gas as it is used. a very ingenious contrivance has been invented for regulating the generation of the gas. a certain amount of the calcium carbide is placed in a gas-tight vessel containing water. as soon as the water comes in contact with the carbide the evolution of the gas begins. when the pressure on the inside of the vessel has reached a certain degree it is made, through mechanical contrivances, to lift the carbide out of the water and thus stop the evolution of the gas. when the pressure is relieved through the consumption of the gas at the burners it allows the carbide to drop into the water, when the evolution of the gas begins again. of course there is the same objection to this mode of lighting that attends all open burners; it is constantly discharging into the air the products of combustion, chiefly carbon dioxide, which is poisonous to animal life. as has been explained in some of the chapters on heat, in volume ii, the illuminating property of any gas is determined by the number of carbon particles that are contained in it, which become heated to incandescence as soon as they come in contact with the oxygen of the air, and remain so, for a brief period, during their passage between the two extremes of the flame. while acetylene equals electricity in its illuminating properties, the latter still stands without a rival when considered from a sanitary standpoint, as the use of electricity does not in any degree vitiate the air in a room where it is used. we have now given somewhat in detail the following processes that are carried on at niagara falls through the agency of electricity, viz.: the reduction of aluminum from its oxide alumina; the production of the new and useful compound called carborundum; the formation of calcium carbide used for the production of acetylene gas, and a large chemical works, where bleaching-powder is made. in addition to these works, there is an establishment for the production of sodium from caustic potash, which is one of the products arising from the decomposition of salt in the bleaching-powder works. there is also another establishment for the production of phosphorus made from the bones and shells obtained from the phosphate beds that abound in some of the southern states, on the coast of the atlantic ocean. there is in process of construction a plant for the purpose of manufacturing chlorate of potash by an electrical process. in addition to these establishments mentioned, the electricity is furnished for power purposes to the niagara electric light company; to the electric railway between niagara and buffalo; to the niagara falls railway, on the opposite side of the river; to the niagara power and conduit company of buffalo, and the niagara development company. this is only a small beginning of the uses to which electricity will be put as an agent for the development of heat, light and power as well as for the production of all substances where electrolysis is the chief factor. sixteen companies or more are now using electricity from the niagara power-house,--the whole amounting to about , horse-power. chapter xxix. the new era. when we consider the number of new products for whose existence we are indebted to electricity, and the number of old products that have heretofore existed experimentally, in the laboratory of the chemist only, that have now been brought into play as useful agents in the various arts and industries, we begin to realize that this is truly an electrical age and the dawning of a new era. how many, many things there are, familiar to the children of to-day, that were not even imagined by the children of twenty-five to fifty years ago. fifty years ago the only useful purpose to which electricity was put was that of transmitting news from city to city by the morse telegraphic code. it will be fifty-seven years the first of april, , since the first telegraph-line was thrown open to the public. less than thirty years ago but little advance had been made in the use of electrical appliances beyond the perfection of certain private-line instruments, and a means for multiple transmission. about twenty years ago there were evidences of the beginning of a new era in electrical development. at no time in the history of the world has wonder succeeded wonder with such rapidity, producing such astounding results that have revolutionized all our modes of doing business and all of the operations of commercial and domestic life, as during the last two decades. we set our watches by time furnished by electricity from one central point of observation. we read the tape from hour to hour, upon which is recorded the commercial pulse of the world, as it throbs in the marts of trade, by means of this same speedy messenger. we enter a street-car that is lighted and heated, and at the same time propelled by the same wonderful agent. in our homes and on our streets night is turned into day by a light that outrivals all other illuminants. when we wish to speak to a friend who may be a mile or a thousand miles away we step to the end of a wire that comes within the walls of our dwelling and we talk to him as though face to face, and means are at hand by which we may write a letter to that same friend and deliver it to him in our own handwriting and over our own signatures, so quickly that it will appear before him in full form and completeness as soon as the last period is made at the end of the last line. one sees, and hears, and lives more in a single day in this age of electricity and steam than he did in twelve months sixty years ago. and yet there are those who cry out against modern inventions and modern civilization, and are constantly quoting the days of their grandfathers and great-grandfathers when "life was simple" and there was "time to rest." "why are we tormented with this thought-stimulating age?" they say. "why are our emotions called into action by modern music and modern art? why are we called upon to help the downtrodden and oppressed, and to help to elevate mankind to a higher level? why cannot we be left alone in peace and quiet, to live in the easiest way?" if this be good philosophy, then the swine, if he were a reasoning being, ought to be ranked among the greatest of philosophers--when he seeks a wallow in the sunshine and sleeps away his useless existence. if he is useful it is because some other being of a higher order uses him to help along his own existence. the man in these days who does not "keep up with the procession" is soon trodden under foot and some other man uses him as a stepping-stone to elevate himself. yet this is a selfish motive, after all. the world is now rapidly advancing in light, in knowledge, in power to use the infinite gifts that the creator has hidden in nature; but hidden only to stimulate and reward our seeking. every man can help in this grand progress,--if not by research and positive thought-power, at least by grateful acceptance and realization of what is gained. _look forward!_ as emerson puts it: "to make habitually a new estimate--that is elevation." index. acetylene gas at niagara, . alexandria, temple with loadstone, . amber--elektron, . ampère, theory of magnetism, . unit of electrical current, . galvanometer, . aluminum at niagara, . arabians, magnetic needle, . arago, germ of electromagnet, . aristotle mentions torpedo, . refers to magnet, . atmospheric electricity, ch. viii, . atoms and molecules, . of substances differ in weight, . relations to heat, . aurora borealis, . bain chemical telegraph register, . barlow on galvanism in telegraphy, . bell, alexander graham, radiophone, . bleaching-powder at niagara, . branly invents the coherer, . cables, submarine. see submarine cables. calcium-carbide at niagara, . capacity of a circuit, , . caustic soda, . chinese, magnetic needle, . chlorine and sodium, . circuit-breaker at niagara, . closed circuit and current, . coherer (wireless telegraphy), . columbus, compass variations, , . condenser in resistance-coil, . in morse relays, . conductors and non-conductors of electricity, . relation to electric light, . different resistances, , . cooke, needle telegraph, . crookes, prof., x-ray, . cuneus and the leyden jar, . curiosities, ch. xx, . daniell battery, . differential magnet, . dinocares and the loadstone, . dolbear, amos e., wireless telegraphy, . dupay discovers positive and negative electricity, . duplex telegraphy, . dynamo-electric machines, . invented by faraday, , . usual construction, . at niagara, . double transmission, . earth electric currents, in telegraphy, , , . earth magnetism, . effects of, on iron, . aurora, . telegraph-lines, . from sun's heat, . edison, thomas, railway telegraphy, . electromotograph, . electric currents, ch. vi, . not currents but atomic motion, . induction of, . guarded against, . at niagara, . electric generators, ch. vii, . frictional, . galvanic batteries, . storage-batteries, . dynamos, , . metal heating, . electricity, science of, . achievements of, . eras in science of, . theory of, ch. v, . not a fluid, a form of energy, . static and dynamic, . measurement of, ch. ix, . electric light, cause of, . electric machines, . frictional, . galvanic or chemical, . mechanical, . electromagnet invented by faraday, . commercial value, . theory of (soft iron), . permanent (steel), . condition of use, . the earth a, . germ of, . differential, . electromotograph, . ellsworth, miss, sends first telegraphic message, . ether, lines of force, . nature of, . ether, impressed by atomic motion, . inducing electric action, . farad, unit of capacity, . faraday, michael, . farmer, moses g., double transmission, . field, cyrus w., lays first atlantic cable, . field of a magnet, . fitzgerald, niagara falls chemist, . franklin catches the lightning, . identity of lightning and electricity, . kite experiment, . electric firing-telegraph, . frode, history of iceland, . gadenhalen uses magnetic needle a.d., . galileo's seed-thought, . galvani, luigi, and galvanism, . galvanic batteries, . author's experience, . galvanometer, , . gilbert, dr., frictional electricity, . gintl, double transmission, . gray, elisha, constructs voltaic pile, . electrically transmits music, . experiments on transmission of music, articulate speech, and multiple messages, . files telephone caveat, . musical experiments, . speech receivers, . boys' telephone, . first telephone specification on record, . dial-telegraph, . automatic-printing telegraph, . telautograph, . electric musical receiver, . gray, stephen, electrician, . grier, john a., quoted, . guyot of provence mentions mariner's compass, . halske, double transmission, . harmonic telegraphy, . receivers, . relay, . hawksbee, francis, electrician, . heat, a mode of motion, . related to atoms, . begins and ends in matter, . electrical and mechanical energy the same, . henry, joseph, first practical telegrapher, . constructs long-distance line, . produces induction, . heraclea and the loadstone, . hertz experiments in ether-waves, . homer refers to loadstone, . horse-power, . house, royal e., printing telegraph, , . hughes, david e., printing telegraph, , . induction, . guarded against, . produced by henry, . keeper of a magnet, . kelvin, lord (sir w. thompson), cable message receiver, . "kick," in telegraphy, , . kleist and the leyden jar, . "let her buzz," . leyden jar invented, . lightning, electricity; franklin, . restoration of equilibrium, . lightning-rods, . dangerous conductors, . loadstone, , . maury, lieut., deep-sea soundings, . magnes, magnesia, . magnet, electro. see electromagnet. magnetic earth poles, , . magnetic lines of force, , , . magnetic needle, . variation of, . dip of, . action of, . magnetism, history of, . and electricity mutually dependent, . theories of, . in iron and steel, . in the earth, , . and sun-spots, . magnetization, limit of, . marconi, wireless telegraphy, - . measurement of electricity, . ampère, unit of, . method of, . mercury luminous by shaking, . micro-farad, unit of capacity, . molecules of iron and steel natural magnets, . and atoms, . morse, s. f. b., devises code of telegraphic signals, . induces congress to construct line, . transmits battery current through water, . motion universal, . causes sound, heat, light, and electricity, . multiple transmission, ch. xiii, . duplex, . quadruplex, . multiple transmission, musical, . musical message receivers, , . musical tones transmitted, , , , . muschenbroeck, prof., and the leyden jar, . newton, sir isaac, electrician, . niagara falls power, chs. xxii to xxviii, to . introduction--rock, water, power, . appliances: tunnel, power-house, . shaft, dynamos, . current, . governor, . water-head, . crane, . circuit-breaker, . transformer, . electromotive force, . electrical products--carborundum, . materials, . furnaces, . electric current, . horse-power, . method of work, . bleaching-powder, . chlorine and sodium, . method of work, . caustic soda, . aluminum, . crucibles and methods, . magnetic effects, . calcium carbide, . process, . acetylene gas, . other products, . oersted, galvanic current on magnetic needle, . ohm, g. s., resistance unit, . patents--caveat and application, . planté, storage-battery plates, . pliny mentions electrical properties of amber, . loadstone, . preece, double transmission, . prescott, geo. b., quoted, , , , . ptolemy philadelphus and loadstones, . pythagoras refers to natural magnets, . radiophone, . railway train telegraphy, . richman, prof., killed, . reiss, metallic telephone transmitters, . resistance, unit of, . -coil, . siemens, double transmission, . selenium in radiophone, . shephard, charles s., induction-coil, . stager, gen. anson, telegrapher, . stearns, joseph b., cures the "kick" in double transmission, . storage-battery, . strada, loadstone telegraph, . submarine cables, ch. xvii, . first lines, - . maury's deep-sea soundings, . first atlantic, . retardations, . receiver, . sun-spots and magnetic storms, . telautograph, ch. xix, . telegraph: heliostat, . semaphore, . loadstone, . franklin's electric firing, . electrically dropped balls, . electric transmission of musical tones, . of signals, . morse register, . first line, . description, . reading by various senses, . bain, chemical recorder, . cooke needle, . wheatstone needle, . house printing, , . hughes printing, , . automatic systems, , . multiple transmission, . musical transmission, . musical receivers, . way duplex, . from moving railway trains, . repeater, . short-line dials, . printing, . wireless, ch. xxi, . telegraphic messages, receiving, . telephone, chs. xv, xvi, , . author's first experiment, . experiments, . caveat, . speech receivers, . boys' telephone, . first specification of, on record, . how telephone talks, . simple construction, . two methods of transmission: magneto and varied resistance, , . limit of transmission, . central station, . affected by heat-lightning, . telephote, . thales of miletus first described electrical properties of amber, . theophrastus mentions amber, . thermo-electric pile, . torpedo, the, . transformers at niagara, . transmission, multiple, ch. xiii, . trowbridge, prof., telephones through the earth, . tunnel at niagara, . tyndall, and gray's experiments, . unrest of the universe, . volt, unit of electrical pressure, . volta, alessandro, and the voltaic pile, . watt, james, . unit of electrical power, . way duplex system, ch. xiv, . wheatstone transmits musical tones mechanically, . needle telegraph, . dial-telegraph, . wireless telegraphy, ch. xxi, . signaling by ether-waves, . morse and henry, . trowbridge, dolbear, hertz, . branly, marconi, . marconi's system, . by earth-currents, . wolimer, king of goths, a natural battery, . _the romance of science._ -------------------------------------------------- sounding the ocean of air _being six lectures_ delivered before the lowell institute of boston in december by a. lawrence rotch, s.b., a.m., director of the blue hill meteorological observatory, massachusetts, u.s.a., and member of the international cloud and aeronautical committees. published under the direction of the general literature committee. london: society for promoting christian knowledge, northumberland avenue, w.c.; , queen victoria street, e.c. brighton: , north street. new york: e. & j. b. young & co. this little book is gratefully dedicated to the late augustus lowell, esq. of boston, u.s.a. who, as trustee of the lowell institute, enabled scientific men of two continents to present the results of their investigations to the public corrigenda page , line , _for_ "isolation" _read_ "insolation." page , line , _before_ "direction" _insert_ "opposite." page , line ; page , lines and , and index, pages and , _for_ "viollé" _read_ "violle." page , line ; page , line , and index, page , _for_ "muntz" _read_ "müntz." page , last line, _for_ " " _read_ " ." index, page , _for_ "cotte (t.)" _read_ "cotte (l.)." index, page , _after_ "hann (j.), ," _add_ " ." index, page , _for_ "hellman (g.)" _read_ "hellmann (g.)." index, page , _after_ "langley (s. p.)" _insert_ " ." [transcriber's note: these corrections have been applied to the current version] contents chap. page i. the atmosphere--ancient and modern knowledge--methods of investigation ii. clouds--formation and classification--measurements at blue hill--the international observations iii. balloons--notable ascents and results obtained--captive balloons iv. _ballons-sondes_ for great altitudes--the international ascents v. kites--history and application to meteorological purposes at blue hill and elsewhere vi. results of the kite-flights at blue hill--future work index list of illustrations page plate i. comparative altitudes plate ii. optical phenomena showing the height of the atmosphere plate iii. temperature at different latitudes and altitudes fig. . nephoscope at blue hill observatory. fig. . cloud theodolite at blue hill observatory plate iv. heights and velocities of clouds plate v. atmospheric circulation at different heights in cyclones and anti-cyclones fig. . german balloon equipped for meteorological observations plate vi. temperatures observed in four high balloon ascents fig. . german kite-balloon. fig. . baro-thermograph of richard fig. . the _aérophile_ rising plate vii. heights and temperatures recorded in eight ascents of the _cirrus_ fig. . oriental tailless kites. fig. . eddy tailless kite fig. . hargrave kite fig. . modified hargrave kites at blue hill fig. . lamson's aero-curve kite fig. . meteorograph lifted by kites at blue hill plate viii. meteorogram from the kite-flight of oct. , , at blue hill plate ix. mean changes with height, and changes during the kite-flight of oct. , plate x. changes with height recorded by kites at blue hill plate xi. kite observations at blue hill, sept. - , plate xii. automatic records during a high kite-flight at blue hill plate xiii. results of kite-flights at blue hill during an anti-cyclone and a cyclone sounding the ocean of air ---------- chapter i the atmosphere--ancient and modern knowledge--methods of investigation concerning this most important element in which we live and move and have our being, pliny, in the first century of our era, wrote as follows: "it is time to consider the other marvels of the heavens; thus our fathers called that immense space where flows the vital fluid to which we give the name of air, and which is not apparent to the senses because of its great rarity. there clouds form, thunder and lightning also; it is the region of tempests and of whirlwinds; from there fall rains, hail, and hoar frost; from there come all those phenomena, astonishing and often disastrous, which follow the combat of nature with herself.... the sun's rays strike the earth on all sides, warming and strengthening it; they are reflected and detach all the particles they can carry away; vapours descend and rise again; the winds come empty and return laden with spoil; animals breathe in from above this vital fluid which animates them, and the earth sends it back to its source as if she would fill the void by this means. so, by nature acting everywhere and in all directions there results an apparent discord from which is born the harmony of the universe; it is this general movement which puts all things in their places; some are preserved by the destruction of others; all move, all act, the struggle is continual, if it ceased an instant everything would fall into chaos...." from the earliest times, as far back as history extends, we find mankind interested in meteorological phenomena. this appears natural if we consider the importance of the weather to the ancient pastoral nations, which, from the open-air life and keen perceptive faculties of their people, were well fitted to study natural phenomena. the beauty and grandeur of many of the phenomena occurring in the atmosphere, and the curiosity excited concerning their causes, probably contributed to interest people in them. meteorology appears to have been first treated systematically, as distinct from astronomy and astrology, by the greek philosopher, aristotle, more than years ago. the word "meteor," derived from the greek "elevated," was applied to certain phenomena having their origin in the atmosphere. these were classified into aërial, aqueous, and luminous meteors, and were all included in the term meteorology. in his treatise by this name aristotle gave a more detailed account of them than any preceding or contemporary writer, and theophrastus, his pupil, wrote two books on the winds and on the signs of rain, which have been translated into latin and english. about the same period aratus incorporated the current weather proverbs in his poem, _diosemeia_. the greek historians and poets frequently alluded to atmospheric phenomena, and their example was followed by the romans, of whom pliny has been quoted. no doubt the desire to ascend into the air always possessed man, but owing to the awe with which mountains seem to have inspired the ancients, there is rarely mention in their writings of climbing mountains, or of the physiological effects which could hardly have failed to be apparent upon high summits. citing one of the few existing narratives, aristotle relates: "those which ascend to the top of the mountain olympus could not keep themselves alive without carrying with them wet sponges, by whose assistance they could respire in that air otherwise too thin for respiration." this mountain of less than , feet was said to be so high that it never rained on its summit, where, it was supposed, the air was always still. a still higher mountain, easily accessible to the ancient world, and which we know was ascended, is etna. concerning the progress of meteorology, from the time of the ancient romans to the revival of knowledge in europe, there is little to say except that during the middle ages meteorology, like other learning, was confined to the monasteries. speculations were current as to the extent of the atmosphere until, in the middle of the eleventh century, alhazen, a learned arab, computed from the duration of twilight that the atmosphere extended nineteen leagues above the earth. the same method was applied with more precision by tycho brahe, kepler, and other astronomers of the sixteenth and seventeenth centuries. the earliest weather chronicles were probably noted by monks from time to time in almanacks or missals, although when this was done first we do not know. the oldest daily chronicles of the weather extant are those kept by william merle in oxford from to . we owe it to the late mr. symons, the english meteorologist and bibliophile, that this ms. and many other old records have been brought to light and published. dr. hellmann has done even more in germany, and this historical research is evidence of the growing importance of the science of meteorology. with the advent of the age of geographical discovery it was seen that the climatic features of our globe depend chiefly upon distance from the equator, proximity to the ocean, and height above it. in the tropics especially, the luxuriant vegetation, which diminishes on mountain slopes and higher up gives place to snow, must have been visible proof of the decrease of temperature with altitude, for, as professor daniell remarked, mountains are a gigantic registering thermometer having for the freezing-point the line of perpetual snow. the invention of instruments for measuring temperature and barometric pressure made possible the quantitative observations that have supplied the data for deducing the laws governing the atmosphere. the oldest meteorological instrument is, no doubt, the weather or wind-vane, which had its origin before the christian era. the next oldest is the hygrometer, or instrument for measuring moisture in the air, the form which acts by absorption dating from the middle of the fifteenth century, and the condensation hygrometer being a century younger. next in chronological order comes the rain-gauge, which appears to have been used by castelli, a friend of galileo, in the year . the history of that important instrument, the thermometer, is obscure, but it is certain that galileo in padua used an air-thermometer in the latter part of the sixteenth century, which rey, a french physician, filled with liquid in . this thermometer, as well as other physical instruments, was perfected by members of the accademia del cimento at florence. these instruments are described in _saggi di naturali esperienza_, written in , and translated into latin and english. the florentine thermometers had one fixed point, that of freezing water, and contained either spirits or mercury. in fahrenheit, in danzig, fixed three points on the scale of the mercurial thermometer, viz. the cold produced by ice and sal-ammoniac which he called °, freezing water or °, and the heat of the human blood which he assumed to be °. this thermometric scale, having ° between freezing and boiling water, and that of celsius, with °, are the only ones in scientific use to-day. it is a remarkable fact in the history of thermometers that neither of these thermometers remained in the country where it was invented; thus the thermometer of fahrenheit, a german, came into use exclusively in england and her colonies, while that of celsius, a swede, is now used on the continent of europe except in germany, where the thermometer of réaumur, a frenchman, is still in popular use. of the four fundamental meteorological instruments, the barometer was the last invented. aristotle had suspected that air had weight, but it was not demonstrated until the middle of the seventeenth century, when the old axiom "that nature abhors a vacuum" was replaced by the rational explanation, given by galileo and torricelli, his pupil, why water will not rise in a suction pump more than thirty-two feet. in torricelli executed this famous experiment: he took a glass tube, sealed at one end, and filled it with mercury, then, closing the open end with his finger, he inverted it in a basin of mercury. the mercury fell to about thirty inches, which was recognized to be the weight of a column having the area of the tube and of the height of the atmosphere. the application of the barometer was due to blaise pascal, who repeated at rouen torricelli's experiment with a much longer tube filled with water, which being thirteen times lighter than mercury, stood thirteen times higher, or thirty-two feet, in the tube. pascal, being himself at paris in , got his brother-in-law perier to carry a barometric tube filled with mercury to the top of the puy de dôme, a mountain in auvergne rising about feet above the city of clermont. the mercury fell in the tube with the ascent, and at the top of the mountain it stood some three inches lower than at the base, showing that the lower layers of the atmosphere are denser than the upper. pascal repeated the experiment on the tower of st. jacques in paris, and it is interesting to note that more than two hundred years afterwards, meteorological stations were established both there and on the puy de dôme. it was soon perceived that not only did the level of the mercury in the tube change with height, but that it oscillated continually at the same place, and from its observed relation to the state of the weather its name "weather-glass" is derived. in the weight of the air was demonstrated in another manner by otto von guericke, burgomaster of magdeburg, who by means of an air-pump of his invention performed the experiment, which aristotle had tried unsuccessfully, of weighing a vessel full of air and the same vessel exhausted of air. he also showed the pressure of the air in all directions by the famous experiment of the magdeburg hemispheres, which, being hollow, were placed together, and after the air was exhausted from the sphere so formed sixteen horses were unable to pull them apart. soon afterwards robert boyle experimented further upon the weight and "spring of the air," as he called it, and gave the name to the barometer. both boyle in england and mariotte in france discovered the law, bearing indifferently their names, that the pressure of gases is proportional to their density. halley, a few years later, showed that the rate of decrease in pressure differed from the rate of increase in height, and developed formulæ for measuring heights by the barometer, which were afterwards perfected by laplace. knowing the heights of the barometer at a high and at a low-level station, and the mean temperature of the air lying between them, it is possible to compute accurately the difference of height of the two stations, or, conversely, given this height, the difference in barometric pressure can be calculated. by the middle of the seventeenth century the most important meteorological instruments had been invented, and not only can italy claim to be their birthplace, but the grand duke ferdinand ii., whose brother leopold founded the accademia del cimento, distributed the new instruments in italy and even beyond the alps, so that in observations several times a day were begun at a dozen stations. the observations in florence from to were preserved and constitute the commencement of instrumental meteorology. it was the conquest of peru which, by leading men over the high passes of the andes, first brought them to great heights, but although we find mention in the history of the expeditions of the so-called mountain sickness, caused by fatigue as well as by cold and rarefied air, it does not appear that scientific observations were made. therefore, while it must be assumed that the atmospheric conditions at considerable altitudes were familiar to travellers, yet not until the middle of the last century did bouguer, one of three french academicians sent to peru on a geodetic mission, fix the height of the freezing point in various latitudes, after observing that the temperature fell below freezing at night upon the mountains near the equator. during the latter part of the century, kirwan, an english chemist, calculated the temperature for various parallels of latitude, and in alexander von humboldt, after a voyage around the world, published his isothermal lines, or lines of equal temperature on the surface of the globe, by which he showed that the deviation from the normal, or calculated, temperature arose from the distribution of land and water, and from the geographical relief of the former. this work of von humboldt formed the basis of all subsequent studies in comparative climatology. meanwhile chemistry had kept pace with physics, and in the old theory, that air was one of the four elements from which all things originated, was rendered untenable by priestley, who proved that oxygen gas, which he discovered, was a constituent part of air. the other constituent, nitrogen, formerly called azote from its destructiveness to life, was discovered soon afterwards, and its proportion in the air determined by the french chemist, lavoisier. in man became possessed of the long-sought-for means of rising freely in the air, and he speedily availed himself of it. the first balloons, filled with heated air, were called _montgolfières_ from the inventors, the brothers montgolfier, living in annonay, france. after animals had been sent up attached to one, pilâtre de rozier ventured to ascend in the aerostatic machine, first tethered captive but then set free, and before the close of the year a balloon, filled with hydrogen gas, or "inflammable air" as it was called, carried m. charles feet above paris. during more than a century the balloon has been the most important agent for the exploration of the atmosphere, and yet, notwithstanding the courage and devotion to science of the early aeronauts, their ascents with unsuitable instruments furnished much discordant and erroneous data. some of the most remarkable balloon voyages and the modern methods of sounding and dredging the atmosphere, to borrow terms from the exploration of the ocean, will be described in two future chapters. perhaps the chief reason for the slow progress of meteorology to the status of a science is the variable character of its phenomena with the place of observation. in this respect it differs from astronomy, which was more easily cultivated in the restricted ancient world. only after many years of observation at different places had contributed a foundation for climatology was it realized that man, in his relation to the atmosphere, resembled marine organisms confined to the bottom of the ocean, and that in order to discover the true conditions of the atmosphere it was necessary to observe them at considerable heights. in the last century the highest point at which physical observations had been made was the summit of mont blanc, less than , feet above the sea. the ascent of this mountain was first accomplished in by h. b. de saussure and his guides with much difficulty and suffering, and the observations, abridged and rendered less accurate by the fatigue and sickness of de saussure, were also influenced by the proximity of the mountain itself. in von humboldt and bonpland reached a height of about , feet in the andes, where they made important observations. the ascent of man was rapid during the first years of the nineteenth century, for in gay-lussac rose in a balloon, without exertion or discomfort, to the height of , feet, and there made observations which were assumed to give the true atmospheric conditions. after an active campaign the conquest of the air by balloons was temporarily abandoned, and the field was left free to the mountaineer. but to-day supremacy rests with the aeronaut, for no one has succeeded in getting higher than , feet on a mountain, while the aeronaut has exceeded this altitude by a mile without great hardship, and lately has sent his unmanned balloons twice as high as the loftiest mountains. plate i., headed the exploration of the atmosphere, represents a vertical section of the lower portion of our atmosphere. on the right is a scale of miles above the sea, and on the left is a scale of barometric pressures corresponding to the height. the right-hand half of the diagram shows the eastern hemisphere with the himalaya mountains, the left-hand half the western hemisphere with the andes. there are seen the heights of the different kinds of clouds, measured at blue hill, as described in the next chapter; the highest meteorological stations, those on mont blanc and el misti in peru; the highest permanently inhabited place, which is a monastery in thibet; and the greatest height to which man has climbed, namely, in the andes. the heights at which observations have been made in balloons, carrying observers, or only recording instruments, may be compared with the height attained by the blue hill kites, to be described hereafter. other altitudes can be noted, such as the height of the snow-line on various mountains, and as a thousand-foot measure, the eiffel tower in paris, the tallest structure erected by man, may be used. [illustration: plate i.--comparative altitudes.] the development of meteorological knowledge to the commencement of the present century has now been traced, but before beginning the consideration of the methods of exploring the atmosphere that form the subject of the book, let us, in order to understand this work better, review the general knowledge which we possess of our atmosphere as regards its origin, composition, extent, and conditions of heat and moisture. first, then, regarding the =origin of the atmosphere=, or vapour envelope which the name means. according to the nebular hypothesis of laplace, our earth, like all existing suns and planets, was condensed from clouds of nebulous matter and became a highly-heated globular mass rotating, like every celestial body, from west to east. as the earth cooled, a crust was formed, and many of the substances that now exist in the earth were suspended as clouds in the cooler atmosphere surrounding it. eventually, these substances were condensed upon the crust; the oxygen, especially, must have been diminished by combining with the rocks, while the lighter gases, such as hydrogen, may have escaped from the earth's atmosphere. no doubt, when vegetable and animal life began, the earth's atmosphere was denser than now and much richer in carbonic acid, which, during the carboniferous period, was absorbed by plants, and is now imprisoned in coal and limestone. within historic times, however, there is no evidence of any change in the composition of our atmosphere, or the climatic conditions as a whole. m. jourdanet, a distinguished french physiologist, maintained that man appeared on the earth at the close of the tertiary period, when the barometric pressure at sea-level was, he supposed, about forty-three inches, or nearly a half more than it is to-day, and owing to the greater density of the air its temperature was also considerably higher. under these circumstances he believed that man first occupied the high regions of central asia, and only emigrated to lower levels when the climatic conditions became ameliorated. in other words, m. jourdanet believed in a literal "descent of man," but if this be true, many of the race have returned to their birthplace, for to-day millions of people dwell on the great asiatic plateau, and on the south american cordillera, at an average altitude of , feet, while a few live throughout the year at extreme heights of , feet. =composition of the atmosphere.=--dry air is a mixture of about one-fifth of a volume of oxygen to four-fifths of a volume of nitrogen, besides a very small quantity ( / , ) of carbonic acid, traces of ammonia, ozone, argon, and other recently discovered gases. the oxygen consumed, and the carbonic acid given off by animal life and by combustion, are maintained in this fixed proportion in the free air by the absorption of the carbonic acid, and the setting free of oxygen by vegetation. by diffusion and the mobility of the air, a thorough mixture is effected, with the result that the fundamental composition of our atmosphere is everywhere nearly the same. in the lower atmosphere the vapour of water is present in a varying quantity, in the average about one per cent. in weight, with a volume depending on the temperature. dust is always suspended in the atmosphere; the coarser particles settle, but the finer ones, that come from volcanoes, may float for a long time in the high atmosphere. dust is an important factor in the production of clouds and rain, and occasions many optical phenomena. [illustration: plate ii.--optical phenomena showing the height of the atmosphere. ] =extent of the atmosphere.=--if the atmosphere were incompressible and had throughout the density that it has at the earth, its height would be about five miles only, but actually it is composed of gases that follow boyle's law and vary in volume inversely as the pressure upon them. since the pressure decreases with height in a geometrical progression, it would be halved for each three and a half miles of ascent were the temperature constant, but as the temperature also decreases with height, the successive intervals, beginning with three and a half miles, become shorter because the volume of a gas depends on its temperature as well as on the pressure upon it. the decrease of pressure with increasing height above the earth is shown by the left-hand scale of plate i., already described, and the subsequent diminution of density to the limits of our measurable atmosphere is indicated on the right of plate ii., optical phenomena showing the height of the atmosphere. the gases composing the atmosphere probably extend to heights proportional to their density; viz. oxygen to about thirty miles and nitrogen to thirty-five miles, although water-vapour nearly disappears at twelve miles. from these considerations it is supposed that the atmosphere, as measurable by the barometer, vanishes at about thirty-eight miles, and this is about the height indicated by twilight, which is the reflected light of the sun when ° below the horizon. after the great eruption of the volcano krakatoa in the south seas in , the brilliant sunset glows and the longer twilight showed that the dust emitted by the eruption remained for more than a year suspended at a height of at least sixty miles. the so-called "luminous clouds" seen at night during the same period, and which were probably these same dust particles still illumined by the sun, were found by trigonometrical measurements to have about the same altitude. although it is computed that at a height of seventy miles the air has less than one-millionth of its density at sea-level--which is about the density of the air remaining in the exhausted bulb of an incandescent electric lamp--it is there sufficiently dense to render meteors luminous by friction after they with great velocity enter our atmosphere. the height of these meteors has been found, from simultaneous trigonometrical measures at two stations, sometimes to exceed one hundred miles, and if we suppose the aurora borealis to be an electrical discharge in highly rarefied air, measures made in the same way indicate as great a height for our atmosphere. the height of the aurora varies enormously, but the average altitude of it and of the other phenomena described, with the corresponding computed density of the air, are shown in the preceding diagram, in which the depth of the ocean of air may be compared with the deepest seas and the highest mountains. while, as professor young says, it cannot be asserted that the atmosphere has any defined upper limit, yet the kinetic theory of gases seems to afford evidence that the molecules of oxygen and nitrogen do not escape from the earth's attraction, and therefore the hypothesis of professor förster is unwarranted, that interplanetary space is filled with _himmelsluft_, or very thin air. =temperature of the atmosphere.=--the warmth of the atmosphere is derived chiefly from the sun's rays which, arrested by the earth's surface, are partly reflected and partly radiated back through the atmosphere. not more than seventy-five per cent.--professor langley says only sixty per cent.--of the heat of the sun, which is received vertically on the upper surface of the atmosphere, penetrates to the earth, and very much less than this when the angle of the sun is low. the reason why temperature diminishes as we ascend, is partly owing to the greater loss of heat by radiation through the thinner envelope of the upper strata, and partly owing to the greater absorption of the heat given off from the earth by the lower and denser strata. in general, it may be said that there is a diminution of ° fahrenheit for each three hundred and thirty feet that we rise vertically, but, this rate varies greatly at different heights, places, and times. for instance, the decrease is not the same on mountains as it is in the free air, and in the northern hemisphere it is greater on the south than on the north sides of mountains; it is usually greatest near the ground, and is faster in summer than in winter. but in the average, the temperature falls as much for three hundred and thirty feet of elevation as it does for a change of seventy miles on the earth's surface north or south of the equator. when dry air rises, because it is heated and thereby is made lighter, the laws of thermo-dynamics show that, by reason of its expansion, its temperature is decreased ° fahrenheit for each one hundred and eighty-three feet that it ascends, and, by compression, its temperature is increased as much if it is made to descend the same distance. this is called the "adiabatic rate of change of temperature," because it is produced by an alteration in the density of the air, due to variation in pressure, without the addition or loss of heat. in the course of this book there will be occasion frequently to refer to this law of heating and cooling. the adiabatic rate of change is seldom observed on mountains because of their influence upon the currents of air in contact with their flanks, or even in balloons, on account of imperfect measurements, but, as will be explained in the closing chapter, the adiabatic change of temperature is confirmed by the observations with kites, which furnish the best method of obtaining the temperature of the free air up to moderate heights. the adiabatic cooling of rising currents of air is another reason for the rapid decrease of temperature with height up to a mile or more. the upper air alters its temperature from diurnal and seasonal causes much more slowly than the lower air, and a mile above the earth the daily change of temperature, apart from the passage of "warm and cold waves," is less than one degree. at a height of six miles above the earth a temperature much below zero constantly prevails, while, at ten miles, ° below zero has been recorded in a balloon--this is approximately the temperature prevailing winter and summer above pole and equator. these facts are expressed graphically in plate iii., temperature at different latitudes and altitudes, which represents half of a section of the earth, from the north pole to the equator, with the superincumbent atmosphere. [illustration: plate iii.--temperature at different latitudes and altitudes.] perhaps it should be explained, that whereas the curvature of the earth with respect to the height of the atmosphere in the previous diagram was not exaggerated, in the present diagram the height of the atmosphere over the radius of the earth is enormously increased. at the north pole the mean annual temperature is about ° fahrenheit, and at the equator it is about °. it is seen that the atmospheric layer having a temperature of ° (here represented in section by a line) touches the earth at ° latitude, but is about two miles above the equator. in the same way the line of freezing ( °) leaves the earth's surface at ° latitude and rises to about three and a half miles over the equator; the line of ° rises from the pole to about seven miles at the equator. this is familiarly illustrated by the fact that only the highest mountains in the tropics are snow-capped, while within the arctic circle the snow-line descends nearly to sea-level. the lines in the diagram show the mean annual temperatures, but the isothermal surfaces rise in summer and sink in winter, the change of altitude being greatest in northern regions and near the ground. frequently there is an inversion of temperature, that is to say, it is warmer above than below. notably, in siberia, where the winter temperature is ° below zero, there can be no immediate decrease of temperature with height, and it is probable that there is a warmer layer of air interposed between the very cold earth and the still colder upper air, so that the temperature first rises rapidly with elevation and then falls slowly to the limits of the atmosphere. in temperate latitudes it often happens, with a high barometric pressure, in winter that the mountain stations enjoy a long period of still and relatively warm weather, as compared to that experienced in the valleys. but the subject of inversions of temperature will be discussed at length in considering the results of the balloon and kite observations. the observations from balloons at great heights are neither sufficiently numerous nor accurate to enable us to form an opinion as to what is the temperature of interplanetary space, which the kinetic theory of gases places at ° fahrenheit below zero. this temperature is called "the absolute zero," and is calculated from the fact that air under a constant pressure contracts / of its volume for each degree fahrenheit it is cooled below the temperature of freezing water, and consequently under no pressure it should have an infinite volume and a temperature of about ° below freezing, or ° below zero. there are other hypotheses regarding the temperature of space, but since it can never be measured directly, it will probably remain a matter of speculation. it is certain, however, that if the earth were deprived of its atmosphere, the temperature would fall very low, and even with our atmosphere as a blanket our earth would be uninhabitable were it not for the aqueous vapour which controls the selective absorption of the solar rays, transforming them into obscure rays so that they cannot escape from the atmosphere. water-vapour, then, is a very important factor in the physics of the atmosphere, but it can only be considered briefly here. =moisture of the atmosphere.=--the air is constantly absorbing moisture from the water on the earth, but the tension of this aqueous vapour decreases with elevation much faster than does the atmospheric pressure. at the height of about a mile and a quarter half the quantity of water-vapour is below, while we must rise about three and a half miles to reduce the quantity of air one-half, as may be seen in plate i. the relative humidity, or the percentage of moisture in the air, as compared to the amount which it could contain at that temperature, is nearly the reverse at low and at high levels. it is found from the kite-observations at blue hill, that up to the height of a mile or two the air is drier during winter and at night, and damper during summer and in the day-time than it is near the ground. at great heights probably the air is always very dry. the condensation of the invisible vapour into a visible form is considered in the next chapter on clouds. it is apparent that our observational knowledge of the atmosphere is gained by two general methods of exploring it, viz. observations made from the earth upon clouds and optical phenomena at a distance, and observations made directly in the air itself. although it was realized at the beginning of this century that meteorological observations were almost all conducted at the very bottom of our atmosphere--"in the shoals and shallows of the ocean of air," von humboldt said--yet only within the past thirty years was it thought necessary to replace the occasional observations on mountains by systematic and long-continued ones, comparable to those so generally carried on at low levels. it is an evidence of the zeal in america to advance the young science of meteorology, that the first mountain-top station in the world was established in upon mount washington, and that both this exposed post of observation, feet above the sea, and the one more than twice as high on pike's peak, which was for a long time the highest in the world, were maintained for many years by the united states signal service. the present highest station in the world is maintained by the harvard observatory upon el misti in peru, where, at a height exceeding , feet, a combination of self-recording instruments was constructed by my assistant, mr. fergusson, to operate during three months without attention. it must be admitted, however, that the addition to our knowledge of the physics of the atmosphere afforded by the american stations has been slight and incommensurate with the expense incurred. more has been gained from the mountain stations in europe, notably from those in the austrian alps, which have furnished data for dr. hann's splendid discussions of the thermo-dynamics of the atmosphere. while mountain stations present the only means of obtaining continuous observations at a considerable and constant height, still they have serious drawbacks. not only is the distribution of mountains over our globe irregular, but since they form part of the earth's crust, terrestrial influences affect all observations made upon them. in the case of plateaux this was at once admitted, but by placing the stations on the summits of high and isolated peaks, it was hoped to approximate to the conditions of the free air. it is now recognized that the equilibrium of the atmosphere is so delicate that for its dynamical study exact and minute measurements of temperature, moisture, and currents are required, and the methods which will be described are intended to give the values of these elements free from terrestrial disturbances. clouds, balloons, and kites naturally supplement one another. while clouds indicate the direction and velocity of the air at different heights, yet the lower clouds often conceal the upper strata, or there may be no clouds at all, in which case balloons or kites will aid us to determine the drift of the currents. when there is little wind at the ground, or to reach heights of several miles, we must employ balloons, but otherwise kites are preferable in most cases. the thermal and hygrometric conditions of the free air can be ascertained only by personal observations in balloons, or by raising self-recording instruments with balloons and kites, and this latter method it is predicted will be the path of greatest progress. chapter ii clouds--formation and classification--measurements at blue hill--the international observations clouds must have been among the earliest observed natural phenomena, and they were used from time immemorial as weather signs. yet their every-day occurrence was very likely the reason why their origin was not studied until about a century ago. father cotte, in his classic work on meteorology, published in , devotes only a couple of paragraphs to clouds, but abbé richard, in his contemporary _histoire naturelle de l'air_, discusses the appearance and theories of clouds in ten chapters. the cause of evaporation was unknown in the last century, and it was not until its close that dalton, the english chemist, proved that water-vapour exists independently in the air, and hutton explained that precipitation was produced by the contact of a current of saturated air with a colder one. although there remains much to be learned about cloud formation, yet it is now pretty well established that its most effective cause is the ascent, and consequent cooling by expansion of the air, rather than the mixture of masses of air having different temperatures. the ascent of the air may result from its being forced up a mountain slope by its horizontal movement, or from its being drawn up in a vortex, but most commonly the air rises from its lessened specific gravity when warmed. if the temperature of the quiescent air decreases faster than ° for each feet of height, which is the adiabatic rate of cooling for dry air, as explained in the last chapter, air warmed locally will rise and cool at this rate until the dew-point is reached. then the vapour in the air will be condensed upon particles of dust, which aitken found to be more numerous in clouds than outside them. the most conspicuous of the clouds formed by rising currents is the cumulus, or rounded summer cloud, which has been aptly termed "the visible capital of an invisible column of air." saturated air cools as it rises more slowly than dry air, consequently the upward motion is maintained through the cloud mass, causing the swelling up of the tops of the cumulus clouds, which reach their highest development in the thunder clouds, or cumulo-nimbus, as they are called. the lower limit of the cloud region is determined therefore by the height at which the rising currents reach their dew-point, and the altitude of the cloud formation depends upon the humidity of the ascending current, the drier it is, so much the higher must it rise to have its vapour condensed. in storms the rising current mingles with the stronger horizontal current above, which carries with it the upper portion of the cloud, and covers the whole sky with a uniform sheet. the wave, or ripple cloud, has been explained by von helmholtz and von bezold to be due to the undulations in a horizontal current producing alternate rarefaction and condensation of its water-vapour through changes of temperature. still another cause of low-lying clouds is the cooling of the air to its dew-point by contact with a cold surface, such as the earth when cooled by radiation during a clear night, or the polar currents of the ocean. fog is often formed in this way, which we call stratus cloud when it rises above us. the highest clouds consist of ice crystals, because the temperature of the air where they are is much below that of freezing water. although it is possible to cool drops of water considerably below ° fahrenheit without congelation, yet it can be told with certainty that the clouds are composed of ice if the sun and moon when seen through them are surrounded by the large rings or halos, which the theory of optics shows can only result from refraction of light by ice crystals, whereas water drops in the clouds produce the smaller coloured rings, which are called coronæ. the old question, why clouds float unless their particles are hollow, is easily answered, for they do not float, and always tend to sink if they are not supported by the currents of air. in sinking into warmer air the particles are vapourized and become invisible, but others rising are condensed and take their places, so that the cloud persists, although its particles change. this is illustrated by the "cloud banners," which frequently stream from mountain peaks, and are caused by the rise of air up the mountain side. even in a strong wind the cloud remains attached to the peak, showing that its particles are being renewed continually; but if, as is often the case, the wind descends on the leeward side of the mountain, the cloud particles disappear. lamarck, the celebrated naturalist, in the opening year of the present century, first proposed a classification of cloud forms. two years later luke howard, a london merchant, published his epoch-making essay on _the modifications of clouds_. the theories there advanced and the nomenclature proposed have been accepted generally to our day, notwithstanding the more complete classifications devised by poëy, ley, and others. howard believed that clouds are formed by the aqueous vapour which rises from the earth, and that the globules which compose them are solid, and are not filled with hydrogen gas as had been maintained by deluc and de saussure. howard classified the clouds as we do to-day, according to their appearance, into three principal types, viz. stratus, cumulus, and cirrus, which represented also low, middle, and high clouds. stratus is the sheet of low-lying cloud which forms at night, and commonly rests on the earth; cumulus is the heaped-up cloud of the day-time; and cirrus is the curl cloud of the high atmosphere. these three types were further divided into four intermediate types, viz. nimbus, cumulo-stratus, cirro-stratus, and cirro-cumulus. howard's nomenclature was used almost exclusively, until in the international meteorological conference that met at paris recommended the adoption of another classification, based on howard's, but modified by two experts, abercromby of england and hildebrandsson of sweden. this classification also disregarded the origin of clouds, and was based only on their appearance. the next year an atlas, with coloured pictures of the clouds, separated according to the new nomenclature, with descriptive text, was prepared by dr. hildebrandsson, assisted by drs. neumayer and köppen of the deutsche seewarte, or german national meteorological observatory. this atlas was adopted by the principal meteorological institutions on the continent of europe for their observers. the preface contained the following statement: "the study of the forms of clouds is daily increasing in importance, both from the standpoints of theory and of weather prediction. observations taken at the bottom of the atmospheric ocean are plainly insufficient to determine its circulation. the clouds, however, furnish information about the condition and motion of the air at various levels. but, a comparison of the observations of different observers is only possible when the same ideas are connected with the same expressions. it is hardly possible to give a sufficient verbal description of such indeterminate and changeable forms as those of the clouds; graphical representations are therefore necessary, with the help of a short description, in order to enable an observer to connect what he sees in the sky with what he finds in the instructions. in order that a cloud picture may be intelligible to non-specialists, the clouds and the blue sky must, at least, be plainly distinguishable from each other." the meeting of the directors of the meteorological institutions in different parts of the world, which was held at munich in , decided to adopt the classification of abercromby and hildebrandsson, and a committee was appointed to prepare an atlas of clouds, which should be cheaper than the preceding one. this committee, of which the writer has the honour to be the american member, met at upsala in . it defined the various forms of clouds, selected typical pictures to illustrate them, and drew up instructions for observing. this atlas, which was published in , is the recognized authority on cloud forms. meanwhile the united states weather bureau had issued a plate of clouds, printed in one colour, to familiarize its observers with the new system. the navy department has also an interest in clouds, for several thousand seamen in various parts of the world send their special logs to the united states hydrographic office. the hydrographer, a few years ago, was captain sigsbee, who, long before he became known to the public as commander of the ill-fated _maine_, had achieved scientific reputation from his investigations upon the depths and the currents of the ocean. captain sigsbee desired to render comparable the observations of clouds which were being made all over the world, and to this end he resolved to publish a coloured atlas of the international cloud types which should be intelligible to seamen, and yet not too costly for his office to supply. after two years of experimenting, during which the writer and his assistant, mr. clayton, were frequently consulted, the _illustrative cloud forms_, with and without descriptive text, were issued in by the hydrographic office, and in several respects this atlas is the best. still, it is impossible for anything but a photograph from the cloud itself to show the extreme delicacy of certain forms. perhaps it should be explained, however, that as the blue sky and the white clouds act with almost equal actinic effect upon the sensitized plate, in order to obtain the proper contrast between sky and cloud it is necessary either to polarize the light from the sky, or, as is most commonly done, to separate the coloured rays by allowing them to pass through a yellow screen, and to fall upon autochromatic plates. before defining the ten principal types of cloud it should be explained that two general classes of clouds are distinguished, separate or globular masses, which are most frequently seen in dry weather, and forms which are widely extended or completely cover the sky, which are typical of wet weather. both these classes of clouds are found at all heights. =cirrus= are thin, fibrous, detached, and feather-like clouds formed of ice-crystals. they are the highest of all the clouds, and move with the greatest velocity. =cirro-stratus= form a thin whitish veil, more or less fibrous, which often produces halos around the sun and moon and other optical phenomena. =cirro-cumulus= are flocks of small detached fleecy clouds, generally white and without shadows. =alto-stratus= is a grey or bluish veil through which the sun and moon are faintly visible, occasionally giving rise to coronæ. its altitude is only about half that of cirro-stratus. =alto-cumulus= are flocks of larger, more or less rounded, white or partially shaded masses, often touching one another, and frequently arranged in lines in one or more directions. =strato-cumulus= are large globular masses or rolls of dark cloud, frequently covering the whole sky, especially in winter. =cumulus= are piled clouds with conical or hemispherical tops and flat bases. they are formed by rising currents of heated air, and are therefore most common in summer and in tropical regions. when broken up by strong winds the detached portions are called fracto-cumulus. =cumulo-nimbus= is the massive thunder shower cloud rising in the form of mountains or turrets, and generally having above a screen of fibrous appearance (false cirrus), and underneath a mass of cloud similar to nimbus from which rain falls. =nimbus= is a dense, dark sheet of ragged cloud from which continued rain or snow generally falls. broken clouds underneath, forming the scud of the sailors, are called fracto-nimbus. =stratus= is a thin uniform layer of cloud at a very low level. when the sheet is broken up into irregular shreds it is called fracto-stratus. having described the origin and appearance of the different clouds, an account will now be given of the measurements made at blue hill observatory and the information which they give about the circulation of the atmosphere. the work there was taken up in in consequence of the interest of the meteorologist, mr. clayton, in the study of clouds; his discussion of the cloud observations, published two years ago with the blue hill observations, has been termed by far the most thorough study of the kind ever undertaken in america if not in the world. most of the conclusions which are stated popularly here have their scientific expression in his work. the first investigation related to the amount of cloud at different hours of the day, and during the various seasons. it is customary to note the degree of cloudiness on a scale of from , when there are no clouds, to , when the whole sky is covered. for twelve years the amount of cloud at each hour of the day has been recorded at blue hill. the personal observations have been supplemented during the day-time by an automatic instrument called a sunshine-recorder, for it has been proved that the cloudiness is very nearly the inverse of the bright sunshine. consequently, if, as is usual there, the sun shines forty-six per cent. of the time when it is above the horizon, the cloudiness is very nearly fifty-four per cent., which is the average for the year. the instrument generally used for this purpose is a glass sphere which acts as a burning-glass, and chars a strip of cardboard placed concentrically around the lower part of the sphere. as the sun moves, the image on the card moves in the opposite direction over the card, burning a line as long as it shines, but leaving the card untouched when it is cloudy. in a similar way a record may be obtained on sensitized "blue paper" by allowing the sun's rays to enter a dark chamber containing the paper. the maintenance of personal observations at each hour of the night is arduous, and, therefore, during ten years an automatic instrument has been used at blue hill which deserves to be better known. it is called the pole-star recorder, and was devised by professor pickering, director of the harvard college observatory. the instrument is very simple, and consists of a telescopic camera focussed on polaris. this star is not at the north pole of the heavens but a little more than a degree distant, and, consequently, it describes a small circle in the heavens during twenty-four hours. when the sky is clear around polaris its trail upon the photographic plate is continuous, but when the sky is partly or entirely covered with clouds the trail is broken or obscured. of course the plate is not exposed until after dark, and a shutter is closed by a clock before dawn. the only hourly records of cloudiness at night in the united states are obtained by this instrument on blue hill and at cambridge. it will be objected, perhaps, that the cloudiness derived from observations of the sun or the pole-star is not the amount over the whole sky, but only that in the region of the luminary. this is true, but it is found that the average of the records for a month or a year agrees very closely with the average of estimates of cloudiness over the whole sky during these periods. the use of the pole-star is preferable to that of the sun, because in our latitude it gives values at a point about half-way between the horizon and the zenith; while since the sun travels at a variable height across the sky, when its altitude is low the same mass of cloud may intercept more sunlight than when it shines vertically. from ten years' observations the following deductions have been made concerning the variation in the amount of cloud at blue hill. for all the months the diurnal amount of cloud is greatest about one o'clock in the afternoon, on account of the frequency of cumulus clouds near the warmest part of the day, while the next greatest amount, due to the frequency of stratus clouds, occurs near sunrise, or at the coldest time of day. all over the world the least cloudiness is in the evening, when the sum of the combined effects of radiation and insolation is least. the annual period in the cloudiness is complex, because the amount of cloud is connected with changes of humidity at many different levels in the atmosphere, but in the northern hemisphere there is most cloud during the first half of the year and least during the latter half, probably because the increasing warmth at the earth's surface produces increased ascending currents until summer, while the chilling of the earth's surface in the autumn becomes unfavourable for ascending currents. the distribution of cloud over the globe is intimately connected with the general atmospheric circulation, being greater where there are rising currents and less where there are downward currents. the reason, naturally, is that as descending air becomes warmer and therefore relatively drier, the clouds in it evaporate and disappear. a cloudy belt encircles the earth at the equator, and on either side are two belts of less cloud, but in higher latitudes the cloudiness increases. if we could see our earth from outside its atmosphere, the light reflected from the upper surfaces of the cloud-belts would probably make them appear bright. from the markings on a planet that are known to be caused by condensation, a french meteorologist, m. teisserenc de bort, believes that the circulation of its atmosphere can be inferred, for wherever on the surface of the planet bright spots are seen, there the vapour of rising currents should be condensed. if this be true, there is a resemblance between jupiter, as we see it, and the earth as it would appear from another planet, the bright bands being cloud surfaces, and the dark patches glimpses of the surface of the planet beneath. observations of the direction of motion, and apparent velocity of clouds at different heights, have been made at blue hill several times a day since . to measure the motion of clouds the nephoscope (fig. ) is used. it consists of a horizontal circular mirror with a concentric circle of azimuths and an eye-piece _c_, movable in a plane _bd_ at right angles to the mirror and also around it, through which the image of the cloud is brought to the centre of the mirror _a_. it can be proved by geometry that the motion of the cloud-image is proportional to the movement of the cloud itself, so by noting in what direction and how far the image is displaced in a given time, we have the true direction of motion of the cloud itself and also its relative velocity, comparable with the velocity of all clouds having the same height. if the height is known, then the relative velocity can be easily converted into absolute velocity, and thus the velocity of currents at different heights in the atmosphere is accurately ascertained. [illustration: fig. .--nephoscope at blue hill observatory.] the height of clouds seems to have been measured trigonometrically from two stations as early as by riccioli and grimaldi, two jesuits of bologna, but notwithstanding these measurements and some conclusions derived from observations on mountains, and in balloons, the altitudes of the different clouds were not known with any accuracy until in ekholm and hagström made a series of trigonometrical measurements upon the different kinds of clouds at upsala, sweden. about the same time attempts were made at kew observatory to measure clouds by photography, and in probably the first trigonometrical measurements in america were made at cambridge, mass., by professor w. m. davis and mr. a. mcadie. in - the swedish methods were employed at blue hill by messrs. clayton and fergusson of the observatory staff, and until recently the measurements there and at upsala comprised all that was known accurately about the heights and velocities of the various species of clouds. [illustration: fig. .--cloud theodolite at blue hill observatory.] the trigonometrical measurements at blue hill were made as follows: at two stations, one at the observatory, the other at the base of the hill about a mile distant, two observers determined simultaneously the angular altitude and azimuth of some point on the cloud which was agreed upon by telephonic conversation. if, as is generally the case, the lines of sight did not meet, the trigonometrical formulæ gave the height of a point midway between the crossing of these lines. such was the accuracy of these measurements that the probable error of the calculated heights of the highest clouds is only a few hundred feet. successive observations at the two stations of the position of the cloud enabled its velocity to be calculated, or, as already explained, this may be got from the relative velocity measured at one station, if the height of the cloud be known. fig. shows the theodolite on the tower of the observatory. five other methods of measuring clouds have been employed at blue hill: ( ) the only method of finding the height of lofty and uniform cloud strata is by means of the light thrown on them from below, and on blue hill the electrical illumination of the surrounding towns is utilized. the angle which the centre of the illumination makes with the horizon is measured, and knowing the distance of the town, the right-angled triangle may be solved. ( ) an accurate method for low and uniform clouds is to send kites into them, as will be explained in the closing chapter. ( ) when the clouds are low enough to cast shadows on the ground, the angles of the cloud and sun as seen from the observatory are measured, and with the distance of the shadow from the hill-top, ascertained by a map, this triangle can be solved. the times of passage of the shadow over known points on the landscape afford another means of calculating its velocity. ( ) a method that was suggested by espy, the pioneer american meteorologist, for getting the altitude of the bases of clouds lying within a mile of the earth, is to find the difference in temperature between the air and the dew-point at the ground, and to compute the height at which this difference should disappear. when the temperature of the rising currents increases, as on warm days, and the level of the dew-point rises higher, the cloud can be seen to ascend, and, in fact, the measurements at blue hill show that the clouds of moderate altitude are highest during the warmest part of the day. ( ) finally, very low stratus or nimbus may be measured by noting the heights of their bases on the sides of the hill. [illustration: plate iv.] the identity of cloud-forms all over the world has been established, and as a result of the measurements at blue hill, the heights and speed of all clouds observed there are known. the averages have been plotted in the five levels into which we separate the clouds in plate iv., heights and velocities of clouds, where ordinates represent heights and abscissæ velocities, and, consequently, the distances of the various forms of clouds above the horizontal base indicate their heights, and the distances from the left-hand vertical line their velocities. for comparison, the velocity of the wind on blue hill, a few hundred feet above the general level of the country, is represented. the mean height of the cirrus is about , feet, but this cloud sometimes reaches , feet. the mean height of the cumulus is about a mile, but the tops of the cumulo-nimbus, or thunder-shower cloud, sometimes penetrate into the cirrus level. generally the base of the nimbus, or rain cloud, is only feet above the ground, and it frequently sinks below the top of blue hill, which is only feet above the sea. the poetic saying, that "earth wraps her garment closer about her in winter," has a scientific basis, for the average height of all the clouds is greatest in summer and least in winter. but the reverse is true of their velocity, for the entire atmosphere moves twice as fast in winter as it does in summer, and at the lower levels the seasonal change is even greater. the average velocity of cirriform clouds is ninety miles an hour in winter, and sixty miles in summer, but occasionally in winter cirrus have been found to have the enormous velocity of two hundred and thirty miles an hour. in the average, the velocity of the currents increases, from the lowest to the highest clouds, at the rate of about three miles an hour for each feet of height, but near the ground the increase with height is faster. it has been found that the velocity of the lower clouds is less than the velocity of the wind on a mountain of the same height, which may, perhaps, be explained on the supposition that the mountain acts like a dam to accelerate the flow of air over it. the measurements in sweden showed that the middle and upper levels of clouds are higher than in america, but that they move less rapidly. this may be because the surfaces of equal temperature in the air are higher in the united states than in sweden, on account of the direction of the upper currents, while the greater velocity of our high clouds corresponds with the more rapid movement of areas of low and high barometric pressure over the united states. these results are suggestive. for instance, the energy of the upper half of the mass of the atmosphere, or that portion which lies above , feet, has been calculated to possess six times the energy of the lower half in which we live, and as yet, none of this enormous store of energy is applied to the use of man. while it appears certain that no navigable balloon or flying machine will ever be able to stem the enormous velocity of the upper atmosphere, rarified though it is, perhaps in the future aërial machines will take advantage of the prevailing currents of the high atmosphere, as our sailing ships do of the trade winds. the observations of cirrus clouds in various parts of the world show that they always move from a general westerly direction, while below this primary drift toward the east occur the relatively permanent or transient differences of pressure which cause the deviations from the normal circulation of the atmosphere, and give rise to the local circulation in storms. in the familiar daily weather map it will be noticed that there is usually some portion marked "low," and another portion marked "high." the former is an area of low barometric pressure, into which the winds at the ground blow spirally inward in the opposite direction that the clock hands turn; the latter is an area of high barometer, out of which the winds at the ground blow in the contrary way. the former when well developed are called "cyclones," and are usually accompanied by stormy weather, and the latter, called "anti-cyclones," bring fair weather. from the observations of the directions from which the clouds move in cyclones and anti-cyclones, we have found that above the cumulus level (at the height of about a mile) the inward inclination of the wind in a cyclone, and the outward inclination in an anti-cyclone, both disappear, and the general drift from the west prevails. the results of the observations are shown in plate v., atmospheric circulation in cyclones and anti-cyclones, representing sections of the atmosphere, concentric to the earth's surface, in the five cloud-levels seen from above. the arrows fly with the wind and are proportional in length to its velocity, the dotted arrows indicating the probable flow of the air through the cyclones and anti-cyclones that are indicated by the circles, their axes being assumed to be nearly perpendicular to the earth's surface. above the cumulus it will be observed that the wind in the cyclone tends to come from the south-west in front and from the north-west in the rear, while in the anti-cyclone the contrary is the case, indicating a deflection of the westerly upper current to the right in cyclones and to the left in anti-cyclones. this sustains the theory that the cyclonic circulation is struggling against a general atmospheric drift from the west which increases with altitude, and above the height of a mile becomes greater than the cyclonic influence. higher than this, the atmospheric circulation is controlled primarily by the permanent temperature gradient between equator and pole, by the seasonal temperature gradient between ocean and continent, and, in the united states, by the passage of "warm and cold waves." mr. clayton's investigations indicate that the motion of the upper clouds is nearly parallel to the lines of equal temperature at the earth's surface. a high temperature, by expanding the air upward, causes in the upper air a high pressure; and a low temperature, by contracting the air towards the ground, causes in the upper air a low pressure, so that the lines of equal pressure in the upper air are parallel to the lower lines of equal temperature, and since there is little friction in the upper air the motion of the wind is nearly parallel to the lines of equal pressure. below the cumulus level the winds follow the normal cyclonic and anti-cyclonic circulation. there are two theories of the origin of these areas of high and low pressure, the "driven theory" which supposes that they derive their energy and drift from the general atmospheric movement from west to east, and the "convectional theory" which attributes their formation and progression to the difference of temperature between them and the adjacent air. while the observations on mountains have favoured the driven theory, yet the inward spiral motion of the cirrus clouds above the anti-cyclone, indicating a lower pressure than in the surrounding air, contradicts the hypothesis, and the recent observations with kites at blue hill strongly support the convectional theory of cyclones. [illustration: atmospheric circulation at different heights in cyclones anti-cyclones plate v.] the relation of the clouds to weather forecasting has been investigated at blue hill. for instance, it is found, in this region at least, and contrary to the general opinion, that cirrus clouds do not indicate rain, but do foretell a change of temperature that is proportional to the rapidity of motion of the clouds. alto-cumulus is followed by rain within twenty-four hours three times in four. rain follows the appearance of all high and intermediate clouds most frequently when the cloud banks are densest toward some westerly point and when they come from the west. mr. sweetland, an assistant, has studied special forms of cloud in their relation to the succeeding weather. he concludes that cirrus plumes precede fair weather, while dense clots of cirro-cumulus are followed by rain. rounded pendants, or mammillated clouds, in the lower levels indicate rain, but in the upper levels fair weather. of all the forms, the dark sheet of stratus, and clouds of lenticular shape, are most frequently followed by rain. of clouds presaging changes in temperature, the turreted cumulus, which is connected with thunder-storms, precedes the greatest fall in temperature, and next in order come lenticular clouds, flaky cirrus, and alto-cumulus. in general, flat and flaky clouds, clouds forming and disappearing rapidly, and clouds changing to forms at a higher level precede dry and cooler weather. it will be seen that this modern study of clouds as prognostics simply adds to the weather proverbs that have come down to us from the time of theophrastus. it does not appear, however, that cloud forms alone can usually serve to predict rain for more than twenty-four hours, but for a few hours in advance the appearance of certain cloud forms frequently furnishes the observer more trustworthy signs of coming rain than does the synoptic weather map. to a forecaster in possession of telegraphic data, the prevalence of rapidly-moving cirrus over a wide area indicates a rapid storm movement, with sudden and marked changes of weather and of temperature, while slowly-moving cirrus indicate slight changes of temperature and dry weather. the direction of the cirrus movements in front and around a storm centre will usually point out the future movement of the storm, which tends to advance in the same general direction. the work done at blue hill shows the importance of cloud observations to elucidate the general movements of the atmosphere, as well as the circulation of the air above barometric maxima and minima, which can result practically in making accurate weather forecasts possible a day or two in advance. the systematic observation of the upper currents was brought to the attention of the international meteorological committee by dr. hildebrandsson in , and at the meeting of the international cloud committee in , besides the adoption of the nomenclature of clouds and instructions for observing them, it was decided that observations of their motion, as well as measurements of their height, should be made in various parts of the world. accordingly, the year commencing may , , was designated as the "international cloud-year," and observations with nephoscopes of the direction of motion and relative velocity of clouds were begun at many stations in europe and asia, and at fifteen stations in the united states. trigonometrical measures of the heights of clouds were undertaken at stations in norway and sweden, russia, finland, prussia, and france, as well as at toronto, manila, and batavia; in the united states the measurements already described were recommenced at blue hill, and the weather bureau equipped a similar station in washington. in europe it is thought that the determination of heights by photogrammeters, as the theodolites with attached photographic cameras are called, possesses advantages over the visual theodolites, and it is true that not only is the kind of cloud recorded on the plates, but there are available for calculation as many points on the cloud as can be identified on the two plates exposed simultaneously at both stations. on the other hand, in the case of nearly uniform or dark cloud-strata, it is easier to see points for measurement on the cloud than to fix them on the photographic plates. for this reason, and from the difficulty of manipulating the photogrammeter, visual instruments were adopted both at blue hill and at washington. the work was successfully carried on until may , , and the observations and measurements were reduced at blue hill according to the plan prescribed by the committee. already the observations and measurements made at upsala, manila, and blue hill are published, and the others will follow. the discussion of the correlated data from the various countries will probably increase our knowledge of the circulation of the atmosphere, which is certainly one of the most interesting and important questions in the physics of the globe. the result will have been reached by international co-operation, of which the benefits to science are everywhere manifest to-day. but for the whole problem to be solved, it is necessary, not only to know the movement of the air, but, as far as possible, to ascertain its conditions of heat and moisture. this may be accomplished by the use of balloons and kites, to be described in the remaining chapters. chapter iii balloons--notable ascents and results obtained--captive balloons in the first chapter the invention of the hot-air and the hydrogen balloon was chronicled, and it was stated that on december , , charles rose from paris to a height of feet. public interest in france was greatly excited by this wonderful extension of the realm of man, and numerous ascensions with _montgolfières_ and _charlières_, as the hot-air and hydrogen balloons were respectively called, took place in paris and the provinces. the uses of the balloon seemed innumerable, and lavoisier was instructed by the academy of sciences to draw up a report on the value of the new discovery. after having described in detail the ascensions which he had witnessed, the great chemist stopped, appalled at the multitude of problems which the balloon could solve. history has shown, however, that no commercial application of the balloon was possible, and that aside from its spectacular attractions, its chief use has been for scientific observations. the first persons in england who devoted themselves to aërial navigation were foreigners. two of them were italians, the philosopher tiberius cavallo, who already in had showed to a london assembly that soap-bubbles filled with hydrogen will rise, and therefore had almost anticipated the invention of the hydrogen balloon, and the diplomatist vincent lunardi, who made some daring balloon ascents in . but the honour of making the first scientific balloon voyage is due to a bostonian, dr. john jeffries. dr. jeffries graduated at harvard college in and then practised medicine in england, where he became a loyalist, and during the revolution was with the british troops. in london he interested himself in aerostation, and, aided by the royal society, ascended in a balloon because, he said, "i wished to see the following points more clearly determined: first, the power of ascending or descending at pleasure, while suspended and floating in the air; secondly, the effect which oars or wings might be made to produce towards the purpose and in directing the course of the balloon; thirdly, the state and temperature of the atmosphere at different heights from the earth; and fourthly, by observing the varying course of the currents of air, or winds, at certain elevations, to throw some new light on the theory of winds in general." a french professional aeronaut named blanchard had made three ascents in france and one in england, and dr. jeffries paid one hundred guineas to accompany blanchard on his fifth ascent, which was made from london november , . he took with him a thermometer, a barometer, a hygrometer, an electrometer, and a mariner's compass, also several numbered bottles, filled with water and provided with glass stoppers, which were to be emptied and corked up at different heights in the atmosphere. it was arranged to record the observations on ruled paper with a silver pen, because the doctor would not trust a common pen or pencil as liable to accident. he also had a map of england to determine the direction which the balloon took. jeffries' english sentiments are shown by this quotation from his narrative: "i had provided a handsome british flag, invidiously represented the next day in one of the public papers to have been the flag of the american states." the barometer and thermometer were observed every few minutes, and the hygrometer occasionally. the electrometer did not change its indications. samples of air were obtained and sent to the royal society, but it does not appear that they were ever analyzed. the balloon rose nearly two miles, and descended safely in kent after an hour and a half. jeffries' observations compare favourably with those made until recently; indeed, for nearly a century there was little improvement in the apparatus. the decrease of temperature which jeffries found, viz. ° for feet rise, and the decreasing humidity with height agree very well with later observations. jeffries and blanchard undertook a more perilous voyage on january , , from dover across the channel, landing in the province of artois, after, so runs the announcement, "we were suspended and floating in the atmosphere two hours over the sea and forty-seven minutes over the land of france." the voyagers were cordially welcomed, and were entertained lavishly in paris as being, jeffries says, "the first who passed across the sea from england into france by the route of the air." no instruments but a barometer and a compass were carried, and the only scientific result worthy of notice was that the balloon seemed to lose buoyancy over the sea, due to what jeffries thought might be "the power of attraction over the water." the height of the balloon was measured trigonometrically by french officers in calais, who found by angular measures, when the balloon was midway across the channel, that its height was feet. jeffries' voyages have been described somewhat at length because the first scientific balloon voyage is generally attributed to the belgian physicist, robertson, who ascended from hamburg in to the improbable height of , feet. robertson made his third ascent the next year from st. petersburg, accompanied by the academician sacharoff. this was a scientific voyage, instituted at the request of the russian academy, to ascertain the physical state of the atmosphere and the component parts of it at different heights, also the difference between the results of vertical ascents and the observations of deluc, de saussure, von humboldt and others on mountains, which it was rightly concluded could not be so free from terrestrial influences as those made in the open air. among the experiments which the academy proposed were the following: change of rate of evaporation of fluids, decrease or increase in the magnetic force, inclination of the magnetic needle, increase of heat of the solar rays, fainter colours in the spectrum, influence of rarefaction of the air on the human body, as well as some other chemical and philosophical experiments. a height of about two miles was reached, and many interesting observations were made, but since the instruments were not easily used in the basket of the balloon, the results were unsatisfactory and required repetition to be conclusive. the academy of sciences of paris now took up the investigation with the special object of proving whether the magnetic force decreased as robertson in a balloon and de saussure in the alps had supposed. two young physicists, biot and gay-lussac, were chosen to carry out the investigations. they ascended from paris on august , , provided with all necessary instruments, but the balloon was too small to rise higher than , feet. gay-lussac ascended alone to a height of , feet on september , , in a balloon filled with hydrogen. his observations confirmed those which he had made with biot, that there was no change in the magnetic force, and from samples of air collected he proved that the chemical constitution of the air is invariable. his observations of temperature seemed to confirm the theory of a decline of temperature of ° in feet of elevation. the air was found to be very dry, and gay-lussac noticed that at the highest altitude the clouds were still far above him. passing over several notable ascents in other countries, it was not until that scientific ballooning was begun again in the land where the balloon originated. then mm. barral and bixio made two ascents from paris in rainy weather to the heights of , and , feet respectively, although they had expected to attain twice these altitudes. their most interesting observations were the great thickness of the cloud mass, which in one case amounted to , feet, and the sudden fall of temperature in it from + ° to - °. some curious optical phenomena were connected with the floating ice crystals, and although the light of the sky was found to be strongly polarized, the light reflected from the clouds was not polarized. the field of operations was now transferred to england, where, under the auspices of the british association, four ascents were made by john welsh of the kew observatory in the great _nassau_ balloon managed by green, the veteran aeronaut. the special object of these investigations, like those in france, was the determination of the temperature and hygrometric condition of the air at different elevations. besides this, samples of air at different heights were collected for analysis and the light reflected from clouds was examined for polarization. recognizing that on account of the calm prevailing in the car of the balloon and the great solar radiation, the readings of the thermometer would be affected, welsh enclosed the thermometers in polished tubes through which air was forced by bellows. this was the first aspirated thermometer, which alone gives the true temperature of the air with the conditions prevailing in a balloon. the instrument fell into oblivion until a few years ago, and to this fact is due the fictitious temperatures generally obtained by aeronauts. welsh reached heights of from , to , feet, and his observations showed that the temperature of the air decreased uniformly with height until at a certain elevation, varying on different days, the decrease is arrested, and for a space of or feet the temperature remains nearly constant, or even increases slightly; the regular diminution being afterwards resumed and generally maintained at a less rapid rate than in the lower air, and commencing from a higher temperature than would have existed but for the interruption. the variation of the decrease with the seasons was also demonstrated. the humidity did not change much with height, and it was nowhere very dry. finally, the light of the clouds was proved not to be polarized, and the permanent composition of the atmosphere was confirmed. in another grant of money was made by the british association for balloon experiments to be performed, under the direction of a committee, by mr. james glaisher, then engaged in geodetic and meteorological work in england. between and glaisher, accompanied by the aeronaut coxwell, made thirty ascents. they attained three times a height exceeding , feet, and once more than , feet, when they believed that the balloon rose to , feet. the primary objects of glaisher's experiments were as follows: determination of the temperature of the air and its hygrometrical conditions up to five miles, comparisons of an aneroid barometer with a mercurial one, determination of the electrical state of the air and of its oxygenic state by means of ozone papers, time of vibration of a magnet at different distances from the earth. secondary objects of study were the composition of the air, the form and thickness of clouds, the atmospheric currents, acoustical phenomena, etc. in order to obtain many observations frequent ascents were necessary, as the insular position of england precluded long voyages. during ascents in a captive balloon up to feet supplemented the employment of the free balloon, which from its rapid rise and fall made observations in it near the earth impossible. glaisher was a good observer; his instruments were excellent, and had been previously tested, but their exposure in the basket of the balloon was bad, and although the thermometer was provided with an aspirator similar to welsh's, glaisher, noticing that the readings agreed with those of a freely exposed thermometer, hastily concluded that the use of the aspirator was unnecessary, and so discarded it. until quite recently glaisher's results were accepted as representing the conditions of the free air up to the greatest height which it was possible to reach. these results showed that the temperature did not fall uniformly with height, but that it fell most rapidly near the earth and much less rapidly at great heights. in cloudy weather up to the height of a mile the mean decrease of temperature in the day-time differed little from the theory of ° per feet, but in clear or partly clear weather the decrease was more rapid, commencing with ° for feet near the ground and diminishing to ° for feet at an elevation exceeding six miles. the observations in the captive balloon up to a third of a mile indicated a daily range in the vertical decrease of temperature. the observations of relative humidity agreed with welsh's in showing a slight increase up to about half-a-mile, then a decrease up to above five miles, where there seemed to be an almost entire absence of water. the other observations were inconclusive, except that the time of vibration of a magnet was found to be somewhat longer than on the earth, which was contrary to gay-lussac's experience. the most remarkable of glaisher's ascents was made from wolverhampton on september , , when in less than one hour he had passed the altitude of five miles, exceeding the greatest height hitherto reached. to quote from glaisher's narrative: "up to this time i had taken observations with comfort and experienced no difficulty in breathing, whilst mr. coxwell, in consequence of the exertion he had to make, had breathed with difficulty for some time. having discharged sand, we ascended still higher; the aspirator became troublesome to work, and i also found a difficulty in seeing clearly.... about hour min., or later, i read the dry-bulb thermometer as minus °; after this i could not see the column of mercury in the wet-bulb thermometer, nor the hands of the watch, nor the fine divisions of any instrument. i asked mr. coxwell to help me to read the instruments. in consequence, however, of the rotatory motion of the balloon, which had continued without ceasing since leaving the earth, the valve-line had become entangled, and he had to leave the car and mount into the ring to readjust it. i then looked at the barometer, and found its reading to be - / inches, still decreasing fast, and implying a height exceeding , feet. shortly after, i laid my arm upon the table, possessed of its full vigour, but on being desirous of using it, i found it powerless.... trying to move the other arm, i found it powerless also. then i tried to shake myself and succeeded, but i seemed to have no limbs.... i dimly saw mr. coxwell, and endeavoured to speak, but could not. in an instant intense darkness overcame me, so that the optic nerve lost power suddenly, but i was still conscious, with as active a brain as at the present moment whilst writing this. i thought i had been seized with asphyxia, and believed i should experience nothing more, as death would come unless we speedily descended; other thoughts were entering my mind, when i suddenly became unconscious.... i cannot tell anything of the sense of hearing, as no sound reaches the air to break the perfect stillness and silence of the regions between six and seven miles above the earth. my last observation was made at hour min., above , feet.... whilst powerless i heard the words, 'temperature' and 'observation,' and i knew mr. coxwell was in the car speaking to and endeavouring to rouse me.... i then heard him speak more emphatically, but could not see, speak, or move. i heard him again say, 'do try; now do!' then the instruments became dimly visible, then mr. coxwell, and very shortly i saw clearly.... mr. coxwell told me that while in the ring he felt it piercingly cold, that hoarfrost was all round the neck of the balloon, and that on attempting to leave the ring he found his hands frozen. he had, therefore, to place his arms on the ring and drop down.... he wished to approach me, but could not; and when he felt insensibility coming over him too, he became anxious to open the valve. but in consequence of having lost the use of his hands he could not do this; ultimately he succeeded, by seizing the cord with his teeth, and dipping his head two or three times, until the balloon took a decided turn downwards. no inconvenience followed my insensibility; and when we dropped, it was in a country where no conveyance of any kind could be obtained, so i had to walk between seven and eight miles.... i have already said that my last observation was made at a height of , feet; at this time ( hour min.) we were ascending at the rate of feet per minute; and when i resumed observations we were descending at the rate of feet per minute. these two positions must be connected, taking into account the interval of time between, viz. minutes, and on these considerations the balloon must have attained the altitude of , or , feet. again, a very delicate minimum thermometer read minus °. , and this would give a height of , feet. mr. coxwell, on coming from the ring, noticed that the centre of the aneroid barometer, its blue hand, and a rope attached to the car were all in the same straight line, and this gave a reading of seven inches and leads to the same result. therefore, these independent means all lead to about the same elevation, viz. fully seven miles." mr. glaisher's circumstantial evidence of the height he reached has been assailed lately, partly from his assumption that the velocity of the balloon while rising and falling during the thirteen minutes was uniform, but principally from the supposition that men could have survived in that region of death, without at least artificial means of respiration. while it is certain that berson's observations, which are described later, were made at a greater height than glaisher's, yet all credit must be given to this nestor of aeronautical and meteorological science in great britain, who is still living at the advanced age of ninety. the example of glaisher was not followed in england, but it stimulated interest in the balloon again in france, where mm. flammarion, de fonvielle, and tissandier have made many ascents for scientific purposes, and have presented the results in a popular form to the public. photography in a balloon is generally a failure on account of the intense reflection from the upper cloud surfaces and the haze which masks the earth. consequently, for scenic effects we must rely upon sketches, of which those in that interesting, but now rather rare book, _travels in the air_, may be referred to. the high atmosphere is often filled with fine ice crystals which, though invisible from below, occasion curious optical phenomena, and some of these have been sketched by m. albert tissandier, who has the advantage of being an artist as well as an aeronaut. of the many narratives of balloon voyages, one of the most thrilling is the tragedy of the _zenith_. in , through the co-operation of the french academy of sciences and other scientific bodies, it was arranged to make two voyages, one of long duration, the other to a great height, in the balloon _zenith_. the long voyage from paris to bordeaux was successfully accomplished in twenty-four hours, and on april the _zenith_ again rose from paris, carrying mm. gaston tissandier and crocé-spinelli, with sivel as aeronaut. by the advice of m. paul bert, the distinguished physiologist, three small balloons of oxygen were provided to assist respiration. the scientific apparatus was as follows: a pump was arranged to draw air through tubes filled with potash in which to store the carbonic acid at different heights in the atmosphere, in order that analysis might determine if its proportion diminished at great heights; a spectroscope was employed to examine the line of water-vapour in the atmosphere, and two aneroid barometers were provided, one giving the pressure corresponding to heights up to , feet, the other the pressure between , and , feet. there were also two barometric tubes registering the lowest pressure, as well as thermometers and other scientific instruments. at , feet the voyagers began to breathe oxygen, which had been used beneficially by sivel and crocé-spinelli in a high ascent the previous year. at , feet tissandier wrote in his notes: "my hands are freezing. i am well. we are all right. haze on horizon with small rounded cirrus. we are rising. crocé pants. we breathe oxygen. sivel shuts his eyes, crocé does the same." five minutes later: "sivel throws out ballast, temperature - ° cent., barometer millimeters." after this, tissandier became so weak that he could not turn his head to look at his companions. he tried to seize the oxygen tube, but was unable to move his arms. his mind was clear, and he saw the barometer sink below millimeters, indicating a height of , feet. then he fainted. after a half-hour of unconsciousness he revived and wrote: "we are falling, temperature - °, barometer millimeters. i discharge ballast. crocé and sivel unconscious in bottom of basket. we fall rapidly." again he fell into a stupor, from which he was roused by crocé shaking his arm, saying, "throw out ballast!" which he did, together with the pump, wraps, etc. what happened after this is unknown, but probably the balloon, thus lightened and the gas in it being warm, rose again nearly as high as before. when tissandier came to his senses the balloon was falling with frightful speed, and in the bottom of the basket, which was oscillating violently from side to side, were crouched his two companions with black faces and bloody mouths. the shock of striking the ground was terrific, but the anchor held, and the balloon soon emptied. from the barometric data it appears probable that the _zenith_ attained twice a height of about , feet, and that asphyxiation from the long deprivation of sufficient oxygen killed the two companions of tissandier and nearly proved fatal to him. this disaster discouraged further attempts to reach high altitudes, and with the exception of the ascent to , feet in france by mm. jovis and mallet, no more were made until the past decade. the results of the meteorological observations were seen to be strangely discordant; for example, the temperature of ° below zero, observed by barral and bixio at a height of , feet, and ° above zero, noted by the american aeronaut wise, at feet. the prophecy "that the balloon-basket would be the cradle of the young science of meteorology" seemed unlikely to be realized, but, nevertheless, observations in balloons continued to be made in france, italy, and russia. in the united states a series of balloon ascents was conducted by the signal service, which then included the weather bureau, and the height of , feet reached by professor hazen in is probably the greatest at which observations in the free air have been made in america. the difficulty of obtaining the true temperature of the air from a balloon is great, and without special precautions the observations give the conditions of the free air even less well than do observations on mountain summits. during a rapid ascent the air is carried up in the balloon basket like water in a well-bucket, and since the balloon drifts with the wind it is relatively in a calm, so that there is no circulation of air; the thermometers, even when screened from direct sunshine, are affected by radiation from the heated gas-bag above, and moreover they are not sufficiently sensitive to follow the changing temperature of the air strata so quickly traversed by the balloon. the aneroid barometer, from which the height of the balloon is calculated, cannot respond to rapid changes of pressure; consequently there is a double source of error in determining the height at which the temperature is measured. ordinarily, the temperature of the air may be obtained quite accurately by slinging in a circle a thermometer attached to a cord, even though this is done in sunshine. during two balloon ascents by the writer, a sling thermometer was found in extreme cases to read ° lower than was recorded by automatic instruments, hung in their usual position from the ring of the balloon. the sling thermometer, however, is influenced by intense insolation, and moreover cannot be swung far enough outside the basket of a balloon to insure good results. the standard instrument for obtaining the temperature of the air under all conditions, adopted for international use in , is a modification of that used by welsh forty-five years before. this instrument, which is the invention of dr. assmann of berlin, is called the aspiration thermometer, and is designed to prevent the casing surrounding the thermometer from being heated by insolation or conduction, and to insure a flow of air past the thermometer bulbs. [illustration: fig. .--german balloon equipped for meteorological observations.] the reorganization of balloon observations was accomplished by the german society for the promotion of aërial navigation, which has been assisted by the prussian meteorological institute, and by officers of the german army balloon corps. the german emperor takes a personal interest in the work, and has aided it by the gift of a considerable sum of money. the first voyage under the direction of the society was made in , and many notable ones followed. in , through the courtesy of the president, dr. assmann, the writer made an ascent from berlin in a balloon equipped for accurate observations, with the special purpose of comparing the sling with the aspiration thermometer. the car of the balloon is shown in fig. . a companion was the now famous dr. berson, who then made his second ascent, but who has now become an expert aeronaut by reason of more than fifty ascensions, some of them to great heights. on december , , he ascended alone from stassfurt, prussia, in the _phoenix_, to probably the greatest height ever reached by man, at least in a conscious state. by breathing oxygen he was able to keep his senses and to read the barometer at · inches, indicating approximately an altitude of , feet, and the aspirated thermometer at ° below zero. an ordinary thermometer read ° below zero in the sun, showing its heat was much diminished in consequence of the haze that prevailed even at this enormous height. the cirriform clouds which surrounded the balloon were found to have the structure of snow-flakes rather than that of ice-crystals. the chief result of this record-breaking ascent was the extraordinarily low temperatures recorded at great heights, as compared with those observed by glaisher, tissandier, and others. an inversion of temperature--that is an increase of temperature with height--prevailed up to a mile, but above that the temperature fell at a rapid and accelerated rate which approached the adiabatic fall above , feet. the wind, which was almost calm at the earth's surface, increased to a gale in the high atmosphere, and carried the balloon along at an average speed of thirty-six miles an hour. wishing to demonstrate conclusively whether the insular position of england influenced the temperature of the high atmosphere, as had been suggested, dr. berson determined to execute a high ascension in england during the prevalence of a barometric maximum in summer, when the air column would be abnormally warmed and the upper isothermal surfaces elevated. an opportunity was afforded berson to follow in glaisher's footsteps on september , , when abnormal heat prevailed in europe. berson, with the aeronaut spencer, in the balloon _excelsior_, rose from the crystal palace in london to the height of , feet, where he observed a temperature of - °. the oxygen inhaled prevented harmful physiological effects except for the discomfort caused by the enormous reduction of temperature from ° at the ground only thirty-five minutes before. the temperature decreased rapidly at first, then moderately up to three miles, and above that it fell almost at the adiabatic rate. even in this hot summer maximum of pressure and notwithstanding the maritime climate and south-westerly currents, a temperature about ° below zero reigned at , feet, being only a few degrees warmer than berson had observed in winter at the same height above germany. yet glaisher, in all his ascents, two of which exceeded , feet, never recorded a temperature of less than ° below zero. these relatively high temperatures, obtained also by welsh, tissandier, and gay-lussac, must be attributed to the insufficient protection of the thermometers against insolation, to the proximity of the instruments to the heated basket and its occupants, and lastly, to the sluggishness of the thermometers themselves, from lack of ventilation, during the rapid passage through air-strata of different temperatures. plate vi. indicates the change of temperature with height observed during the four highest balloon ascents in europe and in the united states. dots indicate the observations while ascending, and crosses the observations while descending; these are connected by full and broken lines respectively, an inclination upward to the left showing a decrease of temperature with height and _vice versâ_. the adiabatic lines, representing a fall of temperature of ° fahrenheit per feet of ascent, serve for comparison. [illustration: plate vi.--temperatures observed in four high balloon ascents.] this account of notable balloon ascents should not be closed without mentioning the most daring and unique of all, the voyage of mr. s. a. andrée towards the north pole in . although his was a voyage of geographical discovery, and not one for the exploration of the air, yet meteorological and other observations were to be made, and andrée had familiarized himself with the instruments and the management of a balloon during several voyages in sweden. the success of the polar voyage depended primarily upon the prevalence of southerly winds, and the ability of the balloon to keep afloat long enough to profit by them, even should they be light and variable at times. therefore the impermeability of the balloon to hydrogen gas was of vital importance, and it was the conviction that the _eagle_, of , cubic feet, was neither sufficiently large nor staunch to sustain itself for thirty days, the time which might be required to reach behring straits, that led dr. nils ekholm, the meteorologist and physicist, to withdraw from the expedition. unfortunately, his fears seem to have been well founded, and it is probable that we must now abandon hope of the safety of the brave andrée and his two companions. a less perilous voyage northward across the alps was attempted in by professor heim, the swiss geologist, and two associates, conducted by the italian aeronaut, spelterini. with an automatic photographic camera, similar to one described in the next chapter, it was hoped to get views of the high alps from above, which would be alike valuable for geologic and topographic study. extensive meteorological observations were made in connection with the sixth international balloon ascent, but only the jura was crossed, at an altitude of , feet, because the balloon travelled in a north-westerly direction, instead of north-east as was expected. many years ago wise and donaldson, the american aeronauts, proposed to cross the atlantic ocean in a balloon. the difficulties which present themselves in such an undertaking are purely technical, and given a balloon which loses its gas so slowly that its buoyancy can be maintained for several days, there seems to be no reason why such a balloon, at a height of four or five miles, could not pass from san francisco to new york, or from the united states to europe, since the motion of the upper clouds proves that the high atmosphere moves almost constantly with great velocity from the west to the east. the dirigible balloon has not been realized except in nearly calm weather, but the aeronaut can often reverse his direction by ascending or descending into a contrary wind to that in which he has been travelling. frequently no clouds separate these opposing currents, which become apparent only when a balloon enters them. it has been mentioned that in glaisher made observations in a captive balloon in england up to the height of feet in order to study the conditions of the air within this distance of the earth, which could not be done in a free and rapidly moving balloon. although captive balloons are frequently used in the european cities to lift people who wish to enjoy the view from a height of or feet, they appear to have been little used by scientific observers since the time of glaisher. in - the aeronautical society at berlin employed a captive balloon in connection with the observations in free balloons which have been described. this captive balloon had a capacity of only cubic feet, but it sufficed to lift an apparatus weighing sixteen pounds, designed by dr. assmann to record atmospheric pressure, as well as the temperature and relative humidity of the air. the balloon, attached to a cable feet long, was drawn down by a steam engine. it was possible in this way to have simultaneous observations at three levels, viz. near the ground, in the free air at a height of about half-a-mile, and at the highest level attained by a free balloon. but the captive balloon is often at a disadvantage, for the wind drives it down, and although the meteorograph mentioned had ingenious devices to neutralize the violent shocks caused by this and by the rebound of the balloon after the gust of wind, yet these impaired the automatic record. the height to which the balloon rose was so much diminished by the wind that instead of feet, which the balloon attained in calm weather when the cable was vertical, the average height of the twenty-four ascents was but half this, and in very windy weather the balloon could not rise at all. [illustration: fig. .--german kite-balloon.] to obviate these difficulties, a few years ago there was invented by two officers of the german army, lieutenants von siegsfeld and von parseval, a captive balloon capable of resisting strong winds, called, from its action as a kite, the _drachen-ballon_ or kite-balloon, and which at the present time is being successfully used in the german army and navy for reconnoitring in all kinds of weather. a smaller kite-balloon, of cubic feet capacity, filled either with hydrogen or with illuminating gas, was first used to lift meteorological instruments at strassburg in , where it remained at a height of several hundred feet during twenty-four hours. as is seen from fig. , the balloon is cylindrical, with hemispherical ends, and is attached to its cable like a kite, so that the wind acts to lift and not to depress it. the cylinder is divided by a diaphragm near its lower end into two chambers, the upper and larger one being filled with gas, while the lower chamber, by means of a valve opening inwards, receives the pressure of the wind which presses against the diaphragm, and preserves the sausage-like form of the balloon in spite of leakage of gas. another wind-bag encircling the bottom of the air-chamber serves as a rudder, and lateral fins or wings give stability to the balloon about its longer axis. the instruments are placed in a basket hung far below the balloon. in cases where there is little or no wind at the ground, captive balloons can render valuable service for meteorological observations, but in all other cases kites are preferable. the reasons for this assertion will be given when we consider kites. from what has been said it will be perceived how much the germans did to advance scientific ballooning, yet their constant rivals, the french, found a way to surpass them in the exploration of the atmosphere. for several years the struggle for supremacy in the attainment of the greatest heights was keen between the scientific men of both countries, but a truce was declared at paris in , and since then both nations have worked together harmoniously. the friendly meeting of french and german physicists at strassburg in to agree upon the details of co-operation, typified the union of nations through science, and while it is true that the atmosphere has no boundaries and cannot be pre-empted, let us hope that the common aims of science will ultimately obliterate even political boundaries. chapter iv _ballons-sondes_ for great altitudes--the international ascents we have seen that the ascent of human beings to heights of six miles is attended with difficulty and danger, and even with apparatus for supplying the life-sustaining oxygen, man can hardly hope to reach much greater altitudes. consequently, to obtain information about the atmospheric strata lying above six miles, that is to say, those facts which require to be ascertained in the medium itself, we must employ the so-called _ballons-sondes_, carrying self-recording instruments but no observers. this method, which was proposed in copenhagen as long ago as , was first put into execution by the french aeronauts, hermite and besançon, who, it may be remarked, suggested attempting to reach the north pole by balloon some time before andrée announced his scheme. a balloon is the best of anemometers, since it takes the direction and speed of the currents in which it floats, and hence it is customary, before a manned balloon starts, to dispatch several small pilot-balloons in order to judge of the direction and strength of the upper winds. even if we do not know the height of the currents in which they float, though this can be ascertained by measuring the height of the balloon trigonometrically or micrometrically, we still obtain a general knowledge of the direction and speed of the currents. with this idea, m. bonvallet in dispatched from amiens, france, ninety-seven paper balloons, each provided with a postal card asking for the time and place of descent. sixty of these cards were returned, almost all the balloons having been carried east by the upper current, ten going beyond one hundred and thirty miles, and one travelling at a speed of almost one hundred miles an hour. the next year the experiment was continued by mm. hermite and besançon with balloons of thirty-five cubic feet contents, and about half of those dispatched from paris were recovered within a radius of one hundred miles. the height to which the balloons could rise is determined by the following considerations: to ascend , feet, where the atmospheric pressure is one-half that at the earth (see plate i.), the balloon when half full of gas must lift itself from the ground; to rise , feet, where the pressure is reduced to one-quarter, it must be able to start upward when one-quarter filled, and so on. in practice the ascensional force usually diminishes at first from various causes, such as the escape of gas, its cooling, and the deposit of moisture on the outside of the balloon. to penetrate the clouds, therefore, it is necessary to have a considerable excess of ascensional force, but above the clouds, since the heating effect of the sun increases greatly with altitude, the gas in the balloon is warmed much above the surrounding air, and so the theoretical altitudes are exceeded. having determined that balloons inflated with one hundred and fifty cubic feet of coal-gas would rise to great heights, simple and light registering instruments, as well as the postal cards, were attached to them. as the pressure diminished, an aneroid barometer traced a line on a smoked glass, and after the descent was placed under the receiver of an air-pump, and the pressure required to reproduce the trace was measured by a manometer. from this the height could be computed approximately. the maximum and minimum thermometer was of the well-known u-form, and instructions appended asked that it be read as soon as found. a slow-match was arranged to detach postal cards successively, so that if they were found and mailed, the track of the balloon could be determined. these balloons at first were called _ballons perdus_, or lost balloons, but when it was known that most of the fourteen balloons liberated from paris were recovered, the name _ballons explorateurs_ was given, which was afterwards changed to _ballons-sondes_, or sounding balloons. the germans call them, _registrir-ballons_, and in english they have been designated unmanned balloons also. one of these paper balloons having reached a height of nearly , feet, mm. hermite and besançon proceeded to construct a balloon of gold-beater's skin, having a capacity of cubic feet, in order to carry a better instrumental equipment. the self-recording instruments made by the french firm of richard brothers were well adapted for this purpose, and a combined barometer and thermometer, registering in ink on an upright drum that is turned by clockwork inside, is shown in fig. . the exhausted pair of boxes b of the barometer actuates the lower pen, while the curved tube c, which is filled with alcohol, by its change of shape moves the upper pen and records the temperature. from the indications of the barometer and the temperature of the mass of air, it is possible by laplace's formula to calculate the height at any hour of the registration. the balloon mentioned was the first of the so-called _aérophiles_, and when inflated with coal-gas it could lift seventy-seven pounds besides its own weight of forty pounds. it carried two of the baro-thermographs described, and a package of information cards arranged to be detached by a slow-match. to mitigate the shock of striking the ground one of the instruments was hung by rubber cords inside a wicker basket that in the first ascent was not screened from the sun. it was decided to liberate these balloons entirely filled with gas (instead of partly full, to allow for its expansion), and to utilize all possible ascensional force at first rather than to weight the balloon with an automatic discharger of ballast (fig. ). the trial of the _aérophile_ occurred march , , and the next day one of the cards was returned announcing its fall in the department of the yonne, where the balloon and the instruments were recovered injured. from the blurred traces of the latter it was computed that at an altitude of about , feet a temperature of - ° fahrenheit had been met with, both pressure and temperature being the lowest measured in a balloon up to that time. although the data secured by this ascent were somewhat doubtful, yet the feasibility of exploring the atmosphere by _ballons-sondes_ was proved. it was seen that the enormous velocity of ascent overcame the wind and permitted the path of the balloon to the summit of its trajectory to be followed, the balloon appearing like a meteor visible in daylight, and so its height could be calculated by trigonometrical measurements; while the descent, caused by the escape and cooling of the gas, was gentle and regular, permitting the delicate instruments to be recovered uninjured. [illustration: fig. .--baro-thermograph of richard.] [illustration: fig. .--the _aérophile_ rising. the left-hand picture shows the deformation caused by the resistance of the air to its rapid ascent, and the right-hand one the violent oscillations when first liberated.] the second ascent of this _aérophile_ was its last, for, after falling in the black forest, it was burned by children. however, m. besançon, not discouraged, constructed the _aérophile ii._ of cubic feet, and improved the instruments as experience suggested. the records had often been interrupted by freezing of the ink, so the pen was replaced by a needle marking with less friction on smoked paper surrounding the record drum. to avoid heating of the thermometers by the sun, they were placed in a wicker cylinder open at both ends and covered with bright metallic paper. this was hung below the balloon with its axis vertical, in order that the draught through the cylinder when the balloon was rising or falling should counteract the insolation, and in the next ascent, at about the same altitude, a temperature lower by ° fahrenheit indicated the effect of the protection. to secure an independent record of the lowest temperature an ingenious device was used, consisting of a thermometer tube filled with alcohol and having black divisions. the lowest point to which the alcohol sank was recorded on photographic paper placed behind the tube, the whole being enclosed in a metallic box that was automatically closed on striking the ground, and so was preserved against the meddling of curious persons. up to the middle of ten voyages had been made by the _aérophiles_, which were now constructed of varnished silk to hold , cubic feet of gas. one of the objects sought was the collection of samples of air at great heights, but this was not accomplished until recently. in the first apparatus for this purpose, an aneroid barometer at a predetermined pressure turned the cock communicating with an exhausted receiver that filled with air and was then closed. the cock leaked, so next the ingenious device of generating heat chemically to seal the glass tube was tried. this, too, failed, but finally, an apparatus of m. cailletet solved the problem. it is advisable to control the height deduced from the barometric records by direct observations so long as the balloon remains visible, and for this purpose micrometric measures were made with a telescope as soon as the balloon left the ground. there was also used a species of registering theodolite which, when kept pointed at the balloon, automatically traced on paper its azimuth and angular altitude. these records, when combined with the barometric height at a known hour, permitted the horizontal distance traversed, and hence the velocity, to be calculated, or, with two such instruments at ends of a base line, the height of the balloon could be found. the first experiments with _ballons-sondes_ in france were soon repeated in germany, where a balloon of rubber-fabric holding cubic feet was obtained by the german society for the promotion of aërial navigation. when inflated with coal-gas it had a lifting force of about two hundred and ninety pounds, in excess of its envelope, etc., weighing ninety-three pounds, and the meteorological apparatus weighing six pounds. the _cirrus_, as it was called, burst on its first trial, but in july it made a remarkable voyage from berlin to the boundary of bosnia, a distance of seven hundred miles, at an average speed of sixty-two miles an hour. a maximum height of , feet and a minimum temperature of - ° fahrenheit were recorded. the _cirrus_ on its third voyage was accompanied by manned balloons in order to have simultaneous observations at different levels, and this time it travelled eighty-three miles an hour and rose , feet. the lowest temperature of - ° fahrenheit was supposed to be too high, for the reason that whereas the ventilation of the thermometers in a rapidly ascending or descending balloon might be sufficient to counteract solar radiation, this would not be the case when the balloon was approaching its culminating point with a diminishing speed. therefore, dr. assmann, under whose supervision the german experiments were conducted, employed the thermometer, which in the captive balloon was aspirated electrically, but now was driven by a weight, and later, because the ink froze, the registration was made photographic. the efficacy of the aspirator was seen in the ascent referred to, for, when its action stopped, a higher temperature was recorded though the balloon continued to ascend. in april the _cirrus_ rose to the extraordinary height of , feet, or more than thirteen and a quarter miles, where the barometric pressure was reduced to one and a half inches of mercury. (in plate i. this extreme and possibly excessive height is not shown as the height of the _ballon-sonde_, but the average of the three highest ascents of the _cirrus_ is indicated.) the comparative warmth (- ° fahrenheit) recorded has led dr. assmann himself to doubt the accuracy of the usual methods of registering temperature at such extremely low pressures. plate vii. shows the heights in metres, and the temperatures in degrees centigrade, during eight voyages from berlin prior to june . notwithstanding the rivalry and difference of opinion between the germans and french as to the methods of exploring the high atmosphere, there was also a sincere desire to co-operate, and the international meteorological conference which was held at paris in september furnished an opportunity to make the arrangements. resolutions were adopted favouring ascents with manned balloons, as well as simultaneous ascents of _ballons-sondes_ in the different countries. the successful use of kites at blue hill to lift self-recording instruments more than a mile into the air led to the wish that similar experiments should be tried elsewhere. an international committee was appointed to carry out these resolutions, of which professor hergesell of strassburg is president, and the veteran parisian aeronaut and journalist, wilfrid de fonvielle, is secretary. [illustration: plate vii.--heights and temperatures recorded in eight ascents of the _cirrus_.] it was agreed to make a night ascent and to use identical instruments, in order that the observations might be made everywhere under the same conditions. accordingly, on the early morning of november , , five balloons manned by observers, and three _ballons-sondes_ with recording instruments, were liberated in france, germany, and russia. by means of the automatic diagrams from the _ballons-sondes_, and the direct observations in the manned balloons, it was sought to determine the decrease of temperature with height in vertical sections of the atmosphere connecting the various centres from which the balloons started. seven such sections were available by connecting paris and strassburg, berlin and st. petersburg, warsaw and munich, etc., but, unfortunately, observations in the highest strata were generally lacking. three more international ascents were made during the year , which were participated in less extensively. at this time it was necessary to decide questions that had arisen, and to make plans for the future, consequently a meeting of the international committee was held at strassburg in . many technical questions were settled, but the chief result accomplished was the dissipation of misunderstandings and prejudices, not only between french and germans, but between the german representatives themselves, for no doubt personal intercourse is the greatest good of such conferences. although it was not a surprise, nevertheless it was regretted that no one came from great britain, where, since glaisher's epoch-making balloon ascensions, little has been done to explore the air. the beneficial results of the conference were apparent at the fifth international ascent, which occurred in the early morning of june , . austria and belgium joined germany, france, and russia, and the field of atmospheric survey was extended over a good part of europe. a veritable aeronautic fleet was launched from paris, brussels, berlin, warsaw, st. petersburg, strassburg, munich, and vienna, consisting of twenty-one balloons, of which thirteen carried observers, who all used the aspiration thermometers, and eight were equipped only with self-recording instruments. some of the latter balloons reached altitudes of , feet, and the former attained extreme heights of one-third this. on the day selected the atmosphere was in a state of repose, with light variable winds, except high up, where they blew, as is usual, from the west or south-west. these observations were sufficiently numerous to form a synoptic chart at a considerable height above europe for comparison with the usual chart drawn from the surface observations. besides the general work of the international committee, special investigations have been undertaken by the french, who formed an aerostatic commission in paris. the services of the eminent physicists, mm. cailletet and violle, have been enlisted, while a generous patron has been found in prince roland bonaparte. the apparatus of m. cailletet to bring down samples of air from the high regions may now be described. when the balloon has reached its greatest height a cock of special construction, turned by clockwork, opens and allows the air to enter a reservoir in which a vacuum exists, and then the cock is automatically and hermetically closed. as it is known that the balloon reaches its extreme height in about an hour and a quarter, the time of opening the cock is so regulated, the closing taking place a little later by its further rotation. in order to protect the moving parts from the extreme cold, a receptacle filled with fused acetate of soda is placed in the box containing the motor, so that, notwithstanding the intense cold of the high regions, this salt in assuming a crystalline state gives out enough heat during several hours at least. during the ascent of an _aérophile_ air was collected at , feet, which when analyzed by m. müntz showed what was supposed, viz. that at this altitude the composition of the air does not differ much from that of the lower air. the slight excess of carbonic acid found in the upper air might be due to the oxidation of the grease used on the cock, and the smaller quantity of oxygen, as compared with normal air, might be caused by the absorption of this gas by the grease or even the absorption by the tinned sides of the copper reservoir. by eliminating all possible sources of error in future ascensions, m. müntz thinks that it can be proved whether there are real differences in the air at different heights, for the methods of analysis are to-day accurate enough to show such differences if they exist. but since it is probable that in the regions which can be explored by the _ballons-sondes_, the air undergoes the same mixing that renders the lower air nearly uniform, only the smallest variations in its composition would naturally be found, requiring minute precautions against errors. this is no doubt why previous measures agreed in showing the invariable composition of the air at lower altitudes. another important contribution of m. cailletet is an apparatus for measuring the height of the balloon by photography in order to verify laplace's formula connecting the barometric pressure with the altitude. the idea was to replace the observers on the ground, who sometimes made the trigonometrical measurements of the balloon described, by a photographic apparatus carried by the balloon itself, and which at frequent intervals should photograph automatically the ground over which it passed, at the same time that an aneroid barometer was photographed on the same sheet. the apparatus is hung vertically below the balloon; in the lower portion of the box is an objective which photographs the ground, and in the upper portion is a second objective which photographs the face of an aneroid barometer placed at the proper distance above. a clock-movement makes exposures every two minutes, and a sensitive film unrolled between the objectives receives the images on each side. if there are known, the focal length of the objective, the distance of two points on the ground, and the distance of two points on the photograph, a simple proportion permits the height of the balloon to be determined at that time, and consequently, from the barometric record, the law connecting the pressure with height can be deduced. the apparatus was successfully used in the voyage of a large balloon with observers, and the accuracy of the determination of height was found to be within / . if the apparatus is to be used at great heights it would be necessary to protect the barometer and the camera from the very low temperatures. besides the use for which it was designed, this apparatus may serve to trace the route of a balloon and to determine the horizontal velocity at the different points of its path. the exploration of the high atmosphere by _ballons-sondes_, which can aid so many investigations, has been utilized by m. violle to obtain actinometric measures, that is, to determine the amount of heat given by the sun, or what is called the "solar constant." this has been done on mountains with varying results, due to the changing amount of atmospheric absorption. in regions traversed by the balloon where the pressure of the air is reduced to a few inches of mercury, where there is a complete absence of water-vapour, and at heights to which terrestrial dust does not extend, the measure of the quantity of heat sent by the sun towards the earth is freed from almost all the errors which we encounter on its surface. the actinometer of m. violle is, in principle, a sphere of copper, blackened externally, and having inside a thermometric apparatus which registers some distance away. under the action of the solar rays the sphere is heated, and assumes equilibrium when the loss by radiation and by contact with the air compensates for the gain by the absorption of the direct heat. while at low levels the atmosphere also contributes to heat the sphere, at great heights the sun shines from an almost black sky and alone heats the sphere. since the balloon follows the wind the apparatus is protected from air currents which would otherwise introduce errors. each twenty minutes a screen cuts off the solar rays from the sphere so that it cools to the temperature of the air, which is also recorded. on account of its weight this apparatus has not yet been carried by a _ballon-sonde_, but it has operated successfully in a balloon with observers. m. teisserenc de bort, who is actively engaged in exploring the air from a meteorological standpoint, has constructed a very sensitive thermometer made of a blade of german silver set in a frame of nickel-steel that does not expand with heat. this may be ventilated by a fan, and, while extremely sensitive to changes of temperature, it is not affected by shocks, and consequently is well adapted for use in _ballons-sondes_ that pass rapidly through air-strata of varying temperature. from this review of the development of the _ballons-sondes_ it is evident that they offer possibilities of obtaining data in the high atmosphere, perhaps up to fifteen miles or more, which, though subject to inaccuracies, are of great interest to the physicist and astronomer. the meteorologist is chiefly concerned with that portion of the atmosphere which lies within two or three miles of the earth, and he requires, moreover, accurate measurements for his conclusions. the new and most satisfactory way of obtaining these data is by kites, and the remaining chapters will treat of this method of exploring the atmosphere and the results. chapter v kites--history and application to meteorological purposes at blue hill and elsewhere kites are supposed to have been invented four hundred years before the christian era by archytas, and at smyrna the flying of kites remains a national sport to this day. we are told that two hundred years later, a chinese general, han sin, employed kites as a means of communication with the garrison of a besieged town, and there is a legend about their use in japan to dislodge and carry away a golden ornament from a tower. whatever may be the truth of these stories, we know that kite-flying in the malay archipelago, in china, and in japan, has been a pastime for all classes during centuries, and that the asiatic people have always been the expert kite-fliers of the world. kites with tails seem to have been introduced into england about two hundred and fifty years ago, and isaac newton when a school-boy made some improvements in them. notwithstanding the fact that generations of boys have flown kites and so eminent a mathematician as euler investigated their theory, until recently kites remained toys unsuited for practical purposes. since the tailless kite has become a familiar object, it has been said facetiously that kites lost their tails by the same process of evolution which deprived man of his caudal appendage; but as kites without tails have been flown in asia for centuries, the truth is that the tailed kites were the ones first brought to europe as playthings. to-day in holland we see boys flying the english bow-kite and the common kite with crossed sticks, both of which require tails, and by the side of them tailless kites imported from the dutch colonies in java. fig. represents a kite from the east coast of java, drawn from a model in a museum at amsterdam, and also a drawing of a chinese bird-kite in the national museum at washington. like most of the oriental kites, they are made flat, but when exposed to the wind the extremities of the wings, which have a frame of split bamboo, bend backward, securing in this way the stability which in our common flat kite is gained by the action of the tail in lowering the centre of gravity and in maintaining the inclination to the wind. [illustration: fig. .--oriental tailless kites.] from historical researches that have been stimulated by the recent practical applications of kites, it appears that their first use for scientific purposes was in , when dr. alexander wilson of glasgow, and his pupil, thomas melvill, used kites to lift thermometers. their kites, from four to seven feet in height, and covered with paper, were fastened behind one another, each kite taking up as much line as could be supported, thereby allowing its companion to soar to an elevation proportionally higher. it is related that "the uppermost one ascended to an amazing height, disappearing at times among the white summer clouds, whilst all the rest, in a series, formed with it in the air below such a lofty scale, and that too affected by such regular and conspiring motions, as at once changed a boyish pastime into a spectacle which greatly interested every beholder.... to obtain the information they wanted they contrived that thermometers, properly secured, and having bushy tassels of paper tied to them, should be let fall at stated periods from some of the higher kites, which was accomplished by the gradual singeing of a match-line." how the thermometers were prevented from changing their readings while falling to the ground is not explained. the account concludes: "when engaged in these experiments, though now and then they communicated immediately with the clouds, yet, as this happened always in fine weather, no symptoms whatever of an electrical nature came under their observation. the sublime analysis of the thunderbolt, and of the electricity of the atmosphere, lay yet entirely undiscovered, and was reserved two years longer for the sagacity of the celebrated dr. franklin." hence it seems that franklin's famous experiment of collecting the electricity of a thunder-cloud by means of a kite, performed at philadelphia in , was not its first scientific application, and therefore america can claim only the later and most remarkable development of this means of exploring the air. about there existed in philadelphia an organization called the franklin kite club that flew kites for recreation. espy, the eminent meteorologist, was a member, and he states "that on those days when columnar clouds form rapidly and numerously the kite was frequently carried upward nearly perpendicularly by columns of ascending air," a phenomenon which is often observed to-day. espy calculated the height at which clouds should form by the cooling of the air to its dew-point, and then employed kites to verify his calculations of the heights of the clouds. it will be remembered that both these methods are utilized in the measurements of cloud-heights at blue hill. kites were employed to get temperatures a hundred or more feet above the arctic ocean early in the present century, and in w. r. birt flew a kite at kew observatory, with which he hoped to obtain measures of temperature, humidity, wind velocity, etc. this kite, hexagonal in shape, required three divergent strings attached to the ground to keep it steady, and the instruments were to be hoisted up to the kite by a pulley. perhaps the first person to soar aloft on a kite was a lady, who, more than fifty years ago, was lifted some hundred feet by a great kite constructed by george pocock, an englishman, to serve as an aërial observatory in warfare, and also to drag carriages along the ground. it was proposed afterwards to make use of kites in shipwrecks to take persons or life-lines ashore, and in sir george nares invented a storm-kite, so called, with a tail made up of hollow cones. this form of tail, subsequently used for both kites and balloons, is very efficient, since it offers increasing resistance as the wind becomes stronger. in mr. douglas archibald in england revived the use of kites for meteorological observations, and outlined a comprehensive scheme of exploring the air with kites which included almost all that has been done since, but his actual work, performed during the next three years, was limited to ascertaining the increase of wind velocity with height. to do this, he attached registering anemometers at four different points on the kite-wire, but since the total wind movements only were registered from the time the anemometers left the ground until they returned, it was impossible to obtain simultaneous records near the ground and at the kite, as is done to-day. still, archibald got differential measurements of the velocity of the wind up to the height of feet. the kites he employed were diamond-shaped, covered with silk, and were flown tandem, with the hollow cones, already mentioned, attached to the tails. although copper and iron wire had been used for flying kites many years before, yet archibald was the first to substitute steel pianoforte wire for the string, thereby increasing the strength while diminishing the weight, size, and cost of the line. mr. archibald in took the first photograph from a kite, a method which mm. batut and wenz developed in france, and messrs. eddy and woglom in the united states. the subsequent progress of kite-flying for meteorological purposes has taken place in the last-named country, and may be chronologically stated as follows: in mr. alexander mcadie (later of the u. s. weather bureau) repeated franklin's kite experiment on blue hill, with the addition of an electrometer; in , and again in , he measured simultaneously the electric potential at the base of blue hill, on the hill, and with kites as collectors several hundred feet above the hill-top, about the same time that dr. weber, in breslau, germany, was making a more extensive use of kites for the same purpose. it was no doubt william a. eddy of bayonne, n. j., who turned the attention of american scientific men to kite-flying, and created the widespread interest in kites which exists to-day. about mr. eddy lifted thermometers with an ordinary kite, but soon afterwards devised a tailless kite, resembling the java kite except that the horizontal cross-piece is nearer the top of the vertical stick, and its ends are bent backward in a bow and connected by a cord. this kite starts upward on being held in the wind at the end of a taut line, and continues to rise until the increasing wind-pressure on the portion above the cross-stick balances the pressure on the larger lower portion. the kite is kept from falling to one side by the looseness of the covering on either side of the backbone, and if there is more material on one side than on the other, or if the covering is too tight to form pockets in the wind, the kite requires a tail.[ ] [ ] a tail will prevent any kite from turning over, or "diving," because its weight keeps the lower end down while the pressure of the wind on the tail also pulls the lower end backward and maintains the necessary angle of the kite to the wind, the most efficient angle being about degrees. bending back the ends of the cross-stick gives stability to a kite because, when, on account of the eddies in the wind, a stronger pressure is exerted on one side of the kite, this side is driven backward, thereby presenting less effective surface to the wind, while as the other side comes forward more nearly at right angles to the wind, it receives greater pressure than before. in this way the equilibrium about the central stick is automatically maintained, the required inclination to the wind being secured by the greater surface presented to the wind below the point of attachment of the bridle. in mr. eddy lifted a minimum thermometer by several of these kites flown tandem, and proposed to obtain in this way data to forecast the weather. in the _proceedings of the aeronautical conference_, held in connection with the chicago exposition, prof. m. w. harrington, then chief of the u. s. weather bureau, quoted mr. eddy's estimate of the cost of exploring the air by means of kites flown in series, and advocated their use. up to this time it does not appear that self-recording instruments--that is to say, those which make continuous graphic records--had been raised by kites. in the days of the early experimenters such instruments were too heavy and cumbersome to be lifted by the more or less unmanageable kites, but within the past few years m. richard of paris has made the simple and light recording instruments described in connection with balloons, which can be attached to kites. in this way it is possible to obtain simultaneous records at the kite and at a station on the ground, and from them to study the differences of temperature and humidity, and this seems to have been done first at blue hill observatory. in august mr. eddy brought his kites to blue hill and with them lifted a richard thermograph, which had been partly reconstructed of aluminium by mr. fergusson so that it weighed but - / lbs., to the height of feet, and so the earliest automatic record of temperature was obtained by a kite. during the next summer, mr. eddy assisted again in the experiments at blue hill, and secured photographs of the observatory and the hill by a camera carried between his kites to the height of a hundred feet or more. now that the possibility of lifting self-recording meteorological instruments to considerable heights had been demonstrated, an investigation of the thermal and hygrometric conditions of the free air was undertaken by the staff of the blue hill observatory, who had already made an investigation of the movements of the clouds by the methods described in the second chapter. the development of the kite and its accessory apparatus, and the acquisition of the knowledge how to use them, required much time, and resulted in the damage or loss of many kites. two meteorographs, as the combination of two or more self-recording instruments is called, were dropped from a great height and no trace of them was found. when, however, by the breaking of the line both kites and instrument are carried away, the kites act as a parachute and bear the instrument gently to the ground, where both are usually recovered uninjured; to facilitate their return should they fall at a distance, the name and address are marked on each. it would be tedious to relate the ups and downs of scientific kite-flying at blue hill before the wind was successfully harnessed to the service of science, and the kites were prevented from kicking over the traces, or from breaking away, so only a brief account of the progress of the work will be given, and then the methods at present used will be described. at first the eddy, or malay kites, as they are also called, covered with paper or with varnished cloth, were coupled tandem to secure greater safety and lifting power. the principle of attaching kites at several points on the line was early adopted at blue hill, for although it can be demonstrated theoretically that a greater height is possible by concentrating all the pull at the end of the line, yet in the case of a line which is not infinitely strong the best results are got by distributing the pull, and in this way, too, kites can be added as the wind conditions aloft warrant. to obviate the frequent breaking of the bowed cross-piece, mr. fergusson made it in two pieces, each being held in a metal socket on the central stick, the two pieces forming a dihedral angle towards the wind. it had the advantage also of being readily taken apart for transportation. this kite, shown in fig. , flew at a high angle above the horizon and through a considerable range of wind velocity, but it could not be kept permanently in balance, or made to adjust itself to great variations in wind velocity, and therefore it was discarded. the first meteorograph, a combined recording thermometer and barometer (from which the height can be calculated), was constructed by mr. fergusson in august , and three months later he united a recording anemometer to the thermometer, which was probably the first apparatus of this kind to be attached to kites. a meteorograph, recording the atmospheric pressure, air temperature, and relative humidity, was ordered from m. richard of paris in , like one already carried by french aeronauts, except that, since for kites lightness is all-essential, m. richard constructed this triple-recorder for the first time of aluminium, and hereby reduced its weight to - / lbs. [illustration: fig. .--eddy tailless kite.] [illustration: fig. .--hargrave kite.] one of these meteorographs was hung to a ring at the point of attachment of the two kite-lines to the main line, a method which was used until recently. in august , besides the eddy kites, there was first used the cellular or box kite, invented by lawrence hargrave of sydney, australia, which bears no resemblance to the conventional forms of kites and which it would not be supposed could fly. as seen from fig. its appearance is that of two light boxes without tops or bottoms, fastened some distance above each other. the wind exerts its lifting force chiefly upon the front and rear sides of the top box, the lower box, which inclines to the rear, and so receives less pressure, preserving the balance. the ends of the boxes, being in line with the wind, keep the kite steady and serve the purpose of the dihedral angle in the malay kite. the japanese are said to fly a single box, which is the prototype of the hargrave double cell. at the present time some form of the hargrave kite is generally employed for scientific purposes. on account of the weight of the large cord necessary to control these kites, and the surface which it presented to the wind, a height of feet could not be reached, so, during the winter of - , following archibald's example and the methods of deep-sea sounding employed by captain sigsbee, u. s. n., steel pianoforte wire was substituted for the cord. this wire is less than half as heavy, and less than one-fourth the size of cord having the same strength, and, moreover, its surface is polished, which reduces the friction of the wind blowing past it. with the wire the height of a mile was reached in july, and a mile and two-thirds above blue hill in october . up to this time a reel turned by two men sufficed to draw down the kites, but the increasing pull and length of wire made recourse to steam-power necessary. in january a grant of money was allotted from the hodgkins fund of the smithsonian institution for the purpose of obtaining meteorological records at heights exceeding ten thousand feet, and no doubt the first application of steam to kite-flying was the winch built by mr. fergusson with ingenious devices for distributing, oiling, and measuring the length of wire. the cumulative pressure of the successive coils of wire finally crushed the drum, and the next apparatus applied the principle of sir william thomson's deep-sea sounding apparatus, in which there is no accumulation of pressure. in october records were brought down from eleven thousand feet, or a thousand feet above the prescribed height. the kites and apparatus at present employed at blue hill will now be described. the kites are all of the multiplane type, and mostly of hargrave's construction with two rectangular cells. these cells are covered with cloth or silk, except at their tops and bottoms, and one is secured above the other by four or more sticks. the wooden frames are as light as possible, but are made rigid by guys of steel wire that bind them in all directions. the average weight is about two ounces a square foot of lifting surface, which is about the same weight a square foot as the eddy kites when all the surface is included in the estimate. the largest of the hargrave kites stands nine feet high, weighs eleven pounds, and contains ninety square feet of lifting surface, which in the recent kites is arched, resembling the curvature of a bird's wings, a construction that was proposed many years ago by phillips (fig. ). these curved surfaces increase the lift, or upward pull, more than the drift, or motion to leeward, and so the angular elevation is augmented without materially adding to the total pull on the wire, which should not exceed one-half its breaking strength. [illustration: fig. .--modified hargrave kite at blue hill.] perhaps the most important factor in the success of the blue hill work was the invention by mr. clayton of the regulating bridle which is applied to every kite. an elastic cord is inserted in the lower part of the bridle, to which the flying-line is attached, and when the wind-pressure increases this cord stretches, and causes the kite to diminish its angle of incidence to the wind until the gust subsides. a kite can be set to pull only a fixed amount in the strongest wind, when the kite will fly nearly horizontal. we are therefore able to calculate the greatest pull which can be exerted on the wire by all the kites. with this device the kites have flown through gales of fifty or sixty miles an hour without breaking loose or injuring themselves. another efficient kite which has been used at blue hill is the so-called "aero-curve kite" made by mr. c. h. lamson of portland, maine. as is seen from fig. , this kite resembles a soaring bird, and it can be taken apart and folded up for storage or transportation. in general, the angle of the flying lines of the blue hill kites is ° or ° above the horizon, and in winds of twenty miles an hour the pull on the line is about one pound for each square foot of lifting surface in the kite. kites can be raised in a wind that blows more than twelve miles an hour at the ground, and as the average velocity of the wind for the year on blue hill is eighteen miles an hour, the days are few when kites will not fly there. [illustration: fig. .--lamson's aero-curve kite.] the wire to which the kites are attached is steel music-wire, / of an inch in diameter, weighing fifteen pounds a mile, and capable of withstanding a pull of three hundred pounds. the wire is spliced in lengths of more than a mile with the greatest care, special pains being taken that no sharp bends or rust-spots occur which would cause it to break. to lift the increasing weight of wire, kites are attached at intervals of a few thousand feet, so that the angle may be maintained as high as is consistent with a safe pull, and this is done by screwing on the wire aluminium clamps, to which the kite-lines are fastened. on account of the greater stability and strength of the new kites, the meteorograph is suspended directly from the top kite. the richard meteorograph, contained in an aluminium cage of about a foot cube, weighs less than three pounds, and it is only necessary to screen the thermometer from the sun's rays to obtain the true temperature of the air, since the wind insures a circulation of air around the thermometer. another meteorograph, constructed by mr. fergusson, records the velocity of the wind in addition to the three other elements, and it weighs no more than the french instrument. the reeling apparatus is an example of how the same apparatus may serve diametrically opposite purposes. in sounding the deep sea the wire must be pulled upwards, whereas in sounding the heights of the atmosphere the wire must be pulled in the reverse direction. therefore the deep-sea sounding apparatus has been altered by mr. fergusson to pull obliquely downwards, the wire passing over a swivelling pulley which follows its direction and registers on a dial the exact length unreeled. next the wire bears against a pulley carried by a strong spiral spring, by which the pull upon it at all times is recorded on a paper-covered drum turned by clockwork. the wire passes now several times around a strain-pulley, and finally is coiled under slight tension upon a large storage-drum. when the kites are to be pulled down, the strain-pulley is connected with a two-horse-power steam-engine, and the wire is drawn in at a speed of from three to six miles an hour; but when the kites are rising the belt is removed, and the pull of the kites unreels the wire. [illustration: fig. .--meteorograph lifted by kites at blue hill.] the method of making a kite-flight for meteorological purposes at blue hill is as follows: a kite, fastened by a long wire to the ring in the main wire, being in the air, and the meteorograph suspended, another kite is attached to the ring by a shorter cord (fig. ). they are then allowed to rise, and to unreel the wire, until its angle with the horizon becomes low, when, by means of the clamps described, other kites are added, the number depending on the size of the kites and the strength of the wind. after a pause at the highest attainable altitude, the winch is connected with the steam-engine and the kites are drawn down. the pauses at the highest point, and when kites are attached or detached, are necessary to allow the recording instruments to acquire the conditions of the surrounding air; and because at these times the meteorograph is nearly stationary, measurements of its angular elevation are made with a surveyor's transit, while observations of azimuth give the direction of the wind at the different heights. the time of making each angular measurement is noted, so that the corresponding point on the trace of the meteorograph may be found. from the length of the wire and its vertical angle, the height of the meteorograph can be calculated, it having been found that the sag of the wire, or its deviation either in a vertical or a horizontal plane from the straight line joining the kite and the reel, does not cause an error exceeding three per cent. in the height so computed. when the meteorograph is hidden by clouds, the height above the last point trigonometrically determined is computed from the barometer record by laplace's formula. at night there is only the barometer from which to determine the height; for although an attempt was made to use a lantern to sight upon, yet it soon became invisible, or, when seen, was confounded with the stars. before and after the flight the meteorograph is hung upon a tripod in the free air, in order that its thermometer and hygrometer may be compared with the standards. heights above sea-level of kite-flights. (_blue hill is feet above the sea_) ----+----+----------------++------------------------------------------- | no.|heights in feet || percentages of records above year| of +-------+--------++-------+--------+--------+--------+-------- |rec-|mean of|absolute|| m.| m.| m.| m.| m. |ords|maximum|maximum || ( | ( | ( | ( | ( | | | || ft.) | ft.) | ft.) | ft.) | ft.) ----+----+-------+--------++-------+--------+--------+--------+-------- | | , | , || | | | | | | , | , || | | | | | | , | , || | | | | | | , | , || | | | | | | , | , || | | | | ----+----+-------+--------++-------+--------+--------+--------+-------- since the use of wire and more efficient kites, the heights have been greatly increased. thus the average height above the hill attained by the meteorograph in thirty-five flights made during was more than a mile and a quarter, whereas the average height of all the ascents prior to was about a quarter of a mile (see table). the average height of the meteorograph above the hill, in all the flights during august , was nearly a mile and a half, and on august the meteorograph was raised feet higher than ever before, its altitude, determined trigonometrically, being , feet above blue hill, or , feet above the neighbouring ocean. the meteorograph was suspended from the topmost kite, one of the lamson pattern, having square feet of lifting surface, and this was increased to a total of square feet by four kites of the modified hargrave type, that were attached at intervals to the wire. the five miles of wire in the air weighed lbs., and the total weight including kites and apparatus was lbs. the meteorograph left the ground at : a.m., attained its greatest height at : p.m., and returned to the ground at : p.m., a feat which it would be difficult for a man to equal on a mountain. the cumulus clouds were traversed three-quarters of a mile from the earth, and above them the air was found to be very dry. on the hill the air temperature was °, when it was ° in the free air , feet above, and the wind velocity increased from twenty-two to forty miles an hour. these figures give an idea of the change of atmospheric conditions which occurs, but the conclusions deduced from the blue hill kite-flights will be discussed in the next chapter. however, the phenomena of atmospheric electricity, which have become noticeable since the use of wire, may be described here. generally, whenever the kites rise above seventeen hundred feet, the wire becomes strongly charged with electricity, and when the great heights are reached the electricity is discharged in long and brilliant sparks at the reel, often to the inconvenience of the attendants. usually, the electrical potential increases with altitude, and it is greatest during snow-storms or when the conditions favour thunder-storms. notwithstanding its intensity, the quantity of electricity in the atmosphere is probably insufficient to make its collection and storage for practical purposes worth while. it must not be imagined that kite-flying for meteorological purposes is a sinecure. at blue hill about two hundred flights have been made in all seasons and in all weathers, with temperatures varying from - ° to + °, in gales, in rain, and in snow-storms, though not in thunder-storms. sometimes the kites are invisible from almost the time they leave the earth until their return, but when the upper kites are visible it is necessary to observe them with theodolites every few minutes. remembering that a high flight occupies ten or twelve hours, and frequently terminates late at night, or even continues until morning, it will be obvious that the work requires skill, energy, and perseverance, which have been shown by my assistants at the blue hill observatory who have conducted the flights. occasionally, for lack of wind or from breakage of the line, the kites fall to the ground, usually intact. if they were visible, trigonometrical measurements on the hill enable the place of descent to be located, and then the kites and meteorograph are sent for and the wire is reeled up. but at night, or when clouds hide the kites, the direction in which they fall is not known, because the azimuth of the wire at the reel often differs from that of the kites; so last autumn several hundred miles of road, path, wood, and swamp were traversed before the aërial apparatus, which had been lost during a flight at night, was found comparatively close at hand. from what has been said it will be evident that a former toy has been proved to be of the greatest importance for meteorological investigation at the blue hill observatory. on account of the success there attained it is coming into use elsewhere for meteorological observations. in the united states weather bureau created seventeen kite stations, chiefly in the mississippi valley, with the intention of obtaining data every day, at the height of a mile or more, with which to plot a synoptic weather map similar to the map that is now drawn from the data at the ground. from a knowledge of the weather conditions prevailing simultaneously in the upper and lower air, it was expected that the weather forecasts could be improved, but unfortunately, on account of the light winds during the summer, it was impossible to make enough simultaneous kite-flights to construct the upper-air map, and therefore the scheme was abandoned. however, the data obtained will no doubt furnish valuable information about the vertical temperature gradient, etc., in various conditions of weather. the chief meteorological bureaus of germany and russia are equipping stations with kites and balloons, and m. teisserenc de bort, who has provided his private observatory near paris with kite apparatus of the blue hill type, has already reached high altitudes. in scotland too, which was the birthplace of scientific kite-flying, experiments have been resumed by a scotchman and an american--a happy union of forces. from these preparations it appears that the resolution of the international aeronautical conference, recommending that all central observatories should employ this method of investigation as being of prime importance for the advancement of meteorological knowledge, is being carried out, and seems likely to produce important results. chapter vi results of the kite-flights at blue hill--future work kites possess several advantages over other methods of exploring the air up to heights of at least , feet whenever there is wind, but their chief merit is, that with them the true conditions of the air may be ascertained. the disadvantages of other methods of exploring the air, as compared with kites, are these: . =mountains= not only affect by contact the adjacent air, but by deflecting the air-currents cause mixture and ascent, which give conditions differing widely from those of the free air. . =free balloons= are more or less surrounded by heated or stagnant air, because they drift with the wind, and on account of the sluggishness of the thermometers, the temperatures observed at a given height in a balloon are generally higher during the ascent, _i.e._ when passing from warm to cold air, than during the descent, when the conditions are reversed. again, it is not possible to study the progressive changes in the atmospheric conditions at one place, because observations in a drifting balloon are not comparable with simultaneous ones made at a station on the ground below. with kites, however, the possibility of making frequent and nearly vertical ascents and descents permits observations to be obtained almost simultaneously in superincumbent strata of air. the height of the kite can usually be determined with an accuracy not attainable by the barometer in a balloon. . =captive balloons=, although constructed so as not to be driven down by wind, cannot rise nearly so high as kites on account of the weight and resistance of the cable necessary to control them, and even the german kite-balloon, on account of its large surface, would hardly withstand the strong winds in which kites can fly. . =the cost= of installing and operating either mountain stations or balloons is much greater than for kites. the exploration of the lower two miles of air with kites flown from blue hill is no doubt the most complete ever made at one place. nearly two hundred records have been obtained in all kinds of weather conditions, and the progressive attainment of greater and greater heights is shown in the table in the preceding chapter. the records from the flights have been discussed by mr. clayton; those until february , with the blue hill observations, in vol. xlii., part i., of the _annals of the astronomical observatory of harvard college_, and later records in two _bulletins_ of the blue hill observatory, in which the changes of temperature and humidity with height, and their relation to the positions of cyclones and anti-cyclones, are investigated. the use of kites for weather predicting, as was said, has been tried by the united states weather bureau, but it is certain that further studies, such as have been made on blue hill, are necessary before the sequence of the conditions at the earth's surface to the phenomena observed in the upper air is definitely known, so that the latter can be utilized in forecasting. some of the deductions from the observations with kites at blue hill follow. plate viii. is a facsimile of the record of the baro-thermo-hygrograph during two flights on october , , when for the first time the height of a mile and a half was attained. the record-sheet, it may be said, is wrapped around a cylinder that turns on its axis in twelve hours, and the curved lines in each of the three horizontal sections divide them into quarter hours. the lower section contains the trace of the barometer, the horizontal lines being the heights in metres and feet that correspond to the barometric pressure with a temperature of ° fahrenheit; in the middle section is the trace of the hygrometer on a scale of relative humidity in percentages, and in the upper section is the trace of the thermometer on a scale of temperatures in fahrenheit and centigrade degrees. it will be observed that the record of the barometer is reversed, _i.e._ the trace rises for falling pressure, and in the second flight when the unexpected height of feet above blue hill was reached, the limit of the altitude scale was exceeded. [illustration: plate viii.--meteorogram from the kite-flight of oct. , , at blue hill.] in order to study the changes of these elements with height during the higher flight, in plate ix., figs. and , the temperature and humidity of the automatic record are plotted as abscissæ, with the heights above sea-level in metres as ordinates. for those not familiar with this unit of length, it may be said that metres are about feet, and that metres equal one mile approximately. when the meteorograph was ascending, dots indicate the recorded temperatures and humidities, which are each connected respectively by continuous lines; when the meteorograph was descending, crosses indicate the observations, which are connected by broken lines. lines inclining upwards to the left indicate decreasing temperature and humidity with increase of height, and lines inclining to the right increasing temperature and humidity with height. the straight dotted lines show the adiabatic decrease of temperature for ascending dry air. the ascent was made during the warmest part of the day, and the descent for the most part after sunset. the two branches of the temperature-lines typify the temperature change with height which usually occurs in fair weather during the day and the night respectively. the continuous line, representing the day observations, shows a uniform fall of temperature at the adiabatic rate to the cloud level. during the night, the lower part of the broken line bends decidedly to the left, showing a body of relatively cold air near the ground, caused by radiation. there is a rise of temperature with increasing altitude above the ground up to a certain height, and afterwards a comparatively uniform fall as high as the clouds, if they exist; but the rate of fall with increasing altitude, shown by the upper part of the diagram, is slower at night than during the day. it appears that the diurnal change of temperature is very small at great altitudes, compared with the change near the earth's surface. the relative humidity (fig. ) up to metres varies inversely with the temperature, and in the present case there was only a slight change in the direction of the wind (fig. ). [illustration: plate ix.--mean changes with height, and changes during the kite-flight of oct. , .] =diurnal changes of temperature at different altitudes.=--the curve representing the diurnal change in the air at some distance above the ground is probably similar to one representing the change near the ground, except that its amplitude is less. if this be true, then the diurnal rate of fall for a given time at any two levels will be proportional to the daily ranges of temperature at the two levels. it is impossible in practice to keep a kite at exactly the same level for twenty-four hours; hence the daily ranges for the different levels must be found by comparing the rates of rise or fall of temperature for given times with the rates found from records near the ground, made simultaneously with those above. in plate ix., fig. , the results for six stations, _i.e._ the kite at and metres, the eiffel tower in paris ( metres), the summit of blue hill, its base, and the valley ( , , and metres respectively), are connected, and a smooth curve is drawn through them. the curve passes approximately through every one of the observed and the computed ranges, except the one at the summit of blue hill, which is too great. this evidently is because insolation and radiation, acting through the soil of the hill, heat and cool the air to a greater extent than the free air is heated and cooled at the same altitude, and this must be true at every mountain station. the smoothed curve passes also very slightly to the left of the data for the eiffel tower, indicating that the range there is about ° greater than the true range on account of the heating and cooling of the tower. from this it appears that the diurnal range of temperature diminishes rapidly with increasing altitude in the free air, and almost disappears in the average at a height of metres. the records of the anemometer show that, as a rule, the wind increases steadily as the kites rise, but the increase is greatest between boston and the top of blue hill, due probably to the retarding of the lower winds by contact with the ground. the results are plotted in plate ix., fig. , together with the mean wind velocity on blue hill ( metres), and the velocity on a tower in boston ( metres). single records of the kite-anemometer differ much, for sometimes the wind velocity diminished with altitude, and at other times it increased so rapidly that the kites were unable to rise higher. on several occasions when the kites passed from one current into another, having a different direction and a different temperature, the wind suddenly increased, and was stronger between the two currents than above or below that plane. =diurnal changes of humidity at different altitudes.=--it is found that as night approaches the humidity at the altitude of metres diminishes, while at the earth it increases. this agrees with the evidence furnished by the cumulus clouds that form during the day between and metres, and disappear at night, thus visibly indicating an increase of humidity by day and a decrease by night. if the trend of the humidity-curve at a height of metres is assumed to be the reverse of its trend at the ground, then the results from the kite-meteorograph show the minimum humidity to be at the coldest and the maximum humidity at the warmest part of the day. the mean daily ranges for different altitudes are plotted in plate ix., fig. . the part of the curve at the left of the zero line shows the range at different altitudes, with the minimum humidity near the warmest time of day, while the part at the right of the zero shows the ranges at different altitudes, with the minimum humidity at the coldest time of day. =types of change of temperature with altitude.=--when the records of temperature and humidity made aloft by the kite-meteorograph and at the stations near the ground are plotted in relation to altitude, they are found to be easily divisible into a few types. in plate x., type represents the decrease of temperature on most fair days from the ground to altitudes of a mile or more, when no clouds are met. the continuous line, plotted from the records of the ascent, represents the day conditions, and the broken line, plotted from the records of the descent, represents the night conditions. this curve shows that with increasing altitude the temperature falls uniformly during the day and approximately at the adiabatic rate represented by the dotted lines. the fall of temperature with increasing altitude during the night is slower than during the day, and in fact, from the earth's surface to an altitude of a few hundred metres, there is often a rise of temperature with height, so that the air at altitudes of from to metres may be considerably warmer than it is at the ground. this was shown in the descent on october , , and is found in type . [illustration: plate x.--changes with height recorded by kites at blue hill.] when clouds are traversed during the flight, the temperature curve assumes the form of type . the continuous curve is plotted from the records of an ascent; the broken curve from the records of the descent, both occurring in the day-time. the temperature falls at the adiabatic rate in unsaturated air till the base of the cumulus cloud is reached. it falls at a slower rate in the cloud, the rate probably being that computed by physicists as the adiabatic rate for air in which condensation is taking place. above the clouds, the fall of temperature appears to be very slow. type is a condition which persists throughout the day and night, and it resembles the night form of type . the temperature rises very rapidly for a short distance above the ground and then falls, with increase of height, somewhat slower than the adiabatic rate. the rise of temperature near the ground with increasing height is more marked after sunset than during the day-time. type was illustrated by the ascent of october . this distribution of temperature is caused by a warmer current overflowing colder air, which is very commonly found at low altitudes in the atmosphere and probably exists usually at some altitude, great or small. recent observations indicate that this type represents the normal condition of the atmosphere in all sorts of weather. frequently there are two or more sudden rises of temperature at different heights, so that the plotted data resemble inverted stair-steps. during the day there is a decrease of temperature at the adiabatic rate ( °· in metres) from the ground to the height of several hundred metres, then a sudden rise of temperature in the next one or two hundred metres, and above this a slow fall of temperature with increasing altitude, usually much less than the adiabatic rate. generally, clouds are found near the plane of meeting of the warm and cold current. the reverse of type , that is, a sudden fall of temperature, due to a colder current overlying a warmer one, is probably impossible, because the colder air, on account of its greater weight, would immediately begin to sink and the warmer air would rise. this should cause a fall of temperature at the adiabatic rate from the ground to the top of the colder current, and is probably the origin of the "cold wave" shown in type . both the continuous and broken curves (representing an ascent and a descent) show a fall of temperature at the adiabatic rate of unsaturated air, from about metres to the highest point reached. up to metres the decrease of temperature is more rapid than the adiabatic rate, due to the rapid moving in of colder air above, whereby air rising from the ground is cooled by contact as well as by its expansion, and also because the air is heated more than usual by contact with the ground, which under these conditions is abnormally warmer. this is the special characteristic of the "cold wave" type of curve during the day hours. the night form of type , notwithstanding the excessive radiation from the ground through the dry air, shows a rapid decrease of temperature with increase of altitude from the ground upward. type shows a less common, but an interesting form, of vertical distribution of temperature, in which the temperature is about the same from metres to metres or more. up to metres there is a fall of temperature with increasing altitude during the day, and a rise with increasing altitude at night. these last conditions can be readily traced to the effects of insolation and radiation near the ground. in the morning, if the temperature of the air be the same from the ground up to metres or more, the heating of the ground by the sun will cause ascending currents, until the warmest part of the day. this air, cooling by expansion at the adiabatic rate, will rise to about metres before it assumes the mean temperature of the upper air column. at night cooling takes place next the ground by radiation and is gradually transferred upward a few hundred metres by conduction, thus producing an increasing temperature with increasing altitude, until sunrise. as a result of the conditions described, it is evident that on certain days the diurnal range of temperature is but little felt above metres. =types of change of relative humidity with altitude.=--as in the temperature types, the continuous lines represent the records of the ascent, and the broken lines the records of the descent, generally under changing conditions. lines inclining upward to the left show a decreasing humidity, and to the right an increasing humidity. type may be called a normal type of curve when there are clouds. a variation of this type was met with in the ascent on october , , and it differed from that now illustrated in indicating in its upper part a fall of humidity rather than a rise. these two types can be taken as the normal change of humidity with change of altitude in cloudy or partly cloudy weather. the humidity increases steadily to the base of the cloud, then there is complete saturation in the cloud, and above it is a sudden fall of humidity, on entering the dry air above the cloud, into which the ascending currents from the ground have not penetrated. type is a clear-weather form of curve in which the humidity increases until a certain altitude is reached, probably at the upper limits of the currents rising from the ground. above this altitude the humidity decreases rapidly. type is also a clear-weather form and accompanies the "cold wave" type of temperature, also numbered . the very dry descending air mingles with air rendered damp by ascent, and the result is a nearly uniform relative humidity at different altitudes, although the absolute humidity diminishes on account of decreasing pressure and temperature. in type both the relative and the absolute humidity decrease rapidly, this type coinciding with the temperature, type . during the week of september to , , kite-flights were made daily on blue hill. twice the kites were maintained in the air, and continuous records were obtained during most of twenty-four hours. these records furnish an example of the small diurnal changes of temperature in the free air at short distances above the ground, which were deduced from the average changes at different hours and at different heights. from p.m. of the fifth to p.m. of the sixth, the altitude of the self-recording instruments varied between and metres above sea-level, averaging about metres and varying little from this height during much of the night. the times when the kite-meteorograph crossed the -metre level in ascending and descending were determined from its barograph trace, and the synchronous temperatures and humidities were read from the records of its thermograph and hygrograph. the results have been plotted in plate xi., figs. and , together with the temperatures recorded simultaneously at the summit and valley stations of the observatory and the humidities at the summit. fig. shows that the diurnal variation of temperature, well marked at the lower levels, is very slight or has entirely disappeared at metres. fig. shows that the course of the relative humidity at metres is exactly opposite in phase to that recorded at lower levels, for at metres the minimum humidity was recorded at night and the maximum during the day, while the opposite conditions prevailed on the hill. repeated kite-flights indicate that these are the normal conditions at the two levels. in plate xi., fig. , is plotted a curve from the hourly readings of the thermograph at the blue hill valley station (fifteen metres) during the week, and also a curve connecting temperatures recorded by the kite-meteorograph once or twice each day during the same week at a level of metres, obtained in the way described or computed from the adiabatic change. all the night records show that it was decidedly warmer at the height of metres during the night than it was at the ground, except during the cool wave on the seventh and eighth. furthermore, the curves in fig. indicate a control of the surface temperatures during the day by those above. for instance, on the seventh there was a distinct flattening of the day curve, evidently because, as the temperature on the ground rose ° above that at metres, the air was in unstable equilibrium, and colder air descended to take the place of the surface air so that its temperature could rise no higher. on the tenth, the temperature at metres was considerably greater than the mean of the day at the ground, and the air at the ground did not acquire the unstable condition in any volume until the warmest part of the day, so that the diurnal curve at the lower station forms a sharp peak. [illustration: plate xi.--kite observations at blue hill, sept. - , .] since there appears to be no appreciable diurnal period in the temperature at and above metres, a better comparison of the relative changes aloft and below during the passage of warm and cold waves is obtained by smoothing out the diurnal period below. this has been done in plate xi., fig. , with the data given by the kites at and metres plotted in curves, which it was necessary to complete by extrapolation. it is seen that there is a much greater range in the temperature from the crest of a warm to the crest of a cold wave at a height of metres than at the ground. at metres the range appears to be slightly greater than at metres, and the crests of the warm and cold waves occur successively earlier than they do at the ground. on the approach, and until the passage of the crest of the cold wave the air is colder aloft than at the ground, the difference being apparently that of the adiabatic cooling of ascending air. after the passage of the crest of the cold wave, the temperature aloft rises much more rapidly than at the ground, and at the crest of the warm wave the air at metres is some ° warmer than the mean daily temperature at the ground. in many kite-flights the difference was found to be even greater than this. taking the mean temperature of twenty-four-hours, it is seen that the average temperature at the ground during a week or more is about the same as it is at metres. fig. shows the change in the vertical distribution of temperature during the oncoming of the warm wave on the eighth and early morning of the ninth, as determined by four ascents, culminating at a.m., p.m., p.m., and a.m. the lines of °, °, °, and ° show that there was a gradual rise of temperature aloft, which extended downwards to metres, or to the top of blue hill. clouds formed at the level of lowest temperature, and these sank also until they covered the top of the hill. plate xii. is a facsimile of the meteorogram during the kite-flight of october , , the lower part showing the trace of the barometer on a scale of heights in metres, the middle section the trace of the hygrometer, and the upper one the trace of the thermometer on a scale of centigrade degrees. the temperature followed the normal change, which is as follows: during the day, up to a certain height, which varies under different conditions, there is a decrease nearly at the adiabatic rate of °· f. per hundred metres. above that height the air suddenly becomes warmer, and then cools with ascent at a rate somewhat less than the adiabatic rate. during the night there is a marked inversion of temperature between the ground and or metres. [illustration: plate xii.--automatic records during a high kite-flight at blue hill.] higher than this, the temperature decreases at a fairly uniform rate, but more slowly than the adiabatic rate. although no clouds were visible, yet the relative humidity increased greatly, both during the ascent and descent, near and metres, these being about the heights at which cumulus and alto-cumulus clouds usually form. during september four kite-flights were made on four successive days when an anti-cyclone and a cyclone passed nearly over blue hill. this is a rare occurrence, and the mechanism of these phenomena was accordingly studied by mr. clayton, some of whose deductions will now be given, illustrated by plate xiii. figs. and give the temperature plotted according to height on september in the anti-cyclone, and on september , when the barometric pressure was falling, the full lines, as in previous diagrams, indicating observations during the ascents, and the broken lines observations during the descents. it is seen that from the ground the lines all incline upward to the left, indicating a fall of temperature, to a certain height when the lines bend to the right sharply, showing a sudden rise of temperature. above this, the temperature again falls, but more slowly than at lower levels. the general prevalence of this phenomenon was noted by welsh in his balloon ascents in england in , and the high kite-flights at blue hill show it to be very frequent below metres. the plane of increased temperature usually determines the height of the tops of cumulus and strato-cumulus clouds. above metres other sudden rises of temperature are found during the highest kite-flights. [illustration: plate xiii.--results of kite-flights at blue hill during an anti-cyclone and a cyclone.] figs. to show the changes in the various elements during the four days at some of the following levels, viz. near sea-level, , , , and metres. fig. shows the changes in the barometer at the four levels, from which it is evident that the fall of pressure was greatest near sea-level. fig. shows temperature changes at the different levels, and indicates that the changes were of the same nature up to metres. the greatest non-diurnal range of temperature is seen to be at metres, and it diminishes both at higher and at lower levels. fig. shows changes in relative humidity at , , and metres. the curves show that the greatest range of humidity was at metres. there the relative humidity rose from almost zero, in the anti-cyclone on the twenty-first, to saturation at the same level in the cyclone. at metres the change is similar to that at , but is less in amount. at metres the relative humidity fell until the twenty-second, but then rose rapidly, showing the very dry air at metres on the twenty-first had descended as low as metres on the twenty-second. fig. gives the change in wind velocity at the different levels. there was an increase of wind at all the levels from the time of the passage of the anti-cyclone to the passage of the cyclone. the minimum of wind at metres was in the anti-cyclone, with a secondary minimum during the passage of the centre of the cyclone. figs. to show the changes in height from day to day of the equal conditions at the different levels. fig. shows the change in level of the isobars, which, although very small, is largest at the lower levels. the light broken lines in fig. and subsequent figures indicate the axes of the anti-cyclone and cyclone. that the axis of the cyclone was inclined backward, and that the high pressure occurred later at high than at low levels, was confirmed by the wind observations on the twenty-first. fig. shows the heights at which the same temperatures were found on successive days. since the isotherms rose until the twenty-third, the temperature of the air up to metres was higher on the day of the cyclone than on the day of the anti-cyclone. previous high flights indicate that this is the normal condition in the moving cyclones and anti-cyclones of the eastern united states. as the light broken lines represent the axes of the anti-cyclone and cyclone up to metres, it is seen that at this level the temperature at the place of maximum pressure is probably higher than at the place of minimum pressure, although this is not true for a vertical column of air above the earth. fig. gives the positions of equal humidities on successive days, saturated and cloudy areas being indicated by crossed shading, and less humidity by single ruling. from the laws of thermo-dynamics the unshaded curves should represent descending currents, and the shaded portions ascending ones. in the first case, increased warmth and a lower relative humidity are produced in the descent to a lower altitude; in the last case, cooling, increasing relative humidity, and condensation are produced by expansion in the ascent to a higher altitude. consequently, two regions of descending air are indicated, one in the centre of the anti-cyclone, the other in the centre of the cyclone. fig. shows the change in height of the lines of equal wind velocity. with ascending currents and precipitation, high wind velocities were found at low levels, because of increased barometric gradient, while with the descending currents in the anti-cyclone and centre of the cyclone, the high velocities were found only at great altitudes. the study of these data indicate that the cyclonic and anti-cyclonic circulations observed in this latitude do not embrace any air-movements at greater altitudes than metres, except in front of the cyclone, when the air appears to be carried upward to a great height. above metres there are probably other weak cyclones and anti-cyclones, or secondary ones, with their centres at different places from those at the earth's surface and producing a different circulation of wind. the observations of the cirrus clouds at blue hill indicate that at their level exists a cyclonic circulation above the anti-cyclone apparent at the earth's surface. the shallowness of our anti-cyclones would be inferred from the great differences in speed of the general atmospheric drift, for since the velocity of the general drift from the west is more than thirty times greater at , metres than it is at metres, a circulation of great depth could not endure long. cyclones and anti-cyclones appear to be but secondary phenomena in the great waves of warm and cold air which sweep across the united states from periodic causes. the origin of cyclones and anti-cyclones is perhaps the most important problem remaining for meteorological study. the theory that they are produced by differences of temperature in adjacent masses of air, or, as it is called, the convectional theory of the american meteorologists, espy and ferrel, is opposed by the observations on mountains in europe which were collected by dr. hann of vienna. if the question can be solved by the use of kites, as seems to be foreshadowed by the results just stated, another foundation-stone will be laid in the science of meteorology and the status of the kite established as an instrument of research. the kite fails when there is little or no wind at the ground, but it seems possible in such cases to lift the kite into the upper air, where there usually is wind, by attaching it to a small balloon that, after the kite can support itself, shall be detached automatically. while the height to which kites can rise is limited, and the limit is probably being approached, judging from the less gain of altitude in recent flights, yet it seems reasonable to expect that, with favourable conditions, a height of at least three miles will be reached. besides lifting the meteorological instruments described, kites can carry apparatus for other investigations in the free air, such as the measurement of atmospheric electricity, and the collection of samples of air, to be examined for cosmic dust and bacteria. cameras have been lifted by kites, as already said, and for the purpose of photographing the upper surfaces of clouds there is being constructed for the blue hill observatory a very light automatic camera, similar in principle to m. cailletet's apparatus for photographing the ground from a balloon. the use of the kite as an aeroplane can only be alluded to in this book, and it may be sufficient to say that if a motor attached to a kite can, by wings or screws, propel it against the wind, the sustaining string is unnecessary, and we shall have the flying machine which professor langley tells us will soon be realized. the surface of our globe has been tolerably well explored; the exploration of the atmosphere by balloons and kites will continue to make great progress during the last year of the century, and at the end of the twentieth century we may confidently expect that as the seas now are a medium for transportation, so the ocean of air will have been brought likewise into man's domain. index a abercromby (r.), classification of clouds, academy of sciences, french, , - academy of sciences, russian, balloon ascent, accademia del cimento, actinometer, viollé's, adiabatic rate of change of temperature, aeronautical conference at chicago, ---- conference at strassburg, , ---- committee; international, _aérophile_ balloons, , aerostatic commission, french, air, collection and analysis of, , , , , ---- weight of, aitken (j.), dust particles, alhazen (b. a.), height of atmosphere, altitudes, comparative, andrée (s. a.), balloon voyage to north pole, anti-cyclones, , aratus, _diosemeia_, archibald (d.), kites for meteorological observations, archytas, supposed inventor of kite, aristotle, , , assmann (r.), , , atmosphere, composition of, ---- energy of upper portion, ---- extent of, ---- methods of exploring same, _et seq._, ---- moisture of, ---- origin of, ---- phenomena showing height of, ---- pliny on, ---- temperature of, atmospheric circulation in cyclones and anti-cyclones, ---- electricity, , , , b balloon ascents, international, _et seq._ ---- crossing the atlantic by, ---- invention of hot-air, ---- kite, balloons, , , , , , _et seq._ ---- captive, , , ---- changes of temperature observed in, , , , , , , ---- changing the direction of, _ballons-sondes_, _et seq._ barometer, , , , baro-thermograph of richard, barral, balloon ascent, , batavia, java, international cloud measurements, batut (a.), photography from kites, berson (a.), balloon ascents, , _et seq._ bert (p.), respiration of oxygen, besançon (g.), , , , bezold (w. von), wave-cloud, biot (j. b.), balloon ascent, birt (w. r.), kite at kew observatory, bixio, balloon ascent, , blanc, mont, , blanchard, balloon ascent with jeffries, , blue hill observatory, , , , , , _et seq._ bonaparte (prince roland), patron of aeronautics, bonpland (a.), ascent in andes, bonvallet (l.), exploring balloons, bouguer (p.), height of freezing-point, boyle (r.), c cailletet (l.), , cambridge, mass., clouds measured at, castelli (b.), invented rain-gauge, cavallo (t.), showed lightness of hydrogen, celsius (a.), thermometer, charles (j. a. c.), ascent in hydrogen balloon, , cimento, accademia del, _cirrus_ balloon, clayton (h. h.), , , , , , , cloud, amount of, _et seq._ ---- atlases, , , ---- committee, international, , ---- -year, international, clouds, classification of, _et seq._ ---- definitions of, ---- formation of, ---- observations of direction and relative velocity, , ---- measurements of height and velocity, , _et seq._, , ---- on jupiter, ---- relation to forecasting, - cotte (l.), on clouds, coxwell (h.), aeronaut for glaisher, _et seq._ crocé-spinelli (j.), ascent in _zenith_, cyclones, , d dalton (j.), water-vapour in the air, daniell (j. f.), mountains a registering thermometer, davis (w. m.), cloud measurements, deluc (j. a.), theory of clouds, de saussure (h. b.), , , ---- (h. b.), ascent of mont blanc, deutsche-seewarte, hamburg, donaldson (w. h.), proposed crossing atlantic in a balloon, e eddy (w. a.), , _et seq._ eiffel tower, paris, , ekholm (n.), , electricity, atmospheric, , , , espy (j. p.), kites to verify calculated height of clouds, etna, ascended by ancients, euler (l.), theory of kites, exploring the atmosphere, methods of, _et seq._, - f fahrenheit (d. g.), thermometer, ferdinand ii. (grand duke), distributed meteorological instruments, fergusson (s. p.), , , , , , ferrel (w.), theory of cyclones, flammarion (c.), balloon ascents, flying machines, future, , fonvielle (w. de), , förster (w.), hypothesis of _himmelsluft_, forecasting by kites, , franklin (b.), experiment with kites, , franklin kite club, g galileo (g.), , gay-lussac (j. l.), balloon ascent, , german emperor (william ii.), patron of aeronautics, ---- society for promotion of aërial navigation, , , glaisher (j.), balloon ascents, _et seq._, green (c.), aeronaut for welsh, grimaldi (f. m.), first measured clouds trigonometrically, guericke (o. von), experiment of magdeburg hemispheres, h hagström (k.), measured clouds, halley (e.), measured heights by barometer, hann (j.), , han sin, employed kites in warfare, hargrave (l.), invented cellular kite, harrington (m. w.), advocated exploring air with kites, harvard college observatory, , hazen (h. a.), highest balloon ascent in america, height of balloon, cailletet's apparatus for obtaining, heights of kite-flights at blue hill, , ---- how measured by barometer, , heim (a.), voyage across the alps, hellmann (g.), historical researches, helmholtz (h. von), wave-cloud, hergesell (h.), president of aeronautical committee, hermite (g.), , , hildebrandsson (h. h.), , hodgkins' fund of smithsonian institution, grant from, howard (l.), cloud nomenclature, humboldt (a. von), , , humidity, changes with altitudes, , , , , , , ---- diurnal changes at different altitudes, ---- types of change with altitudes, _et seq._ hutton (j.), cause of precipitation, hygrometer, invention of, j jeffries (j.), first scientific balloon ascent, ---- first to cross the english channel, jourdanet (d.), hypothesis of descent of man, jovis, balloon ascent, jupiter, analogy between cloudiness on earth and on, k kepler (j.), height of atmosphere, kew observatory, , , köppen (w.), cloud atlas, kirwan (r.), temperature at different latitudes, kite, antiquity of the, ---- eddy or malay, , ---- flights at blue hill, , , , ---- hargrave, , ---- lamson's "aero-curve," ---- photography, , , ---- theory of, , , kites, first scientific use of, ---- first self-recording instruments raised by, ---- oriental tailless, ---- scientific uses of, kite-winch at blue hill, steam, , krakatoa, eruption of volcano, l lamarck (j. b.), first to classify cloud forms, lamson (c. h.), aero-curve kite, langley (s. p.), , laplace (p. s. de), , , lavoisier (a. l.), , ley (w. c.), classification of clouds, lunardi (v.), balloon ascent, m m'adie (a.), , magnetism, variation with height, , mallet (m.), balloon ascent, manila, philippine islands, cloud measurements at, - mariotte (e.), law of gases, melvill (t.), aided in first scientific use of kites, merle (w.), oldest weather chronicles, meteorograph for kites, , meteorological conferences, international, , , meteorology, first treated by aristotle, ---- origin of, misti, el, highest station, montgolfier brothers, invented hot-air balloon, müntz (a.), analysis of air, - n nares (sir g.), storm-kite, nebular hypothesis of laplace, neumayer (g.), cloud-atlas, newton (i.), improved kites, o olympus, mountain ascended by ancients, oxford, oldest weather chronicles, p parseval (a. von), kite-balloon, pascal (b.), experiment with barometer, , perier (f.), _idem_, photography from balloons, , ---- from kites, , , pickering (e. c.), pole-star recorder, pike's peak, meteorological station, pilâtre de rozier (j. f.), first to ascend in balloon, pliny, the atmosphere, pocock (g.), great kite, poëy (a.), classification of clouds, priestley (j.), oxygen in the air, r rain-gauge, invention of, réaumur (r. a. f. de), thermometer, rey (j.), first thermometer filled with liquid, riccioli (g. b.), first to measure clouds trigonometrically, richard (abbé), clouds, ---- (j.), self-recording instruments, , , , robertson (e. g.), , ---- balloon ascent, rotch (a. l.), balloon ascents, , s sacharoff, balloon ascent, siegsfeld (h. b. von), kite-balloon, sigsbee (c. d.), , sivel (t.), ascent in _zenith_, spelterini (e.), balloon voyage with heim, spencer (s.), aeronaut for berson, sweetland (a. e.), prognostics from clouds, symons (g. j.), meteorologist and bibliophile, t teisserenc de bort (l.), , , temperature, change with height, , , , , , , , , , , , , , , , ---- diurnal changes at different altitudes, , - ---- types of change with altitude, _et seq._ theodolite, registering, theophrastus, weather prognostics, thermometer, aspiration, , ---- metallic, ---- sling, thermometers, early, tissandier (a.), sketches of optical phenomena, ---- (g.), ascent in _zenith_, , toronto, canada, international cloud measurements, torricelli (e.), invented barometer, tycho brahe, height of atmosphere, u united states hydrographic office, ---- weather bureau, , , upsala, sweden, clouds measured at, , , v violle (j.), , w washington (mount), meteorological station, weather chronicles, first, ---- forecasting by means of clouds, ---- prognostics of aristotle and theophrastus, ---- vane, oldest meteorological instrument, weber (l.), measured electric potential with kites, welsh (j.), balloon ascents, wenz (e.), photography from kites, wilson (a.), first scientific use of kites, wind at different heights, _et seq._, , , , wire for kite-lines, , , wise (j.), , woglom (j. t.), photography from kites, y young (c. a.), limit of atmosphere, z _zenith_, catastrophe of balloon, zero, absolute, _richard clay & sons, limited, london & bungay._ transcriber's notes the text presented here is essentially that in the original printed volume. the list of corrections (corrigenda) which accompanied the original has been applied. one additional typo (see below) has also been made. there may have been some minor corrections (missing period, commas, etc. added) which are not detailed here. in the original publication, several figures and plates were placed in the middle of paragrahs. here most were moved between paragraphs. the list of illustrations and any other page references still indicate the page number of the original location. on page , the "oe" ligature was replaced with the individual letters. typographical correction page correction ===== ================================== all-assential => all-essential generously made available by the internet archive/american libraries.) a brief account of radio-activity by francis p. venable, ph.d., d.sc., ll.d. professor of chemistry, university of north carolina author of "a short history of chemistry," "periodic law," etc. d. c. heath & co., publishers boston new york chicago copyright, , by d. c. heath & co. ia preface i have gathered the material for this little book because i have found it a necessary filling out of the course for my class in general chemistry. such a course dealing with the composition and structure of matter is left unfinished and in the air, as it were, unless the marvellous facts and deductions from the study of radio-activity are presented and discussed. the usual page or two given in the present text-books are too condensed in their treatment to afford any intelligent grasp of the subject, so i have put in book form the lectures which i have hitherto felt forced to give. perhaps the book may prove useful also to busy men in other branches of science who wish to know something of radio-activity and have scant leisure in which to read the larger treatises. it is needless to say that there is nothing original in the book unless it be in part the grouping of facts and order of their treatment. i have made free use of the writings of rutherford, soddy, and j. j. thomson, and would here express my debt to them--just a part of that indebtedness which we all feel to these masters. i wish also to acknowledge my obligations to professor bertram b. boltwood for his helpful suggestions in connection with this work. contents chapter i discovery of radio-activity page the beginning--radio-active bodies--an atomic property--discovery of new radio-active bodies--discovery of polonium--discovery of radium--other radio-active bodies found chapter ii properties of the radiations ionization of gases--experimental confirmation--application of electric field--size and nature of ions--photographing the track of the ray--action of radiations on photographic plates--discharge of electrified bodies--scintillations on phosphorescent bodies--penetrating power--magnetic deflection--three types of rays--alpha rays--beta rays--gamma rays--measurement of radiations--identifications of the rays chapter iii changes in radio-active bodies radio-activity a permanent property--induced activity--discovery of uranium x--conclusions drawn--search for new radio-active bodies--methods of investigation--nature of the radiations--life-periods--equilibrium series chapter iv nature of the alpha particle disintegrating of the elements--identification of the rays--the alpha rays--alpha rays consist of solid particles--electrical charge--helium formed from alpha particles--discovery of helium--characteristics of helium--table of constants chapter v the structure of the atom properties of radium--energy evolved by radium--necessity for a disintegration theory--disintegration theory--constitution of the atom--rutherford's atom--scattering of alpha particles--stopping power of substances chapter vi radio-activity and chemical theory influence upon chemical theory--the periodic system--basis of the periodic system--influence of positive nucleus--determination of the atomic number--use of x-ray spectra--changes caused by ray-emission--atomic weight losses--lead the end product--changes of position in the periodic system--changes from loss of beta particles--isotopes--radio-activity in nature--radio-active products in the earth's crust--presence in air and soil waters--cosmical radio-activity index a brief account of radio-activity chapter i discovery of radio-activity the object of this brief treatise is to give a simple account of the development of our knowledge of radio-activity and its bearing on chemical and physical science. mathematical processes will be omitted, as it is sufficient to give the assured results from calculations which are likely to be beyond the training of the reader. experimental evidence will be given in detail wherever it is fundamental and necessary to a confident grasp of some of the marvelous deductions in this new branch of science. theories cannot be avoided, but the facts remain while theories grow old and are discarded for others more in accord with the facts. the beginning as so often happens in the history of science, the opening up of this new field with its fascinating disclosures was due to an investigation undertaken for another purpose but painstakingly carried out with a mind open to the truth wherever it might lead. in , röntgen modestly announced his discovery of the _x_ rays. this attracted immediate and intense interest. among those who undertook to follow up these phenomena was becquerel, who, because of the apparent connection with phosphorescence, tried the action of a number of phosphorescent substances upon the photographic plate, the most striking characteristic of the _x_ rays being their effect upon such sensitive plates. in these experiments he obtained no results until he tried salts of uranium, recalling previous observations of his as to their phosphorescence. distinct action was noted. furthermore, he proved that this had no connection with the phenomenon of phosphorescence, as both uranic and uranous salts were active and the latter show no phosphorescence. becquerel announced his discoveries in and this was the beginning of the new science of radio-activity. radio-active bodies the rays given off by uranium and its salts were found to differ from the _x_ rays. they showed no appreciable variation in intensity, no previous exposure of the substance to light was necessary, and neither changes of temperature nor any other physical or chemical agency affected them. at first uranium and its compounds were the only known source of these new radiations, but many other substances were examined and two years later thorium and its compounds were added to the list. in general the discharging action seemed about the same. other elements and ordinary substances show a minute activity. only potassium and rubidium have a greater activity than this, and theirs is only about one-thousandth that of uranium. an atomic property in the examination of uranium and thorium compounds it was found that the activity was determined by the uranium and thorium present; it was proportto the amount ofional these elements present and independent of the nature of the other elements composing the compound. the conclusion was, therefore, that the activity was an inherent property of the atoms of uranium and thorium, that is, an atomic property. this was a long step forward and introduced into science the conception of a new property of matter, or at least of certain forms of matter. discovery of new radio-active bodies in examining a large number of minerals containing uranium and thorium, mme. curie made the important observation that many of these were more active than the elements themselves. in measuring the activity she made use of the electrical method which will be described later. in the following table giving her results for uranium minerals the numbers under _i_ give the maximum current in amperes. they serve simply for comparison. _i_ pitchblende from joachimsthal . × ^{- } clevite . × ^{- } chalcolite . × ^{- } autunite . × ^{- } carnotite . × ^{- } uranium . × ^{- } uranium and potassium sulphate . × ^{- } uranium and copper phosphate . × ^{- } the last three are pure uranium and compounds of that element given for comparison with the first five, which are naturally occurring minerals. the last compound has the same composition as chalcolite and is simply the artificially prepared mineral. it has the activity which would be calculated from the proportion of uranium present, the copper and phosphoric acid contributing no activity. since the activity is not dependent upon the composition but upon the amount of uranium present, the activity in all of the minerals should be less than that of uranium. on the contrary, it is several times greater. natural and artificial chalcolite also show a marked difference in favor of the former. the supposition was a natural one, therefore, that these minerals contained small quantities of an element, or elements, undetected by ordinary analysis and having a much greater activity than uranium. similar results were obtained in the examination of thorium minerals and thorium salts. discovery of polonium following up this supposition, m. and mme. curie set themselves the task of separating this unknown substance. starting with pitchblende, a systematic chemical examination was made. this is an exceedingly complex mineral, containing many elements. the processes were laborious and demanded much time and minute care. they need not be described here. it is sufficient to say that along with bismuth a very active substance was separated, to which mme. curie gave the name of polonium for poland, her native land. its complete isolation is very difficult and sufficient quantities of the pure substance have not been obtained to determine its atomic weight and other properties, but some of the lines of its spectrum have been determined. chemically it is very closely analogous to bismuth. discovery of radium in a similar manner a barium precipitate was obtained from pitchblende which contained a highly active substance. the pure chloride of this body and barium can be prepared together and then separated by fractional crystallization. to the new body thus found the name of radium was given. it is similar in chemical properties to barium. its atomic weight has been determined by several careful investigators and is accepted as . its spectrum has been mapped and its general properties are known. it is a silvery white, oxidizable metal. in one ton of pitchblende about . gram of radium is present; this is about times greater than the amount of polonium present. the activity of the products was depended upon as the guide in these separations. the radium found is relatively enormously more active than the pitchblende or uranium. other radio-active bodies found in the above separations use was made of relationships to bismuth and barium. similarly, by taking advantage of chemical relationship to the iron group of elements, another body was partially separated by debierne, to which he gave the name actinium. boltwood discovered in uranium minerals the presence of a body which he named ionium, and which is so similar to thorium that it cannot be separated from it. it, however, far exceeds thorium in activity. the lead which is present in uranium and thorium minerals--apparently in fairly definite ratio to the amount of uranium and thorium--is found, on separation and purification, to possess radio-active properties. this activity is due to the presence of a very small proportion of an active constituent called radio-lead, which has chemical properties identical with those of ordinary lead. the bulk of the lead obtained from radio-active minerals differs in atomic weight from ordinary lead and appears also to be different according to whether its source is a thorium or a uranium mineral. a large number of other radio-active substances have been separated and some of their properties determined, but these were found by different means and will be noted in their proper place. they number in all more than thirty. the sources or parents of these are the original uranium or thorium, and the products form regular series with distinctive properties for each member. chapter ii properties of the radiations the activity of these radio-active bodies consists in the emission of certain radiations which may be separated into rays and studied through the phenomena which they cause. ionization of gases one of these phenomena is the power of forming ions or carriers of electricity by the passage of the rays through a gas, thus ionizing the gas. the details of an experiment will serve to make the meaning of this ionization clear. [illustration: fig. .--ionization of gases.] when this apparatus is set up a minute current will be observed without the introduction of any radio-active matter. this, as rutherford says, has been found due mainly to a slight natural radio-activity of the matter composing the plates. if radio-active matter is spread on plate _a_, which is connected with one pole of a grounded battery, and if plate _b_ is connected with an electrometer which is also connected with the earth, a current is caused which increases rapidly with the difference of potential between the plates, then more slowly until a value is reached that changes only slightly with a larger increase in the voltage. according to the theory of ionization, the radiation produces ions at a constant rate. the ions carrying a positive charge are attracted to plate _b_, while those negatively charged are attracted to plate _a_, thus causing a current. these ions will recombine and neutralize their charges if the opportunity is given. the number, therefore, increases to a point at which the ions produced balance the number recombining. when an electric field is produced between the plates, the velocity of the ions between the plates is increased in proportion to the strength of the electric field. in a weak field the ions travel so slowly that most of them recombine on the way and consequently the observed current is very small. on increasing the voltage the speed of the ions is increased, fewer recombine, the current increases, and, when the condition for recombination is practically removed, it will have a maximum value. this maximum current is called the saturation current and the value of the potential difference required to give this maximum current is called the saturation p.d. or saturation voltage. the picture, then, is this. the radiations separate the components of the gas into ions, or carriers of electricity, half of which are charged negatively and half positively. in the electric field those negatively charged seek the positive plate and those positively charged seek the negative plate. if time is given, these ions meet and recombine, their charges are neutralized, and there is no current. experimental confirmation this theory of the ionization of gases has been most interestingly confirmed by direct experiment. for instance, the ions may form nuclei for the condensation of water, and in this way the existence of the separate ions in the gas may be shown and the number present actually counted. when air saturated with water vapor is allowed to expand suddenly, the water present forms a mist of small globules. there are always small dust particles in air and around these as nuclei the drops are formed. these drops will settle and thus by repeated small expansions all dust nuclei may be removed and no mist or cloud will be formed by further expansions. if now the radiation from a radio-active body be introduced into the condensation vessel, a new cloud is produced in which the water drops are finer and more numerous according to the intensity of the rays. on passing a strong beam of light through the condensation chamber, the drops can readily be seen. these drops form on the ions produced by the radiation. application of electric field if the condensation chamber has two parallel plates for the application of an electric field like that already described, the ions will be carried at once to the electrodes and disappear. the rapidity of this action depends upon the strength of the electric field and experiment shows that the stronger the field the smaller the number of condensation drops formed. if there is no electric field, a cloud can be produced some time after the shutting off of the source of radiation, showing that time is required for the recombination of the ions. size and nature of ions if the drops are counted (there being special methods for this) and the total current carried accurately measured, then the charge carried by each ion may be calculated. this has been determined. the mass of an ion compared with the mass of the molecules of gas in which it was produced can also be approximately estimated. in the study of these ions the view has been held that the charged ion attracted to itself a cluster of molecules which surrounded the charged nucleus and traveled with it. it is roughly estimated that about thirty molecules of the gas cluster around each charged ion. photographing the track of the ray utilizing the fact that these ions with their clusters of molecules form nuclei for the condensation of water vapor, c. t. r. wilson has by instantaneous photography been able to photograph the track of an ionizing ray through air. the number of the ions produced, and hence the number of drops, is so great that the trail is shown as a continuous line. in the copy of this photograph it will be seen that at some distance from its source the straight trail is slightly but abruptly bent. near the end of its course there is another abrupt and much sharper bend. these bends show where the ionizing ray, in this case an alpha particle, has been deflected by more or less direct collision with an atom. these collisions and the final disappearance of the ray will be discussed later. [illustration: fig. .--photograph of the track of an ionizing ray.] action of radiations on photographic plates taking up now other means of examining these radiations, it is well to consider their action upon a photographic or sensitive plate. it will be recalled that this was the method by which their existence was originally detected. to illustrate the method, the following account of how one such photograph was taken may be given. the plate was wrapped in two thicknesses of black paper. the objects were placed upon this and the radio-active ore, separated by a board one inch thick, was placed above. the exposure lasted five days. the action is much less rapid and the result not so clearly defined as in the case of photographs taken by _x_ rays. of course, the removal of the board and the use of more concentrated preparations of radium would give quicker and better results. the method, however, on account of time consumed and lack of definition is ill adapted to accurate work. [illustration: fig. --photograph of various objects taken by means of pitchblende] discharge of electrified bodies the radiations from radio-active bodies can discharge both positively and negatively electrified bodies by making the air surrounding them a conductor of electricity. to demonstrate this, use is made of an electroscope. if the hinged leaf of such an instrument be electrically charged and a radio-active body be brought into its neighborhood, the electricity will be discharged and the leaf return to its original position. the rapidity of this discharge is used to measure the degree of activity of the body giving off the radiation. [illustration: fig. .--gold-leaf electroscope. the gold-leaf _l_ is attached to a flat rod _r_ and is insulated inside the vessel by a piece of amber _s_ supported from the rod _p_. the system is charged by a bent rod _cc'_ passing through an ebonite stopper. after charging, it is removed from contact with the gold-leaf system. the rods _p_ and _c_ and the cylinder are then connected with the earth.] scintillations on phosphorescent bodies it was found by crookes that a screen covered with phosphorescent zinc sulphide was brightly lighted up when exposed to the radiations. this is due to the bombardment of the zinc sulphide by a type of ray called the alpha ray. under a magnifying glass this light is seen to be made up of a number of scintillating points of light and is not continuous, each scintillation being of very short duration. by proper subdivision of the field under the lens, the number of scintillations can be counted with close accuracy. a simple form of apparatus called the spinthariscope has been devised to show these scintillations. a zinc sulphide screen is fixed in one end of a small tube and a plate carrying a trace of radium is placed very close to it. the scintillations can be observed through an adjustable lens at the other end of the tube. outer light should be cut off, as in a dark room. the screen then appears to be covered with brilliant flashes of light. other phosphorescent substances, such as barium platino-cyanide, may be substituted for the zinc sulphide, but they do not answer so well. penetrating power by penetrating power is meant the power exhibited by the rays of passing through solids of different thicknesses and gases of various depths. this power varies with different radiations and with the nature of the solid or gas. for instance, a sheet of metallic foil may be used and the effect of aluminum will differ from that of gold and the different rays vary in penetrating power. in the case of gases air will differ from hydrogen, and it is noticed that certain rays disappear after penetrating a short distance, while others can penetrate further before being lost. magnetic deflection if the radiations are subjected to the action of a strong magnetic field, it is found that part of them are much deflected in the magnetic field and describe circular orbits, part are only slightly deflected and in the opposite direction from the first, and the remaining rays are entirely unaffected. [illustration: fig. .--showing magnetic deflection of [alpha], [beta], and [gamma] rays.] three types of rays by the use of these methods of investigation it is learned that the radiations consist of three types of rays. these have been named the alpha, beta, and gamma rays, respectively. some radio-active bodies emit all three types, some two, and some only one. the distinguishing characteristic of these types of rays may be summed up as follows: alpha rays the alpha rays have a positive electrical charge and a comparatively low penetrating power. they are slightly deflected in strong magnetic and electric fields. they have a great ionizing power and a velocity about one-fifteenth that of light. beta rays the beta rays are negatively charged and have a greater penetrating power than the alpha rays. they show a strong deflection in magnetic and electric fields, have less ionizing power than the alpha rays, and a velocity of the same order as light. gamma rays the gamma rays are very penetrating and are not deflected in the magnetic or electric fields. they have the least ionizing power and a very great velocity. the penetrating power of each type is complex and varies with the source, so the statements given are but generalizations. the alpha rays are projected particles which lose energy in penetrating matter. as to the power of ionizing gases, if that for the [alpha] rays is taken as , , then the [beta] rays would be approximately and the [gamma] rays . measurement of radiations the rays are examined and measured in several ways: . by their action on the sensitive photographic plates. the use of this method is laborious, consumes time, and for comparative measurements of intensity is uncertain as to effect. . by electrical methods, using electroscopes, quadrant electrometers, etc. these are the methods most used. . by exposure to magnetic and electric fields, noting extent and direction of deflection. . by their relative absorption by solids and gases. . by the scintillations on a zinc sulphide screen. identification of the rays the alpha rays have been identified as similar to the so-called canal rays. these were first observed in the study of the _x_ rays. when an electrical discharge is passed through a vacuum tube with a cathode having holes in it, luminous streams pass through the holes toward the side away from the anode and the general direction of the stream. they travel in straight lines and render certain substances phosphorescent. these rays are slightly deflected by a magnetic field and in an opposite direction from that taken by the cathode rays in their deflection. the rays seem to be positive ions with masses never less than that of the hydrogen atom. their source is uncertain, but they may be derived from the electrodes. the beta rays are identical in type with the cathode rays and are negative electrons. the gamma rays are analogous to the _x_ rays and are of the order of light. they are in general considerably more penetrating than _x_ rays. for example, the gamma rays sent out by milligrams of radium can be detected by an electroscope after passing through centimeters of iron, a much greater thickness than can be penetrated by the ordinary _x_ rays. chapter iii changes in radio-active bodies is radio-activity a permanent property? is this power of emitting radiations a permanent property or is it lost with the passage of time? the first investigations of the activity of uranium and thorium showed no loss of intensity at the end of several years, and radium also seemed to show no decrease in its enormous activity. polonium, however, was found to lose most of its activity in a year, and later it appeared that some radio-active substances lost most of their activity in the course of a few minutes or hours. induced activity a phenomenon called induced or secondary radio-activity was also observed. thus a metal plate or wire exposed to the action of thorium oxide for some hours became itself active. this induced activity was not permanent but decreased to half its value in about eleven hours and practically disappeared within a week. similar phenomena were observed when radium was substituted for thorium. discovery of uranium x in crookes precipitated a solution of an active uranium salt with ammonium carbonate. the precipitate was dissolved so far as possible in an excess of the reagent, leaving an insoluble residue. this residue was many hundred times more active, weight for weight, than the original salt, and the solution containing the salt was practically inactive. at the end of a year the uranium salt had regained its activity while the residue had become inactive. another method of obtaining the same result is to dissolve crystallized uranium nitrate in ether. two layers of solution are formed, one ether and the other water coming from the water of crystallization. the aqueous layer is active, while the water layer is inactive. similarly, by adding barium chloride solution to a solution of a salt of uranium and then precipitating the barium as sulphate, the activity is transferred to this precipitate. these experiments give proof of the formation and separation of a radio-active body by ordinary chemical operations. so, too, in the case of thorium salts a substance can be obtained by means of ammonium hydroxide which is several thousand times more active than an equal weight of the original salt. after standing a month, the separated material has lost its activity and the thorium salt has regained it. here, again, there is the formation, separation, and loss of a radio-active body. conclusions drawn now, these are ordinary chemical processes for the separation of distinct chemical individuals. the results, therefore, lead naturally to the conclusions: ( ) it would seem that uranium and thorium are themselves inactive and the activity is due to some other substance formed by these elements; ( ) this active substance is produced by some transformation in those elements, for on standing the activity is regained. this latter conclusion is startling, for it indicates a change in the atom which, up to the time of this discovery, was deemed unchangeable under the influence of such physical and chemical changes as were known to us. search for new radio-active bodies the search for new radio-active bodies and the study of their characteristics has been systematically and successfully carried on. the bodies obtained in the above experiments were named uranium _x_ and thorium _x_, respectively. further, it became clear from the investigation of uranium minerals that radium, polonium, actinium, and ionium originated from uranium. from thorium minerals a body was separated called mesothorium, which was analogous to radium. both thorium and radium were found to give off a radio-active gas. the first lost half of its activity in less than one minute. the second was more stable and lost half of its activity in about four days. the name radium emanation was given to the latter and it was found chemically and physically to belong to the class of monatomic or noble gases, such as helium, argon, neon, etc., which had been discovered by ramsay. in some cases the chemical action was determined and these new bodies were found analogous to well-known elements, as radium to barium, polonium to bismuth. the physical properties were investigated and, where possible, spectra were mapped and atomic weights determined. it is clear, therefore, that these bodies are elemental in character and as such are made up of distinct, similar atoms, just as the commonly recognized elements are believed to be. in this way more than thirty new elements have been added to the list. these new elements are called radio-active elements, but it is an open question whether all atoms do not possess this property in greater or less degree. certainly, it is possessed in varying degree by four of the old elements widely separated in the periodic system, namely, uranium, thorium, rubidium, and potassium. the last two, while feebly active themselves, do not form any secondary radio-active substance so far as is known. only two of the elements, then, can definitely be said to go through these transformations. it is just possible that radio-activity may be found to be a common property of all atoms and of all matter. methods of investigation it is important to know how these new bodies were discovered and distinguished from one another. two properties are relied upon. one is the nature of the rays emitted and the other is the duration of the activity. of course, knowledge of the physical and chemical properties is also of great importance whenever obtainable. nature of the radiations the nature of the radiation is a distinguishing characteristic, though similarity here does not prove identity of substances. some emit [alpha] rays only, some emit [beta] rays, some emit two of the possible rays, as for instance, [beta] and [gamma], and some emit all three. the rays may also differ in the velocity with which they are emitted by different radio-active substances. thus, in the case of one substance the [alpha] rays may have a slightly greater or less penetrating power than those emitted by some other substance, and this may be true also of the other rays. life periods the duration of the activity is called the life period. this is absolutely fixed for each body and furnishes the most important mode of differentiating among them. it measures the relative stability and is the time which must elapse before their activity is lost and they, changing into something else, entirely disappear. the measure usually adopted is the half-value period. two hypotheses are made use of: . that there is a constant production of fresh radio-active matter by the radio-active body. . that the activity of the matter so formed decreases according to an exponential law with the time from the moment of its formation. these hypotheses agree with the experimental results. the decrease and rise of activity, for example, of uranium and uranium _x_, and also of thorium and thorium _x_, have been measured, plotted, and the equations worked out. manifestly, a state of equilibrium will be reached when the rate of loss of activity of the matter already produced is balanced by the activity of the new matter produced. this equilibrium and the knowledge of the rate of decrease in general will have little value if this rate, like chemical changes, is subject to the influence of chemical and physical conditions. the rate of decrease has been found to be unaltered by any known chemical or physical agency. for instance, neither the highest temperatures applicable nor the cold of liquid air have any appreciable effect. equilibrium series in order to measure the disintegration of a radio-active body in units of time so that the rate may be comparable with that of other radio-active bodies, the relation between the amounts under consideration must be a definite one. for this purpose equal weights of the bodies are not taken, but use is made of the amounts which are in equilibrium with a fixed amount of the parent substance. one gram of radium has been settled upon as the standard for that series and a unit known as the "curie" has been adopted to express the equilibrium quantity of radium emanation. thus, a curie of radium emanation (or niton) is the weight (or, as this is a gas, the volume at standard pressure and temperature) of the emanation in equilibrium with one gram of radium. this, by calculation and experiment, is found to be . cubic millimeter. when this amount has been produced by one gram of radium, the formation and decay will exactly balance one another. this is, therefore, one curie of emanation. the measurement of the rate of decay is difficult but can be carried out with great accuracy, even down to seconds, in the case of certain short-lived bodies. errors crept in at first from the failure to completely separate the substances produced in the series, and sometimes because of the simultaneous production of two substances. as stated, the decay follows an exponential law. the time required for the decay of activity to half-value does not mean, therefore, that there will be total decay in twice that time. thus the half-value period for uranium _x_ is about days. the period for complete decay is about days. this half-value period corresponds to the half-value recovery period of uranium, which is also days. these were the earlier figures obtained for uranium _x_ and they illustrate some of the difficulties surrounding such determinations. it was found later that the body examined as uranium _x_ was really a constant mixture and of course the decay and recovery periods were also composite. it required later and very skilful work to separate them into the bodies indicated in the disintegration series. the half-value period for thorium _x_ is much shorter, namely, a little over four days, and this is also the recovery period for thorium _x_. the plotted decay and recovery curves will intersect at this point. the consecutive disintegration series, with the half-value periods, for the uranium and thorium series as given by soddy are seen in the following tables. they are probably subject to some changes on further and more accurate determination. the nature of the rays emitted is also given. [illustration: uranium ( × ^ years) . -> [alpha] -> [alpha] \/ uranium x ( . days) ( . ) -> [beta]&[gamma] -> ([beta]) \/ \/ \/ ionium ( × ^ to ^ years) ( . ) -> [alpha] \/ radium ( , years) . -> [alpha] \/ emanation ( . days) ( . ) -> [alpha] \/ radium a ( . minutes) ( . ) -> [alpha] \/ radium b ( . minutes) ( . ) -> ([beta]) \/ radium c_{ } ( . minutes) { ( . ) -> [alpha] { -> [beta]&[gamma] { \/ radium c_{ } ( . minutes) { ( . ) -> [beta]&[gamma] \/ radium d ( years?) ( . ) -> ([beta]) \/ radium e ( . days) ( . ) -> [beta]&[gamma] \/ radium f (polonium days) ( . ) -> [alpha] \/ radium g (probably lead) ( . ) actinium (?) \/ radio-actinium ( . days) -> [alpha] -> ([beta]) \/ actinium x ( days) -> [alpha] \/ emanation ( . seconds) -> [alpha] \/ actinium a ( . second) -> [alpha] \/ actinium b ( . minutes) -> ([beta]) \/ actinium c_{ } ( . mins.) { -> [alpha] { \/ actinium c_{ } (?) { -> [alpha] \/ actinium d ( . minutes) -> [beta]&[gamma] \/ actinium e (unknown) thorium ( × ^{ } years?) . -> [alpha](?) \/ mesothorium_{ } ( . years) \/ mesothorium_{ } ( . hours) -> [beta]&[gamma] \/ radiothorium ( . years?) -> [alpha] \/ thorium x ( . days) -> [alpha] \/ emanation ( seconds) -> [alpha] \/ thorium a ( . second) -> [alpha] \/ thorium b ( . hours) -> ([beta]) \/ thorium c_{ } ( minutes) { -> [alpha] { \/ thorium c_{ } (?) { -> [alpha] \/ thorium d ( . minutes) -> [beta]&[gamma] \/ thorium e (unknown) fig. .--disintegration series for uranium, actinium, and thorium, as given by soddy.] chapter iv nature of the alpha particle disintegration of the elements the remarkable disintegrations related in the last chapter, in which the heaviest known elementary atom--that of uranium (at. wt. )--is by successive stages changed into others of lower atomic weight, afford a clue to the nature of the atom and to that goal of the chemist, the final constitution of matter. the composite nature of the atom and some sort of interrelation of the elements had previously been made apparent from a study of the periodic system and data gathered still earlier, but all attempts at working out a so-called genesis of the elements had proved vague and unsatisfactory. identification of the rays to get an understanding of the disintegration occurring in radio-active substances, the nature of the rays produced must be known. these rays are the cause of the activity and their emission accompanies the changes or disintegration. they have for the sake of convenience been called the alpha, beta, and gamma rays. the gamma rays have been identified with the _x_ rays discovered by röntgen and are a form of energy analogous to light. the beta rays are particles of negative electricity or electrons. with these, then, we have some degree of familiarity. but what are the alpha rays? an answer to this question should make clearer the character of the changes taking place, and should give some insight into the composition and mechanism of the atom. the alpha rays it has already been stated that these alpha rays are similar or analogous to the canal rays, but this advances the matter very little, as the nature of these canal rays has not been fully determined. the full identity with them, if proved, should have an important theoretical bearing. alpha rays consist of solid particles in the first place, these alpha rays have been found to be made up of solid particles, that is, of what we are accustomed to call matter. since it has become more and more difficult to draw a clear distinction between matter and energy, it would perhaps be better to say that these particles appear to have some of the properties hitherto attributed solely to matter. the best evidence that these particles are of atomic mass is furnished by their deflection in electric and magnetic fields. electrical charge it is not of first importance to discuss this or other proofs of the material nature of these particles. that they carry a charge of positive electricity is, however, a fact of very great import. the value of this charge has been carefully determined by a number of investigators working with different sources of the alpha particles and has been found to be . × ^{- } electrostatic units (. , , , e.s.). from the consideration of the charge upon an electron previously obtained by j. j. thomson and others, it was concluded that the alpha particle carried two unit positive charges; the fundamental unit charge, therefore, is half this value, or . × ^{- } e.s. helium formed from alpha particles to determine the nature of the alpha particle a crucial experiment was carried out by rutherford and royds, which was described as follows: [illustration: fig. .--apparatus used in experiment by rutherford and royds.] a large quantity of radium emanation was compressed into a fine glass tube _a_, about . cm. long. this tube, which was sealed to a larger capillary tube _b_, was sufficiently thin to allow the alpha particles from the emanation and its products to pass through, but sufficiently thick to withstand atmospheric pressure. the thickness of the glass wall was in most cases less than . mm. on introducing the emanation into the tube, the escape of the alpha particles from the emanation was clearly seen by the scintillations produced at some distance on a zinc sulphide screen. after this test the glass tube _a_ was surrounded by a glass tube _t_ and a small spectrum tube _v_ attached to it. the tube _t_ was exhausted to a charcoal vacuum. by means of the mercury column _h_, the gases in the tube _t_ could at any time be compressed into the spectrum tube _v_ and the nature of the gases which had been produced determined spectroscopically. it was found that two days after the introduction of the emanation into _a_ the spectrum showed the yellow line of helium, and after six days the whole helium spectrum was observed. in order to be certain that the helium, coming possibly from some other source, had not diffused through the thin walls of the tube _a_, the emanation was pumped out and helium substituted. no trace of helium could be observed in the vacuum tube after several days, showing that the helium observed in the first experiment must have originated from the alpha particles which had been propelled through the thin glass tube into the outer tube. most of the alpha particles are propelled with such force that they penetrate some distance into the walls of the outer tube and some of these gradually diffuse out into the exhausted space. the presence of helium in the spectrum tube can be detected after a shorter interval if a thin cylinder of lead is placed over the emanation tube, since the particles fired into the lead diffuse out more rapidly than from glass. a still more definite proof of the identity of the alpha particle with the helium atom was obtained by removing the outer glass tube _t_ and placing a cylinder of lead over the emanation tube in the open air. helium was always detected in the lead after it had remained several hours over the thin tube containing a large quantity of the emanation. in order to test for the presence of helium in the lead, the gases present were released by melting the lead in a closed vessel. there can thus be no doubt that the alpha particle becomes a helium atom when its positive charge is neutralized. thus the chemist was afforded the experience of the building up of at least one element under his observation, and both the analysis and synthesis of matter have been revealed through the discoveries of radio-activity. discovery of helium it is of interest at this point to learn something of the history of helium and its occurrence. in there was discovered by janssen and lockyer a bright yellow line in the spectrum of the sun's chromosphere. because of its origin the name helium was given to the supposed new element causing it. later it was found in the spectra of many of the stars, and because of its predominance in some of these they were called helium stars. its existence on our planet was not detected for nearly thirty years. in , in connection with the discovery of argon in the atmosphere, a search was made to see if the latter element could be obtained from mineral sources. in analyzing certain uranium minerals hillebrand had found considerable quantities of a gas which he took to be a peculiar form of nitrogen. ramsay made a further examination of the gas coming from these minerals and the spectroscope revealed the yellow line of helium, thus at last proving the presence of this element on the earth. it is known now to be present in thorium minerals, in the waters of radio-active wells, and in minute amounts in the atmosphere. its occurrence in every case, in the light of the experiment described above, would seem to be due to the presence of radio-active changes. characteristics of helium helium, on account of its chemical inactivity and physical properties, is classed along with argon, neon, krypton, and xenon in the zero group of the periodic system, and forms with them the monatomic, inert gases. in this class are now placed also the three radio-active gases, emanating respectively from radium, thorium, and actinium. these are generally known as radium emanation, thorium emanation, and actinium emanation. the first mentioned was once called niton. emanium was the name originally proposed by giesel for the body now known as actinium. the calculated rate of production of helium in the series in equilibrium with one gram of radium is cubic millimeters per year. this corresponds quite well with the experimental results. table of constants some of the more important atomic and radio-active constants are given in the following table. they are recorded here to show how helpful the study of radio-activity has been in working out the composition of matter, and to give some idea of the magnitude of the numbers and the minuteness of the quantities dealt with. electric charge carried by each h atom in electrolysis . × ^{- } e.s.[ ] electric charge carried by each [alpha] particle . × ^{- } e.s. number of atoms in gram of h . × ^{ } mass of atom of h . × ^{- } gram number of molecules per cc. of any gas at standard pressure and temperature . × ^{ } number of [alpha] particles expelled per second per gram of radium itself . × ^{ } number of [alpha] particles expelled per second per gram of radium in equilibrium with its products . × ^{ } [ ] the expression ^{- } means multiplying by . , , , ; ^{ } means multiplying by , , , . chapter v the structure of the atom properties of radium a study of the properties of radium will aid in throwing light upon the question as to the building up of the atom. first to be considered are the usual properties which distinguish an elementary body. metallic radium has been prepared by a method similar to that used in the preparation of barium. it is a pure white metal, melting at °, and far more volatile than barium. it rapidly alters on exposure to the air, probably forming a nitride. it energetically decomposes water and the product dissolves in the water. its atomic weight is . radium forms a series of salts analogous in appearance and chemical action to those of barium. in the course of time they become colored, especially if mixed barium salts. the radiations from radium produce marked chemical effects in a number of substances. carbon dioxide is changed into carbon, oxygen, and carbon monoxide, and the latter is changed into carbon and oxygen. ammonia is dissociated into nitrogen and hydrogen; hydrochloric acid into chlorine and hydrogen. oxygen is condensed into ozone. in general, the action upon gases appears to be similar to that of the silent electric discharge. water is decomposed into hydrogen and oxygen. if moist radium chloride or a salt of radium containing water of crystallization is sealed in a glass tube, the gradual accumulation of hydrogen and oxygen will burst the tube. the radiations rapidly decompose organic matter with the evolution of gases. thus grease from stopcocks of apparatus used with radium or paraffin will give off carbon dioxide. under an intense alpha radiation paraffin or vaseline become hard and infusible. white phosphorus is changed into red. the action upon living tissue is most noteworthy, as its possible use as a remedial agent is dependent upon this. a small amount of a radium salt enclosed in a glass tube will cause a serious burn on flesh exposed to it. it therefore has to be handled with care and undue exposure to the radiations must be avoided. cancer sacs shrivel up and practically disappear under its action. whether the destruction of whatever causes the cancer is complete is at least open to serious doubt. the coagulating effect upon globulin is interesting. when two solutions of globulin from ox serum are taken and acetic acid added to one while ammonia is added to the other, the opalescence in drops of the former is rapidly diminished on exposure to radium, showing a more complete solution, whereas the latter solution rapidly turns to a jelly and becomes opaque, indicating a greatly decreased solubility. energy evolved by radium the greater part of the tremendous energy evolved by radium is due to the emission of the alpha particles, and in comparison the beta and gamma rays together supply only a small fraction. this energy may be measured as heat. it was first observed that a radium compound maintained a temperature several degrees higher than that of the air around it. the rate of heat production was later measured by means of an ice calorimeter and also by noting the strength of the current required to raise a comparison tube of barium salt to the same temperature. both methods showed that the heat produced was at the rate of about gram calories per hour. as the emission is continuous, one gram of radium would therefore emit about , , gram calories in the course of a year. at the end of years it would still emit , gram calories per year. such a production of energy so far surpasses all experience that it becomes almost inconceivable. it is futile to speak of it in terms of the heat evolved by the combustion of hydrogen, which is the greatest that can be produced by chemical means. this effect is unaltered at low temperatures, as has been tested by immersing a tube containing radium in liquid air. it should be stated that these measurements were made after the radium had reached an equilibrium with its products; that is, after waiting at least a month after its preparation. the evolution of heat from radium and the radio-active substances is, in a sense, a secondary effect, as it measures the radiant energy transformed into heat energy by the active matter itself and whatever surrounds it. let us repeat, therefore, that the total amount of energy pent up in a single atom of radium almost passes our powers of conception. necessity for a disintegration theory the facts gathered so far justify and necessitate a theory which shall satisfactorily explain them, and since these phenomena are not caused by nor subject to the influence of external agencies, they must refer to changes taking place within the atom--in other words, a theory of disintegration. in the main, these facts may be summed up as the emission of certain radiations from known elemental matter: the material alpha particles with positive charge, the beta particles or negative electrons, and the gamma rays analogous to _x_ rays. the emission of these rays results in the production of great heat. then there is the law of transformations by which whole series of new elements are generated from the original element and maintain a constant equilibrium of growth and decay in the series. lastly, we have the production of helium from the alpha particles. disintegration theory in explanation of these phenomena, rutherford offered the hypothesis that the atoms of certain elements were unstable and subject to disintegration. the only elements definitely known to come under this description are the two having atoms of the greatest known mass, thorium ( ) and uranium ( ). the atoms of uranium, for instance, are supposed to be not permanent but unstable systems. according to the hypothesis, about atom in every ^{ } becomes unstable each second and breaks up with a violent explosion for so small a mass of matter. one, or possibly two alpha particles are expelled with great velocity. this alpha particle corresponds to an atom of helium with an atomic weight of , and its loss reduces the original atomic weight to with the formation of a new element, having changed properties corresponding to the new atomic weight. this new element is uranium x_{ }. these new atoms are far more unstable than those of uranium, and the decomposition proceeds at a new rate of in ^{ } per second. so at a definite, measurable rate this stepwise disintegration proceeds. the explosions are not in all cases equally violent in going from element to element, nor are the results the same. sometimes alpha particles alone are expelled, sometimes beta, or two of them together, as alpha and beta. the new product may remain with the unchanged part of the original matter. thus there would be an accumulation of it until its own decay balances its production, resulting eventually in a state of equilibrium. constitution of the atom in order to explain the electrical and optical properties of matter, the hypothesis was made that the atom consisted of positively and negatively electrified particles. later it was shown that negative electrons exist in all kinds of matter. various attempts were made to work out a model of such an atom in which these particles were held in equilibrium by electrical forces. the atom of lord kelvin consisted of a uniform sphere of positive electrification throughout which a number of negative electrons were distributed, and j. j. thomson has determined the properties of this type as to the number of particles, their arrangement and stability. rutherford's atom according to rutherford, the atom of uranium may be looked upon as consisting of a central charge of positive electricity surrounded by a number of concentric rings of negative electrons in rapid motion. the positively charged centre is made up of a complicated system in movement, consisting in part of charged helium and hydrogen atoms, and practically the whole charge and mass of the atom is concentrated at the centre. the central system of the atom is from some unknown cause unstable, and one of the helium atoms escapes from the central mass as an alpha particle. there are, confessedly, difficulties connected with this conception of the atom which need not, however, be discussed here. much remains to be learned as to the mechanics of the atom, and the hypothesis outlined above will probably have to be materially altered as knowledge grows. perhaps it may have to be entirely abandoned in favor of some more satisfactory solution. until such time it at least suffices as a mental picture around which the known facts group themselves. in this picture energy and matter lose their old-time distinctness of definition. discrete subdivisions of energy are recognized which may be called charged particles without losing their significance. some of these subdivisions charged in a certain way or with neutralized charge exhibit the properties of so-called matter. scattering of alpha particles this conception of the atom would doubtless fail of much support were it not for certain experimental facts which lend great weight to it. certain suppositions can be based on this theory mathematically reasoned out and tested by experiment. predictions thus based on mathematical reasoning and afterward confirmed by experiment give a very convincing impression that truth lies at the bottom. the first of these experimental proofs comes under the head of what is known as the scattering of the alpha particles, a phenomenon which, when first observed, proved hard to explain. if an alpha particle in its escape from the parent atom should come within the influence of the supposed outer electrical field of some other atom, it should be deflected from its course and, the intensity of the two charges being known, the angle of deflection could be calculated. for instance, if it came to what might be called a head-on collision with the positive central nucleus of another atom, it would recoil if it were itself of lesser mass, or would propel the other forward if that were the lighter. the experiment is carried out by placing a thin metal foil over a radio-active body, as radium _c_, which expels alpha particles with a high velocity, and counting the number of alpha particles which are scattered through an angle greater than ° and so recoil toward their source. this has been done by a number of investigators and it has been found that the angle of scattering and the number of recoil particles depend upon the atomic weight of the metal used as foil. for example, if gold is used, the number of recoil atoms is one in something less than , . taking the atomic weight of gold into consideration, rutherford calculated mathematically that this was about the number which should be driven backward. but he went further and calculated also the number which should be returned by aluminum, which has an atomic weight of only about one-seventh that of gold. two investigators determined experimentally the number for aluminum and their results agreed with rutherford's calculations. the metals from aluminum to gold have been examined in this way. the number of recoil particles increases with the atomic weight of the metal. comparing experiment with theory, the central charge in an atom corresponds to about one-half the atomic weight multiplied by the charge on an electron, or, as it is expressed, / ae. there is only one lighter atom than helium, namely, hydrogen, which has a mass only one-fourth as great. when alpha particles are discharged into hydrogen, a few of the latter atoms are found to be propelled to a distance four times as great as that reached by the alpha particles. stopping power of substances parallel with the experiments mentioned, there is what is called the stopping power of substances. this means the depth or thickness of a substance necessary to put a stop to the course of the alpha particles. this gives the range of the alpha particles in such substances and is connected in a simple way with the atomic weight, that is, it is again fixed by the mass of the opposing atom. this stopping power of an atom for an alpha particle is approximately proportional to the square root of its atomic weight. considering gases, for instance, if the range in hydrogen be , then the range in oxygen, the atomic weight of which is , is only ( / )^{ / } or / . generally in the case of metals the weight of matter per unit area required to stop the alpha particle is found to vary according to the square root of the atomic weight of the metal taken. chapter vi radio-activity and chemical theory influence upon chemical theory it can easily be seen that the revelations of radio-activity must have a far-reaching effect upon chemical theory, throwing light upon, and so bringing nearer, the solution of some of the problems which have been long discussed without arriving at any satisfactory solution. the so-called electro-chemical nature of the elements will certainly be made much clearer. the changes in valence should become intelligible and valence itself should be explained. a fuller understanding of the ionization of electrolytes also becomes possible. as these matters are debatable and the details are still unsettled, it is scarcely appropriate to give here the hypotheses in detail or to enter into any discussion of them. but the promise of solution in accord with the facts is encouraging. the periodic system such progress has been made, however, in regard to a better understanding of the periodic system that the new facts and their interpretation may well be given. no reliable clue to the meaning of this system and the true relationship between the elements had been found up to the time when new light was thrown upon it by the discoveries of radio-activity. the underlying principle was unknown and even the statement of what was sometimes erroneously called the periodic law was manifestly incorrect and its terms were ignored. basis of the periodic system the ordinary statement of the fundamental principle of the periodic system has been that the properties of the elements were periodic functions of the atomic weights, and that when the elements were arranged in the order of their atomic weights they fell into a natural series, taking their places in the proper related groups. in accepting this, the interpretation of function was both unmathematical and vague, and the order of the atomic weights was not strictly adhered to but unhesitatingly abandoned to force the group relationship. wherever consideration of the atomic weight would have placed an element out of the grouping with other elements to which it was clearly related in physical and chemical properties, the guidance of these properties was accepted and that of the atomic weights disregarded. such shiftings are noted in the cases of tellurium and iodine; cobalt and nickel; argon and potassium. it was most helpful that, following the order of atomic weights, the majority of the elements fell naturally into their places. otherwise the generalization known as the periodic system might have remained for a long time undiscovered and the progress of chemistry would have been greatly retarded. influence of positive nucleus it is evident that the order of the elements is determined by something else than their atomic weights. from the known facts of radio-activity it would seem that this determining factor is the positive nucleus. and this nucleus also determines the mass or weight of the atom. taking the elements in their order in the periodic series and numbering the positions held by them in this series as , , , etc., we get the position number or what is called the atomic number. this designates the order or position of the element in the series. we must learn that this number marks a position rather than a single element, a statement which will be explained later. determination of the atomic number since the atomic weight is unreliable as a means of settling the position of an element in the series and so fixing its atomic number, how is this number to be determined? of course, one answer to this question is that we may rely upon a consideration of the general properties, as has been done in the past. fortunately, other methods have been found by which this may be confirmed. for instance, the stopping and scattering power of the element for alpha particles has been suggested and successfully used. use of x-ray spectra a most interesting method is due to moseley's observations upon the _x_-ray spectra of the various elements. it has been found that crystals, such as those of quartz, have the power of reflecting and defining the _x_ rays. the spectra given by these rays can be photographed and the wave lengths measured. these _x_ rays are emitted by various substances under bombardment by the cathode rays (negative electrons) and have great intensity and very minute wave lengths. moseley made use of various metals as anti-cathodes for the production of these rays. these metals ranged from calcium to zinc in the periodic system. in each case he observed that two characteristic types of _x_ rays of definite intensity and different wave lengths were emitted. from the frequency of these waves there is deduced a simple relation connected with a fundamental quantity which increases in units from one element to the next. this is due to the charge of the positive central nucleus. the number found in this way is one less than the atomic number. thus the number for calcium is instead of and that for zinc is instead of . so, by adding to the number found the atomic number is obtained. the atomic weight can usually be followed in fixing the atomic number, but where doubt exists the method just given can be resorted to. thus doubt arises in the case of iron and nickel and cobalt. this would be the order according to the atomic weights. the _x_-ray method gives the order as iron, cobalt, and nickel, and this is the accepted order in the periodic system. changes caused by ray emission on studying the properties of the elements in a transformation series in connection with the ray emission which produced them, it was seen that these properties were determined in each case by the nature of the ray emitted from the preceding transformation product or parent element. atomic weight losses each alpha particle emitted means a loss of in the atomic weight. this is the mass of a helium atom. thus from uranium with an atomic weight of to radium there is a loss of three alpha particles. therefore, must be subtracted from , leaving , which agrees closely with the atomic weight of radium as actually determined by the ordinary methods. uranium x_{ }, then, would have an atomic weight of and that of ionium would be . the other intermediate elements, whose formation is due to the loss of beta particles only, show no decrease in atomic weight. lead the end product from uranium to lead there is a loss of alpha particles, or units in atomic weight. this would give for the final product an atomic weight of . the atomic weight of lead is . . it is not at all certain that the final product of this series is ordinary lead. the facts are such that they would lead one to think that it is not. it is known only that the end product would probably be some element closely resembling lead chemically and hence difficult or impossible to separate from it. several accurate determinations of lead coming from uranium minerals, which always carry this element and in an approximately definite ratio to the amount of uranium present, show atomic weights of . ; . ; and . . even the most rigid methods of purification fail to change these results. the lead in these minerals might therefore be considered as coming in the main from the disintegration of the uranium atom and, though chemically resembling lead, as being in reality a different element with different atomic weight. furthermore, in the thorium series alpha particles are lost before reaching the end product, which again is perhaps the chemical analogue of lead. the atomic weight here should be less , or . determinations of the atomic weight of lead from thorite, a thorium mineral nearly free from uranium, gave . . the end product of the actinium series is also an element resembling lead, but both the beginning and ending of this series are still in obscurity. changes of position in the periodic system the loss of units in the atomic weight of an element on the expulsion of an alpha particle is accompanied by a change of chemical properties which removes the new element two groups toward the positive side in the periodic system. thus ionium is so closely related to thorium and so resembles it chemically that it is properly classed along with thorium as a quadrivalent element in the fourth group. ionium expels an alpha particle and becomes radium, which is a bivalent element resembling barium belonging to the second group. radium then expels an alpha particle and becomes the gas, radium emanation, which is an analogue of argon and belongs to the zero group. other instances might be cited which go to show that in all cases the loss of an alpha particle makes a change of two places toward the left or positive side of the system. changes from loss of beta particles the loss of a beta particle causes no change in the atomic weight but does cause a shift for each beta particle of one group toward the right or negative side of the system. two such losses, then, will counterbalance the loss of an alpha particle and bring the new element back to the group originally occupied by its progenitor. thus uranium in the sixth group loses an alpha particle and the product ux_{ } falls in the fourth group. one beta particle is then lost and ux_{ } belonging to the fifth group is formed. with the loss of one more beta particle the new element returns to the sixth group from which the transformation began. the table on page , as adapted from soddy, affords a general view of these changes. isotopes an examination of the table will show a number of different elements falling in the same position in a group of the periodic system irrespective of their atomic weights. these are chemically inseparable so far as the present limitations of chemical analysis are concerned. even the spectra of these elements seem to be identical so far as known. this identity extends to most of the physical properties, but this demands much further investigation. for this new phenomenon soddy has suggested the word isotope for the element and isotopic for the property, and these names have come into general use. [illustration: radio-active elements from uranium and thorium placed in the periodic systems adapted from soddy] manifestly, we have come across a phenomenon here which quite eliminates the atomic weight as a determining factor as to position in the periodic or natural system or of the elemental properties in general. all of the properties of the bodies which we call elements, and consequently of their compounds and hence of matter in general, seem to depend upon the balance maintained between the charges of negative and positive electricity which, according to rutherford's theory, go to make up the atom. it is evident that any study of chemical phenomena and chemical theory is quite incomplete without a study of radio-activity and the transformations which it produces. radio-activity in nature in concluding this outline of the main facts of radio-activity, it is of interest to discuss briefly the presence of radio-active material on this planet and in the stars. facts enough have been gathered to show the probable universality of this phenomenon of radio-activity. whether this means solely the disintegration of the uranium and thorium atoms, or whether other elements are also transformed under the intensity of the agencies at work in the universe, is of course a question as yet unsolved. radio-active products in the earth's crust the presence of uranium and thorium widely distributed throughout the crust of the earth would lead to the conclusion that their disintegration products would be found there also. various rocks of igneous origin have been examined revealing from . × ^{- } to . × ^{- } grams of radium per gram of the rock. aqueous rocks have shown a lesser amount, ranging from . × ^{- } to . × ^{- } grams. as the soil is formed by the decomposition of these rocks, radium is present in varying amounts in all kinds of soil. presence in air and soil waters as radium is transformed into the gaseous emanation, this will escape wherever the soil is not enclosed. for instance, a larger amount of radio-activity is found in the soil of caves and cellars than in open soils. if an iron pipe is sunk into a soil and the air of the soil sucked up into a large electroscope, the latter instrument will show the effect of the rays emitted and will measure the degree of activity. also the interior of the pipe will receive a deposit of the radio-active material and will show appreciable radio-activity after being removed from the soil. this radium emanation is dissolved in the soil waters, wells, springs, and rivers, rendering them more or less radio-active, and sometimes the muddy deposit at the bottom of a spring shows decided radio-activity. the emanation also escapes into the air so that many observations made in various places show that the radium emanation is everywhere present in the atmosphere. neither summer nor winter seems to affect this emanation, and it extends certainly to a height of two or three miles. rain, falling through the air, dissolves some of the emanation, so that it may be found in freshly-fallen rain water and also in freshly-fallen snow. radio-active deposits are found upon electrically charged wires exposed near the earth's surface. as helium is the resulting product of the alpha particles emitted by the emanation and other radio-active bodies, it is found in the soil air, soil waters, and atmosphere. average measurements of the radio-activity of the atmosphere have led to the calculation that about one gram of radium per square kilometer of the earth's surface is requisite to keep up the supply of the emanation. a number of estimates have been given as to the heat produced by the radio-active transformations going on in the material of this planet. actual data are scarce and mere assumptions unsatisfactory, so little that is worth while can be deduced. it is possible that this source of heat may have an appreciable effect upon or serve to balance the earth's rate of cooling. cosmical radio-activity meteorites of iron coming from other celestial bodies have not shown the presence of radium. aerolites or stone meteorites have been found to contain as much as similar terrestrial rock. since the sun contains helium and some stars show its presence as predominating, this suggests the presence of radio-active matter in these bodies. in addition, the spectral lines of uranium, radium, and the radium emanation have been reported as being found in the sun's spectrum and also in the new star, _nova geminorum _. these observations await further investigation and confirmation. so far as the sun's chromosphere is concerned, the possible amount of radium present would seem to be very small. if this is true, radio-active processes could have little to do with the sun's heat. the statement is made by rutherford that indirect evidence obtained from the study of the aurora suggests that the sun emits rays similar in type to the alpha and beta rays. such rays would be absorbed, and the gamma rays likewise, in passing through the earth's atmosphere and so escape ordinary observation. all of this is but further evidence of the unity of matter and of forces in the universe. index actinium, discovery of, activity, induced, alpha particles, effect of loss on atomic weight, electrical charge of, form helium, nature of, penetrating power of, position of element changed by its loss, recoil, scattering of, solid, atom, constitution of, kelvin's, models of, rutherford's, atomic number, determination of, becquerel's experiments, beta particles, change in position of element by loss of, chalcolite, natural and artificial, constants, table of, curie unit, disintegration of the element, disintegration series, disintegration theory, electroscope, equilibrium series, helium, characteristics of, discovery of, ionium, discovery of, ionization, application of electric field to, experimental confirmation, ionization of gases, theory of, ions, size and nature of, isotopes, lead, atomic weight varies with source, radio-active, the end product, life-periods of radio-active bodies, periodic system, basis of, polonium, discovery of, positive nucleus, influence of, potassium, radio-activity of, radiations, action on phosphorescent bodies, action on photographic plates, discharge electrified bodies, magnetic deflection of, measurements of, penetrating power of, , radio-active bodies, elemental nature of, examination of, life periods of, radio-activity, an atomic property, cosmical, influence on chemical theory, products in atmosphere, products in earth's crust, products in soil waters, radium, action on organic matter, etc., amount in pitchblende, discovery of, emanation, energy evolved by, properties of, , rays, alpha, , , beta, , gamma, , identification of, , magnetic deflection of, photographing track of, types of, rubidium, radio-activity of, spinthariscope, stopping power of substances, thorium x, discovery of, , uranium atom, disintegration of, uranium minerals, radio-activity of, uranium x, discovery of, , , x-ray spectra, zinc sulphide screen, transcriber's notes . passages in italics are surrounded by _underscores_. . images have been moved from the middle of a paragraph to the closest paragraph break. . the original text includes certain greek alphabets. for this text version [alpha], [beta], [gamma] indicate first three letters of greek alphabet respectively. . in this version, the number following carat character ^ is to be interpreted as follows. the expression ^{- } means multiplying by . ; ^{ } means multiplying by , , , . . in this version, the subscripted text has been replaced by an underline character _ followed by the same with curly braces { and }. for example, x_{ } indicates x with subscript . . the fractions are indicated with the help of forward character /. for example, / indicates one-fourth. . other than the changes listed above, the original text has been reproduced as such. heroes of science. heroes of science. physicists. by william garnett, m.a., d.c.l., formerly fellow of st. john's college, cambridge; principal of the durham college of science, newcastle-upon-tyne; hon. member of the north of england institute of mining and mechanical engineers. published under the direction of the committee of general literature and education appointed by the society for promoting christian knowledge. london: society for promoting christian knowledge, northumberland avenue, charing cross, w.c.; , queen victoria street, e.c.; , st. george's place, hyde park corner, s.w. brighton: , north street. new york: e. & j. b. young and co. preface. the following pages claim no originality, and no merits beyond that of bringing within reach of every boy and girl material which would otherwise be available only to those who had extensive libraries at their command, and much time at their disposal. in the schools and colleges in which the principles of physical science are well taught, the history of the discoveries whereby those principles have been established has been too much neglected. the series to which the present volume belongs is intended, in some measure, to meet this deficiency. a complete history of physical science would, if it could be written, form a library of considerable dimensions. the following pages deal only with the biographies of a few distinguished men, who, by birth, were british subjects, and incidental allusions only are made to living philosophers; but, notwithstanding these narrow restrictions, the foundations of the royal society of london, of the american philosophical society, of the great library of pennsylvania, and of the royal institution, are events, some account of which comes within the compass of the volume. the gradual development of our knowledge of electricity, of the mechanical theory of heat, and of the undulatory theory of optics, will be found delineated in the biographies selected, though no continuous history is traced in the case of any one of these branches of physics. the sources from which the matter contained in the following pages has been derived have been, in addition to the published works of the subjects of the several sketches, the following:-- "the encyclopædia britannica." "memoir of the honourable robert boyle," by thomas birch, m.a., prefixed to the folio edition of his works, which was published in london in . "life of benjamin franklin," from his own writings, by john bigelow. dr. g. wilson's "life of cavendish," which forms the first volume of the publications of the cavendish society; and the "electrical researches of the hon. henry cavendish, f.r.s.," edited by the late professor james clerk maxwell. "the life of sir benjamin thompson, count rumford," by george e. ellis, published by the american academy of arts and sciences, in connection with the complete edition of his works. "memoir of thomas young," by the late dean peacock. dr. bence jones's "life of faraday;" and professor tyndall's "faraday as a discoverer." "life of james clerk maxwell," by professor lewis campbell and william garnett. it is hoped that the perusal of the following sketches may prove as instructive to the reader as their preparation has been to the writer. wm. garnett. newcastle-upon-tyne, _december, _. contents. page introduction robert boyle benjamin franklin henry cavendish count rumford thomas young michael faraday james clerk maxwell conclusion heroes of science. introduction. the dawn of true ideas respecting mechanics has been described in the volume of this series devoted to astronomers. at the time when the first of the following biographies opens there were a few men who held sound views respecting the laws of motion and the principles of hydrostatics. considerable advance had been made in the subject of geometrical optics; the rectilinear propagation of light and the laws of reflection having been known to the greeks and arabians, whilst willebrod snellius, professor of mathematics at leyden, had correctly enunciated the laws of refraction very early in the seventeenth century. pliny mentions the action of a sphere of rock-crystal and of a glass globe filled with water in bringing light to a focus. roger bacon used segments of a glass sphere as lenses; and in the eleventh century alhazen made many measurements of the angles of incidence and refraction, though he did not succeed in discovering the law. huyghens developed to a great extent the undulatory theory; while newton at the same time made great contributions to the subject of geometrical optics, decomposed white light by means of a prism, investigated the colours of thin plates, and some cases of diffraction, and speculated on the nature, properties, and functions of the ether, which was equally necessary to the corpuscular as to the undulatory theory of light, if any of the phenomena of interference were to be explained. the velocity of light was first measured by roemer, in . the camera obscura was invented by baptista porta, a wealthy neapolitan, in ; and kepler explained the action of the eye as an optical instrument, in . antonio de dominis, archbishop of spalatro, discovered the fringe of colours produced by sunlight once reflected from the interior of a globe of water, and this led, in newton's hands, to the complete explanation of the rainbow. the germ of the mechanical theory of heat is to be found in the writings of lord bacon. the first thermometers which were blown in glass with a bulb and tube hermetically sealed, were made by a craftsman in florence, in the time of torricelli. the graduations on these thermometers were made by attaching little beads of coloured glass to their stems, and they were carried about europe by members of the florentine academy, in order to learn whether ice melted at the same temperature in all latitudes. in electricity the attraction of light bodies by amber when rubbed, was known at least six hundred years before the christian era, and the shocks of the torpedo were described by pliny and by aristotle; but the phenomena were not associated in men's minds until recent times. dr. gilbert, of colchester, physician to queen elizabeth, may be regarded as the founder of the modern science. he distinguished two classes of bodies, viz. electrics, or those which would attract light bodies when rubbed; and non-electrics, or those which could not be so excited. the first electric machine was constructed by otto von guericke, the inventor of the magdeburg hemispheres, who mounted a ball of sulphur so that it could be made rapidly to rotate while it was excited by the friction of the hand. he observed the repulsion which generally follows the attraction of a light body by an electrified object after the two have come in contact. he also noticed that certain bodies placed near to electrified bodies possessed similar powers of attraction to those of the electrified bodies themselves. newton replaced the sulphur globe of otto von guericke by a globe of glass. stephen gray discovered the conduction of electricity, in , when he succeeded in transmitting a charge to a distance of feet along a pack-thread suspended by silk strings so as to insulate it from the earth. desaguliers showed that gilbert's "electrics" were simply those bodies which could not conduct electricity, while all conductors were "non-electrics;" and dufay showed that all bodies could be electrified by friction if supported on insulating stands. he also showed that there were two kinds of electrification, and called one _vitreous_, the other _resinous_. gray, hawksbee, and dr. wall all noticed the similarity between lightning and the electric discharge. the prime conductor was first added to the electric machine by boze, of wittenberg; and winkler, of leipsic, employed a cushion instead of the hand to produce friction against the glass. the accumulation of electricity in the leyden jar was discovered accidentally by cuneus, a pupil of muschenbroeck, of leyden, about , while attempting to electrify water in a bottle held in his hand. a nail passed through the cork, by which the electricity was communicated to the water. on touching the nail after charging the water, he received the shock of the leyden jar. this brings the history of electrical discovery down to the time of franklin. robert boyle. robert boyle was descended from a family who, in saxon times, held land in the county of hereford, and whose name in the doomsday book is written biuvile. his father was richard boyle, earl of cork, to whom the fortunes of the family were largely due. richard boyle was born in the city of canterbury, october , . he was educated at bene't college (now corpus christi college), cambridge, and afterwards became a member of the middle temple. finding his means insufficient for the prosecution of his legal studies, he determined to seek his fortune abroad. in he married, at limerick, one of the daughters of william apsley, who brought him land of the value of £ per annum. in his autobiography the earl of cork writes:-- when first i arrived at dublin, in ireland, the rd of june , all my wealth then was twenty-seven pounds three shillings in money, and two tokens which my mother had given me, viz. a diamond ring, which i have ever since and still do wear, and a bracelet of gold worth about ten pounds; a taffety doublet cut with and upon taffety, a pair of black velvet breeches laced, a new milan fustian suit laced and cut upon taffety, two cloaks, competent linen, and necessaries, with my rapier and dagger. and since, the blessing of god, whose heavenly providence guided me hither, hath enriched my weak estate, in beginning with such a fortune, as i need not envy any of my neighbours, and added no care or burthen of my conscience thereunto. and the rd of june, , i have served my god, queen elizabeth, king james, and king charles, full forty-four years, and so long after as it shall please god to enable me. richard boyle's property in ireland increased so rapidly that he was accused to queen elizabeth of receiving pay from some foreign power. when about to visit england in order to clear himself of this charge, the rebellion in munster broke out; his lands were wasted, and his income for the time destroyed. reaching london, he returned to his old chambers in the middle temple, until he entered the service of the earl of essex, to whom the government of ireland had been entrusted. the charges against him were then resumed, and he was made a prisoner, and kept in confinement until the earl of essex had gone over to ireland. at length he obtained a hearing before the queen, who fully acquitted him of the charges, gave him her hand to kiss, and promised to employ him in her own service; at the same time she dismissed sir henry wallop, who was treasurer for ireland, and prominent among boyle's accusers, from his office. a few days afterwards, richard boyle was appointed by the queen clerk to the council of munster, and having purchased a ship of sir walter raleigh, he returned to ireland with ammunition and provisions. "then, as clerk of the council, i attended the lord president in all his employments, and waited upon him at the siege of kingsale, and was employed by his lordship to her majesty, with the news of that happy victory; in which employment i made a speedy expedition to the court; for i left my lord president at shannon castle, near corke, on the monday morning, about two of the clock, and the next day, being tuesday, i delivered my packet, and supped with sir robert cecil, being then principal secretary of state, at his house in the strand; who, after supper, held me in discourse till two of the clock in the morning; and by seven that morning called upon me to attend him to the court, where he presented me to her majesty in her bed-chamber, who remembered me, calling me by my name, and giving me her hand to kiss, telling me that she was glad that i was the happy man to bring the first news of that glorious victory ... and so i was dismissed with grace and favour." in reading of this journey from cork to london, it is almost necessary to be reminded that it took place two hundred and fifty years before the introduction of steam-boats and railways. at the close of the rebellion, richard boyle purchased from sir walter raleigh all his lands in munster; and on july , , he married his second wife, catharine, the only daughter of sir geoffrey fenton, principal secretary of state, and privy councillor in ireland, "with whom i never demanded any marriage portion, neither promise of any, it not being in my consideration; yet her father, after my marriage, gave me one thousand pounds in gold with her. but that gift of his daughter unto me i must ever thankfully acknowledge as the crown of all my blessings; for she was a most religious, virtuous, loving, and obedient wife unto me all the days of her life." he was knighted by the lord deputy of ireland, sir george carew, on his wedding-day; was sworn privy councillor of state of the kingdom of ireland in ; created lord boyle, baron of youghall, september , ; lord viscount of dungarvon and earl of cork, october , ; one of the lords justices of ireland, with a salary of £ per annum, in ; and lord high treasurer of ireland, november , . robert boyle, the seventh son of the earl of cork, was born january , . his mother died february , . the earl lived in prosperity in ireland till the breaking out of the rebellion in , and died at youghall in september, . it is said that when cromwell saw the vast improvements which the earl had made on his estate in munster, he declared that "if there had been an earl of cork in every province, it would have been impossible for the irish to have raised a rebellion." at a very early age robert was sent by his father to a country nurse, "who, by early inuring him, by slow degrees, to a coarse but cleanly diet, and to the usual passion of the air, gave him so vigorous a complexion that both hardships were made easy to him by custom, and the delights of conveniences and ease were endeared to him by their rarity." making the acquaintance of some children who stuttered in their speech, he, by imitation, acquired the same habit, "so contagious and catching are men's faults, and so dangerous is the familiar commerce of those condemnable customs, that, being imitated but in jest, come to be learned and acquired in earnest." before going to school he studied french and latin, and showed considerable aptitude for scholarship. he was then sent to eton, where his master took much notice of him, and "would sometimes give him unasked play-days, and oft bestow upon him such balls and tops and other implements of idleness as he had taken away from others that had unduly used them." while at school, in the early morning, a part of the wall of the bedroom, with the bed, chairs, books, and furniture of the room above, fell on him and his brother. "his brother had his band torn about his neck, and his coat upon his back, and his chair crushed and broken under him; but by a lusty youth, then accidentally in the room, was snatched from out the ruins, by which [robert] had, in all probability, been immediately oppressed, had not his bed been curtained by a watchful providence, which kept all heavy things from falling on it; but the dust of the crumbled rubbish raised was so thick that he might there have been stifled had not he remembered to wrap his head in the sheet, which served him as a strainer, through which none but the purer air could find a passage." at eton he spent nearly four years, "in the last of which he forgot much of that latin he had got, for he was so addicted to more solid parts of knowledge that he hated the study of bare words naturally, as something that relished too much of pedantry to consort with his disposition and designs." on leaving eton he joined his father at stalbridge, in dorsetshire, and was sent to reside with "mr. w. douch, then parson of that place," who took the supervision of his studies. here he renewed his acquaintance with latin, and devoted some attention to english verse, spending some of his idle hours in composing verses, "most of which, the day he came of age, he sacrificed to vulcan, with a design to make the rest perish by the same fate." a little later he returned to his father's house in stalbridge, and was placed under the tutelage of a french gentleman, who had been tutor to two of his brothers. in october, , robert boyle and his brother were sent into france. after a short stay at lyons, they reached geneva, where robert remained with his tutor for about a year and three quarters. during his residence here an incident occurred which he regarded as the most important event of his life, and which we therefore give in his own words. "to frame a right apprehension of this, you must understand that, though his inclinations were ever virtuous, and his life free from scandal and inoffensive, yet had the piety he was master of already so diverted him from aspiring unto more, that christ, who long had lain asleep in his conscience (as he once did in the ship), must now, as then, be waked by a storm. for at a time which (being the very heat of summer) promised nothing less, about the dead of night, that adds most terror to such accidents, [he] was suddenly waked in a fright with such loud claps of thunder (which are oftentimes very terrible in those hot climes and seasons), that he thought the earth would owe an ague to the air, and every clap was both preceded and attended with flashes of lightning, so frequent and so dazzling that [he] began to imagine them the sallies of that fire that must consume the world. the long continuance of that dismal tempest, where the winds were so loud as almost drowned the noise of the very thunder, and the showers so hideous as almost quenched the lightning ere it could reach his eyes, confirmed him in his apprehensions of the day of judgment's being at hand. whereupon the consideration of his unpreparedness to welcome it, and the hideousness of being surprised by it in an unfit condition, made him resolve and vow that, if his fears were that night disappointed, all his further additions to his life should be more religiously and watchfully employed. the morning came, and a serene, cloudless sky returned, when he ratified his determinations so solemnly, that from that day he dated his conversion, renewing, now he was past danger, the vow he had made whilst he believed himself to be in it; and though his fear was (and he blushed it was so) the occasion of his resolution of amendment, yet at least he might not owe his more deliberate consecration of himself to piety to any less noble motive than that of its own excellence." after leaving geneva, he crossed the alps and travelled through northern italy. here he spent much time in learning italian; "the rest of his spare hours he spent in reading the modern history in italian, and the new paradoxes of the great stargazer galileo, whose ingenious books, perhaps because they could not be so otherwise, were confuted by a decree from rome; his highness the pope, it seems, presuming, and that justly, that the infallibility of his chair extended equally to determine points in philosophy as in religion, and loth to have the stability of that earth questioned in which he had established his kingdom." having visited rome, he at length returned to france, and was detained at marseilles, awaiting a remittance from the earl to enable him to continue his travels. through some miscarriage, the money which the earl sent did not arrive, and robert and his brother had to depend on the credit of the tutor to procure the means to enable them to return home. they reached england in the summer of , "where we found things in such confusion that, although the manor of stalbridge were, by my father's decease, descended unto me, yet it was near four months before i could get thither." on reaching london, robert boyle resided for some time with his sister, lady ranelagh, and was thus prevented from entering the royalist army. later on he returned for a short time to france; visited cambridge in december, , and then took up his residence at stalbridge till may, , where he commenced the study of chemistry and natural philosophy. it was in october, , that boyle first made mention of the "_invisible college_," which afterwards developed into the royal society. writing to a fellow of magdalen college, cambridge, in february, , he says, "the corner-stones of the _invisible_, or, as they term themselves, the _philosophical college_, do now and then honour me with their company." it appears that a desire to escape from the troubles of the times had induced several persons to take refuge in philosophical pursuits, and, meeting together to discuss the subjects of their study, they formed the "invisible college." boyle says, "i will conclude their praises with the recital of their chiefest fault, which is very incident to almost all good things, and that is, that there is not enough of them." dr. wallis, one of the first members of the society, states that mr. theodore hooke, a german of the palatinate, then resident in london, "gave the first occasion and first suggested those meetings and many others. these meetings we held sometimes at dr. goddard's lodging, in wood street (or some convenient place near), on occasion of his keeping an operator in his house, for grinding glasses for telescopes and microscopes, and sometimes at a convenient place in cheapside; sometimes at gresham college, or some place near adjoining. our business was (precluding theology and state affairs) to discourse and consider of philosophical inquiries, and such as related thereunto; as physic, anatomy, geometry, astronomy, navigation, statics, magnetics, chemics, mechanics, and natural experiments, with the state of these studies as then cultivated at home and abroad. about the year - some of us being removed to oxford, first dr. wilkins, then i, and soon after dr. goddard, our company divided. those in london continued to meet there as before, and we with them when we had occasion to be there. and those of us at oxford, with dr. ward, since bishop of salisbury, dr. ralph bathurst, now president of trinity college in oxford, dr. petty, since sir william petty, dr. willis, then an eminent physician in oxford, and divers others, continued such meetings in oxford, and brought those studies into fashion there; meeting first at dr. petty's lodgings, in an apothecary's house, because of the convenience of inspecting drugs and the like, as there was occasion; and after his remove to ireland (though not so constantly) at the lodgings of dr. wilkins, then warden of wadham college; and after his removal to trinity college in cambridge, at the lodgings of the honourable mr. robert boyle, then resident for divers years in oxford. these meetings in london continued, and after the king's return, in , were increased with the accession of divers worthy and honourable persons, and were afterwards incorporated by the name of the _royal society_, and so continue to this day." boyle was only about twenty years of age when he wrote his "free discourse against swearing;" his "seraphic love; or, some motives and incentives to the love of god;" and his "essay on mistaken modesty." "seraphic love" was the last of a series of treatises on love, but the only one of the series that he published, as he considered the others too trifling to be published alone or in conjunction with it. in a letter to lady ranelagh, he refers to his laboratory as "a kind of elysium," and there were few things which gave him so much pleasure as his furnaces and philosophical experiments. in he visited ireland, returning in the following summer. in the autumn he was again obliged to visit ireland, and remained there till the summer of , though residence in that country was far from agreeable to him. he styled it "a barbarous country, where chemical spirits were so misunderstood, and chemical instruments so unprocurable, that it was hard to have any hermetic thoughts in it." on his return he settled in oxford, and there his lodgings soon became the centre of the scientific life of the university. boyle and his friends may be regarded as the pioneers of experimental philosophy in this country. to boyle the methods of aristotle appeared little more than discussions on words; for a long time he refused to study the philosophy of descartes, lest he should be turned aside from reasoning based strictly on the results of experiment. the method pursued by these philosophers had been fully discussed by lord bacon, but at best his experimental methods, though most complete and systematic, existed only upon paper, and it was reserved for boyle and his friends to put the baconian philosophy into actual practice. it was during his residence at oxford that he invented the air-pump, which was afterwards improved for him by hooke, and with which he conducted most of those experiments on the "spring" and weight of the air, which led up to the investigations that have rendered his name inseparably connected with "the gaseous laws." the experiments of galileo and of torricelli had shown that the pressure of the air was capable of supporting a column of water about thirty-four feet in height, or a column of mercury nearly thirty inches high. the younger pascal, at the request of torricelli, had carried a barometer to the summit of the puy de dome, and demonstrated that the height of the column of mercury supported by the air diminishes as the altitude is increased. otto von guericke had constructed the magdeburg hemispheres, and shown that, when exhausted, they could not be separated by sixteen horses, eight pulling one way and eight the other. he was aware that the same traction could have been produced by eight horses if one of the hemispheres had been attached to a fixed obstacle; but, with the instincts of a popular lecturer, he considered that the spectacle would thus be rendered less striking, and it was prepared for the king's entertainment. boyle wished for an air-pump with an aperture in the receiver sufficiently large for the introduction of various objects, and an arrangement for exhausting it without filling the receiver with water or otherwise interfering with the objects placed therein. his apparatus consisted of a large glass globe capable of containing about three gallons or thereabouts, terminating in an open tube below, and with an aperture of about four inches diameter at the top. around this aperture was cemented a turned brass ring, the inner surface being conical, and into this conical seat was fitted a brass plate with a thick rim, but drilled with a small hole in the centre. to this hole, which was also conical, was fitted a brass stopper, which could be turned round when the receiver was exhausted. by attaching a string to this stopper, which was so long as to enter the receiver to the depth of two or three inches, and turning the stopper in its seat, the string could be wound up, and thus objects could be moved within the receiver. the tube at the bottom of the receiver communicated with a stop-cock, and this with the upper end of the pumpbarrel, which was inverted, so that this stop-cock, which was at the top of the barrel, took the place of the foot-valve. the piston was solid, made of wood, and surrounded with sole leather, which was kept well greased. there being no valve in the piston, it was necessary to place an exhaust-valve in the upper end of the cylinder. this consisted of a small brass plug closing a conical hole so that it could be removed at pleasure. the construction of the cylinder was, therefore, similar to that of an ordinary force-pump, except that the valves had to be moved by hand (as in the early forms of the steam-engine). the piston was raised and depressed by means of a rack and pinion. the pumps could be used either for exhausting the receiver or for forcing air into it, according to the order in which the "valves" were opened. if the stop-cock communicating with the receiver were open while the piston was being drawn down, and the brass plug removed so as to open the exhaust-valve when the piston was being forced up, the receiver would gradually be exhausted. if the brass plug were removed during the descent of the piston, and the stop-cock opened during its ascent, air would be forced into the receiver. in the latter case it was necessary to take special precautions to prevent the brass plate at the top of the receiver being raised from its seat. all joints were made air-tight with "diachylon," and when, through the bursting of a glass bulb within it, the receiver became cracked, the crack was rendered air-tight by the same means. other receivers of smaller capacity were also provided, on account of the greater readiness with which they could be exhausted. with this apparatus boyle carried out a long series of experiments. he could reduce the pressure in the large receiver to somewhat less than that corresponding to an inch of mercury, or about a foot of water. squeezing a bladder so as to expel nearly all the air, tying the neck, and then introducing it into the receiver, he found, on working the pump, that the bladder swelled so that at length it became completely distended. in order to account for this great expansibility, boyle pictured the constitution of the air in the following way. he supposed the air to consist of separate particles, each resembling a spiral spring, which became tightly wound when exposed to great pressure, but which expanded so as to occupy a larger circle when the pressure was diminished. each of these little spirals he supposed to rotate about a diameter so as to exclude every other body from the sphere in which it moved. increasing the length of the diameter tenfold would increase the volume of one of these spheres, and therefore the volume of the gas, a thousandfold. possibly this was only intended as a mental illustration, exhibiting a mechanism by which very great expansion might conceivably be produced, and scarcely pretending to be considered a _theory_ of the constitution of the air. boyle's first idea seems to have been derived from a lock of wool in which the elasticity of each fibre caused the lock to expand after it had been compressed in the hand. in another passage he speaks of the air as consisting of a number of bodies capable of striking against a surface exposed to them. he demonstrated the weight of the air by placing a delicate balance within the receiver, suspending from one arm a bladder half filled with water, and balancing it with brass weights. on exhausting the air, the bladder preponderated, and, by repeating the experiment with additional weights on the other arm until a balance was effected in the exhausted receiver, he determined the amount of the preponderance. in another experiment he compressed air in a bladder by tying a pack-thread round it, balanced it from one arm of his balance in the open air; then, pricking the bladder so as to relieve the pressure, he found that with the escape of the compressed air the weight diminished. one of the most important of his experiments with the air-pump was the following. he placed within the receiver the cistern of a mercurial barometer, the tube of which was made to pass through the central hole in the brass plate, from which the stopper had been removed. the space around the tube was filled up with cement, and the receiver exhausted. at each stroke of the pump the mercury in the barometer tube descended, but through successively diminishing distances, until at length it stood only an inch above the mercury in the cistern. the experiment was then repeated with a tube four feet long and filled with water. this constituted the nineteenth experiment referred to later on. a great many strokes of the pump had to be made before the water began to descend. at length it fell till the surface in the tube stood only about a foot above that in the tank. placing vessels of ordinary spring-water and of distilled rain-water in the receiver, he found that, after the exhaustion had reached a certain stage, bubbles of gas were copiously evolved from the spring-water, but not from the distilled water. on another occasion he caused warm water to boil by a few strokes of the pump; and, continuing the exhaustion, the water was made to boil at intervals until it became only lukewarm. the experiment was repeated with several volatile liquids. he also noticed the cloud formed in the receiver when the air was allowed rapidly to expand; but the mechanical theory of heat had not then made sufficient progress to enable him to account for the condensation by the loss of heat due to the work done by the expanding air. the very minute accuracy of his observations is conspicuous in the descriptions of most of his experiments. that the air is the usual medium for the conveyance of sound was shown by suspending a watch by a linen thread within the receiver. on exhausting the air, the ticking of the watch ceased to be heard. a pretty experiment consisted in placing a bottle of a certain fuming liquid within the receiver; on exhausting the air, the fumes fell over the neck of the bottle and poured over the stand on which it was placed like a stream of water. another experiment, the thirty-second, is worthy of mention on account of the use to which it was afterwards applied in the controversy respecting the cause of suction. the receiver, having been exhausted, was removed from the cylinder, the stop-cock being turned off, and a small brass valve, to which a scale-pan was attached, was placed just under the aperture of the tube below the stop-cock. on turning the latter, the stream of air raised the valve, closing the aperture, and the atmospheric pressure supported it until a considerable weight had been placed in the scale-pan. because the receiver could not be exhausted so thoroughly as the pump-cylinder, boyle attempted to measure the pressure of the air by determining what weight could be supported by the piston. he found first that a weight of twenty-eight pounds suspended directly from the piston was sufficient to overcome friction when air was admitted above the piston. when the access of air to the top of the piston was prevented, more than one hundred pounds additional weight was required to draw down the piston. the diameter of the cylinder was about three inches. boyle's style of reasoning is well illustrated by the following from his paper on "the spring of the air:"-- "in the next place, these experiments may teach us what to judge of the vulgar axiom received for so many ages as an undoubted truth in the peripatetick schools, that nature abhors and flieth a vacuum, and that to such a degree that no human power (to go no higher) is able to make one in the universe; wherein heaven and earth would change places, and all its other bodies rather act contrary to their own nature than suffer it.... it will not easily, then, be intelligibly made out how hatred or aversation, which is a passion of the soul, can either for a vacuum or any other object be supposed to be in water, or such like inanimate body, which cannot be presumed to know when a vacuum would ensue, if they did not bestir themselves to prevent it; nor to be so generous as to act contrary to what is most conducive to their own particular preservation for the public good of the universe. as much, then, of intelligible and probable truth as is contained in this metaphorical expression seems to amount but to this--that by the wise author of nature (who is justly said to have made all things in number, weight, and measure) the universe, and the parts of it, are so contrived that it is hard to make a vacuum in it, as if they studiously conspired to prevent it. and how far this itself may be granted deserves to be further considered. "for, in the next place, our experiments seem to teach that the supposed aversation of nature to a vacuum is but accidental, or in consequence, partly of the weight and fluidity, or, at least, fluxility of the bodies here below; and partly, and perhaps principally, of the air, whose restless endeavour to expand itself every way makes it either rush in itself or compel the interposed bodies into all spaces where it finds no greater resistance than it can surmount. and that in those motions which are made _ob fugam vacui_ (as the common phrase is), bodies act without such generosity and consideration as is wont to be ascribed to them, is apparent enough in our thirty-second experiment, where the torrent of air, that seemed to strive to get into the emptied receiver, did plainly prevent its own design, by so impelling the valve as to make it shut the only orifice the air was to get [in] at. and if afterwards either nature or the internal air had a design the external air should be attracted, they seemed to prosecute it very unwisely by continuing to suck the valve so strongly, when they found that by that suction the valve itself could not be drawn in; whereas, by forbearing to suck, the valve would, by its own weight, have fallen down and suffered the excluded air to return freely, and to fill again the exhausted vessel.... "and as for the care of the public good of the universe ascribed to dead and stupid bodies, we shall only demand why, in our nineteenth experiment, upon the exsuction of the ambient air, the water deserted the upper half of the glass tube, and did not ascend to fill it up till the external air was let in upon it. whereas, by its easy and sudden rejoining that upper part of the tube, it appeared both that there was then much space devoid of air, and that the water might, with small or no resistance, have ascended into it, if it could have done so without the impulsion of the readmitted air; which, it seems, was necessary to mind the water of its formerly neglected duty to the universe." boyle then goes on to explain the phenomena correctly by the pressure of the air. elsewhere he accounts for the diminished pressure on the top of a mountain by the diminished weight of the superincumbent column of air. the treatise on "the spring of the air" met with much opposition, and boyle considered it necessary to defend his doctrine against the objections of franciscus linus and hobbes. in this defence he described the experiment in connection with which he is most generally remembered. linus had admitted that the air might possess a certain small amount of elasticity, but maintained that the force with which mercury rose in a barometer tube was due mainly to a totally different action, as though a string were pulling upon it from above. this was his funicular hypothesis. boyle undertook to show that the pressure of the air might be made to support a much higher column of mercury than that of the barometer. to this end he took a glass tube several feet in length, and bent so as to form two vertical legs connected below. the shorter leg was little more than a foot long, and hermetically closed at the top. the longer leg was nearly eight feet in length, and open at the top. the tube was suspended by strings upon the staircase, the bend at the bottom pressing lightly against the bottom of a box placed to receive the mercury employed in case of accident. each leg of the tube was provided with a paper scale. mercury was poured in at the open end, the tube being tilted so as to allow some of the air to escape from the shorter limb until the mercury stood at the same level in both legs when the tube was vertical. the length of the closed tube occupied by the air was then just twelve inches. the height of the barometer was about - / inches. mercury was gently poured into the open limb by one operator, while another watched its height in the closed limb. the results of the experiments are given in the table on the opposite page. in this table the third column gives the result of adding to the second column the height of the barometer, which expresses in inches of mercury the pressure of the air on the free surface of the mercury in the longer limb. the fourth column gives the total pressure, in inches of mercury, on the hypothesis that the pressure of the air varies inversely as the volume. the agreement between the third and fourth columns is very close, considering the roughness of the experiment and that no trouble appears to have been taken to _calibrate_ the shorter limb of the tube, and justified boyle in concluding that the hypothesis referred to expresses the relation between the volume and pressure of a given mass of air. +-----------+---------------+----------------+--------------+ |length of |height of |total pressure |total pressure| |closed tube|mercury in open|on air in inches|according to | |occupied |tube above that|of mercury. |boyle's law. | |by air. |in closed tube.| | | +-----------+---------------+----------------+--------------+ | | | - / | - / | | - / | - / | - / | - / | | | - / | - / | - / | | - / | - / | - / | - / | | | - / | - / | | | - / | - / | | - / | | | - / | - / | - / | | - / | - / | - / | - / | | | - / | - / | - / | | - / | - / | - / | - / | | | - / | - / | | | - / | - / | - / | - / | | | - / | - / | - / | | - / | - / | - / | - / | | - / | - / | - / | - / | | - / | - / | - / | - / | | | - / | - / | | | - / | | - / | - / | | - / | - / | - / | - / | | - / | - / | - / | - / | | | - / | - / | - / | | - / | - / | - / | - / | | - / | - / | - / | - / | | - / | - / | - / | - / | | | - / | - / | - / | +-----------+---------------+----------------+--------------+ to extend the investigation so as to include expansion below atmospheric pressure, a different apparatus was employed. it consisted of a glass tube about six feet in length, closed at the lower end and filled with mercury. into this bath of mercury was plunged a length of quill tube, and the upper end was sealed with wax. when the wax and air in the tube had cooled, a hot pin was passed through the wax, making a small orifice by which the amount of air in the tube was adjusted so as to occupy exactly one inch of its length as measured by a paper scale attached thereto, after again sealing the wax. the quill tube was then raised, and the height of the surface of the mercury in the tube above that in the bath noticed, together with the length of the tube occupied by the air. the difference between the height of the barometer and the height of the mercury in the tube above that in the bath gave the pressure on the imprisoned air in inches of mercury. the result showed that the volume varied very nearly in the inverse ratio of the pressure. a certain amount of air, however, clung to the sides of the quill tube when immersed in the mercury, and no care was taken to remove it by boiling the mercury or otherwise; in consequence of this, as the mercury descended, this air escaped and joined the rest of the air in the tube. this made the pressure rather greater than it should have been towards the end of the experiment, and when the tube was again pressed down into the bath it was found that, when the surfaces of the mercury within and without the tube were at the same level, the air occupied nearly - / inch instead of one inch of the tube. these experiments first established the truth of the great law known as "boyle's law," which states that _the volume of a given mass of a perfect gas varies inversely as the pressure to which it is exposed_. another experiment, to show that the pressure of the air was the cause of suction, boyle succeeded in carrying out at a later date. two discs of marble were carefully polished, so that when a little spirit of turpentine was placed between them the lower disc, with a pound weight suspended from it, was supported by the upper one. the apparatus was introduced into the air-pump, and a considerable amount of shaking proved insufficient to separate the discs. after sixteen strokes of the pump, on opening the communication between the receiver and cylinder, when no mechanical vibration occurred, the discs separated. upon the restoration in , the earl of clarendon, who was lord chancellor of england, endeavoured to persuade boyle to enter holy orders, urging the interest of the church as the chief motive for the proceeding. this made some impression upon boyle, but he declined for two reasons--first, because he thought that he would have a greater influence for good if he had no share in the patrimony of the church; and next, because he had never felt "an inward motion to it by the holy ghost." in an association was incorporated by parliament, to be called "the president and society for the propagation of the gospel in new england," whose object should be "to receive and dispose of moneys in such manner as shall best and principally conduce to the preaching and propagating the gospel among the natives, and for the maintenance of schools and nurseries of learning for the education of the children of the natives; for which purpose a general collection was appointed to be made in and through all the counties, cities, towns, and parishes of england and wales, for a charitable contribution, to be as the foundation of so pious and great an undertaking." the society was revived by special charter in , and boyle was appointed president, an office he continued to hold until shortly before his death. the society afterwards enlarged its sphere of operations, and became the society for the propagation of the gospel in foreign parts. in the same year ( ) boyle published "some considerations on the usefulness of experimental natural philosophy," etc., and in an extremely interesting paper on "experiments and considerations touching colours." in the course of this paper he describes some very beautiful experiments with a tincture of _lignum nephriticum_, wherein the dichroism of the extract is made apparent. boyle found that by transmitted light it appeared of a bright golden colour, but when viewed from the side from which it was illuminated the light emitted was sky blue, and in some cases bright green. by arranging experiments so that some parts of the liquid were seen by the transmitted light and some by the scattered light, very beautiful effects were produced. boyle endeavoured to learn something of the nature of colours by projecting spectra on differently coloured papers, and observing the appearance of the papers when illuminated by the several spectral rays. he also passed sunlight, concentrated by a lens, through plates of differently coloured glass superposed, allowing the light to fall on a white paper screen, and observing the tint of the light which passed through each combination. but the most interesting of these experiments was the actual mixture of light of different colours by forming two spectra, one by means of a fixed prism, the other by a prism held in the hand, and superposing the latter on the former so that different colours were made to coincide. this experiment was repeated in a modified form, nearly two hundred years later, by helmholtz, who found that the mixture of blue and yellow lights produced pink. unfortunately, boyle's spectra were far from pure, for, the source of light being of considerable dimensions, the different colours overlapped one another, as in newton's experiments, and in consequence some of his conclusions were inaccurate. thus blue paper in the yellow part of the spectrum appeared to boyle green instead of black, but this was due to the admixture of green light with the yellow. he concluded that bodies appear black because they damp the light so as to reflect very little to the eye, but that the surfaces of white bodies consist of innumerable little facets which reflect the light in all directions. in the same year he published some "observations on a diamond, which shines in the dark;" and an extensive treatise on "some considerations touching the style of the holy scriptures." next year appeared several papers from his pen, the most important being "occasional reflections upon several subjects," the wide scope of which may be gathered from the title. his "new experiments and observations touching cold" were printed in . in this paper he discussed the cause of the force exerted by water in freezing, methods of measuring degrees of cold, the action of freezing-mixtures, and many other questions. he contended that cold was probably only privative, and not a positive existence. lord bacon had asserted that the "essential self" of heat was probably motion and nothing more, and had adduced several experiments and observations in support of this opinion. in his paper on the mechanical origin of heat and cold, boyle maintained that heat was motion, but motion of the very small particles of bodies, very intense, and taking place in all directions; and that heat could be produced by any means whatever by which the particles of bodies could be agitated. on one occasion he caused two pieces of brass, one convex and the other concave, to be pressed against each other by a spring, and then rubbed together in a vacuum by a rotary motion communicated by a shaft which passed air-tight through the hole in the cover of the receiver, a little emery being inserted between them. in the second experiment the brasses became so hot that he "could not endure to hold [his] hand on either of them." this experiment was intended, like the rubbing of the blocks of ice in vacuo by davy, to meet the objection that the heat developed by friction was due to the action of the air. the following extract from a paper intended to show that the sense of touch cannot be relied upon for the estimation of temperature, shows that boyle possessed a very clear insight into the question:--"the account upon which we judge a body to be cold seems to be that we feel its particles less vehemently agitated than those of our fingers or other parts of the organ of touching; and, consequently, if the temper of that organ be changed, the object will appear more or less cold to us, though itself continue of one and the same temperature." to determine the expansion of water in freezing, he filled the bulb and part of the stem of a "bulb tube," or, as it was then generally called, "a philosophical egg," with water, and applying a freezing-mixture, at first to the bottom of the bulb, he succeeded in freezing the water without injury to the glass, and found that volumes of water expanded to - / volumes of ice--an expansion of about - / per cent. probably air-bubbles caused the ice to appear to have a greater volume than it really possessed, the true expansion being about nine per cent. of the volume of the water at °c. the expansion of water in freezing he employed in order to compress air to a greater extent than he had been able otherwise to compress it. having nearly filled a tube with water, but left a little air above, and then having sealed the top of the tube, he froze the water from the bottom upwards, so that in expanding it compressed the air to one-tenth of its former volume. magnetism and electricity came in for some share of boyle's attention. he carried out a number of experiments on magnetic induction, and found that lodestones, as well as pieces of iron, when heated and allowed to cool, became magnetized by the induction of the earth. his later experiments with exhausted receivers were not made with his first pump, but with a two-barrelled pump, in which the pistons were connected by a cord passing over a large fixed pulley, so that, when the receiver was nearly exhausted, the pressure of the air on the descending piston during the greater part of the stroke nearly balanced that on the ascending piston. in this respect the pump differed only from hawksbee's in having the pulley and cord instead of the pinion and two racks. it also resembled hawksbee's pump in having self-closing valves in the pistons and at the bottom of the cylinders, which, in this pump, had their open ends at the top. the pistons were alternately raised and lowered by the feet of the operator, which were placed in stirrups, of which one was fixed on each piston. the lower portions of the barrels were filled with water, through which the air bubbled, and this, occupying the clearance, enabled a much higher degree of exhaustion to be produced than could be obtained without its employment. in boyle was nominated provost of eton, but declined to accept the appointment. his "hydrostatical paradoxes," published about this time, contain all the ordinary theorems respecting the pressure of fluids under the action of gravity demonstrated experimentally. in boyle printed, at his own expense, five hundred copies of the four gospels and the acts of the apostles in the malayan tongue. this was but one of his many contributions towards similar objects. on november , , the royal society chose boyle for president. he, however, declined to accept the appointment, because he had conscientious objections to taking the oath required of the president by the charter of the society. it appears that very many of boyle's manuscripts, which were written in bound books, were taken away, and others mutilated by "corrosive liquors." in may, , he made this known to his friends, but, though these losses put him on his guard, he complained afterwards that all his care and circumspection had not prevented the loss of "six centuries of matters of fact in one parcel," besides many other smaller papers. his works, however, which have been published are so numerous that it would take several pages for the bare enumeration of their titles, many of them being devoted to medical subjects. the edition published in london in comprises nearly three thousand pages of folio. boyle always suffered from weak eyes, and in consequence he declined to revise his proofs. in the advertisement to the original edition of his works the publisher mentioned this, and at the same time pleaded his own business engagements as an excuse for not revising the proofs himself! it was partly on account of the injury to his manuscripts, and partly through failing health, that in he set apart two days in the week, during which he declined to receive visitors, that he might devote himself to his work, and especially to the reparation of the injured writings. about this time he succeeded in procuring the repeal of an act passed in the fifth year of henry iv. to the effect "that none from thenceforth should use to multiply gold or silver, or use the craft of multiplication; and if any the same do, they should incur the pain of felony." by this repeal it was made legal to extract gold and silver from ores, or from their mixtures with other metals, in this country provided that the gold and silver so procured should be put to no other use than "the increase of moneys." it is curious that boyle seems always to have believed in the possibility of transmuting other metals into gold. his sister, lady ranelagh, died on december , , and boyle survived her but a few days, for he died on december , and his body was interred near his sister's grave in the chancel of st. martin's-in-the-fields. dr. shaw, in his preface to boyle's works, writes, "the men of wit and learning have, in all ages, busied themselves in explaining nature by words; but it is mr. boyle alone who has wholly laid himself out in showing philosophy in action. the single point he perpetually keeps in view is to render his reader, not a talkative or a speculative, but an actual and practical philosopher. himself sets the example; he made all the experiments he possibly could upon natural bodies, and communicated them with all desirable candour and fidelity." the second part of his treatise on "the christian virtuoso," boyle concluded with a number of aphorisms, of which the following well represent his views respecting science:-- "i think it becomes christian philosophers rather to try whether they can investigate the final causes of things than, without trial, to take it for granted that they are undiscoverable." "the book of nature is a fine and large piece of tapestry rolled up, which we are not able to see all at once, but must be content to wait for the discovery of its beauty and symmetry, little by little, as it gradually comes to be more unfolded or displayed." benjamin franklin. among those whose contributions to physics have immortalized their names in the annals of science, there is none that holds a more prominent position in the history of the world than benjamin franklin. at one time a journeyman printer, living in obscure lodgings in london, he became, during the american war of independence, one of the most conspicuous figures in europe, and among americans his reputation was probably second to none, general washington not excepted. professor laboulaye says of franklin: "no one ever started from a lower point than the poor apprentice of boston. no one ever raised himself higher by his own unaided forces than the inventor of the lightning-rod. no one has rendered greater service to his country than the diplomatist who signed the treaty of , and assured the independence of the united states. better than the biographies of plutarch, this life, so long and so well filled, is a source of perpetual instruction to all men. every one can there find counsel and example." a great part of the history of his life was written by franklin himself, at first for the edification of the members of his own family, and afterwards at the pressing request of some of his friends in london and paris. his autobiography does not, however, comprise much more than the first fifty years of his life. the first part was written while he was the guest of the bishop of st. asaph, at twyford; the second portion at passy, in the house of m. de chaumont; and the last part in philadelphia, when he was retiring from public life at the age of eighty-two. the former part of this autobiography was translated into french, and published in paris, in , though it is not known how the manuscript came into the publisher's hands. the french version was translated into english, and published in england and america, together with such other of franklin's works as could be collected, before the latter part was given to the world by franklin's grandson, to whom he had bequeathed his papers, and who first published them in america in . for a period of three hundred years at least franklin's family lived on a small freehold of about thirty acres, in the village of ecton, in northamptonshire, the eldest son, who inherited the property, being always brought up to the trade of a smith. franklin himself "was the youngest son of the youngest son for five generations back." his grandfather lived at ecton till he was too old to follow his business, when he went to live with his second son, john, who was a dyer at banbury. to this business franklin's father, josiah, was apprenticed. the eldest son, thomas, was brought up a smith, but afterwards became a solicitor; the other son, benjamin, was a silk-dyer, and followed josiah to america. he was fond of writing poetry and sermons. the latter he wrote in a shorthand of his own inventing, which he taught to his nephew and namesake, in order that he might utilize the sermons if, as was proposed, he became a presbyterian minister. franklin's father, josiah, took his wife and three children to new england, in , where he practised the trade of a tallow-chandler and soap-boiler. franklin was born in boston on january (o.s.), , and was the youngest of seventeen children, of whom thirteen grew up and married. benjamin being the youngest of ten sons, his father intended him for the service of the church, and sent him to the grammar school when eight years of age, where he continued only a year, although he made very rapid progress in the school; for his father concluded that he could not afford the expense of a college education, and at the end of the year removed him to a private commercial school. at the age of ten young benjamin was taken home to assist in cutting the wicks of candles, and otherwise to make himself useful in his father's business. his enterprising character as a boy is shown by the following story, which is in his own words:-- there was a salt marsh that bounded part of the mill-pond, on the edge of which, at high-water, we used to stand to fish for minnows. by much trampling we had made it a mere quagmire. my proposal was to build a wharf there fit for us to stand upon, and i showed my comrades a large heap of stones, which were intended for a new house near the marsh, and which would very well suit our purpose. accordingly, in the evening, when the workmen were gone, i assembled a number of my play-fellows, and working with them diligently, like so many emmets, sometimes two or three to a stone, we brought them all away and built our little wharf. the next morning the workmen were surprised at missing the stones, which were found in our wharf. inquiry was made after the removers; we were discovered and complained of; several of us were corrected by our fathers; and, though i pleaded the usefulness of the work, mine convinced me that nothing was useful which was not honest. until twelve years of age benjamin continued in his father's business, but as he manifested a great dislike for it, and his parents feared that he might one day run away to sea, they set about finding some trade which would be more congenial to his tastes. with this view his father took him to see various artificers at their work, that he might observe the tastes of the boy. this experience was very valuable to him, as it taught him to do many little jobs for himself when workmen could not readily be procured. during this time benjamin spent most of his pocket-money in purchasing books, some of which he sold when he had read them, in order to buy others. he read through most of the books in his father's very limited library. these mainly consisted of works on theological controversy, which franklin afterwards considered to have been not very profitable to him. "there was another bookish lad in the town, john collins by name, with whom i was intimately acquainted. we sometimes disputed, and very fond we were of argument, and very desirous of confuting one another, which disputatious turn, by the way, is apt to become a very bad habit, making people often very disagreeable in company by the contradiction that is necessary to bring it into practice; and thence, besides souring and spoiling the conversation, is productive of disgusts and perhaps enmities when you may have occasion for friendship. i had caught it by reading my father's books of dispute about religion. persons of good sense, i have since observed, seldom fall into it, except lawyers, university men, and men of all sorts that have been bred at edinburgh." at length franklin's fondness for books caused his father to decide to make him a printer. his brother james had already entered that business, and had set up in boston with a new press and types which he had brought from england. he signed his indentures when only twelve years old, thereby apprenticing himself to his brother until he should attain the age of twenty-one. the acquaintance which he formed with booksellers through the printing business enabled him to borrow a better class of books than he had been accustomed to, and he frequently sat up the greater part of the night to read a book which he had to return in the morning. while working with his brother, the young apprentice wrote two ballads, which he printed and sold in the streets of boston. his father, however, ridiculed the performance; so he "escaped being a poet." he adopted at this time a somewhat original method to improve his prose writing. meeting with an odd volume of the _spectator_, he purchased it and read it "over and over," and wished to imitate the style. "making short notes of the sentiment in each sentence," he laid them by, and afterwards tried to write out the papers without looking at the original. then on comparison he discovered his faults and corrected them. finding his vocabulary deficient, he turned some of the tales into verse, then retranslated them into prose, believing that the attempt to make verses would necessitate a search for several words of the same meaning. "i also sometimes jumbled my collection of hints into confusion, and after some weeks endeavoured to reduce them into the best order, before i began to form the full sentence and complete the paper. this was to teach me method in the arrangement of my thoughts." meeting with a book on vegetarianism, franklin determined to give the system a trial. this led to some inconvenience in his brother's house-keeping, so franklin proposed to board himself if his brother would give him half the sum he paid for his board, and out of this he was able to save a considerable amount for the purpose of buying books. moreover, the time required for meals was so short that the dinner hour afforded considerable leisure for reading. it was on his journey from boston to philadelphia that he first violated vegetarian principles; for, a large cod having been caught by the sailors, some small fishes were found in its stomach, whereupon franklin argued that if fishes ate one another, there could be no reason against eating them, so he dined on cod during the rest of the journey. after reading xenophon's "memorabilia," franklin took up strongly with the socratic method of discussion, and became so "artful and expert in drawing people, even of superior knowledge, into concessions, the consequence of which they did not foresee," that some time afterwards one of his employers, before answering the most simple question, would frequently ask what he intended to infer from the answer. this practice he gradually gave up, retaining only the habit of expressing his opinions with "modest diffidence." in or james franklin began to print a newspaper, the _new england courant_. to this paper, which he helped to compose and print, benjamin became an anonymous contributor. the members of the staff spoke highly of his contributions, but when the authorship became known, james appears to have conceived a jealousy of his younger brother, which ultimately led to their separation. an article in the paper having offended the assembly, james was imprisoned for a month and forbidden to print the paper. he then freed benjamin from his indentures, in order that the paper might be published in his name. at length, some disagreement arising, benjamin took advantage of the cancelling of his indentures to quit his brother's service. as he could get no employment in boston, he obtained a passage to new york, whence he was recommended to go to philadelphia, which he reached after a very troublesome journey. his whole stock of cash then consisted of a dutch dollar and about a shilling's worth of coppers. the coppers he gave to the boatmen with whom he came across from burlington. his first appearance in philadelphia, about eight o'clock on a sunday morning, was certainly striking. a youth between seventeen and eighteen years of age, dressed in his working clothes, which were dirty through his journey, with his pockets stuffed out with stockings and shirts, his aspect was not calculated to command respect. "then i walked up the street, gazing about till near the market-house i met a boy with bread. i had made many a meal on bread, and, inquiring where he got it, i went immediately to the baker's he directed me to, in second street, and ask'd for bisket, intending such as we had in boston; but they, it seems, were not made in philadelphia. then i asked for a threepenny loaf, and was told they had none such. so, not considering or knowing the difference of money, and the greater cheapness, nor the name of his bread, i bad him give me three-penny-worth of any sort. he gave me, accordingly, three great puffy rolls. i was surpriz'd at the quantity, but took it, and having no room in my pockets, walk'd off with a roll under each arm, and eating the other. thus i went up market street as far as fourth street, passing by the door of mr. read, my future wife's father; when she, standing at the door, saw me, and thought i made, as i certainly did, a most awkward, ridiculous appearance. then i turned and went down chestnut street and part of walnut street, eating my roll all the way, and, coming round, found myself again at market street wharf, near the boat i came in, to which i went for a draught of the river water; and, being filled out with one of my rolls, gave the other two to a woman and her child that came down the river in the boat with us, and were waiting to go further." in philadelphia franklin obtained an introduction, through a gentleman he had met at new york, to a printer, named keimer, who had just set up business with an old press which he appeared not to know how to use, and one pair of cases of english type. here franklin obtained employment when the business on hand would permit, and he put the press in order and worked it. keimer obtained lodgings for him at the house of mr. read, and, by industry and economical living, franklin found himself in easy circumstances. sir william keith was then governor of pennsylvania, and hearing of franklin, he called upon him at keimer's printing-office, invited him to take wine at a neighbouring tavern, and promised to obtain for him the government printing if he would set up for himself. it was then arranged that franklin should return to boston by the first ship, in order to see what help his father would give towards setting him up in business. in the mean while he was frequently invited to dine at the governor's house. notwithstanding sir william keith's recommendation, josiah franklin thought his son too young to take the responsibility of a business, and would only promise to assist him if, when he was twenty-one, he had himself saved sufficient to purchase most of the requisite plant. on his return to philadelphia, he delivered his father's letter to sir william keith, whereon the governor, stating that he was determined to have a good printer there, promised to find the means of equipping the printing-office himself, and suggested the desirability of franklin's making a journey to england in order to purchase the plant. he promised letters of introduction to various persons in england, as well as a letter of credit to furnish the money for the purchase of the printing-plant. these letters franklin was to call for, but there was always some excuse for their not being ready. at last they were to be sent on board the ship, and franklin, having gone on board, awaited the letters. when the governor's despatches came, they were all put into a bag together, and the captain promised to let franklin have his letters before landing. on opening the bag off plymouth, there were no letters of the kind promised, and franklin was left without introductions and almost without money, to make his own way in the world. in london he learned that governor keith was well known as a man in whom no dependence could be placed, and as to his giving a letter of credit, "he had no credit to give." a friend of franklin's, named ralph, accompanied him from america, and the two took lodgings together in little britain at three shillings and sixpence per week. franklin immediately obtained employment at palmer's printing-office, in bartholomew close; but ralph, who knew no trade, but aimed at literature, was unable to get any work. he could not obtain employment, even among the law stationers as a copying clerk, so for some time the wages which franklin earned had to support the two. at palmer's franklin was employed in composing wollaston's "religion of nature." on this he wrote a short critique, which he printed. it was entitled "a dissertation on liberty and necessity, pleasure and pain." the publication of this he afterwards regretted, but it obtained for him introductions to some literary persons in london. subsequently he left palmer's and obtained work at watts's printing-office, where he remained during the rest of his stay in london. the beer-drinking capabilities of some of his fellow-workmen excited his astonishment. he says:-- we had an alehouse boy who attended always in the house to supply the workmen. my companion at the press drank every day a pint before breakfast, a pint at breakfast with his bread and cheese, a pint between breakfast and dinner, a pint at dinner, a pint in the afternoon about six o'clock, and another when he had done his day's work. i thought it a detestable custom, but it was necessary, he suppos'd, to drink _strong_ beer, that he might be _strong_ to labour. i endeavoured to convince him that the bodily strength afforded by beer could only be in proportion to the grain or flour of the barley dissolved in the water of which it was made; that there was more flour in a pennyworth of bread; and therefore, if he would eat that with a pint of water, it would give him more strength than a quart of beer. he drank on, however, and had four or five shillings to pay out of his wages every saturday night for that muddling liquor; an expense i was free from. and thus these poor devils keep themselves always under. afterwards franklin succeeded in persuading several of the compositors to give up "their muddling breakfast of beer and bread and cheese," for a porringer of hot-water gruel, with pepper, breadcrumbs, and butter, which they obtained from a neighbouring house at a cost of three halfpence. among franklin's fellow-passengers from philadelphia to england was an american merchant, a mr. denham, who had formerly been in business in bristol, but failed and compounded with his creditors. he then went to america, where he soon acquired a fortune, and returned in franklin's ship. he invited all his old creditors to dine with him. at the dinner each guest found under his plate a cheque for the balance which had been due to him, with interest to date. this gentleman always remained a firm friend to franklin, who, during his stay in london, sought his advice when any important questions arose. when mr. denham returned to philadelphia with a quantity of merchandise, he offered franklin an appointment as clerk, which was afterwards to develop into a commission agency. the offer was accepted, and, after a voyage of nearly three months, franklin reached philadelphia on october , . here he found governor keith had been superseded by major gordon, and, what was of more importance to him, that miss read, to whom he had become engaged before leaving for england, and to whom he had written only once during his absence, had married. shortly after starting in business, mr. denham died, and thus left franklin to commence life again for himself. keimer had by this time obtained a fairly extensive establishment, and employed a number of hands, but none of them were of much value; and he made overtures to franklin to take the management of his printing-office, apparently with the intention of getting his men taught their business, so that he might afterwards be able to dispense with the manager. franklin set the printing-house in order, started type-founding, made the ink, and, when necessary, executed engravings. as the other hands improved under his superintendence, keimer began to treat his manager less civilly, and apparently desired to curtail his stipend. at length, through an outbreak of temper on the part of keimer, franklin left, but was afterwards induced to return in order to prepare copper-plates and a press for printing paper money for new jersey. while working for keimer, franklin formed a club, which was destined to exert considerable influence on american politics. the club met on friday evenings, and was called the junto. it was essentially a debating society, the subject for each evening's discussion being proposed at the preceding meeting. one of the rules was that the existence of the club should remain a secret, and that its members should be limited to twelve. afterwards other similar clubs were formed by its members; but the existence of the junto was kept a secret from them. the club lasted for about forty years, and became the nucleus of the american philosophical society, of which franklin was the first president. this, and the fact that many of the great questions that arose previously to the declaration of independence were discussed in the junto in the first instance, give to the club a special importance. the following are specimens of subjects discussed by the club:-- "is sound an entity or body?" "how may the phenomena of vapours be explained?" "is self-interest the rudder that steers mankind, the universal monarch to whom all are tributaries?" "which is the best form of government? and what was that form which first prevailed among mankind?" "can any one particular form of government suit all mankind?" "what is the reason that the tides rise higher in the bay of fundy than the bay of delaware?" "is the emission of paper money safe?" "what is the reason that men of the greatest knowledge are not the most happy?" "how may the possessions of the lakes be improved to our advantage?" "why are tumultuous, uneasy sensations united with our desires?" "whether it ought to be the aim of philosophy to eradicate the passions." "how may smoky chimneys be best cured?" "why does the flame of a candle tend upwards in a spire?" "which is least criminal--a bad action joined with a good intention, or a good action with a bad intention?" "is it consistent with the principles of liberty in a free government to punish a man as a libeller when he speaks the truth?" on leaving keimer's, franklin went into partnership with one of his fellow-workmen, hugh meredith, whose father found the necessary capital, and a printing-office was started which soon excelled its two rivals in philadelphia. franklin's industry attracted the attention of the townsfolk, and inspired the merchants with confidence in the prospects of the new concern. keimer started a newspaper, which he had not the ability to carry on; franklin purchased it from him for a trifle, remodelled it, and continued it in a very spirited manner under the title of the _pennsylvania gazette_. his political articles soon attracted the attention of the principal men of the state; the number of subscribers increased rapidly, and the paper became a source of considerable profit. soon after, the printing for the house of representatives came into the hands of the firm. meredith never took to the business, and was seldom sober, and at length was bought out by his partner, on july , . the discussion in the junto on paper currency induced franklin to publish a paper entitled "the nature and necessity of a paper currency." this was a prominent subject before the house, but the introduction of paper money was opposed by the capitalists. they were unable, however, to answer franklin's arguments; the point was carried in the house, and franklin was employed to print the money. the amount of paper money in pennsylvania in amounted to £ , ; during the war it rose to more than £ , . "in order to secure my credit and character as a tradesman, i took care not only to be in _reality_ industrious and frugal, but to avoid all appearances to the contrary. i drest plainly; i was seen at no places of idle diversion. i never went out a-fishing or shooting; a book, indeed, sometimes debauch'd me from my work, but that was seldom, snug, and gave no scandal; and, to show that i was not above my business, i sometimes brought home the paper i purchas'd at the stores thro' the streets on a wheelbarrow. thus being esteem'd an industrious, thriving young man, and paying duly for what i bought, the merchants who imported stationery solicited my custom; others proposed supplying me with books, and i went on swimmingly. in the mean time, keimer's credit and business declining daily, he was at last forc'd to sell his printing-house to satisfy his creditors." on september , , franklin married his former _fiancée_, whose previous husband had left her and was reported to have died in the west indies. the marriage was a very happy one, and continued over forty years, mrs. franklin living until the end of . industry and frugality reigned in the household of the young printer. mrs. franklin not only managed the house, but assisted in the business, folding and stitching pamphlets, and in other ways making herself useful. the first part of franklin's autobiography concludes with an account of the foundation of the first subscription library. by the co-operation of the members of the junto, fifty subscribers were obtained, who each paid in the first instance forty shillings, and afterwards ten shillings per annum. "we afterwards obtained a charter, the company being increased to one hundred. this was the mother of all the north american subscription libraries, now so numerous. it is become a great thing itself, and continually increasing. these libraries have improved the general conversation of the americans, made the common tradesmen and farmers as intelligent as most gentlemen from other countries, and perhaps have contributed in some degree to the stand so generally made throughout the colonies in defence of their privileges." ten years ago this library contained between seventy and eighty thousand volumes. franklin's success in business was attributed by him largely to his early training. "my circumstances, however, grew daily easier. my original habits of frugality continuing, and my father having, among his instructions to me when a boy, frequently repeated a proverb of solomon, 'seest thou a man diligent in his business? he shall stand before kings; he shall not stand before mean men,' i from thence considered industry as a means of obtaining wealth and distinction, which encourag'd me, tho' i did not think that i should ever literally _stand before kings_, which, however, has since happened; for i have stood before _five_, and even had the honour of sitting down with one, the king of denmark, to dinner." after his marriage, franklin conceived the idea of obtaining moral perfection. he was not altogether satisfied with the result, but thought his method worthy of imitation. assuming that he possessed complete knowledge of what was right or wrong, he saw no reason why he should not always act in accordance therewith. his principle was to devote his attention to one virtue only at first for a week, at the end of which time he expected the practice of that virtue to have become a habit. he then added another virtue to his list, and devoted his attention to the same for the next week, and so on, until he had exhausted his list of virtues. he then commenced again at the beginning. as his moral code comprised thirteen virtues, it was possible to go through the complete curriculum four times in a year. afterwards he occupied a year in going once through the list, and subsequently employed several years in one course. a little book was ruled, with a column for each day and a line for each virtue, and in this a mark was made for every failure which could be remembered on examination at the end of the day. it is easy to believe his statement: "i am surprised to find myself so much fuller of faults than i had imagined; but i had the satisfaction of seeing them diminish." "this my little book had for its motto these lines from addison's 'cato':-- "'here will i hold. if there's a power above us (and that there is, all nature cries aloud thro' all her work), he must delight in virtue; and that which he delights in must be happy.' "another from cicero:-- "'o vitæ philosophia dux! o virtutum indagatrix expultrixque vitiorum! unus dies ex præceptis tuis actus, peccanti immortalitati est anteponendus.' "another from the proverbs of solomon, speaking of wisdom and virtue:-- "'length of days is in her right hand; and in her left hand riches and honour. her ways are ways of pleasantness, and all her paths are peace.' "and conceiving god to be the fountain of wisdom, i thought it right and necessary to solicit his assistance for obtaining it; to this end i formed the following little prayer, which was prefixed to my tables of examination, for daily use:-- "'o powerful goodness! bountiful father! merciful guide! increase in me that wisdom which discovers my truest interest. strengthen my resolutions to perform what that wisdom dictates. accept my kind offices to thy other children as the only return in my power for thy continual favours to me.' "i used also sometimes a little prayer which i took from thomson's poems, viz.:-- "'father of light and life, thou good supreme! oh teach me what is good; teach me thyself! save me from folly, vanity, and vice, from every low pursuit; and fill my soul with knowledge, conscious peace, and virtue pure; sacred, substantial, never-failing bliss!'" the senses in which franklin's thirteen virtues were to be understood were explained by short precepts which followed them in his list. the list was as follows:-- " . temperance. "eat not to dulness; drink not to elevation. " . silence. "speak not but what may benefit others or yourself; avoid trifling conversation. " . order. "let all your things have their places; let each part of your business have its time. " . resolution. "resolve to perform what you ought; perform without fail what you resolve. " . frugality. "make no expense but to do good to others or yourself; _i.e._ waste nothing. " . industry. "lose no time; be always employed in something useful; cut off all unnecessary actions. " . sincerity. "use no hurtful deceit; think innocently and justly; and, if you speak, speak accordingly. " . justice. "wrong none by doing injuries, or omitting the benefits that are your duty. " . moderation. "avoid extremes; forbear resenting injuries so much as you think they deserve. " . cleanliness. "tolerate no uncleanness in body, clothes, or habitation. " . tranquillity. "be not disturbed at trifles, or accidents common or unavoidable. " . chastity. " . humility. "imitate jesus and socrates." the last of these was added to the list at the suggestion of a quaker friend. franklin claims to have acquired a good deal of the _appearance_ of it, but concluded that in reality there was no passion so hard to subdue as _pride_. "for even if i could conceive that i had completely overcome it, i should probably be proud of my humility." the virtue which gave him most trouble, however, was order, and this he never acquired. in appeared the first copy of "poor richard's almanack." this was prepared, printed, and published by franklin for about twenty-five years in succession, and nearly ten thousand copies were sold annually. besides the usual astronomical information, it contained a collection of entertaining anecdotes, verses, jests, etc., while the "little spaces that occurred between the remarkable events in the calendar" were filled with proverbial sayings, inculcating industry and frugality as helps to virtue. these sayings were collected and prefixed to the almanack of , whence they were copied into the american newspapers, and afterwards reprinted as a broad-sheet in england and in france. in franklin commenced studying modern languages, and acquired sufficient knowledge of french, italian, and spanish to be able to read books in those languages. in he was chosen clerk to the general assembly, an office to which he was annually re-elected until he became a member of the assembly about . there was one member who, on the second occasion of his election, made a long speech against him. franklin determined to secure the friendship of this member. accordingly he wrote to him to request the loan of a very scarce and curious book which was in his library. the book was lent and returned in about a week, with a note of thanks. the member ever after manifested a readiness to serve franklin, and they became great friends--"another instance of the truth of an old maxim i had learned, which says, '_he that has once done you a kindness will be more ready to do you another than he whom you yourself have obliged_.' and it shows how much more profitable it is prudently to remove, than to resent, return, and continue inimical proceedings." in franklin was appointed deputy-postmaster-general for pennsylvania. he was afterwards made postmaster-general of the colonies. he read a paper in the junto on the organization of the city watch, and the propriety of rating the inhabitants on the value of their premises in order to support the same. the subject was also discussed in the other clubs which had sprung from the junto, and thus the way was prepared for the law which a few years afterwards carried franklin's proposals into effect. his next scheme was the formation of a fire brigade, in which he met with his usual success, and other clubs followed, until most of the men of property in the city were members of one club or another. the original brigade, known as the union fire company, was formed december , . it was in active service in . franklin founded the american philosophical society in . the head-quarters of the society were fixed in philadelphia, where it was arranged that there should always be at least seven members, viz. a physician, a botanist, a mathematician, a chemist, a mechanician, a geographer, and a general natural philosopher, besides a president, treasurer, and secretary. the other members might be resident in any part of america. correspondence was to be kept up with the royal society of london and the dublin society, and abstracts of the communications were to be sent quarterly to all the members. franklin became the first secretary. spain, having been for some years at war with england, was joined at length by france. this threatened danger to the american colonies, as france then held canada, and no organization for their defence existed. franklin published a pamphlet entitled "plain truth," setting forth the unarmed condition of the colonies, and recommending the formation of a volunteer force for defensive purposes. the pamphlet excited much attention. a public meeting was held and addressed by franklin; at this meeting twelve hundred joined the association. at length the number of members enrolled exceeded ten thousand. these all provided themselves with arms, formed regiments and companies, elected their own officers, and attended once a week for military drill. franklin was elected colonel of the philadelphia regiment, but declined the appointment, and served as a private soldier. the provision of war material was a difficulty with the assembly, which consisted largely of quakers, who, though they appeared privately to be willing that the country should be put in a state of defence, hesitated to vote in opposition to their peace principles. hence it was that, when the government of new england asked a grant of gunpowder from pennsylvania, the assembly voted £ "for the purchasing of bread, flour, wheat, or _other grain_." pebble-powder was not then in use. when it was proposed to devote £ , which was a balance in the hands of the union fire company, as a contribution towards the erection of a battery below the town, franklin suggested that it should be proposed that a fire-engine be purchased with the money, and that the committee should "buy a great gun, which is certainly a _fire-engine_." the "pennsylvania fireplace" was invented in . a patent was offered to franklin by the governor of pennsylvania, but he declined it on the principle "_that, as we enjoy great advantages from the inventions of others, we should be glad of an opportunity to serve others by any invention of ours; and this we should do freely and generously_." an ironmonger in london made slight alterations, which were not improvements, in the design, and took out a patent for the fireplace, whereby he made a "small fortune." franklin never contested the patent, "having no desire of profiting by patents himself," and "hating disputes." this fireplace was designed to burn wood, but, unlike the german stoves, it was completely open in front, though enclosed at the sides and top. an air-chamber was formed in the middle of the stove, so arranged that, while the burning wood was in contact with the front of the chamber, the flame passed above and behind it on its way to the flue. through this chamber a constant current of air passed, entering the room heated, but not contaminated, by the products of combustion. in this way the stove furnished a constant supply of fresh warm air to the room, while it possessed all the advantages of an open fireplace. subsequently franklin contrived a special fireplace for the combustion of coal. in the scientific thought which he devoted to the requirements of the domestic economist, as in very many other particulars, franklin strongly reminds us of count rumford. the next important enterprise which franklin undertook, partly through the medium of the junto, was to establish an academy which soon developed into the university of philadelphia. the members of the club having taken up the subject, the next step was to enlist the sympathy of a wider constituency, and this franklin effected, in his usual way, by the publication of a pamphlet. he then set on foot a subscription, the payments to extend over five years, and thereby obtained about £ . a house was taken and schools opened in . the classes soon became too large for the house, and the trustees of the academy then took over a large building, or "tabernacle," which had been erected for george whitefield when he was preaching in philadelphia. the hall was divided into stories, and at a very small expense adapted to the requirements of the classes. franklin, having taken a partner in his printing business, took the oversight of the work. afterwards the funds were increased by english subscriptions, by a grant from the assembly, and by gifts of land from the proprietaries; and thus was established the university of philadelphia. having practically retired from business, franklin intended to devote himself to philosophical studies, having commenced his electrical researches some time before in conjunction with the other members of the library company. public business, however, crowded upon him. he was elected a member of the assembly, a councillor and afterwards an alderman of the city, and by the governor was made a justice of the peace. as a member of the assembly, he was largely concerned in providing the means for the erection of a hospital, and in arranging for the paving and cleansing of the streets of the city. in he was appointed, in conjunction with mr. hunter, postmaster-general of america. the post-office of the colonies had previously been conducted at a loss. in a few years, under franklin's management, it not only paid the stipends of himself and mr. hunter, but yielded a considerable revenue to the crown. but it was not only in the conduct of public business that franklin's merits were recognized. by this time he had secured his reputation as an electrician, and both yale college and cambridge university (new england) conferred on him the honorary degree of master of arts. in the same year that he was made postmaster-general of america he was awarded the copley medal and elected a fellow of the royal society of london, the usual fees being remitted in his case. before his election as member, franklin had for several years held the appointment of clerk to the assembly, and he used to relieve the dulness of the debates by amusing himself in the construction of magic circles and squares, and "acquired such a knack at it" that he could "fill the cells of any magic square of reasonable size with a series of numbers as fast as" he "could write them." many years afterwards mr. logan showed franklin a french folio volume filled with magic squares, and afterwards a magic "square of ," which mr. logan thought must have been a work of great labour, though it possessed only the common properties of making in every row, horizontal, vertical, and diagonal. during the evening franklin made the square shown on the opposite page. "this i sent to our friend the next morning, who, after some days, sent it back in a letter, with these words: 'i return to thee thy astonishing and most stupendous piece of the magical square, in which----;' but the compliment is too extravagant, and therefore, for his sake as well as my own, i ought not to repeat it. nor is it necessary; for i make no question that you will readily allow this square of to be the most magically magical of any magic square ever made by any magician." the square has the following properties:--every straight row of sixteen numbers, whether vertical, horizontal, or diagonal, makes . every bent row of sixteen numbers, as shown by the diagonal lines in the figure, makes . if a square hole be cut in a piece of paper, so as to show through it just sixteen of the little squares, and the paper be laid on the magic square, then, wherever the paper is placed, the sum of the sixteen numbers visible through the hole will be . [illustration: ] in war with france appeared to be again imminent, and a congress of commissioners from the several colonies was arranged for. of course, franklin was one of the representatives of pennsylvania, and was also one of the members who independently drew up a plan for the union of all the colonies under one government, for defensive and other general purposes, and his was the plan finally approved by congress for the union, though it was not accepted by the assemblies or by the english government, being regarded by the former as having too much of the _prerogative_ in it, by the latter as being too _democratic_. franklin wrote respecting this scheme: "the different and contrary reasons of dislike to my plan makes me suspect that it was really the true medium; and i am still of opinion that it would have been happy for both sides of the water if it had been adopted. the colonies, so united, would have been sufficiently strong to have defended themselves; there would then have been no need of troops from england; of course, the subsequent pretence for taxing america, and the bloody contest it occasioned, would have been avoided." with this war against france began the struggle of the assemblies and the proprietaries on the question of taxing the estates of the latter. the governors received strict instructions to approve no bills for the raising of money for the purposes of defence, unless the estates of the proprietaries were specially exempted from the tax. the assembly of pennsylvania resolved to contribute £ , to assist the government of massachusetts bay in an attack upon crown point, but the governor refused his assent to the bill for raising the money. at this juncture franklin proposed a scheme by which the money could be raised without the consent of the governor. his plan was successful, and the difficulty was surmounted for the time, but was destined to recur again and again during the progress of the war. the british government, not approving of the scheme of union, whereby the colonies might have defended themselves, sent general braddock to virginia, with two regiments of regular troops. on their arrival they found it impossible to obtain waggons for the conveyance of their baggage, and the general commissioned franklin to provide them in pennsylvania. by giving his private bond for their safety, franklin succeeded in engaging one hundred and fifty four-horse waggons, and two hundred and fifty-nine pack-horses. his modest warnings against indian ambuscades were disregarded by the general, the little army was cut to pieces, and the remainder took to flight, sacrificing the whole of their baggage and stores. franklin was never fully recouped by the british government for the payments he had to make on account of provisions which the general had instructed him to procure for the use of the army. after this, franklin appeared for some time in a purely military capacity, having yielded to the governor's persuasions to undertake the defence of the north-western frontier, to raise troops, and to build a line of forts. after building and manning three wooden forts, he was recalled by the assembly, whose relations with the governor had become more and more strained. at length the assembly determined to send franklin to england, to present a petition to the king respecting the conduct of the proprietaries, viz. richard and thomas penn, the successors of william penn. a bill had been framed by the house to provide £ , for the king's use in the defence of the province. this the governor refused to pass, because the proprietary estates were not exempted from the taxation. the petition to the king was drawn up, and franklin's baggage was on board the ship which was to convey him to england, when general lord loudon endeavoured to make an arrangement between the parties. the governor pleaded his instructions, and the bond he had given for carrying them out, and the assembly was prevailed upon to reconstruct the bill in accordance with the governor's wishes. this was done under protest; in the mean time franklin's ship had sailed, carrying his baggage. after a great deal of unnecessary delay on account of the general's inability to decide upon the despatch of the packet-boats, franklin at last got away from new york, and, having narrowly escaped shipwreck off falmouth, he reached london on july , . on arriving in london, franklin was introduced to lord granville, who told him that the king's instructions were laws in the colonies. franklin replied that he had always understood that the assemblies made the laws, which then only required the king's consent. "i recollected that, about twenty years before, a clause in a bill brought into parliament by the ministry had proposed to make the king's instructions laws in the colonies, but the clause was thrown out by the commons, for which we adored them as our friends and the friends of liberty, till, by their conduct towards us in , it seem'd that they had refus'd that point of sovereignty to the king only that they might reserve it for themselves." a meeting was shortly afterwards arranged between franklin and the proprietaries at mr. t. penn's house; but their views were so discordant that, after some discussion, franklin was requested to give them in writing the heads of his complaints, and the whole question was submitted to the opinion of the attorney- and solicitor-general. it was nearly a year before this opinion was given. the proprietaries then communicated directly with the assembly, but in the mean while governor denny had consented to a bill for raising £ , for the king's use, in which it was provided that the proprietary estates should be taxed with the others. when this bill reached england, the proprietaries determined to oppose its receiving the royal assent. franklin engaged counsel on behalf of the assembly, and on his undertaking that the assessment should be fairly made between the estates of the proprietaries and others, the bill was allowed to pass. by this time franklin's career as a scientific investigator was practically at an end. political business almost completely occupied his attention, and in one sense the diplomatist replaced the philosopher. his public scientific career was of short duration. it may be said to have begun in , when mr. peter collinson presented an "electrical tube" to the library company in philadelphia, which was some time after followed by a present of a complete set of electrical apparatus from the proprietaries, but by franklin's time was so much taken up by public business that there was very little opportunity for experimental work. throughout his life he frequently expressed in his letters his strong desire to return to philosophy, but the opportunity never came, and when, at the age of eighty-two, he was liberated from public duty, his strength was insufficient to enable him to complete even his autobiography. it was on a visit to boston in that franklin met with dr. spence, a scotchman, who exhibited some electrical experiments. soon after his return to philadelphia the tube arrived from mr. collinson, and franklin acquired considerable dexterity in its use. his house was continually full of visitors, who came to see the experiments, and, to relieve the pressure upon his time, he had a number of similar tubes blown at the glass-house, and these he distributed to his friends, so that there were soon a number of "performers" in philadelphia. one of these was mr. kinnersley, who, having no other employment, was induced by franklin to become an itinerant lecturer. franklin drew up a scheme for the lectures, and kinnersley obtained several well-constructed instruments from franklin's rough and home-made models. kinnersley and franklin appear to have worked together a good deal, and when kinnersley was travelling on his lecture tour, each communicated to the other the results of his experiments. franklin sent his papers to mr. collinson, who presented them to the royal society, but they were not at first judged worthy of a place in the "transactions." the paper on the identity of lightning and electricity was sent to dr. mitchell, who read it before the royal society, when it "was laughed at by the connoisseurs." the papers were subsequently published in a pamphlet, but did not at first receive much attention in england. on the recommendation of count de buffon, they were translated into french. the abbé nollet, who had previously published a theory of his own respecting electricity, wrote and published a volume of letters defending his theory, and denying the accuracy of some of franklin's experimental results. to these letters franklin made no reply, but they were answered by m. le roy. m. de lor undertook to repeat in paris all franklin's experiments, and they were performed before the king and court. not content with the experiments which franklin had actually performed, he tried those which had been only suggested, and so was the first to obtain electricity from the clouds by means of the pointed rod. this experiment produced a great sensation everywhere, and was afterwards repeated by franklin at philadelphia. franklin's papers were translated into italian, german, and latin; his theory met with all but universal acceptance, and great surprise was expressed that his papers had excited so little interest in england. dr. watson then drew up a summary of all franklin's papers, and this was published in the "philosophical transactions;" mr. canton verified the experiment of procuring electricity from the clouds by means of a pointed rod, and the royal society awarded to franklin the copley medal for , which was conveyed to him by governor denny. we must now give a short account of franklin's contributions to electrical science. "the first is the wonderful effect of pointed bodies, both in _drawing off_ and _throwing off_ the electrical fire." it will be observed that this statement is made in the language of the _one_-fluid theory, of which franklin may be regarded as the author. this theory will be again referred to presently. franklin electrified a cannon-ball so that it repelled a cork. on bringing near it the point of a bodkin, the repulsion disappeared. a blunt body had to be brought near enough for a spark to pass in order to produce the same effect. "to prove that the electrical fire is _drawn off_ by the point, if you take the blade of the bodkin out of the wooden handle, and fix it in a stick of sealing-wax, and then present it at the distance aforesaid, or if you bring it very near, no such effect follows; but sliding one finger along the wax till you touch the blade, and the ball flies to the shot immediately. if you present the point in the dark, you will see, sometimes at a foot distance or more, a light gather upon it like that of a fire-fly or glow-worm; the less sharp the point, the nearer you must bring it to observe the light; and at whatever distance you see the light, you may draw off the electrical fire, and destroy the repelling." by laying a needle upon the shot, franklin showed "that points will _throw off_ as well as _draw off_ the electrical fire." a candle-flame was found to be equally efficient with a sharp point in drawing off the electricity from a charged conductor. the effect of the candle-flame franklin accounted for by supposing the particles separated from the candle to be first "attracted and then repelled, carrying off the electric matter with them." the effect of points is a direct consequence of the law of electrical repulsion. when a conductor is electrified, the density of the electricity is greatest where the curvature is greatest. thus, if a number of spheres are electrified from the same source, the density of the electricity on the different spheres will vary inversely as their diameters. the force tending to drive the electricity off a conductor is everywhere proportional to the density, and hence in the case of the spheres will be greatest for the smallest sphere. on this principle, the density of electricity on a perfectly sharp point, if such could exist, on a charged conductor, would be infinite and the force tending to drive it off would be infinite also. hence a moderately sharp point is sufficient to dissipate the electricity from a highly charged conductor, or to neutralize it if the point is connected to earth and brought near the conductor so as to be electrified by induction. franklin next found that, if the person rubbing the electric tube stood upon a cake of resin, and the person taking the charge from the tube stood also on an insulating stand, a stronger spark would pass between these two persons than between either of them and the earth; that, after the spark had passed, neither person was electrified, though each had appeared electrified before. these experiments suggested the idea of _positive_ and _negative_ electrification; and franklin, regarding the electric fluid as corresponding to positive electrification, remarked that "you may circulate it as mr. watson has shown; you may also accumulate or subtract it upon or from any body, as you connect that body with the rubber or with the receiver, the common stock being cut off." thus franklin regarded electricity as a fluid, of which everything in its normal state possesses a certain amount; that, by appropriate means, some of the fluid may be removed from one body and given to another. the former is then electrified negatively, the latter positively, and all processes by which bodies are electrified consist in the removal of electricity from one body or system and giving it to another. he regarded the electric fluid as repelling itself and attracting matter. Æpinus afterwards added the supposition that matter, when devoid of electricity, is self-repulsive, and thus completed the "one-fluid theory," and accounted for the repulsion observed between negatively electrified bodies. it had been usual to employ water for the interior armatures of leyden jars, or phials, as they were then generally called. franklin substituted granulated lead for the water, thereby improving the insulation by keeping the glass dry. with these phials he contrived many ingenious experiments, and imitated lightning by discharging them through the gilding of a mirror or the gold lines on the cover of a book. he found that the inner and outer armatures of his leyden jars were oppositely electrified. "here we have a bottle containing at the same time a _plenum_ of electrical fire and a _vacuum_ of the same fire; and yet the equilibrium cannot be restored between them but by a communication _without_! though the plenum presses violently to expand, and the hungry vacuum seems to attract as violently in order to be filled." the charging of leyden jars by cascade, that is by insulating all the jars except the last, connecting the outer armature of the first with the inner armature of the second, and so on throughout the series, was well understood by franklin, and he knew too that by this method the extent to which each jar could be charged from a given source varied inversely as the number of jars. the discharge of the leyden jar by alternate contacts was also carried out by him; and he found that, if the jar is first placed on an insulating stand, it may be held by the hook (or knob) without discharging it. franklin, in fact, appears to have known almost as much about the leyden jar as is known to-day. he found that, when the armatures were removed from a jar, no discharge would pass between them, but when a fresh pair of armatures were supplied to the glass, the jar could be discharged. "we are of opinion that there is really no more electrical fire in the phial after what is called its _charging_ than before, nor less after its _discharging_; excepting only the small spark that might be given to and taken from the non-electric matter, if separated from the bottle, which spark may not be equal to a five-hundredth part of what is called the explosion. "the phial will not suffer what is called a _charging_ unless as much fire can go out of it one way as is thrown in by another. "when a bottle is charged in the common way, its _inside_ and _outside_ surfaces stand ready, the one to give fire by the hook, the other to receive it by the coating; the one is full and ready to throw out, the other empty and extremely hungry; yet, as the first will not _give out_ unless the other can at the same time _receive in_, so neither will the latter receive in unless the first can at the same time give out. when both can be done at once, it is done with inconceivable quickness and violence." then follows a very beautiful illustration of the condition of the glass in the leyden jar. "so a straight spring (though the comparison does not agree in every particular), when forcibly bent, must, to restore itself, contract that side which in the bending was extended, and extend that which was contracted; if either of these two operations be hindered, the other cannot be done. "glass, in like manner, has, within its substance, always the same quantity of electrical fire, and that a very great quantity in proportion to the mass of the glass, as shall be shown hereafter. "this quantity proportioned to the glass it strongly and obstinately retains, and will have neither more nor less, though it will suffer a change to be made in its parts and situation; _i.e._ we may take away part of it from one of the sides, provided we throw an equal quantity into the other." "the whole force of the bottle, and power of giving a shock, is in the glass itself; the non-electrics in contact with the two surfaces serving only to _give_ and _receive_ to and from the several parts of the glass, that is, to give on one side and take away from the other." all these statements were, as far as possible, fully substantiated by experiment. they are perfectly consistent with the views held by cavendish and by clerk maxwell, and, though the phraseology is not that of the modern text-books, the statements themselves can hardly be improved upon to-day. one of franklin's early contrivances was an electro-motor, which was driven by the alternate electrical attraction and repulsion of leaden bullets which discharged leyden jars by alternate contacts. franklin concluded his account of these experiments as follows:-- chagrined a little that we have been hitherto able to produce nothing in this way of use to mankind, and the hot weather coming on, when electrical experiments are not so agreeable, it is proposed to put an end to them for this season, somewhat humorously, in a party of pleasure, on the banks of skuylkil. spirits, at the same time, are to be fired by a spark sent from side to side through the river, without any other conductor than the water--an experiment which we some time since performed, to the amazement of many. a turkey is to be killed for our dinner by the _electrical shock_, and roasted by the _electrical jack_ before a fire kindled by the _electrified bottle_, when the healths of all the famous electricians in england, holland, france, and germany, are to be drunk in _electrified bumpers_, under the discharge of guns from the _electrical battery_. franklin's electrical battery consisted of eleven large panes of glass coated on each side with sheet lead. the electrified bumper was a thin tumbler nearly filled with wine and electrified as a leyden jar, so as to give a shock through the lips. franklin's theory of the manner in which thunder-clouds become electrified he found to be not consistent with his subsequent experiments. in the paper which he wrote explaining this theory, however, he shows some knowledge of the effects of bringing conductors into contact in diminishing their capacity. he states that two gun-barrels electrified equally and then united, will give a spark at a greater distance than one alone. hence he asks, "to what a great distance may ten thousand acres of electrified cloud strike and give its fire, and how loud must be that crack? "an electrical spark, drawn from an irregular body at some distance, is scarcely ever straight, but shows crooked and waving in the air. so do the flashes of lightning, the clouds being very irregular bodies. "as electrified clouds pass over a country, high hills and high trees, lofty towers, spires, masts of ships, chimneys, etc., as so many prominences and points, draw the electrical fire, and the whole cloud discharges there. "dangerous, therefore, is it to take shelter under a tree during a thunder-gust. it has been fatal to many, both men and beasts. "it is safer to be in the open field for another reason. when the clothes are wet, if a flash in its way to the ground should strike your head, it may run in the water over the surface of your body; whereas, if your clothes were dry, it would go through the body, because the blood and other humours, containing so much water, are more ready conductors. "hence a wet rat cannot be killed by the exploding electrical bottle [a quart jar], while a dry rat may." in the above quotations we see, so to speak, the germ of the lightning-rod. this was developed in a letter addressed to mr. collinson, and dated july , . the following quotations will give an idea of its contents:-- "the electrical matter consists of particles extremely subtile, since it can permeate common matter, even the densest metals, with such ease and freedom as not to receive any perceptible resistance.[ ] [footnote : franklin was aware of the resistance of conductors (see p. ).] "if any one should doubt whether the electrical matter passes through the substance of bodies or only over and along their surfaces, a shock from an electrified large glass jar, taken through his own body, will probably convince him. "common matter is a kind of sponge to the electrical fluid. "we know that the electrical fluid is _in_ common matter, because we can pump it _out_ by the globe or tube. we know that common matter has near as much as it can contain, because when we add a little more to any portion of it, the additional quantity does not enter, but forms an electrical atmosphere." to illustrate the action of a lightning-conductor on a thunder-cloud, franklin suspended from the ceiling a pair of scales by a twisted string so that the beam revolved. upon the floor, in such a position that the scale-pans passed over it, he placed a blunt steel punch. the scale-pans were suspended by silk threads, and one of them electrified. when this passed over the punch it dipped towards it, and sometimes discharged into it by a spark. when a needle was placed with its point uppermost by the side of the punch, no attraction was apparent, for the needle discharged the scale-pan before it came near. "now, if the fire of electricity and that of lightning be the same, as i have endeavoured to show at large in a former paper ... these scales may represent electrified clouds.... the horizontal motion of the scales over the floor may represent the motion of the clouds over the earth, and the erect iron punch a hill or high building; and then we see how electrified clouds, passing over hills or high buildings at too great a height to strike, may be attracted lower till within their striking distance; and lastly, if a needle fixed on the punch, with its point upright, or even on the floor below the punch, will draw the fire from the scale silently at a much greater than the striking distance, and so prevent its descending towards the punch; or if in its course it would have come nigh enough to strike, yet, being first deprived of its fire, it cannot, and the punch is thereby secured from its stroke;--i say, if these things are so, may not the knowledge of this power of points be of use to mankind, in preserving houses, churches, ships, etc., from the stroke of the lightning, by directing us to fix, on the highest parts of those edifices, upright rods of iron made sharp as a needle, and gilt to prevent rusting, and from the foot of those rods a wire down the outside of the building into the ground, or down round one of the shrouds of a ship, and down her side till it reaches the water? would not these pointed rods probably draw the electrical fire silently out of a cloud before it came nigh enough to strike, and thereby secure us from that most sudden and terrible mischief?" franklin goes on to suggest the possibility of obtaining electricity from the clouds by means of a pointed rod fixed on the top of a high building and insulated. such a rod he afterwards erected in his own house. another rod connected to the earth he brought within six inches of it, and, attaching a small bell to each rod, he suspended a little ball or clapper by a silk thread, so that it could strike either bell when attracted to it. on the approach of a thunder-cloud, and occasionally when no clouds were near, the bells would ring, indicating that the rod had become strongly electrified. on one occasion franklin was disturbed by a loud noise, and, coming out of his bedroom, he found an apparently continuous and very luminous discharge taking place between the bells, forming a stream of fire about as large as a pencil. a very pretty experiment of franklin's was that of the _golden fish_. a small piece of gold-leaf is cut into a quadrilateral having one of its angles about °, the opposite angle about °, and the other two right angles. "if you take it by the tail, and hold it at a foot or greater horizontal distance from the prime conductor, it will, when let go, fly to it with a brisk but wavering motion, like that of an eel through the water; it will then take place under the prime conductor, at perhaps a quarter or half an inch distance, and keep a continual shaking of its tail like a fish, so that it seems animated. turn its tail towards the prime conductor, and then it flies to your finger, and seems to nibble it. and if you hold a [pewter] plate under it at six or eight inches distance, and cease turning the globe, when the electrical atmosphere of the conductor grows small it will descend to the plate and swim back again several times with the same fish-like motion; greatly to the entertainment of spectators. by a little practice in blunting or sharpening the heads or tails of these figures, you may make them take place as desired, nearer or further from the electrified plate." by the discharge of the battery, franklin succeeded in melting and volatilizing gold-leaf, thin strips of tinfoil, etc. his views on the nature of light are best given in his own words. "i am not satisfied with the doctrine that supposes particles of matter called light, continually driven off from the sun's surface, with a swiftness so prodigious! must not the smallest particle conceivable have, with such a motion, a force exceeding that of a twenty-four pounder discharged from a cannon?... yet these particles, with this amazing motion, will not drive before them, or remove, the least and lightest dust they meet with. "may not all the phenomena of light be more conveniently solved by supposing universal space filled with a subtile elastic fluid, which, when at rest, is not visible, but whose vibrations affect that fine sense in the eye, as those of air do the grosser organs of the ear? we do not, in the case of sound, imagine that any sonorous particles are thrown off from a bell, for instance, and fly in straight lines to the ear; why must we believe that luminous particles leave the sun and proceed to the eye? some diamonds, if rubbed, shine in the dark without losing any part of their matter. i can make an electrical spark as big as the flame of a candle, much brighter, and therefore visible further; yet this is without fuel; and i am persuaded no part of the electrical fluid flies off in such case to distant places, but all goes directly and is to be found in the place to which i destine it. may not different degrees of the vibration of the abovementioned universal medium occasion the appearances of different colours? i think the electric fluid is always the same; yet i find that weaker and stronger sparks differ in apparent colour, some white, blue, purple, red: the strongest, white; weak ones, red. thus different degrees of vibration given to the air produce the seven different sounds in music, analogous to the seven colours, yet the medium, air, is the same." mr. kinnersley having called franklin's attention to the fact that a sulphur globe when rubbed produced electrification of an opposite kind from that produced by a glass globe, franklin repeated the experiment, and noticed that the discharge from the end of a wire connected with the conductor was different in the two cases, being "long, large, and much diverging when the glass globe is used, and makes a snapping (or rattling) noise; but when the sulphur one is used it is short, small, and makes a hissing noise; and just the reverse of both happens when you hold the same wire in your hand and the globes are worked alternately.... when the brush is long, large, and much diverging, the body to which it is joined seems to be throwing the fire out; and when the contrary appears it seems to be drinking in." on october , , franklin wrote to mr. peter collinson as follows:-- as frequent mention is made in public papers from europe of the success of the philadelphia experiment for drawing the electric fire from clouds by means of pointed rods of iron erected on high buildings, etc., it may be agreeable to the curious to be informed that the same experiment has succeeded in philadelphia, though made in a different and more easy manner, which is as follows:-- make a small cross of two light strips of cedar, the arms so long as to reach to the four corners of a large thin silk handkerchief when extended. tie the corners of the handkerchief to the extremities of the cross, so you have the body of a kite; which, being properly accommodated with a tail, loop, and string, will rise in the air like those made of paper; but this being of silk is fitter to bear the wet and wind of a thunder-gust without tearing. to the top of the upright stick of the cross is to be fixed a very sharp-pointed wire, rising a foot or more above the wood. to the end of the twine, next the hand, is to be tied a silk ribbon, and, where the silk and twine join, a key may be fastened. this kite is to be raised when a thunder-gust appears to be coming on, and the person who holds the string must stand within a door or window, or under some cover so that the silk ribbon may not be wet, and care must be taken that the twine does not touch the frame of the door or window. as soon as any of the thunder-clouds come over the kite, the pointed wire will draw the electric fire from them, and the kite, with all the twine, will be electrified, and the loose filaments of the twine will stand out every way, and be attracted by an approaching finger. and when the rain has wetted the kite and twine so that it can conduct the electric fire freely, you will find it stream out plentifully from the key on the approach of your knuckle. at this key the phial may be charged, and from electric fire there obtained spirits may be kindled, and all the other electric experiments be performed which are usually done by the help of a rubbed glass globe or tube, and thereby the sameness of the electric matter with that of lightning completely demonstrated. having, in september, , erected the iron rod and bells in his own house, as previously mentioned, franklin succeeded, in april, , in charging a leyden jar from the rod, and found its charge was negative. on june , however, he obtained a positive charge from a cloud. the results of his observations led him to the conclusion "_that the clouds of a thunder-gust are most commonly in a negative state of electricity, but sometimes in a positive state._" in order to illustrate a theory respecting the electrification of clouds, franklin placed a silver can on a wine-glass. inside the can was placed a considerable length of chain, which could be drawn out by means of a silk thread. he electrified the can from a leyden jar until it would receive no more electricity. then raising the silk thread, he gradually drew the chain out of the can, and found that the greater the length of chain drawn out the greater was the charge which the jar would give to the system, and as the chain was raised, spark after spark passed from the jar to the silver can, thus showing that the capacity of the system was increased by increasing the amount of chain exposed. in franklin observed the effects of induction; for, having attached to his prime conductor a tassel made of damp threads and electrified the conductor, he found that the threads repelled each other and stood out. bringing an excited glass tube near the other end of the conductor, the threads were found to diverge more, "because the atmosphere of the prime conductor is pressed by the atmosphere of the excited tube, and driven towards the end where the threads are, by which each thread acquires more atmosphere." when the excited tube was brought near the threads, they closed a little, "because the atmosphere of the glass tube repels their atmospheres, and drives part of them back on the prime conductor." a number of other experiments illustrating electrical induction were also carried out. in writing to dr. living, of charlestown, under date march , , franklin gave the following extracts of the minutes of his experiments as explaining the train of thought which led him to attempt to obtain electricity from the clouds:-- "_november , ._ electrical fluid agrees with lightning in these particulars: . giving light. . colour of the light. . crooked direction. . swift motion. . being conducted by metals. . crack or noise in exploding. . subsisting in water or ice. . rending bodies it passes through. . destroying animals. . melting metals. . firing inflammable substances. . sulphureous smell. the electric fluid is attracted by points. we do not know whether this property is in lightning. but since they agree in all the particulars wherein we can already compare them, is it not probable they agree likewise in this? let the experiment be made." another experiment very important in its bearing on the theory of electricity was described by franklin in the same letter to dr. living. it was afterwards repeated in a much more complete form by cavendish, who deduced from it the great law that electrical repulsion varies inversely as the square of the distance between the charges. the same experiment was repeated in other forms by faraday, who had no means of knowing what cavendish had done. franklin writes:-- i electrified a silver fruit-can on an electric stand, and then lowered into it a cork ball of about an inch in diameter, hanging by a silk string, till the cork touched the bottom of the can. the cork was not attracted to the inside of the can, as it would have been to the outside, and though it touched the bottom, yet, when drawn out, it was not found to be electrified by that touch, as it would have been by touching the outside. the fact is singular. you require the reason? i do not know it. perhaps you may discover it, and then you will be so good as to communicate it to me. i find a frank acknowledgment of one's ignorance is not only the easiest way to get rid of a difficulty, but the likeliest way to obtain information, and therefore i practise it. i think it is an honest policy. a note appended to this letter runs as follows:-- mr. f. has since thought that, possibly, the mutual repulsion of the inner opposite sides of the electrized can may prevent the accumulating an electric atmosphere upon them, and occasion it to stand chiefly on the outside. but recommends it to the further examination of the curious. the explanation in this note is the correct one, and from the fact that in the case of a completely closed hollow conductor the charge is not only _chiefly_ but _wholly_ on the outside, the law of inverse squares above referred to follows as a mathematical consequence. on writing to m. dalibard, of paris, on june , , franklin complained that, though he always (except once) assigned to lightning-rods the alternative duty of either _preventing_ a stroke or of _conducting_ the lightning with safety to the ground, yet in europe attention was paid only to the _prevention_ of the stroke, which was only a _part_ of the duty assigned to the conductors. this is followed by the description of the effect of a stroke upon a church-steeple at newbury, in new england. the spire was split all to pieces, so that nothing remained above the bell. the lightning then passed down a wire to the clock, then down the pendulum, without injury to the building. "from the end of the pendulum, down quite to the ground, the building was exceedingly rent and damaged, and some stones in the foundation-wall torn out and thrown to the distance of twenty or thirty feet." the pendulum-rod was uninjured, but the fine wire leading from the bell to the clock was vaporized except for about two inches at each end. mr. james alexander, of new york, having proposed to franklin that the velocity of the electric discharge might be measured by discharging a jar through a long circuit of river-water, franklin, in his reply, explained that such an experiment, if successful, would not determine the actual velocity of electricity in the conductor. he compared the electricity in conductors to an incompressible fluid, so that when a little additional fluid is injected at one end of a conductor, an equal amount must be extruded at the other end--his view apparently being identical with that of maxwell, who held that all electric displacements must take place _in closed circuits_. "suppose a tube of any length open at both ends.... if the tube be filled with water, and i inject an additional inch of water at one end, i force out an equal quantity at the other in the very same instant. "and the water forced out at one end of the tube is not the very same water that was forced in at the other end at the same time; it was only one motion at the same time. "the long wire, made use of in the experiment to discover the velocity of the electric fluid, is itself filled with what we call its natural quantity of that fluid, before the hook of the leyden bottle is applied at one end of it. "the outside of the bottle being at the time of such application in contact with the other end of the wire, the whole quantity of electric fluid contained in the wire is, probably, put in motion at once. "for at the instant the hook, connected with the inside of the bottle, _gives out_, the coating or outside of the bottle _draws in_, a portion of that fluid.... "so that this experiment only shows the extreme facility with which the electric fluid moves in metal; it can never determine the velocity. "and, therefore, the proposed experiment (though well imagined and very ingenious) of sending the spark round through a vast length of space, by the waters of susquehannah, or potowmack, and ohio, would not afford the satisfaction desired, though we could be sure that the motion of the electric fluid would be in that tract, and not underground in the wet earth by the shortest way." in his investigations of the source of electricity in thunder-clouds, franklin tried an experiment which has been frequently repeated with various modifications. having insulated a large brass plate which had been previously heated, he sprinkled water upon it, in order, if possible, to obtain electricity by the evaporation of the water, but no trace of electrification could be detected. during his visit to england, franklin wrote many letters to mr. kinnersley and others on philosophical questions, but they consisted mainly of accounts of the work done by other experimenters in england, his public business occupying too much of his attention to allow him to conduct investigations for himself. in one of his letters, speaking of lord charles cavendish, he says:-- it were to be wished that this noble philosopher would communicate more of his experiments to the world, as he makes many, and with great accuracy. when the controversy between the relative merits of points and knobs for the terminals of lightning-conductors arose, franklin wrote to mr. kinnersley:-- here are some electricians that recommend knobs instead of points on the upper end of the rods, from a supposition that the points invite the stroke. it is true that points draw electricity at greater distances in the gradual silent way; but knobs will draw at the greatest distance a stroke. there is an experiment which will settle this. take a crooked wire of the thickness of a quill, and of such a length as that, one end of it being applied to the lower part of a charged bottle, the upper may be brought near the ball on the top of the wire that is in the bottle. let one end of this wire be furnished with a knob, and the other may be gradually tapered to a fine point. when the point is presented to discharge the bottle, it must be brought much nearer before it will receive the stroke than the knob requires to be. points, besides, tend to repel the fragments of an electrical cloud; knobs draw them nearer. an experiment, which i believe i have shown you, of cotton fleece hanging from an electrized body, shows this clearly when a point or a knob is presented under it. the following quotation from franklin's paper on the method of securing buildings and persons from the effects of lightning is worthy of attention, for of late years a good deal of money has been wasted in providing insulators for lightning-rods. a few years ago the vicar and churchwardens of a lincolnshire parish were strongly urged to go to the expense of insulating the conductor throughout the whole height of the very lofty tower and spire of their parish church. happily they were wise enough to send the lightning-rod man about his business. but this is not the only case which has come under the writer's notice, showing that there is still a widespread impression that lightning-conductors should be carefully insulated. franklin says:-- "the rod may be fastened to the wall, chimney, etc., with staples of iron. the lightning will not leave the rod (a good conductor) to pass into the wall (a bad conductor) through these staples. it would rather, if any were in the wall, pass out of it into the rod, to get more readily by that conductor into the earth."[ ] [footnote : see p. .] the conditions to be secured in a lightning-conductor are, firstly, a sharp point projecting above the highest part of the building, and gilded to prevent corrosion; secondly, metallic continuity from the point to the lower end of the conductor; and, thirdly, a good earth-contact. the last can frequently be secured by soldering the conductor to iron water-pipes underground. where these are not available, a copper plate, two or three feet square, imbedded in clay or other damp earth, will serve the purpose. the method of securing a building which is erected on granite or other foundation affording no good earth-connection, will be referred to in a subsequent biographical sketch. the controversy of points _versus_ knobs was again revived in london when franklin was in paris, and the war of independence had begun. franklin was consulted on the subject, the question having arisen in connection with the conductor at the palace. his reply was characteristic. "as to my writing anything on the subject, which you seem to desire, i think it not necessary, especially as i have nothing to add to what i have already said upon it in a paper read to the committee who ordered the conductors at purfleet, which paper is printed in the last french edition of my writings. "i have never entered into any controversy in defence of my philosophical opinions. i leave them to take their chance in the world. if they are _right_, truth and experience will support them; if _wrong_, they ought to be refuted and rejected. disputes are apt to sour one's temper and disturb one's quiet. i have no private interest in the reception of my inventions by the world, having never made, nor proposed to make, the least profit by any of them. the king's changing his _pointed_ conductors for _blunt_ ones is, therefore, a matter of small importance to me. if i had a wish about it, it would be that he had rejected them altogether as ineffectual. for it is only since he thought himself and family safe from the thunder of heaven, that he dared to use his own thunder in destroying his innocent subjects." the paper referred to was read before "the committee appointed to consider the erecting conductors to secure the magazines at purfleet," on august , . it described a variety of experiments clearly demonstrating the effect of points in discharging a conductor. this was a committee of the royal society, to whom the question had been referred on account of dr. wilson's recommendation of a blunt conductor. the committee decided in favour of franklin's view, and when, in , the question was again raised and again referred to a committee of the royal society, the decision of the former committee was confirmed, "conceiving that the experiments and reasons made and alleged to the contrary by mr. wilson are inconclusive." though franklin's scientific reputation rests mainly on his electrical researches, he did not leave other branches of science untouched. besides his work on atmospheric electricity, he devoted a great deal of thought to meteorology, especially to the vortical motion of waterspouts. the gulf-stream received a share of his attention. his improvements in fireplaces have already been noticed; the cure of smoky chimneys was the subject of a long paper addressed to dr. ingenhousz, and of some other letters. one of his experiments on the absorption of radiant energy has been deservedly remembered. "my experiment was this: i took a number of little square pieces of broad-cloth from a tailor's pattern-card, of various colours. there were black, deep blue, lighter blue, green, purple, red, yellow, white, and other colours or shades of colours. i laid them all out upon the snow in a bright, sun-shiny morning. in a few hours (i cannot now be exact as to the time) the black, being warmed most by the sun, was sunk so low as to be below the stroke of the sun's rays; the dark blue almost as low, the lighter blue not quite so much as the dark, the other colours less as they were lighter; and the quite white remained on the surface of the snow, not having entered it at all. "what signifies philosophy that does not apply to some use? may we not learn from hence that black clothes are not so fit to wear in a hot, sunny climate or season, as white ones?" franklin knew much about electricity, but his knowledge of human nature was deeper still. this appears in all his transactions. his political economy was, perhaps, not always sound, but his judgment of men was seldom at fault. "finally, there seem to be but three ways for a nation to acquire wealth. the first is by _war_, as the romans did, in plundering their conquered neighbour: this is _robbery_. the second by _commerce_, which is generally _cheating_. the third by _agriculture_, the only _honest way_, wherein man receives a real increase of the seed thrown into the ground, in a kind of continual miracle wrought by the hand of god in his favour, as a reward for his innocent life and his virtuous industry." when franklin reached london in he took up his abode with mrs. margaret stevenson, in craven street, strand. for mrs. stevenson and her daughter mary, then a young lady of eighteen, he acquired a sincere affection, which continued throughout their lives. miss stevenson spent much of her time with an aunt in the country, and some of franklin's letters to her respecting the conduct of her "higher education" are among the most interesting of his writings. miss stevenson treated him as a father, and consulted him on every question of importance in her life. when she was a widow and franklin eighty years of age, he urged upon her to come to philadelphia, for the sake of the better prospects which the new country offered her boys. in coming to england, franklin brought with him his son william, who entered the middle temple, but he left behind his only daughter, sarah, in charge of her mother. to his wife and daughter he frequently sent presents from london, and his letters to mrs. franklin give a pretty full account of all his doings while in england. during his visit he received the honorary degrees of d.c.l. from the university of oxford, and ll.d. from that of edinburgh. at cambridge he was sumptuously entertained. in august, , he started again for america, and reached philadelphia on november , after an absence of five years. his son william had shortly before been appointed governor of new jersey. from this time william franklin became very much the servant of the proprietaries and of the english government, but no offer of patronage produced any effect on the father. franklin's stay in america was of short duration, but while there he was mainly instrumental in quelling an insurrection in pennsylvania. he made a tour of inspection through the northern colonies in the summer of , to regulate the post-offices. the disorder just referred to in the province caused the governor, as well as the assembly, to determine on the formation of a militia. a committee, of which franklin was a member, drew up the necessary bill. the governor claimed the sole power of appointing officers, and required that trials should be by court-martial, some offences being punishable with death. the assembly refused to agree to these considerations. the ill feeling was increased by the governor insisting on taxing all proprietary lands at the same rate as uncultivated land belonging to other persons, whether the proprietary lands were cultivated or not. the assembly, before adjourning, expressed an opinion that peace and happiness would not be secured until the government was lodged directly in the crown. when the assembly again met, petitions to the king came in from more than three thousand inhabitants. in the mean while the british ministry had proposed the stamp act, which was similar in principle to the english stamp act, which requires that all agreements, receipts, bills of exchange, marriage and birth certificates, and all other legal documents should be provided with an inland revenue stamp of a particular value, in order that they might be valid. as soon as the assembly was convened, it determined to send franklin to england, to take charge of a petition for a change of government. the merchants subscribed £ towards his expenses in a few hours, and in twelve days he was on his journey, being accompanied to the ship, a distance of sixteen miles, by a cavalcade of three hundred of his friends, and in thirty days he reached london. arrived in london, he at once took up his abode in his old lodgings with mrs. stevenson. he was a master of satire, equalled only by swift, and during the quarrels which preceded the war of independence, as well as during the war, he made good use of his powers in this respect. articles appeared in some of the english papers tending to raise an alarm respecting the competition of the colonies with english manufacturers. franklin's contribution to the discussion was a caricature of the english press writers. "it is objected by superficial readers, who yet pretend to some knowledge of those countries, that such establishments [manufactories for woollen goods, etc.] are not only improbable, but impossible, for that their sheep have but little wool, not in the whole sufficient for a pair of stockings a year to each inhabitant; that, from the universal dearness of labour among them, the working of iron and other materials, except in a few coarse instances, is impracticable to any advantage. "dear sir, do not let us suffer ourselves to be amused with such groundless objections. the very tails of the american sheep are so laden with wool that each has a little car or waggon on four little wheels to support and keep it from trailing on the ground. would they caulk their ships, would they even litter their horses with wool, if it were not both plenty and cheap? and what signifies the dearness of labour, when an english shilling passes for five and twenty? their engaging three hundred silk throwsters here in one week for new york was treated as a fable, because, forsooth, they have 'no silk there to throw!' those who make this objection perhaps do not know that, at the same time, the agents for the king of spain were at quebec, to contract for one thousand pieces of cannon to be made there for the fortification of mexico, and at new york engaging the usual supply of woollen floor-carpets for their west india houses. other agents from the emperor of china were at boston, treating about an exchange of raw silk for wool, to be carried in chinese junks through the straits of magellan. "and yet all this is as certainly true as the account said to be from quebec in all the papers of last week, that the inhabitants of canada are making preparations for a cod and whale fishery this summer in the upper lakes. ignorant people may object that the upper lakes are fresh, and that cod and whales are salt-water fish; but let them know, sir, that cod, like other fish when attacked by their enemies, fly into any water where they can be safest; that whales, when they have a mind to eat cod, pursue them wherever they fly; and that the grand leap of the whale in the chase up the falls of niagara is esteemed, by all who have seen it, as one of the finest spectacles in nature." one of franklin's chief objects in coming to england was to prevent the passing of mr. grenville's bill, previously referred to as the stamp act. the colonists urged that they had always been liberal in their votes, whenever money was required by the crown, and that taxation and representation must, in accordance with the british constitution, go hand-in-hand, so that the english parliament had no right to raise taxes in america, so long as the colonists were unrepresented in parliament. "had mr. grenville, instead of that act, applied to the king in council for such requisitional letters [_i.e._ requests to the assemblies for voluntary grants], to be circulated by the secretary of state, i am sure he would have obtained more money from the colonies by their voluntary grants than he himself expected from the sale of stamps. but he chose compulsion rather than persuasion, and would not receive from their good will what he thought he could obtain without it." the stamp act was passed, stamps were printed, distributors were appointed, but the colonists would have nothing to do with the stamps. the distributors were compelled to resign their commissions, and the captains of vessels were forbidden to land the stamped paper. the cost of printing and distributing amounted to £ , ; the whole return was about £ , from canada and the west indies. the passing of the stamp act was soon followed by a change of ministry, when the question again came before parliament. franklin submitted to a long examination before a committee of the whole house. the feeling prevalent in america respecting the stamp act may be inferred from some of his answers. " . _q._ do you think the people of america would submit to pay the stamp duty if it was moderated? "_a._ no, never, unless compelled by force of arms. " . _q._ what was the temper of america towards great britain before the year ?[ ] [footnote : the date of the sugar act.] "_a._ the best in the world. they submitted willingly to the government of the crown, and paid, in their courts, obedience to the acts of parliament. numerous as the people are in the several old provinces, they cost you nothing in forts, citadels, garrisons, or armies to keep them in subjection. they were governed by this country at the expense only of a little pen, ink, and paper; they were led by a thread. they had not only a respect, but an affection, for great britain--for its laws, its customs and manners, and even a fondness for its fashions, that greatly increased the commerce. natives of britain were always treated with particular regard; to be an _old-englandman_ was, of itself, a character of some respect, and gave a kind of rank among us. " . _q._ and what is their temper now? "_a._ oh, very much altered. " . _q._ was it an opinion in america before that the parliament had no right to lay taxes and duties there? "_a._ i never heard any objection to the right of laying duties to regulate commerce; but a right to lay internal taxes was never supposed to be in parliament, as we are not represented there. " . _q._ you say the colonies have always submitted to external taxes, and object to the right of parliament only in laying internal taxes; now, can you show that there is any kind of difference between the two taxes to the colony on which they may be laid? "_a._ i think the difference is very great. an _external_ tax is a duty laid on commodities imported; that duty is added to the first cost and other charges on the commodity, and, when it is offered to sale, makes a part of the price. if the people do not like it at that price, they refuse it; they are not obliged to pay it. but an _internal_ tax is forced upon the people without their consent, if not laid by their own representatives. the stamp act says we shall have no commerce, make no exchange of property with each other, neither purchase, nor grant, nor recover debts; we shall neither marry nor make our wills, unless we pay such and such sums; and thus it is intended to extort our money from us, or ruin us by the consequences of refusing to pay it. " . _q._ don't you think cloth from england absolutely necessary to them? "_a._ no, by no means absolutely necessary; with industry and good management they may very well supply themselves with all they want. " . _q._ will it not take a long time to establish that manufacture among them? and must they not in the mean while suffer greatly? "_a._ i think not. they have made a surprising progress already. and i am of opinion that, before their old clothes are worn out, they will have new ones of their own making. " . _q._ if the act is not repealed, what do you think will be the consequence? "_a._ a total loss of the respect and affection the people of america bear to this country, and of all the commerce that depends on that respect and affection. " . _q._ how can the commerce be affected? "_a._ you will find that, if the act is not repealed, they will take a very little of your manufactures in a short time. " . _q._ is it in their power to do without them? "_a._ i think they may very well do without them. " . _q._ is it their interest not to take them? "_a._ the goods they take from britain are either necessaries, mere conveniences, or superfluities. the first, as cloth, etc., with a little industry they can make at home; the second they can do without till they are able to provide them among themselves; and the last, which are much the greatest part, they will strike off immediately. they are mere articles of fashion, purchased and consumed because the fashion in a respected country; but will now be detested and rejected. the people have already struck off, by general agreement, the use of all goods fashionable in mournings, and many thousand pounds' worth are sent back as unsaleable. " . _q._ what used to be the pride of the americans? "_a._ to indulge in the fashions and manufactures of great britain. " . _q._ what is now their pride? "_a._ to wear their old clothes over again till they can make new ones." the month following franklin's examination, the repeal of the stamp act received the royal assent. thereupon franklin sent his wife and daughter new dresses, and a number of other little luxuries (or toilet necessaries). in franklin visited paris. in the same year his daughter married mr. richard bache. though parliament had repealed the stamp act, it nevertheless insisted on its right to tax the colonies. the duty act was scarcely less objectionable than its predecessor. on franklin's return from the continent, he heard of the retaliatory measures of the boston people, who had assembled in town-meetings, formally resolved to encourage home manufactures, to abandon superfluities, and, after a certain time, to give up the use of some articles of foreign manufacture. these _associations_ afterwards became very general in the colonies, so that in one year the importations by the colonists of new york fell from £ , to £ , , and in pennsylvania from £ , to £ , . the effect of the duty act was to encourage the dutch and other nations to smuggle tea and probably other india produce into america. the exclusion from the american markets of tea sent from england placed the east india company in great difficulties; for while they were unable to meet their bills, they had in stock two million pounds' worth of tea and other goods. the balance of the revenue collected under the duty act, after paying salaries, etc., amounted to only £ for the year, and for this a fleet had to be maintained, to guard the fifteen hundred miles of american coast; while the fall in east india stock deprived the revenue of £ , per annum, which the east india company would otherwise have paid. at length a licence was granted to the east india company to carry tea into america, duty free. this, of course, excluded all other merchants from the american tea-trade. a quantity of tea sent by the east india company to boston was destroyed by the people. the british government then blockaded the port. this soon led to open hostilities. franklin worked hard to effect a reconciliation. he drew up a scheme, setting forth the conditions under which he conceived a reconciliation might be brought about, and discussed it fully with mr. daniel barclay and dr. fothergill. this scheme was shown to lord howe, and afterwards brought before the ministry, but was rejected. other plans were considered, and franklin offered to pay for the tea which had been destroyed at boston. all his negotiations were, however, fruitless. at last he addressed a memorial to the earl of dartmouth, secretary of state, complaining of the blockade of boston, which had then continued for nine months, and had "during every week of its continuance done damage to that town, equal to what was suffered there by the india company;" and claiming reparation for such injury beyond the value of the tea which had been destroyed. the memorial also complained of the exclusion of the colonists from the newfoundland fisheries, for which reparation would one day be required. this memorial was returned to franklin by mr. walpole, and franklin shortly afterwards returned to philadelphia. during this visit to england he had lost his wife, who died on december , ; and his friend miss stevenson had married and been left a widow. in april, , franklin was appointed agent for georgia, in the following year for new jersey, and in for massachusetts, so that he was then the representative in england of four colonies, with an income of £ per annum. in he spent three weeks at twyford, with the bishop of st. asaph, who remained a fast friend of franklin's until his death. in he was nominated by the king of france as foreign associate of the academy of sciences. during his negotiations with the british government franklin wrote two satirical pieces, setting forth the treatment which the american colonists were receiving. the first was entitled "rules for reducing a great empire to a small one," the rules being precisely those which, in franklin's opinion, had been followed by the british government in its dealings with america. the other was "an edict by the king of prussia," in which the king claimed the right of taxing the british nation; of forbidding english manufacture, and compelling englishmen to purchase prussian goods; of transporting prisoners to britain, and generally of exercising all such controls over the english people as had been claimed over america by various acts of the english parliament, on the ground that england was originally colonized by emigrants from prussia. before franklin reached america, the war of independence, though not formally declared, had fairly begun. he was appointed a member of the second continental congress, and one of a committee of three to confer with general washington respecting the support and regulation of the continental army. this latter office necessitated his spending some time in the camp. on october , , he wrote to priestley:-- tell our dear good friend, dr. price, who sometimes has his doubts and despondencies about our firmness, that america is determined and unanimous; a very few tories and placemen excepted, who will probably soon export themselves. britain, at the expense of three millions, has killed a hundred and fifty yankees this campaign, which is £ , a head; and at bunker's hill she gained a mile of ground, half of which she lost again by our taking the post on ploughed hill. during the same time sixty thousand children have been born in america. from these _data_ his mathematical head will easily calculate the time and expense necessary to kill us all and conquer our whole territory. in franklin, then seventy years old, was appointed one of three commissioners to visit canada, in order, if possible, to promote a union between it and the states. finding that only one canadian in five hundred could read, and that the state of feeling in canada was fatal to the success of the commissioners, they returned, and franklin suggested that the next commission sent to canada should consist of schoolmasters. on the th of july franklin took part in the signing of the declaration of independence. when the document was about to be signed, mr. hancock remarked, "we must be unanimous; there must be no pulling different ways; we must all hang together." franklin replied, "yes, we must indeed all hang together, or most assuredly we shall all hang separately." in the autumn of franklin was unanimously chosen a special commissioner to the french court. he took with him his two grandsons, william temple franklin and benjamin franklin bache, and leaving marcus hook on october , crossed the atlantic in a sloop of sixteen guns. in paris he met with an enthusiastic reception. m. de chaumont placed at his disposal his house at passy, then about a mile from paris, but now within the city. here he resided for nine years, being a constant visitor at the french court, and certainly one of the most conspicuous figures in paris. he was obliged to serve in many capacities, and was very much burdened with work. not only were there his duties as commissioner at the french court, but he was also made admiralty judge and financial agent, so that all the coupons for the payment of interest on the money borrowed for the prosecution of the war, as well as all financial negotiations, either with the french government or contractors, had to pass through his hands. perhaps the most unpleasant part of his work was his continued applications to the french court for monetary advances. the french government, as is well known, warmly espoused the cause of the americans, and to the utmost of its ability assisted them with money, material, and men. franklin was worried a good deal by applications from french officers for introductions to general washington, that they might obtain employment in the american army. at last he framed a model letter of recommendation, which may be useful to many in this country in the present day. it was as follows:-- sir, the bearer of this, who is going to america, presses me to give him a letter of recommendation, though i know nothing of him, not even his name. this may seem extraordinary, but i assure you it is not uncommon here. sometimes, indeed, one unknown person brings another equally unknown, to recommend him; and sometimes they recommend one another! as to this gentleman, i must refer you to himself for his character and merits, with which he is certainly better acquainted than i can possibly be. i recommend him, however, to those civilities which every stranger, of whom one knows no harm, has a right to; and i request you will do him all the good offices and show him all the favour that, on further acquaintance, you shall find him to deserve. "i have the honour to be," etc. captain wickes, of the _refusal_, having taken about a hundred british seamen prisoners, franklin and silas deane, one of the other commissioners, wrote to lord stormont, the british ambassador, respecting an exchange. receiving no answer, they wrote again, and ventured to complain of the treatment which the american prisoners were receiving in the english prisons, and in being compelled to fight against their own countrymen. to this communication lord stormont replied:-- the king's ambassador receives no applications from rebels, unless they come to implore his majesty's mercy. to this the commissioners rejoined:-- in answer to a letter, which concerns some of the most material interests of humanity, and of the two nations, great britain and the united states of america, now at war, we received the enclosed _indecent_ paper, as coming from your lordship, which we return for your lordship's more mature consideration. at first the british government, regarding the americans as rebels, did not treat their prisoners as prisoners of war, but threatened to try them for high treason. their sufferings in the english prisons were very great. mr. david hartley did much to relieve them, and franklin transmitted money for the purpose. when a treaty had been formed between france and the states, and france had engaged in the war, and when fortune began to turn in favour of the united armies, the american prisoners received better treatment from the english government, and exchanges took place freely. in april, , mr. hartley visited franklin at passy, apparently for the purpose of preventing, if possible, the offensive and defensive alliance between america and france. very many attempts were made to produce a rupture between the french government and the american commissioners, but franklin insisted that no treaty of peace could be made between england and america in which france was not included. in the other commissioners were recalled, and franklin was made minister plenipotentiary to the court of france. in a letter to mr. david hartley, dated february , , franklin showed something of the feelings of the americans with respect to the english at that time:-- you may have heard that accounts upon oath have been taken in america, by order of congress, of the british barbarities committed there. it is expected of me to make a school-book of them, and to have thirty-five prints designed here by good artists, and engraved, each expressing one or more of the horrid facts, in order to impress the minds of children and posterity with a deep sense of your bloody and insatiable malice and wickedness. every kindness i hear of done by an englishman to an american prisoner makes me resolve not to proceed in the work. while at passy, franklin addressed to the _journal of paris_ a paper on an economical project for diminishing the cost of light. the proposal was to utilize the sunlight instead of candles, and thereby save to the city of paris the sum of , , livres per annum. his reputation in paris is shown by the following quotation from a contemporary writer:-- i do not often speak of mr. franklin, because the gazettes tell you enough of him. however, i will say to you that our parisians are no more sensible in their attentions to him than they were towards voltaire, of whom they have not spoken since the day following his death. mr. franklin is besieged, followed, admired, adored, wherever he shows himself, with a fury, a fanaticism, capable no doubt of flattering him and of doing him honour, but which at the same time proves that we shall never be reasonable, and that the virtues and better qualities of our nation will always be balanced by a levity, an inconsequence, and an enthusiasm too excessive to be durable. franklin always advocated free trade, even in time of war. he was of opinion that the merchant, the agriculturist, and the fisherman were benefactors to mankind. he condemned privateering in every form, and endeavoured to bring about an agreement between all the civilized powers against the fitting out of privateers. he held that no merchantmen should be interfered with unless carrying war material. he greatly lamented the horrors of the war, but preferred anything to a dishonourable peace. to priestley he wrote:-- perhaps as you grow older you may ... repent of having murdered in mephitic air so many honest, harmless mice, and wish that, to prevent mischief, you had used boys and girls instead of them. in what light we are viewed by superior beings may be gathered from a piece of late west india news, which possibly has not yet reached you. a young angel of distinction, being sent down to this world on some business for the first time, had an old courier-spirit assigned him as a guide. they arrived over the seas of martinico, in the middle of the long day of obstinate fight between the fleets of rodney and de grasse. when, through the clouds of smoke, he saw the fire of the guns, the decks covered with mangled limbs and bodies dead or dying; the ships sinking, burning, or blown into the air; and the quantity of pain, misery, and destruction the crews yet alive were thus with so much eagerness dealing round to one another,--he turned angrily to his guide, and said, 'you blundering blockhead, you are ignorant of your business; you undertook to conduct me to the earth, and you have brought me into hell!' 'no, sir,' says the guide, 'i have made no mistake; this is really the earth, and these are men. devils never treat one another in this cruel manner; they have more sense and more of what men (vainly) call humanity.' franklin maintained that it would be far cheaper for a nation to extend its possessions by purchase from other nations than to pay the cost of war for the sake of conquest. two british armies, under general burgoyne and lord cornwallis, having been wholly taken prisoners during the war, at last, after two years' negotiations, a definitive treaty of peace was signed on september , , between great britain and the united states, franklin being one of the commissioners for the latter, and mr. hartley for the former. on the same day a treaty of peace between great britain and france was signed at versailles. the united states treaty was ratified by the king on april , and therewith terminated the seven years' war of independence. franklin celebrated the surrender of the armies of burgoyne and cornwallis by a medal, on which the infant hercules appears strangling two serpents. when peace was at length realized, a scheme was proposed for an hereditary knighthood of the order of cincinnatus, to be bestowed upon the american officers who had distinguished themselves in the war. franklin condemned the hereditary principle. he pointed out that, in the ninth generation, the "young noble" would be only "one five hundred and twelfth part of the present knight," men and women being counted among his ancestors, reckoning only from the foundation of the knighthood. "posterity will have much reason to boast of the noble blood of the then existing set of chevaliers of cincinnatus." on may , , franklin received from congress permission to return to america. he was then in his eightieth year. on july he left passy for havre, whence he crossed to southampton, and there saw for the last time his old friend, the bishop of st. asaph, and his family. he reached his home in philadelphia early in september, and the day after his arrival he received a congratulatory address from the assembly of pennsylvania. in the following month he was elected president of the state, and was twice re-elected to the same office, it being contrary to the constitution for any president to be elected for more than three years in succession. the following extract from a letter, written most probably to tom paine, is worthy of the attention of some writers:-- i have read your manuscript with some attention. by the argument it contains against a particular providence, though you allow a general providence, you strike at the foundations of all religion. for without the belief of a providence that takes cognizance of, guards, and guides, and may favour particular persons, there is no motive to worship a deity, to fear his displeasure, or to pray for his protection. i will not enter into any discussion of your principles, though you seem to desire it. at present i shall only give you my opinion, that, though your reasonings are subtle, and may prevail with some readers, you will not succeed so as to change the general sentiments of mankind on that subject, and the consequence of printing this piece will be a great deal of odium drawn upon yourself, mischief to you, and no benefit to others. he that spits against the wind spits in his own face. but were you to succeed, do you imagine any good would be done by it? you yourself may find it easy to live a virtuous life without the assistance afforded by religion; you having a clear perception of the advantages of virtue and the disadvantages of vice, and possessing strength of resolution sufficient to enable you to resist common temptations. but think how great a portion of mankind consists of weak and ignorant men and women, and of inexperienced, inconsiderate youth of both sexes, who have need of the motives of religion to restrain them from vice, to support their virtue, and retain them in the practice of it till it becomes _habitual_, which is the great point for its security. and perhaps you are indebted to her originally, that is, to your religious education, for the habits of virtue upon which you now justly value yourself. you might easily display your excellent talents of reasoning upon a less hazardous subject, and thereby obtain a rank with our most distinguished authors. for among us it is not necessary, as among the hottentots, that a youth, to be raised into the company of men, should prove his manhood by beating his mother. i would advise you, therefore, not to attempt unchaining the tiger, but to burn this piece before it is seen by any other person; whereby you will save yourself a great deal of mortification by the enemies it may raise against you, and perhaps a good deal of regret and repentance. if men are so wicked _with religion_, what would they be _if without_ it? i intend this letter itself as a _proof_ of my friendship, and therefore add no _professions_ to it; but subscribe simply yours. during the last few years of his life franklin suffered from a painful disease, which confined him to his bed and seriously interfered with his literary work, preventing him from completing his biography. during this time he was cared for by his daughter, mrs. bache, who resided in the same house with him. he died on april , , the immediate cause of death being an affection of the lungs. he was buried beside his wife in the cemetery of christ church, philadelphia, the marble slab upon the grave bearing no other inscription than the name and date of death. in his early days ( ) he had written the following epitaph for himself:-- the body of benjamin franklin, printer, (like the cover of an old book, its contents torn out and stript of its lettering and gilding,) lies here, food for worms. but the work shall not be lost, for it will (as he believed) appear once more in a new and more elegant edition, revised and corrected by the author. when the news of his death reached the national assembly of france, mirabeau rose and said:-- "franklin is dead! "the genius, which gave freedom to america, and scattered torrents of light upon europe, is returned to the bosom of the divinity. "the sage, whom two worlds claim; the man, disputed by the history of the sciences and the history of empires, holds, most undoubtedly, an elevated rank among the human species. "political cabinets have but too long notified the death of those who were never great but in their funeral orations; the etiquette of courts has but too long sanctioned hypocritical grief. nations ought only to mourn for their benefactors; the representatives of free men ought never to recommend any other than the heroes of humanity to their homage. "the congress hath ordered a general mourning for one month throughout the fourteen confederated states on account of the death of franklin; and america hath thus acquitted her tribute of admiration in behalf of one of the fathers of her constitution. "would it not be worthy of you, fellow-legislators, to unite yourselves in this religious act, to participate in this homage rendered in the face of the universe to the rights of man, and to the philosopher who has so eminently propagated the conquest of them throughout the world? "antiquity would have elevated altars to that mortal who, for the advantage of the human race, embracing both heaven and earth in his vast and extensive mind, knew how to subdue thunder and tyranny. "enlightened and free, europe at least owes its remembrance and its regret to one of the greatest men who has ever served the cause of philosophy and liberty. "i propose, therefore, that a decree do now pass, enacting that the national assembly shall wear mourning during three days for benjamin franklin." henry cavendish. it would not be easy to mention two men between whom there was a greater contrast, both in respect of their characters and lives, than that which existed between benjamin franklin and the honourable henry cavendish. the former of humble birth, but of great public spirit, possessed social qualities which were on a par with his scientific attainments, and toward the close of his life was more renowned as a statesman than as a philosopher; the latter, a member of one of the most noble families of england, and possessed of wealth far exceeding his own capacity for the enjoyment of it, was known to very few, was intimate with no one, and devoted himself to scientific pursuits rather for the sake of the satisfaction which his results afforded to himself than from any hope that they might be useful to mankind, or from any desire to secure a reputation by making them known, and passed a long life, the most uneventful that can be imagined. though the records of his family may be traced to the norman conquest, the famous elizabeth hardwicke, the foundress of two ducal families and the builder of hardwicke hall and of chatsworth as it was before the erection of the present mansion, was the most remarkable person in the genealogy. her second son, william, was raised to the peerage by james i., thus becoming baron cavendish, and was subsequently created first earl of devonshire by the same monarch. his great-grandson, the fourth earl, was created first duke of devonshire by william iii., to whom he had rendered valuable services. he was succeeded by his eldest son in , and the third son of the second duke was lord charles cavendish, the father of henry and frederick, of whom henry was the elder, having been born at nice, october , . his mother died when he was two years old, and very little indeed is known respecting his early life. in he entered dr. newcome's school at hackney, where he remained until he entered peterhouse, in . he remained at cambridge until february, , when he left the university without taking his degree, objecting, most probably, to the religious tests which were then required of all graduates. in this respect his brother frederick followed his example. on leaving cambridge cavendish appears to have resided with his father in marlborough street, and to have occasionally assisted him in his scientific experiments, but the investigations of the son soon eclipsed those of the father. it is said that the rooms allotted to henry cavendish "were a set of stables, fitted up for his accommodation," and here he carried out many of his experiments, including all those electrical investigations in which he forestalled so much of the work of the present century. during his father's life, or, at any rate, till within a few years of its close, henry cavendish appears to have enjoyed a very narrow income. he frequently dined at the royal society club, and on these occasions would come provided with the five shillings to be paid for the dinner, but no more. upon his father's death, which took place in , when henry was more than fifty years of age, his circumstances were very much changed, but it seems that the greater part of his wealth was left him by an uncle who had been an indian officer, and this legacy may have come into his possession before his father's death. he appears to have been very liberal when it was suggested to him that his assistance would be of service, but it never occurred to him to offer a contribution towards any scientific or public undertaking, and though at the time of his death he is said to have had more money in the funds than any other person in the country, besides a balance of £ , on his current account at his bank, and various other property, he bequeathed none to scientific societies or similar institutions. throughout the latter part of his life he seems to have been quite careless about money, and to have been satisfied if he could only avoid the trouble of attending to his own financial affairs. hence he would allow enormous sums to accumulate at his banker's, and on one occasion, being present at a christening, and hearing that it was customary for guests to give something to the nurse, he drew from his pocket a handful of guineas, and handed them to her without counting them. after his father's death, cavendish resided in his own house on clapham common. here a few rooms at the top of the house were made habitable; the rest were filled with apparatus of all descriptions, among which the most numerous examples were thermometers of every kind. he seldom entertained visitors, but when, on rare occasions, a guest had to be entertained, the repast invariably consisted of a leg of mutton. his extreme shyness caused him to dislike all kinds of company, and he had a special aversion to being addressed by a stranger. on one occasion, at a reception given by sir joseph banks, dr. ingenhousz introduced to him a distinguished austrian philosopher, who professed that his main object in coming to england was to obtain a sight of so distinguished a man. cavendish listened with his gaze fixed on the floor; then, observing a gap in the crowd, he made a rush to the door, nor did he pause till he had reached his carriage. his aversion to women was still greater; his orders for the day he would write out and leave at a stated time on the hall-table, where his house-keeper, at another stated time, would find them. servants were allowed access to the portion of the house which he occupied only at fixed times when he was away; and having once met a servant on the stairs, a back staircase was immediately erected. his regular walk was down nightingale lane to wandsworth common, and home by another route. on one occasion, as he was crossing a stile, he saw that he was watched, and thenceforth he took his walks in the evening, but never along the same road. there were only two occasions on which it is recorded that scientific men were admitted to cavendish's laboratory. the first was in , when hunter, priestley, romayne, lane, and nairne were invited to see the experiments with the artificial torpedo. the second was when his experiment on the formation of nitric acid by electric sparks in air had been unsuccessfully attempted by van marum, lavoisier, and monge, and he "thought it right to take some measures to authenticate the truth of it." besides his house at clapham, cavendish occupied (by his instruments) a house in bloomsbury, near the british museum, while a "mansion" in dean street, soho, was set apart as a library. to this library a number of persons were admitted, who could take out the books on depositing a receipt for them. cavendish was perfectly methodical in all his actions, and whenever he borrowed one of his own books he duly left the receipt in its place. the only relief to his solitary life was afforded by the meetings of the royal society, of which he was elected a fellow in ; by the occasional receptions at the residence of sir joseph banks, p.r.s.; and by his not infrequent dinners with the royal society club at the crown and anchor; and he may sometimes have joined the social gatherings of another club which met at the cat and bagpipes, in downing street. it was to his visits to the royal society club that we are indebted for the only portrait that exists of him. alexander, the draughtsman to the china embassy, was bent upon procuring a portrait of cavendish, and induced a friend to invite him to the club dinner, "where he could easily succeed, by taking his seat near the end of the table, from whence he could sketch the peculiar great-coat of a greyish-green colour, and the remarkable three-cornered hat, invariably worn by cavendish, and obtain, unobserved, such an outline of the face as, when inserted between the hat and coat, would make, he was quite sure, a full-length portrait that no one could mistake. it was so contrived, and every one who saw it recognized it at once." another incident is recorded of the royal society club which, perhaps, reflects as much credit upon cavendish as upon the society. "one evening we observed a very pretty girl looking out from an upper window on the opposite side of the street, watching the philosophers at dinner. she attracted notice, and one by one we got up and mustered round the window to admire the fair one. cavendish, who thought we were looking at the moon, hustled up to us in his odd way, and when he saw the real object of our study, turned away with intense disgust, and grunted out, 'pshaw!'" in the spring and autumn of , , , and , cavendish made tours through most of the southern, midland, and western counties, and reached as far north as whitby. the most memorable of these journeys was that undertaken in , since during its course he visited james watt at the soho works, and manifested great interest in watt's inventions. this was only two years after the great controversy as to the discovery of the composition of water, but the meeting of the philosophers was of the most friendly character. on all these journeys considerable attention was paid to the geology of the country. allusion has already been made to the two committees of the royal society to which the questions of the lightning-conductors at purfleet, and of points _versus_ knobs for the terminals of conductors, were referred. cavendish served on each of these committees, and supported franklin's view against the recommendation of mr. wilson. on the first committee he probably came into personal communication with franklin himself. cavendish's life consisted almost entirely of his philosophical experiments. in other respects it was nearly without incident. he appears to have been so constituted that he must subject everything to accurate measurement. he rarely made experiments which were not _quantitative_; and he may be regarded as the founder of "quantitative philosophy." the labour which he expended over some of his measurements must have been very great, and the accuracy of many of his results is marvellous considering the appliances he had at disposal. when he had satisfied himself with the result of an experiment, he wrote out a full account and preserved it, but very seldom gave it to the public, and when he did publish accounts of any of his investigations it was usually a long time after the experiments had been completed. one of the consequences of his reluctance to publish anything was the long controversy on the discovery of the composition of water, which was revived many years afterwards by arago's _éloge_ on james watt; but a much more serious result was the loss to the world for so many years of discoveries and measurements which had to be made over again by faraday, kohlrausch, and others. the papers he published appeared in the _philosophical transactions of the royal society_, to which he began to communicate them in . on march , , he was elected one of the eight foreign associates of the institute of france. his _éloge_ was pronounced by cuvier, in , who said, "his demeanour and the modest tone of his writings procured him the uncommon distinction of never having his repose disturbed either by jealousy or by criticism." dr. wilson says, "he was almost passionless. all that needed for its apprehension more than the pure intellect, or required the exercise of fancy, imagination, affection, or faith, was distasteful to cavendish. an intellectual head thinking, a pair of wonderfully acute eyes observing, and a pair of very skilful hands experimenting or recording, are all that i realize in reading his memorials." he appeared to have no eye for beauty; he cared nothing for natural scenery, and his apparatus, provided it were efficient, might be clumsy in appearance and of the cheapest materials; but he was extremely particular about accuracy of construction in all essential details. he reminds us of one of our foremost men of science, who, when his attention was directed to the beautiful lantern tower of a cathedral, behind which the full moon was shining, remarked, "i see form and colour, but i don't know what you mean by beauty." the accounts of cavendish's death differ to some extent in their details, but otherwise are very similar. it appears that he requested his servant, "as he had something particular to engage his thoughts, and did not wish to be disturbed by any one," to leave him and not to return until a certain hour. when the servant came back, at the time appointed, he found his master dead. this was on february , , after an illness of only two or three days. it is mainly on account of his researches in electricity that the biography of cavendish finds a place in this volume. these investigations took place between the years and , and, as already stated, were all conducted in the stables attached to his father's house in marlborough street. it was by these experiments that electricity was first brought within the domain of measurement, and many of the numerical results obtained far exceeded in accuracy those of any other observer until the instruments of sir w. thomson rendered many electrical measurements a comparatively easy matter. the near agreement of cavendish's results with those of the best modern electricians has made them a perpetual monument to the genius of their author. it was at the request of sir w. thomson, mr. charles tomlinson, and others, that cavendish's electrical researches might be given to the public, that the duke of devonshire, in , entrusted the manuscripts to the care of the late professor clerk maxwell. they had previously been in the hands of sir william snow harris, who reported upon them, but after his death, in , the report could not be found. the papers, with an introduction and a number of very valuable notes by the editor, were published by the cambridge university press, just before the death of clerk maxwell, in . sir w. thomson quotes the following illustration of the accuracy of cavendish's work:--"i find already that the capacity of a disc was determined experimentally by cavendish as / · of that of a sphere of the same radius. now we have capacity of disc = ( /[pi])_a_ = _a_/ · !" cavendish adopted franklin's theory of electricity, treating it as an incompressible fluid pervading all bodies, and admitting of displacement only in a closed circuit, unless, indeed, the disturbance might extend to infinity. this fluid he supposed, with franklin, to be self-repulsive, but to attract matter, while matter devoid of electricity, and therefore in the highest possible condition of negative electrification, he supposed, with Æpinus, to be, like electricity, self-repulsive. one of cavendish's earliest experiments was the determination of the precise law according to which electrical action varies with the distance between the charges. franklin had shown that there was no sensible amount of electricity on the interior of a deep hollow vessel, however its exterior surface might be charged. cavendish mounted a sphere of · inches in diameter, so that it could be completely enclosed (except where its insulating support passed through) within two hemispheres of · inches diameter, which were carried by hinged frames, and could thus be allowed to close completely over the sphere, or opened and removed altogether from its neighbourhood. a piece of wire passed through one of the hemispheres so as to touch the inner sphere, but could be removed at pleasure by means of a silk string. the hemispheres being closed with the globe within them, and the wire inserted so as to make communication between the inner and outer spheres, the whole apparatus was electrified by a wire from a charged leyden jar. this wire was then removed by means of a silken string and "the same motion of the hand which drew away the wire by which the hemispheres were electrified, immediately after that was done, drew out the wire which made the communication between the hemispheres and the inner globe, and, immediately after that was drawn out, separated the hemispheres from each other," and applied the electrometer to the inner globe. "it was also contrived so that the electricity of the hemispheres and of the wire by which they were electrified was discharged as soon as they were separated from each other.... the inner globe and hemispheres were also both coated with tinfoil to make them the more perfect conductors of electricity." the electrometer consisted of a pair of pith-balls; but, though the experiment was several times repeated, they shewed no signs of electrification. from this it was clear that, as there could have been no communication between the globe and hemispheres when the connecting wire was withdrawn, there must have been no electrification on the globe while the hemispheres, though themselves highly charged, surrounded it. to test the delicacy of the experiment, a charge was given to the globe less than one-sixtieth of that previously given to the hemispheres, and this was readily detected by the electrometer. from the result cavendish inferred that there is no reason to think the inner globe to be at all charged during the experiment. "hence it follows that the electric attraction and repulsion must be inversely as the square of the distance, and that, when a globe is positively electrified, the redundant fluid in it is lodged entirely on its surface." this conclusion cavendish showed to be a mathematical consequence of the absence of electrification from the inner sphere; for, were the law otherwise, the inner sphere must be electrified positively or negatively, according as the inverse power were higher or lower than the second, and that the accuracy of the experiment showed the law must lie between the - / and the - / power of the distance. with his torsion-balance, coulomb obtained the same law, but cavendish's method is much easier to carry out, and admits of much greater accuracy than that of coulomb. cavendish's experiment was repeated by dr. macalister, under the superintendence of clerk maxwell, in the cavendish laboratory, the absence of electrification being tested by thomson's quadrant electrometer, and it was shown that the deviation from the law of inverse squares could not exceed one in , . the distinction between _electrical charge_ or _quantity of electricity_ and "_degree of electrification_" was first clearly made by cavendish. the latter phrase was subsequently replaced by _intensity_, but _electric intensity_ is now used in another sense. cavendish's phrase, _degree of electrification_, corresponds precisely with our notion of electric _potential_, and is measured by the work done on a unit of electricity by the electric forces in removing it from the point in question to the earth or to infinity. along with this notion cavendish introduced the further conception of the amount of electricity required to raise a conductor to a given degree of electrification, that is, the capacity of the conductor. in modern language, the _capacity_ of a conductor is defined as "the number of units of electricity required to raise it to unit potential;" and this definition is in precise accordance with the notion of cavendish, who may be regarded as the founder of the mathematical theory of electricity. finding that the capacities of similar conductors are proportional to their linear dimensions, he adopted a sphere of one inch diameter as the unit of capacity, and when he speaks of a capacity of so many "inches of electricity," he means a capacity so many times that of his one-inch sphere, or equal to that of a sphere whose diameter is so many inches. the modern unit of capacity in the electro-static system is that of a sphere of _one centimetre radius_, and the capacity of any sphere is numerically equal to its radius expressed in centimetres. cavendish determined the capacities of nearly all the pieces of apparatus he employed. for this purpose he prepared plates of glass, coated on each side with circles of tinfoil, and arranged in three sets of three, each plate of a set having the same capacity, but each set having three times the capacity of the preceding. there was also a tenth plate, having a capacity equal to the whole of the largest set. the capacity of the ten plates was thus sixty-six times that of one of the smallest set. with these as standards of comparison, he measured the capacities of his other apparatus, and, when possible, modified his conductors so as to make them equal to one of his standards. his large leyden battery he found to have a capacity of about , "inches of electricity," so that it was equivalent to a sphere more than five miles in diameter. one of his instruments employed in the measurement of capacities was a "trial plate," consisting of a sheet of metal, with a second sheet which could be made to slide upon it and to lie entirely on the top of the larger plate, or to rest with any portion of its area extending over the edge of the former. this was a conductor whose capacity could be varied at will within certain limits. finding the capacity of two plates of tinfoil on glass much greater than his calculations led him to expect, cavendish compared them with two equal plates having air between, and found their capacity very much to exceed that of the air condenser. the same was the case, though in a less degree, with condensers having shellac or bee's-wax for their dielectrics, and thus cavendish not only discovered the property to which faraday afterwards gave the name of "specific inductive capacity," but determined its measure in these dielectrics. he also discovered that the apparent capacity of a leyden jar increases at first for some time after it has been charged--a phenomenon connected with the so-called residual charge of the leyden jar. another feature on which he laid some stress, and which was brought to his notice by the comparison of his coated panes, was the creeping of electricity over the surface of the glass beyond the edge of the tinfoil, which had the same effect on the capacity as an increase in the dimensions of the tinfoil. the electricity appeared to spread to a distance of · inch all round the tinfoil on glass plates whose thickness was · inch, and · inch in the case of plates · inch thick. his paper on the torpedo was read before the royal society in . the experiments were undertaken in order to determine whether the phenomena observed by mr. john walsh in connection with the torpedo could be so far imitated by electricity as to justify the conclusion that the shock of the torpedo is an electric discharge. for this purpose cavendish constructed a wooden torpedo with electrical organs, consisting of a pewter plate on each side, covered with leather. the plates were connected with a charged leyden battery, by means of wires carried in glass tubes, and thus the battery was discharged through the water in which the torpedo was immersed, and which was rendered of about the same degree of saltness as the sea. cavendish compared the shock given through the water with that given by the model fish in air, and found the difference much greater than in the case of the real torpedo, but, by increasing the capacity of the battery and diminishing the potential to which it was charged, this discrepancy was diminished, and it was found to be very much less in the case of a second model having a leather, instead of a wooden, body, so that the body of the fish itself offered less resistance to the discharge. one of the chief difficulties lay in the fact that no one had succeeded in obtaining a visible spark from the discharge of the torpedo, which will not pass through the smallest thickness of air. cavendish accounted for this by supposing the quantity of electricity discharged to be very great, and its potential very small, and showed that the more the charge was increased and the potential diminished in his model, the more closely did it imitate the behaviour of the torpedo. but the main interest in this paper lies in the indications which it gives that cavendish was aware of the laws which regulate the flow of electricity through multiple conductors, and in the comparisons of electrical resistance which are introduced. it had been formerly believed that electricity would always select the shortest or best path, and that the whole of the discharge would take place along that route. franklin seems to have assumed this in the passage quoted[ ] respecting the discharge of the lightning down the uninsulated conductor instead of through the building. the truth, however, is that, when a number of paths are open to an electric current, it will divide itself between them in the inverse ratios of their resistances, or directly as their conductivities, so that, however great the resistance of one of the conductors, some portion, though it may be a very small fraction, of the discharge will take place through it. but this law does not hold in the case of insulators like the air, through which electricity passes only by disruptive discharges, and which completely prevent its passage unless the electro-motive force is sufficient to break through their substance. in the case of the lightning-conductor, however, its resistance is generally so small in comparison with that of the building it is used to protect, that franklin's conclusion is practically correct. [footnote : page .] in his paper on the torpedo cavendish stated that some experiments had shown that iron wire conducted , , times better than rain or distilled water, sea-water times, and saturated solution of sea-salt about times, better than rain-water. maxwell pointed out that this comparison of iron wire with sea-water would agree almost precisely with the measurements of matthiesen and kohlrausch at °c. the records of the experiments which led to these results were found among cavendish's unpublished papers, and the experiments also showed that the conductivity of saline solutions was very nearly proportional to the percentage of salt contained, when this was not very large--a result also obtained long afterwards by kohlrausch. in making these measurements cavendish was his own galvanometer. the solutions were contained in glass tubes more than three feet long, and a wire inserted to different distances into the solution; thus the discharge could be made to pass through any length of the liquid column less than that of the tube itself. from the leyden battery of forty-nine jars, six jars of nearly equal capacity were selected and charged together, and the charge of one jar only was employed for each shock. the discharge passed through the column of liquid contained in the tube, from a wire inserted at the further end, until it reached the sliding wire, when nearly the whole current betook itself to the wire on account of its smaller resistance, and thence passed through the galvanometer, which was cavendish himself. two tubes were generally compared together, and the jars discharged alternately through the tubes, and the tube which gave the greatest shock was assumed to possess the least resistance. the wires were then adjusted till the shocks were nearly equal, and positions determined which made the first tube possess a greater and then a less resistance than the second. from these positions the length of the column of liquid was estimated which would make the resistances of the two tubes exactly equal. but the result which has the greatest theoretical interest was obtained by discharging the leyden jars through wide and narrow tubes containing the same solutions. by these experiments cavendish found that the resistances of the conductors were independent of the strengths of the currents flowing in them; that is to say, he established ohm's law for electrolytes in a form which carried with it its full explanation. this was in january, . ohm's law was first formally stated in . the physical fact which is expressed by it is that the ratio of the electro-motive force to the current produced is the same for the same conductor, otherwise under the same physical conditions, however great or small that electro-motive force may be. cavendish devoted considerable attention to the subject of heat, especially thermometry. in many of his investigations on latent and specific heat he worked on the same lines as black, and at about the same time; but it is difficult to determine the exact date of some of cavendish's work, as he frequently did not publish it for a long time after its completion, and most of black's results were made public only to his lecture audience. cavendish, however, improved upon black in his mode of stating some of his results. the heat, for instance, which is absorbed by a body in passing from the solid to the liquid, or from the liquid to the gaseous, condition, black called "latent heat," and supposed it to become latent within the substance, ready to reveal itself when the body returned to its original condition. this heat cavendish spoke of as being _destroyed_ or _generated_, and this is in accordance with what we now know respecting the nature of heat, for when a body passes from the solid to the liquid, or from the liquid or solid to the gaseous, condition, a certain amount of work has to be done, and a corresponding amount of heat is used up in the doing of it. when the body returns to its original condition, the heat is restored, as when a heavy body falls to the ground, or a bent spring returns to its original form. cavendish's determination of the so-called latent heat of steam was very slightly in error. about very extraordinary beliefs were current respecting the excessive degree of cold and the rapid variations of temperature which take place in the arctic regions. braun, of st. petersburg, had observed that mercury, in solidifying in the tube of a thermometer, descended through more than four hundred degrees, and it was assumed that the melting point of mercury was about ° below fahrenheit's zero. it then became necessary to suppose that, while the mercury in a thermometer was freezing, there was a variation of temperature to this extent, and thus these wild reports became current. cavendish and black independently explained the anomaly, and each suggested the same method of determining the freezing point of mercury. cavendish, however, had a piece of apparatus prepared which he sent to governor hutchins, at albany fort, hudson's bay. it consisted of an outer vessel, in which the mercury was allowed to freeze, but not throughout the whole of its mass, and the bulb of the thermometer was kept immersed in the liquid metal in the interior. in this way the mercury in the thermometer was cooled down to the melting point without commencing to solidify, and the temperature was found to be between ° and ° below fahrenheit's zero. as a chemist, cavendish is renowned for his eudiometric analysis, whereby he determined the percentage of oxygen in air with an amount of accuracy that would be creditable to a chemist of to-day, and for his discovery of the composition of water; but to the world generally he is perhaps best known by the famous "cavendish experiment" for determining the mass, and hence the mean density, of the earth. the apparatus was originally suggested by the rev. john michell, but was first employed by cavendish, who thereby determined the mean density of the earth to be · . at the request of the astronomical society, the investigation was afterwards taken up by mr. francis baily, who, after much labour, discovered that the principal sources of error were due to radiation of heat, and consequent variation of temperature of parts of the apparatus during the experiment. to minimize the radiation and absorption, he gilded the principal portions of the apparatus and the interior of the case in which it was contained, and his results then became consistent. cavendish had himself suggested the cause of the discrepancy, but the gilding was proposed by principal forbes. as a mean of many hundreds of experiments, mr. baily deduced for the mean density of the earth · . cavendish's apparatus was a delicate torsion-balance, whereby two leaden balls were supported upon the extremities of a wooden rod, which was suspended by a thin wire. these balls were about two inches in diameter, and the experiment consisted in determining the deflection of the wooden arm by the attraction of two large solid spheres of lead brought very near the balls, and so situated that the attraction of each tended to twist the rod horizontally in the same direction. the force required to produce the observed deflection was calculated from the time of swing of the rod and balls when left to themselves. the force exerted upon either ball by a known spherical mass of metal, with its centre at a known distance, being thus determined, it was easy to calculate what mass, having its centre at the centre of the earth, would be required to attract one of the balls with the force with which the earth was known to attract it. dr. wilson sums up cavendish's view of life in these words:-- his theory of the universe seems to have been that it consisted _solely_ of a multitude of objects which could be weighed, numbered, and measured; and the vocation to which he considered himself called was to weigh, number, and measure as many of these objects as his allotted three score years and ten would permit. this conviction biased all his doings--alike his great scientific enterprises and the petty details of his daily life. [greek: _panta metrô, kai arithmô, kai stathmô_], was his motto; and in the microcosm of his own nature he tried to reflect and repeat the subjection to inflexible rule and the necessitated harmony which are the appointed conditions of the macrocosm of god's universe. count rumford. benjamin thompson, like franklin, was a native of massachusetts, his ancestors for several generations having been yeomen in that province, and descendants of the first colonists of the bay. in the diploma of arms granted him when he was knighted by george iii., he is described as "son of benjamin thompson, late of the province of massachusetts bay, in new england, gent." he was born in the house of his grandfather, ebenezer thompson, at woburn, massachusetts, on march , . his father died at the age of twenty-six, on november , , leaving the infant benjamin and his mother to the care of the grandparents. the widow married josiah pierce, junior, in march, , and with her child, now a boy of three, went to live in a house but a short distance from her former residence. young thompson appears to have received a sound elementary education at the village school. from some remarks made by him in after years to his friend, m. pictet, it has been inferred that he did not receive very kind treatment at the hands of his stepfather. it is clear, however, that the most affectionate relationships always obtained between him and his mother, and the latter appears to have had no cause to complain of the treatment she received from her second husband, with whom she lived to a very good old age. that thompson in early boyhood developed some tendencies which did not meet with ready sympathy from those around him is, however, equally clear. his guardians destined him for a farmer, like his ancestors, and his experiments in mechanics, which took up much of his playtime and in all probability not a few hours which should have been devoted to less interesting work, were not regarded as tending towards the end in view. hence he was probably looked upon as "indolent, flighty, and unpromising." later on he was sent to school in byfield, and in , at the age of eleven, "was put under the tuition of mr. hill, an able teacher in medford, a town adjoining woburn." at length, his friends having given up all hope of ever making a farmer of the boy, he was apprenticed, on october , , to mr. john appleton, of salem, an importer of british goods and dealer in miscellaneous articles. he lived with his master, and seems to have done his work in a manner satisfactory on the whole, but there is evidence that he would, during business hours, occupy his spare moments with mechanical contrivances, which he used to hide under the counter, and even ventured occasionally to practise on his fiddle in the store. he stayed with mr. appleton till the autumn of , and during this time he attended the ministry of the rev. thomas barnard. this gentleman seems to have taken great interest in the boy, and to have taught him mathematics, so that at the age of fifteen he was able "to calculate an eclipse," and was delighted when the eclipse commenced within six seconds of his calculated time. thompson, while an apprentice, showed a great faculty for drawing and designing, and used to carve devices for his friends on the handles of their knives or other implements. it was at this time he constructed an elaborate contrivance to produce perpetual motion, and on one evening it is said that he walked from salem to woburn, to show it to loammi baldwin, who was nine years older than himself, but his most intimate friend. like many other devices designed for the same purpose, it had only one fault--it wouldn't go. it was in , while preparing fireworks for the illumination on the abolition of the stamp act, that thompson was injured by a severe explosion as he was grinding his materials in a mortar. his note-book contained many directions for the manufacture of fireworks. during thompson's apprenticeship those questions were agitating the public mind which finally had their outcome in the war of independence. mr. appleton was one of those who signed the agreement refusing to import british goods, and this so affected the trade of the store that he had no further need for the apprentice. hence it was that, in the autumn of , thompson went to boston as apprentice-clerk in a dry goods store, but had to leave after a few months, through the depression in trade consequent on the non-importation agreement. his note-book, containing the entries made at this time, comprised several comic sketches very well drawn, and a quantity of business memoranda which show that he was very systematic in keeping his accounts. his chief method of earning money, or rather of making up the "cr." side of his accounts, was by cutting and cording wood. a series of entries made in july and august, , show the expense he incurred in constructing an electrical machine. it is not easy to determine, from the list of items purchased, the character of the machine he constructed; but it is interesting to note that the price in america at that time of nitric acid was _ s. d._ per ounce; of lacquer, _ s._ per pint; of shellac, _ s._ per ounce; brass wire, _ s._ per pound; and iron wire, _ s. d._ per yard. the nature of the problems which occupied his thoughts during the last year or two of his business life are apparent in the following letters:-- woburn, august , . mr. loammi baldwin, sir, please to inform me in what manner fire operates upon clay to change the colour from the natural colour to red, and from red to black, etc.; and how it operates upon silver to change it to blue. i am your most humble and obedient servant, benjamin thompson god save the king. woburn, august, . mr. loammi baldwin, sir, please to give the nature, essence, beginning of existence, and rise of the wind in general, with the whole theory thereof, so as to be able to answer all questions relative thereto. yours, benjamin thompson. this was an extensive request, and the reply was probably not altogether satisfactory to the inquirer. on the back of the above letter was written:-- woburn, august , . sir, there was but few beings (for inhabitants of this world) created before the airy element was; so it has not been transmitted down to us how the great creator formed the matter thereof. so i shall leave it till i am asked only the natural cause, and why it blows so many ways in so short a time as it does. thompson appears now to have given up business and commenced the study of medicine under dr. hay, to whom for a year and a half he paid forty shillings per week for his board. during this time he paid part of his expenses by keeping school for a few weeks consecutively at wilmington and bradford, and another part was paid by cords of wood. his business capacity, as well as his dislike of ordinary work, is shown by some arrangements which he made for getting wood cut and corded at prices considerably below those at which he was himself paid for it. his note-book made at this time contains, besides business entries, several receipts for medicines and descriptions of surgical operations, in some cases illustrated by sketches. in his work he was methodical and industrious, and the life of a medical student suited his genius far better than that of a clerk in a dry goods store. when teaching at wilmington he seems to have attracted attention by the gymnastic performances with which he exercised both himself and his pupils. while a student with dr. hay, he attended some of the scientific lectures at harvard college. the pleasure and profit which he derived from these lectures are sufficiently indicated by the fact that forty years afterwards he made the college his residuary legatee. thompson won such a reputation as a teacher during the few weeks that he taught in village schools in the course of his student life, that he received an invitation from colonel timothy walker to come to concord, in new hampshire, on the merrimack, and accept a permanent situation in a higher grade school. it was from this place that he afterwards took his title, for the early name of concord was rumford, and the name was changed to concord "to mark the restoration of harmony after a long period of agitation as to its provincial jurisdiction and its relation with its neighbours." the young schoolmaster of concord was soon on very intimate terms with the minister of the town, the rev. timothy walker,[ ] a man who was so much respected that he had thrice been sent to britain on diplomatic business. mr. walker's daughter had been married to colonel rolfe, a man of wealth and position, and, with the exception of the governor of portsmouth, said to have been the first man in new hampshire to drive a curricle and pair of horses. thompson soon married--or, as he told pictet, was married to--the young widow. whatever may have been implied by this other way of putting the question, there is no doubt that thompson always had the greatest possible respect for his father-in-law, and ever remembered him with sincere gratitude. the fortunes of the gallant young schoolmaster now appeared to be made; when the engagement was settled, the carriage and pair were brought out again, and the youth was attired in his favourite scarlet as a man of wealth and position. in this garb he drove to woburn, and introduced his future wife to his mother, whose surprise can be better imagined than described. [footnote : father of the colonel.] the exact date of thompson's marriage is not known. his daughter sarah, afterwards countess of rumford, was born in the rolfe mansion on october , . it is needless to say that the engagement to mrs. rolfe terminated the teaching at the school. thompson now had a large estate and ample means to improve it. he gave much attention to gardening, and sent to england for garden seeds. in some way he attracted the attention of governor wentworth, the governor of portsmouth, who invited him to the government house, and was so taken with the former apprentice, medical student, and schoolmaster, that he gave him at once a commission as major. this appointment was the cause of the misfortunes which almost immediately began to overtake him. he incurred the jealousy of his fellow-officers, over whom he had been appointed, and he failed to secure the confidence of the civilians of concord. public feeling in new england was very much excited against the mother country. representations were sent to the british government, but appeared to be treated with contempt. very many of these documents were found, after the war was over, unopened in drawers at the colonial office. british ministers appeared to know little about the needs of their american dependencies, and relations rapidly became more and more strained. the patriots appointed committees to watch over the patriotism of their fellow-townsmen, and thus the freedom of a free country was inaugurated by an institution bordering in character very closely upon the inquisition; and the committees of correspondence and safety accepted evidence from every spy or eavesdropper who came before them with reports of suspected persons. thompson was accused of "toryism;" the only definite charge against him being that he had secured remission of punishment for some deserters from boston who had for some time worked upon his estate. he was summoned before the committee of safety, but refused to make any confession of acts injurious to his country, on the ground that he had nothing to confess. his whole after-life shows that his sympathies were very much on the side of monarchy and centralization, but at this time there appears to have been no evidence that could be brought against him. the populace, however, stormed his house, and he owed his safety to the fact that he had received notice of their intentions, and had made his escape a few hours before. this was in november, . thompson then took refuge at woburn, with his mother, but the popular ill feeling troubled him here, so that his life was one of great anxiety. while at woburn, his wife and child joined him, and stayed there for some months. at length he was arrested and confined in the town upon suspicion of being inimical to the interests of his country. when he was brought before the committee of inquiry, there was no evidence brought against him. major thompson then petitioned to be heard before the committee of the provincial congress at washington. this petition he entrusted to his friend colonel baldwin to present. the petition was referred by the committee to congress, by whom it was deferred for the sake of more pressing business. at length he secured a hearing in his native town, but the result was indecisive, and he did not obtain the public acquittal that he desired, though the committee of correspondence found that the "said thompson" had not "in any one instance shown a disposition unfriendly to american liberty; but that his general behaviour has evinced the direct contrary; and as he has now given us the strongest assurances of his good intentions, we recommend him to the friendship, confidence, and protection of all good people in this and the neighbouring provinces." this decision, however, does not appear to have been made public; and thompson, on his release, retired to charlestown, near boston. when the buildings of harvard college were converted into barracks, major thompson assisted in the transfer of the books to concord. it is said that, after the battle of charlestown, thompson was introduced to general washington, and would probably have received a commission under him but for the opposition of some of the new hampshire officers. he afterwards took refuge in boston, and it does not appear that he ever again saw his wife or her father. his daughter he did not see again till , when she was twenty-two years of age. on march , , general washington obliged the british troops to evacuate boston; thompson was the first official bearer of this intelligence to london. of course, his property at concord was confiscated to the commonwealth of massachusetts, and he himself was proscribed in the alienation act of new hampshire, in . when thompson reached london with the intelligence of the evacuation of boston, lord george germaine, the secretary for war, saw that he could afford much information which would be of value to the government. an appointment was soon found for him in the colonial office, and afterwards he was made secretary of the province of georgia, in which latter capacity, however, he had no duties to fulfil. throughout his career in the colonial office he remained on very intimate terms with lord george germaine, and generally breakfasted with him. in july, , he was guest of lord george at stoneland lodge, and here, in company with mr. ball, the rector of withyham, he undertook experiments "to determine the most advantageous situation for the vent in firearms, and to measure the velocities of bullets and the recoil under various circumstances." the results of these investigations procured for him the friendship of sir joseph banks, the president of the royal society, and thompson was not the man to lose opportunities for want of making use of them. in he was elected a fellow of the royal society, "as a gentleman well versed in natural knowledge and many branches of polite learning." in the same year he went for a cruise in the _victory_ with sir charles hardy, in order to pursue his experiments on gunpowder with heavy guns. here he studied the principles of naval artillery, and devised a new code of marine signals. in he was made under-secretary of state for the northern department, and in that capacity had the oversight of the transport and commissariat arrangements for the british forces. on the defeat of cornwallis, lord george germaine and his department had to bear the brunt of parliamentary dissatisfaction. lord george resigned his position in the government, and was created viscount sackville. he had, however, previously conferred on thompson a commission as lieutenant-colonel in the british army, and thompson, probably foreseeing the outcome of events and its effect on the ministry, was already in america when lord george resigned. he had intended landing at new york, but contrary winds drove him to charlestown. it is needless to trace the sad events which preceded the end of the war. it was to be expected that many bitter statements would be made by his countrymen respecting thompson's own actions as colonel commanding a british garrison, for at length he succeeded in reaching long island, and taking the command of the king's american dragoons, who were there awaiting him. the spirit of war always acts injuriously on those exposed to its influence, and lieutenant-colonel thompson in long island was doubtless a very different man from that which we find him to have been before and after; nor were the months so spent very fruitful in scientific work. in , before the final disbanding of the british forces, thompson returned to england, and was promoted to the rank of colonel, with half-pay for the rest of his life. still anxious for military service, he obtained permission to travel on the continent, in hopes of serving in the austrian army against the turks. he took with him three english horses, which rendered themselves very objectionable to his fellow-travellers while crossing the channel in a small boat. thompson went to strasbourg, where he attracted the attention of the prince maximilian, then field-marshal of france, but afterwards elector of bavaria. on leaving strasbourg, the prince gave him an introduction to his uncle, the elector of bavaria. he stayed some days at munich, but on reaching vienna learned that the war against the turks would not be carried on, so he returned to munich, and thence to england. m. pictet gives the following as rumford's account of the manner in which he was cured of his passion for war:-- "'i owe it,' said he to me, one day, 'to a beneficent deity, that i was cured in season of this martial folly. i met, at the house of the prince de kaunitz, a lady, aged seventy years, of infinite spirit and full of information. she was the wife of general bourghausen. the emperor, joseph ii., came often to pass the evening with her. this excellent person conceived a regard for me; she gave me the wisest advice, made my ideas take a new direction, and opened my eyes to other kinds of glory than that of victory in battle.'" if the course in life which colonel thompson afterwards took was due to the advice of this lady, she deserves a european reputation. the elector of bavaria, charles theodore, gave thompson a pressing invitation to enter his service in a sort of semi-military and semi-civil capacity, to assist in reorganizing his dominions and removing the abuses which had crept in. before accepting this appointment, it was necessary to obtain the permission of george iii. the king not only approved of the arrangement, but on february , , conferred on the colonel the honour of knighthood. sir benjamin then returned to bavaria, and was appointed by the elector colonel of a regiment of cavalry and general aide-de-camp. a palatial residence in munich was furnished for him, and here he lived more as a prince than a soldier. it was eleven years before he returned, even on a visit, to england, and these years were spent by him in works of philanthropy and statesmanship, to which it is difficult to find a parallel. at one time he is found reorganizing the military system of the country, arranging a complete system of military police, erecting arsenals at mannheim and munich; at another time he is carrying out scientific investigations in one of these arsenals; and then he is cooking cheap dinners for the poor of the country. one great evil of a standing army is the idleness which it develops in its members, unfitting them for the business of life when their military service is ended. thompson commenced by attacking this evil. in he was made major-general of cavalry and privy councillor of state, and was put at the head of the war department, with instructions to carry out any schemes which he had developed for the reform of the army and the removal of mendicity. four years after his arrival in munich he began to put some of his plans into operation. the pay of the soldiers was only threepence per day, and their quarters extremely uncomfortable, while their drill and discipline were unnecessarily irksome. thompson set to work to make "soldiers citizens and citizens soldiers." the soldier's pay, uniform, and quarters were improved; the discipline rendered less irksome; and schools in which the three r's were taught were connected with all the regiments,--and here not only the soldiers, but their children as well as other children, were taught gratuitously. not only were the soldiers employed in public works, and thus accustomed to habits of industry, while they were enlivened in their work by the strains of their own military bands, but they were supplied with raw material of various kinds, and allowed, when not on duty, to manufacture various articles and sell them for their own benefit--an arrangement which in this country to-day would probably raise a storm of opposition from the various trades. the garrisons were made permanent, so that soldiers might all be near their homes and remain there, and in time of peace only a small portion of the force was required to be in garrison at any time, so that the great part of his life was spent by each soldier at home. each soldier had a small garden appropriated to his use, and its produce was his sole property. garden seeds, and especially seed potatoes, were provided for the men, for at that time the potato was almost unknown in bavaria. under these circumstances a reform was quickly effected; idle men began to take interest in their gardens, and all looked on sir benjamin as a benefactor. having thus secured the co-operation of the army, thompson determined to attack the mendicants. the number of beggars may be estimated from the fact that in munich, with a population of sixty thousand, no less than two thousand six hundred beggars were seized in a week. in the towns, they possessed a complete organization, and positions of advantage were assigned in regular order, or inherited according to definite customs. in the country, farm labourers begged of travellers, and children were brought up to beggary from their infancy. of course, the evils did not cease with simple begging. children were stolen and ill treated, for the purpose of assisting in enlisting sympathy, and the people had come to regard these evils as inevitable. thompson organized a regular system of military patrol through every village of the country, four regiments of cavalry being set apart for this work. then on january , , when the beggars were out in full force to keep their annual holiday, thompson, with the other field officers and the magistrates of the city, gave the signal, and all the beggars in munich were seized upon by the three regiments of infantry then in garrison. the beggars were taken to the town hall, and their names and addresses entered on lists prepared for the purpose. they were ordered to present themselves next day at the "military workhouse," and a committee was appointed to inquire into the condition of each, the city being divided into sixteen districts for that purpose. relieved of an evil which they had regarded as inevitable, the townspeople readily subscribed for the purpose of affording systematic relief, while tradesmen sent articles of food and other requisites to "the relief committee." in the military workhouse the former mendicants made all the uniforms for the troops, besides a great deal of clothes for sale in bavaria and other countries. thompson himself fitted up and superintended the kitchen, where food was daily cooked for between a thousand and fifteen hundred persons; and, under sir benjamin's management, a dinner for a thousand was cooked at a cost for fuel of fourpence halfpenny--a result which has scarcely been surpassed in modern times, even at gateshead. that thompson's work was appreciated by those in whose interest it was undertaken is shown by the fact that when, on one occasion, he was dangerously ill, the poor of munich went in public procession to the cathedral to pray for him, though he was a foreigner and a protestant. perhaps it may appear that his philanthropic work has little to do with physical science; but with thompson everything was a scientific experiment, conducted in a truly scientific manner. for example, the lighting of the military workhouse afforded matter for a long series of experiments, described in his papers on photometry, coloured shadows, etc. the investigations on the best methods of employing fuel for culinary purposes led to some of his most elaborate essays; and his essay on food was welcomed alike in london and bavaria at a time of great scarcity, and when famine seemed impending. the emperor joseph was succeeded by leopold ii., but during the interregnum the elector of bavaria was vicar of the empire, and he employed the power thus temporarily placed in his hands in raising sir benjamin to the dignity of count of the holy roman empire, with the order of the white eagle, and the title which the new count selected was the old name of the village in new england where he had spent the two or three years of his wedded life. in count rumford returned to england, in order to publish his essays, and to make known in this country something of the work in which he had been engaged. soon after his arrival he was robbed of most of his manuscripts, the trunk containing them being stolen from his carriage in st. paul's churchyard. on the invitation of lord pelham, he visited dublin, and carried out some of his improvements in the hospitals and other institutions of that city. on his return to london he fitted up the kitchen of the foundling hospital. lady thompson lived to hear of her husband's high position in bavaria, but died on january , . when rumford came to london in , he wrote to his daughter, who was then twenty-one years of age, to meet him there, and on january , , she started in the _charlestown_, from boston. she remained with her father for more than three years, and her autobiography gives much information respecting the count's doings during this time. while in london, count rumford attained a high reputation as a curer of smoky chimneys. one firm of builders found full employment in carrying out work in accordance with his instructions; and in his hotel at pall mall he conducted experiments on fireplaces. he concluded that the sides of a fireplace ought to make an angle of ° with the back, so as to throw the heat straight to the front; and that the width of the back should be one-third of that of the front opening, and be carried up perpendicularly till it joins the breast. the "rumford roaster" gained a reputation not less than that earned by his open fireplace. it was during this stay in london that rumford presented to the royal society of london, and to the american academy of sciences £ three per cent. stock, for the purpose of endowing a medal to be called the rumford medal, and to be given each alternate year for the best work done during the preceding two years in the subjects of heat and light. he directed that two medals, one in gold and the other in silver, should be struck from the same die, the value of the two together to amount to £ . whenever no award was made, the interest was to be added to the principal, and the excess of the income for two years over £ was to be presented in cash to the recipient of the medal. at present the amount thus presented is sufficient to pay the composition fee for life membership of the royal society. the first award of the medal was made in , to rumford himself. the other recipients have been john leslie, william murdock, Étienne-louis malus, william charles wells, humphry davy, david brewster, augustin jean fresnel, macedonio melloni, james david forbes, jean baptiste biot, henry fox talbot, michael faraday, m. regnault, f. j. d. arago, george gabriel stokes, neil arnott, m. pasteur, m. jamin, james clerk maxwell, kirchoff, john tyndall, a. h. l. fizeau, balfour stewart, a. o. des cloiseaux, a. j. Ångström, j. norman lockyer, p. j. c. janssen, w. huggins, captain abney. in the summer of rumford and his daughter left england to return to munich. on account of the war, they were obliged to go by sea to hamburg; whence they drove to munich, where the count was anxiously expected, political troubles having compelled the elector to leave the city. after the battle of friedburg, the austrians retired to munich, and, finding the gates of the city closed, they fortified themselves on an eminence overlooking the city, and, through some misunderstanding with the local authorities, the austrian general threatened to attack the city if any frenchman should be allowed to enter. rumford took supreme command of the bavarian forces, and so gained the respect of the rival generals that neither the french nor the austrians made any attempt to enter the city. the large number of soldiers now in munich gave rumford a good opportunity to exercise his skill in cooking on a large scale, and this he did, adding to the comfort of the soldiers and reducing the cost of the commissariat. on the return of the elector, miss sarah was made a countess, and one-half of her father's pension was secured to her, thus providing her with an income of about £ per annum for life. many of the details of the home life and social intercourse during this period of residence at munich are preserved in the autobiography of the countess, as well as accounts of excursions, including a trip by river to salzburg for the purpose of inspecting the salt-mines. after two years' stay in munich, the count was appointed minister plenipotentiary from bavaria to the court of great britain. after an unpleasant and perilous journey, he reached london, _viâ_ hamburg, in september, , but was terribly disappointed on learning that a british subject could not be accepted as an envoy from a foreign power. as he did not then wish to return to bavaria, he purchased a house in brompton row. but he had been too much accustomed to great enterprises to be content with a quiet life, and was bound to have some important scheme on hand. pressing invitations were sent him to return to america, but he preferred residence in london, and devoted himself to the foundation of the royal institution, though the countess returned to the states in august, . a letter from colonel baldwin to her father shortly after her return contains the following passage:-- in the cask of fruit which your daughter and mr. rolfe have sent you, there is half a dozen apples of the growth of my farm, wrapped up in papers, with the name of _baldwin's apples_ written upon them.... it is (i believe) a spontaneous production of this country; that is, it was not originally engrafted fruit. the history of the remaining period of rumford's residence in london is the early history of the royal institution. for many years rumford had had at his disposal for his philanthropic projects all the resources of the electorate of bavaria, and he had done everything on a royal scale. his original plan for the royal institution appears to embody to a very great extent the work of the science and art department, the city and guilds institute for the advancement of technical education, the national school of cookery, the london society for the extension of university teaching, and, in addition to all this, to have comprehended a sort of perpetual international health exhibition, where every device for domestic purposes, and especially for the improvement of the condition of the poor, could be inspected. how all this was to be carried out with the resources which the count expected to be able to devote to the purpose, does not appear. foremost among the objects of the institution was placed the management of fire; for its promoter was convinced that more than half the fuel consumed in the country might be saved by proper arrangements. the philanthropic objects with which the institution was started are apparent from the fact that it was the society for bettering the condition of the poor which appointed a committee to confer with rumford, to report on the scheme, and to raise the funds necessary for starting the project; and one of rumford's hopes in connection with it was "to make benevolence fashionable." it was arranged that donors of fifty guineas each should be perpetual proprietors of the institution; and that subscribers should be admitted at a subscription of two guineas per annum, or ten guineas for life. the price of a proprietor's share was raised to sixty guineas from may , , and afterwards increased by ten guineas per annum up to one hundred guineas. in a very short time there were fifty-eight fifty-guinea subscribers, and to them rumford addressed a pamphlet, setting forth his scheme in detail. the following are specified as some of the contents of the future institution:--"cottage fireplaces and kitchen utensils for cottagers; a farm-house kitchen with its furnishings; a complete kitchen, with its utensils, for the house of a gentleman of fortune; a laundry, including boilers, washing, ironing, and drying rooms, for a gentleman's house, or for a public hospital; the most improved german, swedish, and russian stoves for heating rooms and passages." as far as possible all these things were to be seen at work. there were also to be ornamental open stoves with fires in them; working models of steam-engines, of brewers' boilers, of distillers' coppers and condensers, of large boilers for hospital kitchens, and of ships' coppers with the requisite utensils; models of ventilating apparatus, spinning-wheels and looms "adapted to the circumstances of the poor;" models of agricultural machinery and bridges, and "of all such other machines and useful instruments as the managers of the institution shall deem worthy of public notice." all articles were to be provided with proper descriptions, with the name and address of the maker, and the price. a lecture-room and laboratory were to be fitted up with all necessary philosophical apparatus, and the most eminent expounders of science were to be engaged for the purpose of "teaching the application of science to the useful purposes of life." the lectures were to include warming and ventilation, the preservation of food, agricultural chemistry, the chemistry of digestion, of tanning, of bleaching and dyeing, "and, in general, of all the mechanical arts as they apply to the various branches of manufacture." the institution was to be governed by nine managers, of whom three were to be elected each year by the proprietors; and there was also to be a committee of visitors, the members of which should not be the managers. the king became patron of the institution, and the first set of officers was nominated by him. the earl of winchelsea and nottingham was president; the earls of morton and of egremont and sir joseph banks, vice-presidents; the earls of bessborough, of egremont, and of morton, and count rumford, were among the managers; the duke of bridgewater, viscount palmerston, and earl spencer the visitors; and dr. thomas garnett was appointed first professor of physics and chemistry. the royal charter of the institution was sealed on january , . the superintendence of the journals of the institution was entrusted to rumford's care. for some time the count resided in the house in albemarle street, which had been purchased by the institution, and while there he superintended the workmen and servants. dr. thomas garnett, the first professor at the institution, was highly respected both as a man and a philosopher, and seems to have been everywhere well spoken of. but rumford and he could not work together, and his connection with the institution was consequently a short one. rumford was then authorized to engage dr. young as professor of natural philosophy, editor of the journals, and general superintendent of the house, at a salary of £ per annum. shortly before this the count's attention had been directed to the experiments on heat, made by humphry davy, and on february , , it was "resolved that mr. humphry davy be engaged in the service of the royal institution, in the capacity of assistant-lecturer in chemistry, director of the chemical laboratory, and assistant-editor of the journals of the institution; and that he be allowed to occupy a room in the house, and be furnished with coals and candles, and that he be paid a salary of one hundred guineas _per annum_." in his personal appearance, davy is said to have been at first somewhat uncouth, and the count was by no means charmed with him at their first interview. it was not till he had heard him lecture in private that rumford would allow davy to lecture in the theatre of the institution; but he afterwards showed his complete confidence in the young chemist by ordering that all the resources of the institution should be at his service. davy dined with rumford at the count's house in auteuil, when he visited paris with lady davy and faraday, in . he commenced his duties at the institution on march , . it was on june , in the same year, that the managers having objected to the syllabus of his lectures, dr. garnett's resignation was accepted; and on july dr. young was appointed in his stead. dr. young resigned after holding the appointment only two years, as he found the duties incompatible with his work as a physician. rumford's life in london now became daily more unpleasant to himself. accustomed, as he had been in bavaria, to carry out all his projects "like an emperor," it was difficult for him to work as one member of a body of managers. one by one he quarrelled with his colleagues, and at length left england, in may, , never to return. when distinguished men of science are placed at the head of an institution like that which rumford founded, there is always a tendency for the _technical_ teaching of the establishment to become gradually merged into scientific research; and in this case, after rumford's departure, the genius of davy gradually converted the royal institution into the establishment for scientific research which it has been for more than three quarters of a century. probably the man who has come nearest to realizing all that count rumford had planned for his institution is the late sir henry cole; but he succeeded only through the resources of the treasury. on leaving england in may, , rumford went to paris, where he stayed till july or august, when he revisited bavaria and remained there till the following year, when he returned to paris. he was again at munich in ; but under the new elector, though an old friend of the count, relationships do not seem to have been all that they were with his uncle, and at length the elector himself was compelled to leave munich, and soon after the bavarian sovereign became a vassal of napoleon. on october , , rumford married madame lavoisier, a lady of brilliant talents and ample fortune. that his position might be nearly equal to hers, the elector of bavaria raised his pension to £ per annum. a house, rue d'anjou, no. , was purchased for six thousand guineas, and rumford expended much thought and energy in making it, with its garden of two acres, all that he could desire. but the union was not so happy as he anticipated. the count loved quiet; madame de rumford was fond of company: to the former the pleasure of the table had no charms; the latter took delight in sumptuous dinner-parties. as time went on, domestic affairs became more and more unpleasant, and at length a friendly separation was agreed upon, after they had lived together for about three years and a half. the count then retired to a small estate which he hired at auteuil, about four miles from paris. the elector of bavaria was crowned king on january , , and in rumford was again at munich, for the purpose of forming, at the king's request, an academy of arts and sciences. at auteuil the count was joined by his daughter in december, , her journey having been much delayed through the capture of the vessel in which she had taken her passage, off bordeaux. an engraving of the house at auteuil, and the room in which rumford carried on his experiments, was published in the _illustrated london news_ of january , . while resident at auteuil, rumford frequently read papers before the institute of france, of which he was a member. he complained very much of the jealousy exhibited by the other members with reference to any discoveries made by a foreigner. he died in his house at auteuil, on august , , in the sixty-second year of his age. in he had made over, by deed of gift to his mother, the sum of ten thousand dollars, that she might leave it by will to her younger children. as before mentioned, harvard college was his residuary legatee, and the property so bequeathed founded the rumford professorship in that institution. cuvier, as secretary of the institute, pronounced the customary eulogy over its late member. the following passages throw some light on the reputation in which the count was held:-- he has constructed two singularly ingenious instruments of his own contriving. one is a new calorimeter for measuring the amount of heat produced by the combustion of any body. it is a receptacle containing a given quantity of water, through which passes, by a serpentine tube, the product of the combustion; and the heat that is generated is transmitted through the water, which, being raised by a fixed number of degrees, serves as the basis of the calculations. the manner in which the exterior heat is prevented from affecting the experiment is very simple and very ingenious. he begins the operation at a certain number of degrees below the outside heat, and terminates it at the same number of degrees above it. the external air takes back during the second half of the experiment exactly what it gave up during the first. the other instrument serves for noting the most trifling differences in the temperature of bodies, or in the rapidity of its changes. it consists of two glass bulbs filled with air, united by a tube, in the middle of which is a pellet of coloured spirits of wine; the slightest increase of heat in one of the bulbs drives the pellet towards the other. this instrument, which he called a thermoscope, was of especial service in making known to him the varied and powerful influence of different surfaces in the transmission of heat, and also for indicating a variety of methods for retarding or hastening at will the processes of heating and freezing.... he thought it was not wise or good to entrust to men, in the mass, the care of their own well-being. the right, which seems so natural to them, of judging whether they are wisely governed, appeared to him to be a fictitious fancy born of false notions of enlightenment. his views of slavery were nearly the same as those of a plantation-owner. he regarded the government of china as coming nearest to perfection, because, in giving over the people to the absolute control of their only intelligent men, and in lifting each of those who belonged to this hierarchy on the scale according to the degree of his intelligence, it made, so to speak, so many millions of arms the passive organs of the will of a few sound heads--a notion which i state without pretending in the slightest degree to approve it, and which, as we know, would be poorly calculated to find prevalence among european nations. as for the rest, whatever were the sentiments of m. rumford for men, they in no way lessened his reverence for god. he never omitted any opportunity in his works of expressing his religious admiration of providence, and of proposing for that admiration by others, the innumerable and varied provisions which are made for the preservation of all creatures; indeed, even his political views came from his firm persuasion that princes ought to imitate providence in this respect by taking charge of us without being amenable to us. in front of the new government offices and the national museum in the maximilian strasse, in munich, stand, on granite pedestals, four bronze figures, ten feet in height. these represent general deroy, fraunhofer, schelling, and count rumford. the statue of rumford was erected in , at the king's private expense. in the english garden which rumford planned and laid out is the monument erected during his absence in england in , and bearing allegorical figures of peace and plenty, and a medallion of the count. the bare enumeration of rumford's published papers would occupy considerable space, but many of them have more to do with philanthropy and domestic economy than with physics. we have seen that, when guest of lord george germaine, he was engaged in experiments on gunpowder. the experiments were made in the usual manner by firing bullets into a ballistic pendulum, and recording the swing of the pendulum. thompson suggested a modification of the ballistic pendulum, attaching the gun-barrel to the pendulum, and observing the recoil, and making allowance for the recoil due to the discharge from the gun of the products of combustion of the powder, the excess enabled the velocity of the bullet to be calculated. afterwards he made experiments on the maximum pressure produced by the explosion of powder, and pointed out that the value of powder in ordnance does not depend simply on the whole amount of gas produced, but also on the rapidity of combustion. while superintending the arsenal at munich, rumford exploded small charges of powder in a specially constructed receiver, which was closed by a plug of well-greased leather, and on this was placed a hemisphere of steel pressed down by a -pounder brass cannon weighing pounds. he found that the weight of the gun was lifted by the explosion of quantities of powder varying from twelve to fifteen grains, and hence concluded that, if the products of combustion of the powder were confined to the space actually occupied by the solid powder, the initial pressure would exceed twenty thousand atmospheres. rumford's calculation of the pressure, based upon the bursting of a barrel, which he had previously constructed, is not satisfactory, inasmuch as he takes no account of the fact that the inner portions of the metal would give way long before the outer layers exerted anything like their maximum tension. when a hollow vessel with thick walls, such as a gun-barrel or shell, is burst by gaseous pressure from within, the inner layers of material are stretched to their breaking tension before they receive much support from the outer layers; a rift is thus made in the interior, into which the gas enters, and the surface on which the gas presses being thus increased, the rift deepens till the fracture is complete. in order to gain the full strength due to the material employed, every portion of that material should be stretched simultaneously to the extent of its maximum safe load. this principle was first practically adopted by sir w. g. armstrong, who, by building up the breech of the gun with cylinders shrunk on, and so arranged that the tension increased towards the exterior, availed himself of nearly the whole strength of the metal employed to resist the explosion. had rumford's barrel been constructed on this principle, he would have obtained a much more satisfactory result. these investigations were followed by a very interesting series of experiments on the conducting power of fluids for heat, and, although he pushed his conclusions further than his experiments warranted, he showed conclusively that convection currents are the principal means by which heat is transferred through the substance of fluids, and described how, when a vessel of water is heated, there is generally an ascending current in the centre, and a descending current all round the periphery. hence it is only when a liquid expands by increase of temperature that a large mass can be readily heated from below. water below ° fahr. contracts when heated. rumford, in his paper, enlarges on the bearing of this fact on the economy of the universe, and the following extracts afford a good specimen of his style, and justify some of the statements made by cuvier in his eulogy:-- i feel the danger to which a mortal exposes himself who has the temerity to undertake to explain the designs of infinite wisdom. the enterprise is adventurous, but it cannot surely be improper. the wonderful simplicity of the means employed by the creator of the world to produce the changes of the seasons, with all the innumerable advantages to the inhabitants of the earth which flow from them, cannot fail to make a very deep and lasting impression on every human being whose mind is not degraded and quite callous to every ingenuous and noble sentiment; but the further we pursue our inquiries respecting the constitution of the universe, and the more attentively we examine the effects produced by the various modifications of the active powers which we perceive, the more we shall be disposed to admire, adore, and love that great first cause which brought all things into existence. though winter and summer, spring and autumn, and all the variety of the seasons are produced in a manner at the same time the most simple and the most stupendous (by the inclination of the axis of the earth to the plane of the ecliptic), yet this mechanical contrivance alone would not have been sufficient (as i shall endeavour to show) to produce that gradual change of temperature in the various climates which we find to exist, and which doubtless is indispensably necessary to the preservation of animal and vegetable life.... but in very cold countries the ground is frozen and covered with snow, and all the lakes and rivers are frozen over in the very beginning of winter. the cold then first begins to be extreme, and there appears to be no source of heat left which is sufficient to moderate it in any sensible degree. let us see what must have happened if things had been left to what might be called their natural course--if the condensation of water, on being deprived of its heat, had followed the law which we find obtains in other fluids, and even in water itself in some cases, namely, when it is mixed with certain bodies. had not providence interfered on this occasion in a manner which may well be considered _miraculous_, all the fresh water within the polar circle must inevitably have been frozen to a very great depth in one winter, and every plant and tree destroyed; and it is more than probable that the region of eternal frost would have spread on every side from the poles, and, advancing towards the equator, would have extended its dreary and solitary reign over a great part of what are now the most fertile and most inhabited climates of the world!... let us with becoming diffidence and awe endeavour to see what the means are which have been employed by an almighty and benevolent god to protect his fair creation. he then goes on to explain how large bodies of water are prevented from freezing at great depths on account of the expansion which takes place on cooling below ° fahr., and the further expansion which occurs on freezing, and mentions that in the lake of geneva, at a depth of a thousand feet, m. pictet found the temperature to be ° fahr. "we cannot sufficiently admire the simplicity of the contrivance by which all this heat is saved. it well deserves to be compared with that by which the seasons are produced; and i must think that every candid inquirer who will begin by divesting himself of all unreasonable prejudice will agree with me in attributing them both to the same author.... "but i must take care not to tire my reader by pursuing these speculations too far. if i have persisted in them, if i have dwelt on them with peculiar satisfaction and complacency, it is because i think them uncommonly interesting, and also because i conceived that they might be of value in this age of _refinement_ and _scepticism_. "if, among barbarous nations, the _fear of a god_, and the practice of religious duties, tend to soften savage dispositions, and to prepare the mind for all those sweet enjoyments which result from peace, order, industry, and friendly intercourse; a _belief in the existence of a supreme intelligence_, who rules and governs the universe with wisdom and goodness, is not less essential to the happiness of those who, by cultivating their mental powers, have learned to know how little can be known." rumford, in connection with his experiments on the conducting power of liquids, tried the effect of increasing the viscosity of water by the addition of starch, and of impeding its movements by the introduction of eider-down, on the rate of diffusion of heat through it. hence he explained the inequalities of temperature which may obtain in a mass of thick soup--inequalities which had once caused him to burn his mouth--and, applying the same principles to air, he at once turned his conclusions to practical account in the matter of warm clothing. after an attempt to determine, if possible, the weight of a definite quantity of heat--an attempt in which very great precautions were taken to exclude disturbing causes, while the balance employed was capable of indicating one-millionth part of the weight of the body weighed--rumford, finding no sensible effect on the balance, concluded that "if the weight of gold is neither augmented nor lessened by _one-millionth part_, upon being heated from the point of _freezing water_ to that of a _bright red heat_, i think we may very safely conclude that all attempts to discover any effect of heat upon the apparent weights of bodies will be fruitless." the theoretical investigations of principal hicks, based on the vortex theory of matter and the dynamical theory of heat, have recently led him to the conclusion that the attraction of gravitation may depend to some extent on temperature. a series of very valuable experiments on the radiating powers of different surfaces showed how that power varied with the nature of the surface, and the effect of a coating of lamp-black in increasing the radiating power of a body. in order to determine the effect of radiation in the cooling of bodies, rumford employed the thermoscope referred to by cuvier. the following passage is worthy of attention, as the truth it expounds in the last thirteen words appears to have been but very imperfectly recognized many years after it was written:-- "all the heat which a hot body loses when it is exposed in the air to cool is not given off to the air which comes into contact with it, but ... a large proportion of it escapes in rays, which do not heat the transparent air through which they pass, but, like light, generate heat only when and where they are stopped and absorbed." rumford then investigated the absorption of heat by different surfaces, and established the law that good radiators are good absorbers; and recommended that vessels in which water is to be heated should be blackened on the outside. in speculating on the use of the colouring matter in the skin of the negro, he shows his fondness for experiment:-- "all i will venture to say on the subject is that, were i called to inhabit a very hot country, nothing should prevent me from making the experiment of blackening my skin, or at least, of wearing a black shirt, in the shade, and especially at night, in order to find out if, by those means, i could contrive to make myself more comfortable." in his experiments on the conduction of heat, rumford employed a cylinder with one end immersed in boiling water and the other in melting ice, and determined the temperature at different points in the length of the cylinder. he found the difficulty which has recently been forcibly pointed out by sir wm. thomson, in the article "heat," in the "encyclopædia britannica," viz. that the circulation of the water was not sufficiently rapid to keep the temperature of the layer in contact with the metal the same as that of the rest of the water; and he also called attention to the arbitrary character of thermometer-scales, and recommended that more attention should be given to the scale of the air thermometer. it was in his visit to edinburgh, in , that, in company with some of the university professors, the count conducted some experiments in the university laboratory on the apparent radiation of cold. rumford's views respecting _frigorific rays_ have not been generally accepted, and prevost's theory of exchanges completely explains the apparent radiation of cold without supposing that cold is anything else than the mere absence of heat. we must pass over rumford's papers on the use of steam as a vehicle of heat, on new boilers and stoves for the purpose of economizing fuel, and all the papers bearing on the nutritive value of different foods. the calorimeter with which he determined the amount of heat generated by the combustion, and the latent heat of evaporation, of various bodies has been already alluded to. of the four volumes of rumford's works published by the american academy of arts and sciences, the third is taken up entirely with descriptions of fireplaces and of cooking utensils. before deciding on the best way to light the military workhouse at munich, rumford made a series of experiments on the relative economy of different methods, and for this purpose designed his well-known shadow-photometer. in the final form of this instrument the shadows were thrown on a plate of ground glass covered with paper, forming the back of a small box, from which all extraneous light was excluded. two rods were placed in front of this screen, and the lights to be compared were so situated that the shadow of one rod thrown by the first light might be just in contact with that of the other rod thrown by the second light. by introducing coloured glasses in front of the lights, rumford compared the illuminating powers of different sources with respect to light of a particular colour. the complementary tints exhibited by the shadows caused him to devise his theory of the harmony of complementary colours. one result is worthy of mention: it is a conclusion to which public attention has since been called in connection with "duplex" burners. rumford found that with wax tapers the amount of light emitted per grain of wax consumed diminished with the diminution of the consumption, so that a small taper gave out only one-sixteenth as much light as an ordinary candle for the same consumption of wax. he says:-- "this result can be easily explained if we admit the hypothesis which supposes light to be analogous to sound.... the particles ... were so rapidly cooled ... that they had hardly time to shine one instant before they became too cold to be any longer visible." an argand lamp, when compared with a lamp having a flat wick, gave more light in the ratio of to for the same consumption of oil. one of the latest investigations of rumford was that bearing on the effect of the width of the wheels on the draught of a carriage. to his own carriage, weighing, with its passengers, nearly a ton, he fitted a spring dynamometer by means of a set of pulleys attached to the under-carriage and the splinter-bar. he used three sets of wheels, respectively - / , - / , and inches wide, and, introducing weights into the carriage to make up for the difference in the weights of the wheels, he found a very sensible diminution in the tractive force required as the width of the wheels was increased, and in a truly scientific spirit, despising the ridicule cast upon him, he persisted in riding about paris in a carriage with four-inch tyres. but the piece of work by which rumford will be best known to future generations is that described in his paper entitled "an inquiry concerning the source of the heat which is excited by friction." it was while superintending the boring of cannon in the arsenal at munich that rumford was struck with the enormous amount of heat generated by the friction of the boring-bar against the metal. in order to determine whether the heat had come from the chips of metal themselves, he took a quantity of the abraded borings and an equal weight of chips cut from the metal with a fine saw, and, heating them to the temperature of boiling water, he immersed them in equal quantities of water at - / ° fahr. the change of temperature of the water was the same in both cases, and rumford found that there was no change which he could discover _in regard to its capacity for heat_ produced in the metal by the action of the borer. in order to prevent the honeycombing of the castings by the escaping gas, the cannon were cast in a vertical position with the breech at the bottom of the mould and a short cylinder projecting about two feet beyond the muzzle of the gun, so that any imperfections in the casting would appear in this projecting cylinder. it was on one of these pieces of waste metal, while still attached to the gun, that rumford conducted his experiments. having turned the cylinder, he cut away the metal in front of the muzzle until the projecting piece was connected with the gun by a narrow cylindrical neck, · inches in diameter and · inches long. the external diameter of the cylinder was · inches, and its length · inches, and it was bored to a depth of · inches, the diameter of the bore being · inches. the cannon was mounted in the boring-lathe, and a blunt borer pressed by a screw against the bottom of the bore with a force equal to the weight of , pounds. a small transverse hole was made in the cylinder near its base for the introduction of a thermometer. the cylinder weighed · pounds, and, with the gun, was turned at the rate of thirty-two revolutions per minute by horse-power. to prevent loss of heat, the cylinder was covered with flannel. after thirty minutes' work, the thermometer, when introduced into the cylinder, showed a temperature of ° fahr. the loss of heat during the experiment was estimated from observations of the rate of cooling of the cylinder. the weight of metal abraded was grains, while the amount of heat produced was sufficient to raise nearly five pounds of ice-cold water to the boiling point. to exclude the action of the air, the cylinder was closed by an air-tight piston, but no change was produced in the result. as the air had access to the metal where it was rubbed by the piston, and rumford thought this might possibly affect the result, a deal box was constructed, with slits at each end closed by sliding shutters, and so arranged that it could be placed with the boring bar passing through one slit and the narrow neck connecting the cylinder with the gun through the other slit, the sliding shutters, with the help of collars of oiled leather, serving to make the box water-tight. the box was then filled with water and the lid placed on. after turning for an hour the temperature was raised from ° to ° fahr., after an hour and a half it was ° fahr., at the end of two hours the temperature was ° fahr., at two hours and twenty minutes it was ° fahr., and at two hours and thirty minutes it actually boiled! "it would be difficult to describe the surprise and astonishment expressed in the countenances of the bystanders on seeing so large a quantity of cold water heated and actually made to boil without any fire. "though there was, in fact, nothing that could justly be considered as surprising in this event, yet i acknowledge fairly that it afforded me a degree of childish pleasure which, were i ambitious of the reputation of a _grave philosopher_, i ought most certainly rather to hide than to discover." rumford estimated the "total quantity of ice-cold water which, with the heat actually generated by the friction and accumulated in two hours and thirty minutes, might have been heated degrees, or made to boil" at · pounds, and the rate of production he considered exceeded that of nine wax candles, each consuming ninety-eight grains of wax per hour, while the work of turning the lathe could easily have been performed by one horse. this was the first rough attempt ever made, so far as we know, to determine the mechanical equivalent of heat. in his reflections on these experiments, rumford writes:-- it is hardly necessary to add that anything which any _insulated_ body or system of bodies can continue to furnish _without limitation_ cannot possibly be _a material substance_; and it appears to me to be extremely difficult, if not quite impossible, to form any distinct idea of anything capable of being excited and communicated in the manner the heat was excited and communicated in these experiments, except it be motion. it has been stated that, if rumford had dissolved in acid the borings and the sawn strips of metal, the capacity for heat of which he determined, and had shown that the heat developed in the solution was the same in the two cases, his chain of argument would have been absolutely complete. considering the amount of heat produced in the experiments, there are few minds whose conviction would be strengthened by this experiment, and it is only those who look for faultless logic that will refuse to rumford the credit of having established the dynamical nature of heat. davy afterwards showed that two pieces of ice could be melted by being rubbed against one another in a vacuum, but he does not appear to have made as much as he might of the experiment. mayer calculated the mechanical equivalent of heat from the heat developed in the compression of air, but he _assumed_, what afterwards was shown by joule to be nearly true, that the whole of the work done in the compression was converted into heat. it was joule, however, who first showed that heat and mechanical energy are mutually convertible, so that each may be expressed in terms of the other, a _given_ quantity of heat always corresponding to the _same amount_ of mechanical energy, whatever may be the intermediate stages through which it passes, and that we may therefore define the mechanical equivalent of heat as _the number of units of energy which, when entirely converted into heat, will raise unit mass of water one degree from the freezing point_. thomas young. "we here meet with a man altogether beyond the common standard, one in whom natural endowment and sedulous cultivation rivalled each other in the production of a true philosopher; nor do we hesitate to state our belief that, since newton, thomas young stands unrivalled in the annals of british science." such was the verdict of principal forbes on one who may not only be regarded as one of the founders of the undulatory theory of light, but who was among the first to apply the theory of elasticity to the strength of structures, while it is to him that we are indebted in the first instance for all we know of egyptian hieroglyphics, and for the vast field of antiquarian research which the interpretation of these symbols has opened up. thomas young was the son of thomas and sarah young, and the eldest of ten children. his mother was a niece of the well-known physician, dr. richard brocklesby, and both his father and mother were members of the society of friends, in whose principles all their children were very carefully trained. it was to the independence of character thus developed that dr. young attributed very much of the success which he afterwards attained. he was born at milverton, in somersetshire, on june , . for the greater part of the first seven years of his life he lived with his maternal grandfather, mr. robert davis, at minehead, in somersetshire. according to his own account, he could read with considerable fluency at the age of _two_, and, under the instructions of his aunt and a village schoolmistress, he had "read the bible twice through, and also watts's hymns," before he attained the age of four. it may with reason be thought that both the schoolmistress and the aunt should have been severely reprimanded, and it is certain that their example is not to be commended; but young's infantile constitution seems to have been proof against over-pressure, and before he was five years old he could recite the whole of goldsmith's "deserted village," with scarcely a mistake. he commenced learning latin before he was six, under the guidance of a nonconformist minister, who also taught him to write. when not quite seven years of age he went to boarding-school, where he remained a year and a half; but he appears to have learned more by independent effort than under the guidance of his master, for privately he "had mastered the last rules of walkinghame's 'tutor's assistant'" before reaching the middle of the book under the master's inspection. after leaving this school, he lived at home for six months, but frequently visited a neighbour who was a land surveyor, and at whose house he amused himself with philosophical instruments and scientific books, especially a "dictionary of arts and sciences." when nearly nine he went to the school of mr. thompson, at compton, in dorsetshire, where he remained nearly four years, and read several greek and latin authors, as well as the elements of natural philosophy--the latter in books lent him by mr. jeffrey, the assistant-master. this mr. jeffrey appears to have been something of a mechanical genius, and he gave young lessons in turning, drawing, bookbinding, and the grinding and preparation of colours. before leaving this school, at the age of thirteen, young had read six chapters of the hebrew bible. during the school holidays the construction of a microscope occupied considerable time, and the reading of "priestley on air" turned young's attention to the subject of chemistry. having learned a little french, he succeeded, with the help of a schoolfellow, in gaining an elementary knowledge of italian. after leaving school, he lived at home for some time, and devoted his energies mainly to hebrew and to turning and telescope-making; but eastern languages received a share of attention, and by the time he was fourteen he had read most of sir william jones's "persian grammar." he then went to youngsbury, in hertfordshire, and resided at the house of mr. david barclay, partly as companion and partly as classical tutor to mr. barclay's grandson, hudson gurney. this was the beginning of a friendship which lasted for life. gurney was about a year and a half junior to young, and for five years the boys studied together, reading the classical works which young had previously studied at school. before the end of these five years young had gained more or less acquaintance with fourteen languages; but his studies were for a time delayed through a serious illness when he was little more than sixteen. to this illness his uncle, dr. brocklesby, referred in a letter, of which the following extract is interesting for several reasons:-- recollect that the least slip (as who can be secure against error?) would in you, who seem in all things to set yourself above ordinary humanity, seem more monstrous or reprehensible than it might be in the generality of mankind. your prudery about abstaining from the use of sugar on account of the negro trade, in any one else would be altogether ridiculous, but as long as the whole of your mind keeps free from spiritual pride or too much presumption in your facility of acquiring language, which is no more than the dross of knowledge, you may be indulged in such whims, till your mind becomes enlightened with more reason. my late excellent friend, mr. day, the author of 'sandford and merton,' abhorred the base traffic in negroes' lives as much as you can do, and even mr. granville sharp, one of the earliest writers on the subject, has not done half as much service in the business as mr. day in the above work. and yet mr. day devoured daily as much sugar as i do; for he reasonably concluded that so great a system as the sugar-culture in the west indies, where sixty millions of british property are employed, could never be affected either way by one or one hundred in the nation debarring themselves the reasonable use of it. reformation must take its rise elsewhere, if ever there is a general mass of public virtue sufficient to resist such private interests. read locke with care, for he opens the avenues of knowledge, though he gives too little himself. with respect to the sugar, no doubt very much may be said on young's side of the question. it appears, however, that in his early manhood there was a good deal in his conduct which to-day would be regarded as _priggish_, though it was somewhat more in harmony with the spirit of his time. he left youngsbury at the age of nineteen, having read, besides his classical authors, the whole of newton's "principia" and "opticks," and the systems of chemistry by lavoisier and nicholson, besides works on botany, medicine, mineralogy, and other scientific subjects. one of young's peculiarities was the extraordinary neatness of his handwriting, and a translation in greek iambics of wolsey's farewell to cromwell, which he sent, written very neatly on vellum, to his uncle, dr. brocklesby, attracted the attention of mr. burke, dr. charles burney, and other classical scholars, so that when, a few months later, young went to stay with his uncle in london, and was thrown into contact with some of the chief literary men of the day, he found that his fame as a scholar had preceded him. this neatness of his handwriting and his power of drawing were of great use in his researches on the egyptian hieroglyphics. he had little faith in natural genius, but believed that anything could be accomplished by persevering application. "thou say'st not only skill is gained, but genius too may be obtained, by studious imitation." in the autumn of young went to london for the purpose of studying medicine. he lived in lodgings in westminster, and attended the hunterian school of anatomy. a year afterwards he entered st. bartholomew's hospital as a medical student. the notes which he took of the lectures were written sometimes in latin, interspersed with greek quotations, and not unfrequently with mathematical calculations, which may be assumed to have been made before the lecture commenced. during his school days he had paid some attention to geometrical optics, and had constructed a microscope and telescope. now his attention was attracted to a far more delicate instrument--the eye itself. young had learned how a telescope can be "focussed" so as to give clear images of objects more or less distant. some such power of adjustment must be possessed by the eye, or it could never form distinct images of objects, whether at a distance of a foot or a mile. the apparently fibrous structure of the crystalline lens of the eye had been noticed and described by leuwenhoeck; and pemberton, a century before young took up the subject, had suggested that the fibres were muscles, by the action of which the eye was "accommodated" for near or distant vision. in dissecting the eye of an ox young thought he had discovered evidence confirmatory of this view, and the paper which he wrote on the subject was not only published in the "philosophical transactions," but secured his election as a fellow of the royal society in june, . this paper was important, not simply because it led to young's election to the royal society, but mainly because it was his first published paper on optical subjects. later on he showed incontestably, by exact measurements, that it is the crystalline lens which changes its form during adjustment; but he was wrong in supposing the fibres of the lens to be muscular. by carefully measuring the distance between the images of two candles formed by reflection from the cornea, he showed that the cornea experienced no change of form. his eyes were very prominent; and turning them so as to look very obliquely, he measured the length of the eye from back to front with a pair of compasses whose points were protected, pressing one point against the cornea, and the other between the back of the eye and the orbit, and showed that, when the eye was focussed for different distances, there was no change in the length of the axis. the crystalline lens was the only resource left whereby the accommodation could be effected. the accommodation is, in fact, brought about by the action of the ciliary muscle. the natural form of the lens is more convex than is consistent with distinct vision, except for very near objects. the tension of the suspensory ligament, which is attached to the front of the lens all round its edge, renders the anterior surface of the lens much less curved than it would naturally be. the ciliary muscle is a ring of muscular fibre attached to the ciliary process close to the circumference of the suspensory ligament. by its contraction it forms a smaller ring, and, diminishing the external diameter, it releases the tension of the suspensory ligament, thus allowing the crystalline lens to bulge out and adapt itself for the diverging rays coming from near objects. it is the exertion of contracting the ciliary muscle that constitutes the effort of which we are conscious when looking at very near objects. it was not, however, till long after the time of dr. young that this complicated action was fully made out, though the change of form of the anterior surface of the crystalline lens was discovered by the change in the image of a bright object formed by reflection. in the spring of young took a holiday tour in cornwall, with hudson gurney, visiting on his way the duke of richmond, who was drinking the waters at bath, under the advice of dr. brocklesby. in cornwall, the mining machinery attracted his attention very much more than the natural beauties of the country. towards the end of the summer he visited the duke of richmond at goodwood, when the duke offered him the appointment of private secretary. he resolved, however, to continue his medical course, one of the reasons which he alleged being his regard for the society of friends, whose principles he considered inconsistent with the appointment of private secretary to the master-general of the ordnance. the following winter he spent as a medical student at edinburgh. here he gave up the costume of the society of friends, and in many ways departed from their rules of conduct. he mingled freely with the university, attended the theatre, took lessons in dancing and playing the flute, and generally cultivated the habits of what is technically known as "society." throughout this change in his life he retained his high moral principles as a guide of conduct, and appears to have acted from a firm conviction of what was right. at the same time, it must be admitted that the breaking down of barriers, however conventional they may be, is an operation attended in most cases by not a little danger. with young, the progress of his scientific education may have been delayed on account of the new demands on his time; but besides the study of german, spanish, and italian, he appears to have read a considerable amount of general literature during his winter session in edinburgh. the following summer he took a tour on horseback through the highlands, taking with him his flute, drawing materials, spirits for preserving insects, boards for drying plants, paper and twine for packing up minerals, and a thermometer; but the geological hammer does not then appear to have been regarded as an essential to the equipment of a philosopher. at aberdeen he stayed for three days, and reported thus on the university:-- some of the professors are capable of raising a university to celebrity, especially copeland and ogilvie; but the division and proximity of the two universities (king's college and marischal college) is not favourable to the advancement of learning; besides, the lectures are all, or mostly, given at the same hour, and the same professor continues to instruct a class for four years in the different branches. were the colleges united, and the internal regulations of the system new modelled, the cheapness of the place, the number of small bursaries for poor or distinguished students, and the merit of the instructors, might make this university a very respectable seminary in some branches of science. the fee to a professor for a five-months' session is only a guinea and a half. i was delighted with the inspection of the rich store of mathematical and philosophical apparatus belonging to professor copeland of marischal college, made in his own house, and partly with his own hands, finished with no less care than elegance; and tending to illustrate every branch of physics in the course of his lectures, which must be equally entertaining and instructive. before leaving the highlands, young visited gordon castle, where he stayed two days; and appears to have distinguished himself by the powers of endurance he exhibited in dancing reels. on leaving he writes: "i could almost have wished to break or dislocate a limb by chance, that i might be detained against my will; i do not recollect that i have ever passed my time more agreeably, or with a party that i thought more congenial to my own dispositions: and what would hardly be credited by many grave reasoners on life and manners, that a person who had spent the whole of his earlier years a recluse from the gay world, and a total stranger to all that was passing in the higher ranks of society, should feel himself more at home and more at ease in the most magnificent palace in the country than in the humblest dwelling with those whose birth was most similar to his own. without enlarging on the duke's good sense and sincerity, the duchess's spirit and powers of conversation, lady madeline's liveliness and affability, louisa's beauty and sweetness, georgiana's _naïveté_ and quickness of parts, young sandy's good nature, i may say that i was truly sorry to part with every one of them." young seems not to have known at this time that it is an essential feature of true gentlefolk to dissipate all sense of constraint or uneasiness from those with whom they are brought into contact and that in this they can be readily distinguished from those who have wealth without breeding. the duchess of gordon gave young an introduction to the duke of argyll, so, while travelling through the western highlands, he paid a visit to inverary castle, and "galloped over" the country with the duke's daughters. speaking of these ladies, he says, "lady charlotte ... is to lady augusta what venus is to minerva; i suppose she wishes for no more. both are goddesses." on his return to the west of england, he visited the coalbrook dale iron works, when mr. reynolds told him "that before the war he had agreed with a man to make a flute a hundred and fifty feet long, and two and a half in diameter, to be blown by a steam-engine and played on by barrels." on the th of the following october young left london, and after spending six days on the voyage from yarmouth to hamburg, he reached göttingen on the th of the same month; two days afterwards he matriculated, and on november he commenced his studies as a member of the university. he continued to take lessons in drawing, dancing, riding, and music, and commenced learning the clavichord. the english students at göttingen, in order to advance their german conversation, arranged to pay a fine whenever they spoke in english in one another's company. on sundays it was usual for the professors to give entertainments to the students, though they seldom invited them to dinner or supper. "indeed, they could not well afford, out of a fee of a louis or two, to give large entertainments; but the absence of the hospitality which prevails rather more in britain, is compensated by the light in which the students are regarded; they are not the less, but perhaps the more, respected for being students, and indeed, they behave in general like gentlemen, much more so than in some other german universities." at göttingen young attended, in addition to his medical lectures, spithler's lectures on the history and constitution of the european states, heyne on the history of the ancient arts, and lichtenberg's course on physics. speaking of blumenbach's lectures on natural history, young says, "he showed us yesterday a laborious treatise, with elegant plates, published in the beginning of this century at wurzburg, which is a most singular specimen of credulity in affairs of natural history. dr. behringen used to torment the young men of a large school by obliging them to go out with him collecting petrifactions; and the young rogues, in revenge, spent a whole winter in counterfeiting specimens, which they buried in a hill which the good man meant to explore, and imposed them upon him for most wonderful _lusus naturæ_. it is interesting in a metaphysical point of view to observe how the mind attempts to accommodate itself; in one case, where the boys had made the figure of a plant thick and clumsy, the doctor remarks the difference, and says that nature seems to have restored to the plant in thickness that which she had taken away from its other dimensions." on april , , young passed the examination for his medical degree at göttingen. the examination appears to have been entirely oral. it lasted between four and five hours. there were four examiners seated round a table provided "with cakes, sweetmeats, and wine, which helped to pass the time agreeably." they "were not very severe in exacting accurate answers." the subject he selected for his public discussion was the human voice, and he constructed a universal alphabet consisting of forty-seven letters, of which, however, very little is known. this study of sound laid the foundation, according to his own account, of his subsequent researches in the undulatory theory of light. the autumn of young spent in travelling in germany; in the following february he returned to england, and was admitted a fellow-commoner of emmanuel college, cambridge. it is said that the master, in introducing young to the tutors and other fellows, said, "i have brought you a pupil qualified to read lectures to his tutors." young's opinion of cambridge, as compared with german universities, was favourable to the former; but as he had complained of the want of hospitality at göttingen, so in cambridge he complained of the want of social intercourse between the senior members of the university and persons _in statu pupillari_. at that time there was no system of medical education in the university, and the statutes required that six years should elapse between the admission of a medical student and his taking the degree of m.b. young appears to have attracted comparatively little attention as an undergraduate in college. he did not care to associate with other undergraduates, and had little opportunity of intercourse with the senior members of the university. he was still keeping terms at cambridge when his uncle, dr. brocklesby, died. to young he left the house in norfolk street, park lane, with the furniture, books, pictures, and prints, and about £ , . in the summer of a slight accident at cambridge compelled young to keep to his rooms, and being thus forcibly deprived of his usual round of social intercourse, he returned to his favourite studies in physics. the most important result of this study was the establishment of the principle of interference in sound, which afforded the explanation of the phenomenon of "beats" in music, and which afterwards led up to the discovery of the interference of light--a discovery which sir john herschel characterized as "the key to all the more abstruse and puzzling properties of light, and which would alone have sufficed to place its author in the highest rank of scientific immortality, even were his other almost innumerable claims to such a distinction disregarded." the principle of interference is briefly this: when two waves meet each other, it may happen that their crests coincide; in this case a wave will be formed equal in height (amplitude) to the sum of the heights of the two. at another point the crest of one wave may coincide with the hollow of another, and, as the waves pass, the height of the wave at this point will be the difference of the two heights, and if the waves are equal the point will remain stationary. if a rope be hung from the ceiling of a lofty room, and the lower end receive a jerk from the hand, a wave will travel up the rope, be reflected and reversed at the ceiling, and then descend. if another wave be then sent up, the two will meet, and their passing can be observed. it will then be seen that, if the waves are exactly equal, the point at which they meet will remain at rest during the whole time of transit. if a number of waves in succession be sent up the string, the motions of the hand being properly timed, the string will appear to be divided into a number of vibrating segments separated by stationary points, or nodes. these nodes are simply the points which remain at rest on account of the upward series of waves crossing the series which have been reflected at the top and are travelling downwards. the division of a vibrating string into nodes thus affords a simple example of the principle of interference. when a tuning-fork is vibrating there are certain hyperbolic lines along which the disturbance caused by one prong is exactly neutralized by that due to the other prong. if a large tuning-fork be struck and then held near the ear and slowly turned round, the positions of comparative silence will be readily perceived. if two notes are being sounded side by side, one consisting of two hundred vibrations per second and the other of two hundred and two, then, at any distant point, it is clear that the two sets of waves will arrive in the same condition, or "phase," twice in each second, and twice they will be in opposite conditions, and, if of the same intensity, will exactly destroy one another's effects, thus producing silence. hence twice in the second there will be silence and twice there will be sound, the waves of which have double the amplitude due to either source, and hence the sound will have four times the intensity of either note by itself. thus there will be two "beats" per second due to interference. later on this principle was applied by young to very many optical phenomena of which it afforded a complete explanation. young completed his last term of residence at cambridge in december, , and in the early part of he commenced practice as a physician at , welbeck street. in the following year he accepted the chair of natural philosophy in the royal institution, which had shortly before been founded, and soon afterwards, in conjunction with davy, the professor of chemistry, he undertook the editing of the journals of the institution. this circumstance has already been alluded to in connection with count rumford, the founder of the institution. he lectured at the royal institution for two years only, when he resigned the chair in deference to the popular belief that a physician should give his attention wholly to his professional practice, whether he has any or not. this fear lest a scientific reputation should interfere with his success as a physician haunted him for many years, and sometimes prevented his undertaking scientific work, while at other times it led him to publish anonymously the results he obtained. this anonymous publication of scientific papers caused him great trouble afterwards in order to establish his claim to his own discoveries. many of the articles which he contributed to the supplement to the fourth, fifth, and sixth editions of the "encyclopædia britannica" were anonymous, although the honorarium he received for this work was increased by per cent. when he would allow his name to appear. the practical withdrawal of young from the scientific world during sixteen years was a great loss to the progress of natural philosophy, while the absence of that suavity of manner when dealing with patients which is so essential to the success of a physician, prevented him from acquiring a valuable private practice. in fact, young was too much of a philosopher in his behaviour to succeed as a physician; he thought too deeply before giving his opinion on a diagnosis, instead of appearing to know all about the subject before he commenced his examination, and this habit, which is essential to the philosopher, does not inspire confidence in the practitioner. his fondness for society rendered him unwilling to live within the means which his uncle had left him, supplemented by what his scientific work might bring, and it was not until his income had been considerably increased by an appointment under the admiralty that he was willing to forego the possible increase of practice which might accrue by appearing to devote his whole attention to the subject of medicine. it was this fear of public opinion which caused him, in , to decline the offer of the appointment of secretary to the royal society, of which, in , he accepted the office of foreign secretary. young's resignation of the chair of natural philosophy was, however, not a great loss to the royal institution; for the lecture audience there was essentially of a popular character, and young cannot be considered to have been successful as a popular lecturer. his own early education had been too much derived from private reading for him to have become acquainted with the difficulties experienced by beginners of only average ability, and his lectures, while most valuable to those who already possessed a fair knowledge of the subjects, were ill adapted to the requirements of an unscientific audience. a syllabus of his course of lectures was published by young in , but it was not till that the complete course of sixty lectures was published in two quarto volumes. they were republished in in octavo, with references and notes by professor kelland. among the subjects treated in these lectures are mechanics, including strength of materials, architecture and carpentry, clocks, drawing and modelling; hydrostatics and hydraulics; sound and musical instruments; optics, including vision and the physical nature of light; astronomy; geography; the essential properties of matter; heat; electricity and magnetism; climate, winds, and meteorology generally; vegetation and animal life, and the history of the preceding sciences. the lectures were followed by a most complete bibliography of the whole subject, including works in english, french, german, italian, and latin. the following is the syllabus of one lecture, and illustrates the diversity of the subjects dealt with:-- "on drawing, writing, and measuring. "subjects preliminary to the study of practical mechanics; instrumental geometry; statics; passive strength; friction; drawing; outline; pen; pencil; chalks; crayons; indian ink; water-colours; body colours; miniature; distemper; fresco; oil; encaustic paintings; enamel; mosaic work. writing; materials for writing; pens; inks; use of coloured inks for denoting numbers; polygraph; telegraph; geometrical instruments; rulers; compasses; flexible rulers; squares; triangular compasses; parallel rulers; marquois's scales; pantograph; proportional compasses; sector. measurement of angles; theodolites; quadrants; dividing-engine; vernier; levelling; sines of angles; gunter's scale; nicholson's circle; dendrometer; arithmetical machines; standard measures; quotation from laplace; new measures; decimal divisions; length of the pendulum and of the meridian of the earth; measures of time; objections; comparison of measures; instruments for measuring; micrometrical scales; log-lines." this represents an extensive area to cover in a lecture of one hour. when newton, by means of a prism, "unravelled all the shining robe of day," he showed that sunlight is made up of light varying in tint from red, through orange, yellow, green, and blue, to violet, and that by recombining all these kinds of light, or certain of them selected in an indefinite number of ways, white light could be produced. subsequently sir wm. herschel showed that rays less refrangible than the red were to be found among the solar radiation; and other rays more refrangible than the violet, but, like the ultra-red rays, incapable of exciting vision, were found by ritter and wollaston. in speaking of newton's experiments, in his thirty-seventh lecture, young says:-- it is certain that the perfect sensations of yellow and of blue are produced respectively by mixtures of red and green and of green and violet light, and there is reason to suspect that those sensations are always compounded of the separate sensations combined; at least, this supposition simplifies the theory of colours. it may, therefore, be adopted with advantage, until it be found inconsistent with any of the phenomena; and we may consider white light as composed of a mixture of red, green, and violet only, ... with respect to the quantity or intensity of the sensations produced. it should be noticed that, in the above quotation, young speaks only of the sensations produced. objectively considered, sunlight consists of an infinite number of differently coloured lights comprising nearly all the shades from one end of the spectrum to the other, though white light may have a much simpler constitution, and may, for example, consist simply of a mixture of homogeneous red, green, and violet lights, or of homogeneous yellow and blue lights, properly selected. but considered subjectively, young implies that the eye perceives three, and only three, distinct colour-sensations, corresponding to pure red, green, and violet; that when these three sensations are excited in a certain proportion, the complex sensation is that of white light; but if the relative intensities of the separate sensations differ from these ratios, the perception is that of some colour. to exhibit the effects of mixing light of different colours, young painted differently coloured sectors on circles of cardboard, and then made the discs rotate rapidly about their centres, when the effect was the same as though the lights emitted by the sectors were mixed in proportion to the breadth of the sectors. this contrivance had been previously employed by newton, and will be again referred to in connection with another memoir. the results of these experiments were embodied by young in a diagram of colour, consisting of an equilateral triangle, in which the colours red, green, and violet, corresponding to the simple sensations, were placed at the angles, while those produced by mixing the primary colours in any proportions, were to be found within the triangle or along its sides; the rule being that the colour formed by the admixture of the primary colours in any proportions, was to be found at the centre of gravity of three heavy particles placed at the angular points of the triangle, with their masses proportioned to the corresponding amounts of light. thus the colours produced by the admixture of red and green only, in different proportions, were placed along one side of the triangle, these colours corresponding to various tints of scarlet, orange, yellow, and yellowish green; another side contained the mixtures of green and violet representing the various shades of bluish green and blue; and the third side comprised the admixtures of red and violet constituting crimsons and purples. the interior of the triangle contained the colours corresponding to the mixture of all three primary sensations, the centre being neutral grey, which is a pure white faintly illuminated. if white light of a certain degree of intensity fall on white paper, the paper appears white, but if a stronger light fall on another portion of the same sheet, that which is less strongly illuminated appears grey by contrast. shadows thrown on white paper may possess any degree of intensity, corresponding to varying shades of neutral grey, up to absolute blackness, which corresponds to a total absence of light. thus considered, chromatically black and white are the same, differing only in the amount of light they reflect. a piece of white paper in moonlight is darker than black cloth in full sunlight. it must be remembered that young's diagram of colours corresponds to the admixture of coloured lights, not of colouring materials or pigments. the admixture of blue and yellow lights in proper proportions may make white or pink, but never green. the admixture of blue and yellow pigments makes a green, because the blue absorbs nearly all the light except green, blue, and a little violet, while the yellow absorbs all except orange, yellow, and green. the green light is the only light common to the two, and therefore the only light which escapes absorption when the pigments are mixed. another point already noticed must also be carefully borne in mind. young was quite aware that, physically, there are an infinite number of different kinds of light differing continuously in wave-length from the ultra-red to the ultra-violet, though colour can hardly be regarded as an attribute of the light considered objectively. the question of colour is essentially one of perception--a physiological, not a physical, question--and it is only in this sense that young maintained the doctrine of three primary colours. in his paper on the production of colours, read before the royal society on july , , he speaks of "the proportions of the sympathetic fibres of the retina," corresponding to these primary colour-sensations. according to this doctrine, white light would always be produced when the three sensations were affected in certain proportions, whether the exciting cause were simply two kinds of homogeneous light, corresponding to two pure tones in music, or an infinite number of different kinds, as in sunlight; and a particular yellow sensation might be excited by homogeneous yellow light from one part of the spectrum, or by an infinite number of rays of different wave-lengths, corresponding to various shades of red, orange, yellow, and green. subjectively, the colours would be the same; objectively, the light producing them would differ exceedingly. but young's greatest service to science was his application of the principle of interference--of which he had already made good use in the theory of sound--to the phenomena of light. the results of these researches were presented to the royal society, and two of the papers were selected as bakerian lectures in and respectively. unfavourable criticisms of these papers, which appeared in the _edinburgh review_, and were said to have been written by mr. (afterwards lord) brougham, seem to have caused their contents to be neglected by english men of science for many years; and it was to arago and fresnel that we are indebted for recalling public attention to them. the undulatory theory of light, which maintains that light consists of waves transmitted through an _ether_, which pervades all space and all matter, owes its origin to hooke and huyghens. huyghens showed that this theory explained, in a very beautiful manner, the laws of reflection and of refraction, if it be allowed that light travels more slowly the denser the medium. according to the celebrated principle of huyghens, every point in the front of a wave at any instant becomes a centre of disturbance, from which a secondary wave is propagated. the fronts of these secondary waves all lie on a surface, which becomes the new surface of the primary wave. when light enters a denser medium obliquely, the secondary waves which are propagated within the denser medium extend to a less distance than those propagated in the rarer medium, and thus the front of the primary wave becomes bent at the point where it meets the common surface. huyghens explained, not only the laws of ordinary refraction in this manner, but, by supposing the secondary waves to form spheroids instead of spheres, he obtained the laws of refraction of the extraordinary ray in iceland-spar. he did not, however, succeed in explaining why light should not diverge laterally instead of proceeding in straight lines. newton supported the theory that light consists of particles or corpuscles projected in straight lines from the luminous body, and sometimes transmitted, sometimes reflected, when incident on a transparent medium of different density. to account for the particle being sometimes transmitted and sometimes reflected, newton had recourse to the hypothesis of "fits of easy transmission and of easy reflection," and, to account for the fits themselves, he supposed the existence of an ether, the vibrations of which affected the particles. the laws of reflection were readily explained, being the same as for a perfectly elastic ball; the laws of refraction admitted of very simple explanation, by supposing that the particles of the denser medium exert a greater attraction on the particles of light than those of the rarer medium, but that this attraction acts only through very short distances, so that when the light-corpuscle is at a sensible distance from the surface, it is attracted equally all round, and moves as though there were no force acting upon it. as a consequence of this hypothesis, it follows that the velocity of light must be greater the denser the medium, while the undulatory theory leads to precisely the opposite result. when foucault directly measured the velocity of light both in air and water, and found it less in the denser medium, the result was fatal to the corpuscular theory. dr. young called attention to another crucial test between the two theories. when a piece of plate-glass is pressed against a slightly convex lens, or a watch-glass, a series of coloured rings is formed by reflected light, with a black spot in the centre. this was accounted for by newton by supposing that the light which was reflected in any ring was in a fit of easy transmission (from glass to air) when it reached the first surface of the film of air, and in a fit of easy reflection when it reached the second surface. by measuring the thickness of a film of air corresponding to the first ring of any particular colour, the length of path corresponding to the interval between two fits for that particular kind of light could be determined. when water instead of air is placed between the glasses, according to the corpuscular theory the rings should expand; but according to the undulatory theory they should contract; for the wave-length corresponds to the distance between successive fits of the same kind on the corpuscular hypothesis. on trying the experiment, the rings were seen to contract. this result seemed to favour the undulatory theory; but the objection urged by newton that rays of light do not bend round obstacles, like waves of sound, still held its ground. this objection young completely demolished by his principle of interference. he showed that when light passes through an aperture in a screen, whatever the shape of the aperture, provided its width is large in comparison with the length of a wave of light (one fifty-thousandth of an inch), no sensible amount of light will reach any point not directly in front of the aperture; for if any point be taken to the right or left, the disturbances reaching that point from different points of the aperture will neutralize one another by interference, and thus no light will be appreciable. when the breadth of the aperture is only a small multiple of a wave-length, then there will be some points outside the direct beam at which the disturbances from different points of the aperture will not completely destroy one another, and others at which they will destroy one another; and these points will be different for light of different wave-lengths. in this way young not only explained the rectilinear propagation of light, but accounted for the coloured bands formed when light diverges from a point through a very narrow aperture. in a similar way he accounted for the hyperbolic bands of colour observed by grimaldi within the shadow of a square near its corners. with a strip of card one-thirtieth of an inch in width, young obtained bands of colour within the shadow which completely disappeared when the light was cut off from either side of the strip of card, showing that they were produced by interference of the two portions of light which had passed, one to the right, the other to the left, of the strip of card. professor stokes has succeeded in showing a bright spot at the centre of the shadow of a circular disc of the size of a sovereign. the narrow bands of colour formed near the edge of the shadow of any object, which newton supposed to be due to the "inflection" of the light by the attraction of the object, young showed to be independent of the material or thickness of the edge, and completely accounted for them by the principle of interference. newton's rings were explained with equal facility. they were due to the interference of light reflected from the first and second surfaces of the film of air or water between the glasses. the black spot at the centre of the reflected rings was due to the difference between reflection taking place from the surface of a denser or a rarer medium, half an undulation being lost when the reflection takes place in glass at the surface of air. if a little grease or water be placed between two pieces of glass which are nearly in contact, but the space between be not filled with the water or grease, but contain air in some parts, and water or grease in others, a series of rings will be seen by transmitted light, which have been called "the colours of mixed plates." young showed that these colours could be accounted for by interference between the light that had passed through the air and that which had passed through the water, and explained the fact that, to obtain the same colour, the distance between the plates must be much greater than in the case of newton's rings. the bands of colour produced by the interference of light proceeding from a point and passing on each side of a narrow strip of card, have already been referred to. the bands are broader the narrower the strip of card. a fine hair gives very broad bands. when a number of hairs cross one another in all directions, these bands form circular rings of colour. if the width of the hairs be very variable, the rings formed will be of different sizes and overlapping one another, no distinct series will be visible; but when the hairs are of nearly the same diameter, a series of well-defined circles of colour, resembling newton's rings, will be seen, and if the diameter of a particular ring be measured, the breadth of the hairs can be inferred. young practically employed this method for measuring the diameter of the fibres of different qualities of wool in order to determine their commercial value. the instrument employed he called the _eriometer_. it consisted of a plate of brass pierced with a round hole about one-thirtieth of an inch in diameter in the centre, and around this a small circle, about one-third of an inch in diameter, of very fine holes. the plate was placed in front of a lamp, and the specimen of wool was held on wires at such a distance in front of the brass plate that the first green ring appeared to coincide with the circle of small holes. the eye was placed behind the lock of wool, and the distance to which the wool had to be removed in front of the brass plate in order that the first green ring might exactly coincide with the small circle of fine holes, was proportional to the breadth of the fibres. the same effect is produced if fine particles, such as lycopodium powder, or blood-corpuscles, scattered on a piece of glass, be substituted for the lock of wool, and young employed the instrument in order to determine the diameter of blood-corpuscles. he determined the constant of his apparatus by comparison with some of dr. wollaston's micrometric observations. the coloured halos sometimes seen around the sun young referred to the existence of small drops of water of nearly uniform diameter, and calculated the necessary diameter for halos of different angular magnitudes. the same principle of interference afforded explanation of the colours of striated surfaces, such as mother-of-pearl, which vary with the direction in which they are seen. viewed at one angle light of a particular colour reflected from different ridges will be in a condition to interfere, and this colour will be absent from the reflected light. at a different inclination, the light reaching the eye from all the ridges (within a certain angle) will be in precisely the same phase, and only then will light of that colour be reflected in its full intensity. with a micrometer scale engraved on glass by coventry, and containing five hundred lines to the inch, young obtained interference spectra. modern gratings, with several thousand lines to the inch, afford the purest spectra that can be obtained, and enable the wave-length of any particular kind of light to be measured with the greatest accuracy. young's dislike of mathematical analysis prevented him from applying exact calculation to the interference phenomena which he observed, such as subsequently enabled fresnel to overcome the prejudice of the french academy and to establish the principle on an incontrovertible footing. young's papers attracted very little attention, and fresnel made for himself many of young's earlier discoveries, but at once gave young the full credit of the work when his priority was pointed out. the phenomena of polarization, however, still remained unexplained. both young and fresnel had regarded the vibrations of light as similar to those of sound, and taking place in the direction in which the wave is propagated. the fact that light which had passed through a crystal of iceland-spar, was differently affected by a second crystal, according to the direction of that crystal with respect to the former, showed that light which had been so transmitted was not like common light, symmetrical in all azimuths, but had acquired sides or poles. such want of symmetry could not be accounted for on the hypothesis that the vibrations of light took place at right angles to the wave-front, that is, in the direction of propagation of the light. the polarization of light by reflection was discovered by malus, in . in a letter written to arago, in , young hinted at the possibility of the existence of a component vibration at right angles to the direction of propagation, in light which had passed through iceland-spar. in the following year fresnel arrived independently at the hypothesis of transverse vibrations, not as constituting a small component of polarized light, but as representing completely the mode of vibration of all light, and in the hands of fresnel this hypothesis of transverse vibrations led to a theory of polarization and double refraction both in uniaxal and biaxal crystals which, though it can hardly be regarded as complete from a mechanical point of view, is nevertheless one of the most beautiful and successful applications of mathematics to physics that has ever been made. to young, however, belongs the credit of suggesting that the spheroidal form of the waves in iceland-spar might be accounted for by supposing the elasticity different in the direction of the optic axis and at right angles to that direction; and he illustrated his view by reference to certain experiments of chladni, in which it had been shown that the velocity of sound in the wood of the scotch fir is different along, and perpendicular to, the fibre in the ratio of to . young was also the first to explain the colours exhibited by thin plates of crystals in polarized light, discovered by arago in , by the interference of the ordinary and extraordinary rays, and fresnel afterwards completed young's explanation in . it is for his contributions to the undulatory theory of light that young will be most honourably remembered. hooke, in , referred to light as a "quick, short, vibrating motion;" huyghens's "traité de la lumière" was published in . from that time the undulatory theory lost ground, until it was revived by young and fresnel. it soon after received great support from the establishment, by joule and others, of the mechanical theory of heat. one remark of young's respecting the ether opens up a question which has attracted much attention of late years. in a letter addressed to the secretary of the royal society, and read january , , he says:-- that a medium, resembling in many properties that which has been denominated ether, does really exist, is undeniably proved by the phenomena of electricity; and the arguments against the existence of such an ether throughout the universe have been pretty sufficiently answered by euler. the rapid transmission of the electrical shock shows that the electric medium is possessed of an elasticity as great as is necessary to be supposed for the propagation of light. whether the electric ether is to be considered as the same with the luminous ether--if such a fluid exists--may perhaps at some future time be discovered by experiment. besides his contributions to optics, young made distinct advances in connection with elasticity, and with surface-tension, or "capillarity." it is said that leonardo da vinci was the first to notice the ascent of liquids in fine tubes by so-called capillary attraction. this, however, is only one of a series of phenomena now very generally recognized, and all of which are referable to the same action. the hanging of a drop from the neck of a phial; the pressure of air required to inflate a soap-bubble; the flotation of a greasy needle on the surface of water; the manner in which some insects rest on water, by depressing the surface, without wetting their legs; the possibility of filling a tumbler with water until the surface stands above the edge of the glass; the nearly spherical form of rain-drops and of small drops of mercury, even when they are resting on a table,--are all examples of the effect of surface-tension. these phenomena have recently been studied very carefully by quincke and plateau, and they have been explained in accordance with the principle of energy by gauss. hawksbee, however, was the first to notice that the rise of a liquid in a fine tube did not depend on the thickness of the walls of the tube, and he therefore inferred that, if the phenomena were due to the attraction of the glass for the liquid, it could only be the superficial layers which produced any effect. this was in . segner, in , introduced the notion of a surface-tension; and, according to his view, the surface of a liquid must be considered as similar to a thin layer of stretched indiarubber, except that the tension is always the same at the surface bounding the same media. this idea of surface-tension was taken up by young, who showed that it afforded explanation of all the known phenomena of "capillarity," when combined with the fact, which he was himself the first to observe, that the angle of contact of the same liquid-surface with the same solid is constant. this angle he called the "appropriate angle." but young went further, and attempted to explain the existence of surface-tension itself by supposing that the particles of a liquid not only exert an attractive force on one another, which is constant, but also a repulsive force which increases very rapidly when the distance between them is made very small. his views on this subject were embodied in a paper on the cohesion of liquids, read before the royal society in . he afterwards wrote an article on the same subject for the supplement of the "encyclopædia britannica." the changes which solids undergo under the action of external force, and their tendency to recover their natural forms, were studied by hooke and gravesande. the experimental fact that, for small changes of form, the extension of a rod or string is proportional to the tension to which it is exposed, is known as hooke's law. the compression and extension of the fibres of a bent beam were noticed by james bernoulli, in , by duhamel and others. the bending of beams was also studied by coulomb and robison, but young appears to have been almost the first to apply the theory of elasticity to the statics of structures. in a letter to the secretary of the admiralty, written in , in reply to an invitation to report on mr. steppings's improvements in naval architecture, young claimed that he was the only person who had published "any attempts to improve the _theory_ of carpentry." it may be here mentioned that young accepted the invitation of the admiralty, and sent in a very exhaustive report, which their lordships regarded as "too learned" to be of great practical value. young's contributions to this subject will be chiefly remembered in connection with his "modulus of elasticity." this he originally defined as follows:-- "the modulus of the elasticity of any substance is a column of the same substance capable of producing a pressure on its base which is to the weight causing a certain degree of compression as the length of the substance is to the diminution of its length." it is not usual now to express young's modulus of elasticity in terms of a length of the substance considered. as now usually defined, young's modulus of elasticity is the force which would stretch a rod or string to double its natural length if hooke's law were true for so great an extension. so much of dr. young's scientific work has been mentioned here because it was during his early years of professional practice that his most original scientific work was accomplished. as already stated, after two years' tenure of the natural philosophy chair at the royal institution, young resigned it because his friends were of opinion that its tenure militated against his prospects as a physician. in the summer of he escorted the great-nephews of the duke of richmond to rouen, and took the opportunity of visiting paris. in march, , he took his degree of m.b. at cambridge, and on june , , he married eliza, second daughter of j. p. maxwell, esq., whose country seat was near farnborough. for sixteen years after his marriage, young resided at worthing during the summer, where he made a very respectable practice, returning to london in october or november. in january, , he was elected one of the physicians of st. george's hospital, which appointment he retained for the rest of his life. in this capacity his practice was considerably in advance of the times, for he regarded medicine as a science rather than an empirical art, and his careful methods of induction demanded an amount of attention which medical students, who preferred the more rough-and-ready methods then in vogue, were slow to give. the apothecary of the hospital stated that more of dr. young's patients went away cured than of those who were subjected to the more fashionable treatment; but his private practice, notwithstanding the sacrifices he had made, never became very valuable. in young was appointed secretary to a commission for determining the length of the second's pendulum. the reports of this commission were drawn up by him, though the experimental work was carried out by captain kater. the result of the work was embodied in an act of parliament, introduced by sir george clerk, in , which provided that if the standard yard should be lost it should "be restored to the same length," by making it bear to the length of the second's pendulum at sea-level in london, the ratio of to · ; but before the standards were destroyed, in , so many sources of possible error were discovered in the reduction of pendulum observations, that the commission appointed to restore the standards recommended that a material standard yard should be constructed, together with a number of copies, so that, in the event of the standard being again destroyed, it might be restored by comparison with its copies. in young was appointed superintendent of the nautical almanac and secretary of the board of longitude. when this board was dissolved in , its functions were assumed by the admiralty, and young, faraday, and colonel sabine were appointed a scientific committee of reference to advise the admiralty in all matters in which their assistance might be required. the income from these government appointments rendered young more independent of his practice, and he became less careful to publish his scientific papers anonymously. in he left worthing and gave up his practice there. the following year, in company with mrs. young, he took a tour through france, switzerland, and italy, and at paris attended a meeting of the institute, where he met arago, who had called on him in worthing, in . at the same time he made the acquaintance of laplace, cuvier, humboldt, and others. in he visited spa, and took a tour through holland. in the same year young was appointed inspector of calculations and medical referee to the palladium insurance company. this caused him to turn his attention to the subject of life assurance and bills of mortality. in , as foreign secretary of the royal society, he had the satisfaction of forwarding to fresnel the rumford medal in acknowledgment of his researches on polarized light. fresnel died, in his fortieth year, a few days after receiving the medal. dr. young died on may , , in the fifty-sixth year of his age, his excessive mental exertions in early life having apparently led to a premature old age. he was buried in the parish church of farnborough, and a medallion by sir francis chantrey was erected to his memory in westminster abbey. but, though young was essentially a scientific man, his accomplishments were all but universal, and any memoir of him would be very incomplete without some sketch of his researches in egyptian hieroglyphics. his classical training, his extensive knowledge of european and eastern languages, and his neat handwriting and drawing, have already been referred to. to these attainments must be added his scientific _method_ and power of careful and systematic observation, and it will be seen that few persons could come to the task of deciphering an unknown language with a better chance of success than dr. young. the rosetta stone was found by the french while excavating at fort st. pierre, near rosetta, in , and was brought to england in . the stone bore an inscription in three different kinds of character--the hieroglyphic, the enchorial or demotic, and the ordinary greek. young's attention was first called to the egyptian characters by a manuscript which was submitted to him in . he then obtained copies of the inscriptions on the rosetta stone and subjected them to a careful analysis. the latter part of the greek inscription was very much injured, but was restored by the conjectures of porson and heyne, and read as follows:--"what is here decreed shall be inscribed on a block of hard stone, in sacred, in enchorial, and in greek characters, and placed in each temple, of the first, second, and third gods." this indicated that the three inscriptions contained the same decree, but, unfortunately, the beginnings of the first and second inscriptions were lost, so that there were no very definitely fixed points to start upon. the words "alexander" and "alexandria," however, occurred in the greek, and these words, being so much alike, might be recognized in each of the other inscriptions. the word "ptolemy" appeared eleven times in the greek inscription, and there was a word which, from its length and position, seemed to correspond to it, which, however, appeared fourteen times in the hieroglyphic inscription. this word, whenever it appeared in the hieroglyphics, was surrounded by a ring forming what champollion called a _cartouche_, which was always employed to denote the names of royal persons. these words were identified by baron sylvestre de sacy and the swedish scholar akerblad. young appears to have started with the idea, then generally current, that hieroglyphic symbols were purely ideographic, each sign representing a word. his knowledge of chinese, however, led him to modify this view. in that language native words are represented by single symbols, but, when it is necessary to write a foreign word, a group of word-symbols is employed, each of which then assumes a phonetic character of the same value as the initial letter of the word which it represents. the phonetic value of these signs is indicated in chinese by a line at the side, or by enclosing them in a square. young supposed that the ring surrounding the royal names in the hieroglyphic inscription had the same value as the phonetic mark in chinese, and from the symbols in the name of ptolemy he commenced to construct a hieroglyphic alphabet. he made an error, however, in supposing that some of the symbols might be syllabic instead of alphabetic. it is true that in the older inscriptions single signs have sometimes a syllabic value, and sometimes are used ideographically, while in other cases a single sign representing the whole word is employed in conjunction with the alphabetic signs, probably to distinguish the word from others spelt in the same way, but in inscriptions of so late a date as the rosetta stone, the symbols were purely alphabetic. another important step made by young was the discovery of the use of _homophones_, or different symbols to represent the same letter. young's work was closely followed up by champollion, and afterwards by lepsius, birsch, and others. the greater part of his researches he never published, though he made careful examinations of several funeral rolls and other documents. it would occupy too much space to give an adequate account of young's researches in this subject; some portion of his work he published in a popular form in the article "egypt," in the supplement of the "encyclopædia britannica," to which supplement he contributed about seventy articles on widely different subjects. perhaps it is not too much to say that to young we owe the foundation of all we now know of hieroglyphics and the egyptian history which has been learned from them; and the obelisk on the thames embankment should call to mind the memory of no one more prominently than that of thomas young. michael faraday. the work of michael faraday introduced a new era in the history of physical science. unencumbered by pre-existing theories, and untrammelled by the methods of the mathematician, he set forth on a line of his own, and, while engaged in the highest branches of experimental research, he sought to explain his results by reference to the most elementary mechanical principles only. hence it was that those conclusions which had been obtained by mathematicians only by the help of advanced analytical methods, and which were expressed by them only in the language of the integral calculus, faraday achieved without any such artificial aids to thought, and expressed in simple language, having reference to the mechanism which he conceived to be the means by which such results were brought about. for a long time faraday's methods were regarded by mathematicians with something more than suspicion, and, while they could not but admire his experimental skill and were compelled to admit the accuracy of his conclusions, his mode of thought differed too widely from that to which they were accustomed to command their assent. in sir william thomson, and in clerk maxwell, faraday at length found interpreters between him and the mathematical world, and to the mathematician perhaps the greatest monument of the genius of faraday is the "electricity and magnetism" of clerk maxwell. michael faraday was born at newington, surrey, on september , , and was the third of four children. his father, james faraday, was the son of robert and elizabeth faraday, of clapham wood hall, in the north-west of yorkshire, and was brought up as a blacksmith. he was the third of ten children, and, in , married margaret hastwell, a farmer's daughter. soon after his marriage he came to london, where michael was born. in james faraday, with his family, moved from newington, and took rooms over a coach-house in jacob's well mews, charles street, manchester square. in looking at this humble abode one can scarcely help thinking that the yorkshire blacksmith and his little family would have been far happier in a country "smiddy" near his native moors than in a crowded london court; but, had he remained there, it is difficult to see how the genius of young michael could have met with the requisites for its development. james faraday was far from enjoying good health, and his illness often necessitated his absence from work, and, as a consequence, his family were frequently in very straitened circumstances. the early education of michael was, therefore, not of a very high order, and consisted "of little more than the rudiments of reading, writing, and arithmetic." like most boys in a similar position in london, he found his amusement for the most part in the streets, but, except that in his games at marbles we may assume that he played with other boys, we have no evidence whether his time was spent mostly by himself, or whether he was one of a "set" of street companions. in , when thirteen years of age, michael faraday went as errand-boy to mr. geo. riebau, a bookseller in blandford street. part of his duty in this capacity was to carry round papers lent on hire by his master, and in his "life of faraday," dr. bence jones tells how anxious the young errand-boy was to collect his papers on sunday morning in time to attend the sandemanian service with the other members of his family. faraday was apprenticed to mr. riebau on october , , and learned the business of a bookbinder. he occasionally occupied his spare time in reading the scientific books he had to bind, and was particularly interested in mrs. marcet's "conversations in chemistry," and in the article on "electricity" in the "encyclopædia britannica." these were days before the existence of the london society for the extension of university teaching, and, though professor anderson in glasgow had shown how the advantages of a university might be extended to those whose fortunes prevented them from becoming regular university students, professor stuart had not yet taught the english universities that they had responsibilities outside their own borders, and that the national universities of the future must be the teachers of all classes of the community. but private enterprise supplied in a measure the neglect of public bodies. mr. tatum, of , dorset street, fleet street, advertised a course of lectures on natural philosophy, to be delivered at his residence at eight o'clock in the evenings. the price of admission was high, being a shilling for each lecture, but michael's brother robert frequently supplied him with the money, and in attending these lectures faraday made many friendships which were valuable to him afterwards. faraday appears to have been aware of the value of skill in drawing--a point to which much attention has recently been called by those interested in technical education--and he spent some portion of his time in studying perspective, so as to be better able to illustrate his notes of mr. tatum's lectures, as well as of some of sir humphry davy's, which he was enabled to hear at the royal institution through the kindness of a customer at mr. riebau's shop. in , before the end of his apprenticeship, faraday was engaged in experiments with voltaic batteries of his own construction. having cut out seven discs of zinc the size of halfpence, and covered them with seven halfpence, he formed a pile by inserting pieces of paper soaked in common salt between each pair, and found that the pile so constructed was capable of decomposing epsom salts. with a somewhat larger pile he decomposed copper sulphate and lead acetate, and made some experiments on the decomposition of water. on july , , in writing to his friend abbott, he mentions the movements of camphor when floating on water, and adds, "science may be illustrated by those minute actions and effects, almost as much as by more evident and obvious phenomena.... my knife is so bad that i cannot mend my pen with it; it is now covered with copper, having been employed to precipitate that metal from the muriatic acid." something of faraday's disposition, as well as of the results of his self-education, may be gathered from the following quotations from letters to abbott, written at this time:-- i have again gone over your letter, but am so blinded that i cannot see any subject except chlorine to write on; but before entering on what i intend shall fill up the letter, i will ask your pardon for having maintained an opinion against one who was so ready to give his own up. i suspect from that circumstance i am wrong.... in the present case i conceive that experiments may be divided into three classes: first, those which are for the old theory of oxymuriatic acid, and consequently oppose the new one; second, those which are for the new one, and oppose the old theory; and third, those which can be explained by both theories--apparently so only, for in reality a false theory can never explain a fact. it is not for me to affirm that i am right and you wrong; speaking impartially, i can as well say that i am wrong and you right, or that we both are wrong and a third right. i am not so self-opinionated as to suppose that my judgment and perception in this or other matters is better or clearer than that of other persons; nor do i mean to affirm that this is the true theory in reality, but only that my judgment conceives it to be so. judgments sometimes oppose each other, as in this case; and as there cannot be two opposing facts in nature, so there cannot be two opposing truths in the intellectual world. consequently, when judgments oppose, one must be wrong--one must be false; and mine may be so for aught i can tell. i am not of a superior nature to estimate exactly the strength and correctness of my own and other men's understanding, and will assure you, dear a----, that i am far from being convinced that my own is always right. i have given you the theory--not as the true one, but as the one which appeared true to me--and when i perceive errors in it, i will immediately renounce it, in part or wholly, as my judgment may direct. from this, dear friend, you will see that i am very open to conviction; and from the manner in which i shall answer your letter, you will also perceive that i must be convinced before i renounce. on october , , faraday's apprenticeship terminated, and immediately afterwards he started life as a journeyman bookbinder. he now found that he had less time at his disposal for scientific work than he had enjoyed when an apprentice, and his desire to give up his trade and enter fully upon scientific pursuits became stronger than ever. during his apprenticeship he had written to sir joseph banks, then president of the royal society, in the hope of obtaining some scientific employment; he now applied to sir humphry davy. in a letter written to dr. paris, in , faraday gave an account of this application. "my desire to escape from trade, which i thought vicious and selfish, and to enter into the service of science, which i imagined made its pursuers amiable and liberal, induced me at last to take the bold and simple step of writing to sir h. davy, expressing my wishes, and a hope that, if an opportunity came in his way, he would favour my views; at the same time, i sent the notes i had taken of his lectures. "the answer, which makes all the point of my communication, i send you in the original, requesting you to take great care of it, and to let me have it back, for you may imagine how much i value it. "you will observe that this took place at the end of the year ; and early in he requested to see me, and told me of the situation of assistant in the laboratory of the royal institution, then just vacant. "at the same time that he thus gratified my desires as to scientific employment, he still advised me not to give up the prospects i had before me, telling me that science was a harsh mistress, and, in a pecuniary point of view, but poorly rewarding those who devoted themselves to her service. he smiled at my notion of the superior moral feelings of philosophic men, and said he would leave me to the experience of a few years to set me right on that matter. "finally, through his good efforts, i went to the royal institution, early in march of , as assistant in the laboratory; and in october of the same year went with him abroad, as his assistant in experiments and in writing. i returned with him in april, , resumed my station in the royal institution, and have, as you know, ever since remained there." sir h. davy's letter was as follows:-- "sir, "i am far from displeased with the proof you have given me of your confidence, and which displays great zeal, power of memory, and attention. i am obliged to go out of town, and shall not be settled in town till the end of january; i will then see you at any time you wish. it would gratify me to be of any service to you; i wish it may be in my power. "i am, sir, "your obedient humble servant, "h. davy." the minutes of the meeting of managers of the royal institution, on march , , contain the following entry:--"sir humphry davy has the honour to inform the managers that he has found a person who is desirous to occupy the situation in the institution lately filled by william payne. his name is michael faraday. he is a youth of twenty-two years of age. his habits seem good, his disposition active and cheerful, and his manner intelligent. he is willing to engage himself on the same terms as those given to mr. payne at the time of quitting the institution. "resolved, that michael faraday be engaged to fill the situation lately occupied by mr. payne, on the same terms." about this time faraday joined the city philosophical society, which had been started at mr. tatum's house in . the members met every wednesday evening, either for a lecture or discussion; and perhaps the society did not widely differ from some of the "students' associations" which have more recently been started in connection with other educational enterprises. magrath was secretary of this society, and from it there sprang a smaller band of students, who, meeting once a week, either at magrath's warehouse in wood street, or at faraday's private rooms in the attics of the royal institution, for mutual improvement, read together, and freely criticized each other's pronunciation and composition. in a letter to abbott six weeks after commencing work at the royal institution, faraday says:-- a stranger would certainly think you and i were a couple of very simple beings, since we find it necessary to write to each other, though we so often personally meet; but the stranger would, in so judging, only fall into that error which envelops all those who decide from the outward appearances of things.... when writing to you i seek that opportunity of striving to describe a circumstance or an experiment clearly; so that you will see i am urged on by selfish motives partly to our mutual correspondence, but, though selfish, yet not censurable. during the summer of faraday, in his letters to abbott, gave his friend the benefit of his experience "on the subject of lectures and lecturers in general," in a manner that speaks very highly of his power of observation of men as well as things. he was of opinion that a lecture should not last more than an hour, and that the subject should "fit the audience." "a lecturer may consider his audience as being polite or vulgar (terms i wish you to understand according to shuffleton's new dictionary), learned or unlearned (with respect to the subject), listeners or gazers. polite company expect to be entertained, not only by the subject of the lecture, but by the manner of the lecturer; they look for respect, for language consonant to their dignity, and ideas on a level with their own. the vulgar--that is to say, in general, those who will take the trouble of thinking, and the bees of business--wish for something that they can comprehend. this may be deep and elaborate for the learned, but for those who are as yet tyros and unacquainted with the subject, must be simple and plain. lastly, listeners expect reason and sense, whilst gazers only require a succession of words." in favour of experimental illustration he says:-- "i need not point out ... the difference in the perceptive powers of the eye and the ear, and the facility and clearness with which the first of these organs conveys ideas to the mind--ideas which, being thus gained, are held far more retentively and firmly in the memory than when introduced by the ear.... apparatus, therefore, is an essential part of every lecture in which it can be introduced.... when ... apparatus is to be exhibited, some kind of order should be observed in the arrangement of them on the lecture-table. every particular part illustrative of the lecture should be in view, no one thing should hide another from the audience, nor should anything stand in the way of or obstruct the lecturer. they should be so placed, too, as to produce a kind of uniformity in appearance. no one part should appear naked and another crowded, unless some particular reason exists and makes it necessary to be so." on october , , faraday left the royal institution, in order to accompany sir humphry davy in a tour on the continent. his journal gives some interesting details, showing the inconveniences of foreign travel at that time. sir humphry davy took his carriage with him in pieces, and these had to be put together after escaping the dangers of the french custom-house on the quay at morlaix, two years before the battle of waterloo. one apparently trivial incident somewhat marred faraday's pleasure throughout this journey. it was originally intended that the party should comprise sir humphry and lady davy, faraday, and sir humphry's valet, but at the last moment that most important functionary declined to leave his native shores. davy then requested faraday to undertake such of the duties of valet as were essential to the well-being of the party, promising to secure the services of a suitable person in paris. but no eligible candidate appeared for the appointment, and thus faraday had throughout to take charge of domestic affairs as well as to assist in experiments. had there been only sir humphry and himself, this would have been no hardship. sir humphry had been accustomed to humble life in his early days; but the case was different with his lady, and, apparently, faraday was more than once on the point of leaving his patron and returning home alone. a circumstance which occurred at geneva illustrates the position of affairs. professor e. de la rive invited sir humphry and lady davy and faraday to dinner. sir humphry could not go into society with one who, in some respects, acted as his valet. when this point was represented to the professor, he replied that he was sorry, as it would necessitate his giving another dinner-party. faraday subsequently kept up a correspondence with de la rive, and continued it with his son. in writing to the latter he says, in speaking of professor e. de la rive, that he was "the first who personally at geneva, and afterwards by correspondence, encouraged and by that sustained me." at paris faraday met many of the most distinguished men of science of the time. one morning ampère, clément, and desormes called on davy, to show him some iodine, a substance which had been discovered only about two years before, and davy, while in paris, and afterwards at montpellier, executed a series of experiments upon it. after three months' stay, the party left paris for italy, _viâ_ montpellier, aix, and nice, whence they crossed the col de tende to turin. the transfer of the carriage and baggage across the alps was effected by a party of sixty-five men, with sledges and a number of mules. the description of the journey, as recorded in faraday's diary, makes us respect the courage of an englishman who, in the early part of this century, would attempt the conveyance of a carriage across the alps in the winter. "from turin we proceeded to genoa, which place we left afterwards in an open boat, and proceeded by sea towards lerici. this place we reached after a very disagreeable passage, and not without apprehensions of being overset by the way. as there was nothing there very enticing, we continued our route to florence; and, after a stay of three weeks or a month, left that fine city, and in four days arrived here at rome." the foregoing is from faraday's letter to his mother. at florence a good deal of time was spent in the academia del cimento. here faraday saw the telescope with which galileo discovered jupiter's satellites, with its tube of wood and paper about three feet and a half long, and simple object-glass and eye-glass. a red velvet electric machine with a rubber of gold paper, leyden jars pierced by the discharge between their armatures, the first lens constructed by galileo, and a number of other objects, were full of interest to the recently enfranchised bookbinder's apprentice; but it was the great burning-glass of the grand-duke which was the most serviceable of all the treasures of the museum. with this glass--which consisted of two convex lenses about three feet six inches apart, the first lens having a diameter of about fourteen or fifteen inches, and the second a diameter of three inches--davy succeeded in burning several diamonds in oxygen gas, and in proving that the diamond consists of little else than carbon. in faraday published a paper on this subject in the _quarterly journal of science_. at genoa some experiments were made with the torpedo, but the specimens caught were very small and weak, and their shocks so feeble that no definite results were obtained. at rome davy attempted to repeat an experiment of signor morrichini, whereby a steel needle was magnetized by causing the concentrated violet and blue rays from the sun to traverse the needle from the middle to the north end several times. the experiment did not succeed in the hands of davy and faraday, and it was left to the latter to discover a relation between magnetism and light. from rome they visited naples and ascended vesuvius, and shortly afterwards left italy for geneva. in the autumn of they returned from switzerland through germany, visiting berne, zurich, the tyrol, padua, venice, and bologne, to florence, where davy again carried out some chemical investigations in the laboratory of the academy. thence they returned to rome, and in the spring went on to naples, and again visited vesuvius, returning to england in april, _viâ_ rome, the tyrol, stuttgart, brussels, and ostend. a fortnight after his return from the continent faraday was again assistant at the royal institution, but with a salary of thirty shillings a week. his character will be sufficiently evident from the quotations which have been given from his diary and letters. henceforth we must be mainly occupied with the consideration of his scientific work. in january, , he gave his first lecture to the city philosophical society. in a lecture delivered shortly afterwards before the same society, the following passage, which gives an idea of one of the current beliefs of the time, occurs:-- "the conclusion that is now generally received appears to be that light consists of minute atoms of matter of an octahedral form, possessing polarity, and varying in size or in velocity.... "if now we conceive a change as far beyond vaporization as that is above fluidity, and then take into account also the proportional increased extent of alteration as the changes rise, we shall, perhaps, if we can form any conception at all, not fall far short of radiant matter;[ ] and as in the last conversion many qualities were lost, so here also many more would disappear. [footnote : not crookes's.] "it was the opinion of newton, and of many other distinguished philosophers, that this conversion was possible, and continually going on in the processes of nature, and they found that the idea would bear without injury the application of mathematical reasoning--as regards heat, for instance. if assumed, we must also assume the simplicity of matter; for it would follow that all the variety of substances with which we are acquainted could be converted into one of three kinds of radiant matter, which again may differ from one another only in the size of their particles or their form. the properties of known bodies would then be supposed to arise from the varied arrangements of their ultimate atoms, and belong to substances only as long as their compound nature existed; and thus variety of matter and variety of properties would be found co-essential. the simplicity of such a system is singularly beautiful, the idea grand and worthy of newton's approbation. it was what the ancients believed, and it may be what a future race will realize." in the closing words of his fifth lecture to the city philosophical society, faraday said:-- "the philosopher should be a man willing to listen to every suggestion, but determined to judge for himself. he should not be biassed by any appearances; have no favourite hypothesis; be of no school; and in doctrine have no master. he should not be a respecter of persons, but of things. truth should be his primary object. if to these qualities be added industry, he may indeed hope to walk within the veil of the temple of nature." many years afterwards he stated that, of all the suggestions to which he had patiently listened after his lectures at the royal institution, only one proved on investigation to be of any value, and that led to the discovery of the "extra current" and the whole subject of self-induction. faraday always kept a note-book, in which he jotted down any thoughts which occurred to him in reference to his work, as well as extracts from books or other publications which attracted his attention. he called it his "commonplace-book." many of the queries which he here took note of he subsequently answered by experiment. for example:-- "query: the nature of sounds produced by flame in tubes." "convert magnetism into electricity." "general effects of compression, either in condensing gases or producing solutions, or even giving combinations at low temperature." "do the pith-balls diverge by the disturbance of electricity through mutual induction or not?" speaking of this book, he says, "i already owe much to these notes, and think such a collection worth the making by every scientific man. i am sure none would think the trouble lost after a year's experience." in a letter dated may , , he writes:-- i have this evening been busy with an atmospherical electrical apparatus. it was a very temporary thing, but answered the purpose completely. a wire, with some small brush-wire rolled round the top of it, was elevated into the atmosphere by a thin wood rod having a glass tube at the end, and tied to a chimney-pot on the housetop; and this wire was continued down (taking care that it touched nothing in its way) into the lecture-room; and we succeeded, at intervals, in getting sparks from it nearly a quarter of an inch in length, and in charging a leyden jar, so as to give a strong shock. the electricity was positive. now, i think you could easily make an apparatus of this kind, and it would be a constant source of interesting matter; only take care you do not kill yourself or knock down the house. on june , , he married miss sarah barnard, third daughter of mr. barnard, of paternoster row--"an event which," to use his own words, "more than any other contributed to his earthly happiness and healthful state of mind." it was his wish that the day should be "just like any other day"--that there should be "no bustle, no noise, no hurry occasioned even in one day's proceeding," though in carrying out this plan he offended some of his relations by not inviting them to his wedding. up to this time faraday's experimental researches had been for the most part in the domain of chemistry, and for two years a great part of his energy had been expended in investigating, in company with mr. stodart, a surgical instrument-maker, the properties of certain alloys of steel, with a view to improve its manufacture for special purposes. it was in that he commenced his great discoveries in electricity. in the autumn of that year he wrote an historical sketch of electro-magnetism for the "annals of philosophy," and he repeated for himself most of the experiments which he described. in the course of these experiments, in september, , he discovered the rotation of a wire conveying an electric current around the pole of a magnet. [oe]rsted had discovered, in , the tendency of a magnetic needle to set itself at right angles to a wire conveying a current. this action is due to a tendency on the part of the north pole to revolve in a right-handed direction around the current, while the south pole tends to revolve in the opposite direction. the principle that action and reaction are equal and opposite indicates that, if a magnetic pole tend to rotate around a conductor conveying a current, there must be an equal tendency for the conductor to rotate around the pole. it was this rotation that constituted faraday's first great discovery in electro-dynamics. on december , in the same year, faraday showed that the earth's magnetism was capable of exerting a directive action on a wire conveying a current. writing to de la rive on the subject, he says:-- i find all the usual attractions and repulsions of the magnetic needle by the conjunctive wire are deceptions, the motions being, not attractions or repulsions, nor the result of any attractive or repulsive forces, but the result of a force in the wire, which, instead of bringing the pole of the needle nearer to or further from the wire, endeavours to make it move round it in a never-ending circle and motion whilst the battery remains in action. i have succeeded, not only in showing the existence of this motion theoretically, but experimentally, and have been able to make the wire revolve round a magnetic pole, or a magnetic pole round the wire, at pleasure. the law of revolution, and to which all the other motions of the needle are reducible, is simple and beautiful. conceive a portion of connecting wire north and south, the north end being attached to the positive pole of a battery, the south to the negative. a north magnetic pole would then pass round it continually in the apparent direction of the sun, from east to west above, and from west to east below. reverse the connections with the battery, and the motion of the pole is reversed; or, if the south pole be made to revolve, the motions will be in the opposite direction, as with the north pole. if the wire be made to revolve round the pole, the motions are according to those mentioned.... now, i have been able, experimentally, to trace this motion into its various forms, as exhibited by ampère's helices, etc., and in all cases to show that the attractions and repulsions are only appearances due to this circulation of the pole; to show that dissimilar poles repel as well as attract, and that similar poles attract as well as repel; and to make, i think, the analogy between the helix and common bar magnet far stronger than before. but yet i am by no means decided that there are currents of electricity in the common magnet. i have no doubt that electricity puts the circles of the helix into the same state as those circles are in that may be conceived in the bar magnet; but i am not certain that this state is directly dependent on the electricity, or that it cannot be produced by other agencies; and therefore, until the presence of electric currents be proved in the magnet by other than magnetical effects, i shall remain in doubt about ampère's theory. the most convenient rule by which to remember the direction of these electro-magnetic rotations is probably that given by clerk maxwell, which will be stated in its place.[ ] if a circular plate of copper and another of zinc be connected by a piece (or better, by three pieces) of insulated wire, so that the zinc is about an inch above the copper, and the combined plates be suspended by a silk fibre in a small beaker of dilute sulphuric acid, which is placed on the pole of a large magnet, the liquid will be seen to rotate about a vertical axis in one direction, and the two plates with their connecting wires in the opposite direction. on reversing the polarity of the magnet, both rotations will be reversed. this is a very simple mode of exhibiting faraday's discovery. a little powdered resin renders the motion of the liquid readily visible. [footnote : see p. .] in faraday published his work on the liquefaction of gases, from which he concluded that there was no difference in kind between gases and vapours. in the course of this work he met with more than one serious explosion. on january , , he was elected a fellow of the royal society, and in , on the recommendation of sir humphry davy, he was appointed director of the laboratory of the royal institution, and in this capacity he instituted the laboratory conferences, which developed into the friday evening lectures. for five years after this, the greater part of faraday's spare time was occupied in some investigations in connection with optical glass, made at the request of the royal society, and at the expense of the government. mr. dollond and sir john herschel were associated with him on this committee, but the results obtained were not of much value to opticians. the silico-borate of lead which faraday prepared in the course of these experiments was, however, the substance with which he first demonstrated the effect of a magnetic field on the plane of polarization of light, and with which he discovered diamagnetic action. faraday's experimental researches were generally guided by theoretical considerations. frequently these theories were based on very slender premises, and sometimes were little else than flights of a scientific imagination, but they served to guide him into fruitful fields of discovery, and he seldom placed much confidence in his conclusions till he had succeeded in verifying them experimentally. for many years he had held the opinion that electric currents should exhibit phenomena analogous to those of electro-static induction. again and again he returned to the investigation, and attempted to obtain an induced current in one wire through the passage of a powerful current through a neighbouring conductor; but he looked for a permanent induced current to be maintained during the whole time that the primary current was flowing. at length, employing two wires wound together as a helix on a wooden rod, the first capable of transmitting a powerful current from a battery, while the second was connected with a galvanometer, he observed that, when the current started in the primary, there was a movement of the galvanometer, and when it ceased there was a movement in the opposite direction, though the galvanometer remained at zero while the current continued steady. hence it was apparent that it is by changes in the primary current that induced currents may be generated, and not by their steady continuance; and it was demonstrated that, when a current is started in a conductor, a temporary current is induced in a neighbouring conductor in the opposite direction, while a current is induced in the same direction as the primary when the latter ceases to flow. before obtaining this result with the wires on a wooden bobbin, he had experimented with a wrought-iron ring about six inches in diameter, and made of / -inch round iron. he wound two sets of coils round it, one occupying nearly half the ring, and the other filling most of the other half. one of these he connected with a galvanometer, the other could be connected at will with a battery. on sending the battery current through the latter coil, the galvanometer needle swung completely round four or five times, and a similar action took place, but in the opposite direction, on stopping the current. here it was clearly the magnetism induced in the iron ring which produced so powerful a current in the galvanometer circuit. next he wound a quantity of covered copper wire on a small iron bar, and connecting the ends to a galvanometer, he placed the little bobbin between the opposite poles of a pair of bar magnets, whose other ends were in contact. as soon as the iron core touched the magnets, a current appeared in the galvanometer. on breaking contact, the current was in the opposite direction. then came the experiment above mentioned, in which no iron was employed. after this, one end of a cylindrical bar magnet was introduced into a helix of copper wire, and then suddenly thrust completely in. the galvanometer connected with the coil showed a transient current. on withdrawing the magnet, the current appeared in the opposite direction; so that currents were induced merely by the relative motion of a magnet and a conductor. a copper disc was mounted so that it could be made to rotate rapidly. a wire was placed in connection with the centre of the disc, and the circuit completed by a rubbing contact on the circumference. a galvanometer was inserted in the circuit, and the large horseshoe magnet of the royal institution so placed that the portion of the disc between the centre and the rubbing contact passed between the poles of the magnet. a current flowed through the galvanometer as long as the disc was kept spinning. then he found that the mere passage of a copper wire between the poles of the magnet was sufficient to induce a current in it, and concluded that the production of the current was connected with the cutting of the "magnetic curves," or "lines of magnetic force" which would be depicted by iron filings. thus in the course of ten days' experimental work, in the autumn of , faraday so completely investigated the phenomena of electro-magnetic induction as to leave little, except practical applications, to his successors. a few weeks later he obtained induction currents by means of the earth's magnetism only, first with a coil of wire wound upon an iron bar in which a strong current was produced when it was being quickly placed in the direction of the magnetic dip or being removed from that position, and afterwards with a coil of wire without an iron core. on february , , he succeeded in obtaining a spark from the induced current. unless the electro-motive force is very great, it is not possible to obtain a spark between two metallic surfaces which are separated by a sensible thickness of air. if, however, the circuit of a wire is broken _while_ the current is passing, a little bridge of metallic vapour is formed, across which for an instant the spark leaps. the induced current being of such short duration, the difficulty was to break the circuit while it was flowing. faraday wound a considerable length of fine wire around a short bar of iron; the ends of the wire were crossed so as just to be in contact with one another, but free to separate if exposed to a slight shock. the ends of the iron bar projected beyond the coil, and were held just over the poles of the magnet. on releasing the bar it fell so as to strike the magnetic poles and close the circuit of the magnet. an induced current was generated in the wire, but, while this was passing, the shock caused by the bar striking the magnet separated the ends of the wire, thus breaking the circuit of the conductor, and a spark appeared at the gap. in this little spark was the germ of the electric light of to-day. subsequently faraday improved the apparatus, by attaching a little disc of amalgamated copper to one end of the wire, and bending over the other end so as just to press lightly against the surface of the disc. with this apparatus he showed the "magnetic spark" at the meeting of the british association at oxford. faraday supposed that when a coil of wire was in the neighbourhood of a magnet, or near to a conductor conveying a current, the coil was thrown into a peculiar condition, which he called the _electro-tonic state_, and that the induced currents appeared whenever this state was assumed or lost by the coil. he frequently reverted to his conception of the electro-tonic state, though he saw clearly that, when the currents were induced by the relative motion of a wire and a magnet, the current induced depended on the rate at which the lines of magnetic force had been cut by the wire. of his conception of lines of force filling the whole of space, we shall have more to say presently. it is sufficient to remark here that, in the electro-tonic state of faraday, clerk maxwell recognized the number of lines of magnetic force enclosed by the circuit, and showed that the electro-motive force induced is proportional to the rate of change of the number of lines of force thus enclosed. it is seldom that a great discovery is made which has not been gradually led up to by several observed phenomena which awaited that discovery for their explanation. in the case of electro-magnetic induction, however, there appears to have been but one experiment which had baffled philosophers, and the key to which was found in faraday's discovery, while the complete explanation was given by faraday himself. arago had found that, if a copper plate were made rapidly to rotate beneath a freely suspended magnetic needle, the needle followed (slowly) the plate in its revolution, though a sheet of glass were inserted between the two to prevent any air-currents acting on the magnet. the experiment had been repeated by sir john herschel and mr. babbage, but no explanation was forthcoming. faraday saw that the revolution of the disc beneath the poles of the magnet must generate induced currents in the disc, as the different portions of the metal would be constantly cutting the lines of force of the magnet. these currents would react upon the magnet, causing a mechanical stress to act between the two, which, as stated by lenz, would be in the direction tending to oppose the _relative_ motion, and therefore to drag the magnet after the disc in its revolution. in the above figure the unfledged arrows show the general distribution of the currents in the disc, while the winged arrows indicate the direction of the disc's rotation. the currents in the semicircle a will repel the north pole and attract the south pole. those in the semicircle b will produce the opposite effect, and hence there will be a tendency for the magnet to revolve in the direction of the disc, while the motion of the disc will be resisted. this resistance to the motion of a conductor in a magnetic field was noticed by faraday, and, independently, by tyndall, and it is sufficiently obvious in the power absorbed by dynamos when they are generating large currents. faraday's next series of researches was devoted to the experimental proof of the identity of frictional and voltaic electricity. he showed that a magnet could be deflected and iodide of potassium decomposed by the current from his electrical machine, and came to the conclusion that the amount of electricity required to decompose a grain of water was equal to , charges of his large leyden battery. the current from the frictional machine also served to deflect the needle of his galvanometer. these investigations led on to a complete series of researches on the laws of electrolysis, wherein faraday demonstrated the principle that, however the strength of the current may be varied, the amount of any compound decomposed is proportional to the whole quantity of electricity which has passed through the electrolyte. when the same current is sent through different compounds, there is a constant relation between the amounts of the several compounds decomposed. in modern language, faraday's laws may be thus expressed:-- _if the same current be made to pass through several different electrolytes, the quantity of each ion produced will be proportional to its combining weight divided by its valency, and if the current vary, the quantity of each ion liberated per second will be proportional to the current._ this is the great law of electro-chemical equivalents. the amount of hydrogen liberated per second by a current of one ampère is about · gramme, or nearly one six-thousandth of a grain. this is the electro-chemical equivalent of hydrogen. that of any other substance may be found by faraday's law. from faraday's results it appears that the passage of the same amount of electricity is required in order to decompose one molecule of any compound of the same chemical type, but it does not follow that the same amount of energy is employed in the decomposition. for example, the combining weights of copper and zinc are nearly equal. hence it will require the passage of about the same amount of electricity to liberate a pound of copper from, say, the copper sulphate as to liberate a pound of zinc from zinc sulphate; but the work to be done is much less in the case of the copper. this is made manifest in the following way:--a battery, which will just decompose the copper salt slowly, liberating copper, oxygen, and sulphuric acid, will not decompose the zinc salt at all so as to liberate metallic zinc, but immediately on sending the current through the electrolyte, polarization will set in, and the opposing electro-motive force thus introduced will become equal to that of the battery, and stop the current before metallic zinc makes its appearance. in the case of the copper, polarization also sets in, but never attains to equality with the electro-motive force of the primary battery. in fact, in all cases of electrolysis, polarization produces an opposing electro-motive force strictly proportional to the work done in the cell by the passage of each unit of electricity. if the strength of the battery be increased, so that it is able to decompose the zinc sulphate, and if this battery be applied to the copper sulphate solution, the latter will be _rapidly_ decomposed, and the excess of energy developed by the battery will be converted into heat in the circuit. one important point in connection with electrolysis which faraday demonstrated is that the decomposition is the result of the passage of the current, and is not simply due to the attraction of the electrodes. thus he showed that potassium iodide could be decomposed by a stream of electricity coming from a metallic point on the prime conductor of his electric machine, though the point did not touch the test-paper on which the iodide was placed. it was in that mr. wm. jenkin, after one of the friday evening lectures at the royal institution, called the attention of faraday to a shock which he had experienced in breaking the circuit of an electro-magnet, though the battery employed consisted of only one pair of plates. faraday repeated the experiment, and found that, with a large magnet in circuit, a strong spark could thus be obtained. on november , , he writes, "the phenomenon of increased spark is merely a case of the induction of electric currents. if a current be established in a wire, and another wire forming a complete circuit be placed parallel to it, at the moment the current in the first is stopped it induces a current in the same direction in the second, itself then showing but a feeble spark. but if the second be away, it induces a current in its own wire in the same direction, producing a strong spark. the strong spark in the current when alone is therefore the equivalent of the current it can produce in a neighbouring wire when in company." the strong spark does, in fact, represent the energy of the current due to the self-induction of its circuit, which energy would, in part at least, be expended in inducing a current in a neighbouring wire if such existed. his time from till was largely taken up with his work on electro-static induction. faraday could never be content with any explanation based on direct action at a distance; he always sought for the machinery through which the action was communicated. in this search the lines of magnetic force, which he had so often delineated in iron filings, came to his aid. faraday made many pictures in iron filings of magnetic fields due to various combinations of magnets. he employed gummed paper, and when the filings were arranged on the hard gummed surface, he projected a feeble jet of steam on the paper, which melted the gum and fixed the filings. several of his diagrams were exhibited at the loan collection at south kensington. he conceived electrical action to be transmitted along such lines as these, and to him the whole electric field was filled with lines passing always from positive to negative electrification, and in some respects resembling elastic strings. the action at any place could then be expressed in terms of the lines of force that existed there, the electrifications by which these lines were produced being left out of consideration. the acting bodies were thus replaced by the field of force they produced. he showed that it was impossible to call into existence a charge of positive electricity without at the same time producing an equal negative charge. from every unit of positive electricity he conceived a line of force to start, and thus, with the origin of the line, there was created simultaneously a charge of negative electricity on which the line might terminate. by the famous ice-pail experiment he showed that, when a charged body is inserted in a closed or nearly closed hollow conductor, an equal amount of the same kind of electricity appeared on the outside of the hollow conductor, while an equal amount of the opposite kind appeared on the interior surface of the conductor. with the ice-pail and the butterfly-net he showed that there could be no free electricity on the interior of a conductor. lines of force cannot pass through the material of a conductor without producing electric displacement. every element of electricity must be joined to an equal amount of the opposite kind by a line of force. such lines cannot pass through the conductor itself; hence the charge must be entirely on the outside of the conductor, so that every element of the charge may be associated with an equal amount of the opposite electricity upon the surfaces of surrounding objects. thus to faraday every electrical action was an exhibition of electric induction. all this work had been done before by henry cavendish, but neither faraday nor any one else knew about it at the time. from the fact that there could be no electricity in the interior of a hollow conductor, cavendish deduced, in the best way possible, the truth of the law of inverse squares as applied to electrical attraction and repulsion, and thus laid the foundation of the mathematical theory of electricity. to cavendish every electrical action was a displacement of an incompressible fluid which filled the whole of space, producing no effect in conductors on account of the freedom of its motion, but producing strains in insulators by displacing the material of the body. faraday, in his lines of force, saw, as it were, the lines along which the displacements of cavendish's fluid took place. faraday thought that, if he could show that electric induction could take place along curved lines, it would prove that the action took place through a medium, and not directly at a distance. he succeeded in experimentally demonstrating the curvature of these lines; but his conclusions were not warranted, for if we conceive of two or more centres of force acting directly at a distance according to the law of inverse squares, the resultant lines of force will generally be curved. of course, this does not prove the possibility of direct action at a distance, but only shows that the curvature of the lines is as much a consequence of the one hypothesis as of the other. it soon appeared to faraday that the nature of the dielectric had very much to do with electric induction. the capacity of a condenser, for instance, depends on the nature of the dielectric as well as on the configuration of the conductors. to express this property, faraday employed the term "specific inductive capacity." he compared the electric capacity of condensers, equal in all other respects, but one possessing air for its dielectric, and the other having other media, and thus roughly determined the specific inductive capacities of several insulators. these results turned out afterwards to be of great value in connection with the insulation of submarine cables. even now the student of electricity is sometimes puzzled by the manner in which specific inductive capacity is introduced to his notice as modifying the capacity of condensers, after learning that the capacity of any system of conductors can be calculated from its geometrical configuration; but the fact is that the intensity of all electrical actions depends on the nature of the medium through which they take place, and it will require more electricity to exert upon an equal charge a unit force at unit distance when the intervening medium has a high than when it possesses a low specific inductive capacity. in faraday received a pension from the civil list; in he was appointed scientific adviser to the elder brethren of the trinity house. in the same year he was made a member of the senate of the university of london, and in that capacity he has exerted no small influence on the scientific education of the country, for he was one of those who drew up the schedules of the various examinations. in his early years, faraday thought that all kinds of matter might ultimately consist of three materials only, and that as gases and vapours appeared more nearly to resemble one another than the liquids or solids to which they corresponded, so each might be subject to a still higher change in the same direction, and the gas or vapour become radiant matter--either heat, light, or electricity. later on, faraday clearly recognized the dynamical nature of heat and light; but his work was always guided by his theoretical conceptions of the "correlation of the physical forces." for a long time he had tried to discover relations between electricity and light; at length, on september , , after experimenting on a number of other substances, he placed a piece of silico-borate of lead, or heavy-glass, in the field of the magnet, and found that, when a beam of polarized light was transmitted through the glass in the direction of the lines of magnetic force, there was a rotation of the plane of polarization. afterwards it appeared that all the transparent solids and liquids experimented on were capable of producing this rotation in a greater or less degree, and in the case of all non-magnetic substances the rotation was in the direction of the electric current, which, passing round the substance, would produce the magnetic field employed. abandoning the magnet, and using only a coil of wire with the transparent substance within it, similar effects were obtained. thus at length a relation was found between light and electricity. on november , employing a piece of heavy-glass and a new horseshoe magnet, faraday noticed that the magnet appeared to have a directive action upon the glass. further examination showed that the glass was repelled by the magnetic poles. three days afterwards he found that all sorts of substances, including most metals, were acted upon like the heavy-glass. small portions of them were repelled, while elongated cylinders tended to set with their lengths perpendicular to the lines of magnetic force. such actions could be imitated by suspending a feebly magnetic body in a medium more magnetic than itself. faraday, therefore, sought for some medium which would be absolutely neutral to magnetic action. filling a glass tube with compressed oxygen, and suspending it in an atmosphere of oxygen at ordinary pressure, the compressed gas behaved like iron or other magnetic substances. faraday compared the intensity of its action with that of ferrous sulphate, and this led to an explanation of the diurnal variations of the compass-needle based on the sun's heat diminishing the magnetic _permeability_ of the oxygen of the air. repeating the experiment with nitrogen, he found that the compressed gas behaved in a perfectly neutral manner when surrounded by the gas at ordinary pressure. hence he inferred that in nitrogen he had found the neutral medium required. repeating his experiments in an atmosphere of nitrogen, it still appeared that most bodies were repelled by the magnetic poles, and set _equatorially_, or at right angles to the lines of force when elongated portions were tested. to this action faraday gave the name of diamagnetism. about a month after his marriage, faraday joined the sandemanian church, to which his family had for several generations belonged, by confession of sin and profession of faith. not unfrequently he used to speak at the meetings of his church, but in he was elected an elder, and then he took his turn regularly in conducting the services. the notes of his addresses he generally made on small pieces of card. he had a curious habit of separating his religious belief from his scientific work, although the spirit of his religion perpetually pervaded his life. a lecture on mental education, given in , at the royal institution, in the presence of the late prince consort, he commenced as follows:-- "before entering on this subject, i must make one distinction, which, however it may appear to others, is to me of the utmost importance. high as man is placed above the creatures around him, there is a higher and far more exalted position within his view; and the ways are infinite in which he occupies his thoughts about the fears, or hopes, or expectations of a future life. i believe that the truth of that future cannot be brought to his knowledge by any exertion of his mental powers, however exalted they may be; that it is made known to him by other teaching than his own, and is received through simple belief of the testimony given. let no one suppose for a moment that the self-education i am about to commend, in respect of the things of this life, extends to any considerations of the hope set before us, as if man by reasoning could find out god. it would be improper here to enter upon this subject further than to claim an absolute distinction between religious and ordinary belief. i shall be reproached with the weakness of refusing to apply those mental operations which i think good in respect of high things to the very highest. i am content to bear the reproach. yet even in earthly matters i believe that 'the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and godhead;' and i have never seen anything incompatible between those things of man which can be known by the spirit of man which is within him, and those higher things concerning his future which he cannot know by that spirit." on more than one occasion the late prince consort had discussed physical questions with faraday, and in the queen offered him a house on hampton court green. this was his home until august , . he saw not only the magnetic spark, which he had first produced, employed in the lighthouses at the south foreland and dungeness, but he saw also his views respecting lines of electric induction examined and confirmed by the investigations of thomson and clerk maxwell. of the ninety-five distinctions conferred upon him, we need only mention that of commandant of the legion of honour, which he received in january, . james clerk maxwell. the story of the life of james clerk maxwell has been told so recently by the able pen of his lifelong friend, professor lewis campbell, that it is unnecessary, in the few pages which now remain to us, to attempt to give a repetition of the tale which would not only fail to do justice to its subject, but must of necessity fall far short of the merits of the (confessedly imperfect) sketch which has recently been placed within the reach of all. looking back on the life of clerk maxwell, he seems to have come amongst us as a light from another world--to have but partly revealed his message to minds too often incapable of grasping its full meaning, and all too soon to have returned to the source from whence he came. there was scarcely any branch of natural philosophy that he did not grapple with, and upon which his vivid imagination and far-seeing intelligence did not throw light. he was born a philosopher, and at every step nature partly drew aside the veil and revealed that which was hidden from a gaze less prophetic. a very brief sketch of the principal incidents in his life may, however, not be out of place. james clerk maxwell was born in edinburgh, on june , . his father, john clerk maxwell, was the second son of james clerk, of penicuik, and took the name of maxwell on inheriting the estate at middlesbie. his mother was the daughter of r. h. cay, esq., of north charlton, northumberland. james was the only child who survived infancy. some years before his birth his parents had built a house at glenlair, which had been added to their middlesbie estate, and resided there during the greater part of the year, though they retained their house in edinburgh. hence it was that james's boyish days were spent almost entirely in the country, until he entered the edinburgh academy in . as a child, he was never content until he had completely investigated everything which attracted his attention, such as the hidden courses of bell-wires, water-streams, and the like. his constant question was "what's the go o' that?" and, if answered in terms too general for his satisfaction, he would continue, "but what's the particular go of it?" this desire for the thorough investigation of every phenomenon was a characteristic of his mind through life. from a child his knowledge of scripture was extensive and accurate, and when eight years old he could repeat the whole of the hundred and nineteenth psalm. about this time his mother died, and thenceforward he and his father became constant companions. together they would devise all sorts of ingenious mechanical contrivances. young james was essentially a child of nature, and free from all conventionality. he loved every living thing, and took delight in petting young frogs, and putting them into his mouth to see them jump out. one of his attainments was to paddle on the duck-pond in a wash-tub, and to make the vessel go "without spinning"--a recreation which had to be relinquished on washing-days. he was never without the companionship of one or two terriers, to whom he taught many tricks, and with whom he seemed to have complete sympathy. as a boy, maxwell was not one to profit much by the ordinary teaching of the schools, and experience with a private tutor at home did not lead to very satisfactory results. at the age of ten, therefore, he was sent to the edinburgh academy, under the care of archdeacon williams, who was then rector. on his first appearance in this fashionable school, he was naturally a source of amusement to his companions; but he held his ground, and soon gained more respect than he had previously provoked ridicule. while at school in edinburgh, he resided with his father's sister, mrs. wedderburn, and devoted a very considerable share of his time and attention to relieving the solitude of the old man at glenlair, by letters written in quaint styles, sometimes backwards, sometimes in cypher, sometimes in different colours, so arranged that the characters written in a particular colour, when placed consecutively, formed another sentence. all the details of his school and home life, and the special peculiarities of the masters at the academy, were thus faithfully transmitted to his father, by whom the letters were religiously preserved. at thirteen he had evidently made progress in solid geometry, though he had not commenced euclid, for he writes to his father, "i have made a tetrahedron, a dodecahedron, and two other hedrons whose names i don't know." in these letters to glenlair he generally signed himself, "your most obedient servant." sometimes his fun found vent even upon the envelope; for example:-- "mr. john clerk maxwell, "postyknowswere, "kirkpatrick durham, "dumfries." sometimes he would seal his letters with electrotypes of natural objects (beetles, etc.), of his own making. in july, , he writes:-- i have got the eleventh prize for scholarship, the first for english, the prize for english verses, and the mathematical medal. when only fifteen a paper on oval curves was contributed by him to the _proceedings of the royal society of edinburgh_. in the spring of he accompanied his uncle on a visit to mr. nicol, the inventor of the nicol prism, and on his return he made a polariscope with glass and a lucifer-match box, and sketched in water-colours the chromatic appearances presented by pieces of unannealed glass which he himself prepared. these sketches he sent to mr. nicol, who presented him in return with a pair of prisms of his own construction. the prisms are now in the cavendish laboratory at cambridge. maxwell found that, for unannealed glass, pieces of window-glass placed in bundles of eight or nine, one on the other, answered the purpose very well. he cut the figures, triangles, squares, etc., with a diamond, heated the pieces of glass on an iron plate to redness in the kitchen fire, and then dropped them into a plate of iron sparks (scales from the smithy) to cool. in maxwell entered the university of edinburgh, and during his course of study there he contributed to the royal society of edinburgh papers upon rolling curves and on the equilibrium of elastic solids. his attention was mostly devoted to mathematics, physics, chemistry, and mental and moral philosophy. in he went to cambridge, entering peterhouse, but at the end of a year he "migrated" to trinity; here he was soon surrounded with a circle of friends who helped to render his cambridge life a very happy one. his love of experiment sometimes extended to his own mode of life, and once he tried sleeping in the evening and working after midnight, but this was soon given up at the request of his father. one of his friends writes, "from to . a.m. he took exercise by running along the upper corridor, _down_ the stairs, along the lower corridor, then _up_ the stairs, and so on until the inhabitants of the rooms along his track got up and laid _perdus_ behind their sporting-doors, to have shots at him with boots, hair-brushes, etc., as he passed." his love of fun, his sharp wit, his extensive knowledge, and above all, his complete unselfishness, rendered him a universal favourite in spite of the temporary inconveniences which his experiments may have occasionally caused to his fellow-students. an undergraduate friend writes, "every one who knew him at trinity can recall some kindness or some act of his which has left an ineffaceable impression of his goodness on the memory--for 'good' maxwell was in the best sense of the word." the same friend wrote in his diary in , after meeting maxwell at a social gathering, "maxwell, as usual, showing himself acquainted with every subject on which the conversation turned. i never met a man like him. i do believe there is not a single subject on which he cannot talk, and talk well too, displaying always the most curious and out-of-the-way information." his private tutor, the late well-known mr. hopkins, said of him, "it is not possible for that man to think incorrectly on physical subjects." in maxwell took his degree at cambridge as second wrangler, and was bracketed with the senior wrangler (mr. e. j. routh) for the smith's prize. during his undergraduate course, he appears to have done much of the work which formed the basis of his subsequent papers on electricity, particularly that on faraday's lines of force. the colour-top and colour-box appear also to have been gradually developing during this time, while the principle of the stereoscope and the "art of squinting" received their due share of attention. shortly after his degree, he devoted a considerable amount of time to the preparation of a manuscript on geometrical optics, which was intended to form a university text-book, but was never completed. in the autumn of he was elected fellow of trinity. about this time the colour-top was in full swing, and he also constructed an ophthalmoscope. in may, , he writes:-- the colour trick came off on monday, th. i had the proof-sheets of my paper, and was going to read; but i changed my mind and talked instead, which was more to the purpose. there were sundry men who thought that blue and yellow make green, so i had to undeceive them. i have got hay's book of colours out of the university library, and am working through the specimens, matching them with the top. the "colour trick" came off before the cambridge philosophical society. while a bachelor fellow, maxwell gave lectures to working men in barnwell, besides lecturing in college. his father died in april, , and shortly afterwards he was appointed professor of natural philosophy in marischal college, aberdeen. this appointment he held until the fusion of the college with king's college in . these four years were very productive of valuable work. during them the dynamical top was constructed, which illustrates the motion of a rigid body about its axis of greatest, least, or mean moment of inertia; for, by the movement of certain screws, the axis of the top may be made to coincide with any one at will. the adams prize essay on the stability of saturn's rings belongs also to this period. in this essay maxwell showed that the phenomena presented by saturn's rings can only be explained on the supposition that they consist of innumerable small bodies--"a flight of brickbats"--each independent of all the others, and revolving round saturn as a satellite. he compared them to a siege of sebastopol from a battery of guns measuring thirty thousand miles in one direction, and a hundred miles in the other, the shots never stopping, but revolving round a circle of a hundred and seventy thousand miles radius. a solid ring of such dimensions would be completely crushed by its own weight, though made of the strongest material of which we have any knowledge. if revolving at such a rate as to balance the attraction of the planet at one part, the stress in other parts would be more than sufficient to crush or tear the ring. laplace had shown that a narrow ring might revolve about the planet and be stable if so loaded that its centre of gravity was at a considerable distance from its centre, and thought that saturn's rings might consist of a number of such unsymmetrical rings--a theory to which some support was given by the many small divisions observable in the bright rings. maxwell showed that, for stability, the mass required to load each of laplace's rings must be four and a half times that of the rest of the ring; and the system would then be far too artificially balanced to be proof against the action of one ring on another. he further showed that, in liquid rings, waves would be produced by the mutual action of the rings, and that before long some of these waves would be sure to acquire such an amplitude as would cause the rings to break up into small portions. finally, he concluded that the only admissible theory is that of the independent satellites, and that the _average_ density of the rings so found cannot be much greater than that of air at ordinary pressure and temperature. while he remained at aberdeen, maxwell lectured to working men in the evenings, on the principles of mechanics. on the whole, it is doubtful whether aberdeen society was as congenial to him as that of cambridge or edinburgh. he seems not to have been understood even by his colleagues. on one occasion he wrote:-- gaiety is just beginning here again.... no jokes of any kind are understood here. i have not made one for two months, and if i feel one coming i shall bite my tongue. but every cloud has its bright side, and, however maxwell may have been regarded by his colleagues, he was not long without congenial companionships. an honoured guest at the home of the principal, "in february, , he announced his betrothal to katherine mary dewar, and they were married early in the following june." professor campbell speaks of his married life as one of unexampled devotion, and those who enjoyed the great privilege of seeing him at home could more than endorse the description. in maxwell accepted the chair of natural philosophy at king's college, london. here he continued his lectures to working men, and even kept them up for one session after resigning the chair in . on may , , he gave his first lecture at the royal institution, on "the theory of the three primary colours." this lecture embodies many of the results of his work with the colour-top and colour-box, to be again referred to presently. while at king's college, he was placed on the electrical standards committee of the british association, and most of the work of the committee was carried out in his laboratory. here, too, he compared the electro-static repulsion between two discs of brass with the electro-magnetic attraction of two coils of wire surrounding them, through which a current of electricity was allowed to flow, and obtained a result which he afterwards applied to the electro-magnetic theory of light. the colour-box was perfected, and his experiments on the viscosity of gases were concluded during his residence in london. these last were described by him in the bakerian lecture for . after resigning the professorship at king's college, maxwell spent most of his time at glenlair, having enlarged the house, in accordance with his father's original plans. here he completed his great work on "electricity and magnetism," as well as his "theory of heat," an elementary text-book which may be said to be without a parallel. on march , , he accepted the chair of experimental physics in the university of cambridge. this chair was founded in consequence of an offer made by the duke of devonshire, the chancellor of the university, to build and equip a physical laboratory for the use of the university. in this capacity maxwell's first duty was to prepare plans for the laboratory. with this view, he inspected the laboratories of sir william thomson at glasgow, and of professor clifton at oxford, and endeavoured to embody the best points of both in the new building. the result was that, in conjunction with mr. w. m. fawcett, the architect, he secured for the university a laboratory noble in its exterior, and admirably adapted to the purposes for which it is required. the ground-floor comprises a large battery-room, which is also used as a storeroom for chemicals; a workshop; a room for receiving goods, communicating by a lift with the apparatus-room; a room for experiments on heat; balance-rooms; a room for pendulum experiments, and other investigations requiring great stability; and a magnetic observatory. the last two rooms are furnished with stone supports for instruments, erected on foundations independent of those of the building, and preserved from contact with the floor. on the first floor is a handsome lecture-theatre, capable of accommodating nearly two hundred students. the lecture-table is carried on a wall, which passes up through the floor without touching it, the joists being borne by separate brick piers. the lecture-theatre occupies the height of the first and second floors; its ceiling is of wood, the panels of which can be removed, thus affording access to the roof-principals, from which a load of half a ton or more may be safely suspended over the lecture-table. the panels of the ceiling, adjoining the wall which is behind the lecturer, can also be readily removed, and a "window" in this wall communicates with the large electrical-room on the second floor. access to the space above the ceiling of the lecture-theatre is readily obtained from the tower. adjoining the lecture-room is the preparation-room, and communicating with the latter is the apparatus-room. this room is fitted with mahogany and plate-glass wall and central cases, and at present contains, besides the more valuable portions of the apparatus belonging to the laboratory, the marble bust of james clerk maxwell, and many of the home-made pieces of apparatus and other relics of his early work. the rest of the first floor is occupied by the professor's private room and the general students' laboratory. throughout the building the brick walls have been left bare for convenience in attaching slats or shelves for the support of instruments. the second floor contains a large room for electrical experiments, a dark room for photography, and a number of private rooms for original work. water is laid on to every room, including a small room in the top of the tower, and all the windows are provided with broad stone ledges without and within the window, the two portions being in the same horizontal plane, for the support of heliostats or other instruments. the building is heated with hot water, but in the magnetic observatory the pipes are all of copper and the fittings of gun-metal. open fireplaces for basket fires are also provided. over the principal entrance of the laboratory is placed a stone statue of the present duke of devonshire, together with the arms of the university and of the cavendish family, and the cavendish motto, "cavendo tutus." maxwell presented to the laboratory, in , all the apparatus in his possession. he usually gave a course of lectures on heat and the constitution of bodies in the michaelmas term; on electricity in the lent term; and on electro-magnetism in the easter term. the following extract from his inaugural lecture, delivered in october, , is worthy of the attention of all students of science:-- science appears to us with a very different aspect after we have found out that it is not in lecture-rooms only, and by means of the electric light projected on a screen, that we may witness physical phenomena, but that we may find illustrations of the highest doctrines of science in games and gymnastics, in travelling by land and by water, in storms of the air and of the sea, and wherever there is matter in motion. the habit of recognizing principles amid the endless variety of their action can never degrade our sense of the sublimity of nature, or mar our enjoyment of its beauty. on the contrary, it tends to rescue our scientific ideas from that vague condition in which we too often leave them, buried among the other products of a lazy credulity, and to raise them into their proper position among the doctrines in which our faith is so assured that we are ready at all times to act on them. experiments of illustration may be of very different kinds. some may be adaptations of the commonest operations of ordinary life; others may be carefully arranged exhibitions of some phenomenon which occurs only under peculiar conditions. they all, however, agree in this, that their aim is to present some phenomenon to the senses of the student in such a way that he may associate with it some appropriate scientific idea. when he has grasped this idea, the experiment which illustrates it has served its purpose. in an experiment of research, on the other hand, this is not the principal aim.... experiments of this class--those in which measurement of some kind is involved--are the proper work of a physical laboratory. in every experiment we have first to make our senses familiar with the phenomenon; but we must not stop here--we must find out which of its features are capable of measurement, and what measurements are required in order to make a complete specification of the phenomenon. we must then make these measurements, and deduce from them the result which we require to find. this characteristic of modern experiments--that they consist principally of measurements--is so prominent that the opinion seems to have got abroad that, in a few years, all the great physical constants will have been approximately estimated, and that the only occupation which will then be left to men of science will be to carry these measurements to another place of decimals. if this is really the state of things to which we are approaching, our laboratory may, perhaps, become celebrated as a place of conscientious labour and consummate skill; but it will be out of place in the university, and ought rather to be classed with the other great workshops of our country, where equal ability is directed to more useful ends. but we have no right to think thus of the unsearchable riches of creation, or of the untried fertility of those fresh minds into which these riches will continually be poured.... the history of science shows that, even during that phase of her progress in which she devotes herself to improving the accuracy of the numerical measurement of quantities with which she has long been familiar, she is preparing the materials for the subjugation of new regions, which would have remained unknown if she had been contented with the rough methods of her early pioneers. maxwell's "electricity and magnetism" was published in . shortly afterwards there were placed in his hands, by the duke of devonshire, the cavendish manuscripts on electricity, already alluded to. to these he devoted much of his spare time for several years, and many of cavendish's experiments were repeated in the laboratory by maxwell himself, or under his direction by his students. the introductory matter and notes embodied in "the electrical researches of the honourable henry cavendish, f.r.s.," afford sufficient evidence of the amount of labour he expended over this work. the volume was published only a few weeks before his death. another of maxwell's publications, which, as a text-book, is unique and beyond praise, is the little book on "matter and motion," published by the s.p.c.k. in maxwell, at the request of the vice-chancellor, delivered the rede lecture in the senate-house. his subject was the telephone, which was just then absorbing a considerable amount of public attention. this was the last lecture which he ever gave to a large public audience. it was during his tenure of the cambridge chair that one of the cottages on the glenlair estate was struck by lightning. the discharge passed down the damp soot and blew out several stones from the base of the chimney, apparently making its way to some water in a ditch a few yards distant. the cottage was built on a granite rock, and this event set maxwell thinking about the best way to protect, from lightning, buildings which are erected on granite or other non-conducting foundations. he decided that the proper course was to place a strip of metal upon the ground all round the building, to carry another strip along the ridge-stay, from which one or more pointed rods should project upwards, and to unite this strip with that upon the ground by copper strips passing down each corner of the building, which is thus, as it were, enclosed in a metal cage. after a brief illness, maxwell passed away on november , . his intellect and memory remained perfect to the last, and his love of fun scarcely diminished. during his illness he would frequently repeat hymns, especially some of george herbert's, and richard baxter's hymn beginning "lord, it belongs not to my care." "no man ever met his death more consciously or more calmly." it has been stated that thomas young propounded a theory of colour-vision which assumes that there exist three separate colour-sensations, corresponding to red, green, and violet, each having its own special organs, the excitement of which causes the perception of the corresponding colour, other colours being due to the excitement of two or more of these simple sensations in different proportions. maxwell adopted blue instead of violet for the third sensation, and showed that if a particular red, green, and blue were selected and placed at the angular points of an equilateral triangle, the colours formed by mixing them being arranged as in young's diagram, all the shades of the spectrum would be ranged along the sides of this triangle, the centre being neutral grey. for the mixing of coloured lights, he at first employed the colour-top, but, instead of painting circles with coloured sectors, the angles of which could not be changed, he used circular discs of coloured paper slit along one radius. any number of such discs can be combined so that each shows a sector at the top, and the angle of each sector can be varied at will by sliding the corresponding disc between the others. maxwell used discs of two different sizes, the small discs being placed above the larger on the same pivot, so that one set formed a central circle, and the other set a ring surrounding it. he found that, with discs of five different colours, of which one might be white and another black, it was always possible to combine them so that the inner circle and the outer ring exactly matched. from this he showed that there could be only three conditions to be satisfied in the eye, for two conditions were necessitated by the nature of the top, since the smaller sectors must exactly fill the circle and so must the larger. maxwell's experiments, therefore, confirmed, in general, young's theory. they showed, however, that the relative delicacy of the several colour-sensations is different in different eyes, for the arrangement which produced an exact match in the case of one observer, had to be modified for another; but this difference of delicacy proved to be very conspicuous in colour-blind persons, for in most of the cases of colour-blindness examined by maxwell the red sensation was completely absent, so that only two conditions were required by colour-blind eyes, and a match could therefore always be made in such cases with four discs only. holmgren has since discovered cases of colour-blindness in which the violet sensation is absent. he agrees with young in making the third sensation correspond to violet rather than blue. maxwell explained the fact that persons colour-blind to the red divide colours into blues and yellows by the consideration that, although yellow is a complex sensation corresponding to a mixture of red and green, yet in nature yellow tints are so much brighter than greens that they excite the green sensation more than green objects themselves can do, and hence greens and yellows are called yellow by such colour-blind persons, though their perception of yellow is really the same as perception of green by normal eyes. later on, by a combination of adjustable slits, prisms, and lenses arranged in a "colour-box," maxwell succeeded in mixing, in any desired proportions, the light from any three portions of the spectrum, so that he could deal with pure spectral colours instead of the complex combinations of differently coloured lights afforded by coloured papers. from these experiments it appears that no ray of the solar spectrum can affect one colour-sensation alone, so that there are no colours in nature so pure as to correspond to the pure simple sensations, and the colours occupying the angular points of maxwell's diagram affect all three colour-sensations, though they influence two of them to a much smaller extent than the third. a particular colour in the spectrum corresponds to light which, according to the undulatory theory, physically consists of waves all of the same period, but it may affect all three of the colour-sensations of a normal eye, though in different proportions. thus, yellow light of a given wave-length affects the red and green sensations considerably and the blue (or violet) slightly, and the same effect may be produced by various mixtures of red or orange and green. for his researches on the perception of colour, the royal society awarded to clerk maxwell the rumford medal in . another optical contrivance of maxwell's was a wheel of life, in which the usual slits were replaced by concave lenses of such focal length that the picture on the opposite side of the cylinder appeared, when seen through a lens, at the centre, and thus remained apparently fixed in position while the cylinder revolved. the same result has since been secured by a different contrivance in the praxinoscope. another ingenious optical apparatus was a real-image stereoscope, in which two lenses were placed side by side at a distance apart equal to half the distance between the pictures on the stereoscopic slide. these lenses were placed in front of the pictures at a distance equal to twice their focal length. the real images of the two pictures were then superposed in front of the lenses at the same distance from them as the pictures, and these combined images were looked at through a large convex lens. the great difference in the sensibility to different colours of the eyes of dark and fair persons when the light fell upon the _fovea centralis_, led maxwell to the discovery of the extreme want of sensibility of this portion of the retina to blue light. this he made manifest by looking through a bottle containing solution of chrome alum, when the central portion of the field of view appears of a light red colour for the first second or two. a more important discovery was that of double refraction temporarily produced in viscous liquids. maxwell found that a quantity of canada balsam, if stirred, acquired double-refracting powers, which it retained for a short period, until the stress temporarily induced had disappeared. but maxwell's investigations in optics must be regarded as his play; his real work lay in the domains of electricity and of molecular physics. in daniel bernouilli published an explanation of atmospheric pressure on the hypothesis that air consists of a number of minute particles moving in all directions, and impinging on any surface exposed to their action. in herapath explained the diffusion of gases on the hypothesis that they consisted of perfectly hard molecules impinging on one another and on surfaces exposed to them, and pointed out the relation between their motion and the temperature and pressure of a gas. the present condition of the molecular theory of gases, and of molecular science generally, is due almost entirely to the work of joule, clausius, boltzmann, and maxwell. to maxwell is due the general method of solving all problems connected with vast numbers of individuals--a method which he called the statistical method, and which consists, in the first place, in separating the individuals into groups, each fulfilling a particular condition, but paying no attention to the history of any individual, which may pass from one group to another in any way and as often as it pleases without attracting attention. maxwell was the first to estimate the average distance through which a particle of gas passes without coming into collision with another particle. he found that, in the case of hydrogen, at standard pressure and temperature, it is about / of an inch; for air, about / of an inch. these results he deduced from his experiments on viscosity, and he gave a complete explanation of the viscosity of gases, showing it to be due to the "diffusion of momentum" accompanying the diffusion of material particles between the passing streams of gas. one portion of the theory of electricity had been considerably developed by cavendish; the application of mathematics to the theory of attractions, and hence to that of electricity, had been carried to a great degree of perfection by laplace, lagrange, poisson, green, and others. faraday, however, could not satisfy himself with a mathematical theory based upon direct action at a distance, and he filled space, as we have seen, with tubes of force passing from one body to another whenever there existed any electrical action between them. these conceptions of faraday were regarded with suspicion by mathematicians. sir william thomson was the first to look upon them with favour; and in he showed that electro-static force might be treated mathematically in the same way as the flow of heat; so that there are, at any rate, two methods by which the fundamental formulæ of electro-statics can be deduced. but it is to maxwell that mathematicians are indebted for a complete exposition of faraday's views in their own language, and this was given in a paper wherein the phenomena of electro-statics were deduced as results of a stress in a medium which, as suggested by newton and believed by faraday, might well be that same medium which serves for the propagation of light; and "the lines of force" were shown to correspond to an actual condition of the medium when under electrical stress. maxwell, in fact, showed, not only that faraday's lines formed a consistent system which would bear the most stringent mathematical analysis, but were more than a conventional system, and might correspond to a state of stress actually existing in the medium through which they passed, and that a tension along these lines, accompanied by an equal pressure in every direction at right angles to them, would be consistent with the equilibrium of the medium, and explain, on mechanical principles, the observed phenomena. the greater part of this work he accomplished while an undergraduate at cambridge. he showed, too, that faraday's conceptions were equally applicable to the case of electro-magnetism, and that all the laws of the induction of currents might be concisely expressed in faraday's language. defining the positive direction through a circuit in which a current flows as the direction in which a right-handed screw would advance if rotating with the current, and the positive direction around a wire conveying a current as the direction in which a right-handed screw would rotate if advancing with the current, maxwell pointed out that the lines of magnetic force due to an electric current always pass round it, or through its circuit, in the positive direction, and that, _whenever the number of lines of magnetic force passing through a closed circuit is changed, there is an electro-motive force round the circuit represented by the rate of diminution of the number of lines of force which pass through the circuit in the positive direction_. the words in italics form a complete statement of the laws regulating the production of currents by the motion of magnets or of other currents, or by the variation of other currents in the neighbourhood. maxwell showed, too, that faraday's electro-tonic state, on the variation of which induced currents depend, corresponds completely with the number of lines of magnetic force passing through the circuit. he also showed that, when a conductor conveying a current is free to move in a magnetic field, or magnets are free to move in the neighbourhood of such a conductor, _the system will assume that condition in which the greatest possible number of lines of magnetic force pass through the circuit in the positive direction_. but maxwell was not content with showing that faraday's conceptions were consistent, and had their mathematical equivalents,--he proceeded to point out how a medium could be imagined so constituted as to be able to perform all the various duties which were thus thrown upon it. assuming a medium to be made up of spherical, or nearly spherical, cells, and that, when magnetic force is transmitted, these cells are made to rotate about diameters coinciding in direction with the lines of force, the tension along those lines, and the pressure at right angles to them, are accounted for by the tendency of a rotating elastic sphere to contract along its polar axis and expand equatorially so as to form an oblate spheroid. by supposing minute spherical particles to exist between the rotating cells, the motion of one may be transmitted in the same direction to the next, and these particles may be supposed to constitute electricity, and roll as perfectly rough bodies on the cells in contact with them. maxwell further imagined the rotating cells, and therefore, _à fortiori_, the electrical particles, to be extremely small compared with molecules of matter; and that, in conductors, the electrical particles could pass from molecule to molecule, though opposed by friction, but that in insulators no such transference was possible. the machinery was then complete. if the electric particles were made to flow in a conductor in one direction, passing between the cells, or _molecular vortices_, they compelled them to rotate, and the rotation was communicated from cell to cell in expanding circles by the electric particles, acting as idle wheels, between them. thus rings of magnetic force were made to surround the current, and to continue as long as the current lasted. if an attempt were made to displace the electric particles in a dielectric, they would move only within the substance of each molecule, and not from molecule to molecule, and thus the cells would be deformed, though no continuous motion would result. the deformation of the cells would involve elastic stress in the medium. again, if a stream of electric particles were started into motion, and if there were another stream of particles in the neighbourhood free to flow, though resisted by friction, these particles, instead of at once transmitting the rotary motion of the cells on one side of them to the cells on the other side, would at first, on account of the inertia of the cells, begin to move themselves with a motion of translation opposite to that of the primary current, and the motion would only gradually be destroyed by the frictional resistance and the molecular vortices on the other side made to revolve with their full velocity. a similar effect, but in the opposite direction, would take place if the primary current ceased, the vortices not stopping all at once if there were any possibility of their continuing in motion. the imaginary medium thus serves for the production of induced currents. the mechanical forces between currents and magnets and between currents and currents, as well as between magnets and currents, were accounted for by the tension and pressure produced by the molecular vortices. when currents are flowing in the same direction in neighbouring conductors, the vortices in the space between them are urged in opposite directions by the two currents, and remain almost at rest; the lateral pressure exerted by those on the outside of the conductors is thus unbalanced, and the conductors are pushed together as though they attracted each other. when the currents flow in opposite directions in parallel conductors, they conspire to give a greater velocity to the vortices in the space between them, than to those outside them, and are thus pushed apart by the pressure due to the rotation of the vortices, as though they repelled each other. in a similar way, the actions of magnets on conductors conveying currents may be explained. the motion of a conductor across a series of lines of magnetic force may squeeze together and lengthen the threads of vortices in front, and thus increase their speed of rotation, while the vortices behind will move more slowly because allowed to contract axially and expand transversely. the velocity of the vortices thus being greater on one side of the wire than the other, a current must be induced in the wire. thus the current induced by the motion of a conductor in a magnetic field may be accounted for. this conception of a medium was given by maxwell, not as a theory, but to show that it was possible to devise a _mechanism_ capable, in imagination at least, of producing all the phenomena of electricity and magnetism. "according to our theory, the particles which form the partitions between the cells constitute the matter of electricity. the motion of these particles constitutes an electric current; the tangential force with which the particles are pressed by the matter of the cells is electro-motive force; and the pressure of the particles on each other corresponds to the tension or potential of the electricity." when a current is maintained in a wire, the molecular vortices in the surrounding space are kept in uniform motion; but if an attempt be made to stop the current, since this would necessitate the stoppage of the vortices, it is clear that it cannot take place suddenly, but the energy of the vortices must be in some way used up. for the same reason it is impossible for a current to be suddenly started by a finite force. thus the phenomena of self-induction are accounted for by the supposed medium. the magnetic permeability of a medium maxwell identified with the density of the substance composing the rotating cells, and the specific inductive capacity he showed to be inversely proportional to its elasticity. he then proved that the ratio of the electro-magnetic unit to the electro-static unit must be equal to the velocity of transmission of a transverse vibration in the medium, and consequently proportional to the square root of the elasticity, and inversely proportional to the square root of the density. if the medium is the same as that engaged in the propagation of light, then this ratio ought to be equal to the velocity of light, and, moreover, in non-magnetic media, the refractive index should be proportional to the square root of the specific inductive capacity. the different measurements which had been made of the ratio of the electrical units gave a mean very nearly coinciding with the best determinations of the velocity of light, and thus the truth underlying maxwell's speculation was strikingly confirmed, for the velocity of light was determined by purely electrical measurements. in the case also of bodies whose chemical structure was not very complicated, the refractive index was found to agree fairly well with the square root of the specific inductive capacity; but the phenomenon of "residual charge" rendered the accurate measurement of the latter quantity a matter of great difficulty. it therefore appeared highly probable that light is an electro-magnetic disturbance due to a motion of the electric particles in an insulating medium producing a strain in the medium, which becomes propagated from particle to particle to an indefinite distance. in the case of a conductor, the electric particles so displaced would pass from molecule to molecule against a frictional resistance, and thus dissipate the energy of the disturbance, so that true (_i.e._ metallic) conductors must be nearly impervious to light; and this also agrees with experience. maxwell thus furnished a complete theory of electrical and electro-magnetic action in which all the effects are due to actions propagated in a medium, and direct action at a distance is dispensed with, and exposed his theory successfully to most severe tests. in his great work on electricity and magnetism, he gives the mathematical theory of all the above actions, without, however, committing himself to any particular form of mechanism to represent the constitution of the medium. "this part of that book," professor tait says, "is one of the most splendid monuments ever raised by the genius of a single individual.... there seems to be no longer any possibility of doubt that maxwell has taken the first grand step towards the discovery of the true nature of electrical phenomena. had he done nothing but this, his fame would have been secured for all time. but, striking as it is, this forms only one small part of the contents of this marvellous work." conclusion. some of the results of faraday's discoveries, and the principle of energy. in early days, _the spirit of the amber_, when aroused by rubbing, came forth and took to itself such light objects as it could easily lift. later on, and the spirit gave place to the _electric effluvium_, which proceeded from the excited, or charged, body into the surrounding space. still later, and a fluid, or two fluids, acting directly upon itself, or upon matter, or on one another, through intervening space without the aid of intermediate mechanism, took the place of the electric effluvium--a step which in itself was, perhaps, hardly an advance. then came the time for accurate measurement. the simple _observation_ of phenomena and of the results of experiment must be the first step in science, and its importance cannot be over-estimated; but before any quantity can be said to be known, we must have learned how to _measure_ it and to reproduce it in definite amounts. the great law of electrical action, the same as that of gravitation--the law of the inverse square--soon followed, as well as the associated fact that the electrification of a conductor resides wholly on its surface, and there only in a layer whose thickness is too small to be discovered. the fundamental laws of electricity having thus been established, there was no limit to the application of mathematical methods to the problems of the science, and, in the hands of the french mathematicians, the theory made rapid advances. george green, of sneinton, nottingham, introduced the term "potential" in an essay published by subscription, in nottingham, in , and to him we are indebted for some of our most powerful analytical methods of dealing with the subject; but his work remained unappreciated and almost unknown until many of his theorems had been rediscovered. but the idea of a body acting where it is not, and without any conceivable mechanism to connect it with that upon which it operates, is repulsive to the minds of most; and, however well such a theory may lend itself to mathematical treatment and its consequences be borne out by experiment, we still feel that we have not solved the problem until we have traced out the hidden mechanism. the pull of the bell-rope is followed by the tinkling of the distant bell, but the young philosopher is not satisfied with such knowledge, but must learn "what is the particular go of that." this universal desire found its exponent in faraday, whose imagination beheld "lines" or "tubes of force" connecting every body with every other body on which it acted. to his mind these lines or tubes had just as real an existence as the bell-wire, and were far better adapted to their special purposes. maxwell, as we have seen, not only showed that faraday's system admitted of the same rigorous mathematical treatment as the older theory, and stood the test as well, but he gave reality to faraday's views by picturing a mechanism capable of doing all that faraday required of it, and of transmitting light as well. thus the problem of electric, magnetic, and electro-magnetic actions was reduced to that of strains and stresses in a medium the constitution of which was pictured to the imagination. were this theory verified, we might say that we know at least as much about these actions as we know about the transmission of pressure or tension through a solid. with regard to the _nature_ of electricity, it must be admitted that our knowledge is chiefly negative; but, before deploring this, it is worth while to inquire what we mean by saying that we know what a thing is. a definition describes a thing in terms of other things simpler, or more familiar to us, than itself. if, for instance, we say that heat is a form of energy, we know at once its relationship to matter and to motion, and are content; we have described the constitution of heat in terms of simpler things, which are more familiar to us, and of which we _think_ we know the nature. but if we ask what _matter_ is, we are unable to define it in terms of anything simpler than itself, and can only trust to daily experience to teach us more and more of its properties; unless, indeed, we accept the theory of the vortex atoms of thomson and helmholtz. this theory, which has recently been considerably extended by professor j. j. thomson, the present occupier of clerk maxwell's chair in the university of cambridge, supposes the existence of a perfect fluid, filling all space, in which minute whirlpools, or vortices, which in a perfect fluid can be created or destroyed only by superhuman agency, form material atoms. these are _atoms_, that is to say, they defy any attempts to sever them, not because they are infinitely hard, but because they have an infinite capacity for _wriggling_, and thus avoid direct contact with any other atoms that come in their way. perhaps a theory of electricity consistent with this theory of matter may be developed in the future; but, setting aside these theories, we may possibly say that we know as much about electricity as we know about matter; for while we are conversant with many of the properties of each, we _know_ nothing of the ultimate nature of either. but while the theory of electricity has scarcely advanced beyond the point at which it was left by clerk maxwell, the practical applications of the science have experienced great developments of late years. less than a century ago the lightning-rod was the only practical outcome of electrical investigations which could be said to have any real value. [oe]rsted's discovery, in , of the action of a current on a magnet, led, in the hands of wheatstone, cooke, and others, to the development of the electric telegraph. sir william thomson's employment of a beam of light reflected from a tiny mirror attached to the magnet of the galvanometer enabled signals to be read when only extremely feeble currents were available, and thus rendered submarine telegraphy possible through very great distances. the discovery by arago and davy, that a current of electricity flowing in a coil surrounding an iron bar would convert the bar into a magnet, at once rendered possible a variety of contrivances whereby a current of electricity could be employed to produce small reciprocating movements, or even continuous rotation, where not much power was required, at a distance from the battery. an illustration of the former is found in the common electric bell; it is only necessary that the vibrating armature should form part of the circuit of the electro-magnet, and be so arranged that, while it is held away from the magnet by a spring, it completes the battery circuit, but breaks the connection as soon as it moves towards the magnet under the magnetic attraction. to produce continuous rotation, a number of iron bars may be attached to a fly-wheel, and pass very close to the poles of the magnet without touching them; when a bar is near the magnet, and approaching it, contact should be made in the circuit, but should be broken, so that the magnet may lose its power, as soon as the bar has passed the poles; or the continuous rotation may be produced from an oscillating armature by any of the mechanical contrivances usually adopted for the conversion of reciprocating into continuous circular motion. but all such motors are extremely wasteful in their employment of energy. faraday's discovery of the rotation of a wire around a magnetic pole laid the foundation for a great variety of electro-motors, in some of which the efficiency has attained a very high standard. about ten years ago, clerk maxwell said that the greatest discovery of recent times was the "reversibility" of the gramme machine, that is, the possibility of causing the armature to rotate between the field-magnets by sending a current through the coils. the electro-motors of to-day differ but little from dynamos in the principles of their construction. the copper disc spinning between the poles of a magnet while an electric current was sent from the centre to the circumference, or _vice versâ_, formed the simplest electro-motor. all the later motors are simply modifications of this, designed to increase the efficiency or power of the machine. similarly, the earliest machine for the production of an electric current at the expense of mechanical power only, but through the intervention of a permanent magnet, was the rotating disc of faraday, described on page . this contrivance, however, caused a waste of nearly all the energy employed, for while there was an electro-motive force from the centre to the circumference, or in the reverse direction, in that part of the disc which was passing between the poles of the magnet, the current so generated found its readiest return path through the other portions of the disc, and very little traversed the galvanometer or other external circuit. this source of waste could be, for the most part, got rid of by cutting the disc into a number of separate rays, or spokes, and filling up the spaces between them with insulating material. the current then generated in the disc would be obliged to complete its circuit through the external conductor. if we can so arrange matters as to employ at once several turns of a continuous wire in place of one arm, or ray, of the copper disc, we may multiply in a corresponding manner the electro-motive force induced by a given speed of rotation. all magneto-electric generators are simply contrivances with this object. the iron cores frequently employed within the coils of the armature tend to concentrate the lines of force of the magnet, causing a greater number to pass through the coils in certain positions than would pass through them were no iron present. the electro-motive force of such a generator depends on the strength of the magnetic field, the length of wire employed in cutting the lines of force, and the speed with which the wire moves across these lines. the point to aim at in constructing an armature is to make the resistance as small as possible consistent with the electro-motive force required. as there is a limit to the strength of the magnetic field, it follows that for strong currents, where thick wire must be employed, the generator must be made of large dimensions, or the armature must be driven at very high speed to enable a shorter length of wire to be used. the so-called "compound-interest principle," by which a very small charge of electricity might be employed to develop a very large one by the help of mechanical power, was first applied about a century ago in the revolving doubler. long afterwards, sir william thomson availed himself of the same principle in the construction of the "mouse-mill," or replenisher. the holtz machine, the voss and wimshurst machines, and the other induction-machines of the same class, all work on this principle. it may be illustrated as follows: take two canisters, call them a and b, and place them on glass supports. let a very small positive charge be given to a, b remaining uncharged. now take a brass ball, supported by a silk string. place it inside a, and let it touch its interior surface. the ball will, as shown by franklin, cavendish, and faraday, remain uncharged. now raise it near the top of the canister, and, while there, touch it. the ball will become negatively electrified, because the small positive charge in a will attract negative electricity from the earth into the ball. take the ball, with its negative charge, still hanging by the silk thread, and lower it into b till it touches the bottom. it will give all its charge to b, which will thus acquire a slight negative charge. raise the ball till it is near the top of b, and then touch it with the finger or a metal rod. it will receive a positive charge from the earth because of the attraction of the negative charge on b. now remove the ball and let it again touch the interior of a. it will give up all its charge to a; and then, repeating the whole cycle of operations, the charge carried on the ball will be greater than before, and increase in each successive operation, the electrification increasing in geometrical progression like compound interest. a leyden jar having one coating connected to a and the other to b, may thus be highly charged in course of time. a pair of carrier balls or plates, or a number of pairs, may be used instead of one. the carriers, just before leaving a and b, may be put in contact with one another instead of being put to earth; they may be mounted on a revolving shaft, and the forms of a and b modified to admit of the revolution of the carriers, and all the necessary contacts may be made automatically. we thus get various forms of the continuous electrophorus, and if the carriers are mounted on glass plates, and rows of points placed alongside the springs or brushes used for making the contacts, when the charges on the carriers become very strong, electricity will be radiated from the points on to the revolving glass plates, which will thus themselves take the place of the metal carriers. such is the action in the voss and other similar machines. but after faraday had shown how to construct a magneto-electric machine, the idea of applying the "compound-interest principle," and thus converting the magneto-electric machine into the "dynamo," occurred apparently simultaneously and independently to siemens, varley, and wheatstone. the first dynamo constructed by wheatstone is still in the museum of king's college, london. wilde employed a magneto-electric machine to generate a current which was used to excite the electro-magnet of a similar but larger machine, having an electro-magnet instead of a permanent steel magnet. the electro-magnet could be made much larger and stronger than the steel magnet, and from its armature, when made to revolve by steam power, a correspondingly stronger current could be maintained. the idea which occurred to siemens, varley, and wheatstone was to use the whole, or a part, of the current produced by the armature to excite its own electro-magnet, and thus to dispense with the magneto-electric machine which served as the separate exciter. when a part only of the current is thus employed, and is set apart entirely for this duty, the machine is a "shunt dynamo;" when the whole of the current traverses the field-magnet coils as well as the external circuit, it is a "series dynamo." the apparent difficulty lies in starting the current, but a mass of iron once magnetized always retains a certain amount of "residual magnetism," unless special means are taken to get rid of it, and even then the earth's magnetism would generally induce sufficient in the iron to start the action. commencing, then, with a slight trace of residual magnetism, the revolution of the armature generates a feeble current, which passing round the magnet coils, strengthens the magnetism, whereupon a stronger current is generated, which in turn makes the magnet still stronger, and so on until the magnet becomes saturated or the limit of power of the engine is reached, and the speed begins to diminish, or a condition of affairs is reached at which an increased current in the armature injures the magnetic field as much as the corresponding increase in the field-magnet coils strengthens it, and then no further increase of current will take place without increasing the speed of rotation. in a true dynamo the whole of the energy, both of the current and of the electro-magnets, is obtained from the source of power employed in driving the machine. but faraday's discovery of electro-magnetic induction led to practical developments in other directions. graham bell placed a thin iron disc in front of the pole of a bar magnet, and wound a coil of fine wire round the bar very near the pole. the ends of the coils of two such instruments he connected together. when the iron disc of one instrument approached the pole of the magnet, the lines of force were disturbed, fewer escaped radially from the bar, and more left it at the end, so as to go straight to the iron disc; thus the number of lines of force passing through the coil was altered, and a current was induced which, passing round the coil of the other instrument, strengthened or weakened its magnet, and caused the iron disc to approach it or recede from it, according to the way in which the coils were coupled. thus the movements of the first disc were faithfully repeated by the second, and the minute vibrations set up in the disc by sound-waves were all faithfully repeated by the second instrument. this was graham bell's telephone, in which the transmitter and receiver were convertible. but another and an earlier application of faraday's discoveries is found in the induction coil. a short length of thick wire and a very great length of thin wire are wound upon an iron bar. the ends of the long thin wire, or secondary coil, form the terminals of the machine; the short thick wire, or primary coil, is connected with a battery, but in the circuit is placed an "interrupter." this is generally a small piece of iron, or hammer, mounted on a steel spring opposite one end of the iron core, the spring pressing the hammer back against a screw the end of which, like the back of the hammer, is tipped with platinum; and this contact completes the battery circuit. when the current starts, the iron core becomes a magnet, attracts the hammer, breaks the contact, stops the current, the magnetism dies away, the hammer is forced back by the spring, and then the cycle of events is repeated. but the starting of the current in the primary causes a great many lines of magnetic force to pass through each of the many thousand turns of wire in the secondary, especially as the iron core conducts most of the lines of force of each turn of the primary almost from end to end of the coil, and thus through nearly all the turns of the secondary. this action might be further increased by connecting the ends of the iron core with an iron tube or series of longitudinal bars placed outside the whole coil. when the primary current ceases, all these lines of force vanish. thus during the starting of the primary current, which, on account of self-induction, occupies a considerable time, there will be an inverse current in the secondary proportional to the rate of increase of the primary; and while the primary is dying away, there will be a direct current in the secondary proportional to its rate of decrease. the primary current cannot be increased at a faster rate than corresponds to the power of the battery, but by making a very sharp break it may be stopped very rapidly. still, however rapidly the circuit is broken, self-induction causes a spark to fly across the gap until the energy of the current is used up. the introduction of the condenser, consisting of a number of sheets of tinfoil insulated by paper steeped in paraffin wax, and connected alternately with one end or the other of the primary coil, serves to increase the rapidity with which the primary current died away, by rapidly using up its energy in charging the condenser, and produces a corresponding diminution in the spark at the contact-breaker. this rapid destruction of the primary current causes a correspondingly great electro-motive force in the secondary coil, and thus very long sparks are produced between the terminals of the secondary coil when the primary current is broken, though no such sparks are produced when the primary current starts. if the secondary coil be connected up with a galvanometer, so that there is a metallic circuit throughout, it will be found that just as much electricity flows in one direction through the circuit at the break of the primary as flows in the other direction at the make, the difference being that the first is a very strong current of great electro-motive force but lasting a very short time, the second a feebler current lasting a correspondingly longer time. * * * * * but though the recent advances in electrical science have been very great, the grandest triumph of this century is the establishment of the principle of the conservation of energy, which has settled for ever the problem of "the perpetual motion," by showing that it has no solution. this problem was not simply to find a mechanism which should for ever move, but one from which energy might be continuously derived for the performance of external work--in fact, an engine which should require no fuel. but in spite of all that has been proved, numbers of patents are annually taken out for contrivances to effect this object. we have seen how rumford showed that heat was motion, and how he approximately determined its mechanical equivalent. séguin, a nephew of montgolfier, endeavoured to show that, when a steam-engine was working, less heat entered the condenser than when the same amount of steam was blown idly through the engine. this hirn succeeded in showing, thus proving that heat was actually used up in doing work. mayer, of heilbronn, measured the work done in compressing air, and the heat generated by the compression, and assumed that the whole of the work done in the compression, and no more, was converted into the heat developed, which was the same thing as assuming that no work was done in altering the positions of the particles of gas. from these measurements he deduced a value of the mechanical equivalent of heat. the assumption which mayer made was shown experimentally by joule to be nearly correct. joule proved that, when air expands from a high pressure into a vacuum, no heat is generated or absorbed on the whole. this he did by compressing air in an iron bottle, which was connected with another bottle from which the air had been exhausted, the connecting tube being closed by a stop-cock. the whole apparatus was immersed in a bath of water, and on allowing the air to rush from one vessel into the other, and then stirring the water, the temperature was found to be the same as before. when the iron bottles were in separate baths of water, that from which the air rushed was cooled, and that into which it rushed was heated to the same extent. joule and thomson afterwards showed that a very small amount of heat is absorbed in this experiment. joule also showed that the heat generated in a battery circuit is proportional to the product of the electro-motive force and the current, or to the product of the resistance and the square of the current, which, in virtue of ohm's law, is the same thing. this relation is often known as joule's law. he also proved that, for the same amount of chemical action in the battery, the heat generated was the same, whether it were all generated within the battery or part in the battery and part in an external wire; and that in the latter case, if the wire became so hot as to emit light, the heat measured was less than before, on account of the energy radiated as light. with a magneto-electric machine he employed mechanical power to produce a current, and the energy of the current he converted into heat. in all cases he found that, _whatever transformations the energy might undergo in its course, a definite amount of mechanical energy, if entirely converted into heat, always produced the same amount of heat_; and he thereby proved, not only that heat is essentially _motion_, but that it corresponds precisely with that particular dynamical quantity which is called _energy_; and thus justified the attempt to find a relation between heat and energy, or to express the mechanical equivalent of heat as so many foot-pounds. joule then set to work to determine, in the most accurate manner possible, the number of foot-pounds of work which, if entirely converted into heat, would raise one pound of water through ° fahr. the best known of his experiments is that in which he caused a paddle to revolve by means of a falling weight, and thereby to churn a quantity of water contained in a cylindrical vessel, the rotation of the water being prevented by fixed vanes. in these experiments he allowed for the work done outside the vessel of water or calorimeter, for the buoyancy of the air on the descending weight, and for the energy still retained by the weight when it struck the floor. from the results obtained he deduced foot-pounds as the mechanical equivalent of heat. expressed in terms of the centigrade scale, joule's equivalent, that is, the number of foot-pounds of work in the latitude of manchester, which, if entirely converted into heat, will raise one pound of water ° c., is . joule's experiments show that the same amount of energy always corresponds to, and can be converted into, the same amount of heat, and that no transformations, electrical or other, can ever increase or diminish this quantity. maxwell expressed this principle as follows:-- _the energy of a system is a quantity which can neither be increased nor diminished by any actions taking place between the parts of the system, though it may be transformed into any of the forms of which energy is susceptible._ this is the great principle of the conservation of energy which is applicable equally to all branches of science. another principle, almost equally general in its applicability, is that of the dissipation of energy, for which we are indebted in the first instance to sir william thomson. all forms of energy may be converted into heat, and heat tends so to diffuse itself throughout all bodies as to bring them to one uniform temperature. this is its ultimate state of degradation, and from that state no methods with which we are acquainted can transform any portion of it. when energy is possessed by a system in consequence of the relative positions or motions of bodies which we can handle, and whose movements we may control, the whole of the energy may be employed in doing any work we please; in fact, it is all _available_ for our purpose, or its _availability_ may be said to be perfect. energy in any other form is limited in its availability by the conditions under which we can place it. for example, the energy of chemical action in a battery may be used to produce a current, and this to drive a motor by which mechanical work is effected, but some of the energy must inevitably be degraded into the form of heat by the resistance of the battery and of the conductor, and this portion will be greater as the rate of doing work is increased. the ratio of the quantity of energy which can be employed for mechanical purposes with the means at our disposal, to the whole amount present, is called the _availability_ of the energy. all forms of energy may be wholly converted into heat, but only a fraction of any quantity of heat can be transformed into higher forms of energy, and this depends on the temperature of the source of heat and of the coldest body which can be employed as a condenser, being greater the greater the difference between the temperatures of the source and condenser, and the lower the temperature of the latter. in every operation which takes place in nature there is a degradation of energy, and though some portion of the energy may be raised in availability, another portion is lowered, so that on the whole the availability is diminished. thus, in the case of the heat-engine, work can be obtained from heat only by allowing another portion of the heat to fall in temperature; and, as originally stated by sir william thomson, "it is impossible, by means of inanimate material agency, to obtain mechanical effect from any portion of matter by cooling it below the temperature of the coldest of the surrounding objects," and to leave the working substance in the same condition in which it was at the commencement of the operations. accepting this principle, professor james thomson showed that increase of pressure must lower the freezing point of water, for otherwise it would be possible to construct an engine which, working by the expansion of water in freezing, would continue to do work by cooling a body below the temperature of any other body available, and he calculated the amount of pressure necessary to lower the freezing point through one degree. the conclusion was afterwards experimentally verified by sir william thomson, and served to explain all the phenomena of regelation. thus, like the principle of the conservation of energy, the principle of the dissipation of energy serves as a guide in the search after truth. but there is this difference between the two principles--no one can conceive of any method by which to circumvent the conservation of energy; but clerk maxwell showed that the principle of dissipation of energy might be overridden by the exercise of intelligence on the part of any creature whose faculties were sufficiently delicate to deal with individual molecules. in the case of gases, the temperature depends on the average energy of motion of the individual particles, and heat consists simply of this motion; but in any mass of gas, whatever the average energy may be, some of the particles will be moving with very great, and some with very small, velocities. by imagining two portions of gas, originally at the same temperature, separated by a partition containing trap-doors which could be opened or closed without expenditure of energy, and supposing a "demon" placed in charge of each door, who would open the door whenever a particle was approaching very rapidly from one side, or very slowly from the other, but keep it shut under other circumstances, he showed that it would be possible to sort the particles, so that those in the one compartment should have a great velocity, and those in the other a small one. hence, out of a mass of gas at uniform temperature, two portions might be obtained, one at a high temperature and the other at a low, and, by means of a heat-engine, work could be obtained until the two portions were again at equal temperatures, when the services of the "demons" might be again taken advantage of, and the operations repeated until all the heat was used up. any theory which is brought forward to explain a phenomenon, or any process which is proposed to effect any operation, must in the first instance submit to the test of the application of these two principles of conservation and dissipation of energy; and any proposal which fails to bear these tests may be at once rejected. the essential feature of the science of to-day is its quantitative character. we must, for instance, not only know that radiant energy comes to us from the sun, but we must learn how much energy is annually received by the earth in this way; and, in the next place, how much energy is radiated by the sun in all directions in the same time. when we have learned this, we want to know what is the source of this energy; and no theory of the sun which does not enable us to explain how this constant expenditure of energy is maintained can be accepted. last century it was possible to believe, with sir william herschel, that the greater part of the sun's mass is comparatively cool, and that it is surrounded by only a thin sheet of flame. to-day such a theory would be rejected at once, simply because the thin shell of flame could not provide energy for the solar radiation for any considerable time. the contact theory of the galvanic cell, as originally enunciated, fell to the ground for a similar reason. the simple contact of dissimilar metals could afford no continuous supply of energy to sustain the current. applied to the steam-engine, the doctrine of energy teaches us, not only that, corresponding to the combustion of a pound of coal, there is a definite quantity of work which is the mechanical equivalent of the heat generated, and is such that no engine of which we can conceive is capable of deriving from the combustion of the pound of coal a greater amount of work, but it teaches us that there is a further limitation fixed to the amount of work obtainable. this limitation depends upon the range of temperature at our command; and, when the range is known, we can express the amount of energy realizable by a perfect engine working through that range as a definite fraction of the whole energy corresponding to the heat of combustion of the fuel. thus, if we find that a particular engine realizes only per cent. of the energy of its fuel in work done, we must not suppose that mechanical improvements in the engine would enable us to realize any considerable portion of the other per cent.; for it may be that a theoretically perfect engine, working with its boiler and condenser at the same temperatures as those of the engine considered, could only realize per cent. of the energy of the fuel, reducing the margin for improvement from to per cent., as long as the range of temperature is unaltered. to improve the efficiency beyond this limit, the range of temperature must be increased, that is, generally, hotter steam must be used. the principles of energy are thus guides, not only to the scientific theorist, but to the practical engineer, and they have been established only through careful measurement. the simple observation of phenomena, and of the conditions under which they occur, could never have led to the establishment of such principles; and, though the carrying out of experiments which do not involve measurements is of great value, it is the careful measurement, however simple, which affords the highest training to the mind and hand, and without which any course of instruction in experimental physics is of little value. the hindoos used to regard the earth as a vast dome carried on the backs of elephants. the elephants themselves, however, required support, and were represented as standing on the back of a gigantic tortoise. it does not, however, appear that any support was provided for the tortoise. in some respects this figure represents the apparently perpetual condition of scientific knowledge. phenomena are investigated, and are shown to depend upon other actions which appear simpler or more fundamental than the phenomena at first observed. these, again, are found to obey laws which are of much wider application, or appear to be still more fundamental; but it may be that we are as far off as ever from discovering the great secret of the universe, the ultimate nature of all things. index. a. abbott, faraday's letters to, , . aberdeen university, maxwell appointed professor in, ; young's report on, . absorption, rumford's experiments on, ; of sun's rays by cloth of different colours, . academy of sciences, franklin nominated foreign associate of, . adjustment of the eye, young's paper on the, . Æpinus's completion of franklin's theory, . air, boyle's conception of the constitution of, . air-pump, boyle's experiments with, ; constructed by boyle, . american independence, declaration of, . american philosophical society, foundation of, . ampère's theory, faraday's views on, . anchor-ring experiment, faraday's, . arago's experiment, . argand lamp, efficiency of, . armstrong gun, principle of the, . atmospheric electricity, faraday's experiments on, ; obtained by a pointed rod, . autobiography of franklin, . availability of energy, . b. baily, francis, repetition of the cavendish experiment by, . beats in music, explanation of, . beggary in bavaria banished by rumford, . bernoulli's, daniel, molecular theory of gases, . boston, blockade of, . =boyle=, hon. robert, birth, ; conversion, ; first air-pump, ; conception of the constitution of the air, ; experiments with the air-pump, , _et seq._; argument on the cause of a vacuum, ; experiments establishing his law, ; statement of his law, ; observations on cold, , and on the expansion of water in freezing, ; experiments on induced magnetism, ; the province of experimental science, . boyle's law, . brocklesby, dr., death of, . brougham's criticisms of thomas young, . bumper, electrical, . c. camera obscura, invention of, . canada balsam, stresses in, . candle-flame, effect of, in discharging electricity, . capacity, electrical, ; franklin's experiments on, , ; cavendish's unit of, ; cavendish's measures of, , ; of disc, measured by cavendish, . capillarity, . cascade method of charging leyden jars, . =cavendish=, hon. henry, f.r.s., birth and parentage, ; social habits, ; appointed member of the r.s. committee on lightning-conductors, ; elected foreign associate of the french institute, ; death, ; proof of the law of inverse squares, ; experiment with the spheres repeated by macalister, ; experiments on the torpedo, ; experiments on the resistance of conductors, ; discovery of ohm's law, ; view of latent heat, ; apparatus for determining the melting point of mercury, ; the cavendish experiment, . cavendish experiment, ; laboratory, ; manuscripts, ; maxwell's work on the manuscripts, . city philosophical society, joined by faraday, ; faraday's lectures to, . cold, boyle's observations on, . collinson, peter, present of, to the library company, . colour-blindness, maxwell's experiments on, . colour-box, maxwell's, . colours, effect of, on absorption of sun's rays, , . colours of the spectrum mixed by boyle, . colour-top, maxwell's, , ; young's, . colour-vision, maxwell's theory of, ; young's theory of, . commonplace-book, faraday's, . compound-interest principle, . condenser, use of, in induction coils, . conduction of heat, rumford's experiments on, . conductors, multiple, flow of electricity through, . conductors necessarily opaque, . conservation of energy, maxwell's statement of the principle of, . copley medal awarded to franklin, , . cork, earl of, autobiography of, . creeping of electricity on glass, . crystalline lens, fibrous structure of, ; mode of adjustment of, . cuneus's discovery of the leyden jar, . d. davy, sir humphry, appointed professor at the royal institution, ; letter of, to faraday, . declaration of american independence signed, . defence of the american colonies against france and spain, . degree of electrification, . de la rive's invitation to faraday, . density of the earth, determinations of the mean, . desaguliers on electrics and non-electrics, . diagram of colour, young's, ; maxwell's, . diamagnetism discovered by faraday, . diamonds burned by davy, . dichroism of _lignum nephriticum_, . discharge, electrical, difference between positive and negative, . dissipation of energy, principle of, . distilled water, resistance of, . double refraction explained by huyghens, . dufay showed that all bodies could be electrified, . dynamical nature of heat, suggested by bacon, , ; maintained by boyle, ; investigated by rumford, ; established by joule, , . dynamical top, maxwell's, . dynamo, constructed by wheatstone, ; action of, ; essential feature of, . e. effect of points in discharging electricity, . electrical picnic, . electrical standards committee, . electric intensity, ; potential, . electricity, first obtained from clouds, ; velocity of, . electrics and non-electrics, . electrolysis, faraday's laws of, . electro-magnetic induction, discovered by faraday, ; maxwell's statement of the laws of, . electro-magnetic theory of light, . electro-motors, . electro-tonic state, conceived by faraday, ; explained by maxwell, . energy of leyden jar resident in the glass, . eriometer, young's, . ether, maxwell's illustration of the possible constitution of, . expansion of water on freezing, . extra current, . f. =faraday=, michael, birth, ; life in jacob's well mews, ; becomes an errand-boy, ; apprenticeship, ; attends lectures at tatum's, ; constructs a voltaic pile, ; letters to abbott, , ; starts as a journeyman, ; application to davy, ; appointed assistant at the royal institution, ; joins the city philosophical society, ; opinions respecting lectures, , ; journey with davy, ; acquaintance with de la rive, ; crosses the alps, ; at the academia del cimento, ; returns from the continent, ; lectures to the city philosophical society, ; commonplace-book, ; atmospheric electricity apparatus, ; marriage, ; discovery of electro-magnetic rotation, ; of the earth's action on a current, ; letter to e. de la rive, ; views on ampère's theory, ; elected f.r.s., ; appointed director of the laboratory at the royal institution, ; work on optical glass, ; discovery of induced currents, ; institutes friday evening lectures, ; anchor-ring experiment, ; magneto-electric machine, ; obtains induced current by action of the earth, ; obtains "magnetic spark," ; explanation of arago's experiment, ; laws of electrolysis, ; proves the identity of frictional and voltaic electricity, ; experiments on self-induction, ; diagrams of lines of magnetic force, ; conception of lines of electric force, ; ice-pail experiment, ; butterfly-net, ; experiments on specific inductive capacity, ; appointed scientific adviser to trinity house, ; appointed member of the senate of the university of london, ; discovery of the electro-magnetic rotation of the plane of polarization, ; investigations in diamagnetism, ; joins the sandemanian church, ; lectures before the prince consort, ; retirement to hampton court, ; death, ; lines of force investigated by thomson and maxwell, . forbes's, principal, opinion of young, . foucault's measurement of the velocity of light, . _fovea centralis_, insensibility of, to blue light, . franciscus linus, funicular hypothesis of, . =franklin=, benjamin, autobiography of, ; birth, ; on the disputatious temper, ; method of learning prose composition, ; tries vegetarianism, ; adopts the socratic method, ; first voyage to england, ; experience as a journeyman in london, ; views on beer as a food, ; marriage, ; endeavours to attain moral perfection, ; method of reconciling an enemy, ; elected f.r.s., ; second voyage to england, ; begins electrical experiments, ; electrical papers ridiculed by the royal society, ; discovers the effect of points, ; one-fluid theory of electricity, ; theory of the leyden jar, ; invention of the lightning-rod, ; golden fish, ; view of the nature of light, ; kite, ; experiments on capacity, , ; experiments on electrical induction, ; proof of the absence of electricity in a hollow conductor, ; third voyage to england, ; examination before the parliamentary committee, ; nominated foreign associate of the academy of sciences, ; signs the declaration of independence, ; sent to paris, ; made minister plenipotentiary to the court of france, ; signs the treaty of peace, ; elected president of pennsylvania, ; death, . fresnel, awarded the rumford medal, . fresnel's repetition of young's experiments, . friction as a source of heat, rumford's experiments on, . friday evening lectures instituted by faraday, . g. galileo and torricelli on the pressure of the air, . garnett, dr. thomas, professor at the royal institution, . gilbert, dr., founder of electrical science, . göttingen, young's university course at, . graham bell's telephone, . gray, stephen, discovers electric conduction, . grimaldi's fringes explained by young, . gunpowder, rumford's experiments on, . h. halos, coloured, young's explanation of, . hawksbee's observations on capillary attraction, . heat, a form of energy, ; generated by friction in vacuum, ; generated by friction, rumford's experiments on, . herapath's explanation of gaseous diffusion, . herschel's, sir john, comments on young's principle of interference, . hicks's, principal, investigations on the influence of temperature on gravitation, . hieroglyphics, young's work on, . hobbes, opposition of, to boyle, . hollow conductor, franklin's experiments on, ; cavendish's experiments on, ; faraday's experiments on, . honorary degrees conferred on franklin, , . hooke's law, . hooke, theodore, founds the royal society, . huyghens's explanation of double refraction, ; principle, . hydrogen, electro-chemical equivalent of, . i. ice-pail experiment of faraday, . identity of frictional and voltaic electricity, . induced currents, discovered by faraday, ; explained by structure of ether, ; from earth's action, . induction coil, . induction, franklin's experiments on, ; self, , . induction machines, principle of, . insulators for lightning-rods, . interference, principle of, discovered by young, ; spectra of, obtained by young, . invisible college, . j. jenkin, william, discovery of the "extra current" by, . joule and thomson's determination of the heat absorbed by air in expanding, . joule, dr., establishment of mechanical theory of heat by, , . joule's law, ; proof that heat and energy are equivalent, ; determination of the mechanical equivalent of heat, . junto club, formation of the, . k. kelland's, professor, edition of young's lectures, . kinnersley commences lecturing, . kite, franklin's, . knobs _versus_ points, . l. laboulaye's comments on franklin, . laplace's theory of saturn's rings, . latent heat, black's theory of, ; cavendish's views on, . leonardo da vinci's observation of capillary attraction, . leyden jar, discovery of, ; energy of, resident in the glass, . leyden jars charged by cascade, . light, franklin's view of nature of, ; maxwell's electro-magnetic theory of, ; rotation of the plane of polarization of, . lightning, effects of, on newbury steeple, . lightning-protectors, maxwell's, . lightning-rod, illustrations of the, . _lignum nephriticum_, dichroism of, . lines of force mathematically investigated by thomson and maxwell, . lines of magnetic force fixed by faraday, . luminiferous ether, the vehicle of electrical action, ; illustration of the possible constitution of, . m. magdeburg hemispheres, experiments with, by otto von guericke, . magic squares, franklin's proficiency in, . "magnetic spark" obtained by faraday, . magnetization by induction, boyle's experiments on, . magneto-electric machine, faraday's, , . magneto-electric machines, wilde's, ; objects to be aimed at in the construction of, . =maxwell=, james clerk, birth and parentage, ; enters edinburgh academy, ; letters to his father, ; early papers before the royal society of edinburgh, ; visit to mr. nicol, ; experiments with unannealed glass, ; enters the university of edinburgh, ; enters peterhouse, ; migrates to trinity, ; degree in cambridge, ; elected fellow of trinity, ; appointed professor at marischal college, ; marriage, ; essay on saturn's rings, ; dynamical top, ; appointed professor at king's college, ; lecture on colour at the royal institution, ; work on the electrical standards committee, ; appointed professor of experimental physics at cambridge, ; plans the cavendish laboratory, ; lectures at cambridge, ; work on the cavendish manuscripts, , ; delivers the rede lecture, ; method of protecting buildings from lightning, ; death, ; colour-top, ; experiments on colour-blindness, ; colour-box, ; awarded the rumford medal, ; wheel of life, ; real-image spectroscope, ; discovery of stresses in canada balsam, ; of the insensibility of the _fovea centralis_ to blue light, ; statistical method, ; explanation of the viscosity of gases, ; investigations of faraday's lines of force, ; statement of the laws of electro-magnetic induction, ; mechanical illustration of the ether, ; explanation of induced currents, ; of the mechanical action between currents and currents, and between magnets and currents, ; of self-induction, ; electro-magnetic theory of light, ; contrivance for overcoming the principle of the dissipation of energy, . maxwell's experiment for showing electro-magnetic rotation, . mayer's determination of the mechanical equivalent of heat, . mechanical equivalent of heat, definition of, ; rumford's determination of, . mercury, melting point of, . mirabeau's declamation on franklin, . mixed plates, colours of, . moral perfection, franklin's endeavour to attain, . mother-of-pearl, young's explanation of the colours of, . n. nautical almanack, young appointed superintendent of the, . newton's analysis and synthesis of white light, ; rings, young's explanation of, ; theory of light, . nicol prisms given to clerk maxwell, . o. [oe]rsted's discovery, . ohm's law, discovered by cavendish, ; meaning of, . optical glass, faraday's work on, . otto von guericke, contributions of, to electricity, ; experiments of, with the magdeburg hemispheres, . p. paris, dr., faraday's letter to, . pascal takes a barometer up the puy de dome, . pennsylvania fireplace invented by franklin, ; _gazette_ published by franklin, . perpetual motion, rumford's contrivances for, ; impossibility of, . philadelphia, franklin's first arrival in, ; library, foundation of the, . photometer, rumford's, . pigments, effects of mixing, . points _versus_ knobs, , . polarization, explained by transverse vibrations, ; of light discovered by malus, . "poor richard's almanack," . pressure of the air the cause of suction, . r. radiation, rumford's experiments on, ; of cold, rumford's experiments on, . rede lecture, delivered by clerk maxwell, . refraction of light, laws of, ; mentioned by pliny, . relative economy of different sources of light, . resistance of conductors, cavendish's experiments on, . roemer, measurement of the velocity of light by, . rosetta stone, discovery of the, ; inscription on, . royal institution, foundation of the, ; young's lectures at the, ; faraday's appointment at the, ; maxwell's lecture on colour at the, . royal society, origin of the, - . =rumford=, count, birth and parentage, ; life as a medical student, ; becomes a schoolmaster at concord, ; marriage, ; summoned before the committee of safety, ; imprisoned at woburn, ; first journey to london, ; receives an appointment in the colonial office, ; experiments on the explosion of gunpowder, , ; elected f.r.s., ; made lieutenant-colonel in the british army, ; promoted to colonel, ; visits elector of bavaria, ; cured of martial ambition, ; enters the service of the elector of bavaria, ; knighted by george iii., ; reforms in the bavarian army, ; attack on the beggars, ; made count of the holy roman empire, ; robbed of his manuscripts, ; visited by his daughter, ; his roaster, ; experiments on fire-places, ; founds the rumford medal, ; appointed minister plenipotentiary to the court of great britain, ; founds the royal institution, ; plans for the institution, ; residence in paris, ; marriage with madame lavoisier, ; death; ; cuvier's _éloge_ on, ; statue at munich, ; experiments on the conduction of heat in fluids, ; on the convection of heat in viscous liquids, ; on the weight of heat, ; on radiation, ; on the conduction of heat, ; on the apparent radiation of cold, ; shadow-photometer, ; experiments on the relative economy of candles and tapers, ; on the traction of carriages, ; on friction as a source of heat, ; determination of the mechanical equivalent of heat, . rumford medal, foundation of the, ; recipients of the, ; awarded to fresnel, ; awarded to clerk maxwell, . rumford roaster, . s. "sandford and merton," influence of, on the negro traffic, . saturn's rings, maxwell's essay on, . sea-water, resistance of, . séguin's attempt to measure loss of heat in the steam-engine, . self-induction, effect of, on sudden discharge, ; of electro-magnet, ; effect of, in induction coil, . sensation of heat, cause of, . seraphic love, boyle's essay on, . shaw's, dr., comments on boyle, . snellius's laws of refraction, . socratic method adopted by franklin, . specific inductive capacity, discovered by cavendish, ; rediscovered by faraday, . spectral colours, mixed by boyle, ; mixed by maxwell, . s.p.g., foundation of the, . spheroidal waves in iceland-spar explained by young, . stamp act, . standards commission, report of, . statistical method, maxwell's, . steeple struck by lightning at newbury, . stereoscope, maxwell's real-image, . stokes's, professor g. g., exhibition of the bright centre in the shadow of a disc, . suction caused by atmospheric pressure, . surface-tension, ; suggested by segner, ; young's investigations on, . t. table of results of experiments on boyle's law, . tatum's lectures on natural philosophy, . telephone, graham bell's, . temperature, its nature, . thermometers first hermetically sealed, . thomson's, professor james, application of the principle of dissipation of energy to the freezing of water under pressure, . thomson's, sir william, statement of the principle of dissipation of energy, ; vortex theory of matter, ; mirror galvanometer, ; replenisher, . thunder-storms, franklin's theory of, . torpedo, cavendish's experiments on the, ; davy's experiments on the, . traction of carriages, rumford's experiments on, . trial plate used by cavendish, . tyres, relative advantages of broad and narrow, . u. undulatory theory founded by hooke and huyghens, . union of the american states, franklin's plan for, . university of philadelphia, foundation of the, . v. vacuum, boyle's argument on the cause of a, . velocity of electricity, ; of light measured by roemer, ; of light deduced from electro-magnetic theory, . viscosity of gases explained by maxwell, . voltaic pile constructed by faraday, . vortex theory of matter, . voss machine, . w. wallis, dr., account of the royal society by, . wealth, ways to acquire, . wheel of life, clerk maxwell's, . wilson, dr., account of cavendish by, , . y. =young=, thomas, principal forbes's opinion of, ; birth and parentage, ; early education, ; becomes a london medical student, ; paper on the power of adjustment of the eye, ; elected f.r.s., ; visit to cornwall, ; first visit to the duke of richmond, ; enters the medical school at edinburgh, ; declines secretaryship to the duke of richmond, ; visits gordon castle, ; visits inverary castle, ; enters the university of göttingen, ; examination in medicine at göttingen, ; enters emmanuel college, ; discovers the principle of interference, ; appointed professor of natural philosophy at the royal institution, , ; lectures at the royal institution, ; theory of colour-vision, ; his colour-top, ; colour-diagram, ; his bakerian lectures, ; explanation of the rectilinear propagation of light, ; of newton's rings, ; eriometer, ; explanation of coloured halos, ; of the colours exhibited by mother-of-pearl, ; interference spectra, ; explanation of spheroidal waves in iceland-spar, ; of the colours of thin plates, ; hypothesis of an electric ether, ; investigations on surface-tension, ; modulus of elasticity, ; his marriage, ; appointed physician in st. george's hospital, ; superintendent of the nautical almanack, ; death, . printed by william clowes and sons, limited, london and beccles. wind and weather [illustration: logo] the macmillan company new york · boston · chicago · dallas atlanta · san francisco macmillan & co., limited london · bombay · calcutta melbourne the macmillan co. of canada, ltd. toronto [illustration: how the wind ruffles the top of a fog bank _frontispiece_] wind and weather by alexander mcadie a. lawrence rotch professor of meteorology, harvard university and director of the blue hill observatory new york the macmillan company _all rights reserved_ copyright, , by alexander mcadie. set up and electrotyped. published november, . list of illustrations how the wind ruffles the top of a fog bank _frontispiece_ page fig. . the tower of the winds " . boreas--the north wind " . kaikias--the northeast wind " . apheliotes--the east wind " . euros--the southeast wind " . notos--the south wind " . lips--the southwest wind " . all storms lead to new england " . zephyros--the west wind " . paths of high and low, january, " . skiron--the northwest wind " . the idealized storm " . turning of wind with altitude " . velocity of summer and winter winds " . blue hill observatory in an ice storm wind and weather the tower of the winds in athens on the north side and near the base of the hill on which the upper city--the acropolis--is built, there is a small temple still standing, altho its walls were completed twenty-two centuries ago. it is known as the tower of the winds; but as a matter of fact, the citizens of athens used it to tell the hour of the day and the seasonal position of the sun. it was a public timepiece. it served as a huge sun dial. water from a spring on the hillside filled the basins of a water clock in the basement of the tower. and so, whether the day was clear or cloudy the measure of the outflow of water indicated the time elapsed. also there were markings or dials on each of the eight walls of the temple, and the position of the shadow of a marker indicated the seasonal advance or retreat of the sun as it moved north from the time of the winter solstice and then south after the summer solstice. the sun is not an accurate time keeper and no one to-day runs his business or keeps engagements on sun time. but the old athenians were quite content to do so; and their tower served excellently for their needs. and they did what we moderns fail to do, namely, give distinctive names to the winds. they represented figuratively the characteristics of the weather as the wind blew from each of the eight cardinal directions. [illustration: fig. . the tower of the winds erected in athens, on the north side of the acropolis, b. c. ] the allegorical figures of the winds used in this little book are reproductions of the eight bas-reliefs in the library of the blue hill observatory, placed there by the late professor a. lawrence rotch. they are copied from the frieze of the tower of the winds at athens. the names of the winds boreas, the north wind, is perhaps the most important of all winds. at athens this a cold, boisterous wind from the mountains of thrace. the noise of the gusts is so loud that the greek sculptor symbolized the tumult by placing a conch shell in the mouth of boreas. his modern namesake, the bora of the adriatic, is the same noisy, blustering, cold wind-rush from the north. the northeast wind kaikias is a trifle more pleasant looking than boreas, but still not much to brag about. master of the squall and thunderstorm, he carries in his shield an ample supply of hailstones, ready to spill them on defenseless humanity. he might well serve as the patron saint of air raiders dropping their bombs on helpless humans below. apheliotes, the east wind, is a graceful youth, with arms full of flowers, fruit and wheat. naturally this was a favorite wind, blowing in from the sea, with frequent light showers. some of us who dwell on the atlantic coast, in more northern latitudes than athens, do not always regard with favor the east wind, associating it with chilly, damp and sombre weather. yet it is the harbinger of good--tempering the cold of winter and the heat of summer. it is an angel of mercy in mid-summer when the temperature is above the nineties and there is no air stirring. then it is, that we all welcome the refreshing wind from the sea. euros, the southeast wind, and neighbor to apheliotes, is a cross old fellow, intent on the business of cloud making. he alone of all the winds carries nothing in his hands. in the new testament he becomes euroclydon, wind of the waves. he is no friend of the sailor; and the seasick traveler prays to be rid of his company. the figure on the south face of the tower, notos, is the master of the warm rain. he carries with him a water jar which has just been emptied. compare his light flowing robes and half-clad neck and arms with the close fitting jacket of old boreas. at his shrine, hydraulic engineers well might worship. next, the mariner's wind, lips, the southwest favoring breeze bringing the ships speedily into harbor; yes, into that piraeus, famed in classic history. incidentally it is the southwest wind which differentiates the climate of great britain from that of labrador. this wind makes northwest europe habitable; while on the other side of the atlantic, in similar latitudes, but under the influence of prevailing northwest winds, we find labrador--a section certainly misnamed, for it is not the abode of farmers, as the name implies--but barren and bleak. what a difference it would make thruout this region if the gulf stream continued north, close to the shore, and the prevailing winds were _from the east_. our north atlantic coast would then be _the land of zephyrs_, using the word in the sense of pleasant, gentle winds. [illustration: fig. . boreas--the north wind] zephyros, the west wind, is represented as a graceful youth, scantily clad, with his arms filled with flowers. in greece this wind traversed the ionian sea and the gulf of corinth before reaching athens. it is quite unlike our west wind which blows across a continent, and is continuously robbed of its water vapor on the long passage. the ionian wind is pleasantly moist and refreshing. last of all, but by no means least important, is skiron, lord of gusty northwest gales. freezing in winter, parching in summer, he carries with him a brazen fire basket and spills a generous stream of hot air on all below. his husky highness might not inappropriately adorn legislative halls and editorial sanctums. he would displace the blindfolded lady holding scales very much out of balance. think of the deep significance of his presence. in our country the northwest is of all winds, except the west, most persistent. for hours in a year, this wind is with us. joining forces with the west wind, these directions prevail one third of the time. these northwest-west winds also have the greatest speed and gustiness. the climate of the united states is essentially determined by the prevalence of the north, northwest and west winds. forecasting the weather in old days, the _haruspices_ (for this is what the romans called weather men in the days of caesar) proclaimed the will of the gods by consulting the entrails of some freshly killed animal. evidently these haruspices did not always make correct forecasts; for there were some romans who openly questioned their worth. cato, the censor, is on record as saying "that he wondered how one haruspex could look another in the face without laughing!" [illustration: fig. . kaikias--the northeast wind] the modern professional forecaster would scorn to consult the entrails. there are however many amateur forecasters who foretell weather by their aches and rheumatic pains. probably there is a high correlation factor between body sensations and dampness; and some individuals are quite sensitive to changes in both relative and absolute humidity. this, however, does not always mean that a storm is approaching. humidity or dampness is only one factor and may be quite local, whereas most storms are wide-spread. the weather map the official forecaster consults a daily weather map and certain auxiliary maps which show changes in pressure and temperature for twelve hours or more. he examines closely the contours of pressure as shown on the map. the synoptic map, as it is called, because it is a glance at weather conditions over a large area at one and the same moment, is a map on which are plotted pressure, temperature, wind direction, velocity and rainfall. the lines of equal pressure or isobars generally curve and inclose what is known as a cyclonic centre, or depression or low. the arrows point in, but not exactly toward the centre of the depression. on the map there will probably appear also an area of high pressure where the surface air flows leisurely outward and away from the place of highest pressure. such an area is called an anticyclone, a word first used by sir francis galton in to designate not only high pressure, but general flow of the air in a reversed or opposite direction to that of the low area or cyclone. the word cyclone was first used by piddington in in describing the flow of the air in the typhoons of the east indian seas. it is from the greek and literally means the coils of a serpent. the word cyclone must possess some special merit in the minds of journalists for it is quite commonly misused for tornado in descriptions of the smaller and more destructive storm. the low cyclone is simply the generic name for a large rotating air mass. it is a barometric depression or low and is characterized by a flow of air inward and around a moving centre. the air circulation is counter-clockwise in the northern hemisphere and clockwise in the south. perhaps if the earth stopped rotating and there was no planetary circulation, with the great west-moving trades and east-moving "westerlies," the arrows on the weather map would all point directly toward the centre of the low; but, as things are, there are some very good reasons why air can not move directly into a low, that is at right angles to the isobars. moreover, the weather map does not indicate the true flow of the air, for observations of the wind made at the ground tell only a part of the story of the balance which the flowing air must maintain under the action of various forces, such as gravitation, rotational deflection, centrifugal tendency, and the various expansion and compression forces. the winds near the ground are modified both in velocity and direction by friction. the free flow is often interfered with by topography. the true air flow one must rise above the ground some distance to get the true air flow, or what is known as the gradient wind, the flow which balances the gradient, i.e. a flow along the isobars. the gradient velocity is found about metres above the ground, and the gradient direction a little higher. the lower clouds as a rule indicate true wind values very well; and so, it is desirable in studying winds to use cloud directions and velocities rather than surface values. in cloud work a nephoscope is essential. the unaided eye, unless properly shielded, suffers from the glare of a sunlit sky; and moreover, there are no fixed points or references. a black mirror, with suitable sighting rods and measuring devices, enables an observer to follow the cloud, estimate its height and determine with accuracy the direction from which it is moving. there is an average difference of degrees between the cloud direction and the surface wind; the upper direction being more to the right. at times the directions may be opposite. [illustration: fig. . apheliotes--the east wind] it may seem surprising but few of us, except at sunrise and sunset, really see what is going on in cloud land. some meteorologists hold that the circulation of air to metres above the ground controls the path and perhaps the intensity of storms. it is therefore important to know something of the flow at high levels if we would improve the forecasts. limitations of map the weather map fails to indicate what shifts of direction and changes in velocity are likely to occur. the forecaster tries to anticipate these, but he bases his conclusions chiefly upon an expected movement of the low area; using the accumulated records of the paths of past storms. but each storm is in reality a law unto itself; and while we know something of the relations between pressure and flow of the air; as yet we know very little about the relations of wind and weather. the problem is complicated by the behavior of the load of water vapor. [illustration: fig. . euros--the southeast wind] the chief forecaster of one of the great national weather services recently wrote: "despite the fact that maps have now been drawn day by day for over half a century, we may safely say that no two maps have been identical." it is perhaps unfortunate that so much attention has been given to the cyclone or depression or low, and comparatively little to the high or anticyclone. for we are now beginning to understand that while there may seem at first to be nothing specially noteworthy about a mass of air where the pressure varies from to kilobars, that is, to per cent _above_ a standard atmosphere, with isobars irregularly curved and feeble surface winds, yet the anticyclone is more important than the cyclone in determining weather sequence; for the progressive motion of the cyclone depends largely upon the strength of the anticyclone. ocean storms sir napier shaw, who has written much on the weather of the british isles, may be quoted here. "anyone who is interested in the weather is always on the lookout for 'lows' and is very keen to know whether he is going to be on the south of the centre or the north of it. he is, of course, interested in the anticyclone too, because as long as an anticyclone is there, there cannot be a depression; but it is the depression which has the life and movement about it, giving it a claim to the attention of everybody who wants to know what the weather and its changes are going to be. "this has been recognized from the very earliest days of weather maps with isobars. the depressions which pass over our shores (great britain) mostly come from the west. some of them come all the way from america; one or two have been traced from the west coast of africa and so have crossed the atlantic twice, first to the westward and then to the eastward. some have come all the way from a sort of parent 'low' in the north pacific ocean. so general is the tendency for 'lows' to go eastward that it was thought at one time, particularly by the 'new york herald,' that their departure from the american coast and subsequent arrival on our own shores could be notified by cable, and we (the british) might thus be forewarned of their approach, some three or four days in advance. the attempt was made by the 'new york herald' acting in co-operation with the meteorological offices of the united kingdom and france. but a depression keeps to no beaten track; it has as many paths for its centre as there are lines in a bundle of hay. though groups can be picked out there are many strays, and, moreover, the depression changes its shape and intensity while it travels, so that if you lose sight of it for a day you cannot be at all sure of its identity." [illustration: fig. . notos--the south wind] transcontinental storms if there is so much uncertainty in forecasting the path of a disturbance at sea, how much more uncertain must it be on land? elaborate statistics of the average daily movement of various types of storms have been officially published. the average speed of storms (not wind speeds) across the united states is metres per second or miles an hour. storms travel more rapidly in winter than in summer, about half again as fast; that is, summer storms travel miles, and winter storms miles, an hour. [illustration: fig. . lips--the southwest wind] the paths vary widely; from the gulf storms moving northeast and west indian hurricanes recurving on the southern coast, to the storms from alberta and the west which move south and east. ten types of storms, classified according to the place of origin, are recognized by the official forecasters of the united states. these are north pacific, alberta, northern rocky mountain, colorado, central, south pacific, texas, east gulf, south atlantic and west indian hurricanes. a better nomenclature would be ( ) alberta, ( ) washington, ( ) kootenay, ( ) utah, ( ) kansas, ( ) california, ( ) texas, ( ) louisiana, ( ) florida, and ( ) hurricanes. hurricanes type is the general class of tropical storms occurring chiefly in the summer and fall which, drifting west, slowly work northward. similar storms are the typhoons and baguios of the east indian and china seas. the path and point of recurvature will be determined by the position of the bermuda hyperbar, that is, the seasonal anticyclone of the atlantic. this accounts for the swinging east and north of these tracks as the season progresses; for the hyperbar is slowly displaced east, the maximum displacement occurring in september. [illustration: base map by goode fig. . all storms lead to new england] individual anticyclones also influence individual hurricanes. thus a hurricane passing west over havana, will go farther west if a vigorous "high" is spreading southeast over the gulf states. and when this "high" passes seaward, the hurricane will work around the southwest quadrant of the "high," recurving and moving northeast. storm rendezvous altho storms originate or are first detected in nine different sections, it is a fact worth mentioning that they all leave the united states in the vicinity of new england or nova scotia. some of the southern depressions starting near the coast, pass to sea south of new york, but in general an observer standing on plymouth rock can virtually encompass within a radius of kilometres, miles, the paths of ninety per cent of the storms that traverse the country. thus a storm that originates in texas ( ) will probably pass close to cape cod. likewise, types ( ) and ( ); while the other types may pass a little to the north or south. see chart, paths of storms. storm paths forecasting then would seem to be very easy; for one would only have to know the place of origin of the storm and the rate of travel, to foretell exactly the time of arrival. unfortunately these are only the average paths; and as with most mean values, represent a value not often experienced in fact. these paths then are not paths which any given storm will follow. one must recall the story of the operating surgeon who gave the average age of his patients in the operating room as . there were but two patients, one years old and the other year old. [illustration: fig. . zephyros--the west wind] as a matter of fact the path of any individual depression depends upon several factors, some of which are:--the prevailing eastward drift of the air; the extent and motion of some anticyclone advancing before the "low"; the duration and speed of relatively dry cold tongues of air from the north; and the supply of water vapor brought from southern waters by south winds. a depression can make little headway if to the north or east the normal path is blocked by what is known as a stagnant "high." so therefore, if the anticyclone is a slow mover, a texas storm, which would normally pass not far from southern new england, may be deflected farther north than when the high moved rapidly east. so too, with the storms which originate in the western part of the country. a slow moving high will prevent the low following it, from moving east at a normal rate along the usual path. anticyclones then, are the real weather controls. there are various types, but all drift from the north or west. occasionally they enter the country from the pacific, but the great majority come from alberta and move leisurely southeast, often reaching the south atlantic states; but more frequently recurving and passing to the north. stagnant highs highs are sometimes reinforced and this results in what is called a stagnant high. a good illustration of such a slow moving high and its consequences occurred during the last week of january, . a surge of cold air from alberta or farther north reached the international boundary january st and spread slowly eastward, reaching the great lakes on the th and the st. lawrence valley two days later. then seemingly it halted or moved slowly westward, retrograding. in three days, that is, on the th, the centre of the high was apparently miles _west_ of where it had been on the th. after the th it followed a normal track, moving slowly southeast, reaching the atlantic near long island. meanwhile a depression on the south coast of texas on the th, moved across the gulf of mexico, passing over southern florida on the th and advanced steadily northeast, reaching cape hatteras in hours. owing to the presence of the anticyclone referred to above, the depression recurved off hatteras. the result was a memorable snow storm in northern virginia and maryland. at p.m. january th, there had been a fall of cms. ( inches). within the following twenty hours the average depth in the city of washington was cms. ( inches). the weight of the snow caused the collapse of the roof of the knickerbocker theatre and the death of persons. the total snowfall in various coast cities was: raleigh cms.* richmond " washington " baltimore " wilmington " philadelphia " trenton " new york " new haven " boston " *note: to convert to inches multiply by . . the table shows clearly how the snow was formed. on the east side of the low a stream of air, relatively warm, carried a load of water vapor, approximately grams in each cubic metre. [illustration: base map by goode fig. . paths of high and low, great snow storm of january - , ] this current was steered around the north side of the low and met the north-northeast wind. under the new conditions the air saturated could hold only or grams; and so condensation and heavy precipitation resulted. the region of maximum snowfall was near washington, and it will be seen that there is a proportional decrease north and south. the snowfall at washington was the heaviest ever known at that city. unlike most storms, there was no strong cold northwest wind blowing into the depression. the temperature rose slowly. it was less a contrast of winds than a steady slow outward push of the anticyclone, and the consequent turning of the path of the cyclone eastward. laws of forecasting buys ballot's law. "if you stand with your back to the wind the pressure decreases toward your left, and increases toward your right." for navigators, this law is more generally expressed in the words of the hydrographic office on "cyclonic storms." "since the wind circulates counter-clockwise in the northern hemisphere, the rule in that hemisphere is to face the wind, and the storm centre will be at the right hand. if the wind traveled in exact circles, the centre would be eight points ( degrees) to the right when looking directly in the wind's eye. but the wind follows a more or less spiral path inward which brings the centre from eight to twelve points ( to degrees), to the right of the wind. the centre will bear more nearly eight points from the direction of the lower clouds than from the surface wind." [illustration: fig. . skiron--the northwest wind] the law given on the preceding page is named after c. h. d. buys ballott, a dutch meteorologist. it was announced in a paper published in the _comptes rendus_ in . two american writers on the winds, j. h. coffin and william ferrell, had however earlier found the law to hold. * * * * * while most of us study storms from a window at home and are not called upon to handle a ship in a storm, yet it may not be out of place to include here the diagram of the winds in an ideal storm and give the rules for maneuvering. see figure . the winds in an idealized storm. the rules apply only to storms in the northern hemisphere. "_right or dangerous semicircle_,--steamers: bring the wind on the starboard bow, make as much way as possible, and if obliged to heave-to, do so head to sea. sailing vessels: keep close-hauled on the starboard tack, make as much way as possible, and if obliged to heave-to, do so on the starboard tack. _left or navigable semicircle_,--steam and sailing vessels: bring the wind on the starboard quarter, note the course and hold it. if obliged to heave-to, steamers may do so stern to sea; sailing vessels on the port tack. _on the storm track in front of center_,--steam and sailing vessels: bring the wind two points on the starboard quarter, note the course and hold it, and run for the left semicircle, and when in that semicircle manoeuvre as above. on the storm track, in rear of center,--avoid the center by the best practicable route, having due regard to the tendency of cyclones to recurve to the southward and eastward." [illustration: from hydrographic office fig. . the winds in an idealized storm] wind and altitude the law of the turning of the wind with altitude. a casual observation of the lower clouds where no means of measuring small angles is available will not usually show any difference between the motion of the clouds and the surface wind; but with the upper clouds the case is different, and one readily detects a difference. several thousand observations with various agencies, such as kites and pilot balloons and more especially measurements made with theodolites and nephoscopes, show that there is a definite twist to the right with elevation. the amount of the deflection is shown in figure . turning of the wind with altitude. here the average yearly values are given for directions and velocities. thus if the mean wind direction at blue hill is from a point a little to the north of west, grads or degrees, and the mean velocity metres per second; the clouds at metres elevation will move from or degrees and at a speed of approximately metres per second ( miles an hour). these however, are average values. in individual cases the difference between surface winds and stratus clouds may be considerably greater. it may be as much as degrees; that is, the cloud may move directly opposite to the wind. in general there will be a difference of to degrees. wind and rain the law of wind direction, approximate cooling and rain. when the lower clouds are moving from the north or northwest, without sharply defined edges, the low is east or northeast of the observer; and rain or snow is not likely unless there is a rapidly falling temperature. [illustration: turning of wind with altitude, blue hill fig. . turning of wind with altitude] when a stream of warm air with a high absolute humidity flows north on the east side of a low, and a cold northwest wind follows quickly after the low, rain or snow may be expected. any rapid chilling of warm, moist air produces cloudiness and rain or snow; but a cold stream blowing into a warm area will not produce as much rain as a warm stream blowing into a cold area. duration of wind the average duration of wind from various directions is as follows: from the north about hours each week; from the northeast, the same; from the east, hours; from the southeast, hours; from the south, hours; from the southwest, hours; from the west, hours; and from the northwest hours. during an individual disturbance lasting about hours, we may have hours of southwest wind; hours of west wind, backing during the next hours to south; hours of south wind; hours of southeast wind; hours of east wind; hours northeast wind and hours north wind, hours northwest, when it may be considered that a new pressure distribution prevails. the above values hold only for a storm moving with normal velocity. lows are often blocked by slow moving highs in advance. in such cases the duration of east winds is greater. the winds of a year the following table shows the marked increase in the prevalence of northwest and west winds during winter months, the decrease in north winds during july, the increase in northeast winds in may, also in east winds; the increase of south and southwest winds in july; and the falling off of southeast winds in december. see table, page . in cities near the atlantic coast, a continuance of northeast wind, especially in the fall and winter months, results in frequent altho not necessarily heavy rains. on the other hand a period of continued northwest and west wind is a dry period. in summer, southeast and east winds bring fog and cooler weather; while southwest winds are favorable for the development of thunderstorms. winds of a year table i.--number of hours the wind blows from different directions. -------------------------------------------------------------------- jan. mar. may july sept. nov. year feb. apr. june aug. oct. dec. boreas (n) kaikias (ne) apheliotes (e) euros (se) notus (s) lips (sw) zephyros (w) skiron (nw) -------------------------------------------------------------------- [illustration: fig. . velocity of summer and winter winds in metres per second] the sea breeze when the weather has been clear and moderately warm for two or more days, and the winds are light and variable, there may occur on the third day a moderate wind from the east, known as the sea-breeze. this occurs during anticyclonic conditions. preceding the sea-breeze, the winds are very light, there are no clouds, and the temperature rises rapidly during the forenoon. this heating is due to a slow dynamic compression as the air slowly descends and the surface air does not flow away. there is no cooling because there is no evaporation due to air movement. the absolute humidity is low, often less than ten grams per cubic metre. cumulus clouds do not form because there is no uplift of the lower air and consequently no chance for condensation of whatever water vapor may be present. no thunder-heads form notwithstanding the heat. the heat, while dry, is nevertheless extremely trying to men and animals. relief comes in the early hours of the afternoon by the arrival of the sea-breeze. the usual explanation of the origin of the sea-breeze is that the land being excessively warm, the air over a relatively cool ocean moves in to take the place of the warm and therefore lighter air, which it is assumed has risen. unfortunately for this explanation, the air over the land has _not_ risen; but on the contrary is falling slowly. again the sea-breeze does not begin at the place where the temperature contrast is greatest, namely, just inside the shore line; but comes in from the sea. nor does the flow extend far inland, which would be the case if there were up-rising currents. the sea-breeze is very shallow, generally not extending upward more than metres, and often not above metres. it does not penetrate far inland, as a rule not more than kilometres, miles. the sea-breeze is probably caused by a slow descent of dry, warm air, on an incline sloping from northeast to southwest. as it reaches the surface it is twisted more to the right; that is, becomes an east wind. it carries inland with it some of the air over the ocean which is much cooler and heavily saturated. muggy days there are certain days, more noticeable in summer than at other times, when the air is heavily laden with water vapor; and there is little or no cooling of the body due to evaporation. we perspire freely but as the sweat does not evaporate, there is a constantly increasing amount of water on the skin. [illustration: fig. . blue hill observatory during ice storm, november - , ] it is not altogether a question of temperature, for another day may have as high or even higher temperature. it is essentially a matter of ventilation. on muggy days we are somewhat in the condition of the unfortunate prisoners in the black hole at calcutta. they did not die by poisoning, as has generally been accepted, that is, lack of sufficient oxygen and an excess of carbon dioxide; but because they were unable to keep the skin sufficiently cool. there was no ventilation; no movement of the air and the body became over-heated and exhaustion followed. no matter how much water there may be on the skin if the surrounding space is saturated, one feels oppressed. a vigorous fanning of the air helps evaporation and cools us. that is why a brisk northwest wind routs a muggy condition. castilian days john hay wrote of such days spent in spain. we who live in a land where the winds are more boisterous, occasionally experience what we call a perfect day. such days have easterly winds of two metres per second or less than five miles an hour. the temperature is midway between freezing and normal body temperature or about ° f. the relative humidity is approximately % and the absolute humidity grams per cubic metre. the table on page explains the paucity of perfect days. the gusty, boisterous winds, skiron and zephyros, blow too frequently. perhaps certain of our national characteristics may be traceable to this flow of the air and our climatic environment. from newton to einstein changing conceptions of the universe by benjamin harrow, ph.d. second edition, revised and enlarged with articles by prof. einstein, prof. j. s. ames (johns hopkins), sir frank dyson (astronomer royal), prof. a. s. eddington (cambridge) and sir j. j. thomson (president of the royal society) portraits and illustrations new york d. van nostrand company eight warren street preface einstein's contributions to our ideas of time and space, and to our knowledge of the universe in general, are of so momentous a nature, that they easily take their place among the two or three greatest achievements of the twentieth century. this little book attempts to give, in popular form, an account of this work. as, however, einstein's work is so largely dependent upon the work of newton and newton's successors, the first two chapters are devoted to the latter. b. h. preface to second edition the preparation of this new edition has made it possible to correct errors, to further amplify certain portions of the text and to enlarge the ever-increasing bibliography on the subject. photographs of professors j. j. thomson, michelson, minkowski and lorentz are also new features in this edition. the explanatory notes and articles in the appendix will, i believe, present no difficulties to readers who have mastered the contents of the book. they are in fact "popular expositions" of various phases of the einstein theory; but experience has shown that even "popular expositions" of the theory need further "popular introductions." i wish to take this opportunity of thanking prof. einstein, prof. a. a. michelson of the university of chicago, prof. j. s. ames of johns hopkins university, and professor g. b. pegram of columbia university for help in various ways which they were good enough to extend to me. prof. j. s. ames and the editor of science have been kind enough to allow me to reprint the former's excellent presidential address on einstein's theory, delivered before the members of the american physical society. b. h. table of contents page i. newton ii. the ether and its consequences iii. einstein iv. appendix time, space and gravitation, by prof. einstein einstein's law of gravitation, by prof. j. s. ames the deflection of light by gravitation and the einstein theory of relativity, by sir frank dyson, prof. a. s. eddington and sir j. j. thomson newton "newton was the greatest genius that ever existed."--lagrange, one of the greatest of french mathematicians. "the efforts of the great philosopher were always superhuman; the questions which he did not solve were incapable of solution in his time."--arago, famous french astronomer. einstein "this is the most important result obtained in connection with the theory of gravitation since newton's day. einstein's reasoning is the result of one of the highest achievements of human thought."--sir j. j. thomson, president of the british royal society and professor of physics at the university of cambridge. "it surpasses in boldness everything previously suggested in speculative natural philosophy and even in the philosophical theories of knowledge. the revolution introduced into the physical conceptions of the world is only to be compared in extent and depth with that brought about by the introduction of the copernican system of the universe."--prof. max planck, professor of physics at the university of berlin and winner of the nobel prize. i newton in speaking of newton we are tempted to paraphrase a line from the scriptures: before newton the solar system was without form, and void; then newton came and there was light. to have discovered a law not only applicable to matter on this earth, but to the planets and sun and stars beyond, is a triumph which places newton among the super-men. what newton's law of gravitation must have meant to the people of his day can be pictured only if we conceive what the effect upon us would be if someone--say marconi--were actually to succeed in getting into touch with beings on another planet. newton's law increased confidence in the universality of earthly laws; and it strengthened belief in the cosmos as a law-abiding mechanism. newton's law. the attraction between any two bodies is proportional to their masses and inversely proportional to the square of the distance that separates them. this is the concentrated form of newton's law. if we apply this law to two such bodies as the sun and the earth, we can state that the sun attracts the earth, and the earth, the sun. furthermore, this attractive power will depend upon the distance between these two bodies. newton showed that if the distance between the sun and the earth were doubled the attractive power would be reduced not to one-half, but to one-fourth; if trebled, the attractive power would be reduced to one-ninth. if, on the other hand, the distance were halved, the attractive power would be not merely twice, but four times as great. and what is true of the sun and the earth is true of every body in the firmament, and, as professor rutherford has recently shown, even of the bodies which make up the solar system of the almost infinitesimal atom. this mysterious attractive power that one body possesses for another is called "gravitation," and the law which regulates the motion of bodies when under the spell of gravitation is the law of gravitation. this law we owe to newton's genius. newton's predecessors. we can best appreciate newton's momentous contribution to astronomy by casting a rapid glance over the state of the science prior to the seventeenth century--that is, prior to newton's day. ptolemy's conception of the earth as the center of the universe held undisputed sway throughout the middle ages. in those days ptolemy was in astronomy what aristotle was in all other knowledge: they were the gods who could not but be right. did not aristotle say that earth, air, fire and water constituted the four elements? did not ptolemy say that the earth was the center around which the sun revolved? why, then, question further? questioning was a sacrilege. copernicus ( - ), however, did question. he studied much and thought much. he devoted his whole life to the investigation of the movements of the heavenly bodies. and he came to the conclusion that ptolemy and his followers in succeeding ages had expounded views which were diametrically opposed to the truth. the sun, said copernicus, did not move at all, but the earth did; and far from the earth being the center of the universe, it was but one of several planets revolving around the sun. the influence of the church, coupled with man's inclination to exalt his own importance, strongly tended against the acceptance of such heterodox views. among the many hostile critics of the copernican system, tycho brahe ( - ) stands out pre-eminently. this conscientious observer bitterly assailed copernicus for his suggestion that the earth moved, and developed a scheme of his own which postulated that the planets revolved around the sun, and planets and sun in turn revolved around the earth. the majority applauded tycho; a small, very small group of insurgents had faith in copernicus. the illustrious galileo ( - ) belonged to the minority. the telescope of his invention unfolded a view of the universe which belied the assertions of the many, and strengthened his belief in the copernican theory. "it (the copernican theory) explains to me the cause of many phenomena which under the generally accepted theory are quite unintelligible. i have collected arguments for refuting the latter, but i do not venture to bring them to publication." so wrote galileo to his friend, kepler. "i do not venture to bring them to publication." how significant of the times--of any time, one ventures to add. galileo did overcome his hesitancy and published his views. they aroused a storm. "look through my telescope," he pleaded. but the professors would not; neither would the body of inquisitors. the inquisition condemned him: "the proposition that the sun is in the center of the earth and immovable from its place is absurd, philosophically false and formally heretical; because it is expressly contrary to the holy scriptures." and poor galileo was made to utter words which were as far removed from his thoughts as his oppressors' ideas were from the truth: "i abjure, curse and detest the said errors and heresies." the truth will out. others arose who defied the majority and the powerful inquisition. most prominent of all of these was galileo's friend, kepler. though a student of tycho, kepler did not hesitate to espouse the copernican system; but his adoption of it did not mean unqualified approval. kepler's criticism was particularly directed against the copernican theory that the planets revolve in circles. this was boldness in the extreme. ever since aristotle's discourse on the circle as a perfect figure, it was taken for granted that motion in space was circular. nature is perfect; the circle is perfect; hence, if the sun revolves, it revolves in circles. so strongly were men imbued with this "perfection," that copernicus himself fell victim. the sun no longer moved, but the earth and the planets did, and they moved in a circle. radical as copernicus was, a few atoms of conservatism remained with him still. not so kepler. tycho had taught him the importance of careful observation,--to such good effect, that kepler came to the conclusion that the revolution of the earth around the sun takes the form of an ellipse rather than a circle, the sun being stationed at one of the foci of the ellipse. to picture this ellipse, we shall ask the reader to stick two pins a short distance apart into a piece of cardboard, and to place over the pins a loop of string. with the point of a pencil draw the loop taut. as the pencil moves around the two pins the curve so produced will be an ellipse. the positions of the two pins represent the two foci. kepler's observation of the elliptical rotation of the planets was the first of three laws, quantitatively expressed, which paved the way for newton's law. why did the planets move in just this way? kepler tried to answer this also, but failed. it remained for newton to supply the answer to this question. newton's law of gravitation. the great plague of drove newton from cambridge to his home in lincolnshire. there, according to the celebrated legend, the philosopher sitting in his little garden one fine afternoon, fell into a deep reverie. this was interrupted by the fall of an apple, and the thinker turned his attention to the apple and its fall. it must not be supposed that newton "discovered" gravity. apples had been seen to fall before newton's time, and the reason for their return to earth was correctly attributed to this mysterious force of attraction possessed by the earth, to which the name "gravity" had been given. newton's great triumph consisted in showing that this "gravity," which was supposed to be a peculiar property residing in the earth, was a universal property of matter; that it applied to the moon and the sun as well as to the earth; that, in fact, the motions of the moon and the planets could be explained on the basis of gravitation. but his supreme triumph was to give, in one sublime generalization, quantitative expression to the motion regulating heavenly bodies. let us follow newton in his train of thought. an apple falls from a tree yards high. it would fall from a tree yards high. it would fall from the highest mountain top several miles above sea level. it would probably fall from a height much above the mountain top. why not? probably the further up you go the less does the earth attract the apple, but at what distance does this attraction stop entirely? the nearest body in space to the earth is the moon, some , miles away. would an apple reach the earth if thrown from the moon? but perhaps the moon itself has attractive power? if so, since the apple would be much nearer the moon than the earth, the probabilities are that the apple would never reach the earth. but hold! the apple is not the only object that falls to the ground. what is true of the apple is true of all other bodies--of all matter, large and small. now there is the moon itself, a very large body. does the earth exert any gravitational pull on the moon? to be sure, the moon is many thousands of miles away, but the moon is a very large body, and perhaps this size is in some way related to the power of attraction? but then if the earth attracts the moon, why does not the moon fall to the earth? a glance at the accompanying figure will help to answer this question. we must remember that the moon is not stationary, but travelling at tremendous speed--so much so, that it circles the entire earth every month. now if the earth were absent the path of the moon would be a straight line, say mb. if, however, the earth exerts attraction, the moon would be pulled inward. instead of following the line mb it would follow the curved path mb'. and again, the moon having arrived at b', is prevented from following the line b'c, but rather b'c'. so that the path instead of being a straight line tends to become curved. from kepler's researches the probabilities were that this curve would assume the shape of an ellipse rather than a circle. the only reason, then, why the moon does not fall to the earth is on account of its motion. were it to stop moving even for the fraction of a second it would come straight down to us, and probably few would live to tell the tale. newton reasoned that what keeps the moon revolving around the earth is the gravitational pull of the latter. the next important step was to discover the law regulating this motion. here kepler's observations of the movements of the planets around the sun was of inestimable value; for from these newton deduced the hypothesis that attraction varies inversely as the square of the distance. making use of this hypothesis, newton calculated what the attractive power possessed by the earth must be in order that the moon may continue in its path. he next compared this force with the force exerted by the earth in pulling the apple to the ground, and found the forces to be identical! "i compared," he writes, "the force necessary to keep the moon in her orb with the force of gravity at the surface of the earth, and found them answer pretty nearly." one and the same force pulls the moon and pulls the apple--the force of gravity. further, the hypothesis that the force of gravity varies inversely as the square of the distance had now received experimental confirmation. the next step was perfectly clear. if the moon's motion is controlled by the earth's gravitational pull, why is it not possible that the earth's motion, in turn, is controlled by the sun's gravitational pull? that, in fact, not only the earth's motion, but the motion of all the planets is regulated by the same means? here again kepler's pioneer work was a foundation comparable to reinforced concrete. kepler, as we have seen, had shown that the earth revolves around the sun in the form of an ellipse, one of the foci of this ellipse being occupied by the sun. newton now proved that such an elliptic path was possible only if the intensity of the attractive force between sun and planet varied inversely as the square of the distance--the very same relationship that had been applied with such success in explaining the motion of the moon around the earth! newton showed that the moon, the sun, the planets--every body in space conformed to this law. the earth attracts the moon; but so does the moon the earth. if the moon revolves around the earth rather than the earth around the moon, it is because the earth is a much larger body, and hence its gravitational pull is stronger. the same is true of the relationship existing between the earth and the sun. further developments of newton's law of gravitation. when we speak of the earth attracting the moon, and the moon the earth, what we really mean is that every one of the myriad particles composing the earth attracts every one of the myriad particles composing the moon, and vice versa. if in dealing with the attractive forces existing between a planet and its satellite, or a planet and the sun, the power exerted by every one of these myriad particles would have to be considered separately, then the mathematical task of computing such forces might well appear hopeless. newton was able to present the problem in a very simple form by pointing out that in a sphere such as the earth or the moon, the entire mass might be considered as residing in the center of the sphere. for purposes of computation, the earth can be considered a particle, with its entire mass concentrated at the center of the particle. this viewpoint enabled newton to extend his law of inverse squares to the remotest bodies in the universe. if this great law of newton's found such general application beyond our planet, it served an equally useful purpose in explaining a number of puzzling features on this planet. the ebb and flow of the tides was one of these puzzles. even in ancient times it had been noticed that a full moon and a high tide went hand in hand, and various mysterious powers, were attributed to the satellite and the ocean. newton pointed out that the height of the water was a direct consequence of the attractive power of the moon, and, to a lesser extent, because further away, of the sun. one of his first explanations, however, dealt with certain irregularities in the moon's motion around the earth. if the solar system would consist of the earth and moon alone, then the path of the moon would be that of an ellipse, with one of the foci of this ellipse occupied by the earth. unfortunately for the simplicity of the problem, there are other bodies relatively near in space, particularly that huge body, the sun. the sun not only exerts its pull on the earth but also on the moon. however, as the sun is much further away from the moon than is the earth, the earth's attraction for its satellite is much greater, despite the fact that the sun is much huger and weighs far more than the earth. the greater pull of the earth in one direction, and a lesser pull of the sun in another, places the poor moon between the devil and the deep sea. the situation gives rise to a complexity of forces, the net result of which is that the moon's orbit is not quite that of an ellipse. newton was able to account for all the forces that come into play, and he proved that the actual path of the moon was a direct consequence of the law of inverse squares in actual operation. the "principia." the law of gravitation, embodying also laws of motion, which we shall discuss presently, was first published in newton's immortal "principia" ( ). a selection from the preface will disclose the contents of the book, and, incidentally, the style of the author: "... we offer this work as mathematical principles of philosophy; for all the difficulty in philosophy seems to consist in this--from the phenomena of motions to investigate the forces of nature, and then from these forces to demonstrate the other phenomena; and to this end the general propositions in the first and second book are directed. in the third book we give an example of this in the explication of the system of the world; for by the propositions mathematically demonstrated in the first book, we there derive from the celestial phenomena the forces of gravity with which bodies tend to the sun and the several planets. then, from these forces, by other propositions which are also mathematical, we deduce the motions of the planets, the comets, the moon and the sea. i wish we could derive the rest of the phenomena of nature by the same kind of reasoning from mechanical principles; for i am induced by many reasons to suspect that they may all depend upon certain forces by which the particles of bodies, by some causes hitherto unknown, are either mutually impelled towards each other, and cohere in regular figures, or are repelled and recede from each other...." at this point we may state that neither newton, nor any of newton's successors including einstein, have been able to advance even a plausible theory as to the nature of this gravitational force. we know that this force pulls a stone to the ground; we know, thanks to newton, the laws regulating the motions due to gravity; but what this force we call gravity really is we do not know. the mystery is as deep as the mystery of the origin of life. "prof. einstein," writes prof. eddington, "has sought, and has not reached, any ultimate explanation of its [that is, gravitation] cause. a certain connection between the gravitational field and the measurement of space has been postulated, but this throws light rather on the nature of our measurements than on gravitation itself. the relativity theory is indifferent to hypotheses as to the nature of gravitation, just as it is indifferent to hypotheses as to the nature of light." newton's laws of motion. in his principia newton begins with a series of simple definitions dealing with matter and force, and these are followed by his three famous laws of motion. the nature and amount of the effort required to start a body moving, and the conditions required to keep a body in motion, are included in these laws. the fundamentals, mass, time and space, are exhibited in their various relationships. of importance to us particularly is that in these laws, time and space are considered as definite entities, and as two distinct and widely separated manifestations. we shall see that in einstein's hands a very close relationship between these two is brought about. both newton and einstein were led to their theory of gravitation by profound studies of the mathematics of motion, but as newton's conception of motion differed from einstein's, and as, moreover, important discoveries into the nature of matter and the relationship of motion to matter were made subsequent to newton's time, we need not wonder that the two theories show divergence; that, as we shall see, newton's is probably but an approximation of the truth. if we confine our attention to our own solar system, the deviation from newton's law is, as a rule, so small as to be negligible. newton's laws of motion are really axioms, like the axioms of euclid: they do not admit of direct proof; but there is this difference, that the axioms of euclid seem more obviously true. for example, when euclid informs us that "things which are equal to the same thing are equal to one another," we have no hesitation in accepting this statement, for it seems so self-evident. when, however, we are told by newton that "the alteration of motion is ever proportional to the motive force impressed," we are at first somewhat bewildered with the phraseology, and then, even when that has been mastered, the readiness with which we respond will probably depend upon the amount of scientific training we have received. "every body continues in its state of rest or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed thereon." so runs newton's first law of motion. a body does not move unless something causes it to move; to make the body move you must overcome the inertia of the body. on the other hand, if a body is moving, it tends to continue moving, as witness our forward movement when the train is brought to a standstill. it may be asked, why does not a bullet continue moving indefinitely once it has left the barrel of the gun? because of the resistance of the air which it has to overcome; and the path of the bullet is not straight because gravity acts on it and tends to pull it downwards. we have no definite means of proving that a body once set in motion would continue moving, for an indefinite time, and along a straight line. what newton meant was that a body would continue moving provided no external force acted on it; but in actual practise such a condition is unknown. newton's first law defines force as that action necessary to change a state of rest or of uniform motion, and tells us that force alone changes the motion of a body. his second law deals with the relation of the force applied and the resulting change of motion of the body; that is, it shows us how force may be measured. "the alteration of motion is ever proportional to the motive force impressed, and is made in the direction of the right line in which that force is impressed." newton's third law runs--"to every action there is always opposed an equal reaction." the very fact that you have to use force means that you have to overcome something of an opposite nature. the forward pull of a horse towing a boat equals the backward pull of the tow-rope connecting boat and horse. "many people," says prof. watson, "find a difficulty in accepting this statement ... since they think that if the force exerted by the horse on the rope were not a little greater than the backward force exerted by the rope on the horse, the boat would not progress. in this case we must, however, remember that, as far as their relative positions are concerned, the horse and the boat are at rest, and form a single body, and the action and reaction between them, due to the tension on the rope, must be equal and opposite, for otherwise there would be relative motion, one with respect to the other." it may well be asked, what bearing have these laws of newton on the question of time and space? simply this, that to measure force the factors necessary are the masses of the bodies concerned, the time involved and the space covered; and newton's equations for measuring forces assume time and space to be quite independent of one another. as we shall see, this is in striking contrast to einstein's view. newton's researches on light. in , when but years old, newton invented the binomial theorem and the infinitesimal calculus, two phases of pure mathematics which have been the cause of many a sleepless night to college freshmen. had newton done nothing else his fame would have been secure. but we have already glanced at his law of inverse squares and the law of gravitation. we now have to turn to some of newton's contributions to optics, because here more than elsewhere we shall find the starting point to a series of researches which have culminated so brilliantly in the work of einstein. newton turned his attention to optics in when he proved that the light from the sun, which appears white to us, is in reality a mixture of all the colors of the rainbow. this he showed by placing a prism between the ray of light and a screen. a spectrum showing all the colors from red to violet appeared on the screen. another notable achievement of his was the design of a telescope which brought objects to a sharp focus and prevented the blurring effects which had occasioned so much annoyance to newton and his predecessors in all their astronomical observations. these and other discoveries of very great interest were brought together in a volume on optics which newton published in . our particular concern here is to examine the views advanced by him as to the nature of light. that the nature of light should have been a subject for speculation even to the ancients need not surprise us. if other senses, as touch, for example, convey impressions of objects, it is true to say that the sense of sight conveys the most complete impression. our conception of the external world is largely based upon the sense of sight; particularly so when we deal with objects beyond our reach. in astronomy, therefore, a study of the properties of light is indispensable. [ ] but what is this light? we open our eyes and we see; we close our eyes and we fail to see. at night in a dark room we may have our eyes open and yet we do not see; light, then, must be absent. evidently, light does not wholly depend upon whether our eyes are open or closed. this much is certain: the eye functions and something else functions. what is this "something else"? strangely enough, plato and aristotle regarded light as a property of the eye and the eye alone. out of the eye tentacles were shot which intercepted the object and so illuminated it. from what has already been said, such a view seems highly unlikely. far more consistent with their philosophy in other directions would have been the theory that light has its source in the object and not in the eye, and travels from object to eye rather than the reverse. how little substance the aristotelian contribution possesses is immediately seen when we refer to the art of photography. here light rays produce effects which are independent of any property of the eye. the blind man may click the camera and produce the impression on the plate. newton, of course, could have fallen into no such error as did plato and aristotle. the source of light to him was the luminous body. such a body had the power of emitting minute particles at great speed, and these when coming in contact with the retina produce the sensation of sight. this emission or corpuscular theory of newton's was combated very strongly by his illustrious dutch contemporary, huyghens, who maintained that light was a wave phenomenon, the disturbance starting at the luminous body and spreading out in all directions. the wave motions of the sea offer a certain analogy. newton's strongest objection to huyghens' wave theory was that it seemed to offer no satisfactory explanation as to why light travelled in straight lines. he says: "to me the fundamental supposition itself seems impossible, namely that the waves or vibrations of any fluid can, like the rays of light, be propagated in straight lines, without a continual and very extravagant bending and spreading every way into the quiescent medium, where they are terminated by it. i mistake if there be not both experiment and demonstration to the contrary." in the corpuscular theory the particles emitted by the luminous body were supposed to travel in straight lines. in empty space the particles travelled in straight lines and spread in all directions. to explain how light could traverse some types of matter--liquids, for example--newton supposed that these light particles travelled in the spaces between the molecules of the liquid. newton's objection to the wave theory was not answered very convincingly by huyghens. today we know that light waves of high frequency tend to travel in straight lines, but may be prevented from doing so by gravitational forces of bodies near its path. but this is einstein's discovery. a very famous experiment by foucault in proved beyond the shadow of a doubt that newton's corpuscular theory was untenable. according to newton's theory, the velocity of light must be greater in a denser medium (such as water) than in a lighter one (such as air). according to the wave theory the reverse is true. foucault showed that light does travel more slowly in water than in air. the facts were against newton and in favor of huyghens; and where facts and theory clash there is but one thing to do: discard the theory. some facts about newton. newton was a cambridge man, and newton made cambridge famous as a mathematical center. since newton's day cambridge has boasted of a clerk maxwell and a rayleigh, and her larmor, her j. j. thomson and her rutherford are still with us. newton entered trinity college when he was and soon threw himself into higher mathematics. in , when but years old, he became professor of mathematics at cambridge, and later represented that seat of learning in parliament. when his friend montague became chancellor of the exchequer, newton was offered, and accepted, the lucrative position of master of the mint. as president of the royal society newton was occasionally brought in contact with queen anne. she held newton in high esteem, and in she conferred the honor of knighthood on him. he died in . "i do not know," wrote newton, "what i may appear to the world, but, to myself, i seem to have been only like a boy playing on the seashore, and directing myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me." such was the modesty of one whom many regard as the greatest intellect of all ages. references an excellent account of newton may be found in sir r. s. ball's great astronomers (sir isaac pitman and sons, ltd., london). sedgewick and tyler's a short history of science (macmillan, ) and cajori's a history of mathematics (macmillan, ) may also be consulted to advantage. the standard biography is that by brewster. ii the ether and its consequences huyghens' wave theory of light, now so generally accepted, loses its entire significance if a medium for the propagation of these waves is left out of consideration. this medium we call the ether. [ ] huyghens' reasoning may be illustrated in some such way as this: if a body moves a force pushes or pulls it. that force itself is exemplified in some kind of matter--say a horse. the horse in pulling a cart is attached to the cart. the horse in pulling a boat may not be attached to the boat directly but to a rope, which in turn is attached to the boat. in common cases where one piece of matter affects another, there is some direct contact, some go-between. but cases are known where matter affects matter without affording us any evidence of contact. take the case of a magnet's attraction for a piece of iron. where is the rope that pulls the iron towards the magnet? perhaps you think the attraction due to the air in between the magnet and iron? but removing the air does not stop the attraction. yet how can we conceive of the iron being drawn to the magnet unless there is some go-between? some medium not readily perceptible to the senses perhaps, and therefore not strictly a form of matter? if we can but picture some such medium we can imagine our magnet giving rise to vibrations in this medium which are carried to the iron. the magnet may give rise to a disturbance in that portion of the medium nearest to it; then this portion hands over the disturbance to its neighbor, the next portion of the medium; and so on, until the disturbance reaches the iron. you see, we are satisfying our sense-perception by arguing in favor of action by actual contact rather than some vague action at a distance; the go-between instead of being a rope is the medium called the ether. foucault's experiment completely shattered the corpuscular theory of light, and for want of any other more plausible alternative, we are thrown back on huyghens' wave theory. it will presently appear that this wave theory has elements in it which make it an excellent alternative. in the meantime, if light is to be considered as a wave motion, then the query immediately arises, what is the medium through which these waves are propagated? if water is the medium for the waves of the sea, what is the medium for the waves of light? again we answer, the medium is the ether. what is this "ether"? balloonists find conditions more and more uncomfortable the higher they ascend, for the density of the air (and therefore the amount of oxygen in a given volume of air) becomes less and less. meteorologists have calculated that traces of the air we breathe may reach a height of some miles. but what is beyond? nothing but the ether, it is claimed. light from the sun and stars reaches us via the ether. but what is this ether? we cannot handle it. we cannot see it. it fails to fall within the scope of any of our senses, for every attempt to show its presence has failed. it is spirit-like in the popular sense. it is lodge's medium for the souls of the departed. helmholtz and kelvin tried to arrive at some properties of this hypothetical substance from a careful study of the manner in which waves were propagated through this ether. if, as the wave theory teaches us, the ether can be set in motion, then according to laws of mechanics, the ether has mass. if so it is smaller in amount than anything which can be detected with our most accurate balance. further--and this is a difficulty not easily explained--if this ether has any mass, why does it offer no detectable resistance to the velocity of the planets in it? why is not the velocity of the planets reduced in time, just as the velocity of a rifle bullet decreases owing to the resistance of the air? lodge, in arguing in favor of an ether, holds that its presence cannot be detected because it pervades all space and all matter. his favorite analogy is to point out the extreme unlikelihood of a deep-sea fish discovering the presence of the water with which it is surrounded on all sides;--all of which tells us nothing about the ether, but does try to tell us why we cannot detect it. [ ] in short, answering the query at the head of this paragraph, we may say that we do not know. waves set up in this ether. the waves are not all of the same length. those that produce the sensation of sight are not the smallest waves known, yet their length is so small that it would take anywhere from one to two million of them to cover a yard. curiously enough, our eye is not sensitive to wave lengths beyond either side of these limits; yet much smaller, and much larger waves are known. the smallest are the famous x-rays, which are scarcely one ten-thousandth the size of light waves. waves which have a powerful chemical action--those which act on a photographic plate, for example--are longer than x-rays, yet smaller than light waves. waves larger than light waves are those which produce the sensation of heat, and those used in wireless telegraphy. the latter may reach the enormous length of , yards. x-ray, actinic, or chemically active ray, light ray, heat ray, wireless ray--they differ in size, yet they all have this in common: they travel with the same speed ( , miles per second). the electromagnetic theory of light. powerful support to the conception that space is pervaded by ether was given when maxwell discovered light to be an electromagnetic phenomenon. from purely theoretical considerations this gifted english physicist was led to the view that waves could be set up as a result of electrical disturbances. he proved that such waves would travel with the same velocity as light waves. as air is not needed to transmit electrical phenomena--for you can pump all air out of a system and produce a vacuum, and electrical phenomena will continue--maxwell was forced to the conclusion that the waves set up by electrical disturbances and transmitted with the same velocity as light, were enabled to do so with the help of the same medium as light, namely, the ether. it was now but a step for maxwell to formulate the theory that light itself is nothing but an electrical phenomenon, the sensation of light being due to the passage of electric waves through the ether. this theory met with considerable opposition at first. physicists had been brought up in a school which had taught that light and electricity were two entirely unrelated phenomena, and it was difficult for them to loosen the shackles that bound them to the older school. but two startling discoveries helped to fasten attention upon maxwell's theory. one was an experimental confirmation of maxwell's theoretical deduction. hertz, a pupil of helmholtz, showed how the discharge from a leyden jar set up oscillations, which in turn gave rise to waves in the ether, comparable, in so far as velocity is concerned, to light waves, but differing from the latter in wave length, the hertzian waves being much longer. at a later date these waves were further investigated by marconi, with the result that wireless messages soon began to be flashed from one place to another. just as there is a close connection between light and electricity, so there is between light and magnetism. the first to point out such a relationship was the illustrious michael faraday, but we owe to zeeman the most extensive investigations in this field. if we throw some common salt into a flame, and, with the help of a spectroscope, examine the spectrum produced, we are struck by two bright lines which stand out very prominently. these lines, yellow in color, are known as the d-lines and serve to identify even minute traces of sodium. what is true of sodium is true of other elements: they all produce very characteristic spectra. now zeeman found that if the flame is placed between a powerful magnet, and then some common salt thrown into the flame, the two yellow lines give place to ten yellow lines. such is one of the results of the effect of a magnetic field on light. the electron. the "zeeman effect" led to several theories regarding its nature. the most successful of these was one proposed by larmor and more fully treated by lorentz. it has already been pointed out that the only difference between wireless and light waves is that the former are much "longer," and, we may now add, their vibrations are much slower. light and wireless waves bear a relationship to one another comparable to the relationship born by high and low-pitched sounds. to produce wireless waves we allow a charge of electricity to oscillate to and fro. these oscillations, or oscillating charges, are the cause of such waves. what charges give rise to light waves? lorentz, from a study of the zeeman effect, ascribed them to minute particles of matter, smaller than the chemical atom, to which the name "electron" was given. the unit of electricity is the electron. electrons in motion give rise to electricity, and electrons in vibration, to light. the zeeman effect gave lorentz enough data to calculate the mass of such electrons. he then showed that these electrons in a magnetic field would be disturbed by precisely the amount to which zeeman's observations pointed. in other words, the assumption of the electron fitted in most admirably with zeeman's experiments on magnetism and light. in the meantime, a study of the discharge of electricity through gases, and, later, the discovery of radium, led, among other things, to a study of beta or cathode rays--negatively charged particles of electricity. through a series of strikingly original experiments j. j. thomson ascertained the mass of such particles or corpuscles, and then the very striking fact was brought out that thomson's corpuscle weighed the same as lorentz's electron. the electron was not merely the unit of electricity but the smallest particle of matter. the nature of matter. all matter is made up of some eighty-odd elements. oxygen, copper, lead are examples of such elements. each element in turn consists of an innumerable number of atoms, of a size so small, that million of them could be placed alongside of one another without their total length exceeding one inch. john dalton more than a hundred years ago postulated a theory, now known as the atomic theory, to explain one of the fundamental laws in chemistry. this theory started out with an old greek assumption that matter cannot be divided indefinitely, but that, by continued subdivision, a point would be reached beyond which no further breaking up would be possible. the particles at this stage dalton called atoms. dalton's atomic hypothesis became one of the pillars upon which the whole superstructure of chemistry rested, and this because it explained a number of perplexing difficulties so much more satisfactorily than any other hypothesis. for nearly a century dalton stood as firm as a rock. but early in the nineties some epoch-making experiments on the discharge of electricity through gases were begun by a group of physicists, particularly crookes, rutherford, lenard, roentgen, becquerel, and, above all, j. j. thomson, which pointed very clearly to the fact that the atoms are not the smallest particles of matter at all; that, in fact, they could be broken up into electrons, of a diameter one one-hundred-thousandth that of an atom. it remained for the illustrious madame curie to confirm this beyond all doubt by her isolation of radium. here, as madame curie showed, was an element whose atoms were actually breaking up under one's very eyes, so to speak. so far have we advanced since dalton's day, that dalton's unit, the atom, is now pictured as a complex particle patterned after our solar system, with a nucleus of positive electricity in the center, and particles of negative electricity, or electrons, surrounding the nucleus. all this leads to one inevitable conclusion: matter is electrical in nature. but now if matter and light have the same origin, and matter is subject to gravitation, why not light also? so reasoned einstein. summary. newton's studies of matter in motion led to his theory of gravitation, and, incidentally, to his conception of time and space as definite entities. as we shall see, einstein in his theory of gravitation based it upon discoveries belonging to the post-newtonian period. one of these is minkowski's theory of time and space as one and inseparable. this theory we shall discuss at some length in the next chapter. other important discoveries which led up to einstein's work are the researches which culminated in the electron theory of matter. the origin of this theory may be traced to studies dealing with the nature of light. here again newton appears as a pioneer. newton's corpuscular theory, however, proved wholly untenable when foucault showed that the velocity of light in water is less than in air, which is the very reverse of what the corpuscular theory demands, but which does agree with huyghens' wave theory. but huyghens' wave theory postulated some medium in which the waves can act. to this medium the name "ether" was given. however, all attempts to show the presence of such an ether failed. naturally enough, some began to doubt the existence of an ether altogether. huyghens' wave theory received a new lease of life with maxwell's discovery that light is an electromagnetic phenomenon; that the waves set up by a source of light were comparable to waves set up by an electrical disturbance. zeeman next showed that magnetism was also, closely related to light. a study of zeeman's experiments led lorentz to the conclusion that electrical phenomena are due to the motion of charged particles called "electrons," and that the vibrations of these electrons give rise to light. the conception of the electron as the very fundamental of matter was arrived at in an entirely different way: from studies dealing with the discharge of electricity through gases and the breaking up of the atoms of radium. if matter and light have the same origin, and if matter is subject to gravitation, why not light also? references for the general subject of light the reader must be referred to a rather technical work, but one of the best in the english language: edwin edser, light for students (macmillan, ). the nature of matter and electricity is excellently discussed in several books of a popular variety. the very best and most complete of its kind that has come to the author's attention is comstock and troland's the nature of matter and electricity (d. van nostrand co., ). two other very readable books are soddy's matter and energy (henry holt and co.) and crehore's the mystery of matter and energy (d. van nostrand co., ). iii einstein "this is the most important result obtained in connection with the theory of gravitation since newton's day. einstein's reasoning is the result of one of the highest achievements of human thought." these words were uttered by sir j. j. thomson, the president of the royal society, at a meeting of that body held on november , , to discuss the results of the eclipse expedition. einstein another newton--and this from the lips of j. j. thomson, england's most illustrious physicist! if ever man weighed words carefully it is this cambridge professor, whose own researches have assured him immortality for all time. what has this albert einstein done to merit such extraordinary praise? with the world in turmoil, with classes and races in a death struggle, with millions suffering and starving, why do we find time to turn our attention to this jew? his ideas have no bearing on europe's calamity. they will not add one bushel of wheat to starving populations. the answer is not hard to find. men come and men go, but the mystery of the universe remains. it is einstein's glory to have given us a deeper insight into the universe. our scientists are huxley's agnostics: they do not deny activities beyond our planet; they merely center their attention on the knowable on this earth. our philosophers, on the other hand, go far afield. some of them soar so high that, like one poet's opinion of shelley, the bubble bursts. einstein, using the tools of the scientist--the experimentalist--builded a skyscraper which ultimately reached the philosophical school. his rôle is the rôle of alcohol in causing water and ether (the anæsthetic) to mix. ether and water will mix no better than oil and water, without the presence of alcohol; in its presence a uniform mixture is obtained. the object of the eclipse expedition. einstein prophesied that a ray of light passing near the sun would be pulled out of its course, due to the action of gravity. he went even further. he predicted how much out of its course the ray would be deflected. this prediction was based on a theory of gravitation which einstein had developed in great mathematical detail. the object of the british eclipse expedition was either to prove or disprove einstein's assumption. the result of the expedition. einstein's prophecy was fulfilled almost to the letter. the significance of the result. since einstein's theory of gravitation is intimately associated with certain revolutionary ideas concerning time and space, and, therefore, with fundamentals of the universe, the net result of the expedition was to strengthen our belief in the validity of his new view of the universe. it is our intention in the following pages to discuss the expedition and the larger aspects of einstein's theory that follow from it. but before we do so we must have a clear idea of our solar system. our solar system. in the center of our system is the sun, a flaming mass of fire, much bigger than our own earth, and very, very far away. the sun has its family of eight planets--of which the earth is one--which travel around the sun; and around some of the planets there travel satellites, or moons. the earth has such a satellite, the moon. now we have good reasons for believing that every star which twinkles in the sky is a sun comparable to our own, having also its own planets and its own moons. these stars, or suns, are so much further away from us than our own sun, that but a speck of their light reaches us, and then only at night, when, as the poets would say, our sun has gone to its resting place. the distances between bodies in the solar system is so immense that, like the number of dollars spent in the great war, the number of miles conveys little, or no impression. but picture yourself in an express train going at the average rate of miles an hour. if you start from new york and travel continuously you would reach san francisco in days. if you could continue your journey around the earth at the same rate you would complete it in days. if now you could travel into space and to the moon, still with the same velocity, you would reach it in days. having reached the moon, you could circumscribe it with the same express train in days, as compared to the days it would take you to circumscribe the earth. if instead of travelling to the moon you would book your passage for the sun you, or rather your descendants, would get there in years, and it would then take them additional years to travel around the sun. immense as these distances are, they are small as compared to the distances that separate us from the stars. it takes light which, instead of travelling miles an hour, travels , miles a second, about minutes to get to us from the sun, and a little over years to reach us from the nearest star. the light from some of the other stars do not reach us for several hundred years. the eclipse of the sun. now to return to an infinitesimal part of the universe--our solar system. we have seen that the earth travels around the sun, and the moon around the earth. at some time in the course of these revolutions the moon must come directly between the earth and the sun. then we get the eclipse of the sun. as the moon is smaller than the earth, only a portion of the earth's surface will be cut off from the sun's rays. that portion which is so cut off suffers a total eclipse. this explains why the eclipse of may, , which was a total one for brazil, was but a partial one for us. einstein's assertion re-stated. einstein claimed that a ray of light from one of the stars, if passing near enough to the surface of the sun, would be appreciably deflected from its course; and he calculated the exact amount of this deflection. to begin with, why should einstein suppose that the path of a ray of light would be affected by the son? newton's law of gravitation made it clear that bodies which have mass attract one another. if light has mass--and very recent work tends to show that it has--there is no reason why light should not be attracted by the sun, or any other planetary body. the question that agitated scientists was not so much whether a ray of light would be deviated from its path, but to what extent this deviation would take place. would einstein's figures be confirmed? of the bodies within our solar system the sun is by far the largest, and therefore it would exert a far greater pull than any of the planets on light rays coming from the stars. under ordinary conditions, however, the sun itself shines with such brilliancy, that objects around it, including rays of light passing near its surface, are wholly dimmed. hence the necessity of putting our theory to the test only when the moon covers up the sun--when there is a total eclipse of the sun. a graphical representation. imagine a star a, so selected that as its light comes to us the ray just grazes the sun. if the path of the ray is straight--if the sun has no influence on it--then the path can be represented by the line ab. if, however, the sun does exert a gravitational pull, then its real path will be ab', and to an observer on the earth the star will have appeared to shift from a to a'. what the eclipse expedition set out to do. photographs of stars around the sun were to be taken during the eclipse, and these photographs compared with others of the same region taken at night, with the sun absent. any apparent shifting of the stars could be determined by measuring the distances between the stars as shown on the photographic plates. three possibilities anticipated. according to newton's assumption, light consists of corpuscles, or minute particles, emitted from the source of light. if this be true these particles, having mass, should be affected by the gravitational pull of the sun. if we apply newton's theory of gravitation and make use of his formula, it can be shown that such a gravitational pull would displace the ray of light by an average amount equal to . (seconds of angular distance.) [ ] on the other hand, where light is regarded as waves set in motion in the "ether" of space (the wave theory of light), and where weight is denied light altogether, no deviation need be expected. finally there is a third alternative: einstein's. light, says einstein, has mass, and therefore probably weight. mass is the matter light contains; weight represents pull by gravity. light rays will be attracted by the sun, but according to einstein's theory of gravitation the sun's gravitational pull will displace the rays by an average amount equal to . (seconds of angular distance). the expeditions. that science is highly international, despite many recent examples to the contrary, is evidenced by this british eclipse expedition. here was a theory propounded by one who had accepted a chair of physics in the university of berlin, and across the english channel were germany's mortal enemies making elaborate preparations to test the validity of the berlin professor's theory. the british astronomical society began to plan the eclipse expedition even before the outbreak of the great war. during the years that followed, despite the destinies of nations which hung on threads from day to day, despite the darkest hours in the history of the british people, our english astronomers continued to give attention to the details of the proposed expedition. when the day of the eclipse came all was in readiness. one expedition under dr. crommelin was sent to sobral, brazil; another, under prof. eddington, to principe, an island off the west coast of africa. in both these places a total eclipse was anticipated. the eclipse occurred on may , . it lasted for six to eight minutes. some photographs, with an average exposure of five to six seconds, were taken. two months later another series of photographs of the same region were taken, but this time the sun was no longer in the midst of these stars. the photographs were brought to the famous greenwich observatory, near london, and the astronomers and mathematicians began their laborious measurements and calculations. on november , at the meeting of the royal society, the result was announced. the sobral expedition reported . ; the principe expedition . . the average was . . einstein had predicted . , newton might have predicted . , and the orthodox scientists would have predicted . there could now no longer be any question as to which of the three theories rested on a sure foundation. to quote sir frank dyson, the astronomer royal: "after a careful study of the plates i am prepared to say that there can be no doubt that they confirm einstein's prediction. a very definite result has been obtained that light is deflected in accordance with einstein's law of gravitation." [ ] where did einstein get his idea of gravitation? in einstein published the first of a series of papers supporting and extending a theory of time and space to which the name "the theory of relativity" had been given. these views as expounded by einstein came into direct conflict with newton's ideas of time and space, and also with newton's law of gravitation. since einstein had more faith in his theory of relativity than in newton's theory of gravitation, einstein so changed the latter as to make it harmonize with the former. more will be said on this subject. let not the reader misunderstand. newton was not wholly in the wrong; he was only approximately right. with the knowledge existing in newton's day newton could have done no more than he did; no mortal could have done more. but since newton's day physics--and science in general--has advanced in great strides, and einstein can interpret present-day knowledge in the same masterful fashion that newton could in his day. with more facts to build upon, einstein's law of gravitation is more universal than newton's; it really includes newton's. but now we must turn our attention very briefly to the theory of relativity--the theory that led up to einstein's law of gravitation. the theory of relativity. the story goes that einstein was led to his ideas by watching a man fall from a roof. this story bears a striking similarity to newton and the apple. perhaps one is as true as the other. [ ] however that may be, the principle of relativity is as old as philosophical thought, for it denies the possibility of measuring absolute time, or absolute space. all things are relative. we say that it takes a "long time" to get from new york to albany; long as compared to what? long, perhaps, as compared to the time it takes to go from new york city to brooklyn. we say the white house is "large"; large when compared to a room in an apartment. but we can just as well reverse our ideas of time and distance. the time it takes to go from new york to albany is "short" when compared to the time it takes to go from new york to san francisco. the size of the white house is "small" when compared to the size of the city of washington. let us take another illustration. every time the earth turns on its axis we mark down as a day. with this as a basis, we say that it takes a little over days for the earth to complete its revolution around the sun, and our days we call a year. but now consider some of our other planets. with our time as a basis, it takes jupiter or saturn hours to turn on its axis, as compared to the hours it takes the earth to turn. saturn's day is less than one-half our day, and our day is more than twice saturn's--that is, according to the calculations of the inhabitants of the earth. mercury completes her circuit around the sun in days; neptune, in years. mercury's year is but one-fourth ours, neptune's, times ours. and observers at mercury and neptune would regard us from their standard of time, which differs from our standard. you may say, why not take our standard of time as the standard, and measure everything by it? but why should you? such a selection would be quite arbitrary. it would not be based on anything absolute, but would merely depend on our velocity around the sun. these ideas are old enough in metaphysics. einstein's improvement of them consists not merely in speculating about them, but in giving them mathematical form. the origin of the theory of relativity. a train moves with reference to the earth. the earth moves with reference to the sun. we say the sun is stationary and the earth moves around the sun. but how do we know that the sun itself does not move with reference to some other body? how do we know that our planetary system, and the stars, and the cosmos as a whole is not in motion? there is no way of answering such a question unless we could get a point of reference which is fixed--fixed absolutely in space. we have already alluded to our view of the nature of light, known as the wave theory of light. this theory postulates the existence of an all-pervading "ether" in space. light sets up wave disturbances in this ether, and is thus propagated. if the ocean were the ether, the waves of the ocean would compare with the waves set up by the ether. but what is this ether? it cannot be seen. it defies weight. it permeates all space. it permeates all matter. so say the exponents of this ether. to the layman this sounds very much like another name for the deity. to sir oliver lodge it represents the spirits of the departed. to us, of importance is the conception that this ether is absolutely stationary. such a conception is logical if the various developments in optics and electricity are considered. but if absolutely stationary, then the ether is the long-sought-for point of reference, the guide to determine the motion of all bodies in the universe. the famous experiment performed by prof. michelson. if there is an ether, and a stationary ether, and if the earth moves with reference to this ether, the earth, in moving, must set up ether "currents"--just as when a train moves it sets up air currents. so reasoned michelson, a young annapolis graduate at the time. and forthwith he devised a crucial experiment the explanation of which we can simplify by the following analogy: which is the quicker, to swim up stream a certain length, say a hundred yards, and back again, or across stream the same length and back again? the swimmer will answer that the up-and-down journey is longer. [ ] our river is the ether. the earth, if moving in this ether, will set up an ether stream, the up stream being parallel to the earth's motion. now suppose we send a beam of light a certain distance up this ether stream and back, and note the time; and then turn the beam of light at right angles and send it an equal distance across the stream and back, and note the time. how will the time taken for light to travel in these two directions compare? reasoning by analogy, the up-and-down stream should take longer. michelson's results did not accord with analogy. no difference in time could be detected between the beam of light travelling up-and-down, and across-and-back. but this was contrary to all reason if the postulate of an ether was sound. must we then revise our ideas of an ether? perhaps after all there is no ether. but if no ether, how are we to explain the propagation of light in space, and various electrical phenomena connected with it, such as the hertzian, or wireless waves? there was another alternative, one suggested by larmor in england and lorentz in holland,--that matter is contracted in the direction of its motion through the ether current. to say that bodies are actually shortened in the direction of their motion--by an amount which increases as the velocity of these bodies approaches that of light--is so revolutionary an idea that larmor and lorentz would hardly have adopted such a viewpoint but for the fact that recent investigations into the nature of matter gave basis for such belief. matter, it has been shown, is electrical in nature. the forces which hold the particles together are electrical. lorentz showed that mathematical formulas for electrical forces could be developed which would inevitably lead to the view that material bodies contract in the direction of their motion. [ ] "but this is ridiculous," you say; "if i am shorter in one direction than in another i would notice it." you would if some things were shortened and others were not. but if all things pointing in a certain direction are shortened to an equal extent, how are you going to notice it? will you apply the yard stick? that has been shortened. will you pass judgment with the help of your eyes? but your retina has also contracted. in brief, if all things contract to the same amount it is as if there were no contraction at all. lorentz's plausible explanation really deepens the mystery. the startling ideas just outlined have opened up several new vistas, but they have left unanswered the two problems we set out to solve: whether there is an ether, and if so, what is the velocity of the earth in reference to this ether? lorentz maintains that there is an ether, but the velocities of bodies relative to it must forever remain a mystery. as you change your position your distances change; you change; everything about you changes accordingly; and all basis for comparison is lost. nature has entered into a conspiracy to keep you ignorant. einstein comes upon the scene. einstein starts with the assumption that there is no possible way of identifying this ether. suppose we ignore the ether altogether, what then? [ ] if we do ignore the ether we no longer have any absolute point of reference; for if the ether is considered stationary the velocity of all bodies within the ether may be referred to it; any point in space may be considered a fixed point. if, however, there is no ether, or if we are to ignore it, how are we to get the velocity of bodies in space? the principle of relativity. if we are to believe in the "causal relationship between only such things as lie within the realm of observation," then observation teaches us that bodies move only relative to one another, and that the idea of absolute motion of a body in space is meaningless. einstein, therefore, postulates that there is no such thing as absolute motion, and that all we can discuss is the relative motion of one body with respect to another. this is just as logical a deduction from michelson's experiment as the attempt to explain michelson's anomalous results in the light of an all-pervading ether. consider for a moment newton's scheme. this great pioneer pictured an absolute standard of position in space relative to which all velocities are measured. velocities were measured by noting the distance covered and dividing the result by the time taken to cover the distance. space was a definite entity; and so was time. "time," said newton, "flows evenly on," independent of aught else. to newton time and space were entirely different, in no way to be confounded. just as newton conceived of absolute space, so he conceived of absolute time. from the latter standard of reference the idea of a "simultaneity of events" at different places arose. but now if there is no standard of reference, if the ether does not exist or does not function, if two points a and b cannot be referred to a third, and fixed point c, how can we talk of "simultaneity of events" at a and b? [ ] in fact, einstein shows that if all you can speak about is relative motion, then one event which takes say one minute on one planet would not take one minute on another. for consider two bodies in space, say the planets venus and the earth, with an observer b on venus and another a on the earth. b notes the time taken for a ray of light to travel from b to the distance m. a on the earth has means of observing the same event. b records one minute. a is puzzled, for his watch records a little more than one minute. what is the explanation? granting that the two clocks register the same time to start with, and assuming further einstein's hypothesis that the velocity of light is independent of its source, the difference in time is due to the fact that the planet venus moves with reference to the observer on the earth; so that a in reality does not measure the path bm and mb, but bm' and m'b', where bb' represents the distance venus itself has moved in the interval. and if you put yourself in b's position on venus the situation is exactly reversed. all of which is simply another way of saying that what is a certain time on one body in space is another time on another body in space. there is nothing definite in time. prof. cohen's illustration. further bewildering possibilities are clearly outlined in this apt illustration: "if when you are going away on a long and continuous journey you write home at regular intervals, you should not be surprised that with the best possible mail service your letters will reach home at progressively longer intervals, since each letter will have a greater distance to travel than its predecessor. if you were armed with instruments to hear the home clock ticking, you would find that as your distance from home keeps on increasing, the intervals between the successive ticks (that is, its seconds) grow longer, so that if you travelled with the velocity of sound the home clock would seem to slow down to a standstill--you would never hear the next tick. "precisely the same is true if you substitute light rays for sound waves. if with the naked eye or with a telescope you watch a clock moving away from you, you will find that its minute hand takes a longer time to cover its five-minute intervals than does the chronometer in your hand, and if the clock travelled with the velocity of light you would forever see the minute hand at precisely the same point. that which is true of the clock is, of course, also true of all time intervals which it measures, so that if you moved away from the earth with the velocity of light everything on it would appear as still as on a painted canvas." your time has apparently come to a standstill in one position and is moving in another! all this seems absurd enough, but it does show that time alone has little meaning. minkowski's conclusion. the relativity theory requires that we thoroughly reorganise our method of measuring time. but this is intimately associated with our method of measuring space, the distance between two points. as we proceed we find that space without time has little meaning, and vice versa. this leads minkowski to the conclusion that "time by itself and space by itself are mere shadows; they are only two aspects of a single and indivisible manner of coordinating the facts of the physical world." einstein incorporated this time-space idea in his theory of relativity. how we measure a point in space. suppose i say to you that the chemical laboratory of columbia university faces broadway; would that locate the laboratory? hardly, for any building along broadway would face broadway. but suppose i add that it is situated at broadway and th street, south-east? there could be little doubt then. but if, further, this laboratory would occupy but part of the building, say the third floor; then the situation would be specified by naming broadway, th street s. e., third floor. if broadway represents length, th street width, and third floor height, we can see what is meant when we say that three dimensions are required to locate a position in space. the fourth dimension. a point on a line may be located by one dimension; a point on a wall requires two dimensions; a point in the room, like the chemical laboratory above ground, needs three. the layman cannot grasp the meaning of a fourth dimension; yet the mathematician does imagine it, and plays with it in mathematical terms. minkowski and einstein picture time as the fourth dimension. to them time occupies no more important position than length, breadth, or thickness, and is as intimately related to these three as the three are to one another. h. g. wells, the novelist, has beautifully caught this spirit when in his novel, "the time machine," he makes his hero travel backwards and forwards along time just as a man might go north or south. when the man with his time machine goes forward he is in the future; when he goes backwards he is in the past. in reality, if we stop to think a minute, there is no valid reason for the non-existence of a fourth dimension. if one, two and three dimensions, why not four--and five and six, for that matter? theoretically at least there is no reason why the limit should be set at three. however, our minds become sluggish when we attempt to picture dimensions beyond three; just as an extraordinary effort on our part is needed to follow einstein when he "juggles" with space and time. our difficulty in imagining four dimensions may be likened to the difficulty two-dimensioned beings would experience in imagining us, beings of the conventional three dimensions. suppose these two-dimensional beings were living on the surface of the earth; what could they see? they could see nothing below and nothing above the surface. they would see shifting surfaces as we walked about, but being sensitive to length and breadth only, and not to height, they could gain no notion whatsoever of what we really look like. it is thus with us when we attempt to picture four-dimensional space. perhaps the analogy of the motion picture may help us somewhat. as everybody knows, these motion pictures consist of a series of photographs which are shown in rapid succession on the screen. each photograph by itself conveys a sensation of space, that is, of three dimensions; but one photograph rapidly following another conveys the sensation of space and time--four dimensions. space and time are interlinked. the time-space idea further developed. we have already alluded to the fact that objects in space moving with different velocities build up different time intervals. thus the velocity of the star arcturus, if compared with reference to the earth, moves at the rate of miles a second. its motion through space is different from ours. objects which, according to lorentz, contract in the direction of their motion to an extent proportional to their velocity, will contract differently on the surface of arcturus than on the earth. our space is not arcturus' space; neither is arcturus' time our time. and what is true of the discrepancies existing between the space and time conceptions of the earth and arcturus is true of any other two bodies in space moving at different velocities. but is there no relationship existing between the space and time of one body in the universe as compared to the space and time of another? can we not find something which holds good for all bodies in the universe? we can. we can express it mathematically. it is the concept of time and space interlinked; of time as the fourth dimension, length, breadth and thickness being the other three; of time as one of four co-ordinates and at right angles to the other three (a situation which requires a terrific stretch of the imagination to visualize). the four dimensions are sufficient to co-ordinate the time-space relationships of all bodies in the cosmos, and hence have a universality which is totally lacking when time and space are used independently of one another. the four components of our time-space are up-and-down, right-and-left, backwards-and-forwards, and sooner-and-later. "strain" and "distortion" in space. the four-dimensional unit has been given the name "world-line," for the "world-line" of any particle in space is in reality a complete history of that particle as it moves about in space. particles, we know, attract one another. if each particle is represented by a world-line these world-lines will be deflected from their course owing to such attraction. imagine a bladder representing the universe, with lines on it representing world-lines. now squeeze the bladder. the world-lines are bent in various directions; they are "distorted." this illustrates the influence of gravity on these world-lines; it is the "strain" brought about due to the force of attraction. the distorted bladder illustrates even more, for it is a true representation of the real world. how einstein's conception of time and space led to a new view of gravitation. in our conventional language we speak of the sun as exerting a "force" on the earth. we have seen, however, that this force brings about a "distortion" or "strain" in world-lines; or, what amounts to the same thing, a "distortion" or "strain" of time and space. the sun's "force," the "force" of any body in space, is the "force" due to gravity; and these "forces" may now be treated in terms of the laws of time and space. "the earth," prof. eddington tells us, "moves in a curved orbit, not because the sun exerts any direct pull, but because the earth is trying to find the shortest way through a space and time which have been tangled up by an influence radiating from the sun." [ ] at this point newton's conceptions fail, for his views and his laws do not include "strains" in space. newton's law of gravitation must be supplanted by one which does include such distortions. it is einstein's great glory to have supplied us with this new law. einstein's law of gravitation. this appears to be the only law which meets all requirements. it includes newton's law, and cannot be distinguished from it if our experiments are confined to the earth and deal with relatively small velocities. but when we betake ourselves to some orbits in space, with a gravitational pull much greater than the earth's, and when we deal with velocities comparable to that of light, the differences become marked. einstein's theory scores its first great victory. in the beginning of this chapter we referred to the elaborate eclipse expedition sent by the british to test the validity of einstein's new theory of gravitation. the british scientists would hardly have expended so much time and energy on this theory of einstein's but for the fact that einstein had already scored one great victory. what was it? imagine but a single planet revolving about the sun. according to newton's law of gravitation, the planet's path would be that of an ellipse--that is, oval--and the planet would travel indefinitely along this path. according to einstein the path would also be elliptical, but before a revolution would be quite completed, the planet would start along a slightly advanced line, forming a new ellipse slightly in advance of the first. the elliptic orbit slowly turns in the direction in which the planet is moving. after many years--centuries--the orbit will be in a different direction. the rapidity of the orbit's change of direction depends on the velocity of the planet. mercury moving at the rate of miles a second is the fastest among the planets. it has the further advantage over venus or the earth in that its orbit, as we have said, is an ellipse, whereas the orbits of venus and the earth are nearly circular; and how are you going to tell in which direction a circle is pointing? observation tells us that the orbit of mercury is advancing at the rate of seconds (of arc) per century. we can calculate how much of this is due to the gravitational influence of other planets. it amounts to seconds per century. what of the remaining seconds? you might be inclined to attribute this shortcoming to experimental error. but when all such possibilities are allowed for our mathematicians assure us that the discrepancy is times greater than any possible experimental error. this discrepancy between theory and observation remained one of the great puzzles in astronomy until einstein cleared up the mystery. according to einstein's theory the mathematics of the situation is simply this: in one revolution of the planet the orbit will advance by a fraction of a revolution equal to three times the square of the ratio of the velocity of the planet to the velocity of light. when we allow mathematicians to work this out we get the figure , which is certainly close enough to to be called identical with it. still another victory? einstein's third prediction--the shifting of spectral lines toward the red end of the spectrum in the case of light coming to us from the stars of appreciable mass--seems to have been confirmed recently (march, ). "the young physicists in bonn," writes prof. einstein to a friend, "have now as good as certainty (so gut wie sicher) proved the red displacement of the spectral lines and have cleared up the grounds of a previous disappointment." summary. velocity, or movement in space, is at the basis of einstein's work, as it was at the basis of newton's. but time and space no longer have the distinct meanings that they had when examined with the help of newton's equations. time and space are not independent but interdependent. they are meaningless when treated as separate entities, giving results which may hold for one body in the universe but do not hold for any other body. to get general laws which are applicable to the cosmos as a whole the fundamentals of mechanics must be united. einstein's great achievement consists in applying this revised conception of space and time to elucidate cosmical problems. "world-lines," representing the progress of particles in space, consisting of space-time combinations (the four dimensions), are "strained" or "distorted" in space due to the attraction that bodies exhibit for one another (the force of gravitation). on the other hand, gravitation itself--more universal than anything else in the universe--may be interpreted in terms of strains on world-lines, or, what amounts to the same thing, strains of space-time combinations. this brings gravitation within the field of einstein's conception of time and space. that einstein's conception of the universe is an improvement upon that of newton's is evidenced by the fact that einstein's law explains all that newton's law does, and also other facts which newton's law is incapable of explaining. among these may be mentioned the distortion of the oval orbits of planets round the sun (confirmed in the case of the planet mercury), and the deviation of light rays in a gravitational field (confirmed by the english solar eclipse expedition). einstein's theories and the inferences to be drawn from them. einstein's theories, supported as they are by very convincing experiments, will probably profoundly influence philosophic and perhaps religious thought, but they can hardly be said to be of immediate consequence to the man in the street. as i have said elsewhere, einstein's theories are not going to add one bushel of wheat to war-torn and devastated europe, but in their conception of a cosmos decidedly at variance with anything yet conceived by any school of philosophy, they will attract the universal attention of thinking men in all countries. the scientist is immediately struck by the way einstein has utilized the various achievements in physics and mathematics to build up a co-ordinated system showing connecting links where heretofore none were perceived. the philosopher is equally fascinated with a theory, which, in detail extremely complex, shows a singular beauty of unity in design when viewed as a whole. the revolutionary ideas propounded regarding time and space, the brilliant way in which the most universal property of matter, gravitation, is for the first time linked up with other properties of matter, and, above all, the experimental confirmation of several of his more startling predictions--always the finest test of scientific merit--stamps einstein as one of those super-men who from time to time are sent to us to give us a peep into the beyond. some facts about einstein himself. albert einstein was born in germany some years ago. at first he was engaged at the patent bureau in berne, and later became professor at the zürich polytechnic. after a short stay at prague university he accepted one of those tempting "akademiker" professorships at the university of berlin--professorships which insure a comfortable income to the recipient of one of them, little university work beyond, perhaps, one lecture a week, and splendid facilities for research. a similar inducement enticed the chemical philosopher, van 't hoff, to leave his amsterdam, and the swedes came perilously near losing their most illustrious scientist, arrhenius. einstein published his first paper on relativity in , when not more than years old. of this paper planck, the nobel laureate in physics this year, has offered this opinion: "it surpasses in boldness everything previously suggested in speculative natural philosophy and even in the philosophical theories of knowledge. the revolution introduced into the physical conceptions of the world is only to be compared in extent and depth with that brought about by the introduction of the copernican system of the universe." einstein published a full exposition of the relativity theory in . during the momentous years of - , einstein quietly pursued his labors. there seems to be some foundation for the belief that the ways of the german high command found little favor in his eyes. at any rate, he was not one of the forty professors who signed the famous manifesto extolling germany's aims. "we know for a fact," writes dr. o. a. rankine, of the imperial college of science and technology, london, "that einstein never was employed on war work. whatever may have been germany's mistakes in other directions, she left her men of science severely alone. in fact, they were encouraged to continue in their normal occupations. einstein undoubtedly received a large measure of support from the imperial government, even when the german armies were being driven back across belgium." quite recently (june, ) the barnard medal of columbia university was conferred on him "in recognition of his highly original and fruitful development of the fundamental concepts of physics through application of mathematics." in acknowledging the honor, prof. einstein wrote to president butler that "... quite apart from the personal satisfaction, i believe i may regard your decision [to confer the medal upon him] as a harbinger of a better time in which a sense of international solidarity will once more unite scholars of the various countries." references for those lacking all astronomical knowledge, an excellent plan would be to read the first pages of w. h. snyder's everyday science (allyn and bacon), in which may be found a clear and simple account of the solar system. this could be followed with bertrand russell's chapter on the nature of matter in his little volume, the problems of philosophy (henry holt and co.). here the reader will be introduced to the purely philosophical side of the question--quite a necessary equipment for the understanding of einstein's theory. of the non-mathematical articles which have appeared, those by prof. a. s. eddington (nature, volume , pages and , ) and prof. m. r. cohen (the new republic, jan. , ) are the best which have come to the author's notice. other articles on einstein's theory, some easily comprehensible, others somewhat confusing, and still others full of noise and rather empty, are by h. a. lorentz, the new york times, dec. , (since reprinted in book form by brentano's, new york, ); j. q. stewart, scientific american, jan. , ; e. cunningham, nature, volume , pages and , ; f. h. loring, chemical news, volume , pages , , , and , ; e. b. wilson, scientific monthly, volume , page , ; j. s. ames, science, volume , page , [ ]; l. a. bauer, science, volume , page ( ), and volume , page ( ); sir oliver lodge, scientific monthly, volume , page , ; e. e. slosson, independent, nov. , dec. , dec. , dec. , (since collected and published in book form by harcourt, brace and howe); isabel m. lewis, electrical experimenter, jan., ; a. j. lotka, harper's magazine, march, , page ; and r. d. carmichael, new york times, march , . einstein himself is responsible for a brief article in english which first appeared in the london times, and was later reprinted in science, volume , page , (see the appendix). a number of books deal with the subject, and all of them are more or less mathematical. however, in every one of these volumes certain chapters, or portions of chapters, may be read with profit even by the non-mathematical reader. some of these books are: erwin freundlich, the foundations of einstein's theory of gravitation (university press, cambridge, ). (a very complete list of references--up to feb., --is also given); a. s. eddington, report on the relativity theory of gravitation for the physical society of london (fleetway press, ltd., london, ); r. c. tolman, theory of the relativity of motion (university of california press, ); e. cunningham, relativity and the electron theory (longmans, green and co., ); r. d. carmichael, the theory of relativity (john wiley and sons, ); l. silberstein, the theory of relativity (macmillan, ); and e. cunningham, the principle of relativity (university press, cambridge, england, ). to those familiar with the german language einstein's book, Ã�ber die spezielle und die allgemeine relativitätstheorie (friedr. vieweg und sohn, braunschweig, ), may be recommended. [ ] the mathematical student may be referred to a volume incorporating the more important papers of einstein, minkowski and lorentz: das relativitätsprinzip, (b. g. teubner, berlin, ). einstein's papers have appeared in the annalen der physik, leipzig, volume , page , , volume , page , , and volume , page , . appendix note (page ) "on this earth there is indeed a tiny corner of the universe accessible to other senses [than the sense of sight]: but feeling and taste act only at those minute distances which separate particles of matter when 'in contact:' smell ranges over, at the utmost, a mile or two, and the greatest distance which sound is ever known to have traveled (when krakatoa exploded in ) is but a few thousand miles--a mere fraction of the earth's girdle."--prof. h. h. turner of oxford. note (page ) huyghens and leibniz both objected to newton's inverse square law because it postulated "action at a distance,"--for example, the attractive force of the sun and the earth. this desire for "continuity" in physical laws led to the supposition of an "ether." we may here anticipate and state that the reason which prompted huyghens to object to newton's law led einstein in our own day to raise objections to the "ether" theory. "in the formulation of physical laws, only those things were to be regarded as being in causal connection which were capable of being actually observed." and the "ether" has not been "actually observed." the idea of "continuity" implies distances between adjacent points that are infinitesimal in extent; hence the idea of "continuity" comes in direct opposition with the finite distances of newton. the statement relating to causal connection--the refusal to accept an "ether" as an absolute base of reference--leads to the principle of the relativity of motion. note (page ) sir oliver lodge goes to the extreme of pinning his faith in the reality of this ether rather than in that of matter. witness the following statement he made recently before a new york audience: "to my mind the ether of space is a substantial reality with extraordinarily perfect properties, with an immense amount of energy stored up in it, with a constitution which we must discover, but a substantial reality far more impressive than that of matter. empty space, as we call it, is full of ether, but it makes no appeal to our senses. the appearance is as if it were nothing. it is the most important thing in the material universe. i believe that matter is a modification of ether, a very porous substance, a thing more analogous to a cobweb or the milky way or something very slight and unsubstantial, as compared to ether." and again: "the properties of ether seem to be perfect. matter is less so; it has friction and elasticity. no imperfection has been discovered in the ether space. it doesn't wear out; there is no dissipation of energy; there is no friction. ether is material, yet it is not matter; both are substantial realities in physics, but it is the ether of space that holds things together and acts as a cement. my business is to call attention to the whole world of etherealness of things, and i have made it a subject of thirty years' study, but we must admit that there is no getting hold of ether except indirectly." "i consider the ether of space," says lodge, in conclusion, "the one substantial thing in the universe." and lodge is certainly entitled to his opinion. note (page ) for the benefit of those readers who wish to gain a deeper insight into the relativity principle, we shall here discuss it very briefly. newton and galileo had developed a relativity principle in mechanics which may be stated as follows: if one system of reference is in uniform rectilinear motion with respect to another system of reference, then whatever physical laws are deduced from the first system hold true for the second system. the two systems are equivalent. if the two systems be represented by $xyz$ and $x'y'z'$, and if they move with the velocity of v along the x-axis with respect to one another, then the two systems are mathematically related thus: $$x' = x - vt, y' = y, z' = z, t' = t,$$ and this immediately provides us with a means of transforming the laws of one system to those of another. with the development of electrodynamics (which we may call electricity in motion) difficulties arose which equations in mechanics of type ( ) could no longer solve. these difficulties merely increased when maxwell showed that light must be regarded as an electromagnetic phenomenon. for suppose we wish to investigate the motion of a source of light (which may be the equivalent of the motion of the earth with reference to the sun) with respect to the velocity of the light it emits--a typical example of the study of moving systems--how are we to coordinate the electrodynamical and mechanical elements? or, again, suppose we wish to investigate the velocity of electrons shot out from radium with a speed comparable to that of light, how are we to coordinate the two branches in tracing the course of these negative particles of electricity? it was difficulties such as these that led to the lorentz-einstein modifications of the newton-galileo relativity equations ( ). the lorentz-einstein equations are expressed in the form: $$x' = \frac{x-vt}{\sqrt{ -\frac{v^ }{c}}}, y' = y, z' = z, t' = \frac{t-\frac{v}{c^ }\cdot x}{\sqrt{ -\frac{v^ }{c^ }}},$$ c denoting the velocity of light in vacuo (which, according to all observations, is the same, irrespective of the observer's state of motion). here, you see, electrodynamical systems (light and therefore "ray" velocities such as those due to electrons) are brought into play. this gives us einstein's special theory of relativity. from it einstein deduced some startling conceptions of time and space. note (page ) the velocity (v) of an object in one system will have a different velocity (v') if referred to another system in uniform motion relative to the first. it had been supposed that only a "something" endowed with infinite velocity would show the same velocity in all systems, irrespective of the motions of the latter. michelson and morley's results actually point to the velocity of light as showing the properties of the imaginary "infinite velocity." the velocity of light possesses universal significance; and this is the basis for much of einstein's earlier work. note (page ) "euclid assumes that parallel lines never meet, which they cannot do of course if they be defined as equidistant. but are there such lines? and if not, why not assume that all lines drawn through a point outside a given line will eventually intersect it? such an assumption leads to a geometry in which all lines are conceived as being drawn on the surface of a sphere or an ellipse, and in it the three angles of a triangle are never quite equal to two right angles, nor the circumference of a circle quite [pi] times its diameter. but that is precisely what the contraction effect due to motion requires." (dr. walker) note (page ) einstein had become tired of assumptions. he had no particular objection to the "ether" theory beyond the fact that this "ether" did not come within the range of our senses; it could not be "observed." "the consistent fulfilment of the two postulates--'action by contact' and causal relationship between only such things as lie within the realm of observation [see note ] combined together is, i believe, the mainspring of einstein's method of investigation...." (prof. freundlich). note (page ) that the conception of the "simultaneity" of events is devoid of meaning can be deduced from equation ( ) [see note ]. we owe the proof to einstein. "it is possible to select a suitable time-coordinate in such a way that a time-measurement enters into physical laws in exactly the same manner as regards its significance as a space measurement (that is, they are fully equivalent symbolically), and has likewise a definite coordinate direction.... it never occurred to anyone that the use of a light-signal as a means of connection between the moving-body and the observer, which is necessary in practice in order to determine simultaneity, might affect the final result, i.e., of time measurements in different systems." (freundlich). but that is just what einstein shows, because time-measurements are based on "simultaneity of events," and this, as pointed out above, is devoid of meaning. had the older masters the occasion to study enormous velocities, such as the velocity of light, rather than relatively small ones--and even the velocity of the earth around the sun is small as compared to the velocity of light--discrepancies between theory and experiment would have become apparent. note (page ) how the special theory of relativity (see note ) led to the general theory of relativity (which included gravitation) may now be briefly traced. when we speak of electrons, or negative particles of electricity, in motion, we are speaking of energy in motion. now these electrons when in motion exhibit properties that are very similar to matter in motion. whatever deviations there are are due to the enormous velocity of these electrons, and this velocity, as has already been pointed out, is comparable to that of light; whereas before the advent of the electron, the velocity of no particles comparable to that of light had ever been measured. according to present views "all inertia of matter consists only of the inertia of the latent energy in it; ... everything that we know of the inertia of energy holds without exception for the inertia of matter." now it is on the assumption that inertial mass and gravitational "pull" are equivalent that the mass of a body is determined by its weight. what is true of matter should be true of energy. the special theory of relativity, however, takes into account only inertia ("inertial mass") but not gravitation (gravitational pull or weight) of energy. when a body absorbs energy equation (see note ) will record a gain in inertia but not in weight--which is contrary to one of the fundamental facts in mechanics. this means that a more general theory of relativity is required to include gravitational phenomena. hence einstein's general theory of relativity. hence the approach to a new theory of gravitation. hence "the setting up of a differential equation which comprises the motion of a body under the influence of both inertia and gravity, and which symbolically expresses the relativity of motions.... the differential law must always preserve the same form, irrespective of the system of coordinates to which it is referred, so that no system of coordinates enjoys a preference to any other." (for the general form of the equation and for an excellent discussion of its significance, see freundlich's monograph, pages - .) time, space, and gravitation [ ] by prof. albert einstein there are several kinds of theory in physics. most of them are constructive. these attempt to build a picture of complex phenomena out of some relatively simple proposition. the kinetic theory of gases, for instance, attempts to refer to molecular movement the mechanical thermal, and diffusional properties of gases. when we say that we understand a group of natural phenomena, we mean that we have found a constructive theory which embraces them. theories of principle.--but in addition to this most weighty group of theories, there is another group consisting of what i call theories of principle. these employ the analytic, not the synthetic method. their starting-point and foundation are not hypothetical constituents, but empirically observed general properties of phenomena, principles from which mathematical formulæ are deduced of such a kind that they apply to every case which presents itself. thermodynamics, for instance, starting from the fact that perpetual motion never occurs in ordinary experience, attempts to deduce from this, by analytic processes, a theory which will apply in every case. the merit of constructive theories is their comprehensiveness, adaptability, and clarity, that of the theories of principle, their logical perfection, and the security of their foundation. the theory of relativity is a theory of principle. to understand it, the principles on which it rests must be grasped. but before stating these it is necessary to point out that the theory of relativity is like a house with two separate stories, the special relativity theory and the general theory of relativity. since the time of the ancient greeks it has been well known that in describing the motion of a body we must refer to another body. the motion of a railway train is described with reference to the ground, of a planet with reference to the total assemblage of visible fixed stars. in physics the bodies to which motions are spatially referred are termed systems of coordinates. the laws of mechanics of galileo and newton can be formulated only by using a system of coordinates. the state of motion of a system of coordinates can not be chosen arbitrarily if the laws of mechanics are to hold good (it must be free from twisting and from acceleration). the system of coordinates employed in mechanics is called an inertia-system. the state of motion of an inertia-system, so far as mechanics are concerned, is not restricted by nature to one condition. the condition in the following proposition suffices; a system of coordinates moving in the same direction and at the same rate as a system of inertia is itself a system of inertia. the special relativity theory is therefore the application of the following proposition to any natural process: "every law of nature which holds good with respect to a coordinate system k must also hold good for any other system k' provided that k and k' are in uniform movement of translation." the second principle on which the special relativity theory rests is that of the constancy of the velocity of light in a vacuum. light in a vacuum has a definite and constant velocity, independent of the velocity of its source. physicists owe their confidence in this proposition to the maxwell-lorentz theory of electro-dynamics. the two principles which i have mentioned have received strong experimental confirmation, but do not seem to be logically compatible. the special relativity theory achieved their logical reconciliation by making a change in kinematics, that is to say, in the doctrine of the physical laws of space and time. it became evident that a statement of the coincidence of two events could have a meaning only in connection with a system of coordinates, that the mass of bodies and the rate of movement of clocks must depend on their state of motion with regard to the coordinates. the older physics.--but the older physics, including the laws of motion of galileo and newton, clashed with the relativistic kinematics that i have indicated. the latter gave origin to certain generalized mathematical conditions with which the laws of nature would have to conform if the two fundamental principles were compatible. physics had to be modified. the most notable change was a new law of motion for (very rapidly) moving mass-points, and this soon came to be verified in the case of electrically-laden particles. the most important result of the special relativity system concerned the inert mass of a material system. it became evident that the inertia of such a system must depend on its energy-content, so that we were driven to the conception that inert mass was nothing else than latent energy. the doctrine of the conservation of mass lost its independence and became merged in the doctrine of conservation of energy. the special relativity theory which was simply a systematic extension of the electro-dynamics of maxwell and lorentz, had consequences which reached beyond itself. must the independence of physical laws with regard to a system of coordinates be limited to systems of coordinates in uniform movement of translation with regard to one another? what has nature to do with the coordinate systems that we propose and with their motions? although it may be necessary for our descriptions of nature to employ systems of coordinates that we have selected arbitrarily, the choice should not be limited in any way so far as their state of motion is concerned. (general theory of relativity.) the application of this general theory of relativity was found to be in conflict with a well-known experiment, according to which it appeared that the weight and the inertia of a body depended on the same constants (identity of inert and heavy masses). consider the case of a system of coordinates which is conceived as being in stable rotation relative to a system of inertia in the newtonian sense. the forces which, relatively to this system, are centrifugal must, in the newtonian sense, be attributed to inertia. but these centrifugal forces are, like gravitation, proportional to the mass of the bodies. is it not, then, possible to regard the system of coordinates as at rest, and the centrifugal forces as gravitational? the interpretation seemed obvious, but classical mechanics forbade it. this slight sketch indicates how a generalized theory of relativity must include the laws of gravitation, and actual pursuit of the conception has justified the hope. but the way was harder than was expected, because it contradicted euclidian geometry. in other words, the laws according to which material bodies are arranged in space do not exactly agree with the laws of space prescribed by the euclidian geometry of solids. this is what is meant by the phrase "a warp in space." the fundamental concepts "straight," "plane," etc., accordingly lose their exact meaning in physics. in the generalized theory of relativity, the doctrine of space and time, kinematics, is no longer one of the absolute foundations of general physics. the geometrical states of bodies and the rates of clocks depend in the first place on their gravitational fields, which again are produced by the material system concerned. thus the new theory of gravitation diverges widely from that of newton with respect to its basal principle. but in practical application the two agree so closely that it has been difficult to find cases in which the actual differences could be subjected to observation. as yet only the following have been suggested: . the distortion of the oval orbits of planets round the sun (confirmed in the case of the planet mercury). . the deviation of light-rays in a gravitational field (confirmed by the english solar eclipse expedition). . the shifting of spectral lines towards the red end of the spectrum in the case of light coming to us from stars of appreciable mass (not yet confirmed). the great attraction of the theory is its logical consistency. if any deduction from it should prove untenable, it must be given up. a modification of it seems impossible without destruction of the whole. no one must think that newton's great creation can be overthrown in any real sense by this or by any other theory. his clear and wide ideas will for ever retain their significance as the foundation on which our modern conceptions of physics have been built. einstein's law of gravitation [ ] by prof. j. s. ames johns hopkins university ... in the treatment of maxwell's equations of the electromagnetic field, several investigators realized the importance of deducing the form of the equations when applied to a system moving with a uniform velocity. one object of such an investigation would be to determine such a set of transformation formulæ as would leave the mathematical form of the equations unaltered. the necessary relations between the new space-coordinates, those applying to the moving system, and the original set were of course obvious; and elementary methods led to the deduction of a new variable which should replace the time coordinate. this step was taken by lorentz and also, i believe, by larmor and by voigt. the mathematical deductions and applications in the hands of these men were extremely beautiful, and are probably well known to you all. lorentz' paper on this subject appeared in the proceedings of the amsterdam academy in . in the following year there was published in the annalen der physik a paper by einstein, written without any knowledge of the work of lorentz, in which he arrived at the same transformation equations as did the latter, but with an entirely different and fundamentally new interpretation. einstein called attention in his paper to the lack of definiteness in the concepts of time and space, as ordinarily stated and used. he analyzed clearly the definitions and postulates which were necessary before one could speak with exactness of a length or of an interval of time. he disposed forever of the propriety of speaking of the "true" length of a rod or of the "true" duration of time, showing, in fact, that the numerical values which we attach to lengths or intervals of time depend upon the definitions and postulates which we adopt. the words "absolute" space or time intervals are devoid of meaning. as an illustration of what is meant einstein discussed two possible ways of measuring the length of a rod when it is moving in the direction of its own length with a uniform velocity, that is, after having adopted a scale of length, two ways of assigning a number to the length of the rod concerned. one method is to imagine the observer moving with the rod, applying along its length the measuring scale, and reading off the positions of the ends of the rod. another method would be to have two observers at rest on the body with reference to which the rod has the uniform velocity, so stationed along the line of motion of the rod that as the rod moves past them they can note simultaneously on a stationary measuring scale the positions of the two ends of the rod. einstein showed that, accepting two postulates which need no defense at this time, the two methods of measurements would lead to different numerical values, and, further, that the divergence of the two results would increase as the velocity of the rod was increased. in assigning a number, therefore, to the length of a moving rod, one must make a choice of the method to be used in measuring it. obviously the preferable method is to agree that the observer shall move with the rod, carrying his measuring instrument with him. this disposes of the problem of measuring space relations. the observed fact that, if we measure the length of the rod on different days, or when the rod is lying in different positions, we always obtain the same value offers no information concerning the "real" length of the rod. it may have changed, or it may not. it must always be remembered that measurement of the length of a rod is simply a process of comparison between it and an arbitrary standard, e.g., a meter-rod or yard-stick. in regard to the problem of assigning numbers to intervals of time, it must be borne in mind that, strictly speaking, we do not "measure" such intervals, i.e., that we do not select a unit interval of time and find how many times it is contained in the interval in question. (similarly, we do not "measure" the pitch of a sound or the temperature of a room.) our practical instruments for assigning numbers to time-intervals depend in the main upon our agreeing to believe that a pendulum swings in a perfectly uniform manner, each vibration taking the same time as the next one. of course we cannot prove that this is true, it is, strictly speaking, a definition of what we mean by equal intervals of time; and it is not a particularly good definition at that. its limitations are sufficiently obvious. the best way to proceed is to consider the concept of uniform velocity, and then, using the idea of some entity having such a uniform velocity, to define equal intervals of time as such intervals as are required for the entity to traverse equal lengths. these last we have already defined. what is required in addition is to adopt some moving entity as giving our definition of uniform velocity. considering our known universe it is self-evident that we should choose in our definition of uniform velocity the velocity of light, since this selection could be made by an observer anywhere in our universe. having agreed then to illustrate by the words "uniform velocity" that of light, our definition of equal intervals of time is complete. this implies, of course, that there is no uncertainty on our part as to the fact that the velocity of light always has the same value at any one point in the universe to any observer, quite regardless of the source of light. in other words, the postulate that this is true underlies our definition. following this method einstein developed a system of measuring both space and time intervals. as a matter of fact his system is identically that which we use in daily life with reference to events here on the earth. he further showed that if a man were to measure the length of a rod, for instance, on the earth and then were able to carry the rod and his measuring apparatus to mars, the sun, or to arcturus he would obtain the same numerical value for the length in all places and at all times. this doesn't mean that any statement is implied as to whether the length of the rod has remained unchanged or not; such words do not have any meaning--remember that we can not speak of true length. it is thus clear that an observer living on the earth would have a definite system of units in terms of which to express space and time intervals, i.e., he would have a definite system of space coordinates (x, y, z) and a definite time coordinate (t); and similarly an observer living on mars would have his system of coordinates (x', y', z', t'). provided that one observer has a definite uniform velocity with reference to the other, it is a comparatively simple matter to deduce the mathematical relations between the two sets of coordinates. when einstein did this, he arrived at the same transformation formulæ as those used by lorentz in his development of maxwell's equations. the latter had shown that, using these formulæ, the form of the laws for all electromagnetic phenomena maintained the same form; so einstein's method proves that using his system of measurement an observer, anywhere in the universe, would as the result of his own investigation of electromagnetic phenomena arrive at the same mathematical statement of them as any other observer, provided only that the relative-velocity of the two observers was uniform. einstein discussed many other most important questions at this time; but it is not necessary to refer to them in connection with the present subject. so far as this is concerned, the next important step to note is that taken in the famous address of minkowski, in , on the subject of "space and time." it would be difficult to overstate the importance of the concepts advanced by minkowski. they marked the beginning of a new period in the philosophy of physics. i shall not attempt to explain his ideas in detail, but shall confine myself to a few general statements. his point of view and his line of development of the theme are absolutely different from those of lorentz or of einstein; but in the end he makes use of the same transformation formulæ. his great contribution consists in giving us a new geometrical picture of their meaning. it is scarcely fair to call minkowski's development a picture; for to us a picture can never have more than three dimensions, our senses limit us; while his picture calls for perception of four dimensions. it is this fact that renders any even semi-popular discussion of minkowski's work so impossible. we can all see that for us to describe any event a knowledge of four coordinates is necessary, three for the space specification and one for the time. a complete picture could be given then by a point in four dimensions. all four coordinates are necessary: we never observe an event except at a certain time, and we never observe an instant of time except with reference to space. discussing the laws of electromagnetic phenomena, minkowski showed how in a space of four dimensions, by a suitable definition of axes, the mathematical transformation of lorentz and einstein could be described by a rotation of the set of axes. we are all accustomed to a rotation of our ordinary cartesian set of axes describing the position of a point. we ordinarily choose our axes at any location on the earth as follows: one vertical, one east and west, one north and south. so if we move from any one laboratory to another, we change our axes; they are always orthogonal, but in moving from place to place there is a rotation. similarly, minkowski showed that if we choose four orthogonal axes at any point on the earth, according to his method, to represent a space-time point using the method of measuring space and time intervals as outlined by einstein; and, if an observer on arcturus used a similar set of axes and the method of measurement which he naturally would, the set of axes of the latter could be obtained from those of the observer on the earth by a pure rotation (and naturally a transfer of the origin). this is a beautiful geometrical result. to complete my statement of the method, i must add that instead of using as his fourth axis one along which numerical values of time are laid off, minkowski defined his fourth coordinate as the product of time and the imaginary constant, the square root of minus one. this introduction of imaginary quantities might be expected, possibly, to introduce difficulties; but, in reality, it is the very essence of the simplicity of the geometrical description just given of the rotation of the sets of axes. it thus appears that different observers situated at different points in the universe would each have their own set of axes, all different, yet all connected by the fact that any one can be rotated so as to coincide with any other. this means that there is no one direction in the four-dimensional space that corresponds to time for all observers. just as with reference to the earth there is no direction which can be called vertical for all observers living on the earth. in the sense of an absolute meaning the words "up and down," "before and after," "sooner or later," are entirely meaningless. this concept of minkowski's may be made clearer, perhaps, by the following process of thought. if we take a section through our three-dimensional space, we have a plane, i.e., a two-dimensional space. similarly, if a section is made through a four-dimensional space, one of three dimensions is obtained. thus, for an observer on the earth a definite section of minkowski's four-dimensional space will give us our ordinary three-dimensional one; so that this section will, as it were, break up minkowski's space into our space and give us our ordinary time. similarly, a different section would have to be used to the observer on arcturus; but by a suitable selection he would get his own familiar three-dimensional space and his own time. thus the space defined by minkowski is completely isotropic in reference to measured lengths and times, there is absolutely no difference between any two directions in an absolute sense; for any particular observer, of course, a particular section will cause the space to fall apart so as to suit his habits of measurement; any section, however, taken at random will do the same thing for some observer somewhere. from another point of view, that of lorentz and einstein, it is obvious that, since this four-dimensional space is isotropic, the expression of the laws of electromagnetic phenomena take identical mathematical forms when expressed by any observer. the question of course must be raised as to what can be said in regard to phenomena which so far as we know do not have an electromagnetic origin. in particular what can be done with respect to gravitational phenomena? before, however, showing how this problem was attacked by einstein; and the fact that the subject of my address is einstein's work on gravitation shows that ultimately i shall explain this, i must emphasize another feature of minkowski's geometry. to describe the space-time characteristics of any event a point, defined by its four coordinates, is sufficient; so, if one observes the life-history of any entity, e.g., a particle of matter, a light-wave, etc., he observes a sequence of points in the space-time continuum; that is, the life-history of any entity is described fully by a line in this space. such a line was called by minkowski a "world-line." further, from a different point of view, all of our observations of nature are in reality observations of coincidences, e.g., if one reads a thermometer, what he does is to note the coincidence of the end of the column of mercury with a certain scale division on the thermometer tube. in other words, thinking of the world-line of the end of the mercury column and the world-line of the scale division, what we have observed was the intersection or crossing of these lines. in a similar manner any observation may be analyzed; and remembering that light rays, a point on the retina of the eye, etc., all have their world-lines, it will be recognized that it is a perfectly accurate statement to say that every observation is the perception of the intersection of world-lines. further, since all we know of a world-line is the result of observations, it is evident that we do not know a world-line as a continuous series of points, but simply as a series of discontinuous points, each point being where the particular world-line in question is crossed by another world-line. it is clear, moreover, that for the description of a world-line we are not limited to the particular set of four orthogonal axes adopted by minkowski. we can choose any set of four-dimensional axes we wish. it is further evident that the mathematical expression for the coincidence of two points is absolutely independent of our selection of reference axes. if we change our axes, we will change the coordinates of both points simultaneously, so that the question of axes ceases to be of interest. but our so-called laws of nature are nothing but descriptions in mathematical language of our observations; we observe only coincidences; a sequence of coincidences when put in mathematical terms takes a form which is independent of the selection of reference axes; therefore the mathematical expression of our laws of nature, of every character, must be such that their form does not change if we make a transformation of axes. this is a simple but far-reaching deduction. there is a geometrical method of picturing the effect of a change of axes of reference, i.e., of a mathematical transformation. to a man in a railway coach the path of a drop of water does not appear vertical, i.e., it is not parallel to the edge of the window; still less so does it appear vertical to a man performing manoeuvres in an airplane. this means that whereas with reference to axes fixed to the earth the path of the drop is vertical; with reference to other axes, the path is not. or, stating the conclusion in general language, changing the axes of reference (or effecting a mathematical transformation) in general changes the shape of any line. if one imagines the line forming a part of the space, it is evident that if the space is deformed by compression or expansion the shape of the line is changed, and if sufficient care is taken it is clearly possible, by deforming the space, to make the line take any shape desired, or better stated, any shape specified by the previous change of axes. it is thus possible to picture a mathematical transformation as a deformation of space. thus i can draw a line on a sheet of paper or of rubber and by bending and stretching the sheet, i can make the line assume a great variety of shapes; each of these new shapes is a picture of a suitable transformation. now, consider world-lines in our four-dimensional space. the complete record of all our knowledge is a series of sequences of intersections of such lines. by analogy i can draw in ordinary space a great number of intersecting lines on a sheet of rubber; i can then bend and deform the sheet to please myself; by so doing i do not introduce any new intersections nor do i alter in the least the sequence of intersections. so in the space of our world-lines, the space may be deformed in any imaginable manner without introducing any new intersections or changing the sequence of the existing intersections. it is this sequence which gives us the mathematical expression of our so-called experimental laws; a deformation of our space is equivalent mathematically to a transformation of axes, consequently we see why it is that the form of our laws must be the same when referred to any and all sets of axes, that is, must remain unaltered by any mathematical transformation. now, at last we come to gravitation. we can not imagine any world-line simpler than that of a particle of matter left to itself; we shall therefore call it a "straight" line. our experience is that two particles of matter attract one another. expressed in terms of world-lines, this means that, if the world-lines of two isolated particles come near each other, the lines, instead of being straight, will be deflected or bent in towards each other. the world-line of any one particle is therefore deformed; and we have just seen that a deformation is the equivalent of a mathematical transformation. in other words, for any one particle it is possible to replace the effect of a gravitational field at any instant by a mathematical transformation of axes. the statement that this is always possible for any particle at any instant is einstein's famous "principle of equivalence." let us rest for a moment, while i call attention to a most interesting coincidence, not to be thought of as an intersection of world-lines. it is said that newton's thoughts were directed to the observation of gravitational phenomena by an apple falling on his head; from this striking event he passed by natural steps to a consideration of the universality of gravitation. einstein in describing his mental process in the evolution of his law of gravitation says that his attention was called to a new point of view by discussing his experiences with a man whose fall from a high building he had just witnessed. the man fortunately suffered no serious injuries and assured einstein that in the course of his fall he had not been conscious in the least of any pull downward on his body. in mathematical language, with reference to axes moving with the man the force of gravity had disappeared. this is a case where by the transfer of the axes from the earth itself to the man, the force of the gravitational field is annulled. the converse change of axes from the falling man to a point on the earth could be considered as introducing the force of gravity into the equations of motion. another illustration of the introduction into our equations of a force by a means of a change of axes is furnished by the ordinary treatment of a body in uniform rotation about an axis. for instance, in the case of a so-called conical pendulum, that is, the motion of a bob suspended from a fixed point by string, which is so set in motion that the bob describes a horizontal circle and the string therefore describes a circular cone, if we transfer our axes from the earth and have them rotate around the vertical line through the fixed point with the same angular velocity as the bob, it is necessary to introduce into our equations of motion a fictitious "force" called the centrifugal force. no one ever thinks of this force other than as a mathematical quantity introduced into the equations for the sake of simplicity of treatment; no physical meaning is attached to it. why should there be to any other so-called "force," which like centrifugal force, is independent of the nature of the matter? again, here on the earth our sensation of weight is interpreted mathematically by combining expressions for centrifugal force and gravity; we have no distinct sensation for either separately. why then is there any difference in the essence of the two? why not consider them both as brought into our equations by the agency of mathematical transformations? this is einstein's point of view. granting, then, the principle of equivalence, we can so choose axes at any point at any instant that the gravitational field will disappear; these axes are therefore of what eddington calls the "galilean" type, the simplest possible. consider, that is, an observer in a box, or compartment, which is falling with the acceleration of the gravitational field at that point. he would not be conscious of the field. if there were a projectile fired off in this compartment, the observer would describe its path as being straight. in this space the infinitesimal interval between two space-time points would then be given by the formula $$ds^ = dx^ _ + dx _ + dx^ _ + dx _ ,$$ where ds is the interval and $x_ , x_ , x_ , x_ $ are coordinates. if we make a mathematical transformation, i.e., use another set of axes, this interval would obviously take the form $$ds^ = g_{ }dx^ _{ } + g_{ }dx^ _ + g_{ }dx^ _ + g_{ }dx _ + g_{ }dx_ dx_ + \rm{etc.},$$ where $x_ , x_ , x_ $ and $x_ $ are now coordinates referring to the new axes. this relation involves ten coefficients, the coefficients defining the transformation. but of course a certain dynamical value is also attached to the g's, because by the transfer of our axes from the galilean type we have made a change which is equivalent to the introduction of a gravitational field; and the g's must specify the field. that is, these g's are the expressions of our experiences, and hence their values can not depend upon the use of any special axes; the values must be the same for all selections. in other words, whatever function of the coordinates any one g is for one set of axes, if other axes are chosen, this g must still be the same function of the new coordinates. there are ten g's defined by differential equations; so we have ten covariant equations. einstein showed how these g's could be regarded as generalized potentials of the field. our own experiments and observations upon gravitation have given us a certain knowledge concerning its potential; that is, we know a value for it which must be so near the truth that we can properly call it at least a first approximation. or, stated differently, if einstein succeeds in deducing the rigid value for the gravitational potential in any field, it must degenerate to the newtonian value for the great majority of cases with which we have actual experience. einstein's method, then, was to investigate the functions (or equations) which would satisfy the mathematical conditions just described. a transformation from the axes used by the observer in the following box may be made so as to introduce into the equations the gravitational field recognized by an observer on the earth near the box; but this, obviously, would not be the general gravitational field, because the field changes as one moves over the surface of the earth. a solution found, therefore, as just indicated, would not be the one sought for the general field; and another must be found which is less stringent than the former but reduces to it as a special case. he found himself at liberty to make a selection from among several possibilities, and for several reasons chose the simplest solution. he then tested this decision by seeing if his formulæ would degenerate to newton's law for the limiting case of velocities small when compared with that of light, because this condition is satisfied in those cases to which newton's law applies. his formulæ satisfied this test, and he therefore was able to announce a "law of gravitation," of which newton's was a special form for a simple case. to the ordinary scholar the difficulties surmounted by einstein in his investigations appear stupendous. it is not improbable that the statement which he is alleged to have made to his editor, that only ten men in the world could understand his treatment of the subject, is true. i am fully prepared to believe it, and wish to add that i certainly am not one of the ten. but i can also say that, after a careful and serious study of his papers, i feel confident that there is nothing in them which i can not understand, given the time to become familiar with the special mathematical processes used. the more i work over einstein's papers, the more impressed i am, not simply by his genius in viewing the problem, but also by his great technical skill. following the path outlined, einstein, as just said, arrived at certain mathematical laws for a gravitational field, laws which reduced to newton's form in most cases where observations are possible, but which led to different conclusions in a few cases, knowledge concerning which we might obtain by careful observations. i shall mention a few deductions from einstein's formulæ. . if a heavy particle is put at the center of a circle, and, if the length of the circumference and the length of the diameter are measured, it will be found that their ratio is not [pi] ( . ). in other words the geometrical properties of space in such a gravitational field are not those discussed by euclid; the space is, then, non-euclidean. there is no way by which this deduction can be verified, the difference between the predicted ratio and [pi] is too minute for us to hope to make our measurements with sufficient exactness to determine the difference. . all the lines in the solar spectrum should with reference to lines obtained by terrestrial sources be displaced slightly towards longer wave-lengths. the amount of displacement predicted for lines in the blue end of the spectrum is about one-hundredth of an angstrom unit, a quantity well within experimental limits. unfortunately, as far as the testing of this prediction is concerned, there are several physical causes which are also operating to cause displacement of the spectrum-lines; and so at present a decision can not be rendered as to the verification. st. john and other workers at the mount wilson observatory have the question under investigation. . according to newton's law an isolated planet in its motion around a central sun would describe, period after period, the same elliptical orbit; whereas einstein's laws lead to the prediction that the successive orbits traversed would not be identically the same. each revolution would start the planet off on an orbit very approximately elliptical, but with the major axis of the ellipse rotated slightly in the plane of the orbit. when calculations were made for the various planets in our solar system, it was found that the only one which was of interest from the standpoint of verification of einstein's formulæ was mercury. it has been known for a long time that there was actually such a change as just described in the orbit of mercury, amounting to '' of arc per century; and it has been shown that of this a rotation of '' was due to the direct action of other planets, thus leaving an unexplained rotation of '' per century. einstein's formulæ predicted a rotation of '', a striking agreement. . in accordance with einstein's formulæ a ray of light passing close to a heavy piece of matter, the sun, for instance, should experience a sensible deflection in towards the sun. this might be expected from "general" consideration of energy in motion; energy and mass are generally considered to be identical in the sense that an amount of energy e has the mass $e c^ $ where c is the velocity of light; and consequently a ray of light might fall within the province of gravitation and the amount of deflection to be expected could be calculated by the ordinary formula for gravitation. another point of view is to consider again the observer inside the compartment falling with the acceleration of the gravitational field. to him the path of a projectile and a ray of light would both appear straight; so that, if the projectile had a velocity equal to that of light, it and the light wave would travel side by side. to an observer outside the compartment, e.g., to one on the earth, both would then appear to have the same deflection owing to the sun. but how much would the path of the projectile be bent? what would be the shape of its parabola? one might apply newton's law; but, according to einstein's formulæ, newton's law should be used only for small velocities. in the case of a ray passing close to the sun it was decided that according to einstein's formula there should be a deflection of ''. whereas newton's law of gravitation predicted half this amount. careful plans were made by various astronomers, to investigate this question at the solar eclipse last may, and the result announced by dyson, eddington and crommelin, the leaders of astronomy in england, was that there was a deflection of ''. . of course the detection of such a minute deflection was an extraordinarily difficult matter, so many corrections had to be applied to the original observations; but the names of the men who record the conclusions are such as to inspire confidence. certainly any effect of refraction seems to be excluded. it is thus seen that the formulæ deduced by einstein have been confirmed in a variety of ways and in a most brilliant manner. in connection with these formulæ one question must arise in the minds of everyone; by what process, where in the course of the mathematical development, does the idea of mass reveal itself? it was not in the equations at the beginning and yet here it is at the end. how does it appear? as a matter of fact it is first seen as a constant of integration in the discussion of the problem of the gravitational field due to a single particle; and the identity of this constant with mass is proved when one compares einstein's formulæ with newton's law which is simply its degenerated form. this mass, though, is the mass of which we become aware through our experiences with weight; and einstein proceeded to prove that this quantity which entered as a constant of integration in his ideally simple problem also obeyed the laws of conservation of mass and conservation of momentum when he investigated the problems of two and more particles. therefore einstein deduced from his study of gravitational fields the well-known properties of matter which form the basis of theoretical mechanics. a further logical consequence of einstein's development is to show that energy has mass, a concept with which every one nowadays is familiar. the description of einstein's method which i have given so far is simply the story of one success after another; and it is certainly fair to ask if we have at last reached finality in our investigation of nature, if we have attained to truth. are there no outstanding difficulties? is there no possibility of error? certainly, not until all the predictions made from einstein's formulæ have been investigated can much be said; and further, it must be seen whether any other lines of argument will lead to the same conclusions. but without waiting for all this there is at least one difficulty which is apparent at this time. we have discussed the laws of nature as independent in their form of reference axes, a concept which appeals strongly to our philosophy; yet it is not at all clear, at first sight, that we can be justified in our belief. we can not imagine any way by which we can become conscious of the translation of the earth in space; but by means of gyroscopes we can learn a great deal about its rotation on its axis. we could locate the positions of its two poles, and by watching a foucault pendulum or a gyroscope we can obtain a number which we interpret as the angular velocity of rotation of axes fixed in the earth; angular velocity with reference to what? where is the fundamental set of axes? this is a real difficulty. it can be surmounted in several ways. einstein himself has outlined a method which in the end amounts to assuming the existence on the confines of space of vast quantities of matter, a proposition which is not attractive. desitter has suggested a peculiar quality of the space to which we refer our space-time coordinates. the consequences of this are most interesting, but no decision can as yet be made as to the justification of the hypothesis. in any case we can say that the difficulty raised is not one that destroys the real value of einstein's work. in conclusion i wish to emphasize the fact, which should be obvious, that einstein has not attempted any explanation of gravitation; he has been occupied with the deduction of its laws. these laws, together with those of electromagnetic phenomena, comprise our store of knowledge. there is not the slightest indication of a mechanism, meaning by that a picture in terms of our senses. in fact what we have learned has been to realize that our desire to use such mechanisms is futile. the deflection of light by gravitation and the einstein theory of relativity. [ ] sir frank dyson the astronomer royal the purpose of the expedition was to determine whether any displacement is caused to a ray of light by the gravitational field of the sun, and if so, the amount of the displacement. einstein's theory predicted a displacement varying inversely as the distance of the ray from the sun's center, amounting to ''. for a star seen just grazing the sun.... a study of the conditions of the eclipse showed that the sun would be very favorably placed among a group of bright stars--in fact, it would be in the most favorable possible position. a study of the conditions at various points on the path of the eclipse, in which mr. hinks helped us, pointed to sobral, in brazil, and principe, an island off the west coast of africa, as the most favorable stations.... the greenwich party, dr. crommelin and mr. davidson, reached brazil in ample time to prepare for the eclipse, and the usual preliminary focusing by photographing stellar fields was carried out. the day of the eclipse opened cloudy, but cleared later, and the observations were carried out with almost complete success. with the astrographic telescope mr. davidson secured out of photographs showing the required stellar images. totality lasted minutes, and the average exposure of the plates was to seconds. dr. crommelin with the other lens had successful plates out of . the unsuccessful plates were spoiled for this purpose by the clouds, but show the remarkable prominence very well. when the plates were developed the astrographic images were found to be out of focus. this is attributed to the effect of the sun's heat on the coelostat mirror. the images were fuzzy and quite different from those on the check-plates secured at night before and after the eclipse. fortunately the mirror which fed the -inch lens was not affected, and the star images secured with this lens were good and similar to those got by the night-plates. the observers stayed on in brazil until july to secure the field in the night sky at the altitude of the eclipse epoch and under identical instrumental conditions. the plates were measured at greenwich immediately after the observers' return. each plate was measured twice over by messrs. davidson and furner, and i am satisfied that such faults as lie in the results are in the plates themselves and not in the measures. the figures obtained may be briefly summarized as follows: the astrographic plates gave ''. for the displacement at the limb when the scale-value was determined from the plates themselves, and ''. when the scale-value was assumed from the check plates. but the much better plates gave for the displacement at the limb ''. , einstein's predicted value being ''. . further, for these plates the agreement was all that could be expected.... after a careful study of the plates i am prepared to say that there can be no doubt that they confirm einstein's prediction. a very definite result has been obtained that light is deflected according to einstein's law of gravitation. professor a. s. eddington royal observatory mr. cottingham and i left the other observers at madeira and arrived at principe on april .... we soon realized that the prospect of a clear sky at the end of may was not very good. not even a heavy thunderstorm on the morning of the eclipse, three weeks after the end of the wet season, saved the situation. the sky was completely cloudy at first contact, but about half an hour before totality we began to see glimpses of the sun's crescent through the clouds. we carried out our program exactly as arranged, and the sky must have been clearer towards the end of totality. of the plates taken during the five minutes of totality the first ten showed no stars at all; of the later plates two showed five stars each, from which a result could be obtained. comparing them with the check-plates secured at oxford before we went out, we obtained as the final result from the two plates for the value of the displacement of the limb ''. ± . .... this result supports the figures obtained at sobral.... i will pass now to a few words on the meaning of the result. it points to the larger of the two possible values of the deflection. the simplest interpretation of the bending of the ray is to consider it as an effect of the weight of light. we know that momentum is carried along on the path of a beam of light. gravity in acting creates momentum in a direction different from that of the path of the ray and so causes it to bend. for the half-effect we have to assume that gravity obeys newton's law; for the full effect which has been obtained we must assume that gravity obeys the new law proposed by einstein. this is one of the most crucial tests between newton's law and the proposed new law. einstein's law had already indicated a perturbation, causing the orbit of mercury to revolve. that confirms it for relatively small velocities. going to the limit, where the speed is that of light, the perturbation is increased in such a way as to double the curvature of the path, and this is now confirmed. this effect may be taken as proving einstein's law rather than his theory. it is not affected by the failure to detect the displacement of fraunhofer lines on the sun. if this latter failure is confirmed it will not affect einstein's law of gravitation, but it will affect the views on which the law was arrived at. the law is right, though the fundamental ideas underlying it may yet be questioned.... one further point must be touched upon. are we to attribute the displacement to the gravitational field and not to the refracting matter around the sun? the refractive index required to produce the result at a distance of ' from the sun would be that given by gases at a pressure of / to / of an atmosphere. this is of too great a density considering the depth through which the light would have to pass. sir j. j. thomson president of the royal society ... if the results obtained had been only that light was affected by gravitation, it would have been of the greatest importance. newton, did, in fact, suggest this very point in his "optics," and his suggestion would presumably have led to the half-value. but this result is not an isolated one; it is part of a whole continent of scientific ideas affecting the most fundamental concepts of physics.... this is the most important result obtained in connection with the theory of gravitation since newton's day, and it is fitting that it should be announced at a meeting of the society so closely connected with him. the difference between the laws of gravitation of einstein and newton come only in special cases. the real interest of einstein's theory lies not so much in his results as in the method by which he gets them. if his theory is right, it makes us take an entirely new view of gravitation. if it is sustained that einstein's reasoning holds good--and it has survived two very severe tests in connection with the perihelion of mercury and the present eclipse--then it is the result of one of the highest achievements of human thought. the weak point in the theory is the great difficulty in expressing it. it would seem that no one can understand the new law of gravitation without a thorough knowledge of the theory of invariants and of the calculus of variations. one other point of physical interest arises from the discussion. light is deflected in passing near huge bodies of matter. this involves alterations in the electric and magnetic field. this, again, implies the existence of electric and magnetic forces outside matter--forces at present unknown, though some idea of their nature may be got from the results of this expedition. notes [ ] see note at the end of the volume. [ ] see note . [ ] see note . [ ] a circle--in our case the horizon--is measured by dividing the circumference into parts; each part is called a degree. each degree is divided into minutes, and each minute into seconds. [ ] see page . [ ] see note . [ ] see note . [ ] see note . [ ] see note . [ ] see note . [ ] see note . [ ] see page . [ ] this has since been translated into english by dr. lawson and published by methuen (london). since the above has been written two excellent books have been published. one is by prof. a. s. eddington, space, time and gravitation (cambridge univ. press, ). the other, somewhat more of a philosophical work, is prof. moritz schlick's space and time in contemporary physics (oxford univ. press, ). though published as early as , bertrand russell's an essay on the foundations of geometry (cambridge univ. press, ) contains a fine account of non-euclidean geometry. [ ] republished by permission from "science." [ ] presidential address delivered at the st. louis meeting of the physical society, december , . republished by permission from "science." [ ] from a report in the observatory, of the joint eclipse meeting of the royal society and the royal astronomical society, november , . generously made available by the internet archive.) a treatise on meteorological instruments. london: printed by williams and strahan, lawrence lane, cheapside, e.c. a treatise on meteorological instruments: explanatory of their scientific principles, method of construction, and practical utility. by negretti & zambra, meteorological instrument makers to the queen, the royal observatory, greenwich, the british meteorological society, the british and foreign governments, etc. etc. etc. london: published and sold at negretti & zambra's establishments: hatton garden, e.c., cornhill, e.c., regent street w., and fleet street, e.c. . _price five shillings._ preface. the national utilisation of meteorology in forewarning of storms, and the increasing employment of instruments as weather indicators, render a knowledge of their construction, principles, and practical uses necessary to every well-informed person. impressed with the idea that we shall be supplying an existing want, and aiding materially the cause of meteorological science, in giving a plain description of the various instruments now in use, we have endeavoured, in the present volume, to condense such information as is generally required regarding the instruments used in meteorology; the description of many of which could only be found in elaborate scientific works, and then only briefly touched upon. every meteorological instrument now in use being fully described, with adequate directions for using, the uninitiated will be enabled to select those which seem to them best adapted to their requirements. with accounts of old or obsolete instruments we have avoided troubling the reader; on the other hand, we were unwilling to neglect those which, though of no great practical importance, are still deserving of notice from their being either novel or ingenious, or which, without being strictly scientific, are in great demand as simple weather-glasses and articles of trade. we trust, therefore, that the work (however imperfect), bearing in mind the importance of the subject, will be acceptable to general readers, as well as to those for whose requirements it has been prepared. the rapid progress made in the introduction of new apparatus of acknowledged superiority has rendered the publication of some description absolutely necessary. the report of the jurors for class xiii. of the international exhibition, , on meteorological instruments, fully bears out our assertion, as shown by the following extract:-- "the progress in the english department has been very great;--in barometers, thermometers, anemometers, and in every class of instruments. at the close of the exhibition of , there seemed to have arisen a general anxiety among the majority of makers to pay every attention to all the essentials necessary for philosophical instruments, not only in their old forms, but also with the view of obtaining other and better forms. this desire has never ceased; and no better idea can be given of the continued activity in these respects, than the number of patents taken out for improvements in meteorological instruments in the interval between the recent and preceding exhibitions, which amount to no less than forty-two." * * * "in addition to numerous improvements patented by messrs. negretti and zambra, there is another of great importance, which they did not patent, viz. enamelling the tubes of thermometers, enabling the makers to use finer threads of mercury in the construction of all thermometers; for the contrast between the opaque mercury and the enamel back of the tubes is so great, that the finest bore or thread of mercury, which at one time could not be seen without the greatest difficulty, is now seen with facility; and throughout the british and foreign departments, the makers have availed themselves of this invention, the tubes of all being made with enamelled backs. it is to be hoped that the recent exhibition will give a fresh stimulus to the desire of improvement, and that the same rate of progress will be continued." to fulfil the desire of the international jury in the latter portion of the above extract will be the constant study of negretti & zambra. _ st january, ._ table of contents. chapter i. instruments for ascertaining the atmospheric pressure. section . principle of the barometer. . construction of barometers. . fortin's barometer cistern. . standard barometer. . correction due to capillarity. . " " temperature. . " " height. . the barometer vernier. . self-compensating standard barometer. . barometer with electrical adjustment. . pediment barometers. . the words on the scale. . correction due to capacity of cistern. . public barometers. . fishery or sea-coast barometers. . admiral fitzroy's words for the scale. . instructions for sea-coast barometer. . french sea-coast barometer. . common marine barometer. . the kew marine barometer. . method of verifying barometers. . fitzroy's marine barometer. . words for its scale. . trials of this barometer under gun-fire. . negretti and zambra's farmer's barometer and domestic weather-glass. . rules for foretelling the weather. . causes which may bring about a fall or a rise in the barometer. . use of the barometer in the management of mines. . use of the barometer in estimating the height of tides. chapter ii. syphon tube barometers. . principle of. . dial, or wheel, barometers. . standard syphon barometer. chapter iii. barographs, or self-registering barometers. section . milne's self-registering barometer. . modification of milne's barometer. . king's self-registering barometer. . syphon, with photographic registration. chapter iv. mountain barometers. . gay lussac's mountain barometer. . fortin's mountain barometer. . newman's mountain barometer. . negretti and zambra's patent mountain and other barometers. . short tube barometer. . method of calculating heights by the barometer; tables and examples. chapter v. secondary barometers. . desirability of magnifying the barometer range. . howson's long-range barometer. . mcneil's long-range barometer. . the water-glass barometer. . sympiesometers. . aneroids. . small size aneroids. . watch aneroid. . measurement of heights by the aneroid; example. . metallic barometer. chapter vi. instruments for ascertaining temperature. . temperature. . thermometric substances. . description of the thermometer. . standard thermometer. . method of ascertaining the exact boiling temperature; tables, &c. . displacement of the freezing point. . the scale. . the method of testing thermometers. . porcelain scale-plates. . enamelled tubes. . thermometers of extreme sensitiveness. . varieties of thermometers. . superheated steam thermometer. . thermometer for sugar boiling. . earth thermometer. . marine thermometer. chapter vii. self-registering thermometers. . importance of. . rutherford's maximum thermometer. . phillips's ditto ditto. . negretti and zambra's patent maximum thermometer. . rutherford's alcohol minimum thermometer. . horticultural minimum thermometer. . baudin's alcohol minimum thermometer. . mercurial minima thermometers desirable. . negretti and zambra's patent mercurial minimum thermometer. . negretti and zambra's second patent mercurial minimum thermometer. . casella's patent mercurial minimum thermometer. . day and night thermometer. . sixe's self-registering thermometer. chapter viii. radiation thermometers. . solar and terrestrial radiation considered. . solar radiation thermometer. . vacuum solar radiation thermometer. . terrestrial radiation thermometer. . Æthrioscope. . pyrheliometer. . actinometer. chapter ix. deep-sea thermometers. . on sixe's principle. . johnson's metallic thermometer. chapter x. boiling-point thermometers. . ebullition. . relation between boiling-point and elevation. . hypsometric apparatus. . precautions to ensure correct graduation. . method of calculating heights from observations with the mountain thermometer; example. . thermometers for engineers. chapter xi. instruments for ascertaining the humidity of the air. . hygrometric substances. . saussure's hygrometer. . dew-point. . drosometer. . humidity. . leslie's hygrometer. . daniel's hygrometer. . regnault's condenser hygrometer. . temperature of evaporation. . mason's hygrometer. . self-registering hygrometer. . causes of dew. . plan of exposing thermometers. chapter xii. instruments used for measuring the rainfall. . howard's rain-gauge. . glaisher's rain-gauge. . rain-gauge with float. . rain-gauge with side tube. . fitzroy's rain-gauge. . self-registering rain-gauge. . the principle of measurement. . position for rain-gauge, &c. . cause of rain. . laws of rainfall. . utility of statistics of rainfall. . new form of rain-gauge. chapter xiii. apparatus employed for registering the direction, pressure, and velocity of the wind. . the vane. . lind's wind-gauge. . harris's wind-gauge. . robinson's anemometer. . whewell's anemometer. . osler's anemometer and pluviometer. . beckley's anemometer. . self-registering wind-gauge. . anemometric observations. chapter xiv. instruments for investigating atmospheric electricity. . atmospheric electroscope. . volta's electrometer. . peltier's electrometer. . bohnenberger's electroscope. . thomson's electrometer. . fundamental facts. . lightning conductors. . precautions against lightning. chapter xv. ozone and its indicators. . nature of ozone. . schonbein's ozonometer. . moffat's ozonometer. . clark's ozone cage. . distribution and effects of ozone. . lancaster's registering ozonometer. chapter xvi. miscellaneous instruments. . chemical weather glass. . leslie's differential thermometer. . romford's differential thermometer. . glaisher's thermometer stand. . thermometer screen, for use at sea. . anemoscope. . evaporating dish, or gauge. . admidometer. . cloud reflector. . sunshine recorder. . set of portable instruments. . implements. . hydrometer. . newman's self-registering tide-gauge. tables. page table of corrections, for capillary depression of the mercury in boiled and in unboiled barometer-tubes tables for deducing heights by means of the barometer:-- no. . approximate height due to barometric pressure no. . correction for mean temperature of air no. . correction due to latitude no. . correction due to approximate elevation tables for determining the temperature of the vapour of boiling water at any place:-- no. . factor due to latitude no. . temperature and tension table of temperature of the soil table of difference of elevation corresponding to a fall of ° in the boiling-point of water table showing proportion of salt for various boiling temperatures of sea-water table for finding the degree of humidity from observations with mason's hygrometer table showing amount and duration of rain at london, in table of average british rainfall in westerly, central, and easterly districts table showing force of wind, for use with lind's wind-gauge tables for correcting observations made with-- brass hydrometers glass hydrometers addenda. page . rule for converting millimetres into inches, et vice versa . old french lineal measure, with english equivalents . rule for finding diameter of bore of barometer tube . wind scales . letters to denote the state of the weather . table of expansion of bodies . table of specific gravity of bodies . important temperatures . table of meteorological elements, forming exponents of the climate of london . list of works on meteorology meteorological instruments. in the pursuits and investigations of the science of meteorology, which is essentially a science of observation and experiment, instruments are required for ascertaining, . the pressure of the atmosphere at any time or place; . the temperature of the air; . the absorption and radiation of the sun's heat by the earth's surface; . the humidity of the air; . the amount and duration of rainfall; . the direction, the horizontal pressure, and the velocity of winds; . the electric condition of the atmosphere, and the prevalence and activity of ozone. chapter i. instruments for ascertaining the atmospheric pressure. [illustration: fig. .] = . principle of the barometer.=--the first instrument which gave the exact measure of the pressure of the atmosphere was invented by torricelli, in . it is constructed as follows:--a glass tube, cd (fig. ), about inches long, and from two to four-tenths of an inch in diameter of bore, having one end closed, is filled with mercury. in a cup, b, a quantity of mercury is also poured. then, placing a finger securely over the open end, c, invert the tube vertically over the cup, and remove the finger when the end of the tube dips into the mercury. the mercury in the tube then partly falls out, but a column, ab, about inches in height, remains supported. this column is a weight of mercury, the pressure of which upon the surface of that in the cup is precisely equivalent to the corresponding pressure of the atmosphere which would be exerted in its place if the tube were removed. as the atmospheric pressure varies, the length of this mercurial column also changes. it is by no means constant in its height; in fact, it is very seldom stationary, but is constantly rising or falling through a certain extent of the tube, at the level of the sea, near which the above experiment is supposed to be performed. it is, therefore, an instrument by which the fluctuations taking place in the pressure of the atmosphere, arising from changes in its weight and elasticity, can be shown and measured. it has obtained the name _barometer_, or measurer of heaviness,--a word certainly not happily expressive of the utility of the invention. if the bore of the barometer tube be uniform throughout its length, and have its sectional area equal to a square inch, it is evident that the length of the column, which is supported by the pressure of the air, expresses the number of cubic inches of mercury which compose it. the weight of this mercury, therefore, represents the statical pressure of the atmosphere upon a square inch of surface. in england the annual mean height of the barometric column, reduced to the sea-level and to the temperature of ° fahrenheit, is about · inches. a cubic inch of mercury at this temperature has been ascertained to weigh · lbs. avoirdupois. hence, · × · = · lbs., is the mean value of the pressure of the atmosphere on each square inch of surface, near the sea-level, about the latitude of degrees. nearer the equator this mean pressure is somewhat greater; nearer the poles, somewhat less. for common practical calculations it is assumed to be lbs. on the square inch. when it became apparent that the movements of the barometric column furnished indications of the probable coming changes in the weather, an attempt was made to deduce from recorded observations the barometric height corresponding to the most notable characteristics of weather. it was found that for fine dry weather the mercury in the barometer at the sea-level generally stood above inches; changeable weather happened when it ranged from to inches, and when rainy or stormy weather occurred it was even lower. hence, it became the practice to place upon barometer scales words indicatory of the weather likely to accompany, or follow, the movements of the mercury; whence the instruments bearing them obtained the name "weather glasses." = . construction of barometers.=--in order that the instrument may be portable, it must be made a fixture and mounted on a support; and, further, to render it scientifically or even practically useful, many precautions are required in its construction. the following remarks apply to the construction of all barometers:--mercury is universally employed, because it is the heaviest of fluids, and therefore measures the atmospheric pressure by the shortest column. water barometers have been constructed, and they require to be at least feet long. oil, or other fluids, might be used. mercury, however, has other advantages: it has feeble volatility, and does not adhere to glass, if pure. oxidised, or otherwise impure mercury, may adhere to glass; moreover, such mercury would not have the density of the pure metal, and therefore the barometric column would be either greater or less than it should be. the mercury of commerce generally contains lead; sometimes traces of iron and sulphur. it is necessary, therefore, for the manufacturer to purify the mercury; and this is done by washing it with diluted acetic, or sulphuric acid, which dissolves the impurities. no better test can be found for ascertaining if the mercury be pure than that of filling a delicate thermometer tube; if, on exhausting the air from this thermometer, the mercury will freely run up and down the bore, which is probably one thousandth of an inch in diameter, the mercury from which this thermometer was made will be found fit for any purpose, and with it a tube may be filled and boiled, not only of one inch, but even of two inches diameter. in all barometers it is requisite that the space above the mercurial column should be completely void of air and aqueous vapour, because these gases, by virtue of their elasticity, would depress the column. to exclude these the mercury is introduced, and boiled in the tube, over a charcoal fire, kept up for the purpose. in this manner the air and vapour which adhere to the glass are expanded, and escape away. one can tell whether a barometer has been properly "boiled," as it is termed, by simply holding the tube in a slanting direction and allowing the mercury to strike the top. if the boiling has been well performed, the mercury will give a clear, metallic sound; if not, a dull, flat sound, showing some air to be present. when the mercury in a barometer tube rises or falls, the level of the mercury in the cup, or _cistern_, as it is generally termed, falls or rises by a proportionate quantity, which depends upon the relative areas of the interior of the tube and of the cistern. it is necessary that this should be taken into consideration in ascertaining the exact height of the column. if a fixed scale is applied to the tube, the correct height may be obtained by applying a correction for capacity. a certain height of the mercury is ascertained to be accurately measured by the scale, and should be marked on the instrument as the _neutral point_. above this point the heights measured are all less, and below, all more, than they should be. the ratio between the internal diameters of the tube and cistern (which should also be stated on the instrument, as, for instance, capac. / ) supplies the data for finding the correction to be applied. this correction is obviated by constructing the cistern so as to allow of the surface of the mercury in it being adjustable to the commencement of the fixed scale, as by fortin's or negretti's plan. it is also unnecessary in barometers constructed on what is now called the "kew method." these will all be detailed in their proper place. the tube, being fixed to the cistern, may have a moveable scale applied to it. but such an arrangement requires the utmost care and skill in observing, and is seldom seen except in first-class observatories. [illustration: fig. .] = . fortin's barometer.=--fortin's plan of constructing a barometer cistern is shown in fig. . the cistern is formed of a glass cylinder, which allows of the level of the mercury within being seen. the bottom of the cylinder is made of sheep-skin or leather, like a bag, so as to allow of being pushed up or lowered by means of a screw, d b, worked from beneath. this screw moves through the bottom of a brass cylinder, c c, which is fixed outside, and protects the glass cylinder containing the mercury. at the top of the interior of the cistern is fixed a small piece of ivory, a, the point of which exactly coincides with the zero of the scale. this screw and moveable cistern-bottom serve also to render the barometer portable, by confining the mercury in the tube, and preventing its coming into the cistern, which is thus made too small to receive it. . standard barometer. fig. represents a standard barometer on fortin's principle. the barometer tube is enclosed and protected by a tube of brass extending throughout its whole length; the upper portion of the brass tube has two longitudinal openings opposite each other; on one side of the front opening is the barometrical scale of english inches, divided to show, by means of a vernier, / th of an inch; on the opposite side is sometimes divided a scale of french millimetres, reading also by a vernier to / th of a millimetre (see directions for reading the vernier, page ). a thermometer, c, is attached to the frame, and divided to degrees, which can be read to tenths; it is necessary for ascertaining the temperature of the instrument, in order to correct the observed height of the barometer. [illustration: fig. .] as received by the observer, the barometer will consist of two parts, packed separately for safety in carriage,-- st, the barometer tube and cistern, filled with mercury, the brass tube, with its divided scale and thermometer; and nd, a mahogany board, with bracket at top, and brass ring with three adjusting screws at bottom. _directions for fixing the barometer._--in selecting a position for a barometer, care should be taken to place it so that the sun cannot shine upon it, and that it is not affected by direct heat from a fire. the cistern should be from two to three feet above the ground, which will give a height for observing convenient to most persons. a standard barometer should be compared with an observatory standard of acknowledged accuracy, to determine its index error; which, as such instruments are graduated by micrometrical apparatus of great exactitude, will be constant for all parts of the scale. it should be capable of turning on its axis by a movement of the hand, so that little difficulty can ever be experienced in obtaining a good light for observation. having determined upon the position in which to place the instrument, fix the mahogany board as nearly vertical as possible, and ascertain if the barometer is perfect and free from air, in the following manner:--lower the screw at the bottom of the cistern several turns, so that the mercury in the tube, when held upright, may fall two or three inches from the top; then slightly incline the instrument from the vertical position, and if the mercury in striking the top elicit a sharp tap, the instrument is perfect. supposing the barometer to be in perfect condition, as it is almost sure to be, it is next suspended on the brass bracket, its cistern passing through the ring at bottom, and allowed to find its vertical position, after which it is firmly clamped by means of the three thumb-screws. _to remove the instrument when fixed to another position._--if it should be necessary to remove the barometer,--first, by means of the adjusting screw, drive the mercury to the top of the tube, turning it gently when it is approaching the top, and cease directly any resistance is experienced; next, remove from the upper bracket or socket; lift the instrument and invert it, carrying it with its lower end upwards. _directions for taking an observation._--before making an observation, the mercury in the cistern must be raised or lowered by means of the thumb-screw, f, until the ivory point, e, and its reflected image in the mercury, d, are just in contact; the vernier is then moved by means of the milled head, until its lower termination just excludes the light from the top of the mercurial column; the reading is then taken by means of the scale on the limb and the vernier. the vernier should be made to read upward in all barometers, unless for a special object, as this arrangement admits of the most exact setting. in observing, the eye should be placed in a right line with the fore and back edges of the lower termination of the vernier; and this line should be made to form a tangent to the apex of the mercurial column. a small reflector placed behind the vernier and moving with it, so as to assist in throwing the light through the back slit of the brass frame on to the glass tube, is advantageous; and the observer's vision may be further assisted by the aid of a reading lens. the object is, in these standard barometers, to obtain an exact reading, which can only be done by having the eye, the fore part of the zero edge of the vernier, the top of the mercurial column, and the back of the vernier, in the same horizontal plane. _uniformity of calibre._--the diameter of that part of the tube through which the oscillations of the mercury will take place is very carefully examined to insure uniformity of calibre, and only those tubes are used which are as nearly as possible of the same diameter throughout. the size of the bore should be marked on the frame of the barometer in tenths and hundredths of an inch. a correction due to capillary action, and depending on the size of the tube, must be applied to the readings. = . correction due to capillarity.=--when an open tube of small bore is plunged into mercury, the fluid will not rise to the same level inside as it has outside. hence, the effect of capillary action is to depress the mercurial column; and the more so the smaller the tube. the following table gives the correction for tubes in ordinary use:-- diameter depression, in depression, in of tube. boiled tubes. unboiled tubes. inch. inch. inch. · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · this correction is always additive to the observed reading of the barometer. = . correction due to temperature.=--in all kinds of mercurial barometers attention must be given to the temperature of the mercury. as this metal expands and contracts very much for variations of temperature, its density alters correspondingly, and in consequence the height of the barometric column also varies. to ascertain the temperature of the mercury, a thermometer is placed near the tube, and is sometimes made to dip into the mercury in the cistern. the freezing point of water, °f., is the temperature to which all readings of barometers must be reduced, in order to make them fairly comparable. the reduction may be effected by calculation, but the practical method is by tables for the purpose; and for these tables we refer the reader to the works mentioned at the end of this book. = . correction due to height above the half-tide level.=--further, in order that barometrical observations generally may be made under similar circumstances, the readings, corrected for capacity, capillarity, and temperature, should be reduced to what they would be at the sea-level, by adding a correction corresponding to the height above the mean level of the sea, or of half-tide. for practical purposes of comparison with barometric pressure at other localities, add one-tenth of an inch to the reading for each hundred feet of elevation above the sea. for scientific accuracy this will not suffice, but a correction must be obtained by means of schuckburg's formula, or tables computed therefrom. = . the barometer vernier.=--the _vernier_, an invaluable contrivance for measuring small spaces, was invented by peter vernier, about the year . the barometer scale is divided into inches and tenths. the vernier enables us to accurately subdivide the tenths into hundredths, and, in first-class instruments, even to thousandths of an inch. it consists of a short scale made to pass along the graduated fixed scale by a sliding motion, or preferably by a rack-and-pinion motion, the vernier being fixed on the rack, which is moved by turning the milled head of the pinion. the principle of the vernier, to whatever instrumental scale applied, is that the divisions of the moveable scale are to those in an equal length of the fixed scale in the proportion of two numbers which differ from each other by unity. [illustration: fig. .] [illustration: fig. .] the scales of standard barometers are usually divided into half-tenths, or · , of an inch, as represented, in fig. , by ab. the vernier, cd, is made equal in length to twenty-four of these divisions, and divided into twenty-five equal parts; consequently one space on the scale is larger than one on the vernier by the twenty-fifth part of · , which is · inch, so that such a vernier shows differences of · inch. the vernier of the figure reading upwards, the lower edge, d, will denote the top of the barometer column; and is the zero of the vernier scale. in fig. , the zero being in line exactly with inches and five-tenths of the fixed scale, the barometer reading would be · inches. it will be seen that the vernier line, _a_, falls short of a division of the scale by, as we have explained, · inch; _b_, by · ; _c_, by · ; _d_, by · ; and the next line by one hundredth. if, then, the vernier be moved so as to make _a_ coincide with _z_, on the scale, it will have moved through · inch; and if on the vernier be moved into line with _y_ on the scale, the space measured will be · . hence, the figures , , , , on the vernier measure hundredths, and the intermediate lines even thousandths of an inch. in fig. , the zero of the vernier is intermediate · and · on the scale. passing the eye up the vernier and scale, the second line above is perceived to lie evenly with a line of the scale. this gives · and · to add to · , so that the actual reading is · inches. it may happen that no line on the vernier _accurately_ lies in the same straight line with one on the scale; in such a case a doubt will arise as to the selection of one from two equally coincident, and the intermediate thousandth of an inch should be taken. for the ordinary purposes of the barometer as a "weather-glass," such minute measurement is not required. hence, in household and marine barometers the scale need only be divided to tenths, and the vernier constructed to measure hundredths of an inch. this is done by making the vernier either or - ths of an inch long, and dividing it into ten equal parts. the lines above the zero line are then numbered from to ; sometimes the alternate divisions only are numbered, the intermediate numbers being very readily inferred. hence, if the first line of the vernier agrees with one on the scale, the next must be out one-tenth of a tenth, or · of an inch from agreement with the next _scale_ line; the following vernier line must be · out, and so on. consequently, when the vernier is set to the mercurial column, the difference shown by the vernier from the tenth on the scale is the hundredths to be added to the inches and tenths of the scale. a little practice will accustom a person to set and read any barometer quickly; an important matter where accuracy is required, as the heat of the body, or the hand, is very rapidly communicated to the instrument, and may vitiate, to some extent, the observation. . self-compensating standard barometer. this barometer has been suggested to messrs. negretti and zambra by wentworth erk, esq. it consists of a regular barometer; but attached to the vernier is a double rack worked with one pinion, so that in setting or adjusting the vernier in one position, the second rack moves in directly the opposite direction, carrying along with it a plug or plunger the exact size of the internal diameter of the tube dipping in the cistern, so that whatever the displacement that has taken place in the cistern, owing to the rise or fall of the mercury, it is exactly compensated by the plug being more or less immersed in the mercury, so that no capacity correction is required. a barometer on this principle is, however, no novelty, for at the royal society's room a very old instrument may be seen reading somewhat after the same manner. [illustration: fig. .] fig. is an illustration of the appearance of this instrument. the cistern is so constructed that the greatest amount of light is admitted to the surface of the mercury. . barometer with electrical adjustment. this barometer is useful to persons whose eyesight may be defective; and is capable of being read off to greater accuracy than ordinary barometers, as will be seen by the following description:--the barometer consists of an upright tube dipping into a cistern, so contrived, that an up-and-down movement, by means of a screw, can be imparted to it. in the top of the tube a piece of platina wire is hermetically sealed. the cistern also has a metallic connection, so that by means of covered copper wires (in the back of the frame) a circuit is established; another connection also exists by means of a metallic point dipping into the cistern. the circuit, however, can be cut off from this by means of a switch placed about midway up the frame; on one side of the tube is placed a scale of inches; a small circular vernier, divided into parts, is connected with the dipping point, and works at right angles with this scale. to set the instrument in action for taking an observation, a small battery is connected by means of two small binding screws at the bottom of the frame. the switch is turned upwards, thereby disconnecting the dipping point; the cistern is then screwed up, so that the mercury in the tube is brought into contact with the platina wire at the top; the instant this is effected the magnetic needle seen on the barometer will be deflected. the switch is now turned down; by so doing the connection with the upper wire or platina is cut off, and established instead only between the dipping point carrying the circular vernier and the bottom of the cistern; the point is now screwed by means of the milled head until the needle is again deflected. we may now be sure that the line on the circular vernier that cuts the division on the scale is the exact height of the barometer. although the description here given may seem somewhat lengthy, the operation itself is performed in less time than would be taken in reading off an ordinary instrument. . pediment barometers. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] these barometers, generally for household purposes, are illustrated by figs. to . they are intended chiefly for "weather glasses," and are manufactured to serve not only a useful, but an ornamental purpose as well. they are usually framed in wood, such as mahogany, rosewood, ebony, oak or walnut, and can be obtained either plain or handsomely and elaborately carved and embellished, in a variety of designs, so as to be suitable for private rooms, large halls, or public buildings. the scales to the barometer and its attached thermometer may be ivory, porcelain, or silvered metal. it is not desirable that the vernier should read nearer than one-hundredth of an inch. two verniers and scales may be fitted one on either side of the mercurial column, so that one can denote the last reading, and thus show at a glance the extent of rise or fall in the interval. the scale and thermometer should be covered with plate glass. a cheap instrument has an open face and plain frame, with sliding vernier instead of rack-and-pinion motion. the barometer may or may not have a moveable bottom to the cistern, with screw for the purpose of securing the mercury for portability. the cistern should not, however, require adjustment to a zero or fiducial point. it should be large enough to contain the mercury, which falls from to inches, without any appreciable error on the height read off on the scale. = . the words on the scale.=--the following words are usually engraved on the scales of these barometers, although they are not now considered of so much importance as formerly:-- at inches very dry. " · " settled fair. " " fair. " · " changeable. " " rain. " · " much rain. " " stormy. the french place upon their barometers a similar formula:-- at millimètres très-sec. " " beau-fixe. " " beau temps. " " variable. " " pluie ou vent. " " grande pluie. " " tempête. manufacturers of barometers have uniformly adopted these indications for all countries, without regard to the elevation above the sea, or the different geographical conditions; and as it can readily be shown that the height and variations of the barometer are dependent on these, it follows that barometers have furnished indications which, under many circumstances, have been completely false. even in this country, and near the sea-level, storms are frequent with the barometer not below ; rain is not uncommon with the glass at ; even fine weather sometimes occurs with a low pressure; while it is evident that at an elevation of a few thousand feet the mercury would never rise to inches; hence, according to the scale, there should never be fair weather there. if tempests happened as seldom in our latitude as the barometer gets down to inches, the maritime portion of the community at least would be happy indeed. these words have long been ridiculed by persons acquainted with the causes of the barometric fluctuations; nevertheless opticians continue to place them on the scales, evidently only because they appear to add to the importance of the instrument in the eyes of those who have not learned their general inutility. in different regions of the world, the indications of the barometer are modified by the conditions peculiar to the geographical position and elevation above the sea, and it is necessary to take account of these in any attempt to found rules of general utility in connection with the barometer as a weather guide. all that can be said in favour of these words is, that within a few hundred feet of the sea-level, when the column rises or falls gradually during two or three days towards "fair" or "rain," the indications they afford of the coming weather are generally extremely probable; but when the variations are quick, upward or downward, they presage unsettled or stormy weather. admiral fitzroy writes:--"the words on the scales of barometers should not be so much regarded, for weather indications, as the rising or falling of the mercury; for if it stands at _changeable_, and then rises a little towards _fair_, it presages a change of wind or weather, though not so great as if the mercury had risen higher; and, on the contrary, if the mercury stands above _fair_ and falls, it presages a change, though not to so great a degree as if it had stood lower; besides which, the direction and force of wind are not in any way noticed. it is not from the point at which the mercury stands that we are alone to form a judgment of the state of the weather, but from its _rising_ or _falling_; and from the movements of immediately preceding days as well as hours, keeping in mind effects of change of _direction_ and dryness, or moisture, as well as alteration of force or strength of wind."[ ] = . correction due to capacity of cistern.=--these barometers, having no adjustment for the zero of the scale, require a correction for the varying level of the mercury in the cistern, when the observations are required for strict comparison with other barometric observations, or when they are registered for scientific purposes; but for the common purpose of predicting the weather, this correction is unnecessary. the neutral point, and the ratio of the bore of the tube to the diameter of the cistern, must be known (see p. ). then the capacity correction, as it is termed, is found as follows:--take the fractional part, expressed by the capacity ratio, of the difference between the observed reading and the height of the neutral point; then, if the mercury stand _below_ the neutral point, _subtract_ this result from the reading; if it stand _above_, _add_ it to the reading. for example, suppose the neutral point to be · inches, and the capacity ratio / , required the correction when the barometer reads · . here · - · = · correction = · / = + · nearly. scale reading · ----- correct reading · ===== of course the correction could as easily be found to three decimal places, if desirable. it is evident that the correction is more important the greater the distance of the top of the mercury from the neutral point. . public barometers. since the increased attention paid to the signs of forthcoming weather of late years, and the good which has resulted therefrom to farmers, gardeners, civil engineers, miners, fishermen, and mariners generally, by forewarning of impending wet or stormy weather, the desirability of having good barometers exposed in public localities has become evident. barometers may now be seen attached to drinking fountains, properly protected, and are frequently consulted by the passers-by. but it is among those whose lives are endangered by sudden changes in the weather, fishermen especially, that the warning monitor is most urgently required. many poor fishing villages and towns have therefore been provided by the board of trade, at the public expense, and through the humane effort of admiral fitzroy, with first-class barometers, each fixed in a conspicuous position, so as to be easily accessible to all who desire to consult it. following this example, the royal national life boat institution has supplied each of its stations with a similar storm warner; the duke of northumberland and the british meteorological society have erected several on the coast of northumberland; and many other individuals have presented barometers to maritime places with which they are connected. these barometers have all been manufactured by messrs. negretti and zambra. the form given to the instrument seems well adapted for public purposes. [illustration: fig. .] = . fishery or sea-coast barometers.=--fig. gives a representation of these coast and fishery barometers. the frame is of solid oak, firmly screwed together. the scales are very legibly engraved on porcelain by negretti and zambra's patent process. the thermometer is large, and easily read; and as this instrument is exposed, it will indicate the actual temperature sufficiently for practical purposes. the barometer tube is three-tenths of an inch in diameter of bore, exhibiting a good column of mercury; and the cistern is of such capacity, in relation to the tube, that the change of height in the surface of the mercury in the cistern corresponding to a change of height of three inches of mercury in the tube, is less than one-hundredth of an inch, and therefore, as the readings are only to be made to this degree of accuracy, this small error is of no importance. the cistern is made of boxwood, which is sufficiently porous to allow the atmosphere to influence the mercurial column; but the top is plugged with porous cane, to admit of free and certain play. = . admiral fitzroy's scale words.=--the directions given on the scales of these barometers were drawn up by admiral fitzroy, f.r.s. they appear to be founded on the following considerations:-- supposing a compass diagram, with the principal points laid down, the n.e. is the wind for which the barometer stands highest; for the s.w. wind it is lowest. this is found to be so in the great majority of cases; but there are exceptions to this, as to all rules. the n.e. and s.w. may therefore be regarded as the poles of the winds, being opposite each other. when the wind veers from the s.w. through w. and n. to n.e., the barometer gradually rises; on the contrary, when the wind veers from n.e. and e. to s.e., s. and s.w., the mercury falls. a similar curious law exists in relation to the veering of the wind, and the action of the thermometer. as the wind veers from the s.w. to w. and n., the thermometer falls; as it veers from n.e. to e. and s., it rises, because the wind gets from a colder to a warmer quarter. the polar winds are cold, dry, and heavy. those from the equatorial regions are warm, moist, and comparatively light. these laws have been clearly developed and expressed by professor dové in his work on the "law of storms." the warm winds of europe are those which bring the greatest quantity of rain, as they blow from the ocean, and come heavily laden with moisture. the cold winds, besides containing less moisture, blow more from the land. the weight of the vapour of the warm winds tends to raise the barometric column; but, at the same time, the increased dilatation of the air tends to lower it. this latter influence being the stronger, the barometer always falls for these winds; and in regions where they traverse a large extent of land, retain their heat, and become necessarily very dry, the fall in the barometer will be greater. admiral fitzroy's words for the scales of barometers for use in northern latitudes, then, are as follows:-- _rise._ _fall._ for for n. ely. s. wly. nw.--n.--e. se.--s.--w. dry wet or or less more wind. wind. ------- ------- except except wet from wet from n. ed. n. ed. ------- ------- long foretold, long last; first rise after low, short notice, soon past. foretells stronger blow. it will be perceived that the exception in each case applies to n.e. winds. the barometer may fall with north-easterly winds, but they will be violent and accompanied with rain, hail, or snow; again, it will rise with these winds accompanied with rain, when they are light, and bring only little rain. it rises, however, highest with the dry and light n.e. winds. these directions are very practically useful; they provide for geographical position--also for elevation above the sea--since they are not appended to any particular height of the column. they are suited to the northern hemisphere generally, as well as around the british isles. the same directions are adapted to the southern hemisphere, by simply substituting for the letter n the letter s, reading south for north, and _vice versa_. south of the equator the cold winds come from the south; the warm, from the north. the s.e. wind in the southern hemisphere corresponds to the n.e. in the northern. the laws there are, while the wind veers from s.e. through e. to n. and n.w., the barometer falls and the thermometer rises. as the wind veers from n.w. through w. and s. to s.e., the barometer rises and the thermometer falls. = . instructions for the sea-coast barometer.=--the directions for fixing the barometer, and making it portable when it has to be removed, should be attended to carefully. the barometer should be suspended against a frame or piece of wood, so that light may be seen _through_ the tube. otherwise a piece of paper, or a _white place_, should be behind the upper or _scale part_ of the _tube_. when suspended on a hook, or stout nail, apply the milled-head key (which will be found just below the scales) to the square brass pin at the lower end of the instrument, and turn _gently_ toward the left hand till the screw stops; then take off the key and replace it for use, near the scale, as it was before. the cistern bottom being thus _let down_, the mercury will sink to its proper level quickly. in removing this barometer it is necessary to _slope it gradually_, till the mercury is at the top of the tube, and then, with the instrument reversed, to screw up the cistern bottom, or bag, by the key, used _gently_, till it stops. it will then be portable, and may be carried with the _cistern_ end _upwards_, or lying flat; but it must not be jarred, or receive a concussion. = . french sea-coast barometer.=--the french have imitated this form of barometer for coast service, and have translated admiral fitzroy's indications for the scale as follows:-- la la hausse baisse indique. indique. --------- --------- des vents de la des vents de la partie du partie du n.e. s.o. (du n.o. á l'e) (du s.e. á l'o.) (par le nord. ) (par le sud. ) de la de sÉcheresse. l'humiditÉ. --------- --------- un vent un vent plus faible plus fort exceptÉ s'il pleut exceptÉ s'il pleut avec de fortes brises avec de petites brises du n.e. du n.e. --------- --------- mouvements lents, le commencement temps durable. de la hausse, --------- après une grande mouvements rapides, baisse présage temps variable. un vent violent. marine barometers. = . the common form.=--the barometer is of great use to the mariner, who, by using it as a "weather glass," is enabled to foresee and prepare for sudden changes in the weather. for marine purposes, the lower portion of the glass tube of the barometer must be contracted to a fine bore, to prevent oscillation in the mercurial column, which would otherwise be occasioned by the movements of the ship. this tube is cemented to the cistern, which is made of boxwood, and has a moveable leathern bottom, for the purpose of rendering the instrument portable, by screwing up the mercury compactly in the tube. the tube is enclosed in a mahogany frame, which admits of a variety of style in shape, finish, and display, to meet the different fancies and means of purchasers. the frame is generally enlarged at the upper part to receive the scales and the attached thermometer, which are covered by plate glass. the cistern is encased in brass for protection, the bottom portion unscrewing to give access to the portable screw beneath the cistern. figs. and illustrate this form of barometer. marine barometers require to be suspended, so that they may remain in a vertical position under the changeable positions of a vessel at sea. to effect this they are suspended in gimbals by a brass arm. the gimbals consist of a loose ring fastened by thumb-screws to the middle part of the frame of the barometer, in front and back. the forked end of the arm supports this ring at the sides, also by the aid of thumb-screws. hence the superior weight of the cistern end is always sufficient to cause the instrument to move on its bearing screws, so as always to maintain a perpendicular position; in fact, it is so delicately held that it yields to the slightest disturbance in any direction. the other end of the arm is attached to a stout plate, having holes for screws, or fitted to slip into a staple or bracket, by which it may be fixed to any part of the cabin of a ship; the arm is hinged to the plate, for the purpose of turning the arm and barometer up whenever it is desirable. [illustration: fig. .] [illustration: fig. .] other forms of barometer (to be immediately described) have superseded this in the british marine, but the french still give the preference to the wooden frames. they think the barometer can be more securely mounted in wood, is more portable, and less liable to be broken by a sudden concussion than if fitted in a metal frame. the english deem the ordinary wooden barometers not sufficiently accurate, owing to the irregular expansion of wood, arising from its hygrometric properties. some of the english opticians have shown that very portable, and really accurate barometers can be made in brass frames, and therefore the preference is now given to this latter material. = . the kew marine barometer.=--the form of barometer so-called, is that recommended by the congress of brussels, held in , for the purpose of devising a systematic plan of promoting meteorological observations at sea. the materials employed in its construction are mercury, glass, iron, and brass. the upper part of the tube is carefully calibrated to ensure uniformity of bore, as this is a point upon which the accuracy of the instrument to some extent depends. at sea, the barometer has never been known to stand above inches, nor below . these extremes have been attained with instruments of undoubted accuracy, but they are quite exceptional. it is not necessary, therefore, to carry the scales of marine barometers beyond these limits, but they should not be made shorter. if the vernier is adjusted to read upward, the scale should extend to inches, to allow room for the vernier to be set to inches at least. cases have occurred in which this could not be done, and rare, but valuable observations have been lost in consequence. if the scale part of the tube be not uniform in bore, the index error will be irregular throughout the scale. whether the bore of the rest of the tube varies in diameter, is of no moment. from two to three inches below the measured part, the bore is contracted very much, to prevent the pulsations in the mercurial column--called "pumping"--which, otherwise, would occur at sea from the motion of the ship. in ordinary marine barometers, this contraction extends to the end of the tube. below the contracted part is inserted a pipette--or gay lussac air-trap--which is a little elongated funnel with the point downwards. its object is to arrest any air that may work in between the glass and the mercury. the bubble of air lodges at the shoulder, and can go up no farther. it is one of those simple contrivances which turn out remarkably useful. if any air gets into the tube, it does not get to the top, and therefore does not vitiate the performance of the barometer; for the mercury itself works up and down through the funnel. below this, the tube should not be unnecessarily contracted. [illustration: fig. .] the open end of the tube is fixed into an iron cylinder, which forms the cistern of the barometer. iron has no action upon mercury, and is therefore used instead of any other metal. one or two holes are made in the top of the cistern, which are covered on the inside with strong sheep-skin leather, so as to be impervious to mercury, but sufficiently porous for the outer air to act upon the column. the cistern is of capacity sufficient to receive the mercury which falls out of the tube until the column stands lower than the scale reads; and when the tube is completely full, there is enough mercury to cover the extremity so as to prevent access of air. there is no screw required for screwing up the mercury. the glass tube thus secured to the cistern is protected by a brass tubular frame, into which the iron cistern fits and screws compactly. cork is used to form bearings for the tube. a few inches above the cistern is placed the attached thermometer. its bulb is enclosed in the frame, so as to be equally affected by heat with the barometric column. the upper end of the frame is fitted with a cap which screws on, and embraces a glass shield which rests in a gallery formed on the frame below the scale, and serves to protect the silvered scale, as well as the inner tube, from dust and damp. a ring, moveable in a collar fixed on the frame above the centre of gravity of the instrument, is attached to gimbals, and the whole is supported by a brass arm in the usual manner; so that the instrument can be moved round its axis to bring any source of light upon it, and will remain vertical in all positions of the ship. the vernier reads to five-hundredths of an inch. no words are placed upon the scale, as the old formulary was deemed misleading. the vernier can be set with great exactness, as light is admitted to the top of the mercury by a front and a back slit in the frame. the lower edge of the vernier should be brought to the top of the mercury, so as just to shut out the light. it is evident that this form of barometer must be more reliable in its indications than those in wooden frames. the graduations can be accurately made, and they will be affected only by well-known alterations due to temperature. some think the tube is too firmly held, and therefore liable to be broken by concussion more readily than that of an inferior instrument. this, however, appears a necessary consequence of greater exactness. it is an exceedingly good portable instrument, and can be put up and taken down very readily. these barometers are preferred to marine barometers in wood, wherever they have been used. in merchant ships, and under careful treatment, they have been found very durable. they may be sent with safety by railway, packed carefully in a wooden box. _directions for packing._--in removing this barometer it is necessary to slope it gradually till the mercury reaches the top of the tube. it is then portable, if carried cistern end upwards or lying flat. if carried otherwise, it will very probably be broken by the jerking motion of the heavy mercury in the glass tube. of course it must not be jarred, or receive concussion. _position for marine barometer._--admiral fitzroy, to whose valuable papers we are much indebted, writes in his "barometer manual":--"it is desirable to place the barometer in such a position as not to be in danger of a side blow, and also sufficiently far from the deck above to allow for the spring of the metal arm in cases of sudden movements of the ship. "if there is risk of the instrument striking anywhere when the vessel is much heeled, it will be desirable to put some soft padding on that place, or to check movement in that direction by a light elastic cord; in fixing which, attention must be paid to have it acting only where risk of a blow begins, not interfering otherwise with the free swing of the instrument: a very light cord attached above, when possible, will be least likely to interfere injuriously." = . method of verifying marine and other barometers.=--"in nearly all the barometers which had been employed at sea till recently the index correction varied through the range of scale readings, in proportion to the difference of capacity between the cistern and the tube. to find the index correction for a land barometer, comparison with a standard, at any part of the scale at which the mercury may happen to be, is generally considered sufficient. to test the marine barometer is a work of much more time, since it is necessary to find the correction for scale readings at about each half inch throughout the range of atmospheric pressure to which it may be exposed; and it becomes necessary to have recourse to artificial means of changing the pressure of the atmosphere on the surface of the mercury in the cistern. "the barometers to be thus tested are placed, together with a standard, in an air-tight chamber, to which an air-pump is applied, so that, by partially exhausting the air, the standard can be made to read much lower than the lowest pressure to which marine barometers are likely to be exposed; and by compressing the air it can be made to read higher than the mercury ever stands at the level of the sea. the tube of the standard is contracted similarly to that of the marine barometer, but a provision is made for adjusting the mercury in its cistern to the zero point. glass windows are inserted in the upper part of the iron air-chamber, through which the scales of the barometers may be seen; but as the verniers cannot be moved in the usual way from outside the chamber, a provision is made for reading the height of the mercury independent of the verniers attached to the scales of the respective barometers. at a distance of some five or six feet from the air-tight chamber a vertical scale is fixed. the divisions on this scale correspond exactly with those on the tube of the standard barometer. a vernier and telescope are made to slide on the scale by means of a rack and pinion. the telescope has two horizontal wires, one fixed and the other moveable by a micrometer screw, so that the difference between the height of the column of mercury and the nearest division on the scale of the standard, and also of all the other barometers placed by the side of it for comparison, can be measured either with the vertical scale and vernier or the micrometer wire. the means are thus possessed of testing barometers for index error in any part of the scale, through the whole range of atmospheric pressure to which they are likely to be exposed; and the usual practice is to test them at every half inch from · to inches. "in this way barometers of various other descriptions have been tested, and some errors found to be so large that a few barometers read half an inch and upwards too high, while others read as much too low. in some cases those which were correct in one part of the scale were found to be from half an inch to an inch wrong in other parts. these barometers were of an old and ordinary, not to say inferior, construction. in some the mercury would not descend lower than about inches, owing to a fault very general in the construction of many common barometers till lately in frequent use:--the _cistern was not large enough_ to hold the mercury which descended from the tube in a _low atmospheric pressure_. "when used on shore, this contraction of the tube causes the marine barometer to be _sometimes_ a little behind an ordinary land barometer, the tube of which is not contracted. the amount varies according to the rate at which the mercury is rising or falling, and ranges from · to · of an inch. as the motion of the ship at sea causes the mercury to pass more rapidly through the contracted tube, the readings are almost the same there as they would be if the tube were not contracted, and in no case do they differ enough to be of importance in maritime use." the cistern of this marine barometer is generally made an inch and a quarter in diameter, and the scale part of the tube a quarter of an inch in bore. the inches on the scale, instead of being true, are shortened by · of an inch, in order to avoid the necessity of applying a correction due to the difference of capacity of the tube and cistern. this is done with much perfection, and the errors of the instruments, when compared with a standard by the apparatus used at kew and liverpool observatories, are determined to the thousandth of an inch, and are invariably very uniform and small. the error so determined includes the correction due to capillarity, capacity, and error of graduation, and forms a constant correction, so that only one variable correction, that due to temperature, need be applied, when the barometer is suspended near the water line of the ship, to make the observations comparable with others. with all the advantages of this barometer, however, it has recently been superseded, to some extent, because it was found to require more care than could ordinarily be expected to be given to it by the commander of a ship. seamen do not exactly understand the value of such nice accuracy as the thousandth part of an inch, but prefer an instrument that reads only to a hundredth part. . the fitzroy marine barometer. admiral fitzroy deemed it desirable to construct a form of barometer as practically useful as possible for marine purposes. one that should be less delicate in structure than the kew barometer, and not so finely graduated. one that could be set at a glance and read easily; that would be more likely to bear the common shocks unavoidable in a ship of war. accordingly, the admiral has devised a barometer, which he has thus described:-- "this marine barometer, for her majesty's service, is adapted to _general_ purposes. "it differs from barometers hitherto made in points of detail, rather than principle:-- . the glass tube is packed with vulcanised india-rubber, which checks vibration from concussion; but does not hold it rigidly, or prevent expansion. . it does not oscillate (or pump), though extremely sensitive. . the scale is porcelain, _very legible_, and not liable to change. . there is no iron anywhere (_to rust_). . every part can be unscrewed, examined, or cleaned, by any careful person. . there is a _spare_ tube, fixed in a cistern, filled with boiled mercury, and _marked_ for adjustment in this, or _any similar_ instrument. "these barometers are graduated to hundredths, and they will be found accurate to _that_ degree, namely the second decimal of an inch. "they are packed with vulcanised caoutchouc, in order that (by this, and by a peculiar strength of glass tube) guns may be fired near these instruments without causing injury to them by ordinary concussion. "it is hoped that all such instruments, for the public service at sea, will be quite similar, so that any spare tube will fit _any_ barometer. "_to shift a tube._--incline the barometer slowly, and then take it down, after allowing the mercury to fill the upper part. lay the instrument on a table, unscrew the outer cap at the joining just below the cistern swell, then unscrew the tube _and_ cistern, by turning the cistern gently, against the sun, or to _the left_, and draw out the tube very carefully _without bending it in the least_, _turning_ it a little, if required, as moved. then insert the new tube very cautiously, screw in, and adjust to the diamond-cut mark for inches. attach the cap, and suspend the barometer for use. "if the mercury does not immediately quit the top of the tube, tap the cistern end rather sharply. in a well-boiled tube, with a good vacuum, the mercury hangs, at times, so adhesively as to deceive, by causing a supposition of some defect. "in about ten minutes the mercurial column should be nearly right; but as local temperature affects the brass, as well as the mercury, slowly and unequally, it may be well to defer any _exact comparisons with other instruments_ for some few hours." messrs. negretti and zambra are the makers of these barometers for the royal navy. fig. is an illustration. [illustration: fig. .] the tube is fixed to a boxwood cistern, which is plugged with very porous cane at the top, to allow of the ready influence of a variation in atmospheric pressure upon the mercury. round the neck of the cistern is formed a brass ring, with a screw thread on its circumference. this screws into the frame, and a mark on the tube is to be adjusted to inches on the scale, the cistern covering screwed on, and the instrument is ready to suspend. the frame and all the fittings are brass, without any iron whatever; because the contact of the two metals produces a galvanic action, which is objectionable. the spare tube is fitted with india-rubber, and ready at any time to replace the one in the frame. the ease with which a tube can be replaced when broken is an excellent feature of the instrument. the spare tube is carefully stowed in a box, which can also receive the complete instrument when not in use. all the parts are made to a definite gauge; the frames are, therefore, all as nearly as possible similar to each other, and the tubes--like rifle bullets--are adjustible to any frame. if, then, the tube in use gets broken, the captain can replace it by the other; but, as it is securely packed with india-rubber, there is very little liability of its being broken by fair usage. every person who knows the importance of the barometer on board ship, will acknowledge that the supplementary tube is a decided improvement. many instruments of this description are afloat in the royal navy, and in a short time it may be expected that all the frames and tubes of barometers in the public service at sea will be similar in size and character; so that should a captain have the misfortune to get both his tubes broken, he would be able to borrow another from any ship he fell in with that had one to spare, which would be perfectly accurate, because it would have been verified before it was sent out. = . admiral fitzroy's words for the scale.=--the graduation of inches and decimals are placed in this barometer on the right-hand side of the tube; and on a similar piece of porcelain, on the left-hand, are engraved, as legibly as they are expressed succinctly, the following words, of universal application in the interpretation of the barometer movements:-- _rise_ _fall_ for for cold warm dry wet or or less more wind. wind. -------- -------- except except wet from wet from cooler side. cooler side. reverting to the explanation of the words on the "coast" barometers (at page ), and comparing and considering them as given for northern latitudes, and as they must be altered for southern latitudes, it will be perceived, that for all _cold_ winds the barometer rises; and falls for _warm_ winds. the mercury also falls for _increased_ strength of wind; and rises as the wind _lulls_. likewise before or with rain the column of mercury falls; but it rises with fine dry weather. putting these facts together, and substituting for the points of the compass the terms "cold" and "warm," the appropriateness of the words on the scale of this barometer is readily perceived. these concise and practical indications of the movements in the barometer are applicable for instruments intended for use in any region of the world, and are in perfect accordance with the laws of winds and weather deduced by dové and other meteorologists. there is nothing objectionable in them, and being founded upon experience and the deductions made from numerous recorded observations of the weather in all parts of the world, as well as confirmed by the theories of science, they may consequently be considered as generally reliable. they involve no conjecture, but express succinctly scientific principles. = . trials of the fitzroy marine barometer under fire of guns.=--some of the first barometers made by messrs. negretti and zambra on admiral fitzroy's principle were severely tried under the heaviest naval gun firing, on board h.m.s. _excellent_; and under all the circumstances, they withstood the concussion. the purpose of the trials was "to ascertain whether the _vulcanized india-rubber packing_ round the glass tube of a _new marine barometer_ did check the vibration caused by firing, and whether guns might be fired close to these instruments without causing injury to them." in the first and second series of experiments, a marine barometer on admiral fitzroy's plan was tried against a marine barometer on the kew principle, both instruments being new, and treated in all respects similarly. they were "hung over the gun, under the gun, and by the side of the gun, the latter both inside and outside a bulkhead,--in fact, in all ways that they would be tried in action with the bulkheads cleared away." the result was that the kew barometer was broken and rendered useless, while the new pattern barometer was not injured in the least. in a third series of experiments, mr. negretti being present, five of the new pattern barometers were subjected to the concussion produced by firing a -pounder gun with shot, and lbs. charge of powder. they were suspended from a beam immediately under the gun, then from a beam immediately over the gun, and finally they were suspended by the arm to a bulkhead, at a distance of only ft. in. from the axis of the gun; and the result was, according to the official report, "that all these barometers, however suspended, would stand, without the slightest injury, the most severe concussion that they would ever be likely to experience in any sea-going man-of-war." these trials were conducted under the superintendence of captain hewlett, c.b., and the guns were fired in the course of his _usual_ instructions. his reports to admiral fitzroy, giving all the particulars of the trials, are published in the "ninth number of meteorological papers," issued by the board of trade.[ ] . negretti and zambra's farmer's barometer and domestic weather-glass. it is a well-known fact that the barometer is as much, or even more affected by a change of wind as it is by rain; and the objection raised against a simple barometer reading, as leaving the observer in doubt whether to expect wind or rain, is removed by the addition of the hygrometer, an instrument indicating the comparative degree of dryness or dampness of the air;--a most important item in the determination of the coming weather. the farmer should not be content to let his crops lie at the mercy, so to speak, of the weather, when he has within his command instruments which may be the means of preventing damage to, and in cases total loss of, his crops. the farmer hitherto has had to depend for his prognostication of the weather on his own unassisted "weather wisdom;" and it is perfectly marvellous how expert he has become in its use. science now steps in, not to ignore this experience, but on the contrary, to give it most valuable assistance by extending it, and enabling it to predict, with an accuracy hitherto unknown, the various changes that take place in this most variable of climates. to the invalid, the importance of predicting with tolerable accuracy the changes that are likely to occur in the weather, cannot be over-rated. many colds would be prevented, if we could know that the morning so balmy and bright, would subside into a cold and cheerless afternoon. even to the robust, much inconvenience may be prevented by a due respect to the indications of the hygrometer and the barometer, and the delicate in health will do well to regard its warnings. [illustration: fig. .] _description of the instrument._--the farmer's barometer, as figured in the margin, consists of an upright tube of mercury inverted in a cistern of the same fluid; this is secured against a strong frame of wood, at the upper end of which is fixed the scale, divided into inches and tenths of an inch. on either side of the barometer, or centre tube, are two thermometers--that on the left hand has its bulb uncovered and freely exposed, and indicates the temperature of the air at the place of observation; that on the right hand has its bulb covered with a piece of muslin, from which depend a few threads of soft lamp cotton; this cotton is immersed in the small cup situated just under the thermometer, this vessel being full of water; the water rises by capillary attraction to the muslin-covered bulb, and keeps it in a constantly moist state. these two thermometers, which we distinguish by the names "wet bulb" and "dry bulb," form the hygrometer; and it is by the simultaneous reading of these two thermometers, and noting the difference that exists between their indications, that the humidity in the atmosphere is determined. admiral fitzroy's words (see p. ) are placed upon the scale of the barometer, as the value of a reading depends, not so much on the actual height of the mercury in the tube, as it does on whether the column is rising, steady, or falling. the moveable screw at the bottom of the cistern is for the purpose of forcing the mercury to the top of the tube when the instrument is being carried from place to place, and it must always be unscrewed to its utmost limit when the barometer is hung in its proper place. after this it should never be touched. the manner in which the hygrometer acts is as follows: it is a pretty well-known fact that water or wine is often cooled by a wet cloth being tied round the bottle, and then being placed in a current of air. the evaporation that takes place in the progressive drying of the cloth causes the temperature to fall considerably below that of the surrounding atmosphere, and the contents of the bottle are thus cooled. in the same manner, then, the covered wet bulb thermometer will be found _invariably_ to read lower than the uncovered one; and the greater the dryness of the air, the greater will be the difference between the indications of the two thermometers; and the more moisture that exists in the air, the more nearly they will read alike. the cup must be kept filled with pure water, and occasionally cleaned out, to remove any dirt. the muslin, or cotton-wick, should also be renewed every few weeks. the hygrometer may be had separate from the barometer, if the combined instruments cannot be sufficiently exposed to the external air, this being essential for the successful use of the hygrometer. this farmer's weather-glass, then, consists of three distinct instruments: the barometer, the thermometer, and the hygrometer. he has thus at command the three instrumental data necessary for the prediction of the weather. and now to describe-- _how to use the instrument._--the observations should be taken twice a day, say at a.m. and p.m.; and should be entered on a slip of paper, or a slate hung up by the barometer. the observer will then be able to see the different values of the readings from time to time, and to draw his conclusions therefrom. the thermometer on the left hand should first be read, and a note made of its indication, which is the temperature of the air. the wet bulb thermometer should now be read, and also noted; and the difference should be taken of these two readings. next read the barometer by moving the small index at the side of the tube until it is on a level with the top of the mercury. having noted the number of inches at which the column stands, compare with the last observation, and see immediately whether the barometer is rising, steady, or falling. now, having taken the observations as above, we naturally ask the question, _what are we to predict from them?_ and, probably, the best way of answering this query will be by giving an example. we will suppose that our readings yesterday were as follows:--temperature, °; wet bulb, °; difference, °; =very moist air. barometer, · , and that rain has fallen. to-day, we read:--temperature, °; wet bulb, °; difference, °; =dryer air. barometer, . we may safely predict that the rain will cease, and probably we may have wind from the northward. in spring or autumn, if the barometric height be steady any where between · and inches, with the temperature about °, fresh to moderate south-westerly winds, with cloudy sky, will probably characterize the weather; the indications of the hygrometer being then specially serviceable in enabling us to foretell rain; but if the mercury become steady at about · inches, with temperature about °, north-easterly winds, dry air, and clear sky, may be confidently expected. many cases will doubtless suggest themselves to the observer where these figures do not occur, and where he might find a difficulty in interpreting the indications of his instruments. we have, therefore, drawn up some concise rules for his guidance; and although they will not prove absolutely infallible guides to this acknowledged most difficult problem, still, they will be found of much service in foretelling the weather, when added to an intelligent observation of ordinary atmospheric phenomena, as force and direction of wind, nature of any particular season, and the time of year. . rules for foretelling the weather. a rising barometer. a "rapid" rise indicates unsettled weather. a "gradual" rise indicates settled weather. a "rise," with dry air, and cold increasing in summer, indicates wind from northward; and if rain has fallen, better weather is to be expected. a "rise," with moist air and a low temperature, indicates wind and rain from northward. a "rise," with southerly wind, indicates fine weather. a steady barometer, with dry air and a seasonable temperature, indicates a continuance of very fine weather. a falling barometer. a "rapid" fall indicates stormy weather. a "rapid" fall, with westerly wind, indicates stormy weather from northward. a "fall," with a northerly wind, indicates storm, with rain and hail in summer, and snow in winter. a "fall," with increased moisture in the air, and the heat increasing, indicates wind and rain from southward. a "fall," with dry air, and cold increasing (in winter), indicates snow. a "fall," after very calm and warm weather, indicates rain with squally weather. = . causes which may bring about a fall or a rise in the barometer.=[ ]--as heat produces rarefaction, a sudden rise of temperature in a distant quarter may affect the weight of the atmosphere over our heads, by producing an aerial current outwards, to supply the place of the lighter air which has moved from its former position; in which case the barometer will fall. now such a movement in the atmosphere is likely to bring about an intermixture of currents of air of different temperatures, and from this intermixture rain is likely to result. on the other hand, as cold produces condensation, any sudden fall of temperature causes the column of air over the locality to contract and sink to a lower level, whilst other air rushes in from above to supply the void; and, accordingly, the barometer rises. should this air, as often happens, proceed from the north, it will contain in general but little moisture; and hence, on reaching a warmer latitude, will take up the vapour of the air, so that dry weather will result. it is generally observed, that wind causes a fall in the instrument; and, indeed, in those greater movements of the atmosphere which we denominate storms or hurricanes, the depression is so considerable as to forewarn the navigator of his impending danger. it is evident, that a draught of air in any direction must diminish the weight of the column overhead, and consequently cause the mercury in the barometer to sink. the connection, therefore, of a sinking of the barometric column with rain is frequently owing to the wind causing an intermixture of the aerial currents which, by their motion, diminish the weight of the atmosphere over our heads; whilst a steady rise in the column indicates the absence of any great atmospheric changes in the neighbourhood, and a general exemption from those causes which are apt to bring about a precipitation of vapour. = . use of the barometer in the management of mines.=--the inflammable and suffocating gases, known to coal-miners as fire-damp and choke-damp, are specifically heavier than air; and as they issue from the fissures of the mine, or are released from the coal, the atmospheric pressure tends to drive them into the lowest and least ventilated galleries. consequently a greatly reduced atmospheric pressure will favour a sudden outflow or advance of gas; whence may result cases of explosion or suffocation. it has been found that these accidents occur for the most part about the time of a low barometric column. a reliable barometer should, therefore, be systematically consulted by those entrusted with the management or control of coal-mines, so that greater vigilance and caution may be enjoined on the miners whenever the mercury falls low, especially after it has been unusually high for some days. = . use of the barometer in estimating the height of tides.=--the pressure of the atmosphere affects the height of the tide, the water being in general higher as the barometer is lower. the expressions of seamen, that "frost nips the tide," and "fog nips the tide," are explained by the high barometer which usually accompanies frost and fog. m. daussy, sir j. c. ross, and others, have established that a rise of one inch in the barometer will have a corresponding fall in the tide of about one foot. therefore navigators and pilots will appreciate the following suggestion of admiral fitzroy:-- "vessels sometimes enter docks, or even harbours, where they have scarcely a foot of water more than their draught; and as docking, as well as launching large ships, requires a close calculation of height of water, the state of the barometer becomes of additional importance on such occasions." chapter ii. syphon tube barometers. = . principle of.=--if some mercury, or any other fluid, be poured into a tube of glass, bent in the form of =u=, and open at both ends, it will rise to the same height in both limbs, the tube being held vertically. if mercury be poured in first, and then water upon it at one end, these liquids will not come to the same level; the water will stand much higher than the mercury. if the height of the mercury, above the line of meeting of the fluids, be one inch, that of the water will be about thirteen-and-a-half inches. the explanation of this is, that the two columns balance each other. the pressure of the atmosphere in each limb is precisely similar; but the one column stands so much higher than the other, because the fluid of which it is composed is so much lighter, bulk for bulk, than the other. if one end of the tube be hermetically closed, the other limb be cut off within a few inches of the bend, and the tube carefully filled with mercury; by placing it in a vertical position, the mercury will fall, if the closed limb be long enough, until it is about thirty inches higher than that in the exposed limb, where it will remain. here the atmosphere presses upon the short column; but not upon the long one. it is this pressure, therefore, which maintains the difference of level. in fact, it forms a barometer without a cistern, the short limb answering the purpose of a cistern. the first barometers on this principle were devised by the celebrated philosopher, dr. hook, as described in the next section. . dial, or wheel barometers. the familiar household "weather glasses" are barometers on the syphon principle. the portions of the two limbs through which the mercury will rise and fall with the varying pressure of the atmosphere are made of precisely the same diameter; while the part between them is contracted. on the mercury, in the exposed limb, rests a round float of ivory or glass; to this a string is attached and passed over and around a brass pulley, the other end carrying another lighter weight. the weight resting on the mercury rises and falls with it. on the spindle of the pulley, which passes through the frame and centre of the dial-plate, is fixed a light steel hand, which revolves as the pulley turns round. when the mercury falls for a decrease of atmospheric pressure, it rises by the same quantity in the short tube, and pushes up the float, the counterpoise falls, and thus moves the hand or pointer to the left. when the pressure increases, the pointer is drawn in a similar manner to the right. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] the dials are generally made of metal silvered over or enamelled, but porcelain may be used. if the circumference of the pulley, or "wheel," be two inches, it will revolve once for an alteration of level amounting to two inches in each tube, or four inches in the height of the barometric column; and as the dial may be from twenty to thirty-six inches in circumference, five to nine inches on the graduated scale corresponds to one inch of the column; and hence the sub-divisions are distinctly perceptible, and a vernier is not necessary. the motion of the pointer alone is visible; and a mahogany, or rosewood, frame, supports, covers, and renders the instrument ornamental and portable. in the back of the frame is a hinged door, which covers the cavity containing the tube and fixtures. the dial is covered by a glass in a brass rim, similar to a clock face. a brass index, working over the dial, moveable by a key or button, may be applied, and will serve to register the position of the hand when last observed. these instruments are usually fitted with a thermometer, and a spirit level; the latter for the purpose of getting the instrument perfectly vertical. they sometimes have, in addition, a hygrometer, a sympiesometer, an aneroid, a mirror, or a clock, &c., singly or combined. the frame admits of much variety of style and decoration. it may be carved or inlaid. the usual adjustment of scale is suited for localities at no considerable elevation above the sea. accordingly, being commercial articles, they have been found frequently quite out of place. when intended for use at high elevations, they should have a special adjustment of scale. as household instruments they are serviceable, and ornamental. but the supply-and-demand principle upon which they are sold, has entailed upon those issued by inferior makers a generally bad adjustment of scale. the illustrations are those of ordinary designs. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] dial barometers required for transmission to distant parts, as india and the colonies, are furnished with a steel stop-cock, to render them portable more effectually than can be done by the method of _plugging_ the tube. . standard syphon barometer. fig. represents the most accurate form of the gay lussac barometer. the short limb is closed at the top, after the mercury is introduced, and a small lateral puncture is made at _a_, which is covered over with a substance which permits the access of air, but prevents the escape of any mercury when the instrument is packed for travelling. the bent part of the tube is contracted to a capillary bore; and just above this, in the long limb, is placed the air-trap, already described (see p. ), and here illustrated (fig. ). when reversed, as it must be for portability, the capillary attraction keeps the mercury in the long branch. should the mercury of the short column get detached, some small quantity of air _may_ pass; but it will be arrested at the pipette, and will not vitiate the length of the barometric column. it can be easily expelled by gently shaking or tapping the instrument before suspending it for observation. in the illustration, the zero of the scale is placed at z, near the middle of the tube; and the graduations extend above and below. in making an observation, it is necessary to take the reading za on the long branch, and zb on the short one. the sum of the two gives the height of the barometer. the zero of the scale in some instruments is placed low down, so as to require the difference of the two readings to be taken. a thermometer is attached to the frame as usual. [illustration: fig. .] [illustration: fig. .] these instruments can be very accurately graduated, and are very exact in their indications, provided great care has been exercised in selecting the tubes, which must be of the same calibre throughout the parts destined to measure the variations of atmospheric pressure. they should be suspended so as to insure their hanging vertically. the syphon barometer does not require correction for capillarity nor for capacity, as each surface of the mercury is equally depressed by capillary attraction, and the quantity of mercury which falls from the long limb of the tube occupies the same length in the short one. the barometric height must, however, be corrected for temperature, as in the cistern barometer. tables containing the temperature corrections to be applied to barometer readings for scales engraved on the glass tube, or on brass or wood frames, are published. chapter iii. barographs, or self-registering barometers. = . milne's self-registering barometer.=--for a long time a good and accurate self-recording barometer was much desired. this want is now satisfactorily supplied, not by one, but by several descriptions of apparatus. the one first to be described was the design of admiral sir a. milne, who himself constructed, in , we believe, the original instrument, which he used with much success. since that time several of these instruments have been made, and have performed satisfactorily. the barometer tube is a syphon of large calibre, provided with a gay lussac pipette, or air-trap; and fitted with a float, a wheel, and a pointer, as in the "dial" barometer. the float is attached to a delicate watch-chain, which passes over the wheel and is adequately counterpoised. behind the indicating extremity of the pointer or hand is a projecting point, which faces the frame of the instrument, and is just within contact with the registering paper. a clock is applied, and fitted with auxiliary mechanism, so as to be able to move the mounted paper with regularity behind the pointer, and at designed equal intervals of time to release a system of levers and springs, so as to cause the marker to impress a dot on the paper, either by puncture or pencil-mark. the paper is ruled with horizontal lines for the range of the mercurial column, and parallel arcs of circles for the hours. thus the barometer is rendered self-recording, by night or day, for a week or more; hence the great value of the instrument. the clock, index, and registering mechanism are protected from dust and interference by a glass front, hinged on and locked. as the temperature of the mercury is not registered, there is fixed to the frame a sixe's thermometer to record the maximum and minimum temperatures, which should be noted at least every twenty-four hours. admiral fitzroy has suggested the name "atmoscope" for admiral milne's barometer; and he has also termed it a "barograph." this latter word appears to be applicable to all kinds of self-registering barometers hitherto designed. of the arrangement under consideration admiral fitzroy writes:--"it shows the alterations in tension, or the pulsations, so to speak, of atmosphere, on a large scale, by hourly marks; and the diagram expresses, to a practised observer, what the 'indicator card' of a steam-cylinder shows to a skilful engineer, or a stethescope to a physician." [illustration: fig. .] = . modification of milne's barometer.=--the great difficulty to be overcome in milne's barometer, is to adjust the mechanism for obtaining registration so that the action of the striker upon the pointer should not in the slightest degree move it from its true position. a different mode of registration, capable of recording accurately the least appreciable movement of the mercurial column, has been effected. in this instrument the registering paper is carried upon a cylinder or drum. by reference to the illustration, fig. , the details of construction will be readily understood. it should, however, be mentioned, that it is not a picture of the outward appearance of the instrument. the position of the barometer should be behind the clock; it is represented on one side merely for the purpose of clearly illustrating the arrangement and principles. the instrument has a large syphon barometer tube, in which the mercurial column is represented. on the mercury at _a_, in its open end, rests a glass float, attached to a watch-chain, or suitable silken cord, the other end of which is connected to the top of the arched head on the short arm of a lever-beam. the long arm of the beam is twice the length of the short arm, for the following reason. as the mercury falls in the long limb, it rises through an equal space in the short limb of the tube, and _vice versa_. but the barometric column is the difference of height of the mercury in the two limbs; hence the rise or fall of the float through half-an-inch will correspond to a decrease or an increase of the barometric column of one inch. in order, then, to record the movements of the barometric column, and not those of the float, the arm of the beam connected with the float is only half the radius of the other arm. both arms of the beam carry circular-arched heads, which are similar portions of the complete circles, the centre of curvature being the fulcrum, or axis. this contrivance maintains the leverage on each extremity of the beam always at the same distance from the fulcrum. from the top of the large arched head a piece of watch-chain descends, and is attached to the marker, _b_, which properly counterpoises the float, _a_, and is capable of easy movement along a groove in a brass bar, so as to indicate the barometric height on an ivory scale, _c_, fixed on the same vertical framing. on the opposite side of the marker, _b_, is formed a metallic point, which faces the registration sheet and is nearly in contact with it. the framing, which carries the scale and marker, is an arrangement of brass bars, delicately adjusted and controlled by springs, so as to permit of a quick horizontal motion, in a small arc, being communicated to it by the action of the hammer, _e_, of the clock, whereby the point of the marker is caused to impress a dot upon the paper. the same clock gives rotation to the hollow wooden cylinder, _d_, upon which is mounted the registering paper. the clock must be rewound when a fresh paper is attached to the cylinder, which may be daily, weekly, or monthly, according to construction; and the series of dots impressed upon the paper shows the height of the barometric column every hour by day and night. the space traversed by the marker is precisely equal to the range of the barometric column. = . king's self-registering barometer.=--mr. alfred king, engineer of the liverpool gas-light company, designed, so long ago as , a barometer to register, by a continuous pencil-tracing, the variations in the weight of the atmosphere; and a highly-satisfactory self-recording barometer, on his principle and constructed under his immediate superintendence, has quite recently been erected at the liverpool observatory. [illustration: fig. .] fig. is the front elevation of this instrument. _a_, the barometer tube, is three inches in internal diameter, and it floats freely (not being fixed as usual) in the fixed cistern, _b_, guided by friction-wheels, _w_. the top end of the tube is fastened to a peculiar chain, which passes over a grooved wheel turning on finely-adjusted friction rollers. the other end of the chain supports the frame, _d_, which carries the tracing pencil. the frame is suitably weighted and guided, and faces the cylinder, _c_, around which the tracing paper is wrapped, and which rotates once in twenty-four hours by the movement of a clock. mr. hartnup, director of the liverpool observatory, in his annual report, , says:--"for one inch change in the mercurial column the pencil is moved through five inches, so that the horizontal lines on the tracing, which are half an inch apart, represent one-tenth of an inch change in the barometer. the vertical lines are hour lines, and being nearly three-quarters of an inch apart, it will be seen that the smallest appreciable change in the barometer, and the time of its occurrence, are recorded." "it has been remarked by persons in the habit of reading barometers with large tubes, that, in squally weather, sudden and frequent oscillations of the mercurial column are sometimes seen. now, to register these small oscillations must be a very delicate test of the sensitiveness of a self-registering barometer, as the time occupied by the rise and fall of the mercury in the tube in some cases does not exceed one minute." mr. hartnup affirms that the tracing of this instrument exhibits such oscillations whenever the wind blows strong and in squalls. as the barometer in this instrument is precisely similar to the "long range barometer" invented by mr. mcneild (and which will be found described at page ), it may be desirable to quote the following, from mr. hartnup's report:--"mr. king constructed a small model instrument to illustrate the principle. this instrument was entrusted to my care for examination, and it was exhibited to the scientific gentlemen who visited the observatory in , during the meeting of the british association for the advancement of science." = . syphon, with photographic registration.=--a continuous self-registering barometer has been constructed, in which photography is employed. those who may wish to adopt a similar apparatus, or thoroughly to understand the arrangements and mode of observation, should consult the detailed description given in the _greenwich magnetical and meteorological observations_, . as the principles are applicable to photographic registration of magnetic and electric as well as meteorologic variations in instrumental indications, it would be beside our purpose to describe fully the apparatus. the barometer is a large syphon tube; the bore of the upper and lower extremities, through which the surfaces of the mercury rise and fall, is - / inch in diameter. the glass float in the open limb is attached to a wire, which moves a delicately-supported light lever as it alters its elevation. the fulcrum of the lever is on one side of the wire; the extremity on the other side, at four times this distance from the fulcrum, carries a vertical plate of opaque mica, having a small aperture. through this hole the light of a gas-jet shines upon photographic paper wrapped round a cylinder placed vertically, and moved round its axis by a clock fixed with its face horizontal. the cylinder is delicately supported, and revolves in friction rollers. a bent wire on the axis is embraced by a prong on the hour hand of the time-piece; therefore the cylinder is carried round once in twelve hours. it might be arranged for a different period of rotation. as the cylinder rotates, the paper receives the action of the light, and a photographic trace is left of the movements of the barometer four times the extent of the oscillations of the float, or twice the length of the variations in the barometric column. certain chemical processes are required in the preparation of the paper, and in developing the trace. the diagram which we give on the next page, with the explanation, taken from drew's _practical meteorology_, will enable the above description to be better understood: [illustration: fig. .] "_q e_ is a lever whose fulcrum is _e_, the counterpoise _f_ nearly supporting it; _s_ is an opaque plate of mica, with a small aperture at _p_, through which the light passes, having before been refracted by a cylindrical lens into a long ray, the portion only of which opposite the aperture _p_ impinges on the paper; _d_ is a wire supported by a float on the surface of the mercury; _g h_ is the barometer; _p_, the vertical cylinder charged with photographic paper; _r_, the photographic trace; _i_, the timepiece, carrying round the cylinder by the projecting arm _t_. it is evident that the respective distances of the float and the aperture _p_ from the fulcrum may be regulated so that the rise and fall of the float may be multiplied to any extent required." when _only_ the lower surface of the mercury in a syphon barometer is read, as in the instrument just described, a correction for temperature is strictly due to the height of the quicksilver in the _short_ tube; but this in so short a column will rarely be sensible. chapter iv. mountain barometers. = . the syphon tube mountain barometer, on gay lussac's principle=, constructed as described at page , and fixed in a metallic tubular frame, forms a simple and light travelling instrument. the graduations are made upon the frame, and it is suspended for reading by a ring at the top, from beneath an iron tripod stand, which is usually supplied with it. considerable care is requisite in adjusting the verniers, so as to keep the instrument steady and vertical. a drawback to the convenience of this barometer is the movement of the mercury in the short limb, which is generally not confined, and hence has every facility for becoming quickly oxidised in travelling. to remedy this, messrs. negretti and zambra so construct the mountain syphon barometer that by a simple half turn of a screw the mercury can be confined for portability, while the lower limb can be taken out for cleaning whenever found requisite. = . mountain barometer on fortin's principle.=--this barometer, with fortin's cistern, as arranged by messrs. negretti and zambra, is an elegant, manageable, and very accurate instrument for travelling purposes, and well adapted for careful measurement of heights. the cistern is made large enough to receive all the mercury that will fall from the tube at the highest attainable elevation. the screw at the bottom confines the mercury securely for carriage, and serves to adjust the surface of the mercury to the zero of the scale when making an observation. the vernier reads to · of an inch, and slides easily on the brass frame, which is made as small in diameter as is compatible with the size of the tube. the tube in this barometer should be altogether without contractions, so that the mercury will readily fall when it is set up for observation. it must be carefully calibrated, and its internal diameter ascertained, in order that correction may be made for capillarity. this correction, however, should be combined with the error of graduation, and form a permanent index error, ascertainable at any time by comparison with an acknowledged standard barometer. the barometer is supported in the tripod stand (furnished as part of the instrument) when used for observation. it is suspended by placing two studs, in the ring on the frame, in slots formed on the top of the stand, so that it hangs freely and vertically in gimbals. to the metal top of the stand, mahogany legs are hinged. to make the barometer portable, it must be lifted out of the stand, sloped gently until the mercury reaches the top, turning the screw at the bottom meanwhile; then invert and screw until the mercury is made tight. the inverted instrument packs in the stand, the legs being formed to fit round the frame; and receptacles are scooped out for the cistern, thermometer, gimbals, and vernier; so that the instrument is firmly surrounded by the wooden legs, which are held fast together by brass rings passed over them. [illustration: fig. .] = . newman's mountain barometer.=--fig. is an illustration of the mountain barometer known as newman's. the cistern consists of two separate compartments;--the top of the lower and the bottom of the upper, being perfectly flat, are pivoted closely together at the centres, so that the lower can move through a small arc, when turned by the hand. this movement is limited by two stops. the top of the lower compartment and the bottom of the upper have each a circular hole, through which the mercury communicates. when the instrument is required for observation, the cistern is turned close up to the stop marked "_open_" or "_not portable_." when it is necessary to pack it for travelling, the mercurial column must be allowed to fill the tube by sloping the barometer gently; then invert it, and move the cistern to the stop marked "_shut_" or "_portable_." in this condition, the upper compartment is completely filled with mercury, and consequently that in the tube cannot move about, so as to admit air or endanger the tube. nor can the mercury pass back to the lower compartment, as the holes are not now coincident, and the contact is made too perfect to allow the mercury to creep between the surfaces. the tube does not enter the lower compartment, which is completely full of mercury when the instrument is arranged for observation. the spare capacity of the upper cistern is sufficient to receive the mercury which descends from the tube to the limit of the engraved scale, which in these barometers generally extends only to about inches. a lower limit could of course be given by increasing the size of the cisterns, which it is not advisable to do unless for a special purpose. this barometer may be had mounted in wood, or in brass frame. if in wood, it has a brass shield, which slides round the scale part of the frame, so as to be easily brought in front of the tube and scale as a protection in travelling; the vernier screw, in this case, being placed at the top of the instrument. when the scale is graduated with true inches, the neutral point, the capacity and capillarity corrections should be marked on the frame. the graduated scales, however, placed on these barometers in brass frames, are usually artificial inches, like the kew plan of graduation; the advantage being that one simple correction only is required, viz. one for index error and capillarity combined, which can always be readily determined by comparison with a standard barometer; moreover, as no adjustment of cistern is required in reading, the instrument can be verified by artificial pressure throughout the scale, by the plan practised at kew, liverpool, &c., and already described (see p. ). . negretti & zambra's patent mountain and other barometers. this invention is intended to make mountain and other barometers of standard accuracy stronger, more portable, and less liable to derangement, when being carried about, than heretofore, by dispensing with the ordinary flexible cistern containing the mercury at the bottom of the instrument, and adapting in lieu thereof a rigid cistern constructed of glass and iron. the cistern is composed of a glass cylinder, which is secured in a metallic tube or frame. in order to render the cistern mercury-tight at top and bottom, metal caps are screwed into the tube or frame, and bear against leather washers placed between them and the edges of the glass cylinder. the upper cap of the cistern is tapped with a fine threaded screw to receive the iron plug or socket, into which the barometer tube is securely fixed. the whole length of this plug has a fine screw cut upon it by which the cistern can be screwed up or down. at the side of this plug or socket, extending from the lower end to within half an inch of the top, is cut a groove for admitting the air to the surface of the mercury within the cistern when the barometer is in use. an ivory point is screwed into the under surface of the plug, carrying the barometer tube. this ivory point is very carefully adjusted by measurement to be the zero point of the instrument, from which the barometer scale of inches is divided. the surface of the mercury in the cistern is adjusted to the zero point by screwing the cistern up or down until the ivory point and its reflected image are in contact. [illustration: fig. .] the instrument (fig. ) is shown in a state of adjustment, ready to take an observation; but _when it is desired to render it portable, it must be inclined, until mercury from the cistern fills the tube; the cistern must then be screwed up on the socket_, so as to bring the face of the upper cap against the under side of the shoulder of the cover immediately above it; the instrument may then be carried without being liable to derangement. _precautions necessary in using the mountain barometer._--on removing the barometer from its case after a journey, allow it to remain with its scale end downward, whilst the cistern is unscrewed to the extent of _one turn of the screw_, after which slightly shake the cistern; the mercury in it will then completely fill the end of the barometer tube, should any portion of it have escaped therefrom. the barometer is then inverted, and if it be desired to make an observation, suspend it vertically from its stand by the ring at top. the cistern must then be unscrewed, until the surface of the mercury is brought just level with the extreme end of the ivory or zero point fixed to the iron plug on which the glass cistern moves up and down. should the elevation of the place where the barometer is to be used be considerably above the sea level, it will be well--after suspending it from the stand--to unscrew the cistern several turns, _holding the barometer in an oblique position_, as at great heights the mercury will fall considerably quicker than the cistern can be unscrewed, thereby filling it to overflowing; but by partly unscrewing the cistern first, room is given for the reception of a fall of mercury to the extent of several inches. the cistern must not be unscrewed when the _instrument is_ inverted _more than_ two turns of the screw, otherwise the mercury will flow out through the groove. it is found safer when travelling to carry the barometer in a horizontal position, or with its cistern end uppermost. _to clean the barometer._--should at any time the mercury in the cistern become oxidised, and reading from its surface be difficult, it can be readily cleaned by removing the cistern and its contained mercury from the barometer frame by unscrewing it _when in a horizontal position_; this precaution is necessary that the mercury in the tube may not escape, and thereby allow air to enter. the cistern must then be emptied, and with a dry clean leather, or silk handkerchief, well cleaned. the operation of cleaning being performed, return the cistern to the frame, and screw it until the face is brought up against the under side of the shoulder, still keeping the instrument _horizontal_. the cistern is now ready for re-filling, to do which stand the barometer on end _head downwards_, and remove the small screw at bottom; through the aperture thus opened, pour in mercury, passing it through a paper funnel with a very small aperture. it is well to pass the mercury through a very small funnel two or three times before returning it to the barometer cistern, as by this process all particles of dust or oxide adhere to the paper, and are effectually removed. should any small quantity of the mercury be lost during the operation of cleaning, it is of no importance so long as sufficient remains to allow of adjustment to the zero point. this latter constitutes one of the great advantages of this new instrument over the ordinary barometer; for, in the majority of cases, after an instrument has been compared carefully with a standard, should mercury be lost, there is no means of correcting the error unless a standard barometer be at hand; the new barometer is, in this respect, independent, a little mercury more or less being unimportant. = . short tube barometer.=--this is simply a tube shorter, as may be required, than that necessary to show the atmospheric pressure at the sea level. it is convenient for balloon purposes, and for use at mountain stations, being of course a special construction. = . method of calculating heights by the barometer.=--the pressure of the atmosphere being measured by the barometer, it is evident that as the instrument is carried up a high mountain or elevated in a balloon, the length of the column must decrease as the atmospheric pressure decreases, in consequence of a stratum of air being left below. the pressure of air arises from its weight, or the attraction of gravitation upon it, and therefore the quantity of air below the barometer cistern cannot influence the height of the column. hence it follows that a certain relation must exist between the difference of the barometric pressure at the foot and at the top of a hill or other elevation, and the difference of the absolute heights above the sea. theoretical investigation, abundantly confirmed by practical results, has determined that the strata of air decrease in density in a geometrical proportion, while the elevations increase in an arithmetical one. hence we have a method of determining differences of level, by observations made on the density of the air by means of the barometer. it is beyond our purpose to explain in detail the principles upon which this method is founded, or to give its mathematical investigation. we append tables, which will be useful to practical persons,--surveyors, engineers, travellers, tourists, &c.,--who may carry a barometer as a travelling companion. table i. is calculated from the formula, height in feet = , (log. · - log. b) + ; where · is the mean atmospheric pressure at ° f., and the mean sea-level in latitude °; and b is any other barometric pressure; the being added to avoid minus signs in the table. table ii. contains the correction necessary for the mean temperature of the stratum of air between the stations of observation; and is computed from regnault's co-efficient for the expansion of air, which is · of its volume at ° for each degree above that temperature. table iii. is the correction due to the difference of gravitation in any other latitude, and is found from the formula, _x_ = + · cos. lat. table iv. is to correct for the diminution of gravity in ascending from the sea-level. to use these tables: the barometer readings at the upper and lower stations having been corrected and reduced to temperature ° f., take out from table i. the numbers opposite the corrected readings, and subtract the lower from the upper. multiply this difference successively by the factors found in tables ii. and iii. the factor from table iii. may be neglected unless precision is desired. finally, add the correction taken from table iv. table i. _approximate height due to barometric pressure._ +----------------------------------------------+ |inches.| feet.||inches.| feet.||inches.| feet.| |-------+------++-------+------++-------+------| | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | +----------------------------------------------+ table i.--_continued_. _approximate height due to barometric pressure._ +-------------------------------------------------+ |inches.| feet. ||inches.| feet. ||inches.| feet. | |-------+-------++-------+-------++-------+-------| | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | | · | || · | || · | | +-------------------------------------------------+ table ii. _correction due to mean temperature of the air._ +-------------------------------------------+ |mean |factor.||mean |factor.||mean |factor.| |temp.| ||temp.| ||temp.| | |-----+-------++-----+-------++-----+-------| | ° | · || ° | · || ° | · | | | · || | · || | · | | | · || | · || | · | | | · || | · || | · | | | · || | · || | · | | | · || | · || | · | | | · || | · || | · | | | · || | · || | · | | | · || | · || | · | | | · || | · || | · | | | · || | · || | · | | | · || | · || | · | | | · || | · || | · | | | · || | · || | · | | | · || | · || | · | | | · || | · || | · | | | · || | · || | · | | | · || | · || | · | | | · || | · || | · | | | · || | · || | · | | | · || | · || | · | | | · || | · || | · | | | · || | · || | · | | | · || | · || | · | | | · || | · || | · | +-------------------------------------------+ table iii. +-------------------------------------------------------+ |latitude.|factor.||latitude.|factor.||latitude.|factor.| |---------+-------++---------+-------++---------+-------| | ° | · || | · || | · | | | · || | · || | · | | | · || | · || | · | | | · || | · || | · | | | · || | · || | · | | | · || | · || | | +-------------------------------------------------------+ table iv. +----------------------------------------------------+ | height in |correction|| height in |correction| |thousand feet.| additive.||thousand feet.| additive.| |--------------+----------++--------------+----------| | | || | | | | || | | | | || | | | | || | | | | || | | | | || | | | | || | | | | || | | | | || | | | | || | | | | || | | | | || | | | | || | | +----------------------------------------------------+ example . on october st, , when mr. welsh ascended in a balloon, at h. m. p.m., the barometer, corrected and reduced, was · , the air temperature °, while at greenwich, feet above the sea, the barometer at the same time was · inches, air temperature °, the balloon not being more than miles s.w. from over greenwich; required its elevation. feet. barometer in balloon · , table i. = " at greenwich · " ----- mean temperature, °, table ii. factor · ----- · ----- latitude - / °, factor from table iii. · ----- correction from table iv. ----- elevation of greenwich ----- " balloon feet. ===== the following examples, from the balloon ascents of j. glashier, esq., f.r.s., will serve for practice.[ ] . ascended from wolverhampton, th august, , at h. m. p.m.; barometer (in all cases corrected and reduced to ° f) was · , the temperature of the air °; at the same time, at wrottesley hall, feet above the sea, in latitude - / ° n, the barometer was · , and the temperature of the air °· ; find the elevation of the balloon above the sea. height, , feet. . from the same place an ascent was made th september, , when at h. m. p.m. barometer was · , air o°; at wrottesley hall · , air °. height, , feet. . from the crystal palace a balloon ascent was made th august, . at h. m. p.m. barometer was · , air °· ; and at the same time at greenwich observatory, at feet above the sea, the barometer was · , air °. height, , feet. . from the same place an ascent was made th september, . at p.m., the balloon being over blackheath, barometer was · , and the air °· , while at greenwich, barometer was · , air °· . height, , feet. chapter v. secondary barometers. = . desirability of magnifying the barometer range.=--the limits within which the ordinary barometric column oscillates, do not exceed four inches for extreme range, while the ordinary range is confined to about two inches; hence it has often been felt that the public utility of the instrument would be greatly enhanced if by any means the scale indications could be increased in length. this object was sought to be obtained by bending the upper part of the tube from the vertical, so that the inches on the scale could be increased in length in proportion to the secant of the angle it made with the vertical. this was called "the diagonal barometer." the upper part of the tube has also been formed into a spiral, and the scale, placed along it, is thus greatly enlarged. but these methods of enlarging the indications cannot be so successfully accomplished, nor so cheaply nor so elegantly, as is done by the principle employed in the dial barometer. hence they are not in use. [illustration: fig. .] = . howson's long range barometer.=--very recently quite a novel design has been patented by mr. howson, for a long range barometer. the construction requires neither distortion of the tube, nor mechanism for converting a short scale into a long one; but the mercury itself rises and falls, through an extended range, naturally, and in simple obedience to the varying pressure of the atmosphere. the tube is fixed, but its cistern is sustained by the mere pressure of the atmosphere. looking at the instrument, it seems a perfect marvel. it appears as though the cistern with the mercury in it must fall to the ground. the bore of the tube is wide, about an inch across. a long glass rod is fixed to the bottom of the glass cistern, where a piece of cork or some elastic substance is also placed. the tube is filled with mercury; the glass rod is plunged into the tube as it is held top downwards, until the cork gets close up to the tube and fits tightly against it. the pressure against the cork simply prevents the mercury from coming out while the instrument is being inverted. when it is inverted, the mercury partly falls, and forms an ordinary barometric column. when the top is held, the cistern and glass rod, instead of falling away, remain perfectly suspended. there is no material support to the cistern; the tube only is fixed, the cistern hangs to it. glass is many times lighter than mercury. when the glass rod is introduced, it displaces an equal volume of mercury. the glass rod, being so much lighter than mercury, floats and sustains the additional weight of the cistern by its buoyancy. in the mean time, the atmosphere is acting upon the mercury, keeping up the ordinary barometric column. supposing there is a rise in the ordinary barometer, the atmosphere presses some more mercury up the tube. this mercury is taken out of the cistern, which of course becomes lighter, and therefore the rod and cistern float up a little higher, which thus causes the column of mercury to rise still more. the increased pressure and buoyancy thus acting together, increase the ascent in the barometric column, as shown by the fixed scale. one inch in the barometer might be represented by two or more inches in this instrument, according to construction. supposing there was a decrease of pressure, the mercury would fall, come into the cistern, make it heavier, and increase the fall somewhat. friction guides, at the top of the rod, prevent it coming into contact with the side of the tube when vertically suspended. the illustration, fig. , shows the appearance of the instrument as framed in wood by the makers, messrs. negretti and zambra. = . mcneild's long range barometer.=--a barometer designed by a gentleman named mcneild is on a directly opposite principle to the one just described. the tube is made to float on the mercury in the cistern. it is filled with mercury, inverted in the usual manner, then allowed to float, being held vertically by glass friction points or guides. by this contrivance, the ordinary range of the barometer is greatly increased. one inch rise or fall in the standard barometer may be represented by four or five inches in this instrument, so that it shows small variations in atmospheric pressure very distinctly. as the mercury falls in the tube with a decrease of pressure, the surface of the mercury in the cistern rises, and the floating tube rises also, which causes an additional descent in the column, as shown by fixed graduations on the tube. with an increase of pressure, some mercury will leave the cistern and rise in the tube, while the tube itself will fall, and so cause an additional ascent of mercury. this barometer is identical in principle with king's barograph (see p. ). the construction of howson's and mcneild's barometers has been assigned to messrs. negretti and zambra. these instruments are usually made for domestic purposes with a scale of from three to five, and for public use from five to eight times the scale of the ordinary standard. their sensitiveness is consequently increased in an equal proportion, and they have the additional advantage of not being affected by differences of level in the cistern. however, these novelties have not been sufficiently tried to determine their practical value for strictly scientific purposes; but as weather-glasses, for showing minute changes, they are superior to the common barometer. = . the water-glass barometer.=--if a florence flask, having a long neck, have a small quantity of water poured into it, and then be inverted and so supported that the open end dips into a vessel containing water, a small column of water will be confined in the neck of the bottle, the pressure of which, upon the surface of the exposed water, will be equal to the difference between the atmospheric pressure and the elasticity of the confined air in the body of the bottle. as the pressure of the atmosphere varies, this column will alter in height. but the elasticity of the confined air is also subject to variations, owing to changes of temperature. it follows, then, that the oscillations of the column are dependent on alterations of temperature and atmospheric pressure. such an arrangement has been called "the water-glass barometer," and bears about the same relative value to the mercurial barometer, as an exponent of weather changes, that a cat-gut hygrometer bears to a thermometric hygrometer, as an indicator of relative moisture. . sympiesometer. nevertheless the instrument now about to be described, depending upon similar principles, but scientifically constructed and graduated, is a very useful and valuable substitute for the mercurial barometer. it consists of a glass tube, varying, according to the purposes for which the instrument is required, from six to twenty-four inches in length. the upper end is closed, and formed into a bulb; the lower is turned up, formed into a cistern, and open at top, through a pipette, or cone. a plug, moveable by a catch from below, can be made to close this opening, so as to render the instrument portable. [illustration: fig. .] the upper portion of the tube is filled with air; the lower portion, and part of the cistern, with sulphuric acid, coloured so as to render it plainly visible. formerly, hydrogen and oil were used. it was found, however, that, by the process known to chemists as _osmosis_, this light gas in time partially escaped, and the remainder became mixed with air, the consequence being that the graduations were no longer correct. they are more durable as at present constructed. the liquid rises and falls in the tube with the variations of atmospheric pressure and temperature acting together. if the pressure were constant, the confined air would expand and contract for temperature only, and the instrument would act as a thermometer. in fact, the instrument is regarded as such in the manufacture; and the thermometric scales are ascertained and engraved on the scale. a good mercurial thermometer is also mounted on the same frame. if, therefore, at any time the mercurial and the air thermometers do not read alike, it must evidently be due to the atmospheric pressure acting upon the air in the tube; and it is further evident that, under these circumstances, the position of the top of the liquid may be marked to represent the barometric pressure at the time. in this manner a scale of pressure is ascertained by comparison with a standard barometer, extending generally from to inches. when made correctly, these instruments agree well with the mercurial barometer for a number of years, and their subsequent adjustment is not a matter of much expense. for use at sea, the liquid column is contracted at the bend. the sympiesometer is very sensitive, and feels the alterations in the atmospheric pressure sooner than the ordinary marine barometer. the scale is usually on silvered brass, mounted on a mahogany or rosewood frame, protected in front by plate glass. it is generally furnished with a revolving register, to record the observation, in order that it may be known whether the pressure has increased or decreased in the interval of observation. small pocket sympiesometers are sometimes fitted with ivory scales, and protected by a neat velvet-lined pasteboard or morocco case. _how to take an observation._--in practice, the indications of the atmospheric pressure are obtained from the sympiesometer by noting, first, the temperature of the mercurial thermometer; secondly, adjusting the pointer of the pressure scale to the same degree of temperature on the scale of the air column; thirdly, reading the height of the liquid on the sliding scale. _directions for use._--the sympiesometer should be carried and handled so as to keep the top always upwards, to prevent the air mechanically mixing with the liquid. care should also be taken to screen it from casual rays of the sun or cabin fire. . aneroids. the beautiful and highly ingenious instrument called by the name _aneroid_, is no less remarkable for the scientific principles of its construction and action, than for the nicety of its mechanism. it is a substitute, and perhaps the best of all substitutes, for the mercurial barometer. as its name implies, it is constructed "without fluid." it was invented by m. vidi of paris. in the general form in which it is made it consists of a brass cylindrical case about four inches in diameter and one and a half inch deep, faced with a dial graduated and marked similarly to the dial-plate of a "wheel-barometer," upon which the index or pointer shows the atmospheric pressure in inches and decimals of an inch in accordance with the mercurial barometer. within the case, for ordinary sizes, is placed a flat metal box, generally not more than half an inch thick and about two inches or a little more in diameter, from which nearly all the air is exhausted. the top and bottom of this box is corrugated in concentric circles, so as to yield inwardly to external pressure, and return when the pressure is removed. the pressure of the atmosphere, acting externally, continually changes, while the elastic pressure of the small quantity of air within can only vary by its volume being increased or decreased, or by change of temperature. leaving out of consideration, for the moment, the effect of temperature, we can readily perceive that as the pressure is lessened upon the outside of the box, the elastic force of the air within will force out the top and bottom of the box; and when the outer pressure is increased they will be forced in. thus with the varying pressure of the atmosphere, the top and bottom of the box approach to and recede from each other by a small quantity; but the bottom being fixed, nearly all this motion takes place on the top. thus the top of the box is like an elastic cushion, which rises and falls according as the compressing force lessens or increases. to the eye these expansions and contractions would not be perceptible, so small is the motion. but they are rendered very evident by a nice mechanical arrangement. to the box is attached a strong piece of iron, kept pressed upon it by a spring at one extremity; so that as the top of the box rises, the motion is made sensible at the point held by the spring, and when the top descends the spring draws the piece of iron into close contact with it. this piece of iron acts as a lever, having its fulcrum at one extremity, the power at the centre of the box-top, and the other extremity controlled by the spring. thus it is evident that the small motion of the centre of the box-top is much increased at the spring extremity. the motion thus obtained is communicated to a system of levers; and, by the intervention of a piece of watch-chain and a fine spring passing round the arbour, turns the index to the right or left, according as the external pressure increases or decreases. thus, when by increase of pressure the vacuum box is compressed, the mechanism transfers the movement to the index, and it moves to the right; when the vacuum box bulges out under diminished pressure, the mechanical motion is reversed, and the index moves to the left. as the index traverses the dial, it shows upon the scale the pressure corresponding with that which a good mercurial barometer would at the same time and place indicate; that is, supposing it correctly adjusted. a different and more elegant arrangement has since been adopted. a broad curved spring is connected to the top of the vacuum box, so as to be compressed by the top of the box yielding inward to increased pressure, and to relax itself and the box as the pressure is lessened. the system of levers is connected to this spring, which augments and transfers the motion to the index, in the manner already described. increase of pressure causes the levers to slacken the piece of watch-chain connected with them and the arbour of the index. the spring now uncoils, winds the chain upon the arbour, and turns the index to the right. decrease of pressure winds the chain off the barrel, tightens the spiral spring, which thus turns the index to the left. the graduations of the aneroid scale are obtained by comparisons with the correct standard reading of a mercurial barometer, under the normal and reduced atmospheric pressure. reduced pressure is obtained by placing both instruments under the receiver of an air pump. [illustration: fig. .] fig. represents the latest improved mechanism of an aneroid. the outer case and the face of the instrument are removed, but the hand is attached by its collet to the arbour. _a_ is the corrugated box, which has been exhausted of air through the tube, _j_, and hermetically sealed by soldering. _b_ is a powerful curved spring, resting in gudgeons fixed on the frame-plate, and attached to a socket behind, _f_, in the top of the box. a lever, _c_, joined to the stout edge of the spring, is connected, by the bent lever at _d_, with the chain, _e_, the other end of which is coiled round, and fastened to the arbour, _f_. as the box, _a_, is compressed by the weight of the atmosphere increasing, the spring, _b_, is tightened, the lever, _c_, depressed, and the chain, _e_, uncoiled from _f_, which is thereby turned so that the hand, _h_, moves to the right. in the mean while the spiral spring, _g_, coiled round _f_, and fixed at one extremity to the frame-work and by the other to _f_, is compressed. when, therefore, the pressure decreases, _a_ and _b_ relax, by virtue of their elasticity; _e_ slackens, _g_ unwinds, turning _f_, which carries _h_ to the left. near _j_ is shown an iron pillar, cast as part of the stock of the spring, _b_. a screw works in this pillar through the bottom of the plate, by means of which the spring, _b_, may be so adjusted to the box, _a_, as to set the hand, _h_, to read on the scale according to the indications of a mercurial barometer. the lever, _c_, is composed of brass and steel, soldered together, and adjusted by repeated trials to correct for the effects of temperature. a thermometer is sometimes attached to the aneroid, as it is convenient for indicating the temperature of the air. as regards the instrument itself, no correction for temperature can be applied with certainty. it should be set to read with the mercurial barometer at ° f. then the readings from it are supposed to require no correction. in considering the effects of temperature upon the aneroid, they are found to be somewhat complex. there is the effect of expansion and contraction of the various metals of which the mechanism is composed; and there is the effect on the elasticity of the small portion of air in the box. an increase of temperature produces greater, a diminution less elasticity in this air. the compensation for effects of temperature is adjusted by the process of "trial and error," and only a few makers do it well. it is very often a mere sham. admiral fitzroy writes, in his _barometer manual_, "the known expansion and contraction of metals under varying temperatures, caused doubts as to the accuracy of the aneroid under such changes; but they were partly removed by introducing into the vacuum box a small portion of gas, as a compensation for the effects of heat or cold. the gas in the box, changing its bulk on a change of temperature, was intended to compensate for the effect on the metals of which the aneroid is made. besides which, a further and more reliable compensation has lately been effected by a combination of brass and steel bars." "aneroid barometers, if often compared with good mercurial columns, are similar in their indications, and valuable; but it must be remembered that they are not independent instruments, that they are set originally by a barometer, require adjustment occasionally, and may deteriorate in time, though slowly." "the aneroid is quick in showing the variation of atmospheric pressure; and to the navigator who knows the difficulty, at times, of using barometers, this instrument is a great boon, for it can be placed anywhere, quite out of harm's way, and is not affected by the ship's motion, although faithfully giving indication of increased or diminished pressure of air. in ascending or descending elevations, the hand of the aneroid may be seen to move (like the hand of a watch), showing the height above the level of the sea, or the difference of level between places of comparison." in the admiral's _notes on meteorology_, he says, "the aneroid is an excellent _weather glass_, if well made. compensation for heat or cold has lately been introduced by efficient mechanism. in its _improved_ condition, when the cost may be about £ , it is fit for measuring heights as far as , feet with approximate accuracy; but even at the price of £ , as a _weather-glass_ only, it is exceedingly valuable, because it can be carried anywhere; and if now and then compared with a good barometer, it may be relied on sufficiently. i have had one in constant use for ten years, and it appears to be as good now as at first. for a ship of war (considering concussion by the fire of guns), for boats, or to put in a drawer, or on a table, i believe there is nothing better than it for use as a common weather-glass." colonel sir h. james, r.e., in his _instructions for taking meteorological observations_, says of the aneroid, "this is a most valuable instrument; it is extremely portable. i have had one in use for upwards of ten years, and find it to be the best form of barometer, as a "weather-glass," that has been made." one of the objects of mr. glaisher's experiments in balloons was "to compare the readings of an aneroid barometer with those of a mercurial barometer up to five miles." in the comparisons the readings of the mercurial barometer were corrected for index-error and temperature. the aneroid readings, says mr. glaisher, "prove all the observations made in the several ascents may be safely depended upon, and also that an aneroid barometer can be made to read correctly to pressures below twelve inches." as one of the general conclusions derived from his experiments he states, "that an aneroid barometer read correctly to the first place, and probably to the second place of decimals, to a pressure as low as seven inches." the two aneroids used by mr. glaisher were by messrs. negretti and zambra. aneroids are now manufactured almost perfectly compensated for temperature. such an instrument therefore ought to show the same pressure in the external air at a temperature say of °, as it would in a room where the temperature at the same time may be °; provided there is no difference of elevation. to test it thoroughly would require an examination and a comparison with barometer readings reduced to ° f., conducted through a long range of temperature and under artificially reduced pressure. a practical method appears to be to compare the aneroid daily, or more often, for a few weeks with the readings of a mercurial barometer reduced to °; and if the error so found be constant, the object of the compensation may be assumed to be attained, particularly if the temperature during the period has varied greatly. _directions for using the aneroid._--aneroids are generally suspended with the dial vertical; but if they be placed with the dial horizontal, the indications differ a few hundredths of an inch in the two positions. hence, if their indications are registered, they should be kept in the same position. the aneroid will not answer for exact scientific purposes, as it cannot be relied upon for a length of time. its error of indication changes slowly, and hence the necessity of its being set from time to time with the reading of a good barometer. to allow of this being done, at the back of the outer case is the head of a screw in connection with the spring attached to the vacuum box. by applying a small turnscrew to this screw, the spring of the vacuum box may be tightened or relaxed, and the index made to move correspondingly to the right or left on the dial. by this means, besides being enabled to correct the aneroid at any time, "if the measure of a height rather greater than the aneroid will commonly show be required, it may be _re-set_ thus: when at the upper station (_within its range_), and having noted the reading carefully, touch the screw behind so as to bring back the hand a few inches (if the instrument will admit), then read off and start again. _reverse the operation when descending._ this may add some inches of measure _approximately_."--_fitzroy._ [illustration: fig. .] = . small size aneroids.=--the patent for the aneroid having expired, admiral fitzroy urged upon messrs. negretti & zambra the desirability of reducing the size at which it had hitherto been made, as well as of improving its mechanical arrangement, and compensation for temperature. they accordingly engaged skilful workmen, who, under their directions, and at their expense, by a great amount of labour and experiment, succeeded in reducing its dimensions to two inches in diameter, and an inch and a quarter thick. the exact size and appearance of this aneroid are shown in fig. . the compensation is carefully adjusted, and the graduations of the dial ascertained under reduced pressure, so that they are not quite equal, but more accurate. = . watch aneroid.=--subsequently the aneroid has been further reduced in size and it can now be had from an inch and a quarter to six inches in diameter. the smallest size can be enclosed in watch cases, fig. , or otherwise, so as to be adapted to the pocket. by a beautifully simple contrivance, a milled rim is adjusted to move round with hand pressure, and carry a fine index or pointer, outside and around the scale engraved on the dial, or face, for the purpose of marking the reading, so that the subsequent increase or decrease of pressure may be readily seen. these very small instruments are found to act quite as correctly as the largest, and are much more serviceable. besides serving the purpose of a weather-glass in the house or away from home, if carried in the pocket, they are admirably suited to the exigencies of tourists and travellers. they may be had with scale sufficient to measure heights not exceeding , feet; with a scale of elevation in feet, as well as of pressure in inches, engraved on the dial. the scale of elevation, which is for the temperature of °, was computed by professor airy, the astronomer royal, who kindly presented it to messrs. negretti and zambra, at the same time suggesting its application. moderate-sized aneroids, fitted in leathern sling cases, are also good travelling instruments, and will be found serviceable to pilots, fishermen, and for use in coasting and small vessels, where a mercurial barometer cannot be employed, because requiring too much space. [illustration: fig. .] admiral fitzroy, in a communication to the _mercantile marine magazine_, december, , says:--"aneroids are now made more portable, so that a pilot or chief boatman may carry one in his pocket, as a railway guard carries his timekeeper; and, thus provided, pilots cruising for expected ships would be able to caution strangers arriving, if bad weather were impending, or give warning to coasters or fishing boats. harbours of refuge, however excellent and important, are not always accessible, even when most wanted, as in snow, rain, or darkness, when neither land, nor buoy, nor even a lighthouse-light can be seen." = . measurement of heights by the aneroid.=--for measuring heights not exceeding many hundred feet above the sea-level by means of the aneroid, the following simple method will suffice:-- divide the difference between the aneroid readings at the lower and upper stations by · ; the quotient will give the approximate height in feet. thus, supposing the aneroid to read at the lower station · inches. upper station · ------ difference · ====== divided gives · /· = feet. as an illustration of the mode in which the aneroid should be used in measuring heights, the following example is given:-- a gentleman who ascended helvellyn, august th, , recorded the following observations with a pocket aneroid by negretti and zambra:-- near a.m., at the first milestone from ambleside, found by survey to be feet above the sea, the aneroid read · inches; about p.m., at the summit of helvellyn, · ; and at p.m., at the milestone again, · . the temperature of the lower air was °, of the upper, °. hence the height of the mountain is deduced as follows:-- inches. reading at a.m. · " p.m. · ------ mean · table i.[ ] upper reading · " ----- difference mean temperature °· , gives in table ii. · ----- lat. ° n., gives in table iii. · ----- table iv. ----- difference of height height of lower station ----- " helvellyn in sir j. herschell's _physical geography_ it is given as ft. so near an agreement is attributable to the excellence of the aneroid, and the careful accuracy of the observer. . metallic barometer. this instrument, the invention of m. bourdon, has a great resemblance to the aneroid, but is much simpler in arrangement. the inventor has applied the same principle to the construction of metallic steam-pressure gauges. we are here, however, only concerned with it as constructed to indicate atmospheric pressure. it consists of a long slender flattened metallic tube, partially exhausted of air, and hermetically closed at each end, then fixed upon its centre, and bent round so as to make the ends face each other. the transverse section of this tube is an elongated ellipse. the principle of action is this: interior pressure tends to straighten the tube, external pressure causes it to coil more. hence as the atmospheric pressure decreases, the ends of the tube become more apart. this movement is augmented and transferred by a mechanical arrangement of small metallic levers to a radius bar, which carries a rack formed on the arc of its circle. this moves a pinion, upon the arbour of which a light pointer, or "hand," is poised, which indicates the pressure upon a dial. when the pressure increases, the ends of the tube approach each other, and the pointer moves from left to right over the dial. the whole mechanism is fixed in a brass case, having a hole at the back for adjusting the instrument to the mercurial barometer by means of a key, which sets the pointer without affecting the levers. the dial is generally open to show the mechanism, and is protected by a glass, to which is fitted a moveable index. this barometer is very sensitive, and has the advantage of occupying little space, although it has not yet been made so small as the aneroid. both these instruments admit of a great variety of mounts to render them ornamental. the metallic barometer can be constructed with a small clock in its centre, so as to form a novel and beautiful drawing-room ornament. admiral fitzroy writes, "metallic barometers, by bourdon, have not yet been tested in very moist, hot, or cold air for a sufficient time. they are dependent, or secondary instruments, and liable to deterioration. for limited employment, when sufficiently compared, they may be very useful, especially in a few cases of electrical changes, _not foretold or shown by mercury_, which these seem to indicate remarkably." they are not so well adapted for travellers, nor for measurements of considerable elevations, as aneroids. chapter vi. instruments for ascertaining temperature. = . temperature= is the energy with which heat affects our sensation of feeling. bodies are said to possess the same temperature, when the amounts of heat which they respectively contain act outwardly with the same intensity of transfer or absorption, producing in the one case the sensation of warmth, in the other that of coldness. instruments used for the determination and estimation of temperatures are called _thermometers_. experience proves that the same body always occupies the same space at the same temperature; and that for every increase or decrease of its temperature, it undergoes a definite dilatation or contraction of its volume. provided, then, a body suffers no loss of substance or peculiar change of its constituent elements or atoms, while manifesting changes of temperature it will likewise exhibit alterations in volume; the latter may, therefore, be taken as exponents of the former. the expansion and contraction of bodies are adopted as arbitrary measures of changes of temperature; and any substance will serve for a thermometer in which these changes of volume are sensible, and can be rendered measureable. = . thermometric substances.=--thermometers for meteorological and domestic purposes are constructed with liquids, and generally either mercury or alcohol, because their alterations of volume for the same change of temperature are greater than those of solids; while being more manageable, they are preferred to gases. mercury is of all substances the best adapted for thermometric purposes, as it maintains the liquid state through a great alteration of heat, has a more equable co-efficient of expansion than any other fluid, and is peculiarly sensitive to changes of temperature. the temperature of solidification of mercury, according to fahrenheit's scale of temperature, is - °; and its temperature of ebullition is about °. sulphuric ether, nitric acid, oil of sassafras, and other limpid fluids, have been employed for thermometers. = . description of the thermometer.=--the ordinary thermometer consists of a glass tube of very fine bore, having a bulb of thin glass at one extremity, and closed at the other. the bulb and part of the tube contains mercury; the rest of the tube is a vacuum, and affords space for the expansion of the liquid. this arrangement renders very perceptible the alterations in volume of the mercury due to changes of temperature. it is true, the glass expands and contracts also; but only by about one-twentieth of the extent of the mercury. regarding the bulb, then, as unalterable in size, all the changes in the bulk of the fluid must take place in the tube, and be exhibited by the expansion and contraction of the column, which variations are made to measure changes of temperature. . standard thermometer. the peculiarities in the construction of thermometers will be best understood by describing the manufacture of a _standard thermometer_, which is one of the most accurate make, and the scale of which is divided independently of any comparison with another thermometer. fig. is an illustration of such an instrument, on a silvered brass scale. [illustration: fig. ] _selection of tube._--in selecting the glass tube, much care is requisite to ascertain that its bore is perfectly uniform throughout. as received from the glass-house, the tubes are generally, in their interior, portions of very elongated cones, so that the bore is wider at one end than at the other. with due care, however, a proper length of tube can be selected, in which there is no appreciable difference of bore. this is ascertained by introducing into the tube a length of mercury of about a half or a third of an inch, and accurately measuring it in various positions in the tube. to accomplish this, the workman blows a bulb at one end of the tube, and heats the bulb a little to drive out some of the air. then, placing the open end in mercury, upon cooling the elasticity of the enclosed air diminishes, and the superior pressure of the atmosphere drives in some mercury. the workman stops the process so soon as he judges sufficient mercury has entered. by cooling or heating the bulb, as necessary, the mercury is made to pass from one end of the tube to the other. should the length of this portion of mercury alter in various parts of the bore, the tube must be rejected. if it is, as nearly as possible, one uniform length, the tube is set aside for filling. the _bulb_ is never blown by the breath, but by an elastic caoutchouc ball containing air, so that the introduction of moisture is avoided. the spherical form is to be preferred; for it is best adapted to resist the varying pressure of the atmosphere. the bulbs should not be too large, or the mercury will take some time to indicate sudden changes of temperature. cylindrical bulbs are sometimes desirable, as they offer larger surfaces to the mercury, and enable thermometers to be made more sensitive. the _mercury_, with which the bulb is to be filled, should be quite pure, and freed from moisture and air by recent boiling. _filling the tube._--the filling is effected by heating the bulb with the flame of a spirit-lamp, while the open end is embedded in mercury. upon allowing the bulb to cool, the atmospheric pressure drives some mercury into it; and the process of heating and cooling is thus continued until sufficient mercury is introduced. the mercury is next boiled in the tube, to expel any air or moisture that may be present. in order to close the tube and exclude all air, the artist ascertains that the tube contains the requisite quantity of mercury; then, by holding the bulb over the spirit flame, he causes the mercury to fill the whole of the tube, and dexterously removing it from the source of heat, he, at the same instant, closes it with the flame of a blow-pipe. if any air remain in the tube, it is easily detected; for if the instrument be inverted, the mercury will fall to the extremity of the tube, if there is a perfect vacuum, unless the tube be so finely capillary that its attraction for the mercury is sufficient to overcome the force of gravity, in which case the mercury will retain its position in every situation of the instrument. if, however, the mercury fall and does not reach quite to the extremity of the bore, some air is present, which must be removed. _the graduation._--the thermometer is now prepared for graduation, the first part of which process is the determination of two fixed points. these are given by the temperatures of melting ice and of the vapour of boiling water. melting ice has always the same temperature in every place and under all circumstances; provided only that the water from which the ice is congealed is free from salts. the temperature of the vapour of boiling water depends upon the pressure of the atmosphere, but is always constant for the same pressure. the fixed point corresponding to the temperature of melting ice is called the _freezing point_. it is obtained by keeping the bulb and the part of the tube occupied by mercury immersed in melting ice, until the mercury contracts to a certain point, where it remains stationary. this position of the end of the mercury is then marked upon the tube. the _boiling point_ is not so easily determined, for the barometer must be consulted about the same time. the boiling apparatus is generally constructed of copper. it consists of a cylindrical boiler, heated from the base by a spirit lamp or charcoal fire. an open tube two or three inches in diameter and of suitable length enters the top of the boiler. this tube is enveloped by another fixed to the top of the boiler but not opening into it, and so that the two tubes are about an inch apart. the object of the outer tube is to protect the inner tube from the cold temperature of the air. the outer tube has an opening at the top for the admission of the thermometer, and a hole near the bottom for the escape of steam through a spout. when the water is made to boil, the steam rises in the inner tube, fills the space between the tubes, and escapes at the spout. the thermometer is then passed down into the inner cylinder, and held securely from the top by means of a piece of caoutchouc. the tubes or cylinders should be of sufficient length to prevent the thermometer entering the water. this is necessary because the temperature of boiling water is influenced by any substance which it holds in chemical solution; and, moreover, its temperature increases with the depth, owing to the pressure of the upper stratum. the thermometer being thus surrounded with steam, the mercury rises in the tube. as it does so, the tube should be depressed so as always to keep the top of the mercury just perceptible. when the temperature of the vapour is attained, the mercury ceases to rise, and remains stationary. the position of the end of the mercury is now marked upon the tube, and the "_boiling-point_" is obtained. = . methods of ascertaining the exact boiling temperature.=--the normal boiling temperature of water all nations have tacitly agreed to fix under a normal barometric pressure of · inches of mercury, having the temperature of melting ice, in the latitude of °, and at the sea-level. if the atmospheric pressure at the time or place of graduating a thermometer does not equal this, the boiling temperature will be higher or lower according as the pressure is greater or less. hence a reading must be taken from a reliable barometer, which must also be corrected for errors and temperature, and reduced for latitude, in order to compare the actual atmospheric pressure at the time with the assumed normal pressure. tables of vapour tension, as they are termed, have been computed from accurate experimental investigations and theory,--giving the temperatures of the vapour of water for all probable pressures; regnault's, the most recent, is considered the most accurate; and his investigations are based upon the standard pressure given above, and are for the same latitude. his table, therefore, will give the temperature on the thermometric scale corresponding to the pressure. the commissioners appointed by the british government to construct standard weights and measures, decided that the normal boiling-point, °, on the thermometer should represent the temperature of steam generated under an atmospheric pressure equal in inches of mercury, at the temperature of freezing water, to · + (cos. latitude × · ) + (· × height in feet above the sea-level). hence, at london, lat. ° ´ n., we deduce · as the barometric pressure representing the normal boiling point of water,--the trifling correction due to height being neglected. if then, in the latitude of london, the barometric pressure, at the time of fixing the boiling point, be not · inches, that point will be higher or lower, according to the difference of the pressure from the normal. near the sea-level about · inch of such difference is equivalent to ° fahrenheit in the boiling point. suppose, then, the atmospheric pressure at london to be · inches, the following calculation gives the corresponding boiling temperature for fahrenheit's scale:-- observed pressure · normal " · ------ difference · ======= as · is to · , so is ° to °· . that is, the water boils at °· above its normal temperature; so that, in this case, the normal temperature to be placed on the scale, viz. °, must be °· lower than the mark made on the tube at the height at which the mercury stood under the influence of the boiling water. the temperature of the vapour of boiling water may be found, at any time and place, as follows:--multiply the atmospheric pressure by the factor due to the latitude, given in the annexed table v., and with the result seek the temperature in table vi. table v. table vi. +----------------------------------------------------------------+ |latitude.| factor. |||temperature|tension.||temperature|tension.| | | ||| of vapour.| || of vapour.| | |---------+---------+++-----------+--------++-----------+--------| |degrees. | ||| degrees. |inches. || degrees. |inches. | | | · ||| | · || | · | | | · ||| | · || | · | | | · ||| | · || | · | | | · ||| | · || | · | | | · ||| | · || | · | | | · ||| | · || | · | | | · ||| | · || | · | | | · ||| | · || | · | | | · ||| | · || | · | | | · ||| | · || | · | | | · ||| | · || | · | | | · ||| | · || | · | | | · ||| | · || | · | | | · ||| | · || | · | | | · ||| | · || | · | | | · ||| | · || | · | | | · ||| | · || | · | | | ||| | · || | · | +----------------------------------------------------------------+ _how to use the tables._--when the _temperature_ is known to decimals of a degree, take out the tension for the degree, and multiply the difference between it and the next tension by the decimals of the temperature, and add the product to the tension, for the degree. required the tension corresponding to °· . ° = · · × · = · = · ° = · ------ ------ difference · · = · ====== ====== when the _tension_ is given, take the difference between it and the next less tension in the table, and divide this difference by the difference between the next less and next greater tensions. the quotient will be the decimals to add to the degree opposite the next less tension. thus, for · inches, required the temperature. given · next greater · · next less · ------ ------ · difference · · and ---- = · · temperature opposite next less · ----- temperature required · ===== a similar method of interpolation in taking out numerical quantities is applicable to almost all tables; and should be practised with all those given in this work. _example._--thus, in liverpool, lat. ° ´ n., the barometer reading · inches, its attached thermometer °, and the correction of the instrument being + · (including index error, capillarity and capacity), what temperature should be assigned for the boiling point marked on the thermometer? observed barometer · correction + · ------ · correction for temperature - · ------ reduced reading · factor from table v. · ------- ----------- equivalent for lat. ° · =========== in table vi., · gives temperature °· . = . displacement of the freezing point.=--either the prolonged effect of the atmospheric pressure upon the thin glass of the bulbs of thermometers, or the gradual restoration of the equilibrium of the particles of the glass after having been greatly disturbed by the operation of boiling the mercury, seems to be the cause of the freezing points of standard thermometers reading from a few tenths to a degree higher in the course of some years, as has been repeatedly observed. to obviate this small error, it is our practice to place the tubes aside for about six months before fixing the freezing point, in order to give time for the glass to regain its former state of aggregation. the making of accurate thermometers is a task attended with many difficulties, the principal one being the liability of the zero or freezing point varying constantly, so much so, that a thermometer that is perfectly correct to-day, if immersed in boiling water, will be no longer accurate; at least, it will take some time before it again settles into its normal state. then, again, if a thermometer is recently blown, filled, and graduated immediately, or, at least, before some months have elapsed, though every care may have been taken with the production of the instrument, it will require some correction; so that the instrument, however carefully made, should from time to time be plunged into finely-pounded ice, in order to verify the freezing point. = . the scale.=--the two fixed points having been determined, it is necessary to apply the scale. the thermometers in general use in the united kingdom, the british colonies, and north america are constructed with fahrenheit's scale. fahrenheit was a philosophical instrument maker of amsterdam, who, about the year , invented the scale which has given his name to the thermometer. the freezing point is marked °, the boiling point °, so that the intermediate space is divided into equal parts, called degrees. "the principle which dictated this _peculiar division_ of the scale is as follows:--when the instrument stood at the greatest cold of iceland, or degree, it was computed to contain equal parts of quicksilver, which, when plunged in melting snow, expanded to parts; hence the intermediate space was divided into equal portions, and was taken as the freezing point of water: when the thermometer was plunged in boiling water, the quicksilver was expanded to ; and therefore ° was marked as the boiling point of that fluid. in _practice_, fahrenheit determined the divisions of his scale from two fixed points, the freezing and boiling of water. _the theory_ of the division, if we may so speak, was derived from the lowest cold observed in iceland, and the expansions of a given portion of mercury" (_professor trail_). the divisions of the scale can be carried beyond the fixed points, if requisite, by equal graduations. fahrenheit's scale is very convenient in some respects. the meteorological observer is seldom troubled with negative signs, as the zero of the scale is much below freezing. again, the divisions are more numerous, and consequently smaller, than on other scales in use; and the further subdivision into tenths of degrees, seems to give all the minuteness usually required. _celcius_, a swede, in , proposed zero for the freezing point, and for the boiling point, all temperatures below zero being distinguishable by the sign (--) minus. this scale is known as the _centigrade_, and is in use in france, sweden, and the southern part of europe. it has the advantage of the decimal notation, with the embarrassment of the negative sign. _reaumur_, a frenchman, proposed zero for the freezing point, and ° for the boiling point, an arrangement inferior to the centigrade. it is, however, in use in spain, switzerland, and germany. it is merely a simple arithmetical operation to change the indications of any one of these scales into the equivalents on the others. to facilitate such conversions, tables are convenient, when a large number of observations are under discussion; and they can be easily formed or obtained. in the absence of such tables, the following formulæ will insure accuracy of method, and save thinking, when occasional conversions are wanted to be made:--f. stands for fahrenheit, c. for centigrade, and r. for reaumur. given. required. solution. f. c. = (f.- ) / f. r. = (f.- ) / c. f. = / c. + c. r. = / c. r. f. = / r. + r. c. = / r. _example._--convert ° of fahrenheit's scale into the corresponding temperature on the centigrade scale. here c. = ( - ) / c. = - / = - · or nearly ° _below_ zero of the centigrade scale. the algebraical sign must be carefully attended-to in the calculations. = . the method of testing thermometers= for meteorological purposes is very simple. such thermometers are seldom required to read above °. in these the freezing point having been determined, the divisions of the scale are ascertained by careful comparisons, with a standard thermometer, in water of the requisite temperature. "for the freezing point, the bulbs, and a considerable portion of the tubes of the thermometers, are immersed in pounded ice. for the higher temperatures, the thermometers are placed in a cylindrical glass vessel containing water of the required heat: the scales of the thermometers intended to be tested, together with the standard with which they are to be compared, are read through the glass. in this way the scale readings may be tested at any required degree of temperature, and the usual practice is to test them at every ten degrees from ° to ° of fahrenheit."--_fitzroy._ = . porcelain scale plates.=--thermometer scales of brass, wood, or ivory, either by atmospheric influence or dipping in sea-water, are very liable to become soiled and discoloured, so much so that after a very little time the divisions are rendered nearly invisible. to obviate this inconvenience, messrs. negretti and zambra were the first to introduce into extensive use thermometer and barometer scale-plates made of porcelain, having the divisions and figures engraved thereon by means of fluoric acid, and permanently burnt-in and blackened, so as always to present a clear legible scale. that these scales have been found superior to all others, may be inferred from the fact that all the thermometers now supplied to the various government departments are provided with such scales. they can be adapted to replace any of the old forms of brass or zinc scales, the divisions and figures of which have become obliterated or indistinct. = . enamelled tubes.=--nearly all thermometer tubes are now made with enamelled backs. this contrivance of enamelling the backs of the tubes enables the makers to use finer threads of mercury than had before been found practicable; for were it not for the great contrast between the dark thread of mercury and the white enamel on the glass, many of the thermometers now in use would be positively illegible. the enamelling of thermometers is an invention of messrs. negretti and zambra. it is necessary to state this, as many persons, from interested motives, are anxious to ignore to whom the credit of the invention is due. = . thermometers of extreme sensitiveness.=--thermometers for delicate experiments are no novelty. thermometers have been made with very delicate bulbs to contain a very small quantity of mercury. such instruments have also been made with spiral or coiled tubular bulbs, but the thickness of glass required to keep these coils or spirals in shape, and in fact to prevent their falling to pieces, served to nullify the effect sought to be produced, viz. instantaneous action; and where a small thin bulb was employed, the indicating column was generally so fine that it was positively invisible except by the aid of a powerful lens. messrs. negretti and zambra have now introduced a new form of thermometer, which combines sensitiveness and quickness of action, together with a good visible column. the bulb of this thermometer is of the gridiron form. care has been taken in constructing the bulb, so that the objections attending spirals and other forms have been overcome; for whilst the reservoir or bulb is made of glass so thin that it is only by a spirit lamp and not a glass blower's blowpipe that it can be formed, yet it is still so rigid (owing to its peculiar configuration) that no variations in its indications can be detected, whether it be held in a horizontal, vertical, or oblique position, nor will any error be detected if it be stood on its own bulb. they have made thermometers with bulbs or reservoirs formed of about nine inches of excessively thin cylindrical glass, whose outer diameter is not more than a twentieth of an inch; so that, owing to the large surface presented, the indications are positively instantaneous. this form of thermometer was constructed expressly to meet the requirements of scientific balloon ascents, to enable thermometrical readings to be taken at the precise elevation. it was contemplated to procure a metallic thermometer, but on the production of this perfect instrument the idea was abandoned. . varieties of thermometers. fig. is an illustration of boxwood scale thermometers for general use and common purposes. fig. , negretti and zambra's travelling thermometer; it is fixed in a plated metal (silver or otherwise) case, similar to a pencil-case, and has the scale divided upon its stem. fig. , thermometer mounted on a slab of glass, upon which the scale is etched, the back being either oak, mahogany, or ebony. fig. , portable thermometer, in a bronzed brass or german silver revolving case. fig. , pocket thermometer, on ivory or metallic scale, in morocco or papier-mâché case. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] fig. , an ornamental drawing-room thermometer, on ebony or ivory stand, with glass shade. fig. , representation of highly carved or engine-turned design for thermometer mounts, in ivory or wood, for the drawing-room. some have the addition of a sundial or compass at the top; they may also be formed for a watch-stand. fig. , =bath thermometer=, having a float to admit of its being kept in the water. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] fig. , thermometer with ivory scale in glass cylinder, mounted on oak bracket with metal top, for out-door use; as at a window. fig. , thermometer for the window, on patent porcelain or glass scale, with oak bracket and convenient brass supports, for placing the instrument at any angle. fig. , =chemical thermometer=, on boxwood scale, jointed near the bulb on a brass hinge, ranging from ° to °. fig. , =chemical thermometer=, for acids, graduated on its own stem, suitable for insertion in the tubulure of retorts; they are also made insulated in glass cylinder to protect the graduated stem; ranging from ° to °. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] = . superheated steam thermometer.=--the great advantage gained by the use of superheated steam in marine and other steam-engines being now generally admitted by engineers, reliable thermometers, reading to ° at least, are of the utmost importance. to meet this want, messrs. negretti and zambra have constructed for the purpose a substantial form of thermometer, on their patent porcelain scales, in strong and convenient metal mountings, with perforated protection to the bulb. the scales cannot be deteriorated by steam, heat, oil, or dirt; and an occasional wiping will be all that is necessary to keep the divisions and figures clean and visible for any length of time; while careful calibration of the thermometer tubes ensures the most accurate indications attainable. these thermometers are illustrated by figs. & . a similar, but cheaper, construction is given to thermometers to be used with hot air, or hot water, apparatus. = . thermometer for sugar boiling= is protected by a metallic frame; and is usually from three to four feet long, the graduations being confined to a space of about twelve inches at the upper part of the instrument, allowing the bulb and greater part of the tube to be immersed in the boiling sugar. the graduations extend to ° or further. an index is sometimes attached to the scale, which may be set to any degree of heat required to be maintained. . earth thermometer. the earth thermometer is for ascertaining the temperature of the soil at various depths. it is protected by a brass frame, pointed and strengthened at the end to facilitate insertion into the ground, as in fig. . [illustration: fig. .] _utility of a knowledge of the temperature of the soil._--the temperature of the soil is an important element in the consideration of climate, as it concerns the vegetable kingdom. dr. daubeny, in his _lectures on climate_, gives the following statement with respect to some temperatures which have been observed just beneath the earth's surface, in different parts of the globe:-- -------------------------------------------------------------------- country. | temperature. | authority. -----------------+-------------------------------------------------- tropics, often | - ° | humboldt. | | egypt | - | edwards & colin. | | orinoco |in white sand, | humboldt. | | chili | - , among dry grass | boussingault. | | cape of good hope| , under the soil of a bulb | herschell. garden | | | bermuda | , thermometer barely covered | emmet. | in earth | | | china |water of the fields, ; | meyer. | adjacent sand, much higher; | | blackened sides of the boat | | at midday, - | | | france | - , and in one instance | arago. -------------------------------------------------------------------- "the importance of this to vegetation may be estimated by the following considerations:-- "it is known that every plant requires a certain amount of heat, varying in the case of each species, for the renewal of its growth, at the commencement of the season. "now when this degree of heat has spurred into activity those parts that are above ground, and caused them to elaborate the sap, it is necessary that the subterranean portions should at the same time be excited by the heat of the ground to absorb the materials which are to supply the plant with nourishment. unless the latter function is provided for, the aerial portions of the plant will languish from want of food to assimilate. indeed, it is even advisable that the roots should take the start of the leaves, in order to have in readiness a store of food for the latter to draw upon." in another place the professor remarks:--"it has been calculated by mr. raikes, from experiments made at chat moss, that the temperature of the soil when drained averages ° more than it does when undrained; and this is not surprising, when we find that lb. of water evaporated from , lbs. of soil will depress the whole by °, owing to the latent heat which it absorbs in its conversion into vapour." . marine thermometer. this instrument is a special construction to meet the requirements of navigation. it consists of a carefully constructed thermometer divided on its stem to degrees, which are sufficiently large to admit of subdivision into tenths of degrees by estimation, and ranging from ° to °. the scale is porcelain, having the degrees etched upon it, and burnt-in a permanent black. the instrument is made to slide into a japanned metallic case, for handy use and protection. it is therefore adapted for almost any ordinary purpose; and cannot be injuriously affected by any chemical action arising from air or sea-water. a set of these thermometers consists of six, carefully packed in a neat box; two having japanned metallic cases (fig. ), the others being designed for use without the case, or to replace a breakage. [illustration: fig. .] this thermometer is employed in the royal navy, and for the observations made at sea for the board of trade. the thermometer is now considered a necessary instrument on board ship. not only is it of invaluable utility in connection with the barometer as a guide to the weather, but its indications are of service in showing the presence of a warm or cold current in the sea; many of the great oceanic currents being characterised by the warmth or coldness of their waters. in seas visited by icebergs, the habitual use of the thermometer would indicate their proximity, as the water is rendered colder for some distance around by the thawing of huge masses of ice. the water over a shoal in the sea is generally colder than the surface-water of the surrounding ocean; which may result from the cold water being brought to the surface by the current of water encountering the shoal. with this fact navigators are well acquainted; and therefore a fall in the sea-water thermometer may forebode that shallow water is at hand. it has been ascertained that fish inhabit regions of the oceans and seas having the peculiar temperature suitable to their habits. the better and firmer sort of fish are found where cold waters exist. those taken in warmer belts or streams of water, even in the same latitude, are far inferior in condition, and less approved by the palate. the fish of the mediterranean, a warm sea, are generally poor and scarce. fish taken in the cold waters between the american shore and the gulf stream are much esteemed; while in and on the other side of the stream they are said to be tasteless, and of no flavour. between the coasts of china and the warm waters of the japanese current, the seas abound with excellent fish; but in the warm waters of the current and beyond, they are never seen in such shoals. in fact, it is clearly ascertained that fishes are adapted to climates, like birds and beasts. it has been even affirmed, after careful investigation, that herrings, which abound in the british seas, and form a most important branch of our fisheries, can only be found in a temperature varying from ° to °. hence the thermometer, if brought into use by the fishermen, would guide them to the spots where they may with the best chance cast their nets on dark nights, when other indications are not perceptible. this thermometer in its metallic case is perfectly suited for dipping overboard, or placing in a bucket of water just taken from the sea, to ascertain its temperature. chapter vii. self-registering thermometers. = . importance of self-registering thermometers.=--heat being apparently the most effective agent in producing meteorological phenomena, the determination of the highest temperature of the day, and the lowest during the night, is a prime essential to enable an estimate of the climate of any place to be formed. to observe these extremes by means of the ordinary thermometer would be impracticable, from the constant watchfulness which would be necessary. hence, the utility and importance of self-recording thermometers are evident. a thermometer constructed to _register_ the highest temperature is usually called a _maximum thermometer_; one to show the lowest temperature is termed a _minimum thermometer_; and if made to record both extremes of temperature, it is designated a _maximum-and-minimum thermometer_. we will, for the sake of method, describe the instruments in use in this order. it would carry us beyond our scope to explain in detail the methods of dealing with temperature observations; but we may remark that half the sum of the maximum and minimum temperature of each day of twenty-four hours, is not what meteorologists designate the _mean daily temperature_, although it very frequently approximates to it. the mean temperature of the day is understood to be the average of twenty-four consecutive hourly readings of a thermometer; and meteorology now supplies formulæ whereby this result can be deduced from two or three observations only in a day. but we would observe that the actual mean temperature of any place has not such an important influence upon life, either animal or vegetable, as the abruptness and magnitude of the variations of temperature. climate, therefore, should be estimated more by the range of the thermometer than by the average of its indications. the registrar general's returns prove that with a wide range of the thermometer, the mortality greatly increases; and it is now becoming apparent to meteorologists that the daily range of the thermometer marks the effects of temperature on the health of men, and the success of crops, better than any other meteorological fact of which we take cognizance. now that self-registering thermometers are constructed with mercury, the most appropriate of all thermometric substances, not only for maxima, but likewise for minima temperatures, the determination of the diurnal range of temperature is rendered more certain, and observations at different places are more strictly comparable. maxima thermometers. = . rutherford's maximum thermometer.=--the maximum thermometer, invented by dr. john rutherford, differs from an ordinary thermometer in having a small cylinder of steel, porcelain, or aluminium, moving freely in the tube beyond the mercury, so as to form an index. the stem of the thermometer is fixed horizontally on the frame, which must be suspended in the same position, as represented in fig. . the instrument is set by holding it bulb downward, so as to allow the index to fall by its own gravity into contact with the mercury. increase of heat produces expansion of the mercury, which consequently pushes forward the index. when the temperature decreases, the mercury recedes from the index, leaving it so that the extremity which was in contact with the mercury indicates upon the scale the highest temperature since the instrument was last set. [illustration: fig. .] as it is easily constructed and is comparatively cheap, it is still employed for ordinary purposes. its disadvantages are, firstly, its liability of soon getting out of order by the index becoming embedded in the mercury, or fixed by oxidation, thus rendering it altogether useless; secondly, the ease with which the index can be displaced by the wind moving the instrument, or other accidental disturbance, so as to cause it to give erroneous indications occasionally; and thirdly, its consequent total unfitness for use at sea. in the part of the tube beyond the mercury, a small quantity of air is enclosed for the purpose of preventing the metal flowing freely in the tube. this necessitates the construction of a larger bulb, which renders the thermometer less sensitive. moreover, as it frequently happens that some mercury passes the index, particles of air insinuate themselves in the metal, and cause separations in the column, which very often can be removed only by a maker. to facilitate this re-adjustment, a small chamber is left at the end of the tube, and the mercury being expanded into it by heat until the index and air bubbles are forced into it, if possible, upon the cooling down again, by a little management, the mercury will contract, leaving the air and index behind. yet sometimes the index cannot be moved in the least from its place of fixture, so that the instrument must be virtually reconstructed. = . phillip's maximum thermometer.=--a maximum thermometer, better perhaps in its action than rutherford's, has been suggested by professor john phillips, of oxford. a small portion of air is introduced into an ordinary thermometer, so as to cut off about half an inch of the mercurial thread near its end in the tube. this forms a maximum thermometer, when the stem is arranged horizontally. the isolated portion is pushed forward by expansion, and is left in this position when the mercury contracts. the end remote from the bulb shows on the scale the maximum temperature. when made with a capillary tube so fine that the attraction arising from capillarity overcomes the force of gravity, and prevents the mercury falling to the end of the tube when the instrument is inverted, it forms a very serviceable thermometer, quite portable and suitable for use on board ship. in such a tube a smart shake from a swing of the hand is required to bring the detached portion back to the column, so as to set the instrument for future observation; no ordinary motion will move it. when the thermometer has not this peculiarity, the mercury will flow to the end, if held bulb downward; and in this state it is not at all a satisfactory instrument, as the air is likely to be displaced, and a great deal of tact is requisite to again get it to divide the column suitably. it has been found in practice that the air bubble at different temperatures assumes different lengths, and if very small it disappears in a few years by oxidation and by diffusion with the mercury, so that the instrument becomes defective and uncertain in action,--results which led to the construction of the self-registering mercurial maximum thermometer, invented and patented by messrs. negretti and zambra. it has been before the public about twelve years; we may therefore, now, safely speak of its merits. = . negretti and zambra's patent maximum thermometer= consists of a glass tube containing mercury fitted on an engraved scale, as shown in fig. . the part of the thermometer tube above the mercury is entirely free from air; and at the point a in the bend above the bulb, is inserted and fixed with the blow-pipe a small piece of solid glass, or enamel, which acts as a valve, allowing mercury to pass on one side of it when heat is applied, but not allowing it to return when the thermometer cools. when mercury has been once made to pass the contraction, which nothing but the expansive force of heat can effect, and has risen in the tube, the upper end of the column registers the maximum temperature. to return the mercury to the bulb, we must apply a force equal to that which raised it in the tube; the force employed is gravity, assisted when necessary by a little agitation of the instrument. [illustration: fig. .] the degrees are generally divided on the stems of these thermometers, but their frames of course bear a scale as well. the makers have various styles of framing in wood, metal, porcelain, and even glass. each material is eligible according to requirements. porcelain scales, having the marks _etched_ upon them by acid and permanently blackened and baked in,--by a process for which the inventors have a separate patent,--will be found very serviceable, as they do not corrode or tarnish by exposure to any kind of weather; while any amount of dust and dirt can readily be cleaned off. the chief recommendation of this thermometer is its simplicity of construction, enabling it to be used with confidence and safety. of no other maximum thermometer can it be said that it is impossible to derange or put it out of order; hence, as regards durability, it surpasses all others. nothing short of actual breakage can cause it to fail. hence it is the most easily portable of all self-registering thermometers, an advantage which renders it suitable for travellers, and for transmission abroad. in the year , the british meteorological society reported this thermometer to be "the best which has yet been constructed for maximum temperature, and particularly for sun observations." since then eleven years have elapsed, and it is still without a rival. _directions for use._ in using this thermometer for meteorological observations, it should be suspended by means of two brass plates b, c, attached for that purpose, in such manner that it hangs raised up a little at c, and so placed that it is in the shade, with the air passing freely to it from all sides; then, on an increase of heat, the mercury will pass up the tube as in an ordinary thermometer, and continue doing so as long as the heat increases. on a decrease of heat, the contraction of mercury will take place _below_ the _bend_ in the tube, leaving the whole column of mercury in the tube, thus registering the highest temperature, and showing such till the instrument is disturbed. to prepare the instrument for future observations, remove and hold it perpendicularly, with the bulb downward, and then shake it. the mercury will then descend in the tube, and indicate the temperature of the air at that time; and, when again suspended, is prepared for future observation. after the temperature has attained a maximum, there will be, with a decrease of heat, a slight contraction of mercury in the tube--as well as of that in the bulb--and hence doubts have arisen as to the accuracy of the registration; but calculation shows, and critical trial has proved, that the greatest daily range of temperature will not produce an error large enough to be appreciable on the scale. a very great advantage of this thermometer is that the mercury may be allowed to flow to the end of the tube without the maximum temperature attained during an experiment being lost. it can be employed with the bulb uppermost. all that is necessary for reading the maximum temperature is to slope the instrument so that the mercury flows gently towards the bulb. it will then stop at the contraction so as to show the maximum temperature on the scale. afterwards the mercury is driven into the bulb by agitating the instrument while held in the hand. hence the instrument is invaluable as a registering thermometer on board ship, as its indications are in no way affected by the motions and tremors of the vessel. for physiological experiments, such as taking the temperature of the mouth in fever, this thermometer is the only one that can be used with certainty, as it can be held in any position, without losing the maximum temperature attained. minima thermometers. = . rutherford's alcohol minimum thermometer=, fig. , consists of a glass tube, the bulb and part of the bore of which is filled with perfectly pure spirits of wine, in which moves freely a black glass index. a slight elevation of the thermometer, bulb uppermost, will cause the glass index to flow to the surface of the liquid, where it will remain, unless violently shaken. on a _decrease_ of temperature the alcohol recedes, taking with it the glass index; on an _increase_ of temperature the alcohol alone ascends in the tube, leaving the end of the index _farthest_ from the bulb indicating the minimum temperature. [illustration: fig. .] _directions for using, &c._--having caused the glass index to flow to the end of the column of spirit, by slightly tilting the thermometer, bulb uppermost, suspend the instrument (in the shade with the air passing freely to it on all sides) by the two brass plates attached for that purpose,--in such manner that the bulb is about half an inch lower than the upper, or the end of the thermometer farthest from the bulb; then, on a decrease of temperature, the spirits of wine will descend, carrying with it the glass index; on an increase of temperature, however, the spirits of wine will ascend in the tube, leaving that end of the small glass index farthest from the bulb indicating the minimum temperature. to reset the instrument, simply raise the bulb end of the thermometer a little, as before observed, and the index will again descend to the end of the column, ready for future observation. _precautions._-- . by no means jerk or shake an alcohol minimum thermometer _when resetting_ it, for by so doing it is liable to disarrange the instrument, either by causing the index to leave the spirit, or by separating a portion of the spirit from the main column. . as alcohol thermometers have a tendency to read lower by age, owing to the volatile nature of the fluid allowing particles in the form of vapour to rise and lodge in the tube, it becomes necessary to compare them occasionally with a mercurial thermometer whose index error is known; and if the difference be more than a few tenths of a degree, examine well the upper part of the tube to see if any alcohol is hanging in the bore thereof; if so, the detached portion of it can be joined to the main column by swinging the thermometer with a pendulous motion, _bulb downwards_. . the spirit column is sometimes much separated by jolting in travelling. if the instrument is in such a condition when received, it should be held by the right hand, bulb downward, and the frame tapped smartly, but cautiously, against the palm of the left hand. the broken thread of spirit will soon begin to join, and by continuing the operation a sufficient time all the bubbles will disappear, and the thermometer become as good as ever. = . horticultural minimum thermometer.=--this instrument, represented in fig. , is a special construction of rutherford's minimum thermometer to meet the requirements of horticulturists. it is desirable, if not essential, that gardeners should have the means of ascertaining to what temperature stoves and greenhouses descend on cold nights, especially in winter. this thermometer is mounted on a strong cast zinc frame, with the divisions and figures of the scale raised. [illustration: fig. .] the sunk surface of the frame is painted dark; the figures and division a bright colour, so that observations can be made without a close inspection of the instrument. the directions for using are the same as those given in the preceding section. it may be used as an ordinary thermometer, by simply hanging it from the top loop, in which position, the coloured liquid will always indicate the present temperature. it was a source of annoyance with the ordinary boxwood and flat metal scales, that after a time, exposure to a damp warm atmosphere favoured the growth of confervæ upon them, and obliterated the divisions; the plan of raising the figures and divisions of the scale has been found to prevent the destruction of the instrument in this way. = . baudin's alcohol minimum thermometer.=--this instrument resembles rutherford's thermometer in appearance; its indications are given by the expansion and contraction of alcohol, and its minimum temperature is likewise registered by a glass index being pulled back and left behind by the alcohol, as in rutherford's instrument. there is, however, a great improvement in baudin's instrument; for whilst rutherford's thermometer can only register in a horizontal position, baudin's can be used either horizontally or vertically, as necessity may require. this important change is effected in the following manner:--instead of the index in the thermometer being loose and free to run up and down according to the position in which the instrument is held, as in rutherford's, the index in the new instrument is made to fit the bore of the tube as nearly tight as possible, so much so that in holding the thermometer even upside down, or shaking it, the index will not shift from its position; but, inasmuch as a minimum thermometer with an immoveable index could not be set when required for observation, and would consequently be useless, the inventor has introduced behind the index a piece of solid glass, about one-and-a-half inch in length, which moves freely in the alcohol. the addition of the weight of this piece of glass on the top of the index, when turned upside down, forces the index down to the edge of the alcohol; and it is there left, as in the case of the ordinary rutherford's thermometer. it is, therefore, by turning the thermometer upside down, and letting the moveable piece of glass fall on the index, that the index is driven to the end of the alcohol; after this operation the thermometer is hung up either horizontally or vertically, and will then be ready for use. the index, although immoveable _per se_, is by the alcohol drawn back, as in the ordinary minimum, and its indications are read off on the scale from the top of the index. = . mercurial minima thermometers desirable.=--alcohol does not expand equally for equal increments of heat, consequently errors are likely to exist in the scale indications unless the graduations are very accurately--not necessarily equally--made. on this account, as well as from the volatility of alcohol, and the intervention of gaseous partitions in the tube, a good and thoroughly reliable minimum thermometer was for a long time a desideratum. it was desirable to obtain a thermometer which should register the lowest temperature by mercury, the fluid in general use for meteorological thermometers. several instruments have recently been invented to meet this requirement, which are suitable and satisfactory for land purposes, but one well adapted for use on board ship is still very much wanted. for very low temperatures, alcohol thermometers will always be required; as mercury freezes at - ° f, and contracts very irregularly much before this point, while alcohol has never yet been frozen. = . negretti and zambra's patent mercurial minimum thermometer=, represented by fig. , has a cylindrical bulb of large size, which, at first sight, might induce the idea that the instrument would not be sufficiently sensitive; but as length is given to the cylinder instead of increasing its diameter, it will be found as sensitive as a globular bulb of the same diameter, and much more so than an ordinary alcohol thermometer. [illustration: fig. .] the reason for having the bulb large is to allow the internal diameter of the thermometer tube to be larger than that generally used for thermometrical purposes, so that a steel index, pointed at both ends, may move freely within when required. the tube is blown, filled and regulated in the usual way, ° of temperature being about half-way up the tube. a small cylindrical bulb is then formed at the upper end of the tube, and then is introduced a steel needle pointed at both ends, that in contact with the mercury being abrupt, the other more prolonged. the open extremity of the tube is now drawn out into a fine capillary tube, and the bulb of the instrument warmed so as to cause the mercury to fill the tube completely. when the mercury reaches the capillary tube, the flame of a blow-pipe is applied; the glass is dexterously melted, the superfluous part taken away, and the tube left hermetically closed. during this operation, the steel index has been embedded in the heated mercury. as the instrument cools, if held upright, the mercury will recede and expose the needle, which will then follow the descending column simply by its own gravity. in this condition the thermometer resembles rutherford's maximum, being a tube of mercury with a steel index floating on its surface; but it possesses these important advantages: it is quite free from air, so that the mercury can move with perfect freedom; and the index is pointed at both ends, to allow the mercury to pass, instead of being ground flat to prevent it. _to use the thermometer_, it is suspended perpendicularly (figure ) with the steel index resting on the surface of the mercurial column. as the mercury in the cylinder contracts, that in the tube descends, and the index, of its own gravity, follows it; on the contrary, as the mercury expands and rises in the tube, it passes the index on one side, and in rising, exerts a lateral pressure on the needle, and jams it to one side of the tube, where it remains firmly fixed, leaving the upper point of the needle indicating the minimum temperature. in this thermometer, the reading is always from the upper point of the needle, and not from the mercury itself. _to extricate the needle_ from the mercury, a magnet is used, when, if the needle is embedded only a few degrees, it can readily be withdrawn without altering the position of the instrument. should the magnet not be sufficient for the purpose, we simply turn the thermometer on its support from the upright position, slightly elevating the bulb (fig. (= =)). the mercury and index will then flow into the small reservoir. should the index not freely leave the tube with the mercury, assist it with a magnet, and when the mercury and index are in the upper bulb (figure = =), apply a magnet outside, which will attract and hold fast the index; and whilst thus holding it, again bring the thermometer to the upright position, when the mercury will immediately fall back into the tube, leaving the index attached to the magnet (figure = =), with which it is guided down to the surface of the mercury, ready for another observation. [illustration: fig. .] care must be taken not to withdraw the magnet until the index is in contact with mercury; for, if released before touching, it might plunge too deeply, and give a false indication. the rule for re-setting it will be to bring the needle-point in contact with the mercury, and then withdraw the magnet, having previously ascertained that no particles of mercury are attached to the index. it may sometimes, though rarely, happen, that from the time a minimum temperature is registered by the index, and by the time an observation is made, the mercury may have risen so high in the tube as to completely pass the index, as shown (figure = =). should it so happen, the space which the index occupies will readily be observed, as it will be pressed to one side of the tube, causing a different appearance in that part, although the point of the needle may not be seen. if such be the case, apply a magnet to the spot where you see the index is fixed: this will hold the needle firmly. then, by slightly tilting the thermometer bulb uppermost, the mercury will flow into the top bulb, leaving the index attached to the magnet, and quite uncovered. having taken the reading, draw the needle into the top bulb, and hold it there whilst you adjust the thermometer by again bringing it to the upright position. by contracting the bore of this thermometer, at the bend of the tube, sufficiently to keep the mercury from flowing out of its bulb with too much freedom by motion, the instrument becomes perfectly safe for transmission abroad. = . negretti & zambra's second patent mercurial minimum thermometer.=--in this thermometer a principle is used that has been long known to scientific men, viz. the affinity of mercury for platinum. if mercury be placed in contact with platinum under ordinary circumstances, no effect will take place; but if the mercury is once made to attack the platinum, the amalgamation is permanent and the contact perfect, so much so, that the principle was made use of in constructing standard barometers. a ring of platinum was fused round the end of the tube, dipping into the mercury; and the contact between the platinum and mercury became so perfect that air could not creep down the tube and up the bore, as in ordinary barometer tubes. this principle of adhesion or affinity of mercury for platinum has been brought into play for the purpose of arresting the mercury after it has reached the minimum temperature in a thermometer. this thermometer is made as follows:--behind the bulb is placed a supplementary chamber; in the space or neck between the bulb of the thermometer and the chamber, is placed a small piece of platinum; this may be of any shape or size, but the smaller the better. this is not to fit in the neck; it must, on the contrary, be rather loose; it may be fastened in position or not. the instrument is represented by fig. . [illustration: fig. .] _directions for using._--having suspended the thermometer in a horizontal position, the mercury is made to stand in exact contact with the platinum plug by slightly elevating the bulb end of the instrument. the thermometer is now ready for observation. on a decrease of temperature, the mercury will endeavour to contract first from the easier passage, viz. behind the bulb; but in consequence of the adhesion of the mercury to the platinum, it cannot recede from here, it is therefore forced to contract from the indicating tube, and will continue to do so as long as the temperature decreases; and as no indices are employed in this thermometer, the extreme end of the mercurial column will show "how cold it has been." on an increase of temperature the mercury will glide over the platinum plug and expand by the easier passage into the supplementary chamber, and there remain until a decrease of temperature again takes place, when the mercury that had gone into the supplementary chamber will be the first to recede, until it reaches the platinum plug, its further progress being arrested; it will then fall in the indicating tube, and there remain until re-set. = . casella's mercurial minimum thermometer.=--the general form and arrangement of this instrument is shown in fig. . a tube with large bore, _a_, has at the end a _flat glass diaphragm_ formed by the abrupt junction of a small chamber, _b c_, the inlet to which at _b_ is larger than the bore of the indicating tube. the result of this is that on setting the thermometer, as described below, the contracting force of the mercury in cooling withdraws the fluid in the indicating stem only; whilst on its expanding with heat, the long column does not move, the increased bulk of mercury finding an easier passage into the small pear-shaped chamber attached. [illustration: fig. .] we believe that a small speck of air must be confined in the chamber, _b c_, to act as a spring to start the mercury from the chamber in the act of setting the thermometer. were this air not present, the mercury would so adhere to the glass that no amount of shaking could induce it to flow from the chamber. _to set the instrument_, place it in a horizontal position, with the back plate, _d_, suspended on a nail, and the lower part supported on a hook, _e_. the bulb end may now be gently raised or lowered, causing the mercury to flow slowly until the bent part, _a_, _is full_ and the chamber, _b c_, _quite empty_. at this point the flow of mercury in the long stem of the tube is arrested, _and indicates the exact temperature_ of the bulb or air at the time. on an increase of temperature the mercury will expand into the small chamber, _b c_; and a return of cold will cause its recession from this chamber only, until it reaches the diaphragm, _b_. any further diminution of heat withdraws the mercury down the bore to whatever degree the cold may attain, where it remains until farther withdrawn by increased cold, or till re-set for future observation. maxima and minima thermometers. = . rutherford's= arrangement for obtaining a complete instrument for the registration of heat and cold was simply mounting a maximum thermometer and a minimum thermometer upon the same frame or slab. thus constructed, they are often called "day and night" thermometers, though somewhat inappropriately; for in temperate climates the temperature of the night sometimes exceeds that of the day, notwithstanding the reverse is the general law of temperature. fig. will explain the arrangement of rutherford's day and night thermometer. [illustration: fig. .] = . sixe's self-registering thermometer.=--the very ingenious and certainly elegant instrument about to be described was invented by james sixe, of colchester. it consists of a long cylindrical bulb, united to a tube of more than twice its length, bent round each side of it in the form of a syphon, and terminated in a smaller, oval-shaped bulb. figure gives a representation of this instrument. the lower portion of the syphon is filled with mercury; the long bulb, the other parts of the tube, and part of the small bulb, with highly rectified alcohol. a steel index moves in the spirit in each limb of the syphon. the two indices are terminated at top and bottom with a bead of glass, to enable them to move with the least possible friction, and without causing separation of the spirit, or allowing mercury to pass easily. they would, from their weight, always rest upon the mercury; but each has a fine hair tied to its upper extremity and bent against the interior of the tube, which acts as a spring with sufficient elasticity to keep the index supported in the spirit in opposition to gravity. [illustration: fig. .] the instrument acts as follows:--a rise of temperature causes the spirit in the long bulb to expand and press some of the mercury into the other limb of the syphon, into which it rises also from its own expansion, and carries the index with it, until the greatest temperature is attained. the lower end of this index then indicates upon the engraved scale the maximum temperature. as the temperature falls the spirit and the mercury contract, and in returning towards the bulb the second index is met and carried up by the mercury until the lowest temperature occurs, when it is left to indicate upon the scale the minimum temperature. the limb of the syphon adjoining the bulb requires, therefore, a descending scale of thermometric degrees; the other limb, an ascending scale. the graduations must be obtained by comparisons with a standard thermometer under artificial temperatures, which should be done in this way for every °, in order to correct for the inequality in the bore of the tube, and the irregular expansion of the spirit. the instrument is set for observation by bringing the indices into contact with the mercury, by means of a small magnet, which attracts the steel through the glass, so that it is readily drawn up or down. they should be drawn nearly to the top of the limbs when it is desired to remove the instrument, which should be carefully carried in the vertical position; for should it be inverted, or laid flat, the spirit may get among the mercury, and so break up the column as to require the skill of a maker to put it in order again. for transmission by ordinary conveyances, it requires that attention be given to keep it vertical. the entanglement of a small portion of mercury with the indices is sometimes a source of annoyance in this instrument, for the readings are thereby rendered somewhat incorrect. small breakages in the mercury, either from intervening bubbles of spirit or adhesion to the indices, may generally be rectified by cautiously tapping the frame of the instrument, so as to cause the mercury to unite by the assistance thus given to its superior gravity. these thermometers, when carefully made and adjusted to a standard thermometer, are strongly recommended for ordinary purposes, where strict scientific accuracy is not required. this is also the only fluid thermometer applicable for determining the temperature of the sea at depths. chapter viii. radiation thermometers. = . solar and terrestrial radiation considered.=--the surface of the earth absorbs the heat of the sun during the day, and radiates heat into space during the night. the envelope of gases and vapour, which we call the atmosphere, exerts highly important functions upon these processes. thanks to the researches of professor tyndall, we are now enabled to understand these functions much more clearly than heretofore. his elaborate, patient, and remarkably sagacious series of experiments upon radiant heat, have satisfactorily demonstrated that _dry_ air is as transparent to radiant heat as the vacuum itself; while air _perfectly saturated_ with aqueous vapour absorbs more than five per cent. of radiant heat, estimated by the thermal unit adopted for the galvanometer indications of the effect upon a thermo-electric pile. aqueous vapour, in the form of fog or mist, as is well known, gives to our sensation a feeling of cold, and interferes with the healthy action of the skin and the lungs; the cause being its property of absorbing heat from our person. air containing moisture in an invisible state likewise exerts a remarkable influence in radiating and absorbing heat. by reason of these properties, aqueous vapour acts as a kind of blanket upon the ground, and maintains upon it a higher temperature than it would otherwise have. "regarding the earth as a source of heat, no doubt at least ten per cent. of its heat is intercepted within ten feet of the surface." thus vapour--whether transparent and invisible, or visible, as cloud, fog, or mist--is intimately connected with the important operations of solar and terrestrial radiation. cloudy, or humid days, diminish the effect upon the soil of solar radiation; similar nights retard the radiation from the earth. a dry atmosphere is the most favourable for the direct transmission of the sun's rays; and the withdrawal of the sun from any region over which the air is dry, must be followed by very rapid cooling of the soil. "the removal, for a single summer night, of the aqueous vapour from the atmosphere which covers england, would be attended by the destruction of every plant which a freezing temperature could kill. in sahara, where 'the soil is fire and the wind is flame,' the refrigeration at night is often painful to bear. ice has been formed in this region at night. in australia, also, the _diurnal range_ of temperature is very great, amounting, commonly, to between and degrees. in short, it may be safely predicted, that wherever the air is _dry_, the daily thermometric range will be great. this, however, is quite different from saying that when the air is _clear_, the thermometric range will be great. great clearness to light is perfectly compatible with great opacity to heat; the atmosphere may be charged with aqueous vapour while a deep blue sky is overhead; and on such occasions the terrestrial radiation would, notwithstanding the 'clearness,' be intercepted." the great range of the thermometer is attributable to the absence of that protection against gain or loss of heat which is afforded when aqueous vapour is present in the air; and during such weather the rapid abstraction of moisture from the surface of plants and animals is very deleterious to their healthy condition. "the nipping of tender plants by frost, even when the air of the garden is some degrees above the freezing temperature, is also to be referred to chilling by radiation." hence the practice of gardeners of spreading thin mats, of bad radiating material, over tender plants, is often attended with great benefit. by means of the process of terrestrial radiation ice is artificially formed in bengal, "where the substance is never formed naturally. shallow pits are dug, which are partially filled with straw, and on the straw flat pans containing water which had been boiled is exposed to the clear firmament. the water is a very powerful radiant, and sends off its heat into space. the heat thus lost cannot be supplied from the earth--this source being cut off by the non-conducting straw. before sunrise a cake of ice is formed in each vessel.... to produce the ice in abundance, the atmosphere must not only be clear, but it must be comparatively free from aqueous vapour." considering, therefore, the important consequences attending both terrestrial and solar radiation, it appears to us that observations from radiation thermometers are of much more utility in judging of climate than is usually supposed. these observations are very scanty; and what few are upon record are not very reliable, principally from bad exposure of the instruments, while the want of uniformity in construction may be another cause. herschell's actinometer and pouillet's pyrheliometer, instruments for ascertaining the absolute heating effect of the sun's rays, should, however, be more generally employed by meteorologists. in comparing observations on radiation it should be kept in mind, that "the difference between a thermometer which, properly confined [or shaded], gives the true temperature of the night air, and one which is permitted to radiate freely towards space, must be greater at high elevations than at low ones;"[ ] because the higher the place, the less the thickness of the vapour-screen to intercept the radiation. = . solar radiation thermometer.=--"as the interchange of heat between two bodies by radiation depends upon the relative temperature which they respectively possess, the earth, by the rays transmitted from the sun during the day, must be continually gaining an accession of heat, which would be far from being counterbalanced by the opposite effect of its own radiation into space. hence, from sunrise till two or three hours after mid-day, the earth goes on gradually increasing in temperature, the augmentation being greatest where the surface consists of materials calculated, from their colour and texture, to absorb heat, and where it is deficient in moisture, which, by its evaporation, would have a tendency to diminish it."[ ] it is, therefore, important to have instruments for measuring the efficacy of solar radiation, apart from those for exhibiting the temperature of the place in the shade. [illustration: fig. .] fig. shows the arrangement of negretti & zambra's maximum thermometer, for registering the greatest heat of the sun's direct rays, hence called a _solar radiation thermometer_. it has a blackened bulb, the scale divided on its own stem, and the divisions protected by a glass shield. in use it should be placed nearly horizontally, resting on y supports of wood or metal, with its bulb in the full rays of the sun, resting on grass, and, if possible, so that lateral winds should not strike the bulb; and at a sufficient distance from any wall, so that it does not receive any _reflected_ heat from the sun. some observers place the thermometer as much as two feet from the ground. it would be very desirable if one uniform plan could be recognized: that of placing the instrument as indicated in the figure appears to be most generally adopted, and the least objectionable. = . vacuum solar radiation thermometer.=--in order that the heat absorbed by the blackened bulb of the solar radiation thermometer may not in part be carried off by the currents of air which would come into contact with it, the instrument has been improved by messrs. negretti and zambra into the _vacuum solar radiation thermometer_, as illustrated by fig. . [illustration: fig. .] this consists of a blackened-bulb radiation thermometer, enclosed in a glass tube and globe, from which all air is exhausted. thus protected from the loss of heat which would ensue if the bulb were exposed, its indications are from ° to ° higher than when placed side by side with a similar instrument with the bulb exposed to the passing air. at times when the air has been in rapid motion, the difference between the reading of a thermometer giving the true temperature of the air in the shade, and an ordinary solar radiation thermometer, has been ° only, whilst the difference between the air temperature and the reading of a radiation thermometer in vacuo has been as large as °. it is also found that the readings are almost identical at distances from the earth varying from six inches to eighteen inches. by the use of this improvement, it is hoped that the amounts of solar radiation at different places may be rendered comparable; hitherto they have not been so; the results found at different places cannot be compared, as the bulbs of the thermometers are under very different circumstances as to exposure and currents of air. important results are anticipated from this arrangement. the observations at different places are expected to present more agreement. observers would do well to note carefully the effect of any remarkable degree of intensity in the solar heat upon particular plants, crops, fruit or other trees. = . terrestrial radiation thermometer= is an alcohol minimum thermometer, with the graduations etched upon the stem, and protected by a glass shield, as shown in figure , instead of being mounted on a frame. the bulb is transparent; that is to say, the spirit is not coloured. [illustration: fig. .] in use, it should be placed with its bulb fully exposed to the sky, resting on grass, the stem being supported by little forks of wood. the precautions required with this thermometer are similar to those for ordinary spirit thermometers, explained at page . [illustration: fig. .] = . Æthrioscope.=--the celebrated experimental philosopher, sir john leslie, was the inventor of this instrument, the purpose of which is to give a comparative idea of the radiation proceeding from the surface of the earth towards the sky. it consists, as represented in fig. , of two glass bulbs united by a vertical glass tube, of so fine a bore that a little coloured liquid is supported in it by its own adhesion, there being air confined in each of the bulbs. the bulb, _a_, is enclosed in a highly polished brass sphere, _d_, made in halves and screwed together. the bulb, _b_, is blackened and placed in the centre of a metallic cup, _c_, which is well gilt on the inside, and which may be covered by a top, _f_. the brass coverings defend both bulbs from solar radiation, or any adventitious source of heat. when the top is on, the liquid remains at zero of the scale. on removing the top and presenting the instrument to a clear sky, either by night or by day, the bulb, _b_, is cooled by terrestrial radiation, while the bulb, _a_, retains the temperature of the air. the air confined in _b_, therefore, contracts; and the elasticity of that within _a_ forces the liquid up the tube, to a height proportionate to the intensity of the radiation. such is the sensitiveness of the instrument, that the smallest cloud passing over it checks the rise of the liquid. sir john leslie says:--"under a clear blue sky, the _æthrioscope_ will sometimes indicate a cold of fifty millesimal degrees; yet, on other days, _when the air seems equally bright_, the effect is hardly °." this anomaly, according to dr. tyndall, is simply due to the difference in the quantity of aqueous vapour present in the atmosphere. the presence of invisible vapour intercepts the radiation from the æthrioscope, while its absence opens a door for the escape of this radiation into space. = . pouillet's pyrheliometer.=--"this instrument is composed of a shallow cylinder of steel, _a_, fig. , which is filled with mercury. into the cylinder a thermometer, _d_, is introduced, the stem of which is protected by a piece of brass tubing. we thus obtain the temperature of the mercury. the flat end of the cylinder is to be turned towards the sun, and the surface, _b_, thus presented is coated with lamp black. there is a collar and screw, _c_, by means of which the instrument may be attached to a stake driven into the ground, or into the snow, if the observations are made at considerable heights. it is necessary that the surface which receives the sun's rays should be perpendicular to the rays; and this is secured by appending to the brass tube which shields the stem of the thermometer, a disk, _e_, of precisely the same diameter as the steel cylinder. when the shadow of the cylinder accurately covers the disk, we are sure that the rays fall, as perpendiculars, on the upturned surface of the cylinder. [illustration: fig. .] "the observations are made in the following manner:--first, the instrument is permitted, not to receive the sun's rays, but to radiate its own heat for five minutes against an unclouded part of the firmament; the decrease of the temperature of the mercury consequent on this radiation is then noted. next, the instrument is turned towards the sun, so that the solar rays fall perpendicularly upon it for five minutes; the augmentation of heat is now noted. finally, the instrument is turned again towards the firmament, away from the sun, and allowed to radiate for another five minutes, the sinking of the thermometer being noted as before. in order to obtain the whole heating power of the sun, we must add to his observed heating power the quantity lost during the time of exposure, and this quantity is the mean of the first and last observations. supposing the letter _r_ to represent the augmentation of temperature by five minutes' exposure to the sun, and that _t_ and _t¹_ represent the reductions of temperature observed before and after, then the whole force of the sun, which we may call _t_, would be thus expressed:--_t = r + / (t + t¹)_. "the surface on which the sun's rays here fall is known; the quantity of mercury within the cylinder is also known; hence we can express the effect of the sun's heat upon a given area, by stating that it is competent, in five minutes, to raise so much mercury so many degrees in temperature."--_dr. tyndall's "heat considered as a mode of motion."_ [illustration: fig. .] = . sir john herschell's actinometer=, for ascertaining the absolute heating effect of the solar rays, in which _time_ is considered one of the elements of observation, is illustrated by fig. . the actinometer consists of a large cylindrical thermometer bulb, with a scale considerably lengthened, so that minute changes may be easily seen. the bulb is of transparent glass filled with a deep blue liquid, which is expanded when the rays of the sun fall direct on the bulb. to take an observation, the actinometer is placed in the shade for one minute and read off; it is then exposed for one minute to sunshine, and its indication recorded; it is finally restored to the shade, and its reading noted. the mean of the two readings in the shade, subtracted from that in the sun, gives the actual amount of expansion of the liquid produced by the sun's rays in one minute of time. for further information, see _report of the royal society on physics and meteorology_; or _kæmtz's meteorology_, translated by c. v. walker; or the _admiralty manual of scientific instructions_. chapter ix. deep-sea thermometers. = . on sixe's principle.=--thermometers for ascertaining the temperature of the sea at various depths are constructed to register either the maximum or minimum temperature, or both. the principle of each instrument is that of sixe. there are very few parts of the ocean in which the temperature below is greater than at the surface, except in the polar seas, where it is generally found to be a few degrees warmer at considerable depths than at the surface. when the instrument is required to register only one temperature, it can be made narrower and more compact--a great advantage in sounding; and with less length of bulb and glass tube, so that the liability of error is diminished. hence, the minimum is the most generally useful for deep-sea soundings. these thermometers must be sufficiently strong to withstand the pressure of the ocean at two or three miles of depth, where there may be a force exerted to compress them exceeding three or four hundred atmospheres (of lbs. to the square inch). many have been the contrivances for obtaining correct deep-sea indications. thermometers and machines of various sorts have been suggested, adopted, and eventually abandoned as only approximate instruments. the principal reason for such instruments failing to give correct or reliable indications, has been that the weight or pressure on the bulbs at great depths has interfered with the correct reading of the instruments. thermometers have been enclosed in strong water-tight cases to resist the pressure; but this contrivance has only had the tendency to retard the action, so much so as to throw a doubt on the indications obtained by the instrument so constructed. the thermometers constructed by messrs. negretti and zambra for this purpose do not differ materially from those usually made under the denomination of sixe's thermometers, except in the following most important particular:--the usual sixe's thermometers have a central reservoir or cylinder containing alcohol; this reservoir, which is the only portion of the instrument likely to be affected by pressure, has been, in negretti and zambra's new instrument, superseded by a strong outer cylinder of glass, containing mercury and rarefied air; by this means the portion of the instrument susceptible of compression, has been so strengthened that no amount of pressure can possibly make the instrument vary. this instrument has been tested in every possible manner, and the results have been highly satisfactory, so much so as to place their reliability beyond any possible doubt. the scales are made of porcelain, and are firmly secured to a back of oak, which holds in a recess the bulb with its protecting shield, and is rounded off so as to fit easily and firmly in a stout cylindrical copper case, in which the thermometer is sent down when sounding (see fig. ). the lid of the case is made to fit down closely, and water-tight. at the bottom of the case is a valve opening upward; and the lid has a similar valve. these allow the water to pass through the case as the instrument sinks, so that the least amount of obstruction is offered to the descent. at the lower end of the case is a stout brass spring, to protect the instrument from a sudden jar if it should touch the bottom while descending rapidly. as the instrument is drawn up, the valves close with the weight of water upon them, and it arrives at the surface filled with water brought up from its lowest position. the deep-sea thermometers used in the royal navy are of this pattern. [illustration: fig. .] = . johnson's metallic deep-sea thermometer.=--the objection to the employment of mercurial thermometers for ascertaining the temperature of the ocean at depths, arising from the compression of the bulbs, which was of such serious consequence previous to the modification made in the construction of the instrument by messrs. negretti and zambra, led to the construction of a metallic thermometer altogether free from liability of disturbance from compression by the surrounding water; which, however, is certainly not so sensitive to changes of temperature as mercury. this instrument is the invention of henry johnson, esq., f.r.a.s., and is thus described by him:-- "during the year some experiments were made by james glaisher, esq., f.r.s., on the temperature of the water of the thames near greenwich at the different seasons of the year; when that gentleman found that the indications of temperature were greatly affected by the pressure on the bulbs of the thermometers. at a depth of feet this pressure would be nearly equal to the presence of three-fourths of an atmosphere. these observations demonstrate the importance of using in deep-sea soundings an instrument free from liability of disturbance from compression by the surrounding water, and have ultimately led to the construction of the thermometer now to be described. "the instrument is composed of solid metals of considerable specific gravity, viz. of brass and steel, the specific gravity of these metals being · and · respectively. they are therefore not liable to compression by the water, which under a pressure of , atmospheres, or at a depth of , fathoms in round numbers, acquires a density or specific gravity of · . in the construction of this instrument, advantage has been taken of the well-known difference in the ratios of expansion and contraction by heat and cold of brass and steel, to form compound bars of thin bars of these metals riveted together; and which will be found to assume a slight curve in one direction when heat has expanded the brass more than the steel, and a slight one in the contrary direction when cold has contracted the brass more than the steel. "the indications of the instrument record the motions under changes of temperature of such compound bars; in which the proportion of brass, the more dilatable metal, is two-thirds, and of steel one-third. [illustration: fig. .] "upon one end of a narrow plate of metal about a foot long, _a_, are fixed three scales of temperature, _h_, which ascend from ° to ° f., and which are shown more clearly in the drawing detached from the instrument. upon one of these scales the present temperature is shown by the pointer, _e_, which turns upon a pivot in its centre. the register index, _g_, to the maximum temperature, and the index, _f_, to the minimum temperature, are moved along the other scales by the pin upon the moving pointer, at _e_, where they are retained by stiff friction. at equal distances from the centre of the pointer are two connecting pieces, _d d_, by which it is attached to the free ends of two compound bars, _b b_, and its movements correspond with the movements of the compound bars under variations of temperature. the other ends of the bars are fastened by the plate, _c_, to the plate, _a_, on which the scales of temperature are fixed. the connection of the bars with both sides of the centre of the pointer prevents disturbance of indication by lateral concussion. the case of the instrument has been improved at the suggestion of admiral fitzroy, and now presents to the water a smooth cylindrical surface, with rounded ends, and without projection of fastenings. "in surveying expeditions, this instrument would be found useful in giving notice of variation of depth of water, and of the necessity for taking soundings. a diminution of the temperature of water has been observed by scientific voyagers to accompany diminution of depth, as on nearing land, or approaching hidden rocks or shoals. attention would also thus be attracted to the vicinity of icebergs." this thermometer might easily be modified to serve for several other important purposes, such as the determination of the temperature of intermittent hot springs, and mud volcanoes. [illustration: fig. .] the principle of this thermometer is not altogether new; but the duplicate arrangement of the bars, which effectually prevents the movement of the indices by any shaking, and the application are certainly novel. professor trail, in the _library of useful knowledge_, writes:--"in , mr. james crighton, of glasgow, published a new 'metallic thermometer,' in which the unequal expansion of zinc and iron is the moving power. a bar is formed by uniting a plate of zinc (fig. ), _c d_, inches long, inch broad, and / inch thick, to a plate of iron, _a b_, of the same length. the lower extremity of the compound bar is firmly attached to a mahogany board at _e e_; a pin, _f_, fixed to its upper end, plays in the forked opening in the short arm of the index, _g_. when the temperature is raised, the superior expansion of the zinc, _c d_, will bend the whole bar, as in the figure; and the index, _g_, will move along the graduated arc, from right to left, in proportion to the temperature. in order to convert it into a _register thermometer_, crighton applied two slender hands, _h h_, on the axis of the index; these lie below the index, and are pushed in opposite directions by the stud, _i_,--a contrivance seemingly borrowed from the instrument of fitzgerald," a complicated metallic thermometer, described by the professor previously. chapter x. boiling-point thermometers. = . ebullition.=--the temperature at which a fluid _boils_ is called the _boiling-point_ of that particular fluid. it is different for different liquids; and, moreover, in the same liquid it varies with certain changes of circumstance. thus the same liquid in various states of purity would have its boiling temperature altered in a slight degree. there is also an intimate connection with the pressure under which a fluid is boiled, and its temperature of ebullition. liquids boiled in the open air are subjected to the atmospheric pressure, which is well known to vary at different times and places; and the boiling-point of the liquid exhibits corresponding changes. when the pressure is increased on the surface of any fluid, the temperature of ebullition rises; and with a decrease of pressure, the boiling goes on at a lower degree of heat. in the case of water, we commonly state the boiling-point to be ° f.; but it is only so at the level of the sea, under the mean pressure of the atmosphere, represented, in the latitude of london, by a column of · inches of mercury, at a temperature of ° f., and when the water is fresh and does not contain any matter chemically dissolved in it. when steam is generated and confined in a boiler, the pressure upon the boiling water may be several times greater than that of the atmosphere. experimentally it has been found, that if the pressure in the boiler be lbs. on the square inch, the temperature of the boiling water, and of the steam likewise, is raised to °; while under the exhausted receiver of an air-pump, water will boil at °, when the pressure is reduced to inches of mercury. = . relation between the boiling-point and elevation.=--now, as the atmospheric pressure is diminished by ascent, as shown by the fall of mercury in the barometer, it follows that in elevated localities water, or any other fluid, heated in the open air, will boil at a temperature lower than at the sea-level. therefore, there must be some relation between the height of a hill, or mountain, and the temperature at which a fluid will boil at that height. hence, the thermometer, as used to determine the boiling-point of fluids, is also an indicator of the atmospheric pressure; and may be used as a substitute for the barometer in measuring elevations. if the atmospheric pressure were constant at the sea-level, and always the same for definite heights, we might expect the boiling-points of fluids also to be in exact accordance with height; and the relation once ascertained, we could readily, by means of the thermometer and boiling water, determine an unknown height, or for a known elevation assert the boiling temperature of a liquid. however, as the atmospheric pressure is perpetually varying at the same place, within certain limits, so there are, as it were, sympathetic changes in the boiling temperatures of fluids. it follows from this, that heights can never be accurately measured, either by the barometer or the boiling-point thermometer, by simply observing at the places whose elevations are required. to determine a height with any approach to accuracy, it is necessary that a similar observation should be made at the same time at a lower station, not very remote laterally from the upper, and that they should be many times repeated. when such observations have been very carefully conducted, the height of the upper station above the lower may be ascertained with great precision, as has been repeatedly verified by subsequent trigonometrical measurement of elevations so determined. if the lower station be at the sea-level, of course the absolute height of the upper is at once obtained. = . mountain thermometer; sometimes called hypsometric apparatus.=--we have now to examine the construction of the boiling-point thermometer, and its necessary appendages, as adapted for the determination of heights. messrs. negretti and zambra's arrangement of the instrument is shown in figures and . [illustration: fig. .] [illustration: fig. .] the thermometer is made with an elongated bulb, so as to be as sensitive as possible. the scale, about a foot long, is graduated on the stem, and ranges from ° to °, each degree being sufficiently large to show the divisions of tenths of a degree. a sliding metallic vernier might perhaps with advantage be attached to the stem, which would enable the observer to mark hundredths of a degree; which, however, he can pretty well do by estimation. the boiler is so contrived as to allow, not only the bulb, but the stem also of the thermometer, to be surrounded by the steam. the arrangement is readily understood by reference to the accompanying diagram, fig. . _c_, is a copper boiler, supported by a tripod stand so as to allow a spirit-lamp, _a_, made of metal to be placed underneath. the flame from the lamp may be surrounded by a fine wire gauze, _b_, which will prevent it being extinguished when experimenting in the external air. _e e e_, is a three-drawn telescope tube, proceeding from the boiler, and open also at top. another tube, similarly constructed, envelops this, as shown by _d d d_. this tube is screwed to the top of the boiler, and has two openings, one at the top to admit the thermometer, the other low down, _g_, to give vent to the steam. as the steam is generated, it rises in the inner tube, passes down between the tubes, and flows away at _g_. the thermometer is passed down, supported by an india-rubber washer, fitting steam tight, so as to leave the top of the mercury, when the boiling-point is attained, sufficiently visible to make the observation. the telescopic movement, and the mode of supporting the thermometer, enable the observer always to keep the bulb near the water, and the double tube gives all the protection required to obtain a steady boiling-point. some boiling-point thermometers are constructed with their scales altogether exposed to the air, which may be very cold, and consequently may contract to some extent the thread of mercury outside the boiler. the steam, having the same temperature as the boiling water, keeps the tube, throughout nearly its whole length, at the same degree of heat, in the apparatus described. the whole can be packed in a tin case very compactly and securely for travelling, as in fig. . _directions for using._--when the apparatus is required for practical use, sufficient water must be poured into the boiler to fill it about one third, through an opening, _f_, which must be afterwards closed by the screw plug. then apply the lighted lamp. in a short time steam will issue from _g_; and the mercury in the thermometer, kept carefully immersed, will rise rapidly until it attains a stationary point, which is the boiling temperature. the observation should now be taken and recorded with as much accuracy as possible, and the temperature of the external air must be noted at the same time by an ordinary thermometer. the water employed should be pure. distilled water would therefore be the best. if a substance is held mechanically suspended in water, it will not affect the boiling-point. thus, muddy water would serve equally as well as distilled water. however, as it cannot be readily ascertained that nothing is dissolved chemically when water is dirty, we are only correct when we employ pure water. = . precautions to ensure correct graduation.=--those who possess a boiling-point thermometer should satisfy themselves that it has been correctly graduated. to do this, it is advisable to verify it with the reading of a standard barometer reduced to ° f. the table of "vapour tension" (given at p. ) will furnish the means of comparison. thus, if the reduced reading of the barometer, corrected also for latitude, be · , the thermometer should show ° as the boiling-point of water at the same time and place; if · , the thermometer should read · ; and so on as per table. in this way the error of the chief point of the scale can be obtained. other parts of the scale may be checked with a standard thermometer, by subjecting both to the same temperature, and comparing their indications. the graduations as fixed by some makers are not always to be trusted; and this essential test should be conducted with the utmost nicety and care. admiral fitzroy writes, in his _notes on meteorology_:--"each degree of the boiling-point thermometer is equivalent to about _feet of ascent_, or one-tenth to feet; therefore, the smallest error in the graduation of the thermometer itself will affect the height deduced materially. "in the thermometer which is graduated from ° (the boiling-point) to °, similarly to those intended for the purpose of measuring heights, there must have been a starting point, or zero, from which to begin the graduation. i have asked an optician in london how he fixed that zero, the boiling-point. 'by boiling water at my house,' he replied. 'where is your house?' in such a part of the town, he answered. i said: 'what height is it above the sea?' to which he replied, 'i do not know;' and when i asked the state of the barometer when he boiled the water, whether the mercury was high or low, he said that he had not looked at it! now, as this instrument is intended to measure heights and to decide differences of some hundred, if not thousand feet upwards, at least one should endeavour to ascertain a reliable starting point. from inquiries made, i believe that the determination of the boiling-point of ordinary thermometers has been very vague, not only from the extreme difficulties of the process itself (which are well known to opticians), but from the radical errors of not allowing for the pressure of the atmosphere at the time of graduation--which may be much, even an inch higher or lower, than the mean, or any _given height_--while the elevation of the place above the level of the sea is also unnoticed. then there is another source of error, a minor one, perhaps: the inner limit, the ° point, is fixed only by comparison with another thermometer; it may be right, or it may be very much out, as may be the intermediate divisions; for the difficulty of ascertaining degree by degree is great: and it must be remembered that the measurement of a very high mountain depends upon those inner degrees from ° down to °, thereabouts. hence, the difficulty of making a reliable observation by boiling water seems to be greater than has been generally admitted." = . method of calculating heights from observations with the mountain thermometer.=--having considered how to make observations with the proper care and accuracy, it becomes necessary to know how to deduce the height by calculation. that a constant intimate relation exists between the boiling temperature of water and the pressure of the air, we have already learned. this knowledge is the result of elaborate experiments made by several scientific experimentalists, who have likewise constructed formulæ and tables for the conversion of the boiling temperatures into the corresponding pressures of vapour, or, which is equivalent, of the atmosphere, when the operation is performed in the open air. as might be expected, there is not a perfect accord in the results arrived at by different persons. regnault is the most recent, and his experiments are considered the most reliable. from regnault's table of vapour tension, we can obtain the pressure in inches of mercury at °, which corresponds to the observed boiling-point; or _vice versa_, if required. from the pressure, the height may be deduced by the method for finding heights by means of the barometer. the following table expresses very nearly the elevation in feet corresponding to a fall of ° in the temperature of boiling water:-- boiling temperatures elevation in feet between. for each degree. ° and -- and -- and and these numbers agree very well with the results of theory and actual observation. the assumption is that the boiling-point will be diminished ° for each feet of ascent until the temperature becomes °, then feet of elevation will lower it one degree until the water boils at °, and so on; the air being at °. let _h_ represent the vertical height in feet between two stations; _b_ and _b_, the boiling-points of water at the lower and upper stations respectively; _f_, the factor found in the above table. then _h_ = _f_(_b_ - _b_) further, let _m_ be the mean temperature of the stratum of air between the stations. now, if the mean temperature is less than °, the column of air will be shorter; and if greater, longer than at °. according to regnault, air expands / · or · of its volume at °, for each degree increase of heat. calling the correction due to the mean temperature of air _c_, its value will be found from the equation, _c_ = _h_ (_m_ - ) · calling the corrected height _h'_, it will be found from the formula, _h'_ = _h_ + _h_ (_m_ - ) · that is, _h'_ = _h_ { + (_m_ - ) · } and substituting the value of _h_, _h'_ = _f_(_b_ - _b_) { + (_m_ - ) · } strictly, according to theoretical considerations, there is a correction due to latitude, as in the determination of heights by the barometer; but its value is so small that it is practically of no importance. if a barometer be observed at one of the stations, the table of vapour tensions (p. ) will be useful in converting the pressure into the corresponding boiling-point, or _vice versa_; so that the difference of height may be found either by the methods employed for the boiling-point thermometer or the barometer. in conclusion, it may be remarked that observers who have good instruments at considerable elevations, as sites on mountains or plateaus, would confer a benefit to science, by registering for a length of time the barometer along with the boiling temperature of water, as accurately as possible. such observations would serve to verify the accuracy of theoretical deductions, and fix with certainty the theoretical scale with the barometer indications. _example, in calculating heights from the observations of the boiling-point of water._-- . at geneva the observed boiling-point of water was °· ; on the great st. bernard it was °· ; the mean temperature of the intermediate air was °· ; required the height of the great st. bernard above geneva. method by formula:-- _h'_ = _f_ (_b_ - _b_) { + (_m_ - °) · } in this case _f_ is between and , or . _b_ = · _m_ = · _b_ = · ------- ----- · · _f_ = · ------- --------- · · · ------- ----- _h'_ = · feet. · ====== method by tables supplied with boiling-point apparatus made by messrs. negretti and zambra:-- · gives in table i. · " " ---- · " · in table ii. ---- height ==== = . thermometers for engineers.=--_ st. salinometer._--under the circumstances at which fresh water boils at °, sea water boils at °· . the boiling temperature is raised by the chemical solution of any substance in the water, and the more with the increase of matter dissolved. from a knowledge of this principle, marine engineers make use of the thermometer to determine the amount of salts held in solution by the water in the boilers of sea-going steamers. common sea-water contains / of its volume of salt and other earthy matters. as evaporation proceeds, the solution becomes proportionally stronger, and more heat is required to produce steam. the following table from the work of messrs. main and brown, on the marine steam-engine, shows the relation between the boiling-point under the mean pressure of the atmosphere, or inches of mercury, and the proportion of matter dissolved in the water:-- proportion of salt in parts of water boiling-point ° " " / " · " " / " · " " / " · " " / " · " " / " · " " / " · " " / " · " " / " · " " / " · " " / " · " " / " · " " / " · when the salts in solution amount to / , the water is saturated. it has also been ascertained that, when a solution of / is attained, incrustation of the substances commences on the boiler. hence, it is a rule with engineers to expel some of the boiling water, when the thermometer indicates a temperature of °, and introduce some more cold water, in order to prevent incrustation, which not only injures the boiler, but opposes the passage of heat to the water. the thermometer used for this purpose should be very accurately graduated, and the scale must be considerably higher than, though it need not read much below °. _ nd. pressure gauge._--the elasticity of gases augments by increase of temperature, and _vice versa_; it follows, therefore, that when steam is generated in a closed boiler, its temperature rises beyond the boiling temperature of °, owing to the increased pressure upon the water. the law connecting the pressure and the corresponding temperature of steam is the same as that upon which the boiling of fluids under diminished atmospheric pressure takes place. hence, the indications of the thermometer become exponents of steam pressure. engineers are furnished, in works on the steam-engine, with tables, from which the pressure corresponding to a given temperature, or the converse, can be obtained by mere inspection. [illustration: fig. .] fig. represents the thermometer employed as a steam-pressure gauge. it is fitted in a brass case, with screw-plug and washers for closing the boiler when the thermometer is not in use. the scale shows the pressure corresponding to the temperature, from to lbs., above the atmospheric pressure, which is usually taken as lbs. on the square inch. chapter xi. instruments for ascertaining the humidity of the air. = . hygrometric substances.=--the instruments devised for the purpose of ascertaining the humidity of the atmosphere are termed _hygrometers_. the earliest invented hygrometers were constructed of substances readily acted upon by the vapour in the air, such as hair, grass, seaweed, catgut, &c., which all absorb moisture, and thereby increase in length, and when deprived of it by drying they contract. toy-like hygrometers, upon the principle of absorption, are still common as ornaments for mantel-pieces. a useful little instrument of this class, formed from the beard of the wild oat, is made to resemble a watch in external appearance, and is designed to prove the dampness or dryness of beds: a moveable hand points out on the dial the hygrometric condition of the clothes upon which the instrument is laid. = . saussure's hygrometer=, formerly used as a meteorologic instrument, but now regarded as an ornamental curiosity, is represented in fig. . its action depends upon a prepared hair, fixed at one end to the frame of the instrument, and wound round a pulley at the other. the pulley carries a pointer which has a counterpoise sufficient to keep the hair stretched. by this means the shrinking and lengthening of the hair cause the pointer to traverse a graduated arc indicating the relative humidity. [illustration: fig. .] such instruments, however ingenious, are not of scientific value; because they do not admit of rigid comparison, are liable to alter in their contractile and expansive properties, and cannot be made to indicate precisely alike. = . dew-point.=--the amount of water which the air can sustain in an invisible form increases with the temperature; but for every definite temperature there is a limit to the amount of vapour which can be thus diffused. when the air is cooled, the vapour present may be more than it can sustain; part will then be condensed as dew, rain, hail or snow, according to the meteorologic circumstances. the temperature which the air has when it is so fully saturated with vapour that any excess will be deposited as dew, is called the _dew-point_. = . drosometer.=--"to measure the quantity of dew deposited each night, an instrument is used called a _drosometer_. the most simple process consists in exposing to the open air bodies whose exact weight is known, and then weighing them afresh after they are covered with dew. according to wells, locks of wool, weighing about eight grains, are to be preferred, which are to be divided [formed] into spherical masses of the diameter of about two inches."--_koemtz._ = . humidity.=--the proportion existing between the amount of vapour actually present in the air at any time, and the quantity necessary to completely saturate it, is called _the degree of humidity_. it is usually expressed in a centesimal scale, being perfect dryness, and complete saturation. the pressure, or tension, of vapour at the dew-point temperature, divided by the tension of vapour at the air temperature and the quotient multiplied by , gives the degree of humidity. (regnault's tables should be used.) hence the utility of instruments for determining the dew-point. = . leslie's hygrometer.=--this instrument consists of a glass syphon tube, terminated with a bulb or ball at each end, turned outwards from each other, as in fig. . the tube is partly filled with concentrated sulphuric acid, tinged by carmine. one of the balls is covered smoothly with fine muslin, and is kept continually moistened with pure water, drawn from a vase placed near it by the capillary attraction of a few strands of clean cotton-wick. the descent of the coloured liquid in the other stem will mark the diminution of temperature caused by the evaporation of the water from the humid surface. the drier the ambient air is, the more rapidly will the evaporation go on; and the cold produced will be greater. when the air is nearly saturated with moisture, the evaporation goes on slowly; the cold produced is moderate, because the ball regains a large portion of its lost heat from surrounding bodies; and the degree of refrigeration of the ball is an index of the dryness of the air. [illustration: fig. .] "should the water become frozen on the ball, this hygrometer will still act; for evaporation goes on from the surface of ice in proportion to the dryness of the air. leslie estimates, that when the ball is moist, air, at the temperature of the ball, will take up moisture equal to the sixteen-thousandth part of its weight, for each degree of his hygrometer; and as ice in melting requires one-seventh of the caloric consumed in converting water into vapour, when the ball is frozen, the hygrometer will sink more than when wet by ° in °; and hence, in the frozen state, we must increase the value of the degrees one-seventh: so that each of them will correspond to an absorption of moisture equal to one-fourteen-thousandth part of the weight of the air. "when this hygrometer stands at °, the air feels damp; from ° to °, we reckon it dry; from ° to °, very dry; and from ° upwards, we should call it intensely dry. a room would feel uncomfortable, and would probably be unwholesome, if the instrument in it did not reach °.[ ] in thick fogs it keeps almost at the beginning of the scale. in winter, in our climate, it ranges from ° to °; in summer often from ° to °; and sometimes attains ° or °. the greatest degree of dryness ever noticed by leslie was at paris, in the month of september, when the hygrometer indicated °."--_professor trail, in "library of useful knowledge."_ in estimating the value of the indications of this hygrometer, it should be borne in mind that the scale adopted by leslie was _millesimal_, that is to say, from the freezing to the boiling-point of water was divided into a thousand parts; ten millesimal degrees are therefore equal to one of the scale of celsius. . daniel's hygrometer. this instrument was invented about the year , by professor daniel, the distinguished author of _meteorological essays_; and it entirely superseded all hygrometers depending upon the absorption of moisture. the form of the instrument is shown in fig. . [illustration: fig. .] it consists of a glass tube, about one-eighth of an inch in diameter of bore, bent twice at right angles, and terminated, at each end, in a bulb about one inch and a quarter in diameter. in one limb of the tube is enclosed a delicate thermometer, which descends to the centre of the adjoining bulb, which is about three-parts filled with sulphuric ether. all the other parts of the tube are carefully freed from air, so that they are occupied by the vapour of the ether. this bulb is generally made of black glass; the other is transparent, but covered with a piece of fine muslin. the support for the tube has a thermometer attached, which shows the temperature of the external air. the tube can be removed from the stand, and the parts are made to pack, with a necessary phial of ether, in a small box, which can easily be got into the pocket. _how to use the hygrometer._--this instrument gives the dew-point by direct observation, which must be made in the following manner:--having fixed the tube upon the stand, with the bulbs vertically downward, the ether is all caused to flow into the lower ball by inclining the tube. the temperature of the air is noted by the exposed thermometer. then some ether is poured, from a dropping tube fitting into the neck of the phial, upon the muslin-covered bulb. the rapid evaporation of this ether cools the bulb and causes condensation of the ethereal vapour in its interior. this gives rise to rapid evaporation of the ether in the lower bulb, whereby its temperature is greatly reduced. the air in the vicinity is deprived of its warmth by the cold bulb, and is soon cooled to the temperature at which it is perfectly saturated with the vapour which it contains. cooled ever so little below this temperature, some aqueous vapour will be condensed, and will form a dew upon the black-glass bulb. at the first indication of the deposit of dew the reading of the internal thermometer is taken: which is the dew-point. this hygrometer has undeniable disadvantages. the surface upon which the dew condenses is small, and requires a peculiar direction of light in which to see it well. the observer, having his attention on the bulb and the thermometer, cannot always fix with precision the dew-point; and hence he is recommended to note the temperature at the appearance and at the disappearance of the dew, in order that the chance of error may be diminished. without doubt, the necessarily long continuance of the observer near the instrument influences, to some extent, the observed temperatures; and the difficulty of not being always able to procure pure ether for the experiments is not the least of the drawbacks to the use of the instrument. some of these disadvantages are obviated in regnault's hygrometer. . regnault's condenser hygrometer (fig. ) consists of a tube, _c_, made of silver, very thin, and perfectly polished; the tube is larger at one end than the other, the large part being · inches in depth, by · in diameter; this is fitted tightly to a brass stand, _b_, with a telescopic arrangement for adjusting when making an observation. [illustration: fig. .] the tube, _c_, has a small lateral tubulure, to which is attached an india-rubber tube, with ivory mouth-piece; this tubulure enters _c_ at right angles near the top, and traverses it to the bottom of the largest part. a delicate thermometer, _d_, is inserted through a cork, or india-rubber washer, at the open end of the tube, _c_, the bulb of which descends to the centre of its largest part. _g_ is an attached thermometer for taking the temperature of the air, and _f_ is a bottle containing ether. _to use the condenser hygrometer_, a sufficient quantity of ether is poured into the silver tube to cover the thermometer bulb: on allowing air to pass bubble by bubble through the ether, by breathing in the tube, _e_, an uniform temperature will be obtained; if the ether continues to be agitated, by breathing briskly through the tube a rapid reduction of temperature will be the result; at the moment the ether is cooled down to the dew-point temperature, the external surface of that portion of the silver tube containing ether will become covered with a coating of moisture, and the degree shown by the thermometer at that instant will be the temperature of the dew-point. this form of hygrometer, for ascertaining by direct observation the dew-point, is so superior to daniell's, both from its being more certain in its indications and economical in use, that messrs. negretti and zambra have been induced to modify it, and reduce its price to little more than that of a good daniell's hygrometer. = . temperature of evaporation.=--when the air is not saturated with vapour, evaporation is going on with more or less activity, according as the temperature is high or low, rising or falling. now vapour cannot be formed without an expenditure of heat; as we invariably find that the process of evaporation lowers the temperature of the liquid from which the vapour is produced, and, by communication, that of contiguous substances also. thus the emigrant, crossing the line under the scorching influence of the vertical sun, wraps a wet towel round his can of water, swings it in the breeze, to evaporate the moisture of the towel, and obtains a glass of cool water. so also, european residents in india, during the hot season, spread out mats in their apartments, and keep them wet, in order that the evaporation may cool the air. this principle has been applied, for the purpose of ascertaining the hygrometric condition of the air, in the instrument known as mason's hygrometer, or psychrometer, which is now in general use, from its simplicity, accuracy, and ease of observing. . mason's hygrometer. =the dry and wet bulb hygrometer, or psychrometer=, known also as mason's hygrometer (fig. ), consists of two parallel thermometers, as nearly identical as possible, mounted on a wooden bracket, one marked _dry_, the other _wet_. the bulb of the wet thermometer is covered with thin muslin, and round the neck is twisted a conducting thread of lamp-wick, which passes into a vessel of water, placed at such a distance as to allow a length of conducting thread, of about three inches; the cup or glass is placed on one side, and a little beneath, so that the water within may not affect the reading of the _dry bulb thermometer_. in observing, the eye should be placed on a level with the top of the mercury in the tube, and the observer should refrain from breathing whilst taking an observation. [illustration: fig. .] the _dry_ bulb thermometer indicates the temperature of the air itself; while the wet bulb, cooled by evaporation, shows a lower temperature according to the rapidity of evaporation. _to find the dew-point._--from the readings of the two thermometers, the dew-point can be deduced by formulæ (that known as apjohn's is considered the most theoretically true), or from the valuable hygrometric tables by j. glaisher, esq., f.r.s. for practical purposes in estimating the comparative humidity, the annexed table, which is a reduction from mr. glaisher's elaborate work, will be sufficient; it will at least serve to assist in familiarising the inexperienced in the value of the psychrometer's indications:-- +------------------------------------------+ | | difference between dry-bulb | | | and wet-bulb readings. | |temperature |-----------------------------| | by the | ° | ° | ° | ° | °| °| | dry bulb |-----------------------------| |thermometer.| degree of humidity. | |------------------------------------------| | ° | | | | .. | .. | .. | | | | | | .. | .. | .. | | | | | | | .. | .. | | | | | | | .. | .. | | | | | | | .. | .. | | | | | | | .. | .. | | | | | | | .. | .. | | | | | | | | .. | | | | | | | | .. | | | | | | | | .. | | | | | | | | .. | | | | | | | | .. | | | | | | | | .. | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | +------------------------------------------+ the total quantity of aqueous vapour which at any temperature can be diffused in the air being represented by , the per-centage of vapour actually present will be found in the table opposite the temperature of the dry thermometer, and under the difference between the dry-bulb and wet-bulb temperatures. the degree of humidity for intermediate temperatures and differences to those given in the table can be easily estimated sufficiently accurately for most practical purposes. the difference between the two thermometer readings taken from the reading of the wet bulb, gives the dew-point very nearly, when the air is at any temperature between freezing and °. this simple rule will be found serviceable to horticulturists, since it will enable them to estimate the chilling effect of dew or hoar-frost on tender plants. _use as an indicator of weather._--in our climate, the usual difference between the thermometer readings,--in the open air, shaded from the sun, reflected heat, and currents of air,--ranges from one to twelve degrees. in hot and dry climates, as india and australia, the range out of doors has been found as much as °, occasionally. when the moisture is frozen, the bulb should be wetted afresh, and the reading taken just before it again freezes; but the observation then is of little value, and for general purposes need not be taken, as the air is known to be dry in frosty weather. the muslin or cotton rag should be washed once or twice a week by pouring water over the bulb; and it should be replaced by a fresh piece at least once a month. accuracy depends very much upon keeping the wet bulb clean, and not _too_ wet. in connection with the barometer, this hygrometer is very useful, not only on land, but especially at sea, where other kinds of hygrometers cannot be practically used. a fall in the barometer is indicative of coming wind or rain: if the hygrometer shows increasing dampness by the difference of the readings becoming smaller,--rain may therefore be anticipated. on the contrary, if the hygrometer shows continuing or increasing dryness, a stronger wind is probable, without rain. _domestic uses._--mason's hygrometer is useful in regulating the moisture of the air of apartments; a difference in the thermometer readings of from ° to ° being considered healthy. many complaints require that the temperature and humidity of the air which the invalid breathes should be carefully regulated. hence it is a valuable household instrument. in a room, it should be placed away from the fire as much as possible, but not exposed to draughts of air. figs. and show cheap arrangements of the instrument for domestic purposes. other arrangements are given to the instrument to make it suitable for exhibiting the hygrometrical state of the air in hot-houses, conservatories, malting-houses, warehouses, manufactories, &c. [illustration: fig. .] [illustration: fig. .] [illustration: fig. .] fig. shows the instrument arranged on brass tripod stand, with folding legs and metal cover, to render it portable. = . self-registering hygrometer.=--a maximum thermometer and a minimum thermometer, each fitted up as a wet-bulb thermometer, record the highest and lowest temperature of evaporation during the interval of observation. negretti's mercurial maximum, and an alcohol minimum, answer best. = . causes of dew.=--"the aqueous vapour of our atmosphere is a powerful radiant; but it is diffused through air which usually exceeds its own mass more than one hundred times. not only, then, its own heat, but the heat of the large quantity of air which surrounds it, must be discharged by the vapour, before it can sink to its point of condensation. the retardation of chilling due to this cause enables good solid radiators, at the earth's surface, to outstrip the vapour in their speed of refrigeration; and hence, upon these bodies, aqueous vapour may be condensed to liquid, or even congealed to hoar-frost, while at a few feet above the surface it still maintains its gaseous state."[ ] the amount of moisture so deposited will vary with different atmospheric conditions. if the sky be decidedly cloudy or misty, the heat radiated from the earth will be partly restored by counter-radiation from the visible vapour; the cooling of the earth's surface will, therefore, take place slowly, and little dew will be deposited. on the other hand, if the air contain transparent vapour, and the sky appear clear, the counter-radiation will be less, the earth will cool rapidly, and the deposit of dew will be copious; provided the night be comparatively calm, for, when the wind blows, the circulating air supplies heat to the radiating substances, and prevents any considerable chilling. the dew which falls in tropical countries greatly exceeds in abundance what we experience in our climate; because the air is there, from the great heat, capable of sustaining a large amount of vapour in the transparent state, and the conditions most favourable for a maximum reduction of temperature by radiation are present. at those places, or upon those substances which cool the lowest and most readily, the dew falls most copiously. [illustration: fig. .] = . plan of exposing thermometers=, &c.--figure is an illustration of a convenient slab for supporting thermometers in an exposed position attached to a stand (such as glaisher's, described in chapter xvi.) for ordinary scientific observations. it has a projecting ledge, _b_, to carry off rain from the instruments, the slab, _a_, being erected vertically. the hygrometer is placed at _e_, with the vase of water at _f_. an alcohol minimum thermometer is represented at _c_, in the position most favourable to its certain action; and at _d_ is shown one of negretti & zambra's maximum thermometers, the position of which may be more nearly horizontal than there exhibited, although a slight depression of the bulb-end of the frame is desirable, but not necessary, as this thermometer can be used in any position. chapter xii. instruments used for measuring the rainfall. the instruments in use for measuring the quantity of rain which falls on a given spot are of very simple construction. perhaps the simplest is:-- = . howard's rain-gauge.=--it consists of a copper funnel, a stout glass or stone bottle, and a measuring glass. the bottle is to be placed upon the ground, with the funnel resting on its neck. a brass band or cylinder fixed upon the outer surface of the funnel envelops the neck of the bottle, and the pipe of the funnel extends nearly to the bottom of the bottle; so that loss by evaporation is avoided as much as possible. the receiving space of the funnel is formed by a brass ring, five inches in diameter, very accurately turned. the measuring vessel enables the observer to note the rainfall in inches, tenths, and hundredths of an inch. [illustration: fig. .] = . glaisher's rain-gauge.=--the rain-gauge designed by mr. glaisher, the well-known meteorologist, and used by most observers of the present day, is arranged for the reception of the water which falls upon its receiving surface only, and for the prevention of loss by evaporation. the rain is first collected in a funnel, _b_, (fig. ,) the receiving surface of which is turned in a lathe. the conical surface of the funnel slopes to the pipe, _e_, at an angle of ° from the horizontal receiving surface. the tube, _e_, is of small aperture, and is bent up, in order to retain the last few drops of rain, so that the only opening for the escape of vapour may be closed as long as possible. the funnel, _b_, fits upon the cylinder, _a_, tightly in the groove, _d_. a copper can is placed inside the cylinder, _a_, to receive the rain from the funnel. once or twice a day, or after a shower, this can should be taken out, and the water measured in the glass measure, _c_, which is graduated to hundredths of an inch, according to the calculated quantity of water, determined by the area of the receiving space. in use, this gauge should be partly sunk in the ground, so that the top may be about five inches above it. thus situated, there will be little or no evaporation from it during any month of the year; and the readings need not be taken daily, although desirable. = . rain-gauge with float.=--in this construction the graduated glass measure is dispensed with. the cylinder of the gauge is made less in diameter than the funnel, and a hollow, very flattened spheroid of copper forming a float, and carrying a vertical graduated boxwood scale which moves through the orifice of the funnel, is placed in it. as the rain accumulates the float rises, and the amount of rain in the gauge is read upon the scale from the top of the gauge, a bar, having a hole at the centre for the passage of the scale, being fixed diametrically across the receiving space of the funnel. the gauge is provided at the bottom with a brass cock, by which the water may be allowed to flow out of it whenever necessary. this form of gauge is not very suitable for the measurement of small quantities; but is admirably adapted for localities where the rainfall is excessive. [illustration: fig. .] = . rain-gauge with side-tube.=--this instrument, as represented in fig. , is a cylindrical vessel, mounted on a base shaped as a frustum of a cone. this base may be filled with sand or gravel to make the instrument stable, so that when placed upon a lawn or in a garden it may have an ornamental appearance. the funnel for collecting the rain is larger in diameter than the cylinder. parallel to the cylinder, and communicating with the lowest part of the interior and extending to its top, is a graduated glass tube, open at both ends. the rain collected will rise as high in this tube as in the cylinder, and its amount can therefore be read off without any trouble. the gauge is emptied by the brass tap at the bottom of the cylinder. = . admiral fitzroy's rain-gauge.=--a form of rain-gauge, very well adapted for expeditious observation at any time, has been designed by admiral fitzroy, and extensively employed by his observers. it is cylindrical in shape, with the funnel let into the top; and the rainfall is collected in an inner and much smaller cylinder, so that a small fall is represented by a considerable depth of water in the gauge. the amount of rain which has fallen is ascertained by a dipping tube, similar in principle to the dipping syphon used by gaugers for taking out specimens of wines or spirits from casks by simply removing the bung. a short, vertical, tubular opening provided with a cap, which is attached to the instrument by a chain that it may not be lost, is formed in the funnel. the measuring tube, which has a small hole at each end, should be placed upright in the gauge; then the thumb should be pressed over the upper aperture, while the tube is lifted gently out, holding in the lower part a quantity of water representing the depth of the rain in the gauge, the upper edge of which is at the mark to be read off. the glass tube is graduated to inches and tenths; hundredths of an inch can be readily estimated by the eye. the marks are fixed by actual trial with a standard gauge, and are artificial, not true, inches. = . self-registering rain-gauge.=--the rain-gauge can be combined with clock-work and other mechanism so as to be self-recording of the amount of rain, the time, and duration of its fall. for the details of construction the reader is referred to the next chapter, where he will find the instrument described in connection with osler's anemometer, as the "pluviometer." to observe and duly record the times of commencement and termination of rain is very desirable. scarcely any observer can attempt to do this even approximately from personal observation. hence the want of a cheap and simple self-recording rain-gauge is much felt, the present construction being too expensive for all but a few individuals. in , mr. r. strachan estimated the duration and amount of rain in london (gray's inn road) as follows:-- +-------------------------------------------------------------------+ | months. |inches.|days.|hours.|| months. |inches.|days.|hours.| |-----------+-------+-----+------++------------+-------+-----+------| |january. | · | | ||july. | · | | | |february. | · | | ||august. | · | | | |march. | · | | ||september. | · | | | |april. | · | | ||october. | · | | | |may. | · | | ||november. | · | | | |june. | · | | ||december. | · | | | +-------------------------------------------------------------------+ "during the year , the rainfall amounted to · inches. rain fell on days, that is, on nearly every other day. the hours of rain were estimated at ; therefore, if the rain had fallen continuously, it would have lasted nearly days and nights."[ ] the value of similar estimates of the rainfall by numerous observers would be very great to meteorology. = . the principle of measurement= in all these gauges is the relation existing between the areas of the collecting and receiving surfaces; that is, between the area of the funnel into which the rain falls, and the area of the cylinder which receives it. in howard's and glaisher's gauges, this cylinder is virtually the measuring glass itself; in the others, above described, the measuring scales show the same depth of water as in the cylinder of the gauge. the cylinder being of less diameter than the funnel, and receiving all the rain collected by the funnel, it follows that its contents will have an increased depth. now equal cylindrical volumes, having different diameters, are to each other in length inversely as the squares of the diameters. hence, if the funnel be inches and the cylinder inches in diameter, a fall of inch of rain will be represented in the gauge by inches; for ² : ² :: : _x_ = . in this case, therefore, a length of nine inches of the measuring glass, tube, or scale, would represent an inch of rainfall, and be divided into tenths and hundredths of the artificial inch. = . position for rain-gauge, &c.=--rain gauges should be placed on the ground, in any position exposed to a free fall of rain, snow, or hail, where neither walls, buildings, nor trees shelter or cause eddies of wind. they should be supported by a frame, or other means, to prevent them being blown down by the wind, but so that they can be readily emptied. during snow or frost, the gauge must be watched, and its contents melted by placing it in a warm room, either when the amount is to be measured, or the funnel is filled up with snow. a tin vessel of equal area to the funnel may at such times be useful as a substitute. rain gauges are constructed of metal, usually copper, which, besides being readily workable, is little affected by atmospheric influences. if made of iron or zinc, they should be well japanned; if of copper, this is not so essential. the capacity of a gauge should be sufficient to contain at least the probable maximum fall of rain in a day at the locality. those required for rainy districts must be of large size. = . causes of rain.=--when the invisible vapour which is diffused in the atmosphere becomes sufficiently cooled, it appears visible as mist or cloud, and a further reduction of temperature causes its precipitation as rain, hail, or snow. the cooling of the higher regions of the atmosphere is doubtless the chief cause of this condensation; but the property which aqueous vapour possesses of radiating heat may also contribute to the result. moreover, the law which regulates the amount of vapour which air at any particular temperature can sustain in a transparent state, determines that when two bodies of air at different temperatures, saturated with vapour, intermix, some moisture must be rendered visible; and hence, it is not only possible, but highly probable, that rain may result from the conflict of different winds. let us imagine two cubic yards of air, both saturated with moisture, but having the respective temperatures of and degrees, to come into contact. there will be a tendency to equalize the temperature to a mean, which is °; and during this process, some of the vapour will be condensed. for in the air at ° there is · grains of vapour[ ] and " " · " ------ total amount of vapour · " but two cubic yards of air at ° can only sustain · " ------ hence there will be deposited · " of rain. ====== it may be conceded, therefore, that when a warm and moist current of air encounters a body of cold air which may not be extremely dry, the mixture is unable to retain the whole of the vapour in an invisible state; so that the excess becomes visible as mist or fog, and, when the temperature has become sufficiently lowered, rain. the british isles are more or less enveloped in fog, or mist, at the commencement of easterly winds, which, with a sudden change of wind, is exhibited even in summer; while the south-westerly winds, warm, and arriving from the ocean, deposit large quantities of rain by the cooling effect of the land, colder by reason of its latitude. when rain occurs with a northerly wind, it is probably due to the deposition from an upper south-westerly current, often apparently proved by the movements of the upper clouds. = . laws of rain-fall.=--tropical countries have a dry and a wet season during the year: _dry_, when the sun is at the opposite side of the equator; _wet_, when the sun is overhead. with reference to the british isles, the statistics collected by mr. g. j. symons indicate that: st. the stations of least rain are inland, or on the east or south-east coasts; the stations of greatest rain are on the western coasts. nd. the rain-fall is very large in the vicinity of mountain chains or groups, unless the station happens to be some miles to the north-eastward. it may be well to illustrate these remarks by quoting[ ] the average fall at a few places, grouping them as-- westerly. inches. bodmin bolton (lancashire) coniston (windermere) seathwaite torosay (i. of mull) killaloe (limerick) central. inches. enfield epping derby york stirling perth easterly. inches. witham (essex) patrington (hull) sunderland inveresk (edinburgh) pittenweem (fife) dublin mr. green, the celebrated aeronaut, has asserted from his experience, "that whenever a fall of rain happens, and the sky is entirely overcast, there will invariably be found to exist another stratum of cloud at a certain elevation above the former;" and the recent scientific balloon ascents by mr. glaisher have tended to confirm this theory. mr. glaisher says, "it would seem to be an established fact, that whenever rain is falling from an overcast sky, there is a second stratum above." "it would also seem that when the sky is overcast without rain, that there is no stratum of cloud above, but that the sun is shining on the upper surface. in every instance in which i have been up under these circumstances, i have found such to be the case, agreeing in this respect also with mr. green's observations." the amount of rain collected in a gauge placed near the surface of the earth is larger than in any gauge placed above it; and the higher the gauge is placed, the less water is collected. mr. glaisher contends that his balloon experiments corroborate this law. = . utility of statistics of rain-fall.=--the utility of knowing the rain-fall of any locality is sufficiently obvious, and little need be said upon the subject. the rain-gauge should be in the hands of every gardener and farmer. in the management of out-door plants and crops, as well as in the construction of cisterns and tanks for the supply of water, a rain gauge is a valuable assistant. by its use, the gardener will be guided in judging how far the supply of moisture to the earth is needed; and he will also see how beneficial is even a hasty shower to growing plants, when he considers that a fall of rain measuring the tenth of an inch in depth, corresponds to the deposit of about forty hogsheads per acre. the study of the rain-fall of a country is of considerable interest to agriculturists. the health and increase of domestic animals, the development of the productions of the land, as well as the daily labours of the farmer, are dependent upon the excess or deficiency of rain. "it must be a subject of great satisfaction and confidence to the husbandman to know at the beginning of a summer, by the certain evidence of meteorological results on record, that the season, in the ordinary course of things, may be expected to be a dry and warm one; or to find, in a certain period of it, that the average quantity of rain to be expected for the month has fallen. on the other hand, when there is reason, from the same source of information, to expect much rain, the man who has courage to begin his operations under an unfavourable sky, but with good ground to conclude, from the state of his instruments and his collateral knowledge, that a fair interval is approaching, may often be profiting by his observations; while his cautious neighbour who waited 'for the weather to settle' may find that he has let the opportunity go by. this superiority, however, is attainable by a very moderate share of application to the subject; and by the keeping of a plain diary of the barometer and rain-gauge, with the hygrometer and vane, under his daily notice."[ ] the statistics of rain-fall are not only valuable and interesting in a meteorological point of view, and for agricultural purposes, but are also highly important in connection with sanitary arrangements for towns, and engineering operations. this is especially evident to the hydraulic engineer. as rain is an important source of water-supply to rivers, canals, and reservoirs, it is evident that a knowledge of the probable fall for any season or month, at a given place, as furnished by averages of the observations of former years, will be the data upon which the engineer will base his plans for providing for floods or droughts; while the measurement of the actual quantity which has just fallen, as gathered from the indications of a series of gauges, will suggest to him the precautions to adopt either to economise or conduct away the in-pouring waters. "when a canal is conducted across an undulating country, its course is necessarily governed by the accidents of the ground, and it alternately rises and falls. in this case, rising by a succession of levels, it necessarily arrives at a certain highest level, which is called by engineers the _summit level_. from this it again descends by a corresponding series of levels. now, it is evident that, supposing the locks to be all equal in magnitude, the ascent of a vessel will require the descent of as much water from the summit to the lowest level as would fill a single lock; for this quantity of water must be discharged from each lock of the series when the vessel passes through it. "the same may be said of the process by which the vessel descends along the series of locks on the other side of the summit. it appears, therefore, that a supply of water must always be maintained on the summit level sufficient to fill a single lock twice for each vessel which crosses the summit. "it happens, fortunately, that by the laws of natural evaporation, rain is precipitated in greater quantities on elevated summits than on the intermediate valleys, so that the moving power, in this case, accommodates itself to the exigencies of intercommunication."--_dr. lardner's "handbook of natural philosophy."_ = . new form of rain-gauge.=--since the foregoing pages were in type, a modification of howard's rain-gauge has been arranged by mr. symons, which is compact in design, convenient in use, and low in price. it combines the advantages of most gauges; having solidity, and facility of measurement. the bottle is placed in a tin case, to the bottom of which are attached stout spikes, which, when forced into the earth, prevent its being upset either by wind or accident. the bottle being transparent, and slits made in the case, the fall of rain is seen at a glance, or with a race-glass, from a window. the funnel being attached to the cover of the case is thereby kept strictly horizontal, and the depth of rain can be accurately measured by lifting the bottle from its case and emptying it into a graduated glass jar. the funnel of this gauge is a very deep cone, to prevent the rain drops outsplashing. when properly placed, the receiving surface will be twelve inches above the ground, which experience has shown to be the most advantageous height. chapter xiii. apparatus employed for registering the direction, pressure, and velocity of the wind. = . the vane.=--the instrument by which the wind's direction is most generally noted, is the vane, or weather-cock, and all that need be said of it here is that the points north, east, south and west, usually attached to it, should indicate the _true_ and not the _magnetic_ directions; and that care should be taken to prevent its setting fast. very complicated instruments are required for ascertaining the pressure and velocity of the wind, and these are called _anemometers_. the simplest is _lind's_. [illustration: fig. .] = . lind's anemometer, or wind-gauge= (fig. ), invented so late as , for showing the pressure of the wind, consists of a glass syphon, the limbs parallel to each other, and each limb the same diameter. one end of the syphon is bent at right angles to the limb, so as to present a horizontal opening to the wind. a graduated scale, divided to inches and tenths, is attached to the syphon tube, reading either way from a zero point in the centre of the scale. the whole instrument is mounted on a spindle, surmounted by a vane, and is moved freely in any direction by the wind, always presenting the open end towards the quarter from which the wind blows. to use the instrument, it is simply filled up to the zero point with water, and then exposed to the wind; the difference in the level of the water gives the force of the wind in inches and tenths, by adding together the amount of depression in one limb, and elevation in the other, the _sum of the two_ being the height of a column of water which the wind is capable of sustaining at that time. table, showing the force of wind on a square foot, for different heights of the column of water in lind's wind-gauge. +-----------------------------------+ |inches.|force in|common designation| | | lbs. | of such wind. | |-------+--------+------------------| | | · | a hurricane. | | | · | a violent storm. | | | · | a great storm. | | | · | a storm. | | | · | a strong wind. | | | · | a high wind. | | · | · | a brisk wind. | | · | · | a fresh breeze. | | · | · | a gentle breeze. | | . | . | a calm. | +-----------------------------------+ = . modification of lind's gauge.=--_sir w. snow harris_ has effected a modification of lind's anemometer, with a view of obtaining a hand instrument for use at sea more especially. at present the force of the wind is estimated at sea by an arbitrary scale, suggested by sir f. beaufort, the late hydrographer; being calm, the strongest hurricane, and the intermediate numerals giving the varying strength of the wind. there has been a long-felt want of instrumental means for obtaining this data at sea, if merely for the sake of checking occasionally personal estimations, which may vary considerably among different observers. harris's wind gauge is intended to be held by hand, while facing the wind, and keeping it in proper position by attending to a spirit-level attached. when in position, and held firmly, the tube has to be opened to the wind by pressure of the thumb acting upon jointed levers, controlled by springs. the pressure of the wind moves the enclosed liquid; and by withdrawing the thumb, the tube is closed so as to keep the liquid in its position; the reading is then taken from its scale, either in pounds on the square foot, miles per hour, or the ordinary designations of wind, as light, fresh, strong, &c. = . robinson's anemometer.=--_dr. robinson_, of armagh, is the inventor of a very successful anemometer, which determines the horizontal velocity of the wind. it was first used in , in the meteorological and tidal observations made on the coast of ireland under the direction of the rev. dr. lloyd. no meteorological observatory should be without this valuable instrument, which is essential in determining the average velocity of the wind of a locality as distinguished from the most frequent wind of the same place. it is represented in fig. . four hollow hemispherical cups, _a a_, are extended upon conjugate diameters, or arms, with their diametrical planes placed vertically, and facing the same way upon a vertical axis, _b_, which has at its lower extremity an endless screw, _d_. the axis is supported at _c_ so as to turn with as little friction as possible. the endless screw is placed in gear with a train of wheels and pinions. each wheel carries an index over a stationary dial in front; or the index is fixed, and the graduations are placed upon the wheels themselves. [illustration: fig. .] dr. robinson has proved, both by theory and experiment, that the centre of any one of the cups so mounted and set in motion by the wind, revolves with one-third of the wind's velocity. if, therefore, the diametrical distance between the centres of the cups be one foot, the circle described by the centres in one revolution is · feet, and the velocity of the wind will be three times this, or · feet, which must be referred to time for the absolute rate. the instrument is sometimes made with the centres of the cups · feet apart, so that the circle described is / of a mile in circumference. hence, to produce one revolution of the cups, the wind must travel three times as fast, or / of a mile. therefore, revolutions will be produced by one mile of wind; so that the dials may be graduated to register the velocity in miles and tenths of miles. the simplest arrangement is with five dials, recording respectively , , , , , and , revolutions. _directions for using robinson's anemometer._--the dials read off in the same manner as the register of a gas meter, commencing with the dial farthest from the endless screw. "the figures on the first dial indicate so many hundreds of thousands of revolutions; those on the second dial so many tens of thousands; those on the third, thousands; those on the fourth, hundreds; and those on the fifth so many tens. "the instrument should be read every morning at o'clock; and, usually, it will only be necessary to read the first three dials. the figures can be entered as they are read off. should the index point _between_ two figures, the less of the two is to be taken. "for example, if the first dial points to , or between and ; while the second dial indicates ; and the third, ; the entry to be made is (indicative of _thousand_ revolutions). "every time the index of the first dial is found to have passed zero ( ), a cross or star is to be prefixed to the next (a lower) reading. "to ascertain how many _thousands_ of revolutions have been made during the month, it will simply be necessary to subtract the first reading from the last, and prefix to the three figures thus obtained a figure corresponding to the number of stars in the column. for every _thousand_ revolutions there are two miles of wind: we have therefore only to multiply by to find how many miles of wind have passed during the month. "two entries must be made for the last day of each month (the one being written under the other), so as to bring the readings down to a.m. on the st of the following month. the same entry which ends one month, will therefore begin the next. this repetition of one entry is necessary, in order to prevent losing a day's wind. "the accompanying example of the readings of an anemometer for days will illustrate the method of making the entries, &c. "in this instance, the first reading ( ) is less than the last ( ). when the first reading is greater than * the last, it will be necessary to borrow , in making the subtractions, and then deduct one from the number of stars. thus, if the first reading of the series on the margin had been , the result would have been instead of . ---- thousands of revolutions. +----- | miles of wind in period. +----- miles of wind per day, on an average. "the foregoing directions are all which require to be regularly attended to. but it may be interesting at times to find the velocity of the wind during a period of a few minutes. this may be ascertained by observing the difference of two readings of all the dials, with an interval of some minutes between them, when a very brief calculation will suffice; but perhaps the simplest method is the following:-- "take two readings, with an interval of minutes between them. the difference of these readings, divided by , is the velocity of the wind in miles per hour. thus--if the reading of the five dials (from left to right) at noon is , and at minutes past is , the velocity of the wind is · miles per hour."--_admiral fitzroy, f.r.s._ a lever and clutch are sometimes fitted to this anemometer, as in fig. , for throwing the train out of gear when not required to register. it may also be connected with clock-work so as to be self-recording, by causing the mechanism to impress a mark upon prepared paper moved by the apparatus, at certain intervals of time. [illustration: fig. .] this anemometer should be fixed in an exposed situation, as high above ground as may be convenient for reading. it may be made very portable, by the arms which carry the cups being fitted to unscrew or to fold down. when fitted in gimbals, it can be used at sea with much advantage. the pressure of the wind has been experimentally proved to vary as the square of the velocity; the relation being _v²_ = × _p_. from this formula, therefore, the pressure can be calculated corresponding to the observed velocity. = . whewell's anemometer.=--this apparatus, the invention of the celebrated dr. w. whewell, registers the horizontal motion of the air with the direction. its mechanism may be described in general terms, as follows:-- a horizontal brass plate is attached to a vertical spindle, which passes through the axis of a fixed cylinder, being supported by a bearing at the lower end, and working in a collar at the upper. a vane is attached, by which the plate is moved about according to the direction of the wind. a fly, having eight fans, each fixed at an angle of ° with the axle, is placed upon the plate so that the axle is in the line of direction of the vane. an endless screw on the axle turns a vertical wheel having one hundred teeth, the axle to which has also an endless screw working into a horizontal wheel, having a like number of teeth, and which communicates motion to a vertical screw fifteen inches long. on this screw is placed a moveable nut, which carries a pencil. round the cylinder is wrapped daily a paper divided for the points of the compass. the wind acting upon the vane will cause the plate to turn; and the screw which carries the pencil will travel with it, so that the pencil will mark upon the paper the direction of the wind. the fly will also be set in motion, and thereby the nut upon the screw will descend, so that the attached pencil will trace a vertical line upon the paper. when the fans on the axle are · inches from axis to end, and · inches wide, and the thread of the screw such that forty-five revolutions will cause the nut to descend two inches, · miles of wind will cause the pencil to descend through a vertical space of two inches; but the actual trace upon the paper will be longer in proportion to the magnitude of change of azimuth, or direction, of the wind. = . osler's anemometer, and pluviometer.=--mr. follet osler is the inventor of a self-recording apparatus which registers the direction and pressure of the wind, and the amount and duration of rain, upon the same sheet of paper. his apparatus has met with very much approbation, and has been erected in many observatories. the mechanism may be modified in various ways, and the following is a description of the simplest and most recent arrangement. [illustration: fig. .] the instrument, of which fig. is a diagram rather than a picture, consists, first, of a vane, _v_, of a wedge-shape form, which is found to answer better than a flat vane; for the latter is always in a neutral line, and therefore is not sufficiently sensitive. a wind-mill governor has been substituted for the vane to get the direction of the wind, with advantage. at the lower end of the tube, _t t_, is a small pinion, working in a rack, _r_, which moves backwards and forwards as the wind presses the vane. to this rack a pencil, _x_, is attached, which marks the direction of the wind on a properly ruled paper, placed horizontally beneath, and so adjusted as to progress at the rate of half an inch per hour, by means of a simple contrivance connecting it with a good clock. the paper is shown in the illustration upon the table of the instrument. the pressure plate, _f_, for ascertaining the force of the wind, is one foot square, placed immediately beneath, and at right angles with the vane; it is supported by light bars, running horizontally on friction rollers, and communicating with flattened springs, , , , so that the plate, when affected by the pressure of the wind, acts upon them, and they transfer such action to a copper chain passing down the interior of the direction tube, and over a pulley at the bottom. a light copper wire connects this chain with the spring lever, _y y_, carrying a pencil which records the pressure upon the paper below. mr. osler much prefers a spring to any other means for ascertaining the force of the wind, because it is of the highest importance to have as little matter in motion as possible, otherwise the momentum acquired will cause the pressure plate to give very erroneous indications. the pressure plate is as light as is consistent with strength. it is kept before the wind by the vane, and is urged out by three or more springs, so that with light winds one only is compressed, and two, or more, according to the strength of the wind. the _pluviometer_ is placed on the right in the figure, _p p_ being the plane of the roof of the building. the rain funnel, _r_, exposes an area of about square inches. the water collected in it is conveyed by a tube through the roof of the building into a glass vessel, _g_, so adjusted and graduated as to indicate a quarter of an inch of rain for every square inches of surface, _i. e._ cubic inches. _g_ is supported by spiral springs, _b b_, which are compressed by the accumulating rain. a glass tube, open at both ends, is cemented into the bottom of _g_, and over it is placed a larger one closed at the top like a bell glass. the smaller tube thus forms the long leg of a syphon, and the larger tube acts as the short leg. the water, having risen to the level of the top of the inner tube, drops over into a little copper tilt, _t_, in the globe, _s_, beneath the reservoir. this tilt is divided into two equal partitions by a slip of copper, and placed upon an axis not exactly balanced, but so that one end or the other preponderates. the water then drops into the end of the tilt which happens to be uppermost, and when quite full it falls over, throwing the water into the globe, _s_, from which it flows away by the waste pipe. in this way an imperfect vacuum is produced in the globe, quite sufficient to produce a draught in the small tube of the syphon, or the long leg; and the whole contents of the reservoir, _g_, immediately run off, and the spiral springs, _b b_, elevate the reservoir to its original position. to produce this action, a quarter of an inch of rain must have fallen. the registration is easily understood. a spring lever, _z_, carrying a pencil, is attached by a cord, _c_, to _s_. this spring always keeps the cord tight, so that as the apparatus descends during the fall of rain, the spring advances the pencil more and more from the zero of the scale upon the paper beneath, until a quarter of an inch has fallen, when the pencil is drawn back to zero by the ascent of the reservoir. the clock movement carries the registering paper forward by one of the wheels working into a rack attached to the frame. the adjustment of the instrument should be carefully made at its first erection. the scale for pressure should be established experimentally, by applying weights of , , , &c., lbs., to move the pressure plate. the registration trace for twenty-four hours is readily understood. the direction is recorded on the centre part; the pressure on one side, and the rain on the other. lines parallel to the length of the paper show no rain, steady wind, and constant pressure. on the rain trace, a line parallel to the width of the paper shows that the pencil had been drawn back to zero, a quarter of an inch of rain having fallen. the hour lines are in the direction of the width of the paper. at the international exhibition , messrs. negretti and zambra exhibited an improved osler's anemometer, having combined with it robinson's cups, so that the pressure and velocity appear on the same sheet, on which a line an inch in length is recorded at every ten miles; thus the complete instrument shows continuously the direction, pressure, and velocity of the wind. = . beckley's anemometer.=--mr. r. beckley, of the kew observatory, has devised a self-registering anemometer, which consists of three principal parts: robinson's cups for the determination of velocity; a double fan, or wind-mill governor, for obtaining the direction; and a clock to move a cylinder, around which registration paper is wrapped. the paper records the time, velocity, and direction of the wind for twenty-four hours, when it must be replaced. it has a cast-iron tubular support, or pedestal to carry the external parts--the cups and the fans,--which must be erected upon the roof of the building upon which it is desired to mount the instrument. the fans keep their axis at right angles to the wind; and with any change of direction they move, carrying with them an outer brass tube, which rests upon friction balls on the top of the pedestal, and is attached to a tubular shaft passing through the interior of the pedestal, and terminating with a mitre wheel. the mitre wheel, working with other cogged wheels, communicates the motion of the direction shaft to a cylinder carrying a pencil, to record the direction. the shaft carrying the cups is supported upon friction balls, placed in a groove formed on the top of the direction shaft, and passing through the interior of that shaft, comes out below the mitre wheel, where it is terminated in an endless screw, or worm. upon the wind moving the cups, motion is given to the innermost shaft, thence to the worm-wheel, whence motion is given to a pencil which registers the velocity. de la rue's metallic paper is used in registration, it having the property of receiving a trace from a brass pencil. the pencils can, therefore, be made in the most convenient form. mr. beckley forms each pencil of a strip of brass wrapped round a cylinder, making a very thin threaded screw, so that the contact of the pencil cylinder and the clock cylinder is a mere point of the metallic thread. the pencil cylinders are placed side by side upon the cylinder turned by the clock, and require no spring or other appliance to keep them to their work, but always make contact with the registration paper by their own gravity. they therefore require no attention, and being as long as the trace which they make, they will last a long time. the velocity pencil has only one turn on the cylinder, and its pitch is equal to a scale of fifty miles upon the paper. the direction pencil has likewise one turn on its cylinder, its pitch being equal to a scale of the cardinal points of the compass upon the paper. the clock gives a uniform motion of half an inch per hour to the cylinder upon which the paper is fastened. the registering mechanism of the instrument is very compact, requiring only a space of about inches by inches. in the report of the british association for , mr. beckley has given a detailed description of his anemometer, with drawings of all the parts. = . self-registering lind's anemometer.=--a lind's wind-gauge, designed to register the maximum pressure, was exhibited at the international exhibition , by mr. e. g. wood. the bend of the syphon is contracted to obtain steadiness. on the leeward limb a hole is drilled corresponding in size with the contracted portion of the tube. the edge of the hole corresponds with the zero of the scale. on the pressure of the wind increasing, as much of the water as would have risen above the aperture flows away, and therefore the quantity left indicates the greatest pressure of the wind since the last setting of the instrument, which is done by filling it with water up to the zero point. = . anemometric observations.=--to illustrate the value of anemometric observations, we quote from a paper by mr. hartnup, on the results obtained from osler's anemometer, at the liverpool observatory. the six years' observations, ending , gave for the yearly average of the winds: north-easterly, on days, at · miles per hour; north-westerly, on days, at · miles per hour; south-easterly, on days, at · miles per hour; south-westerly, on days, at · miles per hour; and one day calm. from the same observations, the average variation in the strength of the wind during the hours is:-- miles per hour, the minimum force, occurring at - / a.m.; until a.m. it remains much the same, being then · miles per hour; at a.m. it is · miles per hour; at - / p.m. the wind is at its maximum strength, being · miles per hour; at p.m. it is again · miles per hour, and at p.m. · miles per hour. hence it appears that the wind falls to its minimum force much more gradually than it rises to its maximum; that the decrease and increase are equal and contrary, so that the curve is symmetrical; and that generally the force of wind is less at night than during the day. "there is evidence," says admiral fitzroy, "in mr. hartnup's very valuable anemometrical results, which seems to prove that to his observatory, in a valley, with buildings and hills to the north-eastward, the real polar current does not blow from n.e., but nearer s.e. by his reliable digest of winds experienced there, it appears that those most prevalent were from w.n.w. and s.s.e. but in england, generally, the _prevailing_ winds are _believed_ to be westerly, inclining to south-westerly, and north-easterly; while of all winds, the south-easterly is about the rarest. "at lord wrottesley's observatory, in staffordshire, about feet above the sea, there appears to be considerably less strength of wind at any given time, when a gale is blowing _generally_, than occurs simultaneously at places along the sea-coast: whence the inference is, that undulations of the land's surface and hills, diminish the strength of wind materially by frictional resistance. "all the synoptic charts hitherto advanced at the board of trade exhibit a marked diminution of force inland compared with that on the sea-coast. indeed, the coast itself offers similar evidence, in its stunted, sloping trees, and comparative barrenness."[ ] chapter xiv. instruments for investigating atmospheric electricity. = . atmospheric electroscope.=--the simplest instrument for ascertaining at any time the electric condition of the atmosphere is an electroscope composed of two equal pieces of gold leaf, suspended from a brass support, and insulated, as well as protected from the movement of the air, by a glass covering. fig. represents such an instrument. the cap of the brass support is fitted for the reception, in the vertical direction, of a metallic rod, not less than two or three feet in length. the top of the rod carries a clip. the instrument acts according to the law, that bodies similarly electrified repel each other; but when dissimilarly electrified, they attract each other. to make an observation, the instrument is placed in the open air, and a lighted piece of cigar fusee, or touch-paper, is fixed in the clip. the electricity of the air is collected by the substance undergoing combustion, and conducted by the rod to the gold leaf; and the pieces, being similarly electrified, separate more or less according to the amount of electricity present. the kind is determined by the effect of either an excited stick of sealing-wax or rod of glass upon the electrified gold leaf. a rod of glass, when rubbed briskly with a silk handkerchief or piece of woollen cloth, becomes positively electrified, or excited, as it is termed. a stick of sealing-wax, similarly treated, acquires the negative state. if, therefore, an excited glass rod be presented to the cap of the instrument, and it cause the pieces of gold leaf to diverge still further, the electric state of the air must be analogous to that of the glass, that is, _positive_; if they approach, it is _negative_. on the contrary, if a stick of sealing-wax be used, the pieces will be repelled more apart if they have acquired negative electricity from the air; and they will converge if they have a positive charge. [illustration: fig. .] by means of this very simple instrument, meteorological observers can readily ascertain the electric condition of the lower air at any time. note.--a book containing strips of gold leaf is sent with the electrometer to replace the gold leaves when torn or broken in use. to mount fresh gold leaves, unscrew the brass plate to which is attached the rod supporting the leaves; then moisten with the breath the flat piece of brass, and press it gently down on one strip of gold, whilst the book is only partly opened; the second leaf is attached in the same manner. = . volta's electrometer= is similar to the instrument just described, except that instead of gold leaf two light pieces of straw, or two pith balls, are freely suspended from the conductor; the amount of the electric charge being estimated from the degrees of divergence, shown by a graduated arc. = . peltier's electrometer= is a much superior instrument in point of sensibility. a tall glass tube an inch or more in diameter, is connected to a glass receiver, mounted on a base fitted with levelling screws. at the top of the tube is formed a globe from four to five inches in diameter, which is thickly gilt on the exterior, so as to form a good conducting surface. a wire passes from the ball down the tube into the receiver, where it is bent up, and ends in a steel point over the centre of the base. a bent wire, carrying a small magnetic needle, is balanced on the steel point, so that the magnet, with the fine wire, arranges itself horizontally in the direction of the magnetic meridian. if any cloud or portion of air in the neighbourhood be in an electrical state, it will act by induction upon the gilt ball, and the needle will be deflected from its north and south direction. a graduated circle indicates the number of degrees of the deflection, which will be greater or less according to the tension of the electricity. to ascertain whether the electricity is positive or negative, a stick of shellac or glass must be employed, as already described. = . bohnenberger's electroscope= may be fitted with a metallic conductor, and used with great advantage for observing atmospheric electricity. "the principal parts of the instrument, as improved by becquérel, are the following:--_a b_, fig. , is a small dry galvanic pile of from to pairs, about a quarter of an inch in diameter; when the plates are pressed together, such a pile will be from to - / inches in length. the wires, which are bent so as to stand above the pile, terminate in two plates, _p_ and _m_, which are the poles of the pile. these plates, which are inches by / an inch, are parallel and opposite to each other. it is convenient for their opposite sides to be slightly convex, for them to be gilded or coated with platinum, and for them to run on the polar wires, by the latter being made to pass through a small hole in them. one of these plates will always be in a state of positive, and the other of negative, electricity; between them suspend the very fine gold leaf, _d g_, which is attached to the conductor, _c d_, of copper wire. if the leaf hang exactly between the two plates, it is equally attracted by each, and will therefore be in a state of repose. the apparatus should be protected by a bell-glass, fitting exactly, and having an opening at the top through which the copper wire, _c d_, passes; the wire, however, is insulated by its being contained in a glass tube, which is made to adhere to the bell-glass by means of a small portion of shellac or gum-lac. screw on a metal ball or plate, to impart to it the electricity you wish to test, which will be conveyed by the copper wire to the gold leaf, and the latter will immediately move towards the plate which has the opposite polarity. this electroscope is, beyond doubt, one of the most delicate ever constructed, and is well adapted to show small quantities of positive and negative electricity. [illustration: fig. .] "to ensure the susceptibility of electroscopes and electrometers placed under bell-glasses, precautions should be taken to render the air they contain as dry as possible, which may be effected by enclosing in a suitable vessel a little melted chloride of calcium beneath the glass." the galvanic pile employed in this electroscope is that invented by zamboni. "it differs from the common hydro-electric batteries principally in this, that the presence of the electromotive liquid is dispensed with, and that in its place is substituted some moist substance of low conducting power, generally paper. the electromotors in these piles are composed for the most part of dutch gold (copper) and silver (zinc) paper pressed one on the other, with their paper sides together, out of which discs are cut with a diameter of from a quarter of an inch to an inch. more powerful pairs of plates may be obtained by using only the silver paper and smearing its paper side with a thin coat of honey, on which some finely pulverized peroxide of manganese has been sprinkled, and all the sides similarly coated are presented one way. powerful pairs of plates may also be made by pasting pure gold leaf on the paper side of zinc-paper. these plates are then to be arranged, just as in the ordinary voltaic pile, one above the other, so that the similar metallic surfaces may all lie one way; press them tightly together; tie them with pretty stout silk threads, and press them into a glass tube of convenient size. the metal rims of the tubes, which must be well connected with the outermost pairs of plates, form the poles of the pile, the negative pole being in the extreme zinc surface, and the positive in the extreme copper or manganese surface. "the electromotive energy called into action in these dry piles is less than that excited in the moist or hydro-electric piles, principally on account of the imperfect conduction of the paper. the accumulation of electricity at their poles also goes on less rapidly, and consequently the electrical tension continues for a long while unaltered; whereas, in all moist piles, even in the most constant of them, the tension is maintained, comparatively speaking, for but a short time, on account of the chemical action and decomposition of the electromotive fluid--causes of disturbance which do not exist in the dry pile."[ ] = . thomson's electrometer.=--professor w. thomson, of glasgow, has devised an atmospheric electrometer, which is likely to become eminently successful, in the hands of skilful observers. it is mainly a torsion balance combined with a leyden-jar. the index is an aluminium needle strung on a fine platinum wire, passing through its centre of gravity, and stretched firmly between two points. the needle and wire are carefully insulated from the greater part of the instrument, but are in metallic communication with two small plates fixed beside the two ends of the needle, and termed the repelling plates. a second pair of larger plates face the repelling plates, on the opposite side of the needle, but considerably farther from it. these plates are in connection with the inner coating of a leyden-jar, and are termed the attracting plates. the whole instrument is enclosed in a metal cage, to protect the glass leyden-jar and the delicate needle. the leyden-jar should be charged when the instrument is used. its effect is two-fold: it increases greatly the sensibility of the instrument, and enables the observer to distinguish between positive and negative electrification. the air inside the jar is kept dry by pumice-stone, slightly moistened with sulphuric acid; by which means very perfect insulation is maintained. electrodes, or terminals, are brought outside the instrument, by which the leyden-jar can be charged, and the needle system connected with the body, the electric state of which is to be tested. for the purpose of testing the electric state of the atmosphere, the instrument is provided with a conductor and support for a burning match, or, preferably, with an arrangement termed a water-dropping collector; by either of which means the electricity of the air is conveyed to the needle system. the needle abuts upon the repelling plates when not influenced by electricity, in which position it is at zero. it can always be brought back to zero by a torsion-head, turning one end of the platinum wire, but insulated from it, and provided with a graduated circle, so that the magnitude of the arc, that the torsion-head is moved through to bring the needle to zero, measures the force tending to deflect it. the action of the instrument is as follows:--the leyden-jar is to be highly charged, say negatively; and the repelling plates are to be connected with the earth. the needle will then be deflected against a stop, under the combined influence of attraction from the leyden-jar, or attracting plates, and repulsion from the repelling plates due to the positive charge induced on the needle and its plates by the leyden-jar plates. the platinum wire must then be turned round by the torsion-head so as to bring back the needle to zero; and the number of degrees of torsion required will measure the force with which the needle is attracted. next, let the needle plates be disconnected from the earth, and connected with the insulated body, the electric state of which is to be tested. in testing the atmosphere, the conductor and lighted match, or water-dropping apparatus, must be applied. if the electricity of the body be positive, it will augment the positive charge in the needle plates, induced by the leyden-jar plates; and consequently the needle will be more deflected than by the action of the jar alone. if the electricity of the body be negative, it will tend to neutralize the positive charge; and the needle will be less deflected. hence the kind of electricity present in the air becomes at once apparent, without the necessity of an experimental test. the platinum wire must then be turned till the needle is brought to zero, and the number of degrees observed; which is a measure of the intensity of the electrification. any loss of charge from the leyden-jar which may from time to time occur, reducing the sensibility inconveniently, may be made good by additions from a small electrophorus which accompanies the instrument.[ ] the instrument may be made self-recording by the aid of clockwork and photography. to effect this, a clock gives motion to a cylinder, upon which photographic paper is mounted. the needle of the electrometer is made to carry a small reflector; and rays from a properly adjusted source of light are thrown by the reflector, through a small opening, upon the photographic paper. it is evident, that as the cylinder revolves, a trace will be left upon the paper, showing the magnitude of, and variations in, the deflection of the needle. = . fundamental facts regarding atmospheric electricity.=--the _general_ electrical condition of the atmosphere is _positive_ in relation to the surface of the earth and ocean, becoming more and more positive as the altitude increases. when the sky is overcast, and the clouds are moving in different directions, it is subject to great and sudden variations, changing rapidly from positive to negative, and the reverse. during fog, rain, hail, sleet, snow, and thunderstorms, the electrical state of the air undergoes many variations. the intensity of the electricity increases with hot weather following a series of wet days, or of wet weather coming after a continuance of dry days. the atmospheric electricity, in fact, seems to depend for its intensity and kind upon the direction and character of the prevailing wind, under ordinary circumstances. it has an annual and a diurnal variation. there is a greater diurnal change of tension in winter than in summer. by comparing observations from month to month, a gradual increase of tension is perceived from july to february, and a decrease from february to july. the intensity seems to vary with the temperature. the diurnal variation exhibits two periods of greatest and two of least intensity. in summer, the _maxima_ occur about a.m. and p.m.; the _minima_ about a.m. and noon. in winter, the _maxima_ take place near a.m. and p.m.; the _minima_ near a.m. and p.m. the researches of saussure, beccaria, crosse, quétèlet, thompson, and fitzroy have tended to show that during the prevalence of polar currents of air positive electricity is developed, and becomes more or less active according to the greater or less coldness and strength of wind; but with winds from the equatorial direction there is little evidence of sensitive electricity, and when observable, it is of the negative kind. storms and gales of wind are generally attended, in places, with lightning and thunder; and as the former are very often attributed to the conflict of polar and equatorial winds, the difference of the electric tension of these winds may account for the latter phenomena. it is not our intention to enter upon the general consideration of thunderstorms; the facts which we have given may be of service to the young observer; and finally, as it is interesting to be able to judge of the locality of a thunderstorm, the following simple rule will be of service, and sufficiently accurate:--note by a second's watch the number of seconds which elapse from the sight of the lightning to the commencement of the thunder; divide them by five, and the quotient will be the distance in miles. thus, if thunder is heard ten seconds after the lightning was seen, the distance from the seat of the storm will be about two miles. the interval between the flash and the roll has seldom been observed greater than seventy-two seconds. = . lightning conductors.=--"the line of danger, whether from the burning or lifting power of lightning, is the line of strong and obstructed currents of air, of the greatest aerial friction."[ ] trees, church spires, wind-mills and other tall structures, obstruct the aerial currents, and hence their exposure to danger. the highest objects of the landscape, especially those that are nearest the thunder cloud, will receive the lightning stroke. the more elevated the object, the more likely is it to be struck. of two or more objects, equally tall and near, the lightning is invariably found to select the best conductor of electricity, and even to make a circuitous path to get to it. hence the application and evident advantage of metallic rods, called _lightning conductors_, attached to buildings and ships. a lightning conductor should be pointed at top, and extend some feet above the highest part of the edifice, or mast. it should be made of copper, which is a better conducting medium than iron, and more durable, being less corrosive. it must be unbroken throughout its length, and extend to the bottom of the building, and even some distance into the ground, so as to conduct the electricity into a well or moist soil. if it be connected with the lead and iron work in the structure of the house, it will be all the better, as affording a larger surface, and a readier means of exit for the fluid. in a ship, the lower end of the conductor should be led into communication with the hull, if of iron, and with the copper sheathing, if a wooden vessel; so that, spread over a large surface, it may escape more readily to the water. = . precautions against lightning.=--experience seems to warrant the assumption that any building or ship, fitted with a substantial lightning conductor, is safe from danger during a thunderstorm. should a house or vessel be undefended by a conductor, it may be advisable to adopt a few precautions against danger. in a house, the fire-place should be avoided, because the lightning may enter by the chimney, its sooty lining being a good conductor. "through chimneys, lightning has a way into most houses; and therefore, it is wise, by opening doors or windows, to give it a way out. wherever the aerial current is fiercest, there the danger is greatest; and if we kept out of the way of currents or draughts, we keep out of the way of the lightning."[ ] lightning evinces as it were a preference for metallic substances, and will fly from place to place, even out of the direct line of its passage to the earth, to enter such bodies. it is therefore well to avoid, as much as possible, gildings, silvered mirrors, and articles of metal. the best place is perhaps the middle of the room, unless a draught passes, or a metallic lamp or chandelier should be hanging from the ceiling. the neighbourhood of bad conductors, such as glass windows, not being open, and on a thick bed of mattrasses, are safe places. the quality of trees as lightning conductors is considered to depend upon their height and moisture, those which are taller and relatively more humid being struck in preference to their fellows; therefore, it is unwise to seek shelter under tall and wet trees during a thunderstorm. in the absence of any other shelter, it would be better to lie down on the ground. chapter xv. ozone and its indicators. = . nature of ozone.=--during the action of a powerful electric machine, and in the decomposition of water by the voltaic battery, a peculiar odour is perceptible, which is considered to arise from the generation of a substance to which the term ozone has been given, on account of its having been first detected by smell, which, for a long time after its discovery, was its only known characteristic. a similar odour is evolved by the influence of phosphorus on moist air, and in other cases of slow combustion. it is also traceable, by the smell, in air,--where a flash of lightning has passed immediately before. afterwards it was established that the same element possessed an oxidising property. it was found to be liberated at the oxygen electrode when water was decomposed by an electric current; and has been regarded by some chemists as what is termed an _allotropic_ form of oxygen, while others speak of it as oxygen in the _nascent_ state, and some even regard it as intimately related to chlorine. so various are the existing notions of the nature of this obscure agent. its oxidising property affords a ready means for its detection, even when the sense of smell completely fails. the methods of noting the presence and measuring the amount of ozone present in the air, are very simple; being the free exposure to the air, defended from rain and the direct rays of the sun, of prepared test-papers. there are two kinds of test-papers. one kind was invented by dr. schonbein, the original discoverer of ozone; and the other, which is more generally approved, by dr. moffat. = . schonbein's ozonometer= consists of strips of paper, previously saturated with a solution of starch and iodide of potassium, and dried. the papers are suspended in a box, or otherwise properly exposed to the air, for a given interval, as twenty-four hours. the presence of ozone is shown by the test-paper acquiring a purple tint when momentarily immersed in water. the amount is estimated by the depth of the tint, according to a scale of ten tints furnished for the purpose, which are distinguished by numbers from to . the ozone decomposes the compound which iodine forms with hydrogen, and, it is presumed, combines as oxygen with hydrogen, while the iodine unites with the starch, giving the blue colour when moist. = . dr. moffat's ozonometer= consists of papers prepared in a somewhat similar manner to schonbein's; but they do not require immersion in water. the presence of ozone is shown by a brown tint, and the amount by the depth of tint according to a scale of ten tints, which is furnished with each box of the papers. moffat's have the advantage of preserving their tint for years, if kept in the dark, or between the leaves of a book; and are simpler to use. [illustration: fig. .] = . sir james clark's ozone cage= (fig. ), consists of two cylinders of very fine wire gauze, one fitting into the other; the wire gauze being of such a fineness as to permit the free ingress of air, at the same time that it shuts out all light that would act injuriously on the test-paper, which is suspended by a clip or hook attached to the upper part of the inner cylinder. = . distribution and effects of ozone.=--mr. glaisher has found that "the amount of ozone at stations of low elevation is small; at stations of high elevation, it is almost always present; and at other and intermediate stations, it is generally so. the presence and amount of ozone would seem to be graduated by the elevation, and to increase from the lowest to the highest ground. the amount of ozone is less in towns than in the open country at the same elevation; and less at inland than at sea-side stations." it seems to abound most with winds from the sea, and to be most prevalent where the air is considered the purest and most salubrious. this may seem, says admiral fitzroy, in _the weather book_, to point to a connection between ozone and chlorine gas, which is in and over sea-water, and which _must_ be brought by any wind that blows from the sea. it prevails more over the ocean and near it than over land, especially land remote from the sea; and, says the admiral, it affects the gastric juice, improves digestion, and has a tanning effect. dr. daubeny, in his _lectures on climate_, writes: "its presence must have a sensible influence upon the purity of the air, by removing from it foetid and injurious organic effluvia. it is also quite possible that ozone may play an important part in regulating the functions of the vegetable kingdom likewise; and although it would be premature at present to speculate upon its specific office, yet, for this reason alone, it may be well to note the fact of its frequency, in conjunction with the different phases which vegetation assumes, persuaded that no principle can be generally diffused throughout nature, as appears to be the case, with this, without having some important and appropriate use assigned for it to fulfil." = . registering ozonometer.=--dr. e. lancaster has contrived an ozonometer, the object of which is to secure the constant registration of ozone, so that the varying quantities present in the atmosphere may be detected and registered. for this purpose, an inch of ozone paper passes in each hour, by clock-work, beneath an opening in the cover of the instrument. chapter xvi. instruments not described in the preceding chapters. = . chemical weather glass.=--this curious instrument appears to have been invented more than a hundred years ago, but the original maker is not known. it is simply a glass vial about ten inches long and three quarters of an inch in diameter, which is nearly filled, and hermetically sealed, with the following mixture:--two drachms of camphor, half a drachm of nitrate of potassium, half a drachm of chlorate of ammonium, dissolved in about two fluid ounces of absolute alcohol mixed with two ounces of distilled water. all the ingredients should be as pure as possible, and each vial filled separately. when the instruments are made in numbers and filled from a common mixture, some get more than the due proportion of the solid ingredients, and consequently such glasses do not exhibit that uniformity of appearance and changes, that undoubtedly should accompany similar influencing circumstances. it is in consequence of a want of precision and fixed principle of manufacture, that these interesting instruments are not properly appreciated, and more generally used. the glass should be kept quite undisturbed, exposed to the north, and shaded from the sun. camphor is soluble in alcohol, but not in water, while both water and alcohol have different solvent powers, according to the temperature; hence, the solid ingredients being in excess for certain conditions of solution, depending upon temperature chiefly, and perhaps electricity and the action of light also, appear as crystals and disappear with the various changes that occur in the weather. the various appearances thus presented in the menstruum have been inferred to prognosticate atmospheric changes. the following rules have been deduced from careful study of the glass and weather:-- . during cold weather, beautiful fern-like or feathery crystallization is developed at the top, and sometimes even throughout the liquid. this is the normal state of the glass during winter. the crystallization increases with the coldness; and if the structure grows downward, the cold will continue. . during warm and serene weather, the crystals dissolve, the upper and greater part of the liquid becoming perfectly clear. this is the normal state of the glass during summer. the less amount of crystallization, that is, the greater the clear portion of the liquid (for there is always some of the composition visible at the bottom), the greater the probability of continued fine dry weather. . when the upper portion is clear, and flakes of the composition rise to the top and aggregate, it is a sign of increasing wind and stormy weather. . in cold weather, if the top of the liquid becomes thick and cloudy, it denotes approaching rain. . in warm weather, if small crystals rise in the liquid, which still maintains its clearness, rain may be expected. . sharpness in the points and features of the fern-like structure of the crystals, is a sign of fine weather; but when they begin to break up, and are badly defined, unsettled weather may be expected. admiral fitzroy, in _the weather book_, writes of this instrument as follows:--"since , we have generally had some of these glasses, as curiosities rather than otherwise; for nothing certain could be made of their variations until lately, when it was fairly demonstrated that if fixed undisturbed in free air, not exposed to radiation, fire, or sun, but in the ordinary light of a well-ventilated room, or, _preferably_, in the outer air, the chemical mixture in a so-called storm-glass varies in character with the _direction_ of the wind--not its force, _specially_ (though it _may_ so vary in _appearance_, only from another cause, _electrical tension_). "as the atmospheric current veers toward, comes from, or is only _approaching_ from the polar direction, this chemical mixture--if closely, even microscopically watched--is found to grow like _fir_, _yew_, fern leaves, or hoar-frost--or like crystallizations. "as the wind, or great body of air, tends more from the _opposite_ quarter, the lines or spikes--all regular, hard, or crisp features--gradually diminish, till they vanish. "before, and in a continued southerly wind, the mixture sinks slowly downward in the vial, till it becomes shapeless, like melting white sugar. "before, or during the continuance of a northerly wind (polar current), the crystallizations are beautiful (if the mixture is correct, the glass a _fixture_, and duly _placed_); but the least motion of the liquid disturbs them. "when the main currents meet, and turn _toward the west_, making _easterly_ winds, stars are more or less numerous, and the liquid dull, or less clear. when, and while they _combine by the west_, making westerly winds, the liquid is clear, and the crystallization well-defined, without loose stars. "while _any hard_ or _crisp_ features are visible below, above, or at the top of the liquid (where they form for polar winds), there is _plus_ electricity in the air; a _mixture_ of polar current co-existing _in that locality_ with the opposite, or southerly. "when nothing but soft, melting, sugary substance is seen, the atmospheric current (feeble or strong as it may be) is southerly with _minus_ electricity, unmixed with, and _uninfluenced_ by, the contrary wind. "repeated trials with a delicate galvanometer, applied to measure electric tension in the air, have proved these facts, which are now found useful for aiding, with the barometer and thermometer, in forecasting weather. "temperature affects the mixture much, but not solely; as many comparisons of winter with summer changes of temperature have fully proved. "a confused appearance of the mixture, with flaky spots, or stars, in motion, and less clearness of the liquid, indicates south-easterly wind, probably strong to a gale. "clearness of the liquid, with more or less perfect crystallizations, accompanies a combination, or a contest, of the main currents, by the _west_, and very remarkable these differences are,--the results of these air currents acting on each other _from_ eastward, or from an entirely opposite direction, the _west_. "the glass should be wiped clean now and then,--and once or twice a year the mixture should be disturbed, by inverting and gently shaking the glass vial." [illustration: fig. .] = . leslie's differential thermometer.=--a glass tube having a large bulb at each extremity, and bent twice at right angles, as represented in figure , containing strong sulphuric acid tinged with carmine, and supported at the centre by a wooden stand, constitutes the differential thermometer as invented by professor leslie. the instrument is designed to exhibit and measure small differences of temperature. each leg of the instrument is usually from three to six inches long, and the balls are about four inches apart. the calibre of the legs is about / inch, not more; the other part of the tube may be wider. the tube is filled with the liquid, the bulbs contain air. when both bulbs are heated alike, each scale indicates zero. the scale is divided so that the space between the freezing and the boiling-points of water is equal to , parts. when one bulb is heated more than the other, the difference of temperature is delicately shown by the descent of the coloured fluid from the heated ball. it is uninfluenced by changes in the temperature of the atmosphere; hence it is admirably adapted for experiments of radiant heat. the theory of the instrument is that gases expand equally for uniform increments of heat. = . rumford's differential thermometer= differs from that just described in simply containing only a small bubble of liquid, which lies in the centre of the tube, when both bulbs are similarly influenced. the bulbs and other parts of the tube contain air. when one bulb is more heated than the other, the bubble moves towards the one less heated; and the scale attached to the horizontal part of the tube affords a measurement of the difference of temperature. [illustration: fig. .] = . glaisher's thermometer stand.=--the thermometer stand consists of a horizontal board as a base, of a vertical board projecting upwards from one edge of the horizontal one, and of two parallel inclined boards, separated from each other by blocks of three inches in thickness, connected at the top with the vertical, and at the bottom with the horizontal board, and the air passes freely about and between them all. to the top of the inclined boards is connected a small projecting roof to prevent the rain falling on the bulbs of the instrument, which are carried on the face of the vertical board, with their bulbs projecting below it, so that the air plays freely on the bulbs from all sides. the whole frame revolves on an upright post firmly fixed to the ground, as shown in the engraving, fig. ; and in use, the inclined side is always turned towards the sun. = . thermometer screen, for use at sea.=--this screen, or shade, was designed by admiral fitzroy, and has been in use for several years on board h.m. vessels and many merchant-ships. it is about twenty-four inches long by twelve wide and eight deep; having lattice-work sides, door, and bottom; with perforation also at top, so contrived that the air has free access to the interior, while the direct rays of the sun, rain, and sea spray are effectually excluded from the thermometers mounted inside. there is ample space for two thermometers placed side by side on brackets, at least three inches from each other or any part of the exterior of the screen. one thermometer should be fitted up as a "wet bulb" (see p. ). a small vessel of water can easily be fixed inside the screen so as to retain its position and contents under the usual motions of the ship; and by means of a piece of cotton-wick, or muslin rag tied round the bulb of the thermometer and trailing into the cup of water, keep the bulb constantly moist. self-registering thermometers should be protected by a similar screen. it has been found that thermometric observations made at sea are not valuable for scientific purposes unless the instruments have been duly protected by such a screen. = . anemoscope=, or portable wind vane for travellers, with compass, bar needle, &c., shows the direct course of the wind to half a point of the compass. [illustration: fig. .] = . evaporating dish, or gauge= (fig. ), for showing the amount of evaporation from the earth's surface. this gauge consists of a brass vessel, the area or evaporating surface of which is accurately determined; and also a glass cylindrical measure, graduated into inches, tenths, and hundredths of inches. in use, the evaporating gauge is nearly filled with water, the quantity having been previously measured by means of the glass cylinder; it is then placed out of doors, freely exposed to the action of the atmosphere; after exposure, the water is again measured, and the difference between the first and second measurement shows the amount of evaporation that has taken place. if rain has fallen during the exposure of the gauge, the quantity collected by it must be deducted from the measured quantity; the amount is shown by the quantity of rain collected in the rain gauge. the wire cage round the gauge is to prevent animals, birds, &c., from drinking the water. = . dr. babington's atmidometer=, or instrument for measuring the evaporation from water, _ice or snow_, consists of an oblong hollow bulb of glass or copper, beneath which and communicating with it by a contracted neck is a second globular bulb, duly weighted with mercury or shot. the upper bulb is surmounted by a small glass or metal stem, having a scale graduated to grains and half-grains; on the top of which is fixed horizontally a shallow metal pan. the bulbs are immersed in a vessel of water having a circular hole in the cover through which the stem rises. distilled water is then gradually poured into the pan above, until the zero of the stem sinks to a level with the cover of the vessel. thus adjusted, as the water in the pan evaporates, the stem ascends, and the amount of evaporation is indicated in grains. this instrument affords a means of measuring evaporation from _ice or snow_. an adjustment for temperature is necessary. = . cloud reflector.=--at the international exhibition , mr. j. t. goddard exhibited a cloud mirror, for ascertaining the direction in which the clouds are moving. the mirror is laid on a horizontal support near a window, and fastened so that the point marked north may coincide with the south point of the horizon,--the several points will consequently be reversed. the edge of a conspicuous cloud is brought to the centre of the mirror, and the observer keeps perfectly still until it passes off at the margin, where the true point of the horizon _from which_ the clouds are coming can be read off. = . sunshine recorder.=--mr. goddard also exhibited an instrument which he calls by this name. it works by letting the sun's rays pass through a narrow slit, and fall on photographic paper wound round a barrel moved by clock-work; the paper being changed daily, and the photographic impression developed and fixed in the usual manner.[ ] . set of portable instruments. in a small box, in. by in. by in., a complete set of meteorological instruments have been packed. the lid of the box, by an ingenious arrangement, is made to take off and hang up; on it are permanently fixed for observation, a maximum and minimum, and a pair of dry and wet bulb thermometers. the interior of the box contains a maximum thermometer in vacuo for solar radiation, and a minimum for terrestrial purposes, one of negretti and zambra's small pocket aneroid barometers, pedometer for measuring distances, pocket compass, clinometer, and lastly a rain gauge. this latter instrument consists of an accurately turned brass ring having an india rubber body fastened to it to receive the rain, which is measured off by a small graduated glass, also contained in the box. gentlemen travelling will find this compact observatory all that can be desired for meteorological observations. . implements. the practical meteorologist will find the following articles very useful, if not necessary. they scarcely require description; an enumeration will therefore suffice:--_weather diagrams_, or prepared printed and ruled forms, whereon to exhibit graphically the readings of the various instruments to render their indications useful in foretelling weather, &c.;--_meteorological registers_, or record books, for recording all observations, and the deductions;--_cloud pictures_, by which the clouds can be readily referred to their particular classification, very necessary to the inexperienced and learners;--cyclone glasses, or horns, outline maps with wind-markers, are also useful, especially in forecasting weather. . hydrometer. a simple kind of hydrometer is very much used at sea, as "a sea-water test;" and as the observations are usually recorded in a meteorological register or the ship's log-book, it may not be altogether out of place to give a description of it here. [illustration: fig. .] [illustration: fig. .] it is constructed of glass. if made of brass, the corrosive action of salt-water soon renders the instrument erroneous in its indications. the shapes usually given to the instruments are shown in figs. and . a globular bulb is blown, and partly filled with mercury or small shot, to make the instrument float steadily in a vertical position. from the neck of the bulb the glass is expanded into an oval or a cylindrical shape, to give the instrument sufficient volume for flotation; finally, it is tapered off to a narrow upright stem which encloses an ivory scale, and is closed at the top. the divisions on the scale read downward, so as to measure the length of the stem which stands above the surface of any liquid in which the hydrometer is floated. the denser the fluid, the higher will the instrument rise; the rarer, the lower it will sink. the indications depend upon the hydrostatic principle, that floating bodies displace a quantity of the fluid which sustains them equal to their own weight. according, therefore, as the specific gravities of fluids differ from each other, so will vary the quantities of the fluids displaced by the same body when floated successively in each. the specific gravity of distilled water, at the temperature of ° _f_, being taken as unity, the depth to which the instrument sinks when gently immersed in such water is the zero of the scale. the graduations extend from to ; the latter being the mark which will be level with the surface when the instrument is placed in water, the specific gravity of which is · . in recording observations, the last two figures only--being the figures on the scale--are written down. sea-water usually ranges from · to · . a small tin, copper, or glass cylinder is useful for containing the water to be tested. it should be wider than the hydrometer, and always filled to the brim. if fitted to a stand, which is supported by gimbals, it will be very convenient. water in a bucket, basin, or other wide vessel, acquires motion at sea, and the eye cannot be brought low enough (on account of the edges) to read off the scale accurately. errors of observation may occur with the hydrometer, if it be put into water without being clean, or without being carefully wiped. the instrument is extremely accurate if correctly used. it should be kept free from contact with the sides of the vessel; and all dust, smears, or greasiness, should be scrupulously avoided, by carefully wiping it with a clean cloth before and after use. whenever the temperature of the water tested differs from °, a correction to the reading is necessary, for the expansion or contraction of the glass, as well as the water itself, in order to reduce all observations to one generally adopted standard. negretti and zambra's hydrometer, with thermometer in the stem, shows the density and temperature in one instrument. for the following tables we are indebted to the kindness of admiral fitzroy:-- table for reducing observations made with a brass hydrometer, assuming the linear expansion of brass to be · for ° f. the correction is additive for all temperatures above °, and subtractive for temperatures below °. +----------------------------------------------------------------------+ |_t°_|correction.||_t°_|correction.||_t°_|correction.||_t°_|correction.| |----+-----------++----+-----------++----+-----------++----+-----------| | | - · || | - · || | + · || | + · | | | · || | · || | · || | · | | | · || | · || | · || | · | | | · || | - · || | · || | · | | | · || | · || | + · || | · | | | · || | · || | · || | + · | | | - · || | · || | · || | · | | | · || | · || | · || | · | | | · || | - · || | · || | · | | | · || | · || | · || | · | | | · || | · || | + · || | + · | | | · || | · || | · || | · | | | - · || | · || | · || | · | | | · || | - · || | · || | · | | | · || | · || | · || | · | | | - · || | + · || | + · || | + · | +----------------------------------------------------------------------+ table for reducing observations made with a glass hydrometer, assuming the linear expansion of glass to be · for ° f. the correction is additive for temperatures above °, and subtractive for temperatures below °. +----------------------------------------------------------------------+ |_t°_|correction.||_t°_|correction.||_t°_|correction.||_t°_|correction.| |----+-----------++----+-----------++----+-----------++----+-----------| | | - · || | - · || | + · || | + · | | | · || | · || | · || | · | | | · || | · || | · || | · | | | · || | - · || | · || | · | | | · || | · || | + · || | · | | | · || | · || | · || | + · | | | - · || | · || | · || | · | | | · || | · || | · || | · | | | · || | - · || | · || | · | | | · || | · || | · || | · | | | · || | · || | + · || | + · | | | · || | · || | · || | · | | | - · || | · || | · || | · | | | · || | - · || | · || | · | | | · || | · || | · || | · | | | - · || | + · || | + · || | + · | +----------------------------------------------------------------------+ . newman's self-registering tide-gauge. at places where the phenomena of the tides are of much maritime importance, a continuous series of observations upon the rise and fall, and times of change, is essentially necessary as a basis for the construction of good tide tables; and as such observations should also be accompanied with the registration of atmospheric phenomena, we have no hesitation in inserting a description of an accurate self-registering tide-gauge. the tide-gauge, as shown in the illustration, consists of a cylinder, _a_, which is made to revolve on its axis once in twenty-four hours by the action of the clock, _b_. a chain, to which is attached the float, _d_, passes over the wheel, _c_, and on the axis of this wheel, _c_ (in about the middle of it) is a small toothed wheel, placed so as to be in contact with a larger toothed wheel carrying a cylinder, _e_, over which passes another smaller chain. this chain, passing along the upper surface of the cylinder, _a_, and round a second cylinder, _f_, at its further end, is acted on by a spring so as to be kept in a constant state of tension. in the middle of this chain a small tube is fixed for carrying a pencil, which, being gently pressed down by means of a small weight on the top of it, performs the duty of marking on paper placed round the cylinder the progress of the rise or fall of the tide as the cylinder revolves, and as it is drawn by the chain forward or backward by the rise or fall of the float. the paper is prepared with lines equidistant from each other, to correspond with the hours of the clock, crossed by others showing the number of feet of rise and fall. [illustration] the cylinder while in action revolves from left to right to a spectator facing the clock, and the pencil is carried horizontally along the top of the cylinder; and the large wheel being made to revolve by the rise and fall of the float, turns the wheel with the small cylinder, _e_, attached to it. if the tide is _falling_, the small chain is wound round the cylinder, _e_, and the pencil is drawn towards the large wheel; but if the tide is _rising_, the small chain is wound on to the cylinder, _f_, by means of the spring contained in it, which constantly keeps it in a state of tension. thus, by means of the rise and fall of the tide, a lateral progress is given to the pencil, while the cylinder is made to revolve on its axis by the clock, so that a line is traced on the paper showing the exact state of the tide continuously, without further attention than is necessary to change the paper once every day, and to keep the pencil carefully pointed; or a metallic pencil may be used, which will require little, if any, attention. a good self-registering tide-gauge is a valuable and important acquisition wherever tidal observations are required, and the only perfectly efficient instrument of this kind is that invented by the late mr. john newman, of regent street, london. it is now in action in several parts of the world, silently and _faithfully_ performing its duty, requiring no other kind of attention than that of a few minutes daily, and thus admitting the employment of the person on any other service whose duty it would otherwise have been to have registered the tide. it has done much by its faithful records in contributing to the construction of good tide tables for many places; for those unavoidable defects dependent on merely watching the surface on a divided scale are set aside by it, all erroneous conclusions excluded, and a true delineation of nature's own making is preserved by it for the theorist. addenda. . french barometers are graduated to millimetres. an english inch is equal to · millimetres. hence, inches on the english barometer scales correspond to millimetres on the french barometer scales. conversions from one scale to another can be effected by the following formulæ:-- ( ) inches = millimetres divided by · ( ) millimetres = inches multiplied by · of course, a table of equivalent values should be drawn up and employed, when a large number of observations are to be converted from one scale to the other. . in germany, barometers are sometimes graduated with old french inches and lines,--the vernier generally indicating the tenth of a line. old french lineal measure. english inches. douzième, or point = · points = ligne = · lignes = pouce = · pouces = pied = · pied = · millimetres. "the germans indicate inches by putting two accents after the number; lines, by putting three accents; " '''· , means inches lines hundredths of a line; more frequently, they give the height in lines, and the preceding number becomes '''· ."--_kaemtz._ . _rule for finding diameter of bore of a barometer tube._ "if the maker has not taken care to measure the interior diameter directly, it may be deduced from the exterior diameter. the exterior diameter is first measured by calipers, and, by deducting from this diameter · of an inch for tubes from · to · of an inch in external diameter, we have an approximation to the interior diameter of the tube."--_kaemtz._ . wind scales. sea scale. wind. land scale. ---------- ----- ----------- to = light = to " = moderate = " " = fresh = " " = strong = " " = heavy = " " = violent = " pressure in velocity in pounds (land scale). miles (avoirdupois) (hourly). ------------- --------- ----------- / = = = = = = = = = = = = . letters to denote the state of the weather. _b_ denotes blue sky, whether with clear or hazy atmosphere. _c_ " cloudy, that is detached opening clouds. _d_ " drizzling rain. _f_ " fog. _h_ " hail. _l_ " lightning. _m_ " misty, or hazy so as to interrupt the view. _o_ " overcast, gloomy, dull. _p_ " passing showers. _q_ " squally. _r_ " rain. _s_ " snow. _t_ " thunder. _u_ " ugly, threatening appearance of sky. _v_ " unusual visibility of distant objects. _w_ " wet, that is dew. a letter repeated denotes much, as _r r_, heavy rain; _f f_, dense fog; and a figure attached denotes duration in hours, as _r_, hours rain. by the combination of these letters, all the ordinary phenomena of the weather may be recorded with certainty and brevity. examples.--_b c_, blue sky with less proportion of cloud. _r r l l t_, heavy rain for two hours, with much lightning, and some thunder. the above methods of recording the force of wind and state of weather were originally proposed by admiral sir francis beaufort. they are now in general use at sea, and by many observers on land. . table of expansion by heat from ° to ° f. platinum · of the length. glass, flint · " " with lead · " brass · " mercury · " water · , from ° to ° alcohol · " ° to ° nitric acid · sulphuric acid · . table of specific gravity of bodies at ° f. except water, which is taken at °· . water · alcohol, pure · " proof · mercury · glass to · brass · to · platinum to · weight of a cubic foot of water, at the temperature of comparison, · lbs. avoirdupois. the pound avoirdupois contains , grains. air is · times lighter than water. the linear expansions are the mean values of the results of various experimentalists. the specific gravities are as given in professor rankine's _applied mechanics_. . important temperatures. under the circumstances of-- ° water boiling at mercury boils at sulphuric acid " oil of turpentine " nitric acid " alcohol " a saturated solution of salt " vital heat olive oil begins to solidify fresh water freezes sea water freezes mercury freezes - . table of meteorological elements, forming exponents of the climate of london. --------------------------------------------------------------------+ |mean height of barometer, reduced to ° f., at the mean | to | sea-level. | . | +--------------------------------------------------| | |mean monthly range of barometer. | months. | | +---------------------------------------------| | | |mean of all the highest temperatures. | | | | +----------------------------------------| | | | |mean of all the lowest temperatures. | | | | | +-----------------------------------| | | | | |mean temperature. | | | | | | +------------------------------| | | | | | |mean temperature of dew-point.| | | | | | | +-------------------------| | | | | | | |mean degree of humidity. | | | | | | | | +----------------------| | | | | | | | |mean number of rainy | | | | | | | | | days. | | | | | | | | | +------------------| | | | | | | | | |average rainfall. | | | | | | | | | | +-------------| | | | | | | | | | |average | | | | | | | | | | |amount of | | | | | | | | | | |cloud ( = | | | | | | | | | | | overcast). | | | | | | | | | | | +---------| | | | | | | | | | | |prevalent| | | | | | | | | | | |winds. | ---------+-------+----+----+----+----+----+--+---+----+---+---------+ |inches.| in.| ° | ° | ° | ° | | | in.| | | ---------+-------+----+----+----+----+----+--+---+----+---+---------+ january | · | · | · | · | · | · | | | · | · |w. to n. | | | | | | | | | | | | | february | · | · | · | · | · | · | | | · | · |s. to w. | | | | | | | | | | | | | march | · | · | · | · | · | · | | | · | · |n. to e. | | | | | | | | | | | | | | | | | | | | | | | | | april | · | · | · | · | · | · | | | · | · |n. to e. | | | | | | | | | | | | | may | · | · | · | · | · | · | | | · | · |s. to w. | | | | | | | | | | | | | june | · | · | · | · | · | · | | | · | · |w. to n. | | | | | | | | | | | | | july | · | · | · | · | · | · | | | · | · |w. to n. | | | | | | | | | | | | | august | · | · | · | · | · | · | | | · | · |w. to n. | | | | | | | | | | | | | september| · | · | · | · | · | · | | | · | · |s. to w. | | | | | | | | | | | | | october | · | · | · | · | · | · | | | · | · |s. to w. | | | | | | | | | | | | | november | · | · | · | · | · | · | | | · | · | s.w. | | | | | | | | | | | | | december | · | · | · | · | · | · | | | · | · | w. | | | | | | | | | | | | | ---------+-------+----+----+----+----+----+--+---+----+---+---------+ year | · | · | · | · | · | · | | | · | · | -- | ---------+-------+----+----+----+----+----+--+---+----+---+---------+ --------------------------------------------------------------------- +-----------------------------------------------------+ |sun above the horizon on middle day. | | +---------------------------------------------| | |remarks. | ---------+-------+---------------------------------------------| | hours.| | ---------+-------+---------------------------------------------| january | - / |the majority of the nights are frosty. | | | | february | | frosty nights on the average. | | | | march | | ditto ditto ditto. | | | strong winds. | | | | april | | ditto ditto ditto. | | | | may | - / |very rarely frost. | | | | june | - / |sun attains greatest north declination, st.| | | | july | | | | | | august | - / | | | | | september| - / | | | | | october | - / |a few frosty nights. heavy gales. | | | | november | | nights frosty. | | | | december | |sun attains greatest south declination, st.| | | | ---------+-------+---------------------------------------------| year | -- | | ---------+-------+---------------------------------------------| | ---------------------------------------------------------------+ in the above table, columns to are results obtained at the royal observatory, greenwich, by j. glaisher, esq., f.r.s. the data contained in columns and , are deduced from observations extending over the years to inclusive, and are copied from edward hughes' _third reading book_; the other columns are results of observations made during the twenty years ending . the rest of the information is from luke howard's _climate of london_. these valuable data indicate the characteristics of the weather in each month in the suburbs of london, and will be found tolerably accurate as indications of weather, and serviceable as standards for comparisons of observed results, at most places in england. standard works on meteorology supplied by negretti & zambra. the weather book: a manual of practical meteorology. by vice-admiral fitzroy, f.r.s., m.i.f., &c. _price_, £ the law of storms, by h. w. dove, f.r.s. translated by r. h. scott, m.a. _price_, £ l. f. kÆmtz's "complete course of meteorology," translated by c. v. walker, esq. _price_, £ practical meteorology, by john drew, ph.d., f.r.a.s. _price_, £ hygrometrical tables, adapted to the use of the wet and dry bulb thermometer, by james glaisher, esq., f.r.s. _price_, £ tables of the corrections for temperatures, to reduce observations to the ° fahrenheit, for barometers with brass scales extending from the cistern to the top of the mercurial column, by james glaisher, esq., f.r.s. _price_, £ table of the diurnal range of the barometer, by james glaisher, esq., f.r.s. _price_, £ tables for calculation of heights from observations on the boiling-point of water, adapted to the use of negretti and zambra's boiling-point apparatus. _price_, £ a thermometrical table, on the scales of fahrenheit, reaumur, and centigrade, by alfred s. taylor, esq., m.d., &c. _price_, in sheet, with explanatory pamphlet, £ meteorological tables, for the reduction of barometrical and hygrometrical observations, determination of heights by the barometer and boiling-point thermometer, &c. by g. harvey simmonds, m.b.m.s. _price_, £ barometer manual, compiled by vice-admiral fitzroy, f.r.s., for the board of trade. _price_, £ pocket meteorological register and note-book, with diagrams for exhibiting the fluctuations of barometer, &c. printed on metallic paper. _price_, with pencil, £ london: printed by strahan and williams, lawrence land, cheapside, e.c. negretti & zambra's patent recording and deep-sea thermometer.[ ] this thermometer differs from all other registering or recording thermometers in the following important particulars:-- i. the thermometer contains only mercury without any admixture of alcohol or other fluid. ii. it has no indices or springs, and its indications are by the column of mercury only. iii. it can be carried in any position, and cannot possibly be put out of order except by actual breakage of the instrument. and lastly, it will indicate and record the exact temperature at any hour of the day or night, or the exact temperature at any depth of the sea, irrespective of either warm or cold currents, or stratum through which the thermometer may have to pass in its descent or ascent, this last very special quality renders this thermometer superior for deep-sea temperatures to any others; for those now being used in the "challenger" sounding expedition are liable to give erroneous indications owing to their indices slipping, and otherwise getting deranged--(this was proved by messrs. negretti and zambra at a meeting of the british meteorological society,) and _under certain conditions of temperature_ it is not possible by the old thermometers to obtain true temperatures at certain depths which might be required. _annexed is a copy of a report to the admiralty from captain g. s. nares, of h.m.s. "challenger," dated melbourne, march th, , which we have taken from nature, july th, , proving the assertion._ "in the report to the admiralty of capt. g. s. nares, of h.m.s. _challenger_ dated melbourne, march , , capt. nares, speaking of the temperature of the ocean, especially near the pack edge of the ice, says:--'at a short distance from the pack, the surface water rose to °, but at a depth of fathoms we always found the temperature to be °; this continued to fathoms, the depth in which most of the icebergs float, after which there is a stratum of slightly warmer water of ° or °. as the thermometers had to pass through these two belts of water before reaching the bottom, the indices registered those temperatures, and it was impossible to obtain the exact temperature of the bottom whilst near the ice, but the observations made in lower latitudes show that it is about °. more exact results could not have been obtained even had mr. siemens's apparatus been on board.' it seems to us that the difficulty mentioned is one which would certainly have been surmounted by messrs. negretti and zambra's new recording thermometers, a description of which appeared in nature, vol. ix. p. ; this being exactly one of the cases to which this instrument is peculiarly adapted. we believe the inventors and makers have greatly improved their thermometer since our description appeared, and no doubt means will be taken by the admiralty to transmit one to the _challenger_." description of the deep-sea recording thermometer. in the first place, it must be observed that the bulb of the thermometer is protected so as to resist the pressure of the ocean, which varies according to depth that of three thousand fathoms being something like three tons pressure on the square inch. the manner of protecting the bulb was invented by messrs. negretti and zambra in , and has been latterly copied by other persons and brought out as a new invention. the manner of protecting the bulb has been described by the late admiral r. fitzroy, in the first number of meteorological papers, page , published july th, , as follows: "referring to the erroneous readings of all thermometers, consequent on their delicate bulbs being compressed by the great pressure of the ocean, he says:--'with a view to obviate this failing, messrs. negretti and zambra undertook to make a case for the weak bulbs, which should transmit temperature, but resist pressure. accordingly a tube of thick glass is sealed outside the delicate bulb, between which and the casing is a space all round, which is nearly filled with mercury. the small space not so filled is a vacuum, into which the mercury can be expanded, or forced by heat or mechanical compression, without doing injury to or even compressing the inner or much more delicate bulb.'" [illustration: fig. .] the construction of this instrument for deep-sea temperatures is as follows:-- in shape it is like a syphon with parallel legs, all in one piece and having a continuous communication, as in the annexed figure. the scale of the thermometer is pivoted on a centre and being attached in a perpendicular position to a simple apparatus (which will be presently described), is lowered to any depth that may be desired. in its descent the thermometer acts as an ordinary instrument, the mercury rising or falling according to the temperature of the stratum through which it passes; but so soon as the descent ceases, and a reverse motion is given to the line, so as to pull the thermometer towards the surface, the instrument turns once on its centre, first bulb uppermost, and afterwards bulb downwards. this causes the mercury, which was in the left-hand column, first to pass into the dilated syphon bend at the top, and thence into the right-hand tube, where it remains, indicating on a graduated scale the exact temperature at the time it was turned over. the woodcut, fig. , shows the position of the mercury _after_ the instrument has been thus turned on its centre. a is the bulb; b the outer coating or protecting cylinder; c is the space of rarefied air, which is reduced if the outer casing be compressed; d is a small glass plug on the principle of negretti and zambra's patent maximum thermometer, which cuts off, in the moment of turning, the mercury in the tube from that of the bulb, thereby ensuring that none but the mercury in the tube can be transferred into the indicating column; e is an enlargement made in the bend so as to enable the mercury to pass quickly from one tube to another in revolving; and f is the indicating tube or thermometer proper. in its action, as soon as the thermometer is put in motion, and immediately the tube has acquired a slightly oblique position, the mercury breaks off at the point d, runs into the curved and enlarged portion e, and eventually falls into the tube f when this tube resumes its original perpendicular position. the contrivance for turning the thermometer over may be described as a frame with a vertical propeller; to this frame the instrument is pivoted. on its descent through the water the propeller is lifted out of gear and revolves freely on its axis; but so soon as the instrument is pulled towards the surface the propeller falls into gear and revolves in the contrary direction, turning the thermometer over once, and then becoming locked and immovable. _directions for adjusting the thermometer previous to its being lowered in the sea._ i. the mercury must all be in the left-hand column. ii. the short peg at the back of the thermometer must be in front of the stop plate s +; in order to effect this, pull the knob which stops the thermometer, and slightly turn the propeller, to make the thermometer advance sufficiently to escape the stop plate. negretti & zambra's patent atmospheric recording thermometer, fig. , differs from the deep-sea thermometer by its not having the double or protected bulb, it not being required for resisting pressures. in this case the instrument is turned over by a simple clock movement, which can be set to any hour it may be desirable; the thermometer is fixed on the clock, and when the hand arrives at the hour determined upon, and to which the clock is set as in setting an alarum clock, a spring is released and the thermometer turns over as before described. [illustration: fig. .] [illustration: fig. .] messrs. negretti and zambra have arranged a wet and dry bulb hygrometer upon the same plan. negretti & zambra's price list of standard meteorological and other philosophical instruments. _the marginal figures in this list and the numbers of the wood engravings refer to paragraphs in "negretti & zambra's treatise on meteorological instruments."_ £ s. d. =standard barometers=, fortin's arrangement, as fig. with mahogany board ditto ditto with millemetre and english scales ditto ditto with tube, · internal diameter and millimetre scale =observatory standard barometers=, extra large tubes and cisterns £ ditto ditto arranged for observations being taken by the cathetometer =cathetometer=, for use with above =self compensating standard barometer=, fig. =standard barometer=, with electrical adjustment =pediment barometers=, fig £ ditto ditto fig. £ ditto ditto fig. £ ditto ditto fig. ditto ditto ditto handsome carved mountings, in mahogany, oak, or walnut wood £ £ =fitzroy's storm or sea coast fishery barometer=, fig. ditto ditto with two verniers ditto ditto mounted in ornamental carved frames, oak, walnut, or mahogany £ =marine barometers=, ordinary forms, figs. and £ s. £ s. ditto ditto best mounted £ s. =the board of trade or kew marine barometer=, fig. , £ s. £ s. =negretti and zambra's fitzroy marine or gun barometer=, fig. , with n. and z.'s patent porcelain scales, as used in her majesty's navy extra tube for ditto =negretti and zambra's farmer's barometer or domestic weather glass=, fig. =negretti and zambra's miner's barometers= £ s. £ s. =dial or wheel barometers=, figs. , , , £ s. £ s. ditto ditto in carved ornamental mountings £ s. £ s. £ s. ditto ditto rosewood, inlaid with pearl or metal. made to order, figs. and . price varying with size, &c. =gay lussac's syphon tube mountain barometer= £ =standard syphon barometer=, gay lussac's arrangement, fig. =negretti and zambra's standard mountain barometer=, with fortin's cistern, with tripod stand and travelling case, fig. =barograph, or self-registering barometer=, with syphon mercurial tube. negretti and zambra's improved arrangement, fig. =negretti and zambra's self-recording aneroid barometer=, with =clock= . =aneroid barometers.= =aneroid barometers,= with card dials - / inches diameter, best quality. ditto ditto with silvered metal dial ditto ditto with ditto and thermometer ditto ditto ditto with corrected scale, as supplied by negretti and zambra to the royal navy =aneroid barometers=, with elegantly-chased dials ditto ditto with raised ring on dial ditto ditto ditto with thermometer =aneroid barometer=, for altitude measurements with revolving ring, carrying index, range of scale , feet - / inches diameter, with magnifier =pocket aneroid barometers.= fig. . =pocket aneroid barometer=, - / inches diameter, with silvered metal scale ditto ditto for measuring altitudes to , feet compensated for temperature, in leather case ditto ditto ditto to , feet, with magnifier =watch-size aneroid barometers= in gilt metal cases (see figure .) =watch-size aneroid barometer=, weather range £ ditto ditto of best construction, extra thin, for meteorological observations or altitude measurements to , feet ditto ditto ditto to , feet, compensated for temperature either of the above watch-size barometers may be had in stout silver cases at a cost of £ s. extra _watch-size aneroid barometers in solid gold, highly-finished cases. £ s. to £ ._ table stands for aneroid barometers of carved oak or other woods, s. d., s., s., to £ s. =ships' aneroid barometers=, in suitable mountings £ s. £ s. £ s. £ s. =sympiesometer=, for ship use £ ditto ditto pocket form, fig. the sympiesometer is now rarely used, the aneroid barometer being found equally sensitive and less liable to derangement. =independent standard thermometers=, fig. =standard thermometers=, for boiling point apparatus =chemists' or brewers' standard reference thermometers= £ s. =chemical thermometers=, divided on the stem. fig. . s. d. =chemical thermometers=, boxwood scale s. d. s. d. =thermometers on boxwood scales=, fig. . s. s. d. s. d. s. d. ditto ditto larger sizes s. d. ditto ditto engraved glass scales, fig. s. £ s. £ s. =pocket thermometers, in various mountings.= fig. s. d. fig. s. d. s. fig. s. d. s. d. s. d. =thermometers of extreme sensitiveness=, s. £ s. =drawing room or mantel thermometers=, various mountings, figs. and . s. d. s. £ s. £ s. £ s. =bracket window thermometers=, fig. s. d. s. £ s. =bath thermometers=, figs. s. d. =sugar boiling thermometers= £ s. £ s. =earth thermometers=, fig. £ s. hot bed ditto s. d. £ s. =marine thermometer=, fig. s. d. s. d. =super heated or steam pressure thermometers=, fig. , figs. and £ s. £ s. £ s. =self-registering thermometers for heat.= =negretti and zambra's patent standard maximum self-registering thermometer=,[ ] fig. =negretti and zambra's patent maximum thermometer=, on boxwood scale ditto, ditto, on negretti and zambra's patent solid porcelain or metal scales on oak mounting =rutherford's maximum thermometer=, on boxwood or metal scale, with steel index s. d. s. d. =phillip's maximum thermometer=, on boxwood or metal scale, with air index s. d. =self-registering thermometers for cold.= =negretti and zambra's standard minimum self-registering thermometer=, fig. =rutherford's minimum thermometer=, on boxwood or metal scale s. d. s. d. s. d. =rutherford's minimum thermometer=, on negretti and zambra's patent solid porcelain scale s. d. ditto, on negretti and zambra's porcelain or metal scales and oak mounting =negretti and zambra's horticultural self-registering thermometer.= the scale is made of stout zinc, enclosing the tube; the figures and divisions are boldly marked for quickly and easily reading the indications, fig. =negretti and zambra's patent solar radiation thermometer=, fig. ditto, ditto, ditto, in vacuo, fig. ditto, ditto, ditto, improved form, with test gauge =negretti and zambra's terrestrial radiation thermometer= brass stands for above, fig. and =negretti and zambra's patent mercurial minimum thermometers= £ s. =maxima and minima thermometers=, on sixe's arrangement, fig. , various forms of mounting s. d. s. s. s. =pocket maxima and minima thermometers=, negretti and zambra's patent, in convenient cases £ s. =deep sea registering thermometer=, with negretti and zambra's improved protected bulb, in copper cylinder, fig. . [ ] =negretti and zambra's improved deep sea thermometer=, with vulcanite mountings, in copper cylinder, with door, small size =negretti and zambra's patent recording deep sea thermometer= ditto, ditto, ditto =recording thermometer= ditto, ditto, ditto =hygrometer= =improved boiling point mountain thermometer=, or hypsometric apparatus, with tables, figs. and , in leather case with strap extra thermometer for ditto =negretti and zambra's standard wet and dry bulb hygrometer=, fig. wet and dry bulb hygrometers, various mountings s. s. s. s. pocket hygrometers, in box £ s. =daniell's hygrometer=, fig. =regnault's hygrometer=, fig. £ s. aspirator for ditto £ s. =howard's rain gauge=, has a -inch copper funnel, with turned brass rim fitted to a stout stone-ware or glass bottle, with a graduated glass measure, divided to ths of an inch =symons' portable rain gauge=, ( -inch) with graduated glass measure, japanned tin ditto ditto in stout copper =glaisher's rain gauge=, the receiving surface is -inches diameter, of stout japanned metal, with graduated glass measure, fig. ditto ditto, of stout copper receiving pots for ditto, extra s. and s. d. =rain gauge=, having a receiving surface of inches diameter, and graduated glass gauge tube, divided to hundredths of an inch, in japanned metal, with brass tap ditto ditto, fig. , in copper ditto ditto, with sliding rod instead of graduated tube, japanned tin =rain gauges=, of any form or area made to order, with suitable measuring glasses. =lind's anemometer=, fig. =robinson's anemometer=, fig. ditto ditto, improved arrangement £ s. ditto ditto, with clutch movement, fig. =negretti and zambra's improved air meter=, of extreme sensitiveness, very portable large air meters made to order. =osler's self-registering anemometer and rain gauge=, fig. £ to =berkley's anemometers= fitted up to order, _to suit the observatory_. =gold leaf electrometer=, fig. =peltier's electrometer= =bohnenberger's electroscope=, fig. =thompson's electrometer=, to order =lightning conductors= fitted up to order. =ozone cage=, fig. ditto ditto, copper =leslie's differential thermometer=, fig. £ s. =thermometer stand (glaisher's)= =thermometer screen= for sea use =anemoscope=, or portable vane, fig. =evaporating dish=, fig. =sea water hydrometers=, board of trade marine, figs. and =newman's self-registering tide gauge=, fig. , fitted to the building to order from _further information as to price, &c., will be found in_ negretti & zambra's encyclopÆdic catalogue of mathematical, philosophical, optical, photographic, and standard meteorological instruments, _containing very numerous comparative tables of reference, and illustrated by upwards of_ eleven hundred engravings. royal vo. cloth, gilt lettered--price s. d. footnotes: [ ] second number of "meteorological papers," issued by the board of trade. [ ] with reference to these barometers, we have received the subjoined testimonial, with permission to use it as we please. "_meteorologic office, th june, ._ "messrs. negretti & zambra, "the barometers which you have lately supplied to her majesty's ships through this office are much approved, being good for general service, afloat or on land. "(signed) r. fitzroy." [ ] _vide_ c. daubeny, f.r.s., "on climate." [ ] _vide_ report of the british association, . [ ] see page for the tables. [ ] the quotations in this section are from tyndall's _heat considered as a mode of motion_. [ ] dr. daubeny, f.r.s., _on climate_. [ ] leslie _on the relations of air, heat, and moisture_. [ ] tyndall's _heat considered as a mode of motion_. [ ] vide _horological journal_, vol. v. [ ] _hygrometrical tables_, by j. glaisher, esq., f.r.s. [ ] vide _report of the british association_, . it may be added, for the information of those who are about to commence observing, that mr. symons, of camden road villas, london, is desirous of securing returns of rain-fall from as many stations as possible, in order to render more complete his annual reports to the british association. [ ] luke howard's _climate of london_. [ ] vide _third number of meteorological papers_, issued by the board of trade. [ ] _elements of physics_, by c. f. peschel. [ ] this description is modified from that in report of the jurors for class xiii. international exhibition, . [ ] _all the year round_, no. . [ ] _all the year round_, no. . [ ] vide _jurors' reports_. [ ] see also page of this treatise. [ ] these instruments are the only maximum thermometers that can be recommended, as unless they be broken, they cannot be put out of adjustment. fully described under the head of standard maximum thermometers in our large catalogue, and page of our _treatise on meteorological instruments_.